A Powerful Transmission Line Model for Analysis of Impedance of Insertion Battery Cells: A Case Study on the NMC-Li System

Abstract

Transmission line modeling (TLM) has become an important approach for analysis of measured impedance spectra of battery cells. Still, the existing models lack some important features which prevents a full and accurate analysis of measured spectra. Here we present a general physics-based TLM that contains all the processes known from treatments of transport/reaction schemes based on the Newman porous electrode model or the corresponding Poisson-Nernst-Planck framework. Compared to previous TLMs, the present model additionally takes into account the movement of non-active mobile ions in all phases. After detailed description of the individual TLM model elements and their physical background we show, step-by-step, how it can be used to interpret the measured response of NMC cathodes, lithium anodes and full NMC-Li cells. Many details are considered the origin of which is identified by performing carefully designed additional experiments, e.g. electrolytes with variable concentration of salt. Besides identifying all the meaningful impedance features of electrodes and full cells, we also show how various materials parameters such as transport number, diffusion coefficient, conductivity etc can be calculated for various phases appearing in a porous battery electrode system. Finally, we use the findings to analyze the development of impedance response during discharge of a NMC-Li cell.