First principles study on the effects of Li deintercalation on the structural properties of a novel mixed transition metal cathode material LiNi0.75Mn0.125Fe0.125PO4 and investigation of its electric properties

Abstract

Identifying and developing new cathode materials for Lithium-ion batteries (LIBs) that can achieve the desired balance between performance and environmental requirements is very challenging. Some of the key factors being highly considered in those searches include energy density, voltage, structural stability, cost, and abundance. In this work, ternary doped cathode material LiNi0.75Mn0.125Fe0.125PO4 was synthesized using sol gel method. The structural analysis using Rietveld refinement showed an orthorhombic structure with Pnma space group. The DFT + U calculations were investigated to study the structural and electronic properties during Li deintercalation in Li1-xNi0.75Mn0.125Fe0.125PO4 (0 ≤x ≤ 1). It was found that the material remained structurally stable throughout the deintercalation process. During deintercalation, the Li1-xNi0.75Mn0.125Fe0.125PO4 showed a semiconducting nature for x = 0 and x = 0.25, respectively with Eg = 1.75 eV and 0.57 eV. Then, the material becomes semi-metal for x = 0.5 and x = 0.75 and returns to the semiconducting behavior for x = 1 with Eg = 0.38 eV. The formation energy calculations indicated that the material formed a solid solution throughout the Li concentration range (0 ≤x≤), with values ranging from −17 eV to −1 eV. The voltage decreases from 4.7 eV to 3.4 eV during deintercalation. The Nyquist plot showed the presence of different relaxation processes for LiNi0.75Mn0.125Fe0.125PO4 material. A semiconducting behavior was confirmed. The ac-conductivity as a function of frequency follows the double Jonscher power law. A low dissipation factor was observed, which also decreased with frequency. This shows that LiNi0.75Mn0.125Fe0.125PO4 material could be a good cathode for LIBs.