Epitaxial Growth of FeF2 on FeF3 by Metal–Organic Framework Etching-Fluorination to Stabilize Low-Temperature Li-Fluoride Conversion Batteries

Conventional intercalation-type cathodes for lithium batteries are plagued by rapid capacity decay and unsatisfactory cycling lifespans when operating at subzero temperatures. Conversion-type iron fluoride seems to compensate for these setbacks due to its dramatic theoretical capacity via a three-electron transfer mechanism, but suffers from an irreversible solid–solid phase transition with the collapse of the charge-transporting tunnel and insulating LiF precipitation over the cathode–electrolyte interphase (CEI) layer. Herein, we propose a metal–organic framework in situ etching-fluorination strategy to construct a porous micron-brick monolithic grain cathode with rich heterojunctions of hexagonal tungsten bronze (HTB) FeF3 and rutile FeF2 with an epitaxial intergrowth mode. Owing to the optimized configuration with a self-healing built-in interfacial electric field, open diffusional channels, as well as topotactic conversion and catalyzing functions of FeF2 on Li–F splitting, this heterostructure fluoride enables a high energy density (1114 Wh kg–1 at 215 W kg–1) and high-rate performance (337 mAh g–1 at 1000 mA g–1) at room temperature. These effects also endow the cathode with outstanding low-temperature performance with an average capacity of 170 mAh g–1 for 100 cycles and a discharge capacity of 50 mAh under a 6-layer pouch cell configuration at −20 °C.