A New Solid Li-ion Electrolyte from the Crystalline Lithium-Boron-Sulfur System

We report a solid-state Li-ion electrolyte predicted to exhibit simultaneously fast ionic conductivity, wide electrochemical stability, low cost, and low mass density. We discover four phases within the crystalline lithium-boron-sulfur (LBS) system, Li5B7S13, Li2B2S5, Li3BS3, and Li9B19S33, with exceptional DFT based single crystal ionic conductivity values at room temperature of approximately 74 mS cm–1, 10 mS cm–1, 2 mS cm–1, and 28 mS cm–1 respectively. To our knowledge, our prediction gives Li5B7S13 the second-highest reported DFT-computed single crystal ionic conductivity of any crystalline material. We compute the thermodynamic electrochemical stability window widths of these materials to be 0.50, 0.16, 0.45, and 0.60 V. Individually, these materials exhibit similar or better ionic conductivity and electrochemical stability than the best known sulfide-based solid-state Li-ion electrolyte materials, including Li10GeP2S12. However, we predict that electrolyte materials synthesized from a range of compositions in LBS system may exhibit even wider thermodynamic electrochemical stability windows of 0.63 V and possibly as high as 3 V or greater. The LBS system also has low elemental cost of approximately 0.05 USD/m2 per 10 μm thickness, significantly lower than that of germanium-containing LGPS, and a comparable mass density below 2 g/cc. These fast conducting phases were initially discovered by a machine learning-based approach to screen over 12,000 solid electrolyte candidates, and the evidence provided here represents an inspiring success for this model.