(Invited) Modeling the Dependence of the Lithium-Sulfur Battery Performance on the Materials and Cell Design Factors

Abstract

Lithium-sulfur (Li-S) batteries have attracted significant research interest in the last couple of decades due to their significantly high theoretical specific capacity in addition to the fact that the cathode active material sulfur is abundant and inexpensive and has a high capacity. However, several mechanisms hinder the Li-S battery performance; the depletion of the lithium metal or the electrolyte in the cell with cycling, the polysulfide shuttle mechanism due to the transport of the soluble reaction intermediates between the cathode and the anode, and the passivation of the cathode surface due to the precipitation of the insoluble product Li2S are some examples of these mechanisms that lead to fast capacity fade and thus limited cycle life of the Li-S batteries. As a result of the highly complicated reaction and degradation mechanisms in the cell, the performance of the Li-S battery significantly depends on the materials and cell design. Consequently, it is of significant importance to study this key relation. Herein, we develop electrochemical models to predict the impact of critical materials and cell design parameters: electrolyte-to-sulfur (E/S) ratio, carbon-to-sulfur (C/S) ratio, sulfur loading, and carbon and electrolyte properties, on the Li-S battery performance. We use zero-dimensional (concentration-independent) and one-dimensional (concentration-dependent) models to project the isothermal, constant-current discharge behavior of the Li-S cell. In our models, a novel definition for the electrochemically active area is proposed, which describes the cathode area as a function of the carbon amount and the porosity in the cathode. Experimental trends on the influence of cathode design parameters on the discharge performance are captured successfully. A mechanistic explanation of these trends is offered with the presented model by introducing the proposed electrochemically active area.