A separator for lithium ion batteries and a method for manufacturing the same

Abstract

The application provides a separator for a lithium ion battery and a preparation method thereof. A main base material is selected as cellulose fiber doped with polyolefin fiber, which can improve the toughness and mechanical properties of the separator, effectively avoid battery short circuit caused by lithium dendrite puncture, and improve the safety performance and service life of the lithium ion battery. Doped ceramic powder can further improve the mechanical properties and high-temperature resistance of the separator, and the coating process is omitted, thereby improving the efficiency. Barium titanate is modified by using KH550, which improves the dispersity of the barium titanate and enhances the interface action of the barium titanate with the resin and the gas production problem of inorganic powder in the battery use process. The addition of perfluoroethylene propylene copolymer can reduce the friction coefficient of the separator and effectively reduce the friction loss in the battery.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium primary batteries, and in particular relates to a diaphragm for lithium ion batteries and a preparation method thereof. Background Art

[0002] While the automotive industry has experienced rapid growth in recent years, it has also brought negative impacts such as environmental pollution and a rapid depletion of oil resources. Consequently, countries around the world are actively developing electric vehicles. Lithium-ion batteries are the primary power source for electric vehicles. Lithium-ion batteries generally consist of three main components: a positive electrode, a negative electrode, and a separator that carries an electrolyte. As a crucial component of lithium-ion batteries, the separator primarily prevents internal short circuits and acts as an electrolyte carrier, providing a pathway for lithium ions to move. The separator significantly impacts the operating environment, specific capacity, safety, and service life of lithium-ion batteries. Therefore, research and development of separators with superior performance, simple preparation methods, and suitability for industrial production are crucial for improving the application of lithium-ion batteries.

[0003] Currently, commercial lithium-ion battery separators are mostly porous polymer membranes made from polypropylene and polyethylene. These polymer materials offer excellent porosity, tensile properties, and strength suitable for existing production methods. However, polyolefin separators shrink at high temperatures, leading to direct contact between the positive and negative electrodes and causing safety concerns. Polyolefin-based membranes also have poor liquid absorption capacity. Membranes made from materials such as fibrous nonwovens offer better electrolyte wettability, but suffer from issues such as poor mechanical strength. Summary of the Invention

[0004] The object of the present invention is to provide a lithium-ion battery separator based on the existing technology, comprising a main substrate and a mixed slurry layer vacuum-impregnated on the surface of the main substrate, wherein the main substrate is a separator paper substrate made of polyolefin fiber, cellulose fiber and a dispersant; the mixed slurry layer is made of phenolic resin, a curing agent, ceramic powder, modified barium titanate and perfluoroethylene propylene copolymer; the main substrate selects cellulose fiber doped with polyolefin fiber to improve the toughness and mechanical properties of the separator, effectively avoid battery short circuit caused by lithium dendrite puncture, and improve the safety performance and service life of the lithium-ion battery; doping with ceramic powder can further improve the mechanical properties and high temperature resistance of the separator, while eliminating the coating process and improving efficiency; KH550 is used to modify the barium titanate to improve the dispersibility of the barium titanate and enhance its interface with the resin and the gas generation problem of the inorganic powder during the use of the battery; adding perfluoroethylene propylene copolymer can reduce the friction coefficient of the separator and effectively reduce the friction loss inside the battery.

[0005] Another object of the present invention is to provide a method for preparing the above-mentioned lithium ion battery separator.

[0006] The technical scheme of the present application is as follows:

[0007] The present application provides a kind of separator for lithium ion battery, including main body substrate and mixed slurry layer vacuum impregnated in the surface of main body substrate, wherein, main body substrate is made of polyolefin fiber, cellulose fiber and dispersant separator paper matrix;Mixed slurry layer is made of phenolic resin, curing agent, ceramic powder, modified barium titanate and perfluoroethylene propylene copolymer (FEP).

[0008] For the present application, dispersant is one or more of polyethylene oxide (FEP), polyacrylamide, methyl cellulose or hydroxyethyl cellulose, preferably polyethylene oxide;Curing agent is hexamethylene tetramine.Ceramic powder is one or more of alumina, boehmite, silicon dioxide, barium sulfate, magnesium oxide or aluminum hydroxide, preferably, ceramic powder is alumina, boehmite or barium sulfate;Modified barium titanate is prepared by chemical reaction of barium titanate and KH550.

[0009] In the present application, the weight ratio of polyolefin fiber and cellulose fiber is 1-0.1:3, preferably 1:0.5-2, which can be but not limited to 1:0.5, 1:1.25 or 1:2.

[0010] The weight ratio of the total amount of polyolefin fiber and cellulose fiber to dispersant is 1-5:15, preferably 1:8-12, which can be but not limited to 1:8, 1:10 or 1:12.

[0011] In a preferred embodiment, the preparation method of modified barium titanate comprises the following steps: uniformly ultrasonic mixing barium titanate, solvent and KH550, chemical reaction at 40-50℃, washing, drying to obtain modified barium titanate.

[0012] For the present application, the mass ratio of barium titanate and KH550 is 1-3:0.1-0.5, preferably 2:0.3.The reaction temperature is 40-50℃, preferably 45℃.The reaction time is 4-8 hours, preferably 6 hours.

[0013] The solvent is one or more of methanol, ethanol, isopropyl ketone or acetone, preferably ethanol.

[0014] In the present application, the present application provides a separator for lithium ion battery, comprising a main substrate and a mixed slurry layer vacuum-impregnated on the surface of the main substrate, wherein the main substrate is a separator paper substrate made of polyolefin fibers, cellulose fibers and a dispersant; the mixed slurry layer is made of phenolic resin, curing agent, ceramic powder, modified barium titanate and perfluoroethylene propylene copolymer, wherein the mixed slurry layer comprises the following components by weight: phenolic resin 15-25 parts, hexamethylenetetramine 0.1-0.8 parts, ceramic powder 15-45 parts, modified barium titanate 4-15 parts and perfluoroethylene propylene copolymer 4-20 parts; wherein the weight ratio of the total amount of polyolefin fibers and cellulose fibers to phenolic resin is 9:15-25.

[0015] In a preferred embodiment, the mixed slurry layer comprises the following components by weight: phenolic resin 20 parts, hexamethylenetetramine 0.3-0.5 parts, ceramic powder 23-35 parts, modified barium titanate 7-8 parts and perfluoroethylene propylene copolymer 8-11 parts; wherein the weight ratio of the total amount of polyolefin fibers and cellulose fibers to phenolic resin is 9:20.

[0016] In a more preferred embodiment, the mixed slurry layer comprises the following components by weight:

[0017] For example, phenolic resin 20 parts, hexamethylenetetramine 0.3 parts, alumina 35 parts, modified barium titanate 7 parts and perfluoroethylene propylene copolymer 8 parts; wherein the weight ratio of the total amount of polyolefin fibers and cellulose fibers to phenolic resin is 9:20.

[0018] Phenolic resin 20 parts, hexamethylenetetramine 0.5 parts, boehmite 23 parts, modified barium titanate 7 parts and perfluoroethylene propylene copolymer 10 parts; wherein the weight ratio of the total amount of polyolefin fibers and cellulose fibers to phenolic resin is 9:20.

[0019] Phenolic resin 20 parts, hexamethylenetetramine 0.5 parts, barium sulfate 23 parts, modified barium titanate 8 parts and perfluoroethylene propylene copolymer 11 parts; wherein the weight ratio of the total amount of polyolefin fibers and cellulose fibers to phenolic resin is 9:20.

[0020] The present application also provides a preparation method of the above-mentioned separator for lithium ion battery, which comprises the following steps:

[0021] (1) uniformly mix barium titanate, solvent and KH550, and perform chemical reaction under the condition of 40-50℃, then wash and dry to obtain modified barium titanate;

[0022] (2) dissolve the dispersant in the solvent to prepare a dispersion liquid, add polyolefin fibers and cellulose fibers to the obtained dispersion liquid, uniformly disperse, then filter out the dispersion liquid to obtain a separator paper substrate;

[0023] (3) adding phenolic resin, curing agent, ceramic powder, modified barium titanate and perfluoroethylene propylene into a solvent, stirring uniformly to obtain a mixed slurry;

[0024] (4) Vacuum impregnation of the diaphragm paper substrate obtained in step (2) and the mixed slurry obtained in step (3) is performed, and drying is performed to obtain a diaphragm.

[0025] In the present invention, in step (1), the mass ratio of barium titanate to KH550 is 1-3:0.1-0.5, preferably 2:0.3. The reaction temperature is 40-50° C., preferably 45° C. The reaction time is 4-8 hours, preferably 6 hours.

[0026] In step (1), the solvent is one or more of methanol, ethanol, isopropyl ketone or acetone, preferably ethanol.

[0027] In step (3), the solvent is one or more of methanol, ethanol, isopropyl ketone or acetone, preferably ethanol.

[0028] For the present invention, the dispersant is one or more of polyethylene oxide, polyacrylamide, methyl cellulose or hydroxyethyl cellulose, preferably polyethylene oxide; the curing agent is hexamethylenetetramine; the ceramic powder is one or more of alumina, boehmite, silica, barium sulfate, magnesium oxide or aluminum hydroxide, preferably, the ceramic powder is alumina, boehmite or barium sulfate.

[0029] Adopt the technical scheme of the present invention, the advantages are as follows:

[0030] The present invention provides a diaphragm for a lithium ion battery, comprising a main substrate and a mixed slurry layer vacuum-impregnated on the surface of the main substrate, wherein the main substrate is a diaphragm paper substrate made of polyolefin fibers, cellulose fibers and a dispersant; the mixed slurry layer is made of phenolic resin, hexamethylenetetramine, ceramic powder, modified barium titanate and perfluoroethylene propylene copolymer; the main substrate is cellulose fibers doped with polyolefin fibers, which can improve the toughness and mechanical properties of the diaphragm, effectively avoid battery short circuits caused by lithium dendrite puncture, and improve the safety performance and service life of the lithium ion battery; doping with ceramic powder can further improve the mechanical properties and high-temperature resistance of the diaphragm, while eliminating the coating process and improving efficiency; KH550 is used to modify the barium titanate to improve the dispersibility of the barium titanate and enhance its interface interaction with the resin, and to prevent the gas generation problem of the inorganic powder during battery use; and perfluoroethylene propylene copolymer is added to reduce the friction coefficient of the diaphragm, effectively reducing the friction loss inside the battery. DETAILED DESCRIPTION

[0031] The present application can be better understood from the following examples. However, it will be readily apparent to those of ordinary skill in the art that the examples described are for illustrative purposes only and should not be construed as limiting upon the application as described in the claims.

[0032] Example 1

[0033] A separator for lithium ion battery, comprising a main substrate and a mixed slurry layer vacuum-impregnated on the surface of the main substrate, wherein the main substrate is a separator paper substrate made of polyolefin fibers, cellulose fibers and a dispersant; the mixed slurry layer is made of phenolic resin, hexamethylenetetramine, ceramic powder, modified barium titanate and perfluoroethylene propylene copolymer, wherein the weight ratio of polyolefin fibers to cellulose fibers is 1:0.5, the weight ratio of the total amount of polyolefin fibers and cellulose fibers to the dispersant is 1:8, and the weight ratio of the total amount of polyolefin fibers and cellulose fibers to phenolic resin is 9:20, and the specific preparation method is as follows:

[0034] (1) 2g of barium titanate was added to 100g of ethanol, stirred uniformly, 6g of 5% KH550 ethanol solution was added to the obtained mixture, ultrasonic mixing was carried out for 1h, after uniform mixing, chemical reaction was carried out at 45℃, the reaction time was 6h, the obtained powder was repeatedly washed with pure water, dried in an oven at 60℃ to obtain modified barium titanate;

[0035] (2) The dispersant polyoxyethylene (FEP) was dissolved in ethanol to prepare a dispersion liquid with a concentration of 4wt%, 6g of polyolefin fibers and 3g of cellulose fibers were added to the obtained 1.8kg of dispersion liquid, and the dispersion was dispersed using a fiber defibrator, after uniform dispersion, the dispersed dispersion liquid was poured into a paper machine, and the dispersion liquid was filtered to obtain a separator paper substrate, which was dried at 60℃.

[0036] (3) 20g of phenolic resin, 0.3g of hexamethylenetetramine, 35g of alumina, 7g of modified barium titanate and 8g of perfluoroethylene propylene copolymer (FEP) were added to 250g of ethanol, stirred uniformly to obtain a mixed slurry;

[0037] (4) The separator paper substrate obtained in step (2) and the mixed slurry obtained in step (3) were placed in a vacuum impregnation kettle container, vacuum was drawn to make the kettle in a negative pressure state, vacuum impregnation was carried out, the impregnation time was 5min, and the separator was obtained after hot pressing and drying.

[0038] Example 2

[0039] A kind of lithium ion battery separator, including main body substrate and mixed slurry layer vacuum impregnated in the surface of main body substrate, wherein, main body substrate is the separator paper matrix made of polyolefin fiber, cellulose fiber and dispersant;Mixed slurry layer is made of phenolic resin, hexamethylenetetramine, ceramic powder, modified barium titanate and perfluoroethylene propylene copolymer, wherein, the weight ratio of polyolefin fiber and cellulose fiber is 1:2, the weight ratio of the total amount of polyolefin fiber and cellulose fiber and dispersant is 1:10, the weight ratio of the total amount of polyolefin fiber and cellulose fiber and phenolic resin is 9:20, and the specific preparation method is as follows:

[0040] (1) 2g barium titanate is added to 100g ethanol, stirred uniformly, then 6g 5% KH550 ethanol solution is added to the obtained mixture, ultrasonic mixing is carried out for 1h, after mixing uniformly, chemical reaction is carried out at 45℃, the reaction time is 6h, the obtained powder is repeatedly washed with pure water, dried in oven at 60℃ to obtain modified barium titanate;

[0041] (2) dispersant polyethylene oxide (FEP) is dissolved in ethanol to prepare a dispersion liquid with a concentration of 5wt%, 3g polyolefin fiber and 6g cellulose fiber are added to the obtained 1.8kg dispersion liquid, fiber defibrator is used for defibration and dispersion, after dispersion, the dispersed dispersion liquid is poured into a paper machine, the dispersion liquid is filtered out, and a separator paper matrix is obtained, which is dried at 60℃.

[0042] (3) 20g phenolic resin, 0.5g hexamethylenetetramine, 23g boehmite, 7g modified barium titanate and 10g perfluoroethylene propylene copolymer (FEP) are added to 220g ethanol, stirred uniformly to obtain mixed slurry;

[0043] (4) the separator paper matrix obtained in step (2) and the mixed slurry obtained in step (3) are placed in a vacuum impregnation kettle container, vacuum is drawn to make the kettle in negative pressure state, vacuum impregnation is carried out, the impregnation time is 5min, and the separator is obtained after taking out and hot-press drying.

[0044] Example 3

[0045] A kind of lithium ion battery separator, including main body substrate and mixed slurry layer vacuum impregnated in the surface of main body substrate, wherein, main body substrate is the separator paper matrix made of polyolefin fiber, cellulose fiber and dispersant;Mixed slurry layer is made of phenolic resin, hexamethylenetetramine, ceramic powder, modified barium titanate and perfluoroethylene propylene copolymer, wherein, the weight ratio of polyolefin fiber and cellulose fiber is 1:1.25, the weight ratio of the total amount of polyolefin fiber and cellulose fiber and dispersant is 1:12, the weight ratio of the total amount of polyolefin fiber and cellulose fiber and phenolic resin is 9:20, and the specific preparation method is as follows:

[0046] (1) 2 g of barium titanate was added to 100 g of ethanol and stirred evenly. Then, 6 g of 5% KH550 ethanol solution was added to the mixture and ultrasonically mixed for 1 h. After mixing evenly, a chemical reaction was carried out at 45° C. for 6 h. The obtained powder was repeatedly washed with pure water and dried in an oven at 60° C. to obtain modified barium titanate.

[0047] (2) The dispersant polyethylene oxide (FEP) is dissolved in ethanol to prepare a dispersion with a concentration of 6 wt%. 4 g of polyolefin fiber and 5 g of cellulose fiber are added to the obtained 1.8 kg of dispersion. A fiber decompressor is used to decompose and disperse the fibers. After uniform dispersion, the decompressed dispersion is poured into a papermaking machine, the dispersion is filtered off, and a diaphragm paper substrate is obtained, which is then dried at 60°C.

[0048] (3) adding 20 g of phenolic resin, 0.5 g of hexamethylenetetramine, 23 g of barium sulfate, 8 g of modified barium titanate, and 12 g of perfluoroethylene propylene copolymer (FEP) to 260 g of ethanol and stirring uniformly to obtain a mixed slurry;

[0049] (4) The diaphragm paper substrate obtained in step (2) and the mixed slurry obtained in step (3) are placed in a vacuum impregnation kettle container, and vacuum is drawn to make the kettle in a negative pressure state, and vacuum impregnation is performed for 5 minutes. The diaphragm is taken out and hot-pressed and dried to obtain the diaphragm.

[0050] Comparative Example 1

[0051] (1) Dispersant polyethylene oxide (FEP) was dissolved in ethanol to prepare a dispersion with a concentration of 4 wt %. 9 g of polyolefin fiber was added to the obtained 1.8 kg dispersion. The fiber was dispersed using a fiber decompressor. After uniform dispersion, the dispersed dispersion was poured into a papermaking machine. The dispersion was filtered to obtain a diaphragm paper substrate, which was then dried at 60°C.

[0052] (2) adding 20 g of phenolic resin and 0.3 g of hexamethylenetetramine to 250 g of ethanol and stirring uniformly to obtain a mixed slurry;

[0053] (4) The diaphragm paper substrate obtained in step (2) and the mixed slurry obtained in step (3) are placed in a vacuum impregnation kettle container, and vacuum is drawn to make the kettle in a negative pressure state, and vacuum impregnation is performed for 5 minutes. The diaphragm is taken out and hot-pressed and dried to obtain the diaphragm.

[0054] The diaphragms prepared in the examples and comparative examples were subjected to a series of performance tests, and the obtained data are shown in Table 1 below.

[0055] Table 1 Performance test results

[0056] Test item Example 1 Example 2 Example 3 Comparative Example 1 Air permeability / s - 100 mL -1 ]] 170 175 176 145 Strength / kg-cm -2 ]] 1500 1700 1450 1000 Friction coefficient 0.69 0.62 0.56 0.8 Heat shrinkage at 150°C / % 1.7 1.5 1.45 3.5

[0057] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that it is still possible to modify the technical solutions described in the aforementioned embodiments, or to make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A separator for a lithium ion battery, characterized in that The diaphragm includes a main substrate and a mixed slurry layer vacuum-impregnated on the surface of the main substrate, wherein the main substrate is a diaphragm paper substrate made of polyolefin fiber, cellulose fiber and a dispersant; the mixed slurry layer is made of phenolic resin, a curing agent, ceramic powder, modified barium titanate and perfluoroethylene propylene copolymer; wherein the dispersant is one or more of polyethylene oxide, polyacrylamide, methyl cellulose or hydroxyethyl cellulose; the curing agent is hexamethylenetetramine; the ceramic powder is one or more of alumina, boehmite, silica, barium sulfate, magnesium oxide or aluminum hydroxide; and the modified barium titanate is prepared by chemical reaction of barium titanate and KH550.

2. The lithium ion battery separator according to claim 1, wherein The dispersant is polyethylene oxide; the ceramic powder is aluminum oxide, boehmite or barium sulfate.

3. The lithium ion battery separator according to claim 2, wherein The preparation method of the modified barium titanate comprises the following steps: uniformly mixing barium titanate, a solvent and KH550 with ultrasound, carrying out a chemical reaction at 40-50° C., washing and drying to obtain the modified barium titanate; wherein the mass ratio of barium titanate to KH550 is 1-3:0.1-0.5; the reaction temperature is 45° C.; the reaction time is 4-8 hours; and the solvent is one or more of methanol, ethanol, isopropyl tone or acetone.

4. The lithium ion battery separator according to claim 3, characterized in that The mass ratio of barium titanate to KH550 is 2:0.3; the reaction time is 6 hours; and the solvent is ethanol.

5. The lithium ion battery separator according to claim 3, characterized in that The weight ratio of the polyolefin fiber to the cellulose fiber is 1-0.1:3; the weight ratio of the total amount of the polyolefin fiber and the cellulose fiber to the dispersant is 1-5:

15.

6. The lithium ion battery separator according to claim 5, characterized in that The weight ratio of the polyolefin fiber to the cellulose fiber is 1:0.5-2; the weight ratio of the total amount of the polyolefin fiber and the cellulose fiber to the dispersant is 1:8-12.

7. The lithium ion battery separator according to claim 5 or 6, characterized in that The mixed slurry layer includes the following components in parts by weight: 15-25 parts of phenolic resin, 0.1-0.8 parts of hexamethylenetetramine, 15-45 parts of ceramic powder, 4-15 parts of modified barium titanate and 4-20 parts of perfluoroethylene propylene copolymer; wherein the weight ratio of the total amount of polyolefin fiber and cellulose fiber to the phenolic resin is 9:15-25.

8. The lithium ion battery separator according to claim 7, wherein The mixed slurry layer includes the following components in parts by weight: 20 parts of phenolic resin, 0.3-0.5 parts of hexamethylenetetramine, 23-35 parts of ceramic powder, 7-8 parts of modified barium titanate and 8-11 parts of perfluoroethylene propylene copolymer; wherein the weight ratio of the total amount of polyolefin fiber and cellulose fiber to the phenolic resin is 9:

20.

9. The lithium ion battery separator according to claim 8, characterized in that The mixed slurry layer includes the following components in parts by weight: 20 parts of phenolic resin, 0.3 parts of hexamethylenetetramine, 35 parts of aluminum oxide, 7 parts of modified barium titanate and 8 parts of perfluoroethylene propylene copolymer; 20 parts of phenolic resin, 0.5 parts of hexamethylenetetramine, 23 parts of boehmite, 7 parts of modified barium titanate and 10 parts of perfluoroethylene propylene copolymer; 20 parts of phenolic resin, 0.5 parts of hexamethylenetetramine, 23 parts of barium sulfate, 8 parts of modified barium titanate and 11 parts of perfluoroethylene propylene copolymer.

10. The method for preparing a lithium ion battery separator according to claim 1, wherein: It includes the following steps: (1) Barium titanate, solvent and KH550 are uniformly mixed by ultrasonication, chemically reacted at 40-50°C, washed and dried to obtain modified barium titanate; (2) dissolving a dispersant in a solvent to prepare a dispersion, adding polyolefin fibers and cellulose fibers to the dispersion, and after uniform dispersion, filtering out the dispersion to obtain a separator paper substrate; (3) adding phenolic resin, curing agent, ceramic powder, modified barium titanate and perfluoroethylene propylene into the solvent, stirring evenly to obtain a mixed slurry; (4) Vacuum impregnation of the diaphragm paper substrate obtained in step (2) and the mixed slurry obtained in step (3) is performed, and drying is performed to obtain a diaphragm.

11. The method for preparing a lithium ion battery separator according to claim 10, wherein: In step (1), the mass ratio of barium titanate to KH550 is 1-3:0.1-0.5; the reaction temperature is 45° C., and the reaction time is 4-8 hours; in step (1) or step (3), the solvent is one or more of methanol, ethanol, isopropyl tone or acetone.

12. The method for preparing a lithium ion battery separator according to claim 11, wherein: In step (1), the mass ratio of barium titanate to KH550 is 2:0.3; the reaction time is 6 hours; in step (1) or step (3), the solvent is ethanol.

13. The method for preparing a lithium ion battery separator according to claim 10, wherein: The dispersant is polyethylene oxide; the curing agent is hexamethylenetetramine; the ceramic powder is alumina, boehmite or barium sulfate; the weight ratio of the polyolefin fiber to the cellulose fiber is 1-0.1:3; the weight ratio of the total amount of the polyolefin fiber and the cellulose fiber to the dispersant is 1-5:15; the mixed slurry layer includes the following components in parts by weight: 15-25 parts of phenolic resin, 0.1-0.8 parts of curing agent, 15-45 parts of ceramic powder, 4-15 parts of modified barium titanate and 4-20 parts of perfluoroethylene propylene copolymer; wherein the weight ratio of the total amount of the polyolefin fiber and the cellulose fiber to the phenolic resin is 9:15-25.

14. The method for preparing a lithium ion battery separator according to claim 13, wherein: The weight ratio of the polyolefin fiber to the cellulose fiber is 1:0.5-2; the weight ratio of the total amount of the polyolefin fiber and the cellulose fiber to the dispersant is 1:8-12.