A trade-off between migration and association energies for hydride-ion conductivity in the SrLiH3–CaLiH3–NaLiH2 system

Abstract

Hydride-ion (H⁻) conductors have garnered much attention owing to their high ionic conductivity and potential applications such as batteries and fuel/electrolysis cells. Perovskite-type H⁻ conductors are known to exhibit relatively high ionic conductivity at room temperature. The present work demonstrated systematic material exploration within the SrLiH₃–CaLiH₃–NaLiH₂ pseudo-ternary system. The Na-substituted system, Sr_1−xNa_xLiH_3−x, exhibited a remarkable H⁻ conductivity of 5.1 × 10^–6 S cm⁻¹ at 25 °C for Sr_0.8Na_0.2LiH_2.8, marking the highest value reported among perovskite-type hydrides to date. Furthermore, we found a clear trend of enhanced H⁻ conductivity with Ca substitution in the Sr_1−xCa_xLiH₃ pseudo-binary system. However, in the Sr_1−xCa_xNa_yLiH_3−y pseudo-ternary system, a negative synergistic effect of Ca and Na co-doping was observed. First-principles calculations revealed that this negative effect arises from a trade-off between migration and association energies in defect pairs of Na⁺ dopants and H⁻ vacancies. These findings provide valuable insights into designing superior anion conductors.