Structure stability, electronic property and voltage profile of LiFe1−nNnP1−mMmO4 olivine cathode material

First-principles computational studies under density functional theory (DFT) framework were used to investigate the structural stability, conductivity and voltage profile of LiFe1−nNnP1−mMmO4 (N, M = Si or S) electrode materials. It is found that the LiFeP7/8Si1/8O4 system has the most stable structure. After doping, the band gap values of the systems decrease gradually, and LiFe7/8S1/8PO4 system has a minimum band gap of 1.553 eV, attributed to the hybridization of the Fe-d and S-p orbital electrons. The LiFeP7/8S1/8O4 system demonstrates the characteristic of n-type semiconductor, and other doping systems have the feature of p-type semiconductor. Charge density difference maps show that the covalent property of Si–O bond is enhanced in the LiFeP7/8Si1/8O4 system. The average distance of Li and O atoms in the S doping systems increases from 0.21026 to 0.21486 and 0.21129 nm, respectively, indicating that doping broadens significantly the channel of Li ion de-intercalation in LiFe7/8S1/8PO4 and LiFeP7/8S1/8O4. Additionally, the results of lithium intercalation potential imply that the voltages of the doping systems fall into the range of 2.23–2.86 V.