Pitfalls in the characterization of sulfur/carbon nanocomposite materials for lithium-sulfur batteries

The preparation of sulfur/carbon composite materials for lithium-sulfur batteries is currently a very active research field. Thereto, nanoporous carbon materials are mixed with or infiltrated by sulfur to provide a close contact between both compounds. The characterization of these often complex and on the nanoscale structured composite materials is usually done by vacuum based methods such as nitrogen physisorption or scanning electron microscopy, for example. In this study we show that results from these measurements can be misinterpreted. The reason is the volatility of sulfur that leads to a rapid migration and continuous redistribution effects, especially at low pressures and/or elevated temperatures. For nitrogen physisorption this means that virtually identical isotherms are found for S8/C samples, irrespective of their pre-treatment, making it impossible to prove intentional nanostructuring by pore filling. Similar effects are found for scanning electron microscopy studies where sulfur migration and contamination of originally sulfur free samples is evidenced in situ. Further evidence is provided by macroscopic experiments combined with elementary analysis. The results show that characterizing the structure of S8/C composite materials or electrodes is very challenging. In addition, the observed rapid sulfur redistribution might also have important consequences for the performance of practical lithium-sulfur batteries.