The advancement of the global new energy revolution has placed stringent demands on energy storage technologies, emphasizing the need for high-energy-density and high-power electrochemical devices to enable the grid-scale and public infrastructure energy storage. All-solid-state batteries (ASSBs) exploiting solid ionic conductors as electrolytes offer advantages of high-energy-density, enhanced safety performance and long cycle life, which are considered as a crucial direction for the development of next-generation electrochemical energy storage. Designing and synthesizing solid-state electrolytes (SSEs) with ultrafast ionic conductivity, wide electrochemical windows and excellent mechanical deformability plays a crucial role to realizing the practical application of ASSBs.To date, various types of inorganic solid electrolytes including oxides and sulfides are extensively investigated, but none of them are successfully combined the advantages of excellent interfacial wettability and outstanding oxidation stability. Oxyhalide SSEs are among the most promising solid electrolytes for the commercialization of ASSBs due to their advantages like good compatibility with high-voltage cathodes, excellent mechanical deformability and remarkable ionic conductivity compared to that of liquid electrolytes. Despite oxyhalide electrolytes use in ASSBs, challenges remain in their practical application, including the scalable preparation, enhancement of humid tolerance and improved compatibility with electrode materials.In this review, a critical overview of the development, synthesis, ionic conduction mechanisms and challenges of oxyhalide electrolytes is given. Different synthesis routes of oxyhalide electrolytes including the promising hydrate-assisted synthesis method are summarized in detail. Furthermore, the framework structures of various types of oxyhalide electrolytes are analyzed, and the ionic conduction mechanisms in crystalline and amorphous oxyhalide electrolytes are elucidated. The interfacial compatibility of oxyhalide electrolytes with lithium metal anodes and various cathode materials is highlighted, and the latest progress of oxyhalide electrolytes in low-temperature ASSBs is also represented. Finally, future perspectives on designing high-performance oxyhalide electrolytes and their practical applications in ASSBs are provided, aiming to offer guidance for the advancement of oxyhalide-based ASSBs in energy conversion and storage.Summary and prospectsMetal oxyhalides ionic conductors represent a highly promising class of inorganic solid electrolytes for realizing the commercialization of high-performance ASSBs due to their outstanding electrochemical stability, excellent mechanical deformability, low-cost preparation and high ionic conductivity, compared to that of liquid electrolytes. Despite certain progress is made for the oxyhalide electrolytes in improving ionic conductivity, hydrate-assisted synthesis and low temperature application in ASSBs, some challenges remain for their practical application, including large-scale preparation, improving of moisture instability and enhancing compatibility with electrode materials. The attractive future research directions and prospects are outlined as follows, i.e., (1) Scale-up preparation: The wet-chemistry synthesis offers several advantages for the preparation of oxyhalide electrolytes, including scalable production, effective size control and shortened reaction time. However, reports on synthesizing oxyhalide electrolytes employing the wet-chemistry method remain scarce. By introducing auxiliary agents into the aqueous solution as a coordination agent to preferentially react with metal halides to form stable intermediates, offering a crucial guidance for the development of wet-chemistry synthesis of oxyhalide electrolytes, (2) Revealing ion-conduction mechanism: Advanced characterization techniques, such as synchrotron X-ray diffraction (SXRD), neutron powder diffraction (NPD), X-ray absorption near-edge structure (XANES) and solid-state nuclear magnetic resonance (SSNMR), can be preferentially utilized to deeply analyze the local structure of oxyhalide electrolytes, and combined with simulation and computational modeling to establish the structure-performance relationships of oxyhalide electrolytes, effectively promoting the design and synthesis of next-generation ultrafast ionic conductive oxyhalide electrolytes, (3) Enhancement of chemical stability: The chemical stability of oxyhalide electrolytes exerts a crucial influence on the implementation in their entire life from preparation, storage and transportation to application. An in-depth understanding of the degradation mechanisms of oxyhalide electrolytes when exposed to humid air and polar solvents can provide a significant theoretical guidance for the development of highly moisture-resistant oxyhalides and solvents with a good compatibility towards oxyhalide electrolytes, (4) Improving electrode-electrolyte interface compatibility. The development of highly ionically conductive oxyhalides with a superior reduction stability that are compatible with alkali metal anodes, and the construction of stable passivation layers at cathode-oxyhalide interfaces to suppress side reactions can offer crucial pathways for achieving high-energy-density ASSBs with oxyhalide electrolytes, and (5) Fabrication of electrolyte/electrode film: Tape casting is considered as a scalable strategy for the preparation of ultra-thin electrolyte membrane and electrode sheet. The screening of solvents and binders compatible with oxyhalides and the optimization of slurry component content can be the promising research directions in future due to the limited compatibility of oxyhalides with conventional polar solvents that are similar to those used in halide electrolytes. In addition, the solvent-free process for preparing self-supporting oxyhalide electrolyte membranes and sheet electrodes presents advantages such as simplified processes and reduced environmental pollution, providing a viable pathway for large-scale applications of oxyhalide electrolytes. The unique dual-anion chemistry of oxyhalide electrolytes grants them outstanding comprehensive performance, positioning them as one of the most promising solid electrolytes for the commercialization of ASSBs. It is foreseeable that ASSBs utilizing oxyhalide electrolytes will unlock new opportunities for the storage and conversion of renewable energy in future.