Electrode contributions to the impedance of a high-energy density Li-ion cell designed for EV applications
Résumé
Electrochemical Impedance Spectroscopy (EIS) measurements are performed on a high energy density Li-ion cell, which consists of a graphite-based composite negative electrode, a LiMn2O4-layered oxide blended composite positive electrode, and a carbonate solvents-based electrolyte. Signal analysis is applied to get information regarding cell internal components. The impedance behavior at different states of charge (SOC) is reproduced in an optimized laboratory scaled 3-electrode cell, which enables to: separate each electrode impedance contribution and understand the impedance variations when changing SOC. In the negative electrode, the impedance decreases when increasing SOC because the charge transfer process on the surface of graphite particles becomes less resistive with the lithium content. In the positive electrode, the impedance decreases rapidly at the beginning of charge because at low SOC the EIS response is governed by the layered oxide particles, whose electronic properties strongly depend on the lithium content. Finally, the negative electrode contribution to the total cell impedance varies between 60 and 74%; whereas the positive electrode contribution, being the complement, varies between 40 and 26%.