Lithium-ion batteries with cathodes made of sulfur can, in theory, store much more energy than rechargeable batteries sold today. That means longer times between charges for electric cars and mobile devices. Now, researchers in Texas have shown that adding a layer of carbon nanotubes to one component of the battery could help overcome a host of practical challenges posed by sulfur cathodes (J. Phys. Chem. Lett. 2014, DOI: 10.1021/jz5006913).
Lithium-sulfur battery cathodes can store five times more energy than the positive electrodes in today’s best commercial cells. When a lithium-ion battery is discharged and recharged, lithium ions move between the two electrodes and drive current through an external circuit. The more lithium ions the cathode can hold, the more energy the battery can store. Sulfur cathodes can store a lot of energy because they can take up and release a large amount of lithium.
However, that means the cathode volume fluctuates by 80%, which can short out the battery and jeopardize safety. What’s more, lithium ions react with sulfur to form lithium polysulfides at the cathode throughout the lifetime of the battery. These compounds travel through the electrolyte to the anode, taking sulfur away from the cathode and thus rapidly depleting its energy-storage capacity.
To solve these problems, many researchers are working on new formulations of the sulfur cathode materials. Arumugam Manthiram, a chemist and director of the Texas Materials Institute at the University of Texas, Austin, has taken a different approach: adding a layer of carbon between the cathode and the separator, a membrane that allows electrons and lithium ions to flow between the two battery electrodes. Carbon films are conductive, stretchy, and strong, and they help block polysulfides. Mantiram’s group first tried putting a piece of carbon paper between the cathode and separator, but that adds weight. So to keep batteries as light and compact as possible, they decided to coat a conventional battery separator with a thin film of commercially available multiwalled carbon nanotubes.
To do this, they suspended the nanotubes in an alcohol solution and used vacuum filtration to pull the nanotubes onto the surface of the separator. They dried the coated separator and then used it to make small battery cells for testing in the lab.
Researchers often blend sulfur, which is an insulator, with carbon to increase the cathode’s conductivity, but this cuts down on energy storage. Because carbon nanotubes are very conductive, the researchers were able to use a total weight percentage of about 65% sulfur in the battery. This high proportion of sulfur led to a battery that discharged 1,324 mA-hours per gram, close to the theoretical storage capacity of a Li-S battery. Importantly, the storage capacity didn’t fade much over time. “For a lab battery, the performance we get is pretty good, and the coating process should be low cost and easily scalable,” Manthiram says. The Li-S batteries were only tested through 300 charge and discharge cycles; a real-world battery needs to last through thousands of such cycles, he notes.
“This is an interesting processing-based approach to improve the performance of Li-S batteries without the need for new materials synthesis for the electrodes or electrolyte,” says Jeffrey Pyun, a chemist at the University of Arizona who is also working on these types of batteries. However, he points out that the cost of the carbon nanotubes has to be taken into account. Manthiram says there may be cost issues with using this particular material, but that it should be possible to make the coatings with a less expensive form of carbon.