Lithium-ion battery characteristics
Lithium is the smallest and most lively metal on the chemical cycle table. Small volume and so high volume density, It is widely popular with consumers and engineers. But, the chemical characteristics is too lively, it brings an extremely high risk. Lithium metal when exposed to air, with oxygen produces intense oxidation and explosion. To improve safety and voltage, scientists have invented materials like graphite and lithium cobaltate to store lithium atoms. The molecular structure of these materials forms a small storage lattice that can be used to store lithium atoms. In this way, even if the battery shell breaks and oxygen enters, oxygen is too large for oxygen molecules to enter these small storage cells, so that lithium atoms do not contact with oxygen and avoid explosion. This principle of lithium-ion batteries enables people to achieve safety purposes while obtaining its high capacity density.
When the lithium ion battery is charged, the positive lithium atoms lose their electrons and oxidize to lithium ions. Lithium ions swim to the negative electrode through the electrolyte, enter the storage lattice of the negative electrode, and obtain an electron, reduced to lithium atoms. Upon discharge, the whole procedure turns upside down. To prevent a short circuit between the positive and negative poles, a diaphragm paper with many thin holes to prevent the 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 through, to waste the martial arts, to prevent danger.
Protective measures
Lithium battery cells can begin to have side effects after overcharging to a voltage above 3.7V. the higher the overcharge voltage ,the higher the risk has followed. After the lithium voltage is higher than 3.7V, the number of lithium atoms in the cathode material is less than half of the cathode material, when the storage lattice often collapses, causing a permanent decline in battery capacity. If charging continues, the subsequent lithium metal will accumulate on the surface of the negative material because the negative storage lattice is already filled with lithium atoms. These lithium atoms grow branch crystallization from the negative surface to the direction of lithium ions. These lithium metal crystals pass through the diaphragm paper to short circuit the positive and negative poles. Sometimes the battery explodes before the short circuit, because in the overcharging process, electrolyte and other materials will break the gas, causing the battery shell or pressure valve swoll to break, allowing oxygen to react with lithium atoms accumulated on the negative surface, and then explode. Therefore, when lithium battery charging, we must set the voltage limit, to take into account the life, capacity, and safety of the battery at the same time. Ideal charging voltage upper limit of 3.6V.
There should also be a low voltage limit for the lithium battery discharge. When the cell voltage is less than 2V, part of the material begins to be destroyed. And because the battery will self-discharge, the longer the battery not charged, the battery voltage will be lower. therefore, it is best not to discharge the cell volt to 2V . Lithium battery discharge capacity from 3.0V to 2.8V is only about 3% of the battery capacity. Thus, 3.0V is an ideal discharge cut off voltage.
In charging and discharge process, the current limit is also necessary. When the current is too large, the lithium ions are too late to enter the storage space and will gather on the surface of the material. These lithium ions obtain electrons, producing lithium atomic crystallization on the surface of the material, as with overcharging, causing dangerous. In case the battery shell breaks up, it explodes.
Therefore, the protection of lithium-ion battery include at least three aspects: charging voltage limit, discharge voltage limit and current limit. Generally, in the lithium battery pack, in addition to the lithium cell, there will be a Battery Management System（ BMS）, the BMS is mainly device to provide these three protection. However, the battery management system is obviously not enough, We have seen that lithium battery explosions offen happens around the world. To ensure the safety of the battery system, there is a more careful analysis of the cause of the battery explosion below:
Battery explosion reasons:
1: bigger Cell internal polarization!
2: Polar plate absorbs water and reacts with the electrolyte.
3: The quality and performance problems of the electrolyte itself.
4: The liquid injection amount does not meet the process requirements.
5: Poor sealing performance of laser welding during the assembly process, air leakage and leakage measurement.
6: Dust, polar dust is first easy to lead to micro short circuit, the specific reason is unknown.
7: The positive and negative electrode sheet is thicker than the process range, and it is difficult to enter the shell.
8: Injection sealing problem, and poor sealing performance of steel beads leads to gas drum.
9: The shell material is thick shell wall, and the shell deformation affects the thickness.
Analysis
The reasons of lithium battery explosion can be summarized as external short circuit, internal short circuit, and overcharging. The exterior here refers to the exterior of the cell, including the short circuit caused by the poor insulation design of the battery pack.
When a short circuit occurs outside ,the battery and the electronic devices fails to cut off the circuit, high heat will occur inside the cell, causing vaporization of some electrolyte and holding the battery shell large. When the internal temperature of the battery is high to 135 degrees Celsius, the quality diaphragm paper will close the fine hole, the electrochemical reaction ends or nearly terminate, the current drops sharply and the temperature will slowly drops, thus avoiding the explosion. However, the fine hole closing rate is too poor, or the diaphragm paper that does not close at all, will make the battery temperature continue to rise, vaporize more electrolyte, and finally break the battery shell, and even raise 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 or the bendlike crystallization of lithium atoms. These tiny needle-like metals can cause a micro short circuit. Because the needle is very fine with a certain resistance, the current will not be very large. Copper and aluminum foil burrs are caused in the production process, the observed phenomenon is that the battery leakage is too fast, most can be screened out by the core factory or assembly plant. Also, the tiny burr are small, sometimes burned, bringing the battery back to normal. Therefore, the probability of explosion caused by burr micro short circuit is not high.
all the lithium ion cell factory will find some bad quality batteries which will go low volt after charged shortly , but few explosions, refered to statistical data. Therefore, the explosion caused by the internal short circuit is mainly caused by overcharging. Because, the overcharged pole plate is full of needle lithium metal crystallization, puncture points are everywhere, which will cause micro short circuit . Therefore, the battery temperature will gradually rise, and finally the high temperature will be cause electrolyte gas. In the process, whether the high temperature makes the material combustion explosion, or the shell was broken first, It will let the air in and the lithium metal fierce oxidation, that will lead to explosion.
But the explosion caused by an internal short circuit did not offen occur at the time of charging. It is because that the battery temperature is not too high to make the material burn or the gas produced is not enough to break the battery shell, the user stops charging . At this time, the heat generated by many micro short circuits slowly increased the battery temperature, after a period of time, the explosion occurred.
Based on the above types of explosion, Torphan technical team focuses on the protection of overcharging, external short circuit prevention, and improving the safety of the cell to proventing explosion . Among them, overcharging protection and external short circuit prevention belong to electronic protection, which is greatly related to the design of battery system and battery pack installation. The focus of the cell safety improvement is chemical and mechanical protection, which is greatly related to the battery cell manufacturers.
Design Specification
Torphan battery management system can provide two safety protection for overcharging, overdischarge and overcurrent respectively, including the charger and battery pack.The Torphan charger turns AC to DC and limits the maximum current and maximum voltage of DC. The battery pack protection contains two part --- the battery management system and battery cell .For the first protection, the battery management system can communicate with the charger , it will send commends such as flow limiting and stopping charging signals to the charger , according to the collected battery information. When the charger receives the signal, the charger will automatically reduce the charging current or stop charging. When the charger fails to commnunicate with battery management system , the battery management system will disconnect the relay inside the battery pack and cut off the whole charging circuit, which is the second protection.This means that even if a circuit fails, the battery will not be overcharged and dangerous.
In short, during the battery system design , electronic protection is the first protection for overcharging, overdischarge and overcurrent. The battery Management system is the second protection.
Although the above methods provide two protection, Sometimes the consumers often buy a non-original charger to charge the batteries when charger is broken, so that they may buy a low quality charger or one charger which cannot communicate with battery management . This will lead first protection lost. Overcharging is the most important factor in the battery explosion, so inferior chargers can be called the culprit of the battery explosion.
Final line of defense
If the electronic protection fails, the last line of defense, will be provided by the cell. The safety level of the cell can be slightly differentiated based on whether the cell can be separated by external short circuit and overcharge. Because, before the battery explodes, the lithium atoms accumulate on the inside on the surface of the material. Moreover, the protection of overcharging is often because consumers use an inferior charger and have only one line of defense, therefore, the cell overcharging resistance is more important than the ability to resist the external short circuit.
