Nanowire battery

Nanowire battery

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A nanowire battery is a lithium-ion battery invented by a team led by Dr. Yi Cui at Stanford University
Stanford University
The Leland Stanford Junior University, commonly referred to as Stanford University or Stanford, is a private research university on an campus located near Palo Alto, California. It is situated in the northwestern Santa Clara Valley on the San Francisco Peninsula, approximately northwest of San...

 in 2007. The team's invention consists of a stainless steel
Stainless steel
In metallurgy, stainless steel, also known as inox steel or inox from French "inoxydable", is defined as a steel alloy with a minimum of 10.5 or 11% chromium content by mass....

 anode
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....

 covered in silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...

 nanowires, to replace the traditional graphite
Graphite
The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal...

 anode. Silicon, which stores ten times more lithium
Lithium
Lithium is a soft, silver-white metal that belongs to the alkali metal group of chemical elements. It is represented by the symbol Li, and it has the atomic number 3. Under standard conditions it is the lightest metal and the least dense solid element. Like all alkali metals, lithium is highly...

 than graphite, allows a far greater energy density
Energy density
Energy density is a term used for the amount of energy stored in a given system or region of space per unit volume. Often only the useful or extractable energy is quantified, which is to say that chemically inaccessible energy such as rest mass energy is ignored...

 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 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
Rechargeable battery
A rechargeable battery or storage battery is a group of one or more electrochemical cells. They are known as secondary cells because their electrochemical reactions are electrically reversible. Rechargeable batteries come in many different shapes and sizes, ranging anything from a button cell to...

 and plateaued to 8x density on subsequent charges. In order to take advantage of this anode advancement, an equivalent cathode
Cathode
A cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .Cathode polarity is not always negative...

 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 cycle
Charge cycle
A charge cycle is the process of charging a rechargeable battery and discharging it as required into a load. The term is typically used to specify a battery's expected life, as the number of charge cycles affects life more than the mere passage of time...

s 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.

Potential problem


The very high surface area of the nanowires, which allows high charging rates, also has a downside: heterogeneous side reactions. 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. The result will be a film made from decomposition products that coats the surfaces of the nanowires. 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. 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. (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.

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.