Crystallization Kinetics as a Design Lever for High‐Performance Halide Solid Electrolytes Obtained by Scalable Wet‐Chemical Route

Halide solid electrolytes (SEs), like Li 3 InCl 6 , are promising for high‐energy all‐solid‐state lithium batteries (ASSLBs) due to their high ionic conductivity and compatibility with high‐voltage cathodes. Although solvent‐mediated synthesis offers a scalable route to phase‐pure Li 3 InCl 6 , a lack of understanding of critical synthetic parameters, specifically crystallization kinetics, generally yields SE materials with inferior properties. This study systematically investigates the influence of evaporative crystallization temperature and environment on the phase purity, microstructure, defect chemistry of Li 3 InCl 6 SE, and how these factors collectively impact its transport properties and electrochemical performance. It is revealed that slow crystallization under ambient conditions and moderate temperatures (20–60 °C) yields phase‐pure Li 3 InCl 6 with the highest ionic conductivity ever reported for the water‐mediated route −3.97 mS cm −1 with carbon contact and 2.98 mS cm −1 without. In contrast, high temperature and non‐ambient processing introduce structural defects, increase grain‐boundary impedance, and promote impurity incorporation, leading to a significant drop in conductivity. Full cells incorporating the optimized Li 3 InCl 6 deliver high capacity even at a low 20 °C, along with excellent stability (>95%) at high areal loading, supported by low and stable cathode interfacial impedance. This work addresses a critical knowledge gap in solvent‐mediated synthesis of halide SEs, providing broadly applicable insights for designing phase‐pure, high‐conductivity materials for next‐generation ASSLBs.