Titolo della tesi: Study of safety issues of Lithium-ion batteries for their application in the automotive sector
Lithium-ion batteries offer a series of advantages compared to other type of batteries of different chemistry. In particular they are characterized by high energy and power density and they have a longer shelf life and high efficiency. Despite its overall advantages, the greatest limitation to lithium-ion technology development is due to safety problems.
Nowadays lithium-ion batteries are widely used, in fact they are the most suitable choice to use in many applications, such as portable electronics, power tools, and hybrid/full electric vehicles. Despite the low failure rate, in recent years, several episodes have been reported in which lithium ion batteries have been the source of fires and dangerous accidents. This is due to their high energy density which imply greater dangers in the event of malfunctions or failures, given the presence of toxic and highly flammable substances.
In normal use conditions lithium-ions move from the anode to the cathode passing through the electrolyte. If accidentally a failure occurs, the power generated can become greater than the heat that the cell is able to dissipate. Consequently, the temperature rapidly increases, thus the polymer separator melts due to the temperature increasing leading to an internal short circuit. The electrolyte reacts with the electrodes producing more and more heat, flammable gases and oxidants. Therefore, no external oxygen is required for the ignition of the fire. The failure mechanism that could lead to a fire or an explosion (or both) is called thermal runaway.
For these reasons, failures of Li-ion batteries that resulted in a fire aroused great attention both for the intensity of the fire and for the combustion products. Moreover, some documented cases are reported of ignition of battery pack subject to fire after its extinction, so the difficulty found in extinguishing these fires is another crucial point.
The aim of this thesis is to study the thermal runaway of Li-ion cells of battery systems for hybrid/full electric vehicles.
To this purpose, fire tests were performed on single NCA cells from Panasonic (NCR 18650). In particular, measurements of the heat release rate (HRR) were made using an ISO 5660 cone calorimeter, varying the state of charge (SoC) and the external radiant heat flux, with the intent to reproduce the conditions that are established when the cells are involved in a fire.
Cell components were investigated using a DSC before and after cone calorimetric tests. Moreover, SEM-EDS and XRD were used to determine the structural, morphological, and compositional changes of Panasonic 18650 Li-ion cell components allowing the identification and the characterization of the main reactions that occur in the cell under thermal abuse conditions and which can lead to thermal runaway. The kinetics of these reactions were assessed using the Kissinger and Ozawa methods.
Then the thermal and kinetics parameters of the reactions occurring during the thermal runaway together with the phenomena involving the electrolyte (i.e., venting) were included in the Battery and Fuel Cell Module of COMSOL Multiphysics simulator, allowing the development of a predictive model. The kinetic studies and the COMSOL simulation were made also on Lithium-ion cells with different chemistry (LTO).