Design Micro-nano Structures Of Metal/Metal Oxides And Their Application In Batteries

The development of lithium-ion battery and nickel-iron battery is important for solving the "energy" and "environment" issues. With the rapid development of mobile devices and energy storage systems and other areas, it is extremely urgent to develop high energy density, high power density, high safety and environmental friendliness electrode materials of lithium-ion battery and nickel-iron battery. Compared to other electrode materials, the metal oxide is a potential lithium ion battery electrode material due to their high specific capacity, low toxicity and long cycle life. In recent years, researchers made a lot of breakthroughs in the metal oxide-based lithium-ion battery anode materials, but it will still exist many of the key problems to be solved in its commercialization. Therefore, lithium storage properties and mechanism of the metal oxide especially ternary metal oxide electrode material is worth to be further exploraed. Comapare to Li-ion battery, rechargeable aqueous battery is more suitable for large-scale use in green grid energy storage due to their environmental friendliness, the intrinsic flame resistance of aqueous electrolytes, most importantly, greatly reduced capital and operating cost, inclusive of recycling. Recently, with the rapid development of nanotechnology, nanomaterials based new generation of nickel-iron battery has attracted wide attention and research in the field of national energy. However, nickel-iron battery still has some shortcomings, such as charging efficiency and poor discharge rate performance, which severely restricted its large-scale application. Therefore, it is particularly important to design navol nickel, iron composites to effectively resolve defect of nickel-iron battery. Based on above analysis, in this paper we designed and synthesized α-Fe₂O₃, SnO₂ and ternary Zn₂GeO₄ material with novel micro-nano structures, which was applied to the electrode materials of lithium-ion battery. We also deeply explore the mechanism of lithium storage. At the same time we design NiO/MWCNT cathode and Fe/MWCNT anode materials for the nickel-iron battery, deeply study the charge and discharge mechanism and understand side reactions which impact the nickel-iron battery performance. These researches provide an important theoretical and practical reference value for metal and metal oxide materials’ applications in the lithium-ion batteries and nickel-iron batteries. The main contents and innovations are as follows:（1） In chapter 2, vertically aligned, single crystalline a-Fe₂O₃ NWAs grown directly on Ni foam with large areas was created via a template-free hydrothermal route. We investigated the possible mechanism for the formation of this structure through timedependent experiments. Coin-type LIBs were directly fabricated from single-layer a-Fe₂O₃ nanowalls without any ancillary materials. They showed high reversible capacities, good cycling performance and high rate capabilities. It deliver a charge capacity of 1063 mAh g^-1 after 50 cycles at 0.1C and 563 mAh g^-1 at 2C and even 440 mAh g^-1 at 5C. The good rate performance could be mainly ascribed to the following two aspects: 1) there is a strong adhesion between the array of nanoplate and the substrate to provide fast electron conduction path; 2) The open network structure constituted of interconnected nanowalls is advantageous in electrolyte diffusion efficiency and volume change release which can accelerate the electrochemical reaction. Therefore, compared to a traditional electrode, this material has reduced electrode polarization originating from high electron transport and electrolyte diffusion efficiency.（2） In chapter 3, SnO₂ materials with different structures were designed and fabricated with a simple hydrothermal method, analyzed and summarized the formation mechanism of two different structural materials by controlling the experimental conditions. We further discussed the mechanism of lithium storage performance of SnO₂ with different structure materials. First discharge and charge capacity of SnO₂ with the array structure are 1350 mAh g^-1 and 1079 mAh g^-1 respectively. For flower-like SnO₂, the first discharge capacity is 1645 mAh g^-1, the charge capacity is 950 mAh g^-1. After 50 charge and discharge cycles, SnO₂ structure of the capacity of the array is maintained at 625 mAh g^-1, SnO₂ capacity flower-like structure is maintained at 566 mAh g^-1. Both of SnO₂ material with different structures exhibit excellent lithium storage performance, indicating that the two structures are designed to favor the diffusion of lithium ions. Flower-like SnO₂ has a high initial discharge capacity, due to its larger surface area, more lithium active site and forming more SEI film which resulting more first irreversible capacity. The nanorods materials have better capacity retention, indicating that this structure is more stable than that of flower-like structure when lithiation and delithiation. The above analysis results indicate that the structure of nanomaterials have very significant impact on its electrochemical properties. This conclusion provides valuable experience for designing and preparing other lithium-ion battery anode materials.（1） In chapter 4, the inorganic-organic hybrid nanobelts Zn₂GeO₄（ethylenediamine） were synthesized via a simple ethylenediamine（en）-based solvothermal method and studied their synthesis mechanism. When used as anode material of lithium-ion battery, it shows a very large increase in stability and performance as compared with the Zn₂GeO₄ nanobelts. Zn₂GeO₄（en） nanobeltsthe have up to 990 mAh g^-1 capacity after 70 cycles, showed a very stable cycle performance while Zn₂GeO₄ maintained only 527 mAh g^-1 reversible capacity with severely degradation. Contrast performance of the two materials, we found that capacities of Zn₂GeO₄（en） at 0.1C, 1C and 5C charge current density are 837, 701 and 601 mAh g^-1, even under a current density up to 10 C, Its capacity remains at 544 mAh g^-1 which is much better than Zn₂GeO₄(474 mAh g^-1). Further study of mechanisms found that organic En molecular contribute to in-situ formation a uniform dense SEI protective film on the surface of the active material, which effectively prevent further reaction of the electrode and electrolyte thus significantly improves the cycle stability of the battery. This work provides a new idea to investigate more novel inorganic-organic composite-based lithium-ion battery anode material.（2） In chapter 5, Fe and NiO nanoparticles were successfully deposited and anchored onto the surface of MWCNT with a solution-based method for use in Ni-Fe battery. On a positive side, the produced composites exhibited high uniformity and their applications in rechargeable alkaline batteries resulted in remarkably high capacity utilization and moderately good stability. On a negative side, the high specific surface area of nanostructured materials in both the Ni cathode and the Fe anode was found to enhance multiple side reactions, which were discovered to deteriorate cell performance. Based on our post-mortem analyses（EIS, TEM, SEM, EDS, XRD, ToF SIMS）, we came to the following conclusions: 1) at high alkaline electrolyte concentration and thus high pH values the Fe dissolution and re-precipitation takes place, which reduces the rate performance and capacity utilization of the nanostructured Fe anodes; 2) at lower pH values Fe dissolution could be mitigated, but HE takes place, which becomes particularly significant if high surface area nanostructured Fe anodes are used; 3) the addition of LiOH to KOH electrolyte enhances the Fe dissolution, but reduces the anode polarization and capacity utilization; these findings correlate well with the formation of porous oxidized Fe in LiOH-comprising electrolytes; 4) the dissolution of Ni cathodes（particularly significant in nanoparticle-based electrodes with higher surface area due to their small dimensions and higher surface energy due to the higher curvature） leads to the deposition of Ni metal onto the anode, which slows down the cell rate performance characteristics by blocking OH- anions. This new knowledge should assist in advancing rechargeable Ni-Fe battery technology further.