New Self-Assembled Silicon-Carbon Nanocomposite Anodes for Li-ion Batteries Offer More Than 5X The Reversible Capacity of Graphite Anodes
15 March 2010
|This scanning electron micrograph shows carbon-coated silicon nanoparticles on the surface of the composite granules used to form the new anode. Source: Georgia Tech. Click to enlarge.|
Researchers have developed a new high-performance anode structure for lithium-ion batteries based on silicon-carbon nanocomposite materials. Produced via large-scale hierarchical bottom-up assembly, the material contains rigid and robust silicon spheres with irregular channels for rapid access of Li ions into the particle bulk.
The large silicon volume changes on lithium ion insertion and extraction—which can cause structural problems leading to rapid capacity loss—are accommodated by the particle’s internal porosity. The researchers have shown reversible capacities more than five times higher than that of the state-of-the-art graphite anodes (1,950 mAh g-1) and stable performance. The synthesis process is simple, low-cost, safe and broadly applicable, they say, providing new avenues for the rational engineering of electrode materials with enhanced conductivity and power.
Details of the new self-assembly approach were published online in the journal Nature Materials on 14 March.
Development of a novel approach to producing hierarchical anode or cathode particles with controlled properties opens the door to many new directions for lithium-ion battery technology. This is a significant step toward commercial production of silicon-based anode materials for lithium-ion batteries.
—Gleb Yushin, an assistant professor in the School of Materials Science and Engineering at the Georgia Institute of Technology
Fabrication of the composite anode begins with formation of highly conductive branching structures made from carbon black nanoparticles annealed in a high-temperature tube furnace. Silicon nanospheres with diameters of less than 30 nanometers are then formed within the carbon structures using a chemical vapor deposition process. The silicon-carbon composite structures resemble “apples hanging on a tree.”
Using graphitic carbon as an electrically-conductive binder, the silicon-carbon composites are then self-assembled into rigid spheres that have open, interconnected internal pore channels. The spheres, formed in sizes ranging from 10 to 30 microns, are used to form battery anodes. The relatively large composite powder size—a thousand times larger than individual silicon nanoparticles—allows easy powder processing for anode fabrication.
The internal channels in the silicon-carbon spheres serve two purposes. They admit liquid electrolyte to allow rapid entry of lithium ions for quick battery charging, and they provide space to accommodate expansion and contraction of the silicon without cracking the anode. The internal channels and nanometer-scale particles also provide short lithium diffusion paths into the anode, boosting battery power characteristics.
The size of the silicon particles is controlled by the duration of the chemical vapor deposition process and the pressure applied to the deposition system. The size of the carbon nanostructure branches and the size of the silicon spheres determine the pore size in the composite.
Production of the silicon-carbon composites could be scaled up as a continuous process amenable to ultra high-volume powder manufacturing, Yushin said. Because the final composite spheres are relatively large when they are fabricated into anodes, the self-assembly technique avoids the potential health risks of handling nanoscale powders, he added.
So far, the researchers have tested the new anode through more than a hundred charge-discharge cycles. Yushin believes the material would remain stable for thousands of cycles because no degradation mechanisms have become apparent.
In addition to Yushin, the paper’s authors included Alexandre Magasinki, Patrick Dixon and Benjamin Hertzberg—all from Georgia Tech—and Alexander Kvit from the Materials Science Center and Materials Science Department at the University of Wisconsin-Madison, and Jorge Ayala from Superior Graphite. The paper also acknowledges the contributions of Alexander Alexeev at Georgia Tech and Igor Luzinov from Clemson University.
The research was partially supported by a Small Business Innovation Research (SBIR) grant from the National Aeronautics and Space Administration (NASA) to Chicago-based Superior Graphite and Atlanta-based Streamline Nanotechnologies, Inc.
A. Magasinki, P. Dixon, B. Hertzberg, A. Kvit, J. Ayala, G. Yushin (2010) High-performance lithium-ion anodes using a hierarchical bottom-up approach. Nature Materials doi: 10.1038/nmat2725
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