Coating slurry and preparation method, separator, lithium battery and electric device

Abstract

The application belongs to the field of batteries, and particularly relates to a coating slurry and a preparation method, a diaphragm, a lithium battery and an electric device. The coating slurry comprises the following components in parts by weight: 1-1.5 parts of an ultraviolet light initiator, 3-4.4 parts of an ultraviolet light crosslinking agent, 56-76 parts of a nano metal oxide, 38-112 parts of PI-COOH, 210-330 parts of deionized water, 12-18 parts of a binder, 1-1.4 parts of a dispersing agent and 30-45 parts of an organic solvent. The application provides a multifunctional coating slurry which can improve the lithium ion migration capacity and enhance the thermal safety performance. The coating slurry is coated on the surface of a substrate, and can form a 3D network crosslinked coating composed of points, lines and surfaces on the surface of the base film, so as to provide high-strength support for the polyolefin base film, and significantly enhance the heat resistance of the diaphragm. The addition of the crosslinking agent improves the bonding strength between the coating and the base film, and reduces the risk of material falling off from the coating.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of batteries, and particularly relates to a coating slurry and a preparation method thereof, a separator, a lithium battery and an electric device. BACKGROUND

[0002] The separator is one of the key components of the lithium ion battery, as it plays a role in preventing direct physical contact between the cathode and the anode and allowing ion conduction, and as the lithium ion battery is developing towards higher energy density, higher power density, greater safety and longer life, the separator thus faces more new challenges. At present, the most widely used separator in the lithium battery system is a porous polyolefin film, such as polyethylene (PE), polypropylene (PP) and their blends (PE-PP), which can meet the requirements of low cost, good flexibility, relatively high mechanical strength and heat-sealed porous structure.

[0003] However, the low melting point of the polyolefin film leads to thermal shrinkage of the separator when used at a higher temperature, causing the positive and negative electrodes inside the battery to be in contact and short-circuit, leading to thermal runaway, and ultimately causing a battery safety accident to occur; in addition, the inherent "inert" hydrophobicity of the polyolefin leads to poor wettability and low absorption capacity of the electrolyte, which is not conducive to the transmission of Li ions through the polyolefin separator.

[0004] In order to improve the thermal stability of the separator, a heat-resistant coating layer of inorganic particles or a high-temperature-resistant organic polymer is usually coated on the polyolefin film, however, the separate particles or linear coating layer is prone to falling off, and the improvement of the heat resistance of the separator is limited, and the heat-resistant coating layer inevitably increases the lithium ion diffusion resistance. In this case, although the thermal stability is improved, the electrochemical performance of the lithium ion battery is usually deteriorated.

[0005] Patent CN202111356632.8 discloses a new type of cross-linked separator, which combines a polyolefin composition with a cross-linking agent and a photoinitiator in a specific manner to form a new type of cross-linked separator with high toughness, ultra-high film breaking temperature and high puncture strength, which is beneficial to improve the heat abuse resistance of the battery; however, the addition of the coating layer and the cross-linking inside the base film inevitably increases the lithium ion diffusion resistance of the separator. In this case, although the thermal stability is improved, the electrochemical performance of the lithium ion battery is usually deteriorated. SUMMARY

[0006] The purpose of the present application is to overcome the shortcomings in the prior art and provide a coating slurry preparation method, a separator, a lithium battery and an electric device.

[0007] To achieve the above-mentioned purpose, the technical scheme adopted by the present application is as follows:

[0008] A coating slurry, comprising the following components in parts by weight: 1-1.5 parts of an ultraviolet light initiator, 3-4.4 parts of an ultraviolet light crosslinking agent, 56-76 parts of modified nano metal oxide; 38-112 parts of PI-COOH; 210-330 parts of water, 12-18 parts of a binder, 1-1.4 parts of a dispersant, and 30-45 parts of an organic solvent.

[0009] Preferably, the mass ratio of the modified nano metal oxide to PI-COOH is 2:1.

[0010] Preferably, the coating slurry comprises the following components in parts by weight: 1 part of an ultraviolet light initiator, 3 parts of an ultraviolet light crosslinking agent, 76 parts of nano metal oxide; 38 parts of PI-COOH; 210 parts of deionized water, 12 parts of a binder, 1 part of a dispersant, and 40 parts of an organic solvent.

[0011] The PI-COOH is obtained by hydrolyzing polyimide fibers PI in an alkaline solution;

[0012] Preferably, the alkaline solution is an aqueous NaOH solution.

[0013] Preferably, the concentration of the aqueous NaOH solution is 1-2 mol / L, and more preferably, the concentration of the aqueous NaOH solution is 1.5 mol / L.

[0014] Preferably, the soaking time is 1-5 min, and more preferably, the soaking time is 3 min.

[0015] The abundant polar groups on the PI-COOH can effectively reduce the influence of the inherent "inert" hydrophobicity of polyolefins on the poor wettability and low absorption capacity of electrolytes. Meanwhile, the -COOH groups with non-shared electron pairs in the PI-COOH help to desolvate lithium ions and increase the lithium ion transmission rate, thereby making the battery have better cycle performance and higher rate capability.

[0016] The modified nano metal oxide is a nano metal oxide with oxygen vacancies,

[0017] Preferably, the nano metal oxide is selected from one or a combination of at least two of TiO2, ZrO2, WO3, CeO2, and Co3O4.

[0018] Preferably, the size of the modified nano metal oxide particles is 200 nm≤D(50)≤600 nm, preferably 300 nm≤D(50)≤550 nm, and more preferably 370 nm≤D(50)≤520 nm.

[0019] The particle size is too small to block the pores of the base film, and the particle size is too large to provide a better support for the base film.

[0020] Preferably, the modified nano-metal oxide is prepared by mixing a reducing agent with the nano-metal oxide, and then performing sintering and annealing treatment in sequence to obtain the modified nano-metal oxide containing oxygen vacancies.

[0021] Preferably, the reducing agent is selected from one or a combination of at least two of CO, NH3, SnCl2, KBH4, NaBH4, Li, Mg, or Al; preferably, the mass ratio of the reducing agent to the nano-metal oxide is 1:1-1:3; preferably 1:2-1:3, and further preferably 1:3.

[0022] Preferably, the sintering temperature is 300-380℃, preferably 350℃, and the sintering time is 1-5h; preferably 3h.

[0023] Preferably, the annealing temperature is 250-300℃, preferably 270℃, and the holding time is 1-4h; preferably 2h.

[0024] The sintering and annealing processes are performed in an argon protective atmosphere.

[0025] The ultraviolet light crosslinking agent is a free radical photoinitiator, preferably a cleavage type or hydrogen abstraction type free radical photoinitiator, and further preferably the ultraviolet light initiator is one or a combination of at least two of α-hydroxyalkyl phenone, α-amine alkyl phenone, arylacyl phosphine oxide, dibenzoyl phenyl phosphine oxide, benzophenone, 2,4-dihydroxybenzophenone, 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone, Michler's ketone, thio propoxy thioxanthone, or isopropyl thioxanthone.

[0026] Preferably, the ultraviolet light crosslinking agent is an allyl-based co-crosslinking agent; further preferably one or a combination of at least two of trimethylolpropane trimethacrylate, trimethylolpropane triacrylate, triallyl cyanurate, and triallyl isocyanurate.

[0027] Preferably, the binder is selected from one or a combination of at least two of polyacrylate and its derivatives, rubber-based water-based adhesive, polyvinyl alcohol and its derivatives, polyvinyl acetate, and other water-based binders.

[0028] Preferably, the dispersant is selected from an anionic surfactant or a cationic surfactant.

[0029] Preferably, the organic solvent is anhydrous ethanol or acetone.

[0030] The application also includes a preparation method of the coating slurry, comprising the following steps: dissolving the ultraviolet light initiator, the ultraviolet light crosslinking agent, the modified nanometer metal oxide, the PI-COOH, the binder and the dispersant in an organic solvent and water to form the coating slurry; preferably, the following steps are adopted: dissolving the water-soluble reagent in the ultraviolet light initiator and the ultraviolet light auxiliary crosslinking agent in water to obtain solution 1A, dissolving the non-water-soluble reagent in the ultraviolet light initiator and the ultraviolet light auxiliary crosslinking agent in an organic solvent to obtain solution 1B, and then stirring the two solutions 1A and 1B uniformly to obtain mixed solution 1C, and then adding the modified nanometer metal oxide, the PI-COOH, the dispersant and the binder into the mixed solution 1C and stirring uniformly to obtain the coating slurry.

[0031] The application also includes a separator prepared by the following method: coating a substrate with the coating slurry and then performing ultraviolet light irradiation crosslinking to obtain the separator.

[0032] The substrate is a polyolefin separator or a polyolefin separator containing a ceramic coating.

[0033] Preferably, the polyolefin is one or a combination of at least two of the following crystalline polymers: polyethylene, polypropylene, poly-1-butene, poly-4-methyl-1-pentene, poly-1-hexene, poly-1-octene or polymethyl methacrylate.

[0034] Preferably, the coating is performed using a microgravure coating process.

[0035] Preferably, the thickness of the coating layer is controlled to be between 1 and 5 um, preferably between 1 and 3 um, and more preferably between 1 and 2 um.

[0036] Preferably, the specific process of the ultraviolet light irradiation is to perform light irradiation treatment on the coated substrate using ultraviolet light with a wavelength range of 254-365 nm.

[0037] Preferably, the ultraviolet light irradiation time is 5-10 min.

[0038] The ultraviolet light with a wavelength range of 254-365 nm ensures both UV light penetration and absorption intensity, thereby improving the initiation efficiency, and the light irradiation time of 5-10 min can achieve the effect of uniform ultraviolet light irradiation. The characteristics of the ultraviolet light have relatively small influence on the performance of the separator.

[0039] The application also comprises a lithium battery comprising the diaphragm; preferably, the lithium battery is prepared in the following manner: s1: the diaphragm is arranged between the positive electrode material and the negative electrode material; s2: the layered component comprising the positive electrode material, the diaphragm and the negative electrode material is laminated to obtain a dry cell; s3: the dry cell is loaded into a battery shell; s4: electrolyte is injected; s5: packaging, baking, injection and formation processes are performed, so as to obtain a finished lithium ion battery; preferably, in s1, the positive electrode material-diaphragm-negative electrode material or the negative electrode material-diaphragm-positive electrode material is assembled in sequence.

[0040] The positive electrode material is obtained by mixing the active positive electrode material lithium nickel cobalt manganese oxide, the conductive CNT and the binder in a mass ratio of 97.1:1.7:1.2 in the solvent NMP to form a positive electrode slurry; the positive electrode slurry is uniformly coated on both sides of the aluminum foil through a coating device; the positive electrode slurry on the plate is dried through an oven to remove the solvent; the positive electrode material on the plate is rolled and sliced for standby; preferably, the negative electrode material is obtained by mixing the negative electrode active material, the conductive agent SP, the thickening agent carboxymethyl cellulose CMC and the binder styrene-butadiene rubber SBR in a mass ratio of 97:0.5:1:1.5 in the solvent deionized water to form a negative electrode slurry; the negative electrode slurry is uniformly coated on both sides of the plate (preferably, a copper foil) through a coating device; the negative electrode slurry on the plate is dried through an oven to remove the solvent; the negative electrode material on the plate is cold-pressed and sliced for standby.

[0041] Compared with the prior art, the application has the following beneficial effects:

[0042] The positive charge oxygen vacancies on the modified nano metal oxide particles in the coating slurry can repel the diffusion of anions, promote the migration of lithium ions, significantly improve the transmission efficiency of lithium ions and ionic conductivity, promote the uniform deposition of lithium ions in the whole cycle of the battery, inhibit the growth of lithium dendrites, and thus improve the cycle stability of the lithium battery; in addition, the rich polar groups on the PI fibers can effectively reduce the influence of the poor wettability and low absorption capacity of the electrolyte caused by the inherent "inert" hydrophobicity of polyolefin, and at the same time, the -COOH group with a non-shared electron pair in PI-COOH helps the desolvation of lithium ions and the increase of lithium ion transmission rate, so that the battery has better cycle performance and higher rate capability.

[0043] The diaphragm provided by the application can improve the lithium ion migration ability and enhance the thermal safety performance, and the special formula coating slurry is coated on at least one side of the substrate to form a 3D network cross-linked coating composed of points, lines and surfaces on the surface of the base film, thereby providing high strength support for the polyolefin base film, and the heat resistance of the diaphragm is significantly enhanced, and the addition of the cross-linking agent improves the bonding strength between the coating and the base film, thereby reducing the risk of coating and material falling off on the coating. BRIEF DESCRIPTION OF DRAWINGS

[0044] Figure 1 A schematic diagram of the 3D cross-linked network coating of the embodiments of the application;

[0045] Figure 2 A photograph comparison diagram of the batteries assembled in Example 2 and Comparative Example 4 after the 180℃ hot box test. DETAILED DESCRIPTION

[0046] In order for those skilled in the art to better understand the technical solutions of the application, the application will be further described in detail below with reference to the drawings and the best embodiments.

[0047] The PI-COOH used in the embodiments of the application is prepared in the following manner: polyimide fibers PI are immersed in an aqueous NaOH solution to achieve alkaline hydrolysis to obtain PI-COOH-1; wherein the concentration of the aqueous NaOH solution is 1.5 mol / L, and the soaking time is 3 min; the concentration of the aqueous NaOH solution is changed and the soaking time is changed, the concentration of the aqueous NaOH solution is 1 mol / L, and the soaking time is 5 min to obtain PI-COOH-2; the concentration of the aqueous NaOH solution is 2 mol / L, and the soaking time is 1 min to obtain PI-COOH-3;

[0048] The modified nano metal oxide is a metal oxide with oxygen vacancies, which is prepared in the following manner: ZrO2, TiO2, WO3, CeO2, Co3O4 nanoparticles are mixed with NaBH4 in a mass ratio of 1:3, and then heated to 350℃ for sintering, and the reaction is carried out for 3h; then annealing treatment is carried out, the annealing temperature is 270℃, and the time is 2h, to obtain modified nano metal oxides Zr-1, Ti-1, W-1, Ce-1 and Co-1 with a particle size of 370nm≤D(50)≤520nm;

[0049] ZrO2 nanoparticles are mixed with KBH4 in a mass ratio of 1:2, and then heated to 300℃ for sintering, and the reaction is carried out for 5h; then annealing treatment is carried out, the annealing temperature is 250℃, and the time is 4h; to obtain modified nano metal oxide Zr-2;

[0050] The ZrO2 nanoparticles and Mg are mixed in a mass ratio of 1:1, heated to 380°C for sintering, and reacted for 1 h; then annealing treatment is performed, the annealing temperature is 300°C, and the annealing time is 1 h; a modified nano metal oxide Zr-3 is obtained;

[0051] Example 1: 220 parts by weight of deionized water are weighed, 1 part by weight of a water-soluble ultraviolet photoinitiator 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone is weighed and dissolved completely in the deionized water to obtain a solution 1A. 30 parts by weight of anhydrous ethanol are weighed, and 3 parts by weight of an ultraviolet light co-crosslinking agent trimethylolpropane trimethacrylate (TMPTMA) is added to the anhydrous ethanol and mixed uniformly. A solution 1B is obtained, and then the two solutions 1A and 1B are uniformly stirred at high speed. A mixed solution 1C containing an ultraviolet crosslinking agent and a co-crosslinking agent is obtained, and 56 parts by weight of the modified nano metal oxide Zr-1, 56 parts by weight of the PI-COOH-2, 1 part by weight of a dispersant dodecyl dimethyl betaine and 12 parts by weight of an acrylate adhesive are sequentially added to the mixed solution 1C containing the ultraviolet crosslinking agent and the co-crosslinking agent, and uniformly stirred at high speed to obtain a coating slurry 1;

[0052] ( Figure 1 A structural schematic diagram is shown, and a 3D network crosslinked coating layer composed of points (modified nano metal oxide), lines (PI-COOH) and surfaces (ultraviolet crosslinking agent crosslinking) is formed on the surface of the base film); the coating slurry 1 is uniformly coated on one side of the base material of a 9-micron wet PE separator, and the coated surface is irradiated with ultraviolet light with a wavelength of 254-365 nm for 5-10 min for crosslinking to obtain a separator sample with a coating thickness of 3 microns, which is named Z-P-UV (1:1).

[0053] The above separator is assembled into a battery, and the battery performance is comprehensively evaluated (Examples 2-7 and Comparative Examples 1-3 have the same assembly method, positive electrode material and negative electrode material as Example 1).

[0054] The separator is assembled into a battery in the following manner:

[0055] s1: The separator is arranged between the positive electrode material and the negative electrode material;

[0056] s2: A layered component containing the positive electrode material, the separator and the negative electrode material is laminated to obtain a dry cell;

[0057] s3: The dry cell is loaded into a battery shell;

[0058] s4: Electrolyte is injected;

[0059] s5: Packaging, baking, injection and formation processes are performed, and a lithium ion battery product is obtained.

[0060] Among them, s1 is assembled in the order of positive material-separator-negative material or negative material-separator-positive material.

[0061] The positive material is obtained by mixing the active positive material lithium nickel cobalt manganese oxide, conductive CNT, and adhesive in a mass ratio of 97.1:1.7:1.2 in the solvent NMP to form a positive electrode slurry; the positive electrode slurry is uniformly coated on both sides of the aluminum foil by a coating device; the positive electrode slurry on the plate is dried by an oven to remove the solvent; the positive material on the plate is rolled and sliced for standby;

[0062] The negative material is obtained by mixing the negative active material, conductive agent SP, thickening agent carboxymethyl cellulose CMC, and adhesive styrene butadiene rubber SBR in a mass ratio of 97:0.5:1:1.5 in the solvent deionized water to form a negative electrode slurry; the negative electrode slurry is uniformly coated on both sides of the plate, preferably copper foil, by a coating device; the negative electrode slurry on the plate is dried by an oven to remove the solvent; the negative material on the plate is cold-pressed and sliced for standby.

[0063] Example 2: 40 parts by weight of anhydrous ethanol, 1 part by weight of ultraviolet initiator benzophenone and 3 parts by weight of ultraviolet assistant crosslinking agent triallyl isocyanurate (TAIC) are added into ethanol and stirred at high speed to mix uniformly; after the ultraviolet crosslinking agent and the assistant crosslinking agent are completely dissolved, 210 parts by weight of deionized water is added and stirred uniformly to obtain a mixed solution 2A containing ultraviolet crosslinking agent and assistant crosslinking agent; then, 76 parts by weight of modified nano metal oxide Zr-1, 38 parts by weight of PI-COOH-1, 1 part by weight of dispersant sodium linear alkyl benzene sulfonate and 12 parts by weight of acrylate adhesive are sequentially added into the mixed solution 2A and stirred at high speed to mix uniformly to obtain a coating slurry 2; the coating slurry 2 is uniformly coated on one side of the base material of a 9-micron wet PE separator, and the coated surface is irradiated and crosslinked by ultraviolet light to obtain a separator sample with a coating thickness of 3 microns, which is named Z-P-UV(2:1). The above separator is assembled into a battery, and the battery performance is comprehensively evaluated.

[0064] Example 3: 330 parts by weight of deionized water was weighed, 1.5 parts by weight of water-soluble UV initiator 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone was weighed and dissolved completely in the deionized water to obtain solution 3A. 45 parts by weight of anhydrous ethanol was weighed, 4.4 parts by weight of UV light-assisted crosslinking agent triallyl isocyanurate (TAIC) was added to the anhydrous ethanol and mixed uniformly. Solution 3B was obtained, and then solutions 1A and 1B were uniformly stirred at high speed. A mixed solution 3C containing UV crosslinking agent and assisted crosslinking agent was obtained, and 56 parts by weight of modified nano metal oxide Zr-1, 112 parts by weight of PI-COOH-3, 1.4 parts by weight of dispersant sodium linear alkyl benzene sulfonate and 18 parts by weight of acrylate adhesive were sequentially added to the mixed solution 3C containing the UV crosslinking agent and the assisted crosslinking agent, and uniformly stirred at high speed to obtain coating slurry 3. The coating slurry 3 was uniformly coated on one side of the substrate of the 9-micron wet PE separator, and the coated surface was irradiated and crosslinked by UV light to obtain a separator sample with a coating thickness of 3 microns, which was named Z-P-UV (1:2). The above separator was assembled into a battery, and the battery performance was comprehensively evaluated.

[0065] Examples 4-9: Examples 4-9 differ from Example 2 only in that different modified nano metal oxides are used, which are Zr-2, Zr-3, Ti-1, W-1, Ce-1 and Co-1, respectively; the above separator was assembled into a battery, and the battery performance was comprehensively evaluated. The results show that the battery performance is weaker than that of Zr-1, but the coating peeling force is still between 479-530 gf, the 150°C heat shrinkage rate in the TD direction and the MD direction is less than 1%, the 180°C heat shrinkage rate in the TD direction is between 1.5%-3.5%, and the MD direction is between 1.2%-3.4%; the lithium ion transfer number is between 0.66-0.71.

[0066] Comparative Example 1: 220 parts by weight of deionized water was weighed, 1 part by weight of water-soluble UV initiator 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone was completely dissolved in the deionized water to obtain solution 4A. 30 parts by weight of anhydrous ethanol was weighed, 3 parts by weight of UV co-crosslinking agent trimethylolpropane trimethacrylate (TMPTMA) was added to the anhydrous ethanol and mixed uniformly. Solution 4B was obtained, then solutions 4A and 4B were uniformly stirred at high speed. A mixed solution 4C containing UV crosslinking agent and co-crosslinking agent was obtained, 112 parts by weight of modified nano metal oxide Zr-1, 1 part by weight of dispersant dodecyl dimethyl betaine sodium and 12 parts by weight of acrylate binder were sequentially added to the mixed solution 4C containing UV crosslinking agent and co-crosslinking agent, and uniformly stirred at high speed to obtain coating slurry 1; the coating slurry 1 was uniformly coated on one side of the substrate of a 9-micron wet-process PE separator, and the coated surface was crosslinked by UV irradiation to obtain a separator sample with a coating thickness of 3 microns, and the sample was named Z-UV. The above separator was assembled into a battery, and the battery performance was comprehensively evaluated.

[0067] Comparative Example 2: 40 parts by weight of anhydrous ethanol was weighed, 1 part by weight of UV initiator benzophenone and 3 parts by weight of UV co-crosslinking agent triallyl isocyanurate (TAIC) were added to the ethanol and uniformly stirred at high speed, and after the UV crosslinking agent and co-crosslinking agent were completely dissolved, 210 parts by weight of deionized water was added and stirred uniformly. A mixed solution 5A containing UV crosslinking agent and co-crosslinking agent was obtained; then, 113 parts by weight of PI-COOH-1, 1 part by weight of dispersant sodium linear alkyl benzene sulfonate and 12 parts by weight of acrylate binder were sequentially added to the mixed solution 5A and uniformly stirred at high speed to obtain coating slurry 5; the coating slurry 5 was uniformly coated on one side of the substrate of a 9-micron wet-process PE separator, and the coated surface was crosslinked by UV irradiation to obtain a separator sample with a coating thickness of 3 microns, and the sample was named P-UV. The above separator was assembled into a battery, and the battery performance was comprehensively evaluated.

[0068] Comparative Example 3: 56 parts by weight of modified nano metal oxide Zr-1, 112 parts by weight of PI-COOH-1, 1.4 parts by weight of dispersant sodium linear alkyl benzene sulfonate and 18 parts by weight of acrylate binder were sequentially added to 330 parts by weight of deionized water and uniformly stirred at high speed to obtain coating slurry 6; the coating slurry 6 was uniformly coated on one side of the substrate of a 9-micron wet-process PE separator, and the coated surface was crosslinked by UV irradiation to obtain a separator sample with a coating thickness of 3 microns, and the sample was named Z-P(1:1). The above separator was assembled into a battery, and the battery performance was comprehensively evaluated.

[0069] Comparative Example 4: A commercial 9-micron wet-process PE base film was coated with a 3-micron Al2O3 separator on one side. The above separator was assembled into a battery, and the battery performance was comprehensively evaluated.

[0070] Test Example 1: The contact angle, thermal shrinkage rate and lithium ion migration number of the above-mentioned diaphragm were tested respectively.

[0071] 1. Lithium ion migration number test: The above-mentioned separator is assembled into a lithium symmetric battery, and the t of the separator is measured by steady current and AC impedance method. Li+ , according to the formula t Li+ =I s (ΔV-I0R0) / I0(ΔV-I s R s ) is calculated. Where I0 and I s are the initial and steady-state currents; ΔV is the applied constant potential difference (10 mV); R0 and R s is the interfacial impedance before and after polarization;

[0072] 2. Coating Peel Test: Refer to the national standard GB / T 4851-1998. Attach transparent tape to a test steel plate, then 3M double-sided tape to the transparent tape. Place the test side of the diaphragm facing up, with the back side facing the double-sided tape. Then, test on a universal tensile testing machine. (Test machine speed: 50 mm / min, sample width: 19 mm.)

[0073] 3. Diaphragm thermal shrinkage: Cut a 100mm*100mm diaphragm into a size and place it between two A4 papers. Seal the edges with staples and then place it in an oven for constant temperature heating for 1 hour. Then take it out and measure the diaphragm thermal shrinkage rate.

[0074] The test results are shown in Table 1;

[0075] Table 1

[0076]

[0077]

[0078] The above test results show that the addition of modified nano-metal oxides and PI-COOH containing oxygen vacancies in the diaphragm coating can effectively improve the migration number of lithium ions on the diaphragm. As shown in Table 1, the lithium ion migration number is generally increased from 0.31 of the commercial diaphragm to about 0.63-0.72. In addition, the successful preparation of the 3D cross-linked coating significantly improves the bonding effect between the base film and the coating. At the same time, it is not difficult to see from the thermal shrinkage data that the synergistic effect of the modified nano-metal oxides and nanofiber materials plays a key role in the support of the base film. From the thermal shrinkage data of Comparative Examples 1-3, it can be seen that the cross-linked coating only contains modified nano-gold. When the inorganic particles are used as points and the polymer fibers are used as lines, and the points and lines are connected into surfaces by adding a cross-linking agent, the modified nano-metal oxide and PI-COOH are connected into a whole to form a cross-linked coating with a 3D network, and the mass ratio of the modified nano-metal oxide to PI-COOH is 2:1, the coating provides the strongest support for the polyolefin-based film, and the coating has the best heat shrinkage resistance.

[0079] Test Example 2: The above diaphragm was subjected to a rate discharge test and a hot box safety performance test, respectively, as follows:

[0080] The rate discharge test conditions are as follows: under the test voltage range of 3.0V-4.2V, the battery is fully charged at constant current and constant voltage, and discharged to 3.0V at current densities of 0.33C, 0.5C, 1C, 2C and 3C respectively.

[0081] The hot box safety performance test conditions are: heating from room temperature to the specified temperature at a rate of 5°C / min, maintaining the temperature for half an hour, then stopping heating and observing for one hour. The evaluation criteria are: no fire, no smoke, and no noticeable voltage drop.

[0082] The test results are shown in Table 2.

[0083] Table 2

[0084]

[0085] Figure 2 For Example 2 ( Figure 2 a)) and Comparative Example 4 ( Figure 2The photo of the battery 180℃ hot box test after assembly in b); the above test results show that the increase of lithium ion migration number is beneficial to the improvement of the discharge rate performance of the battery; as shown in Table 2, the discharge capacity retention rate of the battery with a higher ion migration number is increased by about 0.4-0.8%, 1.3-2.4%, 2-3.4% and 2.3-3.7% at the current density of 0.5C, 1C, 2C and 3C, respectively, compared with the conventional commercial separator; and in combination with the data in Tables 1 and 2, it can be seen that the battery assembled by the separator with good heat resistance has better safety performance, and the battery assembled by the coated film containing the modified nanometer metal oxide and the PI-COOH 3D cross-linked coating can pass the 180℃ hot box, and shows excellent safety performance.

[0086] In addition, Zr-1 in Comparative Examples 1-3 is replaced by Zr-2, Zr-3, Ti-1, W-1, Ce-1 or Co-1 at the same time, and the method of Test Example 1 and Test Example 2 is used for testing, and it is found that the effect is poorer than that of Example 1-9.

[0087] It can be seen that the positive charged oxygen vacancies on the modified nanometer metal oxide particles in the coating slurry can repel the diffusion of anions, promote the migration of lithium ions, significantly improve the transmission efficiency and ionic conductivity of lithium ions, promote the uniform deposition of lithium ions in the whole cycle of the battery, inhibit the growth of lithium dendrites, and thus improve the cycle stability of the lithium battery; in addition, the rich polar groups on the PI fiber can effectively reduce the influence of the poor wettability and low absorption capacity of the electrolyte caused by the inherent "inert" hydrophobicity of polyolefin, and at the same time, the -COOH group with non-shared electron pair in PI-COOH helps to increase the desolvation of lithium ions and the transmission rate of lithium ions, so that the battery has better cycle performance and higher rate capability.

[0088] The separator provided by the application not only helps to improve the lithium ion migration ability, but also enhances the thermal safety performance; the coating slurry with a special formula is coated on at least one side of the substrate, so that a 3D network cross-linked coating composed of points, lines and surfaces is formed on the surface of the base film, high-strength support is provided for the polyolefin base film, the heat resistance of the separator is significantly enhanced, and the addition of the cross-linking agent improves the bonding strength between the coating and the base film, and reduces the risk of material falling off from the coating.

[0089] The above only describes the preferred embodiments of the application, and it should be pointed out that those skilled in the art can make several improvements and refinements without departing from the principles of the application, and these improvements and refinements should also be regarded as the protection scope of the application.

Claims

1. A diaphragm, characterized in that: The diaphragm is prepared by the following method: coating a substrate with a coating slurry, and then irradiating and cross-linking with ultraviolet light; The base material is polyolefin; The coating slurry comprises the following components by weight: 1-1.5 parts of ultraviolet initiator, 3-4.4 parts of ultraviolet crosslinking agent, 56-76 parts of modified nano metal oxide; 38-112 parts of PI-COOH; 210-330 parts of water, 12-18 parts of binder, 1-1.4 parts of dispersant, and 30-45 parts of organic solvent; The PI-COOH is obtained by immersing the polyimide fiber PI in an alkaline solution for hydrolysis; The modified nano-metal oxide is a nano-metal oxide with oxygen vacancies, and the nano-metal oxide is ZrO2; the particle size of the modified nano-metal oxide is 370nm≤D50≤520nm; The modified nano-metal oxide is prepared by the following method: a reducing agent and a nano-metal oxide are mixed and then sintered and annealed in sequence to obtain a modified nano-metal oxide containing oxygen vacancies; the reducing agent is NaBH4, the mass ratio of the reducing agent to the nano-metal oxide is 1:3, the sintering temperature is 350°C, the sintering time is 3 hours, and the annealing temperature is 270°C, and the holding time is 2 hours; the sintering and annealing reaction processes are carried out under an argon protective atmosphere; The ultraviolet light initiator is a free radical light initiator, the ultraviolet light crosslinking agent is an allyl auxiliary crosslinking agent, and the adhesive is an acrylate adhesive.

2. The diaphragm according to claim 1, characterized in that The invention comprises the following components in parts by weight: 1 part of ultraviolet initiator, 3 parts of ultraviolet crosslinking agent, 76 parts of modified nano metal oxide, 38 parts of PI-COOH, 210 parts of deionized water, 12 parts of binder, 1 part of dispersant and 40 parts of organic solvent.

3. The diaphragm according to claim 1, characterized in that The mass ratio of the modified nano metal oxide to PI-COOH is 2:

1.

4. The diaphragm according to claim 1 or 2, characterized in that The alkaline solution is a NaOH aqueous solution; the concentration of the alkaline solution is 1-2 mol / L; and the soaking time is 1-5 minutes.

5. The diaphragm according to claim 1, characterized in that The ultraviolet light initiator is a cleavage type or hydrogen abstraction type free radical photoinitiator.

6. The diaphragm according to claim 1, characterized in that The ultraviolet light initiator is one or a combination of at least two of α-hydroxyalkyl phenone, α-aminoalkyl phenone, aromatic phosphine oxide, bisbenzoylphenyl phosphine oxide, benzophenone, 2,4-dihydroxybenzophenone, 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone, Michler's ketone, thiopropoxythioxanthone or isopropylthioxanthone; The ultraviolet crosslinking agent is one or a combination of at least two of trimethylolpropane trimethacrylate, trimethylolpropane triacrylate, triallyl cyanurate and triallyl isocyanurate; The dispersant is selected from anionic surfactants or cationic surfactants; The organic solvent is anhydrous ethanol or acetone.

7. The diaphragm according to claim 1, characterized in that The preparation method of the coating slurry comprises the following steps: dissolving an ultraviolet light initiator, an ultraviolet light crosslinking agent, modified nano metal oxide, PI-COOH, a binder and a dispersant in an organic solvent and water to form a coating slurry.

8. The diaphragm according to claim 7, characterized in that The method for preparing the coating slurry comprises the following steps: dissolving a water-soluble agent in a UV initiator and a UV cross-linking agent in water to obtain a solution 1A; dissolving a water-insoluble agent in a UV initiator and a UV cross-linking agent in an organic solvent to obtain a solution 1B; and then stirring the two solutions 1A and 1B uniformly; A mixed solution 1C is obtained, and the modified nano metal oxide, PI-COOH, a dispersant and a binder are added to the mixed solution 1C and stirred uniformly to obtain a coating slurry.

9. The diaphragm according to claim 1, wherein The polyolefin is one or a combination of at least two of the following crystalline polymers: polyethylene, polypropylene, poly-1-butene, poly-4-methyl-1-pentene, poly-1-hexene, and poly-1-octene; the coating is performed using a micro-gravure coating process; the coating thickness is controlled to be between 1 and 5 μm; and the specific process of the ultraviolet irradiation is: the coated substrate is irradiated with ultraviolet light having a wavelength range of between 254 and 365 nm; the ultraviolet irradiation time is 5 to 10 minutes.

10. A lithium battery comprising the separator according to any one of claims 1 to 9.

11. An electrical device comprising the lithium battery according to claim 10.

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