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Power Management


Battery strength gets a boost with the use of small holes


Scientists have developed a new battery technology that could see batteries used in cellphones stay charged for more than a week and then take just 15 minutes to recharge. CIE finds out how


S


cientists from Northwestern University in the US have developed a new battery technology for phones and laptops that can recharge ten times faster and hold a charge ten times larger than current technology allows. The new technology could see the development of cellphone batteries that could stay charged for more than a week and then recharged in under 15 minutes.


A team of engineers based at the university has been able to create a new electrode for lithium-ion batteries by changing the materials used in lithium-ion batteries to boost their abilities. The researchers combined two chemical engineering approaches to address two major battery limitations, energy capacity and charge rate. In addition to better batteries for cellphones and iPods, the technology could pave the way for more efficient,


smaller batteries


for electric cars and according to the scientists these new batteries could be in the shops within five years.


One key change to the battery involves


creating millions of tiny holes in it, this has helped accelerate the recharging speed. Because the density and movement of lithium ions are key to the entire recharging process the engineers involved have said that they have been able to find


24 December 2011/January 2012


The recharging speed has been accelerated using a chemical oxidation process which drills small holes - just 20-40 nanometers wide - in the atom-thick sheets of graphene that batteries are made of and this helps the lithium ions to move much quicker and to be stored much faster. Currently lithium-ion batteries charge through a chemical reaction in which


Components in Electronics


a way of placing more ions into the battery and then speeding up their movement by altering the materials used in creating the battery. According to the project leader Dr


Harold Kung and his team at Northwestern the maximum charge has been boosted by replacing sheets of silicon with tiny clusters of the substance to increase the amount of lithium ions a battery can hold on to.


lithium ions are sent between two ends of the battery, the anode and the cathode. As energy in the battery is used, the lithium ions travel from the anode, through the electrolyte, and to the cathode. As the battery is recharged, so they travel in the reverse direction.


Using existing technology, the performance of a lithium-ion battery is limited in two ways. Its energy capacity, that is how long a battery can maintain its charge, is limited by the charge density, or how many lithium ions can be packed into the anode or cathode. The battery’s charge rate, the speed at which it recharges, is currently limited by another factor: the speed at which the lithium ions can make their way from the electrolyte into the anode. In current


rechargeable batteries, the anode, which is made of layer upon layer of carbon- based graphene sheets, can only accommodate one lithium atom for every six carbon atoms.


To increase energy capacity, scientists have previously experimented with replacing the carbon with silicon, as silicon can accommodate much more lithium: four lithium atoms for every silicon atom. However, silicon expands and contracts in the charging process, which can cause fragmentation and the rapid loss of charge capacity. At present the speed of a battery’s


charge rate is hindered by the shape of the graphene sheets: they are extremely thin, just one carbon


atom thick, but by comparison, very long. During the charging process, a lithium ion must travel all the way to the outer edges of the graphene sheet before entering and coming to rest between the sheets. And because it takes so long for lithium to travel to the middle of the graphene sheet, what has been described as a sort of ionic traffic jam tends to appear around the edges of the material.


While the new technology is capable of boosting energy capacity and the charge rate it has been found that the recharging and power gains tend to fall off sharply after a battery has been charged about 150 times. According to Kung, however, "Even after 150 charges, which would be one year or more of operation, the battery is still five times more effective than lithium- ion batteries on the market today." So far, the work done by the team has concentrated on making improvements to anodes - where the current flows into the batteries when they are providing power. The group now plans to study the cathode - where the current flows out - to make further improvements. ■


A paper detailing the work of Prof Kung and his co-workers has been published in the journal Advanced Energy Materials.


www.cieonline.co.uk


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