Method to Quantify Interfacial and Charge Transfer Resistance in the Context of Lithium-Ion Cell Ageing

This paper explains and demonstrates a comprehensive process for extracting and quantifying SEI, anode and cathode charge transfer resistance from Electrochemical Impedance Spectroscopy (EIS) data. Distribution of Relaxation Times (DRT) is combined with gaussian distribution fitting and frequency domain circuit elements to construct equivalent circuit models representing the individual resistance contributions. The process accounts for high frequency ohmic resistance and low frequency diffusion and capacitance effects to isolate the charge transfer and SEI contributions. This gives sufficient information to allow evaluation of the changes in charge transfer and SEI with conditions and ageing. An ageing case study of 28Ah prismatic Liion cells with Nickel-Manganese-Cobalt oxide (NMC) cathode and graphite anodes was used to demonstrate the process. Two cells aged for 9 months at 45°C using through repetitive drive cycles were compared to a new uncycled cell. EIS testing was performed in a full-factorial approach of multiple temperature and voltage points to evaluate the resistance changes with condition. A detailed evaluation of the quantified results was performed using the described approach. This identified that Solid Electrolyte Interphase (SEI) and Anode charge transfer resistance increased with ageing however cathode charge transfer decreases significantly. This cathode effect was theorized to be from stresses during Li-ion intercalation. All evaluated resistances increased in capacitance independent of resistance change, which is not often analyzed in literature but observable through the demonstrated process. Time constant increased of up to 400% were observed. The quantitative yet physically underpinned results were found to be useful in identifying degradation causes and would be valuable insight to assist modelling and control applications. The demonstrated technique easily identifies key parameter changes in individual charge transfer impedances for modeling applications.