An analysis protocol for three-electrode Li-ion battery impedance spectra: Part II. Analysis of a graphite anode cycled vs. LNMO

Lithium-Ion batteries consisting of LNMO (LiNi_0.5Mn_1.5O₄) cathodes and graphite anodes show severe capacity fading at elevated temperatures due to a damage of the solid electrolyte interface (SEI) on the anode. Hence, a detailed investigation of the anode with electrochemical impedance spectroscopy (EIS) can provide valuable insight into the phenomenon of anode degradation. In this study, we use a modified version of our novel impedance procedure (Part I of this study), where the anode impedance is measured at non-blocking conditions (10% SOC) and blocking conditions (0% SOC) in a graphite/LNMO full-cell with a gold wire micro-reference electrode (GWRE). We show that during cycling an ionic contact resistance (R_Cont.Ion) at the separator/anode interface evolves, which is most likely caused by manganese dissolution from the high-voltage cathode (LNMO). By simultaneously fitting EIS spectra in blocking and non-blocking conditions, we can deconvolute the anode impedance evolving over 86 cycles at 40°C into contributions of: a) the separator resistance (R_Sep.), b) the true charge transfer resistance (R_CT), and, c) the ionic contact resistance (R_Cont.Ion) evolving at the separator/anode electrode interface. We also show that the main contributor to a rising anode impedance is the ionic contact resistance (R_Cont.Ion).