Si negative electrodes for Li-ion batteries : Aging mechanism studies by electrochemistry and photoelectron spectroscopy

Sammanfattning: This thesis is focusing on the challenges when using Si as a possible new negative electrode material in Li-ion batteries. The overall aim is to contribute to a general understanding of the processes in the Si electrode, to identify aging mechanisms, and to evaluate how they influence the cycling performance. Another objective is to investigate how photoelectron spectroscopy (PES) can be used to analyze these mechanisms.LiPF6 based electrolytes are aggressive towards the oxide layer present at the surface of the Si particles. With the use of fluoroethylene carbonate (FEC) as an electrolyte additive the cycling performance is improved, but the oxide layer is still affected. A recently developed salt, lithium 4,5-dicyano-2-(trifluoromethyl)imidazolide (LiTDI), is shown not to have any detrimental effects on the oxide. The SEI with FEC and vinylene carbonate (VC) as contains a high concentration of LiF and polymeric carbonate species and this composition seems to be beneficial for the cycling performance, but the results indicate that additional aging mechanisms occur. Therefore, electrochemical analysis is performed and confirms a continuous SEI formation. However, it also reveals a self-discharge mechanism and that a considerable amount of Li is remaining in the Si material after standard cycling.PES is used in this work to analyze the SEI-layers as well as the surface and the bulk of the Si material. With this technique it is hence possible to distinguish changes in the Si material as a function of lithiation. To improve the data interpretation of PES spectra, a range of battery electrode model systems are investigated. These results show shifts of the SEI peaks relative to the electrode specific peaks as a result of the SEI thickness and the presence of a dipole layer. Also other electronically insulating composite electrode components show relative peak shifts as a function of the electrochemical potential.To summarize, these studies investigate a number of well recognized aging mechanisms in detail and also establish additional processes contributing to aging in Si electrodes. Furthermore, this work highlights phenomena that influence data interpretation of PES measurements from battery materials.