Self-discharge behavior of lithium-sulfur batteries at different electrolyte/sulfur ratios

The commercialization of lithium sulfur (Li-S) batteries is hindered by their poor cycling performance, including fast capacity fade, low Coulombic efficiency, and high self-discharge rate. The static electrochemical stability of Li-S batteries, which is usually described in terms of their self-discharge properties, was much less studied compared to the dynamic electrochemical stability under continuous cycling. In this article, a set of experiments designed to understand the correlation between the self-discharge process and various operational conditions were made by using freestanding carbon nanotube foams as cathodes. We found a strong dependence of the self-discharge rate on the depth of discharge and on the electrolyte/sulfur ratio. We show that the effects of the self-discharge on the subsequent discharge capacities of Li-S cells are closely related to the type and concentration of lithium polysulfide species in the electrolyte and their interaction with the Li anode. This relation is analyzed in detail in this article, showing that one should pay special attention to the state-of-charge during self-discharge and the other operational conditions in order to improve the cyclability and capacity of Li-S batteries. In addition, we also highlight the importance of an efficient anode protection to improve the static electrochemical stability of these batteries.