Recently Li-ion batteries (LIBs) had a great development because of their advantages with respect to traditional ones: they present higher energy and power density, moreover they last longer and have higher efficiency. For these reasons, their use has grown above all for portable electronics and hybrid/full electric vehicles.
However, the high energy content of these batteries poses safety problems. In recent years, several episodes have been reported in which LIBs have been the source of fires and dangerous accidents. This is due to their high energy density which imply greater danger in the event of malfunctions or failures, given the presence of toxic and highly flammable substances. In the worst case a thermal runaway can occur. Any abuse, for example exposure to fire, overcharging, short- circuiting or crushing, can be a trigger for spontaneous self-heating reactions which can result in a fire, and eventually an explosion. The variables that affect the fire behavior of the batteries are various, in particular chemical composition of components (e.g. anode, cathode, electrolyte) vary among LIBs.
The purpose of this project is to examine the response to thermal abuse tests of 18650 cells. In particular, the aim is to study the influence of electrical formation cycle (a sequence of charge and discharge cycles that guarantees an adequate stabilization of the cells performance) and its impact on the behavior of the cell when exposed to an external heat flux. Heat release rate, surface temperature, mass loss and VOC concentration are measured using a cone calorimeter and a photoionization detector.
Cell components before and after the tests are then characterized by using different techniques such as DSC, XRD and SEM. With regard to the residues after tests, the purpose is to identify the composition of components and their role in re-ignition of fire, in fact several documented cases are reported of battery pack subject again to fire after its extinction.
In our knowledge the difference in behavior between cells that underwent an electrical formation cycle with cells that had not undergone it is not well investigated. The aim of the project is to better study the influence of electrical formation cycle (and therefore the presence of the SEI) and its impact on the behavior of the cell when exposed to an external heat flux.
Furthermore, very few works in literature are focused on the characterization of the residues after the tests, so, using different techniques like XRD and SEM, the purpose is to identify the chemical composition and the morphology of the components of the cell residues after tests. Further analysis will be carried out by DSC to analyze the possible decomposition and/or exothermic reactions between cell components.
It is very important to understand how to manage the residues produced after a fire for proper waste disposal. In fact, in general, materials present in the batteries are difficult to treat and they can be extremely harmful to the environment. Moreover, an investigation of the cases of fires on electric vehicles shows that one of the main problems is the re-ignition of the fire even after several apparently successful shutdown attempts. The case of July 2018 certified by the investigators of the National Fire Protection Association of a Tesla S model that has undergone two re-enactments following a traffic accident is reported. It is not the only documented case of ignition of an electric vehicle subjected to fire after its extinction. The characterization of the residues would make possible to understand their role in reignition of fire e.
Finally, since the appropriate fire prevention and protection measures have not yet been developed, it is very important to investigate the effects of thermal abuse conditions on Li-ion cells of battery systems, in order to develop safe instructions and procedures for a rescue team that is called to respond to accidents involving these kind of batteries. In this respect HRR data are fundamental, and they are also for the development of mathematical models in order to predict the behavior of cells in thermal abuse conditions.