1. Definition of diaphragm
The separator (Separator film) is an important part of the ion battery. It is located between the positive and negative electrodes inside the battery, ensuring the passage of lithium ions while hindering electron transmission. 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. A separator with excellent performance plays an important role in improving the overall performance of the battery.
2. Requirements for diaphragm
When selecting a separator according to a specific battery and application, several factors must be considered, and the characteristics and use requirements of each separator must be weighed. The basic requirements are that the separator has the following characteristics:
②Easy to be infiltrated by electrolyte, and the resistance after infiltration by electrolyte is the smallest;
③ Good mechanical properties and structural stability;
④Good chemical stability to electrolyte, impurities, positive and negative reactants;
⑤It can effectively prevent the migration of particles, colloids or soluble substances between the two electrodes.
In addition, according to different battery applications, the requirements can be adjusted, in a word, it has good performance, high safety and low cost.
3. Types of diaphragm
Classification of Lithium Battery Separator film
According to the differences in physical and chemical properties, battery separators can be divided into: woven membranes, nonwoven membranes (non-woven fabrics), microporous membranes, composite membranes, separator papers, and laminated membranes.
Although there are many types, polyethylene and polypropylene microporous membranes are mainly used as commercial lithium battery separator materials so far.
Since lithium will react in water solvents, and non-aqueous electrolytes are stable under high voltage, all potassium batteries use non-aqueous electrolytes. Lithium-ion secondary batteries can be divided into three categories, and the corresponding diaphragm materials are roughly as follows:
1. Liquid electrolyte battery (polyolefin PE, PP, PP/PE/PP)
2. Gel electrolyte (polymer and liquid hybrid) type battery (polyvinylidene fluoride or polyolefin)
3. Solid-state electrolyte battery (partial ethylene oxide-lithium salt)
Design advantages of PP/PE/PP
Due to the excellent mechanical properties, chemical stability and low cost of polyolefins, they are widely used, among which PP, PE and their mixed PP/PE/PP are used more. Its thickness is about 20 to several hundred um, and the pore size is 0.05 to 0.5 μm.
PE: Low melting point makes it have the function of thermal insurance. When the temperature is close to the melting point of the polymer, the pores of the separator are closed, which can completely or partially cut off the internal current of the battery, prevent the continuous generation of Joule heat inside the battery, and improve the safety of the battery.
PP: The melting point of 165C is higher than that of PE (135C). Even if PE melts and closes the cells, the current is blocked, and the high melting point of PP maintains the structural integrity of the diaphragm.
Characteristics of Ceramic Separator film
Ceramic separators are formed by combining flexible polyolefin separators with hydrophilic ceramic materials with chemical and thermodynamic resistance advantages.
Ceramic materials such as aluminum, silicon, and zinc are coated and cured on smooth substrate surfaces. Ceramic separators have excellent high temperature stability, excellent chemical inertness and good wettability. In addition, some studies have coated metal oxide sol-gel layers, polymers, surfactants, and hydrophilic monomers on the surface of the substrate. etc.
4. The preparation process of the diaphragm
There are two main types, dry and wet.
Wet method (phase transformation process): wet method is also called phase separation method or thermal phase separation method, mixing liquid hydrocarbon or small molecular substance with polyalkane resin, heating and melting to form a uniform mixture, and then cooling down for phase separation , pressed to obtain a membrane, and then heated to a temperature close to the melting point of the membrane, biaxially stretched to orient the molecular chain, and finally kept for a certain period of time to elute the residual solvent with volatile substances to prepare an interpenetrating microporous membrane. Typical wet Process companies including Asahi Kasei, Tonen, Mitsubishi Chemical, and Nitto Denko have the disadvantage of wet process that they have to deal with a large amount of used solvents, and the recovery of these solvents is very difficult.
Dry method: The basic process is to melt polyolefin resin, extrude to form a film, and make a crystalline polymer film. After crystallization treatment and annealing, a highly oriented multilayer structure is obtained, which is further stretched at high temperature. Exfoliation is performed to form a porous structure, which can increase the pore size of the film.
The dry process does not require any solvent and is technically very simple. Typical dry process companies include Japan Ube Celgard and so on.
Uniaxially stretched separators have high strength in only one direction, while biaxially stretched films have high strength in both machine and transverse directions. However, there are not too many biaxially stretched separators in use, because the shrinkage in the transverse direction at high temperatures can easily lead to a short circuit between the positive and negative electrodes.
5. Characterization of the Separator
Separators are characterized mainly through structural and functional properties. Structural properties include properties of chemical molecules and crystallites, thickness, pore size, pore size distribution, porosity, and other relevant chemical and physical properties, such as chemical stability and liquid absorption capacity. .
Functional properties include resistivity, permeability, ion transport capacity, etc. Each characteristic parameter has a great influence on the performance of the diaphragm (self-understanding), and it is necessary to select appropriate design parameters.
1. The thinner the separator, the better, the higher the battery capacity
2. Ion transport and block electron conduction.
3. Good physical and chemical properties, not easy to break, and resistant to chemical corrosion.
The diaphragm is a diaphragm that hinders electrons. At the same time, it is required to achieve the required ion transmission rate by controlling its thickness and pores. There is no perfect diaphragm, only the most suitable diaphragm. Low-cost, high-performance, and safer diaphragms are always the research and development direction of diaphragms!
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