Electrochemical Impedance Spectroscopic Study of the Electronic and Ionic Transport Properties of NiF2/C Composites

Abstract

NiF2/C composites were prepared through high energy mechanical milling, and the cyclic performance was characterized by discharge-charge measurements. Electrochemical impedance spectra (EIS) for NiF2/C electrode were obtained at different potentials during initial discharge-charge cycle. The results revealed the typical EIS characteristics appeared with three semicircles in the Nyquist diagram at 1.6 V. An equivalent circuit was proposed to simulate the experimental EIS data, and it was found that the high frequency semicircle (HFS) is mainly caused by lithium-ion through the SEI film on the electrode surface, the middle frequency semicircle (MFS) should be related to the Schottky contact between NiF2 and conductive agents, which may be the important feature of such composites materials with big band gap, Besides the low frequency arc (LFA) was observed as the charge transfer step. Consequently a modified model was proposed to explain the impedance response of NiF2/C electrode for lithium ion batteries.