Modeling the Effect of Layer-on-Layer Cathodes in Lithium-Sulfur Batteries

Abstract

Lithium sulfur (Li-S) batteries are promising candidates for next-generation energy storage due to their high theoretical energy density and environmentally abundant chemical inputs. However, well-known challenges such as polysulfide crossover, large volume changes during cycling, and deposition of insulating lithium sulfide limit the capacity and capacity retention of Li-S batteries. A popular approach to address these issues is to control the structure and properties of the carbon cathode in order to influence the transport of polysulfides, surface area for reaction, and lithium sulfide deposition. Towards this direction, we developed a layer-on-layer cathode structure with sulfur-impregnated activated carbon alternating with graphene. The layered structure exhibits improved capacity over the control at low rates but the benefits diminish at high rates and high graphene loadings. To explain this behavior, we use a 1-dimension + time numerical continuum model for the Li-S full cell which is validated against experimental data. The model includes the electrochemical reactions and precipitation of sulfur and intermediate polysulfide species, transport of species through the battery, lithium sulfide nucleation and growth, and adsorption of polysulfides. Experiments and simulations suggest that a combination mass-transfer limitations and surface passivation effects is crucial in describing the performance of the cells, and the effect of cathode properties is further investigated using the model.