The effect of the SEI layer on the electrochemical impedance in the graphite/ Li[Ni0.5Mn0.3Co0.2]O2 lithium-ion full cells

In this work, we investigated the effect of the solid-electrolyte interface (SEI) on the charge transfer reaction on the negative electrode by the electrochemical impedance spectroscopy method. Also, the results were evaluated for the two types of full cells with lithium bis(trifluoromethanesulfonyl)imide and without additives to create different SEI layers on the graphite. Here we employed an equivalent circuit for full cells and evaluated high and low-frequency semicircles in four tests. The middle frequency was considered to the charge transfer and SEI layer resistance. Exposure to longer holding times at constant potential resulted in less change for the middle-frequency component than shorter holding times. The higher and lower resistance changes were attributed to the growth of the inner and outer SEI layers on the graphite surface. Also, the cell containing additive showed a lower rate of resistance change and lower activation energy for the middle-frequency semicircle. Improved SEI layer led to lower electrolyte reduction on the negative electrode surface. The XPS analysis confirmed the growth of the outer layer on the inner layer after a long hold time and the lower growth of the SEI layer in the presence of the additive.