October 2018

In order to develop safe and efficient solid state batteries, solid electrolytes with high ionic conductivity must be developed. It was found that the lattice dynamics of crystalline solid electrolytes, i.e. the dynamic oscillation of the lattice atoms, correlates strongly with the transport of the mobile ions in the solid. The picture shows the structure of a thiophosphatic sodium ion conductor whose sulfur anions (reddish) can be substituted with selenium (yellowish). With the exchange of sulfur with the bigger and more polarizable selenium, a lower activation barrier for an ion jump from one lattice site to an adjacent site could be observed experimentally. This is shown schematically on a flattened 3D potential landscape. In parallel, a frequency minimization of the lattice oscillations (phonon DOS) can be observed, which can be measured experimentally by inelastic neutron scattering, but also predicted theoretically. This frequency minimization goes hand in hand with the observed trends in ionic conductivity and shows that for the screening of possible high-performance solid ion conductors the lattice dynamics as a descriptor must be investigated and understood in more detail. (Picture submitted by Thorben Krauskopf.)