The reliance on Li-ion batteries is increasing as we transition from fossil fuels to renewable energy sources. Despite their widespread use, a gap remains in understanding certain processes within these batteries, especially regarding the solid electrolyte interphase (SEI) and the impact of side reactions on Li-ion batteries. A custom-made Online Electrochemical Mass Spectrometry (OEMS) instrument was designed to explore these aspects. The OEMS instrument was validated through the study of gas-evolving reactions in the classic LiCoO₂ | Graphite system. In-depth studies focusing on the reaction pathways of ethylene carbonate, the archetype Li-ion battery electrolyte solvent, identified the specific reaction pathways contributing to SEI formation. Moreover, ethylene carbonate’s interaction with residual contaminants like OH^– from H₂O reduction was explored. It was revealed that the integrity of the SEI can be compromised by minor amounts of contaminants, establishing a competitive dynamic at the negative electrode surface between ethylene carbonate and residual contaminants such as H₂O and HF. Additionally, the roles of additives like vinylene carbonate and lithium bis(oxolato) borate in SEI formation were explored. Vinylene carbonate was shown to form a layer on the negative electrode, but also scavenge protons and H₂O, revealing that it is a multi-functional additive. Lithium bis(oxolato) borate on the other hand formed an SEI layer before H₂O reduction, blocking the residual contaminant and ethylene carbonate from reaching the electrode surface. By providing insights into the negative electrode’s interphase and SEI formation through a custom-made OEMS instrument, this research underscores the complexity of reaction pathways and the necessity of considering both major and minor, as well as, primary and secondary reactions for a holistic understanding of Li-ion batteries.

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.