Defect Chemistry And Hydrogen Transport In La/Sr-Based Oxyhydrides

Oxyhydrides in the series La2-ySryLiH1+ O-y(3-y) are attractive materials for solid-state hydride electrolytes. However, their chemical stability is poor, and thus far only La2LiHO3 (y = 0) has been demonstrated in a working fuel cell. Using a first-principles approach to study defect chemistry and stability, we find that La2LiHO3 can be stabilized over a range of H-rich, O-poor chemical conditions, while Sr2LiH3O has low stability. Defects are critical for ionic transport and for stability, particularly in Sr2LiH3O, in which anion antisites and H vacancies readily form. In fact, we show that the ground-state structure of Sr2LiH3O is inherently disordered. La2LiHO3 has low conductivity, which we connect to the high formation energy of defects such as V-O and O-H that are necessary for hydride conduction. When Sr2LiH3O is grown under carefully selected conditions, V-H(+) defects can be made prevalent; these defects give rise to much higher ionic conductivity than can be obtained in La2LiHO3. In general, we show that the choice of synthesis conditions is vital when seeking to optimize the stability and ionic conductivity of these oxyhydrides, thereby tailoring them for use in solid-state hydrogen fuel cells.