KR101447565B1 - Porous separating membrane for secondary battery which contains inorganic coating layer method of - Google Patents

Abstract

The present invention relates to a separator for a lithium secondary battery composed of a conventional polyolefin film and a method of manufacturing a separator for separating inorganic particles (hereinafter, referred to as " inorganic particles ") on a polyacrylonitrile nanoweb on which polyacrylonitrile (PAN) is electrospun on a polyolefin substrate, And a polyacrylonitrile-affinity polymer resin by a casting method to obtain a porous separator for a secondary battery having high heat resistance and high temperature, and a method for producing the same.

Description

TECHNICAL FIELD [0001] The present invention relates to a porous separator for a secondary battery including an inorganic coating layer and a method for manufacturing the same.

The present invention relates to a porous separator for a secondary battery comprising an inorganic coating layer and a method of manufacturing the same. More particularly, the present invention relates to a porous separator for a secondary battery comprising a polyacrylonitrile in dimethyl formamide (DMF) or dimethylacetamide (DMAc) The slurry prepared by dissolving the spinning solution by electrospinning on the polyolefin substrate and adding the inorganic particles (SiO ₂ , Al ₂ O _3, etc.) and the polyacrylonitrile-affinity polymer resin (PVDF, PVA, SBR etc.) 0001] The present invention relates to a porous separator and a porous separator that are coated on an acrylonitrile nanofiber by a casting method.

As consumers' demands have changed due to digitization and high performance of electronic products, market demand is changing due to the development of batteries with high capacity due to thinness, light weight and high energy density. In addition, in order to cope with future energy and environmental problems, hybrid electric vehicles and fuel cell vehicles are being actively developed, and it is required to increase the size of batteries for automobile power sources.

Secondary batteries, including high energy density and large capacity lithium ion secondary cells, lithium ion polymer batteries, and super capacitors (electric double layer capacitors and similar capacitors), must have a relatively high operating temperature range and should be continuously used at high rate charge and discharge The separation membrane used in these batteries is required to have higher heat resistance and thermal stability than those required in ordinary separation membranes. In addition, it should have excellent cell characteristics such as rapid charge / discharge and high ion conductivity capable of coping with low temperature.

The separator is disposed between the anode and the cathode of the battery to insulate the electrolyte and maintain the electrolyte to provide a path for ion conduction. When the temperature of the battery becomes excessively high, a part of the separator melts to block the current, Lt; / RTI >

When the temperature rises further and the separator melts, a large hole is formed and a short circuit occurs between the anode and the cathode. This temperature is called SHORT CIRCUIT TEMPERATURE. In general, the separator should have a lower SHUTDOWN temperature and a higher short-circuit temperature. In the case of the polyethylene separator membrane, when the battery is abnormally heated, it contracts at 150 or more, and the electrode portion is exposed, which may cause a short circuit. Therefore, it is very important to have both a closing function and a heat resistance for a high energy density, large-sized secondary battery.

That is, there is a need for a separation membrane having excellent heat resistance, small heat shrinkage, and excellent cycle performance in accordance with high ion conductivity.

The porous nanofiber can be used for various purposes because it has a wide surface area and excellent porosity. For example, it can be used for a water purification filter, an air purification filter, a composite material, and a separator for a battery. Particularly, such a porous nanofiber can be applied to a membrane of a fuel cell for an automobile.

Conventional lithium ion polymer batteries using a conventional polyolefin separator and a liquid electrolyte such as a lithium ion secondary battery or a gel polymer electrolyte membrane or a polymer electrolyte gel coated on a polyolefin separator have a high energy density and a very high Lack. Therefore, it does not satisfy the heat resistance and safety required in a high capacity, large area battery such as automobile.

In order to solve this problem, US Patent Application Publication No. 2006/0019154 A1 discloses a polyolefin-based separator which is impregnated with a polyamide, polyimide or polyamideimide solution having a melting point of 180 ° C or higher and then immersed in a coagulating liquid to extract a solvent, A heat - resistant polyolefin separator with thin layer of resin is proposed. It has a small heat shrinkage, excellent heat resistance and excellent cycle performance. The heat-resistant thin layer is made porous through solvent extraction and the polyolefin separator used is limited to the use of an AIR PERMEABILITY of 200 sec / min or less.

Japanese Patent Application Laid-Open No. 2005-209570 also discloses a heat-resistant resin such as an aromatic polyamide having a melting point of 200 ° C or higher, polyimide, polyether sulfone, polyether ketone, polyether imide, etc., The solution was applied on both sides of the polyolefin membrane, and immersed in a coagulating liquid, washed with water, and dried to give a polyolefin membrane having a heat resistant resin. In order to reduce the deterioration of ionic conductivity, a phase separator for providing porosity is contained in the heat resistant resin solution and the heat resistant resin layer is limited to 0.5-6.0 g / m 2.

However, the immersion in the heat resistant resin blocks the pores of the polyolefin separator to restrict the movement of lithium ions, resulting in a decrease in charge / discharge characteristics, and even if the heat resistance is secured, the capacity required for large capacity batteries such as automobiles is insufficient. Even if the pore structure of the polyolefin porous membrane is not clogged due to the immersion of the heat resistant resin, the porosity of the polyolefin separator generally used is about 40%, and the pore size is also several tens of nanometers, which limits the ion conductivity for a large capacity battery .

U.S. Patent No. 6,447,958 discloses a slurry obtained by dissolving and dispersing a ceramic powder and a heat-resistant nitrogen-containing aromatic polymer in an organic solvent as a support, a porous woven fabric such as polyolefin, rayon, vinylon, polyester, acrylic, polystyrene and nylon, Film and the like, and then removing the solvent to produce a heat-resistant separator. However, in the process of introducing the heat-resistant polymer layer, the application of the heat-resistant resin and the application of the porous heat-resistant resin layer including the immersion, There is a problem that the manufacturing process is very complicated and the cost is greatly increased.

Japanese Patent Application Laid-Open Nos. 2001-222988 and 2006-59717 disclose a gel electrolyte of a polymer such as polyethylene oxide, polypropylene oxide, polyether, polyvinylidene and the like in polyamide, polyimide woven fabric, nonwoven fabric, To produce a heat-resistant electrolyte membrane. However, even in this case, the required heat resistance may be satisfied, but the ion transport in the support or the heat-resistant aromatic polymer layer is still limited in terms of ion conduction, similar to that of a separator or a gel electrolyte of a conventional lithium ion battery.

As a separator for a fuel cell currently in use, there is a perfluorosulfonic acid resin (Nafion, trade name) (hereinafter referred to as "Nafion resin") as a fluororesin. However, since the Nafion resin has a low mechanical strength, pinholes are generated when the Nafion resin is used for a long period of time, thereby deteriorating energy conversion efficiency. There is an attempt to increase the film thickness of the Nafion resin to reinforce the mechanical strength. However, in this case, there is a problem that the resistance loss is increased and the economical efficiency is lowered due to the use of the expensive material.

Therefore, the above-mentioned conventional patented techniques still do not satisfy the heat resistance and the ionic conductivity at the same time, and there is no mention of the shutdown function of the separator, High energy densities and large capacity batteries, such as batteries, are still unsatisfactory.

In addition, polyacrylonitrile-based polymers are excellent in physical and chemical stability, and have excellent chemical resistance and mechanical properties and are widely used as industrial fibers. Especially nylon, polyvinyl alcohol, and the like, and it is known as a highly suitable material as a filter material because it has excellent static electricity holding ability when electrofusion is manufactured by electrofusion. In addition, when the acrylonitrile-based copolymer is appropriately modified with a hydrophilic polymer and electrospun, the nanofiber can be widely used as sanitary materials, agricultural horticulture, food distribution, civil engineering construction, toiletries, pharmaceuticals, electric and electronic materials and the like.

Further, as the battery becomes larger, the problem of wettability with respect to electrolytic solution becomes more serious. If the affinity of the electrolyte solution and the separator membrane is low, the lithium ion transfer capacity in the membrane is deteriorated, leading to a phenomenon in which the output characteristics of the battery are deteriorated. In particular, when the electrolytic solution component has a polarity such as ethylene carbonate (EC) or propylene carbonate (PC), it exhibits poor wettability to the non-polar polyolefin-based separator. Therefore, The problem of aspect appears more clearly.

Therefore, techniques for modifying the surface of the separator to improve the affinity with the electrolyte have been developed. In particular, the inorganic composite separator or the ceramic separator is manufactured by connecting inorganic particles of a very small size with a small amount of a binder. Show high wettability to electrolytes due to their high hydrophilicity and large specific surface area, and excellent wettability in electrolytes such as EC and PC improves battery life and performance. These ceramic separators have very good thermal stability and hardly shrink even at high temperatures.

In the present invention, in order to improve the low thermal stability of the polyolefin substrate and the low affinity to the electrolyte, the polyacrylonitrile nanofiber is prepared, and the inorganic particles are coated thereon to solidly bind the inorganic material, And to provide a method for producing the same.

The present invention relates to a method for producing a polyacrylonitrile nanofiber, comprising: electrospinning a solution of polyolefin resin in a polyacrylonitrile organic solvent on a polyolefin substrate to form polyacrylonitrile nanofiber; And coating an inorganic slurry containing an inorganic material and a polyacrylonitrile affinity binder on the polyacrylonitrile nanofiber to form an inorganic coating layer. The method for producing a porous separator for a secondary battery according to claim 1, wherein the polyacrylonitrile The affinity binder is any one selected from the group consisting of polymethyl methacrylate (PMMA), polyacrylonitrile (PAN), and carboxymethyl cellulose (CMC). The molecular weight of the polyacrylonitrile is 2000 To 4,000.

According to another preferred embodiment of the present invention, there is provided a polyolefin substrate comprising: a polyacrylonitrile nanofiber layer formed by electrospinning on one side of the polyolefin substrate; And an inorganic coating layer formed by coating an inorganic material on one side of the polyacrylonitrile nanofiber, wherein the inorganic material is selected from the group consisting of SiO ₂ , Al ₂ O ₃ , TiO ₂ , Li ₃ PO ₄ , zeolite, MgO, CaO, BaTiO ₃ , Li ₂ O, LiF, LiOH, Li ₃ N, BaO, Na ₂ O, Li ₂ CO ₃ , CaCO ₃ , LiAlO ₂ , SiO, SnO, SnO ₂ , PbO ₂ , ZnO, P ₂ O ₅ , CuO, MoO, V ₂ O ₅ , B ₂ O ₃ , Si ₃ N ₄ , CeO ₂ , Mn ₃ O ₄ , Sn ₂ P ₂ O ₇ , Sn ₂ B ₂ O ₅ , Sn ₂ BPO ₆ , each mixture thereof, and the like.

The separator using the conventional polyolefin film and the inorganic coating layer are firmly bonded by the heat-resistant polymer nanofibers electrified through the separator using the conventional polyolefin film and the inorganic coating layer formed directly on the polyolefin substrate, A separator can be manufactured.

1 is a schematic view of a porous separator of the present invention.

A method of preparing a porous nanofiber by electrospinning a heat-resistant polymer polyacrylonitrile of the present invention and coating inorganic particles on the polyacrylonitrile (PAN) nanofiber will be described.

A spinning solution is prepared which is dissolved in dimethylformamide (DMF) and dimethylacetamide (DMAc), which are solvents mainly used for polyacrylonitrile.

Polyacrylonitrile resins are copolymers made from mixtures of acrylonitrile and monomers. Typical units are butadiene styrene, vinylidene chloride, or other vinyl compounds. Of course, the same acrylic fiber contains at least 85 acrylonitrile, and the mode acrylic contains 35-85 acrylonitrile. If other monomer units are included, desired properties such as increasing the affinity of the fiber to the dye can be obtained.

More particularly, the present invention relates to a method for producing an acrylonitrile-based copolymer and a spinning solution. Acrylonitrile-based copolymer, it is possible to provide a resin composition which is less susceptible to contamination of the nozzle and excellent in electrospinning in the process of preparing ultrafine fibers by electrospinning, can increase solubility in a solvent and can impart better mechanical properties have.

It is preferable to use it in a range that satisfies the usage amount of the acrylonitrile monomer, the hydrophobic monomer and the hydrophilic monomer. The weight% of the acrylonitrile monolaurate in polymer polymerization is too low to be electrospun when the weight% of the hydrophilic monomer and the weight% of the hydrophobic monomer are 3: 4 and the total monomer is less than 60, It is difficult to form a stable jet (JET) at the time of electrospinning as well as to cause contamination of the nozzle. If the ratio is more than 99, the spinning viscosity is too high to spin, and even if an additive capable of lowering the viscosity is added, the diameter of the microfine fibers becomes too large and the productivity of electrospray is too low to achieve the object of the present invention.

In addition, as much amount of comonomer is added to the acrylic polymer, the amount of the crosslinking agent must be increased so that the stability of electrospinning can be secured and deterioration of the mechanical properties of the nanofiber can be prevented.

The polymerization degree of the polyacrylonitrile is 1,000 to 1,000,000, preferably 2,000 to 1,000,000. If the degree of polymerization is lower than 1,000, the efficiency of the battery tends to decrease due to dissolution or swelling in the carbonate-based electrolytic solution and progress of the cycle, leading to desorption of the electrode from the collector. If the degree of polymerization is as high as 1,000,000 or more, , The viscosity of the electrode mixture is increased and it is difficult to handle.

The hydrophobic monomer may be at least one selected from the group consisting of ethylenic compounds such as methyl acrylate, ethyl acrylate, methyl methacrylate, ethyl methacrylate, butyl methacrylate, vinyl acetate, vinyl pyrrolidone, vinylidene chloride, vinyl chloride, It is preferable to use at least one selected.

In the present invention, the hydrophilic monomer may be selected from the group consisting of acrylic acid, allyl alcohol, methallyl alcohol, hydroxyethyl acrylate, hydroxyethyl methacrylate, hydroxypropyl acrylate, butanediol monoacrylate, dimethylaminoethyl acrylate, It is preferable to use any one or more selected from ethylene-based compounds such as acid, vinylsulfonic acid, allylsulfonic acid, methallylsulfonic acid, and para styrenesulfonic acid, and polyvalent acids or derivatives thereof.

In the present invention, an azo compound or a sulfate compound may be used as an initiator for preparing the acrylonitrile-based polymer, but it is generally preferable to use a radical initiator used for the oxidation-reduction reaction.

An example of the electrospinning device used in the present invention will be described as follows.

The electrospinning device includes a spinning liquid main tank for storing a spinning solution, a metering pump for supplying a spinning solution in a fixed amount, a nozzle block for combining a plurality of tubular nozzles composed of a plurality of pins in block form and discharging the spinning solution in a fiber form, A collector for accumulating short fibers emitted at a position corresponding to the nozzle block, a voltage generating device for generating a high voltage, and a spinning solution discharging device connected to the top of the nozzle block.

The thickness of the polymer membrane, the diameter of the fiber, the shape of the fiber, and the mechanical characteristics of the separation membrane can be arbitrarily controlled by controlling the electrospinning process conditions such as the intensity of the applied voltage, the kind of the polymer solution, the viscosity of the polymer solution, Can be adjusted.

The preferred electrospinning process conditions are such that when the spinning solution transferred to the spinning liquid supply tube is discharged to the collector through the multi tubular nozzle to form the fibers, the nanofibers electrospun from the multi-tubular nozzles are ejected from the air- It spreads widely and is collected on the collector to widen the collecting area and to make the density of the particles uniform. The excess flushing liquid which is not fibrous in the multistage nozzle is collected in the overflow removing nozzle and moved to the flushing liquid supply plate through the temporary storage plate of the overflow liquid.

When manufacturing nanofibers, the speed of air in the air supply nozzle is preferably 0.05 to 50 m / sec, more preferably 1 to 30 m / sec. If the velocity of the air is less than 0.05 m / sec, the collecting area of nanofibers is not so low and the collecting area is not greatly improved. If the velocity of the air exceeds 50 m / sec, And the more serious problem is attached to the collector in the form of a thick thread instead of the nanofiber, resulting in a marked deterioration of nanofiber forming ability.

In addition, the spinning solution which is excessively supplied to the top of the nozzle block is forcibly transferred to the spinning liquid main tank by the spinning solution discharging device.

At this time, in order to promote the formation of the fiber by the electric force, a voltage of 1 kV or more, more preferably 20 kV or more, generated by the voltage generator is applied to the conductive plate and collector provided at the lower end of the nozzle block. It is more advantageous in terms of productivity to use an endless belt as the collector. The collector preferably reciprocates a predetermined distance in the left and right directions to uniform the density of the nanofibers.

When the nanofibers formed on the collector are wound on a winding roller through a web supporting roller, the nanofiber manufacturing process is completed.

The manufacturing apparatus can increase the collecting area to uniform the density of the nanofibers and effectively prevent the droplet phenomenon, thereby improving the quality of the nanofibers and enhancing the fiber-forming effect by the electric force, Can be mass-produced. In addition, the width and the thickness of the nanofibers can be freely changed and adjusted by arranging the nozzles composed of a plurality of pins in block form.

In addition, when the heat-resistant polymer is spun as described above, it is most preferable to spin the polymer under environmental conditions of a temperature range of 30 to 40 DEG C and a humidity of 40 to 70% depending on the polymer material.

The total diameter of the membrane of the radiated nanofiber of the present invention is preferably 30 to 2000 nm, more preferably 50 to 1500 nm.

The porosity of the separator is preferably 40 to 80%, and as the diameter of the fiber is smaller, the pore size becomes smaller and the pore size distribution becomes smaller. In addition, since the specific surface area of the fibers is increased as the diameter of the fibers is smaller, the ability to lyse the electrolyte increases and the possibility of leakage of the electrolyte decreases.

After the polyacrylonitrile nanofibers are attached to the polyolefin substrate through the electrospinning method as described above, inorganic particles (SiO ₂ , Al ₂ O _3, etc.) and polyacrylonitrile-affinity polymer resins (PMMA, PVA, CMC Etc.) is added to acetone to prepare a porous separator by coating a slurry on a methacrylonitrile nanofiber by a casting method.

The inorganic material is in the size 0.5㎛ _{SiO 2, Al 2 O 3,} TiO 2, Li 3 PO 4, zeolite, MgO, and wherein at least any one selected from the group consisting of CaO, such as, in particular SiO _2, Al ₂ O ₃ .

The inorganic particles are used for coating the nanofibers using polymethylmethacrylate (PMMA), polyvinyl alcohol (PVA), carboxymethyl cellulose (CMC), or the like as the polyacrylonitrile-affinity polymer resin as the binder resin . Most preferably, the weight ratio of the inorganic material and the binder is 95: 5 to 50:50.

The coating method may be various coating methods such as chemical vapor deposition (CVD), physical vapor deposition (PVD), spray coating, dip coating, spin coating, casting, , In particular a coating by a casting method.

The thickness of the polyolefin microporous membrane used in the composite membrane of the present invention is 5 to 50 μm and the porosity (porosity (%)) = 1- (membrane bulk density / resin density) × 100 is 30 To 80%. The tensile strength was 700 g / cm or more in the mechanical direction (MD), 150 kg / cm or more in the transverse direction (CD), the punching strength was 200 g or more per mil (1 mil = 25.4) Less than 10%, an average pore size of 0.005 to 3 탆, and an electrical characteristic of an electric resistance of 10,000 Ω / cm ² or more at 130 to 185 ° C. is particularly suitable for use in electrochemical devices.

Hereinafter, embodiments of the present invention will be described in more detail. However, the embodiments are only examples of the present invention, and the scope of the present invention is not limited thereto.

[Example 1]

Polyacrylonitrile having a weight average molecular weight of 157,000 (Korea-Japan Synthesis) was dissolved in DMF solvent to prepare a spinning solution. The spinning solution was applied on a polyolefin (celgand 2400) substrate having a thickness of 10 탆 by a distance of 40 cm, Under the conditions of an applied voltage of 15 kV, a spinning solution flow rate of 0.1 mL / h, a temperature of 22 캜 and a humidity of 20%, polyacrylonitrile nanofibers having a thickness of 3 탆 are formed.

(Al ₂ O ₃₎ inorganic particles having a size of 0.5 μm and a polyacrylonitrile-affinity polymeric poly methyl methacrylate (PMMA) (LG IG840) as a binder were added to acetone at a weight ratio of 9: 1 The slurry is coated on polyacrylonitrile nanofibers by a casting method to a thickness of 5 탆.

[Example 2]

Except that Al ₂ O ₃ inorganic particles having a size of 0.5 탆 and polyacrylonitrile-affinity high molecular weight poly (methyl methacrylate) (PMMA) were added to acetone at an 8: 2 weight ratio, , A porous separator for a secondary battery was prepared, and the performance of the separator was evaluated as in the examples.

[Comparative Example 1]

Separating membrane performance evaluation as in the examples was performed using a polyolefin (celgand 2400) film without any additional treatment.

[Comparative Example 2]

A porous separator for a secondary battery was prepared by coating the inorganic slurry of Example 1 on a polyolefin substrate to evaluate the performance of the separator as in the examples.

- Evaluation of Heat Shrinkage of Membrane -

The separation membranes of Examples 1 and 2 and Comparative Examples 1 and 2 were cut into 3 cm x 3 cm, and then stored at 150 ° C for 30 minutes, and the heat shrinkage was evaluated.

Example 1 Example 2 Comparative Example 1 Comparative Example 2 Heat shrinkage (%) 4 5 42 12 Total thickness (탆) 18 18 18 18

Thus, it can be seen that the porous separator provided with the present invention has improved thermal stability compared to a separator coated with an inorganic material on a general polyolefin film and a polyolefin by firmly adhering an inorganic coating layer through fabrication of nanofibers by electrospinning.

Claims (8)

Forming a polyacrylonitrile nanofiber on a polyolefin substrate by electrospinning a solution of polyacrylonitrile having a weight average molecular weight of 2,000 to 1,000,000 in an organic solvent; And
Coating an inorganic slurry containing an inorganic material and a polyacrylonitrile affinity binder on the polyacrylonitrile nanofiber in a ratio of 95: 5 to 50:50 to form an inorganic coating layer, wherein the inorganic material is _{SiO 2, Al 2 O 3,} TiO 2, Li 3 PO 4, _{zeolites, MgO, CaO, BaTiO 3,} Li 2 O, LiF, LiOH, Li 3 N, BaO, Na 2 O, Li 2 CO 3, CaCO ₃ , LiAlO ₂ , SiO, SnO, SnO ₂ , PbO ₂ , ZnO, P ₂ O ₅ , CuO, MoO, V ₂ O ₅ , B ₂ O ₃ , Si ₃ N ₄ , CeO ₂ , Mn ₃ O ₄ , Sn ₂ P ₂ O ₇ , Sn ₂ B ₂ O ₅ , Sn ₂ BPO _6, and mixtures thereof.
delete delete delete delete The method according to claim 1,
Wherein the binder is any one selected from the group consisting of polymethylmethacrylate (PMMA), polyacrylonitrile (PAN), and carboxymethylcellulose (CMC).
Polyolefin substrates;
A polyacrylonitrile nanofiber layer formed on one surface of the polyolefin substrate by electrospinning and having a weight average molecular weight of 2,000 to 1,000,000; And
Wherein the inorganic coating layer comprises at least one of SiO ₂ , Al ₂ O ₃ , TiO ₂ , Li ₃ PO ₄ , zeolite, MgO, CaO, BaTiO 3, ₃ , Li ₂ O, LiF, LiOH, Li ₃ N, BaO, Na ₂ O, Li ₂ CO ₃ , CaCO ₃ , LiAlO ₂ , SiO, SnO, SnO ₂ , PbO ₂ , ZnO, P ₂ O ₅ , CuO, MoO, V ₂ O ₅ , B ₂ O ₃ , Si ₃ N ₄ , CeO ₂ , Mn ₃ O ₄ , Sn ₂ P ₂ O ₇ , Sn ₂ B ₂ O ₅ , Sn ₂ BPO ₆ , Wherein the porous separator is a porous separator for a secondary battery.

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