The electroanalytical behavior of Li x NiO2 and Li x Co0.2Ni0.8O2 was studied by simultaneous application of slow-scan rate cyclic voltammetry (SSCV), potentiostatic and galvanostatic intermittent titration (PITT and GITT), and electrochemical impedance spectroscopy (EIS). Application of a finite-space diffusion model for treating the results obtained by these techniques allowed us to calculate the diffusion coefficient of Li ions (D) and the differential (incremental) capacity (C int) as functions of the electrode’s potential. Our final purpose was to compare D versus E and C int versus E plots for both the electrodes, in order to correlate the observed difference in their electroanalytical behavior with the clear distinction in the related Li-insertion mechanisms deduced from XRD studies. While Li insertion into Li x Co0.2Ni0.8O2 exhibits a single-phase reaction upon charge in the 3.0–4.08V (versus Li/Li+) range, Li intercalation into Li x NiO2 undergoes two-phase transitions in the same potential range. The shape of both plots, D versus E and C int versus E for these electrodes, is discussed in the framework of a finite-space diffusion model and Li-insertion processes that can be described by Frumkin-type intercalation isotherms with short-range attraction interactions among intercalation sites.