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Title: Fundamental studies on charge storage mechanisms in metal oxalate anodes for lithium-ion batteries
Authors: Ang, Elijah Wei An
Keywords: DRNTU::Engineering::Materials::Energy materials
DRNTU::Engineering::Materials::Nanostructured materials
DRNTU::Science::Chemistry::Physical chemistry::Electrochemistry
DRNTU::Engineering::Materials::Functional materials
Issue Date: 2015
Source: Ang, E. W. A. (2015). Fundamental studies on charge storage mechanisms in metal oxalate anodes for lithium-ion batteries. Doctoral thesis, Nanyang Technological University, Singapore.
Abstract: Presently, environmetal disruption and economic recession of burning non-renewable fossil fuels have gained enormous awareness of renewable energy storage systems. By contrast with other energy storage devices, lithium-ion batteries (LIBs) are smaller and lighter, as well as capable of providing higher energy density and longer cycle life, making it one of the most promising choices. Currently, LIBs are used to power portable electronics e.g. laptops and mobile phones. With greater demand and widespread use of large applications such as electrified vehicles (EVs), LIBs with enhanced electrochemical performances i.e. high energy and power densities, reduced cost, lower toxicity and safety features are necessary. In this thesis, a collection of single and mixed metal oxalates by two synthesis routes namely facile chemical precipitation and solvothermal are evaluated as LIBs anodes. Based on our results, the electrochemical performances of chemical precipitated single and mixed metal oxalate are better than synthesis by solvothermal. Moreover, both synthesized metal oxalates yield superior properties compared to other oxalates reported. Highlighting the investigations, Li/FeC2O4 cells delivered an initial discharge capacity > 1288 mAh g-1 at 1C-rate and anhydrous FeC2O4 cocoons exhibited stable cycling (438 mAh g-1; greater than graphite) even at high C-rate (11C). In addition, the synthesis mechanisms i.e. factors influencing materials’ properties/morphologies are examined. Systematic studies on mixed metal oxalates i.e. Fe- and Mn- doped cobalt oxalates are also carried out, indicating improved electrochemical performances. It can be concluded that positive synergistic effects existed in these mixed metal oxalates, whereby the presence of Co is crucial for high capacity while Fe is needed in the structure for cyclability and stability of the cell. Finally, the effects of morphologies, compositional differences and synthesis methodologies on Li storage mechanisms for these metal oxalates are elucidated in detail. In general, properties of the metal oxalate anodes were characterized using field emission scanning electron microscopy (FE-SEM), high resolution transmission electron microscopy (HR-TEM), X-ray diffraction (XRD), thermogravimetric analyses (TGA), among other characterization methods. Furthermore, electrochemical properties of these metal oxalates were studied using cyclic voltammetry, galvanostatic cycling and electrochemical impedance spectroscopy. Additionally, the practical applications of metal oxalate anodes employing ILs (i.e. in mixed electrolytes) and in elevated temperature was presented. Lastly, the fundamental study of complex Li storage mechanism (i.e. involves conversion reactions together with double layer charge storage and reversible reactions with electrolyte) in these metal oxalates by employing CV sweep test and ex-situ techniques such as XPS, AFM, FE-SEM, XRD and FT-IR are investigated in this thesis. Along with, variation of salts and solvents (i.e. electrolytes) to offer new insights of interfacial phenomenon on metal oxalate electrodes are explored as well.
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