Northwestern Univ. researchers report on a new high-power Si–graphene composite anode material for Li-ion batteries in the journal Advanced Energy Materials.
With current technology, the capabilities of a lithium-ion battery are limited in two ways: energy capacity is limited by the charge density, and charge rate is limited by the speed at which the lithium ions can make their way from the electrolyte into the anode.
The Northwestern research team combined two techniques to combat both these problems. First, to stabilize the silicon in order to maintain maximize charge capacity, they sandwiched clusters of silicon between the graphene sheets. This allowed for a greater number of lithium ions in the electrode while utilizing the flexibility of graphene sheets to accommodate the volume changes of silicon during use.
|Rendering of the composite electrode with sandwiched Si clusters and in-plane defects. Click to enlarge.|
The team also used a chemical oxidation process to create minuscule holes (10-20 nanometers) in the graphene sheets—i.e., “in-plane defects”—so the lithium ions would have a “shortcut” into the anode. This reduced the time it takes the battery to recharge by up to 10 times.
Introducing a high density of in-plane, nanometer-sized carbon vacancies in graphene sheets greatly enhances ion diffusion across the sheets in a Si–graphene composite. The flexible, self-supporting three-dimensional conducting graphenic scaffold incorporating Si nanoparticles exhibit excellent rate performance and tolerance to structural deformation, which represents an attractive high power-high capacity anode material for Li-ion batteries.—Zhao et al.
Next the researchers will begin studying changes in the cathode that could further increase effectiveness of the batteries. They will also look into developing an electrolyte system that will allow the battery to automatically shut off at high temperatures.
Zhao, X., Hayner, C. M., Kung, M. C. and Kung, H. H. (2011) In-Plane Vacancy-Enabled High-Power Si–Graphene Composite Electrode for Lithium-Ion Batteries. Advanced Energy Materials DOI: 10.1002/aenm.201100426