A surface-enriched pvdf microsphere composite microporous separator and a method of preparing the same

Abstract

This invention relates to a composite microporous separator with surface-enriched PVDF microspheres and its preparation method. The separator comprises a base film and a coating layer thereon, the coating being composed of a high-temperature resistant polymer, an adhesive polymer, and inorganic ceramic particles. Phase separation technology is used to induce the high-temperature resistant polymer to form a three-dimensional porous network framework, while simultaneously inducing the adhesive polymer to agglomerate into microspheres and enrich on the coating surface. This structure significantly improves the utilization rate of the binder, significantly enhances the adhesion between the separator and the electrode while reducing material usage, and improves the membrane breakage temperature and dimensional stability. The preparation method employs a one-step coating combined with a coagulation bath phase separation process, simplifying the production process and reducing costs. This invention combines the advantages of high safety, high adhesion, and low cost, making it suitable for various types of lithium batteries.

Description

Technical Field

[0001] This invention belongs to the field of lithium battery separator technology, and particularly relates to a composite microporous separator with PVDF microspheres enriched on its surface and its preparation method. Background Technology

[0002] As one of the four main materials in lithium batteries, the lithium battery separator plays a crucial role in battery safety. Functionally, lithium battery separators have two main requirements: first, they must act as electrical insulators to effectively isolate the positive and negative electrodes and prevent internal short circuits; second, they must have a porous structure to allow lithium ions to move freely between the positive and negative electrodes.

[0003] In existing lithium battery assembly processes, conventional polyethylene (PE), polypropylene (PP), or ceramic separators coated with alumina on one or both sides are typically used. However, these separators lack sufficient adhesion to the battery electrodes, resulting in loosely wound cells. This leads to low battery hardness and makes the cells prone to interface delamination after long-term cycling, which in turn causes problems such as shortened cycle life and poor safety performance.

[0004] To address this deficiency, the industry typically applies an additional layer of polyvinylidene fluoride (PVDF) material (usually PVDF-HFP, a copolymer of vinylidene fluoride and hexafluoropropylene) to the ceramic separator surface using spraying or gravure roller coating. After assembly into a battery cell, a hot-pressing process can significantly improve battery rigidity and the separator-electrode interface, thereby enhancing battery cycle life and safety performance. However, current conventional PVDF-containing separators and their preparation methods still face the following pressing issues:

[0005] (1) Insufficient safety: The alumina used in traditional ceramic coatings is mostly nano-sized particles, which are only bonded together by a small amount of adhesive. This structure makes the coating extremely prone to pulverization and breakage at high temperatures. Therefore, although its heat shrinkage performance is significantly improved compared to pure PE base film, its overall safety is still poor.

[0006] (2) The process is complicated and costly: Existing coating processes usually require a ceramic coating to be applied first, and then the PVDF coating to be applied after drying (for example, spraying or roller coating with water-based PVDF emulsion, or roller coating with oil-based PVDF dissolved in organic solvents and supplemented with a coagulation bath to create pores). This process of applying the ceramic layer and PVDF layer in separate steps not only increases the process cost, but also reduces the overall yield, and there is a high risk of loss during intermediate transportation.

[0007] (3) Low PVDF utilization and impaired thermal performance: If PVDF and ceramic particles are mixed into an emulsion and then directly coated, although the process is simplified, it will result in a uniform distribution of PVDF and ceramic particles in the coating. This uniform distribution means that a large portion of the PVDF particles are encased inside the coating and cannot contact the electrode surface after hot pressing, thus failing to bond the separator to the electrode. This not only results in low bonding strength but also causes a serious waste of PVDF material. At the same time, the presence of a large amount of PVDF will also lead to a significant deterioration in the thermal properties of the coated separator, such as thermal shrinkage rate and membrane rupture temperature.

[0008] To address these issues, the industry has attempted to blend PVDF with high-temperature resistant polymers (such as polyimide PI). For example, patent application No. 202410444684.8, entitled "A Bonding Heat-Resistant Composite Polymer Coated Separator and its Preparation Method, Secondary Battery," discloses a scheme in which a bonding polymer (such as PVDF) and a high-temperature resistant polymer (such as PI) are blended and then coated onto both sides of a base membrane. However, in this scheme, the PVDF and PI are relatively evenly distributed in the coating, and only the PVDF on the outermost layer of the coating can truly provide adhesion. The PVDF inside the coating is not only completely wasted, but its relatively low temperature resistance also lowers the overall high-temperature resistance limit of the separator.

[0009] In addition, other existing technologies (such as the patent application "A Composite Polymer Membrane and Its Preparation Method" by Guangdong Jingjin Energy Co., Ltd. with application number 201210582514.3) also disclose composite coatings of PI and PVDF, but their formulation systems do not contain inorganic ceramic particles, resulting in a serious lack of thermal shrinkage performance, which cannot solve the technical contradiction faced by this invention.

[0010] In summary, there is an urgent need for a new lithium battery separator technology that integrates high adhesion, high membrane rupture temperature, low thermal shrinkage, and high process compatibility. Summary of the Invention

[0011] The purpose of this invention is to overcome the shortcomings of the prior art and provide a composite microporous membrane with PVDF microspheres enriched on its surface and its preparation method.

[0012] A composite microporous membrane with PVDF microspheres enriched on its surface includes a base membrane and a coating applied to at least one surface of the base membrane.

[0013] Considering the different thicknesses and weights of the coatings, as well as the different densities of the inorganic particles, the coating is made from raw materials containing the following parts by weight: 100 parts of high-temperature resistant polymer, 50-400 parts of adhesive polymer, and 30-120 parts of inorganic ceramic particles or solid electrolyte particles with ionic conductivity. The main function of the high-temperature resistant polymer is to construct a three-dimensional network structure, which is the main material of the coating network. The inorganic ceramic particles or solid electrolyte particles with ionic conductivity provide better thermal shrinkage, and the particles are evenly distributed between the three-dimensional network structure constructed by the high-temperature resistant polymer.

[0014] The weight percentage of inorganic ceramic particles or solid electrolyte particles with ionic conductivity is close to or slightly greater than that of high-temperature resistant polymers to provide better heat shrinkage performance, while reducing the air permeability of the membrane and improving its air permeability. If the content of inorganic ceramic particles is too high, the heat shrinkage performance is good, but the membrane rupture temperature is low and the coating integration is poor. If the content of inorganic ceramic particles is too low, the heat shrinkage performance will be slightly worse.

[0015] In the coating, the high-temperature resistant polymer forms a three-dimensional porous network structure through phase separation, and the adhesive polymer agglomerates in the coating to form a microsphere structure through phase separation, with at least some of the microsphere structures distributed on the coating surface.

[0016] As a preferred option:

[0017] Adhesive polymers include: polyvinylidene fluoride (PVDF), polyacrylate, polyacrylonitrile, and polyvinylidene fluoride-hexafluoropropylene copolymer; adhesive polymers can also be copolymers of polyvinylidene fluoride and hexafluoroisopropanol (PVDF-HFP), in which the proportion of polyvinylidene fluoride is not less than 75%, ensuring both good adhesion and a low melting point, which is beneficial for hot pressing.

[0018] High-temperature resistant polymers include: soluble polyimide (PI, such as P84), polyetherimide (PEI), polyamideimide (PAI), polyethersulfone (PES), polyarylsulfone, aramid, and polyetheretherketone (PEEK) and other soluble high-temperature resistant polymer materials;

[0019] Inorganic ceramic particles include: fumed silica with a particle size D50 of 20–100 nm, fumed alumina with a particle size D50 of 30–150 nm, zirconium oxide, titanium oxide, nano-alumina with a particle size D50 of 150–1000 nm, and nano-boehmite with a particle size D50 of 30–800 nm.

[0020] The solid electrolyte particles with ionic conductivity are lithium aluminum titanium phosphate (LATP) with a particle size of D50 of 200 nm to 1 μm.

[0021] As a preferred option:

[0022] The coating is made from the following raw materials in parts by weight: 100 parts soluble polyimide (PI), 50-130 parts polyvinylidene fluoride (PVDF), and 20-300 parts inorganic ceramic particles.

[0023] In the coating, soluble polyimide forms a three-dimensional porous network structure (polyimide molecular chains have poor mobility and cannot rearrange themselves quickly enough, so they tend to form a three-dimensional porous network structure, which can effectively improve the coating integration of the diaphragm, increase the diaphragm rupture temperature, reduce the thermal shrinkage of the diaphragm, improve electrolyte wettability, and improve liquid absorption and retention capacity). Polyvinylidene fluoride (PVDF) agglomerates in the coating to form microsphere structures (PVDF acts as an adhesive material, and PVDF molecular chains tend to form microspheres), and at least some of the microsphere structures are distributed on the coating surface; the content of PVDF in the coating located on the base membrane surface is 0.2–0.8 g / m³. 2 Too much PVDF results in excellent bonding performance but also a large increase in air permeability and material waste; too little PVDF results in poor bonding performance and fails to bond the electrodes and separator or harden the battery.

[0024] As a preferred option: without the addition of inorganic particles, the air permeability of oil-coated separators is usually too high, even exceeding 300 s / 100mL. For battery manufacturers, this air permeability is too large and difficult to apply. Adding inorganic ceramic particles or solid electrolyte particles with ionic conductivity can improve the thermal shrinkage and air permeability of the separator. The more inorganic particles, the better it is for reducing thermal shrinkage. However, the proportion of inorganic particles should not be too high to prevent the membrane rupture temperature from being too low. The air permeability of the separator is 100-300 s / 100mL (preferably 150-200 s / 100mL), which is within the normal air permeability range for lithium battery separators. The air permeability of the separator in this range is quite good, even better than that of high-temperature resistant aramid separators. It is especially suitable for situations where lithium batteries are required to have both fast charging performance and safety performance, which is of great significance.

[0025] A composite microporous membrane with PVDF microspheres enriched on its surface includes a base membrane and a coating applied to at least one surface of the base membrane.

[0026] The coating is made from the following raw materials in parts by weight: 100 parts of high-temperature resistant polymer, 30-145 parts of adhesive polymer, and 0-500 parts of inorganic ceramic particles or solid electrolyte particles with ionic conductivity. The main function of the high-temperature resistant polymer is to construct a three-dimensional network structure, which is the main material of the coating network. This raw material ratio can ensure that the slurry itself does not undergo phase separation or hard agglomeration.

[0027] In the coating, the high-temperature resistant polymer forms a three-dimensional porous network structure through phase separation, and the adhesive polymer agglomerates in the coating to form a microsphere structure through phase separation, with at least some of the microsphere structures distributed on the coating surface.

[0028] As a preferred option:

[0029] Adhesive polymers include: polyvinylidene fluoride (PVDF), polyacrylate, polyacrylonitrile, and polyvinylidene fluoride-hexafluoropropylene copolymer; adhesive polymers can also be copolymers of polyvinylidene fluoride and hexafluoroisopropanol (PVDF-HFP), in which the proportion of polyvinylidene fluoride is not less than 75%, ensuring both good adhesion and a low melting point, which is beneficial for hot pressing.

[0030] High-temperature resistant polymers include: soluble polyimide (PI, such as P84), polyetherimide (PEI), polyamideimide (PAI), polyethersulfone (PES), polyarylsulfone, aramid, and polyetheretherketone (PEEK) and other soluble high-temperature resistant polymer materials;

[0031] Inorganic ceramic particles include: fumed silica with a particle size D50 of 20–100 nm, fumed alumina with a particle size D50 of 30–150 nm, zirconium oxide, titanium oxide, nano-alumina with a particle size D50 of 150–1000 nm, and nano-boehmite with a particle size D50 of 30–800 nm.

[0032] The solid electrolyte particles with ionic conductivity are lithium aluminum titanium phosphate (LATP) with a particle size of D50 of 200 nm to 1 μm.

[0033] As a preferred option:

[0034] The coating does not contain inorganic ceramic particles or solid electrolyte particles with ionic conductivity; the coating is made from raw materials containing the following ratio: soluble polyimide (PI, such as P84): polyvinylidene fluoride (PVDF) = (1:1) to (3:1); the bulk density of the PI coating is 1.4 g / (μm*m). 2 If the coating has a porosity of 50%, then the surface density of the coating is approximately 0.7 g / (μm*m). 2 The PI is calculated as follows: 0.7g for a 1μm coating, 1.4g for a 2μm coating, and 2.1g for a 3μm coating. A fixed empirical value of 0.7g / side of PVDF provides adequate adhesion.

[0035] As a preferred option, the air permeability of the separator is 300 s / 100mL. Although this air permeability is relatively high for lithium batteries, it is acceptable in some lithium battery application scenarios where fast charging performance is not required.

[0036] A method for preparing the above-mentioned diaphragm includes the following steps:

[0037] According to the above proportions, the high-temperature resistant polymer, the adhesive polymer, and the inorganic ceramic particles or solid electrolyte particles with ionic conductivity are dispersed in an organic solvent to prepare a solution with a viscosity of 80–2000. A composite oily slurry with a solid content of 5%–20% and good coatability (if the solid content is too high, the slurry viscosity will inevitably be very high, resulting in poor coatability and excessively high porosity and permeability of the coating, leading to high internal resistance of the battery; if the solid content is too low, the slurry viscosity will be low, resulting in a looser coating and poorer thermal performance; if the viscosity is too low, the coating will be too loose after coating, failing to form a stable and firm three-dimensional network structure; if the viscosity is too high, it will lead to poor coatability, poor flowability, and slow coagulation process). The organic solvent should be at least one of the following: N-methyl-2-pyrrolidone (NMP), N,N-dimethylacetamide (DMAc), or N,N-dimethylformamide (DMF).

[0038] A composite oily slurry is applied to at least one surface of the base film (i.e., single-sided or double-sided coating) to form a wet coating.

[0039] The base film with the wet coating is immersed in a coagulation bath for coagulation treatment, which causes phase separation of the wet coating and forms a porous PVDF-based resin coating. If the proportion of organic solvent in the coagulation bath is too high, the phase separation rate is slow, and PVDF cannot quickly aggregate into microspheres or migrate to the surface, resulting in poor overall membrane adhesion. Therefore, the water content of the coagulation bath is 60 wt%~100 wt% (a higher water content in the coagulation bath makes it easier for PVDF and PI to undergo rapid and obvious phase separation due to their different molecular structures, forming a morphology of PVDF diffusing to the surface to form microspheres).

[0040] The base film with the porous coating is cleaned and dried at 50~100 °C to obtain a separator with a dry coating of 0.5~4 μm (the ratio of PI and PVDF in the separator will undergo phase separation, and this phase separation will be used to generate an island structure in the separator obtained after coating).

[0041] Preferably, the separator is used as a separator between the positive and negative electrodes of the lithium battery. The separator is configured to: isolate the positive and negative electrodes by the electrical insulation properties of the coating, and allow lithium ions to shuttle between the positive and negative electrodes by the ion transport channels formed by the three-dimensional porous network structure of the coating and the gaps between the microspheres.

[0042] As a preferred embodiment, in order to address the problem that PI and PVDF are prone to phase separation and are not stable enough in existing slurries, the present invention also slowly stirs the slurry at a speed of 10 to 400 rpm during the slurry storage and coating feeding process to avoid this problem.

[0043] The beneficial effects of this invention are:

[0044] This invention utilizes a three-dimensional porous network structure formed by high-temperature resistant polymers (such as PI) during phase separation as a framework, significantly improving the membrane rupture temperature and heat shrinkage resistance. This structure effectively overcomes the weakness of interparticle bonding in traditional ceramic coatings, maintaining structural integrity at high temperatures, preventing thermal runaway, and greatly enhancing battery safety.

[0045] This invention utilizes the polarity difference between polymers to induce PVDF to actively migrate to the coating surface and agglomerate into microspheres during the curing process. This allows most of the PVDF to be exposed on the coating surface and directly participate in bonding during hot pressing, solving the problem of PVDF being embedded and having low utilization in traditional blend coatings. It achieves stronger electrode adhesion and cell hardness with less material.

[0046] This invention, through the synergy of inorganic ceramic particles and a high-temperature resistant polymer network, significantly reduces the thermal shrinkage rate of the separator while maintaining reasonable porosity and air permeability. This allows the separator to possess both excellent dimensional stability and meet the requirements of lithium batteries for ion conductivity and fast charging performance, thus resolving the contradiction in existing technologies where "thickening the coating reduces shrinkage but worsens air permeability".

[0047] This invention achieves simultaneous enhancement of inorganic particle stacking and enrichment of PVDF microspheres through a single coating combined with coagulation bath phase separation technology. It eliminates the cumbersome process of step-by-step coating and multiple drying, significantly reducing equipment investment, energy consumption and intermediate transfer losses, and improving production yield.

[0048] The technical solution of this invention is not only applicable to the classic combination of PVDF and PI, but can also be extended to the combination of other adhesive polymers (such as polyacrylate) and high-temperature resistant polymers (such as PEI and PEEK), and is compatible with solid electrolyte particles. This allows the invention to adapt to lithium battery systems with different energy density and safety requirements, giving it extremely high industrial application value. Attached Figure Description

[0049] Figure 1SEM image at PI / PVDF=1:1, magnified 50,000 times;

[0050] Figure 2 The image is a SEM image at PI / PVDF = 1:0.6 and magnification of 50,000. Detailed Implementation

[0051] The present invention will be further described below with reference to embodiments. The description of the embodiments below is only for the purpose of helping to understand the present invention. It should be noted that those skilled in the art can make several modifications to the present invention without departing from the principle of the present invention, and these improvements and modifications also fall within the protection scope of the claims of the present invention.

[0052] Example 1

[0053] A composite microporous membrane with PVDF microspheres enriched on its surface includes a base membrane and a coating applied to at least one surface of the base membrane; the coating is made from raw materials comprising the following parts by weight: 5 parts of PI-1 with a molecular weight of 50,000, 5 parts of PVDF-HFP with a molecular weight of 60w, and 0 parts of inorganic ceramic particles or solid electrolyte particles.

[0054] The membrane is prepared as follows: PI-1 and PVDF-HFP are dispersed in NMP according to the above ratio to prepare a composite oily slurry with a viscosity of 164 mPa·s and a solid content of 10%; the composite oily slurry is coated on at least one surface of the base membrane to form a wet coating; the base membrane with the wet coating is immersed in a coagulation bath for coagulation treatment, causing phase separation of the wet coating and forming a porous coating; the base membrane with the porous coating is cleaned and dried to obtain a membrane with a 2 μm thick dry coating; Figure 1 As shown, in this membrane, PVDF microspheres are distributed on the coating surface.

[0055] Example 2

[0056] A composite microporous membrane with PVDF microspheres enriched on its surface includes a base membrane and a coating applied to at least one surface of the base membrane; the coating is made from raw materials comprising the following parts by weight: 7.5 parts of P84 with a molecular weight of 50,000, 4.5 parts of PVDF-HFP with a molecular weight of 60w, and 0 parts of inorganic ceramic particles or solid electrolyte particles.

[0057] P84 and PVDF-HFP were dispersed in NMP according to the above ratio to prepare a composite oily slurry with a viscosity of 185 mPa·s and a solid content of 12%. The composite oily slurry was coated on at least one surface of the base membrane to form a wet coating. The base membrane with the wet coating was immersed in a coagulation bath for coagulation treatment, so that the wet coating underwent phase separation and formed a porous coating. The base membrane with the porous coating was cleaned and dried to obtain a diaphragm with a dry coating of 2 μm thickness.

[0058] Example 3

[0059] A composite microporous membrane with PVDF microspheres enriched on its surface includes a base membrane and a coating applied to at least one surface of the base membrane; the coating is made from raw materials comprising the following parts by weight: 4 parts of PI-1 with a molecular weight of 50,000, 3 parts of PVDF-HFP with a molecular weight of 60w, and 5 parts of Q-SiO2 (Q represents the gas phase);

[0060] PI-1, PVDF-HFP, and Q-SiO2 (Q represents the gas phase) were dispersed in NMP according to the above ratio to prepare a composite oily slurry with a viscosity of 331 mPa·s and a solid content of 12%. The composite oily slurry was coated on at least one surface of the base membrane to form a wet coating. The base membrane with the wet coating was immersed in a coagulation bath for coagulation treatment, so that the wet coating underwent phase separation and formed a porous coating. The base membrane with the porous coating was cleaned and dried to obtain a diaphragm with a 2 μm thick dry coating.

[0061] Example 4

[0062] A composite microporous membrane with PVDF microspheres enriched on its surface includes a base membrane and a coating applied to at least one surface of the base membrane; the coating is made from raw materials comprising the following parts by weight: 4 parts of PEI with a molecular weight of 50,000, 3 parts of PVDF-HFP with a molecular weight of 60w, and 5 parts of Q-alumina (Q represents gas phase);

[0063] PEI, PVDF-HFP, and Q-alumina (Q represents the gas phase) are dispersed in NMP according to the above ratio to prepare a composite oily slurry with a viscosity of 327 mPa·s and a solid content of 12%. The composite oily slurry is coated on at least one surface of the base membrane to form a wet coating. The base membrane with the wet coating is immersed in a coagulation bath for coagulation treatment, so that the wet coating undergoes phase separation and forms a porous coating. The base membrane with the porous coating is cleaned and dried to obtain a diaphragm with a 2 μm thick dry coating.

[0064] Example 5

[0065] A composite microporous membrane with PVDF microspheres enriched on its surface includes a base membrane and a coating applied to at least one surface of the base membrane; the coating is made from raw materials comprising the following parts by weight: 5 parts of polyarylsulfone with a molecular weight of 48,000, 3 parts of PMMA with a molecular weight of 30w, and 4 parts of Q-SiO2 (Q represents gas phase).

[0066] Polyarylsulfone, PMMA, and Q-SiO2 (Q represents the gas phase) were dispersed in NMP according to the above ratio to prepare a composite oily slurry with a viscosity of 308 mPa·s and a solid content of 12%. The composite oily slurry was coated onto at least one surface of the base membrane to form a wet coating. The base membrane with the wet coating was immersed in a coagulation bath for coagulation treatment, causing phase separation of the wet coating and forming a porous coating. The base membrane with the porous coating was cleaned and dried to obtain a separator with a 2 μm thick dry coating. Figure 2 As shown, in this membrane, PVDF microspheres are distributed on the coating surface.

[0067] Example 6

[0068] A composite microporous membrane with PVDF microspheres enriched on its surface includes a base membrane and a coating applied to at least one surface of the base membrane; the coating is made from raw materials comprising the following parts by weight: 1 part of para-aramid with a molecular weight of 200,000, 1 part of PVDF / HFP with a molecular weight of 60w, and 2 parts of Q-SiO2 (Q represents gas phase);

[0069] Para-aramid, PVDF / HFP, and Q-SiO2 (Q represents the gas phase) are dispersed in NMP according to the above ratio to prepare a composite oily slurry with a viscosity of 206 mPa·s and a solid content of 4%. The composite oily slurry is coated on at least one surface of the base film to form a wet coating. The base film with the wet coating is immersed in a coagulation bath for coagulation treatment, so that the wet coating undergoes phase separation and forms a porous coating. The base film with the porous coating is cleaned and dried to obtain a diaphragm with a 2 μm thick dry coating.

[0070] Comparative Example 1

[0071] A diaphragm includes a base membrane and a coating applied to at least one surface of the base membrane; the coating is made of raw materials comprising the following parts by weight: 2 parts of PI-1 with a molecular weight of 50,000, 8 parts of PVDF-HFP with a molecular weight of 60w, and 2 parts of Q-SiO2 (Q represents gas phase);

[0072] PI-1, PVDF-HFP, and Q-SiO2 (Q represents the gas phase) were dispersed in NMP according to the above ratio to prepare a composite oily slurry with a viscosity of 196 mPa·s and a solid content of 12%. The composite oily slurry was coated on at least one surface of the base membrane to form a wet coating. The base membrane with the wet coating was immersed in a coagulation bath for coagulation treatment, so that the wet coating underwent phase separation and formed a porous coating. The base membrane with the porous coating was cleaned and dried to obtain a diaphragm with a 2 μm thick dry coating.

[0073] Comparative Example 2

[0074] A diaphragm includes a base membrane and a coating applied to at least one surface of the base membrane; the coating is made of raw materials comprising the following parts by weight: 8 parts of PI-1 with a molecular weight of 50,000, 2 parts of PVDF-HFP with a molecular weight of 60w, and 2 parts of Q-SiO2 (Q represents gas phase);

[0075] PI-1, PVDF-HFP, and Q-SiO2 (Q represents the gas phase) were dispersed in NMP according to the above ratio to prepare a composite oily slurry with a viscosity of 189 mPa·s and a solid content of 12%. The composite oily slurry was coated on at least one surface of the base membrane to form a wet coating. The base membrane with the wet coating was immersed in a coagulation bath for coagulation treatment, so that the wet coating underwent phase separation and formed a porous coating. The base membrane with the porous coating was cleaned and dried to obtain a diaphragm with a 2 μm thick dry coating.

[0076] Comparative Example 3

[0077] A separator includes a base membrane and a coating applied to at least one surface of the base membrane; the coating is made of raw materials comprising the following parts by weight: 2 parts of para-aramid with a molecular weight of 200,000, 0.5 parts of PVDF-HFP with a molecular weight of 60w, and 1.5 parts of Q-SiO2 (Q represents gas phase);

[0078] Para-aramid, PVDF-HFP, and Q-SiO2 (Q represents the gas phase) were dispersed in NMP according to the above ratio to prepare a composite oily slurry with a viscosity of 217 mPa·s and a solid content of 4%. The composite oily slurry was coated on at least one surface of the base film to form a wet coating. The base film with the wet coating was immersed in a coagulation bath for coagulation treatment, so that the wet coating underwent phase separation and formed a porous coating. The base film with the porous coating was cleaned and dried to obtain a diaphragm with a dry coating of 2 μm thickness.

[0079] In summary, the performance tests of the diaphragms in Examples 1 to 6 and Comparative Examples 1 to 3 are shown in Table 1 below.

[0080] Table 1. Diaphragm Performance Test Table

[0081]

[0082] As can be clearly seen from Table 1 above, comparing the adhesive strength and rupture temperature of the diaphragm under different PI / PVDF ratios, it is undeniable that the technical solution of this invention can obtain an oil-coated diaphragm with suitable adhesive strength and rupture temperature in one step (in the embodiments of this invention, the same slurry is used on both sides of the base membrane, which can provide higher adhesive strength and heat resistance at the same time). However, an excessively high or low PI ratio (correspondingly, an excessively low or high PVDF ratio) will result in either insufficient adhesive strength or a low rupture temperature, respectively.

[0083] Compared to Example 5, Example 1 has a higher PVDF content, resulting in more and larger PVDF microspheres with smaller spacing between them.

[0084] In Example 1, the peel strength of the diaphragm after hot pressing is about 8 N / m, which is slightly higher than the hot pressing peel strength of conventional sprayed PVDF diaphragms (6-7 N / m).

[0085] Based on the inventors' experiments, it has been found that when the ratio of PI to inorganic particles is 1:5, the membrane breaking temperature decreases slightly, falling below 190 degrees Celsius, offering little advantage compared to the breaking temperature of conventional ceramic membranes. Therefore, this invention sets the upper limit of the PI / inorganic particle ratio to 1:5.

[0086] The inventors considered PI and inorganic particles in the coating as a whole. If there are more inorganic particles and less PI, the ratio of PI to PVDF will decrease.

[0087] If the system contains PI / PVDF / inorganic particles, the preferred ratio is set to PI / PVDF / inorganic particles = 1:3:3. In this case, a 1.5 μm coating can also achieve good heat shrinkage / film breaking / adhesion.

Claims

1. A composite microporous membrane with PVDF microspheres enriched on its surface, characterized in that, Includes a base film and a coating applied to at least one surface of the base film; The coating is made from raw materials comprising the following parts by weight: 100 parts of high-temperature resistant polymer, 50-400 parts of adhesive polymer, and 30-120 parts of inorganic ceramic particles or solid electrolyte particles. In the coating, the high-temperature resistant polymer forms a three-dimensional porous network structure through phase separation, and the adhesive polymer agglomerates in the coating to form a microsphere structure through phase separation, with at least a portion of the microsphere structure distributed on the surface of the coating.

2. The composite microporous membrane with PVDF microspheres enriched on its surface according to claim 1, characterized in that: The adhesive polymers include: polyvinylidene fluoride, polyacrylate, polyacrylonitrile, and polyvinylidene fluoride-hexafluoropropylene copolymer; The high-temperature resistant polymer includes at least one of the following: polyimide, polyetherimide, polyamideimide, polyethersulfone, polyarylsulfone, aramid, and polyetheretherketone; The inorganic ceramic particles include: fumed silica with a particle size D50 of 20-100 nm and fumed alumina with a particle size D50 of 30-150 nm. The solid electrolyte particles are lithium titanium aluminum phosphate with a particle size of D50 of 200 nm to 1 μm.

3. The composite microporous membrane with PVDF microspheres enriched on its surface according to claim 2, characterized in that: The coating is made from raw materials comprising the following parts by weight: 100 parts polyimide, 50-400 parts polyvinylidene fluoride, and 30-120 parts inorganic ceramic particles. In the coating, the polyimide forms a three-dimensional porous network structure through phase separation, and the polyvinylidene fluoride agglomerates in the coating to form a microsphere structure through phase separation, and at least a portion of the microsphere structure is distributed on the surface of the coating. The coating on the surface of the base film contains 0.2–0.8 g / m³ of polyvinylidene fluoride. 2 .

4. The composite microporous membrane with PVDF microspheres enriched on its surface according to claim 1, characterized in that: The air permeability of the diaphragm is 100–300 s / 100mL.

5. A composite microporous membrane with PVDF microspheres enriched on its surface, characterized in that: Includes a base film and a coating applied to at least one surface of the base film; The coating is made from raw materials comprising the following parts by weight: 100 parts of high-temperature resistant polymer, 30-145 parts of adhesive polymer, and 0-500 parts of inorganic ceramic particles or solid electrolyte particles. In the coating, the high-temperature resistant polymer forms a three-dimensional porous network structure through phase separation, and the adhesive polymer agglomerates in the coating to form a microsphere structure through phase separation, with at least a portion of the microsphere structure distributed on the surface of the coating.

6. The composite microporous membrane with PVDF microspheres enriched on its surface according to claim 5, characterized in that: The adhesive polymers include: polyvinylidene fluoride, polyacrylate, polyacrylonitrile, and polyvinylidene fluoride-hexafluoropropylene copolymer; The high-temperature resistant polymer includes at least one of the following: polyimide, polyetherimide, polyamideimide, polyethersulfone, polyarylsulfone, aramid, and polyetheretherketone; The inorganic ceramic particles include: fumed silica with a particle size D50 of 20-100 nm and fumed alumina with a particle size D50 of 30-150 nm. The solid electrolyte particles are lithium titanium aluminum phosphate with a particle size of D50 of 200 nm to 1 μm.

7. The composite microporous membrane with PVDF microspheres enriched on its surface according to claim 6, characterized in that: The coating does not contain inorganic ceramic particles or solid electrolyte particles; the coating is made from raw materials containing the following ratio: polyimide: polyvinylidene fluoride = (1:1) to (3:1).

8. The composite microporous membrane with PVDF microspheres enriched on its surface according to claim 7, characterized in that: The air permeability of the diaphragm is 300 s / 100mL.

9. A method for preparing the diaphragm as described in claim 1 or 5, characterized in that, Includes the following steps: The high-temperature resistant polymer, the adhesive polymer, and the inorganic ceramic particles or solid electrolyte particles are dispersed in an organic solvent according to the above proportions to prepare a composite oily slurry with a viscosity of 80-2000 mPa·s and a solid content of 5%-20%. The composite oily slurry is coated onto at least one surface of the base film to form a wet coating. The base film with the wet coating attached is immersed in a coagulation bath for coagulation treatment, causing phase separation of the wet coating and forming a porous coating, wherein the water content of the coagulation bath is 60 wt%~100 wt%; The base membrane with the porous coating is cleaned and dried at 50~100 °C to obtain the diaphragm with a dry coating thickness of 0.5~4 μm.

10. The application of the separator as described in claim 1 or 5 in a lithium battery, characterized in that: The separator is used as a separator between the positive and negative electrodes of a lithium battery.

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