Decoding the Oxygen Activity with Iron through Ligand-to-Metal Charge Transfer in Li-Rich Layered Cathodes

The quest for high-energy-density and cost-effective cathode materials has revitalized lithium-rich iron-based oxides, where Fe³⁺ ions with their stable half-filled d⁵ electronic configuration demonstrate unique potentials in regulating oxygen redox activity through ligand-to-metal charge transfer. Using operando ⁵⁷Fe Mössbauer spectroscopy, synchrotron X-ray techniques, and density functional theory calculations, we unravel the enhanced Fe-O redox activity through the LMCT process and the concomitant evolution of local structure distortion in the Li-rich layered system Li_1.2MO₂ (M = Ni, Mn, Fe). The metastable Fe⁴⁺ intermediates facilitate early activation of oxygen redox through LMCT along with increased delithiation at a lowered potential (below 4.5 V). This process inherently suppresses Jahn-Teller distortion of Fe⁴⁺ and exhibits Fe-O redox dominance over conventional Fe³⁺/Fe⁴⁺ redox in Li-rich oxides. The Fe-O coupling redox exhibits improved initial Coulombic efficiency and 95% capacity retention over 200 cycles at 4.4 V. However, the LMCT process is accompanied by aggravated cation migration and voltage fade at low cutoff potentials, which requires further mitigation for the utilization of Fe-O redox. Overall, these findings demonstrate the regulating mechanism of iron for oxygen redox through LMCT and provide fundamental insights into the design principles for high-performance, cost-effective iron-based cathode materials.