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Predicting calendar aging mechanisms in lithium-ion batteries via float current analysis

Year: 2026
Type: dissertation
Abstract: Calendar aging determines the lifetime of lithium-ion batteries in most long-term applications, yet its underlying mechanisms are difficult to observe without destructive analysis or months of capacity testing. This dissertation establishes float current, the residual current measured during a potentiostatic hold, as an active diagnostic signal of calendar aging. The float current is separated into a transient phase, arising from reversible redistribution such as polarization and the anode overhang effect, and a steady-state phase reflecting irreversible degradation. Voltage-dependent scaling factors derived from open-circuit voltage slopes decompose the steady-state current into its two pathways, SEI growth and cathode lithiation by salt decomposition, validated against capacity fade across three 18650 chemistries (LFP, NCA, NMC) with graphite anodes. This exposes a blind spot of conventional capacity-based testing: at high voltages, lithium trapped in the cathode compensates part of the lithium loss, so capacity fade alone underestimates the true degradation. Temperature variation further separates entropy-induced from aging currents, and benchmarking against open-circuit voltage decay shows that the potentiostatic hold does not induce additional aging. On a further cell, impedance spectroscopy links cathode lithiation to rising cathode charge-transfer resistance, while ICP-OES confirms lithium loss from SEI growth as well as lithium increase and phosphorus deposits on the cathode. By resolving a single, continuously measurable signal into electrode-specific degradation, this work establishes float current analysis as a tool for aging research and lifetime prediction.
DOI: 10.18154/rwth-2026-07889
Language: English
Open Access status: green
OpenAlex ID: W7212754247
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