APPLICATIONS OF TECHNOLOGY:
- Lithium batteries
- Electric car batteries
- Portable electronic devices
- Uses a non-toxic, abundant, low cost material with great potential for making high energy density rechargeable cells
- Prevents polysulfide cathode dissolution
- Overcomes insulating property of sulfur
- Improved charge cycling
A Berkeley Lab team headed by Yuegang Zhang and Elton Cairns has developed a method to fabricate battery cathodes from nanoscale flakes of graphene oxide and sulfur. This innovation solves at once two design problems that have impeded efforts to make commercially viable lithium-sulfur (Li-S) batteries: 1) sulfur is a natural insulator, and designers must find ways to overcome its resistance; 2) Li-S batteries are notoriously short-lived because sulfur that dissolves in the electrolyte can form insulating sulfides that coat and degrade the electrodes during charge/discharge cycles.
The flake-like composites developed at Berkeley Lab are comprised of a thin (~10–30 nm) and uniform coating of sulfur on graphene oxide sheets. They are produced using simple chemical deposition techniques and a relatively low-temperature (155 °C) thermal treatment process. Graphene oxide transforms insulating graphene into a conductor by forming oxygen functional groups on the surface. These groups form anchor points to bind just enough sulfur to promote the charge/discharge reactions. Sulfur is an insulator, but its resistance is overcome when the coatings are very thin, thus the graphene oxide-sulfur composite will carry current in reactions with lithium.
The oxygen groups also immobilize the sulfur, so it does not dissolve in the electrolyte. Li-S cells currently suffer degradation because small amounts of sulfur on conventional cathodes will dissolve during discharge, forming polysulfide ions that migrate to the anode, where they are reduced to insulating solids such as Li2S and Li2S2. The polysulfides can diffuse back to the cathode and coat it as well during recharging. This process shortens the cycle life of Li-S cells. By immobilizing the sulfur, the diffusion problem is minimized. Tests show that this graphene oxide-sulfur composite cathode demonstrated high reversible charge capacity over more than 50 cycles.
Sulfur is a highly desirable material for rechargeable batteries because it is non-toxic, abundant, low cost, and has great potential for making high energy density rechargeable cells. In theory, Li-S cells can store much more energy than state-of-the-art lithium-ion cells, which are being used in current generations of electric cars. The Berkeley Lab invention takes advantage of sulfur’s beneficial qualities for battery technology and tackles its shortcomings.
DEVELOPMENT STAGE: Proof of principle.
FOR MORE INFORMATION:Ji, L., Rao, M., Zheng, H., Zhang, L., Li, Y., Duan, W., Guo, J., Cairns, E.J., Zhang, Y., “Graphene Oxide as a Sulfur Immobilizer in High Performance Lithium/Sulfur Cells,” Journal of the American Chemical Society 2011, Vol. 133, No.46, pp.18522-18525.
REFERENCE NUMBER: IB-3096