Crystal refinement in biaxially stretched capacitor films for enhanced energy storage

Abstract

Polymer-based film capacitors are essential energy storage components in high-power electric devices. Biaxial stretching is a scalable, high-throughput technique widely used for this film fabrication, yet the impact of crystalline structure evolution during stretching on capacitive performance remains insufficiently understood. Here, we report a crystallization regulation strategy by tailoring the initial crystal size in polypropylene through the incorporation of tiny nucleating agent. This approach refines the crystalline morphology of pre-stretched PP sheets and governs subsequent structural evolution during biaxial stretching. A critical grain size threshold of 5.4 μm is identified, beyond which crystal fragmentation and excessive free volume emerge, leading to elevated leakage currents and premature dielectric breakdown. By maintaining crystal sizes below the threshold, crystal fragmentation is suppressed and microstructural integrity preserved, enabling reduced interfacial defects and restrained electron migration. The resulting films simultaneously exhibit enhanced mechanical robustness and dielectric reliability with a record breakdown strength of 958 MV/m and an exceptional discharged energy density of 10.2 J/cm³. This insight redefines the design paradigm from traditional crystallinity enhancement toward precise crystal-size control for optimized dielectric performance. Coupled with low material cost and scalable processability, this crystallization regulation strategy offers a promising route toward next-generation, industrial-scale polymer-based energy storage films.