An Overlooked Parameter in Li-S Batteries: The Impact of Electrolyte-to-Sulfur Ratio on Capacity Fading

The Lithium-Sulfur (Li-S) system has gained a lot of interest as a promising next-generation rechargeable battery due to the high theoretical energy density, high theoretical capacity and abundance of sulfur. Many parameters of Li-S batteries such as sulfur loading, electrolyte-to-sulfur (E/S) ratio, type of the conductive network, electrode design, etc. have a profound impact on the capacity, energy density and cycling performance. But the effect of E/S ratio on the electrochemical performance of Li-S batteries is often neglected, although it is one of the most important parameters. A high electrolyte amount in the cells could decrease the energy density and increase the cost, therefore it could limit the practical use of Li-S batteries. In this work, we first presented a statistical study on the sulfur loading and electrolyte quantity in Li-S cells by reviewing 240 selected papers from the state-of-the-art Li-S research. This analysis revealed that the electrolyte quantity was not reported as often as the sulfur loading in the literature, and the reported E/S values differ by a fair amount from each other. Second, in order to explore the effect of different E/S ratios on the performance of Li-S cells, batteries with 5:1, 10:1, 20:1 and 30:1 E/S ratios were prepared. Galvanostatic cycling with potential limitation (GCPL), electrochemical impedance spectroscopy (EIS) and galvanostatic intermittent titration technique (GITT) were used to analyze the effect of E/S ratio on the capacity fade, impedance, internal resistance and ion diffusivity in Li-S batteries. The effects of different electrolyte penetration conditions on the electrochemical performance of cells with the same E/S ratios were also studied by preparing cells with different resting times. It was shown that E/S ratio has a strong influence on the electrochemical performance of Li-S batteries, and an optimal E/S ratio should be achieved, which is low enough to minimize the free migration of active materials between the electrodes, and at the same time high enough in order to have a sufficient electrolyte wetting of the active material. It is suggested that capacity decay in batteries with low E/S ratios could be originating from electrolyte depletion, whereas the capacity decay in batteries with high E/S ratios could be due to the dissolved lithium polysulfide species in the liquid electrolyte and their diffusion to the lithium anode surface.