Several grades of commercially-available polyacrylonitrile (PAN)-based carbon fibers have been studied for structural lithium-ion batteries to understand how the sizing, different lithiation rates and number of fibers per tow affect the available reversible capacity, when used as both current collector and electrode, for use in structural batteries. The study shows that at moderate lithiation rates, 100 mA g-1, most of the carbon fibers display a reversible capacity close to or above 100 mAh g-1 after ten full cycles. For most of the fibers, removing the sizing increased the capacity to some extent. However, the main factor affecting the measured capacity was the lithiation rate. Decreasing the current by a tenth yielded an increase of capacity of around 100 for all the tested grades. From the measurements performed in this study it is evident that carbon fibers can be used as the active negative material and current collector in structural batteries.

Carbon fibres are particularly well suited for use in a multifunctional lightweight design of a structural composite material able to store energy as a lithium-ion battery. The fibres will in this case act as both a high performance structural reinforcement and one of the battery electrodes. However, the electrochemical cycling consists of insertions and extractions of lithium ions in the microstructure of carbon fibres and its impact on the mechanical performance is unknown. This study investigates the changes in the tensile properties of carbon fibres after they have been subjected to a number of electrochemical cycles. Consistent carbon fibre specimens were manufactured with polyacrylonitrile-based carbon fibres. Sized T800H and desized IMS65 were selected for their mechanical properties and electrochemical capacities. At the first lithiation the ultimate tensile strength of the fibres was reduced of about 20% but after the first delithiation some strength was recovered. The losses and recoveries of strength remained unchanged with the number of cycles as long as the cell capacity remained reversible. Losses in the cell capacity after 1000 cycles were measured together with smaller losses in the tensile strength of the lithiated fibres. These results show that electrochemical cycling does not degrade the tensile properties which seem to depend on the amount of lithium ions inserted and extracted. Both fibre grades exhibited the same trends of results. The tensile stiffness was not affected by the cycling. Field emission scanning electron microscope images taken after electrochemical cycling did not show any obvious damage of the outer surface of the fibres.

Carbon fibres (CFs) can work as lightweight structural electrodes in CF-reinforced composites able to store energy as lithium (Li)-ion batteries. The CF has high stiffness and strength-to-weight ratios and a carbonaceous microstructure which enables Li intercalation. An innovative in situ technique for studying the longitudinal expansion of the CF and the relationship with the amount of intercalated Li is described in the present paper. The polyacrylonitrile-based CFs, T800H and unsized IMS65, were chosen for their electrochemical storage capacities. It was found that the CF expands during lithiation and contracts during delithiation. At the first electrochemical cycle, the expansion is partly irreversible which supports that the first-cycle capacity loss partly relates to Li trapped in the CF structure. For the following cycles, the capacity and the expansion are reversible. The expansion, which might relate to tensile stress, increases up to 1% as the measured capacity approaches the theoretical limit of 372 mAh/g for Li storage in graphite. Minor additional expansions due to the uneven distribution of intercalated Li in the CF structure were measured before and after lithiations. Using scanning electron microscope images the transverse expansion of fully lithiated CFs was estimated to about 10% of the cross-section area.

Carbon fibres (CFs) can be used as lightweight structural electrodes since they have high specific tensile stiffness and ultimate tensile strength (UTS), and high lithium (Li)-intercalation capability. This paper investigates the relationship between the amount of intercalated Li and the changes induced in the tensile stiffness and UTS of polyacrylonitrile-based CF tows. After a few electrochemical cycles the stiffness was not degraded and independent of the measured capacity. A drop in the UTS of lithiated CFs was only partly recovered during delithiation and clearly larger at the highest measured capacities, but remained less than 40% at full charge. The reversibility of this drop with the C-rate and measured capacity supports that the fibres are not damaged, that some Li is irreversibly trapped in the delithiated CFs and that reversible strains develop in the fibre. However, the drop in the strength does not vary linearly with the measured capacity and the drop in the ultimate tensile strain remains lower than the CF longitudinal expansion at full charge. These results suggest that the loss of strength might relate to the degree of lithiation of defectives areas which govern the tensile failure mode of the CFs.

In this paper we have conducted experiments to investigate the coupling between electrochemical and mechanical properties of lithium (Li)-intercalating carbon fibres (CFs). The results show promising potential for new functionalities of CFs for electrochemical actuation, sensing and energy harvesting. Li-intercalation at 1 C-rate in CFs subjected to a constant tensile extension induced a free reversible longitudinal expansion strain of approximately 0.30% which can be used as mechanical actuation. Varying the tensile extension of lithiated CFs resulted in a piezoelectric response of the open-circuit potential, in the range of several mV, enabling strain sensing. If the electrical potential is kept constant during a tensile extension a piezo-electrochemical current response is found with about 50% mechanical to electrical energy conversion efficiency, enabling energy harvesting.