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theses

Rechargeable lithium ion batteries have attracted tremendous attention as “green” technology for electric vehicles and smart grids. In addition, the demand for flexible and high energy batteries has increased exponentially due to the growing need for smartphones and bio-devices over last few years. The conventional inorganic cathode materials (e.g., LiCoO2 and LiFePO4) for Lithium ion batteries are not flexible; they are also restricted by their low theoretical specific capacity. To satisfy the emerging large-scale applications of energy storage, new generation batteries should have high power and energy densities, and a long cycle life. In near term, new inorganic cathode and anode materials are developing to increase their capacity. In long term, the next generation batteries were proposed to be made from inexpensive renewable and/or recyclable resources via low energy consumption processes for energy sustainability with minimal environmental footprint. However, issues such as low electronic conductivity, large volume change during the charge/discharge cycles and dissolution of the active materials, commonly existed in these new electrode materials. These problems not only decrease their energy and power density, charging/discharging rate, but also lead to poor cycling performance, which largely hampered their practical applications. In this thesis, novel assembly approaches were developed to address some of the critical issues for the next generation battery devices with optimum electrochemical performance. Chapter 1 will include a general overview of basic but important information of current rechargeable Lithium ion battery technology, the requirement for the next generation Lithium ion battery for sustainable energy storage and their current issues and challenges. In addition, the structures, physical properties, methods of fabrications and applications of graphene, an important carbon material was employed for assembly with electrode materials in this thesis, will also be discussed. In Chapter 2, a simple, efficient and scalable assembly method was introduced for the controllable fabrication of nano-structured electrochemical active organic material for sustainable energy storage. Croconic acid disodium salt (CADS) as a sustainable organic electrode example to investigate the size effect on the battery performance of organic electrodes. CADS organic wires with different diameters were fabricated through a facile synthetic route using anti-solvent crystallization method. Cracks and pulverization were observed for micromter size CADS and its relative low capacity retention rate revealed that lithiation induced strain was also contributed to the limited cycling performance for organic electrode materials. The CADS nanowire exhibits much better electrochemical performance than its crystal bulk material and microwire counterpart. CADS nanowire with a diameter of 150 nm delivers a reversible capability of 177 mAh g-1 at a current density of 0.2 C, and retains capacity of 170 mAh g-1 after 110 charge/discharge cycles. The nanowire structure also remarkably enhances the kinetics of croconic acid disodium salt. The CADS nanowire retains 50% of the 0.1 C capacity even when the current density increases to 6 C. In contrast, the crystal bulk and microwire material completely lose their capacities when the current density merely increases to 2 C. Such a high rate performance of CADS nanowire is attributed to its short ion diffusion pathway and large surface area, which enable fast ion and electron transport in the electrode. In Chapter 3, we successfully developed a one-step, bottom up method for direct conversion of H2S to sulfur@graphene core-shell composite with various shapes (nanoparticles, nanosheets, and nano-wires), which can be used as cathode materials for the next generation Li batteries. This method employed graphene quantum dots as novel catalytic soft templates, taking advantage of their unique amphiphilicity and catalytic characteristics. We found, for the first time, the graphene quantum dots undergo micelle formation in aqueous solution (various solvents). The size and shape of the graphene micelle can be easily adjusted by changing the solution condition (ionic strength, dielectric constant) and it determines the size and shape of the resulted sulfur@graphene core-shell composite material. Our sulfur source, H2S, a major air pollutant, was directly converted as sulfur based cathode material, which opens up a potential route toward effective pollution control. We developed a general route to fabricate graphene based free standing, carbon black and binder free, flexible electrodes for high energy lithium ion battery in Chapter 4. Various particles (element sulfur, element Tin, Tin oxide and Li1.2Mn0.5Ni0.3Co0.3O2) were wrapped by graphene oxide through a simple solution phase assembly approach, no special interaction was needed. The as-prepared composite can be easily fabricated as free standing, flexible film and directly used as anode/cathode after recover graphene’s conductivity through thermal annealing. A free standing, flexible electrode of SnO2@Graphene was fabricated and used as an example for high energy lithium ion battery. The inter-connected graphene network functions as a conductive buffer matrix for the volume expansion of SnO2 during charge and discharge. A high specific capacity of 726 mAh g-1 (calculated by the total mass) and area specific capacity of 2.2mAh/cm2 was retained after 50 cycles for SnO2@Graphene composite anode at the current density of 500mA g-1. The assembly method developed in this study is general, robust and easy to apply on other functional materials rather than battery material, which opens up an easy path to fabricate flexible devices. In the Chapter 5, we report a new approach to intentionally induce phase transition of Li-excess layered cathode materials for high-performance lithium ion batteries. In high contrast to the limited layered-to-spinel phase transformation that occurred during in-situ electrochemical cycles, we hereby completely convert a Li-excess layered Li[Li0.2Mn0.54Ni0.13Co0.13]O2 (LMNCO) to a Li4Mn5O12-type spinel product via ex-situ ion-exchanges and a post-annealing process. Such a layered-to-spinel phase conversion is examined using in-situ X-ray diffraction (XRD) and in-situ high-resolution transmission electron microscopy (HRTEM). It is found that generation of sufficient lithium ion vacancies within the Li-excess layered oxide plays a critical role for realizing a complete phase transition. The newly-formed spinel material exhibits initial discharge capacities of 313.6, 267.2, 204.0 and 126.3 mAh g-1 when cycled at 0.1, 0.5, 1 and 5 C (1 C = 250 mA g-1), respectively, and can retain a specific capacity of 197.5 mAh g-1 at 1 C after 100 electrochemical cycles, demonstrating remarkably improved rate capability and cycling stability in comparison with the original Li-excess layered cathode materials. This work sheds light on fundamental understanding of phase transitions within Li-excess layered oxides. It also provides a novel route for tailoring electrochemical performance of Li-excess layered cathode materials for high-capacity lithium ion battery. In chapter 6, a facile surfactant-free sonication-induced route is developed to prepare colloidal nanocrystals of lithium-excess transition metal oxide. The sonication process plays a critical role in forming LMNCO nanocrystals in ethanol (ethanol molecules marked as EtOHs) and inducing the interaction between LMNCO and ethanol molecules. The formation mechanism of LMNCO-EtOHs supramolecules in the colloidal dispersion system is proposed and examined by theoretical simulation and Zeta potential measurement. It is suggested that the as-formed supramolecule is composed of numerous ethanol molecules capping at the surface of LMNCO nanocrystal core via hydrogen bonding. Such chemisorption gives rise to dielectric polarization of the absorbed ethanol molecules, resulting in a negative surface charge of LMNCO colloids. Additionally, diverse superstructures are resulted from self-assembly of LMNCO colloids during evaporation of ethanol. Such self-assembly behaviors of colloidal LMNCO nanocrystals are then investigated by tuning the solvent evaporation condition. The assembled LMNCO architecture also exhibits remarkably improved capacity and cycleability compared to original LMNCO particles, demonstrating a very promising cathode material for next-generation lithium-ion batteries. This work thus provides new insight into the formation and self-assembly of multiple-element complex inorganic colloids in common and surfactant-free solvents for enhanced performance in device applications.

ETD_6751

Ph.D.

Includes bibliographical references

by Ruiming Huang

doi:10.7282/T34B33BN

ETD doctoral

The author owns the copyright to this work.