(594b) A Comprehensive Approach to Produce Industrially Relevant Sulfur Cathode for Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries have attracted great attention over the world because of their high theoretical specific energy. However, due to its low conductivity and the shuttling of lithium polysulfides, the performance of Li-S batteries are hindered by the low utilization and rapid capacity fading. In this work, we exam a comprehensive approach to produce a high capacity sulfur cathode by systematic studies of the carbon host structures, multi-functional polymer binders, and the use of electrolyte.

Porous carbon sheet structure was carefully synthesized as the host for sulfur by using graphene oxide as a shape-directing template and conductive filler. These graphene oxide sheets were uniformly coated with carbon on both sides, which provided enough space for sulfur and polysulfide, also shortened the ion transfer distance, ensuring a good performance of the carbon-sulfur composite. Using different CO₂ activation time, the resultant carbon materials have systematic variation of surface area, total pore volume and pore size distribution. The electrochemical characterization clearly shows that a porous carbon sheet structure with balanced surface area (total pore volume) and pore size distribution is essential to achieve high overall capacity of the sulfur-carbon composite.

In addition to the sulfur-carbon composite, we also propose and investigate ionomers with positively charged backbones (polycations) as binder materials to improve the polysulfide sequestration. For the first time, our electrochemical quart crystal microbalance experiments demonstrated that polycation binder significantly improved the sequestration of the lithium polysulfides during the lithiation process, and thus resulting improved capacity and capacity retention.

Finally we demonstrate that the electrolyte/sulfur weight ratio needs to be watched closely since it may be the decisive parameter to determine the achievable practical specific capacity of Li-S batteries. With minimized electrolyte/sulfur ratio, we demonstrate high capacity of a sulfur cathode with high areal loading.