J. Cent. South Univ. Technol. (2007)03-0340-04

DOI: 10.1007/s11771-007-0067-3               

  Synthesis and characterization of triclinic structural LiVPO₄F as possible 4.2 V cathode materials for lithium ion batteries

ZHONG Sheng-kui(钟胜奎)^1,2, YIN Zhou-lan(尹周澜)¹, WANG Zhi-xing(王志兴)³, CHEN Qi-yuan(陈启元)¹

(1. School of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China;

2. Department of Materials and Chemistry, Guilin University of Technology, Guilin 541004, China;

3. School of Metallurgical Science and Engineering, Central South University, Changsha 410083, China)

Abstract: A potential 4.2 V cathode material LiVPO₄F for lithium batteries was prepared by two-step reaction method based on a carbon-thermal reduction (CTR) process. Firstly, V₂O₅, NH₄H₂PO₄ and acetylene black are reacted under an Ar atmosphere to yield VPO₄. The transition-metal reduction is facilitated by the CTR based on C→CO transition. These CTR conditions favor stabilization of the vanadium as V³⁺ as well as leaving residual carbon, which is useful in the subsequent electrode processing. Secondly, VPO₄ reacts with LiF to yield LiVPO₄F product. The property of the LiVPO₄F was investigated by X-ray diffractometry (XRD), scanning electron microscopy (SEM) and electrochemical measurement. XRD studies show that LiVPO₄F synthesized has triclinic structure(space group ), isostructural with the naturally occurring mineral tavorite, LiFePO₄·OH. SEM image exhibits that the particle size is about 2 ?m together with homogenous distribution. Electrochemical test shows that the initial discharge capacity of LiVPO₄F powder is 119 mA?h/g at the rate of 0.2C with an average discharge voltage of 4.2V (vs Li/Li⁺), and the capacity retains 89 mA?h/g after 30 cycles.

Key words: lithium ion batteries; cathode material; LiVPO₄F; carbon-thermal reduction method

1 Introduction

Currently, LiCoO₂ is the most widely used cathode materials for lithium-ion batteries. However it is relatively expensive, especially for large-scale applications such as backup power systems and hybrid electric vehicles^[1]. Many researchers are working to find a cheap, effective replacement for LiCoO₂^[2-7]. Since the demonstration of reversible electrochemical lithium insertion-extraction for LiFePO₄ in 1997^[8], lithium transition metal phosphates have attracted much great interest as promising new cathode materials for lithium-ion batteries^[9-13].

Recently, BARKER et al^[14] extended their investigations to include the electrochemical evaluation of a series of fluorophosphate materials represented by the general formula LiMPO₄F, where M represents a 3d transition metal. Among these materials, LiVPO₄F is the most promising and attractive one because of its relatively high capacity with a plateau at around 4.2 V. In this study, we synthesized LiVPO₄F by two-step reaction method based on a carbothermal reduction(CTR) process. The cathode material LiVPO₄F synthesized has better electrochemical performance than that reported in Ref.[15]. However, improvements must be made for the long-term cycleability in future.

2 Experimental

Appropriate amounts of V₂O₅, NH₄H₂PO₄ and acetylene black were initially ground in mortar and then were thoroughly mixed by ball milling for 2 h for further use. The LiVPO₄F material was prepared by a two-step reaction method based on a CTR method. In the first step of CTR, the mixed precursor was first decomposed at 300 ℃ for 4 h to disperse NH₃ and H₂O, reground, and then calcined at 750 ℃ for 6 h to obtain VPO₄. In the second step, appropriate amounts of VPO₄ and LiF were thoroughly mixed by ball milling for 2 h, and calcined at 750 ℃ for 30 min to yield LiVPO₄F material. To avoid the oxidation of vanadium, each step of the process was carried out under flowing Ar atmosphere. In order to complete vanadium reduction and to leave residual carbon in the product, a 25% mass excess of carbon was used on the stoichiometric conditions based on Reaction (1). LiVPO₄F preparative reaction scheme may be summarized as follows:

V₂O₅+2NH₄H₂PO₄+2C→

2VPO₄+2NH₃+3H₂O+2CO        (1)

LiF+VPO₄→LiVPO₄F            (2)

The powder X-ray diffraction (XRD, Rint-2000, Rigaku) measurement using Cu K_α radiation was employed to identify the crystalline phase and lattice parameters of the synthesized materials, recorded at room temperature. The particle size and morphology of LiVPO₄F powders were observed by scanning electron microscope (JEOL, JSM-5600LV) with an accelerating voltage of 20 kV.

The electrochemical characterizations were performed using CR2025 coin-type cell. For positive electrode fabrication, the prepared powders were mixed with 10%(mass fraction) carbon black and 10%(mass fraction) polyvinylidene fluoride in N-methyl pyrrolidinone until slurry was obtained. And then, the blended slurries were pasted onto an aluminum current collector, and the electrode was dried at 120 ℃ for    10 h in vacuum. The coin cell consisted of the positive electrode and lithium foil negative electrode separated by a porous polypropylene film, and 1 mol/L LiPF₆ in EC+EMC+DMC (1:1:1 in volume) as the electrolyte. The assembly of the cells was carried out in a dry Ar-filled glove box. The cells were charged and discharged over a voltage range of 3.0-4.4 V versus Li/Li⁺ electrode at room temperature. Cyclic voltammograms were tested at a scanning rate of     0.1 mV/s in the voltage range of 3.0-4.9 V.

3 Results and discussion

Fig.1 shows the XRD pattern of VPO₄ powders. All fundamental peaks can be indexed to the orthorhombic structure with space group Cmcm (JCPDS card No.34-1336). The crystallographic parameters of VPO₄ powders: a=0.523 8 nm, b=0.779 0 nm , c=0.628 8 nm, and cell volume of 0.256 6 nm³, are close to literature data of VPO₄ that made by conventional solid-state synthesis methods^[16].

Fig.1 XRD pattern of VPO₄ powder

Fig.2 shows the XRD pattern collected for the prepared LiVPO₄F powders. It’s evident that all fundamental peaks can be indexed to the triclinic structure with space group (JCPDS card No.42-1412). Excess carbon (1.83%, determined by carbon-sulphur analyzer) left in LiVPO₄F powders is not detected in the XRD pattern, indicating that excess carbon exists in amorphous form. As expected, no diffraction peaks are identifiable as LiF, which confirms complete LiF incorporation. The incorporation reaction of the orthorhombic VPO₄ with LiF results in a loss of crystal symmetry so that the LiVPO₄F product phase adopts a triclinic structure. The following crystallographic parameters of the prepared LiVPO₄F powders, a=0.514 9 nm, b=0.529 2 nm, c=0.744 5 nm, α=67.397?, β=67.518?, γ=82.008?, and cell volume of 0.173 0 nm³, are in good accordance with literature sample^[17]. The framework structure of LiVPO₄F comprises a three-dimensional framework built up from [PO₄] tetrahedron and [VO₄F₂] octahedron with the oxygen atoms shared between PO₄ and VO₄F₂. Within this framework structure, there exist two crystallographic positions, within which the lithium ions are statistically distributed.

The SEM image of VPO₄ powders is shown in Fig.3. The irregular and slight agglomeration morphology of VPO₄ is clearly seen in the image.

Fig.2 XRD pattern of LiVPO₄F

Fig.3 SEM image of VPO₄ powder

Fig.4 shows the SEM image of LiVPO₄F powder. The sphere-like particles are observed, and the particle size is about 2 ?m together with homogenous distribution. The notable morphology difference between VPO₄ and LiVPO₄F is that the surfaces of LiVPO₄F are smoother and compacter than those of VPO₄. The composite product morphology can also be seen from the SEM micrograph, and the floc-like particles enclosing LiVPO₄F maybe excess carbon left.

Fig.4 SEM image of LiVPO₄F powder

The first charge-discharge curves of LiVPO₄F cycled between 3.0 V and 4.4 V at the rate of 0.2C are shown in Fig.5. The initial charge and discharge capacities are about 140 mA?h/g and 119 mA?h/g, respectively. It is clear that the columbic efficiency of the initial charge-discharge cycle is only about 85%. The charge-discharge curves of LiVPO₄F powders exhibit the presence of a small inflection in the charge curve, reflecting two energetically inequivalent reactions. This phenomenon presumably corresponds to accessing of  the lithium ions located in the two known crystallographic sites within the LiVPO₄F structure.

Fig.5 First charge-discharge curves of LiVPO₄F at rate of 0.2C

The electrochemical cycling performance of LiVPO₄F compound is evaluated in Li/LiVPO₄F cell configuration in the voltage range of 3.0-4.4 V at room temperature. Fig.6 shows the cyclic charge/discharge profiles for LiVPO₄F cathode materials at 0.2C rate. As seen in Fig.6, the initial discharge capacity of LiVPO₄F is about 119 mA?h/g, and the discharge capacity drops to about 89 mA?h/g after 30 cycles. The capacity loss is about 25.2% after 30 cycles. From above results, although LiVPO₄F synthesized has better cycling performance than the result reported by Ref.[15], the capacity loss should be reduced.

Fig.6 Electrochemical cycling performance of LiVPO₄F

The cyclic voltammetry (CV) for the LiVPO₄F electrode in the first cycle at a scanning rate of 0.1 mV/s is shown in Fig.7. The cyclic voltammetry profile indicates the oxidation/reduction potential, at which lithium ion is extracted from or inserted into the lattice. As shown in Fig.7, the electrode of LiVPO₄F exhibits oxidation and reduction peak at 4.38 and 4.18 V, respectively. The appearance of a small oxidation peak located at 4.27 V presumably corresponds to accessing of Li ions located in the two known crystallographic sites within the LiVPO₄F structure, which is in good agreement with the first charge-discharge results.

Fig.7 CV profile of LiVPO₄F electrode in first cycle

4 Conclusions

LiVPO₄F powder has been prepared by a two-step reaction method based on a carbon-thermal reduction(CTR) process under Ar atmosphere. X-ray diffraction (XRD) patterns show that LiVPO₄F has triclinic structure. Scanning electron microscopic (SEM) images show that the particle size is about 2 ?m. A initial charge and discharge capacity is about 140 mA·h/g and 119 mA·h/g, respectively. In summary, the demonstrated performance of the synthesized LiVPO₄F material offers some properties favorable for commercial application.

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(Edited by YANG Bing) 

Foundation item: Project(50302016) supported by the National Natural Science Foundation of China

Received date: 2006-07-24; Accepted date: 2006-09-27

Corresponding author: ZHONG Sheng-kui, PhD; Tel: +86-731-8836633; E-mail: zskui74@163.com