Reasons analysis of battery drum and explosion

I. Characteristics of lithium-ion batteries

Lithium is the smallest and most active metal on the chemical periodic table. Small size so high capacity density, popular with consumers and engineers. However, the chemical properties are too active and pose a high risk. When lithium metal is exposed to air, it will explode with intense oxidation reaction with oxygen. To boost safety and voltage, scientists have invented materials such as graphite and lithium cobalt to store lithium atoms. the molecular structure of these materials, forming a nanoscale fine storage lattice that can be used to store lithium atoms. That way, even if the battery shell breaks and oxygen enters, the oxygen molecules are too large to enter these tiny storage cells, so that lithium atoms do not contact with oxygen and avoid explosion. This principle of lithium ion battery makes people obtain its high capacity density, but also achieve the purpose of safety.

When lithium ion batteries are charged, the lithium atoms of the positive electrode will lose electrons and oxidize to lithium ions. Lithium ions travel through the electrolyte to the negative electrode, enter the storage cell of the negative electrode, and obtain an electron that is reduced to lithium atoms. When discharging, the whole program is reversed. To prevent the battery from being short-circuited by direct contact between the positive and negative electrodes, a diaphragm paper with many fine holes is added to the battery to prevent short-circuit. Good diaphragm paper can also automatically close the fine hole when the battery temperature is too high, so that lithium ions can not cross, in order to self-waste military work, to prevent danger.

Protection measures

After the lithium battery core is overcharged to a voltage of more than 4.2 V, side effects will begin to occur. The higher the overcharge pressure, the higher the risk. After the lithium core voltage is above 4.2 V, less than half the amount of lithium atoms left in the cathode material often collapses, causing a permanent drop in battery capacity. if continue charging, since the storage cell of the negative electrode is already filled with lithium atoms, the subsequent lithium metal will pile up on the surface of the negative electrode material. These lithium atoms grow dendritic crystals from the negative surface to the lithium ion. these lithium metal crystals will pass through the diaphragm paper, shorting the positive and negative electrodes. Sometimes the battery explodes before a short circuit occurs, because during overcharging, materials such as electrolytes crack to produce gas, causing the battery housing or pressure valve to swell and break, allowing oxygen to enter and react with lithium atoms stacked on the negative surface, and then explode. Therefore, when lithium battery is charged, the upper voltage limit must be set to take into account the battery life, capacity, and safety at the same time. Ideal charging voltage up to 4.2 V..

Lithium core discharge should also have a lower voltage limit. As the core voltage is below 2.4 V, part of the material will start to be destroyed. Because the battery will be self-discharge, the longer the voltage will be lower, so it is best not to discharge 2.4 V to stop. the lithium battery discharged from 3.0 V to 2.4 V during this period, the released energy is only about 3% of the battery capacity. hence ,3.0 V is an ideal discharge cutoff voltage.

charge and discharge, in addition to the voltage limit, the current limit is also necessary. When the current is too large, the lithium ion will not enter the storage cell before it can gather on the surface of the material. When these lithium ions obtain electrons, they produce lithium atomic crystallization on the surface of the material, which is as dangerous as overcharge. In case the battery case breaks, it explodes.

Therefore, the protection of lithium ion battery should include at least three items: upper limit of charging voltage, lower limit of discharge voltage and upper limit of current. In general lithium battery pack, in addition to lithium battery core, there will be a piece of protection board, this piece of protection board is mainly to provide these three protection. However, these three protections of the protection plate are obviously not enough, the global lithium battery explosion or frequency transmission. To ensure the safety of the battery system, the cause of the battery explosion must be analyzed more carefully.

II. Causes of battery explosion:

1： internal polarization is larger!

2： electrode absorbs water and reacts with electrolyte.

3： electrolyte itself quality, performance issues.

4： liquid injection can not meet the process requirements.

5： assembly process laser welding sealing performance is poor, air leakage, leak detection.

6： dust, polar dust first easily lead to micro-short circuit, the specific reasons are unknown.

7： positive and negative plates are thicker than the process range and difficult to enter the shell.

8： liquid injection sealing problem, steel bead sealing performance is not good resulting in gas drum.

9： the shell material is thick, the shell deformation affects the thickness.

III. ANALYSIS OF EXPLOSIVE TYPES

Battery core explosion can be classified into three types: external short circuit, internal short circuit, and overcharge. The outer part of the battery is the outer part of the core, including the short circuit caused by poor internal insulation design of the battery pack.

When the outside of the core short circuit, electronic components can not cut off the circuit, the core will produce high heat, resulting in part of the electrolyte vaporization, will support the battery shell. When the temperature inside the battery is as high as 135 degrees Celsius, the good quality diaphragm paper, will close the fine hole, the electrochemical reaction terminates or almost terminates, the current drops suddenly, the temperature also drops slowly, thus avoiding the explosion. However, the thin hole closure rate is too poor, or the thin hole will not close the diaphragm paper, will continue to increase the temperature of the battery, more electrolyte vaporization, and finally the battery shell burst, or even increase the battery temperature to make the material burn and explode.

The internal short circuit is mainly caused by the burr of copper foil and aluminum foil through the diaphragm, or the dendritic crystallization of lithium atoms through the diaphragm. These fine needle-like metals can cause micro-short circuit. Because the pin is very thin and has a certain resistance value, the current is not necessarily very large. Copper-aluminum foil burr system in the production process, the observed phenomenon is that the battery leakage is too fast, most can be screened by the core factory or assembly plant. Moreover, because the burr is small, sometimes will be burned, so that the battery return to normal. Therefore, the probability of explosion caused by burr micro short circuit is not high.

Such a statement, from each core plant often after charging, the voltage is low bad battery, but rarely explosion events, get statistical support. Therefore, the explosion caused by internal short circuit is mainly caused by overcharge. Because, overcharge the cathode sheet everywhere is needle-like lithium metal crystal, puncture point everywhere, everywhere is happening micro-short circuit. Therefore, the battery temperature will gradually increase, and finally the high temperature will electrolyte gas. In this case, whether the temperature is too high to cause the material to burn and explode, or the shell is first braced, so that the air and lithium metal into the intense oxidation, are the end of the explosion.

But the explosion caused by the internal short circuit caused by overcharging does not necessarily occur at the time of charging. It is possible that when the temperature of the battery is not high enough to let the material burn and the gas produced is not enough to break the battery shell, the consumer stops charging and takes the phone out. At this time, many of the micro-short circuit generated by the heat, slowly increase the temperature of the battery, after a period of time before the explosion. The common description of consumers is to pick up the phone found that the phone is very hot, thrown away and exploded.

Combined with the above types of explosion, we can focus on the prevention of overcharge, the prevention of external short circuit, and enhance the safety of the core. The overcharge and external short circuit protection belongs to electronic protection, which is related to battery system design and battery assembly. The key point of improving the safety of electric core is chemical and mechanical protection, which has great relation with battery core manufacturer.

IV. Design specifications

Because there are hundreds of millions of mobile phones around the world, the failure rate of security protection must be less than one in 100 million to achieve safety. As a result, the circuit board failure rate is generally much higher than one hundred million. Therefore, when the battery system is designed, there must be more than two safety lines. Common wrong design is to use the charger (adaptor) to go directly to the charging pool group. This will overcharge the protection of the heavy responsibility, completely to the battery pack on the protection board. Although the failure rate of the protection board is not high, but even if the failure rate is as low as one million, the probability of the world or every day will have an explosion accident.

If the battery system can provide two safety protection for overcharge, over-discharge and over-current respectively, if the failure rate of each protection is 1/10000, two protection can reduce the failure rate to 1/100000. Common battery charging system block diagram as follows, including charger and battery pack two parts. Charger also contains adapter (Adaptor) and charging controller two parts. The adapter converts AC to DC, while the charging controller limits the maximum current and voltage of DC. A battery pack consists of two parts, a protective panel and a battery core, and a PTC to limit the maximum current.

Charger text box: protection board overcharge, over-discharge over-current protection text box: battery pack text box: cell phone battery system as an example, overcharge protection system using the charger output voltage set at about 4.2 V to achieve the first layer of protection, so that even if the protection board on the battery pack failure, the battery will not be overcharged and dangerous. A second protection is the overcharge protection function on the protection board, which is generally set to V.4.3 In this way, the protection board usually does not have to be responsible for cutting off the charging current, only when the charger voltage is abnormally high, only need to act. Over-current protection is by the protection plate and current-limiting plate to be responsible, which is also two protection against over-current and external short circuit. as overdischarge will only occur in the process of electronic products being used. Therefore, the general design is by the electronic product circuit board to provide the first to protect, the protection board on the battery pack provides the second protection. If the electronic product detects that the supply voltage is less than 3.0 V, it should be shut down automatically. if this function is not designed at the time of the product design, the protection board will close the discharge loop when the voltage is as low as 2.4 V.

In short, the battery system design, must overcharge, over-discharge, and over-current respectively to provide two electronic protection. where the protection plate is the second protection. Take off the protection plate and charge, if the battery will explode, it means bad design.

Although the above method provides two protection, but because consumers often buy non-original factory charger to charge after the charger is broken, and the charger industry, based on cost considerations, often remove the charging controller to reduce costs. As a result, bad money drives out good, and there are many inferior chargers on the market. This makes overcharge and over-defense lose the first and most important line of defense. Overcharging is also the most important factor causing battery explosion, so inferior charger can be called the culprit of battery explosion.

Of course, not all battery systems use the scheme shown above. In some cases, there will also be a charge controller design within the battery group. For example: Many note-taking computers with additional battery rods have charging controllers. This is because note-taking computers generally do charging controllers inside the computer, giving consumers only one adapter. Therefore, the additional battery pack of the note-taking computer must have a charging controller to ensure the safety of the additional battery pack when charging with the adapter. In addition, the product that uses the car cigarette lighter to charge, sometimes also will charge the controller to do in the battery group.

Last line of defense

If electronic protection fails, the last line of defense will be provided by the core. The safety level of the core can be distinguished by whether the core can pass through external short circuit and overcharge. Because, before the battery explodes, if there are lithium atoms inside the material surface, the explosion power will be greater. Moreover, overcharge protection is often left with a line of defense because consumers use inferior chargers, so the ability of the core to resist overcharge is more important than the ability to resist external short circuit.