Carrier bags are made from polyethylene which, when heated to 700°C, transforms into carbon spheres with a hardness approaching that of diamond. These tiny particles could be used to extend the lives of the lithium-ion batteries in consumer electronics and electrically-powered cars. Researchers from Argonne National Laboratory in the USA and the Rutherford Appleton Laboratory have used the Nimrod instrument at the ISIS neutron source to examine the process by which the plastic transforms, and how this controls the properties of the carbon spheres. The results have been published in the journal Carbon.
Lithium-ion batteries are used in a wide range of applications, from mobile phones to electric cars. They are popular because they are light, and they can store a lot of energy – up to 6 times as much as a lead-acid battery. In a lithium-ion battery, lithium ions flow freely between two electrodes, typically a carbon anode and a metal oxide cathode, during charge and discharge. As the battery is used, the electrodes degrade and gradually wear out and the capacity of the battery is reduced. However, research at Argonne National Laboratory has revealed that the carbon spheres, after being heated to an extremely high temperature, e.g., 2800°C, can accommodate and release lithium while maintaining their shape, thereby prolonging operational life.
The properties of carbon materials depend on their structure on the nm scale, which in turn depends on how the material was formed. In this case, the challenge is to understand how the chain-like molecules of polyethylene evolve into carbon spheres. Understanding this process helps to tailor the properties of the carbon end product for a particular technological application.
The researchers used x-ray diffraction to monitor the evolution of polyethylene, and neutron diffraction at ISIS to determine the nm scale internal structure of the carbon spheres. They found that the polyethylene starts to decompose at around 105°C. The hydrocarbon product remains molten until about 500°C, when it rapidly decomposes, accompanied by a sharp increase in pressure. On cooling, the spheres form abruptly at about 350°C. Neutron diffraction showed that on the nm scale the carbon adopts a predominantly layered structure. Heating the carbon spheres to 2800°C, increases the carbon (graphitic) layering which improves their electrochemical properties.
Scanning electron microscopy (a,b) and transmission electron microscopy (c,d) provide information about the evolution of the carbon spheres heated to 700°C (top panels) and 2800°C (bottom panels).
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Vilas Pol led the study. He says, “This research has shown that plastic waste from carrier bags could be transformed into carbon architectures that might have real practical applications, improving the performance of lithium-ion batteries or a lubrication additive to reduce wear and tear in engines.”
Research date: November 2013
Probing the evolution and morphology of hard carbon spheres, Vilas G. Pol, Jianguo Wen, Kah Chun Lau, Samantha Callear, Daniel T. Bowron, Chi-Kai Lin, Sanket A. Deshmukh, Subramanian Sankaranarayanan, Larry A. Curtiss, William I.F. David, Dean J. Miller, Michael M. Thackeray Carbon2013 In press.