Improving the self-discharge behavior of sulfur-polypyrrole cathode material by LiNO3 electrolyte additive

Self-discharge characteristics of Li/S cells using pure sulfur and sulfur-polypyrrole (S-PPy) cathode materials and lithium bis(trifluoromethane)sulfonimide (LiTFSI) as the electrolyte salt were investigated by monitoring the open circuit voltage, the electrochemical impedance change, and the capacity loss during or after storage at room temperature. Corrosion behavior of the aluminum current collector was studied using linear sweep voltammetry and optical microscope observations of aluminum substrate in aged cells. The results showed that the cell with a pure sulfur cathode suffered from severe self-discharge, which is attributed to the corrosion of the current collector by LiTFSI and a rapid shuttle mechanism. However, a PPy coating on the surface of sulfur particles can suppress the shuttle effect, giving better self-discharge performance. LiNO3 was investigated as a suitable electrolyte additive to prevent the self-discharge of Li/S cells. A self-discharge rate of 3.1 % was obtained for a cell with an S-PPy cathode and a modified electrolyte containing 0.4 M LiNO3. It was found that LiNO3 acts both as a corrosion inhibitor and a shuttle inhibitor. This respectively reduces the transformation of solid sulfur to soluble lithium polysulfides and prevents the dissolved sulfur and generated polysulfides from chemical reaction with the Li anode.