Lithium-ion batteries are effective, but they can be a bit on the expensive side. Striking a balance between common materials and efficiency is important, and regular old salt looks like it could fit the bill – after a few kinks are ironed out. Now, researchers at King Abdullah University of Science and Technology (KAUST) have developed a way to make "disordered" graphene that can help improve the sodium-ion battery recipe.
Sodium ions may not be quite as powerful a charge carrier as lithium, but they more than make up for that in their abundance and, as a result, cost. The anode material commonly used in lithium-ion batteries – graphite – is also cheap, but unfortunately it's not great at grabbing hold of sodium ions, which are larger than lithium ones. In the past, scientists have overcome this problem by carbonizing oak leaves, or stuffing the anode full of crumpled graphene balls.
The KAUST team's approach was similar to the latter project. A disordered form of graphite called hard carbon, which is able to store more sodium ions, was the goal, but creating it is usually a tricky process that requires temperatures close to 1,000° C (1,832° F). So the researchers developed a much simpler method that involves creating disordered graphene using a basic laser.
First, the KAUST researchers coated a copper foil with a polymer made up of polyimide and urea. This is then blasted with intense laser light to "carbonize" it, turning it into graphene. The team also introduces nitrogen gas during the process, which replaces some of the carbon atoms in the material. With about 13 percent nitrogen, the end result is 3D graphene that's more conductive, has expanded atomic spacing and is directly bonded to the copper base.
"We wanted to find a way to make three dimensional hard carbons without having to excessively heat our samples," says Fan Zhang, an author of the study. "This way we could form the hard carbon directly on copper collectors."
When this material was used as an anode in sodium-ion batteries, the team found that the device was more efficient and had better capacity than similar batteries using carbon-based anodes.
The research was published in the journal Advanced Energy Materials.