A composite diaphragm and a preparation method thereof and a lithium battery using the same

Abstract

The present invention provides a method for preparing a composite diaphragm, comprising the following operations: mixing inorganic particles A, a binder polymer B and a non-binder polymer C to prepare a coating slurry, coating the coating slurry on the surface of a substrate layer, and sequentially performing a baking treatment and a drying treatment, thereby preparing a composite diaphragm; wherein the inorganic particles A have a D 50 0.1~2.5μm, D of non-binder polymer C 50 The present invention optimizes the preparation process of the composite diaphragm. On the one hand, by controlling the particle size of the inorganic particles A and the particle size and average sphericity of the non-binder polymer C, a porous coating having a dense, complete, uniform thickness and a stable convex structure can be formed on the surface of the composite diaphragm substrate layer. On the other hand, by controlling the overall viscosity of the coating slurry, the coating slurry has good flow properties, and one coating is sufficient to form a complete coating on the surface of the substrate layer.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium batteries, and in particular, relates to a composite diaphragm and a preparation method thereof, and a lithium battery using the composite diaphragm. Background Art

[0002] As one of the key internal components in lithium batteries, the diaphragm has the function of isolating the positive and negative electrodes of the battery to prevent short circuits and transmit lithium ions through its own microporous structure. Traditional diaphragms use inorganic particles A and adhesive polymer B to form a functional coating on at least one surface of the polyolefin substrate layer. The inorganic particles in the functional coating are connected and fixed by the adhesive polymer. However, the disadvantages of this functional coating are: the diaphragm coating has no adhesion to the pole piece, resulting in poor mechanical strength, looseness, wrinkles and other problems of the winding core.

[0003] In response to the shortcomings of traditional diaphragms, there is a new type of lithium battery composite diaphragm, which includes a porous substrate layer and a porous coating coated on at least one side of the porous substrate layer; the porous coating contains inorganic particles A, binder polymer B, and non-binder polymer C; the porous coating has strong adhesion to the pole piece, which can enhance the mechanical strength of the winding core and improve the diaphragm wrinkle problem. At the same time, the non-binder polymer C with a larger particle size protrudes from the surface of the porous coating, which can increase the gap between the composite diaphragm and the pole piece, which is beneficial to electrode wetting. However, in the preparation process of the composite diaphragm, due to the presence of solid particles of inorganic particles A and non-binder polymer C with different particle sizes in the coating slurry, the coating accuracy and coating uniformity during the coating process are difficult to control, which easily leads to uneven thickness and poor quality of the porous coating surface, making it difficult for the composite diaphragm to fully exert its excellent properties. Summary of the invention

[0004] The purpose of the present invention is to provide a method for preparing a composite diaphragm, which is conducive to controlling the coating accuracy of the coating slurry so that a porous coating with uniform thickness and stable structure is formed on the surface of the substrate layer.

[0005] According to one aspect of the present invention, a method for preparing a composite diaphragm is provided, comprising the following operations: mixing inorganic particles A, a binder polymer B and a non-binder polymer C to prepare a coating slurry, coating the coating slurry on the surface of a substrate layer, and sequentially performing a baking treatment and a drying treatment, thereby preparing a composite diaphragm; wherein the inorganic particles A have a D 50 0.1~2.5μm, D of non-binder polymer C 50 The average sphericity of the non-binder polymer C is ≥0.4, and the viscosity of the coating slurry is 20 to 1000 mPa·s.

[0006] The coating slurry formed by mixing inorganic particles A with different particle sizes and non-binder polymer C is easy to form a porous coating with uneven thickness and poor quality on the surface of the composite diaphragm substrate layer after coating. Therefore, the present invention optimizes the preparation process of the composite diaphragm. On the one hand, by controlling the particle size of the inorganic particles A and the particle size and average sphericity of the non-binder polymer C, the inorganic particles A are evenly dispersed around the non-binder polymer C, and during the coating process, the non-binder polymer C can easily slip between the pores of the inorganic particles A due to its high average sphericity, so that the non-binder polymer C is firmly embedded in the base coating composed of the inorganic particles A and the adhesive polymer B. As a result, the surface of the composite diaphragm substrate layer can form a dense, complete, uniformly thick, and porous coating with a stable convex structure; at the same time, the coating convenience and spreading effect of the coating slurry are effectively improved. On the other hand, by controlling the overall viscosity of the coating slurry, the coating slurry has good flow properties, so that the coating slurry can be easily spread evenly and smoothly on the surface of the substrate layer. One coating is sufficient to form a complete coating on the surface of the substrate layer, thereby avoiding multiple coatings and effectively improving the production efficiency of the composite diaphragm.

[0007] In summary, the preparation method of the composite diaphragm provided by the present invention is simple in process, easy to operate, suitable for industrial production, and can ensure that the composite diaphragm prepared thereby has excellent thickness consistency and structural stability.

[0008] Preferably, the inorganic particles A include at least one of alumina, boehmite, titanium dioxide and silicon dioxide.

[0009] Preferably, the binder polymer B includes at least one of polyvinylidene fluoride, polyvinylidene fluoride-co-hexafluoropropylene, polyvinylidene fluoride-co-trichloroethylene, polymethyl methacrylate, polyacrylonitrile, polyvinyl acetate, polyethylene-co-vinyl acetate, polyimide, and polyethylene oxide.

[0010] Preferably, the non-binder polymer C includes at least one of polymethyl acrylate, polyethyl acrylate, polybutyl acrylate, polymethyl methacrylate, butyl acrylate-styrene copolymer, ethylene-acrylic acid copolymer, ethylene-methyl acrylate copolymer, ethylene-ethyl acrylate copolymer, ethylene-butyl acrylate copolymer and ethylene-methyl methacrylate copolymer.

[0011] Preferably, the substrate layer includes at least one of polyethylene, polypropylene, polybutylene and polypentene.

[0012] Preferably, the coating slurry further includes a thickener, and the thickener includes at least one of cyanoethyl cellulose, cyanoethyl sucrose, carboxymethyl cellulose, polyvinyl alcohol, polyacrylic acid, and polymaleic anhydride.

[0013] Preferably, the coating slurry further comprises a wetting agent, and the wetting agent comprises at least one of alkyl glycoside, alkyl polyglycoside, polyvinyl pyrrolidone, sorbitan fatty acid ester and polysorbate.

[0014] Preferably, the particle size concentration of the inorganic particles A (D 90 -D 10 ) / D 50 = 0.5 to 2.0. Controlling the pore size distribution of the inorganic particles A is beneficial to improving the flatness of the base coating composed of the inorganic particles A and the binder polymer B, and is also beneficial to ensuring that the base coating has an appropriate porosity and good air permeability.

[0015] Preferably, the average sphericity of the non-binder polymer C is ≥ 0.7. The non-binder polymer C with the above-mentioned suitable average sphericity can more easily slide into the pores of the inorganic particles A, so that the non-binder polymer C is firmly embedded in the base coating, and is conducive to further making the inorganic particles A and the non-binder polymer C uniformly and tightly dispersed in the coating slurry, thereby effectively improving the density and uniformity of the porous coating, and making the protrusion structure formed by the non-binder polymer C more firm and stable.

[0016] Preferably, the viscosity of the coating slurry is 30-500 mPa·s. The coating slurry within the above viscosity range is easier to spread evenly and smoothly on the surface of the substrate layer, so that the flowability and spreading effect of the coating slurry are further optimized.

[0017] Preferably, the solid content of the coating slurry is 20-40%. By controlling the solid content of the coating slurry, it is beneficial to improve the dispersion effect of the solid particles (inorganic particles A, non-binder polymer C) in the coating slurry and the flowability of the coating slurry, so that a porous coating with uniform thickness and appropriate porosity is formed on the surface of the substrate layer, thereby achieving effective regulation of the thickness consistency and air permeability of the porous coating of the composite diaphragm.

[0018] Preferably, during the coating process, the coating temperature is 15 to 35°C and the coating speed is 10 to 150 m / min. By reasonably setting the coating temperature and coating speed during the coating process, it is beneficial to maintain good stability and uniform dispersion of the coating slurry, so that a smooth, uniform, thin and stable porous coating can be formed on the surface of the substrate layer after one coating, avoiding surface defects such as uneven thickness of the composite diaphragm.

[0019] Preferably, the coating method is micro gravure coating or slot extrusion coating.

[0020] Preferably, the baking temperature of the baking treatment is 40-80°C.

[0021] Preferably, the dew point temperature of the drying treatment is -20 to -80°C, and the drying time of the drying treatment is 12 to 24 hours. By properly setting the conditions of the baking treatment and the drying treatment, it is beneficial to effectively control the moisture in the composite diaphragm, so that the moisture content of the composite diaphragm is less than 2,000 parts per million (2000 ppm).

[0022] According to another aspect of the present invention, a composite membrane is provided, which is prepared by the above-mentioned composite membrane preparation method. The composite membrane provided by the present invention has excellent thickness consistency and structural stability.

[0023] Preferably, the composite separator includes a substrate layer and a porous coating, the porous coating is disposed on at least one surface of the substrate layer, the porous coating includes a base coating and a non-adhesive polymer C embedded in the base coating, and the thickness of the base coating is 0.5 to 6 μm. By reasonably setting the thickness of the base coating, it is beneficial to ensure that the non-adhesive polymer C can be stably embedded in the base coating, and at the same time, the thermal shrinkage rate of the composite separator is reduced.

[0024] According to another aspect of the present invention, a lithium battery is provided, comprising the above-mentioned composite separator. DETAILED DESCRIPTION

[0025] In order to enable those skilled in the art to better understand the technical solutions of the present invention, the technical solutions of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present invention.

[0026] Example 1

[0027] This embodiment provides a lithium battery, and the preparation method thereof comprises the following steps:

[0028] 1. Preparation of composite diaphragm:

[0029] S1. Adding a polyethylene (PE) substrate into a material tank, and obtaining a substrate layer by extrusion, sheet casting, longitudinal stretching, extraction and heat setting.

[0030] S2, the particle size D 50 The inorganic particles A are alumina with a diameter of 0.8 μm, the binder polymer B is PVDF, and the particle size D is 50 Non-binder polymer C polyethyl acrylate with a particle size of 6 μm and an average sphericity of 0.86 was added to a stirring tank, and solvent deionized water was added and stirred to disperse evenly, and then the dispersion was continued at a stirring speed of 12000 rpm for 100 minutes to form a coating slurry. The coating slurry had a viscosity of 102 mPa·s and a solid content of 33%.

[0031] S3, the coating slurry is coated on the surface of the substrate layer by micro gravure coating, the coating speed is 60 m / min, and the coating temperature is 25° C. After coating, the coating is baked in a 20-meter oven, and the baking temperature of the baking treatment is 60° C.

[0032] S4. The diaphragm obtained in S3 is transferred to a drying room for drying. The dew point temperature in the drying room is -45°C. After drying for 18 hours, the moisture content of the diaphragm is less than 2000 ppm, thereby obtaining a composite diaphragm.

[0033] The composite diaphragm comprises a substrate layer and a porous coating layer, wherein the porous coating layer is arranged on the surface of the substrate layer, the porous coating layer comprises a base coating layer and a non-adhesive polymer C embedded in the base coating layer, and the base coating layer has a thickness of 4 μm.

[0034] 2. Preparation of positive electrode sheet:

[0035] The positive electrode active material (lithium iron phosphate), conductive agent SP, and binder PVDF were uniformly mixed in a mass ratio of 94:3:3, dispersed in NMP to obtain a positive electrode slurry, coated the positive electrode slurry on aluminum foil, and dried at 85° C. in a vacuum environment for 24 hours to obtain a positive electrode sheet.

[0036] 3. Preparation of negative electrode sheet:

[0037] The negative electrode active material (graphite), conductive agent SP, and binder CMC were uniformly mixed in a mass ratio of 92:4:4, dispersed in deionized water to obtain a negative electrode slurry, coated the negative electrode slurry on a copper foil, and dried at 100° C. in a vacuum environment for 12 hours to obtain a negative electrode sheet.

[0038] 4. Preparation of electrolyte:

[0039] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) are mixed in a volume ratio of 1:1:1 to obtain an organic solvent, and then fully dried lithium salt LiPF6 is dissolved in the organic solvent to prepare an electrolyte with a concentration of 1 mol / L.

[0040] 5. Preparation of lithium batteries:

[0041] The positive electrode sheet, separator and negative electrode sheet are stacked in order, so that the separator is placed between the positive and negative electrode sheets to play an isolating role, and then wound to obtain a bare battery cell; the bare battery cell is placed in an outer packaging shell, and after drying, the electrolyte is injected, and a lithium battery is prepared through vacuum packaging, standing, formation and constant capacity processes.

[0042] Example 2

[0043] This example prepares a lithium battery with reference to Example 1. The difference between this example and Example 1 is that in the process of preparing the composite diaphragm in this example, the average sphericity of the non-binder polymer C used is 0.4. In addition to the above differences, the materials and process operations used in this example are strictly consistent with those in Example 1.

[0044] Example 3

[0045] This example prepares a lithium battery with reference to Example 1. The difference between this example and Example 1 is that in the process of preparing the composite diaphragm in this example, the average sphericity of the non-binder polymer C used is 0.7. In addition to the above differences, the materials and process operations used in this example are strictly consistent with those in Example 1.

[0046] Example 4

[0047] This example prepares a lithium battery with reference to Example 1. The difference between this example and Example 1 is that in the process of preparing the composite separator in this example, the average sphericity of the non-binder polymer C used is 0.95. In addition to the above differences, the materials and process operations used in this example are strictly consistent with those in Example 1.

[0048] Example 5

[0049] This example prepares a lithium battery with reference to Example 1. The difference between this example and Example 1 is that in the process of preparing the composite diaphragm in this example, the viscosity of the coating slurry is 20 mPa·s. In addition to the above differences, the materials and process operations used in this example are strictly consistent with those in Example 1.

[0050] Example 6

[0051] This example prepares a lithium battery with reference to Example 1. The difference between this example and Example 1 is that in the process of preparing the composite diaphragm in this example, the viscosity of the coating slurry is 30 mPa·s. Except for the above difference, the materials and process operations used in this example are strictly consistent with those in Example 1.

[0052] Example 7

[0053] This example prepares a lithium battery with reference to Example 1. The difference between this example and Example 1 is that in the process of preparing the composite diaphragm in this example, the viscosity of the coating slurry is 500 mPa·s. In addition to the above differences, the materials and process operations used in this example are strictly consistent with those in Example 1.

[0054] Example 8

[0055] This example prepares a lithium battery with reference to Example 1. The difference between this example and Example 1 is that in the process of preparing the composite diaphragm in this example, the viscosity of the coating slurry is 600 mPa·s. Except for the above difference, the materials and process operations used in this example are strictly consistent with those in Example 1.

[0056] Example 9

[0057] This example prepares a lithium battery with reference to Example 1. The difference between this example and Example 1 is that in the process of preparing the composite diaphragm in this example, the solid content of the coating slurry is 15%. In addition to the above differences, the materials and process operations used in this example are strictly consistent with those in Example 1.

[0058] Example 10

[0059] This embodiment prepares a lithium battery with reference to the embodiment 1. The difference between this embodiment and the embodiment 1 is that in the process of preparing the composite diaphragm in this embodiment, the solid content of the coating slurry is 50%. In addition to the above differences, the materials and process operations used in this embodiment are strictly consistent with those in the embodiment 1.

[0060] Embodiment 11

[0061] This embodiment is used to prepare a lithium battery with reference to the embodiment 1. The difference between this embodiment and the embodiment 1 is that in the process of preparing the composite diaphragm in this embodiment, the inorganic particles A used are D 50 0.1μm, non-binder polymer C D 50 Except for the above differences, the materials and process operations used in this embodiment are strictly consistent with those in Example 1.

[0062] Example 12

[0063] This embodiment is used to prepare a lithium battery with reference to the embodiment 1. The difference between this embodiment and the embodiment 1 is that in the process of preparing the composite diaphragm in this embodiment, the inorganic particles A used are D 50 2.5μm, D of non-binder polymer C 50 Except for the above differences, the materials and process operations used in this embodiment are strictly consistent with those in Example 1.

[0064] Comparative Example 1

[0065] This comparative example prepares a lithium battery with reference to Example 1. The difference between this comparative example and Example 1 is that in the process of preparing the composite diaphragm in this example, the viscosity of the coating slurry is 10 mPa·s. In addition to the above differences, the materials and process operations used in this comparative example are strictly consistent with those in Example 1.

[0066] Comparative Example 2

[0067] This comparative example prepares a lithium battery with reference to Example 1. The difference between this comparative example and Example 1 is that in the process of preparing the composite diaphragm in this example, the average sphericity of the non-binder polymer C used is 0.3. In addition to the above differences, the materials and process operations used in this comparative example are strictly consistent with those in Example 1.

[0068] Comparative Example 3

[0069] This comparative example is used to prepare a lithium battery with reference to Example 1. The difference between this comparative example and Example 1 is that in the process of preparing the composite diaphragm in this example, the inorganic particles A used are D 50 Except for the above differences, the materials and process operations used in this comparative example are strictly consistent with those in Example 1.

[0070] Comparative Example 4

[0071] This comparative example is used to prepare a lithium battery with reference to Example 1. The difference between this comparative example and Example 1 is that in the process of preparing the composite diaphragm in this example, the inorganic particles A used are D 50 Except for the above differences, the materials and process operations used in this comparative example are strictly consistent with those in Example 1.

[0072] Test Example 1

[0073] 1. Test object:

[0074] The composite diaphragms and lithium batteries prepared in Examples 1 to 12 and Comparative Examples 1 to 4 were used as test objects in this test example.

[0075] 2. Test items:

[0076] (1) Composite membrane thickness consistency ΔD: stack four layers of composite membrane neatly, take five sampling points at equal distances on the surface of the composite membrane, use a micrometer to measure the thickness of the four layers of composite membrane at each sampling point, and take the average value to obtain the thickness of a single layer of composite membrane at each sampling point; calculate the thickness consistency of the composite membrane according to the following formula: where d i represents the thickness of a single-layer composite membrane at any sampling point, It represents the average value of the thickness of single-layer composite diaphragm at 5 sampling points.

[0077] (2) Thermal shrinkage of composite diaphragm: A 10 cm*10 cm composite diaphragm was clamped with a glass plate and placed in an oven at 130°C for 1 hour. The composite diaphragm was then taken out and the thermal shrinkage in the longitudinal direction (MD) and transverse direction (TD) was tested.

[0078] (3) Composite membrane air permeability: A gas permeability tester is used to test the time required for 100 mL of gas to pass through a composite membrane with a diameter of 4 cm within a specific time.

[0079] (4) High temperature cycling performance of lithium batteries: The lithium batteries were placed in a 60°C incubator and cycled at a charging current of 1C and a discharging current of 1C. The capacity retention rate of the lithium batteries was observed after 800 cycles.

[0080] 3. Test results:

[0081] Table 1 Performance test results of lithium batteries of Examples 1 to 12 and Comparative Examples 1 to 4

[0082]

[0083]

[0084] The test results are shown in Table 1. By comparing the performance test results corresponding to Example 1 and Comparative Example 1, it can be found that the thickness consistency ΔD, thermal shrinkage rate, and air permeability of the composite diaphragm prepared in Comparative Example 1 are higher than those in Example 1, and the high-temperature cycle capacity retention rate of the battery is lower than that in Example 1. The reason is that in the process of preparing the composite diaphragm in Comparative Example 1, the viscosity of the coating slurry is too low, and the solid particles in the coating slurry have a tendency to settle during the curing and molding of the coating, which reduces the surface flatness and thermal stability of the composite diaphragm to a certain extent, and because the bottom layer of the porous coating of the formed composite diaphragm is relatively compact, the air permeability effect of the composite diaphragm and the cycle characteristics of the battery are significantly deteriorated.

[0085] By comparing the performance test results corresponding to Example 1 and Comparative Example 2, it can be found that the thickness consistency ΔD and thermal shrinkage of the composite diaphragm prepared in Comparative Example 2 are higher than those in Example 1, and the high-temperature cycle capacity retention rate of the battery is lower than that in Example 1. The reason is that the average sphericity of the non-binder polymer C used in Comparative Example 2 is too low. As large particles in the porous coating of the composite diaphragm, the low average sphericity of the non-binder polymer C is not conducive to the coating performance and spreading effect of the coating slurry, thereby reducing the surface flatness and thermal stability of the composite diaphragm. In addition, the non-binder polymer C with a low average sphericity reduces the connection strength between the composite diaphragm and the pole piece, thereby further deteriorating the cycle characteristics of the battery.

[0086] By comparing the performance test results of Example 1 and Comparative Example 3, it can be found that the thickness consistency ΔD, thermal shrinkage rate, and air permeability of the composite diaphragm prepared in Comparative Example 3 are higher than those in Example 1, and the high temperature cycle capacity retention rate of the battery is lower than that in Example 1. The reason is that the D of the inorganic particles A used in Comparative Example 3 is 50The dispersion effect of small-sized inorganic particles A and large-sized non-binder polymer C is poor, which reduces the surface flatness and thermal stability of the composite diaphragm formed thereby to a certain extent. In addition, since small-sized inorganic particles A are easily filled in the porous coating of the composite diaphragm, the porous coating of the composite diaphragm is filled tightly and has a low porosity, which may block the pores on the surface of the substrate layer, thereby significantly deteriorating the air permeability of the composite diaphragm and the cycle characteristics of the battery. Comparing the performance test results corresponding to Example 1 and Comparative Example 4, it can be found that the thickness consistency ΔD and thermal shrinkage rate of the composite diaphragm prepared in Comparative Example 4 are higher than those in Example 1, and the high-temperature cycle capacity retention rate of the battery is lower than that in Example 1. The reason is that the D of the inorganic particles A used in Comparative Example 4 is 50 If the diameter is too large, the pores of the porous coating of the composite diaphragm will be too large and too numerous, which is not conducive to the inorganic particles A to play a reinforcing effect on the substrate layer, thereby deteriorating the surface flatness, thermal stability and cycle characteristics of the composite diaphragm.

[0087] The performance test results of Example 1 and Examples 2 to 4 are compared. As shown in Table 1, under the same conditions of other materials and operations for preparing the battery, the average sphericity of the non-binder polymer C used in Example 2 is less than 0.7, and the thickness consistency ΔD and thermal shrinkage rate of the composite diaphragm obtained are slightly higher than those of Examples 1, 3 to 4, and the high temperature cycle capacity retention rate of the battery is slightly lower than that of Examples 1, 3 to 4. This shows that, relative to Example 2, Examples 1, 3 to 4 further reasonably set the average sphericity of the non-binder polymer C, so that the non-binder polymer C is easier to slide between the pores of the inorganic particles A, so that the non-binder polymer C is firmly embedded in the base coating, and it is beneficial to further make the inorganic particles A and the non-binder polymer C uniformly and tightly dispersed in the coating slurry, thereby effectively improving the density and uniformity of the porous active coating, and making the convex structure formed by the non-binder polymer C more firm and stable, so that the thickness flatness and thermal stability of the composite diaphragm are optimized, and the cycle performance of the obtained battery is better.

[0088] The performance test results of Example 1 are compared with those of Examples 5 to 8. As shown in Table 1, under the same conditions of other materials and operations for preparing the battery, the viscosity of the coating slurry in Example 5 during the preparation of the composite diaphragm is lower than 30 mPa·s, and the thickness consistency ΔD, thermal shrinkage rate, and air permeability of the composite diaphragm obtained are slightly higher than those of Example 1; the viscosity of the coating slurry in Example 8 during the preparation of the composite diaphragm is higher than 500 mPa·s, and the thickness consistency ΔD and thermal shrinkage rate of the composite diaphragm obtained are slightly higher than those of Example 1, and the high-temperature cycle capacity retention rate of the battery is slightly lower than that of Example 1. This shows that, compared with Examples 5 and 8, Examples 1, 6 to 7 control the viscosity of the coating slurry within an appropriate range, so that the coating slurry is easier to spread evenly and smoothly on the surface of the substrate layer, so that the flowability and spreading effect of the coating slurry are further optimized, and the sedimentation of the coating slurry is avoided, so that the thickness flatness, thermal stability, and air permeability of the composite diaphragm are optimized, and the cycle performance of the obtained battery is better.

[0089] The performance test results of Example 1 and Examples 9-10 are compared. As shown in Table 1, under the same conditions of other materials and operations for preparing the battery, the solid content of the coating slurry in Example 9 during the preparation of the composite diaphragm is less than 20%, and the thickness consistency ΔD and thermal shrinkage of the composite diaphragm obtained are slightly higher than those of Example 1, and the high temperature cycle capacity retention rate of the battery is slightly lower than that of Example 1; the solid content of the coating slurry in Example 10 during the preparation of the composite diaphragm is higher than 40%, and the thickness consistency ΔD, thermal shrinkage, and air permeability of the composite diaphragm obtained are slightly higher than those of Example 1, and the high temperature cycle capacity retention rate of the battery is slightly lower than that of Example 1. It can be seen that, relative to Example 1, Examples 9-10 are beneficial to improving the dispersion effect of solid particles (inorganic particles A, non-binder polymer C) in the coating slurry and the flowability of the coating slurry by controlling the solid content of the coating slurry, so that the surface of the substrate layer forms a porous active coating with uniform thickness and appropriate porosity, thereby optimizing the thickness flatness, thermal stability, and air permeability of the composite diaphragm, and making the cycle performance of the obtained battery better.

[0090] The above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit the protection scope of the present invention. Although the present invention is described in detail with reference to the above embodiments, ordinary technicians in the relevant field should understand that the technical solutions of the present invention can be modified or equivalently replaced, but these modifications or replacements are all within the protection scope of the present invention.

Claims

1. A method for preparing a composite diaphragm, characterized in that: The following operations are included: The inorganic particles A, the binder polymer B and the non-binder polymer C are mixed to prepare a coating slurry, the coating slurry is applied on the surface of the substrate layer, and the coating slurry is sequentially subjected to a baking treatment and a drying treatment, thereby preparing a composite diaphragm; Wherein, the D of the inorganic particles A 50 is 0.1 to 2.5 μm, and the D of the non-binder polymer C 50 The average sphericity of the non-binder polymer C is ≥ 0.4, and the viscosity of the coating slurry is 20 to 1000 mPa·s; During the coating process, the coating temperature is 15 to 35° C. and the coating speed is 10 to 150 m / min. The baking temperature of the baking treatment is 40 to 80°C; The dew point temperature of the drying process is -20 to -80°C, and the drying time of the drying process is 12 to 24 hours.

2. The method for preparing the composite diaphragm according to claim 1, characterized in that: The average sphericity of the non-binder polymer C is ≥ 0.

7.

3. The method for preparing the composite diaphragm according to claim 1, characterized in that: The viscosity of the coating slurry is 30-500 mPa·s.

4. The method for preparing the composite diaphragm according to claim 1, characterized in that: The solid content of the coating slurry is 20-40%.

5. A composite diaphragm, characterized in that: The composite diaphragm is prepared by the preparation method of any one of claims 1 to 4.

6. The composite diaphragm according to claim 5, characterized in that: The composite diaphragm comprises a substrate layer and a porous coating, wherein the porous coating is arranged on at least one surface of the substrate layer, the porous coating comprises a base coating and a non-adhesive polymer C embedded in the base coating, and the base coating has a thickness of 0.5 to 6 μm.

7. A lithium battery, characterized in that: Comprising the composite diaphragm as claimed in claim 5 or 6.

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