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.