1、 Diaphragm action

Isolate, positive and negative electrodes and prevent electrons from passing freely; Give WayelectrolyteThe ions in the liquid pass freely between the positive and negative electrodes. Its performance determines the interface structure and internal resistance of the battery, and directly affects the capacity, cycle and safety performance of the battery.

2、 Diaphragm characteristics

the diaphragmThe material must have good insulation, good affinity, temperature resistance and wettability to electrolyte, and good liquid retention to electrolyte. The diaphragm can prevent the positive and negative electrodes from contacting with short circuit or being punctured by burrs, particles, lithium dendrites, etc. The tensile strength and puncture strength of the diaphragm are not easy to tear, and the thermal contraction is stable at high temperature, which will not lead to short circuit and thermal runaway of the battery.

In the case of overcharge or temperature rise, it can limit the rise of current, prevent the explosion caused by battery short circuit, and block the current conduction in the battery through the closed hole function, which has the function of micropore self closing protection, and plays a role of safety protection for battery users and equipment. The diaphragm must have high porosity and uniform micropore distribution. The characteristics of the material itself and the pore characteristics after film formation restrict the migration of lithium ions in the battery, that is, high ionic conductivity.

3、 Diaphragm classification

According to the structural characteristics and production technology of lithium-ion battery membrane, it can be divided into microporous polyolefin membrane, modified polyolefin membrane, non-woven fabric membrane, coating composite membrane, nanofiber membrane and solid electrolyte membrane.

1 microporous polyolefin membrane

Through continuous technological updates and practical applications, polyolefin microporous membrane has become the best comprehensive performance and industrialized lithium-ion battery diaphragm at present. According to different production processes, it can be divided into single-layer film and multi-layer film, namely polypropylene (PP) single-layer film, polyethylene (PE) single-layer film and pp/pe/pp three-layer composite film. Polyolefin microporous membranes represented by polypropylene (PP) and polyethylene (PE) have the characteristics of excellent performance, good chemical stability and low cost, and occupy a dominant position in lithium battery membranes.

2 modified polyolefin membrane

PE and PP membranes have poor affinity, temperature resistance and wettability to electrolytes. By adding or compounding materials with hydrophilic properties and high temperature resistance on the single-layer polyolefin membrane, grafting hydrophilic monomers on the surface of PE and PP microporous membranes or changing the organic solvents in the electrolyte, the process includes coating, dipping, spraying, compounding, etc., to obtain composite membranes with excellent performance, which is the current trend of preparing high-performance membranes.

Song and others coated the PE diaphragm with polyarylate material with good heat resistance to form a composite diaphragm with a melting temperature of porous polymer as high as 180 ℃. Cheng Hu and others coated the surface of Celgard 2400 single-layer PP film with polyoxyethylene mixed with nano SiO2, which improved the wettability of the membrane and significantly improved the recyclability. Ryou and others coated dopamine on PE diaphragm by dip coating method, and the modified diaphragm has higher performance of adsorbing electrolyte, effectively improving the high rate circulation performance of the diaphragm. Kim et al. Modified the polyolefin diaphragm with pvdf/sio2 mixture to obtain a composite diaphragm with the hydrophilic properties of PVDF and the high temperature resistance of SiO2. The charging and discharging efficiency of the battery reached 94% at 2C discharge rate. Fang et al. Modified PP membrane with polyethylene glycol grafted polydopamine coating, which increased the liquid absorption rate of the modified membrane and reduced the interface resistance.

3 non woven diaphragm

Compared with polyolefin diaphragms, non-woven diaphragms have better thermal dimensional stability, safety, wettability and porosity. The preparation of non-woven materials usually adopts special fibers for directional or random arrangement, and its structure presents a network, which is reinforced by mechanical, thermal bonding or chemical crosslinking methods.

Fibers include natural and synthetic fiber materials, such as natural cellulose and its derivatives, synthetic polyolefin fibers, polyamide (PA) fibers and polyethylene terephthalate (PET) fibers; The non-woven fabric diaphragm has good mechanical properties and high melting temperature, which better maintains the dimensional stability when used. Zhang Song and others used the nanofibers of bacterial cellulose and nano TiO2 particles to prepare bc/tio2 composite films with polarity, porosity and good thermal stability, which improved the ionic conductivity and battery cycle performance.

4 nanofiber membrane

Miao and others used polyimide as raw materials to prepare nanofiber films with extremely high thermal stability. There was no thermal shrinkage at 250 ℃, and the discharge capacity of battery at 10C was 60% of 0.2C, which was much higher than that of polyolefin films. Jung et al. Made of pmma/ polyvinyl chloride (PVC) composite fiber membrane, the electrochemical stability window of the battery is 4.7V. In the lithium-ion half cell system, the capacity of 0.5c cycle for 100 times has almost no attenuation.

5 coating composite film

The non-woven diaphragm is thick, with large pore diameter and poor uniformity, and the tensile mechanical strength is poor. The coating composite diaphragm is usually made by transfer coating or impregnation to improve the comprehensive performance of the diaphragm. Composite diaphragm is a composite multi-layer diaphragm with dry, wet and nonwovens as the substrate and coated with inorganic ceramic particle layer or composite polymer layer on the substrate.

According to the composition of the coating, it can be divided into four kinds: organic coating composite film, inorganic coating composite film, organic / inorganic hybrid coating composite film, and in-situ composite.

Inorganic coating inorganic composite membrane, also known as ceramic membrane, is a porous membrane composed of a small amount of adhesive and inorganic particles. Inorganic composite membrane has good flexibility, high mechanical strength, high thermal stability, excellent high temperature resistance, excellent electrolyte wetting and adsorption performance. At present, some diaphragm enterprises have been industrialized. Ceramic materials have large thermal resistance, which can prevent the expansion of thermal runaway at high temperature and improve the thermal stability of batteries.

Surface coating Al2O3 series: Yang Baobao takes polyethylene (PE) wet film as the matrix, and evenly coats Al2O3 particles on both sides to obtain a composite coating PE lithium-ion battery diaphragm, which significantly improves the thermal safety performance, ionic conductivity and cycling performance of lithium-ion batteries. Jeong and others used atomic layer deposition technology to deposit an Al2O3 ceramic layer with a thickness of about 6nm on the surface of PP microporous membrane, which effectively improved the heat resistance and hydrophilicity of PP based membrane. 10. Huang mixed the fiber with Al2O3 to prepare a composite diaphragm, and then coated with a layer of PVDF membrane by dip coating method. After treatment, the composite diaphragm has stable cyclic performance and almost no shrinkage at 250 ℃. J. Lee et al. Studied that the surface of polyimide film was coated with al2o3/pvdf-hfp, which improved the wettability of the membrane and delayed the growth of battery impedance.

Surface coating SiO2 series: Yoo and others use the coating process to coat nano SiO2 on the PE diaphragm to obtain the ceramic PE diaphragm with SiO2 layer, and the heat resistance temperature is increased to 170 ℃ (pe135 ℃). H. S.jeong et al. Studied the effect of SiO2 with different particle sizes on the performance of composite membranes. The composite membranes prepared with 40nmsio2 had the highest porosity, and SiO2 did not dissolve after 200 cycles. Yang Yunxia's team from East China University of science and technology coated the PE film with boehmite. After treatment, the diaphragm had almost no thermal shrinkage at 140 ℃, and the thermal shrinkage of 0.5h at 180 ℃ was less than 3%, which significantly improved the thermal stability of the diaphragm. The evenly mixed slurry is coated on the surface of the base membrane with a specific machine or appliance to obtain a composite membrane containing TiO2 /batio3.

The disadvantages of organic coating and inorganic coating are serious hole blockage and large ion transfer resistance, which affect the wettability of diaphragm to electrolyte and the cycle performance of battery. In order to solve these problems, researchers have tried to use polymer nanoparticles, polymer fibers, PVDF, pan, PMMA, PEO, etc. as coating materials to replace the traditional dense coating, nano porous structure with high porosity, so as to improve the wettability of the diaphragm to electrolyte and the ionic conductivity of the battery.

Hu Jiwen's team of the Chinese Academy of sciences applied aramid fiber (ANF) to the surface of PP membrane by multiple impregnation. After coating, the diaphragm has good dimensional stability, and the magnification and cycle performance are significantly improved.

Organic / inorganic composite coating organic / inorganic composite coating diaphragm is the mixture of inorganic nanoparticles and organic polymers, and the evenly mixed slurry is coated on the diaphragm substrate. The Li Weishan research group of South China Normal University coated the composite membrane prepared by Quaternary polymer P (MMA Ba an ST) doped with CeO2 ceramic particles on the surface of PE membrane. Comparing the effects of different ceramic contents (0, 10%, 50%, 100%, 150% and 200%) on electrolyte retention and ionic conductivity, the ceramic content of about 50% is the best.

In situ compoundin situ compounding is to pre disperse ceramic particles or polymer fibers into the film-forming slurry, and make the diaphragm by wet biaxial stretching or electrospinning. Compared with organic or inorganic coatings, in-situ composite membranes solve the problem of coating falling off on the surface, forming a uniform open pore structure.

Donghua University of technology proposed to add ceramic nanoparticles to the electrospun pvdf/pan diaphragm by suction filtration. The ceramic load of the prepared composite diaphragm reached 67.5%, and the ceramic particles were evenly distributed, which had excellent comprehensive properties.

6 solid electrolyte membrane

Traditional lithium-ion batteries use volatile organic electrolytes, which has potential safety hazards. Solid electrolytes (mainly inorganic electrolytes and polymer electrolytes) are more safe for all solid-state lithium-ion batteries.

Inorganic solid electrolytes inorganic solid electrolytes include crystalline and amorphous forms. At present, LiPON electrolytes and sulfide electrolytes have good practical application prospects. These electrolyte materials are generally prepared by sputtering or powder sintering process. Li et al. Prepared pt/licoo2/lipon/snxny/pt with a thickness of only 7.6 by sputtering μ M thin film battery. When ≤ 150 ℃, the battery has high capacity retention rate and good high-temperature performance. The discharge capacity at 150 ℃ is 87% of that at 20 ℃.

Polymer electrolyte polymer electrolyte is an ionic conductive composite system composed of polymer and lithium salt. In recent years, there are mainly four categories: all solid polymer electrolyte, gel electrolyte, microporous gel polymer electrolyte and composite polymer electrolyte. All solid polymer electrolyte (SPE) is a combination of polymer and lithium salt that can dissolve lithium salt and migrate ions.

4、 Diaphragm preparation

Diaphragm preparation methods the mainstream lithium battery diaphragm production processes in the market are mainly divided into dry process and wet process, namely, dry process (melting lifting process) and wet process (thermally induced phase separation process). The pore forming mechanism of diaphragm micropores is different.

1 dry process

Dry method is to melt, extrude and blow polyolefin resin into crystalline polymer film. After crystallization and annealing, a highly oriented multilayer structure is obtained, and the crystalline interface is further stretched at high temperature to peel off, forming a porous structure, which can increase the pore size of the film.

According to different stretching directions, dry stretching can be divided into one-way stretching and two-way stretching. Among them, the core patents of unidirectional drawing process are mainly owned by enterprises in the United States and Japan; The Institute of chemistry of the Chinese Academy of Sciences has domestic patents on biaxially oriented PP. The film prepared by dry unidirectional stretching process is stretched at low temperature to form crazing and other defects, and then the defects are pulled open at high temperature to form micropores. At present, Celgard company of the United States and Yubu company of Japan all use this process to produce single-layer PE, PP and three-layer pp/pe/pp composite films.

The diaphragm produced by this process has a flat and long microporous structure. Because it is only stretched in one direction, the transverse strength of the diaphragm is relatively poor, but there is almost no heat shrinkage in the transverse direction. The product is thicker than the diaphragm produced by the wet method, and it is easy to tear in the longitudinal direction. Compared with unidirectional stretching, the strength of the dry biaxial stretching process in the transverse direction is improved, and the required performance can be obtained by appropriately changing the transverse and longitudinal stretching ratio according to the strength requirements of the diaphragm, and the micropore aperture of the biaxial stretching is more uniform and the air permeability is better. The dry stretching process is simple and pollution-free, which is a common method for preparing lithium-ion battery diaphragm. However, the pore diameter and porosity of this process are difficult to control, and the stretching is relatively small. At the same time, the diaphragm is easy to be perforated at low temperature, and the product is thick.

2 wet process

Wet method, i.e. phase separation method or thermally induced phase separation method, mixes liquid hydrocarbon or some small molecular substances with polyolefin resin, heats and melts to form a uniform mixture, then cools down for phase separation, and presses the membrane; Then the membrane was heated to a temperature close to the melting point, and the molecular chain was oriented by biaxial stretching. After heat preservation, the microporous membrane material was prepared by solvent extraction to form micropores. Enterprises such as Asahi Kasei, dongran, Nitto in Japan and Entek in the United States use this method to produce single-layer PE battery diaphragms.

The diaphragm produced by the wet biaxial stretching method has uniform pore forming dispersion, good wettability to electrolyte, isotropy, high transverse tensile strength, high puncture strength, normal process flow will not cause perforation and is not easy to tear, and the product can be made thinner to make the battery energy density higher. Domestic power and energy storage batteries mainly use PP diaphragm, while 3C batteries mainly use PE diaphragm. From the perspective of cost and technology, dry process will dominate the domestic power diaphragm Market in the short term. In the long run, wet process will be the mainstream trend of technology in the future.

3 electrospinning process

Electrospinning can produce uniform, small pore size, high specific surface area and high porosity fibers and fiber felt materials. The diameter of the fibers ranges from tens to thousands of nanometers, and the diameter of the fibers affects the pore diameter of the diaphragm. Electrospinning technology is to mix polymer and ceramic materials evenly to make slurry, and then use electrospinning equipment to prepare ceramic diaphragm. Ceramic particles are embedded in the fiber, which can significantly improve the thermal stability and electrolyte wettability of the diaphragm. Zhang Zihao reviewed the technology of electrospinning nanofiber membrane to prepare single polymer membrane, modified polymer membrane and organic / inorganic composite membrane. It mainly includes PVDF, PA, pet, pan, PMMA and PPESK.

4 wet process

Wet manufacturing is a common method for manufacturing diaphragm materials. The short and fine fibers are mixed and dispersed in the slurry, and the slurry is coated on the carrier with transfer coating. Finally, the diaphragm is obtained by dehydration / solvent, drying and winding. Zhang et al. Prepared the flame-retardant cellulose composite membrane with excellent wettability and liquid absorption rate by using the wet manufacturing process, which significantly reduced the cost of preparing the membrane. Cui Guanglei and others invented a paper-making technology to prepare non-woven diaphragms, which is simple, low-cost and can be produced on a large scale.

5 melt blown spinning process

Melt blown process is a method to directly spin the resin into a net to produce superfine fiber nonwovens, which has excellent impermeability and filtration performance. Deng Rongjian introduced that the melt blown spinning process has the advantages of mature technology, good safety and low cost. The polyester or polyamide nonwoven membranes prepared by melt blown method have excellent stability.

6-phase conversion method

The phase transformation method is to use the casting solution to carry out the mass transfer exchange of solvent and non solvent, so that the original steady-state solution will undergo phase transformation, and finally the phase separation structure will solidify into a film. A series of polyvinylidene fluoride (PVDF), polyethylene oxide (PEO) and polystyrene (PS) crosslinked composite gel polymer electrolyte films were prepared by using the reverse conversion method and in-situ crosslinking of polystyrene. The results showed that when the content of PS reached 25% of the total content of peo/ps, the thin film had high porosity, liquid absorption and conductivity.

Looking forward to the future,Lithium diaphragmWith the expansion of production capacity, the diaphragm with low cost, high wettability, good thermal stability, high safety and excellent cycle life will occupy the leading position in the industry. In recent years, domestic diaphragm enterprises have made rapid technological progress, and domestic diaphragms have gradually replaced imported diaphragms to occupy the middle and low-end lithium-ion battery market. With the maturity of various diaphragm preparation technologies, a hundred flowers will bloom and a hundred schools of thought will contend in the diaphragm Market in the future. In particular, solid electrolyte will become an important development direction, and solid-state batteries will be industrialized in the next 10 years.

     
Article source:Battery China