PROBING CHANGES IN THE SOLID ELECTROLYTE INTERPHASE IN LI-ION CELLS
In Li-ion cells, a passivating film called the solid electrolyte interphase (SEI) develops at the interface between the electrolyte and the electrode particles. The SEI is essential to the functionality of the cell, but the SEI can also have detrimental effects on the cell. The SEI impacts the internal cell impedance and capacity retention over the lifetime of the cell. Despite being a vital part of Li-ion cells, the properties of the SEI are not well known or understood. This thesis studies several properties of the SEI through a variety of techniques. Cells containing different electrolyte additives were studied using electrochemical impedance spectroscopy (EIS) and X-ray photoelectron spectroscopy (XPS) after formation and after long-term cycling. Positive and negative electrodes were examined separately to study the SEI at each electrode. EIS measurements were taken versus temperature, and activation energies (Ea) related to Li+ transport through the SEI were calculated. Ea values for the negative electrode SEI varied depending on the choice of electrolyte additive, but values did not vary substantially for the positive electrode SEI. After cycling, the Ea differed depending on electrolyte additive, electrode type, and voltage limits. Charge transfer resistance was also compared after formation and cycling and did not always correlate with Ea trends, suggesting that multiple factors influence SEI properties. XPS was used to study the chemical composition and thickness of the SEI. Electrolyte additives affected the quantity of inorganic materials in the SEI, and more inorganic material appears to correlate with lower Ea values. Cells containing lithium difluorophosphate electrolyte additive had the best lifetime of the cells studied in this work. These cells also showed the lowest SEI activation energy values, lowest charge transfer resistance, and most inorganic SEI composition after cycling.