Rich‐Mixed‐Valence Ni<i>_x</i>Co<i>_3−x</i>P<i>_y</i> Porous Nanowires Interwelded Junction‐Free 3D Network Architectures for Ultrahigh Areal Energy Density Supercapacitors

Abstract

Abstract Herein, novel phosphide composites (Ni x Co 3 −x P y ) are reported with well‐defined hexagonal thin‐plate morphology and a hieratically porous but robust junction‐free 3D network constructed by interwelding porous NWs, which are achieved by first synthesizing 2D Ni‐Co precursors with tunable Ni/Co molar ratios and top‐down etching, followed by phosphorization. Owing to enhanced electron/ion transfer, increased availability of active sites/interfaces, rich mixed valences of bimetals and P, and strong intercomponent synergy, the optimized NiCoP‐CoP shows a specific capacitance reaching 1969 F g −1 , much larger than those of Ni‐Co sulfides and other similar phosphides. An asymmetric supercapacitor employing the advanced NiCoP‐CoP as positive electrode exhibits a remarkable cycling stability with 93% retention after 5000 cycles at 8 A g −1 , which is mainly attributed to such architecture allowing strong mechanical stability and to effectively buffer the strain/volume expansion during fast Faradaic reactions. A single device having both an output voltage as high as 7.2 V and an ultrahigh areal energy density of 639 mWh cm −2 at 48 W cm −2 is realized by a designed configuration in which four capacitors in series connection are stacked with solid‐state electrolyte. An almost linear increase in output voltage with the layer number indicates possibility to meet various output requirements for miniaturized electronics.