Development of substitute cells to replicate the thermal behaviour of lithium-ion cells

Summary:

The performance as well as the lifetime of lithium-ion battery cells are strongly temperature-dependent. To achieve optimal results in both aspects, efficient thermal management systems are needed in the automotive application. Along the development process of these systems, manufacturers face certain obstacles considering the validation of battery systems. An extensive experimental infrastructure is required and high safety regulations have to been met considering the investigation of lithium-ion cells. The thermal behaviour of the cells changes as a result of cell degradation, which leads to a reduced reproducibility considering the experimental validation of the thermal management systems. Finally extreme situation tests are hardly possible because of the danger of cell damage or thermal runaway.

To overcome the mentioned obstacles this work focuses on the development of thermal substitute cells, which exactly replicate the thermal cell behaviour but no longer contain any electrochemical storage function. These substitute cells have no electrochemical components and therefore do not succumb to degradation. They require no complex equipment, have low safety limitations and enable the possibility of extreme situation investigations. Furthermore, they provide the possibility of flexible design adaptations and are highly available even in early development stages. With these advantages they enable a reduction of development time and costs and improve the efficiency and quality of thermal management systems.

In this contribution, the development of thermal substitute cells is presented including the following steps: characterisation and investigation of the thermal behaviour of the reference cell, simulation-based design of the substitute cell concept and validation of substitute cell prototypes.

The chosen reference cell is a high-energy automotive prismatic hardcase cell. To investigate the thermal behaviour of the reference cell a 3D simulation model was developed and parameterised by in-house characterisation. In comprehensive simulation studies the interaction of the inner cell components with the outer thermal boundary condition were evaluated and the critical heat transfer paths identified. Different thermal boundary conditions representing different thermal management applications, like bottom or side cooling are considered.

Through the simulation-aided design process a substitute cell concept was developed to replicate the thermal behaviour of the reference cell. Additional requirements like a comparable mechanical resilience as the battery cell as well as cost efficient construction of the substitute cell were taken into account. The simulative verification between substitute and reference cell shows a very good replication of the battery behaviour with substitute cells. Not only is the transient thermal behaviour in good agreement also the inner temperature distribution can be approximated with sufficient accuracy. Additional to the simulative comparison the replication quality is also proven with the experimental validation based on substitute cell prototypes.