lithium battery separator

1、What is lithium battery separator

Lithium battery separator, from the appearance, it is a thin film material with a microporous structure, usually made of high polymer. Its thickness is extremely thin, generally between 5 microns and 30 microns, to meet the needs of lightweight and high-performance lithium batteries. For example, in some high-end consumer electronics lithium batteries, the separator thickness can even be as low as 5 microns, but it can effectively shoulder the task of separating the positive and negative electrodes of the lithium battery.

Although the separator is thin, it shoulders the crucial mission of building an ion conduction channel between the positive and negative electrodes, while preventing the direct passage of electrons to ensure that the lithium battery can be charged and discharged normally. It can be said that although the lithium battery separator is small, it is related to the "life and death" of the lithium battery. Any subtle defects, such as uneven pores, impurities, etc., may affect the performance of the battery, and even cause the battery to fail.

2、Core role

(1) Isolate the positive and negative poles

The most basic and key role of the lithium battery celgard is to physically isolate the positive and negative electrodes and prevent short circuit caused by direct contact of the positive and negative materials. Once the positive and negative terminals are short circuited, there will be a large current through the battery, which will not only lead to an instant failure of the battery, but also may cause serious safety accidents such as overheating, fire and even explosion.

Take electric vehicles as an example, its lithium battery pack contains a large number of cells, if one of the cell separator problems, resulting in the positive and negative short circuit, short circuit generated by high heat may be quickly transferred to the surrounding cell, triggering a chain reaction, resulting in the entire battery pack fire burning, the consequences are unimaginable. The separator is like a strong "firewall" that effectively prevents the conduction of electrons between the positive and negative electrodes, ensuring that the battery operates in a safe environment. According to statistics, lithium battery safety accidents caused by separator isolation failure account for up to 30% of all battery safety accidents.

(2) ion conduction channels

While isolating the positive and negative electrodes, the lithium battery separator also needs to have good ion permeability to provide a smooth channel for the migration of lithium ions between the positive and negative electrodes. In the process of lithium battery charging, lithium ions emerge from the positive electrode, pass through the separator, and embed in the negative electrode; When discharged, the lithium ions migrate in the opposite direction.

The ion conduction performance of the separator directly affects the charge and discharge efficiency and power characteristics of the lithium battery. The high-quality separator ensures the rapid and stable shuttle of lithium ions, enabling the battery to store and release energy efficiently. For example, in fast-charge technology, the high ion conductivity separator allows a large number of lithium ions to migrate in a short period of time, achieving fast charging of the battery. Studies have shown that for every 10% increase in the ionic conductivity of the separator, the charge and discharge speed of the lithium battery can be increased by about 15%.

3、Working principle

The working principle of the lithium battery separator is based on its special microporous structure. The diameter of these microholes is usually 10 nanometers to 1000 nanometers, and the size is precisely designed to prevent the passage of electrons, prevent the short circuit of the positive and negative electrodes, and allow smaller lithium ions to migrate smoothly between the positive and negative electrodes through the microholes with the help of the medium of the electrolyte.

When the battery is in the state of charge and discharge, under the action of electric field force, lithium ions are diffused from the high concentration area to the low concentration area, and the microporous structure of the separator is like a precision "ion screen", guiding the orderly passage of lithium ions to complete the electrochemical reaction inside the battery. At the same time, the chemical stability of the separator material itself ensures that it will not degrade or other chemical reactions in the complex chemical environment inside the battery, and always maintains good isolation and ion conduction properties. For example, in the case of highly corrosive lithium salts in the electrolyte, the separator material must be able to withstand this chemical environment to ensure the stability of its own structure and performance.

4、Common types

(1) Polyethylene (PE) separator

Polyethylene separator is one of the most widely used lithium battery separators. It has a high melting point, generally between 130 ° C and 140 ° C, and has good chemical stability, which can withstand the high temperature environment inside the battery to a certain extent. The production process of PE separator is relatively mature, and the film with uniform microporous structure can be prepared by bidirectional drawing and other processes.

Its advantages are high mechanical strength, tensile strength can reach more than 100MPa, can effectively prevent positive and negative electrode contact, and the cost is relatively low, suitable for large-scale production. However, the high temperature stability of the PE separator has certain limitations, when the temperature exceeds 160 ° C, it may shrink, and the shrinkage rate can reach 10%-20%, which will affect the safety of the battery, resulting in a shortening of the distance between the positive and negative electrodes, increasing the risk of short circuit.

(2) Polypropylene (PP) separator

Polypropylene membranes are also widely used in the field of lithium batteries. Compared with PE separators, PP separators have a higher melting point, generally between 160 ° C and 170 ° C, so they have better high temperature stability and can maintain the integrity of the separator at higher temperatures, reducing battery safety risks caused by high temperatures.

PP separator can also be prepared by drawing and other processes, and its micropore structure is uniform and ion permeability is good. However, the disadvantage of PP separator is that the mechanical strength is relatively weak, the tensile strength is generally between 60MPa-80MPa, and in some application scenarios that require high mechanical properties of the separator, such as high-capacity power batteries, it may be necessary to use it with other materials.

5、Performance requirements

(1) Good mechanical properties

The lithium battery separator needs to have sufficient tensile strength and puncture strength to ensure that the separator will not break or be pierced during the winding, assembly and other processes in the battery production process and the battery use process, so as to ensure its function of isolating the positive and negative poles. In the winding process of battery production, the separator must withstand a certain tension, if the tensile strength is insufficient, it is easy to break.

In general, the tensile strength of the separator used for power batteries needs to reach more than 80MPa, and the puncture strength needs to be greater than 100mN. Good mechanical properties help to improve the production efficiency and yield of the battery, while extending the service life of the battery. According to production practice data, the defective rate of battery production due to poor mechanical properties of the separator can be as high as 5%-10%.

(2) High ion transmittance and low resistance

In order to achieve efficient charging and discharging of lithium batteries, the separator needs to have a high ion transmittance, so that lithium ions can pass through quickly. At the same time, the resistance of the separator should be as low as possible to reduce the energy loss of the battery during the charging and discharging process and improve the energy conversion efficiency of the battery. A good balance between high ion transmittance and low resistance is needed, which puts a high requirement on the design of the microporous structure of the separator and the selection of materials.

For example, by optimizing the size, shape and distribution of micropores, as well as selecting suitable polymer materials and additives, the ionic conductivity of the separator can be increased and the resistance reduced.

(3) Excellent chemical stability

The electrolyte inside the lithium battery is usually highly corrosive, and the battery will undergo complex electrochemical reactions during charging and discharging. Therefore, the lithium battery separator must have excellent chemical stability, can maintain long-term stability in the chemical environment inside the battery, and does not react with the electrolyte, positive and negative electrode materials, etc., to ensure that the performance of the separator will not be reduced by chemical action.

6、 Future outlook

With the acceleration of the global energy transition, lithium batteries as the core energy storage equipment in the field of new energy, its market prospects will be broader. As one of the key components of lithium batteries, lithium battery separator will also usher in greater development opportunities. In the future, the lithium battery separator will develop in the direction of high performance, low cost and green environmental protection.