Faster, more durable lithium-ion batteries

Dr. Neil Canter, Contributing Editor | TLT Tech Beat June 2010

A new anode material prepared by coating titanium disilicide with silicon provides superior performance.

KEY CONCEPTS
• Two-dimensional, network-Web heteronanostructures or nanonets have been prepared that can provide superior performance as an anode material in a lithium battery.
• The anode material is prepared by coating titanium disilicide with silicon.
• Use of this technology may enable lighter-weight batteries to be used in applications such as computer laptops.

The potential for using lithium-ion batteries in a number of applications is due to their superior performance, which includes lower power loss compared to other battery types. But problems have been encountered with slow cycling (charge and discharge) rates of lithium ions and safety. In the latter case, lithium-ion batteries have been found to catch on fire during use.

In a previous TLT article, a layered lithium battery was developed featuring a transitional cathodic material, which contains high levels of nickel in the core and high concentrations of manganese on the surface (1). The former generates high energy but is unstable while the latter is more stable. This cathode material provides the combination of high energy with better stability.

An appealing material for lithium-ion battery anodes is crystalline silicon. Dunwei Wang, assistant professor of chemistry at Boston College in Chestnut Hill, Mass., says, “Silicon has the highest, theoretical specific capacity to hold lithium cations. But an undesirable volume expansion ranging from 300% to 400% occurs because silicon becomes amorphous, and lithium tetrasilicide is formed. This process causes structural and electronic degradation that can lead to cracking and pulverization of the bulk material.”

Moving to smaller silicon nanowires enables accommodation of the volume expansion. Wang says, “With a thin film, space is available to expand upward.” But high capacity, long capacity life and fast cycling rates are not seen because charge transport is impeded.

There is need for a new anode material that can provide improved performance. Such a material has now been developed.

HETERONANOSTRUCTURES
Wang and his associates have developed an anode material in which silicon is coated on to titanium disilicide. The result is the formation of two-dimensional, network-Web heteronanostructures designated as nanonets. Wang explains, “These nanonets are Web-like structures that are similar to a network. The titanium disilicide has the advantage of being able to readily pack in a perpendicular direction, which generates an ideal structural support or scaffolding for the silicon coating.”

The silicon coating contains a monolayer of particles with diameters between 10 and 20 nanometers. Wang notes that the particles do not continuously cover the surface of the titanium disilicide.

A transmission electron microscopic image of the heteronanostructure is shown in Figure 1. The dotted red line shows the boundary point between the crystalline silicon coating and the titanium disilicide structural support.


Figure 1. A coating of silicon on titanium disilicide produces a heteronanostructure that displays a charge/recharge rate estimated to be five to 10 times greater than a typical anode material used in a lithium-ion battery. (Courtesy of Boston College)

Addition of lithium ions into the silicon allows for the volumetric expansion. During a discharge cycle, the lithium ions move out of the silicon coating and then move through the titanium disilicide, which acts as a highway. In charging, the lithium ions move from the titanium disilicide highway back into the silicon coating.

In experiments run at a charge/discharge rate of 8,400 milliamps per gram, the specific capacity of this heteronanostructure is 1,000 milliamps-hour per gram (mA-h/g), which is a charge/recharge rate estimated to be five to 10 times greater than a typical anode material.

Wang indicates that a battery for a typical computer laptop is rated between 4,000 and 12,000 mA/h. This means that it only takes four to 12 grams of this new heteronanostructure to produce a battery with a comparable capacity to what is currently used.

In testing over 100 cycles, the heteronanostructure achieves greater than 99% capacity retention per cycle over 100 cycles conducted at a level above 1,000 mAh/g.

Titanium disilicide is prepared by the reaction between titanium tetrachloride and silane in a chemical vapor deposition process. Silane and hydrogen are next introduced at 650 C for 12 minutes to produce the silicon nanoparticle coating. The heteronanostructure is treated finally at 900 C for 30 seconds in forming gas (5% hydrogen in nitrogen) to conclude the process. Wang says, “The silane is pyrolyzed in this process in a similar manner to how graphite is produced.”

The heteronanostructure has a number of advantages over existing anode materials. Wang says, “The siliconcoated titanium disilicide is composed of nanoparticles instead of something continuous. This provides sufficient volume to enable the silicon coating to expand when lithium ions are inserted. A three-dimensional nanostructure, with a high surface area, is created to facilitate good transport of the lithium ions.”

The nature of the chemistry enables the researchers to just insert lithium into the silicon layer and not into the titanium disilicide. This is accomplished through control of the voltage. Lithium will react with silicone at a voltage of 120 millivolts, while the reaction with titanium disilicide occurs in a voltage range between 60 and 70 millivolts.

Wang says, “We have conducted a proof-of-concept demonstration but now need to learn more about how the process works.” Future work involves gaining a better understanding of the mechanism of how lithium ions can be inserted into the silicon coating and not the titanium disilicide scaffolding.

Wang also would like to determine if this heteronanostructure could be used also as the material for a cathode. Further information on this research can be found in a recent article (2) or by contacting Wang at dunwei.wang@bc.edu. 

REFERENCES
1. Canter, N. (2009), “Layered Lithium Batteries,” TLT, 65 (11), pp. 12–13.
2. Zhou, S., Liu, X. and Wang, D. (2010), “Si/TiSi2 Heteronanostructures as High-Capacity Anode Material for Li Ion Batteries,” Nano Letters, 10 (3), pp. 860–863.

Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.