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

LiMn 0.8 Fe 0.2 PO 4 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, LiMn 0.8 Fe 0.2 PO 4 /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 LiMn 0.8 Fe 0.2 PO 4 /C cathodes are obtained in the initial lithiation and delithiation process. It is found that the Nyquist plots of LiMn 0.8 Fe 0.2 PO 4 /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.