Modeling the Effect of Electrolyte-to-Sulfur Ratio in the Cathode on the Systems-Level Performance of a Li-S Battery

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Lithium-Sulfur (Li-S) batteries have received significant interest as a result of their high theoretical specific energy and non-toxic, naturally abundant and low-cost active material [1, 2]. Because of the complexity of the processes taking place in the sulfur cathode, performance of a Li-S battery is highly sensitive to the cathode design [1, 2]. Electrolyte-to-sulfur ratio (E/S) is one of the key design factors in a Li-S cathode defining both the electrochemical performance and the systems-level energy density of the battery. Increasing the E/S ratio improves the cathode kinetics and hence the discharge capacity and cyclability of the battery. On the other hand, an excess of electrolyte has an adverse impact on the energy density of the battery at the systems level. In this study, we present a systems-level analysis for a Li-S battery demonstrating the effect of E/S ratio on the energy density of the battery. The systems-level performance model for the Li-S battery has been developed based on the BatPaC model [2]. The performance model uses a 1-D electrochemical model in which the sulfur kinetics has been treated with a single kinetic parameter, cathode exchange current density (i0,PE) [1]. The model predicts the impact of the E/S ratio on the energy density firstly through this kinetic model parameter; i0,PE is defined as a linear function of the electrolyte amount in the electrochemical model. More importantly, cathode specific capacity is described with an empirical linear dependence on the E/S ratio. The dependence of the systems-level energy density on the E/S ratio has been predicted as a function of other critical design parameters such as the maximum cathode thickness, carbon-to-sulfur (C/S) ratio in the cathode and excess Li% in the anode. Figure 1 presents the effect of E/S ratio on the systems-level energy density of a Li-S battery at different C/S ratios. It can be seen in the figure that increasing the E/S ratio improves the energy density until around 9 mL/g S. Any further increase in the E/S ratio results in a significant decrease at the systems-level energy density after this point. This trend can be explained by the dependence of the specific capacity on the electrolyte amount. As mentioned earlier, cathode specific capacity or the sulfur utilization in other words, has been defined as a linear function of the E/S ratio in the model. Therefore, at low E/S ratios increasing discharge capacity with increasing E/S ratio results in higher energy densities. However, when the theoretical capacity has been reached, any further increase in the E/S ratio brings a volume penalty to the pack and thus decreases the energy density. Increasing E/S ratio also improves the cathode kinetics through increasing i0,PE; however, this influence is less significant. It can also be seen from the figure that this trend is independent of the C/S ratio. However, it is clear from the figure that energy density is more sensitive to the E/S ratio for higher C/S ratios. Figure 1.The effect of E/S ratio in the cell on the calculated energy density of a Li-S battery for different C/S ratios. In the model, specific capacity is defined as a linear function of the E/S ratio. Acknowledgment This work was partially supported by the Scientific and Technological Research Council of Turkey (TUBITAK), Grant No: 116M574.