A Physically-Based Equivalent Circuit Model for the Impedance of a LiFePO₄/Graphite 26650 Cylindrical Cell

Abstract

In this work an Equivalent Circuit Model (ECM) is developed and used to model impedance spectra measured on a commercial 26650 LiFePO 4 /Graphite cylindrical cell. The ECM is based on measurements and modeling of impedance spectra recorded separately on cathode (LiFePO 4 ) and anode (Graphite) samples, harvested from the commercial cell. Modeling of the single-electrode impedance spectra provided information about the electronic and ionic resistance in the porous composite electrodes, as well as the solid state diffusion. Focused Ion Beam (FIB)/Scanning Electron Microscopy (SEM) of anode and cathode samples was used to make 3-D maps of the electrode microstructures and to obtain microstructural data for the ECM. The cylindrical cell continues to be one of the most widely used packaging styles for primary and secondary batteries. The advantages are ease of manufacture and good mechanical stability. The tubular cylinder can withstand high internal pressures without deforming.

Electrochemical Impedance and its Applications in Energy‐Storage Systems

“…A good recent example was to use AC impedance and the TML model to investigate the LiFePO 4 cathode and graphite anode recovered from a commercial 26650 cylindrical cell . Figure shows the AC impedance spectra and fitting results of a fully charged LiFePO 4 cathode (Figure d) and in a 100% discharged state (FePO 4 , Figure e).…”

“…d,e) Nyquist plots of the Li 1− x FePO 4 at 0% SOC ( x = 0) (d), and 100% SOC ( x = 1) (e). a–e) Reproduced with permission . Copyright 2017, The Electrochemical Society.…”

“…The semiinfinite Warburg impedance can be applied. Then Equation (31) or (33) can be used to calculate the diffusion coefficient, D.…”

“…It is obvious that the finite diffusion model fitted the experimental data better, especially in the low-frequency range and for the partially discharged electrode. The Warburg prefactor, σ can be obtained by fitting the impedance spectra using either Equation (33) or (43) can be measured through coulometric titration and practically it can be calculated from the slope of an OCP vs depth of discharge curve. However, diffusion length (δ), the interfacial surface area (A) and the molar volume (V m ) at various stages of the discharge need to be determined.…”

“…A good recent example was to use AC impedance and the TML model to investigate the LiFePO 4 cathode and graphite anode recovered from a commercial 26650 cylindrical cell. [33] Figure 17 shows the AC impedance spectra and fitting results of a fully charged LiFePO 4 cathode (Figure 17d) and in a 100% discharged state (FePO 4 , Figure 17e). Figure 17a Figure 17c represents the electrolyte resistance and the layer the active material which was intimately in contact with the Al current collector, which can be considered as 2D interface.…”