Higher Energy Density Mediated Lithium-Sulfur Flow Batteries

There is a need for safe, reliable, high-capacity storage for long duration energy storage. The low cost and high capacity of sulfur make Li-S batteries ideal for this purpose. However, sulfur has poor electrical conductivity and Li-S batteries are prone to polysulfide shuttling that decreases the battery life. Additionally, lithium metal cannot be cycled at high rates or dendritic growth is produced. We have previously addressed the issues with the S by combining aspects of a static Li-S battery with aspects of a redox targeting system and flow battery. With this system we demonstrated that fundamental Li-S chemistry and novel SEI engineering strategies can be adapted to the hybrid redox flow battery architecture, obviating the need for ion-selective membranes or flowing carbon additives, and offering a potential pathway for inexpensive, scalable, safe MWh scale Li-S energy storage. However, these tests were done at lab scale with low S loadings and limited charge rates, limiting both the energy and power density of the proof-of-concept system. In this study we present recent progress scaling up the system from 2.5 mg_S cm^-2 to over 50 mg_Scm^-2and increasing the current density from 0.5 mA cm^-2 to 10 mA cm^-2 to decrease the charge/discharge time. The increase in S loading results in an increase in energy density and the increase in applied current increases the power density of the system.<br/> To scale up the flow cell we address limitations of the small-scale architecture including examining the flow field used with the catholyte and the structure of the Li metal anode. We first tested high surface area scaffolds in Swagelok cells to examine the effect of the increased effective surface area and lithiophilic seed materials like ZnO on Li metal deposition independent of the flow cell or S chemistry. Using a high effective surface area anode support enables us to increase the applied current density from 0.5 mA cm^-2 to 10 mA cm^-2 greatly increasing the charge/discharge speed. Furthermore, the addition of a lithiophilic seed layer decreases the nucleation overpotential and encourages uniform Li electrodeposition. A tailored flow field also improves the uniformity of Li deposition on the anode by improving the uniformity of the catholyte flow velocity. These improvements are first evaluated separately and then combined in a mediated Li-S flow battery where cycling rate and capacity retention are compared against a traditional planar Li anode and open flow field.<br/>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.