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

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, Li_0.5Pr_0.455Ta_0.179Zr_0.06Cl₃ 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.