Northwestern University engineers have developed a silicon-graphene composite electrode that makes lithium-ion batteries to retain their charge to a period 10 folds more than that of existing technology and also allows them to be recharged 10 folds quicker than that of existing batteries.
The novel technology paves the way to make superior rechargeable batteries for iPods and mobile phones, and highly efficient compact batteries for electric cars. According to the researchers, this technology will be available in the marketplace within three to five years. They have published their research in the Advanced Energy Materials journal.
To maintain energy capacity in rechargeable batteries, researchers tried to use silicon in place of carbon, as every silicon atom can attach with four lithium atoms. However, silicon swells and contracts drastically during the charging process, resulting in fragmentation and quick loss of charge capacity. The charging rate of the battery is affected by the shape of the graphane sheets used as the anode. Even though the sheets have one-carbon-atom thickness, due their length, a lithium ion must travel a long distance to reach and relax between the graphene sheets, causing an ionic traffic jam around the corners of the material.
In order to maintain the energy capacity and increase the charging rate of the current rechargeable batteries, the Northwestern University engineers have used two techniques. To retain optimal charge capacity, the researchers stabilized the silicon by sandwiching silicon clusters between the graphene sheets. This enabled the electrode to accommodate more number of lithium atoms while using the graphene sheet flexibility to hold the silicon’s volume changes during use.
The researchers have also utilized a chemical oxidation method to form minute holes of 10-20 nm size on the graphene sheets, which is known as in-plane defects that act as a ‘shortcut’ to the lithium ions to reach the anode where they stored after their reaction with silicon. This process increases the charging rate of the battery by up to 10 folds. The scientists will extend their research by investigating the changes occurring in the cathode, which can further improve the battery efficiency. They also plan to design an electrolyte system as a safety mechanism that will make the battery shut off reversibly and automatically at high temperatures for electric car applications.
Source: http://www.northwestern.edu