Lithium ion battery diaphragm and preparation method thereof

Abstract

The invention discloses a lithium ion battery diaphragm and a preparation method thereof, the lithium ion battery diaphragm comprises a base membrane and a polyester type resin binder loaded on the base membrane, the polyester type resin binder comprises a polymer and an additive, the polymer is formed by polymerizing carboxyl-terminated polyester, carbon dioxide-based polyester diol and modified diol, and the additive is an additive. The additive is a mixture of a plasticizer and an antioxidant. The lithium ion battery diaphragm provided by the invention contains the polyester resin binder, and when the lithium ion battery diaphragm is prepared into a battery, a cell is compacted by cold pressing instead of hot pressing in the prior art, so that the adhesion of the diaphragm and a pole piece can be realized at room temperature, and the energy consumption in the production process is effectively reduced.

Description

Technical Field

[0001] The present invention belongs to the technical field of batteries, and specifically relates to a lithium-ion battery separator and a preparation method thereof. Background Art

[0002] With the development of new energy vehicles, the requirements for long endurance and energy density of power batteries are also getting higher and higher. As one of the key components of the battery, the separator should prevent the positive and negative electrodes from contacting while allowing ions to migrate between the positive and negative electrodes. As the battery continuously cycles, the battery core will continuously expand, resulting in misalignment between the separator and the electrode sheet or poor wrapping of the electrode sheet, increasing potential safety hazards. In order to improve the bonding performance between the positive and negative electrode sheets, glue is coated on the separator, and the formed glue layer can keep good bonding performance between the separator and the positive and negative electrode sheets, effectively suppressing the expansion of the battery core, avoiding misalignment between the separator and the electrode sheet, and reducing the risk of battery short circuit. The prior art uses a hot pressing process at 60-80 °C to bond the separator and the electrode sheet. However, this method not only consumes energy and time, increases the preparation cost of the battery core, but also does not significantly improve the bonding strength between the separator and the electrode sheet. Summary of the Invention

[0003] Aiming at the deficiencies of the prior art, the purpose of the present invention is to provide a lithium-ion battery separator.

[0004] Another purpose of the present invention is to provide a preparation method of the above-mentioned lithium-ion battery separator.

[0005] Another purpose of the present invention is to provide a battery.

[0006] The purpose of the present invention is achieved by the following technical solutions.

[0007] A lithium-ion battery separator includes: a base film and a polyester resin binder loaded on the base film.

[0008] The polyester resin binder includes: a polymer and an additive. By mass, the ratio of the polymer to the additive is (30-90):(0.1-10). The polymer is polymerized from carboxyl-terminated polyester, carbon dioxide-based polyester diol, and modified diol. By mole, the ratio of carboxyl-terminated polyester, carbon dioxide-based polyester diol, and modified diol is (1-2):(1-4):(1-2). The additive is a mixture of a plasticizer and an antioxidant.

[0009] In the above technical solution, the method for preparing the polyester resin binder includes: mixing the polymer and the additive until uniform to obtain the polyester resin binder.

[0010] In the above technical solution, by mass parts, the ratio of the plasticizer to the antioxidant is 9:1. The plasticizer is tributyl citrate (TBC), and the antioxidant is butylated hydroxytoluene (BHT).

[0011] In the above technical solution, the carboxyl-terminated polyester is polybutylene succinate, and the number-average molecular weight of polybutylene succinate is 1500 - 3500 g / mol.

[0012] In the above technical solution, the carbon dioxide-based polyester diol is poly(propylene carbonate) diol, and the number-average molecular weight of poly(propylene carbonate) diol is 1000 - 2000 g / mol.

[0013] In the above technical solution, the modified diol is obtained by modifying diethanolamine with butyl acrylate.

[0014] In the above technical solution, the method for preparing the polymer includes: mixing the carboxyl-terminated polyester, the carbon dioxide-based polyester diol, and the modified diol at 25 - 30 °C until homogeneous, heating to 130 - 170 °C, adding a catalyst, and performing vacuum polycondensation at 130 - 170 °C for 2 - 5 h, then cooling to room temperature to obtain the polymer. By mole parts, the ratio of the carboxyl-terminated polyester, the carbon dioxide-based polyester diol, and the modified diol is (1 - 2):(1 - 4):(1 - 2).

[0015] In the above technical solution, the vacuum polycondensation is carried out at 1 - 100 Pa (absolute pressure).

[0016] In the above technical solution, the catalyst is tetrabutyl titanate, and the mass of the catalyst is 3‰ - 5‰ of the total mass of the carboxyl-terminated polyester, the carbon dioxide-based polyester diol, and the modified diol.

[0017] In the above technical solution, the synthesis method of the modified diol includes: mixing diethanolamine and a third solvent, then dropping butyl acrylate into the mixed system of diethanolamine and the third solvent, reacting at 60 - 90 °C for 10 - 20 h under a nitrogen or inert gas atmosphere, performing extraction, and then performing vacuum distillation to obtain the modified diol. By mole parts, the ratio of diethanolamine to butyl acrylate is (1 - 2):2.

[0018] In the above technical solution, the third solvent is N,N-dimethylformamide (DMF).

[0019] In the above technical solution, by mole parts, the ratio of the sum of diethanolamine and butyl acrylate to the third solvent is 1:(5 - 10).

[0020] In the above technical solution, both the first solvent and the second solvent are water.

[0021] A method for preparing a lithium-ion battery separator, comprising: coating a coating liquid on at least one side of a base film, drying, and obtaining a glue-coated layer on the base film to obtain a lithium-ion battery separator, wherein the coating liquid comprises: a polyester resin binder.

[0022] In the above technical solution, the coating liquid is one of a first slurry, a second slurry, and a third slurry;

[0023] The first slurry is a polyester resin binder;

[0024] The second slurry comprises: a polyester resin binder, a first solvent, and a first polyacrylic acid. In the second slurry, by mass, the ratio of the polyester resin binder, the first solvent, and the first polyacrylic acid is (20-50):(40-70):(3-15);

[0025] The third slurry comprises: a polyester resin binder, a second solvent, polyvinylidene fluoride (PVDF), a thickening agent, a second polyacrylic acid, and a dispersant. In the third slurry, by mass, the ratio of the polyester resin binder, the second solvent, polyvinylidene fluoride (PVDF), the thickening agent, the second polyacrylic acid, and the dispersant is (10-50):(30-60):(4-9):(5-10):(5-15):(0.002-0.005);

[0026] In the above technical solution, the thickening agent is sodium carboxymethyl cellulose, and the dispersant is stearamide.

[0027] In the above technical solution, the coating method is dot coating.

[0028] In the above technical solution, the drying temperature is 40-80 °C.

[0029] A battery, comprising a lithium-ion battery separator.

[0030] Use of a polymer in improving the adhesion between the separator and the electrode in a battery.

[0031] Use of a polymer as a binder between the separator and the electrode in a battery.

[0032] In the above technical solution, reduce the temperature during compaction between the separator and the electrode.

[0033] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0034] The lithium-ion battery separator of the present invention contains a polyester resin binder. When preparing the battery, cold pressing (room temperature pressing) is used for compacting the battery core instead of hot pressing in the prior art. The present invention can achieve the bonding between the separator and the electrode at room temperature, effectively reducing the energy consumption in the production process; the coating is applied by dot coating, making the separator thickness, coating amount and coverage uniform and controllable, and the adhesion force at each position is uniform during the pressing process of the battery core, avoiding the misalignment of the electrode and the separator during the charge and discharge process, and enhancing the safety performance of the battery.

[0035] The cold pressing process can be used in the present invention to closely bond the battery electrode and the separator, solving the problem that the separator and the electrode can only be bonded after hot pressing, improving the adhesion force (electrode bonding strength) between the separator and the electrode, ensuring the safety during the operation of the battery and improving the production efficiency. Description of the Drawings

[0036] Figure 1 SEM image of the lithium-ion battery separator prepared in Example 1;

[0037] Figure 2 SEM image of the lithium-ion battery separator prepared in Example 3;

[0038] Figure 3 SEM image of the lithium-ion battery separator prepared in Comparative Example 1. Detailed Embodiments

[0039] The technical solutions of the present invention will be further described below with specific embodiments.

[0040] In the following embodiments, the base film is a PE film with a thickness of 7 microns.

[0041] In the following embodiments, the modified diol is obtained by modifying diethanolamine with butyl acrylate. The synthesis method of the modified diol includes: adding diethanolamine to a third solvent (DMF), then dropping butyl acrylate into the mixed system of diethanolamine and the third solvent, reacting at 70 °C for 15 h under an argon atmosphere, extracting with ethyl acetate, and distilling off DMF under reduced pressure to obtain the modified diol. By mole fraction, the ratio of diethanolamine to butyl acrylate is 2:2, and by mole fraction, the ratio of the sum of the amounts of diethanolamine and butyl acrylate to DMF is 1:5.

[0042] The conventional PVDF slurries used in Comparative Examples 1 to 3 were the same. The method for preparing the conventional PVDF slurry was as follows: PVDF, water, sodium carboxymethyl cellulose, polyacrylic acid, and stearamide were mixed in a double-planet high-speed disperser and stirred at a rotation speed of 200 r / min and a revolution speed of 40 r / min for 120 min until uniform to obtain the conventional PVDF slurry. Among them, by mass, the ratio of PVDF, water, sodium carboxymethyl cellulose, polyacrylic acid, and stearamide was 10:60:10:5:0.003.

[0043] The positive electrode sheet includes a positive electrode current collector (aluminum foil) and a positive electrode coating loaded on the positive electrode current collector. The positive electrode coating includes: lithium iron phosphate, a conductive agent (Super P), and a binder (PVDF). By mass, the ratio of lithium iron phosphate, the conductive agent (Super P), and the binder (PVDF) is 8:1:1.

[0044] The negative electrode sheet includes a negative electrode current collector (copper foil) and a negative electrode coating loaded on the negative electrode current collector. The negative electrode coating includes: graphite, a conductive agent (Super P), and a binder (the binder is a mixture of sodium carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR)). By mass, the ratio of graphite, the conductive agent (Super P), CMC, and SBR is 100:1:2:3.

[0045] The bonding strength of the electrode sheet (the bonding strength between the separator and the positive electrode sheet): The separator and the positive electrode sheet were both cut into standard sheets of 2 cm * 15 cm, stacked together, and pressed at a pressure of 900 kg for 120 s at T °C. Then, the upper clamp of the tensile machine was clamped to the separator, and the lower clamp of the tensile machine was clamped to the positive electrode sheet, and peeled at a speed of 300 mm / s (the effective length of peeling was 20 mm, and the peeling degree was 180°) to obtain the bonding strength of the electrode sheet (N / m). When testing the bonding strength of the electrode sheets of Examples 1 to 3, the separator was one of the lithium-ion battery separators prepared in Examples 1 to 3, and T °C = 25 °C. When testing the bonding strength of the electrode sheets of Comparative Examples 1 to 3, the separator was one of the lithium-ion battery separators prepared in Comparative Examples 1 to 3, and T °C = 80 °C.

[0046] Capacity retention rate: The battery was charged to 3.8 V at a current of 0.5C (equivalent to 0.5 times the battery capacity), and then discharged to 2.0 V at the same current. The discharge capacity was recorded as C1; the constant-current charge and discharge were repeated 500 times according to the above steps, and the 500th discharge capacity was recorded as C500. (C500 / C1) × 100% was the capacity retention rate after 500 cycles.

[0047] Coating coverage: Cut the separator into a 40mm * 40mm sample piece (coating points are formed on the separator, the coating points are circular, and the diameter of the coating points is 400 - 500μm). Fix the sample piece under the lens of the Keyence microscope stage. Adjust the objective magnification to 50 times. The ratio of the sum of the areas of all coating points within the lens shooting area to the total area of the lens shooting area is the coating coverage.

[0048] The electrolyte is a mixture of an electrolyte and a solvent. The electrolyte is LiPF6, and the solvent in the electrolyte is ethylene carbonate and diethyl carbonate. By mass fraction, the ratio of LiPF6, ethylene carbonate, and diethyl carbonate is 3:5:12.

[0049] Example 1

[0050] A method for preparing a lithium-ion battery separator includes: coating a coating liquid on one side of a base film by dot coating (coating rate is 100m / min), drying in an oven at 60°C for 2min, obtaining a glue coating layer with a thickness of 2.6μm on the base film, and obtaining a lithium-ion battery separator. The coating liquid is a first slurry, and the first slurry is a polyester-type resin binder. The polyester-type resin binder includes: a polymer and an additive. By mass fraction, the ratio of the polymer to the additive is 90:10. The polymer is polymerized from a carboxyl-terminated polyester, a carbon dioxide-based polyester diol, and a modified diol. By mole fraction, the ratio of the carboxyl-terminated polyester, the carbon dioxide-based polyester diol, and the modified diol is 1:3:2. The carboxyl-terminated polyester is polybutylene succinate (number-average molecular weight is 3200g / mol), and the carbon dioxide-based polyester diol is poly(propylene carbonate) diol (molecular weight is 1800g / mol).

[0051] The method for preparing the polyester-type resin binder includes: mixing the polymer and the additive until uniform to obtain the polyester-type resin binder. Among them, the additive is a mixture of a plasticizer and an antioxidant. The plasticizer is tributyl citrate, and the antioxidant is butylated hydroxytoluene. By mass fraction, in the additive, the ratio of the plasticizer to the antioxidant is 9:1.

[0052] The method for preparing the polymer is: In a reaction kettle, mix the carboxyl-terminated polyester (improving the cohesive strength) as the hard segment, the carbon dioxide-based polyester diol as the soft segment, and the modified diol with the function of reducing the glass transition temperature at 25°C, stir at a speed of 100rpm for 30min until uniform, heat to 160°C, add a catalyst, perform vacuum polycondensation at 160°C (absolute pressure is 50Pa) for 3.5h, and cool to room temperature to obtain the polymer. The catalyst is tetrabutyl titanate, and the mass of the catalyst is 4‰ of the total mass of the carboxyl-terminated polyester, the carbon dioxide-based polyester diol, and the modified diol.

[0053] Example 2

[0054] A method for preparing a lithium-ion battery separator includes: coating a coating liquid on one side of a base film by dot coating (coating rate: 100 m / min), drying it in an oven at 60 °C for 2 min, obtaining a coated glue layer with a thickness of 1.6 μm on the base film, and obtaining the lithium-ion battery separator. The coating liquid is a second slurry, and the second slurry is a mixture of a polyester resin binder prepared in Example 1, a first solvent (water), and a first polyacrylic acid. In the second slurry, by mass, the ratio of the polyester resin binder, the first solvent, and the first polyacrylic acid is 30:60:7. The method for preparing the second slurry is: mixing the polyester resin binder prepared in Example 1, the first solvent, and the first polyacrylic acid in a double-planet high-speed disperser, stirring at a rotation speed of 200 r / min and a revolution speed of 40 r / min for 30 min until uniform to obtain the second slurry.

[0055] Example 3

[0056] A method for preparing a lithium-ion battery separator includes: coating a coating liquid on one side of a base film by dot coating (coating rate: 100 m / min), drying it in an oven at 60 °C for 2 min, obtaining a coated glue layer with a thickness of 5.5 μm on the base film, and obtaining the lithium-ion battery separator. The coating liquid is a third slurry, and the third slurry is a mixture of a polyester resin binder prepared in Example 1, a second solvent (water), polyvinylidene fluoride (PVDF), a thickening agent, a second polyacrylic acid, and a dispersant. The thickening agent is sodium carboxymethyl cellulose, and the dispersant is stearamide. By mass, the ratio of the polyester resin binder, the second solvent, polyvinylidene fluoride (PVDF), the thickening agent, the second polyacrylic acid, and the dispersant is 20:60:8:6:6:0.003. The method for preparing the third slurry is: mixing the polyester resin binder prepared in Example 1, the second solvent, polyvinylidene fluoride (PVDF), the thickening agent, the second polyacrylic acid, and the dispersant in a double-planet high-speed disperser, stirring at a rotation speed of 2000 r / min and a revolution speed of 40 r / min for 120 min until uniform to obtain the third slurry.

[0057] Comparative Example 1

[0058] A method for preparing a lithium-ion battery separator is basically the same as that in Example 1, except that: “the first slurry” is replaced with “conventional PVDF slurry”.

[0059] Comparative Example 2

[0060] A method for preparing a lithium-ion battery separator is basically the same as that in Example 2, except that: “the second slurry” is replaced with “conventional PVDF slurry”.

[0061] Comparative Example 3

[0062] A method for preparing a lithium-ion battery separator is substantially the same as that of Example 3, except that: the "third slurry" is replaced with "conventional PVDF slurry".

[0063] Examples 4 to 6

[0064] A method for preparing a battery includes: sequentially placing a positive electrode sheet, a separator, and a negative electrode sheet, winding them to obtain an electrode core, pressing the electrode core at a pressure of 3 MPa for 120 s at room temperature, putting it into a shell (putting it into a battery shell made of aluminum alloy), and injecting an electrolyte to obtain a battery, and the separator is one of those in Examples 1 to 3.

[0065] Table 1

[0066] Battery Separator used in preparing the battery Example 4 Example 1 Example 5 Example 2 Example 6 Example 3

[0067] Comparative Examples 4 to 6

[0068] A method for preparing a battery includes: sequentially placing a positive electrode sheet, a separator, and a negative electrode sheet, winding them to obtain an electrode core, hot-pressing the electrode core at a pressure of 3 MPa for 120 s at 80 °C, putting it into a shell (putting it into a battery shell made of aluminum alloy), and injecting an electrolyte to obtain a battery, and the separator is one of those in Comparative Examples 1 to 3.

[0069] Table 2

[0070] Battery Separator used in preparing the battery Comparative Example 4 Comparative Example 1 Comparative Example 5 Comparative Example 2 Comparative Example 6 Comparative Example 3

[0071] The test results of the lithium-ion battery separators prepared in Examples 1 to 3 and Comparative Examples 1 to 3 are shown in Table 3:

[0072] Table 3

[0073]

[0074] It can be seen from Table 3 that under the same conditions, the electrode sheet adhesion strength of Examples 1 to 3 is basically the same as or even higher than that of Comparative Examples 1 to 3, indicating that it is difficult to separate the separator from the electrode sheet in Examples 1 to 3, and the misalignment of the electrode sheet during battery assembly can be prevented. In addition, the wettability of the lithium-ion battery separators prepared in Examples 1 to 3 is also better than that of the comparative examples.

[0075] The capacity retention rate of the battery prepared in Example 4 is 87%, the capacity retention rate of the battery prepared in Example 5 is 84%, the capacity retention rate of the battery prepared in Example 6 is 86%, the capacity retention rate of the battery prepared in Comparative Example 4 is 84%, the capacity retention rate of the battery prepared in Comparative Example 5 is 83%, and the capacity retention rate of the battery prepared in Comparative Example 6 is 84%. The capacity retention rate of the battery prepared in Example 4 is higher than that of Examples 5 to 6 and Comparative Examples 4 to 6.

[0076] In summary: The separator prepared from the slurry containing the polyester resin binder of the present invention can be bonded to the electrode sheet at room temperature, effectively reducing the energy consumption during the production process. The dot coating method combined with cold pressing (pressing at room temperature) makes the adhesion force uniform at each position during the pressing process of the battery cell, avoiding the occurrence of misalignment between the electrode sheet and the separator, and enhancing the safety performance of the battery.

[0077] Figure 1 SEM image of the lithium-ion battery separator prepared in Example 1 Figure 1 The average particle size of the spherical particles (formed after drying the polyester resin binder) is 6 μm; Figure 2 SEM image of the lithium-ion battery separator prepared in Example 3; Figure 3 SEM image of the lithium-ion battery separator prepared in Comparative Example 1.

[0078] Comparative Examples 7-9

[0079] A method for preparing a battery, comprising: sequentially placing a positive electrode sheet, a separator, and a negative electrode sheet, winding them to obtain a battery cell, compacting at a pressure of 3 MPa for 120 s at room temperature of 20-25 °C, putting it into a shell (putting it into a battery shell made of aluminum alloy), and injecting an electrolyte to obtain a battery, wherein the separator is one of Comparative Examples 1-3. The capacity retention rate and the electrode sheet bonding strength of the battery are shown in Table 4.

[0080] Table 4

[0081]

[0082] The above has made an exemplary description of the present invention. It should be noted that without departing from the core of the present invention, any simple deformation, modification, or equivalent substitution that can be made by those skilled in the art without creative labor falls within the protection scope of the present invention.

Claims

1. A polymer, characterized in that, The polymer is formed by polymerizing carboxyl-terminated polyester, carbon dioxide-based polyester diol, and modified diol. In terms of the number of moles, the ratio of carboxyl-terminated polyester, carbon dioxide-based polyester diol, and modified diol is (1 - 2):(1 - 4):(1 - 2).

2. The polymer according to claim 1, wherein The carboxyl-terminated polyester is polybutylene succinate, and the carbon dioxide-based polyester diol is poly(propylene carbonate) diol. The modified diol is obtained by modifying diethanolamine with butyl acrylate.

3. A method for preparing the polymer according to claim 1 or 2, characterized in that, It includes: Mix the carboxyl-terminated polyester, carbon dioxide-based polyester diol, and modified diol at 25 - 30 °C until homogeneous, heat to 130 - 170 °C, add a catalyst, and carry out vacuum polycondensation at 130 - 170 °C for 2 - 5 h, then cool to room temperature to obtain the polymer. In terms of the number of moles, the ratio of carboxyl-terminated polyester, carbon dioxide-based polyester diol, and modified diol is (1 - 2):(1 - 4):(1 - 2).

4. A polyester-based resin binder, characterized in that, It includes: The polymer as claimed in claim 1 or 2 and an additive. In terms of the number of mass parts, the ratio of the polymer to the additive is (30 - 90):(0.1 - 10). The additive is a mixture of a plasticizer and an antioxidant.

5. The polyester resin binder according to claim 4, characterized in that, The plasticizer is tributyl citrate, and the antioxidant is butylated hydroxytoluene.

6. A lithium-ion battery separator, characterized in that, It includes: A base film and the polymer as claimed in claim 1 or 2 loaded on the base film.

7. A method for preparing a lithium-ion battery separator, characterized in that, It includes: Coat the coating liquid on at least one side of the base film, dry it, and obtain a coated glue layer on the base film to obtain a lithium-ion battery separator. The coating liquid includes: the polymer as claimed in claim 1.

8. A battery, characterized in that, It includes the lithium-ion battery separator as claimed in claim 6.

9. Use of the polymer as claimed in claim 1 in improving the adhesion between the separator and the electrode in a battery.

10. Use of the polymer as claimed in claim 1 as a binder between the separator and the electrode in a battery, characterized in that, The polymer is used to reduce the temperature during the compaction between the separator and the electrode.

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