A high porosity wet separator and method of making same

Abstract

The application discloses the technical field of battery diaphragm and relates to a high-porosity wet diaphragm and a preparation method thereof. The high-porosity wet diaphragm is composed of a porous base film and a diaphragm coating layer, and the porous base film is prepared from the following components: polyolefin powder, glycidyl methacrylate, an initiator, a diluent and an extractant. The application introduces an epoxy group by modifying the polyolefin, and then introduces an imidazole group, realizes in-situ polymerization of an organic metal framework material on the surface of the base film, generates the diaphragm coating layer covalently combined with the base film, and thus improves the porosity and thermal stability of the diaphragm, and further improves the cycle performance of the diaphragm.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of battery separator, and particularly relates to a high-porosity wet-process separator and a preparation method thereof. BACKGROUND

[0002] The common range of the core performance index porosity of a lithium battery separator is between 30% and 55%, and the size directly determines the ability of the separator to retain electrolyte and the migration rate of lithium ions, which is crucial to the internal resistance, charge-discharge efficiency and cycle life of the battery; the microporous structure of the current mainstream wet-process polyethylene separator is derived from bidirectional stretching of the structure of the alternating crystalline region and amorphous region, but the process is limited by the crystallization behavior of the material itself, and the higher the molecular weight, the lower the crystallinity, but the molecular chain movement of the high-molecular-weight polyethylene is difficult, so that the porosity that can be created by stretching is difficult to break through the upper limit of 55% in the existing system, which seriously restricts its application in high-end batteries; the organic metal framework material becomes a front research direction due to its inherent, super-high and adjustable porosity, the organic metal framework material is formed by self-assembly of metal nodes and organic ligands, the highly regular nanoscale channels can not only store a large amount of electrolyte and realize extremely low ion transmission impedance, but also have excellent thermal stability and functionalizable channel surface, providing a highly potential solution for developing the next generation of high-performance and high-safety lithium battery separators.

[0003] The current prior art mainly has the following problems: 1. low porosity of the separator, 2. poor thermal stability of the separator, and 3. poor cycle performance of the separator. SUMMARY

[0004] In view of the above problems, in order to overcome the defects of the prior art, the application provides a high-porosity wet-process separator and a preparation method thereof, in order to solve the problems of low porosity and poor thermal stability of the separator, the application introduces an epoxy group by modifying the polyolefin, and then introduces an imidazole group, realizes in-situ polymerization of the organic metal framework material on the surface of the base film, generates a separator coating covalently combined with the base film, thereby improving the porosity and thermal stability of the separator, and further improving the cycle performance of the separator.

[0005] In order to achieve the above purpose, the technical scheme adopted by the application is as follows: the application provides a high-porosity wet-process separator, which is composed of a porous base film and a separator coating.

[0006] Preferably, the porous base film is prepared from the following components: polyolefin powder, glycidyl methacrylate, initiator, diluent and extractant.

[0007] Preferably, the polyolefin powder is polyethylene powder, the initiator is benzoyl peroxide, the diluent is paraffin oil, and the extractant is dichloromethane.

[0008] Preferably, the preparation method of the porous base film specifically comprises the following steps:

[0009] (1) adding polyolefin powder into a reactor, purging air with nitrogen, then mixing glycidyl methacrylate and initiator uniformly and dropping into the reactor, stirring uniformly, heating reaction to obtain modified polyolefin;

[0010] (2) mixing the modified polyolefin obtained in step (1) with diluent, melt blending, extruding into a sheet, cooling on a casting roll, then performing synchronous bidirectional stretching, extracting with an extractant to remove the diluent, secondary transverse stretching, winding, slitting to obtain a porous base film.

[0011] Preferably, in step (1), the mass ratio of polyolefin, glycidyl methacrylate and initiator is 100:8-10:0.3-0.5;

[0012] Preferably, in step (1), the heating reaction is performed at a temperature of 100-110℃ for 2.2-2.5h.

[0013] Preferably, in step (2), the mass ratio of modified polyolefin and diluent is 1:2-2.4; the extrusion temperature is 190-200℃;

[0014] Preferably, in step (2), the casting roll temperature control medium is water, the speed is 4.2-4.6m / min, the temperature is 25-28℃, and the casting thickness is 1.5mm;

[0015] Preferably, in step (2), the synchronous bidirectional stretching is performed at a temperature of 114-116℃ with a stretching ratio of 7.8-8.0 times;

[0016] Preferably, in step (2), the extraction time is 100-110s;

[0017] Preferably, in step (2), the secondary transverse stretching is performed at a temperature of 120-122℃ with a stretching ratio of 1.2-1.3 times.

[0018] Preferably, the separator coating is a metal-organic framework material covalently bonded to the base film;

[0019] Further, the separator coating is a ZIF-67 coating obtained by introducing imidazole groups on the base film through the reaction of epoxy groups and amino groups, thereby generating in situ on the surface of the base film and covalently bonding to the porous base film.

[0020] The application also provides a preparation method of a high-porosity wet separator, specifically comprising the following steps:

[0021] S1, immerse the porous base film into 1-(3-aminopropyl)imidazole methanol solution, water bath reaction, then take out, wash with methanol until neutral to obtain the modified base film;

[0022] S2, immerse the modified base film obtained in S1 into cobalt nitrate methanol solution, react for 20-30 min, then take out, immerse into 2-methyl imidazole methanol solution, react for 30-40 min as a cycle, repeat 3-4 times, wash and dry to obtain the high porosity wet diaphragm.

[0023] Preferably, in S1, the concentration of 1-(3-aminopropyl)imidazole methanol solution is 1-1.2 mol / L;

[0024] Preferably, in S1, the water bath reaction is at a temperature of 60-70℃ for 10-12 h.

[0025] Preferably, in S2, the concentration of cobalt nitrate methanol solution is 20-25 mmol / L, and the concentration of 2-methyl imidazole methanol solution is 20-25 mmol / L.

[0026] The beneficial effects obtained by the present application are as follows: the present application modifies polyethylene by glycidyl methacrylate, introduces epoxy groups into the modified polyethylene by solid phase grafting, and then prepares the porous base film by wet two-way stretching process, which has uniform material mixing, uniform micropore size distribution, and uniform distribution of epoxy groups on the surface; the imidazole groups are introduced on the surface of the base film by the reaction of the amino group of 1-(3-aminopropyl)imidazole with the epoxy group on the surface of the porous base film, the in-situ synthesis of the organic metal framework material is further carried out on the surface of the base film, and then the diaphragm coating covalently combined with the base film is obtained, and finally the high porosity wet diaphragm is obtained; the organic metal framework material has good heat resistance, and is covalently combined with the base film to enhance the interface stability of the diaphragm and improve the thermal stability of the diaphragm, while reducing the use of adhesive and the thickness of the coating; the microporous structure and high porosity help to adsorb electrolyte, promote the rapid transmission of lithium ions, reduce the side reaction with electrolyte, and further inhibit the growth of lithium dendrites, the anion adsorption effectively inhibits the migration of anions, reduces the side reaction, reduces the consumption of lithium, and significantly improves the cycle performance. BRIEF DESCRIPTION OF DRAWINGS

[0027] Figure 1 The results of the porosity test of the present application examples 1-3 and comparative examples 1-2 are shown in the figure;

[0028] Figure 2 The results of the thermal stability test of the present application examples 1-3 and comparative examples 1-2 are shown in the figure;

[0029] Figure 3 The results of the cycle test of the present application examples 1-3 and comparative examples 1-2 are shown in the figure.

[0030] The accompanying drawings are included to provide a further understanding of the application and are incorporated in and constitute a part of this specification, illustrate embodiments of the application and are intended to provide a further understanding of the application, and are made a part of the specification. The drawings are not to scale. DETAILED DESCRIPTION

[0031] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by a person of ordinary skill in the art without creative work fall within the protection scope of the present application.

[0032] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as is commonly understood by one of ordinary skill in the art. Any methods and materials similar or equivalent to those described herein can be used in the practice of the present application. The preferred methods and materials are presented by way of example only and not by way of limitation.

[0033] The experimental methods in the following embodiments are all conventional methods unless otherwise specified. The experimental materials used in the following embodiments are all purchased from commercial channels unless otherwise specified.

[0034] Embodiment 1

[0035] A high porosity wet separator is composed of a porous base film and a separator coating.

[0036] The porous base film is prepared from the following components: polyethylene powder, glycidyl methacrylate, benzoyl peroxide, paraffin oil and dichloromethane.

[0037] The method for preparing the porous base film specifically comprises the following steps:

[0038] (1) The polyethylene powder is added to a reactor, nitrogen is introduced to remove air, then the glycidyl methacrylate and benzoyl peroxide are mixed uniformly and added dropwise into the reactor, the mass ratio of the polyethylene powder, the glycidyl methacrylate and the benzoyl peroxide is 100:8:0.3, stirred uniformly, heated at 100°C for 2.2h to obtain the modified polyethylene;

[0039] (2) The modified polyethylene obtained in step (1) is mixed with paraffin oil at a mass ratio of 1:2, melt blended, extruded into a sheet, the extrusion temperature is 190℃, the cast roll is cooled, the cast roll temperature control medium is water, the speed is 4.2m / min, the temperature is 25℃, the cast thickness is 1.5mm, then synchronous bidirectional stretching is carried out, the temperature is 114℃, the stretching ratio is 7.8 times, dichloromethane is used for 100s extraction to remove paraffin oil, secondary transverse stretching is carried out, the temperature is 120℃, the stretching ratio is 1.2 times, winding, slitting, and a porous base film is obtained.

[0040] The application further provides a preparation method of the high-porosity wet-process diaphragm.

[0041] S1, immersing the porous base film into a 1-(3-aminopropyl)imidazole methanol solution with a concentration of 1mol / L, reacting for 10h in a 60℃ water bath, then taking out, washing to neutral with methanol, and obtaining a modified base film;

[0042] S2, immersing the modified base film obtained in S1 into a 20mmol / L cobalt nitrate methanol solution, reacting for 20min, then taking out, immersing into a 20mmol / L 2-methyl imidazole methanol solution, reacting for 30min, as one cycle, repeating for 3 times, washing and drying, and obtaining the high-porosity wet-process diaphragm.

[0043] Example 2

[0044] The high-porosity wet-process diaphragm is composed of a porous base film and a diaphragm coating.

[0045] The porous base film is prepared from the following components: polyethylene powder, glycidyl methacrylate, benzoyl peroxide, paraffin oil and dichloromethane.

[0046] The preparation method of the porous base film specifically comprises the following steps:

[0047] (1) polyethylene powder is added into a reactor, nitrogen is introduced to discharge air, then glycidyl methacrylate and benzoyl peroxide are uniformly mixed and added dropwise into the reactor, the mass ratio of polyethylene powder, glycidyl methacrylate and benzoyl peroxide is 100:9:0.4, stirring is uniformly carried out, heating is carried out at 105℃ for 2.4h, and modified polyethylene is obtained;

[0048] (2) The modified polyethylene obtained in step (1) is mixed with paraffin oil at a mass ratio of 1:2.2, melt-blended, extruded into a sheet, the extrusion temperature is 195℃, the cast roll is cooled, the cast roll temperature control medium is water, the speed is 4.4m / min, the temperature is 27℃, the cast thickness is 1.5mm, then synchronous bidirectional stretching is carried out, the temperature is 115℃, the stretching ratio is 7.9 times, dichloromethane is used for 105s extraction to remove paraffin oil, secondary transverse stretching is carried out, the temperature is 121℃, the stretching ratio is 1.25 times, winding, slitting, and a porous base film is obtained.

[0049] The application further provides a preparation method of the high-porosity wet-process diaphragm.

[0050] S1, immersing the porous base film into a 1-(3-aminopropyl)imidazole methanol solution with a concentration of 1.1mol / L, reacting for 11h in a 65℃ water bath, then taking out, washing to neutral with methanol, and obtaining a modified base film;

[0051] S2, immersing the modified base film obtained in S1 into a 22.5mmol / L cobalt nitrate methanol solution, reacting for 25min, then taking out, immersing into a 22.5mmol / L 2-methyl imidazole methanol solution, reacting for 35min, as one cycle, repeating for three times, washing and drying, and obtaining the high-porosity wet-process diaphragm.

[0052] Example 3

[0053] The high-porosity wet-process diaphragm is composed of a porous base film and a diaphragm coating.

[0054] The porous base film is prepared from the following components: polyethylene powder, glycidyl methacrylate, benzoyl peroxide, paraffin oil and dichloromethane.

[0055] The preparation method of the porous base film specifically includes the following steps:

[0056] (1) polyethylene powder is added into a reactor, nitrogen is introduced to discharge air, then glycidyl methacrylate and benzoyl peroxide are uniformly mixed and added dropwise into the reactor, the mass ratio of polyethylene powder, glycidyl methacrylate and benzoyl peroxide is 100:10:0.5, stirring is uniformly carried out, heating is carried out at 110℃ for 2.5h, and modified polyethylene is obtained;

[0057] (2) The modified polyethylene obtained in step (1) is mixed with paraffin oil at a mass ratio of 1:2.4, melt-blended, extruded into a sheet, the extrusion temperature is 200℃, the cast roll is cooled, the cast roll temperature control medium is water, the speed is 4.6m / min, the temperature is 28℃, the cast thickness is 1.5mm, then synchronous bidirectional stretching is carried out, the temperature is 116℃, the stretching ratio is 8.0 times, paraffin oil is removed by dichloromethane extraction for 110s, secondary transverse stretching is carried out, the temperature is 122℃, the stretching ratio is 1.3 times, winding, slitting, and a porous base film is obtained.

[0058] The application also provides a preparation method of the high-porosity wet-process diaphragm, specifically comprising the following steps:

[0059] S1, the porous base film is immersed in a 1-(3-aminopropyl) imidazole methanol solution with a concentration of 1.2mol / L, and reacted in a 70℃ water bath for 12h, then taken out, washed with methanol until neutral, and a modified base film is obtained;

[0060] S2, the modified base film obtained in S1 is immersed in a 25mmol / L cobalt nitrate methanol solution, and reacted for 30min, then taken out, immersed in a 25mmol / L 2-methyl imidazole methanol solution, and reacted for 40min, as one cycle, repeated for 4 times, washed, and dried, and a high-porosity wet-process diaphragm is obtained.

[0061] Comparative Example 1

[0062] The comparative example provides a battery diaphragm, which is different from Example 1 only in that 1-(3-aminopropyl) imidazole is replaced by an equal amount of 2-methyl imidazole in the components, and the rest of the components and component contents are the same as those in Example 1.

[0063] Comparative Example 2

[0064] The comparative example provides a battery diaphragm, which is different from Example 1 only in that 1-(3-aminopropyl) imidazole is not contained in the components, and a coating layer is formed by coating an equal amount of ZIF-67 with polyvinylidene fluoride as an adhesive, the mass ratio of ZIF-67 to adhesive is 4:1, and the rest of the components and component contents are the same as those in Example 1.

[0065] Experimental Example

[0066] 1. Porosity test

[0067] The diaphragms obtained in Examples 1-3 and Comparative Examples 1-2 of the application are immersed in n-butanol solution, taken out after 1h, the excess liquid on the surface of the diaphragm is wiped off, and weighed again, and the porosity is calculated according to the following formula:

[0068] Porosity = (mass of diaphragm after immersion-mass of diaphragm before immersion) / (density of n-butanol x volume of diaphragm before immersion) x 100%.

[0069] Figure 1 The results of the porosity test of the present application examples 1-3 and comparative examples 1-2 are shown in the figure; as shown, the porosities of examples 1-3 and comparative examples 1-2 were 61.1%, 62.3%, 62.6% and 44.9%, 52.2% respectively, which were tested by liquid immersion method using n-butanol; the porosities of examples 1-3 were obviously higher than those of comparative examples 1-2, indicating that the use of 1-(3-aminopropyl) imidazole and the preparation of covalently bound coating on the membrane effectively improved the porosity of the membrane.

[0070] 2. Thermal stability test

[0071] The membranes obtained from the present application examples 1-3 and comparative examples 1-2 were placed in an oven at 150℃, with A4 paper on top and bottom, heated for 1 hour, and then the size change before and after heat treatment was compared to evaluate the thermal stability of the longitudinal (MD) and transverse (TD) directions respectively.

[0072] Figure 2 The results of the thermal stability test of the present application examples 1-3 and comparative examples 1-2 are shown in the figure; as shown, the thermal shrinkage rates MD of examples 1-3 and comparative examples 1-2 were 1.42%, 1.43%, 1.39% and 26.2%, 8.3% respectively, and the thermal shrinkage rates TD of examples 1-3 and comparative examples 1-2 were 1.21%, 1.19%, 1.16% and 21.8%, 7.1% respectively; the thermal shrinkage rates of examples 1-3 were obviously lower than those of comparative examples 1-2, indicating that the use of 1-(3-aminopropyl) imidazole and the preparation of covalently bound coating on the membrane effectively improved the thermal stability of the membrane.

[0073] 3. Cycle test

[0074] Full cells were assembled with lithium iron phosphate as the positive electrode, lithium metal as the negative electrode, and the membranes obtained from the present application examples 1-3 and comparative examples 1-2, respectively, and after 5 cycles of activation at a rate of 0.2C, constant current charge and discharge long cycle tests were carried out at a rate of 0.5C in the voltage range of 2.8~3.9V, the discharge capacity of the 6th cycle was recorded as the initial capacity, and after 300 cycles, the discharge capacity was recorded as the final capacity, and the capacity retention rate was calculated using the following formula:

[0075] Capacity retention rate = final capacity / initial capacity x 100%.

[0076] Figure 3The results of the cycle test for the inventive examples 1-3 and comparative examples 1-2 are shown in the figure; as shown, the capacity retention rates of the inventive examples 1-3 and comparative examples 1-2 are 93.1%, 92.6%, 92.7% and 43.5%, 79.8% respectively after 300 cycles; the capacity retention rate of the inventive examples 1-3 is obviously higher than that of the comparative examples 1-2, which indicates that the use of 1-(3-aminopropyl) imidazole and the preparation of the covalently bonded coating on the membrane effectively improve the capacity retention rate of the separator.

[0077] Although the embodiments of the present application have been shown and described, it is to be understood that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the present application.

[0078] The above description of the present application and its embodiments is not restrictive, and the embodiments shown in the drawings are only one of the embodiments of the present application, and the actual application is not limited thereto. In summary, if a person skilled in the art is inspired by it, without departing from the purpose of the present application, without creative design, similar ways and embodiments of the technical solution should belong to the protection scope of the present application.

Claims

1. A high-porosity wet-process separator, wherein the high-porosity wet-process separator is composed of a porous base membrane and a separator coating, characterized in that: The porous base membrane is prepared from the following components: polyolefin powder, glycidyl methacrylate, initiator, diluent, and extractant; the membrane coating is an organometallic framework material covalently bonded to the base membrane. The polyolefin powder is polyethylene powder, the initiator is benzoyl peroxide, the diluent is paraffin oil, and the extractant is dichloromethane; the membrane coating is a ZIF-67 coating that is generated in situ on the surface of the porous base membrane and covalently bonded to the porous base membrane. The method for preparing the high-porosity wet-process membrane specifically includes the following steps: S1. Immerse the porous base membrane in a 1-(3-aminopropyl)imidazolium methanol solution and react in a water bath. Then remove it and wash it with methanol until neutral to obtain the modified base membrane. S2. Immerse the modified base membrane obtained in S1 in a cobalt nitrate methanol solution and react for 20-30 min. Then remove it and immerse it in a 2-methylimidazolium methanol solution and react for 30-40 min. This is one cycle. Repeat 3-4 times. Wash and dry to obtain a high-porosity wet-process membrane.

2. The high-porosity wet-process membrane according to claim 1, characterized in that: In S1, the concentration of 1-(3-aminopropyl)imidazolium methanol solution is 1-1.2 mol / L; the reaction is carried out in a water bath at a temperature of 60-70℃ for 10-12 h.

3. The high-porosity wet-process membrane according to claim 2, characterized in that: In S2, the concentration of cobalt nitrate methanol solution is 20-25 mmol / L, and the concentration of 2-methylimidazolium methanol solution is 20-25 mmol / L.

4. The high-porosity wet-process membrane according to claim 3, characterized in that: The method for preparing the porous base membrane specifically includes the following steps: (1) Add polyolefin powder to the reactor, purge the air with nitrogen, then mix glycidyl methacrylate and initiator evenly and add dropwise to the reactor, stir evenly, heat to react, and obtain modified polyolefin; (2) The modified polyolefin obtained in step (1) is mixed with a diluent, melt-blended, extruded into sheets, cooled on a casting roller, and then subjected to synchronous biaxial stretching. The diluent is removed by extraction with an extractant, followed by secondary transverse stretching, winding, and slitting to obtain a porous base film.

5. The high-porosity wet-process membrane according to claim 4, characterized in that: In step (1), the mass ratio of polyolefin, glycidyl methacrylate and initiator is 100:8-10:0.3-0.5, and the reaction is heated at 100-110℃ for 2.2-2.5h.

6. The high-porosity wet-process membrane according to claim 5, characterized in that: In step (2), the mass ratio of modified polyolefin to diluent is 1:2-2.4; the extrusion temperature is 190-200℃.

7. The method for preparing a high-porosity wet-process membrane according to claim 6, characterized in that: In step (2), the temperature control medium for the casting roller is water, the speed is 4.2-4.6 m / min, the temperature is 25-28℃, and the thickness of the casting is 1.5 mm.

8. The high-porosity wet-process membrane according to claim 7, characterized in that: In step (2), the simultaneous biaxial stretching is performed at a temperature of 114-116℃ with a stretching ratio of 7.8-8.0 times; the extraction time is 100-110s; the secondary transverse stretching is performed at a temperature of 120-122℃ with a stretching ratio of 1.2-1.3 times.

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