Sodium-ion battery cathode materials: towards improved energy-based descriptors 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 material stability and water–material interactions. These descriptors are assessed through density functional theory (DFT) calculations, focusing on the onset of water insertion in NaTm_xNi_1−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 bonds and charge transfer between water and the oxide layers greatly stabilize the NaTm_xNi_1−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.