Fast divalent conduction in MB12H12·12H2O (M = Zn, Mg) complex hydrides: effects of rapid crystal water exchange and application for solid-state electrolytes

This work investigates the divalent conduction of Zn2+ and Mg2+ in hydrated closo-type complex hydrides MB12H12·nH2O (M = Zn, Mg) and the effects of crystal water exchange in the structure for enhancing divalent ionic conduction. MB12H12·nH2O (M = Zn, Mg), which exhibited different hydrous phases, was synthesised as a single phase and characterised using X-ray diffraction and Raman spectroscopy for determining the number of crystal water molecules and the cationic water coordination environment. Among the anhydrous and hydrated MB12H12·nH2O (n = 0–12 and M = Zn, Mg), ZnB12H12·12H2O and MgB12H12·12H2O exhibited exceptional ionic conductivities of 3.8 × 10−5 and 6.1 × 10−5 S cm−1 at 50 °C, respectively, thereby being of greater significance than less-hydrated MB12H12·nH2O (n ≤ 6), and a few of the highest solid-state Zn2+ and Mg2+ conductivities reported so far. 1H, 11B, and 67Zn NMR measurements show narrow peaks indicating rapid crystal water exchange, fast [B12H12]2− anion rotation, and rapid Zn2+ isotropic motion in ZnB12H12·12H2O. Electrochemical cells fabricated using the ZnB12H12·12H2O solid electrolyte exhibited reversible Zn2+ migration through the material, indicating that ZnB12H12·12H2O shows favourable potential for application as a Zn2+ conductor in all-solid-state Zn batteries.