In Situ Well-Aligned Microfibrils and Mother–Daughter Crystals as Promising Blocks to Suppress Carrier Transport in Polypropylene Dielectric Films

High-performance dielectric capacitors are essential for advanced electronics and electrical power systems. Nonetheless, the enhanced discharged energy density (U_d) of polymer-based dielectric films is frequently accompanied by increased conduction and polarization losses as well as reduced charge–discharge efficiency (η). In this study, a scalable “melting extrusion–hot stretching–solid-state stretching” technique was proposed to fabricate high-performance polypropylene (PP)/poly(ethylene terephthalate) (PET) all-organic dielectric films. During hot stretching, PET microdroplets deformed into microfibrils in situ, establishing massive parallel interfaces. The subsequent solid-state stretching induced a mesophase-to-α-crystal transition, forming a mother–daughter crystalline structure. The synergistic effect of well-aligned microfibrillar interfaces and the dense mother–daughter crystalline structure could significantly enhance the breakdown strength and capacitive storage capability by suppressing carrier transport. As a result, the as-prepared PP/PET dielectric films demonstrated an exceptional breakdown strength of 672.2 MV m^–1 and a maximum U_d of 4.11 J cm^–3 with an η of as high as 92.8%. The proposed technique is demonstrated to be highly effective for structuring in situ well-aligned microfibrils and mother–daughter crystalline structures, thus promoting the development of next-generation, high-performance PP-based film capacitors.