CN110943194B

CN110943194B - Preparation method and coating application of lithium battery diaphragm with controllable surface structure

Info

Publication number: CN110943194B
Application number: CN201911217542.3A
Authority: CN
Inventors: 张叶青; 翁星星; 金骋; 陈朝晖; 赵蒙晰
Original assignee: Jiangsu Housheng New Energy Technology Co Ltd
Current assignee: Jiangsu Housheng New Energy Technology Co Ltd
Priority date: 2019-12-02
Filing date: 2019-12-02
Publication date: 2022-02-22
Anticipated expiration: 2039-12-02
Also published as: CN110943194A

Abstract

The invention provides a preparation method of a lithium battery diaphragm with a controllable surface structure, which is characterized by comprising the following steps: 1) mixing polyethylene, paraffin oil plasticizer and processing aid, and then melting and extruding by a double-screw extruder; 2) the material flows out of a die head of the double-screw extruder, the drawing ratio of the die head is 92-112, the material is cast and molded by a casting roller, the casting speed of the casting roller is 5-6m/min, and the temperature of the casting roller is 15-25 ℃; 3) longitudinally drawing by a roller, improving the tensile strength of the diaphragm by transversely drawing, extracting high-boiling-point solvent white oil by using a volatile reagent dichloromethane, secondarily drawing after extraction, and drying to obtain a high-molecular microporous membrane to form the diaphragm. The invention provides a coating application of a diaphragm. The invention can effectively solve the problem of the adhesive force between the coating slurry and the base film in the prior art.

Description

Preparation method and coating application of lithium battery diaphragm with controllable surface structure

Technical Field

The invention relates to the field of lithium battery diaphragms, in particular to a preparation method and a coating application of a lithium battery diaphragm with a controllable surface structure.

Background

In the lithium battery industry, in the demand of lithium battery separators in the market in recent years, the proportion of coating films is continuously improved, and the coated separators not only have improved heat shrinkage, but also can improve tensile strength and liquid absorption rate, and can improve the controllability of battery core manufacture.

However, the method has certain problems that the adhesive force between the coating slurry and the base film is not in a better controllable range all the time, the adhesive force is not good, the phenomena of false adhesion, degumming, glue failure and the like are caused, the safety of the battery is further influenced, and the problem of the adhesive force is caused.

Disclosure of Invention

In view of the above, the invention provides a preparation method and a coating application of a lithium battery diaphragm with a controllable surface structure.

Therefore, in one aspect, the invention provides a preparation method of a lithium battery diaphragm with a controllable surface structure, which comprises the following steps:

1) mixing polyethylene, paraffin oil plasticizer and processing aid, and then melting and extruding by a double-screw extruder;

2) the material flows out through a die head of a double-screw extruder, the drafting ratio of the die head is 92-112, the material is cast and molded through a casting roller, the casting speed of the casting roller is 5-6m/min, and the temperature of the casting roller is 15-25 ℃;

3) longitudinally drawing by a roller, improving the tensile strength of the diaphragm by transversely drawing, extracting high-boiling-point solvent white oil by using a volatile reagent dichloromethane, secondarily drawing after extraction, and drying to obtain a high-molecular microporous membrane to form the diaphragm.

Further, in the step 1), the molecular weight of the polyethylene is 1 to 500 ten thousand, and the ratio of the polyethylene to the paraffin oil plasticizer is 10:90-50:50, the specific gravity of the processing aid is 200 and 1000 ppm.

Further, in the step 2), the draft ratio of the die head is 102-112, the casting speed of the casting roll is 5-5.5m/min, and the temperature of the casting roll is 20-25 ℃.

Further, in the above step 2), the draft ratio of the die was 112, the casting speed of the casting roll was 5m/min, and the temperature of the casting roll was 25 ℃.

On the other hand, the invention provides a coating application of a preparation method of a lithium battery diaphragm with a controllable surface structure, the prepared diaphragm is subjected to ceramic coating, and the preparation process comprises the following steps: after the dispersing agent is mixed with water, ceramic powder and glue solution are premixed in a mixer, then grinding is carried out, then a binder and a wetting agent are added for secondary uniform dispersion, finished product slurry is obtained, and the slurry is coated on a diaphragm to obtain a coating film.

Further, the mass ratio of the dispersant is 0.5-5 wt%.

Furthermore, the mass ratio of the binder is 0.5-3 wt%, and the mass ratio of the wetting agent is 0.5-3 wt%.

Further, the rotation speed of the mixer is 500-.

The invention provides a preparation method and a coating application of a lithium battery diaphragm with a controllable surface structure, and the diaphragm is more beneficial to coating slurry on a base film surface and can improve the adhesive force between the slurry and the base film;

the specific advantages of the invention include:

(1) the base film is prepared through certain process conditions, and experiments show that the non-roll-sticking surface of the film from the die head is suitable for coating the sizing agent on the base film, and the adhesive force between the non-roll-sticking surface (hereinafter referred to as the orange peel surface) and the sizing agent is stronger than that between the roll-sticking surface (hereinafter referred to as the non-orange peel surface) and the sizing agent;

(2) polyethylene and white oil with certain molecular weight are used as main raw materials, and a melt with certain crystal structure and lamella thickness is obtained by adjusting a preparation process, including adjusting casting draft ratio, casting roller temperature, casting speed and the like, so that the diaphragm with a controllable surface structure is prepared; the diaphragm prepared by the method has certain friction coefficient and surface tension coefficient, is suitable for the coating field, and the prepared coating film has higher peel strength and tensile strength.

Drawings

Various other advantages and benefits will become apparent to those of ordinary skill in the art upon reading the following detailed description of the preferred embodiments. The drawings are only for purposes of illustrating the preferred embodiments and are not to be construed as limiting the invention. Also, like reference numerals are used to refer to like parts throughout the drawings. In the drawings:

FIG. 1 is a flow chart of a preparation process of a wet-process separator for a lithium battery in an embodiment of the invention;

FIG. 2 is a flow chart illustrating the preparation of a coated separator in accordance with an embodiment of the present invention;

FIG. 3 is a schematic illustration of the melt flow of a die in an embodiment of the present invention;

FIG. 4 is a schematic representation of DSC curves for various embodiments of the present invention;

fig. 5 is a second comparison of orange peel noodles and non-orange peel noodles in accordance with an embodiment of the present invention.

Detailed Description

Exemplary embodiments of the present disclosure will be described in more detail below. While exemplary embodiments of the present disclosure have been shown, it should be understood that the present disclosure may be embodied in various forms and should not be limited by the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art.

The preparation flow of the lithium battery wet-process separator of the embodiment is shown in fig. 1:

the basic preparation process of the wet diaphragm is that polyethylene with a certain molecular weight (1-500 ten thousand), paraffin oil plasticizer and processing aid are melted and extruded by a double-screw extruder according to a certain proportion (10: 90-50: 50), the melted and extruded polyethylene flows out from a die head, the melted and extruded polyethylene flows out from the die head, the melted and extruded polyethylene flows through a casting roll to be cast into a sheet, the sheet is longitudinally pulled through a roller, the tensile strength of the diaphragm is improved through transverse pulling, a high-boiling-point solvent white oil is extracted by using a volatile reagent dichloromethane, secondary stretching is carried out after extraction, and a polymer microporous membrane with a certain structural shape is obtained through drying.

The preparation process of the lithium battery coating separator of the present embodiment is shown in fig. 1 and 2 as follows:

the prepared basement membrane is subjected to ceramic coating, and the preparation flow is described as follows: mixing 0.5-5 wt% of dispersing agent with water, premixing ceramic powder and glue solution in a high-speed mixer (500-2000 rpm), grinding, adding 0.5-3 wt% of binder and 0.5-3 wt% of wetting agent, dispersing uniformly for the second time to obtain finished slurry, and coating the slurry on a base film to obtain a coating film.

The following is a description of experimental protocols for each example, including protocols one to nine:

TABLE 1

Nine schemes are designed according to the preparation method, wherein the scheme I, the scheme IV and the scheme VII are the same scheme. The first, second and third schemes research the influence of different casting roller temperatures, the fourth, fifth and sixth schemes research the influence of different melt draft ratios, and the seventh, eighth and ninth schemes research the influence of different casting sheet speeds. Wherein, the thickness of basement membrane is 9um, and the coating film is two-sided coating, and the coating thickness is 3 um.

The following experimental test conditions were used:

TABLE 2

Kind of product	Test items	Test unit	Test conditions
				Base film	Tensile strength	kgf/cm2	Rate: 250 mm/min; intercept: 100 mm; sample width: 15mm
Base film	Degree of crystallinity	％	DSC: the heating rate is 10K/min; the cooling rate is 20K/min
				Base film	Coefficient of surface tension	mN/m	Dyne pen
Base film	Surface topography	--	--
				Coating film	Tensile strength	kgf/cm2	Rate: 250 mm/min; intercept: 100 mm; sample width: 15mm
Coating film	Peel strength	mN/m	Rate: 100 mm/min; intercept: 100 mm; sample width: 15mm

The tensile properties of the base film were as follows:

TABLE 3

Scheme(s)	MD tensile Strength/kgf/cm 2	TD tensile Strength/kgf/cm 2
			Scheme one	3340	3410
Scheme two	3230	3320
			Scheme three	2980	3104
Scheme four	3340	3410
			Scheme five	3102	3210
Scheme six	2812	3042
			Scheme seven	2790	2982
Scheme eight	3340	3410
			Scheme nine	2670	2765

As can be seen from the stretch property data of the base film, the MD tensile strength and TD tensile strength of the base film tend to increase with increasing temperature of the casting roll; with the increase of the melt draft ratio, the MD tensile strength and the TD tensile strength of the base film are in an increasing trend, but the influence degree of the casting roller temperature is larger than that of the melt draft ratio; as the casting speed decreases, the MD tensile strength and TD tensile strength of the base film tend to increase.

The crystallinity of the base film is as follows:

TABLE 4

Scheme(s)	Degree of crystallization/%)	Enthalpy value/J/g
			Scheme one	77.33	226.6
Scheme two	69.42	203.4
			Scheme three	65.64	192.3
Scheme four	77.33	226.6
			Scheme five	67.61	198.1
Scheme six	64.44	188.8
			Scheme seven	64.25	188.3
Scheme eight	77.33	226.6
			Scheme nine	64.18	188

5-10mg of the diaphragm was taken, and enthalpy test was performed with a differential calorimeter, and the crystallinity of the film was calculated. The test procedure was as follows: heating from 25 deg.C to 200 deg.C at 10K/min, maintaining for 5min, and cooling to 25 deg.C at 20K/min.

From the data of the first, second and third crystallinities of the above scheme in combination with fig. 3 and 4, it can be seen that when the temperature of the casting roll is low, the casting film is rapidly cooled by the cold roll, and the crystallization is limited, resulting in low crystallinity and platelet thickness of the film; when the temperature of the casting roller is increased, on one hand, the chain section which is not crystallized in the casting film continues to be crystallized at a higher temperature, meanwhile, the crystal region defect is improved, and the thickness and the crystallinity of the lamella of the casting film are improved. On the other hand, when the temperature of the casting roller is higher, the temperature of the film which is not attached to the surface of the casting roller is also increased to be close to the crystallization temperature of polyethylene, so that molecular chains are favorably crystallized in a stretching state, and the crystal region orientation in the film is improved.

In both cases, it is found that the tensile strength of the base film is improved to some extent when the casting roll temperature is high. The optimum crystallization temperature of polyethylene is 0.80-0.85Tm (Tm is the melting point of the polyethylene material).

As can be seen from the data of the fourth, fifth and sixth crystallinity in the above schemes, increasing the melt draft ratio in the casting film forming process can induce PP crystallization, is beneficial to the arrangement of molecular chains, and improves the orientation of the molecular chains, thereby improving the thickness and crystallinity of the lamella of the film, therefore, the film prepared under the condition has higher tensile strength. On the other hand, when the draft ratio is increased, the orientation in the stretching direction is increased, which is beneficial to the crystallization in the direction perpendicular to the stretching direction, and the orderly arranged lamellar structure is formed, thereby being beneficial to the formation of micropores after stretching and having better distribution uniformity of the pores.

As can be seen from the data of the seventh, eighth and ninth crystallinity of the above schemes, the speed of casting the sheet is reduced, the cooling time of the casting film is increased, and the arrangement and crystallization of molecular chains are facilitated, so that the crystallinity of the film and the thickness of the lamella are improved.

The surface tension coefficient of the base film is as follows:

TABLE 5

The dyne value, which means the value of the surface tension of the film measured by a dyne pen, is also a decisive criterion for the surface treatment of the film, and generally, the higher the dyne value, the more easily the film is colored or adhered. If the dyne value of the material is lower than 34 dyne, the adhesion force is poor, and if the dyne value of the material is higher than 34 dyne or the material is subjected to physical and chemical treatment, the adhesion force is enough, otherwise, the risk of flicking, false sticking, degumming and glue opening can exist after compounding. In the experiment, dyne values of two surfaces of the base film of different schemes are tested by adopting 32#, 34#, 36#, 38#, 40# and 42 #.

As shown in fig. 3, when the melt flows out from the die head to the casting roller, one surface of the melt is not attached to the surface of the casting roller, and crystallization occurs under certain conditions, and the base film generated in the later stage of the process forms a surface similar to orange peel, which is called orange peel surface; one surface of the base film is attached to the surface of the casting roller, and the surface of the base film generated in the later stage process is called a non-orange peel surface. The orange peel surface and the non-orange peel surface have different surface morphologies, different surface roughness, and different surface tension coefficients measured with a dyne pen. The different degrees of orange peel can be adjusted by casting process conditions, including casting roll temperature, casting draft ratio, casting sheet speed, etc.

From the above dyne values, it can be seen that the dyne value of the orange peel noodles is larger than that of the non-orange peel noodles, which is beneficial to the bonding of the subsequent coating process.

The temperature rise of the casting roller, the increase of the melt draft ratio and the reduction of the casting speed are beneficial to the crystallization behavior of the melt from the die head, the thickness of the lamella and the crystallinity of the base film are improved to a certain extent, and thus the surface tension is correspondingly improved.

The surface topography of the base film, as shown in fig. 5, includes an orange peel side and a non-orange peel side. The left graph is the appearance of the orange peel side and the right graph is the appearance of the non-orange peel side. As can be seen from the above figures, the surface of the orange peel side is rougher and the surface of the non-orange peel side is smoother. Also, the surface tension coefficient of the orange peel side is larger than that of the non-orange peel side.

The tensile properties and release properties of the coated film are as follows:

TABLE 6

Coating films of different schemes are obtained from the obtained base film according to the preparation method of 4.1, mechanical properties of various coating films are tested, and MD tensile strength, TD tensile strength, orange peel strength and non-orange peel strength are listed in a table.

As can be seen from the tensile and peel strength data for the above coated films, the peel strength test results for the 9+3+3 coated films were greater for the orange side than for the non-orange side. In addition, the casting roll temperature increases, the melt draft ratio increases, the casting speed decreases, and the tensile and peel strength of the corresponding coating film increases.

From the crystallinity data, the temperature rise of the casting roller, the increase of the melt draft ratio and the reduction of the casting speed are beneficial to the crystallization behavior of the melt from the die head, and the thickness of the lamella and the crystallinity of the base film are improved to a certain extent, so that the tensile strength of the corresponding coating film is also larger; from the surface tension coefficient results, it is found that the increase in the casting roll temperature, the increase in the melt draft ratio, the decrease in the casting speed, and the increase in the surface tension coefficient are advantageous for the coating adhesion on the surface of the base film, and the peel strength of the coating film is relatively high.

Therefore, specific advantages of this embodiment include:

It will be apparent to those skilled in the art that various changes and modifications may be made in the present invention without departing from the spirit and scope of the invention. Thus, if such modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to include such modifications and variations.

Claims

1. A preparation method of a lithium battery diaphragm with a controllable surface structure is characterized by comprising the following steps:

2) the material flows out of a die head of the double-screw extruder, the drawing ratio of the die head is 92-112, the material is cast and molded by a casting roller, the casting speed of the casting roller is 5-6m/min, and the temperature of the casting roller is 15-25 ℃;

3) longitudinally pulling by a roller, improving the tensile strength of the diaphragm by transversely pulling, extracting high-boiling-point solvent white oil by using a volatile reagent dichloromethane, performing secondary stretching after extraction, drying to obtain a high-molecular microporous membrane to form the diaphragm, and rolling;

the prepared diaphragm is subjected to ceramic coating, and the preparation process comprises the following steps: and after the dispersing agent is mixed with water, premixing the ceramic powder and the glue solution in a mixer, grinding, adding the binder and the wetting agent, uniformly dispersing for the second time to obtain finished slurry, and coating the slurry on the diaphragm to obtain the coating film.

2. The method for preparing the lithium battery separator with the controllable surface structure as claimed in claim 1, wherein in the step 1), the molecular weight of the polyethylene is 1-500 ten thousand, the ratio of the polyethylene to the paraffin oil plasticizer is 10:90-50:50, and the specific gravity of the processing aid is 200-1000 ppm.

3. The method as claimed in claim 1, wherein in the step 2), the drawing ratio of the die head is 102-112, the casting speed of the casting roll is 5-5.5m/min, and the temperature of the casting roll is 20-25 ℃.

4. The method for preparing a lithium battery separator with a controllable surface structure according to claim 1, wherein in the step 2), the draw ratio of the die head is 112, the casting speed of the casting roll is 5m/min, and the temperature of the casting roll is 25 ℃.

5. The method for preparing a lithium battery separator with a controllable surface structure as claimed in claim 1, wherein the dispersant is present in an amount of 0.5 to 5 wt%.

6. The method for preparing a lithium battery separator with a controllable surface structure as claimed in claim 1, wherein the binder is 0.5-3 wt% by mass, and the wetting agent is 0.5-3 wt% by mass.

7. The method as claimed in claim 1, wherein the rotation speed of the mixer is 500-2000 rpm.