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

초록

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 PS43− 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.

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