WO2012146126A1

WO2012146126A1 - Method of preparing separator

Info

Publication number: WO2012146126A1
Application number: PCT/CN2012/073701
Authority: WO
Inventors: Miaoyun LIN; Zhen You; Weicheng Yu; Mingjun Luo; Huiquan Liu
Original assignee: Shenzhen Byd Auto R&D Company Limited; Byd Company Limited
Priority date: 2011-04-27
Filing date: 2012-04-10
Publication date: 2012-11-01
Also published as: CN102757577A; CN102757577B

Abstract

A method for preparing a separator is provided. The method comprises steps of: (a) mixing an ultra-high molecular weight polyethylene with a first solvent to form a first mixture, and heating, stirring and filtrating the first mixture to obtain a pre-swollen ultra-high molecular weight polyethylene; (b) dissolving and plasticizing the pre-swollen ultra-high molecular weight polyethylene, a high density polyethylene and a second solvent in a twin screw extruder to obtain a plasticized melt; and (c) casting and cooling the plasticized melt to obtain a cast slab, and stretching, extracting and heat setting the cast slab to obtain the separator, in which the first solvent is a good solvent for a polyolefin, and the second solvent is a plasticizer.

Description

METHOD OF PREPARING SEPARATOR

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority to and benefits of Chinese Patent Application No. 201110106416.8, filed with the State Intellectual Property Office, P. R. C. on April 27, 2011, the content of which is incorporated herein by reference in its entirety.

FIELD

The present disclosure relates to the field of Li-ion battery, especially relates to a method for preparing a separator.

BACKGROUND

A separator is an indispensable part to form a battery. Taking a Li-ion battery as an example, the separator not only needs to conduct Li-ions, but also needs to separate a positive electrode from a negative electrode in order to prevent short circuit between the positive and negative electrodes or self-discharge inside the battery to a certain degree. Therefore, the performances such as security, permeability, porosity and thickness of the separator also affect the performance of a battery largely.

At present, microporous polyolefin films as separators are broadly used in the commercial Li-ion battery, but the polyolefin film has common strength, so that the obtained separator may have defects such as lower separator rupture temperature or weak puncture strength at high temperature. Therefore, the microporous polyolefin films may be only used in those batteries for cellphones, cameras, notebooks or other minitype electronic products, but the microporous polyolefin films may not be used in those power batteries with high capacity.

In order to meet the requirement of power batteries, an ultra-high molecular weight polyethylene may be usually adapted to improve the performance of the separator. Like a high density polyethylene, an ultra-high molecular weight polyethylene is a linear polyethylene with a weight average molecular weight of at least 7.5>< 10⁵, and when the weight average molecular weight of the ultra-high molecular weight polyethylene reaches above 2x 10⁶, the ultra-high molecular weight polyethylene may have better mechanical performance. However, with the increase of the molecular weight, molecular chains of the ultra-high molecular weight polyethylene may twist with each other seriously and further affect the plasticization of the ultra-high molecular weight polyethylene. Therefore, during the separator preparing process, molecules may not be uniformly dissolved due to regular molecular chains, high crystallinity degree and high molecular weight of the ultra-high molecular weight polyethylene; and the viscosity of a solution is very high due to random twisting among long molecular chains of the ultra-high molecular weight polyethylene. Therefore, serious Weissenberg effect may be caused if the ultra-high molecular weight polyethylene is directly dissolved with stirring, which is difficult to industrially control. Furthermore, the ultra-high molecular weight polyethylene may be not completely plasticized, and a jelly wound together may not be measured. Even when a twin screw extruder is adopted during the separator preparing process, the performances such as air permeability, thermal shrinkage percent, puncture strength and high-temperature separator rupture temperature of the separator may also not meet the requirements.

SUMMARY

In order to solve at least one of the above defects existing in prior art, there is provided a method for preparing a separator with good performances such as air permeability, thermal shrinkage percent, puncture strength and high-temperature separator rupture temperature. Moreover, the ultra-high molecular weight polyethylene may be uniformly plasticized.

According to an aspect of the present disclosure, a method for preparing a separator is provided. The method comprises steps of: (a) mixing an ultra-high molecular weight polyethylene with a first solvent to form a first mixture, and heating, stirring and filtrating the first mixture to obtain a pre-swollen ultra-high molecular weight polyethylene; (b) dissolving and plasticizing the pre-swollen ultra-high molecular weight polyethylene, a high density polyethylene and a second solvent in a twin screw extruder to obtain a plasticized melt; and (c) casting and cooling the plasticized melt to obtain a cast slab, and stretching, extracting and heat setting the cast slab to obtain the separator, in which the first solvent is a good solvent for a polyolefin, and the second solvent is a plasticizer.

It has been found by the inventors that with a conventional method of first mixing an ultra-high molecular weight polyethylene, a high density polyethylene and a solvent to form a mixture and then adding the mixture into an extruder for plasticization, although the problem of non-uniform separator micropores caused by non-uniform dissolving of raw materials may be solved to a certain degree, the performances such as air permeability, thermal shrinkage percent, puncture strength and high-temperature separator rupture temperature of the obtained separator may be poor. The main reason for this may be as follows: directly dissolving and plasticizing an ultra-high molecular weight polyethylene, a high density polyethylene and a solvent in a twin screw extruder may cause the separator prepared at a common processing temperature at a common shear rate to have many unplasticized powders, which may increase defects of the separator and reduce the performance of the separator; and the ultra-high molecular weight polyethylene is sensitive to the shear, molecular chains of the ultra-high molecular weight polyethylene may be easily broken when high-temperature shearing, the ultra-high molecular weight polyethylene may not be processed at a high shear rate at a high temperature, and consequently a separator with good performances such as air permeability, thermal shrinkage percent, puncture strength and high-temperature separator rupture temperature may not be obtained.

It has been unexpectedly found by the inventors that with the method for preparing the separator according to an embodiment of the present disclosure, if the ultra- high molecular weight polyethylene is first pre-swollen in a first solvent and then the pre-swollen ultra-high molecular weight polyethylene, a second solvent and a high density polyethylene are dissolved and plasticized in a twin screw extruder, the ultra-high molecular weight polyethylene may be uniformly plasticized at a common processing temperature at a common shear rate, and the performances such as air permeability, thermal shrinkage percent, puncture strength and high-temperature separator rupture temperature of the obtained separator may be significantly improved.

It is supposed by the inventors that pre-swelling of the ultra-high molecular weight polyethylene may cause a solvent to permeate and diffuse into an interior of the polymer to the largest extent, and permeation of the solvent may weaken strong interaction between long molecular chains of the ultra-high molecular weight polyethylene so as to untwist the molecular chains of the ultra-high molecular weight polyethylene to a certain extent. Therefore, the ultra-high molecular weight polyethylene is in a substable state. The more sufficient the solvation, the more easily the ultra-high molecular weight polyethylene is dissolved, and the more uniformly the ultra-high molecular weight polyethylene is dissolved in a twin screw extruder. The good solvent for a polyolefin has strong permeability and strong diffusivity, so that the solvation of this solvent may be more sufficient, and the ultra-high molecular weight polyethylene may be more uniformly dissolved.

In method for preparing the separator according to an embodiment of the present disclosure, if it is required that molecules of the sufficiently pre-swollen ultra-high molecular weight polyethylene are freely dispersed into a solvent, the pre-swollen ultra-high molecular weight polyethylene, a second solvent and a high density polyethylene are added into an extruder together, and dissolved and plasticized at a high temperature.

With the separator prepared by the method according to an embodiment of the present disclosure, not only may the problem of non-uniform plasticization of the ultra-high molecular weight polyethylene be solved, but also the obtained separator may have good performances such as air permeability, thermal shrinkage percent, puncture strength and high-temperature separator rupture temperature. Moreover, the degradation of the ultra-high molecular weight polyethylene may be decreased, thus maintaining the long molecular chains of the ultra-high molecular weight polyethylene, reducing the curvature radius of the separator, and enhancing the permeation ability of Li ions. Meanwhile, the long molecular chains of the ultra-high molecular weight polyethylene may act as a firm support for the separator so as to enhance high-temperature separator rupture temperature of the separator, and the ultra-high molecular weight polyethylene has good thermal stability, thus reducing the thermal shrinkage of the separator and enhancing the safety of a battery.

Additional aspects and advantages of the embodiments of the present disclosure will be given in part in the following descriptions, become apparent in part from the following descriptions, or be learned from the practice of the embodiments of the present disclosure.

DETAILED DESCRIPTION

Reference will be made in detail to embodiments of the present disclosure. The embodiments described herein are explanatory and illustrative, which are used to generally understand the present disclosure. The embodiments shall not be construed to limit the present disclosure.

According to an embodiment of the present disclosure, the method for preparing the separator comprises steps of:

(a) mixing an ultra-high molecular weight polyethylene with a first solvent to form a first mixture, and heating, stirring and filtrating the first mixture to obtain a pre-swollen ultra-high molecular weight polyethylene; (b) dissolving and plasticizing the pre-swollen ultra-high molecular weight polyethylene, a high density polyethylene and a second solvent in a twin screw extruder to obtain a plasticized melt; and

(c) casting and cooling the plasticized melt to obtain a cast slab, and stretching, extracting and heat setting the cast slab to obtain the separator,

in which the first solvent is a good solvent for a polyolefin, and the second solvent is a plasticizer.

Particularly, the method for preparing the separator comprises steps of:

(a) mixing an ultra-high molecular weight polyethylene with a first solvent to form a first mixture, heating, stirring and filtrating the first mixture to obtain a pre-swollen ultra-high molecular weight polyethylene;

(b) dissolving and plasticizing the pre-swollen ultra-high molecular weight polyethylene, a high density polyethylene and a second solvent in a twin screw extruder to obtain a plasticized melt;

(c) casting and cooling the plasticized melt to obtain a cast slab;

(d) stretching the cast slab to obtain a stretched film;

(e) extracting the stretched film to obtain an extracted film; and

(f) drying and heat setting the extracted film to obtain the separator.

In some embodiments, the first solvent is a good solvent for a polyolefin which has a polarity similar to the polyolefin. Preferably, the first solvent may be at least one selected from the group consisting of decalin, coal oils, diphenyl ether, diisodecyl phthalate, benzene, toluene, chloroform, diethyl ether, trichloroethylene or amyl acetate; and the second solvent may be at least one selected from the group consisting of liquid paraffins, paraffins, fatty oils or plant oils.

In the method for preparing the separator according to an embodiment of the present disclosure, the ultra-high molecular weight polyethylene is first mixed with the first solvent to form the first mixture, then the first mixture is heated and stirred in a stirring tank, the temperature of which is gradually increased to a desired temperature, and finally the heated mixture is filtrated to recycle the first solvent and obtain the pre-swollen ultra-high molecular weight polyethylene. Preferably, the heating temperature is about 80°C to about 120°C. More preferably, the heating temperature is about 90°C to about 110°C. Therefore, insufficient solvation of the ultra-high molecular weight polyethylene caused by over low swelling temperature may be avoided, and a high- viscosity layer formed on the surface of the ultra-high molecular weight polyethylene grains due to over high swelling temperature may be prevented from hindering a solvent from permeating through the high-viscosity layer so as to avoid a jelly thus improving uniform plasticization of the ultra-high molecular weight polyethylene, enhancing the molecular weight of the ultra-high molecular weight polyethylene, and enhancing the performances such as air permeability, puncture strength and separator rupture temperature of the separator. Preferably, the stirring rate may be about 20r/min to about 300r/min; more preferably, the stirring rate is about 30r/min to about 200r/min, thus avoiding aggregation of the ultra-high molecular weight polyethylene due to over low stirring rate which may cause the ultra-high molecular weight polyethylene not to be uniformly dispersed in a solvent, and preventing too much shear degradation of the ultra-high molecular weight polyethylene due to over high stirring rate and over high stirring shear. Preferably, the heating time is about lh to about 20h; more preferably, the heating time is about lh to lOh. Therefore, insufficient solvation of the ultra-high molecular weight polyethylene due to too short heating time may be avoided, and too much thermal oxidative degradation of the ultra-high molecular weight polyethylene due to too long heating time may be prevented, thus enhancing the performances such as air permeability, puncture strength, separator rupture temperature and thermal shrinkage performance of the separator.

In the method for preparing the separator according to an embodiment of the present disclosure, preferably, the weight average molecular weight of the ultra-high molecular weight polyethylene may be about l x lO⁶ to about 7x 10⁶, more preferably, about l x lO⁶ to about 5^χ 10⁶, most preferably, about l x lO⁶ to about 3 ^χ 10⁶. Therefore, the ultra-high molecular weight polyethylene may be easy to process while significantly improving the performance of the separator. Preferably, the molecular weight distribution of the ultra-high molecular weight polyethylene may be about 3 to about 30, more preferably, about 5 to about 20. Preferably, the weight average molecular weight of the high density polyethylene may be about 2^χ 10⁵ to about 8x l0⁵, more preferably, about 2^χ 10⁵ to about 5^χ 10⁵. Preferably, the molecular weight distribution of the high density polyethylene may be about 5 to about 40, more preferably, about 10 to about 30.

In the method for preparing the separator according to an embodiment of the present disclosure, based on the total weight of the separator, preferably, the amount of the ultra-high molecular weight polyethylene may be about 1 weight part to about 30 weight parts, the amount of the first solvent may be about 40 weight parts to about 90 weight parts, the amount of the high density polyethylene may be about 5 weight parts to about 40 weight parts, and the amount of the second solvent may be about 50 weight parts to about 90 weight parts. More preferably, the amount of the ultra-high molecular weight polyethylene may be about 5 weight parts to about 15 weight parts, the amount of the first solvent may be about 70 weight parts to about 85 weight parts, the amount of the high density polyethylene may be about 5 weight parts to about 20 weight parts, and the amount of the second solvent may be about 70 weight parts to about 85 weight parts.

In the method for preparing the separator according to an embodiment of the present disclosure, preferably, during the ultra-high molecular weight polyethylene pre-swelling process, an additive may be added to the first mixture of the ultra-high molecular weight polyethylene and the first solvent, thus avoiding thermal oxidative degradation of the ultra-high molecular weight polyethylene during the processing process, ensuring the large molecular weight of the ultra-high molecular weight polyethylene, and enhancing the performances such as air permeability, puncture strength, high-temperature separator rupture temperature and thermal shrinkage performance of the separator. The additive may be at least one selected from the group consisting of an antioxidant, a heat stabilizer or an ultraviolet absorber. The antioxidant may mainly avoid the oxidative degradation of the ultra-high molecular weight polyethylene, and the heat stabilizer may mainly avoid the high-temperature degradation of the ultra-high molecular weight polyethylene. More preferably, the additive may be a mixture of an antioxidant and a heat stabilizer, which may coordinate with each other and enhance the performance of the separator. Most preferably, the additive may be a mixture of an antioxidant, a heat stabilizer and an ultraviolet radiation absorber, in which as an auxiliary additive, a small amount of added ultraviolet radiation absorber may improve the performances such as air permeability and puncture strength of the separator.

In some embodiments, the antioxidant may be at least one selected from 2,5-di-tert-butylhydroquinone, 2,6-di-tert-butyl-4-cresol, 3-(3,5-di-tert-butyl-4-hydroxyl)octadecyl acrylate or pentaerythrite tetra[(3,5-di-tert-butyl-4-hydroxyphenyl)-propionate; the heat stabilizer may be at least one selected from the group consisting of tris(2,4-di-tert-butylphenyl)phosphite, triphenyl phosphite, phenyl diisooctyl phosphite and tri(nonylphenyl) phosphite; and the ultraviolet absorber is at least one selected from the group consisting of benzophenones, benzotriazoles, hindered amines or aromatic esters, for example, at least one of 2-(2'-hydroxy-3',5'-di-tert-butyl-phenyl)-5-chlorobenzotriazole, 2,4-dihydroxy benzophenone or 2-hydroxy-4-n-octyloxybenzophenone.

In some embodiments, based on the total weight of the separator, the amount of the additive may be about 0.1 weight parts to about 1 weight part; preferably, the amount of the additive may be about 0.2 weight parts to about 0.5 weight parts, which may further decrease the thermal oxidative degradation of the ultra-high molecular weight polyethylene.

In the method for preparing the separator according to an embodiment of the present disclosure, the pre-swollen ultra-high molecular weight polyethylene, the high density polyethylene and the second solvent may be added into the twin screw extruder, and then dissolved and plasticized to obtain the plasticized melt. Preferably, the aspect ratio of the twin screw extruder may be about 20 to about 70, the processing temperature of the twin screw extruder may be about 120°C to about 300°C, and the shear rate of the twin screw extruder may be about 20r/min to 500r/min. More preferably, the aspect ratio of the twin screw extruder may be about 30 to about 50, the processing temperature of the twin screw extruder may be about 150°C to about 280°C, and the shear rate of the twin screw extruder may be about 40r/min to 200r/min. A too law aspect ratio may not be beneficial to uniform plasticization of the ultra-high molecular weight polyethylene, while a higher aspect ratio may bring difficulty to processing; a lower processing temperature also may not be benefit to uniform plasticization of the ultra-high molecular weight polyethylene, while a higher processing temperature may accelerate the thermal oxidative degradation of the ultra-high molecular weight polyethylene; and a lower shear rate leads to too large load on the twin screw extruder so that the ultra-high molecular weight polyethylene is difficult to process, while a higher shear rate may accelerate the shear degradation of the ultra-high molecular weight polyethylene.

In the method for preparing the separator according to an embodiment of the present disclosure, the plasticized melt may be cast by a die and cooled to form a cast slab. Preferably, the adopted die may be in a 'T' shape, a clothes rack shape or a fish tail shape, and the thickness of the obtained cast slab may be about 0.5mm to about 6mm; more preferably, the thickness of the obtained cast slab may be about 1mm to 5mm; most preferably, the thickness of the obtained cast slab may be about 1.2mm to 3mm. Preferably, the cooling rate is at least about 50°C/min. When the temperature of the cast slab is cooled down to a temperature below 25°C, the cooling rate may be mainly related to phase separation of solids and solvents in the cast slab. Different cooling rates may lead to different phase separation mechanisms, and therefore the obtained separator may also be different in the air permeability. It is found by the inventors that when the cooling rate is no less than about 50°C/min, the air permeability of the separator may be better.

In the method of preparing the separator according to an embodiment of the present disclosure, after cooled, the cast slab is stretched to obtain a stretched film. The stretching mode may be biaxial stretching, or first longitudinal stretching and then transverse stretching, preferably, first longitudinal stretching and then transverse stretching. Preferably, the area stretching ratio may be about 10 to about 200, and the stretching temperature may be about 50°C to about 140°C. More preferably, the area stretching ratio may be about 20 to about 80, which may prevent the phenomena such as stretching rupture or clip dropping while maintaining good air permeability and high puncture strength. Preferably, the stretching temperature may be about 70°C to about 130°C, in order to ensure higher puncture strength while maintaining good tensile property.

In the method for preparing the separator according to an embodiment of the present disclosure, an extractant may be used to extract the stretched film, in order to reduce the residual rate of the solvents in the extracted film, since high residual rate of the solvents may affect air permeability and other performances of the separator. Preferably, the extractant may be at least one selected from the group consisting of hexane, heptane, octane or methylene chloride. Preferably, after extraction, the residual rate of the solvents in the extracted film may be not more than 5%; more preferably, the residual rate of the solvents in the extracted film may be not more than 1%, thus avoiding poor performances such as air permeability, tensile strength, puncture strength and separator rupture temperature of the separator due to over high residual rate of the solvents.

In the method for preparing the separator according to an embodiment of the present disclosure, the extracted film may be dried and heat set. Preferably, the drying temperature may be about 70°C to about 120°C, and the heat setting temperature may be about 100°C to about 140°C; more preferably, the drying temperature may be about 85°C to about 115°C, and the heat setting temperature may be about 120°C to about 135°C. The heat setting process comprises the step of micro-stretching the dried film. Preferably, the area stretching ratio of micro-stretching may be about 1 to about 5; more preferably, the area stretching ratio of micro-stretching may be about 1.2 to about 2.

In the following, particular embodiments of the present disclosure will be described in detail. These embodiments should not be construed to limit the scope of the present disclosure in any way. Embodiment 1

The separator in this embodiment may be obtained by the following steps of:

(a) mixing 40 kg of an ultra-high molecular weight polyethylene with a weight average molecular weight of 3.9x 10⁶ and a molecular weight distribution of 12 with 160 kg of decalin to form a first mixture, adding the first mixture into a stirring tank, at a stirring rate of 50r/min, rising the temperature to about 100°C, heating and stirring the first mixture at about 100°C for about 5h, and filtrating the first mixture to obtain a pre-swollen ultra-high molecular weight polyethylene;

(b) mixing the pre-swollen ultra-high molecular weight polyethylene with 60 kg of a high density polyethylene with a weight average molecular weight of 3.8>< 10⁵ and a molecular weight distribution of 12.5 and 300 kg of liquid paraffins to form a second mixture, gradually adding the second mixture into a twin screw extruder with an aspect ratio of about 40, and dissolving and plasticizing the second mixture in the twin screw extruder at a processing temperature of about 150°C to about 280°C at a shear rate of about lOOr/min to obtain a plasticized melt;

(c) casting the plasticized melt into a cast slab by a die in a 'T' shape, and then cooling the cast slab to about 20°C at a cooling rate of about 60°C/min to obtain the cast slab with a thickness of about 1.5mm;

(d) stretching the cast slab to obtain a stretched film, in which the cast slab is first longitudinally stretched and then transversely stretched at a stretching temperature of about 110°C with a area stretching ratio of about 25;

(e) extracting the stretched film with hexane to obtain an extracted film, in which the residual rate of the solvents in the extracted film is about 0.5%; and

(f) drying the extracted film at a temperature of about 85°C, then heat setting the dried film at a temperature of about 120°C to obtain a separator SI, in which during the heat setting process, the area stretching ratio of micro-stretching is about 1.5.

Embodiment 2

This embodiment is substantially the same as Embodiment 1, except that: in the step (a), 40kg of the ultra-high molecular weight polyethylene and 60kg of decalin were mixed and added into a mixing tank. The separator obtained in Embodiment 2 is recorded as S2.

Embodiment 3 This embodiment is substantially the same as Embodiment 1, except that: in the step (a), 40kg of the ultra-high molecular weight polyethylene, 160kg of decalin., and 0.6kg of 3-(3,5-di-tert-butyl-4-hydroxyl)octadecyl acrylate were added into a mixing tank. The separator obtained in Embodiment 3 is recorded as S3.

Embodiment 4

This embodiment is substantially the same as Embodiment 1, except that: in the step (a), 40kg of the ultra-high molecular weight polyethylene, 160kg of decalin, and 0.15kg of 3-(3,5-di-tert-butyl-4-hydroxyl)octadecyl acrylate were added into a mixing tank. The separator obtained in Embodiment 4 is recorded as S4.

Embodiment 5

This embodiment is substantially the same as Embodiment 1, except that: in the step (a), 40kg of the ultra-high molecular weight polyethylene, 160kg of decalin, 0.5kg of 3-(3,5-di-tert-butyl-4-hydroxyl)octadecyl acrylate, and 0.2kg of tris(2,4-di-tert-butylphenyl)phosphite were added into a mixing tank. The separator obtained in Embodiment 5 is recorded as S5.

Embodiment 6

This embodiment is substantially the same as Embodiment 1, except that: in the step (a), 40kg of the ultra-high molecular weight polyethylene and 160kg of coal oils were added into a mixing tank. The separator obtained in Embodiment 6 is recorded as S6.

Embodiment 7

This embodiment is substantially the same as Embodiment 1, except that: in the step (a), 40kg of the ultra-high molecular weight polyethylene and 160kg of diisodecyl phthalate were added into a mixing tank. The separator obtained in Embodiment 7 is recorded as S7.

Embodiment 8

This embodiment is substantially the same as Embodiment 1, except that: in the step (a), the heating temperature was about 80°C, the stirring rate was about 300r/min, and the heating time was about 15h. The separator obtained in Embodiment 8 is recorded as S8. Embodiment 9

This embodiment is substantially the same as Embodiment 1, except that: in the step (a), the heating temperature was about 106°C, and the heating time was about lh. The separator obtained in Embodiment 9 is recorded as S9.

Embodiment 10

This embodiment is substantially the same as Embodiment 1, except that: in the step (a), 40kg of the ultra-high molecular weight polyethylene, 160kg of decalin, 0.5kg of 3-(3,5-di-tert-butyl-4-hydroxyl)octadecyl acrylate, 0.2kg of tris(2,4-di-tert-butylphenyl)phosphite, and 0.4kg of 2-(2'-hydroxy-3 ',5'-di-tert-butyl-phenyl)-5-chlorobenzotriazole were added into a mixing tank. The separator obtained in Embodiment 10 is recorded as S10. Comparative Embodiment 1

This embodiment is substantially the same as Embodiment 1, except that: the ultra-high molecular weight polyethylene was not pre-swollen, but 40kg of the ultra-high molecular weight polyethylene, 60kg of the high density polyethylene, and 300kg of liquid paraffins were directly added into a twin screw extruder, and then were dissolved and plasticized to form a separator Dl.

Comparative Embodiment 2

The separator in this embodiment is prepared by a method similar to that in Chinese Patent No. CN101020759A. This embodiment is substantially the same as Embodiment 1, except that, 40kg of the ultra-high molecular weight polyethylene, 60kg of the high density polyethylene and 300kg of liquid paraffin were first mixed uniformly and then added into a twin screw extruder to be dissolved and plasticized to form a separator D2.

Performance Test

The obtained separators S1-S10, and D1-D2 all have a thickness of about 25μπι. The following performances of the separators will be tested and shown in table 1. Air Permeability

Using a 4011 type Gurley air permeability tester according to the GB/T5402-2003 test standard, under conditions of an average pressure difference of about 1.23kpa and a compressed area of the separator inside a cylinder of about 6.41cm², based on the time for which air with a volume of 100ml passed through the separator, air permeability of the separators S1-S10 and D1-D2 was tested.

Puncture Strength

A puncture instrument was used to test the puncture strength of the separators S1-S10 and D1-D2. Particularly, each of the separators S1-S10 and D1-D2 was vertically pierced using a slick pin with a diameter of 1mm at a speed of 2m/min, and the results were recorded by a FCN-5B type data recorder.

Thermal Shrinkage Percent

According to the GB/T12027-2004/ISO 11501 : 1995 test standard, a lOOmmx lOOmm region on each of the 120mmx 120mm separators S1-S10 and D1-D2 was marked, each of the separators S1-S10 and D1-D2 was spread in an oven and coated with a layer of preheated kaolin, the temperature in the oven was about 90°C, and the heating time was 2h. 2h later, each sample was taken out and maintained at room temperature for about 30min. Before each sample was tested again, a lOOmmx 100mm region was marked. The thermal shrinkage percent was calculated according to a formula of ΔΤ=(Τ-Τ0)/Τ0^χ 100%, where T is the length of a marked region after heating, T₀ is the initial length of a marked region, and the thermal shrinkage percent is the absolute value of ΔΤ.

Separator Rupture Temperature

Each of the separators S1-S10 and D1-D2 was placed in a simulation battery, the positive and negative electrodes of the simulation battery were made from stainless steel sheet respectively, the volume of an electrolyte in the simulation battery was about 1.2ml, and the contact area between each separator and the electrolyte was about 6.42 cm². The simulation battery was heated from about 30°C to about 200°C. When the resistance of the simulation battery suddenly drops for the first time, if the resistance difference is more than 50 ohms, the temperature at which the resistance suddenly drops is the separator rupture temperature.

The test results are shown in Table 1.

Table 1

It may be seen from Table 1 that compared with Comparative Embodiment 1 and Comparative Embodiment 2, with the separator prepared by the method according to an embodiment of the present disclosure, air permeability and thermal shrinkage percent are significantly decreased, while puncture strength and separator rupture temperature are significantly enhanced. That is, with the separator prepared by the method according to an embodiment of the present disclosure, not only may the problem of non-uniform plasticization of the ultra-high molecular weight polyethylene be solved, but also the performances such as air permeability, puncture strength, high-temperature separator rupture temperature and thermal shrinkage performance may be improved, thus enhancing the safety of a battery.

Although explanatory embodiments have been shown and described, it would be appreciated by those skilled in the art that changes, alternatives, and modifications may be made in the embodiments without departing from spirit and principles of the disclosure. Such changes, alternatives, and modifications all fall into the scope of the claims and their equivalents.

Claims

WHAT IS CLAIMED IS:

1. A method for preparing a separator, comprising steps of:

(a) mixing an ultra-high molecular weight polyethylene with a first solvent to form a first mixture, and heating, stirring and filtrating the first mixture to obtain a pre-swollen ultra-high molecular weight polyethylene;

(b) dissolving and plasticizing the pre-swollen ultra-high molecular weight polyethylene, a high density polyethylene and a second solvent in a twin screw extruder to obtain a plasticized melt; and

wherein the first solvent is a good solvent for a polyolefin, and the second solvent is a plasticizer.

2. The method of claim 1, wherein the first solvent is at least one selected from a group consisting of decalin, coal oils, diphenyl ether, diisodecyl phthalate, benzene, toluene, chloroform, diethyl ether, trichloroethylene or amyl acetate.

3. The method of claim 1, wherein based on the total weight of the separator, the amount of the ultra-high molecular weight polyethylene is about 1 weight part to about 30 weight parts, and the amount of the first solvent is about 50 weight parts to about 90 weight parts.

4. The method of claim 1, wherein in step (a), the heating temperature is about 80°C to about 120°C, the stirring rate is about 20r/min to about 300r/min, and the heating time is about lh to about 20h.

5. The method of claim 1, wherein step (a) further comprises a step of mixing an additive with the ultra-high molecular weight polyethylene and the first solvent, and the additive is at least one selected from a group consisting of an antioxidant, a heat stabilizer or an ultraviolet absorber.

6. The method of claim 5, wherein the additive is a mixture of an antioxidant and a heat stabilizer.

7. The method of claim 5, wherein the antioxidant is at least one selected from a group consisting of 2,5-di-tert-butylhydroquinone, 2,6-di-tert-butyl-4-cresol, 3-(3,5-di-tert-butyl-4-hydroxyl)octadecyl acrylate or pentaerythrite tetra[(3,5-di-tert-butyl-4-hydroxyphenyl)-propionate;

the heat stabilizer is at least one selected from a group consisting of tris(2,4-di-tert-butylphenyl) phosphite, triphenyl phosphite, phenyl diisooctyl phosphite and tri(nonylphenyl) phosphite; and

the ultraviolet absorber is at least one selected from a group consisting of benzophenones, benzotriazoles, hindered amines or aromatic esters.

8. The method of claim 5, wherein the amount of the additive is about 0.1 weight parts to about 1 weight part.

9. The method of claim 1, wherein the cooling rate is at least about 50°C/min.

10. The method of claim 1, wherein the weight average molecular weight of the ultra-high molecular weight polyethylene is about l x lO⁶ to about 7x 10⁶, and the molecular weight distribution of the ultra-high molecular weight polyethylene is about 3 to about 30.

11. The method of claim 1, wherein the second solvent is at least one selected from a group consisting of liquid paraffins, paraffins, fatty oils and plant oils;

the amount of the high density polyethylene is about 5 weight parts to about 50 weight parts, and the amount of the second solvent is about 50 weight parts to about 90 weight parts; and

the weight average molecular weight of the high density polyethylene is about 2x l0⁵ to about 8x l0⁵, and the molecular weight distribution of the high density polyethylene is about 5 to about 40.