Synthesis and electrochemical characterization of novel electroactive materials for lithium-ion batteries

Lithium-ion batteries (LIBs) have become ubiquitous as energy storage devices for mobile electronics, electric vehicles, and are beginning to be used for electric grid-scale energy storage. Lithium-ion batteries offer higher efficiencies, energy density, and longer life compared to incumbent technologies such as lead-acid and nickel metal hydride. Applications in which LIBs are used are continuing to demand better performing batteries at lower cost, which requires improvement in electroactive materials. This dissertation investigates the low temperature synthesis and modification of LiCoPO₄ as a potential high-voltage and therefore higher energy density polyanion cathode material for LIBs, as well as a new class of interdigitated metal foil anodes which promises to be an inexpensive, higher energy density, alternative to graphite. Chapter 1 is a brief introduction to lithium-ion batteries and the principle of operation of intercalation type electrochemical energy storage devices. The components of lithium ion batteries are introduced, specifically the anode, cathode, separator, and electrolyte. Some of the shortfalls of the current technologies are discussed and areas of research interest are highlighted. Chapter 2 is a brief overview of the various experimental methods that are generally applicable to more than one of the subsequent chapters. Methods which are specific to a given study are discussed in their respective chapters. Chapter 3 presents work on the low temperature microwave-assisted solovthermal synthesis (MW-ST) of three unique polymorphs of LiCoPO₄, specifically the polymorphs belonging to the Pnma, Cmcm, and Pn2₁a space groups. Prior to this work, only the Pnma polymorph had been reported via MW-ST method, and electrochemistry had not yet been reported for either the Pn2₁a or Cmcm polymorph. The dependence of the polymorphs on both the water content, and ammonium hydroxide content of the solvent was shown. Although, the electrochemistry of both the Pn2₁a and Cmcm polymorphs was found to be inferior to the Pnma polymorph, the ability to synthesize phase pure materials was crucial to conducting the work presented in chapters 4 and 5. Chapter 4 presents the aliovalent substitution of V³⁺ for Co²⁺ in LiCoPO₄ via a low-temperature MW-ST process. Substitution of up to 7% vanadium for cobalt was demonstrated and verified by changes in the lattice parameters with vanadium content. Both the ionic and electronic conductivity of LiCoPO₄ was enhanced with increasing vanadium substitution, which was attributed to the introduction of both charge carriers as well as inter-tunnel cobalt vacancies. Finally, the first cycle capacity was enhanced (from 69 mAh/g to 115 mAh/g) as well as the capacity retention over cycling. Chapter 5 demonstrates a novel technique of MW-ST assisted coating of a thin (2-5nm) conformal coating of LiFePO₄ on vanadium substituted LiCoPO₄. Although the vanadium substitution was able to independently increase the performance of LiCoPO₄, the materials still suffers from severe side reactions with the electrolyte. The coating of LiFePO₄ effectively raises the Fermi energy of the cathode material above the high occupied molecular orbital (HOMO) of the electrolyte preventing side reactions and increase the coulombic efficiency to nearly 100%. Chapter 6 introduces a novel method of producing high surface area, electrically conductive, metal nanofoams via a MW-ST process. Nickel, copper, and silver metal nanofoams are made via an inexpensive yet scalable process whereby metal acetates are reduced by polyglycol under microwave irradiation. The nanofoams were characterized via BET, SEM, XRD, EDS, and TEM. The nanofoams have potential uses in many clean energy applications, particularly lithium-ion batteries. Chapter 7 introduces a new framework for making a new class of high capacity, low-cost alloying anodes for lithium ion batteries. A novel interdigitated metal foil anode (IMFA) in which a nanosized active material, such as tin, is interdigitated with an electrically conductive matrix, such as aluminum, is presented. The foils are formed by the rolling of a eutectic Al-Sn alloy into a foil, which is an extremely cheap and scalable process. The anodes demonstrate an approximately 70% increase in capacity compared to graphite over 100 cycles, at reasonably fast rates (C/5), and high coulombic efficiency (>99%). Finally, Chapter 8 gives a brief overview of the results of the prior work and proposes areas for future research.