KR20100072825A - Porous membrane having advanced heat resistance and electrochemical device using the same - Google Patents

Abstract

PURPOSE: A porous separator and an electrochemical device including thereof are provided to secure the chemical stability and the affinity with an electrolyte without sealing pores on a polymeric porous separator substrate. CONSTITUTION: A porous separator comprises a polymeric porous separator substrate with a pore unit, and an inorganic layer coated to the fibrous surface of the substrate. A manufacturing method of the porous separator comprises the following steps: forming an inorganic precursor solution by dissolving an inorganic precursor to a solvent; spraying the inorganic precursor solution to the polymeric porous separator substrate; and curing and drying the solution.

Description

Porous membrane having advanced heat resistance and electrochemical device using the same

The present invention relates to a porous separator and an electrochemical device including the same, and more particularly, to a porous separator and an electrochemical device having high temperature stability and durability improved by using a heat resistant inorganic material as a coating component.

Recently, interest in energy storage technology is increasing due to the limitation of fossil resources and the expansion of IT technology. As the field of application extends to the energy of mobile phones, camcorders, notebooks and PCs, and even electric vehicles, efforts for research and development of batteries have become increasingly concrete. The electrochemical device is the most attracting field in this respect, and among them, the development of a secondary battery that can be charged and discharged is becoming important.

Among the secondary batteries currently applied, lithium secondary batteries developed in the early 1990s have higher operating voltage and significantly higher energy density than conventional batteries such as Ni-MH, Ni-Cd, and sulfuric acid-lead batteries using aqueous electrolyte solution. There is an advantage. However, since the lithium secondary battery uses an organic electrolyte, safety problems such as ignition and explosion exist, and manufacturing is difficult. Recently, the lithium ion polymer battery has been considered as one of the next generation batteries by improving the weakness of the lithium secondary ion battery, but the capacity of the battery is relatively low compared to the lithium secondary ion battery, and in particular, the discharge capacity at low temperatures is insufficient. Improvements are urgently needed.

Lithium secondary batteries are coated with a positive electrode active material (eg, LiCoO ₂ ) and a negative electrode active material (eg, graphite) having a vacant space on aluminum foil and copper foil, which are current collectors, respectively. After the preparation, the separator is interposed between the electrodes, and the electrolyte is injected. When the battery is charged, lithium embedded in the positive electrode active material crystal is detached and enters the negative electrode active material crystal structure. On discharge, lithium in the negative electrode active material is detached and inserted into the positive electrode crystal.

As the charge and discharge as described above, lithium ions move energy between the positive electrode and the negative electrode to transfer energy, and thus, are called rocking chair batteries. These batteries are produced by many companies, but their safety characteristics are different. Evaluating the safety of the battery and ensuring safety are the most important considerations. In particular, in order to prevent user injury due to battery malfunction, safety standards of lithium secondary batteries such as fire and smoke in the battery are strictly regulated. Many methods have been proposed to solve such battery safety problems, but battery ignition due to a forced internal short circuit caused by an external shock or a user-abused has not yet been clearly solved.

Recently, a technique of coating inorganic layers such as calcium carbonate and silica on a polyolefin-based separator for the purpose of preventing internal short circuits caused by dendrite growth inside a cell has been disclosed in US Pat. However, in the case of coating such an inorganic particle layer on the polymer membrane substrate, there is a disadvantage that the performance of the battery is reduced by blocking the pores formed in the polymer membrane substrate. In particular, the portion occupied by the non-porous inorganic particles constituting the inorganic layer hinders the movement of lithium ions which determine the performance of the battery, so that a significant decrease in battery performance is fundamentally inevitable.

In order to improve the problem, the use of porous inorganic particles to improve the performance of the battery is disclosed in Korean Patent Publication No. 10-2007-0055979. The patent uses porous inorganic particles in which a large number of macropores of uniform size and shape exist in the particles themselves as components of the organic / inorganic hybrid porous membrane, and heat-resistant inorganic particles are used to form pores on the surface of the membrane. Although the blocking phenomenon is partially improved, even when the porous inorganic particles are used, the inorganic particles block the pores of the separator or the pores of the inorganic particles are partially closed by the binder resin, thereby reducing the porosity.

Accordingly, an object of the present invention is to provide a porous separator and an electrochemical device including the same, which do not block pores of the polymer porous separator substrate and have excellent heat resistance, chemical stability, and affinity with an electrolyte solution.

In order to achieve the above object, the present invention, the porous membrane substrate having a pore; And it provides a porous separator comprising an inorganic coating is coated on the surface of the fibrous tissue of the substrate. Here, the inorganic coating is made of polysiloxane (including derivatives thereof), and selected from the group consisting of polysilazane, polymethylhydrosilazane, polymethylvinylhydrosilazane, polydimethylhydrosilazane, and mixtures thereof. It is preferable that it is a hardening product of the inorganic precursor used.

In addition, the present invention comprises the steps of (a) diluting an inorganic precursor capable of forming a polymer in a solvent to prepare an inorganic precursor solution; (b) applying the inorganic precursor solution to the porous separator substrate; And (c) provides a method for producing a porous membrane comprising the step of curing and drying the applied inorganic precursor solution. In addition, the present invention is a positive electrode and a negative electrode; A porous separator substrate interposed between the positive electrode and the negative electrode and having pores; And an inorganic membrane coated on the surface of the fibrous tissue of the substrate; And it provides an electrochemical device comprising an electrolyte.

The porous separator according to the present invention and the electrochemical device including the same are thinly coated only on the surface of the fibrous tissue of the separator substrate while maintaining the pores of the porous separator to sufficiently transfer the lithium ions in the solvated state with the electrolyte molecules. A large number of pore structures can be uniformly present, and they have excellent affinity with general electrolytes to express good mobility of lithium ions, and have similar mobility performance and excellent heat resistance as compared to a conventional polyolefin-based separator. Have

Hereinafter, the present invention will be described in detail.

The porous membrane according to the present invention is a thin inorganic film formed by using an inorganic precursor solution on the surface of a fibrous tissue of a porous membrane substrate having pores, and the inorganic film does not block pores of the porous membrane substrate. Do not. In the present specification, the fibrous tissue is a microstructure in the form of a net forming the surface of the porous membrane and the sidewalls of the pores, the inorganic coating is coated on the surface of the porous membrane and the sidewalls of the pores, pores formed in the porous separator Do not block it. Therefore, although the inorganic particles introduced to increase the heat resistance improves the safety of the battery, it solves the disadvantage of reducing the battery performance by blocking the pores of the polymer porous separator.

Since the pores of the porous separator are the movement paths of active components, such as lithium ions (Li ⁺ ), which cause the battery reaction in the electrochemical device, the pore size and porosity may be an important factor in controlling the ion conductivity in the battery. In addition, it directly affects battery performance. That is, in the case where lithium ions causing battery reaction in the lithium secondary battery move to the positive electrode, the pores in the separator located between the positive electrodes are theoretically larger than the diameter of the lithium ions, Can play a role. For reference, the diameter of lithium ions is a few kV units. However, when lithium ions actually move to the positive electrode, instead of moving alone, they move in a state in which they are solvated in an electrolyte, for example, a large number of carbonate-based compound molecules. When the porosity has a range similar to the diameter of the lithium ions, the performance of the battery may not be properly exhibited due to a decrease in lithium ion mobility and a decrease in ion conductivity in the battery. For example, when ethylene carbonate (EC), dimethyl carbonate (DMC), or the like is used as an electrolyte component, lithium ions are solvated in a form surrounded by four molecules of relatively large EC and DMC, and move to the electrode. It may have a size of about 1 to 2 nm or more. Therefore, in order to improve battery performance, the size of the lithium ions and the electrolyte molecules must be considered together. In addition, in order for the lithium ion molecules solvated in the electrolyte to move the positive electrode and the negative electrode well, the interface of the separator and the compatibility of the electrolyte should be excellent.

The porous membrane substrate used in the present invention is not particularly limited as long as it is a porous membrane substrate having pores. For example, a porous polymer commonly used in the art, preferably, a heat resistant porous polymer having a melting temperature of 140 ° C. or higher may be used. In particular, in the case of the heat-resistant porous polymer, since membrane shrinkage caused by external and / or internal heat stimulation is prevented, thermal stability of the organic / inorganic composite porous membrane can be ensured. Non-limiting examples of the porous membrane substrate, high density polyethylene (HDPE), low density polyethylene (LDPE), linear low density polyethylene (LLDPE), ultra high molecular weight polyethylene (UHMWPE), polypropylene (polypropylene), polyethylene terephthalate (polyethyleneterephthalate), Polybutyleneterephthalate, polyester, polyacetal, polyamide, polycarbonate, polyimide, polyetheretherketone, polyethersulfone (polyethersulfone), polyphenyleneoxide, polyphenylenesulfidro, polyethylene naphthalene, mixtures thereof, and the like, and other heat resistant engineering plastics.

The thickness of the porous membrane substrate is 1 to 100㎛, preferably 5 to 50㎛. If the thickness of the porous membrane substrate is less than 1㎛, it is difficult to maintain the mechanical properties, if it exceeds 100㎛, there is a risk of acting as a resistance layer during the movement of lithium ions (Li ⁺ ) and the like. The pore size (diameter) of the porous membrane substrate is 0.01 to 50㎛, preferably 0.1 to 20㎛, porosity is not particularly limited, but preferably 5 to 95%. If the pore size and porosity are less than 0.01 μm and 5%, respectively, it may act as a resistive layer during the movement of lithium ions (Li ⁺ ). If the pore size and porosity exceeds 50 μm and 95%, Mechanical properties can be difficult to maintain. The porous membrane substrate may be in the form of a fiber or membrane, and in the case of the fiber, a nonwoven fabric forming a porous web, and is formed of spunbond, melt blown, or electric material composed of long fibers. It is preferably in the form of electro spinning or the like, and combinations thereof are also possible.

The inorganic coating is formed by coating an inorganic precursor solution capable of forming a polymer on the surface of the fibrous tissue of the porous separator substrate, followed by curing and drying to form a thin coating having a thickness of 10 to 100 nm, wherein the inorganic material is the porous separator substrate. It is coated on the surface of the fibrous tissue of the, the porous membrane is electrochemically stable because it is not oxidized and / or reduced in the operating voltage range (eg, 0 ~ 5V based on Li / Li ⁺ ) of the electrochemical device to which the porous separator is applied, the separator A material that does not damage the substrate, ensures stability and affinity (wetting) with respect to the electrolyte, and does not block pores of the porous separator substrate may be used without limitation.

The method of forming the inorganic coating is (a) diluting an inorganic precursor capable of forming a polymer in a solvent to prepare an inorganic precursor solution (b) applying the inorganic precursor solution to the surface of the fibrous tissue of the porous separator substrate Step and (c) curing and drying the applied inorganic precursor solution to form an inorganic coating.

, n=1~1,000) 및 그 유도체를 들 수 있고, 상기 폴리실라잔 유도체의 예로는, 폴리메틸하이드로실라잔(polymethyl(hydro)silazane,

, n=1~1,000), 폴리메틸하이드로실라잔과 폴리메틸비닐하이드로실라잔(polymethylvinyl(hydro)silazane/polymrthylvinylsilazane)의 혼합물(

, m=1~1,000, n=1~1,000), 폴리메틸하이드로실라잔과 폴리다이메틸실라잔(polymethylvinyl(hydro)silazane/polydimethylsilazane)의 혼합물(

, m=1~1,000, n=1~1,000) 등이 있다.A representative example of the inorganic coating is polysiloxane (including a derivative thereof), and the inorganic precursor is a material that is cured to form the inorganic coating, for example, polysilazane (polysilazane,

, n = 1 to 1,000) and derivatives thereof, and examples of the polysilazane derivative include polymethyl (hydro) silazane,

, n = 1 to 1,000), a mixture of polymethylhydrosilazane and polymethylvinyl (hydro) silazane / polymrthylvinylsilazane (

, m = 1 to 1,000, n = 1 to 1,000), a mixture of polymethylhydrosilazane and polydimethylsilazane (polymethylvinyl (hydro) silazane / polydimethylsilazane)

, m = 1 to 1,000, n = 1 to 1,000).

In particular, the polysilazane oligomer has excellent lithium ion transfer ability, and is applied to the surface of the fibrous tissue of the porous membrane substrate, and then reacted with moisture in the air as shown in Scheme 1 to form an inorganic film, that is, a polysiloxane film. Form. The polysiloxane film thus formed has an advantage of having good affinity with the electrolyte solution, ensuring a good migration path for lithium ions, and facilitating an electrolyte injection process. The inorganic precursors such as polysilazane may be purchased or manufactured by a conventional method.

The content of the inorganic precursor can be variously adjusted within the range of 0.05 to 30% by weight, preferably 0.1 to 20% by weight with respect to the total inorganic precursor solution. When the content of the inorganic precursor is less than 0.05% by weight, there is no effect of improving the heat resistance expressed by the inorganic coating, and when the content of the inorganic precursor exceeds 30% by weight, the pores of the porous membrane substrate may be prevented from exhibiting the performance of the membrane.

In addition, the coating amount of the inorganic precursor may vary depending on the material, porosity, type of inorganic precursor, etc. of the porous membrane substrate, but is generally 0.005 to 10 parts by weight, preferably 0.01 to 8, based on 100 parts by weight of the porous membrane substrate. Parts by weight. If the coating amount is less than 0.005 parts by weight, the inorganic coating may be insufficiently formed on the porous separator, and sufficient heat resistance may not be expressed. If the coating amount is more than 10 parts by weight, the pores of the porous separator may be blocked.

The solvent of the inorganic precursor solution is not particularly limited as long as it has sufficient solubility in the inorganic precursor and does not damage the substrate of the separator, and a general organic solvent may be widely used. Non-limiting examples of such organic solvents include acetone, tetrahydrofuran, dioxane, methylenechloride, chloroform, dimethylformamide, N-methyl- 2-pyrrolidone (N-methyl-2-pyrrolidone, NMP), cyclohexane, cyclohexanone, butyrolactone, methylethyleketone, normal-hexane (n- hexane, diethylether, dibutylether, ethyl acetate, mixtures thereof, and the like.

Application of the inorganic precursor solution may be applied to conventional coating methods used in the art, for example, dip coating, die coating, roll coating, comma coating, It may be applied by spray coating or a mixture thereof.

The curing is performed by leaving the separator coated with the inorganic precursor solution in the air. For example, as shown in Scheme 1, the inorganic precursor (polysilazane) absorbs moisture in the air and the inorganic film (polysiloxane film). ) Is cured (crosslinked). The curing may be heated to increase the curing rate.

The drying may be performed by standing in an air at room temperature, or may be dried by heating, but a method of heating and drying is preferable in terms of shortening a manufacturing process time. The drying temperature is 20 to 300 ℃, preferably 30 to 280 ℃, if the drying temperature is less than 20 ℃, the drying rate is slow to increase the process speed, if it exceeds 300 ℃, the pores of the porous membrane substrate to heat The clogging phenomenon may occur.

The porous separator formed with the inorganic coating of the present invention prepared as above is useful as a separator of an electrochemical device, preferably a lithium secondary battery.

The electrochemical device according to the present invention includes a positive electrode, a negative electrode, a porous separator and an electrolyte solution in which the inorganic coating of the present invention is interposed between the positive electrode and the negative electrode. The electrochemical device includes all devices that undergo an electrochemical reaction, and specific examples thereof include all kinds of primary, secondary cells, fuel cells, solar cells, or capacitors. In particular, a lithium secondary battery is preferable among secondary batteries, and specific examples thereof include a lithium metal secondary battery, a lithium ion secondary battery, a lithium polymer secondary battery, or a lithium ion polymer secondary battery.

The electrochemical device may be manufactured according to a conventional method known in the art, for example, assembled through the electrode and the separator, and then prepared by injecting electrolyte into the assembly. The cathode, the anode, and the electrolyte are not particularly limited, and conventional ones that may be used in the conventional electrochemical device may be used.

Hereinafter, the present invention will be described in more detail with reference to specific examples. The following examples are intended to illustrate the invention, and the invention is not limited by the following examples.

Preparation Example Preparation of Polysilazane and Polysilazane Solution

After the gas introduction tube, mechanical stirrer and dewar condenser were mounted in a 300 ml four-necked flask, the interior of the flask was replaced with high purity nitrogen. 150 ml of degassed pyrimidine (Py) was added to the flask, cooled to 0 ° C. in an ice bath, and then 16 g of dichlorosilane (SiH ₂ Cl ₂ ) was slowly added over 50 minutes. Thus, a white solid compound (SiH ₂ Cl ₂ Py) was formed. The reaction mixture was vigorously stirred in an ice bath and then passed through a soda lime tube and activated carbon tube and bubbled for 1 hour with a mixture of 10.9 g of previously purified nitrogen gas and ammonia. After the reaction was completed, the reaction solution was filtered, and the filtrate was dried in vacuo (50 ° C, 5 mmHg, 2 hours) to remove the solvent, thereby obtaining 5.52 g of a glassy solid polysilazane. The molecular weight of the polysilazane measured by the vacuum pressure drop method was 1,500. In an oxygen-blocked glove box, 0.5 g of the polysilazane powder was mixed with 99.5 g of dibutylether to prepare 100 g of a 0.5% by weight polysilazane solution.

EXAMPLES Preparation of Porous Separator with Polysiloxane Film

Polyolefin porous membrane substrate having a thickness of 20 μm and a porosity of 40% was dip coated on the polysilazane solution prepared (or commercialized) according to the above preparation, and then cured for 30 minutes in a convection oven at 50 ° C. Crosslinking) and dried to prepare a porous separator in which a polysiloxane film was formed.

Evaluation of Porous Membranes

The porous separator prepared with the polysiloxane film prepared according to the above example and the conventional porous separator without the coating treatment (comparative example) were measured by an electron scanning microscope (SEM, JEOL Co., Ltd.) Example), after cutting the porous separator of the above Examples and Comparative Examples to 5 cm in width, 10 minutes in a hot plate (103 ℃ hot plate), and after heating the picture of Figure 3 (Example ) And FIG. 4 (comparative example). In addition, the length before and after heating the porous separator of the Examples and Comparative Examples was measured three times at the measurement position shown in Figure 5, the results are shown in Table 1 below.

Example Comparative example 1 time Episode 2 3rd time 1 time Episode 2 3rd time Measuring position ① 4.55 cm 4.56 cm 4.57 cm 3.8 cm 3.8 cm 3.7 cm Measurement position ② 4.55 cm 4.54 cm 4.53 cm 3.9 cm 3.7 cm 3.8 cm Measuring position ③ 4.6 cm 4.65 cm 4.54 cm 3.7 cm 3.7 cm 3.7 cm Measurement position ④ 4.6 cm 4.6 cm 4.58 cm 3.6 cm 3.6 cm 3.75 cm Measurement position ⑤ 4.5 cm 4.54 cm 4.55 cm 3.5 cm 3.4 cm 3.5 cm Measuring position ⑥ 4.7 cm 4.6 cm 4.7 cm 4.1cm 3.9 cm 3.5 cm Average 4.6 cm 4.6 cm 4.6 cm 3.8 cm 3.7 cm 3.7 cm Shrinkage 8.3% 8.4% 8.4% 24.7% 26.3% 26.8% Average shrinkage 8.4% 25.9%

Table 1 shows that the porous membrane formed with the inorganic coating (polysiloxane coating) of the present invention has an average shrinkage rate of 8.4% after heating, and has excellent heat resistance as compared with a conventional porous separator (comparative example, average shrinkage rate of 25.9%). have.

In addition, from FIG. 1, the inorganic coating of the present invention is formed from the inorganic precursor solution (polysilazane solution), so that the polysiloxane coating is thin and uniformly distributed on the surface of the fibrous tissue substrate of the radially crosslinked porous separator substrate, and the coating is porous. It can be confirmed that it does not prevent the. Therefore, the polymer porous membrane in which the inorganic particles are introduced to increase the heat resistance solves the disadvantage of blocking pores in spite of the safety of the battery. Inorganic materials such as polysiloxane have good affinity with general electrolytes and have good lithium ions. Since the mobility is expressed, ion migration performance is comparable to that of a conventional polyolefin-based separator.

1 is a SEM photograph of 20,000 magnification of a porous separator prepared in an embodiment of the present invention.

Figure 2 is a SEM picture of 20,000 magnification of a conventional porous separator.

Figure 3 is a photograph showing the state after heating of the porous separator prepared in the embodiment of the present invention.

Figure 4 is a photograph showing a state after heating of a conventional porous separator.

5 is a view showing a length measurement position for measuring the shrinkage of the porous separator.

Claims (6)

Porous separator substrate having pores; And Porous separator comprising an inorganic coating is coated on the surface of the fibrous tissue of the substrate. The porous membrane of claim 1, wherein the inorganic coating is made of polysiloxane (including a derivative thereof). 3. The inorganic coating according to claim 2, wherein the inorganic coating is a cured product of an inorganic precursor selected from the group consisting of polysilazane, polymethylhydrosilazane, polymethylvinylhydrosilazane, polydimethylhydrosilazane and mixtures thereof. Phosphorus porous membrane. The method of claim 1, wherein the porous membrane substrate is high density polyethylene, low density polyethylene, linear low density polyethylene, ultra high molecular weight polyethylene, polypropylene, polyethylene terephthalate, polybutylene terephthalate, polyester, polyacetal, polyamide, polycarbonate, Polyimide, polyether ether ketone, polyether sulfone, polyphenylene oxide, polyphenylene sulfide, and porous separator is selected from the group consisting of polyethylene naphthalene and mixtures thereof. (a) diluting an inorganic precursor capable of forming a polymer in a solvent to prepare an inorganic precursor solution; (b) applying the inorganic precursor solution to the porous separator substrate; And (c) curing and drying the applied inorganic precursor solution. Anode and cathode; A porous separator substrate interposed between the positive electrode and the negative electrode and having pores; And an inorganic membrane coated on the surface of the fibrous tissue of the substrate; And An electrochemical device comprising an electrolyte.

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