Abstract: The comparative study of LiNi_0.8Co_0.2O₂ and LiNi_0.75Al_0.25O₂ was carried out by X-ray diffraction (XRD) and electrochemical methods. The results show that Co and Al doping suppress the phase transition during charge-discharge. The experiments indicate that LiNi_0.75Al_0.25O₂ has the better cycle-ability and over-charge resistance comparing with LiNi_0.8Co_0.2O₂. The interfacial behavior was studied by use of electrochemical impedance spectroscopy (EIS). The results show that LiNi_0.75Al_0.25O₂ has a slightly larger polarization character than LiNi_0.8Co_0.2O₂.