Reports on lithium ion batteries have caused fire or even explosion accidents in recent years. Lithium ion batteries are mainly composed of negative electrode materials, electrolytes and positive poles. The chemical activity of the litter of the negative material is close to the metal lithium during charging state. The SEI membrane on the surface at high temperature is decomposed. The lithium ion and electrolytic solution, and the adhesive polar vinyl of the graphite will react to a large amount of heat.
Electrolytic solution is generally adopted by alkyl carbonate organic solution, which is flammable. The positive pole material is usually a transitional metal oxide, which has strong oxidation properties during charging. It is easy to decompose oxygen at high temperature.
Therefore, from the perspective of materials, lithium -ion batteries have strong danger, especially when abuse, safety issues are more prominent.
1. Analysis of thermal stability of lithium -ion battery material
The fire risk of lithium -ion batteries is mainly determined by chemical reactions in the inside of the battery. The fire risk of lithium -ion batteries depends on the thermal stability of the battery material, and the thermal stability of the battery material depends on the chemical reactions between the internal parts. At present, people mainly use the differential scanning heat meters (DSC), thermal analyzer (TGA), thermal heating acceleration heat meter (ARC), etc. to study the thermal stability of battery -related materials.
1 The influencing factors of the thermal stability of the negative pole material:
The starting temperature of the maintenance of the negative material increases with the increase of the particle size.
Use DSC to study the thermal stability of the lithium -embedded natural graphite of different particles. It was found that there were three hot peaks in all samples. The first peak of the sample was located near 150 ° C. The position of the latter two peaks was obviously different. The start temperature of the latter two peaks increased with the size of the particle size. The study showed that the first split peak was the decomposition of the SEI membrane, and the latter two hot peaks reaches the reaction of lithium graphite with PVDF and electrolyte.
The relationship between the surface area and thermal stability of graphite materials was studied with ARC. It was found that when the surface area of graphite materials increased from 0.4 square meters/gram to 9.2 square meters/gram, the reaction rate increased by two orders of magnitude. Therefore, the reaction rate of carbon negative materials increases with the increase of the surface area.
Different structured carbon materials have different heat production heat production, and graphite structures are much more calories than non -fixed carbon structures.
The thermal stability of carbon fiber, hard carbon, soft carbon and MCMB was used with DSC. Studies have found that the first peaks of four kinds of carbon appear at 100 ° C. This hot peak is considered to be produced by the decomposition of the SEI membrane; as the temperature rises to 230 ° C, the carbon structure and the surface area of the surface are hot and stable. The influence of sexuality gradually appears that the carbon electrode material (carbon fiber, MCMB) of the graphite structure (carbon fiber, MCMB) produces more calories from carbon electrode materials (soft carbon and hard carbon) with non -fixed structures. XRD is displayed at about 230 ° C, and the total loss of lithium embedded amount has become a linear relationship with the carbon ratio area.
2 The influencing factors of the thermal stability of the positive pole material:
The start temperature of the positive electrode material and electrolyte reaction increases with the reduction of chemical measurement.
Use DSC to study the effects of the changes in x on the reaction of the reaction of the ivory materials LIXCOO2, LIXNIO2, LIXMN2O4 and LIXC6 and the electrolyte. The conclusion is concluded through research: there is a widespread heat -reaction between electrolyte and positive materials. When the X value decreases, the reaction temperature rises to the range of 200-230 ° C. Essence
Use ARC to study the thermal stability of LixCoo2. Above the critical temperature, lixCoo2 has an oxygen reaction and a large amount of heat is released. When x = 0.25, the starting temperature of the insulation reaction is about 230 ° C. Li Yi and other natural response temperatures of the 18650 LICOO2 were measured in the heat resistance test, indicating that the start temperature of the decomposition reaction was lower. Therefore, it can be seen that the start temperature of the decomposition reaction of the positive material increases with the decrease of X.
The higher the Ni content in the positive material, the more unstable, the higher the MN content, the more stable.
I studied the thermal stability of different component materials of Li1-XNI1-2xCoxMnxo2 with DSC. As a result, it was found that as the Ni content decreased, the insulation start temperature and peak temperature of the LI1-XNI1-2xCoxMNXO2 were higher and the heat produced was less. Maeneil et al. Research several propelities of the positive pole material to the 1mol Lipf6 EC/DEC reaction.
3 The influencing factor of thermal stability of electrolyte:
Organic solvent DMC is an important factor that causes unstable electrolyte. The higher the content of the DMC, the more unstable the electrolyte.
Use the EC+DEC, EC+DMC, PC+DMC, PC+DEC and PC+DMC hybrid solvents with 1MOL/LIPF6 of DSC to study in a closed container. It is more likely to react.
The electrolyte can react at a lower temperature, and different solvents and lithium salts in the electrolyte are suitable for different positive electrode materials.
Use the ARC and XRD methods to study the insulation reactions between LI0.5COO2 and LIMN2O4 charging positive poles and electrolytes, respectively. Studies have shown that the decomposition reaction occurs when the temperature of Li0.5COO2 powder is greater than 200 ° C, and oxygen is precipitated, and the extension reactions with the EC/DEC solvent appear at 130 ° C. After adding LIPF6 to the solvent, the reaction is inhibited. For the Limn2O4 material, the crystal transformation occurs at 160 ° C. The existence of the solvent has no effect on this reaction. After adding LIPF6 to the electrolyte, with the increase of the LIPF6 concentration, the response between LIMN2O4 and electrolytes intensified.
Second, the safety analysis of the abuse of lithium -ion batteries
The safety of lithium -ion batteries mainly depends on the thermal stability of the battery material, and it is also closely related to the abuse conditions such as the battery over charge, acupuncture, squeezing and high temperature.
1 Overcoming security analysis:
The over -charging test is an analog when there is an error when the voltage detection of the charger, the battery may occur when the charger is faulty or the battery may occur when the charger is faulty.
The heat out of control caused by over -filling may come from two aspects: on the one hand, the scorched ear heat generated by the current, and on the other hand, the reaction heat generated by the side reaction occurred in the positive and negative electrode. When the battery is over -charged, the negative electrode voltage gradually increases. When the amount of lithium loss of the negative electrode is too large, the process of lithium loss is becoming more and more difficult. This has led to a sharp increase in the battery internal resistance. The multiplier is more obvious when charging. The high -voltage positive oxide agent in the overcomplaction state releases a lot of heat, and the negative electrode after the temperature is rising. When the heat dissipation rate is greater than the battery, the temperature rises to a certain degree, and the heat is out of control.
TOBISHIM et al. Compared the overcharge performance of aluminum -shell square batteries based on LICOO2 and Limn2O4 as the positive electrode material. The research results show that the LICOO2 battery will explode when the current is 2C charging to 10V, and the Limn2O4 battery cells are 2C with 2C respectively. /10V, 3C/10V are not smoking, fire or explosion when they are overcharged, and only swelling occurs, which shows that MN has better overcoming performance than CO. LEISING et al. The effects of different graphite ratio on the overcoming performance of LICOO2 battery components. The results show that the overcoming performance of the battery cell mainly depends on the positive pole material, which does not change with the increase of graphite. This shows that the precipitation of metal lithium in the negative electrode during the overcharge process is not the key to affecting overcharge performance, but the thermal stability of the excessively lithium LICOO2 or the oxidation of the electrolyte on its surface.
2 High temperature safety analysis:
The simulation environment high temperature test can be performed in the hot box test. The heat box test is the case where the simulation battery is used in improper use. For example, put the mobile phone in a sun -exposed car, or put the mobile phone or electronic product in a microwave oven, the temperature can reach 130 ° C or even 150 ° C. When the heat abuse, in addition to the positive and negative electrode materials inside the battery and its reaction with the electrolyte, the melting and contraction of the isolation membrane at high temperatures cause positive and negative pole short circuits. Coccustment generated by short circuit is also an important heat source when the hot box test is tested. Essence
When the temperature is 90 ~ 120 ° C, the sub -stabilization layer of the solid -state electrolytes (SEI) formed on the surface of the carbon negative surface (SEI) is multiple times. The temperature is 180 ~ 500 ° C, and the positive pole and the electrolyte have a supporting heat reaction and generate gas; the SEI membrane can prevent the interaction between lithium carbon carbon and the mechanical electrolyte. The negative electrode material may begin to react with the solvent and generate gas. When the temperature rises to 240-350 ° C, the fluoride adhesive begins to react violent chain growth with lithium carbon. Lithium may be exhausted, and this reaction will not occur; if the temperature continues to rise to 660 ° C, the AL set will fuse. These situations are very dangerous for large lithium -ion power batteries, affecting the life and safety of the battery.
3 Short -circuit security analysis:
The short -circuit of batteries is divided into external short circuits and internal short circuits. The external short -circuit generally refers to the short circuit caused by the direct contact of the positive and negative electrode; the internal short -circuit refers to the short circuit of the battery in the area of the foreign object when the battery is punctured or collided and squeezed by the sharp object.
External short -circuit security analysis
External short -circuit security studies are tested by direct connection of positive and negative poles on the outside. The study of the external short -circuit of the battery will study the lithium cobalt lithium lithium -type lithium -ion battery, 6 -core notebook battery (6 18650 batteries, 3 series into 1 group, 2 groups, remove the protection circuit) Positive and negative pole short circuit, short circuit, Paste the thermocouple on the surface of the battery to detect changes in the surface temperature of the battery. Record the temperature curve of the battery surface with a paperless recorder.
Internal short -circuit security analysis
Short -circuit security studies inside the battery generally use acupuncture, extrusion and other methods to test. The purpose is to simulate the battery's puncture, collision, and squeezing of the battery. Acupuncture caused the battery to short -circuit at the acupuncture point. The short -circuit area formed a local hot zone due to a large amount of scorched ear heat. When the temperature of the hot area exceeds the critical point, it will cause heat loss, and the danger of smoking, fire and even explosion. Squeeze is similar to acupuncture, which causes short circuit in the local area and may cause heat out of control. The difference is that squeezing does not necessarily cause damage to the battery shell. If the shell does not damage, it means that the flammable electrolyte will not leak from the hot area. The heat dissipation effect at the hot area is worse.
Tests that cause short -circuit internal internal internal internal internal internal circuit through squeezing and acupuncture are often more rare than passing the short -circuit testing outside the battery. This is because the battery is often uniformly put on the battery in the external short -circuit of the battery. Directly touched the thermal discharge of the battery.
Test conditions such as acupuncture and extrusion have a greater impact on the test results. This is because the internal short circuit conditions caused by acupuncture and extrusion tests under different conditions are different. Impact.
There are 4 types of short circuit in the battery:
(1) Between Al set and negative electrode materials (LIC6, C6);
(2) Between Al sets and CU sets;
(3) Between the positive material and LIC6;
(4) Between the positive material and the CU set.
By establishing a battery electrochemical finite element heating model, the inside the battery and the battery temperature of the battery were systematically simulated and analyzed, and the corresponding tests were systematically simulated and analyzed in these 4 short circuit conditions. The results show that the short -circuit between the AL set flow and the charging graphite is the most dangerous, because in this case, the short -circuit resistance is small, the current, the high heat power, the heat transmission, the heat dissipation is relatively slow, and the carbon negative activity is high, so it is easy to be easy Causes a series of electrical and chemical reactions to cause accidents.
Three, conclusion
By analyzing thermal stability of lithium -ion battery negative materials, positive poles and electrolytes, the main factors affecting the thermal stability of lithium -ion batteries are summarized. Sexual analysis is analyzed in detail, providing a reference for the safe use of lithium ion batteries. When more people pay attention to the danger of the lithium ion battery itself, and at the same time, the safety management of lithium ion electronics production, storage and use of lithium ion electronics will greatly reduce the lithium -ion battery fire.