Hydrated alkali-B₁₁H₁₄salts as potential solid-state electrolytes

Metal boron-hydrogen compounds are considered as promising solid electrolyte candidates for the development of all-solid-state batteries (ASSB), owing to the high ionic conductivity exhibited bycloso- andnido-boranes. In this study, an optimised low cost preparation method of MB₁₁H₁₄·(H₂O)_n, (M = Li and Na) and KB₁₁H₁₄is proposed and analysed. The formation of the B₁₁H₁₄⁻salt is pH-dependent, and H₃O⁺competes with small ionic radii cations, such as Li⁺and Na⁺, to produce a hydronium salt of B₁₁H₁₄⁻, which forms B₁₁H₁₃OH⁻upon heating. The use of diethyl ether to extract B₁₁H₁₄⁻salt from the aqueous medium during synthesis is an important step to avoid hydrolysis of the compound upon drying. The proposed method of synthesis results in LiB₁₁H₁₄and NaB₁₁H₁₄coordinated with water, whereas KB₁₁H₁₄is anhydrous. Hydrated LiB₁₁H₁₄·(H₂O)_nand NaB₁₁H₁₄·(H₂O)_nexhibit exceptional ionic conductivities at 25 °C, 1.8 × 10⁻⁴S cm⁻¹and 1.1 × 10⁻³S cm⁻¹, respectively, which represent some of the highest solid-state Li⁺and Na⁺conductivities at room temperature. The salts also exhibit oxidative stability of 2.1 Vvs.Li⁺/Li and 2.6 Vvs.Na⁺/Na, respectively. KB₁₁H₁₄undergoes a reversible polymorphic structural transition to a metastable phase before decomposing. All synthesisednido-boranes decompose at temperatures greater than 200 °C.