Lithium-intercalated Carbon Fibres : Towards the Realisation of Multifunctional Composite Energy Storage Materials

Abstract: Lightweight design is a major improvement path for sustainable transport asit contributes to lower vehicles energy consumption and gas emissions. Anovel solution to weight savings is to store energy directly in the mechanicalstructure of the vehicle with a multifunctional material, called structural battery,which could simultaneously bear mechanical loads and store electricalenergy. This is especially possible because the carbon fibre is a high performancemechanical reinforcement for polymer composites and can also be usedas a lithium-intercalating electrode in lithium-ion batteries. In this thesis, theperformance of carbon fibres for use as a lithium-intercalating structural electrodeis investigated.Electrochemical characterisation has shown that intermediate modulus polyacrylonitrile- based carbon fibres which have the highest strength also offerthe most promising electrochemical capacities when compared to other fibregrades with different microstructures. The measured capacity of fibre bundleswas highly dependent on the current rate and at low rate the capacitiesclose to that of graphite electrodes were measured. In a mechanical characterisationthe carbon fibre was not affected by the number of electrochemicalcycles, up to 1000 cycles, but rather by the amount of intercalated lithium.The tensile stiffness appeared to remain unchanged, but during lithation thetensile strength dropped and partly recovered during delithiation due to afirst-cycle irreversible drop. A longitudinal expansion of the carbon fibre wasalso measured during lithiation. An irreversible expansion in the delithiatedfibres highlighted that the first cycle-capacity loss is partly due to intercalatedlithium which is trapped in the carbon fibre. From these results, the carbonfibre is without doubts suitable for structural battery applications.A mechanical-electrochemical coupling in lithium-intercalated carbon fibreswas also measured, highlighting a piezo-electrochemical transducer effect resultingin new functionalities for lithium-intercalated carbon fibres. The longitudinalexpansion strain can be used for mechanical actuation. A responseof the cell open-circuit potential to an applied mechanical strain can be usedfor strain sensing.