Contribution of Ti-Doping to the Cyclic Stability of LiFe_0.6Mn_0.4PO₄/C

Abstract

Li(Fe 0.6 Mn 0.4 ) 1−x Ti x PO 4 /C cathode materials, with x values of 0, 0.01, 0.02, 0.03, and 0.04, were fabricated through a dual-stage synthesis process, incorporating both coprecipitation and high-temperature solid-phase techniques. The composition, microstructure, and surface morphology of these materials were thoroughly characterized using a suite of analytical techniques. These analyses confirmed the successful doping of Ti ions into the olivine lattice, resulting in a decrease in unit cell volume and the formation of an amorphous carbon layer on the particles' surfaces, which also improved particle dispersion. The electrochemical performance of the Li(Fe 0.6 Mn 0.4 ) 1−x Ti x PO 4 /C samples was assessed using techniques including constant current charge−discharge testing, cyclic voltammetry, and electrochemical impedance spectroscopy. The findings showed that Ti-doping markedly diminishes potential polarization in these materials and the strong Ti−O coordination suppresses the Jahn−Teller effect of Mn 3+ , effectively enhancing the stability and lithium-ion diffusion rate of the material. Additionally, density functional theory (DFT) calculations were conducted to assess the impact of Ti-doping on LFMP. The findings reveal that Ti-doping reduces the bandgap of the material and increases the bond length of Li−O, thereby further confirming that Ti-doping can enhance electronic conductivity. Among them, the Li(Fe 0.6 Mn 0.4 ) 1−x Ti x PO 4 /C-3%Ti cathode material exhibited the best electrochemical performance. The optimized sample demonstrated a specific discharge capacity of 163.53 mAh•g −1 at 0.1C, accompanied by an initial coulombic efficiency of 93.18%. At 1C, it provided a capacity of 140.59 mAh•g −1 , sustaining a capacity retention of 93.58% after 500 cycles, and delivered a discharge capacity of 94.08 mAh•g −1 at 5C.