New insights into ion transport in next-generation solid electrolytes

The results of a new study show how materials scientists may be able to fine-tune the ionic transport properties to meet specific technological needs in catalysis and­ solid-state energy devices.

Mixed-anion materials are chemical compounds that contain positive ions (cations) and more than one kind of negative ions (anions) within a single crystal structure. This offers a greater tunability of their physical and chemical properties than conventional single-anion compounds. The most widely studied class of such compounds are oxyhydrides, containing both oxide ions (O²⁻) and hydride ions (H^-). This mixing of anions modifies bond strengths, enhances chemical reactivity and enables faster anion diffusion. These unique properties make oxyhydrides promising for diverse applications in both energy storage and energy conversion.

In some oxyhydrides, electrons and hydride ions are both mobile, which make them attractive for use as catalysts in various chemical reactions. In particular, the oxyhydride of barium titanate (BaTiO_3-2xH_x□_x, where □ denotes oxygen vacancies) has drawn attention for its remarkable catalytic activity in ammonia synthesis ^[1,2].

Hydride ions move by hopping into neighboring oxygen vacancies, that is, missing oxide-ions in the crystal structure, however the mechanistic detail of such hops and the response of the electronic sublattice have remained unclear ^[2]. A study now published shows that how easily a hydride ion can jump depends critically on what happens to the electrons associated with those vacancies ^[1].

Using density functional theory calculations, researchers from the ILL and Chalmers University of Technology (Sweden) modeled how hydride ions diffuse under different conditions. By comparing several scenarios ranging from strongly localised to fully delocalised electrons, they quantified how electron localisation affects the energy barrier that a hydride ion must overcome to move from one site to another.

When the electrons in the material are fully delocalised, corresponding to a metal-like behavior of the material, hydride ions move easily, with a low diffusion barrier of only 0.29 eV. In contrast, when the electrons become localised at a vacancy, corresponding to a semiconductor-like electronic conductivity, the barrier rises to 0.83 eV, significantly slowing hydride ion motion. Intermediate electronic states produce moderate barriers of about 0.4–0.6 eV.