Packing More into Lithium Batteries

Researchers at Stanford University have developed an electrode that can be used to make more energy-dense lithium-sulfur batteries. If issues surrounding life-cycle deterioration can be addressed, the battery could resolve performance and safety issues limiting the spread of longer-lasting batteries in hybrid and electric vehicles.

In 2007, researchers at Stanford University, led by materials science professor Yi Cui, devised an electrode made of silicon nanowires that could hold 10 times as much charge as conventional lithium-ion batteries. But for the device to realize its full potential, battery developers sought a corresponding cathode that could store electrons in similarly high densities.

Power pack: The lithium-sulfur battery shown here has the potential to last four times as long as lithium-ion batteries.

Now the same Stanford team thinks they have found their answer: a proof-of-concept lithium-sulfide cathode with 10 times the power density of conventional lithium-ion cathodes. Together, the anode and cathode could yield a battery that lasts four times as long and is significantly safer than existing lithium-ion batteries. The new battery cannot realize 10 times the energy storage capacity because the new cathode has significantly lower conductivity than the lithium metals used in conventional batteries.

But by using lithium sulfide, a non-metallic form of lithium, instead of a lithium metal, the researchers have overcome a key safety issue that has plagued lithium-metal batteries. During normal battery use, lithium metal can grow branchlike structures that can penetrate a thin polymer layer that separates the battery's two electrodes. When this occurs, the battery can short-circuit and potentially explode. With lithium sulfide, the branching does not occur.

To fabricate their lithium-sulfide cathode, the researchers started with a novel carbon-sulfur nanostructure cathode that was recently developed by researchers at Waterloo University in Ontario. Then they heated the carbon sulfur nanostructure in the presence of n-butyl lithium to form the lithium-sulfide cathode. Others have tried using lithium-sulfide cathodes in the past, but experienced serious problems with the material's conductivity. These were partly overcome with the new nanostructure design.

By combining the new cathode with the previously developed silicon anode, the team created a battery with an initial discharge of 630 watt-hours per kilogram of active ingredients. This represents an approximately 80 percent increase in the energy density over commercially available lithium-ion batteries, according to Stanford's Cui, who was a coauthor of a paper describing the work published last month in Nano Letters. Further increases in energy density--as much as four times that of lithium-ion batteries--are theoretically achievable by optimizing the battery's electrodes, Cui says.

The new battery still has significant issues, particularly in maintaining capacity. After just five discharge and recharge cycles, the cells lost one-third of their initial energy storage capacity and ceased to function after 40 to 50 cycles. The loss is likely due to polysulfides, chemicals that form during normal discharging and recharging. If allowed to dissolve into the battery's liquid electrolyte, polysulfides can poison the battery by blocking future charging and discharging. "This is a huge issue," Cui says. "We are making some great progress, but we certainly aren't there yet to compete with current technology in terms of cycle life."

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."

By Phil McKenna

From Technology Review


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