Quantitative separation of thermal and electrochemical contributions in nonlinear EIS measurements

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 1 kHz 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 3.5 Ah cylindrical lithium nickel manganese cobalt oxide (NMC) cell quantify contributions: at 5 °C and 10 Hz, 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 20 mV linearity criterion may be too strict: we observed linear behavior up to 280 mV, 14 times the threshold.