Temperature Dependent EIS Studies Separating Charge Transfer Impedance from Contact Impedance in Lithium-Ion Symmetric Cells

Abstract

Measuring electrochemical impedance spectra of lithium-ion symmetric cells at low temperatures allows for unambiguous separation of charge transfer impedance contributions from other cell impedance features. Electrodes from dry Li[Ni0.5Mn0.3Co0.2]O2 (NMC 532)/artificial graphite (AG) pouch cells were used to make blocking electrode configuration symmetric cells and blocking electrode full coin cells. NMC532/AG cells with 1.2M LiPF6 in EC:DMC 3:7 (w:w) electrolyte also underwent the formation process before the electrodes were used to make non-blocking configuration symmetric and full coin cells. Through the use of temperature dependent measurements on blocking and non-blocking cells, the impedance contribution due to faradaic, charge transfer interactions can be distinguished from non-faradaic, high frequency impedance contributions. Fitting the EIS spectra allowed both the charge transfer resistances and the related capacitances to be determined for the solid electrolyte interphase layers on the negative and positive electrodes. The charge transfer resistances were strongly temperature dependent while the capacitances were not. The temperature dependence of the charge transfer resistance was used to determine the activation energies for lithium transport through the solid electrolyte interphase layers on the negative and positive electrodes. Surprisingly, the activation energies were approximately 0.6 eV for both positive and negative electrodes extracted from cells after formation.