C anode material for Li-ion batteries

C anode material for Li-ion batteries

Journal of Electroanalytical Chemistry 745 (2015) 1–7 Contents lists available at ScienceDirect Journal of Electroanalytical Chemistry journal homep...

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Journal of Electroanalytical Chemistry 745 (2015) 1–7

Contents lists available at ScienceDirect

Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem

The low and high temperature electrochemical performance of Li3VO4/C anode material for Li-ion batteries Zhiyong Liang a, Yanming Zhao b,c,⇑, Youzhong Dong c, Quan Kuang c, Xinghao Lin a, Xudong Liu a, Danlin Yan a a b c

School of Material Science and Engineering, South China University of Technology, PR China State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, PR China School of Physics, South China University of Technology, Guangzhou 510640, PR China

a r t i c l e

i n f o

Article history: Received 11 January 2015 Received in revised form 9 March 2015 Accepted 10 March 2015 Available online 10 March 2015 Keywords: Anode Carbon-coated Li3VO4 Low and high temperature Electrochemical performance

a b s t r a c t The carbon-coated Li3VO4 (Li3VO4/C) sample was synthesized by simple solid-state reaction method using glucose as carbon source. Rietveld refinement, XPS and element analysis results show that, though it is synthesized in the presence of carbon and reducing atmosphere, both the single-phase Li3VO4/C and the valence of vanadium of +5 can be retained. The SEM and TEM images reveal that Li3VO4/C composite has uniform particles with size less than 1 lm. Electrochemical testing results show that Li3VO4/C at high operation temperatures holds both higher specific capacity and cyclic performance than that of low temperatures. The initial discharge capacities for the Li3VO4/C electrodes at temperatures of 20, 0, 25 and 50 °C are 312, 600, 760 and 721 mAh g1 with the coulombic efficiency of 40.45%, 72.09%, 74.34% and 73.41%, respectively. Even at a high discharge/charge rate of 15 C, the capacities of the Li3VO4/C electrodes at 20, 0, 25 and 50 °C still can retain about 20, 120, 370 and 450 mAh g1, respectively. The CV results demonstrate that the higher operation temperature can decrease the voltage polarization of the electrode, thus benefit the electrochemical performance of the Li3VO4/C electrode. In addition, the EIS results indicate that larger charge-transfer resistance and smaller lithium diffusion coefficient can be obtained at low operation temperatures, which should be one of the major reasons for its poor low-temperature performance of the Li3VO4/C electrode. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction Rechargeable Li-ion batteries (LIBs) have become very important components in power portable electronic devices and will have more extensive application prospects as power sources in hybrid electric vehicles (HEVs), electric vehicles (EVs) and smart grids [1,2]. During the passed twenty years, graphite has remained the dominant anode in rechargeable LIBs, operating by intercalation of Li ions between the graphite sheets. However, graphite intercalates at a low potential close to that of the Li-plating (Li dendrite), which is a potential cause of short circuits, especially for fast intercalation. Great efforts have been devoted to search for graphite alternatives with both large capacities and slightly more positive intercalation voltages compared to Li/Li+ [3,4]. More recently, it has been reported that Li3VO4 can be using as a potential anode for LIBs with a relatively lower Li intercalation ⇑ Corresponding author at: State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, PR China. E-mail address: [email protected] (Y. Zhao). http://dx.doi.org/10.1016/j.jelechem.2015.03.013 1572-6657/Ó 2015 Elsevier B.V. All rights reserved.

potential (mainly between 0.5 1.0 V vs. Li/Li+) and a comparable capacity to that of graphite [5,6]. In spite of these advantages, Li3VO4 as an anode material still meets some drawbacks when it comes to practical implementation. The most prominent problem is low electronic conductivity due to its intrinsic character [5]. In order to overcome this drawback and improve the electrochemical performance of Li3VO4, many efforts have been performed, such as wrapping with graphite nanosheets, coating conductive Ni and fabricating by facile hydrothermal method [7–9], etc. However, these methods are difficult to realize when it comes to practical implementation. Fortunately, our previous work reveals that the existences of residual carbon under reducing atmosphere would not change the valence of vanadium (+5) as well as the structure of Li3VO4 even at high sintering temperature (750 °C) [10]. This is a significant breakthrough because amorphous carbon can be in-situ coated on the surface of Li3VO4 which results in uniform carbon coating and particle size distribution at a proper sintering temperature. Moreover, our further research gives a totally new understanding on lithium insertion behavior of Li3VO4 as anode for LIBs [11]. The results show that the maximum embeddable

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Li-ion number in a Li3VO4 single cell is 3 corresponding to the change of V5+ to V2+ (theoretical capacity of 590 mAh g1), and the Li-inserted sites are predicted by first-principles calculations. On the other hand, in practical applications, it is proposed that the LIBs can be operated favorably at various temperatures. At low temperatures, the problems have been summarized as the increased charge-transfer resistance on the electrolyte–electrode interface, high polarization, limited diffusivity of lithium ions, reduced ionic conductivity of the electrolyte and solid electrolyte interface (SEI) formed on the electrodes [12]. At high temperatures, however, the LIBs may also suffer from capacity fading due to the non-uniformity of the SEI layer, electrolyte decomposition, current collector corrosion and/or nanocrystalline deposits and phase segregation in the electrode [13]. It is believed that studies of temperature influence on a novel electrode are necessary and meaningful. Up to this time, the electrochemical performance of Li3VO4/C at various temperatures has not been reported. In the present work, we prepare a well crystalline, in-situ carbon-coated Li3VO4 (Li3VO4/C) by simple solid-state reaction method. The structure characteristics, element compositions, and morphology of the Li3VO4/C samples were characterized using Powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM). Also, the electrochemical performance of the Li3VO4/C at various temperatures (20, 0, 25 and 50 °C) in the potential range of 0.1–3.0 V were systematically investigated. 2. Experimental The Li3VO4/C sample was prepared via simple solid-state reaction method. Stoichiometric amounts of pure V2O5 and Li2CO3 powders (Li:V = 3:1 in mol) were mixed together, and 35 wt.% glucose was added to the mixed powders as carbon source. The mixture (including the glucose) was ball milled for 4 h, and then presintered at 350 °C for 4 h in 70% Ar + 30% H2 atmosphere to expel CO2 and H2O. The obtained precursors were reground carefully, and then sintered at 750 °C for 8 h in 70% Ar + 30% H2 gas. The phase identification was carried out by Powder X-ray diffraction (XRD) using X’Pert PRO (PANalytical, the Netherlands) with Cu Ka radiation (k = 1.5418 Å). X-ray photoelectron spectroscopy (XPS) was obtained for the sample using a Kratos Axis Ultra spectrometer. The carbon content of Li3VO4/C was verified by Vario EL CHNS elemental analyzer. The morphologies of the samples were characterized by a scanning electron microscopy (SEM, Hitachi S4800) and high resolution transmission electron microscopy (HRTEM, Hitachi 7650) operating at 300 kV. The electrochemical tests of Li3VO4/C were carried out with two-electrode electrochemical cell by an automatic battery tester system (LandÒ, China) at various temperatures (20, 0, 25 and 50 °C). And the testing temperatures were regulated and controlled via high–low temperature test chamber with about ±2 °C fluctuation. The working electrodes were prepared by mixing the as-synthesis sample with conducting agent and polyvinylidene fluoride (PVDF) binder in a weight ratio of 8:1:1, and then coating the obtained slurry on Cu foil uniformly and drying at 90 °C. The weight of active material of the electrode was 2 mg cm2. Lithium metal foils were used as counter electrodes, CelgardÒ 2320 as separator, and the electrolyte was a solution of 1 M LiPF6 in a 1:1 vol.% mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC). The specific capacity was calculated based on the active material of the electrode. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were performed on an AUTOLAB PGSTAT302N (Metrohm, Netherlands) at various temperatures. The CV curves were recorded at a scanning rate of

0.1 mV s1 in the voltage range of 0.1–3.0 V, and the EIS were recorded by applying an ac voltage of 5 mV in the frequency range of 0.01 Hz–1 MHz. 3. Results and discussion As small amounts of impurities can affect the electrochemical properties of semiconductors, a XRD analysis was adopted for the Li3VO4/C powder and the resulting diffraction intensity data were refined using the GASAS program with a space group of Pnm21 as the refinement model (Fig. 1). The weighted factor Rwp obtained in the refinement results reflects the fitting degree between the experimental value and the theoretical value (the smaller the better). The reasonable small Rwp factor of 7.17% of our experimental value suggests that the single phase of Li3VO4/C can be obtained under our experimental process. The cell parameters with a = 6.3289(0) Å, b = 5.4472(0) Å, c = 4.9498(2) Å and V = 170.644 Å3 obtained from the result of the Rietveld refinement agree well with the values of previously reported [5,10–11]. The structure of Li3VO4 based on the crystal structure parameters obtained from Rietveld refinement is presented in the inset of Fig. 1. The orthorhombic structure comprises a framework of many regular corner-shared VO4 and LiO4 tetrahedrons. This configuration forms a three-dimensional network which leaves many empty sites and possible Li-insertion pathways [10–11]. The atomic sites and coordinates of Li3VO4/C sample determined from the Rietveld refinement of high-power XRD are presented in Table 1. The XPS spectrum of the Li3VO4/C composite is illustrated in Fig. 2. According to the published reports [14–16], the binding energy of 54.95, 284.79, 517.99 and 530.77 eV corresponds to Li 1s, C 1s, V 2p3 and O 1s, respectively. So, it is reasonable to conclude that the product only consisted of Li1+, V5+, O2 and C. And the carbon should be amorphous since no peaks attributed to crystalline carbon can be detected in the XRD patterns (Fig. 1). The content of residual carbon measured by element analysis is 6.37% in the final product of Li3VO4/C composite. In order to elucidate the structural and morphological aspects of Li3VO4/C more detail, its SEM and TEM images are characterized. The SEM images shown in Fig. 3a indicate that the powder has uniform particle size distribution (less than 1 lm) with little agglomeration. Fig. 3b and c shows the TEM and HRTEM images of Li3VO4/C powder where, in order to obtain high resolution

Fig. 1. XRD pattern and Rietveld analysis of single-phase Li3VO4/C, the inset is the crystal structure of Li3VO4 projected along the c-axis.

Z. Liang et al. / Journal of Electroanalytical Chemistry 745 (2015) 1–7 Table 1 The atomic sites and coordinates of Li3VO4/C determined from the Rietveld refinement of high-power XRD. Atoms

Wyckoff sites

x

y

z

Occupancy

Li (1) Li (2) V O (1) O (2) O (3)

2a 4b 2a 4b 2a 2a

0.243539 1/2 0 0.218175 0 1/2

0.338679 0.828508 0.829785 0.681862 0.121494 0.175245

0.994213 0.990757 0.011483 0.911363 0.915250 0.854464

1.00 1.00 1.00 1.00 1.00 1.00

O 1s

Intensity (a.u.)

V 2p3 C 1s Li 1s

0

200

400

600

Binding energy (eV) Fig. 2. XPS spectrum of Li3VO4/C composite.

800

3

(HRTEM) images, a much thinner particle for Li3VO4/C particle was examined. The lattice spacing is measured to be 0.366 nm, which corresponds to the d-spacing of the (011) faces. In the inset of Fig. 3c, the selected area electron diffraction (SAED) image taken along the [1 0 0] crystal zone axis shows regular and bright diffraction spots, which confirms that Li3VO4 particles adopt a single crystal structure and the lattice constant obtained here agrees well with our X-ray diffraction measurement result. In addition, an amorphous carbon layer with thickness about 5 nm can be observed coating on the particles (Fig. 3d). These results are consistent with our previous obtained value. The in-situ coated carbon provides good conductivity as well as fast lithium ion diffusion, and thus results in good electrochemical performance [17]. Fig. 4a and b shows the capacity–voltage curves of Li3VO4/C electrodes within the initial and the second cycles at various temperatures under 0.2 C rate (90 mA g1). As it can be seen from Fig. 4a and b, the electrochemical behaviors of the electrode tested under 25 °C are well consistent with previously reported results [5–11]. At higher temperatures (25 and 50 °C), the electrode exhibits two discharge platforms around 0.75 and 0.6 V, and two charge platforms around 1.1 and 1.3 V, which are identified as the two-phase processes during the electrochemical reactions [5,10–11]. As the operation temperature declines to 0 and 20 °C, the platforms and discharge/charge capacities decrease, along with an increasing electrochemical polarization. The initial discharge capacities for the Li3VO4/C electrodes at temperatures of 20, 0, 25 and 50 °C are 312, 600, 760 and 721 mAh g1 with the coulombic efficiency of 40.45%, 72.09%, 74.34% and 73.41%, respectively (Fig. 4a). Hence, at 20 °C, the cell only retains 22.3% of the charge capacity (reversible capacity) obtained at 25 °C (565 mAh g1). The dramatically reduction of energy and

Fig. 3. (a) SEM images of Li3VO4/C composite, (b) TEM image of Li3VO4/C composite, and (c and d) HRTEM image of Li3VO4/C composite, the inset is the corresponding SAED pattern indexed using the powder electron diffraction technique.

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Fig. 4. (a) The first cycle discharge/charge curves, (b) the second cycle discharge/charge curves, (c) rate capabilities and (d) cyclic performance of Li3VO4/C electrode at various temperatures between 0.1 and 3.0 V.

power capability at low temperature is probably attributed to the decreased ionic conductivity of the electrolyte and the solid electrolyte interface (SEI) film formed on the lithium metal surface, the related polarization of the Li3VO4/C electrodes and the substantially increased charge-transfer resistance on the electrolyte–electrode interfaces [12]. The relatively low initial coulombic efficiency of the Li3VO4/C electrodes may be attributed to the irreversible capacity lost, including side reactions such as formation of solid electrolyte interface (SEI) film and decomposition of electrolyte, which is similar to Fe3O4 anode material [10–11,18]. On the other hand, the second capacity–voltage curves show a fairly consistent electrochemical behavior compared to the initial curves, indicating a similar lithium insertion/de-insertion process to the first cycle (as shown in Fig. 4b). And it is worth to mention that, in the second cycle, the reversible capacities (charge capacities) for the Li3VO4/C electrodes at temperatures of 20, 0, 25 and 50 °C are 175, 430, 551 and 537 mAh g1 with the coulombic efficiencies turn back to 75.3%, 90%, 92.7% and 94.9%, respectively, which are much larger than that of the first cycle results. These results indicate that irreversible capacity lost of this material mainly occurs in the first cycle, and low operation temperature can aggravate the irreversible capacity lost, especially in the first cycle. In order to investigate the influence of operation temperature on rate capability and cycle performance, Li3VO4/C electrodes were cycle at various discharge/charge rates in the voltage of 0.1–3.0 V (Fig. 4c and d). As shown in Fig. 4c, the Li3VO4/C electrode exhibits better rate capability along with the increasing operation temperature. In the five cycles at low rate (0.2 C), the electrodes tested at lower temperatures of 20 and 0 °C display only around 200 and 440 mAh g1 discharge/charge capacities, respectively,

while about 550 mAh g1 discharge/charge capacity can be obtained for the electrodes operated at 25 and 50 °C. Compared to electrode operated at 25 °C, the electrode operated at 50 °C shows no evident improvement at low rate. However, as the discharge/charge rates increase, the higher operation temperature electrode exhibit superior capacity retention and rate capability. At 15 C rate (3000 mA g1), the discharge/charge capacities of the electrodes at 20, 0, 25 and 50 °C still can retain about 20, 120, 370 and 450 mAh g1, respectively. And it should be noted that, when the current density turn back to low rate of 0.2 C, discharge/charge capacities of about 200, 450, 500 and 520 mAh g1 can be recovered for the electrode operated at 20, 0, 25 and 50 °C, respectively. These values are almost the same as the first 5 cycles, showing a good capacity recovered ability of the Li3VO4/C electrode. The electrochemical performances of the Li3VO4/C electrode at room temperatures (25 °C) are also superior to those of graphite and the previous reported [5–9]. Cyclic performances of Li3VO4/C electrodes at various temperatures are also presented in Fig. 4d. As shown in Fig. 4d, all these electrodes operated at different temperatures can exhibit good cyclic performances and the discharge/charge capacities increase along with the increasing of operation temperature. After 80 cycles discharge/charge at 1 C rate (350 mA g1), discharge/charge capacities of about 49, 283, 398 and 416 mAh g1 can be retain for the electrode operated at 20, 0, 25 and 50 °C, respectively, which is about 118.07%, 104.43%, 90.44% and 93.48% of the value obtained in the second cycle. The larger coulombic efficiency at low operation temperatures (20 and 0 °C) indicates that there is an activation process during electrochemical reaction at low temperature.

Z. Liang et al. / Journal of Electroanalytical Chemistry 745 (2015) 1–7

a

b'

0.6

1st

a'

0.4

c

0.2 Current (mA)

5

0.0 -0.2

-20 oC 0 oC 25 oC 50 oC

-0.4 -0.6 -0.8 -1.0

b 0.0

a

0.5

1.0

1.5

2.0

2.5

Voltage (V)

b

0.6

d' 2nd

0.4

c'

Current (mA)

0.2 0.0

-20 oC 0 oC 25 oC 50 oC

-0.2 -0.4

d -0.6 0.0

0.5

c 1.0

1.5

2.0

Fig. 6. (a) Nyquist plots for the Li3VO4/C electrode at a stage of charge (3.0 V) at various temperatures and, (b) equivalent circuit used for simulating the experimental impedance data.

2.5

Voltage (V) Fig. 5. (a) The first cycle CV curves and (b) the second cycle CV curves of Li3VO4/C electrode at various temperatures between 0.1 and 3.0 V.

Table 2 Potential differences between the oxidation and reduction peaks for Li3VO4/C electrode. T (°C)

DEa0 –a (V)

DEb0 –b (V)

DEc0 -c (V)

DEd0 –d (V)

20 0 25 50

– 0.930 0.666 0.412

1.364 1.091 0.976 0.805

– 0.491 0.228 0.183

1.384 0.994 0.844 0.818

CV curves of the Li3VO4/C electrode in the voltage ranges of 0.1–3.0 V at different temperatures are shown in Fig. 5. As it can be seen, the CV curves of high temperatures (25 and 50 °C) in the first cycle show two domain reduction peaks (marked as a and b) corresponding to lithium insertion process and three domain oxidation peaks (marked as a0 , b0 and c) corresponding to lithium extraction process (as depicted in Fig. 5a). The peak of current of the electrode decreases significantly along with decreasing the operation temperature. At low temperature of 20 °C, only one oxidation peak at about 1.364 V can be observed during all insertion/de-insertion process. Meanwhile, the potential differences between oxidation and reduction peaks tend to small with increasing the temperature, as summarized in Table 2. Considering that the electrochemical process of the Li3VO4/C electrode may involve in some side reactions, such as the formation of SEI film, CV curves in the second cycle are also presented in Fig. 5b. The CV curves of the second cycle are similar to the first cycle except that two

reduction peaks (marked as d and e) separate to much more evident and one oxidation peak (marked as d0 ) becomes weaker. These results are in accordance with the discharge/charge curves of the Li3VO4/C electrode presented in Fig. 4a and b. And it should be emphasized that, as shown in Table 2, the potential differences of the CV peaks in the second cycle also decrease along with the increasing temperature. Combined with these results, it can be concluded that higher operation temperature can decrease the voltage polarization of the electrode, thus benefit the electrochemical performance of the Li3VO4/C electrode (see Fig. 4c and d). In order to understand impedance changes of Li3VO4/C electrode at different operation temperatures, EIS measurements were conducted at various temperatures after 4 cycles. Fig. 6a presents the Nyquist plots of the Li3VO4/C electrode at the stage of full charge (3.0 V) at different operation temperatures. From Fig. 6a, it is apparently seen that the impedance spectrums are composed of two partially overlapped semicircles at high and medium frequency regions, and a straight slopping line at low frequency regions. Fig. 6b presents an equivalent circuit to simulate the electrochemical impedance data. Rel represents the solution resistance; Rsl and Csl stand for the Li ion migration resistance and the capacity of surface layer, respectively; Rct and Cdl designate the related charge-transfer resistance and double-layer capacitance, respectively; ZW represents the diffusion-controlled Warburg impedance. The lithium diffusion coefficient, DLi, could be calculated from the Warburg region using the following equation [19]:

DLi ¼

1 2



 2 Vm dE  dx FSr

ð1Þ

where Vm is the molar volume of Li3VO4 (102.78 cm3 mol1), F is the Faraday constant, S is the surface area of the electrode (1 cm2), the

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a

Table 4 Comparison on Li diffusion coefficients of modified Li3VO4 synthesized by different methods.

10000

Sample

Methods

DLi (cm2 s1)

T (°C)

References

Li3VO4 particles

Solid-state reaction Sol–gel method Hydrothermal process Solid-state reaction

3.68  109 4.40  108 9.18  1013

RT RT RT

[20] [20] [7]

3.77  107

RT

Spray pyrolysis technique Solid-state reaction

4  105

RT

Present work [21]

300 700

[22] [22]

8000 Inset -20 C 0 oC 25 oC 50 oC o

1200

6000

Zre (Ohms)

Zre (Ohms)

1600

4000

800

400

[email protected] Li3VO4/C Li4GeO4–Li3VO4

0 3

2000

4

5

6

7

8

9

10

Li3VO4 particles

ω-1/2 (s-1/2)

6

3  10 5.7  103

0 3

4

5

6

7

8

9

at low temperature should be two of the major causes for its poor performance during the low temperature operation [12]. Furthermore, as showed in Table 4, the Li diffusion coefficients at room temperature (25 °C, RT) of Li3VO4/C obtained at our present work is much higher than those already reported using similar methods [7,20], but lower than that of Li4GeO4–Li3VO4 system which using spray pyrolysis technique and those obtained at higher temperatures [21,22]. These results show that synthetic method and test condition are two key factors which directly affect the Li diffusion coefficients of Li3VO4. To see the temperature effect on DLi more clearly, the logarithmic was plotted against the inverse of temperature (as shown in Fig. 7b), and the resultant plots shows a format as: log DLi = 0.8717  (2223.61/T), which follows the conventional temperature Arrhenius equation:

10

ω-1/2 (s-1/2) b

Log (DLi cm2 s-1)

-5.5

Log DLi = 0.8717-(2223.61/T)

-6.0 -6.5 -7.0 -7.5

DLi ¼ D0 exp

-8.0 -8.5 3.0

3.2

3.4

3.6 -1

3.8

4.0

-1

1000T / K

Fig. 7. (a) The linear fitting of the Z0 vs. x1/2 relationship and the inset is a magnification image, (b) Arrhenius plot of the apparent diffusion coefficients of lithium ions in Li3VO4/C composite.

Table 3 Evaluated impedance parameters according to the equivalent circuit of Fig. 6b and the apparent diffusion coefficients of Li ions at various operation temperatures. T (°C) 20 0 25 50

Rel (O) 6.3 3.17 3.09 2.80

Rsl (O) 65.84 54.82 12.65 15.91

Rct (O) 8997.24 251.52 29.44 42.9

DLi (cm2 s1) 9

8.92  10 7.10  108 3.77  107 6.84  107

value of dE/dx is the slope of the open-circuit potential vs. Li ion concentration x and r is the Warburg factor which obeys the following relationship:

Z 0 ¼ RD þ RL þ rx1=2

ð2Þ 0

The slope r can be obtained based on a linear fitting of Z vs. x1/2 as shown in the Fig. 7a, and the inset of Fig. 7a is a magnifying image. Using the value of Eqs. (1) and (2), the lithium diffusion coefficients of Li3VO4/C electrode at various temperatures can be calculated. The evaluated impedance parameters according to the equivalent circuit of Fig. 6b and the apparent diffusion coefficients of Li ions at various temperatures are shown in Table 3. It can be seen that the larger charge-transfer resistance on the electrolyte– electrode interfaces and the smaller lithium diffusion coefficient

  Ea RT

ð3Þ

where D0 is the pre-exponential factor (a temperature-independent coefficient), Ea is the activation energy, R is the gas constant, and T is the absolute temperature. The apparent activation energies (Ea) for lithium intercalation can be estimated by EIS [23]. The apparent activation energy (Ea = Rk ln 10, R is the gas constant and k is the slope of the fitting line shown in Fig. 7b) of the Li3VO4/C electrode is calculated to be 42.57 kJ mol1, which is close to Li4Ti5O12 (44.4 kJ mol1) using the same binder (polyvinylidene fluoride) [24]. 4. Conclusion Carbon-coated Li3VO4 (Li3VO4/C) sample has been synthesized by simple solid-state method using glucose as a carbon source. Rietveld refinement, XPS and element analysis results show that, though it is synthesized in the presence of carbon and reducing atmosphere, both the single-phase Li3VO4/C and the valence of vanadium of +5 can be retained. The SEM and TEM images reveal that Li3VO4/C composite has uniform particles with size less than 1 lm. The electrochemical test results show that Li3VO4/C at high operation temperatures holds both higher specific capacity and cyclic performance than that of low temperatures. Even at a high discharge/charge rate of 15 C, the discharge/charge capacities of the Li3VO4/C electrodes at 20, 0, 25 and 50 °C still can retain about 20, 120, 370 and 450 mAh g1, respectively. More importantly, the CV results demonstrate that the higher operation temperature can decrease the voltage polarization of the electrode, thus benefit the electrochemical performance of the Li3VO4/C electrode. In addition, the EIS results indicate that low operation temperatures lead to larger charge-transfer resistance and smaller lithium diffusion coefficient, which should be two major reasons for its poor performance of the Li3VO4/C electrode at low operation temperature. In brief, this study gives a valuable investigation of

Z. Liang et al. / Journal of Electroanalytical Chemistry 745 (2015) 1–7

low and high temperature electrochemical performance of Li3VO4/ C anode material for Li-ion batteries and further investigations about low-temperature performance improvement of this material still need to be done in the next work. Conflict of interest There is no conflict of interest. Acknowledgments This work was funded by NSFC Grant (Nos. 51172077 & 51372089) supported through NSFC Committee of China, the Foundation of (No. S2011020000521) supported through the Science and Technology Bureau of Guangdong Government and the Foundation of (No. 2014ZB0014) supported through the Fundamental Research Funds for the Central Universities. References [1] J.B. Goodenough, K.-S. Park, J. Am. Chem. Soc. 135 (2013) 1167. [2] L. Su, Y. Jing, Z. Zhou, Nanoscale 3 (2011) 3967. [3] A. Manthiram, J. Phys. Chem. Lett. 2 (2011) 176.

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