Nanowire battery

A nanowire battery is a lithium-ion battery invented by a team led by Dr. Yi Cui at Stanford University in 2007. The team's invention consists of a stainless steel anode covered in silicon nanowires, to replace the traditional graphite anode. Silicon, which stores ten times more lithium than graphite, allows a far greater energy density on the anode, thus reducing the mass of the battery. The large surface area further allows for fast charging and discharging.

Design

Traditional silicon anodes had been researched and dismissed because the silicon tended to crack and become unusable as it swelled with lithium during operation. Silicon nano-wires do not suffer from this, according to Dr. Cui the battery reached 10x density on the first charge and plateaued to 8x density on subsequent charges. In order to take advantage of this anode advancement, an equivalent cathode advancement is required to achieve the increased storage density.

Commercialization is expected to occur in 2012 with the batteries costing the same or less per watt hour than conventional lithium-ion batteries. The next milestone, life cycle testing, should be completed and the team expects to achieve at least one thousand charge cycles from nano-wire batteries.

In September 2010, Dr. Yi Cui's team demonstrated that 250 charge cycles are possible before the charge capacity drops below 80 percent of its initial storage capacity. The team expects to reach 3,000 charge cycles by 2014. Reaching this goal would make nano-wire batteries viable for use in electric vehicles. A prototype for use in cellular phones and other electronic devices was expected to be delivered by the first quarter of 2011.

Potential problem

The very high surface area of the nanowires, which allows high charging rates, also has a downside: heterogeneous side reactions.^{[citation needed]} These will occur as the nanowires on the negative electrode are not allowed to fully discharge before charging again. The electrolyte becomes thermodynamically unstable, and begins getting reduced.^{[citation needed]} The result will be a film made from decomposition products that coats the surfaces of the nanowires.^{[citation needed]} This coating, called a "solid electrolyte interphase (SEI)," is present in all Li-ion batteries that use conventional electrolytes and low voltage electrodes such as graphite or silicon.^{[citation needed]} Typically, the active particles on the negative electrode side (graphite) are around 10 microns in diameter. While such large sizes extract a penalty by lowering the surface area and power, that size is necessary in order to reduce the amount of SEI formed (which is proportional to the surface area). Even so, 5-10% of all of the Li in a Li-ion battery ends up incorporated into the SEI, leading to an irreversible capacity loss (ICL) of that amount.^{[citation needed]} (The source of the Li in a cell is mainly the positive electrode, such as LiFePO4.) Fortunately, the SEI formation reactions are self-limiting, and after the first cycle ICL can be very small.

On the other hand, a nanowire might have a couple of orders of magnitude more surface area per unit volume than a 10 micron particle, which would result in a couple of orders of magnitude more SEI formed—except that there is not enough Li in the positive electrode to make this much SEI. The result of this loss of accessible Li would be a drastic loss of capacity. For example, if the coulombic efficiency is 99.9%, far better than claimed, then 0.1% of the Li is lost on each cycle to the growing SEI film. For 5,000 cycles (minimum required for a plug-in hybrid vehicle), the remaining active Li would be reduced to well under 1% of the amount of Li present in the cathode initially.

Nanowire cells can nevertheless cycle hundreds of times in half-cells. In a half cell, an electrode made from a piece of Li metal would be cycled against the nanowires. Since in a half cell there is a nearly unlimited supply of Li, capacity need never decline. Such half cells, however, would have no commercial value.^{[citation needed]}

There are tricks that can be employed to reduce ICL—for example, by pre-forming the SEI before assembling the cell. However, this process is not done commercially because of the high cost of adding such a processing step. However, discovery of a cheap and effective (coulombic efficiency > 99.99%) artificial SEI would make nanowires a very viable way to increase the capacity of the negative electrode substantially. This would yield a modest but still very significant improvement in the capacity of the overall cell.^{[citation needed]}

Application to Radio Control

In the world of radio control (RC), batteries which (appear to) utilize nanowire technology were introduced in approximately March 2009, and have since become extremely popular. As of the beginning of 2013, they are widespread, and nanowire batteries are ever-increasing in availability and selection among the top RC product vendors. These new Lithium Polymer (LiPo) batteries boast extremely high charge and discharge rates up to 5~15C charge rates, and 65C continuous discharge rates, with 130C burst discharge (for example, see; note, however, that smaller batteries with lower C-ratings, such as this one [45C continuous discharge, 90C burst] are more common:) while still getting up to 250 cycles lifetime use. As the technology continues to develop, these numbers will likely continue to dramatically increase. Examples of these extremely high performance LiPo batteries utilizing this "nano" technology include the renowned Hyperion "G3" series, the very popular HobbyKing Nano-tech series, ThunderPower's "G6" series, and BananaHobby's "Genesis Power" Pete Signature Series. These batteries continue to push the limits of LiPo battery technology in the ever-increasing demand for more and more power in the highly competitive world of RC.

See also

• List of emerging technologies
• EEStor

External links

• "Stanford's nanowire battery holds 10 times the charge of existing ones". Stanford News Service. 2007-12-18. 
• "Nanowire battery can hold 10 times the charge of existing lithium-ion battery". Stanford News Service. 2007-12-18. 
• "Nanowire battery holds 10 times the charge of existing ones". Physorg.com. 2007-12-18. 
• "High-performance lithium battery anodes using silicon nanowires". Nature Nanotechnology. 2007-12-16. 
• "A Short Introduction to Lithium-Ion Batteries". Amprius. 2009-12-11.