1. Mechanism of thermal runaway process in lithium-ion batteries
Lithium batteries are formed by embedding lithium ions into carbon (petroleum coke and graphite) to form a negative electrode. LixCoO2 is commonly used as the positive electrode material, while LixNiO2 and LixMnO4 are also used. LiPF6+diethylene carbonate (EC)+dimethyl carbonate (DMC) is used as the electrolyte. The main triggering factors for thermal runaway include mechanical damage, overcharging, internal short circuits, etc. Under the influence of various factors, the active materials inside lithium-ion batteries undergo violent exothermic reactions, and the internal temperature of the battery exceeds the controllable range, ultimately leading to thermal runaway. The exothermic chemical reactions occurring inside the lithium-ion battery include the decomposition of the solid electrolyte interface facial mask SEI, the reaction between the negative active material and the electrolyte, the reaction between the negative active material and the binder, and the oxidation decomposition reaction of the electrolyte.
During the charging and discharging process of lithium-ion batteries, the vinyl carbonate on the solid-phase interface of the electrode active material will react with the negative electrode lithium, forming a layer of SEI film on the graphite adhesion surface. This membrane can directly slow down or even prevent the reaction between the electrolyte and the active materials on both sides of the electrode, significantly reducing its exothermic rate and improving the stability of the positive and negative electrode materials.
As the temperature rises to 90-120 ℃, the SEI film begins to decompose, followed by an exothermic reaction between the electrolyte and the negative electrode active material. Taking vinyl carbonate as an example, the reaction process is shown in equations (1) and (2):
During the exothermic reaction, the internal temperature of the battery gradually increases. Based on the use of different diaphragm materials, their melting points also vary. The common polypropylene diaphragm has a melting point of 165 ℃ and the polyethylene material has a melting point of 135 ℃. After reaching the melting point temperature of the separator material, the internal separator undergoes local contraction, causing direct contact between the positive and negative electrode materials inside the battery, resulting in a short circuit and generating a large amount of heat. The large amount of heat generated by the short circuit causes the diaphragm to rapidly contract, further exacerbating the exothermic reaction.
At the same time, in the temperature range where the SEI film decomposes and undergoes exothermic reactions, lithium salts also undergo intense exothermic reactions with the electrolyte. Common types of active materials for lithium-ion batteries include lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), etc. Lithium hexafluorophosphate decomposes at high temperatures to produce PF5, which further reacts with the solvent to take up the oxygen atoms of the C-O bond and undergo a violent exothermic reaction, further accelerating the decomposition of the electrolyte. At the same time, the oxidation-reduction reaction between lithium hexafluorophosphate and solvent also releases the highly toxic gas hydrofluoric acid (HF). The specific reaction process is shown in equations (3) to (5):
Within the same temperature range, the electrolyte itself undergoes a decomposition reaction and releases a small amount of combustible gas. When using rate calorimetry to analyze the thermal runaway process, it was found that the gases produced by electrolyte decomposition are mainly composed of C2H4, CO, and H2. The electrolyte is rapidly vaporized and increases the internal pressure of the battery. When the internal pressure reaches the limit of the pressure relief valve, a large amount of combustible gas will be ejected, further exacerbating the spread of thermal runaway. The heat generated by complete combustion of electrolyte is much greater than the heat released by decomposition reaction. Taking ethylene carbonate (EC) and propylene carbonate (PC) as examples, the reaction processes of electrolyte oxidation (6)~(7) and incomplete oxidation (8)~(9) are as follows:
As the internal temperature of the battery gradually increases, the active material of the positive electrode begins to decompose. Based on the use of different active materials, the temperature at which exothermic reactions occur also varies. The decomposition of the positive electrode active material produces oxygen, which then participates in the reaction with the internal active material, generating a large amount of gas inside the battery. The reaction process is as follows:
When the temperature exceeds 136 ℃, the binder polyvinylidene fluoride (PVDF) will react with lithium to produce hydrogen gas. The reaction process is as follows:
Except for the SEI film melting and absorbing heat, the above chemical reactions are all exothermic reactions. The heat release of electrolyte decomposition, separator, battery active material, and adhesive accounts for 43.5%, 30.3%, 20.1%, and 6.2% of the total heat release, respectively. The reaction between the positive and negative active materials of the battery and the electrolyte is the largest heat source.
2. Inducing factors of thermal runaway in lithium-ion batteries
The triggering factors of thermal runaway in lithium-ion batteries can be classified into three categories: mechanical abuse (needle puncture, compression deformation, external collision), electrical abuse (overcharge and over discharge, short circuit), and thermal abuse (thermal management system failure). Mechanical abuse can easily induce internal short circuits in lithium batteries, leading to thermal runaway; In the abuse of electricity, overcharging and discharging of batteries can cause internal side reactions, leading to local overheating of the battery cells and causing thermal runaway; External short circuit is a dangerous state of rapid discharge of batteries, where extremely high currents cause rapid heating and even fuse the battery terminals; In the state of thermal abuse, the failure of the thermal management system often triggers the contraction and decomposition of the internal diaphragm, ultimately leading to internal short circuits and thermal runaway.
In addition, the battery's own state is also one of the important factors causing thermal runaway. With the increase of battery charge and discharge cycles and the induction of impurities mixed in during dendrite production, adverse side reactions such as metal dendrites are generated, which are easy to pierce the separator and cause local short circuits in the battery.
2.1 Research on battery thermal runaway caused by thermal abuse
According to the electrochemical thermal coupling overcharge heat escape model of lithium-ion batteries established in literature, lithium-ion batteries usually start to self heat up when the temperature reaches 80 ℃. When the battery heat overflows and cannot be effectively released, the thermal management of the battery will lead to an uncontrollable increase in battery temperature, which will diffuse from local individual cells to the power battery pack, causing a series of side reactions and thermal runaway.
Thermal abuse does not spontaneously occur inside the battery. Often, due to mechanical abuse or other reasons, the internal temperature of the battery rises to a threshold, and local areas of the battery are heated up, leading to thermal abuse and further triggering temperature control and self ignition of the battery.
At the same time, thermal runaway has also been used as a research method for testing experimental battery runaway processes and detecting safety characteristics during battery thermal runaway. In 1999, KITOH et al. conducted research on monitoring the thermal runaway safety characteristics of high specific energy power batteries based on external heating methods. Since then, the adiabatic energy method has been widely used to test the thermal runaway temperature threshold of lithium-ion batteries. The current research on thermal abuse is mainly based on external radiation ignition of batteries. Liu Mengmeng established a multi endogenous transient heat generation model and an electrochemical thermal coupling model. Based on the radiation heating method, the safety characteristics of batteries after self ignition caused by thermal abuse were studied. It was found that battery combustion can be divided into three stages, namely injection combustion, stable combustion, and secondary injection combustion. LI et al. studied the effect of discharge current on temperature under the background of thermal runaway caused by thermal abuse. It was found that when the discharge current is constant, the quality loss, safety characteristic parameters, thermal runaway initiation temperature, and peak temperature during the thermal runaway process all depend on the battery capacity.
2.2 Research on battery thermal runaway caused by electrical abuse
Common causes of battery thermal runaway include overcharging and discharging, internal short circuits, external short circuits, etc.
(1) Overcharge and over discharge
During the completion of a charge discharge cycle in a lithium-ion battery, the BMS battery management system will normally block the charging current based on the state of charge. When the BMS system fails, overcharging the battery can easily cause serious self ignition accidents. After reaching the SOC threshold during charging, lithium metal will adhere to the surface of the negative electrode active material, and the attached lithium will react with the electrolyte at a certain temperature, releasing a large amount of high-temperature gas. At the same time, the positive electrode active material begins to melt due to excessive lithium removal and a large potential difference with the negative electrode. Once the positive electrode potential exceeds the safe voltage of the electrolyte, the electrolyte will also undergo an oxidation reaction with the positive electrode active material. During the overcharging process, a series of side reactions such as Ohmic heating and gas overflow may occur, exacerbating the occurrence of thermal runaway.
The gas released during overcharging of lithium-ion batteries is mainly composed of CO2, CO, H2, CH4, C2H6, and C2H4, and the gas volume and heat increase with the increase of charging current. By using an accelerated calorimeter and a battery cycle analyzer for joint analysis, the experiment shows that the danger of overcharging based on constant current constant voltage is much greater than that of overcharging directly with constant current. Based on the overcharge performance of composite positive electrode and graphite negative electrode in different experimental environments, Ren et al. comprehensively considered the effects of charging current, separator material, and heat dissipation system. The study found that the amount of heat released during overcharging of NCM batteries is not closely related to the magnitude of charging current. The melting point of different separator materials and the deformation and swelling of the battery are the main factors causing thermal runaway of lithium-ion batteries. Wang et al. analyzed the thermal propagation path and high-temperature gas overflow path of overcharged lithium batteries, and found that the heat generated by the reaction between lithium deposition and electrolyte during battery overcharge accounted for more than 43%. Zhang et al. studied the degradation mechanism of battery pack capacity based on incremental capacitance differential voltage and found that a single overcharge had little effect on battery capacity, but after overcharging until the positive electrode active material delithiated, it would seriously affect the thermal stability of the battery pack.
The harm caused by over discharge is much smaller. Early over discharge is difficult to cause thermal runaway of the battery, but it can affect the battery capacity. Zhou Ping et al. studied the discharge characteristics of nickel cobalt manganese NCM ternary lithium batteries after overdischarging. During the static discharge process, the degree of short circuit inside the NCM lithium battery decreases, the resistance increases, and the discharge current decreases. Experiments have shown that the greater the depth of discharge, the greater the degree of attenuation of individual cells inside the battery pack. Ma et al. found in the overdischarge experiment of lithium batteries that overdischarge does not change the structure of the battery's active materials, but can cause the dissolution of the negative electrode current collector, increase the thickness of the SEI film, and accelerate the aging of the battery. The behavior characteristics of lithium-ion battery over discharge process are shown in the figure.
(2) External short circuit
External short circuits are also an important cause of thermal runaway in power batteries. Chen et al. developed a new electric thermal coupling model based on the combination of heat generation, distribution, and propagation models. Research has shown that the peak temperature of lithium-ion batteries under external short circuit conditions exists at the edge of the electrode ear. Ma Taixiao et al. found that in the external short-circuit state of power batteries, the heat generated by side reactions is much smaller than the heat generated by electrochemistry, and the heat generated by electrochemistry is positively correlated with the initial SOC, but negatively correlated with the temperature peak thermal stress.
(3) Internal short circuit
Internal short circuit, which occurs inside the battery and is difficult to detect by the BMS system, is the main cause of thermal runaway in lithium-ion batteries. When the battery is overcharged or overdischarged, lithium dendrites gradually grow to penetrate the SEI film, causing internal short circuits and rapidly leading to uncontrollable temperature rise and thermal runaway. In addition, lattice damage or current collector burrs caused by rough manufacturing processes of batteries may also lead to internal short circuits.
2.3 Research on battery thermal runaway caused by mechanical abuse
In the application of automotive power batteries, mechanical failures are inevitably caused by accidents. If the battery pack is deformed by external forces such as puncture and compression, it can cause internal structural changes and even lead to thermal runaway due to direct contact between the positive and negative poles under extreme stress. Therefore, it is necessary to conduct research on battery thermal runaway caused by mechanical abuse, among which Fan Wenjie and Xu Huiyong have conducted research on thermal runaway caused by mechanical abuse based on finite element modeling and numerical monitoring analysis.
Wang et al. conducted a study on the cross-sectional changes of the battery pack after collision based on soft pack lithium-ion batteries. The puncture experiment found that a large number of local deformations and shear fracture layers appeared inside the battery pack during the puncture process, and the tearing of the current collector and positive electrode active material, as well as the rearrangement of the internal structure of the battery pack, caused by the puncture of the separator, were the fundamental reasons for the short circuit thermal runaway inside the battery. Lamb et al. studied the deformation state of 18650 cylindrical lithium-ion batteries under puncture conditions based on computer tomography technology. The experiment found that the infiltration phenomenon between the positive and negative electrodes exacerbates the occurrence of internal short circuits. During the short circuit, the attached aluminum foil melts, forming a large number of metal beads at the puncture crack. Li et al. established finite element analysis models for various states of mechanical abuse based on puncture, compression, etc., and developed a learning algorithm for predicting the thermal runaway process of batteries using parameters of waste batteries. The impact of mechanical abuse on the safety of lithium-ion batteries was analyzed based on eight types of parameters, including impact force, collision angle, and deformation range, significantly reducing computational complexity.
The mechanical abuse that occurs in practical applications is more complex than single experiments such as puncture and compression. Relying solely on experimental simulation cannot deeply study the safety characteristics of battery mechanical abuse. The fundamental solution is to optimize the battery installation position, set a reliable BMS system, and optimize the design of the vehicle frame while designing the power battery pack, in order to minimize deformation and compression of the power battery pack in the event of a collision.
3. Preventive measures and methods for thermal runaway of lithium-ion batteries
With the goal of blocking, delaying, and preventing thermal runaway of power batteries, many scholars have conducted research on battery pack thermal management, high-strength battery pack structure design, and other aspects.
3.1 Safety design of individual batteries
(1) Research on the Safety of Diaphragm Design
The core of improving the safety of the diaphragm lies in increasing the temperature at which the diaphragm contracts and melts, enhancing its high-temperature isolation ability. The high-temperature isolation ability of the diaphragm ensures that its micropores are sealed in a high-temperature environment, blocking the flow of lithium ions. The widely used diaphragm materials are generally covered with ceramic coatings or other materials with closed cell effects.
(2) Research on the Safety of Positive Electrode Materials
The most common lithium-ion positive electrode active materials used in the power battery market are generally LiCoO2, LiFePO4, LiMn2O4, LiNixCoyMnzO2 (NCM), etc. Using materials to cover the positive electrode to block and alleviate thermal runaway side reactions, improve battery cycling and thermal stability, such as ZrO2 and AlF3. Zhang et al. developed a layered ternary NCM material based on gradient distribution of atomic concentration, with Ni as the core and Mn covering the outer layer of the attached particles. Tests have shown that it can maintain good cycling and thermal stability even under multiple high temperature and overcharge conditions.
(3) Research on the Safety of Negative Electrode Materials
The improvement of negative electrode safety is mainly achieved through material coating or adding additives to the electrolyte to enhance the thermal stability of the SEI film. Xu et al. added liquid alloy GaSnIn to the electrolyte to improve the thermal stability of the battery. The experiment shows that the prepared gradient SEI layer greatly reduces voltage polarization and improves Coulomb efficiency to 99.06%. Zheng et al. prepared an ultra-thin aramid nanofiber (ANF) membrane to suppress lithium dendrite growth. In the experimental test, under a high current density environment of 50mA/cm2, the capacity of the ANF-Li | LiFePO4 full battery decreased to 80.2% after 1200 cycles. And for the first time, its research discovered fibrous lithium deposition, and the ANF membrane prepared with nanoscale pores promoted electrolyte diffusion, accelerated lithium transport efficiency, and eliminated the drawbacks of micrometer sized lithium dendrites penetrating the membrane.
(4) Research on the Safety of Electrolytes
Most thermal runaway accidents involve electrolyte, and improving electrolyte safety to prevent thermal runaway is crucial. Flame retardants, solid polymer substances, or ionic liquids are often added to the electrolyte as anti overcharge additives. Fluorinated ethylene carbonate (FEC) is the most common electrolyte additive, which has the advantage of improving the Coulombic efficiency of reversible lithium removal in the negative electrode by changing the SEI film composition. Li et al. designed a double-layer crystalline and polymer solid electrolyte interphase SEI film using lithium difluoroborate (LiDFOB) as the main salt in a mixed phosphate electrolyte. The flame retardant experiment showed that the self extinguishing time of the flame retardant electrolyte was 6.1 seconds, and the reversible efficiency of Li was 98.2%. After 150 charge discharge cycles, it still maintained 89.7% of the battery capacity.
3.2 Safety protection and optimization design of power battery system
(1) Optimization design of battery pack structure
The design of battery pack structure and optimization of vehicle installation position are crucial for improving safety. Chen et al. conducted a classification experiment on the impact of thermal runaway range based on the 18650 battery layout. The experiment shows that the ignition time is shorter, and the spreading speed and range are larger in areas with larger heating areas. But its experiment only considered the overall heating of the power battery module and did not take into account the local overheating caused by internal short circuits. Liu Zhenjun et al. optimized the design of the battery pack based on a three-dimensional heat dissipation model of the power battery pack and conducted heat dissipation simulation. The experiment showed that the peak temperature of the optimized lithium-ion battery decreased from 46 ℃ to 34 ℃, and the temperature difference between individual cells was controlled within 5 ℃.
(2) Design of Battery Thermal Management System
Lithium ion batteries have strong thermal sensitivity, and improving low-temperature discharge efficiency and high-temperature safety is the core of battery thermal management systems. The cooling methods for battery packs include liquid cooling and air cooling. Tesla's electric vehicles all use liquid cooling technology, while electric buses generally use air cooling. In recent years, such as aerogels, phase change materials and hybrid materials have been used in battery thermal management systems due to their excellent heat absorption efficiency. Wu et al. have developed a flexible material for battery thermal management system based on hydrogel. Low cost sodium polyacrylate material is used. Its extremely strong plasticity can be made into a variety of shapes and stacked in the battery pack, which can economically realize the heat dissipation effect of traditional air cooling and liquid cooling.
(3) Design of cooling, extinguishing, blocking, and gas guidance for battery thermal runaway
When battery thermal runaway is unavoidable, it is particularly important to promptly block and cool down the heat spread and guide high-temperature gases to avoid affecting batteries installed in close proximity.
The main ways to block the spread of thermal runaway include: filling with flame-retardant media, using insulation materials to isolate thermal runaway batteries, or guiding flames and high-temperature gases out of the battery pack through pathways. Xu et al. developed a high-temperature gas heat dissipation tube with a rectangular cross-sectional shape arranged along the battery as shown in Figure 5. Although it cannot prevent the occurrence of thermal runaway in individual batteries, it can effectively prevent the spread of local thermal runaway in battery packs. Li Haoliang et al. designed a thermal spread blocking system and integrated control system based on inert gases and mixed refrigerants. Based on the heat dispersion diagram and heating acceleration, a threshold is set for the blocking system. The experiment shows that it can effectively block heat propagation when the battery pack is locally overheated.
4. Conclusion
The article summarizes the literature on the triggering mechanism, causes, and safety monitoring management of thermal runaway in lithium-ion power batteries.
(1) In the research of thermal runaway mechanism, the thermal stability and heat release law of the main components of lithium-ion batteries were analyzed, and the principles of reaction heat release processes such as electrolyte decomposition, separator, battery active materials, and adhesives were mainly explained.
(2) In the research on triggering factors of thermal runaway, the characteristics and reasons of different triggering conditions were classified and summarized, namely mechanical abuse, electrical abuse, and battery thermal runaway caused by thermal abuse.
(3) In terms of preventing and monitoring thermal runaway, this article elaborates on the research to improve the safety of lithium-ion power battery thermal runaway from three aspects: optimization design of lithium-ion battery cells, optimization of power battery systems, and battery thermal management and monitoring warning systems.
Although significant progress has been made in the study of thermal runaway in lithium-ion batteries, there are still gaps in some areas of research. The research on the impact of aging on safety caused by the superposition of cycle times on lithium-ion batteries has only begun in recent years, especially the experimental study of the aging path and mechanism on thermal stability is still relatively scarce. At the same time, there are only a few experimental studies on the prediction and modeling of flame propagation after thermal runaway occurs, and there is still a lack of numerical simulation analysis of flame propagation. It can be seen that the safety management of thermal runaway in lithium-ion power batteries is still in the development stage, especially in the direction of warning and blocking, which requires further research.