Bio-based interfacial engineering enables extreme-temperature dielectric energy storage in polymer nanocomposites

Dielectrics capable of simultaneous high energy density (U_d), high efficiency (η), and reliable operation at elevated temperatures are urgently needed for advanced electronics and power systems, yet remain a formidable challenge. Here, we report a bio-based interfacial engineering strategy that employs wide-bandgap, polar cellulose to conformally coat high-k TiO₂ nanosheets, creating a core-shell structured filler. When incorporated into a polyetherimide (PEIs) matrix, the cellulose shell simultaneously enhances filler dispersion and introduces deep charge traps at the interface. This synergistic effect yields a remarkable combination of high dielectric polarization and strongly suppressed conduction loss at high temperatures. The resulting nanocomposite with only 2 wt% filler achieves an electrical breakdown strength of 600 MV m⁻¹ and a U_d of 4.05 J cm⁻³ at 150 °C, retaining a high U_d of 2.89 J cm⁻³ with 90% η even at 200 °C. Combining thermally stimulated depolarization current analysis and finite-element modelling, we reveal the mechanism by which the cellulose interlayer homogenizes the electric field distribution and immobilizes space charges. This work demonstrates a nature-derived interfacial strategy for high-performance dielectrics under extreme conditions, offering a sustainable pathway to high-energy-density capacitive energy storage.