| In the past decades rechargeable lithium ion batteries have been widely used in portable electronic devices, however, they can hardly meet the demands of high power and energy densities for electric and hybrid electric vehicles. New lithium ion batteries with more advantages, such as low cost, excellent safety, high energy density and good cycling stability, attract specialists’more and more attention nowadays. In this dissertation, the structure, component and electrochemical performance of lithium ion batteries are introduced, together with the crystal structure, lithiation-delithiation mechanism and recent research development of commercially used electrode materials. Especially, a cathode material of layered LiCoO2, spinel Li2MnO4, olivine LiFePO4or lithium-enriched solid solution and an anode material of nanostructured carbon, alloy-based compounds or transition metal oxides have been briefly summarized. Herein, inorganic crystals of olivine LiFePO4and copper-based compounds (i.e., Cu2O, CuO and CUC2O4·xH2O) are selected as lithium ion battery electrode active substances, which have been systematically investigated in this thesis.It has been generally accepted that an ingenious fabrication of nanostructured electrode materials may partly satisfy the challenge of high energy density and good cycling stability of lithium ion batteries. Taking the controlling synthesis of an inorganic electrode material as an example, its solution-based precipitation has been widely adopted to construct nanostructures. On one hand, nanofabrication and/or nanocrystallization of an electrode active substance deals with a hot research topic on its structure-function relationship, which can greatly increase the contact areas between organic electrolyte and inorganic crystallites, reduce the solid-state diffusion routes of lithium ions and improve the electrochemical properties (or lithium storage capability) of assembled electrodes. On the other hand, nanofabrication or nanocrystallization of an electrode active substance concentrates on another hot research topic on the formation or assembly mechanism of nanostructures, which includes the effect of weak-polarity solvent or crystal modifier, the nucleation and crystal growth of inorganics, the oriented attachment of tiny nanoparticles, etc.In a word, the nanofabrication, formation mechanism and electrochemical properties of olivine LiFePO4and copper-based compounds (i.e., Cu2O, CuO and CUC2O4·xH2O) have been focused in this thesis. According to the selected four active substances, their research backgrounds, experimental results, relative discussion, conclusions and outlooks can be simply divided into four sections, shown as below.(1) Hydro-and/or solvo-thermal nanocrystallization of lithium iron phosphate (LiFePO4) has been well recognized as an effective pathway to improve its electrochemical performances. Herein, it is reported that high-performance LiFePO4single-crystalline nanoparticles have been successfully prepared in an additive-free solvothermal reaction and subsequent glucose-assisted calcination. In comparison with the similar hydrothermal reaction, the presence of ethylene glycol can unexpectedly induce the formation of gel-like intermediates at the time interval of0.5h, resulting in LiFePO4nanoparticles with a three-dimensional (3D) lattice structure and an amorphous nanocoating for a reaction time of2h. Interestingly, the lattice structure of growing LiFePO4crystals can be thoroughly damaged under the irradiation of an electron beam. Furthermore, after the continuous crystal growth and subsequent heattreatment, nanocrystalline LiFePO4can become stable under the electron beam and the discharge capacity achieves165mAh g-1at0.1C in assembled LiFePO4/Li half-cells, proving a successful nanocrystal-forming engineering of LiFePO4for a lithium-ion battery cathode.(2) Crystalline materials with a well-defined morphology and/or a narrow size distribution might exhibit a specific shape-and/or size-dependent performances. In the first instance, the catanionic surfactants of anionic sodium dodecyl sulfate (SDS) and cationic cetyltrimethylammonium hydroxide (CTAOH) are added as crystal modifiers into the60℃reaction systems of copper chloride and sodium hydroxide for the syntheses of cuprous oxide (Cu2O). Then, the reversible reaction activity of crystalline Cu2O with metal lithium is conducted to investigate its electrochemical performance as rechargeable lithium ion battery anodes. The presence of SDS-rich catanionic surfactants can induce the formation of polyhedral Cu2O structures with8triangular{111},6square{100}, and12rectangular{110} faces outside, while the presence of CTAOH-rich catanionic surfactants, especially the doping methanol in CTAOH, lead to the generation of hexapod-shaped Cu2O mesostructures with tiny nanoparticles on these symmetrical branches. At a discharge-charge cycling current of80mA g-1, the26-faceted Cu2O crystals with rough{110} faces display an initial capacity of756mAh g-1and a reversible capacity of280mAh g-1in the first cycle. In comparison with the electrochemical performance of hexapod-shaped Cu2O mesocrystals at the same cycling current, the26-faceted crystals of Cu2O could be capable of remaining a relatively high capacity (145mAh g-1) and keeping an excellent coulombic efficiency (100%) over50discharge-charge cycles. As a whole, the catanionic surfactants at different anionic/cationic molar ratios are used as additives to highlight the secondary nucleation and growth mechanism for the formation of Cu2O, and then the resulting26-faceted crystallites and hexapod-shaped mesoparticles are separately used as active materials in the assembled Cu2O/Li half-cells to study their shape-dependent electrochemical performances.(3) For the preparation of a composite electrode, active components and polymer binders should be uniformly mixed to ensure the electrochemical stability of subsequently assembled lithium ion batteries. In this paper, the size-controlled syntheses of CuO nanocrystals (30.4±5.3or44.1±6.2nm) are obtained through the decomposition of copper oxalate precursors which are prepared in the solvothermal reactions. And their water-based interactions with natural binder sodium alginate have been considered to explain the high and fast lithium storage capability of CuO-alginate nanocomposites. CuO has a theoretical capacity of674.0mAh g-1as a lithium ion battery anode, while the initial discharge capacity of water-based CuO-alginate nanocomposites can reach1304.9mAh g-1at0.1C. Even at a higher rate of2C or5C, the reversible capacity can still retain665.8or524.2mAh g-1after100discharge-charge cycles. By contrast, an oil-based composite of nanocrystalline CuO and oil-soluble binder poly(vinylidene fluoride) PVDF deals merely with the mechanical interaction between them, possessing an initial capacity of734.4mAh g-1 but achieving a low retention ratio of18%at2C over100cycles. Anyway, aside from the effect of average particle size on the electrochemical properties of CuO nanocrystals, it is the previously involved electrostatic interaction between CuO and sodium alginate that greatly reduce the electrochemical polarization of composite electrodes and effectively improve the lithium storage capability of CuO.(4) In a hydrothermal and solvothermal system at120℃, cylinder-like and rod-like superstructures of hydrate copper oxalate (CuC2O4·xH2O) can be thoroughly synthesized in the absence of any shape-controlling additives, respectively. The self-assembly of primary nanocrystals has been investigated considering the polarity of reaction medium, and in the chemical formula of CuC2O4·xH2O the average x value of avoidable crystal water is estimated to discuss the superior lithium storage capability of hydrate products. The results show that cylinder-like superstructure of CuC2O4·xH2O (x～0.14) possesses an initial discharge capacity of920.3mAh g-1with a residual value of970.0mAh g-1at200mA g-1over100discharge-charge cycles, while rod-like superstructure with an x value of～0.53per chemical formula exhibits a higher initial capacity of1211.3mAh g-1and a lower retention of849.3mAh g-1under the same conditions. Furthermore, time-dependent measurements present a novel crystal-to-amorphous transformation of active substances, suggesting a positive effect of unavoidable crystal water on the superior lithium storage capability of nanostructured CuC2O4·xH2O. |
PDF Full Text Request