WO2017107435A1 - 一种pvdf涂覆锂离子电池隔膜及其制备方法 - Google Patents

一种pvdf涂覆锂离子电池隔膜及其制备方法 Download PDF

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WO2017107435A1
WO2017107435A1 PCT/CN2016/088407 CN2016088407W WO2017107435A1 WO 2017107435 A1 WO2017107435 A1 WO 2017107435A1 CN 2016088407 W CN2016088407 W CN 2016088407W WO 2017107435 A1 WO2017107435 A1 WO 2017107435A1
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pvdf
coating
slurry
ion battery
base film
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PCT/CN2016/088407
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English (en)
French (fr)
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赵中雷
武跃
邵培苓
孙卫佳
于中彬
王庆通
庄浩然
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沧州明珠隔膜科技有限公司
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M50/00—Constructional details or processes of manufacture of the non-active parts of electrochemical cells other than fuel cells, e.g. hybrid cells
    • H01M50/40—Separators; Membranes; Diaphragms; Spacing elements inside cells
    • H01M50/403—Manufacturing processes of separators, membranes or diaphragms
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M50/00—Constructional details or processes of manufacture of the non-active parts of electrochemical cells other than fuel cells, e.g. hybrid cells
    • H01M50/40—Separators; Membranes; Diaphragms; Spacing elements inside cells
    • H01M50/409—Separators, membranes or diaphragms characterised by the material
    • H01M50/443—Particulate material
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M50/00—Constructional details or processes of manufacture of the non-active parts of electrochemical cells other than fuel cells, e.g. hybrid cells
    • H01M50/40—Separators; Membranes; Diaphragms; Spacing elements inside cells
    • H01M50/489—Separators, membranes, diaphragms or spacing elements inside the cells, characterised by their physical properties, e.g. swelling degree, hydrophilicity or shut down properties
    • H01M50/491—Porosity
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/10—Energy storage using batteries

Definitions

  • the invention relates to a lithium ion battery technology, in particular to a PVDF coated lithium ion battery separator and a preparation method thereof, and belongs to the technical field of batteries.
  • Lithium-ion batteries have the advantages of high specific energy, long cycle life, no memory effect, safety, reliability, and rapid charge and discharge. They have become the research hotspots of new power supply technologies in recent years.
  • the composition of the lithium ion battery includes a positive electrode, a negative electrode, a separator and an electrolyte.
  • the separator acts as a barrier between the positive and negative electrodes and plays a vital role in the performance of the lithium ion battery. Its performance directly affects the capacity and circulation of the battery, especially It is an important factor affecting the safety performance of the battery. Coating on the surface of the diaphragm is an effective method to improve the safety of the diaphragm.
  • the PVDF (polyvinylidene fluoride) coated lithium ion battery separator is a surface-coated PVDF material which has been specially treated on the basis of a conventional separator.
  • the PVDF coating layer can be combined with the electrolyte in the lithium ion battery to form a stable gelled conductive polymer, which significantly improves the performance of the lithium ion battery.
  • the PVDF coated diaphragm has the following advantages: 1.
  • the PVDF coated diaphragm can increase the liquid absorption and liquid retention of the electrolyte, and significantly improve the safety of the battery; 2.
  • the PVDF coated diaphragm significantly increases the electricity.
  • PVDF coated diaphragm to reduce battery expansion rate, with low leakage rate
  • 4 PVDF coated diaphragm can increase the cycle performance of the battery
  • 5 PVDF coating can bond the diaphragm and pole piece, The hardness of the pole piece is made higher, the battery is thinner and stronger, and it is easy to process and transport.
  • the coating technology of the existing lithium ion battery PVDF separator generally adopts an oil coating process, and the oil coating process mostly uses acetone as a solvent.
  • the problem is that acetone is flammable, explosive, and has great harm to the human body. There are unsafe factors in the production process; at the same time, acetone is the raw material for the manufacture of ice, and its use is strictly controlled by the relevant departments; in addition, the coating made of an oily solvent such as acetone, after the surface coating of the separator, due to the oily slurry
  • the material has good compatibility with the separator material, and the slurry will penetrate into the micropores of the base film, resulting in a large loss of gas permeability, which easily causes the membrane to block the pores, affects the performance of the battery, and reduces the effective utilization space of the positive and negative materials in the battery, so that the battery Capacity and performance are limited.
  • the invention aims to overcome the defects of the prior art, and provides a PVDF coated lithium ion battery separator which is non-polluting, high in safety, thin in coating, and beneficial to improving the effective utilization space of the battery.
  • the present invention also provides a method of preparing the PVDF coated lithium ion battery separator.
  • a PVDF coated lithium ion battery separator is composed of a base film and a coating coated on one side or both sides of the base film, the coating being obtained by coating and drying the slurry, the coating thickness Between 0.1 and 0.5 ⁇ m, the coating contains aligned PVDF spherical particles.
  • the slurry is a low solid content aqueous PVDF slurry, the slurry contains 1-2.5% by weight of the base material, and the balance is deionized water, the base material is composed of The following mass parts of the composition: 65-75 parts of PVDF resin powder, 3-7 parts of aqueous binder, 1.5-3 parts of surfactant, 8-15 parts of dispersant, and the dispersing agent is triethyl phosphate.
  • the PVDF coated lithium ion battery separator is one of a vinylidene fluoride homopolymer powder or a vinylidene fluoride and a hexafluoropropylene copolymer powder.
  • the PVDF resin powder has spherical particles and a particle size of 100.
  • the aqueous binder is styrene-butadiene latex, styrene-acrylic latex, pure benzene latex, polymethyl methacrylate, polybutyl methacrylate, polyethyl acrylate, polyvinyl alcohol, ethylene-vinyl acetate copolymerization a mixture of one or more of polyvinyl acetate or polyurethane;
  • the surfactant is a fluoroalkyl methoxy ether alcohol, a fluoroalkyl ethoxy ether alcohol, a polyoxyethylene alkyl amide a mixture of one or more of fatty alcohol polyoxyethylene ethers.
  • the base film is a polyethylene base film, a polypropylene base film, a polypropylene/polyethylene/polypropylene composite base film, a polyimide base film, a polyvinylidene fluoride film, One of a polyethylene nonwoven fabric base film, a polypropylene nonwoven fabric base film or a polyimide nonwoven fabric base film having a base film thickness of 5 to 40 ⁇ m and a porosity of 30 to 60%.
  • a method of preparing a PVDF coated lithium ion battery separator as described above is carried out as follows:
  • the slurry coating method is one of gravure coating, narrow slit coating, dip coating or spray coating, and the drying temperature is 40- 70 ° C.
  • the present invention discards the conventional process of using a PVDF coated lithium ion battery separator with an oily substance such as acetone as a solvent, using water as a solvent for the PVDF material, and adding no thickener to obtain a low viscosity aqueous PVDF coating slurry. After coating with the slurry, an ultra-thin coating with a tight and tight arrangement of PVDF particles is obtained, and the coating thickness is only 0.1-0.5 ⁇ m.
  • the above ultra-thin coating can effectively bond the separator and the pole piece, thereby improving the hardness of the pole piece and the effective use space of the battery, and reducing the gas permeability loss caused by the thickness of the coating.
  • the present invention is directed to the fact that the PVDF resin has extremely strong hydrophobicity and is highly agglomerated in an aqueous system, and a reasonable ratio of triethyl phosphate is selected as a dispersing agent for PVDF.
  • Triethyl phosphate is added as an intermediate medium between water and PVDF.
  • triethyl phosphate is hydrolyzed to form diethyl phosphate, a large amount of hydroxyl groups are formed around the PVDF molecule, so that PVDF is well dispersed in water, effectively increasing the adhesion of PVDF.
  • Sexuality improves the cracking phenomenon caused by the evaporation of water during the drying process.
  • triethyl phosphate is inexpensive, simple to operate, reduces production costs, and is advantageous for industrial production.
  • the invention adopts water as the solvent of the PVDF resin, and the production process is friendly, high in safety, low in production cost, and on the other hand, water as a solvent can be greatly reduced, which is advantageous for industrial promotion.
  • the PVDF resin in the separator coating material of the invention can swell in the electrolyte, has good liquid absorption and liquid retention property to the electrolyte, increases battery cycle life, and significantly improves cell consistency and battery safety.
  • FIG. 1 is a graph showing constant current discharge cycle test data of a lithium ion battery prepared by a lithium ion battery prepared by a PVDF coated lithium ion battery separator of the present invention and a conventional oily PVDF coated lithium ion battery separator;
  • FIG. 2 is an electron scanning imaging diagram (SEM image) of an apparent morphology of a PVDF coated lithium ion battery separator of the present invention
  • FIG. 3 is an electron scanning imaging diagram of the apparent morphology of an oil-based PVDF coated lithium ion battery separator prepared by using triethyl phosphate as a solvent.
  • the PVDF coated lithium ion battery separator of the present invention is composed of a base film and a coating applied to one side or both sides of the base film, and the coating slurry is obtained after coating and drying.
  • the slurry used was a low-solids slurry of aqueous solvent PVDF.
  • the slurry contained 1-2.5% by weight of the base material, and the balance was deionized water.
  • the ultra-thin coating made of the slurry has a thickness of only 0.1-0.5 ⁇ m, and the coating contains uniformly arranged and uniformly dispersed PVDF spherical particles, as shown in FIG.
  • the above ultra-thin coating can effectively bond the separator and the pole piece, thereby improving the hardness of the pole piece and the effective use space of the battery, and reducing the gas permeability loss caused by the thickness of the coating.
  • PVDF is formed by the interposition of CH 2 bonds and CF 2 bonds. This chemical bond structure allows PVDF to have both the stability of a typical fluoropolymer and the unique interaction of its molecular chains.
  • the polarity which affects the interaction between PVDF and lithium ions, active materials and metal current collectors, ie the adhesion between the PVDF coating and the separator and the battery pole piece.
  • the symmetric distribution of fluorine atoms on the PVDF molecular chain leads to the problem that the surface energy of the material is low, hydrophobic, and difficult to disperse in water.
  • the material is hydrophilic depends on whether the material itself has polar groups such as hydroxyl, carboxyl, amino, etc., and PVDF does not have such a group, so if it has permanent stability, it must introduce a hydrophilic large group on its surface. .
  • the invention has long-term research and trial and error to select triethyl phosphate as a dispersing agent, on the one hand, the PVDF resin is dispersed, and on the other hand, the PVDF adhesion is improved.
  • Triethyl phosphate can be miscible with water in any proportion, stable at room temperature, slowly hydrolyzed to produce diethyl phosphate and ethanol when heated, and the hydrophilic structure of hydrophilic acid and ethyl ester in both molecular structure of diethyl phosphate and ethanol.
  • Ester group and hydrocarbon group are adsorbed on the surface of the PVDF particles, which reduces the interfacial tension between PVDF and water, and makes the surface of the PVDF particles easy to wet.
  • the PVDF particles when the PVDF particles are close, due to the steric hindrance effect of the adsorbed diethyl phosphate and ethanol on the surface, the PVDF particles will slide and stagger with each other, and it is difficult to re-agglomerate, so that the PVDF is well dispersed, the working area is increased, and the bonding is performed. Sexual improvement.
  • the volatilization temperature of water is low. When water is used as a solvent, the coating will be cracked due to excessive evaporation of water during the coating and drying process. The addition of triethyl phosphate will slow down the drying process and make the coating. The crack phenomenon disappeared. Furthermore, the replacement of the traditional dispersant with triethyl phosphate is simple and easy to operate.
  • Triethyl phosphate is added as a dispersing agent in the base material, and the ratio of the mixture needs to be controlled within a certain range. If the amount is higher, the higher the boiling point of triethyl phosphate, the longer the drying time of the wet film is added, and when added When the amount is very large, the system will tend to prepare oily PVDF with triethyl phosphate as solvent; if the amount is insufficient, it will weaken the dispersion of triethyl phosphate on PVDF, and on the other hand, it will lead to wet film drying process. The water volatilizes too quickly and the coating cracks.
  • the formation mechanism is as follows: the aqueous slurry is coated and heated to volatilize the water, and the remaining PVDF solid particles are stacked together in a layered arrangement, and the gap between the particles and the particles is formed.
  • the hole provides a lithium ion transport channel.
  • the spongy structure exists in a dissolution system using an organic solvent as a solvent, and is formed by phase inversion.
  • the formation mechanism is as follows: firstly, PVDF is dissolved into a slurry by using a suitable solvent, and after coating, it is heated or immersed in a coagulating solution, and mass transfer is carried out.
  • the process changes the composition of the slurry to cause phase separation, forming a dense phase of PVDF and a dilute phase, and finally the dense phase solidifies into a skeleton, and the dilute phase forms pores.
  • the main differences between the two pore forming mechanisms are as follows: the spherical structure adopts the aqueous slurry of the dispersion system, and the water is volatilized and then accumulated into pores.
  • the pore diameter is mainly affected by the size and distribution of the particle size.
  • the pore size of the pores is small and the pore distribution is uniform, which is advantageous for preparing a high-performance battery with uniform and stable charging and discharging current; while the sponge-like structure adopts an oily slurry of a dissolved system, and the pore forming process is mainly determined by the phase separation process. Since the phase separation process control is controlled by both thermodynamics and mass transfer dynamics, the pore size is large and the distribution range is wide, resulting in relatively poor uniformity of the coating. After assembly into a battery, the local charge and discharge is uneven. The uneven charging and discharging phenomenon greatly affects the safety performance and the performance of the large-capacity battery by superimposing and amplifying.
  • triethyl phosphate is used as a solvent to prepare oily PVDF coated separator.
  • Triethyl phosphate is a high boiling point solvent. The wet film after coating needs to be dried in a vacuum drying oven for more than 20 hours. It has a long production cycle and should not be used. In industrial production.
  • the binder of the present invention is composed of the following parts by mass: 65-75 parts of PVDF resin powder, 3-7 parts of aqueous binder, 1.5-3 parts of surfactant, 8-15 parts of dispersant, and phosphoric acid dispersant Triethyl ester.
  • the PVDF resin powder is a vinylidene fluoride homopolymer powder or one of a vinylidene fluoride and a hexafluoropropylene copolymer powder, the PVDF resin powder is spherical particles having a particle diameter of 100-150 nm;
  • the aqueous binder It is styrene-butadiene latex, styrene-acrylic latex, pure benzene latex, polymethyl methacrylate, polybutyl methacrylate, polyethyl acrylate, polyvinyl alcohol, ethylene-vinyl acetate copolymer, polyvinyl acetate, polyurethane.
  • the surfactant is a fluoroalkyl methoxy ether alcohol, a fluoroalkyl ethoxy ether alcohol, a polyoxyethylene alkyl amide, a fatty alcohol polyoxyethylene ether a mixture of one or several.
  • the base film of the present invention is a polyethylene separator, a polypropylene separator, a polypropylene/polyethylene/polypropylene composite separator, a polyimide separator, a polyvinylidene fluoride separator, a polyethylene nonwoven membrane, a polypropylene nonwoven fabric.
  • One of the separator and the polyimide nonwoven fabric separator has a base film thickness of 5 to 40 ⁇ m and a porosity of 30 to 60%.
  • the preparation method of the PVDF coated lithium ion battery separator of the invention is as follows: a. weighing each substance constituting the slurry according to the ratio; b. taking the proportion of the dispersant, deionized water, mixing the two, stirring 10-30 Minutes, heating to a temperature of 50 ° C -70 ° C, to make a mixture; c. adding a proportion of PVDF resin powder to the above mixture, grinding 1-2 hours to obtain a mixture of two; d.
  • the specific amount of the aqueous binder and surfactant is evenly stirred, and then filtered with a 400 mesh stainless steel mesh to obtain a low solid content aqueous PVDF slurry having a viscosity of 3-10 mPa ⁇ s; e.
  • the PVDF coated lithium ion battery separator was prepared by drying on one side or both sides of the base film.
  • Example 1 Weigh 6.5 kg of vinylidene fluoride homopolymer powder, 0.3 kg of styrene-butadiene latex, 1 kg of triethyl phosphate, 0.2 kg of fluoroalkyl methoxyether alcohol, 792 kg of deionized water, and triethyl phosphate Mixing with deionized water, stirring for 10 minutes, heating to 50 ° C to prepare a mixture 1; adding a vinylidene fluoride homopolymer powder to the mixture 1 for 1 hour to obtain a mixture 2; adding styrene-butadiene latex, fluorinated to the above mixture 2 After the alkyl methoxy ether alcohol was uniformly stirred, the PVDF slurry was prepared by filtration through a 400 mesh stainless steel mesh, and the viscosity of the slurry was 3 Pa ⁇ s; the PVDF slurry was applied to a thickness of 20 ⁇ m by a gravure coating method.
  • the rate was 38 m/min on both sides of the polyethylene film, and the coating rate was 20 m/min.
  • the drying was performed in a three-stage oven at 55 ° C, 70 ° C, and 60 ° C, and the PVDF coating was obtained after drying.
  • a lithium ion battery separator was coated, the separator having a thickness of 20.2 ⁇ m and a thickness of each side coating of 0.1 ⁇ m.
  • Example 2 Weigh 7.5 kg of vinylidene fluoride and hexafluoropropylene copolymer powder, 0.3 kg of styrene-acrylic latex, 0.4 kg of pure benzene latex, 1.5 kg of triethyl phosphate, 0.15 kg of polyoxyethylene alkylamide, and fatty alcohol Oxygen 0.15kg of vinyl ether, 390kg of deionized water; triethyl phosphate mixed with deionized water, stirred for 30 minutes, heated to 60 ° C to make a mixture 1; adding vinylidene fluoride and hexafluoropropylene copolymer powder to the mixture Grinding for 2 hours to obtain the mixture 2; adding styrene-acrylic latex, pure benzene latex, polyoxyethylene alkylamide, fatty alcohol polyoxyethylene ether to the above mixture 2, stirring uniformly, and filtering with a 400-mesh stainless steel mesh to obtain a PVDF slurry.
  • the slurry viscosity was 10 Pa ⁇ s; the PVDF slurry was applied to one side of a polyvinylidene fluoride film having a thickness of 30 ⁇ m and a porosity of 60% by a slit coating method at a coating rate of 25 m/min. Drying is carried out in a three-stage oven at 60 ° C, 65 ° C, and 55 ° C, respectively. After drying, a PVDF coated lithium ion battery separator is obtained. The thickness of the separator is 30.5 ⁇ m, and the thickness of the coating is 0.5 ⁇ m. .
  • Example 3 Weigh 7 kg of vinylidene fluoride homopolymer powder, 0.5 kg of polyvinyl acetate, 0.8 kg of triethyl phosphate, 0.15 kg of fluoroalkyl ethoxy ether alcohol, 500 kg of deionized water; The ethyl ester is mixed with deionized water, stirred for 20 minutes, and heated to 70 ° C to prepare a mixture 1; the mixture of the vinylidene fluoride homopolymer powder is added to the mixture for 1.5 hours to obtain a mixture 2; and polyvinyl acetate is added to the above mixture 2 After the fluoroalkyl methoxy ether alcohol was uniformly stirred, the PVDF slurry was prepared by filtration through a 400 mesh stainless steel mesh, and the viscosity of the slurry was 5.6 Pa ⁇ s; the PVDF slurry was applied by spray coating.
  • Example 4 Weigh 7.1 kg of vinylidene fluoride homopolymer powder, 0.18 kg of polymethyl methacrylate, 0.22 kg of polyurethane, 1.1 kg of triethyl phosphate, 0.2 kg of fluoroalkyl methoxy ether alcohol, go Ionized water 600kg; triethyl phosphate mixed with deionized water, stirred for 18 minutes, heated to 58 ° C to make a mixture 1; a mixture of vinylidene fluoride homopolymer powder was added to the mixture for 2 hours to obtain a mixture of two; to the above mixture After adding polymethyl methacrylate, polyurethane, fluoroalkyl methoxy ether alcohol and stirring uniformly, the PVDF slurry is prepared by filtering with a 400 mesh stainless steel mesh, and the viscosity of the slurry is 4.5 Pa ⁇ s; The PVDF slurry was applied to both sides of a polyimide nonwoven fabric base film having a thickness of 30 ⁇ m and a
  • the oven temperatures of the various stages were 55 ° C, 65 ° C, and 60 ° C, respectively.
  • an aqueous PVDF ultrathin coated lithium ion battery separator was obtained.
  • the thickness of the separator was 30.4 ⁇ m, and the thickness of each side coating layer was 0.2 ⁇ m.
  • Example 5 Weigh 7.2 kg of vinylidene fluoride and hexafluoropropylene copolymer powder, 0.7 kg of polyethyl acrylate, Triethyl phosphate 1.3kg, polyoxyethylene alkylamide 0.25kg, deionized water 461kg; triethyl phosphate mixed with deionized water, stirred for 15 minutes, heated to 65 ° C to make a mixture; Adding a vinylidene fluoride homopolymer powder for 1 hour to obtain a mixture 2; adding polyethyl acrylate and polyoxyethylene alkyl amide to the above mixture 2, stirring uniformly, and filtering with a 400 mesh stainless steel mesh to obtain a PVDF slurry.
  • the viscosity of the slurry was 8 Pa ⁇ s; the PVDF slurry was applied to both sides of a polyethylene film having a thickness of 12 ⁇ m and a porosity of 38% by a gravure coating method at a coating rate of 20 m/min; using a three-stage oven Drying, oven temperature of each stage is 50 ° C, 60 ° C, 65 ° C, after drying to obtain an aqueous PVDF ultra-thin coated lithium ion battery separator, the thickness of the separator is 12.6 ⁇ m, the thickness of each side coating is 0.3 ⁇ m.
  • the PVDF coated separator prepared according to the conventional oil coating process was taken as a comparative example.
  • the comparative membrane thickness was 14 ⁇ m
  • the double-sided coating the coating thickness was 2 ⁇ m
  • the base film was a polyethylene film
  • the base film thickness was 12 ⁇ m
  • the porosity was 38. %.
  • the separator prepared in Example 5 and the comparative separator were respectively fabricated into a soft-package lithium ion battery by a winding process using a prepared nickel-cobalt-manganese ternary material (type 523) positive electrode tab and graphite (FSN-1) negative electrode.
  • the battery capacity, internal resistance and the bond strength between the separator and the pole piece are compared.
  • the results are shown in Table 2:
  • the lithium ion battery prepared by using the separator of the present invention has higher capacity and lower internal resistance than the lithium ion battery prepared by the proportional diaphragm, and the superior bonding performance can reduce the external force of the battery.
  • the cycle test data of the above two batteries using 0.5C constant current constant voltage charging / 1.0C constant current discharge is shown in Figure 1.
  • the cycle performance of the lithium ion battery prepared by the present invention is better than the comparative example.

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Abstract

一种PVDF涂覆锂离子电池隔膜及其制备方法，所述PVDF涂覆锂离子电池隔膜由基膜和涂布于基膜单侧或双侧的涂层构成，所述涂层由浆料经涂布、烘干后获得，所述涂层厚度为0.1-0.5μm，涂层中包含排列均匀的PVDF球状颗粒。本发明摒弃现有PVDF涂覆锂离子电池隔膜以丙酮等油性物质作溶剂的传统工艺，采用水作为PVDF材料的溶剂，且不添加任何增稠剂，得到低粘度水性PVDF涂覆浆料，使用该浆料涂覆后得到PVDF颗粒排布整齐且相对疏松的超薄涂层，上述超薄涂层在能够有效粘接隔膜和极片的同时，提升了极片硬度和电池有效利用空间，降低了因涂层厚而带来的透气损失。

Description

一种PVDF涂覆锂离子电池隔膜及其制备方法 技术领域
本发明涉及一种锂离子电池技术，特别是一种PVDF涂覆锂离子电池隔膜及其制备方法，属电池技术领域。
背景技术
锂离子电池具有高比能量、长循环寿命、无记忆效应的特性以及安全、可靠且能快速充放电等优点，成为近年来新型电源技术的研究热点。锂离子电池的构成包括正极、负极、隔膜和电解质，隔膜作为正负极之间的阻隔物对锂离子电池的性能起到至关重要的作用，其性能直接影响到电池的容量和循环，特别是影响到电池安全性能的重要因素，在隔膜表面进行涂覆是提高隔膜安全性的有效方法。PVDF(聚偏氟乙烯)涂覆的锂离子电池隔膜是在传统隔膜的基础上，经过特殊工艺处理表面涂覆PVDF材料。PVDF涂覆层可与锂离子电池中电解质结合为稳定的凝胶质导电聚合物，显著提高锂离子电池的性能。与普通隔膜相比，PVDF涂覆隔膜具有以下优点：1、PVDF涂覆隔膜可以增加对电解液的吸液性和保液性，显著提高电池的安全性；2、PVDF涂覆隔膜明显提高电芯一致性；3、PVDF涂覆隔膜减少电池膨胀率，具有低的漏液气胀率；4、PVDF涂覆隔膜能够增加电池的循环性能；5、PVDF涂层可以粘接隔膜和极片，使得极片硬度变高，电池更薄更结实，方便加工和运输。
现有锂离子电池PVDF隔膜的涂层技术，普遍采用油性涂覆工艺，油性涂覆工艺多是采用丙酮做溶剂，存在的问题是：丙酮易燃、易爆，且对人体有很大伤害，在生产过程中存有不安全因素；同时丙酮是制造***的原料，其使用受到有关部门的控制严格；此外，采用丙酮等油性溶剂制作的涂层，隔膜进行表面涂覆后，由于油性浆料与隔膜材质相容性较好，浆料会渗透到基膜的微孔中，导致透气损失很大，容易造成隔膜堵孔，影响电池性能，缩小了电池中正负极材料的有效利用空间，使电池容量及性能受到限制。
发明内容
本发明旨在克服现有技术的缺陷，提供一种生产过程无污染、安全性高、涂层薄、有利于提升电池有效利用空间的PVDF涂覆锂离子电池隔膜。
此外，本发明还提供了所述PVDF涂覆锂离子电池隔膜的制备方法。
为达到上述目的，本发明采用的技术方案是这样的：
一种PVDF涂覆锂离子电池隔膜，由基膜和涂布于基膜单侧或双侧的涂层构成，所述涂层由浆料经涂布、烘干后获得，所述涂层厚度为0.1-0.5μm，涂层中包含排列整齐的PVDF球状颗粒。
上述PVDF涂覆锂离子电池隔膜，所述浆料为低固含量水性PVDF浆料，浆料中按照重量百分比计含有1-2.5％的基料，余量为去离子水，所述基料由以下质量份的物质组成：PVDF树脂粉末65-75份、水性粘合剂3-7份、表面活性剂1.5-3份、分散剂8-15份，分散剂为磷酸三乙酯。
上述PVDF涂覆锂离子电池隔膜，所述PVDF树脂粉末为偏氟乙烯均聚物粉末或偏氟乙烯与六氟丙烯共聚物粉末中的一种，PVDF树脂粉末呈球形颗粒，颗粒粒径为100-150nm；所述水性粘合剂为丁苯乳胶、苯丙乳胶、纯苯乳胶、聚甲基丙烯酸甲酯、聚甲基丙烯酸丁酯、聚丙烯酸乙酯、聚乙烯醇、乙烯-醋酸乙烯共聚物、聚醋酸乙烯酯或聚氨酯中的一种或几种的混合物；所述表面活性剂为氟代烷基甲氧基醚醇、氟代烷基乙氧基醚醇、聚氧乙烯烷基酰胺、脂肪醇聚氧乙烯醚中的一种或几种的混合物。
上述PVDF涂覆锂离子电池隔膜，所述基膜为聚乙烯基膜、聚丙烯基膜、聚丙烯/聚乙烯/聚丙烯复合基膜、聚酰亚胺基膜、聚偏氟乙烯基膜、聚乙烯无纺布基膜、聚丙烯无纺布基膜或聚酰亚胺无纺布基膜中的一种，基膜厚度为5-40μm，孔隙率为30-60％。
一种制备如上所述PVDF涂覆锂离子电池隔膜的方法，按下述步骤进行：
a.按照配比称取组成浆料的各物质；
b.取配比量的分散剂、去离子水，两者混合，搅拌10-30分钟，加热至温度50℃-70℃，制成混合物一；
c.向上述混合物一中加入配比量的PVDF树脂粉末，研磨1-2小时得到混 合物二；
d.向上述混合物二中加入配比量的水性粘合剂、表面活性剂，搅拌均匀后，用400目不锈钢筛网过滤即制得低固含量水性PVDF浆料，浆料粘度为3-10mPa·s；
e.将浆料涂布于基膜的单侧或双侧，干燥后即制得PVDF涂覆锂离子电池隔膜。
上述PVDF涂覆锂离子电池隔膜的制备方法，浆料涂布方式为凹版式涂布、窄缝式涂布、浸涂式涂布或喷涂式涂布中的一种，烘干温度为40-70℃。
本发明PVDF涂覆锂离子电池隔膜与现有技术相比，具有以下主要优点：
1、本发明摒弃现有PVDF涂覆锂离子电池隔膜以丙酮等油性物质作溶剂的传统工艺，采用水作为PVDF材料的溶剂，且不添加任何增稠剂，得到低粘度水性PVDF涂覆浆料，使用该浆料涂覆后得到PVDF颗粒排布整齐紧密的超薄涂层，涂层厚度仅为0.1-0.5μm。上述超薄涂层在能够有效粘接隔膜和极片的同时，提升了极片硬度和电池有效利用空间，降低了因涂层厚而带来的透气损失。
2、本发明针对PVDF树脂具有极强疏水性、在水性体系中团聚严重的特点，选择合理配比的磷酸三乙酯作为PVDF的分散剂。磷酸三乙酯作为水和PVDF的中间介质加入，当磷酸三乙酯水解生成磷酸二乙酯时，在PVDF分子周围生成大量的羟基，使PVDF较好的分散在水中，有效提高PVDF的粘结性，改善了涂层在烘干过程中因水分挥发太快造成的龟裂现象。此外，磷酸三乙酯价格低廉，操作简单，降低了生产成本，利于工业化生产。
3、本发明采用水作为PVDF树脂的溶剂，生产过程环境友好、安全性高，生产成本低，另一方面用水作溶剂能够极大地降低，利于工业推广。
4、本发明隔膜涂层材料中的PVDF树脂在电解液中能够溶胀，对电解液有良好的吸液性和保液性，增加电池循环寿命，显著提高电芯一致性和电池安全性。
附图说明
图1是以本发明PVDF涂覆锂离子电池隔膜制备的锂离子电池与常规油性PVDF涂覆锂离子电池隔膜制备的锂离子电池的恒流放电循环测试数据图；
图2是本发明PVDF涂覆锂离子电池隔膜表观形貌电子扫描显像图(SEM图)；
图3是采用磷酸三乙酯作为溶剂制备的油性PVDF涂覆锂离子电池隔膜表观形貌电子扫描显像图。
具体实施方式
本发明所述PVDF涂覆锂离子电池隔膜，由基膜和涂布于基膜单侧或双侧的涂层构成，所述涂层浆料经涂布、烘干后获得。采用的浆料为水性溶剂PVDF低固含量的浆料，浆料中按照重量百分比计含有1-2.5％的基料，余量为去离子水。以该浆料制成的超薄涂层厚度仅为0.1-0.5μm，涂层中包含排列整齐、分散均匀的PVDF球状颗粒，如图2所示。上述超薄涂层在能够有效粘接隔膜和极片的同时，提升了极片硬度和电池有效利用空间，降低了因涂层厚而带来的透气损失。在结构上，PVDF由CH2键和CF2键相间连接而成，这种化学键结构使PVDF既具有典型的含氟聚合物的稳定性，又使其分子链上的交互基团能产生一个独特的极性，该极性影响着PVDF与锂离子、活性材料和金属集流器之间的相互作用力，即PVDF涂层与隔膜和电池极片间的粘接力。但同时PVDF分子链上氟原子的对称分布又导致了该材料表面能低、疏水性强、在水中难分散的问题。
材料是否亲水，取决于材料本身是否带极性基团如羟基、羧基、氨基等，而PVDF不具备此类基团，因此若具备永久稳定性，必须在其表面引入亲水性大基团。本发明经长期研究、反复试验选用磷酸三乙酯作为分散剂，一方面分散PVDF树脂，另一方面提高PVDF粘接性。磷酸三乙酯可与水以任何比例混溶，常温下稳定，加热时慢慢水解生成磷酸二乙酯和乙醇，磷酸二乙酯和乙醇分子结构中均含有亲水的羟基，又含有亲油的酯基和烃基。酯基和烃基吸附在PVDF颗粒表面，降低了PVDF与水之间的界面张力，使PVDF颗粒表面易于湿润。同时，当PVDF颗粒接近时，由于表面吸附的磷酸二乙酯和乙醇产生空间位阻效应，PVDF颗粒之间会相互滑动错开，难以再发生团聚，使PVDF分散良好，作用面积增大，粘接性提升。此外，水的挥发温度较低，以水为溶剂时，隔膜涂覆烘干过程中会因水分挥发太快造成涂层龟裂，加入适量磷酸三乙酯后会减慢干燥过程，使涂层龟裂现象消失。再者，以磷酸三乙酯替代传统分散剂，操作简单， 分散效果好，降低了成本，利于工业化生产。磷酸三乙酯作为分散剂添加在基料中，其配比量需控制在一定的范围，若过量，由于磷酸三乙酯沸点较高，则会增加最后湿膜烘干的时间，且当加入量非常大时，体系会倾向于以磷酸三乙酯为溶剂制备油性PVDF；若加入量不足时，一方面会减弱磷酸三乙酯对PVDF的分散作用，另一方面会导致湿膜烘干过程中水分挥发太快涂层产生龟裂。
现有技术，有将磷酸三乙酯作为溶剂制备油性PVDF涂覆隔膜的工艺方法，通过PVDF溶解在磷酸三乙酯中制得浆料，经涂布、烘干后获得PVDF涂覆隔膜。以该工艺方法制得浆料为溶解体系，破坏了PVDF的球形结构，涂层形貌呈海绵状，如图3所示。本发明PVDF涂层的球形结构与现有技术的海绵状结构属于两种完全不同的多孔结构，这取决于二者不同的成孔机理。球形结构存在于以水为溶剂的分散体系，形成机理如下：水性浆料经涂覆后加热使水分挥发，剩下的PVDF固体颗粒堆积在一起成层状排布，颗粒与颗粒间的间隙成孔，提供锂离子传输通道。海绵状结构存在于以有机溶剂为溶剂的溶解体系，通过相转化形成，形成机理如下：先使用合适的溶剂将PVDF溶解制成浆料，涂覆后经加热或浸凝固液处理，经传质过程使浆料组分产生变化导致分相，形成PVDF浓相和稀相，最终浓相固化成骨架，稀相成孔。通过比较可以发现二者成孔机理主要差别如下：球形结构采用分散体系的水性浆料，水分挥发后堆积成孔，其孔径主要受颗粒粒径的大小和分布影响，由于PVDF粒径较小，故成孔的孔径小且孔隙分布均匀，这就有利于制备充放电流均匀、稳定的高性能电池；而海绵状结构则采用了溶解体系的油性浆料，成孔过程主要由分相过程决定，由于分相过程控制受热力学和传质动力学两方面的共同控制，孔径较大且分布范围宽，导致涂层均匀性相对较差，组装成电池以后导致局部充放电不均匀，这种局部不均匀的充放电现象通过叠加放大对大容量电池的安全性能以及性使用性能造成极大影响。此外，将磷酸三乙酯作为溶剂制备油性PVDF涂覆隔膜，磷酸三乙酯为高沸点溶剂，涂布后的湿膜需在真空干燥箱中干燥20h以上，具有很长的生产周期，不宜用于工业化生产。
本发明所述基料由以下质量份的物质组成：PVDF树脂粉末65-75份、水性粘合剂3-7份、表面活性剂1.5-3份、分散剂8-15份，分散剂为磷酸三乙酯。 所述PVDF树脂粉末为偏氟乙烯均聚物粉末或偏氟乙烯与六氟丙烯共聚物粉末中的一种，PVDF树脂粉末呈球形颗粒，颗粒粒径为100-150nm；所述水性粘合剂为丁苯乳胶、苯丙乳胶、纯苯乳胶、聚甲基丙烯酸甲酯、聚甲基丙烯酸丁酯、聚丙烯酸乙酯、聚乙烯醇、乙烯-醋酸乙烯共聚物、聚醋酸乙烯酯、聚氨酯中的一种或几种的混合物；所述表面活性剂为氟代烷基甲氧基醚醇、氟代烷基乙氧基醚醇、聚氧乙烯烷基酰胺、脂肪醇聚氧乙烯醚中的一种或几种的混合物。
本发明所述基膜为聚乙烯隔膜、聚丙烯隔膜、聚丙烯/聚乙烯/聚丙烯复合隔膜、聚酰亚胺隔膜、聚偏氟乙烯隔膜、聚乙烯无纺布隔膜、聚丙烯无纺布隔膜、聚酰亚胺无纺布隔膜中的一种，基膜厚度为5-40μm，孔隙率为30-60％。
本发明PVDF涂覆锂离子电池隔膜的制备方法如下：a.按照配比称取组成浆料的各物质；b.取配比量的分散剂、去离子水，两者混合，搅拌10-30分钟，加热至温度50℃-70℃，制成混合物一；c.向上述混合物一中加入配比量的PVDF树脂粉末，研磨1-2小时得到混合物二；d.向上述混合物二中加入配比量的水性粘合剂、表面活性剂，搅拌均匀后，用400目不锈钢筛网过滤即制得低固含量水性PVDF浆料，浆料粘度为3-10mPa·s；e.将浆料涂布于基膜的单侧或双侧，干燥后即制得PVDF涂覆锂离子电池隔膜。
以下提供几个本发明的具体实施例：
实施例1：称取偏氟乙烯均聚物粉末6.5kg，丁苯乳胶0.3kg，磷酸三乙酯1kg，氟代烷基甲氧基醚醇0.2kg，去离子水792kg；将磷酸三乙酯与去离子水混合，搅拌10分钟，加热至50℃制成混合物一；向混合物一中加入偏氟乙烯均聚物粉末研磨1小时得到混合物二；向上述混合物二中加入丁苯乳胶、氟代烷基甲氧基醚醇搅拌均匀后，用400目不锈钢筛网过滤即制得PVDF浆料，浆料粘度为3Pa·s；采用凹版式涂布方式将PVDF浆料涂布于厚度20μm、孔隙率为38％的聚乙烯基膜的双侧，涂布速率为20m/min；使用三级烘箱进行烘干，各级烘箱温度分别为55℃、70℃、60℃，干燥后制得PVDF涂覆锂离子电池隔膜，所述隔膜的厚度为20.2μm，各侧面涂层的厚度为0.1μm。
实施例2：称取偏氟乙烯与六氟丙烯共聚物粉末7.5kg，苯丙乳胶0.3kg，纯苯乳胶0.4kg，磷酸三乙酯1.5kg，聚氧乙烯烷基酰胺0.15kg、脂肪醇聚氧 乙烯醚0.15kg，去离子水390千克；将磷酸三乙酯与去离子水混合，搅拌30分钟，加热至60℃制成混合物一；向混合物一中加入偏氟乙烯与六氟丙烯共聚物粉末研磨2小时得到混合物二；向上述混合物二中加入苯丙乳胶、纯苯乳胶、聚氧乙烯烷基酰胺、脂肪醇聚氧乙烯醚搅拌均匀后，用400目不锈钢筛网过滤即制得PVDF浆料，浆料粘度为10Pa·s；采用窄缝式涂布方式将PVDF浆料涂布于厚度30μm、孔隙率为60％的聚偏氟乙烯基膜的单侧，涂布速率为25m/min；使用三级烘箱进行烘干，各级烘箱温度分别为60℃、65℃、55℃，干燥后制得PVDF涂覆锂离子电池隔膜，隔膜的厚度为30.5μm，涂层的厚度为0.5μm。
实施例3：称取偏氟乙烯均聚物粉末7kg，聚醋酸乙烯酯0.5kg，磷酸三乙酯0.8kg，氟代烷基乙氧基醚醇0.15kg，去离子水500千克；将磷酸三乙酯与去离子水混合，搅拌20分钟，加热至70℃制成混合物一；向混合物一中加入偏氟乙烯均聚物粉末研磨1.5小时得到混合物二；向上述混合物二中加入聚醋酸乙烯酯、氟代烷基甲氧基醚醇搅拌均匀后，用400目不锈钢筛网过滤即制得PVDF浆料，浆料粘度为5.6Pa·s；采用喷涂式涂布方式将PVDF浆料涂布于厚度14μm、孔隙率为55％的聚酰亚胺基膜的单侧，涂布速率为20m/min；使用三级烘箱进行烘干，各级烘箱温度分别为45℃、55℃、50℃，干燥后制得PVDF涂覆锂离子电池隔膜，所述隔膜的厚度为14.4μm，涂层的厚度为0.4μm。
实施例4：称取偏氟乙烯均聚物粉末7.1kg，聚甲基丙烯酸甲酯0.18kg，聚氨酯0.22kg，磷酸三乙酯1.1kg，氟代烷基甲氧基醚醇0.2kg、，去离子水600kg；将磷酸三乙酯与去离子水混合，搅拌18分钟，加热至58℃制成混合物一；向混合物一中加入偏氟乙烯均聚物粉末研磨2小时得到混合物二；向上述混合物二中加入聚甲基丙烯酸甲酯、聚氨酯、氟代烷基甲氧基醚醇搅拌均匀后，用400目不锈钢筛网过滤即制得PVDF浆料，浆料粘度为4.5Pa·s；采用浸涂式涂布方式将PVDF浆料涂布于厚度30μm、孔隙率为60％的聚酰亚胺无纺布基膜的双侧，涂布速率为20m/min；使用三级烘箱进行烘干，各级烘箱温度分别为55℃、65℃、60℃，干燥后制得水性PVDF超薄涂覆锂离子电池隔膜，所述隔膜的厚度为30.4μm，各侧面涂层的厚度为0.2μm。
实施例5：称取偏氟乙烯与六氟丙烯共聚物粉末7.2kg，聚丙烯酸乙酯0.7kg， 磷酸三乙酯1.3kg，聚氧乙烯烷基酰胺0.25kg，去离子水461千克；将磷酸三乙酯与去离子水混合，搅拌15分钟，加热至65℃制成混合物一；向混合物一中加入偏氟乙烯均聚物粉末研磨1小时得到混合物二；向上述混合物二中加入聚丙烯酸乙酯、聚氧乙烯烷基酰胺搅拌均匀后，用400目不锈钢筛网过滤即制得PVDF浆料，浆料粘度为8Pa·s；采用凹版式涂布方式将PVDF浆料涂布于厚度12μm、孔隙率为38％的聚乙烯基膜的双侧，涂布速率为20m/min；使用三级烘箱进行烘干，各级烘箱温度分别为50℃、60℃、65℃，干燥后制得水性PVDF超薄涂覆锂离子电池隔膜，所述隔膜的厚度为12.6μm，各侧面涂层的厚度为0.3μm。
取按照常规油性涂覆工艺制备的PVDF涂覆隔膜作为对比例，对比例隔膜厚度为14μm，双侧涂层，涂层厚度2μm，基膜为聚乙烯基膜，基膜厚度12μm，孔隙率38％。
实施例5制备的隔膜与对比例隔膜性能测试数据参见表1：
表1：
Figure PCTCN2016088407-appb-000001
由表1可见，本发明所述隔膜的透气性、接触角及吸液率均优于对比例。使用相同基膜时，透气性好说明涂层对隔膜微孔影响小；接触角小、吸液率高说明隔膜的吸液保液性能更优异。
取实施例5制备的隔膜和对比例隔膜，分别与制备的镍钴锰三元材料(523型)正极极片和石墨(FSN-1)负极采用卷绕工艺，制成软包装锂离子电池，进行电池容量、内阻及隔膜与极片间粘接强度对比试验，所得结果见表2：
表2：
Figure PCTCN2016088407-appb-000002
Figure PCTCN2016088407-appb-000003
由表2可见，采用本发明隔膜制备的锂离子电池水相比对比例隔膜制备的锂离子电池具有更高的容量和更低的内阻，同时更加优异的粘接性能可减少电池在外力作用下极片与隔膜错位导致的短路风险。上述两种电池采用0.5C恒流恒压充电/1.0C恒流放电进行的循环测试试数据如图1所示，由图可见，以本发隔膜制备的锂离子电池的循环性能优于对比例隔膜制备的锂离子电池。

Claims (6)

  1. 一种PVDF涂覆锂离子电池隔膜，由基膜和涂布于基膜单侧或双侧的涂层构成，所述涂层由浆料经涂布、烘干后获得，其特征在于：所述涂层厚度为0.1-0.5μm，涂层中包含排列整齐的PVDF球状颗粒。
  2. 根据权利要求1所述的PVDF涂覆锂离子电池隔膜，其特征在于，所述浆料为低固含量水性PVDF浆料，浆料中按照重量百分比计含有1-2.5％的基料，余量为去离子水，所述基料由以下质量份的物质组成：PVDF树脂粉末65-75份、水性粘合剂3-7份、表面活性剂1.5-3份、分散剂8-15份，分散剂为磷酸三乙酯。
  3. 根据权利要求2所述的PVDF涂覆锂离子电池隔膜，其特征在于，所述PVDF树脂粉末为偏氟乙烯均聚物粉末或偏氟乙烯与六氟丙烯共聚物粉末中的一种，PVDF树脂粉末呈球形颗粒，颗粒粒径为100-150nm；所述水性粘合剂为丁苯乳胶、苯丙乳胶、纯苯乳胶、聚甲基丙烯酸甲酯、聚甲基丙烯酸丁酯、聚丙烯酸乙酯、聚乙烯醇、乙烯-醋酸乙烯共聚物、聚醋酸乙烯酯、聚氨酯中的一种或几种的混合物；所述表面活性剂为氟代烷基甲氧基醚醇、氟代烷基乙氧基醚醇、聚氧乙烯烷基酰胺、脂肪醇聚氧乙烯醚中的一种或几种的混合物。
  4. 根据权利要求3所述的PVDF涂覆锂离子电池隔膜，其特征在于，所述基膜为聚乙烯基膜、聚丙烯基膜、聚丙烯/聚乙烯/聚丙烯复合基膜、聚酰亚胺基膜、聚偏氟乙烯基膜、聚乙烯无纺布基膜、聚丙烯无纺布基膜、聚酰亚胺无纺布基膜中的一种，基膜厚度为5-40μm，孔隙率为30-60％。
  5. 一种制备如权利要求1、2、3或4所述的PVDF涂覆锂离子电池隔膜的方法，其特征在于，按照下述步骤进行：
    a.按照配比称取组成浆料的各物质；
    b.取配比量的分散剂、去离子水，两者混合，搅拌10-30分钟，加热至温度50℃-70℃，制成混合物一；
    c.向上述混合物一中加入配比量的PVDF树脂粉末，研磨1-2小时得到混合物二；
    d.向上述混合物二中加入配比量的水性粘合剂、表面活性剂，搅拌均匀后，用400目不锈钢筛网过滤即制得低固含量水性PVDF浆料，浆料粘度为3-10mPa·s；
    e.将浆料涂布于基膜的单侧或双侧，干燥后即制得PVDF涂覆锂离子电池隔膜。
  6. 根据权利要求6所述的PVDF涂覆锂离子电池隔膜的制备方法，其特征在于，涂布方式为凹版式涂布、窄缝式涂布、浸涂式涂布或喷涂式涂布中的一种，烘干温度为40-70℃。
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