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Saturday, December 3, 2011

New and improved lithium ion batteries



Lithium ion batteries are a common component of many consumer electronic products, such as cell phones.  Unfortunately, current lithium ion batteries don’t last as long as we’d like.  Innovations by Harold Kung and his colleagues from Northwestern University may soon change that.  They’ve developed a battery that lasts ten times as long.  It recharges much faster as well.

Briefly, lithium ion batteries work by shuttling lithium ions from the anode (negative end) to the cathode (positive end). Once all the lithium atoms have arrived at the cathode, the battery is dead and must be recharged by reversing the flow (which is done by applying a higher voltage to the battery).  How long a battery lasts is limited by the number of lithium ions available to move through that battery.

Currently, anodes are constructed from layers of graphene, which is a form of carbon.  Although carbon has the advantage of being stable, it takes six carbon atoms to hold each lithium ion.  In contrast, you can pack four lithium ions onto every atom of silicon, a considerable improvement.  Regrettably, pure silicon tends to shatter during charging.  The solution? Combine the two materials.

The researchers placed clusters of silicon between layers of graphene.  The new anode can hold many more lithium ions than plain graphene, but is much more stable than pure silicon.  By adding some silicon to the anode, the scientists have created a much more powerful battery. 

Not only have the Northwestern researchers greatly increased the charge capacity of their batteries, but they also decreased the recharging time required.  As I said, lithium ion batteries are recharged by forcing the flow of the lithium ions back from the cathode to the anode.  The scientists effectively gave those ions a shortcut by cutting tiny holes in the graphene/silicon lattice through which the ions could plunge.

Taken together, we may soon have a battery that lasts much longer and recharges much more quickly than anything on the market today.  Well, I say soon, but the authors predict that it will be at least three years.