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Modeling of reaction and degradation mechanisms in lithium-sulfur batteries

Erişen, Nisa
Lithium-sulfur batteries are promising alternatives for the energy storage systems beyond Li-ion batteries due to their high theoretical specific energy (2567 Wh/kg) in addition to the natural abundancy, non-toxicity and low cost of sulfur. The reaction and degradation mechanisms in a Li-S battery include various electrochemical and precipitation/dissolution reactions of sulfur and polysulfides; however, the exact mechanism is still unclear. In this study, the effect of critical cathode design parameters such as carbon to sulfur (C/S) and electrolyte to sulfur (E/S) ratios in the cathode, on the electrochemical performance of a Li-S battery is modeled by defining each of the two discharge plateaus with a single electrochemical reaction. In the first part, a 1-D, concentration-independent model is developed which estimates the electrochemical performance through the cell voltage at 60% depth of discharge (corresponds to the low voltage plateau) and validated with experimental results. In the second part, a 1-D, concentration-dependent, spatiotemporal model is developed to predict the entire discharge behavior of the Li-S cell. As the C/S ratio increases, cell voltage of the Li-S battery also increases up to a certain limit. While further increase in the C/S ratio causes no change in the cell voltage, it slightly decreases the capacity due to the reduction in the active material. The increase in the E/S ratio in the cathode ends up with a significant increase in the cell voltage and capacity. Due to limited active material utilization, the discharge capacity becomes very low at low E/S ratios. Finally, a sensitivity analysis is performed to understand the effect of diffusion coefficients of the species, electrochemical reaction kinetics and precipitation/dissolution kinetics on the predicted discharge curve.