The increasing demand for lithium-ion batteries in electric mobility and stationary storage highlights the importance of accurately measuring their physical and electrochemical health. Advanced diagnostic methods are now being used for the early detection of defective cells, from quality control during production to operando monitoring of aging processes. In this study, sensitive magnetic measurement techniques are combined with electrochemical impedance spectroscopy (EIS) and thermal analysis to investigate the behavior of lithium-ion pouch cells throughout their lifetime.
Magnetic imaging provides spatially and temporally resolved information from battery cells. Arrays of fluxgate sensors, optically pumped magnetometers (OPMs), and superconducting quantum interference devices (SQUIDs) are used to study the magnetic response of the cells during operation and relaxation for different states of health and aging parameters. Fluxgates are suited for measurements at high operating currents (up to 1 C, 30 A), while OPMs and SQUIDs are able to detect weaker relaxation currents with high magnetic sensitivity. The combination of classical and quantum sensors enables measurements of the cells both during operation and in dynamic states, specifically during relaxation.
In addition to magnetic characterization, the internal physical and electrochemical processes of the cells were analyzed using EIS. An equivalent circuit model (ECM) is applied to the spectra to interpret characteristic parameters and their changes over time. In this study, an in-house spatially resolving isoperibolic calorimeter is used to characterize the thermal behavior of the cell during operation. This highly sensitive calorimeter is used to analyze the total and local heat flow rates generated or absorbed by the cell under various operating conditions. Three different cycling conditions have been applied to age a number of 30 Ah, lithium-iron-phosphate (LFP) cells: cycling at low temperature of 0 °C, high temperature of 45 °C, and varying depths of discharge (DoD) of 0-100% and 0-50%. Magnetic, electrochemical, and calorimetric measurements were carried out at ambient temperature. The results show that different cycling conditions cause distinct, spatially resolved degradation patterns that can be identified non-destructively through magnetic-field mapping in combination with electrochemical and calorimetric analyses. The magnetic-field measurements show the change in static magnetic field during aging of the cell, and current-density-shift results show the effect of aging on the current pathways in the cell. Studies of magnetic-field relaxation following a discharge pulse are used to investigate the spatial distribution of aging processes in the cell. In this study, we investigate the correlations between spatially resolved magnetic field measurements and degradation processes identified through electrochemical and calorimetric analyses.