MULTIFUNCTIONAL COMPOSITE MATERIALS USING LITHIUM ION FUNCTIONALIZATION

In this paper we show how one can functionalise carbon fibres by using them as an electrochemical electrode. The electrochemical process is the same as in a lithium-ion battery cell so that the carbon fibres act as an active electrode in future structural battery concepts. The functionalization of carbon fibres using lithium ion intercalation reveals three novel and interesting possibilities enabling carbon fibres composites to obtain several other multi-functionalities. These are strain sensing, actuation and energy harvesting. We have found that by intercalating lithium ions into the nano-/micro-structure of carbon fibres a piezo-electrochemical effect is revealed. This is observed as a change in the potential of the carbon fibre electrode when applying a mechanical load. The response is direct and easily measurable being in the order of several mV. This can be utilised as a strain sensor since there is a relation between the potential change and the strain in the carbon fibre. Secondly, we have measured substantial axial expansion of carbon fibres when intercalated with lithium ions. The strain measured is as high as 1%. Since the stiffness of carbon fibres is very high, this corresponds 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 converting mechanical work to electrical energy. Applying a tensile force to carbon fibre bundles used as Li-intercalating electrodes results in a response of the electrode potential of a few mV which allows, at low charge rates, discharge at higher electrode potential than at charge. More electrical energy is thereby released from the cell at discharge than provided at charge, harvesting energy from the mechanical work of the applied force.