Lithium batteries are the fastest growing battery system in the past 20 years and are widely used in electronic products. The recent explosion of mobile phones and laptops is essentially a battery explosion. What cell phone and laptop batteries look like, how they work, why they explode, and how to avoid them.
Side effects begin to occur when the lithium cell is overcharged to a voltage higher than 4.2V. The higher the overcharge pressure, the higher the risk. At voltages higher than 4.2V, when less than half the lithium atoms are left in the cathode material, the storage cell often collapses, causing a permanent decline in battery capacity. If the charge continues, subsequent lithium metals will pile up on the surface of the cathode material, since the cathode’s storage cell is already full of lithium atoms. These lithium atoms grow dendritic crystals from the cathode surface in the direction of the lithium ions. The lithium crystals will pass through the diaphragm paper, shorting the anode and cathode. Sometimes the battery explodes before a short circuit occurs. That’s because during the overcharge process, materials such as electrolytes crack to produce gas that causes the battery casing or pressure valve to swell and burst, allowing oxygen to react with lithium atoms accumulated on the surface of the negative electrode and explode.
Therefore, when lithium battery charging, it is necessary to set the voltage upper limit, to take into account the battery life, capacity, and safety. The ideal charging voltage upper limit is 4.2V. There should also be a lower voltage limit when lithium cells discharge. When the cell voltage falls below 2.4V, some of the material begins to break down. And because the battery will self-discharge, put the longer the voltage will be lower, therefore, it is best not to discharge 2.4V to stop. From 3.0V to 2.4V, lithium batteries release only about 3% of their capacity. Therefore, 3.0V is an ideal discharge cut-off voltage. When charging and discharging, in addition to the voltage limit, the current limit is also necessary. When the current is too high, lithium ions do not have time to enter the storage cell, will accumulate on the surface of the material.
As these ions gain electrons, they crystallize lithium atoms on the surface of the material, which can be as dangerous as overcharging. If the battery case breaks, it will explode. Therefore, the protection of lithium ion battery should at least include the upper limit of charging voltage, lower limit of discharging voltage and upper limit of current. In general, in addition to the lithium battery core, there will be a protection plate, which is mainly to provide these three protection. However, the protection plate of these three protection is obviously not enough, the global lithium battery explosion events or frequent. To ensure the safety of battery systems, a more careful analysis of the cause of battery explosions is needed.
Cause of explosion:
1. Large internal polarization;
2.The pole piece absorbs water and reacts with the electrolyte gas drum;
3.The quality and performance of the electrolyte itself;
4.The amount of liquid injection can not meet the process requirements;
5. The laser welding seal performance is poor in the preparation process, and the air leakage is detected.
6. Dust and pole-piece dust are easy to cause microshort circuit first;
7.Positive and negative plate thicker than the process range, difficult to shell;
8. Sealing problem of liquid injection, poor sealing performance of steel ball leads to gas drum;
9.Shell incoming material shell wall is too thick, shell deformation affects the thickness;
10. The high ambient temperature outside is also the main cause of the explosion.
The explosion type
Explosion type Analysis The types of battery core explosion can be classified as external short circuit, internal short circuit, and overcharge. The external here refers to the external of the cell, including the short circuit caused by the poor insulation design of the internal battery pack. When a short circuit occurs outside the cell, and the electronic components fail to cut off the loop, the cell will generate high heat inside, causing part of the electrolyte to vaporize, the battery shell. When the internal temperature of the battery is high to 135 degrees Celsius, the diaphragm paper of good quality will close the fine hole, the electrochemical reaction is terminated or almost terminated, the current plunges, and the temperature also drops slowly, thus avoiding the explosion. But a diaphragm paper with a poor closing rate, or one that doesn’t close at all, will keep the battery warm, vaporize more electrolyte, and eventually burst the battery casing, or even raise the battery temperature to the point where the material burns and explodes. The internal short circuit is mainly caused by the burr of copper foil and aluminum foil piercing the diaphragm, or the dendritic crystals of lithium atoms piercing the diaphragm.
These tiny, needle-like metals can cause microshort circuits. Because the needle is very thin and has a certain resistance value, the current is not necessarily very large. The burrs of copper aluminum foil are caused in the production process. The observed phenomenon is that the battery leaks too fast, and most of them can be screened out by cell factories or assembly plants. And because the burrs are small, they sometimes burn off, making the battery back to normal. Therefore, the probability of explosion caused by burr micro short circuit is not high. Such a view, can often charge from the inside of each cell factory, the voltage on the low bad battery, but rarely explosion, get statistical support. Therefore, the explosion caused by internal short circuit is mainly caused by overcharge. Because there are needle-like lithium metal crystals everywhere on the overcharged rear electrode sheet, puncture points are everywhere, and micro-short circuit occurs everywhere. Therefore, the cell temperature will gradually rise, and finally the high temperature will electrolyte gas. This situation, whether the temperature is too high to make the material combustion explosion, or the shell was first broken, so that the air in and lithium metal fierce oxidation, are the end of the explosion.
But 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 consumers will stop charging and take their phones out before the battery is hot enough to burn materials and produce enough gas to burst the battery casing. The heat generated by the numerous short circuits slowly warms the battery and, after some time, explodes. The common description of consumers is that they picked up the phone and found it was very hot, then threw it away and exploded. Based on the above types of explosion, we can focus on the prevention of overcharge, the prevention of external short circuit, and improve the safety of the cell. Among them, the prevention of overcharge and external short circuit belongs to electronic protection, which is greatly related to the design of battery system and battery pack. The key point of cell safety improvement is chemical and mechanical protection, which has a great relationship with cell manufacturers.
Safe hidden trouble
The safety of lithium ion battery is not only related to the nature of the cell material itself, but also related to the preparation technology and use of the battery. Mobile phone batteries frequently explode, on the one hand, due to the failure of the protection circuit, but more importantly, the material aspect has not fundamentally solved the problem.
Cobalt acid lithium cathode active material is a very mature system in small batteries, but after a full charge, there are still a lot of lithium ions at the anode, when overcharge, remaining in the anode of lithium ion are expected to flock to the anode, is formed on the cathode dendrite is using cobalt acid lithium battery overcharge corollary, even in the normal charge and discharge process, There may also be excess lithium ions free to the negative electrode to form dendrites. The theoretical specific energy of lithium cobalate material is more than 270 mah/g, but the actual capacity is only half of the theoretical capacity to ensure its cycling performance. In the process of use, due to some reason (such as damage to the management system) and the battery charging voltage is too high, the remaining part of lithium in the positive electrode will be removed, through the electrolyte to the negative electrode surface in the form of lithium metal deposition to form dendrites. Dendrites Pierce the diaphragm, creating 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 overheats, it will lead to the oxidation and reduction of the carbonate in the electrolyte, resulting in a lot of gas and more heat. If there is no safety valve or the gas is not released through the safety valve, the internal pressure of the battery will rise sharply and cause an explosion.
Polymer electrolyte lithium ion battery does not fundamentally solve the safety problem, lithium cobalt acid and organic electrolyte are also used, and the electrolyte is colloidal, not easy to leak, will occur more violent combustion, combustion is the biggest problem of polymer battery safety.
There are also some problems with the use of the battery. An external or internal short circuit can produce a few hundred amperes of excessive current. When an external short circuit occurs, the battery instantly discharges a large current, consuming a large amount of energy and generating huge heat on the internal resistance. The internal short circuit forms a large current, and the temperature rises, causing the diaphragm to melt and the short circuit area to expand, thus forming a vicious cycle.
Lithium ion battery in order to achieve a single cell 3 ~ 4.2V high working voltage, must take the decomposition of the voltage is greater than 2V organic electrolyte, and the use of organic electrolyte in high current, high temperature conditions will be electrolyzed, electrolytic gas, resulting in increased internal pressure, serious will break through the shell.
Overcharge may precipitate lithium metal, in the case of shell rupture, direct contact with air, resulting in combustion, at the same time ignition electrolyte, strong flame, rapid expansion of gas, explosion.
In addition, for mobile phone lithium ion battery, due to improper use, such as extrusion, impact and water intake lead to battery expansion, deformation and cracking, etc., which will lead to battery short circuit, in the discharge or charging process caused by heat explosion.
Safety of lithium batteries:
In order to avoid overdischarge or overcharge caused by improper use, triple protection mechanism is set in single lithium ion battery. One is the use of switching elements, when the temperature of the battery rises, its resistance will rise, when the temperature is too high, will automatically stop the power supply; The second is to choose the appropriate partition material, when the temperature rises to a certain value, the micron pores on the partition will automatically dissolve, so that lithium ions can not pass, the battery internal reaction stops; The third is to set up the safety valve (that is, the vent hole on the top of the battery). When the internal pressure of the battery rises to a certain value, the safety valve will open automatically to ensure the safety of the battery.
Sometimes, although the battery itself has safety control measures, but because of some reasons caused by the control failure, the lack of safety valve or gas has no time to release through the safety valve, the internal pressure of the battery will rise sharply and cause an explosion. Generally, the total energy stored in lithium-ion batteries is inversely proportional to their safety. As the capacity of the battery increases, the volume of the battery also increases, and its heat dissipation performance deteriorates, and the possibility of accidents will greatly increase. For lithium-ion batteries used in mobile phones, the basic requirement is that the probability of safety accidents should be less than one in a million, which is also the minimum standard acceptable to the public. For large-capacity lithium-ion batteries, especially for automobiles, it is very important to adopt forced heat dissipation.
The selection of safer electrode materials, lithium manganese oxide material, in terms of molecular structure to ensure that in full charge state, the lithium ions in the positive electrode have been completely embedded into the negative carbon hole, fundamentally avoid the generation of dendrites. At the same time, the stable structure of lithium manganese acid, so that its oxidation performance is far lower than lithium cobalt acid, decomposition temperature of lithium cobalt acid more than 100℃, even because of external external short-circuit (needling), external short-circuit, overcharging, also can completely avoid the danger of combustion and explosion caused by precipitated lithium metal.
In addition, the use of lithium manganate material can also greatly reduce the cost.
To improve the performance of the existing safety control technology, we must first improve the safety performance of lithium ion battery core, which is particularly important for large capacity batteries. Choose a diaphragm with good thermal closing performance. The role of the diaphragm is to isolate the positive and negative poles of the battery while allowing the passage of lithium ions. When the temperature rises, the membrane is closed before it melts, raising the internal resistance to 2,000 ohms and shutting down the internal reaction. 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 lead to shell burst. Improve the control sensitivity, select more sensitive control parameters and adopt the combined control of multiple parameters (which is particularly important for large capacity batteries). For large capacity lithium ion battery pack is a series/parallel multiple cell composition, such as notebook computer voltage is more than 10V, large capacity, generally using 3 to 4 single battery series can meet the voltage requirements, and then 2 to 3 series of battery pack parallel, in order to ensure large capacity.
The high-capacity battery pack itself must be equipped with a relatively perfect protection function, and two kinds of circuit board modules should also be considered: ProtecTIonBoardPCB module and SmartBatteryGaugeBoard module. The whole battery protection design includes: level 1 protection IC (prevent battery overcharge, overdischarge, short circuit), level 2 protection IC (prevent second overvoltage), fuse, LED indicator, temperature regulation and other components. Under the multi-level protection mechanism, even in the case of abnormal power charger and laptop, the laptop battery can only be switched to the automatic protection state. If the situation is not serious, it often works normally after being plugged and removed without explosion.
The underlying technology used in lithium-ion batteries used in laptops and mobile phones is unsafe, and safer structures need to be considered.
In conclusion, with the progress of material technology and the deepening of people’s understanding of the requirements for the design, manufacture, testing and use of lithium ion batteries, the future of lithium ion batteries will become safer.
Post time: Mar-07-2022