Despite numerous studies presenting advances in tomographic imaging and analysis of lithium ion batteries, graphite-based anodes have received little attention. Weak X-ray attenuation of graphite and, as a result, poor contrast between graphite and the other carbon-based components in an electrode pore space renders data analysis challenging. Here we demonstrate operando tomography of weakly attenuating electrodes during electrochemical (de)lithiation. We use propagation-based phase contrast tomography to facilitate the differentiation between weakly attenuating materials and apply digital volume correlation to capture the dynamics of the electrodes during operation. After validating that we can quantify the local electrochemical activity and microstructural changes throughout graphite electrodes, we apply our technique to graphite-silicon composite electrodes. We show that microstructural changes that occur during (de)lithiation of a pure graphite electrode are of the same order of magnitude as spatial inhomogeneities within it, while strain in composite electrodes is locally pronounced and introduces significant microstructural changes.

This paper presents an approach to quantify microstructural inhomegeneity in lithium ion battery electrodes over multiple length scales and examines the impact of this microstructural inhomogeneity on electrochemical performance. Commerical graphite anodes are investigated because graphite remains the anode material of choice due to its low cost, mechanical robustness, and suitable electrochemical properties. At the same time, the graphite anode often plays a role in cell degradation and failure, as lithium plating can occur on the graphite anode during charge, when unfavorable microstructure in the graphite electrode leads to a large overpotential. Here, three-dimensional representations of four different commercial anodes obtained with X-ray tomographic microscopy are statistically analyzed to quantify the microstructural inhomogeneity that is commonly present in lithium ion battery electrodes. Electrochemical simulations on the digitalized microstructures are performed to isolate and understand the influence of different types of microstructural inhomogeneity on battery performance. By understanding how distributions in particle size and shape or slurry and electrode processing cause microstructural inhomogeneity and impact performance, it is possible to determine the extent to which homogeneity should be prioritized for specific applications and how homogeneity could be achieved through smart material selection and processing.

Water management is an important factor for optimizing polymer electrolyte fuel cells (PEFC) under high current density conditions as required for the automotive application. The characteristics of the local liquid saturation of the gas diffusion layer (GDL) is of particular interest. Here we report on the development of in-situ X-ray tomographic microscopy (XTM) with a pixel sizes in the order of 2 μm and sensitivity for carbon and liquid water for the quantitative analysis of liquid water in GDLs. In-situ XTM of PEFC is a major experimental challenge. A complete cell needs to be operated under realistic conditions in the constraint space of the small field of view on the beamline sample stage. Further phase segmentation of the images is required to successfully analyze the quantitative properties of the different phases. For this a workflow, applying differential images between dry and wet structures has been developed. Cells with Toray TGP-H-060 GDLs were analyzed in-situ. Droplets that appear on the GDL surface are connected to a significant water structure inside the GDL. Further the water cluster size distribution in the GDL shows that while small droplets (<100 pl) are numerous, most of the water is contained in few larger clusters.

Operating Polymer Electrolyte Fuel Cells (PEFC) under high current density conditions, causes significant losses related to liquid water saturation in the gas diffusion layer (GDL). The blockage of pores inside the material has a strong influence on its effective gas transport properties. Here we report on the combination of in-situ X-ray tomographic microscopy (XTM) of PEFC and the numerical determination of gas transport properties using Lattice Boltzmann and finite difference methods. The GDL domains (Toray TGP-H-060) of two identical cells, each with 11 mm² active area, were analyzed in sections of about 0.3 to 0.8 mm² size. Saturation levels between 0.1 and 0.4 were found, with higher saturation under the ribs. The saturated and the non-saturated states of the GDL samples were compared in order to quantify the dependence of gas phase permeability and effective relative diffusivity on liquid water saturation. Both these relative measures were found to follow power relationships of (1 − s)^λ, where the exponent λ was approximately 3 for all cases except for the in-plane diffusivity where it was closer to 2.