Reversible potassium storage in ultrafine CF : A superior cathode material for potassium batteries and its mechanism

Current studies of cathodes for potassium batteries (PBs) mainly focus on the intercalation-type materials. The conversion-type materials that possess much higher theoretical capacities are rarely discussed in previous literatures. In this work, carbon fluoride (CFx) is reported as a high capacity conversion-type cathode for PBs for the first time. The material delivers a remarkable discharge capacity of >250 mAh g−1 with mid-voltage of 2.6 V at 20 mA g−1. Moreover, a highly reversible capacity of around 95 mAh g−1 is achieved at 125 mA g−1 and maintained for 900 cycles, demonstrating its excellent cycling stability. The mechanism of this highly reversible conversion reaction is further investigated by nuclear magnetic resonance spectra, X-ray diffraction, and transmission electron microscopy studies. According to the analyses, the C-F bond in the cycled material is different from that in the pristine state, which presents relatively higher reversibility. This finding offers important insights for further improving the performance of the CFx. This work not only demonstrates the CFx as a high performance cathode for PBs, but also paves a new avenue of exploring conversion-type cathodes for high energy density PBs.