High-oxidation-resistant and high-weather-resistant battery separator and its preparation method

Abstract

This invention discloses a high-oxidation-resistance and high-weather-resistance battery separator and its preparation method, relating to the field of lithium battery separator technology. The technical solution is as follows: the high-oxidation-resistance and high-weather-resistance battery separator has a three-layer co-extruded structure, including an intermediate layer and surface layers on both sides of the intermediate layer; the intermediate layer is composed of maleic anhydride-grafted polyethylene, a silane crosslinking agent, an initiator, and white oil; the surface layers are composed of plasma-pretreated ultra-high molecular weight polyethylene, hydroxylated silica, antioxidants, ultraviolet absorbers, and white oil. This invention solves the problem that traditional wet-process polyethylene separators have poor weather resistance, inevitably leading to separator aging and brittleness when applied to scenarios requiring longer service life, such as energy storage cells, thus rendering the cells unusable.

Description

Technical Field

[0001] This invention relates to the field of lithium battery separator technology, specifically to a high-oxidation-resistant and high-weather-resistant battery separator and its preparation method. Background Technology

[0002] As a key component of liquid lithium-ion batteries, the battery separator plays a crucial role. The separator is an insulating membrane with a large number of micropores between the positive and negative electrodes of a lithium battery, primarily composed of insulating olefin polymer materials. The separator has two main functions: first, to isolate the positive and negative electrodes, preventing direct contact and short circuits, while also being as thin as possible to minimize the distance between the electrodes and reduce internal resistance; second, to store and maintain sufficient electrolyte, the microporous structure allowing Li-O2 in the electrolyte to flow freely. + Freedom of passage, realizing Li + Rapid transport between the positive and negative electrodes. Therefore, the performance of the battery separator can directly affect key performance characteristics of lithium batteries such as capacity, cycle performance, and charge / discharge current density.

[0003] There are three main methods for preparing battery separators: dry uniaxial stretching, dry biaxial stretching, and wet biaxial stretching. With the large-scale application of energy storage, higher requirements are placed on the cycle life of batteries. Traditional wet-process separators mainly use polyethylene as a raw material, but polyethylene itself has weak oxidation resistance, and the carbon-hydrogen bonds in its molecular structure are easily affected by oxidation, leading to material aging and performance degradation. Therefore, when applied to energy storage cells, due to the long service life of these cells, after thousands of charge-discharge cycles, the polyethylene separator will inevitably yellow and become brittle, leading to battery failure. At the same time, the surface of polyethylene is inert and non-polar, resulting in poor compatibility with polar polymers and inorganic fillers, and it is difficult to improve the weather resistance of polyethylene by directly adding antioxidants. Therefore, it is necessary to develop a battery separator with high oxidation resistance and high weather resistance. Summary of the Invention

[0004] To address the issues of yellowing and degradation during cycling of traditional separators used in long-cycle energy storage cells, leading to decreased strength and compromised battery safety, this invention provides a high-oxidation-resistant and high-weather-resistant battery separator and its preparation method. This solves the problem that traditional wet-process polyethylene separators have poor weather resistance and inevitably age and become brittle when used in scenarios requiring longer service life, such as energy storage cells, rendering the cells unusable.

[0005] The technical solution of this invention is as follows:

[0006] On one hand, the present invention provides a high-oxidation-resistance and high-weather-resistance battery separator, which is a three-layer co-extruded structure, including an intermediate layer and surface layers on both sides of the intermediate layer; the intermediate layer is composed of maleic anhydride-grafted polyethylene, silane crosslinking agent, initiator and white oil, and the surface layer is composed of plasma-pretreated ultra-high molecular weight polyethylene, hydroxylated silica, antioxidant, ultraviolet absorber and white oil.

[0007] Preferably, in the intermediate layer, the mass ratio of maleic anhydride-grafted polyethylene, silane crosslinking agent, initiator and white oil is (30-40):(1-5):(0.1-0.5):(54.5-68.9).

[0008] Preferably, the maleic anhydride grafting rate of the maleic anhydride-grafted polyethylene is 0.5-2%; the silane crosslinking agent is methyltriacetoxysilane, methyltrimethoxysilane, or vinyltrimethoxysilane; and the initiator is dicumyl peroxide, di-tert-butyl peroxide, or benzoyl peroxide.

[0009] Preferably, in the surface layer, the mass ratio of plasma-pretreated ultra-high molecular weight polyethylene, hydroxylated silica, antioxidant, ultraviolet absorber and white oil is (30-39):(1-5):(0.2-1):(0.2-1):(54-69.6).

[0010] Preferably, the plasma-pretreated ultra-high molecular weight polyethylene has a weight-average molecular weight of 1 million to 2 million g / mol and a melt index of <0.5 g / 10 min at 190°C; the hydroxylated silica has a particle size of 5-20 nm and a hydroxyl content of 2-4%; the antioxidant is tert-butylhydroxyanisole, butylated hydroxytoluene, or N-phenyl-α-naphthylamine; and the ultraviolet absorber is benzotriazole, 2,4-dihydroxybenzophenone, or hexamethylphosphoric acid triamine.

[0011] Preferably, the white oil has an initial boiling point ≥220℃ and a viscosity >40 mm at 40℃. 2 / s.

[0012] Preferably, the longitudinal tensile strength of the high-oxidation-resistance and high-weather-resistance battery separator is >2000 kgf / cm. 2 Transverse tensile strength > 2000 kgf / cm 2 Puncture strength > 600gf, oxidation induction time > 50min, environmental stress resistance test F50 > 1200h.

[0013] On the other hand, the present invention provides a method for preparing the above-mentioned high-oxidation-resistance and high-weather-resistance battery separator, comprising the following steps:

[0014] S1 Pretreatment: Ultra-high molecular weight polyethylene is pretreated by plasma bombardment to obtain plasma-pretreated ultra-high molecular weight polyethylene.

[0015] S2 Extruded Casting: The materials for the surface layer and the intermediate layer are mixed separately, and then extruded, cooled, and drawn to obtain a casting.

[0016] S3 stretching film formation: The cast sheet is stretched longitudinally and then transversely in sequence to obtain a polyolefin film;

[0017] S4 Extraction and Drying: The polyolefin membrane is extracted with dichloromethane and dried to obtain the membrane precursor;

[0018] S5 Pore Expanding and Winding: After the separator precursor undergoes two transverse stretching, heat setting, and winding, a high-oxidation-resistant and high-weather-resistant battery separator is obtained.

[0019] Preferably, in step S1, the plasma is a mixture of oxygen and nitrogen, with a volume ratio of oxygen to nitrogen of 1:(2-3), a power of 100-150W, and a processing time of 30-60s; in step S2, the extrusion temperature is 160-200℃, the die temperature is 180-220℃, the cooling temperature is 50-80℃, and the traction speed is 10-40m / min.

[0020] Preferably, in step S3, the longitudinal stretching temperature is 80-120℃, the stretching ratio is 6-10, the transverse stretching temperature is 100-130℃, and the stretching ratio is 6-10; in step S4, the extraction temperature is 20-30℃, and the drying temperature is 30-50℃; in step S5, the secondary transverse stretching temperature is 100-130℃, the stretching ratio is 1.1-1.5, and the heat setting temperature is 110-140℃.

[0021] Compared with the prior art, the present invention has the following advantages:

[0022] 1. Compared to traditional wet-process polyethylene separators, the battery separator of this invention adopts a three-layer co-extrusion structure. The middle layer is made of grafted modified polyethylene, with anhydride groups introduced onto the polyethylene backbone. The grafted polymer hardly changes the polyethylene skeleton structure. Since polyethylene itself is a non-polar polymer, it has poor compatibility with polar materials such as antioxidants and ultraviolet absorbers. Therefore, by grafting polar anhydride groups onto the polyethylene backbone, this invention can improve the polarity of polyethylene and enhance the compatibility of polyethylene with antioxidants and ultraviolet absorbers at the interface of the middle and surface layers. At the same time, the oxygen-containing groups on the anhydride can react with amino, hydroxyl, and silane groups during extrusion melting to form chemical bonds, thereby constructing a three-dimensional network structure. This further improves the weather resistance and resistance to environmental stress cracking of the separator, while generating carboxylic acid groups, providing active sites for subsequent cross-linking reactions. To enable crosslinking of modified polyethylene, this invention introduces a silane crosslinking agent and an initiator. Under the action of the initiator, the siloxy groups on the silane crosslinking agent generate silanols, which then react and crosslink with the carboxylic acid groups on the grafted modified polyethylene to form a three-dimensional Si-O-Si network structure. The three-dimensional network structure constructed through the crosslinking reaction is more stable than that of traditional polyethylene, thus exhibiting higher weather resistance and resistance to environmental stress cracking.

[0023] 2. This invention incorporates plasma-pretreated ultra-high molecular weight polyethylene (UHMWPE) into the surface layer, resulting in higher regularity, higher crystallinity, and the formation of more crystalline regions. Therefore, compared to conventional polyethylene, plasma-pretreated UHMWPE exhibits a more stable molecular structure and better oxidation resistance. Plasma pretreatment introduces polar groups such as hydroxyl and carboxyl groups onto the surface of UHMWPE, thereby improving its compatibility with antioxidants and UV absorbers. Simultaneously, the plasma-pretreated UHMWPE reacts with the maleic anhydride-grafted polyethylene and silane crosslinking agent in the intermediate layer, enhancing the interlayer forces between the surface and intermediate layers and preventing delamination due to material differences. To further enhance the diaphragm's antioxidant capacity, this invention incorporates antioxidants, UV absorbers, and hydroxylated silica into the surface layer through blending. The antioxidants can terminate the oxidation chain reaction by capturing free radicals or decomposing peroxides, thereby inhibiting oxidation during use and preventing performance degradation due to oxidation. The UV absorbers selectively absorb ultraviolet light with wavelengths of 280-400 nm, converting it into harmless heat energy and preventing UV damage to the material's molecular structure. The combined use of antioxidants and UV absorbers simultaneously inhibits oxidation and photodegradation, further improving the diaphragm's weather resistance and delaying yellowing and embrittlement of polyethylene. Furthermore, the hydroxyl groups on the surface of the hydroxylated silica can form hydrogen and covalent bonds with the polar groups on the surface of plasma-pretreated ultra-high molecular weight polyethylene, maleic anhydride-grafted polyethylene, and the siloxy groups on the silane crosslinking agent. This enhances the strength of the three-dimensional network structure, improving the diaphragm's environmental stress resistance; and through the light scattering effect of nanoparticles, it assists in blocking UV rays, forming a synergistic weather protection with the surface layer UV absorbers. Detailed Implementation

[0024] To enable those skilled in the art to better understand the technical solutions of this invention, the technical solutions of this invention will be clearly and completely described below in conjunction with the embodiments of this invention.

[0025] The high-oxidation-resistance and high-weather-resistance battery separator of the present invention includes an intermediate layer, and surface layers are disposed on both sides of the intermediate layer to form a three-layer co-extrusion structure of surface layer / intermediate layer / surface layer; the intermediate layer is composed of maleic anhydride-grafted polyethylene, silane crosslinking agent, initiator and white oil, and the surface layer is composed of plasma-pretreated ultra-high molecular weight polyethylene, hydroxylated silica, antioxidant, ultraviolet absorber and white oil.

[0026] The method for preparing the high-oxidation-resistance and high-weather-resistance battery separator of the present invention includes the following steps:

[0027] S1 Pretreatment: Ultra-high molecular weight polyethylene (weight average molecular weight of 1 million to 2 million g / mol, melt index of <0.5 g / 10 min at 190℃) is pretreated by plasma bombardment. The plasma is a mixture of oxygen and nitrogen in a volume ratio of 1:(2-3), the power is 100-150W, and the treatment time is 30-60s, to obtain plasma-pretreated ultra-high molecular weight polyethylene.

[0028] S2 Extrusion Casting: 30-39 parts of plasma-pretreated ultra-high molecular weight polyethylene, 1-5 parts of hydroxylated silica (particle size 5-20nm, hydroxyl content 2-4%), 0.2-1 parts of antioxidant, 0.2-1 parts of UV absorber, and 54-69.6 parts of white oil are weighed by an electronic scale and mixed in a mixing hopper, then fed into the first twin-screw extruder; 30-40 parts of maleic anhydride-grafted polyethylene, 1-5 parts of... The silane crosslinking agent, 0.1-0.5 parts of initiator, and 53.5-68.9 parts of white oil are weighed by an electronic scale and mixed in a mixing hopper. Then, the mixture is fed into the second twin-screw extruder. The temperature of the extruder is adjusted to 160-200℃. After melting, the mixture is filtered and extruded through a die at a die temperature of 180-220℃. The melt extruded through the die is cooled at 50-80℃ and cast at a traction speed of 10-40m / min.

[0029] S3 stretching film formation: The cast sheet is first stretched longitudinally at a temperature of 80-120℃ with a stretching ratio of 6-10, and then stretched transversely at a temperature of 100-130℃ with a stretching ratio of 6-10 to obtain a polyolefin film.

[0030] S4 Extraction and Drying: The polyolefin membrane is extracted in a dichloromethane extraction tank at a temperature of 20-30℃, and then dried at a temperature of 30-50℃ to obtain the membrane precursor;

[0031] S5 pore enlargement and winding: The separator precursor is subjected to secondary transverse stretching at a temperature of 100-130℃ with a stretching ratio of 1.1-1.5; then it is heat-set at a temperature of 110-140℃, and after traction and thickness measurement, it is wound to obtain a high-oxidation-resistant and high-weather-resistant battery separator.

[0032] Example 1

[0033] The preparation method of the high-oxidation-resistance and high-weather-resistance battery separator in this embodiment includes the following steps:

[0034] S1 pre-treats ultra-high molecular weight polyethylene (weight average molecular weight of 1 million g / mol, melt index of 0.2 g / 10 min at 190℃) by plasma bombardment. The plasma is a mixture of oxygen and nitrogen in a volume ratio of 1:2, the power is 100W, and the treatment time is 60s, to obtain plasma-pretreated ultra-high molecular weight polyethylene.

[0035] S2 contains 30 parts plasma-pretreated ultra-high molecular weight polyethylene, 1 part hydroxylated silica (particle size 5 nm, hydroxyl content 2%), 0.2 parts tert-butylhydroxyanisole, 0.2 parts benzotriazole, and 69.6 parts white oil (initial boiling point 400℃, viscosity 60 mmHg at 40℃). 2 The mixture is weighed by an electronic scale and mixed in a mixing hopper before being fed into the first twin-screw extruder. 30 parts of maleic anhydride-grafted polyethylene (maleic anhydride grafting rate of 0.5%), 1 part of methyltriacetoxysilane, 0.1 parts of dicumyl peroxide, and 68.9 parts of white oil are weighed by an electronic scale and mixed in a mixing hopper before being fed into the second twin-screw extruder. The extruder temperature is adjusted to 160°C. After melting and filtering, the mixture is extruded through a die at a die temperature of 180°C. The extruded melt is cooled at 60°C and cast at a traction speed of 15 m / min.

[0036] S3 first stretches the cast sheet longitudinally at 90°C with a stretching ratio of 8, and then stretches it transversely at 110°C with a stretching ratio of 8 to obtain a polyolefin film.

[0037] S4 extracts the polyolefin membrane in a dichloromethane extraction tank at 25°C and then dries it at 40°C to obtain the membrane precursor.

[0038] S5 performs a second transverse stretching of the separator precursor at 120°C with a stretching ratio of 1.3; then it is heat-set at 130°C, and after traction and thickness measurement, it is wound up to obtain a high-oxidation-resistant and high-weather-resistant battery separator.

[0039] Example 2

[0040] The preparation method of the high-oxidation-resistance and high-weather-resistance battery separator in this embodiment includes the following steps:

[0041] S1 pre-treats ultra-high molecular weight polyethylene (weight average molecular weight of 1 million g / mol, melt index of 0.2 g / 10 min at 190℃) by plasma bombardment. The plasma is a mixture of oxygen and nitrogen in a volume ratio of 1:3, the power is 130 W, and the treatment time is 45 s, to obtain plasma-pretreated ultra-high molecular weight polyethylene.

[0042] S2 contains 30 parts plasma-pretreated ultra-high molecular weight polyethylene, 1 part hydroxylated silica (particle size 10 nm, hydroxyl content 3%), 0.6 parts dibutylhydroxytoluene, 0.6 parts 2,4-dihydroxybenzophenone, and 67.8 parts white oil (initial boiling point 400℃, viscosity 60 mmHg at 40℃). 2 The mixture is weighed by an electronic scale and mixed in a mixing hopper before being fed into the first twin-screw extruder. 30 parts of maleic anhydride-grafted polyethylene (maleic anhydride grafting rate of 0.5%), 1 part of methyltriacetoxysilane, 0.1 parts of dicumyl peroxide, and 68.9 parts of white oil are weighed by an electronic scale and mixed in a mixing hopper before being fed into the second twin-screw extruder. The extruder temperature is adjusted to 180°C. After melting, the mixture is filtered and extruded through a die at a die temperature of 200°C. The extruded melt is cooled at 50°C and cast at a traction speed of 40 m / min.

[0043] S3 first stretches the cast sheet longitudinally at 80°C with a stretching ratio of 10, and then stretches it transversely at 100°C with a stretching ratio of 6 to obtain a polyolefin film.

[0044] S4 extracts the polyolefin membrane in a dichloromethane extraction tank at 30°C and then dries it at 50°C to obtain the membrane precursor.

[0045] S5 performs a second transverse stretching of the separator precursor at 100°C with a stretching ratio of 1.5; then it is heat-set at 140°C, and after traction and thickness measurement, it is wound up to obtain a high-oxidation-resistant and high-weather-resistant battery separator.

[0046] Example 3

[0047] The preparation method of the high-oxidation-resistance and high-weather-resistance battery separator in this embodiment includes the following steps:

[0048] S1 pre-treats ultra-high molecular weight polyethylene (weight average molecular weight of 1.5 million g / mol, melt index of 0.03 g / 10 min at 190℃) by plasma bombardment. The plasma is a mixture of oxygen and nitrogen in a volume ratio of 1:3, the power is 130W, and the treatment time is 45s, to obtain plasma-pretreated ultra-high molecular weight polyethylene.

[0049] S2 contains 35 parts plasma-pretreated ultra-high molecular weight polyethylene, 3 parts hydroxylated silica (particle size 10 nm, hydroxyl content 3%), 0.6 parts butylated hydroxytoluene, 0.6 parts 2,4-dihydroxybenzophenone, and 60.8 parts white oil (initial boiling point 400℃, viscosity 60 mmHg at 40℃). 2 The mixture is weighed by an electronic scale and mixed in a mixing hopper before being fed into the first twin-screw extruder. 30 parts of maleic anhydride-grafted polyethylene (maleic anhydride grafting rate of 0.5%), 1 part of methyltriacetoxysilane, 0.1 parts of dicumyl peroxide, and 68.9 parts of white oil are weighed by an electronic scale and mixed in a mixing hopper before being fed into the second twin-screw extruder. The extruder temperature is adjusted to 180°C. After melting, the mixture is filtered and extruded through a die at a die temperature of 200°C. The extruded melt is cooled at 50°C and cast at a traction speed of 40 m / min.

[0050] S3 first stretches the cast sheet longitudinally at 80°C with a stretching ratio of 10, and then stretches it transversely at 100°C with a stretching ratio of 6 to obtain a polyolefin film.

[0051] S4 extracts the polyolefin membrane in a dichloromethane extraction tank at 30°C and then dries it at 50°C to obtain the membrane precursor.

[0052] S5 performs a second transverse stretching of the separator precursor at 100°C with a stretching ratio of 1.5; then it is heat-set at 140°C, and after traction and thickness measurement, it is wound up to obtain a high-oxidation-resistant and high-weather-resistant battery separator.

[0053] Example 4

[0054] The preparation method of the high-oxidation-resistance and high-weather-resistance battery separator in this embodiment includes the following steps:

[0055] S1 pre-treats ultra-high molecular weight polyethylene (weight average molecular weight of 1.5 million g / mol, melt index of 0.03 g / 10 min at 190℃) by plasma bombardment. The plasma is a mixture of oxygen and nitrogen in a volume ratio of 1:3, the power is 130W, and the treatment time is 45s, to obtain plasma-pretreated ultra-high molecular weight polyethylene.

[0056] S2 contains 35 parts plasma-pretreated ultra-high molecular weight polyethylene, 3 parts hydroxylated silica (particle size 10 nm, hydroxyl content 3%), 0.6 parts butylated hydroxytoluene, 0.6 parts 2,4-dihydroxybenzophenone, and 60.8 parts white oil (initial boiling point 400℃, viscosity 60 mmHg at 40℃). 2The mixture is weighed by an electronic scale and then mixed in a mixing hopper before being fed into the first twin-screw extruder. 35 parts of maleic anhydride-grafted polyethylene (maleic anhydride grafting rate of 1%), 3 parts of methyltrimethoxysilane, 0.3 parts of di-tert-butyl peroxide, and 61.7 parts of white oil are weighed by an electronic scale and then mixed in a mixing hopper before being fed into the second twin-screw extruder. The extruder temperature is adjusted to 180°C. After melting and filtering, the mixture is extruded through a die at a die temperature of 200°C. The extruded melt is cooled at 50°C and cast at a traction speed of 40 m / min.

[0057] S3 first stretches the cast sheet longitudinally at 80°C with a stretching ratio of 10, and then stretches it transversely at 100°C with a stretching ratio of 6 to obtain a polyolefin film.

[0058] S4 extracts the polyolefin membrane in a dichloromethane extraction tank at 30°C and then dries it at 50°C to obtain the membrane precursor.

[0059] S5 performs a second transverse stretching of the separator precursor at 100°C with a stretching ratio of 1.5; then it is heat-set at 140°C, and after traction and thickness measurement, it is wound up to obtain a high-oxidation-resistant and high-weather-resistant battery separator.

[0060] Example 5

[0061] The preparation method of the high-oxidation-resistance and high-weather-resistance battery separator in this embodiment includes the following steps:

[0062] S1 pre-treats ultra-high molecular weight polyethylene (weight average molecular weight of 1.8 million g / mol, melt index of 0.002 g / 10 min at 190℃) by plasma bombardment. The plasma is a mixture of oxygen and nitrogen in a volume ratio of 1:3, the power is 150W, and the treatment time is 30s, to obtain plasma-pretreated ultra-high molecular weight polyethylene.

[0063] S2 contains 39 parts plasma-pretreated ultra-high molecular weight polyethylene, 5 parts hydroxylated silica (particle size 20 nm, hydroxyl content 4%), 1 part N-phenyl-α-naphthylamine, 1 part hexamethylphosphoric triamine, and 54 parts white oil (initial boiling point 400℃, viscosity 60 mmHg at 40℃). 2The mixture is weighed by an electronic scale and then mixed in a mixing hopper before being fed into the first twin-screw extruder. 35 parts of maleic anhydride-grafted polyethylene (maleic anhydride grafting rate of 1%), 3 parts of methyltrimethoxysilane, 0.3 parts of di-tert-butyl peroxide, and 61.7 parts of white oil are weighed by an electronic scale and then mixed in a mixing hopper before being fed into the second twin-screw extruder. The extruder temperature is adjusted to 200℃. After melting, the mixture is filtered and extruded through a die at a die temperature of 220℃. The extruded melt is cooled at 80℃ and cast at a traction speed of 10 m / min.

[0064] S3 first stretches the cast sheet longitudinally at 120°C with a stretching ratio of 6, and then stretches it transversely at 130°C with a stretching ratio of 10 to obtain a polyolefin film.

[0065] S4 extracts the polyolefin membrane in a dichloromethane extraction tank at 20°C and then dries it at 30°C to obtain the membrane precursor.

[0066] S5 performs a second transverse stretching of the separator precursor at 130°C with a stretching ratio of 1.1; then it is heat-set at 110°C, and after traction and thickness measurement, it is wound up to obtain a high-oxidation-resistant and high-weather-resistant battery separator.

[0067] Example 6

[0068] The preparation method of the high-oxidation-resistance and high-weather-resistance battery separator in this embodiment includes the following steps:

[0069] S1 pre-treats ultra-high molecular weight polyethylene (weight average molecular weight of 1.8 million g / mol, melt index of 0.002 g / 10 min at 190℃) by plasma bombardment. The plasma is a mixture of oxygen and nitrogen in a volume ratio of 1:3, the power is 150W, and the treatment time is 30s, to obtain plasma-pretreated ultra-high molecular weight polyethylene.

[0070] S2 contains 39 parts plasma-pretreated ultra-high molecular weight polyethylene, 5 parts hydroxylated silica (particle size 20 nm, hydroxyl content 4%), 1 part N-phenyl-α-naphthylamine, 1 part hexamethylphosphoric triamine, and 54 parts white oil (initial boiling point 400℃, viscosity 60 mmHg at 40℃). 2The mixture is weighed by an electronic scale and then fed into a mixing hopper for mixing. After mixing, it is fed into the first twin-screw extruder. 40 parts of maleic anhydride-grafted polyethylene (maleic anhydride grafting rate of 2%), 5 parts of vinyltrimethoxysilane, 0.5 parts of benzoyl peroxide, and 54.5 parts of white oil are weighed by an electronic scale and fed into a mixing hopper for mixing. After mixing, it is fed into the second twin-screw extruder. The extruder temperature is adjusted to 200℃. After melting, the mixture is filtered and extruded through a die at a die temperature of 220℃. The extruded melt is cooled at 80℃ and cast at a traction speed of 10 m / min.

[0071] S3 first stretches the cast sheet longitudinally at 120°C with a stretching ratio of 6, and then stretches it transversely at 130°C with a stretching ratio of 10 to obtain a polyolefin film.

[0072] S4 extracts the polyolefin membrane in a dichloromethane extraction tank at 20°C and then dries it at 30°C to obtain the membrane precursor.

[0073] S5 performs a second transverse stretching of the separator precursor at 130°C with a stretching ratio of 1.1; then it is heat-set at 110°C, and after traction and thickness measurement, it is wound up to obtain a high-oxidation-resistant and high-weather-resistant battery separator.

[0074] Comparative Example 1

[0075] The difference from Example 1 is that in step S2, hydroxylated silica is not added to the surface layer, and the amount of plasma-pretreated ultra-high molecular weight polyethylene added is 31 parts.

[0076] Comparative Example 2

[0077] The difference from Example 1 is that, in step S2, tert-butylhydroxyanisole and benzotriazole are not added to the surface layer.

[0078] Comparative Example 3

[0079] The difference from Example 1 is that in step S2, methyltriacetoxysilane and dicumyl peroxide are not added to the intermediate layer, and the amount of white oil added is 70 parts.

[0080] Comparative Example 4

[0081] The difference from Example 1 is that in step S2, the intermediate layer uses polyethylene (weight average molecular weight of 1 million g / mol, melt index of 0.2 g / 10 min at 190°C) instead of maleic anhydride-grafted polyethylene.

[0082] Comparative Example 5

[0083] The difference from Example 1 is that in step S1, the ultra-high molecular weight polyethylene is not subjected to plasma pretreatment.

[0084] The battery separators prepared in Examples 1-6 and Comparative Examples 1-5 were cut into A4 size and subjected to mechanical and thermal performance tests. The test methods are as follows:

[0085] (1) Tensile strength

[0086] The longitudinal tensile strength (MD) and transverse tensile strength (TD) of the battery separator were tested using the Xieqiang CTM universal testing machine. Five specimens were tested in each direction, and their average values ​​were calculated.

[0087] (2) Puncture strength

[0088] The puncture strength of the battery separator was tested using the Xieqiang CTM universal testing machine. Five specimens were tested in each direction, and the average value was calculated.

[0089] (3) Oxidation induction time

[0090] The oxidation induction time of the battery separator was tested using a differential scanning calorimeter. Three samples were tested, and their average values ​​were calculated.

[0091] (4) Environmental stress resistance test F50

[0092] The test was conducted in accordance with GB / T 1842-2008 "Environmental Stress Cracking Test Method for Polyethylene Plastics": The battery separator was bent and fixed in a fixture, immersed in a sodium alkyl aryl sulfonate solution, and continuously observed under a constant temperature of 50±0.5℃. The time when visible cracks appeared on the sample was recorded. The time corresponding to the stress cracking failure of 50% of the samples was obtained by plotting the results, which is F50.

[0093] The battery separator was bent and fixed in a fixture, immersed in a sodium alkyl aryl sulfonate solution, and continuously observed under a constant temperature of 50±0.5℃. The time when visible cracks appeared on the sample was recorded. Three samples were tested and their average value was calculated.

[0094] The test results are shown in Table 1:

[0095] Table 1. Performance test results of the battery separators in Examples 1-6 and Comparative Examples 1-5.

[0096]

[0097] As shown in Table 1, the performance of the membranes prepared in Comparative Examples 1-5 was significantly lower than that in Examples 1-6. Specifically, Comparative Example 1 lacked hydroxylated silica in its surface layer, resulting in insufficient three-dimensional network strength. The hydroxyl groups on the surface of the hydroxylated silica could not form hydrogen bonds and covalent bonds with the polar groups of the plasma-pretreated ultra-high molecular weight polyethylene, maleic anhydride-grafted polyethylene, and silane crosslinking agent, weakening the internal structural support of the membrane and leading to lower tensile and puncture strengths than in Example 1. Furthermore, due to the loss of the light-scattering-assisted blocking effect of the hydroxylated silica nanoparticles, Comparative Example 1 relied solely on the UV absorber for protection, resulting in a decrease in the environmental stress resistance test F50 to 951 h and a shortened oxidation induction time to 48 min.

[0098] Comparative Example 2 did not have antioxidants or UV absorbers added to its surface layer. Without antioxidants to capture free radicals and decompose peroxides, the polyethylene molecular chains were prone to oxidative breakage, with an oxidation induction time of only 39 minutes, far shorter than that of Example 1. At the same time, without UV absorbers, the material absorbed 280-400nm ultraviolet light, and the material's molecular structure was damaged by ultraviolet light. The environmental stress resistance test F50 was only 728 hours. Oxidation and photodegradation led to the destruction of the molecular chain integrity, and the tensile strength was slightly lower than that of Example 1, while the puncture strength was close to but did not reach the level of Example 1.

[0099] In Comparative Example 3, no silane crosslinking agent or initiator was added to the intermediate layer, which prevented the formation of silanol by the silane crosslinking agent. As a result, the silanol and the carboxylic acid groups of maleic anhydride grafted onto polyethylene could not form a Si-O-Si three-dimensional network, resulting in extremely poor membrane structure stability. Without the support of the crosslinking network, the tensile strength and puncture strength were significantly reduced, far below those of Example 1. Without a stable three-dimensional network to block oxidation and stress erosion, the oxidation induction time was only 36 min, and the environmental stress resistance test F50 was only 615 h, which could not meet the requirements for long-term use.

[0100] Comparative Example 4 replaced the maleic anhydride-grafted polyethylene in the intermediate layer with ordinary polyethylene. Since ordinary polyethylene does not have the active groups introduced by the anhydride graft, it cannot react with the silane crosslinking agent to form a Si-O-Si three-dimensional network structure, resulting in a significant decrease in the internal structural stability of the membrane. At the same time, due to the lack of the support of the crosslinking network, the tensile strength and puncture strength are significantly reduced compared with Example 1. Moreover, without a stable three-dimensional network to block oxidative erosion, the oxidation induction time is shortened to 47 min, and the environmental stress resistance test F50 drops to 1022 h, which cannot resist oxidative aging during long-term use.

[0101] Comparative Example 5 did not undergo plasma pretreatment of ultra-high molecular weight polyethylene. The surface of untreated ultra-high molecular weight polyethylene lacks polar groups such as hydroxyl and carboxyl groups, resulting in poor compatibility with antioxidants, ultraviolet absorbers, and hydroxylated silica, and the functional additives cannot fully exert their effects. The lack of reactive bonding between the surface layer and the intermediate layer weakens the interlayer forces, leading to a decrease in tensile strength and puncture strength. Insufficient compatibility reduces the protective effect of antioxidants and ultraviolet absorbers, with the oxidation induction time decreasing to 46 minutes and the environmental stress resistance test F50 being only 916 hours, indicating significantly insufficient weather resistance and anti-aging ability.

[0102] In summary, this invention utilizes maleic anhydride-grafted polyethylene and a silane crosslinking agent in the intermediate layer. Without altering the original skeleton, the anhydride grafting provides new active sites for the polyethylene. Under high temperature and the action of an initiator, the anhydride groups on the polyethylene crosslink with the silane crosslinking agent to form a three-dimensional network structure. This three-dimensional network structure, constructed through the crosslinking reaction, is more stable than the traditional polyethylene structure, thus exhibiting better weather resistance and resistance to environmental stress cracking. The surface layer employs plasma-pretreated ultra-high molecular weight polyethylene and hydroxylated silica. The hydroxylated silica enhances the strength of the three-dimensional network structure, improving the membrane's resistance to environmental stress testing. Furthermore, through the light scattering effect of nanoparticles, it assists in blocking ultraviolet rays, forming a synergistic weather protection with the surface layer's UV absorber. The plasma-pretreated ultra-high molecular weight polyethylene possesses better crystallinity and higher molecular structural stability, thus exhibiting superior oxidation and weather resistance compared to traditional polyethylene. Combined with the antioxidants and UV absorbers in the surface layer, it significantly inhibits oxidation and photodegradation of the membrane, further enhancing its weather resistance and delaying yellowing and embrittlement of the polyethylene.

[0103] The battery separator prepared by this invention possesses high tensile strength and puncture strength, while effectively improving the oxidation induction time and environmental stress resistance. Compared with existing technologies, the battery separator prepared by this invention solves the problem that traditional wet-process polyethylene separators have poor weather resistance and inevitably age and become brittle when applied to scenarios requiring longer service life, such as energy storage cells, rendering the cells unusable.

Claims

1. A high-antioxidant and high-weather-resistant battery separator, characterized in that, The high-antioxidant and high-weather-resistant battery separator has a three-layer co-extruded structure, including an intermediate layer and surface layers on both sides of the intermediate layer. The intermediate layer is made of maleic anhydride-grafted polyethylene, silane crosslinking agent, initiator and white oil mixed together. The surface layer is made of plasma-pretreated ultra-high molecular weight polyethylene, hydroxylated silica, antioxidant, ultraviolet absorber and white oil mixed together.

2. The high-oxidation-resistance and high-weather-resistance battery separator as described in claim 1, characterized in that, In the intermediate layer, the mass ratio of maleic anhydride-grafted polyethylene, silane crosslinking agent, initiator and white oil is (30-40):(1-5):(0.1-0.5):(54.5-68.9).

3. The high-oxidation-resistance and high-weather-resistance battery separator as described in claim 1, characterized in that, The maleic anhydride grafting rate of the maleic anhydride-grafted polyethylene is 0.5-2%; the silane crosslinking agent is methyltriacetoxysilane, methyltrimethoxysilane, or vinyltrimethoxysilane; the initiator is dicumyl peroxide, di-tert-butyl peroxide, or benzoyl peroxide.

4. The high-oxidation-resistance and high-weather-resistance battery separator as described in claim 1, characterized in that, In the surface layer, the mass ratio of plasma-pretreated ultra-high molecular weight polyethylene, hydroxylated silica, antioxidant, ultraviolet absorber and white oil is (30-39):(1-5):(0.2-1):(0.2-1):(54-69.6).

5. The high-oxidation-resistance and high-weather-resistance battery separator as described in claim 1, characterized in that, The plasma-pretreated ultra-high molecular weight polyethylene has a weight-average molecular weight of 1 million to 2 million g / mol and a melt index of <0.5 g / 10 min at 190 °C; the hydroxylated silica has a particle size of 5-20 nm and a hydroxyl content of 2-4%; the antioxidant is tert-butylhydroxyanisole, butylated hydroxytoluene, or N-phenyl-α-naphthylamine; and the ultraviolet absorber is benzotriazole, 2,4-dihydroxybenzophenone, or hexamethylphosphoric acid triamine.

6. The high-oxidation-resistance and high-weather-resistance battery separator as described in claim 1, characterized in that, The white oil has an initial boiling point ≥220℃ and a viscosity >40 mm at 40℃. 2 / s.

7. The high-oxidation-resistance and high-weather-resistance battery separator as described in claim 1, characterized in that, The longitudinal tensile strength of the high-oxidation-resistant and high-weather-resistant battery separator is >2000 kgf / cm. 2 Transverse tensile strength > 2000 kgf / cm 2 Puncture strength > 600gf, oxidation induction time > 50min, environmental stress resistance test F50 > 1200h.

8. The method for preparing a high-oxidation-resistant and high-weather-resistant battery separator according to any one of claims 1-7, characterized in that, Includes the following steps: S1 Pretreatment: Ultra-high molecular weight polyethylene is pretreated by plasma bombardment to obtain plasma-pretreated ultra-high molecular weight polyethylene. S2 Extruded Casting: The materials for the surface layer and the intermediate layer are mixed separately, and then extruded, cooled, and drawn to obtain a casting. S3 stretching film formation: The cast sheet is stretched longitudinally and then transversely in sequence to obtain a polyolefin film; S4 Extraction and Drying: The polyolefin membrane is extracted with dichloromethane and dried to obtain the membrane precursor; S5 Pore Expanding and Winding: After the separator precursor undergoes two transverse stretching, heat setting, and winding, a high-oxidation-resistant and high-weather-resistant battery separator is obtained.

9. The method for preparing a high-oxidation-resistance and high-weather-resistance battery separator as described in claim 8, characterized in that, In step S1, the plasma is a mixture of oxygen and nitrogen, with a volume ratio of oxygen to nitrogen of 1:(2-3), a power of 100-150W, and a processing time of 30-60s; in step S2, the extrusion temperature is 160-200℃, the die temperature is 180-220℃, the cooling temperature is 50-80℃, and the traction speed is 10-40m / min.

10. The method for preparing a high-oxidation-resistance and high-weather-resistance battery separator as described in claim 8, characterized in that, In step S3, the longitudinal stretching temperature is 80-120℃, the stretching ratio is 6-10, and the transverse stretching temperature is 100-130℃, with a stretching ratio of 6-10. In step S4, the extraction temperature is 20-30℃, and the drying temperature is 30-50℃. In step S5, the secondary transverse stretching temperature is 100-130℃, the stretching ratio is 1.1-1.5, and the heat setting temperature is 110-140℃.

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