CN114883742A - Preparation method of porous low-shrinkage polypropylene diaphragm for lithium ion battery - Google Patents
Preparation method of porous low-shrinkage polypropylene diaphragm for lithium ion battery Download PDFInfo
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- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M50/00—Constructional details or processes of manufacture of the non-active parts of electrochemical cells other than fuel cells, e.g. hybrid cells
- H01M50/40—Separators; Membranes; Diaphragms; Spacing elements inside cells
- H01M50/403—Manufacturing processes of separators, membranes or diaphragms
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Abstract
The invention relates to a preparation method of a porous low-shrinkage polypropylene diaphragm for a lithium ion battery, which comprises the steps of melting, blending and extruding modified polypropylene with a reversible covalent crosslinking structure and polypropylene, stretching to obtain a polypropylene prefabricated membrane with the reversible covalent crosslinking structure, and then stretching to obtain the porous low-shrinkage polypropylene diaphragm for the lithium ion battery; the modified polypropylene with the reversible covalent crosslinking structure is prepared by blending, melting, extruding and granulating hydroxylated polypropylene, dioctyl terephthalate and zinc acetylacetonate. The preparation method of the porous low-shrinkage polypropylene diaphragm for the lithium ion battery is simple and feasible, the porosity of the prepared diaphragm is effectively improved, the high breaking strength can be kept, and the thermal stability of the diaphragm can be kept for a long time.
Description
Technical Field
The invention belongs to the technical field of lithium ion battery diaphragms, and relates to a preparation method of a porous low-shrinkage polypropylene diaphragm for a lithium ion battery.
Background
In recent years, lithium ion batteries are widely used in the field of electric vehicles, and people have put higher demands on energy density of the lithium ion batteries, wherein a diaphragm is an important factor for restricting the energy density. The commercial polypropylene diaphragm has stable chemical property and low price, but has low porosity, is not beneficial to the transmission of lithium ions, and is easy to shrink when heated, thereby causing short circuit of the battery.
According to the document 1(Stretching-Induced uniformity microporous Formation: An in Situ SAXS/WAXS Study [ J ]. Macromolecules,2018,51(9), 3433-. Therefore, the amount of microphase formation in the amorphous region of the pre-formed film will determine the amount of porosity. Document 2(MicroStructure of polypropylene-based porous structures with biomodal micro-pore structure for water fluorine enhancement and emission performance requirement [ J ] Separation and Purification Technology,2019,213,328-338.) an ethylene-vinyl alcohol copolymer/polypropylene membrane was prepared, which, by inducing phase Separation in polypropylene, creates a weak point of stress, increases the porosity of the membrane, but the pore size increases greatly and the breaking strength decreases drastically. In contrast, document 3(Effect of Hydroxyl-Functionalization on the Structure and Properties of Polypropylene [ J ]. Macromolecules,2013,46,5455-5463.) synthesizes a hydroxylated Polypropylene, and researches show that Hydroxyl groups form hydrogen bonding in an amorphous region, thereby enhancing the breaking strength of Polypropylene. A crosslinked maleic anhydride-grafted polypropylene was prepared by transesterification in document 4(Scalable overcycling of thermoplastic polyolefins in one of the polymers through transesterification [ J ]. Journal of Materials Chemistry A,2020,8(45), 24137-24147.). The cross-linking bonds promote the aggregation of amorphous zone subchains, and the elastic modulus of the polypropylene at high temperature is enhanced.
Patent CN 110444717A discloses a reinforced polypropylene diaphragm, a preparation method and application thereof, wherein a long-chain alkyl modified graphene oxide, a cross-linking agent and polypropylene are melted, blended and formed into a film, and the film is irradiated and cross-linked to prepare a covalent cross-linked polypropylene diaphragm. The covalent cross-linked structure can enhance the strength of the diaphragm, but reduces the crystallization capacity of the polypropylene, and meanwhile, the high cross-linking density limits the movement of molecular chains, so that the porosity is reduced.
Patent CN 108400272 a discloses a lithium battery polypropylene diaphragm compounded with kenyaite-silica aerogel, and the method prepares a polypropylene diaphragm compounded with kenyaite-silica aerogel. In the stretching process, the kenyaite-silicon dioxide migrates to the surface layer to block heat transfer, improve the thermal stability of the diaphragm and resist thermal shrinkage, but the kenyaite-silicon dioxide and polypropylene have no interaction and are easy to fall off from the surface, so that the thermal stability of the diaphragm is lost.
Therefore, it is of great commercial interest to develop a polypropylene separator with high porosity and low heat shrinkage.
Disclosure of Invention
The invention aims to solve the problems in the prior art and provides a preparation method of a porous low-shrinkage polypropylene diaphragm for a lithium ion battery.
In order to achieve the purpose, the technical scheme adopted by the invention is as follows:
a preparation method of a porous low-shrinkage polypropylene diaphragm for a lithium ion battery comprises the steps of melting, blending and extruding modified polypropylene with a reversible covalent crosslinking structure and polypropylene, stretching to obtain a polypropylene prefabricated membrane with the reversible covalent crosslinking structure, and then stretching to obtain the porous low-shrinkage polypropylene diaphragm for the lithium ion battery;
the modified polypropylene with the reversible covalent crosslinking structure is prepared by blending, melting, extruding and granulating hydroxylated polypropylene, dioctyl terephthalate and zinc acetylacetonate;
the reversible covalent crosslinking structure is characterized in that hydroxylated polypropylene and dioctyl terephthalate are subjected to ester exchange reaction under the catalysis of zinc acetylacetonate to form ester-based crosslinking and octanol, and the ester-based crosslinking and octanol are subjected to ester exchange reaction at high temperature to destroy the crosslinking structure and reform the hydroxylated polypropylene and the dioctyl terephthalate.
As a preferred technical scheme:
according to the preparation method of the porous low-shrinkage polypropylene separator for the lithium ion battery, the thickness of the polypropylene separator is 10-22.0 mu m, the crystallinity is 34.0-45.0%, the porosity is 62.0-75.0%, the pore size distribution is 0.150-0.200 mu m, the hydrophilic contact angle is 50.0-65.0 degrees, and the breaking strength is 95.0-105 MPa; and heating at 130 ℃ for 30min, wherein the size shrinkage of the diaphragm is 0.5-1% (adopting a TMA thermal mechanical analyzer, applying 0.015N to two ends of the film, heating from 30 ℃ to a thermal deformation temperature at a heating rate of 1 ℃/min, heating at the thermal deformation temperature for 30min, and testing the strain of the longitudinal size of the diaphragm to obtain the size shrinkage of the diaphragm).
The preparation method of the porous low-shrinkage polypropylene diaphragm for the lithium ion battery comprises the following specific steps:
(1) blending, melting, extruding and granulating hydroxylated polypropylene, dioctyl terephthalate and zinc acetylacetonate to prepare modified polypropylene with a reversible covalent crosslinking structure;
the reversible covalent crosslinking reaction is:
(2) melting and blending the modified polypropylene with the reversible covalent crosslinking structure and polypropylene, and forming a film from the melt at a high stretching ratio to prepare a polypropylene prefabricated film with the reversible covalent crosslinking structure; the high stretch ratio is 40-120;
(3) and (3) post-stretching the polypropylene prefabricated membrane with the reversible covalent crosslinking structure to obtain the porous low-shrinkage polypropylene diaphragm for the lithium ion battery.
The preparation method of the porous low-shrinkage polypropylene diaphragm for the lithium ion battery comprises the following steps of (1) and the structural formula of the hydroxylated polypropylene is as follows:
wherein the number average molecular weight of the hydroxylated polypropylene is 40.8-82.3 kg/mol, x is 855-1632, y is 28-80, and the hydroxyl insertion rate is 3.20-4.60%.
The preparation method of the porous low-shrinkage polypropylene diaphragm for the lithium ion battery comprises the following steps of (1) melting and extruding at the temperature of 200-210 ℃; the mass ratio of the hydroxylated polypropylene to the dioctyl phthalate to the zinc acetylacetonate is 80-90: 9-19: 1.
According to the preparation method of the porous low-shrinkage polypropylene diaphragm for the lithium ion battery, when the melting and blending are carried out in the step (2), the temperatures of the five regions of the screw are respectively 160 ℃, 190 ℃, 200 ℃, 220 ℃ and 200 ℃.
According to the preparation method of the porous low-shrinkage polypropylene diaphragm for the lithium ion battery, the mass ratio of the modified polypropylene with the reversible covalent crosslinking structure to the polypropylene in the step (2) is 5.00-15.0: 85.0-95.0.
According to the preparation method of the porous low-shrinkage polypropylene diaphragm for the lithium ion battery, in the step (2), the thickness of the polypropylene prefabricated film with the reversible covalent crosslinking structure is 25.0-40.0 mu m, the crystallinity is 25.0-34.0%, and the breaking strength is 110-130 MPa.
According to the preparation method of the porous low-shrinkage polypropylene diaphragm for the lithium ion battery, the temperature of the neck ring mold in the film forming process in the step (2) is 200-220 ℃.
According to the preparation method of the porous low-shrinkage polypropylene diaphragm for the lithium ion battery, the post-stretching in the step (3) sequentially comprises cold stretching and hot stretching, wherein the cold stretching temperature is 25 ℃, the cold stretching strain is 25.0%, the hot stretching temperature is 130 ℃, and the hot stretching strain is 100-180%.
The invention mechanism is as follows:
the invention starts from the design of a polypropylene molecular structure and an aggregation state structure, and firstly, in order to improve the porosity of the diaphragm, reversible covalent crosslinking is utilized to induce an amorphous region to form microphase aggregation. At high temperature, the molecular chains of the polypropylene amorphous region are orderly arranged under the stretching action, the molecular chains are gathered, the free volume between the molecular chains is rejected, and the free volume is gathered to form a diaphragm pore. The invention introduces reversible covalent cross-linking points in the amorphous region, controls the cross-linking density, reduces the influence of chain motion, and further aggregates molecular chains, thereby releasing more free volume and greatly improving the porosity of the diaphragm. The reversible covalent cross-linking point is introduced by reacting hydroxylated polypropylene with dioctyl terephthalate, performing ester exchange between hydroxyl and ester group to form ester group cross-linking, and controlling the cross-linking density by the content of hydroxyl and dioctyl terephthalate (the invention uses the reaction between hydroxylated polypropylene and dioctyl terephthalate, and performs ester exchange reaction between hydroxyl and ester group to limit the ester group in the amorphous region, i.e. the reversible covalent cross-linking point can be introduced in the amorphous region, the hydroxyl itself destroys the regularity of the molecular chain, the segment containing hydroxyl is limited in the amorphous region, and the cross-linked segment is naturally in the amorphous region after the ester exchange reaction.
At high processing temperature (the temperature of melt extrusion granulation in the step (1) and the temperature of melt blending in the step (2)), the ester exchange is carried out reversely, and the crosslinking is broken, so that the polypropylene can be melt processed; when the temperature is reduced from 200 ℃ to 180 ℃ after the melt is extruded from the die (when the temperature is lower than 200 ℃, the forward speed of the ester exchange reaction is increased, the reverse speed is reduced, when the temperature is lower than 180 ℃, the forward speed of the ester exchange reaction is maximum, and when the temperature is lower than 180 ℃, the cross-linking is formed again), under the action of the catalyst, the ester exchange reaction is carried out in the forward direction, and the cross-linking is established again.
Secondly, in order to reduce the thermal shrinkage rate of the diaphragm, the polypropylene amorphous region should be cross-linked with the sub-chains. The diaphragm is heated and shrunk because of the oriented molecular chain of the amorphous region, and the molecular chain is shrunk due to the high-temperature orientation decomposition. Therefore, the invention distinguishes the molecular chain by crosslinking part of amorphous, so that the molecular chain keeps a straight state at a higher temperature, and the thermal shrinkage rate of the diaphragm is greatly reduced. In the prior art, inorganic matters are blended with polypropylene and migrate to the surface to isolate heat transfer, but the inorganic matters are incompatible with the polypropylene and are easy to fall off, so that the heat stability is lost. According to the invention, through the molecular chain crosslinking of the copolymer, the falling-off condition can not occur, and the thermal stability of the diaphragm can be maintained for a long time.
Finally, due to the hydrogen bonding of the hydroxyl groups, the crystallinity of the hydroxylated polypropylene is lower and, after crosslinking, the crystallinity of the reversibly crosslinked polypropylene is further reduced. And part of chain segments of the reversible crosslinked polypropylene can form cocrystallization with the polypropylene, and a certain degree of crystallinity can be maintained. The invention adopts a mode of melt blending polypropylene and reversible crosslinked polypropylene to prepare the diaphragm which has certain crystallinity and reversible crosslinking in an amorphous area.
Has the advantages that:
(1) the porosity of the polypropylene diaphragm prepared in the prior art is lower and is 40-55%, and the porosity of the diaphragm prepared by the method is effectively improved, and the high breaking strength can be kept.
(2) In the prior art, inorganic particles are compounded with polypropylene, inorganic matters are used as a diaphragm coating to isolate heat transfer, but the inorganic matters are incompatible with the polypropylene and are easy to fall off, so that the thermal stability is lost; the invention can keep the thermal stability of the diaphragm for a long time by the molecular chain crosslinking of the copolymer.
(3) The polypropylene copolymer adopted by the invention has good compatibility with polypropylene; the melt blending is adopted to prepare the membrane, so that the cost is lower.
Detailed Description
The invention will be further illustrated with reference to specific embodiments. It should be understood that these examples are for illustrative purposes only and are not intended to limit the scope of the present invention. Further, it should be understood that various changes or modifications can be made by those skilled in the art after reading the contents of the present invention, and those equivalents also fall within the scope of the invention defined by the appended claims.
Example 1
A preparation method of a porous low-shrinkage polypropylene diaphragm for a lithium ion battery comprises the following specific steps:
(1) blending hydroxylated polypropylene, dioctyl terephthalate and zinc acetylacetonate, and performing melt extrusion granulation at 210 ℃ to prepare modified polypropylene with a reversible covalent crosslinking structure;
wherein the mass ratio of the hydroxylated polypropylene to the dioctyl phthalate to the zinc acetylacetonate is 80:19:1, the number average molecular weight of the hydroxylated polypropylene is 82.3kg/mol, and the structural formula of the hydroxylated polypropylene is as follows:
wherein, the hydroxyl insertion rate is 4.6 percent;
(2) melting and blending the modified polypropylene with the reversible covalent cross-linking structure prepared in the step (1) and polypropylene, and forming a film by using the melt at a high stretching ratio to prepare a polypropylene prefabricated film with the reversible covalent cross-linking structure;
wherein the mass ratio of the modified polypropylene with the reversible covalent crosslinking structure to the polypropylene is 5.0: 95.0; during melt blending, the temperature of the fifth screw region is respectively 160 ℃, 190 ℃, 200 ℃, 220 ℃ and 200 ℃; a high draw ratio of 120; the temperature of a neck ring mold during film forming is 220 ℃;
the thickness of the prepared polypropylene prefabricated film with the reversible covalent crosslinking structure is 25 mu m, the crystallinity is 28.2 percent, and the breaking strength is 116 MPa;
(3) post-stretching the polypropylene prefabricated film with the reversible covalent cross-linking structure prepared in the step (2) to prepare a porous low-shrinkage polypropylene diaphragm for the lithium ion battery;
wherein, the post-stretching comprises cold stretching and hot stretching in sequence, the cold stretching temperature is 25 ℃, the cold stretching strain is 25.0 percent, the hot stretching temperature is 130 ℃, and the hot stretching strain is 180 percent;
the prepared porous low-shrinkage polypropylene diaphragm for the lithium ion battery has the thickness of 10 mu m, the crystallinity of 40 percent and the porosity of 68 percent, the pore size distribution of 0.150 to 0.200 mu m, the hydrophilic contact angle of 60 degrees and the breaking strength of 101 MPa; heating at 130 deg.C for 30min to reduce the size shrinkage of the membrane by 0.8%.
Example 2
A preparation method of a porous low-shrinkage polypropylene diaphragm for a lithium ion battery comprises the following specific steps:
(1) blending hydroxylated polypropylene, dioctyl terephthalate and zinc acetylacetonate, and performing melt extrusion granulation at 210 ℃ to prepare modified polypropylene with a reversible covalent crosslinking structure;
wherein the mass ratio of the hydroxylated polypropylene to the dioctyl phthalate to the zinc acetylacetonate is 85:14:1, the number average molecular weight of the hydroxylated polypropylene is 82.3kg/mol, and the structural formula of the hydroxylated polypropylene is as follows:
wherein, the hydroxyl insertion rate is 4.6 percent;
(2) melting and blending the modified polypropylene with the reversible covalent cross-linking structure prepared in the step (1) and polypropylene, and forming a film by using the melt at a high stretching ratio to prepare a polypropylene prefabricated film with the reversible covalent cross-linking structure;
wherein the mass ratio of the modified polypropylene with the reversible covalent crosslinking structure to the polypropylene is 10.0: 90.0; during melt blending, the temperature of the fifth screw region is respectively 160 ℃, 190 ℃, 200 ℃, 220 ℃ and 200 ℃; a high draw ratio of 120; the temperature of a neck ring mold during film forming is 220 ℃;
the thickness of the prepared polypropylene prefabricated film with the reversible covalent crosslinking structure is 25 mu m, the crystallinity is 25 percent, and the breaking strength is 110 MPa;
(3) post-stretching the polypropylene prefabricated film with the reversible covalent cross-linking structure prepared in the step (2) to prepare a porous low-shrinkage polypropylene diaphragm for the lithium ion battery;
wherein, the post-stretching comprises cold stretching and hot stretching in sequence, the cold stretching temperature is 25 ℃, the cold stretching strain is 25.0 percent, the hot stretching temperature is 130 ℃, and the hot stretching strain is 160 percent;
the prepared porous low-shrinkage polypropylene diaphragm for the lithium ion battery has the thickness of 12 mu m, the crystallinity of 35.2 percent, the porosity of 72 percent, the pore size distribution of 0.150-0.200 mu m, the hydrophilic contact angle of 58 degrees and the breaking strength of 97 MPa; heating at 130 deg.C for 30min to reduce the size shrinkage of the membrane by 0.6%.
Example 3
A preparation method of a porous low-shrinkage polypropylene diaphragm for a lithium ion battery comprises the following specific steps:
(1) blending hydroxylated polypropylene, dioctyl terephthalate and zinc acetylacetonate, and performing melt extrusion granulation at 210 ℃ to prepare modified polypropylene with a reversible covalent crosslinking structure;
wherein the mass ratio of the hydroxylated polypropylene to the dioctyl phthalate to the zinc acetylacetonate is 90:9:1, the number average molecular weight of the hydroxylated polypropylene is 82.3kg/mol, and the structural formula of the hydroxylated polypropylene is as follows:
wherein, the hydroxyl insertion rate is 4.6 percent;
(2) melting and blending the modified polypropylene with the reversible covalent cross-linking structure prepared in the step (1) and polypropylene, and forming a film by using the melt at a high stretching ratio to prepare a polypropylene prefabricated film with the reversible covalent cross-linking structure;
wherein the mass ratio of the modified polypropylene with the reversible covalent crosslinking structure to the polypropylene is 10.0: 90.0; during melt blending, the temperature of the fifth screw region is respectively 160 ℃, 190 ℃, 200 ℃, 220 ℃ and 200 ℃; a high draw ratio of 100; the temperature of a neck ring mold during film forming is 210 ℃;
the thickness of the prepared polypropylene prefabricated film with the reversible covalent crosslinking structure is 28 mu m, the crystallinity is 25.4 percent, and the breaking strength is 112 MPa;
(3) post-stretching the polypropylene prefabricated film with the reversible covalent cross-linking structure prepared in the step (2) to prepare a porous low-shrinkage polypropylene diaphragm for the lithium ion battery;
wherein, the post-stretching comprises cold stretching and hot stretching in sequence, the cold stretching temperature is 25 ℃, the cold stretching strain is 25.0 percent, the hot stretching temperature is 130 ℃, and the hot stretching strain is 140 percent;
the prepared porous low-shrinkage polypropylene diaphragm for the lithium ion battery has the thickness of 13 mu m, the crystallinity of 34 percent, the porosity of 75 percent, the pore size distribution of 0.150 to 0.200 mu m, the hydrophilic contact angle of 58 degrees and the breaking strength of 95 MPa; heating at 130 deg.C for 30min to reduce the size shrinkage of the membrane by 0.6%.
Example 4
A preparation method of a porous low-shrinkage polypropylene diaphragm for a lithium ion battery comprises the following specific steps:
(1) blending hydroxylated polypropylene, dioctyl terephthalate and zinc acetylacetonate, and performing melt extrusion granulation at 205 ℃ to prepare modified polypropylene with a reversible covalent crosslinking structure;
wherein the mass ratio of the hydroxylated polypropylene to the dioctyl phthalate to the zinc acetylacetonate is 85:14:1, the number average molecular weight of the hydroxylated polypropylene is 40.8kg/mol, and the structural formula of the hydroxylated polypropylene is as follows:
wherein, the hydroxyl insertion rate is 3.2%;
(2) melting and blending the modified polypropylene with the reversible covalent cross-linking structure prepared in the step (1) and polypropylene, and forming a film by using the melt at a high stretching ratio to prepare a polypropylene prefabricated film with the reversible covalent cross-linking structure;
wherein the mass ratio of the modified polypropylene with the reversible covalent crosslinking structure to the polypropylene is 5.0: 95.0; during melt blending, the temperature of the fifth screw region is respectively 160 ℃, 190 ℃, 200 ℃, 220 ℃ and 200 ℃; a high draw ratio of 100; the temperature of a neck ring mold during film forming is 200 ℃;
the thickness of the prepared polypropylene prefabricated film with the reversible covalent crosslinking structure is 28 mu m, the crystallinity is 34 percent, and the breaking strength is 130 MPa;
(3) post-stretching the polypropylene prefabricated film with the reversible covalent cross-linking structure prepared in the step (2) to prepare a porous low-shrinkage polypropylene diaphragm for the lithium ion battery;
wherein, the post-stretching comprises cold stretching and hot stretching in sequence, the cold stretching temperature is 25 ℃, the cold stretching strain is 25.0 percent, the hot stretching temperature is 130 ℃, and the hot stretching strain is 140 percent;
the prepared porous low-shrinkage polypropylene diaphragm for the lithium ion battery has the thickness of 13 mu m, the crystallinity of 45 percent, the porosity of 62 percent, the pore size distribution of 0.150 to 0.200 mu m, the hydrophilic contact angle of 65 degrees and the breaking strength of 105 MPa; the membrane is heated for 30min at 130 ℃, and the size shrinkage rate of the membrane is 1%.
Example 5
A preparation method of a porous low-shrinkage polypropylene diaphragm for a lithium ion battery comprises the following specific steps:
(1) blending hydroxylated polypropylene, dioctyl terephthalate and zinc acetylacetonate, and performing melt extrusion granulation at 205 ℃ to prepare modified polypropylene with a reversible covalent crosslinking structure;
wherein the mass ratio of the hydroxylated polypropylene to the dioctyl phthalate to the zinc acetylacetonate is 90:9:1, the number average molecular weight of the hydroxylated polypropylene is 40.8kg/mol, and the structural formula of the hydroxylated polypropylene is as follows:
wherein, the hydroxyl insertion rate is 3.2%;
(2) melting and blending the modified polypropylene with the reversible covalent cross-linking structure prepared in the step (1) and polypropylene, and forming a film by using the melt at a high stretching ratio to prepare a polypropylene prefabricated film with the reversible covalent cross-linking structure;
wherein the mass ratio of the modified polypropylene with the reversible covalent crosslinking structure to the polypropylene is 10.0: 90.0; during melt blending, the temperature of the fifth screw region is respectively 160 ℃, 190 ℃, 200 ℃, 220 ℃ and 200 ℃; a high stretch ratio of 80; the temperature of a neck ring mold during film forming is 210 ℃;
the thickness of the prepared polypropylene prefabricated film with the reversible covalent crosslinking structure is 35 mu m, the crystallinity is 32.6 percent, and the breaking strength is 126 MPa;
(3) post-stretching the polypropylene prefabricated film with the reversible covalent cross-linking structure prepared in the step (2) to prepare a porous low-shrinkage polypropylene diaphragm for the lithium ion battery;
wherein, the post-stretching comprises cold stretching and hot stretching in sequence, the cold stretching temperature is 25 ℃, the cold stretching strain is 25.0 percent, the hot stretching temperature is 130 ℃, and the hot stretching strain is 160 percent;
the prepared porous low-shrinkage polypropylene diaphragm for the lithium ion battery has the thickness of 14 mu m, the crystallinity of 44.5 percent, the porosity of 65 percent, the pore size distribution of 0.150-0.200 mu m, the hydrophilic contact angle of 62 degrees and the breaking strength of 103 MPa; heating at 130 deg.C for 30min to reduce the size shrinkage of the membrane by 0.8%.
Example 6
A preparation method of a porous low-shrinkage polypropylene diaphragm for a lithium ion battery comprises the following specific steps:
(1) blending hydroxylated polypropylene, dioctyl terephthalate and zinc acetylacetonate, and performing melt extrusion granulation at 205 ℃ to prepare modified polypropylene with a reversible covalent crosslinking structure;
wherein the mass ratio of the hydroxylated polypropylene to the dioctyl phthalate to the zinc acetylacetonate is 90:9:1, the number average molecular weight of the hydroxylated polypropylene is 40.8kg/mol, and the structural formula of the hydroxylated polypropylene is as follows:
wherein, the hydroxyl insertion rate is 3.2%;
(2) melting and blending the modified polypropylene with the reversible covalent crosslinking structure prepared in the step (1) and polypropylene, and forming a film by using a melt at a high draw ratio to prepare a polypropylene prefabricated film with the reversible covalent crosslinking structure;
wherein the mass ratio of the modified polypropylene with the reversible covalent crosslinking structure to the polypropylene is 15.0: 90.0; during melt blending, the temperature of the fifth screw region is respectively 160 ℃, 190 ℃, 200 ℃, 220 ℃ and 200 ℃; a high draw ratio of 40; the temperature of a neck ring mold during film forming is 210 ℃;
the thickness of the prepared polypropylene prefabricated film with the reversible covalent crosslinking structure is 40 mu m, the crystallinity is 28 percent, and the breaking strength is 120 MPa;
(3) post-stretching the polypropylene prefabricated film with the reversible covalent cross-linking structure prepared in the step (2) to prepare a porous low-shrinkage polypropylene diaphragm for the lithium ion battery;
wherein, the post-stretching comprises cold stretching and hot stretching in sequence, the cold stretching temperature is 25 ℃, the cold stretching strain is 25.0 percent, the hot stretching temperature is 130 ℃, and the hot stretching strain is 160 percent;
the prepared porous low-shrinkage polypropylene diaphragm for the lithium ion battery has the thickness of 15 mu m, the crystallinity of 39 percent, the porosity of 70 percent, the pore size distribution of 0.150 to 0.200 mu m, the hydrophilic contact angle of 55 degrees and the breaking strength of 101 MPa; heating at 130 deg.C for 30min to reduce the size shrinkage of the membrane by 0.5%.
Example 7
A preparation method of a porous low-shrinkage polypropylene diaphragm for a lithium ion battery comprises the following specific steps:
(1) blending hydroxylated polypropylene, dioctyl terephthalate and zinc acetylacetonate, and performing melt extrusion granulation at 205 ℃ to prepare modified polypropylene with a reversible covalent crosslinking structure;
wherein the mass ratio of the hydroxylated polypropylene to the dioctyl phthalate to the zinc acetylacetonate is 80:14:1, the number average molecular weight of the hydroxylated polypropylene is 61.7kg/mol, and the structural formula of the hydroxylated polypropylene is as follows:
wherein, the hydroxyl insertion rate is 3.8%;
(2) melting and blending the modified polypropylene with the reversible covalent cross-linking structure prepared in the step (1) and polypropylene, and forming a film by using the melt at a high stretching ratio to prepare a polypropylene prefabricated film with the reversible covalent cross-linking structure;
wherein the mass ratio of the modified polypropylene with the reversible covalent crosslinking structure to the polypropylene is 5.0: 95.0; during melt blending, the temperature of the fifth screw region is respectively 160 ℃, 190 ℃, 200 ℃, 220 ℃ and 200 ℃; a high stretch ratio of 80; the temperature of a neck ring mold during film forming is 205 ℃;
the thickness of the prepared polypropylene prefabricated film with the reversible covalent crosslinking structure is 35 mu m, the crystallinity is 30.6 percent, and the breaking strength is 125 MPa;
(3) post-stretching the polypropylene prefabricated film with the reversible covalent cross-linking structure prepared in the step (2) to prepare a porous low-shrinkage polypropylene diaphragm for the lithium ion battery;
wherein, the post-stretching comprises cold stretching and hot stretching in sequence, the cold stretching temperature is 25 ℃, the cold stretching strain is 25.0 percent, the hot stretching temperature is 130 ℃, and the hot stretching strain is 120 percent;
the prepared porous low-shrinkage polypropylene diaphragm for the lithium ion battery has the thickness of 18 mu m, the crystallinity of 38.4 percent, the porosity of 71 percent, the pore diameter distribution of 0.150-0.200 mu m, the hydrophilic contact angle of 63 degrees and the breaking strength of 99 MPa; heating at 130 deg.C for 30min to reduce the size shrinkage of the membrane by 0.8%.
Example 8
A preparation method of a porous low-shrinkage polypropylene diaphragm for a lithium ion battery comprises the following specific steps:
(1) blending hydroxylated polypropylene, dioctyl terephthalate and zinc acetylacetonate, and performing melt extrusion granulation at 210 ℃ to prepare modified polypropylene with a reversible covalent crosslinking structure;
wherein the mass ratio of the hydroxylated polypropylene to the dioctyl phthalate to the zinc acetylacetonate is 90:9:1, the number average molecular weight of the hydroxylated polypropylene is 61.7kg/mol, and the structural formula of the hydroxylated polypropylene is as follows:
wherein, the hydroxyl insertion rate is 3.8%;
(2) melting and blending the modified polypropylene with the reversible covalent cross-linking structure prepared in the step (1) and polypropylene, and forming a film by using the melt at a high stretching ratio to prepare a polypropylene prefabricated film with the reversible covalent cross-linking structure;
wherein the mass ratio of the modified polypropylene with the reversible covalent crosslinking structure to the polypropylene is 15.0: 85.0; during melt blending, the temperature of the fifth screw region is respectively 160 ℃, 190 ℃, 200 ℃, 220 ℃ and 200 ℃; a high draw ratio of 40; the temperature of a neck ring mold during film forming is 215 ℃;
the thickness of the prepared polypropylene prefabricated film with the reversible covalent crosslinking structure is 40 mu m, the crystallinity is 26.5 percent, and the breaking strength is 115 MPa;
(3) post-stretching the polypropylene prefabricated film with the reversible covalent cross-linking structure prepared in the step (2) to prepare a porous low-shrinkage polypropylene diaphragm for the lithium ion battery;
wherein, the post-stretching comprises cold stretching and hot stretching in sequence, the cold stretching temperature is 25 ℃, the cold stretching strain is 25.0 percent, the hot stretching temperature is 130 ℃, and the hot stretching strain is 100 percent;
the prepared porous low-shrinkage polypropylene diaphragm for the lithium ion battery has the thickness of 22 mu m, the crystallinity of 35 percent, the porosity of 73 percent, the pore size distribution of 0.150 to 0.200 mu m, the hydrophilic contact angle of 50 degrees and the breaking strength of 97 MPa; heating at 130 deg.C for 30min to reduce the size shrinkage of the membrane by 0.5%.
Claims (10)
1. A preparation method of a porous low-shrinkage polypropylene diaphragm for a lithium ion battery is characterized by comprising the following steps: melting, blending and extruding the modified polypropylene with the reversible covalent crosslinking structure and polypropylene, stretching to obtain a polypropylene prefabricated membrane with the reversible covalent crosslinking structure, and then stretching to obtain the porous low-shrinkage polypropylene diaphragm for the lithium ion battery;
the modified polypropylene with the reversible covalent crosslinking structure is prepared by blending, melting, extruding and granulating hydroxylated polypropylene, dioctyl terephthalate and zinc acetylacetonate.
2. The preparation method of the porous low-shrinkage polypropylene separator for the lithium ion battery according to claim 1, wherein the polypropylene separator has a thickness of 10-22.0 μm, a crystallinity of 34.0-45.0%, a porosity of 62.0-75.0%, a pore size distribution of 0.150-0.200 μm, a hydrophilic contact angle of 50.0-65.0 ° and a breaking strength of 95.0-105 MPa; heating the membrane at 130 ℃ for 30min, wherein the size shrinkage rate of the polypropylene membrane is 0.5-1%.
3. The preparation method of the porous low-shrinkage polypropylene separator for the lithium ion battery according to claim 2, which is characterized by comprising the following specific steps:
(1) blending, melting, extruding and granulating hydroxylated polypropylene, dioctyl terephthalate and zinc acetylacetonate to prepare modified polypropylene with a reversible covalent crosslinking structure;
(2) melting and blending the modified polypropylene with the reversible covalent crosslinking structure and polypropylene, and forming a film from the melt at a high stretching ratio to prepare a polypropylene prefabricated film with the reversible covalent crosslinking structure; the high stretch ratio is 40-120;
(3) and (3) post-stretching the polypropylene prefabricated membrane with the reversible covalent crosslinking structure to obtain the porous low-shrinkage polypropylene diaphragm for the lithium ion battery.
4. The preparation method of the porous low-shrinkage polypropylene separator for the lithium ion battery as claimed in claim 3, wherein the structural formula of the hydroxylated polypropylene in the step (1) is as follows:
wherein the number average molecular weight of the hydroxylated polypropylene is 40.8-82.3 kg/mol, x is 855-1632, y is 28-80, and the hydroxyl insertion rate is 3.20-4.60%.
5. The preparation method of the porous low-shrinkage polypropylene separator for the lithium ion battery according to claim 3, wherein the melt extrusion temperature in the step (1) is 200-210 ℃; the mass ratio of the hydroxylated polypropylene to the dioctyl phthalate to the zinc acetylacetonate is 80-90: 9-19: 1.
6. The preparation method of the porous low-shrinkage polypropylene membrane for the lithium ion battery according to claim 3, wherein the five zones of the screw are respectively 160 ℃, 190 ℃, 200 ℃, 220 ℃ and 200 ℃ during the melt blending in the step (2).
7. The preparation method of the porous low-shrinkage polypropylene separator for the lithium ion battery according to claim 3, wherein the mass ratio of the modified polypropylene with the reversible covalent crosslinking structure to the polypropylene in the step (2) is 5.00-15.0: 85.0-95.0.
8. The preparation method of the porous low-shrinkage polypropylene separator for the lithium ion battery according to claim 3, wherein the thickness of the polypropylene prefabricated film with the reversible covalent crosslinking structure in the step (2) is 25.0-40.0 μm, the crystallinity is 25.0-34.0%, and the breaking strength is 110-130 MPa.
9. The preparation method of the porous low-shrinkage polypropylene separator for the lithium ion battery according to claim 3, wherein the die temperature during film formation in the step (2) is 200-220 ℃.
10. The preparation method of the porous low-shrinkage polypropylene membrane for the lithium ion battery according to claim 3, wherein the post-stretching in the step (3) sequentially comprises cold stretching and hot stretching, wherein the cold stretching temperature is 25 ℃, the cold stretching strain is 25.0%, the hot stretching temperature is 130 ℃, and the hot stretching strain is 100-180%.
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