Enabling Long Cycle Life and High Rate Iron Difluoride Based Lithium Batteries by In Situ Cathode Surface Modification.

Metals fluorides (MFs) are potential conversion cathodes to replace commercial intercalation cathodes. However, the application of MFs is impeded by their poor electronic/ionic conductivity and severe decomposition of electrolyte. Here, a composite cathode of FeF and polymer-derived carbon (FeF @PDC) with excellent cycling performance is reported. The composite cathode is composed of nanorod-shaped FeF embedded in PDC matrix with excellent mechanical strength and electronic/ionic conductivity. The FeF @PDC enables a reversible capacity of 500 mAh g with a record long cycle lifetime of 1900 cycles. Remarkably, the FeF @PDC can be cycled at a record rate of 60 C with a reversible capacity of 107 mAh g after 500 cycles. Advanced electron microscopy reveals that the in situ formation of stable Fe O layers on the surface of FeF prevents the electrolyte decomposition and leaching of iron (Fe), thus enhancing the cyclability. The results provide a new understanding to FeF electrochemistry, and a strategy to radically improve the electrochemical performance of FeF cathode for lithium-ion battery applications.