Master of Science, The Ohio State University, 2019, Chemistry

Since its discovery in 1979, the solid electrolyte interface (SEI) has drawn attention due to its importance for efficient battery performance. Despite its significance, there is still some ambiguity with regards to how certain electrolytes form a protective interface to aid Li+ intercalation whilst others irreversibly degrade the anode structure. One of the most debated examples is the “EC-PC disparity” towards the graphite anode, where ethylene carbonate (EC) results in protective SEI formation whilst propylene carbonate (PC) leads to destructive graphite exfoliation. This study focuses on the correlation between the concentration of the Li+ salt, LiPF6 dissolved in PC and the growth of SEI on graphite. In-situ and in-operando electrochemical scanning tunneling microscope (STM) observation of the basal plane of highly oriented pyrolytic graphite (HOPG) is performed in 1 M, 2.5 M and 3 M LiPF6 dissolved in propylene carbonate (PC) in sequence to cyclic voltammetry (CV) and potential hold experiments. This technique allows the study of the topographic evolution of the basal plane and edge sites on HOPG with changes in the potential, as a result of solvent co-intercalation, reduction, graphite exfoliation and SEI formation. STM images obtained whilst holding the potential at selected values, gives an insight into the nature of SEI formation, in connection to the extent of regeneration of the graphite surface. It is found that below 0.9 V vs Li, solvent decomposition, followed by extensive graphite exfoliation takes place in the 1 M and to a lesser degree in 2.5 M LiPF6 electrolyte solutions. However, in the 3 M salt solution, graphite exfoliation is scarce and SEI formation is observed at potentials as high as 1.1 V vs Li, which clearly indicates a concentration dependent SEI formation on graphite. CV experiments conducted in parallel to EC-STM, provide further confirmation. The HOPG is cycled between 3 to 0.005 V vs Li in all three salt concentrations within Swa (open full item for complete abstract)

Committee: Anne Co Dr. (Advisor); Zachary Schultz Dr. (Committee Member) Subjects: Chemistry