Lithium-ion Battery: Structure, Working Principle and Package

Topics covered in this article:

Ⅰ. What is a lithium-ion battery?

Ⅱ. How do lithium-ion batteries work？

Ⅲ. Lithium-ion battery structure

Ⅳ. Lithium-ion battery package technology

Ⅴ. Main equipment for lithium-ion battery production

Ⅰ. What is a lithium-ion battery?

Lithium batteries are divided into lithium batteries and lithium-ion batteries. Both mobile phones and laptops use lithium-ion batteries, commonly known as lithium batteries. Real lithium batteries are rarely used in daily electronic products because of their great danger.

Figure. 1

Figure. 1

Lithium-ion batteries are rechargeable batteries that mainly rely on lithium ions moving between the positive and negative electrodes to work. In the process of charging and discharging, Li+ is embedded and de-embedded back and forth between the two electrodes: when charging the battery, Li+ is de-embedded from the positive electrode and embedded in the negative electrode through the electrolyte, which is in a lithium-rich state; when discharging, the opposite is true. Generally, lithium-containing materials are used as electrodes for batteries, which are representative of modern high-performance batteries.

Ⅱ. How do lithium-ion batteries work？

Lithium-ion batteries use carbon materials as the negative electrode and lithium-containing compounds as the positive electrode. There is no lithium metal, only lithium-ion, which is a lithium-ion battery. Lithium-ion batteries refer to batteries with lithium-ion embedded compounds as cathode materials. The charging and discharging process of lithium-ion batteries is the embedding and de-embedding process of lithium ions. During the embedding and de-embedding of lithium ions, it is accompanied by the embedding and de-embedding of electrons equivalent to lithium ions (habitually, the positive electrodes are represented by embedding or de-embedding, while the negative electrodes are represented by insertion or de-inserting). During charging and discharging, lithium ions are embedded/deemed and inserted/unplugged back and forth between positive and negative electrodes, which is vividly called "rocking chair batteries".

Figure. 2

Figure. 2

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. As an anode, the carbon is layered. It has many micropores. Lithium ions that reach the negative electrode are embedded in the micropores of the carbon layer. The more lithium ions embedded, the higher the charging capacity. Similarly, when the battery is discharged (that is, the process we use the battery), the lithium ions embedded in the negative carbon layer are released and move back to the positive pole. The more lithium ions return to the positive electrode, the higher the discharge capacity.

Generally, the charging current of lithium batteries is set between 0.2C and 1C. The greater the current, the faster the charging, and the greater the heating of the battery. Moreover, if the current is too large to charge, the capacity is not enough, because the electrochemical reaction inside the battery takes time. Just like pouring beer, if you pour it too fast, it will cause bubbles and be dissatisfied.

For batteries, normal use is the process of discharge.

Lithium battery discharge needs to pay attention to a few points:

First, the discharge current should not be too large. Excessive current will cause heat inside the battery, which may cause permanent damage. On the mobile phone, there is no problem with this. You can not consider it.

Second, never overcharge! Lithium batteries are most afraid of over-discharge. Once the discharge voltage is lower than 2.7V, it may cause the battery to be scrapped. Fortunately, a protection circuit has been installed inside the mobile phone battery. The voltage is not low enough to damage the battery, and the protection circuit will work and stop discharging. As can be seen from the figure, the greater the discharge current of the battery, the smaller the discharge capacity, and the faster the voltage drop.

Ⅲ. Lithium-ion battery structure

Figure. 3

Positive electrode: active substance, conductive, solvent, adhesive, matrix.

Figure. 4

When the battery discharges, the electron electrode is obtained from the external circuit, and the electrode is reduced at this time. It is usually a high-potential electrode. Lithium cobaltate, lithium manganate electrodes, etc. in lithium-ion batteries.

Negative electrode: active substances (graphite, MCMB, CMS), adhesives, solvents, matrix.

Figure. 5

When the battery discharges, the electrode transmits electrons to the external circuit, at which time the electrode has an oxidation reaction. Usually low-potential electrodes, graphite electrodes in lithium-ion batteries.

Estrangement

Figure. 6

A diaphragm is a device placed between the poles as an isolation electrode to avoid a short circuit inside the battery caused by direct contact with the active substances on the poles. However, the diaphragm still needs to allow charged ions to pass through to form a pathway.

Diaphragm requirements:

1. Large ion transmittance

2. Appropriate mechanical strength

3. It is an insulator itself

4. Do not react with electrolytes and electrodes

Material: single-layer PE (polyethylene) or three-layer composite PP (polypropylene) + PE+PP

Thickness: The single layer is generally 0.016-0.020mm, and the third layer is generally 0.020-0.025mm.

Electrolyte

Case hardware (steel case, aluminum case, cover plate, pole ear, insulator, insulating tape)

Lithium-ion battery cell raw materials

Cathode material

Cathode materials have the largest market capacity and high added value in lithium batteries, accounting for about 30% of the cost of lithium batteries, and the gross profit margin ranges from 15% to more than 70%.

At present, the cathode materials that have been applied to lithium batteries in batches mainly include lithium cobaltate, lithium manganate, lithium nickel, lithium cobalt-nickel manganate, and lithium iron phosphate.

Lithium nickelate batteries are the least safe (excessive charging is easy to catch fire), have the lowest high-temperature tolerance (high-temperature decomposition), and are the most difficult synthesis.

Lithium cobaltate was first commercially applied. The technology has matured so far and has been widely used in small and low-power portable electronic products, such as mobile phones, laptops, and digital electronic products.

Lithium iron phosphate, as a cathode material for lithium-ion batteries, has good electrochemical properties. The charging and discharging platform is very stable and the structure is stable during the charging and discharging process. At the same time, the material is non-toxic, non-polluting, has good safety performance, can be used in a high-temperature environment, and has a wide range of raw materials. It is a hot topic in the battery industry at present.

Negative electrode material

Anode materials account for a low proportion of lithium electricity costs, mainly carbon anode materials and non-carbon anode materials.

Carbon anode material: widely used in commercial lithium-ion batteries at present.

Advantages: safe, long cycle life, low price, and non-toxic.

Disadvantages: The mass is lower than the energy.

Non-carbon anode materials: According to their composition, it is divided into lithium transition metal nitrides, transition metal oxides and nanoalloy materials.

Advantages: It has a high volume energy density.

Disadvantages: poor cyclic stability, large irreversible capacity, high preparation cost, and not yet industrialized.

In the future, the goal of anode materials is to improve capacity and cyclic stability. Carbon materials are composited with various high-capacity non-carbon anode materials to develop high-capacity and non-carbon composite anode materials.

Diaphragm material

Market-oriented diaphragm materials are mainly polyolefin diaphragms dominated by polyethylene (PE) and polypropylene (PP). Among them, PE products are mainly made by wet processes, and PP products are mainly Made by dry processes.

Figure. 7

Figure. 7

Comparison of the characteristics of PE products and PP products:

1. PP is relatively resistant to high temperature, and PE is relatively resistant to low temperature;

2. PP density is smaller than PE;

3. The melting point and closed-hole temperature of PP are higher than PE;

4. PP products are crisper than PE products;

5. PE is more sensitive to environmental stress.

The main diaphragm material products are single-layer PP, single-layer PE, PP+ ceramic coating, PE+ ceramic coating, double-layer PP/PE, double-layer PP/PP, and three-layer PP/PE/PP, etc. Among them, the first two types of products are mainly used in the field of 3C small batteries, and the latter types of products are mainly used for power lithium batteries. Field.

Among the products of diaphragm materials for power lithium batteries, double-layer PP/PP diaphragm materials are mainly produced by Chinese enterprises and used in mainland China, mainly because there is no technology and ability of Chinese enterprises to make PP and PE into double-layer composite films at this stage. The diaphragm used by lithium automotive power batteries around the world is mainly three-layer PP/PE/PP, double-layer PP/PE, PP+ ceramic coating, PE+ ceramic coating and other diaphragm material products. At the same time, some other new diaphragm material products are also emerging and starting to be applied. However, due to low quantity and high price, they are mainly used in the field of power lithium battery manufacturing. These products mainly include: coated polyester film (PET, Polyethylene Terephthalate), cellulose film, polyimide film (PI), polyamide film (PA), spandex, or aramid film, etc. These diaphragms have the advantages of high-temperature resistance, low-temperature output, long charging cycle life and moderate mechanical strength. Generally speaking, lithium battery diaphragm material products show an obvious diversified development trend.

Electrolyte

Lithium-ion battery electrolyte materials focus on high safety and high environmental adaptability. The main development will focus on: new solvents (widening the range of working temperature), ionic liquids, new lithium salts (improving environmental adaptability), additives (flame retardant, redox shuttle, protection of positive and negative electrode film formation, etc.), and the new positive, anode materials are matched to improve safety, power and capacity, and eventually be safely and conveniently applied to electric vehicles, energy storage, aerospace and a wider range of fields.

The manufacturing process of lithium-ion batteries

Figure. 8

Figure. 8

Ⅳ. Lithium-ion battery package technology

In addition to raw materials, packaging technology also has a significant impact on the final performance of lithium batteries. Even if the material formulation is the same, different processing processes can produce different finished products in terms of safety, energy density, and cycle life.

Currently, packaging technologies can be divided into three categories:

A square battery is a square single battery. The core gap of this type of battery is smaller, the internal material is closer, the battery is not easy to expand under the limit of high hardness, and the safety is relatively high. At the same time, the shell adopts aluminum-magnesium alloy with lower density, lighter weight, and higher strength to further strengthen the internal protection, but the corresponding production process is not complicated. But the consistency of the square battery is poor, and because it can be customized according to the needs of production, there are many models on the market, the process is not unified.

Consistency refers to that the initial performance of each battery string is similar, such as capacity, temperature, and circulation. If the performance of a single battery is too different, the service life of the battery string will be severely affected.

Although the round battery and the square battery belong to the same hard shell packaging route, the size is smaller, the cell consistency is good, the energy density of the single cell is relatively high, the group is more flexible, the production process is mature and the cost is low. The defect is that the overall performance in general, the number of cells in the battery pack is large, the weight is large, and the cylindrical form is not good for space utilization, resulting in low energy density.

The performance of the soft-pack battery is the best of the three routes, with flexible size, high energy density and light weight. But the mechanical strength is not high, the production process is more complex, the production cost is high, the cost performance is general.

In addition to the three mature packaging technologies, there are currently new CTP technologies for lithium batteries, and derived from "blade battery" and "CTP battery" two new products, both of which are the upgraded form of square batteries.

The Cell To Pack (CTP) technology enables cells To be grouped directly, skipping the intermediate step of battery modules. On the one hand, this technology improves the utilization rate of space in the battery pack and increases the amount of power; On the other hand, the weight is reduced, and the energy density of the battery pack increases dramatically.

At present, the blade battery represented by BYD chooses to completely cancel the module; Nind era CTP batteries, on the other hand, took the route of integrating small modules into large modules.

These two routes have their own advantages and disadvantages, but both are in the early stage of commercialization, and the manufacturing technology and large-scale production still need to be improved, so they cannot replace the traditional technology on a large scale in a short time.