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

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 p $\overline 1$), isostructural with the naturally occurring mineral tavorite, LiFePO₄·OH. SEM image exhibits that the particle size is about 2 μm 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.