Microstructure Characteristics of Blend Cathodes Assessed By 3D-Tomography Methods

Abstract

Electrode performance of lithium ion battery cathodes is strongly affected by its microstructure parameters. FIB (focused ion beam) / SEM (scanning electron microscopy) tomography is a powerful tool for determining microstructure parameters. The application of this method and processing of images has been optimized for battery electrodes [1], enabling an accurate detection of the three components (1) carbon black, (2) pore phase and (3) active material. A special challenge is to distinguish between different active materials which are combined in blend electrodes for optimizing their performance and energy density. In this study, a dual beam modus is applied for analyzing the composition and microstructure of a blended battery cathode consisting of three active materials (NMC, NCA and LMO). By combining the information of InLens and Everhart-Thornley detector it was possible to reconstruct a volume of 35µm x 37µm x 20µm consisting of 1010 x 1070 x 570 voxel (isotropic voxelsize of 35 nm). The pore phase and the carbon black were segmented by evaluating images acquired using the Everhart-Thornley detector, due to lower falsification by local charging, whereas the active material was separated into its three components interpreting information of InLens detector which provides a better contrast of specific material. It was possible to clearly assign the three materials, confirmed by EDX analysis, and to calculate microstructure parameters for every specific component, as particle size distribution, active surface area, porosity and tortuosity. Limiting factor for an application of this method is the representative volume element size which is dependent of the number of active materials as well as their maximum particle size, as reported in [2] for solid oxide fuel cells. For prospective studies, an increased reconstruction volume is necessary while maintaining high resolution of images, which is challenging with regard to FIB-procedure as well as processing of big data amount. Alternative tomography methods as X-ray tomography enable the reconstruction of larger volumes, but suffer from low contrast of carbon black and limited resolution. Advantages and disadvantages of the different tomography methods are discussed and assed. [1] M. Ender, J. Joos, T. Carraro and E. Ivers-Tiffee, Journal of the Electrochemical Society, 159, A972-A980 (2012) [2] J. Joos, M. Ender, T. Carraro, A. Weber and E. Ivers-Tiffée, Electrochemica Acta, 82, 268-276 (2012)