Corner-Sharing PS$_4$-BS$_4$ Modes Facilitate Fast Ion Conduction in Lithium Thioborophosphate Iodide Glassy Solid Electrolytes

Abstract: Glassy solid electrolytes (GSEs), with their amorphous nature and the absence of grain boundaries, make them highly attractive for applications in all-solid-state lithium batteries (ASSLBs), a leading candidate for next-generation energy storage technologies. A recently developed lithium thioborophosphate iodide GSE, composed of 30Li$_2$S-25B$_2$S$_3$-45LiI-5P$_2$S$_5$ (LBPSI), has demonstrated excellent room-temperature ionic conductivity and low activation energy. Despite this exciting finding, the underlying mechanism behind this ultrafast ion transport remains ambiguous. Here, we accurately fine-tune the foundational MACE-MP-0 model and perform large-scale machine learning molecular dynamics simulations to investigate the structural and ion dynamics in LBPSI GSE. Our results reveal that B$_2$S$_3$ glass formers primarily form multi-bridged B$_x$S$_y$ long-chain networks that impede Li$^+$ conduction. In contrast, P$_2$S$_5$ gives rise to mono-tetrahedral PS$_4$$^{3-}$ and di-tetrahedral P$_2$S$_7$$^{4-}$ tetrahedra, which engage in distinctive corner-sharing modes with BS$_4$$^{5-}$ tetrahedra, effectively disrupting the B$_x$S$_y$ chains and enhancing Li$^+$ mobility. Furthermore, the polyhedral anion rotations of PS$_4$$^{3-}$ and BS$_4$$^{5-}$ in the corner-sharing PS$_4$-BS$_4$ motifs may further promote fast Li$^+$ conduction.