The global transition toward renewable energy and electrified transportation has intensified the demand for advanced energy storage systems with high energy density, intrinsic safety, and long-term durability. Conventional lithium-ion batteries (LIBs) in the market face critical limitations, i.e., organic liquid electrolytes are flammable, posing significant safety risks (i.e., thermal runaway and combustion), and their energy density is approaching theoretical limits (i.e., ~300 W·h·kg^-1). These shortcomings hinder their applicability in emerging technologies such as electric vehicles (EVs), grid-scale storage, and high-power portable electronics. All-solid-state batteries (ASSBs), which replace liquid electrolytes with solid-state electrolytes (SSEs), offer a revolutionary solution. SSEs eliminate flammability risks, enable the use of lithium metal anodes (i.e., theoretical capacity of 3860 mA·h·g^-1), and facilitate compact cell designs through bipolar stacking. Among SSE candidates, halide-based electrolytes emerge as a game-changing material class due to their unique advantages like ultrahigh ionic conductivity, high voltage compatibility, mechanical adaptability, and scalable synthesis. The urgency to develop halide SSEs is further underscored due to the growing need for safe, high-energy-density batteries in critical applications. This article provides a comprehensive and updated review of the classification, structure, synthesis methods, air stability, mechanical stability, and applications of halide electrolytes at both cathode and anode interfaces.

Structural design is pivotal in developing superionic conductors as it serves as a primary determinant of ionic conductivity. Effective strategies to enhance lithium-ion conductivity include regulating the concentration and rational occupation of lithium ions and lithium vacancies within the electrolyte. Halide-based electrolytes exhibit an exceptional structural tunability, providing a foundation for designing novel electrolytes with optimized properties. Among these strategies, elemental doping stands out as one of the most common approaches to improve ionic conductivity in halide electrolytes. In recent years, high-entropy materials, characterized by their adjustable chemical compositions and unique properties, indicate widespread applications in various fields. Introducing high-entropy strategies into SSEs can effectively boost ionic conductivity through chemical disorder and local lattice distortions, while reducing reliance on rare-earth elements. In addition, amorphous SSEs have attracted much attention due to their outstanding mechanical flexibility and rapid ion transport capabilities, which arise from the decoupling of charge carriers from the supporting matrix. This review also highlights the synthesis methods for halide-based electrolytes. Simple, cost-effective, and environmentally friendly synthetic routes are crucial for advancing the industrialization of these materials. The existing approaches primarily involve solid-state, liquid-phase, and vapor-phase synthesis techniques, each of which significantly affects the resulting electrolyte structure and ionic conductivity.

Air stability is crucial for SSEs, impacting their entire lifecycle from synthesis to application. Halide-based electrolytes, derived from hygroscopic halide salts, are prone to moisture-induced degradation during synthesis and storage. To enhance their air stability, some methods like surface coating and structural modification are employed. For instance, creating physical barriers, superhydrophobic membranes, or doping with In³⁺ can significantly improve moisture resistance. In ASSBs, inorganic SSEs must have good electrochemical properties and excellent mechanical properties. Bulk modification as a common approach to improving halide-based electrolytes performance also boosts mechanical stability. For instance, O^2- doping can disrupt the crystal structure of electrolyte, making it more ductile. In addition, adjusting the particle size of halide-based electrolytes and combining them with binders to form organic-inorganic composite membranes can also enhance their mechanical properties.

As a component in direct contact with the positive and negative electrodes, understanding the electrochemical stability and interfacial reactions of halide SSEs is crucial for achieving high-energy-density and long-cycle-life halide-based ASSBs. For high-voltage cathodes, halide SSEs are among the most promising. The oxidation potential of halide SSEs mainly depends on the halogen anion, especially for chlorine- and fluorine-based halides, whose high electronegativity prevents oxidation at battery operating voltages. To further enhance the cathode interface stability of halide SSEs, element doping can broaden the thermodynamic electrochemical window or form a stable cathode-electrolyte interphase (CEI) in-situ. Structural control, like reducing the SSE's Young's modulus and particle size or coating the SSE on the cathode material surface to improve solid-to-solid contact in composite cathodes, is also effective. The instability of halide SSEs with metal lithium mainly stems from the transition metals in the halide. The high potential difference between halide SSEs and lithium metal makes direct contact with lithium or lithium alloy anodes extremely unstable. This leads to high-resistance interfacial layers that impede Li⁺ diffusion and cause SSE degradation, triggering lithium dendrite growth. To stabilize the halide SSE/anode interface, constructing interfacial buffer layers, optimizing SSE structure, and developing lithium alloy anodes are essential strategies.