A solid-state electrolyte modified wet polyethylene separator and a method for preparing the same

Abstract

The application discloses a kind of solid electrolyte modified wet polyethylene diaphragm and preparation method thereof, belong to energy storage material technical field.The application is with ultra-high molecular weight polyethylene and high molecular weight polyethylene as matrix, and lithium titanium phosphate aluminum LATP solid electrolyte is modified auxiliary agent, cooperate high activity dispersing agent and white oil pore-forming agent, by premixing, double screw extrusion, two-stage stretching, dichloromethane extraction and heat setting, and the modified diaphragm is obtained.The diaphragm is formed by LATP occupation Large-pore pore structure, with high electrolyte wettability and high ionic conductivity, can improve lithium ion transmission rate, form uniform SEI layer and inhibit dendrite generation, adapt high specific capacity, fast charge-discharge lithium ion battery.

Description

Technical Field

[0001] This invention relates to the field of energy storage materials technology, and in particular to a solid electrolyte modified wet-process polyethylene diaphragm and its preparation method. Background Technology

[0002] The separator is a crucial component of a battery. Its low electronic conductivity prevents direct contact between the positive and negative electrodes, while its high ionic conductivity allows lithium ions to pass between them through the microporous structure, facilitating charging and discharging. Its excellent strength also helps prevent the formation of metal dendrites on the electrode surfaces, ensuring safety. The performance of the separator is critical for the development of novel high-capacity, fast-charge-discharge batteries. Traditional wet-process polyethylene separators suffer from low affinity for the electrolyte and poor ionic conductivity, hindering the development of new high-capacity, fast-charge-discharge batteries.

[0003] Currently, wet-process polyethylene separators have the following disadvantages: First, polyethylene separators have poor electrolyte wettability, resulting in uneven distribution of lithium ions on the separator surface. This makes it difficult to form a stable and uniform SEI layer during battery cycling, leading to concentrated dendrite regions on the electrode surface and causing safety issues. Second, the pore structure of polyethylene separators is mainly micropores, which hinder the rapid transport of lithium ions and make it difficult to meet the requirements of rapid charge and discharge. Summary of the Invention

[0004] The purpose of this invention is to provide a solid electrolyte modified wet-process polyethylene separator and its preparation method. The solid electrolyte is used as an additive in the extrusion production of wet-process lithium-ion battery separators. As an inorganic filler, the solid electrolyte can occupy the space in the separator, serve as a supporting skeleton to improve the mechanical strength of the separator, improve the pore structure of the separator, enhance the wettability of the electrolyte, optimize the uniformity of the electric field on the surface of the separator during battery operation, and form a uniform SEI layer. At the same time, the solid electrolyte has a high ionic conductivity and can serve as a lithium ion transport medium to improve the electrochemical performance of the separator.

[0005] To achieve the above objectives, the present invention provides a method for preparing a solid electrolyte modified wet-process polyethylene membrane, comprising the following steps: S1. Raw material preparation: Polyethylene is used as the matrix, solid electrolyte powder is added as a modifier, and pore-forming agent and dispersant are compounded. S2. Premixing: First, mix the solid electrolyte powder with the pore-forming agent, add the dispersant and stir until uniform and without sedimentation, then add polyethylene and continue stirring to obtain the mixed raw material; S3. Extrusion: The mixed raw materials are fed into a twin-screw extruder for melt extrusion to obtain a cast film. The cast film is then rapidly cooled by cold rollers to undergo phase separation, resulting in a cast film. S4. Stretching and Shaping: The casting sheet is stretched longitudinally and then stretched laterally for the first time. S5. Extraction: Use an extractant to rinse and remove the pore-forming agent from the diaphragm; S6. Heat treatment for shaping: The extracted membrane is subjected to a second transverse stretching, followed by heat shaping to obtain a solid electrolyte modified wet-process polyethylene membrane.

[0006] In an optional embodiment, in S1, the solid electrolyte powder is lithium titanium aluminum phosphate (LATP) powder.

[0007] In an optional embodiment, in S1, the polyethylene includes ultra-high molecular weight polyethylene and high molecular weight polyethylene; the ultra-high molecular weight polyethylene has a viscosity-average molecular weight of 1.8 million to 3 million, and the high molecular weight polyethylene has a viscosity-average molecular weight of 500,000 to 1 million.

[0008] In one optional embodiment, the polyethylene has a purity of 99%; and the amount of LATP powder added is 20wt% to 60wt%.

[0009] In one optional embodiment, the pore-forming agent is white oil, and the dispersant is YY-5021 high-activity filler-coated dispersant; the mass ratio of the dispersant to LATP powder is 1:30.

[0010] In an optional embodiment, in S2, the stirring speed is 500~800 rpm, the stirring ambient temperature is room temperature, the stirring time of the solid electrolyte powder and the pore-forming agent is 30 min, and stirring continues for 1 h after adding polyethylene.

[0011] In an optional embodiment, in S3, the feed rate of the twin-screw extruder is 250 kg / h to 500 kg / h, the extrusion speed is 70 r / min to 200 r / min, the melt temperature is 215°C to 230°C, and the extrusion temperature is 160°C to 230°C; the ratio of feed rate to extrusion speed is 2.8 to 3.6.

[0012] In an optional embodiment, in S4, the longitudinal stretching ratio is 9 to 12 times, the longitudinal stretching preheating temperature is 80°C to 90°C, the longitudinal stretching temperature is 90°C to 100°C, and the longitudinal stretching preheating temperature is lower than the longitudinal stretching temperature. The first transverse stretching ratio is 7 to 16 times, the preheating temperature for the first transverse stretching is 105℃ to 125℃, and the stretching temperature is 95℃ to 120℃. The preheating temperature for the first transverse stretching is higher than the temperature for the first transverse stretching.

[0013] In an optional embodiment, in S6, the second transverse stretching ratio is 1.2 to 1.8 times, the heat setting temperature is 120°C to 135°C, and the heat setting time is 20 to 30 seconds.

[0014] The present invention also provides a solid electrolyte modified wet-process polyethylene diaphragm, which is prepared by the aforementioned preparation method; the diaphragm uses polyethylene as a matrix and is doped with solid electrolyte, and the interior has a large-pore structure formed by the solid electrolyte occupying sites, which has high electrolyte wettability and high ionic conductivity.

[0015] Therefore, the present invention employs the above-mentioned solid electrolyte modified wet-process polyethylene diaphragm and its preparation method, which has the following technical advantages: 1) Improves the problem of poor wettability of traditional wet-process PE separators. Solid electrolyte is distributed as particles on the surface of the base membrane, which improves the liquid absorption and retention rate, optimizes the electrode / separator interface, and improves battery performance.

[0016] 2) Improve the ionic conductivity of traditional wet-process PE separators. Solid electrolytes, as the medium for lithium-ion transport, significantly improve the transport rate when the electrolyte passes through the separator and migrates between the positive and negative electrodes, thereby accelerating the charging and discharging efficiency and improving battery performance.

[0017] 3) Optimize the surface morphology of traditional wet-process PE separators. Traditional wet-process PE separators are mainly composed of micropores. The addition of solid electrolytes plays a role in occupying space and forming a large-pore structure, which helps to improve the lithium-ion transport rate, accelerate the charging and discharging efficiency, and improve battery performance.

[0018] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0019] Figure 1 This is a flowchart illustrating the preparation process of the solid electrolyte modified wet-process polyethylene diaphragm according to an embodiment of the present invention; Figure 2 This is a SEM image of the diaphragm sample obtained by the present invention; Figure 3 This is a contact angle diagram of the diaphragm sample of the present invention. Detailed Implementation

[0020] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0021] The following embodiments are provided to better understand the present invention and are not limited to the described embodiments. They do not constitute a limitation on the content and scope of protection of the present invention. Any product that is the same as or similar to the present invention, derived by any person under the guidance of the present invention or by combining the features of the present invention with other prior art, falls within the protection scope of the present invention.

[0022] For experiments not specifically described in the examples, the procedures or conditions should be followed according to the conventional experimental procedures described in the literature in this field. Reagents or instruments whose manufacturers are not specified are all commercially available products.

[0023] The raw materials used in the following embodiments and comparative examples of this invention are as follows: Ultra-high molecular weight polyethylene (viscosity-average molecular weight 2 million, purity 99%) serves as the basic framework of the diaphragm, improving its mechanical strength. High molecular weight polyethylene (viscosity-average molecular weight 800,000, purity 99%) improves the plasticity of the raw materials, enhances the plasticizing effect, and solves the problem of difficult extrusion of ultra-high molecular weight polyethylene mixed with inorganic particles. Lithium aluminum titanium phosphate (LATP) solid electrolyte (50-100 nm) is used as a modifying agent in the extrusion of the diaphragm casting, optimizing the pore structure of the diaphragm, improving the wettability and ionic conductivity of the electrolyte, occupying sites to form large-diameter channels, and simultaneously serving as a supporting framework to strengthen mechanical properties. YY-5021 high-activity filler coating dispersant is used to break up LATP powder agglomeration, improve the dispersion uniformity of LATP in white oil and polyethylene, and enhance system compatibility. White oil serves as a pore-forming agent in the wet process, providing a basis for the micropore formation of the diaphragm. Dichloromethane is used as an extractant to wash away the white oil pore-forming agent in the diaphragm. All the above raw materials and reagents are commercially available conventional raw materials. Equipment used: twin-screw extruder, stretching machine, extraction equipment, heat setting equipment, Mahr thickness gauge, contact angle measuring instrument, electrochemical workstation, etc.

[0024] Example 1 A method for preparing a solid electrolyte modified wet-process polyethylene membrane, the overall process is as follows: Figure 1 As shown, the specific steps are as follows: S1. Raw material preparation: By mass percentage, 16wt% ultra-high molecular weight polyethylene, 4wt% high molecular weight polyethylene, 10wt% solid electrolyte LATP powder, and the remainder is white oil; the mass ratio of dispersant YY-5021 to LATP powder is 1:30.

[0025] S2. Premixing: Add LATP powder and white oil to a buffer tank, add dispersant, set the speed to 600 rpm, and stir at room temperature for 30 minutes until uniform and without sedimentation; then add the two types of polyethylene powder, and continue stirring at 600 rpm for 1 hour to obtain the mixed raw materials.

[0026] S3. Extrusion: The mixed raw materials are fed into a twin-screw extruder at a feed rate of 300 kg / h, an extrusion speed of 200 r / min, a melt temperature of 220℃, and an extrusion temperature of 170~210℃. Specifically, the length-to-diameter ratio of the extruder barrel is 60, and the temperature is 170~210℃ (barrel section 2: 170℃, section 3: 190℃, section 4: 210℃, and sections 5-13: 195℃). The extrusion produces a transparent melt cast film. When the film is rapidly cooled by a 15℃ cold roller, phase separation occurs, resulting in a cast sheet.

[0027] S4. Stretching and Shaping: The casting sheet is first stretched longitudinally, with a preheating temperature of 85℃, a stretching temperature of 95℃, and a stretching ratio of 11 times; then it is stretched laterally for the first time, with a preheating temperature of 115℃, a stretching temperature of 110℃, and a stretching ratio of 10 times.

[0028] S5. Extraction: The diaphragm is flushed with dichloromethane to remove the pore-forming agent, white oil.

[0029] S6. Heat treatment for shaping: After extraction, the membrane is stretched laterally for the second time with a stretching ratio of 1.5 times; then heat-set at 125℃ for 25s and dry at 50℃ to obtain a solid electrolyte modified wet-process polyethylene membrane.

[0030] Example 2 The only difference from Example 1 is that the amount of LATP solid electrolyte added is 20wt%, ultra-high molecular weight polyethylene is 16wt%, high molecular weight polyethylene is 4wt%, and the remainder is white oil; the functions of each raw material remain unchanged, and the remaining preparation steps and process parameters are exactly the same.

[0031] Example 3 The only difference from Example 1 is that the amount of LATP solid electrolyte added is 50wt%, ultra-high molecular weight polyethylene is 16wt%, high molecular weight polyethylene is 4wt%, and the remainder is white oil; the functions of each raw material remain unchanged, and the remaining preparation steps and process parameters are exactly the same.

[0032] Comparative Example 1 No LATP solid electrolyte was added. The remaining raw materials and functions were: 16 wt% ultra-high molecular weight polyethylene, 4 wt% high molecular weight polyethylene, and the remainder white oil. The preparation steps and parameters were the same as in Example 1.

[0033] Comparative Example 2 The LATP solid electrolyte was replaced with alumina (20 wt%), ultra-high molecular weight polyethylene was 16 wt%, high molecular weight polyethylene was 4 wt%, and the remainder was white oil; the remaining steps were the same as in Example 1.

[0034] Comparative Example 3 Silica was used to replace the LATP solid electrolyte (20 wt%) as an inorganic filler; 16 wt% ultra-high molecular weight polyethylene, 4 wt% high molecular weight polyethylene, and the remainder white oil were used; the remaining steps were the same as in Example 1.

[0035] Performance test description: (a) Testing methods Thickness testing: A Mar thickness gauge was used. Static electricity on the sample was removed by an ion blower, and the instrument was calibrated and zeroed before testing.

[0036] Air permeability test: The average time t for 100 mL of gas to pass through the diaphragm is measured, and the unit is s / 100 mL.

[0037] Porosity test: Sample size 4cm × 6cm, polyethylene density 0.955g / cm³ 3 Calculation formula: Porosity = (1 - 10 × membrane weight / 0.955 / (6 × 4 × membrane thickness)) × 100%.

[0038] Pore ​​size test: The average pore size of the diaphragm was measured using the bubble compression method.

[0039] Ionic conductivity test: The resistance of the membrane in 1 mol / L LiPF6 electrolyte (DMC:EC:EMC=1:1:1) was tested using the AC impedance method; the calculation formula is: σ=d / (R×S) (d=membrane thickness, R=impedance resistance, S=effective contact area).

[0040] Wettability test: The electrolyte is dropped onto the diaphragm surface using the seat drop method, and the contact angle is recorded (the smaller the contact angle, the better the wettability).

[0041] (II) Test Results The test results are shown in the table below. The addition of LATP effectively improves the ionic conductivity of the finished membrane. The ionic conductivity of LATP10, LATP20, and LATP50 is 0.037, 0.079, and 0.103 mS / cm higher than that of the membrane without LATP, respectively. This is because LATP effectively accelerates the migration of lithium ions. The physical properties of the membrane also change significantly. After adding LATP, the porosity, average pore size, and thickness of the membrane increase. The porosity of LATP10, LATP20, and LATP50 is 2.8%, 5%, and 6.7% higher than that of the membrane without LATP (Comparative Example 1), respectively; the average pore size is 6 nm, 12 nm, and 19 nm higher than that of the membrane without LATP, respectively. As the proportion of LATP increases, the membrane thickness also increases. These results demonstrate the space-occupying effect of LATP during the polyethylene extrusion film formation process. LATP induces more porous structures and larger pore size distribution in the membrane. The data from Comparative Examples 2 and 3 further illustrate that the addition of inorganic particles does indeed improve the porosity and pore size of the membrane, but LATP exhibits higher particle conductivity compared to alumina and silica materials.

[0042] Table 1 Performance Test Results

[0043] In addition, SEM electron microscopy tests were performed on the diaphragm. Figure 2 ) and electrolyte contact angle test ( Figure 3 ). Figure 2 In the image, a, b, and c are optical photographs of the surface of a polyethylene (PE) extruded sheet, respectively. Figure 2Photo a is a typical image of PE-LATP0, showing the surface of the cast sheet as pure white PE. Figure 2 b、 Figure 2 c shows typical photographs of PE-LATP20 and PE-LATP50. It can be observed that the surface of the casting is dark yellow with LATP electrolyte, and the color of PE-LATP50 is darker than that of PE-LATP20, indicating that there is more LATP in the PE-LATP50 casting. Figure 2 In the image, d, e, and f are SEM images of the surface of the finished membrane, respectively. Figure 2 Figures g, h, and i show cross-sectional SEM images of the finished membranes. Figures d, e, and f show that, compared to the membrane without LATP (Figure d), the LATP on the surface of the PE-LATP20 and PE-LATP50 membranes is aggregated, with noticeable upward protrusions at these aggregation points (Figures e and f). The protrusions are even larger on membranes with higher LATP content. Combined with the SEM cross-sectional images, it can be seen that the LATP aggregation points create cavities within the membrane (Figures h and i). The higher the LATP content, the more cavities are created, which corroborates the conclusions in Table 1 and explains how the addition of LATP improves the porosity and pore structure of the membrane.

[0044] To verify the beneficial effect of LATP addition on the electrolyte wettability of the separator, electrolyte contact angle tests were conducted on the separators. The contact angles of PE-LATP0, PE-LATP20, and PE-LATP50 were 40.4°, 36.9°, and 34.1°, respectively. This indicates that the addition of LATP effectively improves electrolyte wettability. Good wettability ensures uniform distribution of the electrolyte on the separator surface, which reduces impedance when the separator is applied to batteries in the future. The uniformly distributed electrolyte optimizes the electrode / electrolyte interface and reduces the formation of lithium dendrites.

[0045] Therefore, the present invention adopts the above-mentioned solid electrolyte modified wet polyethylene separator and its preparation method. By using LATP solid electrolyte modified wet polyethylene separator, the wet polyethylene separator improves the wettability and ionic conductivity of electrolyte, optimizes the pore structure to form large-diameter channels, accelerates lithium-ion transport to meet the fast charging requirements, can form a uniform SEI layer, suppress dendrite formation, improve battery safety, and enhance the mechanical strength of the separator.

[0046] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method for preparing a solid electrolyte-modified wet-process polyethylene diaphragm, characterized in that, Includes the following steps: S1. Raw material preparation: Polyethylene is used as the matrix, solid electrolyte powder is added as a modifier, and pore-forming agent and dispersant are compounded. S2. Premixing: First, mix the solid electrolyte powder with the pore-forming agent, add the dispersant and stir until uniform and without sedimentation, then add polyethylene and continue stirring to obtain the mixed raw material; S3. Extrusion: The mixed raw materials are fed into a twin-screw extruder for melt extrusion to obtain a cast film. The cast film is then rapidly cooled by cold rollers to undergo phase separation, resulting in a cast film. S4. Stretching and Shaping: The casting sheet is stretched longitudinally and then stretched laterally for the first time. S5. Extraction: Use an extractant to rinse and remove the pore-forming agent from the diaphragm; S6. Heat treatment for shaping: The extracted membrane is subjected to a second transverse stretching, followed by heat shaping to obtain a solid electrolyte modified wet-process polyethylene membrane.

2. The preparation method according to claim 1, characterized in that, In S1, the solid electrolyte powder is lithium titanium aluminum phosphate (LATP) powder.

3. The preparation method according to claim 2, characterized in that, In S1, the polyethylene includes ultra-high molecular weight polyethylene and high molecular weight polyethylene; the viscosity-average molecular weight of the ultra-high molecular weight polyethylene is 1.8 million to 3 million, and the viscosity-average molecular weight of the high molecular weight polyethylene is 500,000 to 1 million.

4. The preparation method according to claim 3, characterized in that, The polyethylene has a purity of 99%; the amount of LATP powder added is 20wt%~60wt%.

5. The preparation method according to claim 2, characterized in that, The pore-forming agent is white oil, and the dispersant is YY-5021 high-activity filler-coated dispersant; the mass ratio of the dispersant to LATP powder is 1:

30.

6. The preparation method according to claim 1, characterized in that, In S2, the stirring speed is 500~800 rpm and the stirring environment temperature is room temperature; the stirring time of solid electrolyte powder and pore-forming agent is 30 min, and stirring continues for 1 h after adding polyethylene.

7. The preparation method according to claim 1, characterized in that, In S3, the feed rate of the twin-screw extruder is 250kg / h~500kg / h, the extrusion speed is 70r / min~200r / min, the melt temperature is 215℃~230℃, and the extrusion temperature is 160℃~230℃; the ratio of feed rate to extrusion speed is 2.8~3.

6.

8. The preparation method according to claim 1, characterized in that, In S4, the longitudinal stretching ratio is 9 to 12 times, the longitudinal stretching preheating temperature is 80℃ to 90℃, the longitudinal stretching temperature is 90℃ to 100℃, and the longitudinal stretching preheating temperature is lower than the longitudinal stretching temperature. The first transverse stretching ratio is 7 to 16 times, the preheating temperature for the first transverse stretching is 105℃ to 125℃, and the stretching temperature is 95℃ to 120℃. The preheating temperature for the first transverse stretching is higher than the temperature for the first transverse stretching.

9. The preparation method according to claim 1, characterized in that, In S6, the second transverse stretching ratio is 1.2 to 1.8 times, the heat setting temperature is 120℃ to 135℃, and the heat setting time is 20s to 30s.

10. A solid electrolyte modified wet-process polyethylene separator, characterized in that, The membrane is prepared by the preparation method according to any one of claims 1 to 9; the membrane is based on polyethylene and doped with solid electrolyte, and the interior is formed by the solid electrolyte occupying the sites to form a large-pore structure, which has high electrolyte wettability and high ionic conductivity.

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