Electrochemical impedance spectroscopy of Li-ion negative electrodes based on titanium oxyfluoride and titanium trifluoride

The processes of lithium insertion into TiOF2 and TiF3 electrodes were studied using electrochemical impedance spectroscopy. The measurements were carried out at different degrees of lithiation during the first two charge/ discharge cycles. Electrical equivalent circuits were proposed to model the resulting impedance spectra. They reflect the processes of Li+ migration through SEI, lithium ion insertion/extraction stages with the change of titanium oxidation degree and lithium ion diffusion in the material volume. A comparative evaluation of kinetic parameters of TiF3 and TiOF2 electrodes depending on the degree of lithiation was carried out for the first time. It is shown that the main process of SEI formation takes place at the first cycle. SEI has a lower impedance value on TiF3 electrodes. The concentration dependence of the charge transfer resistance and diffusion coefficient for TiF3 and TiOF2 materials is observed: with increasing lithium content the interfacial charge transfer resistance decreases and the Li+ diffusion coefficient increases. The concentration dependences of these parameters for TiOF2 electrodes are more pronounced.