How Fast Do Elevated Temperatures Reach the Cell Interior?

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The performance and durability of lithium-ion (Li-ion) batteries are heavily influenced by their operating temperature. Their performance decreases at low temperatures while the battery degrades quickly at high temperatures. This means that overall reliability is compromised, creating a potential safety issue. Industry research has led to standards regulating the ability of a battery to withstand fluctuations in temperature when it is in operation. In contrast, there has been much less focus on the temperatures that batteries are exposed to during the manufacturing process, which includes plasma pretreatment, UV curing, laser welding, ultrasonic joining, hot stacking, and hot gluing. A Li-ion battery may contain thousands of individual cells, which have to be stacked together. This is typically done through an assembling procedure that may involve various heat treatments, some of which can be extremely intense and expose the casing or other parts to high temperatures for short times. Gerd Liebig of NEXT ENERGY EWE Research Centre for Energy Technology at the University of Oldenburg, Germany, explained, “It is already well known that certain processes such as welding greatly increase the temperature within a battery. What is not known is the extent to which such elevated temperatures could propagate within and compromise a cell.”

NEXT ENERGY EWE Research Centre for Energy Technology at the University of Oldenburg, Germany, is a leading research institution focused on energy technology. The center collaborates closely with various academic and industrial partners to advance the understanding and development of energy systems, including lithium-ion batteries. The research team, led by Gerd Liebig and Pamina Bohn, is dedicated to investigating the thermal behavior of batteries during the manufacturing process. Their goal is to ensure the stability and capacity of batteries by understanding how elevated temperatures during manufacturing can affect the cells. The team employs advanced numerical simulation techniques to model different operating scenarios and validate their findings, providing valuable insights for the battery manufacturing industry.

The research team from NEXT ENERGY EWE Research Centre for Energy Technology at the University of Oldenburg set out to investigate whether the manufacturing process could cause irreversible damage to lithium-ion cells due to elevated temperatures. They used numerical simulation to model different operating scenarios and placed probes to inspect results at any point in the model. The first step was to set up a physical experiment to measure temperatures reached inside a prismatic lithium dummy cell when subjected to short-term thermal stress. The goal was to collect data to validate the mathematical model and investigate the effect of various processes during cell manufacturing. The team created a 3D replica of a commercial prismatic lithium cell in Autodesk Inventor software and imported it into the COMSOL Multiphysics software. They modeled heat transfer by conduction due to an external heat source at different positions on the cell corresponding to different manufacturing processes, and the natural convective cooling on other areas of the cell surface. The physical and thermal properties of the individual materials were experimentally defined and mathematically homogenized into one jelly roll domain within a prismatic steel housing. Adaptive mesh refinement was used to adopt a finer discretization in regions where temperature gradients were higher, ensuring high-accuracy results.

• The multiphysics model closely replicated the behavior of the dummy cell, allowing the team to simulate temperature propagation within the cell during various manufacturing processes.
• The team confirmed that the high power density of a laser beam enables high welding rates while limiting heat input into the battery cell, ensuring safety.
• The simulation showed that heat propagated along the cell housing, causing moderate temperatures that did not exceed 36ºC, which were not a danger to battery components.

• The temperature within the cell continued to rise, reaching 138ºC in the jelly roll four seconds after the external heating ended.
• During laser welding simulation, the temperature did not exceed 36ºC, ensuring the safety of battery components.