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Ni–Zn Batteries www.advmat.de

An Ultrastable and High-Performance Flexible Fiber-Shaped

Ni–Zn Battery based on a Ni–NiO Heterostructured
Nanosheet Cathode
Yinxiang Zeng, Yue Meng, Zhengzhe Lai, Xiyue Zhang, Minghao Yu, Pingping Fang,
Mingmei Wu,* Yexiang Tong, and Xihong Lu*

knittability and widespread applications

Currently, the main bottleneck for the widespread application of Ni–Zn in on-body equipment. Of the energy-
batteries is their poor cycling stability as a result of the irreversibility of storage devices developed, Ni–Zn battery
the Ni-based cathode and dendrite formation of the Zn anode during the is a sustainable and promising alterna-
charging–discharging processes. Herein, a highly rechargeable, flexible, fiber- tive to lithium-ion battery owing to its
merits of intrinsically good safety, abun-
shaped Ni–Zn battery with impressive electrochemical performance is ration-
dant resource, high capacity, and output
ally demonstrated by employing Ni–NiO heterostructured nanosheets as the voltage (≈1.8 V).[11–14] Unfortunately, the
cathode. Benefiting from the improved conductivity and enhanced electro­ commercialization of Ni–Zn battery is
activity of the Ni–NiO heterojunction nanosheet cathode, the as-fabricated plagued by its poor cycling stability in
fiber-shaped Ni–NiO//Zn battery displays high capacity and admirable rate terms of the irreversibility of Ni-based
capability. More importantly, this Ni–NiO//Zn battery shows unprecedented cathode and dendrite formation of the Zn
anode during the charging–discharging
cyclic durability both in aqueous (96.6% capacity retention after 10 000 cycles)
processes.[15–17] To solve this issue, tre-
and polymer (almost no capacity attenuation after 10 000 cycles at 22.2 A g−1) mendous efforts have been devoted to
electrolytes. Moreover, a peak energy density of 6.6 µWh cm−2, together with a design nanostructured Ni-based cath-
remarkable power density of 20.2 mW cm−2, is achieved by the flexible quasi- odes with well-defined morphology and
solid-state fiber-shaped Ni–NiO//Zn battery, outperforming most reported composition.[14,17–20] Various Ni-based
nanoarchitectures such as NiO nano-
fiber-shaped energy-storage devices. Such a novel concept of a fiber-shaped
flakes,[14] NiO nanosheets,[18] Ni(OH)2,[17]
Ni–Zn battery with impressive stability will greatly enrich the flexible energy- Ni3S2 nanosheets,[19] and Co3O4@NiO
storage technologies for future portable/wearable electronic applications. nanostrip@nanorod arrays[20] have been
extensively explored as cathodes for Ni–Zn
batteries and shown enhanced electro-
To keep pace with the ever-growing pursuit of wearable and chemical performance and stability. For instance, Dai and his
portable electronics, it is scientifically important to develop co-workers reported a kind of alkaline rechargeable Ni–Zn
matchable energy-storage devices with tiny volume, light battery based on an NiAlCo-layered double hydroxide/carbon
weight, good flexibility, and wearability.[1–4] Particularly, fiber- nanotube (CNT) hybrid cathode that could retain about 90%
shaped energy-storage devices (FESDs), such as fiber super- capacity retention after 600 cycles.[16] When using Co-doped
capacitors,[3,5,6] fiber Li-ion batteries,[7,8] and fiber metal-air Ni(OH)2 on nickel nanowire arrays as cathode, the aqueous
batteries,[9,10] have attracted increasing interests for their Ni/Zn battery exhibited a remarkable capacity (≈0.37 mA h cm−2
at 4 A g−1) and good cycling durability (about ≈12% capacity
losses after 5000 cycles).[17] Despite these achievements, most
Y. X. Zeng, Y. Meng, Z. Z. Lai, X. Y. Zhang, M. H. Yu, Prof. P. P. Fang, Ni–Zn batteries are focused on aqueous electrolytes, which
Prof. M. M. Wu, Prof. Y. X. Tong, Prof. X. H. Lu
MOE of the Key Laboratory of Bioinorganic and Synthetic Chemistry
is not desirable for flexible electronic devices.[16–18,20] Until
KLGHEI of Environment and Energy Chemistry recently, Liu et al. developed a flexible quasi-solid-state Ni–Zn
School of Chemistry battery with an NiO/carbon nanofiber cathode, which was pre-
Sun Yat-Sen University pared by electrochemical polymerization of pyrrole on carbon
Guangzhou 510275, P. R. China cloth followed by carbonization and chemical bath deposition
E-mail: ceswmm@mail.sysu.edu.cn; luxh6@mail.sysu.edu.cn
of Ni(OH)2 nanoparticles.[14] This quasi-solid-state Ni–Zn bat-
Prof. X. H. Lu
Key Laboratory of Advanced Energy Materials Chemistry tery presented a remarkable capacity of ≈0.23 mA h cm−2 at
(Ministry of Education) 20 mA cm−2 (10.4 A g−1) and capacity retention of ≈72.9% after
Nankai University 2400 cycles. Yet, the long-term durability of Ni–Zn battery is
Tianjin 300071, China still far from satisfactory, especially at high discharge current
The ORCID identification number(s) for the author(s) of this article density.[21] Besides, the complicated preparation processes of
can be found under https://doi.org/10.1002/adma.201702698. these cathodes is less impressive for large-scale application.
DOI: 10.1002/adma.201702698 To this end, the search for economical, environmental, and

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feasible approaches to synthesize high-performance Ni-based free-standing Ni-based precursor nanosheets with thickness of
cathode materials for flexible quasi/all-solid-state rechargeable ≈30 nm were grown on carbon fibers via a precipitation reac-
Ni–Zn battery is highly challenging and pursued. tion during the hydrothermal process, and there is no obvious
In this work, we constitute the first demonstration of an morphological transformation after calcination in air (denoted
ultrastable and high-performance flexible quasi-solid-state as NiO, Figure S1, Supporting Information). To introduce
fiber-shaped Ni–Zn battery based on Ni–NiO heterostructured metallic Ni into the sample, the as-prepared Ni-based pre-
nanosheet cathode. The binder-free porous Ni–NiO nanosheets cursor was annealed under N2 atmosphere at 340 °C (denoted
were obtained by a facile, efficient hydrothermal and low- as Ni–NiO-X, X presents the different amount of oxalic acid).
temperature thermal treatment process. The interconnected Scanning electron microscopy (SEM) images reveal that the
Ni/NiO heterojunction nanoparticles and tunable metallic nanosheets turn rough and porous as the oxalic acid increases
Ni content of such porous Ni–NiO nanosheets enable them (Figure 1a and Figure S2, Supporting Information). To further
remarkably enhanced conductivity, fast ion diffusion rate, and investigate the structure of the Ni–NiO-3 sample, the high-
high electrolyte penetration. As a consequence, the fiber-shaped resolution transmission electron microscopy (HRTEM) test
Ni–Zn battery with this Ni–NiO cathode yields an extraordinary was conducted in Figure 1c. Specifically, the neighboring lattice
cyclic durability (3.4% capacity decay after 10 000 cycles) with fringe spacings of 0.209 and 0.241 nm correspond, respectively,
considerable capacity (237.8 µAh cm−3 at 3.7 A g−1) and superior to the (200) and (111) planes of NiO (JCPDS (Joint Committee
rate capability (74.4% capacity retention at a huge current den- on Powder Diffraction Standards) No. 47-1049). Another inter-
sity of 37 A g−1) in aqueous electrolyte. More encouragingly, the planar spacing of 0.203 nm matches well with the d111 spacing
as-fabricated quasi-solid-state fiber-shaped Ni–Zn battery not of metal Ni (JCPDS No. 65-2865), revealing that the sample is
only possesses outstanding flexibility but also owns prominent composed of the NiO matrix with some embedded metallic Ni
long-term stability even at different current densities (almost nanoparticles (Figure 1b). The corresponding energy-dispersive
no capacity attenuation after 10 000 cycles at 22.2 A g−1). Fur- spectroscopy (EDS) mapping images clearly verify the homo-
thermore, a remarkable power density of 20.2 mW cm−2 is also geneous spatial distribution of Ni and O elements (Figure 1d).
achieved by the fiber-shaped Ni–Zn battery, even much higher But for the NiO sample, only interplanar spacing of polycrys-
than most of the carbon-based fiber supercapacitors (SCs). talline NiO (JCPDS No. 47-1049) can be identified (Figure S3,
The Ni–NiO heterostructured nanosheets are prepared by Supporting Information). To examine the specific surface area
calcination of Ni-based sheet-like precursor. First, uniform and pore size distribution of the NiO and Ni–NiO-3 samples,

Figure 1. a) SEM images of the Ni–NiO-3 sample; the inset is the corresponding magnified SEM image. b) TEM image of the Ni–NiO-3 sample.
c) HRTEM image of the Ni–NiO-3 sample. d) EDS mapping of Ni and O elements of the Ni–NiO-3 sample. e–g) XRD spectra (e), normalized core–level
Ni 2p XPS spectra (f), and O 1s XPS spectra (g) of the NiO and Ni–NiO-3 samples.

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N2 adsorption–desorption isothermal measurements were acid amount (Figure S5a, Supporting Information), while an
performed. As displayed in Figure S4a (Supporting Informa- opposite trend is observed for that of NiO peak (Figure S5b,
tion), the NiO sample shows a more apparent hysteresis loop Supporting Information), indicating that the metallic Ni content
in sorption isotherm than that of the Ni–NiO-3 sample, indi- can be easily adjusted by changing the amount of oxalic acid in
cating more mesoporous structure. The dominant pore sizes the Ni-based precursor synthesis, which is further confirmed by
of both samples originated from the Barrett–Joyner–Halenda the XRD analysis (Figure S6, Supporting Information).
pore size distribution curves are centered around 60 nm and The electrochemical performances of the fiber-shaped NiO
smaller pores with sizes of about 2–6 nm are also detected and Ni–NiO-X electrodes were measured in a three-electrode
(Figure S4b, Supporting Information). By calculation, the system with 1 m KOH as aqueous electrolyte. Figure 2a com-
Brunauer–Emmett–Teller surface area of the NiO and Ni–NiO-3 pares the discharge curves of the NiO and Ni–NiO-X electrodes
samples is 92.6 and 27.6 m2 g−1, respectively. collected at a current density of 3.7 A g−1. Evidently, the Ni con-
More evidence for the Ni–NiO heterostructures was con- tent exerts profound effect on the charge–discharge properties
firmed by the X-ray diffraction (XRD) and X-ray photoelectron of the Ni–NiO-X electrodes. The discharge time prolongs first
spectroscopy (XPS) analyses. As shown in Figure 1e, only NiO with the increment of the Ni content and the Ni–NiO-3 elec-
peaks (JCPDS No. 47-1049) are observed for the NiO sample trode yields the optimal performance. Figure 2b displays the
that annealed in air, while both NiO (JCPDS No. 47-1049) and specific capacities of these NiO and Ni–NiO-X electrodes, which
metal Ni (JCPDS No. 65-2865) peaks can be detected for the are extracted from their discharge profiles at various current
Ni–NiO-3 sample. This confirms the hybrid composition of the densities (Figure S7, Supporting Information). The capacities of
Ni–NiO-3 sample. Figure 1f compares the core level Ni 2p XPS all Ni–NiO-X electrodes outperform that of NiO at all the cur-
spectra of the NiO and Ni–NiO-3 samples. The peaks centered rent densities, especially for the Ni–NiO-3 electrode, indicating
at 853.7, 855.6, 860.9, 872.7, and 879.5 eV can be assigned to the that the introduction of metallic Ni nanoparticles can consider-
characteristic peaks of Ni2+,[22,23] whereas the peaks located at ably boost the capacity of the NiO electrode. Remarkably, the
852.5 and 869.7 eV represent the characteristic Ni 2p3/2 and Ni Ni–NiO-3 delivers a remarkable capacity of 5.78 mA h cm−3 at a
2p1/2 peaks of Ni0, respectively.[24] Apart from the peaks of Ni2+, high current density of 3.7 A g−1, while the corresponding capac-
there are noticeable characteristic peaks of Ni0 for the Ni–NiO-3 ities of the NiO, Ni–NiO-1, Ni–NiO-2, and Ni–NiO-4 are 1.62,
sample, indicating the existence of metal Ni. In comparison 2.74, 5.37, and 4.84 mA h cm−3, respectively. Cyclic volta­mmetry
with the NiO sample, the Ni–NiO-3 sample shows substantially (CV) study was also conducted to evaluate the performance of
lower intensity of the Ni-O peak in the core level O 1s XPS the NiO and Ni–NiO-X electrodes collected at various scan rates.
spectra, again demonstrating the lower content of NiO in the As the scan rate increases from 10 to 100 mV s−1, all the NiO
Ni–NiO-3 sample (Figure 1g).[25] Moreover, the intensity of the and Ni–NiO-X electrodes share similar CV curves with little dis-
Ni0 characteristic peaks strengthens with the increasing oxalic tortion, revealing good pseudocapacitive behavior and high-rate

Figure 2. a) Discharge curves of the NiO and Ni–NiO-X electrodes at 3.7 A g−1. b) Volumetric capacity of the NiO and Ni–NiO-X electrodes as a func-
tion of current density obtained from the galvanostatic charge–discharge curves. c) The anodic and cathodic peak current density obtained from the CV
curves of the Ni–NiO-3 electrode as a function of square root of scan rates. d,e) Nyquist plots (d) and cycling performance and Coulombic efficiency
(e) of the NiO and Ni–NiO-3 electrodes collected at 18.5 A g−1.

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capability. As shown in Figure S8 (Supporting Information), was assembled. CV curves of the NiO//Zn and Ni–NiO//Zn
both the CV curves present a pair of obvious redox peaks, indi- batteries show a pair of redox peaks with minor change at dif-
cating a Faradaic redox reaction between Ni2+ and Ni3+. Besides, ferent scan rates, which is indicative of good rate capability
contrast to the NiO electrode, the Ni–NiO-3 electrode exhibits a of the Ni–Zn batteries (Figure S12, Supporting Information).
substantial augment of redox current density, again manifesting The comparison of the discharge profiles of both batteries is
its higher reaction activity and better electrochemical property depicted in Figure 3a. Compared to the NiO//Zn battery, the
(Figure S9, Supporting Information). Ni–NiO//Zn battery delivers a much lower voltage hysteresis
To gain insight into the electrochemical kinetics of the and longer discharge plateaus, suggesting its less polarization
Ni–NiO-3 electrode, the anodic and cathodic peak current and higher capacity. The discharge profiles of the NiO//Zn
densities (i) are plotted versus square root of scan rate (ν1/2) and Ni–NiO//Zn batteries manifest that both batteries have
according to the power-law relationship.[26,27] Both plots an operating voltage of 1.7–1.8 V at various current densities
exhibit a linear relationship regardless of scan rate (Figure 2c), (Figure S13, Supporting Information). Besides, much higher
implying a semi-infinite diffusion controlled process during capacity (237.8 µA h cm−3 at 3.7 A g−1) and capacity retention
cycling. Such improved performance was further elucidated by (74.4%) were achieved by the Ni–NiO//Zn battery, whereas
the electrochemical impedance spectroscopy test (Figure 2d). the NiO//Zn battery only owns a relatively low capacity of
In the Nyquist plot, there is a positive correlation between the 133.7 µA h cm−3 and capacity retention of 58.2% (Figure 3b).
diameter of the semicircle in the high frequency range and More importantly, the Ni–NiO//Zn battery affords an unprece­
charge-transfer resistance of the electrode. The Ni–NiO-3 elec- dented long-term cyclability with less than 4% capacity
trode (≈1.2 Ω) shows a substantially smaller charge-transfer attenuation after 10 000 cycles, substantially better than that
resistance than that of the NiO electrode (≈27.5 Ω), verifying the of the NiO//Zn battery (69.7% retention after 10 000 cycles)
greatly improved electrical conductivity after the introduction and most reported Ni–Zn batteries, such as Ni3S2-Ni//Zn
of metallic Ni nanoparticles. For the Ni–NiO-3 electrode, the (≈83.3% retention after 100 cycles),[19] Co3O4@NiO//Zn (≈89%
abundant embedded Ni nanoparticles can promote the electron retention after 500 cycles),[20] NiO//Zn (≈89% retention after
transfer between the electroactive sites and the current collector 500 cycles),[18] NiAlCo LDH-CNT//Zn foil (≈90% retention after
during the charge–discharge process, and thus eventually boost 600 cycles),[16] and Co doped Ni(OH)2//Zn (≈90% retention after
the electrochemical activity. It is worth noting that the specific 5000 cycles).[17] As far as we know, this is the best cycling per-
surface area of the NiO sample (92.6 m2 g−1) is much higher formance reported for Ni–Zn batteries so far.[11,16–20] Also, the
than that of the Ni–NiO-3 electrode (27.6 m2 g−1). Actually, average Coulombic efficiency of NiO//Zn and Ni–NiO//Zn bat-
besides surface area, the capacity of an electrode is also greatly teries is calculated to be 94.6% and 96.7%, respectively. More-
affected by the charge-transfer rate during the Faradaic redox over, the overpotential of the Ni–NiO//Zn battery is still almost
reaction. This indicate the electrochemical performance of NiO unchanged after 10 000 cycles, while the overpotential of the
electrode is mainly suppressed by the poor conductivity owing NiO//Zn battery obviously increases after 10 000 cycles, demon­
to the absence of metallic Ni content, and the introduction of strating the higher reversibility of Ni–NiO//Zn battery than that
metallic Ni nanoparticles could significantly boost the conduc- of NiO//Zn battery (Figure S14, Supporting Information).
tivity of NiO electrode, hence enhancing its electrochemi­ cal As a demonstration, a quasi-solid-state fiber-shaped Ni–
properties. The cycling property of the NiO and Ni–NiO-3 NiO//Zn battery was assembled. Figure 4a illustrates the sche-
electrodes was also evaluated (Figure 2e). Intriguingly, the matic diagram of the fiber-shaped Ni–NiO//Zn battery, where a
Ni–NiO-3 electrode presents an impressive durability with Zn fiber anode is twined tightly by Ni–NiO fiber cathode. Such
98.9% capacity retention after charge–discharge 10 000 cycles, structure was further verified by the SEM image and photograph
whereas the NiO electrode retains only 87.5%. Such cycling of the fiber-shaped Ni–NiO//Zn battery device (Figure 4b and
performance is superior to or comparable with those of the Figure S15, Supporting Information). The as-fabricated fiber-
reported NiO-based electrodes.[25,28–30] Except for the first cycle, shaped Ni–NiO//Zn battery possesses a high voltage of 1.9 V
the Coulombic efficiency of the charging/discharging process and its discharge profiles witness a plateau around 1.7–1.8 V
for both NiO and Ni–NiO-3 electrodes is more than 90%, indi- (Figure 4c and Figure S16a, Supporting Information). By cal-
cating that both electrodes have high reversibility and stability. culation, a decent capacity of 116.1 µAh cm−3 was obtained by
Furthermore, the charge–discharge profiles of the first and the Ni–NiO//Zn battery at a high current density of 3.7 A g−1.
10 000th cycles both for the NiO and Ni–NiO-3 electrodes were As current density rises tenfold, it still has a rate capability of
also analyzed. As depicted in Figure S10 (Supporting Infor- 38.4% (Figure S16b, Supporting Information). It is expect that
mation), the overpotential of NiO electrode increases a little, the performance of the Ni–NiO//Zn battery could be further
whereas the overpotential of Ni–NiO-3 electrode has barely enhanced by optimizing Ni–NiO electrode nanostructures and
changed after long-term cycling, revealing the highly reversible mass loading, as well as using other ionic liquid electrolytes.
reaction of the Ni–NiO-3 electrode. SEM images present that Furthermore, such this quasi-solid-state fiber-shaped Ni–NiO//
the Ni–NiO-3 nanosheet structure is well preserved after cycling Zn battery also owns an unexpectedly excellent stability even
10 000 cycles, further verifying the superior cyclic stability of at different current densities (Figure 4d). Specially, this bat-
the Ni–NiO electrode (Figure S11, Supporting Information). tery can retain more than 99.6% of its initial capacity after
To further demonstrate the potential application of NiO and 10 000 charge–discharge cycles at a high current density of
Ni–NiO-3 electrodes for Ni–Zn batteries, an aqueous Ni–Zn 22.2 A g−1, which outperforms most recently reported quasi-
battery (denoted as NiO//Zn and Ni–NiO//Zn) using a soft Zn solid-state SCs and batteries, such as Li1.1Mn2O4//LiTi2(PO4)3
fiber as anode and an NiO or Ni–NiO-3 electrode as cathode Li-ion battery (72% after 100 cycles),[31] NiO//ZnO battery

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Figure 3. a) Charge–discharge curves of the NiO//Zn and Ni–NiO//Zn batteries at 3.7 A g−1. b) Volumetric capacity and capacity retention of the
NiO//Zn and Ni–NiO//Zn batteries as a function of current density obtained from the galvanostatic charge/discharge curves. c) Cycling performance
and Coulombic efficiency of the NiO//Zn and Ni–NiO//Zn batteries collected at 18.5 A g−1.

Figure 4. a) Schematic diagram of the flexible quasi-solid-state fiber-shaped Ni–NiO//Zn battery. b) SEM image of the quasi-solid-state fiber-shaped
Ni–NiO//Zn battery. c,d) Galvanostatic discharge curves (c) and cycling performance (d) of the quasi-solid-state fiber-shaped Ni–NiO//Zn battery at
various current densities. e) The 1st and 10 000th discharge profiles of the quasi-solid-state fiber-shaped Ni–NiO//Zn battery when cycling at 22.2 A g−1.

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(≈72.9% after 2400 cycles),[14] graphene oxide//Na2Ti3O7 Na-ion fiber-shaped Ni–NiO//Zn battery, the charge–discharge perfor-
battery (≈80.3% after 2500 cycles),[32] MnO2@TiN//N-MoO3-x mance of the device was tested under various bending condi-
fiber SC (80.3% after 5000 cycles),[33] C//Na2Ti3O7 Na-ion tions (Figure 5b). The discharge profiles remained almost
capacitor (86% after 3000 cycles),[34] MnO2//Fe2O3@C cable SC unchanged with the increment of bending angels, indicative
(93% after 4000 cycles),[35] and RuO2•0.4H2O@CF fiber SC of its high flexibility and the reliability of the fiber-shaped
(94% after 5000 cycles).[36] As shown in Figure 4e, the corre- Ni–NiO//Zn battery for application in the state-of-the-art port-
sponding discharge curve after 10 000 cycles at 22.2 A g−1 is able electronics. More encouragingly, the fiber-shaped Ni–NiO//
similar to the first one, again clarifying the extraordinary long- Zn battery can effectively power a 1.5 V commercial light-emit-
term stability of the quasi-solid-state Ni–NiO//Zn battery. ting diode under normal and bending conditions (Figure S17,
Energy density and power density of our as-fabricated quasi- Supporting Information), illustrating its high potentials for
solid-state fiber-shaped Ni–NiO//Zn battery are also impressive future flexible energy storage.
when compared to other reported fiber-shaped energy-storage In conclusion, the first paradigm of a flexible stable quasi-
devices. As depicted in Figure 5a, a remarkable power density solid-state fiber-shaped Ni–NiO//Zn battery was demon-
of 20.2 mW cm−2 (0.22 W cm−3; based on the entire device) strated based on the Ni–NiO heterostructured cathode. Due
was delivered by the quasi-solid-state fiber-shaped Ni–NiO//Zn to the boosted electrical conductivity and fast ion diffusion of
battery, which is about two times that of pen-ink-based fiber the porous Ni–NiO heterostructured nanosheets, the fiber-
supercapacitor, about 40 times that of the CNT-based fiber shaped Ni–NiO//Zn battery exhibited an enhanced capacity
supercapacitor, and about 118 times that of the graphene-based (237.8 µAh cm−3 at 3.7 A g−1) and rate capability and cycling
fiber supercapacitor. Meanwhile, the peak energy density of performance (96.6% capacity retention after 10 000 cycles)
the device reached 6.6 µW h cm−2 (0.67 mW h cm−3), much when compared to the NiO//Zn battery. More importantly,
higher than that of most reported fiber-shaped devices, such as when using poly(vinyl alcohol) (PVA) gel electrolyte, this flex-
ZnO-MnO2 (0.027 µW h cm−2),[37] CNT (0.226 µW h cm−2),[38] ible quasi-solid-state fiber-shaped Ni–NiO//Zn battery deliv-
MnO2-CNT (2.6 µW h cm−2),[39] pen ink (2.7 µW h cm−2),[40] and ered an unprecedented long-term cyclic durability without
RGO-CNT@CMC (5.91 µW h cm−2),[41] and so on.[42–46] To fur- obvious capacity decay when charged–discharged up to
ther examine the flexibility of the as-fabricated quasi-solid-state 10 000 cycles. Moreover, the flexible fiber-shaped Ni–NiO//Zn

Figure 5. a) Ragone plots of the quasi-solid-state fiber-shaped Ni–NiO//Zn battery. The values reported for other FESDs are added for comparison.[5,37–46]
b) Galvanostatic discharge curves of the quasi-solid-state fiber-shaped Ni–NiO//Zn batter under different bending conditions at 7.4 A g−1. The inset is
the corresponding digital photos of the device.

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battery achieved an impressive power density of 20.2 mW cm−2, Acknowledgements

which is several to hundreds times that of the other FESDs. The
Y.X.Z. and Y.M. contributed equally to this work. The authors
strategy of introducing Ni in NiO presented in this study may acknowledge the financial support of this work received by the National
be extended for the fabrication of other metal oxides to opti- Natural Science Foundation of China (Grant Nos. 21403306, 51672315,
mize the performance. Considering the combined high power and 31530009), Guangdong Natural Science Funds for Distinguished
density and long lifespan properties, the fiber-shaped Ni–Zn Young Scholar (Grant No. 2014A030306048), National Key R&D Program
battery may open a new avenue for the high-end wearable and of China (Grant No. 2016YFA0202604), Tip-top Scientific and Technical
miniaturized electronics. Innovative Youth Talents of Guangdong Special Support Program
(Grant No. 2015TQ01C205), Pearl River Nova Program of Guangzhou
(Grant No. 201610010080), Technology Planning Project of Guangdong
Province (Grant No. 2015B090927007), the Government of Guangzhou
Experimental Section city for international joint-project (Grant No. 201704030020), Open
Fund of Jiangsu Key Laboratory of Materials and Technology for Energy
Preparation of Ni–NiO Heterostructured and NiO Nanosheets: The Conversion (Grant No. MTEC-2015M05), and Science and Technology
Ni-based precursor was first synthesized through a facile hydrothermal Program of Guangzhou, China (Grant No. 201604010124).
method. A piece of carbon cloth was immersed in ethanol and sonicated
for 10 min. A certain amount of H2C2O4·H2O, i.e., 0.10, 0.20, 0.30,
and 0.40 g, was dissolved in 10 mL of 2 m hexamethylenetetramine
aqueous solution, and was then slowly dropped into 100 mL of 0.1 m Conflict of Interest
Ni(NO3)2·6H2O aqueous solution under stirring. The obtained green
solution together with the cleansed carbon cloth was transferred into The authors declare no conflict of interest.
a Teflon-lined stainless-steel autoclave of 25 mL capacity and then
heated in the electric oven at 95 °C for 6 h. After cooling to room
temperature, the carbon cloth with a light green layer of Ni-based
precursor was washed with deionized water and dried at 70 °C in air.
Keywords
Subsequently, to synthesize the Ni–NiO heterostructure, the obtained flexible, heterostructures, Ni–NiO, Ni–Zn batteries, ultrastable
precursor was annealed in N2 atmosphere at 340 °C for 2 h. Ni–NiO-X
(X = 1, 2, 3, 4) correspond to the amount of H2C2O4·H2O of 0.10, 0.20, Received: May 15, 2017
0.30, and 0.40 g, respectively. For comparison, the NiO nanosheets were Revised: July 24, 2017
obtained by annealing the precursor of Ni–NiO-3 in air at 340 °C for 2 h. Published online:
The mass of NiO and Ni–NiO-X nanosheets was obtained by electronic
scales (BT25S, 0.01 mg).
Fabrication of Quasi-Solid-State Fiber-Shaped Ni–Zn Batteries: The PVA
gel electrolyte was prepared by dissolving 2.0 g of PVA into 15 mL of
distilled water at 85 °C under vigorous stirring. Then 10 mL of aqueous [1] J. Liu, M. Chen, L. Zhang, J. Jiang, J. Yan, Y. Huang, J. Lin, H. J. Fan,
solution with 40 mmol of KOH and 2 mmol of ZnO was added into the Z. X. Shen, Nano Lett. 2014, 14, 7180.
well-dissolved PVA solution. [2] N. Feng, P. He, H. Zhou, ChemSusChem 2015, 8, 600.
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