Modified polyethylene for preparing high-safety lithium battery diaphragm as well as preparation method and application of modified polyethylene

Abstract

The invention provides modified polyethylene for preparing a high-safety lithium battery diaphragm. The modified polyethylene is polyethylene grafted by taking five-membered ring, six-membered ring or heterocyclic anhydride or four-membered ring lactone as a main body structure and taking a cyclic carboxylic acid derivative coupled olefine acid ester compound of a coupled olefine acid ester group as a modified grafting agent. The invention further provides a preparation method of the modified polyethylene, which comprises the following steps: melting polyethylene in a solvent, adding the modified grafting agent, and mixing and reacting in a molten state. A series of cyclic carboxylic acid derivative coupling olefine acid ester based grafting agents are designed to modify a polyethylene product, and the grafting agents have more polar groups, can effectively improve the wettability, tensile strength and other properties of the polyethylene diaphragm, can be effectively compatible with other high-performance polymers, and further improve the polyethylene properties; compared with a conventional maleic anhydride or olefine acid ester grafting agent, the maleic anhydride or olefine acid ester grafting agent is small in dosage and high in grafting rate.

Description

Technical Field

[0001] The present invention belongs to the technical field of polyethylene preparation, and particularly relates to a modified polyethylene for preparing a high-safety lithium battery separator, and a preparation method and application of the modified polyethylene. Background Art

[0002] With the accelerating adjustment of the global energy structure, the installed capacity of lithium-ion batteries in the fields of new energy vehicles and energy storage has shown an exponential growth trend. However, the incidence of safety accidents caused by the inherent thermodynamic instability of the electrochemical system cannot be ignored. According to statistical data, 78.6% of the global lithium battery safety accidents in 2022 originated from separator failure. In the structure of a lithium battery, a lithium battery mainly consists of a positive electrode, a negative electrode, a separator, and an electrolyte. Among them, the performance of the separator is crucial for the safety of the lithium battery. This is because during the daily charge and discharge process, Li + frequently transfers between the positive and negative electrodes through the separator. With the passage of time, unreasonable cell design, and illegal charge and discharge will all promote the formation of lithium dendrites. When the lithium dendrites grow to a certain scale, they will pierce the separator, resulting in the contact between the positive and negative electrodes and causing safety accidents. The solutions to this problem are, one is to increase the thickness of the separator; the other is to improve the performance of the separator, such as increasing the puncture resistance, increasing the melting point, and increasing the wettability, etc. However, different from the above-mentioned strategies for improving the safety of lithium batteries by improving the performance of the separator, the current downstream application scenarios of lithium batteries require the lithium batteries to have fast charge and discharge and high output power, which requires the lithium battery separator to reduce the thickness as much as possible and improve the Li + transport efficiency, which fundamentally conflicts with improving the safety of the separator.

[0003] High molecular weight and ultra-high molecular weight polyethylene (Ultra-High Molecular Weight Polyethylene, UHMWPE), as special polymer materials, exhibit characteristic parameters that are significantly different from those of conventional polyethylene (PE) in terms of crystal structure integrity, molecular chain entanglement density, and mechanical properties. Especially in the field of lithium-ion battery separators, its unique nano-scale porous topological structure (porosity ≥ 40%, average pore diameter ≤ 100 nm) realizes excellent Li + transference number, while maintaining excellent electronic insulation properties, which makes it an excellent separator material for high-power density lithium battery systems.

[0004] Based on the above considerations, improving the comprehensive performance of polyethylene by modifying polyethylene or combining it with other materials is expected to become an effective way to solve the safety problem of polyethylene diaphragms. In the prior art, surface functionalization and blending modification are the mainstream directions for improving the safety of polyethylene diaphragms. For example, Patent CN105185936A discloses a safe and high-performance lithium-ion battery diaphragm, which improves the closed-hole and flame-retardant properties of the lithium-ion battery diaphragm by setting a nano-metal hydroxide coating on the surface of the polyolefin microporous membrane. However, this method requires setting a coating in the subsequent diaphragm processing link, increasing the complexity of the production process. Patent CN110635092A discloses a polar polyolefin diaphragm composed of a blend of a polar polymer and a polyolefin material, which improves the electrolyte wettability, liquid absorption and retention rate, and ionic conductivity of the polyolefin diaphragm. However, the polar polymers used in this method mainly include polyether / polyolefin block polymers, block polyether amide resins, polyvinyl acetate, secondary alkyl sulfonic acid compounds, and polyvinyl alcohol, etc., which have poor combination with polyolefins and limited modification effects. Therefore, further research is still needed on how to obtain a lithium-ion battery diaphragm with high safety. Summary of the Invention

[0005] The present invention aims to provide a high-safety lithium-ion battery diaphragm, a modified polyethylene for preparing the high-safety lithium-ion battery diaphragm, and a preparation method of the modified polyethylene to solve the above defects existing in the background technology.

[0006] The present invention is implemented by adopting the following technical solutions:

[0007] The first aspect of the present invention relates to a modified polyethylene for preparing a high-safety lithium-ion battery diaphragm, comprising at least one polyethylene grafted with a modified grafting agent, and the modified grafting agent is a cyclic carboxylic acid derivative-coupled acrylic acid ester compound. The cyclic carboxylic acid derivative-coupled acrylic acid ester compound uses a five-membered ring, a six-membered ring or a heterocyclic anhydride, or a four-membered ring lactone as the main structure and couples an acrylic acid ester group.

[0008] The modified grafting agent is selected from cyclic carboxylic acid derivative-coupled acrylic acid ester compounds having any one of the general formula structures of Formula A, Formula B, Formula C or Formula D, wherein Y is an acrylic acid ester group; R1, R2, R3, R4, and R5 are each independently selected from H, halogen, C1-C3 alkyl, C1-C3 alkoxy, phenyl or Y.

[0009]

[0010] The present invention designs a modified grafting agent with a special structure, uses a cyclic carboxylic acid derivative as the main framework, and at the same time introduces an acrylic acid ester group. The cyclic carboxylic acid derivative can open the ring, and the acrylic acid ester group has a double bond that can be grafted into the polyethylene main chain. Moreover, both the acrylic acid ester end and the cyclic carboxylic acid derivative have polar groups, which can improve the hydrophilicity of the polyethylene main chain, improve the processing performance of polyethylene, and improve the puncture resistance and wettability of the lithium battery diaphragm.

[0011] The enoate group Y coupled to the cyclic carboxylic acid derivative framework can be represented by the following structural formula: -C=C-(CH2) n -COOR6, where n = 0 - 3 and R6 is selected from C1 - C3 alkyl groups. The carbon chain length of the enoate group Y should not be too long, otherwise the grafting rate of the modified polyethylene will decrease and the hydrophilicity will also decline. Preferably, n = 0 - 1 and R6 is selected from methyl or ethyl. The enoate group Y is more preferably a methyl acrylate group, i.e., -C=C-COOCH3.

[0012] The substituents R1 - R5 on the cyclic carboxylic acid derivative framework are preferably groups with less steric hindrance, which is more conducive to the grafting modifier being grafted onto the polyethylene main chain. Preferably, R1 - R5 are each independently selected from H, methyl, ethyl, methoxy or ethoxy.

[0013] As a preferred embodiment of the present invention, the modified grafting agent is selected from any one of the following compounds of formula A1, formula B1 - B4, formula C1 or formula D1:

[0014]

[0015]

[0016] Furthermore, the preparation method of the cyclic carboxylic acid derivative-coupled enoate compound is to react a halogenated cyclic carboxylic acid derivative with an enoate under the action of a catalyst.

[0017] Specifically, the preparation method of the cyclic carboxylic acid derivative-coupled enoate compound includes the following steps: Dissolve the halogenated cyclic carboxylic acid derivative and the enoate in an organic solvent at a molar ratio of 1:1.1 - 2, add a catalytic amount of palladium salt, react at 110 - 130 °C for 3 - 12 hours, remove the solvent after the reaction, and purify the solid product to obtain the cyclic carboxylic acid derivative-coupled enoate compound.

[0018] The halogenated cyclic carboxylic acid derivative is preferably a bromide, such as but not limited to 2-bromopropiolactone, 2-bromomaleic anhydride, 3-bromo-4-methylmaleic anhydride, 2,3-dibromomaleic anhydride, 3-bromoglutaric anhydride, 4-bromophthalic anhydride.

[0019] The enoate is represented by the following structural formula: C=C-(CH2) n -COOR6, where n = 0 - 3 and R6 is selected from C1 - C3 alkyl groups. Preferably, n = 0 - 1 and R6 is selected from methyl or ethyl. The enoate is preferably methyl acrylate, i.e., C=C-COOCH3.

[0020] The catalyst used is a palladium salt commonly used in the art, such as but not limited to palladium acetate and palladium triphenylphosphine. The organic solvent can be a solvent commonly used in the art, such as but not limited to DMF. The solvent removal step, product purification step, etc. after the reaction are all conventional operations in the art. For example, the solvent is removed by vacuum distillation, and the product is purified by column chromatography. Details are not described here.

[0021] Taking the compound of formula B1 as an example, the preparation method of the cyclic carboxylic acid derivative coupling enoate compound is specifically described as follows. 2-Bromomaleic anhydride and methyl acrylate are dissolved in an organic solvent at a molar ratio of 1:1.1 - 2, a catalytic amount of palladium salt is added, and the reaction is carried out at 110 - 130 °C for 3 - 12 hours. After the reaction is completed, the solvent is removed, and the solid product is purified to obtain the compound of formula B1.

[0022] Preferably, the grafting amount of the modified grafting agent is 0.1 - 10% of the mass of polyethylene.

[0023] Preferably, in the modified polyethylene for preparing the high-safety lithium battery separator, an appropriate amount of polymers such as polypropylene, polyethylene wax or EVA can also be added to improve the mechanical properties such as puncture strength and tensile yield strength of the separator made of the material. However, the addition ratio of these polymers should not be too high, otherwise the wettability of the separator may be reduced. The appropriate addition amount is 1 - 20% of the mass of polyethylene, preferably 10 - 20%.

[0024] The second aspect of the present invention relates to a preparation method of the above-mentioned modified polyethylene for preparing a high-safety lithium battery separator, including the following steps: melting polyethylene in a solvent, adding a modified grafting agent, and mixing and reacting in a molten state to obtain.

[0025] Further, the addition amount of the modified grafting agent is 1 - 15% of the mass of polyethylene, and the preferred addition amount is 5 - 10% of the mass of polyethylene.

[0026] The polyethylene preferably uses high molecular weight polyethylene or ultra-high molecular weight polyethylene, and its average molecular weight is 500,000 - 2,000,000. For example, high molecular weight polyethylene or ultra-high molecular weight polyethylene of models such as LPF-8008V, LPF-8010U, LPF-8009V, LPF-8006V, and LPF-8020U of Puxijing New Energy Materials (Shanghai) Co., Ltd. are used.

[0027] The solvent can be an organic solvent commonly used in the art, such as but not limited to xylene, chlorobenzene, etc.

[0028] Preferably, an appropriate amount of polymers such as polypropylene, polyethylene wax or EVA can also be added during the melting step of polyethylene. The addition amount is preferably 1-20% of the mass of polyethylene, more preferably 10-20%. Of course, in order to improve the compatibility of polyethylene and the above polymers and enhance the mechanical properties of the blend, an appropriate amount of compatibilizer can also be added, such as SEBS or other conventional compatibilizers in the art. Its addition amount can be obtained according to common sense and through conventional experiments. In some preferred embodiments, the addition amount of the compatibilizer is 0.5-2.0% of the total mass of the blend polymers.

[0029] Preferably, the time for the above reaction is generally 3-12 hours.

[0030] The third aspect of the present invention relates to a high-safety lithium battery separator, comprising a film made of the modified polyethylene.

[0031] The high-safety lithium battery separator can be prepared by conventional methods in the art, such as dry unidirectional stretching process, dry bidirectional stretching process or wet bidirectional stretching process. In addition, inorganic or organic materials can be further coated on the film surface to improve the thermal stability and mechanical strength.

[0032] Compared with the prior art, the present invention has the following beneficial effects:

[0033] The present invention modifies the polyethylene product by designing and synthesizing a cyclic carboxylic acid derivative-coupled acrylate-based grafting agent to obtain a lithium battery separator. The grafting agent has many polar groups, which can effectively improve the wettability, tensile strength and other properties of the polyethylene separator. In addition, the cyclic carboxylic acid derivative and acrylate group can be compatible with other high-performance polymers to further improve the properties of polyethylene and expand the application fields of polyethylene. Compared with a single maleic anhydride or acrylate-based grafting agent, the amount used in the grafting process of the present invention is less and the grafting rate is high. Detailed Embodiments

[0034] The following description is used to disclose the present invention so that those skilled in the art can implement the present invention. The preferred embodiments in the following description are only examples and do not limit the scope of the present invention. Those skilled in the art can think of other obvious variations. The basic principles defined in the following description can be applied to other embodiments, variations, improvements, equivalent solutions and other technical solutions without departing from the spirit and scope of the present invention.

[0035] For the instruments or reagents in the embodiments of the present invention that are not specified by the manufacturer, they are all conventional commercial instruments or reagents. The main reagents used in the embodiments are specifically as follows:

[0036] 2-bromopropiolactone (CAS: 1309866-05-4 Sigma-Aldrich)

[0037] 2-Bromo-maleic anhydride (CAS: 5926-51-2 Sigma-Aldrich)

[0038] 3-Bromo-4-methylmaleic anhydride (CAS: 59107-74-3 Shanghai Macklin Biochemical Co., Ltd.)

[0039] 2,3-Dibromomaleic anhydride (CAS: 1122-12-9 Shanghai Macklin Biochemical Co., Ltd.)

[0040] 3-Bromoglutaric anhydride (CAS: 50901-12-7 Sigma-Aldrich)

[0041] 4-Bromophthalic anhydride (CAS: 86-90-8 Sigma-Aldrich)

[0042] Methyl acrylate (CAS: 96-33-3 Sigma-Aldrich)

[0043] Methyl 3-butenoate (CAS: 3724-55-8 Sigma-Aldrich)

[0044] Cyclobutane-1,2-dicarboxylic anhydride (CAS: 4462-96-8 Aladdin Reagent Co., Ltd.)

[0045] N,N-Dimethylformamide (CAS: 68-12-2 Sigma-Aldrich)

[0046] Ethyl acetate (CAS: 141-78-6 Sigma-Aldrich)

[0047] n-Hexane (CAS: 110-54-3 Sigma-Aldrich)

[0048] Palladium(II) acetate (CAS: 3375-31-3 Sigma-Aldrich)

[0049] Polyethylene: Puxijing New Energy Materials (Shanghai) Co., Ltd. LPF-8008V

[0050] Polypropylene: (CAS: 9003-07-0 Exxon mobil)

[0051] SEBS: Yueyang Petrochemical YH-501

[0052] All solvents involved in the present invention need to be treated with anhydrous treatment without special instructions.

[0053] Examples 1-7

[0054] Preparation of modified grafting agents. Modified grafting agents A1, B1 - B4, C1 and D1 were prepared respectively from the cyclic carboxylic acid derivatives and acrylic esters listed in Table 1.

[0055] Table 1 Structures and preparation raw materials of modified grafting agents

[0056]

[0057]

[0058] Preparation of modified grafting agent A1:

[0059] In a 250 ml three-necked flask, 2-bromopropiolactone (0.75 g, 5 mmol) and methyl acrylate (0.52 g, 6 mmol) were dissolved in 100 ml of DMF solution, and catalytic amount of palladium acetate (2.25 μg, 10 μmol) was added. Water was separated and refluxed at 120 °C for 12 h. After the reaction was completed, the solvent DMF was removed by means of vacuum distillation, and the solid product was purified by column chromatography (eluent = EA:HEX = 2:5), with a final yield of 40%. (1H 300 MHz, DMSO): 7.42 (s, 1H, C=C-H); 6.23 - 4.02 (m, 7H, C=C-H, -CH3, -CH2, -CH). C7H8O4: Calculated values of elemental analysis (%): C, 53.85; H, 5.16; O, 40.99. Experimental measured values (%): C, 53.97; H, 5.25; O, 41.12.

[0060] Preparation of modified grafting agent B1:

[0061] In a 250 ml three-necked flask, 2-bromomaleic anhydride (0.99 g, 5 mmol) and methyl acrylate (0.52 g, 6 mmol) were dissolved in 100 ml of DMF solution, and catalytic amount of palladium acetate (2.25 μg, 10 μmol) was added. Water was separated and refluxed at 120 °C for 12 h. After the reaction was completed, the solvent DMF was removed by means of vacuum distillation, and the solid product was purified by column chromatography (eluent = EA:HEX = 1:5), with a final yield of 88%. (1H 300 MHz, DMSO): 7.50 (s, 1H, C=C-H); 7.13 (s, 1H, ring H); 5.38 (s, 1H, C=C-H); 3.94 - 3.71 (m, 3H, -CH3). C8H6O5: Calculated values of elemental analysis (%): C, 52.76; H, 3.32; O, 43.92. Experimental measured values (%): C, 52.90; H, 3.39; O, 44.03.

[0062] Preparation of modified grafting agent B2:

[0063] In a 250 ml three-necked flask, 3-bromo-4-methylmaleic anhydride (0.95 g, 5 mmol) and methyl acrylate (0.52 g, 6 mmol) were dissolved in 100 ml of DMF solution, and a catalytic amount of palladium acetate (2.25 μg, 10 μmol) was added. The mixture was refluxed with water separation at 120 °C for 12 h. After the reaction was completed, the solvent DMF was removed by means of vacuum distillation, and the solid product was purified by column chromatography (eluent = EA: HEX = 1:5), with a final yield of 76%. (1H 300 MHz, DMSO): 7.52 (s, 1H, C═C-H); 5.33 (s, 1H, C═C-H); 3.86 - 2.61 (m, 6H, CH3). C8H8O5: Calculated values (%) for elemental analysis: C, 55.11; H, 4.11; O, 40.78. Experimental values (%): C, 55.20; H, 4.23; O, 40.83.

[0064] Preparation of modified grafting agent B3:

[0065] In a 250 ml three-necked flask, 2,3-dibromomaleic anhydride (1.27 g, 5 mmol) and methyl acrylate (0.52 g, 6 mmol) were dissolved in 100 ml of DMF solution, and a catalytic amount of palladium acetate (2.25 μg, 10 μmol) was added. The mixture was refluxed with water separation at 120 °C for 12 h. After the reaction was completed, the solvent DMF was removed by means of vacuum distillation, and the solid product was purified by column chromatography (eluent = EA: HEX = 1:1), with a final yield of 27%. (1H 300 MHz, DMSO): 7.51 (s, 2H, C═C-H); 5.35 (s, 2H, C═C-H); 3.81 (s, 6H, -CH3). C 12 H 10 O7: Calculated values (%) for elemental analysis: C, 54.14; H, 3.79; O, 42.07. Experimental values (%): C, 54.14; H, 3.80; O, 42.11.

[0066] Preparation of modified grafting agent C1:

[0067] In a 250 ml three-necked flask, 3-bromoglutaric anhydride (0.95 g, 5 mmol) and methyl acrylate (0.52 g, 6 mmol) were dissolved in 100 ml of DMF solution, and a catalytic amount of palladium acetate (2.25 μg, 10 μmol) was added. The mixture was refluxed with water separation at 120 °C for 12 h. After the reaction was completed, the solvent DMF was removed by means of vacuum distillation, and the solid product was purified by column chromatography (eluent = EA:HEX = 1:3), with a final yield of 80%. (1H 300 MHz, DMSO): 7.15 (s, 1H, C═C-H); 6.02 (s, 1H, C═C-H); 3.77 (s, 3H, -CH3); 3.03 - 1.79 (m, 5H, ring H). C9H 10 O5: Calculated values (%) for elemental analysis: C, 54.55; H, 5.09; O, 40.37. Experimental values (%): C, 54.57; H, 5.19; O, 40.42.

[0068] Preparation of modified grafting agent D1:

[0069] In a 250 ml three-necked flask, 4-bromophthalic anhydride (1.13 g, 5 mmol) and methyl acrylate (0.52 g, 6 mmol) were dissolved in 100 ml of DMF solution, and a catalytic amount of palladium acetate (2.25 μg, 5 μmol) was added. The mixture was refluxed with water separation at 120 °C for 12 h. After the reaction was completed, the solvent DMF was removed by means of vacuum distillation, and the solid product was purified by column chromatography (eluent = EA:HEX = 1:4), with a final yield of 85%. (1H 300 MHz, DMSO): 8.37 (s, 1H, Ar-H); 7.93 (s, 1H, Ar-H); 7.74 (s, 1H, Ar-H); 7.52 (s, C═C-H); 6.32 (s, C═C-H); 3.82 (s, 3H, -CH3). C 12 H8O5: Calculated values (%) for elemental analysis: C, 62.07; H, 3.47; O, 34.45. Experimental values (%): C, 62.11; H, 3.48; O, 34.60.

[0070] Preparation of modified grafting agent B4:

[0071] In a 250 ml three-necked flask, 2-bromomaleic anhydride (0.88 g, 5 mmol) and methyl 3-butenoate (0.60 g, 6 mmol) were dissolved in 100 ml of DMF solution, and a catalytic amount of palladium acetate (2.25 μg, 10 μmol) was added. The mixture was refluxed with water separation at 120 °C for 12 h. After the reaction was completed, the solvent DMF was removed by means of vacuum distillation, and the solid product was purified by column chromatography (eluent = EA:HEX = 1:5), and the final yield was 74%. (1H 300 MHz, DMSO): 8.37 (s, 1H, Ar-H); 7.93 (s, 1H, Ar-H); 7.74 (s, 1H, Ar-H); 7.52 (s, C═C-H); 6.32 (s, C═C-H); 3.82 (s, 3H, -CH3). C 12 H8O5: Calculated values (%) for elemental analysis: C, 55.11; H, 4.11; O, 40.78. Found values (%): C, 55.22; H, 4.26; O, 40.93.

[0072] Examples 8 - 14

[0073] Preparation of modified polyethylene samples 1 - 7.

[0074] According to the raw materials and dosages in Table 2, the obtained modified grafting agent was used for grafting modification with high molecular weight polyethylene. The specific steps were as follows: High molecular weight polyethylene and xylene (50 ml) were added to a container with stirring and N2 was introduced. The mixture was heated to 140 °C, and after complete melting, the modified grafting agent was added for grafting modification. After reacting for 1 h, the mixture was poured into a large amount of ethanol, stirred, and filtered to obtain modified polyethylene samples 1 - 7.

[0075] The grafting rate was determined by infrared spectroscopy. Since the cyclic carboxylic acid derivative has obvious characteristic peaks (C═O, 1780 cm -1 ), by collecting the infrared spectra of the original polyethylene and the modified polyethylene, the grafting rate was calculated according to the Lambert-Beer law based on the characteristic peak intensity.

[0076] Table 2 Ratios of different modified grafting agents to polyethylene and grafting rates

[0077]

[0078]

[0079] Example 15

[0080] Preparation of modified polyethylene sample 8:

[0081] In a container with N₂ being introduced and equipped with stirring, add polyethylene (10 g) and xylene (50 ml), heat to 140 °C. After complete melting, add 1 g of modified grafting agent B1 and 1 g of polypropylene for graft modification, add 0.1 g of SEBS as a compatibilizer, and react for 1 h. After the reaction, pour the mixture into a large amount of ethanol, stir, and filter to obtain modified polyethylene sample 8.

[0082] Example 16

[0083] Preparation of modified polyethylene sample 9:

[0084] In a container with N₂ being introduced and equipped with stirring, add polyethylene (10 g) and xylene (50 ml), heat to 140 °C. After complete melting, add 1 g of modified grafting agent B1 and 2 g of polypropylene for graft modification, add 0.1 g of SEBS as a compatibilizer, and react for 1 h. After the reaction, pour the mixture into a large amount of ethanol, stir, and filter to obtain modified polyethylene sample 9.

[0085] Comparative Example 1

[0086] Preparation of modified polyethylene sample 10:

[0087] In a container with N₂ being introduced and equipped with stirring, add polyethylene (10 g) and xylene (50 ml), heat to 140 °C. After complete melting, add 1 g of grafting agent cyclobutane-1,2-dicarboxylic anhydride and 1 g of polypropylene for graft modification, add 0.1 g of SEBS as a compatibilizer, and react for 1 h. After the reaction, pour the mixture into a large amount of ethanol, stir, and filter to obtain modified polyethylene sample 10.

[0088] Comparative Example 2

[0089] Preparation of modified polyethylene sample 11:

[0090] In a container with N₂ being introduced and equipped with stirring, add polyethylene (10 g) and xylene (50 ml), heat to 140 °C. After complete melting, add 1 g of grafting agent glutaric anhydride and 1 g of polypropylene for graft modification, add 0.1 g of SEBS as a compatibilizer, and react for 1 h. After the reaction, pour the mixture into a large amount of ethanol, stir, and filter to obtain modified polyethylene sample 11.

[0091] Application Example

[0092] Preparation of lithium-ion battery separator:

[0093] Samples 1-11 of modified polyethylene were made into diaphragms and their properties were tested. The specific preparation method is as follows: 1 kg of modified polyethylene and white oil were mixed evenly (the mass fraction of polyethylene was 40%), and an oil-containing cast film was obtained through a twin-screw extruder (extrusion temperature 160 °C). The oil-containing cast film was longitudinally and transversely stretched to obtain a diaphragm about 10 μm thick. The diaphragm was dried and evaporated, and cut into appropriate sizes for use.

[0094] The diaphragm determination standard was tested by the following method:

[0095] 1. Closed-hole melting point determination

[0096] Tested with reference to standard GB / T 19466.1-2004

[0097] 2. Puncture strength determination

[0098] The puncture strength determination was carried out with reference to standard GB / T 36363-2018

[0099] 3. Tensile yield strength determination

[0100] The tensile yield strength was determined with reference to standard GB / T1040.2-2006.

[0101] 4. Wettability determination

[0102] Refer to standard GB / T 30447-2013. The wetting angle (i.e., the contact angle) (θ) less than 90° is the liquid-loving state. The smaller the angle, the better the wettability; θ greater than 90° is the liquid-repellent state. The larger the angle, the better the hydrophobicity, and the liquid is not easy to wet the solid.

[0103] The performance determination results of each modified polyethylene diaphragm sample are shown in Table 3.

[0104] Table 3 Performance determination of modified polyethylene diaphragm samples

[0105]

[0106]

[0107] Note: The blank group is high molecular weight polyethylene without modification.

[0108] As can be seen from Table 3, the modified grafting agent obtained in the embodiments of the present invention helps to improve the properties of polyethylene, increase the closed pore melting point, puncture strength, tensile yield strength and wettability of the polyethylene separator. In particular, the performance of B2 is the best, with a high grafting rate, far exceeding that of commercially available grafting agents. This may be because the modified grafting agent of the present invention contains a large number of polar carboxylic acid groups that can form a network structure, which improves the structural strength of polyethylene to a certain extent, resulting in an increase in the closed pore melting point, puncture strength and tensile strength. On the other hand, the introduction of polar groups greatly improves the hydrophilicity of polyethylene and reduces the wetting angle of polyethylene, which are all performance urgently needed to be improved in the current development of lithium battery separators. In addition, the modified grafting agent also helps to combine and graft other modified components (such as polypropylene) with polyethylene, further improving or expanding the performance of the polyethylene separator. Compared with other types of grafting agents, the present invention can significantly improve the wettability of the separator.

Claims

1. A modified polyethylene for preparing a high-safety lithium battery separator, comprising at least one polyethylene grafted by a modified grafting agent, wherein the modified grafting agent is a cyclic carboxylic acid derivative coupled with an alkenoate compound with a five-membered ring, a six-membered ring or a heterocyclic acid anhydride, or a four-membered ring lactone as the main structure.

2. The modified polyethylene according to claim 1, characterized in that The modified grafting agent is selected from a cyclic carboxylic acid derivative coupled olefinic acid ester compound having any one of the general structures of Formula A, Formula B, Formula C or Formula D, wherein Y is an olefinic acid ester group; R1, R2, R3, R4, and R5 are independently selected from H, halogen, C1-C3 alkyl, C1-C3 alkoxy, phenyl or Y; 3. The modified polyethylene according to claim 2, characterized in that The acrylate group Y is represented by the following structural formula: -C=C-(CH2) n -COOR6, wherein n=0-3, R6 is selected from C1-C3 alkyl.

4. The modified polyethylene according to claim 2, characterized in that The acrylate group Y is a methyl acrylate group.

5. The modified polyethylene according to claim 2, characterized in that R1 to R5 are each independently selected from H, methyl, ethyl, methoxy or ethoxy.

6. The modified polyethylene according to claim 2, characterized in that The modified grafting agent is selected from any one of the following compounds: A1, B1-B4, C1 or D1:

7. The modified polyethylene according to claim 1, characterized in that It also contains at least one of polypropylene, polyethylene wax or EVA in an amount of 1-20% by mass of the polyethylene.

8. The modified polyethylene according to claim 1, characterized in that The method for preparing the cyclic carboxylic acid derivative coupled enoate compound is to obtain the compound by reacting the halogenated cyclic carboxylic acid derivative with the enoate under the action of a catalyst.

9. The method for preparing the modified polyethylene according to any one of claims 1 to 8, characterized in that: The method comprises the following steps: melting polyethylene in a solvent, adding a modified grafting agent, and mixing and reacting the polyethylene in a molten state to obtain the polyethylene.

10. A high-safety lithium battery separator, characterized in that: A film comprising the modified polyethylene according to any one of claims 1 to 8.

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