Supersaturation-Driven Co-Precipitation Enables Scalable Wet-Chemical Synthesis of High-Purity Na₃InCl₆ Solid...

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 Na₃InCl₆ SSE is reported, achieving high phase purity and unprecedented ionic conductivity. Highly pure P3₁c Na₃InCl₆ phase (<1% NaCl impurity) and inter-site exchange within two Na sites are confirmed, as characterized by Rietveld refinement and solid-state ²³Na 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 Na₃InCl₆ 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 Na₃InCl₆ but also indicates the potential applicability of the method to other SSE systems, offering both synthesis and structural insights under realistic conditions.