Dilute nanocomposites for capacitive energy storage: progress, challenges and prospects

Electrostatic capacitors (ECs) are critical components in advanced electronics and electric power systems due to their rapid charge-discharge rate and high power density. While polymers are ideal for ECs due to their high voltage tolerance and mechanical flexibility, their low dielectric constants (K) and limited energy density remain significant limitations. Traditional polymer nanocomposites, which incorporate high-K ceramic fillers, have shown promise in enhancing dielectric properties but often at the cost of electric breakdown strength and scalability. In this perspective, we explore a pioneering approach that utilizes ultralow loadings of small-sized inorganic nanofillers to significantly improve dielectric constants without compromising other key properties. We delve into the unconventional effects observed in these polymer nanocomposites, including dielectric enhancements, charge trapping, mechanical reinforcements, and microstructural changes, and highlight the impressive energy storage performance achieved with minimal filler contents. We discuss innovative design strategies from viewpoints of polymer and filler structures and showcase recent advancements in nanoscale characterization and theoretical modelling for understanding the crucial role of polymer-filler interfaces. Finally, we stress fundamental challenges and prospects, providing insights into the transformative potential of these nanocomposites for next-generation energy storage applications.