Ageing behaviour of a commercial 18650 Li-ion cell with a SiOx- containing negative electrode

Abstract

This thesis investigated the cycling performance and ageing behaviour of commercial 18650 Li-ion cells with a SiOx-containing negative electrode. Lifetime testing was conducted under different voltage window placements, voltage window widths, and charge C-rates to evaluate the influence of operating conditions on cell age ing. Electrochemical characterisation included capacity measurements, incremental capacity analysis (ICA), and electrochemical impedance spectroscopy (EIS). Non destructive characterisation was performed using computed tomography (CT), while post-mortem analysis included scanning electron microscopy (SEM), energy disper sive X-ray spectroscopy (EDX), and laboratory-scale PAT-cells. The results demonstrated that voltage window placement had a greater influence on ageing than voltage window width or charge C-rate under the investigated conditions. Cells cycled within a centred voltage window exhibited comparatively mild degradation, whereas cells cycled closer to the upper or lower voltage limits showed significantly more severe ageing behaviour. In particular, cells cycled at lower voltage limit exhibited substantially faster capacity fade despite operating within similar voltage window widths. The degradation was primarily associated with the negative electrode and was linked to high delithiation, Li-plating, SEI instability, mechanical degradation, and loss of cyclable lithium. Post-mortem analysis revealed heterogeneous ageing across the jellyroll, where the inner, middle, and outer electrode regions exhibited different degradation behaviour. Electrochemical characterisation through ICA and EIS revealed increased resistance, peak broadening, peak shifts, and increased polarisation with ageing, consistent with interfacial degradation and non-uniform lithiation. Furthermore, visible Li-plating was not directly proportional to capacity fade, indicating that irreversible degradation processes contributed significantly to the ageing behaviour. In addition, an in-house developed teardown tool significantly reduced the cell disassembly time. However, the cutting process damaged the outer three jellyroll layers, indicating that further refinement of the tool design is required.