Reducing dielectric loss by nanoconfined impurity ion transport in multilayer films under low electric fields

Abstract

Despite higher dielectric constants than nonpolar polymers, polar polymers exhibit a high dielectric loss, which is detrimental for electric energy storage applications. To mitigate the ionic conduction loss, we have fabricated multilayer films (MLFs) with alternating layers of a high dielectric constant polar polymer and a high breakdown/low loss nonpolar polymer. In this study, we fabricated high temperature polycarbonate (HTPC)/poly (vinylidene fluoride) (PVDF) MLFs. The ionic transport behavior confined in the PVDF nanolayers under a low electric field was investigated using broadband dielectric spectroscopy (BDS). Direct analytical simulation was implemented to determine the concentration (n0) and diffusion coefficient (D0) for the confined impurity ions in PVDF at different temperatures and with and without the presence of lamellar crystals. It was found that both n0 and D0 increased upon increasing temperature. Using the simulated n0 and D0, nanoconfined transport of impurity ions was found for the PVDF layers with decreasing the layer thickness. Namely, a crossover PVDF layer thickness was predicted, below which the conduction loss from impurity ions substantially decreased. However, experimental BDS results systematically deviated from the theoretical predictions. One important reason for this deviation was attributed to the presence of edge-on lamellar crystals confined in the PVDF layers, which increased the tortuosity of the ion transport pathway. The knowledge gained in this study can be useful to understand the nanoconfinement and crystallinity effects on the ionic transport in MLFs, which is beneficial for the future development of MLFs for capacitor applications.