Sodium and magnesium ionic conduction in complex hydrides

Sodium and magnesium ionic conduction in complex hydrides as a new category of solid-state fast ionic conductors is presented. We first investigated experimentally the sodium ionic conductivities of the representative Na-based complex hydrides NaBH4, NaNH2, NaAlH4, and Na3AlH6, all of which have single complex anion [BH4]−, [NH2]−, [AlH4]− and [AlH6]3−, respectively. Na3AlH6 showed much higher ionic conductivity of 5 × 10−7 S/cm at 300 K than NaBH4, NaNH2, and NaAlH4 (their ionic conductivities were around 10−10 S/cm). We obtained the sodium fast-ionic conductor Na2(BH4)(NH2) with the [BH4]− and [NH2]− complex anions by combining NaBH4 and NaNH2 with a molar ratio of 1:1. It exhibited the most superior sodium ionic conductivity of 3 × 10−6 S/cm at 300 K because of the specific antiperovskite-type structure with vacancies in the Na+ site. Furthermore, first-principles molecular dynamics simulations suggested the possible magnesium ionic conduction in the high-temperature phase of Mg(BH4)2 by substituting partly [BH4]− with [AlH4]−.

• We investigate experimentally sodium ionic conduction in complex hydrides. • Sodium fast-ionic conductor Na2(BH4)(NH2) is obtained by combining NaBH4 and NaNH2. • We simulate possible magnesium ionic conduction in Mg(BH4)2.