Rational design of redox mediator for fast and energy-efficient charging of sulfur cathodes

Stanford researchers in Zhenan Bao and Yi Cui's labs have developed an organic redox mediator that could make Lithium Sulfur batteries charge faster with less energy. An energy efficient Li₂S electrode design is crucial for industrial manufacturing of Li-S batteries, a lightweight and high performance alternative to Li-ion batteries. The redox mediator (RM) acts as a charge shuttle (see figure 1). It 'patrols' the cathode to react with Li₂S particles that are not in contact with current collectors, facilitates electron transport, and improves efficiency.

Figure 1 Direct oxidation of Li₂S vs. mediated oxidation

This specially designed RM is compatible and soluble with the electrolyte, making it easy to integrate into the conventional cell design.

Stage of Research
Stanford researchers designed and employed a small molecule in the electrolyte as a redox mediator. This RM molecule facilitates charge transport during oxidation of Li₂S, at a 20% lower first charge potential than state of the art approaches, at a voltage that is only slightly above the oxidation potential of Li₂S.

Figure 2 Cycle stability of bare Li₂S vs. AQT redox mediated Li₂S

The new RM demonstrated good compatibility and solubility with the electrolyte, and cycle stability retention improved from 60% to 81%, with 76% retention over 700 cycles at a decay rate of 0.034 % per cycle – outperforming other methods.