Hybrid-coated separator, and preparation method therefor and use thereof

Abstract

A preparation method for a hybrid-coated separator. The preparation method comprises the following operations: mixing raw materials comprising an inorganic powder, a hollow polymer microsphere emulsion, a binder and a solvent to obtain a mixed slurry; and coating the mixed slurry onto the surface of a base membrane, and then drying same to obtain the hybrid-coated separator. In the mixed slurry, the mass proportion of the inorganic powder is 20-35%, and the mass proportion of the hollow polymer microsphere emulsion is 5-10%; the viscosity of the mixed slurry is 100-120 mPa·s; and the hollow polymer microsphere emulsion comprises hollow polymer particles, and the density of the hollow polymer particles is 0.8-0.9 g / cm3.

Description

A mixed-coating diaphragm, its preparation method and application

[0001] This application claims priority to Chinese Patent Application No. 202410931649.9, filed with the Chinese Patent Office on July 11, 2024, the entire contents of which are incorporated herein by reference. Technical Field

[0002] This application relates to the field of lithium battery technology, and in particular to a mixed-coating separator, its preparation method and application. Background Technology

[0003] As one of the key inner components in lithium batteries, the separator's main function is to separate the positive and negative electrodes of the lithium battery, preventing them from coming into contact and short-circuiting.

[0004] While commonly used alumina-coated separators offer good mechanical strength and high-temperature resistance, their adhesion to the positive and negative electrodes is poor, leading to issues such as loose bonding between the electrodes and the separator, and a softer cell. To address this, newly developed separator materials primarily involve adding an adhesive coating to the alumina coating layer. This not only increases the adhesion between the separator and the electrodes but also improves the core rigidity, thereby enhancing the performance of lithium-ion batteries. Technical issues

[0005] The high coverage of polyvinylidene fluoride (PVDF) in the adhesive coating leads to severe pore blockage, resulting in fewer lithium-ion transport channels and affecting the performance of lithium batteries. Technical solutions

[0006] In a first aspect, this application provides a method for preparing a mixed-coated diaphragm, comprising the following operations: mixing raw materials including inorganic powder, hollow sphere emulsion, binder, and solvent to obtain a mixed slurry; coating the mixed slurry onto the surface of a base membrane, and drying it to obtain the mixed-coated diaphragm; wherein in the mixed slurry, the inorganic powder accounts for 20%-35% by mass, and the hollow sphere emulsion accounts for 5%-10% by mass; the viscosity of the mixed slurry is 100-120 mPa·s; the hollow sphere emulsion comprises hollow granular spheres, and the density of the hollow granular spheres is 0.8-0.9 g / cm³. 3 .

[0007] Secondly, this application provides a mixed-coating diaphragm prepared by the method described above.

[0008] Thirdly, this application provides a lithium-ion battery, including the co-coated separator as described above. Beneficial effects

[0009] First, by selecting inorganic powder and hollow sphere emulsion to prepare a mixed slurry, and adjusting the proportions of inorganic powder and hollow sphere emulsion in the mixed slurry, it is possible not only to obtain a mixed slurry with a suitable viscosity and adjust the spreading performance of the mixed slurry on the base film, so that the mixed slurry can form a complete coating on the surface of the base film with a single coating, avoiding multiple coatings and thus improving the production efficiency of the mixed-coated diaphragm; but also to improve the dispersion uniformity of hollow sphere particles in the hollow sphere emulsion, which helps to improve the bonding uniformity between the base film and the electrode in different areas, and helps to improve the mechanical strength of different areas of the core, preventing wrinkles from appearing in the core due to weak strength in some areas.

[0010] Secondly, by controlling the viscosity of the mixed slurry and the density of the hollow granule balls, the hollow granule balls can gradually move towards the surface of the coating formed by the mixed slurry as the water evaporates during the molding process. At this time, the part of the hollow granule balls protruding from the coating can form a good adhesion with the electrode under hot pressing conditions. Specifically, the hollow granule balls melt under hot pressing conditions, so that the coating and the electrode form a physical bond (adsorption through intermolecular forces on the material surface, forming force accumulation to achieve the bonding effect). This helps to improve the thermal peel strength and mechanical strength of the separator, thereby improving the problem of electrode wrinkling caused by the weak strength of traditional battery cells.

[0011] Third, by controlling the density of the hollow granule balls to remain within the above-mentioned range, it is possible to ensure that a sufficient number of hollow granule balls float to the surface of the coating formed by the mixed slurry, and that the hollow granule balls can still maintain sufficient strength and not collapse during the hot pressing process.

[0012] Fourth, the protruding part of the hollow granular spheres can form a supporting effect between the electrode and the base film, increasing the gap between the electrode and the base film. This not only provides a buffer space for the release of expansion force during the charging and discharging of the negative electrode, but also improves the liquid absorption performance of the separator, thereby improving the wetting performance of the electrolyte on the electrode and helping to improve the overall performance of the lithium battery. Embodiments of the present invention

[0013] In some embodiments, the hollow sphere emulsion is prepared by a method comprising the following steps: S1, mixing water and emulsifier to obtain an emulsion, taking a first portion of the emulsion and stirring it, adding a mixture of monomer and initiator dropwise during the stirring process, and obtaining a pretreatment solution after the dropwise addition process is completed; S2, adding a second portion of the emulsion to the pretreatment solution, and obtaining the hollow sphere emulsion after stirring, heat preservation, and cooling.

[0014] In some embodiments, the mass ratio of water to emulsifier is (2-4):98; the mass ratio of the first portion of emulsion to the second portion of emulsion is 1:(1-2); and the mass ratio of monomer to initiator is 100:(0.5-1).

[0015] In some embodiments, in S1, the temperature during the stirring process is 25±2℃, the speed during the dropping process is uniform, and the duration is 1.5-2h; in S2, the stirring time is 1-2h, and after the stirring process is completed, the temperature is raised to 55-60℃ and kept at that temperature for 5-6h.

[0016] By controlling the proportion of raw materials and preparing the hollow sphere emulsion using the above steps, hollow spheres with suitable density can be obtained. After the mixed slurry is coated on the surface of the base film, a sufficient number of hollow spheres can float and move to the surface of the coating. This not only increases the contact area between the hollow spheres and the electrode, thus enabling higher bonding strength between the hollow spheres and the electrode under hot pressing conditions, improving the mechanical strength of the cell and preventing electrode wrinkling, but also generates a stable support structure between the electrode and the separator to form pores that can accommodate more electrolyte, improving the wetting degree of the electrolyte on the electrode and the separator, and helping to improve the overall performance of the lithium battery.

[0017] In some embodiments, the monomer includes at least one of unsaturated nitrile monomer unit copolymers, vinyl monomer unit copolymers, alkenylamine monomer unit copolymers, acrylate monomer unit copolymers, methacrylate monomer unit copolymers, vinyl chloride monomer unit copolymers, and modifying compounds of the above copolymers.

[0018] In some embodiments, the unsaturated nitrile monomer unit comprises at least one of acrylonitrile and methacrylonitrile;

[0019] The vinyl monomer unit comprises at least one of styrene and α-methylstyrene monomer;

[0020] The alkenylamine monomer unit comprises at least one of acrylamide, methacrylamide, phenylmaleimide and its derivatives;

[0021] The acrylate monomer units include at least one of methyl acrylate, ethyl acrylate, propyl acrylate, isopropyl acrylate, n-butyl acrylate, tert-butyl acrylate, n-pentyl acrylate, n-hexyl acrylate, heptyl acrylate, isooctyl acrylate, 2-ethylethyl acrylate, nonyl acrylate, decyl acrylate, lauryl acrylate, and their alkali metal salts.

[0022] The methacrylate monomer unit includes at least one of the following: methyl methacrylate, ethyl methacrylate, propyl methacrylate, n-hexyl methacrylate, heptyl methacrylate, isooctyl methacrylate, 2-ethylethyl methacrylate, nonyl methacrylate, and their alkali metal salts.

[0023] The vinyl chloride monomer unit includes vinylidene chloride.

[0024] By selecting the aforementioned monomers to participate in the preparation of hollow sphere emulsion, the hollow sphere particles in the prepared emulsion have high mechanical strength, which can prevent the hollow sphere particles from cracking when bonded to the electrode under hot pressing conditions. This maintains the supporting effect of the hollow sphere particles on the electrode, preserves the stability of the pore structure between the electrode and the base film, helps to improve the wetting degree of the electrolyte on the separator and electrode, and provides buffer space for the expansion of the negative electrode, thus helping to improve the overall performance of the lithium battery.

[0025] In some embodiments, the mixed-coating diaphragm further comprises, by weight percentage: 0.2%-0.4% dispersant, 5%-10% thickener, 5%-10% binder, and the solvent to make up to 100%.

[0026] In some embodiments, the dispersant includes at least one selected from polyethylene glycol, sodium polyacrylate, potassium polyacrylate, ammonium polyacrylate, and polyacrylate.

[0027] In some embodiments, the thickener includes at least one of sodium carboxymethyl cellulose, polymethacrylic acid emulsion, and polyacrylic acid emulsion.

[0028] In some embodiments, the adhesive includes at least one of polyacrylic acid, polyvinyl alcohol, and polyacrylamide.

[0029] In some embodiments, the specific steps include: Step 1, mixing the inorganic powder with the solvent and performing a first stirring operation, then adding the dispersant and performing a second stirring operation, then adding the hollow sphere emulsion and performing a third stirring operation, then adding the thickener and performing a fourth stirring operation, and finally adding the binder and performing a fifth stirring operation to obtain the mixed slurry; Step 2, coating the mixed slurry onto the surface of the base film using microgravure coating, and then drying it to obtain the mixed-coated diaphragm.

[0030] By combining inorganic powder and hollow sphere emulsion, on the one hand, it helps to obtain a mixed slurry with a suitable viscosity, improves the spreadability of the mixed slurry on the base film surface, and enhances the dispersion uniformity of hollow sphere particles in the slurry, thereby improving the bonding uniformity between the hollow sphere particles and the electrode. On the other hand, it helps to improve the mechanical strength of the coating formed by the mixed slurry.

[0031] In some embodiments, in the first stirring operation, the stirring speed is 500-600 rpm and the stirring time is 40-60 min; in the second stirring operation, the stirring speed is 500-600 rpm and the stirring time is 40-60 min; in the third stirring operation, the stirring speed is 200-300 rpm and the stirring time is 30-60 min; in the fourth stirring operation, the stirring speed is 200-300 rpm and the stirring time is 30-60 min; and in the fifth stirring operation, the stirring speed is 200-300 rpm and the stirring time is 30-60 min.

[0032] In some embodiments, the coating includes a base film and a coating disposed on at least one surface of the base film, the coating including a base layer and hollow granular balls embedded in the base layer, the base layer having a thickness ≥3 μm.

[0033] In some embodiments, the base membrane includes at least one of polyethylene membrane, polypropylene membrane, and polypropylene-polyethylene composite membrane.

[0034] In some embodiments, the particle size D50 of the hollow granular spheres is 1-2 μm.

[0035] By controlling the particle size D50 of the hollow granular gel balls and the thickness of the base layer, the ratio of the volume of the hollow granular gel balls protruding from the surface of the base layer to the volume embedded in the base layer can be adjusted and kept within a reasonable range. On the one hand, this ensures that the portion of the hollow granular gel balls protruding from the surface of the base layer has good adhesion to the electrode sheet. On the other hand, it ensures that the portion of the hollow granular gel balls embedded in the base layer can form a stable interlocking relationship with the base layer, balancing the uniformity of stress on the part of the hollow granular gel balls bonded to the electrode sheet and the part interlocked with the base layer, improving the volume stability of the hollow granular gel balls, and thus improving the overall performance of the lithium battery.

[0036] In some embodiments, the base layer further includes the inorganic powder, wherein the particle size D50 of the inorganic powder is ≤1μm.

[0037] In some embodiments, the inorganic powder includes at least one of alumina powder, magnesium oxide, and boehmite.

[0038] Example 1

[0039] 1. Preparation of hollow gel ball emulsion

[0040] S1, mix water and emulsifier (propylene glycol) at a mass ratio of 2:98 to obtain an emulsion. Take the first part of the emulsion and stir it at 25°C. During the stirring process, a mixture of monomer (styrene) and initiator (azobisisobutyronitrile) is added dropwise at a uniform rate (the mass ratio of total monomer to initiator is 100:1). The dropwise addition process lasts for 2 hours. After the dropwise addition process is completed, a pretreated solution is obtained.

[0041] S2, add the second part of the emulsion (the mass ratio of the first part of the emulsifier to the second part of the emulsifier is 1:1) to the above pretreatment solution, stir for 2 hours, raise the temperature to 55-60℃ and keep it at that temperature for 5-6 hours, and then cool it down to 25℃ to obtain the hollow granule emulsion (the particle size of the hollow granule granules is D50=1.5μm).

[0042] 2. Preparation of Coated Diaphragms

[0043] Step 1: Mix alumina powder (particle size D50=1μm) with water and stir at 500 rpm for 60 min. Then add dispersant (polyethylene glycol) and stir at 500 rpm for 60 min. Next, add hollow gel ball emulsion and stir at 300 rpm for 60 min. Then, add thickener (sodium carboxymethyl cellulose) and stir at 300 rpm for 60 min. Finally, add binder (polyacrylic acid) and stir at 300 rpm for 60 min. After filtering to remove iron, a mixed slurry is obtained.

[0044] The proportions of each raw material in the mixed slurry, by percentage, are as follows: alumina powder 30%, hollow sphere emulsion 5%, dispersant 0.3%, thickener 5%, binder 5%, and water 54.7%.

[0045] Step 2: The above mixed slurry is coated onto the two surfaces of the base film (PE separator) by microgravure coating, and then dried to obtain the mixed-coated separator (the thickness of the base layer in the mixed-coated separator is 3μm).

[0046] 3. Preparation of positive electrode sheet

[0047] The positive electrode active material (lithium iron phosphate), conductive agent SP, and binder PVDF are mixed evenly in a mass ratio of 94:3:3 and dispersed in N-methylpyrrolidone to obtain a positive electrode slurry. The positive electrode slurry is coated on aluminum foil and dried in a vacuum environment at 85°C for 24 hours to obtain a positive electrode sheet.

[0048] 4. Preparation of negative electrode sheet

[0049] The negative electrode active material (artificial graphite), conductive agent SP, and binder CMC are mixed evenly in a mass ratio of 92:4:4 and dispersed in deionized water to obtain a negative electrode slurry. The negative electrode slurry is coated on copper foil and dried in a vacuum environment at 100°C for 12 hours to obtain a negative electrode sheet.

[0050] 5. Preparation of electrolyte

[0051] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a volume ratio of 1:1:1 to obtain an organic solvent. Then, fully dried lithium salt LiPF6 was dissolved in the organic solvent to prepare an electrolyte with a concentration of 1 mol / L.

[0052] 6. Preparation of lithium batteries

[0053] The positive electrode, separator, and negative electrode are stacked in sequence, with the separator acting as a separator between the positive and negative electrodes. Then, they are wound to obtain a bare cell. The bare cell is placed in an outer packaging shell, dried, and then injected with the electrolyte. After vacuum sealing, it is left to stand and undergoes formation (at a temperature of 45°C and a pressure of 0.6 MPa, under which the hollow granular gel balls melt and create physical adhesion between the electrode and the coating). Finally, a volume-fixing process is performed to prepare a lithium battery.

[0054] Example 2

[0055] The difference between this embodiment and Embodiment 1 lies in the preparation of the hollow gel ball emulsion and the preparation of the mixed-coating diaphragm;

[0056] 1. Preparation of hollow gel ball emulsion

[0057] S1, mix water and emulsifier (propylene glycol) at a mass ratio of 4:98 to obtain an emulsion. Take the first part of the emulsion and stir it at 25°C. During the stirring process, a mixture of monomer (styrene) and initiator (azobisisobutyronitrile) (total mass of monomer to mass ratio of initiator is 100:1) is added dropwise at a uniform rate. The dropwise addition process lasts for 1.5 hours. After the dropwise addition process is completed, a pretreated solution is obtained.

[0058] S2, add the second part of the emulsion (the mass ratio of the first part of the emulsifier to the second part of the emulsifier is 1:1) to the above pretreatment solution, stir for 1 hour, heat to 60°C and keep warm for 6 hours, cool down to 25°C to obtain hollow granule emulsion (the particle size of the hollow granule granules is D50=1μm).

[0059] 2. Preparation of Coated Diaphragms

[0060] Step 1: Mix alumina powder (particle size D50=0.5μm) with water and stir at 600 rpm for 40 min. Then add dispersant (polyethylene glycol) and stir at 600 rpm for 60 min. Next, add hollow gel ball emulsion and stir at 300 rpm for 60 min. Then, add thickener (sodium carboxymethyl cellulose) and stir at 200 rpm for 60 min. Finally, add binder (polyacrylic acid) and stir at 300 rpm for 30 min. After filtering to remove iron, a mixed slurry is obtained.

[0061] The proportions of each raw material in the mixed slurry, by percentage, are as follows: alumina powder 20%, hollow sphere emulsion 6%, dispersant 0.2%, thickener 5%, binder 5%, and water 63.8%.

[0062] Step 2: The above mixed slurry is coated onto the two surfaces of the base film (PE separator) by microgravure coating, and then dried to obtain the mixed-coated separator (the thickness of the base layer in the mixed-coated separator is 3μm).

[0063] All other steps and parameter settings are consistent with those in Example 1.

[0064] Example 3

[0065] The difference between this embodiment and Embodiment 1 lies in the preparation of the hollow gel ball emulsion and the preparation of the mixed-coating diaphragm;

[0066] 1. Preparation of hollow gel ball emulsion

[0067] S1. A mixture of water and emulsifier (propylene glycol) at a mass ratio of 3:98 was prepared to obtain an emulsion. The first part of the emulsion was stirred at 25°C, and a mixture of monomer (styrene) and initiator (azobisisobutyronitrile) was added dropwise at a uniform rate during the stirring process (the mass ratio of total monomer mass to initiator was 100:0.5). The dropwise addition process lasted for 1.5 hours. After the dropwise addition process was completed, a pretreated solution was obtained.

[0068] S2, add the second part of the emulsion (the mass ratio of the first part of the emulsifier to the second part of the emulsifier is 1:2) to the above pretreatment solution, stir for 1 hour, raise the temperature to 55°C and keep it at that temperature for 8 hours, and then cool it down to 25°C to obtain hollow granule emulsion (the particle size of the hollow granule granules is D50=2μm).

[0069] 2. Preparation of Coated Diaphragms

[0070] Step 1: Mix alumina powder (particle size D50 = 0.8 μm) with water and stir at 600 rpm for 40 min. Then add dispersant (polyethylene glycol) and stir at 600 rpm for 40 min. Next, add hollow gel ball emulsion and stir at 200 rpm for 30 min. Then, add thickener (sodium carboxymethyl cellulose) and stir at 200 rpm for 60 min. Finally, add binder (polyacrylic acid) and stir at 200 rpm for 60 min. After filtering to remove iron, a mixed slurry is obtained.

[0071] The proportions of each raw material in the mixed slurry, by percentage, are as follows: alumina powder 35%, hollow sphere emulsion 10%, dispersant 0.4%, thickener 8%, binder 7%, and water 39.6%.

[0072] Step 2: The above mixed slurry is coated onto the two surfaces of the base film (PE separator) by microgravure coating, and then dried to obtain the mixed-coated separator (the thickness of the base layer in the mixed-coated separator is 3μm).

[0073] All other steps and parameter settings are consistent with those in Example 1.

[0074] Example 4

[0075] The difference between this embodiment and Embodiment 1 lies in the preparation of the hollow gel ball emulsion;

[0076] 1. Preparation of hollow gel ball emulsion

[0077] S1. A mixture of water and emulsifier (propylene glycol) at a mass ratio of 1:98 was prepared to obtain an emulsion. The first part of the emulsion was taken and stirred at 25°C. During the stirring process, a mixture of monomer (styrene) and initiator (azobisisobutyronitrile) was added dropwise at a uniform rate (the mass ratio of total monomer to initiator was 100:1). The dropwise addition process lasted for 0.5 hours. After the dropwise addition process was completed, a pretreated solution was obtained.

[0078] S2, add the second part of the emulsion (the mass ratio of the first part of the emulsifier to the second part of the emulsifier is 1:3) to the above pretreatment solution, stir for 0.5 h, raise the temperature to 50°C and keep it at that temperature for 6 h, and then cool it down to 25°C to obtain the hollow gel ball emulsion.

[0079] All other steps and parameter settings are consistent with those in Example 1.

[0080] Example 5

[0081] The difference between this embodiment and Example 1 is that butadiene is used as the monomer in the preparation of the hollow gel ball emulsion; all other steps and parameter settings are consistent with Example 1.

[0082] Example 6

[0083] The difference between this embodiment and Embodiment 1 lies in the preparation of the mixed-coating diaphragm;

[0084] 2. Preparation of Coated Diaphragms

[0085] Step 1: Mix alumina powder (particle size D50=1μm) with water and stir at 300 rpm for 60 min. Then add dispersant (polyethylene glycol) and stir at 300 rpm for 60 min. Next, add hollow gel ball emulsion and stir at 600 rpm for 30 min. Then, add thickener (sodium carboxymethyl cellulose) and stir at 600 rpm for 30 min. Finally, add binder (polyacrylic acid) and stir at 600 rpm for 30 min to obtain a mixed slurry.

[0086] The proportions of each raw material in the mixed slurry, by percentage, are as follows: alumina powder 30%, hollow sphere emulsion 5%, dispersant 0.3%, thickener 5%, binder 5%, and water 54.7%.

[0087] Step 2: The above mixed slurry is coated onto the surface of the PE membrane using microgravure coating, and then dried to obtain the mixed-coated membrane.

[0088] All other steps and parameter settings are consistent with those in Example 1.

[0089] Example 7

[0090] The difference between this embodiment and Embodiment 1 is that the thickness of the base layer is 1 μm; all other steps and parameter settings are consistent with Embodiment 1.

[0091] Example 8

[0092] The difference between this embodiment and Embodiment 1 is that the particle size D50 of the hollow granular gel balls is 0.5 μm; the other steps and parameter settings are consistent with Embodiment 1.

[0093] Example 9

[0094] The difference between this embodiment and Embodiment 1 is that the particle size D50 of the hollow granular gel balls is 4μm; the other steps and parameter settings are consistent with Embodiment 1.

[0095] Example 10

[0096] The difference between this embodiment and Embodiment 1 is that the particle size D50 of the alumina powder is 2μm; the other steps and parameter settings are consistent with Embodiment 1.

[0097] Comparative Example 1

[0098] The difference between this embodiment and Embodiment 1 is that the coating on the surface of the base film (PE membrane) includes an overlaid alumina coating and an organic coating; the alumina coating is obtained by drying a slurry containing alumina powder, and the organic coating is obtained by drying a slurry containing hollow sphere emulsion.

[0099] 1. Slurry preparation

[0100] (1) Add alumina powder to water and stir at 500 rpm for 60 min. Add dispersant (polyethylene glycol) and stir at 500 rpm for 60 min. Continue to add thickener (sodium hydroxymethyl cellulose) and stir at 300 rpm for 60 min. Add 5% binder (polyacrylic acid) and stir at 300 rpm for 60 min. Filter to remove iron and set aside for later use. The resulting slurry containing alumina powder is called alumina slurry.

[0101] The raw materials, by mass percentage, are: alumina powder 30%, dispersant 0.3%, thickener 5%, binder 5%, and water 59.7%.

[0102] (2) Add the hollow rubber ball emulsion to water and stir at 500 rpm for 60 min. Add the dispersant (polyethylene glycol) and stir at 500 rpm for 60 min. Continue to add the thickener (sodium carboxymethyl cellulose) and stir at 300 rpm for 60 min. Add 5% binder (polyacrylic acid) and stir at 300 rpm for 60 min. Filter to remove iron and set aside for later use. The resulting slurry containing the hollow rubber ball emulsion is called organic slurry.

[0103] The raw materials, by mass percentage, are: hollow sphere emulsion 30%, dispersant 0.3%, thickener 5%, binder 5%, and water 59.7%.

[0104] 2. Diaphragm preparation

[0105] The above-mentioned alumina slurry of glue balls is coated onto the surface of the base film (PE separator) by micro-gravure coating, and an inorganic alumina coating is obtained after drying. The above-mentioned organic slurry is coated onto the surface of the alumina coating by high-speed rotary spraying, and after drying, it is rolled up for later use.

[0106] All other steps and parameter settings are consistent with those in Example 1.

[0107] Comparative Example 2

[0108] The difference between this embodiment and Embodiment 1 is that the mixed slurry does not contain hollow gel ball emulsion; all other steps and parameter settings are consistent with Embodiment 1.

[0109] Comparative Example 3

[0110] The difference between this embodiment and Embodiment 1 is that solid rubber ball emulsion of equal weight is used instead of hollow rubber ball emulsion.

[0111] 1. Preparation of solid rubber ball emulsion

[0112] The monomer (styrene) and the initiator (azobisisobutyronitrile) were mixed to obtain a mixture; the mixture was added dropwise into a three-necked flask at a uniform rate, and the mixture was added completely within 2 hours. After the addition was completed, the mixture was stirred at a uniform rate for 2 hours, then heated to 50°C and kept at that temperature for 6 hours. After the temperature dropped to 25°C, the mixture was discharged to obtain a solid rubber ball emulsion containing large solid particles.

[0113] All other steps and parameter settings are consistent with those in Example 1.

[0114] Comparative Example 4

[0115] The difference between this embodiment and Example 1 is that the mass percentage of alumina powder in the mixed slurry is 40%, and the mass percentage of hollow sphere emulsion is 2%.

[0116] All other steps and parameter settings are consistent with those in Example 1.

[0117] Comparative Example 5

[0118] The difference between this embodiment and Example 1 is that the mass percentage of alumina powder in the mixed slurry is 10%, and the mass percentage of hollow sphere emulsion is 20%.

[0119] All other steps and parameter settings are consistent with those in Example 1.

[0120] Test methods

[0121] I. Adhesion test between diaphragm and electrode

[0122] The diaphragms prepared in the above examples and comparative examples were subjected to adhesion force tests between the diaphragm and the electrode. The test method was as follows: under a pressure of 0.3 MPa, at 80°C and for 60 s, the adhesion force between the diaphragm and the electrode was tested using an electronic tensile test.

[0123] II. Electrolyte absorption test:

[0124] Take a diaphragm sample with an area of ​​100 cm² from the diaphragm samples prepared in the above embodiments and comparative examples. 2Electrolyte absorption capacity was tested. The specific test steps were as follows: First, the weight of the diaphragm sample was measured using a precision density balance and recorded as M1. The diaphragm sample was then immersed in the electrolyte (electrolyte composition EC:PC:EMC = 30:5:65) at 85℃ for 24 hours. After immersion, the sample was removed, the surface residual electrolyte was wiped dry with lint-free paper, and the sample was weighed and recorded as M2. The absorption rate was calculated as an indicator of the electrolyte absorption capacity of the diaphragm. The absorption rate was calculated as follows: Absorption rate (%) = (M2 - M1) / M1 × 100%, where M1: the initial weight of the diaphragm sample; M2: the weight of the diaphragm sample after immersion in the electrolyte.

[0125] III. Lithium-ion battery cycle performance test

[0126] The lithium batteries prepared in the above examples and comparative examples were subjected to high-temperature cycling performance tests. The specific test steps were as follows: the lithium batteries were placed in a 60°C temperature chamber, and the charging current was 1C and the discharging current was 1C for cycling. The capacity retention rate of the lithium batteries was observed after 500 cycles.

[0127] Table 1

[0128] Serial Number | Adhesion Strength / Nm⁻¹ | Liquid Absorption Rate / % | Cyclic Capacity Retention Rate / % | Example 1 | 13.78 | 1.09 | 5.4 | Example 2 | 13.68 | 0.59 | 5.2 | Example 3 | 13.68 | 0.89 | 5.3 | Example 4 | 12.37 | 8.49 | 3.1 | Example 5 | 11.87 | 6.49 | 2.5 | Example 6 | 12.07 | 8.29 | 3.0 | Example 7 | 11.57 | 6.29 | 2.3 | Example 8 | 10.47 | 6.09 | 2.0 | Example 9 | 11.87 | 6.39 | 2.1 | Example 10 | 11.67 | 6.29 | 2.2 | Comparative Example 1 | 10.27 | 4.69 | 0.5 | Comparative Example 2 | 8.56 | 0.38 | 5.4 | Comparative Example 3 | 9.56 | 5.38 | 8.4 | Comparative Example 4 | 9.76 | 6.38 | 8.5 | Comparative Example 5 | 11.87 | 4.09 | 1.3

[0129] Based on Examples 1-3, Comparative Examples 1-3, and Table 1, it can be seen that by using hollow sphere emulsion and alumina powder to prepare a mixed slurry for forming a coating on the surface of the base film, the mixed slurry has a suitable viscosity, which facilitates its spread on the surface of the base film. On the other hand, the hollow sphere particles in the hollow sphere emulsion can be uniformly dispersed in the mixed slurry, thereby uniformly embedded in the coating, significantly improving the bonding stability between the electrode and the separator, increasing the mechanical strength of the core, and preventing electrode wrinkling. Moreover, during the coating formation process of the mixed slurry, the hollow sphere particles can float to the surface of the coating and protrude from the surface of the coating by means of their cavities, which helps to form pores between the electrode and the separator to accommodate the electrolyte. This not only improves the wettability of the electrolyte to the electrode and the separator, but also provides a buffer space for the expansion of the negative electrode during lithium battery cycling, thereby helping to improve the cycle performance of the lithium battery.

[0130] In Comparative Example 1, the coating was formed by layering alumina powder and hollow granular gel balls, resulting in poor coating stability. In Comparative Examples 2-3, hollow granular gel balls were not used, or solid granular gel balls (the density of solid granular gel balls is greater than that of hollow granular gel balls) were used. In these cases, sufficient pores could not be created between the separator and the electrode to accommodate the electrolyte, which was detrimental to improving the overall performance of the lithium battery. Compared to Comparative Examples 1-3, in this embodiment, some of the hollow granular gel balls were embedded in the base layer, while others protruded from the surface of the base layer. The base layer also contained a certain amount of alumina powder, which greatly improved the adhesion between the electrode and the mixed-coated separator. Furthermore, a stable porous structure for accommodating the electrolyte was formed between the separator and the electrode. Therefore, the coating stability on the base film surface in this embodiment was better, which helped improve the cycle performance of the lithium battery.

[0131] Based on Example 1, Comparative Examples 4-5, and Table 1, it can be seen that Example 1, by adjusting the ratio of alumina powder to hollow granular balls, can maximize the number of hollow granular balls floating while ensuring the mixed slurry achieves a suitable viscosity. This increases the contact area between the hollow granular balls and the electrode, which helps improve the stability of the coating, the bonding stability between the diaphragm and the electrode, and thus improves the adhesion between the diaphragm and the electrode, as well as the liquid absorption performance of the diaphragm.

[0132] Based on Examples 1, 4, and 6 and Table 1, it can be seen that the hollow sphere emulsion and mixed-coating separator prepared by the method of this application have excellent compatibility between alumina powder and the hollow sphere particles. While achieving stable adhesion between the base layer and the base film, and stable adhesion between the hollow sphere particles and the electrode, it also improves the intercalation stability of the hollow sphere particles in the base layer, which helps to improve the stability of the coating and the cycle performance of the lithium battery.

[0133] Based on Examples 1 and 5 and Table 1, it can be seen that by selecting appropriate monomer types and using them in combination, and by using limited monomers, large-sized granular spheres can be synthesized, which can improve the performance of the membrane.

[0134] Combining Examples 1, 7-10, and Table 1, it can be seen that by controlling the thickness of the base layer and the particle size D50 of the alumina powder and the hollow granular spheres, the alumina powder and the hollow granular spheres can meet a suitable range. This helps the alumina powder to be uniformly dispersed around the hollow granular spheres, which produces a better "anchoring" effect on the hollow granular spheres, improves the intercalation stability of the hollow granular spheres in the base layer, helps to improve the stability of the coating, and improves the cycle performance of the lithium battery.

Claims

1. A method for preparing a mixed-coated diaphragm, comprising the following operations: Raw materials including inorganic powder, hollow sphere emulsion, binder and solvent are mixed to obtain a mixed slurry; the mixed slurry is coated on the surface of a base film and dried to obtain the mixed-coated separator; In the mixed slurry, the inorganic powder accounts for 20%-35% by mass, and the hollow sphere emulsion accounts for 5%-10% by mass. The viscosity of the mixed slurry is 100-120 mPa·s; The hollow sphere emulsion comprises hollow granular spheres, the density of which is 0.8-0.9 g / cm³. 3 .

2. The method for preparing the mixed-coated diaphragm according to claim 1, wherein: The hollow gel ball emulsion is prepared by a method comprising the following steps: S1, mix water and emulsifier to obtain emulsion, take the first part of emulsion and stir it, add a mixture of monomer and initiator dropwise to it during the stirring process, and obtain a pretreatment solution after the dropwise addition process is completed; S2, add the second part of the emulsion to the pretreatment solution, and obtain the hollow gel ball emulsion after stirring, keeping warm and cooling.

3. The method for preparing the mixed-coated diaphragm according to claim 2, wherein: The mass ratio of water to emulsifier is (2-4):98; the mass ratio of the first emulsion to the second emulsion is 1:(1-2); and the mass ratio of monomer to initiator is 100:(0.5-1).

4. The method for preparing the mixed-coated diaphragm according to claim 2, wherein: In S1, the temperature during the stirring process is 25±2℃, the dropping speed is uniform, and the duration is 1.5-2h; in S2, the stirring time is 1-2h, and after the stirring process is completed, the temperature is raised to 55-60℃ and kept at that temperature for 6-8h.

5. The method for preparing the mixed-coated diaphragm according to claim 2, wherein: The monomers include at least one of unsaturated nitrile monomer unit copolymers, vinyl monomer unit copolymers, alkenylamine monomer unit copolymers, acrylate monomer unit copolymers, methacrylate monomer unit copolymers, vinyl chloride monomer unit copolymers, and modified compounds of the above copolymers.

6. The method for preparing the mixed-coated diaphragm according to claim 1, wherein: The mixed coating membrane further comprises, by weight percentage: 0.2%-0.4% dispersant, 5%-10% thickener, 5%-10% binder, and solvent to make up to 100%.

7. The method for preparing the mixed-coated diaphragm according to claim 6, comprising the following steps: Step 1: After mixing the inorganic powder with the solvent, perform a first stirring operation, add the dispersant and perform a second stirring operation, add the hollow sphere emulsion and perform a third stirring operation, add the thickener and perform a fourth stirring operation, add the binder and perform a fifth stirring operation to obtain the mixed slurry; Step 2: The mixed slurry is coated onto the surface of the base film using a microgravure coating process, and then dried to obtain the mixed-coated diaphragm.

8. The method for preparing the mixed-coated diaphragm according to claim 7, wherein: In the first stirring operation, the stirring speed is 500-600 rpm and the stirring time is 40-60 min; in the second stirring operation, the stirring speed is 500-600 rpm and the stirring time is 40-60 min; in the third stirring operation, the stirring speed is 200-300 rpm and the stirring time is 30-60 min; in the fourth stirring operation, the stirring speed is 200-300 rpm and the stirring time is 30-60 min; in the fifth stirring operation, the stirring speed is 200-300 rpm and the stirring time is 30-60 min.

9. A mixed-coating diaphragm, prepared by the method according to any one of claims 1-8.

10. The mixed-coating diaphragm according to claim 9, wherein: It includes a base film and a coating disposed on at least one surface of the base film, the coating comprising a base layer and hollow granular balls embedded in the base layer, the base layer having a thickness ≥3μm.

11. The mixed-coating diaphragm according to claim 10, wherein: The hollow granular gel balls have a particle size D50 of 1-2 μm.

12. The mixed-coating diaphragm according to claim 10, wherein: The base layer also includes the inorganic powder, wherein the particle size D50 of the inorganic powder is ≤1μm.

13. A lithium-ion battery comprising the co-coated separator as described in any one of claims 9-12.