Beijing (China) - Rechargeable batteries are a great return business for battery companies and a continuous source of inconvenience for users of pretty much device that uses such batteries: Ongoing use will cause the battery to lose its capacity over time: While you can influence the degree of capacity loss, you will eventually need to replace the battery at some point. New research now indicates that a carbon nanotube anode coating on may cut the capacity almost in half.
Researchers at the Shenyang National Laboratory for Materials Science in China have discovered that carbon nanotubes enable battery manufacturers to sue silicon as an anode material in Li-ion products, without previously found disadvantages of the material.
Li-ion batteries, which are used for example in cellphones or laptop computers, typically suffer from a strong degradation in capacity especially when they get too hot or too cold. This problem is mainly due to the formation of a solid electrolyte interphase film that increases the batteries internal resistance and prevents a full recharge. This problem is even more apparent when silicon is used within the anode: While is generally believed to provide improved charge capacity overall, silicon apparently causes an accelerated loss in capacity.
We don't know why or how Shengyang's Hui-Ming Cheng came up with the idea to enhance silicon with carbon nanotubes as the battery to avoid the problem of a large volume change during alloying and de-alloying during charge and discharge cycles. However, the idea brought some results indicating that carbon nanotubes in fact may provide some improvement.
Hui-Ming Cheng's team grew carbon nanotubes on the surface of tiny particles of silicon using a technique described as chemical vapor deposition in which a carbon-containing vapor decomposes and then condenses on the surface of the silicon particles forming nanoscopic tubes. The group 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.
In a 20-cycle test-run, the researchers found that the sugar-coated silicon-carbon-nanotube material achieve a discharge capacity of 727 milliamp hours per gram. Without carbon nanotubes, the charge capacity had dropped to 363 mAh per gram.
The research group did not provide further details on their findings, if this technology is suitable for mass production and when it could become available in commercial products.
However, before you get too excited about carbon nanotubes, we also should mention a non-related study Desirée Plata and her mentors, chemists Phil Gschwend of the Massachusetts Institute of Technology and Chris Reddy of the Woods Hole Oceanographic Institution. Plata discovered that the process of making carbon nanotubes "may have unintentional and potentially harmful impacts on the environment".
Plata said they analyzed ten commercially made carbon nanotubes to identify the chemical byproducts of the manufacturing process and to help track them in the environment. She found that the ten different carbon nanotubes had vastly different compositions: Most previous toxicity studies have generally assumed that all nanotubes are the same. According to Plata, this diversity of chemical signatures will make it harder to trace the impacts of carbon nanotubes in the environment. In previous work, Plata and her colleagues found that the process of nanotube manufacturing produced emissions of at least 15 aromatic hydrocarbons, including four different kinds of toxic polycyclic aromatic hydrocarbons similar to those found in cigarette smoke and automobile tailpipe emissions.