Xanthate-Mediated Oxidation of Li₂S as the Lithium-Containing Cathode in Lithium-Sulfur Batteries with Extremely Low...

Lithium-sulfide (Li₂S) 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 Li₂S 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 Li₂S, ethanol, and CS₂ 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 Li₂S (2.25 V vs Li⁺/Li), which enables fast Li₂S oxidation reaction kinetics, and more importantly, lowers the Li₂S 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^-1 at 0.2 mA cm^-2 and long cycle life of 1400 cycles with ∼73% capacity retention is achieved, outperforming those of other Li₂S-based cathode materials from recent literature.