Outstanding discharge energy density and efficiency of the bilayer nanocomposite films with BaTiO3-dispersed PVDF polymer and polyetherimide layer

•Novel bilayer polymer nanocomposites were prepared via tape casting and hot-pressing.•PEI-3 vol% BT/PVDF bilayer composite exhibits the highest η of 84.48% and Ud of 8 J·cm−3.•The Bilayer composites have excellent charging-discharging cycling stability, which is beneficial for actual applications. Dielectric polymer nanocomposite films have been widely studied because of their high polarization and electric breakdown strength, leading to high discharge energy density (Ud). However, nanocomposites with high Ud often show low discharge energy efficiency (η). In order to achieve both high Ud and η simultaneously, a series of novel bilayer polymer nanocomposite films composed of pure polyetherimide (PEI) layer and BaTiO3-dispersed poly(1,1-difluoroethylene) (BT/PVDF) layer were prepared by hot-pressing method in this work. The bilayer composites exhibit high η > 84% at 600 MV⋅m−1, which are much larger than most reported values (η