Silicon nanowires : synthesis and use as lithium-ion battery anodes
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As the power demands of mobile technologies continue to increase, lithium-ion batteries are needed with greater power and energy density. Silicon anodes offer an alternative to commercial graphite with much greater gravimetric and volumetric Li storage. Si nanowires are particularly compelling anode materials because they provide short Li diffusion paths due to their narrow diameter combined with long continuous paths for electron transport down their length. To achieve reasonable battery performance in Si nanowire anodes, conductive carbon particles must be added to provide sufficient electrical conductivity through the anode layer. This lowers the capacity of the anode, but more importantly the carbon particles can segregate in the electrode layer during processing or as a result of mechanical stresses during cycling, leading to unreliable performance. Better performance can be achieved by altering the structure of the Si nanowire to improve electrical conductivity. Si nanowires with a conductive carbon coating were synthesized in a supercritical organic solvent using an organometallic tin precursor to seed growth. The coating eliminated the need for additional conductive additives and improved Si nanowire anode performance. In situ TEM experiments showed that the coated nanowires exhibit higher lithiation rates than bare Si nanowires, but the coating restricts volume expansion limiting the amount of Li storage. Nanowires with a crystalline Si core and amorphous Si shell were also synthesized. The thickness of the core and shell were controlled by altering the Si:Sn precursor ratio. Sn was found to incorporate strongly within the crystalline core, but not at all in the amorphous shell, creating nanowires with varying conductivity. The addition of tin improved Si nanowire performance in Li-ion batteries, eliminating the need for conductive additives. Lastly, the low-temperature limit on the solution synthesis of Si nanowires via in situ seeding was explored using tin, gallium, and indium seeds.