Experimental corroboration of theory for impedance response of solid electrolytes: Doped cubic garnet LLZO

We corroborate phenomenological theory for the electrochemical impedance response of solid state electrolyte (SSE) with experimental data for the garnet structured oxide LLZO synthesized using conventional solid-state method. Theoretical analysis of experimentally recorded impedance spectrum indicates three characteristic frequency regimes, viz. (i) high frequency regime, ion relaxation dynamics in grain (g) and space charge layer (SCL) in grain boundary (gb) controls the impedance response, (ii) intermediate frequency regime, depicts the ion transfer and reorganization across heterogeneous grain and grain boundary and (iii) low frequency regime, response is attributed to the capacitive behavior due to the formation of double layer at solid electrolyte/partially blocking metal interface. Theory identifies phenomenological time scales associated with different electrochemical processes occurring in grain, grain boundary and their interface. The material characterization of samples is carried out using powder X-ray diffraction, scanning electron microscopy and Raman spectroscopy. The characteristic phenomenological time scales are extracted from the impedance response of Ta-doped and Al-doped LLZO unraveling the influence of doping. Finally, theory provides an alternative approach as it has rigor of phenomenological ab initio methodology as well as captures the simplicity of equivalent circuit models.