Accelerated lithium ions diffusion at the interface between LiFePO4 electrode and electrolyte by surface-nitride treatment

Abstract

Improving the rate performance of lithium-ion batteries is important for the widespread utilization of electric vehicles and energy grids. Because the rate-determining step for cathode materials with a two-phase reaction is nucleation reaction at the material surface, surface modification is a promising approach for achieving this goal. However, the cause of this improvement in the reaction rate at the interface between the surface-modified cathode and the electrolyte is not clearly understood. In this study, we prepared a surface-nitrided LiFePO4 thin film and investigated its electrochemical properties. In addition, we examined its surface structure using surface-sensitive X-ray absorption spectroscopy measurements and first-principles calculations, and discussed the correlation between the rate performance and the interfacial reaction. The experiments revealed the formation of a new energy level and the increase of the FeO bond distance at the surface of LiFePO4 due to nitrogen doping. The electronic and local structural changes accelerated lithium ion diffusion at the interface between surface-nitrided LiFePO4 and the electrolyte, improving the rate performance.