Why ionic conductivity does not predict durability: coordination-transport-interface design rules for sodium batteries across liquid and solid electrolytes

초록

A central obstacle in sodium battery development is that bulk ionic conductivity, the metric most often used to rank electrolytes, is a weak predictor of cell-level durability because performance is usually limited not by bulk transport but by interfacial resistance evolution under operating stress. Progress across ionic liquids, high-concentration and localized high-concentration electrolytes, polymers, oxides, sulfides, halides, hydroborates, and hybrid architectures is reported with heterogeneous metrics and evidence standards that prevent fair cross-class comparison, and no transferable principle yet links Na+ coordination chemistry to interfacial outcomes across the liquid-solid continuum. This review closes that gap by recasting the Coordination-Transport-Interface (CTI) framework as an explicit causal chain in which the Na+ coordination environment biases interphase chemistry, sets the transport boundary condition, and thereby governs interface evolution and the dominant failure mode. The framework 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, chloride-halide and closo-hydroborate conductors, 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.

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