Multifunctional Composite Materials using Lithium Ion Functionalization

In this paper we show how one can functionalise carbon fibres by using them as an electrochemicalelectrode. The electrochemical process is the same as in a lithium-ion battery cell so that the carbonfibres act as an active electrode in future structural battery concepts. The functionalization of carbonfibres using lithium ion intercalation reveals three novel and interesting possibilities enabling carbonfibres composites to obtain several other multi-functionalities. These are strain sensing, actuation andenergy harvesting.We have found that by intercalating lithium ions into the nano-/micro-structure of carbon fibres apiezo-electrochemical effect is revealed. This is observed as a change in the potential of the carbon fibreelectrode when applying a mechanical load. The response is direct and easily measurable being in theorder of several mV. This can be utilised as a strain sensor since there is a relation between the potentialchange and the strain in the carbon fibre.Secondly, we have measured substantial axial expansion of carbon fibres when intercalated withlithium ions. The strain measured is as high as 1%. Since the stiffness of carbon fibres is very high, thiscorresponds to very large forces. This can be used for actuation or morphing.Thirdly, the newly found piezo-electrochemical effect can be used to harvest energy by convertingmechanical work to electrical energy. Applying a tensile force to carbon fibre bundles used as Liintercalatingelectrodes results in a response of the electrode potential of a few mV which allows, at lowcharge rates, discharge at higher electrode potential than at charge. More electrical energy is therebyreleased from the cell at discharge than provided at charge, harvesting energy from the mechanical workof the applied force.QC 20150828