In this experimental research, in order to obtain the effect of excess lithium in the cathode lithium ionization stage in a nonstoichiometric manner, the cathode precursor with (Ni0.3Mn0.5Co0.2) composition was synthesized as hydroxide using the co-precipitation process. Then, lithium was added with different amounts of lithium hydroxide (LiOH) to investigate the effect of excess lithium in the Li-x(Ni0.3Mn0.5Co0.2)O-2 composition on the electrochemical properties of the cathode. The results of battery charge-discharge tests for all three synthesized samples at rates of 5C-0.5 indicate that the sample Li-1.5(Ni0.3Mn0.5Co0.2)O-2 has the best electrochemical performance. So that at 1C discharge rate, its capacity was 200 mAh/g, and after 30 cycles, its capacity reached 138 mAh/g with a 5C discharge rate. The electrochemical impedance analysis showed that the sample Li-1.5(Ni0.3Mn0.5Co0.2)O-2 has the lowest internal resistance. The results show that increasing the amount of excess lithium is optimal for improving battery performance. In nonstoichiometric mode, increasing the amount of lithium to Li-1.5 has been associated with improved performance and above this value leads to a decrease in battery performance.