Lithium ion battery separator, method for preparing the same, and lithium battery

Abstract

The present application relates to the technical field of lithium battery, and more particularly to a lithium ion battery separator, a preparation method thereof and a lithium battery. The lithium ion battery separator comprises a base film and a coating layer arranged on the surface of the base film, and the coating layer contains an ionic liquid grafted organic coating material. The preparation method comprises the following steps: (1) performing a grafting reaction between an ionic liquid and an organic coating material to obtain an ionic liquid grafted organic coating material; (2) preparing a slurry of the ionic liquid grafted organic coating material, coating the slurry on the surface of the base film, and drying, thereby obtaining the product. The prepared separator has significantly improved electrolyte wettability and ionic conductivity compared with a conventional coated modified separator, and can improve the cycle stability of the battery.

Description

Technical Field

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

[0002] As a porous, high-performance material, the lithium-ion battery separator not only transports lithium ions but also isolates the positive and negative electrodes, preventing direct contact and short circuits. As a channel for lithium-ion transport within the battery, the separator is crucial to the speed of lithium-ion transport and diffusion. Its performance plays a key role in the battery's characteristics. A separator with better electrolyte wetting ability has higher ionic conductivity, which is more conducive to lithium-ion transport within the separator, increasing ion conduction efficiency and facilitating rapid charging and discharging of the battery.

[0003] Currently, there are two main methods to improve the electrolyte wetting rate and ionic conductivity of the separator. One is to improve the pore structure of the base membrane by increasing its porosity and reducing its tortuosity to increase the number of through-holes. The greater the porosity, the more pores in the separator, making it easier for lithium ions to migrate. The smaller the tortuosity, the more through-holes there are, resulting in a shorter lithium ion transport path and faster migration speed. However, a large number of pores and an increase in through-holes can lead to a decrease in the strength of the separator, increasing the risk of short circuits caused by foreign objects puncturing the separator during the manufacturing process. The second method is to modify the surface of the base membrane by coating. Common coatings include inorganic coatings such as alumina or boehmite, and organic coatings such as PVDF. Although traditional coating modifications can improve the liquid absorption and retention capacity to some extent, they still have the problems of slow electrolyte wetting and low ionic conductivity.

[0004] In view of this, the present invention is hereby proposed. Summary of the Invention

[0005] The primary objective of this invention is to provide a lithium-ion battery separator that, compared to conventional coated and modified separators, exhibits significantly improved electrolyte wettability and ionic conductivity, as well as better cycle stability.

[0006] The second objective of this invention is to provide a method for preparing a lithium-ion battery separator as described above, wherein an organic coating material is grafted and modified using an ionic liquid, and the grafted and modified organic coating material is made into a slurry and coated onto a base film to form a separator, thereby improving the lithium-ion conductivity and wettability of the separator to the electrolyte.

[0007] A third objective of the present invention is to provide a lithium battery, including the lithium-ion battery separator as described above.

[0008] In order to achieve the above-mentioned objectives of the present invention, the following technical solution is adopted:

[0009] A lithium-ion battery separator includes a base film and a coating disposed on the surface of the base film, the coating containing an ionic liquid grafted modified organic coating material.

[0010] Preferably, the ionic liquid-grafted modified organic coating material includes ionic liquid-grafted modified PVDF and / or ionic liquid-grafted modified PMMA.

[0011] Preferably, the grafting rate of the ionic liquid-grafted modified organic coating material is 4%-6%.

[0012] Preferably, the base film includes a PE base film or a PP base film.

[0013] Preferably, the thickness of the coating is 1-3 μm.

[0014] A method for preparing a lithium-ion battery separator as described above includes the following steps:

[0015] (1) Grafting reaction of ionic liquid with organic coating material to obtain ionic liquid grafted modified organic coating material.

[0016] (2) The organic coating material grafted with the ionic liquid is made into a slurry, coated onto the surface of the base film, and dried to obtain the lithium-ion battery separator.

[0017] Preferably, the organic coating material includes PVDF and / or PMMA.

[0018] Preferably, the cation of the ionic liquid includes one of imidazole salt ions, pyrrole salt ions, quaternary ammonium salt ions, or quaternary phosphate salt ions containing polymerizable groups;

[0019] And / or, the anion of the ionic liquid includes one of bis(trifluoromethanesulfonyl)imide ion, tetrafluoroborate ion, or hexafluorophosphate ion.

[0020] Preferably, the grafting reaction method includes one of plasma treatment grafting, irradiation grafting, or chemical treatment grafting.

[0021] More preferably, the grafting reaction method is plasma treatment grafting, comprising the following steps:

[0022] First, the organic coating material is subjected to plasma treatment to form reactive sites on its surface. Then, it is mixed and reacted with an ionic liquid to obtain the ionic liquid-grafted modified organic coating material.

[0023] Preferably, in step (2), the slurry comprises, by mass parts, 8-12 parts of the ionic liquid grafted modified organic coating material, 4-6 parts of binder, 1-3 parts of wetting and dispersing agent and 80-90 parts of solvent.

[0024] More preferably, the adhesive includes at least one of acrylate and acrylate copolymer.

[0025] Preferably, the wetting and dispersing agent comprises ammonium polycarboxylate and / or sodium polycarboxylate.

[0026] Preferably, the solvent includes water.

[0027] A lithium battery, comprising a lithium-ion battery separator as described above.

[0028] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0029] (1) Compared with traditional coated and modified diaphragms, the diaphragm provided by the present invention has significantly improved electrolyte wettability and ionic conductivity, and has better cycle stability.

[0030] (2) The method of the present invention uses ionic liquid to graft and modify organic coating material, and then coats the grafted and modified organic coating material into a slurry on the base film to form a separator with a high ionic conductivity coating. This can effectively improve the ionic conductivity of the separator, enhance the wettability of the electrolyte, and improve the cycle performance of the battery. Detailed Implementation

[0031] The technical solution of the present invention will be clearly and completely described below with reference to specific embodiments. However, those skilled in the art will understand that the embodiments described below are some embodiments of the present invention, but not all embodiments, and are only used to illustrate the present invention, and should not be regarded as limiting the scope of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall be followed. Where the manufacturers of reagents or instruments are not specified, they are all conventional products that can be purchased commercially.

[0032] A first aspect of the present invention provides a lithium-ion battery separator, comprising a base film and a coating disposed on the surface of the base film, the coating comprising an ionic liquid grafted modified organic coating material.

[0033] This invention improves the lithium-ion conductivity of the coating by setting a coating containing an ionic liquid-grafted modified organic adhesive material on the base membrane, thereby enhancing the ionic conductivity and electrolyte wettability of the separator. Compared with traditional separators coated with PVDF (polyvinylidene fluoride) and PMMA (polymethyl methacrylate), the separator provided by this invention has a faster electrolyte wettability, higher ionic conductivity, and better cycle stability.

[0034] In some specific embodiments of the present invention, the ionic liquid grafted modified organic coating material includes ionic liquid grafted modified PVDF and / or ionic liquid grafted modified PMMA. The membrane prepared by grafting ionic liquid modified PVDF or PMMA has higher ionic conductivity, which can effectively improve the lithium ion transfer efficiency of the membrane and accelerate the wetting of the electrolyte.

[0035] In some specific embodiments of the present invention, the grafting rate of the ionic liquid grafted modified organic coating material is 4%-6%, for example, any one value or a range of any two values ​​among 4%, 4.5%, 5%, 5.5%, and 6%.

[0036] When the grafting rate is too low, there is less grafted ionic liquid, and the effect of improving ionic conductivity is not obvious. When the grafting rate is too high, it will cause greater damage to the molecular chains of the matrix material, affect the structural integrity, and lead to a weakening of the bonding effect.

[0037] In some specific embodiments of the present invention, the base film includes a PE base film or a PP base film.

[0038] In some specific embodiments of the present invention, the thickness of the coating is 1-3 μm, for example, any one value or a range of any two values ​​among 1 μm, 1.5 μm, 2 μm, 2.5 μm, and 3 μm.

[0039] If the coating is too thin, the improvement effect will not be obvious; if the coating is too thick, it will affect the air permeability of the separator and have a significant impact on the battery cycle performance. Therefore, it is necessary to reasonably control the thickness of the coating.

[0040] A second aspect of the present invention provides a method for preparing the lithium-ion battery separator as described above, comprising the following steps:

[0041] (1) Grafting reaction of ionic liquid with organic coating material to obtain ionic liquid grafted modified organic coating material.

[0042] (2) The organic coating material grafted with the ionic liquid is made into a slurry, coated onto the surface of the base film, and dried to obtain the lithium-ion battery separator.

[0043] The method of this invention utilizes the grafting reaction between ionic liquid and organic coating material to modify the organic coating material, and then coats the grafted and modified organic coating material into a slurry on a base membrane to form a separator with a high lithium-ion conductivity coating. This effectively improves the ionic conductivity of the separator and the wettability of the electrolyte, thereby improving the cycle performance of the battery. Furthermore, the method of this invention is simple and easy to scale up for production.

[0044] In some specific embodiments of the present invention, the organic coating material includes PVDF and / or PMMA.

[0045] In some specific embodiments of the present invention, the cation of the ionic liquid includes one of imidazole salt ions, pyrrole salt ions, quaternary ammonium salt ions or quaternary phosphate salt ions containing polymerizable groups.

[0046] And / or, the anion of the ionic liquid includes one of bis(trifluoromethanesulfonyl)imide ion, tetrafluoroborate ion, or hexafluorophosphate ion.

[0047] As an example, the polymerizable group can be a group containing unsaturated double bonds, such as vinyl or acryloyl groups, etc.

[0048] In some specific embodiments of the present invention, the grafting reaction method includes one of plasma treatment grafting, irradiation grafting, or chemical treatment grafting.

[0049] In some specific embodiments of the present invention, the grafting reaction method is plasma treatment grafting, which includes the following steps:

[0050] First, the organic coating material is subjected to plasma treatment to form reactive sites on its surface. Then, it is mixed and reacted with an ionic liquid to obtain the ionic liquid-grafted modified organic coating material.

[0051] Plasma grafting involves reactivating the surface of an organic coating material to form active free radicals. These free radicals then initiate the grafting of functional monomers (ionic liquids) onto the surface of the organic coating material. After plasma treatment, the organic coating material directly undergoes a grafting reaction with the ionic liquid. The plasma treatment parameters can be arbitrarily selected to achieve the desired treatment purpose.

[0052] In some specific embodiments of the present invention, in step (2), the composition of the slurry, by mass parts, includes: 8-12 parts of the ionic liquid grafted modified organic coating material, 4-6 parts of binder, 1-3 parts of wetting and dispersing agent, and 80-90 parts of solvent.

[0053] In different embodiments, the ionic liquid-grafted modified organic coating material can be any one value or a range of any two values ​​from 8 parts, 9 parts, 10 parts, 11 parts, and 12 parts; the binder can be any one value or a range of any two values ​​from 4 parts, 5 parts, and 6 parts; the wetting and dispersing agent can be any one value or a range of any two values ​​from 1 part, 2 parts, and 3 parts; and the solvent can be any one value or a range of any two values ​​from 80 parts, 83 parts, 85 parts, 88 parts, and 90 parts.

[0054] In some specific embodiments of the present invention, the adhesive includes at least one of acrylates and acrylate copolymers, such as methyl acrylate or ethyl acrylate.

[0055] In some specific embodiments of the present invention, the wetting and dispersing agent includes ammonium polycarboxylate salt and / or sodium polycarboxylate salt.

[0056] In some specific embodiments of the present invention, the solvent includes water, and to avoid impurities, the water is deionized water as an example.

[0057] The present invention does not impose specific restrictions on the drying conditions in step (2), and can select them according to actual conditions, with the aim of removing the solvent from the slurry. In some specific embodiments of the present invention, the drying temperature can be 60-75℃, for example, any one value or a range of any two values ​​among 60℃, 62℃, 65℃, 68℃, 70℃, 72℃, and 75℃; the drying time can be 2-5 min, for example, any one value or a range of any two values ​​among 2 min, 3 min, 4 min, and 5 min.

[0058] A third aspect of the present invention provides a lithium battery comprising the lithium-ion battery separator described in any of the foregoing embodiments or the lithium-ion battery separator prepared by the method described in any of the foregoing embodiments.

[0059] The lithium-ion battery provided by this invention has good cycle stability, short liquid injection time in the cell preparation process, and high production efficiency.

[0060] The following detailed description of some embodiments of the present invention is provided in conjunction with specific examples. Unless otherwise specified, all raw materials used in the embodiments are commercially available.

[0061] Example 1

[0062] PVDF was selected as the organic coating material. First, the PVDF was plasma-treated, and then grafted with 1-vinyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt to obtain grafted modified PVDF with a grafting rate of 5%. A slurry was prepared using the grafted modified PVDF. By weight, 10 parts of grafted modified PVDF, 83 parts of deionized water, 5 parts of methyl acrylate, and 2 parts of polycarboxylate ammonium salt were used to prepare the separator coating slurry. The slurry was coated onto both sides of a 9μm PE base film using a gravure roller coating method, with a coating thickness of 1μm. After drying at 70℃ for 5 minutes, the lithium-ion battery separator was obtained.

[0063] Example 2

[0064] The organic coating material is a non-fluorinated PMMA polymer. The PMMA is first plasma-treated, and then grafted with 1-vinyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt to obtain grafted modified PMMA with a grafting rate of 5%. A slurry is prepared using the grafted modified PMMA. By weight, 10 parts of grafted modified PMMA, 83 parts of deionized water, 5 parts of methyl acrylate, and 2 parts of polycarboxylate ammonium salt are used to prepare the separator coating slurry. The slurry is coated onto both sides of a 9μm PE base film using a gravure roller coating method, with a coating thickness of 1μm. After drying at 70℃ for 5 minutes, the lithium-ion battery separator is obtained.

[0065] Comparative Example 1

[0066] The difference between Comparative Example 1 and Example 1 is that the organic coating material used is the same PVDF as in Example 1, without any treatment, and untreated PVDF is used instead of the grafted modified PVDF in Example 1 to prepare the slurry for preparing the lithium-ion battery separator. The other conditions are the same as in Example 1.

[0067] Comparative Example 2

[0068] The difference between Comparative Example 2 and Example 2 is that the organic coating material used is the same PMMA as in Example 2, without any treatment, and untreated PMMA is used instead of the grafted modified PMMA in Example 2 to prepare the slurry for preparing the lithium-ion battery separator. The other conditions are the same as in Example 2.

[0069] Test case

[0070] 1. The thickness, air permeability, areal density, ionic conductivity and electrolyte contact angle of the lithium-ion battery separators prepared in each embodiment and each comparative example were tested respectively.

[0071] The test methods for thickness, air permeability, areal density and ionic conductivity are in accordance with GB / T 36363-2018;

[0072] Electrolyte contact angle test: Fix the diaphragm flat on the sample stage, draw up the electrolyte with a syringe, slowly add the liquid to form a droplet of a certain volume at the end of the syringe, keep the position of the syringe unchanged, move the sample stage up and continuously approach the droplet hanging at the end of the syringe, drop a certain volume of electrolyte onto the surface of the diaphragm, take a picture and calculate the contact angle with software.

[0073] The test results of Example 1, Comparative Example 1, Example 2 and Comparative Example 2 are shown in Tables 1, 2, 3 and 4, respectively.

[0074] Table 1. Performance results of the diaphragm in Example 1

[0075]

[0076]

[0077] Table 2 shows the performance results of the diaphragm in Comparative Example 1.

[0078]

[0079] Table 3 Performance results of the diaphragm in Example 2

[0080]

[0081] Table 4 shows the performance results of the diaphragm in Comparative Example 2.

[0082]

[0083] As shown in Tables 1 and 2, compared with Comparative Example 1, the thickness, air permeability, and areal density of the diaphragm in Example 1 did not change significantly. The electrolyte contact angle decreased from 13.3° to 2.5°, indicating a significant improvement in electrolyte wettability. The ionic conductivity increased from 9.599 × 10⁻⁶. -4 S / cm increased to 13.61×10 -4 The S / cm ratio is increased by approximately 42%, which is far higher than the lithium-ion conductivity of existing separators.

[0084] As shown in Tables 3 and 4, compared with Comparative Example 2, the thickness, air permeability, and areal density of the diaphragm in Example 2 did not change significantly. The electrolyte contact angle decreased from 14.9° to 2.4°, indicating a significant improvement in electrolyte wettability. The ionic conductivity increased from 9.467 × 10⁻⁶. -4 S / cm increased to 13.15×10 -4 The S / cm ratio is increased by approximately 39%, which is far higher than the lithium-ion conductivity of existing separators.

[0085] The results above show that, compared with traditional PVDF or PMMA coated separators, the electrolyte wettability and lithium-ion conductivity of the lithium-ion battery separator prepared by this invention are significantly improved.

[0086] 2. The lithium-ion battery separators prepared in each embodiment and each comparative example were assembled into lithium-ion batteries. The assembled batteries were identical except for the separators used. The electrolyte injection time and electrochemical performance were then tested.

[0087] Battery assembly:

[0088] (1) Preparation of positive electrode sheet: Lithium iron phosphate, binder (PVDF) and conductive agent (CNT) are used to prepare slurry. The slurry is coated on carbon-coated aluminum foil and then processed to obtain positive electrode sheet. The positive electrode material accounts for 96% of the coating, and the rest is binder, conductive agent, etc.

[0089] (2) Preparation of negative electrode sheet: Artificial graphite, binder (CMC) and conductive agent (CNT) are used to prepare slurry. The slurry is coated on copper foil and processed to obtain negative electrode sheet. The negative electrode material accounts for 95.5% of the coating, and the rest is binder, conductive agent, etc.

[0090] (3) Assembly: The prepared negative electrode, positive electrode, separator and electrolyte are assembled. The electrolyte injection time of the cell is recorded and its capacity retention rate is tested after 500 cycles at 25℃ and 45℃. The cycle mode is 45min step charging from 0-80% SOC and 1C discharge. The test results are shown in Table 5.

[0091] Table 5

[0092]

[0093] As shown in Table 5, the capacity retention rate of the battery assembled with the separator prepared in the embodiments of the present invention is higher than that of the comparative example after 500 cycles at different temperatures, indicating that the separator prepared by the method of the present invention can improve the cycle stability of lithium-ion batteries. Furthermore, the electrolyte injection time of the separator prepared in the embodiments of the present invention is significantly shorter than that of the comparative example during the cell assembly process, with the injection time shortened by about 32%, which can significantly improve the production efficiency of the battery.

[0094] Although the present invention has been illustrated and described with specific embodiments, it should be understood that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; those skilled in the art should understand that modifications can be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein, without departing from the spirit and scope of the present invention; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention; therefore, this means that all such substitutions and modifications that fall within the scope of the present invention are included in the appended claims.

Claims

1. A lithium-ion battery separator, characterized in that, The system includes a base film and a coating disposed on the surface of the base film, wherein the coating contains an ionic liquid-grafted modified organic coating material; the ionic liquid-grafted modified organic coating material includes ionic liquid-grafted modified PVDF and / or ionic liquid-grafted modified PMMA; the grafting rate of the ionic liquid-grafted modified organic coating material is 4%-6%; The cation of the ionic liquid includes one of imidazole salt ions, pyrrole salt ions, quaternary ammonium salt ions, or quaternary phosphate salt ions containing polymerizable groups; the anion of the ionic liquid includes one of bis(trifluoromethanesulfonyl)imide ions, tetrafluoroborate ions, or hexafluorophosphate ions.

2. The lithium-ion battery separator according to claim 1, characterized in that, It includes at least one of the following features (1) to (2): (1) The base film includes a PE base film or a PP base film; (2) The thickness of the coating is 1-3 μm.

3. The method for preparing the lithium-ion battery separator according to claim 1 or 2, characterized in that, Includes the following steps: (1) The ionic liquid was grafted onto the organic coating material to obtain the ionic liquid grafted modified organic coating material. (2) The organic coating material grafted with the ionic liquid is made into a slurry, coated onto the surface of the base film, and dried to obtain the lithium-ion battery separator.

4. The method for preparing the lithium-ion battery separator according to claim 3, characterized in that, The grafting reaction method includes one of plasma treatment grafting, irradiation grafting, or chemical treatment grafting.

5. The method for preparing the lithium-ion battery separator according to claim 4, characterized in that, The grafting reaction method is plasma-treated grafting, including the following steps: First, the organic coating material is subjected to plasma treatment to form reactive sites on its surface. Then, it is mixed and reacted with an ionic liquid to obtain the ionic liquid-grafted modified organic coating material.

6. The method for preparing the lithium-ion battery separator according to claim 3, characterized in that, The slurry comprises, by weight, 8-12 parts of the ionic liquid grafted modified organic coating material, 4-6 parts of binder, 1-3 parts of wetting and dispersing agent, and 80-90 parts of solvent.

7. The method for preparing a lithium-ion battery separator according to claim 6, characterized in that, It includes at least one of the following features (1) to (3): (1) The adhesive includes at least one of acrylate and acrylate copolymer; (2) The wetting and dispersing agent includes ammonium polycarboxylate and / or sodium polycarboxylate; (3) The solvent includes water.

8. A lithium battery, characterized in that, Includes the lithium-ion battery separator as described in claim 1 or 2.

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