Graphene-Coated Separators As Effective Polysulfide Traps in Lithium-Sulfur Batteries

The increasing demand for rechargeable lithium batteries with higher energy densities for applications such as electric vehicles and grid-scale energy storage systems has led to the intensive search for advanced energy storage systems beyond the current lithium-ion battery technology.  Lithium-sulfur (Li-S) batteries have become the preferable choice among the next-generation energy storage systems due to their high theoretical capacity (1675 mAh g^-1), high specific energy density (2600 Wh kg^-1) and abundant availability in the earth’s crust. In addition, the elemental sulfur is non-toxic and cheap.  However, the inherent disadvantages of Li-S batteries such as the highly insulating (both ionically and electronically) nature of sulfur that requires a large of amount of inactive conductive additives, dissolution of lithium polysulfides into the electrolyte solution and the migration of lithium polysulfides to the anode side, need to be overcome to realize their practical applications.  These problems result in low utilization of sulfur and rapid capacity fading during cycling. Various strategies such as encapsulating sulfur using conductive matrices (carbon, conducting polymers and metal oxides), using polysulfide traps on the cathode side and covering the anode with a protective layer have been adopted to mitigate these problems.  In our study, graphene-coated separators have been prepared and investigated as effective lithium polysulfide traps to prevent the migration of polysulfides to the anode side. Detailed characterization of the graphene-coated separator and its performance as a polysulfide trap in Li-S cells will be presented.  Also, the effect of coating thickness and the surface functional groups on the Li-S cell performance will be discussed.