CN111599969A - PVDF (polyvinylidene fluoride) -coated lithium ion battery diaphragm and preparation method thereof - Google Patents
PVDF (polyvinylidene fluoride) -coated lithium ion battery diaphragm and preparation method thereof Download PDFInfo
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M50/00—Constructional details or processes of manufacture of the non-active parts of electrochemical cells other than fuel cells, e.g. hybrid cells
- H01M50/40—Separators; Membranes; Diaphragms; Spacing elements inside cells
- H01M50/409—Separators, membranes or diaphragms characterised by the material
- H01M50/411—Organic material
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M10/00—Secondary cells; Manufacture thereof
- H01M10/05—Accumulators with non-aqueous electrolyte
- H01M10/052—Li-accumulators
- H01M10/0525—Rocking-chair batteries, i.e. batteries with lithium insertion or intercalation in both electrodes; Lithium-ion batteries
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M50/00—Constructional details or processes of manufacture of the non-active parts of electrochemical cells other than fuel cells, e.g. hybrid cells
- H01M50/40—Separators; Membranes; Diaphragms; Spacing elements inside cells
- H01M50/403—Manufacturing processes of separators, membranes or diaphragms
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M50/00—Constructional details or processes of manufacture of the non-active parts of electrochemical cells other than fuel cells, e.g. hybrid cells
- H01M50/40—Separators; Membranes; Diaphragms; Spacing elements inside cells
- H01M50/409—Separators, membranes or diaphragms characterised by the material
- H01M50/449—Separators, membranes or diaphragms characterised by the material having a layered structure
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
- Y02E60/10—Energy storage using batteries
Abstract
The invention discloses a PVDF-coated lithium ion battery diaphragm, which adopts a high-molecular dispersant, a high-molecular emulsifier and a solvent, and prepares a water-based PVDF slurry which has uniform dispersion and good stability in the manufacturing, transportation and use processes according to the synergistic cooperation of the three components, and the water-based PVDF coating formed after the water-based PVDF slurry is coated on the diaphragm has high peel strength; after the PVDF-coated lithium ion battery diaphragm is prepared into a battery, the hot-pressing formation condition is mild, the bonding force with a pole piece is excellent, and the good battery hardness and the long-acting cycle life of the soft package battery are ensured.
Description
Technical Field
The invention relates to the field of lithium ion batteries, in particular to a PVDF (polyvinylidene fluoride) -coated lithium ion battery diaphragm and a preparation method thereof.
Background
The existing water-based PVDF slurry for the lithium battery diaphragm on the market generally has the following problems: firstly, the coating on the surface of the diaphragm is not uniform due to the non-uniform slurry; secondly, coating the slurry on the surface of the diaphragm, and drying the coating to ensure that the adhesion of the coating is poor; and thirdly, the diaphragm containing the PVDF coating is manufactured into a battery core, the battery is softened after hot pressing formation, and the battery is disassembled to find that the PVDF coating is bonded with a pole piece badly, so that the soft package battery has the problems of low yield, poor battery performance and the like.
Most of the existing aqueous PVDF slurry is prepared by adding aqueous auxiliary agents, such as wetting agents, dispersing agents, defoaming agents, thickening agents, emulsifying agents and the like, hydrophobic PVDF powder is dispersed in water through a strong mechanical stirring process, and some products can also be added with functional powder and fillers to prepare mixed slurry, but the products often have the following problems: firstly, the types and the addition amounts of the auxiliary agents are large, and the auxiliary agents have the problem of dissolution and migration in the battery electrolyte, so that the cycle life of the battery is short; secondly, due to the hydrophobic characteristic of PVDF, an amphiphilic auxiliary agent needs to be added to emulsify and disperse PVDF into water, the suspension dispersion performance of the PVDF of the slurry just prepared is good, but the quality guarantee period of the product is short, and the sedimentation problem of the slurry in transportation exists; during production and use, the stability of the slurry is easily damaged due to mechanical external forces such as stirring, high-speed coating and the like, and the problems of emulsion breaking, agglomeration and coarsening, particle sedimentation and the like are presented; and thirdly, the PVDF slurry prepared by the process has good integrity of PVDF particles and no damage to the crystal structure, so that the crystal structure of the PVDF is difficult to damage when the PVDF is subjected to hot pressing (conditions: electrolyte infiltration, certain temperature and pressure), and the problems of poor swelling, poor bonding and the like are presented, and the application effect of the battery is seriously influenced. Therefore, the water-based PVDF slurry with good stability and uniform dispersion is urgently needed in the market at present, and the peeling strength is high after the water-based PVDF slurry is coated on a diaphragm; after the battery is manufactured, the hot-pressing formation condition is mild, and the product with excellent bonding force is used for ensuring the good battery hardness and the long-acting cycle life of the soft package battery.
Chinese patent CN201510452494 discloses an aqueous PVDF slurry for lithium ion batteries and a preparation and use method thereof, and the technical key points are that, calculated by 100% of total mass, the mass percentages of the components are as follows: 0.05-20% of PVDF powder, 0.1-10% of dispersing agent, 0.2-5% of wetting agent and the balance of water. The aqueous PVDF slurry prepared by the method is a product prepared by the conventional process sold in the market at present, and as mentioned above, the aqueous PVDF slurry has various problems, such as poor slurry stability, poor coating adhesion, poor adhesion with a pole piece and the like, and the hardness and long-acting cycle life of the battery required by a middle-high-end soft package battery are difficult to meet.
Disclosure of Invention
In order to overcome the defects of the prior art, the invention aims to provide a PVDF-coated lithium ion battery diaphragm.A base material is coated with aqueous PVDF slurry which has good stability, uniform dispersion and high peel strength after being coated on the diaphragm; after the battery is manufactured, the hot-pressing formation condition is mild, and the product with excellent bonding force is used for ensuring the good battery hardness and the long-acting cycle life of the soft package battery.
The purpose of the invention is realized by adopting the following technical scheme:
a PVDF coated lithium ion battery separator, wherein one side or two sides of a substrate are coated with aqueous PVDF slurry, and the aqueous PVDF slurry consists of the following components in percentage by weight: 1-40 wt% of PVDF base material, and the balance of deionized water; wherein the PVDF base material consists of the following raw materials in percentage by weight: 0.1-5% of a polymer emulsifier, 0.5-10% of a polymer dispersant, 1-10% of a solvent and the balance of PVDF resin powder; the weight average molecular weight of the macromolecular emulsifier is more than 5 ten thousand and less than MwLess than 20 ten thousand, and the HLB value of the macromolecular emulsifier is 3-18; the weight average molecular weight of the macromolecular dispersant is more than 10 ten thousand and less than MwLess than 50 ten thousand; the solvent is soluble in water.
Since C-C in PVDF belongs to nonpolar groups and C-H belongs to weakly polar groups, these are hydrophobic groups; the C-F bond is a polar bond, but the surface energy of the C-F bond is very low, and the whole macromolecular structure is symmetrical, non-polar and hydrophobic. PVDF powder is difficult to wet and disperse with water. The preparation method comprises the following steps of preparing an aqueous PVDF slurry, wherein the problem of wetting and emulsifying of PVDF is solved, so that the aqueous PVDF slurry adopts a high-molecular emulsifier with the HLB value of 3-18, the surface tension of an aqueous medium can be reduced, and the wettability of PVDF powder in the aqueous medium is improved, so that the PVDF powder is infiltrated and solvated by deionized water; the interfacial tension is reduced, and simultaneously, a layer of polymer interfacial film is formed, the interfacial film has a protection effect on PVDF particles, has a strong hydrogen bonding effect and high mechanical strength, and can ensure the suspension stability of the PVDF slurry in the manufacturing, transporting and using processes.
Wherein the weight average molecular weight of the macromolecular emulsifier is more than 5 ten thousand and less than MwLess than 20 ten thousand, and the molecular weight is too small, the electrolyte resistance of the material is insufficient, the interface emulsification protective capability of PVDF particles is insufficient, and the stability of slurry storage, transportation and use is poor; if the molecular weight is too large, the dissolution and dispersion become difficult, and meanwhile, the film forming property of the material is too good, so that the problem that the pores of the diaphragm are blocked after the subsequent slurry is coated is easily caused, and the application rate and the cycle performance of the product are poor. The dosage of the polymer wetting emulsifier is 0.1-5% of the mass percent of the PVDF base material, and if the dosage is too small, the wetting emulsifying capacity is insufficient, and the PVDF resin powder cannot be fully wetted and dispersed by water, so that the pulping failure is caused; if the amount of the slurry is too much, the polymer material can gather, and the problem that the pores of the diaphragm are blocked after subsequent slurry coating is easily caused.
After the PVDF resin powder is wetted and emulsified in the deionized water, most of the PVDF resin powder suspended in the water still exists in the form of aggregates, so that the particle size distribution is wide, the particle size is large, the application requirement cannot be met, and further dispersion is needed to open the PVDF particle aggregates. The water-based PVDF slurry does not use a small molecular auxiliary agent, adopts a macromolecular dispersing agent with the weight-average molecular weight of more than 10 ten thousand and less than 50 ten thousand, has an amphiphilic group, a hydrophilic group and a lipophilic group in a macromolecular structure, and the lipophilic group of the macromolecular dispersing agent forms an adsorption layer on the surface of PVDF particles, so that the charge on the surface of solid particles is increased, and the reaction force between the particles forming the three-dimensional obstruction is improved; the hydrophilic group has stronger affinity with water, so that the wetting degree of the solid particles by water is increased, and the solid particles are far away from each other due to steric hindrance repulsion; meanwhile, the macromolecular dispersant has obvious thickening effect due to the large molecular weight (the weight average molecular weight is more than 10 ten thousand), so that the system is uniform, the suspension performance is improved, no precipitation is caused, and the whole slurry system is stable and uniform. If the weight average molecular weight of the high molecular dispersant is less than 10 ten thousand, the molecular chain of the dispersant is short, the dispersant cannot form enough steric hindrance when being adsorbed on the surface of PVDF particles, the product has the risk of re-agglomeration after dispersion, and the dispersant with small molecular weight has slightly poor thickening and suspending properties and insufficient electrolyte resistance, which is not preferable; if the weight average molecular weight of the high molecular dispersant is more than 50 ten thousand, the thickening effect on the slurry is too obvious, so that the slurry has poor fluidity and the dispersing effect is influenced.
Wherein, the dosage of the macromolecular dispersant accounts for 0.5 to 10 percent of the mass percent of the PVDF base material. If the addition amount is small, the polymer dispersant cannot effectively coat and modify the PVDF resin powder, so that poor dispersion is caused; if the amount of the polymeric dispersant added is large, the thickening effect on the slurry is too remarkable due to the large viscosity of the polymeric aqueous solution, resulting in poor fluidity of the slurry and difficulty in dispersion.
The aqueous PVDF slurry is calculated according to the total mass of PVDF base material and deionized water as 100%, the PVDF base material accounts for 30-40%, the content of the PVDF base material is too little, the solid content of the prepared slurry is too low, and an enough coating thickness cannot be formed, so that the application value cannot be reached; if the PVDF base material content is too much, the dispersion becomes very difficult, and the viscosity of the prepared slurry is very high, so that the uniform coating cannot be realized during production, and the application requirement cannot be met.
Because the PVDF resin powder is a semi-crystalline high polymer material, the crystallinity is 60-80%; the degree of crystallinity determines the swelling degree of the PVDF-slurry-coated lithium ion battery diaphragm in electrolyte, the size of the binding power of a pole piece and the stability at high temperature; the high crystallinity of PVDF material causes that PVDF is difficult to damage the crystal structure when being hot-pressed (conditions: electrolyte infiltration, certain temperature and pressure), so that the problems of poor swelling, poor bonding and the like are presented, the application effect of the battery is seriously influenced, and the problems are the common application problems of the aqueous PVDF slurry for the current commercial lithium battery diaphragm, which are not solved all the time and become the pain point of the industry. Therefore, the aqueous PVDF slurry is pre-swelled by using a solvent which is an active solvent relative to PVDF resin powder, is soluble in water or soluble in water, has a low boiling point or is volatile, and can be removed by mild heating or reduced pressure drying means. The PVDF resin powder is pre-swelled by adding the solvent, the crystal structure of a part of the PVDF resin powder is destroyed, and the crystallinity of the material is reduced, so that the swelling effect of the PVDF-slurry-coated lithium ion battery diaphragm in the electrolyte is improved, the binding force with a pole piece is improved, the battery hardness is improved, and the cycle life is prolonged.
Wherein, the addition amount of the solvent accounts for 1 to 10 percent of the mass percentage of the PVDF base material. If the addition amount of the solvent is too high, the PVDF resin powder is too large in swelling and even dissolved, the damage to the crystal structure is serious, and the quality problems of large viscosity of the PVDF aqueous slurry, adhesion and agglomeration of PVDF particles, sedimentation and the like are caused; if the addition amount of the solvent is too small, the swelling of the PVDF resin powder is small, the reduction of the crystallinity is not obvious, and the bonding effect with the pole piece cannot be improved.
Further, the polymer emulsifier is one or more of polyacrylic acid, polyethylene glycol, polyvinyl alcohol, polyethylene oxide, hydroxyethyl cellulose, polydimethylsiloxane and polyether modified siloxane. The polymer emulsifier has excellent electrolyte resistance, does not migrate in a battery system, has excellent film forming property, can improve the adhesive force between the PVDF coating and the diaphragm, and helps the PVDF coating film to exert better battery application performance.
Still further, the polymeric dispersant is one or more of modified polyacrylic acid, carboxylated modified styrene-butadiene latex, polyacrylonitrile modified copolymer, polyacrylamide, polyvinyl acetamide, ammonium polyacrylate, polyether derivative and polycarboxylate. The polymer dispersant has good film forming property, adhesive force and electrolyte resistance, improves the adhesive force of the PVDF coating and the diaphragm, is insoluble in electrolyte, does not have the migration problem, and has no side effect on the battery.
Further, the solvent is one or more of acetone, methyl ethyl ketone, 4-methyl-2-pentanone, tetrahydrofuran, methyl carbonate, ethyl carbonate, methyl ethyl carbonate, ethylene glycol monomethyl ether, ethylene glycol dimethyl ether, ethylene glycol monoethyl ether, 1, 4-dioxane and dimethylformamide.
The preparation method of the PVDF-coated lithium ion battery diaphragm comprises the following steps:
1) adding a high-molecular emulsifier into deionized water, stirring and dissolving the mixture until the mixture is fully and uniformly dissolved, then adding PVDF resin powder, and stirring the mixture until the PVDF resin powder is fully wetted by the deionized water to obtain a first mixture;
2) adding a high molecular dispersant into the mixture I, and performing wet ball milling to obtain a mixture II;
in order to open up the PVDF particle agglomerates, a physical dispersion method, which includes a high-speed shear mixing dispersion method and an ultrasonic dispersion method in addition to wet ball milling, is combined by using a polymeric dispersant (chemical modification) and a physical dispersion phase. The physical dispersion means is preferably a wet ball milling process, the ball milling process utilizes beads with proper sizes to physically act on PVDF powder particles through shearing, impacting and the like, the soft agglomeration of the particles can be effectively opened by high energy, the opened particles are acted on the surfaces of the particles by a high molecular dispersant to form effective coating and chemical action so as to be stabilized, and steric hindrance is formed, so that the particles cannot be agglomerated again.
3) Adding a solvent into the mixture II, uniformly stirring, heating and preserving heat, and drying to remove most of the solvent to obtain aqueous PVDF slurry; the crystallinity of the PVDF resin powder contained in the aqueous PVDF slurry is reduced.
4) And coating the aqueous PVDF slurry on one side or two sides of the base material to form an aqueous PVDF coating, and drying to obtain the PVDF-coated lithium ion battery diaphragm.
Further, in the step 2), the size of beads used for ball milling is 0.2-2.0 mm. The dispersion time of the wet ball milling is 0.1-3.0 h. The material of the beads used for ball milling is one or more of zirconia, alumina, yttria and silica. The ball mill may be selected from an attritor mill, a horizontal ball mill or a basket ball mill.
And further, in the step 3), the heating rate is 1-10 ℃/min, the swelling temperature is 30-80 ℃, the heat preservation time is 0.1-5.0h, the decompression drying time is 0.1-3.0h, the decompression drying temperature is 20-50 ℃, and the vacuum degree is-0.1 MPa.
The PVDF-coated lithium ion battery diaphragm can be applied to lithium batteries.
Compared with the prior art, the invention has the beneficial effects that:
(1) according to the invention, the water-based PVDF slurry which is uniform and consistent in dispersion and good in stability in the manufacturing, transporting and using processes is prepared by adopting the polymer dispersing agent, the polymer emulsifying agent and the solvent according to the synergistic cooperation of the three, and the water-based PVDF coating formed after the water-based PVDF slurry is coated on the diaphragm has high peel strength; after the PVDF-coated lithium ion battery diaphragm is made into a battery, the battery is thermally pressed into a product with mild conditions and excellent bonding force with a pole piece, so that the good battery hardness and the long-acting cycle life of the soft package battery are ensured.
(2) The invention applies the pre-swelling process to carry out pre-treatment on the PVDF resin powder, destroys the crystal structure of a part of the PVDF resin powder, reduces the crystallinity of the material, and improves the swelling degree of the PVDF-coated lithium ion battery diaphragm in the electrolyte, thereby improving the binding power of the PVDF-coated lithium ion battery diaphragm and a pole piece, improving the hardness of the battery and prolonging the cycle life of the battery.
Drawings
Fig. 1 is a schematic layer structure of a PVDF coated lithium ion battery separator of the present invention.
In the figure: 1. a substrate; 2. a water-based PVDF coating.
Detailed Description
The present invention will be further described with reference to the accompanying drawings and the detailed description, and it should be noted that any combination of the embodiments or technical features described below can be used to form a new embodiment without conflict.
A PVDF coated lithium ion battery separator, wherein one side or two sides of a substrate 1 are coated with aqueous PVDF slurry, and the aqueous PVDF slurry consists of the following components in percentage by weight: 1-40 wt% of PVDF base material, and the balance of deionized water; wherein the PVDF base material consists of the following raw materials in percentage by weight: 0.1-5% of a polymer emulsifier, 0.5-10% of a polymer dispersant, 1-10% of a solvent and the balance of PVDF resin powder; the weight average molecular weight of the macromolecular emulsifier is more than 5 ten thousand and less than MwLess than 20 ten thousand, and the HLB value of the macromolecular emulsifier is 3-18; the weight average molecular weight of the macromolecular dispersant is 10 ten thousand <MwLess than 50 ten thousand; the solvent is soluble in water.
Further, the polymer emulsifier is one or more of polyacrylic acid, polyethylene glycol, polyvinyl alcohol, polyethylene oxide, hydroxyethyl cellulose, polydimethylsiloxane and polyether modified siloxane.
Still further, the polymeric dispersant is one or more of modified polyacrylic acid, carboxylated modified styrene-butadiene latex, polyacrylonitrile modified copolymer, polyacrylamide, polyvinyl acetamide, ammonium polyacrylate, polyether derivative and polycarboxylate.
Further, the solvent is one or more of acetone, methyl ethyl ketone, 4-methyl-2-pentanone, tetrahydrofuran, methyl carbonate, ethyl carbonate, methyl ethyl carbonate, ethylene glycol monomethyl ether, ethylene glycol dimethyl ether, ethylene glycol monoethyl ether, 1, 4-dioxane and dimethylformamide.
The preparation method of the PVDF-coated lithium ion battery diaphragm comprises the following steps:
1) adding a high-molecular emulsifier into deionized water, stirring and dissolving the mixture until the mixture is fully and uniformly dissolved, then adding PVDF resin powder, and stirring the mixture until the PVDF resin powder is fully wetted by the deionized water to obtain a first mixture;
2) adding a high molecular dispersant into the mixture I, and performing wet ball milling to obtain a mixture II;
3) adding a solvent into the mixture II, uniformly stirring, heating and preserving heat, and drying to remove most of the solvent to obtain aqueous PVDF slurry;
4) the PVDF-coated lithium ion battery separator shown in fig. 1 is obtained by coating the aqueous PVDF slurry on one or both sides of the substrate 1 to form the aqueous PVDF coating 2 and drying the aqueous PVDF coating.
Further, in the step 2), the size of beads used for ball milling is 0.2-2.0 mm. The dispersion time of the wet ball milling is 0.1-3.0 h. The material of the beads used for ball milling is one or more of zirconia, alumina, yttria and silica. The ball mill may be selected from an attritor mill, a horizontal ball mill or a basket ball mill.
And further, in the step 3), the heating rate is 1-10 ℃/min, the swelling temperature is 30-80 ℃, the heat preservation time is 0.1-5.0h, the decompression drying time is 0.1-3.0h, the decompression drying temperature is 20-50 ℃, and the vacuum degree is-0.1 MPa.
Example 1
A PVDF-coated lithium ion battery diaphragm is characterized in that one side of a base material 1 is coated with aqueous PVDF slurry, the aqueous PVDF slurry contains 40% of PVDF base material according to weight percentage, and the balance is deionized water. The PVDF base material consists of the following substances in parts by mass: 100g of PVDF resin powder, 1g of a polymer wetting emulsifier, 3g of a polymer dispersant and 5g of a solvent. Specifically, the macromolecular emulsifier is selected from polyethylene glycol with HLB value of 15 and Mw of 8 ten thousand; the polymer dispersant is modified polyacrylic acid with Mw of 25 ten thousand; the solvent is dimethylformamide.
The preparation method of the PVDF-coated lithium ion battery diaphragm comprises the following steps:
1) adding a high-molecular emulsifier into deionized water, stirring and dissolving the mixture until the mixture is fully and uniformly dissolved, then adding PVDF resin powder, slowly adding the PVDF resin powder and stirring the mixture until the PVDF resin powder is completely added, and continuously stirring the mixture until the PVDF resin powder is fully wetted by the deionized water to obtain a mixture I;
2) adding a high molecular dispersant into the mixture I, and performing wet ball milling to obtain a mixture II; wherein the size of the beads used for ball milling is 0.2 mm. The dispersion time of the wet ball milling is 1.0 h. The material of the beads used for ball milling is zirconia. The ball mill is an upright ball mill.
3) Adding a solvent into the mixture II, uniformly stirring, heating to a specific temperature under a certain stirring process, preserving the temperature for a period of time, then cooling, decompressing and drying for a certain period of time, and removing most of the solvent to obtain aqueous PVDF slurry; wherein the heating rate is 10 ℃/min, the swelling temperature is 70 ℃, the heat preservation time is 5.0h, the decompression drying time is 3.0h, the decompression drying temperature is 50 ℃, and the vacuum degree is-0.1 MPa;
4) the PVDF-coated lithium ion battery separator shown in fig. 1 is obtained by coating the aqueous PVDF slurry on one side of the substrate 1 to form the aqueous PVDF coating layer 2 and drying the aqueous PVDF coating layer.
Example 2
A PVDF-coated lithium ion battery diaphragm is characterized in that one side of a base material 1 is coated with aqueous PVDF slurry, the aqueous PVDF slurry contains 1% of PVDF base material in percentage by weight, and the balance is deionized water. The PVDF base material consists of the following substances in parts by mass: 100 parts of PVDF resin powder, 0.5g of emulsifier, 5g of dispersant and 10g of solvent. Specifically, the polymeric emulsifier is polydimethylsiloxane with HLB value of 3 and Mw of 10 ten thousand; the high molecular dispersant is selected from ammonium polyacrylate with Mw of 15 ten thousand; the solvent is methyl carbonate.
The PVDF-coated lithium ion battery separator was prepared by the same method as in example 1, except that step 3) is as follows. In the step 3), the heating rate is 1 ℃/min, the swelling temperature is 30 ℃, the heat preservation time is 10h, the decompression drying time is 0.1h, the decompression drying temperature is 20 ℃, and the vacuum degree is-0.1 MPa.
Example 3
The PVDF-coated lithium ion battery diaphragm is characterized in that two sides of a base material 1 are coated with aqueous PVDF slurry, the aqueous PVDF slurry contains 20% of PVDF base material in percentage by weight, and the balance is deionized water. The PVDF base material consists of the following substances in parts by mass: 100 parts of PVDF resin powder, 6.7g of emulsifier, 13.3g of dispersant and 13.3g of solvent. Specifically, the macromolecular emulsifier is polyvinyl alcohol with HLB value of 15 and Mw of 18 ten thousand; the macromolecular dispersant is polyvinyl acetamide with Mw of 20 ten thousand; the solvent is glycol dimethyl ether.
The remaining steps in the process for making the PVDF-coated lithium ion battery separator were the same as in example 1.
Example 4
The PVDF-coated lithium ion battery diaphragm is characterized in that two sides of a base material 1 are coated with aqueous PVDF slurry, the aqueous PVDF slurry contains 5% of PVDF base material in percentage by weight, and the balance is deionized water. The PVDF base material consists of the following substances in parts by mass: 100 parts of PVDF resin powder, 0.1g of emulsifier, 0.5g of dispersant and 1g of solvent. Specifically, the high molecular emulsifier is polyether modified siloxane with HLB value of 7 and Mw of 17 ten thousand; the macromolecular dispersant is polyvinyl acetamide with Mw of 40 ten thousand; the solvent is tetrahydrofuran.
The remaining steps in the process for making the PVDF-coated lithium ion battery separator were the same as in example 1.
Comparative example 1
The separator of the comparative example was prepared in a manner lacking step 3) as compared to example 1, i.e., the comparative example was made without adding a solvent.
The PVDF-coated lithium ion battery separators prepared in examples 1-4 and comparative example 1 have performance test data shown in Table 1:
TABLE 1 separator prepared in examples 1-4 and comparative example 1 separator Performance test data
The PVDF-coated lithium ion battery diaphragm of the embodiment 1-4 and the diaphragm prepared in the comparative example 1 are taken, NCM is taken as a positive pole piece and graphite is taken as a negative pole respectively, a winding process is adopted to prepare the flexible package lithium ion battery, the battery capacity, internal resistance, hardness, cycle life and bonding strength between the diaphragm and the pole piece are compared, and the obtained results are shown in a table 2:
TABLE 2 Performance parameters of lithium ion batteries prepared in examples 1-4 and comparative example 1
Safety inspection tests of the lithium ion batteries of examples 1 to 4 and comparative examples according to the safety requirements and test methods of the power storage batteries for electric automobiles are carried out, and the results are shown in table 3:
TABLE 3 safe pass rates and cycle performance of the lithium ion batteries of examples 1 to 4 and comparative example 1
| Safe passing rate% | Cycle performance | |
| Example 1 | 100 | 93 |
| Example 2 | 100 | 91 |
| Example 3 | 100 | 92 |
| Comparative example | 90 | 88 |
The cycle performance is the capacity retention rate of 800 times of 3C charging and 1C discharging.
According to the data in tables 1-3, the contact angles of the PVDF-coated lithium ion battery separators prepared in examples 1-4 are all smaller than that of comparative example 1, which shows that the wettability of the separators prepared in examples 1-4 is better than that of comparative example 1; the liquid absorption rates of the diaphragms of the examples 1 to 4 are higher than that of the diaphragm of the comparative example 1, which shows that the diaphragms prepared in the examples 1 to 4 have higher porosity and better wettability than that of the diaphragm of the comparative example 1; under the conditions of similar battery capacity and internal resistance, the bonding strength of the lithium ion batteries prepared in the embodiments 1-4 is higher than that of the comparative example 1, which shows that the coating formed by the aqueous PVDF slurry used in the invention has good bonding force and good bonding with the pole piece; the safety pass rate and the cycle performance of the lithium ion batteries prepared in the examples 1 to 4 are superior to those of the comparative example 1, which shows that the lithium ion batteries prepared by using the PVDF-coated lithium ion battery separator have long-acting cycle life and good safety performance.
The above embodiments are only preferred embodiments of the present invention, and the protection scope of the present invention is not limited thereby, and any insubstantial changes and substitutions made by those skilled in the art based on the present invention are within the protection scope of the present invention.
Claims (10)
1. The PVDF-coated lithium ion battery separator is characterized in that one side or two sides of a base material are coated with aqueous PVDF slurry, and the aqueous PVDF slurry consists of the following components in percentage by weight: 1-40 wt% of PVDF base material, and the balance of deionized water; wherein the PVDF base material consists of the following raw materials in percentage by weight: 0.1-5% of a polymer emulsifier, 0.5-10% of a polymer dispersant, 1-10% of a solvent and the balance of PVDF resin powder; the weight average molecular weight of the macromolecular emulsifier is more than 5 ten thousand and less than MwLess than 20 ten thousand, and the HLB value of the macromolecular emulsifier is 3-18; the weight average molecular weight of the macromolecular dispersant is more than 10 ten thousand and less than MwLess than 50 ten thousand; the solvent is soluble in water.
2. The PVDF-coated lithium ion battery separator of claim 1, wherein the polymeric emulsifier is one or more of polyacrylic acid, polyethylene glycol, polyvinyl alcohol, polyethylene oxide, hydroxyethyl cellulose, polydimethylsiloxane, and polyether-modified siloxane.
3. The PVDF-coated lithium ion battery separator of claim 1, wherein the polymeric dispersant is one or more of modified polyacrylic acid, carboxylated modified styrene-butadiene latex, polyacrylonitrile-modified copolymer, polyacrylamide, polyvinyl acetamide, ammonium polyacrylate, polyether derivatives and polycarboxylate.
4. The PVDF-coated lithium ion battery separator as in claim 1, wherein the solvent is one or more of acetone, methyl ethyl ketone, 4-methyl-2-pentanone, tetrahydrofuran, methyl carbonate, ethyl carbonate, methyl ethyl carbonate, ethylene glycol monomethyl ether, ethylene glycol dimethyl ether, ethylene glycol monoethyl ether, 1, 4-dioxane and dimethylformamide.
5. The method of making the PVDF-coated lithium ion battery separator of any of claims 1-4, comprising the steps of:
1) adding a high-molecular emulsifier into deionized water, stirring and dissolving the mixture until the mixture is fully and uniformly dissolved, then adding PVDF resin powder, and stirring the mixture until the PVDF resin powder is fully wetted by the deionized water to obtain a first mixture;
2) adding a high molecular dispersant into the mixture I, and performing wet ball milling to obtain a mixture II;
3) adding a solvent into the mixture II, uniformly stirring, heating and preserving heat, and drying to remove most of the solvent to obtain aqueous PVDF slurry;
4) and coating the aqueous PVDF slurry on one side or two sides of the base material to form an aqueous PVDF coating, and drying to obtain the PVDF-coated lithium ion battery diaphragm.
6. The method for preparing the PVDF-coated lithium ion battery separator as defined in claim 5, wherein in the step 2), the size of the beads used for ball milling is 0.2-2.0 mm.
7. The method for preparing the PVDF-coated lithium ion battery separator as defined in claim 5, wherein in the step 2), the dispersion time of the wet ball milling is 0.1-3.0 h.
8. The method for preparing the PVDF-coated lithium ion battery separator as defined in claim 5, wherein in step 2), the material of the beads used for ball milling is one or more of zirconia, alumina, yttria and silica.
9. The method for preparing the PVDF-coated lithium ion battery separator as defined in claim 5, wherein in step 3), the heating rate is 1-10 ℃/min, the swelling temperature is 30-80 ℃, the heat preservation time is 0.1-5.0h, the decompression drying time is 0.1-3.0h, the decompression drying temperature is 20-50 ℃, and the vacuum degree is-0.1 MPa.
10. Use of the PVDF-coated lithium-ion battery separator as defined in any one of claims 1 to 5 in a lithium battery.
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