Highly Conductive Iodine and Fluorine Dual-Doped Argyrodite Solid Electrolyte for Lithium Metal Batteries

Sulfide-type argyrodite solid electrolytes (SEs) with halide doping have attracted serious interest. Although other halides such as Cl and Br have been found to enhance Li-ion transport in argyrodites, the direct synthesis of efficiently conductive Li6PS5I without postprocessing has rarely been investigated. In this work, we report the one-step synthesis of highly conductive Li6PS5I with an impressive ionic conductivity of 2.5 × 10–4 S cm–1 at room temperature through a solvent-based method. Moreover, by introducing F– to partially replace I–, hybrid-doped argyrodites Li6PS5FxI1–x (x = 0.25, 0.5, 0.75) have been synthesized. Li6PS5F0.25I0.75 achieves the highest conductivity of 3.5 × 10–4 S cm–1 due to the energetic preference for anion-disordering among F–/I– and S2–, which facilitate faster Li transport as supported by density functional theory (DFT) calculations. With higher F content in argyrodites, Li6PS5F0.75I0.25 displays the best electrochemical stability toward Li metal, as evidenced by long-term stable cycling in Li symmetric cells up to 1100 h. Solid-state Li metal batteries with an active cathode of Li4Ti5O12 (LTO) display an initial specific capacity of 140 mAh g–1 and remain at 105 mAh g–1 after 200 cycles, suggesting great battery cycling performance. This research has developed new compositions in the argyrodite SE family which could lead to advancements in the development of solid-state Li metal batteries.