<p>Recently, the degradation mechanisms of commercial sodium-ion batteries were investigated by Liu et al., who proposed a temperature-resistant method for state of health (SOH) estimation using electrochemical impedance spectroscopy (EIS). However, their analysis was restricted to the impedance response (Z*) over the frequency range of 0.005 Hz to 2500 Hz. To advance this investigation, the present study provides a deeper analysis by extrapolating the impedance data over a broader frequency range (10⁻⁴ – 10⁴ Hz). Our theoretical approach enabled the identification of relaxation processes through the analysis of the imaginary component (Z″) and extends this investigation to complex conductivity (σ*). This analysis confirms the existence of three relaxation processes in the complex conductivity (σ*) spectra. A deconvolution procedure coupled with our theoretical approach enabled the identification of each process in the high, medium, and low-frequency domains, corresponding to SEI dynamics, charge-transfer, and solid-state diffusion, respectively. In addition, key parameters obtained from each relaxation, including the relaxation time (τ_σ), the equivalent capacitance (C_eq), and the conductivity values at various frequencies, show excellent agreement with the charge dynamics at each specific interface from the anode to the cathode. Notably, the identification of a low-frequency relaxation (τ_σ = 100 to 110s) provides new insights into ion transport and structural fatigue, validated by recent experimental literature. This work demonstrates that complex conductivity (σ*) is a powerful tool for monitoring interfacial behavior and aging mechanisms across multiple timescales.</p>