Conductivity in Li-ion gel polymer electrolytes: Role of ultrafast solution dynamics and agreement between Onsager predictions and measurements

In this paper, we studied the role of ultrafast medium dynamics in determining the conductivity of a representative gel polymer electrolyte system composed of propylene carbonate, lithium perchlorate (LiClO4), and polypropylene glycol (PPG), with a fixed PC:LiClO4 ratio of 10.7, and explored the possible reasons behind the breakdown of Onsager theory in successfully predicting the composition-dependent measured conductivities at different temperatures. For this purpose, we measured the solution dynamical response by employing frequency dependent dielectric relaxation (DR) experiments and a streak camera-based fluorescence dynamics setup. These measurements indicated fast solution dynamics facilitated ion transport, while the slow diffusive dynamics offered frictional resistance. These opposite roles for the fast and the slow dynamics led to viscosity decoupling of conductivity and suggested the presence of non-hydrodynamic modes for ion transport. Our dynamic light scattering measurements suggested the presence of polymer-induced aggregated structures (∼2000-5000 nm) in these solutions, hinting at a negligible contribution from the center-of-mass motions of these nanoaggregates to the measured conductivities. Raman spectroscopic measurements indicated PC-PPG interaction and suppression of ion-pair formation upon addition of PPG. Comparison of experimental conductivities to Hubbard-Onsager (HO) predictions reveals a strong sensitivity to the high-frequency dielectric constant (ε∞) and average DR times τDR. Use of a relatively faster τDR in the HO theory significantly improves the predictions, emphasizing the importance of the ultrafast medium dynamics. This demonstrates the overwhelming dominance of the long-wavelength collective medium polarization modes in governing ion transport and suggests a possible route for optimization of solution composition for battery applications.