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

Abstract

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.

Keywords: solid-state electrolyte, Density functional theory, machine learning, superionic conductor, lithium thioboride