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Thermal runaway process of lithium ion battery

Battery thermal runaway is caused by the fact that the heat generation rate of the battery is much higher than the heat dissipation rate, and the heat is accumulated in a large amount but not dissipated in time. In essence, "thermal runaway" is a positive energy feedback cycle process: the rising temperature will cause the system to become hot, and the temperature will rise after the system becomes hot, which in turn will make the system become hotter. Without strict division, battery thermal runaway can be divided into three stages:

Stage 1: Thermal runaway stage inside the battery

Due to internal short circuit, external heating, or self heating of the battery during high current charging and discharging, the internal temperature of the battery rises to about 90 ℃~100 ℃, and lithium salt LiPF6 starts to decompose; For the charged carbon anode, the chemical activity is very high, close to the lithium metal. At high temperatures, the SEI film on the surface decomposes, and the lithium ion embedded in the graphite reacts with the electrolyte and the binder, further pushing the battery temperature to 150 ℃. At this temperature, a new severe exothermic reaction occurs, such as a large number of electrolytes decompose to generate PF5, and PF5 further catalyzes the decomposition reaction of organic solvents.

Stage 2: Battery Bulging Stage

When the battery temperature reaches above 200 ℃, the cathode material decomposes, releasing a large amount of heat and gas, and continuously heating up. The lithium embedded anode began to react with the electrolyte at 250-350 ℃.

Stage 3: battery thermal runaway, explosion failure stage

During the reaction, the charged cathode material starts to undergo a violent decomposition reaction, and the electrolyte undergoes a violent oxidation reaction, releasing a large amount of heat, generating high temperature and a large amount of gas, causing combustion and explosion of the battery.

Safety of Lithium Ion Battery Materials

Negative electrode material

Although the anode material is relatively stable, the carbon anode in the lithium embedded state will react with the electrolyte at high temperatures. The reaction between negative electrode and electrolyte includes the following three parts: the decomposition of SEI; The reaction of lithium embedded in cathode with electrolyte; The reaction between lithium embedded in cathode and binder. The electronic insulated SEI film at room temperature can prevent further decomposition reaction of electrolyte. However, the decomposition reaction of SEI membrane will occur at about 100 ℃. The reaction formula of SEI exothermic decomposition reaction is as follows:

Although the decomposition reaction heat of SEI is relatively small, its initial reaction temperature is low, which will increase the "combustion" diffusion rate of negative electrode to a certain extent.

Temperature range and reaction enthalpy of various exothermic reactions of lithium ion batteries

At higher temperatures, the anode surface loses the protection of the SEI film, and the lithium embedded in the anode will react directly with the electrolyte solvent to produce C2H4O, which may be acetaldehyde or ethylene oxide. The graphite embedded with lithium reacts with the molten PVDF – HPF copolymer at above 300 ℃ as follows:

The reaction heat increases with the increase of lithium intercalation degree, and varies with the type of binder. The thermal stability is increased by film forming additives or lithium salts. The ways to reduce the reaction heat of lithium embedded in the negative electrode with the electrolyte include the following two aspects: reducing the lithium embedded in the negative electrode and reducing the specific surface area of the negative electrode. The reduction of lithium embedded in the negative electrode means that the ratio of positive and negative electrodes must be appropriate, and the negative electrode should be excessive by about 3%~8%. Reducing the specific surface area of the negative electrode can also effectively improve the safety of the battery. It has been reported that when the specific surface area of the carbon negative electrode material increases from 0.4 m2 · g – 1 to 9.2 m2 · g – 1, the reaction rate increases by two orders of magnitude.

However, if the specific surface area is too low, it will reduce the rate performance and low-temperature performance of the battery. It is necessary to improve the diffusion rate of lithium ions in the solid phase of anode and obtain SEI films with good ionic conductivity through reasonable design of anode structure and optimization of electrolyte formula. In addition, although the weight ratio of the binder in the negative electrode is very small, its reaction heat with the electrolyte is very considerable. Therefore, reducing the amount of binder or selecting appropriate binder will help to improve the safety performance of the battery.

Through the analysis of patents, the literature also believes that the main methods to solve the safety of carbon anode materials are to reduce the specific surface area of anode materials and improve the thermal stability of SEI films. In the existing domestic patent applications, the technology related to improving the anode material and structure to improve the safety performance of the battery.

Research on Improvement of Anode Materials and Structure in Patent Documents

cathode material

Common cathode materials are stable when the temperature is lower than 650 ℃, and metastable when charging. The following reactions occur when the temperature rises.

The released oxygen will oxidize the solvent:

Is there an exact statement whether the positive electrode reacts directly with electrolyte or reacts after oxygen is released?

DSC test results of common positive materials:

The following conclusions can be drawn from the analysis of thermal stability of cathode materials:

First, the reaction mechanism between cathode materials and solvents needs further study;

Secondly, the decomposition reaction of the positive electrode and its reaction with the electrolyte have relatively large heat release, which is the main cause of battery explosion in most cases;

Third, the use of ternary or LFP cathode materials can improve the safety of the battery compared with LCO.

electrolyte

The electrolyte of lithium ion battery is basically organic carbonate, which is a kind of flammable substance. The common electrolyte salt lithium hexafluorophosphate has exothermic reaction of thermal decomposition. Therefore, it is very important to improve the safety of electrolyte for the safety control of power lithium ion battery.

The thermal stability of LiPF6 is the main factor affecting the thermal stability of electrolyte. Therefore, the main improvement method is to use lithium salt with better thermal stability. However, because the reaction heat of electrolyte decomposition is very small, the impact on battery safety is very limited. The flammability has a greater impact on battery safety. The main way to reduce the flammability of electrolyte is to use flame retardant additives.

At present, lithium salts that attract people's attention include LiFSI bis (fluorosulfonic acid) imine lithium] and boron based lithium salts. Among them, lithium bis oxalate borate (LiBOB) has a high thermal stability, with a decomposition temperature of 302 ℃, which can form a stable SEI film on the negative electrode. As lithium salt and additive, LiBOB can improve the thermal stability of batteries. In addition, lithium difluoroxalate borate (LiODFB) combines the advantages of LiBOB and lithium tetrafluoroborate (LiBF4), and is also expected to be used in the electrolyte of lithium batteries.

In addition to the improvement of electrolyte salts, flame retardant additives should also be used to improve the safety of batteries. The reason why the solvent in the electrolyte burns is that it has chain reaction itself. If flame retardant with high boiling point and high flash point can be added to the electrolyte, the safety of lithium ion battery can be improved.

The reported flame retardant additives mainly include three categories: organic phosphorus, fluorocarbonate and composite flame retardant additives. Although the organophosphorus flame retardant additive has good flame retardancy and oxidation stability, its reduction potential is high, it is incompatible with graphite cathode, and its viscosity is also high, leading to the reduction of electrolyte conductivity and poor low-temperature performance. The addition of co solvent or film forming additive such as EC can effectively improve its compatibility with graphite, but reduce the flame retardancy of electrolyte. Composite flame retardant additives can improve their comprehensive properties by halogenation or introduction of multifunctional groups. In addition, fluorocarbonate has good application prospects because of its high flash point or no flash point, favorable for film formation on the surface of negative electrode, and low melting point.

In the above figure, a nano dendritic polymer compound (STOBA) is used to coat NCM (424). When the lithium battery is abnormal and high temperature occurs, a film will be formed to block the flow of lithium ions, stabilize the lithium battery, and improve the battery safety. It can be seen from the figure below that during the acupuncture test, the internal temperature of the battery with positive material without STOBA coating rose to 700 ℃ within a few seconds, while the battery with STOBA coating positive material had a maximum temperature of only 150 ℃.

the diaphragm

At present, there are three main types of commercialized lithium-ion battery separators, namely PP/PE/PP multilayer composite microporous membrane, PP or PE single-layer microporous membrane and coating membrane. The widely used membrane is mainly polyolefin microporous membrane, which has stable chemical structure, excellent mechanical strength and good electrochemical stability.

The higher the mechanical strength of the diaphragm in the vertical direction, the smaller the probability of micro short circuit of the battery; The smaller the thermal shrinkage of the diaphragm, the better the safety performance of the battery. The micropore closing function of the diaphragm is another method to improve the safety of the power battery; The gel polymer electrolyte has good liquid retention, and the battery with this electrolyte has better safety than the conventional liquid battery; In addition, the ceramic diaphragm can also improve the safety of the battery. The types of preparation and treatment of lithium battery separator in common domestic patent documents are shown in the table below.

Improvement of diaphragm in patent literature

Process design and thermal runaway

The production process of battery is very complex. Even if strictly controlled, metal impurities or burrs in the production process cannot be completely avoided. If impurities, burrs or dendrites appear in the battery, the conductivity and temperature will increase after amplification and deterioration, and the heat generated by chemical reaction and discharge heating will accumulate continuously, which may eventually lead to the thermal runaway of the battery.

Insufficient negative capacity

When the capacity of the negative pole opposite the positive pole is insufficient or there is no capacity at all, part or all of the lithium produced during charging cannot be inserted into the interlayer structure of the negative pole graphite, and will be separated on the surface of the negative pole, forming a protuberant "dendrite". The protuberant part is more likely to cause lithium precipitation during the next charging. After tens to hundreds of cycles of charging and discharging, the "dendrite" will grow up and finally pierce the diaphragm, Make internal short circuit. The electric core discharges rapidly, generating a lot of heat, burning the diaphragm, causing greater short circuit. The high temperature will cause the electrolyte to decompose into gas, and the negative carbon and diaphragm paper will burn, causing excessive internal pressure. When the shell of the electric core cannot withstand this pressure, the electric core will explode.

The moisture content is too high

The water can react with the electrolyte in the electric core to produce gas. During charging, it can react with the lithium generated to generate lithium oxide, which will cause the capacity loss of the electric core. It is easy to overcharge the electric core to generate gas. The decomposition voltage of the water is low, and it is easy to decompose to generate gas during charging. When this series of generated gases will increase the internal pressure of the electric core, and when the shell of the electric core cannot bear, the electric core will explode.

Internal short circuit

Due to the internal short circuit phenomenon, the electric core discharges with large current, generating a large amount of heat, burning the diaphragm, resulting in a larger short circuit phenomenon. In this way, the electric core will generate high temperature, causing the electrolyte to decompose into gas, resulting in excessive internal pressure. When the shell of the electric core cannot withstand this pressure, the electric core will explode. During laser welding, the heat is transmitted to the positive electrode lug through the shell to make the positive electrode lug hot. If the upper adhesive tape does not separate the positive electrode lug and the diaphragm, the hot positive electrode lug will burn or shrink the diaphragm paper, causing internal short circuit and explosion.

High temperature adhesive tape wraps the negative electrode lug

When the negative electrode lug is spot welded, the heat is transmitted to the negative electrode lug. If the high-temperature adhesive tape is not properly attached, the heat on the negative electrode lug will burn the diaphragm, causing internal short circuit and explosion.

The adhesive at the bottom is not completely covering the bottom

When customers spot weld the aluminum nickel composite strip at the bottom, a lot of heat will be generated on the bottom shell wall. If the high temperature adhesive tape does not completely cover the diaphragm, it will burn the diaphragm, causing internal short circuit and explosion.

Overcharge

When the battery core is overcharged, the excessive discharge of lithium from the positive pole will change the structure of the positive pole, and too much lithium will easily be unable to be inserted into the negative pole, which will also easily cause lithium precipitation on the negative pole surface. Moreover, when the voltage reaches above 4.5V, the electrolyte will decompose and produce a large amount of gas. All of the above may cause an explosion.

External short circuit

External short circuit may be caused by improper operation or misuse. Due to external short circuit, the battery discharge current is very large, which will cause the heating of the electric cell. High temperature will cause the diaphragm inside the electric cell to shrink or completely damage, causing internal short circuit and explosion.

Station with insufficient negative capacity

The negative pole cannot wrap the positive pole, the positive and negative poles are wrongly paired in different grades, the negative pole is pressed dead when pressing, the negative pole particles, the negative pole exposed foil, the negative pole concave, the negative pole scratch, the negative pole dark mark, the negative pole coating is uneven, the positive pole head and tail are stacked, the positive pole coating is uneven, the positive pole dressing amount is too large, the positive and negative poles are mixed unevenly, the negative pole incoming material capacity is low, the positive pole incoming material capacity is too high, and the negative pole capacity is insufficient.

Station with excessive moisture content

The sealing is too slow and absorbs moisture. It absorbs moisture during aging. The electrolyte contains too much moisture. The baking is not dried or absorbed moisture before liquid injection. The baking is not dried during assembly and baking. The positive and negative electrodes are not dried during coating. The positive electrode is not fully baked and the moisture content is too high.

Station with internal short circuit

The adhesive on the bottom does not completely cover the bottom, the high temperature adhesive tape covers the negative electrode ear, the upper adhesive is not in the right position, the temperature is too high during baking, and the diaphragm is damaged, the laser welding short circuit core is not detected, the assembly of the micro short circuit core flows down, the assembly short circuit core is not detected, the pressure is too large when flattening, the diaphragm paper has sand holes, the winding is uneven, the negative electrode riveting is not flattened, there are burrs, the positive and negative electrodes are divided into small pieces of burrs, the positive and negative electrodes are divided into small pieces of material, and the internal short circuit occurs.

Station with possible overcharge

The charger voltage is too high when the user is using it, the voltage at individual points is too high during detection, the current setting is too large during detection, the cell capacity is insufficient, the current at individual points of the pre charging cabinet is too large, the current setting is too large during pre charging, and the charge is too high.

Possible station for external short circuit

The protective circuit board fails, the positive and negative poles are short circuited when the user is using it, the electric core is ignited in the process of turnover, and the electric core is not properly aligned, resulting in positive and negative pole contact and external short circuit.

Measures to prevent explosion of lithium ion battery

The safety of lithium ion batteries is a complex and comprehensive problem. The biggest hidden danger of battery safety is the random internal short circuit of the battery, resulting in on-site failure and out of control heating. Therefore, the development and use of materials with high thermal stability is the fundamental way and direction of efforts to improve the safety performance of lithium ion batteries in the future.

Improve the thermal stability of battery materials

The cathode materials can be synthesized into materials with good thermal stability by optimizing the synthesis conditions and improving the synthesis methods; Or use composite technology (such as doping technology) and surface coating technology (such as coating technology) to improve the thermal stability of cathode materials.

The thermal stability of negative electrode materials is related to the type of negative electrode materials, the size of material particles and the stability of SEI film formed by the negative electrode. If the size particles are made into negative electrode according to a certain proportion, the purpose of expanding the contact area between particles, reducing electrode impedance, increasing electrode capacity and reducing the possibility of active metal lithium precipitation can be achieved.

The quality of SEI film formation directly affects the charging and discharging performance and safety of lithium ion batteries. Weakly oxidizing the surface of carbon materials, or reducing, doping, and surface modified carbon materials, as well as using spherical or fibrous carbon materials, will help to improve the quality of SEI film.

The stability of electrolyte is related to the type of lithium salt and solvent. Lithium salt with good thermal stability and solvent with wide potential stability window can improve the thermal stability of the battery. Adding some high boiling point, high flash point and nonflammable solvents to the electrolyte can improve the safety of the battery.

The type and quantity of conductive agent and binder also affect the thermal stability of the battery. The binder reacts with lithium at high temperatures to generate a large amount of heat. The heat value of different binders is different. The heat value of PVDF is almost twice that of fluorine free binder. Replacing PVDF with fluorine free binder can improve the thermal stability of the cell.

Improve battery overcharge protection

To prevent overcharge of lithium ion battery, special charging circuit is usually used to control the charging and discharging process of battery, or safety valve is installed on a single battery to provide greater overcharge protection; Secondly, a positive temperature coefficient resistor (PTC) can also be used. Its mechanism is that when the battery heats up due to overcharge, the internal resistance of the battery will be increased to limit the overcharge current; A special diaphragm can also be used. When the temperature of the diaphragm is too high due to the abnormality of the battery, the pores of the diaphragm will shrink and block, preventing the migration of lithium ions and overcharging of the battery.

Prevent short circuit of battery

For the diaphragm, the porosity is about 40%, and the distribution is uniform. The diaphragm with a pore diameter of 10 nm can prevent the movement of positive and negative micro particles, thereby improving the safety of lithium ion batteries;

The insulation voltage of the diaphragm is directly related to its prevention of positive and negative pole contact. The insulation voltage of the diaphragm depends on the material, structure of the diaphragm and the assembly conditions of the battery.

The composite diaphragm (such as PP/PE/PP) with large difference between thermal closing temperature and melting temperature can prevent thermal runaway of the battery. The membrane surface is coated with a ceramic layer to improve the temperature resistance of the membrane. The low melting point PE (125 ℃) is used to close the hole at low temperature, while PP (155 ℃) can maintain the shape and mechanical strength of the diaphragm, prevent positive and negative pole contact, and ensure the safety of the battery.

As we all know, the graphite anode is used to replace the metal lithium anode, so that the deposition and dissolution of lithium on the cathode surface during the charging and discharging process become the insertion and removal of lithium in carbon particles, preventing the formation of lithium dendrites. However, this does not mean that the safety of the lithium-ion battery has been solved. During the charging process of the lithium-ion battery, if the positive electrode capacity is too much, metal lithium will deposit on the negative electrode surface. If the negative electrode capacity is too much, the battery capacity will be seriously lost.

The coating thickness and its homogeneity also affect the insertion and removal of lithium ions in active substances. For example, the density of the negative pole surface is thick and uneven, so the polarization size is different everywhere during the charging process, and it is possible to deposit metal lithium locally on the negative pole surface.

In addition, improper use conditions will also cause short circuit of the battery. Under low temperature conditions, the deposition speed of lithium ions is greater than the embedding speed, which will cause short circuit when metal lithium is deposited on the electrode surface. Therefore, the key to prevent lithium dendrite formation is to control the proportion of positive and negative electrode materials and enhance the uniformity of coating.

In addition, the crystallization of the binder and the formation of copper dendrites will also cause internal short circuits in the battery. In the coating process, the solvent in the slurry is completely removed by coating, baking and heating. If the heating temperature is too high, the binder may also crystallize, which will peel off the active substance and short circuit the battery.

Under the over discharge condition, when the battery is over discharged to 1-2V, the copper foil as the negative collector will start to dissolve and separate out on the positive electrode. When the battery is less than 1V, copper dendrites will appear on the positive electrode surface, causing internal short circuit of the lithium ion battery.

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