October 2014The lithium-sulfur battery currently attracts worldwide attention due to its high energy density and the low cost components. A central aspect of current research is the preparation of special sulfur/carbon composite materials for the positive electrode that ought to improve the cycle life. Usually, the characterization of these nanostructured composites is done by vacuum-based methods, mainly by nitrogen physisorption and scanning electron microscopy (SEM). However, we could show by systematic analyses of some sulfur/porous carbon model composites that the obtained results from these methods can be misinterpreted easily. For nitrogen physisorption for example the expected result would be that identical isotherms were found for the pure carbons and their mixed composites with sulfur, since sulfur itself is non-porous and the results are normalized on the amount of the porous carbon. Yet the figure shows that rather an extreme loss of porosity was observed for the mixed composite compared to the pure carbon. In situ SEM studies with energy-dispersive X-ray spectroscopy (EDS) could prove that an unexpected, rapid sulfur redistribution with remarkable extent takes place in the carbon material during this measurements. For example 13 wt% sulfur could be detected in the originally sulfur-free carbon material after three hours at SEM vacuum, which also give an explanation for the observed physisorption results. This shows that characterizing the structure of sulfur/carbon composites or electrodes is very challenging and the observed rapid sulfur migration might also have important consequences for the performance of lithium-sulfur batteries.(Picture submitted by Christine Eufinger)https://www.uni-giessen.de/en/faculties/f08/departments/physchem/janek/gallerypotm/pom2014/PoM1014/viewhttps://www.uni-giessen.de/@@site-logo/logo.png
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October 2014
The lithium-sulfur battery currently attracts worldwide attention due to its high energy density and the low cost components. A central aspect of current research is the preparation of special sulfur/carbon composite materials for the positive electrode that ought to improve the cycle life. Usually, the characterization of these nanostructured composites is done by vacuum-based methods, mainly by nitrogen physisorption and scanning electron microscopy (SEM). However, we could show by systematic analyses of some sulfur/porous carbon model composites that the obtained results from these methods can be misinterpreted easily. For nitrogen physisorption for example the expected result would be that identical isotherms were found for the pure carbons and their mixed composites with sulfur, since sulfur itself is non-porous and the results are normalized on the amount of the porous carbon. Yet the figure shows that rather an extreme loss of porosity was observed for the mixed composite compared to the pure carbon. In situ SEM studies with energy-dispersive X-ray spectroscopy (EDS) could prove that an unexpected, rapid sulfur redistribution with remarkable extent takes place in the carbon material during this measurements. For example 13 wt% sulfur could be detected in the originally sulfur-free carbon material after three hours at SEM vacuum, which also give an explanation for the observed physisorption results. This shows that characterizing the structure of sulfur/carbon composites or electrodes is very challenging and the observed rapid sulfur migration might also have important consequences for the performance of lithium-sulfur batteries.(Picture submitted by Christine Eufinger)
