KR20200061719A - Surface treated separation film for lithium secondary battery and surface treatment method of separation film - Google Patents

Abstract

The present invention relates to a surface treated separation film for a lithium secondary battery and to a method for treating a surface of a separation film for a lithium secondary battery. By treating a surface of a polyolefin separation film with a substance having a polar functional group, a hydrophilic property is given to the polyolefin separation film so as to improve charge/discharge characteristics, the life, and rate characteristics.

Description

Surface treated separation film for lithium secondary battery and surface treatment method of separation film}

The present invention relates to a method for surface treatment of a separator for a lithium secondary battery and a separator for a lithium secondary battery, which is a hydrophilic modification of a hydrophobic polyolefin separator through surface treatment.

Recently, the lithium secondary battery market is expanding from a small IT application market such as mobile phones and notebooks to a mid-to-large market such as ESS and EV. The separator, which is one of the components of the secondary battery, is also in increasing demand.

The separator for the secondary battery has a function of actively moving lithium ions through pores and an important function of preventing physical contact between the positive electrode and the negative electrode to solve the safety problem of the battery such as an electrical short circuit.

The middle and large-sized lithium secondary battery has a very high proportion of safety, durability, and production cost compared to the existing small-sized lithium secondary battery. Therefore, there is a possibility of overheating ignition due to overcharge, high temperature use, short circuit, and external shock during battery operation, and efforts to significantly improve the safety are being conducted around the separator. In addition, as the research and development of lithium and secondary batteries focusing on electrodes and electrolytes have reached the limit, the development of separators has become more important.

Polyolefin-based porous separators, which are currently commercialized as separators for lithium secondary batteries, have thermal safety problems at high temperatures due to material characteristics and manufacturing process characteristics. In addition, the polyolefin porous separator is hydrophobic due to the nature of the material, and has a low affinity for a lithium secondary battery electrolyte having a high polarity, and thus has a low impregnation property of the electrolyte.

In recent years, research and development have been actively conducted to satisfy the required characteristics of the separator, such as thermal safety, mechanical properties, and improved affinity to the electrolyte, as required characteristics for the new separator. Therefore, in order to satisfy the thermal safety, research and development of organic and inorganic composite separators produced by coating ceramic particles and a binder polymer on a polyolefin-based porous separator or a PET nonwoven fabric has been actively conducted.

Representatively, Korean Patent Registration No. 0775310 developed thermal and electrochemical safety and performance by developing an organic-inorganic composite porous separator comprising an active layer coated with a mixture of inorganic particles and a binder polymer on a polyolefin-based separator substrate and applying it to a lithium secondary battery. It is disclosed that improvement can be simultaneously achieved.

However, since the existing polyolefin-based separator has very hydrophobic properties, adhesion with other polymer binders is poor, so even if a heat-resistant polymer material and a nanofiber form are bonded to the existing polyolefin-based separator, they are separated when put in a lithium secondary battery electrolyte. Symptoms may occur. When the ceramic material is coated on the separator substrate as described above, the thermal and mechanical properties are improved to show the safe characteristics of the lithium secondary battery even under excessive conditions such as high temperature and overcharge, but the thermal stability of the porous membrane, which is poor in thermal safety, is suppressed and the mechanical safety is satisfied. In order to do so, ceramic particles having a certain proportion or more of the binder should be contained. However, as the amount of ceramic particles increases compared to such a binder, adhesion to the porous substrate is weak, and thus, coated ceramic particles have a problem of detachment from the porous substrate. In addition, ceramic particles coated by stress generated in the battery assembly process can be easily detached. In particular, the ceramic material contributes to the improvement of the thermal stability and mechanical properties of the separator, but also has a problem of deteriorating battery characteristics as the thickness of the ceramic coating layer increases as it acts as a resistance from the viewpoint of battery characteristics.

Republic of Korea Registered Patent No. 1470696 Republic of Korea Registered Patent No. 0775310

An object of the present invention is to provide a separator for a lithium secondary battery, which is a hydrophilic modification of a hydrophobic polyolefin separator through surface treatment.

In addition, another object of the present invention to provide a surface treatment method of the separator for a lithium secondary battery.

The separator for a lithium secondary battery of the present invention for achieving the above object may be treated with a material having a polar functional group on the surface of the polyolefin separator.

The material having the polar functional group may be used by mixing with distilled water.

The material having the polar functional group and distilled water may be mixed in a weight ratio of 1:99 to 10:90.

The polyolefin separator may be a polyethylene separator or a polypropylene separator.

The material having a polar functional group may be at least one selected from the group consisting of ammonium persulfate, hydrogen peroxide, sodium hypochlorite, potassium chlorate, potassium periodate, silver nitrate and potassium permanganate.

In addition, the surface treatment method of the separator for a lithium secondary battery of the present invention for achieving the other object described above is (A) primary immersing the polyolefin separator in an aqueous solution of a substance having a functional group of polarity for 1 to 2.5 hours; (B) washing the immersed polyolefin membrane with distilled water; (C) secondary immersing the washed polyolefin separator in an aqueous solution of a substance having a polar functional group for 1 to 2.5 hours; And (D) washing the immersed polyolefin membrane with distilled water and then drying it.

The immersion in steps (A) and (C) can be carried out under 70 to 95 °C.

The present invention is currently widely used as a separator for lithium secondary batteries, but to improve the electrochemical properties of the polyolefin separator having poor life stability due to hydrophobic surface properties, the surface property of the polyolefin separator is made hydrophilic by chemical surface treatment with a material having a polar functional group. Changed.

The separator for a lithium secondary battery whose surface is modified with the hydrophilicity of the present invention can exhibit excellent electrochemical properties because it has high electrolyte impregnation and ion conductivity. In particular, it is possible to significantly improve the long-term deterioration of the lithium secondary battery.

1 is a view showing a photograph and a contact angle value of a polyethylene membrane having a surface modified according to Example 2 of the present invention and a polyethylene membrane having a surface unmodified according to Comparative Example 1 with a contact angle measuring device.
2 is a graph measuring the electrolyte impregnation rate of a polyethylene membrane having a surface modification according to Examples 1 and 2 of the present invention and a polyethylene membrane having a surface modification according to Comparative Example 1;
3 is a graph measuring the ionic conductivity of the separators prepared according to Examples 1 to 2 and Comparative Example 1 of the present invention.
Figure 4 is a photograph taken with a scanning electron microscope (SEM) of the separator prepared according to Examples 1 to 2 and Comparative Example 1 of the present invention.
5 is a graph showing charging and discharging characteristics of a battery manufactured using separators prepared according to Examples 1 to 2 and Comparative Example 1 of the present invention.
Figure 6a is a graph measuring the cyclability of the battery produced by using the separators prepared according to Examples 1 to 2 and Comparative Example 1 of the present invention, Figure 6b is Examples 1 to 2 and It is a graph showing the life cycle of a battery produced using a separator prepared according to Comparative Example 1.

The present invention relates to a method for surface treatment of a separator for a lithium secondary battery and a separator for a lithium secondary battery, which is a hydrophilic modification of a hydrophobic polyolefin separator through surface treatment.

Hereinafter, the present invention will be described in detail.

Due to the advantages of strong physical properties and easy thinning, the separators currently commercialized, such as the polyolefin separators, are used in many lithium secondary batteries, but have hydrophobicity in material properties. The hydrophobicity of the separator causes many problems in the impregnation and retention of the electrolyte, and studies such as plasma treatment, grafting reaction, fluorination, etc. have been conducted as a method of surface modification of the separator to overcome this problem. have. However, the surface modification method has a problem in that the process is complicated or the processing cost increases.

In the present invention, the surface of the polyolefin membrane is treated with a material having a polar functional group to change the hydrophobic polyolefin membrane to hydrophilicity, so that the surface of the polyolefin membrane is processed quickly and simply without high processing cost.

The material having the polar functional group is used as an aqueous solution mixed with distilled water so that the surface of the polyolefin separator can be easily modified. When a substance having a functional group having a polarity other than an aqueous solution is used as it is, modification to hydrophilicity is difficult and physical properties of the polyolefin separation membrane present in porosity may be reduced.

The material having the polar functional group and distilled water are mixed in a weight ratio of 1:99 to 10:90, preferably a weight ratio of 2:98 to 7:93, and more preferably 2.5:97.5 to 5:95. When the weight ratio of the material having the polar functional group and distilled water is outside the preferred range, hydrophilicity may not be imparted to the hydrophobic polyolefin separator. In particular, when the content of the substance having a polar functional group in the weight ratio of 7:93 in the preferred range (substance having a polar functional group: distilled water) is greater than the above range, the separator is damaged, and its lifespan is rather shortened and rate characteristics are reduced. It may degrade.

The polyolefin separator used in the present invention is not particularly limited, but may preferably be a polyethylene separator or a polypropylene separator; The substance having a polar functional group is not particularly limited as long as it is a substance capable of imparting hydrophilicity to the surface of the hydrophobic substance, but is preferably not limited to ammonium persulfate, hydrogen peroxide, sodium hypochlorite, potassium chlorate, potassium periodate, potassium nitrate, and permanganese And one or more selected from the group consisting of potassium acid.

In addition, the present invention provides a surface treatment method of a separator that changes the hydrophilicity of the surface of the hydrophobic polyolefin separator.

The surface treatment method of the separator for a lithium secondary battery of the present invention comprises the steps of: (A) first dipping the polyolefin separator in an aqueous solution of a substance having a polar functional group for 1 to 2.5 hours; (B) washing the immersed polyolefin membrane with distilled water; (C) secondary immersing the washed polyolefin separator in an aqueous solution of a substance having a polar functional group for 1 to 2.5 hours; And (D) washing the immersed polyolefin membrane with distilled water and then drying it.

First, in step (A), the polyolefin separation membrane is first immersed in an aqueous solution of a substance having a polar functional group for 1 to 2.5 hours.

The polyolefin separation membrane is immersed in an aqueous solution in which distilled water is mixed with a substance having a polar functional group, and is immersed for 1 to 2.5 hours, preferably 1 to 1.5 hours. When the immersion time is less than the lower limit of the preferred range, the hydrophilicity may not be modified, and when it exceeds the upper limit, the quality of the separation membrane may be deteriorated.

At this time, the temperature of the aqueous solution of the substance having a functional group of the polarity is 70 to 95°C, preferably 75 to 85°C. If the temperature of the aqueous solution is outside the above-mentioned preferred range, the hydrophobic polyolefin separator may not be modified to be hydrophilic.

Next, in step (B), the immersed polyolefin separator is washed with distilled water.

Next, in step (C), the washed polyolefin separation membrane is immersed in an aqueous solution of a substance having a polar functional group for 1 to 2.5 hours.

When the polyolefin separation membrane is immersed in the aqueous solution for a long time at a time, the entire surface of the separation membrane may be uniformly hydrophilic and unmodified, as well as deteriorate the physical properties of the porous separation membrane. It is preferred that the polyolefin separator is immersed in the aqueous solution. Specifically, the number of times the polyolefin separation membrane is immersed in the aqueous solution may be two to four times, but in the case of further immersion, battery performance may be deteriorated.

Even in step (C), the temperature of the aqueous solution of the substance having a polar functional group is 70 to 95°C, preferably 75 to 85°C. When the temperature of the aqueous solution is outside the above-mentioned preferred range, the hydrophobic polyolefin separation membrane may not be modified to be hydrophilic.

Next, in the step (D), the immersed polyolefin separator is washed with distilled water and dried to obtain a separator for a lithium secondary battery of the present invention having a hydrophilic surface.

The surface-modified polyolefin separator has a very low loss weight of 1 to 5% by weight, preferably 1 to 3% by weight.

Hereinafter, a preferred embodiment is provided to help the understanding of the present invention, but the following examples are only illustrative of the present invention, and it is apparent to those skilled in the art that various changes and modifications are possible within the scope and technical scope of the present invention. It is no wonder that such variations and modifications fall within the scope of the appended claims.

Example 1. Ammonium persulfate: distilled water = weight ratio of 2.5:97.5

After dissolving ammonium persulfate (APS) in distilled water and a weight ratio of 2.5: 97.5, a polyethylene separation membrane was immersed in the prepared aqueous solution and treated at 80° C. for 1 hour 30 minutes. After the treatment was completed, the polyethylene separation membrane was taken out, washed with distilled water, and then the polyethylene separation membrane was turned over and immersed in the aqueous solution, and then treated at 80°C for 1 hour 30 minutes. After the two dipping processes were completed, the polyethylene separation membrane was taken out, washed with distilled water, and dried to obtain a polyethylene separation membrane having a hydrophilic surface.

Example 2. Weight ratio of ammonium persulfate: distilled water = 5:95

The same procedure as in Example 1 was carried out, but ammonium persulfate and distilled water were mixed in a weight ratio of 5:95 to obtain a polyethylene separation membrane having a hydrophilic surface.

Comparative Example 1. Surface untreated polyethylene separator

The general polyethylene separator is washed with distilled water and then dried.

<Test Example>

Test Example 1. Contact angle measurement

The contact angle of the surface of the separator prepared according to the above Examples and Comparative Examples was measured using a contact angle meter (Holmarc).

1 is a view showing a photograph and a contact angle value of a polyethylene membrane having a surface modified according to Example 2 of the present invention and a polyethylene membrane having a surface unmodified according to Comparative Example 1 with a contact angle measuring device.

As shown in FIG. 1, as a result of measuring the contact angle between the separation membrane and the droplet by dropping droplets on the separation membrane of Example 2 and the separation membrane of Comparative Example 1, the separation membrane of Example 2 has a contact angle of 63.57° and is hydrophilic. It was confirmed that the separation membrane of 1 was hydrophobic as 100.41 MPa.

Test Example 2. Electrolyte uptake measurement

The separators prepared according to the above Examples and Comparative Examples were cut into 3 cm X 3 cm, respectively, and then the weight (mb) was measured, and 1 hour in 1M LiPF6 in EC/DMC (1:1, V/V) (electrolyte). After soaking, the weight (ma) was measured to obtain an electrolyte impregnation rate of the separator using Equation 1 below.

[Equation 1]

2 is a graph measuring the electrolyte impregnation rate of a polyethylene membrane having a surface modification according to Examples 1 and 2 of the present invention and a polyethylene membrane having a surface modification according to Comparative Example 1;

As shown in FIG. 2, the separator of Comparative Example 1 has low affinity with the electrolyte due to hydrophobic properties, and the electrolyte impregnation rate is low at 79.84%, whereas the separator prepared according to Examples 1 and 2 is Comparative Example 1 It was confirmed that the impregnation rate of the electrolyte was increased than that of the membranes of 90.48% and 111.95%, respectively.

Test Example 3. Measurement of Ionic conductivity

Ion conductivity was measured by immersing the separator prepared according to the above Examples and Comparative Examples in an electrolyte solution and then taking it out again. At this time, the electrolyte solution is the same solution as in Test Example 2.

3 is a graph measuring the ionic conductivity of the separators prepared according to Examples 1 to 2 and Comparative Example 1 of the present invention.

3, the example 1 and the ionic conductivity of the resulting separation membrane according to the second are each ^{3.16X10 -4 S / cm, 3.60X10 -4} S / cm as compared 2.66X10 ^-4 S of the membrane of Example 1 It was confirmed that it was higher than /cm. This means that the separators of Examples 1 and 2 have a very low resistance to lithium ion migration compared to the separators of Comparative Example 1.

The reason that the separators prepared according to Examples 1 and 2 has superior ionic conductivity characteristics compared to the separators of Comparative Example 1 is that air permeability characteristics and electrolyte impregnation characteristics are improved.

Test Example 4. Scanning electron microscope (SEM) analysis

Figure 4 is a photograph taken with a scanning electron microscope (SEM) of the separator prepared according to Examples 1 to 2 and Comparative Example 1 of the present invention.

As shown in FIG. 4, the separators of Examples 1 and 2 and Comparative Example 1 are porous, and the separators prepared according to Examples 1 and 2 are modified with a hydrophilic surface unlike Comparative Example 1.

Test Example 5. Charge/discharge cycle evaluation _ half cell

Preparation of anode

LiNi0.6Co0.2Mn0.2 92% by weight as a positive electrode active material, 4% by weight of Super P as a conductive agent, and 4% by weight of PVdF kf1100 as a binder are added to the solvent N-methyl-2pyrrolidone (NMP) to prepare a slurry of the positive electrode mixture. It was prepared. The positive electrode mixture slurry was applied to an aluminum thin film having a thickness of 10 μm and dried to prepare a positive electrode, followed by roll press.

Production of lithium secondary battery

The positive electrode and each of the separators of Examples 1 to 2 and Comparative Example 1 were assembled using a stacking method, and ethylene carbonate in which 1 M lithium hexafluorophosphate was dissolved in the assembled battery: ethyl methyl carbonate = 3:7 A lithium secondary battery was prepared by injecting (volume ratio) electrolyte.

Each of the prepared secondary batteries using the separators of Examples 1 to 2 and Comparative Example 1 thus manufactured was charged and discharged at a current density of 0.5 C in the range of 2.8 to 4.2 V.

5 is a graph showing charging and discharging characteristics of a battery manufactured using separators prepared according to Examples 1 to 2 and Comparative Example 1 of the present invention.

As shown in FIG. 5, the separation membranes prepared according to Examples 1 to 2 and Comparative Example 1 were not found to have a large difference in 1, 10, 20, and 50 cycles.

However, at 90 cycles, the battery using the separator of Example 1 was 152/152 mA h g-1, the battery using the separator of Example 2 was 164/160 mA h g-1, and the battery using the separator of Comparative Example 1 was It was confirmed that the charge/discharge capacities of 143/139 mA h g-1 were respectively shown. This means that due to the improved ion conductivity and electrolyte impregnation properties of the separators of Examples 1 to 2, it shows superior charge/discharge capacity compared to the battery using the separator of Comparative Example 1.

Test Example 6. Rate characteristic evaluation and life cycle evaluation

Rate characteristics and life cycle of the battery prepared according to Test Example 5 were confirmed.

Figure 6a is a graph measuring the cyclability of the battery produced by using the separators prepared according to Examples 1 to 2 and Comparative Example 1 of the present invention, Figure 6b is Examples 1 to 2 of the present invention and It is a graph showing the life cycle of a battery manufactured using a separator prepared according to Comparative Example 1.

As shown in Figure 6a, the battery using the separator of Comparative Example 1 showed a discharge capacity of 176 mA h g-1 in the first cycle, but gradually decreased capacity as the cycle progressed to 139 mA h g in 90 cycles. It confirmed that it fell to -1.

On the other hand, each of the batteries using the separators of Examples 1 and 2 had a discharge capacity of 176 mA h g-1 and 177 mA h g-1, respectively, in the first cycle, and a discharge capacity similar to that of the battery using the separator of Comparative Example 1 above. Although visible, the discharge capacity of 152 mA h g-1 and 164 mA h g-1 at 90 cycles, respectively, was confirmed to significantly improve the rate characteristics compared to the battery using the separator of Comparative Example 1. This shows excellent rate characteristics compared to the battery using the separator of Comparative Example 1 due to the improved ionic conductivity and electrolyte impregnation properties of the separators of Examples 1 and 2. The reason is that when the impregnation of the electrolyte during the charging/discharging of the secondary battery is incomplete, the non-uniformity of the electrode state is maximized, the electrode reaction is concentrated, and as a result, the capacity of the battery is lowered.

In addition, as shown in Figure 6b, it was confirmed that the battery using the separator of Example 2 shows a higher discharge capacity than the battery using the separator of Example 1 and Comparative Example 1 at all C-rates. This phenomenon can also be explained by the improved ionic conductivity and electrolyte impregnation properties of the separator.

Claims (7)

A separator for a lithium secondary battery, characterized in that the surface of the polyolefin separator is treated with a material having a polar functional group. The separator for a lithium secondary battery according to claim 1, wherein the material having the polar functional group is used by mixing with distilled water. [3] The separator for a lithium secondary battery according to claim 2, wherein the material having the polar functional group and distilled water are mixed in a weight ratio of 1:99 to 10:90. The separator for a lithium secondary battery according to claim 1, wherein the polyolefin separator is a polyethylene separator or a polypropylene separator. According to claim 1, wherein the material having a functional group of the polar is characterized in that at least one member selected from the group consisting of ammonium persulfate, hydrogen peroxide, sodium hypochlorite, potassium chlorate, potassium periodate, silver nitrate and potassium permanganese acid Separator for lithium secondary battery. (A) first immersing the polyolefin separator in an aqueous solution of a substance having a polar functional group for 1 to 2.5 hours;
(B) washing the immersed polyolefin membrane with distilled water;
(C) secondary immersing the washed polyolefin separator in an aqueous solution of a substance having a polar functional group for 1 to 2.5 hours; And
(D) washing the immersed polyolefin membrane with distilled water and then drying it. The method of claim 1, wherein the immersion in steps (A) and (C) is performed under 70 to 95°C.

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