Chemical reaction characteristics, structural transformation and electrochemical performances of new cathode LiVPO 4 F/C synthesized by a novel one-step method for lithium ion batteries.

A new cathode LiVPO 4 F/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 V 2 O 5 is successfully reduced to V 3+ . The reaction thermodynamics indicates that LiVPO 4 F/C is formed when the sintering temperature is higher than 650 °C, while the accompanying impurity phase Li 3 V 2 (PO 4 ) 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 LiVPO 4 F/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 Li 3 V 2 (PO 4 ) 3 at a high temperature of 850 °C is demonstrated directly, which is mainly due to the volatilization of VF 3 . The electrochemical performances of the cathode are related to the crystallite content of LiVPO 4 F/C and Li 3 V 2 (PO 4 ) 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.