Lithium Thioborate Solid-State Electrolytes for Practical, High-Energy All-Solid-State Lithium Metal Batteries

Solid-state batteries (SSBs) using a lithium metal anode have the potential to transform the energy density and safety of rechargeable batteries for EVs, consumer electronics, and the grid. There is a strong need to improve the chemical/electrochemical stability, ionic conductivity, and fundamental understanding of new materials for the solid-state electrolyte (SSE) before commercialization. Lithium thioborates (LBS) are a new class of SSEs with ionic conductivity predicted to be up to 80 mS cm^-1, making them exciting SSE candidates with unexplored potential. Our work focuses on developing and studying LBS compounds for application in all-solid-state batteries.<br/>Through careful optimization, we have developed a multigram-scale, solvent-free synthesis for high-purity LBS compounds. We have synthesized and fully characterized pure-phase LBS materials Li₂B₂S₅, Li₃BS₃, and Li₁₀B₁₀S₂₀ using X-ray diffraction, FTIR, and electrochemical analysis. Of all LBS stoichiometries, Li₁₀B₁₀S₂₀ is the most promising candidate for SSBs, with experimental Li⁺ conductivity of 0.34 mS cm^-1 and negligible electronic conductivity. Li₁₀B₁₀S₂₀ demonstrates an exceptional voltage window of 1.3-3.0 V vs. Li/Li⁺, wider than most sulfur-based SSEs. At room temperature, in Li-LBS-Li symmetric cells, Li₁₀B₁₀S₂₀ has shown high critical current density of 1.0 mA cm^-2 and stable cycling for hundreds of hours at reasonable current densities as high as 0.3 mA cm^-2. In NMC-LBS-Li full cells using a solid composite cathode, we have demonstrated cycling at C/10 rates with minimal capacity degradation at room temperature and reasonable pressure. These initial results using Li₁₀B₁₀S₂₀ are highly encouraging for future studies on this exciting new class of electrolytes.