Roll-to-roll boosting the energy storage of ultrathin biaxially-oriented polypropylene film by soft-photon-induced crosslink implanted at industry scale

Abstract

Compact and lightweight modern electronics suffering harsh service environment pose urgent demands on miniaturization and robustness of energy-storage film capacitors and ever thinner dielectric films. However, the energy storage of ultrathin dielectric films is bottlenecked by the end of Helgee-Bjellheim’s scale-law for sub-10 µm range and there currently exists no interface-free method to boost their performance to be implemented at industry-scale. This work proposes monochromatic and soft ultra-violet (UV) irradiation to enhance the breakdown strength of 2.4 µm biaxially-oriented polypropylene (BOPP) film from 374 to 484 V μm-1 by 29.4%, its discharge energy density to competitive 2.41 J cm-3 by 53% with an impressive charge-discharge efficiency of 96% at 125 ℃, and the charge-discharge cycle-lifetime by more than 2.5 times. In-situ electron spin resonance spectra illustrate that UV-activated peroxyl radicals crosslink BOPP and form deep traps suppressing charge carrier migration, without decreasing its bandgap even under strong electric field. However, currently line-implanted corona introduces harmful C=O bonds, significantly deteriorating BOPP’s bandgap. This work demonstrates one-step and high-throughput UV irradiation matching the existing drafting-roll production line of ultrathin capacitor films to promote their energy storage.