Rational Design and General Synthesis of High-Entropy Metallic Ammonium Phosphate Superstructures Assembled by Nanosheets

Three-dimensional (3D) superstructure nanomaterials with special morphologies and novel properties have attracted considerable attention in the fields of optics, catalysis, and energy storage. The introduction of high entropy into ammonium phosphate (NPO·nH₂O) has not yet attracted much attention in the field of energy storage materials. Herein, we systematically synthesize a series of 3D superstructures of NPOs·nH₂O ranging from unitary, binary, ternary, and quaternary to high-entropy by a simple chemical precipitation method. These materials have similar morphology, crystallinity, and synthesis routes, which eliminates the performance difference caused by the interference of physical properties. Subsequently, cobalt-nickel ammonium phosphate (Co_xNi_y-NPO·nH₂O) powders with different cobalt-nickel molar ratios were synthesized to predict the promoting effect of mixed transition metals in supercapacitors. It is found that the Co_xNi_y-NPO·nH₂O 3D superstructures with a Co/Ni ratio of 1:1 show the best electrochemical performance for energy storage. The aqueous device shows a high energy density of 36.18 W h kg^-1 at a power density of 0.71 kW kg^-1, and when the power density is 0.65 kW kg^-1, the energy density of the solid-state device is 13.83 W h kg^-1. The work displays a facile method for the fabrication of 3D superstructures assembled by 2D nanosheets that can be applied in energy storage.