Introduction
Battery Separator Material Article states the effect of separator thickness and porosity on the performance of Lithium Iron Phosphate batteries. In recent years there have been intensive efforts to improve the performance of the lithium-ion batteries. Separators are important component of lithium-ion batteries since they isolate the electrodes and prevent electrical short-circuits. Separators are also used as an electrolyte reservoir which is used as a medium for ions transfer during charge and discharge. Electrochemical performance of the batteries is highly dependent on the material, structure, and separators used.
Lithium Battery Industry
A lithium-ion battery consists of four parts: a positive electrode material, a negative electrode material, an electrolyte, and a separator. Figure 1 is a schematic diagram of the working principle and structure of the lithium-ion battery. The separator is a functional membrane material with a microporous structure, and its thickness is generally 8-40 μm. It separates the positive and negative electrodes in the battery system, blocks the passage of electrons in the circuit during charging and discharging, and allows the free passage of lithium ions in the electrolyte. It can selectively close the micropores when the battery is charged and discharged or the temperature rises to limit excessive current and prevent short circuits. The quality of its performance directly determines the overall performance of the battery.
Preparation Method of Traditional Lithium-ion Separator
Traditional lithium-ion battery separators are polyolefin separators, mostly single-layer or three-layer structures, such as single-layer PE, single-layer PP, and PP/PE/PP composite films. According to the conventional preparation process, it can be divided into dry process and wet process.
Dry Process
The dry process is the most commonly used method.
- The molten polyolefin resins made into a sheet-like crystalline film by extrusion and film blowing, and slit-shaped porous films are formed by uniaxial stretching or biaxial stretching at high temperature structure.
- The microporous structure of the film prepared by the uniaxial stretching process is flat, long and interconnected, and has good conductivity; no solvent is used in the production process, and the process is environmentally friendly;
- The longitudinal strength of the film is better than the transverse direction, and there is basically no heat shrinkage in the transverse direction; representative of the company
- The film prepared by the biaxial stretching process has a certain strength in both longitudinal and transverse directions, and the size and distribution of micropores are uniform.
Wet Process
Wet process is also known as phase separation method or thermally induced phase separation method in industry.
- The mixture is formed into a uniformly mixed liquid state, and the microporous membrane material is obtained by cooling and phase separation pressing.
- The three-dimensional structure of the wet film is more complex than that of the dry film, and the micropore buckling degree is higher
- The wet method is relatively less environmentally friendly than the dry method due to the use of solvents in the production process, and the thermal stability is poor, and the process flow Also relatively complex
- According to the different stretching process when pressing the film, it can be divided into bidirectional synchronous stretching and bidirectional asynchronous stretching. The properties of the film prepared by bidirectional synchronous stretching, such as tensile strength, thermal shrinkage, etc., are basically the same in the longitudinal and transverse directions; bidirectional asynchronous stretching is to cool the molten polymer to obtain a film, and then stretch longitudinally first. Lateral stretching is carried out, because the stretching force cannot be guaranteed to be completely consistent during the step-by-step stretching, and the properties of the prepared films vary greatly in the longitudinal and transverse directions.
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Research Status of Lithium-Ion Separators at Home and Abroad
Multilayer Composite Diaphragm
The multi-layer composite diaphragm is a PP/PE two-layer composite diaphragm or a PP/PE/PP three-layer composite diaphragm independently developed by Celgard Company in the United States. It combines the good mechanical properties of PP film, high melting temperature and PE film. The advantage of low pore temperature increases the safety performance of the battery; however, the PE and PP membranes have poor affinity for the electrolyte, and the fiber structure of the PP/PE/PP triple-layer separator is linear, once a short circuit occurs, it will cause a short circuit The area expands rapidly in an instant, the heat rises sharply and it is difficult to discharge, and there is a potential explosion.
- Organic/inorganic composite separator
The organic/inorganic composite separator is to coat inorganic materials (such as Al₂O₃, SiO₂ and other particles) on polyolefin films or non-woven fabrics, and improve the safety and high-power fast charge and discharge of lithium-ion batteries through the complementation of organic and inorganic materials. It not only has the flexibility of organic materials and the effective closed-cell function to prevent short-circuit of the battery, but also has the functions of low heat transfer rate of inorganic materials, the thermal runaway point in the battery is not easy to expand, and it can absorb a small amount of water in the electrolyte to prolong the service life of the battery.
- Nanofiber Coated Separator
Nanofiber-coated separator refers to coating nanofibers on the base membrane to modify the surface of the existing separator or non-woven fabric. On the one hand, it can improve the high temperature shrinkage resistance of the separator, and on the other hand, it can improve the battery The electrode compatibility and adhesion of the separator, and the absorbency and affinity of the separator to the electrolyte are increased.
- Electrospinning Separator
Electrospinning is a technology in which an electric field is applied to a polymer solution or melt to atomize to form micro-jets, which are finally solidified into nano-scale fibers. The battery separator prepared by electrospinning technology has a wide range of raw materials, the prepared separator has a large specific surface area, a high porosity, a small fiber pore size and a large aspect ratio. F CROCE et al. prepared PVDF-CTFE fiber membranes by electrospinning technology. The results show that this kind of separator has good ionic conductivity in a wide temperature range and can better block the positive and negative electrodes.
- Cellulose based separators
Cellulose-based separator is a lithium-ion battery separator material prepared from cellulose fiber as raw material by non-woven and other processing technologies. Cellulose fibres are the most widely distributed and the largest natural polymer in nature. Compared with synthetic polymers, cellulose fibres have the advantages of being environmentally friendly, renewable, and biocompatible, and the cellulose substrate has pores. It has the advantages of large structure, good wettability, good thermal stability and good chemical stability.
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Characteristics and Technical Requirements of Lithium-Ion Battery Separator:
The performance requirements of lithium-ion battery separators mainly include:
- Electronic insulation;
- Appropriate pore size and porosity;
- Good electrochemical stability, corrosion resistance of electrolyte;
- Good thermal stability, low closed-cell temperature and high fusing temperature;
- Good affinity with electrolyte, with Certain liquid absorption rate;
- Sufficient mechanical properties and small thickness;
- Good spatial stability and flatness.
- Test standards and methods
Referring to the American Advanced Battery Alliance’s regulations on the performance parameters of lithium-ion battery separators, the performance of battery separators can be divided into physical and chemical properties, mechanical properties, thermal properties and electrochemical properties.
- Physical and chemical properties
Physical and chemical properties include thickness, porosity, average pore size and distribution, air permeability, tortuosity, wettability, liquid absorption, and chemical stability. As the most basic parameter of the battery separator, the thickness is inversely proportional to the permeability of lithium ions. Therefore, if the mechanical properties meet the actual needs, the thickness should be as small as possible; porosity refers to the volume of micropores in the material accounting for the total volume of the material. The percentage of pore size is closely related to the gas permeability, liquid absorption rate and electrochemical impedance of the battery separator. The porosity can be obtained by the liquid absorption method, calculation method and instrument measurement method; It can also be measured using an instrument combined with the Laplace equation; wettability and liquid absorption are the ability of the separator to retain electrolyte to reduce battery internal resistance and improve battery performance.
- Mechanical properties
Mechanical properties mainly include puncture strength, mixed puncture strength and tensile strength. The diaphragm material must not only withstand the piercing force of the electrode mixture during the working process of the battery, but also meet the physical impact, puncture, abrasion, compression and tensile force during the production process due to curling, packaging, and manufacturing. Short circuits play an important role.
- Thermal properties
Thermal properties mainly include thermal closed cell temperature, fusing temperature and thermal shrinkage. The closed-cell temperature is set by the special protection mechanism of the separator for the battery, that is, when the temperature exceeds the closed-cell temperature, the micropores in the separator are closed, preventing the passage of lithium ions and reducing the risk of short circuit to a certain extent; and the melting temperature is Refers to the temperature at which the diaphragm ruptures and short-circuits occur at high temperatures. The higher the temperature, the lower the risk of short-circuit.
- Electrochemical performance
The electrochemical performance mainly includes linear voltammetry test, electrochemical impedance spectroscopy test, cycle performance, ionic conductivity and resistance value.
Summary
As one of the key materials for lithium-ion batteries, the market demand for separators is also growing rapidly. In the future, the development of lithium-ion battery separators will mainly focus on:
- Diversification of types of film materials. Biomass composite materials and special polymer materials are gradually used in battery separator products; the output power and safety performance of battery separators are improved by compounding various separators or adding inorganic particles and PE micro powders.
- Diversification of membrane microporous structure and preparation methods. For example, the extraction method is used to extract the soluble substances in the substrate after film formation to prepare a microporous film; the electrospinning method is used to obtain a separator with smaller micropores and higher porosity; The method prepares membranes with uniform distribution and upper and lower pore passages.
- Focus on high-performance diaphragms with low cost and simple production process. At present, the commercialized separators are mainly PE and PP films. Due to their own structure and cost constraints, their status as commercial battery separators cannot be shaken. Surface modification, coating and other methods are used to seek diaphragm materials with simple manufacturing process and greatly improved performance.
- Pay attention to the comprehensive performance and evaluation system of diaphragm materials. With the increasing attention to the physical and chemical properties, mechanical properties, thermal properties, and electrochemical properties of separators, it is also an important development direction to further improve the application evaluation of battery separator materials.
Conclusion
The content of the Lithium Battery Industry Official Account includes lithium battery industry chain exchanges, information, and connections. The materials, auxiliary materials, additives, consumables, and related equipment involved in the production of positive and negative electrode materials, separators, electrolytes, packaging materials, and testing and assembly, as well as related equipment, are introduced. If you want to know more information about lithium batteries, you are welcome to pay attention to this public account, and you can get relevant information as soon as possible!
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