Unraveling the Foreign-Cation Effect in UCl₃-Type Halide Solid Electrolytes for Low-Temperature All-Solid-State Batteries

UCl₃-based halide solid electrolytes have garnered increasing interest for application in all-solid-state batteries, yet their structural characteristics, chemical composition, and ion transport mechanisms remain under debate. These uncertainties hamper their rational design and broader application. Taking PrCl3 as a model system, we present a comprehensive structural investigation and reveal that foreign cations (e.g., Ta5+, Zr4+ and In3+) preferentially reside in amorphous matrix rather than substituting for Pr³⁺ in crystalline PrCl₃, owing to substantial mismatches in both ionic radii and coordination numbers. Importantly, the dominant pathway for fast Li⁺ conduction lies within the amorphous phase, rather than the PrCl₃ nanocrystals or their interfacial regions. Guided by these insights, Li0.5Pr0.455Ta0.179Zr0.06Cl3 is rationally designed, realizing high ionic conductivity (3.10 mS cm⁻¹) and a low activation energy (0.236 eV). These improved ion-conducting properties enables battery with a capacity retention of 71.7% at 20 mA g-1 and –20 °C, and a prolonged cycle life of 1350 cycles at 100 or 200 mA g-1 and –10 °C. These results underscore the critical role of amorphous phase engineering in halide electrolytes and the potential of UCl₃-type systems for low-temperature all-solid-state batteries. UCl₃-type electrolytes show promise for all-solid-state batteries, yet their structure and Li⁺ transport remain debated. Here, authors reveal foreign cations reside in the amorphous phase, which serve as the dominant Li⁺ pathway yielding enhanced conductivity and low-temperature battery performance.