Exploring Impedance Growth in High Voltage NMC/Graphite Li-Ion Cells Using a Transmission Line Model

Abstract

A discrete transmission line model (TLM) for the impedance of the positive electrode in a Li-ion cell was studied to investigate causes of impedance increase for Li[Ni0.42Mn0.42Co0.16]O2 (NMC442) positive electrodes operated at high voltage (> 4.4 V vs. Li/Li+). The TLM included contact resistance between the conductive carbon and the active particles (Rc), electrical path resistance through the carbon network (Re), ionic path resistance through the bulk electrolyte (Ri) and transfer resistance/capacitance (Rs, C) through the SEI layers formed on the active particles. It was found that an increase in any of Re, Ri or Rc was necessary to increase the high frequency intercept of the impedance spectra. A limited increase in the spectrum diameter of the TLM was achievable by increasing Re or Ri, but an unlimited increase was only possible by increasing the SEI resistance Rs. Comparison with experiment concluded that the high voltage impedance growth observed in NMC442/graphite cells is primarily due to increases in Rs, while minor increases in Re, Ri or Rc may occur. A brief investigation of inhomogeneous SEI capacitance/resistance produced impedance spectra with a range of heights and asymmetries. This can explain in part the variety of shapes of impedance spectra from real impedance measurements of Li-ion cells.