Based on recent discoveries by University of Waterloo University scientists from Ontario, a team of Stanford researchers developed an electrode that can provide more energy density in lithium-sulfur batteries. Their experiments promise an improvement of 4 times the capacity of a standard lithium-ion battery, if current issues are overcame.
The lithium-sulfide cathode can have 10 times the power density of conventional lithium-ion cathodes. Though, the overall capacity of the battery cannot be increased tenfold, because the new cathode has a significantly lower conductivity than the lithium metals used in conventional batteries.
By using lithium sulfide (a non-metallic form of lithium), the researchers overcame the biggest issue of standard Li-Ion batteries: the branch-like structures that grow inside them and that can penetrate the thin polymer layer separating the battery’s two electrodes, causing the battery to short-circuit, heat itself and explode.
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.
There are two main approaches towards making the lithium-sulfur batteries commercially viable: the first is to place an additive in the electrolyte that would protect both electrodes from the negative effects of polysulfides. The second would be to place a polymer or ceramic membrane between the two electrodes, to only allow lithium ions to pass between the electrodes, as the battery is being charged or discharged.
Added to the above-mentioned issues, there is the problem of lithium-sulfide’s sensitivity to the air. Current prototypes are being made in an argon-rich environment, which would be expensive to replicate in large-scale production facilities. There’s a long way until lithium-sulfur batteries would reach the phones in our pockets or the batteries in our cars, but there’s hope to that. Even if the experiment doesn’t succeed after all, it may teach others and give more clues as to what needs to be done to have a successful battery made.
