The role of ultrafast phonon-ion coupling in solid-state ion conductors

Ion hopping on the picoseconds timescale makes solid-state materials effective electrolytes in batteries. We measure time domain correlations between ions, phonons, and electrons to understand the complex interplay behind such short ion hopping timescales. This includes measurements of multiple ion hopping following excitation of THz rocking modes compared to acoustic and optical phonons in Li_0.5La_0.5TiO₃. We also measure the role of dynamic anion polarizability during a hopping event through a 4-O bottleneck in the same material. The results are compared to the dynamics of ion hopping in other solid-state ion conductors, such as Li₇La₃Zr₂O₁₂, where cubic and tetragonal symmetries change phonon-ion coupling, as well as the multivalent ion conductor ZnPS₃. The measurements are enabled by a new technique termed ultrafast impedance spectroscopy, wherein a laser is used to perturb many-body interactions in the solid-state electrolyte while high-speed electronics measure the resulting ion motion. The measurements give new insight into design rules for solid-state electrolytes. The measurement technique broadly applies to ultrafast ion hopping in a range of battery, fuel cell, and biological samples.