JP2007179846A - Manufacturing method of microporous polyolefin-based diaphragm - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method capable of easily controlling size of minute pores by using a silicon compound having a hydrophobic group and a hydrophilic group as a pore formation additive, and used for highly distributing a ratio of open cells directly related to ventilation capability, in a manufacturing method of a microporous polyolefin-based diaphragm. <P>SOLUTION: This manufacturing method of a microporous polyolefin-based diaphragm includes steps of: mixing 20-80 wt.% of a silicon compound having a hydrophobic group and a hydrophilic group in a polyethylene-mixed resin having a melt index of 0.01-0.5; melting the mixture at a temperature of 200-270°C by mixing it in an extruding screw and thereafter passing it through a T-die and a cast roll to form a sheet having a thickness of 300-600 μm; stretching the sheet four times to seven times in both the vertical and horizontal axis directions to manufacture a film having a thickness of 10-25 μm; and removing the silicon compound from the film. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a method for producing a microporous polyolefin diaphragm, and more specifically, in producing a microporous polyolefin diaphragm used in a high performance secondary battery such as a lithium ion battery, a hydrophobic group and a hydrophilic group. By using a silicon compound having a pore-forming additive, it is possible to more easily control the size of fine pores formed in and on the surface of the diaphragm, and in particular, an open cell directly related to air permeability ( The present invention relates to a method for producing a diaphragm capable of distributing a high percentage of open cells.

  In recent years, with the development of the electronic industry, the demand for high performance secondary batteries such as lithium ion batteries and lithium ion polymer batteries has been greatly increasing. For example, mobile phones, portable information terminal devices, PDAs, digital cameras, notebook computers, Bluetooth, etc. all use such high-performance secondary batteries as power sources. The application field of secondary batteries is expected to expand into various fields.

  In general, a rechargeable secondary battery is mainly composed of four components such as a positive electrode material, a negative electrode material, an electrolytic solution, and a diaphragm. Here, the diaphragm (separator) is a thin film in which fine pores, that is, micropores are widely distributed on the surface and inside of the membrane, and prevents a short-circuit phenomenon caused by direct contact between the positive electrode material and the negative electrode material. It plays the role of allowing only ionic substances to freely pass through the electrolyte. In addition, when a short circuit occurs, a so-called shutdown function that quickly blocks pores inside the diaphragm is exhibited to prevent the possibility of ignition or explosion due to temperature rise. In this way, the diaphragm functions to improve the safety and life of the battery by allowing the ionic substance to pass smoothly and maintaining the electrolyte in a stable manner.

  In light of the current technical level, it is recognized that among the components of the secondary battery, the positive and negative electrode materials and the electrolyte are technically almost stabilized or optimized. However, quality improvement is still required for many parts of the diaphragm. Therefore, it can be said that the most important technical factor that determines the life and quality of high-performance secondary batteries currently used is the diaphragm, and the quality and life of the battery depend on the performance of the diaphragm. Currently.

  Until now, as a main material of a microporous diaphragm for a secondary battery, a polyolefin-based resin that is stable against an electrolytic solution and excellent in shutdown characteristics and insulating characteristics has been frequently used. In particular, when a microporous diaphragm having a single layer structure is manufactured by adopting a wet process manufacturing method, a polyethylene resin having a very large molecular weight is mainly used among polyolefin polymer resins.

  A conventionally known method for producing a microporous polyolefin diaphragm is as follows. First, after adding a plasticizer or wax as a pore forming additive in an appropriate ratio to a main material, for example, a high molecular weight polyethylene resin, this is charged into an extrusion screw via a hopper and melted. A sheet having a predetermined width and thickness is formed by passing through a T-die and a casting roll. The sheet is then stretched in the vertical and horizontal directions to obtain a diaphragm film having a desired width and thickness, and then the film is deposited in a solvent to remove the pore-forming additive. . As a result, fine pores are formed at the positions while the pore-forming additive is eluted, and as a result, a diaphragm having a microporous structure is obtained. The microporous diaphragm is cut into a predetermined size through processes such as heat fixation and corona treatment, and then used as a secondary battery diaphragm.

  On the other hand, the pore characteristics, which are the most important factors determining the performance of the secondary battery diaphragm, are determined by the pore size, the porosity, the ratio of open cells, and the like. Here, the open cell means a pore penetrating the diaphragm, and is directly related to air permeability. However, such pore characteristics are largely determined at an early stage in the manufacturing process of the diaphragm. That is, most of the pore characteristics are determined in the process of blending and kneading the main material and the pore-forming additive, and in the step of forming the sheet by the melting and extrusion process. In particular, the pore-forming additive is most closely related to the pore properties. However, in the past, organic liquid phases such as paraffin wax, liquid paraffin, and process oil have been mainly used as pore-forming additives. However, in such conventional methods, technically fine pore size and distribution are precisely measured. There was a limit to control.

  Accordingly, the present invention has been made in view of the above problems, and its purpose is to use a silicon compound having a hydrophobic group and a hydrophilic group as a pore-forming additive in a method for producing a microporous polyolefin-based diaphragm. It is an object of the present invention to provide a method for controlling the size of fine pores more easily, and in particular, distributing a high proportion of open cells directly related to air permeability.

  In order to achieve the above object, the present invention provides a silicon compound having a hydrophobic group and a hydrophilic group in a polyethylene mixed resin having a melt index of 0.01 to 0.5 to 20 wt% to 80 wt% based on the polyethylene mixed resin. Mixing, mixing the mixture into an extrusion screw, melting the mixture at a temperature of 200 ° C. to 270 ° C., and passing through a T die and a cast roll to form a sheet having a thickness of 300 μm to 600 μm. A step of producing a film having a thickness of 10 μm to 25 μm by stretching the sheet 4 to 7 times in the vertical and horizontal directions, and a step of depositing the film in an organic solvent to remove the silicon compound. It is characterized by including these.

  According to the present invention, by using a silicon compound having a hydrophobic group and a hydrophilic group as a pore-forming additive, the size of the fine pores formed on the surface and inside of the diaphragm can be more easily controlled. There is an effect of improving so that the proportion of open cells directly related to the air permeability is highly distributed.

  Therefore, the present invention is expected to be greatly used to improve the performance of high performance secondary batteries, particularly lithium batteries.

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

  The present invention is characterized by using a silicon compound having a hydrophobic group and a hydrophilic group as a pore-forming additive. The silicon compound is represented by the following chemical formula 1 or chemical formula 2.

In Formula 1, R ₁ and R ₂ were each selected from —H, —OH, —COOH, —CH ₃ COOH, —NH ₂ , —SCO ₃ H, —CH ₃ , and —C ₆ H ₅ . In any one of them, R ₁ and R ₂ may be the same or different, and the degree of polymerization 1 is 10 to 1,000.

In the chemical formula 2, R ₃ is a hydrophilic group and is any one selected from —OH, —COOH, —CH ₃ COOH, and —SCO ₃ H. But you can. R ₄ is a hydrophobic group and is any one selected from —H, —CH ₃ , —C ₆ H ₅ , and —NH ₂ , and may be the same or different substituents. Good. Furthermore, the sum of polymerization degrees m and n is 10 to 1,000.

  In the chemical compounds 1 and 2, the hydrophobic group enhances the water repellency of the silicon compound, and the hydrophilic group enhances the hygroscopicity and the infiltration property. Therefore, when the substitution ratio of hydrophilic groups in the silicon compound is increased, the pore size is increased. When the substitution ratio of hydrophobic groups is increased, the pore size is decreased. Since the polyethylene resin which is the main material of the present invention is a hydrophobic substance, when it is mixed with a hydrophilic group, it is not mixed well and becomes a lump and the pore size increases. In particular, since the silicon compound of Chemical Formula 2 has a structure in which a hydrophilic group and a hydrophobic group are blocked, depending on the degree of substitution of the hydrophilic group and the hydrophobic group, the pore size in the inside and the surface of the diaphragm, the distribution of open cells, the porosity, etc. Can be controlled efficiently.

  The main material of the present invention is preferably a mixed resin composed of two or more polyethylene resins having different molecular weights and having a melt index of 0.01 to 0.5. Further, the mixing ratio of the silicon compound can be 20 to 80% by weight with respect to the polyethylene mixed resin. However, if the ratio of the silicon compound is 20% by weight or less, the pore distribution becomes extremely poor. If the function of the diaphragm cannot be performed, and if it is 80% by weight or more, the strength of the diaphragm is too weak. In the present invention, in order to maintain a stable mechanical strength while the diaphragm shows a high pore distribution rate and an excellent open cell formation rate, it is preferable to add 50 to 60% by weight of a silicon compound. . In this way, the pore distribution rate as a whole inside the diaphragm film is about 50% to 80%, and the proportion of open cells directly related to air permeability among these pores is maintained at a level of 90% to 95%. become.

  Thus, after mixing the silicon compound as the pore forming additive with the polyethylene mixed resin as the main material, an additive such as an antioxidant is added in the same manner as in the conventional method, and this is mixed into the screw for extrusion. After melting at a temperature of 270 ° C. to 270 ° C., a sheet having a thickness of 300 μm to 600 μm is formed by passing through a T-die, a cast roll and the like by a normal method.

  Next, the sheet is stretched 4 to 7 times in the vertical and horizontal directions to form a film having a thickness of 10 to 25 μm. At this time, the stretching ratio is determined by the thickness of the sheet before stretching and the thickness of the final film. Finally, the sheet stretched to a predetermined thickness is deposited in an organic solvent such as hexane, cyclohexane, methylene chloride, methyl ethyl ketone, and the silicon compound is extracted, followed by a biaxial stretching process and a heat setting process. A microporous membrane for a battery is completed.

  50% by weight of a silicon compound (degree of polymerization 1 is 10 to 1,000) represented by the following chemical formula 3 is added to a polyethylene mixed resin having a Melt Index of 0.01 to 0.5, This mixture was melt-kneaded in an extrusion screw at a temperature of 240 ° C to 250 ° C.

  Next, the mixture is extruded through a gear pump and a T die to form a sheet having a thickness of 500 μm, and this sheet is continuously passed through a cast roll at a temperature of 80 ° C. or less at a speed of 10 m / min, and a thickness of 450 μm. Sheet was formed.

  The silicon compound is stretched at a temperature of 110 to 120 ° C. at a stretching ratio of 5 times in the longitudinal direction and 7 times in the lateral direction, and the stretched sheet is deposited in a hexane solution of 90% or more for 2 to 3 minutes. Removed.

  The stretched sheet was taken out about 10 minutes after being deposited in a sedimentation tank in which water and alcohol were mixed at a ratio of 7: 3, and then stretched in a heat setting chamber at a stretching ratio of 25% and a heat setting temperature of 120 degrees. And heat setting to complete a microporous diaphragm.

  As a result of measuring various pore characteristics with respect to the microporous diaphragm, the film thickness was 10 to 12 μm, the pore size was 100 to 300 nm, the porosity was 65%, and the open cell formation rate was 93%. It was recognized that there was.

  A microporous diaphragm was produced in the same manner as in Example 1 except that a silicon compound represented by the following chemical formula 4 was used as a pore-forming additive. At this time, the polymerization degree of the silicon compound (Chemical Formula 4) was that m was 10 to 100 and n was 300 to 400.

  As a result of measuring various pore characteristics with respect to the microporous diaphragm, the film thickness was 10 to 12 μm, the pore size was 50 to 200 nm, the porosity was 63%, and the open cell formation rate was 91%. It was recognized that there was.

Claims (5)

In a method for producing a microporous polyolefin diaphragm used in a high performance secondary battery,
Mixing a silicon compound having a hydrophobic group and a hydrophilic group in a polyethylene mixed resin having a melt index of 0.01 to 0.5 with 20 wt% to 80 wt% with respect to the polyethylene mixed resin;
The mixture mixed in the mixing step is mixed in an extrusion screw, melted at a temperature of 200 ° C. to 270 ° C., and then passed through a T die and a cast roll to form a sheet having a thickness of 300 μm to 600 μm. And the stage of
Stretching the sheet 4 to 7 times in the vertical and horizontal directions, respectively, to produce a film having a thickness of 10 μm to 25 μm;
Depositing the film in an organic solvent to remove the silicon compound, and a method for producing a polyolefin-based microporous diaphragm. 2. The method for producing a polyolefin microporous diaphragm according to claim 1, wherein the silicon compound is a compound represented by the following chemical formula 1.

(Where R ₁ and R ₂ are each selected from —H, —OH, —COOH, —CH ₃ COOH, —NH ₂ , —SCO ₃ H, —CH ₃ , —C ₆ H _5. R ₁ and R ₂ may be the same or different from each other, and the degree of polymerization 1 is 10 to 1,000.) 2. The method for producing a polyolefin microporous diaphragm according to claim 1, wherein the silicon compound is a compound represented by the following chemical formula 2.

(Here, R ₃ is any one selected from —OH, —COOH, —CH ₃ COOH, and —SCO ₃ H, and R ₄ is —H, —CH ₃ , —C ₆ H _5. , —NH ₂ , wherein R ₃ and R ₄ may be the same or different from each other, and the degree of polymerization m and n is 10 to 1,000 in total. .)   2. The method for producing a polyolefin-based microporous membrane according to claim 1, wherein the organic solvent is one or more selected from hexane, cyclohexane, methylene chloride, and methyl ethyl ketone.   2. The method for producing a polyolefin-based microporous diaphragm according to claim 1, wherein the silicon compound is mixed in an amount of 50% to 60% by weight with respect to the polyethylene mixed resin.