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Electrical storage technologies for high-density energy and power are crucial for the elaboration of electric vehicles with increased driving ranges. Not only improved charging while being connected to the electricity grid is demanded to facilitate a higher driving range, but also technologies for energy harvesting during driving offer great potential. Re-using braking energy during re-acceleration of the vehicle is currently one of these promising harvesting technologies being investigated. Because of the high-power currents generated during a braking event, the recovered energy can only be efficiently stored by a capacitor compared to traditional Li-ion batteries. However, the limited energy density of currently used capacitors prevent them from being a large-scale alternative to electrochemical batteries. This research is addressing this shortcoming and aims to establish fast, light and reliable capacitive energy storage while improving the energy density by boosting the maximal allowable voltage on top of an optimized capacitance. Multifunctional composite materials offer a unique integration method, which allows to embed the functional capacitor materials into structural fibre-reinforced polymers. In this concept, the composite material is not only the structural component, but also the storage capacitor, avoiding adding extra weight. Potential materials to be addressed are carbon fibres as structural electrode material and ceramic nanoparticles embedded in a polymer as solid dielectric in order to create a percolation network. The absence of any electrolyte allows significantly higher voltages to be applied, which can be fully exploited by the current trend in automotive of storing electrical energy at 100s of volts. Structural composite capacitors have been produced by optimized ultrasonication, solution casting and warm pressing of the dielectric polymer/nanoparticles film in between carbon weaves as illustrated in Figure 1a. Next to the commonly used conductive nanoparticles as percolative network, novel core-shell nanoparticles have been manufactured to introduce an insulating shell, as shown by figure 1b. The shell significantly reduces dielectric self-discharge currents while improved dielectric breakdown strength is expected. The electrical performance has been assessed by broadband dielectric spectroscopy at quasi-DC frequencies. As a result of the boost in dielectric constant also the energy density of percolation-based structural composite capacitors was increased significantly. [ABSTRACT FROM AUTHOR]