CN114024100B - Separator for nonaqueous electrolyte lithium secondary battery and nonaqueous electrolyte lithium secondary battery - Google Patents
Separator for nonaqueous electrolyte lithium secondary battery and nonaqueous electrolyte lithium secondary battery Download PDFInfo
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- CN114024100B CN114024100B CN202210002819.6A CN202210002819A CN114024100B CN 114024100 B CN114024100 B CN 114024100B CN 202210002819 A CN202210002819 A CN 202210002819A CN 114024100 B CN114024100 B CN 114024100B
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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
- H01M50/40—Separators; Membranes; Diaphragms; Spacing elements inside cells
- H01M50/409—Separators, membranes or diaphragms characterised by the material
- H01M50/449—Separators, membranes or diaphragms characterised by the material having a layered structure
- H01M50/451—Separators, membranes or diaphragms characterised by the material having a layered structure comprising layers of only organic material and layers containing inorganic material
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- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M10/00—Secondary cells; Manufacture thereof
- H01M10/05—Accumulators with non-aqueous electrolyte
- H01M10/052—Li-accumulators
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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
- 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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- H01M50/40—Separators; Membranes; Diaphragms; Spacing elements inside cells
- H01M50/409—Separators, membranes or diaphragms characterised by the material
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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 present invention relates to a separator for a nonaqueous electrolyte lithium secondary battery and a nonaqueous electrolyte lithium secondary battery, including: a polyolefin porous base film; and a porous coating layer containing inorganic particles formed on at least one surface of the polyolefin porous base film; wherein the inorganic particles have a Mohs hardness D of 2.5 to 9, a sphericity coefficient S of 0.2 to 1, and a ratio of the Mohs hardness D/the sphericity coefficient S in the range of 5 to 20, and the inorganic particles have an areal density per unit thickness of 0.7 to 1.5g/m in the porous coating layer2Mum, the separator using the defined inorganic particles can reduce damage of the inorganic particles to the polyolefin porous layer during coating, and is excellent in mechanical properties, air permeability, heat shrinkage properties, and adhesive properties.
Description
Technical Field
The present invention relates to a separator for an electrochemical device and an electrochemical device. In particular, the present invention relates to a separator for a nonaqueous electrolyte lithium secondary battery and a nonaqueous electrolyte lithium secondary battery.
Background
In the construction of lithium batteries, the separator is one of the key internal components. The performance of the diaphragm determines the interface structure, internal resistance and the like of the battery, directly influences the capacity, circulation, safety performance and other characteristics of the battery, and the diaphragm with excellent performance plays an important role in improving the comprehensive performance of the battery. The separator has a main function of separating the positive electrode and the negative electrode of the battery to prevent short circuit due to contact between the two electrodes, and also has a function of allowing electrolyte ions to pass therethrough. The separator material is non-conductive, and the physical and chemical properties of the separator have a great influence on the performance of the battery. In the lithium secondary battery, since the electrolyte is an organic solvent system, a separator material resistant to an organic solvent is required, and a polyolefin porous film having high strength and a thin film is generally used.
Since such polyolefin-based separators have a melting point of 200 ℃ or less, they have a drawback in that they shrink or melt when the temperature of the battery is increased by internal and/or external factors, resulting in volume change. Therefore, there is a high possibility of short circuit between the positive electrode and the negative electrode caused by shrinkage or melting of the separator, thereby causing an unexpected accident such as explosion of the battery caused by discharge. In order to prevent the polyolefin porous film from heat shrinkage at high temperature, in the prior art, poor heat shrinkage performance of the conventional polyolefin-based separator (e.g., CN 100502097C) has been initially improved by using an organic/inorganic composite porous film formed of (1) a heat-resistant porous base film, (2) inorganic particles, and (3) a binder polymer.
The composite porous film is mainly formed by coating a mixture of inorganic particles and a binder polymer on the surface of a base film or on an active layer formed in a part of pores in the base film, but the inorganic particles have a certain pressure on the membrane during the coating process, and thus scratch the surface of the base film, which affects the mechanical strength, especially the puncture strength, of the membrane, and in addition, insufficient closing performance at the scratch is also generated, thereby causing a problem of high temperature safety.
Disclosure of Invention
Thus, up to now, there is no concern in the prior art about the damage that may be caused to the polyolefin porous layer in the coating of the inorganic particles, which in turn brings about adverse effects on mechanical properties and safety properties.
The present invention has been made in view of the above problems, and it is an object of the present invention to provide a separator for a nonaqueous electrolyte lithium secondary battery, which is formed by compounding a porous coating layer containing inorganic particles and a polyolefin porous layer, and which can reduce damage of the inorganic particles to the polyolefin porous layer during the coating process by controlling the irregularity and hardness ratio of the inorganic particles within a suitable range, and a nonaqueous electrolyte lithium secondary battery. The specific scheme is as follows:
a separator for a nonaqueous electrolyte lithium secondary battery, comprising:
a polyolefin porous base film;
and a porous coating layer containing inorganic particles formed on at least one surface of the polyolefin porous base film;
wherein the Mohs hardness D of the inorganic particles is 2.5-9, the sphericity coefficient S is 0.2-1, and the ratio of the Mohs hardness D/the sphericity coefficient S is in the range of 5-20, preferably 6.5-18;
wherein the sphericity coefficient S is defined as follows:
wherein, VpIs the volume of the particles, SpIs the particle surface area;
and the inorganic particles have an areal density per unit thickness of 0.7 to 1.5g/m in the porous coating layer2Mu.m, preferably from 0.8 to 1.2g/m2/μm。
The content of the inorganic particles in the porous coating layer is 20wt% to 95wt%, preferably 30wt% to 80wt%, and more preferably 40wt% to 70 wt%.
The porous coating also contains a binder, and the binder comprises one or more selected from polyamide, polyacrylonitrile, acrylic resin, vinyl acetate-ethylene copolymer, sodium carboxymethyl cellulose, aramid fiber, polyvinyl butyral, polyvinyl pyrrolidone, polyvinylidene fluoride, polytetrafluoroethylene, polyhexafluoropropylene, vinylidene fluoride-hexafluoropropylene copolymer, epoxy resin, siloxane, modified polyolefin, polyurethane, polyvinyl alcohol, polyvinyl ether and styrene-butadiene rubber; among them, polyvinylidene fluoride and/or a copolymer of vinylidene fluoride-hexafluoropropylene are preferable.
In one embodiment, the total thickness of the porous coating layer containing inorganic particles is 0.5 to 20 μm, preferably 3 to 12 μm.
The inorganic particles comprise one or more materials selected from the group consisting of:
based on alumina, silica, zirconia, magnesia, ceria, titania, zinc oxide, iron oxide in oxide ceramics;
silicon nitride, titanium nitride, boron nitride based on nitride materials;
and boehmite, aluminum hydroxide, magnesium hydroxide, barium sulfate, calcium carbonate, wollastonite, silicon carbide, among which alumina or boehmite is preferable.
The inorganic particles have an average particle diameter of 0.01 to 5 μm, preferably 0.1 to 2 μm, and more preferably 0.3 to 1.5. mu.m.
In one embodiment, the polyolefin porous base film comprises a copolymer or a plurality of blends formed by co-polymerizing monomers corresponding to one or more selected from polyethylene, polypropylene, polybutylene and poly-4-methylpentene.
The invention also discloses a preparation method of the diaphragm for the non-aqueous electrolyte lithium secondary battery, which comprises the following steps:
a. preparing a polyolefin porous base membrane;
b. a porous coating layer coating slurry containing inorganic particles configured to be formed on at least one surface of a polyolefin porous base film, the coating slurry comprising:
inorganic particles having a Mohs hardness D of 2.5 to 9, a sphericity coefficient S of 0.2 to 1, and a ratio of the Mohs hardness D/the sphericity coefficient S in the range of 5 to 20, preferably 6.5 to 18; wherein the sphericity coefficient S is defined as follows:
wherein, VpIs the volume of the particles, SpIs the particle surface area;
a binder;
and a solvent;
c. uniformly coating the porous coating layer coating slurry on one side or two sides of a polyolefin porous base membrane; d. and drying to obtain the separator for the non-aqueous electrolyte lithium secondary battery coated with the porous coating layer, wherein the total thickness of the porous coating layer after drying is 0.5-20 μm, preferably 3-12 μm.
The invention also discloses a lithium secondary battery, which comprises a positive electrode, a negative electrode, a nonaqueous electrolyte and the separator for the nonaqueous electrolyte lithium secondary battery or the separator for the nonaqueous electrolyte lithium secondary battery obtained by the preparation method.
The invention has the beneficial effects that:
according to the separator for a nonaqueous electrolyte lithium secondary battery obtained in the present invention, the inorganic particles are uniformly dispersed in the porous coating layer, and therefore, the separator is imparted with heat resistance and heat shrinkage inhibition, and at the same time, when the irregularity of the inorganic particles and the ratio of hardness are controlled within a suitable range, damage of the inorganic particles to the polyolefin porous layer during coating can be reduced, and a balance of ion permeability, heat shrinkage inhibition, mechanical properties, and thermal runaway shutdown uniformity is achieved.
Drawings
Fig. 1 is an SEM image showing alumina particles in example 1;
fig. 2 is a plan SEM view showing the inorganic particle-containing porous coating membrane in example 1;
fig. 3 is a cross-sectional SEM image showing the inorganic particle-containing porous coating membrane in example 1.
Detailed Description
In order to better explain the invention, refer to the implementation of the invention detailed description, and combine the specific examples to further clarify the main content of the invention, but the content of the invention is not limited to the following examples only. The examples, where specific techniques or conditions are not indicated, are to be construed according to the techniques or conditions described in the literature in the art or according to the product specifications. The reagents or instruments used are not indicated by the manufacturer, and are all conventional products commercially available.
[ porous polyolefin base film ]
The polyolefin porous base film may be selected from conventional polyolefin porous base films suitable for separators for nonaqueous electrolyte lithium secondary batteries, and may include a copolymer or a blend of plural kinds of monomers corresponding to one or more kinds selected from polyethylene, polypropylene, polybutene, and poly-4-methylpentene.
The polyolefin microporous membrane preferably contains polyethylene from the viewpoint of exhibiting a shutdown function, and the content of polyethylene is preferably 95% by mass or more.
In one embodiment, the polyolefin porous base film is a single-layer polyolefin microporous film, and in another embodiment, the polyolefin porous base film is a polyolefin microporous film having a laminated structure of 2 or more layers.
The polyolefin contained in the polyolefin porous base film preferably has a weight average molecular weight (Mw) of 10 to 500 ten thousand. When the weight average molecular weight is 10 ten thousand or more, sufficient mechanical properties can be secured. On the other hand, when the weight average molecular weight is 500 ten thousand or less, the shutdown property is good and the film formation is easy.
The thickness of the polyolefin porous base film is not particularly limited, and is preferably 5 to 30 μm. The polyolefin porous base film is a porous polymer film formed mainly by stretching.
There is no limitation on the method of manufacturing the polyolefin porous base film according to the exemplary embodiment of the present invention as long as the polyolefin porous base film is manufactured by one skilled in the art, and in the exemplary embodiment, the polyolefin porous base film may be manufactured by a dry method or a wet method. The dry method is a method of forming micropores by forming a polyolefin film and then stretching the film at a low temperature, which results in microcracks between sheets that are crystalline portions of the polyolefin. The wet process is a process in which a polyolefin-based resin and a diluent are kneaded at a high temperature at which the polyolefin-based resin is melted to form a single phase, the polyolefin and the diluent are phase-separated during cooling, and then the diluent is extracted to form pores therein. The wet process is a method of imparting mechanical strength and transparency by a stretching/extracting process after a phase separation treatment. The wet method is more preferable because it is thinner in film thickness, uniform in pore diameter, and excellent in physical properties, compared to the dry method.
From the viewpoint of obtaining an appropriate membrane resistance and shutdown function, the porosity of the porous substrate is preferably 20 to 60%, and the average pore diameter is 15 to 100 nm.
The puncture strength of the polyolefin porous base film is preferably 200g or more from the viewpoint of improving the production yield.
[ porous coating layer containing inorganic particles ]
The porous coating layer containing inorganic particles, which is provided in the separator for a nonaqueous electrolyte lithium secondary battery of the present invention, is an assembly layer containing inorganic particles and a binder resin and having porous pore diameters, which is provided on one or both surfaces of a polyolefin porous base film.
Wherein the proportion of the inorganic particles in the porous coating layer is 20 to 95% by weight, preferably 30 to 80% by weight, and more preferably 40 to 70% by weight. If the content of the inorganic particles is less than 20wt%, the heat resistance, the unit cell strength and the safety of the separator are not well embodied, while the content of the inorganic particles is more than 95%, which easily causes deterioration of the adhesiveness of the porous coating layer.
The total thickness of the porous coating layer containing inorganic particles is 0.5-20 μm, preferably 3-12 μm, and if less than 1 μm, the effect of thermal shrinkage and adhesion is not obtained, and if more than 20 μm, the ion conductivity is remarkably reduced.
[ inorganic particles ]
The selection and use of the inorganic particles are critical to solve the technical problems of the present invention, and the selection of the shape, hardness, and coating density of the inorganic particles affects various properties of the final separator.
The inorganic particle species may be selected from any inorganic filler that is stable to the electrolyte and electrochemically stable, and may specifically be selected to include one or more of the following materials:
based on alumina, silica, titania, zirconia, magnesia, ceria, titania, zinc oxide, iron oxide in oxide ceramics;
silicon nitride, titanium nitride, boron nitride based on nitride materials;
aluminum hydroxide, magnesium hydroxide, calcium hydroxide, chromium hydroxide, zirconium hydroxide, cerium hydroxide, nickel hydroxide based on metal hydroxide;
and boehmite, magnesium carbonate, magnesium sulfate, barium sulfate, calcium carbonate, wollastonite, silicon carbide and the like, and among them, alumina or boehmite is preferable.
The above inorganic particle species generally have a wide range of Mohs hardness D, from 2.5 to 9. The mohs hardness D refers to a value measured based on the hardness of the following 10 minerals according to the general definition of mohs hardness D:
1: sulfur stone, 2: gypsum, 3: calcite, 4: fluorite, 5: apatite, 6; orthoclase, 7: quartz, 8: topaz, 9: corundum, 10: diamond.
It is to be noted that even for the same inorganic particle type, there is a difference in the mohs hardness D due to the relationship between the crystal form and the degree of hydration.
The inorganic particles express their shape parameters by the sphericity coefficient S, which is specifically defined as follows:
wherein, VpIs the volume of the particles, SpIs the particle surface area;
by this definition, the sphericity factor S of a standard geometry particle is as follows:
| geometric name | Sphericity coefficient S |
| Tetrahedron | 0.671 |
| Cube | 0.806 |
| Octahedron | 0.846 |
| Dodecahedron | 0.910 |
| Icosahedron | 0.939 |
| Ideal cone | 0.794 |
| Hemisphere (sphere) | 0.840 |
| Ideal cylinder | 0.874 |
| Ideal torus | 0.894 |
| Hexagonal icosahedron | 0.986 |
| Ball body | 1 |
The inorganic particles may have a spherical shape, a needle shape, a plate shape, a spindle shape, or the like, and according to the experience of the prior art, it is generally considered that the plate-shaped inorganic particles are selected to increase the path between the positive electrode and the negative electrode, which has a good effect of suppressing the dendrite short circuit.
In the present invention, from the viewpoint of scratch property and mechanical properties of the base film, the range of the ratio of mohs hardness D/sphericity coefficient S is defined such that when the ratio is less than 5, the puncture strength of the separator becomes poor, the mechanical properties deteriorate seriously, the improvement of the heat shrinkage property is insignificant, and when the ratio is more than 20, scratch of the base film easily occurs, and even in the case where the hardness is not too high, if there is a relatively sharp non-spherical portion, irregular scratch is caused to the base film in coating, thereby decreasing the service life of the separator.
In the present invention, the average particle diameter of the inorganic particles is 0.01 to 5 μm, preferably 0.1 to 2 μm, and more preferably 0.3 to 1.5. mu.m.
In the present invention, another aspect is to provide the inorganic particles having an areal density per unit thickness of 0.7 to 1.5g/m in the porous coating layer2Mu.m, preferably from 0.8 to 1.2g/m2Mu m, inorganic particles with a specific thickness surface density higher than 1.5g/m2At/. mu.m, the air permeability is deteriorated, while at less than 0.7, the heat shrinkability is greatly affected.
As a method of adjusting the sphericity and the particle size distribution of the inorganic particles as described above, there is included crushing the inorganic particles using a ball mill, a bead mill, a jet mill, or the like to obtain the corresponding sphericity, particle size, and particle size distribution.
[ Binders ]
The binder resin of the porous coating layer in the present invention includes one or more selected from the group consisting of polyamide, polyacrylonitrile, acrylic resin, vinyl acetate-ethylene copolymer, sodium carboxymethyl cellulose, aramid, polyvinyl butyral, polyvinyl pyrrolidone, polyvinylidene fluoride, polytetrafluoroethylene, polyhexafluoropropylene, a copolymer of vinylidene fluoride-hexafluoropropylene, epoxy resin, siloxane, modified polyolefin, polyurethane, polyvinyl alcohol, polyvinyl ether, and styrene-butadiene rubber; among them, polyvinylidene fluoride and/or a copolymer of vinylidene fluoride-hexafluoropropylene are preferable.
The binder is selected to have a better fixing effect on the inorganic particles, so that the inorganic particles are prevented from falling off during the preparation of the separator or during the storage and use of an electrochemical device having the separator for a nonaqueous electrolyte lithium secondary battery of the present invention.
When the PVDF-based binder is selected, the molecular weight is preferably 80,000-800,000 in view of the binding strength, pore-forming property, and ion permeability.
The content of the binder and the inorganic particles in the coating liquid is preferably 6wt% to 20wt%, preferably 8wt% to 15wt%, from the viewpoint of forming a good porous structure.
[ solvent ]
The solvent used for preparing the coating liquid preferably contains a phase separation agent that induces phase separation from the viewpoint of forming a porous layer having a good porous structure. Therefore, the solvent used for preparing the coating liquid is preferably a mixed solvent of a good solvent and a phase-separating agent. Preferably, the phase separating agent is mixed with the good solvent in an amount that can ensure a viscosity suitable for coating. Examples of the good solvent include acetone, methyl ethyl ketone, N-methylpyrrolidone, and polar amide solvents such as dimethylacetamide, diethylformamide, and examples of the phase separating agent include water, methanol, ethanol, propanol, butanol, butanediol, ethylene glycol, propylene glycol, and tripropylene glycol.
The solvent used for preparing the coating liquid preferably contains 60 wt% or more of a good solvent and 40wt% or less of a phase separating agent, from the viewpoint of forming a good porous structure.
[ coating method ]
After the inorganic particles, the binder, and the solvent are blended to form a coating slurry, a method of first coating the porous substrate with a coating composition by a method such as a dip coating method, an air knife coating method, a curtain coating method, a roll coating method, a wire bar coating method, a gravure coating method, or a die coating method to form a coating film is preferable. Among these coating methods, gravure coating method or die coating method is preferable as the coating method of the coating liquid.
Then, the coating film formed on the substrate by the coating step is dried. The drying conditions are not particularly limited as long as the substrate does not shrink due to softening, the binder component and the particles are sufficiently bonded, and when the coating composition contains thermoplastic particles, the temperature is within a temperature range in which the thermoplastic particles do not melt.
Examples of the drying method include heat transfer drying (adhesion to a high-heat object), convection heat transfer (hot air), radiation heat transfer (infrared ray), and other methods (microwave, induction heating, and the like). Among them, in the above-mentioned production method, since it is necessary to have a precise and uniform drying speed in the width direction, it is preferable to use a method of convection heat transfer or radiation heat transfer. In order to achieve a uniform drying speed in the width direction during constant rate drying, it is preferable to use a method of reducing the total mass transfer coefficient during drying while maintaining a controlled air velocity in the case of using a convection heat transfer drying method. Specifically, a method of feeding hot air in a direction parallel to the supporting substrate, parallel to the feeding direction of the substrate, or perpendicular thereto may be used.
[ lithium Secondary Battery ]
The lithium secondary battery of the present invention has a positive electrode, a negative electrode, an electrolyte solution, and the separator of the present invention disposed between the positive electrode and the negative electrode, and specifically has a structure in which a battery element in which the negative electrode and the positive electrode are opposed to each other with the separator interposed therebetween and the electrolyte solution are sealed in an exterior material.
The positive electrode has a structure in which an active material layer containing, for example, a positive electrode active material and a binder resin is formed on a current collector.
Examples of the positive electrode active material include positive electrode active materials commonly used in the art, such as lithium-containing transition metal oxides, and specific examples thereof include LiCoO2、LiNiO2、LiMn1/2Ni1/2O2、LiNi0.5Co0.2Mn0.3O2、LiMn2O4、LiFePO4、LiC01/2Ni1/2O2、LiAl1/4Ni3/4O2And the like. As the binder resin, there may be mentioned, for example, polyvinylidene fluorideOlefinic resins, styrene-butadiene copolymers, and the like. The conductive additive may be contained, and examples thereof include carbon materials such as acetylene black, ketjen black, and graphite powder. Examples of the current collector include an aluminum foil, a titanium foil, and a stainless steel foil having a thickness of 5 to 20 μm.
As an example of the embodiment of the negative electrode, there is a structure in which an active material layer containing a negative electrode active material and a binder resin is molded on a current collector. The active material layer may further include a conductive aid. Examples of the negative electrode active material include materials capable of electrochemically occluding lithium, and specifically, for example: a carbon material; alloys of silicon, tin, aluminum, etc. with lithium; wood's alloy (Wood); and so on. The binder resin, the conductive assistant, and the current collector are substantially the same as the positive electrode portion. In addition, a metal lithium foil may be used as the negative electrode instead of the negative electrode.
The electrolyte is a solution obtained by dissolving a lithium salt in a nonaqueous solvent. As an example of the electrolyte, an electrolyte system commonly used in the art may be used. Examples of the lithium salt include LiPF6、LiBF4、LiClO4And the like. Examples of the nonaqueous solvent include cyclic carbonates such as ethylene carbonate, propylene carbonate, fluoroethylene carbonate, difluoroethylene carbonate, and vinylene carbonate; chain carbonates such as dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, ethylene carbonate, and fluorine substitutes thereof; and cyclic esters such as γ -butyrolactone and γ -valerolactone, which may be used alone or in combination.
Examples
Hereinafter, the separator and the lithium secondary battery including the separator according to the present invention will be described in more detail with reference to examples. However, the embodiments of the present invention are not limited to the following examples.
< evaluation method >
(1) Film thickness
And testing the thickness of the isolation base film by adopting a micrometer, then testing the thickness after coating, and removing the thickness of the isolation base film to obtain the thickness of the porous coating.
(2) Average particle diameter
The particle size was measured using a particle size measuring apparatus (MicrotracUPA 150, manufactured by Nikkiso K.K.). The measurement conditions were set to load index 0.15 to 0.3 and measurement time 300 seconds, and the particle size was expressed as a value of 50% of the particle size in the obtained data.
(3) Average pore diameter of base film
The pore size distribution of the porous membrane is tested by using an AAQ-3K-A-1 type water pressing instrument of PMI instruments of America, water is used as a test liquid, the water is extruded into pore channels of the membrane under the action of pressure, and the pressure corresponding to the water extruded into different pore diameters follows the Washburn equation, so that a series of pore structure parameters of the membrane are calculated.
(4) Mohs hardness D
The Mohs hardness D of the fine particles was measured by the method shown in CN111397993A, and the scratch was rated with reference to a standard substance.
(5) Sphericity coefficient S
The SEM image of the inorganic particles was magnified 20,000 times and input into the photo imaging software, and the contour (two-dimensional) of each particle was followed. In this analysis, particles that are in close proximity to each other but not attached to each other should be considered as individual particles. The contour particles are then filled with color, and the IMAGEs are introduced into particle characterization software (e.g., IMAGE-PRO PLUS, available from Media Cybernetics, Inc. (Bethesda, Md.)) that is capable of determining the perimeter and area of the particles. Then, the sphericity coefficient S of the particles can be calculated according to the following formula: sphericity ═ length of circumference2/(4 π × area), where perimeter is the software-measured perimeter derived from the outline trace of the particle, and where area is the software-measured area within the trace perimeter of the particle.
The measurement method approximates the definition of the sphericity coefficient S.
(6) Gurley gas permeability value
A 100mm x 100mm sample of the separator with the porous membrane was cut and tested using a test gas mode of 100cc using a u.s.gurley 4110N permeability tester, and the time for all of the test gas to pass through the sample of the separator with the porous membrane was recorded as the Gurley value. The Gurley value of the porous membrane is the Gurley value of the separator provided with the porous membrane minus the Gurley value of the separator not provided with the porous membrane (i.e., a purely porous substrate).
(7) Peel strength between porous substrate and porous layer
An adhesive tape (manufactured by Scotch, model 550R-12) having a width of 12mm and a length of 15cm was attached to one porous layer surface of the separator, and the separator was cut so that the width and the length thereof were consistent with those of the adhesive tape to prepare a measurement sample. When the adhesive tape is bonded to the separator, the longitudinal direction is aligned with the MD direction of the separator. The adhesive tape is used as a support for peeling off one of the porous layers.
The measurement sample was left to stand in an atmosphere at a temperature of 23. + -. 1 ℃ and a relative humidity of 50. + -. 5% for 24 hours or more, and the following measurement was carried out in the same atmosphere.
The adhesive tape was peeled off by about 10cm together with the porous layer immediately below the adhesive tape, and the laminate (1) of the adhesive tape and the porous layer was separated by about 10cm from the laminate (2) of the porous substrate and the other porous layer. The end of the laminate (1) was fixed to the upper chuck of TENSILON (RTC-1210A manufactured by Orientec corporation), and the end of the laminate (2) was fixed to the lower chuck of TENSILON. The measurement sample was suspended in the direction of gravity so that the stretching angle (the angle of the laminate (1) with respect to the measurement sample) was 180 °. The laminate (1) was stretched at a stretching speed of 50mm/min, and the load at which the laminate (1) was peeled from the porous base material was measured. The load of 10mm to 40mm from the start of measurement was taken at intervals of 0.4mm, and the average value thereof was taken as the peel strength.
(8) Electrode bonding strength
The test was carried out with reference to the requirements of GB/T2792.
1) Stacking a4 paper and a separator in the order a4 paper/separator/a 4 paper, wherein the separator coating is opposite the separator coating;
2) carrying out thermoplastic treatment on the stacked A4 paper and the diaphragm at the temperature of 100 ℃;
3) the thermoplastic diaphragm is cut into strips with the length of 200mm and the width of 25mm, the distance between the clamps is (100 +/-5) mm, and the test speed is (50 +/-10) mm/min.
(9) Tensile strength
A test sample with a fixed thickness T is respectively punched and cut into pieces of 100mm multiplied by 15mm along MD (length direction)/TD (width direction) by a cutting die, then the pieces are perpendicular to a chuck of a high-speed rail tension machine, the initial height of the chuck is 5cm up and down, the tensile rate is set to be 50mm/min, and the maximum tensile force is measured to be F.
Tensile strength F/9.8/(15mm × T).
(10) Shrinkage rate
The test was carried out with reference to the requirements of GB/T12027-2004.
1) Cutting a diaphragm with the size of 15 x 15cm, marking the longitudinal direction and the transverse direction on the surface of the diaphragm, and measuring the longitudinal length and the transverse length of a sample by using a ruler;
2) placing the sample in a fixture in a flat state, and then placing the fixture in an oven, and keeping the fixture at the temperature of 130 ℃ for 60 min;
3) after heating, taking out the samples, after the temperature is returned to room temperature, measuring the lengths of the longitudinal mark and the transverse mark again, respectively calculating the shrinkage rate according to the following formula, and finally taking the average value of the samples as the shrinkage rate.
Δ L — heat shrinkage in the longitudinal direction of the sample, expressed in%;
L0-the length of the sample in the longitudinal direction before heating, in millimeters (mm);
l-the length of the sample in the longitudinal direction after heating, in millimeters (mm);
Δ T — heat shrinkage in the transverse direction of the sample, expressed in%;
T0-the length of the sample in the transverse direction before heating, in millimeters (mm);
t-the length of the sample in the transverse direction after heating, in millimeters (mm).
(11) Puncture strength of diaphragm
Preparing a sheet sample, fixing the sheet sample under a test fixture, using a high-speed iron tensile machine and a needling fixture, using a pricking pin with the diameter of 1mm on a pricking tester, performing pricking at the speed of 50mm/min, measuring the top pricking force F after the data is stable, and calculating the pricking strength (unit gf) to be F/9.8 x 1000.
(12) Performance of battery
The anode piece of the invention uses the layered lithium transition metal oxide LiNi0.5Co0.2Mn0.3O2Mixing with acetylene black (SP) serving as a conductive agent and polyvinylidene fluoride (PVDF) serving as a binder in a weight ratio of 96: 2: and 2, adding a solvent N-methyl pyrrolidone, and mixing and stirring uniformly to obtain the anode slurry. And uniformly coating the positive electrode slurry on a positive electrode current collector aluminum foil, drying at 85 ℃, then carrying out cold pressing, trimming, cutting into pieces and slitting, and drying for 4 hours at 85 ℃ under a vacuum condition to obtain a positive electrode piece.
The negative pole piece comprises the following components in parts by weight: 1: 2: 1, adding solvent deionized water, and stirring and mixing uniformly to obtain the cathode slurry. And uniformly coating the negative electrode slurry on a copper foil of a negative electrode current collector, drying at 80-90 ℃ after coating, carrying out cold pressing, trimming, cutting into pieces and slitting, and then drying for 4h under the vacuum condition of 110 ℃ to obtain a negative electrode pole piece.
Preparing a basic electrolyte, wherein the basic electrolyte comprises dimethyl carbonate (DMC), Ethyl Methyl Carbonate (EMC) and Ethylene Carbonate (EC), and the mass ratio of the dimethyl carbonate to the ethyl methyl carbonate to the ethylene carbonate is 5:1: 4. Then adding electrolyte salt to lead LiPF in the electrolyte6The concentration is 1 mol/L.
And (3) stacking the negative pole piece, the diaphragm prepared in each embodiment of the invention and the positive pole piece in sequence, wherein the isolating film is positioned between the positive pole piece and the negative pole piece, the coating on one side surface of the isolating film faces the positive pole piece, and then winding the isolating film into a square bare cell with the thickness of 10mm, the width of 50mm and the length of 120 mm. And (2) filling the bare cell into an aluminum foil packaging bag, baking for 10h at 75 ℃, injecting a non-aqueous electrolyte, carrying out vacuum packaging, standing for 24h, charging to 4.2V by using a constant current of 0.1C (160mA), then charging to 0.05C (80mA) by using a constant voltage of 4.2V until the current is reduced to 0.05V, then discharging to 3.0V by using a constant current of 0.1C (160mA), repeating the charging and discharging for 3 times, and finally charging to 3.8V by using a constant current of 0.1C (160mA), thus completing the preparation of the lithium ion secondary battery.
And (3) testing the cycle performance:
charging the obtained lithium ion secondary battery to 4.2V by a 1C constant current and constant voltage, and standing for 10 min; discharging to 3.0V at constant current of 1C, standing for 10min, and recording the discharged electricity as Q1. The above steps were used as one cycle of charge and discharge, and 200 cycles were performed. The amount of electricity discharged in the 200 th cycle is recorded as Q2. The result of the cycle performance test is Q2/Q1 multiplied by 100 percent.
Example 1:
40 parts by weight of PVDF/HPF (95: 5, weight average molecular weight 500,000) was added to a Dimethylacetamide (DMA) dispersion system and dissolved at 35 ℃ for about 4 hours to prepare a binder polymer solution. The inorganic particles were alumina particles, having a Mohs hardness D8.8, an average particle diameter of 0.5 μm, as measured by PVDF/HPF: alumina particles were added to the binder polymer solution at a weight ratio of 40:60 to form a porous coating layer coating slurry, wherein the alumina particles had a sphericity coefficient S of 0.71 as measured by ball milling. The slurry for forming a porous coating layer was coated on both surfaces of a polyethylene porous substrate (thickness 9 μm, porosity 55%, average pore diameter 70nm, puncture strength 300gf) by a gravure roll under conditions of 23 ℃ and 20% relative humidity, coagulated by 40% dimethylacetamide/water coagulation liquid, washed by pure water, and dried at 70 ℃ to manufacture a separator having a porous coating layer such that each layer on both surfaces of the porous substrate was coated in a dry thickness of 2 μm and the surface density of an inorganic particle monolayer was 2.2g/m2. Fig. 1 is an SEM image of the alumina inorganic particles used in example 1, and fig. 2 and 3 are SEM images of the plane and cross-section of the porous coating membrane of example 1, in which the porous coating layer is interfacially engaged with the porous base membrane and the inorganic particles are embedded at a certain depth in the surface of the base membrane.
Example 2:
the inorganic particles were selected as magnesium hydroxide particles, Mohs hardness D5, average particle size 1 μm, as PVDF/HPF: magnesium hydroxide particles in a weight ratio of 40:60 were added to the binder polymer solution to form a porous coating layer coating slurry, wherein the magnesium hydroxide particles had a sphericity coefficient S of 0.77 as measured by ball milling. The areal density of the inorganic particle monolayer was 1.71g/m, with all other conditions remaining unchanged2。
Example 3:
the inorganic particles were selected as boehmite particles, having a mohs hardness D3.5, an average particle size of 0.68 μm, as PVDF/HPF: boehmite particles in a weight ratio of 40:60 were added to a binder polymer solution to form a porous coating layer coating slurry, wherein the boehmite particles had a sphericity coefficient S of 0.45 as measured by ball milling. The surface density of the inorganic particle monolayer was 2.08g/m under otherwise unchanged conditions2。
Example 4:
the inorganic particles were selected as silica particles, Mohs hardness D6.5, average particle size 0.03 μm, as PVDF/HPF: silica particles in a weight ratio of 40:60 were added to the binder polymer solution to form a porous coating layer coating slurry, wherein the silica particles had a sphericity coefficient S of 0.98 as measured by ball milling. The areal density of the inorganic particle monolayer was 1.66g/m, with other conditions remaining unchanged2。
Example 5:
the inorganic particles were selected as alumina particles, mohs hardness D8.6, average particle size 0.47 μm, as PVDF/HPF: alumina particles in a weight ratio of 15:85 were added to the binder polymer solution to form a porous coating layer coating slurry, wherein the alumina particles had a sphericity coefficient S of 0.79 as measured by ball milling. The surface density of the inorganic particle monolayer was 2.01g/m under otherwise unchanged conditions2。
Example 6:
the inorganic particles were selected as alumina particles, mohs hardness D8, average particle size 0.1 μm, as PVDF/HPF: adding alumina particles with weight ratio of 65:35 into adhesive for polymerizationA porous coating layer coating slurry was formed in the solution, wherein the alumina particles had a sphericity coefficient S of 0.65 as measured by ball milling. The areal density of the inorganic particle monolayer was 1.93g/m, with other conditions remaining unchanged2。
Example 7:
the inorganic particles were selected as alumina particles, mohs hardness D8.8, average particle size 0.8 μm, as PVDF/HPF: alumina particles in a weight ratio of 40:60 were added to the binder polymer solution to form a porous coating layer coating slurry, wherein the alumina particles had a sphericity coefficient S of 0.53 as measured by ball milling. The surface density of the inorganic particle monolayer was 2.21g/m under otherwise unchanged conditions2。
Example 8:
the inorganic particles were selected as alumina particles, mohs hardness D8.8, average particle size 1.5 μm, as PVDF/HPF: alumina particles in a weight ratio of 40:60 were added to the binder polymer solution to form a porous coating layer coating slurry, wherein the silica particles had a sphericity coefficient S of 0.94 as measured by ball milling. The surface density of the inorganic particle monolayer was 2.14g/m under otherwise unchanged conditions2。
Comparative example 1:
the inorganic particles were selected as alumina particles, having a mohs hardness D8.8, an average particle size of 0.5 μm, as PVDF/HPF: alumina particles in a weight ratio of 40:60 were added to the binder polymer solution to form a porous coating layer coating slurry, wherein the alumina particles had a sphericity coefficient S of 0.24 as measured by ball milling. The surface density of the inorganic particle monolayer was 2.30g/m under otherwise unchanged conditions2。
Comparative example 2:
the inorganic particles were selected as hydrated aluminum hydroxide particles, Mohs hardness D3, average particle size 0.5 μm, as a PVDF/HPF: the hydrated aluminum hydroxide particles in a weight ratio of 40:60 were added to the binder polymer solution to form a porous coating layer coating slurry, wherein the hydrated aluminum hydroxide particles had a sphericity coefficient S of 0.98 as measured by ball milling. The areal density of the inorganic particle monolayer was 1.69g/m, with other conditions remaining unchanged2。
Comparative example 3:
the inorganic particles were selected as alumina particles, having a Mohs hardness D7.5, an average particle size of 0.03 μm, as a PVDF/HPF: alumina particles in a weight ratio of 40:60 were added to the binder polymer solution to form a porous coating layer coating slurry, wherein the alumina particles had a sphericity coefficient S of 0.18 as measured by ball milling. The areal density of the inorganic particle monolayer was 1.88g/m, with other conditions remaining unchanged2。
Comparative example 4:
the inorganic particles were selected as alumina particles, having a mohs hardness D8.8, an average particle size of 0.5 μm, as PVDF/HPF: alumina particles in a weight ratio of 10:90 were added to the binder polymer solution to form a porous coating layer coating slurry, wherein the alumina particles had a sphericity coefficient S of 0.71 as measured by ball milling. The areal density of the inorganic particle monolayer was 3.2g/m, with other conditions remaining unchanged2。
Comparative example 5:
the inorganic particles were selected as alumina particles, having a mohs hardness D8.8, an average particle size of 0.5 μm, as PVDF/HPF: alumina particles in a weight ratio of 80:20 were added to the binder polymer solution to form a porous coating layer coating slurry, wherein the alumina particles had a sphericity coefficient S of 0.71 as measured by ball milling. The areal density of the inorganic particle monolayer was 1.26g/m, with other conditions remaining unchanged2。
Table 1 shows performance indexes of the separators of examples 1 to 8 and comparative examples 1 to 5 and corresponding batteries, and it can be seen from table 1 that examples 1 to 8 are excellent in air permeability, peeling/adhesion, heat shrinkage and mechanical properties when the mohs hardness D/sphericity coefficient S is secured within the range of 5 to 20 in spite of the change in the kind of inorganic particles, hardness and shape, and corresponding batteries exhibit good cycle performance, while the puncture strength is deteriorated when the ratio is excessively high in comparative examples 1 and 3; when the ratio is too low in comparative example 2, the thermal shrinkage and the adhesive strength are remarkably deteriorated, while when the areal density per unit thickness is high in comparative example 4, the adhesive property is relatively poor and the air permeability is deteriorated, and when the areal density per unit thickness is low in comparative example 5, the thermal shrinkage is greatly affected. The cycle performance of the corresponding batteries was also inferior to that of examples 1-8.
Each document cited herein, including any cross referenced or related patent or patent application and any patent application or patent to which this application claims priority or its benefits, is hereby incorporated by reference in its entirety unless expressly excluded or otherwise limited. The citation of any document is not an admission that it is prior art with any disclosure of the invention or the claims herein or that it alone, or in combination with any one or more of the references, teaches, suggests or discloses any such invention. Further, to the extent that any meaning or definition of a term in this document conflicts with any meaning or definition of the same term in a document incorporated by reference, the meaning or definition assigned to that term in this document shall govern.
Claims (9)
1. A separator for a nonaqueous electrolyte lithium secondary battery, characterized by comprising:
a polyolefin porous base film;
and a porous coating layer containing inorganic particles formed on at least one surface of the polyolefin porous base film;
wherein the inorganic particles have a Mohs hardness D of 2.5 to 9, a sphericity coefficient S of 0.2 to 1, and a ratio of the Mohs hardness D/the sphericity coefficient S in the range of 5 to 20;
wherein the sphericity coefficient S is defined as follows:
wherein Vp is the particle volume and Sp is the particle surface area;
and of inorganic particlesThe average particle diameter is 0.01-2 μm, and the surface density per unit thickness in the porous coating layer is 0.8-1.2g/m2/μm。
2. The separator for a nonaqueous electrolyte lithium secondary battery as claimed in claim 1, wherein the content of the inorganic particles in the porous coating layer is 20% by weight to 95% by weight.
3. The separator for a nonaqueous electrolyte lithium secondary battery according to claim 1, wherein the porous coating layer further contains a binder, and the binder comprises one or more selected from the group consisting of polyamide, polyacrylonitrile, acrylic resin, vinyl acetate-ethylene copolymer, sodium carboxymethyl cellulose, aramid, polyvinyl butyral, polyvinyl pyrolidone, polyvinylidene fluoride, polytetrafluoroethylene, polyhexafluoropropylene, a copolymer of vinylidene fluoride-hexafluoropropylene, epoxy resin, siloxane, modified polyolefin, polyurethane, polyvinyl alcohol, polyvinyl ether, and styrene-butadiene rubber.
4. The separator for a nonaqueous electrolyte lithium secondary battery according to claim 1, wherein the total thickness of the porous coating layer containing inorganic particles is 0.5 to 20 μm.
5. The separator for a nonaqueous electrolyte lithium secondary battery as claimed in claim 1, wherein the inorganic particles comprise one or more selected from the group consisting of:
based on alumina, silica, zirconia, magnesia, ceria, titania, zinc oxide, iron oxide in oxide ceramics;
silicon nitride, titanium nitride, boron nitride based on nitride materials;
and boehmite, aluminum hydroxide, magnesium hydroxide, barium sulfate, calcium carbonate, wollastonite, silicon carbide.
6. The separator for a nonaqueous electrolyte lithium secondary battery according to claim 5, wherein the inorganic particles are alumina or boehmite.
7. The separator for a nonaqueous electrolyte lithium secondary battery as claimed in claim 1, wherein the polyolefin porous base film comprises a copolymer or a blend of plural kinds of monomers corresponding to one or plural kinds selected from polyethylene, polypropylene, polybutene, and poly-4-methylpentene, which are co-polymerized.
8. The method for producing a separator for a nonaqueous electrolyte lithium secondary battery according to claim 1, characterized by comprising the steps of:
a. preparing a polyolefin porous base membrane;
b. a porous coating layer coating slurry containing inorganic particles configured to be formed on at least one surface of a polyolefin porous base film, the coating slurry comprising:
inorganic particles having a Mohs hardness D of 2.5 to 9, a sphericity coefficient S of 0.2 to 1, and a ratio of the Mohs hardness D/the sphericity coefficient S ranging from 5 to 20;
wherein the sphericity coefficient S is defined as follows:
wherein Vp is the particle volume and Sp is the particle surface area;
a binder;
and a solvent;
c. uniformly coating the porous coating layer coating slurry on one side or two sides of a polyolefin porous base membrane;
d. drying the resultant to obtain a separator for a non-aqueous electrolyte lithium secondary battery coated with a porous coating layer, wherein the porous coating layer after drying has a total thickness of 0.5 to 20 μm, an average particle diameter of the inorganic particles is 0.01 to 2 μm, and an area density per thickness in the porous coating layer is 0.8 to 1.2g/m2/μm。
9. A lithium secondary battery comprising a positive electrode, a negative electrode, a nonaqueous electrolyte solution, and the separator for a nonaqueous electrolyte solution lithium secondary battery according to any one of claims 1 to 7 or the separator for a nonaqueous electrolyte solution lithium secondary battery obtained by the method according to claim 8.
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| PCT/CN2022/101851 WO2023130683A1 (en) | 2022-01-05 | 2022-06-28 | Separator for non-aqueous electrolyte lithium secondary battery, and non-aqueous electrolyte lithium secondary battery |
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| CN114243093B (en) * | 2022-02-25 | 2022-05-13 | 湖南中锂新材料科技有限公司 | High-breakdown-strength aramid fiber diaphragm and preparation method and application thereof |
| CN114709565B (en) * | 2022-06-07 | 2022-09-02 | 中材锂膜(宁乡)有限公司 | Organic/inorganic composite layer porous diaphragm, preparation method thereof and electrochemical device |
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| KR20190067351A (en) * | 2017-12-07 | 2019-06-17 | 주식회사 엘지화학 | A seperator for lithium-sulfur battery and lithium-sulfur battery comprising the same |
| WO2019146137A1 (en) * | 2018-01-24 | 2019-08-01 | パイオトレック株式会社 | Separator-less conductive polymer solid electrolyte secondary battery |
| JP2021054876A (en) * | 2019-09-26 | 2021-04-08 | 日本ゼオン株式会社 | Thermal conductive sheet |
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| US9818999B2 (en) * | 2012-06-29 | 2017-11-14 | Mitsubishi Chemical Corporation | Multilayer porous film, separator for nonaqueous electrolyte secondary battery, and nonaqueous electrolyte secondary battery |
| CN104659313B (en) * | 2015-02-26 | 2017-12-08 | 广东烛光新能源科技有限公司 | A kind of composite porous isolating membrane, the battery and preparation method thereof of the isolation film preparation |
| WO2016152718A1 (en) * | 2015-03-24 | 2016-09-29 | 日本電気株式会社 | Lithium ion secondary battery |
| CN104868086B (en) * | 2015-05-26 | 2018-05-25 | 广东烛光新能源科技有限公司 | A kind of composite porous isolating membrane and preparation method thereof |
| KR102187519B1 (en) * | 2015-08-27 | 2020-12-07 | 도레이 카부시키가이샤 | Battery separator and its manufacturing method |
| CN106784529A (en) * | 2016-12-27 | 2017-05-31 | 深圳中兴创新材料技术有限公司 | A kind of lithium ion battery separator and preparation method thereof |
| CN110212140A (en) * | 2018-02-28 | 2019-09-06 | 宁德新能源科技有限公司 | Isolation film and lithium ion battery |
| CN108423699B (en) * | 2018-04-20 | 2020-01-07 | 淄博鹏丰铝业有限公司 | Preparation method of lithium battery diaphragm coated with special alumina |
| KR102374143B1 (en) * | 2018-12-21 | 2022-03-14 | 주식회사 엘지에너지솔루션 | Separator and electrochemical device comprising the same |
| CN110357135B (en) * | 2019-06-19 | 2020-11-24 | 苏州盛曼特新材料有限公司 | Preparation method of special alumina for high-purity lithium battery diaphragm |
| CN114024100B (en) * | 2022-01-05 | 2022-04-15 | 湖南中锂新材料科技有限公司 | Separator for nonaqueous electrolyte lithium secondary battery and nonaqueous electrolyte lithium secondary battery |
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| KR20190067351A (en) * | 2017-12-07 | 2019-06-17 | 주식회사 엘지화학 | A seperator for lithium-sulfur battery and lithium-sulfur battery comprising the same |
| WO2019146137A1 (en) * | 2018-01-24 | 2019-08-01 | パイオトレック株式会社 | Separator-less conductive polymer solid electrolyte secondary battery |
| JP2021054876A (en) * | 2019-09-26 | 2021-04-08 | 日本ゼオン株式会社 | Thermal conductive sheet |
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