Dry goods sharing: 18650 lithium battery knowledge analysis

More than three hundred years ago, Benjamin Franklin began to capture lightning, which opened up people's awareness of lightning and opened up people's understanding of electricity. From that time on, many people were considering how to store the captured electricity, so Gradually the battery has been produced. Indeed, as a very important invention in history, the emergence of batteries has indeed solved many problems, greatly improving various life efficiencies, and the emergence of lithium batteries has made people more The understanding of electricity is getting deeper and deeper. The 18650 lithium battery is the lithium battery with the longest use time and still exists. The shadow of 18650 lithium battery can be seen in many local areas, because this battery is compared with other batteries. Battery, the advantages are very obvious, and the security is relatively good, even has been used on large equipment, this article will introduce you to the knowledge of 18650 battery, and explain why there will be an explosion.

18650 lithium battery

First, the advantages of 18650 lithium battery

It is generally believed that the lithium battery's no-load voltage is below 3.0V and it is considered that the power is used up. (The specific value needs to look at the threshold of the battery protection board, such as low to 2.8V and 3.2V). Most lithium battery discharges can not put the no-load voltage below 3.2V, otherwise excessive discharge will damage the battery (usually the lithium battery on the market is basically used with a protective plate, so over-discharge will also cause the protection board to detect To the battery, it is impossible to charge the battery).

4.2V is the highest limit voltage for battery charging. It is generally considered that charging the lithium battery's no-load voltage to 4.2V is considered to be full of electricity. During battery charging, the battery voltage gradually rises to 4.2V at 3.7V, and the lithium battery cannot be charged. Charge the no-load voltage to 4.2V or higher, otherwise it will damage the battery. This is the special place for lithium batteries. Generally speaking, the 18650 lithium battery has the following advantages.

1, a wide range of uses

Electronic devices such as laptops, walkie-talkies, portable DVDs, instrumentation, audio equipment, model airplanes, toys, video cameras, digital cameras, etc.

2, series

It can be combined in series or in parallel to form a 18650 lithium battery pack.

3, the internal resistance is small

The internal resistance of the polymer battery core is smaller than that of the general liquid battery. The internal resistance of the domestic polymer battery can even be less than 35mΩ, which greatly reduces the self-consumption of the battery and prolongs the standby time of the mobile phone. The level of international integration. This polymer lithium battery that supports large discharge currents is an ideal choice for remote control models and is the most promising alternative to nickel-metal hydride batteries.

4, no memory effect

It is not necessary to empty the remaining power before charging, which is convenient to use.

5, high voltage

The voltage of the 18650 lithium battery is generally 3.6V, 3.8V and 4.2V, which is much higher than the 1.2V voltage of NiCd and NiMH batteries.

6, high security performance

18650 lithium battery has high safety performance, no explosion, no burning; non-toxic, non-polluting, RoHS-certified; all kinds of safety performance in one go, the number of cycles is more than 500 times; high temperature resistance is good, discharge efficiency is up to 100% under 65 degrees . In order to prevent battery short circuit, the positive and negative poles of the 18650 lithium battery are separated. So the possibility of its short circuit has been reduced to the extreme. A protective plate can be added to prevent the battery from overcharging and over-discharging, which also extends the battery life.

7, long service life

The 18650 lithium battery has a long service life, and the cycle life can reach more than 500 times in normal use, which is more than twice that of ordinary batteries.

8, large capacity

The capacity of 18650 lithium battery is generally between 1200mah and 3600mah, and the general battery capacity is only about 800. If you combine 18650 lithium battery to 18650 lithium battery pack, then the 18650 lithium battery pack can break through 5000mah casually.

Second, 18650 lithium battery comparison polymer battery

The so-called polymer lithium battery refers to a lithium ion battery using a polymer as an electrolyte, and is specifically classified into two types of "semi-polymer" and "all-polymer". "Semi-polymer" means applying a layer of polymer (typically PVDF) to the separator to make the cell more adhesive, the battery can be made harder, and the electrolyte is still a liquid electrolyte. By "all polymer" is meant the use of a polymer to form a gel network inside the cell and then injecting an electrolyte to form an electrolyte. Although "all-polymer" batteries still use liquid electrolytes, they are used in much smaller quantities, which greatly improves the safety performance of lithium-ion batteries. As far as I know, only SONY is now mass-produced "all-polymer" lithium-ion batteries.

On the other hand, a polymer battery refers to a lithium ion battery using an aluminum plastic packaging film as an outer package, which is also known as a soft pack battery. The packaging film is composed of three layers, a PP layer, an Al layer and a nylon layer. Since PP and nylon are polymers, such a battery cell is called a polymer battery.

1, the price is lower

The international price of 18650 is about 1USD/pcs, which is about 3RMB/Ah according to 2Ah. The price of polymer lithium battery, the low-end cottage factory is 4RMB/Ah, the middle end is 5~7RMB/Ah, and the middle and high end is above 7RMB/Ah. Like ATL, Lishen, it can usually sell around 10RMB/Ah, and you are not willing to pick up.

2, can not be customized

SONY has always wanted to make lithium-ion batteries the same as alkaline batteries. There are certain industry standards, like the 5th and 7th batteries, which are basically the same in the world. However, because of the great advantage of lithium-ion battery, it can be designed according to the customer's needs. It is impossible to have a unified standard. So now, the lithium-ion battery industry is basically only a standard model of 18650. Others are based on customers. The requirements are designed.

3, poor security

We know that lithium-ion batteries in extreme cases (such as overcharge, high temperature, etc.), there will be a violent chemical reaction inside, generating a large amount of gas. The 18650 battery uses a metal casing with a certain strength. When the internal air pressure reaches a certain level, the steel shell will rupture and explode, causing a terrible safety accident.

This is why rooms that test 18650 batteries generally must be protected in layers, and they must not be tested when tested. Polymer batteries do not have this problem. Even in extreme cases, because of the low strength of the packaging film, as long as the air pressure is slightly higher, it will rupture and will not explode. The worst case is burning. Therefore, the polymer battery is superior to the 18650 battery in terms of safety.

4, low energy density

The average 18650 battery capacity can be about 2200mAh, so the energy density is about 500Wh/L, and the energy density of the polymer battery can be close to 600Wh/L.

But polymer batteries also have their own drawbacks. Mainly because of the higher cost, because it can be designed according to customer needs, the research and development costs inside should be counted. Moreover, the variety of shapes and types, resulting in various fixtures in the manufacturing process are non-standard parts, and correspondingly increased costs. The versatility of the polymer battery itself is poor, which is also brought about by the flexible design. It is often necessary to redesign the customer for the difference of 1mm.

Third, the application of 18650 lithium battery

The application of lithium secondary batteries in the energy battery industry benefits from its superior performance. With the continuous development of lithium secondary battery technology, it has been widely used in our daily life; rechargeable batteries that are used repeatedly are increasingly favored by consumers. Pay attention to the energy battery industry, fully understand the battery and let us do it for me. At present, the 18650 lithium battery has certain applications in the following fields.

1, energy storage class

Mainly used in base station power, clean energy storage, grid power storage, home optical storage system.

2, power class

Mainly refers to electric vehicles, electric bicycles, new energy vehicles and so on.

3, digital class

Mobile phones, tablets, laptops, electric toys, MP3/MP4, headphones, charging treasures, model airplanes, mobile power supplies, etc.

Fourth, 18650 lithium battery classification

18650 lithium battery production needs to have a protection circuit to prevent the battery from being overcharged and discharged. Of course, this is a must for lithium batteries, which is also a disadvantage of lithium batteries, because the materials used in lithium batteries are basically lithium cobalt oxide materials, and lithium batteries of lithium cobalt oxide materials can not discharge at high current, safety Poor, from the classification point of view, the classification of 18650 lithium batteries can be classified by the following methods.

1, according to the battery practical performance classification

Power battery and energy battery. Energy-based batteries are characterized by high energy density and are mainly used for high-energy output. Power-type batteries are characterized by high power density and are mainly used for batteries with instantaneous high-power output and output. Power-energy lithium batteries have emerged with the advent of plug-in hybrid vehicles. It requires the battery to store higher energy, can support pure electric driving for a distance, and also has better power characteristics, and enters the hybrid mode when the battery is low.

Simple understanding, the energy type is similar to the marathon player. It requires endurance, that is, it requires high capacity, and it does not require high current discharge performance. Then the power type is a sprinter, and it is a bursting force, but there is also endurance, otherwise the capacity is too Small will not run far.

2, according to electrolyte materials

Lithium-ion batteries are classified into liquid lithium ion batteries (LIB) and polymer lithium ion batteries (PLB).

Liquid lithium-ion batteries use liquid electrolytes (mostly current power batteries). Polymer lithium-ion batteries are replaced by solid polymer electrolytes, which can be either "dry" or "colloidal", and most currently use polymer gel electrolytes. With regard to solid state batteries, strictly speaking, both the electrodes and the electrolyte are solid.

3, by product appearance classification

Divided into: cylindrical, soft bag, square.

Cylindrical and square outer packagings are mostly steel or aluminum. The outer packaging of the soft bag is an aluminum plastic film. In fact, the soft bag is also a square. The market is accustomed to packaging the aluminum plastic film as a soft bag, and some people refer to the soft battery as a polymer battery.

For cylindrical lithium-ion batteries, the model is typically 5 digits. The first two digits are the diameter of the battery and the middle two digits are the height of the battery. The unit is millimeters. For example, a 18650 lithium battery has a diameter of 18 mm and a height of 65 mm.

4, according to the polar material classification

Cathode material: lithium iron phosphate battery (LFP), lithium cobalt oxide battery (LCO), lithium manganese oxide battery (LMO), (binary battery: lithium nickel manganese oxide / lithium nickel cobaltate), (ternary: nickel cobalt manganese Lithium acid battery (NCM), nickel cobalt aluminum aluminate battery (NCA).

Anode material: lithium titanate battery (LTO), graphene battery, nano carbon fiber battery.

Regarding the graphene concept on the market, it mainly refers to a graphene-based battery, that is, a graphene paste is added to a pole piece, or a graphene coating layer is added to a separator. Lithium nickelate and magnesium based batteries are basically non-existent in the market.

5. Charge and discharge of 18650 lithium battery

According to industry standards, for lithium batteries, the nominal capacity is the minimum capacity. This capacity is a batch of batteries. At room temperature of 25 degrees, it is first filled with cc/cv0.5c and allowed to stand for a while ( Usually 12 hours). With a constant current discharge of 0.2c current to 3.0v (there is also a standard of 2.75V, but the effect is not big, 3v to 2.75v drops quickly and not much capacity), the capacity value released, because a batch of batteries must have individuals The difference is the actual capacity of the battery with the lowest capacity. That is to say, the actual capacity of any one of these batteries should be greater than or equal to the nominal capacity.

1, 18650 lithium battery charging process

Some chargers are implemented using inexpensive solutions, which are not good enough in control accuracy, which may cause abnormal battery charging and even damage the battery. When purchasing the charger, try to choose the 18650 lithium-ion battery charger of the big brand. The quality and after-sales guarantee, and extend the service life of the battery. The brand-assured 18650 lithium-ion battery charger has four protections: short circuit protection, over current protection, over voltage protection, and battery reverse connection protection. Overcharge protection: When the charger overcharges the Li-ion battery, it is necessary to terminate the charging state in order to prevent the internal pressure from rising due to temperature rise.

To this end, the protection device needs to monitor the battery voltage. When it reaches the battery overcharge voltage, it activates the overcharge protection function and stops charging. Over-discharge protection: In order to prevent over-discharge of the lithium-ion battery, when the lithium-ion battery voltage is lower than its over-discharge voltage detection point, the over-discharge protection is activated, the discharge is stopped, and the battery is kept in a standby mode with low quiescent current. Overcurrent and short circuit protection: When the discharge current of the lithium ion battery is too large or a short circuit occurs, the protection device activates the overcurrent protection function.

Lithium battery charging control is divided into two stages. The first stage is constant current charging. When the battery voltage is lower than 4.2V, the charger will charge at a constant current. The second stage is the constant voltage charging phase. When the battery voltage reaches 4.2V, due to the characteristics of the lithium battery, if the voltage is higher, it will be damaged. The charger will fix the voltage at 4.2V, and the charging current will gradually decrease. When it is reduced to a certain value (generally 1/10 set current), the charging circuit is cut off, the charging completion indicator lights up, and charging is completed.

Excessive charging and discharging of lithium-ion batteries can cause permanent damage to the positive and negative electrodes. Over-discharge leads to collapse of the negative carbon sheet structure, and collapse can cause lithium ions to be inserted during charging; excessive charging causes excessive lithium ions to be embedded in the negative carbon structure, and some of the lithium ions can no longer be released.

2, 18650 lithium battery charging principle

The working principle of lithium-ion battery refers to its charging and discharging principle. 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. The 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.

By the same token, when the battery is discharged (ie, 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. What we usually call battery capacity refers to the discharge capacity.

It is not difficult to see that during the charging and discharging process of a lithium ion battery, lithium ions are in a state of motion from a positive electrode to a negative electrode to a positive electrode. If we compare the lithium-ion battery to a rocking chair, the two ends of the rocking chair are the two poles of the battery, and the lithium ion is like an excellent sportsman, running back and forth at both ends of the rocking chair. Therefore, the experts gave the lithium-ion battery a cute name rocking chair battery.

6. Why does the 18650 lithium battery have a history of explosion?

Most of the 18650 batteries are packaged in a steel case. The battery with poor quality does not have a protection function. In the case of overcharging (overcharging), the internal pressure will suddenly increase, and when it exceeds the value, it will explode. Problems such as short circuits, excessive temperatures, or battery deformation or even puncture may cause the battery to explode.

After 30 years of development, the 18650 battery preparation process has been very mature, in addition to the performance has been greatly improved, its security is also very perfect. In order to avoid explosion of the sealed metal casing, the 18650 battery is now equipped with a safety valve at the top. The safety valve is standard for every 18650 battery and is the most important explosion-proof barrier.

When the internal pressure of the battery is too large, the top safety valve will open the exhaust pressure to avoid explosion. However, when the safety valve is opened, the chemical leaking inside the battery will chemically react with the oxygen in the air under high temperature conditions, and there is still a possibility of fire. In addition, some 18650 batteries also come with a protective board, with overcharge and over-discharge and short-circuit protection, and the safety performance is very high.

The previous incidents of mobile power explosions were all caused by unscrupulous manufacturers using unqualified 18650 batteries for cost savings or even second-hand waste. At present, the 18650 batteries produced by major battery manufacturers such as Panasonic, Sony, Samsung, etc. are actually very safe, and the 18650 battery usage rate is very high. We can use it properly in daily use and avoid battery short circuit, damage or The temperature is too high, so you don't have to worry about the battery exploding. We can't overwhelm a boatman with a bamboo raft to characterize the safety of the 18650 with a few inferior products.

Seven, summary: In fact, for lithium batteries, the cause of the explosion is understandable, after all, in the past, the stability of this stability will not be so powerful, but 18650 as a performance of very high contrast lithium Battery, its application will certainly become more and more extensive, after all, for this indispensable battery, 18650 lithium battery is the best choice.

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