Alternative Anode Materials for a Highly Stable Lithium Sulfur Battery

The lithium-sulfur battery system is expected to achieve more than double the gravimetrical energy density as state-of-the-art lithium-ion systems. At the same time the material costs can be significantly decreased due to the abundance and low price of sulfur.

What has kept the lithium-sulfur battery from commercialization is its low cycle stability of the system. In our work we demonstrate that the main problem is the metallic lithium anode, which is currently used in most of the publications. The aprotic solvent molecules of the electrolyte are instable versus lithium. When a low amount of electrolyte is used, the cell runs dry and reversible cycling is limited to less than 200 cycles.

Using hard carbon anodes we are able to demonstrate that prolonged, reversible cycling can be achieved for more than 1000 cycles. This system enables balanced lithium-sulfur full-cells with a lithium excess of only 10% and a low amount of electrolyte. The sulfur utilization can be increased to 1470 mAh g^-1_sulfur (720 mAh g⁻¹_cathode) by tuning the porosity of the cathode.[1] Our current generation hard carbon anodes achieve a stable areal capacity of 3 mAh cm^-2 enabling a combination with high capacity cathodes. Dry processed sulfur cathodes exhibiting an areal loading of 2.5 to 3 mg_sulfur cm^-2and sulfur mass loading in the cathode of 68 wt% (including carbon matrix, conductive additive and binder) were applied and resulting full cell results will be discussed.

In addition the stable hard carbon anodes can be used to investigate degradation mechanisms in the lithium sulfur battery system. We present results that point toward the fact that nanometer-sized carbon host materials on the cathode side may suffer from increased mechanical degradation. In contrast micrometer-sized porous particles exhibit outstanding cycling performance.

Finally new approaches to transfer these results to high capacity anodes, based on Si/C composites will be discussed.

FIGURE1

Fig. 1: Cycle stability for a metallic lithium and all-carbon anode versus the same carbon-sulfur cathode at 836 mA g^-1_sulfurand a limited amount of electrolyte.

[1] J. Brückner et al., Adv. Funct. Mater., 2013. DOI: 10.1002/adfm.201302169.