Lithium-ion Battery Thermal Runaway Process
Battery thermal runaway occurs when the rate of heat generation in the battery far exceeds the rate of heat dissipation, resulting in a large accumulation of heat that is not dissipated in time. Essentially, thermal runaway is a positive energy feedback loop: increased temperature leads to system heating, which in turn causes the temperature to rise further, making the system even hotter. Loosely speaking, battery thermal runaway can be divided into three stages:
[Diagram of lithium-ion battery thermal runaway process]
[Research on the kinetic mechanism of thermal runaway reaction in different types of lithium batteries]
Stage 1: Internal Thermal Runaway Stage
Due to internal short circuits, external heating, or self-heating during high-current charging and discharging, the internal temperature of the battery rises to approximately 90℃～100℃, and the lithium salt LiPF6 begins to decompose. The carbon anode in the charging state has very high chemical activity, approaching that of metallic lithium. At high temperatures, the SEI film on the surface decomposes, and the lithium ions embedded in the graphite react with the electrolyte and binder, further pushing the battery temperature up to 150℃. At this temperature, new and violent exothermic reactions occur, such as the large-scale decomposition of the electrolyte, generating PF5, which further catalyzes the decomposition of organic solvents.
Stage 2: Battery Swelling Stage
When the battery temperature reaches above 200℃, the positive electrode material decomposes, releasing a large amount of heat and gas, and the temperature continues to rise. At 250-350℃, the lithium-intercalated anode begins to react with the electrolyte.
Stage 3: Battery Thermal Runaway and Explosion Failure
During the reaction, the charged positive electrode material begins a violent decomposition reaction, and the electrolyte undergoes a violent oxidation reaction, releasing a large amount of heat, generating high temperatures and a large amount of gas, leading to battery combustion and explosion.
Safety of Lithium-ion Battery Materials
Anode Materials
Although anode materials are relatively stable, the lithium-intercalated carbon anode will react with the electrolyte at high temperatures. The reaction between the anode and the electrolyte includes the following three parts: decomposition of the SEI (Selenium Electrode Ion); reaction of the lithium intercalated in the anode with the electrolyte; and reaction of the lithium intercalated in the anode with the binder. At room temperature, the electronically insulating SEI film can prevent further decomposition of the electrolyte. However, at around 100℃, the SEI film decomposes. The exothermic decomposition reaction of the SEI is shown below:
Although the heat of SEI decomposition is relatively small, its reaction initiation temperature is low, which to some extent increases the diffusion rate of the "combustion" of the anode sheet.
Temperature Ranges and Enthalpies of Various Exothermic Reactions in Lithium-ion Batteries
At higher temperatures, the negative electrode surface loses the protection of the SEI film, and the lithium intercalated in the negative electrode reacts directly with the electrolyte solvent to produce C₂H₄O, which may be acetaldehyde or ethylene oxide. Lithium-intercalated graphite reacts with molten PVDF–HPF copolymer above 300℃ as follows:
The heat of reaction increases with the degree of lithium intercalation and varies with the type of binder. Thermal stability can be increased by using film-forming additives or lithium salts. The ways to reduce the heat of reaction between lithium intercalated in the negative electrode and the electrolyte include two aspects: reducing the amount of lithium intercalated in the negative electrode and reducing the specific surface area of the negative electrode. Reducing the amount of lithium intercalated in the negative electrode means that the ratio of positive to negative electrodes must be appropriate, with the negative electrode in excess by about 3% to 8%. Reducing the specific surface area of the negative electrode can also effectively improve battery safety. Literature reports that when the specific surface area of carbon negative electrode materials increases from 0.4 m²·g⁻¹ to 9.2 m²·g⁻¹, the reaction rate increases by two orders of magnitude. However, an excessively low specific surface area will reduce the battery's rate performance and low-temperature performance. This necessitates optimizing the negative electrode structure and electrolyte formulation to improve the lithium-ion diffusion rate in the solid phase of the negative electrode and obtain an SEI film with good ionic conductivity. Furthermore, although the binder's weight proportion in the negative electrode is very small, its reaction heat with the electrolyte is considerable. Therefore, reducing the amount of binder or selecting a suitable binder will help improve battery safety performance.
Literature, through patent analysis, also suggests that methods to address the safety of carbon negative electrode materials mainly include reducing the specific surface area of the negative electrode material and improving the thermal stability of the SEI film. Existing domestic patent applications highlight technologies related to improving negative electrode materials and structures to enhance battery safety performance.
Patent Literature Research on Improvements to Negative Electrode Materials and Structures
Positive Electrode Materials
Common positive electrode materials are stable below 650℃, are in a metastable state during charging, and undergo the following reactions as the temperature rises.
The released oxygen will oxidize the solvent:
Is there a definitive answer as to whether the positive electrode reacts directly with the electrolyte or reacts after releasing oxygen?
DSC test results of common positive electrode materials:
Analysis of the thermal stability of positive electrode materials yields the following conclusions:
First, the reaction mechanism between the positive electrode material and the solvent requires further investigation;
Second, the decomposition reaction of the positive electrode and its reaction with the electrolyte release a relatively large amount of heat, which is the main cause of battery explosions in most cases;
Third, using ternary or LFP positive electrode materials can improve battery safety compared to LCO.
Electrolyte
Lithium-ion battery electrolytes are basically organic carbonate substances, which are flammable. The commonly used electrolyte salt, lithium hexafluorophosphate, undergoes a thermal decomposition and exothermic reaction. Therefore, improving the safety of the electrolyte is crucial for the safety control of power lithium-ion batteries.
The thermal stability of LiPF6 is the main factor affecting the thermal stability of the electrolyte. Therefore, the main improvement method currently is to use lithium salts with better thermal stability. However, since the heat of reaction of the electrolyte itself is very small, its impact on battery safety performance is very limited. Flammability has a greater impact on battery safety. The main way to reduce the flammability of the electrolyte is by using flame-retardant additives.
Currently, lithium salts attracting attention include lithium bis(fluorosulfonic acid)imide (LiFSI) and boron-based lithium salts. Among them, lithium bis(oxalato)borate (LiBOB) has high thermal stability, a decomposition temperature of 302℃, and can form a stable SEI film at the negative electrode. LiBOB, as a lithium salt and additive, can improve the thermal stability of the battery. In addition, lithium difluorooxalato borate (LiODFB) combines the advantages of LiBOB and lithium tetrafluoroborate (LiBF4) and also shows promise for use in lithium battery electrolytes.
Besides improving the electrolyte salt, flame-retardant additives should also be used to improve battery safety. The solvent in the electrolyte burns because of a chain reaction. Adding high-boiling-point, high-flash-point flame retardants to the electrolyte can improve the safety of lithium-ion batteries.
Reported flame-retardant additives mainly include three categories: organophosphorus compounds, fluorocarbonates, and composite flame-retardant additives. Although organophosphorus flame retardants possess good flame retardant properties and excellent oxidation stability, their high reduction potential makes them incompatible with graphite anodes, and their high viscosity leads to reduced electrolyte conductivity and poor low-temperature performance. Adding co-solvents such as EC or film-forming additives can effectively improve their compatibility with graphite, but this reduces the flame retardant properties of the electrolyte. Composite flame retardant additives can improve their overall performance through halogenation or the introduction of multifunctional groups. Additionally, fluorocarbonates, due to their high or no flash point, favorable film formation on the anode surface, and low melting point, also have promising application prospects.
The above figure shows a nanoscale dendritic polymer compound (STOBA) coating NCM (424). When a lithium battery malfunctions and generates high temperatures, a thin film forms, blocking the flow of lithium ions and stabilizing the battery, thereby improving battery safety. As shown in the figure below, during the needle penetration test, the internal temperature of the battery without a STOBA coating on the positive electrode material rose to 700℃ within seconds, while the temperature of the battery with a STOBA-coated positive electrode material only reached a maximum of 150℃.
Separator
Currently, there are three main types of commercially available lithium-ion battery separators: PP/PE/PP multilayer composite microporous membranes, PP or PE single-layer microporous membranes, and coated membranes. The most widely used separator is the polyolefin microporous membrane, which has a stable chemical structure, excellent mechanical strength, and good electrochemical stability.
The higher the mechanical strength of the separator in the vertical direction, the lower the probability of a micro-short circuit in the battery; the lower the thermal shrinkage rate of the separator, the better the battery's safety performance. The micropore-closing function of the separator is also another method to improve the safety of power batteries; gel polymer electrolytes have good liquid retention properties, and batteries using this electrolyte have better safety than conventional liquid batteries; in addition, ceramic separators can also improve battery safety. Common domestic patent literature on the preparation and processing types of lithium battery separators is shown in the table below.
Improvements to the separator in patent literature:
Process Design and Thermal Runaway
Battery manufacturing processes are extremely complex. Even with strict control, it's impossible to completely avoid metallic impurities or burrs during production. If impurities, burrs, or dendrites appear inside the battery, their amplification and deterioration lead to increased conductivity, rising temperature, and the continuous accumulation of heat generated by chemical reactions and discharge, potentially causing thermal runaway.
Insufficient Negative Electrode Capacity
When the negative electrode opposite the positive electrode has insufficient capacity, or no capacity at all, some or all of the lithium generated during charging cannot insert into the interlayer structure of the negative electrode graphite. Instead, it precipitates on the surface of the negative electrode, forming protruding "dendrites." During the next charge, these protrusions are more prone to lithium deposition. After dozens to hundreds of charge-discharge cycles, the "dendrites" grow and eventually puncture the separator paper, causing an internal short circuit. Rapid discharge of the battery cell generates a large amount of heat, burning the separator and causing a larger short circuit. The high temperature causes the electrolyte to decompose into gas, and the negative electrode carbon and separator paper to burn, resulting in excessive internal pressure. When the cell's casing cannot withstand this pressure, the cell will explode.
High Moisture Content
Moisture can react with the electrolyte in the battery cell to produce gas. During charging, it can react with the generated lithium to form lithium oxide, causing capacity loss and making the cell prone to overcharging and gas generation. Moisture has a low decomposition voltage, making it easy to decompose and generate gas during charging. This series of gas generation increases the internal pressure of the cell. When the cell's casing cannot withstand this pressure, the cell will explode.
Internal Short Circuit
Due to an internal short circuit, the cell discharges with a large current, generating a large amount of heat, burning the separator and causing a larger short circuit. This results in high temperatures, causing the electrolyte to decompose into gas, creating excessive internal pressure. When the cell's casing cannot withstand this pressure, the cell will explode. During laser welding, heat is conducted through the casing to the positive electrode tab, causing it to reach a high temperature. If the upper adhesive tape does not separate the positive electrode tab from the diaphragm, the heat from the positive electrode tab will burn or shrink the diaphragm, causing an internal short circuit and potentially leading to an explosion.
High-temperature adhesive tape covering the negative electrode tab
During spot welding of the negative electrode tab, heat is conducted to it. If the high-temperature adhesive tape is not properly applied, the heat from the negative electrode tab will burn the diaphragm, causing an internal short circuit and potentially leading to an explosion.
Bottom adhesive tape not completely covering the bottom
When customers spot weld at the bottom aluminum-nickel composite strip, a large amount of heat is generated on the bottom shell wall, conducted to the bottom of the electrode core. If the high-temperature adhesive tape does not completely cover the diaphragm, it will burn the diaphragm, causing an internal short circuit and potentially leading to an explosion.
Overcharging
When a battery cell is overcharged, excessive lithium release from the positive electrode can alter its structure. Excessive lithium release can also prevent it from inserting into the negative electrode, leading to lithium deposition on the negative electrode surface. Furthermore, when the voltage reaches 4.5V or higher, the electrolyte decomposes, producing a large amount of gas. All of these factors can potentially cause an explosion.
External Short Circuit
External short circuits may be caused by improper operation or misuse. Due to an external short circuit, the battery discharge current is very large, causing the cell to heat up. The high temperature can cause the internal separator of the cell to shrink or completely fail, resulting in an internal short circuit and ultimately an explosion.
Station with Insufficient Negative Electrode Capacity
Issues include: the negative electrode not fully covering the positive electrode; incorrect pairing of positive and negative electrodes; the negative electrode being crushed during pressing; negative electrode particles; exposed foil on the negative electrode; negative electrode dents; scratches on the negative electrode; dark marks on the negative electrode; uneven negative electrode coating; material buildup at the beginning and end of the positive electrode; uneven positive electrode coating; excessive positive electrode material; uneven mixing of positive and negative electrodes; low incoming negative electrode capacity; high incoming positive electrode capacity; and insufficient negative electrode capacity.
Workstations with excessive moisture content:
Slow sealing leading to moisture absorption; moisture absorption during aging; excessive electrolyte moisture content; inadequate drying or moisture absorption during pre-filling baking; inadequate drying during assembly baking; inadequate drying of positive and negative electrodes during coating; moisture absorption during positive electrode adhesive preparation; insufficient positive electrode baking; excessive moisture content.
Workstations with internal short circuits:
Incomplete bottom adhesive coverage; high-temperature adhesive tape covering the negative electrode tab; incorrect upper adhesive placement; excessively high baking temperature damaging the separator; undetected laser-welded short-circuited cells; downstream assembly of micro-short-circuited cells; undetected assembly of short-circuited cells; excessive pressure during flattening; pinholes in the separator paper; uneven winding; improperly flattened negative electrode riveting; burrs; small burrs on positive and negative electrodes; small pieces of material falling off positive and negative electrodes; internal short circuits.
Possible Overcharge Locations
Overcharging can occur due to: charger voltage being too high during user operation; voltage being too high at individual points during testing; excessive current setting during testing; insufficient cell capacity; excessive current at individual points in the pre-charge cabinet; or excessive current setting during pre-charge.
Possible External Short Circuit Locations
External short circuits can occur due to: protection circuit board failure; short circuit between positive and negative terminals during user operation; arcing during cell handling; or improper cell alignment causing contact between positive and negative terminals, resulting in an external short circuit.
Measures to Prevent Lithium-ion Battery Explosions
Lithium-ion battery safety is a complex and multifaceted issue. The greatest hidden danger to battery safety is the random occurrence of internal short circuits, leading to on-site failure and thermal runaway. Therefore, developing and using materials with high thermal stability is the fundamental approach and direction for improving the safety performance of lithium-ion batteries in the future.
Improving the Thermal Stability of Battery Materials
Positive electrode materials can be improved by optimizing synthesis conditions, improving synthesis methods to synthesize materials with good thermal stability, or by using composite technologies (such as doping technology) and surface coating technologies (such as coating technology).
The thermal stability of anode materials is related to the type of anode material, the size of the material particles, and the stability of the SEI film formed by the anode. For example, by mixing particles of different sizes in a certain ratio to form the anode, the contact area between particles can be increased, electrode impedance reduced, electrode capacity increased, and the possibility of active lithium metal deposition reduced.
The quality of the SEI film formation directly affects the charge-discharge performance and safety of lithium-ion batteries. Weakly oxidizing the surface of carbon materials, or reducing, doping, or surface-modifying carbon materials, as well as using spherical or fibrous carbon materials, helps improve the quality of the SEI film.
The stability of the electrolyte is related to the type of lithium salt and solvent. Using lithium salts with good thermal stability and solvents with a wide potential stability window can improve the thermal stability of the battery. Adding some high-boiling-point, high-flash-point, and non-flammable solvents to the electrolyte can improve battery safety.
The type and quantity of conductive agents and binders also affect the thermal stability of the battery. The binder reacts with lithium at high temperatures, generating a large amount of heat. Different binders generate different amounts of heat; PVDF generates almost twice the heat of fluorine-free binders. Replacing PVDF with fluorine-free binders can improve the battery's thermal stability.
Improving Battery Overcharge Protection
To prevent overcharging of lithium-ion batteries, dedicated charging circuits are typically used to control the charging and discharging process, or safety valves are installed on individual cells to provide greater overcharge protection. Alternatively, positive temperature coefficient (PTC) resistors can be used. Their mechanism is that when the battery heats up due to overcharging, they increase the battery's internal resistance, thereby limiting the overcharge current. Dedicated separators can also be used; when a battery malfunction causes the separator temperature to rise excessively, the separator pores shrink and close, preventing lithium ion migration and thus preventing overcharging.
Preventing Battery Short Circuits
For separators, a porosity of approximately 40% with uniform distribution and a pore size of 10nm can prevent the movement of small particles from the positive and negative electrodes, thus improving the safety of lithium-ion batteries.
The insulation voltage of the separator is directly related to its ability to prevent contact between the positive and negative electrodes. The insulation voltage depends on the separator's material, structure, and battery assembly conditions.
Using composite separators with a large difference between their thermal closure temperature and melting temperature (such as PP/PE/PP) can prevent thermal runaway. Coating the separator surface with a ceramic layer improves its temperature resistance. Low-melting-point PE (125℃) acts as a pore-closing agent at lower temperatures, while PP (155℃) maintains the separator's shape and mechanical strength, preventing contact between the positive and negative electrodes and ensuring battery safety.
It is well known that replacing the metallic lithium anode with a graphite anode transforms the deposition and dissolution of lithium on the anode surface during charging and discharging into the insertion and extraction of lithium into carbon particles, preventing the formation of lithium dendrites. However, this does not mean that the safety of lithium-ion batteries has been resolved. During the charging process, if the positive electrode capacity is too high, metallic lithium will deposit on the negative electrode surface. Excessive negative electrode capacity leads to significant battery capacity loss.
Coating thickness and uniformity also affect the insertion and extraction of lithium ions into the active material. For example, if the negative electrode surface density is uneven, the polarization will vary during charging, potentially causing localized lithium deposition on the negative electrode surface.
Furthermore, improper operating conditions can cause short circuits. At low temperatures, the deposition rate of lithium ions exceeds the insertion rate, leading to metallic lithium deposition on the electrode surface and causing a short circuit. Therefore, controlling the ratio of positive and negative electrode materials and enhancing coating uniformity are crucial to preventing lithium dendrite formation.
In addition, binder crystallization and copper dendrite formation can also cause internal short circuits. In the coating process, the solvent in the slurry is removed through baking. If the heating temperature is too high, the binder may crystallize, causing the active material to peel off and resulting in an internal short circuit.
Under over-discharge conditions, when the battery is over-discharged to 1-2V, the copper foil, which serves as the negative electrode current collector, will begin to dissolve and precipitate on the positive electrode. When the voltage is less than 1V, copper dendrites will begin to appear on the surface of the positive electrode, causing a short circuit inside the lithium-ion battery.
