Effects of oxidation on structure and performance of LiVPO4F as cathode material for lithium-ion batteries

In this paper, a series of LiVPO4F-based samples are prepared through sintering LiVPO4F at different temperatures under air. Experimental results show that the pristine sample is oxidized to a new compound Li(1-x)VPO(4)F(1-y)Oz (0 <x < 0.2, 0.9 <y < 1, 0.9 <z <1) with similar structure of LiVPO4O under air at 550 C or higher one. In-situ X-ray diffraction patterns indicate that the original material LiVPO4F undergoes two two-phase structural evolutions upon Li+ electrochemical extraction at average operating potentials at 4.26 and 4.30 V, corresponding to the continuous transformation of LiVPO4F -> Li0.72VPO4F -> VPO4F in the first charge process. In the reverse discharge process, there is only one twophase structural transition VPO4F -> VPO4F without the appearance of the intermediate phase Li0.72VPO4F on Li+ insertion reaction at 4.18 V. Therefore, the extraction/insertion process of LiVPO4F is an asymmetrical phase transformation. When the sintering temperature is raised to 550 degrees C, Li(1-x)VPO(4)F(1-y)Oz 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 under air. However, Li(1-x)VPO(4)F(1-y)Oz has a very stable structure during the whole process of galvanostatic charge/discharge cycles as confirmed by in-situ X-ray diffraction technique. (C) 2013 Elsevier B.V. All rights reserved.