To extend the range of electric cars, automakers need new types of batteries that can store large amounts of energy. Lithium-sulfur batteries can, in theory, store two to three times more energy in a given weight than today’s lithium-ion batteries. But, in practice, it’s difficult to make a long-lasting sulfur cathode. Now, researchers at the Chinese Academy of Sciences have developed a cathode material for Li-S batteries out of sulfur and graphene that lasts through 2,000 charging cycles, one of the longest lifetimes for these batteries to date (Nano Lett. 2014, DOI: 10.1021/nl5020475).
When a lithium-ion battery is charged and discharged, lithium ions move between the two electrodes. In lithium-sulfur batteries, unwanted reactions at the cathode can produce lithium polysulfides that dissolve in the electrolyte, thus depleting the sulfur and reducing the cathode’s storage capacity. What’s more, when lithium moves into the cathode to react with sulfur, the electrode’s volume expands by 76%, which can cause it to crack apart and separate from the rest of the battery.
In recent years, researchers have tried all kinds of sulfur-trapping tricks to make Li-S batteries that last, such as mixing sulfur with carbon nanotubes. Yuegang Zhang, a chemist at the Chinese Academy of Sciences, took the approach of wrapping sulfur nanoparticles in graphene oxide to try to trap polysulfides and to withstand the problematic volume expansion. However, in a previous study, Zhang’s team had to add carbon black to the materials to achieve a cathode with a high enough conductivity (J. Am. Chem Soc. 2011, DOI: 10.1021/ja206955k).
In the new study, Zhang and his colleagues found a better conductivity solution: dope the graphene oxide with nitrogen. They exposed graphene oxide to NH3 at 750 °C for 30 minutes to replace most of the oxygen atoms with nitrogen. The researchers then mixed the nitrogen-doped graphene with a solution of sulfur salts at low temperature, which resulted in doped graphene wrapped around 25-nm-diamter particles of sulfur. His group found that this material is conductive enough to make a battery, without the need for black carbon. The nitrogen also seems to form strong ionic bonds with lithium-polysulfides, trapping them in the cathode.
The battery remained functional through 2,000 charging and discharging cycles, compared with 1,500 for the previous graphene-oxide-sulfur device. Although functional, the new battery lost 0.028% of its storage capacity with each cycle.
Yi Cui, a materials scientist at Stanford University who is also working on new battery materials, says the performance of Zhang’s sulfur batteries is among the best demonstrated so far. But he believes these batteries need to do better still to be commercially viable because even a 0.028% loss during each charge cycle adds up.
Two thousand charges, Zhang says, will be sufficient for electric-car batteries. “We have tried a lot of forms of graphene, and we think that nitrogen-doping is the best approach,” he says.
The key will be scaling up the technology. So far they’ve only tested the cathodes in a coin cell, essentially a watch battery. Zhang says that when they make a thicker electrode for a car battery, the performance is likely to suffer, but it’s not clear by how much.