Degradation Diagnosis of Li(Ni_0.5Mn_0.2Co_0.3)O₂/Li Half-cell by Identifying Physical Parameter Evolution Profile Using Impedance Spectra During Cycling

Electrochemical Impedance Spectroscopy (EIS) is considered as a quick and nondestructive diagnostic tool to characterize the degradation of LIBs within a short testing timeframe. In this study, to obtain the relationship between EIS spectra and cell capacity degradation, our previously reported physics-based EIS model is further utilized to interpret EIS spectra of Li-NMC(5:2:3) half-cell measured under cycling. The EIS spectra show that the polarization impedance (R_p) increases with the number of cycles under different open circuit voltages (OCVs), especially in the mid-frequency range. After interpreting EIS spectra by the physics-based model, we found that the diffusion coefficient, ionic conductivity, and cathode reaction rate at different OCVs all decrease with the number of cycles. The impedance variation caused by the change of cathode reaction rate during cycles is much more significant than that caused by the other two parameters. Furthermore, the cell capacity degradation is linearly related to the charge transfer resistance (R_ct2) caused by cathode/electrolyte interface reaction rate at specific OCVs and it could serve as the indicator of cell capacity fade beyond 10 cycles.