Synthesis And Electrochemical Properties Of Nano/Micro Structured Metal Oxides As Negative Electrode Materials

Batteries for high power applications and energy storage are important directions for lithium-ion batteries in the future. The development of cathode and anode materials with high safety, high energy density, long life and low cost of the has become a key target of lithium-ion battery technology. Nanotechnology is an important way for the research and development of high performance electrode materials, and it has made great progress. Compared with the micron-sized electrode materials, nano-electrode materials have larger surface area and higher reaction activity, with better conducting pathways due to the full infiltration between electrolyte and electrode materials and sufficient porosity that can effectively alleviate the volume change of electrode materials in the charge and discharge processes. In addition, it can also greatly increase the contact area of electrochemical reaction and shorten the conduction distance for lithium ions and electrons, so that the high rate capability and long cycle life of electrode materials are significantly improved.But nano-electrode materials also have some drawbacks:(1) the large contact area between the electrode materials and electrolyte results in larger irreversible capacity los;(2) the nano electrode particles are easy to agglomerate and destroy the electrode structure in the cycling process.(3) Compared with the micron-sized electrode materials, the preparation process of nano electrode material is more complicated and the production cost is higher. In order to solve these problems, the main research goal of this thesis is the exploration and improvement of preparation technologies for nano electrode materials, including the designs of three-dimension (3D) porous structure and nano/micro composite structure electrode materials. These materials can combine both the advantages of nanomaterials and micron-sized materials with good cycling performance and rate performance.Chapter1gives a brief introduction of lithium-ion batteries on working principle, typical cathode materials, anode materials and electrolyte. The importance and necessity of nanotechnology and the electrode materials with nano/micro structure for the development of lithium-ion batteries are given in detail.Chapter2presents the experimental reagents and equipments used in this thesis. The procedure of test cells fabrication and the characterization methods for electrode materials have been elaborated. In Chapter3, an improved electrostatic spray deposition (ESD) setup is firstly designed and built in order for the thin film materials to be prepared in an inert atmosphere or special reaction atmosphere. It not only can be used for direct preparation of easily oxidized metal oxides or carbon-based composite films, but can also avoid the negative effects brought by possible substrate oxidation. By using this setup, MnO thin films with3D porous structure are synthesized successfully on different substrates (nickel foam and stainless steel), and used as anode materials for lithium-ion batteries. They show stable cycling performance with improved rate capability. In particular, the MnO thin film deposited on the nickel foam substrate (MnO@Ni) exhibits the best electrochemical performance with high initial columbic efficiency (83.9%) and low initial capacity loss (14.2%) with very good cycling performance (955mAh/g after50cycles at0.1C) and high rate capability (498mAh/g at1.6C). In addition, the MnO thin film electrodes prepared by the improved ESD technique can avoid the influence of the substrates on the electrochemical performance and prove the advantage of3D porous structure in the lithium-ion batteries.In Chapter4, MnO/multi-walled CNT (MWCNT) composite films electrodes with3D porous structure are synthesized by the improved ESD setup. The conductivities of the MnO film electrodes are significantly improved by introducing MWCNT into the MnO films. The effect of MWCNT on the morphology of the electrodes and the electrochemical performances is studied. It is found that the electrostatic repulsion between the charged spray droplets may be reduced by adding MWCNT, which leads to the breakdown of the3D porous morphology of the MnO film electrodes. With increasing the amount of MWCNT, the3D porous structure is destroyed more seriously, and the electrochemical performance becomes worse. The content of5wt%MWCNT is foun to be optimal, which can improve the conductivities of MnO film electrodes without destroying their3D porous structure and obtain good cycling performance (588mAh/g after100cycles at1C) and high rate capabilities (373mAh/g at5C).In Chapter5, based on the preparation of the MnO@Ni and MnO/MWCNT thin film electrodes, MnO/RGO composite film electrodes are also synthesized using the improved ESD setup. The morphology of the film electrode and electrochemical performance are studied through varying the RGO content. The results show that the issue of3D-structure collapse can be prevented in the MnO/RGO composite film electrodes when appropriate content of the RGO is added (16.58wt%). Therefore, a further improvement in the cycling performance (772mAh/g after100cycles at1C) and rate capability (425mAh/g at6C) are achieved due to the excellent electrical conductivity of MnO/RGO composite film electrodes.In Chapter6, we combine the electrostatic atomization principle with solution precipitation reaction and put forward a new preparation method, namely electrostatic spray-precipitation method, for nanomaterials. Using this novel method, we prepare MnO nanoparticles for lithium-ion batteries, and study the effect of Mn(Ac)2·4H2O solution concentration on the morphology and electrochemical performance of MnO nanomaterial. The results show that MnO nanoparticles gradually grow up and agglomeration take place with increasing the concentration of Mn(Ac)2·4H2O solution.0.01M is the optimal concentration to obtain monodispersed MnO nanoparticles with the smallest particle size (120nm) and good cycling performance (766mAh/g after95cycles at0.25C) and high rate capabilities (448mAh/g at3C).In Chapter7, we develop a simple wet-chemical process for the synthesis of rod-like and dandelion-like Cu(OH)2and CuO particles at a low temperature without using any surfactants. It is found that the amount of the Cu(Ac)2solution plays an important role in the formation of Cu(OH)2and CuO nanostructure. When the ratio V(Cu(Ac)2) to V(NaOH) was4:10, the prepared dandelion-like CuO electrode shows the best rate performance with a high capacity of511mAh/g at4C and stable cycling performance with a reversible capacity of over587mAh/g after50cycles.Based on the advantage of CuO electrode with nano/micro structure in electrochemical performance, in Chapter8, the monodisperse α-Fe2O3particles are synthesized via a simple alcoholysis method. By tailoring the concentration of ferric acetylacetonate (Fe(acac)3) solution, we obtain α-Fe2O3samples with different nano/micro structures. When the concentration of ferric acetylacetonate solution was0.02M, the prepared flower-like α-Fe2O3electrode with high specific surface area (33.65g/m2) and the small particle size (3μm) exhibited a high capacity-retention of88.5%after40cycles at0.2C and rate capacity-retention of55%at4.7C (with the discharge capacity of the second cycle as standard).Finally, Chapter9gives a brief summary on the achievements and the deficiency in this thesis. Some prospects and suggestions of the possible future research are pointed out.