MicroCT uses the absorption of X-rays to create a 3D reconstruction. The Field of View is large enough to be representative with regard to the grain sizes (1). However, the submicron porosity in the electrodes is typically not well captured by microCT because of its limited resolution.
The FIBSEM technique which uses a Galium ion beam for sectioning and an electron beam for imaging has high enough resolution to resolve all porosity. However, the Field of View is typically of the order of 10 micrometer; which is not big enough to create a representative model of the pore space.
In this study we have used a plasmabeam for sectioning and an electron beam for imaging. The plasmabeam typically removes a layer of 100 nm followed by imaging by the electron beam. The material removal rate with the plasmabeam is magnitudes higher than with the Ga focused ion beam, allowing to expand the field of view to hundreds of micrometers.
A 3D reconstruction of the anode (Field of View 200micrometer) and the cathode (Field of View 100 micrometer) make it possible to calculate parameters such as the tortuosity on a representative volume.
Segmentation of the pore space is not straightforward due to minimal grey level differences between the different phases. Also, it is not uncommon to see details from layer N+1 through the pores in layer N. This so-called pore-back effect must be taken into account otherwise the permeability will be greatly underestimated.
Direct imaging of light elements such as lithium in the crystal structure of a compound by Transmission Electron Microscopy is typically a difficult task. In most cases the position of the light elements is deduced from image simulations using different defocus and thickness values.
Starting from the theory of scanning transmission electron microscopy imaging we have proven that it is possible to image the phase of a thin sample directly and linearly when use is made of the center of mass (COM) movement of the convergent beam electron diffraction pattern. Integrating the two components of the COM image gives an image which is directly proportional to the phase shift caused by the sample.
Experiments show that both light and heavy elements are visible and that low frequency information is fully transferred. Examples are shown where lithium is directly visualized in the structure of a typical cathode active material.
- Cai et al, Electrochimica Acta 56 (2011) 5804–5814