Coming dissertations at Uppsala university

Li-ion batteries (LIBs) play a crucial role in the transition from fossil fuel to renewable energy sources by providing energy storage solutions for numerous applications. The modern high energy density LIBs were realised largely by the stabilisation of the electrode/electrolyte interface at the negative electrode with the solid electrolyte interphase (SEI). The SEI forms through electrolyte reduction during the first couple of charge/discharge cycles. Despite extensive optimisation of LIBs through electrolyte and SEI development, fundamental understanding of the formation, composition, structure and operating mechanism of the SEI is yet to be established. In order to address this gap, operando analytical techniques and model electrode/electrolyte systems were developed and applied to systematically investigate the formation and evolution of gaseous, soluble and solid reaction products. The thesis provides insights into the mechanisms behind SEI formation as driven by several important electrolyte solvents and additives, namely ethylene carbonate (EC), propylene carbonate (PC), vinylene carbonate (VC), lithium bis(oxalato)borate (LiBOB), 1,3,2-dioxathiolane 2,2-dioxide (DTD), and prop-1-ene-1,3-sultone (PES). Key findings include that EC and PC are reduced via identical reaction pathways, but with product of different solubilities, which in turn explains the difference in their ability to passivate the electrode surface. For the layer-forming additives, reduction pathways and their impact on the SEI passivation properties were identified and discussed. Although VC significantly reduces the layer thickness, the SEI is much denser and therefore more resistive to ion transport. LiBOB, DTD and PES were all found to have similar influence, namely passivate the electrode and suppress EC reduction, but each with their own peculiarities. LiBOB was found to be reduced already at 1.8 V vs. Li+/Li to form primarily Li-oxalate based SEI along with electrolyte soluble borates. DTD reduces to primarily gaseous and solid products with a comparatively thicker SEI at 1.4 V vs. Li+/Li. PES leads primarily to electrolyte-soluble product at 1.4 V vs. Li+/Li, which in a subsequent step generates a comparatively thinner SEI. Both VC and PES reduced H2 evolution, which was explained by the H2O and H scavenging ability of their unsaturated carbon-carbon bonds. Based on the collective findings herein, an effective layer-former suppresses the reduction of both the electrolyte and its impurities and generates a 10 nm thick and dense SEI comprised primarily of inorganic Li-species. The insights into the SEI formation process on a fundamental level of the aforementioned solvents and additives showcase the ability of complementary operando analytical techniques to unravel the complexity of the elusive SEI. The insights thus gained guide the optimisation and development of electrolytes for current and future LIB chemistries.