Electrochemical and safety measurements

Battery measurements on one page: electrochemical impedance from the perturbation to the equivalent circuit, a three-electrode cell with a lithium reference, online gas analysis by mass spectrometry, accelerating rate calorimetry by heat–wait–seek, and titration gas chromatography of dead lithium. The methods and numbers are quoted from the cited papers; no panel is measured data.

1. Impedance in three steps: EIS perturbs a system at equilibrium or steady state with a sinusoidal voltage or current over a wide range of frequencies and records the response; the system must be linear and time-invariant. The perturbation is commonly 5–10 mV on a dc potential, usually the open-circuit potential; analysers span 10 µHz to 1 MHz, and sixty frequencies from 100 kHz to 0.1 Hz, ten per decade, take about 2–3 min (Lazanas 2023). In-situ EIS of cells used a 10 mV amplitude and ten points per decade (Shao 2025). 2. The equivalent circuit: Each part of a lithium-ion cell forms its own interfaces and equivalent circuit: a bulk resistance of electrolyte, electrode and separator; a semicircle from the interface and charge transfer of lithium ions, with a Warburg impedance from their diffusion; and an additional semicircle at higher frequencies from the SEI. On real solid electrodes a constant phase element replaces the ideal capacitor, its exponent running from 0 (a resistor) through 0.5 (a Warburg element) to 1 (an ideal capacitor) (Lazanas 2023). 3. A lithium reference: Across the two terminals the voltage includes cathode and anode, which are coupled; a reference electrode with a stable potential, such as lithium metal, decouples them so each is measured on its own, and watching the anode potential in a three-electrode 2032 coin cell detects lithium plating (Minter 2018). In cells of a Li counter, a graphite working and a Li reference electrode from 30 °C to −40 °C, minima read as the plating overpotential must be disentangled from lithium stripping at the counter electrode (Chen 2025). 4. Online gas analysis: Online continuous-flow DEMS lets a cell cycle under realistic conditions over long tests; electrolyte decomposition was followed in an NMC532/graphite cell over 20 cycles (Berkes 2015). OEMS with only 40 µL of electrolyte and intermittent headspace sampling performed comparably to flooded coin cells (Sim 2023); in high-temperature OEMS argon at 0.65 mL min⁻¹ carries the gas through a capillary at 200 °C to the mass spectrometer, and the cell is heated at 2 °C min⁻¹ to 132 °C (Schmidt 2026). 5. Heat–wait–seek: Accelerating rate calorimetry: the cell is heated step by step to a set point and the instrument looks for heat generated by the cell; once the cell heats itself, the calorimeter follows its temperature so it stays adiabatic and all the heat from the failing cell is captured. A test gives the onset of thermal runaway, the endotherm of separator melting, the pressure release, gas for analysis and the maximum temperature (Klein 2021). In 1.2-Ah cells the maximum thermal-runaway temperature fell from 417 °C with LiPF₆ to 285 °C with LiFSI and succinonitrile (Ali 2024). 6. Counting dead lithium: Inactive lithium is both Li⁺ in SEI compounds and isolated metallic Li⁰; titration gas chromatography measures the metallic Li⁰ and found it the dominant source of inactive lithium and capacity loss (Fang 2019). The cycled cell is opened in the glovebox, its Cu foil and separator sealed in a 5 mL headspace vessel, 0.1 mL of water injected and 6 µL of the gas taken into the GC; LiH also gives hydrogen, so after long cycling TGC alone reads more dead lithium, while D₂O titration tells lithium (D₂, m/z 4) from LiH (HD, m/z 3) (Xiang 2021).

Lazanas & Prodromidis, “Electrochemical Impedance Spectroscopy-A Tutorial”, ACS Meas. Sci. Au 2023 (doi:10.1021/acsmeasuresciau.2c00070); Shao et al., “Multiscale interfacial stabilization via prelithiation separator engineering for Ah-level anode-free lithium batteries”, Nat. Commun. 2025 (doi:10.1038/s41467-025-59521-8); Minter et al., “Three-electrode Coin Cell Preparation and Electrodeposition Analytics for Lithium-ion Batteries”, J. Vis. Exp. 2018 (doi:10.3791/57735); Chen et al., “Mechanistic Understanding of Lithium-Ion Adsorption, Intercalation, and Plating during Charging of Graphite Electrodes”, ACS Electrochem. 2025 (doi:10.1021/acselectrochem.4c00079); Berkes et al., “Online Continuous Flow Differential Electrochemical Mass Spectrometry with a Realistic Battery Setup for High-Precision, Long-Term Cycling Tests”, Anal. Chem. 2015 (doi:10.1021/acs.analchem.5b01237); Sim, Langdon & Manthiram, “Design of an Online Electrochemical Mass Spectrometry System to Study Gas Evolution from Cells with Lean and Volatile Electrolytes”, Small Methods 2023 (doi:10.1002/smtd.202201438); Schmidt et al., “High-temperature chemical oxidation pathways in lithium-ion batteries: mechanistic insights into ethylene carbonate decomposition”, Chem. Sci. 2026 (doi:10.1039/d6sc00426a); Klein, Carter & Love, “Accelerating Rate Calorimetry and Complementary Techniques to Characterize Battery Safety Hazards”, J. Vis. Exp. 2021 (doi:10.3791/60342); Ali et al., “Thermal stability analysis of nitrile additives in LiFSI for lithium-ion batteries: An accelerating rate calorimetry study”, Heliyon 2024 (doi:10.1016/j.heliyon.2024.e29397); Fang et al., “Quantifying inactive lithium in lithium metal batteries”, Nature 2019 (doi:10.1038/s41586-019-1481-z); Xiang et al., “Quantitatively analyzing the failure processes of rechargeable Li metal batteries”, Sci. Adv. 2021 (doi:10.1126/sciadv.abj3423).

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