CN113904061B

CN113904061B - High-safety diaphragm and preparation method and application thereof

Info

Publication number: CN113904061B
Application number: CN202111479753.1A
Authority: CN
Inventors: 李爱军; 黄杜斌; 王春源; 杨扬
Original assignee: Beijing Jinyu New Material Technology Co ltd
Current assignee: Beijing Jinyu New Material Technology Co ltd
Priority date: 2021-12-07
Filing date: 2021-12-07
Publication date: 2022-02-11
Anticipated expiration: 2041-12-07
Also published as: CN113904061A

Abstract

The invention relates to a high-safety diaphragm and a preparation method and application thereof, wherein the preparation method comprises the following steps: (I) mixing fluorocarbon polymer, inorganic ceramic material and plasticizer, and heating to obtain mixed granules; (II) laminating the mixture to obtain a sheet-shaped blank; (III) rolling the sheet-shaped blank to obtain a membrane; (IV) coating a release agent on the surface of the membrane, stacking and laminating a plurality of membranes coated with the release agent on the surface to obtain a multi-layer membrane; (V) rolling the multilayer film to obtain a plurality of films; (VI) extracting, pore-forming and heating each film to obtain the high-safety diaphragm. The preparation method is not required by the fluidity of the slurry, can generate the diaphragm with higher inorganic ceramic material content, and effectively improves the safety of the diaphragm; and the yield can be improved, and the thickness and the pores of the diaphragm are easy to control.

Description

High-safety diaphragm and preparation method and application thereof

Technical Field

The invention relates to the technical field of batteries, in particular to a high-safety diaphragm and a preparation method and application thereof.

Background

The lithium ion battery has the characteristics of high energy density and long service life. Has wide application prospect. In particular, in recent years, the demand for electric vehicles has increased explosively, and the demand for lithium ion batteries has further increased. In particular the safety performance of lithium ion batteries.

The safety performance of lithium ion batteries is closely related to the energy density carried by the batteries. The higher the energy density, the worse the safety performance. When the battery is in a higher temperature condition, the diaphragm is easy to shrink and melt, so that the anode and the cathode are short-circuited, and combustion and explosion are easily caused.

Inorganic ceramic materials are generally high-temperature resistant and non-flammable, safety performance can be effectively improved by adding the inorganic ceramic materials into the diaphragm base material, and the content of the inorganic ceramic in diaphragms like patents CN113013552A, CN102544414B and CN110690388A is not higher than 30%, and the improvement of thermal stability is limited; the separators of patents CN108470875A, CN104485438B have high ceramic content, but their tensile strength is low; patent CN109103396B prepared ceramic/polymer substrates as non-woven type separators with larger pore size with risk of internal short circuit.

In addition, in the preparation method of the high-safety diaphragm, since the inorganic ceramic material is rigid, the flexibility of the diaphragm is reduced when the inorganic ceramic material is added into the diaphragm substrate, and the diaphragm is difficult to form by using a stretching method, a flat coating method with low production efficiency is more adopted, and the method cannot be used for large-scale industrial production and seriously hinders the production efficiency of the diaphragm. Therefore, a preparation method for preparing a separator having both high thermal stability and mechanical properties is urgently needed to meet the current demand for high safety performance of lithium ion batteries.

Disclosure of Invention

The invention provides a method for preparing a high-safety diaphragm through rolling, which aims at the defects of the prior art, and the high-safety diaphragm is obtained by adopting a fluorocarbon polymer and an inorganic ceramic material, rolling mixed particles of the fluorocarbon polymer and the inorganic ceramic material, and then carrying out pore-forming through operations such as extraction, heating and the like. The preparation method has good industrial productivity, can greatly reduce the high requirement and high energy consumption of industrial production, has the effect of simple and easy operation, and improves the efficiency of industrial production; compared with the processes of coating, stretching and the like, the process can not be required by the fluidity of slurry, can generate the diaphragm with higher inorganic ceramic material content, and effectively improves the safety of the diaphragm; and the yield can be improved, and the thickness and the gap of the diaphragm are easy to control.

According to one aspect of the present application, there is provided a method of manufacturing a high-safety separator by roll pressing.

Alternatively, the method for preparing a high safety separator by roll pressing includes:

(I) mixing fluorocarbon polymer, inorganic ceramic material and plasticizer, and heating to obtain mixed granules; (II) laminating the mixture to obtain a sheet-shaped blank; (III) rolling the sheet-shaped blank to obtain a membrane; (IV) coating a release agent on the surface of the membrane, stacking and laminating a plurality of membranes coated with the release agent on the surface to obtain a multi-layer membrane; (V) rolling the multilayer film to obtain a plurality of films; (VI) extracting, pore-forming and heating each film to obtain the high-safety diaphragm.

Optionally, in step (IV), the release agent separates the plurality of membranes to prevent blocking.

Optionally, the release agent is a mixture containing solid particles and an oil agent; the solid particles and the oil agent do not react chemically, and the solid particles are insoluble in the oil agent.

Optionally, the release agent is immiscible with the membrane and does not chemically react.

Optionally, the mass ratio of the polyfluoroolefin to the inorganic ceramic material is (10-70): (90-30).

Optionally, the mass ratio of the polyfluoroolefin to the inorganic ceramic material is 10: 90. 20: 80. 30: 70. 40: 60. 50: 50. 60: 40. 70: 30, or any number between any two ratios.

The preparation method can be used for the diaphragm with higher content of inorganic ceramic materials, and is not limited by the flowing property of slurry compared with the prior flat plate coating process.

Generally, the higher the content of the inorganic ceramic material is, the better the heat resistance of the separator is, but in order to ensure certain mechanical strength and film forming property, a certain proportion of polymer must be contained, and the polymer needs to have good mechanical toughness and high melting point, so the inorganic ceramic material is selected to be fluorocarbon polymer; through the experiments of the inventor, the weight proportion of the fluorocarbon polymer cannot be lower than 10%. Otherwise the film would be particularly brittle and inflexible and could not be roll-to-roll processed.

Optionally, the mass of the plasticizer is 20% to 50% of the total mass of the fluorocarbon polymer, the inorganic ceramic material and the plasticizer.

The plasticizer is used for improving the flexibility of the polyfluorocarbon polymer, so that the materials can be fully and uniformly mixed.

Optionally, the plasticizer volatilizes upon heating in step (I).

Optionally, in the mixed granule material, the mass ratio of the plasticizer is 5-30%.

The plasticizer is added to enable the fluorocarbon polymer and the inorganic ceramic material to be mixed more uniformly, and then the plasticizer is volatilized in a heating mode, so that the content is reduced, but a certain content is still kept, namely the content of the plasticizer before heating is larger than that of the plasticizer after heating.

Optionally, the fluorocarbon polymer contains fluorocarbon chains.

Optionally, the repeating structural unit in the fluorocarbon polymer contains fluorocarbon chains.

Optionally, the repeating structural unit is C₂~C₁₀The compound of (1).

Optionally, the repeating structural unit is C₂~C₆The compound of (1).

Optionally, at least one fluorine atom in the repeating structural unit and a carbon atom form a fluorocarbon chain.

Optionally, the fluorocarbon polymer comprises at least one of polyvinyl fluoride,

poly

1,2 difluoroethylene, polyvinylidene fluoride, polytrifluoroethylene, polytetrafluoroethylene, polyvinylidene fluoride-hexafluoropropylene, polytetrafluoroethylene-hexafluoropropylene.

Preferably, the fluorocarbon polymer is polytetrafluoroethylene.

Preferably, the polymerization degree or molecular weight of the fluorocarbon polymer is 10000-50000, and the fluorocarbon polymer has low surface energy and high ductility.

Optionally, the plasticizer comprises at least one of paraffin oil, decalin, dioctyl phthalate, methylene chloride.

Preferably, the molecular weight of the polytetrafluoroethylene is 10000-50000.

Preferably, the inorganic ceramic material comprises an oxide and/or silicate compound.

Optionally, the oxide is at least one of alumina, zirconia, titania, and silica.

Optionally, the silicate compound is at least one of montmorillonite, halloysite, attapulgite, kaolinite.

Optionally, the oxide and the silicate are in the form of one or more of particles, rods and sheets, and have a particle size of 50 to 3 μm.

Optionally, the plasticizer volatilizes upon heating in step (I).

Optionally, the heating temperature is 50-90 ℃. The temperature and the plasticizer type may be synergistically matched.

The plasticizer is partially removed in the heating and stirring process, so that the mixture forms solid granules, and the fluorocarbon polymer (such as polytetrafluoroethylene) has higher flexibility and plasticity than most polymers and can be processed and molded in a solid state.

Optionally, the thickness of the sheet-shaped blank is 0.1-3 mm.

Preferably, in step (II), two laminations are performed.

The first lamination aims at forming the granules, and the formed primary flaky blanks have more pores, so that the plurality of primary flaky blanks are subjected to secondary lamination forming, the porosity is reduced, and the compaction degree is improved; and adjusting the porosity of the secondary sheet blank according to the porosity parameter requirement of the target diaphragm, and adjusting the porosity of the secondary sheet blank according to the relation between the stacking layer number and the thickness.

Optionally, the pressure for laminating in the step (II) is 10-50 MPa.

Optionally, the particle materials are laminated to form a primary sheet-shaped blank, and the thickness of the primary sheet-shaped blank is 0.3-3 mm; stacking and laminating 3-10 layers of primary flaky blanks to form secondary flaky blanks, wherein the thickness of each layer of secondary flaky blanks is 0.1-1 mm, and cutting edges of each layer of secondary flaky blanks to form the flaky blanks.

Optionally, the thickness of the membrane is 0.05-0.6 mm.

In the step (III), the sheet-shaped blank is rolled into a membrane by rolling, and the effective rolling thickness is large due to the precision limitation of a rolling machine, so that a membrane with the target thickness cannot be formed at one time; due to the fact that the ductility of the fluorocarbon polymer (such as polytetrafluoroethylene) is high, a plurality of films can be stacked and then rolled, and the tolerance of a rolling machine (such as a roll-to-roll machine) is evenly distributed into each layer of the plurality of films, so that the effect of reducing errors is achieved, and the film with the target thickness and the controllable precision can be obtained. However, it was found through the inventors' experiments that the film sheets of the present invention are easily stuck after being stacked and rolled, and cannot be separated into individual films, thereby resulting in step (IV) of the present invention.

In the step (IV), the release agent is used for preventing the adhesion between the membranes, and the release agent comprises solid particles and an oil agent; the solid particles form a supporting framework of the separant to physically separate the two diaphragms, and the oil agent wraps and lubricates the inorganic material particles and the diaphragms to prevent the diaphragms from being adhered to the solid particles in the separant.

And (V) rolling the multilayer diaphragm into a multilayer film, wherein each layer of the multilayer film has the same thickness, and the thickness of the diaphragm can be regulated and controlled by adjusting the number of layers of the multilayer diaphragm and the width of the rolling seam. After obtaining a multilayer film, uncovering the multilayer film to form a single-layer film, and then forming a coiled material through a rolling device; and (5) carrying out extraction pore-forming and heating on the obtained film according to the step (VI).

The plasticizer and the separant added in the previous step still remain in the single-layer film, so that the incidental substances are removed by extraction and cleaning of an organic solvent, and then the residual organic solvent is removed by drying to prepare the high-safety diaphragm; the plasticizer occupies a certain volume in the single-layer film, and a large amount of space is left after the plasticizer is removed to form pore channels of the diaphragm and form ion migration channels, so that the step comprises pore-forming of the diaphragm; the organic solvent used for extraction is an organic solvent commonly used in the chemical industry, such as NMP, DMSO, ethyl acetate and the like, and the organic solvent is used for dissolving solvent naphtha and plasticizer.

Optionally, in step (IV), the release agent separates the plurality of membranes to prevent blocking; the separant is a mixture containing solid particles and an oil agent; the solid particles and the oil agent do not have chemical reaction, and the solid particles are insoluble in the oil agent; the isolating agent is not compatible with the membrane and does not have chemical reaction.

Optionally, the solid particles comprise particles of an inorganic material.

Optionally, the inorganic material comprises at least one of a carbonate, an oxide, a layered mineral material.

Optionally, the inorganic material is selected from at least one of calcium carbonate, magnesium carbonate, alumina, zinc oxide, magnesium oxide, montmorillonite powder, talcum powder and lithium soap powder.

Optionally, the oil agent comprises at least one of solvent oil, vegetable oil and mineral oil.

Optionally, the solvent oil comprises at least one of a paraffinic solvent oil, a naphthenic solvent oil, and an aromatic solvent oil.

Optionally, the solvent oil is selected from one of dichloromethane, No. 6 solvent oil, No. 200 solvent oil, toluene, xylene, kerosene, paraffin.

Optionally, the release agent is a mixture of inorganic material particles and mineral spirits.

Optionally, the inorganic material is carbonate, oxide and/or layered mineral material, specifically calcium carbonate, magnesium carbonate, aluminum oxide, zinc oxide, magnesium oxide, montmorillonite powder, talcum powder and lithium soap stone powder, and the particle size range of the inorganic material is 50 nm-3 μm.

Optionally, the mineral spirit is an aromatic mineral spirit.

Optionally, the coating thickness of the release agent is 3-100 μm.

Optionally, the release agent is coated at a thickness of 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 20 μm, 30 μm, 40 μm, 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, 100 μm, or any one of any two values.

The excessively thick release agent can cause the membrane to be excessively thin and even to crack, so that the subsequent complete peeling of the membrane is not facilitated; the thinner release agent can not play a corresponding role in release, so that the films are adhered to each other and the subsequent peeling is not facilitated.

Optionally, the multilayer membrane comprises 3-12 layers of membranes.

In the above steps, the raw material for preparing the diaphragm is in a solid state, and only a small amount of plasticizer is contained for subsequent pore-forming, so that the controllability of film formation is better, and the adjustment of the proportion of the inorganic ceramic material and the fluorocarbon polymer (such as polytetrafluoroethylene) does not cause great influence on the processability of each step. In some disclosed technologies, an extruder is used to extrude the polymer into a membrane, and although the method improves the production efficiency, the membrane substrate of the present invention contains a large amount of inorganic ceramic materials, which has a great influence on the fluidity of the mixture, so that the high-safety membrane of the present invention is not suitable for the preparation method using extrusion molding.

Optionally, the thickness of the high-safety diaphragm is 12-40 μm; the ignition point is more than 400 ℃; the elongation at break is more than 50 percent.

The high-safety diaphragm has the advantages that the diaphragm base material contains a large amount of inorganic ceramic materials, the ignition point of the high-safety diaphragm is more than 400 ℃, the ignition point is related to the addition amount of the inorganic ceramic materials in the diaphragm base material, the higher the mass percent of the inorganic ceramic materials is, the higher the ignition point is, and when the mass percent of the inorganic ceramic materials is increased to more than 80%, the safety diaphragm is not combustible even if exposed fire is met, so that the diaphragm can become a flame retardant of a battery, and the possibility of burning of the battery is reduced.

Further, the high safety separator has excellent thermal stability, and has a thermal shrinkage rate of not more than 15% after being maintained at 150 ℃ for 1 hour, a thermal shrinkage rate of not more than 15% after being maintained at 180 ℃ for 0.5 hour, and a thermal shrinkage rate of not more than 15% even after being maintained at 210 ℃ for 15 min.

The high-safety diaphragm provided by the invention has good elongation at break which is more than 50%, so that the high-safety diaphragm has good extensibility, and can be extended to the maximum extent under the condition that foreign matters exist in the battery and the condition that the foreign matters grow or the battery is extruded, so that the safety risk that the battery is thermally out of control due to the fact that the diaphragm is broken under the action of stress to cause short circuit in the battery is avoided.

The thickness of the high-safety diaphragm is 12-40 mu m; the porosity is 30-65%.

In the aspect of the requirement of energy density of the battery, the thickness of the high-safety diaphragm is as low as possible in principle, but in the prior art, in order to ensure certain mechanical strength, the thickness of the diaphragm needs to be more than 12 μm, and the higher the thickness is, the better the safety performance of the battery is, preferably, the thickness of the high-safety battery diaphragm is 12-40 μm, and in order to ensure the migration of lithium ions between the diaphragms, the porosity is preferably 30-65%.

According to another aspect of the present application, a high safety diaphragm is provided.

Optionally, the high-safety separator is selected from the high-safety separators prepared by any one of the methods.

Further, the high safety separator has excellent thermal stability, and has a thermal shrinkage rate of not more than 10% after being maintained at 150 ℃ for 1 hour, a thermal shrinkage rate of not more than 12% after being maintained at 180 ℃ for 0.5 hour, and a thermal shrinkage rate of not more than 15% even after being maintained at 210 ℃ for 15 min.

According to still another aspect of the present application, there is provided a battery including the high-safety separator prepared according to any one of the above-described methods, and at least one of the above-described high-safety separators.

Optionally, the battery is a lithium ion battery.

Optionally, the lithium ion battery comprises a positive plate, a negative plate and a diaphragm which is arranged between the adjacent positive plate and the negative plate.

The invention has the following main beneficial effects:

(1) the invention provides a method for preparing a high-safety diaphragm through rolling, which adopts fluorocarbon polymer and inorganic ceramic material, and obtains the diaphragm with high safety by rolling mixed particles of the fluorocarbon polymer and the inorganic ceramic material and then carrying out pore-forming through operations such as extraction, heating and the like. In the manufacturing process, a plurality of layers of films are stacked and then rolled to manufacture the film, and a separant is contained among the films to prevent the films from being adhered, so that the diaphragm with higher thickness precision is obtained; the method is not required by the fluidity of the slurry, and can generate the diaphragm with higher inorganic ceramic material content. The high-safety diaphragm provided by the invention has good thermal stability and tensile strength, and can greatly reduce the probability of thermal runaway and combustion of a battery when being used in a lithium ion battery.

(2) The fluorocarbon polymer and the inorganic ceramic material are selected as raw materials, and the fluorocarbon polymer has good mechanical toughness, so that the prepared diaphragm has certain flexibility and film-forming property and is easy to process; and the inorganic ceramic material can increase the heat resistance of the diaphragm, and the diaphragm with high safety is obtained. In addition, a plasticizer and a release agent are also used in the preparation method, and the plasticizer is added to improve the flexibility of the polyfluorocarbon polymer, so that the materials are fully and uniformly mixed; the separant is used for separating and preventing the adhesion between the membranes, is convenient for separating the subsequently generated films and simultaneously prevents the adhesion between the membranes and the inorganic material particles. Moreover, the plasticizer and the separant also play a role in pore forming; the plasticizer and the separant added in the previous step still remain in the single-layer film, and the plasticizer occupies a certain volume in the single-layer film, so that a large amount of space is left after the plasticizer is removed to form pore channels of the diaphragm to form a channel for ion migration, and the high-safety diaphragm with a certain void ratio is obtained.

Drawings

FIG. 1 is a Scanning Electron Microscope (SEM) image of a separator prepared in example 1 of the present application;

FIG. 2 is a Scanning Electron Microscope (SEM) image of a separator prepared in comparative example 6 of the present application;

FIG. 3 is a graph of the voltage after needling and the temperature of the battery surface versus time for a battery assembled with a separator prepared in example 1 of the present application; the abscissa is time in units of s; the ordinate is temperature in units of; the ordinate is voltage in V;

FIG. 4 is a graph of voltage and cell surface temperature versus time after needling for a separator assembled cell made according to comparative example 6 of the present application; the abscissa is time in units of s; the ordinate is temperature in units of; the ordinate is voltage in V;

FIG. 5 is a graph of voltage and cell surface temperature versus time after needling for a separator assembled cell made according to comparative example 7 of the present application; the abscissa is time in units of s; the ordinate is temperature in units of; the ordinate is voltage in V;

fig. 6 is a graph showing the cell cycle of the separator-assembled batteries prepared in example 1 and comparative example 7 of the present application; the abscissa is the cycle number, and the unit is times; the ordinate is the cell capacity in Ah.

Detailed Description

In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below with reference to specific embodiments of the present invention, and it is obvious that the described embodiments are some, but not all embodiments of the present invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention. The reagents, materials and procedures used herein are those widely used in the corresponding fields and are generally available on the market.

Example 1

Mixing polytetrafluoroethylene (molecular weight of 30000) and granular alumina (particle size of 50 nm) at a ratio of 40% to 60%, uniformly dispersing for 60min, adding plasticizer (volume ratio of paraffin oil to dichloromethane is 3: 1), and controlling solid content (content of polytetrafluoroethylene and alumina) at 65%. After stirring for 60min under heating (50 ℃), the solvent was evaporated to form a mixed granule, at which point the mass of the plasticizer was 20%.

Laminating by an oil press, wherein the laminating pressure is 20MPa, and the thickness of the primary laminated sheet blank is 1 mm; stacking and laminating 4 layers of the primary sheet-shaped blanks to form secondary sheet-shaped blanks, wherein the thickness of each layer of the secondary sheet-shaped blanks is 1mm, and the thickness of each layer of the secondary sheet-shaped blanks is 0.25 mm; and cutting edges of the secondary sheet-shaped blanks to form the sheet-shaped blanks.

And rolling the sheet blank by a rolling machine to form a membrane, wherein the thickness of the membrane is 0.1 mm.

A release agent (composition of magnesium carbonate and kerosene, mass ratio of 2: 3) was coated on the surface of the membrane at a release agent coating thickness of 50 μm, and then 6 membranes coated with the release agent on the surface were stacked and laminated to obtain a multilayer membrane.

And rolling the multilayer film sheet into a multilayer film by a double-roller machine, and then respectively uncovering the multilayer film to form the film. And (3) removing the plasticizer and the separant on the surface of the film by extraction and cleaning, and then heating and drying to remove the residual solvent to form the high-safety diaphragm, wherein the thickness of the high-safety diaphragm is 25 mu m, and the porosity of the high-safety diaphragm is 41%.

Example 2

Mixing polytetrafluoroethylene (molecular weight of 30000) and fibrous alumina (particle size of 200 nm) at a ratio of 10% to 90%, uniformly dispersing for 60min, adding plasticizer (volume ratio of paraffin oil to dichloromethane is 3: 1), and controlling solid content at 50%. After heating (70 ℃) and stirring for 60min, the solvent was evaporated to form a mixed granule, at which point the mass of the plasticizer was 23%.

A release agent (composition of alumina and toluene in a mass ratio of 1: 1) was applied to the surface of the membrane at a thickness of 50 μm, and then 5 membranes coated with the release agent on the surface were stacked and laminated to obtain a multilayer membrane.

And rolling the multilayer film sheet into a multilayer film by a double-roller machine, and then respectively uncovering the multilayer film to form the film. And (3) removing the plasticizer and the separant on the surface of the film by extraction and cleaning, and then heating and drying to remove the residual solvent to form the high-safety diaphragm, wherein the thickness of the high-safety diaphragm is 20 microns, and the porosity of the high-safety diaphragm is 48%.

Example 3

Mixing polytetrafluoroethylene (molecular weight 40000), polyvinylidene fluoride (molecular weight 20000), granular alumina (particle size of 50 nm) and sheet montmorillonite (particle size of 2 μm) at a ratio of 40%, 20%, 25%, 15%, uniformly dispersing for 60min, adding plasticizer (dichloromethane), and controlling solid content at 50%. After heating (70 ℃) and stirring for 60min, the solvent is evaporated to form a mixed granule, and the mass ratio of the plasticizer is 15%.

A release agent (composition of No. 6 solvent oil and talcum powder in a mass ratio of 3: 2) is coated on the surface of the membrane, the coating thickness of the release agent is 50 mu m, and then 8 membranes coated with the release agent on the surfaces are stacked and laminated to obtain the multilayer membrane.

And rolling the multilayer film sheet into a multilayer film by a double-roller machine, and then respectively uncovering the multilayer film to form the film. And (3) removing the plasticizer and the separant on the surface of the film by extraction and cleaning, and then heating and drying to remove the residual solvent to form the high-safety diaphragm, wherein the thickness of the high-safety diaphragm is 30 mu m, and the porosity of the high-safety diaphragm is 43%.

Example 4

Mixing polytetrafluoroethylene (molecular weight of 30000), polytetrafluoroethylene-hexafluoropropylene (molecular weight of 50000, D-618, Guangjie Kogyo Co., Ltd.) and rod-shaped attapulgite (particle size of 400 nm) at a ratio of 40%, 30%, uniformly dispersing for 60min, adding plasticizer (dichloromethane), and controlling solid content at 66%. After heating (65 ℃) and stirring for 60min, the solvent is evaporated to form a mixed granule, and the mass percentage of the plasticizer is 30%.

Laminating by an oil press, wherein the laminating pressure is 20MPa, and the thickness of the primary laminated sheet blank is 1 mm; stacking and laminating 4 layers of the primary sheet-shaped blanks to form secondary sheet-shaped blanks, wherein the thickness of each layer of the secondary sheet-shaped blanks is 0.1mm, and the thickness of each layer of the secondary sheet-shaped blanks is 0.25 mm; and cutting edges of the secondary sheet-shaped blanks to form the sheet-shaped blanks.

The surface of the diaphragm is coated with a release agent (the composition is paraffin oil and calcium carbonate, the mass ratio is 3: 2), the coating thickness of the release agent is 50 μm, and then 9 diaphragms coated with the release agent on the surface are stacked and laminated to obtain the multilayer diaphragm.

And rolling the multilayer film sheet into a multilayer film by a double-roller machine, and then respectively uncovering the multilayer film to form the film. And (3) removing the plasticizer and the separant on the surface of the film by extraction and cleaning, and then heating and drying to remove the residual solvent to form the high-safety diaphragm, wherein the thickness of the high-safety diaphragm is 25 mu m, and the porosity of the high-safety diaphragm is 45%.

Comparative example 1

The same procedure as in example 1 was repeated except that alumina, an inorganic ceramic material, was not added.

Comparative example 2

As in example 1, the fluorocarbon polymer was replaced with polyethylene

Because the flexibility and ductility of polyethylene are inferior to those of fluorocarbon polymer, the diaphragm cannot be manufactured by the method of the invention, and the diaphragm with high ceramic content cannot be formed.

Comparative example 3

Like example 1, the fluoropolymer was polyurethane.

The ductility of polyurethane is slightly lower than that of fluorine-containing polymer, and the molecule has strong polar functional groups, higher surface energy and adhesiveness; after the inorganic ceramic material is added, the ceramic is adhered by polyurethane to form a cross-linked structure, so that the integral rigidity is improved, the flexibility and the tensile strength are reduced, and a diaphragm cannot be formed.

Comparative example 4

The same as in example 1, except that no release agent was added.

Because the release agent is not added, the multilayer films are mutually adhered after being rolled, cannot be separated, and cannot obtain thinner films.

Comparative example 5

The plasticizer was completely volatilized as in example 1 except that the heating was conducted, that is, the mass ratio of the plasticizer in the mixed granules was 0%.

At this time, the pellet cannot be laminated to form a sheet-like blank, and thus film formation is impossible.

Comparative example 6

The obtained separator was prepared according to the method of example 1 of patent CN 101260216A.

As can be seen from fig. 2, the separator prepared in comparative example 6 has the film forming effect of a microporous film, and has a honeycomb-like morphology. The diaphragm of example 1 of the present application is heated and mechanically mixed, so that the polytetrafluoroethylene in a plastic state is mixed with the inorganic material, and the diaphragm substrate gradually changes into a fibrous state during the stirring process, so that it can be seen from the microscopic morphology of the formed film that the polymer substrate forms a fibrous filamentous structure (as shown in fig. 1), and this structure can provide the diaphragm with higher mechanical properties, such as tensile strength and tensile strain.

Comparative example 7

The separator of the present comparative example was a polyethylene film having a single-sided alumina ceramic coating layer, and was a commercially available ceramic-coated separator (12 +4 ceramic separator, Li-Sha New energy science and technology Co., Ltd., Zhuhai), the coating thickness was 4 μm, the polyethylene film was 12 μm, the total thickness was 16 μm,

performance testing

The performance of the separators prepared in the examples and comparative examples was tested by the following specific test methods:

(1) elongation at break test: a separator having a length of 100mm and a width of 15mm was taken, and a test was carried out using a tensile tester at a tensile rate of 250mm/min until stretch-broken, and the length of the separator was measured. The elongation at break is obtained by dividing the length after breaking by the initial length.

(2) And (3) ignition point test: a burning point tester is adopted to roll a 5cm multiplied by 20cm diaphragm sample into a coil to be placed in a hole of a copper ingot furnace, a cover is covered, certain temperature is adjusted, and flame is placed above the hole. If there is no continuous flame for 5 seconds above the nozzle of the hole within 5 minutes, the furnace temperature is raised by 5 degrees and a new sample is taken and re-run until a flame for more than 5 seconds above the nozzle is measured. The temperature at this time was recorded as the ignition point.

(3) And (3) testing thermal stability: the thermal shrinkage of the separator at different temperatures was recorded separately.

Table 1 elongation at break and ignition point of separator prepared in example and comparative example

The test results are shown in table 1, the rupture elongation of the diaphragm in the embodiments 1 to 4 of the present invention is significantly higher than that of the comparative example at the ignition point, the ignition point is significantly improved with the increase of the ceramic content in the diaphragm, the incombustible property of the diaphragm is realized with 90% of the ceramic content in the embodiment 2, and compared with other comparative examples, the diaphragm has excellent rupture elongation and higher ignition point, which indicates that the diaphragm prepared by the present invention has better safety performance.

Table 2 heat shrinkage rates of separators prepared in examples and comparative examples

The test results are shown in table 2, and the results show that the separators of examples 1 to 4 of the present invention all have good thermal stability, do not shrink at a high temperature of 210 ℃, and provide a higher safety guarantee for the battery. The comparative example 1 shows certain thermal stability by adopting PTFE with good heat resistance as a diaphragm substrate, but the thermal shrinkage rates of the PTFE exceed 15 percent, which is higher than those of the PTFE in the examples 1 to 4, so that the thermal stability of the diaphragm can be obviously improved by the combination of the fluorocarbon polymer and the inorganic ceramic material, and the higher the ceramic content is, the better the thermal stability is. Comparative example 7 is a commercially used ceramic-coated separator (zhhai, li-share new energy technology ltd, 12+4 ceramic separator), which was found to have far less thermal stability than the high safety separator of the present invention.

(4) Safety test

After the separators prepared in example 1, comparative example 6 and comparative example 7 were assembled into batteries, safety tests were performed, specifically, as follows:

the lithium ion batteries were assembled by using the separators prepared in example 1, comparative example 6 and comparative example 7, respectively, together with a positive electrode, a negative electrode, an electrolyte and a case, in order to fully illustrate the safety performance of the high-safety separator according to the present invention. The lithium nickel cobalt manganese ternary positive electrode with poor safety performance generally considered in the industry is selected as the positive electrode (the content ratio of nickel, cobalt and manganese is 88:5: 7). Meanwhile, the capacity of the lithium ion battery is 60 Ah. It is currently generally accepted that the safety performance of batteries decreases as the capacity of lithium ion batteries increases. The lithium ion battery is subjected to a needling test: the high-temperature resistant steel needle with the diameter of 5mm (the conical angle of the needle tip is 45-60 degrees, the surface of the needle is smooth and has no embroidery corrosion, oxidation layer and oil stain) penetrates into the battery pole piece from the direction vertical to the battery pole piece at the speed of 25mm/s, the temperature and the voltage are observed for 1h, and the temperature and the voltage are recorded.

Fig. 3, 4 and 5 are plots of temperature and voltage versus time during assembly of the separator of example 1 and the separators of comparative examples 6 and 7, respectively, into a battery for a needle test. It can be seen that the high safety separator proposed by the present invention has no explosion and fire after the battery is needle-punched and the battery is left to stand for 1h, and the battery temperature and voltage are maintained at a relatively stable level with little variation from before the needle-punching. In contrast, in the batteries of the separators of comparative examples 6 and 7, ignition and explosion occurred in the batteries within half a minute after the needling, and the battery voltage rapidly decreased to about 0V along with a rapid increase in temperature. The microstructure of the diaphragm prepared by the method is fibrillar and has excellent ductility, so that when the high-safety film provided by the invention is inserted by a steel needle, the diaphragm can deform and wrap the steel needle, the short-circuit resistance of a short-circuit point at the steel needle is greatly improved, the internal short-circuit current is limited in a very low range, meanwhile, the influence of short circuit can be controlled at the short-circuit point due to the good heat resistance of the diaphragm, the electrode material in the battery is inactivated only at the short-circuit point, and chain reaction cannot be caused, so that the thermal runaway of the battery is caused, and the safety performance of the battery is effectively improved. The separator prepared by the method of comparative example 6 has component similarity with the separator prepared by the method of the invention, and the microstructure is a honeycomb microporous structure, so that the extensibility is poor, the deformation amplitude is small when the steel needle is inserted, the steel needle cannot be wrapped, and the punctured battery is rapidly subjected to internal short circuit and thermal runaway.

(5) Electrical Performance testing

As shown in table 3 and fig. 6, it can be found that the separator of the present invention can obtain the same cycle performance as that of a commercial separator when used in a lithium ion battery, and can ensure the discharge performance of the battery while improving the safety of the battery, and the separator of the present invention has excellent application effects compared to the commercial separator.

Table 3 electrical properties of the separator assembled batteries of example 1 and comparative example 7

In summary, the high-safety diaphragm prepared by the method of the invention has the characteristics that the fluorocarbon polymer gradually forms a fiber net shape in the preparation process, and the fluorine-containing polymer has low surface energy and low adhesion, so that the ceramic material exists in the diaphragm in a loading rather than adhesion mode, the diaphragm can keep enough flexibility and mechanical strength, the risk caused by the penetration of the battery can be reduced, the diaphragm can not shrink under the high-temperature environment by taking a large amount of thermally stable ceramic material as a framework, and the diaphragm has excellent safety performance under the comprehensive action of the factors.

Finally, it should be noted that: the above-mentioned embodiments are only used for illustrating the technical solution of the present invention, and not for limiting the same; although the present invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some or all of the technical features may be equivalently replaced; and the modifications or the substitutions do not make the essence of the corresponding technical solutions depart from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method of making a high safety diaphragm, comprising:

(I) mixing fluorocarbon polymer, inorganic ceramic material and plasticizer, and heating to obtain mixed granules;

(II) laminating the mixed granules to obtain a sheet-shaped blank;

(III) rolling the sheet-shaped blank to obtain a membrane;

(IV) coating a release agent on the surface of the membrane, stacking and laminating a plurality of membranes coated with the release agent on the surface to obtain a multi-layer membrane;

(V) rolling the multilayer film to obtain a plurality of films; peeling a plurality of the films apart to form a single film; the single film contains a plasticizer;

(VI) extracting and pore-forming the single film, and heating to obtain the high-safety diaphragm;

in the step (IV), the release agent separates the plurality of membranes to prevent adhesion;

the separant is a mixture containing solid particles and an oil agent; the solid particles and the oil agent do not have chemical reaction, and the solid particles are insoluble in the oil agent;

the isolating agent and the membrane are not mutually soluble and do not have chemical reaction.

2. The method according to claim 1, wherein the mass of the plasticizer is 20% to 50% of the total mass of the fluoropolymer, the inorganic ceramic material and the plasticizer;

the plasticizer volatilizes when heated in step (I);

in the mixed particle material, the mass ratio of the plasticizer is 5-30%.

3. The method according to claim 1, wherein in the step (I), the heating temperature is 50-90 ℃;

the plasticizer comprises at least one of paraffin oil, decalin, dioctyl phthalate and dichloromethane.

4. The method of claim 1, wherein the fluorocarbon polymer comprises at least one of polyvinyl fluoride, poly 1,2 difluoroethylene, polyvinylidene fluoride, polytrifluoroethylene, polytetrafluoroethylene, polyvinylidene fluoride-hexafluoropropylene, polytetrafluoroethylene-hexafluoropropylene;

the inorganic ceramic material comprises an oxide and/or silicate compound.

5. The method of claim 4, wherein the oxide is selected from at least one of alumina, zirconia, titania, silica;

the silicate compound is at least one of montmorillonite, halloysite, attapulgite and kaolinite.

6. The method according to claim 4, wherein the mass ratio of the fluorocarbon polymer to the inorganic ceramic material is (10-70): (90-30).

7. The method according to any one of claims 1 to 6, wherein the high-safety separator has a thickness of 12 to 40 μm; the ignition point is more than 400 ℃, and the elongation at break is more than 50%.

8. A high-safety separator prepared by the method according to any one of claims 1 to 7.

9. A battery comprising the high-safety separator produced by the method according to any one of claims 1 to 7.