Perspective

pubs.acs.org/JPCL

High-Energy Cathode Materials (Li2MnO3−LiMO2) for Lithium-Ion

Batteries
Haijun Yu† and Haoshen Zhou*,†,‡
†
Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Umezono 1-1-1,
Tsukuba, 305-8568, Japan
‡
National Laboratory of Solid state Microstructures & Department of Energy Science and Engineering, Nanjing University, Nanjing
210093, China

ABSTRACT: Lithium-rich layered oxide materials xLi2MnO3·(1−x)LiMO2 (M = Mn, Ni,

Co, Fe, Cr, etc.) have attracted much attention for the use of cathode materials in lithium-
ion batteries in recent years. However, there are many issues still unclear (the structure
and reaction mechanism are ambiguous until now), and numerous scientific challenges
(low initial Coulombic efficiency, poor rate capability, and voltage degradation during
cycling) of these materials that must be overcome to realize their utilization in commercial
lithium-ion batteries. This Perspective focuses on the challenges and prospects associated
with the current researching results of these lithium-rich layered cathode materials.
Specifically, their average/local structures, reaction mechanisms, and electrochemical
properties are discussed.

A s the problems of fossil energy exhaustion, global

warming, and environment pollution plague modern
society, sustainable energies have gradually become a world-
wide topic. There have also been increasing demands for wind
or solar power stations and low-emission or zero-emission
electric vehicles. Lithium-ion batteries are of great significance
as power sources to satisfy these demands and realize a low-
carbon society.1−5 However, the energy density of current
lithium-ion batteries is still not enough for market require-
ments, and their cost and environment-related issues should be
also considered for much broader market penetration.6,7
Owing to the key roles of cathode materials on energy
density and the cost of current lithium-ion batteries, several
alternative cathode materials, such as LiCoO 2 , Li-
Ni0.8Co0.15Al0.05O2, LiNi0.33Co0.33Mn0.33O2, spinel LiMn2O4,
olive LiFePO4, and so on, have been commercially used in
lithium ion batteries.3,4 However, the available rechargeable Figure 1. Voltage and capacity of the main cathode materials for
capacity for all of these materials almost approaches their limits lithium-ion batteries.
(120−200 mAh/g), thus cathode materials associated with
higher specific capacity are needed to meet the demand for
further energy density enhancement of lithium-ion batteries. notations are equal to the same material and have been used
During the past two decades, much effort on exploiting new extensively in the published literature. For example, the
cathode materials has been done (Figure 1).3,7−10 0.5Li2MnO3·0.5LiMn0.42Ni0.42Co0.16O2 material can also be
Among the reported cathode materials so far, the lithium-rich described as Li[Li0.2Mn0.567Ni0.166Co0.067]O2.13−15 In this
layered oxide materials (LLOs) have attracted much attention Perspective, we will focus on the challenges and prospects of
in recent years because their capacities can be larger than 280 these LLOs with three sections for the next-generation lithium-
mAhg−1 with 3.6 V or larger operating voltages when these ion batteries. In the first section, the design theory and average/
materials are charged to over 4.6 V at room temperature.11−15
These LLOs can be described with two completely different Received: January 7, 2013
notations: xLi2MnO3·(1−x)LiMO2 (M = Mn, Ni, Co, Fe, Cr, Accepted: March 28, 2013
etc.) and Li1+(x/(2+x))M′1−(x/(2+x))O2 (M′ = Mn+M). Both Published: March 28, 2013

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The Journal of Physical Chemistry Letters Perspective

local structures are discussed. The second section discusses the inspired by Hunter’s discovery that acid treatment of the spinel
reaction mechanisms, especially the first charge and discharge LiMn2O4 yielded λ-MnO2 with a Mn2O4 spinel framework,
processes. The third section describes the electrochemical Thackeray et al. synthesized the layered lithium−manganese
performance, current main problems, and improving methods oxide compound Li2−xMnO3−x/2 (0 < x < 2) with a cubic-close-
of these LLOs. packed oxygen anion array by chemical leaching of Li2O from
the rock salt phase Li2MnO3 (Li2O·MnO2) with acid at 25 °C,
The rechargeable capacity and and got the compound Li1.09Mn0.91O2 or
energy density of LLO materials at 0.2Li2MnO3·0.8LiMnO2 after relithiation in an electrochemical
cell.16,17 The structure stability of this compound is much
room temperature can be close to better than that of the pure layered LiMnO2 cathode material
280 mAhg−1 and 1000 Whkg−1, during electrochemical cycling, and then the xLi2MnO3·(1−
respectively, which are about x)LiMnO2 material concept is first introduced. When Kalyani
twice that of current commercial et al. first found that the monoclinic Li2MnO3 material could be
activated electrochemically by charging the Li/Li2MnO3 cell to
cathode materials for lithium ion 4.5 V, the xLi2MnO3·(1−x)LiMO2 (M = Mn, Ni, Co, Fe, Cr,
batteries. etc.) materials became more and more attractive.18 This
notation can not only describe the electrochemical processes
These LLOs were first researched as cathode materials for of these LLOs combining with the single LiMO2 (M = Mn, Ni,
rechargeable lithium batteries by Thackeray et al. when they Co, Fe, Cr, etc.) and Li2MnO3 component electrochemical
were researching the LiMnO2 layered materials.16,17 In 1991, process, but also indicates that the cathode materials for lithium

Figure 2. Crystal structure of the (a) rhombohedral LiMO2 structure (space group: R3̅m, M = Ni, Co, Mn, Fe, Cr, etc.) and (b) monoclinic
Li2MnO3 structure (space group: C2/m) viewed from the [100] crystallographic direction. (c) Synchrotron powder X-ray diffraction pattern and
Rietveld refinement profile of the 0.5Li2MnO3·0.5LiMn0.42Ni0.42Co0.16O2 material with rhombohedral and monoclinic structures. Reprinted with
permission from ref 13. Copyright 2012 Royal Society of Chemistry. (d) Bragg filtered high-angle annular dark field scanning transmission electron
microscopy (STEM-HAADF) image of the Li1.2Mn0.61Ni0.18Mg0.01O2 material, containing Li2MnO3 parts (blue) and LiNi0.45Mn0.525Mg0.025O2 ones
(green). Reprinted with permission from ref 33. Copyright 2012 American Chemical Society.

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The Journal of Physical Chemistry Letters Perspective

Figure 3. (a) XRD patterns and (b) hexagonal lattice parameters of the xLi2MnO3·(1−x)LiNi1/2Mn1/2O2 (x = 0, 1/3, 1/2, and 1) materials.
Reprinted with permission from ref 40. Copyright 2012 Royal Society of Chemistry. (c) Homogeneous solid solution structure with partial ordered
C2/m monoclinic phase viewed from the [100] crystallographic direction. (d) Aberration-corrected scanning transmission electron microscopy
(STEM) image of the Li[Li0.2Ni0.2Mn0.6]O2 crystal. Reprinted with permission from ref 37. Copyright 2011 American Chemical Society.

ion batteries can be designed with different contents of LiMO2 Li2MnO3 structure (space group: C2/m) viewed from their
(M = Mn, Ni, Co, Fe, Cr, etc.) and Li2MnO3 components, [100] crystallographic direction, respectively. As the Li2MnO3
realizing the variational electrochemical performances (re- structure can be reformulated with Li[Li1/3Mn2/3]O2, the
chargeable capacity, rate performance, and cycle stability) of monoclinic Li 2 MnO 3 structure is very similar to the
lithium ion batteries.11,19−24 Following this materials desig- rhombohedral LiMO2 structure, and can be considered as a
nation proposition, many other composite materials between particular case of LiMO2 with an M layer consisting of a
Li2NO3 (N = Mn, Ti, and Zr) and layered LiMO2 (M = Mn, periodic sequence of one Li and two Mn atoms. Thus, both of
Ni, Co, Fe, Cr, etc.) or spinel LiMn2O4 have also been these two structures can be considered layered α-NaFeO2-type
proposed and researched during the past decade.25−28 Among rock salt structures, and all the octahedral sites of their close-
them, these LLOs are hot topics of research for their high packed oxygen arrays are occupied. The experimental
energy density and low cost. synchrotron X-ray diffraction (SXRD) patterns of the LLO
Owing to the importance of the relationship between the (0.5Li2MnO3·0.5LiMn0.42Ni0.42Co0.16O2) is shown as the cyan
structure and electrochemical performance of electrode circles in Figure 2 c.13 It is clear that all peaks can be well
materials for lithium-ion batteries, it is necessary to reveal the indexed on the basis of LiNiO2 structure with a space group
actual structure of these LLOs to deeply understand and R3̅m except for those weak peaks around 6.4−8°. These peaks
precisely control their electrochemical performance. Until now, can be indexed to the (020), (110), and (1̅11) lattice planes of
there has been an ongoing debate in the literature on whether a Li2MnO3-like unit cell with monoclinic (C2/m) symmetry,
these LLOs form homogeneous solid solutions or Li2MnO3 indicating the existence of Li2MnO3-like phase structure. On
domains within a LiMO2 matrix.3,20,27,29−42 In this Perspective, the basis of the Rietveld structure refinement of this material
first, we will discuss the pristine structures of these LLOs based with different models by the RIETAN-FP program, the whole
on the average and local structures analysis. diffraction pattern, including the weak peaks around 6.4−8°,
Figure 2a,b shows the rhombohedral LiMO2 structure (space can be refined well if the two-phase model consisting of
group: R3̅m, M = Co, Ni, Mn, Fe, Cr, etc.) and monoclinic rhombohedral LiMn0.42Ni0.42Co0.16O2 (space group R3̅m) and
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Figure 4. The expected morphology evolution of the TM plane in xLi2MnO3·(1−x)LiCoO2, showing the coexistence of Co and LiMn2 domains: (a)
x = 0.15; (b) x = 0.45; (c) x = 0.75; and (d) x = 0.90. The rhombohedral (R) and monoclinic (M) unit cells are indicated in the figure. Reprinted
with permission from ref 29. Copyright 2011 American Chemical Society.

monoclinic Li2MnO3 (space group C2/m) structures are worth noting that the X-ray diffraction techniques (XRD or
chosen. The phase fractions of the rhombohedral and SXRD) can only provide key information on the average crystal
monoclinic components are 43% and 57%, respectively, and structure. As a matter of fact, the large difference between the
v e r y c l o s e t o t h e c o m p o s i t i o n o f t h e s t u d i ed atomic number, size, and tendency for like or unlike atom
0.5Li2MnO3·0.5LiMn0.42Ni0.42Co0.16O2 material.13 clusters of the elements (Li, Mn, Ni, Co, Fe, Cr) inside these
By the high-resolution transmission electron microscopy LLOs can induce large lattice distortions, although these LLOs
(HRTEM) technique combined with electron energy-loss can preserve the periodical long-range structure. There are
spectroscopy (EELS) technique, Wen, Abraham, Tabuchi et many examples of multistructural phases existing inside one
al. find that the locally monoclinic (Li2MnO3-like) regions are material although their long-range structures can be considered
existed in the parent rhombohedral structure of these a solid solution.29,44,45
LLOs,22,29,31,36 and there are obvious Mn-rich nanodo- Thus, the local structure studies on these LLOs are very
mains.22,32,43 The recent research on these LLOs by high- important to investigate their actual structure. Jarvis et al.
angle annular dark field scanning transmission electron carefully investigated the Li[Li0.2Ni0.2Mn0.6]O2 material with a
microscopy (HAADF-STEM) also reveals the coexistence of diffraction scanning transmission electron microscopy (D-
two phases inside the Li1.2Mn0.61Ni0.18Mg0.01O2 STEM) technique, and indicated that this material formed a
(0.6Li2MnO3·0.4LiNi0.45Mn0.525Mg0.025O2) material; the C2/m partial ordered solid solution (Figure 3c,d).37 At the same time,
structure with accentuated contrast slab and the R3m ̅ structure they also concluded that, although two phases have not been
with attenuated contrast slab are encountered in the blue and observed for their studied material, other compositions,
green parts (Figure 2d), respectively, and the proportion of especially those with less excess lithium, may result in two-
these two structures are about 55% and 45%, respectively, phase regions.37 For Li2MnO3 material, there is no doubt that
which is in agreement with the component composition of their this material has been evidenced as the monoclinic structure
studied material.33 In addition, both extended X-ray absorption with C2/m space group.46 The ratio of Li and Mn content
fine structure (EXAFS) and Li magic-angle spinning (MAS) inside this material is 2. In particular, there are about 0.33 Li
nuclear magnetic resonance (NMR) spectroscopy techniques and 0.67 Mn atoms located at the monoclinic ordering of
studies on these LLOs also state that most Mn4+ in Li2MnO3- LiMn2 planes with Li−Mn−Mn periodic arrangement. For
like atomic environments and Mn+ in LiMO2-like (M = Co, Ni, Li[Li0.2Mn0.567Ni0.166Co0.067]O2, Li[Li0.2Co0.4Mn0.4]O2, or other
Mn, Fe, Cr; 2 ≤ n ≤ 4) atomic environments are contained LLOs associated with less lithium, the composition of these
inside these LLOs, and the locally monoclinic Li2MnO3-like materials can be described as the common formulation
structures are probably quasi-random distributed within the Li[LixM1−x]O2 (M = Mn, Ni, Co, etc.; x < 0.33).13,29,36 It is
rhombohedral α-NaFeO2 framework.27,29,38,39 obvious that there are not enough lithium sources to support
Although there is much two-phase evidence of these LLOs the overall LiM2 periodic ordering. Therefore, two-phase
by average and local structure studies, on the other hand, some domains with M−M (M = Mn, Ni, Co, Fe, and Cr) and Li−
researchers think that these LLOs are homogeneous solid M′−M′ (M′ = Mn, etc.) periodic ordering in local regions most
solutions between the two components Li2MnO3 and LiMO2 probably exist inside these LLOs when their compositions are
(M = Co, Ni, Mn, Fe, Cr), because their lattice parameters vary located between the LiMO2 (M = Mn, Ni, Co, Fe, and Cr) and
linearly with the composition of its end members (Figure 3 Li2MnO3 components. Experiments by Dahn et al. also
b).20,35,37,40 This indicates that these samples follow Vegard’s confirmed that more and more Li atoms occupied the Ni and
rule. These weak peaks around 25−35° (XRD data) are also Mn layer with the increase of y at fixed x = 1.1 for the
proposed as the result from long-rang Li ordering with a √3ahex LixMnyNixO2 (0.9 ≤ x ≤ 1.2; 0.1 ≤ y ≤ 0.5) material, the solid
× √3bhex superstructure in the transition-metal layer (Figure 3 solution series LixMnyNixO2 as a single phase could only be
a).35 Note that, although some researchers take the solid prepared for x near 1 and 0 ≤ y ≤ 0.5, and more impurities
solution opinion, the crystal symmetries of these LLOs are also (especially Li2MnO3) were shown with more larger y (y =
being debated. Some researchers consider these LLOs as being 0.6).47 In addition, the raw materials, preparation methods, and
composed of a solid solution with R3̅m rhombohedral calcination temperatures are also the important influence
symmetry,20,35 while others indicate that these LLOs belong factors for determining the structure of these LLOs with
to a solid solution with C2/m monoclinic symmetry.37 It is homogeneous solid solution or two phases. Therefore, the
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Figure 5. (a) Reaction pathways diagram through controlling the activation of the Li2MnO3 phase inside the 0.5Li2MnO3·0.5LiMn0.42Ni0.42Co0.16O2
material based on the three-dimensional compositional phase diagram. (b) The charge and (c) discharge curves with three different current densities.
(a−c) Reprinted with permission from ref 13. Copyright 2012 Royal Society of Chemistry.

domains with Li2MnO3-like components most probably exist these LLOs during lithium extraction and insertion processes
inside these LLOs, increased with the lithium and manganese can be well explained.
content (in proportion to x in the xLi2MnO3·(1−x)LiCoO2 For the 0.5Li2MnO3·0.5LiMn0.42Ni0.42Co0.16O2 electrode
equation) increasing, which is described by the simulated figure material, the reaction pathways and phase composition changes
in Figure 4.29 during the first charge region below 4.4 V with different current
The reaction mechanisms of these LLOs are very densities vary along the green lines in Figure 5a,b, from point 1
complicated, and have been extensively researched and to point 2. During this period, maximum 0.5 Li+ ions can be
discussed in the past decade.6,11,12,25,35,40,42,48−55 However, extracted from the lithium layer of the
these reaction mechanisms proposed are still being debated, 0.5LiMn0.42Ni0.42Co0.16O2 component associated predominantly
and most of them cannot explain all of the electrochemical with the oxidation of nickel ions from Ni2+ to Ni4+ and followed
phenomena or are merely supposition. by the trivalent cobalt oxidation process at higher voltage. The
In order to understand the complex electrochemical or practical charge capacity is 111, 108, and 103 mAh/g,
chemical reaction processes during the first and following cycles respectively, associated with 5, 20, and 50 mA/g initial current
of these LLOs, one of the reaction mechanisms associated with density at room temperature, which is a little smaller than the
an integrated three-dimensional compositional phase diagram theoretical capacity (126 mAh/g, assuming all the nickel and
(Figure 5a) is introduced based on Thackeray’s two-dimen- cobalt are oxidized to tetravalent).
sional phase diagram and our previous structure studies of these When the electrochemical potential of the Li/
LLOs.13 Following this phase diagram, the reaction pathways, 0.5Li2MnO3·0.5LiMn0.42Ni0.42Co0.16O2 cell increases from 4.4
phase composition change, and the reaction mechanism of to 4.8 V during the first charge process, presented with the blue
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long voltage plateaus in Figure 5 b, more lithium ions can be gas quantity in Figure 6a is associated with the voltage
extracted from the Li2MnO3 component together with the loss increasing, and the large amount of oxygen gas is emitted above
of oxygen and structure rearrangement, and there is probably a 4.5 V, corresponding to the charge plateau of these LLOs.
new phase (MnO2) formed. The oxygen accompanied with Through first-principles calculations, the oxygen 2p electron
lithium ions extraction and structure rearrangement phenom- clouds change significantly with the lithium extraction during
the first charge process, indicating that extra electrons that
ena during this process have been confirmed by Armstrong
cannot be provided by the transitional metal redox couples are
using in situ differential electrochemical mass spectrometry coming from oxygen ions (Figure 6b−e).34 However, it is very
(DEMS) (Figure 6a), Yabuuchi using SXRD, and Lu using difficult to unambiguously determine whether the new phase is
Rietveld analysis.50,52,56 It is clear that the evolution of oxygen MnO2, although new phases created during the first charge
process have been reported by Simonin and Gray in
Li[Li0.2Mn0.61Ni0.18Mg0.01]O2 and Li[Li1/9Ni1/3Mn5/9]O2 mate-
rials, respectively.57,58 In our opinion, based on electrochemical
performances, dQ/dV curves and kinetic analysis of these LLOs
during cycling, the new phase (MnO2) is most likely to appear
during the first charge process above 4.4 V, and transfer to the
cubic spinel-like framework (MnMO4, M = Ni, Co, and Mn)
during the following cycles.13,15 The theoretical capacity of this
process (from 4.4 V to 4.8 V) is calculated to be 251 mAh/g if
we suppose all of the lithium (0.5 Li2O) can be extracted from
the 0.5Li2MnO3 component. The practical charged capacities of
this process are 205, 162, and 139 mAh/g, respectively,
corresponding to different current densities (5, 20, and 50 mA/
g) at room temperature. All of the practical charged capacities
are close but smaller than those of theoretical capacity. Thus,
the reaction pathways and compositional changes of this
process at room temperature can be described with the blue
lines in Figure 5a, from point 2 to points 3, 3′, and 3″,
respectively. During these charge regions, when all of Li2O are
extracted from the Li2MnO3 component, the oxidized electrode
material will be Mn0.712Ni0.208Co0.08O2 with α = 0.208 and β =
0.08 in MO2 (M = Mn1−α−βNiαCoβ; 0≤ α ≤ 5/12, 0≤ β ≤1/6),
and the electrode composition changes along the blue dashed
line until it reaches the apex C of the tie-triangle in Figure 5a.

The reaction mechanisms associ-

ated with the mysterious anom-
alous capacity of these LLOs at
high temperature may be differ-
ent compared with those at room
temperature, and are still unclear.

During the first discharge process, lithium ions will insert

into the Mn0.42 Ni 0.42Co 0.16 O2 and newly formed MnO2
components, respectively, while the unactivated Li2MnO3
component still exist in these “composite” layered materials.
Our previous and Yabuuchi’s research results show that the
activated manganese redox reaction (Mn3+/Mn4+) occurs after
the first cycle of these LLOs.13,50 Therefore, the electro-
chemical reaction pathways and composition change of the first
discharge process is not suitable to be located in the Li2MnO3−
Figure 6. (a) Mass spectrometry analysis of O2 evolved on the 1st LiMO2 (M = Mn, Ni, and Co) tie-line in Figure 4 of ref 11, and
charging process of the Li/Li[Ni0.2Li0.2Mn0.6]O2 cell. Reprinted with should be located in the face compositing Li2MnO3, LiMnO2,
permission from ref 52. Copyright 2006 American Chemical Society. and LiMO2 (M = Mn, Ni, and Co) components. The reaction
(b) First-principle calculation sketch of partial oxygen layer in pathways and compositional changes during the first discharge
Lix/14Ni1/4Mn7/12O2 (pink balls: oxygen ions; colored polyhedrons:
adjacent TM slab) and its calculated spin density at (c) x = 14, (d) x =
processes of these LLOs with different current densities can
8, and (e) x = 0. Reprinted with permission from ref 34. Copyright follow the red lines in Figure 5a. During this process, the
2011 Royal Society of Chemistry. (f) Schemes of the proposed surface theoretical discharge capacity is calculated to be 269 mAh/g
reaction mechanisms in the Li1.2Ni0.13Co0.13Mn0.54O2 material. based on the weight of the
Reprinted with permission from ref 50. Copyright 2011 American 0.5LiMnO2·0.5LiMn0.42Ni0.42Co0.16O2, while the practical dis-
Chemical Society. charge capacity, corresponding to 5, 20, and 50 mA/g initial
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Figure 7. Charge and discharge curves of the Li/Li[Li1/5Ni1/5Mn3/5]O2 cell at (a) 55 °C and (b) 85 °C. Reprinted with permission from ref 40.
Copyright 2011 Royal Society of Chemistry.

charge/discharge currents, is 272, 224, and 184 mAh/g,

respectively (Figure 5c). It is obvious that all of the discharge Currently, the low initial coulom-
capacity except for the value with small current density (≤5 bic efficiency, unsatisfied rate
mA/g) can be explained by the proposed reaction mechanism. performance, and cycle stability
On the basis of surface reaction investigation of these LLOs, it
is suggested that some of the extra discharge capacity for these
of these LLOs are still the main
LLOs originated from the electrochemical reduction reaction of problems preventing their uti-
reduction
the oxygen molecules at the electrode surface (O2 ⎯⎯⎯⎯⎯⎯⎯⎯→ O−2 ), lization in practical lithium ion
but these contribution are suppressed by the accumulated batteries.
lithium carbonate formation at the electrode surface (Figure
6f).50
Nevertheless, with the temperature increased to 55 and 85 presenting a serious trade-off in lithium-ion battery design. At
present, most of the initial Coulombic efficiencies of these
°C, the rechargeable charge/discharge capacity of the Li/
LLOs in the published literature are smaller than 80% at room
Li[Li1/5Ni1/5Mn3/5]O2 can reach 300 mAh/g (Figure 7a), and
temperature, and the main reason is due to the irreversible
even 350 mAh/g (Figure 7b), which are much larger than the
reaction resulting from the first charge plateau above 4.4 V. In
theoretical capacity if we just consider the nickel (Ni2+/Ni4+)
order to improve the initial Coulombic efficiency, the
and manganese (Mn3+/Mn4+) valence variation.40 Ozhuku et al. preconditioned methods with NH3 and HNO3,53 surface
speculated that the highest rechargeable theoretical capacity at modification with nanostructured Al2O3, AlPO4, or RuO2,70
high temperature was contributed by the possible “cation” and ruthenium substitution14 for manganese on these LLOs
redox reaction (Mn4+/Mn5+, Mn5+/Mn6+) or “anion” redox have been conducted. The experiment results of ruthenium
(O2−/O22−) in a solid matrix in terms of lithium insertion substitution for manganese (Figure 8) show that all of the
scheme.40 The reaction mechanism of these LLOs at high initial columbic efficiency increased with the ruthenium content
temperature may be different and more complex compared increasing, and the highest initial columbic efficiency is 86%
with those at room temperature, and more experimental with 284 mAh/g discharge capacity at room temperature when
evidence or theoretical calculations for supporting these the content of substituted ruthenium is 5 mol %.14 The initial
hypotheses need to be conducted in the future to explain the Coulombic efficiency improvements are most probably
high mysterious rechargeable capacity of these LLOs. contributed by the content decrease of the Li 2 MnO 3
Although there are many debates on these LLOs currently, component or Li2MnO3 component, which can be activated
their large electrochemical capacities are still very attractive for inside these materials.
utilization as cathode materials in lithium ion batteries. In the Although most of the published literature shows that the
past, different composition, preparation methods, first or cycle performance based on the charge and discharge capacity
second crystalline grain morphology, surface treatments and of these LLOs is good, their voltage degradation after long
doping elements of these LLOs have been extensively cycling is extremely serious, which can largely lower the energy
studied.40,42,59−67 However, many problems of these LLOs output and efficiency of the lithium-ion batteries.69,71 In order
still exist at present. For example, the initial coulombic to investigate the cycle stability of these materials in detail, long
efficiency is low,14,68 and the cycle stability13,69 and rate time cycling of the Li/0.5Li2MnO3·0.5LiMn0.42Ni0.42Co0.16O2
performance15,62 of these materials still need to be amended to cell has been conducted.13 It is obvious that two cycling stages
satisfy the application requirements. (Stage I in Figure 9a and Stage II in Figure 9c) exist during
Low initial Coulombic efficiency associated with large long time cycling. During Stage I (from about the second cycle
irreversible capacity can result in mass ionized lithium, and to the 25th cycle), the charge and discharge capacities
the formation of a solid electrolyte interface (SEI) layer, thus (especially below 3.5 V) increase with cycling, while those
reducing the energy density of the lithium-ion battery and decrease during Stage II (from about the 26th cycle to the
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reduction peaks shifting to the lower voltage region (Figure

9d), indicating that the structures of these LLOs are not stable
during long cycling.
Observing Figure 9b,d, we found that there are obvious
swellings on the oxidation curves of dQ/dV between 3.0 and
3.2 V during Stage I, and this swelling become more and more
obvious during Stage II. These phenomena indicate that the
cubic spinel-like phase transformation of layered phase in local
regions may arise during stage I and gradually completed during
Stage II. In order to confirm these phase transformation, the
Raman spectroscopies of these materials with different initial
charge/discharge states were investigated. From observing
Figure 9e−i, it is clear that the shoulders characterizing cubic
spinel-like phase spectroscopy between 630 and 670 cm−1
appear and become more and more obvious with the cycle
number and initial charge/discharge current density increasing.
In addition, X-ray absorption spectroscopy (XAS),55 HRTEM
techniques,33,72 and dQ/dV curves42 studies on these LLOs
Figure 8. The initial Coulombic efficiency and the charge/discharge also show that the cubic spinel-like phases appear after long
capacity of the Li1.2Mn0.567−xRuxNi0.166Co0.067O2 (x = 0.00, 0.03, 0.05, cycling.
and 0.07) materials. Adapted from ref 14. Copyright 2012 Royal Surface coating with Al2O3, CeO2, ZrO2, SiO2, ZnO, AlPO4,
Society of Chemistry. and Li−Ni−PO4 and mildly acidic treatment on these LLOs
can enhance the cycling stability, but these coatings cannot
151st cycle). The increased capacity in Stage I is obvious and adequately overcome the voltage decay.53,73,74 That means that
corresponds to the new redox peaks (Ox3/Re3) increasing after the phase transitions from layered structure into cubic spinel-
the first cycle (Figure 9b). This indicates that the content of like structure of these LLOs not only occurs on the particle
activated manganese increases step by step during Stage I. surface, but also inside the particle bulk. As a matter of fact, the
Consequently, the charge/discharge capacity increases, and the transformation of layered Li0.5MO2 (delithiation) into the ideal
charge voltage decreases gradually, attributed to the lower cubic spinel phase (Li)8a[M2]16dO4 just requires a migration of
redox reaction voltage of Mn3+/Mn4+ with respect to that of one-fourth of the transition metal ion from the octahedral sites
nickel and cobalt. During Stage II, both the charge/discharge (3b sites) of the M planes into the empty octahedral sites (3a
capacity and discharge plateaus decrease with cycling. The sites) of the lithium planes and to what become 16d positions
decreased discharge plateaus are mostly contributed to the of spinel without changing the framework of closed-paced

Figure 9. Charge/discharge and dQ/dV profiles at different cycling stages ((a,b), Stage I: from the 2nd cycle to the 25th cycle; (c,d), Stage II: from
the 26th cycle to the 151st cycle) of the Li/0.5Li2MnO3·0.5LiMn0.42Ni0.42Co0.16O2 cell. Raman profiles of the 0.5Li2MnO3·0.5LiMn0.42Ni0.42Co0.16O2
materials with different testing conditions: (e) pristine material; (f,g,h) electrode materials after 51 electrochemical cycles with 50 mA/g, 20 mA/g
and 5 mA/g initial charge/discharge current density and 2.0−4.8 V cutoff voltage: (i) electrode material after 151 electrochemical cycles with 20
mA/g initial charge/discharge current density and 2.0−4.6 V cutoff voltage. (a−i) Reprinted with permission from ref 13. Copyright 2012 Royal
Society of Chemistry.

1275 dx.doi.org/10.1021/jz400032v | J. Phys. Chem. Lett. 2013, 4, 1268−1280

The Journal of Physical Chemistry Letters Perspective

Figure 10. Galvanostatic intermittent titration technique (GITT) in the first, second, and third (a) charge and (b) discharge processes, Li+ diffusion
coefficients during the first three (c) charge and (d) discharge processes, and interface activation energy of different states during the first charge (e),
discharge (f), and the second charge (g) processes of the 0.5Li2MnO3·0.5LiMn0.42Ni0.42Co0.16O2 electrode material. The inset figures in panels e, f,
and g are the dQ/dV curves during the first charging, discharging, and second charging processes, respectively. (a) Reprinted with permission from
ref 15. Copyright 2012 Royal Society of Chemistry.

Figure 11. (a) TEM image and (b) rate capabilities of Li[Ni0.25Li0.15Mn0.6]O2 nanowires. Reprinted with permission from ref 77. Copyright 2009
Royal Society of Chemistry. (c) SEM image and (d) discharge curves with different rates (6, 3, 1, 0.5, and 0.1C) of the Li[Li1/3−2x/3NixMn2/3−x/3]O2
habit-tuned nanoplate material. Reprinted with permission from ref 78. Copyright 2010 Wiley-VCH.

oxygen arrays.75 Thus, the voltage degradations of these LLOs by the migration of transition metal ions. At present, the
during cycling are also believed to be associated with both stepwise precycling treatment on these LLOs is used and can
internal and surface phase transition to a cubic spinel-like phase improve their cyclic durability.76 The reason may be
1276 dx.doi.org/10.1021/jz400032v | J. Phys. Chem. Lett. 2013, 4, 1268−1280
The Journal of Physical Chemistry Letters Perspective

Figure 12. Current debates on structure and reaction mechanism, problems on electrochemical properties, and keys to the study in the future of
Li2MnO3-based lithium-rich layered cathode materials.

contributed by the weakened structure damage and little components and interfaces (between electrode and electrolyte)
structure arrangement after electrochemical cycling of these are very consistent with the two-phase models proposed and
materials with this pretreatment method. confirmed in Figure 2. Therefore, the electrochemical kinetics
In order to reveal the kinetically controlled charge and of the lithium ion extraction and insertion reactions in these
discharge processes of these cathode materials, the lithium ion LLOs is mainly controlled by the Li2MnO3 component inside
diffusion in active material and lithium ion transfer at the these LLOs. Even though this component can be activated after
electrode/electrolyte interface of the LLO the first charge process, the novel possible MnO2 and LiMnO2
(0.5Li2MnO3·0.5LiMn0.42Ni0.42Co0.16O2) during the first three components still have lower lithium ion diffusion coefficients
cycles were studied in detail by means of galvanostatic and higher interface reaction barriers with large activation
intermittent titration (GITT; Figure 10a−d) and electro- energy.
chemical impedance spectroscopy for interface activation Thus, in order to improve the rate performance of these
energies (Figure 10e−g) methods.15 Fifteen points in the first LLOs, the materials with low Li2MnO3 component proportion,
charge process, and 11 points in the first discharge and second short lithium ion diffusion pathway, and small interface reaction
and third charge/discharge processes were chosen for lithium barrier should be introduced to these materials. Kim et al. have
ion diffusion coefficient calculation of this LLO (Figure 10a,b). prepared Li[Ni0.25Li0.15Mn0.6]O2 nanowires with an aspect ratio
Results show that the variation of lithium ion diffusion of several hundreds and a diameter of about 30 nm (Figure
coefficient can be separated into two stages in all three charge 11a), exhibiting a rate capability of 95% at 4C (=1200 mA/g)
and discharge processes (Figure 10c,d). On the basis of the (Figure 11b).77 Meanwhile, Wei et al. have reported that a
two-phase model of this LLO, the lithium ion diffusion crystal habit-tuned nanoplate materials of Li[Li0.17Ni0.25Mn0.58]-
coefficients associated with LiMO2/MO2 (M = O2, associated with significantly increased (010) nanoplates
Mn0.42Ni0.42Co0.16) components are much larger than those (Figure 10 c), exhibits high rate performance (Figure 10 d).78
associated with Li2MnO3, LiMnO2/MnO2 components. In Through surface modification with insulating materials (Al2O3
addition, it is also obvious that the interface activation energy and AlPO4), Manthiram et al. also found that the rate capability
associated with LiMO2 (20 kJ/mol) and MO2 (31 kJ/mol, M = of these LLOs can be improved, which may be contributed to
Mn0.42Ni0.42Co0.16) components (Figure 10e−g) is small, while the lower charge-transfer resistance with small activation energy
that associated with Li2MnO3 (35 kJ/mol), LiMnO2 (32 kJ/ compared with the unmodified sample.70 Thus, the crystal grain
mol), and MnO2 (35 kJ/mol) components is large. The lithium and particle surface modification of these LLOs are very useful
ion diffusion coefficient variations during the first three charge/ to improve their rate performance.
discharge processes and interface activation energy variations of The LLOs are very attractive for utilization as cathode
the electrode materials with different states, corresponding to materials for lithium ion batteries. Although researchers have
lithium ions extraction/insertion from/into the different put forth much effort in studying these materials in the past,
1277 dx.doi.org/10.1021/jz400032v | J. Phys. Chem. Lett. 2013, 4, 1268−1280
The Journal of Physical Chemistry Letters Perspective

many debates and issues over these materials still exist, and Biographies
need to be clarified and solved in the future (Figure 12). Haijun Yu received his Ph.D. (2007) in Metallurgy Science and
(1) The structures of these LLOs are still currently being Engineering from the Northeastern University. In 2007-10, he worked
debated (yellow region in Figure 12). The local structures are as senior engineer at the General Research Institute for Nonferrous
very important to the electrochemical performance, especially Metals (GRINM) in China on batteries and battery-related materials
the rate performance of these LLOs. After synthesizing a lot of research. He is currently researching electrode materials and novel
these LLOs, we found that the electrochemical properties of batteries for the next-generation energy storage system at the National
these LLOs were extremely sensitive to the preparation Institute of Advanced Industrial Science and Technology (AIST),
conditions and composition change, although their XRD or Japan.
SXRD patterns are very similar. These phenomena may be Haoshen Zhou is now a prime senior researcher of the Energy
contributed to the imperceptible variation of local structures, Technology Research Institute (ETRI), National Institute of Advanced
because the local environments govern properties such as the Industrial Science and Technology (AIST), and leading the Energy
activation barriers, strain fields, and steric hindrances of the Interface Technology Group, ETRI, AIST, Japan. He is a guest invited
electrode materials, and can affect the lithium ion transport professor at both The University of Tokyo and Nanjing University.
inside the electrode materials by blocking or opening the His research interests include the synthesis of functional materials and
lithium ion pathways. The investigation with some novel their applications in lithium ion batteries, metal-air batteries, new type
analysis techniques, especially in situ testing methods (in situ batteries/cells. Web page: http://unit.aist.go.jp/energy/groups/eit_e.
TEM, Raman, neutron, etc.), on these LLOs will help us to htm
reveal the more detailed nature of these LLOs, understand the
reason for their sensitive electrochemical performance, and find
the relationship between local structure and electrochemical
■ ACKNOWLEDGMENTS
This work was partially supported financially by the Funding
properties. Program for World-Leading Innovative R&D on Science and
(2) It is very useful to understand the reaction mechanism Technology (FIRST Program).
(blue region in Figure 12) of these materials at room
temperature by the three-dimensional phase diagram. However,
it is still difficult to understand the large mysterious abundant
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