Impedance Response Model of a Lithium Ion Cell with a Phase Change Electrode

Abstract

Modeling the impedance response of a porous electrode composed of spherical intercalation particles that undergo phase transition, requires taking into account the physics that describes the two-phase coexistence. In the present work, electrode particles with two-phase coexistence are modeled as shell-core structures. It is observed that the presence of phase boundary within the particle gives rise to an impedance response distinct from that of a single-phase particle, at low frequencies. A closed form, analytical solution for the diffusion impedance is obtained for electrode particles that exhibit phase transitions. Using this single particle model, the impedance response of a porous electrode composed of spherical electrode particles is determined. The dependence of the diffusion impedance on the position of the two-phase interface and in turn on the electrode state of charge (SOC) is studied. It is demonstrated that the features in the low frequency response have unique signatures that can be developed as a state estimator. Additionally, the impedance response of a cell consisting of a phase-change positive electrode and a non-phase change negative electrode is obtained at various SOCs. The proposed model is used to study the sensitivity of parameters on the cell impedance.