| The research on new types of green secondary batteries with high specific energy density, and related energy materials has become an international research hotspot in energy field. As an efficient and reusable way of energy conversion and storage, it has become a major demand for the future development in a series of high technology, and will be an important technological approach which can alleviate energy, resources and environment problems, as well as a key link for the applications such as electric vehicles and energy storage. So far, various rechargeable battery systems have been developed and commercialized over the past few decades. Among these systems, the alkali metal-air battery has attracted much attention due to their outstanding specific energy density. The theoretical energy density of Li-air battery is approximately 11,680 Wh kg-1, nearly equivalent to gasoline. The theoretical energy density of Na-air battery also reaches 1,600 Wh kg-1. Therefore, alkali metal-air battery is a pivotal research area for next-generation power source with high specific energy density.Currently, there are four systems of alkali metal-air batteries being pursued, which are categorized based on the applied electrolyte species (aprotic, aqueous, hybrid, and all solid-state electrolytes). The aprotic system is advantageous because it has been proved that the reduction product of peroxide can be reversed into the original reagents of the oxygen reduction reaction (ORR). This is aptly named the oxygen evolution reaction (OER) and signifies the rechargeability of the aprotic lithium-oxygen battery. Because only the aprotic lithium-air battery has shown promise of electrical rechargeability, this configuration has attracted the most effort worldwide to date compared to other electrolyte systems.However, this new technology is in a very early stage of development, and several challenges must be overcome before there will be a commercially viable product. These challenges include materials research such as the designing of air electrode structure and catalysts with high performance, and fundamental problems of electrochemical reaction process in the battery and the generation/decomposition of electrochemical product during discharge/charge process.In our work, precursors of cobaltite mixed oxide catalysts were deposited on the surface of porous substrate by hydrothermal reaction, after a heat treating process in a muffle furnace, three-dimensional cobaltite mixed oxide-substrate composite materials were obtained. The composite materials are with much pore structure and high electrical conductivity that is beneficial to the transmission of active materials and charges during electrochemical reactions. The composites were used as air electrodes in nonaqueous alkali metal-air batteries. The capacity density, catalytic performance and cycling ability were analyzed by electrochemical test such as galvanostatic charge/discharge measurement and cyclic voltammetry. SEM, TEM and FTIR technologies were used to analyze the morphology changes of the electrode and the composition of discharge product. The formation mechanism of discharge product and the relationship between Li-air and Na-air batteries were also discussed. These research results are helpful to understand the fundamental questions in alkali metal-air batteries and provide some suggestion to this battery system for the further research and improvement.This thesis specifically includes the following sections:1. A three-dimensional NiCo2O4 nanowire array/carbon cloth as an air electrode for nonaqueous Li-air batteries. The composite electrode was synthesized via a two-step process-a hydrothermal synthesis process in a Teflon-lined stainless autoclave and a heat treating process in a muffle furnace. Based on the special structure of the air electrode, the direct observation of Li2O2 growth at different discharge-charge states is possible and the influence of polymeric binder on the nature of the growth process of Li2O2 is avoided. This electrode is also a good carrier to analyze the performance of catalyst. The SEM, TEM and SAED results indicate that the polycrystalline NiCo2O4 nanowires on the surface of carbon fibers are composed with nanoparticles. The composite as a cathode of Li-air cells was investigated under galvanostatic cycling conditions. By compared with pure carbon cloth electrode, the NiCo204 nanowire was proved to be a fine catalyst for OER. At the current density of 80 mA g-1, the first discharge is found to be 862 mAh g-1 and 82% of the initial discharge capacity is reserved after the 12th cycle. By renewing the electrolyte, the cycling ability of the battery with 62.5% depth of discharge has been improved to more than 100 times. SEM images of the electrode indicate that novel porous ball-like Li2O2 was found to be deposited on the tip of NiCo2O4 nanowires after discharge. During the charge process, the Li2O2 shrunk and disappeared. In order to explain the observed morphological changes of Li2O2 mentioned above, we proposed a possible mechanism for the growth of Li2O2. From the directions of surface-charge density on NiCo2O4 nanowire and the oxygen density distribution on the surface of electrode, the special growth process of Li2O2 was analyzed.2. A ZnCo2O4 nanoneedle -Ni foam air electrode for rechargeable nonaqueous Li- air batteries. The carbon-free and binder-free porous electrode was synthesized by the same method with work 1. The growth of ZnCo2O4 nanoneedles on the surface of Ni foam was uniform and large amount of small pores was found inside the nanoneedles. Under galvanostatic cycling test, the first discharge is 1865 mAh g-1 at the current density of 20 mA g-1 and 62% of the initial discharge capacity is reserved after the 25th cycle. The morphology and composition of the discharge product were analyzed by SEM, TEM, XRD, FTIR and Raman spectra. Our results indicate that ZnCo2O4 nanoneedle-Ni foam air electrode as carbon-free and binder-free electrode is a promising candidate for rechargeable Li-air batteries of high energy density.3. NiCo2O4 nanosheets/nanowires supported on Ni foam for rechargeable nonaqueous sodium-air batteries. Two composite electrodes were synthesized via a similar two-step process with work 1 and used as air electrodes to study their electrochemical performance and the discharge product in sodium-air batteries. The experimental results indicate that the electrochemical performance and discharge product of electrodes with NiCo2O4 nanosheets or nanowires are similar. The discharge product was confirmed mainly to be Na2O2 by FTIR. The morphology of the discharge product is sheet-like and this indicate that the growth process nature of Na2O2 is sim ilar with that of Li2O2 as discharge product in their corresponding air batteries. However, Na2O2 nanosheets are more likely to form on the outer surface of the electrode, rather than the inner space constructed with nanocatalyst and the size of Na2O2 nanosheets clusters is larger than that of Li2O2. This result indicates that it is helpful to design new air electrodes with enough void volumes and open frameworks in their architecture structures for accommodating the discharge product so that the electrochemical performance of Na-air batteries could be improved.The research on fundamental research of alkali metal-air batteries with three-dimensional composite electrode promotes the understanding of the electrochemical reactions and the formation/decomposition of electrochemical product in the batteries. This study is also conducive to the structure designing of air electrode and the development of catalysts with high performance.In addition, appendix research works I made are introduced. It contains two aspects:1. Interfacial sodium storage in NaF-Ti nanocomposites. A heterogeneous nanocomposite material of NaF-Ti thinfilm is prepared by pulsed laser deposition and it was analyzed by TEM, SAED and XPS. Under galvanostatic cycling condition, the typical sodium-storage capacity of interfacial charging is about 52.8 mAh g-1 based on the weight of nanocomposite NaF-Ti. By potential step measurements, the sodium chemical diffusion coefficients in NaF-Ti nanocomposites are approximately estimated to be in the range from 6.5 to 46.4×10-15 cm2 s-1. The interfacial sodium storage mechanism should be constructive to design nanocomposite electrodes with large surface and abundant grain boundaries for SIB.2. In the 3 years doctoral study, the author also did many work in the preparation research of CIGS thin film solar cells, which was supported by the National 973 Project. The preparation processes in work include Mo back contact, CIGS absorption layer, CdS buffer layer, i-ZnO layer and top contact. Many techniques are used in this research, such as direct current sputtering, radio-frequency sputtering, thermal evaporation, hydrothermal precipitation and co-evaporation. |
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