Sodium-Ion Battery Cathode Materials: Towards an improved energy-based descriptor for investigating material stability against moisture

Abstract

The poor stability of transition metal (TM) layered oxide cathode materials upon exposure to moisture poses a significant challenge, hindering their widespread practical use in sodium-ion batteries. To facilitate the selection of suitable dopants for enhancing air-stability, we propose energy-based descriptors to assess the material stability and the water-material interactions. These descriptors are assessed through density functional theory (DFT) calculations, focusing on the onset of water insertion in NaTmxNi1-xO2 (Tm= Ti, Mn) cathode materials. The importance of energy-based descriptors is highlighted by examples discussed where despite having large sodium layer disruption and expansion of the surface layers due to water insertion, the formation of hydrogen bond and charge transfer between water and the oxide layers greatly stabilizes the NaTmxNi1-xO2 structure, thus promoting water insertion. The energy descriptors are used in a materials screening protocol to predict the water stability trends in sodium-ion battery materials and to understand the effect of dopants in mitigating the air-stability issue.