In Situ Well-Aligned\nMicrofibrils and Mother–Daughter\nCrystals as Promising Blocks to Suppress Carrier Transport in Polypropylene\nDielectric Films

High-performance dielectric capacitors are essential\nfor advanced\nelectronics and electrical power systems. Nonetheless, the enhanced\ndischarged energy density (<i>U</i><sub>d</sub>) of polymer-based\ndielectric films is frequently accompanied by increased conduction\nand polarization losses as well as reduced charge–discharge\nefficiency (η). In this study, a scalable “melting extrusion–hot\nstretching–solid-state stretching” technique was proposed\nto fabricate high-performance polypropylene (PP)/poly(ethylene terephthalate)\n(PET) all-organic dielectric films. During hot stretching, PET microdroplets\n deformed into microfibrils in situ, establishing massive parallel\ninterfaces. The subsequent solid-state stretching induced a mesophase-to-α-crystal\ntransition, forming a mother–daughter crystalline structure.\nThe synergistic effect of well-aligned microfibrillar interfaces and\nthe dense mother–daughter crystalline structure could significantly\nenhance the breakdown strength and capacitive storage capability by\nsuppressing carrier transport. As a result, the as-prepared PP/PET\ndielectric films demonstrated an exceptional breakdown strength of\n672.2 MV m<sup>–1</sup> and a maximum <i>U</i><sub>d</sub> of 4.11 J cm<sup>–3</sup> with an η of as high\nas 92.8%. The proposed technique is demonstrated to be highly effective\nfor structuring in situ well-aligned microfibrils and mother–daughter\ncrystalline structures, thus promoting the development of next-generation,\nhigh-performance PP-based film capacitors.