Reversible Li⁺ Storage in CF-Based Cathodes through Transition Metal Decomposition

Graphite fluorides (CF) have been commercially applied in primary lithium batteries for decades with high specific capacity and low self-discharge rate, but the electrode reaction of CF with Li⁺ is basically irreversible compared to that of transition metal fluorides (MF, M = Co, Ni, Fe, Cu, etc.). In this work, rechargeable CF-based cathodes are fabricated by introducing transition metals, which reduces the R_ct of the CF electrode during the primary discharge process and participates in the re-conversion process of LiF under high voltage, which generates MF (confirmed by ex situ X-ray diffraction measurements) for subsequent Li⁺ storage. A CF-Cu (F/Cu = 2/1 by mol) electrode, for example, delivers a primary capacity as high as 898 mAh g_(CF0.56)^-1 (∼2.35 V vs Li/Li⁺) and a reversible capacity of 383 mAh g_(CF0.56)^-1 (∼3.35 V vs Li/Li⁺) in the second cycle. Furthermore, excessive transition metal decomposition during the charge process is harmful to electrode structure stability. Methods such as building a compact counter electrolyte interface (CEI) and obstructing the electron transport of transition metal atoms will contribute to finite and local transition metal oxidation that benefits cathode reversibility.