Abstract:Halide solid state electrolytes (HSSEs) have emerged as key strategic materials for next-generation solid-state batteries due to their superior ionic conductivity, wide electrochemical stability window, and excellent compatibility with high-voltage cathode materials. In recent years, significant progress has been made in the design and performance optimization of novel HSSEs; however, substantial research gaps remain in the optimization of their synthesis processes. Refining synthesis routes to reduce production costs and achieve efficient large-scale synthesis is crucial for advancing the industrialization of halide electrolytes. This article comprehensively reviews recent research advances in two primary synthesis routes of HSSEs, elucidates the relationship between synthesis methodology and electrolyte properties, and provides insights for the scalable production of HSSEs.Solid-state synthesis methods encompass comelting method and mechanochemical synthesis method. The comelting method achieves homogenized synthesis via high-temperature melting of precursors under inert atmosphere, yet its applicability for electrolyte synthesis is constrained by the requirement for solid solution formation based on phase diagrams. Mechanochemical synthesis, typically referring to high energy ball milling, is the most prevalent method for synthesizing HSSEs. During the ball milling process, intense shear and collision forces between the precursors and milling balls drive mechanochemical reactions, leading to the formation of new products. It is noteworthy that the annealing following ball milling require careful optimization depending on the space group of the target product. The mechanochemical method offers advantages of simplicity and low impurity incorporation and exhibits broad applicability for synthesizing diverse HSSEs. Overall, solid-state synthesis methods provide high versatility and purity, but its high energy consumption and limited sample homogeneity significantly hinder its application in industrial-scale production. Leveraging the advantages of solid-state synthesis, strategies doping with cationic and anionic species can effectively enhance the ionic conductivity, widen the electrochemical stability window, and improve interfacial stability of solid electrolytes.In contrast to the poor mixing uniformity in solid-state routes, the solution-based synthesis method offers unique technical advantages by achieving molecular-scale mixing and leveraging solvent-mediated chemical regulation. Besides, this method adopts a "dissolution-drying" green synthesis approach, which not only avoids the high-energy and high-temperature equipment demands and energy consumption typical of solid-state synthesis, but also facilitates uniform dispersion of precursors in the solvent, resulting in finer particle size and improved ionic conductivity. To address the limited applicability of conventional solution synthesis, the ammonium-assisted method utilizes coordination effects to suppress hydrolysis of intermediate species, significantly expanding the range of HSSEs accessible by solution-based routes. However, the large-scale production of HSSEs via solution synthesis still encounters challenges such as complex solvent selection, uncontrollable side reactions, and residual solvents. Unlike the solid-state approach, solution synthesis is not limited to elemental doping but is also applicable to the fabrication of composite cathode materials and ultrathin electrolyte membranes, contributing to the industrial-scale realization of all-solid-state batteries.By summarizing the research progress in solid-state and solution-based synthesis pathways, this review provides a comprehensive overview of current progress in solid-state synthesis and solution-based synthesis methods for HSSEs, systematically comparing different approaches in terms of their merits, limitations, representative applications, and resultant material properties. Furthermore, the relationship between synthesis processes and electrolyte performance is elucidated, aiming to provide a practical framework for the scalable, high-performance, and economically viable manufacturing of HSSEs, ultimately accelerating the commercialization of halide-based all-solid-state batteries.
