Integrating ultrathin and modified NiCoAl-layered double-hydroxide nanosheets with N-doped reduced graphene oxide for high-performance all-solid-state supercapacitors

As one class of important electroactive materials, layered double-hydroxide (LDH) nanostructures show great promise for application in the fields of electrocatalysis, secondary batteries, and supercapacitors. Nonetheless, the synthesis of ultrathin multi-metallic-based LDH nanosheets or related nanohybrids (NHs) remains a challenge, and their supercapacitive performances need to be further improved for achieving both high energy and high power densities. Herein, ultrathin and modified NiCoAl-LDH (m-LDH) nanosheets and N-doped reduced graphene oxide (NRG) NHs were synthesized by an alkaline etching of pre-synthesized ultrathin NiCoAl-LDH nanosheets, followed by electrostatic assembly with NRG. The alkaline etching could efficiently modulate the chemical states of the active Ni/Co elements and create more oxygen vacancies in the m-LDH nanosheets. After integrating m-LDH with NRG, the strong interaction or efficient electronic coupling of those two constituents further mediated the surface electronic structure of the m-LDH nanosheets, improving the interfacial charge transport and offering more available electrochemical active sites for surface faradaic reactions. Thus, the obtained m-LDH/NRG NHs manifested greatly enhanced specific capacitance (1877.0 F g^-1 at 1 A g^-1), which was superior to that of pure m-LDH and most other reported electrode materials. Moreover, using such NHs as the positive electrode and activated carbon as the negative electrode, a fabricated asymmetric all-solid-state supercapacitor device delivered a high energy density of 19.9 W h kg^-1 at a power density of 319.8 W kg^-1 together with good cycling stability (76.5% capacitance retention after over 5000 cycles). Remarkably, even at a power density up to 1637.5 W kg^-1, it could still retain an energy density of 13.1 W h kg^-1, superior to recently reported asymmetric supercapacitors devices based on Ni, Co, and other transition metal compounds.