AC conductivity studies of polyethylene oxide-garnet-type Li7La3Zr2O12 hybrid composite solid polymer electrolyte films
By Bashiri, Parisa; Rao, T. Prasada; Naik, Vaman M.; Nazri, G. A.; Naik, Ratna
Published in Solid State Ionics
2019
Abstract
We have investigated both AC dielectric permittivity and ionic conductivity of hybrid composite solid polymer electrolyte (CSPE) films ( 100 μm) comprised of sub-micron sized aluminum substituted cubic Li7La3Zr2O12 (LLZO) particles dispersed in polyethylene oxide-LiClO4 (PEO-LiClO4) matrix with [EO]:[Li] = 15:1. Homogeneous CSPE films with 50 wt% LLZO (PEO-LLZO-LiClO4) exhibit good flexibility and mechanical robustness up to 120 °C. The electrochemical potential stability window measured for these films was 5.2 V vs. Li+/Li. The complex AC permittivity and conductivity were determined using the measured electrical impedance spectra in the range of 1 Hz to 300 kHz and found to be heavily affected by the electrode polarization in low frequency region. We have analyzed the data using an expression for complex AC conductivity that generalizes the power-law dependence taking into account the effect of electrode polarization and involving ionic and polymer segmental relaxation times in its description. This expression describes very well the observed behavior of AC dielectric permittivity and conductivity in the entire measured frequency region. The ionic and segmental relaxation times obtained from fitting the experimental data indicate a strong coupling between the ionic motion and segmental dynamics. Further, the observed temperature dependent conductivity follows the Vogel-Tammann-Fulcher (VTF) behavior implying a close correlation between ionic conductivity and segmental relaxation in these polymer electrolytes. The VTF model yields an activation energy of 0.32 eV for PEO-LiClO4 film, and 0.07 eV for PEO-LLZO-LiClO4 composite film, consistent with the observed enhancement in conductivity by two orders of magnitude at 30 °C upon the addition of LLZO particles.