The growing demand for portable electronic devices with power-hungry features has led manufacturers to invest in the highest-capacity batteries. But packing so much energy into a small package can be dangerous, as shown by the recent, massive recalls of Sony lithium-ion batteries for Dell and Apple laptops.
We asked MIT materials scientist and battery expert Yet-Ming Chiang, who cofounded the battery startup A123 Systems of Watertown, MA, what makes designing safe, high-energy batteries so difficult–and whether battery capacity can continue to improve without consumer products spontaneously bursting into flames.
Technology Review: Portable electronic devices have been improving quickly, but batteries haven’t been improving nearly as fast. And now we’ve learned that some of the highest-performing batteries can be dangerous. Why has it been so difficult to design high-capacity, yet safe batteries?
Yet-Ming Chiang: One reason it’s hard is that the chemistries that have been most desirable from the energy density point of view require redundant safety systems. But that often comes at the expense of either lower energy or higher manufacturing cost, because you then add protective components that take up some of the space.
TR: So how can batteries be improved?
YC: There really are two routes to as-high or higher energy systems that are safer and lower cost. One is better control of manufacturing quality. Based on what I’ve read in the press about these two recalls [Dell and Apple], it was a manufacturing problem that resulted in metal particles that created some internal short [circuit] problems. So that is quite simply a manufacturing issue.
The alternative approach is to try to make the chemistries intrinsically safe, or at least safer. People are working on this in many laboratories around the world.
Even [with] the current materials that have been used up until now, the general trend is toward alloys and modified compositions that are safer than what had been used in the past. And then there are the more radical changes in chemistry, such as the phosphate chemistry that you just wrote about [in “Safer Lithium-Ion Batteries”].
TR: But the phosphate-based batteries sacrifice energy storage. Are there materials that promise to both increase capacity and safety? Or are higher-capacity batteries inevitably more dangerous?
YC: It’s not inevitable. Having more electrical energy, you can always think of that as having more energy to dissipate. But the difference in safety of different systems is so chemistry specific, so element specific, that it’s possible to have a higher-energy-density system that is at the same time safer.
As an example, if you look on the negative electrode side, there are some tin- and silicon-based alloys that are being studied that will store more lithium per volume by a considerable margin, but do not appear to be any less safe.
TR: But for that to be of any use, you’ve also got to have the positive electrode with a higher capacity. On that side, are there examples of high-capacity, safe chemistries?
YC: I’m going to be a little cryptic and say that we believe there are. There are definitely materials systems that we are interested in on that side, that we believe can be both higher energy and safer.
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.