Facile preparation of monodisperse NiCo2O4 porous microcubes as a high capacity anode material for lithium ion batteries

Yanming Wang ab, Jia Li a, Sheng Chen ab, Bing Li a, Guangping Zhu a, Fei Wang *ab and Yongxing Zhang *a
aCollaborative Innovation Center of Advanced Functional Composites, Huaibei Normal University, Huaibei 235000, P. R. China. E-mail: zyx07157@mail.ustc.edu.cn; wangfeichem@126.com
bAnhui Key Laboratory of Energetic Materials, Huaibei Normal University, Huaibei 235000, P. R. China

Received 17th October 2017 , Accepted 25th December 2017

First published on 28th December 2017


Binary transition metal oxides have attracted great attention as high-performance electrode materials for lithium-ion batteries in recent years. Herein, monodisperse NiCo2O4 porous microcubes were prepared for the first time via a simple urea-assisted solvothermal method followed by a thermal decomposition process. The porous microcubes assembled by nanoparticles with a size of ca. 35 nm have an average edge length of 1.5 μm. Nitrogen sorption isotherms show that this structure possesses a high surface area of 26.26 m2 g−1 with an average pore diameter of 22.57 nm. The rich mesopores among NiCo2O4 microcubes not only provide a large electrode/electrolyte reaction interface, but also provide enough void space to accommodate the volume change and prevent electronic disconnection in the electrode material during cycling. Furthermore, primary nanoparticles with a smaller size within microcubes can facilitate rapid Li-ion transport. So, when the as-prepared porous NiCo2O4 microcubes are used as anode materials for Li-ion batteries, they exhibit high-rate capability and outstanding cyclability.


1. Introduction

In view of the low theoretical capacity (372 mA h g−1) and inferior rate capability, graphite as a commercial anode material can not satisfy the increasing requirements of high power and energy densities for current lithium ion batteries.1 Recently, silicon, tin and transition metal oxide anode materials with improved electrochemical performances have been explored.2–4 Among these potential anode materials, nanostructured transition metal oxides exhibit outstanding electrochemical properties owing to their high surface-to-volume ratio and short Li-ion diffusion pathway.5–8 More recently, binary metal oxides, such as NiCo2O4, MnCo2O4, ZnCo2O4, CuCo2O4 and ZnMn2O4, have attracted a great deal of attention due to their high reversible capacity (ca. 1000 mA h g−1).9–16 Note that the two different metal cations in a binary metal oxide offer synergistic and interfacial effects, resulting in high electrochemical activity.17,18 In particular, NiCo2O4 is regarded as a promising electrode material for lithium/sodium ion batteries and supercapacitors due to its good electronic/ionic conductivity, structural stability, electrochemical activity, and easy electrolyte penetration.19–25

Until now, various morphologies of NiCo2O4, such as nanoparticles, nanosheets, nanoarrays, microflowers, nanocages, nanorods, and microspheres, have been designed and synthesized.26–38 In particular, the reported porous NiCo2O4 materials displayed higher rate capabilities, longer cycle life, and larger reversible capacities as compared to nonporous bulk materials.39,40 The porous structure provides large useable electrode/electrolyte reaction areas, which is beneficial to obtain high specific capacities at high charge/discharge rates. Moreover, the interior void space can effectively tolerate volume changes during cycling, thereby restraining the crack and pulverization of the electrode materials to enhance the cycling stability.41 Generally, porous materials are obtained from translating the hard templates, such as mesoporous silica or carbon.37,38,42,43 These complicated procedures restrict the large-scale application. Fortunately, a solvothermal method has been successfully developed to design regular NiCo2O4 porous micro/nanostructures. For example, Xu et al.44 prepared a rose-like porous NiCo2O4 structure through a poly(vinylpyrolidone)-assisted solvothermal method, which retains a reversible capacity of 915 mA h g−1 at a high current density of 1000 mA g−1. Fu et al.45 fabricated porous NiCo2O4 microspheres and microrods by a solvothermal method through varying the solvents, and microrods give superior battery performance to microspheres on account of their one-dimensional porous hierarchical structure. Li et al.46 obtained NiCo2O4 mesoporous microspheres with high electrochemical performance from the pyrolysis of spherical Ni0.33Co0.67CO3 precursors synthesized via a simple template-free solvothermal process. Graphene loaded with mesoporous NiCo2O4 nanoneedles were fabricated using a solvothermal reaction together with annealing treatment, which exhibited outstanding cycling stability.47 However, exploring an effective solvothermal method for the large-scale controlled synthesis of monodisperse NiCo2O4 porous microcubes with high-rate capability and outstanding cyclability is still a challenge faced by chemists and material scientists.

In this study, monodisperse NiCo2O4 porous microcubes assembled by nanoparticles have been fabricated via a simple urea-assisted solvothermal method followed by a thermal decomposition process. The former process was to prepare highly uniform Ni0.33Co0.67CO3 carbonate precursors with microcube morphology. A subsequent thermal decomposition process was employed to convert carbonate precursors into NiCo2O4 porous microcubes. To the best of our knowledge, this is the first report on the fabrication and electrochemical performance of monodisperse NiCo2O4 porous microcubes. Most importantly, the regular micro/nanostructures have been well preserved without pulverizing during cycles due to the sufficient pores against drastic volume changes. As a result, the synthesized NiCo2O4 porous microcubes as anode materials for lithium ion batteries display excellent electrochemical performance in terms of high-rate capability and cyclability.

2. Experimental section

2.1 Preparation of monodisperse porous NiCo2O4 microcubes

All reagents with analytical grade were purchased from Sinopharm Chemical Reagent Co., Ltd (China), and used without further purification. The highly uniform carbonate precursors (Ni0.33Co0.67CO3) with microcube morphology were synthesized by a urea-assisted solvothermal method, and the details are as follows: Ni(CH3COO)2·4H2O (1 mmol), Co(CH3COO)2·4H2O (2 mmol) and urea (0.06 mol) were dissolved in 30 mL of ethylene glycol and deionized water (5[thin space (1/6-em)]:[thin space (1/6-em)]1, v/v) and stirred sufficiently. Then, the resulting solution was transferred into a Teflon-skinned autoclave and heated at 120 °C for 12 h. After the reaction system was naturally cooled to room temperature, the purple precipitates were separated from solution and thoroughly washed several times with deionized water and absolute ethanol, and then dried in a vacuum oven at 60 °C for 12 h. The monodisperse NiCo2O4 porous microcubes were obtained from the as-prepared highly uniform Ni0.33Co0.67CO3 carbonate precursors with microcube morphology via calcining in air at 450 °C for 3 h.

2.2 Characterization

X-ray diffraction (XRD) patterns were obtained in the 2θ range of 10–80° using a Philips X'Pert Pro X-ray diffractometer with Cu Kα radiation (1.5418 Å). Field emission scanning electron microscopy (FESEM) images were taken on a FESEM (S-4800) operated at an accelerating voltage of 1.0 kV. Transmission electron microscopy (TEM) images were obtained on a JEOL JEM-2010 high resolution transmission electron microscope, equipped with X-ray energy dispersive spectroscopy (EDS) capabilities, working at an acceleration voltage of 200 kV. Thermogravimetric analysis (TGA-2050 (TA Corp.)) was conducted to determine the composition of the samples. The TGA measurements were carried out at a heating rate of 10 °C min−1 from 20 to 800 °C with an air flow rate of 100 mL min−1. Surface analysis of the samples was performed using XPS (VGESCA-LABMKIIX-ray photoelectronic spectrometer). The specific surface areas of the samples were measured with Micromeritics ASAP 2020 M + C Brunauer–Emmett–Teller (BET) equipment by using nitrogen adsorption and desorption.

2.3 Electrochemical measurements

The electrochemical behaviors of porous NiCo2O4 microcubes were investigated using CR2016-type coin cells with lithium-foil as the counter electrode and Celgard 2400 membrane as the separator. The working electrode was fabricated by blending 75 wt% NiCo2O4 active material, 15 wt% Super P conductive carbon, and 10 wt% polyvinylidene difluoride binder onto copper foil. The electrode sheets with a diameter of 12 mm were punched out and dried at 80 °C for 12 h in a vacuum oven. The test electrolyte solution was 1 M LiPF6 in a 1[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]1 (v/v/v) mixture of ethylene carbonate (EC), dimethylcarbonate (DMC), and diethylcarbonate (DEC). A cyclic voltammogram (CV) was recorded on a CHI650 electrochemistry workstation at a scan rate of 0.1 mV s−1. Galvanostatic discharge/charge cycling was performed using a battery testing system (LANHE CT2001) between 0.01 and 3.0 V at room temperature.

3. Results and discussion

The strategy for synthesizing monodisperse NiCo2O4 porous microcubes is schematically depicted in Fig. 1. First, a urea-assisted solvothermal method for the preparation of highly uniform Ni0.33Co0.67CO3 carbonate precursors with microcube morphology is developed. A subsequent step thermal decomposition process is introduced to convert carbonate precursors into NiCo2O4 porous microcubes. The XRD pattern (Fig. S1 in the ESI) of the microcube precursors reveals the diffraction of calcite Ni0.33Co0.67CO3. The SEM image shows that the Ni0.33Co0.67CO3 precursors are composed of uniform and monodisperse microcubes with a size of 1.5–2.0 μm (Fig. S2 in the ESI). In addition, it is also clear that the surface of the microcubes is very smooth. Thermogravimetric analysis (TGA, Fig. S3 in the ESI) shows that thermal decomposition of Ni0.33Co0.67CO3 into NiCo2O4 in air is completed at 450 °C, and the calculated value of weight loss is very close to the theoretical value (32.78% vs. 32.54%), which is consistent with the previous report.46 Thus, we chose 450 °C as the calcination temperature for the synthesis of the NiCo2O4 phase in our experiment.
image file: c7qi00648a-f1.tif
Fig. 1 Schematic illustration of the formation of monodisperse NiCo2O4 porous microcubes.

The XRD pattern of the NiCo2O4 product obtained is shown in Fig. 2a. All of the diffraction peaks can be indexed as cubic spinel NiCo2O4 (JCPDS no. 01-073-1702, space group: Fd3m, lattice constant a = 8.269 Å). Compared with the standard diffraction patterns, no characteristic peaks are from impurities. It is generally believed to adopt a spinel-related structure in which nickel occupies the octahedral sites and cobalt is distributed over both octahedral and tetrahedral sites,46,48,49 as illustrated in Fig. 2b. XPS analyses are carried out to confirm the elemental composition of NiCo2O4 and the valence states of the metal ions (Fig. S4 in the ESI). The survey spectrum in Fig. S4a proves the presence of Ni, Co and O elements. The total atomic ratio of Ni and Co elements is ca. 1[thin space (1/6-em)]:[thin space (1/6-em)]1.99, which is very close to the theoretical value (1[thin space (1/6-em)]:[thin space (1/6-em)]2). With the Gaussian fitting method, the Ni 2p spectrum (Fig. S4b) is well fitted to two spin–orbit doublets characteristic of Ni2+ and Ni3+, and two shakeup satellites. Similarly, the Co 2p spectrum (Fig. S4c) also displays the presence of Co2+ and Co3+. These results indicate the co-existence of Co2+, Co3+, Ni2+, and Ni3+ in NiCo2O4, which is in accordance with the reported results in the literature.31,32,46


image file: c7qi00648a-f2.tif
Fig. 2 (a) XRD pattern of the NiCo2O4 porous microcubes. (b) Schematic representation of NiCo2O4 cubic spinel (ICSD no. 02241).

The morphology and structure of NiCo2O4 obtained by calcination of the precursors at 450 °C are investigated by SEM as shown in Fig. 3. The overall morphology of the samples, as shown Fig. 3a, indicates that large quantities of microcubes with good monodispersity are obtained. As seen from the low magnification SEM image (Fig. 3b), all microcubes have edge lengths ranging from 0.8 to 2 μm with an average length of 1.5 μm. Close observation (Fig. 3c) illustrates that the peripheral surface of the microcube is not smooth. It appears that the microcube is composed of many small nanoparticles. Fig. 3d displays a typical high-magnification SEM image of the corner of the microcube. There are many pore structures in the surface of NiCo2O4 microcubes. In addition, it further appears that the microcube is composed of numerous nanoparticles with an average size of ca. 35 nm.


image file: c7qi00648a-f3.tif
Fig. 3 SEM images of the as-prepared monodisperse NiCo2O4 porous microcubes. (a) Overall morphology; (b) low-magnification; (c) close observation; (d) high-magnification.

The as-prepared NiCo2O4 porous microcubes are further studied using TEM images. Fig. 4a shows the low-magnification TEM image of the NiCo2O4 microcubes, indicating the mesopores that are uniformly distributed in the microcubes. The magnified image (Fig. 4b) provides clear observation of the nanoparticles with a diameter of about 35 nm that are assembled into the microcubes, which is consistent with the SEM observation (Fig. 3d). The EDS spectrum of the NiCo2O4 microcubes, as presented in Fig. 4d, demonstrates that the sample just contains the elements of Ni, Co and O, while no other impurities exist. In order to further confirm the obtained samples which are NiCo2O4 microcubes, samples are analyzed by electron mapping image analysis (Fig. 4(e–h)). The images are acquired by visualizing the inelastically scattered electrons in the energy loss windows for elemental Ni, Co and O. The different color areas shown in parts (f–h) of Fig. 4 indicate Ni-, Co-, and O-enriched areas of the sample, respectively. The images also show that Ni, Co and O elements are well dispersed in the NiCo2O4 microcubes.


image file: c7qi00648a-f4.tif
Fig. 4 (a), (e) Low-magnification, (b) magnified TEM images, (c) HRTEM image and (d) EDS spectrum of the monodisperse porous NiCo2O4 submicrocubes. Electron energy loss: (f) “Ni”, (g) “Co” and (h) “O” elemental mapping images of the monodisperse NiCo2O4 porous microcubes.

Nitrogen sorption isotherms are provided to investigate the mesoporous structures and the BET surface areas of the monodisperse NiCo2O4 porous microcubes, and the data are shown in Fig. 5. The isotherm of monodisperse NiCo2O4 porous microcubes can be classified as type IV with a type H2 hysteresis loop, determining this structure to be mesoporous.50 According to the corresponding Barrett–Joyner–Halenda (BJH) plots (the inset in Fig. 5) recorded from the nitrogen isotherms of the monodisperse porous NiCo2O4 microcube samples, the average pore size is ca. 22.57 nm, further confirming that the sample is characteristic of mesoporous materials. The BET specific surface areas and pore volumes of the samples are as high as 26.26 m2 g−1 and 0.148 cm3 g−1, respectively. The microcubes with a mesoporous structure permit the electrolyte to easily penetrate through the mesopores and make compact contact with the surface of numerous primary nanoparticles assembled into the NiCo2O4 microcubes, possibly resulting in a considerable improvement in the electrochemical performance.


image file: c7qi00648a-f5.tif
Fig. 5 Nitrogen adsorption–desorption isotherm and pore size distribution (insets) of the monodisperse NiCo2O4 porous microcubes.

The electrochemical behaviors of the NiCo2O4 porous microcubes as anode materials were investigated via cyclic voltammetry and galvanostatic charge/discharge tests. Fig. 6a displays the CV curves of the NiCo2O4 porous microcube electrode in the initial three cycles at a scan rate of 0.1 mV s−1. As is seen, the shape of the CV curve for the first cycle is different from those of the second and third cycles. In the original cathodic scan, the sharp reduction peaks located at 1.1 and 0.95 V can be deemed to the reduction of Co3+ to Co2+, then Ni2+ and Co2+ to metallic Ni and Co, respectively.17,45 In the following anodic scan, the broad oxidation peak located at 2.1 V can be ascribed to the oxidation of metallic Ni and Co to nickel oxides and cobalt oxides.18 In the subsequent scans, the main reduction peak is negatively shifted to 0.90 V, whereas the oxidation peak is positively shifted to 2.3 V, which might be due to the structure changes during the first lithiation process and the pulverization of the electrode.34,43 Notably, the CV curves for the second and third cycles are well overlapped, suggesting a good reversibility of the electrode reaction. From the above discussion and previous reports, the lithium insertion/extraction reactions in the NiCo2O4 microcubes can be expressed as follows:32–35

 
NiCo2O4 + 8Li+ + 8e → Ni + 2Co + 4Li2O(1)
 
Ni + Li2O ↔ NiO + 2Li+ + 2e(2)
 
Co + Li2O ↔ CoO + 2Li+ + 2e(3)
 
CoO + 1/3Li2O ↔ 1/3Co3O4 + 2/3Li+ + 2/3e.(4)


image file: c7qi00648a-f6.tif
Fig. 6 Electrochemical characterization of the NiCo2O4 electrode: (a) CV curves for the initial three cycles at a scan rate of 0.1 mV s−1; (b) discharge/charge curves at a current density of 100 mA g−1; (c) cycling performance at a current density of 400 mA g−1; (d) rate capability at various current densities; (e) long-term cycling performance at a higher current density of 1000 mA g−1.

Fig. 6b presents the discharge/charge curves of the NiCo2O4 porous microcube electrode for the 1st, 2nd, and 5th cycles at a current density of 100 mA g−1. In the first discharge curve, there are a short voltage plateau at ca. 1.26 V and a long voltage plateau at ca. 1.15 V, followed by a gradual voltage decrease, corresponding to the decomposition of NiCo2O4 (eqn (1)) and the formation of a solid electrolyte interface (SEI) layer. Then, the consequent charge curve provides a declining voltage plateau around 2.1 V, which is associated with various oxidation reactions (eqn (2)–(4)). Compared to the first cycle, the second and fifth cycles illustrate a slightly lower discharge voltage plateau and a higher charge voltage plateau, which are in accordance with the CV results. The initial discharge and charge capacities of the NiCo2O4 electrode are 1412 mA h g−1 and 1053 mA h g−1, achieving a coulombic efficiency of 75%. The irreversible capacity loss in the first cycle is mainly due to the formation of the SEI layer and the inactivation of some inserted Li-ions.28 The reversible discharge capacity retains 1120 mA h g−1 at the second cycle and 1164 mA h g−1 at the fifth cycle with the ideal coulombic efficiency of 95% and 97%, respectively. The practical capacity of NiCo2O4 is larger than the theoretical capacity (890 mA h g−1), which can be explained by the reversible interfacial reaction, thus resulting in reversible lithium storage during cycling. The reaction consists of the formation/dissolution of an organic polymeric/gel-like film and the pseudo-capacitive interfacial storage reaction on the grain boundary in porous materials.14,29 As a result, the porous NiCo2O4 porous microcube electrode provides high electrochemical activity and reversibility due to its unique micro/nanostructures.

The cyclability of the NiCo2O4 porous microcube electrode at 400 mA g−1 after the first cycle at 100 mA g−1 is presented in Fig. 6c. The discharge capacity of the NiCo2O4 electrode gradually increases from 1120 mA h g−1 at the second cycle to 1164 mA h g−1 after the fifth cycle, which may be attributed to the activation process of the porous active material for sufficient contact with the electrolyte.18,32 Then the discharge capacity slowly decreases and finally remains at 1067 mA h g−1 over 300 cycles with 95% of the second reversible capacity. Moreover, the coulombic efficiency remains at ca. 98% upon cycling, indicating excellent cycling stability and reversibility. Apart from the outstanding cycling performance and high reversible capacity, the NiCo2O4 porous microcube electrode exhibits competitive rate capability. As shown in Fig. 6d, the reversible discharge capacities are 1135, 1105, 1042, 1008, 860, and 733 mA h g−1 at current densities of 200, 400, 800, 1000, 2000, and 4000 mA g−1, respectively. When the current density returned to 200 mA g−1, the discharge capacity could recover as high as 1023 mA h g−1, indicating good electrochemical reversibility. Compared to previous reports, the NiCo2O4 porous microcubes depict superior rate capability. A comparison of the rate capability and cycling performance of our NiCo2O4 electrodes and the previous reports is summarized in Table 1. The distinctive micro/nanostructures with abundant pores provide a large reaction interface and a short Li-ion diffusion pathway, leading to the remarkable rate capability of the NiCo2O4 porous microcubes. The cycling performance of NiCo2O4 porous microcubes was further evaluated at a high current density of 1000 mA g−1, as demonstrated in Fig. 6e. The current density of 100 mA g−1 was used to activate the electrode for the first cycle. As is seen, the discharge capacity undergoes a rapid decrease up to the 50th cycle, which may be attributed to the structure cracking and reorganization as well as pulverization of the electrode caused by a high discharge/charge current.45 However, the reversible capacity gradually increases in the subsequent 50 cycles and stabilizes at 650 mA h g−1 after 300 cycles with a high coulombic efficiency of 97–99%. The increase of capacity with cycling could be attributed to the gradual formation of a polymeric gel-like film and the continuous high-rate lithiation induced reactivation owing to the reconstruction of the porous cube and optimized stable SEI.11,28,29,45

Table 1 Comparison of the rate capability and cycling performance of NiCo2O4 materials in this work and in previous reports
Morphology of NiCo2O4 Discharge capacity (mA h g−1) Cycling performance Ref.
0.2 A g−1 1 A g−1 2 A g−1 Reversible capacity (mA h g−1) Current density (A g−1)
Porous microcubes 1135 1008 860 1067/300th 0.4 Our work
650/300th 1
Carbon-coated microspheres 982 849 1225/190th 0.5 10
Mesoporous nanoribbons 1050 710 1000/50th 0.5 17
Porous nanoflakes 1100 902 ∼800 981/100th 0.5 18
500/100th 0.8
Carbon-coated nanoparticles 980 805 612 880/50th 0.5 26
Porous microflowers 1200 ∼700 ∼500 720/100th 0.5 30
Nanowires 1201 810 1011/50th 0.1 33
Porous microflowers ∼900 ∼500 420 939/60th 0.1 35
Mesoporous nanorods ∼1070 764 653 863/200th 0.5 36
Hollow nanocubes 1160 ∼750 1058/200th 0.1 39
Plum-like particles 735 525 769/50th 0.1 40
Hollow nanoboxes 950 919 1080/150th 0.5 42
Hollow nanospheres 1210 923 659 695/200th 2 43
Porous microflowers 1397 1205 1100 860/100th 1 44
Porous microrods 988 925 701 719/600th 0.5 45
Mesoporous microspheres 1003 ∼550 705/500th 0.8 46
Mesoporous nanoneedles 1850 675 404 886/450th 0.5 47


The morphological evolution of the NiCo2O4 porous microcube electrode before cycling and after 100 cycles at the current density of 400 mA g−1 was verified by the SEM images (Fig. 7). Fig. 7a reveals that the regular NiCo2O4 porous microcubes are surrounded by conductive carbon and a binder in the fresh electrode. After 10 and 30 discharge/charge cycles, the morphology of the microcube is well preserved, whereas its size slightly increases due to the irreversible volume expansion caused by lithium insertion (Fig. 7b and c). Fig. 7d illustrates that the microcubes expand to nearly twice their original volume after 100 cycles. However, the porous micro/nanostructure could be perfectly held without obvious crack and pulverization, demonstrating excellent structural stability of the NiCo2O4 porous microcubes. Moreover, the integrity and steadiness of the NiCo2O4 porous microcube electrode over the repeated discharge/charge process can be distinctly observed from Fig. 7e and f. Thus, the stable micro/nanostructure of porous microcubes is favorable for good cycling stability of the NiCo2O4 electrode.


image file: c7qi00648a-f7.tif
Fig. 7 SEM images of the NiCo2O4 electrodes: (a) fresh electrode; (b) after 10 cycles; (c) after 30 cycles; (d–f) after 100 cycles.

Based on the abovementioned results, the porous NiCo2O4 microcubes as anode materials for lithium ion batteries exhibit outstanding cyclability and high-rate capability. The good lithium storage property of the NiCo2O4 can be ascribed to its unique porous micro structure assembled with nanoparticles. The rich mesopores among NiCo2O4 microcubes not only provide a large electrode/electrolyte reaction interface, but also provide enough void space to accommodate the volume change and prevent electronic disconnection in the electrode material during cycling.41–43 Furthermore, the primary nanoparticles with a smaller size within the microcubes can facilitate rapid Li-ion transport, resulting in high rate capacity.

4. Conclusion

In summary, we have devised a two-step method to prepare monodisperse NiCo2O4 porous microcubes assembled by nanoparticles with a size of ca. 35 nm. The first step involves simple urea-assisted solvothermal processing for the synthesis of binary metal carbonate microcubes. In the second step, Ni0.33Co0.67CO3 carbonate precursors are facilely annealed in air to generate the NiCo2O4 microcubes with porous structures. The porous microcubes have an average edge length of 1.5 μm and a pore size of 22.57 nm. Due to the unique porous micro/nanostructure, the NiCo2O4 microcube electrode exhibits a large initial discharge capacity of 1412 mA h g−1 at a current density of 100 mA g−1 and maintains a reversible discharge capacity of 733 mA h g−1 even at a high current density of 4000 mA g−1. Moreover, the porous microcube-like morphology of NiCo2O4 is well retained upon cycling without obvious crack and pulverization. The superior rate capability and cyclability enable such porous NiCo2O4 microcubes to be a competitive anode material for high-power lithium ion batteries.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (no. 21401061, 51302102 and 11504120), the Natural Science Foundation of Anhui Province (1708085ME96 and 1608085QE90), the Key Natural Science Research Project for Colleges and Universities of Anhui Province (KJ2016A638 and KJ2016SD53), the Key Project of Anhui Universities Support Program for Outstanding Youth, China (no. gxyqZD2016111), and the Huaibei Scientific Talent Development Scheme (20140305). We also thank the support of the Innovation Team of Design and Application of Advanced Energetic Materials from Huaibei Normal University.

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c7qi00648a

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