Ultrafast Ionic Hopping, Electron, and Phonon Correlations in Solid State Electrolytes

Superionic conductors are solid-state materials that allow ion diffusion at speeds that approach those of polymer and liquid solvents (tilde 1 mS/cm). The mobile ions move through channels in the lattice cage by picosecond interactions with vibrational modes, referred to as phonons. The superionic conductor can be nanometers to micrometers thick while preventing dendrite formation, allowing for compact and safe battery packaging with fast charging times. An ionic species moving through a solid state lattice may induce many-body correlations with charge carriers, phonons, and other ions - just as an electron moving through a lattice does. After discovering superionic conductors like RbAg4I5, lattice-gas model models predicted that high ionic conductivity could only exist if the ion and phonons correlated on vibrational timescales. As the ionic conductivity of synthesized materials increased, later theoretical work posited that ion-electron cloud and ion-ion correlations must also be present on a picosecond time scale for high ionic conductivity. A model based on the geometric hindrance of the lattice channel, ignoring many-body correlations, fails to describe most superionic conductors. This proposal aims to temporally resolve the electron-ion and phonon-ion correlations theorized to be essential for solid-state ionic conduction. Current measurements of battery dynamics are usually limited to microseconds and longer times with traditional impedance techniques. Nuclear magnetic resonance studies can access faster dynamics with site specificity through linewidth changes. Neutron scattering can measure pair-correlations, which are then interpreted to short time scale dynamics. The hopping time is extrapolated from Arrhenius-type plots in these non-picosecond techniques, leading to a reported nine orders of magnitudes read in the fit hopping frequency.