Graphene:

Graphene is a relatively new material, discovered in 2004, though its existence was theorized for over sixty years. It is essentially a two-dimensional crystal of carbon atoms arranged in a hexagonal lattice or flat sheet hundreds of atoms wide, but only one atom thick. Because of this unique arrangement, graphene possesses a high surface area-to-volume ratio (making it a good material for membranes), excellent electric conductivity, and high tensile strength when in the form of graphene oxide. Such graphene papers are used as lithium battery anodes either alone or in a form of graphene-silicon composite paper. We will use graphene because its high strength will allow it to maintain its structure as the silicon expands and contracts. In addition, its high electrical conductivity will improve the electrochemical performance of our silicon anode.

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Electrochemical Deposition:

Electrochemical deposition is a process in which particles of solid metal, e.g. silicon, are deposited onto an electrically conducting surface. These particles normally merge into a film, but if they are electrochemically deposited in short, controlled bursts, the particles can stay on the surface as separate entities, rather than a continuous sheet. This method, which we plan to use to precisely control the size of our silicon nanoparticles, is known as pulse deposition. The combination of silicon tetrachloride (SiCl₄) as the solute with propylene carbonate (PC) as the solvent produces the fewest amount of impurities, which would otherwise interfere with our results.

In order to improve the cycle life, the structure of the electrodes must remain intact even after repeated charge and discharge of the battery. However, one solution to the issue of silicon pulverization would be to reduce and control deposition time with pulse deposition so that small amounts of silicon nanoparticles are deposited onto the anode. Separating these smaller ‘dots’ of silicon with graphene will prevent expansion and will retain the overall structure of the individual silicon deposits.

Aqueous Dispersion:

Aqueous dispersion is a method of combining two materials by suspending them in particulate form in water and then applying pressure and heat to remove the water and form a composite. Silicon and graphite are both oxidized to allow for their dissolution in water. Graphite is placed in concentrated H₂SO₄, while silicon merely needs to be placed in contact with air. Silicon oxide, suspended in water, is added to the graphite solution and thoroughly mixed by sonication. We will use the thin graphene-silicon composite paper produced in this process as a control anode and compare it to an anode made of silicon electrodeposited on graphene, to see if and how the manufacturing method affects the performance of a silicon-graphene anode.