GITT Limitations and EIS Insights into Kinetics of NMC622

The galvanostatic intermittent titration technique (GITT) was applied to NMC622 positive electrodes, with Electrochemical Impedance Spectroscopy (EIS) performed at quasi-equilibrium conditions determined by cutoff criteria based on relaxation rates. Below an open-circuit voltage (OCV) of 3.8 V, the cutoff criterion of 0.1 mV h−1 was reached after approximately 8 hours. However, above 3.8 V, a non-saturating voltage decay was observed, increasing up to ∼0.56 mV h−1 above 4.1 V during charging steps. This persistent voltage decay upon subsequent discharging steps led to non-monotonic relaxation behavior. A pulse time of 10 minutes did not satisfy the t dependence required for GITT kinetic analysis. Instead, the initial 36-second transients were extended for chemical diffusivity evaluation, aligning with the Warburg-like response observed in EIS, consistent with the sequential reaction-diffusion assumption. GITT analysis for solid-state diffusivity is ineffective for spherical active particles dispersed in porous electrodes and performs even worse due to liquid-phase diffusion within the pores, where t+=0.3. The apparent SOC-independent chemical diffusivity obtained from GITT across both low and high OCV ranges suggests that the process is dominated by liquid-phase diffusion. The application of the physics-based three-rail transmission line model (TLM) developed by Gaberšček et al. in EIS holds practical potential for deconvoluting the two diffusion kinetics.