Lithium ion battery diaphragm with high safety performance and preparation method thereof

Abstract

The invention discloses a lithium ion battery diaphragm with high safety performance and a preparation method thereof, the diaphragm comprises an aramid fiber base membrane and a ceramic coating, the ceramic coating is of a double-layer composite structure and comprises an inner layer close to the aramid fiber base membrane and including ZrO2-Al2O3-SiO2 composite ceramic particles with the particle size of 0.01-0.05 [mu] m, an outer layer close to the aramid fiber base membrane and an outer layer close to the aramid fiber base membrane and including ZrO2-Al2O3-SiO2 composite ceramic particles with the particle size of 0.01-0.05 [mu] m; the outer layer covers the surface of the inner layer and comprises Al2O3-MgAl2O4 composite ceramic particles with the particle size of 0.1 to 0.3 mu m; wherein the particle size and porosity of the ceramic coating are gradually increased from the inner layer to the outer layer. By designing a double-layer composite ceramic coating structure and optimizing the particle size and composition of inner and outer layer ceramic particles, a gradient structure in which the particle size and porosity of the particles are gradually increased from inside to outside is realized, and the problems of insufficient heat resistance, poor binding force, low ion transmission efficiency and the like of a diaphragm in the prior art are solved; therefore, the safety performance of the lithium ion battery is obviously improved.

Description

TECHNICAL FIELD

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

[0002] Lithium ion batteries are widely used in consumer electronics, electric vehicles and energy storage systems due to their high energy density, long cycle life, no memory effect and other advantages. With the continuous improvement of the energy density of lithium ion batteries and the increasing expansion of application scenarios, battery safety problems have become increasingly prominent.

[0003] The separator, as a key internal component of lithium ion batteries, plays an important role in isolating the positive and negative electrodes, preventing internal short circuit, and allowing lithium ions to migrate freely. The thermal stability and mechanical strength of the separator directly affect the safety performance of the battery. Currently, commercial lithium ion batteries generally use polyolefin separators, such as polyethylene (PE), polypropylene (PP) and their composites. However, such separators have serious safety hazards: when the temperature exceeds 135-165℃, the separator will shrink severely or even melt, causing the positive and negative electrodes to come into direct contact and causing internal short circuit, which may in turn lead to thermal runaway, combustion or even explosion and other serious safety accidents.

[0004] To improve the safety performance of the separator, researchers have developed a variety of high-temperature resistant separator materials in recent years, among which aramid separators have attracted widespread attention due to their excellent thermal stability (thermal decomposition temperature > 400℃) and high mechanical strength. However, single aramid-based films may still affect ion transport due to pore structure collapse under high-rate charging and discharging and high-temperature environments, and their preparation process is complex and costly, limiting their large-scale application.

[0005] Another important strategy to improve the safety of the separator is to coat a ceramic layer on the surface of the traditional separator. The ceramic coating not only significantly improves the heat resistance and mechanical strength of the separator, but also optimizes the battery performance by adjusting the coating structure. However, existing ceramic coating technologies still have many shortcomings: first, most ceramic coatings are single-layer structures, which cannot simultaneously consider the adhesion to the base film and ion transport efficiency; second, single-component ceramic materials cannot meet the multiple requirements of thermal stability, mechanical strength and ion conductivity of the separator; third, the traditional ceramic coating and the base film mainly rely on physical adsorption for bonding, which may easily fall off under long-term cycling and high-temperature environments, affecting the battery performance and safety.

[0006] Therefore, it is urgent to develop a separator scheme with optimized structure, strong adhesion and higher thermal stability to meet the urgent needs of high-safety lithium ion batteries. SUMMARY

[0007] The application aims to provide a lithium ion battery separator with high safety performance and a preparation method thereof, and by designing a double-layer composite ceramic coating structure, optimizing the particle size and composition of the inner and outer layers of the ceramic particles, and realizing a gradient structure with gradually increasing particle size and porosity from the inside to the outside, the problems of poor heat resistance, poor bonding force and low ion transmission efficiency of the separator in the prior art are solved, thereby significantly improving the safety performance of the lithium ion battery.

[0008] To achieve the above-mentioned purpose, the application provides the following technical scheme. A lithium ion battery separator with high safety performance, comprising an aramid-based film and a ceramic coating layer located on at least one side of the aramid-based film, wherein the ceramic coating layer is a double-layer composite structure, comprising: an inner layer close to the aramid-based film, wherein the inner layer comprises ZrO2-Al2O3-SiO2 composite ceramic particles with a particle size of 0.01-0.05 μm; an outer layer covering the surface of the inner layer, wherein the outer layer comprises Al2O3-MgAl2O4 composite ceramic particles with a particle size of 0.1-0.3 μm; wherein the particle size of the ceramic coating layer gradually increases from the inner layer to the outer layer, and the porosity of the ceramic coating layer gradually increases from the inner layer to the outer layer.

[0009] Preferably, the porosity of the inner layer is 25%-35%, and the porosity of the outer layer is 40%-55%.

[0010] Preferably, the thickness of the inner layer is 0.2-2 μm, and the thickness of the outer layer is 0.5-4 μm.

[0011] Preferably, the molar ratio of ZrO2, Al2O3 and SiO2 in the inner layer is (1-3):(1-2):(0.5-2), and the molar ratio of Al2O3 and MgAl2O4 in the outer layer is (1-5):(0.5-3).

[0012] Preferably, the surface of the ceramic particles in the inner layer and the outer layer is coated with at least one of Li 1.3 Al 0.3 Ti 1.7 (PO4)3 or polyimide, and the thickness of the coating layer is 5-50 nm.

[0013] Preferably, the aramid-based film is a non-woven film formed by compounding aramid fibers and nanocellulose at a mass ratio of (90:10)-(70:30).

[0014] Preferably, the inner layer and the outer layer both further comprise a binder selected from at least one of polyvinylidene fluoride, polyimide, polyacrylic acid, acrylate copolymer and polyurethane. The mass ratio of ceramic particles to binder in the inner layer is (95-85):(5-15), and the mass ratio of ceramic particles to binder in the outer layer is (92-80):(8-20).

[0015] Preferably, the total thickness of the ceramic coating is 1-6 μm.

[0016] Preferably, the inner layer further contains a chemical crosslinking agent for initiating in-situ crosslinking reaction, so that the inner layer forms a chemical bond with the aramid-based film.

[0017] Preferably, the chemical crosslinking agent is selected from at least one of epoxy resin, polyisocyanate, and amino resin, and the content of the chemical crosslinking agent is 1%-8% of the total mass of the inner layer.

[0018] Preferably, a thermal conductive material with a thermal conductivity of 10-300 W / (m·K) is added to the ceramic coating, the thermal conductive material is selected from at least one of boron nitride, aluminum nitride, carbon nanotube, and graphene, and the content of the thermal conductive material is 0.5%-5% of the total mass of the ceramic coating.

[0019] Preferably, the surface of the aramid-based film is treated by plasma or ultraviolet ozone to increase the density of surface polar groups and surface roughness.

[0020] Preferably, the membrane breaking temperature of the membrane is not lower than 420℃, and the puncture strength is not lower than 1.5 N (3 mm diameter puncture needle).

[0021] In addition, the present application also provides a preparation method of a high-safety-performance lithium ion battery membrane, comprising the following steps: (1) preparing an aramid-based film; (2) depositing a mixture of ZrO2-Al2O3-SiO2 composite ceramic particles with a particle size of 0.01-0.05 μm and a binder on the surface of the aramid-based film by electrostatic spraying to form an inner layer; (3) generating a mixture of Al2O3-MgAl2O4 composite ceramic particles with a particle size of 0.1-0.3 μm and a binder on the surface of the inner layer by sol-gel in-situ reaction to form an outer layer; (4) heat treatment and curing at 150-250℃ to obtain the high-safety-performance lithium ion battery membrane.

[0022] Preferably, the voltage of electrostatic spraying in the step (2) is 10-30 kV, the spraying distance is 5-20 cm, the solid content of the inner layer ceramic slurry is 10%-40%, and the spraying temperature is 20-80℃.

[0023] Preferably, the sol-gel reactant in step (3) contains a lithium source, a titanium source, and an aluminum source to form Li in situ on the surface of ceramic particles. 1.3 Al 0.3 Ti 1.7 (PO4)3 coating layer; the lithium source is LiOH or LiNO3, the titanium source is tetrabutyl titanate, and the aluminum source is aluminum nitrate or aluminum acetylacetonate.

[0024] Preferably, step (3) specifically includes the following steps: (3.1) The magnesium aluminate precursor, binder and solvent are mixed to form a sol; (3.2) The sol is applied to the inner layer surface by dip coating, spraying or scraping. (3.3) Pre-curing at 50-120℃ to form a gel layer; The magnesium aluminate precursor is obtained by co-precipitation of magnesium salt and aluminum salt under alkaline conditions, wherein the molar ratio of magnesium salt to aluminum salt is (0.8-1.2):2.

[0025] Preferably, step (1) specifically includes the following steps: (1.1) Aramid fibers and nanocellulose are dispersed in an aqueous medium at a mass ratio of (90:10)-(70:30) to form a slurry; (1.2) Composite nonwoven membranes were prepared using a wet film-forming process; (1.3) The nonwoven film is subjected to hot pressing or hot rolling to control its thickness to be 15-25 μm and its porosity to be 60%-75%; (1.4) The surface of the nonwoven film is subjected to plasma or ultraviolet ozone treatment to increase the density of surface polar groups and surface roughness.

[0026] In addition, the present invention also provides a lithium-ion battery, including the aforementioned high-safety-performance lithium-ion battery separator.

[0027] Preferably, the lithium-ion battery exhibits a size shrinkage rate of no more than 1% after 1 hour at 180°C and shows no thermal runaway under 5.0V overcharge conditions.

[0028] Compared with the prior art, the present invention has at least the following beneficial effects: 1) This invention, through the design of a double-layer composite ceramic coating structure and the use of ceramic materials with different particle sizes and compositions, achieves a gradient structure in which the particle size and porosity of the ceramic coating gradually increase from the inside to the outside. This solves the problem in existing technologies where single-layer ceramic coatings cannot simultaneously achieve both adhesion to the substrate and ion transport efficiency. The inner layer uses smaller (0.01-0.05 μm) ZrO2–Al2O3–SiO2 composite ceramic particles to achieve close contact and good bonding with the aramid substrate. The outer layer uses larger (0.1-0.3 μm) Al2O3–MgAl2O4 composite ceramic particles to form a structure with higher porosity, which is beneficial for efficient lithium-ion transport. This gradient structure design enables the separator to maintain excellent thermal stability and mechanical strength while also possessing good ion conductivity, thereby significantly improving the safety and electrochemical performance of the battery.

[0029] 2) The ceramic material combination selected in this invention has a synergistic effect: In the inner layer of ZrO2–Al2O3–SiO2 composite ceramic particles, ZrO2 has excellent thermal stability and chemical corrosion resistance, Al2O3 provides mechanical strength and tensile strength, and SiO2 helps improve affinity and adhesion to the aramid-based membrane; in the outer layer of Al2O3–MgAl2O4 composite ceramic particles, Al2O3 provides heat resistance and mechanical strength, while the spinel structure of MgAl2O4 has good ion conductivity and chemical stability. This rational combination of multi-component composite ceramic materials enables the membrane to simultaneously possess ultra-high thermal stability (rupture temperature ≥420℃) and excellent mechanical strength (puncture strength ≥1.5N), far superior to traditional polyolefin membranes.

[0030] 3) This invention coats the surface of ceramic particles with Li 1.3 Al 0.3 Ti 1.7 Materials such as (PO4)3 or polyimide further enhance the interfacial stability and ion conductivity of the membrane. 1.3 Al 0.3 Ti 1.7 (PO4)3, as a solid electrolyte material, exhibits excellent lithium-ion conductivity, enabling improved lithium-ion transport efficiency without sacrificing the mechanical strength of the separator. The polyimide coating layer combines high-temperature resistance and flame retardancy, further enhancing the thermal stability and safety performance of the separator. This core-shell structure design allows the separator to maintain stable electrochemical performance under high-temperature environments, ensuring battery safety.

[0031] 4) This invention significantly improves the adhesion and interfacial stability between the ceramic coating and the base film by adding a chemical crosslinking agent to the inner layer, thereby forming a chemical bond between the inner layer and the aramid base film. Traditional ceramic coatings and base films primarily rely on physical adsorption for bonding, which is prone to detachment under long-term cycling and high-temperature environments. The chemical bonding strategy of this invention solves this problem, ensuring that the separator maintains structural integrity and performance stability during long-term battery use, thus extending the battery's safe service life.

[0032] 5) This invention uses a nonwoven membrane formed by aramid fiber and nanocellulose composite as the base membrane, which fully utilizes the excellent heat resistance and mechanical strength of aramid, while the introduction of nanocellulose improves the flexibility and processing performance of the base membrane. The aramid base membrane itself has ultra-high thermal stability (thermal decomposition temperature > 400℃), and works synergistically with the ceramic coating to ensure that the dimensional shrinkage rate of the separator does not exceed 1% after 1 hour at 180℃, which is far superior to traditional polyolefin separators, effectively preventing the risk of internal short circuits caused by separator shrinkage in the battery at high temperatures.

[0033] 6) The membrane preparation method of this invention features innovative processes. It employs electrostatic spraying to prepare the inner layer and a sol-gel in-situ reaction to form the outer layer, effectively controlling the particle size and porosity distribution of the ceramic coating. Electrostatic spraying can form a uniform and dense inner layer on the aramid-based membrane surface, while the sol-gel method facilitates the in-situ generation of controllable-size outer ceramic particles on the inner layer surface, thus achieving an ideal gradient structure. This preparation process not only improves the quality stability of the membrane but also has excellent potential for industrial production. Attached Figure Description

[0034] Figure 1 This is a schematic diagram of the high-safety lithium-ion battery separator of the present invention.

[0035] In the figure: 1-aramid base film; 2-ceramic coating; 21-inner layer; 22-outer layer. Detailed Implementation

[0036] 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 some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0037] like Figure 1 As shown, the present invention provides a high-safety lithium-ion battery separator, including an aramid base film 1 and a ceramic coating 2 located on at least one side of the aramid base film 1. The ceramic coating 2 is a double-layer composite structure, including an inner layer 21 close to the aramid base film 1 and an outer layer 22 covering the surface of the inner layer 21.

[0038] The core innovation of this invention lies in the design of a gradient-structured double-layer composite ceramic coating, which achieves a harmonious balance between ultra-high thermal stability, excellent mechanical strength, and good ion transport performance of the membrane. Specifically, the inner layer 21 comprises ZrO2–Al2O3–SiO2 composite ceramic particles with a particle size of 0.01-0.05 μm and a binder, while the outer layer 22 comprises Al2O3–MgAl2O4 composite ceramic particles with a particle size of 0.1-0.3 μm and a binder. The particle size and porosity of the ceramic coating gradually increase from the inner to the outer layer. This gradient structure design enables the membrane to possess both strong adhesion to the base membrane and excellent ion transport performance, while maintaining ultra-high thermal stability and mechanical strength.

[0039] The technical solution of the present invention will be described in detail below with reference to specific embodiments. It should be understood that these embodiments are only for illustrating the present invention and are not intended to limit the scope of the present invention. Experimental methods in the following embodiments that do not specify specific conditions are generally performed under conventional conditions or as recommended by the manufacturer.

[0040] The key parameters of the diaphragm are tested using the following methods: (1) Test of membrane breakage temperature: The test was conducted in accordance with GB / T 36473-2018 "Test method for thermal shrinkage performance of lithium-ion battery separator". A 10cm×10cm separator sample was placed on a hot plate with a temperature control accuracy of ±1℃. The temperature was increased from room temperature to the point where the sample broke or carbonized at a heating rate of 5℃ / min. The temperature at this point was recorded as the membrane breakage temperature.

[0041] (2) Puncture strength test: The puncture tester was used to test the diaphragm sample according to ASTM D3763-18 standard. The diaphragm sample was fixed on the test fixture, and a cylindrical puncture needle with a diameter of 3 mm was used to puncture the diaphragm vertically at a speed of 50 mm / min. The maximum force value during the puncture process was recorded as the puncture strength.

[0042] (3) Porosity test: The liquid displacement method was used for determination. A known mass of dried diaphragm sample was immersed in n-hexane for 24 hours. After removal, excess liquid on the surface was wiped off, and the sample was weighed quickly. The porosity was calculated according to the following formula: Porosity (%) = [(W2-W1) / (ρ·V)]×100% Where W1 is the mass of the dried sample (g), W2 is the mass of the sample after soaking (g), ρ is the density of n-hexane (g / cm³), and V is the sample volume (cm³).

[0043] (4) Ionic conductivity test: Electrochemical impedance spectroscopy (EIS) was used for determination. The membrane sample was immersed in 1 mol / L LiPF6 / EC:DMC (1:1, volume ratio) electrolyte and placed in a stainless steel / stainless steel symmetric cell. The conductivity was measured at a frequency range of 10 ppm. 6 -10 -2 The impedance spectrum was measured at Hz, and the membrane impedance R was obtained by fitting the equivalent circuit. The ionic conductivity was calculated according to the formula σ = L / (R·A), where L is the membrane thickness (cm) and A is the electrode area (cm²).

[0044] (5) Dimensional shrinkage rate test: The test was conducted in accordance with GB / T 36472-2018 "Test method for heat shrinkage rate of lithium-ion battery separator". A 10cm×10cm separator sample was placed in a constant temperature oven at 180℃ for 1 hour. After taking it out, the dimensional changes of the sample in the longitudinal (MD) and transverse (TD) directions were measured, and the dimensional shrinkage rate was calculated.

[0045] (6) Particle size determination: The surface and cross-sectional morphology of the ceramic coating were observed using a field emission scanning electron microscope (FESEM), and the particle size distribution of the ceramic particles was determined by a laser particle size analyzer.

[0046] In the following examples, unless otherwise stated, all chemical reagents used are of analytical grade and the water is deionized water. Example 1

[0047] A method for preparing a high-safety-performance lithium-ion battery separator includes the following steps: (1) Preparation of aramid-based films (1.1) Aramid fibers (average length 3 mm, diameter 12 μm) and nanocellulose (diameter 50-100 nm, length 1-3 μm) were dispersed in water at a mass ratio of 80:20. 0.5% of dispersant (polyvinyl alcohol) was added, and the mixture was sheared at high speed for 30 minutes to obtain a uniform slurry with a solid content of 1.5 wt%. (1.2) A composite nonwoven membrane was prepared by a wet film-forming process and dried at 80°C after being formed. (1.3) The nonwoven membrane was subjected to hot pressing treatment at a pressure of 10 MPa, a temperature of 180℃, and a treatment time of 5 minutes, with its thickness controlled at 20 μm and porosity at 65%. (1.4) The surface of the nonwoven film is treated with oxygen plasma at a power of 50W for 60 seconds to increase the density of polar groups and surface roughness.

[0048] (2) Preparation of the inner layer (2.1) Preparation of inner layer ceramic slurry: ZrO2, Al2O3 and SiO2 ceramic powders with a particle size of 0.02μm are mixed in a molar ratio of 2:1:1, polyimide binder (10wt%) and epoxy resin crosslinking agent (3wt%) are added, and N,N-dimethylacetamide (DMAc) is used as solvent. The mixture is stirred and dispersed for 24 hours to obtain a uniform slurry with a solid content of 25wt%. (2.2) The inner ceramic slurry was deposited on the surface of the aramid base film by electrostatic spraying. The voltage of electrostatic spraying was 20kV, the spraying distance was 10cm, and the spraying temperature was 50℃. (2.3) Pre-dry at 120°C for 30 minutes to form an inner layer with a thickness of 1 μm and a porosity of 30%.

[0049] (3) Preparation of the outer layer (3.1) Preparation of magnesium aluminate precursor: magnesium nitrate and aluminum nitrate were dissolved in water at a molar ratio of 1:2. Ammonia was slowly added dropwise under stirring until pH=9.5 to form a coprecipitate. After washing, drying and grinding, magnesium aluminate precursor was obtained. (3.2) Preparation of outer sol: Magnesium aluminate precursor, Al2O3 powder with a particle size of 0.15μm, polyimide binder (15wt%), lithium source (LiNO3), titanium source (tetrabutyl titanate) and aluminum source (aluminum nitrate) are mixed in an appropriate ratio, and the mixture is stirred and dispersed for 24 hours with ethanol / water (7:3, volume ratio) as solvent to obtain a uniform sol; (3.3) The sol was coated onto the inner layer surface by dip coating, and the dip coating speed was controlled at 5 mm / s; (3.4) Pre-curing at 80°C for 2 hours to form a gel layer.

[0050] (4) Heat treatment curing The coated diaphragm was vacuum heat-treated at 200℃ for 2 hours to achieve the curing of the ceramic coating and chemical bonding with the base film, resulting in a double-layer ceramic coating with a total thickness of 3μm (1μm inner layer and 2μm outer layer), with an inner layer porosity of 30% and an outer layer porosity of 45%.

[0051] The main properties of the high-safety lithium-ion battery separator obtained are as follows: Film breaking temperature: 430℃; Puncture strength: 1.65N; Total thickness: 23μm (base film 20μm + ceramic coating 3μm); Ionic conductivity: 1.2 mS / cm; Dimensional shrinkage rate at 180℃ for 1 hour: 0.8% in the MD direction and 0.7% in the TD direction. Example 2

[0052] The difference from Example 1 is that: (1) The mass ratio of aramid fiber to nanocellulose is 85:15; (2) The molar ratio of ZrO2, Al2O3 and SiO2 in the inner layer is 1:1:2; (3) The inner layer thickness is 0.5 μm, and the outer layer thickness is 3 μm; (4) 2 wt% of sheet boron nitride (thermal conductivity 200 W / (m·K)) was added to the outer layer as a heat conduction material.

[0053] The main properties of the high-safety lithium-ion battery separator obtained are as follows: Film breaking temperature: 425℃; Puncture strength: 1.58N; Total thickness: 23.5μm (base film 20μm + ceramic coating 3.5μm); Ionic conductivity: 1.3 mS / cm; Dimensional shrinkage rate at 180℃ for 1 hour: 0.7% in the MD direction and 0.6% in the TD direction. Example 3

[0054] The difference from Example 1 is that: (1) The mass ratio of aramid fiber to nanocellulose is 75:25; (2) The molar ratio of ZrO2, Al2O3 and SiO2 in the inner layer is 3:2:1; (3) The inner layer thickness is 1.5 μm, and the outer layer thickness is 1.5 μm; (4) The surface of the ceramic particles is coated with polyimide, and the coating thickness is 30nm; (5) 5 wt% of polyisocyanate was added to the inner layer as a chemical crosslinking agent.

[0055] The main properties of the high-safety lithium-ion battery separator obtained are as follows: Film breaking temperature: 435℃; Puncture strength: 1.72N; Total thickness: 23μm (base film 20μm + ceramic coating 3μm); Ionic conductivity: 1.1 mS / cm; Dimensional shrinkage rate at 180℃ for 1 hour: 0.6% in the MD direction and 0.5% in the TD direction. Example 4

[0056] The difference from Example 1 is that: (1) The mass ratio of aramid fiber to nanocellulose is 70:30; (2) The molar ratio of Al2O3 to MgAl2O4 in the outer layer is 2:1; (3) The inner layer porosity is 25%, and the outer layer porosity is 50%; (4) Li is formed in situ on the surface of ceramic particles using the sol-gel method. 1.3 Al 0.3 Ti 1.7 (PO4)3 coating layer with a thickness of 15 nm; (5) The electrostatic spraying voltage is 15kV and the spraying distance is 15cm.

[0057] The main properties of the high-safety lithium-ion battery separator obtained are as follows: Film breaking temperature: 440℃; Puncture strength: 1.80N; Total thickness: 24μm (base film 20μm + ceramic coating 4μm); Ionic conductivity: 1.4 mS / cm; Dimensional shrinkage rate at 180℃ for 1 hour: 0.5% in the MD direction and 0.4% in the TD direction. Example 5

[0058] The difference from Example 1 is that: (1) The ceramic particle size in the inner layer is 0.01 μm, and the ceramic particle size in the outer layer is 0.25 μm; (2) The mass ratio of inner ceramic particles to binder is 90:10, and the mass ratio of outer ceramic particles to binder is 85:15; (3) Hot rolling is used instead of hot pressing for the preparation of the base film; (4) The inner layer thickness is 2μm and the outer layer thickness is 4μm; (5) 1 wt% and 3 wt% of graphene were added to the inner and outer layers, respectively, as heat-conducting materials.

[0059] The main properties of the high-safety lithium-ion battery separator obtained are as follows: Film breaking temperature: 445℃; Puncture strength: 1.85N; Total thickness: 26μm (base film 20μm + ceramic coating 6μm); Ionic conductivity: 1.0 mS / cm; Dimensional shrinkage rate at 180℃ for 1 hour: 0.4% in the MD direction and 0.3% in the TD direction. Example 6

[0060] The difference from Example 1 is that: (1) The aramid-based film has a thickness of 18 μm and a porosity of 70%; (2) The molar ratio of ZrO2, Al2O3 and SiO2 in the inner layer is 2.5:1.5:1; (3) The molar ratio of Al2O3 to MgAl2O4 in the outer layer is 4:2; (4) The inner layer thickness is 0.8 μm, and the outer layer thickness is 2.2 μm; (5) The outer layer is coated by spraying instead of dip coating; (6) The base film surface is treated with ultraviolet ozone instead of plasma treatment for 120 seconds.

[0061] The main properties of the high-safety lithium-ion battery separator obtained are as follows: Film breaking temperature: 432℃; Puncture strength: 1.67 N; Total thickness: 21μm (base film 18μm + ceramic coating 3μm); Ionic conductivity: 1.25 mS / cm; Dimensional shrinkage rate at 180℃ for 1 hour: 0.7% in the MD direction and 0.6% in the TD direction. Example 7

[0062] The difference from Example 1 is that: (1) The mass ratio of aramid fiber to nanocellulose is 90:10; (2) 2wt% carbon nanotubes were added to the inner layer as a heat-conducting material; (3) 8 wt% amino resin was added to the outer layer as a chemical crosslinking agent; (4) The heat treatment temperature is 180℃ and the treatment time is 3 hours; (5) Polyvinylidene fluoride is used as an adhesive in both the inner and outer layers.

[0063] The main properties of the high-safety lithium-ion battery separator obtained are as follows: Film breaking temperature: 428℃; Puncture strength: 1.60 N; Total thickness: 22.5μm (base film 20μm + ceramic coating 2.5μm); Ionic conductivity: 1.15 mS / cm; Dimensional shrinkage rate at 180℃ for 1 hour: 0.9% in the MD direction and 0.8% in the TD direction.

[0064] Comparative Example 1: Traditional polyolefin separator It adopts a commercially available PP / PE / PP three-layer composite membrane with a thickness of 25μm and a porosity of 40%.

[0065] The main properties of this diaphragm are as follows: Film breaking temperature: 165℃; Puncture strength: 0.35N; Thickness: 25μm; Ionic conductivity: 0.8 mS / cm; Dimensional shrinkage rate at 180℃ for 1 hour: 100% in the MD direction (completely melted), 100% in the TD direction (completely melted).

[0066] Comparative Example 2: Single-layer ceramic-coated polyolefin separator A single layer of Al2O3 ceramic coating was coated on a PP / PE / PP three-layer composite membrane. The ceramic coating had a thickness of 3μm and a porosity of 40%.

[0067] The main properties of this diaphragm are as follows: Film breaking temperature: 220℃; Puncture strength: 0.65N; Total thickness: 28μm (base film 25μm + ceramic coating 3μm); Ionic conductivity: 0.7 mS / cm; Dimensional shrinkage rate at 180℃ for 1 hour: 45% in the MD direction and 40% in the TD direction.

[0068] Comparative Example 3: Aramid separator without ceramic coating An aramid-based film was prepared using the same method as in Example 1, but without a ceramic layer.

[0069] The main properties of this diaphragm are as follows: Film breaking temperature: 410℃; Puncture strength: 1.20N; Thickness: 20μm; Ionic conductivity: 0.9 mS / cm; Dimensional shrinkage rate at 180℃ for 1 hour: 2.5% in the MD direction and 2.2% in the TD direction.

[0070] Comparative Example 4: Single-layer ceramic-coated aramid membrane A single layer of Al2O3 ceramic coating was coated on the same aramid-based film as in Example 1. The ceramic coating had a thickness of 3 μm and a porosity of 40%.

[0071] The main properties of this diaphragm are as follows: Film breaking temperature: 420℃; Puncture strength: 1.35N; Total thickness: 23μm (base film 20μm + ceramic coating 3μm); Ionic conductivity: 0.95 mS / cm; Dimensional shrinkage rate at 180℃ for 1 hour: 1.8% in the MD direction and 1.5% in the TD direction; Comparative Example 5: A non-gradient structure double-layer ceramic-coated aramid membrane.

[0072] A double-layer ceramic coating was applied to the same aramid-based film as in Example 1, but both the inner and outer layers used Al2O3 ceramic particles with the same particle size (0.1 μm), both the inner and outer layers had a thickness of 1.5 μm, and both had a porosity of 40%.

[0073] The main properties of this diaphragm are as follows: Film breaking temperature: 425℃; Puncture strength: 1.45N; Total thickness: 23μm (base film 20μm + ceramic coating 3μm); Ionic conductivity: 1.0 mS / cm; Dimensional shrinkage rate at 180℃ for 1 hour: 1.5% in the MD direction and 1.3% in the TD direction.

[0074] 3. Preparation and Testing of Lithium-ion Batteries This invention also provides a lithium-ion battery employing the aforementioned high-safety-performance separator. The method for preparing the lithium-ion battery is as follows: (1) Preparation of positive electrode: Lithium nickel cobalt manganese oxide (NCM811), conductive carbon black and polyvinylidene fluoride (PVDF) are mixed in a mass ratio of 92:3:5, N-methylpyrrolidone (NMP) solvent is added to make a slurry, which is coated on an aluminum foil with a thickness of 15μm, dried, rolled and cut into appropriate sizes to obtain the positive electrode.

[0075] (2) Preparation of negative electrode sheet: Artificial graphite, conductive carbon black and PVDF are mixed in a mass ratio of 94:2:4, NMP solvent is added to make a slurry, which is coated on a copper foil with a thickness of 10μm, dried, rolled and cut into appropriate sizes to obtain negative electrode sheet.

[0076] (3) Battery assembly: In a glove box under an argon atmosphere, the negative electrode, the separator prepared in the embodiments or comparative examples of the present invention, and the positive electrode are stacked in sequence, wound up and placed into an aluminum-plastic film battery case, electrolyte (1M LiPF6 in EC:DMC:EMC=1:1:1 volume ratio solution) is injected, and the battery is packaged to form a soft pack battery.

[0077] (4) Battery performance test: a) Rate performance test: Constant current charge and discharge tests were conducted at 25℃ with current densities of 0.2C, 0.5C, 1C, 2C, 5C and 10C respectively, and the test cutoff voltage was 4.2-2.8V; b) Cyclic performance test: 500 cycles at 25°C and 1C rate; c) High-temperature safety performance test: The battery charged to 100% SOC is placed in an environment of 180℃ for 1 hour, and the morphological changes and temperature changes of the battery are observed. d) Overcharge test: Charge the battery to 5.0V at a rate of 0.5C and observe whether thermal runaway occurs.

[0078] Table 1 Comparison of main performance characteristics of different diaphragms

[0079] Table 2. Dimensional shrinkage rate of the diaphragm prepared in Example 4 at different temperatures.

[0080] 4. The experimental results above are analyzed as follows: (1) Thermal stability analysis As shown in Table 1, the membranes prepared in the seven embodiments of the present invention all exhibit excellent thermal stability, with rupture temperatures all above 425°C. In contrast, the rupture temperature of Comparative Example 1 (conventional polyolefin membrane) is only 165°C, and the rupture temperature of Comparative Example 2 (single-layer ceramic-coated polyolefin membrane) is 220°C. This indicates that the aramid-based double-layer gradient ceramic-coated membrane of the present invention has extremely superior high-temperature resistance, far exceeding that of traditional polyolefin-based membranes.

[0081] In the dimensional shrinkage test at 180℃ / 1h, the shrinkage rate of the separator of the present invention was less than 1% in both the MD and TD directions, while Comparative Example 1 completely melted at this temperature, and Comparative Example 2 showed a shrinkage rate as high as 40%-45%. Even Comparative Example 3 (aramid separator without ceramic coating) and Comparative Example 4 (aramid separator with single-layer ceramic coating) showed significantly higher shrinkage rates than the separator of the present invention. This result indicates that the double-layer gradient ceramic coating structure of the present invention has a significant enhancing effect on the dimensional stability of the aramid-based membrane.

[0082] Furthermore, as can be seen from Table 2, the diaphragm prepared in Example 4, even after being kept at a high temperature of 300°C for 1 hour, still has a dimensional shrinkage rate of less than 1.5%, and after a long-term heat treatment at 180°C (10 hours), the dimensional shrinkage rate does not exceed 1%, indicating that the diaphragm of the present invention has excellent thermal stability and dimensional stability.

[0083] (2) Mechanical performance analysis Puncture strength is an important indicator for evaluating the mechanical properties of a diaphragm. The puncture strength of the diaphragm of this invention is between 1.58 and 1.85 N, while the puncture strengths of Comparative Example 1 and Comparative Example 2 are only 0.35 N and 0.65 N, respectively. The puncture strength of Comparative Example 3 (aramid diaphragm without ceramic coating) is 1.20 N, the puncture strength of Comparative Example 4 (single-layer ceramic-coated aramid diaphragm) is 1.35 N, and the puncture strength of Comparative Example 5 (non-gradient structure double-layer ceramic-coated aramid diaphragm) is 1.45 N. This result shows that the double-layer gradient ceramic coating structure of this invention significantly improves the mechanical strength of the diaphragm and enhances its resistance to external puncture.

[0084] Among them, Example 5 exhibited the highest puncture strength (1.85 N), primarily attributed to its use of finer inner ceramic grains (0.01 μm) and a thicker ceramic coating (total thickness 6 μm), along with the addition of graphene as a reinforcing material. Example 4 also demonstrated excellent puncture strength (1.80 N), characterized by the use of Li... 1.3 Al 0.3 Ti 1.7 The (PO4)3 coating not only enhances the bonding force between particles, but also improves the stability of the overall structure.

[0085] These results demonstrate that by optimizing the structural design and material composition of the ceramic coating, the mechanical properties of the separator can be effectively improved, thereby enhancing the safety performance of the battery under extreme conditions.

[0086] (3) Analysis of ion conduction performance Ionic conductivity directly affects the rate performance and internal resistance of lithium-ion batteries. The ionic conductivity of the separator in this invention is between 1.00 and 1.40 mS / cm, while the ionic conductivity of comparative examples 1-5 are 0.80, 0.70, 0.90, 0.95, and 1.00 mS / cm, respectively. This indicates that the double-layer gradient ceramic coating structure of this invention not only does not reduce the ionic conductivity of the separator, but also improves the ionic conductivity to a certain extent.

[0087] Example 4 exhibited the highest ionic conductivity (1.40 mS / cm), primarily due to the Li coating on the surface of its ceramic particles. 1.3 Al 0.3 Ti 1.7 The (PO4)3-layer material, as a solid electrolyte, exhibits good lithium-ion conductivity. Example 2 also shows a high ionic conductivity (1.30 mS / cm), which may be related to the high SiO2 content in its inner ceramic composition (ZrO2:Al2O3:SiO2=1:1:2). The hydroxyl groups on the SiO2 surface are beneficial for lithium-ion transport.

[0088] It is worth noting that although the ceramic coating of Example 5 is the thickest (6 μm), its ionic conductivity still reaches 1.00 mS / cm. This indicates that the gradient pore structure of the present invention can effectively ensure the rapid transport of lithium ions and maintain good ion conduction performance even under thicker coating conditions.

[0089] (4) Battery performance analysis In battery rate performance testing, the battery using the separator of this invention achieved a capacity retention rate of 82.5%-89.0% at a high rate of 10C, significantly better than the batteries in Comparative Examples 1-5 (60.5%-80.0%). This indicates that the separator of this invention has excellent ion conduction performance and can meet the ion transport requirements of the battery under high-rate charge and discharge conditions.

[0090] In cycle performance testing, the battery using the separator of this invention maintained a capacity retention of 91.0%-95.0% after 500 cycles, while the batteries in Comparative Examples 1-5 maintained a capacity retention of 78.0%-89.0%. This indicates that the separator of this invention has good structural and electrochemical stability and can maintain good performance during long-term cycling.

[0091] (5) Safety performance analysis In the 180℃ / 1h high-temperature safety test, batteries using the separators of Comparative Example 1 and Comparative Example 2 both experienced short circuits and fires, while the battery using the separator of Comparative Example 3 experienced a short circuit but did not fire. However, batteries using the separator of this invention and the separators of Comparative Examples 4 and 5 did not experience short circuits or fires. This indicates that the aramid-based film and ceramic coating structure have a significant effect on improving the high-temperature safety of batteries.

[0092] In the 5.0V overcharge test, batteries using separators from Comparative Examples 1-3 all experienced thermal runaway, batteries using separators from Comparative Examples 4 and 5 partially experienced thermal runaway, while batteries using the separator of this invention did not experience thermal runaway. This indicates that the dual-layer gradient ceramic coating structure of this invention can effectively improve the safety performance of batteries under overcharge conditions.

[0093] In summary, the high-safety lithium-ion battery separator of the present invention exhibits significant advantages in terms of thermal stability, mechanical strength, ion conduction performance, and battery safety, especially in terms of safety performance under high-temperature and overcharge conditions, providing important technical support for the development of high-safety lithium-ion batteries.

[0094] 5. Working Mechanism Analysis The superior performance of the high-safety lithium-ion battery separator of this invention mainly stems from the synergistic effect of the following aspects: (1) Synergistic effect of material combination The inner layer utilizes a ZrO2–Al2O3–SiO2 composite ceramic. ZrO2 exhibits excellent thermal stability and chemical corrosion resistance, with a melting point as high as 2715℃, enabling it to maintain structural stability at high temperatures. Al2O3 provides good mechanical strength and insulation, while SiO2 possesses good hydrophilicity and affinity with the aramid-based film, contributing to enhanced adhesion between the coating and the base film. This composite of three materials gives the inner layer high thermal stability, high mechanical strength, and excellent interfacial bonding performance.

[0095] In the outer layer, an Al2O3–MgAl2O4 composite ceramic is used. Al2O3 provides basic thermal stability and mechanical strength, while the spinel structure of MgAl2O4 provides good ion conductivity and electrochemical stability. This combination allows the outer layer to maintain sufficient strength while also exhibiting excellent ion transport performance.

[0096] (2) Optimization design of gradient structure One of the core innovations of this invention is the design of a gradient structure in which the particle size and porosity gradually increase from the inside out. The inner layer uses ceramic particles with a smaller particle size (0.01-0.05 μm) and a lower porosity (25%-35%), which allows the inner layer to form a tight bond with the aramid-based film, providing a solid foundation support; the outer layer uses ceramic particles with a larger particle size (0.1-0.3 μm) and a higher porosity (40%-55%), forming a channel network that is conducive to lithium-ion transport.

[0097] This gradient structure design resolves the contradiction between the traditional ceramic coating's difficulty in simultaneously achieving adhesion to the substrate and ion transport efficiency, thus realizing the optimal balance between structural stability and functionality.

[0098] (3) Interface design and chemical bonding This invention significantly improves interfacial adhesion and stability by adding a chemical crosslinking agent (such as epoxy resin, polyisocyanate, etc.) to the inner layer, enabling chemical bonding between the ceramic coating and the aramid base film. This chemical bonding strategy solves the problem that traditional ceramic coatings and base films mainly rely on physical adsorption for bonding, and are prone to detachment under long-term cycling and high-temperature environments.

[0099] In addition, by coating the surface of ceramic particles with Li 1.3 Al 0.3 Ti 1.7 Materials such as (PO4)3 or polyimide not only enhance the bonding force between ceramic particles, but also improve the electrochemical stability and ion conduction performance of the overall structure.

[0100] (4) Thermal management strategy This invention incorporates high thermal conductivity materials (such as boron nitride, aluminum nitride, carbon nanotubes, graphene, etc.) into the ceramic coating, effectively improving the thermal conductivity of the separator. This allows the separator to dissipate heat more quickly during battery operation, preventing localized overheating and enhancing battery safety under high temperature and overcharge conditions.

[0101] In summary, this invention has successfully developed a high-safety lithium-ion battery separator with ultra-high thermal stability, excellent mechanical strength, and good ion conductivity through the synergistic effect of material combination, optimized design of gradient structure, interface design and chemical bonding, and comprehensive application of thermal management strategies, providing a new technical path for the development of high-safety lithium-ion batteries.

[0102] Therefore, this invention provides a high-safety lithium-ion battery separator and its preparation method. By designing a double-layer gradient composite ceramic coating structure, the separator achieves a unified balance of ultra-high thermal stability, excellent mechanical strength, and good ion conductivity. Compared with traditional separators, the separator of this invention has the following significant advantages: (1) The membrane breaking temperature is ≥425℃, which is much higher than that of traditional polyolefin membranes (165℃) and single-layer ceramic coated membranes (220℃); (2) The puncture strength is ≥1.58N, which is significantly better than that of traditional diaphragms (0.35N) and single-layer ceramic-coated diaphragms (0.65N); (3) The dimensional shrinkage rate is less than 1% after 1 hour at a high temperature of 180℃, while traditional diaphragms completely melt under the same conditions; (4) The ionic conductivity is as high as 1.00-1.40 mS / cm, which is superior to traditional diaphragms and single-layer ceramic coating diaphragms; (5) The battery using the separator of the present invention does not exhibit thermal runaway under 5.0V overcharge conditions, demonstrating excellent safety performance.

[0103] These superior properties make the separator of this invention particularly suitable for use in lithium-ion battery fields with high safety requirements, such as electric vehicles, energy storage systems, special batteries, and high-safety consumer electronics, with broad prospects for industrial applications.

[0104] Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the invention. Any person skilled in the art can make various modifications and alterations without departing from the spirit and scope of the invention; therefore, the scope of protection of the present invention should be determined by the scope defined in the claims.

Claims

1. A high-safety-performance lithium-ion battery separator, characterized in that, The product includes an aramid-based film and a ceramic coating located on at least one side of the aramid-based film, wherein the ceramic coating is a two-layer composite structure, comprising: The inner layer near the aramid-based film includes ZrO2–Al2O3–SiO2 composite ceramic particles with a particle size of 0.01-0.05 μm. An outer layer covering the surface of the inner layer, the outer layer comprising Al2O3–MgAl2O4 composite ceramic particles with a particle size of 0.1-0.3 μm; The particle size of the ceramic coating gradually increases from the inner layer to the outer layer, and the porosity of the ceramic coating gradually increases from the inner layer to the outer layer.

2. The high-safety-performance lithium-ion battery separator according to claim 1, characterized in that, The porosity of the inner layer is 25%-35%, and the porosity of the outer layer is 40%-55%; and / or, The thickness of the inner layer is 0.2-2 μm, and the thickness of the outer layer is 0.5-4 μm.

3. The high-safety-performance lithium-ion battery separator according to claim 1, characterized in that, The molar ratio of ZrO2, Al2O3 and SiO2 in the inner layer is (1-3):(1-2):(0.5-2), and the molar ratio of Al2O3 to MgAl2O4 in the outer layer is (1-5):(0.5-3).

4. The high-safety-performance lithium-ion battery separator according to claim 1, characterized in that, The ceramic particles in the inner and outer layers are coated with Li. 1.3 Al 0.3 Ti 1.7 The coating layer has a thickness of 5-50 nm and is made of at least one of (PO4)3 or polyimide.

5. The high-safety-performance lithium-ion battery separator according to claim 1, characterized in that, The aramid-based membrane is a nonwoven membrane formed by combining aramid fibers and nanocellulose in a mass ratio of (90:10)-(70:30).

6. The high-safety-performance lithium-ion battery separator according to claim 1, characterized in that, Both the inner layer and the outer layer further include an adhesive, which is selected from at least one of polyvinylidene fluoride, polyimide, polyacrylic acid, acrylate copolymer, and polyurethane. The mass ratio of ceramic particles to binder in the inner layer is (95-85):(5-15), and the mass ratio of ceramic particles to binder in the outer layer is (92-80):(8-20).

7. A method for preparing a high-safety-performance lithium-ion battery separator as described in any one of claims 1-6, characterized in that, Includes the following steps: (1) Preparation of aramid-based films; (2) An inner layer is formed by depositing a mixture of ZrO2–Al2O3–SiO2 composite ceramic particles with a particle size of 0.01-0.05μm and a binder on the surface of the aramid-based film using an electrostatic spraying method. (3) An outer layer is formed by a mixture of Al2O3–MgAl2O4 composite ceramic particles with a particle size of 0.1-0.3 μm and a binder on the inner layer surface through sol-gel in-situ reaction; (4) Heat treatment and curing at 150-250℃ to obtain the high-safety-performance lithium-ion battery separator.

8. The preparation method according to claim 7, characterized in that, In step (2), the voltage of electrostatic spraying is 10-30kV, the spraying distance is 5-20cm, the solid content of the inner layer ceramic slurry is 10%-40%, and the spraying temperature is 20-80℃. The sol-gel reactant in step (3) contains lithium, titanium, and aluminum sources to form Li in situ on the surface of ceramic particles. 1.3 Al 0.3 Ti 1.7 (PO4)3 coating layer; the lithium source is LiOH or LiNO3, the titanium source is tetrabutyl titanate, and the aluminum source is aluminum nitrate or aluminum acetylacetonate.

9. The preparation method according to claim 7, characterized in that, Step (3) specifically includes the following steps: (3.1) The magnesium aluminate precursor, binder and solvent are mixed to form a sol; (3.2) The sol is applied to the inner layer surface by dip coating, spraying or scraping. (3.3) Pre-curing at 50-120℃ to form a gel layer; The magnesium aluminate precursor is obtained by co-precipitation of magnesium salt and aluminum salt under alkaline conditions, wherein the molar ratio of magnesium salt to aluminum salt is (0.8-1.2):

2.

10. The preparation method according to claim 7, characterized in that, Step (1) specifically includes the following steps: (1.1) Aramid fibers and nanocellulose are dispersed in an aqueous medium at a mass ratio of (90:10)-(70:30) to form a slurry; (1.2) Composite nonwoven membranes were prepared using a wet film-forming process; (1.3) The nonwoven film is subjected to hot pressing or hot rolling to control its thickness to be 15-25 μm and its porosity to be 60%-75%; (1.4) The surface of the nonwoven film is subjected to plasma or ultraviolet ozone treatment to increase the density of polar groups and surface roughness.

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