Composite separator, method for manufacturing the same, and secondary battery

Abstract

The application belongs to the technical field of secondary batteries, and particularly relates to a composite diaphragm, which comprises a base film and a conductive adsorption layer arranged on at least one side surface of the base film, and the conductive adsorption layer is a hollow Zn-Fe double hydroxide layer. The composite diaphragm has the hollow Zn-Fe double hydroxide layer, can adsorb polysulfide, inhibit polysulfide shuttle effect, improve utilization of active material, and improve electrochemical performance and capacity retention performance.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of secondary batteries, and particularly relates to a composite diaphragm, a preparation method thereof and a secondary battery. BACKGROUND

[0002] Lithium-sulfur batteries have the advantages of high theoretical energy density, low cost and environmental friendliness, and have attracted extensive attention and research from the scientific community and the industry.

[0003] However, the lithium-sulfur battery still has problems such as low active material utilization, poor electrochemical reversibility and rapid capacity decay. SUMMARY

[0004] One of the purposes of the application is to provide a composite diaphragm with a hollow Zn-Fe double hydroxide layer that can adsorb polysulfides, inhibit polysulfide shuttle effect, improve the utilization rate of active materials, and improve the electrochemical performance and capacity retention performance, in view of the deficiencies of the prior art.

[0005] In order to achieve the above purpose, the application adopts the following technical scheme:

[0006] A composite diaphragm comprises a base film and a conductive adsorption layer arranged on at least one side surface of the base film, and the conductive adsorption layer is a hollow Zn-Fe double hydroxide layer.

[0007] Preferably, the composite diaphragm further comprises a liquid absorbing and retaining layer, which is arranged on the side surface of the base film away from the conductive adsorption layer, or the liquid absorbing and retaining layer is arranged on the side surface of the conductive adsorption layer away from the base film.

[0008] Preferably, the liquid absorbing and retaining layer is a polyethylene oxide-cellulose layer.

[0009] The second purpose of the application is to provide a preparation method of a composite diaphragm, which has good controllability and can be mass-produced, in view of the deficiencies of the prior art.

[0010] In order to achieve the above purpose, the application adopts the following technical scheme:

[0011] A preparation method of a composite diaphragm comprises the following steps:

[0012] Step S1: adding an iron source into a solvent, adding an acid to stir and dissolve, heating, centrifuging and cleaning to obtain an iron source powder;

[0013] Step S2: adding a zinc source and urea into a solvent to stir to obtain a first solution;

[0014] Step S3: dissolving the iron source powder in ethanol to obtain a second solution, mixing the first solution with the second solution, pressurizing and heating to react, and centrifuging and cleaning to obtain a mixed powder.

[0015] Step S4, heating the mixed powder to obtain Zn-Fe double hydroxide powder;

[0016] Step S5, adding the binder and the Zn-Fe double hydroxide powder into the oily solvent, and stirring and grinding to obtain a first slurry;

[0017] Step S6, coating the first slurry to at least one surface of the base film, and heating and drying to form a hollow Zn-Fe double hydroxide layer, thereby obtaining the composite separator.

[0018] Preferably, the method for preparing the composite separator further comprises the following steps:

[0019] Step S7, ball-milling the cellulose in a solvent to obtain a first liquid;

[0020] Step S8, stirring the polyethylene oxide in a solvent to obtain a second liquid;

[0021] Step S9, mixing the first liquid and the second liquid, and adding a solvent to obtain a second slurry;

[0022] Step S10, coating the second slurry to a side of the base film away from the conductive adsorption layer, or to a side of the conductive adsorption layer away from the base film, and heating and drying to form a liquid-absorbing and liquid-retaining layer, thereby obtaining the composite separator.

[0023] Preferably, in the step S1, the weight ratio of the iron source to the acid is 0.1-1:0.2-0.6, the heating temperature is 80-120℃, and the heating time is 2-5 hours.

[0024] Preferably, in the step S3, the weight ratio of the first solution to the second solution is 1-3:1-2, the heating temperature is 80-120℃, and the heating time is 10-15 hours.

[0025] Preferably, in the step S4, the heating temperature is 400-600℃, and the heating time is 1-5 hours.

[0026] Preferably, in the step S5, the weight ratio of the binder, the Zn-Fe double hydroxide powder and the oily solvent is 0.05-0.5:0.5-2:1-5.

[0027] The third object of the present application is to provide a secondary battery with good electrochemical performance and cycle stability.

[0028] In order to achieve the above objects, the present application adopts the following technical solutions:

[0029] The secondary battery comprises the composite separator.

[0030] Compared with the prior art, the beneficial effects of the present application are that the composite diaphragm has a hollow Zn-Fe double hydroxide layer, can adsorb polysulfide compounds, inhibit polysulfide shuttle effect, improve the utilization rate of active substances, and improve the electrochemical performance and capacity retention performance. BRIEF DESCRIPTION OF DRAWINGS

[0031] Figure 1 is an SEM image of the Zn-Fe layered double hydroxide of the present application.

[0032] Figure 2 is an effect comparison chart of the Zn-Fe layered double hydroxide of the present application adsorbing polysulfide.

[0033] Figure 3 is a cycle stability comparison chart of the battery prepared by the composite diaphragm of the present application and the battery prepared by the comparative example 1.

[0034] Figure 4 is a preparation flow chart of the Zn-Fe layered double hydroxide of the present application. DETAILED DESCRIPTION

[0035] The present application will be described in further detail below with specific embodiments and the accompanying drawings of the specification, but the embodiments of the present application are not limited thereto.

[0036] The composite diaphragm of the present application comprises a base film and a conductive adsorption layer provided on at least one side surface of the base film, and the conductive adsorption layer is a hollow Zn-Fe double hydroxide layer.

[0037] The composite diaphragm has a hollow Zn-Fe double hydroxide layer, can adsorb polysulfide compounds, such as Figure 4 As shown in the figure, after the Zn-Fe layered double hydroxide is put into a yellowish polysulfide solution, the color of the solution disappears, and a colorless and clear liquid appears, as shown in the figure Figure 2 , which shows that the Zn-Fe layered double hydroxide of the present application has good adsorption of polysulfide compounds. Therefore, when the composite diaphragm is assembled with the positive electrode sheet into a battery cell, the composite diaphragm can adsorb polysulfide compounds, avoid the accumulation of polysulfide compounds in the negative electrode, inhibit the polysulfide shuttle effect, improve the utilization rate of active substances, and improve the electrochemical performance and capacity retention performance. The Zn-Fe layered double hydroxide of the present application has a hollow structure, can accommodate and store polysulfide, and the metal oxide can further utilize the stored oxide, thereby improving the cycle stability of the lithium-sulfur battery.

[0038] In some embodiments, the composite separator further comprises a liquid-absorbing and liquid-retaining layer, which is arranged on the side of the base film away from the conductive adsorption layer, or arranged on the side of the conductive adsorption layer away from the base film. The liquid-absorbing and liquid-retaining layer can increase the liquid-absorbing rate of the composite separator, so that the composite separator has better conductivity and electrochemical performance, and the cycle performance of the composite separator is improved. When the conductive adsorption layer is arranged on only one side of the base film, the liquid-absorbing and liquid-retaining layer can be arranged on the side of the base film away from the conductive adsorption layer, or arranged on the side of the conductive adsorption layer away from the base film.

[0039] In some embodiments, the liquid-absorbing and liquid-retaining layer is a polyethylene oxide-cellulose layer. The wood cellulose has abundant carboxyl and hydroxyl groups, and the hydroxyl groups in the PEO can be crosslinked with the carboxyl groups in the cellulose. The crosslinked product has properties different from those of any of the original substances, including crystallinity, thermal stability, and the morphology of the precipitate. The crosslinked product can improve the liquid-retaining performance of the separator and provide cycle stability.

[0040] The preparation method of the composite separator has good controllability and can be mass-produced.

[0041] A preparation method of a composite separator comprises the following steps:

[0042] In step S1, an iron source is added to a solvent, an acid is added and stirred to dissolve, heated, centrifuged, and washed to obtain an iron source powder;

[0043] In step S2, a zinc source and urea are added to a solvent, and stirred to obtain a first solution;

[0044] In step S3, the iron source powder is dissolved in ethanol to obtain a second solution, the first solution and the second solution are mixed, and pressure heating reaction is performed, and the mixture is centrifuged and washed to obtain a mixed powder;

[0045] In step S4, the mixed powder is heated to obtain a Zn-Fe double hydroxide powder;

[0046] In step S5, a binder and the Zn-Fe double hydroxide powder are added to an oily solvent, and stirred and ground to obtain a first slurry;

[0047] In step S6, the first slurry is coated on at least one surface of the base film, and heated and dried to form a hollow Zn-Fe double hydroxide layer, to obtain a composite separator.

[0048] The hollow Zn-Fe layered double hydroxide is prepared by using a hydrothermal method and high-temperature carbonization, the Zn-Fe layered double hydroxide prepared has a more stable structure, a larger hollow structure, can accommodate and store more polysulfides, and the metal oxide can be further utilized to store the oxide, thereby improving the cycle stability of the lithium-sulfur battery. In step S1, the solvent is deionized water, the acid is fumaric acid, and the stirring time is 0.5 hours. In step S2, the solvent is deionized water.

[0049] Preferably, the iron source is selected as ferric chloride, which is a covalent inorganic compound with a chemical formula of FeCl3. It is a black-brown crystal, also in the form of a flake, has a melting point of 306 DEG C and a boiling point of 316 DEG C, is easily soluble in water and has strong water absorption, can absorb moisture in the air and deliquesce. When FeCl3 is precipitated from an aqueous solution, it has six crystal waters as FeCl3·6H2O, and the hexahydrate ferric chloride is an orange-yellow crystal. Preferably, the zinc source is selected as zinc acetate dihydrate, which is a white monoclinic flaky crystal, has a pearl luster, has a slight acetic acid taste, and loses crystal water at 100 DEG C.

[0050] Preferably, the preparation method of the composite diaphragm further comprises the following steps:

[0051] Step S7, cellulose is added to a solvent for ball milling to obtain a first liquid;

[0052] Step S8, polyethylene oxide is added to a solvent for stirring to obtain a second liquid;

[0053] Step S9, the first liquid and the second liquid are mixed, and a solvent is added for mixing to obtain a second slurry;

[0054] Step S10, the second slurry is coated to a side of the base film away from the conductive adsorption layer, or to a side of the conductive adsorption layer away from the base film, and is heated and dried to form a liquid-absorbing and liquid-storing layer, thereby obtaining a composite diaphragm.

[0055] The application uses the mixed slurry prepared by mixing the ball-milled cellulose and the ball-milled polyethylene oxide, the ball-milled cellulose has abundant groups and can react with the polyethylene oxide to obtain a liquid-liquid absorbing layer with better performance. Preferably, the cellulose is selected from lignocellulose, which is a natural renewable wood organic fiber material obtained by chemical treatment and mechanical processing, and is non-toxic, odorless, pollution-free and non-radioactive. The lignocellulose includes coniferous wood fiber, broad-leaved wood fiber and grass wood fiber. The coniferous wood fiber has long fibers, compact structure and less miscellaneous cells, and the miscellaneous cells in the chemical pulp are lost during washing, so the pulp quality is good and the formed paper has strong mechanical properties. Under the electron microscope, the coniferous wood fiber can be clearly observed to be arranged densely and compactly, and the ball-milled coniferous wood fiber is of different lengths and is intertwined with each other, and the thinnest fiber is only about several tens of nanometers. The lignocellulose (BWF) has strong film-forming property and is rich in carboxyl and hydroxyl groups. The polyethylene oxide (PEO) is also called polyethylene oxide, which is a crystalline and thermoplastic polymer. The molecular weight of the industrial product can vary in a large range. The product with a relative molecular mass of 200-20000 is called polyethylene glycol (PPG), which is a viscous liquid or a waxy solid. The main functional group is ether bond, and there is a clear characteristic peak at 1100 cm -1 The solvent in step S7 is deionized water, the ball-to-material ratio of ball milling is 50:1, and the rotation speed is 1000-2000 rpm / min. The solvent in step S8 is deionized water, and the concentration of the second liquid is 10-30 mg / ml. The solvent in step S9 is deionized water, and the amount added is 10-50 ml.

[0056] In some embodiments, the weight ratio of the iron source to the acid in step S1 is 0.1-1:0.2-0.6, the heating temperature is 80-120℃, and the heating time is 2-5 hours. The weight ratio of the iron source to the acid is 0.1-0.5:0.2-0.6, 0.3-0.6:0.2-0.6, 0.6-1:0.2-0.6, 0.1-0.5:0.2-0.4, 0.1-0.5:0.4-0.6, specifically, the weight ratio of the iron source to the acid is 0.1:0.2, 0.2:0.3, 0.1:0.2, 0.4:0.3, 0.5:0.2, 0.5:0.6, 0.5:0.3, 0.5:0.25, 0.8:0.6, 0.9:0.6. The heating temperature is 80℃, 85℃, 90℃, 95℃, 100℃, 105℃, 110℃, 115℃, 120℃, and the heating time is 2 hours, 3 hours, 4 hours, 5 hours.

[0057] In some embodiments, the weight ratio of the first solution and the second solution in step S3 is 1-3:1-2, the heating temperature is 80-120℃, and the heating time is 10-15 hours. The weight ratio of the first solution and the second solution is 1:1, 1:2, 1:1.5, 2:1, 3:1, the heating temperature is 80℃, 90℃, 100℃, 110℃, 120℃, and the heating time is 10 hours, 11 hours, 12 hours, 13 hours, 14 hours, 15 hours.

[0058] In some embodiments, the heating temperature in step S4 is 400-600℃, and the heating time is 1-5 hours. The heating temperature is 400℃, 430℃, 450℃, 470℃, 490℃, 500℃, 530℃, 550℃, 580℃, 600℃, and the heating time is 1 hour, 2 hours, 3 hours, 4 hours, 5 hours.

[0059] In some embodiments, the weight ratio of the binder, the Zn-Fe double hydroxide powder, and the oily solvent in step S5 is 0.05-0.5:0.5-2:1-5. The weight ratio of the binder, the Zn-Fe double hydroxide powder, and the oily solvent is 0.07:0.5:1, 0.09:0.6:1, 0.1:0.7:3, 0.1:0.8:4, 0.2:0.8:3, 0.4:0.7:5, 0.4:0.9:5, 0.5:0.9:5, 0.5:1:5, 0.4:1.5:5, 0.4:2:5.

[0060] A secondary battery comprising the composite separator described above. The secondary battery of the present application has good electrochemical performance and cycle stability. The secondary battery can be a lithium ion battery, a sodium ion battery, a magnesium ion battery, a calcium ion battery, a potassium ion battery, etc. Preferably, the following secondary battery takes the lithium ion battery as an example, the lithium ion battery comprises a positive electrode sheet, a negative electrode sheet, a separator, an electrolyte, and a shell, the separator separates the positive electrode sheet and the negative electrode sheet, and the shell is used to accommodate the positive electrode sheet, the negative electrode sheet, the separator, and the electrolyte. The separator is the composite separator described above.

[0061] Positive electrode

[0062] The positive electrode sheet comprises a positive electrode current collector and a positive electrode active material layer arranged on at least one surface of the positive electrode current collector, and the positive electrode active material layer comprises a positive electrode active material. The positive electrode active material can be, but is not limited to, a material with a chemical formula such as Li a Ni x Co y M z O 2-b N bLi2MnO4, LiCoO2, LiNiO2, LiVO2, LiCrO2, LiMn2O4, LiCoMnO4, Li2NiMn3O8, LiNi 0.5 Mn 1.5 Li2MnO4, LiCoO2, LiNiO2, LiVO2, LiCrO2, LiMn2O4, LiCoMnO4, Li2NiMn3O8, LiNi

[0063] Negative electrode

[0064] The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer disposed on the surface of the negative electrode current collector, and the negative electrode active material layer includes a negative electrode active material, which can be one or more of, but not limited to, graphite, soft carbon, hard carbon, carbon fiber, mesocarbon microbead, silicon-based material, tin-based material, lithium titanate, or other metal capable of forming an alloy with lithium. The graphite can be selected from one or more of artificial graphite, natural graphite, and modified graphite; the silicon-based material can be selected from one or more of elemental silicon, silicon oxide compound, silicon-carbon composite, and silicon alloy; and the tin-based material can be selected from one or more of elemental tin, tin oxide compound, and tin alloy. The negative electrode current collector is generally a structure or part that collects current, and the negative electrode current collector can be any material suitable for use as a negative electrode current collector in a lithium ion battery, for example, the negative electrode current collector can be, but not limited to, a metal foil, and more specifically, can be, but not limited to, a copper foil.

[0065] Electrolyte

[0066] The lithium ion battery further comprises an electrolyte, the electrolyte comprising an organic solvent, an electrolyte lithium salt and an additive. The electrolyte lithium salt can be LiPF6 and / or LiBOB used in high-temperature electrolyte, at least one of LiBF4, LiBOB and LiPF6 used in low-temperature electrolyte, at least one of LiBF4, LiBOB, LiPF6 and LiTFSI used in overcharge-preventing electrolyte, and at least one of LiClO4, LiAsF6, LiCF3SO3 and LiN(CF3SO2)2. The organic solvent can be cyclic carbonate including PC and EC, chain carbonate including DFC, DMC or EMC, and carboxylic acid ester including MF, MA, EA and MP. The additive includes, but is not limited to, at least one of film-forming additive, conductive additive, flame-retardant additive, overcharge-preventing additive, additive for controlling H2O and HF content in electrolyte, additive for improving low-temperature performance, and multifunctional additive.

[0067] Preferably, the material of the shell is one of stainless steel, aluminum plate and aluminum plastic film.

[0068] Embodiment 1

[0069] A preparation method of a composite diaphragm, comprising the following steps:

[0070] In step S1, 0.648 g of iron chloride hexahydrate and 0.464 g of fumaric acid are weighed and added into a beaker containing 15 ml of water, and stirred for 0.5 h to fully dissolve; the above solution is placed in a reaction kettle at 100 ℃ for 3 h, and then centrifuged and washed to prepare powder A, i.e. iron source powder;

[0071] In step S2, 80 mg of powder zinc acetate and 10 mg of urea are weighed and added into a beaker to dissolve in 25 ml of deionized water, and stirred to obtain solution B, i.e. first solution;

[0072] In step S3, 40 mg of powder A (iron source powder) is weighed and added into a beaker to dissolve in 10 ml of ethanol, and stirred to obtain solution A, i.e. second solution; solution A (second solution) and solution B (first solution) are mixed in a high-pressure reaction kettle at 90 ℃ for 12 h; the reacted material in step 2 is centrifuged and washed three times by a high-speed centrifuge to prepare powder B, i.e. mixed powder;

[0073] In step S4, powder B is placed in a high-temperature furnace at 450 ℃ for 2 h to prepare powder C, i.e. hollow Zn-Fe layered double hydroxide powder, as shown in FIG. 1; Figure 1

[0074] ​Step S5, weigh 0.1 g of PVDF powder, add 2 ml, 3 g of NMP, grind uniformly, then weigh 0.9 g of powder C into the ground slurry, prepare slurry A, i.e. the first slurry;

[0075] Step S6, evenly lay the cut 20 μm PP base film on a glass plate, evenly coat the slurry A on the base film with a 75 μm spatula, transfer the glass plate with the coated separator to a 60°C hot plate, bake for 3 h to prepare a ~ 25 μm separator A, then transfer to a 45°C vacuum oven for storage;

[0076] Step S7, first weigh 2 g of BWF and add 100 mL of deionized water in a ball mill jar, ball mill at a speed of 1032 r / min for 1 h at a ball-to-material ratio of 50:1, then collect and reserve, which is referred to as A liquid, i.e. the first liquid;

[0077] Step S8, weigh a certain amount of PEO and add deionized water to prepare a 20 mg / mL PEO solution, which is referred to as B liquid, i.e. the second liquid;

[0078] Step S9, take another 100 mL beaker, take 20 mL of A liquid (first liquid) with a 20 mL syringe, take 20 mL of B liquid (second liquid) with a 20 mL syringe, mix A liquid (first liquid) and B liquid (second liquid) at a weight ratio of 3:2, add 20 mL of deionized water, mix and stir for half an hour to prepare slurry B, i.e. the second slurry;

[0079] Step S10, coat the second slurry to the side of the base film away from the conductive adsorption layer, heat and dry to form a liquid absorbing and retaining layer, to obtain a composite separator.

[0080] Preparation of positive electrode sheet:

[0081] Mix lithium cobaltate, conductive agent super carbon (Super-P) and binder polyvinylidene fluoride (PVDF) at a mass ratio of 97:1.5:1.5 to prepare a lithium ion battery positive electrode slurry with a certain viscosity, coat the slurry on the current collector aluminum foil, dry at 85°C, then cold press; then edge cutting, sheet cutting, striping, after striping, dry at 110°C for 4 hours under vacuum condition, weld the tabs, to prepare the positive electrode sheet.

[0082] Preparation of negative electrode sheet:

[0083] The graphite, conductive agent Super-P, thickening agent sodium carboxymethyl cellulose (CMC), and binder styrene-butadiene rubber (SBR) are mixed in a mass ratio of 96:2.0:1.0:1.0 to form a slurry, which is coated on a current collector copper foil and dried at 85°C, followed by edge cutting, piece cutting, and striping. After striping, the strips are dried at 110°C for 4 hours under vacuum, and the tabs are welded to form a negative electrode sheet.

[0084] Preparation of electrolyte:

[0085] Lithium hexafluorophosphate (LiPF6) is dissolved in a mixed solvent composed of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) (mass ratio of 1:2:1) to obtain an electrolyte with a concentration of 1 mol / L.

[0086] Preparation of lithium ion battery:

[0087] The above positive electrode sheet, composite separator, and negative electrode sheet are wound into an electric core, with the composite separator between the positive electrode sheet and the negative electrode sheet. The positive electrode is welded with an aluminum tab for lead-out, and the negative electrode is welded with a nickel tab for lead-out. Then, the electric core is placed in an aluminum plastic packaging bag, and the above electrolyte is injected. After packaging, formation, capacity, and other processes, a lithium ion battery is obtained.

[0088] Example 2

[0089] The difference from Example 1 is that the weight ratio of the binder, Zn-Fe double hydroxide powder, and oily solvent in step S5 is 0.05:0.5:1.

[0090] The rest is the same as Example 1, which is not repeated here.

[0091] Example 3

[0092] The difference from Example 1 is that the weight ratio of the binder, Zn-Fe double hydroxide powder, and oily solvent in step S5 is 0.08:0.7:2.

[0093] The rest is the same as Example 1, which is not repeated here.

[0094] Example 4

[0095] The difference from Example 1 is that the weight ratio of the binder, Zn-Fe double hydroxide powder, and oily solvent in step S5 is 0.1:0.9:3.

[0096] The rest is the same as Example 1, which is not repeated here.

[0097] Example 5

[0098] The difference from Example 1 is that the weight ratio of the binder, Zn-Fe double hydroxide powder and oily solvent in the step S5 is 0.4:1.5:4.

[0099] The rest is the same as Example 1, which will not be repeated here.

[0100] Example 6

[0101] The difference from Example 1 is that the weight ratio of the first solution and the second solution in the step S3 is 1:1.

[0102] The rest is the same as Example 1, which will not be repeated here.

[0103] Example 7

[0104] The difference from Example 1 is that the weight ratio of the first solution and the second solution in the step S3 is 1:2.

[0105] The rest is the same as Example 1, which will not be repeated here.

[0106] Example 8

[0107] The difference from Example 1 is that the weight ratio of the first solution and the second solution in the step S3 is 1:1.5.

[0108] The rest is the same as Example 1, which will not be repeated here.

[0109] Example 9

[0110] The difference from Example 1 is that the weight ratio of the first solution and the second solution in the step S3 is 2:1.

[0111] The rest is the same as Example 1, which will not be repeated here.

[0112] Example 10

[0113] The difference from Example 1 is that the weight ratio of the first solution and the second solution in the step S3 is 3:1.

[0114] The rest is the same as Example 1, which will not be repeated here.

[0115] Comparative Example 1

[0116] The difference from Example 1 is that a conventional PP separator is used.

[0117] The rest is the same as Example 1, which will not be repeated here.

[0118] Comparative Example 2

[0119] The difference from Example 1 is that a composite separator includes a base film and a liquid-absorbing and liquid-retaining layer disposed on one side surface of the base film, and the liquid-absorbing and liquid-retaining layer is a polyethylene oxide-cellulose layer.

[0120] The rest is the same as Example 1, which is not described here.

[0121] Performance test: the above prepared separator is applied to a secondary battery for performance test, and the test results are recorded in Table 1. At the same time, the separators of Example 1 and Comparative Example 1 are soaked for 1 hour to test the liquid absorption rate, which is recorded in Table 2, and the liquid retention rate is tested for 1 hour, 2 hours, 4 hours, 8 hours, 24 hours and 48 hours, which is recorded in Table 3.

[0122] Capacity retention rate test: at 25℃, the lithium ion secondary battery is charged at 1C constant current to 4.25V, then charged at 4.25V constant voltage to the current of 0.05C, and then discharged at 1C constant current to 2.8V, which is a charge and discharge cycle process, and the discharge capacity of this time is the discharge capacity of the first cycle. The lithium ion secondary battery is tested by the above method for 400 cycles of charge and discharge, and the discharge capacity of each cycle is recorded. Cycle capacity retention rate (%) = discharge capacity of the 400th cycle / discharge capacity of the first cycle x 100%.

[0123] Table 1

[0124] Item Capacity retention rate (%) Item Capacity retention rate (%) Example 1 89 Example 2 87 Example 3 86 Example 4 86 Example 5 86 Example 6 85 Example 7 87 Example 8 86 Example 9 86 Example 10 87 Comparative Example 1 73 Comparative Example 2 76

[0125] From the above Table 1, it can be concluded that the prepared separator of the application has better performance than Comparative Examples 1 and 2 when applied to a secondary battery, and has better capacity retention rate.

[0126] From the comparison of Examples 1-5, it can be concluded that when the weight fraction ratio of the binder, Zn-Fe double hydroxide powder and oily solvent in step S5 is 0.1:0.9:3, the prepared separator has better performance, and the appropriate binder and oily solvent make the prepared conductive adsorption layer have good conductive performance and good bonding performance.

[0127] From the comparison of Examples 1, 6-10, it can be concluded that when the weight fraction ratio of the first solution and the second solution in step S3 is 3:2, the prepared separator has better performance, and the appropriate weight fraction ratio of the wood cellulose solution and the polyethylene oxide solution can obtain a better liquid-absorbing and liquid-retaining layer.

[0128] Table 2

[0129] Sample As-received mass (mg) After 1 h of absorption Absorption rate PP 4.0 6.7 69％ Modified composite separator 8.0 30.5 181.2％

[0130] From the above Table 2, it can be seen that the separator of Example 1 of the present application has a better liquid absorption rate than the separator of Comparative Example 1, and the liquid absorption rate is as high as 181.2%, which is nearly three times higher than the liquid absorption rate of 69% of Comparative Example 1. In combination with Figure 3 After 297 charge-discharge cycles at 1C rate, the composite separator of the present application still has a capacity retention rate of 83%, while the separator of Comparative Example 1 only has a capacity retention rate of 56% after 297 charge-discharge cycles at 1C rate, and has a large performance decay.

[0131] Table 3

[0132] Sample 1h 2h 4h 8h 24h 48h PP 5％ 5％ 2.5％ 2.5％ 2.5％ 2.5％ Modified composite separator 175.8％ 166.0％ 152％ 133％ 99.2％ 96.7％

[0133] From the above Table 3, it can be seen that the separator of Example 1 of the present application has a better liquid retention rate than the separator of Comparative Example 1, and still has a retention rate of 96.7% after 48 hours, while the liquid retention rate of the separator of Comparative Example 1 is only 2.5%, and the separator of Comparative Example 1 is prone to insufficient electrolyte infiltration, capacity decline and cycle performance decline after multiple charge-discharge cycles.

[0134] According to the disclosure and teaching of the above description, those skilled in the art of the present application can also make changes and modifications to the above embodiments. Therefore, the present application is not limited to the above specific embodiments, and any obvious improvements, replacements or modifications made by those skilled in the art on the basis of the present application all fall within the protection scope of the present application. In addition, although some specific terms are used in the present specification, these terms are only for convenience of explanation and do not constitute any limitation on the present application.

Claims

1. A method for preparing a composite diaphragm, characterized in that, Includes the following steps: Step S1: Add the iron source to the solvent, add acid and stir to dissolve, heat, centrifuge, and wash to obtain iron source powder; Step S2: Add zinc source and urea to solvent and stir to obtain the first solution; Step S3: Dissolve the iron source powder in ethanol to obtain a second solution, mix the first solution and the second solution, pressurize and heat to react, and centrifuge and wash to obtain a mixed powder; Step S4: Heat the mixed powder to react and obtain Zn-Fe double hydroxide powder. The heating temperature is 400~600℃ and the heating time is 1~5 hours. Step S5: Add the binder and Zn-Fe double hydroxide powder to an oily solvent, stir and grind to obtain the first slurry; Step S6: Coat the first slurry onto at least one surface of the base membrane, heat and dry to form a hollow Zn-Fe double hydroxide layer, and obtain a composite membrane.

2. The method for preparing the composite diaphragm as described in claim 1, characterized in that, The method for preparing the composite diaphragm further includes the following steps: Step S7: Add cellulose to a solvent and ball mill to obtain the first liquid; Step S8: Add polyethylene oxide to the solvent and stir to obtain the second liquid; Step S9: Take the first liquid and the second liquid, mix them, add solvent and mix to obtain the second slurry; Step S10: Coat the second slurry onto the side of the base film away from the conductive adsorption layer, or coat the conductive adsorption layer away from the base film, heat and dry to form a liquid-absorbing and liquid-retaining layer, and obtain a composite membrane.

3. The method for preparing the composite diaphragm as described in claim 1, characterized in that, In step S1, the weight ratio of iron source to acid is 0.1~1:0.2~0.6, the heating temperature is 80~120℃, and the heating time is 2~5 hours.

4. The method for preparing the composite diaphragm as described in claim 1 or 3, characterized in that, In step S3, the weight ratio of the first solution to the second solution is 1~3:1~2, the heating temperature is 80~120℃, and the heating time is 10~15 hours.

5. The method for preparing the composite diaphragm as described in claim 1, characterized in that, In step S5, the weight ratio of the binder, Zn-Fe double hydroxide powder, and oily solvent is 0.05~0.5:0.5~2:1~5.

6. The method for preparing the composite diaphragm as described in claim 1, characterized in that, The prepared composite membrane includes a base membrane and a conductive adsorption layer disposed on at least one side surface of the base membrane, wherein the conductive adsorption layer is a hollow Zn-Fe double hydroxide layer.

7. The method for preparing the composite diaphragm as described in claim 6, characterized in that, The composite membrane further includes a liquid-absorbing and liquid-retaining layer, which is disposed on the side of the base membrane away from the conductive adsorption layer, or the liquid-absorbing and liquid-retaining layer is disposed on the side of the conductive adsorption layer away from the base membrane.

8. The method for preparing the composite diaphragm as described in claim 7, characterized in that, The liquid-absorbing and liquid-retaining layer is a polyethylene oxide-cellulose layer.

9. A secondary battery, characterized in that, Includes the composite membrane described in any one of claims 6 to 8.

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