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

Abstract

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 (≈2.3V vs Li+/Li) close to the electrochemical oxidation potential of Li2S (2.25V vs Li+/Li), which enables fast Li2S oxidation reaction kinetics, and more importantly, lowers the Li2S oxidation potential from ≈3.6 to ≈2.3V. When further integrated with an Ni-NC catalyst in a tandem catalysis scheme, a remarkable specific capacity of ≈1100 mAh g-1 at 0.2mA 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.