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.

Solid‐state Li–S and Li–Se batteries are promising devices that can address the safety and electrochemical stability issues that arise from liquid‐based systems. However, solid‐state Li–Se/S batteries usually present poor cycling stability due to the high resistance interfaces and decomposition of solid electrolytes caused by their narrow electrochemical stability windows. Here, an integrated solid‐state Li–Se battery based on a halide Li3HoCl6 solid electrolyte with high ionic conductivity is presented. The intrinsic wide electrochemical stability window of the Li3HoCl6 and its stability toward Se and the lithiated species effectively inhibit degeneration of the electrolyte and the Se cathode by suppressing side reactions. The inherent thermodynamic mechanism of the lithiation/delithiation process of the Se cathode in solid is also revealed and confirmed by theoretical calculations. The battery achieves a reversible capacity of 402 mAh g−1 after 750 cycles. The electrochemical performance, thermodynamic lithiation/delithiation mechanism, and stability of metal‐halide‐based Li–Se batteries confer theoretical study and practical applicability that extends to other energy‐storage systems.

Inorganic solid-state electrolytes (SSEs) have gained significant attention for their potential use in high-energy solid-state batteries. However, there is a lack of understanding of the underlying mechanisms of fast ion conduction in SSEs. Here, we clarify the critical parameters that influence ion conductivity in SSEs through a combined analysis approach that examines several representative SSEs (Li3YCl6, Li3HoCl6, and Li6PS5Cl), which are further verified in the xLiCl-InCl3 system. The scaling analysis on conductivity spectra allowed the decoupled influences of mobile carrier concentration and hopping rate on ionic conductivity. Although the carrier concentration varied with temperature, the change alone cannot lead to the several orders of magnitude difference in conductivity. Instead, the hopping rate and the ionic conductivity present the same trend with the temperature change. Migration entropy, which arises from lattice vibrations of the jumping atoms from the initial sites to the saddle sites, is also proven to play a significant role in fast Li+ migration. The findings suggest that the multiple dependent variables such as the Li+ hopping frequency and migration energy are also responsible for the ionic conduction behavior within SSEs.