J. Cent. South Univ. (2015) 22: 2043-2051

DOI: 10.1007/s11771-015-2727-z

Effects of temperature variation on Li_xFePO₄/C (0＜X＜1) process

XIAO Zheng-wei(肖政伟)¹, ZHANG Ying-jie(张英杰)¹, HU Guo-rong(胡国荣)²

1. Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology,Kunming 650093, China;

2. School of Metallurgy and Environment, Central South University, Changsha 410083, China

Central South University Press and Springer-Verlag Berlin Heidelberg 2015

Abstract: LiFePO₄/C was prepared via solid state reaction and characterized with X-ray powder diffraction and charge–discharge test. As-prepared LiFePO₄/C has a triphylite structure and exhibits an excellent rate capability and capacity retention. Electrochemical impedance spectroscopy (EIS) was applied to investigate Li_xFePO₄/C (0+ ion accumulation and consumption in the electrode reaction. The surface layer impedance needs to be included in the equivalent circuit when LiFePO₄/C is deeply delithiated at a relatively high temperature. EIS examination indicates that a temperature rise leads to a better reversibility, lower charge transfer resistance, higher exchange current density J₀ and greater Li⁺ ion diffusion coefficient for the Li_xFePO₄/C electrode process. The Li⁺ ion concentration in Li_xFePO₄/C is potential to impact the Li⁺ ion diffusion coefficient, and a decrease in the former results in an increase in the latter.

Key words: lithium ion cell; LiFePO₄/C; electrode process; electrochemical impedance spectroscopy; equivalent circuit; Li⁺ ion diffusion coefficient

1 Introduction

The olivine-structured LiFePO₄ owns the favorable attributes of abundant raw materials, non-toxicity, a high thermal stability, a medium voltage of 3.45 V (vs Li⁺/Li) and an attainable theoretical capacity of 170 mA·h/g. These desired properties have qualified LiFePO₄ as a suitable cathode for the long life and safe lithium ion cells [1]. However, due to the poor electronic conductivity of pristine LiFePO₄, nano-sizing [2], supervalent ion doping [3] and/or carbon coating/mixing [4-6] are/is requested for an enhanced electrochemical performance. Preparation of LiFePO₄/C composite is by all means a simple but effective strategy for LiFePO₄ achieving a practical capacity.

In order to make better use of the energy storage potential for LiFePO₄/C, it is necessary to know the effects of related factors on its electrode process. The electronic and Li⁺ ion ionic conductivities are the key figures to determine the electrochemical behavior of an electrode material for lithium ion cells. As the electronic conductivity for LiFePO₄/C is not so sensitive to temperature variation, the study of the Li⁺ ion diffusion in LiFePO₄/C electrode reaction becomes a practical way. Temperature exerts influences on the LiFePO₄/C electrode process, and its effect on the Li⁺ ion diffusion has been investigated [7]. It has also been reported that a higher temperature leads to higher charge–discharge capacities for LiFePO₄ [8].

Electrochemical impedance spectroscopy (EIS) is an efficient technique to study the kinetics involved in reversible and irreversible electrode processes [9-12]. EIS possesses the advantageous traits of a wide frequency range and a negligible disturbance on tested systems. It is usually adopted to study the kinetics, electronic conductivity and ion diffusion coefficient in an electrode reaction. In particular, EIS is adequate for the electrochemical system containing a Warburg resistance.

In this work, EIS is used to investigate Li_xFePO₄/C (0xFePO₄/C electrode process are introduced according to the electrode process and the criteria for EIS circuit design. Then, a fitting is carried out on the EIS data of LiFePO₄/C electrode to determine the valid circuit. Finally, fittings are performed on the EIS data of Li_xFePO₄/C electrode.

2 Experimental

Stoichiometric Fe₂O₃, NH₄H₂PO₄ and Li₂CO₃ and a certain amount of C₆H₁₂O₆·H₂O were mixed in acetone and ball-milled using a speed of 300 r/min for 4 h to achieve homogenous mixing and thorough mechanical activation. After being dried at 80 °C overnight, the mixture was pulverized and sintered at 700 °C in a high-purity argon atmosphere to prepare LiFePO₄/C.

As-prepared LiFePO₄/C was characterized with X-ray powder diffraction (XRD) performed on a Philips X-pert powder diffractometer using Cu K_α radiation. The specific surface area of the as-prepared LiFePO₄/C powder was obtained with the Brunauer–Emmett–Teller (BET) analysis on an Autosorb-1-C.

Coin cells comprising LiFePO₄/C cathode electrodes were assembled in a glove box. For the coin cell fabrication, the electrolyte was 1 mol/L LiPF₆ in a mixed solvent of ethylene carbonate (EC), dimethylene carbonate (DMC) and ethylmethyl carbonate (EMC) at a volume ratio of 1:1:1. The lithium disc was used as the anode electrode. The membrane was Celgard 2400 microporous polypropylene. LiFePO₄/C, PVdF binder and acetylene black at a mass ratio of 8:1:1 were ground in N-methyl pyrrolidinone (NMP) solvent and evenly spread on an aluminum foil current collector. The wet cathode was dried at 120 °C under vacuum. Round cathode discs were punched for the coin cell assembly. The charge–discharge measurement was conducted galvanostatically on a LAND BTI-40 in the voltage window of 2.5-4.1 V at room temperature.

For both EIS and CV tests, the working electrode was LiFePO₄/C or Li_xFePO₄/C which had already been charged–discharged for ten cycles. The metallic lithium was used as the counter and reference electrode. The electrolyte was the same as that used above. The EIS measurement of Li_xFePO₄/C electrode was carried out in the temperature range of 28-61 °C on a PAR Potentiostat/ Galvanostat, Model 273A, coupled to a PAR lock-in amplifier, Model 5210. The EIS test input was a sinusoidal alternate current with a frequency from 10 mHz to 100 kHz and a voltage amplitude of 5 mV. Each LiFePO₄/C electrode was left to stand for 24 h prior to the EIS measurement. The CV measurement of LiFePO₄/C electrode was performed with a PAR Potentiostat/Galvanostat, Model 273A, in the voltage window of 2.5-4.2 V at 34-60 °C with a scan rate of 0.5 mV/s.

3 Results and discussion

Figure 1 shows the XRD pattern for the as-prepared LiFePO₄/C. The diffraction result is indexed to orthorhombic, space group Pnma. The characteristic peaks in the diffraction pattern are sharp and perfect, suggesting a high degree of crystallinity for the LiFePO₄ in as-prepared LiFePO₄/C. The pattern agrees well with that for triphylite (JCPDS NO. 40–1499), and no impurity phases consisting of lithium, iron and/or phosphorus have been detected, indicating a high purity for the as-prepared LiFePO₄/C.

Fig. 1 XRD pattern for as-prepared LiFePO₄/C

Figure 2(a) shows that at 0.1C the as-prepared LiFePO₄/C exhibits an appreciable discharge capacity of 153.8 mA·h/g. Meanwhile, as indicated in Fig. 2(b),excellent rate capacity retentions without any fade are achieved by as-prepared LiFePO₄/C at 0.1, 1 and 2C with average discharge capacities of 153.8, 128.3 and 121.0 mA·h/g, respectively, in the first 50 cycles.

Fig. 2 Typical charge–discharge profiles at 0.1C (a) and rate capability (b) for LiFePO₄/C electrode

Figure 3 illustrates the EIS and CV patterns for the LiFePO₄/C electrode tested at different temperatures. It is evident that temperature has a great impact on the EIS and CV of LiFePO₄/C electrode. The semicircle of EIS profile in the high frequency region represents the charge transfer resistance (R_ct) in the LiFePO₄/C electrode process. As indicated in Fig. 3(a), with the increase in temperature, R_ct becomes smaller, which is beneficial to charge transfer. As shown in Fig. 3(b), with the elevation of temperature, the redox peaks for Fe³⁺/Fe²⁺ couple become sharper and closer, and the peak currents become bigger. The information indicates that an increase in temperature leads to a better reversibility and faster Li⁺ ion intrusion and extrusion for LiFePO₄/C electrode process. The EIS and CV profiles in Fig. 3 show the same variation trend for the kinetics involved in the LiFePO₄/C electrode process on temperature change. It can be safely concluded that a higher temperature results in a lower polarization, smoother redox reaction for Fe³⁺/ Fe²⁺ couple and smaller charge transfer resistance in LiFePO₄/C electrode process.

In the Li_xFePO₄/C electrode process, the main reaction is Fe³⁺/Fe²⁺ redox behavior, along with the Li⁺ ion intrusion and electron gain or Li⁺ ion extrusion and electron loss. As shown in Fig. 4, several basic steps are involved when Li⁺ ion is intercalated into the Li_xFePO₄: 1) The Li⁺ ion dissolved from lithium anode migrates through bulk electrolyte to the electrolyte/cathode interface. 2) The Li⁺ ion is transferred across the electrolyte–cathode electrical double layer. 3) The Li⁺ ion enters the possibly existent surface layer and then the active material particle. 4) The Li⁺ ion diffuses in the LiFePO₄ and FePO₄ phases. 5) The Fe³⁺/Fe²⁺ redox couple undergoes reduction reaction. 6) The electron is gained from external circuit to maintain charge balance in the cathode. The Li⁺ ion extrusion process is reverse to the Li⁺ ion intrusion process.

Fig. 3 EIS (a) and CV (b) patterns obtained at different temperatures for LiFePO₄/C electrode

Fig. 4 Schematic diagram of Li_xFePO₄/C electrode process on Li⁺ ion intrusion

Step 4) is usually rate-determining in a solid electrode process. The electron transfer, however, may become a rate-determining step when the electrode material exhibits a very poor electronic conductivity. In this work, however, as the carbon coating/mixing has greatly improved the electronic conductivity of LiFePO₄/C composite, this parameter may be excluded to be rate-determining to a certain extent. All the basic steps affect the overall kinetics of Li_xFePO₄/C electrode process to different degrees. As a matter of fact, electrode kinetics is mainly determined by the nature of electrode material and the properties of electrolyte/ electrode interface. Exchange current density, the indicator of reversibility, depends on the charge transfer resistance which is dependent on the electrolyte– electrode properties.

In an electrode process, the impedance contributed by an electrolyte can be represented by a resistor (R_e), a resistor of an electrical double layer (R_ct) and a capacitor (C_ct) in parallel for charge transfer, a Warburg resistance of a concentration polarization (or a resistor and a capacitor in series), and another electrochemical double layer of an surface or absorption layer. Different equivalent circuits have been put forward for investigating LiFePO₄ electrode process [7-8, 13-14]. In order to reflect the LiFePO₄/C electrode process properly, the electrode process shown in Fig. 4 and the criteria for equivalent circuit design [15-16] are followed rigorously.

Based on the above discussion, equivalent circuits for the LiFePO₄/C electrode process are introduced, as shown in Fig. 5, in which R_e is the electrolyte resistance, R_sl is the surface layer resistance, R_ct is the charge transfer resistance, C_sl is the surface layer capacitance, C_ct is the charge transfer capacitance, Z_W and are the Warburg resistances for the faradic and non-faradic currents, respectively, C_int is the intercalation capacitance caused by Li⁺ ion accumulation and consumption in active material particles, Q_sl is the surface layer CPE (constant phase element), and Q_ct is the charge transfer CPE.

The EIS spectra in Fig. 3(a) are fitted with the possible equivalent circuits in Fig. 5 using Zsimpwin software and the results are given in Table 1. It can be seen that there is a negligible difference between the fitted values for R_e by using the possible equivalent circuits to fit the same EIS spectrum. Due to the smallest R_e among all the ohmic and non-ohmic resistances, the Circuit II can be excluded to be viable. From the value of errors, only the results by using the Circuit V have errors less than 10% each. Figure 3(a) shows that an increase in temperature reduces the charge transfer resistance in the LiFePO₄/C electrode process, which is in agreement with the fitted results by using the Circuit V. For the Li⁺ ion intrusion–extrusion in LiFePO₄/C electrode process, there exist the Li⁺ ion accumulation and consumption in the active material particles because of its sluggish diffusion kinetics. Thus, the inclusion of C_int in the Circuit V is reasonable and essential which is chosen to fit the EIS spectra of Li_xFePO₄/C electrode.

Fig. 5 Possible equivalent circuits for LiFePO₄/C electrode process

As shown in Fig. 5, an important characteristic with regard to the Circuit V is the exclusion of surface layer resistance and capacitance. The products from solvent reactions are not detected at the carbon-treated LiFePO₄ surface, contrary to the findings for other cathode materials, such as LiCoO₂, LiMn₂O₄ and LiNiO₂ [17]. The surface layer components for the LiFePO₄/C electrode have been found to be mainly LiF, LiPF₆, Li_xF_y and Li_xPO_yF_z [17–20]. The Li⁺ ion diffusion in the fluorides is different from that in the solvent reaction products and Li₂CO₃. Moreover, no evidence of the formation of a resistive film is deduced from the evolution of EIS measurements for LiFePO₄/C and LiFePO₄ electrodes [20]. Hence, it is practicable not to include a surface layer resistance and capacitance in the Circuit V.

Figure 6 shows the EIS spectra for the Li_xFePO₄/C electrode examined at different temperatures which are fitted by using the equivalent Circuit V. It must be noted that the EIS spectrum for the Li_0.1FePO₄/C at 67 °C contains two arcs in the high and middle frequency regions, which is singled out for clarity as the inset of the figure of the Li_0.1FePO₄/C EIS spectra. The equivalent circuit for fitting this exceptional spectrum should be a revision of equivalent Circuit V. Substantial fitting indicates that on the basis of the equivalent Circuit V, a good fitting effect can be achieved by the addition of a surface layer resistance R_sl and CPE Q_sl and the replacement of Q_ct by C_ct. This demonstrates that the surface layer impedance to Li⁺ ion migration must be considered when the Li_xFePO₄/C is deeply delithiated at a relatively high temperature. A possible reason is that under the conditions, the thickness of surface layer may be greatly increased or surface layer composition may be significantly changed. The fitted result for the EIS spectra in Fig. 6 is given in Table 2. The errors for all fitted values are less than 10%.

Table 1 Fitted results based on possible equivalent circuits for EIS spectra of LiFePO₄/C electrode

From Table 2, it can be seen that R_e exhibits different values for the Li_xFePO₄/C electrode with different Li⁺ ion concentrations. This can be ascribed to the different cells used for each set of EIS tests on the cathode with same Li⁺ ion concentration in the temperature range of 37-69 °C. In fact, it is not scientific to just regard the fitted values as the physical resistance or capacitance for the physical parts in an electrode process, such as electrolyte, electrical double layer, surface layer and FePO₄–LiFePO₄ phase. However, it is valid to investigate the effect of temperature on the electrode process through analyzing the change trend for the value of each element in the equivalent circuit.Table 2 shows that for any x value in Li_xFePO₄/C with the increase in temperature the charge transfer resistance R_ct decreases, indicative of a temperature elevation favoring Li⁺ ion migration across the electrical double layer. Exchange current density J₀ is inversely proportional to R_ct:

                               (1)

where R is the gas constant; T is the absolute temperature; n is the number of electron transferred; F is the Faraday constant. According to Eq. (1), with the rise in temperature, J₀ increases even more rapidly as R_ct simultaneously decreases. A higher exchange current density means a better reversibility. Here, it can be safely concluded that temperature is a potential factor to affect the reversibility of a LiFePO₄/C electrode. As proved by the EIS spectra fitting, a temperature increase can greatly improve the reversibility of LiFePO₄/C electrode, which agrees with the conclusion drawn from CV data analysis.

Fig. 6 EIS spectra for Li_xFePO₄/C (0＜X＜1) electrode at different temperatures

Table 2 Fitted results for EIS spectra of Li_xFePO₄/C electrode by using equivalent Circuit V

Plot the real part of Warburg impedance Re(Z) as the function of (2πf)^–1/2 to obtain Fig. 7. As can be seen, except for very few cases, most curves in Fig. 7 exhibit a linear pattern. OriginLab8 software is used to find the slope (σ) for each curve and the following equation is applied to calculating the Li⁺ ion diffusion coefficient D_Li⁺:

                       (2)

where R is the gas constant; T is the absolute temperature; n is the number of electrons transferred; A is the effective area for Li_xFePO₄/C electrode; F is the Faraday constant; is the concentration of Li⁺ ion in Li_xFePO₄/C.

While an electrode reaction only takes place at the points where a LiFePO₄ particle, conductive carbon and electrolyte meet, Li⁺ ion charge transfer proceeds at the sites where at least a LiFePO₄ particle and electrolyte meet. In as-prepared LiFePO₄/C, the lighter carbon accounts for 8.03% by mass and its contribution to the surface area must be considerable. Thus, it is impossible to know the real effective surface area for LiFePO₄ in LiFePO₄/C electrode. The BET test shows that the specific surface area for the as-prepared LiFePO₄/C is 1.5×10⁵ cm²/g. In this work, the specific effective area for LiFePO₄/C is determined to be 5×10⁴ cm²/g due to the [010] direction-oriented Li⁺ ion diffusion in FePO₄- LiFePO₄ system and intentional ommision of carbon’s contribution. Using Eq. (2), the Li⁺ ion diffusion coefficient is calculated for the Li_xFePO₄/C electrode tested at different temperatures and given in Table 3.

The Li⁺ ion diffusion coefficient obtained from the EIS spectra is 10^-19-10^{-17 cm2}/s, which is close to the reported value [21]. Under the test conditions in this work, at the same temperature, the Li⁺ ion diffusion coefficient increases with the decrease in Li⁺ ion concentration in Li_xFePO₄/C electrode. With the increase in temperature, Li⁺ ion diffusion coefficient increases for Li_xFePO₄/C electrode with the same Li⁺ ion concentration. In addition, at any temperature, the decrease in x from 0.9 to 0.1 gives rise to a (10–20)-fold increase in Li⁺ ion diffusion coefficient. Thus, Li⁺ ion concentration in Li_xFePO₄/C electrode is another important factor to affect the Li⁺ ion diffusion in the electrode process. To any x in Li_xFePO₄/C electrode, however, the increase in the temperature from 37 to 69 °C leads to only a 2-fold increase in Li⁺ ion diffusion coefficient.

Fig. 7 Relation between real part of Warburg impedance and reciprocal of square root of angular frequency for Li_xFePO₄/C electrode

Table 3 Li⁺ ion diffusion coefficients (D_Li⁺/(10^-18 cm²·s^-1)) for Li_xFePO₄/C electrode at different temperatures

The above discussion shows that at any state of charge, an elevation of temperature results in a better reversibility, smaller charge transfer resistance, larger exchange current density and higher Li⁺ ion diffusion coefficient for the Li_xFePO₄/C electrode process. Faster Li⁺ diffusion is beneficial to prompting the deep charge– discharge ability of a LiFePO₄/C electrode at higher rates. Here, the study also gives the direct evidence for higher Li⁺ ion diffusion coefficient at an elevated temperature accounting for a capacity enhancement of LiFePO₄ electrode.

This work demonstrates that temperature elevation is effective to tackle the sluggish kinetics in LiFePO₄/C electrode process. However, the interface of LiFePO₄/ electrolyte based on LiPF₆ is sensitive to temperature variation and the side reactions resulting in Fe²⁺ dissolution become severe in temperature increasing period [22]. Therefore, the strategy of temperature elevation for improving the slow kinetics of LiFePO₄/C electrode can be adopted on the condition that a suitable electrolyte is used. It is anticipated that a solid electrolyte compatible with LiFePO₄/C will be a viable choice.

4 Conclusions

1) LiFePO₄/C cathode material for lithium ion cells is synthesized through solid state reaction. Superior rate capability of 153.8, 128.3 and 121.0 mA·h/g at 0.1C, 1C and 2C, respectively, and very good capacity retentions are achieved by the as-prepared LiFePO₄/C.

2) The equivalent circuit containing the intercalation capacitance for Li⁺ ion accumulation and consumption is valid for fitting the EIS spectra of LiFePO₄/C electrode. The surface layer should be included in the equivalent circuit when Li_xFePO₄/C is deeply delithiated at a relatively high temperature.

3) A temperature elevation results in an increased exchange current density, decreased charge transfer resistance and higher Li⁺ ion diffusion coefficient for Li_xFePO₄/C electrode. Li⁺ ion concentration in Li_xFePO₄/C electrode is potential to impact the Li⁺ ion diffusion coefficient, and a decrease in the former brings about an increase in the latter.

4) The adoption of temperature elevation to circumvent the slow kinetics in LiFePO₄/C electrode reaction needs the realization of a suitable electrolyte capable of reducing its reaction with LiFePO₄ to an acceptable low level at an elevated temperature.

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(Edited by YANG Bing)

Foundation item: Project(2010ZC051) supported by the Natural Science Foundation of Yunnan Province, China; Project(20140439) supported by Analysis and Testing Foundation from Kunming University of Science and Technology, China; Project(14118245) supported by Starting Research Fund from Kunming University of Science and Technology, China

Received date: 2014-08-12; Accepted date: 2014-12-10

Corresponding author: ZHANG Ying-jie, Professor, PhD; Tel: +86-18808804528; E-mail: zhangyingjie09@126.com; HU Guo-rong, Professor, PhD; Tel: +86-731-88830474; E-mail: hgrhsj@263.net