Llewellyn, Alice; (2024) Using Multi-Scale Imaging and Diffraction Techniques to Understand the Degradation Mechanisms of Lithium Ion Batteries. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Advanced lithium-ion batteries adopting new cathode chemistries are required for successful transition to electric vehicles. Promising commercial candidates for cathode materials include nickel rich NMC (Li(NixMnyCo1-x-y¬)O2 where x ≥ 0.8) due to their high specific capacity. However, they can suffer from rapid degradation, the mechanisms of which are still poorly understood. A multi-scale approach is required to gain a more comprehensive understanding of the degradation mechanisms at play and how they initiate and propagate. In this work, synchrotron diffraction methods were employed at the crystal, particle and cell scale using a variety of techniques including operando Bragg coherent diffraction imaging, x-ray diffraction computed tomography and operando high-resolution XRD. Intra-particle, inter-particle and electrode level heterogeneities were observed during cycling, both in pristine and aged samples. It is postulated that these heterogeneities in crystal structure accelerate the cell performance decay by inducing crack formation. The cracks formed during cycling were then studied in detail using x-ray computed tomography in simultaneous lab studies. Imaging was performed ex situ after different cycling protocols, including high voltage cycling and extended cycle life (up to 1000 cycles) whilst also studying the influence of the particle morphology on crack formation. It was found that cracks form at high voltages and close during discharge in early cycles, whilst more permanent cracks form upon extended cycling. Polycrystalline materials also have a higher propensity to form cracks than their single crystal counterparts. By studying NMC811 using both diffraction and imaging methods, the interplay between crystal structure strain and crack formation and how this manifests in the electrochemical performance was investigated. The overarching scientific goal of this work was to add to the understanding of the complex degradation mechanisms for Ni-rich cathodes, ultimately resulting in practical recommendations for the development and optimal operating conditions of future battery electrode materials.

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