This work introduces a sophisticated impedance-based equivalent circuit model of the electrochemical processes inside a lithium-ion battery cell. The influence on the electrical voltage response is derived and merged into a mathematical calculation framework describing all fundamental phenomena inside a battery. The parameters, whose sole influences on the electric behaviour cannot be separated at the cell level, are summarised to derive a model with purely electrical quantities. We significantly reduce the model order compared to a physicochemical model while ensuring a minimal approximation error. Utilising the findings from the model derivation, we develop a parameterisation procedure to separate the individual processes occurring in the battery and to support a hypothesis of the assignment to positive and negative electrodes based on several indicia. For this purpose, electrochemical impedance spectroscopy and correlation analysis are used to calculate the distribution of the time constants. The final parameterised model has physics-based parameter variations, which ensures that the simulation over broad ranges of temperatures and states of charge results in a reasonable voltage response. The model’s physical basis enables extrapolation beyond the measured operation area, and the model verification shows less than a 10 mV root mean square error over a wide range of operations.

Sodium-ion battery cells represent a promising and increasingly viable alternative to conventional lithium-ion technology, particularly when critical raw materials such as nickel or cobalt are eliminated from the cell chemistry. Na₄Fe₃(PO₄)₂P₂O₇ (NFPP)-based sodium-ion cells are rapidly entering the commercial market, driven by the promise of high cycle stability, significantly reduced material costs, and inherently sustainable, nickel-free composition. While these cells exhibit lower energy density compared to state-of-the-art layered oxide sodium-ion and lithium iron phosphate systems, their resource efficiency could position the technology as an attractive option for stationary energy storage, grid stabilization, starter batteries and emerging applications where cost and longevity are prioritized over gravimetric energy density.

Electrochemical impedance spectroscopy (EIS) is a powerful method for battery diagnostics that enables characterization of internal processes. However, one challenge is the dependence of EIS spectra on the battery cell’s relaxation, which limits its practical use. Since ionic and concentration gradients are not immediately equilibrated after discharge, the impedance drifts over time. Thus, it is important to understand how long different battery chemistries require to reach a stable impedance state, and which frequency ranges and battery model parameters are most sensitive to insufficient relaxation.