Lithium-ion battery thermal runaway process
Battery thermal runaway occurs because the rate of heat generation in the battery far exceeds its 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 a hotter system, which in turn raises the temperature, making the system even hotter. Loosely speaking, battery thermal runaway can be divided into three stages:
Schematic diagram of thermal runaway process of lithium-ion battery
Research on the kinetic mechanism of thermal runaway reaction in different types of lithium batteries
Stage 1: Internal thermal runaway stage of the battery
Due to internal short circuits, external heating, or the battery itself generating heat during high-current charging and discharging, the internal temperature of the battery rises to around 90℃～100℃, causing the lithium salt LiPF6 to begin decomposition. 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 to generate PF5, which further catalyzes the decomposition of organic solvents.
Phase 2: Battery Swelling Phase
When the battery temperature reaches above 200℃, the positive electrode material decomposes, releasing a large amount of heat and gas, causing the temperature to continue to rise. At 250-350℃, the lithium-intercalated negative electrode begins to react with the electrolyte.
Stage 3: Battery thermal runaway and explosion failure stage
During the reaction, the charged positive electrode material begins to undergo a violent decomposition reaction, the electrolyte undergoes a violent oxidation reaction, releasing a large amount of heat, generating high temperature and a large amount of gas, and the battery burns and explodes.
Safety of lithium-ion battery materials
Anode material
Although the anode material is relatively stable, the lithium-intercalated carbon anode will react with the electrolyte at high temperatures. The reaction between the anode and electrolyte includes three parts: decomposition of the SEI (Sediment Intercalation Layer); 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 in the following equation:
Although the heat of SEI decomposition reaction is relatively small, its reaction initiation temperature is low, which will increase the "combustion" diffusion rate of the negative electrode to some extent.
Temperature ranges and enthalpy 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 will react 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°C as follows:
The heat of reaction increases with the degree of lithium intercalation and varies depending on the type of binder. Thermal stability can be improved 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 constitutes a very small weight proportion in the negative electrode, its reaction heat with the electrolyte is considerable. Therefore, reducing the amount of binder or selecting a suitable binder will help improve the battery's safety performance.
Literature analysis of patents also suggests that methods to improve the safety of carbon anode materials mainly include reducing the specific surface area of the anode material and improving the thermal stability of the SEI film. Existing domestic patent applications also highlight technologies for improving anode materials and structures to enhance battery safety performance.
Research on improvements to negative electrode materials and structures in patent literature
cathode materials
Common cathode materials are stable at temperatures below 650°C, are in a metastable state during charging, and undergo the following reaction when 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 cathode materials:
The following conclusions can be drawn from the thermal stability analysis of the cathode material:
First, the reaction mechanism between the cathode material and the solvent needs further investigation;
Secondly, the decomposition reaction of the positive electrode and its reaction with the electrolyte release a large amount of heat, which is the main cause of battery explosions in most cases.
Third, using ternary or LFP cathode materials can improve battery safety compared to LCO.
electrolyte
Lithium-ion battery electrolytes are primarily composed of organic carbonates, which are flammable. The commonly used electrolyte salt, lithium hexafluorophosphate, undergoes thermal decomposition and an 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 current main improvement method is to use lithium salts with better thermal stability. However, since the heat of reaction of electrolyte decomposition itself is very small, its impact on battery safety performance is very limited. The flammability of the electrolyte has a greater impact on battery safety. The main way to reduce the flammability of the electrolyte is to use 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) exhibits high thermal stability with a decomposition temperature of 302℃ and can form a stable SEI film on the negative electrode. LiBOB, as a lithium salt and additive, can improve the thermal stability of batteries. Additionally, lithium difluorooxalatoborate (LiODFB) combines the advantages of LiBOB and lithium tetrafluoroborate (LiBF4) and also shows promise for use in lithium battery electrolytes.
In addition to improvements in electrolyte salts, flame-retardant additives should be used to enhance battery safety. The solvent in the electrolyte ignites due to 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 fall into three categories: organophosphorus compounds, fluorocarbonates, and composite flame retardant additives. Although organophosphorus flame retardant additives possess good flame retardant properties and excellent oxidative 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 show promising application prospects.
The image above shows a coating of NCM (424) with a nanoscale dendritic polymer compound (STOBA). 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. The image below shows that during a nail penetration test, the internal temperature of the battery without the STOBA coating on the positive electrode material rose to 700°C within seconds, while the temperature of the battery with the STOBA-coated positive electrode material only reached a maximum of 150°C.
diaphragm
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 exhibits 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 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 diaphragm in patent literature
Process Design and Thermal Runaway
The battery manufacturing process is extremely complex, and even with strict control, it is impossible to completely avoid metallic impurities or burrs during production. If impurities, burrs, or dendrites appear inside the battery, they will amplify and worsen, leading to increased conductivity, rising temperature, and the continuous accumulation of heat generated by chemical reactions and discharge, which may eventually cause thermal runaway of the battery.
Insufficient negative electrode capacity
When the capacity of the negative electrode opposite the positive electrode is insufficient, or even nonexistent, some or all of the lithium generated during charging cannot insert into the interlayer structure of the negative electrode graphite. Instead, it deposits on the surface of the negative electrode, forming protruding "dendritic structures." During the next charge cycle, these protrusions are more prone to lithium deposition. After dozens to hundreds of charge-discharge cycles, the "dendritic structures" grow and eventually pierce the separator paper, causing an internal short circuit. Rapid discharge of the 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 casing cannot withstand this pressure, the cell explodes.
Excessive 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 in the battery cell and making it prone to overcharging and gas generation. Moisture has a low decomposition voltage, so it easily decomposes and generates gas during charging. When this series of generated gases increases the internal pressure of the battery cell, the battery cell will explode when the outer casing cannot withstand it.
Internal short circuit
Due to an internal short circuit, the battery cell discharges a large current, generating a large amount of heat that burns the separator, causing a larger short circuit. This results in high temperatures within the battery cell, causing the electrolyte to decompose into gas, leading to excessive internal pressure. When the battery cell's casing cannot withstand this pressure, the cell explodes. 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 separator, the hot positive electrode tab can burn or shrink the separator, causing an internal short circuit and ultimately an explosion.
High-temperature adhesive tape wrapped around the negative electrode ear
During spot welding of the negative electrode tab, heat is conducted to the negative electrode tab. If the high-temperature adhesive tape is not properly applied, the heat on the negative electrode tab will burn the diaphragm, causing an internal short circuit and potentially leading to an explosion.
The adhesive on the bottom did not completely cover the bottom.
When customers spot weld the aluminum-nickel composite strip at the bottom, a large amount of heat will be generated on the bottom shell wall, which will conduct heat 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 resulting in an explosion.
overcharge
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, which will cause the battery cell to heat up. The high temperature will cause the diaphragm inside the battery cell to shrink or completely break down, resulting in an internal short circuit and thus an explosion.
Workstations with insufficient negative electrode capacity
The following issues are listed: the negative electrode cannot cover the positive electrode; the positive and negative electrodes are incorrectly matched; the negative electrode is crushed during tableting; there are negative electrode particles; the negative electrode foil is exposed; there are negative electrode dents; there are negative electrode scratches; there are negative electrode dark marks; the negative electrode coating is uneven; there is material accumulation at the beginning and end of the positive electrode; the positive electrode coating is uneven; the positive electrode coating amount is too large; the positive and negative electrodes are not mixed evenly; the negative electrode incoming material capacity is too low; the positive electrode incoming material capacity is too high; and the negative electrode capacity is insufficient.
Workstations with excessive moisture content
The sealing process is too slow and the electrode absorbs moisture; moisture is absorbed during aging; the electrolyte has too high a moisture content; the electrode was not dried before injection or absorbed moisture; it was not dried during assembly baking; the positive and negative electrodes were not dried during coating; moisture was absorbed during the preparation of adhesive for the positive electrode; the positive electrode was not baked sufficiently; and the moisture content was too high.
Workstation with internal short circuit
The bottom adhesive was not completely covering the bottom; high-temperature adhesive tape covered the negative electrode tab; the upper adhesive was not positioned correctly; the baking temperature was too high and damaged the separator; the laser-welded short-circuited cell was not detected; the assembled micro-short-circuited cell flowed downstream; the assembled short-circuited cell was not detected; the pressure was too high when flattening; the separator paper had pinholes; it was not wound evenly; the negative electrode riveting was not flattened and had burrs; the positive and negative electrodes had small pieces of burrs; the positive and negative electrodes had small pieces of material falling off; and there was an internal short circuit.
Overcharge possible workstations
When using the charger, the voltage is too high; during testing, the voltage at some points is too high; during testing, the current setting is too high; the battery cell capacity is insufficient; during pre-charging, the current at some points is too high; during pre-charging, the current setting is too high; and overcharging occurs.
Workstations with potential external short circuits
The protection circuit board failed, causing a short circuit between the positive and negative terminals during use. The battery cell sparked during turnover, and the battery cell was not properly aligned, resulting in contact between the positive and negative terminals and an external short circuit.
Measures to prevent lithium-ion battery explosions
The safety of lithium-ion batteries is a complex and multifaceted issue. The greatest potential safety hazard is the random occurrence of internal short circuits, which can lead to field failures 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
The thermal stability of cathode materials can be improved by optimizing synthesis conditions and improving synthesis methods; 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, can help 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 the safety of the battery.
The type and amount of conductive agents and binders also affect the thermal stability of the battery. The binder reacts with lithium at high temperatures to generate a large amount of heat. Different binders generate different amounts of heat. The heat generated by PVDF is almost twice that of fluorine-free binders. Replacing PVDF with fluorine-free binders can improve the thermal stability of the battery.
Improve battery overcharge protection capabilities
To prevent overcharging of lithium-ion batteries, a dedicated charging circuit is typically used to control the charging and discharging process, or a safety valve is installed on a single battery to provide greater overcharge protection. Alternatively, a positive temperature coefficient resistor (PTC) can be used, which works by increasing the battery's internal resistance when the battery heats up due to overcharging, thereby limiting the overcharge current. A dedicated separator can also be used; when a battery malfunctions and the separator temperature becomes too high, the separator pores shrink and close, preventing lithium ion migration and thus preventing overcharging.
Prevent battery short circuits
For the separator, a porosity of around 40% with uniform distribution and a pore size of 10nm can prevent the movement of small particles from the positive and negative electrodes, thereby 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 of the separator depends on the material and structure of the separator, as well as the assembly conditions of the battery.
Using composite separators with a large difference between their thermal closure temperature and melting temperature (such as PP/PE/PP) can prevent battery thermal runaway. Coating the separator surface with a ceramic layer improves its temperature resistance. Low-melting-point PE (125℃) acts as a cell-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 lithium metal anode with a graphite anode changes the deposition and dissolution of lithium on the anode surface during charging and discharging to 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 solved. During the charging process of lithium-ion batteries, if the positive electrode capacity is too high, metallic lithium will deposit on the surface of the anode, resulting in significant battery capacity loss.
The coating thickness and its uniformity also affect the insertion and extraction of lithium ions in the active material. For example, if the surface density of the negative electrode is thick and uneven, the polarization will be different in different places during charging, which may result in localized deposition of metallic lithium on the surface of the negative electrode.
Furthermore, improper usage conditions can also cause short circuits in batteries. Under low-temperature conditions, the deposition rate of lithium ions exceeds the insertion rate, leading to the deposition of metallic lithium on the electrode surface and causing a short circuit. Therefore, controlling the ratio of positive and negative electrode materials and enhancing the uniformity of coating are key to preventing lithium dendrite formation.
In addition, the crystallization of the binder and the formation of copper dendrites can also cause internal short circuits in the battery. During the coating process, the solvent in the slurry is completely removed by heating and baking. If the heating temperature is too high, the binder may also crystallize, causing the active material to peel off and resulting in an internal short circuit in the battery.
Under over-discharge conditions, when the battery is over-discharged to 1-2V, the copper foil, which acts as the negative electrode current collector, will begin to dissolve and precipitate on the positive electrode. Below 1V, copper dendrites will begin to appear on the positive electrode surface, causing an internal short circuit in the lithium-ion battery. (Lithium Battery Alliance Chairman)
