Accelerated polysulfide redox kinetics revealed by ternary sandwich-type S@Co/N-doped carbon nanosheet for high-performance lithium-sulfur batteries

Lithium-sulfur (Li-S) batteries show significant advantages for next-generation energy storage systems due to the high theoretical energy density and cost effectiveness. The main challenge for developing long-life and high-performance Li-S batteries is to simultaneously restrain the shuttle of soluble polysulfides while accelerating the redox kinetics of sulfur-related species during cell operation. Herein, a sandwich-type sulfur@Co/N-doped carbon (S@Co-NC) ternary composite is synthesized, combining in a simple way the advantages of synergetic physical and chemical confinement on sulfur-related species. A further electrochemical redox kinetic study confirms that the incorporated conductive Co mediators and doped N species serve as dual electrocatalysts, substantially accelerating the kinetics of the polysulfide redox reactions. By relieving the sluggishness of polysulfide redox reactions, the as-obtained sandwich-type S@Co-NC composite has significantly promoted electrochemical performance including enhanced rate capability, lower polarization and higher Coulombic efficiency. It delivers extremely high discharge capacity of 1401 mAh g(-1) at 0.05 C with an area sulfur loading of 1.3 mg cm(-2). More importantly, the ternary electrode retains a high rate capability of 694 mAh g(-1) over extensive 600 cycles at 1 C, with nearly 100% Coulombic efficiency maintenance. These results suggest that the S@Co-NC ternary composite provides possibility of realizing the industrially practical Li-S batteries. (C) 2017 Elsevier Ltd. All rights reserved.