Abstract
In lithium-ion batteries, solid electrolyte interphase (SEI) formed on carbon electrodes (negative electrodes) has been studied intensively due to its crucial impact on cycling performance of the cells. This dissertation presents the effects of formation potential range, heat treatment, and the thermal additive, vinylene carbonate (VC), on the formation and stability of SEI by electron spectroscopy for chemical analysis (ESCA), fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), nuclear magnetic resonance (NMR), AC impedance, and certain standard electrochemical techniques. A faster formation process is developed in this study. In addition, the source of thermal instability of SEI and the functions of VC are clearly identified. The formation process presently used in the manufacture of lithium-ion batteries includes the solid electrolyte interphase (SEI) growth process and another process for lithium intercalation into carbon. The latter process is both time and energy consuming. This study proposes a new formation concept that can shorten the formation time by narrowing the potential range and bypassing the intercalation step during formation. The optimal cut-off voltage is found to be 3.7 V for industrial LiCoO2/C cells, and the formation time is markedly reduced to less than one-third of that required in the conventional formation procedure. Cycle performance is not affected significantly because the desired SEI growth is mainly completed in this potential range. These results suggest that our new formation method is superior to the conventional one. The type of lithium salts (LiPF6 and LiClO4) was found to have a strong impact on the thermal stability of the SEI layer formed on graphite electrodes. According to FTIR spectra, the dominant species of the SEI layer is the EC reduction product. Hence, the change of the impedance of the SEI layer is determined by its structure. The oscillating phenomena of the thickness of SEI layer in the LiPF6-system can be attributed to an alternating deterioration and reformation of SEI, illustrating its inherent thermal instability. As for LiClO4 system, because it is less reactive, the build-up of the SEI layer is stable and gradual. In addition, PF5, a decomposition product of LiPF6, was identified to be a chief source of the thermal instability of SEI in the LiPF6-system. Finally, the role of VC as a thermal additive to electrolytes in lithium ion batteries are studied in two aspects: the protection of liquid electrolyte species and the thermal stability of SEI formed from VC on graphite electrodes at elevated temperatures. The NMR spectra indicate VC can not protect LiPF6 salt from thermal decomposition. Hence, the thermally decomposed product PF5 still has the chance to attack SEI. However, the function of VC on SEI can be observed via impedance and ESCA. These results clearly show VC-induced SEI comprises polymeric species and is stable enough to resist thermal damage. It has been confirmed that VC can suppress the formation of resistive LiF, and thus reduce the interfacial resistance. These advantages leads to the improved cycling performance at elevated temperatures.