KR101490890B1 - Separator for secondary battery and manufacturing method thereof - Google Patents

Abstract

The method for preparing a separation membrane for a secondary battery includes forming a nanofiber nonwoven fabric by using a solution of a polymer dissolved in a polymer, and coating a functional material on the nanofiber nonwoven fabric. The polymer nanofiber nonwoven fabric can be coated with a functional material to prevent a voltage delay phenomenon. By pressing the polymer nanofiber nonwoven fabric, it is possible to manufacture a separation membrane having desired pore size, porosity, permeability, and thermal properties.

Description

Technical Field [0001] The present invention relates to a separator for a secondary battery,

The present invention relates to a separation membrane for a secondary battery and a manufacturing method thereof.

Lithium secondary batteries are widely used in mobile devices and IT devices due to their high energy density. Recently, researches have been made to apply lithium secondary batteries to large-capacity high power applications such as HEV, PHEV, EV, Development is progressing all over the world. However, the lithium secondary battery is constantly incurred in safety related accidents such as ignition explosion, thus causing doubts about safety in application of a large capacity high power field, and efforts are being made to dramatically improve the safety.

Conventionally, polyolefin-based separator membranes are widely used, and it is necessary to secure a high-performance membrane element technology that can replace the polyolefin-based separator membrane, which has poor thermal stability at high temperatures and chemical stability when used for a long period of time. Since the mechanical and thermal stability of the separation membrane physically separating the positive electrode and the negative electrode is improved, heat generation and ignition explosion due to short-circuiting of the battery can be prevented in advance. Therefore, the separation membrane is a core part having close relation with the safety improvement of the lithium secondary battery to be.

The polyolefin series membranes that are currently in commercial use show extreme heat shrinkage at temperatures of 150 or more, and the mechanical strength is lowered, which is pointed out as the biggest problem related to the safety accident of the battery. Therefore, the development of a new separation membrane material and nano-ceramic coating technology to improve the fundamental thermal stability and mechanical stability of the polyolefin-based separation membrane are being studied.

LG Chem, a domestic company, has developed a composite membrane coated with ceramic particles on a polyolefin-based membrane, and is currently being applied to a lithium secondary battery mounted on an Avante LPI hybrid vehicle developed by Hyundai Motor. Motorola USA filed a number of patents in the field of using a porous membrane as a support, and a representative example is US Pat. No. 5,681,357. In this patent, a cell is prepared by coating with a poly (vinylidene fluoride) solution on a polyethylene-based separator well known as a Celgard, and then an electrolyte is injected and gelated at a high temperature to form lithium A method of manufacturing a secondary battery is proposed.

As described above, a technique of coating a functional material such as ceramic particles or polyvinylidene fluoride on a conventional polyolefin-based separation membrane to improve thermal shrinkage and mechanical strength at a temperature of 150 ° C or higher is effective in reducing the adhesion force of the coating material, And has the same problem. Conventional polyolefin-based membranes are hydrophobic and have a very smooth surface, so that the adhesion strength of the coating material is low, so that the selection and coating of the coating material is very difficult. In the process of assembling the secondary battery or in contact with the electrolyte in the secondary battery There is a possibility that the function is lost. In addition, conventional polyolefin-based membranes have a pore size of less than 0.5 μm and a porosity of about 40%, so that as the amount of the coating increases, the pore size and porosity decrease, and the membrane function is lost.

When heat-resistant nonwoven fabric is used as a separator, thermal stability and porosity of 60% or more are excellent. However, in the case of a nonwoven fabric having a thickness of about 20 탆, the pore size is about several tens of 탆 larger than that of the polyolefin-based separator, and therefore, the nonwoven fabric has a limit to be applied as a separator for a lithium secondary battery. Techniques for controlling the pore size, which is a disadvantage of the nonwoven fabric separator, have been introduced at home and abroad by coating functional materials such as ceramic particles and polyvinylidene fluoride on the nonwoven fabric.

Evonics, Germany, has developed a composite membrane that is coated with ceramic particles on polyethylene terephthalate (PET) nonwoven fabric, and is currently in production of prototype products. However, there are no examples applied to lithium secondary batteries. Mitsubishi Chemical of Japan is developing products coated with ceramic particles on a nonwoven fabric made of aramid fibers. However, there are no commercialized products to date, and Asahi, Tonen in Japan and Celgard in the US are also developing high-safety separators have.

The present invention has been made to solve the above problems, and it is an object of the present invention to provide a separation membrane for a secondary battery having a desired pore size, porosity, permeability, thermal properties, and the like.

The method for manufacturing a separation membrane for a secondary battery according to an embodiment of the present invention includes the steps of forming a nanofiber nonwoven fabric by using a solution of a polymer dissolved in polymer, coating a functional material on the nanofiber nonwoven fabric, And pressing the nanofiber nonwoven fabric coated with the functional material.

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The method for manufacturing a separator for a secondary battery according to another embodiment of the present invention may further include pressing the nanofiber nonwoven fabric so that physical properties of the nanofiber nonwoven fabric may be controlled before coating the functional material.

The polymer used in the production of the nanofibers may be a polymer containing an OH group in a molecular structure such as polyvinyl alcohol (PVA), a cellulose derivative (cellulose, cellulose acetate), chitin, chitosan or alginate, a poly sulfone ), Polyether imide (PEI), polyimide (PI), polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), poly ethylene oxide (PEO) poly ethylene terephthalate), and mixtures thereof.

The polyvinyl alcohol may have a degree of polymerization of 500 or less.

The polymer solution may contain a crosslinking agent.

The nanofiber nonwoven fabric may be formed through electro-spinning using the polymer solution. In addition, melt spinning, electro-blowing, melt-blowing, (Composite yarn, split yarn), spun-bonded, air laid, or? Can be prepared by a wet laid process.

The nanofiber nonwoven fabric may have a thickness of 10 to 100 μm, and the nanofibers constituting the nanofiber nonwoven fabric may have a diameter of 100 to 3000 nm.

The functional material used for the coating may be at least one selected from the group consisting of PEO (poly ethylene oxide) polymer, PVDF (polyvinylidene fluoride) polymer, polyester-polyol polymer, PAN (polyacrylonitrile) polymer, PMMA ) Based polymer, and a mixture thereof.

The functional material may be coated in an amount exceeding 20% of the coating amount.

At the pressing step, at a pressing pressure of 245 Kgf / cm or less and at a pressing temperature of 70 DEG C or less.

The separation membrane for a secondary battery according to an embodiment of the present invention is formed by coating a functional material on a nanofiber nonwoven fabric formed by using a solution of a polymer dissolved in a polymer.

The nanofiber nonwoven fabric may be pressed to adjust its physical properties.

The polymer may be a polymer containing an OH group in the molecular structure such as polyvinyl alcohol (PVA), a cellulose derivative (cellulose, cellulose acetate), chitin, chitosan or alginate, a PSU (poly sulfone), a PEI imide), polyimide (PI), polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), poly ethylene oxide (PEO), poly ethylene (PE), polypropylene (PP), poly ethylene terephthalate And mixtures thereof.

The polyvinyl alcohol may have a degree of polymerization of 500 or less.

The polymer solution may contain a crosslinking agent.

The nanofiber nonwoven fabric may be formed through electro-spinning using the polymer solution. In addition, melt spinning, electro-blowing, melt-blowing, (Composite yarn, split yarn), spun-bonded, air laid, or? Can be prepared by a wet laid process.

The nanofiber nonwoven fabric may have a thickness of 10 to 100 μm, and the nanofibers constituting the nanofiber nonwoven fabric may have a diameter of 100 to 3000 nm.

The functional material may be at least one selected from the group consisting of PEO (poly ethylene oxide), PVDF, polyester-polyol, PAN, And a mixture thereof.

The functional material may be coated in an amount exceeding 20% of the coating amount.

The pressing may be performed at a pressing pressure of 245 Kgf / cm or less and a pressing temperature of 70 DEG C or less.

According to the embodiment of the present invention, a functional material can be coated on the polymer nanofiber nonwoven fabric and the pressed polymer nanofiber nonwoven fabric to prevent a voltage delay phenomenon. In addition, the coated functional material plays a role in adjusting the tensile strength, pore size, porosity, permeability, heat shrinkage, and nonwoven fabric thickness of the porous nonwoven fabric and can also increase the ionic conductivity of lithium ion in the lithium secondary battery electrolyte .

Further, by pressing the polymer nanofiber nonwoven fabric and the coated polymer nanofiber nonwoven fabric, it is possible to control the desired tensile strength, pore size, porosity, permeability, heat shrinkage and nonwoven fabric thickness.

FIGS. 1 and 2 show that the PVA nanofiber nonwoven fabrics were compressed by a pressing device using a pressing device, and then pressure was measured at the same line pressure (245 Kgf / cm) This is a graph showing the results respectively.
FIGS. 3 and 4 are graphs showing the results of measuring the thickness and tensile strength of the coated PVA nanofiber non-woven fabric and the coated PVA non-woven fabric non-woven fabric according to coating amount of the functional material, respectively.
FIGS. 5 to 9 are graphs showing results of coin cell evaluation for a PVA non-woven fabric produced by electrospinning and a separator formed through the coating and pressing steps using the same.
FIGS. 10 to 12 are graphs showing results of coin cell evaluation for the pressed PVA nanofiber nonwoven fabric and the separation membrane formed through the coating step using the same.
13 is a graph showing the results of an LSV test of a PVA nanofiber nonwoven fabric and a conventional PE separator according to an embodiment of the present invention.
FIG. 14 is a graph showing an experimental result of a voltage drop of a LiB cell to which a PVA nanofiber nonwoven fabric and a conventional PE separator according to an embodiment of the present invention are applied.
15 is a graph showing changes in anode and cathode potentials through a 4-electrode experiment using a PVA nanofiber nonwoven fabric as a separator.
16 is a graph showing the result of the coin cell evaluation when a conventional PE separator is applied to an anode and a PVA separator is applied.
17 is a graph showing the results of coin cell evaluation of the PVA nanofiber separation membrane after coating the PVA nanofibers formed by electrospinning with PVDF and TPU mixed solution.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

The separation membrane for a secondary battery according to an embodiment of the present invention can be used as a separation membrane for a secondary battery such as a lithium secondary battery. Hereinafter, a method for manufacturing a separation membrane for a secondary battery according to an embodiment of the present invention and a separation membrane for a secondary battery Explain.

The method for preparing a separator for a secondary battery according to an embodiment of the present invention includes forming a nanofiber nonwoven fabric by using a solution of a polymer dissolved in a polymer, and coating a functional material on the formed nanofiber nonwoven fabric. It is possible to obtain an effect of preventing the voltage delay of the separation membrane by coating the functional material.

The method for manufacturing a separation membrane for a secondary battery according to another embodiment of the present invention may further include pressing the nanofiber nonwoven fabric. The step of pressing the nanofiber nonwoven fabric may include coating the functional material Or may be performed before the step of coating the functional material. According to another embodiment of the present invention, the step of pressing the nanofiber nonwoven fabric may be performed before and after the step of coating the functional material, respectively. By pressing the nanofiber nonwoven fabric, the properties of the nanofiber nonwoven fabric can be controlled so that the separator has desired properties.

Polymers forming nanofiber nonwoven fabrics include cellulose derivatives such as polyvinyl alcohol (PVA), cellulose and cellulose acetate, those containing hydroxyl group (OH group) in the molecular structure such as chitin, chitosan and alginate [the first group], PSU polyimide, polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), poly ethylene oxide (PEO), poly ethylene (PE) Polypropylene (PP), poly ethylene terephthalate (PET) [third group], and mixtures thereof. The mixture means a mixture of two or more polymers selected randomly among the listed polymers.

Particularly, when polyvinyl alcohol is used, polyvinyl alcohol having a degree of polymerization of 500 or less is preferable, and for example, polyvinyl alcohol having a degree of polymerization of 500 or 300 is a polymer forming a nanofiber nonwoven fabric Can be used. For example, PVA-103, PVA-105, PVA-203, PVA-205, PVA-403, PVA-103 of KURARAY POVAL can be used as polyvinyl alcohol (PVA) of KURARAY POVAL -405 may be used. On the other hand, it has been confirmed that the required characteristics of the separator are secured regardless of the degree of saponification of the PVA, which will be described later through experimental examples.

A nanofiber nonwoven fabric is formed by using the polymer solution obtained by dissolving the above-mentioned polymer.

On the other hand, the polymer solution may contain a crosslinking agent. The crosslinking agent may be a generally known material. For example, as the crosslinking agent of PVA, peroxide such as dibenzoyl peroxide, tetraethyl orthosilicate and 3,3-diethoxypropyltriethoxysilane , An aldehyde such as glutaraldehyde, an inorganic acid such as boric acid, a polyacrylic acid, a diisocyanate, a diacid and its substituent, an organic acid containing a sulfone group, or a mixture thereof . The crosslinking agent may be used for the purpose of minimizing the dissolution of the nanofibers due to the influence of water in the long-term preservation of the nanofiber, or in the post-treatment such as coating or pressing, in addition to the ordinary crosslinking purpose.

The nanofiber nonwoven fabric may be formed by electro-spinning the above polymer solution, and may be formed by melt spinning, electro-blowing, melt-blowing, Spun-bonded, air laid, or? Or may be formed through wet laid or the like.

According to an embodiment of the present invention, the nanofiber nonwoven fabric may have a thickness of 10 to 100 mu m, and the polymer nanofibers constituting the nanofiber nonwoven fabric may have a diameter of 100 to 3000 nm.

The functional material coated on the nanofiber nonwoven fabric may be a polymer or a blend thereof having heat resistance, stability against organic electrolytic solution, mechanical properties, high lithium ion conductivity, or the like. For example, the functional material may be PEO ethylene oxide polymer, polyvinylidene fluoride (PVDF) polymer, polyester-polyol polymer, PAN (polyacrylonitrile) polymer, PMMA (poly methyl methacrylate) polymer or mixtures thereof .

For example, the functional material may be coated on the nanofiber nonwoven fabric by a method of spraying the functional material comprising the polymer solution onto the nanofiber nonwoven fabric, or a method of impregnating the nanofiber nonwoven fabric with the polymer solution.

Meanwhile, the method for manufacturing a separation membrane for a secondary battery according to an embodiment of the present invention may include pressing the nanofiber nonwoven fabric, and the properties of the nanofiber nonwoven fabric can be controlled by pressing the nonwoven fabric, .

For example, pressing of the nanofiber nonwoven fabric can be performed by pressing a polymer nanofiber nonwoven fabric or a polymer nanofiber nonwoven fabric coated with a functional material using a high-pressure press machine. At this time, the pressing can be performed at a pressing pressure of 245 kgf / cm (line pressure) or less and a pressing temperature of 70 DEG C or less.

By forming a composite separator composed of the nanofiber nonwoven fabric obtained by the coating of the polymer nanofiber nonwoven fabric or the combination of the coating and pressing as described above, the thickness and the pore size can be reduced as compared with the polymer nanofiber nonwoven fabric before the treatment And the tensile strength can be increased, and the heat resistance and permeability can be improved as compared with the conventional polyolefin-based separator.

Hereinafter, experimental examples according to embodiments of the present invention will be described.

Experimental Example 1 : PVDF  Coating and Pressing  Processed PVA  Properties of Nanofiber Membranes and Their Applications The lithium secondary battery  Cell evaluation result

A polyvinyl alcohol (having a degree of polymerization of 500 and a degree of saponification of 85-89%) was dissolved in distilled water, and a crosslinking agent was added thereto. The obtained mixed solution was electrospun to prepare a nanofiber nonwoven fabric having a fiber diameter of about 0.8 탆.

FIGS. 1 and 2 show that the PVA nanofiber nonwoven fabrics were compressed by a pressing device using a pressing device, and then pressure was measured at the same line pressure (245 Kgf / cm) This is a graph showing the results respectively. It can be seen that as the line pressure increases, the thickness of the nanofiber nonwoven fabric decreases as the compression temperature increases.

FIGS. 3 and 4 are graphs showing the results of measurement of thickness and tensile strength according to coating amount of the functional material of the manufactured nanofiber nonwoven fabric, respectively. Referring to FIGS. 3 and 4, as the amount of coating increases, thickness and tensile strength increase. When the coated sample is pressed, the thickness decreases and the tensile strength increases as described above.

5 to 9 are graphs showing results of coin cell evaluation for a separation membrane formed of a PVA nanofiber nonwoven fabric produced by electrospinning. Fig. 5 shows the results of evaluation of the PVA non-woven fabric produced by electrospinning. Fig. 6 shows the evaluation results of the PVA non-woven fabric coated with PVDF. Figs. 7 to 9 show the results of evaluation of the PVA non- Were pressed at a line pressure of 94 Kgf / cm, 195 Kgf / cm and 245 Kgf / cm, respectively.

In the case of the PVA nanofiber nonwoven fabric (FIG. 5), it was confirmed that a voltage delay occurs during charging. In the case of the coated PVA nanofiber nonwoven fabric (FIG. 6), the voltage delay phenomenon disappeared and showed normal charge / discharge characteristics such as the conventional PE separator. In the case of FIGS. 7 to 9 pressed for the purpose of reducing the thickness of the PVDF-coated PVA nanofiber nonwoven fabric and increasing the tensile strength, the sample pressed at 94 Kgf / cm with a low line pressure (FIG. 7) In the samples of FIGS. 8 and 9 pressed at 195, 245 Kgf / cm, which exhibited a high behavior of the line pressure, a voltage delay occurred during charging.

5 to 9, the same results as those of Figs. 5 to 9 were obtained even when PVA having a degree of saponification of 98 to 99% (degree of polymerization of 500) was used instead of PVA of 85 to 89% (degree of polymerization: 500).

Table 1 below shows the results of measuring the physical properties of the samples of Figs. 5, 6, 7, and 9. The PVA nanofiber nonwoven fabric prepared by electrospinning has increased the basis weight, tensile strength and heat resistance through the coating. Also, since the coating material is coated with penetration from the surface to the backside, a sample having a thickness of 30 탆, which is similar to the thickness of 29 탆 before coating, was prepared even if a coating amount of 50% Compared with existing PE membrane, it has high pore size and excellent porosity. Therefore, it has very high permeability value considering that it has a permeability value of 1.5 seconds even though a large amount of coating is applied and the existing PE membrane is about 250 seconds.

Non-woven coating Pressing Basis weight
(g / m ² ) thickness
(탆) The tensile strength
(Kgf / cm ² ) Specularity
(sec / 100 ml) Heat shrinkage
(%, 150 degrees / 1 hr) 전압 지연 PVA
(Degree of polymerization
500) PVDF Line pressure (Kgf / cm) at 30 ° C, 5 m / min X X 8 29 70 0.4 13 O (bad) O X 12 30 120 1.5 8 X (normal) O O (94) 13 25 140 2.4 8 X (normal) O O (245) 13.6 20 180 4.4 5 O (bad)

As shown in Table 1, in the case of a sample pressed at two linear pressure conditions in order to reduce the thickness of PVDF-coated PVA nanofiber nonwoven fabric and increase the tensile strength, the thickness decreased and the tensile strength increased as the results of FIGS. 1 to 4 In the case of the sample pressed at high linear pressure, the thickness reduction and the tensile strength increase effect were large, but there was a problem in using as a separator due to a voltage delay phenomenon during charging.

10 to 12 are graphs showing results of coin cell evaluation for a separation membrane formed of pressed PVA nanofiber nonwoven fabric. Fig. 10 shows the results of evaluation of the PVA nanofiber nonwoven fabric pressed at 245 kgf / cm using PVA nanofiber nonwoven fabric produced by electrospinning. Figs. 11 and 12 show results of evaluation of the nonwoven fabric of Fig. And 50% of PVA non-woven fabric.

In the case of the pressed PVA nanofiber nonwoven fabric (FIG. 10) and the PVA nanofiber nonwoven fabric (FIG. 11) coated with 20% PVDF coating amount (FIG. 11) (Fig. 12), it showed normal collapse characteristic behavior.

It was confirmed that the same results could be obtained even when PVA having a degree of saponification of 98 to 99% (degree of polymerization of 500) was used instead of 85 to 89% (degree of polymerization 500) PVA in FIGS. 10 to 12.

Table 2 below shows the results of measuring the physical properties of the samples of Figs. 10 to 12. PVA nanofibers prepared by electrospinning decreased in thickness, tensile strength and permeability were increased through pressing process and heat shrinkage rate was same. In the case of the PVA nanofiber separation membrane coated with 20% of the coating amount after the pressing, the heat shrinkage was decreased and the permeability was increased while the thickness and tensile strength were maintained at a similar level to that of the PVA nanofibers before coating. The thickness of PVA nanofiber separator coated with 50% coating amount was slightly increased from 23 ㎛ to 25 ㎛. However, the thickness was decreased and the tensile strength and heat shrinkage were greatly improved compared with PVA nanofiber without coating and press process As a result, as shown in FIGS. 10 to 12, the lithium secondary battery exhibited normal charge / discharge characteristics as a separator.

Non-woven Pressing coating Basis weight
(g / m ² ) thickness
(탆) The tensile strength
(Kgf / cm ² ) Specularity
(sec / 100 ml) Heat shrinkage
(%, 150 degrees / 1 hr) 전압 지연 PVA
(Degree of polymerization
500) (245 Kgf / cm) at 30 < 0 > C, 5 m / min PVDF O X 8 29 70 0.4 13 O (bad) X 8 23 135 1.0 13 O (bad) O 10 23 130 3.7 8 O (bad) O 12 25 170 10.4 5 X (good)
When PVA nonwoven fabric of Table 1 and 2 is used as a separator, there is a limit to be applied to a lithium secondary battery separator immediately due to a voltage delay phenomenon. In this case, there is a limitation on the coating amount and pressing pressure, And an appropriate coating amount and a pressing pressure depending on physical properties of the nonwoven fabric such as thickness, air permeability (or pore size), it is possible to manufacture a separation membrane having no voltage delay phenomenon.
In the case of the PVA nanofiber separation membrane made of PVA having a degree of polymerization of 500 and the PVA nanofiber separation membrane made of PVA having a polymerization degree of 500 and having a degree of polymerization of 500, due to a side reaction with the active material or the electrolyte of the anode (or the cathode) (Fig. 5 and Fig. 10).

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On the other hand, when PVDF-coated PVA nanofibers or pressed PVA nanofibers are used, the PVA nanofibers are prevented from coming into direct contact with the active material or electrolyte of the positive electrode (or negative electrode) 6 and 12).

Further, even when PVDF-coated PVA nanofibers are used, the PVA nanofiber can be used as a positive electrode (see FIG. 8) when the pressing line pressure is 195 Kgf / cm or more (see FIG. 8) or when the PVDF coating amount is 20% Or the cathode), so that a voltage delay occurs upon charging.

Table 3 below shows the physical properties of the composite nonwoven fabric membrane prepared by coating the PVDF on the PVDF nonwoven fabric followed by the pressing process and the evaluation result of the lithium secondary battery cell.
Non-woven coating Pressing Basis weight
(g / m ² ) thickness
(탆) The tensile strength
(Kgf / cm ² ) Specularity
(sec / 100 ml) Heat shrinkage
(%, 150 degrees / 1 hr) 전압 지연 PVDF PVA
(Degree of polymerization: 1,700) (245 Kgf / cm) at 30 < 0 > C, 5 m / min X X 10 21 155 One 36 x (normal) X O 10 15 - 11 24 x (normal) O O 12.5 13 210 87 16 x (normal) O O 15 17 240 - 6 x (normal)
Table 4 shows physical properties of the composite nonwoven fabric membrane prepared by coating PVDF on a PVDF nonwoven fabric and evaluation results of the lithium secondary battery cell.
Non-woven Pressing coating Basis weight
(g / m ² ) thickness
(탆) The tensile strength
(Kgf / cm ² ) Specularity
(sec / 100 ml) Heat shrinkage
(%, 150 degrees / 1 hr) 전압 지연 PVDF (245 Kgf / cm) at 30 < 0 > C, 5 m / min PVA
(Degree of polymerization: 1,700) O X 10 15 - 11 24 x (normal) O 12.5 13 210 87 16 x (normal) O 15 17 240 - 6 x (normal)
As shown in Table 3 and Table 4, there is no voltage delay phenomenon like PVDF nonwoven fabric, and therefore, there is no limitation on the coating amount and pressing pressure in the case of a material which can be directly applied to the lithium secondary battery separation membrane. It is possible to manufacture an optimal lithium secondary battery separation membrane having high tensile strength and excellent heat resistance.

Experimental Example 2 : PVA  Analysis of Mechanism of Nano Fiber Membrane Voltage Delay Phenomenon

From the above experimental results, it was confirmed that when several kinds of PVA nanofiber separation membranes were applied to a lithium secondary battery, a voltage delay phenomenon was observed at 4.1 V or more. When this phenomenon occurs, it is difficult to apply it to the lithium secondary battery cell, so that additional analysis is performed.

The electrochemical stability of electrospun PVA nanofiber was evaluated in order to clarify the cause of voltage delay phenomenon. An LSV (Linear Sweep Voltammetry) experiment was performed to evaluate electrochemical stability and the results are shown in FIG.

The LSV experiment was performed between 3.0 and 4.5V and the LSV of the commercial PE membrane was also run for comparison. As shown in FIG. 13, no specific difference was observed between the PE separator, which is a commercial separation membrane, and the PVA nanofiber separation membrane. This shows that the PVA nanofibers formed by electrospinning are electrochemically similar to the PE membranes and that the voltage delay phenomenon is not caused by the electrochemical instability of the PVA nanofibers.

Next, the influence of the internal short circuit, which is mentioned in the conventional literature, is examined. In the case of the electrospun PVA nanofiber, the pore size is larger than 1.0 μm, so that an internal short circuit occurs in the full cell and a voltage delay phenomenon can be observed by the internal short circuit. Therefore, in order to observe the effect of the internal short-circuit of the PVA nanofiber separation membrane formed by electrospinning, a voltage drop phenomenon was observed and the results are shown in FIG.

In order to observe the voltage drop phenomenon, the cell was first charged to 4.2 V, and the voltage change phenomenon was observed for 100 hours. If an internal short occurs, the voltage should continue to decrease over time. However, as can be seen from the results, in the lithium secondary battery cell using the PVA membrane similar to the PE membrane, it was confirmed that the actual voltage drop was not observed even after 100 hours. This result is a direct proof that the voltage delay phenomenon is not an effect of an internal short circuit.

Next, a four-electrode experiment was carried out with the expectation that this voltage delay phenomenon may be related to surface reactions at the anode and cathode, and the results are shown in FIG.

As shown in FIG. 15, it was confirmed that the voltage delay phenomenon was observed in the first charge even in the four-electrode experiment, and this voltage delay phenomenon is caused by the potential change on the anode side. That is, this voltage delay phenomenon is considered to be caused by any surface reaction between the anode and the separator interface.

In addition, it is necessary to verify whether the phenomenon caused by the surface reaction causes a voltage delay phenomenon. The PVA nanofiber formed by electrospinning the PE separator on the cathode surface was used to prepare a coin cell, The results are shown in Fig. It was confirmed that the voltage fluctuation phenomenon when the PVA separator was applied alone did not occur when the PE separator was applied on the anode side and the PVA separator was applied.

From the above results, it can be confirmed that there is a limit to apply the electropolymerized PVA nanofiber separator having a polymerization degree of 500 to the lithium secondary battery immediately, and to suppress the surface reaction caused by the contact between the positive electrode and the PVA nanofiber And the coating of a functional material such as PVDF should be applied. It was also found that the PVDF - coated PVA nanofibers should have a proper amount of coating and the pressing for controlling the thickness and tensile strength should be treated under the appropriate line pressure conditions. For example, even with PVDF-coated PVA nanofibers, when the pressing line pressure is 195 Kgf / cm or more, or when the amount of PVDF coating is 20% or less, the PVA nanofiber is in direct contact with the active material of the anode This can cause voltage delays during charging.

Experimental Example 3 : Polyurethane( polyurethane ) Based coating and Pressing  Process PVA  Of nanofiber separator Lithium secondary battery  Cell evaluation result

PVDF and TPU (thermoplastic polyurethane) were applied as coating materials to PVA nanofibers formed by electrospinning, and samples were pressed at a linear pressure of 245 Kgf / cm 2 to evaluate properties and coin cells. The mixing ratio of PVDF and TPU was 4: 1 and the coating amount was 50%. Through the pressing process after coating, the thickness was reduced from 36 ㎛ to 22 ㎛ by 40%, tensile strength increased, and specularity increased from 1.5 sec to 8 sec Respectively.

As shown in FIG. 17, the voltage delay phenomenon disappears when charging the coin cell, and the charge / discharge characteristic behavior is normal. This is because, even if the TPU having elasticity is mixed with PVDF during coating and pressed at a strong line pressure, the coating layer is not destroyed, thereby preventing direct contact between the PVA nanofiber and the cathode active material.

Experimental Example 4 : PVA  Non-woven fabric separator made of other polymer Lithium secondary battery  Cell evaluation result

Raw material Electrolyte stability 전압 지연 용점 점
(Tm, ° C) Polyvinyl alcohol (PVA) O O  230 Cellulose O O  250 Cellulose acetate O O  250 Chitin, Chitosan O O Alginate O O PSU (Poly sulfone) O O  190 PEI (Polyether imide) O O  350 PI (Poly imide) O X  500 PVDF (polyvinylidene fluoride, homopolymer) O X  170 PVDF-HFP (polyvinylidene fluoride-co-hexafluoropropene) come X  140 PAN (Poly acrylonitrile) come X PEO (Polyethylene oxide) come X  98 PE, PP, PET O X 130, 170, 250 Polyether sulfone (PES) X Can not test  340

Like the polyvinyl alcohol used in this experiment, lithium secondary battery electrolyte (carbonate organic solvent such as ethylene carbonate or propylene carbonate containing lithium salt) is stable without shrinkage or decomposition, but when used as a lithium secondary battery separator, Examples of such polymers include hydroxyl groups (OH groups) in the molecular structure such as cellulose, cellulose acetate, chitin, chitosan and alginate. These polymers have a high melting temperature of 200 ° C or higher and are cheap enough to overcome the disadvantages of existing polyolefin-based separators because of their low cost. However, they can not be applied directly to lithium secondary battery separators due to voltage delays.

As a result of using the nonwoven fabric prepared from the polymer in place of the PVA nonwoven fabric used in this experiment, the results similar to those obtained by using the nonwoven fabric of PVA nanofibers were obtained as in Experimental Examples 1 and 3. A coated nonwoven fabric having a coating amount exceeding 20%, a nonwoven fabric coated with more than 20% and pressed at a line pressure of less than 195 Kgf / cm, a coated amount exceeding 20% The nonwoven fabric pressed at a line pressure of less than 195 Kgf / cm was found to have no voltage delay phenomenon, and it was confirmed that the nonwoven fabric having elasticity such as polyurethane When the coating solution was used, it was confirmed that the voltage delay phenomenon disappeared in the nonwoven fabric pressed at a line pressure of 195 Kgf / cm or more after coating the coating amount exceeding 20% with the above results.

In the case of PSU (poly sulfone) and PEI (polyether imide), the results are the same as those of polymers containing hydroxyl groups in the molecular structure, such as PVA nanofiber nonwoven fabric and cellulose.

Nonwoven fabrics made of polyimide (PI), PVDF, PAN, PEO, poly ethylene, polypropylene (PP), and polyethylene terephthalate (PET) are stable in lithium secondary battery electrolyte, It can be used as lithium secondary battery separator. However, in order to satisfy the submicron pore size for eliminating the internal shots and increasing the stability, the lithium ion secondary battery has a thickness of 25 탆 or more, which leads to a disadvantage that the ion conductivity of the lithium ion increases in the lithium secondary battery. By combining the appropriate level of coating and pressing process obtained from the above results, it is possible to manufacture a membrane having a thickness of 25 μm or less and improved tensile strength and heat shrinkage.

While the present invention has been particularly shown and described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments, but, on the contrary, And all changes and modifications to the scope of the invention.

Claims (23)

A step of forming a nanofiber nonwoven fabric by using a polymer solution in which a polymer is dissolved,
Coating a functional material on the nanofiber nonwoven fabric, and
And pressing the nanofiber nonwoven fabric so that physical properties of the nanofiber nonwoven fabric are controlled,
The polymer may be a polymer containing an OH group in the molecular structure such as polyvinyl alcohol (PVA), a cellulose derivative (cellulose, cellulose acetate), chitin, chitosan or alginate, a PSU (poly sulfone), a PEI imide), polyimide (PI), polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), poly ethylene oxide (PEO), poly ethylene (PE), polypropylene (PP), poly ethylene terephthalate A mixture thereof, and a mixture thereof.
The functional material may be at least one selected from the group consisting of PEO (poly ethylene oxide), PVDF, polyester-polyol, PAN, And mixtures thereof. ≪ / RTI > delete delete The method of claim 1,
Wherein the pressing step is performed before or after the coating of the functional material, or both before and after the coating of the functional material. delete The method of claim 1,
Wherein the polyvinyl alcohol has a degree of polymerization of 500 or less. The method of claim 1,
Wherein the polymer solution contains a crosslinking agent. The method of any one of claims 1, 4, 6, and 7,
The nanofiber nonwoven fabric may be formed through electro-spinning using the polymer solution. In addition, melt spinning, electro-blowing, melt-blowing, (Composite yarn, split yarn), spun-bonded, air laid, or? A method for manufacturing a separation membrane for a secondary battery, which is manufactured by a wet laid method. The method of any one of claims 1, 4, 6, and 7,
Wherein the nanofiber nonwoven fabric has a thickness of 10 to 100 mu m, and the nanofibers constituting the nanofiber nonwoven fabric have a diameter of 100 to 3000 nm. delete The method of any one of claims 1, 4, 6, and 7,
Wherein the functional material is coated in a coating amount of more than 20%. The method according to claim 1 or 4,
Wherein the pressing step is performed at a pressing pressure of 245 Kgf / cm or less and a pressing temperature of 70 DEG C or less in the pressing step. The non-woven fabric is formed by coating a functional material on a nanofiber nonwoven fabric formed by using a solution of a polymer dissolved in a polymer,
The nanofiber nonwoven fabric is pressed to control physical properties,
The polymer may be a polymer containing an OH group in the molecular structure such as polyvinyl alcohol (PVA), a cellulose derivative (cellulose, cellulose acetate), chitin, chitosan or alginate, a PSU (poly sulfone), a PEI imide), polyimide (PI), polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), poly ethylene oxide (PEO), poly ethylene (PE), polypropylene (PP), poly ethylene terephthalate A mixture thereof, and a mixture thereof.
The functional material may be at least one selected from the group consisting of PEO (poly ethylene oxide), PVDF, polyester-polyol, PAN, And a mixture thereof. delete delete The method of claim 13,
Wherein the polyvinyl alcohol has a degree of polymerization of 500 or less. The method of claim 13,
Wherein the polymer solution contains a crosslinking agent. The method of claim 13,
Wherein the nanofiber nonwoven fabric is manufactured by electro-spinning, electro-blowing, or melt-blowing the polymer solution. The method according to any one of claims 13 to 17,
The nanofiber nonwoven fabric may be formed through electro-spinning using the polymer solution. In addition, melt spinning, electro-blowing, melt-blowing, (Composite yarn, split yarn), spun-bonded, air laid, or? A separator for a secondary battery produced by a wet laid process. The method according to any one of claims 13 to 18,
Wherein the nanofiber nonwoven fabric has a thickness of 10 to 100 mu m and the nanofiber nonwoven fabric has a diameter of 100 to 3000 nm. delete The method according to any one of claims 13 to 18,
Wherein the functional material is coated in a coating amount exceeding 20%. The method of claim 13,
Wherein the pressing is performed at a pressing pressure of 245 Kgf / cm or less and a pressing temperature of 70 DEG C or less.

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