Delocalized Metal-Oxygen π-Redox Is the Origin of Anomalous Nonhysteretic Capacity in Li-Ion and Na-Ion Cathode Materials.

The anomalous capacity of Li-excess cathode materials has ignited a vigorous debate over the nature of the underlying redox mechanism, which promises to substantially increase the energy density of rechargeable batteries. Unfortunately, nearly all materials exhibiting this anomalous capacity suffer from irreversible structural changes and voltage hysteresis. Nonhysteretic excess capacity has been demonstrated in Na2Mn3O7 and Li2IrO3, making these materials key to understanding the electronic, chemical, and structural properties that are necessary to achieve reversible excess capacity. Here, we use high-fidelity random-phase-approximation (RPA) electronic structure calculations and group theory to derive the first fully consistent mechanism of nonhysteretic oxidation beyond the transition metal limit, explaining the electrochemical and structural evolution of the Na2Mn3O7 and Li2IrO3 model materials. We show that the source of anomalous nonhysteretic capacity is a network of π-bonded metal-d and O-p orbitals, whose activity is enabled by a unique resistance to transition metal migration. The π-network forms a collective, delocalized redox center. We show that the voltage, accessible capacity, and structural evolution upon oxidation are collective properties of the π-network rather than that of any local bonding environment. Our results establish the first rigorous framework linking anomalous capacity to transition metal chemistry and long-range structure, laying the groundwork for engineering materials that exhibit truly reversible capacity exceeding that of transition metal redox.