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The separator is one of the important components of lithium-ion batteries, usually also known as battery separator, separator paper, ion separation membrane, etc. The main function of the separator in lithium-ion battery materials is to isolate the positive and negative poles to prevent self discharge and short circuit of the two poles. Due to the safety, permeability, porosity, and membrane thickness of the separator, they will affect the ion conductivity and mechanical strength of each other. Therefore, different design methods must be applied to manufacture different products, such as single-layer, double-layer, and three-layer isolation membranes.

What is a lithium battery separator?

In the structure of lithium batteries, the separator is one of the key inner components. The performance of the separator determines the interface structure, internal resistance, and other characteristics of the battery, directly affecting its capacity, cycling, and safety performance. A high-performance separator plays an important role in improving the overall performance of lithium batteries.

The main function of a separator is to separate the positive and negative electrodes of a battery, preventing short circuits caused by contact between the two poles. In addition, it also has the function of allowing electrolyte ions to pass through. The diaphragm material is non-conductive, and its physical and chemical properties have a significant impact on the performance of the battery. Different types of batteries use different separators. For the lithium battery series, due to the electrolyte being an organic solvent system, it is necessary to have a separator material that is resistant to organic solvents. Generally, high-strength thin film polyolefin porous membranes are used.

Requirements for lithium battery separators

1. It has electronic insulation properties, ensuring mechanical isolation between positive and negative electrodes.

2. It has a certain pore size and porosity, ensuring low resistance and high ionic conductivity, and has good permeability to lithium ions.

3. Due to the strong polarity of organic compounds in the electrolyte solvent, the separator must be resistant to electrolyte corrosion and have sufficient chemical and electrochemical stability.

4. Good wettability of electrolyte and sufficient ability to absorb and retain moisture.

5. Having sufficient mechanical properties, including puncture strength, tensile strength, etc., but with a thickness as small as possible.

6. Good spatial stability and flatness.

7. Good thermal stability and automatic shutdown protection performance. Power lithium batteries have higher requirements for separators and usually use composite membranes.

Preparation method of lithium battery separator

The preparation methods of lithium battery separators are divided into two categories: dry and wet methods.

Dry process is the process of melting, extruding, and blowing polyolefin resin into a crystalline polymer film. After crystallization treatment and annealing, a highly oriented multilayer structure is obtained, which is further stretched at high temperatures to peel off the crystalline interface and form a porous structure, which can increase the pore size of the film. There are two types of dry methods: uniaxial stretching and biaxial stretching.

Wet process, also known as phase separation method or thermally induced phase separation method, involves mixing liquid hydrocarbons or some small molecular substances with polyolefin resin, heating and melting them to form a uniform mixture, then cooling down for phase separation, pressing the membrane, heating the membrane to a temperature close to the melting point, biaxial stretching to orient the molecular chains, and finally holding it for a certain period of time. Volatile substances are used to wash away residual solvents, which can prepare interconnected microporous membrane materials.

Development trend of lithium iron phosphate battery separator industry

The market prospects of the lithium iron phosphate battery separator industry are broad and the development potential is enormous. Currently, low-end products in the industry have entered the competitive stage, and enterprise profits are becoming increasingly even. Industry integration and market segmentation are also about to be completed. In the future, the overall product prices of the lithium iron phosphate battery separator industry will further decrease, and more product innovations will meet the different needs of downstream industries.

With the advancement of technology and the development of society, safety issues will inevitably receive increasing attention from society. During the charging and discharging process, high-temperature resistant composite membranes that can maintain the integrity of the membrane even after a large area of positive and negative short circuits will be favored by the market. To improve the safety of lithium iron phosphate batteries, coating ceramic or other nanomaterials on polyolefin separators or using new matrix materials has become a future technological development trend.

Develop ultra-thin membranes to improve unit energy density. With the development of the market, thinner and smaller 3C electronic products will continue to enter our lives. These products require thinner lithium-ion battery separators to make batteries. At the same time, the pursuit of capacity for lithium-ion batteries also requires thin films to develop towards slimness. However, the production and preparation of ultra-thin separators require high equipment and process requirements.

In lithium battery materials, the barrier to technology and gross profit margin of the separator are relatively high, and it is also the last material to achieve localization. With the improvement of domestic lithium battery separator equipment technology, the gap in product quality with imported equipment will be further narrowed. The cost-effectiveness advantage of domestic equipment and its substitution effect on imported equipment will become increasingly apparent.

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