Tuning a small electron polaron in FePO₄ by P-site or O-site doping based on DFT+U and KMC simulation

Due to the existence of a small polaron, the intrinsic electronic conductivity of olivine-structured LiFePO₄ is quite low, limiting its performance as a cathode material for lithium-ion batteries (LIBs). Previous studies have mainly focused on improving intrinsic conductivity through Fe-site doping while P-site or O-site doping has rarely been reported. Herein, we studied the formation and dynamics of the small electron polaron in FeP_1-XO₄ and FePO_4-Z by employing the density functional theory with the on-site Hubbard correction terms (DFT+U) and Kinetic Monte Carlo (KMC) simulation, where X and Z indicate the doping elements (X = S, Se, As, Si, V; Z = S, F, Cl), and α and β indicate the light doping at the P position (α = 0.0625) and O position (β = 0.015625), respectively. We confirmed the small electron polaron formation in pristine FePO₄ and its doped systems, and the polaron hopping rates for all systems were calculated according to the Marcus-Emin-Holstein-Austin-Mott (MEHAM) theory. We found that the hopping process is adiabatic for most cases with the defects breaking the original symmetry. Based on the KMC simulation results, we found that the doping of S at the P site changes the polaron's motion mode, which is expected to increase the mobility and intrinsic electronic conductivity. This study attempts to provide theoretical guidance to improve the electronic conductivity of LiFePO₄-like cathode materials with better rate performance.