Free volume design promoting high-temperature capacitive energy storage of all-organic polymer films

Abstract

Dielectric polymer capacitors are widely used in advanced electric power systems, while their discharged energy density and efficiency under elevated temperatures are hindered by a low dielectric constant and increased energy loss, especially beyond 150 °C. Conventional strategies, such as nanofiller incorporation, improve the dielectric constant but face challenges, including interfacial mismatch and limited processability. Here, we report a fully organic dielectric blend simply composed of fluorinated polyester (FPE) and polyetherimide (PEI), designed to overcome these limitations through the proposed free volume method. The introduction of PEI expands the interchain spacing within the FPE matrix, generating additional free volume that facilitates dipole movement while preserving high insulating properties. The optimized 50 wt% FPE/PEI blend achieves a dielectric constant of 3.90, a low loss tangent (≤0.005), and excellent hightemperature performance, thereby delivering a discharged energy density of 5.24 J cm -3 and efficiency of approximately 80% at 150 °C. Structural analyses based on molecular dynamics simulations and density functional theory calculations confirm that enhanced free volume in the glassy state facilitates the dipole mobility and promotes the dielectric constant, enabling superior energy storage under high electric fields. This work presents a scalable, filler-free strategy for developing high-performance polymer dielectrics with robust thermal and electrical properties.