INVESTIGATION OF NOVEL LOW TEMPERATURE ELECTROLYTES FOR LITHIUM-ION BATTERIES

Abstract

Improving energy storge technology is vital to the worldwide adoption of renewable energy sources as well as the growth of electromobility. Therefore, recent research has been focused on developing rechargeable lithium-ion batteries enabling high energy and power density over a wide temperature range with improved safety. Unfortunately, the performance of lithium-ion batteries is dependent upon the operating temperatures. At subzero temperature, the performance of lithium-ion battery in carbonate-based electrolytes is decreased by increased cell resistance limiting lithium-ion transportation. Therefore, adjusting the electrolyte composition through use of novel solvents, electrolyte additives, alternative lithium salts and optimized solvent blends has been reported to improve the low temperature performance of lithium-ion batteries. This dissertation is focused on understanding the performance of lithium-ion batteries in relation to the structure and composition of surface films generated with low temperature electrolytes. Galvanostatic cycling was used to characterize the electrochemical performance of half and full cells constructed using graphite and NCM523 as anode and cathode, with X-ray Photoelectron Spectroscopy (XPS), Infra-Red spectroscopy (IR), and Scanning Electron Microscopy (SEM) to investigate the surface of the graphite anode and NCM523 cathode. In chapter 2, a novel co-solvent, isoxazole (IZ), is introduced into novel electrolyte systems composed of lithium difluoro(oxalato)borate (LiDFOB) in fluoroethylene carbonate (FEC) and LiDFOB in ethylene carbonate (EC) to improve reversible cycling at low temperature, using Li/graphite cells. Using this electrolyte systems, in combination with above mentioned analytical methods, chapter 3 attempts to elucidate the relationship between the structure and composition of the Solid Electrolyte Interphase (SEI) and cycling performance of Li/graphite half cells. Finally, in chapter 4 investigates carboxylate esters, methyl acetate (MA) and methyl propionate (MP), as co-solvent in electrolyte systems composed of carbonate/LiPF6 in LiNi0.5Co0.2Mn0.3O2 (NCM523)/graphite cells with and without electrolyte additives and the effect of surface composition and structure in electrochemical performance over a wide temperature range (-20 ˚C to 45 ˚C).