Thermal inhomogeneities and thermal design of lithium-ion batteries

• Applicant: Dr.-Ing. Sabine Paarmann
• Subject Area: Chemical and Thermal Process Engineering
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 506629963

Lithium-ion batteries as energy storage systems are highly relevant in mobile devices and are essential to electrifying mobility. With their high efficiency and low emissions, they contribute to the fight against the climate crisis. To fully exploit the potential of the batteries, an increase in performance and lifetime is crucial. Industry has so far focused entirely on optimising the energy and power density of lithium-ion batteries, which are electrochemical metrics, and has not given due attention to the thermal behaviour. However, the temperature within a cell is of key importance as it influences the performance, usable energy and lifetime. To quantify its impact is even more challenging when the temperature gradients in cells are considered caused by external temperature control or self-heating. Those inevitable temperature inhomogeneities are the main contributors to non-uniform cell parameters, which lead to non-uniform material usage, reduced usable energy and even cause non-uniform ageing, leading to a reduced lifetime. The first objective of this project is to establish a Cell Inhomogeneity Coefficient as a thermal metric to quantify the orientation and magnitude of temperature gradients – parallel or perpendicular to the electrode layers – and benchmark the impact on the battery performance and lifetime. This will allow the determination of the maximum temperature difference a cell can tolerate.During battery operation, heat is generated internally due to losses. This heat generation depends on the current and resistance, which in turn are influenced by the local temperature. The local heat generation depends on numerous interdependent factors, so it cannot be easily described with simple equations. Still, it is relevant for performance and may be crucial for safety. Thus, as the second objective, the local heat generation and its interaction with temperature will be determined to identify the most critical operating conditions.Besides the internal heat generation, the heat rejection determines the temperature level as well as the magnitude of temperature gradients. In this context, cell design is key to obtaining a beneficial temperature distribution. However, most cell designs are optimised regarding only the electrical performance, although this often reduces the usable energy density when the chosen design negatively impacts thermal behaviour. Against this background, the third objective in this project aims to identify thermal design rules of battery cells for enhanced performance and extended lifetime. Thus, an optimum thermal design of the cell and its components can be derived as well as an optimised dimensioning of the external thermal management system.Overall, this project will contribute tremendously to understanding and assessing temperature distributions in lithium-ion batteries and provide practical design rules to improve its performance and lifetime significantly.

• DFG Programme: WBP Fellowship
• International Connection: United Kingdom
• Host: Professor Dr. Gregory Offer