With the growth of intermittent renewable power sources, 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. To address these problems, we are developing a highly-scalable mediated Li-S flow battery using a Li metal anode, solid state Li+ conductor separator, sulfur cathode, and a redox mediator (RM). During discharge, the Li metal is oxidized to Li+ and the 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 at the counter electrode, and the process is repeated until all available S has reacted. A second RM with a potential higher 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+). Benefits of this design include: 1) The solid state Li+ conductor mitigates polysulfide shuttling, thereby increasing the cycle life. 2) The RM can easily diffuse through the electrolyte to reach the sulfur, eliminating the need for excess conductive carbon additives to address sulfur’s poor conductivity. 3) The physical separation of the anode and cathode mitigates the potential for short circuiting and thermal runaway even if the Li does puncture the separator. 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.

Mediated lithium-sulfur flow batteries for grid-scale energy storage

ACS Spring Meeting 2021

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.