  This work presents a method to produce structural composites capable of
energy storage. They are produced by integrating thin sandwich structures of
CNT fiber veils and an ionic liquid-based polymer electrolyte between carbon
fiber plies, followed by infusion and curing of an epoxy resin. The resulting
structure behaves simultaneously as an electric double-layer capacitor and a
structural composite, with flexural modulus of 60 GPa and flexural strength of
153 MPa, combined with 88 mF/g of specific capacitance and the highest power
(30 W/kg) and energy (37.5 mWh/kg) densities reported so far for structural
supercapacitors. In-situ electrochemical measurements during 4-point bending
show that electrochemical performance is retained up to fracture, with minor
changes in equivalent series resistance for interleaves under compressive
stress. En route to improving interlaminar properties we produce grid-shaped
interleaves that enable mechanical interconnection of plies by the stiff epoxy.
Synchrotron 3D X-ray tomography analysis of the resulting hierarchical
structure confirms the formation of interlaminar epoxy joints. The manuscript
discusses encapsulation role of epoxy, demonstrated by charge-discharge
measurements of composites immersed in water, a deleterious agent for ionic
liquids. Finally, we show different architectures free of current collector and
electrical insulators, in which both CNT fiber and CF act as active electrodes.
