Characterization of the Impact of Carbon-to-Sulfur Ratio on the Lithium-Sulfur Battery Performance
Authors: Can Hatice , Eroglu Damla 
Published: December 2020

Rechargeable Lithium-Sulfur (Li-S) batteries have attracted great attention since they can potentially provide high theoretical energy with low cost [1]. Due to the highly complex reaction and shuttle of soluble polysulfides mechanisms, cathode design plays a key role on the Li-S battery performance [2]. Among design parameters, carbon-to-sulfur (C/S) ratio is one of the key parameters, which highly affects the electrochemical and the system-level performance of the battery. C/S ratio is closely related to the electrochemically active area and the electronic conductivity in the cathode; hence, it significantly impacts the discharge capacity, capacity retention and cycle life of the battery. However, higher C/S ratios may result in a decay in the system-level performance due to an increasing inactive material amount in the battery.
In this work, the impact of the C/S ratios on the cell- and system-level performance of a Li-S battery is investigated by applying an integrated research methodology coupling electrochemical characterization and modeling techniques. First, the electrochemical performance is examined experimentally for Li-S cells with different C/S ratios. Conventional cathode preparation method is used to prepare the cathodes with carbon black, sulfur and PVDF (binder). Cathodes with different C/S ratios are prepared at a constant thickness and cells are established with a constant E/S ratio. Figure 1 shows the cycling behavior of three different cells with varying C/S ratios of 7/2, 1 and 2 at a C-rate of C/10. It is clearly seen in the figure that the highest initial discharge capacity (mAh g-1 S) is achieved with the cell having the highest C/S ratio. This might be explained by the improvement in the electronic conductivity and electrochemically active area in the cathode with increasing carbon amount. When the C/S ratio increases, capacity retention increases up to a point, however further increase in the C/S ratio results in a lower cycling performance. For instance, the best capacity retention is observed for the Li-S cell with C/S=2. The decrease in the active material loading with increasing carbon amount creates an adverse effect on the performance.
Following the experimental characterization, an electrochemical performance model is also developed, in which experimentally measured discharge capacity, cell voltage, E/S ratio and current density are used as model inputs. The proposed model predicts the cell- and system-level specific energy and energy density of the Li-S battery as a function of the C/S ratio in the cathode. As a conclusion, the impact of the C/S ratio on the Li-S battery performance is characterized not only in terms of the discharge capacity and capacity retention but also the cell- and system-level energy density.
Figure 1. Cycling behavior of the Li-S cells with different C/S ratios at a rate of C/10. In all cells, E/S ratio is 35 ml/mg and the cathode thickness is 100 mm.

Acknowledgment

This work was supported by the Bogazici University Research Fund, Grant No: BAP-14443SUP.

References

[1] Gao, J. and Abruña, H. D. Key Parameters Governing the Energy Density of Rechargeable Li/S Batteries. The Journal of Physical Chemistry Letters
2014, 5, 882–885.
[2] Emerce, N. B. and Eroglu, D. Effect of Electrolyte-to-Sulfur Ratio in the Cell on the Li-S Battery Performance. Journal of The Electrochemical Society
2019, 166, A1490–A1500.

Figure 1