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.

No abstract

Few hydride-ion conductors exhibit chemical stability and hydride-ion conductivity, which are essential properties in developing hydride-ion-driven electrochemical devices. Herein, we successfully synthesized a novel hydride-fluoride system, SrMgH4 − xFx, possessing high atmospheric stability and relatively high hydride-ion conductivity. The correlation between anion composition, structure, hydride-ion conductive properties, and atmospheric stability in the hydride-fluoride system was elucidated. The doping of fluoride ions at hydrogen sites resulted in high atmospheric stabilities while retaining hydride-ion conductivities of SrMgH2F2 and SrMgHF3 of 3.4 × 10− 4 and 1.4 × 10− 4 S cm− 1 at 400°C, respectively. For SrMgHF3, in particular, 84% ionic conductivity was retained after exposure to the atmosphere. Hydride-fluoride systems enable the balancing of the ionic conductivity and atmospheric stability by controlling the H/F ratio. Our findings provide insight for use in effective material design to expand the use of metal hydrides in practical solid electrolytes for use in hydride-ion-driven devices.