Degradation of LSB Electrodes investigated with X-ray Phase Contrast Tomography 

L. Zielke1*, C. Barchasz2, S. Waluś2,3,4, F. Alloin3,4, R. Bouchet3,4, J.-C. Leprêtre3,4, A. Spettl5, V. Schmidt5, A. Hilger6, I. Manke6, J. Banhart6, R. Zengerle1,7 and S. Thiele1,8

1 Laboratory for MEMS Applications, Department of Microsystems Engineering - IMTEK, University of Freiburg, Georges-Koehler-Allee 103, 79110 Freiburg, Germany
2 French Atomic Energy and Alternative Energy Agency (CEA), Laboratory of Innovation for New Energy Technologies and Nanomaterials (LITEN), 17 Rue des Martyrs, 38054 Grenoble Cedex 9, France
3 University Grenoble Alpes, LEPMI, F-38000 Grenoble, France
4 CNRS, LEPMI, F-38000 Grenoble, France
5 Institute of Stochastics, Ulm University, Helmholtzstr. 18, 89069 Ulm, Germany
6 Helmholtz Zentrum Berlin, Hahn-Meitner-Platz 1, 14109 Berlin, Germany
7 HSG-IMIT Institut für Mikro- und Informationstechnik der Hahn-Schickard-Gesellschaft e.V., Georges-Koehler-Allee 103, 79110 Freiburg, Germany
8 FIT, University of Freiburg, Stefan-Meier-Straße 21, 79104 Freiburg, Germany

Lithium sulfur batteries (LSB) are a promising battery system, e.g. for electrical vehicles. The reasons for that are: a theoretical capacity of 1.675 mAhg-1, compared to 130-200 mAhg-1 for conventional Li-ion battery materials as well as its non-toxic nature,  its low cost and availability.12 
However, there are severe degradation effects which still need to be understood in detail. They mostly come from the fact that the active material (sulfur and lithium polysulfides (LPS)) is soluble in most electrolytes. Therefore the reactions take place in solution, on the surface of the electron conducting network (current collector and carbon binder domain) of the electrode.3,4 Since sulfur itself does hardly conduct electrons, the conducting network needs to be well percolated. To further reduce shuttle effects of LPS to the anode and passivation, the conducting network also needs to have a large specific surface and suitable porosity.5
There is a need to validate novel designs of electrode microstructures, which address the above mentioned degradation effects. Therefore the microstructure evolution in the electrodes during cycling can be investigated. In the past this was done for different charging states (in operando), using 2D X-ray diffraction.6,7 We however employed three-dimensional X-ray phase and absorption contrast tomography in a representative volume of LSB electrodes during various cycles (Figure 1).8
Since the absorption of X-rays in sulfur and carbon are rather low, a differentiation between all phases (sulfur, carbon binder domain, current collector) is possible. This is not the case for metal-based Li-ion battery electrodes, where the strong absorption of the metal atoms hinders a reliable separation of active material and conductive network. As a consequence, further modelling of the missing, conducting phase is required.9,10   
Using X-ray phase and absorption contrast tomography, sulfur particle sizes, spatial distributions of the different phases and sulfur loading for different cycling steps could be quantified. Further we connected quantified spatio-statistical information to electrochemical measurements. On this way importance of different morphological factors could be ruled out or identified as important. This was done for electrodes, fabricated on 3D current collectors, namely on non-woven carbon paper. Electrochemical methods were used to validate the positive influence of the 3D current collector on both discharge capacity and capacity fading of the cell. We found that using the carbon paper, compared to conventional al-foil as current collector, the discharge capacity was higher while capacity fading was lower.11 Based on electrochemical and tomography results, we found that one reason could be the microstructural evolution of the conducting network and especially the carbon binder domain. 

Figure 1 Three-dimensional representation of the pristine and regrown sulfur particles after various cycles. The conducting network is shown partially transparent. The colors indicate the ten largest, connected sulfur clusters.

It is important to further discuss the advantages/disadvantages of in-situ measurements compared to ex-situ measurements. Also worthwhile combinations between different investigation methods are important to discuss, e.g. EDX to detect nano-sized sulfur in the electrode, combined with X-ray tomography, which would allow quantifying the sulfur content during cycling in both the nano- and micro sizes. Finally, investigation of the active material soluble/solid phase in the electrolyte (i.e. LPS dissolved/precipitated in the separator) would also bring additional information regarding failure mechanisms.

References
1.	Tarascon, J.-M. & Armand, M. Issues and challenges facing rechargeable lithium batteries, Nature 414, 359–367 (2001).
2.	Ji, X. & Nazar, L. F. Advances in Li-S batteries, J. Mater. Chem. 20, 9821–9826 (2010).
3.	Zhang, S. S. Liquid electrolyte lithium/sulfur battery: Fundamental chemistry, problems, and solutions, Journal of Power Sources 231, 153–162 (2013).
4.	Manthiram, A. Fu, Y. Chung, S.-H. Zu, C. & Su, Y.-S. Rechargeable Lithium–Sulfur Batteries, Chem. Rev. 114, 11751–11787 (2014).
5.	Mikhaylik, Y. V. & Akridge, J. R. Polysulfide Shuttle Study in the Li/S Battery System, Journal of The Electrochemical Society 151, A1969 (2004).
6.	Nelson, J. et al. In Operando X-ray Diffraction and Transmission X-ray Microscopy of Lithium Sulfur Batteries, J. Am. Chem. Soc. 134, 6337–6343 (2012).
7.	Waluś, S. et al. New insight into the working mechanism of lithium–sulfur batteries: in situ and operando X-ray diffraction characterization, Chem. Commun. 49, 7899–7901 (2013).
8.	Zielke, L. Degradation of Li/S Battery Electrodes On 3D Current Collectors Studied Using X-ray Phase Contrast Tomography, submitted.
9.	Zielke, L. et al. A Combination of X-Ray Tomography and Carbon Binder Modeling: Reconstructing the Three Phases of LiCoO2 Li-Ion Battery Cathodes, Adv. Energy Mater. 4, 1301617 (2014).
10.	Zielke, L. et al. Three-Phase Multiscale Modeling of a LiCoO2 Cathode: Combining the Advantages of FIB–SEM Imaging and X-Ray Tomography, Adv. Energy Mater. (2014).
11.	Sylwia Walús. PhD Thesis, ongoing publication, Grenoble University (2015).