Ultrathin battery diaphragm with high binding power and safety as well as preparation method and application of ultrathin battery diaphragm

Abstract

The invention discloses an ultrathin battery diaphragm with high cohesiveness and safety as well as a preparation method and application of the ultrathin battery diaphragm. The battery diaphragm comprises a polyolefin base material and polyvinylidene fluoride coatings arranged on the two sides of the polyolefin base material, wherein each polyvinylidene fluoride coating comprises aluminum oxide particles, a first polyvinylidene fluoride copolymer and a second polyvinylidene fluoride copolymer; the particle size of the aluminum oxide particles is below 0.4 micron; both the first polyvinylidene fluoride copolymer and the second polyvinylidene fluoride copolymer contain vinylidene fluoride and hexafluoropropylene monomer units, and the molar percentage of the hexafluoropropylene monomer unit in the first polyvinylidene fluoride copolymer is lower than that of the hexafluoropropylene monomer unit in the second polyvinylidene fluoride copolymer. The battery diaphragm has ultra-thin thickness, has high binding power with a metal electrode plate, and is high in battery safety when being used for a battery.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium ion battery separator, in particular to a kind of high adhesion and safety ultra-thin battery separator and its preparation method and application. BACKGROUND

[0002] Lithium ion battery has been widely used in many fields such as electric vehicles, portable electronic devices, etc. Battery separator, as a key component of lithium ion battery, plays a crucial role in the performance and safety of the battery.

[0003] Polyolefin separator is the most widely used lithium battery separator, but the existing polyolefin separator on the market has the problem of insufficient adhesion and electrolyte affinity. In order to improve the adhesion and electrolyte wettability of polyolefin separator, the main solution at present is to coat polyvinylidene fluoride (PVDF) coating on one side or both sides of the polyolefin separator. The coating can improve the adhesion of the separator to some extent, and has good wettability with electrolyte. However, the thickness of the polyolefin separator containing PVDF coating is not thin enough, resulting in a thicker battery sheet containing the separator, which ultimately leads to a larger thickness of the battery stacked by the battery sheet. In practical application, when there is a certain requirement for the thickness of the battery, the thicker the battery sheet, the fewer the number of battery sheets that can be stacked, resulting in a decrease in the energy density of the battery. However, for the consumer electronics market, the battery has always pursued the improvement of energy density, which in turn requires the separator of each battery sheet to be as thin as possible.

[0004] In addition, the higher the adhesion of the battery separator to the metal electrode sheet, the more firmly the separator and the electrode sheet are bonded, and the less likely the separator and the electrode sheet are separated in practical application (in electrolyte) (after separation, the gas generated by the side reaction of the battery at high temperature is easy to cause the battery to bulge, etc.), improving the safety of the battery in use.

[0005] How to balance the thickness of the thinner separator and the high adhesion of the separator to the electrode sheet, and further ensure the high safety of the battery is a difficulty. SUMMARY

[0006] The technical problem to be solved by the present application is to provide an improved battery separator with ultra-thin thickness and high adhesion to metal electrode sheet, which has high safety when used in battery.

[0007] To solve the above technical problems, the technical scheme adopted by the present application is as follows:

[0008] A battery separator includes a polyolefin base material, polyvinylidene fluoride coating layers provided on both sides of the polyolefin base material, the polyvinylidene fluoride coating layers including alumina particles, a first polyvinylidene fluoride copolymer, and a second polyvinylidene fluoride copolymer; the alumina particles have a particle size of 0.4 microns or less; the first polyvinylidene fluoride copolymer and the second polyvinylidene fluoride copolymer each contain vinylidene fluoride and hexafluoropropylene monomer units, and the first polyvinylidene fluoride copolymer has a lower mole percentage of hexafluoropropylene monomer units than the second polyvinylidene fluoride copolymer.

[0009] In some embodiments, the alumina particles have a particle size of 0.1-0.4 microns. The alumina particles have such a small particle size, and thus a super-thin battery separator can be prepared, the key being how to ensure uniform dispersion when using small-particle-size alumina particles, so that the coating layer surface is uniform.

[0010] In some embodiments, the first polyvinylidene fluoride copolymer has a mole percentage of hexafluoropropylene monomer units of 4%-6%, and the second polyvinylidene fluoride copolymer has a mole percentage of hexafluoropropylene monomer units of 8%-10%.

[0011] In some embodiments, the mass ratio of the first polyvinylidene fluoride copolymer to the second polyvinylidene fluoride copolymer is 1:1-1:9.

[0012] In some embodiments, the ratio of the mass of the alumina particles to the total mass of the first polyvinylidene fluoride copolymer and the second polyvinylidene fluoride copolymer is 3-6:4-7.

[0013] In some embodiments, the battery separator is prepared by a preparation method comprising the following steps: 1) dissolving a first polyvinylidene fluoride copolymer in an organic solvent to obtain solution A; 2) dissolving a second polyvinylidene fluoride copolymer in an organic solvent to obtain solution B; 3) dispersing alumina particles in an organic solvent to obtain dispersion C after grinding; 4) mixing dispersion C and solution A, and then adding solution B to obtain a mixture; 5) grinding the mixture to obtain an oil-based slurry; and 6) coating the oil-based slurry on both sides of the polyolefin substrate to obtain the battery separator after drying. When dispersing the alumina particles in the organic solvent, they are ground and dispersed. The grinding does not reduce the particle size of the alumina particles, but makes them uniformly dispersed and non-aggregated in the organic solvent. By first mixing the alumina dispersion and the solution of the first polyvinylidene fluoride copolymer, and then adding the solution of the second polyvinylidene fluoride copolymer, the three components can be more uniformly mixed. Furthermore, the mixture is ground again after being mixed to obtain a mixture, which ensures that the small-particle-size alumina particles are uniformly dispersed and non-aggregated in the slurry. If the mixture is not ground again, the alumina particles in the battery separator prepared may be aggregated, resulting in uneven surfaces of the separator, which cannot be bonded with metal electrode sheets, and affecting the performance of the battery.

[0014] The present application also provides a preparation method of the aforementioned battery separator, which comprises the steps of dispersing the alumina particles in an organic solvent, dissolving the first polyvinylidene fluoride copolymer and the second polyvinylidene fluoride copolymer in the organic solvent, mixing the three components to obtain a mixture, grinding the mixture to obtain an oil-based slurry, and coating the oil-based slurry to obtain the battery separator.

[0015] In some embodiments, the preparation method comprises the following steps: 1) dissolving a first polyvinylidene fluoride copolymer in an organic solvent to obtain solution A; 2) dissolving a second polyvinylidene fluoride copolymer in an organic solvent to obtain solution B; 3) dispersing alumina particles in an organic solvent to obtain dispersion C after grinding; 4) mixing dispersion C and solution A, and then adding solution B to obtain a mixture; 5) grinding the mixture to obtain an oil-based slurry; and 6) coating the oil-based slurry on both sides of the polyolefin substrate to obtain the battery separator after drying. This preparation method can ensure that even with the small-particle-size alumina particles of the present application, they can be uniformly dispersed in the PVDF copolymer coating, thereby improving the performance of the battery separator and the final lithium ion battery.

[0016] In some embodiments, in steps 3) and 5), the grinding is performed using grinding beads with a particle size of 0.4-0.6 microns. This further improves the uniformity of the dispersion of the alumina particles.

[0017] In some embodiments, the grinding time in step 3) is 30-90 minutes.

[0018] In some embodiments, the grinding time in step 5) is 30-90 minutes.

[0019] In some embodiments, the mass percentage of the first polyvinylidene fluoride copolymer in solution A is 6%-10%.

[0020] In some embodiments, the second polyvinylidene fluoride copolymer in solution B is 6%-10% by mass.

[0021] In some embodiments, the mass percentage of the alumina particles in the dispersion C is 10%-14%.

[0022] In some embodiments, the organic solvent is dimethylacetamide.

[0023] The present invention also provides an application of the aforementioned battery separator in lithium-ion batteries.

[0024] Due to the implementation of the above technical solutions, the present invention has the following advantages compared with the prior art:

[0025] 1) The battery separator of the present invention can have an ultra-thin thickness because it uses alumina particles with ultra-small particle size. At the same time, since the alumina particles can be uniformly dispersed in the PVDF coating without agglomeration, and the combination of two PVDF copolymers with different hexafluoropropylene contents can combine the performance advantages of the two PVDFs, the battery separator and the metal electrode sheet can have high adhesion, and the battery is safe when used in the battery.

[0026] 2) The thickness of the battery separator of the present invention can be as low as about 7 micrometers.

[0027] 3) The battery separator of the present invention has excellent heat resistance, shrinkage resistance, electrolyte wettability, mechanical strength and safety (low shrinkage rate, high needle strength and strong adhesion). Attached Figure Description

[0028] Figure 1 Microscopic images of the battery separator of Example 1;

[0029] Figure 2 This is a microscope image of the battery separator in Comparative Example 1. Detailed Implementation

[0030] Currently, the thickness of polyolefin separators containing PVDF coatings is not thin enough, resulting in thinner solar cells containing the separators, and ultimately a thicker battery when the cells are stacked. In practical applications, when there are certain requirements for battery thickness, the thicker the solar cells, the fewer solar cells can be stacked, leading to a decrease in the battery's energy density.

[0031] In the prior art, when alumina particles are used in PVDF coatings, their particle size is usually above 0.7 micrometers. Although smaller particle size is desired, alumina particles are prone to agglomeration at small particle size, resulting in uneven membrane coating and making it unusable in practice.

[0032] This invention first combines two PVDF copolymers with different hexafluoropropylene monomer unit contents. The first PVDF copolymer with a lower hexafluoropropylene content has relatively low adhesion to the metal electrode sheet, but also low swelling performance in the electrolyte. The second PVDF copolymer with a higher hexafluoropropylene content has relatively high adhesion to the metal electrode sheet, but also high swelling performance in the electrolyte. By using the two PVDF copolymers simultaneously, this invention achieves strong adhesion of the PVDF coating to the metal electrode sheet while minimizing swelling in the electrolyte, resulting in excellent electrical properties of the separator and improved battery safety.

[0033] Secondly, the alumina particles of this invention have a very small particle size, making them difficult to disperse uniformly in PVDF binders. However, this application employs a specific preparation process: first, the alumina particles are dispersed and ground in an organic solvent to obtain a dispersion; then, this dispersion is mixed with a solution of a first polyvinylidene fluoride copolymer; finally, a solution of a second polyvinylidene fluoride copolymer is added to obtain a mixed solution. This mixed solution is then ground again to ensure that the alumina particles remain uniformly dispersed in the mixture, preventing agglomeration. This results in a more uniform PVDF coating on the separator, improving its various performance characteristics. This stepwise mixing and co-dispersion process allows for more complete dissolution or dispersion of the components, avoiding poor dissolution of the two PVDFs in the solvent. It synergistically leverages the high-temperature adhesion and low swelling rate of the first polyvinylidene fluoride copolymer, and the low-temperature adhesion of the second polyvinylidene fluoride copolymer, thereby achieving both low-temperature and high-temperature adhesion of the separator, balancing cell swelling, improving ionic conductivity, and enhancing the overall electrical and safety performance of the battery.

[0034] Finally, the alumina particles of this invention have a small particle size and can be uniformly dispersed, thus reducing the thickness of the separator and obtaining an ultra-thin separator. When used in batteries, this can increase the energy density of the battery. The strong adhesion between the separator and the metal electrode sheets ensures the dimensional stability of the cell, reduces its thermal shrinkage rate at high temperatures, and decreases the risk of short circuits caused by separator thermal deformation, thereby improving battery safety. Simultaneously, due to the small particle size and large specific surface area of ​​the alumina, it can better improve the wettability of the separator to the electrolyte, promoting the transport efficiency of lithium ions inside the battery, thereby improving the charge-discharge performance and cycle life of the battery.

[0035] For oil-based slurry coating, an automatic oil-based coating batching system can be used. This system employs one-time double-sided coating, resulting in high coating efficiency and the ability to produce 1000mm wide diaphragms. After the slurry is prepared, it undergoes foreign matter filtration and metal filtration processes before being directly piped from the slurry preparation workshop into the slurry tank in the coating workshop. A dual-station automatic tape feeding and roll changing system allows for roll material replacement without stopping the machine, significantly reducing equipment downtime and improving overall production efficiency. The coating slurry is gravity-fed; the prepared slurry is first pumped into a higher storage tank, and then fed into the gravure coating hopper by an automatic feeding device below the storage tank. The gravure roller uses a 160PSI ceramic anilox roller. 160PSI is a relatively moderate pressure, reducing wear between the anilox roller and the doctor blade and other related components, minimizing heat and mechanical damage caused by friction, thereby extending the service life of the ceramic anilox roller and reducing production costs. Two sets of defect detectors and static electricity elimination devices are installed at the oven outlet to monitor adhesive application defects in real time.

[0036] During coating, first select a suitable coating substrate, such as a polyolefin diaphragm (e.g., polyethylene (PE) or polypropylene (PP) diaphragm). Mount the substrate onto the unwinding device of the coating machine. Automatic roll changing and fully automatic tape splicing are performed at dual stations, and the unwinding tension is adjusted to ensure smooth and uniform unwinding of the substrate. Start the coating machine via the fully automatic touchscreen panel. The prepared oil-based slurry is fed to the coating head through the feeding system. The dual coating heads coat both sides simultaneously. A 160PSI ceramic anilox roller is used for the gravure roller to evenly transfer the slurry to the substrate surface.

[0037] The technical solutions of the present invention will be described in detail below with reference to specific embodiments, so that those skilled in the art can better understand and implement the technical solutions of the present invention, but the present invention is not limited to the scope of the examples described.

[0038] PVDF-A: Commercially available from Arkema LBG, it is a copolymer of vinylidene fluoride and hexafluoropropylene, with a molar percentage of hexafluoropropylene monomer units of approximately 5% in the copolymer.

[0039] PVDF-B: Commercially available from Solvay 75130, it is a copolymer of vinylidene fluoride and hexafluoropropylene, with a molar percentage of hexafluoropropylene monomer units of approximately 8% in the copolymer.

[0040] Example 1:

[0041] This embodiment provides a battery separator, the preparation process of which is as follows:

[0042] 1) Preparation of PVDF-A solution: The PVDF-A copolymer was slowly added to dimethylacetamide (DMAC) solvent and dispersed at a stirring speed of 20-40 rpm for 60-130 minutes until the PVDF-A was completely dissolved, yielding solution A. The mass percentage of PVDF in solution A was 8%.

[0043] 2) Preparation of PVDF-B solution: The PVDF-B copolymer was slowly added to dimethylacetamide (DMAC) solvent and dispersed at a stirring rate of 20-40 rpm for 60-130 minutes until the PVDF-B was completely dissolved, yielding solution B. The mass percentage of PVDF in solution B was 8%.

[0044] 3) Alumina suspension dispersion: Alumina particles with a D50 of 0.4 μm were slowly added to DMAC solvent and dispersed at a stirring rate of 20-40 rpm for 60-130 minutes. After uniform dispersion, the particles were then ground using zirconia beads with a particle size of 0.4 mm for 60 minutes to obtain dispersion C. The mass percentage of alumina in dispersion C was 12%.

[0045] 4) Mix the dispersion C obtained in step 3) with the solution A obtained in step 1) and disperse again at a stirring speed of 20-50 rpm for 30-60 minutes. Then add the solution B obtained in step 2) and disperse again at a stirring speed of 20-50 rpm for 30-60 minutes to obtain the final mixture.

[0046] In the mixture, the mass ratio of alumina to the total mass of the two PVDFs is 3:7; the mass ratio of PVDF-A to PVDF-B is 1:9.

[0047] 5) Zirconia was then used as grinding beads with a particle size of 0.4 mm. The final mixture was ground again for 60 minutes to obtain a stable oil-based slurry.

[0048] 6) Separator Coating: The above-mentioned oil-based slurry is coated onto the polyolefin separator using an oil-based coating process. The coating thickness is controlled at 0.5-1 micrometer, and uniform thickness is ensured. The final battery separator is obtained.

[0049] The final microscopic image of the battery separator is as follows: Figure 1 As shown, the membrane coating is very uniform, the alumina is evenly dispersed, and there is no agglomeration or aggregation.

[0050] Example 2:

[0051] Example 2 is basically the same as Example 1, except that in step 4), the mass ratio of alumina to the total mass of the two PVDFs in the mixture is controlled to be 4:6; and the mass ratio of PVDF-A to PVDF-B is 1:1.

[0052] Example 3:

[0053] Example 3 is basically the same as Example 1, except that in step 4), the mass ratio of alumina to the total mass of the two PVDFs in the mixture is controlled to be 6:4; and the mass ratio of PVDF-A to PVDF-B is 1:1.

[0054] The performance of the final battery separators prepared in Examples 1-3 was tested, including heat resistance (ASTM D1204-14(2020)), air permeability (GB / T 458-2008), electrical insulation resistance (Gb / T13542.12-2009), mechanical strength (e.g., tensile strength GB / T 13022, puncture strength GB / T 23318-2009), adhesion, and peel strength. The results are shown in Table 1 below. Adhesion refers to the bond between the battery separator and the metal electrode sheet, while peel strength refers to the peel strength between the battery separator and the polyolefin substrate.

[0055] Table 1

[0056]

[0057]

[0058] As can be seen, the battery separator of the present invention has an ultra-thin thickness (due to the small alumina particle size and the coating can achieve a very thin and uniform degree), and has a high adhesion to the metal electrode sheet (although the adhesion of Example 3 appears to be lower than that of Examples 1-2, it is still a very high level relative to the water-based PVDF coating). When used in a battery, it provides high battery safety (low shrinkage rate, high needle strength, and strong adhesion).

[0059] Comparative Example 1:

[0060] The process is basically the same as in Example 1, except that step 5 is omitted, i.e., the final mixture is not ground again, and the diaphragm coating is applied directly. A microscopic image of the resulting diaphragm coating is shown below. Figure 2As shown, without further grinding after mixing, the alumina is unevenly dispersed in the mixture, exhibiting agglomeration. This results in an uneven coating and particle aggregation in the final battery separator. When used as a battery separator, this coating performs poorly and does not bond firmly to the metal electrode sheets (the alumina agglomeration is so high that its poor performance and unusable condition can be determined without testing the separator's performance). The re-grinding method of this invention further improves the dispersion performance of alumina, enabling the use of small-particle-size alumina to prepare ultra-thin, uniform battery separators.

[0061] The above description of the embodiments is only for the purpose of helping to understand the method and core ideas of the present invention. It should be noted that those skilled in the art can make several improvements and modifications to the present invention without departing from the principles of the present invention, and these improvements and modifications also fall within the protection scope of the claims of the present invention.

Claims

1. A battery separator, comprising a polyolefin substrate and a polyvinylidene fluoride coating disposed on both sides of the polyolefin substrate, characterized in that: The polyvinylidene fluoride coating comprises alumina particles, a first polyvinylidene fluoride copolymer, and a second polyvinylidene fluoride copolymer; the alumina particles have a particle size of less than 0.4 micrometers; both the first and second polyvinylidene fluoride copolymers contain vinylidene fluoride and hexafluoropropylene monomer units, and the molar percentage of hexafluoropropylene monomer units in the first polyvinylidene fluoride copolymer is lower than the molar percentage of hexafluoropropylene monomer units in the second polyvinylidene fluoride copolymer.

2. The battery separator according to claim 1, characterized in that: The alumina particles have a particle size of 0.1-0.4 micrometers.

3. The battery separator according to claim 1, characterized in that: The molar percentage of hexafluoropropylene monomer units in the first polyvinylidene fluoride copolymer is 4%-6%, and the molar percentage of hexafluoropropylene monomer units in the second polyvinylidene fluoride copolymer is 8%-10%.

4. The battery separator according to claim 1, characterized in that: The mass ratio of the first polyvinylidene fluoride copolymer to the second polyvinylidene fluoride copolymer is 1:1 to 1:

9.

5. The battery separator according to claim 1, characterized in that: The mass ratio of the alumina particles to the total mass of the first polyvinylidene fluoride copolymer and the second polyvinylidene fluoride copolymer is 3-6:4-7.

6. The battery separator according to claim 1, characterized in that: The battery separator is prepared by a method comprising the following steps: 1) dissolving a first polyvinylidene fluoride copolymer in an organic solvent to obtain solution A; 2) dissolving a second polyvinylidene fluoride copolymer in an organic solvent to obtain solution B; 3) dispersing alumina particles in an organic solvent and grinding them to obtain dispersion C; 4) mixing dispersion C and solution A, and then adding solution B to obtain a mixture. 5) Grind the mixture to obtain an oil-based slurry; 6) The oil-based slurry is coated on both sides of the polyolefin substrate and dried to obtain the battery separator.

7. The method for preparing the battery separator according to any one of claims 1-6, characterized in that: The preparation method includes dispersing the alumina particles in an organic solvent, dissolving the first polyvinylidene fluoride copolymer and the second polyvinylidene fluoride copolymer in an organic solvent, mixing the three to obtain a mixture, grinding the mixture to obtain an oil-based slurry, and coating the oil-based slurry to obtain a battery separator.

8. The method for preparing the battery separator according to claim 7, characterized in that: The preparation method includes the following steps: 1) dissolving the first polyvinylidene fluoride copolymer in an organic solvent to obtain solution A; 2) dissolving the second polyvinylidene fluoride copolymer in an organic solvent to obtain solution B; 3) dispersing alumina particles in an organic solvent and grinding them to obtain dispersion C; 4) mixing dispersion C and solution A, and then adding solution B to obtain a mixture; 5) grinding the mixture to obtain an oil-based slurry; 6) coating the oil-based slurry on both sides of the polyolefin substrate and drying it to obtain the battery separator.

9. The method for preparing the battery separator according to claim 8, characterized in that: In steps 3) and 5), the grinding is performed using grinding beads with a particle size of 0.4-0.6 micrometers; and / or, in step 3), the grinding time is 30-90 minutes; and / or, in step 5), the grinding time is 30-90 minutes.

10. The method for preparing the battery separator according to claim 8, characterized in that: In solution A, the mass percentage of the first polyvinylidene fluoride copolymer is 6%-10%; and / or, in solution B, the mass percentage of the second polyvinylidene fluoride copolymer is 6%-10%; and / or, in dispersion C, the mass percentage of the alumina particles is 10%-14%; and / or, the organic solvent is dimethylacetamide.

11. Use of the battery separator according to any one of claims 1-6 in a lithium-ion battery.

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