Charge of the lighter brigade

Batteries used in electric cars, laptops and mobile phones could double their capacity, thanks to the development of a next-generation rechargeable lithium-ion battery.

Researchers at the US Department of Energy’s Sandia National Laboratories in California have created negatively charged battery anodes from silicon and graphite composite materials to replace current graphite-only technology. The resulting system has shown promise in creating a smaller battery with increased power and longer life.

The breakthrough will be particularly beneficial in the creation of microsystems used to create technologies such as wireless radiation detectors and sensors, where researchers are currently limited by the short life of the batteries when shrunk to small scale.

The new anode materials will improve microsystems’ performance as well as reducing the size and weight of the overall component, making them easier to integrate into smaller devices.

In lithium batteries when the terminals are connected lithium ions move from the negative anode to the positive cathode to produce an electric current. When the battery is being recharged the reaction is reversed and lithium is moved back to the anode ready to be transferred across again.

For years researchers have been attempting to overcome the size limits of lithium batteries by using silicon anodes. However, the material tends to lose power quickly, meaning it must be recharged more often.

But if graphite is added to the silicon the Sandia team discovered that it held more lithium ions for longer, meaning battery life could be prolonged.

The Sandia cell also shows promise in minimising the fading of power capacity that occurs in silicon-based electrodes after they have been recharged a number of times.

The composite is produced using a milling process common within the battery industry, while the raw materials are relatively cheap.

‘Silicon does not cost much, while the system does not require the use of high-grade graphite,’ said Sandia analytical materials science manager Jim Wang. ‘The technology should deliver cheap, small powerful cells that can be recharged again and again.’

Note: NEC has demonstrated a fuel cell for mobile applications with three times the energy density of existing lithium batteries. The design relies on carbon nanohorns, a type of carbon nanotube, as a support structure for platinum catalyst particles within the cell, allowing the particles to be finer and maximising the catalyst’s available surface area where a reaction can occur.

In a demonstration units successfully powered a mobile phone and an A4-sized PC notebook. NEC said the system is likely to be taken up by the mobile phone market and should be available by 2005. By then the cost of nanohorns will have fallen, making the fuel cells cheaper.