Transition metal migration and O2 formation underpin voltage hysteresis in oxygen-redox disordered rocksalt cathodes

Abstract

Lithium-rich disordered rocksalt cathodes display high capacities arising from redox chemistry on both transition-metal and oxygen ions and are potential candidates for next-generation lithium-ion batteries. The atomic-scale mechanisms governing this O-redox behaviour, however, are not fully understood. In particular, it is not clear to what extent transition metal migration is required for O-redox and what role this may play in explaining voltage hysteresis in these materials. Here, we reveal an O-redox mechanism linking transition metal migration and O2 formation in the disordered rocksalt Li2MnO2F. At high states of charge, O-ions dimerise to form molecular O2 trapped in the bulk structure, leaving vacant O sites surrounding neighbouring Mn ions. This undercoordination drives Mn movement into new fully-coordinated octahedral sites. Mn displacement can occur irreversibly, which results in voltage hysteresis, with a lower voltage upon discharge as observed experimentally. Alternatively, Mn displacement may take place into interstitial octahedral sites, which permits a reversible return of the Mn ion to its original site upon discharge, recovering the original Li2MnO2F structure and resulting in reversible O-redox without voltage loss. These new findings suggest that reversible transition metal ion migration provides a possible design route to retain the high energy density of O-redox disordered rocksalt cathodes on cycling.

Unraveling the Dynamic Correlations between Transition Metal Migrations and the Oxygen Dimer Formation in the Layered LiCoO2 Cathode

“…Furthermore, the effect of the Ti dopant on the kinetics of the cathode material's structural change is discussed based on insights provided by the DeePMD simulations. Here, we note that although the generation of oxygen molecules from the cathode surfaces is easier in batteries, the TM cation migrations in the bulk were reported by experiments, [46][47][48][49][50][51] which also could lead to the oxygen release. For instance, the layered-to-rocksalt-phase transition, usually induced by TM cation migrations, was found in the bulk LixCoO2 phase by Li et al 47 , which draws attention to the dynamic behaviors of the TM and oxygen atoms in a highly delithiated bulk-phase cathode material.…”

Electrode/electrolyte additives for practical sodium-ion batteries: a mini review

“…1 The schematic illustration of the development of LIB and SIB timelines. 38,[44][45][46][47] inexpensiveness. From the most basic concept of the electrolyte, the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) are the two most important factors (Fig.…”