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 °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 °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 °C or higher one, the pristine sample is oxidized to Li1−xVPO4F1−yOz at 550 °C and Li1−xVPO4O at 650 °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 °C is associated with the highly reversible process between Li1−xVPO4O and Li4VPO4O in 0.0–3.0 V.