Scientific Highlight: Integration of energy storage devices in structural laminates

One of the current scientific and technological trends in structural composites is the incorporation of new functionalities as thermal/electrical conductivity or energy harvesting capacities to classical laminates. However, the inclusion of any internal conductive layer with capacitive storage will produce undoubtedly detrimental effects as delaminations and lack of strength.

Structural laminates with integrated electric double layer capacitive storage were designed and produced at IMDEA Materials using resin infusion of dry fibre preforms, Figure a), b) and c). An internal carbon nanotube non-woven
fabric working as electrodes/current collectors and a polymer ionic liquid membrane was used as energy storing layer. To avoid/mitigate the effect caused by the internal layer acting as a delamination, the energy storing layer was patterned with holes which after infusion act as resin plugs for mechanical interconnection between layers, a procedure similar to riveting. Finite element modelling was used to optimize rivet shape and areal density on the interlaminar shear properties of the laminate. It should be highlighted
that the aforementioned method of inclusion of CNTF fabric and polymer 

electrolyte for energy storage is carried out without any metallic current collectors.

The robustness of the structural supercapacitor composite was evaluated by three-point bending (see Figure d) tests including progressive deflection, repeated load/unload cycles up to fracture, combined with in/ex situ electrochemical measurements. The multifunctional material developed showed no appreciable degradation of its electrochemical properties, a consequence of the use of the CNTF fabric electrode as current collector. The use of CNT fibre current collector increases energy/power density and the overall figures of merit in energy-storing of the structural composite.
But very importantly, it makes the electrochemical elements tolerant to mechanical deformations by preserving electrical contact of electrochemically-active elements. This feature is critical for continuous operation of batteries, for example, and indicates that the multifunctional materials approach used is a key enabler for the safe operation of structural energy-storing composites 

 a) and b) Energy storage device made up with CNTF fabric as current collector, c) Resin infused glass fiber composite beams containing energy storage devices with and without shear connectors, d) Three-point bending of the beams containing energy storage devices.

Damage-tolerant, laminated structural supercapacitor composites enabled by integration of carbon nanotube fibres
M. Rana, Y. Ou, C. Men, F. Sket, C. Gonzalez and J.J. Vilatela
Multifunctional Materials 3, 015001, 2020