Lithium-ion batteries (LiBs) are now the most employed power source for portable electronic devices and fully electric and hybrid engines [
1‐
6] since they can provide high energy and power per unit of the battery weight, as they are lighter and smaller than other rechargeable batteries [
7,
8]. During the charge and discharge cycles, a significant quantity of heat is created inside of LiBs, due to the exothermic chemical reactions [
9]. This generated heat negatively influences the performance, life span, and safety of LiBs [
10‐
12]. Furthermore, the electric vehicles are increasingly required to use higher energy density batteries than ever before and to accumulate more cells in the pack, in order to increase the mileage that can be covered before the next charging. This trend further increases the internal heat generation and accumulation that might be expected during normal operations [
13]. Further, the exothermic reactions can be triggered in the case of abuse of LiBs, such as mechanical abuse, overcharging or high temperature operation [
14]. Battery failure can lead to the release of a combustible gas mixture (hydrogen, methane, organic carbonates, or propane) and under certain conditions thermal runaway can occur [
15]. Also, the heat released by the exothermic reactions can cause a thermal runaway, manifested as an uncontrollable rise in the reaction rate. The mechanism of thermal runaway can be described as a chain of chemical reactions [
16] that starts in a single battery cell, usually with the decomposition of the solid electrolyte interphase (SEI) layer followed by a reaction between the anode and the electrolyte, the decomposition of electrolyte, and the melting of the separator made from polyethylene (PE) or polypropylene (PP) [
17]. This can result in the ejection of a large amount of dark smoke and hot sparks. The main risk is that when this process happens within the individual cells, it can propagate throughout the entire battery and it can cause an explosion [
15,
18]. During the burning of LiBs, the generated toxic smoke can contain chemical components such as carbon monoxide (CO) and hydrogen fluoride (HF) [
11,
19‐
21]. The main source of the flammable substances in the battery is the electrolyte. Generally, the electrolyte is based on halogens and organic solvents such as diethyl carbonate, polypropylene carbonate, and ethylene carbonate and salts. The most common salt is lithium hexafluorophosphate (LiPF
6) but also other Li-salts (LiBF
4, LiClO
4 or LiSO
3CF
3) can be used. When overheated, the electrolyte will decompose and be released from the battery cells. The gases do not need to be ignited instantly. At higher temperature hydrogen fluoride (HF), phosphorus pentafluoride (PF
5) and phosphoryl fluoride (POF
3) can be formed as a consequence of the electrolyte and binder—polyvinylidene fluoride (PVDF) decomposition. Compounds with fluorine content can also be applied as flame retardant materials for the components such as electrolyte or separator. They can even be used as the additives for cathode and anode materials, usually in a form of e.g. fluorophosphates. The decomposition of LiPF
6 salt is promoted by the presence of water/humidity according to the following reactions [
21]:
$${\text{LiPF}}_{6} \leftrightarrow {\text{LiF}} + {\text{PF}}_{5}$$
(1)
$${\text{PF}}_{5} + {{\text{H}}}_2{\text{O}} \leftrightarrow {\text{POF}}_{3} + 2{\text{HF}}$$
(2)
$${\text{LiPF}}_{6} + {{\text{H}}}_2{\text{O}} \leftrightarrow {\text{LiF}} + {\text{POF}}_{3} + 2{\text{HF}}$$
(3)
To avoid thermal runaway and keep the temperature in a range that does not negatively influence the performance of the battery, the LiBs are provided with a battery thermal management system [
18,
20,
22‐
24]. Generally, the surface of the battery cells in electric vehicles have been cooled with forced directed ambient or cooled air. An alternative method consists of an indirect liquid-cooled system in which a cooled water–glycol mixture is pumped through pipes and so-called cold plates close to the cells [
20,
23]. A third and more effective thermal management system consist of the direct immersion in a cooling single-phase dielectric liquid [
20]. These liquids are forced to circulate with a pump to ensure a constant flow to make continuous contact with all the battery modules [
20,
25]. Dielectric coolants have higher thermal conductivity, density, and specific heat capacity than air and therefore perform more effectively as cooling media than air. Many of the commercial dielectric liquids used as refrigeration liquids (RL) contain halogens due to their performances in avoiding or significantly delaying thermal runaway. Cooling liquids are expected to have chemical stability at higher temperatures. However, there is limited knowledge about the formation of hydrofluoric acid in the case of high temperature accidents involving LiBs using a cooling system based on refrigeration liquids containing halogens. It is unknown if the use of these RLs could increase the quantity of toxic gas released. Such data can contribute to a more sustainable design of future batteries and knowledge about the risks associated with accidents with LiBs thermal runaway. This study aimed to determine the formation of HF in the case of a high temperature accident (at 700 ºC) involving the Li-ion batteries using the cooling system based on four commercial refrigeration liquids containing fluorine. The main goal of the work was to determine quantitively how much the selected liquids contribute to the formation of HF during the fire of LiBs.