Larger lanthanide ions result in a greater barrier to H− migration in LnSrLiH2O2 because a large ξ requires a greater deviation from the optimum Li–H distance at the saddle point.

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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.