Jnawali, Anmol; (2023) Investigating Degradation and Deformation Mechanisms in Cylindrical Li-ion Batteries. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

There has been a great focus on the study of the microstructure of lithium-ion (Li-ion) batteries in order to elucidate the effect of changing morphology on performance. However, just as important is the need to understand the macroscopic structural changes during battery cycling, and the effect of the changes.This work has focused on characterising the changes in the internal structure of Li-ion batteries as a function of cycling. Investigations into how differing geometries and internal structure affect the dynamic changes within electrode rolls were also conducted. Extending this into a quasi-4D in-situ study, the magnitude of the changes occurring during individual charge cycles was elucidated. Finally, the effect of these macro-structural changes on battery performance was discussed.Jelly rolls of cylindrical cells experience volume changes during each charge and discharge, which accumulates into irreversible changes over long periods, resulting in a collapse. The collapse is generally located at existing points of weaknesses in the electrode rolls. Also, variability in the internal structures of the cells (resulting from manufacturing variability) manifests as minor differences in deformation characteristics. Other influencing factors have a profound effect on the jelly roll deformation, for example, it was made clear that deformation is accelerated at lower temperatures and for cells containing silicon additives (SiOx). Identifying characteristics to qualify cells at a rapid pace in production lines and more accurately predict the lifetime of batteries is essential in meeting the increasing demand on Li-ion battery production. The effectiveness of X-ray computed tomography combined with image analysis tools such as the ones developed in or adapted to this work (virtual unrolling and optical flow), have been proven in characterising and quantifying macro-structural changes within batteries leading to failure. With some improvements and augments, these techniques could readily be developed into tools for quality assurance and cell life prediction.

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