The main aim of this thesis is to reveal the chemical structures of the solid-liquid interphase in lithium ion batteries by NMR spectroscopy in order to understand the working mechanism of electrolyte additives for achieving stable cycling performance.

In the first part, a combination of solution and solid-state NMR techniques, including dynamic nuclear polarization (DNP) are employed to monitor the formation of the solid electrolyte interphase (SEI) on next-generation, high-capacity Si anodes in conventional carbonate electrolytes with and without fluoroethylene carbonate (FEC) additives. A model system of silicon nanowire (SiNW) electrode is used to avoid interference from the polymeric binder. To facilitate characterization via one- and two-dimensional NMR, ^13C-enriched FEC was synthesized and used, ultimately allowing a detailed structural assignment of the organic SEI. FEC is found to first defluorinated to form soluble vinylene carbonate (VC) and vinoxyl species, which react to form both soluble and insoluble branched ethylene-oxide-based polymers.

In the second part, the same methodology is applied to study the decomposition products of pure FEC or VC electrolytes containing 1 M LiPF_6. The pure FEC/VC system simplifies the electrolyte solvent formulation and avoids the interaction between different solvent molecules. Polymeric SEIs formed in pure FEC or VC electrolytes consist mainly of cross-linked PEO and aliphatic chain functionalities along with additional carbonate and carboxylate species. The presence of cross-linked PEO-type polymers in FEC and VC correlates with good capacity retention and high Coulombic efficiencies of the SiNWs anode. Using ^29Si DNP NMR, the interfacial region between SEI and the Si surface was probed for the first time with NMR spectroscopy. Organosiloxanes form upon cycling, confirming that some of the organic SEI is covalently bonded to the Si surface. It is suggested that both the polymeric structure of the SEI and the nature of its adhesion to the redox-active materials are important for electrochemical performance.

Finally, the soluble decomposition products of EC formed during electrochemical cycling have been thoroughly analyzed by solution NMR and mass spectrometry, in order to explain the capacity-fading of Si anodes in a conventional EC-based electrolyte and address questions that arose when studying the additive-containing electrolytes. The detailed structures for the EC-degradation products are determined: a linear oligomer consist of ethylene oxide and carbonate units is observed as the major degradation product of EC.