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 Fe[sbnd]O 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.