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New Crystalline-Amorphous Core-Shell Silicon Nanowires for High Capacity and High Current Li-Ion Electrodes

21 January 2009

Cui2009
Top: Schematic illustration of the lithiation of the Si c-a core-shell NWs. Bottom: Capacity and Coulombic efficiency over 100 cycles. Credit: ACS. Click to enlarge.

A team of researchers from Stanford University and Università degli Studi di Milano-Bicocca, led by Stanford professor Yi Cui, have developed a core-shell design of crystalline-amorphous (c-a) silicon nanowires (NW) to enable higher power and longer-life lithium-ion battery electrodes. A paper describing the work was published in the 14 January issue of the ACS journal Nano Letters.

Silicon is an attractive alloy-type anode material for lithium-ion batteries because of its highest known capacity (4,200 mAh g-1). However, silicon’s large volume change upon lithium insertion and extraction, which causes pulverization and capacity fading, has limited its applications.

In a paper published online in the journal Nature Nanotechnology in 2007 , Yi Cui and his colleagues reported the use of single crystalline silicon nanowires as the anode material could offer good electrical connection and ability to overcome the mechanical breaking caused by up to 300% volume expansion. The nanowires achieved the theoretical charge capacity for silicon anodes and maintain a discharge capacity of 75% of the maximum, with little fading under 10 charge/discharge cycles.

The new study builds on that by demonstrating a Si crystalline-amorphous (c-a) core-shell NW design resulting in significant improvement over power rate and cycling life.

Both crystalline Si (c-Si) and amorphous Si (a-Si) can store Li+ ions with similar specific capacity. Previous studies, mostly involving thin films, have suggested that a-Si has a superior cycling performance compared to c-Si. One explanation for this is that the volume expansion of a-Si upon Li insertion is homogeneous and causes less pulverization as in the crystalline material. Another interesting behavior of a-Si is that it reacts with lithium at slightly higher potential (~220 mV) than c-Si does (~120 mV), which leads to our idea of using c-a core-shell Si NWs as anode material. When limiting the charging potential, it should be possible to utilize only the amorphous shell material for Li+ storage while preserving the crystalline core as mechanical support and efficient electron transport pathways.

—Cui et al. (2009)

Cui and his colleagues developed a simple method to grow c-a core-shell Si NWs directly on stainless steel (SS). The c-a core-shell NWs were assembled into half-cells with Li metal as counter electrode. No binders or conducting carbon were used. The half-cells were studied using electrochemical potential spectroscopy (EPS) and galvanostatic charge/discharge.

A comparison of c-a NW half-cells at the low-voltage cutoffs of 10 mV, 70 mV, and 150 mV showed that with charge cutoff at 150mV, the reaction involves just the external amorphous shell of the NWs and preserved the c-Si core for mechanical supports and efficient conductive pathways.

...c-a core-shell NWs with a 150 mV cutoff and the same current density [0.85 A g-1 (0.2C according to the theoretical capacity of Si)] show dramatically improvement of cycle life. The c-a NW electrode was charged to 150 mV in 1.2 h, showing a capacity of ¡~1060 mAh g-1 with 85% discharge capacity retention over 100 cycles. Despite the discharge capacity of 150 mV cutoff is lower than that (2500 mAh g-1) of 10 mV cutoff, the value is 3 times of carbon and is already high enough to be technologically significant. What is more important is to have good capacity retention over long cycle life. In addition, the Coulombic efficiency (86.2% for the first cycle, 99-99.7% for cycles 5-70, 98.4-99% for cycle 70-100) is significantly higher for the 150 mV cutoff, which provides a strong evidence of little fatigue of NWs during cycling.

—Cui et al. (2009)

The team also found that the c-a core-shell NWs showed show excellent electrochemical performance at high rate of charging and discharging (6.8 A g-1, ~20 times of carbon at 1 h rate).

The research was supported by the Global Climate and Energy Project at Stanford; US Office of Naval Research; and King Abdullah University of Science and Technology (KAUST), along with graduate fellowships from the National Science Foundation graduate fellowship and Stanford.

In March 2008, KAUST named Dr. Yi Cui as one of its 12 Global Research Partnership (GRP) Investigators. (Earlier post.)

Resources

  • Candace K. Chan, Hailin Peng, Gao Liu, Kevin McIlwrath, Xiao Feng Zhang, Robert A. Huggins & Yi Cui; “High-performance lithium battery anodes using silicon nanowiresNature Nanotechnology, Published online: 16 December 2007 | doi:10.1038/nnano.2007.411

  • Li-Feng Cui, Riccardo Ruffo, Candace K. Chan, Hailin Peng, and Yi Cui (2009) Crystalline-Amorphous Core-Shell Silicon Nanowires for High Capacity and High Current Battery Electrodes. Nano Lett., 9 (1), 491-495 doi: 10.1021/nl8036323

January 21, 2009 in Batteries, Nanotech | Permalink | Comments (8) | TrackBack (0)

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Comments

So...good. We need cathodes and annodes that perform better if we're going to double the charge density of Li-ion batteries. This study seems to only address annodes. Do we just go with 5x as much cathode material per cell, or what?

How soon can these move to mass production?

I haven't been able to find the relative mass distribution of the components of a Li-Ion battery. However, assuming the anode and cathode have about the same mass and the separator is somewhat smaller, then if the anode were made 90 percent smaller, then the battery may be 30 - 40% smaller for the same capacity. This would be a very significant reduction in size, weight, and probably cost.

If I recall, LiSi batteries have a theoretical specific energy of 1100wh/kg. Long time to commercialization for these.

This is not a LiSi battery. The silicon nano wires are only used to store lithium ions and are not involved in any electrochemical reaction.

"3 times better than carbon", is that all? Current state-of-the-art Lithium Manganese Polymer batteries are almost 3 times better than carbon. The http://www.sionpower.com/ are Lithium-Sulpher, they are high energy due to their superior Cathode performance, however they have a dismal cycle life. Another company http://www.superlatticepower.com/20080602/ is a Lithium-ion Polymer design with a superior Cathode. Either of these might be a good match for Cui's better Anode.

The Arabs are not interested in just getting patents to block battery development, they want to use their wealth so when we get off burning oil, we will still pay them for batteries! They are very smart. They will still sell lots of oil for manufacturing plastics and other synthetics, this is a much better use of this precious resource than burning it.

When Saudi Arabia's oil runs out, it will be around the time that PV panels really come into their own. They will then be able to use their expanses of barren desert to once again create energy. However, by that time the rest of the world will be energy independent using our own PV panels so the Arabs can use their solar energy domestically to desalinate sea water and cultivate the desert. Their primary export will remain tomatoes.

Mark BC,

Solar cells are an eco-disaster just waiting to happen. The thermal efficiency is at best 10-12% less than half an ICE. The "thermal pollution" is extraordinary as a result.
The other pollution effects from making active devices such as Solar cells capturing as much of the Sun's energy as possible, is predictable but unrecognized.

Albedo is the scientific measure of what proportion of the sun's energy is reflected, back into Space verses captured n thr Earth. Lowering the Albedo captures more energy, warming the Earth. The effect of an active device a solar cell, which can correspond to a perfect Capture, or zero Albedo, can be very significant, as compared to the Earth average of 31%. An active device like a solar cell, captures more energy, then moves some energy off the cell, to capture even more, or possibly producing an effective less than zero Albedo, over its surface area.

This can be and is TERRIBLE when in widespread use.
GHGs can purportedly alter the climate by at most tenths or hundredths of one degree. Albedo reductions by solar cells, when collectively and cumulatively used over large areas, can produce thermal warming and climate alterations as much as tens to hundreds of degrees.

Many Greens have little or no Science training, as well as no appreciation of "unintended consequences" for their nostrums. Sometimes they only dimly understand, the alternatives to their proposals. It does NOT mean such devices as they propose, are pollution free, at all.

Nor does it matter whether PVs are centralized in solar farms, or distributed over millions of rooftops. The cumulative area is what counts for Albedo reduction and thermal pollution.

TANSTAAFL! Every energy source has accompanying pollution of one sort or another. Only Green-wackos think otherwise, that there is a free lunch. Pick your poison and choose the least offensive one. It isn't Solar.

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