June 28, 2010
This secondary electron microscopy image shows the surface of silicon-carbon nanocomposite spheres with silicon nanoparticles on the carbon surface.
Lithium-ion batteries are as ubiquitous as the electronic devices they power, such as cells phones, laptops, and even some medical products. To create the current that powers these devices, lithium ions are transferred through a liquid electrolyte between two electrodes: a cathode and an anode. Traditionally based on graphite anodes, lithium-ion batteries could have much greater capacity if their anodes were made from silicon and carbon. Until now, however, silicon-carbon-based anodes have been too unstable for real-world electronic applications. But that may be changing.
"A major shortcoming of traditional graphite-based lithium-ion batteries is their limited capacity: 320 to 360 mAh/g," remarks Gleb Yushin, an assistant professor in the school of materials science and engineering at the Georgia Institute of Technology (Atlanta). "Anodes based on silicon have ten times more capacity than graphite--4200 mAh/g--which results in major improvements in the battery capacity."
With most silicon-based anodes, the silicon particles typically expand and contract as lithium ions enter and leave the silicon. "Silicon expands nearly fourfold in volume when lithium is inserted, and contracts fourfold when it is extracted," Yushin explains. "These huge volume changes eventually fracture the polymeric material that binds the silicon particles, breaking the electrical connection between them and causing them to separate." This fracture prevents the generation of an electrical current.
To overcome the silicon anode's instability, the new silicon-carbon anode is based on a bottom-up, self-assembly technique. It is fabricated by creating highly conductive branching structures made from carbon black nanoparticles annealed in a tube furnace. Using a chemical vapor deposition process, silicon nanospheres are formed within the carbon. With the aid of graphitic carbon as an electrically conductive binder, the silicon-carbon nanoparticles then self-assemble into rigid spheres ranging in size from 10 to 30 µm, forming the battery anodes. The spheres have open, interconnected internal pore channels that admit liquid electrolyte containing lithium ions, enabling quick battery charging and accommodating expansion and contraction of the silicon.
The rate of electrochemical reaction--and thus battery power--depends on the rate at which lithium ions diffuse into the active material, Yushin explains. Since the pore channels enable the lithium ions to diffuse rapidly into the center of the large, rigid spheres, the overall lithium diffusion rate is limited only by the silicon nanoparticles. But since the silicon particles are small, the time required to insert or extract lithium is short. Thus, the overall electrochemical reaction is fast, enabling the battery to be charged and discharged quickly at a high current.
"The silicon-carbon nanocomposite anodes will improve anode capacity over six times compared with today's state-of-the art graphite-based anodes," Yushin comments. "This means batteries that will last longer and cost less. It also means that portable electronic devices will last longer, cost less, and have lower weight and volume." He foresees that any portable medical device using rechargeable lithium-ion batteries could benefit from this silicon-carbon self-assembling technique.
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