Impedance studies of LiMn0.8Fe0.2PO4/C cathodes for lithium-ion batteries

LiMn0.8Fe0.2PO4 by coating with carbon and reducing the particle size to nanometric scale can be used as a superb cathode material for lithium-ion batteries. In this study, LiMn0.8Fe0.2PO4/C composite is synthesized by high-energy milling method, and the charge and discharge test results show that it has high capacity and good cycle stability. The electrochemical impedance spectra (EIS) of LiMn0.8Fe0.2PO4/C cathodes are obtained in the initial lithiation and delithiation process. It is found that the Nyquist plots of LiMn0.8Fe0.2PO4/C cathodes mainly consist of three parts in the lithiation and delithiation process, namely, high-frequency semicircle, middle-frequency semicircle, and low-frequency line. The above three parts are assigned to Schotty contact problem, charge transfer step, and lithium-ion solid-state diffusion process, respectively. A quantitative analysis of the variations of the resistance and barrier capacitance of Schotty contact with the polarization potential of the electrode and a reasonable explanation is given, as well as the resistance and space charge layer capacitance of charge transfer. Moreover, the change of n (a parameter in constant phase angle element (CPE) with a value between − 1 and 1) value is briefly discussed.