Mitigating Jahn–Teller Distortion in LiMn _0.4 Fe _0.6 PO ₄ via Hafnium Doping for Enhanced High‐Voltage Cycling Stability

ABSTRACT Lithium manganese iron phosphate (LiMn 0.4 Fe 0.6 PO 4 , LMFP) offers a significant improvement in operating voltage and energy density compared to Li (lithium) iron phosphate (LiFePO 4 , LFP), garnering considerable research attention in recent years. However, LMFP suffers from low electronic conductivity and sluggish ion diffusion kinetics, resulting in poor performance under high current densities. Furthermore, the Jahn–Teller effect associated with Mn 3+ in LMFP leads to Mn (manganese) dissolution during electrochemical reactions, which compromises structural stability and leads to suboptimal long‐term cycling stability. In this study, a simple ball milling‐sintering method was employed to successfully incorporate Hf 4+ (Hafnium) into the transition metal sites of LMFP. The higher bond energy of Hf–O compared to Mn–O enables the construction of a stable Mn–O framework through Hf doping, thereby stabilizing the lattice structure, reducing Mn dissolution, and significantly enhancing the long‐term cycling performance of the material. Furthermore, Hf 4+ doping improves the redox reaction kinetics of the material, increasing both the lithium‐ion diffusion rate and electronic conductivity. Among the tested materials, LMFP‐3%Hf exhibited the most outstanding cycling stability (with a capacity retention rate of 89.7% after 400 cycles at 1C) and rate capability (delivering a discharge specific capacity of 70 mAh g −1 at 10C).