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A battery mystery solved

Experiments with x-ray diffraction explain why an important cathode material works as well as it does.

When a lithium-ion battery is charged or discharged, Li+ ions move between the electrodes inside the battery and are balanced by the electron flow in the external circuit. Good electrode materials must therefore reversibly store and release Li+ over many charge–discharge cycles. In one attractive candidate, lithium iron phosphate, Li+ moves into and out of voids in the FePO4 lattice without changing the lattice framework; batteries with LiFePO4 cathodes have been commercialized for use in electronics and electric cars. However, calculations and experiments show that a partially charged electrode at equilibrium segregates into almost pure regions of LiFePO4 and FePO4; intermediate phases of LixFePO4 are thermodynamically unstable. That phase separation should limit the rate at which Li+ can be inserted or removed, but nanostructured LiFePO4 electrodes can be made to charge or discharge much more quickly than expected. That discrepancy has now been resolved by two independent groups: one led by Clare Grey (Cambridge University in the UK) and the other by Marnix Wagemaker (Delft University of Technology in the Netherlands). Both groups, working at synchrotron facilities, used time-resolved x-ray diffraction to study the structure of LiFePO4 electrodes charged at different rates. As shown in the figure (adapted from the paper by X. Zhang et al.), when the electrode was charged slowly, both teams saw discrete diffraction peaks corresponding to LiFePO4 and FePO4; as one peak grew, the other shrank. At faster charging rates, though, the diffraction signal was smeared between the two peaks, indicative of a nonequilibrum region of LixFePO4. Learning more about the material’s behavior away from equilibrium could lead to better designs for high-performance batteries. (H. Liu et al., Science 344, 6191, 2014; X. Zhang et al., Nano Lett. 14, 2279, 2014.)

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