To fabricate the polymer-based nanocomposites with excellent energy storage performances for high-temperature applications, hierarchical x wt % Ag@ Al 2 O 3 nanofibers (Ag@AO NFs) are designed and prepared, where various Ag nanoparticles (x wt %) are filled into the Al 2 O 3 nanofibers. By adjusting the weight fraction of Ag nanoparticles, the coupling effects of interfacial polarizations and interfacial barrier at Ag/AO interfaces are explored and optimized; excellent energy storage performance was obtained in the 0.50 wt % Ag@AO/PEI nanocomposite. Owing to the high insulation and high thermal conductivity of Al 2 O 3 nanofibers, a stable energy storage performance is realized within 25−150 °C. For example, the 0.50 wt % Ag@AO/PEI nanocomposite delivers an excellent discharge energy density of 6.1 J/cm 3 with charge−discharge efficiency of 89.2% at 413.4 MV/m, which is twice as large as that of the pure PEI (3.0 J/cm 3 at 382.4 MV/m, 74.6%). This structural design might provide a feasible solution for realizing enhanced energy storage performance at low filler fraction for high-temperature application.

Designing and regulating the microstructure of core−shell fillers are effective ways to fabricate polymer-based nanocomposites with excellent energy storage performances. Along this line, the unique structure combination of 0D metallic Ag nanoparticles (NPs) and 1D bark-like TiO 2 nanowires (NWs) were successfully prepared. Through regulating the volume fraction of Ag NPs (x vol %) in the Ag@TiO 2 NWs, the polarization and breakdown strength of Ag@TiO 2 /PVDF nanocomposites have been optimized, which is evidenced both by the experimental results and finite element simulation. As a result, remarkably enhanced energy storage performances are realized in these nanocomposites. For instance, the 6 vol % Ag@TiO 2 /PVDF nanocomposite shows an excellent performance of the highest discharged energy density of 16.3 J/cm 3 at 376 MV/m, increased by 222% compared with that of pure PVDF (7.4 J/cm 3 , 335 MV/m). Meanwhile, its charge and discharge efficiency remain up to 77% owing to the reduction of the leakage current density caused by the Coulomb blocking effect of small-sized Ag NPs and the prolonged breakdown path resulted by TiO 2 NWs. This proof-of-concept study provides an innovative design thinking of the hybrid structure for obtaining nanocomposites with extraordinary energy storage performances.

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