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Rice University researchers develop inexpensive silicon-based anode for Li-ion batteries with good capacity and cycle life
2 November 2012
|Comparison of the discharge capacity and coulombic efficiency of MPSPs/PPAN anodes at various ratios versus cycle number. Thakur et al. Click to enlarge.|
Researchers at Rice University have created an inexpensive silicon-based anode material for Li-ion batteries consisting of macroporous silicon particulates (MPSPs) created by crushing porous silicon films they had earlier developed. (Earlier post.) The new anode material can achieve more than 1,000 mAh/g capacity over more than 600 charge-discharge cycles. The team, led by Rice engineer Sibani Lisa Biswal, reports on their work in Nature’s open access journal Scientific Reports.
After being mixed with polyacrylonitrile (PAN) and pyrolyzed, MPSPs can alloy with lithium. These sponge-like MPSPs with pyrolyzed PAN (PPAN) can accommodate the large volume expansion associated with silicon lithiation. This performance combined with low cost processing yields a competitive anode material that will have an immediate and direct application in lithium ion batteries, according to the research team.
The new work by Rice through the long-running Lockheed Martin Advanced Nanotechnology Center of Excellence at Rice (LANCER) is the next and biggest logical step since the partners began investigating batteries four years ago.
We previously reported on making porous silicon films. We have been looking to move away from the film geometry to something that can be easily transferred into the current battery manufacturing process. Madhuri [Thakur, lead author] crushed the porous silicon film to form porous silicon particulates, a powder that can be easily adopted by battery manufacturers.—Lisa Biswal
In the quest to develop a material that leverages the high theoretical capacity of silicon in a Li-ion battery without succumbing to the significant volume changes caused by charge/discharge with the resulting rapid capacity fade, researchers have been developing a variety of silicon structures and silicon-carbon based composites as well as—in a different approach— composite materials of porous silicon and carbon.
In the latter approach, the pores in the Si-C composite have been shown to provide the volume needed for the silicon expansion and allow fast transport of the lithium ions to the silicon. The carbon, in turn, improves the stability of the solid electrolyte interface (SEI), offers structural integrity and high electric conductivity.
Professor Biswal and her colleagues earlier developed a freestanding macroporous silicon films as an Li-ion anode material offering a large surface area to volume ratio with controllable pore diameters.
But these films cannot be conveniently incorporated into current battery processing techniques, which utilize material slurries and roll-to-roll processing. To design a more processable material, we changed from a film structure to a particulate structure that can be combined with PAN, or any binder, to form slurry that can be processed with standard coating technologies. Here we report an inexpensive wet etch processing technique that can be used to generate kilogram quantities of macroporous silicon particulates (MPSPs).—Thakur et al.
In their newly reported process, the researchers first fabricate freestanding macroporous silicon films that are on the order of 50-100 microns thick with pore diameters greater than 50 nm. The process allows control of the thickness, pore diameter and the porosity by controlling the etching parameters such as current applied, wafer resistivity, concentration of electrolyte and doping of the wafer.
The freestanding macroporous silicon film is then ultrasonically fractured to create particulates with a nominal size range of 10-50 microns. The macroporous silicon particulates (MPSPs) are then mixed with PAN to form a slurry, which is coated onto a current collector using a drop cast method and pyrolyzed to form an anode. This wet-etch process does not require costly vacuum or deposition processing, making it less costly compared to other silicon structures.
Furthermore, because the underlying bulk silicon substrate can be reused to create another layer of macroporous silicon, there is little silicon waste in the process, they noted. MPSP with pyrolyzed PAN (PPAN) costs $0.024 A-1h-1, which is competitive with existing graphitic carbon anodes, which is priced at $0.013 A-1h-1. Because these particulates are on the scale of tens of microns, this material does not suffer from safety and health concerns associated with silicon nanoparticles, they said.
Comparing the electrochemical testing results of various ratios of MPSPs/PPAN indicated that there is an optimum ratio.
...these results show that an anode comprised of MPSPs with PPAN significantly improves the cycle life compared to that of an anode fabricated from silicon particulates that do not contain pores. The PPAN functions as both a binder and conductive additive. The improved performance is attributed to the porosity of the MPSPs, which is able to accommodate the volume expansion associated with the lithiation of silicon. With use of the FEC electrolyte, cyclability increases by more than 600 cycles. The specific capacity and cycle life is comparable or oftentimes better than reported silicon micro and nanostructures. Combined with the low cost processing for large quantities of active material, MPSPs have the potential to transform the energy storage landscape. In the future, the cyclability of macroporous silicon micro-particulates will be further optimized by changing the electrochemical testing conditions, sonication time, pore size and binding material.—Thakur et al.
The team’s next step will be to test the porous silicon powder as an anode in a full battery, Biswal said. Preliminary results with cobalt oxide as the cathode appear very promising, she said, adding that there are new cathode materials that they would like to investigate.
Madhuri Thakur, Steve Sinsabaugh, Mark J. Isaacson, Michael S. Wong, and Sibani Lisa Biswal (2012) Inexpensive method for producing macroporous silicon particulates (MPSPs) with pyrolyzed polyacrylonitrile for lithium ion batteries. Scientific Reports. [Online access may be delayed until next week due to repercussions from Sandy. — Ed.]
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