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 operando 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 S₈ and Li₂S₆ so that it is thermodynamically and kinetically unstable. Meanwhile, the poor adsorption behavior of Li₂S_n on carbon surface further prevents the oxidization of Li₂S_n back to S₈ 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 (n = 4–8). This approach allows the higher Li₂S_n to be further oxidized into S₈, 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.