Lithium Sulfur Battery Oxidation/Reduction Mechanisms of Polysulfides in THF Solutions

The redox processes of Li₂S_n(6 ≤ n ≤ 12) at a glassy carbon electrode in THF was studied by programmed cyclic voltammetry in the range of +1300 to —2000 mV (vs. polysulfide reference electrode) at sweep rates of 2–200 mV/s. One anodic and up to three cathodic peaks were detected. The anodic peak seems to result from the oxidation of all PS's through the same intermediate to elemental sulfur. The first cathodic peak is caused by the reduction of all PS (n > 6) to Li₂S₆a diffusion controlled reaction. The second reduction peak most likely arises from the reduction of S₆ ^2-to S₅ ^2-. This is apparently preceded by a chemical step. The third reduction peak is caused by the reduction of S₅ ^2-to S₂ ^2-or S^2-or a mixture of both in a diffusion-controlled reaction. The high Tafel slope of the third peak apparently results from passivation of the electrode by the precipitation of Li₂S and Li₂S₂.