Supersaturation‐Driven Co‐Precipitation Enables Scalable Wet‐Chemical Synthesis of High‐Purity Na ₃ InCl ₆ Solid Electrolyte for Sodium‐Ion Batteries

Abstract Solid‐state sodium batteries emerge as a compelling paradigm for next‐generation energy storage, combining intrinsic safety, cost efficiency, and long‐term sustainability. However, the absence of a scalable route for synthesizing phase‐pure sodium solid‐state electrolytes (SSEs) remains a major barrier to practical application. Here, a supersaturation‐driven co‐precipitation synthesis of Na ₃ InCl ₆ SSE is reported, achieving high phase purity and unprecedented ionic conductivity. Highly pure P 3 ₁ c Na ₃ InCl ₆ phase (<1% NaCl impurity) and inter‐site exchange within two Na sites are confirmed, as characterized by Rietveld refinement and solid‐state ²³ Na nuclear magnetic resonance spectroscopy. First‐principles calculations further support the distinct Na environments and dynamic stability of the structure, reinforcing its suitability as an SSE. In situ synchrotron X‐ray diffraction and projection X‐ray microscopy assessed structural and 3D morphological evolution under various atmospheric conditions. The Na ₃ InCl ₆ fabricated by the supersaturation‐driven co‐precipitation method exhibits an ionic conductivity of 1.09 × 10 ⁻⁴ and 3.13 × 10 ⁻³ mS cm ⁻¹ at room temperature and 80 °C, respectively, which is two orders of magnitude higher than the literature. This study not only demonstrates the potential for scalable production of highly phase‐pure Na ₃ InCl ₆ but also indicates the potential applicability of the method to other SSE systems, offering both synthesis and structural insights under realistic conditions.