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Lithium-ion Battery Composition And Short Circuit Protection
- Sep 05, 2018 -

Lithium-ion batteries (Li-ion Batteries) are the development of lithium batteries. Before introducing Li-ion, you should introduce the lithium battery. For example, the button battery used in the previous camera was a lithium battery. The positive electrode material of the lithium battery is manganese dioxide or thionyl chloride, and the negative electrode is lithium. After the battery is assembled, the battery has voltage and does not need to be recharged. Such a battery may also be charged, but the cycle performance is not good. Lithium dendrites are easily formed during the charge and discharge cycle, causing internal short circuits in the battery, so in general, such batteries are prohibited from being charged.

Later, Sony Corporation of Japan invented a carbon material as a negative electrode and a lithium-containing compound as a positive electrode. In the process of charge and discharge, no metal lithium exists, only lithium ions, which is a lithium ion battery. When the battery is charged, lithium ions are generated on the positive electrode of the battery, and the generated lithium ions move to the negative electrode through the electrolyte. The carbon as the negative electrode has a layered structure, and it has many micropores, and lithium ions reaching the negative electrode are embedded in the micropores of the carbon layer, and the more lithium ions are embedded, the higher the charging capacity. Similarly, when the battery is discharged (i.e., the process we use the battery), the lithium ions embedded in the carbon layer of the negative electrode come out and move back to the positive electrode. The more lithium ions return to the positive electrode, the higher the discharge capacity.

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What we usually call battery capacity refers to the discharge capacity. During the charge and discharge of Li-ion, lithium ions are in a state of motion from positive electrode to negative electrode to positive electrode. The Li-ion Batteries are like a rocking chair. The two ends of the rocking chair are the two poles of the battery, and the lithium ion runs back and forth in the rocking chair like an athlete. So Li-ion Batteries is also called a rocking chair battery.

The composition of the lithium ion battery:

Steel shell / aluminum shell / cylinder / flexible packaging series:

(1) Positive electrode - the active material is generally lithium manganate or lithium cobalt oxide, lithium nickel cobalt manganese oxide material, electric bicycle is generally used nickel cobalt cobalt manganate (commonly known as ternary) or ternary + small amount of lithium manganate, pure Lithium manganate and lithium iron phosphate are gradually faded out due to their large size, poor performance or high cost. The electrode fluid is an electrolytic aluminum foil having a thickness of 10 to 20 μm.

(2) Separator - a specially formed polymer film with a microporous structure that allows lithium ions to pass freely without electrons passing.

(3) Negative electrode - The active material is graphite, or carbon of approximately graphite structure, and the conductive current collector uses an electrolytic copper foil having a thickness of 7-15 μm.

(4) Organic electrolyte - a carbonate solvent in which lithium hexafluorophosphate is dissolved, and a polymer electrolyte is used as the polymer.

battery case - divided into steel shell (square type is rarely used), aluminum shell, nickel-plated iron shell (used in cylindrical battery), aluminum plastic film (soft packaging), etc., as well as the battery cap, but also the battery is positive Negative terminal.

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Diaphragm

At present, among all the raw materials used in the manufacture of lithium-ion batteries, only the diaphragm needs to be imported. The rest of the materials are produced domestically. So? Here we first introduce the following knowledge about the diaphragm.

In lithium-ion batteries, the role of the diaphragm is:

 

1. The positive electrode and the negative electrode are separated to avoid short circuit when the two electrodes are in contact;

2. Providing sufficient pores for the lithium ions, so that the lithium ions are deintercalated from the active material of the positive electrode during charging, and flow to the negative electrode through the separator; while discharging, the lithium ions are de-intercalated from the negative electrode through the diaphragm to the positive electrode. For this reason, there must be a gap in the diaphragm.

Since lithium-ion batteries are constantly moving toward large capacity, miniaturization and light weight, the diaphragm is required to meet the following requirements:

From the ability to improve the conduction of lithium ions:

(1) The diaphragm should have a large enough void ratio to facilitate the passage of lithium ions from the pores. The porosity is generally 30-60%?

(2) The separator material has a certain affinity for the electrolyte solution to accommodate the electrolyte solution. The above two points are to reduce the internal resistance of the battery to facilitate the flow of lithium ions, thereby reducing the internal resistance of the battery;

From the safety requirements of the battery:

(3) The pores should not be too large to avoid micro-short circuits;

(4) Have sufficient anti-puncture ability to avoid being pierced by some particles and causing partial short circuit;

(5) The separator material has sufficient chemical stability and does not chemically react with the electrolyte;

(6) The diaphragm should have the function of closing the pores. When the battery heats up in the event of an accident, the gap of the diaphragm is heated and melted and closed, preventing the lithium ions from flowing, that is, cutting off the current and preventing the fire and explosion. a big accident happened;

(7) The thermal shrinkage and deformability of the diaphragm should be as small as possible;

Consider the capacity of the battery:

(8) The diaphragm should be as thin as possible to reduce the space occupied by the diaphragm in the battery; the thickness of the diaphragm is generally 10-50 μm. For the sake of safety, the larger the capacity of the lithium-ion battery, the thicker the diaphragm used. For example, a general cell phone battery can use a 16 μm diaphragm, a 18650 battery uses a 25 μm diaphragm, and a power battery uses a 40 μm diaphragm.

From the perspective of processing performance:

(9)There is enough tensile strength so that the diaphragm will not be broken or torn when the battery is assembled.

The difficulty in making lithium-ion battery separators is that many of the above requirements are contradictory. For example, in order to reduce the internal resistance, the porosity of the separator is required to be high, but the higher the porosity of the separator, the worse the strength of the separator, and the more likely the short circuit occurs. Similarly, in order to increase the capacity, the diaphragm should be as thin as possible, but the thinner the strength. The worse.

Short circuit protection for lithium ion batteries:

Lithium-ion batteries have the risk of internal short-circuit due to many factors such as material system and manufacturing process. Although lithium-ion batteries have undergone strict aging and self-discharge screening at the factory, due to process failure and other unpredictable use factors, there is still a certain probability of failure leading to internal short circuits during use. For the power battery, the lithium-ion battery in the battery pack has several hundred or even tens of thousands of knots, which greatly enlarges the probability that the battery pack will be short inside. Due to the great energy contained in the power battery pack, the occurrence of internal short circuit is easy to induce a malignant accident, resulting in casualties and property losses.

For a parallel lithium-ion battery module, when one or more of the batteries are short-lived, other batteries in the battery module will discharge them, and the energy of the battery pack will rapidly increase the temperature of the short battery. It is easy to induce thermal runaway, which eventually causes the battery to explode. As shown in Figure 1

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Conventional temperature detection can inform the IC to cut off the main circuit when the battery is warming up, but it cannot prevent the continuous discharge inside the parallel battery module, and since the main circuit is cut off, all the energy of the battery module is concentrated on the inner short-circuit battery, but instead increases. The chance of a thermal runaway.

The ideal solution is to cut off the connection circuit between the battery and other batteries in the module when it is found that a certain battery is short and warm. As shown in Figure 2, the TE PPTC or MHP-TA series products are assembled on a single battery. When an internal short circuit occurs, the TE protection device can effectively block the connection between the internal short-circuit battery and other batteries in the module to prevent a serious accident. occur. For a power battery pack with a large number of single cells, the consistency of the internal resistance of the battery and the device is required when the group is assembled, and the MHP-TA has a very good consistency of the device resistance due to its internal bimetal structure, which can greatly satisfy the Battery internal resistance requirements.

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The system composition and actual road conditions of lithium-ion power batteries are complex, and the protection of passive devices is indispensable.


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