Impact of Carbon-to-Sulfur and Electrolyte-to-Sulfur Ratios on the Energy Density of Lithium-Sulfur Batteries

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Lithium-sulfur (Li-S) batteries have been considered to be good candidates among rechargeable batteries due to their high theoretical specific capacity and specific energy of 1675 mAh/g S and 2567 Wh/kg, respectively. The performance of Li-S batteries can be impacted by various vital cell design parameters such as carbon-to-sulfur (C/S) ratio, electrolyte-to-sulfur (E/S), and sulfur loading. These critical design parameters should be carefully selected since they have a direct effect on the electrochemical and system-level performance of Li-S batteries 1,2. Therefore, in this work the effect of C/S and E/S ratios in the cathode on the system-level energy density of a Li-S battery was examined by developing a system-level performance model, which contains an electrochemical model predicting the current-voltage relationship. The system-level energy density and specific energy of the Li-S battery are calculated as a function of the C/S and E/S ratios in the cathode by modifying the publicly available Battery Performance and Cost (BatPac) model. The proposed system-level performance model contains a one-dimensional electrochemical model to calculate the area specific impedance (ASI) and overpotential for each cell component at rated power and energy. Cell design factors such as the C/S ratio, E/S ratio, carbon, sulfur and binder wt%, and sulfur loading are fed to the model along with the experimentally measured cell capacities. The experimental discharge capacities were measured for Li-S cells with varying C/S ratios of 3.5, 2, 1 and 0.5 at a constant E/S ratio of 35 µL/mg and cathode thickness of ≈ 90 µm and with varying E/S ratios of 35, 20, 13 and 6 µL/mg at a constant C/S ratio of 1 and cathode thickness of ≈ 90 µm. For the experimentally obtained capacities, the average of three replicates were taken and fed into the performance model. In the proposed cell-to-system design, the components of packaging and thermal management are also taken into consideration.Figure 1 presents the impact of C/S and E/S ratios on the system-level energy density of the Li-S battery. It can be seen in the figure that the system-level energy density decreases with increasing E/S ratios whereas it shows a slight increase with increasing C/S ratio. Increasing the carbon amount leads to an increase in the discharge capacity to some level due to higher electronic conductivity. Even though the discharge capacity increases with increasing C/S ratio, the energy density may be affected poorly due to an increase in the inactive materials amount in the cell. However, this behavior is less likely to be observed at high E/S ratios since significantly high pack mass and volume due to high amount of electrolyte dominates the performance. Furthermore, increasing E/S ratio worsens the energy density. This suggests that the increase in the cell volume with increasing E/S ratio is more significant than the enhancement in the discharge capacity.