Efficient Synthesis of Hierarchical Porous Carbons for Application in Li-S Cells

The Li-S battery is a promising candidate to meet the demands on future generation mobile power supplies due to the high theoretical gravimetrical energy density and low price of sulfur. Unfortunately, the active material sulfur exhibits a very low conductivity, which limits the performance. A nanostructured carbon matrix is required. Up to date, large numbers of novel, elaborate host structures have been explored. However, in particular, suitable and effective production processes need to be established.

To overcome the existing drawbacks we focus on the development of a scalable and efficient approach for the synthesis of nanostructured porous carbon materials by using of inexpensive, commercially available ZnO nanoparticles as hard template.[1] One major benefit of the process is the in-situ removal of all pore building components through carbothermal reduction. Thus, no toxic/reactive gases are required and purification of the as-prepared porous carbon is not necessary. Adjusting the ZnO-NP template size, porous carbons with internal pore volumes as high as 3.9 cm³ g^-1 and tailored pore sizes reaching from 20 nm to 200 nm with ideal template replication are obtained. The carbothermal reduction process, furthermore, facilitates the access of a hierarchical pore system with distinctive microporosity leading to very high specific surface areas even exceeding 3000 m^-2 g^-1. The synergetic advantage of that bimodal porosity renders them particularly suitable for the application in high energy density lithium-sulfur batteries. A high capacity of > 1200 mAh g^-1-sulfur (> 750 mAh g^-1-electrode) at a high sulfur loading of ≥ 3 mg cm^-2 and good cycle stability was achieved even using a low amount of electrolyte. This allows for further weight reductions and maintenance of high energy density on cell level.

[1] Strubel et al., Adv. Funct. Mater. 2015, 25, 287–297. doi: 10.1002/adfm.201402768.