Harvesting sustainable and low-hysteresis anion redox chemistry in Na layered oxide cathodes through mild ligand-to-metal charge transfer

Triggering oxygen anionic redox (OAR) in layered transition metal (TM) oxides (Na_xTMO₂) is a paradigmatic strategy to boost the energy density of sodium-ion batteries. Regrettably, non-hysteresis and long-term cyclability have not been concurrently accomplished until now for the OAR reaction in Na_xTMO₂. Herein, we unmasked the presence of ligand-to-metal charge transfer (LMCT) in two typical compounds with redox-active Ni²⁺ and Cu²⁺, Na_2/3Ni_1/6Mg_1/6Mn_2/3O₂ and Na_2/3Cu_1/6Mg_1/6Mn_2/3O₂. Through multiple characterizations it is shown that the phase evolution is not as essential as redox chemistry and/or local structural rearrangements in dictating their cyclability and voltage hysteresis. We further discovered that a mild LMCT in Na_2/3Ni_1/6Mg_1/6Mn_2/3O₂ contributes to low-hysteresis and long-term stable OAR reaction by favoring less local structural distortion, Mg migration and TM malposition, bringing about more TM redox but less OAR and less O₂ release, while a strong LMCT in Na_2/3Cu_1/6Mg_1/6Mn_2/3O₂ does not. This result contrasts with the conventional wisdom on manipulating OAR stability through enhancing the TM−O covalency. Additional first-principles calculations indicate a delocalized TM−O bonding in Na_2/3Ni_1/6Mg_1/6Mn_2/3O₂ to harden the oxygen lattice, which is conducive to the stabilization of OAR. These findings provide valuable guidelines towards non-hysteresis and sustainable OAR chemistry in OAR-based high-energy batteries through regulating the strength of LMCT.