Highly Reversible Conversion-Type CoSn₂ Cathode for Fluoride-Ion Batteries

An all-solid-state fluoride-ion battery (FIB) is one of the promising candidates for the next-generation battery owing to its high energy density and high safety. For the practical application of FIBs, it is an urgent task to operate FIBs at lower temperatures. However, there are still two major difficulties in conventional conversion-type pure metal cathodes: low F^- ion conductivities and poor cycle stabilities. Here, the conversion-type Sn-based intermetallic alloy is proposed as a new cathode that can overcome the above issues. The present CoSn₂ cathode retains the discharge capacity of 229 mAh g^-1 after 250 cycles, even at 60 °C. CoSn₂ is decomposed into CoF₂ and SnF₂ nanocrystals in the charging process, and the nanoscale network structure of SnF₂ provides the fast F^- ion conduction path throughout the cathode, facilitating the battery operation at lower temperatures. Moreover, the formed CoF₂ and SnF₂ phases are merged into the original CoSn₂ phase in the discharging process, leading to a highly reversible redox reaction and the high cycle stability of CoSn₂. These findings should pave the way to enhance the performance of all-solid-state FIBs at lower temperatures.