Characterization of coupled transport phenomena in large-format lithium-ion batteries
Keywords: mechanical
thermal
non-invasive
LFP
lithium-ion
batteries
diagnostics
modeling
thermography

Large-format lithium-ion batteries are inherently complicated systems, consisting of many stacks of repeating current-collector/cathode/separator/anode/current- collector structures. When a current is applied to either charge or discharge the battery, a multitude of processes occur, which are, more often than not, coupled to each other. The movement of lithium ions from one electrode to the other during charge/discharge can also generate large amounts of heat, and the positioning of the tabs can make the temperature distribution highly non-uniform. The same process can also cause the electrodes to volumetrically expand and contract when the lithium ions move in or out of it. This non-uniformity, although being an unwanted side effect, offers a method of looking at how lithium-ions move through the battery from an external viewpoint. Taking local measurements and assembling them into a spatial map – whether of temperature or strain – provides insight into the coupled transport processes.
By tracking the temperature non-uniformity and a few characteristic features of the voltage signal that result in response to a squarewave input signal, a streamlined 3D model was proposed to simulate large-format lithium-ion batteries of two different chemistries and shapes in Chapter 2. The electrochemical behavior was described by the Newman-Tobias porous-electrode theory, which was coupled to a local heat balance. Dimensional analysis was done to justify a simplification in which the many-layered geometry could be represented by a single stack of the individual components, dramatically speeding up computation. By matching the simulation output to the experimental data, material property values were extracted that adequately describe the observable thermal and electrical behavior of the batteries. Providing a method of parameter estimation that can resolve many parameters at once through a single experiment. In Chapter 3, the inverse modelling approach was used to study battery degradation by tracking the changes to the extracted material property values. By looking at which parameters changed and by how much, the degradation mechanism that can result in these variations can be inferred.
Intercalation/deintercalation-induced mechanical strain and stress effects in batteries are easily convoluted with temperature effects because of thermally induced volume change. Typically, experiments are designed with the intention of minimizing thermal contributions, so that a single point measurement of temperature describes the whole cell. But cells do not always satisfy this assumption in practice, consequently, in Chapter 4, an experimental rig that simultaneously measures cell surface temperature and dilation non-invasively was designed and built. The apparatus consisted of multiple non-contact sensors, such as an IR sensor/camera and two laser profilers. The setup measured both temperature and dilation simultaneously under the effects of various high-rate current signals and the relaxation that followed, which provided a means to probing the relative contributions of intercalation and temperature induced volume change.