Co,Ni-based Oxide/Hydroxide Nanostructured Array Designs And Their Energy-storage Mechanism

As energy crisis emerges in the world, building better power sources (typically like lithium-ion batteries and supercapacitors) becomes more and more essential. Metal oxide/hydroxide (MxOy/M(OH)x) nanostructures are promising electrode materials for both lithium-ion batteries and supercapacitors because of their high specific capacity/capacitance, typically2-3times higher than that of carbon/graphite-based materials. However, the cycling stability and rate performance of these maiterials in power forms is still far from perfect to meet the requirements of practical applications. It is therefore urgent to improve their overall device performance, which depends on not only the development of advanced electrode materials but also in a large part "how to design the superior electrode architectures". Among electrode designs, the arrays of X-dimensional (X=1,2,3, short for XD) MxOy/M(OH)x nanostructures have been focused since they have superior geometric and morphologic characteristics when compared to bulky materials or nanopowders. On one hand, the establishment of arrayed and integrated nano-architectures with a robust adhesion to substrate provides numerous fast electron-transport accesses to the current collector; conductive electrons can be quickly transferred from the active redox sites to the current collector along "superhighways" rather than randomly walk in the disordered nanocrystalline networks, which can greatly benefit the rate performance of electrode. On the other hand, the arrayed design realizes a binder-free electrochemistry. Using nanostructured arrays design can save the complex technological processes, for example, mixing active material with ancillary materials such as carbon black and polymer binders, and the high-pressure press post-treatment. Additionally, the arrayed nanostructures can help to accommodate the detrimental strains especially caused by lithium insertion/removal. The current thesis aims to develop simple and efficient methods to fabricate MxOy/M(OH)x (M=Co, Ni) nanostructured arrays with various desirable architectures on current collectors. Significantly, we also systematically study their energy-storage mechanisms on lithium-ion batteries and supercapacitors. The main contents are included as follows:1.1D Co-based oxide/hydroxide nanowire arrays:synthesis and their energy-storage mechanisms(a) Primarily, the fabrication of large-scale1D CO3O4nanostructured arrays grown on various substrates is realized by a facile two-step template-free method. After going through a controllable hydrothermal process followed by a calcination treatment in air atmosphere, CO3O4nanowire arrays can grow firmly on insulating substrates like glass slides and ceramics, which were reported previously not suitable for the growth of MxOy. We find that F-ions play an important role within the growth of Co3O4nanowire arrays. Besides, this direct-growth approach can be readily extended to conductive substrates (ITO, Ti, Fe-Co-Ni alloy). Compared to template-based methods, our approach is general, high-efficiency and low-cost.(b) We furthermore develop a facile modified method to fabricate porous CoO nanowire arrays with robust mechanical adhesion to flexible Ti foil. Also, a typical synthesis of single-phased CoO from the complete pyrolysis of cobalt hydroxide carbonate precursors has been demostrated. When used as anode electrodes for additive-free lithium-ion batteries, porous CoO nanowire arrays exhibit good high-rate capability at a rate of1C (716mA/g),2C (1432mA/g),4C (2864mA/g) and6C (4296mA/g), respectively, because of both their highly reversible electrochemical properties and unique advantages from the integrated1D nanostructured architecture.(c) Large-scale a-Co(OH)2nanowire arrays with a length of-20μm are synthesized on graphite by a hydrothermal method, and further applied as binder-free electrodes for supercapacitors. Robust mechanical adhesion between the arrayed products and graphite substrate is confirmed by an ultrasonication test. Electrochemical testing results show that a-Co(OH)2nanowire arrays have a high specific capacitance of～642.5F/g, remarkable rate capability and good capacity retention, which could be attributed to their architectural advantages and good electronic contacts. Besides, a new stride has been made on the coaxial growth of α-Co(OH)2nanowire arrays on commercial carbon cloth scaffolds for bendable supercapacitor applications. Testing results show α-Co(OH)2nanowire arrays/carbon cloth can function as a high-performance electrode, with a high areal capacitance up to～1.13F/cm2, excellent cycling performance (nearly100%capacitance retention after programmed4000cycles) and good rate capability (retaining46.7of its maximum specific capacitance at an extremely high rate of40.8mA/cm2). The bend tests further demonstrate that the electrode possesses a good mechanical endurance. Even in the event of an extreme bend radius, the majority of nanowire arrays can still survive and the electrode shows little changes in capacitance (only7.5%loss) and impendence (lower than11%) compared to the unbent.2.1D hybrid CoO/CoTiO3nanotube arrays:synthesis and their lithium-storage mechanismWe propose a Kirkendall effect-based solid-state reaction route to evolve the simplex core-shell Co(OH)x(CO3)y@TiO2nanowire arrays into CoO/CoTiO3integrated all-oxide hybrid nanotube arrays with preserved morphology. Within the evolution process, the decomposition of Co(OH)x(CO3)y nanowire arrays into chains of CoCO3nanoparticles facilitates the nucleation of Kirkendall voids and promotes the interfacial solid-solid diffusion reaction even at a low temperature of450℃. The resulting CoO/CoTiO3nanotube arrays possess well-defined sealed tubular geometries and a special "inner-outer" hybrid nature. As a proof-of-concept demonstration of the functions of such hybrid products in lithium-ion batteries, CoO/CoTi03nanotube arrays can exhibit both high capacity (-600mAh/g still remained after250continuous cycles) and much better cycling performance (no capacity fading within250total cycles) than CoO nanowire arrays.3.2D Co-Fe mixed oxide nanowall arrays:synthesis and their lithium-storage mechanismCo-Fe layered double hydroxide (LDH) nanowall arrays are grown directly from Fe-Co-Ni alloy substrate by a simple hydrothermal method. An ultrasonication test of30min towards Co-Fe LDH nanowall arrays demonstrates their ultra-robust mechanical adhesion to the substrate. In addition, the carbon-source coating on Co-Fe LDH products is achieved during their in situ growing process conducted in a glucose-containing reaction solution. After heating treatment in an argon atmosphere, carbon coated Co-Fe mixed oxide nanowall arrays are evolved from the thermal decomposition of LDH precursors, and further investigated as anode materials for lithium-ion batteries. When tested, such arrayed products exhibit superior electrochemical performance on specific capacity and cyclability compared to carbon-free samples and samples made by previous carbon-coating methods.4.3D hybrid nanostructured arrays:synthesis and their energy-storage mechanisms(a) A methodology of fabricating CNT/Ni hybrid nanostructured arrays on a stainless steel substrate by using nullaginite nanowall arrays as the starting materials. The formation mechanism is put forward based on the monitoring of the entire fabrication process. Porous Ni nanowall arrays are initially evolved from nullaginite precursors followed by in situ CNT growth on their surface via a CVD process, forming an interesting "CNTs pillars-on-nanowall foundation" hybrid structure. The electrode design concept can be readily extended by selecting other Ni-containing hydroxides as precursors. When activated, CNT/Ni hybrid nanostructured arrays are used as high-performance electrode materials for supercapacitors. They exhibit well-defined pseudocapacitive capabilities with a high areal capacitance (up to～0.901F cm-2), excellent cyclability (nearly100%capacitance retention after5000cycles) and outstanding rate capability. The improvement is ascribed to the hybrid nanostructure as well as a synergetic effect between CNTs and nanowall arrays.(b) An interconnected3D core-shell hybrid electrode of MnO2/Ni nano frameworks is made via the evolution of nullaginite nanowall arrays. The evolution mechanism is studied by monitoring the entire fabrication process. When used as a binder-free electrode, the hybrid MnO2/Ni nanoframeworks exhibit good rate capabilities in lithium-ion storage with high specific capacities and a stable cycling performance.