Influence of Cathode Composition and Fabrication Parameters of Li-S Batteries

Kurzfassung

Demands in energy storage for electric vehicles and in storage of electricity from intermittent renewable energies require a significant improvement in the energy density of battery systems. Among available electrical energy storage systems, rechargeable lithium-ion batteries represent the state-of-the-art technology and remain an important solution as power source in portable electronics. Nevertheless, the performance of current lithium-ion batteries is starting to approach its theoretical limit. Therefore innovative systems with a substantially higher specific energy and improved cycle life have to be developed. Lithium-sulfur based systems are one of the appealing power sources with a high theoretical energy density of 2600 Wh/kg. This fact promises a three- to five-fold increased specific energy and greatly reduced costs compared to lithium-ion technology. Sulfur is available in almost unlimited quantities and is thus cheaper. However, these high energy densities are not yet achievable for the system since most of the sulfur cathodes proposed in the literature have an inherently lower active material loading than commercially cathodes of lithium-ion batteries. The actual energy density of such a positive electrode does not exceed the current level of lithium-ion batteries [1]. To benefit from the low cost raw material the additional cathode components should be inexpensive and abundantly available and the manufacturing process easy to scale-up with industrial relevance [2]. In this work, sulfur-cathodes were fabricated with commercially available raw materials (sulfur, carbon based material and binder). Through subsequent increase of active material content from 50 wt.% to 70 wt.% and coating thickness a sulfur content of the cathode from 0.2 mg(Sulfur)/cm² to 2.5 mg(Sulfur)/cm² is achieved. The standard battery set-up is realized using a lithium metal anode, a polymeric separator, a liquid aprotic electrolyte and a slurry-processed cathode. With this experimental set-up the influence of layer thickness, sulfur content (50 to 70 wt.%) and loading (0.2 mg(Sulfur)/cm² to 2.5 mg(Sulfur)/cm²), porosity and slurry processing of the cathode is investigated. Results show a nonlinear impact of increased sulfur content and loading on the specific capacity of the lithium-sulfur cell. It is possible to reduce the decrease of the cell’s specific energy due to the increased sulfur content and loading by the choice of electrolyte solvent. Such cathodes maintain a specific capacity of 600 Ah/kg(Sulfur) after 100 cycles at 1C, which is 75 % of the initial capacity of the cell.