Ultraviolet curing cross-linking type lithium ion battery diaphragm, preparation method thereof and lithium ion battery

Abstract

The invention discloses an ultraviolet light curing cross-linked lithium ion battery diaphragm and a preparation method thereof and a lithium ion battery, the ultraviolet light curing cross-linked lithium ion battery diaphragm comprises a base membrane and an ultraviolet light curing cross-linked coating coated on the surface of the base membrane, the surface hydroxyl density of the base membrane is greater than or equal to 5 / nm, the contact angle is less than or equal to 35 degrees, and the ultraviolet light curing cross-linked coating is coated on the surface of the base membrane. The ultraviolet curing cross-linked coating comprises a polymer matrix, nanoparticles, an ultraviolet initiator and a polyfunctionality acrylate cross-linking agent with the functionality greater than or equal to 4. The peel strength of the diaphragm coating reaches 78N / m, the ionic conductivity is 4.28 * 10 <-4 > S / cm (25 DEG C), and the 150 DEG C thermal shrinkage rate is lt; and 2%.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion battery materials, and particularly relates to an ultraviolet-curable crosslinked lithium-ion battery separator, a preparation method thereof, and a lithium-ion battery. Background Art

[0002] As a core component of the battery, the lithium-ion battery separator needs to have excellent ion conductivity and mechanical stability while ensuring high safety. Currently, the industry generally modifies polyethylene (PE) or polypropylene (PP) base films with alumina (Al2O3) or polyvinylidene fluoride (PVDF) coatings. However, due to the difficulty of chemical bonding between the non-polar base film and the polar coating, the traditional coating peel strength is generally lower than 50 N / m, and there is a risk of interface delamination after long-term cycling; at the same time, the high-temperature crosslinking process (>120 °C) is likely to cause the thermal shrinkage rate of the base film to exceed 5%, seriously restricting the battery safety. In addition, fast ion conductor materials (such as LLZO) have poor compatibility with the polymer matrix, and are prone to particle agglomeration and interfacial side reactions, resulting in the ion conductivity being difficult to exceed 10 -4 S / cm. Although existing technologies have tried to optimize through silane coupling or thermal crosslinking, they still cannot achieve high interfacial bonding strength, high porosity, and low-temperature process compatibility simultaneously. Developing a new coating technology with comprehensive performance advantages has become a key topic that the industry urgently needs to overcome. Although existing technologies such as the Chinese patent with the publication number CN107180938B adopt plasma treatment and nano-coating modification, its aqueous binder system results in limited coating thickness (30 - 70 nm), and no crosslinked structure is introduced, and the interfacial bonding strength is still lower than 50 N / m; while the Chinese patent application with the publication number CN114243207A and the Chinese patent with the publication number CN104157810B adopt ultraviolet curing technology. The Chinese patent with the publication number CN104157810B includes a polymer matrix, nanoparticles, an ultraviolet initiator, and a multi-functional acrylic resin, but the Chinese patent application with the publication number CN114243207A does not combine the directional dispersion of fast ion conductors (such as LLZO) and the dual-mode pore design, and the ion conductivity only reaches 9.3×10 - 4 S / cm, and the high-temperature thermal shrinkage rate is still higher than 3%. Summary of the Invention

[0003] The technical problem to be solved by the present invention is to provide an ultraviolet-curable crosslinked lithium-ion battery separator, a preparation method thereof, and a lithium-ion battery in view of the deficiencies of the existing technology, so as to solve the problems of insufficient interfacial bonding force, poor high-temperature stability, and low ion conduction efficiency of the traditional separator coating, and is applicable to high-energy-density lithium-ion battery systems.

[0004] To solve the above technical problems, the technical solutions adopted by the present invention are as follows: An ultraviolet-curable crosslinked lithium-ion battery separator, comprising a base film and an ultraviolet-curable crosslinked coating coated on the surface of the base film, wherein the hydroxyl density on the surface of the base film is ≥5 per nm², and the contact angle is ≤35°, and the ultraviolet-curable crosslinked coating comprises a polymer matrix, nanoparticles, an ultraviolet initiator, and a multi-functional acrylate crosslinking agent with a functionality ≥4.

[0005] In the present invention, by making the hydroxyl density on the surface of the base film ≥5 per nm², the contact angle ≤35°, and using a multi-functional acrylate crosslinking agent with a functionality ≥4 in the ultraviolet-curable crosslinked coating, the interfacial bonding force of the separator coating is enhanced, the high-temperature stability is improved, and the ion conduction efficiency is increased, which is applicable to high-energy-density lithium-ion battery systems.

[0006] Experiments show that when the functionality of the crosslinking agent is insufficient (such as TMPTA with a functionality of 3) or the hydroxyl density of the base film is lower than 5 per nm², the peel strength drops to 48 N / m and 42 N / m respectively, and the interfacial impedance deteriorates significantly (the growth rate in 30 days reaches 20%), verifying the necessity of the parameter thresholds of this application.

[0007] In the present invention, by making the hydroxyl density on the surface of the base film ≥5 per nm² and the contact angle ≤35°, the surface energy of the base film can be increased to 50 - 60 mJ / m². Preferably, in the present invention, argon-oxygen mixed plasma treatment can be used to make the hydroxyl density on the surface of the base film ≥5 per nm² and the contact angle ≤35°.

[0008] In a preferred embodiment of the present invention, the peel strength of the ultraviolet-curable crosslinked lithium-ion battery separator is 65 - 85 N / m, the ionic conductivity at 25°C is 3×10 -4 ~5.5×10 -4 S / cm, and the thermal shrinkage rate at 150°C <2.5%.

[0009] The porosity of the composite coating is 40 - 50%, and the pore size shows a bimodal distribution, including macropores of 200 - 1000 nm and mesopores of 50 - 200 nm.

[0010] In a preferred embodiment of the present invention, the mass ratio of the polymer matrix, nanoparticles, multi-functional acrylate crosslinking agent with a functionality ≥4, and ultraviolet initiator is 100:25 - 50:5 - 30:0.3 - 5.

[0011] In a preferred embodiment of the present invention, the nanoparticles include lithium lanthanum zirconium oxide nanoparticles and a coating particle layer coated on the surface of the lithium lanthanum zirconium oxide nanoparticles, and the coating particle layer includes one or two of alumina and lithium metaaluminate.

[0012] Preferably, the thickness of the coated particle layer is 2 - 5 nm; more preferably, the particle size of the nanoparticles is 50 - 300 nm.

[0013] Surface coating modification (Al2O3 / LiAlO2, 2 - 5 nm) and particle size regulation (50 - 300 nm) of lithium lanthanum zirconium oxide (LLZO) nanoparticles, combined with the design of continuous ion channels, increase the room-temperature ionic conductivity of the composite coating to 4.28×10 -4 S / cm, and the capacity retention rate exceeds 92% after 500 cycles at a 5C rate.

[0014] In a preferred embodiment of the present invention, the lithium lanthanum zirconium oxide nanoparticles are Ta- or Nb-doped, with the chemical formula Li 6.4 La3Zr 1.4 Ta 0.6 O 12 or Li7La3Zr 1.5 Nb 0.5 O 12 。

[0015] In a preferred embodiment of the present invention, the polymer matrix includes polyvinylidene fluoride-hexafluoropropylene, and the multifunctional acrylate crosslinking agent includes one or more of dipentaerythritol hexaacrylate, trimethylolpropane triacrylate, or pentaerythritol tetraacrylate.

[0016] The present invention relates to a separator and its preparation method for constructing a crosslinked polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) / lithium lanthanum zirconium oxide (LLZO) composite coating on the surface of a base film through ultraviolet light (UV) curing technology.

[0017] The molecular weight of polyvinylidene fluoride-hexafluoropropylene is 300 - 500 kDa, and the crystallinity is 20 - 30%.

[0018] The ultraviolet light initiator includes one or more of 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone (PI-2959), 1-hydroxycyclohexyl phenyl ketone (UV-184), or bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide (PI-819).

[0019] The present invention also discloses a preparation method for the ultraviolet light-cured crosslinked lithium-ion battery separator, including the following steps: S1. Activate the base film using argon-oxygen plasma to increase the hydroxyl density on the surface of the base film to ≥5 per nm² and the contact angle to ≤35°, obtaining the activated base film; S2. Mix the solvent, polymer matrix, nanoparticles, multifunctional crosslinking agent and ultraviolet initiator, disperse them evenly to obtain a slurry, coat the slurry on the surface of the activated base film in S1, and then form a three-dimensional crosslinked network coating by ultraviolet curing, so that the crosslinking density ≥ 85%; S3. Adopt a gradient phase separation process for the three-dimensional crosslinked network coating in S2 to form a porous coating with a bimodal pore size distribution, so that the porosity is 40 - 50%; S4. Dry the porous coating in S3 in sections to obtain an ultraviolet-cured crosslinked lithium-ion battery separator.

[0020] In a preferred embodiment of the present invention, the argon-oxygen plasma activation of the base film in S1 uses an argon-oxygen mixed gas with an Ar / O2 volume ratio of 3:1 - 5:1, a power density of 0.8 - 1.2 W / cm², and a time of 20 - 60 s.

[0021] In a preferred embodiment of the present invention, the gradient phase separation process in S3 includes sequentially immersing the ultraviolet-cured separator in a first gradient mixture, a second gradient mixture and a third gradient mixture including an organic solvent and water. The organic solvent is at least one of N-methylpyrrolidone, N,N-dimethylformamide, and tetrahydrofuran. The volume ratio of the organic solvent to water in the first gradient mixture is 7:3 - 8:2, and the immersion time is 5 - 7 min. The volume ratio of the organic solvent to water in the second gradient mixture is 4:6 - 5:5, and the immersion time is 3 ± 0.5 min. The volume ratio of the organic solvent to water in the third gradient mixture is 0:10, and the immersion time is 15 ± 5 s. The first gradient mixture further includes a pore-forming agent, and the mass percentage of the pore-forming agent is 1 - 3 wt%.

[0022] The addition amount of the pore-forming agent is 1 - 3 wt% to induce a bimodal pore size distribution.

[0023] The pore-forming agent is polyethylene glycol or polyvinylpyrrolidone, and the addition amount accounts for 1 - 3 wt% of the total mass of the mixture.

[0024] In a preferred embodiment of the present invention, the section drying in S4 includes three-stage heat treatment: the temperature of the first-stage heat treatment is 60 - 80 °C, and the time is 30 - 60 min; the temperature of the second-stage heat treatment is 80 - 100 °C, the time is 1 - 3 h, and the vacuum degree is 5 ± 1 kPa; the temperature of the third-stage heat treatment is 100 - 120 °C, and the time is 5 - 10 min.

[0025] The first-stage heat treatment removes the surface free solvent. The second-stage heat treatment removes the internal residual solvent. The second-stage heat treatment eliminates the internal stress, so that the solvent residue amount < 200 ppm.

[0026] The three - stage heat treatment process (vacuum stage pressure 5 ± 1 kPa) can control the solvent residue amount below 200 ppm, avoiding problems such as battery cycle attenuation or gas swelling caused by excessive solvent residue (≥500 ppm).

[0027] The present invention also discloses a lithium - ion battery, including the ultraviolet - curable cross - linked lithium - ion battery separator described above, or a battery separator prepared by the preparation method of the ultraviolet - curable cross - linked lithium - ion battery separator. The lithium - ion battery has a capacity retention rate ≥85% after 500 cycles at a 5C rate, a thermal shrinkage rate ≤2.5% at 150 °C, and an interfacial impedance growth rate ≤5% / month in a 1M LiPF6 EC / DMC electrolyte.

[0028] Preferably, in S1, the base film is made of polyethylene (PE) or polypropylene (PP) base film, with a thickness of 4 - 20 μm and a porosity of 40 - 60%.

[0029] Preferably, in S2, the solvent includes other solvents and acetone with a volume ratio of 5:5 to 7:3. The other solvents include at least one of N - methylpyrrolidone (NMP), N,N - dimethylformamide (DMF), and tetrahydrofuran (THF).

[0030] Preferably, in S2, the dispersion includes centrifugal stirring and ultrasonic treatment. The rotation speed of centrifugal stirring is 2000 - 4000 rpm, the time is 30 - 60 min, and the ultrasonic treatment is carried out at 400 - 600 W for 1 - 3 hours.

[0031] Preferably, the ultrasonic treatment adopts a pulse mode, with a working cycle of 2 s on / 1 s off, a power of 200 - 500 W, and a total time of 30 min - 2 h.

[0032] Preferably, the viscosity of the slurry prepared in S2 at 25 °C is 3000 - 5000 mPa·s, and the light transmittance at a wavelength of 365 nm is >75%.

[0033] Preferably, S2 adopts micro - gravure coating, and the coating thickness is 1 - 2 μm.

[0034] Preferably, the wavelength of ultraviolet curing in S2 is 350 - 380 nm, the temperature ≤60 °C, the light intensity is 40 - 60 mW / cm², and the time is 20 - 40 s. The double - bond conversion rate ≥95% during the ultraviolet curing process. The surface temperature of the coating after ultraviolet curing ≤60 °C.

[0035] The low - temperature characteristic (≤60 °C) of the ultraviolet curing process is highly compatible with the roll - to - roll production line, which can significantly improve the coating speed and product yield, and has significant industrial application value.

[0036] Preferably, the solvent residue amount after the S3 gradient phase - separation process treatment is <200 ppm.

[0037] Preferably, S4 further includes interface optimization treatment: a hexagonal boron nitride buffer layer is constructed on the surface of the coating by using aerosol deposition technology. The degree of orientation of the 002 crystal plane of the hexagonal boron nitride buffer layer is ≥80%, the XRD full width at half maximum is ≤0.5°, the thickness of the buffer layer is 2 - 10 nm, and the interface impedance is ≤50 Ω·cm².

[0038] Preferably, the surface density and thickness of the h-BN buffer layer satisfy the relational formula: ρ = 0.1 + 0.08t (ρ: mg / cm², t: nm), and the correlation coefficient R² ≥ 0.95.

[0039] The following are the abbreviations of chemical formulas: Polyvinylidene fluoride - hexafluoropropylene (PVDF - HFP); Aluminum oxide (Al2O3), lithium metaaluminate (LiAlO2), lithium lanthanum zirconium oxide (LLZO); Dipentaerythritol hexaacrylate (DPHA), trimethylolpropane triacrylate (TMPTA) or pentaerythritol tetraacrylate (PET4A).

[0040] A polyvinylidene fluoride - hexafluoropropylene (PVDF - HFP) / lithium lanthanum zirconium oxide (LLZO) composite coating is constructed on the surface of the separator in the present invention, which is realized through the following steps: 1) Activate the base film by using argon - oxygen plasma to increase the surface hydroxyl density to ≥5 per nm²; 2) Prepare a slurry containing LLZO nanoparticles, polyfunctional crosslinking agent and ultraviolet light initiator, and form a three - dimensional cross - linked network (cross - linking density ≥85%) through ultraviolet curing after micro - gravure coating; 3) Combine the gradient phase separation process with a pore - forming agent to form a dual - mode pore structure (porosity 40 - 50%); 4) Perform segmented drying and interface optimization treatment.

[0041] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0042] Through the synergistic optimization of plasma activation and ultraviolet curing technologies, the present invention significantly improves the interface bonding strength and high - temperature stability of the separator coating. Specifically, the surface hydroxyl density of the base film treated by argon - oxygen plasma reaches ≥5 per nm². Combining with the three - dimensional cross - linked network (cross - linking density ≥85%) constructed by a polyfunctional crosslinking agent (functionality ≥4), the peeling strength of the coating is increased to 78 N / m, which is more than 50% higher than that of the traditional wet - coating process, effectively inhibiting the interface delamination phenomenon during the cycling process. Through the coupling of the gradient dual - mode pore structure design and the ultraviolet curing process, the thermal shrinkage rate of the separator at 150 °C can be controlled within 2.5%, which is more than 60% lower than that of the uncrosslinked system, significantly enhancing the thermal safety of the battery.

[0043] The present invention shows significant advantages in terms of interfacial bonding strength, ionic conduction efficiency, and industrial production adaptability, and is applicable to high-performance lithium-ion batteries with high-nickel ternary positive electrodes and silicon-carbon negative electrode systems. Description of the Drawings

[0044] Figure 1 Schematic diagram of the composite coating separator structure prepared in an embodiment of the present invention.

[0045] Figure 2 Process flow chart of slurry preparation in an embodiment of the present invention.

[0046] Figure 3 Schematic diagram of the ultraviolet curing and gradient phase separation process in an embodiment of the present invention.

[0047] Figure 4 SEM image of the battery separator prepared in an embodiment of the present invention, forming a crosslinked network and pore structure.

[0048] Figure 5 Comparison chart of the interfacial impedance of the battery separators prepared in the examples and comparative examples of the present invention.

[0049] Figure 6 Test result chart of the thermal shrinkage rate of the battery separators prepared in Examples 1-3 and Comparative Example 1 of the present invention.

[0050] Figure 7 Comparison chart of the cycling performance of the battery separators prepared in Examples 1-3 and Comparative Example 1 of the present invention.

[0051] Figure 8 Ionic conductivity test chart of the battery separator prepared in Example 1 (25 °C) of the present invention.

[0052] Figure 9 Ionic conductivity test chart of the battery separator prepared in Example 2 (25 °C) of the present invention.

[0053] Figure 10 Ionic conductivity test chart of the battery separator prepared in Example 3 (25 °C) of the present invention.

[0054] Figure 11 Ionic conductivity test chart of the battery separator prepared in Example 3 (-40 °C) of the present invention.

[0055] Figure 12 Ionic conductivity test comparison chart of the battery separator prepared in Comparative Example 1 (25 °C) of the present invention.

[0056] Figure 13 Appearance comparison chart of the soft-pack batteries of the battery separators prepared in Example 1 and Comparative Example 4 of the present invention; wherein Figure 13 a is Example 1,Figure 13 b is Comparative Example 4 (battery swelling). Detailed implementation manners

[0057] Aiming at the technical defects of traditional lithium-ion battery separators, such as low coating interface bonding strength (peeling strength < 50 N / m), large high-temperature thermal shrinkage rate (> 5%), and insufficient ionic conductivity (< 1×10 -4 S / cm), etc., the present invention provides a preparation method of a composite coating separator based on ultraviolet light curing cross-linking technology, including the following steps:

[0058] Step 1. Substrate film pretreatment and slurry preparation.

[0059] The surface of a polyethylene (PE) or polypropylene (PP) substrate film is surface-modified with an argon-oxygen mixed plasma (Ar / O2 volume ratio 3:1 - 5:1, power density 0.8 - 1.2 W / cm²) for 20 - 60 seconds, so that the surface hydroxyl density ≥ 5 per nm² (XPS analysis), and the contact angle ≤ 35°. Subsequently, polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP, weight-average molecular weight 400 kDa ± 10%) is dissolved in a mixed solvent (volume ratio 5:5 to 7:3) composed of at least one of N-methylpyrrolidone (NMP), N,N-dimethylformamide (DMF), tetrahydrofuran (THF) and acetone, and lithium lanthanum zirconium oxide (LLZO) nanoparticles (particle size 50 - 300 nm) with a surface coating of 2 - 5 nm alumina (Al2O3) or lithium metaaluminate (LiAlO2), a multi-functional acrylate cross-linking agent and an ultraviolet light initiator are added. After high-speed centrifugal dispersion (2000 - 4000 rpm, 30 - 60 min) and pulsed ultrasonic treatment (200 - 500 W, 2 s on / 1 s off, 30 min - 2 h), a stable slurry with a viscosity of 4000 ± 500 mPa·s (25 °C) is prepared.

[0060] Step 2. Ultraviolet light curing and gradient phase separation

[0061] A 1 - 2 μm coating is formed on the surface of the activated substrate film through a microgravure coating process (anilox roll line count 200 ± 10 lines / inch), and then irradiated with a 350 - 380 nm ultraviolet light source (irradiation intensity 40 - 60 mW / cm²) for 20 - 40 seconds in a nitrogen atmosphere (oxygen content < 200 ppm) to initiate the photopolymerization reaction of the cross-linking agent (double bond conversion rate > 95%) and form a three-dimensional network structure with a cross-linking density ≥ 85%.

[0062] Subsequent gradient solvent replacement was carried out: Immersed successively in a mixed solution composed of at least one of NMP, DMF, and THF and water, with the volume ratios of the gradient mixed solutions being 7:3 - 8:2, 4:6 - 5:5, and 0:10 respectively, the treatment times at each stage being 5 - 7 min, 3 ± 0.5 min, and 15 ± 5 s respectively, and a pore-forming agent (1 - 3 wt%) was introduced synchronously. Finally, a porous coating with a bimodal pore size distribution (macropores: 200 - 1000 nm, mesopores: 50 - 200 nm) was obtained, with a porosity of 40 - 50%.

[0063] Step 3. Thermal densification and interface modification

[0064] A three-stage heat treatment process was adopted: In the first stage (60 - 80 °C, 30 - 60 min), the surface free solvent was removed; in the second stage (80 - 100 °C, 1 - 3 h, vacuum degree 5 ± 1 kPa), the internal residual solvent was removed; in the third stage (100 - 120 °C, 5 - 10 min), the internal stress was eliminated, and the solvent residue was < 200 ppm. Finally, a 2 - 10 nm thick hexagonal boron nitride (h-BN) nano buffer layer was constructed on the coating surface by aerosol deposition method. The areal density and thickness of the h-BN buffer layer satisfied the relationship: ρ = 0.1 + 0.08t (ρ: mg / cm², t: nm), and the correlation coefficient R² ≥ 0.95. Its (002) crystal plane orientation degree was ≥ 80% (XRD full width at half maximum ≤ 0.5°), reducing the interface impedance to below 50 Ω·cm².

[0065] Example 1:

[0066] This example provides a preparation method for an ultraviolet-curable crosslinked lithium-ion battery separator, and the specific implementation steps are as follows:

[0067] A PE base film with a thickness of 12 μm and a porosity of 45% was selected and subjected to plasma treatment for 30 seconds under the condition of a power density of 1.0 W / cm² using an argon-oxygen mixed gas (Ar / O₂ volume ratio 4:1). Analyzed by X-ray photoelectron spectroscopy (XPS), the hydroxyl density on the surface of the base film reached 5.2 per nm², the contact angle decreased from the initial 105° to 32°, and the surface energy increased to 52.3 mJ / m² (measured by the Owens-Wendt method).

[0068] The sol-gel method was used to prepare a coating on the surface of lithium lanthanum zirconium tantalum oxide (Li 6.4 La3Zr 1.4 Ta 0.6 O 12, an alumina (Al2O3) layer is coated on the surface of LiLa3ZrTaO (LLZO) nanoparticles, which specifically includes the following steps: First, aluminum isopropoxide is dissolved in absolute ethanol to prepare a precursor solution with a concentration of 0.1 mol / L, and 0.5 wt% acetylacetone is added as a chelating agent, and stirred until the solution becomes transparent; Subsequently, LiLa3ZrTaO (LLZO) nanoparticles with a particle size of 200 nm are dispersed in ethanol at a solid content of 10 wt%, and ultrasonically treated for 30 minutes at a power of 300 W to obtain a uniformly dispersed LLZO suspension; The suspension is added dropwise to the precursor solution, and the molar ratio of aluminum to LLZO is controlled to be 1:50, and continuously stirred at a constant temperature of 60 °C for 6 hours, and during this period, the pH value of the reaction system is adjusted to 4.0 by acetic acid; After the reaction is completed, the mixed system is aged at room temperature for 12 hours, centrifuged, dried in vacuum at 60 °C, and then placed in an argon atmosphere, and calcined at a heating rate of 5 °C / min to 400 °C for 2 hours to obtain LLZO@Al2O3 composite nanoparticles with an alumina layer of 2.0 ± 0.3 nm coated on the surface. High-resolution transmission electron microscopy (HR-TEM) characterization shows that the thickness of the alumina coating layer is uniformly distributed in the range of 2.0 ± 0.3 nm, and energy-dispersive X-ray spectroscopy (EDS) analysis confirms that aluminum elements are continuously distributed on the surface of LLZO particles. 6.4 La3Zr 1.4 Ta 0.6 O 12 (LLZO) nanoparticles are dispersed in ethanol at a solid content of 10 wt%, ultrasonically treated for 30 minutes at a power of 300 W to obtain a uniformly dispersed LLZO suspension; The suspension is added dropwise to the precursor solution, and the molar ratio of aluminum to LLZO is controlled to be 1:50, and continuously stirred at a constant temperature of 60 °C for 6 hours, and during this period, the pH value of the reaction system is adjusted to 4.0 by acetic acid; After the reaction is completed, the mixed system is aged at room temperature for 12 hours, centrifuged, dried in vacuum at 60 °C, and then placed in an argon atmosphere, and calcined at a heating rate of 5 °C / min to 400 °C for 2 hours to obtain LLZO@Al2O3 composite nanoparticles with an alumina layer of 2.0 ± 0.3 nm coated on the surface. High-resolution transmission electron microscopy (HR-TEM) characterization shows that the thickness of the alumina coating layer is uniformly distributed in the range of 2.0 ± 0.3 nm, and energy-dispersive X-ray spectroscopy (EDS) analysis confirms that aluminum elements are continuously distributed on the surface of LLZO particles.

[0069] NMP and acetone are mixed at a volume ratio of 6:4 as a solvent system, and PVDF-HFP with a weight-average molecular weight of 400 kDa is added, and stirred at a constant temperature of 60 °C until completely dissolved. The LLZO@Al2O3 composite nanoparticles (D50 = 200 nm) with 2.0 nm Al2O3 coated on the surface, DPHA (the addition amount accounts for 20% of the polymer mass), and PI-2959 (the addition amount is 5%) are added in sequence, and centrifugally dispersed at 3000 rpm for 30 minutes and ultrasonically treated with 500 W pulsed ultrasound for 2 hours to obtain a homogeneous slurry with a viscosity of 4000 mPa·s (25 °C) and a light transmittance > 75% (wavelength of 365 nm).

[0070] A 2-μm coating is formed on the surface of the activated base film by microgravure coating process, and then irradiated with ultraviolet light with a wavelength of 365 nm and an irradiation intensity of 50 mW / cm² for 30 seconds in a nitrogen protection environment (oxygen concentration < 200 ppm). Fourier transform infrared spectroscopy analysis shows that the conversion rate of C=C double bonds reaches 96.5%, forming a three-dimensional network structure with a crosslinking density of 86.2%, and the surface temperature of the coating during the curing process is controlled below 58 °C.

[0071] The solidified separator was successively immersed in a mixture of NMP / water with a volume ratio of 7:3 (i.e., the volume ratio of NMP to water is 7:3) containing 1% PEG400 for 5 minutes, and then in a mixture of NMP / water with a volume ratio of 4:6 (i.e., the volume ratio of NMP to water is 4:6) for 3 minutes, and finally rinsed with pure water for 10 seconds. Mercury intrusion porosimetry tests showed that the porosity of the obtained coating was 48%, presenting a bimodal pore size distribution: macropores of 500 - 1000 nm and mesopores of 100 - 200 nm, and the pore tortuosity was 1.75.

[0072] A three - stage drying procedure was implemented: hot air drying at 60 °C for 30 minutes to remove surface solvents, vacuum drying at 80 °C for 2 hours (vacuum degree 5 kPa) to remove residual internal solvents, and annealing at 100 °C for 10 minutes to eliminate internal stress. The final solvent residue was < 180 ppm (analyzed by gas chromatography - mass spectrometry). An h - BN buffer layer was constructed on the coating surface using aerosol deposition technology, with a controlled thickness of 5.0 nm and a surface density of 0.5 mg / cm². X - ray diffraction analysis showed that the orientation degree of the (002) crystal plane was 83%, and the full width at half maximum was 0.47°. The interface impedance between the electrode and the separator decreased to 48 Ω·cm². After soaking in a 1 M LiPF6 EC / DMC electrolyte for 30 days, the interface impedance was 50 Ω·cm², with a growth rate of 4.2%.

[0073] Commercially available SiC (gravimetric capacity 600 mAh / g) and NCM622 were selected as the positive and negative active materials, and an electrolyte of 1 M LiPF6 in EC / DMC (volume ratio 1:1) with 2 wt% FEC and 1 wt% VC was used. It was assembled into a button cell or a soft - pack battery.

[0074] The raw materials and reagents used in Example 1 are shown in Table 1.

[0075] The implementation effects are shown in Table 2.

[0076] Example 2

[0077] In this example, the following optimization adjustments were made on the basis of Example 1 to reduce production costs while maintaining core performance:

[0078] Parameter adjustment: TMPTA and DPHA were compounded in a mass ratio of 3:1, and the total addition amount was reduced to 15% (TMPTA 11.25%, DPHA 3.75%); UV - 184 was used, and the addition amount was adjusted to 3%; the surface coating layer of LLZO nanoparticles retained Al2O3 (2.0 nm), but the mass ratio of LLZO decreased to 25%; the pore - forming agent used was PVP K30, and the addition amount was increased to 2%; the ultraviolet curing time was shortened to 25 seconds, and the irradiation intensity was reduced to 45 mW / cm². The other parameters were the same as those in Example 1.

[0079] The implementation effects are shown in Table 3.

[0080] Example 3

[0081] Based on Example 1, the following optimization adjustments are made in this example to target the high-rate charge and discharge scenario. The specific adjustment plan is as follows:

[0082] Parameter adjustment: Ta-doped LLZO (Li 6.4 La3Zr 1.4 Ta 0.6 O 12 ), with a 3.0 nm LiAlO2 layer coated on the surface and the mass ratio increased to 45%; PET4A is selected as the main cross-linking agent with an addition amount of 25%, and 0.3% hindered amine light stabilizer (Tinuvin 770) is added synchronously; PI-819 is used, with an addition amount of 1.5%, and a 395 nm high-power LED light source is matched; the water extraction gradient is adjusted to DMF / water: 8:2, 5:5, 0:10, and the treatment time in the first stage is extended to 7 minutes; the thickness of the h-BN buffer layer is controlled at 3.0 ± 0.5 nm, and the areal density is 0.3 - 0.38 mg / cm²; the other parameters are the same as those in Example 1.

[0083] The implementation effects are shown in Table 4.

[0084] Comparative Example 1

[0085] This comparative example uses the same substrate and basic process as Example 1, but the photo-curing cross-linking step is cancelled to simulate the traditional coating system. The specific implementation differences are as follows:

[0086] DPHA and ultraviolet photoinitiator are not added to the slurry; the ultraviolet photo-curing step is cancelled, and the wet film is directly dried; the coating is a physical mixture structure without a three-dimensional cross-linking network; the surface hydroxyl density maintains the initial value of the base film.

[0087] The performance comparison is shown in Table 5.

[0088] Comparative Example 2

[0089] The difference between this comparative example and Example 1 is that the cross-linking agent is replaced with trimethylolpropane triacrylate (TMPTA) with a functionality of 3 and an addition amount of 20%, and the other parameters are the same as those in Example 1. The test results show that the double bond conversion rate is 89.2%, the cross-linking density is 78%, the peel strength is 48 N / m, the thermal shrinkage rate at 150 °C is 4.1%, and the ionic conductivity is 3.1×10 -4S / cm, the interfacial impedance increases from 62 Ω·cm² (initial) to 87 Ω·cm² (30 days), and the capacity retention rate after 500 cycles at 5C is 85%.

[0090] Comparative Example 3

[0091] The difference from Example 1 is that the plasma treatment time is shortened to 10 seconds, the hydroxyl density on the surface of the base film is 2.8 per nm², and the other parameters are the same as those in Example 1. The test results show that the crosslinking density is 85%, the coating peel strength is 42 N / m, the thermal shrinkage rate at 150 °C is 2.0%, and the ionic conductivity is 3.8×10 -4 S / cm, the interfacial impedance increases from 75 Ω·cm² (initial) to 90 Ω·cm² (30 days), and the capacity retention rate after 500 cycles at 5C is only 71%. Due to insufficient hydroxyl density, the interface deteriorates, and the interfacial impedance increases to 127 Ω·cm² after 500 cycles at 5C, with a growth rate of 56%.

[0092] Comparative Example 4

[0093] The difference from Example 1 is only that the argon-oxygen plasma treatment in step S1 is cancelled, and the hydroxyl density on the surface of the base film remains at the original value (1.2 per nm², contact angle 105°), and the other parameters are exactly the same as those in Example 1. The hydroxyl density on the surface of the base film without plasma activation is low (1.2 per nm²), resulting in ineffective chemical bonding between the coating and the base film, a sharp drop in the interfacial bonding force (peel strength 18 N / m), and at the same time, due to an increase in interface defects, ion transport is blocked (the conductivity drops to 6.5×10 -5 S / cm), and severe delamination is caused by the difference in the thermal expansion coefficients of the base film and the coating at high temperatures (thermal shrinkage rate 8.5%).

[0094] The performance comparison between Comparative Example 2, Comparative Example 3 and Comparative Example 4 and Example 1 is shown in Table 6.

[0095] Comparative Example 5

[0096] The difference from Example 1 is that the vacuum degree in the three-stage heat treatment is 3 kPa, and the other parameters are the same as those in Example 1. The test results show that the solvent removal is not complete, the residual amount ≥ 500 ppm, and the capacity retention rate of the battery after 100 cycles at 1C is only 67%. Excessive solvent residue leads to an increase in side reactions inside the battery, and the battery bulges ( Figure 13 ).

[0097] Verification of Technical Effects

[0098] 1. Peel strength test: According to ASTM D903, an Instron 5967 universal material testing machine is used, and the tensile speed is 50 mm / min.

[0099] 2. High-temperature stability verification: The separator is placed in an oven for 1 h, and the dimensional change rate is measured by a laser rangefinder.

[0100] 3. Ion conductivity test method: The composite coating is peeled off from the surface of the base film, and independent film samples with a diameter of 16 mm are prepared by a precision slicing machine. Using the Chenhua CH760e electrochemical workstation, it is calculated through the formula σ = L / (R·A), where L is the coating thickness, R is the impedance measured by EIS, and A is the electrode area. The test frequency range is 0.1 Hz - 1 MHz.

Claims

1. An ultraviolet-curable crosslinked lithium-ion battery separator, comprising a base film and an ultraviolet-curable crosslinked coating coated on the surface of the base film, characterized in that, The hydroxyl density on the surface of the base film is ≥5 per nm², and the contact angle is ≤35°. The ultraviolet-curable crosslinked coating includes a polymer matrix, nanoparticles, a photoinitiator, and a multifunctional acrylate crosslinking agent with a functionality ≥4.

2. The ultraviolet light-curing crosslinkable lithium ion battery separator according to claim 1, wherein The peel strength of the ultraviolet-curable crosslinked lithium-ion battery separator is 65-85 N / m, and the ionic conductivity at 25 °C is 3×10 -4 ~5.5×10 -4 S / cm, and the thermal shrinkage rate at 150 °C is <2.5%.

3. The ultraviolet light-curing crosslinkable lithium-ion battery separator according to claim 1, wherein The mass ratio of the polymer matrix, nanoparticles, multifunctional acrylate crosslinking agent with a functionality ≥4, and photoinitiator is 100:25 - 50:5 - 30:0.3 - 5.

4. The ultraviolet-curable crosslinked lithium-ion battery separator according to claim 1, characterized in that, The nanoparticles include lithium lanthanum zirconium oxide nanoparticles and a coating particle layer coated on the surface of the lithium lanthanum zirconium oxide nanoparticles. The coating particle layer includes one or both of alumina and lithium metaaluminate.

5. The ultraviolet light-curing crosslinkable lithium-ion battery separator according to claim 1, wherein, The polymer matrix includes polyvinylidene fluoride - hexafluoropropylene, and the multifunctional acrylate crosslinking agent includes one or more of dipentaerythritol hexaacrylate, trimethylolpropane triacrylate, or pentaerythritol tetraacrylate.

6. A method for preparing an ultraviolet-curable crosslinked lithium-ion battery separator according to any one of claims 1-5, characterized in that, It includes the following steps: S1. Activate the base film with argon - oxygen plasma to increase the hydroxyl density on the surface of the base film to ≥5 per nm² and the contact angle to ≤35°, obtaining the activated base film. S2. Mix the solvent, polymer matrix, nanoparticles, multifunctional crosslinking agent, and photoinitiator, and disperse them evenly to obtain a slurry. Coat the slurry on the surface of the activated base film in S1, and then form a three - dimensional crosslinked network coating by ultraviolet curing, so that the crosslinking density is ≥85%. S3. Use a gradient phase separation process on the three - dimensional crosslinked network coating in S2 to form a porous coating with a bimodal pore size distribution, so that the porosity is 40 - 50%. S4. Perform segmented drying on the porous coating in S3 to obtain an ultraviolet - curable crosslinked lithium - ion battery separator.

7. The preparation method of the ultraviolet light-curing crosslinked lithium ion battery separator according to claim 6, characterized in that, In S1, the argon - oxygen plasma activation of the base film uses an argon - oxygen mixed gas with an Ar / O₂ volume ratio of 3:1 - 5:1, a power density of 0.8 - 1.2 W / cm², and a time of 20 - 60 s.

8. The preparation method of the ultraviolet-curable crosslinked lithium ion battery separator according to claim 6, characterized in that, The gradient phase separation process in S3 includes sequentially immersing the ultraviolet - cured separator into a first gradient mixture, a second gradient mixture, and a third gradient mixture including an organic solvent and water. The organic solvent is at least one of N - methylpyrrolidone, N,N - dimethylformamide, and tetrahydrofuran. The volume ratio of the organic solvent to water in the first gradient mixture is 7:3 - 8:2, and the immersion time is 5 - 7 min. The volume ratio of the organic solvent to water in the second gradient mixture is 4:6 - 5:5, and the immersion time is 3 ± 0.5 min. The volume ratio of the organic solvent to water in the third gradient mixture is 0:10, and the immersion time is 15 ± 5 s. The first gradient mixture further includes a pore - forming agent, and the mass percentage of the pore - forming agent is 1 - 3 wt%.

9. The preparation method of the ultraviolet-curable crosslinked lithium-ion battery separator according to claim 6, characterized in that, The segmented drying in S4 includes three - stage heat treatment: the temperature of the first - stage heat treatment is 60 - 80 °C, and the time is 30 - 60 min; the temperature of the second - stage heat treatment is 80 - 100 °C, the time is 1 - 3 h, and the vacuum degree is 5 ± 1 kPa; the temperature of the third - stage heat treatment is 100 - 120 °C, and the time is 5 - 10 min.

10. A lithium-ion battery, characterized in that, Including the ultraviolet-curable crosslinked lithium-ion battery separator according to any one of claims 1-5, or the battery separator prepared by the preparation method of the ultraviolet-curable crosslinked lithium-ion battery separator according to any one of claims 6-9, the lithium-ion battery has a capacity retention rate of ≥85% after 500 cycles at a 5C rate, a thermal shrinkage rate of ≤2.5% at 150°C, and an interfacial impedance growth rate of ≤5% / month in a 1M LiPF6 EC / DMC electrolyte.

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