The increase demand on energy storage devices require further development of established systems, such as Li-ion batteries. Understanding the link between the complex electrode microstructure and the electrochemical performance is of utmost importance to optimize their energy density, performance and reveal key features regarding macroscopic failures. However, the study of structural electrode inhomogeneities which influence the transport processes taking place is not a trivial task, since they occur at different length scales. Consequently one single technique could provide meaningful information up to a certain extent, but is not enough to gain information about the battery as a whole.
Up to date, the two most frequently employed techniques to characterize the nano- and micro-structure of battery electrodes are focused ion beam-secondary electron microscopy (FIB-SEM) and X-ray tomography. By FIB-SEM tomography, a very high resolution (~7 nm) can be obtained. For instance the three cathode domains i.e. active material particles, carbon and pores phases can be distinguished. However, only small volumes (~5·104 μm3) can be analysed besides the high cost and long time needed for imaging acquisition and post-processing. On the other hand, conventional X-ray tomography depicts larger volumes (~7·1014 μm3) which are thus more representative of the whole electrode, but is limited to materials with good absorption properties and to the lower resolution (~700 nm).
In this work, LiNixMnyCo1-x-y (NMC)-based cathodes are characterized by combination of both 3D techniques, X-ray and FIB-SEM tomographies. The microstructure parameters: material fraction, porosity, surface area, tortuosity are calculated at the different investigated length scales and arising challenges, possibilities and limits of these methods will be critically discussed. Moreover, different representative volume sizes (RVE) are investigated and the minimum size to determine accurately each microstructure parameter will be presented.