Lithium-ion battery thermal runaway process
The thermal runaway of the battery is caused by the fact that the heat generation rate of the battery is much higher than the heat dissipation rate, and a large amount of heat is accumulated and not dissipated in time. Essentially, “thermal runaway” is an energy-positive feedback loop process: increased temperature causes the system to heat up, which in turn makes the system 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 itself during high current charging and discharging, the internal temperature of the battery rises to about 90°C to 100°C, and the lithium salt LiPF6 begins to decompose; the chemical activity of the carbon negative electrode in the charged state is very high, Close to metallic lithium, the SEI film on the surface decomposes at high temperature, and the lithium ions embedded in graphite react with the electrolyte and the binder, further pushing the battery temperature to 150 °C, and a new violent exothermic reaction occurs at this temperature. For example, a large amount of electrolyte is decomposed to generate PF5, and PF5 further catalyzes the decomposition reaction of organic solvents.
Stage 2: Battery Bulge Stage
When the temperature of the battery reaches above 200°C, the cathode material decomposes, releasing a large amount of heat and gas, and the temperature continues to rise. At 250-350°C, the lithium intercalated negative electrode begins to react with the electrolyte.
Stage 3: Battery thermal runaway, explosion failure stage
During the reaction process, the charged cathode material begins 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, and the battery will burn and explode.
Measures to prevent explosion of lithium-ion batteries
Improve the thermal stability of battery materials
Cathode materials can improve the thermal stability of cathode materials by optimizing synthesis conditions, improving synthesis methods, and synthesizing materials with good thermal stability; or using composite technology (such as doping technology) and surface coating technology (such as coating technology).
The thermal stability of the negative electrode material is related to the type of the negative electrode material, the size of the material particles and the stability of the SEI film formed by the negative electrode. If the particles are made into a negative electrode according to a certain proportion, the purpose of expanding the contact area between the particles, reducing the electrode impedance, increasing the electrode capacity, and reducing the possibility of active metal lithium precipitation can be achieved.
The quality of SEI film formation directly affects the charge-discharge performance and safety of lithium-ion batteries. Weak oxidation of the surface of carbon materials, or reduction, doping, and surface modification of carbon materials and the use of spherical or fibrous carbon materials are helpful. Improvement of SEI film quality.
The stability of the electrolyte is related to the type of lithium salt and solvent. The thermal stability of the battery can be improved by using a lithium salt with good thermal stability and a solvent with a wide potential stability window. 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 quantity of conductive agent and binder also affect the thermal stability of the battery. The binder and lithium react at high temperature to generate a lot of heat. Different binders have different calorific values. The calorific value of PVDF is almost fluorine-free. The thermal stability of the battery can be improved by replacing PVDF with a fluorine-free binder.
Improve battery overcharge protection capability
In order to prevent overcharging of lithium-ion batteries, a dedicated charging circuit is usually used to control the charging and discharging process of the battery, or a safety valve is installed on a single battery to provide a greater degree of overcharge protection; secondly, a positive temperature coefficient resistor (PTC) can also be used. PTC), its mechanism of action is that when the battery heats up due to overcharging, the internal resistance of the battery is increased, thereby limiting the overcharge current; a special diaphragm can also be used, when the abnormal temperature of the battery causes the diaphragm temperature to be too high, the diaphragm pores shrink Blocking, preventing the migration of lithium ions and preventing 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 size of 10nm can prevent the movement of small particles of positive and negative electrodes, thereby improving the safety of lithium-ion batteries;
The insulation voltage of the separator is directly related to the contact between the positive and negative electrodes. The insulation voltage of the separator depends on the material and structure of the separator and the assembly conditions of the battery.
The use of composite separators (such as PP/PE/PP) with a large difference between the thermal closure temperature and the melting temperature can prevent the thermal runaway of the battery. The surface of the separator is coated with a ceramic layer to improve the temperature resistance of the separator. PE (125°C) with low melting point is used to close the pores at low temperature, while PP (155°C) can maintain the shape and mechanical strength of the separator, prevent the contact between the positive and negative electrodes, and ensure the safety of the battery.
It is well known that the graphite negative electrode is used to replace the metal lithium negative electrode, so that the deposition and dissolution of lithium on the surface of the negative electrode during the charging and discharging process becomes the intercalation and extraction of lithium in the carbon particles, which prevents 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 large, metal lithium will be deposited on the surface of the negative electrode, and the negative electrode capacity will be too large, and the battery capacity loss will be serious.
The coating thickness and its uniformity also affect the intercalation and deintercalation of lithium ions in the active material. For example, the surface density of the negative electrode is thick and non-uniform, so the polarization size is different everywhere during the charging process, and metal lithium may be locally deposited on the negative electrode surface.
In addition, improper use conditions can also cause a short circuit of the battery. Under low temperature conditions, the deposition rate of lithium ions is greater than the insertion rate, which leads to the deposition of metallic lithium on the electrode surface and causes a short circuit. Therefore, controlling the ratio of positive and negative materials and enhancing the uniformity of coating are the keys to prevent the formation of lithium dendrites.
In addition, the crystallization of the binder and the formation of copper dendrites can also cause internal short circuits in the battery. In the coating process, all the solvent in the slurry is removed by coating, baking and heating. If the heating temperature is too high, the binder may also crystallize, which will cause the active material to peel off and short-circuit the battery.
Under overdischarge conditions, when the battery is overdischarged to 1-2V, the copper foil as the negative electrode current collector will begin to dissolve and precipitate on the positive electrode. Internal short circuit.