KR20100036136A - Manufacturing method for vinylfluoride polymer separator using electrospinning and lithium secondary battery polymer separator manufactured by same method - Google Patents

Abstract

PURPOSE: A polymer separator for a lithium secondary battery is provided to maximize wettability and electrolyte storage, and to ensure improved electric conductivity by using electrospinning compared with an existing separation film. CONSTITUTION: A method for manufacturing a polymer separator for a lithium secondary battery comprises the steps of: forming a polymer matrix with a vinyl fluoride-based polymer solution using phase inversion; and coating the polymer matrix with a hydrophilic polymer in a nanoweb form using an electrospinning method. The electrospinning method has the conditions in which spinning voltage is 7~10 kV, spinning distance is 7~10 cm, and spinning speed is 0.5~1.5 ml/hr.

Description

Manufacture method of vinyl fluoride polymer separator using electrospinning and polymer separator for lithium secondary battery manufactured by the above method

The present invention relates to a method for producing a vinyl fluoride polymer separator using electrospinning, and a polymer separator for a lithium secondary battery prepared by the method.

In the early 1980s, polymer membranes used for secondary batteries introduced lithium salts into linear polymers such as polyethylene oxide (PEO), polypropylene oxide (PPO), polyethylenimine, polyethersulfone (PES), and polyvinylacylate. It was designed to run at the same time. Due to the weak covalent transfer between the lithium salt and the polymer, there was a disadvantage of having a low ion conductivity of 10 ⁻⁶ S / cm or less at room temperature. In addition, there are disadvantages such as the use of a high molecular resistance between the polymer material and the electrode that performs the function of the membrane-electrolyte and the use of amorphous polymer.

Since the late 1980s, membranes in which lithium salts were introduced into PEO-based graft polymers, cross-linked polymers, and other modified or modified polymers such as random or block copolymers have low ionic conductivity at room temperature but high temperature (about 60 ° C). Above) showed high ionic conductivity. However, in this case, the dimensional stability and physical properties tended to decrease due to the decrease in crystallinity of the polymer at high temperature. Since the early 1990s, the polymer electrolyte was prepared in the form of a gel or a solution in which an organic solvent was introduced together with a lithium salt in the polymer substrate, thereby greatly improving the ionic conductivity (10 ^-5 S / cm or more). Separators used here are low cost polymers with low reactivity to organic solvents such as polyethylene (PE) and polypropylene (PP). However, these polymers caused problems of stability such as somewhat unstable mechanical strength and electrode corrosion.

The following shows the characteristics required for the separator for the secondary battery.

1. Separate the positive electrode material and negative electrode material from short circuit by physical contact.

2. Membrane material itself should have electrical insulation

3. In the state containing electrolyte, electrolyte and ion permeability are good and electrical resistance is low.

4. Be stable chemically and electrochemically with respect to electrolyte

5. Easy to get wet with electrolyte and good content of electrolyte

6. Thin film thickness enables high density filling in battery case

7. Have sufficient mechanical properties required for battery assembly and use

8. When abnormal heating occurs due to external short circuit, the pores of the membrane are closed to ensure battery stability.

Membrane manufacturing techniques include phase inversion using phase separation, which is caused by differences in diffusion rates of polymer solutions, sintering to produce polymer powders using pressure and temperature, polymer films or hollows. There is a stretching method that pulls yarns and the like to impart porosity, and a film prepared using such a method exhibits high porosity. However, these methods are complicated and require a lot of effort and time to have a certain pore. As a new technology to solve these disadvantages, there is an electrospinning method, there is an advantage that it can have a high specific surface area and volume when the film is produced by the electrospinning method and the pore control is easy.

The microporous separator used in the secondary lithium ion battery has a function of having an electrical resistance and containing a liquid electrolyte in the lithium ion battery. In addition, the positive electrode material and the negative electrode material are basically isolated to prevent short-circuit of the battery due to contact of the positive electrode, and also to pass electrolyte or ions. Structurally, the separator requires sufficient porosity to contain the electrolyte in order to exhibit high ionic conductivity. In addition, the separator must have electrical resistance and be inserted in a limited space in the battery, while having high energy density, electrical density, charge-discharge cycle characteristics, and stability. In order to have high energy and electrical density, the membrane is required to have a thin form of high porosity, but faces the limitation of reduced mechanical strength. For stability of the battery, when the temperature in the cell is rapidly increased due to an external short circuit, micropores should be closed to completely block the positive electrode and the negative electrode, and a multilayer structure is preferred for this purpose.

An object of the present invention is to form a hydrophilic polymer nanoweb using an electrospinning method on a polymer matrix prepared by a phase transfer method using a phase separation phenomenon of a polymer solution.

Method for producing a polymer separator for a lithium secondary battery according to the present invention comprises the steps of: forming a polymer matrix in a lithium secondary battery using a vinyl fluoride polymer solution using a phase transition method; And applying a hydrophilic polymer on the polymer matrix in the form of a nanoweb using an electrospinning method.

The phase transfer method utilizes precipitation of a polymer by exchanging a solvent and a non-solvent in a polymer solution. Hereinafter, the phase transition method will be described in detail. In the membrane-forming membrane, first, a polymer solution prepared by dissolving the polymer in a suitable solvent is cast with a casting knife and precipitated in a coagulation bath containing a non-solvent. At this time, the polymer in the polymer solution forms a matrix and the solvent is removed to form pores. This pore forming process can be thermodynamically represented as phase separation of the Samsung system as shown in FIG.

Submerging the polymer solution in a coagulation bath to allow the exchange of solvents and nonsolvents results in a change in composition along route A, and thermodynamically into the unstable or metastable region as it enters the polymer. Precipitation occurs. In this process, the part where the polymer solution and the non-solvent contact is rapidly exchanged with the solvent and the non-solvent, and the membrane structure appears to be sponge-like or finger-like depending on the exchange rate. do. Also, the pores on the surface are formed by this phase transfer method, and mainly liquid-liquid phase separation and solid-liquid phase separation are performed. The mechanism of the Samsung system using the phase transition method is divided into two phases: instantaneous phase separation and slow phase separation. Instantaneous phase separation occurs as soon as settling into the coagulation bath after film formation, passes the binodal curve during the migration path in the Samsung system and is divided into two phases. However, when slow phase separation occurs, it has a measurable time before precipitation in the coagulation bath and does not cross the binodal curve. In addition, the phase transition path can be divided into two phases, liquid-liquid phase separation and solidification to explain the formation of pores. Liquid-liquid phase separation means that the free energy of the mixture decreases when the fully mixed polymer solution becomes thermodynamically unstable and is divided into two phases having different compositions. At this time, the nucleus of the polymer-poor phase forms initial pores, and the polymer-rich phase surrounds the nucleus to form pores of a constant size. Solidification means that the polymer chain is limited in motion as the viscosity of the polymer increases, and the polymer solution is converted from a liquid to a solid to form a film.

The polymer matrix prepared by the phase transition method compensates for the problem of deterioration in form stability of the web which may occur when the nanofiber web is produced by electrospinning and deterioration in processability. In addition, the hydrophilic polymer nanoweb coated thereon can provide high porosity, which can contain a large amount of electrolyte, and consequently allow the production of a separator having high ionic conductivity.

Electrospinning is a recently discovered simple and versatile process technology capable of producing micro-sized fibers in nanoscale using low viscosity polymers by electrostatic forces. Hereinafter, the electrospinning method will be described in detail. Electrospun fiber mats have many properties, such as high specific surface area, high aspect ratio, and porosity in the disordered weaving of the fibers. And various potential applications such as optoelectronics, sensor technology, catalysts, purification and medicine. A typical electrospinning process involves feeding polymer solutions through very thin nozzles and simultaneously spinning the fibers onto a collecting plate at a distance of 10 to 25 cm with a high voltage of 10 to 50 kV applied to the nozzle. Under the influence of the voltage, the solution from the nozzle proceeds in the form of a cone, and when a higher voltage is applied, it becomes thinner and thinner. The fiber shape depends on the properties of the polymer (molecular weight, molecular structure, glass transition temperature, solubility) and the properties of the solvent (viscosity, elasticity, concentration, surface tension, conductivity) and also depends on the ambient pressure and humidity. With good control of these parameters, fibers smaller than a few nanometers in diameter can be broken down into tiny droplets, making them stable without spraying. The optimum conditions in the electrospinning method preferably has a radiation voltage of 7 to 10 kV, a radiation distance of 7 to 10 cm, and a spinning speed of 0.5 to 1.5 ml / hr.

Electrospinning method is different from Electrostatic Spray, which is a phenomenon in which a low viscosity liquid is sprayed into a micro drop under a high voltage electric field above a threshold voltage, when a polymer solution or a melt having sufficient viscosity is subjected to high voltage electrostatic force. Microfiber is formed.

The vinyl fluoride polymer may be selected from the group consisting of perfluoroalkyl (PFA), polytetrafluoroethlene (Teflon), polyvinylidene fluoride (PVDF), polyvinylidene fluoride hexafluoro-propylene (PVDF-HFP), and polyvinyl fluoride (PVF). Preferably PVdF can be used.

When the wettability of the microporous separator is improved, the liquid electrolyte may be stably maintained in the battery cell, thereby improving stability. In addition, the wettability of the microporous separator may be improved by applying a microporous separator made of a vinyl fluoride polymer material or replacing the polyolefin-based microporous membrane, or by manufacturing.

PVdF used in the present invention improves the wettability by plasticization by the electrolyte itself. In addition, by maintaining a porosity of more than 70% when manufacturing by a simple phase transition method it can achieve the stability of the lithium ion secondary battery.

The hydrophilic polymer is characterized in that selected from the group consisting of polyethylene oxide (PEO), polyacrylic acid (PAA), polyvinyl alcohol (PVA), polyurethane (PU) and polycaprolactone (PCL).

Another polymer separator for a lithium secondary battery according to the present invention is characterized in that the vinyl fluoride-based polymer solution forms a polymer matrix by using a phase transfer method, and the hydrophilic polymer is coated on the polymer matrix in the form of a nanoweb by an electrospinning method. do.

As described above, in the separator according to the present invention, PVdF is plasticized in the electrolyte to improve wettability and at the same time, the porosity of 70% or more of the applied nanoweb maximizes electrolyte content. In addition, by introducing an electrospinning method rather than a coating on the separator, the electrical conductivity of the existing separator is improved. In addition, by applying a nano-sized web, the thin thickness of the film is maintained, thereby enabling high density filling at high capacity.

Hereinafter, specific examples and comparative examples showing high porosity and improved electrical conductivity by modifying the surface of the polymer separator for a lithium secondary battery according to the present invention will be described. Details not described herein are omitted because they can be sufficiently inferred by those skilled in the art.

EXAMPLES Preparation of Polymer Membrane Using Phase Transfer and Electrospinning

In order to prepare a porous membrane having a constant thickness, a membrane having a desired thickness was obtained by adjusting the viscosity of the solution and the guide thickness of the casting knife. In the present invention, there are guide faces on both sides of the casting knife, and a Gardner knife having a thickness of 10 μm per scale was used. The thickness of the guide was adjusted between about 80-100 μm to obtain a separator having a thickness of 25-30 μm.

In obtaining the separation membrane as described above, first, a phase transfer method was used. PVdF (Solvey, Solef6008) polymer solution was dissolved in N, N-dimethylformamide (DMF) in a hot water bath at 70 ° C using a round bottom flask until it became a single phase at 250 rpm for 5 hours using a mechanical stirrer. Prepared by stirring. When the solution became a single phase, it was sufficiently degassed in an oven at 70 ° C. for 12 hours before the completely degassed solution was cast on a clean glass plate. The cast film was impregnated for 6 hours in a coagulation bath filled with a poor solvent. In the coagulation bath, pores were formed by solvent-poor solvent exchange. The obtained membrane was impregnated with methanol, washed, and dried at room temperature to remove residual solvent to obtain a desired porous separator.

Hydrophilic polymers such as polyethylene oxide (PEO), polyacrylic acid (PAA), polyvinyl alcohol (PVA), polyurethane (PU), and polycaprolactone (PCL) were respectively applied to the prepared porous PVdF membrane by electrospinning. It was. The polymer solution was placed in a 22 gage, 5 mL syringe with a tip inner diameter to completely remove bubbles, and then the solution supply rate was controlled using KD100 (KD Scientific Inc.). CPS60K02VIT (CHUNGPA EMT Co., Ltd.) was used as the high voltage device. A nanoweb having a uniform diameter was prepared with a radiation voltage of 10.0 kV, a spinning distance of 10 cm, and a spinning speed of 1.0 mL / hr.

Table 1 below shows the molecular weight of the hydrophilic polymer used in this example and the electrical conductivity according to the concentration of the polymer solution used. As the molecular weight of the polymer increases, the electrical conductivity increases, and when the polymer of the same molecular weight is used, the electrical conductivity increases as the concentration increases. The porosity of the prepared composite membrane was maintained at 5% of the support PVdF porosity, indicating that the mechanical properties of the membrane also depend on the strength of the support.

[Table 1] Hydrophilic polymer, solvent, polymer concentration, molecular weight and electrical conductivity used

Polymer menstruum Molecular Weight (g / mol) density(%) Conductivity σ (mS / m) PEO Ethanol / water (40/60) 6 × 10 ⁵ 2 1.05 3 1.58 6 1.87 10 ⁶ 2 1.02 3 1.38 4 × 10 ⁶ 2 1.61 PEO water 10 ⁶ 2 9.75 PAA Ethanol / water (40/60) 2.5 × 10 ⁵ 6 25.34 PVA Ethanol / water (50/50) 10 ⁵ 6 3.91 PU THF / Ethanol (50/50) Tecoflex 80A 6 0.13 PCL Acetone 8 × 10 ⁴ 10 0.34 Methylene chloride / dimethyl formamide (75/25) 8 × 10 ⁴ 10 0.41 Methylene chloride / dimethyl formamide (40/60) 10 0.59

[Comparative Example 1]

As in Example, the process of manufacturing the porous separator using the phase transition method was the same, and then the process of applying the polymer by the electrospinning method was not performed.

Comparative Example 2

The polymer was applied to the porous separator using only the same electrospinning method as in Example.

[Experiment result]

1. Porosity and pore size measurement

The porosity of the prepared membrane was measured by the following process.

1) The dried membrane was cut into 3 ㅧ 3cm (length ㅧ) and the thickness was measured to calculate the volume and weigh.

2) The cut separator was impregnated with n-butanol for 2 hours.

3) The membrane completely impregnated with n-butanol was taken out, and the surface of the n-butanol was carefully wiped using a filter paper and weighed.

4) Finally, porosity was calculated by the following equation.

M _b : weight of membrane impregnated with n-butanol

M _p : Weight of dried polymer membrane

ρ _b : density of n-butanol

ρ _p : the density of the polymer

The pore size was measured using a capillary flow porometer manufactured by Porous Materials. The instrument is a fully automated system for analyzing maximum porosity, pore distribution, average pore size and gas permeability, with a pore measuring range of 0.013-600 microns and an available pressure range of 0-500 PSI, 0.15% accuracy. It has a flowmeter range of 10cc / min-500L / min and a resolution of 1 in 20,000. The principle of the device is to calculate the pore size by measuring the gas pressure and flow through the dried separator and the wet separator by filling liquid into the pores of the separator and then passing an inert gas through the separator. The size of a sample is 50 mm x 50 mm (horizontal length), and the formula used is as follows.

   D = 4γcosθ / p

   D: pore diameter γ: surface tension of liquid

   θ: contact angle of liquid p: pressure difference of gas

2. Morphology

 Scanning electron microscopy (SEM) was used to observe the morphology of the surface and cross-section of the separator. The instrument used was a Jeol JSM-6380LV (Japan) model. The sample was coated with gold foil for 10 seconds at 10 mA, and the sample for observing the cross section of the separator was prepared by instantly breaking the sample in liquid nitrogen so as not to break the cross sectional structure of the separator. The morphology of the membrane was experimented at the magnification that could be observed best and observed at 15kV of available voltage.

3. Mechanical strength

The mechanical properties of the membrane were measured by tensile strength, modulus and elongation using Universal testing machine (UTM) of Llody. Dumbbell-type specimens were made and measured using a rubber surface jig at a rate of 5 mm / min with a 100 N load cell.

4. electrical conductivity

Ion conductivity of the prepared composite membrane was measured in the range of 1Hz-1MHz at room temperature using a complex impedance analyzer (ZAHNER IM-6). The equation used to calculate conductivity is

σ: electrical conductivity l: film thickness

A: film area R: electrolyte resistance

5. Experimental Results

(1) Comparative Example 1

1) (a) the top (b) the bottom (c) of the morphology of the PVdF support prepared by the phase transition method is shown in FIG.

2) Porosity and pore size distribution are shown in FIG.

The porosity was 74% and the pore size distribution was 0.07 μm.

3) Table 2 shows the mechanical strength measurement results.

TABLE 2

Tensile strength (MPa) Elongation (%) Modulus (MPa) 80.3 90.9 88.4

(2) Comparative Example 2

1) The morphology of the PVdF nanoweb prepared by electrospinning is shown in FIG. 4.

2) The diameter distribution (400-500 nm) of PVdF fiber is shown in FIG.

3) The porosity of PVdF nanoweb was measured at 80%.

4) Table 3 shows the mechanical strength measurement results.

[Table 3]

Tensile strength (MPa) Elongation (%) Modulus (MPa) 7.84 17.3 0.19

The same polymer PVdF was prepared by the phase transition method and the electrospinning method. As a result, membranes having high mechanical strength and porosity of 70wt% or more were prepared by the phase transition method. Was prepared by. From this result, as in the present invention, a support that provides stable mechanical strength by a phase transition method is prepared, and a hydrophilic polymer that can increase affinity with an electrolyte by using an electrospinning method is coated on the nano-web to form a support over the conventional separator. It can be seen that an improved polymer separator for secondary batteries can be manufactured.

1 illustrates a phase separation diagram of a Samsung demarcation.

Figure 2 shows the (a) the top (b) the bottom (c) of the morphology of the PVdF support prepared by the phase transition method.

Figure 3 shows the porosity and pore size distribution of the separator prepared by the phase transition method.

4 shows the morphology of the PVdF nanoweb prepared by electrospinning.

5 shows the diameter distribution of PVdF fibers.

Claims (8)

Forming a polymer matrix using a vinyl fluoride polymer solution using a phase transfer method; And A method of manufacturing a polymer separator for a lithium secondary battery comprising applying a hydrophilic polymer on the polymer matrix in the form of a nanoweb by using an electrospinning method. The method of claim 1, The electrospinning method is a method for producing a polymer separator for a lithium secondary battery, characterized in that the radiation voltage has a condition of 7 ~ 10kV, radiation distance 7 ~ 10cm, spinning rate 0.5 ~ 1.5ml / hr. The method of claim 1, The vinyl fluoride polymer is selected from the group consisting of perfluoroalkyl (PFA), polytetrafluoroethlene (Teflon), polyvinylidene fluoride (PVdF), polyvinylidene fluoride hexafluoro-propylene (PVDF-HFP), and polyvinyl fluoride (PVF). Method for producing a polymer separator for lithium secondary battery. The method of claim 1, The hydrophilic polymer is a method of producing a polymer separator for lithium secondary battery, characterized in that selected from the group consisting of polyethylene oxide (PEO), polyacrylic acid (PAA), polyvinyl alcohol (PVA), polyurethane (PU) and polycaprolactone (PCL). A polymer separator for lithium secondary battery in which a vinyl fluoride-based polymer solution forms a polymer matrix by using a phase transfer method, and a hydrophilic polymer is coated on the polymer matrix in the form of a nanoweb by an electrospinning method. The method of claim 5, The nanoweb is a lithium secondary battery polymer separator, characterized in that formed under the conditions of radiation voltage 7 ~ 10kV, radiation distance 7 ~ 10cm, spinning speed 0.5 ~ 1.5ml / hr. The method of claim 5, The vinyl fluoride polymer is selected from the group consisting of perfluoroalkyl (PFA), polytetrafluoroethlene (Teflon), polyvinylidene fluoride (PVdF), polyvinylidene fluoride hexafluoro-propylene (PVDF-HFP), and polyvinyl fluoride (PVF). A polymer separator for a lithium secondary battery. The method of claim 5, The hydrophilic polymer is a lithium secondary battery polymer separator, characterized in that selected from the group consisting of polyethylene oxide (PEO), polyacrylic acid (PAA), polyvinyl alcohol (PVA), polyurethane (PU) and polycaprolactone (PCL).

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