Engineering Disorder: Entropy as a Design Lever for Stable Interfaces in Solid-state Sodium-metal Batteries

Abstract

Solid-state sodium-metal batteries (SSSMBs) promise a rare combination of high energy density, enhanced safety, and elemental abundance. Yet, their progress is repeatedly stalled not by bulk properties, but by a far more elusive barrier: the interface. Across oxide, sulfide, and halide electrolytes, electrochemical degradation, space-charge polarization, and contact failure continue to limit performance; often irreversibly. This review reframes the interface not as a passive boundary to be tolerated, but as an active, designable medium that dictates long-term stability. We critically dissect how ion transport, chemical reactivity, and interphase mechanics evolve under realistic cycling conditions, and show that conventional fixes, such as thin film coatings, wetting agents, and buffer layers, offer only fragmented relief. In contrast, we spotlight configurational entropy as a powerful, underexploited tool to systematically modulate interfacial behaviour. Through compositional complexity, entropy rich architectures can flatten potential gradients, localize electron exclusion, and foster self-limiting passivation, while preserving Na⁺ mobility. We unify these insights across electrolyte chemistries, benchmark interfacial performance under practical constraints, and offer a roadmap for building reproducible, scalable, and high rate SSSMBs. This review does not merely summarize a field; it sharpens its focus and proposes a new scientific language for tackling one of energy storage’s most persistent bottlenecks.