High-capacitance BiPO4 material with monoclinic/hexagonal crystalline phase heterostructure for aqueous asymmetric supercapacitors

Aqueous asymmetric supercapacitor devices generally have a fairly high power density, but their practical application is still limited by low energy density due to lack of high-capacity electrode materials, particularly anode materials. Herein, a novel anode material, BiPO4 material with monoclinic/hexagonal crystalline phase heterostructure was synthesized by a simple solvothermal approach. This unique heterostructure, composed of hexagonal and monoclinic nanoparticles, exhibits a large specific surface area, numerous active sites and high ion diffusion rate, all of which contribute to an enhanced specific capacitance in the energy storage process. Besides, the heterogeneous interface formed between two different crystalline phase nanoparticles is regarded as an excellent ion channel, accelerating diffusion and reaction of electrolyte ions. The acquired BiPO4 material serves as anode for aqueous supercapacitors, displaying a superior specific capacitance of 954 F g−1 (265 mAh g−1) at 1 A g−1 current density, maintaining up to 600 F g−1 (166.7 mAh g−1) at 10 A g−1. In order to match the BiPO4 anode, high-capacity (3662 F g−1 (508.61 mAh g−1) at 1 A g−1) cobalt-nickel phosphate micron-sheets (NiCo2(PO4)2) synthesized via hydrothermal method followed by calcination under argon were selected as cathode material. The assembled aqueous asymmetric supercapacitor employing NiCo2(PO4)2 as cathode and BiPO4 as anode yields a high energy density of 98.17 Wh kg−1 at a power density of 846.49 W kg−1.