Nanowire battery

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.



Traditional silicon anodes were researched and dismissed due to the tendency of silicon to crack and become unusable because it swelled with lithium during operation. The nano-wires do not suffer from this flaw. 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[1] 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 2012. 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.[2]

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 brought below around +0.8 V, where the electrolyte becomes thermodynamically unstable and starts 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.[citation needed]

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]

See also


External links

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