Preparation And Characterization Of LiVPO₄F As Cathode Material For Lithium-ion Batteries

In this paper, we adopted a carbon-thermal reduction method to prepare LiVPO4F, and explored its electrochemical performance and the effects of oxidation and exposure to H2O on the structure and electrochemical behaviors of LiVPO4F as cathode material for lithium-ion batteries. The detailed contents are as follows:Firstly, electrochemical tests show that the initial reversible capacity of as-prepared LiVPO4F is112mAh g-1. After40cycles, the reversible capacity can be kept at95.7mAh g-1with a coulombic efficiency of96.8%, which indicates the good reversibility of LiVPO4F. In-situ X-ray diffraction (XRD) confirms that the lithiation/delithiation process of LiVPO4F is associated with an asymmetric structural evolution. Galvanostatic intermittent titration technique (GITT) reveals that the Li+diffusion coefficiet is about10-9～10-13cm2s-1, and the curve of DLi+～V exhibits a twist "W" shape with the minimum value of10-13cm2s-1at4.25V.Secondly, a series of LiVPO4F-based samples were prepared through sintering LiVPO4F at different temperatures under air. LiVPO4F has no changes when the calcining temperature is350℃, but it is oxidized to a new compound Li1-xVPO4F1-yOz(0<x<0.2,0.9<y<1,0.9<z<1) with similar structure of LiVPO4O at550℃or higher one. However, Li1-xVPO4F1-yOz exhibits extremely poor electrochemical performance, which is attributed to the volatilization loss of lithium and the replacement of fluorine by oxygen in the structure during the sintering process. Besides, Li1-xVPO4F1-yOz displays stable structure during cycles as confirmed by in-situ XRD.Lastly, an exposure of LiVPO4F to H2O at room temperature and80℃for different times was undertaken. It is found the effect of H2O on LiVPO4F at room temperature is slight. In contrast, proton/lithium exchange appears at80℃, which results in the formation of a cohesionless surface for LiVPO4F. Electrochemical tests show that LiVPO4F after being soaked in H2O for15days exhibits higher initial reversible capacity. However, it deteriorates rapidly in the following cycles due to the air-sensitive surface, which induces the formation of Li3PO4film on the surface, leading to the higher charge transfer impedance and the degradation of electrochemical performance.