Interview with Carlos Micó, Research Professor at Universitat Politècnica de València
Can you explain what safety tests UPV conducts on cell prototypes and why these are crucial for the NEXTCELL project?
The Battery Research Group from CMT Research Institute at UPV is specialist on the study of the thermal runaway process. One of the most important sources of data to understand the causes leading to this kind of failures and its consequences are the safety tests. In this type of experiments, cells undergo extreme operating conditions and/or even provoked damages, representing different scenarios that the cells could face when reaching the market. The objective is identifying if under these conditions the thermal runaway can be triggered and, if that is the case, how it develops.
In projects that seek to produce a new cell, such as NEXTCELL, safety tests are very useful to identify and understand the potential hazards of the new product, related with the thermal runway event. In addition, the information gathered allows to add the safety dimension to the design process, together with others like the electrical performance, chemical stability, or recyclability.
What types of tests do you perform on the cells?
At UPV we are experts on thermal abuse tests. In those, cells are submitted to a heat source that progressively rises their internal temperature. However, we have equipment and capabilities to apply other types of extreme operating conditions or abuse such as overcharging with a voltage higher than the one defined in the specifications of the cell; overdischarge with a current higher than the one defined in the specifications of the cell; external short circuit that forces the circulation of a very large current inside the cell; or mechanical deformations and even damage (penetration), which causes a deterioration of the internal components of the cell.
All these circumstances force the heat up of the internal components of the cells, that could lead to thermal degradation and generating chemical reactions which release large amounts of energy and even can cause fire out of the cell. These tests can be done at cell level with very controlled conditions, which provide detailed information of the process. However, they can also be performed with full battery modules or battery packs. In this case, test provide less detail but a closer look to the real hazards of thermal runaway in a commercial application.
What kind of data does UPV collect from testing, and how is this data used to understand the batteries better?
One of the key aspects of the thermal runaway process is the temperature at which the thermal degradation and the chemical reactions can start. The difference between causing this type of failure or not, under an abuse scenario, can be determined by the temperature reached inside the cells. This limit is usually known as “Onset Temperature” and can be measured with high accuracy during the safety test.
The large amount of heat generated during a thermal runaway can cause the failure to propagate from one cell to others within the same battery pack. In addition, hot vapours and particles are usually ejected by the cell during the event. All this contribute to the potential propagation and could cause even fire. The safety tests allow to measure the heat generated by the cell. Besides, it is possible to gather information about the materials ejected like the number of gases, its temperature, its composition or the solid particle characteristics.
All these information can be used to evaluate how safe or dangerous different cell designs are. In addition, it supports the development of safer battery system by providing a better understanding of the process. This allows developing mitigation measures in the design process. When combined with simulation tools, the experimental data from the safety test provides a reference to be able to virtually reproduce the consequences of a thermal runaway. This is very useful to evaluate these design of new battery systems without the need of laboratory tests.