Li4B10H10B12H12 as solid electrolyte for solid-state lithium batteries

Abstract

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 Pa) 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 × 10⁻⁴ S cm⁻¹ at 25 °C and 4 × 10⁻³ S cm⁻¹ at 60 °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 °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 closo-hydridoborates as ionic conductors in solid-state lithium batteries.