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ZnMn2O4 applications in batteries and

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Cite this: DOI: 10.1039/d5ta00815h

supercapacitors: a comprehensive review
Open Access Article. Published on 02 April 2025. Downloaded on 4/28/2025 6:48:17 AM.

Joel Kingston Ramesh,†a Sasan Rostami,†b Jayaprakasan Rajesh,a

R. Margrate Bhackiyavathi Princess,d Radhika Govindaraju,e Jinho Kim,c
Rainer Adelung, fg Palanisamy Rajkumar *c and Mozaffar Abdollahifar *fg

ZnMn2O4 (ZMO) has emerged as a promising material for energy storage applications due to its high
theoretical capacity, low cost, and environmental friendliness. This review comprehensively explores the
structure, synthesis methods, and performance of ZMO in various energy storage systems, including
supercapacitors and batteries such as lithium-ion (LIBs), sodium-ion (SIBs) and zinc-ion (ZIBs) batteries,
due to its exceptional electrochemical properties. The influence of various synthesis techniques on the
structural and morphological features of ZMO, which directly impact its electrochemical performance
will be discussed. The review also delves into the charge storage mechanism of ZMO in supercapacitors,
examining the effects of morphology, composites, and doping on its performance. Additionally, the use
of ZMO as an anode material for LIBs and SIBs and its potential as a cathode material in ZIBs are
Received 30th January 2025
Accepted 31st March 2025
discussed. The review also addresses key challenges and proposes strategies to enhance performance
including incorporating conductive materials, synergizing with other materials, and doping. An outlook
DOI: 10.1039/d5ta00815h
on the current challenges, future directions, and potential pathways for performance enhancement is
rsc.li/materials-a also presented.

a
Department of Chemistry, Indian Institute of Technology Madras, Chennai, 600036, e
Department of Physics, Rajalakshmi Institute of Technology, Chennai, 600 124,
Tamil Nadu, India Tamil Nadu, India
b
Department of Physics and Energy Engineering, Amirkabir University of Technology f
Chair for Functional Nanomaterials, Department of Materials Science, Faculty of
(Tehran Polytechnique), Tehran, Iran Engineering, Kiel University, Kaiserstr. 2, 24143, Kiel, Germany. E-mail: moza@tf.
c
Department of Mechanical Engineering, Yeungnam University, Gyeongsan-si, uni-kiel.de
Gyeongbukdo 38541, Republic of Korea. E-mail: rajkumar@yu.ac.kr g
Kiel Nano, Surface and Interface Science (KiNSIS), Kiel University, Germany
d
Department of Chemistry, Lady Doak College, Tallakulam, Madurai, 625002, Tamil † These authors have contributed equally to this work.
Nadu, India

Joel Kingston Ramesh completed Sasan Rostami received his

his Bachelor of Science in Master's degree in Condensed
Physics and Master of Science in Matter Physics from Amirkabir
Physics from The American University of Technology in
College, Tamil Nadu, India. Joel 2020, focusing on fabricating
is serving as a Project Associate electrodes based on two-
at the Indian Institute of Tech- dimensional materials for
nology Madras (IIT-Madras), lithium-ion batteries. Aer
where he contributed to graduation, he continued his
research in materials science. He research at the same university,
also worked as a Research specializing in polymer-based
Assistant at the Photovoltaic solid-state batteries. In 2023,
Laboratory at SSN Institute, he began collaborating with Kiel
Joel Kingston Ramesh Sasan Rostami
Chennai (in 2021), and partici- University (Germany) on some
pated in a Summer Internship at the IIT-Madras (in 2019). His joint publications. His research interests include synthesizing and
scientic research focuses on solar cells and energy storage mate- modifying various materials and optimizing their structures,
rials, particularly Li-ion, Na-ion, Li–sulfur and solid-state morphologies, and properties for use in battery anodes and
batteries. cathodes.

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economically viable, environmentally friendly, and capable of

1 Introduction meeting the evolving needs of our energy-intensive society.6,7
Electrochemical energy storage (EES) systems,1 namely, EES systems utilize a range of electrode materials, including
batteries, supercapacitors, and fuel cells have emerged as the transition metal oxides (TMOs),8 carbon-based materials, con-
primary means of storing energy.2 Electrode performance is ducting polymers, metal suldes/selenides, metal nitrides/
vital for these systems, making electrode materials a key focus phosphides, metal–organic frameworks (MOFs), and hybrid
of global research and development initiatives.3 Researchers are composite materials. Among these materials, TMOs have
actively exploring new electrode materials with key character- garnered signicant interest due to their diverse and tunable
istics, including low production cost, improved specic properties, contributing to the development of high-
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capacity, extended cycle life and environmentally benign prop- performance energy storage devices.9,10 Composed of transi-
tion metal cations and oxygen anions, these compounds exhibit
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erties. Pursuing such materials is fueled by the desire to

enhance energy storage technologies, serve the increasing a wide range of structural motifs, electronic congurations, and
demand for efficient and sustainable energy storage, and pave redox activities; making them versatile candidates for various
the way for the widespread implementation of renewable energy applications.11 TMOs oen exhibit variable oxidation states,
sources. Achieving breakthroughs in electrode materials holds allowing for redox reactions and high catalytic activity. This
tremendous potential for revolutionizing energy storage and property makes them ideal catalysts in various chemical reac-
advancing various applications, ranging from portable elec- tions, where they can enhance reaction rates.12–15 Moreover, the
tronics to electric vehicles and grid-scale energy storage.4,5 By redox behavior and stability of TMOs make them excellent
addressing the limitations of current electrode materials, candidates for energy storage and conversion devices, such as
researchers aim to usher in a new era of energy storage that is lithium (Li)-ion batteries (LIBs)10 and fuel cells. To highlight

Dr Govindaraju Radhika is an Dr Palanisamy Rajkumar is

Assistant Professor at the currently working as an Inter-
Department of Physics at Raja- national Research Professor in
lakshmi Institute of Technology, the Department of Mechanical
India. She earned her PhD from Engineering, Yeungnam Univer-
Alagappa University, Karaikudi, sity, South Korea. He acquired
India. Her research focuses on his PhD degree at Alagappa
creating electrocatalysts for University, Karaikudi, India. He
advanced energy storage has completed his post-doctoral
devices, such as batteries and research at Kunsan National
supercapacitors. University, South Korea. His
research interests are focused on
development of electrocatalysts
Radhika Govindaraju Palanisamy Rajkumar
for next-generation energy
storage devices particularly batteries, supercapacitors, fuel cells
and water splitting devices.

Dr Rainer Adelung is a Professor Dr Mozaffar Abdollahifar

and holder of the Chair of received his doctorate in chem-
Functional Nanomaterials at the ical engineering, focused on
Department of Materials Science energy storage materials, from
at the Faculty of Engineering at National Taiwan University
Kiel University (Germany). He (NTU) in 2018. Before becoming
specializes in various nano- a battery group leader at Kiel
structures, mainly in the University (Germany), he
synthesis and design of porous worked as a scientist for several
materials. Applications range years at NTU and then at the
from energy technology such as Battery LabFactory Braunsch-
batteries and supercapacitors to weig (BLB, TU Braunschweig,
sensor devices, antiviral agents Germany). He is interested in
Rainer Adelung Mozaffar Abdollahifar
and advanced adhesion tech- developing supercapacitors and
nology in engineering. battery materials for Li, Na and sulfur chemistries, anode-free
batteries, and electrode engineering, as well as recycling end-of-
life batteries.

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a few examples, lithium cobalt oxide (LiCoO2) serves as the most battery applications of ZMO in detail. In the supercapacitor
successful commercial cathode material in LIBs, providing high section, we will discuss the charge storage mechanism, and
energy density and exceptional cycling stability.16 Similarly, effects of morphology, composites and doping on ZMO perfor-
TMOs such as MnO2, Mn2O3, and Mn3O4 are well-studied mance, while in the battery section, we will focus on ZMO as an
materials for supercapacitors.17–19 However, the main issues anode material for LIBs and SIBs, as well as a cathode material
preventing the commercialization of supercapacitors based on in ZIBs. Finally, the review will present an outlook on the
Mn are their low electrical conductivity (10−7–10−8 S cm−2), current challenges and future directions in the eld of ZMO-
poor cyclability, and inferior rate capability.19 By creating based electrode materials for energy storage applications. We
composites of manganese-based materials with various con- will discuss potential pathways for performance enhancement,
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ducting materials such as graphene, activated carbon, carbon integration with other materials, and the future prospects of
nanotubes, and polymers, numerous attempts have been made ZMO in emerging energy storage technologies.
Open Access Article. Published on 02 April 2025. Downloaded on 4/28/2025 6:48:17 AM.

to overcome these problems. Apart from this, substituting one

manganese (Mn) cation from a tetrahedral or octahedral loca-
tion in the Mn oxide spinel structures with other metal cations 2 Evolution of ZMO synthesis and
can further improve the electrochemical performance of applications in energy storage
Mn3O4.20 These Mn oxide substituted metal cations may func-
tion as a buffer matrix to absorb strain and stress brought on by Fig. 1 provides a comprehensive overview of the historical
prolonged cycling. Therefore, it will be advantageous to development and current trends in ZMO synthesis and its
substitute one Mn in Mn3O4 with cations such as nickel (Ni), applications in energy storage devices. This gure is divided
zinc (Zn), cobalt (Co), etc.21 into three sections: Fig. 1(a) highlights the rapid growth in ZMO
Mn-based ternary metal oxides (e.g., ZnMn2O4 (ZMO), research in recent years, as evidenced by the increasing number
LiMn2O4, NiMn2O4, MnCo2O4, CuMn2O4, and FeMn2O4) have of publications (from a few papers before 2000 to over 1100
attracted signicant attention for diverse applications such as published reports in 2023 and 2024 alone). Fig. 1(b) illustrates
catalysis, gas sensing, photodetection, and energy storage.20,22–24 the current distribution of ZMO applications, highlighting the
For supercapacitors, mixed TMOs are particularly promising dominance of LIBs which account for 55% of the total. Super-
due to their enhanced redox activity and high electrical capacitors and SIBs represent smaller portions, with 16% and
conductivity which contribute to superior capacitive perfor- a minor share, respectively. Fig. 1(c) traces the evolution of ZMO
mance.25 According to the Shanghai Metals Market (SMM), by synthesis methods, starting from the rst report in 1965.27 It
December 2024, the market prices for Mn, Zn, Ni, and Co will be showcases the rise of hydrothermal (HT) synthesis as the
approximately 2000, 2800, 9000, and 60 000 USD per metric ton, primary method (Fig. 1(d)) by 2007.28 Alongside this, the
respectively.26 It is clear that Mn and Zn are much more applications of ZMO expanded from fundamental structural
affordable than Co and Ni. From a cost-performance perspec- studies to photocatalysis,29 semiconductors,30 and ultimately
tive, the combination of Mn and Zn in ZMO presents energy storage.31,32
a compelling option for supercapacitor and battery applica- Fig. 1(d) details the specic synthesis techniques, high-
tions. Building upon its cost-effectiveness and promising lighting the major contribution of HT, which is employed for
properties, among various ternary metal oxides, ZMO stands various applications. Furthermore, recent research explores
out due to its unique combination of high theoretical capacity, ZMO with anionic S-doping and oxygen vacancies for ZIBs.33,34
structural stability, and cost-effectiveness. Its spinel crystal Overall, Fig. 1 demonstrates a clear trend towards diversica-
structure provides robust mechanical integrity, which helps tion in both ZMO synthesis methods and applications.
accommodate volume expansion during cycling,20 a critical Although LIBs remain the dominant application, the growing
challenge for many transition metal oxides. Additionally, ZMO interest in utilizing ZMO for supercapacitors, SIBs and ZIBs is
benets from the synergistic redox activity of both Mn and Zn; driving the development of new synthesis approaches and
offering enhanced charge storage capability.25 In LIBs and material modications to optimize performance in these
sodium-ion batteries (SIBs), ZMO operates through a conver- emerging energy storage technologies.
sion–intercalation mechanism, enabling high lithium/sodium The diverse applications of ZMO in energy storage stem from
storage capacity. As a Zn-ion battery (ZIB) cathode, its ability its unique structural characteristics, which are explored in the
to reversibly intercalate Zn2+ ions in aqueous electrolytes following section.
ensures stable cycling performance. Furthermore, ZMO exhibits
relatively high electronic conductivity compared to other 3 ZMO structure and properties
manganese-based oxides, facilitating improved charge trans-
port.25 These attributes make ZMO a highly versatile electrode Spinel compounds, represented by the general formula AB2O4,
material, capable of delivering high capacity, long cycle life, and form a notable category of mixed metal oxides. In these struc-
improved rate performance across multiple energy storage tures, the divalent cation (A2+) can include Mn, Ni, Zn, Co, iron
systems. (Fe), or similar elements, while the trivalent cation (B3+) oen
Given the promising attributes of ZMO highlighted above, includes Mn, Co, Fe, or others. A spinel unit cell consists of 56
this review will explore the structure, properties, synthesis ions: 24 metal ions and 32 oxygen ions, corresponding to
methods, and energy storage, including supercapacitor and a general formula of A8B16O32, which is equivalent to 8 AB2O4

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Fig. 1 Overview of ZMO research and applications: (a) number of publications vs. year from 1960 to 2024 (obtained from Google Scholar using
the keyword “ZnMn2O4”); (b) distribution of ZMO applications across energy storage systems, showing the percentage contribution to super-
capacitors, lithium-ion batteries (LIBs), sodium-ion batteries (SIBs), and zinc-ion batteries (ZIBs); (c) timeline highlighting the development of
ZMO, including advancements in morphological evolution, and applications in energy storage devices; (d) distribution of synthesis methods for
ZMO, illustrating the percentage contribution of various techniques to its fabrication.

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units. Spinel ZMO possesses a tetragonal structure with the The degree of inversion in ZMO spinel structures can vary,
bivalent Zn2+ occupying the tetrahedral sites and the trivalent described by using the formula (Zn1−xMnx)(ZnxMn2−x)O4,
Mn3+ occupying the octahedral sites in the spinel structure with where 0 # x # 1. This parameter (x) denes the extent of
space group I41/amd (Fig. 2(a)). Oxygen ions create two types of inversion, which depends on factors such as particle size,
vacancies in the structure: a tetrahedral vacancy with a [ZnO4] morphology, and synthesis method. In a normal spinel, Zn
unit and an octahedral vacancy with a [MnO6] unit; the octa- occupies the A sites and Mn occupies the B sites, while in an
hedrally coordinated Mn3+ exhibits a strong Jahn–Teller inverse spinel, half of the Mn occupies the A sites and all Zn
distortion effect. resides in the octahedral B sites. The partially inversed structure
is described using the x-parameter to indicate the occupancy of
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Fig. 2 ZMO characterization: (a) illustration of the crystal structure, (b) crystal structure details based on JCPDS No. 24-1133, and (c) XRD pattern
with the reference JCPDS No. 24-1133. Reproduced from ref. 35 with permission from ACS, copyright 2023. (d) XPS spectra showing Zn 2p, Mn
2p, and O 1s peaks. Reproduced from ref. 36 with permission from ACS, copyright 2020. (e) Raman spectrum. Reproduced from ref. 37 with
permission from ACS, copyright 2013. (f) XANES spectra of ZMO and LiMn2O4 with tetragonal (TLMO) and cubic (CLMO) structures. Reproduced
from ref. 38 with permission from Elsevier, copyright 2019. (g) FTIR spectra. Reproduced from ref. 39 with permission from ACS, copyright 2018.

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Mn on A sites.40–42 Three JCPDS reference patterns are typically techniques. When water is used as the solvent, the process is
used for ZMO (24-1133, 01-071-2499, and 01-077-0470). As referred to as HT, while the use of other solvents (like ethanol or
a reference, the data for JCPDS No. 24-1133 are shown in methanol) classies it as ST. Autoclaves, commonly used in this
Fig. 2(b). Additionally, characteristic tetragonal ZMO features synthesis, have applications in elds such as microbiology,
are illustrated with XRD patterns, XPS, FTIR, XANES, and geothermal studies, and interdisciplinary scientic
Raman spectra in Fig. 2(c–g). Spinel ZMO exhibits intriguing research.45–47 These methods offer scalability for industrial
electronic, magnetic, and transport properties, making it highly production, enabling control over reaction stoichiometry, low
suitable for a wide range of applications, particularly in sensors aggregation, and producing highly pure powders.48,49 HT/ST
and energy storage. These compelling properties have drawn synthesis also allows for the precise manipulation of size
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considerable attention from the scientic community. distribution, shape, morphology, and chemical reactions,
The interaction between metal cations in ZMO inuences giving researchers the exibility to tailor metal oxide particles
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the electronic structure and redox behavior, enabling multiple for specic applications by controlling precursor composition
redox reactions and increased charge storage capacity when and reaction conditions, along with the use of seed
used as an energy storage material.43 Furthermore, the lattice templates.50–52
distortion induced by incorporating two metal ions modies A variety of morphologies of ZMO are achievable via HT/ST
ion diffusion pathways and inuences the electrochemical synthesis such as nanoparticles, nanorods, nanowires, nano-
kinetics, ultimately impacting the material's performance as an sheets, nanobers, hollow microspheres, nanoplates, ake-
electrode in energy storage devices.44 shaped, loaf-like structures and hierarchical structures. All
The synthesis of bimetallic TMOs necessitates precise these morphologies with varying surface areas, pore width
control over the composition, stoichiometry, and crystal struc- distribution, pore volume, and hierarchy of the porous system
ture to achieve desired properties. Various techniques, such as directly impact the electrochemical device performance. For
solid-state reactions, hydrothermal (HT), solvothermal (ST), example, porosity not only increases the surface area for elec-
sol–gel, and chemical vapor deposition (CVD) have been trode–electrolyte reactions, but also alleviates the mechanical
employed to fabricate tailored ternary oxides. These synthesis stress by accommodating the volume variation from the
methods allow the manipulation of metal–metal interactions repeated cycling.53
and the creation of heterostructures, resulting in improved 4.1.1 Hydrothermal (HT) synthesis. Before delving into
properties for a particular application. In the following section, more complex HT synthesis routes, we will rst examine
we discuss the inuence of synthesis methods and corre- a simple example of HT synthesis for obtaining nanostructured
sponding parameters (e.g., calcination temperature and ZMO. Park et al.54 synthesized ZMO by heating an autoclave
surfactant) on the structural and morphological properties of containing a vigorously stirred precursor solution consisting of
ZMO. KMnO4, KMnO4, and Zn(NO3)2. The chemical reaction inside
the autoclave with the precursor solution can be described as
follows:
4 Methods for ZMO fabrication
MnSO4 + 2KMnO4 / 3MnO2 + K2SO4 + O2 (1)
In this section, we introduce various synthesis methods utilized
for ZMO fabrication for supercapacitor and battery applica- Zn(NO3)2 / ZnO + NO2 + NO + O2 (2)
tions. Based on reported studies, the HT and ST methods are
predominantly employed due to their ability to provide better MnO2 + NO / MnO + NO2 (3)
control over particle size, morphology, and crystallinity. Other
techniques, such as co-precipitation (PC), sol–gel, and 2ZnO + 2MnO + O2 / 2ZnMnO3 (4)
microwave-assisted synthesis, are also used. These methods
enable precise control over the structural and morphological ZnMnO3 + MnO / ZnMn2O4 (5)
features of ZMO, which directly impact its electrochemical
performance. For example, calcination temperature, reaction Another common precursor other than KMnO4 for Mn in the
time, and surfactant selection are critical parameters that HT/ST route is Mn nitrate. Usually, the precursor metal nitrate
inuence the nal properties of ZMO. The following section solution decomposes upon heating, reacting with oxygen
details the inuence of these synthesis techniques and param- present in the system to form metal oxides and other byprod-
eters on the material's performance. ucts such as nitrogen oxides. Ren et al.55 uniquely synthesized
ZMO with rugby ball morphology by adding NH4F (shaping the
material) as an additive to the precursor solution before heating
4.1 Hydrothermal (HT) and solvothermal (ST) synthesis the autoclave. NH4F controls the release of carbonate ions,
HT/ST synthesis methods are one-step processes that offer inuencing how the material forms. When heated, NH4F played
several advantages, such as environmental friendliness, effec- a major role in how the material adhered to the substrate. By
tive solution dispersion, and the ability to operate under high changing factors such as surfactants and additives, they ob-
pressure. These methods are particularly appealing due to their tained a variety of structures and composites through the HT
mild operating conditions (typically under 200 °C), and they can process. For instance, Zhang et al.56 synthesized porous ZMO
be cost-effective compared to other solution-based synthesis nanowires by using a different alkaline agent. This agent

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reacted with zinc and manganese ions to form complex sources such as egg whites and peanut shells to create similar
compounds, which then formed long-chain polymers. These composites. For example, Lin et al.60 used milk as a carbon source.
polymers self-assembled into nanowires, which were then They added milk to the precursor solution and applied heat,
heated to create porous ZMO nanowires. However, the resulting creating a carbon matrix with embedded ZMO nanoparticles.
material had a low surface area, 15.8 m2 g−1. Zhu et al.56 showed Other efforts for synthesizing modied ZMO were conducted
how different surfactants could lead to different shapes even by various researchers using HT methods. Wang et al.61 synthe-
when using the same starting materials and process. They sized Cd-doped ZMO microspheres at 200 °C for 18 hours,
tested this with several surfactants. With cetyl- resulting in uniform spherical structures with an average diam-
trimethylammonium bromide (CTAB), they obtained a honey- eter of approximately 2 mm. Bera et al.62 developed honeycomb-
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comb-like structure made of densely packed nanoparticles. like ZMO@Ni(OH)2 core–shell structures, featuring nanosheets
With PEG, they observed porous microspheres with a rough and interconnected honeycomb walls that formed a hierarchical
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surface. With polysorbate-80, they observed ower-like micro- architecture. Rosaiah et al.63 prepared ZMO/rGO composites by
spheres with a loose texture made of nanosheets. combining HT treatment with subsequent annealing. Through
HT treatment is a versatile technique, effective not only for precise control of synthesis parameters and calcination temper-
creating pure metal oxides but also for producing composites. atures, they achieved needle-like surface architectures on ZMO/
By adding materials such as graphene, carbon nanotubes rGO microspheres. The inclusion of RGO enhanced the surface
(CNTs), reduced graphene oxide (rGO) sheets, or other compo- area and contributed to the formation of porous, interconnected
nents to the initial solution, researchers can synthesize a variety structures. These examples illustrate the versatility of HT
of ZMO-based composites. For instance, to improve ZMO's synthesis in producing diverse and tailored morphologies for
conductivity, Le et al.57 combined it with rGO and a conductive ZMO-based materials. Jiu et al.64 recently synthesized hierarchical
polymer using an HT method. This created an aerogel with a 3D mesoporous ZMO@Mo6S9.5 microowers using a two-step
structure where ZMO nanoparticles were dispersed within the hydrothermal-calcination process. This method facilitated the
graphene oxide sheets. The added polymer coated the surface, growth of ultrathin Mo6S9.5 nanosheets on ZMO microspheres,
enhancing the material's properties. forming a three-dimensional architecture. The hierarchical
Guan et al.58 synthesized a sandwich-like ZMO composite by structure was characterized by its interconnected nanosheets,
using high temperatures to turn a glucose polymer into carbon which provided a porous framework and enhanced morpholog-
nanosheets that encase ZMO nanoparticles. This unique ical features suitable for various applications.
composite features a porous carbon outer layer. Chen et al.59 4.1.2 Solvothermal (ST) synthesis. In a conventional ST
developed a method for creating porous ZMO/carbon micro- synthesis, the cations Mn2+ and Zn2+ interact with carbonate
spheres using leover microalgae. They coated the microalgae ions (CO32−) in an ethylene glycol (EG) solvent at high temper-
with manganese oxide and heated it to create a porous carbon atures. The solubility products ZnCO3 and MnCO3 lead to
template. This template was then used to create the nal ZMO/ precipitation due to their analogous crystal structures, forming
carbon spheres. Other researchers have used various biomass ZnxMnxCO3. To reduce the surface energy, the ZnxMnxCO3

Fig. 3 ZMO morphologies produced by HT/ST synthesis (conditions for every morphology are reported in Table 1): (a) reproduced from ref. 72
with permission from Elsevier, copyright 2015, (b) reproduced from ref. 73 with permission from Elsevier, copyright 2022, (c) reproduced from
ref. 74, (d) reproduced from ref. 75 with permission from Elsevier, copyright 2020, (e) reproduced from ref. 76 with permission from Elsevier,
copyright 2021, (f) reproduced from ref. 70 with permission from Elsevier, copyright 2018, (g) reproduced from ref. 55 with permission from
Elsevier, copyright 2018, (h) reproduced from ref. 77 with permission from Wiley Online Library, copyright 2022, (i) reproduced from ref. 78 with
permission from Elsevier, copyright 2021, (j) reproduced from ref. 79 with permission from Elsevier, copyright 2019, (k) reproduced from ref. 53
with permission from Elsevier, copyright 2015, (l) reproduced from ref. 39 with permission from ACS, copyright 2018, and (m) reproduced from
ref. 65 with permission from Elsevier, copyright 2014.

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particles undergo aggregation during the subsequent calcina- 4.2 Co-precipitation (CP)
tion process, resulting in the formation of microspherical
CP is a chemical synthesis technique where nucleation, growth,
morphologies.65 Xu et al.39 synthesized a triple-shelled ZMO
coarsening, and agglomeration processes occur simultaneously.
structure by adding salicylic acid before the ST treatment. This
It leads to the formation of insoluble species under high
caused the material to shrink inwards during heating, forming
supersaturation conditions. The process involves nucleation,
multiple shells. They found that a slower heating rate allowed
resulting in the formation of numerous small particles, and
more time for the inner layers to detach, creating additional
secondary processes such as Ostwald ripening and aggregation
shells. Similarly, Yuan et al.66 produced mesoporous micro- that signicantly impact the nal product's size, morphology,
spheres with a hierarchical structure. By heating the mixture and properties. Thus, this facile method nds applications in
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twice, they created hollow spheres. However, heating it only

synthesizing metal oxides, chalcogenides, and nanoparticles for
once resulted in a “ball-in-ball” structure.67 Researchers oen
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various applications including EES. Compared to HT/ST tech-

use PVP in the HT synthesis of ZMO to create hollow micro-
niques, CP offers a simpler, quicker process, requiring no high-
spheres. However, Rong et al.68 used CTAB to create non-hollow
pressure or high-temperature equipment, and providing decent
microspheres. Various other ZMO nanostructures, like nano-
control over particle size and composition, thus making it more
cubes69 and interconnected nano-peanut-like structures,70 have
suitable for large-scale production.
also been synthesized using this method under different In a typical CP procedure of ZMO, a metal precursor solu-
conditions. Zhang et al.71 investigated how the choice of solvent tion is mixed with a solvent (ethanol/water) and a stabilizing
signicantly affects the morphology of ZMO prepared through
agent (e.g., oxalic acid), forming metal precipitates (TC2O4-
a ST approach followed by calcination. Their study revealed that
$xH2O, where T = Zn, Mn). Aer the solvents evaporate, the
when ethylene glycol was used as the solvent, it facilitated the
precipitates are heated to high temperatures in air to obtain
formation of a unique pomegranate-like ZMO structure. This
ZMO. For instance, Soundharrajan et al.80 synthesized ZMO
morphology consisted of micron-scale spheres made up of
microrods using Mn2+ as an additive, following the same
closely packed nanoparticles, likely due to the high viscosity of
procedure with an increased concentration of the Mn
ethylene glycol, which limited particle mobility and encouraged precursor. Zhang et al.81 controlled the morphology of ZMO
compact aggregation during the growth process. Conversely, using the same CP method, altering only the stabilizing
using water as the solvent resulted in a more uniform micro-
agents. They found that CTAB, n-hexane, and cyclohexane
spherical ZMO morphology, with larger, smoother particles
produced large microspheres, small microspheres, and hex-
and less-dened secondary structures. The lower viscosity of
ahedral morphologies, respectively. In another study, Zhang
water allowed for more unrestricted particle migration and
et al.82 synthesized ZMO microspheres by mixing metal
growth, producing these distinct differences in the nal struc-
nitrates with sodium bicarbonate, forming metal carbonate
ture. Fig. 3 presents the resulting ZMO morphologies obtained
precipitates. Heating these precipitates caused them to
under various conditions by HT and ST methods, and the decompose from the inside out, creating hollow microspheres.
details are provided in Table 1. The study found that higher temperatures (800 °C) destroyed

Table 1 Summary of ZMO morphologies and synthesis conditions produced by HT/ST

Caption in
Fig. 3 Synthesis method Precursors T (°C) Duration Morphology Ref.

(a) HT MnCl2, ZnCl2, and urea 200 24 h + calcination Nanoparticles 72

(b) HT KMnO4 and Zn(NO3)2$6H2O 160 2h Nanosheets 73
(c) HT Zn(CH3COO)2$2H2O, Mn(NO3)2$4H2O, 100 12 h Nanoakes 74
and FeCl3$6H2O
(d) HT MXene, Zn(CH3COO)2, Mn(CH3COO)2, 180 12 h Nanosheets 75
ethanol, and ammonia
(e) HT Zn(CH3COO)2 and Mn(CH3COO)2 180 24 h Nanoowers 76
(f) HT CTAB, Zn(acac)2 and Mn(acac)2 180 13 h + 3 h Nano-peanuts 70
(g) ST Zn(NO3)2$6H2O, Mn(NO3)2$4H2O, NH4F, 120 5 h + 2 h calcination Hierarchical porous 55
and urea rugby-ball
(h) ST Mn(CH3COO)2$4H2O, 180 8 h + calcination Microspheres 77
Zn(CH3COO)2$2H2O, and PVP
(i) HT Zn(NO3)2$6H2O and Mn(NO3)2 160 9 h + 2 h calcination Nanoparticles 78
(j) ST Zn(CH3COO)2$2H2O, 160 12 h + 3 h calcination Nanocubes 79
Mn(CH3COO)2$4H2O, HMTA, and NH4F
(k) HT ZnCl2, MnCl2$2H2O, and triacetic acid 180 24 h + 2 h calcination Nanowires 53
(l) ST Zn(CH3COO)2$2H2O, 160 14 h + 12 h calcination Hollow microspheres 39
Mn(CH3COO)2$4H2O, salicylic acid, and
PVP
(m) ST MnCl2$4H2O, ZnCl2, and urea 200 24 h + 2 h calcination Microspheres 65

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the microsphere structure, while lower temperatures (400 °C) 4.3 Combustion method
resulted in a dense material lacking porosity. The ideal
The combustion method for metal oxide synthesis offers several
temperature (600 °C) produced porous, hollow microspheres.
advantages. First, it is a simple and cost-effective technique,
Besides creating carbon-based ZMO composites, doping
requiring minimal equipment and operating at atmospheric
with metal atoms such as tin (Sn) is another way to improve
pressure. The rapid and self-sustaining nature of the combus-
ZMO's conductivity and electrochemical properties for better
tion reaction enables fast synthesis with high yields, resulting in
energy storage performance. Chen et al.83 created Sn-doped
shorter reaction times and increased productivity. Additionally,
ZMO microspheres using a CP method. By adding tin oxide to the combustion method oen yields nanoparticles with
the initial mixture and then heating it, they produced porous
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a narrow size distribution and high phase purity, thanks to the

microspheres with increased surface area and larger pores
homogeneous nucleation and growth facilitated by the
compared to the undoped version. They could also increase the
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exothermic reaction. The method can be easily scaled up for

amount of tin to create heavily doped ZMO. Chen et al.84 further
large-scale production and allows for control over particle size
improved the material by adding carbon to their Sn-doped
and composition, providing exibility for tailoring properties to
ZMO. They used a combination of CP and HT methods, add-
specic applications. Furthermore, the combustion method is
ing glucose during the HT step to create an amorphous carbon
energy-efficient, utilizing the exothermic reaction for heat
layer on the porous microspheres. This carbon coating signi- generation. These combined advantages make the combustion
cantly boosted performance, especially at high current density. method an attractive choice for metal oxide nanoparticle
The versatility of CP in synthesizing diverse ZMO structures was
synthesis.86
further emphasized by Zhou et al.,85 who successfully fabricated
Huang et al.87 used a combustion method with different fuels
hollow microrods with consistent particle sizes and high
to create hierarchical porous ZMO. They found that using
porosity. These hollow structures formed due to CO2 release
sucrose as fuel resulted in pure ZMO nanoparticles. Sucrose
during metal oxalate decomposition, creating voids that
acts as both a fuel and a complexing agent, reacting with metal
increase the material's surface area. Surfactants such as mmol
nitrates to form a gel. During combustion, the organic compo-
sodium citrate (SDS) promoted the organization of precursors nents decompose, leaving behind hierarchical porous ZMO
into rod-like structures, with the degree of hollowness and with macropores and mesopores. The combustion reaction can
uniformity depending on SDS concentration and reaction
be written as follows:
parameters. Calcination at 700 °C preserved the hollow archi-
tecture and ensured high crystallinity, while higher tempera- Zn(NO3)2 + 2Mn(NO3)2 + C12H22O11 + 5O2 = ZnMn2O4
tures led to grain coarsening and structural collapse. These + 12CO2 + 11H2O + 3N2 (6)
results demonstrate CP's adaptability in tailoring ZMO nano-
structures for various applications. Kommu et al. synthesized rGO/ZMO using a similar proce-
dure with the use of sucrose as the fuel.88 Furthermore, creating

Fig. 4 Overview of ZMO morphologies and their synthesis methods (conditions for every morphology are reported in Table 2): (a) reproduced
from ref. 102 with permission from Elsevier, copyright 2015, (b) reproduced from ref. 80 with permission from Elsevier, copyright 2020, (c)
reproduced from ref. 61 with permission from Wiley Online Library, copyright 2021, (d) reproduced from ref. 103 with permission from Elsevier,
copyright 2024, (e) reproduced from ref. 95 with permission from ACS, copyright 2018, (f) reproduced from ref. 104 with permission from
Elsevier, copyright 2021, (g) reproduced from ref. 34 with permission from Elsevier, copyright 2019, (h) reproduced from ref. 105 with permission
from Elsevier, copyright 2021, (i) reproduced from ref. 62 with permission from Elsevier, copyright 2022, (j) reproduced from ref. 106 with
permission from Elsevier, copyright 2017, (k) reproduced from ref. 32 with permission from Elsevier, copyright 2015, (l) reproduced from ref. 107
with permission from ACS, copyright 2021, (m) reproduced from ref. 108 with permission from Elsevier, copyright 2022, (n) reproduced from ref.
109 with permission from Wiley Online Library, copyright 2022, and (o) reproduced from ref. 110 with permission from ACS, copyright 2016.

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carbon composites is also very facile using the combustion a high surface area material with an average pore size of 5 nm.
technique. Sim et al.89 used pineapple peel as a carbon source Additionally, several noteworthy ZMO morphologies have been
for a ZMO/C composite and achieved a high specic surface developed for energy storage applications, including, the aero-
area material (976.12 m2 g−1). Sim et al.90 synthesized a meso- gel, MOF-derived, ower-like, nanopyramids, MXene-based
porous carbon/ZMO composite by utilizing green waste as composite and nanocages.96–101 Fig. 4 presents a selection of
a sustainable carbon source. The resulting material exhibited ZMO morphologies synthesized using various methods, except
a high surface area of about 800 m2 g−1, along with promising for the HT and ST methods, and the details are provided in
electrochemical performance. The observed improvements in Table 2.
specic capacitance were due to the synergistic interaction Table 3 provides a summary of various synthesis methods
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between the carbon matrix, which provided abundant active employed in the fabrication of ZMO, categorized by their
sites, and ZMO, which enhanced electrical conductivity. Hasan underlying principles. For each method, a specic example with
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et al. synthesized Ni substituted ZMO through combustion and precursors, conditions, and resulting morphology is included.
found that the grain size depends on the amount of Ni substi- The advantages and disadvantages of each method are
tution. With high amounts of Ni in ZMO, larger non-uniform compared, considering factors such as cost, complexity, scal-
size grains were formed with an average size of 250 nm.91 ability, and control over morphology. In the following section,
Controlling the pore structure in combustion synthesis is we explore how synthesized ZMO with varying morphologies
challenging due to the rapid, high-temperature nature of the performs in supercapacitor and battery applications. Addition-
process. Abdollahifar et al.92 tackled this by using PEG, which ally, Fig. 5 shows a polygon radar chart that compares different
acts as both a pore-structuring agent and a source of carbon. synthesis techniques according to important metrics including
This resulted in carbon-coated ZMO with well-dened nano- crystallinity, specic surface area, morphology controllability,
crystallites and improved electronic conductivity. equipment complexity, and reaction time. This visual repre-
sentation facilitates an easy assessment of each method's
strengths and limitations.
4.4 Other synthesis methods
Electrospinning is a versatile and cost-effective method for
creating one-dimensional ZMO nanowires with high aspect 5 Supercapacitor application of ZMO
ratios. This technique allows for control over diameter, surface
area, and porosity. Joshi et al.93 used electrospinning to create ZMO has emerged as a highly promising material for super-
a binder-free ZMO/carbon nanober with improved mechanical capacitor applications, garnering signicant attention from
stability and electrode–electrolyte contact. Similarly, Radha- researchers and resulting in an extensive body of literature as
mani et al.94 synthesized Mn2O3/ZMO nanobers by varying the summarized in Table 4 at the end of this section. The excep-
compositions and diameters (50 to 250 nm) via electrospinning. tional performance of ZMO in supercapacitors can be attributed
Recently, biomorphic materials, which use biological to several key properties: its spinel structure with voids that
templates, are gaining popularity due to their simple synthesis facilitate ion accommodation, its bi-metallic nature that
process. Luo et al.95 created ZMO microtubules using cotton as increases the number of active sites, its pseudocapacitive
a template. By immersing cotton in a metal precursor solution behavior in neutral electrolytes, depending on the morphology
and then heating it, they were able to replicate the cotton's and its specic surface area. The electrochemical performance
structure, creating mesoporous ZMO tubes. This resulted in of ZMO is notably inuenced by various factors, including

Table 2 Overview of ZMO morphologies using various synthesis methods

Caption in Fig. 4 Synthesis method Morphology Ref.

(a) Template method Honeycomb 102

(b) Co-precipitation Microrods 80
(c) Cation exchange Hollow octahedra 61
(d) Microemulsion Submicron cubic 103
(e) Biomorphic Microtubules 95
(f) Electrospinning Hollow nanobers 104
(g) Electrodeposition Fiber-like 34
(h) Coprecipitation + calcination 3D skeleton structure nanorods 105
(i) Sol–gel Nano block-like 62
(j) Co-precipitation 1D nanostructures 106
(k) Electrospinning + calcination Nanobers 32
(l) Low-temperature synthesis Particle-like morphology 107
(m) Carbon gel-combustion + hard template Hollow porous panpipe-like 108
(n) In situ electrochemical Quantum dots 109
(o) Precipitation Nanoparticles 110

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Table 3 Evaluation of the strengths and weaknesses of various ZMO fabrication techniques

Synthesis method Advantages Drawbacks

Hydrothermal (HT) - One-step synthesis, allowing for high purity - Requires high-pressure equipment (autoclave)
and low aggregation
- Mild operating conditions (<200 °C) - Relatively longer reaction times
- Scalability to industrial demands - Complex optimization of conditions may be
required
- Good solution dispersion and control over - May have limited scalability
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reaction stoichiometry
- Variety of morphologies achievable
(e.g., nanoparticles, nanowires, and hollow
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microspheres)
Solvothermal (ST) -Similar to HT but allows for more versatile - Higher temperatures may be required
solvents (e.g., ethanol and methanol)
- Good control over morphology and surface - Potential environmental concerns based on
area solvents used
- Fast synthesis with simultaneous formation of
solid solutions
Co-precipitation (CP) - Simple and cost-effective and requires no high- - Low crystallinity, oen requires post-synthesis
temperature equipment treatment
- Faster reactions compared to HT/ST methods - Limited control over morphology
- Good scalability for large-scale production
- Allows for precise control over particle size and
composition
Combustion - Simple and cost-effective with minimal - Rapid synthesis can make controlling
equipment requirements morphology challenging
- High phase purity and narrow size distribution - Limited ability to control particle size
- Scalable for large-scale production
Electrospinning - High aspect ratio nanobers, enhancing - Requires specialized equipment for
electrode performance electrospinning
- Versatile method for producing a wide range of - Process complexity and potential difficulties in
materials scaling up
Biomorphic synthesis - Simple synthesis with natural templates - Limited to specic organic templates
leading to unique morphologies
- High surface area and porosity due to structure - The process may be slow and dependent on
replication template availability

ZMO is mainly synthesized using HT methods and tested in two

types of electrolytes: alkaline (mainly KOH) and neutral (mainly
Na2SO4). Pure ZMO materials exhibit capacitances ranging from
150 to 1000 F g−1, but when combined with carbon composites
or doped, capacitances can even reach up to 1800 F g−1. The
electrode–electrolyte interface plays a crucial role in governing
electrochemical reactions, which in pseudocapacitor materials
can occur through surface adsorption/desorption, metal cation
(de)intercalation, and faradaic redox reactions.
The reaction mechanisms of ZMO in a KOH electrolyte can
Fig. 5 Polygon radar chart comparing various synthesis methods
based on key parameters, including morphology controllability, be described as follows:125
specific surface area, crystallinity, equipment complexity, and reaction
time, which are extracted and explained in Table 3. (ZnMn2O4)surface + K+ + e− = [KZnMn2O4]surface (7)

(ZnMn2O4) + K+ + e− = [ZnMn2O4]K (8)

shape/morphology, phase purity, and surface area. Several
The electrolyte also inuences the reaction mechanism,
innovative approaches have been employed to enhance device
while in the case of ZMO, reversible conversion reactions occur
performance, including designing materials with porous nano-
when ZMO interacts with hydroxide ions. The conversion
architectures to shorten ion-diffusion paths, fabricating nano-
reaction mechanisms of ZMO in an alkaline electrolyte (e.g.,
composites to leverage synergistic effects, doping to improve
KOH) can be expressed as follows:79
electrical conductivity, and synthesizing novel structures to
enhance shell porosity and surface area. Based on the literature, ZnMn2O4 + OH− + H2O 4 ZnOOH + 2MnOOH + e− (9)

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Table 4 Electrochemical characterization of ZMO in supercapacitors

Capacity retention
Method Morphology Electrolyte Capacitance (F g−1) (cycles) Ref.

Hydrothermal Nanocubes 2 M KOH 776 at 5 mV s−1 91% (5000) 69

Microspheres 2 M KOH 155 at 2 mV s−1 99% (1100) 72
Cd-doped ZMO 2 M KOH 364 at 2 mV s−1 — 111
Sn doped C/ZMO microspheres 1 M KOH 1010 at 1 A g−1 83% (2000) 84
Composite with Mn2O4 2 M KOH 380 at 0.5 A g−1 92% (2000) 76
1024 mF at 10 mA cm−2
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Microspheres/ZnFe2O4 composite 3 M KOH 95% (3000) 74

Composite with rGO 3 M KOH 628 at 1 A g−1 95% (10 000) 63
ZnO@ZMO/rGO 6 M KOH 276 mF at 0.5 mA cm−2 88% (10 000) 54
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Composite with Ni(OH)2 1 M KOH 2577 at 1 A g−1 90% (10 000) 112
Nanoparticles 0.5 M KCL 675 at 5 mV s−1 61% (1000) 113
Composite with Zn-birnessite 1 M Na2SO4 1903 mF at 3 mA cm−2 93% (16 000) 73
Ni coated ZMO 1 M Na2SO4 193 at 5 mV s−1 98% (1200) 114
ZMO aerogel with rGO 1 M Na2SO4 297 at 0.2 A g−1 93% (5000) 57
Microspheres 0.5 M Na2SO4 191 at 5 mV s−1 98% (1000) 56
Composite with MWCNTs 1 M Na2SO4 103 at 1 mV s−1 56% (1000) 115
Solvothermal Co doped ZMO nanocubes 1 M KOH 1196 at 1 A g−1 80% (4000) 79
Co-precipitation Composite with SnO2 6 M KOH 658 at 1 A g−1 — 116
Pomengranate-like 1 M KOH 447 at 1 A g−1 52% (800) 117
Nanoparticles 2 M KOH 545 at 1 A g−1 — 118
Plate-like 1 M Na2SO4 1093 at 1 A g−1 96% (5000) 119
Sn doped ZMO microspheres 1 M KOH 530 at 1 A g−1 77% (2000) 83
Elec activation Composite with CNTs 2 M KOH 443.9 at 1 A g−1 88% (10 000) 120
Combustion Composite with carbon 6 M KOH 119 at 0.3 A g−1 97% (5000) 89
Composite with carbon 6 M KOH 122.94 at 0.3 A g−1 90% (5000) 90
Auto-combustion Composite with rGO 1 M LiOH 783 at 5 mV s−1 75% (10) 88
Electrospinning Composite with carbon 6 M KOH 1080 at 1 A g−1 92% (10 000) 93
1D hallow nanobers 1 M KOH 1026 at 1 A g−1 100% (5000) 104
Composite Mn2O3 1 M Na2SO4 360 at 0.1 A g−1 98% (3000) 94
Nanobers 1 M Na2SO4 240 at 1 A g−1 99% (2000) 121
Sol–gel Composite with MgFe2O4 6 M KOH 450 at 10 mV s−1 100% (1000) 122
Electrodeposition Nanosheets 0.5 M Na2SO4 457 at 1 A g−1 92% (4000) 123
Composite with Mn3O4 1 M Na2SO4 321 at 1 mV s−1 93% (2000) 124
MOF-derived Composite with carbon nanorods 1 M Na2SO4 589 at 1 A g−1 98% (5000) 71

MnOOH + OH− 4 MnO2 + H2O + e− (10) Galvanostatic charge–discharge (GCD) tests conrmed this
behavior. ZMO exhibited good cycling stability over 500 cycles
Similarly, the reaction mechanisms of ZMO in neutral elec- with 100% coulombic efficiency. This rst ZMO supercapacitor
trolytes (e.g., Na2SO4) can be described as follows:80 achieved an energy density of 18 W h kg−1 and a power density
of 185 W kg−1. Recently, Guo et al.72 were the rst to synthesize
(ZnMn2O4)surface + Na+ + e− = [NaZnMn2O4]surface (11) ZMO microspheres (average size 10 mm) on a Ti sheet, carbon
cloth, and nickel foam using HT synthesis. ZMO on nickel foam
ZnMn2O4 + Na+ + e− = [ZnMn2O4]Na (12)
showed the highest capacitance (about 155 F g−1 at 2 mV s−1)
and low contact resistance (0.2 U) due to improved adhesion,
In the following sections, we will investigate the effect of
enhancing charge transfer kinetics.
these two types of electrolytes on the electrochemical perfor-
However, distinguishing between electric double-layer
mance of ZMO.
capacitance (EDLC) and pseudocapacitance can be chal-
lenging. While both exhibit a linear dependence of stored
5.1 ZMO in KOH electrolyte charge on the potential window width, their charge storage
5.1.1 Early work and challenges. Sahoo et al.125 were the mechanisms differ. Their CV proles can appear similar with
rst to investigate ZMO for supercapacitors. Their ZMO had minor variations. Also, an EDLC electrode with non-linearity in
a near-spherical morphology (50 nm average particle size) and the CV prole (blunt and slanted CV prole) due to high ESR
a porous structure, with a specic surface area of 2.6 m2 g−1. and EPR can exactly depict a typical pseudocapacitance CV
Despite the low surface area, the interconnected mesoporous prole.126 Therefore, it is advisable to verify other typical pseu-
structure facilitated ion diffusion, resulting in a specic docapacitance signatures and conduct quantitative kinetics
capacitance of about 160 F g−1 at 3 mV s−1. The cyclic voltam- analysis (calculation of b by formula i(V) = avb),92 to check
metry (CV) curve showed pseudocapacitive behavior, attributed whether the material exhibits EDLC (b = 1) or
to K+ ion adsorption/desorption and (de)intercalation.

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pseudocapacitance (b nearly 1) behavior.127,128 Additionally, through HT, with a carbon content of 7.8% which can barely
there are also reports79,117 which display the ZMO CV curve with decrease the capacitance of ZMO. The intensity ratio of graph-
a battery-type signature of a distinct redox peak. This peak rises itized carbon to disordered carbon peaks (Ig/Id) of ZMO/C from
from the non-capacitive faradaic process (eqn (9) and (10)) the Raman spectra is found to be nearly 1, indicating the
between the ZMO and the electrolyte (KOH), which does not formation of well-graphitized carbon in the composite. This
come under pseudocapacitance behavior, but can be classied addresses the poor electrical conductivity of disordered carbon
as hybrid supercapacitors if paired with a carbon electrode. via the combustion route. In addition to the above attributes of
5.1.2 ZMO/carbon composites. Most pseudocapacitive conducting carbon phases, a high surface area of 343.2 m2 g−1
materials always possess low conductivity, which limits the and the unique morphology of ZMO encapsulated in sandwich-
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rate capability of the electrode at high current densities. like carbon nanosheets resulted in an impeccable capacitance
Additionally, the pseudocapacitive process has comparatively value of 1786 F g−1 at 1 A g−1, yielding 95% capacity retention.
Open Access Article. Published on 02 April 2025. Downloaded on 4/28/2025 6:48:17 AM.

sluggish kinetics compared to EDLC, leading to lower power ZMO/C nanober composites were synthesized by Yun et al.
density. Combining materials with different charge storage via electrospinning.104 The capacitance value for the given
mechanisms can synergistically enhance the performance of nanober composite was 1026 F g−1 at 1 A g−1 using 1 M KOH as
supercapacitor devices. In this regard, Shen et al.120 synthe- the electrolyte. Typically, KOH concentrations for electrolytes
sized a composite of ZMO/CNT with abundant oxygen vacan- range from 1 to 6 M. The electrolyte concentration plays
cies by using spent zinc–carbon battery powder as the raw a crucial role in inuencing electrochemical parameters, as an
material. Without any reduction process, the oxygen vacancies increase in concentration results in a higher number of ions
were generated by tuning Mn2+/Mn3+ via the electrochemical available for (de)intercalation or (de)adsorption at the surface
activation method, which increased the electronic conduc- and electrode–electrolyte interface, thereby enhancing both
tivity of the material and resulted in a specic capacitance of diffusion and capacitive contributions. However, surpassing an
443 F g−1 at 1 A g−1. Additionally, the oxygen-decient ZMO/ optimal concentration may lead to reduced ionic resistance
CNT composite electrode displayed a prominent cycling leading to higher solution resistance (Rs). The specic conduc-
performance by retaining 96.7% of its initial capacitance aer tivity increased from 0.33 to 0.67 S cm−1 as KOH concentration
10 000 cycles at 1 A g−1. Similarly, Rosaiah et al.63 monitored increased from 1.5 to 6 M, followed by a slight decrease to
and tuned the synthesized parameters to obtain a ZMO–rGO 0.61 S cm−1 at 9 M.129 Therefore, most of the cases listed in
composite with a special spike-like morphology, achieving Table 4 do not exceed 6 M for KOH.
a maximum specic capacitance of 628 F g−1 at a current Joshi et al.93 fabricated a exible supercapacitor using zinc–
density of 1 A g−1. Upon subjecting the composite electrode to manganese oxide coated carbon nanobers. The composite
10 000 charge/discharge cycles at 1 A g−1, it retained 95.2% of nanobers were synthesized using terephthalic acid, which
its initial capacitance. improved ber exibility and ion diffusion, and sodium dodecyl
However, the cost of CNTs, multi-walled CNTs (MWCNTs), sulfate, which ensured uniform ZnO distribution on the nano-
graphene, and rGO is comparatively high compared to that of ber surface during annealing. This surface modication with
other conducting carbon materials. However, there has been ZnO increased the electrochemical activity of the composite. An
signicant interest in utilizing organic waste as a source of optimal zinc/manganese acetate ratio of 0.75 resulted in high
conductive carbon phases in composites for energy storage specic capacitances: 1080 F g−1 and 817 F g−1 at current
applications. Inspired by this approach, Sim et al.89 created densities of 1 A g−1 and 10 A g−1 within a 1.6 V potential
a porous zinc manganese oxide/carbon (ZMO/C) composite window. Furthermore, the device demonstrated excellent
from pineapple peel using a combustion technique. The cycling stability, retaining 92% capacitance aer 10 000 charge–
composite had a high surface area (976 m2 g−1) but a relatively discharge cycles. Bending tests conrmed the structural integ-
low specic capacitance (104.89 F g−1 at 300 mA g−1). However, rity and exibility of the freestanding carbon nanober elec-
it showed excellent capacity retention (97% aer 5000 cycles) trodes, showing minimal changes in the CV curve even at
and rate capability due to its unique pore structure. Analysis signicant bending angles.
revealed that the capacitance was mainly (62%) due to 5.1.3 Doped ZMO. While incorporating carbon phases has
diffusion-controlled processes, with the remaining 38% was shown signicant potential in increasing electrochemical
attributed to surface-induced capacitive behavior from the performance, doping offers an intriguing approach to augment
added carbon. In another study, Sim et al.90 synthesized a ZMO/ electrochemical functionality. Doping modies the electronic
C composite using the combustion method, yielding a specic structure of the material, leading to improved properties. Khaja
capacitance of 123 F g−1 at 0.3 A g−1 in 6 M KOH electrolyte. The Hussain et al.79 synthesized Co-doped ZMO nanocubes via the
low capacitance in both cases may be due to the presence of ST route, achieving a high specic capacitance of 1196 F g−1 at
poorly conductive amorphous carbon. Adding more carbon 1 A g−1, signicantly higher than the 267 F g−1 observed for
content to the composite can increase the electronic conduc- undoped ZMO (Fig. 6). Although the Co2+ ions can incorporate
tivity of the composite, but higher weight percentages can affect and replace the Zn2+ ions due to a similar ionic radius, the
the specic capacitance of the composite due to carbon's inclusion of a larger concentration of Co2+ ions (e.g., 7 mol%)
inherent low specic capacitance. will result in lattice structure distortion. It is also worth noting
To address the challenge of optimizing carbon content in the a signicant difference in Rct between pristine ZMO and Co-
composite, Guan et al.58 synthesized the ZMO/C composite doped ZMO with optimal doping concentrations (5 mol%) of

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Fig. 6 Performance and application of ZMO and Co-doped ZMO PNC-based HSC devices. (a) Illustration of the synthesis process for ZMO and
Co-doped ZMO PNCs; (b) diagram of the HSC device setup; (c) CV profiles of the ZMO:5Co PNCs and AC electrodes; (d) CV curves and (e) GCD
curves of the HSC device recorded at various scan rates and current densities within the 0–1.45 V potential range. (f) Long-term cycling stability
of the HSC device. Reproduced from ref. 79 with permission from Elsevier, copyright 2019.

6.51 and 1.21 ohms, respectively. However, exceeding this Wang et al.111 synthesized Cd-doped ZMO, achieving
optimal doping concentration increased Rct due to impurity ion a specic capacitance of 364 F g−1 at 2 mV s−1. This lower
scattering. capacitance compared to that of Co-doped ZMO can be

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attributed to the nature of the Cd dopant. Unlike Co, Cd does Other researchers have explored various ZMO composites for
not signicantly enhance electrical conductivity or contribute to enhanced supercapacitor performance. For example, Pearline
the charge storage mechanism. The larger size of Cd ions et al.116 synthesized a ZMO/SnO2 composite with a high specic
compared to Zn ions increases the lattice constant, reducing the capacitance of 658 F g−1 at 1 A g−1 in a 6 M KOH electrolyte. Park
formation of high-density defects. However, Cd doping also et al.54 developed a exible composite of ZnO nanospheres,
results in a smaller grain size and increased grain boundaries. ZMO nanorods, and rGO, achieving 276.3 mF cm−2 at 0.5 mA
This increase in grain boundaries, evidenced by a larger semi- cm−2 with good energy and power densities and excellent
circle in the electrochemical impedance spectroscopy (EIS) cycling stability. Bera et al.112 created a ZMO/Ni(OH)2 composite
data, leads to a higher Rct exceeding 20 U. Also, Chen et al.84 with a remarkable specic capacitance of 2577 F g−1 at 1 A g−1,
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interestingly combined the advantage of doping and incorpo- attributed to the increased surface area, synergistic effects, and
rating carbon phases by synthesizing Sn-doped ZMO via the CP, unique core–shell morphology. This composite also demon-
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followed by carbon coating over Sn-doped ZMO via the HT strated excellent capacitance retention (94.8% aer 3000
synthesis. Sn doping served to increase both the electrical cycles). Furthermore, a exible asymmetric supercapacitor
conductivity and the surface area of ZMO, providing more active (ASC) device fabricated using this ZMO/Ni(OH)2 composite and
sites for electrochemical reactions. Furthermore, the synthesis activated carbon (AC) electrodes with a PVA-KOH electrolyte
process created a shell around the ZMO microspheres, which, in achieved a specic capacitance of 138 F g−1 at 0.5 A g−1 and
addition to the outer carbon coating, helped to buffer volume a high energy density of 43 W h kg−1.
expansion during cycling. This combination of Sn doping,
carbon coating, and a protective shell resulted in a high specic
capacitance of 580 F g−1 at 5 A g−1 and excellent cycling 5.2 ZMO in neutral electrolytes
stability. 5.2.1 Inuence of morphology and synthesis conditions.
5.1.4 ZMO composites with other metal oxides. Instead of Several studies have investigated the impact of neutral electro-
doping, which replaces the metal cation within the structure, lytes on the electrochemical performance of ZMO for super-
adding separate phases to the active material can enhance capacitor applications. For instance, Bhagwan et al.121
overall electrochemical performance. Sannasi et al.76 synthe- synthesized one-dimensional nanobers via electrospinning,
sized a ZMO/Mn2O3 nanostructured composite which reacted taking advantage of the increased interfacial area and inter-
with KOH similar to Co-doped ZMO to store extra capacitance connected particle network inherent in this morphology. The
through a faradaic reaction. Though the ZMO/Mn2O3 composite 1D nanobers also offer the benet of reduced surface energy,
underwent a 2-way faradaic reaction (eqn (3) and (4)), the which minimizes active material aggregation and unwanted
highest specic capacitance achieved was 173 F g−1 at 1 A g−1. side reactions with the electrolyte. Their study focused on the
The relatively low capacitance compared to that of other electrical properties of the ZMO nanobers, reporting an
composites can be attributed to the lack of a synergistic effect improved conductivity of 2 × 10−7 S cm−2 and a high Na+ ion
between the active material and the additive phases. diffusion coefficient of 3.48 × 10−11 cm2 s−1. These efficient
To showcase an example of the synergistic effect in ZMO ionic pathways contributed to the impressive performance of
composites, Heiba et al.122 synthesized a (1 − x)ZMO/(x) the nal supercapacitor device, which achieved an energy
MgFe2O4 composite using sol–gel, with different ratios of x = 0, density of 25 W h kg−1 at a high-power density of 2.5 kW kg−1.
0.1, 0.5, 0.9, and 1. Among the different ratios of x, the highest Zhu et al.56 studied the morphology impact on super-
capacitance of 502 F g−1 at 1 A g−1 was obtained for the sample capacitor performance and found that a simple microsphere
0.1ZMO/0.9MgFe2O4 due to the synergy between the ZMO and morphology showed better specic capacitance compared to
MgFe2O4 components, which surpassed the individual compo- honeycomb and ower-like morphologies. The honeycomb,
nents of the composite. They also investigated the dielectric microsphere and ower-like morphologies exhibited a bandgap
characteristics and AC conductivity of the composites in detail. of 2.29 eV, 2.23 eV, and 2.18 eV respectively. These differences in
With lower ZMO in the composite, the dielectric constant the bandgap were attributed to structural components and the
increases and the AC conductivity increased for the 0.1ZMO/ abundance of defects in agglomerated nanoparticles. The
0.9MgFe2O4 sample due to the small polaron hopping mecha- porous microsphere morphology sample exhibited a specic
nism, in contrast to the correlated barrier hopping mechanism capacitance of 191 F g−1 with an impeccable capacity retention
exhibited by the other samples. of 98.8% aer 1000 cycles.
In a similar vein, Sivaguru et al.130 synthesized nano- With the use of deep eutectic solvents for the synthesis of
composites where irregular sheet-like ZMO and cube-like metal oxides by functioning as both a precursor and a template,
morphology of Cu1.5Mn1.5O4 coalesce to establish a hetero- ZMO was synthesized by Samage et al.131 The interactions
structure that amplies active sites and facilitates ion transport. occurring between hydrogen bond acceptors and donors are
It also demonstrated a battery-type charge storage mechanism, modiable through the introduction of water to terminate the
with signicant inner capacity derived from ion intercalation, reaction. This capability permits the rapid synthesis of ZMO in
and its reversibility is conrmed by R2 values approaching unity a mere 1 minute, thereby establishing it as a more expedient,
for both oxidation and reduction peaks. This resulted in efficient, and economically viable approach for the production
a specic capacitance of 468 F g−1 at 0.5 A g−1 and 84% capacity of ZMO. Nevertheless, extending the reaction duration to 10
retention aer 20 000 cycles. minutes yielded superior performance, with the resultant

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asymmetric supercapacitor attaining a specic capacitance of temperature (450 °C) but varied the calcination time (6 h and 12
331 F g−1 at 0.2 A g−1, alongside an energy density of 74.5 W h h). Both samples exhibited a plate-like morphology, but the 12 h
kg−1 and a power density of 5.4 kW kg−1 measured at 4 A g−1. calcination produced a larger crystallite size and a lower surface
Furthermore, the device retained 80% of its initial capacitance area. The 6 h calcined ZMO, with its higher surface area, ach-
aer enduring 30 000 charge–discharge cycles at 6 A g−1. ieved a specic capacitance of 1093 F g−1 at 1 A g−1, and
Barkhordari et al.123 studied the role of calcination temper- excellent cyclability (96.1% aer 5000 cycles). This highlights
ature in ZMO properties which affects the performance of the the importance of optimizing both calcination temperature and
electrode material. They synthesized ZMO via cathodic electro- time to achieve the desired ZMO properties for supercapacitor
deposition, followed by calcination at different temperatures applications.
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(250, 450, and 650 °C). Increasing the calcination temperature 5.2.2 ZMO/carbon composites. Researchers have also
led to higher crystallinity, as evidenced by XRD patterns, which explored incorporating carbon phases into ZMO to enhance its
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typically improves conductivity. However, the 650 °C sample conductivity and electrochemical performance in neutral elec-
showed limited protonation reactions, hindering efficient ionic trolytes. For example, Abdollahifar et al.92 developed a ZMO/
transport. Furthermore, increasing the calcination temperature carbon composite that exhibited exceptional cycling stability,
decreased the surface area. The 450 °C sample achieved the best with no capacitance fading aer 10 000 cycles and over 99%
balance between crystallinity and surface area, resulting in the coulombic efficiency in 1.7 M Na2SO4. However, the composite
highest specic capacitance (456.8 F g−1 at 1 A g−1) and good had a specic capacitance of 150 F g−1 at 2 mV s−1. This high
cycling stability (92.5% capacitance retention aer 4000 cycles). stability but relatively low capacitance can be attributed to the
In a related study, Gao et al.119 maintained the same calcination dominant surface capacitive contributions in the composite,

Fig. 7 Electrochemical characteristics and structural analysis of ZMO-6-1; (a) CV profiles of the synthesized samples at a scan rate of 2 mV s−1; (b
and c) GCD curves recorded at various current densities; (d) dependence of coulombic efficiency and specific capacitance on current density; (e)
long-term cycling performance and coulombic efficiency over 10 000 cycles at 2.5 A g−1, with insets showing GCD profiles at the 40th, 4000th,
7000th, and 10 000th cycles. (f) SEM image and elemental mapping showcasing its structural features and elemental composition. Reproduced
from ref. 92 with permission from Elsevier, copyright 2018.

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which were quantied as 76% at 1 mV s−1 and 89% at 5 mV s−1 In contrast, Zhu et al.132 reported the highest specic
through kinetic analysis (Fig. 7). A similar trend was observed in capacitance for ZMO/C composites in neutral electrolytes,
the aerogel-based ZMO/rGO studied by Le et al.57 which achieving 589 F g−1 at 1 A g−1. They synthesized ZMO/carbon
demonstrated a lower specic capacitance of 108 F g−1 at nanorods by pyrolysis of Zn–Mn MOF, which resulted in
0.2 A g−1 in 1 M Na2SO4, but achieved a capacity retention of uniform dispersion of ZMO on the carbon matrix. This
93.27% aer 5000 charge–discharge cycles. The lower specic composite also exhibited good rate capability, retaining
capacitance compared to that of bare ZMO in some studies a specic capacitance of 278 F g−1 at 20 A g−1, and outstanding
might be due to a lack of synergy between the carbon and ZMO cycling stability, with 98.1% retention aer 2000 cycles at
phases. 10 A g−1. The observed increase in capacitance aer the rst 500
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Fig. 8 Fabrication process and electrochemical properties of the OD-ZMO electrode: (a) diagram depicting the synthesis steps of the OD-ZMO
electrode; (b) CV curves of the OD-ZMO electrode across different potential ranges at 20 mV s−1; (c) CV curves comparing ZMO and OD-ZMO
electrodes at 10 mV s−1; (d) analysis of the capacitive charge contribution (indicated by the orange region) at 5 mV s−1; (e) areal capacitance
values obtained at current densities from 3 to 50 mA cm−2; (f) Nyquist plots with an equivalent circuit model highlighting Rs (series resistance), Rct
(charge transfer resistance), W (Warburg impedance), and Cdl (double-layer capacitance); (g) cycling stability results recorded at 50 mA cm−2.
Reproduced from ref. 73 with permission from Elsevier, copyright 2022.

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cycles was attributed to an activation effect. Compared to bare compared to pure ZMO (28.4%), attributed to the increased
ZMO, the ZMO/C hybrid material showed signicant improve- electrochemically active surface area. These advantages resulted
ments in specic capacitance, cyclability, and charge transfer in a high areal capacitance of 1903 mF cm−2 at 3 mA cm−2 and
resistance, highlighting the synergistic effect of the homoge- excellent cycling stability (93.7% capacitance retention aer 16
neous ZMO distribution on the carbon matrix. 000 cycles). Furthermore, a exible asymmetric supercapacitor
5.2.3 ZMO-based hybrid composites and modied ZMO. assembled using this oxygen-decient ZMO cathode and a V2O5
Combining ZMO with other metal oxides to create hybrid anode achieved a high operating voltage of 2.4 V and delivered
composites with enhanced electrochemical properties has also an energy density of 6.24 mW h cm−3 (Fig. 8).
been explored. In hybrid composites, the ratio of components For real-world supercapacitor applications, Chen et al.134
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plays a crucial role in achieving optimal synergy. For instance, implemented a high-performance thermally charging super-
Zhao et al.133 achieved excellent results with a ZMO/MnOOH capacitor, designed to integrate efficient thermoelectric
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composite using a 55 : 45 ratio while Radhamani et al.94 conversion with exceptional electrochemical storage, tailored
synthesized a Mn2O3/ZMO nanober composite (99 : 1), for wearable electronics. ZMO@Ti3C2Tx MXene composite
achieving a maximum specic capacitance of 360 F g−1 at electrodes were synthesized with porous ZMO nanosheets,
0.1 A g−1. The addition of just 1 wt% ZMO increased the elec- which play a crucial role in preventing the common issue of
tronic conductivity of the composite, and the synergy between MXene restacking and increase the interlayer gap of Ti3C2Tx
the two materials enhanced the rate capability of the electrode. MXene, hence inhibiting the collapse or aggregation of its
Likewise, Ameri et al.124 addressed the bottleneck of low elec- layered structure. This integration complements interlayer
trical conductivity in Mn3O4, one of the most stable forms of space, facilitates ion diffusion paths, and promotes structural
manganese oxide, by synthesizing a Mn3O4/ZMO composite stability, hence optimizing charge transport and energy storage.
using ZMO nanorods via cathodic electrodeposition. In addi- The electrodes incorporate proton-donating groups (–OH) that
tion to cation intercalation and de-intercalation into the voids enhance H+ ion mobility, hence increasing electrochemical
of the ZMO crystal structure, a different process involving Na+ double-layer capacitance. The interaction between the electrode
ions and Mn3O4 occurs in Na2SO4 electrolyte. This composite, and electrolyte materials yielded a specic capacitance of 326.5
with its unique combination of charge storage mechanisms, F g−1 at 1.0 A g−1, with a retention of 94.2% aer 5000 charge–
achieved a high specic capacitance of 321 F g−1 at 1 mV s−1, discharge cycles. Also, the device attained a maximum energy
signicantly higher than that of bare Mn3O4 (∼248 F g−1). This density of 10.4 W h kg−1 and a power density of 1324 W kg−1.
improvement was attributed to the lower Rct (0.76 U) of the This section has explored the diverse applications of ZMO in
composite compared to bare ZMO (2.57 U). The composite also supercapacitors, highlighting how morphology, composites,
exhibited excellent cycling stability, retaining 93% of its initial and doping inuence performance. We discussed various ZMO
capacity aer 2000 cycles. Beyond compositing, Li et al.114 morphologies and their performance in KOH and Na2SO4
modied ZMO by coating it with Ni. This coating altered the electrolytes. The benets of carbon-based composites and
ZMO structure, enhancing its surface area and pore structure, doping strategies were also examined, emphasizing the
and increased cationic ion penetration, leading to improved importance of synergistic interactions between ZMO and the
electrochemical reactions with the electrolyte. The resulting Ni- composite material. Table 5 summarizes the electrochemical
coated ZMO electrode demonstrated a high specic capacitance performance of ZMO supercapacitors across different electro-
of 193 F g−1 in 1 M NaSO4 at 200 mV s−1 and excellent cycling lyte systems (KOH and Na2SO4) based on reclassied data from
stability, maintaining 98.8% of its initial capacitance aer 1200 Table 4. The data indicate that KOH electrolytes generally
cycles. Lyu et al.73 synthesized a Zn-birnessite@spinel ZMO enable higher peak capacitance values (120–2600 F g−1) in ZMO
nanocrystal composite using a HT process followed by chemical supercapacitors, likely due to favorable alkaline conditions and
reduction. They observed an interesting phase transformation the effectiveness of doping and conductive composites (e.g.,
mechanism from Zn-birnessite to spinel ZMO, driven by the with Ni(OH)2 and carbon). Na2SO4 systems demonstrate
formation of oxygen vacancies and Mn ion migration. These a moderate capacitance range (100–1100 F g−1) but offer good
oxygen vacancies, conrmed by EPR analysis, led to a lower overall performance with a potential emphasis on stability and
bandgap (1.06 eV for bare ZMO vs. 1.54 eV for the composite) cost-effectiveness. Strategies for Na2SO4 oen involve
and enhanced electrical conductivity, as evidenced by a low morphology control and stable carbon composites, and these
charge transfer resistance of 0.63 U. The oxygen vacancy-rich systems exhibit a wide range of cycle stability (1000–16 000
ZMO also exhibited a higher capacitive contribution (71.6%) cycles). Overall, electrolyte choice signicantly impacts ZMO

Table 5 Electrolyte-type dependent performance summary of ZMO supercapacitors from Table 4

Typical capacitance test

Electrolyte Capacitance range conditions Capacity retention Typical capacity
type (F g−1) (scan rate/current density) range (%) retention cycles

KOH ∼120–2600 10 mV s−1/0.3 A g−1 52–100% 800–10 000

Na2SO4 ∼100–1100 1–5 mV s−1/0.1–1 A g−1 56–99% 1000–16 000

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supercapacitor performance. KOH appears advantageous for 6.1 ZMO in LIBs

maximizing capacitance, while Na2SO4 provides a balance of
In the past few decades, there has been notable progress in LIB
good performance and potentially enhanced stability. Further
technology, enabling its widespread application in various
research should explore the mechanisms behind these differ-
devices ranging from portable electronics to electric vehicles.
ences and address unit inconsistencies in the original data.
The high energy density and favorable cycle life of LIBs have
garnered signicant attention, driving further exploration of
6 Battery applications of ZMO this technology. Graphite135 remains the primary choice for
anode materials in commercial batteries due to its cost effi-
This section provides a comprehensive overview of ZMO's
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ciency, stability, and extended cycle longevity. Nonetheless, the

applications in three battery systems (LIBs, SIBs and ZIBs), limited theoretical capacity of graphite at 372 mA h g−1
highlighting recent advancements and addressing the chal-
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emphasizes the necessity for alternative anode materials, such

lenges that hinder its widespread adoption. We will explore the as silicon/graphite (carbon)136–139 composites, as well as some
diverse strategies employed to overcome these limitations, such alloy and conversion (typically TMOs) anodes.140,141 Among
as morphology control, compositing with conductive materials, TMOs, spinel ZMO is a promising anode material for LIBs due
and electrolyte optimization.

Fig. 9 CV curves of the first three cycles for (a) CA, (b) the 50% ZMO/CA hybrid, and (c) pure ZMO samples at a scan rate of 0.1 mV s−1 in the
voltage range of 0.01–3.0 V. Initial charge–discharge curves of (d) CA, (e) the 50% ZMO/CA hybrid, and (f) pure ZMO samples at a current density
of 100 mA g−1. Reproduced from ref. 98 with permission from Wiley Online Library, copyright 2014.

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to its high theoretical capacity (897 mA h g−1), abundance, (CA) using a solution immersion chemical approach. This
environmental friendliness, and low working voltage. ZMO's hybrid structure, prepared with various ZMO incorporation
lithium storage mechanism involves both conversion and ratios (15, 35, 50, and 75 wt%), takes advantage of the high
alloying reactions during lithiation.142 Initially, a conversion surface area and extensive porosity of the CA, combined with
reaction occurs as ZMO reacts with incoming Li, forming the high electrochemical properties of ZMO, to create a highly
metallic nanograins dispersed in a Li2O matrix (eqn (13)). conductive 3D network. The optimized 50% ZMO/CA composite
Subsequently, Li alloying takes place with the Zn nanograins demonstrated enhanced electrochemical performance, signi-
(eqn (14)), contributing to the overall Li storage. In addition to cantly exceeding the capacities of both pure ZMO and pure CA
the alloying mechanism, Zn and Mn also react with Li2O to form materials. This improvement is primarily due to the synergistic
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MnO and ZnO (eqn (15) and (16)).82,142 interaction between the ZMO nanocrystals and the porous CA
matrix, which effectively mitigates volume changes during
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ZnMn2O4 + 8Li+ + 8e− / Zn + 2Mn + 4Li2O (13) charge–discharge cycles, enhances electrical conductivity,
facilitates ion diffusion, and reduces charge-transfer resistance
Zn + Li+ + e− / LiZn (14) (Fig. 9).
6.1.1.2 Porous and hollow ZMO architectures. The electro-
2Mn + 2LiO2 / 2MnO + 4Li+ + 4e− (15)
chemical performance of the cell is strongly inuenced by the
Zn + LiO2 / ZnO + 2Li+ + 2e− (16) structural features of the electrodes. Lithium-ion diffusion
channels, electrolyte wettability, and rate of the electrochemical
Despite the several advantages of ZMO, there are few reactions can be controlled by careful engineering of the
inherent aws which retard the practical application or materials. Recognizing the importance of morphology in
commercialization. Poor electronic conductivity of ZMO (∼2.0 inuencing electrochemical performance, Zhang et al.67 devel-
× 10−7 S cm−1)143 is considered a common problem for both oped ZMO microspheres with a unique “ball-in-ball” hollow
battery and supercapacitor applications. To resolve this architecture that enhanced their performance as an anode
problem, various conductive materials such as conductive material in LIBs. This structure achieved a high initial charge
polymers, CNTs, graphene, and MXenes are composited with capacity (662 mA h g−1) and excellent coulombic efficiency
ZMO to enhance the conductive channel for electron transfer. (nearly 100% aer several cycles). The enhanced performance
Another major problem with ZMO based anodes is the drastic was attributed to the small nanoparticle size, facilitating Li+
volumetric change during cycling, which, along with poor diffusion, and the hollow structure, which acted as an electro-
kinetics, induces pulverization and progressive aggregation of lyte reservoir. The “ball-in-ball” design also provided structural
the active material, leading to capacity fading and reduced rate integrity, mitigating strain during cycling. Interestingly, the
capability.35,70 To alleviate this problem, fabricating novel nano- capacity increased to 750 mA h g−1 aer 120 cycles, suggesting
or porous structures can act as a buffer to absorb the stress an activation process typical of transition metal oxides.
induced by the volume change. Additionally, synthesizing Luo et al.95 synthesized mesoporous ZMO microtubules
porous structures with different morphologies can increase the using cotton ber templates; but their performance was limited
contact area between the active material and electrolyte, thereby by the high calcination temperature. Zhang et al.82 conrmed
improving overall device performance. In the following para- the detrimental effects of high calcination temperatures on
graphs, we will discuss the progress made with ZMO as an ZMO morphology and surface area. They optimized the
anode material for LIBs. synthesis process, achieving a ZMO variant (ZMO-600) with
6.1.1 Structure design of ZMO a high capacity of 999 mA h g−1 and excellent capacity retention
6.1.1.1 Nanocrystalline ZMO. To enhance the performance (99.2% aer 50 cycles). This improved performance was attrib-
of ZMO as an anode material, researchers have explored various uted to a novel redox mechanism involving reversible Mn2+/
nanostructured forms. Yang et al.31 synthesized nanocrystalline Mn3+ conversion, interfacial storage, and a unique 3D porous
ZMO using a polymer-pyrolysis method, achieving well- core–shell structure that facilitated lithium-ion diffusion and
crystallized nanoparticles with optimal interaction with the accommodated volume changes of Mn. Additionally, part of the
current collector. This nanocrystalline ZMO exhibited a high improved capacity can be attributed to an interfacial storage
initial discharge capacity of 1302 mA h g−1, exceeding the mechanism. Furthermore, the enhanced stability is linked to
theoretical value. Aer 50 cycles, the capacity was still the unique 3D porous core–shell structure of the material,
a respectable 569 mA h g−1, outperforming similar materials. which facilitates lithium-ion diffusion and enables effective
This enhanced performance was attributed to the stable inter- volume accommodation during cycling, thereby reducing
action between the LiZn–Zn–ZnO and Mn–MnO composites structural degradation. Xu et al.39 controlled the morphology of
formed during cycling, which mitigated volume changes. While ZMO with double or triple-shelled hallow microspheres by
the reversible capacity was lower than that of ZnCo2O4 simply varying the annealing ramp rate. These studies highlight
(900 mA h g−1), ZMO offered advantages at average discharge the critical inuence of calcination temperature and the
and charge voltages (0.5 V and 1.2 V, respectively). annealing ramp rate on the resulting ZMO morphology and
Yin and colleagues98 synthesized spinel ZMO nanocrystals electrochemical properties. Wang et al.65 also recognized the
incorporated into a three-dimensional porous carbon aerogel importance of morphology, developing porous ZMO micro-
spheres through a solution-based method followed by

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calcination. This interconnected porous architecture enhanced effect, boosting energy storage capabilities. Cheng et al.105
lithium storage, achieving a specic capacity of 800 mA h g−1 developed oxygen vacancy-enriched ZMO nanorods (OZMO)
aer 300 cycles. The improved performance was attributed to that demonstrated a high reversible capacity of 1566 mA h g−1
efficient charge transfer, reduced pulverization, and better aer 50 cycles at 0.1 A g−1 when used as an anode in LIBs.
accommodation of volume changes during cycling. OZMO also exhibited outstanding cycling performance
6.1.1.3 Defect engineering and doping. Introducing anion (380 mA h g−1 aer 1000 cycles at 10 A g−1) and favorable
defects, such as oxygen vacancies, in transition metal oxides can environmental adaptability, maintaining high capacities at
signicantly enhance their electrochemical activity. Oxygen both −5 °C and 55 °C (Fig. 10). Du et al.43 further explored the
vacancies reduce the bandgap and amplify the pseudocapacitive concept of anionic vacancies in ZMOs derived from MOFs. They
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Fig. 10 (a) CV of OZMO at a scan rate of 0.2 mV s−1; (b) GCD profile of the OZMO electrodes at 0.1 A g−1; (c) cycling stability of OZMO at 10 A g−1;
(d) rate performance of the OZMO and ZMO electrodes at 0.1–10 A g−1; (e) cycling stability of the OZMO and ZMO electrodes at 0.1 A g−1; (f)
high/low-temperature performance at 2 A g−1. Reproduced from ref. 105 with permission from Elsevier, copyright 2021.

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synthesized three types of ZMOs with varying initial charge 2D architecture increased the electrochemically active surface
capacities and coulombic efficiencies. The relatively low initial area, facilitating lithium-ion intercalation and deintercalation.
coulombic efficiencies were attributed to SEI formation and It also shortened Li+ diffusion pathways, improving charge and
incomplete ZMO oxidation. However, the specic capacity discharge rates. The exible rGO sheets provided structural
improved in subsequent cycles due to self-optimization of the support for the ZMO nanoparticles, accommodating volume
active materials, the formation of a polymer gel-like lm, and changes during cycling. Furthermore, thermal annealing
increased electrolyte wetting. enhanced crystallinity and stability.
Combining ZMO with other active materials can create Zhang et al.147 addressed the limitations of ZMO by using
hybrid nanostructures with improved electrochemical perfor- CMK-3, a conductive carbon source, to create a ZMO@CMK-3
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mance. Jiu et al.64 introduced Mo6S9.5 ultra-thin nanosheets on hybrid with a high surface area (129 m2 g−1). This composite
the outer layer of ZMO spheres, forming ZMO@Mo6S9.5 hier- exhibited a high reversible capacity (997 mA h g−1 at 100 mA g−1
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archical mesoporous micro-owers (HMMs). These HMMs aer 100 cycles), good rate capability, and exceptional cycling
exhibited high initial discharge and charge capacities (1203 and performance (94% capacity retention over 1600 cycles). The
846 mA h g−1, respectively) and a reversible capacity of robust structural integrity of the composite, with its rod-like
731 mA h g−1 aer high-current cycling. This good performance morphology and mesoporous structure, contributed to its
was attributed to the 3D hierarchical architecture and the excellent cycling stability. The growing interest in biomass as an
synergistic interaction between Mo6S9.5 and ZMO. advanced energy material has led to the development of bio-
6.1.2 Composite design of ZMO. To address the inherent template techniques for creating controlled nano- and micro-
low electronic conductivity of ZMO, researchers have investi- structures. Natural materials offer unique advantages due to
gated compositing it with conductive carbon materials. This their intricate hierarchical morphologies and porous structures.
approach enhances electrical contact, prevents exfoliation from In this context, Chen et al.59 developed a ZMO/carbon composite
the current collector, and improves the overall conductivity and using residual broken microalgae as a cost-effective carbon
morphological stability of ZMO. A uniform dispersion of ZMO source. The resulting composite, with its hierarchical porous
on carbon matrices also reduces agglomeration, which is architecture and nanocapsule-like structure, demonstrated
particularly important at high charge/discharge rates. However, remarkable electrochemical performance, including high
beyond improving electronic conductivity, the choice of carbon reversible capacity and excellent capacity retention, even at high
material plays a crucial role in inuencing the initial coulombic current densities. This was attributed to the synergistic effects
efficiency (ICE) and solid electrolyte interphase (SEI) formation, of carbon and a novel binder derived from sea algae. Similar to
both of which signicantly impact the long-term performance the low-cost carbon source approach, Lin et al.60 synthesized
of ZMO-based anodes. A high ICE is essential for maximizing a ZMO/carbon composite with milk as a carbon source. The
the energy density of full cells, as low ICE leads to the irre- milk-derived carbon was co-doped with heteroatoms (N, P, B,
versible consumption of active Li ions in the rst cycle. Ni, and S), enhancing conductivity and increasing the density of
Carbonaceous materials with high surface areas or abundant the carbon material. This co-doping strategy synergistically
defects tend to form thick SEI layers, leading to excessive elec- improved the capacity, cycling performance, and mechanical
trolyte decomposition and alkali-ion loss, thereby reducing the properties of the composite. The ZMO/milk-derived carbon
ICE. Hence, optimizing the pore structure, heteroatom doping, hybrid achieved a high reversible capacity (1352 mA h g−1 aer
and surface functionalization of carbon materials in ZMO 400 cycles) and sustained long-term cyclability. These studies
composites is critical for mitigating these effects.144 highlight the potential of utilizing natural resources and har-
Yuan et al.145 developed core–shell ZMO@CNT coaxial nessing their synergistic effects to develop high-performance
nanocables as an innovative anode material for LIBs. The 3D battery materials.
CNT network provided a exible buffer to accommodate volume Besides carbon composites, carbon coatings have proven to
changes in ZMO during cycling, hindered ZMO nanosheet be an effective strategy for enhancing the electrochemical
aggregation, and improved electron transport. These coaxial performance of ZMO anodes by improving conductivity and
nanocables demonstrated a high initial discharge capacity stabilizing the electrode structure. A study on carbon-coated
(1033 mA h g−1) and maintained a signicant capacity of ZMO demonstrated that the carbon layer suppresses electrode
652 mA h g−1 aer 100 cycles. polarization, reduces capacity uctuations, and enhances rate
Xiong et al.146 proposed a facile two-step synthesis approach capability. The coating prevents excessive SEI growth and phase
to address the challenges associated with ZMO anodes, specif- transitions, particularly stabilizing the MnO phase and miti-
ically targeting improvements in capacity, rate capability, and gating the formation of unwanted Mn3O4,148 which contributes
cycling stability. Their method involved a polyol process fol- to long-term cycling stability. Besides carbon, metal oxide
lowed by thermal annealing, resulting in a unique 2D archi- coatings like Al2O3 have also been explored for ZMO, offering
tecture where ZMO nanoparticles were uniformly integrated further protection against electrolyte decomposition and HF-
onto rGO sheets. This structure signicantly enhanced the induced corrosion, as well as improved SEI stability.149
electrochemical performance, achieving a specic capacity of Various coating materials, including TiO2, ZnO, and Li3PO4,
approximately 650 mA h g−1 over 1500 cycles at a high current have been successfully applied to different anodes, demon-
density of 2000 mA g−1. This improved performance was strating the potential to mitigate structural degradation and
attributed to the synergistic combination of ZMO and rGO. The enhance Li-ion transport. Exploring new coating strategies

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tailored for ZMO anodes could further optimize their perfor- Despite the progress in ZMO research, a notable study by
mance, paving the way for more durable and high-capacity Zhang et al.81 demonstrated exceptional performance with
lithium-ion batteries.150 porous ZMO with a core–shell microsphere morphology. This
6.1.3 Electrolyte design and optimization. While much material achieved a remarkably high specic capacity of
research has focused on ZMO synthesis and morphology 1600 mA h g−1 aer 100 cycles and excellent rate performance,
control, understanding the impact of electrolyte formulations maintaining a capacity of 1208 mA h g−1 aer 250 cycles at
on ZMO's electrochemical behavior is also crucial. Kumar 500 mA g−1. This exceeded the theoretical capacity, likely due to
et al.35 explored ZMO performance with different electrolyte the partially reversible Mn2+ 4 Mn3+ redox process and inter-
combinations, nding that lithium bis(tri- facial lithium storage within the SEI layer.82,146 While the
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uoromethanesulfonyl)imide (LiTFSI) in ethylene carbonate mechanism for this capacity increase has been identied,
and dimethyl carbonate (EC-DMC) yielded the highest initial further research is needed to understand the specic factors
Open Access Article. Published on 02 April 2025. Downloaded on 4/28/2025 6:48:17 AM.

capacity (649 mA h g−1) and best cycling stability. This high- related to ZMO morphology and electrolyte that favor this
lights the importance of efficient ion desolvation for rapid mechanism. Table 6 presents a summary of electrochemical
charge-transfer reactions in conversion-type materials like characterization data for ZMO used as an anode material in
ZMO. LiTFSI and ether-based electrolytes are typically preferred LIBs, drawn from selected reports. Although ZMO offers
when ion–solvent co-intercalation is desired, owing to strong promising characteristics as an anode material for LIBs, it also
ion–solvent interactions. Efficient desolvation of ions is crucial presents certain challenges. Table 7 summarizes these chal-
for rapid charge-transfer reactions at the electrode–electrolyte lenges and the modication strategies employed to overcome
interface, particularly in conversion-type materials such as them, highlighting the resulting benets.
ZMO.

Table 6 Electrochemical characterization of ZMO anodes in LIBs from selected reports

Synthesis Morphology Specic capacity (mA h g−1) Capacity retention (cycles) Ref.

Solvothermal Ball-in-ball 1094 at 0.1 A g−1 70% (250) at 0.5 A g−1 77

Microspheres 800 at 0.5 A g−1 ∼44% (300) at 0.5 A g−1 65
Pomogranate-like 596 at 1 A g−1 ∼91% (100) at 2 A g−1 78
Multi-shell hollow 537 at 0.4 A g−1 ∼100% (150) at 0.4 A g−1 39
Hydrothermal Hollow spheres 1207 at 0.78 A g−1 78% (565) at 0.78 A g−1 66
Nanowires 869 at 0.5 A g−1 ∼84% (50) at 0.5 A g−1 53
Microspheres 723 at 0.4 A g−1 ∼84% (350) at 0.4 A g−1 68
Peanut-like 812 at 0.1 A g−1 ∼90% (200) at 0.1 A g−1 70
Hierarchical porous rugby-balls 1584 at 0.1 A g−1 41.3% (100) at 0.1 A g−1 55
Porous rod-like structure 702 at 1.58 A g−1 ∼70% (1000) at 1.568 A g−1 151
Hierarchical mesoporous 731 at 0.1 A g−1 ∼100% (100) at 0.1 A g−1 64
microowers
Nanosheets@carbon nanotubes 1033 at 1.2 A g−1 ∼55% (100) at 1.224 A g−1 145
Sphere-like shape in a porous 399 at 1 A g−1 60% (400) at 0.1 A g−1 60
carbon matrix
Sol–gel Nanoparticles 874 at 0.1 A g−1 67% (100) at 1 A g−1 35
Nanoblocks 110 at 1 A g−1 ∼33% (40) at 0.1 A g−1 62
Polyol ZMO–graphene 800 at 0.5 A g−1 81% (1500) at 2 A g−1 146
Ball-in-ball 683 at 0.6 A g−1 ∼83% (120) at 0.4 A g−1 67
Coprecipitation Microspheres 999 at 0.1 A g−1 99.2% (50) at 0.1 A g−1 82
Hollow microrods 379 at 1 A g−1 133% (302) at 0.5 A g−1 85
Nanorods 270 at 0.1 A g−1 75% (200) at 0.2 A g−1 106
O decient nanorods 1566 at 0.1 A g−1 ∼100% (50) at 0.1 A g−1 105
Nanoparticles 638 at 1 A g−1 ∼61% (300) at 1 A g−1 35
Solution immersion ZMO@carbon aerogel 833 at 0.1 A g−1 ∼88% (50) at 0.1 A g−1 98
Micro emulsion Microspheres 870 at 0.1 A g−1 158% (250) at 0.5 A g−1 81
Carbongel combustion Hollow porous panpipe 458.7 at 1 A g−1 70% (500) at 0.5 A g−1 108
and hard template
Electrospun Nanobers 428 at 1 A g−1 47.3% (60) at 0.05 A g−1 32
Microwave Micro-rhombus 1057 at 1 A g−1 ∼30% (40) at 0.1 A g−1 152
Reactive template Tubular array 1198 at 0.1 A g−1 ∼66% (100) at 0.1 A g−1 37
Template synthesis Porous 3D interconnected carbon 760 at 0.1 A g−1 ∼55% (550) at 1 A g−1 102
framework
2D-on-3D architecture 693 at 2 A g−1 94% (1600) at 2 A g−1 147
Biotemplating method Microspheres 1450 at 0.2 A g−1 750% (250) at 1 A g−1 59
Polymer pyrolysis Nanocystals 766 at 0.1 A g−1 ∼75% (50) at 0.1 A g−1 31
Biomorphic route Microbelt 738 at 0.2 A g−1 ∼95% (300) at 2 A g−1 95

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Table 7 Strategies to overcome challenges in ZMO anodes for LIBs

Challenge Modication strategy Key benets

Low electrical conductivity - Composite with conductive materials (CNTs, graphene, etc.) - Enhanced charge transfer
- Derived carbon with heteroatom co-doping - Improved rate capability
- Oxygen vacancy engineering - Increased capacity
Volume changes during cycling - Nanostructured ZMO - Accommodate volume
expansion/contraction
- Porous/hollow architectures (“ball-in-ball,” microtubules, - Maintain structural integrity
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core–shell, etc.)
- Control of calcination temperature and the annealing - Improve cycling stability
ramp rate
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Electrolyte limitations - Optimize Li-based electrolytes (LiTFSI in EC-DMC) - Enhance ion desolvation
- Improve ionic conductivity
- Improve cycling stability

6.2 ZMO in SIBs ZnMn2O4 + 8Na+ + 8e− / 2Mn + Zn + 4Na2O (17)

SIBs have emerged as a potential alternative to LIBs, particularly
2Mn + 2Na2O 4 2MnO + 4Na+ + 4e− (18)
for large-scale energy storage applications such as grids, due to
the abundance and low cost of sodium. However, the larger Zn + Na2O / ZnO + 2Na+ + 2e− (19)
ionic radius of Na ions (0.102 nm, about 50% larger than that of
Li-ions) makes it challenging to use commonly used anode The following reaction mechanism could also be valid for
materials of LIBs with SIB anodes. ZMO has also been widely ZMO as a cathode in SIBs:154
studied in SIBs due to its environmental friendliness, energy
efficiency, and low cost. Although fewer studies have explored ZnMn2O4 + Na+ + e− / NaZnMn2O4 (20)
its application in SIBs, some progress has been made in this
area. The reaction mechanism of ZMO with Na in SIBs based on NaZnMn2O4 + Na+ + e− / ZnO + 2MnO + Na2O (21)
the study by Chandra Sekhar et al.153 is as follows:

Fig. 11 ZMO microtubules (ZMO-MTs): (a) SEM image, (b) HR-TEM image, (c) elemental mapping, (d) CV curves at a scan rate of 0.1 mV s−1 within
the voltage range of 0.01–3.0 V, (e) GCD profiles after different cycles, and (f) GCD profiles at various current densities ranging from 100 to
2000 mA g−1. Reproduced from ref. 95 with permission from ACS, copyright 2018.

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ZnO + xNa+ + xe− 4 (1 − x) ZnO + xZn stable performance over extended cycling. In another study on
+ xNa2O (1 > x> 0) (22) mitigating the volume changes and enhancing the conductivity
of ZMO, Yu et al.103 developed a composite material by
2MnO + yNa+ + ye− 4 Mn2O3 + yNa2O (1 > y> 0) (23) anchoring submicron cubic ZMO onto a porous carbon frame-
work derived from jute biomass through a mixed solvent
Chandra Sekhar et al.153 conducted the rst study of ZMO as thermal method. The three-dimensional porous structure of the
an anode for SIBs in 2017. A composite of ZMO and a nitrogen- jute-derived carbon signicantly contributed to improving the
doped graphene sheet anode was used in a half cell, exhibiting composite's performance by enhancing its electronic conduc-
a high capacity of 170 mA h g−1 over 1000 cycles, with a rate tivity. Additionally, this structure helped mitigate the mechan-
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capability of up to 10 A g−1. The superior electrochemical ical stress caused by the volume changes of ZMO during Na+
performance was due to the synergistic effect produced by the cycling. The interconnected pores within the carbon framework
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presence of ZMO nanoparticles decorated with the sheets of not only facilitated efficient ion diffusion but also increased the
nitrogen-doped graphene, the higher electrode conductivity, as number of available sites for ion storage. Additionally, the
well as maintained structural integrity and accommodated composite's hierarchical design allowed for uniform particle
volume changes during cycling. Then, in 2020, Luo et al.95 dispersion, mitigating aggregation and preserving the struc-
fabricated ZMO microtubules (ZMO-MTs) with an inner diam- tural integrity of the anode. These synergistic features enabled
eter of 8.5 mm and wall thickness of 1.5 mm, using a biomorphic this composite to maintain a capacity of 244 mA h g−1 aer 1500
approach. These microtubules were used as anodes for both cycles at a current density of 1 A g−1. Further improvements
SIBs and LIBs. ZMO-MT demonstrated a discharge capacity of were achieved by Muruganantham et al.,154 who developed
102 mA h g−1 aer 300 cycles at 100 mA g−1 and a rate capability porous ZMO microspheres using a PVP-assisted ST method. The
of 58 mA h g−1 at 2 A g−1 (Fig. 11). The exceptional electro- inclusion of PVP facilitated the formation of uniform micro-
chemical performance of ZMO-MT can be attributed to its spheres with enhanced porosity, which not only promoted
distinctive one-dimensional mesoporous microtubular archi- efficient Na+ ion diffusion but also helped mitigate the volume
tecture. This architecture provides a large contact area between changes during cycling. This porous microstructure, coupled
the electrolyte and electrode, as well as a short diffusion with the reversible redox transitions between Mn3+/Mn2+ and
distance for both ions and electrons. As a result, it buffers the Zn2+ reduction, contributed signicantly to the material's stable
volume variation that arises from repeated cycling. In the electrochemical performance. The controlled nanostructure
context of LIBs, ZMO-MT demonstrated a capacity of design of these microspheres enhanced charge transfer and
750 mA h g−1 aer 300 cycles at 200 mA g−1. facilitated capacity retention over extended cycles, highlighting
Defect engineering in transition metal oxides, particularly the critical role of structural engineering in improving the
through the creation of oxygen vacancies, has emerged as sodium-ion storage capabilities of ZMO anodes. Table 8
a promising strategy to enhance the performance of SIBs. These summarizes electrochemical characterization data for ZMO as
vacancies, created by selectively removing oxygen atoms from an anode material in SIBs, based on selected studies.
the crystal lattice, enhance the material's electronic conduc-
tivity, provide additional sites for ion storage, and reduce the 6.3 ZMO in ZIBs
energy barriers for ion diffusion.155–158 Cheng et al.105 demon-
A study by Sousa et al.159 used computer simulations to inves-
strated this approach by synthesizing oxygen vacancy-enriched
tigate ZMO as a potential material for batteries. They compared
ZMO nanorods via a coprecipitation and chemical reduction
ZMO to a similar material used in LIBs, lithium manganese
process. By narrowing the bandgap, oxygen vacancies increase
oxide (TLMO). While ZMO showed promise, the simulations
electrical conductivity and facilitate faster charge transfer. They
revealed some drawbacks. Specically, ZMO shrinks signi-
also contribute to a more porous structure with a larger surface
cantly when zinc ions are removed, suggesting it might not hold
area, which improves the interaction between the electrolyte
up well on repeated use. Also, ZMO lacks stable intermediate
and the electrode, shortens diffusion distances for ions, and
phases during the charge–discharge cycle, unlike TLMO. This
accommodates the volume changes that occur during sodium-
could limit how much energy it can store. Despite these limi-
ion insertion and extraction. These structural and electronic
tations, ZMO has other properties that make it attractive for
modications also encourage a pseudocapacitive behavior,
energy storage. It could be useful in supercapacitors or SIBs,
enabling the material to store more sodium ions and maintain
where the stability issues might be less important. However, to

Table 8 Electrochemical characterization of ZMO in SIBs from selected reports

Method Morphology Specic capacity (mA h g−1) Capacity retention (cycles) Ref.

Hydrothermal Nanoakes 425 at 0.05 A g−1 68% (150) at 0.05 A g−1 153
PVP-assisted solvothermal Mesoporous 112 at 0.2 A g−1 — 154
Biomorphic approach Mesoporous microtubules ∼58 at 2 A g−1 88% (300) at 0.2 A g−1 95
Coprecipitation/chemical reduction Nanorods 266 at 0.1 A g−1 81% (200) at 0.2 A g−1 105
Microemulsion Submicron cubic 392 at 0.1 A g−1 81% (150) at 1 A g−1 103

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make ZMO a viable option for ZIBs, further research is needed 1.35 V,80 large theoretical capacity (224 mA h g−1), and high
to improve its stability. redox potential (Fig. 12).161 Throughout the charging process,
Rechargeable ZIBs are considered to be promising next- Zn ions are gradually removed from the tetrahedral positions of
generation batteries because metallic Zn is stable in water the ZMO cathode. This causes the oxidation of Mn3+ to Mn4+
and can undergo a reversible stripping/plating reaction in mild and results in the formation of MnO2, which then releases two
acidic aqueous electrolytes allowing for the use of metallic Zn as electrons. In contrast, during the discharge process, Zn ions are
the anode in ZIBs.160 In addition, Zn has the merits of high present on the surface of the zinc anode and capture two elec-
natural abundance, low redox potential (−0.76 V vs. the stan- trons. This results in a reversible reaction during the (dis)
dard hydrogen electrode), and high theoretical specic capacity charging processes, respectively. To summarize, the corre-
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(820 mA h g−1). Unfortunately, compared with the large theo- sponding electrochemical reactions occur as follows:162
retical specic capacity of the zinc negative electrode, the re-
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ported capacity of positive electrode materials is still low, and Cathode: ZnMn2O4 4 Zn1−xMn2O4 + 2xe− + xZn2+ (24)
the cycling stability also needs to be further improved.
Considering the success of LiMn2O4 as one of the main lithium Anode: xZn2+ + 2xe− 4 xZn (25)
cathode materials and the close ionic radius of Zn2+ and Li+
Full cell (overall): ZnMn2O4 4 Zn1−xMn2O4 + xZn (26)
(0.06 vs. 0.059 nm), ZMO is a plausible candidate for a cathode
material in aqueous ZIBs. Extensive research has been con-
ducted on ZMO and it has shown promise as a positive material Despite the promising advantages of ZMO as a cathode
for ZIBs due to its low cost, a high average working potential of material, several challenges hinder its practical

Fig. 12 Electrochemical performance of ZMO NTAs and N-ZMO NTAs: (a) CV results; (b) GCD curves; (c) evaluation of rate capability; (d)
capacity performance comparison with previously reported ZMO cathodes; (e) cycling durability; (f) Nyquist impedance plots of ZMO and N-
ZMO NTA electrodes. Electrochemical analysis of the quasi-solid-state N-ZMO//Zn full cell device: (g) schematic representation; (h) charge–
discharge curves; (i) rate performance assessment. Reproduced from ref. 162 with permission from Elsevier, copyright 2021.

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implementation. Firstly, irreversible structural transformations of high-capacity ZIBs. Thirdly, the inherently low electrical
and Mn3+ disproportionation reactions induce capacity degra- conductivity of ZMO (∼1.0 × 10−5 S cm−1) restricts cycle life,
dation during cycling. Secondly, strong electrostatic interac- particularly at high rates. Finally, limited active site accessibility
tions between the host crystal and Zn ions limit the realization and signicant volume changes impede rate performance and
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Fig. 13 (a) Illustration of charge-transfer mechanisms at the interface between ZMO and the carbon matrix in ZMO QD@C; (b) analysis of the Mn
valence state distribution in ZMO QD@C; (c) depiction of the structural stability mechanism in ZMO QD@C; (d) CV curves recorded at a scan rate
of 0.1 mV s−1; (e) charge–discharge profiles at a current density of 200 mA g−1; (f) cycling performance at 200 mA g−1; (g) rate performance.
Reproduced from ref. 109 with permission from Wiley Online Library, copyright 2022. Electrochemical evaluation of the ZnO–MnO@C elec-
trode: (h) charge–discharge curves at 100 mA g−1 within the voltage range of 0.8–1.9 V; (i) rate performance at various current densities; (j)
capacitive contribution integrated within the CV curve; (k) cycling performance at a high current density of 1000 mA g−1. Reproduced from ref.
164 with permission from Wiley Online Library, copyright 2021.

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long-term stability, resulting in suboptimal ZMO//Zn battery heterostructures which had a similar effect of mitigating the
performance. Consequently, research efforts are focused on disproportionation reaction of Mn3+. Kang et al.163 illustrated
developing novel ZMO-based cathodes with enhanced intrinsic the efficacy of this method by synthesizing ZnMn2−xNixO4
conductivity and structural stability. The following sections samples (x = 0, 0.5, 1.0, and 1.5), noting a transition from
provide a comprehensive review of advancements in both a tetragonal to cubic spinel structure for all Ni doping concen-
aqueous and organic ZIBs. trations. Following 100 activation cycles, the ICP measurements
6.3.1 Aqueous electrolytes of the Zn anode indicated that Ni concentration negatively
6.3.1.1 Mn dissolution and conductive additives. The primary correlates with Mn deposition, implying that elevated Ni levels
cause of capacity fading in ZMO is the disproportionation markedly diminish Mn accumulation. Ni-doped ZMO surpassed
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reaction of Mn3+, driven by its unstable orbital conguration, the theoretical capacity of ZMO (224 mA h g−1) and attained
which induces Jahn–Teller distortion and promotes its disso- 277–278 mA h g−1. Furthermore, Ni-doped ZMO retained 80%
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lution in the aqueous electrolyte. This reaction predominantly of its capacity aer 1000 cycles.
transpires at the electrode/electrolyte contact and is represented CNTs, as carbon conductive materials, offer numerous
by using the following eqn (27): advantages, including a large surface area, high electrical
conductivity, and efficient electrolyte channels. Chen et al.169
2Mn3+ + 3H2O / Mn2+(aq) + MnO2(s) + 4H+ (27) utilized these properties by compositing CNTs with ZMO/C
hollow microspheres as a cathode material for ZIBs. In addi-
In this process, Mn3+ is converted into Mn2+ and Mn4+, with tion to the rate performance, cyclability was also improved due
Mn2+ diffusing into the electrolyte and Mn4+ precipitating as to the hollow mesoporous microsphere structure's ability to
MnO2 on the electrode surface.163 buffer volume expansion caused by Zn ion (de)intercalation.
There are proven methods for suppressing Mn dissolution Similarly, Gao et al.143 synthesized 20 nm ZMO particles on
including the use of highly concentrated colloidal electrolytes, CNTs which again increased the cyclability and rate perfor-
the strategic inclusion of preinserted cations, and carbon mance. Along with the CNTs' advantages, the smaller particle
protective layer coating for Mn-based cathodes.107 In particular, size contributed to a shorter ion diffusion path and the strong
the use of Mn2+ as an electrolyte additive has been an effective interface interaction of Mn–O–C buffers the structural degra-
way to prolong cycle life. Soundharrajan et al.80 added 0.1 M dation. There are additional reports on using graphene and
MnSO4 in 1 M ZnSO4 electrolyte, which suppressed the release porous carbon with ZMO for increasing the electrical conduc-
of Mn2+ from Mn3+ disproportion reactions by providing tivity of the cathode.170 Instead of using costly conductive
a dynamic equilibrium between the Mn2+ suspension and the agents, Li et al.171 came up with a one-step solution combustion
following oxidation of Mn2+ in the electrolyte. Although there method to directly grow porous ZMO on carbon cloth, creating
are many reports that use MnSO4 as an electrolyte additive, binder-free porous electrodes. The resulting electrodes
Deng et al. implied that the use of MnSO4 overestimates the demonstrated fast electrochemical kinetics, with an impressive
performance of the Mn oxide-based cathode, and Mn oxide will 1st discharge-specic capacity of 281 mA h g−1 at 100 mA g−1,
undergo phase transformation, as Mn2+ can be electro-oxidized making them ideal for rapid charge/discharge applications.
into active MnOx during the charge operations. The Jahn–Teller 6.3.1.2 Electrolyte optimization and cation-decient struc-
effect and Mn3+ disproportionation in Mn oxide cathodes oen tures. Although the inclusion of Mn2+ does not affect the solu-
cause irreversible structural changes and Mn2+ dissolution, bility of ZnSO4, Soundharrajan et al. reported that Mn2+
compromising cycling stability. Deng et al.109 introduced ZMO enhances capacity through the electro-deposition and dissolu-
quantum dots into a porous carbon framework, forming Mn–O– tion of MnOx on the electrode surface, enabling the subsequent
C bonds at the interface. These bonds effectively suppress the insertion and de-insertion of Zn-ions from the deposited
Jahn–Teller effect and Mn dissolution, leading to enhanced MnOx.80 In addition to the primary charge storage mechanism
electrochemical performance (Fig. 13(a–g)). Islam et al.164 also involving reversible Zn2+ intercalation/deintercalation within
investigated the structural changes during electrochemical the ZMO spinel structure itself,33,107,171 which contains Mn in
studies through in situ XRD analysis and observed that there is mixed oxidation states, a surface phase of MnOx is formed
a new phase formation of ZnMn3O7$3H2O from the reaction of through the electro-deposition of Mn2+ from the electrolyte.80
the Mn2+ additive and Zn4(OH)6SO4$xH2O during the charging Studies have provided direct evidence for Zn2+ insertion into the
process. The remaining Mn2+ ions deposit as MnOx on the ZMO lattice through techniques such as XRD and XPS.107,171
electrode surface. Thus, in addition to the intercalation of Zn2+, While proton insertion in ZMO cathodes might be overlooked
a combination of both conversion and deposition of Mn2+ in some studies, reversible proton insertion into the electro-
increases the capacitance of the cell (Fig. 13(h–k)). deposited MnOx layer could contribute to additional
To improve the stability of ZMO, a prevalent strategy is to capacity.172 It's plausible that both Zn2+ intercalation in ZMO
substitute Mn3+ with other elements to eradicate its dissolving and proton insertion in MnOx contribute to the overall charge
source. Dopants such as Fe2+, Mg2+, or Cu2+, incorporated into storage. Mn2+ plays a key role in the mildly acidic electrolyte by
the ZMO structure, convert Mn3+ into Mn4+ to uphold charge stabilizing the Mn oxidation states, suppressing Mn dissolu-
neutrality, thereby diminishing Mn dissolution.165–167 Hawari tion, and facilitating Zn2+ intercalation, potentially both directly
et al.168 added an iron (Fe) precursor during synthesis and ob- into ZMO and indirectly through the MnOx deposition/
tained the Zn0.5Mn0.5Fe2O4 phase within ZMO forming dissolution cycle.80,173 Furthermore, acidity can improve

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electrochemical reversibility by facilitating proton-assisted with a high current response, low overpotential, and enhanced
intercalation/de-intercalation of ions into the cathode. Under stability at high currents in stripping/plating experiments. This
mildly acidic conditions, protons aid charge compensation, indicates improved reversibility and faster Zn deposition/
reducing polarization and improving reaction kinetics.173 The dissolution kinetics with Zn(CF3SO3)2. Liu and colleagues174
presence of H+ ions can also contribute to the formation of investigated the impact of different valence ions (e.g., Na+, Mg2+,
a stable MnOx layer, enhancing redox activity and supporting and Al3+) as electrolyte additives to impede zinc corrosion and
long-term structural stability.80,173 However, excess acidity can dendrite growth. The presence of Na+ ions was found to effec-
accelerate H2 evolution and induce parasitic reactions at the tively restrict zinc dendrite growth due to its high adsorption
anode, leading to Zn corrosion and a decline in coulombic energy of approximately −0.39 eV, prolonging the duration of
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efficiency.173 Maintaining an optimal pH balance is therefore zinc dendrite formation to 500 h. Furthermore, they also
essential for ZMO cathodes, as it ensures efficient Zn2+ storage applied a polyaniline (PANI) coating on ZMO cathode materials,
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within the ZMO structure and potentially the MnOx layer, resulting in a narrow band gap of around 0.097 eV, thereby
mitigates Mn dissolution, and supports long-term cycling enhancing the kinetics of charge transfer.
stability.172,173 The ideal spinel structure presents challenges for ZIB cath-
Strong acidic electrolytes cause H2 evolution, while alkaline odes due to poor Zn2+ diffusion caused by electrostatic repul-
KOH electrolytes exhibit high polarization, limiting cyclability. sion within the lattice.175 To enhance diffusion, cation-decient
Mild acidic solutions with zinc salts (ZnCl2, Zn(NO3)2, ZnSO4 structures are employed, reducing repulsion and providing
and Zn(CF3SO3)2) were investigated. ZnCl2 and Zn(NO3)2 elec- vacancy-mediated transport pathways.110,143,164 This strategy has
trolytes suffer from Cl− and NO3− instability. Although both been shown to improve Zn2+ ion diffusion in spinel structures.
ZnSO4, and Zn(CF3SO3)2 have wide electrochemical stability Galvanostatic intermittent titration technique (GITT) analysis
windows, Zn(CF3SO3)2 demonstrated superior performance revealed that Zn2+ ion diffusion coefficients in Mn-decient

Fig. 14 (a) Diagram illustrating the synthesis process for ZMO/NCNTs and ZXMO/NCNTs; (b) CV curves of ZN0.5MO/NCNTs during the first three
cycles at a scan rate of 0.2 mV s−1; (c) GCD profiles and (d) cycling stability comparison of ZMO/NCNTs and ZN0.5MO/NCNTs at current densities
of 0.2 and 1.0 A g−1. Reproduced from ref. 177 with permission from Wiley Online Library, copyright 2024; (e) CV curves for the initial five cycles of
the MO–ZMO HOs electrode at a scan rate of 0.2 mV s−1; (f) long-term cycling performance of MO–ZMO HOs, MO HOs, and ZMO HOs
electrodes at a current density of 3 A g−1. Reproduced from ref. 61 with permission from Wiley Online Library, copyright 2021.

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ZMO range from 10−9 to 10−11 cm2 s−1, signicantly higher than transfer between Zn/Mn and coordinating atoms. These modi-
the 10−13 cm2 s−1 observed in pure ZMO. These Mn vacancies cations markedly enhance the electronic structure and
also hinder Mn3+ disproportionation into soluble Mn2+ by stability of the material, further illustrating the substantial
reducing Mn3+ concentration. Furthermore, Mn-decient ZMO inuence of sulfur on ZMO's electrochemical performance.
exhibits higher electronic conductivity compared to pure Wu et al.107 investigated the inuence of structural water in
ZMO.164 ZMO on Zn2+ intercalation energy barriers. Structural water
Mallick et al.176 synthesized defect-rich ZMO with Mn and Zn plays a crucial role in Zn2+ insertion kinetics by (1) enlarging
deciencies and Ni doping, where Ni partially substituted Mn interlayer spacing and stabilizing the host material, (2) acting as
and Zn sites. This resulted in a cubic spinel structure, a charge shield for metal ions, facilitating Zn2+ transport, and
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enhancing Zn2+ diffusion due to increased tunnel size. Ni (3) lowering the Zn2+ diffusion energy barrier, promoting fast
doping transformed the semiconducting ZMO into a metallic interfacial kinetics and preventing irreversible structural
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one, improving electronic conductivity for Zn2+ ion storage. The changes during cycling. This is supported by the calculated Zn2+
introduction of Ni2+ led to the formation of stable [MO6] octa- diffusivity in ZMO, which is signicantly higher with structural
hedra. Additionally, cationic doping promoted oxygen vacan- water (1.51 × 10−11 cm2 s−1) compared to ZMO without struc-
cies, further enhancing charge carrier mobility and electrical tural water (8.99 × 10−13 cm2 s−1). This highlights the impor-
conductivity. These oxygen vacancies act as shallow donors, tance of considering structural water in the design of ZMO
modifying the electronic conguration and increasing the cathodes for enhanced performance.
specic capacity of the material. A high specic capacity of 6.3.1.3 ZMO-based composites and nanostructures. Although
265 mA h g−1 was achieved at 100 mA g−1, with no capacity Mn2O3 offers high theoretical capacity, its practical application
fading and a subsequent two-fold increase in the specic is limited by reversible phase transitions during cycling, leading
capacity noted aer 5000 cycles. to decreased cycle life. ZMO, though exhibiting lower capacity,
In a similar vein, Wang et al.177 employed a microstructure maintains its spinel phase during cycling. To combine the
strain strategy, doping Ni into ZMO to enhance lattice stability advantages of both, Saadi-motaallegh et al.179 synthesized
for improved cycling performance. The introduction of Ni dis- a ZMO/Mn2O3 nanocomposite. By optimizing crystallite size
torted the MnO6 microstructure, creating asymmetrical path- and introducing Ni doping, they achieved a specic capacity of
ways that facilitated Zn2+ ion transport and (de)intercalation. 235 mA h g−1 (0.2 A g−1), exceeding that of the undoped
This led to enhanced reaction rates and structural reversibility. nanocomposite (215 mA h g−1). The Ni-doped nanocomposite
Temperature-dependent EIS spectral analysis using the Arrhe- also demonstrated superior capacity retention (91.32% aer
nius equation revealed that Ni-doped ZMO exhibited lower 3000 cycles at 2 A g−1) compared to the undoped composite
activation energy and higher Zn2+ ion diffusion coefficients (64.54% retention under the same conditions). This improve-
(conrmed via the GITT) compared to undoped ZMO. This ment is attributed to enhanced conductivity and specic surface
demonstrates that Ni substitution facilitates the de-solvation of area facilitated by Ni2+ doping. Furthermore, the hetero-
hydrated Zn ions, lowers the Zn ion transfer energy barrier, and structure formed at the ZMO/Mn2O3 interface creates a built-in
improves ionic diffusion kinetics (Fig. 14(a–d)). electric eld, accelerating interfacial reactions and charge
Yang et al.33 and Yuan et al.178 illustrated the advantages of transport. Similarly, Qin et al.,180 utilized a CuO coating on ZMO
sulfur doping in improving the efficacy of the ZMO material in to suppress Mn dissolution and improve cycle life. The CuO
ZIB applications. They demonstrated that the incorporation of coating also actively contributes to Zn2+ storage through
sulfur into a ZMO/CNT composite enlarges the ZMO lattice and a conversion reaction mechanism, enhancing conductivity and
enhances Zn2+ ion transport. Strong Mn–S interactions, facili- capacity. Zeng et al.61 utilized MOFs, known for their tunable
tated by electron density accumulation around Mn atoms, structures and chemical versatility, as templates to synthesize
promote charge redistribution and improve structural stability. Mn2O3–ZMO hollow octahedra. This strategy effectively tackled
Density functional theory (DFT) simulations indicated that the the challenges of limited cycling stability and rate capability in
Gibbs free energy for Zn2+/H+ adsorption near sulfur atoms in S- Mn-based oxides. The hollow architecture provided space to
ZMO/CNT approaches thermal neutrality, signifying enhanced accommodate volume changes during Zn2+ insertion and
reversibility relative to undoped ZMO/CNT. Sulfur doping extraction, preventing structural damage. Additionally, the
decreases the bandgap from 1.1844 eV to 0.7432 eV, enhancing synergistic effects at the Mn2O3–ZMO interface enhanced ion
electronic conductivity and diminishing Zn2+ interactions with transport and facilitated highly reversible electrochemical
the ZMO framework, leading to accelerated reaction kinetics, reactions (Fig. 14(e and f)).
enhanced ion diffusion, facilitated desorption of adsorbed Very recently, Katsuyama et al.181 developed 5 nm ZMO
Zn2+/H+ ions, a greater electrochemically active surface area, nanoparticles integrated with graphene, inhibiting nano-
and increased capacity.71 Furthermore, Yuan et al.178 performed particle aggregation and enabling a two-electron redox process.
theoretical and experimental investigations to enhance sulfur This resulted in a near-theoretical capacity of 406 mA h g−1 at
substitution in the ZMO lattice, based on the principle of 2000 mA g−1 and excellent rate performance. With a stoichio-
minimizing electrostatic repulsion during Zn2+ diffusion metrically equivalent zinc anode, the ZMO/G cathode achieved
leading to sluggish kinetics. Sulfur establishes covalent an energy density of 371 W h kg−1, potentially reaching 504 W h
connections with Zn and Mn, resulting in charge density kg−1 with increased active material loading, making ZIBs
accumulation within these bonds and facilitating charge competitive with LIBs. This highlights the potential of ZIBs as

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a cost-effective, safe, and environmentally friendly alternative to However, research on hybrid electrolytes for ZMO cathodes
LIBs. remains limited.
6.3.2 Organic/hybrid and quasi-solid-state electrolytes. Zhang et al.110 pioneered the use of a 3 M Zn(CF3SO3)2
Aqueous electrolytes are widely used in ZIBs, but they present acetonitrile–water hybrid electrolyte with a cation-decient
challenges such as a narrow electrochemical stability window, ZMO/C cathode, achieving a high reversible capacity
a limited operating temperature range (0.0–100 °C), and (150 mA h g−1 at 50 mA g−1) and excellent cycling stability. This
cathode dissolution. Zinc anode corrosion, exacerbated by H3O+ performance is attributed to the synergistic effects of the cation-
ions and the self-ionization of water, further hinders perfor- decient ZMO and the Zn(CF3SO3)2 electrolyte, which provides
mance. Dissolution of zinc salts lowers the pH, and Zn2+ a wide ESW and high Zn plating/stripping efficiency (Fig. 15).
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interactions with water weaken O–H bonds, promoting Similarly, Cai et al.183 demonstrated the effectiveness of 0.5 M
hydrogen evolution and accelerating corrosion. To address Zn(CF3SO3)2 in AN : H2O (8 : 2) hybrid electrolyte with an Al-
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these issues, “water-in-salt” electrolytes have been explored, doped ZMO cathode. Al3+ doping increased the (101) crystal
conning water molecules and minimizing Zn2+–H2O interac- plane spacing, facilitating Zn2+ diffusion. Despite the relatively
tions. Hybrid electrolytes, combining aqueous and organic modest capacity compared to aqueous systems, organic/hybrid
components, offer another promising approach. For example, electrolytes offer improved thermodynamic stability and
Wang et al.182 demonstrated that polar solvents such as DMAC reduced dendrite formation, warranting further investigation
and TMP in a hybrid electrolyte enhance dipole–dipole inter- for ZMO-based ZIBs.
actions with water, stabilizing O–H bonds and mitigating zinc 6.3.2.2 Gel and quasi-solid-state electrolytes. Gel and quasi-
corrosion. solid electrolytes offer enhanced safety and are particularly
6.3.2.1 Organic and hybrid electrolytes. While organic elec- suitable for exible ZIBs. However, research has explored ZMO
trolytes offer advantages such as a wider electrochemical in solid-state or quasi-solid-state congurations using oxygen-
stability window (ESW) and operational temperature range decient ZMO (OD-ZMO). Zhang et al.34 fabricated the rst
compared to aqueous electrolytes, they suffer from slow Zn2+ quasi-solid-state ZIB using oxygen-decient OD-ZMO which
migration kinetics due to strong Coulomb interactions with the exhibited p-type semiconductor characteristics with increased
solvent. This limits charge/discharge capability. Although non- carrier density. While Zn2+ diffusion in OD-ZMO was slightly
aqueous electrolytes can prevent water-induced side reactions, lower than in pristine ZMO, DFT calculations revealed a lower
high desolvation energies and low ionic conductivity hinder energy barrier (0.24 eV) for Zn atom mobility near oxygen
reaction kinetics. Hybrid electrolytes have shown promise in vacancies. OD-ZMO displayed a two-step intercalation process,
mitigating these issues, improving Zn anode compatibility by attributed to H+ and Zn2+ intercalation, leading to a higher
suppressing hydrogen evolution and dendrite formation. capacity (174 mA h g−1) compared to ZMO (98 mA h g−1).

Fig. 15 (a) CV curves of the Zn electrode in an aqueous solution of 1 M Zn(CF3SO3)2; (b) CV curves of the Zn electrode in 1 M ZnSO4 solution, both
obtained at a scan rate of 0.5 mV s−1 within the voltage range of −0.2 to 2.0 V; (c) schematic representation of Zn2+ insertion and extraction
mechanisms in the extended three-dimensional ZMO spinel structure; (d) conceptual illustration of Zn2+ diffusion pathways in the ZMO spinel
lattice, comparing scenarios with and without Mn vacancies. Reproduced from ref. 110 with permission from ACS, copyright 2016.

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Table 9 Electrochemical performance of various ZMO-based cathodes for ZIBs

Material composition Specic capacity (mA h g−1) Capacity retention (cycles) Reference

Zn/ZMO QD@C 320 at 0.1 A g−1 86% (1500) at 1 A g−1 109

ZMO/C 150 at 0.05 A g−1 94% (500) at 0.5 A g−1 110
Zn0.65Ni0.58Mn1.75O4 265 at 0.1 A g−1 100% (5000) at 2 A g−1 176
ZMO0.94H2O 230 at 0.5 A g−1 75% (2000) at 4 A g−1 107
OD-ZMO@PEDOT 221 at 0.08 A g−1 93.8% (300) at 1.29 A g−1 34
N-ZMO NTAs 223 at 0.1 A g−1 92.1% (1500) at 2 A g−1 162
220 at 0.1 A g−1 97% (2000) at 3 A g−1
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ZMO/CNT 143
S-ZMO/CNTs 175 at 0.5 A g−1 94.1% (800) at 1.5 A g−1 33
ZMO–CNT/C 209 at 0.5 A g−1 48% (1000) at 1 A g−1 169
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N-ZMO 225 at 0.3 A g−1 ∼40% (1000) at 3 A g−1 97

Zn/Mn-d-ZMO@C 194 at 0.1 A g−1 84% (2000) at 3 A g−1 164
Mn2O3 + ZMO 247 at 0.1 A g−1 93.3% (2000) at 3 A g−1 61
ZN0.5MO/NCNT 239 at 0.1 A g−1 78.5% (100) at 0.2 A g−1 177
ZMO/CuO 150 at 0.3 A g−1 ∼52% (1000) at 0.2 A g−1 180
ZMO@Ti3C2Tx 126 at 0.1 A g−1 92.4 (5000) at 1 A g−1 75
ZMO@PCP 176 at 0.1 A g−1 90.3 (2000) at 1 A g−1 184
ZMO-MnOOH/C 336 at 0.1 A g−1 79.1% (1000) at 1 A g−1 185
ZMO/Mn2O3 216 at 0.2 A g−1 97.8% (2000) at 2 A g−1 186
ZMO/Mn2O3 82 at 0.5 A g−1 ∼135% (300) at 0.5 A g−1 187
Ni-doped ZMO/Mn2O3 235 at 0.2 A g−1 91.3% (3000) at 2 A g−1 179

Coating with PEDOT further enhanced Zn2+ intercalation, Additionally, the 1D hollow structure alleviates mechanical
resulting in a total capacity of 221 mA h g−1. Shi et al.75 reported stress from zinc ion cycling, leading to exceptional long-term
detailed in situ and ex situ experiments on 3D assembly of durability (92.1% retention aer 1500 cycles). Signicantly,
ZMO@Ti-MXene (Ti3C2Tx) with a gelatin-based electrolyte. The the quasi-solid-state N-ZMO NTAs//Zn device was successfully
conductive MXene improved structural stability by inhibiting fabricated, achieving an impressive energy density of 214.6 W h
ZMO aggregation and dissolution. This cathode achieved a high kg−1 and a peak power density of 4 kW kg−1.162 Huang et al.97
specic capacity (172 mA h g−1) and excellent cycling stability synthesized N-doped mesoporous ZMO nanocages with abun-
(∼92.4% aer 5000 cycles). Post-mortem analysis revealed that dant oxygen vacancies to overcome the inherent sluggish rate
capacity fading was primarily due to the formation of the irre- capacitance. N-doped ZMO exhibits excellent ability to store
versible ZnO byproduct. Zn2+ with high specic capacity (225 mA h g−1 at 0.3 A g−1), good
Qiu et al. developed exible quasi-solid-state ZIBs utilizing rate performance, and cycling stability (85.7% aer 1000 cycles
modulated N-doped coupled oxygen vacancies as the cathode, at 3 A g−1). A exible quasi-solid-state device was constructed
enhancing conductivity, ion transport, active sites, and surface with a high energy density of 261.6 W h kg−1, demonstrating
capacitive contribution. The quasi-solid-state device exhibited long-lasting durability.
similar electrochemical behavior to aqueous systems, indi- Table 9 presents a comprehensive overview of various ZMO-
cating good Zn2+ insertion/extraction kinetics. The PVA/LiCl– based cathode materials for ZIBs, highlighting their specic
ZnCl2–MnSO4 gel electrolyte played a critical role, with LiCl capacities, capacity retention, and corresponding references.
enhancing conductivity, MnSO4 mitigating ZMO dissolution, The table showcases the diverse approaches employed to
and ZnCl2 improving discharge voltage, resulting in improved enhance the performance of ZMO in ZIBs, including the
energy and power densities. Moreover, the one-dimensional incorporation of conductive materials, doping, and the creation
(1D) hollow NTA nanoarchitecture facilitated rapid zinc ion of cationic vacancies.
diffusion, yielding a high specic capacity (223 mA h g−1 at Table 10 compares the impact of aqueous and organic elec-
0.1 A g−1) and favorable rate capability (133 mA h g−1 at 4 A g−1). trolytes on ZMO performance in ZIBs. Aqueous electrolytes offer

Table 10 Impact of electrolyte on ZMO performance in ZIBs

Electrolyte
type Advantages Disadvantages

Aqueous - High ionic conductivity - Mn dissolution

- Cost-effective - Dendrite formation
- Limited electrochemical window
Organic - Wider electrochemical window - Lower ionic conductivity
- Improved stability - Higher cost
- Reduced dendrite formation - Sluggish Zn2+ migration kinetics

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Table 11 Strategies to overcome challenges in ZMO battery applications

Challenge Battery type Modication strategy Key benets

Low electrical conductivity LIBs, SIBs, and ZIBs - Composite with conductive materials - Enhanced charge transfer
(CNTs, graphene, MXenes, etc.)
- Doping (Ni, S, N, etc.) - Improved rate capability
- Introduce oxygen vacancies - Increased capacity
Volume changes during cycling LIBs and ZIBs - Nanostructured ZMO - Accommodate volume
expansion/contraction
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- Porous/hollow architectures - Maintain structural integrity

- Core–shell structures - Improve cycling stability
Mn dissolution ZIBs - Mn2+ additives in electrolyte - Suppress Mn dissolution
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- Mn–O–C bond formation - Improve cycling stability

- Cation-decient ZMO
- pH modication
Zn2+ diffusion limitations ZIBs - Cation-decient ZMO - Enhanced Zn2+ diffusion kinetics
- Lattice expansion through doping - Improved rate capability and
capacity
- Lattice expansion through S doping
- Structural water incorporation
- Al3+ doping to increase interlayer spacing
Electrolyte limitations LIBs and ZIBs - “Water-in-salt” electrolytes - Enhance ion desolvation
- Optimize Li-based electrolytes (LiTFSI in - Improve ionic conductivity
EC-DMC)
- Explore Zn-based bulky-anion electrolytes - Widen the electrochemical
(Zn(CF3SO3)2) stability window
- Hybrid electrolytes (aqueous/organic) - Suppress dendrite formation and
corrosion
- Gel and quasi-solid-state electrolytes - Enhance safety and exibility
- Additives to suppress Zn dendrite growth - Improve Zn2+ migration kinetics

high ionic conductivity but can lead to Mn dissolution and Although ZMO is a promising cathode material for ZIBs due
dendrite formation. Organic electrolytes provide wider electro- to its cost-effectiveness and high theoretical capacity, several
chemical windows and improved stability but have lower ionic challenges hinder its practical application. Table 11 outlines the
conductivity. Despite these promising results, gel electrolytes major challenges encountered in utilizing ZMO as a battery
still face stability challenges and oen exhibit low ionic material and the corresponding modication strategies
conductivity. Further research is needed to improve their employed to address these limitations.
performance and enable the development of efficient, exible To sum up the battery section, ZMO has shown promise as
ZIBs for wearable technologies. an electrode material in various battery systems, including LIBs,

Table 12 ZMO in different battery systems: advantages, disadvantages, and mitigation strategies

Battery type Advantages Drawbacks Mitigation strategies

LIBs - High theoretical capacity - Low electrical conductivity - Composite with conductive materials
(CNTs and graphene)
- Low working voltage - Volume changes during cycling - Design unique nanostructures
(such as hollow spheres and core–shell)
- Abundant - Optimize electrolyte formulations
SIBs - Environmentally friendly - Larger ionic radius of Na+ compared to Li+ - Develop suitable anode materials
- Fewer studies and less mature technology - Composite with conductive materials
compared to LIBs
- Design 3D hierarchical architectures
ZIBs - Irreversible structural transformation and - Create cation-decient structures
Mn3+
disproportionation reaction
- Low electrical conductivity - Introduce elemental doping
- Composite with conductive materials
(CNTs and MXenes)
- Explore organic/water cosolvent
electrolytes

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Table 13 Summary of ZMO applications in various battery systems

Battery type ZMO role Modication strategy Key benets observed

LIBs and SIBs Anode - Nanostructured ZMO - High capacity (exceeding the theoretical value in some cases)
- Porous/hollow architectures - Improved cycling stability and rate capability
- ZMO/carbon composites - Enhanced conductivity and Li+ diffusion
- Defect engineering and doping - Mitigated volume changes
- Electrolyte optimization - High capacity and cycling stability
- ZMO/nitrogen-doped graphene composites - Good rate capability
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- ZMO microtubules
ZIBs Cathode - Conductive additives (CNTs and graphene) - Improved capacity and cycling stability
- Mn dissolution suppression strategies - Enhanced Zn2+ diffusion and conductivity
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- Electrolyte optimization - Mitigated Zn dendrite growth and corrosion

- Cation-decient ZMO - Wider electrochemical stability window
- ZMO-based composites and nanostructures
- Organic/hybrid and quasi-solid-state electrolytes

Table 14 Summary of ZMO performance in energy storage applications

Application Key ndings Challenges

Supercapacitors - ZMO/carbon composites show promise - Achieving synergy in ZMO/carbon composites

- Synergy between ZMO and the composite material is crucial - Balancing conductivity and active sites
- High specic capacitance achieved with cost-effective porous ZMO
LIBs - ZMO/carbon composites show promise as anode materials - Low electrical conductivity
- High specic capacity and long-term cyclability achieved - Volume changes during cycling
SIBs - ZMO with microtubular morphology shows potential - Poor cyclability
- Carbon-based materials help mitigate stress during cycling
ZIBs - ZMO shows potential as a cathode material - Low specic capacity
- Further exploration needed to achieve higher specic capacities

SIBs, and ZIBs. Its properties, such as high theoretical capacity, composites have garnered signicant attention, showing
low cost, and environmental friendliness, make it an attractive promise for supercapacitor applications despite complex
option for next-generation energy storage. However, ZMO also synthesis and high costs. A notable breakthrough is the cost-
presents challenges such as low electrical conductivity and effective porous ZMO with high specic capacitance. However,
volume changes during cycling, which can affect its perfor- achieving synergy in ZMO/carbon composites for super-
mance and long-term stability. To address these issues, capacitors remains a challenge, as carbon can reduce active
researchers have explored various strategies, including sites while improving cycling stability.
compositing ZMO with conductive materials such as CNTs and Beyond supercapacitors, ZMO demonstrates versatility in
graphene, and designing unique nanostructures to accommo- various battery types. ZMO/carbon composites show promise as
date volume changes and enhance ion diffusion. Table 12 anode materials in LIBs and SIBs, exhibiting high specic
summarizes the advantages and disadvantages of using ZMO in capacity and long-term cyclability. ZMO also shows potential in
different battery types. As the table illustrates, ZMO offers ZIBs, though further exploration is needed to achieve higher
distinct advantages and faces specic challenges depending on specic capacities. This versatility makes ZMO a compelling
the battery type. In LIBs, the focus is on enhancing conductivity solution for energy storage, contributing to the development of
and mitigating volume changes, while in SIBs, the research sustainable and cost-effective energy solutions. Table 13
aims to address the limitations associated with sodium-ion size provides a concise overview of ZMO's applications in different
and advance the technology's maturity. For ZIBs, the emphasis battery types, highlighting the key modication strategies
is on improving structural stability, preventing manganese employed and the resulting benets.
dissolution, and enhancing Zn2+ diffusion kinetics. By under-
standing these nuances, researchers can tailor their strategies 7.1 Key ndings and challenges
to optimize ZMO's performance in each battery system, paving
In supercapacitors, ZMO exhibits pseudocapacitive behavior,
the way for its successful integration into next-generation
offering high energy density and power density. The electro-
energy storage devices.
chemical performance is strongly inuenced by factors such as
morphology, surface area, and the incorporation of conductive
7 Conclusion and perspectives additives or dopants. HT and ST synthesis methods have been
widely employed to control these parameters, leading to the
This review has explored the extensive research on ZMO, fabrication of ZMO with diverse morphologies, including
focusing on its use in energy storage devices. ZMO/carbon nanowires, nanosheets, and hollow microspheres. While

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signicant progress has been made, challenges remain in potential for higher energy density and reduced reliance on
achieving high cycling stability and rate capability, particularly critical materials like lithium.188–190 For ZMO cathodes in ZIBs,
at high current densities. Future research should focus on anode-free congurations present unique opportunities and
developing strategies to enhance the electrical conductivity and challenges. Research should focus on mitigating issues such as
structural integrity of ZMO-based electrodes, such as opti- zinc dendrite formation and uneven deposition, which can lead
mizing synthesis conditions, surface modications, and the to battery failure. Strategies may include electrolyte optimiza-
formation of robust composites with conductive materials such tion, surface modication of the current collector, and
as graphene and carbon nanotubes. advanced characterization techniques to understand the
In batteries, ZMO has been investigated as an electrode mechanisms of zinc deposition in anode-free systems. This
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material for LIBs, SIBs, and ZIBs. Its high theoretical capacity, could unlock ZMO's full potential in applications ranging from
low cost, and environmental friendliness make it an attractive portable electronics to grid-scale energy storage.
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alternative to conventional electrode materials. However, chal-  Flexible and wearable devices: the development of exible
lenges such as low electrical conductivity and volume changes and wearable energy storage devices is a rapidly growing area.
during cycling need to be addressed to realize its full potential. ZMO, with its potential for fabrication into thin lms and
Table 14 summarizes the key ndings and challenges for ZMO nanostructures, can be integrated into exible substrates for
in various energy storage applications, highlighting the need for applications in wearable electronics, sensors, and implantable
further research to fully realize its potential. medical devices. Research should focus on developing scalable
fabrication methods for ZMO-based exible electrodes and
optimizing device design for exibility, durability, and high
7.2 Outlooks and future directions energy density.
To further advance ZMO research and its practical applications  Theoretical modeling and simulation: utilizing DFT
in energy storage, several key areas warrant attention: calculations and other computational methods can provide
 Emerging energy storage technologies such as hybrid valuable insights into the electronic structure, charge transfer
supercapacitors and metal–air batteries, warrant further inves- mechanisms, and ion diffusion pathways in ZMO. This theo-
tigation. In hybrid supercapacitors, ZMO can be combined with retical understanding can guide material design and optimi-
high-surface-area carbon materials to enhance energy and zation, leading to the development of ZMO with enhanced
power density, necessitating research focused on optimizing electrochemical properties.
electrode design and electrolyte to balance these characteristics.  Machine learning and articial intelligence: the applica-
For metal–air batteries, ZMO holds promise as a bifunctional tion of machine learning (ML) and articial intelligence (AI) can
catalyst for oxygen reduction and evolution reactions, requiring accelerate the discovery and optimization of ZMO-based mate-
efforts to enhance its catalytic activity and stability. rials for energy storage. AI algorithms can analyze large datasets
 Advanced characterization: employing advanced charac- from experiments and simulations to identify patterns and
terization techniques, such as in situ X-ray diffraction (XRD), X- predict the performance of new materials, guiding the design of
ray absorption spectroscopy (XAS), and electron microscopy, high-performance electrodes and devices.
can provide deeper insights into the structural evolution of  Sustainability and life cycle assessment: as the demand for
ZMO during electrochemical processes. This understanding is energy storage technologies grows, it is essential to consider the
crucial for optimizing material design and synthesis strategies sustainability and environmental impact of material production
to enhance performance and stability. and disposal. Research should focus on developing environ-
 Electrolyte optimization: the development of electrolytes mentally friendly synthesis methods for ZMO, utilizing abun-
tailored to specic battery systems is essential. For LIBs and dant and non-toxic precursors, and minimizing waste
SIBs, research should focus on electrolytes that form stable SEIs generation. Life cycle assessments can help evaluate the envi-
to minimize capacity fading and enhance cycling stability. In ronmental impact of ZMO-based energy storage devices,
ZIBs, the focus should be on electrolytes that suppress Mn guiding the development of sustainable and responsible
dissolution and dendrite formation, which are major contrib- manufacturing practices.
utors to performance degradation. Exploring novel electrolyte By pursuing these research directions, we can unlock the full
additives and organic/water cosolvent systems can lead to potential of ZMO, contributing to the development of high-
signicant improvements in battery performance and longevity. performance, sustainable, and cost-effective energy storage
 Solid-state batteries (SSBs): investigating the application of solutions for a wide range of applications. The future of ZMO in
ZMO in SSBs is a promising direction. SSBs offer advantages in the energy storage landscape is promising, and continued
terms of safety and stability compared to liquid electrolytes. research efforts will undoubtedly lead to further breakthroughs
However, challenges such as interfacial resistance and ion and innovations in this eld.
transport need to be addressed. Research efforts should focus
on developing compatible solid-state electrolytes and opti- Data availability
mizing electrode/electrolyte interfaces to enable efficient ion
transport and enhance the performance of ZMO-based SSBs. No primary research results, soware or code have been
 Anode-free application: anode-free batteries are a prom- included and no new data were generated or analysed as part of
ising technology for next-generation energy storage due to their this review.

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12 J. Baneshi, M. Haghighi, N. Jodeiri, M. Abdollahifar and

Author contributions
H. Ajamein, Energy Convers. Manage., 2014, 87, 928–937.
Joel Kingston Ramesh and Sasan Rostami: investigation, visu- 13 M. Abdollahifar, M. Haghighi and A. A. Babaluo, J. Ind. Eng.
alization, writing – original dra, writing – review & editing. Chem., 2014, 20, 1845–1851.
Rajesh Jayaprakasan and R. Margrate Bhackiyavathi Princess: 14 M. Abdollahifar, M. Haghighi, A. A. Babaluo and
investigation, writing – original dra. Radhika Govindaraju: S. K. Talkhoncheh, Ultrason. Sonochem., 2016, 31, 173–183.
investigation, writing – original dra, writing – review & editing. 15 S. K. Talkhoncheh, M. Haghighi, S. Minaei, H. Ajamein and
Jinho Kim: funding acquisition, project administration, writing M. Abdollahifar, RSC Adv., 2016, 6, 57199–57209.
– review & editing. Rainer Adelung: funding acquisition, project 16 N. Nitta, F. Wu, J. T. Lee and G. Yushin, Mater. Today, 2015,
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.

administration, writing – review & editing. Palanisamy Rajku- 18, 252–264.

mar: investigation, visualization, writing – original dra, 17 S.-M. Chen, R. Ramachandran, V. Mani and R. Saraswathi,
Open Access Article. Published on 02 April 2025. Downloaded on 4/28/2025 6:48:17 AM.

writing – review & editing. Mozaffar Abdollahifar: conceptuali- Int. J. Electrochem. Sci., 2014, 9, 4072–4085.
zation, investigation, visualization, writing – original dra, 18 M. Abdollahifar, H.-W. Liu, C.-H. Lin, Y.-T. Weng,
writing – review & editing, project administration, supervision. H.-S. Sheu, J.-F. Lee, M.-L. Lu, Y.-F. Liao and N.-L. Wu,
Energy Environ. Mater., 2020, 3, 405–413.
19 M. Chiku, M. Abdollahifar, T. Brousse, G. Z. Chen,
Conflicts of interest O. Crosnier, B. Dunn, K. Fic, C.-C. Hu, P. JeŻOwski,
The authors declare that they have no known competing A. MaĆKowiak, K. Naoi, N. Ogihara, N. Okita, M. Okubo,
nancial interests or personal relationships that could have W. Sugimoto and N.-L. Wu, Electrochemistry, 2024, 92,
appeared to inuence the work reported in this paper. 074002.
20 M. Abdollahifar, P. Lannelongue, H.-W. Liu, H. Chen,
C.-H. Liao, H.-S. Sheu, J.-F. Lee, Y.-F. Liao and N.-L. Wu,
Acknowledgements ACS Sustain. Chem. Eng., 2021, 9, 13717–13725.
21 R. Sagar and A. S. Gandhi, Appl. Phys. A, 2021, 127, 84.
Funding for this work was provided by the German Federal 22 J. Zia, E. S. Aazam and U. Riaz, J. Mater. Res. Technol., 2020,
Ministry for Education and Research (BMBF) for the funding of 9, 9709–9719.
the research project of SiLiNE (Reference No. 03XP0419B) and 23 L. Jiang, J. Hu, S. Yan, Y. Xue, S. Tang, L. Zhang and Y. Lv,
by the Deutsche Forschungsgemeinscha (DFG, Project No. Microchem. J., 2022, 172, 106990.
546658628). Also, this research was supported by the “Korea 24 S. Agrohiya, S. Dahiya, I. Rawal, P. K. Goyal, A. Ohlan,
Medical Device Development Fund (KMDF) grant funded by the R. Punia and A. S. Maan, J. Mater. Sci.: Mater. Electron.,
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