Abstract: All-solid-state batteries are a promising energy storage technology owing to their high energy density and safety. Exploring solid electrolytes with high room-temperature ionic conductivity, good electrochemical stability, and excellent cathode/anode compatibility is key to realizing the practical application of all-solid-state batteries. Lithium metal halide solid electrolytes have attracted extensive research attention because of their excellent electrochemical windows, high positive electrode stabilities, and acceptable room-temperature Li-ion conductivities of up to 10^-3S·cm^-1. In this paper, the chemical compositions, structural details, lithium-ion conduction pathways, and synthesis routes of lithium metal halide solid electrolytes are reviewed based on recently published papers and our studies. The lithium metal halide Li_a-M-X₆ can be classified as Li_a-M-Cl₆, Li_a-M-Cl₄, and Li_a-M-Cl₈ based on the substitution of the Li ions with different transition metal elements. Among these, the Li_a-M-X₆ and Li_a-M-X₄ electrolytes have been widely investigated because of their high ionic conductivities of up to 10^-3S·cm^-1. Li_a-M-X₆ electrolytes exhibit three types of structure: trigonal, orthorhombic, and monoclinic. Li⁺ diffusion in lithium metal halide electrolytes with different structures follows a vacancy mechanism. When transition metal cations with larger ionic radii and higher valances are used to substitute Li⁺ in the structure, vacancies are generated and larger Li⁺ transport channels are produced, both of which are helpful for achieving faster Li-ion conductivities in the modified electrolytes. The typical synthetic route for lithium metal halide electrolytes is mechanical milling and subsequent sintering. Moreover, recent studies have reported that a pure phase with high conductivity can be obtained via water-mediated synthesis, which is a promising method for mass production. The electrochemical stability of lithium metal halide electrolytes with temperature, humidity, and active electrode materials is also summarized herein. Some lithium halide electrolytes suffer from a low phase-transition temperature close to room temperature, making it difficult to prepare the pure phase and limiting their applications. Owing to the high sensitivity of halides to moisture, lithium halide electrolytes suffer poor stability during storage and operation in the open air. The wide electrochemical window and excellent stability of high-voltage cathode materials of lithium metal halide electrolytes enable the construction of all-solid-state lithium batteries with a high energy density and long lifespan. Moreover, this property makes it possible to introduce carbon conductive additives into the cathode without a surface coating layer on the active materials, which is helpful for designing highly conductive frameworks for thick electrodes used in solid-state batteries. However, lithium metal halide electrolytes exhibit poor stability with bare lithium metal or lithium alloys because of their high reduction potentials. Therefore, another solid electrolyte layer requires the isolation of the direct contact between the lithium metal halide electrolytes and Li-related anodes. Finally, this review summarizes the application of these electrolytes in all-solid-state batteries in recent years and highlights the challenges and research directions of lithium halide electrolytes.

Key words: Lithium halide solid electrolyte, Structure, Conduction mechanism, Synthesis route, Modification, Electrochemical performance