Effect of Co0.58Ni0.42 oxide nanoneedles coating on the electrochemical properties of LiV3O8 cathode

Composites Science and Technology 72 (2012) 344–349

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Effect of Co0.58Ni0.42 oxide nanoneedles coating on the electrochemical properties of LiV3O8 cathode Fanghua Tian a, Li Liu a,b,⇑, Xingyan Wang a, Zhenhua Yang a,b, Meng Zhou a, Xianyou Wang a,⇑ a b

School of Chemistry, Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, Xiangtan University, Xiangtan 411105, China Faculty of Materials, Optoelectronics and Physics, Xiangtan University, Xiangtan 411105, China

a r t i c l e

i n f o

Article history: Received 12 July 2011 Received in revised form 18 October 2011 Accepted 20 November 2011 Available online 27 November 2011 Keywords: A. Coating A. Nano composites B. Surface treatments D. Scanning electron microscopy (SEM) D. X-ray diffraction (XRD)

a b s t r a c t LiV3O8 has been coated by various amounts of Co0.58Ni0.42 oxide nanoneedles using a chemical co-precipitation method. The influences of the coating on the structure, morphology, and electrochemical properties of LiV3O8 have been characterized by X-ray diffraction spectroscopy (XRD), scanning electron microscopy (SEM), and electrochemical measurements. Co0.58Ni0.42 oxide coated LiV3O8 materials exhibit distinct surface morphology. The coating has been found to improve the electrochemical performance of LiV3O8 significantly. Especially, 5.0 wt.% Co0.58Ni0.42 oxide nanoneedles coated LiV3O8 shows much better electrochemical performance than uncoated LiV3O8. It has been proved that Co0.58Ni0.42 oxide coating suppresses phase transitions and decreases the charge-transfer impedance of LiV3O8 effectively. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction Recently, vanadium oxide-based materials as cathode materials for lithium ion batteries have attracted significant attention due to their low-cost, high discharge capacity, and high specific energy [1–3]. Among them, the layered lithium trivanadate (LiV3O8) can reversibly insert over three Li per formula unit at the tetrahedral sites and deliver special discharge capacity above 300 mAh g 1, making it one of the promising substitutes for expensive LiCoO2 cathode used commercially in lithium ion batteries [4–6]. In spite of its good lithium insertion behavior, the two-phase transition occurring upon lithiation in the range Li3–Li4V3O8 corresponds to a slight rearrangement of the oxygen packing and induces some capacity loss upon cycling due to a sudden change in structural parameters [7]. To overcome this defect, many research efforts have been focused on improving the electrochemical properties of LiV3O8 by either developing new synthetic method [2,8] or structural/morphological modification [9–11]. Surface modification by coating is an effective method to improve the cycling performance of LiV3O8 [11–14]. For example, Idris et al. [12] have reported that LiV3O8 /carbon composite pre⇑ Corresponding authors. Address: School of Chemistry, Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, Xiangtan University, Xiangtan 411105, China. Tel.: +86 731 58292206; fax: +86 731 58292477. E-mail addresses: [email protected] (L. Liu), [email protected] (X. Wang). 0266-3538/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2011.11.024

sents enhanced cycling performance. Jiao et al. have demonstrated that 1 wt.% AlPO4 nanowires coating improved the cycling performance of LiV3O8 effectively [11]. Cao et al. [13] have showed that the cycling performance of LiV3O8 is improved by ZnO coating. We have investigated the effect of polypyrrole coating on the electrochemical performance of LiV3O8 previously [14], and the results show that polypyrrole coated LiV3O8 shows much better cycling performance than pristine LiV3O8 [14,15]. However, the studies about surface modification on the surface of LiV3O8 are still few compared with studies of doping and synthetic method. It is well known that selecting good coating materials and an optimal coating amount is important to coating process. Transition metal-oxide coating of the cathode material to suppress the capacity-fading has been reported extensively in the literature: NiO-coated Li(Ni1/2Mn1/2)O2 [16], CoO-coated LiMnO2 [17], Co3O4-coated and ZnO-coated LiMn2O4 [18–19], TiO2-coated Li(Ni0.8Co0.2)O2 [20]. In recent years, transition metal oxides, such as CoO, NiO, and Co3O4, which exhibit large reversible capacity, have been extensively investigated as potential alternatives to carbon anode materials for lithium-ion batteries. However, such as most of the transition metal oxides, they usually suffer from the problems of poor electronic conduction and poor capacity retention performance. One major effort has been used to decrease the dimension of transition metal oxides, thus, allow for short solid state diffusion path for Li ions and large contact area between materials and electrolyte [21]. So nickel oxides and cobalt oxides with nanoscale morphology have attracted many attentions. Recently, binary Ni–Co oxides have been developed as alternative

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∗ NiV 3 O 8 ♦ CoV3 O 8

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2 wt.% coated

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Co0.58 Ni0.42oxide

003 011

The structures of synthesized samples were characterized by X-ray diffraction spectroscopy (XRD). X-ray powder diffraction data were obtained using a Rigaku D/MAX-2500 powder diffractometer with a graphite monochromatic and Cu Ka radiation (k = 0.15418 nm) in the 2h range of 10–80°. Scanning election microscope (SEM) images of the samples were collected on the FEG SEM Sirion scanning election microscope, which was used to observe the particle morphology, particle size and particle size distribution. The cathodes for lithium cells were fabricated by mixing the cathode material, carbon black, and polytetrafluoroethylene (PTFE) binder in a weight ratio of 85:10:5. The testing cells were assembled with the cathodes thus fabricated, metallic lithium anode, celgard 2300 film separator and 1 M LiPF6 in 1:1 ethylene carbonate (EC)/dimethyl carbonate (DMC) electrolyte. The assembly of the testing cells was carried out in an argon-filled glove box, where water and oxygen concentration were kept less than 5 ppm. The discharge–charge cycle tests were run at different current densities of 0.5 C, 1 C (300 mA g 1 was assumed to be 1 C rate) between 4.0 and 1.8 V. All the tests were performed at room temperature. Cyclic voltammetry (CV) tests and EIS experiments were performed on a Zahner Zennium electrochemical workstation. CV tests were carried out at a scan rate of 0.1 mV s 1 or 1 mV s 1 on the

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2.2. Structure characterization and electrochemical measurements

XRD patterns of synthesized compounds are shown in Fig. 1. The pristine LiV3O8 shows good crystal structure. All of the diffractive peaks are well attributed (JCPDS: 72-1193). This compound has monoclinic structures with the space group P21/m. XRD pattern of Co0.58Ni0.42 oxide is also shown in Fig. 1 to facilitate the study. It is found that Co0.58Ni0.42 oxide is a composite with less crystallization and is composed of NiO (JCPDS: 22-1189) and Co3O4 (JCPDS: 42-1467), which is consistent with previous report [23]. Co0.58Ni0.42 oxide coated materials also can be attributed to LiV3O8 structure, and the Co0.58Ni0.42 oxide phase is not observed in these patterns. This is maybe due to low crystallization and small amount of Co0.58Ni0.42 oxide. It is noticeable that new phases of NiV3O8 (JCPDS: 22-0454) and CoV3O8 (JCPDS: 22–0599 can be found in XRD patterns of coated materials. The presence of NiV3O8 and CoV3O8 means Co0.58Ni0.42 oxide may have reacted with LiV3O8. Moreover, it is shown that weak X-ray diffraction peaks are observed for Co0.58Ni0.42 oxide coated materials. The Co0.58Ni0.42 oxide coated materials exhibit microcrystal structures. It is known that LiV3O8 as a cathode material will benefit from lower crystallization [24]. Better performance of the coated samples can be expected. Fig. 2 shows the morphology of Co0.58Ni0.42 oxide, LiV3O8, and 5.0 wt.% Co0.58Ni0.42 oxide coated LiV3O8. As seen in Fig. 2a, Co0.58Ni0.42 oxide powders are composed of many nanoneedles. The atomic percentages (atom %) of the elements Co and Ni in nickel–cobalt oxides were obtained by means of EDX spectroscopy (See Fig. 2b). The result shows that the Co0.58Ni0.42 oxide material is obtained when the molar ratio of the CoCl2
6H2O:NiCl2
6H2O in mixed solution was 1:3. As shown in Fig. 2c, the range of pristine LiV3O8 particles size is 3–6 lm. More fine powders could be obtained to some extent after Co0.58Ni0.42 oxide coating as seen in Fig. 2e. The size of Co0.58Ni0.42 oxide coated LiV3O8 particles is smaller and evenlier than pristine LiV3O8. Especially, from the comparison of Fig. 2d and Fig. 2f, the morphology of Co0.58Ni0.42 oxide coated LiV3O8 is much different from pristine LiV3O8. Co0.58Ni0.42 oxide coated LiV3O8 shows more rough surface, and

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Pristine LiV3O8 powders were synthesized by a classic peroxide sol–gel method [7]. The Co0.58Ni0.42 oxide nanoneedles were prepared by a facile chemical coprecipitation method followed by a simple calcinations process [23]. To prepare Co0.58Ni0.42 oxide coated LiV3O8 powders, 5% ammonia was slowly added to 0.1 M cobalt chloride and nickel chloride solutions (3:1 by molar ratio) until the pH of the resultant solutions reached 9. Keep the pH value of the mixture solution in 9 for 1 h, and then LiV3O8 was slowly poured into the solution. The suspension containing the active material was constantly stirred for 3 h, filtered, washed by distilled water for several times, and dried in a vacuum oven at 80 °C. CoxNi1 x(OH)2 coated LiV3O8 powders were obtained. Then powders were calcined for 6 h at 250 °C in air to form Co0.58Ni0.42 oxide coated LiV3O8. Through change the volumes of 0.1 M cobalt chloride and nickel chloride solutions can control the amount of Co0.58Ni0.42 oxide coating. The coated samples with ratio Co0.58Ni0.42 oxide/LiV3O8 = 0.02, 0.05, and 0.08 in weight were synthesized.

3.1. Characterization of Co0.59Ni0.41 oxide coated LiV3O8 materials

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2.1. Preparation of cathode materials

3. Results and discussion

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2. Experimental

potential interval 1.8–4.0 V (vs. Li+/Li). The ac perturbation signal was ±5 mV, and the frequency range was from 100 mHz to 105 Hz.

002

electrode materials for supercapacitors, and their electrochemical properties in terms of the oxygen evolution reaction (OER) have been widely studied in an alkaline medium [22].Synergistic electrocatalytic behavior has been found. It also has been reported that nickel cobalt binary metal oxide (Co0.56Ni0.44 oxide nanoflake materials) have great electronic conductivity and electrochemical activity. Co0.56Ni0.44 oxide is expected to offer richer redox chemistry than the two single component oxides due to the combined contributions from both nickel and cobalt ions [23]. However, to the best of our knowledge, the binary Ni–Co oxide coated LiV3O8 has not been prepared and also has not been studied as a cathode material in lithium ion batteries. Co0.58Ni0.42 oxide nanoneedles coating has been utilized on the surfaces of LiV3O8 in this work. The effect of the Co0.58Ni0.42 oxide coating on the electrochemical properties of LiV3O8 was investigated in detail.

bare LiV3O8

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2θ Fig. 1. XRD patterns of LiV3O8 and Co0.58Ni0.42 oxide coated LiV3O8.

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(a)

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O

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0 0

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Fig. 2. SEM images of Co0.58Ni0.42 oxide (a), LiV3O8 (c) (d) and 5 wt.% Co0.58Ni0.42 oxide coated LiV3O8 (e) (f); EDX image of Co0.58Ni0.42 oxide (b).

some nanoneedles can be obviously found on its surface. It has been proved by several researchers that the morphology can make an important effect on electrochemical performance of samples. It has been reported [25] that smaller grains show better electrochemical performance. The intercalation process of Li+ between the layers of cathode is a diffusion process. Therefore, the smaller grains would lead to a short path for Li+, which are advantageous to Li+ intercalation. In addition, rough surface is of benefit to electrolyte penetrating into electrode material particles. As mentioned above, it can be expected that Co0.58Ni0.42 oxide coated LiV3O8 would show better electrochemical performance than pristine LiV3O8. The fabrication process of Co0.58Ni0.42 oxide nanoneedles coated LiV3O8 can be concluded through SEM results combined with XRD results. Fig. 3 schematically shows the strategy for the fabrication of the Co0.58Ni0.42 oxide nanoneedles coated LiV3O8. Firstly, 5% ammonia was slowly added to 0.1 M cobalt chloride and nickel chloride solutions got the CoxNi1 x(OH)2[23,26], and then LiV3O8 was slowly poured into the solution. The suspension containing the active material was constantly stirred for 3 h, LiV3O8 particles are coated by CoxNi1 x(OH)2, but this is a physical process, and the contact between LiV3O8 bulk and CoxNi1 x(OH)2 is incompact. After sintering at 250 °C for 6 h, Co0.58Ni0.42 oxide nanoneedles

coated LiV3O8 is obtained. CoxNi1 x(OH)2 particles are decomposed to Co0.58Ni0.42 oxide nanoneedles [23,26], at the same time, and a shell composed of NiV3O8 and CoV3O8 is formed on the surface of LiV3O8 bulk. The shell on the surface of LiV3O8 bulk will protect cathode active material from direct contact with electrolyte and undesirable increases in electrochemical resistance, and will suppresses phase transition during cycling. The Co0.58Ni0.42 oxide nanoneedles on the surface of LiV3O8 particle will enhance its electronic conductivity and induce better electrochemical performance. These properties will be proved later by electrochemical measurements. 3.2. Electrochemical characterizations of Co0.58Ni0.42 oxide coated LiV3O8 materials 3.2.1. Charge/discharge studies The second charge/discharge curves of LiV3O8 and Co0.58Ni0.42 oxide coated LiV3O8 are illustrated in Fig. 4a. The shapes of these curves are similar. It can be seen that a decrease of discharge specific capacity with an increase of coated amount. The second discharge specific capacity decreases from 308.9 mAh g 1 for pristine LiV3O8 to 237.2 mAh g 1 for 8.0 wt.% Co0.58Ni0.42 oxide coated LiV3O8. The features of charge/discharge curves shown in Fig. 4a

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LiV3O8

LiV3O8 Co0.58 Ni 0.42 oxide

CoxNi1-x (OH)2

NiV3O8/ CoV3O8 Heat treatment at 250

Fig. 3. Schematic illustrations of the fabrication process of Co0.58Ni0.42 oxide nanoneedles coated LiV3O8.

(a)

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Voltage/ V Fig. 4. Second charge / discharge curves of LiV3O8 and Co0.58Ni0.42 oxide coated LiV3O8 at a current density of 0.5 C (a); Corresponding incremental capacity curves vs. voltage.

are better emphasized when they are converted to derivative plots. The plateau in the Q (Special capacity) vs. V plot is transformed into a peak in the dQ/dV vs. V plot, as shown in Fig. 4b. There are two peaks at about 2.8 V and 2.5 V in dQ/dV vs. V plots of all samples during the discharge processes, which are corresponding to two main discharge plateaus. It has been reported that the discharge plateau of 2.8 V is ascribed to single-phase lithium insertion process for LiV3O8, and the 2.5 V plateau reveals the two-phase transformation between Li1+xV3O8 (1 6 x 6 2) and Li4V3O8 [8,27]. However, during charge processes, the dQ/dV vs. V plots of Co0.58Ni0.42 oxide coated LiV3O8 are much different from that of pristine LiV3O8. Four peaks at 2.44, 2.69, 2.80, and 2.89 V can be observed in dQ/dV vs. V plot of LiV3O8 during charge process, while only two peaks at 2.70 and 2.87 V can be found in those plots of

Co0.58Ni0.42 oxide coated LiV3O8 samples. This means that coated materials have lesser plateaus than the uncoated LiV3O8 during the charge process. LiV3O8 shows multiple plateaus in charge–discharge profiles, which is due to lithium extraction/insertion processes and in agreement with other reports [2,5]. This is a characteristic performance for LiV3O8 and make against its application in practice. Co0.58Ni0.42 oxide coated LiV3O8 samples show less plateaus in charge–discharge profiles, which is of benefit to application in practice. This is maybe due to the formation of NiV3O8 and CoV3O8 on the surface of LiV3O8 suppress the phase transformation during charge/discharge processes. Both of NiV3O8 and CoV3O8 have good electrochemical properties, and CoV3O8 exhibited small irreversible capacity and flat discharge plateau has been proved [28,29]. Besides, as seen in Fig. 4b, the main discharge plateau of uncoated LiV3O8 is located at 2.51 V and corresponding charge plateau is located at 2.70 V, but main discharge plateaus of coated materials are located at about 2.55 V and corresponding main charge plateaus are located at 2.70 V. The potential difference between charge plateau and corresponding discharge plateau of LiV3O8 decreases obviously after coating. This implies the decrease of electrochemical polarization. Better electrochemical reversibility of the coated samples can be expected. The cycling performance of the LiV3O8 and Co0.58Ni0.42 oxide coated LiV3O8 materials cycled between 1.8 and 4.0 V at 0.5 C are shown in Fig. 5a. Pristine LiV3O8 shows the highest initial discharge capacity of 319.7 mAh g 1, but the discharge capacity decreased acutely with subsequent cycles, the discharge capacity only remains at 129.1 mAh g 1 after 40 cycles. The cycling performance of Co0.58Ni0.42 oxide coated materials improved remarkably. At 2.0 and 8.0 wt.% Co0.58Ni0.42 oxide coatings, the discharge capacities remain 203.0 and 203.3 mAh g 1 after 40 cycles, respectively. At 5.0 wt.% Co0.58Ni0.42 oxide coating, the sample shows the excellent cycling performance, and the first discharge capacity was 243.1 mAh g 1, it remains at 216.1 mAh g 1 after 40 cycles. Although Co0.58Ni0.42 oxide coating affects the initial discharge capacity, the cycle stability distinctly improved, which is the most important property for secondary batteries. The cycling performance of LiV3O8 and Co0.58Ni0.42 oxide coated LiV3O8 materials cycled between 1.8 and 4.0 V at larger current density (1 C) are shown in Fig. 5b. The initial discharge capacity of uncoated LiV3O8 is only 248.5 mAh g 1, which is much lower than that at 0.5 C. And the discharge capacity sharply decreases to 100.4 mAh g 1 at the 60th cycle. It is shown that uncoated LiV3O8 has poor rate capability and cyclability, which is consistent with theresults of Jiao et al. [11] and Xu et al. [6]. Co0.58Ni0.42 oxide coated LiV3O8 materials show much better cycling performance and rate capability than pristine LiV3O8. Among these samples, 5.0 wt.% Co0.58Ni0.42 oxide coated LiV3O8 shows the best electrochemical performance. The initial discharge capacity of 5.0 wt.% Co0.58Ni0.42 oxide coated LiV3O8 is 243.3 mAh g 1 at 1 C, which is nearly the same as that at 0.5 C. After 60 cycles, the

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1.0

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Fig. 6. Cyclic voltammograms of LiV3O8 and 5.0 wt.% Co0.58Ni0.42 oxide coated LiV3O8.

Fig. 5. Cycling stability curves of LiV3O8 and Co0.58Ni0.42 oxide coated LiV3O8 at 0.5 C (a) and 1 C (b) in the range of 1.8–4.0 V.

200 180 160

discharge capacity of 5.0 wt.% Co0.58Ni0.42 oxide coated LiV3O8 remains at 203.9 mAh g 1. It has been found that 5.0 wt.% Co0.58Ni0.42 oxide coated LiV3O8 shows excellent rate capability and cyclability.

3.2.3. Electrochemical Impedance spectroscopic studies The Nyquist plots of coin cells with LiV3O8 and 5.0 wt.% Co0.58Ni0.42 oxide coated LiV3O8 cathodes after the first discharge are illustrated in Fig. 7. The impedance spectra of LiV3O8 and Co0.58Ni0.42 oxide coated LiV3O8 show a high frequency semicircle and a low frequency tail. The former indicates the double-layer

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-Z''/Ω

3.2.2. Cyclic voltammetry Cyclic voltammograms of LiV3O8 and 5.0 wt.% Co0.58Ni0.42 oxide coated LiV3O8 at different cycles between 4.0 and 1.8 V, are presented in Fig. 6 (Scan rate is 1 mV s 1). The current densities of LiV3O8 decrease with cycle increase while the current densities of Co0.58Ni0.42 oxide coated LiV3O8 are almost invariable. The larger current densities in the cyclic voltammograms agree with the larger capacities for the respective materials. In addition, for uncoated LiV3O8, the anodic peak potentials shift toward positive obviously with the increase of cycle number. However, the anodic peak potentials Co0.58Ni0.42 oxide coated LiV3O8 is nearly identical with the increase of cycle number, which indicates Co0.58Ni0.42 oxide coated LiV3O8 has good reversibility upon cycling. This phenomenon is in agreement with the better charge–discharge performance of Co0.58Ni0.42 oxide coated LiV3O8 as shown in Fig. 5.

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100 80 60 40 LiV3 O 8 5wt.% Co0.58 Ni 0.42O coated LiV3 O8 Fitting line

20 0 -20 0

50

100

150

200

250

300

Z'/Ω Fig. 7. Nyquist plots of LiV3O8 and 5.0 wt.% Co0.58Ni0.42 oxide coated LiV3O8, and the equivalent circuit model (inset).

response at the electrode/sample interface and the latter shows diffusion of lithium ions in the solid matrix. The impedance plots are fitted using the equivalent circuit model (the inset in Fig. 7), and the fitted impedance parameters are listed in Table 1. The fitting data is in good agreement with experimental data as shown in Fig. 7.The equivalent circuit model includes electrolyte resistance Rs, a constant phase element (CPE) associated with the interfacial resistance, charge transfer resistance Rct, and the Warburg

F. Tian et al. / Composites Science and Technology 72 (2012) 344–349 Table 1 Impedance parameters for LiV3O8 and 5.0 wt.% Co0.58Ni0.42 oxide coated LiV3O8. Samples

Rs (X)

Rct (X)

LiV3O8 5 wt.% coated

3.1 1.8

150.7 102.4

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No. 10C1250), the Postdoctoral Foundation of Hunan Province (Grant No. 2010RS4027), the Natural Science Foundation of Hunan Province (Grant No.11JJ4038), and the Scientific Research Fund of Xiangtan University (Grant No. 09XZX10). References

impedance (Zw), which is related to the diffusion of lithium ions in the solid oxide matrix. It is clear that The Rs and Rct of Co0.58Ni0.42 oxide coated LiV3O8 are 1.8 and 102.4 X respectively, which are much smaller than pristine LiV3O8. According to Chen et al.’s [30] studies on EIS of lithium-ion cells, the cell impedance is mainly attributed to cathode impedance, especially charge-transfer resistance. Therefore, it can be assumed that the electrochemical impedance of LiV3O8 is suppressed by the presence of Co0.58Ni0.42 oxide nanoneedles. 4. Conclusions

[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]

Co0.58Ni0.42 oxide nanoneedles coated LiV3O8 composites have been successfully synthesized. The 5 wt.% Co0.58Ni0.42 oxide coated LiV3O8 electrode shows much better cycling behavior and rate capability compared to those of pristine material. The slow degradation of the electrochemical performance of Co0.58Ni0.42 oxide coated LiV3O8 during cycling can be explained by the better cycle reversibility and lower electrochemical resistance than pristine LiV3O8. These improved properties are attributed to the change of structure and morphology after coating. Acknowledgements This work was supported financially by the National Natural Science Foundation of China (Grant No. 20871101), the Scientific and Technological Foundation of Ministry of Science and Technology of China (Grant No. 2009GJD20021), the China Postdoctoral Science Foundation (Grant No. 20100480954), the Scientific Research Fund of Hunan Provincial Education Department (Grant

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