Hydrated alkali-B11H14 salts as potential solid-state electrolytes

Abstract

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 by closo- and nido-boranes. In this study, an optimised low cost preparation method of MB₁₁H₁₄·(H₂O)_n, (M = Li and Na) and KB₁₁H₁₄ is 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 LiB₁₁H₁₄ and NaB₁₁H₁₄ coordinated with water, whereas KB₁₁H₁₄ is anhydrous. Hydrated LiB₁₁H₁₄·(H₂O)_n and NaB₁₁H₁₄·(H₂O)_n exhibit exceptional ionic conductivities at 25 °C, 1.8 × 10⁻⁴ S cm⁻¹ and 1.1 × 10⁻³ S 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 V vs. Li⁺/Li and 2.6 V vs. Na⁺/Na, respectively. KB₁₁H₁₄ undergoes a reversible polymorphic structural transition to a metastable phase before decomposing. All synthesised nido-boranes decompose at temperatures greater than 200 °C.