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[Ni0.42Mn0.42Co0.16]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. (C) The Author(s) 2015 Published by ECS. All rights reserved.