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The effect of Ca doping on the Li-ion conductivity and phase stability of the rock-salt-type LiBH phase emerging under high pressures in the range of gigapascals has been investigated. In situ electrochemical measurements under high pressure were performed using a cubic-anvil-type apparatus. Ca doping drastically enhanced the ionic conductivity of the rock-salt-type phase: the ionic conductivity of undoped and 5 mol %Ca-doped LiBH was 2.2 × 10 and 1.4 × 10 S·cm under 4.0 GPa at 220 °C, respectively. The activation volume of LiBH-5 mol %Ca(BH), at 3.2 cm·mol, was comparable to that of other fast ionic conductors, such as lithium titanate and NASICONs. Moreover, Ca-doped LiBH showed lithium plating-stripping behavior in a cyclic voltammogram. These results indicate that the conductivity enhancement by Ca doping can be attributed to the formation of a LiBH-Ca(BH) solid solution; however, the solid solution decomposed into the orthorhombic LiBH phase and the orthorhombic Ca(BH) phase after unloading the high pressure.

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