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

Abstract Fluorographites (CF_x) are ultrahigh‐energy‐density cathode materials for alkaline‐metal primary batteries. However, they are generally not rechargeable. To elucidate the reaction mechanism of CF_x 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 CF_x 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/CF_x 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 CF_x to be partly rechargeable in solid‐state Li batteries.