Conductive Lewis Base Matrix to Recover the Missing Link of Li2S8 during the Sulfur Redox Cycle in Li-S Battery

Sulfur and polysulfides play important roles on the environment and energy storage systems, especially in the recent hot area of high energy density of lithium-sulfur (Li-S) batteries. However, the further development of Li-S battery is still retarded by the lack of complete mechanistic understanding of the sulfur redox process. Herein we introduce a conductive Lewis base matrix which has the ability to enhance the battery performance of Li-S battery, via the understanding of the complicated sulfur redox chemistry on the electrolyte/carbon interface by a combined in operand Raman spectroscopy and density functional theory (DFT) method. The higher polysulfides, Li2S8, is found to be missing during the whole redox route, whereas the charging process of Li-S battery is ended up with the Li2S6. DFT calculations reveal that Li2S8 accepts electrons more readily than S-8 and Li2S6 so that it is thermodynamically and kinetically unstable. Meanwhile, the poor adsorption behavior of Li2Sn on carbon surface further prevents the oxidization of Li2Sn back to S-8 upon charging. Periodic DFT calculations show that the N-doped carbon surface can serve as conductive Lewis base "catalyst" matrix to enhance the adsorption energy of Li2S (n = 4-8). This approach allows the higher Li2Sn to be further oxidized into S-8, which is also confirmed by in operando Raman spectroscopy. By recovering the missing link of Li2S8 in the whole redox route, a significant improvement of the S utilization and cycle stability even at a high sulfur loading (70%, m/m) in the composite on a simple super P carbon.