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Ni composite material and its electrochemical performance as anode for Li-ion battery
Electrochimica Acta 130 (2014) 800–804
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
The fabrication of Li3 VO4 /Ni composite material and its electrochemical performance as anode for Li-ion battery Shibing Ni ∗ , Xiaohu Lv, Jianjun Ma, Xuelin Yang ∗ , Lulu Zhang College of Materials and Chemical Engineering, Collaborative Innovation Center for Energy Equipment of Three Gorges Region, Three Gorges University, 8 Daxue Road, Yichang, Hubei 443002, China
a r t i c l e
i n f o
Article history: Received 24 January 2014 Received in revised form 17 March 2014 Accepted 17 March 2014 Available online 1 April 2014 Keywords: Lithium vanadate Lithium ion battery Anode Electrochemical reaction kinetics Electrochemical reconstruction
a b s t r a c t Uniform Li3 VO4 film was successfully grown on porous Ni foam, which shows good electrochemical performance as anode for Li-ion battery due to the improved reaction kinetics. The as-prepared Li3 VO4 /Ni electrode can deliver discharge and charge capacity of 379 and 378 mAh g−1 after 100 cycles at a charge/discharge rate of 0.3 C. After various charge/discharge rates from 0.4 to 15 C, the discharge capacity of the Li3 VO4 /Ni electrode can restore to 404 mAh g−1 when lowering the charge/discharge rate to 0.4 C. The reaction kinetics of the Li3 VO4 /Ni electrode was studied by cyclic voltammetry (CV) measurement at various scan rate and electrochemical impedance spectroscopy (EIS). Linear dependence between anodic/cathodic peak currents and the square root of scan rate suggests a lithium ion diffusion controlled mechanism of Li3 VO4 /Ni electrode in charge/discharge process, and the symmetrical slope of the two straight lines indicates a highly reversible lithiation/delithiation process. EIS measurements of the Li3 VO4 /Ni electrode show low and stable contact and charge-transfer resistances in cycling, suggesting highly stable charge transfer process and Li-ion diffusion coefficient. © 2014 Elsevier Ltd. All rights reserved.
1. Introduction Li-ion batteries have been becoming not only the main power source in today’s portable electronic devices but also the ideal candidate for power sources of electric vehicles and hybrid electric vehicles. One of the key issues in the development of Li-ion batteries is to explore new advanced materials with higher capacity and better electrochemical performance. As a new kind of anode, Lithium vanadate (Li3 VO4 ) shows potential application in Li-ion batteries due to its relative high volumetric capacity than graphite and relative low voltage plateau than Li4 Ti5 O12 [1–3]. In our previous study, we found a novel morphology variation of Li3 VO4 before and after cycling test, suggesting a phase transition in charge/discharge process [3], which is similar to that of transition metal oxides (TMOS). As we know, TMOS exhibit high theoretical capacity (500∼1000 mAh g−1 ) based on redox reaction mechanisms, which undergo morphology and structure destruction in cycling, leading to unsatisfied electrochemical performance [4–6]. Much work on improving the electrochemical performance of TMOS has been done on growing TMOS directly on conductive collector and/or combining TMOS with matrix phase
∗ Corresponding author. Fax: +86 717 6397559. E-mail addresses: [email protected] (S. Ni), [email protected] (X. Yang). http://dx.doi.org/10.1016/j.electacta.2014.03.120 0013-4686/© 2014 Elsevier Ltd. All rights reserved.
such as carbon [7–10], which has gained impressive results. For Li3 VO4 anode, the electrochemical performance can be improved via combining it with graphene, which shows reduced charge transfer resistance and enhanced lithium diffusion coefficient [2]. It is reasonable to believe that growing Li3 VO4 directly on electric collector may be another feasible way to improve the electrochemical performance. Ni foam shows 3D porous architecture with good structure stability and fine electronic conductivity, which can be adopted as an ideal deposition substrate [11,12]. Here in this paper, we report the preparation of Li3 VO4 /Ni composite and its electrochemical performance as anode for Li-ion battery. The main objective of this paper is to explore the way to improve the electrochemical performance of Li3 VO4 , and to study the factors that affect the electrochemical performance of Li3 VO4 . 2. Experimental 2.1. Fabrication procedure The chemicals were analytical grade and purchased from Shanghai Chemical Reagents. Ni foam (100 PPI pore size, 380 g m−2 surface density, 1.5 mm thick) was purchased from Changsha Lyrun New Material corporation. In a typical procedure, 1 mmol V2 O5 , 3 mmol Li2 CO3 and 5 mmol hexamethylenetetramine were dissolved in 30 ml distilled water. After stirring for 20 minutes, the
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homogeneous yellowy 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 120 ◦ C for 24 h. The final transparent solution was transferred in a culture dish, Ni discs were immersed into the solution for 1 h. After that, the Ni discs were dried and annealed in N2 atmosphere at 400 ◦ C for 5 h. The weight of active Li3 VO4 on Ni disc can be estimated according to the weight difference between the Ni disc and the obtained Li3 VO4 /Ni (mLi3 VO4 = mLi3 VO4 /Ni -mNi ). 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 ˚ and field-emission scanning electron K␣ radiation = 1.5406 A) microscopy (FE-SEM JSM 7500F, JEOL). Fig. 1. XRD pattern of the obtained electrode.
2.3. Electrochemical characterization For fabricating of lithium ion battery, the as-prepared Li3 VO4 /Ni foam discs (diameter of 14 mm) were dried at120 ◦ C for 24 h in vacuum oven. Coin-type cells (2025) of Li/1 M LiPF6 in ethylene carbonate, dimethyl carbonate and diethyl carbonate (EC/DMC/DEC, 1:1:1 v/v/v)/Li3 VO4 /Ni were assembled in an argon-filled dry box (MIKROUNA, Super 1220/750, H2 O < 1.0 ppm, O2 < 1.0 ppm). A Celgard 2400 microporous polypropylene was used as the separator membrane. Galvanostatic charge/discharge test was characterized on a multichannel battery test system (LAND CT2001A) in the voltage region between 0.02 and 3 V. The Cyclic voltammetry measurement of the electrodes (vs. Li+ /Li) was carried out on a CHI660 C electrochemical workstation at a scan rate of 0.2 mV s−1 between 0 and 3 V. Electrochemical impedance spectroscopy measurements were performed on CHI660 C 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. Equivalent circuit fitting of EIS data was carried out via ZsimpWin software. 3. Results and Discussion Typical XRD pattern of the electrode obtained by annealing the immersed Ni foam in N2 atmosphere at 400 ◦ C for 5 h is shown in Fig. 1. As seen, the diffraction peaks (marked by *) located at 16.3◦ , 21.6◦ , 22.9◦ , 24.4◦ , 28.2◦ , 32.7◦ , 36.4◦ , 37.6◦ , 58.6◦ , and 66.2◦ can be attributed to the (100), (110), (011), (101), (111), (200), (002), (201), (320) and (203) faces of orthorhombic Li3 VO4 with lattice constants ˚ b = 5.448 A˚ and c = 4.940 A, ˚ which is in good agreement a = 6.319 A, with JCPDS, No. 38-1247. In addition, three typical diffraction peaks (marked by 䊉) located at 44.4◦ , 51.7◦ and 76.4o can be attributed to
Ni (111), (200) and (220) faces, respectively (JCPDS, No. 04-0850). The diffraction peaks correspond to V4 O9 are not obvious, which may be relevant to its low intensity compared with that of Li3 VO4 . Fig. 2(a) is a low magnification SEM image of the as-prepared Li3 VO4 /Ni, from which uniform film-like morphology can be observed. A low magnification SEM image of Ni foam is shown in the insert of Fig. 2(a). As seen, the surface of the Ni foam is smooth, exhibiting clear grain boundaries. The obvious morphology variation suggests that Li3 VO4 film is successfully grown on Ni foam. High magnification SEM image is provided for further clarifying the microstructure of the Li3 VO4 /Ni. As shown in Fig. 2(b), the Li3 VO4 film exhibits a large number of cracks on the surface, which may originate from the volume variation from precursor to Li3 VO4 . The width of these cracks ranges from tens of nanometers to 150 nm, and the length of these cracks ranges from several tens of nanometers to several micro meters. Galvanostatic charge/discharge cycling was carried out in the potential window of 0.02∼3.0 V versus Li. Fig. 3(a) shows the capacity retention and the 1st, 2nd and 100th charge/discharge voltage profiles of the Li3 VO4 electrode at a rate of 0.3 C (1 C means accomplishing discharge or charge in an hour). As seen, the initial discharge curve differs slightly from the 2nd one, showing a sloping potential region from 1.5 to 0.02 V for the initial lithiation process. The 2nd and the 100th discharge curves show similar profile with two sloping potential regions (1.5∼0.4, and 0.4∼0.02 V), accompanied by the attenuation of discharge capacity. The 1st, 2nd and 100th charge curves exhibit similar profile with a sloping potential region (0.8∼2.5 V), which accompanies by the attenuation of charge capacity. The initial discharge capacity is 841 mAh g−1 , which is bigger than the initial charge capacity (531 mAh g−1 ) owing to
Fig. 2. SEM images of the as-prepared Li3 VO4 /Ni with low (a) and high (b) magnification. The insert of (a) is a low magnification SEM image of Ni foam.
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Fig. 3. Electrochemical performance and rate capability of Li3 VO4 /Ni electrode. (a) Capacity retention of the galvanostatic test runs at a rate of 0.3 C; The inset shows the galvanostatic charge/discharge voltage profiles for the 1st, 2nd and 100th cycle. (b) Cyclic voltammograms at a scan rate of 0.2 mV s−1 . (c) Representative charge and discharge voltage profiles at various C rates. (d) Capacity retention at various C rates.
the formation of solid electrolyte interface (SEI) [1,3]. As seen, the specific capacity of the Li3 VO4 /Ni electrode is bigger than that of Li3 VO4 powder [3], which may be relevant to the special 3D porous architecture of Ni substrate. Similar results have been reported for NiO/Ni foam composite electrodes [13,14]. The discharge and charge capacity decrease slowly along with the increasing of cycle number in the first few cycles and then gradually reach stable values. After 100 cycles, the discharge and charge capacity maintain of 379 and 378 mAh g−1 , respectively. The cyclic voltammetric (CV) curves of the Li3 VO4 electrode were tested over a voltage region from 0 to 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 first and subsequent two cycles is found. In the 1st cathodic scan, two reduction peaks at around 0.34 and 0.72 V are attributed to the lithiation process that can be described as: xLi+ + Li3 VO4 + xe− → Li3+x VO4 [1–3], which is in accordance with the initial discharge curve. The reduction peaks shift to 0.54 and 0.86 V in the 2nd cathodic scan and 0.60 and 0.92 V in the 3rd cycle, which can be ascribed to the activation of Li3 VO4 . The activation process can be understood as the transformation of Li3 VO4 into more active nanosized particles [1], which is similar to that of TMOS [7]. The profiles for the initial three anodic scan are similar, showing an oxidation peak near 1.3 V, which is attributed to the delithiation process that can be described as: Li3+x VO4 → xLi+ + Li3 VO4 + xe− [3]. Fig. 3(c) shows the discharge and charge curves of the Li3 VO4 electrode at various C rates from 0.4 to 15 C. Along with the increasing of charge/discharge rate, the discharge potential decreases and the charge potential increases due to the enhanced polarization, which is similar to that reported in literature [13–15]. As shown in the rate capability in Fig. 3(d), the 10th discharge capacity is 415, 358, 302, 256 and 204 mAh g−1 at rates of 0.4, 1, 3, 6 and 15 C, respectively. After that, the discharge capacity can restore to 404 mAh g−1 when reverting the discharge/charge
rate to 0.4 C. In the subsequent 50 cycles, the discharge capacity shows no capacity attenuation, suggesting good rate capability and cycle stability of the Li3 VO4 /Ni electrode. Fig. 4 shows the XRD pattern of the Li3 VO4 /Ni electrode after 100 cycles with charge state. As seen, diffraction peaks located at 21.6◦ , 22.9◦ , 32.7◦ 36.4◦ and 58.6◦ are found, which correspond to (110), (011), (200), (002) and (320) faces of Li3 VO4 (JCPDS, No. 38-1247). In addition, the diffraction peaks located at 30.5◦ and 31.7◦ can be indexed as (311) and (112) faces of orthorhombic V4 O9 (JCPDS, No. 24-1391). This observation is similar to our previous study, suggesting a reversible charge/discharge process. The cycled Li3 VO4 /Ni electrode shows reduced diffraction peaks compared with the asprepared Li3 VO4 /Ni, which may be relevant to the electrochemical activation effect [3]. Fig. 5(a) is a low magnification SEM image of the Li3 VO4 /Ni electrode after 100 cycles with charge state, which shows uniform
Fig. 4. XRD pattern of the Li3 VO4 /Ni electrode after 100 cycles with charge state.
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Fig. 5. SEM images of the Li3 VO4 /Ni electrode after 100 cycles with low (a) and high (b) magnification.
Fig. 6. (a) CV curves of the Li3 VO4 /Ni electrode at different scan rate between 0 and 3 V. (b) Dependence of peak current on the square root of scan rate for the Li3 VO4 /Ni electrode.
film-like morphology, suggesting good structure stability of the Li3 VO4 /Ni electrode. However, the morphology of the cycled electrode is much different from that before. High magnification SEM image of the Li3 VO4 /Ni electrode is shown in Fig. 5(b). As seen, the film-like Li3 VO4 are composed of a large number of nanosized particles. In addition, cracks on the surface of the Li3 VO4 film disappear after cycling. This morphology variation can be understood as a novel electrochemical reconstruction that consists of the formation of a large number of nanosized particles and the assembly of these nanoparticles into secondary architecture, which is similar to NiO/Ni and Cux O/Cu that reported in literature [7,16]. CV curves of the Li3 VO4 /Ni electrode at scan rates from 0.2 to 3.0 mV s−1 are shown in Fig. 6(a). As seen, the reduction peak shifts to low potential region along with the increasing of scan rate, whereas the oxidation peaks shift to high potential region,
suggesting the high polarization under a high scan rate [3,17,18]. Fig. 6(b) shows the relationship between peak current and the square root of scan rate obtained from the experimental data in Fig. 6(a). As found, both the anodic and cathodic peak currents show linear dependence on the square root of the scan rate from 0.2 to 3 mV s−1 , suggesting a lithium ion diffusion controlled mechanism in the charge and discharge process [17,18]. However, the fitted curve shows no zero interpret, which may be relevant to the phase transformation of the electrode, being consistent with the SEM observation in Fig. 5. Previous study for LiFePO4 has shown that phase transformation leads to a deviation of peak current from a linear relationship versus square root of scan rate [18]. The slope of the anodic and cathodic curve shows close absolute value, which suggest a highly reversible lithiation/delithiation process. Fig. 7 shows the electrochemical impedance spectra of the Li3 VO4 /Ni under different state. The semicircle 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 Li-ion diffusion process within electrodes [18]. The Nyquist plots were fitted via an equivalent circuit, and the contact and charge-transfer resistances of the electrode (Re and Rct) were obtained from the fitting results (Table 1).
Table 1 Electrode kinetic parameters obtained from equivalent circuit fitting of Nyquist plots for Li3 VO4 /Ni electrode at different state.
Fig. 7. AC impedance spectra of the Li3 VO4 /Ni electrode under different state.
Li3 VO4 /Ni electrode
Re ()
Rct ()
fresh electrode after 5 cycles testing after 100 cycles testing
6.04 8.37 12.47
25.02 34.96 34.41
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S. Ni et al. / Electrochimica Acta 130 (2014) 800–804
As seen, Re before charge/discharge test is little smaller than that after test, which may be due to the formation of SEI in the discharge process. The little difference of Re after 5 and 100 cycles suggests the formation of stable SEI in cycling. In addition, Rct after 5 and 100 cycles test shows nearly the same value, suggesting a highly stable charge transfer process in the charge/discharge testing. Furthermore, the inclined lines in low-frequency for the Li3 VO4 /Ni electrode after 5 and 100 cycles show close slope, suggesting a highly stable Li-ion diffusion process. 4. Conclusions In summary, Li3 VO4 film that directly grows on 3D porous Ni foam was prepared via a unique two-step method, which shows excellent electrochemical performance as anode for Li-ion batteries. The good electrochemical performance of the Li3 VO4 /Ni is relevant to its stable charge transfer and Li-ion diffusion coefficient as well as the highly reversible lithiation/delithiation process in cycling. The attractive electrochemical performance of Li3 VO4 as well as the feasible way to improve its electrochemical performance endows it with potential application in Li-ion battery. Acknowledgement We gratefully acknowledge the financial support from Natural Science Foundation of China (NSFC, 51272128, 51302152, and 51302153). Moreover, the authors are grateful to Dr. Jianlin Li at Three Gorges University for his kind support to our research. References [1] H.Q. Li, X.Z. Liu, T.Y. Zhai, D. Li, H.S. Zhou, Li3 VO4 : A Promising Insertion Anode Material for Lithium-Ion Batteries, Advanced Energy Materials 3 (2012) 428–432. [2] Z. Shi, J.Z. Wang, S.L. Chou, D. Wexler, H.J. Li, K. Ozawa, H.K. Liu, Y.P. Wu, Hollow Structured Li3 VO4 Wrapped with Graphene Nanosheets in Situ Prepared by a One-Pot Template-Free Method as an Anode for Lithium-Ion Batteries, Nano Letters 13 (2013) 4715–4720. [3] S.B. Ni, X.H. Lv, J.J. Ma, X.L. Yang, L.L. Zhang, Electrochemical characteristics of lithium vanadate, Li3 VO4 as a new sort of anode material for Li-ion batteries, Journal of Power Sources 248 (2014) 122–129.
[4] J.Y. Xiang, J.P. Tu, L. Zhang, Y. Zhou, X.L. Wang, S.J. Shi, Self-assembled synthesis of hierarchical nanostructured CuO with various morphologies and their application as anodes for lithium ion batteries, Journal of Power Sources 1 (2010) 313–319. [5] X.F. Zheng, G.F. Shen, Y. Li, H.N. Duan, X.Y. Yang, S.Z. Huang, H.G. Wang, C. Wang, Z. Deng, B.L. Su, Self-templated synthesis of microporous CoO nanoparticles with highly enhanced performance for both photocatalysis and lithium-ion batteries, Journal of Materials Chemistry A 1 (2013) 1394–1400. [6] B. Wang, J.S. Chen, H.B. Wu, Z.Y. Wang, X.W. Lou, Quasiemulsion-Templated Formation of ␣-Fe2 O3 Hollow Spheres with Enhanced Lithium Storage Properties, Journal of the American Chemical Society 133 (2011) 17146–17148. [7] 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, Journal of Power Sources 195 (2010) 7432–7437. [8] S.B. Ni, X.H. Lv, T. Li, X.L. Yang, L.L. Zhang, Preparation of Cu2 O-Cu anode for high performance Li-ion battery via an electrochemical corrosion method, Electrochimica Acta 109 (2013) 419–425. [9] W.M. Zhang, X.L. Wu, J.S. Hu, Y.G. Guo, L.J. Wan, Carbon Coated Fe3 O4 Nanospindles as a Superior Anode Material for Lithium-Ion Batteries, Advanced Functional Material 18 (2008) 3941–3946. [10] Y. Huang, X.L. Huang, J.S. Lian, D. Xu, L.M. Wang, X.B. Zhang, Self-assembly of ultrathin porous NiO nanosheets/graphene hierarchical structure for highcapacity and high-rate lithium storage, Journal of Materials Chemistry 22 (2012) 2844–2847. [11] 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, Electrochimica Acta 91 (2013) 267–274. [12] S.B. Ni, T. Li, X.L. Yang, Fabrication of NiO nanoflakes and its application in lithium ion battery, Materials Chemistry and Physics 132 (2012) 1108–1111. [13] X.H. Wang, X.W. Li, X.L. Sun, F. Li, Q.M. Liu, Q. Wang, D.Y. He, Nanostructured NiO electrode for high rate Li-ion batteries, Journal of Materials Chemistry 21 (2011) 3571–3573. [14] B. Varghese, M.V. Reddy, Z. Yanwu, C.S. Lit, T.C. Hoong, G.V.S. Rao, B.V.R. Chowdari, A.T.S. Wee, C.T. Lim, C.H. Sow, Fabrication of NiO Nanowall Electrodes for High Performance Lithium Ion Battery, Chemistry of Materials 20 (2008) 3360–3367. [15] S.B. Ni, X.L. Yang, L. Tao, Fabrication of a porous NiS/Ni nanostructured electrodevia a dry thermal sulfuration method and its application in a lithium ion battery, Journal of Materials Chemistry 22 (2012) 2395–2397. [16] S.B. Ni, X.H. Lv, T. Li, X.L. Yang, L.L. Zhang, Yong Ren, A novel electrochemical activation effect induced morphology variation from massif-like Cux O to forest-like Cu2 O nanostructure and the excellent electrochemical performance as anode for Li-ion battery, Electrochimica Acta 96 (2013) 253–260. [17] C.C. Li, X.M. Yin, L.B. Chen, Q.H. Li, T.H. Wang, Porous Carbo Nanofibers Derived from Conducting Polymer: Synthesis and Application in Lithium-Ion Batteries with High-Rate Capability, The Journal of Physical Chemistry C 113 (2009) 13438–13442. [18] Y.J. Zhu, C.S. Wang, Novel CV for Phase Transformation Electrodes, The Journal of Physical Chemistry C 115 (2011) 823–832.
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
The fabrication of Li3 VO4 /Ni composite material and its electrochemical performance as anode for Li-ion battery Shibing Ni ∗ , Xiaohu Lv, Jianjun Ma, Xuelin Yang ∗ , Lulu Zhang College of Materials and Chemical Engineering, Collaborative Innovation Center for Energy Equipment of Three Gorges Region, Three Gorges University, 8 Daxue Road, Yichang, Hubei 443002, China
a r t i c l e
i n f o
Article history: Received 24 January 2014 Received in revised form 17 March 2014 Accepted 17 March 2014 Available online 1 April 2014 Keywords: Lithium vanadate Lithium ion battery Anode Electrochemical reaction kinetics Electrochemical reconstruction
a b s t r a c t Uniform Li3 VO4 film was successfully grown on porous Ni foam, which shows good electrochemical performance as anode for Li-ion battery due to the improved reaction kinetics. The as-prepared Li3 VO4 /Ni electrode can deliver discharge and charge capacity of 379 and 378 mAh g−1 after 100 cycles at a charge/discharge rate of 0.3 C. After various charge/discharge rates from 0.4 to 15 C, the discharge capacity of the Li3 VO4 /Ni electrode can restore to 404 mAh g−1 when lowering the charge/discharge rate to 0.4 C. The reaction kinetics of the Li3 VO4 /Ni electrode was studied by cyclic voltammetry (CV) measurement at various scan rate and electrochemical impedance spectroscopy (EIS). Linear dependence between anodic/cathodic peak currents and the square root of scan rate suggests a lithium ion diffusion controlled mechanism of Li3 VO4 /Ni electrode in charge/discharge process, and the symmetrical slope of the two straight lines indicates a highly reversible lithiation/delithiation process. EIS measurements of the Li3 VO4 /Ni electrode show low and stable contact and charge-transfer resistances in cycling, suggesting highly stable charge transfer process and Li-ion diffusion coefficient. © 2014 Elsevier Ltd. All rights reserved.
1. Introduction Li-ion batteries have been becoming not only the main power source in today’s portable electronic devices but also the ideal candidate for power sources of electric vehicles and hybrid electric vehicles. One of the key issues in the development of Li-ion batteries is to explore new advanced materials with higher capacity and better electrochemical performance. As a new kind of anode, Lithium vanadate (Li3 VO4 ) shows potential application in Li-ion batteries due to its relative high volumetric capacity than graphite and relative low voltage plateau than Li4 Ti5 O12 [1–3]. In our previous study, we found a novel morphology variation of Li3 VO4 before and after cycling test, suggesting a phase transition in charge/discharge process [3], which is similar to that of transition metal oxides (TMOS). As we know, TMOS exhibit high theoretical capacity (500∼1000 mAh g−1 ) based on redox reaction mechanisms, which undergo morphology and structure destruction in cycling, leading to unsatisfied electrochemical performance [4–6]. Much work on improving the electrochemical performance of TMOS has been done on growing TMOS directly on conductive collector and/or combining TMOS with matrix phase
∗ Corresponding author. Fax: +86 717 6397559. E-mail addresses: [email protected] (S. Ni), [email protected] (X. Yang). http://dx.doi.org/10.1016/j.electacta.2014.03.120 0013-4686/© 2014 Elsevier Ltd. All rights reserved.
such as carbon [7–10], which has gained impressive results. For Li3 VO4 anode, the electrochemical performance can be improved via combining it with graphene, which shows reduced charge transfer resistance and enhanced lithium diffusion coefficient [2]. It is reasonable to believe that growing Li3 VO4 directly on electric collector may be another feasible way to improve the electrochemical performance. Ni foam shows 3D porous architecture with good structure stability and fine electronic conductivity, which can be adopted as an ideal deposition substrate [11,12]. Here in this paper, we report the preparation of Li3 VO4 /Ni composite and its electrochemical performance as anode for Li-ion battery. The main objective of this paper is to explore the way to improve the electrochemical performance of Li3 VO4 , and to study the factors that affect the electrochemical performance of Li3 VO4 . 2. Experimental 2.1. Fabrication procedure The chemicals were analytical grade and purchased from Shanghai Chemical Reagents. Ni foam (100 PPI pore size, 380 g m−2 surface density, 1.5 mm thick) was purchased from Changsha Lyrun New Material corporation. In a typical procedure, 1 mmol V2 O5 , 3 mmol Li2 CO3 and 5 mmol hexamethylenetetramine were dissolved in 30 ml distilled water. After stirring for 20 minutes, the
S. Ni et al. / Electrochimica Acta 130 (2014) 800–804
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homogeneous yellowy 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 120 ◦ C for 24 h. The final transparent solution was transferred in a culture dish, Ni discs were immersed into the solution for 1 h. After that, the Ni discs were dried and annealed in N2 atmosphere at 400 ◦ C for 5 h. The weight of active Li3 VO4 on Ni disc can be estimated according to the weight difference between the Ni disc and the obtained Li3 VO4 /Ni (mLi3 VO4 = mLi3 VO4 /Ni -mNi ). 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 ˚ and field-emission scanning electron K␣ radiation = 1.5406 A) microscopy (FE-SEM JSM 7500F, JEOL). Fig. 1. XRD pattern of the obtained electrode.
2.3. Electrochemical characterization For fabricating of lithium ion battery, the as-prepared Li3 VO4 /Ni foam discs (diameter of 14 mm) were dried at120 ◦ C for 24 h in vacuum oven. Coin-type cells (2025) of Li/1 M LiPF6 in ethylene carbonate, dimethyl carbonate and diethyl carbonate (EC/DMC/DEC, 1:1:1 v/v/v)/Li3 VO4 /Ni were assembled in an argon-filled dry box (MIKROUNA, Super 1220/750, H2 O < 1.0 ppm, O2 < 1.0 ppm). A Celgard 2400 microporous polypropylene was used as the separator membrane. Galvanostatic charge/discharge test was characterized on a multichannel battery test system (LAND CT2001A) in the voltage region between 0.02 and 3 V. The Cyclic voltammetry measurement of the electrodes (vs. Li+ /Li) was carried out on a CHI660 C electrochemical workstation at a scan rate of 0.2 mV s−1 between 0 and 3 V. Electrochemical impedance spectroscopy measurements were performed on CHI660 C 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. Equivalent circuit fitting of EIS data was carried out via ZsimpWin software. 3. Results and Discussion Typical XRD pattern of the electrode obtained by annealing the immersed Ni foam in N2 atmosphere at 400 ◦ C for 5 h is shown in Fig. 1. As seen, the diffraction peaks (marked by *) located at 16.3◦ , 21.6◦ , 22.9◦ , 24.4◦ , 28.2◦ , 32.7◦ , 36.4◦ , 37.6◦ , 58.6◦ , and 66.2◦ can be attributed to the (100), (110), (011), (101), (111), (200), (002), (201), (320) and (203) faces of orthorhombic Li3 VO4 with lattice constants ˚ b = 5.448 A˚ and c = 4.940 A, ˚ which is in good agreement a = 6.319 A, with JCPDS, No. 38-1247. In addition, three typical diffraction peaks (marked by 䊉) located at 44.4◦ , 51.7◦ and 76.4o can be attributed to
Ni (111), (200) and (220) faces, respectively (JCPDS, No. 04-0850). The diffraction peaks correspond to V4 O9 are not obvious, which may be relevant to its low intensity compared with that of Li3 VO4 . Fig. 2(a) is a low magnification SEM image of the as-prepared Li3 VO4 /Ni, from which uniform film-like morphology can be observed. A low magnification SEM image of Ni foam is shown in the insert of Fig. 2(a). As seen, the surface of the Ni foam is smooth, exhibiting clear grain boundaries. The obvious morphology variation suggests that Li3 VO4 film is successfully grown on Ni foam. High magnification SEM image is provided for further clarifying the microstructure of the Li3 VO4 /Ni. As shown in Fig. 2(b), the Li3 VO4 film exhibits a large number of cracks on the surface, which may originate from the volume variation from precursor to Li3 VO4 . The width of these cracks ranges from tens of nanometers to 150 nm, and the length of these cracks ranges from several tens of nanometers to several micro meters. Galvanostatic charge/discharge cycling was carried out in the potential window of 0.02∼3.0 V versus Li. Fig. 3(a) shows the capacity retention and the 1st, 2nd and 100th charge/discharge voltage profiles of the Li3 VO4 electrode at a rate of 0.3 C (1 C means accomplishing discharge or charge in an hour). As seen, the initial discharge curve differs slightly from the 2nd one, showing a sloping potential region from 1.5 to 0.02 V for the initial lithiation process. The 2nd and the 100th discharge curves show similar profile with two sloping potential regions (1.5∼0.4, and 0.4∼0.02 V), accompanied by the attenuation of discharge capacity. The 1st, 2nd and 100th charge curves exhibit similar profile with a sloping potential region (0.8∼2.5 V), which accompanies by the attenuation of charge capacity. The initial discharge capacity is 841 mAh g−1 , which is bigger than the initial charge capacity (531 mAh g−1 ) owing to
Fig. 2. SEM images of the as-prepared Li3 VO4 /Ni with low (a) and high (b) magnification. The insert of (a) is a low magnification SEM image of Ni foam.
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Fig. 3. Electrochemical performance and rate capability of Li3 VO4 /Ni electrode. (a) Capacity retention of the galvanostatic test runs at a rate of 0.3 C; The inset shows the galvanostatic charge/discharge voltage profiles for the 1st, 2nd and 100th cycle. (b) Cyclic voltammograms at a scan rate of 0.2 mV s−1 . (c) Representative charge and discharge voltage profiles at various C rates. (d) Capacity retention at various C rates.
the formation of solid electrolyte interface (SEI) [1,3]. As seen, the specific capacity of the Li3 VO4 /Ni electrode is bigger than that of Li3 VO4 powder [3], which may be relevant to the special 3D porous architecture of Ni substrate. Similar results have been reported for NiO/Ni foam composite electrodes [13,14]. The discharge and charge capacity decrease slowly along with the increasing of cycle number in the first few cycles and then gradually reach stable values. After 100 cycles, the discharge and charge capacity maintain of 379 and 378 mAh g−1 , respectively. The cyclic voltammetric (CV) curves of the Li3 VO4 electrode were tested over a voltage region from 0 to 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 first and subsequent two cycles is found. In the 1st cathodic scan, two reduction peaks at around 0.34 and 0.72 V are attributed to the lithiation process that can be described as: xLi+ + Li3 VO4 + xe− → Li3+x VO4 [1–3], which is in accordance with the initial discharge curve. The reduction peaks shift to 0.54 and 0.86 V in the 2nd cathodic scan and 0.60 and 0.92 V in the 3rd cycle, which can be ascribed to the activation of Li3 VO4 . The activation process can be understood as the transformation of Li3 VO4 into more active nanosized particles [1], which is similar to that of TMOS [7]. The profiles for the initial three anodic scan are similar, showing an oxidation peak near 1.3 V, which is attributed to the delithiation process that can be described as: Li3+x VO4 → xLi+ + Li3 VO4 + xe− [3]. Fig. 3(c) shows the discharge and charge curves of the Li3 VO4 electrode at various C rates from 0.4 to 15 C. Along with the increasing of charge/discharge rate, the discharge potential decreases and the charge potential increases due to the enhanced polarization, which is similar to that reported in literature [13–15]. As shown in the rate capability in Fig. 3(d), the 10th discharge capacity is 415, 358, 302, 256 and 204 mAh g−1 at rates of 0.4, 1, 3, 6 and 15 C, respectively. After that, the discharge capacity can restore to 404 mAh g−1 when reverting the discharge/charge
rate to 0.4 C. In the subsequent 50 cycles, the discharge capacity shows no capacity attenuation, suggesting good rate capability and cycle stability of the Li3 VO4 /Ni electrode. Fig. 4 shows the XRD pattern of the Li3 VO4 /Ni electrode after 100 cycles with charge state. As seen, diffraction peaks located at 21.6◦ , 22.9◦ , 32.7◦ 36.4◦ and 58.6◦ are found, which correspond to (110), (011), (200), (002) and (320) faces of Li3 VO4 (JCPDS, No. 38-1247). In addition, the diffraction peaks located at 30.5◦ and 31.7◦ can be indexed as (311) and (112) faces of orthorhombic V4 O9 (JCPDS, No. 24-1391). This observation is similar to our previous study, suggesting a reversible charge/discharge process. The cycled Li3 VO4 /Ni electrode shows reduced diffraction peaks compared with the asprepared Li3 VO4 /Ni, which may be relevant to the electrochemical activation effect [3]. Fig. 5(a) is a low magnification SEM image of the Li3 VO4 /Ni electrode after 100 cycles with charge state, which shows uniform
Fig. 4. XRD pattern of the Li3 VO4 /Ni electrode after 100 cycles with charge state.
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Fig. 5. SEM images of the Li3 VO4 /Ni electrode after 100 cycles with low (a) and high (b) magnification.
Fig. 6. (a) CV curves of the Li3 VO4 /Ni electrode at different scan rate between 0 and 3 V. (b) Dependence of peak current on the square root of scan rate for the Li3 VO4 /Ni electrode.
film-like morphology, suggesting good structure stability of the Li3 VO4 /Ni electrode. However, the morphology of the cycled electrode is much different from that before. High magnification SEM image of the Li3 VO4 /Ni electrode is shown in Fig. 5(b). As seen, the film-like Li3 VO4 are composed of a large number of nanosized particles. In addition, cracks on the surface of the Li3 VO4 film disappear after cycling. This morphology variation can be understood as a novel electrochemical reconstruction that consists of the formation of a large number of nanosized particles and the assembly of these nanoparticles into secondary architecture, which is similar to NiO/Ni and Cux O/Cu that reported in literature [7,16]. CV curves of the Li3 VO4 /Ni electrode at scan rates from 0.2 to 3.0 mV s−1 are shown in Fig. 6(a). As seen, the reduction peak shifts to low potential region along with the increasing of scan rate, whereas the oxidation peaks shift to high potential region,
suggesting the high polarization under a high scan rate [3,17,18]. Fig. 6(b) shows the relationship between peak current and the square root of scan rate obtained from the experimental data in Fig. 6(a). As found, both the anodic and cathodic peak currents show linear dependence on the square root of the scan rate from 0.2 to 3 mV s−1 , suggesting a lithium ion diffusion controlled mechanism in the charge and discharge process [17,18]. However, the fitted curve shows no zero interpret, which may be relevant to the phase transformation of the electrode, being consistent with the SEM observation in Fig. 5. Previous study for LiFePO4 has shown that phase transformation leads to a deviation of peak current from a linear relationship versus square root of scan rate [18]. The slope of the anodic and cathodic curve shows close absolute value, which suggest a highly reversible lithiation/delithiation process. Fig. 7 shows the electrochemical impedance spectra of the Li3 VO4 /Ni under different state. The semicircle 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 Li-ion diffusion process within electrodes [18]. The Nyquist plots were fitted via an equivalent circuit, and the contact and charge-transfer resistances of the electrode (Re and Rct) were obtained from the fitting results (Table 1).
Table 1 Electrode kinetic parameters obtained from equivalent circuit fitting of Nyquist plots for Li3 VO4 /Ni electrode at different state.
Fig. 7. AC impedance spectra of the Li3 VO4 /Ni electrode under different state.
Li3 VO4 /Ni electrode
Re ()
Rct ()
fresh electrode after 5 cycles testing after 100 cycles testing
6.04 8.37 12.47
25.02 34.96 34.41
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As seen, Re before charge/discharge test is little smaller than that after test, which may be due to the formation of SEI in the discharge process. The little difference of Re after 5 and 100 cycles suggests the formation of stable SEI in cycling. In addition, Rct after 5 and 100 cycles test shows nearly the same value, suggesting a highly stable charge transfer process in the charge/discharge testing. Furthermore, the inclined lines in low-frequency for the Li3 VO4 /Ni electrode after 5 and 100 cycles show close slope, suggesting a highly stable Li-ion diffusion process. 4. Conclusions In summary, Li3 VO4 film that directly grows on 3D porous Ni foam was prepared via a unique two-step method, which shows excellent electrochemical performance as anode for Li-ion batteries. The good electrochemical performance of the Li3 VO4 /Ni is relevant to its stable charge transfer and Li-ion diffusion coefficient as well as the highly reversible lithiation/delithiation process in cycling. The attractive electrochemical performance of Li3 VO4 as well as the feasible way to improve its electrochemical performance endows it with potential application in Li-ion battery. Acknowledgement We gratefully acknowledge the financial support from Natural Science Foundation of China (NSFC, 51272128, 51302152, and 51302153). Moreover, the authors are grateful to Dr. Jianlin Li at Three Gorges University for his kind support to our research. References [1] H.Q. Li, X.Z. Liu, T.Y. Zhai, D. Li, H.S. Zhou, Li3 VO4 : A Promising Insertion Anode Material for Lithium-Ion Batteries, Advanced Energy Materials 3 (2012) 428–432. [2] Z. Shi, J.Z. Wang, S.L. Chou, D. Wexler, H.J. Li, K. Ozawa, H.K. Liu, Y.P. Wu, Hollow Structured Li3 VO4 Wrapped with Graphene Nanosheets in Situ Prepared by a One-Pot Template-Free Method as an Anode for Lithium-Ion Batteries, Nano Letters 13 (2013) 4715–4720. [3] S.B. Ni, X.H. Lv, J.J. Ma, X.L. Yang, L.L. Zhang, Electrochemical characteristics of lithium vanadate, Li3 VO4 as a new sort of anode material for Li-ion batteries, Journal of Power Sources 248 (2014) 122–129.
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