Studies of the SEI layers in Li(Ni0.5Mn0.3Co0.2)O2/Artificial Graphite Cells after Formation and after Cycling

Li(Ni0.5Mn0.3Co0.2)O2/artificial graphite cells containing different electrolyte additives were studied using electrochemical impedance spectroscopy (EIS) and X-ray photoelectron spectroscopy (XPS) after formation and after long-term charge-discharge cycling. Positive and negative electrodes were examined separately in symmetric cells to study the solid electrolyte interphase (SEI) at each electrode. EIS measurements were taken vs temperature, and activation energies (Ea) related to Li+ transport through the SEI were calculated. After cycling, Ea differed depending on electrolyte additive, electrode type, and cycling 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 appeared 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.