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

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.