Chemical reaction characteristics, structural transformation and electrochemical performances of new cathode LiVPO4F/C synthesized by a novel one-step method for lithium ion batteries

Abstract

A new cathode LiVPO₄F/C with a high working voltage of around 4.2 V was synthesized by a novel one-step method. The color of the solution turns green, which implies that V₂O₅ is successfully reduced to V³⁺. The reaction thermodynamics indicates that LiVPO₄F/C is formed when the sintering temperature is higher than 650 °C, while the accompanying impurity phase Li₃V₂(PO₄)₃/C is also generated. The reaction kinetics proves that the reaction is third order and the activated energy is 208.9 kJ mol⁻¹. X-ray photoelectron spectra imply that the components of LiVPO₄F/C prepared at 800 °C (LVPF800) are in their appropriate valence. LVPF800 is composed of micron secondary particles aggregating from nano subglobose. The structural transformation shows that the V : P : F ratio in LVPF800 is close to 1 : 1 : 1. The reason behind generation of impurity Li₃V₂(PO₄)₃ at a high temperature of 850 °C is demonstrated directly, which is mainly due to the volatilization of VF₃. The electrochemical performances of the cathode are related to the crystallite content of LiVPO₄F/C and Li₃V₂(PO₄)₃/C. The specific capacities at 0.2 and 5C of LVPF800 are as high as 139.3 and 116.5 mA h g⁻¹. Electrochemical analysis reveals that LVPF800 possesses an excellent reversibility in the extraction and insertion process and minimum charge transfer resistance.