Titolo della tesi: Fire risk management of Li-ion batteries
The growing use of Lithium-ion batteries (LIBs) in various applications, such as small portable devices, electric vehicles (EVs), and energy storage systems (ESS), is due to the better performance of these devices compared to the previous technologies available. However, this phenomenon also begins to show the disadvantages of these devices. In fact, the number of accidents along the whole life chain of LIBs, from production to use up to final disposal, is increasing. The fire hazard of LIBs, in terms of severity of the fire and products emitted, is strictly dependent on the internal chemical composition, the type of abuse perpetrated, and the energy stored inside, the so-called state of charge (SoC). For this reason, it is important to evaluate the thermal stability of LIBs and to investigate not only the exothermic reactions that occur inside them, but also the composition and the properties of the hazardous products. These are both gases, such as hydrofluoric acid (HF) and carbon monoxide (CO), and solid, such as metallic particle aerosol, that can be released during the thermal runaway (TR).
The thermal stability of LIBs is defined by the reactions that can occur inside the cell between the internal components and the mechanisms of the safety device, such as the current interrupt device (CID), during a TR. From previous studies is possible to define three key events that occur during the abuse of LIBs: the activation of the safety devices, the venting, and the TR. Unfortunately, there are many variables that influence the temperatures and the products emitted during those events, such as the internal composition, the SoC, and the type of abuse, which is either electrical, mechanical, or thermal. Even if the internal composition is one of the fundamental parameters, it is difficult to find as it is only partially expressed on the product safety data sheets (SDS) and even a slight change in the composition, such as the electrolyte solution mixture, affects the final products ejected. The limitation of the available studies on TR behavior is due to the fact that they all refer to works conducted on different cells characteristics, such as shapes, chemistries, SoC, with a different instrumentation to perpetrate the abuse, such as direct flame or electrical heating, and different sensors and techniques to characterize the products.
In this optic, the present work was carried out with the aim of standardizing the information on the thermal stability and the TR behavior of different cylindrical cells, 18650, currently available on the market, i.e., NCA, Lithium Nickel Cobalt Aluminum Oxide (NCA), Lithium Titanate Oxide (LTO), Lithium Iron Phosphate (LFP), Lithium Nickel Manganese Cobalt Oxide (NMC), and Lithium Cobalt Oxide (LCO). To do that, thermal stability tests, as per UN regulation, and thermal abuse tests were conducted in the same reactor and under the same conditions of abuse to have comparable data for the different chemistries analyzed. The validation of the reactor was carried out to obtain the best test conditions and to define a unique procedure to monitor all the parameters and to analyze the collected products, such as gases, solid, and liquid. So, for each thermal abuse test it was possible to obtain information on the temperature and pressure of the key events by using thermocouples and a manometer, the composition and the quantification of the gaseous species by continuous Fourier-transform infrared spectroscopy (FT-IR) analysis, and the composition of the solid and liquid residues by FT-IR, scanning electron microscope coupled with energy dispersive X-ray analysis (SEM-EDX), inductively coupled plasma (ICP) and atomic absorption spectroscopy (AAS) analyses. The characterization procedure was not applied only to the abuse test residues but also to the new LIBs to have a more precise characterization of the internal composition.
This way, it is possible to compare the data obtained and highlight common or significantly different behaviors for the LIBs depending on the internal chemistries, not only in terms of the temperature reached during the TR, but also by the dangerousness of the substances that can be emitted and dispersed in the environment in the short and long term. In fact, these released substances can have extremely dangerous effects on the environment and the people: be it intervening firefighters or people involved. In the case of gases, the concentration values of all toxic species, such as CO and HF, during the TR must be evaluated. In fact, these substances can present toxic effects even in case of short-term exposures (30 min); therefore, already in the stages of the fire. In the case of solids, however, the parameters to be evaluated are the composition and the size. The composition can be traced back to the transition metals used to make the cathode, while the dimensions of the aerosol can vary in the respirable range.
With time other issues started to emerge, such as the search for new materials to enhance the LIBs performance and the choice of a fire extinguishing agent to suppress a LIB fire. The research is now aimed to increase performance by optimizing the active materials of LIBs, using for example nanomaterials (NMs). The main NMs currently under investigation are silicon (Si), graphite, and LTO. Though, even if the NMs show an increase in performance, the reduction in the size induces a more explosive behavior and more toxic effects on aquatic organisms. For this reason, a new project on the safety of nanomaterials was launched with the aim of evaluating the physicochemical characteristics, thermal behavior, explosivity risk, and ecotoxicity of pure materials. These assessments can lead to a more informed choice of manufacturer when selecting materials for cell assembly.The choice of an extinguishing agent to suppress a LIB fire is much debated due to the different nature of this fire. In fact, the extinguishing agents currently used are classified based on the type of fire, but the LIB fire is not actually classified due to the very exothermic reactions that occur inside, the metallic nature of most of the components, and the electrical energy storage inside. Preliminary fire tests were conducted to evaluate the effectiveness of three different extinguishing agents, at different physical states. A unique procedure was also defined for the collection of the residues, both solid and liquid, remaining after the fire that must be characterized to be properly treated and disposed of, to limit the risk of release of dangerous substances into the environment.