Which Process Limits the Operation of a Li–S System?

The impact of the solid film deposit (mainly Li₂S) on the complex electrochemistry of a Li–S cell is studied in detail. Already a simple, straightforward experiment strongly indicates that this impact might be much smaller than usually assumed. Notably, a similar phenomenon is demonstrated for another battery operated on the same basic principle: the magnesium–sulfur battery. In order to better detect the surface-specific phenomena associated with formation and properties of the solid surface deposit, we construct special electrochemical cells with a flat glassy carbon disc or other well-defined materials. Different model systems are prepared in which crucial variables such as the electrode configuration, separator type, and state of charge are varied in a systematic and controlled way. Electrochemical results are supplemented with data from microstructural analysis, in particular focused ion beam–scanning electron microscopy (FIB-SEM) imaging and X-ray diffraction analysis. We show that the growth of the surface film is more complex than generally assumed and that its defect-rich morphology hardly represents any obstacle for electrochemical reaction(s) to take place. Rather, the cell operation is limited by diffusional processes and depletion of polysulfide concentration in electrolyte. The new insight into the occurrence, properties, and especially the impact of solid film deposits on operation of the Li–S system is expected to have important implications for future design of Li–S practical cells.