We present a comprehensive and versatile methodology for the acquisition, preliminary interpretation, and advanced modeling of impedance spectra of lithium-ion insertion electrodes. Using a Ni-rich NMC cathode as a model system, we outline the essential experimental requirements for obtaining reliable impedance data and demonstrate how rigorous characterization of active material particles, electrode morphology, and porosity contributes to the consistency and interpretability of electrochemical impedance spectroscopy (EIS) results. Particular attention is given to common experimental pitfalls and typical misinterpretations of impedance features that often arise in studies of porous insertion electrodes.

A key contribution of this work is the introduction and application of a scaling-based normalization approach for impedance parameters.[1] We show that simple mass-normalized Nyquist plots, together with the assumption of ideal capacitive behavior, enable a fast preliminary estimation of the total chemical insertion capacitance, C_total. Importantly, we experimentally confirm for Ni-rich NMC that C_total obtained from sufficiently low-frequency impedance measurements is quantitatively equivalent to the differential capacitance C_d(x) extracted from the equilibrium OCV curve.[1] This demonstrates that low-frequency EIS can directly probe the thermodynamic properties of single-phase insertion materials in realistic porous electrodes.

In the second step, we systematically analyze impedance spectra of a dedicated series of NMC cathodes with increasing electrode mass loading using an advanced physics-based transmission line model (TLM). We verify theoretically predicted scaling laws for all model parameters and demonstrate how electrode geometry governs the detectability and magnitude of individual impedance contributions.[2] The results highlight a critical practical implication: for a given mass loading, only a subset of impedance processes emerge above the measurement sensitivity threshold. Consequently, accurate parameter extraction typically requires impedance spectra collected from electrodes spanning a wide and well-controlled range of masses. Moreover, the analysis reveals an unexpected additional diffusion feature—predominantly visible in thin electrodes—which we attribute to ion diffusion within micropores inside the NMC aggregates. Overall, this integrated methodology provides experimenters with directly applicable tools, scaling relations, and modeling strategies for developing intuitive and physically grounded interpretations of impedance responses in porous Li-ion insertion cathodes.

A crucial additional consideration arises when aiming to quantitatively interpret impedance parameters in terms of fundamental physical properties. All passive TLMs constructed from R-C elements are grounded in the Nernst–Planck (NP) framework. Accordingly, each circuit element represents a linear response of the part of studied system to the corresponding driving forces associated with gradients of electric potential and salt concentration. It is, however, well established that the classical Nernst–Planck formulation is strictly valid only under two limiting conditions: (i) thermodynamically ideal behavior and (ii) infinitely dilute electrolytes.[3] Real non-aqueous battery electrolytes - used in Li-ion, Na-ion, and analogous insertion systems - substantially deviate from these ideal assumptions. As a result, TLM parameters derived under NP-based assumptions do not necessarily map directly onto physically meaningful transport and thermodynamic quantities.

A more realistic description of battery electrolytes is provided by the Concentrated Solution Theory (CST) framework, pioneered by Newman.[4] CST accounts for non-ideal thermodynamic factors, salt–solvent and ion-ion interactions, and composition-dependent transport coefficients. In this work, we discuss the conceptual and practical implementation of a correction (alignment) step in which NP-derived electrolyte parameters are translated into their CST-consistent counterparts. One of the most prominent examples is the cation transference number, which appears both in NP and CST descriptions but differs quantitatively due to non-ideal thermodynamic and transport behavior. Using representative Li-ion insertion electrolytes, we demonstrate that this alignment step is essential for obtaining physically sound and quantitatively meaningful interpretations of impedance-derived electrolyte parameters in insertion electrodes and full cells.