Tunable Lithium-Ion Transport in Mixed-Halide Argyrodites Li6-xPS5-xClBrx: An Unusual Compositional Space

Argyrodites, with fast lithium-ion conduction, are promising for applications in rechargeable solid-state lithium-ion batteries. We report a new compositional space of argyrodite superionic conductors, Li6-xPS5-xClBrx [0 {$\leq$} x {$\leq$} 0.8], with a remarkably high ionic conductivity of 24 mS/cm at 25 {$^\circ$}C for Li5.3PS4.3ClBr0.7. In addition, the extremely low lithium migration barrier of 0.155 eV makes Li5.3PS4.3ClBr0.7 highly promising for low-temperature operation. Average and local structure analyses reveal that bromination (x {$>$} 0) leads to (i) retention of the parent Li6PS5Cl structure for a wide range of x in Li6-xPS5-xClBrx (0 {$\leq$} x {$\leq$} 0.7), (ii) co-occupancy of Cl-, Br- , and S2- at 4a/4d sites, and (iii) gradually increased Li+-ion dynamics, eventually yielding a ``liquid-like'' Li-sublattice with a flattened energy landscape when x approaches 0.7. In addition, the diversity of anion species and Li-deficiency in halogen-rich Li6-xPS5-xClBrx induce hypercoordination and coordination entropy for the Li-sublattice, also leading to enhanced Li+-ion transport in Li6-xPS5-xClBrx. This study demonstrates that mixed-anion framework can help stabilize highly conductive structures in a compositional space otherwise unstable with lower anion diversity.