Dr Carlos Ziebert of The Battery Safety Center at the Karlsruhe Institute of Technology’s (KIT) Institute for Applied Materials – Applied Materials Physics (IAM-AWP) – outlines how research and testing in calorimeters helps to improve the performance and safety of batteries
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 centre. It has six accelerating rate calorimeters (ARCs) from Thermal Hazard Technology (THT) in combination with cyclers and enables the measurement of the thermal behaviour of lithium-ion batteries under adiabatic, isothermal, and isoperibolic environmental conditions under both normal and abuse conditions (thermal, electrical, mechanical).
The researchers also model the thermal behaviour of the batteries and use their experimental results as input data. As a result of the investigations, quantitative and system-relevant data for temperature, heat, and pressure development of batteries are provided. These data are used by the research and industrial partners for the design of thermal management and safety systems.
Scitech Europa spoke to the centre’s head, Dr Carlos Ziebert, about the importance of the work that takes place there.
Could you outline the importance of battery calorimetry in light of the need for new battery technologies, and battery safety technologies, particularly with regard to the automotive sector?
The development of safe cells is of upmost 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.
Calorimetry is thus 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 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 safety and 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.
In recent research, you have determined for the first time the heat generation data for a large format pouch cell using both isoperibolic and adiabatic conditions. What is the significance of this?
Isoperibolic conditions – the cell sees a constant temperature, and the temperature change at the surface of the cell is measured. 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). In our experiment we were measuring the heat generation of the cell under both of these (isoperibolic and adiabatic) conditions.
We were also looking for the irreversible and reversible heat – the former comes mainly from the resistance of the cell, and the latter from the electrochemical reactions that are occurring. In order to do this it was necessary to introduce additional measurement steps. We compared the data from all of these methods and obtained a very good agreement.
Indeed, from our results we were able to show that, in principle, it is possible to use all three measurement methods to get similar data. However, adiabatic methods are much faster than isoperibolic ones, where it is also necessary to, for example, measure the heat capacity and the heat transfer from the cell surface to the surroundings. Furthermore, in order to measure the reversible heat, you need to look for the entropy over all the state of charge, which takes a week. Therefore, in principle, the fastest solution in this case would be the adiabatic measurement. This is an important finding and can be used to speed up measurement taking.
What methods have you developed for the measurement of cell pressure?
We can measure how the internal pressure in the cell increases during operation. This is important and could be used as a signal to be used by an additional sensor in the battery management system to make sure that no thermal runaway happens or, indeed, to detect any thermal event which might later lead to thermal runaway.
We are also working on developing ways to model such thermal runaway behaviour, and this also of course has to predict and prevent thermal runaway. We have currently developed this model for small cells, and if it is to be scaled-up so that it can be applied to the larger batteries such as those used in the automotive industry, then the large-scale battery calorimetry will be needed in order to validate the models.
Do you feel enough support for research in your areas is offered at the European level?
No, I don’t think that there is enough support for the research into this area. Indeed, nor is there a single call in Horizon 2020 specifically focused on the thermal and safety properties of batteries. There are, of course, some more general calls in which the importance of battery safety and so on receives a mention, but it really needs to become a focus in its own right.
This, it seems, is something of a reflection of the general attitude towards financial support in this area – for instance on two occasions I have attempted to establish a European infrastructure network for cell and battery testing but have been unsuccessful.
I was the manager of the Joint Programme on Energy Storage (JPES) at the European Energy Research Alliance (EERA) for two years, and during that time (2011-13) we did not receive any funding from the EU; all our contributions were provided by the leading large-scale research centres in Europe on battery and battery safety technology, with whom we collaborated. Much more could be gained from this collaboration if the funding were in place to support it.
Yet, perhaps there are signs that things are getting better: the EERA has now become a legal entity, meaning that it might be easier to get EU funding. We are involved in the planning for FP9 – and, of course, we are hoping that this will help to improve the funding situation in the coming years.
Moving forwards, where will your priorities lie – both in terms of research areas and for the institute more generally?
Our future priorities are, on the one hand, improving battery safety at the cell and pack levels of current technologies, and we do this by measuring additional parameters such as cell pressure. This means that sensors will have to be developed which can measure this pressure and which can be integrated into the battery management system.
Currently, for the automotive industry, the testing of thermal runaway propagation becomes more and more important. At the moment, there is no real standard for how to test in a battery pack different mitigation measures to prevent thermal runaway from propagating from one cell to the next cell. We are therefore discussing this with our colleagues at the European Commission’s Joint Research Centre, where, at the European level, they are establishing new regulations where such thermal propagation tests will also be defined.
It is very important, especially for the automotive industry, to make sure that in the case of an accident there will be enough time for the passenger to escape or to be rescued from the damaged vehicle.
In future we also want to assess the thermal and safety properties of advanced materials such as solid state batteries or other systems which are being used instead of lithium such as sodium or magnesium. Of course, there are other safety issues that could emerge, too, and this has to be investigated now on the smaller scale level. Then, we have to ensure that the cells can be up-scaled and remain safe.
There are still many challenges that have 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.