Nickel hydroxide with a unique α/β mixed phase structure is synthesized by partially substituting Al3+ for Ni2+ with chemical coprecipitation method. The physical properties of the synthesized Al-substituted α/β-nickel hydroxide are characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), thermogravimetry analysis (TG), scanning electron microscopy (SEM), inductively coupled plasma atomic emission spectroscopy (ICP-AES), and tap density testing. The results show that the Al-substituted α/β-nickel hydroxide exhibits an irregular shape and contains many intercalated water molecules. In particular, the tap density of the Al-substituted α/β-nickel hydroxide reaches 2.02gcm−3, which is significantly higher than that of α-nickel hydroxide. The electrochemical performances of the prepared samples are characterized by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and charge/discharge tests. The results demonstrate that the Al-substituted α/β-nickel hydroxide has much higher electrochemical activity, better electrochemical reversibility, lower electrochemical reaction impedance, and higher discharge voltage than the pure β-nickel hydroxide. The specific discharge capacity of the Al-substituted α/β-nickel hydroxide maintains to be 325.4mAhg−1 (or 657.3mAhcm−3) after 100cycles at a charging/discharging current density of 200mAg−1 under ambient temperature.