Fluoride cathode materials that undergo conversion reactions such as FeF3 have attracted increasing interest owing to their high energy density. However, the ability of FeF3 to maintain a high capacity over repeated cycles and the decisive factor dominating conversion reactions have yet to be elucidated. By optimizing the choice of lithium salts and electrolyte solutions, we are able to achieve good capacity retention for an FeF3-based cathode, that is, 4.5-fold enhancement compared with a model electrode (372 mAh/g after 30 cycles). The relationship between the capacity and the particle size of the Fe nanoparticles after discharge is well explained by the order parameter description. These findings provide exciting guidelines for achieving higher capacities for FeF3 batteries.
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