Batteries with lithium-sulfur electrodes theoretically could store four times as much energy as conventional lithium-ion batteries. They promise longer play times for electronics and more miles between charges for electric cars. Now, researchers claim that commercialization of these experimental batteries is within reach.
With three new materials designs, researchers show that they can extend the batteries’ lifetimes while maintaining a high energy-storage capacity (Nano Lett. 2013, DOI: 10.1021/nl402793z).
The lifetimes of lithium-sulfur batteries are normally cut short by physical and chemical defects created as the batteries charge and discharge. The lithium-sulfur cathodes absorb and release large quantities of lithium ions, which cause the electrodes to expand and contract in volume by about 80%, says Yuegang Zhang, a materials scientist at the Chinese Academy of Sciences. The changes can cause the cathode to break, stopping the current flow.
Also when lithium and sulfur react, they form lithium polysulfides that are soluble in conventional electrolytes. As this sulfur dissolves from the cathode, the battery’s energy-storage capacity drops. The polysulfides, which are insulators, then deposit on the anode and decrease current flow through the battery.
To address these problems, Zhang and his colleagues first used a composite cathode material made by coating sulfur on graphene oxide. The carbon material, which they reported previously, is conductive and strong, so it increases the cathode’s durability as it expands and contracts (J. Am. Chem. Soc. 2011, DOI: 10.1021/ja206955k). Graphene oxide also binds well to sulfur, preventing the sulfur from dissolving when it reacts with lithium.
Next, the researchers used a rubberlike material for the binder that holds parts of the battery together. The conventional binder, polyvinylidene fluoride, is too brittle to withstand the cathode volume changes. Finally, the team picked a new electrolyte consisting of a conventional organic solvent mixed with an ionic liquid, which prevents sulfur from leaching from the cathode.
A test battery made with these materials could be charged and discharged 1,500 times with little degradation in performance. Its initial energy-storage capacity was 500 watt-hours per kilogram of battery material. After 1,000 cycles, the storage capacity dropped to 300 Wh/kg. Conventional lithium-ion batteries store about 200 Wh/kg, and the U.S. Department of Energy’s target for electric vehicle batteries is 400 Wh/kg.
On the basis of these results, Zhang believes that this technology is ready for commercialization, and he’s seeking industrial partners to scale up the battery design.
The work “is a big step forward,” says Jie Liu, a chemist at Duke University, because it shows that the poor long-term stability of lithium-sulfur batteries can be overcome. The next step, he says, is to find a practical anode material for the batteries.