Stable electronic structure related with Mn⁴⁺[sbnd]O^−• coupling determines the anomalous nonhysteretic behavior in Na₂Mn₃O₇

Oxygen redox is a double-edged sword for battery cathodes as it furnishes a substantial increase in energy density at the expense of bringing additional detrimental issues, such as irreversible local structural transformation and substantial voltage hysteresis. Intriguingly, Na₂Mn₃O₇ can access an oxygen-redox capacity with an abnormal small voltage hysteresis (∼0.05 V), making it a rare model system to identify a reaction mechanism that suitable for engineering oxygen-redox cathodes with high energy efficiency. Herein, we firstly corroborate by operando electron paramagnetic resonance (EPR) that the local electronic structure evolution of Na₂Mn₃O₇ upon oxygen redox is highly reversible. By ex situ low-temperature EPR measurements, we also confirm that O 2p localized holes (O^−•) is the chemical state of oxidized oxygen species in Na_2-xMn₃O₇, contributing to a highly reversible local structural evolution around Na nucleus. The stable electronic structure stimulated by the absence of in-plane Mn migration and the dispersive oxygen redox without O[sbnd]O dimerization in Na_2-xMn₃O₇, is thus firstly proposed as the origin of abnormal small voltage hysteresis. This study provides the rationale for achieving nonhysteretic behavior in the large family of oxygen-redox cathodes.