Lithium-Sulfur (Li-S) battery performance is greatly sensitive to cell design as a result of the highly complex reaction and shuttle mechanisms within the cathode. Electrolyte-to-sulfur (E/S) ratio is one of the key design parameters that have a great impact on the performance of Li-S batteries. Here, an integrated research methodology coupling experimental characterization and electrochemical modeling is applied to forecast the relation between the E/S ratio and the discharge capacity, cycling performance and cell- and system-level specific energy and energy density of the Li-S battery. The highest initial discharge capacity is achieved with an E/S ratio of 20 μl mg−1, whereas, the best capacity retention is observed for 13 μl mg−1. This experimentally obtained link between the E/S ratio and the discharge performance is taken into consideration in the proposed cell- and system-level performance models. Lower E/S ratios lead to higher battery performance at the cell and system level. Consequently, an E/S ratio of 13 μl mg−1 presents the best performance as the impact of E/S ratio not only on the peak discharge capacity and capacity retention but also on the specific energy and energy density at the cell and system level are all considered.

Summary Cell design is a compelling feature to attain lithium‐sulfur (Li‐S) batteries with superior performance and carbon‐to‐sulfur (C/S) ratio is a vital design parameter with a critical influence on the battery performance. Herein, the dependence of the Li‐S battery performance on the C/S ratio is examined for various electrolyte‐to‐sulfur (E/S) ratios based on experimentally measured peak discharge capacities and cycling performance aside from the gravimetric and volumetric energy densities projected by the cell‐ and system‐level performance models. C/S ratio has a great influence on the cycling performance and discharge capacity, particularly for cells with a limited amount of electrolyte. The Li‐S cell having a C/S ratio of 2 and an E/S ratio of 13 μL mg−1 has provided the highest initial capacity in addition to the best capacity retention. Model predictions suggest that increasing C/S ratio worsens the battery metrics at the pack level, particularly at low E/S ratios. Assessment of the performance based on the energy density is highly important; the best battery performance at the system level is calculated for the Li‐S battery with the lowest E/S and C/S ratios despite that a lower discharge capacity has been achieved with this cell.

Since lithium‐sulfur (Li−S) batteries often employ an excess of carbon in the cathode to obtain high electrical conductivity and surface area, the kind and qualities of the carbon in the cathode significantly impact battery performance. Sulfur loading is an essential design element with a considerable influence on battery performance. To anticipate the relationship between the sulfur loading and the cycling performance, discharge capacity, and energy density/specific energy at the cell/pack level of the Li−S battery for different carbon types, an integrated research technique that couples experimental characterization and system‐level performance modeling is used. The capacity retention of Li−S cells with acetylene black is insensitive to S loading, while Li−S cells with carbon black present higher capacity retention at higher S loadings. Li−S cells with Ketjen Black cannot perform well at higher S loadings. At moderate S loadings, when discharge capacities are at maximum, Li−S cells achieve the highest system‐level metrics.

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. References 1.D. Eroglu, K. R. Zavadil, and K. G. Gallagher, J. Electrochem. Soc., 162, A982–A990 (2015).2.H...