High-intensity compact ultrasound assisted synthesis of porous N-doped graphene thin microsheets with well-dispersed near-spherical Ni2P nanoflowers for energy storage

Severe restacking of single-atom-thick two-dimensional graphene and too long ion diffusion path in a thick three-dimensional porous graphene film are both main current problems limiting performance maximization of graphene as an energy storage material. Herein, we report an advance toward the design and fabrication of novel hierarchically porous N-doped graphene thin microsheets (PNGTMSs) with a honeycomb-like network structure allowing good dispersion of near-spherical Ni2P nanoflowers (NFs) by high-intensity compact ultrasound assisted technique. New insights into ultrasonic activation inducing a preferential self-assembly in a planar direction are proposed. High porosity and thin thickness of <500 nm allow the whole interior of isolated microsheets to be ion-accessible, which endows PNGTMSs with advantages of both two-dimensional and three-dimensional graphene as well as enhanced electrical conductivity. Combined with Ni2P nanosheet-built NFs and their good dispersion as well as rich exposed interfaces, Ni2P-PNGTMSs exhibits specific capacitance of 1100 F g−1 at 1.0 A g−1. Asymmetric solid-state device delivers an energy density of 39.8 Wh kg−1 (at 450.6 W kg−1) and remarkable cycling stability with 97% capacitance retention after 5000 charge-discharge cycles. This work provides a convenient and scalable strategy for control over graphene nanostructures and their composites for highly enhanced energy storage.