Nanotechnology improves battery life

Researchers at the Shenyang National Laboratory for Materials Science in China have been investigating how to improve the kind of rechargeable batteries that are used in mobile phones, MP3 players, personal digital assistants and laptop computers.

They found that after 20 cycles of the semi-cell experiments, the sugar-coated Si-CNT composite material achieved a discharge capacity of 727 mAh per g. In contrast, the charge capacity of the simple sugar-coated particles had dropped to 363 mAh per g.

Hui-Ming Cheng and colleagues have turned to carbon nanotubes (CNTs) to help them use silicon (Si) as the battery anode but avoid the material's usual problem of large volume change during alloying and de-alloying that can lead to faster capacity loss.

Li-Ion batteries suffer from degradation, especially when they get too hot or too cold, and eventually lose the capacity to be fully recharged. The problem of the slow degradation of Li-Ion batteries is usually due to the formation of a solid electrolyte interphase film that increases the batteries' internal resistance and prevents a full recharge.

The researchers grew carbon nanotubes on the surface of tiny particles of silicon using a technique known as chemical vapour deposition, in which a carbon-containing vapour decomposes and then condenses on the surface of the silicon particles forming the nanoscopic tubes.

They then coated these particles with carbon released from sugar at a high temperature in a vacuum. A separate batch of silicon particles produced using sugar but without the CNTs was also prepared.

With the Si-CNT anode material to hand, the team then investigated how well it functioned in a prototype Li-Ion battery and compared the results with the material formed from sugar-coated silicon particles.

The growth of carbon nanotubes on silicon suppresses the structure destruction of the composite during charge cycling, resulting in the improvement of cyclability, according to the researchers.