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

Abstract

Graphite fluorides (CFx) 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 (MFx, M = Co, Ni, Fe, Cu, etc.). In this work, rechargeable CFx-based cathodes are fabricated by introducing transition metals, which reduces the Rct of the CFx electrode during the primary discharge process and participates in the re-conversion process of LiF under high voltage, which generates MFx (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.