Hydrated alkali-B11H14 salts 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 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.