The intercalation process of small cations from both aqueous and non-aqueous electrolytes in nonstoichiometric nickel oxide NiO x thin-film electrodes and the related electrochromic effect have been studied quite consitently. The electrochemical and optical properties clearly indicate that nickel oxide thin films can be proposed as new, highly reversible electrochromic electrodes for the realization of improved technological application (display, optical active filters and smart windows). Nevertheless, the exact nature and mechanism of the electrochromic process in these films is not yet fully clear and it has appeared to us of importance to complete our previous work in the area by examining in more details the electrochemical intercalation process of lithium ions in the NiO x host structure, as well as the chemical-physical characteristics of the lithiated Li y NiO x compounds.