The electroanalytical behavior of LixNiO2 and LixCo0.2Ni0.8O2 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 LixCo0.2Ni0.8O2 exhibits a single-phase reaction upon charge in the 3.0-4.08 V (versus Li/Li+) range, Li intercalation into LixNiO2 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 Frtmkin-type intercalation isotherms with shea-range attraction interactions among intercalation sites.