Defect-Engineered 3D Cross-Network Co₃O_4–xN_x Nanostructure for High-Performance Solid-State Asymmetric Supercapacitors

A 3D cross-network Co3O4–xNx nanostructure is obtained from the cobalt–alanine complex (Co(C3H5O2N)2) by amino acid self-assembly and subsequent pyrolysis. In this process, Co(C3H5O2N)2 forms a 2D sheet structure by intermolecular self-assembly; then, the 2D sheet structure self-assembles to form a 3D cross-network nanostructure, which undergoes pyrolysis to form Co3O4–xNx. The resulting Co3O4–xNx has a 3D cross-network nanostructure with a high specific surface area and suitable pore diameter. The unique 3D cross-network nanostructure of Co3O4–xNx can supply a large amount of active sites, and the pores can promote ion migration, which can reduce volume expansion during cycling. Importantly, the amino acid self-assembly strategy is performed at the molecular level so that the N atoms in alanine (C3H7O2N) migrate in the molecule to form in-situ doping and simultaneously create defects. Defect engineering can facilitate rapid transport of electrons in the electrode, further effectively improving the conductivity of the material, which has been further proved by density functional theory calculations. N-doping can also improve the hydrophilicity of the material. Hence, the Co3O4–xNx-300 electrode shows high specific capacitance (666 Fg–1 at 1 Ag–1) and outstanding cycling stability (a capacitance retention rate of 74.1% after 10,000 cycles). In addition, an all-solid-state asymmetric supercapacitor based on the Co3O4–xNx-300 cathode and active carbon anode displays a high energy density of 33.2 W h kg–1 at 751.1 W kg–1 and long cycling stability with a capacitance loss of 15.8% after 5000 cycles. All the experiment results show that the Co3O4–xNx material has great potential as a supercapacitor material.