Cathodes for Lithium Sulfur Batteries

Abstract

Lithium sulfur (Li-S) batteries are a promising candidate to replace state-of-the-art lithium-ion batteries with an expected 3-5 fold increased capacity. Even though they have been studied for decades the final breakthrough to make the system feasible for application for example in the transportation sector has not yet been achieved. However, many improvements have been made during the last years. One of many obstacles that needs to be overcome is the design of suitable cathodes, efficiently preventing capacity fading. Here, different carbon nanostructures have been tested as sulfur containing matrices in Li-S batteries. Examples are sulfur-doped ordered mesoporous carbons (S-doped OMC) or helical carbon nanofibers (HCNFs). With both materials we achieve high sulfur utilization and good capacity retention. Sulfur-doping of the carbon matrix shows positive effects on cyclability and capacity per cathode mass. We find that the doping also influences the electrochemical behavior of the whole system and conclude that there exists a strong interaction between dopant and loaded sulfur utilized during charge and discharge of the battery. In the system using helical nanofibers we find that the performance of HCNFs is substantially better compared to straight fibers or nanotubes. We explain this with the specific properties of HCNFs making activation efficient without major deterioration of the overall structure. All materials have been extensively characterized using a variety of methods such as electron microscopy and electrochemical testing.