Characterization of different negative electrodes (Li or Li4C6) cycled vs. CoO was performed on three-electrode cells by electrochemical impedance spectroscopy and cyclic voltammetry. The unexpected behavior of the negative electrode, especially the electrolyte reactivity occurring at the end of CoO discharge, was correlated with the reactions, taking place at the positive electrode. This electrolyte involvement affects the electrochemical behavior of the negative electrode, and, more precisely, the charge transfer process. which becomes limited. Nevertheless, the latter is recovered on the following positive electrode oxidation. These observations are discussed and assumptions are made to explain the surprising and complex cobalt oxide system.