Nanowires in charge

Lithium metal oxide nanowires could significantly improve the capacity and reduce the time required to fully charge lithium ion rechargeable batteries.

Although the capacity of rechargeable batteries has steadily increased over the past ten years, it has only just kept pace with the power requirements of modern devices such as cell phones and portable computers. A new type of storage material based on lithium nanowires developed by researchers in South Korea could enable substantial improvements in the capacity, power and charging speed of rechargeable lithium ions batteries.¹

Lithium ion batteries are one of the most popular types of rechargeable battery because of their high-energy-to-mass ratio, along with the fact that they retain their charge well when not in use, and do not suffer from the so-called memory effect exhibited by nickel-cadmium batteries. However, they do have one important failing. If they are charged or discharged too quickly, their storage capacity rapidly degrades.

Fig. 1: Scanning electron micrograph of the authors' layered Li_0.8[Li_0.18Co_0.33Mn_0.49]O₂nanowire cathode material.

One of the reasons for this is the difficulty with which the ions move in and out of a lithium ion battery's cathode. This is because such cathodes are usually made from bulk materials. To try address this, Jaephil Cho and colleagues at the Pohang Accelerator Laboratory have developed a low temperature technique for growing cathode materials based on layered lithium-oxide nanowires (Fig.1).

Because of their high surface area and one-dimensional nature, a cathode made from nanowires should allow ions to move more freely between it and a battery's electrolyte. This enabled the researchers to realize charging speeds of around six times greater than those achieved using conventional cathode materials, without significant degradation in storage capacity.

This could enable significant improvements in the performance of modern devices. “It currently requires two and a half hours to fully charge a cellular phones,” says Cho. “With batteries made using our nanowires, this charging time could be reduced to just 30 minutes.”

The potential of nanowire storage materials goes well beyond making better batteries for consumer electronics. In future work the authors hope to integrate their nanowire cathodes into the miniaturized powered sources of new generation of micro-portable devices, from active RF-ID tags for keeping track of supermarket groceries to the wirelessly connected micro-sensor particles of ‘smart dust’.