Crushed, porous silicon with pyrolyzed polyacrylonitrile anode material, able to handle 600 charge-discharge cycles. (via Rice University)
Working hard on a project only to find that it is not adequate for application would make you want to crush it. Well, that is what Rice University research scientist Madhury Thakur did, although she did it deliberately, not out of anger. But, in the process, she has found one of the most promising methods for producing batteries that last three times longer or more.
The trick was finding a way to incorporate silicon into the cathodes; the more ions that can be held by the cathode the bigger the charge capacity. Currently, graphite is used because it does a decent job of holding lithium ions and is resilient after hundreds of charge/discharge cycles. Silicon can hold 10 times more ions per same volume, but it expands to 3 times the size when fully lithiated (treated with lithium or one of its compounds). This swelling and shrinking results in defective cathodes after just a few cycles and so silicon is not used.
To combat the damage of silicon cathodes, Thakur and her team developed thin, porous films of silicon that could handle the expansion and shrinkage. These films were difficult to scale up and handle and ultimately inadequate for manufacturing processes.
So she purposely did what I may have done out of fury, she crushed it. Immediately she realize there was a difference between regular crushed silicon and a crushed porous film. The crushed porous film provided an astounding 46m^2/g of surface area compared to the 0.71m^2/g of regular, crushed silicon (more than 50 times the surface area) plus the grains had ample space to expand into when saturated with lithium ions. They also mixed the powder with a binder, pyrolyzed polyacrylonitrile (PAN), that helps conductivity and adds structural support.
Thakur made a half-cell battery using lithium as the anode and saturated the silicon cathode with 1,000 mAh/g of ions, around three times the saturation in conventional batteries (350 mAh/g) but only a third of the theoretical capacity of silicon. The anodes lasted 600 C/2 cycles (2 hr charge/2 hr discharge) and a projected 700 C/5 cycles, while retaining its 1,000 mAh/g capacity which is on par with regular batteries.
Considering this porous film is easy to synthesize, the powder can be used in roll-to-roll processing and 28% of Earth is silicon, integrating this method into manufacturing processes could be done in a cheap, painless manner.
The team still wants to manufacture a full-cell battery and do more testing with cathode materials. They stated that tests performed with cobalt oxide cathodes have been promising, more experiments to follow I am sure. The research was done through the Lockheed Martin Advance Nanotechnology Center of Excellence at Rice (LANCER).
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