Novel synthesis of nickel-based bimetallic compounds using ammonium tetrafluoroborate and ammonium bifluoride as structure directing agents for energy storage application

Nickel-based compounds are widely applied as the active material for energy storage devices, owing to high theoretical capacity and easy-fabricated process. Bimetallic compounds having multiple redox states can provide abundant redox reactions for charge storage. Structure directing agent (SDA) can promote the preferable growth of active materials and to enhance energy storage abilities. In this work, NH₄BF₄ and NH₄HF₂ are firstly used as SDA to synthesize nickel-based compounds along with 2-methylmidozole in the one-step solution process for energy storage applications. The bimetallic compounds are synthesized by incorporating Al, Mn, Co, Cu and Zn salts in the precursor solution including the Ni salt. The system with Al, Cu or Zn possesses single metal compound and barely nickel compound. Mn and Co-incorporated systems present clear bimetallic components with nickel hydroxide. The highest specific capacitance (C_F) of 1336.3 F/g (297.0 mAh/g) are achieved at 20 mV/s for the NiCo-based electrode, owing to the bimetallic feature and favorable two-dimensional sheet-like structures. An energy storage device is assembled using the NiCo-based compound and reduced graphene oxide electrodes. The maximum energy density of 10.5 Wh/kg at 0.3 kW/kg, and the C_F retention of 78.5% and Coulombic efficiency of 85.2% are achieved after 10,000 times charge/discharge cycles.