Multilayered hierarchical polymer composites for high energydensity capacitors

There is an urgent need to develop advanced energy storage materials to meet the ever-increasing demands of modern electronics and electrical power systems. Polymer-based dielectric materials are one of the most promising energy materials due to their unique combination of high breakdown strength, low dielectric loss, light weight and ultrahigh power density. However, their energy densities are severely limited by their low dielectric constants (K) and thus fall short of the demands of compact and efficient energy storage devices. Remarkable efforts have been performed to improve K, and consequently, energy densities of polymers, e.g. introducing high-K inorganic fillers into the polymer matrix to form polymer composites. However, a general drawback is that the increased K is usually achieved at the cost of substantially decreased breakdown strength, thus leading to a moderate improvement of energy density. More recently, the polymer dielectrics with optimized hierarchically layered structures has become an emerging approach to resolve the existing paradox between high K and high breakdown strength in single-layered composite films, which resulted in substantial improvement in their capacitive energy storage performance. It is demonstrated that the electric field distribution, breakdown strength and capacitive performance can be readily adjusted by systematically varying the interfaces, chemical structures and ratios of the constituent layers. This review, for the first time, outlines the contemporary models and theories, and summarizes the research advances of multilayered hierarchical polymer composites (MHPCs), including inorganic particle/organic MHPCs and all-organic layered films in the field of high-energy-density capacitors. The efficient strategies for improving the energy storage performance of MHPCs have been highlighted. To conclude, the remaining challenges and the promising opportunities for the development of MHPCs for capacitive energy storage applications are presented.