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Silicon Nanowires for Anodes of Rechargeable Li Power Cells

Charge capacities could be increased substantially over those of carbon-based anodes.

Silicon nanowires have been investigated as alternatives to the graphite heretofore widely used as an anode material in rechargeable lithium-ion power cells. The theoretical specific charge capacity of graphite, corresponding to the maximum Li content (at a composition of LiC6) is 372 mA•hr/g. In contrast, the theoretical specific charge capacity of Si corresponding to the maximum Li content (at a composition of Li4.4Si) is much greater — 4.2 A•hr/g. In previous studies in which thin films of silicon on substrates were investigated for use as high-capacity anodes, it was found that charge capacities faded rapidly in charge/discharge cycling because large changes in volume (as much as 310 percent) associated with insertion and extraction of lithium ions caused cracking and crumbling of the films, as well as delamination of the films from the substrates. The basic idea of the present nanowire approach is to disperse the active anode material into regions having small volumes in order to reduce the sizes of the changes in volume in order to reduce the adverse effects of those changes.

Figure 1. These Scanning Electron Micrographs show silicon nanowires that were grown from a nickel-coated silicon wafer.
In the investigation, silicon nanowires were synthesized by a variety of processes from a variety of starting materials. For example, in one experiment, a 5-nm-thick layer of nickel was sputtered onto a p-doped (111)-oriented silicon wafer to serve as a catalyst for growth of silicon nanowires, then the nickel-coated silicon substrate was heated to a temperature of 990 °C in a flowing atmosphere of argon containing 10 percent of water vapor for two hours. This process resulted in the formation of silicon nanowires having diameters from 10 to 50 nm (see figure). The nanowires were dispersed in ethanol by use of sonication, then anodes were fabricated by spraying the suspension onto copper foils that had been cleaned by etching in sulfuric acid, rinsed in deionized water, and dried. Fabrication was completed by drying the anodes in a vacuum oven at 120° for 12 hours.

Figure 2. Specific Capacity of Silicon Nanowires was measured in charge/discharge cycling.
The specific capacity of the silicon nanowires was measured by charge-discharge testing of one of the anodes at a rate of 0.25 C (where C is a constant electric current that, if integrated for one hour, would amount to the nominal charge capacity). The results of these measurements showed a large irreversible decrease in specific capacity upon the first insertion (charge) followed by smaller irreversible decreases in specific capacity on subsequent cycles (see Figure 2). This behavior has tentatively been attributed to the presence of oxygen on the surfaces of the silicon nanowires: A stable Li2O phase that forms upon exposure to oxygen may be responsible for the high initial irreversible decrease in specific capacity. On the other hand, Li2O is believed to have merit in alleviating the adverse effects of the change in volume as a means of binding silicon particles during cycling, and, hence, may be responsible for limiting the loss of capacity after the first cycle.

Coating the silicon nanowires with carbon has been proposed as a way of reducing the loss of capacity with cycling. Beneficial effects of coating with carbon include stabilization of the surfaces of the silicon nanowires by blocking oxygen, increasing electrical conductivity, and providing a cushion material to relieve the stress of volume expansion. An investigation of the effects of coating with carbon was in progress at the time of reporting the information for this article.

This work was done by KunHong Lee of Pohang University of Science and Technology for the Air Force Research Laboratory.

AFRL-0078

Topics:
Batteries Coatings, colorants and finishes Coatings, colorants, and finishes Conductivity Drying Electrical systems Energy storage systems Fabrication Graphite Lithium-ion batteries Manufacturing processes Materials Materials properties Nanomaterials Nanotechnology
This Brief includes a Technical Support Package (TSP).
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Silicon Nanowires for Anodes of Rechargeable Li Power Cells

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    Defense Tech Briefs Magazine

    This article first appeared in the October, 2008 issue of Defense Tech Briefs Magazine (Vol. 2 No. 5).

    Read more articles from the archives here.

    Overview

    The document presents a research study focused on the synthesis of silicon (Si) nanowires intended for use as an anode material in next-generation lithium-ion (Li-ion) batteries. Conducted by Dr. Kun-Hong Lee at Pohang University of Science and Technology, the project aims to explore new alloy anode materials that can significantly improve battery performance.

    The research highlights the potential of Li alloys, particularly Li4.4Si, which boasts a theoretical capacity of 4200 mAhr/g—substantially higher than that of conventional graphite anodes. However, the study also addresses the challenges associated with using silicon in batteries, particularly the significant volume expansion that occurs during charging and discharging cycles, which can lead to a decrease in capacitance. The investigation focuses on understanding the morphological and volumetric changes of Li-Si alloy electrodes to mitigate these issues and enhance the overall performance of Li alloy anodes.

    The document details the synthesis methods employed for creating Si nanowires, particularly through microwave heating techniques. Two primary approaches are discussed: synthesizing Si nanowires from Si wafers and from thin Si films. In the first method, a P-type Si (100) wafer is coated with a thin gold film and subjected to microwave irradiation in an inert atmosphere, resulting in the formation of fibrous structures with diameters ranging from 10 to 50 nm. The second method involves a similar process using a thin Si film on a quartz wafer, yielding nanowires with diameters around 25 nm.

    The document emphasizes the advantages of microwave synthesis, which allows for rapid, clean production of high-quality nanowires without the limitations associated with traditional chemical vapor deposition (CVD) methods. This technique is noted for its ability to grow nanowires on various substrates, including glasses and organic polymers, which expands the potential applications of Si nanowires in battery technology.

    Overall, the research aims to contribute to the development of improved anode materials for Li-ion batteries, addressing the challenges of capacity loss and volume expansion while leveraging the unique properties of silicon nanowires. The findings are expected to pave the way for advancements in battery technology, potentially leading to batteries with significantly enhanced performance.

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