Modeling of High Energy Density Li-S Cell Designs and Testing of Li-S Cells Based on Solid Polymer Electrolytes

Abstract

Sulfur is one of the most promising conversion-type electrodes for rechargeable battery systems because of its high theoretical capacity (1674 mA/gSulfur) and low cost. In recent years, the combination of a sulfur cathode and a lithium metal anode has attracted enormous interest as a viable alternative to Li-ion batteries for electrical vehicles. However, despite intensive research, Li-S batteries have not yet achieved widespread commercialization. The most important limitations to this cell technology are the low electronic conductivity of sulfur and its solid reaction products (Li2S2 and Li2S); the high solubility of intermediate products in commonly used liquid electrolytes; and large volume changes during conversion reactions (ca. 80% in volume) due to the reaction of elemental sulfur (density of 2.03 g/cm3) to Li2S (density of 1.66 g/cm3). As a result, Li-S batteries universally suffer from rapid capacity fade, severe self-discharge, low practical capacity and poor cycle life. In the first part of this work, the performance of Li-S cells with realistic electrode thickness are modeled to establish more practical estimates of the sensitivities to a variety of design factors, including 3 anode variations (graphite, SiGr, lithium metal), 3 electrolyte types (Celgard, gel polymer electrolyte (GPE) and solid state electrolyte (SSE)), and 2 cathode variations (NCA and C-S). These cell designs are evaluated against the USABC specific energy (350 Wh/kg) and energy density (750 Wh/L) targets. The key design factors included are electrode thickness, electrode porosity and the electrolyte/active material ratio (mass electrolyte/ mass of active material). In order to be comparable with the commercial lithium-ion battery systems, the upper limits for the electrode thickness and porosity are set at 100 mm and 30%, respectively. For the sulfur cathode, the model assumes a sulfur utilization of 60% (corresponding to a useable capacity of ca. 1000 mAh/gSulfur), and the electrolyte/sulfur ratio (E/S, g/g) are varied within ranges that meet the electrolyte thickness and porosity limits. The modelling results reveal that a graphite anode paired with a sulfur cathode cannot achieve the USABC specific energy or energy density goals. For an NCA cathode to meet these goals, a high capacity anode, e.g., Li metal, is needed. The modelling also suggests that Li-S battery based on a liquid electrolyte cannot meet the USABC energy density targets, due to the high electrolyte/sulfur ratio and the volume change during charge/discharge cycling. In fact, this volume change may be responsible for “squeezing” electrolyte out of the cathode during cycling. Solid electrolytes, either polymeric or ceramic, theoretically can achieve the specific energy and energy density goals. In the second part of this work, a solid-state Li-S cell based on a lithiated perfluorinated sulfonate membrane (Nafion) was investigated. In this work, cells were fabricated using a Li-substituted Nafion membrane (25 mm) as the electrolyte, a carbon-sulfur composite (> 2 mgSulfur/cm2) cathode with a Li-Nafion ionomer, and a Li metal anode. The electrochemical performance of the solid-state Li-S cell was tested and the results are compared to the estimates from the model described in the first part.