Hydride Ion Conductors with Polyanionic Complex Anions

Hydride ion (H^-)-conducting solid-state materials have lately received great attention for advanced electrochemical energy storage/conversion systems, including hydride ion-based batteries, electrolysis, and fuel cells. However, the highly reactive nature of hydride ions has posed challenges in diversifying anion systems, which are crucial for designing their effective transport. This study reports perovskite-type hydride ion conductors, Sr_1-NaLiH_3-(BH₄) (0 ≤ x ≤ 1 and 0 ≤ y ≤ 0.1), employing polyanionic borohydride (BH₄^-) as the anions in the structure. Structural characterization indicates that single-phase hydride ion conductors, in which H^- and BH₄^- coexist in the cubic perovskite structure, are stabilized at the low-x region. In addition, incorporating H^- vacancies (an increase in y) enhances the disorder of H^- and BH₄^-, improving hydride ion conductivity by 3 orders of magnitude. Neutron powder diffraction analysis reveals that the polyanionic BH₄^- interacts asymmetrically with the cations (Sr²⁺ and Na⁺), thereby facilitating hydride ion conduction through pathways where these interactions are weaker. This unusual structure allows for a high hydride ion conductivity of over 10^-4 S cm^-1 at 100 °C. The current study suggests that many complex anions can be promising candidates as novel anion systems for hydride ion conductors.