Reversible and Fast (De)fluorination of High‐Capacity Cu₂O Cathode: One Step Toward Practically Applicable All‐Solid‐State Fluoride‐Ion Battery

Abstract All‐solid‐state fluoride‐ion batteries (FIBs) are regarded as promising energy storage devices; however, currently proposed cathodes fail to meet the requirements for practical applications in terms of high energy density and high rate capability. Herein, the first use of stable and low‐cost cuprous oxide (Cu₂O) as a cathode material for all‐solid‐state FIBs with reversible and fast (de)fluorination behavior is reported. A phase‐transition reaction mechanism involving Cu⁺/Cu²⁺ redox for charge compensation is confirmed, using the combination of electrochemical methods and X‐ray absorption spectroscopy. The first discharge capacity is approximately 220 mAh g⁻¹, and fast capacity fading is observed in the first five cycles, which is ascribed to partial structural amorphization. Compared with those of simple metal/metal fluoride systems, the material shows a superior rate capability, with a first discharge capacity of 110 mAh g⁻¹ at 1 C. The rate‐determining step and probable structural evolutions are investigated as well. It is believed that the comprehensive investigations of Cu₂O as a cathode material described in this work can lead to an improved understanding of all‐solid‐state FIBs.