Lightweight multifunctional composites : An investigation into ion-inserted carbon fibres for structural energy storage, shape-morphing, energy harvesting & strain-sensing
Abstract: The transport sector accounts for around 26% of all greenhouse gas emissions in the European Union and United Kingdom. By reducing the mass of vehicles it is possible to reduce these emissions substantially.One way to reduce vehicle mass is to use multifunctional structures.An investigation is conducted into the development of lightweight multifunctional composites based on carbon fibres (CFs) that have been activated using ion-insertion. CF composites are already known to have good structural properties - and this research focusses on augmenting fibres with the added functionalities of energy storage, shape-morphing, energy harvesting, and strain-sensing.This work builds on previous research focusing on structural batteries using lithium-inserted CFs. Here, sodium and potassium insertion in intermediate modulus polyacrylonitrile (PAN)-based CFs is investigated. These elements are more abundant globally than lithium, and so can be considered more sustainable. They also have larger atomic radii than lithium, which could give rise to desirable functionalities. The galvanostatic profiles and capacities of sodium and potassium insertion in CFs are reported, as well as the effect on the CF mechanical properties. It is found that sodium and potassium insert with lower capacities than lithium in PAN-based CFs. The mechanical stiffness remains largely unchanged by sodium and potassium insertion, although the mechanical strength drops by up to 28% at full charge for sodium insertion. This strength drop is partially reversible when the CFs are discharged, recovering to 94% of their original strength.Axial expansion of the CFs during ion-insertion is also measured. It is found that expansions up to 0.09% and 0.24% occur for sodium and potassium insertion respectively. This is less than the 0.7% observed previously for lithium insertion in the same CFs.An analytical model to simulate ion-expansions in laminated structural battery composites is developed based on classical laminate plate theory (CLPT). Ion-expansions are treated as analogous to thermal expansions. It is found that it is possible to suppress global deformations and reduce interlaminar stresses by altering the layup sequence of negative and positive electrode layers. It is also found that it is possible to tailor the kinds deformations that can be achieved by altering the ply orientations.A concept for a shape-morphing composite laminate is developed, consisting of two layers of CF either side of a separator, embedded in a structural battery electrolyte matrix. Lithium ions are transferred from one CF layer to the other, causing one layer to contract and the other to expand. This creates a bending deformation. It is demonstrated that the morphing laminate is capable of deforming at low voltages and currents, and exhibiting a zero-power hold. The frequency of deformation is, however, limited by the ion diffusion process.A voltage-strain coupling arises when CFs are inserted with ions known as the piezoelectrochemical transducer (PECT) effect. It is has been proposed that the PECT effect could be used to create strain-sensing and energy harvesting structures. This coupling is measured in sodium and potassium inserted CFs and found to reach a maximum coupling factor of 0.26 V/unit strain for potassiated fibres. This is considerably lower than the coupling factor for lithiated fibres. The same laminated composite structure used for the morphing study is used to investigate the PECT effect in a structural composite. Using this setup it is possible to characterise the compressive PECT effect in CFs for the first time. It is found that the compressive PECT effect is equal in magnitude but opposite in sign to the tensile effect. This means that by using bending it is possible to achieve a larger potential difference between a simultaneously compressed and tensioned CF layer. This is advantageous for applications in strain-sensing and energy harvesting.Energy harvesting using a lithium-activated CF laminate is also demonstrated. The study uses the same laminate structure used for the morphing study. By enforcing known constant-curvature bending on the laminate, a current is generated. A specific power of 18 nW/g is achieved, showing promise for structural energy harvesting composites.Overall, it is shown that ion-inserted CFs can be used to create several different functionalities in composite structures. By incorporating such multifunctional components into vehicles it will be possible to save mass on a systems-level. Although there is still much development to be done, this study shows the diverse potential in multifunctional composites using ion-insertion, and lays the foundation from which these technologies can be progressed.
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