Nanoscale X-ray imaging of ageing in automotive lithium ion battery cells

Due to an increasing number of electric vehicles, battery pack endurance over the cell lifetime is an important issue of public interest, because it determines to a large degree the value of the vehicle. The exact state of health of a battery is still a major challenge for analysis, especially since the individual ageing factors are not well understood and depend on the cell chemistry and design. In this analysis, X-ray computer tomographic (CT) images of new and aged battery electrodes are compared to determine structural changes over lifetime. The X-ray CT images with sub-micrometer resolution show the internal pore structure of the electrodes and allow the identification of the morphology of the materials. Organic residues and depositions responsible for reduced porosity in aged anodes are quantified and visualized. Furthermore, particle cracking and current collector corrosion is detected on the cathode side. All these are ageing effects, which might influence the cell capacity and impedance. A quantitative analysis of the lithium distribution completes the interpretation of the ageing study confirming that the degradation can be attributed to the loss of cyclable lithium which is bound in the particle surface layer of the anode.