Construction Of Hierarchical Nano-arrays And Solid-solution-like Composites And Their Electrochemical Energy Storage

The explosive growth of portable electronic devices and the increasing demand for hybrid electric vehicles require the development of high-performance energy storage systems such as lithium ion batteries（LIBs） and supercapacitors（SCs）. The development of high performance energy storage devices depends on the progress of the electrodes. Furthermore, the electrode material is one of the key factors that determines the performance of the energy storage devices. Up to now, commercial graphite and carbon black have been widely used for the current LIBs and SCs anodes. However, the energy storage capability of the carbon-based materials is limited by their energy storage mechanism, which greatly restrict the futher improvement of the specific capacity of the LIBs and SCs. In order to achieve energy storage devices with both high capacity and high rate performance. One one hand, we can optimize the structure of the existing material system to upgrate the capacity, rate performance and cycling performance; on the other hand, we can develop new electrode materials with high capacity to replace the traditional materials. The transition metal oxides and tin-based electrode materials own high theoretical capacities which is almost 2-3 times as high as that of carbon-based materials. They may be the candidates for carbon. Unfortunately, the cycling stability and the conductivity are unfavorable. Minimizing the size of the paticles, constructing the porous strcture and uniformly coating the nano-carbon layer may address the above probl ems. In addition, the important development trend of the portable electronic products is lightweight and flexible. Energy storage devices as the key parts of the portable electronic products play the most important role. Hence, it is urgent to develop ener gy storage devices with superior mechanical flexibility and stability.In view of the above bottleneck problem,the present dessertation will focus on the design of micro-nano structure, synthesis methodology, synthesis mechanism, controllable preparation and structure-activity relationship of electrode materials. The main research results are summarized as follows:（1） Uniform core–shell ZnO-ZIF-8 NRAs were achieved after the in situ ion exchange process, ZnO/C core–shell nanostructure was fabricated by su bsequently annealing in inert gas atmosphere. During the thermal decomposition process, the shell layer ZIF-8 was transformed into amorphous carbon framework and ZnO nanoparticles, resulting in a stronger adhesive force between carbon and ZnO nanocrystals. ZnO/C core–shell nanostructure displayed an excellent cycling stability, the anode still had a reversible discharge capacity of 699 m A h g-1 after 100 cycles, reaching 89% of the first discharge capacity. Moreover, the electrode showed excellent reversibility, resulting in a high first-cycle Coulombic efficiency of around 92%.（2） Sub-5 nm Sn nanocrystal uniformly dispersed in nitrogen doped carbon fibers were synthesized by a simple and reliable electrospinning and carbon thermal reduction process. Ultrafine-Sn/C electrode exhibited a high reversible capacity of 887 m A h g-1 at a current density of 0.1 A g-1 after 200 cycles. Moreover, it showed good rate capability even when cycled at 0.2 A g-1 about 685 m A h g-1 after 500 cycles and 508 m A h g-1 at 0.4 A g-1 after 200 cycles. Furthermore, the synthetic methods can be directly used to produce transition metal oxides /C in large scale.（3） We proposed and demonstrated a solid-solution-like（SSL） composites concept in which the ZnO in molecular level atomically “dissolved” in carbon matrix. The effects of different annealing processes on the structure of the products were compared. The amorphous SSL ZnO/C hybrids result in high capacity by creating the most possible metal oxide/carbon interfaces and defects. Mean while, all atomic-scale ZnO are fully protected by carbon matrix, which thoroughly eliminate the swelling of metal oxides during the charge/discharge process, the eletrode showed excellent cycling stability. Moreover, the electrochemical performance of the SSL ZnO/C electrode superiored to that of crystalline-ZnO NPs@C. The reversible capacity of SSL ZnO/C NFs electrode maintained at 813.3 m A h g-1 after 100 cycles with the decrease rate of 0.4% per cycle. Moreover, the discharge capacity still maintained 53.5% of the original value, even when the current density increased to 4 A g-1.（4） Three-dimensional mesoporous Ni O nanostructures were synthesized by a simple nickel alkoxide-mediated self-assembly route and subsequent calcination process. The formation mechanism of mesoporous nano spheres was speculated. It can be inferred that several factors（surface area, grain size, crystallinity and stru cture stability） are competing with each other, which may affect the lithium-ion storage performance. The sample anealed at 500°C exhibited a high initial reversible capability of 1220 m A h g-1 at the current density of 0.1 A g-1. It could deliver a superior cycling performance, the capacity were maintained at 518 m A h g-1 in the 60 th cycle.（5） Hierarchical Co₃O₄@Ni Co₂O₄ nanowire arrays were synthesized via a facile hydrothermal process and subsequent co-electrochemical deposition process. The as-fabricated core–shell heterostructured array electrode delivered high areal capacitance. When the current density increased from 10, 20 to 30 m A cm-2, then gradually turned back to 30 m A cm-2, 83.7% of the initial areal capacitance could be recovered（1.18 F cm-2）. The great performance of this hierarchical architecture can be understood from the following two aspects. On the one hand, Ni Co ₂O₄ nanoflakes enhanced the diffusion effiency of ion, and prov ided more reaction sites for the redox reaction. And on the other hand, the further introduction of Ni Co ₂O₄ reduced the charge-transfer resistance of Co₃O₄, leading to fast electron transport within active materials.（6）A novel and scalable synthesis approach to produce hierarchically aligned porous carbon nanotube arrays（PCNTAs） on flexible carbon fibers（CFs） was developed. The PCNTAs were obtained by catalytic conversion of ethanol on ZnO nanorod arrays and then reduction-evaporation of ZnO nanorods, resulting in uniform and controllable wall thicknesses of the final PCNTAs. The three-dimensional arrangement, the diameters and the lengths of the PCNTAs c ould be tuned by adjusting the synthesis protocols of the ZnO nanorod arrays. The PCNTAs@CFs exhibited a high specific capacitance of 182 F g-1 at 40 A g-1（188 F g-1 at 20 A g-1） in 6 M KOH. The symmetric supercapacitor showed an excellent cycling stability with only 0.0016% loss per cycle after 10,000 continuous cycles at the current density of 12 A g-1. These excellent electrochemical performances were ascribed to the unique structural design of hierarchical PCNTAs which provide d not only appropriate channels for enhanced electronic and ionic transport, but also increased surface area for accessing more electrolyte ions. The design concept and the synthesis approach are of guiding significance for the synthesis of porous carbon structure.