Voltage Hysteresis Induced by Fragile Quasi-Flat Bands in Oxygen-Redox Cathodes.

Cathodes exploiting oxygen redox chemistry underpin the realization of lithium battery systems with specific energies beyond 600 Wh kg-1. However, voltage hysteresis exceeding 1 V, especially during the first cycle, remains a long-standing and unresolved challenge, severely limiting energy efficiency and practical deployment. Here, we propose a quasi-flat-band framework in which oxygen 2p states near the Fermi level are intrinsically sensitive to perturbations. Even weak perturbations, such as minor lattice distortions during delithiation or thermal vibrations at room temperature, can disproportionately amplify band dispersion and induce hysteresis. This insight reframes the long-standing design strategy by extending it beyond suppressing structural reconstructions to include control of perturbation-driven electronic instability. Future materials design should pursue two principles: stabilizing symmetry to sustain quasi-flat-band characteristics and engineering orbital hybridization to enhance their robustness against perturbations. This work rethinks the origin and mitigation of voltage hysteresis, bringing oxygen-redox cathodes closer to reliable operation at high voltages.