Deciphering Key Features Determining Electrochemical Stability and Conductivity of Halide Solid‐State Electrolytes

Abstract Halide solid‐state electrolytes (HSSEs) are promising electrolytes for all‐solid‐state batteries (ASSBs), benefiting from their high ionic conductivity and compatibility with high‐voltage cathodes. However, HSSEs exhibit poor interfacial stability against anodes (such as lithium (Li) metal and Li alloys), and interlayers are commonly introduced to circumvent the interfacial instability, which would reduce the overall energy density. Meanwhile, the underlying factors governing the stability of HSSEs against anodes remain poorly understood. Herein, key predictors of stability against anodes and ionic conductivity are identified using a series of Li a ZrCl b O c Br d F e (LZOXs) solid‐state electrolytes through multi‐anion substitution and chemistry‐informed machine learning. As a result, the optimal composition, Li 2 ZrCl 4.4 O 0.4 Br 0.4 F 0.4 (LZOX) enables 1500 h of stable cycling in LiAl symmetric cells and achieves 200 cycles at 0.5 C and 600 cycles at 2 C in LiCoO 2 ASSBs without anode interlayers, effectively doubling the cycling life. More importantly, chemistry‐informed machine learning deciphers key features—especially electronegativity for interfacial stability and cationic polarization for ionic conductivity—in the design of HSSEs, offering a strategy to develop HSSEs with enhanced compatibility toward high‐performance ASSBs.