Extending Non-Hysteretic Electrochemistry Beyond Conventional Transition Metal Capacity in Li and Na-Ion Cathodes Using Pi-Redox

Abstract

The anomalous electrochemical capacity observed in some Li- and Na-excess materials, extending beyond the conventional limit of transition metal redox, represents one of the most promising sources of untapped energy density in intercalation cathodes. Unfortunately, the majority of materials known to exhibit this behavior suffer from voltage hysteresis and structural degradation. A prominent exception is Na2Mn3O7, which shows no voltage hysteresis despite operating beyond the conventional limit of Mnoxidation. The redox mechanism of this material and the source of the non-hysteretic behavior remain controversial with significant inconsistencies between experimental observations and the commonly invoked "oxygen-redox" theories. We use high-level electronic structure calculations to demonstrate that the origin of the anomalous capacity in Na2Mn3O7 is the oxidation of a delocalized redox center consisting of pi-bonded Mn-d and O-p orbitals, which we term pi-redox. This hybridized redox is unique in that capacity, voltage, and stability are determined by the long-range structure of the metal-oxygen network, rather than any local bonding environment. The pi-redox mechanism competes with oxidation-driven transition metal migration which underlies the hysteresis seen in most excess-capacity compounds, such as those based on Li2MnO3. The new pi-redox mechanism is the first to reconcile all experimental observations of non-hysteretic anomalous capacity, and sets forth rigorous materials design principles for extending this behavior from model systems to high-performance compounds.