Battery separator and method of making the same

Abstract

The application belongs to the technical field of batteries, and discloses a battery diaphragm and a preparation method thereof. The battery diaphragm comprises a non-woven fabric film, and a ceramic slurry coating is coated on the outer wall surface of the non-woven fabric film, wherein the components of the ceramic slurry coating comprise a solvent, an adhesive and ion liquid modified halloysite tubes. The ion liquid modified halloysite tubes are used for slurry preparation, which can realize uniform dispersion of nanoparticles in the polymer adhesive and avoid the problem that the nanoparticles are prone to aggregation. The entanglement effect of the peripheral organic chain of the nanoparticles and the chain segment of the polymer adhesive can also greatly improve the structural stability and mechanical properties of the coating, and improve the puncture resistance of the diaphragm. At the same time, the quaternary ammonium salt contained in the modified halloysite tubes can also have an adsorption effect on the anions in the electrolyte, thereby improving the transfer behavior of lithium ions, increasing the lithium ion transference number, weakening the concentration polarization phenomenon and inhibiting the growth of lithium dendrites, and further improving the cycle life of the corresponding battery.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and in particular to a battery separator and its preparation method. Background Technology

[0002] With the rapid development of new energy vehicles and energy storage, power lithium-ion battery technology has gradually become a research hotspot, which places higher demands on various battery components. The separator, located between the positive and negative electrodes, does not participate in the electrochemical reaction, but plays a role in isolating the positive and negative electrodes and allowing lithium ions to pass through quickly, thus determining the battery's charging, discharging, and safety performance.

[0003] Nonwoven membranes offer advantages such as low cost and high porosity, and can achieve high-temperature resistance through raw material selection, making them a future trend in power lithium-ion battery membrane development. However, nonwoven membranes formed by arbitrary fiber stacking suffer from problems such as large and uneven pore size and poor mechanical properties. Appropriate methods are needed to optimize pore size and mechanical properties before they can be used as lithium-ion battery membranes. Otherwise, direct use will cause battery short circuits. Introducing a slurry prepared by mixing adhesives such as polyvinylidene fluoride (PVDF) and nanoparticles onto the nonwoven surface is a common method for optimizing membrane pore size.

[0004] However, this traditional coating still has the following problems: under the long-term swelling effect of the electrolyte, the coating is prone to peeling off, leading to micro-short circuits in the battery and degradation of battery performance; the introduction of traditional coatings has limited improvement on the mechanical properties of the nonwoven membrane, which is not conducive to the suppression of lithium dendrites and the stability of battery performance; and it lacks the ability to regulate lithium-ion transfer behavior. In summary, traditional coatings are prone to peeling and have poor mechanical properties. Summary of the Invention

[0005] The main objective of this invention is to provide a battery separator and its preparation method, which aims to solve the problems of easy peeling and poor mechanical properties of traditional coatings.

[0006] To achieve the above objectives, the present invention provides a battery separator comprising a nonwoven membrane, wherein a ceramic slurry coating is coated on the outer wall surface of the nonwoven membrane, wherein the components of the ceramic slurry coating include a solvent, an adhesive, and an ionic liquid-modified halloysite tube.

[0007] In one embodiment, the mass concentration of the adhesive ranges from 5% to 15%.

[0008] The mass concentration range of the ionic liquid-modified halloysite tube is 0.5%-4.5%.

[0009] In one embodiment, the solvent is one or two of acetone, N,N-dimethylformamide, or N,N-dimethylacetamide.

[0010] In one embodiment, the adhesive is one of polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene, polyvinylidene fluoride-trichloroethylene, or polytetrafluoroethylene.

[0011] The present invention also provides a method for preparing a battery separator, characterized by comprising the following steps:

[0012] The adhesive and ionic liquid-modified halloysite tubes were dispersed in a solvent and stirred evenly to obtain a physically cross-linked ceramic slurry.

[0013] The ceramic slurry is coated onto the surface of the nonwoven membrane;

[0014] The nonwoven membrane is dried to obtain a nonwoven-based composite separator, which forms the battery separator.

[0015] In one embodiment, the method for preparing the ionic liquid-modified halloysite tube includes:

[0016] Grafting of halloysite tubes onto the surface of dimethyloctadecyl[3-(trimethoxysilyl)propyl]ammonium chloride;

[0017] The halloysite tube modified with the ionic liquid was obtained by ion exchange of grafted halloysite tubes with sodium α-sulfonyl-ω-nonylphenoxy polyoxyethylene ether.

[0018] The feeding ratio of the halloysite tube, the dimethyloctadecyl[3-(trimethoxysilyl)propyl]ammonium chloride, and the sodium α-sulfonyl-ω-nonylphenoxy polyoxyethylene ether salt is 1:2 to 5:1 to 3.

[0019] In one embodiment, the coating method for coating the ceramic slurry onto the surface of the nonwoven membrane includes any one of immersion coating, blade coating, cast coating, or roller coating.

[0020] In one embodiment, the drying process for the nonwoven membrane includes either vacuum drying or forced-air drying.

[0021] In one embodiment, after drying the nonwoven film, the process further includes:

[0022] The nonwoven film is hot-pressed and leveled.

[0023] This invention provides a battery separator by using halloysite tubes modified with ionic liquid in slurry preparation. This achieves uniform dispersion of nanoparticles in the polymer binder, avoiding the problems of nanoparticle aggregation and weak interfacial interactions. Simultaneously, the entanglement effect between the organic chains surrounding the nanoparticles and the polymer binder segments significantly improves the structural stability and mechanical properties of the coating. This not only solves the problem of coating detachment during long-term cycling but also improves the puncture resistance of the separator, thereby enhancing its long-term cycling stability. The halloysite tubes modified with ionic liquid used in this invention contain quaternary ammonium salts that adsorb anions in the electrolyte, thereby improving lithium-ion transport behavior, increasing lithium-ion transference number, reducing concentration polarization, and inhibiting lithium dendrite growth, thus further enhancing the cycle life of the corresponding battery. Attached Figure Description

[0024] Figure 1 These are SEM images of the membranes prepared in Example 2 and Comparative Example 1 of this invention;

[0025] Figure 2 These are the stress-strain curves of the diaphragms prepared in Example 2 and Comparative Example 1 of this invention;

[0026] Figure 3 The figures show the chronocurrent curve and lithium-ion transference number of the separator prepared in Example 2 of this invention (the inset shows the interfacial impedance spectrum before and after battery polarization).

[0027] Figure 4 The figures show the chronocurrent curves and lithium-ion transference numbers of the separator prepared in Comparative Example 1 of this invention (the inset shows the interfacial impedance spectra before and after battery polarization).

[0028] Figure 5 These are the cycle stability curves of the batteries assembled with the separators prepared in Example 2 and Comparative Example 1 of this invention.

[0029] The realization of the objective, functional features and advantages of the present invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0030] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0031] It should be noted that if the embodiments of the present invention involve directional indicators (such as up, down, left, right, front, back, etc.), the directional indicators are only used to explain the relative positional relationship and movement of the components in a certain specific posture (as shown in the figure). If the specific posture changes, the directional indicators will also change accordingly.

[0032] Furthermore, if the embodiments of this invention involve descriptions such as "first" or "second," these descriptions are for descriptive purposes only and should not be construed as indicating or implying their relative importance or implicitly specifying the number of technical features indicated. Therefore, a feature defined with "first" or "second" may explicitly or implicitly include at least one of those features. Additionally, the meaning of "and / or" throughout the text includes three parallel solutions; for example, "A and / or B" includes solution A, solution B, or a solution where both A and B are satisfied simultaneously. Furthermore, the technical solutions of the various embodiments can be combined with each other, but this must be based on the ability of those skilled in the art to implement them. When the combination of technical solutions is contradictory or impossible to implement, it should be considered that such a combination of technical solutions does not exist and is not within the scope of protection claimed by this invention.

[0033] A battery separator is provided; comprising a nonwoven membrane, wherein a ceramic slurry coating is coated on the outer wall surface of the nonwoven membrane, wherein the components of the ceramic slurry coating include a solvent, an adhesive, and an ionic liquid-modified halloysite tube.

[0034] In the battery separator provided by this invention, halloysite tubes modified with ionic liquid are used in slurry preparation, which can achieve uniform dispersion of nanoparticles in the polymer binder, avoiding the problems of easy aggregation of nanoparticles and weak interfacial interaction. At the same time, the entanglement effect between the organic chains on the periphery of the nanoparticles and the polymer binder segments can also significantly improve the structural stability and mechanical properties of the coating. This not only solves the problem of easy coating detachment during long-term cycling, but also improves the puncture resistance of the separator, thereby improving the long-term cycling stability of the separator. The halloysite tubes modified with ionic liquid used in this invention contain quaternary ammonium salts that can adsorb anions in the electrolyte, thereby improving the lithium-ion transport behavior, increasing the lithium-ion transference number, weakening concentration polarization, and inhibiting the growth of lithium dendrites, thereby further improving the cycle life of the corresponding battery.

[0035] Furthermore, the mass concentration of the adhesive ranges from 5% to 15%; the mass concentration of the ionic liquid-modified halloysite tube ranges from 0.5% to 4.5%.

[0036] On the other hand, the solvent is one or two of acetone, N,N-dimethylformamide or N,N-dimethylacetamide.

[0037] On the other hand, the adhesive is one of polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene, polyvinylidene fluoride-trichloroethylene, or polytetrafluoroethylene.

[0038] Based on the above-described battery separator, the present invention also provides a method for preparing a battery separator, comprising the following steps:

[0039] S1. Disperse the adhesive and ionic liquid-modified halloysite tube in a solvent and stir evenly to obtain a physically cross-linked ceramic slurry;

[0040] S2. Coat the ceramic slurry onto the surface of the nonwoven fabric membrane;

[0041] S3. The nonwoven membrane is dried to obtain a nonwoven composite separator, which forms the battery separator.

[0042] Furthermore, the preparation method of the ionic liquid-modified halloysite tube includes:

[0043] S01, grafting dimethyloctadecyl[3-(trimethoxysilyl)propyl]ammonium chloride onto the surface of halloysite tubes;

[0044] S02. Ion exchange was performed on the grafted halloysite tube using sodium α-sulfonyl-ω-nonylphenoxy polyoxyethylene ether to obtain the ionic liquid modified halloysite tube.

[0045] The feeding ratio of the halloysite tube, the dimethyloctadecyl[3-(trimethoxysilyl)propyl]ammonium chloride, and the sodium α-sulfonyl-ω-nonylphenoxy polyoxyethylene ether salt is 1:2 to 5:1 to 3.

[0046] Specifically, the coating method for coating the ceramic slurry onto the surface of the nonwoven film includes any one of immersion coating, blade coating, cast coating, or roller coating.

[0047] On the other hand, the drying method for the nonwoven film includes either vacuum drying or forced-air drying.

[0048] Furthermore, after drying the nonwoven film, the process further includes:

[0049] The nonwoven film is hot-pressed and leveled.

[0050] It should be noted that the hot pressing leveling temperature is 60-120℃ and the pressure is 5-30Mpa.

[0051] Example 1

[0052] (1) Preparation of physically cross-linked ceramic slurry

[0053] Polyvinylidene fluoride (PVDF) adhesive and halloysite tube modified with ionic liquid were dissolved in DMF (N,N-dimethylformamide) and stirred evenly to obtain a physically cross-linked ceramic slurry.

[0054] In the ceramic coating slurry, the mass concentration of the binder is 5%, and the mass concentration of the ionic liquid-modified halloysite tube is 0.5%.

[0055] The halloysite tube modified by the ionic liquid was prepared by grafting dimethyloctadecyl[3-(trimethoxysilyl)propyl]ammonium chloride (DC5700) onto the surface of the nanotube and then by ion exchange with sodium α-sulfonyl-ω-nonylphenoxy polyoxyethylene ether (NPEP), wherein the ratio of halloysite tube, DC5700 and NPEP was 1:2:1.

[0056] (2) Preparation of nonwoven fabric-based composite membrane

[0057] The ceramic slurry prepared in (1) was coated onto the surface of the PP nonwoven membrane by dip coating. After drying at 60°C, it was hot-pressed and leveled under a pressure of 10 MPa and a temperature of 60°C to obtain a nonwoven composite membrane.

[0058] Example 2

[0059] (1) Preparation of physically cross-linked ceramic slurry

[0060] The adhesive polyvinylidene fluoride-hexafluoropropylene and the halloysite tube modified by ionic liquid were dissolved in a mixed solvent of DMF and acetone (ratio of 10:1) and stirred evenly to obtain a physically cross-linked ceramic slurry.

[0061] In the ceramic coating slurry, the mass concentration of the binder is 10%, and the mass concentration of the ionic liquid-modified halloysite tube is 2%.

[0062] The halloysite tube modified by the ionic liquid was prepared by grafting dimethyloctadecyl[3-(trimethoxysilyl)propyl]ammonium chloride (DC5700) onto the surface of the nanotube and then by ion exchange with sodium α-sulfo-ω-nonylphenoxy polyoxyethylene ether (NPEP), wherein the ratio of halloysite tube, DC5700 and NPEP was 1:4:2.

[0063] (2) Preparation of nonwoven fabric-based composite membrane

[0064] The ceramic slurry prepared in (1) was coated onto the surface of the PET nonwoven membrane by a doctor blade coating method. After drying at 80°C, it was hot-pressed and leveled under a pressure of 20 MPa and a temperature of 80°C to obtain a nonwoven composite membrane.

[0065] Example 3

[0066] (1) Preparation of physically cross-linked ceramic slurry

[0067] The adhesive polyvinylidene fluoride-trichloroethylene and the halloysite tube modified by ionic liquid were dissolved in DMF and stirred evenly to obtain a physically cross-linked ceramic slurry.

[0068] In the ceramic coating slurry, the mass concentration of the binder is 15%, and the mass concentration of the ionic liquid-modified halloysite tube is 4.5%.

[0069] The halloysite tube modified by the ionic liquid was prepared by grafting dimethyloctadecyl[3-(trimethoxysilyl)propyl]ammonium chloride (DC5700) onto the surface of the nanotube and then by ion exchange with sodium α-sulfonyl-ω-nonylphenoxy polyoxyethylene ether (NPEP), wherein the ratio of halloysite tube, DC5700 and NPEP was 1:5:3.

[0070] (2) Preparation of nonwoven fabric-based composite membrane

[0071] The ceramic slurry prepared in (1) was coated onto the surface of the PPS nonwoven membrane by roller coating. After drying at 100°C, it was hot-pressed and leveled under a pressure of 25 MPa and a temperature of 100°C to obtain a nonwoven composite membrane.

[0072] Based on the above-described battery separator, the present invention provides a comparative embodiment of a prior art technology.

[0073] Comparative Example 1

[0074] (1) Preparation of slurry

[0075] The adhesive polyvinylidene fluoride and halloysite tube are dissolved in DMF and stirred evenly to obtain a coating slurry.

[0076] In the ceramic coating slurry, the mass concentration of the adhesive is 10%, and the mass concentration of the halloysite tube is 2%.

[0077] (2) Preparation of nonwoven fabric-based composite membrane

[0078] The slurry prepared in (1) was coated onto the surface of the PET nonwoven membrane by dip coating, dried at 80°C, and then hot-pressed and leveled at a pressure of 20 MPa and a temperature of 80°C to obtain the nonwoven composite membrane C.

[0079] Performance testing

[0080] The membranes prepared in Example 2 and Comparative Example 1 of this invention were imaged using a scanning electron microscope. Figure 1 (Left) is a SEM image of the diaphragm prepared in Example 2 of the present invention. Figure 1 (Right) is a SEM image of the membrane surface of Comparative Example 1 of this invention. Using halloysite tubes modified with ionic liquids to prepare the slurry can significantly reduce the aggregation of nanoparticles in the matrix. Simultaneously, the introduction of organic segments onto the surface of the halloysite tubes facilitates the regulation of polymer matrix segments during the solvent evaporation and pore-forming process, resulting in richer pores that are beneficial for electrolyte adsorption and storage.

[0081] The mechanical properties of Embodiment 2 and Comparative Example 1 of the present invention were tested, wherein the length and width of the sample were 4mm × 15mm, and the tensile rate during the test was 10mm / min. Figure 2 These are stress-strain curves of Example 2 and Comparative Example 1 of the present invention. The separator prepared in Example 2 exhibits a higher tensile strength of approximately 15 MPa, which is significantly higher than that of the separator prepared in Comparative Example 1. This is mainly due to the effective physical entanglement between the polymer segments in the polymer binder and the organic segments on the surface of the halloysite tube, thereby improving the cohesiveness and tensile strength of the coating. This interaction helps the coating maintain structural stability during long-term swelling by the electrolyte, preventing it from easily detaching from the substrate surface and thus ensuring the electrochemical performance during battery cycling.

[0082] The separators prepared in Example 2 and Comparative Example 1 of this invention were assembled into simulated batteries with lithium sheets as positive and negative electrodes. The current-time curves were monitored using an electrochemical workstation under a bias voltage of 10 mV. Furthermore, electrochemical impedance spectroscopy was performed on the batteries before and after polarization. Combining the test results, the lithium-ion transference number of the separator was calculated using the following formula:

[0083]

[0084] Where: I0 is the initial current obtained by the chronoamperometry method; I s R0 is the steady-state current obtained by the chronoamperometry method; R0 is the interface impedance at the initial moment obtained by EIS; R0 s ΔV is the interface impedance at steady state obtained by EIS; ΔV is the voltage applied by the chronoamperometry method, with a magnitude of 10mV. Figure 3 and Figure 4 The chronocurrent curves and lithium-ion transference numbers of the separators prepared in Example 2 and Comparative Example 1 are shown respectively. The lithium-ion transference number of the separator prepared in Example 2 is 0.51, which is much higher than that of the separator prepared in Comparative Example 1 (0.31). This is mainly because the organic chains on the surface of the halloysite tube contain quaternary ammonium salt functional groups, which can adsorb anions in the electrolyte, thereby promoting the ability of lithium ions to transport charge during battery operation and thus increasing the lithium-ion transference number. This will help to reduce concentration polarization during battery operation, homogenize the distribution of lithium ions, and inhibit the growth of lithium dendrites.

[0085] The separators prepared in Example 2 and Comparative Example 1 of the present invention were assembled into button batteries with lithium iron phosphate as the positive electrode and lithium foil as the negative electrode in an argon-filled glove box. The lithium iron phosphate positive electrode was obtained by coating an aluminum foil with a slurry formed from polyvinylidene fluoride binder, acetylene black, and lithium iron phosphate (mass ratio 1:1:8). The batteries were subjected to 100 charge-discharge cycles at a set 0.5C / 0.5C rate. Figure 5 The battery cycle stability curves for Example 2 and Comparative Example 1 of this invention are shown. The separator prepared in Example 2 provides more stable cycle performance for the battery, with a performance retention rate of 98.5% after 100 charge-discharge cycles. In contrast, the cycle performance curve of the separator prepared in Comparative Example 1 shows a significant downward trend, with a performance retention rate of only 81.3% after 100 charge-discharge cycles. This is mainly due to the effective physical entanglement between the polymer chains of the binder and the organic chains on the surface of the halloysite tube, which enhances the structural stability of the coating and its resistance to electrolyte swelling, reducing the risk of coating detachment and thus ensuring the charge-discharge performance of the battery during cycling.

[0086] The above are merely preferred embodiments of the present invention and do not limit the scope of the patent. Any equivalent structural or procedural transformations made based on the description and drawings of the present invention, or direct or indirect applications in other related technical fields, are similarly included within the scope of patent protection of the present invention.

Claims

1. A battery separator, characterized by, The application relates to a non-woven fabric membrane coated with a ceramic slurry coating on the outer wall surface, wherein the components of the ceramic slurry coating include a solvent, a binder and ion liquid modified halloysite tubes. The preparation method of the ion liquid modified halloysite tubes comprises the following steps: Grafting dimethyloctadecyl [3-(trimethoxysilyl) propyl] ammonium chloride on the surface of halloysite tubes; Ion exchange of the grafted halloysite tubes by alpha-sulfo-omega-nonyl phenoxy polyoxyethylene ether sodium salt to obtain the ion liquid modified halloysite tubes. The feeding ratio of the halloysite tubes, the dimethyloctadecyl [3-(trimethoxysilyl) propyl] ammonium chloride and the alpha-sulfo-omega-nonyl phenoxy polyoxyethylene ether sodium salt is 1:2-5:1-3.

2. The battery separator of claim 1, wherein, The mass concentration of the binder ranges from 5% to 15%. The mass concentration of the ion liquid modified halloysite tubes ranges from 0.5% to 4.5%.

3. The battery separator of claim 1, wherein, The solvent is one or two of acetone, N,N-dimethylformamide or N,N-dimethylacetamide.

4. The battery separator of claim 1, wherein, The binder is one of polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene, polyfluorodiene-trichloroethylene or polytetrafluoroethylene.

5. A method for producing a battery separator for producing the battery separator according to any one of claims 1 to 4, characterized by, The application comprises the following steps: The binder and the ion liquid modified halloysite tubes are dispersed in the solvent and stirred uniformly to obtain a physically crosslinked ceramic slurry; The ceramic slurry is coated on the surface of the non-woven fabric membrane; The non-woven fabric membrane is dried to obtain a non-woven fabric-based composite diaphragm to form the battery diaphragm.

6. The method of claim 5, wherein the polymeric material is a polyolefin. The coating mode of the ceramic slurry on the surface of the non-woven fabric membrane comprises any one of immersion coating, doctor blade coating, flow coating or roll coating.

7. The method of claim 5, wherein the polymeric material is a polyolefin. The drying mode of the non-woven fabric membrane comprises any one of vacuum drying or air blowing drying.

8. The method of claim 5, wherein the battery separator is prepared by the steps of: After the drying treatment of the non-woven fabric membrane, the following steps are further included: Hot pressing and flattening the non-woven fabric membrane.

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