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

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[Ni 0.42 Mn 0.42 Co 0.16 ]O 2 (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 (R c ), electrical path resistance through the carbon network (R e ), ionic path resistance through the bulk electrolyte (R i ) and transfer resistance/capacitance (R s , C) through the SEI layers formed on the active particles. It was found that an increase in any of R e , R i or R c 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 R e or R i , but an unlimited increase was only possible by increasing the SEI resistance R s . Comparison with experiment concluded that the high voltage impedance growth observed in NMC442/graphite cells is primarily due to increases in R s , while minor increases in R e , R i or R c 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.