Perovskite Fluoride Anode Stabilized via Ligand-Field Engineering for High-Performance Lithium-Ion Batteries

Perovskite fluorides (AMF₃, A = alkali metal ions; M═Fe, Mn, etc.) have emerged as promising high-capacity anode materials for lithium-ion batteries (LIBs). However, their practical application is hindered by an intrinsic coupling of structural degradation and interfacial instability, primarily arising from the dynamic evolution of transition-metal (TM) electronic states during cycling. Conventional extrinsic modification strategies have proven inadequate in addressing this intrinsic limitation. Herein, we propose a universal ligand-field engineering strategy to intrinsically regulate the TM electronic environment, with KFeF₃ employed as a representative model system. Isovalent Mn³⁺ doping effectively tunes the Fe-centered ligand field, suppressing Jahn-Teller distortions and mitigating spin-state fluctuations. The resulting KFe_0.5Mn_0.5F₃@C composite exhibits outstanding cycling stability, demonstrating negligible capacity decay after 500 cycles at 0.5 A g^-1 and an unprecedented capacity retention of 94.65% after 1700 cycles at 1 A g^-1. Theoretical calculations further reveal that Mn doping stabilizes a low-spin Fe state, which mitigates crystal-field distortions while simultaneously facilitating the formation of a robust LiF-rich solid-electrolyte interphase (SEI). This work offers an electronic-state-driven solution to the coupled mechanical-chemical degradation, thereby establishing ligand-field regulation as a fundamental design principle for developing advanced conversion-type electrodes.