Halide solid-state electrolytes (SSEs) with high room-temperature ionic conductivity (>10⁻³ S cm⁻¹), wide electrochemical windows, and good compatibility toward oxide cathode materials have achieved impressive progress and attracted significant attention for application in all-solid-state lithium batteries (ASSLBs). This review presents an overview of halide SSEs, including their development, structure, ionic conductivity, chemical stability, and current limitations. Firstly, we give a brief overview of the historical development of halide-based SSEs, followed by an introduction to the different types of halide SSEs. From a practical point of view, the synthesis methods, especially scalable liquid-phase synthesis, are intensively discussed. Then, the associated stability issues involving basic structure stability, air/humidity stability, and electrochemical stability (electrolyte/SSE interface and electrochemical stability window) are also discussed in detail. Comprehensive coverage and thorough understanding of the properties of halide SSEs are provided and it is expected to help guide the development of future SSEs towards ASSLBs for energy storage applications.

Ambient-air-stable Li₃InCl₆ halide solid electrolyte, with high ionic conductivity of 1.49 × 10⁻³ S cm⁻¹ at 25 °C, delivers essential advantages over commercial sulfide-based solid electrolyte.

Sulfide electrolyte (SE)-based all-solid-state lithium batteries (ASSLBs) have gained worldwide attention because of their instrinsic safety and higher energy density over conventional lithium-ion batteries (LIBs). However, poor air stability of SEs, detrimental interfacial reactions, insufficient solid–solid ionic contact, and the large gap between fundamental study and practical engineering have impeded the commercialization of SE-based ASSLBs. This review aims to combine fundamental and engineering perspectives to rationally design practical SE-based ASSLBs with high energy density, covering SEs, interface, and practical all-solid-state pouch cells. First, the latest progress of typical pseudo-binary, pseudo-ternary, and pseudo-quaternary SEs is summarized, and effective strategies to improve ionic conductivity and chemical and electrochemical stability are highlighted. Moreover, challenges and strategies at the cathode and anode interfaces are reviewed separately. Furthermore, advanced in situ characterization techniques are examined to better understand the interface of ASSLBs. Encouraging demonstrations of SE-based all-solid-state lithium-ion and all-solid-state lithium–sulfur batteries are exemplified. Most importantly, energy-density-oriented all-solid-state pouch cells are designed using practical engineering parameters. The proposed design can serve as a quantitative framework to predict the practical energy density of SE-based all-solid-state pouch cells in future. Finally, future directions and our perspectives in SE-based ASSLBs are presented.

The safety of lithium-ion batteries has caused notable concerns about their widespread adoption in electric vehicles. A nascent but promising approach to enhancing battery safety is using solid-state electrolytes (SSEs) to develop all-solid-state batteries, which exhibit unrivaled safety and superior energy density. A new family of SSEs based on halogen chemistry has recently gained renewed interest because of their high ionic conductivity, high-voltage stability, good deformability, and cost-effective and scalable synthesis routes. Here, we provide a comprehensive review of halide SSEs concerning their crystal structures, ion transport kinetics, and viability for mass production. Furthermore, their moisture sensitivity and interfacial challenges are summarized with corresponding effective strategies. Last, halide-based all-solid-state Li-ion and Li-S pouch cells with energy density targets of 400 and 500 Wh kg ⁻¹ are projected to guide future endeavors. This work serves as a comprehensive guideline for developing halide SSEs from material design to practical application.