Visualized redox reaction guides polysulfide synthesis with electrochemical approach for long-cycle lithium-sulfur batteries

Lithium-sulfur (Li-S) batteries are promising next-generation energy storage systems. However, low active material utilization and sluggish reaction kinetics restrict their practical applications. Here, the redox reaction of these batteries was visualized, and their polysulfide component was established with their ultraviolet-visible absorption spectra through in-situ discharge observations. Liquid polysulfides with controllable component and concentration can be obtained by discharging solid sulfur into a determined potential. Fast and timely electrochemical conversion kinetics will occur due to the excellent wettability between polysulfides and carbon nanotube sponges (CNSs). In addition, the robust chemical interaction between liquid polysulfides and oxygen-containing groups inhibit the shuttle effect to a certain extent. Accordingly, the polysulfides-CNSs cathode demonstrates high sulfur utilization that exceeds 60% after 200 cycles. The obtained Li-S battery exhibits remarkably long cycling stability for 1500 cycles at 1 C with nearly 100% coulombic efficiency. This work provides valuable concepts for fabricating cathodes for Li-S batteries and for achieving their high-level services.