Suppressing void formation in all-solid-state batteries: the role of interfacial adhesion on alkali metal vacancy transport

Abstract

All-solid-state batteries containing a solid electrolyte and a lithium (Li) or sodium (Na) metal anode are a promising solution to simultaneously increase the energy density and safety of rechargeable batteries. However, problems remain with the stripping of alkali metal from the alkali metal/solid-state electrolyte interface during discharge in which void formation and loss of contact can occur. A novel bond breaking model is developed in this work to understand the relationship between alkali metal vacancy segregation and interfacial adhesion at the alkali metal/solid-state electrolyte interface. The bond breaking approach is tested against density functional theory (DFT) calculations of pristine Li and Na metal surfaces and interfaces between Li and Na metal and model substrate structures (LiCl, Li₃OCl, LiMg, Li₂O, γ-Li₃PO₄, AlSc, NaCl and NaBr). The activation barrier for surface to subsurface vacancy diffusion was found to be considerably larger than bulk diffusion in Li and Na slabs. At the alkali metal/solid-state electrolyte interface, the preference for alkali metal vacancy segregation is shown to be intimately linked to the interfacial work of adhesion (W_ad) and alkali metal surface energy, σ_m. Suppression of alkali vacancy segregation to the interface is found to occur when W_ad ≥ 2σ_m. The role of interfacial structure on the vacancy segregation energy is demonstrated for both coherent and incoherent Li/LiCl interfaces. This work provides novel guidelines for the materials engineering of new solid-state electrolyte and interlayer materials that can suppress void formation in all-solid-state batteries with alkali metal anodes.