High-capacity or high-power-density capacitors are being actively investigated for portable electronics, electric vehicles, and electric power systems. The dielectric nanocomposite with a small loading of carboxylic polystyrene (PS-COOH) nanoparticles in poly(vinylidene fluoride-chlorotrifluoroethylene) [ P(VDF-CTFE)] matrix, followed by chemical crosslinking has been described. Combination of these two methods significantly improved the capacity of electric energy storage at low electric field. Specially, the nanocomposite with 2 wt % nanoparticles and 15 wt % crosslinking agent achieved a dielectric constant of 17.2 and a discharged energy density of 17.5 J/cm(3) (4.9 Wh/L) at an electric field as high as 324 MV/m, while corresponding values for pristine P(VDF-CTFE) are 9.6 and 13.3 J/cm(3) (3.7 Wh/L), respectively. Fundamental physics underlying the enhancement in the performance of the nanocomposites with respect to P(VDF-CTFE) is illustrated by solidstate F-19 nuclear magnetic resonance of direct excitation or F-19{H-1} cross polarization. It revealed different dynamics behavior between crystalline/amorphous regions, and PS-COOH nanoparticles favored the formation of polar c-form crystals. Small-angle X-ray scattering studies revealed the contribution of the interface to the extraordinary storage of electric energies in the nanocomposites. This approach provided a facile and straightforward way to design or understand PVDF-based polymers for their practical applications in high-energy-density capacitors. (C) 2016 Wiley Periodicals, Inc.