Investigation of the temperature and DOD effect on the performance-degradation behavior of lithium–sulfur pouch cells during calendar aging

High-energy density sulfur cathodes are one of the most promising possibilities to replace currently used intercalation cathodes in lithium-ion batteries in future applications. However, lithium–sulfur batteries are still the subject of research due to unsatisfactory capacity retention and cycle performance. The cause of insufficient properties is the shuttle effect of higher polysulfides which are formed in a high voltage plateau. In an effort to optimize storage conditions of lithium–sulfur (Li–S) batteries, long-term calendar aging tests at various temperatures and depth-of-discharge were performed on pre-commercial 3.4 Ah Li–S pouch cells. The decrease in performance over two years of calendar aging in five stationary conditions was analyzed using non-destructive electrochemical tests. The negative effect on Li–S cell performance was more pronounced for temperature than for depth-of-discharge. The analyses of self-discharge and shuttle current were performed and as expected, the highest values were measured in a fully charged state where higher polysulfides are present. Furthermore, internal resistance was analyzed where an increase of resistance was observed for a discharged state due to the formation of a passivation layer from discharge products (Li2S2, Li2S). To maximize the life of the Li–S battery, storage at high temperatures and in a fully charged state should be avoided.