Nitrogen-Enriched, Nanostructured Carbon Materials for Application in High-Loading Lithium-Sulfur Cathodes

The development of efficient energy storage systems is of exceptional importance considering the increasing demand in electrical energy for both, stationary and mobile applications. Among a wide range of possible cathode materials elemental sulfur has the highest theoretical capacity of 1675 mAh/g which makes it a promising candidate as a positive electrode material.[1] However, the poor cycle life and rate capability of sulfur cathodes have limited the practical application of this attractive technology. Sulfur is highly electronically and ionically insulating which makes the use of conductive additives such as carbon black inevitable. Polysulfides, being generated during cyclization, dissolve in the liquid electrolyte and shuttle between the anode and cathode to react chemically and cause a loss of active material. Furthermore, the final product of the sulfur reduction, Li₂S, is highly insulating and partially responsible for the rising electrode resistance during cycling. Precipitation of solid lithium sulfide is assumed to proceed both, on cathode and on anode side.

Different carbon materials are widely used as cathode additives as they offer a high electronic conductivity coupled with low mass and a high surface area.[2] Recently, significant progress has been achieved using structured meso- and microporous carbons as host materials for sulfur. In order to prevent the loss of active material by dissolution, the conductive matrix is assumed to constrain the highly soluble polysulfide inside the cathode by chemical or physical interactions. Therefore, the parasitic polysulfide shuttle and undesired side reactions with the lithium anode can be mitigated. Moreover, structured carbons have a stabilizing impact on the structural integrity of the cathode matrix, as they are able to compensate the mechanical stress arising from the volume expansion during the chemical conversion of sulfur into lithium sulfide. [3]

Here, we present novel, nitrogen-enriched, nanostructured carbons (MPC) as cathode materials for lithium-sulfur batteries. Synthesized via hard-template approach, the obtained materials exhibit a well-defined mesoporous structure with a nitrogen content of approx. 10 %, enabling the possibility of sulfur infiltration. Due to a high specific surface area (400 – 500 m²/g), enhanced polysulfide adsorption and good mechanical stability, cathodes with a high active material loading (above 2 mg/cm²) and improved cycling performance can be fabricated.

Significantly improved specific and areal capacities were achieved over 200 cycles with an increased sulfur loading. It can be shown that the synergetic effects of sulfur confinement and carbon modification can crucially improve the performance of lithium-sulfur batteries.

References

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

[2] Ji, X. L.; Nazar, L. F. Nat. Mater. 2009, 8, 500–506.

[3] Sun, F. ACS Appl. Mater. Interfaces, 2013, 5, 5630–5638