Effect of Carbon Properties on the Lithium-Sulfur Battery Performance

Lithium-Sulfur (Li-S) batteries have attracted great attention as a beyond Li-ion chemistry because of their high theoretical specific capacity in addition to the natural abundance and low cost of sulfur, the active material in the cathode. Yet, due to the highly complex reaction and shuttle mechanisms in the cell, cell design, specifically the design of cathode materials, is highly determinative of the Li-S battery performance. For instance, the battery performance is vastly sensitive to the type and properties of the carbon in the cathode since Li-S batteries typically use an excess of carbon in the cathode to achieve high electronic conductivity and surface area. In this study, the impact of carbon type and properties on the electrochemical, cell- and system-level performance of the Li-S battery was investigated by combining experimental and modeling techniques. First, the influence of carbon properties on the discharge behavior and cycling performance of Li-S batteries were inspected experimentally for cathodes prepared at different thicknesses (in other words, different S loadings) with three different carbons exhibiting various pore volumes, particle sizes, and specific surface areas: Acetylene Black (AB), Ketjen Black (KB), and Carbon Black (Super C65). Moreover, the influence of S loading on the battery performance was studied for these three different carbons. Following experimental characterization, a system-level performance model projecting the specific energy and energy density of the battery pack as a function of the carbon type and S loading was proposed by modifying the publicly available Battery Performance and Cost (BatPac) model. The performance model considers the experimental dependence of the discharge capacity on the carbon type. It also includes a one-dimensional electrochemical model predicting the area-specific impedance (ASI) and overpotential for each cell component. Subsequently, the variance of the current-voltage relation with the carbon properties was also reflected in the model.<br/>The cycling behavior of Li-S cells with varying sulfur loadings indicates that the carbon used in the cathode has a decisive impact on how the S loading affects battery performance. For instance, Li-S cells with AB present no significant dependence on the S loading, whereas Li-S cells with Super C65 show higher capacity retention at higher S loadings. On the other hand, Li-S cells with KB cannot achieve high performance at higher S loadings, which is quite unexpected due to its considerably higher surface area. The inhomogeneous distribution of S in the cathode may cause this inferior performance. Our results also suggest that carbon properties are more determinative of the cycling performance at higher S loadings. Moreover, the highest pack performance is predicted for Super C65. Interestingly, Li-S cells with both AB and Super C65 cathodes achieve the highest system-level metrics at medium S loadings, where the discharge capacities are maximized. This suggests that the decrease in the cell weight and volume with increasing S loading has a less dominant effect than improving the cell capacity. Our results clearly show that maximizing the energy density and specific energy of the battery pack is critical, and optimizing Li-S cell design based solely on the discharge performance may be misleading.<br/><br/><b>Acknowledgments</b><br/>This study was funded by the Bogazici University Research Fund, Grants No: BAP-14443SUP and BAP-20A05D5.