High-wettability separator and preparation method thereof

Abstract

Disclosed in present invention are a high-wettability separator and a preparation method therefor. The separator comprises an ethylene copolymer, a grafting polyolefin, an ultrahigh molecular weight polyethylene having a molecular weight ranging from 1.0×10.sup.6 to 10.0×10.sup.6, and a high-density polyethylene having a density ranging from 0.940 g/cm.sup.3 to 0.976 g/cm.sup.3, the content of the ethylene copolymer is 1-5 parts by weight, and the content of the grafting polyolefin is 0-5 parts by weight, on the basis that the total weight of the ultrahigh molecular weight polyethylene and the high-density polyethylene is 100 parts. The separator has a contact angle with lithium ion battery electrolyte of 20° to 40°.

Claims

1. A lithium ion battery separator, wherein the separator consists of: an ethylene copolymer, a grafted polyolefin, an ultrahigh molecular weight polyethylene having a molecular weight of 3.5×10.sup.6 to 10.0×10.sup.6, and a high-density polyethylene having a density in the range of 0.940 to 0.976 g/cm.sup.3; wherein the content of the ethylene copolymer is 1-5 parts by weight and the content of the grafted polyolefin is 1-5 parts by weight, on the basis that the total weight of the ultrahigh molecular weight polyethylene and the high-density polyethylene is 100 parts; wherein the ethylene copolymer is one or more selected from the group consisting of ethylene-vinyl acetate copolymer, ethylene-acrylate copolymer, ethylene-acrylic acid copolymer, and ethylene-methyl methacrylate copolymer; and wherein the grafted polyolefin is one or more selected from the group consisting of maleic anhydride grafted polyethylene, and glycidyl methacrylate grafted polyethylene.

2. The separator according to claim 1, wherein the separator has a contact angle with a lithium ion battery electrolyte of 20° to 40°.

3. The separator according to claim 1, wherein the weight ratio of the ultrahigh molecular weight polyethylene to the high-density polyethylene is 1:1 to 1:20.

4. A method for preparing a lithium ion battery separator according to claim 1, wherein the method comprises the steps of: mixing an ethylene copolymer, a grafted polyolefin, an ultrahigh molecular weight polyethylene having a molecular weight of 3.5×10.sup.6 to 10.0×10.sup.6, a high-density polyethylene having a density of 0.940 to 0.976 g/cm.sup.3, an antioxidant, and a pore-forming agent to form a mixture; wherein the content of the ethylene copolymer is 1-5 parts by weight and the content of the grafted polyolefin is 1-5 parts by weight, on the basis that the total weight of the ultrahigh molecular weight polyethylene and the high-density polyethylene is 100 part; wherein the ethylene copolymer is one or more selected from the group consisting of ethylene-vinyl acetate copolymer, ethylene-acrylate copolymer, ethylene-acrylic acid copolymer, and ethylene-methyl methacrylate copolymer; and wherein the grafted polyolefin is one or more selected from the group consisting of maleic anhydride grafted polyethylene, and glycidyl methacrylate grafted polyethylene; extruding the mixture into a strip by an extruder; extracting the strip with an organic solvent; stretching the extracted strip into a film by a stretching machine; and subjecting the film to heat setting and winding to obtain the lithium ion battery separator.

5. The preparation method according to claim 4, wherein the ethylene copolymer is added in an amount of 1-5 parts by weight, on the basis that the total weight of the ultrahigh molecular weight polyethylene and the high-density polyethylene is 100 parts.

6. The preparation method according to claim 4, wherein the weight ratio of the ultrahigh molecular weight polyethylene to the high-density polyethylene is 1:1 to 1:20.

7. A lithium ion battery comprising a positive electrode, a negative electrode, a separator between the positive electrode and the negative electrode, and an electrolyte, wherein the separator is a separator according to claim 1.

Description

Example 1

(1) 220 g of high-density polyethylene having a density of 0.956 g/cm.sup.3, 100 g of ultrahigh molecular weight polyethylene having a molecular weight of 5.0×10.sup.6, 6.4 g of n-octadecyl-β-(3,5-di-tert-butyl-4-hydroxyphenyl)-propionate (antioxidant), 9.6 g of ethylene-vinyl acetate copolymer, and 2200 g of mineral oil were fed to a continuous mixing and charging kettle, and stirred at a speed of 50 rpm to mix the raw materials uniformly.

(2) The mixture was continuously fed to a twin-screw extruder, and the ultrahigh molecular weight polyethylene, the high-density polyethylene, the antioxidant and the ethylene-vinyl acetate copolymer were continuously dissolved in the mineral oil in the twin-screw extruder at 180° C., and continuously extruded by the twin-screw extruder at a speed of 200 rpm. The mixture continuously entered a slit die, and was extruded through the slit die to a casting cooling roller and cast into a strip at 80° C.

(3) The obtained strip was placed in an extraction tank containing dichloromethane for extraction to remove the mineral oil from the strip. The extracted strip was continuously fed into a biaxial stretching machine at 120° C. to be stretched into a film, then the resulting film material was subjected to secondary extraction with dichloromethane, and the resulting film was washed with deionized water, heat set at 120° C. for 15 min, and wound at a speed of 20 m/min to obtain a separator. Its specific performance parameters were tested as shown in the following table:

(4) TABLE-US-00002 Performance Thickness   12 μm Resistance (MacMullin) 7 Transmission (Gurley value) 216 Porosity 45% Pore size 0.48 μm Contact angle 26.2° Puncture strength 550 g .sup.  Tensile strength longitudinal: 177 MPa transverse: 159 MPa

Example 2

(5) 220 g of high-density polyethylene having a density of 0.956 g/cm.sup.3, 100 g of ultrahigh molecular weight polyethylene having a molecular weight of 5.0×10.sup.6, 6.4 g of n-octadecyl-β-(3,5-di-tert-butyl-4-hydroxyphenyl)-propionate (antioxidant), 4.8 g of maleic anhydride grafted polyethylene, 4.8 g of ethylene-vinyl acetate copolymer, and 2200 g of mineral oil were fed to a continuous mixing and charging kettle, and stirred at a speed of 50 rpm to mix the raw materials uniformly.

(6) The mixture was continuously fed to a twin-screw extruder, and the ultrahigh molecular weight polyethylene, the high-density polyethylene, the antioxidant, the maleic anhydride grafted polyethylene and the ethylene-vinyl acetate copolymer were continuously dissolved in the mineral oil in the twin-screw extruder at 180° C., and continuously extruded by the twin-screw extruder at a speed of 200 rpm. The mixture continuously entered a slit die, and was extruded through the slit die to a casting cooling roller and cast into a strip at 80° C.

(7) The obtained strip was placed in an extraction tank containing dichloromethane for extraction to remove the mineral oil from the strip. The extracted strip was continuously fed into a biaxial stretching machine at 120° C. to be stretched into a film, then the resulting film material was subjected to secondary extraction with dichloromethane, and the resulting film was washed with deionized water, heat set at 120° C. for 15 min, and wound at a speed of 20 m/min to obtain a separator. Its specific performance parameters were tested as shown in the following table:

(8) TABLE-US-00003 Performance Thickness   12 μm Resistance (MacMullin) 7.5 Transmission (Gurley value) 210 Porosity 46% Pore size 0.43 μm Contact angle 35.1° Puncture strength 570 g .sup.  Tensile strength longitudinal: 185 MPa transverse: 163 MPa

Example 3

(9) 220 g of high-density polyethylene having a density of 0.956 g/cm.sup.3, 100 g of ultrahigh molecular weight polyethylene having a molecular weight of 5.0×10.sup.6, 6.4 g of n-octadecyl-β-(3,5-di-tert-butyl-4-hydroxyphenyl)-propionate (antioxidant), 3.2 g of maleic anhydride grafted polyethylene, 6.4 g of ethylene-vinyl acetate copolymer, and 2200 g of mineral oil were fed to a continuous mixing and charging kettle, and stirred at a speed of 50 rpm to mix the raw materials uniformly.

(10) The mixture was continuously fed to a twin-screw extruder, and the ultrahigh molecular weight polyethylene, the high-density polyethylene, the antioxidant, the maleic anhydride grafted polyethylene and the ethylene-vinyl acetate copolymer were continuously dissolved in the mineral oil in the twin-screw extruder at 180° C., and continuously extruded by the twin-screw extruder at a speed of 200 rpm. The mixture continuously entered a slit die, and was extruded through the slit die to a casting cooling roller and cast into a strip at 80° C.

(11) The obtained strip was placed in an extraction tank containing dichloromethane for extraction to remove the mineral oil from the strip. The extracted strip was continuously fed into a biaxial stretching machine at 120° C. to be stretched into a film, then the resulting film material was subjected to secondary extraction with dichloromethane, and the resulting film was washed with deionized water, heat set at 120° C. for 15 min, and wound at a speed of 20 m/min to obtain a separator. Its specific performance parameters were tested as shown in the following table:

(12) TABLE-US-00004 Performance Thickness   12 μm Resistance (MacMullin) 7 Transmission (Gurley value) 201 Porosity 48% Pore size 0.45 μm Contact angle 23.5° Puncture strength 550 g .sup.  Tensile strength longitudinal: 181 MPa transverse: 168 MPa

Example 4

(13) 220 g of high-density polyethylene having a density of 0.956 g/cm.sup.3, 100 g of ultrahigh molecular weight polyethylene having a molecular weight of 5.0×10.sup.6, 6.4 g of n-octadecyl-β-(3,5-di-tert-butyl-4-hydroxyphenyl)-propionate (antioxidant), 6.4 g of maleic anhydride grafted polyethylene, 3.2 g of ethylene-vinyl acetate copolymer, and 2200 g of mineral oil were fed to a continuous mixing and charging kettle, and stirred at a speed of 50 rpm to mix the raw materials uniformly.

(14) The mixture was continuously fed to a twin-screw extruder, and the ultrahigh molecular weight polyethylene, the high-density polyethylene, the antioxidant, the maleic anhydride grafted polyethylene and the ethylene-vinyl acetate copolymer were continuously dissolved in the mineral oil in the twin-screw extruder at 180° C., and continuously extruded by the twin-screw extruder at a speed of 200 rpm. The mixture continuously entered a slit die, and was extruded through the slit die to a casting cooling roller and cast into a strip at 80° C.

(15) The obtained strip was placed in an extraction tank containing dichloromethane for extraction to remove the mineral oil from the strip. The extracted strip was continuously fed into a biaxial stretching machine at 120° C. to be stretched into a film, then the resulting film material was subjected to secondary extraction with dichloromethane, and the resulting film was washed with deionized water, heat set at 120° C. for 15 min, and wound at a speed of 20 m/min to obtain a separator. Its specific performance parameters were tested as shown in the following table:

(16) TABLE-US-00005 Performance Thickness   12 μm Resistance (MacMullin) 7.5 Transmission (Gurley value) 220 Porosity 43% Pore size 0.41 μm Contact angle 38.1° Puncture strength 560 g .sup.  Tensile strength longitudinal: 188 MPa transverse: 165 MPa

Example 5

(17) 220 g of high-density polyethylene having a density of 0.956 g/cm.sup.3, 100 g of ultrahigh molecular weight polyethylene having a molecular weight of 5.0×10.sup.6, 6.4 g of n-octadecyl-β-(3,5-di-tert-butyl-4-hydroxyphenyl)-propionate (antioxidant), 3.2 g of maleic anhydride grafted polyethylene, 6.4 g of ethylene-acrylic acid copolymer, and 2200 g of mineral oil were fed to a continuous mixing and charging kettle, and stirred at a speed of 50 rpm to mix the raw materials uniformly.

(18) The mixture was continuously fed to a twin-screw extruder, and the ultrahigh molecular weight polyethylene, the high-density polyethylene, the antioxidant, the maleic anhydride grafted polyethylene and the ethylene-acrylic acid copolymer were continuously dissolved in the mineral oil in the twin-screw extruder at 180° C., and continuously extruded by the twin-screw extruder at a speed of 200 rpm. The mixture continuously entered a slit die, and was extruded through the slit die to a casting cooling roller and cast into a strip at 80° C.

(19) The obtained strip was placed in an extraction tank containing dichloromethane for extraction to remove the mineral oil from the strip. The extracted strip was continuously fed into a biaxial stretching machine at 120° C. to be stretched into a film, then the resulting film material was subjected to secondary extraction with dichloromethane, and the resulting film was washed with deionized water, heat set at 120° C. for 15 min, and wound at a speed of 20 m/min to obtain a separator. Its specific performance parameters were tested as shown in the following table:

(20) TABLE-US-00006 Performance Thickness   12 μm Resistance (MacMullin) 7 Transmission (Gurley value) 203 Porosity 48% Pore size 0.47 μm Contact angle 24.2° Puncture strength 545 g .sup.  Tensile strength longitudinal: 180 MPa transverse: 170 MPa

Comparative Example 2

(21) 220 g of high-density polyethylene having a density of 0.956 g/cm.sup.3, 100 g of ultrahigh molecular weight polyethylene having a molecular weight of 5.0×10.sup.6, 6.4 g of n-octadecyl-β-(3,5-di-tert-butyl-4-hydroxyphenyl)-propionate (antioxidant), and 2200 g of mineral oil were fed to a continuous mixing and charging kettle, and stirred at a speed of 50 rpm to mix the raw materials uniformly.

(22) The mixture was continuously fed to a twin-screw extruder, and the ultrahigh molecular weight polyethylene, the high-density polyethylene, and the antioxidant, were continuously dissolved in the mineral oil in the twin-screw extruder at 180° C., and continuously extruded by the twin-screw extruder at a speed of 200 rpm. The mixture continuously entered a slit die, and was extruded through the slit die to a casting cooling roller and cast into a strip at 80° C.

(23) The obtained strip was placed in an extraction tank containing dichloromethane for extraction to remove the mineral oil from the strip. The extracted strip was continuously fed into a biaxial stretching machine at 120° C. to be stretched into a film, then the resulting film material was subjected to secondary extraction with dichloromethane, and the resulting film was washed with deionized water, heat set at 120° C. for 15 min, and wound at a speed of 20 m/min to obtain a separator, and its specific performance parameters were tested as shown in the following table:

(24) TABLE-US-00007 Performance Thickness   12 μm Resistance (MacMullin) 9 Transmission (Gurley value) 212 Porosity 45% Pore size 0.47 μm Contact angle 88.1° Puncture strength 550 g .sup.  Tensile strength longitudinal: 180 MPa transverse: 162 MPa

Comparative Example 3

(25) 220 g of high-density polyethylene having a density of 0.956 g/cm.sup.3, 100 g of ultrahigh molecular weight polyethylene having a molecular weight of 5.0×10.sup.6, 6.4 g of n-octadecyl-β-(3,5-di-tert-butyl-4-hydroxyphenyl)-propionate (antioxidant), 16.0 g of ethylene-vinyl acetate copolymer, and 2200 g of mineral oil were fed to a continuous mixing and charging kettle, and stirred at a speed of 50 rpm to mix the raw materials uniformly.

(26) The mixture was continuously fed to a twin-screw extruder, and the ultrahigh molecular weight polyethylene, the high-density polyethylene, the antioxidant, and the ethylene-vinyl acetate copolymer were continuously dissolved in the mineral oil in the twin-screw extruder at 180° C., and continuously extruded by the twin-screw extruder at a speed of 200 rpm. The mixture continuously entered a slit die, and was extruded through the slit die to a casting cooling roller and cast into a strip at 80° C.

(27) The obtained strip was placed in an extraction tank containing dichloromethane for extraction to remove the mineral oil from the strip. The extracted strip was continuously fed into a biaxial stretching machine at 120° C. to be stretched into a film, then the resulting film material was subjected to secondary extraction with dichloromethane, and the resulting film was washed with deionized water, heat set at 120° C. for 15 min, and wound at a speed of 20 m/min to obtain a separator, and its specific performance parameters were tested as shown in the following table:

(28) TABLE-US-00008 Performance Thickness   12 μm Resistance (MacMullin) 7 Transmission (Gurley value) 221 Porosity 46% Pore size 0.49 μm Contact angle 27.1° Puncture strength 500 g .sup.  Tensile strength longitudinal: 170 MPa transverse: 155 MPa

Comparative Example 4

(29) 220 g of high-density polyethylene having a density of 0.956 g/cm.sup.3, 100 g of ultrahigh molecular weight polyethylene having a molecular weight of 5.0×10.sup.6, 6.4 g of n-octadecyl-β-(3,5-di-tert-butyl-4-hydroxyphenyl)-propionate (antioxidant), 32.0 g of ethylene-vinyl acetate copolymer, and 2200 g of mineral oil were fed to a continuous mixing and charging kettle, and stirred at a speed of 50 rpm to mix the raw materials uniformly.

(30) The mixture was continuously fed to a twin-screw extruder, and the ultrahigh molecular weight polyethylene, the high-density polyethylene, the antioxidant, and the ethylene-vinyl acetate copolymer were continuously dissolved in the mineral oil in the twin-screw extruder at 180° C., and continuously extruded by a twin-screw extruder at a speed of 200 rpm. The mixture continuously entered a slit die, and was extruded through the slit die to a casting cooling roller and cast into a strip at 80° C.

(31) The obtained strip was placed in an extraction tank containing dichloromethane for extraction to remove the mineral oil from the strip. The extracted strip was continuously fed into a biaxial stretching machine at 120° C. to be stretched into a film, then the resulting film material was subjected to secondary extraction with dichloromethane, and the resulting film was washed with deionized water, heat set at 120° C. for 15 min, and wound at a speed of 20 m/min to obtain a separator, and its specific performance parameters were tested as shown in the following table:

(32) TABLE-US-00009 Performance Thickness   12 μm Resistance (MacMullin) 7 Transmission (Gurley value) 210 Porosity 48% Pore size 0.50 μm Contact angle 28.0° Puncture strength 480 g .sup.  Tensile strength longitudinal: 160 MPa transverse: 151 MPa

Comparative Example 5

(33) 220 g of high-density polyethylene having a density of 0.956 g/cm.sup.3, 100 g of ultrahigh molecular weight polyethylene having a molecular weight of 5.0×10.sup.6, 6.4 g of n-octadecyl-β-(3,5-di-tert-butyl-4-hydroxyphenyl)-propionate (antioxidant), 3.2 g of triethoxyvinylsilane grafted polyethylene, 6.4 g of ethylene-vinyl acetate copolymer, and 2200 g of mineral oil were fed to a continuous mixing and charging kettle, and stirred at a speed of 50 rpm to mix the raw materials uniformly.

(34) The mixture was continuously fed to a twin-screw extruder, and the ultrahigh molecular weight polyethylene, the high-density polyethylene, the antioxidant, the triethoxyvinylsilane grafted polyethylene and the ethylene-vinyl acetate copolymer were continuously dissolved in the mineral oil in the twin-screw extruder at 180° C., and continuously extruded by the twin-screw extruder at a speed of 200 rpm. The mixture continuously entered a slit die, and was extruded through the slit die to a casting cooling roller and cast into a strip at 80° C.

(35) The obtained strip was placed in an extraction tank containing dichloromethane for extraction to remove the mineral oil from the strip. The extracted strip was continuously fed into a biaxial stretching machine at 120° C. to be stretched into a film, then the resulting film material was subjected to secondary extraction with dichloromethane, and the resulting film was washed with deionized water, heat set at 120° C. for 15 min, and wound at a speed of 20 m/min to obtain a separator, and its specific performance parameters were tested as shown in the following table:

(36) TABLE-US-00010 Performance Thickness   12 μm Resistance (MacMullin) 8 Transmission (Gurley value) 207 Porosity 47% Pore size 0.46 μm Contact angle 35.3° Puncture strength 530 g .sup.  Tensile strength longitudinal: 175 MPa transverse: 160 MPa

(37) As can be seen from above results, the separator prepared without adding ethylene copolymer has a relatively large contact angle with the electrolyte, correspondingly the resistance is relatively large (Comparative Example 2); after addition of ethylene copolymer, the contact angle with the electrolyte and the resistance are significantly reduced. This may be mainly due to the fact that the ethylene copolymer contains a large amount of ester groups or carboxylic acid functional groups, which are similar in polarity to the main components of the lithium ion battery electrolyte; as a result, improved wettability can be obtained. The separator prepared by solely adding maleic anhydride grafted polyethylene exhibits a relatively large contact angle with the electrolyte (Comparative Example 1).

(38) As also can be seen from above results, addition of less than 5 parts by weight of ethylene copolymer, on the basis that the total weight of the ultrahigh molecular weight polyethylene and the high-density polyethylene is 100 parts, has no significant effect on the pore size and strength of the separator, thereby maintaining the original properties of the separator material while obtaining improved wettability. When the amount of ethylene-vinyl acetate copolymer was 5 parts by weight (Comparative example 3), the puncture strength and tensile strength of the separator begin to decrease; when the amount of ethylene-vinyl acetate copolymer was 10 parts by weight (Comparative example 4), the puncture strength and tensile strength of the separator significantly decrease, which may be due to the low strength of such copolymer per se. Moreover, as can be seen from Comparative example 5, when the grafted polymers without ester groups or carboxylic acid functional groups are used in combination with ethylene-vinyl acetate copolymer, a contact angle of less than 30° cannot be obtained, and the puncture strength and the tensile strength of the separator are also slightly reduced, which may be due to poor compatibility between such grafted polymer and ethylene-vinyl acetate copolymer.

(39) The above are only preferred examples of the invention and are not intended to limit the scope of the substantive technical content of the invention. The substantive technical content of the invention is broadly defined in the scope of the claims as attached. Any technical entity or method that is completed by others, if it is exactly the same as defined in the scope of the claims of the application, or an equivalent change, is considered to be within the scope of the claims.