A method for preparing a high-wettability polyolefin separator for lithium-ion batteries

Abstract

The present application aims at the problem of low lithium ion battery diaphragm wettability, and provides a preparation method of high wettability polyolefin diaphragm for lithium ion battery, belonging to the technical field of diaphragm preparation for ion battery, the preparation method of high wettability polyolefin diaphragm comprises six steps of synthesizing polyalkoxysilane-methacryl ethyl sulfobetaine random copolymer, raw material melt extrusion, sheet longitudinal stretching, sheet transverse stretching, additive extraction and secondary transverse stretching, wherein a certain amount of polyalkoxysilane and methacryl ethyl sulfobetaine random copolymer is added in the selection of raw materials. The polyolefin diaphragm provided by the present application has super high wettability and liquid absorption rate for electrolyte, and significantly improves the puncture strength of the diaphragm, and the modification process is simple, easy to operate and realize, and easy to industrialize and popularize.

Description

Technical Field

[0001] This invention belongs to the field of membrane preparation technology for lithium-ion batteries, and specifically relates to a method for preparing a highly wettable polyolefin membrane for lithium-ion batteries. Background Technology

[0002] The separator is one of the four essential materials in lithium-ion batteries, and polyolefin porous membranes are widely used as lithium battery separators. The separator in a lithium-ion battery has two main functions: first, to provide electron barrier, physically separating the positive and negative electrodes to prevent short circuits and ensure battery safety; second, to enable ion conduction, the separator should have a high lithium-ion conductivity rate to ensure normal battery operation. The separator itself does not possess ion conduction properties; it achieves the second function by being impregnated with an ion-conducting liquid electrolyte.

[0003] However, polyolefin membranes lack polar groups, resulting in poor electrolyte wettability. Currently, the main approach is to coat the membrane surface with an inorganic particle layer to improve wettability. However, coating cannot alter the wettability of the membrane's internal porous structure. This porous structure has a high specific surface area, and improving its wettability ensures sufficient electrolyte levels within the battery, which helps reduce ion migration resistance.

[0004] Therefore, improving the wettability of the porous structure inside the separator in lithium-ion batteries is an urgent problem that needs to be solved in current technology. Summary of the Invention

[0005] This invention addresses the problem of low wettability in lithium-ion battery separators by providing a method for preparing a high-wetting-ability polyolefin separator for lithium-ion batteries.

[0006] The present invention adopts the following technical solution:

[0007] A method for preparing a highly wettable polyolefin separator for lithium-ion batteries includes the following steps:

[0008] (1) Preparation of polyalkoxysilane-methacryloylethyl sulfobetaine random copolymer:

[0009] Polyalkoxysilane, methacryloylethyl sulfobetaine and an initiator are added to an inert organic solvent, heated and polymerized under an inert atmosphere to obtain a solution of polyalkoxysilane-methacryloylethyl sulfobetaine random copolymer. The solution is then freeze-dried to obtain polyalkoxysilane-methacryloylethyl sulfobetaine random copolymer powder.

[0010] (2) Melt extrusion of raw materials:

[0011] Polyolefin resin, plasticizer and polyalkoxysilane-methacryloylethyl sulfobetaine random copolymer powder are mixed and the resulting mixture is heated and melted in a twin-screw extruder. The melt is extruded through a T-die and cooled into a cast sheet.

[0012] (3) Longitudinal stretching of the sheet:

[0013] The cast sheet obtained in step (2) is longitudinally stretched using a longitudinal hot roller stretching method, with a stretching ratio of 3-15 times, to obtain a diaphragm.

[0014] (4) Lateral stretching of the sheet:

[0015] The diaphragm obtained in step (3) is then stretched laterally and uniformly using a chain clamp, with a stretching ratio of 3-15 times, to obtain a fully stretched diaphragm.

[0016] (5) Extraction with additives:

[0017] The fully stretched diaphragm obtained in step (4) is immersed in dichloromethane liquid for multiple extractions to remove the plasticizer, undecomposed antioxidant and pore-forming agent from the membrane, forming a highly wettable polyolefin diaphragm with a uniform microporous structure.

[0018] (6) Secondary transverse stretching:

[0019] The diaphragm obtained in step (5) is stretched horizontally and uniformly again in a horizontal stretching machine to obtain a complete diaphragm.

[0020] Further, the polyalkoxysilane in step (1) includes at least one of vinyltrimethoxysilane, vinyltriethoxysilane, and methylvinyldiethoxysilane.

[0021] Further, the initiator in step (1) includes at least one of azobisisobutyronitrile, azobisisoheptanenitrile, benzoyl peroxide, dodecyl peroxide, and di-tert-butyl peroxide.

[0022] Further, the inert organic solvent mentioned in step (1) includes at least one of toluene, benzene, triethyl phosphate, dimethyl sulfoxide, ethanol, methanol, and N-methylpyrrolidone.

[0023] Further, in step (1), the ratio of polyalkoxysilane, methacryloxyethyl sulfobetaine, initiator and inert organic solvent is (1g~30g): (1g~30g): (0.1g~0.5g): 100mL.

[0024] Furthermore, the polymerization reaction in step (1) is carried out at a temperature of 60~100℃ and for a reaction time of 3~36h.

[0025] Further, the polyolefin resin in step (2) includes one or a mixture of two of polyethylene or polypropylene; the plasticizer includes organic acid esters, phosphate esters, liquid paraffin or mineral oil.

[0026] Further, the polyolefin resin in step (2) includes polyethylene; the plasticizer includes white oil.

[0027] Further, in step (2), the mass percentage of the polyolefin resin is 10-50 wt%, the mass percentage of the plasticizer is 50-90 wt%, and the amount of the polyalkoxysilane-methacryloylethyl sulfobetaine random copolymer is 0.1-5 wt% of the mass of the polyolefin resin.

[0028] The beneficial effects of this invention are as follows:

[0029] Polyalkoxysilanes on the molecular chain segments of the polyalkoxysilane-methacryloylethyl sulfobetaine random copolymer are adsorbed on the surface of polyolefins. Under high temperature, they undergo a condensation reaction to form Si-O-Si bonds and generate oligomeric silanes. The polyalkoxysilanes self-polymerize and crosslink on the surface of polyolefins to form a silane film. The polar groups of the methacryloylethyl sulfobetaine segments in the random copolymer endow the polyolefin with hydrophilicity, thereby improving the wettability of the polyolefin membrane.

[0030] This invention, through a specially designed formulation, achieves hydrophilic modification of polyolefins during the extrusion mixing stage at the front end of production. The preparation method provided by this invention is simple, easy to operate and implement, and the prepared polyolefin film exhibits extremely high wettability. Detailed Implementation

[0031] The present invention will be further described below with reference to specific embodiments, but is not limited to the contents of the embodiments of the present invention.

[0032] The preparation method of the high wettability polyolefin membrane of the present invention is carried out according to the following steps. In specific implementation, step (2) is changed, and steps (1), (3), (4), (5), and (6) are all carried out according to the following steps.

[0033] (1) Preparation of vinyltrimethoxysilane-methacryloylethyl sulfobetaine random copolymer

[0034] 10g of vinyltrimethoxysilane, 5g of methacryloylethyl sulfobetaine and 0.2g of azobisisoheptanenitrile were added to 100mL of dimethyl sulfoxide, industrial nitrogen gas was introduced, and the mixture was reacted at 80℃ for 6 hours to obtain a solution of vinyltrimethoxysilane-methacryloylethyl sulfobetaine random copolymer. The mixed solution was freeze-dried to obtain vinyltrimethoxysilane-methacryloylethyl sulfobetaine random copolymer powder.

[0035] (2) Melt extrusion of raw materials

[0036] A certain amount of polyolefin resin, plasticizer, and vinyltrimethoxysilane-methacryloylethyl sulfobetaine random copolymer are mixed, and the resulting mixture is heated and melted in a twin-screw extruder; the melt is extruded through a T-die and cooled into a cast sheet;

[0037] (3) Longitudinal stretching of sheet

[0038] The cast sheet obtained in step (2) is longitudinally stretched using a longitudinal hot roller stretching method, with a stretching ratio of 6.5 times, to obtain a diaphragm;

[0039] (4) Lateral stretching of sheet

[0040] The diaphragm obtained in step (3) is then stretched laterally and uniformly using a chain clamp, with a stretching ratio of 10 times, to obtain a fully stretched diaphragm.

[0041] (5) Extraction with additives

[0042] The fully stretched diaphragm obtained in step (4) is immersed in dichloromethane liquid for multiple extractions, so that the plasticizer, undecomposed antioxidant and pore-forming agent in the membrane are removed from the membrane, forming a highly wettable polyolefin diaphragm with a uniform microporous structure.

[0043] (6) Secondary transverse stretching

[0044] The diaphragm obtained in step (5) is stretched horizontally and uniformly again in a horizontal stretching machine to obtain a complete diaphragm.

[0045] Example 1

[0046] (2) Melt extrusion of raw materials:

[0047] 20 wt% polyethylene, 80 wt% white oil and 0.1 wt% (based on polyethylene mass) of vinyltrimethoxysilane-methacryloylethyl sulfobetaine random copolymer were mixed and the resulting mixture was heated and melted in a twin-screw extruder. The melt was extruded through a T-die and cooled into a cast sheet.

[0048] Other steps shall be carried out in accordance with the above steps (1), (3), (4), (5), and (6).

[0049] Example 2

[0050] (2) Melt extrusion of raw materials:

[0051] 20 wt% polypropylene, 80 wt% white oil and 1 wt% (based on polyethylene mass) vinyltrimethoxysilane-methacryloylethyl sulfobetaine random copolymer were mixed and the resulting mixture was heated and melted in a twin-screw extruder. The melt was extruded through a T-die and cooled into a cast sheet.

[0052] Other steps shall be carried out in accordance with the above steps (1), (3), (4), (5), and (6).

[0053] Example 3

[0054] (2) Melt extrusion of raw materials:

[0055] 20 wt% polyethylene, 80 wt% white oil and 2 wt% (based on polyethylene mass) vinyltrimethoxysilane-methacryloylethyl sulfobetaine random copolymer were mixed and the resulting mixture was heated and melted in a twin-screw extruder. The melt was extruded through a T-die and cooled into a cast sheet.

[0056] Other steps shall be carried out in accordance with the above steps (1), (3), (4), (5), and (6).

[0057] Example 4

[0058] (2) Melt extrusion of raw materials:

[0059] 50 wt% polyethylene, 50 wt% white oil and 3 wt% (based on polyethylene mass) vinyltrimethoxysilane-methacryloylethyl sulfobetaine random copolymer were mixed and the resulting mixture was heated and melted in a twin-screw extruder. The melt was extruded through a T-die and cooled into a cast sheet.

[0060] Other steps shall be carried out in accordance with the above steps (1), (3), (4), (5), and (6).

[0061] Comparative Example 1

[0062] (2) Melt extrusion of raw materials:

[0063] 20 wt% polyethylene and 80 wt% white oil are mixed, and the resulting mixture is heated and melted in a twin-screw extruder. The melt is extruded through a T-die and cooled into a cast sheet.

[0064] Other steps shall be carried out in accordance with the above steps (1), (3), (4), (5), and (6).

[0065] The diaphragms obtained in the above embodiments were tested for water contact angle, liquid absorption rate and puncture test results, and the specific test results are shown in the table below.

[0066]

[0067] As shown in Table 1, the addition of vinyltrimethoxysilane-methacrylethyl sulfobetaine random copolymer significantly reduced the water contact angle of the diaphragm, indicating that the compatibility between the diaphragm and the electrolyte was improved. This is beneficial for further increasing the absorption of electrolyte by the diaphragm and also improves the puncture resistance. The improvement effect is more significant with the increase of vinyltrimethoxysilane-methacrylethyl sulfobetaine random copolymer content.

Claims

1. A method for the production of a high-wettability polyolefin separator for lithium-ion batteries, characterized by: The method comprises the following steps: (1) preparing a polyalkoxysilane-methacryl ethyl sulfobetaine random copolymer: adding polyalkoxysilane, methacryl ethyl sulfobetaine and initiator into an inert organic solvent, heating and performing a polymerization reaction under an inert atmosphere to obtain a solution of the polyalkoxysilane-methacryl ethyl sulfobetaine random copolymer, and obtaining the polyalkoxysilane-methacryl ethyl sulfobetaine random copolymer powder after freeze-drying of the solution; the ratio of the polyalkoxysilane, methacryl ethyl sulfobetaine, initiator and inert organic solvent is (1g-30g):(1g-30g):(0.1g-0.5g):100mL; (2) melt extrusion of raw materials: mixing polyolefin resin, plasticizer and polyalkoxysilane-methacryl ethyl sulfobetaine random copolymer powder to obtain a mixture, heating and melting the mixture in a twin-screw extruder, extruding the melt through a T-shaped die and cooling into a cast sheet; the mass percentage content of the polyolefin resin is 10-50wt%, the mass percentage content of the plasticizer is 50-90wt%, and the amount of the polyalkoxysilane-methacryl ethyl sulfobetaine random copolymer is 0.1-5wt% of the mass of the polyolefin resin; (3) longitudinal stretching of the sheet: performing longitudinal stretching on the cast sheet obtained in step (2) by using a longitudinal hot roller stretching method, and the stretching ratio is 3-15 times to obtain a separator; (4) transverse stretching of the sheet: performing transverse uniform stretching on the separator obtained in step (3) by using a chain clamp, and the stretching ratio is 3-15 times to obtain a stretched complete separator; (5) extraction of additives: immersing the complete stretched separator obtained in step (4) into dichloromethane liquid for multiple extractions to separate the plasticizer, un-decomposed antioxidant and pore-forming agent in the membrane body from the membrane body, thereby forming a high-wetting polyolefin separator with a uniform microporous structure; (6) secondary transverse stretching: performing transverse uniform stretching on the separator obtained in step (5) in a transverse stretching machine to obtain a complete separator.

2. The method for preparing a highly wettable polyolefin separator for lithium-ion batteries according to claim 1, characterized in that: The polyalkoxysilane in step (1) comprises at least one of vinyl trimethoxysilane, vinyl triethoxysilane and methyl vinyl diethoxysilane.

3. The method for preparing a highly wettable polyolefin separator for lithium-ion batteries according to claim 1, characterized in that: The initiator in step (1) comprises at least one of azobisisobutyronitrile, azobisisoheptyl nitrile, dibenzoyl peroxide, dilauryl peroxide and di-tert-butyl peroxide.

4. The method for preparing a highly wettable polyolefin separator for lithium-ion batteries according to claim 1, characterized in that: The inert organic solvent in step (1) comprises at least one of toluene, benzene, triethyl phosphate, dimethyl sulfoxide, ethanol, methanol and N-methyl pyrrolidone.

5. The method for preparing a highly wettable polyolefin separator for lithium-ion batteries according to claim 1, characterized in that: The reaction temperature of the polymerization reaction in step (1) is 60-100℃, and the reaction time is 3-36h.

6. The method for preparing a highly wettable polyolefin separator for lithium-ion batteries according to claim 1, characterized in that: The polyolefin resin in step (2) comprises one or both of polyethylene and polypropylene; and the plasticizer comprises organic acid ester, phosphate ester, liquid paraffin or mineral oil.

7. The method for preparing a highly wettable polyolefin separator for lithium-ion batteries according to claim 6, characterized in that: The polyolefin resin in step (2) comprises polyethylene; and the plasticizer comprises white oil.

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