Oxidation Mechanisms of Electrolyte and Fire Gas Generation Laws During a Lithium-Ion Battery Thermal Runaway

Abstract

Lithium-ion batteries (LIBs) have come to hold ever greater significance across diverse fields. However, thermal runaway and associated fire incidents have undeniably constrained the application and development of LIBs. Consequently, gaining a profound understanding of the reaction mechanisms of LIB electrolytes during thermal runaway is of critical importance for ensuring the fire protection of LIBs. In this study, quantum chemical calculations were employed to construct oxidation reaction models of electrolytes, and a comprehensive summary of the sources of fire gas generation during the thermal runaway of LIBs is presented. During the sequence of oxidation reactions, the -COH functional group emerged as the most critical intermediate product. Under conditions of low oxygen availability, it was prone to decompose into CO, whereas in the presence of sufficient oxygen, it could undergo further oxidation to form -COOH and subsequently decompose into CO₂. Moreover, the reaction chains associated with electrolyte oxidation were found to be highly intricate, characterized by multiple branches and a wide variety of intermediate products. Furthermore, an in-depth analysis was carried out on the generation mechanisms of several typical fire gases. The analysis revealed that CH₃OH and C₂H₅OH could be considered as the characteristic products of the oxidation reactions of DMC and DEC, respectively. It is anticipated that this research will provide a robust theoretical foundation for elucidating the complex reactions involved in LIB fires and offer reaction models for fire simulation purposes, thereby contributing to the enhancement of the safety and reliability of LIBs in various applications.

1. Introduction

In recent years, the lithium-ion battery (LIB) industry has seen rapid development. In 2024, China’s production of LIBs reached 1170 GWh, representing a 24% year-on-year increase [1]. Correspondingly, the electric vehicle and energy storage industries have also made great progress. In 2024, the global sales volume of electric vehicles reached 18.23 million [2]. However, fires in scenarios involving the use of LIBs have become increasingly frequent. From January to May 2024, there were 10,051 electric bicycle fires in China [3]. Notably, the number of such fires has been increasing at an average rate of 20% per year over the past three years. According to incomplete statistics, from September to November 2024, more than 10 large-scale fires broke out in lithium battery factories or energy storage facilities globally. Currently, the safety performance of LIBs has emerged as a crucial issue that hinders the rapid development of the lithium battery industry, the electric vehicle sector, and the energy storage industry. Urgent breakthroughs and innovative solutions are required to address this situation [4,5,6,7].

Thermal runaway is the direct cause of LIB fires. During the thermal runaway process, the thermal decomposition and oxidation reactions of the electrolyte not only determine the rate of self-heating of LIBs, but also govern the types, quantities, and release rates of the fire gases produced [8,9,10]. Understanding its combustion mechanism is crucial for the inherent safety design of LIBs and for effective firefighting strategies [11,12,13,14,15]. Currently, LIBs commonly utilize a mixture of ethylene carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate (DEC) in specific proportions, with lithium hexafluorophosphate (LiPF₆) serving as the lithium salt electrolyte. Relevant experimental studies have shown that during the thermal runaway of LIBs, the electrolyte undergoes thermal decomposition and oxidation reactions first. These reactions set the initial conditions for self-heating. Virtually all the fire gases released during a fire originate from the reactions of the electrolyte [16,17,18,19,20]. At present, fire experiments combined with gas chromatography or Fourier transform infrared analysis are the primary methods employed to investigate the mechanisms of the thermal decomposition and oxidation reactions of the electrolyte, as well as the release patterns of fire gases from LIBs. Experimental research has revealed that the main fire gases generated during the combustion of LIBs include electrolyte vapor; hydrogen fluoride (HF); carbon monoxide (CO); carbon dioxide (CO₂); and hydrocarbons, such as methane (CH₄), ethylene (C₂H₄), ethane (C₂H₆), propylene (C₃H₆), and propane (C₃H₈) [21,22,23,24]. And the amount of fire gases generated and the concentration of each component are closely related to the battery system and the state of charge (SOC). The higher the SOC, the higher the proportion of H₂ and hydrocarbons in the fire gases; conversely, the proportions of CO and CO₂ increase when the SOC is lower [4]. Moreover, substances such as PF₅ and POF₃ are generated from the thermal decomposition of LiPF₆ in the electrolyte. Although the contents of these substances are low, they can react with components in the electrolyte, such as EC, DMC, and DEC, thereby promoting the thermal decomposition reaction of the electrolyte [4,18]. However, the current experimental results still fall short in accurately predicting the gas release patterns during LIB fires. The key problem lies in the lack of a combustion kinetic mechanism model for LIBs. Owing to the absence of a comprehensive and detailed chemical reaction mechanism model for the thermal decomposition and oxidation of the electrolyte, it is currently impossible to conduct a quantitative analysis of the generation of combustion products during the fire process of LIBs.

One of the key issues in the field of combustion reaction kinetics is how to obtain accurate elementary reactions to improve the precision of models. At present, combustion elementary reactions are mainly obtained through theoretical calculations and experimental measurements. Theoretical calculations here refer to quantum chemical calculations. When constructing combustion elementary reaction processes using quantum chemistry, the first step is to model the molecules participating in the reaction and search for transition state species during the reaction. These are then subjected to geometric structure optimization to obtain the potential energy surface, reaction barrier, vibration frequency, and other thermodynamic parameters of the elementary reaction. Then a chain reaction model is constructed by those elementary reactions. Employing quantum chemistry simulations to construct detailed chemical reaction models represents an efficient and precise approach for investigating combustion mechanisms [25,26]. Quantum chemistry simulations are capable of offering in-depth insights into the energy changes, reaction pathways, and reaction intermediates associated with combustion reactions. This information is of paramount importance to comprehensively understand and accurately predict combustion processes. Building upon this methodology, it becomes feasible to pinpoint the active sites and ascertain the thermodynamic parameters of various reactive species that participate in the thermal decomposition and oxidation reactions [27,28]. Moreover, it is also possible to accurately delineate the reaction pathways of each reactive species at the molecular level and subsequently construct the elementary reactions that govern thermal decomposition and oxidation. Such efforts lay a solid theoretical foundation for the kinetic analysis and numerical simulation studies of fires in LIBs.

In this study, we utilized quantum chemical calculations to elucidate the oxidation mechanisms of the principal constituents of lithium-ion battery (LIB) electrolytes. Our investigation centered on deciphering the oxidation pathways of EC, DMC, and DEC. We analyzed the patterns of their side reactions and the associated energy variations. Additionally, we provided a comprehensive summary of the mechanisms underlying fire gas generation during LIB thermal runaway. By clarifying these mechanisms, which represent the core contributions and innovative aspects of this research, we aimed to establish a robust theoretical framework. This framework will support further fire mechanism inquiry and numerical modeling efforts directed at enhancing LIB fire prevention.

2. Computational Details

During the thermal runaway process of LIBs, there are two scenarios of electrolyte combustion: The first scenario is that an electrolyte will undergo thermal decomposition reactions directly in a liquid environment and generate small-molecule substances and gaseous components when the temperature rises rapidly. The other scenario is that the electrolyte will evaporate into gaseous molecules first, which then continue to undergo thermal decomposition and oxidation reactions when the temperature rises slowly. In another work, we already studied and discussed the thermal decomposition reaction mechanisms of the electrolyte in liquid and air environments [29]. Therefore, in this work, we focused on the oxidation reaction mechanisms of the electrolyte gas molecules and analyzed their reaction paths.

2.1. Computational Contents

All calculations in this work were performed using Gaussian 16 [30] and GaussView 6.0 [31] software. In this work, we focused on a blended electrolyte formulated with lithium hexafluorophosphate (LiPF₆) as the lithium salt at a molar concentration of 1 mol·L⁻¹ and the primary solvents were a mixture of EC, DMC, and DEC in a volume ratio of 1:1:1. The study examined the oxidation reaction mechanisms of EC, DMC, and DEC molecules in gas environments. Given the analogous reaction mechanisms between DMC and DEC, the analysis did not differentiate between them in the main text to maintain conciseness. And an in-depth exploration of the oxidation paths is presented in the Supplementary Materials Figure S1 and Table S4.

2.2. Computational Methods

In this work Density Functional Theory (DFT) methods were used for quantum chemical simulations. The main molecular models of the electrolyte are displayed in Figure 1.

The quantum chemical calculation process detailed in this paper encompassed the following principal steps. Initial step: Geometric optimization of the molecules or groups was meticulously conducted. The overarching goal was to derive its most stable configuration, corresponding to the state of minimum energy, and concomitantly determine the relevant thermodynamic correction parameters. This geometric optimization step served as the cornerstone for subsequent calculations, as the accurate determination of molecular geometry significantly impacted the precision of all ensuing energy-related computations. Second step: Leveraging the optimally determined geometric configuration, electronic energy calculations were executed. Employing advanced quantum chemical methodologies, these calculations provided profound insights into the electronic structure of the system, and thus, elucidated the fundamental factors that governed the reactivity of the molecules or groups. Third step: The calculation of the Gibbs free energy for the molecule or group was carried out. This was achieved through the application of appropriate thermodynamic algorithms, which took into account relevant factors such as temperature and pressure. The Gibbs free energy, a crucial thermodynamic quantity, plays a pivotal role in predicting the spontaneity and equilibrium position of chemical reactions. Fourth step: An elementary reaction model was constructed. This involved an exhaustive search for the presence of reaction transition state substances. Rigorous verification of the reactivity of these putative transition state species was then performed, followed by the calculation of their respective Gibbs free energies. The accurate identification and characterization of transition states are central to understanding reaction mechanisms and reaction rates. Final step: Chain chemical reaction models were formulated. This model integrated the knowledge gleaned from the elementary reactions by considering the sequential and interactive nature of multiple reaction steps. By constructing such a model, a more comprehensive understanding of the kinetic behavior and reaction pathways within complex chemical reaction systems could be achieved.

During the computational process, the electrolyte molecules and functional groups were geometrically optimized by using the B3LYP density functional and the 6-311++G(d,p) basis set. And the thermodynamic and dispersion forces corrections were taken into account simultaneously. Subsequently, high-accuracy electronic energy calculations were performed based on the geometrically optimized molecules and functional groups by using the M06-2X/def2tzvp basis set. Finally, the Gibbs free energies of the reactions were calculated by the following formulas [29,32]:

$$\Delta G=E+G_{\left(\mathrm{corr}\right)}$$

(1)

$$\Delta G_{\mathrm{R}}=\Delta G_{\mathrm{Pro}1}+\Delta G_{\mathrm{Pro}2}-\Delta G_{\mathrm{Rea}1}-\Delta G_{\mathrm{Rea}2}$$

(2)

In the equations, ∆G represents the Gibbs free energy in kJ/mol; E represents the electronic energy at the M06-2X/def2tzvp level in kJ/mol; G_(corr) represents the Gibbs free energy correction factor at the B3LYP/6-311++G(d,p) level in kJ/mol; ∆G_R represents the reaction Gibbs free energy of the reaction in kJ/mol; and ∆G_Rea and ∆G_Pro represent the Gibbs free energy of reactants and products, respectively, in kJ/mol.

2.3. Transition State Search and Intrinsic Reaction Coordinate Pathway Analysis

In this study, the transition state (TS)–Berny method was used to analyze the transition states involved during the reactions. And the transition state reaction pathways were traced by using the Intrinsic Reaction Coordinate (IRC) in order to verify their links to the respective reactants and productions. And the function and basis used for the transition states search remained consistent with the aforementioned calculations.

3. Results and Discussion

3.1. EC Thermal Decomposition with Oxidation

Figure 2 and Table S1 show the thermal decomposition with oxidation pathways of EC molecules. The symbol TS in the figure represents the transition state species. The reason why it was called the thermal decomposition with oxidation reaction pathways was that the EC molecules first underwent a thermal decomposition reaction, and then started to undergo an oxidation reaction with oxygen. These two reactions were coupled together. These reactions represent a possible scenario for the combustion process of EC molecules in the electrolyte during the battery thermal runaway. This may occur when the oxygen supply is insufficient.

As the temperature rose, the EC molecules absorbed heat and underwent structural transformations with the carbon–oxygen single bond between the No.2 carbon atom and the the No.6 oxygen atom, as well as the No.5 oxygen atom and the No.7 carbon atom, which both elongated simultaneously. During this process it absorbed energy of 295.29 kJ/mol, which led the EC molecules to transform into TS1, which was a transition state. Subsequently, TS1 degraded into gaseous acetaldehyde (CH₃CHO) and carbon dioxide (CO₂), which released energy of −379.17 kJ/mol. Then acetaldehyde could undergo further thermal decomposition or oxidation, with its reactions divided into three concurrent pathways:

(1) In the first pathway, the acetaldehyde dissociated into a methyl radical (-CH₃) and a formyl radical (-CHO). The -CHO further decomposed into a free hydrogen atom (-H) and a CO molecule in the gas.

(2) In the second pathway, the hydrogen atom on the aldehyde group of the CH₃CHO combined with the carbon atom on the methyl radical to form TS2. TS2 then decomposed into a CH₃CO- radical and a CH₄.

(3) In the third pathway, the hydrogen atom on the aldehyde group of the CH₃CHO combined with the oxygen molecule to form TS3. And then TS3 decomposed into a CH₃CO- radical and a -OOH radical.

With CH₃CO- as the same key intermediate product, the subsequent reaction processes of pathway 2 and pathway 3 combined into one pathway. And the CH₃CO- could undergo further reactions in two reaction pathways: one was thermal decomposition, while the other was oxidation. In the thermal decomposition pathway, the carbon–carbon single bond in the CH₃CO- elongated to form TS4. Then TS4 decomposed into -CH₃ and CO. In the oxidation pathway, the CH₃CO- combined with an oxygen molecule to form CH₃COOO-. Then the CH₃COOO- combined with a CH₄ molecule to form TS5. And TS5 decomposed into CH₃COO-, -OH, and -CH₃. The carbon–carbon single bond in the CH₃COO- elongated to form TS6. Finally, TS6 decomposed into -CH₃ and CO_2.

In this reaction pathway, the EC molecule first underwent a thermal decomposition reaction. Then, the intermediate products generated in the reaction process reacted with the oxygen molecule in oxidation reactions. CH₃CHO, as the most critical intermediate product, had its oxidation reaction as the most core chemical reaction in this reaction pathway.

3.2. EC Oxidation

Figure 3 and Table S2 show the oxidation pathways of the EC molecules. During this reaction process, in the EC molecule, one hydrogen atom (No.3 or No.4) on carbon atom number No.1 and one hydrogen atom (No.8 or No.9) on carbon atom No.7 each combined with an oxygen atom in the oxygen molecule to form an intermediate species TS1. TS1 then decomposed into a radical of -OCOO(CH-)(CH-)- and H₂O_2. And another oxygen molecule combined with the radical of -OCOO(CH-)(CH-)- at its one hydrogen atom on the No.1 carbon atom to form a carbon–oxygen bond. And another intermediate species TS2 was produced. Then TS2 decomposed into a radical of -OCOO(C-)(CH-)- and -OOH. After this the -OCOO(C-)(CH-)- could undergo further oxidation reactions into two concurrent pathways.

(1) In the first pathway, it reacted in an incomplete oxidation style. The -OCOO(C-)(CH-)- radical combined with a -OOH radical at the only hydrogen atom on its No.7 carbon atom to form a hydrogen–oxygen bond. It produced TS3. And TS3 broke down at the hydrogen–carbon bond between No.7 carbon to form -OCOO(-C)(-C)- and H₂O₂. Then the -OCOO(-C)(-C)- transformed into TS4 by carbon–carbon and carbon–oxygen bond elongation. And TS4 decomposed into three COs, which released energy of 427.72 kJ/mol.

(2) In the second pathway, it reacted in a complete oxidation style. The -OCOO(C-)(CH-)- radical combined with a -OOH radical at the No.7 carbon atom to form a carbon–oxygen bond and produce a new radical of -OCOO(CH-)(COOH)-. And the -OCOO(CH-)(COOH)- broke down at its oxygen–oxygen bond to form -OCC(CH-)(CO)- and -OH. After this, another oxygen molecule combined with the -OCC(CH-)(CO)- at its No.7 carbon atom to form a radical of -OCC(CHOO)(CO)-. Then on the radical of -OCC(CHOO)(CO)-, the -OO group and -H connected to the number 7 carbon atom to combine together to form TS5, which decomposed into -OCC(CO)(CO)- and -OH, which released energy of 365.18 kJ/mol. The -OCC(CO)(CO)- transformed into TS6 by carbon–carbon and carbon–oxygen bond elongation. And TS6 decomposed into one CO and two CO₂, which released energy of 262.18 kJ/mol.

In the early stage of the thermal runaway of LIBs, before the battery pressure relief valve was opened and there was insufficient oxygen inside the battery, the EC molecules first underwent thermal decomposition reactions, which produced free radicals and CO in gas. As the thermal runaway continued to progress, the decomposition of the cathode material generated oxygen. When the thermal decomposition products of EC encountered this oxygen, oxidation reactions occurred, which produced H₂O, CH₄, and CO₂. After the battery pressure relief valve was opened and air entered the interior of the battery, the EC molecules continued to undergo oxidation reactions with more oxygen, which produced CO, CO₂, and free radicals while releasing energy, which sustained the combustion chain reaction.

3.3. DMC Oxidation

Figure 4 and Table S3 show the oxidation pathways of DMC molecules. The oxidation reactions of DMC molecules are more complex than those of EC molecules, with multiple side reaction pathways and the formation of a greater number of intermediate products. The essence of the oxidation reaction of DMC molecules is the oxidation of the methyl groups at both ends of the molecule. After the methyl groups are oxidized to aldehyde groups, the aldehyde groups can dissociate to form CO or be further oxidized to carboxyl groups, which then dissociate to form CO₂. Because DMC molecules have a symmetrical structure with a methyl group on each side, the oxidation process can start from one side or from both sides simultaneously.

First, the hydrogen atom on the -CH₃ group (No.1 and No.8 carbon atoms are equivalent) in the DMC molecules combined with an oxygen molecule to form TS1. And TS1 broke down at the hydrogen–carbon bond to form CH₃OCOOCH₂- and -OOH. Then the CH₃OCOOCH₂- combined with another oxygen molecule at the carbon atom of its -CH₂- to form a carbon–oxygen bond and produce CH₃OCOOCH₂OO-. After absorbing heat, the oxygen atom and hydrogen atom at the terminal end of CH₃OCOOCH₂OO- combined to form TS2. Then TS2 broke down into CH₃OCOOCHO and -OH. After this, CH₃OCOOCHO could undergo further reactions divided into three concurrent pathways, one of which was a thermal decomposition reaction, and the other two were oxidation reactions.

(1) In the first pathway, it reacted in a thermal decomposition style. After absorbing heat, the hydrogen atom of -CHO combined with the No.6 oxygen atom on the CH₃OCOOCHO to form TS3. Then TS3 broke into CH₃OCOOH and CO. After this, the hydrogen atom of -COOH combined with the No.7 oxygen atom on the CH₃OCOOH to form species TS4. And TS4 decomposed into CH₃OH and CO_2.

(2) In the second pathway, it reacted in an oxidation style and began with the -CHO group in CH₃OCOOCHO. The hydrogen atom of the -CHO combined with an oxygen molecule to form TS5. And TS5 broke into CH₃OCOOCO- and -OOH. As the CH₃OCOOCO- was a key intermediate product, there were three reaction branches. The first reaction branch was that the carbon atom on the end of CH₃OCOOCO- combined with an oxygen molecule to form a CH₃OCOOCOOO-. Subsequently, a -CH₃ combined with the oxygen atom at the end of CH₃OCOOCOOO- to form a CH₃OCOOCOOOCH₃. After absorbing heat, the CH₃OCOOCOOOCH₃ broke into CH₃OCOOCOO- and -OCH₃ at the oxygen–oxygen bond. Then the CH₃OCOOCOO- transformed into TS20 by the carbon–oxygen bond due to the end elongating. And TS20 broke into CH₃OCOO- and CO₂. The CH₃OCOO- transformed into TS21 by its carbon–oxygen bond elongating. Then TS21 decomposed into CH₃O- and CO₂. And the second reaction branch was that the CH₃OCOOCO- transformed into TS12 by the carbon–oxygen bond between the No.8 carbon atom and the No.7 oxygen atom elongating. Then TS12 broke into CH₃OCOO- and CO. And the subsequent reactions of CH₃OCOO- were the same as those in the first reaction branch. The third reaction branch was that the CH₃OCOOCO- transformed into TS13 by the carbon–oxygen bond between the No.5 carbon atom and the No.7 oxygen atom elongating. Then TS13 decomposed into CH₃OCO- and CO₂. After absorbing heat, the CH₃OCO- transformed into TS14 by the carbon–oxygen bond of the No.1 carbon atom and No.4 oxygen atom elongating. TS14 then broke into -CH₃ and CO₂.

(3) In the third pathway, it reacted in an oxidation style and began with the -CH₃ group in CH₃OCOOCHO. And this process was essentially the same as the oxidation reactions of the methyl group on one side of the DMC molecule to an aldehyde group, as previously described. After a series of reactions, another key intermediate product, OHCOCOOCHO, was generated. And then there were two reaction branches that started with OHCOCOOCHO.

In the first branch, the hydrogen atom on the -CHO migrated to the No.6 oxygen atom, while the carbon–oxygen bond between the No.1 carbon atom and the No.2 oxygen atom of OHCOCOOCHO elongated to form TS8. Then TS9 decomposed into OHCOCOOH and CO. And the hydrogen atom on the -OH migrated to the No.4 oxygen atom, while the carbon–oxygen bond between the No.5 carbon atom and the No.4 oxygen atom of OHCOCOOCHO elongated to form TS11. Then TS11 broke into HCOOH and CO₂.

In the other branch, an oxygen molecule combined with the hydrogen atom of the -CHO of the OHCOCOOCHO to form TS9. And TS9 broke into OHCOCOOCO- and -OOH. As a key intermediate product, the reaction pathway of OHCOCOOCO- divided into three directions.

In the first direction, an oxygen molecule combined with the -CO- at the end of the OHCOCOOCO- to form OHCOCOOCOOO-. And a free -CH₃ combined with the oxygen atom of the end of OHCOCOOCOOO- to form OHCOCOOCOOOCH₃. Then it broke into OHCOCOOCOO- and -OCH₃. And the OHCOCOOCOO- transformed into TS20 by the carbon–oxygen bond between the No.5 carbon atom and the No.7 oxygen atom elongating. Then TS20 broke into OHCOCOO- and CO₂. After this, the OHCOCOO- transformed into TS21 by the carbon–oxygen bond between the No.5 carbon atom and the No.4 oxygen atom elongating. TS21 decomposed into HOCO- and CO₂.

In the second direction, the OHCOCOOCO- transforms into TS17 by the carbon–oxygen bond between the No.8 carbon atom and the No.7 oxygen atom elongating. Then TS17 broke into OHCOCOO- and CO. And then the reactions of OHCOCOO- were the same as previously described.

In the third direction, the OHCOCOOCO- transformed into TS18 by the carbon–oxygen bond between the No.5 carbon atom and the No.7 oxygen atom elongating. TS18 decomposed into OHCOCO- and CO₂. Then the OHCOCO- transformed into TS19 by the carbon–oxygen bond between the No.1 carbon atom and the No.4 oxygen atom elongating. Then TS19 broke into HOC- and CO₂.

The oxidation reaction pathway of DMC was very complex, with several reaction pathways. The oxidation process of DMC had several important characteristics:

(1) The products of CO were produced by the dehydrogenation of the aldehyde group (-CHO), while CO₂ was generated by the decomposition of carboxyl groups (-COOH or-COO⁻), which came from the further oxidation of the aldehyde group (-CHO). Therefore, the aldehyde group was the most important key intermediate of the formation of both CO and CO₂.

(2) In the oxidation process of DMC under the conditions of insufficient oxygen, by-products such as methanol, formaldehyde, and formic acid gases could be present. These gases could serve as characteristic products of the DMC oxidation reaction and could be used as indicators to determine the progress of the DMC combustion.

3.4. DEC Oxidation

The oxidation reactions of DEC molecules were similar to those of DMC but more complex, with more reaction pathways and intermediate products. The basic principle of the DEC molecular oxidation reactions were that the -CH₃ at the α site (the carbon atom at the end of -C₂H₅) was oxidized to -CHO, which could dissociate to produce CO or be further oxidized to -COOH or -COO-. And then the -COOH or -COO- dissociated to produce CO₂. After this, the -CH₂- at the β site continued to be oxidized to -CHO or -COOH groups, which dissociated to form CO or CO₂, respectively. Similar to the DMC molecule, the oxidation process of DEC could start from one side or from both sides simultaneously. For the DEC molecule with more carbon atoms, the combination of oxidation reactions of methyl and methylene groups at different sites made the reaction pathway of DEC more complex and extensive. And the oxidation pathways of DMC molecules were shown in Figure S1 and Table S4 (in the Supplementary Materials).

3.5. Main Mechanism of Gas Generation in LIB Fire

During the fire thermal runaway and fire process of LIBs, the electrolyte undergoes thermal decomposition and oxidation reactions, producing large amounts of gases. And these gas products can serve as marks as a warning of thermal runaway and fire. Therefore, the combustion mechanism of the gas productions of electrolytes has important theoretical value and practical significance for LIB fire safety. Shown as

Table 1. Source and typical fire gas during thermal runaway of LIBs.

No.	Source of Fire Gas	Typical Fire Gas
1	LiPF6 thermal decomposition in solution or gas	PF5, POF3, HF, CH3F
2	EC thermal decomposition in solution	H2, CO, CO2, CH4, C2H6, CH3CHO
3	EC + Li+ thermal decomposition in solution	C2H4,
4	EC + PF5 oxidation reaction in solution or gas	C2H5F, CO2, POF3
5	EC + POF3 oxidation reaction in solution or gas	C2H3F, CO2
6	EC oxidation reaction in gas	CH4, CH3CHO, C2H6, CO, CO2
7	DMC thermal decomposition in solution or gas	H2, CH4, C2H6, CO2
8	DMC + Li+ thermal decomposition in solution	CH4, C2H6
9	DMC + PF5 oxidation reaction in gas	CH4, C2H6, CH3F, CO2, POF3
10	DMC + POF3 oxidation reaction in gas	CH4, C2H6, CH3F, CO2
11	DMC oxidation reaction in gas	H2, CH4, C2H6, CH3COH, CH3COOH, CO, CO2
12	DEC thermal decomposition in solution or gas	H2, CH4, C2H6, C3H8, C4H10, C2H4, CO2, HCHO, C2H5OH
13	DEC + Li+ thermal decomposition in solution	C2H6, C4H10
14	DEC + PF5 oxidation reaction in gas	C2H5F, C2H6, C4H10, CO2, POF3
15	DEC + POF3 oxidation reaction in gas	C2H5F, C2H6, C4H10, CO2
16	DEC oxidation reaction in gas	H2, CH4, C2H6, C3H8, C4H10, C2H5OH, HCHO, CH3COH, CH3COOH, CO, CO2

, based on the thermal decomposition and oxidation reaction mechanisms of the main components of LIB electrolytes, it can be inferred that the thermal runaway process of LIBs may generate a variety of fire gases, such as CH₄, C₂H₄, C₂H₆, C₃H₈, C₄H₁₀, CO, CO₂, HCHO, CH₃CHO, CH₃OH, C₂H₅OH, CH₃COH, HCOOH, CH₃COOH, HF, PF₅, POF₃, CH₃F, and C₂H₃F [4,10,18,28].

(1) Generation mechanism of C_xH_y fire gas

The main components of the electrolytes were carbonate esters, such as EC, DMC, and DEC. And there were a large amount of hydrocarbon groups, such as -CH₃- C₂H₅ and -CH₂-, in these molecules. During the thermal runaway of LIBs, whether the electrolytes underwent thermal decomposition or an oxidation reaction after evaporation, a large number of free radicals was generated. And these radicals combined with each other to form alkane and alkene gas products.

Furthermore, the alkanes mainly originated from the combination of free radicals generated by thermal decomposition reactions. In the EC, DMC, and DEC molecules, groups such as -H, -CH₃, -C₂H₅, and -CH₂- broke away from the main chain to form free hydrogen atoms or groups. These free radicals were the key to initiating combustion chain reactions. Free -H combined with other free groups to form CH₄ and C₂H₆. And free -CH₃ and -C₂H₅ groups combined together to form C₂H₆, C₃H₈, and C₄H₁₀. And the main alkene of the fire gases of LIBs was C₂H₄, which was generated from the thermal decomposition of EC + Li⁺ or DEC and the combination of free methylene groups. Its detailed reaction mechanism is shown in Table S5.

(2) Generation mechanism of C_xH_yO_z fire gas

① Generation mechanism of alcohol gas: During the thermal runaway process, the alcohol gases generated were CH₃OH and C₂H₅OH, which were intermediate products produced by the thermal decomposition and oxidation reactions of DMC and DEC, respectively. CH₃OH and C₂H₅OH could originate from three reaction processes, as shown in the following equations:

(3)

(4)

(5)

(6)

(7)

The first process was that the alkyl group (-CH₃ or -C₂H₅) of DMC or DEC detached to form CH₃OCOO- or C₂H₅OCOO-. Then the CH₃OCOO- or C₂H₅OCOO- broke into CO₂ and CH₃O- or C₂H₅O-. Finally, the CH₃O- or C₂H₅O- combined with a free hydrogen atom to form CH₃OH or C₂H₅OH.

The second process was that the alkyl group (-CH₃ or -C₂H₅) of DMC or DEC was oxidized to -CHO. Then the hydrogen atom of -CHO migrated to the carbonyl oxygen atom of the carbonate group to form CH₃OCOHO or C₂H₅OCOHO, while a CO was released. And the hydrogen atom continued to migrate to the other oxygen atom of the previous carbonate group to form CH₃OH or C₂H₅OH, while a CO₂ was released.

And the third process was that the free CH₃- or C₂H₅- generated from the reactions of DMC or DEC combined with the free -OH group to form CH₃OH or C₂H₅OH.

② Generation mechanism of aldehyde gas: The aldehyde gases generated were HCHO and CH₃CHO, which were intermediate products produced by the thermal decomposition and oxidation reactions of EC, DMC, and DEC. And the HCOH could originate from three reaction processes, as shown in the following equations:

(8)

(9)

(10)

(11)

The first process was that the -CH₃ of the DMC lost a hydrogen atom and then it decomposed into CH₃OCO- and HCHO. The second process was that the intermediate product C₂H₅OCOCH₂- of DEC broke into C₂H₅OCO- and HCHO. And the third process was that the CH₄, CH₃OH, and free -CH₃ were oxidized to form HCHO.

The CH₃COH could originate from two reaction processes, shown as Equations (12)–(14): one was that EC broke into CH₃COH and CO₂, and the other one was that C₂H₆, C₂H₅OH, and a free -C₂H₅ were oxidized to form CH₃CHO.

(12)

(13)

(14)

③ Generation mechanism of carboxylic gas: The carboxylic gases generated were HCOOH and CH₃COOH, which were intermediate products produced by the oxidation reactions of EC, DMC, and DEC. The HCOOH could be generated by two pathways, shown as Equations (15)–(19):

(15)

(16)

(17)

(18)

(19)

One pathway is that the intermediate product OHCOCOOCHO of DMC and DEC underwent a reaction in which the hydrogen atom on the -CHO migrated to the carbonyl oxygen atom of carbonate group to form OHCOCOOH, while a CO was released. And then the hydrogen atom on the -COOH migrated again to the other aldehyde group oxygen atom to form HCOOH and CO₂. And the other pathway was that CH₃OH and CH₃CHO were oxidized to form HCOOH.

The CH₃COOH was generated by the oxidation reactions of C₂H₅OH and CH₃CHO, as Equations (20)–(22) show:

(20)

(21)

(22)

④ Generation mechanism of CO and CO₂: During the LIB thermal runaway, CO and CO₂ were the largest quantities of fire gases. They were generated by the thermal decomposition and oxidation reactions of EC, DMC, and DEC, and the main generation mechanisms are shown in Table S6. In the thermal decomposition reactions, CO and CO₂ came from the carbonate groups in the EC, DMC, and DEC molecules. Meanwhile, in the oxidation reactions, the hydrocarbon groups in the molecules were oxidized to form carbonyl groups. And then, under a low-oxygen state, the carbonyl groups decomposed into CO. With enough oxygen, the carbonyl groups were further oxidized to carboxyl groups, and the carboxyl groups broke into CO₂.

(3) Generation mechanism of fluorine containing fire gas

LiPF₆ is one of the most widely used electrolyte lithium salts in LIBs. During the thermal runaway process, LiPF₆ was extremely prone to undergo thermal decomposition reactions and produced PF₅ and POF₃, which have strong oxidizing properties. When the PF₅ and POF₃ reacted with the intermediate products of the electrolyte, fluorine-containing fire gases were generated, such as HF, CH₃F, C₂H₃F, C₂H₅F, and C₂H₄F₂. The generation mechanisms of these fluorine-containing fire gases are shown in the following equations:

(23)

(24)

(25)

(26)

(27)

(28)

(29)

(30)

(31)

(32)

(33)

(34)

(35)

(36)

(37)

(38)

(39)

The gas of PF₅ was derived from the thermal decomposition of LiPF₆. And the gas of POF₃ was produced from the reactions of PF₅ with substances of EC, DMC, DEC, and H₂O. The gas of HF came from the reactions of H₂O or -OH and PF₅ or POF₃. For the gases of CH₃F, C₂H₃F, C₂H₅F, and C₂H₄F₂, they were generated from the reactions of EC, DMC, DEC, -CH₃, and -C₂H₅ with PF₅ or POF₃. In conclusion, the fluorine-containing fire gases generated during the combustion of the LIB originated from LiPF₆. In the reaction process, PF₅ and POF₃ were the most important key intermediate substances.

4. Conclusions

In this study, we used quantum chemistry calculation methods to explore the oxidation processes of the main constituents of EC, DMC, and DEC in gas and analyzed the generation mechanisms of the main fire gases of LIBs. This enabled us to build detailed oxidation reaction models for EC, DMC, and DEC. And the main findings were as follows: In the oxidation reaction process of the electrolyte, -COH was the most crucial intermediate product. When the oxygen concentration was low or the heating rate was fast, -COH underwent thermal decomposition reactions, which, in turn, generated CO. When the oxygen was enough, -COH was further oxidized to form -COOH. Subsequently, -COOH underwent a decomposition reaction to produce CO₂. Furthermore, the oxidation process of the electrolyte was complex, with multiple reaction pathways and various intermediate products.

And the fire gas could include CH₄, C₂H₄, C₂H₆, C₃H₈, C₄H₁₀, CO, CO₂, HCHO, CH₃CHO, CH₃OH, C₂H₅OH, CH₃COH, HCOOH, CH₃COOH, HF, PF₅, POF₃, CH₃F, C₂H₃F, and C₂H₅F. During the oxidation of DMC and DEC, alcohol substances could be produced. CH₃OH and C₂H₅OH could serve as the characteristic products of the oxidation reactions of DMC and DEC, respectively, which can be used to infer the progress of the reactions. The quantum calculations in this study could provide a theoretical basis for understanding the complex reactions during thermal runaway and fire process of LIBs and these models of oxidation reactions can be used for the fire simulation of LIBs.

5. Research Prospects

Constructing combustion reaction mechanism models by using quantum chemical calculations method is a theoretical molecular work. In this work, we only gave the calculation results under standard conditions. In this study, our core aim was to construct a detailed chemical reaction model for the oxidation reaction of the electrolyte in lithium-ion batteries, that is, chemical reaction chain models composed of elementary reactions.

By using DFT calculations, the conditions of pressure and temperature cannot affect the elementary reaction. The processes of optimizing the group structure and calculating the single-point energy were, in essence, aimed at solving the electronic Schrödinger equation for the group system. It should be noted that the solution of the Schrödinger equation does not incorporate temperature and pressure terms. The reaction course of a group was governed by the group potential energy surface, which was defined as the set of single-point energies that corresponded to different geometric configurations of the group. Significantly, the potential energy surface was entirely independent of both the temperature and pressure.

In our future work, we will conduct a thermodynamic analysis of each elementary reaction in the reaction chain models to obtain the kinetic parameters. And there are some methods for obtaining thermodynamic parameters at different temperatures using the calculation data of elementary reactions and groups under standard conditions [32]. As for future applications, these chain reaction models and parameters can serve as a fundamental model to provide theoretical support for combustion numerical simulations. For example, these data can be applied to the combustion kinetics analysis software Chemkin, and then be used in ANSYS FLUENT for the numerical simulations of fires.