KR20190094728A - Multilayer composite separator for secondary battery and manufacturing method thereof - Google Patents

Abstract

The invention relates to a multilayer composite separator for a secondary battery and a manufacturing method thereof. In a separator for a secondary battery prepared by laminating a plurality of polyolefin sheets, an inner layer is polyethylene: polypropylene 98: 2 to 90: 10% by weight. A skin layer is polypropylene: polyethylene 98: 2 to 90:10 weight percent. It is possible to improve interlayer adhesion and mechanical property and uniformity.

Description

Multilayer composite separator for secondary battery and manufacturing method

The present invention relates to a multilayer composite membrane and a method for manufacturing the same, which can satisfy heat resistance, mechanical properties, and quality stability at the same time, and more specifically, by adding a certain amount of resin components adjacent to the skin layer and the inner layer, The present invention relates to a multilayer composite separator and a method of manufacturing the same having enhanced contact force.

A secondary battery is a chemical battery that can be used semi-permanently by repeatedly charging and discharging using an electrochemical reaction, and may be classified into a lead acid battery, a nickel-cadmium battery, a nickel-hydrogen battery, and a lithium secondary battery. Among these, lithium

Secondary batteries lead the secondary battery market with superior voltage and energy density characteristics compared to other batteries. Lithium ion secondary batteries using liquid electrolytes, and lithium ion polymers using solid electrolytes, depending on the type of electrolyte It is classified as a secondary battery. Such a lithium secondary battery has a positive electrode, a negative electrode, an electrolyte, and a separator. Among them, a required characteristic of the separator is to isolate the positive electrode and the negative electrode and electrically insulate, thereby increasing the permeability of lithium ions based on high porosity, thereby improving ion conductivity. To raise.

Conventional lithium secondary battery separators have few pores and do not have a uniform structure, making high power and high speed charging difficult. In particular, there is a problem that the conventional separation membrane is not compatible with the electrolyte solution can not use a variety of electrolyte solutions. In addition, the coating material used in the existing membrane is expensive, can cause environmental pollution, there is a problem that is harmful to the human body.

As the polymer substrate of the separator, which is generally used, polyolefin (PO) film which is advantageous for pore formation and excellent in chemical resistance, mechanical properties, and thermal properties and inexpensive is mainly used. However, when the separator is overcharged beyond the normal charging conditions or when the positive electrode and the negative electrode of the electrode assembly are shorted, the electrolyte in which the lithium salt and the solvent are mixed is decomposed at the positive electrode, and lithium metal is precipitated at the negative electrode to deteriorate battery characteristics. In addition, due to the physical properties of the stretching process and materials included in the manufacture of the separator exhibits extreme heat shrinkage behavior at a temperature of about 100 ℃ or more may cause internal short circuit of the positive electrode and the negative electrode.

In order to solve this problem, the polymer resin layer having different melting points is laminated, and the polymer resin layer having a low melting point performs a shutdown function to prevent thermal runaway of the electrochemical device, and the polymer resin layer having a high melting point Research has been conducted to improve the safety of electrochemical devices by showing high thermal stability and preventing short circuits due to melting at high temperatures.

As a part of this research, a method of forming a porous polyethylene (PE) resin layer and a porous polypropylene (PP) resin layer, respectively, and then applying heat and pressure in a three-layer structure of PP / PE / PP is proposed. In this case, after forming the porous polymer resin layer, in the step of laminating by applying heat and pressure, there is a possibility that the pre-formed micropores are collapsed or deformed to deteriorate porosity or air permeability characteristics.

In addition, each of the polyethylene precursor and the polypropylene precursor is extruded to form a composite precursor having a three-layer structure in which the PE precursor is sandwiched between the two PP precursors, and then laminated by applying heat and pressure to bond the three layers of precursors. Although a method of manufacturing a multilayer separator prepared by heat-treating and stretching a precursor of is proposed, two polymer resins having different thermal properties are heat-treated under one condition, and thus, a heat treatment cannot be performed under optimum conditions suitable for each polymer resin. And, there is a problem that the uniform heat treatment and stretching is not achieved by trapping the air does not escape between each resin layer in the lamination process.

On the other hand, the low adhesion between layers of PP-PE layer causes problems such as quality unevenness or peeling during stretching in the process of manufacturing multilayer composite separator, and the yield is lowered. In addition, the tension in the driving direction also occurs in the battery manufacturing process. There is a possibility of causing a defect, which is the cause of problems such as a decrease in yield or speed up production.

In order to solve the problem, the present invention includes an appropriate amount of polyethylene (PE) in the polypropylene (PP) layer of the skin layer, and an appropriate amount of polypropylene in the polyethylene layer of the inner layer (core). An improved, stable production is possible, and provides a multilayer composite membrane with excellent mechanical properties and high uniformity.

In order to solve the said subject, one Embodiment of this invention is In the separator for secondary batteries manufactured by stacking a sheet of polyolefin material in multiple layers, the inner layer is made of polyethylene: polypropylene in a range of 98: 2 to 90: 10% by weight, and the skin layer is made of polypropylene: polyethylene of 98: 2 to Provided is a multilayer composite separator for secondary batteries composed of 90:10 wt%.

At this time, the interlayer adhesion of the different layers may be 5 to 20 (gf / 15mm). In addition, the Gurley value of the separator may be 100 to 300 (s / 100cc).

Meanwhile, talc, sodium benzoate, phosphite, sorbitol, sorbitol, N, N'-dicyclohexyl-2,6-naphthalene dicarboxamide (N, N'-dicyclohexyl-2,6-naphthalene dicarboxamide) may be additionally added one or more nucleating agents selected from the group consisting of.

Another embodiment of the present invention, a step of preparing a predetermined amount of two resin precursor to the first resin precursor to prepare an inner layer composite resin precursor;

Preparing a skin layer composite resin precursor by adding a predetermined amount of one resin precursor to the second resin precursor;

An extrusion step of forming a multilayer composite resin sheet by laminating the skin layer composite resin precursor on both sides of the inner layer composite resin precursor by co-extrusion in three stages;

A heat treatment step of heat-treating the coextruded multilayer composite resin sheet to improve crystallinity;

A first stretching step of stretching the heat treated multilayer composite resin sheet in an MD direction; And

It provides a method for producing a multilayer composite separator comprising a second stretching step of stretching the first stretched multilayer composite resin sheet in the TD direction.

In this case, polyethylene may be used as the first resin precursor.

The second resin precursor may be polypropylene.

In addition, in the multilayer composite resin sheet, the thickness ratio of the skin layer to the inner layer may be 1: 3 to 3: 4, and the two skin layers stacked on both sides of the inner layer may have the same thickness.

Meanwhile, the inner layer composite resin precursor may be made of polyethylene: polypropylene in a range of 98: 2 to 90:10 wt%, and the skin layer composite resin precursor may be made of polypropylene: polyethylene in a range of 98: 2 to 90:10 wt%. .

The polyethylene may employ polyethylene having a weight average molecular weight of 50,000 or more and 2 million or less.

In addition, it is possible to adopt that the isotacticity of the polypropylene is 96% or more.

On the other hand, the extrusion step, the skin layer composite resin precursor and the inner layer composite resin precursor may adopt a draft ratio of 100 to 500 in the T-die (T-die), respectively.

In addition, the coextrusion composite resin sheet may be heat-treated at 100 to 120 ° C. to improve crystallinity.

In addition, the heat-treated coextruded composite resin sheet may be stretched 200 to 400% by MD stretching at 80 to 100 ° C. as the first stretching process.

In addition, the first stretched coextruded composite resin sheet may be stretched 200 to 400% by TD stretching at 120 to 140 ° C. as a second stretching process.

According to an embodiment of the present invention, the resin layer of the inner layer is added to the skin layer and the resin component of the skin layer is added to the inner layer, and coextruded in an optimal process, under conditions such as temperature and draw ratio suitable for each polymer resin sheet. Heat treatment and stretching are performed to form micro pores, respectively.

Therefore, the multilayer composite separator according to the present invention increases the compatibility between the layers, it is possible to manufacture a multilayer composite separator having high interlayer adhesion and stable adhesion even during processing, excellent mechanical properties and high porosity distribution. .

The electrochemical device having such a multilayer composite separator prevents thermal runaway and prevents short circuit due to melting at high temperature, thereby improving safety.

1 is a cross-sectional view of a multilayer composite separator for a secondary battery according to an embodiment of the present invention.

Hereinafter, the present invention will be described in more detail.

The present invention is a secondary battery separator prepared by laminating a polyolefin sheet in a multi-layer, the inner layer is polyethylene (PE): polypropylene (PP) in the range of 98: 2 to 90: 10% by weight, the skin layer is Polypropylene: Polyethylene relates to a multilayer composite separator for secondary batteries consisting of 98: 2 to 90: 10% by weight.

That is, by using the PP / PE / PP structure for each layer as described above, the PE layer has the advantage of lowering the shutdown temperature and the PP layer can increase the melt fracture temperature. In addition, by mixing the other layer constituent material to each layer of PP, PE, there is an advantage that the adhesion between the layers is improved to increase the processability of the post-process. If the composition ratio is outside the above range, the intrinsic physical properties of each layer may be weakened, and thus the function as the separator may be degraded.

At this time, the interlayer adhesion of the different layers is preferably 5 to 20 (gf / 15 mm). Such adhesive force must be maintained to prevent peeling and non-uniformity of stretching in a later step.

In addition, the Gurley value of the separation membrane is preferably 100 to 300 (s / 100cc), although it may be used even if it is 100 or less, but it is not preferable because the permeability is too high and there is a risk of short circuiting. Not enough and not suitable for high capacity / high efficiency batteries.

Meanwhile, talc, sodium benzoate, phosphite, sorbitol, sorbitol, N, N'-dicyclohexyl-2,6-naphthalene dicarboxamide (N, N'-dicyclohexyl-2,6-naphthalene dicarboxamide) may be additionally added one or more nucleating agents selected from the group consisting of. Such nucleating agent increases the crystallization rate of PP to make the crystal structure denser, and has the advantage of improving the rigidity of the separator.

Another embodiment of the present invention, a step of preparing a predetermined amount of two resin precursor to the first resin precursor to prepare an inner layer composite resin precursor;

Preparing a skin layer composite resin precursor by adding a predetermined amount of one resin precursor to the second resin precursor;

An extrusion step of forming a multilayer composite resin sheet by laminating the skin layer composite resin precursor on both sides of the inner layer composite resin precursor by co-extrusion in three stages;

A heat treatment step of heat-treating the coextruded multilayer composite resin sheet to improve crystallinity;

A first stretching step of MD stretching the heat-treated multilayer composite resin sheet; And

It provides a method for producing a multilayer composite separator comprising a second stretching step of TD stretching the primary stretched multilayer composite resin sheet.

In this case, the first resin precursor may be polyethylene.

In addition, the second resin precursor may adopt polypropylene.

In addition, the thickness ratio of the skin layer: the inner layer in the multilayer composite resin sheet is 1: 3 to 3: 4, and the two skin layers laminated on both sides of the inner layer may be manufactured to have the same thickness.

As such, the thickness of the skin layer: inner layer: skin layer should be within the range of 1: 3: 1 to 3: 4: 3, so that the PE layer lowers the shutdown temperature and the PP layer increases the melt fracture temperature. appear. On the other hand, if the PE layer ratio is less than 40%, the shutdown characteristics are lost, and if it exceeds 60%, the melt breaking temperature is lowered.

Meanwhile, the inner layer composite resin precursor may be made of polyethylene: polypropylene in a range of 98: 2 to 90:10 wt%, and the skin layer composite resin precursor may be made of polypropylene: polyethylene in a range of 98: 2 to 90:10 wt%. .

The polyethylene has a weight average molecular weight of 50,000 or more and 2 million or less, and when the weight average molecular weight is less than 50,000, the physical properties of the membrane become weak, and when the weight average molecular weight exceeds 2 million, the extrusion kneading property It will worsen your productivity.

In addition, it is preferable to adopt an isotacticity of the polypropylene that is 96% or more, and when 96% or more, PP crystals are uniformly formed, thereby improving membrane stiffness and forming uniform pores.

On the other hand, the extrusion step, the skin layer composite resin precursor and the inner layer composite resin precursor may adopt a draft ratio of 100 to 500 in the T-die (T-die), respectively.

In general, in the case of a highly crystalline polyolefin polymer such as polyethylene or polypropylene, a spherulite is formed in the form of spherulite, especially when no stimulus such as tension is applied from the outside. However, as described above, when extruding at a high draft ratio of 100 to 500 in the extrusion step, a large tension is applied to the molten polymer chain, so that orientation crystallization is performed, and lamellas arranged in a folded chain form well. Crystals of the row lamella structure are produced. The crystals of the lamellar structure arranged in such a manner increases the degree of crystallinity as the crystals grow in the subsequent heat treatment, and the pores are separated between the crystal lamellars in the stretching process.

In the heat treatment step, the coextrusion composite resin sheet may be heat treated at 100 to 120 ° C. to improve crystallinity. As such, the extruded composite resin sheet extruded at a high draft ratio to obtain a well-arranged lamellar structure is heat-treated to be transformed into an unstable β-crystal or γ-crystal with a stable α-crystal. As the crystal grows, crystal perfection occurs. Thus, the degree of crystallization of the coextruded composite resin sheet is improved through the heat treatment process, thereby forming uniform pores while separating between the lamellae and the lamellae in the subsequent stretching process.

In the first stretching step, the heat-treated coextruded composite resin sheet may be stretched in the machine direction (MD) at 200 to 400% at 80 to 100 ° C.

 In the second stretching step, the first stretched coextruded composite resin sheet may be stretched 200 to 400% in a transverse direction (TD) stretching at 120 to 140 ° C.

Thus, by stretching the co-extruded composite resin sheet in two steps in the machine direction and in the transverse direction, micro-sized cracks can be formed in the whole sheet, and the micro-sized cracks are deformed into micropores again in the two-stage stretching process to make porous It is possible to prepare a separator.

The finally prepared multilayer composite separator has high interlayer adhesion, excellent mechanical properties, and high pore distribution, resulting in excellent durability.

Hereinafter, specific examples will be presented to aid in understanding the present invention. However, the following examples are merely provided to more easily understand the present invention, and the contents of the present invention are not limited by the examples.

<Example 1>

The polyethylene layer used for the inner layer was a polyethylene resin (Hanhwa, LDPE 5318) with a melting point of 110 ° C. and a melt index of 3 g / 10 min., And a polypropylene resin with a melting point of 160 ° C. and a melt index of 3 g / 10 min. Hanwha, HU300) was added 2wt%.

The three-layer coextrusion sheet which consists of the inner layer (PE) / skin layer (PP) using the single screw extruder of diameter 30mm and L / D = 30 was prepared for the raw material prepared as mentioned above.

The resin melted / extruded from the extruder of an inner layer and an outer skin layer was produced into the sheet | seat using the T-die of width 300mm and Lip Gap 0.8mm.

The structure of the layers was adjusted to PP / PE / PP (3: 4: 3) structure using a feed block (three layers) between the extruder and the die, and the thickness of each layer was adjusted by adjusting the discharge amount of the extruder.

 Cylinder temperature of 130-140 degreeC of the PE layer extruder, the cylinder temperature of the extruder of the PP layer were set to 180-190 degreeC, and die temperature to 190 degreeC.

While cooling the molten resin as described above using a casting roll at a temperature of 120 ° C., a multilayer composite separator was prepared such that the total thickness of the coextruded sheet was 200 μm by adjusting the casting roll speed.

The films prepared above were sequentially biaxially stretched, that is, three times stretched at 90 ° C. in the MD direction, and three times stretched at 140 ° C. in the TD direction to form pores. The thickness of the finished film was 22 μm, and the interlayer adhesion and gas permeability of the film sheet thus prepared were measured.

Cut the specimen to 15mm width in the direction of the machine's MD (Machine Direction), then peel off the layers and fix each layer with a grip to peel it off in the form of T-Peel. The interlaminar adhesion was measured using.

On the other hand, gas permeability was measured using G-B2C (Gurley type densometer, Toyoseki Co., Ltd.) according to JIS P7117, was measured for a sample of 22㎛ thickness composed of PP / PE / PP composite layer.

<Example 2>

A multilayer composite separator was prepared in the same manner as in Example 1 except that 2 wt% of PE was added to the epidermal layer (PP), and physical properties were evaluated.

<Example 3>

A multilayer composite membrane was prepared in the same manner as in Example 1 except that 2 wt% of PP was added to the inner layer (PE) and 2 wt% of PE was added to the epidermal layer (PP), and physical properties were evaluated.

<Example 4>

Except for adding 10wt% of PP to the inner layer (PE), a multilayer composite separator was prepared in the same manner as in Example 1 and the physical properties were evaluated.

Example 5

Except that 10wt% PE was added to the epidermal layer (PP), a multilayer composite separator was prepared in the same manner as in Example 1 and the physical properties were evaluated.

<Example 6>

A multilayer composite membrane was prepared in the same manner as in Example 1 except that 10 wt% of PP was added to the inner layer (PE) and 10 wt% of PE was added to the epidermal layer (PP), and physical properties were evaluated.

Comparative Example 1

PE was used as the inner layer and PP was used as the epidermal layer, and a multilayer composite membrane was prepared in the same manner as in Example 1 except that the resin component of the adjacent layer was not added to each layer, and the physical properties were evaluated. It was.

Comparative Example 2

Except for adding 1wt% of PP to the inner layer (PE), a multilayer composite separator was prepared in the same manner as in Example 1 and the physical properties were evaluated.

Comparative Example 3

Except that 1wt% PE was added to the epidermal layer (PP), a multilayer composite separator was prepared in the same manner as in Example 1 and physical properties were evaluated.

<Comparative Example 4>

A multilayer composite separator was prepared in the same manner as in Example 1 except that 1 wt% of PP was added to the inner layer (PE) and 1 wt% of PE was added to the epidermal layer (PP), and physical properties were evaluated.

Comparative Example 5

Except for adding 15wt% of PP to the inner layer (PE), a multilayer composite separator was prepared in the same manner as in Example 1 and the physical properties were evaluated.

Comparative Example 6

A multilayer composite separator was prepared in the same manner as in Example 1 except that 15 wt% of PE was added to the epidermal layer (PP), and physical property evaluation was performed.

Comparative Example 7

A multilayer composite membrane was prepared in the same manner as in Example 1 except that 15 wt% of PP was added to the inner layer (PE) and 15 wt% of PE was added to the epidermal layer (PP).

Experimental examples and physical property test results of the above Examples and Comparative Examples are shown in Table 1 below.

Inner layer Epidermal layer Adhesive force (gf / 15mm) Gurley value (s / 100cc) Example 1 PE + PP (2%) PP 5.4 168 Example 2 PE PP + PE (2%) 5.4 172 Example 3 PE + PP (2%) PP + PE (2%) 6.0 174 Example 4 PE + PP (10%) PP 12.3 225 Example 5 PE PP + PE (10%) 13.1 217 Example 6 PE + PP (10%) PP + PE (10%) 14.2 230 Comparative Example 1 PE PP 0.4 140 Comparative Example 2 PE + PP (1%) PP 3.1 141 Comparative Example 3 PE PP + PE (1%) 3.5 145 Comparative Example 4 PE + PP (1%) PP + PE (1%) 3.7 145 Comparative Example 5 PE + PP (15%) PP 25.0 410 Comparative Example 6 PE PP + PE (15%) 25.7 434 Comparative Example 7 PE + PP (15%) PP + PE (15%) 26.3 440

As can be seen from the above table, when the component (PP or PE) of the layer adjacent to the main component (PE or PP) of each layer constituting the multilayer separator is added in an amount of 2% or more and 10% or less, It was more than 5 (gf / 15mm), and Gurley value was less than 300 (s / 100cc), which showed proper physical properties.

In other words, the components of each layer should be configured within the appropriate range as described above, the adhesion between the inner layer and the skin layer is 5gf / 15mm or more, and does not cause problems such as peeling, stretching unevenness in the post-processing, such as stretching, less than 5 gf / 15mm In this case, problems such as delamination and stretching unevenness occurred in a later step, resulting in deterioration of quality.

In addition, the Gurley value related to the transmittance showed a value of 300 sec / 100 cc or less in the case of Examples 1 to 6, but when it exceeds 300 sec / 100 cc, the transmittance was insufficient, resulting in deterioration of characteristics when used as a separator, resulting in a problem in quality.

Therefore, it was evaluated that it is desirable to add components of the adjacent layer in an appropriate range, that is, in the range of 2 to 10% by weight, in order to enhance adhesion and compatibility of both layers in the PP and PE used in the separator.

Claims (15)

In the separator for secondary batteries manufactured by stacking a sheet of polyolefin material in multiple layers, the inner layer is made of polyethylene: polypropylene in a range of 98: 2 to 90: 10% by weight, and the skin layer is made of polypropylene: polyethylene of 98: 2 to A multilayer composite separator for secondary batteries composed of 90:10 wt%. The method of claim 1,
A multilayer composite separator for secondary batteries, wherein the interlayer adhesion of the different layers is 5 to 20 (gf / 15 mm). The method of claim 1,
Gurley value of the separator is 100 to 300 (s / 100cc) multilayer composite separator for secondary batteries. The method of claim 1,
Talc, sodium benzoate, sodium benzoate, phosphite, sorbitol, sorbitol, N, N'-dicyclohexyl-2,6-naphthalene dicarboxamide (N, N ') in each layer -Dicyclohexyl-2,6-naphthalene dicarboxamide) Multi-layer composite separator for secondary batteries prepared by adding at least one nucleating agent selected from the group consisting of. Preparing an inner layer composite resin precursor by adding a predetermined amount of 2 resin precursor to the first resin precursor;
Preparing a skin layer composite resin precursor by adding a predetermined amount of one resin precursor to the second resin precursor;
An extrusion step of forming a multilayer composite resin sheet by laminating the skin layer composite resin precursor on both sides of the inner layer composite resin precursor by co-extrusion in three stages;
A heat treatment step of heat-treating the coextruded multilayer composite resin sheet to improve crystallinity;
A first stretching step of stretching the heat treated multilayer composite resin sheet in an MD direction; And
And a second stretching step of stretching the first stretched multilayer composite resin sheet in a TD direction. The method of claim 5,
The first resin precursor is a method for producing a multilayer composite separator. The method of claim 5,
The second resin precursor is a polypropylene manufacturing method of a multilayer composite separator. The method of claim 5,
A method of manufacturing a multilayer composite separator in which the thickness ratio of the skin layer: inner layer in the multilayer composite resin sheet is 1: 3 to 3: 4, and the two skin layers stacked on both sides of the inner layer have the same thickness. The method of claim 5,
The inner layer composite resin precursor is composed of polyethylene: polypropylene in the range of 98: 2 to 90:10 wt%, and the outer layer composite resin precursor is made of polypropylene: polyethylene in the range of 98: 2 to 90:10 wt%. Manufacturing method. The method of claim 6,
The polyethylene is a method of producing a multilayer composite separator having a weight average molecular weight of 50,000 to 2 million. The method of claim 6,
A method for producing a multilayer composite separator having an isotacticity of at least 96% of the polypropylene. The method of claim 5,
The extruding step is a method for manufacturing a multilayer composite separator in which the skin layer composite resin precursor and the inner layer composite resin precursor are each extruded at a draft ratio of 100 to 500 in a T-die. The method of claim 5,
In the heat treatment step, the coextruded composite resin sheet is heat-treated at 100 to 120 ℃ to improve the degree of crystallinity manufacturing method of a multilayer composite separator. The method of claim 5,
In the first stretching step, the heat-treated coextruded composite resin sheet is stretched 200 to 400% by MD stretching at 80 to 100 ℃ manufacturing method of a multilayer composite separator. The method of claim 5,
In the second stretching step, the first stretched co-extruded composite resin sheet is stretched 200 to 400% by TD stretching at 120 to 140 ℃ manufacturing method of a multilayer composite separator.

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