Quantitative separation of thermal and electrochemical contributions in nonlinear EIS measurements

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Sailer, Norbert ; Totschnig, Lukas ; Steffan, Christoph ; Schmidt, Jan Philipp:
Quantitative separation of thermal and electrochemical contributions in nonlinear EIS measurements.
In: Journal of Power Sources. Bd. 691 (2026) . - 240924.
ISSN 0378-7753
DOI: https://doi.org/10.1016/j.jpowsour.2026.240924

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Abstract

Impedance decrease during high–current excitation as in Nonlinear Electrochemical Impedance Spectroscopy (NLEIS) can arise from electrochemical nonlinearity, self–heating, or both. We developed two measurement protocols separating these contributions by exploiting distinct time scales: electrochemical nonlinearity responds quickly, while self–heating evolves slowly. The first protocol extends NLEIS frequency sweeps with auxiliary 1kHz measurements for continuous internal temperature tracking via impedance phase. The second protocol employs single–frequency measurements with alternating excitation amplitudes, enabling direct observation of fast versus slow impedance changes. Experiments on a 3.5Ah cylindrical lithium nickel manganese cobalt oxide (NMC) cell quantify contributions: at 5°C and 10Hz, impedance decrease is entirely electrochemical at 0.57C, while at 2.57C, temperature dependence contributes 15% and electrochemical nonlinearity 85%. At 25°C, the cell remained linear. The widely-adopted 20mV linearity criterion may be too strict: we observed linear behavior up to 280mV, 14 times the threshold.