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

摘要

In this paper, the structural evolutions of different oxidized LiVPO_4F-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 LiVPO_4F-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 LiVPO_4F corresponding to the continuous phase transformation between LiVPO_4F, Li_2VPO_4F and Li_3VPO_4F. Increasing the sintering temperature to 550 °C or higher one, the pristine sample is oxidized to Li_(1-x)VPO_4F_(1-y)O_z at 550 °C and Li_(1-x)VPO_4O at 650 °C. In-situ X-ray diffraction data demonstrate that the electrochemical reaction of oxidized LiVPO_4F-based sample at 550 °C with Li continuously results in the formation of Li_(1.4-x)VPO_4F_(1-y)O_z, Li_(1.7-x)VPO_4F_(1-y)O_z, Li_(2.0-x)VPO_4F_(1-y)O_z, Li_(3.0-x)VPO_4F_(1-y)O_z, Li_(4.0-x)VPO_4F_(1-y)O_z. and Li_3PO_4 after a lithiation reaction to 0.0 V. However, these dclithiation compounds for oxidized LiVPO_4F-based sample at 550 °C cannot return to the original Li_(1-x)VPO_4F_(1-y)O_z, in the reverse recharge process. For comparison, the insertion/extraction mechanism of oxidized LiVPO_4F-based sample at 650 °C is associated with the highly reversible process between Li_(1-x)VPO_4O and Li_4VPO_4O in 0.0-3.0 V.