Electropositive Metal Doping into Lanthanum Hydride for H⁻ Conducting Solid Electrolyte Use at Room Temperature

<jats:title>Abstract</jats:title><jats:p>Hydride ion conductors have made remarkable progress in recent years; in particular, the fluorite‐type LaH<jats:sub>3‐δ</jats:sub> series exhibits high conductivity around room temperature. However, its intrinsic character of hydrogen non‐stoichiometry still makes its application as a solid electrolyte challenging, for which high electronic insulation is essential. Here, Sr‐substituted LaH<jats:sub>3‐δ</jats:sub> with slight O<jats:sup>2−</jats:sup> incorporation, represented as La<jats:sub>1‐</jats:sub><jats:italic><jats:sub>x</jats:sub></jats:italic>Sr<jats:italic><jats:sub>x</jats:sub></jats:italic>H<jats:sub>3‐</jats:sub><jats:italic><jats:sub>x</jats:sub></jats:italic><jats:sub>‐2</jats:sub><jats:italic><jats:sub>y</jats:sub></jats:italic>O<jats:italic><jats:sub>y</jats:sub></jats:italic> (0.1 ≤ <jats:italic>x</jats:italic> ≤ 0.6, <jats:italic>y</jats:italic> ≤ 0.171), is synthesized, which exhibits H<jats:sup>−</jats:sup> conductivity of 10<jats:sup>−4</jats:sup> – 10<jats:sup>−5</jats:sup> S cm<jats:sup>−1</jats:sup> at room temperature. The galvanostatic discharge reaction using an all‐solid‐state cell composed of Ti|La<jats:sub>1‐</jats:sub><jats:italic><jats:sub>x</jats:sub></jats:italic>Sr<jats:italic><jats:sub>x</jats:sub></jats:italic>H<jats:sub>3‐</jats:sub><jats:italic><jats:sub>x</jats:sub></jats:italic><jats:sub>‐2</jats:sub><jats:italic><jats:sub>y</jats:sub></jats:italic>O<jats:italic><jats:sub>y</jats:sub></jats:italic>|LaH<jats:sub>3‐δ</jats:sub> shows that the Ti electrode is completely hydrogenated to TiH<jats:sub>2</jats:sub> for <jats:italic>x</jats:italic> ≥ 0.2, whereas a short circuit occurs for <jats:italic>x</jats:italic> = 0.1. These experimental observations, together with calculation studies on the density of states and the defect formation energy, provide clear evidence that electropositive cation, such as Sr, doping critically suppresses the electron conduction in LaH<jats:sub>3‐δ</jats:sub>. Achieving a superior H<jats:sup>−</jats:sup> conducting solid electrolyte is a novel milestone in the development of electrochemical devices that utilize its strong reducing ability (<jats:italic>E</jats:italic>°(H<jats:sup>−</jats:sup>/H<jats:sub>2</jats:sub>) = −2.25 V vs SHE), such as batteries with high energy density and electrolysis/fuel cells with high efficiency.</jats:p>