Lithium-rich layered oxide is known to be one of the most promising positive electrode materials for high energy Li-ion batteries. Some publications report specific capacities higher than 250 mAh/g but the fade of the average potential during electrochemical cycling is an important drawback for commercial use of this material. The objective of this work is to study the origin of this potential decay during cycling of lithium-rich compounds. For this purpose, the electrochemical behavior of xLi2MnO3·(1−x)LiNi0.5Mn0.5O2 (with x = 1, 0.7, 0.5, 0.27) materials synthesized using coprecipitation route have been compared. For this study, complete or partial charge and discharge cycles have been performed in the 2.5 V–4.8 V voltage range. The results can be briefly summarized as follows: the potential decay is mainly provoked by several phenomena occurring at low potential, this growth being induced by the charging of lithium-rich at high voltage (>4.15 V). Whatever the compositions, same phenomena are involved, only the kinetics is modified.