CN114883742B - 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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- H—ELECTRICITY
- 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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- H01M50/40—Separators; Membranes; Diaphragms; Spacing elements inside cells
- H01M50/409—Separators, membranes or diaphragms characterised by the material
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- H01M50/414—Synthetic resins, e.g. thermoplastics or thermosetting resins
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- H01M50/00—Constructional details or processes of manufacture of the non-active parts of electrochemical cells other than fuel cells, e.g. hybrid cells
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- H01M50/40—Separators; Membranes; Diaphragms; Spacing elements inside cells
- H01M50/489—Separators, membranes, diaphragms or spacing elements inside the cells, characterised by their physical properties, e.g. swelling degree, hydrophilicity or shut down properties
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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 the 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 document 1 (spinning-Induced uniformity microporous Formation: an in Situ SAXS/WAXS Study [ J ]. Macromolecules,2018,51 (9), 3433-3442.), the amorphous zone subchain of the polypropylene prefabricated membrane is oriented along the direction of external force and shrinks along the direction perpendicular to the external force, so that microphase separation is formed and initial pores are generated. 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 membranes with a bimodal micro pore structure for water fluorine enhancement and emission performance determination [ J ] Separation and Purification Technology,2019,213, 328-338.) ethylene-vinyl alcohol copolymer/polypropylene membranes were prepared, and by inducing phase Separation in polypropylene, stress weak points were made, increasing the porosity of the membrane, but the pore size was greatly increased and the breaking strength was drastically reduced. 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 the Polypropylene. Crosslinked maleic anhydride-grafted polypropylene was prepared by transesterification in reference 4 (Scalable overcycling of thermoplastic polyolefins in 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 long-chain alkyl modified graphene oxide, a cross-linking agent and polypropylene are melted, blended and formed into a film, and are irradiated and cross-linked to prepare the polypropylene diaphragm which is covalently cross-linked. 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 108400272A 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 has no interaction with polypropylene and is 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 diaphragm 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 microns, the crystallinity is 34.0-45.0%, the porosity is 62.0-75.0%, the pore size distribution is 0.150-0.200 microns, the hydrophilic contact angle is 50.0-65.0 degrees, and the breaking strength is 95.0-105 MPa; heating at 130 ℃ for 30min, wherein the size shrinkage of the diaphragm is 0.5-1% (adopting TMA thermal mechanical analyzer, applying 0.015N to two ends of the film, heating from 30 ℃ to 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 draw 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.
According to the preparation method of the porous low-shrinkage polypropylene diaphragm for the lithium ion battery, when the step (2) is performed with melt blending, 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 separator 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.
In the preparation method of the porous low-shrinkage polypropylene separator for the lithium ion battery, 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.
According to the preparation method of the porous low-shrinkage polypropylene diaphragm for the lithium ion battery, the die temperature 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 crosslinking points in the amorphous region, controls the crosslinking density, reduces the influence of chain segment motion, and further aggregates molecular chains, thereby releasing more free volume and greatly improving the porosity of the diaphragm. The mode of introducing reversible covalent crosslinking points is that hydroxylated polypropylene reacts with dioctyl terephthalate, ester exchange is carried out between hydroxyl and ester group to form ester group crosslinking, and the crosslinking density is controlled by the content of hydroxyl and dioctyl terephthalate (the invention uses hydroxylated polypropylene to react with dioctyl terephthalate, and ester exchange reaction is carried out between hydroxyl and ester group to limit ester group in amorphous region, i.e. reversible covalent crosslinking points can be introduced in amorphous region, hydroxyl per se destroys the regularity of molecular chain, chain segment containing hydroxyl is limited in amorphous region, and crosslinked chain segment is naturally in amorphous region after 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 crosslinked with a sub-chain. 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. The invention can keep the thermal stability of the diaphragm for a long time without falling off through the molecular chain crosslinking of the copolymer.
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 crosslinking polypropylene to prepare the membrane with 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 between 40 and 55 percent, and the porosity of the diaphragm prepared by the method is effectively improved, and higher 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 crosslinking the molecular chains 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 to the present invention by those skilled in the art after reading the contents of the present invention, and these 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:
wherein, the hydroxyl insertion rate is 4.6 percent;
(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 5.0; 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 the 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 116MPa;
(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, 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 101MPa; 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:
wherein, the hydroxyl insertion rate is 4.6%;
(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; during melt blending, the temperature of the fifth screw zone 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 110MPa;
(3) Post-stretching the polypropylene prefabricated membrane 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 to 0.200 mu m, the hydrophilic contact angle of 58 degrees and the breaking strength of 97MPa; 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:
wherein, the hydroxyl insertion rate is 4.6 percent;
(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 10.0; 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 the 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 112MPa;
(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 95MPa; 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:
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; 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 130MPa;
(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 105MPa; the membrane is heated for 30min at 130 ℃, and the size shrinkage rate of the membrane is 1 percent.
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:
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 10.0; 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 126MPa;
(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 to 0.200 mu m, the hydrophilic contact angle of 62 degrees and the breaking strength of 103MPa; 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:
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 15.0; 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 120MPa;
(3) Post-stretching the polypropylene prefabricated membrane 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 101MPa; 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:
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; during melt blending, the temperature of the fifth screw region is respectively 160 ℃, 190 ℃, 200 ℃, 220 ℃ and 200 ℃; a high draw ratio of 80; the temperature of the 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 125MPa;
(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 size distribution of 0.150-0.200 mu m, the hydrophilic contact angle of 63 degrees and the breaking strength of 99MPa; 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:
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; 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 the neck ring mold during film forming is 215 ℃;
the prepared polypropylene prefabricated film with the reversible covalent crosslinking structure has the thickness of 40 mu m, the crystallinity of 26.5 percent and the breaking strength of 115MPa;
(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 97MPa; heating at 130 deg.C for 30min to reduce the size shrinkage of the membrane by 0.5%.
Claims (9)
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;
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%.
2. The preparation method of the porous low-shrinkage polypropylene membrane for the lithium ion battery according to claim 1, wherein the polypropylene membrane 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 at 130 ℃ for 30min, wherein the size shrinkage rate of the polypropylene diaphragm 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 draw ratio to prepare a polypropylene prefabricated film with the reversible covalent crosslinking structure; the high stretch ratio is 40 to 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 membrane for the lithium ion battery, according to claim 3, is characterized in that 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.
5. 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 at temperatures of 160 ℃, 190 ℃, 200 ℃, 220 ℃ and 200 ℃ respectively during melt blending in the step (2).
6. 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.
7. The method for preparing 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.
8. The method for preparing the porous low-shrinkage polypropylene membrane for the lithium ion battery according to claim 3, wherein the die temperature during the membrane formation in the step (2) is 200-220 ℃.
9. 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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