Lithium-sulfur batteries present a promising alternative to current, widely commercialized lithium-ionbattery technology. Lithium-sulfur batteries take advantage of the high specific capacity (~1675 Ah/kgsulfur)and abundance of sulfur in an attempt to produce lighter, cheaper batteries for portable applications.Despite promising theoretical capacity, commercialization of lithium-sulfur batteries is limited by lowconductivity of cathode end products and low battery cyclability. The first issue requires an electricallyconductive cathode host network, typically carbon, which reduces the specific capacity. The second issueis due to polysulfide shuttling, which can lead to precipitation of insoluble intermediates and loss of activematerial.Current research is largely focused on either preventing the dissolution of intermediate products into theelectrolyte or preventing their migration to the anode where they can be reduced to insoluble species [1-3].Concurrent efforts to reduce the electroltyte/sulfur ratio will maximize the battery’s gravimetric energydensity. For any of these strategies, it is critical to understand and control the impact on the intermediatespecies concentrations in the electrolyte, both to optimize performance and to minimize capacity fade dueto precipitation of species which exceed their solubility limits.The proposed presentation will demonstrate a physico-chemical model of a lithium-sulfur battery withdetailed intermediate chemistry, thermodynamic consistency, and physically derived parameters. Atpresent, most models trade off one or more of these model components for others [4-6]. Model results arevalidated against operando measurements of dissolved polysulfide concentrations in the electrolyte [7,8].Results will demonstrate how C-rate, electrolyte/sulfur ratio, sulfur loading, and cation-selectivemembranes impact cell performance and battery lifetime.Figures 1 and 2 show preliminary model results for varying C-rates. The discharge profiles in Figure 1indicate only moderate losses in potential with increasing C-rate. However, these are associated with ashift toward lower-order, less-soluble polysulfides, as shown in Figure 2, the irreversible precipitation ofwhich leads to capacity fade over multiple cycles. In this manner, the model serves as a quantitative guideto the rational design and operation of next-generation Li-S batteries. In this presentation, we will discussimplications from the model results regarding the battery design and operating strategies to mitigate Li-Sbattery degradation while improving metrics such as gravimetric energy and power density.