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.
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.”