Development of high purity lithium Bis(oxalate)borate, LiB(C2O4)2 (LiBOB)and its effect on the stability of standard and new generation electrode materials

Among other battery technologies, lithium-ion batteries have been extensively used in portable electronics, hybrid/electric vehicles (HEVs/EVs) and energy storage devices because of their high energy density, high capacity, long lifetime, low self-discharge rate, and design flexibility. Over the past decades, many attempts have been made to develop new battery materials towards higher energy density, longer life, safety, environmental friendliness, and sustainability. Currently, most commercial lithium-ion batteries have been used graphite as an anode, LiCoO2 as a cathode and LiPF6 in ethyl carbonate (EC) – dimethyl carbonate (DMC) as an electrolyte solution. However, graphite anode may suffer from its limited capacity and the dendrite formation, whereas LiPF6 salt is susceptible to decompose into dangerous byproducts resulting in safety issues and poor electrochemical performances at high operating voltages. Therefore, various electrolyte additives have been introduced to prevent electrolyte degradation, structural changes, HF attack and impedance rise at the electrode surface by developing a solid electrolyte interface (SEI). Among alternative electrolyte additives, lithium bis(oxalate)borate, LiB(C2O4)2 (LiBOB) has received significant interest owing to its high thermal stability, good solubility, high conductivity, low cost, and safety in a wide potential window. In this thesis, high purity LiBOB was developed as an electrolyte additive to enhance the electrochemical performances of standard and new generation electrode materials. First, LiBOB was synthesized using low-cost starting chemicals and fewer processing steps under a protective atmosphere. The detrimental effects of humidity and remaining impurities of LiBOB were evaluated through physical and chemical characterizations. Secondly, a practical recrystallization process was carried out to eliminate lithium oxalate (Li2C2O4) impurity, which is susceptible to precipitate in the electrolyte solution resulting in high cell impedance and poor cycling stability. After obtaining a high purity product, the desired amount of LiBOB was dissolved in the base electrolyte to increase the stability of LiCoO2 cathode at high potential. Thus, LiCoO2 showed superior cycling stability, rate capability, and high energy density related to the enhanced interfacial stability by LiBOB originated SEI layer. As a new generation anode material, surface modified TiO2 / reduced graphite oxide (RGO) nanocomposite was fabricated due to its promising features like structural stability, cycling stability, environmental friendliness, safety, and low cost. First, anatase TiO2 nanoparticles with an average crystallite size below 20 nm was synthesized by a sol-gel method. Coating with RGO as a conductive network and a surface modification process were applied to enhance overall electrochemical performance related to the better dispersion of nanoparticles and mechanical stability. Consequently, the surface modified TiO2/RGO anode showed high capacity, superior cycling stability, high coulombic efficiency (> 99 %) and outstanding rate capability compared to the pristine composite and TiO2 anodes. Finally, the effects of LiBOB on the stability of surface modified TiO2/RGO anode was studied via galvanostatic cycling tests and various post-mortem analyses. It was found that the applied current rate has a large impact on the correct formation of a SEI layer to ensure cycling stability. With the addition of LiBOB, surface modified TiO2/RGO anode showed improved capacity retention and high coulombic efficiency at lower current density. Ex-situ scanning electron microscopy (SEM) images and X-ray photoelectron spectroscopy (XPS) evaluated the development of a chemically and mechanically stable SEI on the anode surface. However, after first discharging, the huge irreversible capacity loss was occurred due to the preferential oxidation of LiBOB that could not be avoided even at low current densities. The promising results given in this study for the high voltage LiCoO2 cathodes and TiO2/RGO anodes with high purity LiBOB could open new pathways for other high voltage cathodes and new generation anodes resulting in higher energy densities, cycling stability and safety for state-of-the-art lithium-ion batteries.