Influence of Lattice Dynamics on Na⁺Transport in the Solid Electrolyte Na₃PS4–xSex

Li + - and Na + -conducting thiophosphates have attracted much interest because of their intrinsically high ionic conductivities and the possibility to be employed in solid-state batteries. Inspired by the recent finding of the influence of changing lattice vibrations and induced lattice softening on the ionic transport of Li + -conducting electrolytes, here we explore this effect in the Na + conductor Na 3 PS 4– x Se x . Ultrasonic speed of sound measurements are used to monitor a changing lattice stiffness and Debye frequencies. The changes in the lattice dynamics are complemented by X-ray diffraction and electrochemical impedance spectroscopy. With systematic alteration of the polarizability of the anion framework, a softening of the lattice can be observed that leads to a reduction of the activation barrier for migration as well as a decreased Arrhenius prefactor. This work shows that, similar to Li + transport, the softening of the average vibrational frequencies of the lattice has a tremendous effect on Na + -ionic transport and that ion–phonon interactions need to be considered in solid electrolytes.