By Kjell Schroder, The University of Texas at Austin, Stevenson Research Group and MRS Student Chapter
Big Picture – Lithium-Sulfur Batteries
Lithium-sulfur batteries are often touted as a promising next generation lithium battery chemistry, and with good reason. The theoretical gravimetric density (energy per unit weight) of a sulfur cathode is 1,675 mAh g-1, an order of magnitude higher than current lithium iron phosphate positive electrodes at ~160 mAh g-1. This could translate into a battery with a longer charge.
Overview – The Polysulfide Shuttle Effect
Some major challenges face lithium-sulfur based batteries before they can be commercialized and implemented at scale. One major hurdle has been that as sulfur is lithiated and de-lithiated, it forms highly soluble lithium polysulfide intermediates (stoichiometric compositions of Li2Sx, with 2< x < 8). These lithium polysulfide products are then dissolved into the battery’s electrolyte, which not only removes active material from the positive sulfur electrode, but also poisons the battery’s anode surface.
Work by the Nazar group1 at the University of Waterloo and Cui group2 at Stanford has been done on confining lithiated sulfur (particularly in mesoporous and low dimensional carbon materials) to limit the degradation from this polysulfide effect. Alternatively, the Manthiram group3 at the University of Texas has contained the polysulfide shuttle effect by adding an interlayer between the cathode and separator. These efforts have extended the cycling life of full battery cells, but complicate the system by introducing more materials processing and/or fabrication steps..
More in Depth – Hybrid Graphite-Lithium Anodes
Recently however, a group at Pacific Northwest National Labs led by Dr. Jun Liu has suggested an alternative method for limiting the poisoning of negative electrodes by the polysulfide effect. In a recent Nature Communications article4, the group has suggested using lithium metal coated with compacted graphite ﬁlms as negative electrodes paired with sulfur cathodes. The result is a long cycling lithium-sulfur battery. The graphite films prevent many of the issues such as dendrite formation and surface roughening associated with the use of lithium metal electrodes. Additionally, it limits contact between soluble polysulfides and the metal surface.