PNNL

Abstract

Li-S battery is considered as one of the next generation energy storage devices due to its high theoretical capacity and volumetric energy density. To further increase its volumetric energy density, low cathode porosity and high sulfur loading are desired. However, in this study, we demonstrated experimentally that the capacity and cycle performance of Li-S battery deteriorated quickly when the cathode porosity dropped from ~70% to 40%. In addition, the two plateaus on the discharge voltage curves exhibited different shapes depending on the porosity. To further understand the design limitations, a mechanism-based analytical model was developed to quantify the experimental observations. It was determined that the sulfur utilization was mainly limited by the amount of soluble polysulfide in the first plateau and limited by the electronically-accessible surface area of the carbon matrix in the second plateau. Thus, the relationship between the discharge curve and the porosity was connected via the amount of electrolyte and the surface area in the model, which was used to predict that an optimal porosity of 52% will maximize the volumetric energy density without limiting the sulfur utilization nor losing the electrochemical performance. The consistency of the analytical model and the experimental results validated the dominating mechanisms associated with cathode porosity reduction.