Beyond Li-O-Li Configuration in Oxygen Anionic Redox: Amorphization-Driven O-O Dimerization in Fluorinated Li-V-O Cathodes
Oxygen anionic redox (OAR) is pivotal for achieving extra lithium storage in high-energy-density Li-ion batteries, yet its activation and stabilization remain challenging. Traditionally, OAR is studied in crystalline layered oxides with ordered frameworks and transition metal (TM)-centered octahedral coordination, where the Li-O-Li configuration is considered a prerequisite for creating unhybridized O 2p states. However, recent findings indicate that the presence of unhybridized O 2p states, rather than a specific configuration, is essential for oxygen activation. This study reports a novel OAR mechanism in an amorphous Li-V-O-F cathode, operating at a moderate voltage of 4.1 V, distinct from conventional Li-O-Li configurations. The cathode, initially crystalline LiVO2.98F0.02 (F2), undergoes amorphization after the first charge-discharge cycle, as evidenced by ex situ XRD, HRTEM, and EXAFS. Resonant inelastic X-ray scattering (RIXS) and X-ray absorption spectroscopy (XAS) reveal that the initial charge involves O-O formal redox without oxidized oxygen features, indicating electron holes are accommodated via O-O interactions. Reversible OAR activity emerges in the second cycle, confirming O-O dimerization in the amorphous phase. Ab initio molecular dynamics (AIMD) simulations further elucidate the mechanism. This work challenges the conventional Li-O-Li paradigm and opens new avenues for designing high-capacity cathode materials through amorphization and tetrahedral coordination.