KR101013534B1 - Graft mesoporous separator with anion recepting compounds, a method for preparation thereof and lithium secondary batteries using the same - Google Patents

Abstract

The present invention relates to a porous separator grafted with an anion immobilization material, a method for preparing the same, and a lithium secondary battery using the same. The present invention relates to a wettability of an electrolyte by grafting a compound monomer that serves as anion immobilization represented by Formula 1 below. At the same time, it can be used to improve the energy density, performance and safety of the lithium secondary battery since it can exhibit improved electrochemical stability than the conventional separator.

<Formula 1>

 (Substituents in the above formula are as described in the specification.)

Anion immobilization material, polyethylene glycol borate acrylate (PEGBA), grafting, porous separator, lithium secondary battery

Description

Graft mesoporous separator with anion recepting compounds, a method for preparations and lithium secondary batteries using the same}

The present invention relates to a porous separator in which an anion immobilization material is grafted, a method for preparing the same, and a lithium secondary battery using the same. More specifically, the compound monomer represented by Formula 1 serving as anion immobilization on the surface of the polymer separator is graphed. The present invention relates to a separator for a lithium secondary battery, which improves the wettability of the separator by electrolyte and has improved electrochemical stability than the conventional separator.

Recently, with the rapid development of information and communication technology and the development of various products, the market for small portable electronic devices such as wireless mobile phones, notebook computers, personal digital assistants (PDAs), MP3 players, digital cameras, camcorders, etc. is rapidly growing. . As the portable electronic device market grows rapidly, as the demand and supply of portable electronic devices increase, the demand for secondary batteries used as power sources for these electronic devices also increases.

The weight of the battery in various portable electronic devices accounts for 10-20% of the total weight of the notebook computer and 50% of the total weight of the wireless mobile phone, and the secondary battery greatly affects the compact and light weight of the main body of the portable electronic device. In addition, the long-term continuous use has become an important competition of portable electronic devices.

In particular, with the development of a system capable of transmitting data to each other in the electromagnetic period, and the increase in the transmission speed of the electronic device system that transmits data, conventional secondary batteries such as nickel-cadmium batteries and nickel-hydrogen alloy batteries have been used as wireless devices for portable electronic devices. There is a problem that it is difficult to meet the energy consumption due to the high functionality of the Internet or wireless data communication services. Because of this problem, high energy density, high performance, and high safety of small secondary batteries are continuously required. To this end, advanced countries such as Japan and the United States have been actively pursuing R & D of state- or private-led secondary batteries for a long time. One of the world's most advanced high-performance, next-generation advanced new batteries is lithium secondary batteries.

Lithium secondary battery has higher energy density per unit weight than conventional battery and can be manufactured in various forms. It is easy to develop high voltage and high capacity batteries by lamination, and it is environmentally friendly because it does not use heavy metals that pollute the environment such as cadmium or mercury. Have

Lithium secondary battery having the above advantages is one of the chemical energy conversion device that can lead to the change of free energy generated in the oxidation and reduction reaction to the electrical energy, the cathode (cathode) consisting largely of carbon or graphite, LiMO ₂ (M It is composed of an anode, an electrolyte and a separator made of Co, or Ni). The electrolyte serves as a path through which lithium ions can move freely between the positive electrode and the negative electrode, and the separator prevents physical contact between the positive electrode and the negative electrode and passes lithium ions in the electrolyte between the pores of the separator.

The polyolefin resin commercially used as a separator for a lithium secondary battery is widely adopted as a separator because of its excellent physical properties and economical efficiency. However, since polyolefin is inherently hydrophobic, it has low wettability with respect to an electrolyte.

When assembling the lithium battery, the separator is placed between the positive electrode and the negative electrode, wound into a film, placed in a case of the battery, and the electrolyte is injected into the separator. In this case, since the electrolyte penetrates the separator from both ends to the center direction, if the electrolyte does not penetrate rapidly, a three-phase interface is formed, and side effects such as gas generation on the lithium surface as the anode are likely to occur. Therefore, in order to improve the performance of the lithium battery, it is required to quickly infiltrate the electrolyte into the separator and to maintain the wet state uniformly even after the separator is wet with the electrolyte. Therefore, in order to solve this problem, the conventional polymer membrane reforming technique has focused on supplementing the hydrophobicity of the membrane.

As a surface modification method of a conventional polymer membrane, the wettability was improved by irradiating an ion beam under vacuum in Korean Patent 10-1998-0052484, and in Korean Patent 10-2003-0010677, acrylic acid, vinyl acetate, methyl meta, Graft reaction of a monomer having a polar group such as methacrylate (methyl methacrylate) was performed to increase the surface energy of the porous polymer membrane to improve wettability. However, when used as a battery material for a high voltage of 5 V or more, there is a problem of stability.

In addition, asymmetric pore structure membranes for use in electrochemical and electrical (Korea Patent Application No. 10-2005-7009531), adhesive supporting porous film for battery separator and its use (Korea Patent Application No. 10-2005-7019130), separation membrane Lithium secondary battery (Korean Patent Application No. 10-2006-0006617) containing a surfactant in the coating layer of the film, Lithium ion polymer battery (Korean Patent Application No. 10-2006-0066644) using a photosensitive polymer as a separator, gel polymer layer There is a battery separator (Korean Patent Application No. 10-2007-0040290) and the like, but the technical configuration is different from the present invention.

In addition, the present inventors have developed a microporous separator (Korea Patent No. 10-0785490) coated with an anion-immobilized material, which is a method of coating the anion-immobilized material on the separator and the grafting method of the present invention. Itself is totally different.

Accordingly, the present inventors have developed a method for grafting an anion-immobilized material, manufacturing a separator and a secondary battery using the same, and improving the high energy density of the secondary battery while improving the low wettability which is a disadvantage of the conventional polymer separator. The present invention was completed by confirming that high performance and high safety can be ensured.

An object of the present invention is to provide a porous separator grafted with an anion immobilization material as a monomer.

Another object of the present invention is to provide a method for producing a porous separator grafted with anion immobilization material.

Still another object of the present invention is to provide a lithium secondary battery to which a porous separator grafted with anion immobilization material is applied.

In order to achieve the above object, the present invention provides a porous separator grafted with a borate-based or aluminum-based compound monomer that serves as anion immobilization as a monomer.

In another aspect, the present invention provides a method for producing a porous separator grafted with a borate-based or aluminum compound monomer that serves as anion immobilization.

Furthermore, the present invention provides a lithium secondary battery to which a porous separator in which a borate-based or aluminum compound monomer grafted as anion is immobilized is applied.

According to the present invention, by grafting borate-based or aluminum-based compound monomers which act as anion immobilization on the surface of the polymer membrane, the wettability of the membrane is improved and the anion decomposition is delayed, thereby improving the conduction yield of the cation and at the same time, the conventional membrane It can be used to improve the energy density, performance and safety of the lithium secondary battery because it can exhibit improved electrochemical stability.

Hereinafter, the present invention will be described in detail.

The present invention provides a porous separator in which an anion immobilization material is grafted as a monomer.

The anion immobilization material is preferably a boron (B) -based compound or an aluminum (Al) -based compound having an acrylate group at its terminal. More preferably, it may be a monomer compound represented by the following formula (1).

In Formula 1, R ₁ , R ₂ And R ₃ are each hydrogen or a methyl group and m is an integer from 0 to 30. X is boron (B) or aluminum (Al).

The anion immobilization material is used to improve the wettability of the conventional organic solvent of the porous separator of the present invention and to improve the cation conduction yield and the electrochemical stability.

The porous separator is not particularly limited and conventionally known ones can be used. For example, olefin resins such as polyethylene and polypropylene, fluorine resins such as polyvinylidene fluoride and polytetrafluoroethylene, polyester resins such as polyethylene terephthalate, and nonwoven fabrics such as cellulose can be used. Do.

The porosity of the porous separator is 5 to 90%, the thickness is preferably 5 to 100 ㎛. If the porosity of the porous separator is less than 5%, the content of the electrolyte filling the pores is insufficient, which is disadvantageous in terms of performance, and in the case of more than 90%, it may cause a safety problem due to a short circuit. In addition, when the thickness of the porous separator is less than 5 μm, a short circuit may occur due to tearing due to the formation of lithium dendrites generated during charging and discharging. When the thickness of the porous separator exceeds 100 μm, the capacity of the battery per unit volume decreases. have.

In another aspect, the present invention comprises the steps of impregnating the organic solvent with the compound of Formula 1 and the porous membrane as a monomer in the irradiation device (step 1); Replacing oxygen in the irradiation apparatus with nitrogen (step 2); And it provides a method for producing a porous separator grafted with an anion-immobilized compound comprising the step (step 3) of grafting by irradiating the impregnated porous separator of step 1.

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

Step 1 according to the present invention is a step of impregnating an organic solvent with an anion-immobilized compound represented by Formula 1 and a porous separator as a monomer in a radiation device.

Borate-based or aluminum-based compounds represented by the formula (1) is for the role of dissociation and anion immobilization of lithium salts. It is most preferred to use polyethyleneglycol borate acrylate (PEGBA) compounds among the compounds of formula (1).

In the case of the grafting monomer, 0.1 to 95% by weight of the composition of the present invention may be contained, preferably 5 to 70% by weight, more preferably 10 to 50% by weight.

The porous separator is not particularly limited and conventionally known ones may be used. For example, olefin resins such as polyethylene and polypropylene, fluorine resins such as polyvinylidene fluoride and polytetrafluoroethylene, polyester resins such as polyethylene terephthalate, and nonwoven fabrics made of paper such as cellulose can be used. Can be.

In addition, the organic solvent of step 1 is not particularly limited as long as it is suitable for dissolution of the anion-immobilized compound monomer, preferably acetone, toluene, methanol, ethanol, N-methylpyrrolidone, dimethylformamide, dimethyl sulfoxide Said, gamma butyrolactone, tetrahydrofuran, decalin and the like may be used alone or in combination, and more preferably a mixed solvent of methanol and decarin may be used.

Next, step 2 according to the present invention is a step of replacing the oxygen in the irradiation apparatus with nitrogen.

When oxygen is present in the radiation device, since the reactivity of oxygen is large, a side reaction may occur, and therefore, the inside of the device may be replaced with a weak and stable nitrogen atmosphere.

Next, step 3 according to the present invention is a step of grafting by irradiating the impregnated porous separator of step 1.

The radiation is for grafting the anion-immobilized monomer to the porous separator, and preferably irradiated with one or more selected from the group consisting of gamma rays, ultraviolet rays and electron beams. In addition, the radiation is preferably irradiated with an irradiation dose of 1 to 300 kGy. In this case, if the radiation dose of the radiation is less than 1 kGy, there is a problem that grafting does not occur, and if it exceeds 300 kGy, there is a problem that the sample may burn and damage the product.

Additionally, after performing step 3, the grafted separator from which the unreacted monomer or homopolymer is removed may be obtained by washing and drying the reaction product of step 3.

In order to remove the homopolymer, preferably, the organic solvent used in step 1 may be used, and the drying may be performed at a temperature of 45 to 80 ° C. for at least 2 hours, although there is a slight difference depending on the organic solvent used. desirable.

Furthermore, the present invention provides a lithium secondary battery using a porous separator in which an anion-immobilized compound monomer of Chemical Formula 1 is grafted.

The structure of the lithium secondary battery according to the present invention is not particularly limited, but preferably, a battery such as a square, a cylindrical, a pouch, a coin, or the like can be exemplified.

The positive electrode and the negative electrode if can be used the electrode active material, the electrode active material of the positive electrode active material is a material capable of absorbing and releasing lithium, but are by no means the particularly limited, preferably _{LiCoO 2, LiMn 2 O 4,} LiNi 1 - y Co _y O ₂ (0 <y <1), Li _{1 + x} Ni _a Mn _b Co _c O ₂ (−0.05 ≦ _x ≦ 0.1, 0 ≦ a ≦ 1, 0 ≦ _b ≦ 1, 0 ≦ _c ≦ 1), Composite metal oxides such as LiFe _{1 - x} M _x PO ₄ (M = Mg, Mn or Co, 0 ≦ _x ≦ 0.1) may be used alone or in combination.As a negative electrode active material among the electrode active materials, lithium may be used as a non-limiting example. Lithium alloy, carbon, coke, activated carbon, graphite silicon, tin, etc. which can occlude and release can be used.

In addition, the conductive material is for imparting conductive contact between electrode materials, and may be used without limitation as long as the material has high electrical conductivity and a specific surface area, but is preferably acetylene black, ketjen black, panes black, thermal black, or the like. Carbon black, natural graphite, artificial graphite and the like can be used.

The binder used to form the electrode is used to impart the adhesion and binding strength of the active material to the electrode plate, and may be used as a thermoplastic resin or thermosetting resin or a mixture thereof, preferably polyvinylidene fluoride, polytetrafluoroethylene, SBR (styrene-butadiene rubber) may be used, and as the dispersion medium, isopropyl alcohol, N-methylpyrrolidone, acetone, water, or the like may be used.

The current collector collects electrons generated by the electrochemical reaction of the electrode active material or supplies electrons required for the electrochemical reaction. Generally, a highly conductive metal is used, preferably aluminum, copper, or nickel stainless steel. Meshes or foils made of steel or the like can be used.

Among lithium secondary batteries, a lithium ion battery refers to a battery using an organic solvent that is a liquid as an electrolyte. In the present invention, a nonaqueous electrolyte including a lithium salt and an organic solvent may be used as an electrolyte. LiClO ₄ , LiCF ₃ SO ₃ , LiAsF ₆ , LiBF ₄ , LiN (CF ₃ SO ₂ ) ₂ , LiPF ₆ , LiSCN, LiC (CF ₃ SO ₂ ) ₃ , LiBOB, and the like are preferred. Furnace is ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), gamma butyrolactone (γBL), ethyl methyl carbonate (EMC), dimethoxyethane (DME), die It is preferable to use methoxyethane (DEE), 2-methyltetrahydrofuran (2-MeTHF), dimethyl sulfoxide, or a mixed solvent thereof.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, the following examples are merely to illustrate the invention, but the content of the present invention is not limited by the following examples.

Example  One: Polyethylene glycol Borate Of acrylate (Formula 1)  synthesis

After the stirring apparatus, the thermometer, the magnetic bar, and the nitrogen injection apparatus were installed in the 500 mL three-neck flask, nitrogen was flowed for 2 hours or more before the start of the reaction to remove impurities present in the flask. 20 g of polyethylene glycol acrylate (Mn = 375) was added to 100 mL of tetrahydrofuran dried with metal sodium / benzophenone, followed by 0.619 g of boron oxide (B ₂ O ₃ ), followed by reaction at 65 ° C. for 24 hours. (Scheme 1). After completion of the reaction, the tetrahydrofuran solvent was removed under reduced pressure to obtain polyethylene glycol borate acrylate. Unreacted material in the obtained product was removed using gel chromatography to obtain pure polyethylene glycol borate acrylate.

<Scheme 1>

Example  2: Polyethylene glycol Borate Acrylate ( Poly ( ethylene glycol ) borate acrylate , PEGBA )end Grafted  Preparation of Polyethylene Separator

<Step 1> The anion immobilization compound and the porous separator in an organic solvent Impregnated  step

40 g of polyethylene glycol borate acrylate and 0.05 g of iron oxide represented by Chemical Formula 1 were added to 60 g of a solvent in which methanol and decalin were mixed at a volume ratio of 2: 3 to prepare a mixed solution. The polyethylene solution was cut into 6 cm long and 15 cm long to impregnate the mixed solution. Methanol as a solvent was used as HPLC grade reagent without any special purification, and the separator used was a polyethylene separator having a porosity of 40% and a thickness of 20 μm.

<Step 2> replacing oxygen in the irradiation apparatus with nitrogen

The mixed solution impregnated with the polyethylene membrane of step 1 was placed in a reactor, and the reactor was replaced with high purity nitrogen gas for 10 minutes to completely remove oxygen.

<Step 3> Impregnated  By irradiating the porous membrane with radiation Grafting  step

In the mixed solution impregnated with the polyethylene separation membrane of step 2, the graft reaction was caused by setting the total radiation dose to 100 kGy by adjusting the acceleration dose to 1 MeV and the electron current to 5 mA. The electron beam accelerator (EB tech, ELV-12) used for the grafting reaction was used to control the acceleration dose and irradiate under nitrogen atmosphere.

<Step 4> wash the polyethylene membrane and Desiccant  step

After the completion of the grafting reaction of step 3, to remove the unreacted monomer and homopolymer from the product once again washed with methanol, and the grafting membrane was dried for 12 hours at a temperature of 50 ℃ in a vacuum oven.

Experimental Example  1: of the separator of the present invention Grafting  Check whether

Infrared spectroscopy ( FT - IR ) Analysis

In order to qualitatively check the grafting reaction of the separator prepared in Example 1, the separator was subjected to infrared spectroscopy (FT-IR) using a resolution of 4 cm ⁻¹ and a wavelength range of 4000 to 600 cm ^−1. 64 scans were performed. The results of analyzing the sample using the ATR mode (reflection mode) are shown in FIG. 1. Referring to FIG. 1, unlike the pure polyethylene separator (Comparative Example), in the case of the polyethylene separator (Example) grafted with polyethylene glycol borate acrylate, the BOR peak was confirmed at 1310 cm ⁻¹ to 1380 cm ⁻¹ and also CO stretching It was observed that the peak of B (OR) ₃ was formed at 1040 cm ^-1 ~ 1070 cm ^-1 by, it was confirmed that the grafting.

Scanning electron microscope SEM ) Analysis

In order to confirm the grafting reaction of the separator prepared in Example 1, it was confirmed by scanning electron microscopy (SEM) equipped with energy dispersive x-ray spectroscopy (EDS) capable of elemental analysis, the results are shown in Figure 2 It was. As can be seen in Figure 2, it was confirmed that the boron element is detected in the grafted polyethylene separator.

Experimental Example  2: Measurement of the electrochemical stability of the separator of the present invention

The electrochemical stability of the separator prepared in Example 1 and the conventional polyethylene separator were measured and compared, respectively. The separator prepared in Example 1 was placed in a glove box of argon atmosphere, and impregnated with a liquid electrolyte (EC / DMC (1: 1 by volume) / 1M LiPF ₆ ) to prepare a polymer electrolyte, followed by a stainless steel electrode and a lithium electrode. Bonded between and sealed with a polyethylene coated aluminum cloth and then measured for electrochemical stability, the results are shown in FIG. Referring to FIG. 3, in the case of a membrane in which a polyethylene glycol borate acrylate is not grafted (comparative example), the polyethylene glycol borate acrylate is grafted separator (electrode) which is electrochemically stable to 4.4V but serves as anion immobilization. In the case of the electrochemical stability up to 4.9V it was confirmed that the electrochemical stability is improved.

Experimental Example  3: Measurement of ion conductivity and cation yield of the separator of the present invention

After the polymer electrolyte prepared in Experimental Example 2 was laminated between stainless steel symmetric electrodes to assemble a cell, the resistance of the electrolyte was measured by using an alternating current impedance method, and ionic conductivity was calculated using the same. The cation yield was measured by using the difference between the initial and final currents when the polymer electrolyte prepared by the above method was subjected to a constant voltage for a long time. At this time, the electrode uses a lithium symmetric electrode. The measured ion conductivity values, cation yield, cation conductivity and electrochemical stability are shown in Table 1 below.

Comparison of Characteristics of Membrane Grafted with Polyethylene Glycol Borate Acrylate (Example 2) and Membrane Ungrafted (Comparative Example) Ion Conductivity (S / cm) Cation yield Cationic Conductivity (S / cm) Electrochemical Stability (V) Comparative example 4.9 x 10 ^-4 0.24 1.18 x 10 ^-4 4.4 Example 4.3 x 10 ^-4 0.45 1.95 x 10 ^-4 4.9

In the case of lithium batteries, the mobility of lithium cations should be high to help improve performance. In the case of the present invention, as shown in the above results, it can be seen that the cation yield and the cation conductivity of the embodiment using the separator in which the anion-immobilized material is grafted are improved than the comparative example. This may be due to the increase in dissociation of lithium ions and delayed decomposition of the anions due to the interaction between boron, which acts as a Lewis acid in the grafted separator, and anions generated by dissociation of lithium salts.

Example  3: lithium applying the separator of the present invention Of secondary battery  Produce

The polyethylene glycol membrane grafted with polyethylene glycol borate acrylate prepared in Example 1 was applied to prepare a coin-type lithium secondary battery by a conventional method in the art. The positive electrode used LiCoO ₂ , the negative electrode used graphite (MCMB), and the electrolyte EC / DMC (1: 1, by volume) / 1M LiPF ₆ was used. Also Polyvinylidene fluoride (PVdF) was used as the binder, diethylformamide (DMF) was used as the dispersion medium, aluminum foil was used as the positive electrode current collector, and copper foil was used as the negative electrode current collector.

Experimental Example  4: lithium using the separator of the present invention Of secondary battery Charge and discharge  Speed measurement

The charge / discharge rate of the lithium secondary battery to which the polyethylene glycol grafted polyethylene separator prepared in Example 3 was applied was measured using a charge / discharger (Toyo system, TOSCAT-3000).

Charge and discharge rate was 0.1 C-rate, the results are shown in FIG. Referring to FIG. 4, the lithium secondary battery employing the example at 100 cycles shows better charge and discharge characteristics than the lithium secondary battery employing the comparative example. From this, it can be seen that the lithium secondary battery using the modified polymer separator according to the present invention shows better charge and discharge results than the conventional lithium secondary battery.

1 is a graph showing the results of the FT-IR to determine whether the grafted separator (comparative example) and the separator prepared by the present invention (Example).

Figure 2 is a graph showing the results of EDS analysis of the separator prepared by the present invention.

3 is a graph showing the electrochemical stability of the grafted separator (comparative example) and the separator prepared in accordance with the present invention (Example).

Figure 4 is a graph showing the discharge curve of the grafted separator (comparative example) and the separator prepared in accordance with the present invention (Example).

Claims (14)

R ₁ , R ₂ and R ₃ are each a hydrogen or methyl group, m is an integer from 0 to 30, X is a boron (B) is a porous separator membrane grafted with an anion-immobilized compound monomer represented by the formula (1). <Formula 1>

The porous membrane of claim 1, wherein the porous membrane is at least one selected from the group consisting of an olefin resin, a fluorine resin, a polyester resin, and a cellulose nonwoven fabric. The porous membrane of claim 2, wherein the olefin resin is polyethylene or polypropylene, the fluorine resin is polyvinylidene fluoride or polytetrafluoroethylene, and the polyester resin is polyethylene terephthalate. . The porous membrane of claim 1, wherein the porous separator has a porosity of 5 to 90% and a thickness of 5 to 100 μm. Impregnating the organic solvent with an anion immobilizing compound monomer represented by Formula 1 of claim 1 and a porous separator in a radiation device (Step 1); Replacing oxygen in the irradiation apparatus with nitrogen (step 2); Method for producing a porous separator membrane grafted anion immobilization compound monomer comprising the step (step 3) of grafting by irradiating the impregnated porous membrane of step 1. The method of claim 5, wherein the porous membrane of step 1 is at least one selected from the group consisting of olefin resin, fluorine resin, polyester resin and cellulose nonwoven fabric. The method of claim 6, wherein the olefin resin is polyethylene or polypropylene, the fluorine resin is polyvinylidene fluoride or polytetrafluoroethylene, the polyester resin is a polyethylene terephthalate, the preparation of a porous separator Way. The method of claim 5, wherein the porous membrane of step 1 has a porosity of 5 to 90% and a thickness of 5 to 100 ㎛. The method of claim 5, wherein the organic solvent of step 1 is a group consisting of acetone, toluene, methanol, ethanol, N-methylpyrrolidone, dimethylformamide, dimethyl sulfoxide, gamma butyrolactone, tetrahydrofuran and decalin Method for producing a porous membrane, characterized in that any one or a mixture thereof selected from. 10. The method of claim 9, wherein the organic solvent is a mixed solvent of methanol / decalin. The method of claim 5, wherein the radiation of step 3 is at least one selected from the group consisting of gamma rays, ultraviolet rays and electron beams. The method of claim 11, wherein the radiation is irradiated with a radiation dose of 1 to 300 kGy. The method of claim 5, further comprising washing using the organic solvent used in Step 1 after Step 3. In the lithium secondary battery, a lithium secondary battery, characterized in that the porous separator is grafted with the anion-immobilized compound monomer represented by Formula 1 of claim 1 as a separator.

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