JP7266013B2 - Polyethylene resin composition and separation membrane for secondary battery produced therefrom - Google Patents

Description

The present invention provides a polyethylene resin composition for a separation membrane, which is capable of maintaining excellent mechanical properties of the separation membrane and ensuring thermal stability by lowering the shutdown temperature, and a secondary battery produced from the polyethylene resin composition. It relates to separation membranes.

Separation membranes for secondary batteries, especially lithium secondary batteries, are porous thin films that exist between the positive and negative electrodes of secondary batteries, and facilitate the permeation of the electrolyte and lithium cations during the charging and discharging process of the battery. , is used to prevent a direct short circuit between positive and negative electrodes.

A separator for a lithium secondary battery is required to separate and electrically insulate a positive electrode and a negative electrode, and to increase lithium ion permeability through high porosity to enhance ionic conductivity. In addition, it must have mechanical strength that can withstand external shocks and high-speed winding processes during battery assembly, and the battery will ignite due to heat shrinkage of the separator due to overcharging, high temperature exposure, etc.・Do not explode.

A separator is required to have a shutdown function as a battery safety function to prevent a thermal runaway phenomenon when a lithium secondary battery is overheated. The shutdown function is that when the battery is overheated, the pores are closed before decomposition of the cathode material occurs to prevent lithium cation transfer and prevent further overheating. At this time, the temperature at which shutdown occurs is called the shutdown temperature. In general, it can be said that the lower the shutdown temperature, the higher the stability, which is one of the important characteristics of the separation membrane.

Therefore, research continues on a separation membrane composition capable of lowering the shutdown temperature while maintaining the mechanical properties of the microporous film.

An object of the present invention is to improve the performance of a secondary battery to which the separation membrane is applied by lowering the shutdown temperature and securing thermal stability while maintaining excellent mechanical properties of the separation membrane. It is an object of the present invention to provide a polyethylene resin composition for a separation membrane that can be obtained and a separation membrane for a secondary battery produced therefrom.

In order to achieve the above object, the present invention provides 20 to 80% by weight of an alpha-olefin copolymer (A) having a relatively high high-load melt flow index; and ethylene having a relatively low high-load melt flow index. 20 to 80% by weight of the homopolymer (B); and the alpha olefin copolymer (A) contains 90 to 99% by weight of ethylene with respect to 100 weight of the alpha olefin copolymer (A); It is obtained by copolymerizing olefins having 3 to 20 carbon atoms, and the high load melt flow index (190 ° C., 21.6 kg) of the alpha olefin copolymer (A) and the ethylene homopolymer (B) A ratio (A/B) of 3 to 500 provides a polyethylene resin composition.

The polyethylene resin composition according to the present invention maintains excellent mechanical properties of the separation membrane and lowers the shutdown temperature to ensure thermal stability, thereby further improving the performance of the secondary battery to which it is applied. can be improved.

The present invention will now be described in more detail.
The present invention provides 20 to 80% by weight of an alpha olefin copolymer (A) having a relatively high high load melt flow index; and an ethylene homopolymer (B) having a relatively low high load melt flow index of 20 to 80% by weight, and the ratio (A/B) of the high load melt flow index (190 ° C., 21.6 kg) of the alpha olefin copolymer (A) and the ethylene homopolymer (B) is 3 to 500 is provided.

More preferably, the alpha-olefin copolymer (A) has a high-load melt flow index of 0.3 to 5.0 g/10 min (190°C, 21.6 kg) and a melting temperature of 125 to 130°C. It can be a copolymer of some ethylene and an alpha olefin having 3 to 20 carbon atoms.

The ethylene may be contained in an amount of 90 to 99% by weight based on 100 weights of the alpha-olefin copolymer (A).

The alpha olefin may be propylene, butene, hexene, octene, etc. As an example, the alpha olefin may be propylene. Specifically, the copolymer (A) may be ethylene-butene, ethylene-hexene, ethylene-octene, ethylene-propylene, etc., preferably ethylene-propylene copolymer.

If the melt flow index ratio of the alpha-olefin copolymer (A) and the ethylene homopolymer (B) is less than 3, processability may be degraded, and if it exceeds 500, mechanical properties may be degraded.

If the 21.6 kg melt flow index of the alpha-olefin copolymer (A) is less than 0.3 g/10 minutes, the flowability of the resin may decrease during extrusion processing of the film, and processability may decrease. The surface of the extruded film is rough, and there is a risk that the film will break during stretching. If the melt flow index exceeds 5.0 g/10 minutes, mechanical properties such as puncture strength and tensile strength of the film may deteriorate.

The alpha-olefin copolymer (A) may be contained in an amount of 20-80% by weight, for example 40-75% by weight, based on 100% by weight of the total composition. If the alpha-olefin polymer (A) is less than 20% by weight, the mechanical properties such as puncture strength and tensile strength of the film are excellent, but the workability may deteriorate and the shutdown temperature may not be lowered. % or more, the workability is excellent and the shutdown temperature is low, but the mechanical properties may be deteriorated.

The ethylene homopolymer (B) has a melt flow index of 0.01 g/10 min to 0.1 g/10 min when measured at 190° C. under a load of 21.6 kg. If the 21.6 kg melt flow index of the ethylene homopolymer (B) is less than 0.01 g/10 minutes, the processability may deteriorate and the film appearance such as silver spots (fish eye) may become poor. If the melt flow index is 0.1 g/10 minutes or more, the mechanical properties may deteriorate.

The ethylene homopolymer (B) may be contained in an amount of 20-80% by weight, for example 20-60% by weight, based on 100% by weight of the composition. When the ethylene homopolymer (B) is less than 20% by weight, the processability is excellent and the shutdown temperature is low, but the mechanical properties may deteriorate. Although it has excellent mechanical properties such as tensile strength, workability may deteriorate and the shutdown temperature may not be lowered.

The polyethylene resin composition of the present invention may be a mixture of the alpha-olefin copolymer (A) and the ethylene homopolymer (B). The polyethylene resin composition has the effect of lowering the shutdown temperature while maintaining the mechanical properties of the separator as compared to the case of using only the ethylene homopolymer. As an example, the composition can reduce the shutdown temperature of the separation membrane to 140°C or less, eg, 135°C or less, eg, in the range of 130°C to 135°C.

The polyethylene resin composition may have a double melting point with two peaks of melting temperature measured by a differential scanning calorimeter. For example, the composition may have a first melting temperature of 130° C. or lower and a second melting temperature of 133° C. or higher. A temperature difference between the first melting temperature and the second melting temperature may be 3° C. or more. If the polyethylene resin composition has a single melting point or the difference between the first melting temperature and the second melting temperature is less than 3°C, the shutdown temperature cannot be lowered to 135°C or less, which is not preferable.

The polyethylene resin composition has a melt flow index of 0.1 g/10 minutes or more to 5.0 g/10 minutes, for example, 0.3 g/10 minutes or more to 2 when measured at 190° C. under a load of 21.6 kg. 0 g/10 min.

The polyethylene resin composition may have a density of 0.940-0.950 g/cm ³ . If the density is lower than the above range, the mechanical strength of the film may be weakened.

The polyethylene resin composition according to the present invention contains 0.01 to 0.5 parts by weight, preferably 0.05 to 0.2 parts by weight, of an antioxidant and 0.01 to 0.01 part by weight of a neutralizing agent for 100 parts by weight of the total composition. It can further contain 3 parts by weight, preferably 0.05-0.2 parts by weight.

If the content of the antioxidant is less than 0.01 parts by weight, problems such as viscosity change and non-uniformity of the film surface may occur during processing. can cause problems such as poor film surface appearance and roll contamination.

Representative examples of the antioxidant include 1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl)benzene(1,3,5 -Trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl)benzene), 1,6-bis[3-(3,5-di-tert-butyl-4-hydroxy Phenyl)propionamido]hexane (1,6-Bis[3-(3,5-di-tert-butyl-4 hydroxyphenyl)propionamido]hexane), 1,6-bis[3-(3,5-di-tert -butyl-4-hydroxyphenyl)propionamido]propane (1,6-Bis[3-(3,5-ditert-butyl-4-hydroxyphenyl)propionamido]propane), tetrakis[methylene(3,5-di-tert -butyl-4-hydroxyhydrocinnamate)]methane Phenyl)pentaerythritol-di-phosphite (Bis(2,6-di-tert-butyl-4-methylphenyl)pentaerythritoldi-phosphite), bis(2,4-di-tert-butylphenyl)pentaerythritol-di-phosphite Phyto (Bis(2,4-di-tert-butylphenyl) Pentraerythritol-di-phosphate) can be exemplified. As an antioxidant, one or more selected from these can be used.

The polyethylene resin composition according to the present invention may further include 0.01-0.3 parts by weight of the neutralizing agent with respect to 100 parts by weight of the total composition. If the content of the neutralizing agent is less than 0.01 parts by weight, discoloration and viscosity change may occur during processing. Problems such as poor surface appearance and roll contamination can occur.

Representative examples of the neutralizing agent may include calcium stearate, zinc stearate, magnesium aluminum hydroxycarbonate, zinc oxide, magnesium hydroxystearate, or mixtures thereof.

There is no particular limitation on the method for producing the polyethylene resin composition, and a commonly known method for producing a polyethylene resin composition can be used as it is or by appropriately modifying it. For example, it can be produced by the method for preparing ultra-high molecular weight polyethylene referred to in Korean Patent No. R10-1826447.

The polyethylene resin composition according to the present invention may be manufactured into a microporous separator and used as a separator for a secondary battery. The secondary battery may be, for example, a lithium secondary battery. The separation membrane may have a thickness of 1-100 μm, eg, 1-50 μm, and a porosity of 20-99%, eg, 40-70%, but is not limited thereto.

A separation membrane for a secondary battery using the polyethylene resin composition can be easily manufactured by a person skilled in the art by a method known in the art.

For example, (1) a step of extruding a polyethylene resin composition together with a paraffin-based oil to pass between a casting roll and a nip roll to produce a gel-like sheet; (3) forming micropores in the film; and (4) heat setting.

In the step (1), for example, a twin-screw extruder is used, and the resin composition is added together with paraffin oil at a temperature of 180 to 250° C. to melt, and a gel sheet is produced using a T-die. be able to.

In the step (2), the gel-like sheet prepared in the step (1) is successively or simultaneously stretched by 5 to 15 times in the machine direction and the transverse direction to prepare a film. can do.

In step (3), the stretched film is deposited in an extracting solvent such as hydrocarbons such as pentane, hexane and heptane, chlorinated hydrocarbons such as methylene chloride and carbon tetrachloride, fluorohydrocarbons and diethyl ether. Removal of the paraffinic oil results in the formation of micropores within the film.

In step (4), heat setting is performed at 110 to 150° C. to remove residual stress.

Hereinafter, the present invention will be described in detail by taking preferred examples, but the following examples are only presented to aid understanding of the present invention, and the scope of the present invention is the following implementations. Examples are not limiting.

Production Example of Separation Membrane: Microporous Films Produced Using Polyethylene Resin The polyethylene resin compositions used in Examples 1 and 2, Reference Examples 1 and 2, and Comparative Examples 1 to 5 are shown in Tables 1 and 2 below. organized into Additives Irganox 1010 (i-1010), Irgafos 168 (i-168), and calcium stearate at 2,000, 2,000, and 2,000 weight parts, respectively, based on 100 weight parts of the total composition Each ppm was added to the Henschel mixer and kneaded. The kneaded powdery resin composition was put into a kneading extruder (32mm twin extruder, EM, Korea) together with paraffin oil (KUKDONG emulsified LP350F) (resin 30 wt%, paraffin oil 70 wt%) and kneaded at 200°C. A gel-like sheet was produced by extrusion with a T-die. The gel-like sheet was simultaneously stretched 8 times in the machine direction and the transverse direction to produce a film, and then deposited in a methylene chloride extraction solvent to remove the paraffinic oil to obtain a microporous film. A film (separator) was produced.

Battery Preparation Example: Lithium Secondary Battery Preparation _LiFePO4 (Hanhwa chemical Co., Ltd., Korea), Super P (TIMCAL, Switzerland), and Polyvinylidene Fluoride (PVDF) (Aldrich, Korea) 8:1: After preparing a mixture mixed at a weight ratio of 1, the mixture and N-methyl-2-pyrrolidone (NMP) (Aldrich, Korea) were dispersed at a weight ratio of 2:1 to prepare slurry. An aluminum thin film was coated with the slurry using a doctor blade to prepare a positive electrode. The loading density of LFP was 1.4 mg/cm ² . It was fabricated into a coin cell to confirm its electrochemical properties. A Li thin film (thickness=600 μm, Honjo, Japan) was used as the negative electrode. A coin cell was manufactured by using the separators manufactured according to the above examples and comparative examples between the prepared negative electrode and positive electrode. The electrolyte was a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1, v/v, Soulbrain Co., Ltd., Korea) in which LiPF ₆ of 1 M concentration was dissolved. liquid was used.

Measurement/Evaluation Items of Physical Properties and Test Methods Therefor The physical properties of the separation membranes of Examples 1 and 2, Reference Examples 1 and 2, and Comparative Examples 1 to 5 are measured as follows.

High load melt flow index (HLMI)
Measured by ASTM D1238 at 190° C. with a load of 21.6 kg.

density
Measured according to ASTM D1505.

Propylene content (wt%)
Measured by C13 NMR.

bulk density
Measured according to ASTM D1895.

Average particle size (APS)
Particle size value at volume 50% measured by a particle size distribution analyzer (Maker: MALVERN), D (v, 0.5)

Particle size distribution index (SPAN)
(D (v, 0.9) - D (v, 0.1)) / D (v, 0.5) measured by a particle size distribution analyzer (Maker: MALVERN)

Thickness The thickness of the film was measured according to ASTM D374.

Permeability By the Japanese Industrial Standard (JIS) Gurley measurement method, it takes 100 mL of air at room temperature to pass through a 1 inch ² microporous film under a constant pressure of 4.8 inch _H2O . Time (seconds) was measured.

The porous film is cut at a porosity of 50 m width/length, and the thickness and weight are measured to calculate the density. That is, volume is measured by width x length x thickness, and density (ρ ₁ ) is calculated by dividing the measured weight by the volume. The porosity (P) was calculated by the following formula through the true density (ρ ₀ ) of the resin and the film density (ρ ₁ ) measured above. The true density of polyethylene confirmed in the present invention was 0.946 g/cm ³ .
P (%) = (ρ ₀ - ρ ₁ )/ρ ₀ × 100

Shutdown temperature The shutdown temperature was defined as the temperature (°C) at which the impedance of the battery first increased to 100 times its initial value in the process of increasing the temperature at a rate of 10°C/min.

Puncture strength (Puncture)
Puncture strength was measured using a KES-G5 instrument from Kato Tech, Japan at a speed of 10 mm/sec with a 1 mm diameter tip at the end.

Tensile strength was measured on an Instron universal testing machine (UTM) according to ASTM D3763.

Referring to Tables 1 and 2, when the separation membranes of Examples and Comparative Examples were compared, the Examples had similar mechanical properties, while maintaining the mechanical properties compared to the Comparative Examples. I was able to confirm that the temperature was lower. In particular, when comparing Examples 1 and 2 with Reference Examples 1 and 2 , and Comparative Examples 1 to 3, the ratio of the melt flow index of the alpha olefin copolymer (A) and the melt flow index of the ethylene homocopolymer (B) is in the range of 3 to 500, but if it is within the preferred melt flow index range, the mechanical properties are improved and the shutdown temperature is lowered to ensure stability. In addition, Comparative Example 4, in which the ethylene content of the copolymer (A) is less than 90%, and Comparative Example 5, in which the copolymer (B) is not an ethylene homopolymer, have significantly higher mechanical properties than those of Examples. I was able to confirm that it was inferior.

Claims (9)

A polyethylene resin composition for a separation membrane,
20 to 80% by weight of an alpha olefin copolymer (A) having a relatively high high load melt flow index; and 20 to 80% by weight of an ethylene homopolymer (B) having a relatively low high load melt flow index. including ;
The alpha-olefin copolymer (A) is obtained by copolymerizing 90 to 99% by weight of ethylene and an olefin having 3 to 20 carbon atoms with respect to 100 weight of the alpha-olefin copolymer (A). ,
The alpha-olefin copolymer (A) and the ethylene homopolymer (B) have a high load melt flow index (190° C., 21.6 kg) ratio (A/B) of 3 to 500,
The alpha-olefin copolymer (A) includes ethylene having a high load melt flow index of 0.3 to 5.0 g/10 minutes (190°C, 21.6 kg) and a melting temperature of 125 to 130°C, It is a copolymer of alpha olefins having 3 to 20 carbon atoms,
A polyethylene resin composition for a separation membrane, wherein the ethylene homopolymer (B) has a high load melt flow index of 0.01 to 0.10 g/10 min (190°C, 21.6 kg). 2. The polyethylene for separation membrane according to claim 1, wherein the polyethylene resin composition for separation membrane has a high-load melt flow index of 0.3 to 2.0 g/10 minutes (190° C., 21.6 kg). Resin composition. [Claim 2] The polyethylene resin composition for a separator according to claim 1, wherein the polyethylene resin composition for a separator exhibits two peaks of melting temperature in a differential scanning calorimeter. The polyethylene resin composition for separation membranes according to claim 1, wherein the polyethylene resin composition for separation membranes has a density of 0.940 to 0.950 g/cm ³ . At least one of 0.01 to 0.5 parts by weight of an antioxidant and 0.01 to 0.3 parts by weight of a neutralizing agent is further included with respect to 100 parts by weight of the polyethylene resin composition for a separation membrane. The polyethylene resin composition for a separation membrane according to claim 1. The antioxidant is 1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl)benzene (1,3,5-Trimethyl-2,4 ,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl)benzene), 1,6-bis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionamido]hexane (1,6-Bis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionamido]hexane), 1,6-bis[3-(3,5-di-tert-butyl-4- Hydroxyphenyl)propionamido]propane (1,6-Bis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionamido]propane), tetrakis[methylene(3,5-di-tert-butyl- 4-hydroxyhydrocinnamate)]methane Erythritol-di-phosphite (Bis(2,6-di-tert-butyl-4-methylphenyl)pentaerythritol-di-phosphite) and bis(2,4-di-tert-butylphenyl)pentaerythritol-di-phosphite (Bis(2,4- di -tert-butylphenyl)Pentraerythritol-di-phosphate). 6. The polyethylene resin composition for a separation membrane according to claim 5 , wherein the neutralizing agent is calcium stearate, zinc stearate, magnesium aluminum hydroxycarbonate, zinc oxide, magnesium hydroxystearate or a mixture thereof. . A separation membrane for a secondary battery produced from the polyethylene resin composition for a separation membrane according to any one of claims 1 to 7. The separation membrane for a secondary battery according to claim 8, wherein the separation membrane has a shutdown temperature of 130 to 140°C.