Polysulfides form on the cathode when lithium ions bond with sulfur. The sulfur-carbon cathode that the Stanford researchers used as a starting point for their cathode was designed to trap polysulfides on its surface, preventing them from dissolving into the battery’s electrolyte. Tests of the cathode in its initial form show significantly less reduction in capacity, suggesting later modifications made by the Stanford team may have diminished the cathode’s ability to trap polysulfides.
To be competitive with lithium-ion batteries, the batteries developed at Stanford would have to operate for 300 to 500 charge cycles for consumer electronics applications and as many as 1,000 cycles for vehicle use, according to Cui.
Cui will not say what his group is doing to reduce losses in capacity, but two likely approaches exist. The first is to place additives in the battery’s liquid electrolyte that protect both electrodes from the negative effects of polysulfides. John Affinito, chief technical officer of Sion Power Corporation, a leading developer of lithium-sulfur batteries, says his company has achieved a roughly 200-fold decrease in self-discharge rates (discharge that occurs when the batteries are not in use) due to polysulfides, through the use of electrolyte additives. Changes to the electrolyte have to be made carefully, however, since they can also affect electron conductivity and lithium-ion bond formation at both electrodes.
Another option is to place a polymer or ceramic membrane between the two electrodes, to only allow lithium ions to pass back and forth between the electrodes as the battery is being charged and discharged. Such barriers exist already and could also help limit the movement of polysulfides within the battery. This would mean that two different electrolyte solutions, one surrounding each electrode, could be used to further optimize performance, although such membranes tend to be prohibitively expensive.
An additional challenge for the Stanford team, should they try to commercialize the new battery, will be scaling up for mass production. At issue is lithium sulfide’s instability in the presence of air. The cathodes used in the current study were fabricated in a sealed container filled with argon gas, an environment that would be difficult to replicate in large-scale production facilities, says Jeffrey Dahn, professor of physics and chemistry at Dalhousie University in Halifax, Canada.
“The silicon nanowires and the lithium-sulfide combination are a good idea,” says Affinito. “But a lot of good ideas don’t work in the end; they will have to work hard at it to make it commercially viable.”