Blend electrodes are widely used as lithium-ion battery cathodes and are getting more and more prominent on the anode side. The idea of blend electrodes is to combine characteristic properties of different active materials with the aim of improving overall cell performance or to adjust specific performance requirements according to the application of the cell. While being beneficial in this regard, blend electrodes pose challenges for the parametrization of corresponding electrochemical models, especially regarding the state-of-charge (SoC) -dependent charge transfer kinetics. Most reported models in literature for blend electrodes either use an averaged kinetics approach and/or fit the charge transfer of a multi-particle model to experimental discharge curves.
In this contribution we present a way to extract the SoC-dependent charge transfer resistance of a commercial NMC-LMO blend cathode by combining FIB/SEM microstructure characterization, electrochemical impedance spectroscopy (EIS) and transmission line modelling (TLM). The relevant processes are identified from the EIS-spectra with the help of the distribution of relaxation times (DRT) and quantified by fitting to an adapted transmission line model. The contribution of the single active materials, NMC and LMO, to the overall charge transfer resistance is then broken down by applying commonly used theories for SoC-dependent charge transfer kinetics for intercalation electrodes. Here it was observed that a thermodynamic consistent approach for the SoC-dependent kinetics leads to a closer match to the experimental results than the widely used Newman-type approach. The obtained values for the charge transfer resistances of NMC and LMO respectively are in the range of previously reported charge transfer resistances for the single active materials at SoC 50, endorsing the validity of the presented method.