Accepted Manuscript Title: The electrochemical performance of nickel chromium oxide as a new anode material for lithium ion batteries Author: Jianjun Ma Shibing Ni Jicheng Zhang Xuelin Yang Lulu Zhang PII: DOI: Reference:
S0013-4686(15)30138-9 http://dx.doi.org/doi:10.1016/j.electacta.2015.07.071 EA 25353
To appear in:
Received date: Revised date: Accepted date:
21-6-2015 7-7-2015 13-7-2015
Please cite this article as: Jianjun Ma, Shibing Ni, Jicheng Zhang, Xuelin Yang, Lulu Zhang, The electrochemical performance of nickel chromium oxide as a new anode material for lithium ion batteries, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2015.07.071 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
The electrochemical performance of nickel chromium oxide as a new anode material for lithium ion batteries Jianjun Maa,c, Shibing Nia,b1, Jicheng Zhanga, Xuelin Yanga,b2, Lulu Zhanga,b a
College of Materials and Chemical Engineering, China Three Gorges University, 8
Daxue Road, Yichang, Hubei 443002, China b
Hubei Provincial Collaborative Innovation Center for New Energy Microgrid, China
Three Gorges University c
College of Mechanical and Power Engineering, China Three Gorges University, 8
Daxue Road, Yichang, Hubei 443002, China 1
Corresponding author. Fax: +86 717 6397505 E-mail address: [email protected]
(S. Ni) Corresponding author. Fax: +86 717 6397505 E-mail address: [email protected]
Graphical abstract Research highlights ＞ ＞ ＞ ＞
NiCr2O4 is fabricated and used as a new anode material for lithium ion batteries NiCr2O4 electrode is prepared via a facile and low cost way the NiCr2O4 electrode undergoes a novel electrochemical reconstruction in cycling the NiCr2O4 electrode exhibits good electrochemical performance
Abstract NiCr2O4 is successfully prepared via hydrothermal pretreatment and subsequent sintering, which shows excellent electrochemical performance as a new anode material for lithium ion batteries with natural graphite adding and sodium alginate binder. At a specific current of 70 mA g-1, it delivers charge and discharge capacities of 465.5 and 919.8 mAh g-1 in the initial cycle, which gradually increases along with
cycle number owing to an electrochemical reconstruction in cycling. After 100 cycles, the charge and discharge capacities are 582.9 and 592.5 mAh g-1, respectively. Furthermore, it is testified that natural graphite adding can effectively improve the electronic conductivity of the NiCr2O4 electrode, and an appropriate amount of natural graphite is beneficial to improve the specific capacity and cycle stability of the electrode owing to a coordinated electrochemical reconstruction between NiCr2O4 and natural graphite in cycling. Key Words: Nickel chromium oxide; Anode; Lithium ion battery; Coordinated electrochemical reconstruction; Sodium alginate 1. Introduction Transition metal oxides (TMOS) are becoming promising anode materials for lithium ion batteries owing to their combined advantages over other anode materials in terms of low cost, environmental friendliness, abundance in nature and high theoretical capacity (500-1000 mAh g-1) . The main restrictions of its practical application in lithium ion batteries come from two aspects. Firstly, the electronic conductivity of TMOS is low, which is not beneficial for the charge transfer in electrochemical reactions. Combining TMOS with carbon materials can effectively resolve this problem. Among different carbon materials, natural graphite shows the advantages of low cost, high electric conductivity and good lithium ion storage performance, which has been testified to be an effective carbon component in TMOS/C composites [2-5]. Secondly, the charge and discharge plateaus of TMOS are usually high, which will lower the energy density of lithium ion batteries when matching with cathode materials. To improve the energy density of lithium ion batteries, searching for new TMOS anode with lower charge and discharge plateaus becomes a key issue. Among them, CrxOy becomes an ideal candidate owing to the satisfied charge and discharge
plateaus and high specific capacity [6-9]. However, the electrochemical performance of CrxOy is not satisfied owing to the low electronic conductivity. Recently, it is demonstrated that introducing of a secondary metallic element in TMOS can improve their electrochemical performance owing to the enhanced electronic conductivity and electrochemical activity [10-15]. For example, ZnFe2O4 [16-18], CuFe2O4 [19-22], CoFe2O4 [23-25], NiFe2O4 , MgFe2O4 , MgCo2O4 , FeCo2O4 [28, 29], ZnCo2O4 [30-32], NiCo2O4 [33-35], CuCo2O4 , MnCo2O4 [37, 38] and ZnMn2O4 [39-41] have shown attractive electrochemical performance as anode materials for lithium ion batteries. Thus it is significant to introduce a secondary metallic element in CrxOy and optimize its electrochemical performance as anode for lithium ion batteries, which will be valuable for future research on high performance MCrxOy (M=Zn, Co, Ni, Cu etc.) anode materials. Here in this paper, we report the preparation of NiCr2O4 via facile hydrothermal pretreatment and subsequent sintering. Meanwhile, its electrochemical performance as a new anode material for lithium ion batteries was firstly studied. During the electrode preparation, low cost natural graphite was added to improve the electronic conductivity of the electrode, and water soluble sodium alginate was adopted as the binder. The superior electrochemical performance of the NiCr2O4 electrode and the facile way of material and electrode preparation endow NiCr2O4 with potential application in lithium ion batteries.
2. Experimental section 2.1 Fabrication procedure The chemicals were analytical grade and purchased from Shanghai Chemical Reagents. Natural graphite (NG) was obtained from Yichang Hengda graphite 3
company (99.9%). In a typical procedure, 1 mmol Ni(CH3COO)2•4H2O, 2 mmol Cr(NO3)3•9H2O and 5 mmol hexamethylenetetramine were dissolved in 30 ml distilled water. After stirring for 20 minutes, the homogeneous green suspension was transferred into a 50 ml teflonlined autoclave, distilled water was subsequently added to 80% of its capacity. The autoclave was at last sealed and placed in an oven, heated at 160℃ for 24 h. The final products were washed by distilled water and ethanol both four times and dried in an oven, then sintered in air atmosphere at 600℃ for 5h. For preparing NiCr2O4/NG, different amounts of NiCr2O4 and NG with weight ratio of 1:1, 2:1 (named as S1, S2) were manually rubbed for 30 min. 2.2 Structure and morphology characterization The structure and morphology of the resulting products were characterized by X-Ray powder diffraction (Rigaku Ultima IV Cu K radiation =1.5406 Å), field-emission scanning electron microscopy (FE-SEM JSM 7500F, JEOL). For the morphology and structure characterization of the electrode after charge and discharge tests, the cell was disassembled in an argon-ﬁlled dry box and the electrode was washed with DMC. 2.3 Electrochemical characterization Before the assembly of lithium ion battery, a mixture of NiCr2O4/NG (80 wt%), acetylene black (10 wt%), and sodium alginate (SA, dissolved in deionized water, 0.02 g mL−1, 10 wt%) were coated on copper foil and cut into disc electrodes with a diameter of 14 mm using a punch. Coin-type cells (2025) of Li/1 M LiPF6 in ethylene carbonate, dimethyl carbonate and diethyl carbonate (EC/DMC/DEC, 1:1:1v/v/v)/ NiCr2O4/NG disc electrode were assembled in an argon-ﬁlled dry box (MIKROUNA, Super 1220/750, H2O<1.0 ppm, O2<1.0 ppm). A Celgard 2400 microporous polypropylene was used as the separator membrane. The cells were tested in the 4
voltage region between 0.02 and 3 V with a multichannel battery test system (LAND CT2001A). When calculating the specific capacity of the electrode, the weight of both NiCr2O4 and NG was considered as the total weight of active materials. The cyclic voltammetry (CV) measurement of the electrodes was carried out on a CHI660C electrochemical workstation at a scan rate of 0.2 mV s−1 between 0 and 3 V. Electrochemical impedance spectroscopy (EIS) measurement was performed on CHI660C electrochemical workstation under open circuit conditions over a frequency range from 0.01 Hz to 100 kHz by applying an AC signal of 5 mV in amplitude throughout the tests. 3. Results and discussion Fig. 1 Fig. 1 presents the typical XRD pattern of the sample obtained at 600℃ for 5h in air atmosphere. As seen, all the diffraction peaks can be indexed to cubic NiCr2O4 (JCPDS, no.75-0198) with lattice constants a=8.299Å, b=8.299 Å, c=8.299 Å. The diffraction peaks located at 18.5o, 30.4o, 35.9o, 37.5o, 43.6o, 54.1o, 57.7o, 63.3o, 71.9o, 74.9o and 80o can ascribe to the (111), (220), (311), (222), (400), (422), (511), (440), (620), (533) and (444) faces of cubic NiCr2O4. The results suggest NiCr2O4 is successfully prepared. Fig. 2 The morphology and size of the obtained NiCr2O4 are studied via FE-SEM. Fig. 2(a) is a low magnification SEM image of the as-prepared NiCr2O4, which consists of a large number of nanoparticles. A high magnification SEM image of the NiCr2O4 is shown in Fig. 2(b), which suggests the mean size of these nanoparticles is about 100 nm. Fig. 3 5
Galvanostatic charge and discharge cycling of the NiCr2O4/NG electrode (S2) is carried out in the potential window of 0.02~3.0 V (vs. Li/Li+). Fig. 3(a) shows the capacity retention and the initial three and the 100th charge and discharge curves at a specific current of 70 mA g-1. As seen, the initial discharge curve is different from the subsequent two, showing three sloping potential regions (2.75~0.68, 0.68~0.23 and 0.23~0.02 V) owing to the reduction of NiCr2O4 into Ni and Cr, the formation of solid electrolyte interface (SEI), and the insertion of lithium ions into NG [3, 5, 6, 9, 45, 47]. The sloping potential regions in the subsequent two discharge curves show little shift (2.75~0.73 and 0.73~0.02 V) owing to the activation of the electrode. All the charge curves show similar profiles with three sloping potential regions (0.02~0.28, 0.28~2.03 and 2.03~3 V), which correspond to the extraction of lithium ions from NG and the oxidation of Ni and Cr into NiO and Cr2O3 [4, 5, 6, 7, 45]. The initial discharge capacity is 919.8 mAh g-1, which is bigger than the initial charge capacity of 465.5 mAh g-1. The capacity loss may be relevant to irreversible lithium ions consumption during the formation of SEI, which are common for most anode materials [3, 43, 44]. In the subsequent cycles, both charge and discharge capacities increase slowly along with cycle owing to the activation of the electrode and then gradually reach stable values [45, 46]. After 100 cycles, the charge and discharge capacities maintain of 582.9 and 592.5 mAh g-1, respectively. The cyclic voltammetric (CV) curves of the NiCr2O4/NG electrode was tested in voltage region 0~3.0 V at a scan rate of 0.2 mV s-1. As shown in Fig. 3(b), the proﬁles of CV curves for the 2nd and 3rd cycle are similar, whereas an obvious difference between the 1st and the subsequent two cycles can be found. In the 1st cathodic scan, two reduction peaks near 0.44 and 0.16 V are observed, which are relevant to the reduction of NiCr2O4 into Ni and Cr, the formation of the SEI, and the insertion of lithium ions
into NG [3, 4, 7, 13, 44]. As seen, the reduction peak near 0.16 V shows no obvious variation in the subsequent two cycles, which suggest stable structure of NG in cycling. In the qubsequent cathodic scan, two reduction peaks near 1.04 and 0.17 V correspond to the reduction of NiCr2O4 into Ni and Cr and the insertion of lithium ions into NG. In the anodic scan, a strong oxidation peak near 0.21 V can be attributed to the extraction of lithium ions from NG [2-5], and two continuous oxidation peaks near 1.42 and 2.18 V can ascribe to the oxidation of Cr and Ni into Cr2O3 and NiO [6, 7, 42, 45, 47]. The electrochemical reaction of the NiCr2O4 in lithiation is similar to that of NiFe2O4, NiCo2O4 and CuCrO2 [13, 44, 48-50], whch is likely to be as follows: NiCr2O4 + 8Li+ + 8e- → Ni + 2Cr + 4Li2O
During the charging process, the electrochemical reactions are likely to be: Ni + Li2O → NiO + 2e- + 2Li+
2Cr + 3Li2O → Cr2O3 + 6e- + 6Li+
(3) Fig. 4
The cycle performance of electrode material usually shows close relationship with the morphology variation of electrode in cycling. For further understanding the little increasing of capacity in the first few cycles and the excellent cycle stability of the NiCr2O4/NG electrode in cycling, the morphology variation of the electrode after cycle test was studied. Fig. 4(a) is a low magnification SEM image of the NiCr2O4/NG electrode (S2) after cycling test, which shows porous architecture, consisting of a large number of particles. The morphology of the cycled NiCr2O4/NG differs much from both NG and NiCr2O4, which suggests an electrochemical reconstruction [52, 53]. The morphology variation of the NiCr2O4/NG electrode can generate new active sites for lithium ions storage, leading to capacity increasing in the 7
first few cycles . For further studying the microstructure of the cycled electrode, a high magnification SEM image is shown in Fig. 4(b). As seen, no NG can be observed, and the porous architecture of the cycled NiCr2O4/NG electrode originates from the assembly of a large number of particles ranges from 50 to 500 nm. Such morphology observation of the cycled electrode may be relevant to the electrochemical reconstruction in cycling, which leads to the reassembly of NiCr2O4 on the surface of NG. As results, NG was covered by NiCr2O4 particles. The formation of an integrated porous architecture in cycling suggests well coordinated electrochemical reconstruction between NiCr2O4 and NG , which is responsible for the excellent cycle stability. Fig. 5 EIS measurement was employed to further study the causing of the excellent electrochemical performance of the NiCr2O4/NG electrode (S2). The intercept in high-frequency can be attributed to the SEI ﬁlm and/or contact resistance, the medium-frequency semicircle is due to the charge-transfer impedance on electrode/electrolyte interface, and the inclined line in low-frequency corresponds to the lithium ion diffusion process within electrodes [4, 55]. Fig. 5 is the Nyquist plots of NiCr2O4/NG electrode (S2) under different states. According to the fitted results (Tab. 1), it can be seen that Re increases slightly in cycling, which may be relevant to the gradual formation of SEI accompanied by the morphology variation of NiCr2O4. Meanwhile, Rct decreases after 5 cycles, which may be relevant to the electrochemical reconstruction that leads to the formation of an integrated porous architecture in cycling, facilitating the electron transfer between NiCr2O4 and NG [47, 52]. Remarkably, Rct for the NiCr2O4/NG electrode after 10 cycles shows close value than that after 5 cycles, indicating stable charge transfer process in cycling. Such
observation suggests the reconstructed NiCr2O4/NG electrode is electrochemically stable in cycling, which is responsible for the excellent cycle stability. Tab. 1 Fig. 6 The effect of NG on the electrochemical perforamnce of the NiCr2O4 electrode was contrastively studied. Fig. 6(a) shows the capacity retention and the initial three and the 100th charge and discharge curves at a specific current of 100 mA g-1. As seen, the charge and discharge curves for the NiCr2O4/NG electrode (S1) are similar to that in Fig. 3(a), suggesting similar electrochemical reactions in discharging and charging process. It delivers charge and discharge capacities of 376.8 mAh g-1 and 638 mAh g-1, respectively, and the extra capacity may be relevant to the formation of SEI [3, 4, 44]. The NiCr2O4/NG electrode (S1) also exhibits good cycle stability. However, the specific capacities are little smaller than that of S2 electrode, which delivers charge and discharge capacities of 525.8 and 534.8 mAh g-1 after 100 cycles. Such difference was also reflected by cyclic voltammetric (CV) curves. As shown in Fig. 6(b), the relative intensity of the reduction peaks near 0.14 and 0.44 V and oxidation peak near 0.22 V in the 1st CV curve increases slightly in the S1 electrode, which suggest increased capacity contribution of NG. Meanwhile, the relative intensity of reduction peak near 0.43 V and oxidation peaks near 1.42 and 2.18 V in the first CV curve decrease slightly in the S1 electrode, which suggest reduced capacity contribution of NiCr2O4. In the subsequent CV curves, similar results accompanied by the reduction of peak current of NiCr2O4 and the increase of peak current of NG can also be observed. On the basis of CV curves, the little capacity reduction along with the increasing of NG can be understood. Fig. 7
The electrochemial performance of pristine NiCr2O4 was also contrastively studied. Fig. 7(a) shows the capacity retention of the initial three and the 100th charge and discharge curves at a specific current of 85 mA g-1. The initial discharge curve shows two sloping potential regions (1.2~0.44 and 0.44~0.02 V), corresponding to the reduction of NiCr2O4 into Ni and Cr and the formation of SEI [6, 7, 45]. The sloping potential regions shift to 1.5~0.55 and 0.55~0.02 V in the subsequent discharge curves owing to the activation of the electrode. All the charge curves show similar profiles with two sloping potential regions (0.7~1.8 and 1.8~2.7 V), which correspond to the oxidation of Ni and Cr into NiO and Cr2O3 [6, 7, 45]. The initial charge and discharge capacities are 576.1 and 1016.6 mAh g-1, and the large irreversible capacity loss originates from the formation of SEI. The charge and discharge capacities of the NiCr2O4 electrode decrease sharply after 10 cycles, being 114.8 and 115.5 mAh g-1 after 100 cycles. The cyclic voltammetric (CV) curves of pristine NiCr2O4 are shown in Fig. 7(b). As seen, the proﬁles of CV curves for the 2nd and 3rd cycle are similar, whereas an obvious difference between the1st and subsequent two cycles is found. In the 1st cathodic scan, a weak reduction peak near 1.05 V and a strong reduction peak near 0.51 V are observed, which are relevant to the reduction of NiCr2O4 into Ni and Cr and the formation of SEI [4, 6, 9, 13, 42, 44]. In the anodic scan, two oxidation peaks near 1.42 and 2.18 Vcan be attributed to the oxidation of Ni and Cr into NiO and Cr2O3 [4, 6, 7, 47]. Fig. 8 Tab. 2 By comparison, it can be seen that NG can distinct improve the cycle stability of the NiCr2O4 electrode, and an approriate amount of NG is beneficial to improve the specific capacity of the electrode. For further clarifying the effect of NG on the
electrochemical performance of the NiCr2O4 electrodes, EIS spectra of NiCr2O4 electrodes with different amount of NG were provided (Fig. 8) and fitted via R(C(RW)) equivalent circuit (Tab. 2). As seen, Rct decreases obviously along with the increasing of NG amount in the NiCr2O4/NG electrodes, suggesting distinct improved electronic conductivity of the electrode. Maybe improving the electronic conductivity is an effective way to enhance the electrochemical performance of NiCr2O4 to some extent, but other factors such as the morphology variation of the integrated electrode in cycling should also be considered when designing a high performance NiCr2O4 electrode.
4. Conclusions In conclusion, NiCr2O4 was fabricated via a facile method, which shows good electrochemical performance as anode for lithium ion batteries with natural graphite adding and sodium alginate binder. It was testified that natural graphite adding can effectively improve the electronic conductivity of NiCr2O4, and an appropriate amount of natural graphite is beneficial to improve the specific capacity and cycle stability of the composite electrode owing to a coordinated electrochemical reconstruction between NiCr2O4 and natural graphite in cycling. The good electrochemical performance of the NiCr2O4/NG electrodes and the facile ways of material fabrication and electrode preparation can be easily scale up, which are beneficial for the practical application of NiCr2O4 in lithium ion batteries. Acknowledgements We gratefully acknowledge the financial support from Natural Science Foundation of China (NSFC, 51302152, 51272128 and 51302153). Moreover, the authors are
grateful to Dr Jianlin Li at Three Gorges University for his kind support of our research.
References  P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J.M. Tarascon, Nano-sized transition-metal oxides as negative-electrode materials for lithium-ion batteries, Nature 407 (2000) 496-499.  S.B. Ni, X.H. Lv, J.C. Zhang, J.J. Ma, X.L. Yang, L.L. Zhang, The electrochemical performance of lithium vanadate/natural graphite composite material as anode for lithium ion batteries, Electrochim. Acta 145 (2014) 327-334.  S.B. Ni, J.J. Ma, J.C. Zhang, X.L. Yang, L.L. Zhang, The electrochemical performance of commercial ferric oxide anode with natural graphite adding and sodium alginate binder, Electrochim. Acta 153 (2015) 546-551.  T. Li, S.B. Ni, X.H. Lv, X.L. Yang, S. Duan, Preparation of NiO-Ni/natural graphite composite anode for lithium ion batteries, J. Alloys Compd. 553 (2013) 167-171.  S.B. Ni, J.J. Ma, J.C. Zhang, X.L. Yang, L.L. Zhang, Electrochemical performance
of cobalt vanadium oxide/natural graphite as anode for lithium ion batteries, J. Power Sources 282 (2015) 65-69.  J. Hu, H. Li, X.J. Huang, Cr2O3-Based Anode Materials for Li-Ion Batteries, Electrochem. Solid-State Lett. 8 (2005) A66-A69.  Z.Q. Cao, M.L. Qin, B.R. Jia, L. Zhang, Q. Wan, M.S. Wang, A.A. Volinsky, X.H. Qu, Facile route for synthesis of mesoporous Cr2O3 sheet as anode materials for Li-ion batteries, Electrochim. Acta 139 (2014) 76-81.  L.Y. Jiang, S. Xin, X. L. Wu, H. Li, Y.G. Guo, L.J. Wan, Non-sacrificial template synthesis of Cr2O3-C hierarchical core/shell nanospheres and their application as anode materials in lithium-ion batteries, J. Mater. Chem. 20 (2010) 7565-7569.  F. Wang, W. Li, M.Y. Hou, C. Li, Y.G. Wang, Y.Y. Xia, Sandwich-like Cr2O3-graphite intercalation composites as high-stability anode materials for lithium-ion batteries, J. Mater. Chem. A 3 (2015) 1703-1708.  J. Cabana, L. Monconduit, D. Larcher, M.R. Palacín, Beyond Intercalation-Based Li-Ion Batteries: The State of the Art and Challenges of Electrode Materials Reacting Through Conversion Reactions, Adv. Mater. 22 (2010) E170-E192.  H.B. Wu, H. Pang, X.W. Lou, Facile synthesis of mesoporous Ni0.3Co2.7O4 hierarchical structures for high-performance supercapacitors, Energy Environ. Sci. 6 (2013) 3619-3626.  Y.G. Liu, Y.Y. Zhao, Y.L. Yu, M. Ahmad, H.Y. Sun, Facile synthesis of single-crystal mesoporous CoNiO2 nanosheets assembled flowers as anode materials for lithium-ion batteries, Electrochim. Acta 132 (2014) 404-409.
 Y.S. Fu, Y.H. Wan, H. Xia, X. Wang, Nickel ferrite-graphene heteroarchitectures: Toward high-performance anode materials for lithium-ion batteries, J. Power Sources 213 (2012) 338-342.  P.F. Teh, Y. Sharma, S.S. Pramana, M. Srinivasan, Nanoweb anodes composed of one-dimensional, high aspect ratio, size tunable electrospun ZnFe2O4 nanofibers for lithium ion batteries, J. Mater. Chem. 21 (2011) 14999–15008.  S.J. Peng, L.L. Li, M. Srinivasan, Electrospun CuFe2O4 nanotubes as anodes for high-performance lithium-ion batteries, J. Energ. Chem. 23 (2014) 301-307.  X.W. Guo, X. Lu, X.P. Fang, Y. Mao, Z.X. Wang, L.Q. Chen, X.X. Xu, H. Yang, Y.N. Liu, Lithium storage in hollow spherical ZnFe2O4 as anode materials for lithium ion batteries, Electrochem. Commun. 12 (2010) 847-850.  Y. Sharma, N. Sharma, G.V.S. Rao, B.V.R. Chowdari, Li-storage and cyclability of urea combustion derived ZnFe2O4 as anode for Li-ion batteries, Electrochim. Acta 53 (2008) 2380-2385.  Y.F. Deng, Q.M. Zhang, S.D. Tang, L.T. Zhang, S.N. Deng, Z.C. Shi, G.H. Chen, One-pot synthesis of ZnFe2O4/C hollow spheres as superior anode materials for lithium ion batteries, Chem. Commun. 47 (2011) 6828-6830.  M. Bomio, P. Lavela, J.L. Tirado, Electrochemical evaluation of CuFe2O4 samples obtained by sol–gel methods used as anodes in lithium batteries, J. Solid State Electrochem. 12 (2008) 729-737.  L.M. Jin, Y.C. Qiu, H. Deng, W.S. Li, H. Li, S.H. Yang, Hollow CuFe2O4 spheres encapsulated in carbon shells as an anode material for rechargeable lithium-ion
batteries, Electrochim. Acta 56 (2011) 9127-9132.  Z. Xing, Z.C. Ju, J. Yang, H.Y. Xu, Y.T. Qian, One-step solid state reaction to selectively fabricate cubic and tetragonal CuFe2 O4 anode material for high power lithium ion batteries, Electrochim. Acta 102 (2013) 51-57.  Y. Ding, Y.F. Yang, H.X. Shao, Synthesis and characterization of nanostructured CuFe2O4 anode material for lithium ion battery, Solid State Ion. 217 (2012) 27-33.  Y.Q. Chu, Z.W. Fu, Q.Z. Qin, Cobalt ferrite thin films as anode material for lithium ion batteries, Electrochim. Acta 49 (2004) 4915-4921.  C.T. Zhao, C. Yu, S.H. Liu, J.Y. Yang, X.M. Fan, J.S. Qiu, Facile Fabrication of Bicomponent CoO/CoFe2O4-N-Doped Graphene Hybrids with Ultrahigh Lithium Storage Capacity, Part. Part. Syst. Charact. 32 (2015) 91-97.  Z.H. Li, T.P. Zhao, X.Y. Zhan, D.S. Gao, Q.Z. Xiao, G.T. Lei, High capacity three-dimensional ordered macroporous CoFe2O4 as anode material for lithium ion batteries, Electrochim. Acta 55 (2010) 4594-4598.  P.Y. Zhu, S.Y. Liu, J. Xie, S.C. Zhang, G.S. Cao, X.B. Zhao, Facile Synthesis of NiFe2O4/Reduced Graphene Oxide Hybrid with Enhanced Electrochemical Lithium Storage Performance, J. Mater. Sci. Technol. 30 (2014) 1078-1083.  H.W. Liu, H.F. Liu, Synthesis of Nanosize Quasispherical MgFe2O4 and Study of Electrochemical Properties as Anode of Lithium Ion Batteries, J. Electron. Mater. 43 (2014) 2553-2558.  Y. Sharma, N. Sharma, G. Subbarao, B. Chowdari, Studies on spinel cobaltites, FeCo2O4 and MgCo2O4 as anodes for Li-ion batteries, Solid State Ion. 179 (2008)
587-597.  S.G. Mohamed, C.-J. Chen, C.K. Chen, S.-F. Hu, R.-S. Liu, High-Performance Lithium-Ion Battery and Symmetric Supercapacitors Based on FeCo2O4 Nanoflakes Electrodes, ACS Appl. Mat. Interfaces 6 (2014) 22701-22708.  Y. Sharma, N. Sharma, G.V. Subba Rao, B.V.R. Chowdari, Nanophase ZnCo2O4 as a High Performance Anode Material for Li-Ion Batteries, Adv. Funct. Mater. 17 (2007) 2855-2861.  H.W. Liu, J. Wang, One-pot synthesis of ZnCo2O4 nanorod anodes for high power Lithium ions batteries, Electrochim. Acta 92 (2013) 371-375.  G.X. Gao, H.B. Wu, B.T. Dong, S.J. Ding, X.W. Lou, Growth of Ultrathin ZnCo2O4 Nanosheets on Reduced Graphene Oxide with Enhanced Lithium Storage Properties, Adv. Sci. 2 (2015) 1400014-1400019.  J. Liu, C.P. Liu, Y. Wan,L. W. Liu, Z.S. Ma, S.M. Ji, J.B. Wang, Y.C. Zhou, P. Hodgson, Y.C. Li, Facile synthesis of NiCo2O4 nanorod arrays on Cu conductive substrates as superior anode materials for high-rate Li-ion batteries, Crystengcomm 15 (2013) 1578-1585.  A.K. Mondal, D.W. Su, S.Q. Chen, X.Q. Xie, G.X. Wang, Highly Porous NiCo2O4 Nanoflakes and Nanobelts as Anode Materials for Lithium-Ion Batteries with Excellent Rate Capability, ACS Appl. Mat. Interfaces 6 (2014) 14827-14835.  T. Li, X.H. Li, Z.X. Wang, H.J Guo, Y. Li, A novel NiCo2O4 anode morphology for Lithium-ion batteries, J. Mater. Chem. A (2015) DOI:10.1039/C5TA01928A.  Y. Sharma, N. Sharma, G.V.S. Rao, B.V.R. Chowdari, Lithium recycling
behaviour of nano-phase-CuCo2O4 as anode for lithium-ion batteries, J. Power Sources 173 (2007) 495-501.  C.C. Fu, G.S. Li, D. Luo, X.S. Huang, J. Zheng, L.P. Li, One-Step Calcination-Free Synthesis of Multicomponent Spinel Assembled Microspheres for High-Performance Anodes of Li-Ion Batteries: A Case Study of MnCo2O4, ACS Appl. Mat. Interfaces 6 (2014) 2439-2449.  A.K. Mondal, D.W. Su, S.Q. Chen, A. Ung, H.-S. Kim, G.X. Wang, Mesoporous MnCo2O4 with a Flake-Like Structure as Advanced Electrode Materials for Lithium-Ion Batteries and Supercapacitors, Chem. Eur. J. 21 (2015) 1526-1532.  G. Wang, R.H. Chen, Y.Q. Zhou, H. Wang, J.T. Bai, One-pot template-free fabrication of ZnMn2O4 hollow microspheres as high-performance lithium-ion battery anodes, J. Nanopart. Res. 16 (2014) 2300-2308.  M.S. Song, Y.J. Cho, D.Y. Yoon, S. Nahm, S.H. Oh, K. Woo, J.M. Ko, W.I. Cho, Solvothermal synthesis of ZnMn2O4 as an anode material in lithium ion battery, Electrochim. Acta 137 (2014) 266-272.  P.F. Teh, Y. Sharma, Y.W. Ko, S.S. Pramana, M. Srinivasan, Tuning the morphology of ZnMn2O4 lithium ion battery anodes by electrospinning and its effect on electrochemical performance, RSC Adv. 3 (2013) 2812-2821.  F. Li, Q.C. Zhuang, X.Y. Qiu, Z. Sun, Investigation of Lithiation Mechanism of LiCr3O8 as Potential Anode Materials for Lithium-ion Batteries, Int. J. Electrochem. Sci. 8 (2013) 3551-3563.  M.M. Lao, J. Shu, L.Y. Shao, X.T. Lin, K.Q. Wu, M. Shui, P. Li, N.B. Long, Y.L.
Ren, Enhanced electrochemical performance of Ag-coated CuCr2O4 as anode material for lithium-ion batteries, Ceram. Int. 40 (2014) 11899-11904.  X.D. Zhu, J. Tian, S.R. Le, N.Q. Zhang, K.N. Sun, Improved electrochemical performance of CuCrO2 anode with CNTs as conductive agent for lithium ion batteries, Mater. Lett. 97 (2013) 113-116.  X.H. Wang, Z.B. Yang, X.L. Sun, X.W. Li, D.H. Wang, P. Wang, D.Y. He, NiO nanocone array electrode with high capacity and rate capability for Li-ion batteries, J. Mater. Chem. 21 (2011) 9988-9990.  Y.M. Sun, X.L. Hu, W. Luo, F.F. Xia, Y.H. Huang, Reconstruction of Conformal Nanoscale MnO on Graphene as a High-Capacity and Long-Life Anode Material for Lithium Ion Batteries, Adv. Funct. Mater. 23 (2013) 2436-2444.  S.B. Ni, T. Li, X.H. Lv, X.L. Yang, L.L. Zhang, Designed constitution of NiO/Ni nanostructured electrode for high performance lithium ion battery, Electrochim. Acta 91 (2013) 267-274.  L.F. Shen, L. Yu, X. Y. Yu, X.G. Zhang, X.W. Lou, Self-Templated Formation of Uniform NiCo2O4 Hollow Spheres with Complex Interior Structures for Lithium-Ion Batteries and Supercapacitors, Angew. Chem. Int. Ed. 53 (2014) 1-6.  L.F. Shen, Q. Che, H.S. Li, X.G. Zhang, Mesoporous NiCo2O4 Nanowire Arrays Grown on Carbon Textiles as Binder-Free Flexible Electrodes for Energy Storage, Adv. Funct. Mater. 24 (2014) 2630-2637.  G. Huang, F.F. Zhang, X.C. Du, J.W. Wang, D.M. Yin, L.M. Wang, Core-Shell [email protected]
: An Anode Material with Enhanced Electrochemical
Performance for Lithium-Ion Batteries, Chem. Eur. J. 20 (2014) 11214-11219.  F. Han, D. Li, W.C. Li, C. Lei, Q. Sun, A.H. Lu, Nanoengineered Polypyrrole-Coated [email protected]
Multifunctional Composites with an Improved Cycle Stability as Lithium-Ion Anodes, Adv. Funct. Mater. 23 (2013) 1692-1700.  S.B. Ni, X.H. Lv, T. Li, X.L. Yang, L.L. Zhang, Y. Ren, A novel electrochemical activation effect induced morphology variation from massif-like CuxO to forest-like Cu2O nanostructure and the excellent electrochemical performance as anode for Li-ion battery, Electrochim. Acta 96 (2013) 253-260.  C. Wang, D.L. Wang, Q.M. Wang, H.J. Chen, Fabrication and lithium storage performance of three-dimensional porous NiO as anode for lithium-ion battery, J. Power Sources 195 (2010) 7432-7437.  S.B. Ni, J.J. Ma, J.C. Zhang, X.L. Yang, L.L. Zhang, Excellent electrochemical performance of NiV3O8/natural graphite anodes via novel in situ electrochemical reconstruction, Chem. Commun. 51 (2015) 5880-5882.  Y.J. Zhu, C.S. Wang, Novel CV for Phase Transformation Electrodes, J. Phys. Chem. C 115 (2011) 823-832.
Figure captions Fig. 1 XRD pattern of the as-prepared sample. Fig. 2 SEM images of the
as-prepared NiCr2O4 with (a) low and (b) high
magnification. Fig. 3 Electrochemical performance of the NiCr2O4/NG electrode (sample S2). (a) Cycle performance at a specific current of 70 mA g-1.The inset shows the initial three and the 100th charge and discharge curves; (b) Cyclic voltammograms at a scan rate of 0.2 mV s-1. Fig. 4 SEM images of S2 electrode after cycle test with low (a) and high (b) magnification. Fig. 5 Nyquist plots for NiCr2O4/NG electrode (S2) under different states. Fig. 6 Electrochemical performance of NiCr2O4/NG (sample S1). (a) Cycle performance at a specific current of 100 mA g-1. The inset shows the initial three and the 100th charge and discharge curves. (b) Cyclic voltammograms at a scan rate of 0.2 mV s-1. Fig. 7 Electrochemical performance of pristine NiCr2O4 electrode. (a) Cycle performance at a current density of 85 mA g-1. The inset shows the initial three and
the 100th charge and discharge curves. (b) Cyclic voltammograms at a scan rate of 0.2 mV s-1. Fig. 8 Nyquist plots of fresh NiCr2O4 electrodes with different NG adding. Table captions Table. 1 EIS parameters obtained from R(C(RW)) equivalent circuit fitting of Nyquist plots for NiCr2O4/NG electrode (S2) under different states.
NiCr2O4/NG electrode (S2)
after 5 cycles
after 10 cycles
Table. 2 EIS parameters obtained from R(C(RW)) equivalent circuit fitting of Nyquist plots for NiCr2O4/NG electrodes with different NG adding.