Investigating ageing mechanisms in electric vehicle batteries : A multiscale approach to material analysis

Sammanfattning: Electrifying passenger transport is a key strategy in combating global warming, with Li-ion batteries (LIBs) being the current go-to technology. Despite LIB’s satisfactory performance and carbon-neutral operation, lifetime and safety are still public concerns. A thorough understanding of battery ageing is crucial for improving LIBs and advancing the overall sustainability of LIB technology. This thesis bridges a gap between academic and industrial research by combining commercial battery investigation with a multiscale approach using a combination of in-house and synchrotron characterization methods used with the implementation of method development to study commercial batteries.The multiple degradation mechanisms were identified at various scales in the aged commercial cells. Specifically, the results show that the studied cells exhibit significant and distinct ageing heterogeneity in prismatic and cylindrical cell formats, where the area with the highest degradation is found on the side of the positive tab, where the current and temperature gradients are expected to be the strongest. After decoupling the performance on the electrode level, the Ni-rich layered oxide positive electrodes show a significant increase in Li+ diffusion resistance in the aged materials as a function of the State of Charge (SoC) range and temperature. Furthermore, heterogeneity is an issue relevant also on a secondary particle scale, where identified SoC gradients ranging from the centre to the surface of the particle might induce kinetic limitations and cause an increase in Li+ diffusion resistance. On a single particle level, the formation of a large number of voids within the grains was found. Such degradation can additionally contribute to the resistance increase in the material by changing tortuosity for Li-ions. Finally, at the atomic level, Ni was found to be the dominant charge compensator, which can decrease up to 25% of the redox activity after ageing. Compared to Ni, Co was found to be less redox-active, but more involved in charge compensation through changes in hybridization with the oxygen atom. The oxygen, in turn, was revealed to participate in anionic redox reactions at low SoC by both hybridization to TM and also through the formation of molecular oxygen at lower potentials than previously reported. The observed decrease in oxygen anion redox activity follows with material losing performance.The results presented in the thesis demonstrate the importance of the multiscale approach in order to form a more complete understanding of the degradation processes which have effects within different scales.