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Analysis Of The Safety Of Lithium Batteries
- Sep 05, 2018 -

Lithium-based batteries are the fastest-growing battery system in the past 20 years and are now widely used in electronic products. The recent explosion of mobile phones and laptops is essentially a battery explosion. What kind of mobile phone and laptop battery is, how to work, why there is an explosion, how to avoid an explosion.

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When the lithium battery is overcharged to a voltage higher than 4.2V, it will start to have side effects. The higher the overcharge voltage, the higher the risk. When the lithium battery voltage is higher than 4.2V, the amount of lithium atoms remaining in the positive electrode material is less than half. At this time, the storage cell often collapses, causing a permanent drop in battery capacity. If the charging continues, since the cell of the negative electrode is already filled with lithium atoms, the subsequent lithium metal will accumulate on the surface of the negative electrode material. These lithium atoms grow dendrites from the surface of the negative electrode toward the direction of lithium ions. These lithium metal crystals pass through the separator paper, shorting the positive and negative electrodes. Sometimes the battery first explodes before the short circuit occurs. This is because during the overcharging process, the electrolyte and other materials will crack and generate gas, causing the battery casing or pressure valve to bulge and rupture, allowing oxygen to enter and react with the lithium atoms deposited on the surface of the negative electrode. Then exploded.

Therefore, when charging a lithium battery, you must set the upper voltage limit to take into account the battery life, capacity, and safety. The optimal charging voltage is limited to 4.2V. The lithium battery has a lower voltage limit when it is discharged. When the cell voltage is lower than 2.4V, some materials will start to be destroyed. Since the battery will self-discharge, the voltage will be lower for a longer period of time. Therefore, it is best not to put it at 2.4V until it stops. During the period from the 3.0V discharge to the 2.4V lithium battery, the energy released is only about 3% of the battery capacity. Therefore, 3.0V is an ideal discharge cutoff voltage. In charge and discharge, in addition to the voltage limit, current limitation is also necessary. When the current is too large, lithium ions cannot enter the cell and will accumulate on the surface of the material.

When these lithium ions obtain electrons, crystals of lithium atoms are generated on the surface of the material, which is dangerous as overcharge. In case the battery case is broken, it will explode. Therefore, the protection of the lithium ion battery must include at least three factors: the upper limit of the charging voltage, the lower limit of the discharging voltage, and the upper limit of the current. In the general lithium battery pack, in addition to the lithium battery core, there will be a protective plate, which is mainly to provide these three protections. However, these three protections of the protection board are obviously not enough, and the global lithium battery explosion is still frequent. To ensure the safety of the battery system, a more careful analysis of the cause of the battery explosion must be performed.

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Cause of the explosion:

1. Internal polarization is large;

2. The pole piece absorbs water and reacts with the electrolyte to form an air drum;

3. The quality and performance of the electrolyte itself;

4. The amount of liquid injection during the injection is not up to the process requirements;

5. The laser welding sealing performance is poor in the assembly process, and the air leakage is detected when the air leakage is detected;

6, dust, pole flakes are easy to cause micro-short circuit;

7. The positive and negative electrode sheets are thicker than the process range, and it is difficult to enter the shell;

8. Injecting and sealing problems, the sealing performance of the steel ball is not good, resulting in an air drum;

9. The shell material has a thick shell wall, and the shell deformation affects the thickness;

The high ambient temperature outside is also the main cause of the explosion.

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Explosion type

Explosion type analysis The type of battery core explosion can be summarized as external short circuit, internal short circuit, and overcharge. The external term here refers to the outside of the cell, and includes a short circuit caused by poor insulation design of the battery pack. When a short circuit occurs outside the cell, and the electronic component fails to cut off the circuit, high heat is generated inside the cell, causing some of the electrolyte to vaporize and the battery case is enlarged. When the internal temperature of the battery is as high as 135 degrees Celsius, the good quality diaphragm paper will close the pores, the electrochemical reaction will terminate or nearly terminate, the current will drop suddenly, and the temperature will slowly drop, thus avoiding the explosion. However, if the pore closing rate is too poor, or the separator paper whose pores are not closed at all, the battery temperature will continue to rise, more electrolyte will vaporize, and finally the battery casing will be broken, and even the battery temperature will be raised to The material burns and explodes. The internal short circuit is mainly caused by the burrs of the copper foil and the aluminum foil piercing the diaphragm, or the dendrite of lithium atoms piercing the ruthenium film.

These tiny needle-like metals can cause micro short circuits. Since the needle is very thin and has a certain resistance value, the current is not necessarily large. Copper and aluminum foil burrs are caused by the production process. The observable phenomenon is that the battery leaks too fast, and most of them can be screened by the battery factory or assembly plant. And because the burrs are small, they are sometimes blown off, causing the battery to return to normal. Therefore, the probability of an explosion caused by a burr micro-short circuit is not high. In this way, it is possible to obtain a poor battery with a low voltage soon after charging in each of the battery cells, but there is little explosion and statistical support. Therefore, the explosion caused by the internal short circuit is mainly caused by overcharge. Because, after overcharging, the lenticular lithium metal crystals are everywhere on the pole piece, and the piercing point is everywhere, and micro short circuits occur everywhere. Therefore, the battery temperature will gradually increase, and finally the high temperature will be the electrolyte gas. In this case, whether the temperature is too high, the material burns and explodes, or the outer shell is broken first, so that the air enters and the lithium metal is violently oxidized, which is the end of the explosion.

However, such an explosion caused by an internal short circuit caused by overcharging does not necessarily occur at the time of charging. It is possible that when the battery temperature is not high enough for the material to burn and the gas generated is not enough to break the battery casing, the consumer terminates charging and takes the mobile phone out. At this time, the heat generated by the numerous micro short circuits slowly increases the temperature of the battery, and after a period of time, the explosion occurs. The common description of consumers is that when the phone is picked up, the phone is found to be very hot, and it will explode after being thrown away. In combination with the above types of explosions, we can focus on the prevention of overcharge, the prevention of external short circuits, and the safety of battery cells. Among them, overcharge prevention and external short circuit prevention are electronic protection, which has a great relationship with battery system design and battery assembly. The focus of battery safety improvement is chemical and mechanical protection, which has a great relationship with battery manufacturing plants.

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Security risks

The safety of lithium-ion batteries is not only related to the nature of the cell material itself, but also to the battery preparation technology and use. Mobile phone batteries frequently explode, on the one hand, due to the failure of the protection circuit, but more importantly, there is no fundamental solution to the problem in terms of materials.

 

Lithium cobaltate cathode active material is a very mature system in small cells, but after charging, a large amount of lithium ions remain in the positive electrode. When overcharged, lithium ions remaining in the positive electrode will flock to the negative electrode. The formation of dendrites on the negative electrode is an inevitable result of overcharging of a battery using lithium cobaltate. Even during normal charge and discharge, there may be excess lithium ions released to the negative electrode to form dendrites. Theoretical ratio of lithium cobaltate material The energy is more than 270 mAh per gram, but to ensure its cycle performance, the actual use capacity is only half of the theoretical capacity. During use, for some reason (such as damage to the management system), the battery charging voltage is too high, and a part of the lithium remaining in the positive electrode will be removed, and dendrites are deposited in the form of metallic lithium through the electrolyte to the surface of the negative electrode. The dendrites pierce the membrane and form an internal short circuit.

The main component of the electrolyte is carbonate, which has a low flash point and a low boiling point. It will burn or even explode under certain conditions. If the battery is overheated, it will cause the carbonate in the electrolyte to be oxidized and reduced, and a large amount of gas and more heat will be generated. If the safety valve or gas is not enough to be released through the safety valve, the internal pressure of the battery will rise sharply and cause an explosion.

Polymer electrolyte lithium-ion batteries do not fundamentally solve the safety problem. Lithium cobaltate and organic electrolytes are also used, and the electrolyte is gel-like, which is not easy to leak, and will burn more violently. The combustion is safe for polymer batteries. The biggest problem.

There are also some problems in use. An external short circuit or an internal short circuit of the battery will generate excessive current of several hundred amperes. When the external short circuit occurs, the battery instantaneously discharges a large current, which consumes a large amount of energy on the internal resistance and generates a large amount of heat. The internal short circuit forms a large current, and the temperature rise causes the diaphragm to melt, and the short-circuit area is enlarged, thereby forming a vicious circle.

In order to achieve a high working voltage of 3 to 4.2V for a single cell, a lithium ion battery must adopt an organic electrolyte with a decomposition voltage greater than 2V, and the organic electrolyte will be electrolyzed under high current and high temperature conditions to generate gas. This causes the internal pressure to rise and severely breaks through the casing.

Overcharge may precipitate metallic lithium. When the casing is broken, it will directly contact with the air, causing combustion. At the same time, the electrolyte will be ignited, a strong flame will occur, and the gas will rapidly expand and explode.

In addition, for mobile phone lithium-ion batteries, battery expansion, deformation and cracking due to improper use, such as extrusion, impact and water ingress, etc., will cause the battery to be short-circuited, causing an explosion during the discharge or charging process.

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Lithium battery safety:

In order to avoid over-discharging or over-charging of the battery due to improper use, a triple protection mechanism is provided in the single-cell lithium ion battery. First, the switching element is used. When the temperature inside the battery rises, its resistance value rises. When the temperature is too high, the power supply will be automatically stopped. Second, the appropriate separator material is selected. When the temperature rises to a certain value, The micron-sized micropores on the separator will be dissolved automatically, so that the lithium ions will not pass, and the internal reaction of the battery will stop. The third is to set the safety valve (that is, the vent hole at the top of the battery). When the internal pressure of the battery rises to a certain value, the safety valve Automatically open to ensure the safety of the battery.

Sometimes, although the battery itself has safety control measures, if the control fails due to some reasons, and the safety valve or gas is not enough to be released through the safety valve, the internal pressure of the battery will rise sharply and cause an explosion. Under normal circumstances, the total energy stored in a lithium-ion battery is inversely proportional to its safety. As the battery capacity increases, the battery volume also increases, and its heat dissipation performance deteriorates, and the possibility of an accident increases greatly. For lithium-ion batteries for mobile phones, the basic requirement is that the probability of a safety accident is less than one in a million, which is the minimum standard acceptable to the public. For large-capacity lithium-ion batteries, especially for large-capacity lithium-ion batteries such as automobiles, it is particularly important to use forced cooling.

Choosing a safer electrode material and selecting lithium manganate material ensure that the lithium ion of the positive electrode is completely embedded in the carbon hole of the negative electrode in the molecular structure, which fundamentally avoids the generation of dendrites. At the same time, the stable structure of lithium manganate makes its oxidation performance far lower than that of lithium cobaltate. The decomposition temperature exceeds 100 °C of lithium cobaltate. Even if internal short circuit (acupuncture) due to external force, external short circuit, overcharge, it is fully capable. The danger of burning and explosion due to precipitation of metallic lithium is avoided.

 

In addition, the use of lithium manganate materials can also significantly reduce costs.

To improve the performance of existing safety control technology, it is first necessary to improve the safety performance of lithium-ion battery cells, which is especially important for large-capacity batteries. The separator with good thermal shutdown performance is selected. The function of the separator is to allow the passage of lithium ions while isolating the positive and negative electrodes of the battery. When the temperature rises, it is closed before the separator melts, so that the internal resistance rises to 2000 ohms, and the internal reaction is stopped. When the internal pressure or temperature reaches the preset standard, the explosion-proof valve will open and begin to relieve pressure to prevent excessive accumulation of internal gas, deformation, and eventually the housing burst. Improve control sensitivity, select more sensitive control parameters, and combine control with multiple parameters (this is especially important for high-capacity batteries). For large-capacity lithium-ion battery packs, it is composed of multiple cells in series/parallel. For example, the voltage of a notebook computer is 10V or more, and the capacity is large. Generally, 3 to 4 single cells can be connected in series to meet the voltage requirement, and then 2 ~3 series battery packs are connected in parallel to ensure a large capacity.

The large-capacity battery pack itself must have a relatively complete protection function. Two circuit board modules should also be considered: the ProtecTIon Board PCB module and the Smart Battery Gauge Board module. The complete battery protection design includes: level 1 protection IC (to prevent battery overcharge, over discharge, short circuit), level 2 protection IC (to prevent the second overvoltage), fuses, LED indications, temperature adjustment and other components. Under the multi-level protection mechanism, even in the case of abnormal power chargers and laptops, the laptop battery can only be switched to the automatic protection state. If the situation is not serious, it will still work normally after re-plugging. An explosion will occur.

The underlying technology used in lithium-ion batteries used in laptops and cell phones is unsafe and requires a safer structure.

In short, with the advancement of materials technology and the growing understanding of the requirements for the design, manufacture, testing and use of lithium-ion batteries, lithium-ion batteries in the future will become safer.


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