Composition, microstructure and performance of cobalt nickel phosphate as advanced battery-type capacitive material

Highlights: • Cobalt nickel phosphates are prepared. • The composition and microstructure are modulated with variation of Co/Ni ratio. • The electrode properties as battery-type material are optimized. • The flower-like CoNi₂(PO₄)₂ shows high-performance hybrid capacitor application. A series of cobalt nickel phosphates are prepared via a mild chemical precipitation method followed by calcination at a low temperature. With variation of Co/Ni molar ratio, the compositions and microstructures of the as-prepared samples are modulated and their electrochemical performances as battery-type materials are optimized. Electrochemical measurements by cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) technique and electrochemical impedance spectroscopy (EIS) reveal that the flower-like CoNi₂(PO₄)₂ with the molar ratio of Co/Ni = 1:2 exhibits a high specific capacity of 630.4 C g⁻¹ at the current density of 1 A g⁻¹, and an excellent cycling stability of 84.3% capacity retention after 1000 cycles. Furthermore, a hybrid supercapacitor, fabricated with the CoNi₂(PO₄)₂ as positive and graphene as negative electrode material, exhibits a high specific capacitance of 103 F g⁻¹ at the current density of 1 A g⁻¹, along with a high energy density of 32.2 Wh kg⁻¹ at the power density of 377.6 W kg⁻¹, and even 21.7 Wh kg⁻¹ at 4.9 kW kg⁻¹. These figures demonstrate that cobalt nickel phosphates as electrode materials with the optimized composition and microstructure may hold promising application in energy storage devices.