Common electroanalytical behavior of Li intercalation processes into graphite and transition metal oxides

This paper compares the electroanalytical behavior of lithiated graphite, Li₂CoO₂, Li₂NiO₂, and Li₂Mn₂O₄ spinel electrodes. Slow scan rate cyclic voltammetry (SSCV), potentiostatic intermittent titration (PITT), and electrochemical impedance spectroscopy (EIS) were applied in order to study the potentiodynamic behavior, the variation of the solid-state diffusion coefficient, and the impedance of these electrodes. In addition, X-ray diffractometry and Fourier transform infrared (FTIR) spectroscopy were used in order to follow structural and surface chemical changes of these electrodes upon cycling. It was found that all four types of electrodes behave very similarly. Their SSCV are characterized by narrow peaks which may reflect phase transition between intercalation stages, and the potential-dependent Li chemical diffusion coefficient is a function with sharp minima in the vicinity of the CV peak potentials, in which the differential electrode capacity is maximal. The impedance spectra of these electrodes reflect an overall process of various steps in series. These include Li⁺ ion migration through surface films, charge transfer which depends strongly on the potential, solid-state diffusion and, finally, accumulation of the intercalants in their sites in the bulk of the active mass, which appears as a strongly potential-dependent, low-frequency capacitive element. It is demonstrated that the above electroanalytical response, which can be considered as the electrochemical fingerprint of these electrodes, may serve as a good in situ tool for the study of capacity fading mechanisms.