Web Date: March 25, 2016
Batteries perform better without the strain
Using high-intensity X-rays to study the crystal structure of a common battery material while it charges and discharges, researchers have filled in new details about how mechanical strain hinders battery performance. These insights could help battery chemists tailor the composition of electrode materials to make better high-power batteries (Nano Lett. 2016, DOI: 10.1021/acs.nanolett.5b05146).
Lithium iron phosphate batteries have been in commercial products for more than a decade. These batteries are particularly well suited to applications like power tools or vehicle drivetrains that require quick bursts of power or fast recharges. Improved versions of these batteries could also expand the availability of microhybrids—a type of energy-efficient car that can turn its engine on and off rapidly at stoplights to save fuel. Batteries for microhybrids need to withstand this frequent switching.
In any lithium-ion battery, the cathode undergoes what’s called transformational strain during use: It expands and contracts in volume as the lithium ions move in and out. To improve on cathode materials, researchers need a more detailed understanding of how the ratio of elemental ingredients influences how the battery handles strain during operation.
Along with Dorthe B. Ravnsbæk of the University of Southern Denmark, and colleagues at Massachusetts Institute of Technology, Yet-Ming Chiang, a battery chemist at MIT and founder of company A123, collaborated with researchers at Argonne National Laboratory to do such a study. The Argonne group has developed an electrochemical cell for studying battery materials with X-ray techniques (J. Appl. Crystallogr. 2012, DOI: 10.1107/S0021889812042720). Using a synchrotron, the team studied different formulations of the cell’s lithium iron manganese phosphate cathode material with X-ray diffraction while operating the battery under high-power conditions. By watching the expansion and contraction of cathodes with differing amounts of manganese, the team found that strain correlated with performance. Cathodes that exhibit less strain can discharge more of the energy they store.
Chiang says battery chemists can keep this connection between strain and performance in mind when developing new materials. For example, he says, potential low-cost cathode materials that currently exhibit lower performance might be improved by adjusting their formulations to reduce strain in the crystal structure.
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