First-principles investigation of the electronic and Li-ion diffusion properties of LiFePO4 by sulfur surface modification

We present a first-principles calculation for the electronic and Li-ion diffusion properties of the LiFePO4 (010) surface modified by sulfur. The calculated formation energy indicates that the sulfur adsorption on the (010) surface of the LiFePO4 is energetically favored. Sulfur is found to form Fe-S bond with iron. A much narrower band gap (0.67 eV) of the sulfur surface-modified LiFePO4 [S-LiFePO4 (010)] is obtained, indicating the better electronic conductive properties. By the nudged elastic band method, our calculations show that the activation energy of Li ions diffusion along the one-dimensional channel on the surface can be effectively reduced by sulfur surface modification. In addition, the surface diffusion coefficient of S-LiFePO4 (010) is estimated to be about 10−11 (cm2/s) at room temperature, which implies that sulfur modification will give rise to a higher Li ion carrier mobility and enhanced electrochemical performance.