Hierarchically Structured N-Doped Carbon/Sulfur Composite Electrodes with a High Sulfur Loading for Application in Li/S Batteries

Rechargeable batteries have been receiving increasing attention over the past decade, in particular with regard to grid-storage and electric vehicle energy storage applications. Among the broad range of cathode materials, elemental sulfur (S) has the highest theoretical specific capacity of 1675 mAh/g, thereby making it one of the most promising positive electrode materials today.^[1] The use of sulfur for next-generation lithium batteries is also highly desirable from the viewpoint of costs and sustainability. However, there are several issues related to safety and cyclability that have been preventing commercial application of the Li/S technology. For example, deleterious side reactions on the lithium metal anode and dissolution of reactive intermediates (polysulfides) during cycling have been shown to cause severe capacity fading.

Several approaches to addressing the problem of the “quasi-liquid” cathode have been reported and the use of lightweight nanostructured materials such as carbon to trap the generated polysulfides seems to provide a viable solution. In recent years, many such carbon/sulfur nanocomposite electrodes have been described in the literature. In the vast majority, however, the sulfur content and loading, and thus, also the areal capacity was too low for practical applications.^[2] Both of these key parameters strongly affect the overall performance of Li/S batteries.

Here, we report on the preparation and electrochemical properties of hierarchically structured N-doped carbon/sulfur composite electrodes. Highly conductive (ionic liquid-derived) carbon monoliths with a nitrogen content of approx. 10% have been fabricated by facile hard templating and cathodes thereof with a sulfur content of 60% and loadings in the range of 1–4 mg/cm² tested both in coin- and pouch-type cells. Stable specific capacities of greater than or equal to 700 mAh/g could be achieved over 200 cycles at a rate of C/5. For electrodes with the highest loading, this corresponds to an areal capacity of approx. 3.0 mAh/cm². Literature reports on sulfur cathodes with similar areal capacities are scarce.^[3] Besides the cycling performance, the cell chemistry has also been investigated, for example, by means of in operando synchrotron-based X-ray diffraction. Possible failure mechanisms will be discussed. 

[1] Manthiram, A. Acc. Chem. Res. 2013, 46, 1125–1134.

[2] Lee, S. W., Shao-Horn Y. Nat. Nanotechnol. 2010, 5, 531–537.

[3] Song, J.; Wang, D. Adv. Funct. Mater. 2013, 24, 1243–1250.