Wednesday, August 30, 2006

How Future Batteries Will Be Longer-Lasting and Safer

Continued from page 1

By Kevin Bullis

TR: How much better can batteries get?

YC: One thing we have to keep in mind is you can't really conceive of anything like Moore's Law for electrochemical energy storage. Moore's Law was based on being able to perform similar functions [for computing] using either fewer electrons or, more recently, fewer photons. But energy is constrained by chemistry and the periodic table. Expecting Moore's Law from battery chemistry is like expecting steel next year to weigh half as much and be twice as strong.

People who are working on better batteries are very optimistic. There's definitely room for growth; there are many avenues for improvement. If you look at it realistically, I'd say a factor of two improvement in the next decade is quite realistic. A factor of 10 is not.

TR: As a materials scientist, what can you do to increase battery capacity?

YC: In the order of things you can do, you first have [to increase] the voltage. A higher voltage system will have higher energy, because the energy is the capacity of the battery times the voltage.

The second [option is to find] new host materials that can pack more ions into a given space or weight.

A third option is to increase the charge per ion that's transported, which is a more difficult challenge. Basically, if you have the same storage capacity (the same number of ions being stored) and the same voltage, if you had a divalent cation such as magnesium, you would have twice the energy of the lithium counterpart. But the difficulty is that the materials that would make a magnesium-based battery work have not yet been developed. And physically there have been concerns, for example, over the rate at which you could move [the magnesium].

TR: There seem to be a lot of options. What has kept progress on batteries relatively slow?

YC: The key challenge is meeting all of the multiple demands. For instance, there are active materials that have been designed that have higher voltages. The difficulty then is that the rest of the system cannot keep up. In particular, electrolytes are not available that will function over the necessary periods of time at those high voltages. There are smart chemists who are working on designing electrolytes that will operate at these higher voltages, but we're not there yet. That's one of the limitations.

TR: Given these difficulties, would we be better off focusing on other ways of storing energy for portable electronics, such as fuel cells?

YC: I think fuel cells are definitely worth studying. There's no arguing with the metrics that suggest the run-time that you can get from devices is currently higher for fuel cells than the battery chemistries we have today.

Even though on an energy density basis they still look promising, there are a number of engineering challenges. [One concern is] the byproduct that comes out of fuel cells, water, for example, or carbon dioxide. A battery doesn't have any chemical byproducts that come out of the battery.

And then there's the fuel itself. If you look at what you can bring on an airplane now, that may cause some additional concerns for fuel cells.

TR: Are you and others in the battery community concerned that the recent recalls will give lithium-ion batteries a bad name?

YC: There is both concern and opportunity. On the one hand, I don't think that this realistically challenges the use of batteries in portable devices. But it does provide additional impetus to accelerate the research and development in this area.