A systematic way to find battery materials

Lithium-ion batteries have become a leading energy source for everything from smartphones and laptops to power tools and electric cars, and researchers around the world are actively seeking ways to nudge their performance toward ever-higher levels. Now, a new analysis by researchers at MIT and the University of California at Los Angeles (UCLA) has revealed why one widely used compound works particularly well as the material for one of these batteries' two electrodes - an understanding they say could greatly facilitate the process of searching for even better materials.

Lithium-ion batteries' energy and power density - that is, how much electricity they can store for a given weight, and how fast they can deliver that power - are determined mostly by the material used for the cathode (the positive electrode). When such batteries are being used, lithium atoms are stored within the crystal structure of the cathode; when the battery is being recharged, lithium ions flow back out of it. Many different materials are currently used for these cathodes.

But one of those materials has been a bit of a mystery. Lithium iron phosphate (LiFePO4) performs well as a cathode, but this performance has been hard to explain because unlike other cathode materials, lithium ion phosphate seemed to require a two-phase process to store lithium - something that should theoretically reduce its efficiency, but for some reason does not.

That anomaly has now been explained. A more detailed analysis showed that, in fact, the compound was following a single-phase process after all, but doing so in an unusual way - which might point the way to discovery of many other such compounds that had previously been overlooked. The new analysis was carried out by Gerbrand Ceder, the Richard P. Simmons (1953) Professor of Materials Science and Engineering at MIT, his graduate student Rahul Malik, and postdoc Fei Zhou of UCLA, and published in the journal Nature Materials.

According to accepted theory, lithium iron phosphate ''should have been a low-rate'' cathode material, Ceder says - meaning that it could produce electricity only at a very low current, suitable for use with very-low-power devices. Instead, ''it has become one of the highest-rate materials in use,'' something that ''always puzzled us,'' he says.

Most cathode materials are porous, absorbing lithium ions during charging like water going into a sponge. But it was thought that lithium iron phosphate required a two-phase process, first forming one compound, which then morphed into a final, stable compound. The extra step was expected to add complexity and reduce the reaction's speed.