Structure-performance relationships of redox mediators in the lithium-sulfur battery.

Abstract

The lithium-sulfur battery is one of the most promising next-generation battery technologies for high-energy applications due to its high gravimetric energy density of 2567 Wh kg-1. Despite its advantages, widespread application of the lithium-sulfur battery has been hindered by poor cycle life and incomplete sulfur utilization. Homogeneous redox mediators (RMs) have emerged as effective additives to enhance the capacity of the battery, yet their mechanistic roles and the links between their molecular properties and cell performance remain poorly understood. Here, we describe the structure-performance relationship of a range of redox mediators in these systems, revealing for the first time the sulfur redox reactions driven by the action of the redox mediators. We show that for quinone-based mediators, the potential of the second reduction is crucial in directing the interconversion between sulfur and lithium sulfide. Our results also demonstrate that mediators are not innocent in degradation processes and can induce parasitic reactions at lithium and/or degrade spontaneously. Incorporation of bulky substituents on the mediator structure enhances operational stability by suppressing these degradation routes. We identify 2,6-di-tert-butyl-1,4-benzoquinone as an especially promising mediator, delivering the greatest performance gains of the quinone-based mediators we studied, and providing a 16% capacity improvement over the standard after 100 cycles. Our mechanistic insights explain this optimal structure-performance relationship and rationalise why previous use of mediators have resulted in poor coulombic inefficiencies. This work enables predictive design of future RMs with improved stability and electrochemical selectivity, and offers a toolbox for rapid evaluation prior to cycling studies.

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