Reaction Mechanism and Structural Evolution of Fluorographite Cathodes in Solid‐State K/Na/Li Batteries

Abstract Fluorographites (CFx) are ultrahigh‐energy‐density cathode materials for alkaline‐metal primary batteries. However, they are generally not rechargeable. To elucidate the reaction mechanism of CFx cathodes, in situ transmission electron microscopy characterizations and ab initio calculations are employed. It is found that it is a two‐phase mechanism upon K/Na/Li ion insertion; crystalline KF (crystalline NaF nanoparticles and amorphous LiF) is generated uniformly within the amorphous carbon matrix, retaining an unchanged volume during the discharge process. The diffusivity for K/Na/Li ion migration within the CFx is ≈2.2–2.5 × 10–12, 3.4–5.3 × 10–12 , and 1.8–2.5 × 10–11 cm2 s–1, respectively, which is comparable to the diffusivity of K/Na/Li ions in liquid‐state cells. Encouraged by the in situ transmission electron microscopy (TEM) results, a new rechargeable all‐solid‐state Li/CFx battery is further designed that shows a part of the reversible specific discharge capacity at the 2nd cycle. These findings demonstrate that a solid‐state electrolyte provides a different reaction process compared with a conventional liquid electrolyte, and enables CFx to be partly rechargeable in solid‐state Li batteries.