Interfacial properties and reactions of ionic liquids on silicon and graphite surfaces studied byX-ray Photoelectron Spectroscopy
In this work, a fundamental scientific research in affiliation to ionic liquids and lithium ion batteries technologies was carried out. X-ray photoelectron spectroscopy (XPS) was used to study the interface properties of ILs in contact with Si-based and HOPG support materials as well as the formation process of SEI layer with lithium and lithium salt. In the first stage of this research, several surface-related thin film IL studies were addressed including their chemical component as well as chemical environment identification, growth behavior on different silicon surfaces and beam damage under Al Kα radiation analysis. Thin films of IL ([OMIm]Tf2N) were thermally evaporated onto different silicon surfaces including H-Si, Cl-Si, CH3-Si and clean Si with different coverage, from sub-monolayer up to 8 nm. The silicon support was subjected to different surface modification procedures, which were shown to positively influence the IL growth. These studies also revealed some differences of the initial adsorption behavior and further growth of IL depending on the surfaces. The thickness of thin film of IL was approximately proportional to evaporation time at its initial growth stage on all surfaces, which suggests that the average of the first to four nanometers is grown with lay-by-layer structure. The thin films of the ILs then form islands structure rather than a homogeneous film upon increasing the amount of ILs as the interaction between IL species and solid surface gradually decays. With respect to SEI layer formation study, we compared the interaction of lithium and of an IL with each other and with surface-passivated Si(111) and with clean Si(111), respectively, by XPS. This study gives an insight into the interfacial properties and reactions of ILs with lithium on a silicon surface, providing information on the formation of SEI layers on semiconductor surfaces in general. The interaction of lithium and the IL [OMIm]Tf2N with each other and with surface-passivated Si(111) and with clean Si(111) were studied by XPS. The deposition of Li on [OMIm]Tf2N thin-film-covered silicon surface results in the decomposition of the IL and the intercalation of lithium into silicon. Possible decomposition products are LiF, LixO, CxHy , LiCxHyNz , F3C−O2S−N−Li+ , and F3C−O2S−Li+ . In addition, the formation of a stable Si/IL interface and of Si/Li surface alloys was proved to be an effective strategy in stabilizing Li for next-generation Li-ion batteries. Furthermore, the interaction of the ionic liquid [EMIm]FSI and of LiFSI with the lithiated graphite surface was studied by XPS. Their chemical reactions were investigated by stepwise evaporation of an IL and a lithium salt on lithiated graphite and monitored their decomposition process as well as the composition of the passivation layer by XPS. The results indicate that the interaction of an IL adlayer with lithiated graphite leads to a stable passivation layer, which inhibits the deintercalation of Li. The passivation layer is composed of the decomposed products from both the [EMIm]+ cation and the FSI– anion, such as methyl-, ethyl-imidazole, LiF, Li2O, LiNSO2, and LiSO2F. Our work can provide valuable information about designing a suitable electrolyte to get a stable SEI layer for battery applications. In conclusion, this thesis documents fundamental research addressing several surface-related investigation of macroscopically thin IL films, and the SEI layer formation mechanism on IL/solid interface in the context of IL-coated and lithiated silicon, lithium-intercalated HOPG surfaces. The presented results contribute to a large number of IL-related research fields. Furthermore, the detailed understanding of the IL/solid interface at a molecular level is expected to be beneficial for various research areas such as heterogeneous catalysis, electrochemistry, energy storage devices and coating technologies.