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What is separator in battery?

What is separator in battery?


In the structure of a lithium battery, the separator is one of the key inner layer components. The performance of the separator determines the interface structure and internal resistance of the battery, which directly affects the capacity, cycle and safety performance of the battery. The separator with excellent performance plays an important role in improving the overall performance of the battery.

The main function of the separator is to separate the positive and negative electrodes of the battery, prevent the two poles from contacting and short-circuit, and also have the function of allowing electrolyte ions to pass. The separator material is non-conductive and its physicochemical properties have a great influence on the performance of the battery. The type of battery is different, and the separator used is different. In the lithium battery series, since the electrolytic solution is an organic solvent system, a separator material resistant to an organic solvent is required, and a polyolefin porous film having a high strength thin film is generally used.




Lithium battery separator requirements

1. it has electronic insulation to ensure the mechanical isolation of the positive and negative electrodes.

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

3. since the solvent of the electrolyte is a highly polar organic compound, the separator must be resistant to electrolyte corrosion and have sufficient chemical and electrochemical stability.

4. the electrolyte has good wettability and sufficient liquid absorption and moisturizing ability.

5. Have sufficient mechanical properties, including puncture strength, tensile strength, etc., but the thickness is as small as possible.

6. space stability and good integrity.

7. thermal stability and automatic shutdown protection performance is good. Power batteries have higher requirements for separators, usually with composite membranes.




The main performance requirements of lithium ion battery separator are: thickness uniformity, mechanical properties (including tensile strength and puncture strength), gas permeability, physical and chemical properties (including wettability, chemical stability, thermal stability, safety), etc. Four major performance indicators.




Separator performance of lithium ion batteries


The separator is located between the positive electrode and the negative electrode, and the main function is to separate the positive and negative active materials to prevent short circuit of the two electrodes due to contact; in addition, in the electrochemical reaction, the necessary electrolyte can be maintained to form a channel for ion movement. The separator material is non-conductive, the type of battery is different, and the separator used is different. For a lithium ion battery, since the electrolyte is an organic solvent system, the separator is required to have the following properties.

1 In the battery system, its chemical stability is good, and the materials used can resist organic solvents.

2 High mechanical strength and long service life.

3 The ionic conductivity of the organic electrolyte is lower than that of the aqueous solution. In order to reduce the electrical resistance, the electrode area must be as large as possible, so the separator must be thin.

4 When the battery system is abnormal, the temperature rises. To prevent the danger, at the beginning of the rapid heating temperature (120-140 °C), the thermoplastic separator melts, the micropores close, become insulators, and the electrolyte is prevented from passing. The purpose of interrupting the current.

5 From the perspective of the lithium battery, it should be fully impregnated with the organic electrolyte and maintain a high degree of impregnation during repeated charge and discharge.

The separator material commonly used in batteries is generally a microporous membrane made of cellulose or a woven fabric or a synthetic resin. Lithium-ion batteries generally use high-strength, thin-film polyolefin-based porous membranes. Commonly used separators include polypropylene (PP) and polyethylene (PE) microporous membranes, as well as copolymers of propylene and ethylene, polyethylene homopolymers, etc.


Wide application of lithium ion batteries

In recent years, the use of polymer electrolytes for lithium ion batteries has been commercialized. Polymer electrolytes are both channels for ion transport in lithium ion batteries and act as separators between positive and negative materials. Polymer electrolytes can be divided into solid polymer electrolytes and gel polymer electrolytes. As a practical polymer electrolyte membrane, the following necessary conditions must be met:

1 has high ionic conductivity to reduce the internal resistance of the battery;

2 The transfer coefficient of lithium ions is basically unchanged to eliminate concentration polarization;

3 negligible electronic conductivity to ensure effective isolation between the electrodes;

4 electrode materials have high chemical and electrochemical stability;

5 low price, suitable chemical composition, to ensure environmental friendliness.


Since the solid polymer electrolyte has a low room temperature conductivity, it is difficult to commercialize. The gel polymer electrolyte realizes ion conduction through liquid electrolyte molecules immobilized in a polymer network, which has both stability of a solid polymer and high ion conductivity of a liquid electrolyte, and shows a good application prospect.

The polymer electrolyte is combined with polyethylene and polypropylene film to form a polymer lithium ion battery separator. The colloid polymer is covered or filled in the microporous membrane, and has superior performance compared with the polymer electrolyte lithium ion battery without diaphragm. For example, 1 can provide better protection when internal short circuit; 2 can reduce the thickness of electrolyte layer; 3 can provide sufficient safety when overcharged; 4 has better mechanical properties and thermal stability. It can be seen that polyethylene and polypropylene films occupy an important position in ion battery separators due to their special structure and properties, unless there is a true liquid-free polymer electrolyte.



According to different physical and chemical properties, lithium battery separator materials can be divided into: woven film, non-woven film (non-woven fabric), microporous film, composite film, diaphragm paper, laminated film and so on. Polyolefin materials have excellent mechanical properties, chemical stability and relatively inexpensive characteristics. Therefore, polyolefin microporous membranes such as polyethylene and polypropylene have been used as lithium battery separators in the early stage of research and development of lithium batteries. Although in recent years, research has been conducted to prepare lithium battery separators from other materials, such as phase separation method using polyvinylidene fluoride (PVDF) as a bulk polymer to prepare lithium battery separators, and to study cellulose composite membranes as lithium battery separator materials. However, commercial lithium battery separator materials still mainly use polyethylene and polypropylene microporous membranes.




The market-oriented separator materials are mainly polyethylene (polyethylene), polypropylene (polypropylene) (PP)-based polyolefin (Polyolefin) separators, of which PE products are mainly produced by wet process, PP products are mainly dry Made by the process. As for the properties of the two materials PE and PP.


1PP is relatively more resistant to high temperatures, and PE is relatively resistant to low temperatures;

2PP density is smaller than PE;

3PP melting point and closed cell temperature are higher than PE;

4PP products are more brittle than PE;

5PE is more sensitive to environmental stresses.


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, among which the first two products Mainly used in the field of 3C small batteries, the latter types of products are mainly used in the field of power lithium batteries. In the diaphragm material products for power lithium batteries, the double-layer PP/PP diaphragm material is mainly produced by Chinese companies and used in mainland China. This is mainly because there is no Chinese company that can make PP and PE into a double-layer composite film at the current stage. Technology and capabilities. The diaphragms used in global automotive power lithium batteries are mainly made up of three layers of PP/PE/PP, double layer PP/PE and PP+ceramic coating, and PE+ceramic coating.



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