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Effect of electrolyte/sulfur ratio in the cathode on the electrochemical performance of Li-S batteries

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2019
Emerce, Nur Ber
In this study, the effect of electrolyte to sulfur (E/S) ratio in the cathode, which is an important cell design parameter, on the electrochemical and cell- and system-level performance of a Lithium-Sulfur (Li-S) battery is investigated through modeling efforts. First, a 1-D electrochemical model is developed for an isothermal, constant-current discharge of a Li-S cell to predict the voltage at 60% discharge depth. In the model, cathode exchange current density is defined as a linear function of the electrolyte amount. Increasing the E/S ratio improves the electrochemical performance at different current densities. Next, cell- and system-level performance models based on the developed electrochemical model are proposed. In these models, the cathode specific capacity is either defined as a linear function of the E/S ratio or taken constant. The model, in which the cathode specific capacity depends on the E/S ratio, predicts that increasing E/S ratio increases the cell- and system-level specific energy and energy density until 10 mL gsulfur-1. However, when the cathode specific capacity is kept constant at 1200 mAh gs-1 in the model, the specific energy and energy density at the cell and system level decrease significantly with increasing electrolyte amount. In the cell- and system-level performance analysis, the effect of other critical cell parameters such as the cathode thickness, carbon to sulfur ratio, S loading and excess Li amount are also considered. As a result, Li-S batteries with high cell- and system-level performance can be designed with the optimum E/S ratio and specified cell design parameters.