Briefly describe the protective measures and explosion causes of lithium batteries

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 is the mobile phone and laptop battery, how it work, why there is an explosion and how to avoid an explosion?

When the lithium battery cell is overcharged to a voltage higher than 4.2V, it will start to have side effects. The higher the overcharge voltage, the higher is 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 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, lithium batteries, be sure to set the upper limit of voltage, can simultaneously to the battery life, capacity, and security. The ideal maximum charging voltage of 4.2 V. Lithium batteries discharge voltage will be lower limit. When the cell voltage is lower than 2.4V, some materials will start to be destroyed. And because the battery self-discharge, put the more the longer the voltage is low, therefore, discharge when had better not to 2.4 V. Lithium battery discharge from 3.0 V to 2.4 V during this period, the release of energy accounted for only about 3% of the battery capacity. Therefore, is an ideal discharge cut-off voltage of 3.0 V. Charge and discharge, in addition to the limit of voltage and current limits can also be necessary. When the current is too large, lithium ions to enter the store, would gather in the material surface.

When these lithium ions acquire electrons, crystals of lithium atoms are generated on the surface of the material, which is dangerous as overcharge. In the event of a broken battery case, 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 board, 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.

Explosion reasons:

1, internal polarization is larger;

2, water absorption, react with the electrolyte gas drum;

3, electrolyte itself quality, and performance issues;

4, when liquid injection liquid quantity cannot meet the technical requirements;

5, preparation process of laser welding sealing performance is poor, when measuring leak leakage;

6, dust, dust sheet first can cause micro short circuit;

7, is a process from the cathode scope thick, hard into the shell;

8, filler sealing problem, steel ball seal performance is bad to cause gas drum;

9, the incoming exist shell wall thick, shell thickness deformation effect;

10, the outside temperature is the main cause of the explosion.

The explosion type

Explosion type analysis battery core type can be summarized as external short circuit, internal short circuit and overcharge 3 kinds. The external refers to the batteries of the external, contains a battery pack and poor internal insulation design caused by short circuit. When external short-circuited batteries, electronic components and failed to cut off the circuit, batteries inside will produce heat, caused part of the electrolyte vaporization, the battery shell support. When high battery internal temperature to 135 degrees Celsius, good quality paper, diaphragm will close pores, electrochemical reaction termination or near end, current, temperature is falling slowly, then avoid the explosion occurred. But the pores close rate is poor, or pores do not close the diaphragm paper, can let the battery temperature continues to rise, more electrolyte vaporization, finally the battery shell burst, and even to increase the battery temperature to make material combustion and explosion. 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 get a bad battery with a low voltage shortly after charging in each battery factory, but there are few explosion events and statistical support. Therefore, internal short circuit caused by the explosion, mostly because the overcharge. Because, after filling is everywhere on the pole piece needle lithium metal crystallization, Pierce point is everywhere, everywhere, in the event of a micro short circuit. As a result, the battery temperature rise gradually, and finally high-temperature electrolyte gas. This kind of situation, whether high temperature materials burning explosion, or the shell was broken, the air inside with lithium oxide, are explosive.

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.

Safe hidden trouble

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.

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, is also likely to have spare lithium ion cathode dendrite formation, cobalt acid lithium materials than the theory of energy is more than 270MAH/g, but in order to guarantee the cycle performance, the actual use capacity only half theoretical capacity. In use process, for some reason, such as management system damage caused by the battery voltage is too high, the remaining part of the positive electrode lithium will emerge, the electrolyte to the negative electrode surface in the form of metallic lithium dendrite deposit formation. Dendrite pierced diaphragm, form the 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, generating a large amount of gas and more heat. 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 for lithium ion batteries have not fundamentally solve the problem of security, also use lithium cobalt acid and organic electrolyte, and electrolyte as a gel, not easy to leak, will happen more violent combustion, the combustion is the biggest safety issue for polymer batteries.

Also there are some problems in using aspects, battery in external short circuit or internal short circuit will produce hundreds of ampere of large current. External short circuit instantaneous large-current discharge the battery, spend a lot of energy on internal resistance, produce huge quantity of heat. Large current internal short circuit, and melt temperature rising leads to the diaphragm, short circuit area is expanding, and form a vicious circle.

Lithium ion batteries to achieve single batteries only 3 ~ 4.2 V high working voltage, must be taken to decompose organic electrolyte voltage is greater than 2 V, and use organic electrolyte under the condition of high current, high temperature will be electrolysis, electrolytic produce gas, result in higher internal pressure, serious will break the shell.

Overcharge lithium may precipitate in the case of shell burst, direct contact with air and cause combustion, ignition electrolyte at the same time, a strong fire, gas rapid expansion, an explosion.

Lithium ion batteries for mobile phones and, moreover, because of improper use, such as extrusion, impact inflow result in cell expansion, deformation and crack, etc., these could lead to short circuit batteries, an explosion caused by heating in the discharge or charge process.

The safety of the lithium battery:

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), the battery internal pressure rises to a certain value, the valve opens automatically, to ensure the safety of the battery.

Sometimes the battery itself, although there is a safety control measures, but for some reason cause control failure, lack of relief valve or gas through the relief valve to release the battery internal pressure will rise sharply caused by the explosion. In general, the total energy of the lithium ion battery storage and its security is inversely proportional to, with the increase of battery capacity, battery volume is increasing, its thermal performance is poor. The possibility of accident will increase dramatically. With lithium ion batteries for mobile phones and the probability of safety accidents are the basic requirements to less than one over one million, this is also the social public can accept the lowest standard. For the large capacity lithium ion battery, especially with large capacity lithium ion battery such as cars, adopts the forced cooling is especially important.

Safer electrode material, selection of manganese acid lithium materials, in terms of molecular structure to ensure the full charge state, the anode of lithium ion has been completely embedded in the cathode carbon hole, fundamentally avoids the production of dendrite. Manganese acid lithium solid structure at the same time, make its performance is much lower than cobalt acid lithium oxide, decomposition temperature over 100 ℃, cobalt acid lithium even due to external force, internal short circuit (acupuncture), external short circuit, over charge, also can completely avoid the burning and explosion danger caused by precipitation of metal lithium.

In addition, with manganese acid lithium materials can greatly reduce the cost.

To improve the performance of existing safety control technology, we must first improve the safety performance of lithium-ion battery cells, which is especially important for large-capacity batteries. Diaphragm choose hot close good performance, the function of the diaphragm is battery is negative at the same time, the isolation of lithium ion will be allowed to pass. When the temperature rises, it is closed before the diaphragm melts, so that the internal resistance rises to 2000 ohms. Let internal response to stop it. When the internal pressure or temperature reaches preset criteria, explosion-proof valve will open and started unloading. In order to prevent the internal gas accumulation is overmuch, deformation, eventually led to the shell burst. Improve control sensitivity, and the choice of more sensitive control parameters and controlled by a combination of multiple parameters (this was especially important for large capacity battery).For large capacity lithium ion battery pack is composed of series/parallel multiple batteries, such as a laptop for more than 10 v voltage, large capacity, usually use 3 ~ 4 single battery series can meet the requirements of voltage, and then will be 2 ~ 3 series battery pack parallel, in order to make sure the larger capacity.

The large-capacity battery pack itself must have a relatively complete protection function. Two circuit board modules should also be considered: the Protect Circuit Board (ProtecTIonBoardPCB) 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 2protection 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, future lithium-ion batteries will become safer.

The page contains the contents of the machine translation.