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Researchers in Korea have developed a novel material for the anode in rechargeable batteries, which they say could make them much more efficient and extend significantly the length of time between charges. While rechargeable lithium batteries have revolutionised the use of portable electronic gadgets allowing users of mobile technology to keep in touch, listen to music, take photos, browse the web, and much more besides, they still fit into the "could do better" category. They out-perform nickel-cadmium batteries in various ways, including lower density, lower environmental disposal impact, and on number of recharge cycles, but their capacity and so running time remain limited. For instance, an average laptop computer whose power is supplied by a new fully charged lithium battery will run for 2-3 hours, less if the battery is older. Fundamentally the graphite anode use with current lithium batteries limits the number of lithium ions that can be pumped into it during recharging. Lithium ion accumulator batteries generate a current as lithium ions migrate out of the anode towards the cathode, which is usually composed of a mixed metal oxide, such as lithium cobalt oxide. Now, Jaephil Cho of Hanyang University in Korea, and his team believe they have cleared the way for a new generation of rechargeable batteries. The new anode material they report in Angewandte Chemie is based on a highly porous three-dimensional silicon structure that overcomes many of the problems, particularly lithium ion capacity of graphite anodes Cho explains that silicon represents an intriguing alternative to graphite as an anode material as it has the potential, in its porous form to store lithium ions in much greater numbers than is possible with carbon in the form of layered graphite. But, there was a problem with using neat silicon. When this material absorbs lithium ions during charging it expands significantly and shrinks again when it releases them during the discharge process. The net effect is that after just a few charge-discharge cycles the thin silicon layers in such an anode are damaged irreparably and can no longer be charged. The Korean team has now developed a new method for the production of a porous silicon anode that can withstand this strain. Their production process involves annealing silicon dioxide nanoparticles with silicon particles whose outermost silicon atoms have short hydrocarbon chains attached to them at 900 Celsius under an atmosphere of the noble gas argon. On completion, the silicon dioxide particles are removed from the bulk composite by chemical etching. What remains are carbon-coated silicon crystals in a continuous, three-dimensional and highly porous structure. The team used Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy and energy dispersive X-ray spectroscopy (EDXS) in their analysis of their new anode material. Cho and his colleagues have now tested their material and found it to have a high charge capacity for lithium ions. Moreover, the lithium ions are transported and stored more rapidly than with conventional anodes, which makes charging faster. As to any swelling, they explain that the changes in volume on charging and discharging is very small. The 70 nanometre thick pore walls change only negligibly and certainly not enough to damage the internal structure of the anode. The first, running in, charging cycle actually results in the formation of amorphous silicon around any residual nanocrystals in the pore walls, which means that even after 100 charge-discharge cycles, the stress in the pore wall is not noticeable in the material. The prize for success is considerable. A porous silicon anode has a theoretical charge capacity of approximately 4140 milliamp hours per gram, which is eleven times higher than the capacity of graphite, which is just 372 mAhg-1. This leap in capacity would not correlate directly with an increase in battery run time, but certainly an increase of that order could be possible. Given such a boost the next problem to solve would be what to do if your battery has not discharged once you have listened to the thousands of songs on your mp3 player. Related links:
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