Multihierarchical Structure of Hybridized Phosphates Anchored on Reduced Graphene Oxide for High Power Hybrid Energy Storage Devices

A multihierarchical structure with (NH₄)­(Ni, Co)­PO₄·0.67H₂O microplatelets and (Ni, Co)₃­(PO₄)₂·8H₂O ultrathin nanopieces anchored on reduced graphene oxide (NCNP/RGO) is synthesized via a mild hydrothermal approach. This unique interface-rich structure is suitable for a high power energy storage device by providing efficient pathways for both electronic conduction and ionic transportation, which are effective ways to improve the electrochemical performance. Specifically, a specific capacity of 993 F g^–1 is obtained in the three-electrode measurement, with ultrahigh capacity retention of 81.2% (807 F g^–1) from 0.5 to 32 A g^–1. The hybrid device constructed with the as-prepared NCNP/RGO as anode and a hierarchical porous carbon (HPC) as cathode offers a very superior energy density of 42.1 Wh kg^–1 at a power density of 73 W kg^–1, which remains 32 Wh kg^–1 at 14 kW kg^–1. Meanwhile, the as-prepared hybrid capacitor exhibits a remarkable cycling stability (96.5% capacitance retention after 10 000 cycles). The capacity contribution of capacitive behavior for the hybrid device is analyzed as 91.1% at 25 mV s^–1.