Tin, silicon, and a few other elements have long been languishing on chemists' list of electrode materials that could, in theory, help lithium-ion batteries hold more energy. A new way of structuring these materials could at last allow them to be used in this way.
Researchers at the Lawrence Berkeley National Laboratory made tin electrodes by using layers of graphene to protect the normally fragile tin. These first tin electrodes are a sign that materials scientists have made a great deal of progress in using nanoscale structures to improve batteries.
Power pillars: This battery electrode, shown in cross section under an electron microscope, consists of nanoscale tin pillars sandwiched between sheets of graphene.
Making battery electrodes from tin or silicon can boost the battery's overall energy storage. That's because such materials can take in more lithium during charging and recharging than carbon, which is normally used. But silicon and tin tend to be unstable as electrodes. Tin takes up so much lithium that it expands in volume by a factor of two to three during charging. "This forms cracks, and the tin leaks into the electrolyte and is lost," says Yuegang Zhang, a scientist at Lawrence Berkeley.
Zhang's clever solution is to layer the tin between sheets of graphene, single-atom-thick sheets of carbon mesh. Graphene is highly conductive, and while it's flexible, it's also the strongest material ever tested.
The tin-graphene electrode consists of two layers of tin nanopillars sandwiched between three sheets of graphene. The pillars help the electrode remain stable: instead of fracturing, the tin expands and contracts during charging without breaking. The space between the pillars means there's plenty of room for the battery's electrolyte to move around, which ensures fast charging speeds.
Zhang's group has made prototype batteries featuring these electrodes. The prototype tin-graphene batteries can charge up in about 10 minutes and store about 700 milliamp-hours per gram of charge. This storage capacity is maintained over 30 charge cycles. The batteries will ultimately need to hold their performance for hundreds of charge cycles. "The performance they have is quite reasonable, and this has a pretty clear application in existing batteries," says Yi Cui, associate professor of materials science at Stanford University. Cui was not involved with the work.
Several other research groups are working on promising battery materials that include nanoscale structures. Cui has founded a company called Amprius to commercialize another kind of battery anode that features silicon nanowires. The nanostructure of these wires also helps the fragile material remain stable as it takes up and releases lithium. Another group, led by Pulickel Ajayan at Rice, recently built a nanostructured battery that incorporates a tin electrode, in this case integrating the electrodes and the electrolyte on individual nanowires. Arrayed together, these nanowires could make long-lasting microbatteries for small devices such as sensors.
Zhang is working to demonstrate the use of the nanopillar structure with other fragile electrode materials, including silicon. The process may add to the cost of battery production, but the performance gains could offset the potential additional cost. "People typically assume that a fancy nanoscale structure will cost more, but it may not," says Zhang.
By Katherine Bourzac
From Technology Review