Effects of S doping and S/N co-doping on electronic structure and ion diffusion of LiFePO₄

First-principle calculations were performed to investigate the electronic structure and dynamics properties of S single doped and S/N co-doped LiFePO₄. The un-doped LiFePO₄ was a semiconductor with large band gap, while doping made the band gap of LiFePO₄ become smaller. With the increase of S-doping concentration, the band gap decreased gradually and sulfur atoms formed covalent bond with its adjacent iron atoms and the covalency of Fe-S bond was enhanced. The configuration of the Li₈Fe₈P₈O₂₉S₃ had the least diffusion energy barrier (0.31049 eV), which improved its ion transport properties. For sulfur and nitrogen co-doped LiFePO₄ systems, the configuration of Li₈Fe₈P₈O₂₉NS₂ had the least band gaps (0.095 eV), showing the best electronic conductive properties. And co-doping could also improve the diffusion of lithium ions.