Xanthate-Mediated Oxidation of LiS as the Lithium-Containing Cathode in Lithium-Sulfur Batteries with Extremely Low Overpotential.

Lithium-sulfide (LiS) has long been pursued as a lithium-containing cathode material for high-energy-density lithium-sulfur (Li-S) batteries. Unfortunately, its direct oxidation generally has a large overpotential, giving rise to low energy efficiency. The use of redox mediators to accelerate the conversion of solid LiS to polysulfides represents a possible solution to lower the initial oxidation overpotential. However, most reported redox mediators exhibit significantly higher redox potentials than the desirable value. Herein, it is serendipitously found that lithium ethyl xanthate (LiEX) formed from the reaction among LiS, ethanol, and CS at room temperature is an efficient redox mediator. It has a redox potential (≈2.3 V vs Li/Li) close to the electrochemical oxidation potential of LiS (2.25 V vs Li/Li), which enables fast LiS oxidation reaction kinetics, and more importantly, lowers the LiS oxidation potential from ≈3.6 to ≈2.3 V. When further integrated with an Ni-NC catalyst in a tandem catalysis scheme, a remarkable specific capacity of ≈1100 mAh g at 0.2 mA cm and long cycle life of 1400 cycles with ∼73% capacity retention is achieved, outperforming those of other LiS-based cathode materials from recent literature.