Fast ion conduction in garnet-type metal borohydrides Li 3 K 3 Ce 2 (BH 4 ) 12 and Li 3 K 3 La 2 (BH 4 ) 12

Abstract Complex hydrides are a family of compounds which have attracted a lot of attention in the last decade for various clean energy-related purposes, from solid state hydrogen storage to materials suitable in Li-ion batteries. We present two new garnet-type borohydride materials suitable as solid state electrolytes. Li 3 K 3 Ce 2 (BH 4 ) 12 and Li 3 K 3 La 2 (BH 4 ) 12 show unexpectedly high room temperature Li + ionic conductivity (compared to the reported isostructural garnet oxide Li-conductor) of σ Li 3 × 10 −7 and 6 × 10 −7  S/cm with corresponding activation energies of E a  = 0.79 and E a  = 0.67 eV, respectively, which result from large bottleneck windows in the conduction path. The effect of heterovalent cation substitution is investigated as means of tailoring ionic conductivity. Doping with divalent Sr 2+ and Eu 2+ shows that σ Li can be increased by one order of magnitude in the whole temperature range measured.