Friday, May 20, 2011

Nano for batteries

Improved batteries would be of enormous benefit and utility in many sectors of technology.  A factor of 10 improvement in battery capacity (with good charging rate, safety, etc.) would mean electric cars that get 1000 miles per charge, laptops that run for days w/o charging, electrical storage to help with the use of renewable energy, and a host of other changes.  This rate of performance enhancement is completely commonplace in semiconductor electronics and magnetic data storage, yet batteries have lagged far, far behind.

There is real hope that nanostructured materials can help in this area.  Three examples illustrate this well.  Conventional lithium ion batteries have an anode (usually graphitic carbon, into which lithium ions may be intercalated) and a cathode (such as cobalt oxide), with an intervening electrolyte, and a separator barrier to prevent the two sides from shorting together.  A reasonable figure of merit is the capacity of the electrodes, in units of mA-h/g.  The materials described above, anode and cathode, have capacities on the order of 200-300 mA-h/g.  It is known that silicon can take up even more lithium than carbon, with a possible capacity of more than 3000 mA-h/g (!).  Complicating matters, Si swells dramatically when taking in Li, meaning that bulk single-crystal Si cracks and self-pulverizes when taken through a few charge/discharge cycles.  However, Si nanowires have been observed to be much better behaved - they have large surface specific surface area, and have enough free surface to swell and shrink without destroying themselves - see here.  Very recently, this paper has spectacular electron micrographs of the swelling of such nanowires.

A second example:  nanostructured cobalt oxide particles, self-assembled using selectively modified virus proteins, have been put forward as high capacity Li ion battery cathodes.  This approach has also been extended to iron phosphate cathode material.

A third example:  dramatically improved charging rates may be possible using nanostructured electrode geometries, such as these inverse-opal shapes.

There is real hope that nanostructured materials may enable true breakthroughs in battery technology, even though batteries have been studied exhaustively for many decades.  The ability to engineer materials at previously inaccessible scales may bear fruit soon.


DanM said...

Hey Doug,
You ought to have mentioned that your Rice colleagues Lisa Biswal and Mike Wong have demonstrated some incredible results with nanostructured Si battery anodes, exceeding in both capacity and cycles any other published result. It turns out that nano-porous silicon is superior to silicon nanowires for various reasons, including the ability to make good (and non-degrading) electrical contact to the anode material.
Look here for more information.

Doug Natelson said...

Dan - I know, and Lisa and I have been talking a lot about the area. I tried to stick to published papers (rather than patents) on this post, but yes, the nanoporous Si is good stuff!

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Ronnie Deaver said...

I find this very interesting, its more or less going to be my focus once I actually get into college (denied to Rice but trying NY since that are ranked top in nano! Fingers crossed)! I mean a lot of beautiful technologies are being bottle necked by batteries lacking so far behind. Electric cars could run as long as gas cars if they were better, or the Hulk Exoskeleton could be used for military/commercial use if it could have a more reliable source of power, and solar energy and other sources would have a fighting chance if we could store it rather than just throw it all into the grid and watch it dissipate into heat as it travels down the lines. I heard that Rice has made some decent developments on making Nano Cables that could be used for power lines. Imagine! If powerlines were that efficient, energy could be sent without having to boost it or lose as much! We would have soooo much excess that could be stored in huge nano efficient batteries instead of just losing it all! Everyone would be happy :) Thanks for posting this by the way, it confirmed my desire to further my desire to jump into the field of Nanoscale Sciences/Engineering and work on this very issue!

Ronnie Deaver said...

they... not that xD