Doctor of Philosophy (PhD)
Novel battery technologies are being developed due to the high global demand for energy in many fields where different specifications are required. The battery electrolyte provides a medium for ions to diffuse between electrodes and its composition determines the compatibility with the electrode pair. While many works focus on characterizing novel electrolyte systems to advance new batteries, the understanding of the microscopic structure and dynamics of speciations present in these electrolytes is limited. Many experiment methods have obtained incomplete time-averaged structure information due to lack of instrumental time resolution compared to the molecular intrinsic time sales of electrolytes that range from femtoseconds to sub-picosecond. Work described in this dissertation utilized linear and nonlinear infrared spectroscopic techniques in conjugation with computational tools to investigate the structure and picoseconds dynamics of novel battery electrolytes. Glyme based electrolyte systems were identified as potential candidates for sodium-ion battery systems and the first project focused on the microscopic structural and dynamical changes of the ion pairs with the length of the glyme. The preliminary study of Fourier transform infrared (FTIR) spectroscopy revealed a strong ionic association in these electrolytes regardless of the glyme length. Two dimensional infrared (2DIR) spectroscopy and IR pump-probe spectroscopy deduced the characteristic time constants of the motions associated with the anion. The experimental findings were later validated with numerical simulation and density functional theory (DFT) calculations. A similar approach was followed to investigate the interactions and motions associated with highly concentrated systems that are composed of bis(trifluoromethanesulfonyl)imide (LiTFSI) and carbonyl-containing solvents. While the overall speciations present in each solvent remain the same, an additional interaction was identified in the cyclic solvent, which was later determined to affect the macroscopic properties of the electrolyte. Finally, the ionic concentration gradient and changes of the solvation structure and dynamics of electrolytes in presence of an external electric field were studied by probing the anion with linear and nonlinear IR methods. A decrease of the anion near the negative electrode indicated the building of concentration gradient across the electrolyte system and breaking of ionic species was also observed.
Galle Kankanamge, Susith Rajitha Abeythunga, "Investigation of the Structure and Dynamics of Novel Battery Electrolytes by Utilizing Multidimensional Infrared Spectroscopic Techniques" (2021). LSU Doctoral Dissertations. 5455.