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, Li_7-PS_6-(BH₄)X (X = Cl, Br, I; α + β ≤ 1.8), have been synthesized using a two-step ball-milling method without post-annealing. Among the various compositions, Li_5.35PS_4.35(BH₄)_1.15Cl_0.5 exhibits the highest ionic conductivity of 16.4 mS cm^-1 at 25 °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 Li_5.35PS_4.35(BH₄)_1.15Cl_0.5 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 Li_5.35PS_4.35(BH₄)_1.15Cl_0.5 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.