| Energy depletion and environmental pollution have become global problems. It is of great significance to develop renewable energy, which can effectively reduce the use of fossil energy and improve the environment. However, new energy has to be store as electrical energy for convenient utilization. Therefore, developing energy storage systems which are low-cost, highly efficient, safe and environmental friendly is of great importance. Particularly, lithium ion batteries and supercapacitors are good examples and have attracted great interests. For lithium ion batteries and supercapacitors, the electrode material is one of the most important parts, which determines the performance of the device. Therefore, the electrode material with comprehensive performance are eagerly desired. Metal oxides are believed to be very promising electrode materials because of their high theoretical specific capacitance, which is several times higher than that of carbon materials. However, the cycling performance and rate performance of these materials are still unsatisfactory, limiting their development in energy storage systems. Design suitable micro/nano structures for metal oxides can significantly improve their electrochemical performance. This paper aims to explore simple and feasible approaches to obtain innovative micro/nano metal oxides and reveal their growth mechanisms which contributes to their controllable synthesis. More importantly, deeply and systematically study the relationship between the structure and the electrochemical performance of the micro/nano metal oxides to obtain the structural optimization strategy. This is to provide new thoughts for the structural optimization strategies of other hierarchical micro/nano materials. The main points are list below:1. Three-dimensional (3D) porous frameworks have shown great promise in the field of lithium-ion batteries (LIBs). However, the size effects of 3D porous frameworks on the structural and functional optimization are rarely reported. Herein, porous single-crystal a-Fe2O3 microrices synthesized through a facile one-pot hydrothermal method have been developed as a model system to investigate the correlations between the pore structure and LIB performance. A top-down chemical etching method was used to control the pore size and porosity of ?-Fe2O3 microrices simultaneously over a wide range.?-Fe2O3 porous microrices were further coated with carbon to stabilize the structure. Electrochemical characterization shows that the increase of the pore size and total porosity leads to a higher specific capacity but poorer cycling performance. Carbon coating on the surface of ?-Fe2O3 microrices significantly enhances the structural stability of particles and improves the cyclability of batteries. The obtained ?-Fe2O3@C porous microrices exhibit a high capacity of ?1107 mA h g-1 at a current density of 200 mA g-1,83% capacity retention after 100 cycles and an excellent rate capability, which are among the best ones reported so far for ?-Fe2O3 electrodes. Our results provide a general structural optimization strategy for porous oxides for high performance LIB anodes.2. Self-supported MnO2 nanoflakes arrays with pre-interaction of K+ ions are fabricated on the graphene foam. The amount of pre-interacted K+ ions are controlled by adjusting the reaction parameters. The relationship between the amount of pre-interacted K+ ions and the electrochemical performance of the micro/nano metal oxides are systematically investigated to obtain the optimal amount of pre-interacted K+ ions.3. Three-dimensional hierarchical core/shell nanoarrays:synthesis, structural optimization and supercapacitor appilication. Firstly, hierarchical MnO2 core/shell nanoarrays have been fabricated via a two-step hydrothermal method. The as-synthesized products are consist of ?-MnO2 nanotubes as the core and ?-MnO2 nanoflakes as the shell. Furthermore, a pretreatment is conducted to obtain hierarchical core/shell nanoarrays with better structural stability based on the selective dissolution mechanism. The electrochemical performance of the products before and after the treatment are compared. The capacitances of both products are nearly the same but the cycling stability of the products afer being treated is much better than that of the core/shell nanoarrays obtained before the pretreatment.4. Innovative hierarchical ?-MnO2 tube-on-tube arrays (HMNTAs) have been controllably synthesized by a facile hydrothermal route. The resultant HMNTA is comprised of single-crystal [001]-oriented tetragonal nanotubes, where the branch nanotubes are subtly and precisely assembled onto backbone nanotube along the certain crystallographic direction, showing a unique edge-to-edge structure. Time-dependent evolution of the morphology reveals that the formation of HMNTAs undergo a mesoscale transformation from unordered ?-MnO2 nanoflakes@?-MnO2 nanotube core/shell structure to highly ordered ?-MnO2 tube-on-tube structure, where the selective dissolution of ?-MnO2 nanoflakes and the coherent growth of ?-MnO2 nanotube along (110) plane play key roles in "bottom-up" assembly and organization of those branched nanotubes. In addition, the spatial structure of HMNTAs can be easily controlled from 4-fold-symmetry to 2-fold-symmetry by varying the diameter of branch nanotubes from 80 to 180 nm. Owing to remarkable structural features, the 4-fold-symmetry HMNTAs exhibit a specific capacitance of 780 F/g at 1 A/g and 98% capacitance retention after 5000 cycles at 10 A/g, which is superior to that of 2-fold-symmetry HMNTAs and ?-MnO2 nanotube arrays. Furthermore, the prototype symmetric supercapacitor (SSC) device based on 4-fold-symmetry HMNTAs electrode exhibits high specific capacitance (213 F/g), higher than that of the SSCs based on 2-fold-symmetry HMNTAs (182 F/g) and MNT arrays (80F/g). This work demonstrates a previously undescribed level of structural and functional complexity in hierarchical nanoarrays and brings new thoughts for designing novel hierarchical nanoarrays for various structure-sensitive applications in the future. |
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