Ionic conduction and interfacial stability in Na_1+xZr₂Si_xP_3−xO₁₂ solid electrolytes: Past, present, and future perspectives.

This article focuses on the structure-chemistry-property relationships and interfacial challenges of NaSICON (natrium super-ionic conductor) solid electrolytes (SEs) for sodium-based energy storage technologies. NaSICON SEs exhibit promising features such as high ionic conductivity (up to 4–6.7 mS cm⁻¹), chemical and mechanical stability, and low manufacturing cost, making them suitable for all-solid-state sodium batteries and aqueous redox flow batteries. The review details the atomic-scale ionic conduction mechanisms in both rhombohedral and monoclinic NaSICON phases, the impact of grain and phase boundaries on ion transport and stability, and the interfacial issues with sodium metal anodes and cathodes that affect battery performance. It also highlights the role of advanced microscopy and spectroscopy techniques in elucidating these phenomena and outlines key open questions regarding microstructural effects on conductivity, solid electrolyte interphase (SEI) formation, and degradation mechanisms relevant to the development of durable, high-performance sodium energy storage systems.