Lithium-ion batteries constitute a leading technology that plays a major role in the transition towards sustainable transportation and power generation. The stability of modern batteries relies on a passivation layer formed on the negative electrode known as the solid electrolyte interphase (SEI). Despite concerted efforts to comprehend the various processes taking place during SEI formation, monitoring the reaction pathways in real-time is still very challenging. This is due to the complex interactions within the multicomponent electrochemical system, aggravated by the wide range of electrolyte compositions, electrode materials, and operating conditions.

In this thesis, operando surface enhanced Raman spectroscopy is explored to elucidate the progressive formation of the SEI on the negative electrode surface when the electrode is negatively polarised in a spectro-electrochemical cell. Complementary online-electrochemical mass spectrometry is employed to identify the associated gaseous products formed during the process. The work illustrates that the electrolyte as well as contaminants, such as O₂, CO₂, and H₂O, contribute in electro-/chemical processes that build up the SEI. The thesis then explores reaction pathways involving a SEI-forming electrolyte additive, namely vinylene carbonate (VC), emphasizing its role as a H₂O scavenging agent. In comparison to the conventional electrolyte solvent ethylene carbonate, VC exhibits a faster reaction with water impurities, particularly in presence of hydroxide ions. This results in the formation of products that are less likely to impact cell performance.

In the later part, the thesis delves into understanding the stability of electrolyte in an environment of Lewis bases (LB) typically found in the SEI. For that, individual LB (e.g., OH^- and OCH₃^-) are mixed with typical carbonate-based solvents and the products formed as a result of the reaction are analysed. Furthermore, tris(trimethylsilyl)phosphate (TMSPa), a representative of the silyl-functionalised electrolyte additive and known for its reactivity, especially towards fluorides, is used as a means to chemically probe its reactivity towards several LB residues. This investigation aims to establish a more simplified and generally applicable reaction mechanism thereof. The products that are soluble in the electrolyte have been investigated by nuclear magnetic resonance spectroscopy and those in the gas phase is characterised by mass spectrometry. The work highlights that the residues that remain active even after the SEI formation may lead to unwanted side-reactions.

The thesis contributes to a deeper fundamental understanding of the myriad of processes that take place in batteries during SEI formation providing insights crucial for designing next-generation battery materials.