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

RWTH Publications (RWTH Aachen)

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.

Authors 1

1. Mohamed Azzam corresponding Aachen

   RWTH Aachen University

   Affiliation as printed

   RWTH Aachen