WO2019129217A1

WO2019129217A1 - Cross-linked polymer separators for electrochemical devices and preparation methods thereof

Info

Publication number: WO2019129217A1
Application number: PCT/CN2018/124973
Authority: WO
Inventors: Alex Cheng; Lei XIONG
Original assignee: Shanghai Energy New Materials Technology Co., Ltd.
Priority date: 2017-12-29
Filing date: 2018-12-28
Publication date: 2019-07-04
Also published as: CN108192116A; CN108192116B

Abstract

Disclosed are a method for preparing a separator for an electrochemical device, comprising (1) mixing at least one polyethylene, at least one antioxidant, at least one auxiliary cross-linking agent, at least one photo-initiator and at least one pore-forming agent to obtain a mixture; (2) extruding the mixture to obtain an extruded mixture; (3) producing a ribbon from the extruded mixture; (4) removing the at least one pore-forming agent from the ribbon; (5) stretching the ribbon into a film; (6) thermoforming and winding the film to obtain an uncross-linked polymer separator; (7) applying a light irradiation to the uncross-linked polymer separator to obtain the cross-linked polymer separator; and (8) optionally annealing the cross-linked polymer separator; a cross-linked polymer separator for an electrochemical device prepared by the method; and an electrochemical device comprising the separator.

Description

CROSS-LINKED POLYMER SEPARATORS FOR ELECTROCHEMICAL DEVICES AND PREPARATION METHODS THEREOF

CROSS REFERENCE TO RELATED APPLICATION

The present application claims the benefit of priority to Chinese Application No. 201711474230.1, filed on December 29, 2017, the content of which is incorporated by reference.

TECHNICAL FIELD

The present disclosure relates to electrochemistry field, and especially relates to cross-linked polymer separators for electrochemical devices and preparation methods thereof.

BACKGROUND

With rapid growing market of energy storage, batteries and other forms of electrochemical devices are receiving more and more attentions. For example, lithium secondary batteries have been extensively used as energy sources in mobile phones, laptops, power tools, electrical vehicles, etc. Generally, a lithium ion battery mainly comprises a positive electrode, a negative electrode, a permeable membrane (i.e., separator) interposed between the positive electrode and the negative electrode, electrolyte, and a battery shell. The separator is one of the key inner components in the structure of the lithium ion battery. The main function of the separator is to separate the positive electrode from the negative electrode of the battery and prevent direct contact of the two electrodes, thereby avoiding internal short circuit. Meanwhile, the separator allows the ionic charge carriers (e.g., lithium ions) to pass through smoothly to form current during charge and discharge processes of the battery. The separator can also shut down the migration channel of the electrolyte ions when the working temperature of the battery increases abnormally and cut off the current to ensure battery safety. Thus, the properties of the separator may determine the battery’s interfacial structure and internal resistance, which directly affect the characteristics of the battery, such as capacity, cycle performance and safety. Therefore, a separator with excellent performance plays an important role in improving the comprehensive performance of the battery. A separator is generally formed by a polymeric microporous membrane. For example, polyolefin-based porous membranes have been widely used as commercial separators for lithium secondary batteries in the market.

The main performance parameters of a battery separator include thickness, porosity, pore size, pore diameter distribution, strength, thermal shrinkage percentage, shutdown temperature and breakdown temperature, etc. In order to reduce internal resistance of the battery, the area of the electrodes should be as large as possible, and the separator should be as thin as possible. Although the battery separator itself is not conductive, the conductive ions need to migrate through the separator, which requires the separator have certain degree of porosity. However, a porosity that is too high can lead to a decrease in separator’s strength, which may affect the overall reliability of the battery. In addition, the wettability of the electrolyte on the separator can directly affect the resistance of the ion migration. The better the wettability is, the smaller the resistance against the ions migrating through the separator is and the smaller the internal resistance of the battery is. In general, in the case that the pore size of a separator is not very large, the more uniformly the pore diameter distributes, the better the wettability of the electrolyte is. In addition, the separator needs to be pulled during the manufacture and assembly of the battery components and it is required to make sure that the separator is not punctured by the electrode material after the assembly. Therefore, the separator needs to have not only sufficient tensile strength, but also certain puncture strength.

A polymer separator may experience thermal shrinkage under certain heating condition. In order to avoid the internal short circuit due to direct contact between the positive electrode and the negative electrode caused by the thermal shrinkage, the separator should have a low thermal shrinkage percentage. Under abnormal conditions, for example, when short circuit occurs on the external circuit of a lithium ion battery, the internal temperature of the battery rises sharply because of excessive current, which requires the separator be able to shut down the migration channel of the conductive ions in time. Thus, the temperature at which the separator melt and shut down the migration channels of the conductive ions is called “shutdown temperature. ” When the temperature continues to rise, the separator fuses and breaks. The temperature at which the separator fuses and breaks is called “breakdown temperature. ” From the safety point of the lithium ion battery, there needs certain temperature difference between the shutdown temperature and the breakdown temperature of the separator to ensure enough temperature buffer zone to prevent the separator from breaking, even if the temperature continues to rise after the migration channels of the conductive ions are shut down and the current is cut off.

In order to improve the safety of the separator when using the lithium ion battery, one of the most common methods is to coat the polyolefin porous membrane with ceramic slurry. Although the coating treatment can significantly improve the breakdown temperature of the polyolefin porous membrane, it cannot reduce the shutdown temperature of the membrane at the same time. Besides, the coating treatment process has high standard requirement of the ceramic slurry, and the cost of overall raw material and process is relatively high.

SUMMARY OF THE INVENTION

The present disclosure provides a method for preparing a cross-linked polymer separator for an electrochemical device. In some embodiments, the method comprises:

(1) mixing at least one polyethylene, at least one antioxidant, at least one auxiliary cross-linking agent, at least one photo-initiator, and at least one pore-forming agent to obtain a mixture, wherein the at least one polyethylene has an average molecular weight ranging, for example, from 1.0×10 ⁵ to 1.0×10 ⁷ and a density ranging, for example, from 0.940 and 0.976 g/cm ³;

(2) extruding the mixture through, for example, a twin-screw extrusion process, to obtain an extruded mixture;

(3) producing a ribbon from the extruded mixture through, for example, a tape casting process;

(4) removing the at least one pore-forming agent from the ribbon;

(5) stretching the ribbon into a film;

(6) thermoforming and winding the film to obtain an uncross-linked polymer separator; and

(7) applying a light irradiation to the uncross-linked polymer separator to obtain the cross-linked polymer separator.

In some embodiments, the process further comprises:

(8) annealing the cross-linked polymer separator.

The present disclosure further provides a cross-linked polymer separator for an electrochemical device prepared by the method disclosed herein.

The present disclosure further provides an electrochemical device, which comprises a positive electrode, a negative electrode, and the cross-linked polymer separator disclosed herein interposed between the positive electrode and the negative electrode.

DETAILED DESCRIPTION

The present disclosure provides some exemplary embodiments of a method for preparing a cross-linked polymer separator for an electrochemical device. In one embodiment of the present disclosure, the method comprises:

(1) mixing at least one polyethylene, at least one antioxidant, at least one auxiliary cross-linking agent, at least one photo-initiator, and at least one pore-forming agent to obtain a mixture;

(2) extruding the mixture through, for example, a twin-screw extrusion process to obtain an extruded mixture;

(4) removing the at least one pore-forming agent from the ribbon;

(5) stretching the ribbon into a film;

(6) thermoforming and winding the film to obtain an uncross-linked polymer separator;

(7) applying a light irradiation to the uncross-linked polymer separator to obtain the cross-linked polymer separator; and

(8) optionally annealing the cross-linked polymer separator.

In the step (1) , the at least one polyethylene, the at least one antioxidant, the at least one auxiliary cross-linking agent, the at least one photo-initiator, and the at least one pore-forming agent are mixed to obtain a mixture. In some embodiments, the mixture may be stirred to be well blended. The stirring speed may range, for example, from 45 rpm to 55 rpm.

The at least one polyethylene present in the mixture may have an average molecular weight ranging, for example, from 1.0×10 ⁵ to 1.0×10 ⁷, such as, from 1.0×10 ⁵ to 5.0×10 ⁶, further such as from 1.0×10 ⁵ to 2.0×10 ⁶. The at least one polyethylene may have a density ranging, for example, from 0.940 to 0.976 g/cm ³, such as from 0.940 to 0.966 g/cm ³, further such as from 0.950 to 0.966 g/cm ³. The mixture prepared in the step (1) may comprise two or more than two types of polyethylene having different molecular weight or different densities.

The at least one antioxidant present in the mixture may be chosen, for example, from 4, 4-thiobis (6-tert-butyl-m-cresol) , butylated hydroxytoluene, phosphite, tert-butylhydroquinone, n-octadecyl β- (3, 5-di-tert-butyl-4-hydroxyphenyl) propionate, 1, 1, 3-tris (2-methyl-4-hydroxy-5-tert-butylphenyl) butane, 2-tert-butyl-6-methylphenol, N, N’-bis (β-naphthyl) -p-phenylenediamine, dilauryl thiodipropionate, tris (nonylphenyl) phosphite and triphenyl phosphite.

The at least one auxiliary cross-linking agent present in the mixture may be chosen, for example, from mercaptobenzothiazole, dibenzothiazole disulfide, N-cyclohexylbenzothiazole sulfenamide, oxydiethylenebenzothiazole sulfenamide, tetramethyl thiuram monosulfide, tetramethyl thiuram disulfide, zinc dimethyldithiocarbamate, zinc diethyl dithiocarbamate, diphenylguanidine, di-o-tolylguanidine, ethylene thiourea, N, N’-diethylthiourea, hexamethylenetetramine, zinc isopropyl xanthate, trimethylolpropane trimethacrylate, trimethylolpropane triacrylate, triallyl cyanurate and triallyl isocyanurate.

The at least one photo-initiator present in the mixture may be chosen, for example, from benzoin, benzoin dimethyl ether, benzoin ethyl ether, benzoin isopropyl ether, benzoin butyl ether, diphenylethanone, α, α-dimethoxy-α-phenylacetophenone, α, α-diethoxyacetophenone, α-hydroxyalkylphenone, α-aminoalkylphenone, aroylphosphine oxide, bis-benzoylphenylphosphine oxide, benzophenone, 2, 4-dihydroxy benzophenone, Michler’s ketone, thiopropoxy thioxanthone, isopropyl thioxanthone, diaryl iodonium salt, triaryl iodonium salt, alkyl iodonium salt and cumene ferrocene hexafluorophosphate.

The at least one pore-forming agent present in the mixture will be removed in the following steps to form a porous structure in the cross-linked polymer separator. The at least one pore-forming agent may have a kinematical viscosity ranging, for example, from 10 to 100 mm ²/s, such as from 20 to 80 mm ²/s, further such as from 30 to 70 mm ²/s at 40 ℃. The at least one pore-forming agent may have an initial boiling point of, for example, 110 ℃ or above. Examples of the at least one pore-forming agent include natural mineral oil, C _6-15 alkanes, C _8-15 aliphatic carboxylic acids, C _1-4 alkyl C _8-15 aliphatic carboxylates, C _2-6 haloalkanes, phthalates, trimellitates, adipates, sebates, maleates, benzoates, epoxidized vegetable oil, benzsulfamide, phosphotriesters, glycol ethers, acetylated monoglycerides, citrates and diisononyl cyclohexane-1, 2-dicarboxylate.

In the step (1) , the raw materials, i.e., the at least one polyethylene, the at least one antioxidant, the at least one auxiliary cross-linking agent, the at least one photo-initiator, and the at least one pore-forming agent, may be mixed in a specific weight ratio. In some embodiments, the mixture may comprise:

100 parts by weight of the at least one polyethylene;

from 0.1 to 10 parts, such as from 0.2 to 8 parts, and further such as from 0.5 to 5 parts, by weight of the at least one antioxidant;

from 0.1 to 10 parts, such as from 0.2 to 8 parts, and further such as from 0.5 to 5 parts, by weight of the at least one auxiliary cross-linking agent;

from 0.1 to 10 parts, such as from 0.2 to 5 parts, and further such as from 0.5 to 3 parts, by weight of the at least one photo-initiator; and

from 100 to 500 parts, such as from 200 to 500 parts, and further such as from 200 to 400 parts, by weight of the at least one pore-forming agent.

In the step (2) , the mixture is extruded through a twin-screw extrusion process to obtain an extruded mixture. Before the extruding, the temperature of the mixture may be risen to a value ranging, for example, from 170 ℃ to 230 ℃, to dissolve the at least one polyethylene, the at least one antioxidant, the at least one auxiliary cross-linking agent, and the at least one photo-initiator in the at least one pore-forming agent. The twin-screw extrusion process may be carried out at a speed ranging, for example, from 150 rpm to 250 rpm.

In the step (3) , the ribbon is produced from the extruded mixture through a tape casting process. In some embodiments, the tape casting process may comprise: feeding the extruded mixture into a slot die continuously; extruding the extruded mixture via the slot die onto a casting and cooling roller; and forming a ribbon at a temperature ranging, for example, from 70 ℃ to 90 ℃.

In the step (4) , the at least one pore-forming agent in the ribbon is removed. In some embodiments, the at least one pore-forming agent may be removed through a solvent extraction process. During the solvent extraction process, the ribbon may be immersed in or passed through an organic solvent, for example, dichloromethane. The at least one pore-forming agent in the ribbon may transfer into the organic solvent, thereby forming a plurality of pores in the ribbon.

In the step (5) , the ribbon is stretched into a film. In some embodiments, the ribbon may be stretched into a film at a temperature ranging, for example, from 115 ℃ to 125 ℃. The obtained film may be subjected to a solvent extraction process to remove a residue of the at least one pore-forming agent therefrom. During the solvent extraction process, the film may be immersed in or passed through an organic solvent, for example, dichloromethane. After the solvent extraction, the film may be washed using deionized water.

In the step (6) , the film obtained in the step (5) is thermoformed and winded to obtain the uncross-linked polymer separator. The thermoforming may be carried out by keeping the film at a temperature ranging, for example, from 115 ℃ to 125 ℃, for a predetermined time period ranging, for example, from 15 to 20 minutes. The thermoformed film may be winded at a speed ranging, for example, from 20 to 50 m/minute.

In the step (7) , the light irradiation is applied to the uncross-linked polymer separator to obtain the cross-linked polymer separator. In some embodiments, the light irradiation comprises at least one of an ultraviolet light having a wavelength ranging from 250 to 420 nm and a visible light having a wavelength ranging from 400 to 800 nm. The light irradiation may be applied on the uncross-linked separator for a predetermined time period ranging, for example, from 5 to 60 minutes, such as from 5 to 30 minutes, further such as from 10 to 30 minutes. In some embodiments, the light irradiation is through vacuum irradiation that includes placing the separator in a vacuum container or vacuum-packaging the separator prior to light irradiation, such that the separator is isolated from air and oxidizing gases, such as ozone, that are generated during the light irradiation process. Vacuum irradiation can reduce the contact between free radicals in the separator and the oxidizing atmosphere to avoid the oxidative degradation of the separator during the light irradiation as much as possible. In some embodiments, the light irradiation is through protective atmosphere irradiation that includes placing the separator to be irradiated in a container filled with protective atmosphere, or packaging the separator to be irradiated under the condition of protective atmosphere, or continuously introducing a protective gas into an irradiation chamber for light irradiation. The protective atmosphere irradiation can reduce the contact between free radicals in the separator and the oxidizing atmosphere to avoid the oxidative degradation of the separator during the light irradiation as much as possible. In some embodiments, in order to increase the efficiency of free radicals participating in the cross-linking process to obtain a more optimized cross-linked structure, the separator may be preheated to a temperature prior to light irradiation or maintained at a temperature during the light irradiation, wherein the temperature ranges, for example, from 70 ℃ to 120 ℃, such as from 85 ℃ to 110 ℃.

In the step (8) , the annealing is applied to the cross-linked polymer separator. Annealing refers to a method in which a material is kept at a certain temperature for a certain period to improve the performance of the material. Separators that have been annealed are more stable and more sufficiently cross-linked than those without being annealed. The annealing disclosed herein is carried out after irradiation and may improve the performance of the cross-linked polymer separator by: (A) eliminating internal stresses caused by possible uneven irradiation during the irradiation; (B) reducing or eliminating residual free radicals after the irradiation, to improve the internal crystallization and cross-linking state; and/or (C) minimizing reduction in strength caused by irradiation cross-linking, such that the heat shrinkage is less and the separator is more stable. In some embodiments, the annealing may be chosen, for example, from vacuum annealing, annealing in air, and protective atmosphere annealing. Vacuum annealing may be carried out in a vacuum oven. Annealing in air may be carried out in a common oven. Protective atmosphere annealing may be carried out in an oven filled with protective gas such as nitrogen, helium, or argon.

In some embodiments, the annealing disclosed herein may be near melting point annealing, wherein the temperature is held at a temperature for a period of time ranging, for example, from 6 to 48 hours, such as from 8 to 24 hours. The temperature ranges, for example, from 70 ℃ to 120 ℃, such as 110 ℃. The near melting point annealing is followed by furnace cooling, wherein the power supply of an oven is turned off without opening the oven to let the product and the oven cool down to room temperature. In some embodiments, the annealing may be near melting point continuous and repeated annealing, wherein the temperature is held at a temperature for a period of time ranging, for example, from 8 to 24 hours, followed by furnace cooling disclosed herein and such steps are repeated three times. The temperature ranges, for example, from 70 ℃ to 120 ℃, such as 110 ℃. The advantages of near melting point continuous and repeated annealing after light irradiation include: (A) free radicals generated after irradiation can be eliminated to an undetectable extent; and/or (B) the polymer separator is more sufficiently cross-linked.

Further disclosed herein are some exemplary embodiments of cross-linked polymer separators for electrochemical devices prepared by the methods disclosed above. The cross-linked polymer separator may have a porous structure. In some embodiments, the cross-linked polymer separator may have a pore size ranging from 0.01 to 0.1 μm, and a porosity ranging from 30%to 60%. The pore size herein refers to an average pore size that is measured according to the testing method disclosed in the following section of Test Methods.

There is no particular limitation for the thickness of the cross-linked polymer separator disclosed herein, and the thickness of the separator can be controlled in view of the requirements of electrochemical devices, e.g., lithium-ion batteries. In some embodiments, the cross-linked polymer separator may have a thickness ranging, for example, from 5 to 30 μm.

As the cross-linked polymer separator disclosed herein comprises cross-linked bonds that are introduced in the step (7) , it may have improved high-temperature properties, such as a higher breakdown temperature than that of the commercial non-crosslinked polyolefin separator. In addition, the difference between the shutdown temperature and the breakdown temperature of the cross-linked polymer separator disclosed herein may range, for example, from 30 ℃ to 60 ℃.

The cross-linked polymer separator disclosed herein may also have good thermal stability. In some embodiments, the cross-linked polymer separator may have a thermal shrinkage percentage of 0.5%or less after the light irradiation period of at least 30 minutes.

Because of the presence of the cross-linked bonds in the polymer structure, the cross-linked polymer separator disclosed herein can have good porous structure, good mechanical strength, and improved heat-resistance. Thus, the electrochemical devices employing the cross-linked polymer separator may have improved mechanical strength, low internal resistance, improved cycle performance and safety. The separators disclosed herein can have a wide range of applications and can be used for making high-energy density and/or high-power density batteries in many stationary and portable devices, e.g., automotive batteries, batteries for medical devices, and batteries for other large devices.

Further, the present disclosure provides embodiments of an electrochemical device. The electrochemical device comprises a positive electrode, a negative electrode, and a separator disclosed herein that is interposed between the positive electrode and the negative electrode. An electrolyte may be further included in the electrochemical device of the present disclosure. The separator is sandwiched between the positive electrode and the negative electrode to prevent physical contact between the two electrodes and the occurrence of a short circuit. The porous structure of the separator ensures a passage of ionic charge carriers (e.g., lithium ions) between the two electrodes. In addition, the separator may also provide a mechanical support to the electrochemical device. Such electrochemical devices include any devices in which electrochemical reactions occur. For example, the electrochemical device disclosed herein includes primary batteries, secondary batteries, fuel cells, solar cells and capacitors. In some embodiments, the electrochemical device disclosed herein is a lithium secondary battery, such as a lithium metal secondary battery, a lithium ion secondary battery, a lithium polymer secondary battery, and a lithium sulfur secondary battery.

The electrochemical device disclosed herein may be manufactured by a method known in the art. In one embodiment, an electrode assembly is formed by placing a separator of the present disclosure between a positive electrode and a negative electrode, and an electrolyte is injected into the electrode assembly. The electrode assembly may be formed by a process known in the art, such as a winding process or a lamination (stacking) and folding process.

Reference is now made in detail to the following examples. It is to be understood that the following examples are illustrative only and the present disclosure is not limited thereto.

The property parameters of the prepared separators in Examples 1-3 and Comparative Example 1 were tested according to the following testing methods.

Testing Methods

Test 1 Thickness

The thickness of a separator was measured using Mahr Millimar 1216 according to GB/T6672-2001 Plastics Film and Sheeting –Determination of Thickness by Mechanical Scanning.

Test 2 Air permeability

The Air permeability of a separator was measured using a Gurley Densometer 4110 according to GB/T1037 Test Method for Water Vapor Transmission of Plastic Film and Sheet.

Test 3 Porosity

The porosity of a separator was measured using a Water Intrusion Porosimeter (AAQ-3K-A-1, produced by Porous Materials Inc. ) .

Test 4 Pore size

The pores size of a separator was measured using a Water Intrusion Porosimeter (AAQ-3K-A-1, produced by Porous Materials Inc. ) .

Test 5 Puncture Strength

The puncture strength of a separator was measured using a universal testing machine (QJ210A, produced by Shanghai Qingji Instrumentation Technology Co., Ltd. ) according to GB/T 2679.7 Board-Determination of Puncture Resistance.

Test 6 Tensile Strength

The tensile strength of a separator was measured using a universal testing machine (QJ210A, produced by Shanghai Qingji Instrumentation Technology Co., Ltd. ) according to ASTM d882-2002 Standard Test Method for Tensile Properties of Thin Plastic Sheeting.

Test 7 Thermal Shrinkage Percentage

The distance L ₀ of two points on a separator was measured at room temperature. Then the separator was placed in an oven of 120℃±1℃ for one hour. After the separator cooled to room temperature, the distance L ₁ of the two points was measured. The thermal shrinkage percentage was calculated by: S = (L ₀ -L ₁) /L ₀×100%.

Example 1

100 g high molecular weight polyethylene (average molecular weight: 5.0×10 ⁵, density: 0.957 g/cm ³) , 0.5 g antioxidant of n-octadecyl β- (3, 5-di-tert-butyl-4-hydroxyphenyl) propionate, 0.5 g auxiliary cross-linking agent of triallyl isocyanurate, 0.5 g photo-initiator of benzophenone, and 300 g pore-forming agent of mineral oil (kinematical viscosity: 50 mm ²/s at 40℃) were added into a continuous dosing tank. The mixture in the continuous dosing tank was stirred at a speed of 50 rpm to achieve well mixing.

The mixture was continuously fed into a twin-screw extruder where the high molecular weight polyethylene, the antioxidant, the auxiliary cross-linking agent, and the photo-initiator were dissolved in the pore-forming agent at a temperature of 180℃. The dissolved mixture was then extruded via the twin-screw extruder at a speed of 200 rpm. The extruded mixture was continuously fed into a slot die and extruded via the slot die onto a casting and cooling roller to form a ribbon at a temperature of 80℃.

The ribbon was placed in an extraction tank containing dichloromethane to remove the pore-forming agent. After the solvent extraction, the ribbon was stretched into a film at a temperature of 120℃. The film was extracted using dichloromethane to remove the residue of the pore-forming agent and then washed with deionized water. The film was thermoformed at 120℃for 15 minutes, and then winded at a speed of 20 m/minute to obtain an uncross-linked separator. Four samples of the uncross-linked separator were irradiated with an ultraviolet light having a wavelength ranging from 250 nm to 420 nm for different time periods, i.e., 10 minutes, 15 minutes, 20 minutes, and 30 minutes, respectively, to obtain four cross-linked polymer separators. The properties of the four cross-linked polymer separators after the testing were shown in Table 1.

Table 1.

Example 2

100 g high molecular weight polyethylene (average molecular weight: 5.0×10 ⁵, density: 0.957 g/cm ³) , 0.5 g antioxidant of n-octadecyl β- (3, 5-di-tert-butyl-4-hydroxyphenyl) propionate, 0.5 g auxiliary cross-linking agent of triallyl isocyanurate, 1.0 g photo-initiator of benzophenone, and 300 g pore-forming agent of mineral oil (kinematical viscosity: 50 mm ²/s at 40℃) were added into a continuous dosing tank. The mixture in the continuous dosing tank was stirred at a speed of 50 rpm to achieve well mixing.

The mixture was continuously fed into a twin-screw extruder where the high molecular weight polyethylene, the antioxidant, the auxiliary cross-linking agent and the photo-initiator were dissolved in the pore-forming agent at a temperature of 180℃. The dissolved mixture was then extruded via the twin-screw extruder at a speed of 200 rpm. The extruded mixture was continuously fed into a slot die and extruded via the slot die onto a casting and cooling roller to form a ribbon at a temperature of 80℃.

The ribbon was placed in an extraction tank containing dichloromethane to remove the pore-forming agent. After the solvent extraction, the ribbon was stretched into a film at a temperature of 120℃. The film was extracted using dichloromethane to remove the residue of the pore-forming agent and then washed with deionized water. The film was thermoformed at 120℃for 15 minutes, and then winded at a speed of 20 m/minute to obtain an uncross-linked separator. The uncross-linked separator was irradiated with an ultraviolet light having a wavelength ranging from 250 nm to 420 nm for 30 minutes to obtain a cross-linked polymer separator. The properties of the cross-linked polymer separator after the testing were shown in Table 2.

Table 2.

Example 3

100 g high molecular weight polyethylene (average molecular weight: 5.0×10 ⁵, density: 0.957 g/cm ³) , 0.5 g antioxidant of n-octadecyl β- (3, 5-di-tert-butyl-4-hydroxyphenyl) propionate, 1.0 g auxiliary cross-linking agent of triallyl isocyanurate, 0.5 g photo-initiator of benzophenone, and 300 g pore-forming agent of mineral oil (kinematical viscosity: 50 mm ²/s at 40℃) were added into a continuous dosing tank. The mixture in the continuous dosing tank was stirred at a speed of 50 rpm to achieve well mixing.

The mixture was continuously fed into a twin-screw extruder where the high molecular weight polyethylene, the antioxidant, the auxiliary cross-linking agent and the photo- initiator were dissolved in the pore-forming agent at a temperature of 180℃. The dissolved mixture was then extruded via the twin-screw extruder at a speed of 200 rpm. The extruded mixture was continuously fed into a slot die and extruded via the slot die onto a casting and cooling roller to form a ribbon at a temperature of 80℃.

The ribbon was placed in an extraction tank containing dichloromethane to remove the pore-forming agent. After the solvent extraction, the ribbon was stretched into a film at a temperature of 120℃. The film was extracted using dichloromethane to remove the residue of the pore-forming agent and then washed with deionized water. The film was thermoformed at 120℃for 15 minutes, and then winded at a speed of 20 m/minute to obtain an uncross-linked separator. The uncross-linked separator was irradiated with an ultraviolet light having a wavelength ranging from 250 nm to 420 nm for 30 minutes to obtain a cross-linked polymer separator. The properties of the cross-linked polymer separator after the testing were shown in Table 3.

Table 3.

Example 4

100 g high molecular weight polyethylene (average molecular weight: 5.0×10 ⁵, density: 0.957 g/cm ³) , 0.5 g antioxidant of n-octadecyl β- (3, 5-di-tert-butyl-4-hydroxyphenyl) propionate, 0.5 g auxiliary cross-linking agent of triallyl isocyanurate, 0.5 g photo-initiator of diphenylethanone, and 300 g pore-forming agent of mineral oil (kinematical viscosity: 50 mm ²/s at 40℃) were added into a continuous dosing tank. The mixture in the continuous dosing tank was stirred at a speed of 50 rpm to achieve well mixing.

The ribbon was placed in an extraction tank containing dichloromethane to remove the pore-forming agent. After the solvent extraction, the ribbon was stretched into a film at a temperature of 120℃. The film was extracted using dichloromethane to remove the residue of the pore-forming agent and then washed with deionized water. The film was thermoformed at 120℃for 15 minutes, and then winded at a speed of 20 m/minute to obtain an uncross-linked separator. The uncross-linked separator was irradiated with an ultraviolet light having a wavelength ranging from 250 nm to 420 nm for 30 minutes to obtain a cross-linked polymer separator. The properties of the cross-linked polymer separator after the testing were shown in Table 4.

Table 4.

Example 5

100 g high molecular weight polyethylene (average molecular weight: 5.0×10 ⁵, density: 0.957 g/cm ³) , 0.5 g antioxidant of n-octadecyl β- (3, 5-di-tert-butyl-4-hydroxyphenyl) propionate, 0.5 g auxiliary cross-linking agent of triallyl cyanurate, 0.5 g photo-initiator of diphenylethanone, and 300 g pore-forming agent of mineral oil (kinematical viscosity: 50 mm ²/s at 40℃) were added into a continuous dosing tank. The mixture in the continuous dosing tank was stirred at a speed of 50 rpm to achieve well mixing.

The ribbon was placed in an extraction tank containing dichloromethane to remove the pore-forming agent. After the solvent extraction, the ribbon was stretched into a film at a temperature of 120℃. The film was extracted using dichloromethane to remove the residue of the pore-forming agent and then washed with deionized water. The film was thermoformed at 120℃for 15 minutes, and then winded at a speed of 20 m/minute to obtain an uncross-linked separator. The uncross-linked separator was irradiated with an ultraviolet light having a wavelength ranging from 250 nm to 420 nm for 30 minutes to obtain a cross-linked polymer separator. The properties of the cross-linked polymer separator after the testing were shown in Table 5.

Table 5.

Example 6

The ribbon was placed in an extraction tank containing dichloromethane to remove the pore-forming agent. After the solvent extraction, the ribbon was stretched into a film at a temperature of 120℃. The film was extracted using dichloromethane to remove the residue of the pore-forming agent and then washed with deionized water. The film was thermoformed at 120℃for 15 minutes, and then winded at a speed of 20 m/minute to obtain an uncross-linked separator. The uncross-linked separator was irradiated with an ultraviolet light having a wavelength ranging from 250 nm to 420 nm for 30 minutes to obtain a cross-linked polymer separator.

Protective atmosphere annealing with different time settings, i.e., near melting point annealing, near melting point continuous and repeated annealing respectively, was then applied to the cross-linked polymer separator and was carried out in an oven filled with nitrogen. The near melting point annealing was carried out at 110 ℃ for 24 hours, followed by furnace cooling. The near melting point continuous and repeated annealing was carried out at 110 ℃ for 8 hours, followed by furnace cooling, and repeated such steps for three times. The properties of the cross-linked polymer separator after the testing were shown in Table 6.

Table 6.

Comparative Example 1

100 g polyethylene (average molecular weight: 5.0×10 ⁵, density: 0.957 g/cm ³) , 0.5 g antioxidant of n-octadecyl β- (3, 5-di-tert-butyl-4-hydroxyphenyl) propionate, and 300 g pore-forming agent of mineral oil (kinematical viscosity: 50 mm ²/s at 40℃) were added into a continuous dosing tank. The mixture in the continuous dosing tank was stirred at a speed of 50 rpm to achieve well mixing.

The ribbon was placed in an extraction tank containing dichloromethane to remove the pore-forming agent. After the solvent extraction, the ribbon was stretched into a film at a temperature of 120℃. The film was extracted using dichloromethane to remove the residue of the pore-forming agent and then washed with deionized water. The film was thermoformed at 120℃for 15 minutes, and then winded at a speed of 20 m/minute to obtain an uncross-linked separator. The properties of the uncross-linked polymer separator after the testing were shown in Table 7.

Table 7.

Compared with the uncross-linked polymer separator prepared in Comparative Example 1, the cross-linked polymer separators prepared in Examples 1-6 showed lower thermal shrinkage percentage and higher difference between the shutdown temperature and the breakdown temperature. Specifically, the cross-linked polymer separators prepared in Examples 1-6 had a thermal shrinkage percentage of less than 3.2%in machine direction and a thermal shrinkage percentage of less than 1.8%in transverse direction. At a light irradiation time length of 30 minutes, the cross-linked polymer separators prepared in Examples 1-6 had a thermal shrinkage percentage of less than 1.9%in machine direction and a thermal shrinkage percentage of less than 0.6%in transverse direction. The difference between the shutdown temperature and the breakdown temperature of the cross-linked polymer separators prepared in Examples 1-6 was 30℃ or above, indicating the cross-linked polymer separators prepared in Examples 1-6 had improved safety in high-temperature environments.

As shown in Table 1, the light irradiation time length may affect the high temperature properties of the cross-linked polymer separators. With the light irradiation time length increasing from 10 minutes to 30 minutes, the thermal shrinkage of the cross-linked polymer separator decreased, and the difference between the shutdown temperature and the breakdown temperature increased. This is because more cross-linking bonds were formed when the uncross-linked polymer separator was exposed to light radiation for a longer time, and the polymer structure with more cross-linked bonds may contribute to the thermal properties of the separator.

As shown by comparing Examples 1-5 and Example 6, the annealing after light irradiation may affect the thermal stability and strength of the cross-linked polymer separators. With the annealing after light irradiation, the thermal shrinkage of the cross-linked polymer separator decreased, the puncture strength and the tensile strength of the cross-linked polymer separator increased. This is likely because the annealing helped (A) eliminate internal stresses caused by possible uneven irradiation during the irradiation; (B) reduce or eliminate residual free radicals after the irradiation, to improve the internal crystallization and cross-linking state; and (C) minimize reduction in strength caused by irradiation.

Claims

A method for preparing a cross-linked polymer separator for an electrochemical device, comprising:

(1) mixing at least one polyethylene, at least one antioxidant, at least one auxiliary cross-linking agent, at least one photo-initiator and at least one pore-forming agent to obtain a mixture, wherein the at least one polyethylene has an average molecular weight ranging from 1.0×10 ⁵ to 1.0×10 ⁷ and a density ranging from 0.940 and 0.976 g/cm ³;

(2) extruding the mixture to obtain an extruded mixture;

(3) producing a ribbon from the extruded mixture;

(4) removing the at least one pore-forming agent from the ribbon;

(5) stretching the ribbon into a film;

(6) thermoforming and winding the film to obtain an uncross-linked polymer separator; and

(7) applying a light irradiation to the uncross-linked polymer separator to obtain the cross-linked polymer separator.
The method according to claim 1, further comprising:

(8) annealing the cross-linked polymer separator.
The method according to claim 1, wherein in the step (2) , the mixture is extruded through a twin-screw extrusion process.
The method according to claim 1, wherein in the step (3) , the ribbon is produced from the extruded mixture through a tape casting process.
The method according to claim 1, wherein in the step (1) , the mixture comprises: 100 parts by weight of the at least one polyethylene, from 0.1 to 10 parts by weight of the at least one antioxidant, from 0.1 to 10 parts by weight of the at least one auxiliary cross-linking agent, from 0.1 to 10 parts by weight of the at least one photo-initiator, and from 100 to 500 parts by weight of the at least one pore-forming agent,
The method according to claim 1, wherein the at least one antioxidant is chosen from 4, 4-thiobis (6-tert-butyl-m-cresol) , butylated hydroxytoluene, phosphite, tert-butylhydroquinone, n-octadecyl β- (3, 5-di-tert-butyl-4-hydroxyphenyl) propionate, 1, 1, 3-tris (2-methyl-4-hydroxy-5-tert-butylphenyl) butane, 2-tert-butyl-6-methylphenol, N, N’ -bis (β-naphthyl) -p-phenylenediamine, dilauryl thiodipropionate, tris (nonylphenyl) phosphite and triphenyl phosphite.
The method according to claim 1, wherein the at least one auxiliary cross-linking agent is chosen from mercaptobenzothiazole, dibenzothiazole disulfide, N-cyclohexylbenzothiazole sulfenamide, oxydiethylenebenzothiazole sulfenamide, tetramethyl thiuram monosulfide, tetramethyl thiuram disulfide, zinc dimethyldithiocarbamate, zinc diethyl dithiocarbamate, diphenylguanidine, di-o-tolylguanidine, ethylene thiourea, N, N’ -diethylthiourea, hexamethylenetetramine, zinc isopropyl xanthate, trimethylolpropane trimethacrylate, trimethylolpropane triacrylate, triallyl cyanurate and triallyl isocyanurate.
The method according to claim 1, wherein the at least one photo-initiator is chosen from benzoin, benzoin dimethyl ether, benzoin ethyl ether, benzoin isopropyl ether, benzoin butyl ether, diphenylethanone, α, α-dimethoxy-α-phenylacetophenone, α, α-diethoxyacetophenone, α-hydroxyalkylphenone, α-aminoalkylphenone, aroylphosphine oxide, bis-benzoylphenylphosphine oxide, benzophenone, 2, 4-dihydroxy benzophenone, Michler’s ketone, thiopropoxy thioxanthone, isopropyl thioxanthone, diaryl iodonium salt, triaryl iodonium salt, alkyl iodonium salt and cumene ferrocene hexafluorophosphate.
The method according to claim 1, wherein the at least one pore-forming agent has a kinematical viscosity ranging from 10 to 100 mm ²/s at 40℃, an initial boiling point of 110℃ or above, and is chosen from natural mineral oil, C _6-15 alkanes, C _8-15 aliphatic carboxylic acids, C _1-4 alkyl C _8-15 aliphatic carboxylates, C _2-6 haloalkanes, phthalates, trimellitates, adipates, sebates, maleates, benzoates, epoxidized vegetable oil, benzsulfamide, phosphotriesters, glycol ethers, acetylated monoglycerides, citrates and diisononyl cyclohexane-1, 2-dicarboxylate.
The method according to claim 1, wherein in the step (2) , the at least one polyethylene, the at least one antioxidant, the at least one auxiliary cross-linking agent, and the at least one photo-initiator are dissolved in the at least one pore-forming agent at a temperature ranging from 170℃ to 230℃ before and during the extruding.
The method according to claim 4, wherein the tape casting process comprises:

feeding the extruded mixture into a slot die continuously;

extruding the extruded mixture via the slot die onto a casting and cooling roller; and

forming a ribbon at a temperature ranging from 70℃ to 90℃.
The method according to claim 1, wherein in the step (4) , the at least one pore-forming agent in the ribbon is removed through a solvent extraction process using dichloromethane.
The method according to claim 1, wherein in the step (5) , the ribbon is stretched into a film at a temperature ranging from 115℃ to 125℃, and the step (5) further comprises:

removing a residue of the at least one pore-forming agent from the film through a solvent extraction process using dichloromethane; and

washing the film using deionized water.
The method according to claim 1, wherein in the step (6) , the thermoforming is carried out by keeping the film at a temperature ranging from 115℃ to 125℃ for 15 to 20 minutes.
The method according to claim 1, wherein in the step (7) , the light irradiation comprises at least one of an ultraviolet light having a wavelength in a range of 250 nm to 420 nm and a visible light having a wavelength in a range of 400 nm to 800 nm, and is applied on the uncross-linked polymer separator for a time period ranging from 5 to 60 minutes.
The method according to claim 2, wherein in the step (8) , the annealing is chosen from vacuum annealing, annealing in air, and protective atmosphere annealing.
The method according to claim 2, wherein in the step (8) , the annealing is near melting point annealing or near melting point continuous and repeated annealing, wherein (A) the near melting point annealing is carried out at a temperature ranging from 70℃ to 120℃ for a time ranging from 6 to 48 hours, followed by furnace cooling; or (B) the near melting point continuous and repeated annealing is carried out at a temperature ranging from 70℃ to 120℃ for a time ranging from 8 to 24 hours, followed by furnace cooling, and such steps are repeated for three times.
The method according to claim 2, wherein in the step (8) , the annealing is near melting point annealing or near melting point continuous and repeated annealing, wherein (A) the near melting point annealing is carried out at 110℃ for 24 hours, followed by furnace cooling; or (B) the near melting point continuous and repeated annealing is carried out at 110℃ for 8 hours, followed by furnace cooling, and such steps are repeated for three times.
A cross-linked polymer separator for an electrochemical device prepared by the method of claim 1 or 2.
An electrochemical device comprising a positive electrode, a negative electrode, and a separator of claim 19 interposed between the positive electrode and the negative electrode.