Compared to conventional commercial lithium-ion batteries, all-solid-state lithium batteries (ASSLBs) with inflammable inorganic solid electrolytes (SEs) can elevate energy density via choosing the combination of high output voltage and electrodes with a high capacity (i.e., high-nickel ternary cathode and lithium metal anode) and ensure safety. It is crucial to use solid electrolytes with a good electrode compatibility and a high ion conductivity to realize stable cycling at room temperature. Chloride SEs have been developed rapidly in recent years since their preparation in 2018 due to the success of Li₃YCl₆-based InLi/LiCoO₂ ASSLBs without extra interface modifications. This review highlights the excellent cathode compatibility of chloride SEs, which is the most attractive advantage of these SEs. This review also discusses that the easy reduction of central elements causes an unstable interface towards an anode. For the design of chloride SEs, the non-closed packing anion sublattice to enable faster ion transport with a lower migration barrier is described. Finally, this review summarizes the typical annual progress of chloride SEs in the past five years and discusses some challenges to be addressed for the future large-scale application of chloride SEs.

The advances and limitations of chloride SEs from electrode compatibility and ionic conductivity perspectives are analyzed. To better understand recent development of chloride SEs and inspire further explorations and improvements, a typical annual progress is summarized. Chloride SEs were brought back into the spotlight in 2018 due to the stable cycling of uncoated 4 V class cathode LiCoO₂ enabled by Li₃YCl₆ at room temperature. In 2019, a facile and scalable water-mediated synthetic route based on Li₃InCl₆ was developed to overcome the shortcomings of traditional time-consuming mechanical methods. AIMD simulations on LiScCl_3+x is carried out to systematically explore the influence of lithium content on the structure and Li⁺ diffusivity. Note that the above-mentioned chloride SEs (i.e., Li³YCl⁶, Li³InCl⁶ and Li_xScCl_3+x) all use expensive and low-abundance rare-earth elements, which cause high cost and sustainability concerns. Cost-effective Li₂ZrCl₆ with cheap Zr as a central element is developed. As a breakthrough to the close packing anion sublattice of conventional chloride SEs in 2023, a non-close packing style for superionic conduction was realized via selecting large central cation, i.e., LaCl₃-based SE, or employing partial anion replacement, i.e., LiNbOCl₄. The non-close packing anion sublattice with more distorted sites for a low ion migration barrier can theoretically or experimentally realize a high room-temperature ionic conductivity of over 10 mS/cm, compared to those sulfide superionic conductors.

Although a remarkable progress is made on chloride SEs, several crucial challenges have yet to be resolved. The first is still the insufficient interface stability towards the anode. LaCl₃-based SE and Li₃YBr_5.7F_0.3 can enable the cycling of all-solid-state lithium metal battery without extra interface modifications. The interface stabilization mechanisms are still not thoroughly identified. Meanwhile, the 100 cycles and capacity of around 1 mA·h/cm² are insufficient to meet the demand of practical applications. A deeper understanding of interface evolution, accompanied by an artificial interfacial layer to enhance anode interface stability, is needed. The second is that rare and expensive elements like In, Ta, and Sc are still mostly used for high-performance chloride SEs despite Zr-based chloride SEs. It is important for sustainability perspectives and large-scale production to apply cheap and high-abundance elements like Mg, Ca, Al to construct an ion conductive framework and ensure high ionic conductivity and electrode compatibility. The third is that the atmosphere tolerance of chloride SEs needs enhancement to restrain the ionic conductivity loss during ASSLB fabrication due to their easy reaction or combination with water. It is anticipated that chloride SEs can be pushed from laboratory study towards practical applications via taking the intrinsic advantages of chloride SEs and overcoming their shortcomings.