Interfacial failure mechanisms and design principles in solid-state sodium batteries

Solid-state sodium batteries (SSSBs) have attracted increasing attention as a promising alternative for large-scale energy storage owing to their intrinsic safety, material abundance, and potential cost advantages. Significant progress has been made in developing diverse solid-state electrolytes, including polymers, inorganic ceramics, and hybrid systems, many of which exhibit impressive bulk ionic conductivity. However, the translation of these materials-level properties into durable, high-performance solid-state sodium batteries remains limited, indicating that bulk ion transport alone does not govern practical cell behavior. In this review, we adopt an interface-centered and issue-driven perspective to analyze the key challenges in SSSBs. Rather than providing a materials-category-based summary, we focus on dominant interfacial failure mechanisms and their sodium origins. Chemical and electrochemical instability, electrical blocking associated with space-charge effects and grain boundaries, mechanical degradation arising from elastic and thermal mismatch, and defect-assisted sodium dendrite penetration are discussed within a unified mechanistic framework. These interfacial processes are shown to be intrinsically coupled, collectively controlling effective ion transport, critical current density, and long-term cell stability. Building on this understanding, we critically assess why high bulk ionic conductivity has not translated into robust full-cell performance and emphasize the limitations of conductivity as a single performance metric. We further discuss general design principles for interface engineering across different electrolyte families and revisit lessons from technologically mature sodium battery systems to clarify realistic pathways toward practical implementation. By linking interfacial chemistry, defect physics, and mechanical properties, this Review aims to provide a coherent framework and forward-looking guidance for the rational design of next-generation SSSBs.