Increasing the energy storage of an ultrathin biaxially oriented polypropylene film via soft-photon-induced crosslinks implanted at industrial scale using roll-to-roll processing

Abstract

Compact and lightweight modern electronics suffering from harsh service environments result in urgent demands for the miniaturization and robustness of energy-storage film capacitors and ever-thinner dielectric films. However, the energy storage of ultrathin dielectric films is limited by the breakdown of the Helgee-Bjellheim scale law in the sub-10 µm range, and no interface-free method currently exists to boost their performance for industrial-scale implementation. This work proposes monochromatic and soft ultra-violet (UV) irradiation to increase the breakdown strength of 2.4 µm biaxially-oriented polypropylene (BOPP) films from 374 to 484 V µm⁻¹ (by 29.4%), their discharge energy density to a competitive 2.41 J cm⁻³ (by 53%) with an impressive charge–discharge efficiency of 96% at 125 °C, 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 a strong electric field. However, current production lines using corona treatment introduce harmful CO bonds, significantly deteriorating BOPP's bandgap. This work demonstrates a one-step and high-throughput UV irradiation method matching the existing drafting-roll production line of ultrathin capacitor films to promote their energy storage.