Borohydride and halide dual-substituted lithium argyrodites

Solid electrolyte is a crucial component of all-solid-state batteries, with sulphide solid electrolytes such as lithium argyrodite being closest to commercialization due to their high ionic conductivity and formability. In this study, borohydride/halide dual-substituted argyrodite-type electrolytes, Li7-alpha-beta PS6-alpha-beta(BH4)alpha X beta (X = Cl, Br, I; alpha + beta <= 1.8), have been synthesized using a two-step ball-milling method without post-annealing. Among the various compositions, Li5.35PS4.35(BH4)1.15Cl0.5 exhibits the highest ionic conductivity of 16.4 mS cm-1 at 25 degrees C when cold-pressed, which further improves to 26.1 mS cm-1 after low temperature sintering. The enhanced conductivity can be attributed to the increased number of Li vacancies resulting from increased BH4 and halide occupancy and site disorder. Li symmetric cells with Li5.35PS4.35(BH4)1.15Cl0.5 demonstrate stable Li plating and stripping cycling for over 2,000 hours at 1 mA cm-2, along with a high critical current density of 2.1 mA cm-2. An all-solid-state battery prepared using Li5.35PS4.35(BH4)1.15Cl0.5 as the electrolyte and pure Li as the anode exhibits an initial coulombic efficiency of 86.4%. Although these electrolytes have limited thermal stability, it shows a wide compositional range while maintaining high ionic conductivity. We have successfully synthesized new borohydride/halide dual-substituted lithium argyrodites, which have higher ionic conductivity and electrochemical stability and critical current density compared to other halide-only substituted argyrodites.