Oxidation process of polysulfides in charge process for lithium–sulfur batteries

The oxidation of polysulfides to element sulfur in charge process was studied by solution thermodynamic analysis and means of cyclic voltammetry (CV), X-ray diffraction (XRD), and charge–discharge test. Basing on the solution thermodynamic analysis, the oxidation process of polysulfides to element sulfur would arise only if the charge voltage exceeds 3.36 V in a lithium–sulfur cell employing 1.0 M LiN(CF3SO2)2 in 1,2-dimethoxy ethane. Furthermore, the minimum of charge voltage which can push the oxidation would fall down with the increasing solubility of elemental sulfur in electrolyte solution. These analyses were confirmed by practical measurements. One new anodic peak corresponding to the oxidation process of polysulfides to solid sulfur was observed by CV. Both XRD patterns and charge–discharge test showed that the element sulfur appeared in the cathode after the battery was charged over 3.4 V. Hence, the lithium–sulfur cell charged over 3.4 V exhibited an improved cycle life since the capacity degradation between the first cycle and the second was depressed. In order to improve the energy efficiency, carbon disulfide was added in the electrolyte solution of lithium–sulfur cell to increase the solubility of sulfur.