Na vs. Li metal anodes for batteries: unraveling thermodynamic and electronic origins of voids and developing descriptors for artificial surface coatings

Abstract

Techno-economic, humanitarian, and safety concerns limit the possible uses of conventional lithium-ion and lithium-metal batteries. Sodium-based batteries constitute a promising alternative to address these issues; however, due to the similarities between the two alkali metals, they present similar failure modes as their lithium counterparts. In this work, we focus on one of such failure mechanisms: the thermodynamically-driven accumulation of vacancies on the surface of the metallic anode, which leads to the formation of voids and pits, detrimental to battery performance and cycle life. We investigate the differences in behavior between anode/coating interfaces of both lithium and sodium. Adhesion energy, a descriptor previously argued to be a reliable design principle for lithium metal anodes, is found to not exhibit the same predictive power for sodium metal architectures: in cases where vacancy congregation is not thermodynamically favorable for isolated sodium slabs, we find strong interfacial interactions to have adverse effects on void formation. By studying select coating materials, we also reveal that these material interactions at alkali/coating interfaces are highly nuanced, and that the field of surface science and engineering is ripe with opportunities for further discovery and tuning of surface properties via coating selection.