Achieving High Stability and Capacity in Micron‐Sized Conversion‐Type Iron Fluoride Li‐Metal Batteries

Abstract Iron fluoride, a conversion‐type cathode material with high energy density and low‐cost iron, holds promise for Li‐ion batteries but faces challenges in synthesis, conductivity, and cycling stability. This study addresses these issues by synthesizing micron‐sized iron‐fluoride using a simple solid‐state synthesis. Despite a large particle size, a high capacity of 571 mAh g⁻¹ is achieved, which is attributed to the unique surface and internal pores within the iron‐fluoride particles, which provided a large surface area. This is the first study to demonstrate the feasibility of using large iron fluoride particles to enhance the energy density of the electrode and achieve an iron fluoride full cell with high capacity. Also, the cause of the capacity fading is investigated. Electrode delamination from the current collector, which is the main cause of capacity fading in early cycles, is resolved using a carbon‐coated aluminum (C/Al) current collector. Moreover, iron (Fe) dissolution and the deposition of dissolved Fe on the Li metal also contributed significantly to the degradation. Localized high‐concentration electrolytes (LHCEs) suppress iron dissolution and Li dendrite growth, resulting in long‐cycle stability for 300 cycles. This study provides insights into the further development of conversion‐type metal fluorides across various compositions.