Preparation method of natural fiber modified polyethylene separator and its application as a lithium-ion battery separator

Abstract

This invention discloses a method for preparing a natural fiber-modified polyethylene separator and its application as a lithium-ion battery separator. Sisal fibers are heated and stirred in an aqueous solution of a strong alkali and natural clay, then dried, pulverized, and sieved to obtain modified fibers with good compatibility with the PE resin matrix. The modified fibers, composite antioxidants, and composite lubricants are dispersed in white oil at a predetermined ratio and added together with polyethylene into the feed inlet of a co-rotating twin-screw extruder. Under high temperature and high shear force in the barrel, the mixture is uniformly dispersed. After filtering out impurities from the melt, the melt flow rate is controlled, and the mixture is extruded from a T-die. The extruded melt undergoes rapid cooling to achieve phase separation, yielding a cast sheet. The cast sheet is then subjected to asynchronous biaxial stretching, extraction, drying, and heat treatment to obtain a natural fiber-modified polyethylene separator, which exhibits advantages such as high strength, good high-temperature dimensional stability, slightly enhanced liquid absorption and retention capacity, and biodegradability.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and in particular to a method for preparing a natural fiber modified polyethylene separator and its application as a lithium-ion battery separator. Background Technology

[0002] Lithium-ion batteries have become the main energy storage device in modern society, widely used in various new energy devices, from portable electronic devices (including digital cameras, mobile phones, and laptops) to electric vehicles and emerging smart grids. The separator, as a key component of lithium-ion batteries, plays a crucial role in preventing short circuits caused by direct contact between the positive and negative electrodes and allowing lithium ions to migrate during charging and discharging. It is critical to key parameters such as battery safety and charge / discharge efficiency. Polyolefin membranes are widely used in the manufacture of lithium battery separator substrates due to their good chemical stability, excellent corrosion resistance, high tensile strength, and economic advantages. However, they have the following problems: 1. Because the substrate of polyolefin separators is linear polyethylene, a thermoplastic resin with a simple molecular structure and weak intermolecular forces, the viscous deformation caused by the relative slippage of molecular chains results in poor creep resistance and thermal stability of the separator, as well as poor dimensional stability at high temperatures, affecting the final safety of the lithium battery; 2. As a strongly non-polar material, polyolefin separators have poor affinity for polar dielectrics and weak liquid absorption and retention capabilities, affecting the final charge / discharge efficiency of the lithium battery.

[0003] Natural fibers (including flax, hemp, jute, sisal, kenaf, coconut fiber, kapok, banana, etc.) can be used to replace traditional fibers (such as glass fiber, aramid fiber, carbon fiber, etc.) to reinforce polyolefins due to their low cost, high specific strength, good mechanical properties, environmental friendliness and biodegradability. This can give polyolefin membranes higher strength, better dimensional stability and make them more environmentally friendly.

[0004] However, natural fiber reinforced polyethylene has two problems: First, natural fibers usually contain more hydrophilic groups such as hydroxyl groups, which have poor compatibility with non-polar polyolefin resin matrix. This leads to poor fiber-matrix interface and agglomeration when added directly, resulting in defects and a decrease in the overall strength of the diaphragm instead of an increase. Second, the polyethylene commonly used in diaphragm production usually has a high molecular weight. Adding fibers and other substances makes it difficult to disperse and increases the melt viscosity, affecting the processing and having a significant impact on production equipment and process conditions, resulting in unpredictable performance changes in the produced diaphragm. Summary of the Invention

[0005] The purpose of this invention is to address the technical deficiencies in the prior art by providing a natural fiber modified polyethylene diaphragm and its preparation method.

[0006] Another object of the present invention is to provide the application of the aforementioned natural fiber modified polyethylene separator as a lithium-ion battery separator.

[0007] The technical solution adopted to achieve the purpose of this invention is:

[0008] A method for preparing a polyethylene separator modified with natural fibers includes the following steps:

[0009] Step 1: Place the sisal fiber in an aqueous solution of strong alkali and natural clay, heat and stir, remove, dry, pulverize and sieve to obtain modified fiber with good compatibility with PE resin matrix.

[0010] Step 2: The modified fiber, composite antioxidant, and composite lubricant obtained in Step 1 are added to white oil in a predetermined ratio and dispersed to obtain a mixed system. This mixed system is then added to the feed port of a co-rotating twin-screw extruder along with polyethylene. Under the action of high temperature and high shear force in the barrel, the mixture is evenly dispersed. After filtering out impurity particles from the melt, the melt flow rate is metered and controlled, and the mixture is extruded from a T-die. After being extruded through the die, the mixed melt undergoes rapid cooling to achieve phase separation, resulting in a cast sheet.

[0011] Step 3: The cast sheet obtained in Step 2 is subjected to asynchronous biaxial stretching, extraction, drying, and heat treatment to obtain a polyethylene diaphragm modified with natural fibers.

[0012] In step 1, natural fibers are placed in an aqueous solution of alkaline clay of a certain concentration for treatment. The degree of degradation of natural fibers and the degree of reaction with -OH on the fiber surface are controlled by controlling the concentration of alkali solution, treatment temperature, and treatment time, thereby obtaining modified fibers with micron-sized fibers and low polarity.

[0013] In step 2, the composite antioxidant and lubricant have a synergistic effect, mainly preventing the degradation of macromolecular chains by inhibiting the generation of free radicals and reducing melt viscosity by entering the middle of chain segments to prevent macromolecular chain entanglement, thereby ensuring processing stability. The modified fibers are dispersed into the PE melt under the shearing action of a twin-screw extruder, forming a good fiber-resin matrix interface with the PE resin matrix. After cooling, stretching, extraction, and heat treatment to form a film, the fibers are uniformly dispersed on the film.

[0014] When subjected to external forces, the polyethylene membrane modified with natural fibers transmits the force through the resin matrix to the high-strength fibers, thus dispersing the force and increasing its overall strength. The cellulose present on the membrane surface, due to its hydrophilic groups, has a strong affinity for polar dielectrics, enhancing the membrane's liquid absorption and retention capacity. Simultaneously, the biodegradability of natural cellulose makes waste membranes easier to handle, making it more environmentally friendly.

[0015] In the above technical solution, in step 1, the ratio of sisal fiber, strong alkali, natural clay and water is 1:(7-9):(1-2):20, the strong alkali is NaOH or KOH, the natural clay is natural montmorillonite or kaolin, the heating temperature is 60-75℃, the stirring speed is 50-100rpm, and the heating and stirring time is 45-90 minutes.

[0016] In the above technical solution, in step 1, a filtration device is used for separation, the drying temperature is 60-75℃, the drying time is 3-4 hours, and a pulverizer is used for pulverization with a screen mesh size of 500-1000 mesh.

[0017] In the above technical solution, in step 2, the coarse filter and the fine filter filter impurity particles in the melt, and the gear pump metering and controlling the melt flow rate.

[0018] In the above technical solution, in step 2, the polyethylene is high-density polyethylene with an average molecular weight of 500,000 to 2,000,000, the composite antioxidant is a 1:1 composite antioxidant of antioxidant 1010 and antioxidant 168, and the lubricant is a 1:1 composite lubricant of calcium stearate and zinc stearate.

[0019] In the above technical solution, in step 2, in every 100 parts by weight of the mixed system, the mass parts of modified fiber, composite antioxidant, composite lubricant and polyethylene are 1-5, 0.05-0.3, 0.05-0.3 and 15-25 respectively, and the balance is white oil.

[0020] In the above technical solution, in step 2, the length-to-diameter ratio of the co-rotating twin-screw extruder is 50-60, the extruder barrel has 13 barrel sections, section 1 is the feed port, sections 2-13 are heating sections, the temperature is set to 170-210℃, and the screw speed is 100-240rpm.

[0021] In the above technical solution, in step 2, the rapid cooling temperature is 15-30℃, the biaxial stretching ratio is 6-15 times, the stretching temperature is 90-120℃, the extraction solution is dichloromethane, the drying temperature is 40-60℃, and the heat treatment temperature is 70-130℃.

[0022] Another aspect of the present invention includes a natural fiber modified polyethylene separator prepared using the method.

[0023] Another aspect of the invention includes the application of the natural fiber-modified polyethylene separator as a battery separator.

[0024] In another aspect of the present invention, a lithium-ion battery includes a positive electrode, a negative electrode, and a separator, wherein the separator is a polyethylene separator modified with natural fibers.

[0025] Compared with the prior art, the beneficial effects of the present invention are:

[0026] 1. This invention enhances the interfacial compatibility between the fiber and resin matrix by pre-treating natural fibers, thereby enabling the fiber and other reinforcing materials to be uniformly dispersed in the polyethylene melt.

[0027] 2. By adding composite additives with synergistic effects, such as composite antioxidants and composite lubricants, to ensure stability during processing, a natural fiber-reinforced lithium battery separator was finally prepared using a conventional wet-process lithium battery separator production line.

[0028] 3. The natural fiber modified polyethylene diaphragm prepared by this invention has advantages such as high strength, good high-temperature dimensional stability, slightly enhanced liquid absorption and retention capacity, and biodegradability, and has good application prospects and economic benefits. Attached Figure Description

[0029] Figure 1 It is the basic reaction principle equation for sisal fiber surface treatment.

[0030] Figure 2 This is the preparation process of polyethylene separators modified with natural fibers. Detailed Implementation

[0031] The present invention will be further described in detail below with reference to specific embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0032] The equipment used in the specific embodiments of the present invention is as follows:

[0033] Crusher: CR600 model, Weifang Jinghua Powder Engineering Equipment Co., Ltd.;

[0034] Co-rotating twin-screw extruder: CTE 135PLUS model, Coperon (Nanjing) Machinery Co., Ltd. (screw diameter 135, L / D=56);

[0035] Electronic universal testing machine: AGS-X-10KN model, Shimadzu Instruments (Suzhou) Co., Ltd.;

[0036] The relevant pharmaceuticals used in the specific embodiments of this invention are as follows:

[0037] Ultra-high molecular weight polyethylene: Daehan Oil & Chemical (Shanghai) Co., Ltd.;

[0038] Sisal fiber: Guangdong Oriental Sisal Group Co., Ltd.;

[0039] Sodium hydroxide (NaOH): Tianjin Damao Reagent Co., Ltd., analytical grade;

[0040] Antioxidant 1010, Antioxidant 168: BASF (China) Co., Ltd.;

[0041] Natural montmorillonite clay, calcium stearate, zinc stearate: Shanghai Maclean Biochemical Technology Co., Ltd.

[0042] Example 1

[0043] A method for preparing a polyethylene separator modified with natural fibers includes the following steps:

[0044] Step 1, Natural Fiber Pretreatment: Add 100L (100kg) of deionized water, 35kg of NaOH, and 5kg of montmorillonite clay to a 200L mixing tank. Set the mixing temperature to 60℃ and the mixing speed to 50rpm, and mix for 15 minutes. After mixing evenly, add 5kg of sisal fiber, and continue heating and stirring for 45 minutes. Filter the mixture three times, recover the liquid, and dry the solid at 60℃ for 3 hours. Remove the dried material and place it in a pulverizer, then sieve it using a 500-mesh screen to obtain modified sisal fiber. Repeat the above steps to obtain sufficient modified sisal fiber.

[0045] Step 2: Add 75kg white oil, 23kg polyethylene (average molecular weight 1 million), 2kg modified sisal fiber, 50g antioxidant 1010, 50g antioxidant 168, 50g zinc stearate, and 50g calcium stearate to a premixing tank. Set the tank temperature to 80℃ and the stirring speed to 50rpm. After stirring for 20 minutes, a homogeneous suspension is formed. The suspension is then pumped into the extruder feed inlet via a gear pump. The mixture is melted, plasticized, and mixed in a co-rotating twin-screw extruder with a length-to-diameter ratio of 60, a barrel temperature of 170-210℃ (barrel section 2: 170℃, section 3: 190℃, section 4: 210℃, sections 5-13: 195℃), and a screw speed of 200rpm to obtain a homogeneous melt. The melt is then filtered, pumped through a gear pump, and extruded through a T-die to obtain a transparent melt cast sheet. This sheet is then rapidly cooled by a 15℃ cold roller to undergo phase separation, resulting in a cast sheet.

[0046] Step 3: The cast sheet is subjected to asynchronous biaxial stretching, extraction, drying, and heat treatment to obtain a natural fiber modified lithium battery separator. In the asynchronous stretching, the longitudinal stretching ratio is 11 times and the stretching temperature is 110℃, the transverse stretching ratio is 10 times and the stretching temperature is 120℃, the extraction solution is dichloromethane, the drying temperature is 50℃, and the heat treatment temperature is 110℃.

[0047] Based on the above conditions, the modified membrane exhibits the following properties: its strength is approximately 5% higher than the unmodified membrane, with a tensile strength of 2433 kgf / cm². 2 TD tensile strength 2374 kgf / cm 2The needle penetration strength is 0.45 N / μm; the shrinkage at high temperature is reduced by about 10%, the MD shrinkage is 18% at 130℃ for 1 hour, and the TD shrinkage is 4.5%; the wettability is improved, the contact angle is reduced by about 5%, and the contact angle test value is 33°.

[0048] Example 2

[0049] A method for preparing a polyethylene separator modified with natural fibers includes the following steps:

[0050] Step 1, Natural Fiber Pretreatment: Add 100L (100kg) of deionized water, 45kg of NaOH, and 5kg of montmorillonite clay to a 200L mixing tank. Set the mixing temperature to 75℃ and the mixing speed to 100rpm for 15 minutes. After mixing evenly, add 5kg of sisal fiber and continue heating and stirring for 60 minutes. Filter the mixture three times, recovering the liquid and drying the solid at 60℃ for 4 hours. Remove the dried material and place it in a pulverizer, then sieve it using a 500-mesh screen to obtain modified sisal fiber. Repeat the above steps to obtain sufficient modified sisal fiber.

[0051] Step 2: Add 75kg white oil, 20kg polyethylene (average molecular weight 1 million), 5kg modified sisal fiber, 100g antioxidant 1010, 100g antioxidant 168, 100g zinc stearate, and 100g calcium stearate to a premixing tank. Set the tank temperature to 80℃ and the mixing speed to 50rpm. After mixing for 20 minutes, a homogeneous suspension is formed and fed into the extruder feed inlet via a gear pump. The mixture is melted, plasticized, and mixed in a co-rotating twin-screw extruder with a length-to-diameter ratio of 60, a barrel temperature of 170-210℃ (section 2: 170℃, section 3: 190℃, section 4: 210℃, sections 5-13: 195℃), and a screw speed of 200rpm to obtain a homogeneous melt. This melt is then filtered, pumped through a gear pump, and extruded through a T-die to obtain a transparent melt cast sheet. Rapid cooling with a 15℃ cold roller causes phase separation, resulting in a cast sheet.

[0052] Step 3: The cast sheet is subjected to asynchronous biaxial stretching, extraction, drying, and heat treatment to obtain a natural fiber modified lithium battery separator. In the asynchronous stretching, the longitudinal stretching ratio is 11 times and the stretching temperature is 110℃, the transverse stretching ratio is 10 times and the stretching temperature is 120℃, the extraction solution is dichloromethane, the drying temperature is 50℃, and the heat treatment temperature is 110℃.

[0053] Based on the above conditions, the modified membrane exhibits the following properties: overall strength performance is improved by approximately 10% compared to the unmodified test membrane, with an MD tensile strength of 2647 kgf / cm², a TD tensile strength of 2554 kgf / cm², and a needle penetration strength of 0.52 N / μm; shrinkage at high temperatures is reduced by approximately 20%, with an MD shrinkage of 16% and a TD shrinkage of 4% after 1 hour at 130℃; wettability is improved, and the contact angle is reduced by approximately 10%, with a contact angle test value of 30°.

[0054] Example 3

[0055] A method for preparing a polyethylene separator modified with natural fibers includes the following steps:

[0056] Step 1, Natural Fiber Pretreatment: Add 100L (100kg) of deionized water, 40kg of NaOH, and 5kg of montmorillonite clay to a 200L mixing tank. Set the mixing temperature to 65℃ and the mixing speed to 75rpm for 20 minutes. After mixing evenly, add 5kg of sisal fiber and continue heating and stirring for 70 minutes. Filter the mixture three times, recover the liquid, and dry the solid at 60℃ for 4 hours. Remove the dried material and place it in a pulverizer. Sift it through an 800-mesh sieve to obtain modified sisal fiber. Repeat the above steps to obtain sufficient modified sisal fiber.

[0057] Step 2: Add 75kg white oil, 20kg polyethylene (average molecular weight 1 million), 5kg modified sisal fiber, 150g antioxidant 1010, 150g antioxidant 168, 150g zinc stearate, and 150g calcium stearate to a premixing tank. Set the tank temperature to 80℃ and the mixing speed to 50rpm. After mixing for 20 minutes, a homogeneous suspension is formed and fed into the extruder feed inlet via a gear pump. The mixture is melted, plasticized, and mixed in a co-rotating twin-screw extruder with a length-to-diameter ratio of 60, a barrel temperature of 170-210℃ (section 2: 170℃, section 3: 190℃, section 4: 210℃, sections 5-13: 195℃), and a screw speed of 200rpm to obtain a homogeneous melt. This melt is then filtered, pumped through a gear pump, and extruded through a T-die to obtain a transparent melt cast sheet. Rapid cooling with a 15℃ cold roller causes phase separation, resulting in a cast sheet.

[0058] Step 3: The cast sheet is subjected to asynchronous biaxial stretching, extraction, drying, and heat treatment to obtain a natural fiber modified lithium battery separator. In the asynchronous stretching, the longitudinal stretching ratio is 11 times and the stretching temperature is 110℃, the transverse stretching ratio is 10 times and the stretching temperature is 120℃, the extraction solution is dichloromethane, the drying temperature is 50℃, and the heat treatment temperature is 110℃.

[0059] Based on the above conditions, the modified membrane exhibits the following properties: overall strength performance is improved by approximately 20% compared to the unmodified test membrane, with an MD tensile strength of 2890 kgf / cm², a TD tensile strength of 2554 kgf / cm², and a needle penetration strength of 0.56 N / μm; shrinkage at high temperatures is reduced by approximately 40%, with an MD shrinkage of 12% and a TD shrinkage of 3% after 1 hour at 130℃; wettability is improved, and the contact angle is reduced by approximately 20%, with a contact angle test value of 26°.

[0060] The above description is only a preferred embodiment of the present invention. It should be noted that, for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for producing a natural fiber-modified polyethylene separator, characterized by, Includes the following steps: Step 1: Place the sisal fiber in an aqueous solution of strong alkali and natural clay, heat and stir, remove, dry, pulverize and sieve to obtain modified fiber with good compatibility with PE resin matrix. Step 2: The modified fiber, composite antioxidant, and composite lubricant obtained in Step 1 are added to white oil in a predetermined ratio and dispersed to obtain a mixed system. This mixed system is then added to the feed port of a co-rotating twin-screw extruder along with polyethylene. Under the action of high temperature and high shear force in the barrel, the mixture is evenly dispersed. After filtering out impurity particles from the melt, the melt flow rate is metered and controlled, and the mixture is extruded from a T-die. After being extruded through the die, the mixed melt undergoes rapid cooling to achieve phase separation, resulting in a cast sheet. Step 3: The cast sheet obtained in Step 2 is subjected to asynchronous biaxial stretching, extraction, drying, and heat treatment to obtain a polyethylene diaphragm modified with natural fibers. In step 1, the ratio of sisal fiber, strong alkali, natural clay, and water is 1:(7-9):(1-2):20, the strong alkali is NaOH or KOH, the natural clay is natural montmorillonite or kaolin, the heating temperature is 60-75℃, the stirring speed is 50-100 rpm, and the heating and stirring time is 45-90 minutes.

2. The method for producing a natural fiber-modified polyethylene separator according to claim 1, characterized by, In step 1, a filtration device is used for separation, the drying temperature is 60-75℃, the drying time is 3-4 hours, and the material is pulverized using a pulverizer with a screen mesh size of 500-1000 mesh.

3. The method for preparing a natural fiber-modified polyethylene separator according to claim 1, characterized by, In step 2, the polyethylene is high-density polyethylene with an average molecular weight of 500,000 to 2 million, the composite antioxidant is a 1:1 composite antioxidant of antioxidant 1010 and antioxidant 168, and the lubricant is a 1:1 composite lubricant of calcium stearate and zinc stearate.

4. The method for preparing a natural fiber-modified polyethylene separator according to claim 1, characterized by, In step 2, in every 100 parts by weight of the mixed system, the modified fiber, composite antioxidant, composite lubricant, and polyethylene are in the amounts of 1-5, 0.05-0.3, 0.05-0.3, and 15-25 parts by weight, respectively, with the remainder being white oil.

5. The method for preparing a natural fiber-modified polyethylene separator according to claim 1, wherein In step 2, the length-to-diameter ratio of the co-rotating twin-screw extruder is 50-60, the extruder barrel has 13 barrel sections, section 1 is the feed port, sections 2-13 are heating sections, the temperature is set to 170-210℃, and the screw speed is 100-240rpm.

6. The method for preparing a natural fiber-modified polyethylene separator according to claim 1, wherein In step 2, the rapid cooling temperature is 15–30°C, the biaxial stretching ratio is 6–15 times, the stretching temperature is 90–120°C, the extraction solution is dichloromethane, the drying temperature is 40–60°C, and the heat treatment temperature is 70–130°C.

7. A natural fiber modified polyethylene diaphragm obtained by the preparation method according to any one of claims 1-6.

8. The application of the natural fiber modified polyethylene separator as described in claim 7 as a battery separator.

9. A lithium-ion battery, comprising a positive electrode, a negative electrode, and a separator, characterized in that, The diaphragm is a natural fiber modified polyethylene diaphragm as described in claim 8.

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