Electrochemical impedance spectroscopy investigation of the FeF3/C cathode for lithium-ion batteries

The FeF3/C composites were prepared by high energy milling, and its cyclic performance was characterized by discharge–charge measurements. The electrochemical impedance spectra (EIS) of FeF3/C cathode were obtained at different potentials during the initial discharge–charge cycle. The results show that the FeF3/C cathode exhibits an enhanced initial discharge capacity of 710 mAh/g (4.5–1.5 V) but shows a capacity retention of 40% after 100 cycles at room temperature. EIS of the FeF3/C cathode have revealed that the Nyquist plots of the typical EIS consist of two semicircles and one arc in an order of decreasing frequency during lithiation/delithiation. An equivalent circuit was proposed to simulate the experimental EIS data. The kinetic parameters of lithium ion and electron across the porous FeF3/C electrode in the first discharge–charge cycle are discussed in detail, offering some evidence that both the unstable SEI film on the electrode surface and the Schottky contact between metal-fluorides and conductive agents, can eventually affect the cyclic performance.

► We test and analyze the impedance of FeF3/C cathode by EIS.
► EIS consist of three parts (HFS, MFS and LFA).
► HFS is attributed to Li+ transport through the SEI film on the electrode.
► MFS is related to Schottky contact between FeF3 and conductive agents.
► Both SEI film and Schottky contact affect the cyclic performance.