Stable electronic structure related with Mn4+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 Na2-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 OO dimerization in Na2-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.