Ultrafast Measurements of Site-to-Site Ion Hopping in Solid State Ionic Conductors

Ion hopping in batteries is a cutting-edge realm for ultrafast spectroscopy. We present a new experimental technique that can directly measure ultrafast ion hopping on its inherent picosecond and longer timescale in terms of the ion-phonon and ion-electron correlations. The method works by measuring the time-resolved perturbation to a GHz impedance signal when potential ion-coupling interactions are driven with UV to THz irradiation. High-bandwidth, real-time electronics allow synchronization of the impedance measurement to ultrafast laser pulses for varying carrier frequencies. The measurement result is the relative strength and temporal dynamics of ion-electron and ion-phonon correlations. We demonstrate the technique on Li_0.5La_0.5TiO₃ (LLTO), a solid-state Li⁺ conductor. The ultrafast laser-driven impedance measurements reveal that the dominant ion hopping mechanism in LLTO is through a coupled phonon-ion THz rocking mode. Although this higher frequency mode is less than one-quarter of the overall phonon density of states, it leads to most ion hops. Next, the role of the oxygen anion in the sub-lattice polarizability is investigated. A charge transfer transition depletes an electron on the oxygen atoms in the 4-O window gateway, and an increase in ion hopping is measured. The new technique generally applies to any complex ion conducting system, such as polymers, fuel cells, supercapacitors, and membranes.