Magnesium Borohydride Pyridine Derivatives as Solid Electrolytes for All-Solid-State Batteries

Abstract

All-solid-state batteries based on abundant elements may pave the way for safer and cheaper energy storage. Magnesium borohydride derivatives with neutral ligands are a new emerging class of solid-state Mg2+ ionic conductors, and here we report the discovery of two new pyridine derivatives, Mg(BH4)2∙xN(CH)5 (x = 2 or 3). Magnesium tetrahydridoborate tripyridine, Mg(BH4)2∙3N(CH)5, crystallizes in the monoclinic space group C2/c and is built from molecular units consisting of trigonal bipyramidal [Mg(N(CH)5)3(BH4)2] complexes, where the pyridine molecules are packed efficiently as a result of π−π stacking. Magnesium tetrahydridoborate dipyridine, Mg(BH4)2∙2N(CH)5, crystallizes in the monoclinic space group Cc and is built from tetrahedral [Mg(N(CH)5)2(BH4)2] complexes. The highest ionic conductivity is observed for Mg(BH4)2∙3N(CH)5 with σ(Mg2+) = 7.2∙10-5 S∙cm-1 at 24 °C, increasing to (Mg2+) = 1.4∙10-4 S∙cm-1 at 49 °C. The low activation energy of 0.25 eV shows promise for low temperature conductivity. A low electronic conductivity of σe = 2.5∙10-10 S cm-1 was determined for Mg(BH4)2∙3N(CH)5 at 24 °C, providing a high ionic transport number of tion = 0.999997.