From Multi-Physics to Electrical Circuit Models of Aged Graphite Electrodes of Lithium-Ion Battery

Abstract

A two-step up-scaling methodology is established to determine a temporal Electric Equivalent Circuit (EEC) model of aged graphite electrodes. The methodology is fed with original experimental results of graphite impedance spectra obtained upon aging on graphite-NMC pouches instrumented with reference electrodes. An analytical expression is first derived which includes the impedance contributions from interfacial kinetics, double-layer adsorption, solid-phase diffusion processes in the graphite particles as well as lithium diffusion through the SEI layer. The derivation of the temporal EEC model from this analytical expression using Foster series allows to link the EEC model parameters to materials properties and to tune the accuracy of the model by choosing the number of RC elements in the series. The validity of the model is ascertained by comparison with the experimental data obtained at different aging time and for two cycling temperature conditions. In particular, accounting for lithium diffusion through the SEI is necessary to capture the evolution of the impedance spectra of significantly aged graphite electrodes.