A team of scientists at Stanford University and the Department of Energy's SLAC National Accelerator Laboratory has come up with a possible answer to the question of how to make lithium-ion battery anodes out of silicon, as these tend to swell and crack, as well as react with the battery electrolyte to form a coating that harms their performance.
The scientists wrapped each silicon anode particle in a custom-fit "cage" made of graphene, in a simple, three-step method for building microscopic graphene cages of just the right size: roomy enough to let the silicon particle expand as the battery charges, yet tight enough to hold all the pieces together when the particle falls apart, so it can continue to function at high capacity. The strong, flexible cages also block destructive chemical reactions with the electrolyte.In testing, the graphene cages actually enhanced the electrical conductivity of the particles and provided high charge capacity, chemical stability and efficiency, according to one of the researchers who led the study. The method can be applied to other electrode materials, too, making energy-dense, low-cost battery materials a realistic possibility.
This new method is said to allow to use much larger silicon particles that are one to three microns, or millionths of a meter, in diameter, which are cheap and widely available. In fact, the particles that were used are very similar to the waste created by milling silicon ingots to make semiconductor chips. Particles this big have never performed well in battery anodes before, so this is considered to be a very exciting new achievement.
For the graphene cages to work, they have to fit the silicon particles exactly. The scientists accomplished this in a series of steps: First they coated silicon particles with nickel, which can be applied in just the right thickness. Then they grew layers of graphene on top of the nickel; The nickel acts as a catalyst to promote graphene growth. Finally they etched the nickel away, leaving just enough space within the graphene cage for the silicon particle to expand. Researchers have tried a number of other coatings for silicon anodes, but they all reduced the anode's efficiency. The form-fitting graphene cages are the first coating that maintains high efficiency, and the reactions can be carried out at relatively low temperatures.
Next the team will work on fine-tuning the process, and on producing caged silicon particles in large enough quantities to build commercial-scale batteries for testing.