Microporous diaphragm for low-electrical-impedance lithium battery and preparation method of microporous diaphragm

Abstract

The invention discloses a microporous diaphragm for a low-electrical-impedance lithium battery and a preparation method of the microporous diaphragm. The diaphragm comprises: a base film layer, wherein the base film layer is of a microporous structure formed by biaxially stretching polypropylene and / or polyethylene; the surface of the base film layer is coated with the functional coating, and the functional coating contains mixed slurry of inorganic nanoparticles and a binder; the surface modification layer is formed by introducing a hydrophilic electrolyte group on the surface of the functional coating through plasma treatment or chemical grafting. The base membrane layer forms a uniform micropore structure through a two-way stretching process, the functional coating is formed by compounding nano silicon dioxide or aluminum oxide and polyvinylidene fluoride according to a specific proportion, a porous layer with gradient pore diameter is formed, the pore size of the porous layer is smaller than that of the base membrane layer, and efficient permeation of electrolyte is ensured. Electrolyte-philic groups such as sulfonic groups are introduced into the surface modification layer, so that the wettability of the diaphragm to an electrolyte is remarkably improved. During preparation, the base membrane is formed through melt blending, two-way stretching and pore-forming agent extraction, and then the functional layer is constructed through slurry coating and a low-temperature plasma treatment or sulfonation process. The diaphragm combines gradient aperture design, inorganic filler enhancement and surface chemical modification, and realizes low impedance, high thermal stability and excellent mechanical strength.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium battery materials, and in particular to a microporous diaphragm for a low-resistance lithium battery and a preparation method thereof. Background Art

[0002] As lithium-ion batteries expand their applications in high-rate charge and discharge, high-energy-density applications, the ionic conductivity and thermal stability of separators, key battery components, have become key performance constraints. Currently, mainstream separators are based on polyolefin (e.g., polyethylene (PE) and polypropylene (PP)) microporous membranes, formed into a porous structure through uniaxial or biaxial stretching. However, these separators exhibit significant inherent defects: High ion migration resistance: Traditional polyolefin separators exhibit poor micropore uniformity and a wide pore size distribution, leading to inadequate electrolyte infiltration, a tortuous ion transport path, and increased battery internal resistance. Inadequate high-temperature safety: Pure polyolefin separators are prone to melt shrinkage at high temperatures, causing direct contact and short circuits between the positive and negative electrodes. Existing improvements often utilize ceramic coatings, but coating particles can easily clog the pores of the base membrane, exacerbating ion transport resistance and making it difficult to balance safety and electrochemical performance. Process complexity and cost constraints: To achieve multifunctional composite separators, existing technologies typically rely on multiple coating or composite processes, resulting in cumbersome production processes and reduced yields. Furthermore, the interfacial bonding between the coating and the base membrane is insufficient, making delamination likely to occur during long-term cycling, further impacting battery life.

[0003] To address these challenges, the industry urgently needs a novel separator design that can simultaneously improve electrolyte wettability, thermal stability, and process economics while maintaining high porosity and uniform pore size distribution. This invention, through the innovative combination of a gradient pore size composite structure and surface chemical modification, overcomes the bottleneck of traditional technologies that struggle to reconcile low resistance with high safety, providing a reliable solution for high-power lithium-ion batteries. Summary of the Invention

[0004] The purpose of the present invention is to provide a low-resistance microporous diaphragm for lithium batteries and a preparation method thereof, which solves the problems of poor micropore distribution uniformity, low electrical resistance, electrolyte wettability and thermal stability of existing diaphragms.

[0005] The present invention achieves the above-mentioned purpose through the following technical solutions:

[0006] A microporous diaphragm comprising:

[0007] A base film layer, wherein the base film layer is formed into a microporous structure by biaxially stretching polypropylene and / or polyethylene;

[0008] and a functional coating layer coated on the surface of the base film layer, wherein the functional coating layer comprises a mixed slurry of inorganic nanoparticles and a binder;

[0009] The surface modification layer is formed by introducing electrophilic electrolyte groups on the surface of the functional coating through plasma treatment or chemical grafting.

[0010] According to a preferred embodiment of the present invention, the polypropylene is purchased from Fujian Zhongjing Petrochemical Co., Ltd.

[0011] According to a preferred embodiment of the present invention, the polyethylene is purchased from Shandong Zhongyuan Pipeline Technology Co., Ltd.

[0012] According to a preferred embodiment of the present invention, the pore size of the microporous structure is 50-500 nm, and the porosity is 40%-60%.

[0013] In the present invention, polypropylene (PP) and polyethylene (PE) are crystalline polymers. During biaxial stretching (longitudinal and transverse), the molecular chains are oriented and form a lamellar structure. The amorphous regions between the lamellars form uniform micropores under tensile stress, with a pore size range of 50-500nm and a porosity of 40%-60%.

[0014] According to a preferred embodiment of the present invention, the inorganic nanoparticles are SiO2 or Al2O3.

[0015] According to a preferred embodiment of the present invention, the nano-SiO2 is purchased from Zhejiang Yamei Nano Technology Co., Ltd. as AM-SiO2-N-03 model.

[0016] According to a preferred embodiment of the present invention, the nano-Al2O3 is purchased from Zhejiang Yamei Nano Technology Co., Ltd. as an α-Al2O3 nano-alumina model.

[0017] In the present invention, after the nanoparticles are SiO2 or Al2O3 mixed with PVDF, the nanoparticles form a pore gradient through van der Waals forces, optimize the electrolyte infiltration path, and reduce the tortuosity of ion transmission; the high melting point of the nanoparticles forms a physical barrier at high temperatures, inhibiting the shrinkage of the basement membrane while maintaining a stable pore structure.

[0018] According to a preferred embodiment of the present invention, the binder is PVDF.

[0019] According to a preferred embodiment of the present invention, the PVDF is purchased from Solvay, USA. 1015 model.

[0020] In the present invention, PVDF is dissolved in NMP solvent to form a viscous slurry. During the curing process, the solvent evaporates to generate micropores, which together with the inorganic particles form a porous network. The C-F bonds in the PVDF molecular chain form intermolecular forces with the surface of the base film, ensuring that the coating and the base film fit tightly and avoiding peeling during the circulation process.

[0021] According to a preferred embodiment of the present invention, the NMP solvent is purchased from Shandong Pulaihua Chemical Co., Ltd.

[0022] According to a preferred embodiment of the present invention, the electrophilic electrolyte group is a sulfonic acid group.

[0023] In the present invention, concentrated sulfuric acid is used as a sulfonating agent to react with the PVDF molecular chain to introduce sulfonic acid groups, whose strong hydrophilicity significantly improves the adsorption capacity of the electrolyte; the negative charge characteristics of the sulfonic acid groups attract Li + Directed migration reduces interface impedance and improves conductivity.

[0024] According to a preferred embodiment of the present invention, one or both sides of the base membrane layer are coated with the mixed slurry to form a porous layer with a pore size gradient distribution, and the pore size of the functional coating layer is smaller than the pore size of the base membrane layer.

[0025] In the present invention, the large pore size of the base membrane layer provides a fast transmission channel, the small pore size of the functional coating blocks dendrite penetration, and the gradient pore size reduces interface concentration polarization; the high-rigidity nanoparticle layer of the functional coating complements the flexible microporous structure of the base membrane, inhibiting the anisotropic shrinkage of the overall diaphragm at high temperatures.

[0026] The present invention also provides a method for preparing the microporous membrane, comprising the following steps:

[0027] S1, melt-blending polypropylene or polyethylene with a pore-forming agent, extruding into a film, first stretching it longitudinally and then stretching it transversely to form uniform micropores; extracting to remove the pore-forming agent, and drying;

[0028] S2, dispersing nano-SiO2 and PVDF binder in NMP solvent to prepare a slurry; applying the slurry to the surface of the base film; pre-baking at 60°C for 5 minutes, and then curing at 120°C for 10 minutes to form a porous coating;

[0029] S3, performing low-temperature plasma treatment on the coated membrane under a nitrogen atmosphere; or immersing the membrane in a sulfonating agent solution.

[0030] According to a preferred embodiment of the present invention, in step S1, the pore-forming agent is liquid paraffin, the stretching ratio of the longitudinal stretching is 1:3, the temperature during stretching is 120°C, the stretching ratio of the transverse stretching is 1:4, and the temperature during stretching is 135°C; in step S2, the particle size of nano-SiO2 is 50nm, and the mass ratio of nano-SiO2 to PVDF binder is 8:2; the coating thickness on the base film surface is 2-5μm.

[0031] According to a preferred embodiment of the present invention, the liquid paraffin is purchased from Hubei Yunmei Technology Co., Ltd.

[0032] According to a preferred embodiment of the present invention, the nitrogen is purchased from Shenzhen Yuejia Gas Co., Ltd.

[0033] In the present invention, two-stage stretching (longitudinal stretching ratio of 1:3, transverse stretching ratio of 1:4) is used to regulate the ratio of crystalline and amorphous regions, forming uniform and penetrating microporous channels and reducing the resistance to lithium ion transmission; longitudinal stretching initially opens the polymer lamellae structure to form initial pores; transverse stretching further expands the pores and improves connectivity, avoiding the uneven pore size distribution caused by uniaxial stretching.

[0034] In the present invention, the NMP solvent is slowly evaporated at low temperature (60° C.) to prevent the coating surface from forming a skin too quickly and causing closed pores; the high temperature (120° C.) promotes the rearrangement of PVDF molecular chains to form a stable bonding network and fix the distribution of SiO2 particles.

[0035] During melt blending in the present invention, liquid paraffin acts as a porogen to fill the gaps between polymer chains, and after stretching and extraction, an open-pore structure is formed to increase the porosity.

[0036] According to a preferred embodiment of the present invention, in step S3, the power of the low-temperature plasma treatment is 200 W and the time is 30 s; the sulfonating agent is concentrated sulfuric acid.

[0037] According to a preferred embodiment of the present invention, the concentrated sulfuric acid is purchased from Yangzhou Huafu Chemical Co., Ltd.

[0038] According to a preferred embodiment of the present invention, the twin-screw extruder is purchased from Nanjing Ruiya Extrusion Machinery Manufacturing Co., Ltd. and is a CPM series.

[0039] In the present invention, plasma bombards the surface of the functional coating, generates free radicals and introduces oxygen-containing polar groups (-OH, -COOH), reduces the surface energy, and reduces the contact angle of the electrolyte;

[0040] The specific steps of the plasma treatment in the present invention include placing the sample to be treated in a reaction chamber, ensuring the chamber is sealed to prevent gas leakage from interfering with plasma stability; starting a vacuum pump to reduce the chamber pressure to a range of 5-50 Pa to reduce the probability of gas molecule collisions and improve plasma uniformity; introducing an inert gas or reactant gas, with the gas flow rate typically controlled at 10-100 sccm (standard milliliters per minute); applying an electric field using a radio frequency (RF) or microwave power source to excite the gas ionization to form a plasma. Parameters include: power: 200-500 W; frequency: 13.56 MHz (RF) or 2.45 GHz (microwave); plasma stabilization: adjusting the gas flow rate, pressure, and power to set values, monitoring the plasma glow state to ensure uniform discharge (no arcing or local overheating); setting the treatment time based on the material type and the target modification effect; and monitoring the plasma active species concentration (such as free radicals and electron density) using a spectrometer or Langmuir probe to ensure reaction consistency. Turn off the power supply and stop the gas supply in sequence, and keep the vacuum pump running until the chamber reaches room temperature; slowly restore the chamber to normal pressure, and immediately proceed to the next process (such as chemical grafting or assembly) after taking out the sample to avoid failure of the surface modification layer due to exposure.

[0041] According to a preferred embodiment of the present invention, the plasma generator is purchased from Beijing Rongyuan Technology Co., Ltd.

[0042] According to a preferred embodiment of the present invention, the n-hexane is purchased from Shandong Yukang Chemical Co., Ltd.

[0043] The present invention also provides the microporous membrane and the use of the microporous membrane prepared by the preparation method of the microporous membrane in a low-resistance lithium battery.

[0044] The beneficial effects of the present invention are:

[0045] The present invention achieves a comprehensive improvement in the ion transmission efficiency, thermal stability and interface compatibility of the lithium-ion battery separator by combining the synergistic effect of the microporous structure of the base membrane layer formed by biaxial stretching of polypropylene or polyethylene and the inorganic nanoparticle composite functional coating coated on its surface with a surface chemical modification process.

[0046] The base film layer adopts a uniform through-microporous structure formed by a biaxial stretching process, combined with a gradient pore size design, which optimizes the lithium ion transmission path while ensuring the porosity and significantly reduces the interfacial impedance; the porous network constructed by nano-silica or alumina particles and polyvinylidene fluoride binder in the functional coating not only suppresses high-temperature thermal shrinkage and dendrite puncture risks through physical barrier effects, but also enhances electrolyte wettability with the help of the high specific surface area of nanoparticles, and the gradient coating process further balances pore connectivity and mechanical strength.

[0047] The surface modification layer introduces electrolyte-philic groups through low-temperature plasma bombardment or sulfonation agent treatment, significantly reducing the surface energy of the diaphragm. The strong hydrophilicity of the sulfonic acid groups promotes rapid adsorption of the electrolyte and forms a stable interfacial film, effectively reducing the occurrence of side reactions. During the preparation process, a gradient drying process ensures the uniformity and structural stability of the coating. Low-temperature plasma treatment achieves surface chemical activation while preserving the integrity of the micropores, avoiding damage to the substrate caused by traditional high-temperature modification. Its unique gradient pore size design not only ensures rapid ion conduction, but also enhances safety through multi-level physical barriers. The high-temperature resistance of the composite coating and the synergistic effect of surface chemical modification are also achieved. DETAILED DESCRIPTION

[0048] The following specific implementation methods are only used to further illustrate the present application and should not be understood as limiting the scope of protection of the present application. Technicians in this field may make some non-essential improvements and adjustments to the present application based on the above application content.

[0049] 1. Implementation

[0050] Example 1

[0051] 1. Preparation of basement membrane layer

[0052] Polyethylene is used as the base material, melt-blended with liquid paraffin in a suitable proportion, and extruded into a film through a twin-screw extruder at 200°C. Longitudinal stretching is performed at 120°C with a 1:3 stretch ratio, and transverse stretching is performed at 135°C with a 1:4 stretch ratio to form a uniform, interconnected microporous structure. Residual paraffin is removed using n-hexane extraction. After drying, the base film has a pore size of 300 nm and a porosity of 50%.

[0053] 2. Functional coating application

[0054] Slurry preparation: 50nm of silica (SiO2) and polyvinylidene fluoride (PVDF) binder were mixed in a mass ratio of 8:2 and dispersed in N-methylpyrrolidone (NMP) solvent. High-speed shearing (2000 rpm, 30 minutes) was applied to form a uniformly dispersed slurry. The slurry was coated on both sides of the base film using a micro-gravure coater to a thickness of 4μm. Pre-baking was performed at 60°C for 5 minutes to allow the solvent to evaporate slowly and prevent cracking. Thermal curing was then performed at 120°C for 10 minutes to form a porous functional layer with a pore size of 70nm.

[0055] 3. Surface modification

[0056] Under nitrogen atmosphere, the plasma generator power was 200W, the processing time was 30s, and the chamber pressure was 20Pa; the plasma bombarded the surface SiO2-PVDF coating, introduced sulfonic acid electrophilic groups, and reduced the surface contact angle to ≤5°.

[0057] Example 2

[0058] The specific implementation method is the same as Example 1, except that the following steps are used: Base membrane preparation: Polypropylene biaxially stretched (1:3 longitudinally, 125°C; 1:4 transversely, 140°C); base membrane pore size 150 nm, porosity 48%. Functional coating: Al2O3 nanoparticles (50 nm) and PVDF (mass ratio 8:2) coated on one side, 3 μm thick, functional layer pore size 60 nm. Surface modification: Impregnation with a sulfonating agent (concentrated sulfuric acid).

[0059] Example 3

[0060] The specific implementation method is the same as that of Example 1, except that the base membrane preparation is a pore size gradient base membrane (functional layer coated on one side). The functional coating is a SiO2 / PVDF (7:3 mass ratio) coated on one side, with a thickness of 5 μm and a functional layer pore size of 80 nm. Surface modification: oxygen plasma treatment (200 W, 40 s).

[0061] Comparative Example 1

[0062] The specific implementation method is the same as that of Example 1, except that: base film preparation: same as that of Example 1, only uncoated polyethylene base film (pore size 500 nm) is used. Surface modification: None.

[0063] Comparative Example 2

[0064] The specific implementation method is the same as that of Example 1, except that the base film and the functional coating are the same as those of Example 1, but the plasma treatment is omitted.

[0065] Comparative Example 3

[0066] The specific implementation method is the same as that of Example 1, except that the base film and the functional coating have a pore size of 50 nm and a pore size of 200 nm.

[0067] 3. Performance Testing

[0068] The microporous membranes prepared in Examples 1-3 and Comparative Examples 1-3 were tested for performance according to the following method:

[0069] 1. Thickness test

[0070] Steps: Use a contact thickness gauge (accuracy ±0.1μm) to test in an environment with a temperature of 25±2℃ and a humidity of 50±5%; lay the diaphragm flat on a hard surface, apply standard pressure (0.1MPa), and measure the thickness at 10 points evenly selected along the transverse direction of the diaphragm; calculate the average thickness and standard deviation, and the standard deviation <5% is considered qualified.

[0071] 2. Curvature test

[0072] Steps: Cut the diaphragm into 10mm wide strip samples and lay them flat on an optical platform; use a laser displacement sensor to scan the edge of the sample and measure the maximum offset (camber) between the actual edge and the theoretical straight line; if the offset is greater than 0.5mm, the curvature is judged to be unqualified.

[0073] 3. Air permeability test

[0074] Procedure: Using a membrane air permeability tester according to ASTM D726, cut a 25mm diameter circular sample, clamp it in the test chamber, and apply 100mL of air. Record the time (in seconds) required for air to pass through the membrane, repeat three times, and take the average value.

[0075] 4. Pore size distribution test

[0076] Procedure: Use a mercury intrusion porosimeter or gas adsorption method (BET method); Mercury intrusion method: According to standard ISO 15901-1, gradually increase the pressure in the range of 0.1-400 MPa and calculate the pore size distribution according to the Washburn equation; Gas adsorption method: Analyze the mesopore (2-50 nm) distribution through nitrogen adsorption-desorption isotherms.

[0077] 5. Contact angle test

[0078] Steps: Use a contact angle tester to drop 1 μL of electrolyte (1M LiPF6 / EC-DEC) onto the surface of the diaphragm; a high-speed camera (1000 frames / s) records the droplet morphology, and the software fits the contact angle (θ). Repeat 5 times and take the average value.

[0079] 6. Thermal shrinkage test

[0080] Steps: Cut a 100mm x 100mm square sample and mark the initial size (L1); place it in a 150°C oven for 1 hour, and measure the size (L2) after cooling; calculate the thermal shrinkage: thermal shrinkage = (L1-L2) / L1.

[0081] 7. Puncture strength test

[0082] Steps: Use a universal material testing machine and install a 1mm diameter spherical probe; clamp the diaphragm in the fixture and puncture the probe vertically at a speed of 50mm / min; record the maximum puncture force value (N) and calculate the unit thickness strength (N / mm 2 ).

[0083] 8. Ionic conductivity test

[0084] Steps: Soak the membrane in 1M LiPF6 / EC-DEC electrolyte for 24 hours; assemble symmetrical stainless steel (SS) blocking electrodes and measure the impedance spectrum using an electrochemical workstation (frequency range: 100kHz-0.1Hz); calculate the ionic conductivity using the formula: σ=L / (R×A) (L is the membrane thickness, R is the impedance spectrum fitting resistance, and A is the effective contact area).

[0085] 9. Performance test results:

[0086] Table 1: Test results of various embodiments and comparative examples

[0087]

[0088]

[0089] As can be seen from Table 1, the embodiment separator significantly optimizes performance indicators through gradient pore size design and surface modification technology. Its air permeability (Gurley value 205-220s) is 50-70% lower than that of the comparative example (400-650s). This shows that the gradient pore size (150-300nm) of the base membrane formed by the biaxial stretching process and the fine pores (60-80nm) of the functional layer improve the micropore penetration, solving the problem of uneven electrolyte penetration caused by traditional separators due to single pore size (such as the 500nm base membrane without functional layer in Comparative Example 1) or pore size mismatch (50nm base membrane + 200nm functional layer in Comparative Example 3). The lyophilic groups (-SO3H) introduced by surface plasma treatment make the contact angle of the embodiment ≤5° (comparative example ≥30°), combined with the low air permeability resistance (Gurley value ≤220s), verifying the effect of surface chemical modification on the improvement of infiltration dynamics. The introduction of inorganic filler (SiO2 / PVDF=8:2) makes the ionic conductivity of the embodiment reach 1.7-1.9×10 - 3 S / cm (the comparative example is only 0.7-1.0×10 -3 S / cm), the gradient pore size shortens the ion migration path and reduces the interface impedance to achieve low electrical impedance characteristics, which is 2-3 times higher than the comparative example. The thermal shrinkage rate (1.5-1.8%) is much lower than the comparative example (8-18%), and the puncture strength reaches 200-215N / mm 2 (Comparative example only 75-180N / mm 2), demonstrating that the physical cross-linking effect of the inorganic coating inhibits high-temperature deformation of the substrate while enhancing mechanical barrier capabilities. In particular, Comparative Example 3, due to the inverted pore size of the base membrane (50nm) and the functional layer (200nm), has the highest air permeability (650s) and a sharp increase in thermal shrinkage (18%), further highlighting the necessity of gradient pore size design. The examples systematically address the problems of disordered micropore distribution, wetting hysteresis, and thermomechanical defects in traditional diaphragms through a three-dimensional synergistic mechanism of pore size gradient, surface lyophilic modification, and inorganic filler composites.

[0090] The above-described embodiments merely illustrate several implementations of the present invention. While the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that a person skilled in the art would be able to make numerous variations and improvements without departing from the spirit of the present invention, and all such variations and improvements fall within the scope of protection of the present invention.

Claims

1. A microporous membrane, characterized in that: include: A base film layer, wherein the base film layer is formed into a microporous structure by biaxially stretching polypropylene and / or polyethylene; and a functional coating layer coated on the surface of the base film layer, wherein the functional coating layer comprises a mixed slurry of inorganic nanoparticles and a binder; The surface modification layer is formed by introducing electrophilic electrolyte groups on the surface of the functional coating through plasma treatment or chemical grafting.

2. The microporous membrane according to claim 1, characterized in that The microporous structure has a pore diameter of 50-500 nm and a porosity of 40%-60%.

3. The microporous membrane according to claim 1, characterized in that The inorganic nanoparticles are SiO2 or Al2O3.

4. The microporous membrane according to claim 1, characterized in that The binder is PVDF.

5. The microporous membrane according to claim 1, characterized in that The electrophilic electrolyte group is a sulfonic acid group.

6. The microporous membrane according to any one of claims 1 to 5, characterized in that: After coating one or both sides of the base membrane layer with the mixed slurry, a porous layer with a pore size gradient distribution is formed, and the pore size of the functional coating layer is smaller than the pore size of the base membrane layer.

7. A method for preparing a microporous membrane according to any one of claims 1 to 6, characterized in that: The following steps are involved: S1, melt-blending polypropylene or polyethylene with a pore-forming agent, extruding into a film, first stretching it longitudinally and then stretching it transversely to form uniform micropores; Extraction to remove the pore-forming agent and drying; S2, dispersing nano-SiO2 and PVDF binder in NMP solvent to prepare a slurry; applying the slurry to the surface of the base film; pre-baking at 60°C for 5 minutes, and then curing at 120°C for 10 minutes to form a porous coating; S3, low-temperature plasma treatment of the coated membrane under nitrogen atmosphere; Or immersed in a sulfonating agent solution.

8. The preparation method according to claim 7, characterized in that In step S1, the pore-forming agent is liquid paraffin, the longitudinal stretching ratio is 1:3, the temperature during stretching is 120°C, the transverse stretching ratio is 1:4, and the temperature during stretching is 135°C; in step S2, the particle size of nano-SiO2 is 50nm, and the mass ratio of nano-SiO2 to PVDF binder is 8:2; the coating thickness on the base film surface is 2-5μm.

9. The preparation method according to claim 7, characterized in that In step S3, the power of the low-temperature plasma treatment is 200 W and the time is 30 s; the sulfonating agent is concentrated sulfuric acid.

10. Use of the microporous membrane prepared according to any one of claims 1 to 6 and the method for preparing the microporous membrane according to any one of claims 7 to 9 in a low-impedance lithium battery.

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