Boron-hydrogen based compounds such as borohydrides (M(BH4)x, M=Li, Na, Mg, …) or closo-boranes (Mx(ByHy), y=10, 12) are extensively discussed as lightweight hydrogen storage materials or as unwanted, stable side-phases in the hydrogen sorption cycle, respectively. Recently, especially the stable closo-boranes gained attention as superionic lithium and sodium conductors in their respective high temperature phases. Here we report the discovery of (i) lithium-ion conductivity of 6.4 mS/cm at 40 °C in BH4-rich lithium amide-borohydride Li(BH4)1−x(NH2)x (see figure 1a) and of (ii) sodium-ion conductivity of 0.9mS/cm at 20 °C in the mixed anion system Na2(B12H12)0.5(B10H10)0.5. Especially the large electrochemical stability of 3 V including the stability against metallic sodium makes the latter compound a promising candidate for an electrolyte in an all solid state sodium battery. We discuss the conduction mechanism in these compounds on the basis of structural data and density functional theory calculations and present symmetrical electrochemical half cells, demonstrating the high AC conductivity can be translated in fast DC lithium (or sodium) transport (see figure 1b). Finally, the development of a solid state sodium battery with metallic sodium as anode and a transition metal oxide cathode will be outlined.

Borohydride-based solid-state electrolytes with liquid-like sodium and lithium conductivity at room temperature

255th National Meeting (2018)

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