A Mediated Lithium-Sulfur Flow Battery

Abstract

With the growth of intermittent renewable power sources, such as solar and wind energy, there is an increasing need for inexpensive, high capacity, grid-scale energy storage. The high capacity (1675 mAh/g) and low cost (~0.1 $/kg) of sulfur make Li-S batteries an ideal candidate to fill this need. However, sulfur has poor electrical conductivity and Li-S batteries are prone to polysulfide shuttling that decreases the battery life. In an effort to address these problems, we are developing a highly-scalable mediated Li-S flow battery using a lithium metal anode, solid state Li ion conductor separator, sulfur cathode, and a redox mediator. During discharge, the Li metal is oxidized to Li+ and the redox mediator (RM) is reduced. The RM will then react with the S returning to its oxidized form and enabling the formation of Li2S without the need for electrical contact between the S and counter electrode. The oxidized RM is then reduced again, and the process is repeated until all available S has reacted. A second RM with a potential lower than that of the Li-S reaction is used to enable the charge process. Two such mediators are decamethylferrocene (2.86 V vs Li/Li+) and cobaltocene (2.06 V vs. Li/Li+), Figure 1. The flow battery design has multiple benefits. First, the use of a solid state Li ion conductor mitigates polysulfide shuttling, thereby increasing the cycle life. Second, the issue of sulfur conductivity is addressed through use of the redox mediator, where the mediator can easily diffuse through the electrolyte reaching the sulfur and eliminating the need for excess conductive carbon additives. Third, the flow battery design enables physical separation of the anode and cathode, mitigating the potential for short circuiting and thermal runaway even if the Li does puncture the solid state electrolyte. Using this cell design, we examine the role of the solvent, anion, and polysulfide concentration on Li+ transport across the solid electrolyte interphase as well as the role of the mediation potential and interactions between the polysulfide and mediator.Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525. Figure 1