Dr Carlos Ziebert, head of IAM-AWP’s Battery Safety Center, KIT, outlines how research and testing in calorimeters is crucial to improve thermal management and safety of batteries.
Established in 2011, the Battery Safety Center at the Karlsruhe Institute of Technology’s (KIT) Institute for Applied Materials – Applied Materials Physics (IAM-AWP) now operates Europe’s largest battery calorimeter laboratory. It provides six Accelerating Rate Calorimeters (ARCs) of different sizes – from coin to large pouch or prismatic automotive format – in combination with cyclers, which allow the evaluation of thermodynamic, thermal and safety data for lithium-ion cells on material, cell and pack level under adiabatic and isoperibolic environments for both normal and abuse conditions (thermal, electrical, mechanical). With these facilities, and the established technical and methodological expertise, the IAM-AWP is now seen worldwide as being one of the few institutions that investigate both the thermodynamics and the battery safety and their materials.
Battery safety First
Safety comes first – this is the mission of the Center’s head, Dr Carlos Ziebert. The development of safe cells is of the utmost importance for a breakthrough of the electrification of transport and for stationary storage, because an uncontrollable increase in temperature of the entire system (so-called ‘thermal runaway’) can cause an ignition or even explosion of the battery with simultaneous release of toxic gases. Thermal runaway is, of course, something that nobody wants, especially in an electric car or another electric vehicle, and the causes and effects of this can be very diverse and complex. As such, it is crucial to conduct thermodynamic studies of the thermal effects together with the material and cell development for advanced and even post-lithium systems.
In the current state of Li-ion technology, the range of properties is still significantly dependent on the respective operating and usage conditions (state of charge, age) as well as the ambient conditions (temperature, charge/discharge operation). The influence of ageing on the safety of the cells is a very critical factor for their commercial use. Even the regular use of batteries with varying charge and discharge cycles under different operating and environmental conditions leads to the release of heat and sometimes critical temperature increases in the system. These effects are caused by electrochemical reactions, phase transformations, mixing and Joule heating processes. The active materials can initiate highly exothermic reactions in dependence on the respective battery operating conditions and the ambient temperature by internal and external short circuit or by mechanical action. This can be followed by the thermal runaway.
To avoid this, the system must be designed optimally with respect to material and cell level. In addition, the battery and thermal management systems must be optimised. Thus, the complete scientific and technical understanding of these effects is of upmost importance.
Benefits of battery calorimetry
Battery calorimetry is therefore very important as it is fundamental to obtain quantitative data on the thermal behaviour – you need to know how many Watts a cell will produce under certain conditions, and this can be investigated in the calorimetry tests. This information can then be used to adapt the battery and thermal management systems.
Sophisticated battery calorimetry combined with thermography allows for the discovery of new and quantitative correlations between different critical safety and thermally related parameters. The temperature, heat and internal pressure evolution can be studied, while operating cells under conditions of normal use, abuse or accidents. These data are essential for battery management, thermal management and safety system design. Combined with multiscale electrochemical-thermal modelling they provide a powerful tool for thermal runaway prevention and ageing prediction.
Measurement of thermal data in battery calorimeters
Figure One shows two of the Accelerating Rate Calorimeters (ARC) at the IAM-AWP Battery Calorimeter Center. In these ARCs the temperature, heat and internal pressure evolution can be studied, while operating cells under conditions of normal use, abuse or accidents. The cell is inserted into the calorimeter chamber, which has heaters and thermocouples located in the lid, bottom and side wall. These adjust the required ambient conditions, which can be either isoperibolic or adiabatic. Under isoperibolic conditions, the temperature of the calorimeter chamber is kept constant and the temperature change at the surface of the cell is measured. This reflects the ideal conditions of a single cell or an edge cell in pack. In this case the cell temperature reaches its initial temperature again after each cycle.
Adiabatic conditions represent something of a worst case scenario, in that the calorimeter heats up together with the cell meaning that the latter cannot transfer heat to its surroundings (a situation similar to that which the cell would experience should the cooling system fail in a real world application). Such data makes it possible to optimise charge and discharge management and adapt it to the cells. In addition, they can be used to analyse ageing processes in the cells. By measuring the specific heat capacity and the heat transfer coefficient the measured temperature data can be converted into generated and dissipated heat data, which are needed for the adjustment of the thermal management systems.
Safety testing in battery calorimeters
Apart from important data that can be implemented into the battery management system (BMS), the battery calorimeters provide thermal stability data on the materials level, e.g. the anodes, cathodes or electrolytes or their combinations and allow to perform safety tests on cell and pack level by applying:
- Electrical abuse: external/internal short circuit test, overcharge test, overdischarge test;
- Mechanical abuse: nail penetration test; and
- Thermal abuse: Heat-Wait-Seek test, ramp heating test, thermal propagation test.
As a result of the different tests, quantitative and system relevant data for the temperature, heat and pressure development of materials and cells are provided. These data can be used at all levels of the value chain, from safe design at the materials level up to the thermal management and adaption of safety systems or implementation into modelling and simulation tools.
Currently, thermal propagation testing is becoming a very hot topic, because a standardised procedure is needed to develop and qualify suitable countermeasures, such as heat protection barriers. For example, a global technical regulation (GTR) on electric vehicle safety is being developed by all relevant stakeholders.
In the future, battery calorimetry will also be needed to assess the thermal and safety properties of advanced materials such as solid state batteries or other systems which could replace lithium, such as sodium or magnesium. This has to be started now at the smaller scale level and has to be continued in order to ensure that the cells can be up-scaled and remain safe.
There are still numerous challenges that need to be overcome, and we hope that our Battery Safety Center will help the European industry to make further progress in the battery field, which is urgently needed to reach a low-carbon future, to foster European leadership, and to create new jobs.
Dr Carlos Ziebert
Head of the Battery Safety Center
Karlsruhe Institute of Technology
Institute of Applied Materials – Applied Materials Physics
This article will appear in SciTech Europa Quarterly issue 28, which will be published in September, 2018.