Enhancing Li ⁺ Ion Transport via Dynamic Coupling With Borohydride Reorientation in Li ₆ PS ₅ X Argyrodites

ABSTRACT All‐solid‐state batteries employing solid electrolytes offer improved safety and thermal stability compared to conventional lithium‐ion batteries. Among various solid electrolytes, Li‐argyrodites are particularly attractive due to their high ionic conductivity and favorable mechanical properties. In this study, we investigate the effect of borohydride (BH ₄ ⁻ ) substitution on the structure and Li ⁺ ion dynamics of Li‐argyrodites (Li ₆ PS ₅ X, X = Cl ⁻ , BH ₄ ⁻ ) using solid‐state NMR, pulsed field gradient NMR, and ab initio molecular dynamics to clarify the underlying mechanism of conductivity enhancement. We show that BH ₄ ⁻ occupancy at both non‐bridging sulfur sites (Wyckoff 4a and 4d) promotes Li ⁺ transport by inducing cage‐size uniformity, which reduces energetic mismatch between interconnected Li ⁺ cages and enables continuous, long‐range inter‐cage diffusion. Moreover, BH ₄ ⁻ reorientation at these sites is strongly coupled to Li ⁺ transport, facilitating long‐range diffusion via a paddle‐wheel–like mechanism, whereas PS ₄ ³⁻ reorientation does not significantly contribute to Li ⁺ ion mobility. Finally, we demonstrate that strong interfacial contact and efficient Li ⁺ exchange at the cathode–electrolyte interphase are critical for optimizing solid electrolytes and achieving high‐performance all‐solid‐state batteries. Our findings elucidate the structural and dynamic origins of conductivity enhancement and provide design principles for advanced solid electrolytes.