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

One of the most promising future generation candidates for safe, low-cost and long-lasting high energy density rechargeable energy storage systems is without any doubt the lithium-sulfur battery. However, despite more than two decades of intensive scientific research focusing on all cell components the commercialization of a viable lithium-sulfur battery has not been realized so far due to several challenges coupled with the electrochemical conversion of sulfur. One of the major drawbacks is the repeated dissolution and deposition of highly insulating sulfur compounds during cycling, which is furthermore accompanied by an extensive expansion and shrinking of the active material. A nanostructured conductive matrix is required. Porous carbons has been considered as one of the most promising conductive host structure, due to their low weight, high conductivity as well as adjustable properties (specific surface area, pore size, pore volume). In particular, suitable and especially 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 with pore properties specially tuned for an application in high capacity Li-S batteries. The large benefit of the process presented here is the removal of all pore building components by simply pyrolysing the MO_x/carbon composite without any need for toxic/reactive gases and, moreover, purification of the as-prepared porous carbon. The obtained material provides not only good electrical conductivity but also high internal porosity (total pore volume up to 3.8 cm³/g) as well as a large number of electrochemical reaction sites due to its high internal surface area (up to 3000 m²/g). Applied in the Li-S cell, the porous carbon/sulfur composite exhibits a stable capacity of > 1000 mAh g^-1-sulfur (> 600 mAh g^-1-electrode) over 60 cycles at a high sulfur loading of 3.5 mg cm^-2. Both high sulfur utilization and reversible transformation are attributable to the tuned properties of the carbon matrix.