Sodium-deficient nickel–manganese oxides with a layered type of structure are, nowadays, of great interest as electrode materials for both lithium- and sodium-ion batteries since they are able to intercalate lithium and sodium ions reversibly within a broad concentration range. Herein, we report new data on the effects of the particle sizes and of the electrolyte salt on the intercalation properties of Na2/3Ni0.5Mn0.5O2 with a P3-type of structure. The morphology of layered Na2/3Ni0.5Mn0.5O2 oxides has been varied by changing the type of the precursor used: from Na–Ni–Mn acetates to Na–Ni–Mn mixed nitrate acetates. The structure, particle dimensions, and particle size distribution of oxides have been determined by means of powder X-ray diffraction (XRD), transmission electron microscopy (TEM), dynamic light-scattering measurements, and X-ray photoelectron spectroscopy (XPS). The intercalation properties of Na2/3Ni0.5Mn0.5O2 have been studied in model electrochemical cells versus Li metal as the anode. We used two kinds of lithium salts dissolved in organic solutions as the electrolytes: 1 M LiPF6 in EC:DMC and 1 M LiBF4 in EC:DMC. The mechanism of the lithium intercalation into Na2/3Ni0.5Mn0.5O2 is discussed on the basis of e x situ XRD, HRTEM, and X-ray photoelectron spectroscopy analyses. It has been discovered that the lithium salt in the electrolyte salt contributes to the mechanism of the electrochemical reaction, while particle dimensions determine the capacity stability during continuous cycling, as well as the surface reactivity of oxide electrodes.