A battery mystery solved

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 Li_xFePO₄ 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 Li_xFePO₄. 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 .)