Trans. Nonferrous Met. Soc. China 23(2013) 1718-1722

Synthesis and performance of LiVPO₄F/C-based cathode material for lithium ion battery

Jie-xi WANG, Zhi-xing WANG, Li SHEN, Xin-hai LI, Hua-jun GUO, Wei-jia TANG, Zhen-guo ZHU

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

Received 25 July 2012; accepted 17 January 2013

Abstract: LiVPO₄F/C-based material was prepared by heating a precursor, which was obtained through ball milling with slurry of H₂C₂O₄·2H₂O, NH₄H₂PO₄, NH₄VO₃ and LiF. In this process, H₂C₂O₄·2H₂O was used as a reducing agent as well as carbon source. The properties of the precursor and the prepared sample were characterized by TG-DTA, XRD, SEM, TEM and C-S analysis. XRD analysis shows that the precursor is amorphous and the prepared LiVPO₄F sample coexists with Li₃V₂(PO₄)₃, as well as a small amount of V₂O₃. Homogeneous powders with an average particle size of 2 μm are observed by TEM. HRTEM image shows that the crystal particles are coated by amorphous carbon uniformly, with a coating layer of about 2 nm. The synthesized material shows the specific discharge capacities of 151.3 and 102.5 mA·h/g at 0.1C and 10C respectively, in the voltage range of 3.0-4.4 V, and it keeps 90.4% of initial discharge capacity at 10C after 50 cycles. V³⁺/V⁴⁺ redox peaks corresponding to both LiVPO₄F and Li₃V₂(PO₄)₃ are seen clearly in the CV curve.

Key words: lithium ion battery; LiVPO₄F; ball milling; cathode material; carbon coating

1 Introduction

Framework materials based on the phosphate polyanion, such as LiFePO₄, LiMnPO₄ and Li₃V₂(PO₄)₃, have been firmly believed to be potential electro-active materials for lithium-ion battery applications [1-7]. Fluorophosphate, represented by the general formula RMPO₄F, where R represents lithium or sodium and M represents a 3d transition metal, was reported as electro- active material by BARKER et al [8,9]. LiVPO₄F, namely lithium vanadium fluorophosphate, owing to its good electrochemical performance, high voltage platform, high energy density and excellent thermal stability, is considered to a novel 4 V cathode material to substitute LiCoO₂ commonly used in the first-generation lithium-ion batteries [10,11]. It is isostructural with the naturally occurring minerals tavorite LiFePO₄·OH, or amblygonite LiAlPO₄F, crystallizing with a triclinic structure (space group ) [12-14]. The reversible Li⁺ extraction/insertion reaction for Li_1-xVPO₄F based on the V^3+/4+ redox couple operates at about 4.2 V versus Li, which is 0.2 or 0.7 V higher than that exhibited by LiCoO₂ or LiFePO₄, respectively [11,15]. Usually, LiVPO₄F is synthesized by conventional two-step carbon thermal reaction (CTR), which generally operates at high temperatures for a long time. BARKER et al [8] first introduced the two-step CTR method for preparing LiVPO₄F/C successfully, and did much to improve its electrochemical properties by two-step CTR method [10,16,17]. Unfortunately, though the utilization of lithium ion is increased, the poor rate performance still hinders its further development as the next commercial high-potential cathode material. One-step CTR method is also employed to prepare LiVPO₄F/C. ZHONG et al [18] employed humic acid as reduction agent as well as carbon source, and prepared porous LiVPO₄F/C particles by one-step method successfully. A sol-gel method was also employed to synthesize LiVPO₄F. LI et al [19] reported a novel sol-gel method to prepare LiVPO₄F/C, using H₂O₂ as coordinating agent to prepare V₂O₅·nH₂O precursor, which has an important significance. ZHONG et al [15] and QIAO et al [20] employed citric acid to prepare Li-V-PO₄-F gel and then sintered it at high temperatures to obtain crystal LiVPO₄F [15,20]. But it may not be an effective way for large scale production.

In this work, a simple method is presented to synthesize carbon coated LiVPO₄F, namely chemical reduction assisted with mechanical activation followed by short-time calcinations. The V(+5) is reduced to V(+3) by oxalic acid at room temperature and it is not necessary to prepare VPO₄ intermediate at high temperatures compared with traditional CTR method. The short-time calcination can save a large amount of energy. The carbon is decomposed from gas-phase reaction and can be deposited on the surface of the crystal particle uniformly. The electrochemical performance of prepared samples is evaluated.

2 Experimental

The LiVPO₄F sample was prepared as following steps. Firstly, stoichiometric NH₄VO₃ (AR, 99%), NH₄H₂PO₄ (AR, 98.5%), LiF (AR, 99.5%), 50% excess oxalic acid dihydrate (H₂C₂O₄·2H₂O, AR, 98.5%) were added to a sealing can, and ball milled in the alcohol media at a revolving speed of 250 r/min for 4 h. The ball mill type was ND6-2L with rated power of 0.75 kW. The oxalic acid dihydrate here was introduced as reducing agent as well as carbon source. Secondly, the obtained slurry was dried in a drying oven at 80 °C for 6 h. Finally, the dried powders were sintered at 350 °C for 5 h, and then at 750 °C for 90 min under argon atmosphere.

The TG-DTA analysis was tested by SDTQ600 with a step time of 10 °C/min under argon atmosphere. The powder X-ray diffraction (XRD, Rint-2000, Rigaku) using Cu K_α radiation was employed to identify the crystalline phase of the synthesized material. The mass fractions of the crystal phases were evaluated by multiphase refinement with Maud Program. The morphology of the LiVPO₄F powders was observed on a scanning electron microscope (SEM, JEOL, JSM- 5612LV) with an accelerating voltage of 20 kV, and on a transmission electron microscope (TEM, Tecnai G12). Carbon content was tested by a C-S analyzer (Eltar, Germany).

The electrochemical characterizations were conducted with CR2025 coin-type cell. For positive electrode fabrication, the prepared powders were mixed with 10% acetylene black and 10% polyvinylidene fluoride in N-methyl pyrrolidinone until slurry was obtained. And then, the blended slurry was pasted onto an aluminum current collector, followed by drying at 120 °C for 12 h in the air. The test cell consisted of the positive electrode and lithium foil negative electrode separated by a porous polypropylene film, using 1 mol/L LiPF₆ in EC, EMC and DMC (1:1:1 in volume) as the electrolyte. The assembly of the cells was carried out in a dry Ar-filled glove box. Electrochemical tests were carried out using an automatic galvanostatic charge- discharge unit, NEWARE battery circler, between 3.0 V and 4.4 V versus Li/Li⁺ at room temperature. A constant current-constant voltage condition was employed for charge process and a galvanostatic condition was conducted for discharge process. The cyclic voltammetry measurement at a scan rate of 0.05 mV/s between 3.0 V and 4.4 V was carried out with a CHI660D electrochemical analyzer.

3 Results and discussion

In the process of ball milling for raw materials NH₄VO₃, NH₄H₂PO₄, LiF and H₂C₂O₄·2H₂O at room temperature, the pentavalent vanadium is reduced to trivalent vanadium by H₂C₂O₄·2H₂O, and the color of the material changes from creamy white to green. Figure 1(a) shows the XRD pattern of LiVPO₄F precursor prepared by ball milling at room temperature. It can be seen that there is no obvious evidence of diffraction peaks in Fig. 1(a), which indicates that the precursor is nearly amorphous. The TG-DTA curves of the precursor are shown in Fig. 1(b). It is clear that three endothermic peaks and mass loss stages appear at 75-135 °C, 250-390 °C and 450-700 °C, which represent the loss of absorbed water, decomposition of oxalic acid as well as ammonium dihydrogen phosphate, formation accompanied crystallization of LiVPO₄F, respectively. Two distinct changes are observed at 450 and 630 °C. The change at 450 °C may correspond to further decomposition of intermediate product H₃PO₄ and H₄P₂O₇ to (HPO₃)_x and H₂O(g), and the mass loss at 630 °C may result from formation of crystal LiVPO₄F, in which the volatilization of H₂O(g) may induce HF sublimation. According to the above analysis, two-step calcination schedule should be introduced to ensure sufficient decomposition and contact among the reactants, and finally materials with good electrochemical performance can be obtained.

Fig. 1 XRD pattern (a) and TG-DTA curves (b) of LiVPO₄F precursor

Figure 2 shows the XRD pattern of the prepared material after heat treatment under argon atmosphere. The XRD pattern shows a series of diffraction peaks, indicating that a transformation of the precursor from amorphous to a crystalline phase occurs during the calcination process. A multiphase refinement method was employed to analyze the XRD based on the models of LiFePO₄(OH,F) (ICSD #20808), Li₃V₂(PO₄)₃ (ICSD #98362) and V₂O₃ (ICSD #1473). The result shows that the mass fractions of LiVPO₄F, Li₃V₂(PO₄)₃ and V₂O₃ are 79.2%, 17.6%, 3.2%, respectively. Generally, the fitting inequality level factors R_w value should be less than 15%. Herein, the value of R_w is 8.86%, indicating the refined results are credible. The existence of Li₃V₂(PO₄)₃ in the prepared material may be on account of reactions between fluorochemicals and H₂O(g) at high temperatures, causing formation of HF. The volatilization of HF results in decrease of fluorine and vanadium contents in solid phase, causing the formation of Li₃V₂(PO₄)₃.

Fig. 2 XRD pattern of prepared material after heat treatment

Figure 3(a) shows the SEM image of LiVPO₄F sample. In the micrograph, particles with the average size of about 2 μm show good crystallinity and uniformity and the grains are even compared with those synthesized by sol-gel method [19]. The morphology and surface state of the prepared LiVPO₄F particles were observed by TEM and HRTEM, as shown in Figs. 3(b) and (c). From the result, it can be seen that a carbon layer of about 2 nm adheres on the surface of the prepared particles. The fast Fourier transform (FFT) (inset in Fig. 3(c)) shows the lattice planes according to (110) and (010) planes of LiVPO₄F crystal. C-S analysis shows that the carbon content is about 3.7%. The carbon derives from the decomposition of excessive oxalic acid dihydrate (H₂C₂O₄·2H₂O), and is helpful to electronic conductivity during electrode process. It is worth taking a moment to flag the fact that H₂C₂O₄ is decomposed to C and CO₂ at low temperature indirectly, which can be described as the following chemical equation:

H₂C₂O₄=CO(g)+CO₂(g)+H₂O(g)                (1)

2CO(g)=C(s)+CO₂(g)                         (2)

When the temperature is lower than 400 °C, CO gas is unstable and decomposes to C(s) and CO₂ gas. But the stability of CO increases with elevating the temperature. So, it is necessary to keep a period of time at lower temperatures to ensure production of carbon.

Fig. 3 SEM (a), TEM (b) and HRTEM (c) images of prepared sample after heat treatment

Figure 4(a) shows the discharge profiles of the LiVPO₄F/C cathode at different rates. In the discharge curves, there is a long plateau at about 4.2 V, which corresponds to Li⁺ insertion in Li_1-xVPO₄F (0≤x≤1). A short plateau at about 4.0 V following the long one indexes with the reduction of V⁴⁺ to V³⁺ in Li_3-xV₂(PO₄)₃ (1≤x≤2). Two short continuous plateaus at 3.7-3.5 V are in accordance with Li⁺ further insertion into Li_3-xV₂(PO₄)₃ (0≤x≤1). From Fig. 4(a), it can be seen that the initial discharge capacity of LiVPO₄F sample is about 151.3 mAh/g at 0.1C (1C equals to 150 mA/g). Judged from the plateaus of the discharge curve, the capacity is about 120 mA·h/g delivered by LiVPO₄F and 30 mA·h/g displayed by Li₃V₂(PO4)₃, which is consistent with the XRD analysis of prepared sample, as shown in Fig. 2. And the utilization of the active material decreases by the increase of rate, performing 150.9, 147.6, 122.8, and 102.5 mA·h/g, at 0.5C, 2C, 5C and 10 C, respectively. The above results appear excellent compared with those synthesized through traditional two-step CTR method [8,12] and sol-gel method [15]. The prepared material possesses a relatively good cycle performance at a high discharge rate and the capacity retention keeps 90.4% after 50 cycles at 10C, as shown in Fig. 4(b).

Fig. 4 Initial discharge curves (a) and cycle performance (b) at various rates

The relatively high capacity and good cycle performance of the material may be attributed to structural stability of LiVPO₄F [14]. Due to the strong electro-negativity of fluorine, the addition of fluorine atoms makes the PO₄ tetrahedron and VO₄F₂ octahedron more stable, thus offering more pourable passageways for Li⁺ insertion/extraction. It is also attributed to the micro-size particles and carbon coating. The small particles have a large contact area between the electrode material and electrolyte solution in an electrochemical cell. As is well known, the coated carbon is a favorable conductive agent, and it can obviously improve the electronic conductivity in the electrochemical process. In addition, Li₃V₂(PO₄)₃ impurity may affect the electro- chemical performance of the prepared materials [21,22].

For further investigating the effects of Li₃V₂(PO4)₃ on the electrochemical performance of the prepared electrode, a cyclic voltammogram (CV) test was conducted at the scan rate of 0.05 mV/s between 3.0 and 4.4 V. From the CV curves in Fig. 5, V³⁺/V⁴⁺ redox peaks for both Li_3-xV₂(PO₄)₃ and Li_1-xVPO₄F are observed clearly in their curves, which are consistent with the initial charge-discharge test. This indicates that two independent electro-active materials, LiVPO₄F and Li₃V₂(PO₄)₃, coexist in the prepared sample. The peaks for Li_1-xVPO₄F are sharper than those for Li_3-xV₂(PO₄)₃, indicating that LiVPO₄F is the main capacity contributor.

Fig. 5 CV curve of prepared sample at scan rate of 0.05 mV/s

4 Conclusions

1) Carbon coated LiVPO₄F, coexisting with Li₃V₂(PO4)₃ and V₂O₃, is prepared by chemical reduction assisted with mechanical activation followed by short-time calcination. The oxalic acid is used as reducing agent as well as carbon source, and the pentavalent vanadium is reduced to trivalent vanadium by oxalic acid.

2) The synthesized crystal particles are adhered by an amorphous carbon layer. The high electronic conductive carbon film is deposited from CO at low temperatures, which is decomposed from the excess oxalic acid.

3) Both LiVPO₄F and Li₃V₂(PO4)₃ act as electro-active materials and contribute to the capacity.

4) The prepared sample possesses good electrochemical performance, which is attributed to the stable structure of LiVPO₄F, micro-size particles and coated carbon layer.

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锂离子电池用LiVPO₄F/C 基正极材料的制备和性能

王接喜，王志兴，沈 力，李新海，郭华军，唐维佳，朱振国

中南大学 冶金科学与工程学院，长沙 410083

摘  要：将H₂C₂O₄·2H₂O, NH₄H₂PO₄, NH₄VO₃ 和 LiF通过球磨反应、烧结，合成了 LiVPO₄F/C 基正极材料。在这个过程中，草酸起还原剂和碳源的作用，利用热重、 X射线衍射、 扫描电镜、透射电镜和碳-硫分析等手段对合成的前驱体和材料进行检测和分析。XRD分析表明，球磨反应后所得到的前驱体为无定形态，而烧结后的材料中除了LiVPO₄F的衍射峰外，还存在Li₃V₂(PO₄)₃和V₂O₃衍射峰。材料颗粒均匀，尺寸约2 μm。透射电镜分析表明，合成的材料颗粒表面包裹着一层约2 nm厚的无定形碳。在截止电压3.0~4.4 V时，合成的材料在0.1C和10C倍率下的放电比容量分别为151.3和102.5 mA·h/g。在10C倍率下循环50次后容量保持率为90.4%。在LiVPO₄F 和Li₃V₂(PO₄)₃ 的循环伏安曲线中可以明显看到V³⁺/V⁴⁺的氧化还原峰。

关键词：锂离子电池；LiVPO₄F；球磨；正极材料；碳包覆

(Edited by Xiang-qun LI)

Foundation item: Project (2011FJ1005) supported by Science and Technology Major Projects of Hunan Province, China; Project supported by Hunan Provincial Innovation Foundation for Postgraduate, China

Corresponding author: Zhi-xing WANG; Tel/Fax: +86-731-88836633; E-mail: zxwang.csu@hotmail.com

DOI: 10.1016/S1003-6326(13)62653-9