Lithium-ion battery characteristics

Lithium is the smallest and most active metal on the chemical periodic table. Because of its small size and high capacity density, it is widely welcomed by consumers and engineers. However, the chemical properties are too active, which brings extremely high risks. When lithium metal is exposed to air, it will react violently with oxygen and explode. In order to improve safety and voltage, scientists invented materials such as graphite and lithium cobalt oxide to store lithium atoms. The molecular structure of these materials forms a nano-level small storage grid that can be used to store lithium atoms. In this way, even if the battery shell ruptures and oxygen enters, the oxygen molecules will be too large to enter these small storage cells, so that lithium atoms will not come into contact with oxygen and avoid explosion. This principle of lithium-ion batteries enables people to achieve safety while achieving high capacity density.

An electric explosion-proof test

When a lithium-ion battery is charged, the lithium atoms in the positive electrode lose electrons and are oxidized to lithium ions. Lithium ions swim to the negative electrode through the electrolyte, enter the storage cell of the negative electrode, and obtain an electron, which is reduced to lithium atoms. When discharging, the whole procedure is reversed. In order to prevent the positive and negative poles of the battery from directly touching and short-circuiting, a diaphragm paper with many pores is added to the battery to prevent short-circuiting. A good diaphragm paper can also automatically close the pores when the battery temperature is too high, so that lithium ions cannot pass through, so that they can use their own martial arts to prevent danger.

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After the lithium battery cell is overcharged to a voltage higher than 4.2V, side effects will begin to occur. The higher the overcharge voltage, the higher the risk. When the voltage of the lithium battery cell is higher than 4.2V, the number of lithium atoms remaining in the positive electrode material is less than half. At this time, the cell often collapses, causing a permanent decrease in battery capacity. If you continue to charge, since the cell of the negative electrode is already filled with lithium atoms, subsequent lithium metal will accumulate on the surface of the negative electrode material. These lithium atoms will grow dendrites from the surface of the negative electrode toward the direction of the lithium ions. These lithium metal crystals will pass through the separator paper and short-circuit 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 to produce gas, causing the battery shell or pressure valve to swell and rupture, allowing oxygen to enter and react with the lithium atoms accumulated on the surface of the negative electrode. And then exploded. Therefore, when charging a lithium battery, the upper voltage limit must be set so that the battery life, capacity, and safety can be taken into account at the same time. The most ideal upper limit of the charging voltage is 4.2V. There is also a lower voltage limit when discharging lithium batteries. When the cell voltage is lower than 2.4V, some materials will begin to be destroyed. Also, since the battery will self-discharge, the longer it is left, the lower the voltage will be. Therefore, it is best not to stop when the battery is discharged to 2.4V. During the period when the lithium battery is discharged from 3.0V to 2.4V, the energy released only accounts for about 3% of the battery 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 large, lithium ions will not have time to enter the storage cell and will accumulate on the surface of the material. After these lithium ions obtain electrons, they will produce lithium atom crystals on the surface of the material, which is the same as overcharging, which is dangerous. If the battery casing ruptures, it will explode.

Therefore, the protection of lithium-ion batteries must include at least three items: the upper limit of the charging voltage, the lower limit of the discharge voltage, and the upper limit of the current. In general, in a lithium battery pack, in addition to the lithium battery core, there will be a protective board. This protective board mainly provides these three protections. However, these three protections of the protection board are obviously not enough, and there are still frequent explosions of lithium batteries around the world. To ensure the safety of the battery system, the cause of the battery explosion must be analyzed more carefully.

Explosion type analysis

The types of battery cell explosion can be classified into three types: external short circuit, internal short circuit, and overcharge. The outside here refers to the outside of the battery cell, including short circuits caused by poor internal insulation design of the battery pack.

When a short circuit occurs on the outside of the cell and the electronic components fail to cut off the circuit, high heat will be generated inside the cell, which will cause part of the electrolyte to vaporize and expand the battery shell. When the internal temperature of the battery is as high as 135 degrees Celsius, a good quality diaphragm paper will close the pores, the electrochemical reaction will be terminated or almost terminated, the current will drop sharply, and the temperature will slowly drop, thus avoiding an explosion. However, the pore closure rate is too poor, or the pores are not closed at all. The diaphragm paper will cause the battery temperature to continue to rise, more electrolyte will vaporize, and finally the battery shell will be broken, or even the battery temperature will be increased to The material burns and explodes. The internal short circuit is mainly caused by the burrs of the 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 micro short circuits. Since the needle is very thin and has a certain resistance value, the current is not necessarily large.

The copper and aluminum foil burrs are caused during the production process. The observable phenomenon is that the battery leaks too fast, most of which can be screened by the battery cell factory or the assembly factory. Moreover, due to the small burrs, they will sometimes be burned, causing the battery to return to normal. Therefore, the probability of explosion caused by burr micro-short circuit is not high. This statement can be seen from the fact that there are often bad batteries with low voltage shortly after charging in various battery cell factories, but there are few explosions, which is supported by statistics. Therefore, the explosion caused by the internal short circuit is mainly caused by overcharge. Because after overcharge, there are needle-like lithium metal crystals everywhere on the pole piece, the puncture point is everywhere, and the micro short circuit occurs everywhere. Therefore, the battery temperature will gradually rise, and finally the high temperature will cause the electrolyte to gas. In this case, whether the temperature is too high to cause the material to burn and explode, or the outer shell is first broken, causing the air to enter and oxidize the lithium metal, it is an explosion.

However, the 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 to burn the material and the gas generated is not enough to break the battery casing, the consumer will stop charging and take the mobile phone out. At this time, the heat generated by numerous micro-short circuits slowly raises the temperature of the battery, and it explodes after a period of time. The common description of consumers is that when they pick up the phone, they find that the phone is very hot and explode after throwing it away.

Based on the above types of explosions, we can focus on three aspects of explosion protection: the prevention of overcharge, the prevention of external short circuits, and the improvement of cell safety. Among them, overcharge prevention and external short circuit prevention belong to electronic protection, which has a greater relationship with battery system design and battery assembly. The focus of battery cell safety enhancement is chemical and mechanical protection, which has a greater relationship with battery cell manufacturers.