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Lithium ion batteries are a type of secondary battery that relies heavily on the movement of lithium ions between the positive and negative electrodes for operation. A lithium-ion battery consists of five parts: positive electrode material, negative electrode material, organic electrolyte, separator, and battery shell. Among them, the separator is one of the key components of lithium-ion batteries and has two crucial uses:

1. Use of electronic insulation between positive and negative electrodes: A separator with good insulation provides good safety protection for the battery, and good puncture resistance and tensile strength can prevent the occurrence of internal short circuits in the battery caused by burrs and dendrites piercing the separator. In addition, the thickness and thermal stability of the separator are also important factors affecting the safety of lithium-ion batteries.

2. Supply migration microporous channels for lithium ions, which determine the charging, discharging, and cycling performance of the battery. Therefore, the separator should have a high porosity and uniform distribution of micropores.

1、 Research status of separators for lithium-ion batteries

According to different physical and chemical characteristics, lithium-ion battery separator materials can be divided into the following types: microporous separator, modified microporous separator, non-woven separator, composite membrane, and electrolyte membrane. Microporous membranes can be divided into single-layer and multi-layer microporous membranes, depending on the number of layers. Modified microporous membranes are obtained by modifying the surface of traditional microporous membranes, typically using plasma and radiation induced grafting or coating a different layer of polymer on the surface. The non-woven membrane is composed of wrapped fibers combined to form a network structure, which is prepared using melt blown method, wet laying method, and electrospinning technology. Due to its small fiber diameter, the non-woven membrane has a higher porosity compared to other membranes. Composite membranes are prepared by coating or filling inorganic materials with microporous or non-woven membranes, thus possessing excellent thermal stability and wettability.

1. Microporous membrane

The research on microporous membranes mainly focuses on PE, PP monolayer membranes, and multi-layer composite membranes such as PE/PP and PP/PE/PP. The reason why these polyolefin microporous membranes are widely used is that they can provide good mechanical properties and chemical stability. However, due to the generally low melting point of this type of polymer (PE melts at around 130 ℃), when the temperature of the battery increases due to long-term operation, the microporous polyolefin membrane is prone to thermal shrinkage, leading to large positive and negative electrode contact, causing a short circuit and causing the battery to ignite and explode. In addition, the poor electrolyte wettability of polyolefin microporous membranes makes it difficult to further improve the electrochemical performance of the battery.

Regarding the characteristics of such materials, researchers aim to improve the comprehensive performance of microporous membranes from the perspective of improving preparation processes. Generally speaking, there are two methods for preparing microporous membranes, namely dry method (melt extrusion stretching method, MSCS) and wet method (thermally induced phase separation method, TIPS). The dry process is simple and has high production efficiency, without the occurrence of pollutants, but it cannot accurately control the pore size and porosity of the diaphragm. The microporous membrane prepared by wet process has a small and uniform pore size, but the process is complex and costly, making it difficult to achieve industrial production.

In recent years, the development of ultra-high molecular weight polyethylene membranes (UHMWPE) has received great attention. The following advantages greatly improve the safety of batteries:

① Excellent resistance to external force puncture reduces the short circuit rate of the battery;

② Good heat resistance improves the closing temperature and film breaking temperature;

③ Dimensional stability and corrosion resistance in high-temperature environments.

2. Modified microporous membrane

At present, the widely used separator in lithium-ion batteries is made of polyolefin, especially microporous PE and PP membranes. However, the thermal stability and wettability of polyolefin membranes are poor. In order to improve these properties, a series of modification methods are needed to change the structure of microporous polyolefin membranes. One efficient and simple method is to graft hydrophilic monomers onto the membrane surface. Currently, the commonly used grafting techniques include plasma, UV irradiation, and electron irradiation.

3. Non woven fabric diaphragm

Non woven fabrics are usually made by bonding randomly oriented fibers through chemical and mechanical methods. The traditional methods for preparing non-woven fabrics include dry method (melt blown method), wet method (wet laying method), and paper making method. The membranes prepared by traditional methods have relatively large fiber diameters and pore sizes, and are usually used as separators for lead-acid batteries and are not suitable for use in lithium-ion batteries. In order to reduce fiber diameter and pore size, electrospinning technology was used to prepare non-woven fabric separators suitable for lithium-ion batteries. The nanofiber non-woven fabric prepared by electrospinning method has the advantages of small pore size and uniform distribution, high porosity, high liquid absorption rate, and high specific surface area.

Many polymers can be used to prepare electrospun fiber membranes, such as polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), and polyisophthaloyl isophthalamide (PMIA).

4. Composite diaphragm

Generally speaking, nanofiber membranes are disorderly stacked during the electrospinning preparation process, resulting in low mechanical strength and unfavorable anti penetration and thermal stability of the membranes. In order to improve its mechanical properties, researchers introduced second phase inorganic particles into the fiber membrane to form a composite material, thereby improving its mechanical strength. Commonly used nano inorganic particles include aluminum oxide (Al2O3), silicon dioxide (SiO2), and titanium dioxide (TiO2), which can significantly improve mechanical strength and thermal stability, and improve the safety performance of lithium-ion batteries. At the same time, adding inorganic particles to the polymer membrane can reduce crystallization tendency and improve lithium ion migration ability. It can also utilize the high hydrophilicity and large specific surface area of inorganic particles to improve the wettability of the membrane.

2、 Summary

The lithium-ion battery industry, which conforms to the green and environmentally friendly environment, is rapidly developing. As one of the key materials for lithium-ion batteries, the market demand for separators is also rapidly increasing. The development direction of lithium-ion battery separators in the future is to have high porosity, high melting point, high mechanical strength, good thermal stability, and electrolyte wettability. The separator of lithium-ion batteries can be improved in two aspects:

1. The coating technology of polyolefin modified membranes is relatively simple, and the process and equipment are already very mature and cost-effective. It is an effective way to improve the heat resistance of polyolefin membranes and improve the wettability of electrolytes;

2. Change the substrate material of the diaphragm and study a new material system. For example, polyimide (PI) has the characteristics of high temperature resistance and high mechanical strength. It can replace traditional polyolefin materials with PI, but the cost of PI is high. It can be considered to use PI and PE together and develop corresponding production technology.

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