Pseudocapacitance-Enhanced Storage Kinetics of 3D Anhydrous Iron (III) Fluoride as a Cathode for Li/Na-Ion Batteries

Transition metal fluoride (TMF) conversion cathodes, with high energy density, are recognized as promising candidates for next-generation high-energy Li/Na-ion batteries (LIBs/SIBs). Unfortunately, the poor electronic conductivity and detrimental active material dissolution of TMFs seriously limit the performance of TMF-LIBs/SIBs. A variety of FeF₃-based composites are designed to improve their electrochemical characteristics. However, the storage mechanism of the conversion-type cathode for Li⁺ and Na⁺ co-storage is still unclear. Here, the storage mechanism of honeycomb iron (III) fluoride and carbon (FeF₃@C) as a general cathode for LIBs/SIBs is analyzed by kinetics. In addition, the FeF₃@C cathode shows high electrochemical performance in a full-cell system. The results show that the honeycomb FeF₃@C shows excellent long-term cycle stability in LIBs (208.3 mA h g^-1 at 1.0 C after 100 cycles with a capacity retention of 98.1%). As a cathode of SIBs, the rate performance is unexpectedly stable. The kinetic analysis reveals that the FeF₃@C cathode exhibit distinct ion-dependent charge storage mechanisms and exceptional long-durability cyclic performance in the storage of Li⁺/Na⁺, benefiting from the synergistic contribution of pseudocapacitive and reversible redox behavior. The work deepens the understanding of the conversion-type cathode in Li⁺/Na⁺ storage.