Lignin- and PAN-based carbon fibres as negative electrodes for alkali-ion batteries

Abstract: The development of sodium-ion batteries (SIBs) and potassium-ion batteries (KIBs) have accelerated since they can now reach similar gravimetric energy densities as lithium-ion batteries (LIBs) but with a lower environmental impact. Hard carbon is the most common negative electrode for SIBs and KIBs and can be made from renewable resources such as lignin. Lignin can be then manufactured into fibres which can then be used as free-standing electrodes to push even further the sustainability by reducing the amount of current collector and additives needed in the battery. The concept of structural batteries is defined as a system that can simultaneously carry mechanical load as well as store the electrical energy in form of a battery to decrease the total weight. Polyacrylonitrile-based (PAN-based) carbon fibres are some of the most adapted materials thanks to their outstanding mechanical properties as well as their ability to be used as negative electrode for LIBs. However, a structural model and insertion model for alkali-ion insertion in the PAN-based carbon fibres is still lacking and is necessary to be able to understand the dynamics and fundamentals. This thesis focuses on the use of lignin-based carbon fibres (LCFs) and PAN-based carbon fibres as negative electrodes. The potential of using LCFs as negative electrode for SIBs and KIBs is evaluated by using a combination of electrochemical techniques and material characterization methods. The LCFs have high specific capacity and high initial coulombic efficiency when used as negative electrode for SIBs. The diffusion of potassium-ions into the LCFs is investigated by implementing a numerical model. The investigation on the open circuit voltage curves and the entropy change for potassium-ion insertion suggests that the LCFs structure contains two domains which can explain why the numerical model cannot fully fit the experimental data. The PAN-based carbon fibres are investigated as negative electrode for LIBs and SIBs. For SIBs, the axial expansion is investigated during charge/discharge and shows a staged expansion between the slope region and the plateau region of the charge/discharge profile. For LIBs, a combination of ex-situ Li-NMR and ex-situ wide-angle X-ray scattering isused to determine the insertion mechanism and structure of the PAN-based carbon fibres. A structural model and insertion model for lithium-ions is suggested from our experimental results consisting of three different types of sites: disordered domain in the carbon structure, ordereddomain in the carbon structure, and pore filling.