Stabilization of Oxygen‐Dependent Fe3+/4+ Redox in Li‐Excess DRX Cathode Exhibiting Anionic Redox via Transition Metal Combination

Abstract

AbstractDeveloping sustainable Li‐ion batteries requires high‐energy cathodes based on low‐cost, earth‐abundant elements, moving away from low‐reserve nickel and cobalt. Fe‐based oxide cathodes with Fe3+/4+ and O2−/n− redox couples offer potential but face low initial Coulombic efficiency and significant voltage hysteresis. This study investigates Li‐excess Fe‐based disordered rock‐salt (DRX) oxyfluorides (Li2Fe0.5M0.5O2F; M = Fe, Ti, Mn) using combined electrochemical/spectroscopic characterization and first‐principles calculation. Oxygen‐dependent Fe3+/4+ redox, related to Fe 3d–O 2p hybrid state, can be stabilized when combined with Mn3+/4+ redox in DRX structure owing to the unusual decrease in its redox potential. The moderately high charge transfer gap stabilizes Fe4+ against ligand‐to‐metal charge transfer (LMCT) on charge, reduces the amount of oxygen oxidation, thereby increasing Coulombic efficiency. On discharge, it allows metal‐to‐ligand charge transfer (MLCT) without substantial overpotential, reducing hysteresis in oxygen redox. The resulting composition exhibits high capacity (309 mAh g−1) and energy density (998 Wh kg−1), providing insights for next‐generation Ni‐ and Co‐free cathode materials.