Abstract

Electrolyte limitations increasingly set the practical ceiling of sodium batteries, yet progress across ionic liquids, high-concentration and localized high-concentration electrolytes, polymers, oxides, sulfides, and hybrid architectures is reported with heterogeneous metrics and evidence standards that prevent fair cross-class comparison. This review organizes the sodium electrolyte landscape through a Coordination-Transport-Interface (CTI) framework that links Na⁺ solvation chemistry to transport descriptors and interfacial resistance evolution under matched operating conditions. The framework is operationalized into six design rules, a minimal descriptor set, and a failure-mode-to-mitigation decision logic. Liquid-side extremes are compared by their ability to drive anion-rich coordination, sustain ion flux, and suppress cathode electrolyte interphase growth. Solid-state conductors, including NASICON-type oxides, beta-alumina, Na₃PS₄-based and Na₃SbS₄-based sulfides, and polymer electrolytes, are compared by dominant bottleneck rather than nominal conductivity. A minimum benchmarking protocol with a quantitative cross-class performance table and a protocol ladder with worked examples is proposed. The central conclusion is that interface evolution, not bulk ionic conductivity, most often governs practical sodium electrolyte performance.