In-situ X-ray diffraction study on the structural evolutions of oxidized fluorophosphates as anode materials for lithium-ion batteries

In this paper, the structural evolutions of different oxidized LiVPO4F-based anode materials formed at 350, 550 and 650 degrees C are studied and compared by in-situ X-ray diffraction technique in 0.0-3.0 V. It is found that oxidized LiVPO4F-based sample obtained at 350 degrees C not only displays stable structure upon heat-treatment but also shows the same lithium ion insertion/extraction mechanism as the one of the pristine LiVPO4F corresponding to the continuous phase transformation between LiVPO4F, Li2VPO4F and Li3VPO4F. Increasing the sintering temperature to 550 degrees C or higher one, the pristine sample is oxidized to Li1-xVPO4F1-yOz, at 550 degrees C and Li1-xVPO4O at 650 degrees C. In-situ X-ray diffraction data demonstrate that the electrochemical reaction of oxidized LiVPO4F-based sample at 550 C with Li continuously results in the formation of Li1.4-xVPO4F1-yOz, Li1.7-xVPO4F1-yOz, Li2.0-xVPO4F1-yOz, Li3.0-xVPO4F1-yOz, Li4.0-xVPO4F1-yOz and Li3PO4 after a lithiation reaction to 0.0 V. However, these delithiation compounds for oxidized LiVPO4F-based sample at 550 C cannot return to the original Li1-xVPO4F1-yOz in the reverse recharge process. For comparison, the insertion/extraction mechanism of oxidized LiVPO4F-based sample at 650 degrees C is associated with the highly reversible process between Li1-xVPO4O and Li4VPO4O in 0.0-3.0 V. (C) 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.