INVESTIGATION OF ELECTROLYTE ADDITIVES AND LITHIUM METAL ANODES FOR LITHIUM ION BATTERIES

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The continued development of energy storage technology is of high importance in order to facilitate the widespread adoption of intermittent renewable energy sources as well as the expansion of electromobility (for example, fully electric vehicles). These applications require a rechargeable cell with high energy density with a long cycling life, based on the electrochemical cycling of lithium ion batteries, which can be improved by modifying the cell chemistry and construction. Enabling the reversible plating and stripping of lithium metal on the negative electrode substrate – a lithium metal anode – allows for a higher gravimetric capacity necessary for a lightweight battery. However, the application of the lithium metal anode in carbonate-based electrolytes is plagued by the highly reactive nature of lithium metal, causing poor coulombic efficiency and the growth of potentially unsafe lithium dendrites. Another route to achieve a high energy density lithium ion battery is to increase the nickel or lithium content of the layered lithium nickel- manganese - cobalt -oxide positive electrode (NMC cathode) materials. Unfortunately, alongside the improved capacity, these compositional changes result in new challenges to overcome such as surface reconstruction, gas evolution, and transition metal dissolution. Electrolyte engineering and surface modifications to the cathode material can help alleviate detrimental reactions, however, the source of the improvements remains unclear. This dissertation attempts to elucidate the relationship between the molecular composition of the solid electrolyte interphase (SEI) and cycling performance of a lithium ion battery, as well as understanding the role of the cathode in anode SEI formation. Galvanostatic voltammetry was used to characterize the electrochemistry of the lithium metal anode, with Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (ATR-FT-IR), X-ray Photoelectron Spectroscopy (XPS), Electrochemical Impedance Spectroscopy (EIS), and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) to investigate the surface of the lithium metal anode, graphite anode cycled with modified and unmodified high energy NMC cathodes, and the cathode materials themselves. Chapter 1 is a perspective-style review which brings together many different studies to propose a scheme by which the anode SEI evolves throughout cycling and offers an explanation behind the varying reports from different research groups. Chapter 2 is a perspective-style review on the interaction of the electrolyte with the NMC cathode material, and its implications in the cycling performance. In chapter 3, novel electrolyte additives, difluoroacetic anhydride (DFAA) and trifluoroacetic anhydride (TFAA), are investigated in carbonate-based electrolytes which help to improve the reversibility of lithium metal