Lithium-ion batteries are mainly composed of five parts: positive electrode material, negative electrode material, diaphragm, electrolyte and packaging material. The lithium-ion battery diaphragm is a porous film with uniformly distributed micropores. It is located between the positive electrode material and the negative electrode material of lithium battery. It plays a role in preventing direct contact between positive and negative electrodes, preventing battery short circuit and transmitting ions. It is a key material to ensure battery safety and affect battery performance. Although the diaphragm does not directly participate in the electrochemical reaction of the battery, its performance affects the interface structure, internal resistance and other properties of the battery, and thus affects the battery's energy density, cycle life and rate performance; the thermal stability of the diaphragm also determines the battery's operating temperature tolerance range and battery safety. An ideal battery diaphragm should have good insulation, mechanical strength, electrochemical stability and thermal stability, as well as high porosity and appropriate pore size, and good wettability and adsorption properties for the electrolyte.
▲Lithium-ion battery (cylindrical) structure and lithium battery diaphragm
The chemical resistance and electrochemical properties of the diaphragm and its mechanical durability are crucial to battery safety. The diaphragm should not be dissolved or reacted by the electrolyte solution, which is mainly composed of organic carbonates and esters mixed with lithium salts, such as lithium hexafluorophosphate. The commercialized lithium-ion battery diaphragm materials are polyolefin diaphragms mainly composed of polyethylene (PE) and polypropylene (PP). Among them, PE products are mainly made by wet process, and PP products are mainly made by dry process. The important lithium-ion battery diaphragm material products include 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, etc. Among them, the first two types of products are mainly used in the field of 3C small batteries, and the latter types of products are mainly used in the field of power lithium-ion batteries.
▲Scanning electron micrographs of (PP/PE/PP) separators: (a) surface and (b) cross section;
Note: In the field of high-end three-layer PP/PE/PP composite diaphragms, only a few countries such as the United States have mature production technology and corresponding large-scale industries. This type of diaphragm is produced by three-layer co-extrusion technology to produce cast base film. It has the advantages of high pore size uniformity and high melting point temperature of ordinary dry-process single-drawn PP diaphragms, and low closed-cell temperature of wet-process PE diaphragms, which improves the safety performance of batteries.
1. Diaphragm production process
The core process of preparing lithium-ion battery separators is microporous preparation technology, which is mainly divided into dry process, wet process and spinning process according to its process. The difference between dry process and wet process mainly lies in whether solvent is required in the production process.
▲Classification of general diaphragm varieties
▲SEM micrograph of the surface of microporous polyolefin membrane
(a) Uniaxially stretched dry PP separator.
(b) Biaxially stretched dry-process β-nucleated PP separator.
(c) Biaxially stretched wet-processed PE separator. (Scale bar = 5 µm)
▼Comparison of dry and wet processes for lithium battery diaphragms
parameter
Dry uniaxial stretching
Dry biaxial stretching
Wet method
Pore formation mechanism
Wafer separation
Crystal conversion
Phase separation
Applicable Materials
Single-layer PP film, single-layer PE film, double-layer film, multi-layer film
Thicker single-layer PP film
Single layer PE film
01Dry stretching
The dry process is the most commonly used method in the preparation of diaphragms. Currently, the dry process mainly includes ① dry uniaxial stretching and ② biaxial stretching (also known as β crystal method).
▲Dry process diaphragm
Dry unidirectional stretching process
The dry uniaxial stretching process is a method of producing hard elastic fibers. In the casting stage, the melt is stretched at a high multiple and rapidly cooled to obtain a polyolefin casting with high orientation and low crystallinity, and then high-temperature annealing is performed to improve its crystal structure. Finally, the final diaphragm is obtained by longitudinal low-temperature and high-temperature stretching. This method can produce microporous membranes with good pore size uniformity and uniaxial orientation, but its disadvantage is that the transverse mechanical strength of the diaphragm is low, and the production is a multi-unit production process with limited production efficiency.
Process flow:
a Feeding: PE or PP and additives and other raw materials are pre-treated according to the formula and then transported to the extrusion system.
b Casting: The pretreated raw materials are melted and plasticized in the extrusion system, and then the melt is extruded from the die head. Then, it is cooled and crystallized under the stress field of high-speed stretching of the casting roller to obtain a cast base film with a lamellar structure perpendicular to the extrusion direction and arranged in parallel. The base film has good hard elastic properties.
c Heat treatment: The base film is heat treated to eliminate crystal defects, further improve the crystal structure, and increase the crystallinity of the film.
d Stretching: The hard elastic polypropylene cast base film is first stretched at low temperature to form micro defects such as silver streaks (cold stretching), and then the defects are pulled apart at high temperature (hot stretching) to form micropores.
e Cutting: Cut the nanoporous membrane into finished membranes according to customer's specifications.
Key points of the process:
According to the pore-forming mechanism of the microporous membrane prepared by uniaxial stretching of polypropylene, the perfection of the oriented lamella structure of the cast base membrane is the key to determining the stretching pore-forming performance, and the biggest factor affecting the perfection of the oriented lamella structure of the cast base membrane is the key process parameters such as temperature and draw ratio during the preparation of the base membrane. These parameters affect the entire crystallization process of the melt. It can be said that whether the casting process parameters are properly controlled is the decisive condition for preparing a polypropylene microporous membrane with excellent performance.
▲Dry uniaxially stretched polypropylene diaphragm
Biaxial stretching (also known as β crystal method)
The dry biaxial stretching process adds a β-crystal modifier with nucleating effect to polypropylene, and utilizes the density difference between different phases of polypropylene to transform the polypropylene from the crystal form to form micropores during the stretching process. (The dry biaxial stretching process is a process with independent intellectual property rights developed by the Institute of Chemistry of the Chinese Academy of Sciences in the early 1990s.)
Basic principle:
The β crystal form of PP is a hexagonal crystal system. β spherulites are usually formed by single crystal nucleation and grow radially into a divergent beam-like lamellar structure. The crystals are loosely arranged and do not have a complete spherulite structure. Under the action of heat and stress, they will transform into a more dense and stable α crystal, which will absorb a large amount of impact energy and produce holes inside the material. (Using the difference in density between different phases of PP, the crystal form transformation occurs during the stretching process to form micropores)
Process flow:
a tape casting sheet, to obtain a PP tape casting sheet with high β crystal content and good β crystal morphology uniformity;
b. Longitudinal stretching: the casting is stretched longitudinally at a certain temperature, and the pores are formed by taking advantage of the property of β crystals that they are prone to form pores under tensile stress;
c Transverse stretching: the sample is stretched transversely at a higher temperature to expand the pores and improve the uniformity of pore size distribution;
d. Shaping and rolling: by heat treating the diaphragm at high temperature, its thermal shrinkage rate is reduced and its dimensional stability is improved.
▲β-PP biaxially stretched film after annealing
Process characteristics: low cost
The dry biaxial stretching process has the following main characteristics: the production process is continuous, the process is simple, no solvent is required, and the production cost is lower than that of dry single stretching and wet method. Its production can be carried out on the basis of the existing biaxial stretching film (such as BOPP) production platform. my country has a good foundation in related equipment, technology, operation control, etc. For China, this method is relatively low as a technical threshold for manufacturing lithium-ion battery separators. On the other hand, the products prepared by this process still have problems such as too wide pore size distribution, poor thickness uniformity, and low product quality stability. Most products can only be used in low-end fields, and it is difficult to expand to high-end fields such as power vehicle batteries that require higher pore size consistency and thickness uniformity of separators. Process optimization still needs to be explored. In this process, the uniformity of the β crystal morphology of the β-PP cast sheet determines its stretching porosity, which in turn affects the pore size consistency and thickness uniformity of the final separator. Therefore, how to quickly and efficiently characterize the stretching porosity of the cast sheet during the production process, and then predict the quality of its final separator product, has important theoretical and practical significance.
02Wet Process
The wet process is also known as the thermally induced phase separation method. Its principle is to mix a high-boiling point hydrocarbon liquid with polyethylene, use the microphase separation phenomenon that occurs in the mixture melt during the cooling process, stretch the cast sheet, and finally extract the liquid with a volatile solvent to prepare a microporous membrane material. The diaphragm prepared by the wet process is suitable for high-power batteries and has a high permeability in power batteries. The wet process products have high bidirectional mechanical strength, a large pore size distribution, and excellent performance, but the process is long, the equipment requires high precision, and a large amount of organic solvents are required. There are problems such as high production costs, limited production efficiency, and the need to recover solvents.
▲Wet process diaphragm
The wet process is suitable for producing thinner single-layer PE diaphragms. It is a preparation process with better thickness uniformity, better physical and chemical properties and better mechanical properties of diaphragm products. Depending on whether the orientation is simultaneous during stretching, the wet process can also be divided into two types: wet bidirectional asynchronous stretching process and bidirectional synchronous stretching process.
The process flow of wet asynchronous stretching is as follows: 1) Feeding: PE, pore-forming agent and other raw materials are pre-treated according to the formula and transported to the extrusion system. 2) Casting: The pre-treated raw materials are melt-plasticized in a twin-screw extrusion system and then extruded from the die head to form a cast thick sheet containing a pore-forming agent. 3) Longitudinal stretching: The cast thick sheet is stretched longitudinally. 4) Transverse stretching: The cast thick sheet after longitudinal stretching is stretched transversely to obtain a base membrane containing a pore-forming agent. 5) Extraction: The base membrane is extracted with a solvent to form a base membrane without a pore-forming agent. 6) Shaping: The base membrane without a pore-forming agent is dried and shaped to obtain a nanoporous membrane. 7) Cutting: The nanoporous membrane is cut into finished membranes according to the customer's specifications.
The process flow of wet synchronous stretching technology is basically the same as that of asynchronous stretching technology, except that it can be oriented in both the horizontal and vertical directions during stretching, eliminating the process of longitudinal stretching alone, and enhancing the uniformity of the diaphragm thickness. However, the problems of synchronous stretching are firstly slow speed and secondly poor adjustability, as only the transverse stretching ratio is adjustable, while the longitudinal stretching ratio is fixed.
▲Ultra-high molecular weight polyethylene (UHMWPE) diaphragm made by wet process
The wet process is favored by ternary batteries that pursue high energy density because it is thinner and has smaller and more uniform pores. However, its "thermal stability and safety" are its main shortcomings. Therefore, wet-process diaphragms are generally coated with adhesives such as ceramic alumina, PVDF, aramid, etc. after the base film is made to make up for its shortcomings of "thermal stability and safety". "Wet process + coating" diaphragm is considered to be the optimal solution for lithium battery diaphragm materials at present.
▲Comparison of dry and wet processes for lithium battery separators
03Spinning process--non-woven membrane
Non-woven membranes are new membranes that are made by using non-woven manufacturing processes such as electrospinning, wet non-woven process, and melt-blown process to orient or randomly arrange evenly dispersed fibers to form a three-dimensional network structure, and then reinforced by physical methods. Non-woven membranes can be made of synthetic fibers and natural cellulose, including cellulose derivatives. Commonly used non-woven membrane materials include bacterial cellulose(BC),polyethyleneterephthalate(PET),PI,PVDF,PVDF-HFP,polytetrafluoroethylene (PTFE), etc.
▲Wet-laid nonwoven fabric
Electrospun membranes can significantly improve the thermal stability and electrolyte wetting performance of separators. However, they also have drawbacks such as low fiber strength, difficulty in fiber bonding, and limited production capacity.
The meltblowing spinning process uses a single polymer or a blend of multiple polymers as raw materials, which are melt-blown and then heat-bonded to form a web. The porosity and safety performance of the diaphragm products are greatly improved, but there is a defect of poor heat resistance.
2. Polyolefin surface modification technology
At present, commercially available polyethylene (PE) and polypropylene (PP) microporous films are widely used as lithium-ion battery (LIBs) separators due to their low cost, high mechanical strength, good electrochemical stability and suitable microporous structure. However, due to the small temperature difference between the melting and closing temperatures of polyolefin separators (such as the closing temperature of PE is about 130°C and the melting and breaking temperature is about 140°C), the residual heat generated after the closure will still cause the separator temperature to continue to rise, and then the separator will melt and cause serious accidents. In addition, polyolefin separators also have some problems such as dimensional stability and liquid affinity. In order to solve these problems, the shortcomings of polyolefin separators are improved by modifying the surface of the separator.
The popular method is to coat or graft heat-resistant inorganic or organic materials on the surface of commercial polyolefin separators. The surface coating method is to coat a layer of heat-resistant material on the surface of the polyolefin separator by coating, spraying or atomic layer deposition; while the surface grafting method is to generate a large number of free radicals on the surface of the polyolefin separator by electron beam radiation, thereby quickly inducing the grafting of heat-resistant materials to the surface of the separator. Practice has shown that the polyolefin-based composite separator prepared by coating or grafting technology has a wide temperature range, reduces the shrinkage rate of the separator at high temperature, and improves the safety of the battery.
▼Surface modification has three main purposes: improving thermal stability, increasing liquid absorption and retention, and improving adhesion
Coating modification to improve thermal stability and liquid absorption
In addition to the ceramic coating modification that is currently widely used, there are also aramid coating modification, polyimide coating modification, etc.
Modification to improve the adhesion between the diaphragm and the pole piece
Including: PVDF coating, acrylic coating, polyacrylonitrile coating, etc.
Modification to achieve both thermal stability and adhesion
1. Apply ceramic first and then PVDF; 2. Mix ceramic and PVDF
3. Super hydrophobic coating
In this paper, a facile and scalable strategy to prepare multifunctional self-similar superhydrophobic coatings is proposed and demonstrated.
introduce
Using natural cellulose as raw material, a hydrophobic cationic cellulose derivative, cellulose 1-butylimidazole bis(trifluoromethane)sulfonyl imide (CITf), was synthesized through simple homogeneous chemical modification. Subsequently, CITf was used as a macromolecular surfactant to disperse one-dimensional (1D) multi-walled carbon nanotubes (MWCNTs) and two-dimensional (2D) reduced graphene oxide (rGO) through cation-π and π-π interactions (Figure 1(a)) to obtain a three-component nanodispersion. Then, through a simple spraying process, the nanodispersion spontaneously formed an extensible superhydrophobic coating with a self-similar micro-nanostructure, which is similar to the surface of a lotus leaf (Figure 1(b)-1(e)). The ionic part and the superhydrophobic structure can reduce the freezing temperature of water and the adhesion strength of ice, achieving passive anti-icing. MWCNTs and rGO have strong solar energy absorption capacity and high conductivity, so the CITf/MWCNTs/rGO (CCG) coating exhibits excellent photothermal and Joule heating properties, achieving active deicing performance. This simple and easy preparation method and multifunctional superhydrophobic coating have broad application prospects.
Figure 1. Self-similar superhydrophobic coating. (a) Schematic diagram of CCG three-component dispersion. (b) Schematic diagram of superhydrophobic coating. (c) Large-area superhydrophobic coating. (d) Flexibility of superhydrophobic coating. (e) Hydrophobicity of superhydrophobic coating on different substrates (left: water droplet; middle: acidic water droplet (pH=2); right: alkaline water droplet (pH=12).
Preparation and mechanism of superhydrophobic coating
The obtained CITf was used as a macromolecular surfactant to disperse 1D nanofiber MWCNT and 2D nanosheet rGO. The imidazolium cations on CITf can form "cation-π" and/or "π-π" interactions with MWCNT and rGO, and thus a multicomponent nanodispersion containing CITf, MWCNT and rGO was obtained (Figure 1(a)). Through a simple spraying process, the CITf/MWCNT/rGO dispersion spontaneously formed a superhydrophobic coating with self-similar micro-nanostructure from the inside out (Figures 1(b)-1(e) and Figure S3 in ESM). CITf acted as a binder to effectively bond 1DMWCNT and 2DrGO together, resulting in a lotus leaf-like micro-nanostructure (Figure 1(b)). When the content of MWCNT/rGO was 10wt%, there was almost no micro-nanostructure on the surface. The superhydrophobic phenomenon did not occur (Figure S3). As the content of MWCNT/rGO increases, the number of protruding micro-nano structures increases significantly. When the content of MWCNT/rGO is higher than 30wt%, the superhydrophobic surface is formed by countless protruding micro-nano structures. The water contact angle is greater than 150° and the sliding angle is less than 6°. More importantly, through a simple spraying process, large-scale superhydrophobic coatings can be spontaneously formed on various substrates, such as aluminum, plywood, filter paper, and cotton fabric (Figure 1(c)-1(e)). In addition, the coating exhibits superhydrophobic behavior to neutral water (pH=7), acidic water (pH=2), and alkaline water (pH=12) (Figure 1(e)). Therefore, CITf/MWCNT/rGO superhydrophobic coatings can be prepared on a large scale through a simple process, have a wide range of applicability to different substrates, and exhibit good acid and alkali resistance.
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