| In recent years, Lithium ion batteries (LIBs) are the dominant powersources for portable electronic devices such as cameras, mobile phones and laptops, and will gradually extend to high-power systems such as electric vehicles/hybrid vehicles and large-scale energy storageengineering. Therefore, the demand for the improvement of LIB performances is increased, and it is higly urgent to develop novel electrode materials to meet the high energy, highpower, high stability, and security requirements. As a new type of anode materials, metal oxides (including SnO2 and some transition metal oxide) have gained great concern since discovery, owing to their much higher theory specific capacity than the traditional commercial graphite anode. However, the volume change of SnO2 is enormous during lithium ion intercalation/extraction processes, resulting in mechanical failure, electrical disconnection, and fast capacity fading. Additionaly, the low intrinsic electronic conductivity of SnO2 leads to poor rate performance.Moreover, the large initial irreversible capacity loss of SnO2 anode material heavily restricts its practical applications. Currently, through the design and preparation of nanostructured or composite materials,the electrochemical properties of SnO2-based anode materials can be improved to some extent, but massive efforts are still needed to improve its initial Coulomb efficiency, cycling stability and rate performances to meet the commercial application requirments. On the other hand, manganese oxides (e.g., MnO, Mn3O4, Mn2O3, and MnO2) have attracted much attention as promisinganode materials for LIBs owing to their high theoretical capacity, low conversion potential,environmental benignity and natural abundance. Current research mainly focuses on thedesign of nanostructures or carbon coated electrodes, rate performanceremains to be a great challenge because of the intrinsically slow solid-state lithium diffusion.Capacitive-like lithium storage behaviors have been previously observed in some TMO-based anodes, shedding light on attaining high power LIBs. Unfortunately, such surface capacitive storage has been limitedly exposed and rarely utilized since it was normally shadowed and influenced by the structure design and surface morphological/chemical evolution of the electrode.Therefore, it will be of great importance and necessityto develop a robust TMO-based anode structure for delivering durable and high-rate lithium storage capability by tuning the surface pseudocapacitance.This thesismainlyfocuses on two aspects, and a series of research work has been carried out. On one hand, a variety of porous SnO2-based compositefilm anode forLIBs was fabricated using the ESDtechnique, and the electrochemical properties of the composite anode materials are improved by synergistic effect between different components.On the other hand, theRGO-MnO-RGO sandwich nanostructures have been fabricated by layer-by-layer electrodepositon, in which dynamic equilibrium between surface pseudocapacitance and diffusion-controlled lithium storage is achievedafter a novel cycle-induced microstructure evolution, yielding excellent cycling performance and superior rate capability.Besides, a series of three-dimensional porous manganese oxidewere prepared by the ESD technique, confirming the universality of pseudocapacitance effect on improving the electrochemical performance of lithium-ion anode materials,which might open the window for developing next-generation anode materials for high energy/power-density battery applications.Chapter 1 first introduces the development history of LIBs, the working principle and the feature of LIBs, and then briefly summarizes the characteristics of different types of anode materials, and focuses on the research status and problems of SnO2 and transition metal oxide-based anode materials.In chapter 2, the experimental reagents, processes and equipmentsused in this dissertation are briefly introduced, followed by followed by a description of the material characterization methodsand electrochemical meatruments for LIBs.In chapter 3, SnO2 thin films were prepared by the ESD technique to explore the effect of different experimental parameters on the film morphologies, and obtained a three-dimensional porous structure under appropriate experimental conditions.On this basis, SnO2/graphene composite film was prepared, the reversible specific capacity and cycling stability were significantly improved due to its unique microstructure. At a currentdensity of 200 mA g-1, it showed good capacityretention with a capacity of 551.0 mA h g-1 after 100 cycles.The anode also exhibited excellent rate capability with an800 mA g-1 rate capacity up to 507.7 mA h g-1.SnO2/Ag composite films were prepared to study the effects of different compositing methods on the morphology and electrochemical properties. As with thein-situ Ag compositing can enhance the conductivity of the electrode, and ease SnO2volume expansion, SnO2/Ag (5:1) film can diliver a discharge capacity of up to 609.5 mAhg-1after 100 cycles, dramatically improved its cycle stability. EX-situ Ag coating leadsto the decreasing of initial capacity, while not effectively improve the cycle stability.In chapter 4, three-dimensional porous SnO2-Fe2O3 compositefilm anode for lithium ion batteries was fabricated using the ESD technique. Compared topure SnO2film, SnO2-Fe2O3 compositefilm exhibitsgreatly enhanced electrochemical performance and cycling stability. The enhancedlithium storage performanceshould be attributed to the synergistic effect betweenSnO2 and Fe2O3, as well as the three-dimensional hierarchical porous structure.This hierarchical porous SnO2-Fe2O3 anode possesses a high reversible capacity (1034.1 mAhg-1(and a high initial coulombic efficiency of 82.9% at a current density of 0.2 A g-1. At the same time, it shows good capacity retention with a capacity of 1025.6 mAhg-1 after 240 cycles and good rate performance.In chapter 5, theRGO-MnO-RGO sandwich nanostructures have been fabricated by layer-by-layer electrodepositon for enhanced lithium storage.The RGO layers enable fantastic protection of the active MnO layer against aggregation and volume changes, and increase the current collecting ability of the electrode as well, which is comfirmed by SEM, TEM, XPS, EIS and CV meatruments.Benefitted from the designed sandwich structure, pseudocapacitance effect was generated by the surface redox reactions through the microstructure evolution of RGO-MnO-RGO electrode, yielding excellent cycling performance (1115.7 mAh g-1 over 200 cycles at 0.5 A g-1, 946.7 mAh g-1 over 500 cycles at 5 A g-1) and superior rate capability (331.9 mAh g-1 at 40 A g-1,379 mAh g-1 after 4000 cycles at 15 A g’1). The unique design of the RGO-MnO-RGO electrode with ever-increasing pseudocapacitance effect might open the window for developing next-generation anode materials for high energy/power-density battery applications.In chapter 6, a series of three-dimensional porous manganese oxidesincluding amorphous MnxOy, MnO, Mn2O3 and hierarchical MnO were prepared by ESD and subsequent heat treatment as LIB anode materials. The results indicate that all of these three-dimensional porous manganese oxides have the dual lithium storage mechanism ofpseudocapacitance behavior and lithium-ion intercalation/deintercalation process,confirmingthe universalitythat pseudocapacitance effect can improve the reversible specific capacity, cycling stability and rate performance if the electrode structure design is reasonable.By focusing on porousMn2O3, it is found that benefitied from the cycle-induced microstructure evolution, "honeycomb" nanostructure transformed into interconnected manganese oxide nanorods, resulting in the increased pseudocapacitive effect and yielding excellent cycling performance and superior rate capability.Finally, in chapter 7, an overview and the deficiency of the dissertation are summerized. Some prospects and suggestions on the possible future research are presented. |
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