Combined operando X-ray diffraction–electrochemical impedance spectroscopy detecting solid solution reactions of LiFePO4 in batteries

Lithium-ion batteries are widely used for portable applications today; however, often suffer from limited recharge rates. One reason for such limitation can be a reduced active surface area during phase separation. Here we report a technique combining high-resolution operando synchrotron X-ray diffraction coupled with electrochemical impedance spectroscopy to directly track non-equilibrium intermediate phases in lithium-ion battery materials. LiFePO 4 , for example, is known to undergo phase separation when cycled under low-current-density conditions. However, o perando X-ray diffraction under ultra-high-rate alternating current and direct current excitation reveal a continuous but current-dependent, solid solution reaction between LiFePO 4 and FePO 4 which is consistent with previous experiments and calculations. In addition, the formation of a preferred phase with a composition similar to the eutectoid composition, Li 0.625 FePO 4 , is evident. Even at a low rate of 0.1 C , ∼20% of the X-ray diffractogram can be attributed to non-equilibrium phases, which changes our understanding of the intercalation dynamics in LiFePO 4 . A clear understanding of the phase behaviour of lithium-ion electrode materials is essential for the development of the field. Here, the authors report a combined X-ray diffraction–electrochemical impedance spectroscopy method to detect solid solution reactions of LiFePO 4 .