First principles study of the electronic structure and Li-ion diffusion properties of co-doped LIFex-1MxPyNy-1O4 (M=Co/Mn, NS/Si) Li-ion battery cathode materials

Abstract

In this work, a first-principles method based on density functional theory was systematically employed to investigate the stability, electronic properties, lithium-ion migration rates, and capacity-voltage curves of the LiFe_x-1M_xP_yN_y-1O₄ (M = Co/Mn, NS/Si) system. The results indicate that the lattice constants of the LiFe_x-1M_xP_yN_y-1O₄ (M = Co/Mn, NS/Si) system show little variation, and the system exhibits low formation and binding energies. Among the investigated systems, LFP-Mn/S demonstrates the best structural and thermodynamic stability. The bandgap of the doped systems decreases, leading to enhanced electronic conductivity. The LiFe_0.875Co_0.125P_0.875Si_0.125O₄ and LiFe_0.875Mn_0.125P_0.875Si_0.125O₄ systems remain semiconductors, while the LiFe_0.875Co_0.125P_0.875S_0.125O₄ and LiFe_0.875Mn_0.125P_0.875S_0.125O₄ systems exhibit semi-metallic properties due to the introduction of sulfur. Differential charge density calculations reveal changes in the covalent bond strength of the doped systems, with the introduction of Si and S respectively increasing and decreasing the covalency of their bonds with surrounding oxygen atoms. Additionally, doping reduces the Li-ion diffusion energy barriers, with the LiFe_0.875Co_0.125P_0.875Si_0.125O₄ system exhibiting the lowest migration energy barrier. The Li-ion diffusion rate is four orders of magnitude faster than that of the intrinsic system. This is attributed to changes in the average lengths of Li–O, Co–O, and Fe–O bonds. Finally, doping also alters the de-lithiation voltage, with values ranging from 2.69 V to 3.65 V for the doped systems, and the LiFe_0.875Co_0.125P_0.875Si_0.125O₄ system shows the highest complete de-lithiation voltage of 3.65 V. The overall performance improvements of the doped system have significant implications for enhancing the performance of Li-ion batteries.