Effect of electrolyte-to-sulfur ratio on the performance of lithium-sulfur batteries with different electrolyte systems
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Date
2023
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Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2023.
Abstract
Lithium-sulfur (Li-S) batteries, one of the most promising alternatives for next-generation battery systems, have been a top research topic due to their high theoretical specific capacity and energy density. In Li-S batteries, battery performance is closely tied to certain materials and cell design parameters, particularly electrolyte design, due to the complex mechanisms in the cell. Herein, the effect of the electrolyte-to-sulfur (E/S) ratio, a critical electrolyte design parameter, on the performance of the Li-S batteries is investigated with various electrolyte systems. In order to understand the effect of the design parameters on battery performance, at first, experimental characterization is conducted by using galvanostatic cycling and electrochemical impedance spectroscopy (EIS). In the experimental study, seven different types of electrolytes with four different E/S ratios of 6 µL/mg, 9 µL/mg, 16 µL/mg, and 23 µL/mg are used. In general, the lowest E/S ratio, 6 µL/mg, has a better performance compared to the highest E/S ratio because the high amount of electrolyte escalates the polysulfide shuttle mechanism, affecting the battery performance adversely. In addition, EIS characterization is used to investigate the resistance of cells with an E/S ratio of 6 µL/mg and 9 µL/mg. Then, zero-dimensional and one-dimensional electrochemical models are developed to mechanistically study the effect of the E/S ratio on the discharge profile. The 0-D model can capture the expected trend of the variance of the discharge profile with changing E/S ratio; the capacity decreases significantly with decreasing E/S ratio. To model the impact of the E/S ratio on the discharge profile of Li-S for different electrolyte systems, selected model parameters were changed systematically. At the different values of kS8, both the 0-D and 1-D models predict other trends for the dependence of discharge profiles on the E/S ratio. Consequently, both models can capture the effect of the E/S ratio on the discharge behavior for different electrolyte systems implicitly through the variation of kS8. It is seen clearly from this study that the impact of the electrolyte amount on battery performance changes according to the types of electrolytes.