Density functional theory study of lithium diffusion at the interface between olivine-type LiFePO₄ and LiMnPO₄

Coating LiMnPO₄ with a thin layer of LiFePO₄ shows a better electrochemical performance than the pure LiFePO₄ and LiMnPO₄, thus it is critical to understand Li diffusion at their interfaces to improve the performance of electrode materials. Li diffusion at the (1 0 0)${}_{\text{LiFeP}{\text{O}}_4}$//(1 0 0)${}_{\text{LiMnP}{\text{O}}_4}$, (0 1 0)${}_{\text{LiFeP}{\text{O}}_4}$//(0 1 0)${}_{\text{LiMnP}{\text{O}}_4}$, and (0 0 1)${}_{\text{LiFeP}{\text{O}}_4}$//(0 0 1)${}_{\text{LiMnP}{\text{O}}_4}$ interfaces between LiFePO₄ and LiMnPO₄ was investigated using density functional theory. The calculated diffusion energy barriers are 0.55 eV for Li to diffuse along the (0 0 1) interface, 0.44 and 0.49 eV for the Li diffusion inside the LiMnPO₄ and along the (1 0 0) interface, respectively. When Li diffuses from the LiFePO₄ to LiMnPO₄ by passing through the (0 1 0) interfaces, the diffusion barriers are 0.45 and 0.60 eV for the Li diffusions in both sides. The diffusion barriers for Li to diffuse in LiMnPO₄ near the interfaces decrease compared with those in the pure LiMnPO₄. The calculated diffusion coefficient of Li along the (1 0 0) interface is in the range of 3.65  ×  10⁻¹¹–5.28  ×  10⁻¹² cm² s⁻¹, which is larger than that in the pure LiMnPO₄ with a value of 7.5  ×  10⁻¹⁴ cm² s⁻¹. Therefore, the charging/discharging rate performance of the LiMnPO₄ can be improved by surface coating with the LiFePO₄. (paper)