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Batteries have come a long way since their invention more than 200 years ago, but they still have limitations. On the plus side, they tend to have a high energy density — that is, they last a long time. However, they also have a low power rate, making them slow to both charge and discharge. This means that they cannot deliver bursts of power, such as those needed for rapid acceleration of a car. Gerbrand Ceder, a materials scientist at the Massachusetts Institute of Technology in Cambridge, and his graduate student Byoungwoo Kang wanted to see whether they could create high-energy-density lithium batteries that also have a high power rate. Rapid charging and discharging was thought to be possible only using a device called a supercapacitor, but these have lower energy densities, and are costly and large, typically 50 to 100 times the size of a battery. Through theoretical modelling, the two calculated that they could get high power from a battery (see page 190). Ceder tells Nature how they tested their theory.

How did you get from theory to reality?

When you charge or discharge a lithium battery, you're moving lithium ions back and forth between two electrodes inside it. According to our model, using a common lithium ion as the charge carrier was very fast. But in our experiment, the lithium's movement was really slow. So it was being limited by something. We found that the surface reaction by which the lithium transfers from the electrolyte — an electrically conductive medium that the ions flow through — into the active material that stores the charge was the slow part. So we needed to increase the rate at which the lithium could move across the electrolyte's surface.

How did you accomplish that?

We changed the composition of a common electrode, lithium iron phosphate. Our calculations indicated that we needed a different ratio of iron, phosphorus and oxygen, and that the mix needed to be heated.

What was the result of these changes?

We ended up with a glassy top layer of lithium pyrophoshate that allowed the lithium ions to move around ultrafast. I didn't believe the results when I first saw them. I don't think I've ever seen anything do that, and I've been working with batteries for 14 years.

What does this mean for commercial batteries?

Hybrid cars could have smaller, lighter engines because their smaller batteries could deliver more energy and accelerating power. Less weight in the car would also mean better fuel economy.