Optimizing the performance of the Lithium ion batteries requires further understanding of the relationship between the electrode microstructure and the electrochemical performance. Advanced microscopy methods like focused ion beam-secondary electron microscopy (FIB-SEM) and X-ray tomography have been lately used to characterize the battery electrodes [1,2]. However, the study of the complex morphology of the electrodes is not a trivial task, since relevant details take place at different length scales. One single technique is thus meaningful up to a certain extend but not enough to achieve information of the battery as a whole [3].

By a correlative approach the central issues arising during post-processing the image data can be investigated: (1) segmentation of the carbon (carbon black-binder) domain limited by the lower X-ray tomography resolution, (2) reliability of the small volume fractions of the active material and/or other large features taken by FIB-SEM tomography and (3) impact of inner porosity, fissured texture or agglomerated particles, which demand a high resolution technique, in the transport properties.

In this work, LiNi_xMn_yCo_1-x-y (NMC) cathodes are characterized by correlation of both FIB-SEM and X-ray tomography. Relevant microstructural information of each individual phase (i.e. active material, carbon, and pore phases) is determined with their respective volume fraction, surface area, particle size distribution and tortuosity at the different relevant length scales.

In addition, the electrochemical performance of the cathodes is studied by impedance spectroscopy (EIS) combined with distribution of relaxation times (DRT) depicting information about the ohmic resistance (R₀), charge transfer (e^-) at the interface cathode/current collector (R_CC), charge transfer (Li⁺) at the interface cathode/electrolyte (R_CT) and the ionic and electronic transport [4].

As a whole, the possibilities, limits and prospects of both microstructural characterization methods will be critically discussed along with the impact on the results.

References

1. M. Ender, J. Joos, T. Carraro, E. Ivers-Tiffée, J. Electrochem. Soc. 159 (2012) A972-A980.
2. P.R. Shearing, L.E. Howard, P.S. Jorgensen, N.P. Brandon, S.J. Harris, Electrochem. Comm., 12 (2010) 374-377.
3. R. Moroni, M. Börner, L. Zielke, M. Schroeder, S. Nowak, M. Winter, I. Manke, R. Zengerle, S. Thiele, Sci. Rep., 6 (2016) 30109.
4. J. Illig, J.P. Schmidt, M. Weiss, A. Weber, E. Ivers-Tiffée, Journal of Power Sources, 239 (2012) 670-672.