Characterizing the lithiation and failure mechanisms of transition metal phosphates and phosphides for lithium ion batteries
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In this dissertation the lithiation and failure mechanisms of some promising transition metal phosphide and phosphate materials are discussed for application in lithium ion batteries (LIBs). More specifically, the materials investigated include the intercalation cathode Li₃V₂(PO₄)₃ and the conversion anode FeP₂. For FeP₂, a nano amorphous material obtained through a novel, low-temperature synthetic reaction was galvanostatically characterized and the correlation between its morphology and lithiation properties is discussed. For Li₃V₂(PO₄)₃, Raman microscopy and X-ray photoelectron spectroscopy (XPS) are primarily used as the analytical techniques to characterize the bulk and surface chemistry of this material. In the first chapter the advancements and current challenges of LIBs are discussed. A brief overview on the different lithiation mechanisms (intercalation, alloying and conversion) is presented along with the cathode and anodes that historically have been of interest. The potential of Li₃V₂(PO₄)₃ and FeP₂ as next generation LIB electrodes is also discussed. In the second chapter we present the first reports of a nano, amorphous FeP₂ material obtained through a novel, low-temperature reaction between a σ-bonded alkly Fe complex with PH₃. Electrochemical lithiation of nano, amorphous FeP₂ showed superior performance to the bulk, crystalline morphology and a competing lithiation mechanism between classical intercalation and conversion is proposed. The third chapter discusses the monoclinic phase of Li₃V₂(PO₄)₃ which is characterized via Raman microscopy and compared to the spectrum calculated through density functional theory (DFT) providing groundwork for future in situ experiments. The fourth chapter reports on XPS measurements of composite Li₃V₂(PO₄)₃ electrodes after complete intercalation/deintercalation reactions and specifically examines the role that the carbon black additive plays on the interface of the composite electrode and electrolyte. Finally, the fifth chapter presents a novel design for an in situ Raman microscopy test cell for LIBs along with a detailed explanation of the important component and design criteria for optimal scattering and electrochemical measurements. Future in situ Raman microscopy experiments for Li₃V₂(PO₄) and other LIB materials of interest are discussed.