A 3D cross-network Co 3 O 4−x N x nanostructure is obtained from the cobalt−alanine complex (Co(C 3 H 5 O 2 N) 2 ) by amino acid self-assembly and subsequent pyrolysis. In this process, Co(C 3 H 5 O 2 N) 2 forms a 2D sheet structure by intermolecular selfassembly; then, the 2D sheet structure self-assembles to form a 3D cross-network nanostructure, which undergoes pyrolysis to form Co 3 O 4−x N x . The resulting Co 3 O 4−x N x has a 3D cross-network nanostructure with a high specific surface area and suitable pore diameter. The unique 3D cross-network nanostructure of Co 3 O 4−x N x 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 (C 3 H 7 O 2 N) 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 Co 3 O 4−x N x -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 Co 3 O 4−x N x -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 Co 3 O 4−x N x material has great potential as a supercapacitor material. KEYWORDS: supercapacitor, amino acid self-assembly, Co 3 O 4−x N x , defect-engineered, all-solid-state asymmetric device
In this work, a series of LiNi 0.8 Co 0.1 Mn 0.1 O 2 oxides with dual-functional heterostructure are acquired from a trace tungsten (W) modification strategy; such a heterostructure contains a gradient tungsten distribution structure and a fast ion-conductive Li x WO y coating layer that are in situ formed via thermodynamic diffusion during the calcination process. The gradient doping can increase the lattice parameters because of the large radius of tungsten, then further to expand interlayer spacing and facilitate the lithium-ion diffusion coefficient which can enhance the capacity and rate performance. Besides, the strong W−O bonds can improve the stability of the lattice and promote the structural integrity and thermostability. Meanwhile, the fast ion-conductive Li x WO y layer can not only facilitate the rate of Li + deintercalation, but also act as a defensive layer to suppress side reactions and then improve the cycling property. As a result of the dual-functional heterostructure, the W-modified LiNi 0.8 Co 0.1 Mn 0.1 O 2 with the mass fraction of 4500 ppm (W4500) shows a superior discharge capacity of 165.5 mAh g −1 at 5.0 C and an enhanced cycle retention of 88.4% (25 °C) after 100 cycles. Especially, the positive electrode of W4500 delivers a better structural integrity than that of pristine. The investigation elaborates the characters of the dual-functional tungsten modification and offers a path to prepare better Ni-rich layered oxides for lithium-ion batteries.
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