Amorphous Halide Solid-State Electrolytes: Synthesis Strategies, Ion Transport Mechanisms, and Application Progress

Amorphous halide solid-state electrolytes have emerged as a promising but still immature class of ionic conductors for all-solid-state batteries. In contrast to the conventional view that crystallinity is required for fast ion transport, these materials combine long-range disorder with short-range ordered coordination motifs, which can create broadened migration bottlenecks, diverse local environments, and in some cases fast ion pathways associated with amorphous/crystalline interfaces. This review summarizes recent progress in amorphous halide electrolytes with emphasis on their structural features, synthesis strategies, ion transport mechanisms, and electrochemical applications. Structurally, amorphous halides are commonly built from [MX6]- derived polyhedra that retain short-range chemical order while forming a disordered network through variable linkage modes. Such structural heterogeneity can lower migration barriers, but its benefits are highly composition-dependent and not yet fully understood. We compare three main synthesis routes-mechanical ball milling, self-propagating synthesis, and melt-quenching-in terms of compositional versatility, energy input, scalability, and attainable amorphous fraction. We further discuss ion transport models involving local coordination tuning, vacancy regulation, anion-framework flexibility, and amorphous/crystalline interfacial conduction, while distinguishing experimentally supported evidence from still-speculative mechanistic interpretations. Recent battery demonstrations show that amorphous halides can enable attractive combinations of ionic conductivity, cathode compatibility, wide-temperature operation, and long cycling life. However, major challenges remain, including moisture sensitivity, interfacial instability against alkali-metal anodes, uncertain resistance to dendrite penetration, the cost of Ta-containing chemistries, and limited evidence for scalable manufacturing. A more rigorous understanding of structure-property relationships, combined with low-cost synthesis and robust interface engineering, will be essential for translating amorphous halide electrolytes from laboratory discoveries into practically deployable solid-state battery materials.