When the lithium battery is charged, lithium ions move from the positive electrode (cathode) to the negative (anode). Silicon is one of the most promising materials for the anode, because it can store more than 10 times more ions compared to graphite with the same mass. But, absorbing the charge, silicon increases in volume up to 4 times, becoming unusable after several working cycles. Nanocoders come to the rescue.
Kyu, together with his colleagues, created a new material by placing amorphous silicon on carbon nanowires. As a result, the latter acquired the ability to hold a charge of about 2,000 mAh per gram, whereas for graphite anodes, this figure does not exceed 360 mAh. In addition, the carbon base makes the electrodes rigid. Kyu describes the advantages: “Lithium ions are also absorbed by carbon, but its volume increases by 10% or less.” During the tests, nanowires easily passed 50 recharge cycles. Previously, researchers tried to produce electrodes from nanowires in the form of pure crystalline silicon. They were three times superior in efficiency to graphite, but they could not stand more than 20 cycles.
Silicon Carbon Nanowires are easy to manufacture: there is no need for high temperatures typical of pure silicon structures. “Carbon nanofibres are already available as commercial products that can be produced in tons,” says Kyu. For use in electrical transport, electrodes in lithium batteries must be able to go through at least 300 recharge cycles. In this case, they will be able to qualify for competitiveness. In December 2008, a group of scientists from Hanyang University in Ansan, South Korea, demonstrated silicon anodes with nanopores, which broke the milestone of 100 cycles. Chemist Chefil Cho (Jaephil Cho), who led the work, described the development as being more advanced with respect to nanowires, since per unit volume there is more silicon, and hence the charge. However, according to him, “the production of carbon fibers is easy to expand, so Kyu technology is very practical.”
Meanwhile, General Motors and Applied Science are also working on anodes with nanowires. Their technique involves coating the carbon nanofibers with silicon particles as opposed to amorphous silicon; their capacity ranges from 1 000 to 1 500 mAh per gram. The head of research, Golem-Abbas Nazri (Gholam-Abbas Nazri) believes that the anode capacity can be increased by thickening the silicon layer, but the best stability is achieved at 1,000 mAh per gram. Further improvement needs to develop new cathodes.
Kyu is confident in the choice of silicon as the material for the anode of lithium batteries: “Over the next five years we will see batteries with silicon anodes.” Anyway, cost will remain the decisive factor. Ultimately, it all depends on “the emergence of a cheap, large-scale manufacturing process that produces products based on new technologies.”