Lithium-Air Batteries

June 26, 2009

The Technology Review, published by MIT, has a story today about a potentially important breakthrough in battery technology.  As those of you who have a laptop or cell phone may have observed, the battery technology of choice for mobile devices today is the lithium-ion battery, because it provides a high energy density.  The voltage produced by any battery is determined by the two materials used as electrodes (for example, carbon and zinc in traditional flashlight batteries).  Lithium is a logical choice for use in a battery, because it has a very high half-cell potential (about 3 volts), but lithium metal is extremely reactive — explosively so with water.  (You may  remember seeing a science class demonstration of sodium or potassium metal being put into water, and the violent reaction that follows.  Lithium is even more reactive.)

One of the ways in which the energy density of a battery can be increased is to reduce its weight.  One possibility for doing this is to use air as one of the reactants.  This is done in some batteries used in hearing aids, for example, which use a zinc-air reaction.  However, these batteries have the disadvantage that they self-discharge over time.  (New zinc-air batteries come with an airtight seal that is removed when the battery is installed.)

A company in Berkeley CA, called PolyPlus has developed a new electrode technology that they claim will make lithium-air batteries a reality.  Their electrode uses lithium metal enclosed in a substance that is impermeable to water, but allows ions to pass through:

PolyPlus has solved this problem by developing what the company calls a “protected lithium electrode.” The device consists of a flat, rectangular piece of lithium metal overlaid on either side with a ceramic electrolyte material called lisicon. The solid electrolyte is impermeable to water but lets lithium ions pass through.

This is potentially quite significant. Using lithium metal as one of the reactants should allow a considerable improvement in energy density:

Lithium-metal batteries approach the energy density of fuel cells without the plumbing needed for these devices; in theory, the maximum energy density is more than 5,000 watt-hours per kilogram, or more than 10 times that of today’s lithium-ion batteries.

Although electricity has many advantages as an energy source, the sticking point in using it, especially for any application that is not fixed in one place, has been how to store it.   Battery technology today has come a long way, but still imposes limitations.  This development has the potential to give a real boost to the use of electric power for transportation.

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