Li₄B₁₀H₁₀B₁₂H₁₂ as solid electrolyte for solid-state lithium batteries

Hydridoborates are a promising class of solid electrolytes for solid-state batteries, combining liquid-like room temperature ionic conductivity, high (electro-)chemical stability, low gravimetric density, easy processability, and low toxicity. We show that cation exchange is a feasible method to prepare Li2B10H10 and Li2B12H12 from Na2B10H10 and Na2B12H12, respectively, with high yields. Ball milling of an equimolar mixture of Li2B10H10 and Li2B12H12 yields a single phase, isomorphic to the low temperature Li2B12H12 phase (space group Pa3) with ordered [B12H12](2-) and disordered [B10H10](2-) anions sharing the same position in the structure. The ionic conductivity of the equimolar mixture Li4B10H10B12H12 exhibits 4 x 10(-4) S cm(-1) at 25 degrees C and 4 x 10(-)3 S cm-1 at 60 degrees C, respectively, exceeding those of the unmixed phases by several orders of magnitude. The electrolyte possesses an oxidative stability >3 V vs. Li+/Li and thermal stability beyond 300 degrees C, and is evaluated in proof-of-concept solid-state batteries with a lithium metal anode and with titanium disulfide (TiS2) or lithium iron phosphate (LiFePO4) as a cathode active material. Discharge capacities of 83% and 73% of the theoretical capacity were achieved for TiS2 and LiFePO4, respectively, at the end of the first dis-/charge cycle. For LiFePO4, the de-/lithiation potential lies outside the electrochemical stability window of the electrolyte, requiring additional measures to protect the electrolyte from decomposition. Our study demonstrates the feasibility of using closohydridoborates as ionic conductors in solid-state lithium batteries.