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Safety performance analysis of lithium battery

Return Listsource:Gratene date: 2022-06-17

Safety performance analysis of lithium batteryLithium battery 26650, currently mainly used to replace traditional nickel-chromium, nickel-hydrogen batteries for mine lamps, flashlights, electric tools, toys, instruments, UPS back power supplies, communication equipment, medical equipment, and military lights.

The definition of its model is: 26650, that is, the battery is 26 mm, the length of 65 mm, and a cylindrical type. Generally used for screaming lithium batteries, including lithium primary batteries and lithium ion batteries. Common useful nickel-wateng manganese positive materials, lithium batteries made of lithium iron phosphate materials – INR26650-3.6V-4500MAH, IFR26650-3.2V-3200MAH.

26650 advantages of lithium battery

26650 lithium battery, mainly used to replace traditional nickel-chromium, nickel-hydrogen batteries, for mine lamps, flashlights, electric tools, toys, instruments, UPS back power supplies, communication equipment, medical equipment, and military lights. Its advantage, relative to nickel-chromium, nickel-hydrogen batteries, mainly reflected in the following aspects:

1, high energy density, low discharge rate

The capacity of 26650 lithium battery is 1.5-2 times the same quality nickel-hydrogen battery. At the same time, the domestic 26,650 lithium battery is less than 60mΩ, which greatly reduces the self-consuming power of the battery, and can extend the battery while extending the time of use. Life.

2, charge and discharge performance stable

The 26650 lithium battery has no memory effect. It is not decomposed in the heat, the safety performance is high, and the life is circulating.

3, high voltage

26650 lithium battery voltages are generally above 3.6, 3.7V, which is much higher than nickel-chromium, and nickel-hydrogen batteries.

4, can be connected in series or in parallel into 18650 lithium battery pack

5, environmentally friendly non-pollution

32650 lithium battery safety? _32650 Safety Analysis of Lithium Battery

26650 lithium battery charge and discharge principle and process
The 26650 lithium battery can be charged and discharge, and is carried out with the active lithium ion movement on its positive electrode. That is, when charging the battery, there is an active lithium ion form on the positive electrode of the lithium battery, movement to the negative electrode, embedded to the laminated structure of the negative electrode. The material system of the negative electrode is graphite, which has a layered structure, which has a lot of micropores. When the lithium ion moves to the negative electrode, it is embedded in the micropore, the more lithium ions embedded in the micropores, the higher the charging capacity.

In the same way, when the battery is discharged, the lithium ions embedded in the negative electrode carbon layer are removed, the more the lithium ions moved back to the positive electrode, the higher the discharge capacity. And the capacity of the 26650 lithium battery we usually refer to, that is, the discharge capacity.

Dynamic lithium ion batteries generally refer to lithium ion batteries that can power to devices, instruments, vehicles, etc. The power lithium-ion battery has a wide range of energy than energy, high current charge, and long cycle life. The power lithium-ion battery is divided into ternary, lithium cobaltate, lithium manganese acid, lithium iron phosphate, a cylindrical, a cylindrical, and the like according to the different profiles. In order to improve the endless mileage, the energy of the power lithium-ion battery is generally larger, and the capacity is not equal to a few hundred hours of time, and the voltage is from ten volts to several hundred volts. As carrying energy increases, the potential danger of batteries also increases. Therefore, how to improve the safety of the power battery has become an important premise for the continued development of electric vehicles. In the development of the power lithium battery, there is always two development directions. One direction is a large monomer battery, through a small amount of parallel; one direction is a small unit battery, through a large number of parallel combination. South Korea’s LG, domestic BYD represented by companies is a generic route; the United States A123, domestic Waterma represents a small cylindrical route. These two routes are currently undecided, different power battery manufacturers choose different process routes according to their understanding. But in terms of facing security, the results of the two process routes are very large. This paper compares the dynamic battery structure, performance, especially safety, to explain the safety advantages of small cylindrical batteries in the application of electric vehicles.

Cylindrical batteries and square batteries are currently the mainstream direction of the industry. The basic structure of the cylindrical battery is shown in Figure 1. The positive and negative electrode is separated from the diaphragm, and the core is formed by winding. Usually the positive negative pole sheet is welded to have a positive and negative pole ear, and the two sides are respectively introduced. Ear soldering in positive and negative housing. Electrolytic solution is added to the housing. Figure 2 is a square battery structure. Structure of square batteries and winding structure and winding structure. The laminated square lithium ion battery consists of a battery core between the N-piece positive electrode and the N + 1 coupled, and the positive and negative pole sheet is separated from the side of the positive and negative electrode sheets, respectively. Positive, negative euro zone, laminated into a core, the negative ear is symmetrical from both sides of the core. The winding structure of the square battery and the winding structure of the cylindrical battery are similar, and the difference is that the canx is flat-shaped rather than cylindrical type. Due to the different shape of the cylindrical battery and the square battery, the structure is large. Under normal circumstances, the cylindrical battery cannot be done too much due to the limitation of the core current density and heat dissipation. By increasing length, the square battery guarantees to increase capacity by increasing longness. Its monomer capacity can generally exceed more than 10 times more of the cylindrical battery. Table 1 is a comparison of the performance of cylindrical and square batteries. It can be seen that the two batteries have their own features. The cylindrical battery structure is simple, the positive and negative interface is close, the production line is mature, low cost, and well, and excellent safety performance. The disadvantage is that the internal resistance is relatively high, and the set of high-volume requirements is high. The advantages of square batteries are simple in large combinations of monomer capacity. Its disadvantage is that the production process is complex, and the consistency of large-capacity battery monomer is difficult to control. In addition, the square housing is easily concentrated, and the housing is easy to rupture, and the electrolyte splashes to cause safety hazards.

From the global application market, large-capacity-based batteries and small-capacity cylindrical batteries are available in the field of power. At present, the benchmark enterprise in the lithium iron phosphate battery industry, the main product of the US A123 is 18650/26650/32113 three types of cylindrical batteries, and the monomer capacity is 2.5ah-5ah, respectively.

Safety contrast analysis

1. Safety comparison in extreme cases
The security of the power battery under extreme conditions such as serious accidents in vehicles is the most concerned issue because it is directly related to life and property safety. The cylindrical battery capacity is small, and the capacity of the power battery pack is achieved by the series and combinations. Take the current 32650 battery as an example, the battery capacity is only 5ah. And generic battery monomers generally exceed dozens of security, some reach more than 100AH. In the extreme danger of collision, extrusion, etc., the energy of the cylindrical small battery is far less than the generic battery monomer. At present, the electrolyte of Watema 5ah batteries is only 20 grams, while generic batteries, such as 50AH monomers, which have more than 200 grams of electrolyte. The amount of electrolytic liquid of the square cell is more than 10 times the small cylindrical battery. Once a single cell is leakage in an accident, the degree of combustion caused by the electrolyte leakage will also be more than 10 times the small cylindrical battery. In this regard, the safety of small cylindrical batteries is much better than large batteries. When the small cylindrical battery is destroyed, its burning power is far less than a large-scale battery. By separating the separation of a single battery, there is a problem with a single battery, which does not affect other batteries. By dispersing energy, the safety of the battery is greatly improved.

In terms of impact, cylindrical batteries and square batteries have a large difference. Cylindrical batteries have better antibody-resistant capacity relative to square batteries, and all directions are uniform, and the deformation retention capacity is the best in all cells. Large improvement. Even during high speed collision extrusion, the cylindrical cell has a certain deformation, but it will not be burned. For square batteries, the larger area is easy to deform, during high speed collision extrusion, the electrical core housing does not guarantee the internal structure of the battery, which is easy to cause the internal positive and negative polar displacement short circuit; for this instant The impact, the square battery cannot be reacted quickly.

In addition, for large square cells, due to its larger side area, the probability of being impacted by other objects is much higher, and therefore, in the safety accident, the monomer battery is hurt to cause short circuits. Small cylindrical cells are much larger. For small cylindrical batteries, once the battery case is violently impacted, the small cylindrical battery is first disconnected may be the riveting point of each monomer battery, and due to the small volume of the battery, the large possibility is hit The battery pack is invalid. This is of great significance for improving the safety of the power battery pack. Therefore, the battery pack combined with small capacity cylindrical batteries can provide longer escape time when there is an accident. According to Watma’s test, the battery incinerates in the fire, and the electrolyte sprayed the time of strenuous burning after 10 minutes.
2. Comparison of heat dissipation
In terms of monomer heat dissipation, since the shape of the cylindrical battery and the square battery is different, the heat dissipation effect is different. Take a 50AH-type battery as an example, its surface area capacity is approximately 1 x 10-3 m2 / Ah; and the surface area capacity of the 32650-5AH cylindrical battery is approximately 1.6 x 10 to 3 m2 / Ah; which is 60% compared to below. The cylindrical small cell has a natural advantage in the heat dissipation in the case of the external conditions.

When the cylindrical battery is in combination, there is a longitudinal gap between the batteries, which provides a natural heat dissipation path for the heat dissipation of the battery. Theoretically heat radial heat radial is at least 15.9% (tight arrangement) and 21.9% (cubic arrangement). Figure 3 is a combined structure of the Watema cylindrical battery, which can be seen after a combination has a good heat dissipation channel. The natural heat dissipation channel of the cylindrical battery combination ensures the heat dissipation effect of the battery, and the battery is improved.

3. Comparison of safety mechanisms
Both battery structures have explosion-proof safety valves. Safety valves of square batteries are typically located on the end side, and the valve area is greater than the safety valve of the cylindrical battery. However, if the capacity of the battery is taken into account, the size of the unit capacity is much smaller than the cylindrical battery. Once the battery has failed, especially extreme impact conditions, the effectiveness of the square battery safety valve is behind a small cylindrical battery. The cylindrical battery combination cap has an explosion-proof safety valve and a currentinterruptdevice, as shown in Figure 4. This is rarely applied to square batteries. When an external short circuit is blocked or when the battery is reached a 1.2MPa security warning value, first, the CID device is started, the positive and negative pole is disconnected, and the current is cut off, and the internal circuit is turned off; when the battery is pressed to 1.8MPa security alert At the time, the pressure relief member safety valve opens, the gas is discharged, and the risk of explosion is avoided. At present, the safety combination cap process matures, so that the safety of the cylindrical cell is well guaranteed.

4. Comparison of group consistency
It is well known that the consistency of the monomer battery has a greater impact on the life, safety of the battery pack. The complexity of generic batteries has determined that the consistency of the current monomer battery is poor. The problem of consistency in the group is directly affected directly. Because there are few capacities, the battery is high, and the risk of being charged. Cylindrical batteries are mature, and the battery consistency is higher. After the battery is combined, the chances of low capacity batteries have a lower chance. Even if a low capacity battery occurs, the inconsistent impact is eliminated by multiple parallelism.

5. Cylindrical battery safety test
In order to verify the safety of the cylindrical battery, the cylindrical cell and battery packs are subject to safety testing. Figure 5 is a cylindrical battery group acupuncture experience photo. When the steel staple is penetrating the battery, the electrolyte in the battery is leaked. The temperature of the battery is rapidly increased. The voltage is slowly declining. The electrolytic liquid has an small amount of white smoke. The insulation plastic jacket for wrap the battery is melted by high temperature, the process lasts about 10 min. The post-phenomenon disappears, and the final short circuit battery voltage drop is zero, the process is up to 141 ° C. Battery does not explode, do not burn. Fully compliant with UL2580 (and SAEJ2464) standards. Figure 6 is a photo before and after the cylindrical battery pack. After the battery pack is hit by the heavy object, the battery is significantly damaged, but the battery does not fire, no leakage, no smoke or explosion, battery voltage is basically no change. Compliance with UL1642 standards. Table 2 is a safety test item for the battery pack. From the measured results, the cylindrical battery exhibits a good safety performance. Figure 7 is a more demanding incineration testing process photo. From the measured results, the surface temperature of the battery’s explosion-proof sheet opened is around 240 ° C. The battery releases the gas, the electrolyte is partially combined, and there is no explosion. After the flame is extinguished, the battery housing structure has not been destroyed. According to the UL1642 Requirements “The whole and part of the monomer battery should not penetrate the wire mesh during the experiment”, indicating the safety of the UL standard from the experimental results.

At present, small cylindrical batteries have been applied to electric vehicles, in order to verify the safety of batteries in automotive accidents, collision testing. The collision criteria are carried out by C-NCAP. The speed 50 km of speed is 50 kilometers, respectively, the rigid fixation is 100% overlap, 56 kilometers of the speed of 56 kilometers to the front side bias collision of the overlap rate of the deformable disorder, 50 km of deformable movement 50 km from the side of the vehicle. Figure 8 shows the photo after the whole vehicle collision. Remove the battery from the collision, the battery pack is basically intact, no smoke, no combustion. The whole vehicle collision experiment verifies the safety advantages of small cylindrical batteries to electric vehicles. 

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