Lithium ion battery diaphragm as well as preparation method and application thereof

Abstract

The invention provides a lithium ion battery diaphragm and a preparation method and application thereof.The diaphragm comprises a three-dimensional cross-linked network and multifunctional filler, the three-dimensional cross-linked network is formed by polymerization of a skeleton and a cross-linking agent, the skeleton is a high polymer material containing vinyl, the cross-linking agent contains carbon-carbon double bonds, and the multifunctional filler is a polymer material containing vinyl. The mass ratio of the cross-linking agent to the skeleton is (2-4): 5; the multifunctional filler comprises a ceramic material and / or a solid electrolyte, and the mass ratio of the cross-linking agent to the multifunctional filler is (2-5): 1. The diaphragm disclosed by the invention has high liquid absorption rate, high wettability, high ion conduction and low thermal shrinkage, and is applied to the lithium ion battery, so that the capacity retention ratio can be up to 80% or above after the lithium ion battery is circulated for 500 circles under the condition of high-rate 2C charge / 10C discharge, and a new scheme is provided for the lithium ion battery which is environment-friendly and high in safety.

Description

Technical Field

[0001] The present application relates to the field of new energy technology, and in particular to a lithium-ion battery separator and a preparation method and application thereof. Background Art

[0002] As one of the core components of lithium-ion batteries, the separator not only bears the key responsibility of physically isolating the positive and negative electrodes and preventing short circuits, but also directly restricts the safety, cycle life, and rate performance of the battery. Currently, commercial separators are mainly made of polyolefin materials such as microporous polyethylene and polypropylene. Their advantages are high mechanical strength and good electrochemical stability, but they have two inherent defects: (1) poor electrolyte affinity: the non-polar nature of the molecular chain leads to insufficient wettability (contact angle > 25°) and low liquid absorption rate (commercial PE film is about 110%); (2) insufficient thermal stability: it is easy to melt and shrink at high temperatures (shrinkage rate > 18% above 160°C), causing the risk of thermal runaway. These defects make polyolefin separators prone to safety hazards such as electrolyte leakage and battery explosion under extreme conditions such as overcharging and extrusion.

[0003] To overcome the above limitations, gel polymer electrolyte (GPE) separators have become a research hotspot due to their high safety, structural flexibility and multifunctional properties. However, traditional GPE relies on petroleum-based polymers (such as PVDF and PAN), and its non-degradability and high environmental footprint make it difficult to meet the requirements of green and sustainable development. Cellulose acetate has excellent electrolyte affinity due to its large number of polar groups (acetyl and hydroxyl groups). Existing technologies have been used to prepare lithium-ion battery separators. However, on the one hand, linear cellulose acetate molecules are easily swollen and dissolved in ester electrolytes, resulting in a sudden drop in mechanical strength. On the other hand, the cyclic charge and discharge stability of the battery used in lithium-ion batteries is limited, especially the cyclic stability at high rates is unpredictable. Summary of the Invention

[0004] In order to overcome the above problems, the present application provides a lithium-ion battery separator, which is constructed by constructing a three-dimensional cross-linked network structure formed by the polymerization of a skeleton and a cross-linking agent, and a multifunctional filler. There is a synergistic effect between the multifunctional filler and the three-dimensional cross-linked network structure. The prepared separator has high liquid absorption rate, high wettability, high ion conductivity and low thermal shrinkage. When applied to lithium-ion batteries, it can achieve a capacity retention rate of more than 80% after 500 cycles at a high rate of 2C / 10C, ​​providing a new solution for lithium-ion batteries that are both environmentally friendly and highly safe.

[0005] In order to achieve the above objectives, this application adopts the following technical solutions:

[0006] On the one hand, the present application provides a lithium-ion battery separator, comprising a three-dimensional cross-linked network and a multifunctional filler; wherein, the three-dimensional cross-linked network is formed by polymerization of a skeleton and a cross-linking agent, the skeleton is a polymer material containing vinyl groups, the cross-linking agent contains carbon-carbon double bonds, and the mass ratio of the cross-linking agent to the skeleton is (2 to 4):5; the multifunctional filler comprises a ceramic material and / or a solid electrolyte, and the mass ratio of the cross-linking agent to the multifunctional filler is (2 to 5):1.

[0007] In some embodiments, the ceramic material includes one or more of alumina, silica, zirconia, boehmite, nitride, boride, and carbide.

[0008] In some embodiments, the solid electrolyte includes one or more of lithium aluminum titanium phosphate, lithium aluminum zirconium phosphate, lithium lanthanum titanate, tantalum-doped lithium lanthanum zirconate, lithium tin phosphate, and lithium aluminum chromium phosphate.

[0009] In some embodiments, the particle size D50 of the multifunctional filler is 0.1-1 μm.

[0010] In some embodiments, the backbone comprises one or more of polyethylene, polytetrafluoroethylene, polyvinylidene fluoride, comonomer-modified polyvinylidene fluoride, and vinyl-functionalized cellulose acetate.

[0011] In some embodiments, the preparation method of the vinyl-functionalized cellulose acetate comprises: dissolving equal masses of cellulose acetate, methacryloyl chloride, and triethylamine in N,N'-dimethylacetamide under a nitrogen atmosphere at -10 to -20°C, and then transferring the mixture to room temperature for stirring and reaction.

[0012] In some embodiments, the crosslinking agent includes one or more of acrylonitrile-butadiene-styrene copolymer, polymethacrylic acid, acrylic acid and its derivatives, acrylamide and its derivatives, ethylene ethyl acrylate copolymer and ethylene acrylate copolymer.

[0013] In some embodiments, the liquid absorption rate of the lithium ion separator is greater than 400%, the ionic conductivity of the lithium ion separator is greater than or equal to 3.8 mS / cm at 25° C., the contact angle is less than or equal to 20°, and the shrinkage rate is less than or equal to 4% after heat treatment at 180° C. for 30 minutes.

[0014] On the other hand, the present application also provides a method for preparing the above-mentioned lithium-ion battery separator, comprising first dissolving the multifunctional filler, cross-linking agent and skeleton in an organic solvent in proportion, adding 1 to 3 wt% of a photoinitiator, and then applying the obtained casting solution on the base film, placing it under ultraviolet irradiation at a wavelength of 350 to 400 nm and a power of 300 to 20,000 W for 5 to 10 minutes to initiate a polymerization reaction, wherein the organic solvent includes one or more of N,N'-dimethylacetamide, dimethyl sulfoxide, N-methylpyrrolidone and tetrahydrofuran, and the base film includes polyethylene and / or polypropylene.

[0015] In some embodiments, after the polymerization reaction is completed, a film curing step is further included: the base film covered with the diaphragm coating is placed at 60-80° C. and vacuum dried for 8-12 hours to obtain a solid cross-linked film with a thickness of 30±5 μm.

[0016] The present application also provides a lithium-ion battery, comprising the aforementioned lithium-ion battery separator or the lithium-ion battery separator prepared by the aforementioned preparation method, and a positive electrode and a negative electrode located on both sides of the lithium-ion battery separator. The 21700 cylindrical battery assembled with the lithium-ion battery separator is cycled 500 times at a 2C charge / 10C discharge rate, and the capacity retention rate is greater than 80%.

[0017] Compared with the existing technology, the present application has the following advantages: First, the present application provides a lithium-ion battery separator. By constructing a separator comprising a three-dimensional cross-linked network structure formed by polymerization of a skeleton and a cross-linking agent, and a multifunctional filler, the separator exhibits high liquid absorption, high wettability, high ion conductivity, and low thermal shrinkage by utilizing the synergistic effect between the multifunctional filler and the three-dimensional cross-linked network structure, combined with the regulation of the cross-linking agent, skeleton, and the ratio of the cross-linking agent to the multifunctional filler. Specifically, the separator exhibits a liquid absorption rate greater than 400%, an ionic conductivity of ≥3.8 mS / cm at 25°C, a contact angle of ≤20°, and a shrinkage of ≤4% after heat treatment at 180°C for 30 minutes. Second, the present application provides a method for preparing a lithium-ion battery separator. By incorporating ultraviolet light curing technology, the method significantly improves the production efficiency, environmental friendliness, and thermal stability of the lithium-ion battery separator, while simplifying the process flow and forming a high-strength three-dimensional network structure through rapid cross-linking. Thirdly, the present application also provides a lithium-ion battery, in which the prepared diaphragm with high liquid absorption rate, high wettability, high ion conductivity and low thermal shrinkage is applied to the lithium-ion battery, so that after 500 cycles at a high rate of 2C charge / 10C discharge rate, the capacity retention rate can still be as high as 80% or more, providing a new solution for lithium-ion batteries that are both environmentally friendly and highly safe. DETAILED DESCRIPTION

[0018] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions of the present invention. Obviously, the embodiments described are part of the embodiments of this application, not all of the embodiments. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application. It should be understood that the specific embodiments described are only used to explain this application and are not used to limit this application.

[0019] In the description of the embodiments of the present application, it should be noted that all ranges disclosed in the present application will be understood to encompass any and all sub-ranges included therein. For example, the stated range "60-80°C" should be deemed to include any and all sub-ranges that start with a minimum value of 60°C or greater and end with a maximum value of 80°C or less, for example, 60 to 65°C, or 65 to 80°C, or 70 to 80°C. At the same time, all ranges disclosed in the present application are also deemed to include the endpoints of the ranges, unless otherwise expressly stated. For example, the ranges "between 60 and 80" or "60 to 80" or "60-80" should generally be deemed to include the endpoints 60 and 80.

[0020] This application addresses the potential problems of swelling and dissolution when directly using cellulose acetate to prepare lithium-ion battery separators, resulting in a sudden drop in mechanical strength and limited cyclic charge and discharge stability when the separator is used in lithium-ion batteries. The present application proposes a technical solution, namely, constructing a three-dimensional cross-linked network structure formed by the polymerization of a skeleton and a cross-linking agent, and a separator containing a multifunctional filler. On the one hand, there is a synergistic effect between the multifunctional filler and the three-dimensional cross-linked network structure, which enables the prepared separator to have high liquid absorption rate, high wettability, high ion conductivity and low thermal shrinkage. At the same time, when the separator of this solution is applied to lithium-ion batteries, the capacity retention rate can still be as high as 80% or more after 500 cycles at a high charge rate of 2C / discharge rate of 10C, providing a new solution for lithium-ion batteries that are both environmentally friendly and highly safe.

[0021] In order to achieve the above objectives, this application adopts the following technical solutions:

[0022] The present application provides a lithium-ion battery separator, characterized in that the lithium-ion battery separator includes a three-dimensional cross-linked network and a multifunctional filler, wherein the three-dimensional cross-linked network is formed by polymerization of a skeleton and a cross-linking agent, the skeleton is a polymer material containing a vinyl group, the cross-linking agent contains a carbon-carbon double bond, and the mass ratio of the cross-linking agent to the skeleton is (2 to 4):5; the multifunctional filler includes a ceramic material and / or a solid electrolyte, and the mass ratio of the cross-linking agent to the multifunctional filler is (2 to 5):1.

[0023] It should be noted that the formation mechanism of the lithium-ion battery separator of the present application is to select a polymer skeleton containing vinyl to provide active sites for the subsequent process, and to form a covalent bond between the cross-linker containing carbon-carbon double bonds and the vinyl skeleton through free radical polymerization or condensation reaction to form a chemical cross-linking point. Subsequently, ultraviolet light curing technology is combined to promote the polymerization of vinyl and double bond monomers to construct a three-dimensional network. During the process, the multifunctional filler is dispersed in the cross-linker and loaded on the surface of the skeleton to form a uniform coating, which also serves as part of the formation of the separator.

[0024] Based on the above mechanism, this application requires strict control of the ratio of crosslinker to backbone in the polymerization reaction. The reason is that the role of the crosslinker in the reaction is to provide chemical bonds to connect the polymer chains, forming a three-dimensional network structure, and improving the tensile strength, puncture strength and heat resistance of the coating. Under this effect, if the amount of crosslinker is too little, the crosslinking density will be insufficient, the network structure will be loose, and the coating will be prone to cracking and the high-temperature shrinkage rate will increase; if the crosslinker is too much, the network will be over-crosslinked, the material will increase brittleness, and the toughness will decrease, which may cause the coating to peel off or increase the ion transport resistance. At the same time, the amount of crosslinker added also needs to be balanced with the multifunctional filler. If the crosslinker is insufficient, the interfacial bonding will be weakened, and the crosslinker will not be able to fully anchor the filler, resulting in physical adsorption between the filler and the polymer matrix, which is prone to interfacial delamination; if the crosslinker is too much, a thick coating will be formed on the filler surface, which will hinder the exposure of active sites and reduce functional efficiency. Too high a crosslinking density will also increase the brittleness of the coating, easily generate microcracks during dynamic cycling, and accelerate battery failure.

[0025] In some embodiments, the ceramic material includes one or more of aluminum oxide, silicon oxide, zirconium oxide, boehmite, nitride, boride and carbide; the solid electrolyte includes one or more of lithium aluminum titanium phosphate, lithium aluminum zirconium phosphate, lithium lanthanum titanate, tantalum-doped lithium lanthanum zirconate, lithium tin phosphate and lithium aluminum chromium phosphate.

[0026] In this application, the reason for selecting a specific type of multifunctional filler is that by adding a solid electrolyte to the three-dimensional cross-linked network, the pores of the diaphragm can be filled and a penetrating solid-state ion path can be formed, thereby reducing the ion transmission resistance. Especially at high-rate charge and discharge, it can alleviate the performance degradation of the liquid electrolyte caused by concentration polarization; in addition, the mechanical strength of the solid electrolyte is relatively high (such as the Young's modulus of the oxide electrolyte is above 100GPa), which can to a certain extent prevent lithium dendrites from penetrating the diaphragm and reduce the risk of short circuit.

[0027] By adding ceramic particles dispersed in a three-dimensional cross-linked network, a high mechanical strength can be formed as a physical barrier to prevent the tips of lithium dendrites from penetrating the diaphragm. At the same time, the polar groups (such as hydroxyl groups) on the surface of the ceramic particles can adsorb lithium ions in the electrolyte, guide the uniform deposition of lithium, and reduce the initiation of dendrites. In addition, the porous structure formed by the ceramic particles can store more electrolyte, alleviating the problem of "electrolyte depletion" at high rates. Ceramic particles can form a stable interface layer with the electrode material (such as Al2O3 reacting with Li2O on the surface of the positive electrode material to form LiAlO2), inhibiting the direct reaction between the electrolyte and the electrode.

[0028] In some embodiments, the particle size D50 of the multifunctional filler is 0.1-1 μm.

[0029] It should be noted that in addition to the need to specially select the type of multifunctional filler in this application, its particle size must also be moderate, which is conducive to the formation of uniform pores between the filler particles, providing continuous ion transmission channels, and reducing internal resistance. Large particles will extend the lithium ion migration path, and the high-rate discharge capacity will decay significantly. It will also result in low thermal conductivity, and heat accumulation will accelerate the thermal shrinkage of the diaphragm; ultrafine fillers are easy to fill the original micropores of the diaphragm, resulting in a decrease in effective porosity and a decrease in ionic conductivity.

[0030] In some embodiments, the backbone comprises one or more of polyethylene, polytetrafluoroethylene, polyvinylidene fluoride, comonomer-modified polyvinylidene fluoride, and vinyl-functionalized cellulose acetate.

[0031] In some embodiments, the preparation method of the vinyl-functionalized cellulose acetate comprises: dissolving equal masses of cellulose acetate, methacryloyl chloride, and triethylamine in N,N'-dimethylacetamide under a nitrogen atmosphere at -10 to -20°C, and then transferring the mixture to room temperature for stirring and reaction.

[0032] In this application, room temperature is generally defined as 25°C according to conventional interpretation.

[0033] In some embodiments, the crosslinking agent includes one or more of acrylonitrile-butadiene-styrene copolymer, polymethacrylic acid, acrylic acid and its derivatives, acrylamide and its derivatives, ethylene ethyl acrylate copolymer and ethylene acrylate copolymer.

[0034] In some embodiments, the liquid absorption rate of the lithium ion separator is greater than 400%, the ionic conductivity of the lithium ion separator is greater than or equal to 3.8 mS / cm at 25° C., the contact angle is less than or equal to 20°, and the shrinkage rate is less than or equal to 4% after heat treatment at 180° C. for 30 minutes.

[0035] The present application also provides a method for preparing the aforementioned lithium-ion battery separator, comprising first dissolving the multifunctional filler, cross-linking agent and skeleton in an organic solvent in proportion, adding 1 to 3 wt% of a photoinitiator, and then applying the obtained casting solution on the base film, exposing it to ultraviolet radiation with a wavelength of 350 to 400 nm and a power of 300 to 20,000 W for 5 to 10 minutes to initiate a polymerization reaction, wherein the organic solvent includes one or more of N,N'-dimethylacetamide, dimethyl sulfoxide, N-methylpyrrolidone and tetrahydrofuran, and the base film includes polyethylene and / or polypropylene.

[0036] In some possible implementations, the wavelength of the ultraviolet radiation is 365 nm and the power is 300 W.

[0037] In some embodiments, the preparation method further comprises a film curing step after the polymerization reaction is completed: the base film covered with the diaphragm coating is placed in a vacuum drying at 60-80°C for 8-12 hours to obtain a solid cross-linked film with a thickness of 30±5 μm.

[0038] The present application also provides a lithium-ion battery, comprising the lithium-ion battery separator described above or the lithium-ion battery separator prepared by the preparation method described above, and a positive electrode and a negative electrode located on both sides of the lithium-ion battery separator. The 21700 cylindrical battery assembled with the lithium-ion battery separator is cycled 500 times at a 2C charge / 10C discharge rate, and the capacity retention rate is greater than 80%.

[0039] In this application, 500 cycles at a 2C charge / 10C discharge rate refers to 500 cycles under 2C charging and 10C discharging conditions.

[0040] The present application is further explained below with reference to examples.

[0041] The raw materials used in the examples of this application are all commercially available.

[0042] Example 1

[0043] This embodiment provides a method for preparing a positive electrode material, comprising:

[0044] Preparation of diaphragm

[0045] (1) Vinyl functionalization of cellulose acetate: Equal amounts of cellulose acetate, methacryloyl chloride, and triethylamine were dissolved in N,N'-dimethylacetamide under a nitrogen atmosphere at -10°C, and then stirred at room temperature to obtain vinyl functionalized cellulose acetate.

[0046] (2) Acrylamide, alumina ceramics, and vinyl-functionalized cellulose acetate were dissolved in N,N'-dimethylacetamide at a mass ratio of 4:1:10, and then 1 wt% of photoinitiator 2959 was added. The particle size of the alumina ceramics was 0.5 μm.

[0047] (3) The casting liquid was evenly coated on a PE flat plate using a film applicator to form a liquid coating. The coating was then irradiated under an ultraviolet lamp (wavelength 365 nm, power 300 W) for 5 minutes to initiate a polymerization reaction. The PE plate covered with the primary film was transferred to a 70°C vacuum oven and dried for 12 hours. After the solvent was completely removed, a cross-linked film 1 was obtained with a thickness of 30±5 μm.

[0048] Positive electrode sheet preparation method

[0049] The positive electrode active material (high nickel ternary material LiNi 0.8 Co 0.1 Mn 0.1 O2), conductive carbon black, and binder polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 96:1:3, and then N-methylpyrrolidone (NMP) was added, stirred and mixed to form a stable positive electrode slurry with a solid content of 70%. The positive electrode slurry was evenly coated on a 12μm aluminum foil of the positive electrode current collector, and then dried and cold pressed to obtain a positive electrode sheet with a compaction density of 3.5g / cm 3 .

[0050] Negative electrode sheet preparation method

[0051] Nitrogen-doped carbon-coated graphite, silicon-carbon material, conductive agent carbon nanotubes, thickener sodium carboxymethyl cellulose (CMC) and binder polyacrylic acid (PAA) were mixed in a mass ratio of 86:10:1.5:1:1.5, and then deionized water was added and stirred to form a uniform and stable negative electrode slurry with a solid content of 40%. The negative electrode slurry was evenly coated on an 8μm copper foil of the negative electrode current collector, and then dried and cold pressed to obtain a negative electrode sheet with a compaction density of 1.6g / cm 3 .

[0052] Electrolyte preparation

[0053] The electrolyte was obtained by mixing lithium salt lithium hexafluorophosphate (LiPF6), organic solvent ethylene carbonate (EC), dimethyl carbonate (DMC), fluoroethylene carbonate (FEC), vinyl sulfate (DTD) and vinylene carbonate (VC) in a mass percentage ratio of 10.0:20.0:55.0:2.0:8.0:5.0.

[0054] Lithium-ion battery assembly

[0055] The positive and negative electrode sheets are rolled and slit, respectively, and then wound together with the separator to obtain a 21700 cylindrical battery core. The battery core is then welded to the connecting sheet and loaded into the battery casing. After completing the injection, sealing, and formation processes, the lithium-ion battery of Example 1 is obtained. The casing of the lithium-ion battery is cylindrical, and its dimensional parameters are: diameter: 21.0 mm, length: 70.0 mm.

[0056] Example 2

[0057] The only difference between this embodiment and embodiment 1 is that acrylamide, lithium aluminum titanium phosphate (LATP) and vinyl-functionalized cellulose acetate are in a mass ratio of 3:1:5, and the rest are the same as embodiment 1.

[0058] Example 3

[0059] The only difference between this embodiment and embodiment 2 is that acrylamide, lithium aluminum titanium phosphate (LATP) and vinyl-functionalized cellulose acetate are in a mass ratio of 4:1:5, and the rest are the same as embodiment 2.

[0060] Example 4

[0061] The only difference between this embodiment and embodiment 2 is that acrylamide, lithium aluminum titanium phosphate (LATP) and vinyl-functionalized cellulose acetate are in a mass ratio of 2:1:5, and the rest are the same as embodiment 2.

[0062] Example 5

[0063] The only difference between this embodiment and embodiment 2 is that acrylamide, lithium aluminum titanium phosphate (LATP) and vinyl-functionalized cellulose acetate are in a mass ratio of 5:1:12.5, and the rest are the same as embodiment 2.

[0064] Example 6

[0065] The only difference between this embodiment and embodiment 1 is that the particle size of the multifunctional filler is 0.1 μm, and the rest is the same as embodiment 1.

[0066] Example 7

[0067] The only difference between this embodiment and embodiment 1 is that the particle size of the multifunctional filler is 1 micron, and the rest is the same as embodiment 1.

[0068] Comparative Example 1

[0069] The only difference between this comparative example and Example 1 is that the mass ratio of acrylamide, ceramic alumina and vinyl-functionalized cellulose acetate is 2:1:10, and the rest is the same as Example 1.

[0070] Comparative Example 2

[0071] The only difference between this comparative example and Example 1 is that acrylamide, ceramic alumina and vinyl-functionalized cellulose acetate are in a mass ratio of 2:1:2, and the rest are the same as Example 1.

[0072] Comparative Example 3

[0073] The only difference between this comparative example and Example 1 is that the mass ratio of acrylamide, ceramic alumina and vinyl-functionalized cellulose acetate is 1:1:5, and the rest is the same as Example 1.

[0074] Comparative Example 4

[0075] The only difference between this comparative example and Example 1 is that the mass ratio of acrylamide, ceramic alumina and vinyl-functionalized cellulose acetate is 6:1:5, and the rest is the same as Example 1.

[0076] Comparative Example 5

[0077] The only difference between this comparative example and Example 1 is that the D50 of the ceramic alumina is 2 μm, and all other aspects are the same as Example 1.

[0078] Test Method

[0079] 1. Determination of diaphragm liquid absorption rate

[0080] Cut the diaphragm into small discs with a diameter of 14 mm and soak them in electrolyte until the mass no longer changes. Measure the mass before and after soaking. The mass before soaking is recorded as M1, and the mass after soaking is recorded as M2. The liquid absorption rate = (M2-M1) / M1×100%.

[0081] 2. Determination of ionic conductivity

[0082] (1) Sample preparation: The prepared three-layer composite membrane was immersed in an electrolyte (e.g., 1 M LiPF6, EC / DEC = 1:1, v / v) for at least 12 h to ensure adequate wetting.

[0083] (2) Assembling a symmetrical cell: In an inert atmosphere (such as a glove box), two stainless steel electrodes (SS) and a diaphragm soaked in electrolyte are clamped together to form a symmetrical structure of SS|diaphragm|SS, i.e., an impedance test device with “inactive electrodes”.

[0084] (3) Test equipment and parameters: An electrochemical workstation was used to perform the AC impedance spectroscopy (EIS) test. The test conditions were as follows: frequency range: 1 MHz to 0.1 Hz, AC disturbance voltage: 5 to 10 mV, and test temperature: 25°C (temperature controllable in a thermostat).

[0085] Conductivity calculation formula:

[0086] Where: σ is the ionic conductivity (S / cm), L is the thickness of the diaphragm (cm), R is the high-frequency semicircle intercept in the Nyquist diagram (Ω), and A is the effective area of ​​the peripheral electrode (cm 2 ).

[0087] 3. Diaphragm contact angle measurement

[0088] Use a microsyringe to add 2-5 μL of trace electrolyte (EC:DMC=1:1) to the treated clean diaphragm surface, take a side view image of the droplet, fit the droplet contour using software (such as the Young-Laplace equation or ellipse fitting method), and calculate the contact angle after automatic baseline detection.

[0089] 4. Determination of thermal shrinkage of diaphragm

[0090] Cut the above-mentioned diaphragm into rectangular specimens of 10mm×100mm; spread the sample flat between A4 paper to prevent curling at high temperature, place it in an oven with a temperature control accuracy of ±1°C, set the temperature to 180°C, heat for 1 hour without external force, and then cool to room temperature; use a vernier caliper (accuracy 0.01mm) or a laser rangefinder to measure the initial length L0 (length direction) and the length L1 after heating, respectively, and calculate the shrinkage rate according to the formula (L0-L1) / L0×100%. Three samples need to be tested in each direction to take the average value, and retesting is required if the deviation exceeds 5%.

[0091] 5. Cycle performance test

[0092] Place the lithium battery in a 25°C constant temperature box for 6 hours and test it according to the following steps:

[0093] (1) First cycle constant current and constant voltage charging: Charge at a constant current of 0.1C to 4.2V, then switch to constant voltage charging until the current drops to 0.1A.

[0094] (2) After charging is complete, let it sit for 30 minutes.

[0095] (3) Perform constant current discharge at a rate of 0.1C to 2.5V.

[0096] (4) Cyclic charge and discharge process: Charge at a constant current of 2C to 4.2V, then switch to constant voltage charging until the current drops to 0.1A. Let stand for another 30 minutes. Discharge at a constant current of 10C to 2.5V.

[0097] (5) Repeat the above charge and discharge process for a total of 500 cycles.

[0098] Statistical analysis of the battery discharge capacity Q1 and Q after 1 cycle and 500 cycles 500 , statistics of battery capacity retention rate: Q 500 / Q1×100%.

[0099] The results of the performance tests of the embodiments and comparative examples are shown in Table 1:

[0100] Table 1

[0101]

[0102] As shown in Table 1, by comparing Examples 1-7 and Comparative Examples 1-4, it can be seen that when the cross-linking agent content is too low, the cross-linking network becomes sparse (the skeleton ratio is too high), the intermolecular chain forces are weakened, the segment motion is aggravated at high temperatures, and the thermal shrinkage rate increases; at the same time, the network pore structure collapses, reducing the electrolyte wettability and liquid storage capacity, hindering the transmission of lithium ions, resulting in an increase in contact angle, a decrease in liquid absorption rate, and an increase in ionic conductivity. When the cross-linking agent content is too high, a dense network is formed due to the excessive cross-linking agent. Although the thermal stability is slightly improved, the pore space is over-compressed, which limits the electrolyte absorption and prolongs the ion migration path, resulting in a decrease in conductivity.

[0103] By comparing the data of Examples 1-7 and Comparative Example 5, it can be seen that the submicron fillers used in Examples 1-7 are uniformly dispersed in the cross-linked network, and their high specific surface area enhances the affinity of the electrolyte (contact angle ≤ 20°) and forms a continuous ion conduction channel (conductivity ≥ 3.80 mS / cm); while the ultra-large particle size filler with D50 = 2 μm used in Comparative Example 5 destroys the network uniformity and produces interface defects, which not only deteriorates the wettability (contact angle 42°), but also induces stress concentration at high temperature (shrinkage rate 12.5%), while blocking the ion path (conductivity 1.30 mS / cm).

[0104] It can be seen from Examples 1-7 that the ion activation effect of the solid electrolyte filler (LATP) or ceramic material can further optimize the battery performance. Its intrinsic lithium ion conductivity and the three-dimensional cross-linked network produce a synergistic effect. Further combined with the regulation of the cross-linker, skeleton, and the ratio of the cross-linker to the multifunctional filler, the prepared diaphragm has high liquid absorption rate, high wettability, high ion conductivity and low thermal shrinkage, which can be used in lithium-ion batteries to significantly improve the battery's conductivity and cycle stability.

[0105] Although examples of the present embodiment have been shown and described, those skilled in the art will appreciate that various changes, modifications, substitutions, and variations may be made to these embodiments without departing from the principles and intent of the present embodiment, and the scope of the present embodiment is defined by the claims and their equivalents.

Claims

1. A lithium ion battery separator, characterized in that The lithium ion battery separator comprises a three-dimensional cross-linked network and a multifunctional filler; The three-dimensional cross-linked network is formed by polymerization of a skeleton and a cross-linking agent, the skeleton is a polymer material containing vinyl groups, the cross-linking agent contains a carbon-carbon double bond, and the mass ratio of the cross-linking agent to the skeleton is (2-4):5; The multifunctional filler includes ceramic material and / or solid electrolyte, and the mass ratio of the cross-linking agent to the multifunctional filler is (2-5):

1.

2. The lithium-ion battery separator according to claim 1, characterized in that The ceramic material includes one or more of aluminum oxide, silicon oxide, zirconium oxide, boehmite, nitride, boride and carbide; the solid electrolyte includes one or more of lithium aluminum titanium phosphate, lithium aluminum zirconium phosphate, lithium lanthanum titanate, tantalum-doped lithium lanthanum zirconate, lithium tin phosphate and lithium aluminum chromium phosphate.

3. The lithium-ion battery separator according to claim 2, characterized in that The particle size D50 of the multifunctional filler is 0.1-1 μm.

4. The lithium-ion battery separator according to claim 1, characterized in that The skeleton comprises one or more of polyethylene, polytetrafluoroethylene, polyvinylidene fluoride, comonomer-modified polyvinylidene fluoride and vinyl-functionalized cellulose acetate.

5. The lithium-ion battery separator according to claim 4, characterized in that The preparation method of the vinyl-functionalized cellulose acetate comprises: dissolving equal masses of cellulose acetate, methacryloyl chloride and triethylamine in N,N'-dimethylacetamide under a nitrogen atmosphere at -10 to -20°C, and then transferring the mixture to room temperature for stirring and reaction.

6. The lithium-ion battery separator according to claim 1, characterized in that The crosslinking agent includes one or more of acrylonitrile-butadiene-styrene copolymer, polymethacrylic acid, acrylic acid and its derivatives, acrylamide and its derivatives, ethylene ethyl acrylate copolymer and ethylene acrylate copolymer.

7. The lithium-ion battery separator according to any one of claims 1 to 6, characterized in that: The liquid absorption rate of the lithium ion separator is greater than 400%, the ion conductivity of the lithium ion separator is greater than or equal to 3.8 mS / cm at 25° C., the contact angle is less than or equal to 20°, and the shrinkage rate is less than or equal to 4% after heat treatment at 180° C. for 30 minutes.

8. A method for preparing a lithium-ion battery separator according to any one of claims 1 to 7, characterized in that: The method comprises first dissolving the multifunctional filler, crosslinking agent and skeleton in an organic solvent in proportion, adding 1 to 3 wt% of a photoinitiator, then applying the obtained casting solution on the base film, and irradiating the film under ultraviolet light with a wavelength of 350 to 400 nm and a power of 300 to 20,000 W for 5 to 10 minutes to initiate a polymerization reaction, wherein the organic solvent comprises one or more of N,N'-dimethylacetamide, dimethyl sulfoxide, N-methylpyrrolidone and tetrahydrofuran, and the base film comprises polyethylene and / or polypropylene.

9. The preparation method according to claim 8, characterized in that After the polymerization reaction is completed, the process also includes a step of curing to form a film: placing the base film covered with the diaphragm coating at 60-80° C. and vacuum drying for 8-12 hours to obtain a solid cross-linked film with a thickness of 30±5 μm.

10. A lithium ion battery, characterized in that: The invention comprises a lithium-ion battery separator according to any one of claims 1 to 7 or a lithium-ion battery separator prepared by the preparation method according to claim 8 or 9, and a positive electrode and a negative electrode located on both sides of the lithium-ion battery separator. The 21700 cylindrical battery assembled with the lithium-ion battery separator is cycled 500 times at a 2C charge / 10C discharge rate, and the capacity retention rate is greater than 80%.

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