Enhancing Li⁺ Ion Transport via Dynamic Coupling With Borohydride Reorientation in Li₆PS₅X Argyrodites

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 (BH₄ ^-) substitution on the structure and Li⁺ ion dynamics of Li-argyrodites (Li₆PS₅X, X = Cl^-, BH₄ ^-) using solid-state NMR, pulsed field gradient NMR, and ab initio molecular dynamics to clarify the underlying mechanism of conductivity enhancement. We show that BH₄ ^- 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, BH₄ ^- reorientation at these sites is strongly coupled to Li⁺ transport, facilitating long-range diffusion via a paddle-wheel-like mechanism, whereas PS₄ ^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.