JP2007179847A - Microporous polyolefin-based diaphragm for secondary battery, and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a microporous polyolefin-based diaphragm capable of ultimately improving charge and discharge efficiency of a secondary battery by preventing formation of a dendrite around a negative electrode, and of improving a battery life and having a new structure; and to provide its manufacturing method. <P>SOLUTION: In this microporous polyolefin-based diaphragm for a secondary battery, pores each having a size of 80 nm to 2 μm are distributed 90-97% in a negative electrode-side surface layer, and pores each having a size of 30 nm to 1 μm are distributed 90-97% in a positive electrode-side surface layer. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a microporous polyolefin diaphragm for a secondary battery and a method for producing the same, and more particularly, in a diaphragm used for a high performance secondary battery, the size of pores distributed in the negative electrode side surface layer is large. The secondary battery has a larger thickness than the pores of the positive electrode side surface layer and has a large thickness to prevent dendrite from being formed inside the battery, thereby significantly extending the battery life. The present invention relates to a porous polyolefin diaphragm and a method for producing the same.

  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 separator (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. Moreover, when a short circuit occurs, a so-called shutdown function that quickly shuts off pores in 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, since secondary batteries undergo a chemical reaction and a natural chemical reaction due to charging and discharging without interruption, undesired dendrites are formed particularly at the negative electrode portion. When dendrites are formed inside the battery in this way, the pores of the diaphragm are blocked and the movement of ionic substances is blocked, thereby resulting in poor charge and discharge efficiency of the battery, that is, the life of the secondary battery is reduced. Results in shortening. Also, too much dendrite formation can cause damage to the diaphragm, leading to a fatal problem of weighting the risk of a short circuit.

  In order to suppress the formation of dendrites mainly occurring at the negative electrode part of the battery, it is necessary to form a larger pore size of the diaphragm inserted into the negative electrode part of the battery. If the dimensions are increased to some extent, the movement of the ionic material is easy, but the charging and discharging proceed so rapidly that there is a problem of shortening the battery life. Therefore, in order to suppress the formation of dendrites and extend the life of the battery, it is necessary to precisely control the size and distribution of the pores by the sectional structure of the diaphragm. However, in the conventional method, the pore size and distribution can be controlled according to the amount of pore-forming additives, that is, paraffins and waxes, so that the pore characteristics of the diaphragm cannot be precisely controlled. was there.

  Therefore, the present invention has been made in view of the above problems, and its purpose is to suppress dendrite formation around the negative electrode and ultimately improve the charging and discharging efficiency of the secondary battery. Another object of the present invention is to provide a microporous polyolefin diaphragm having a new structure capable of improving battery life and a method for producing the same.

  In order to achieve the above object, the microporous polyolefin diaphragm for a secondary battery according to the present invention has 90 to 97% of pores having a size of 80 nm to 2 μm distributed in the negative electrode side surface layer, and the positive electrode side surface layer. Is characterized in that 90 to 97% of pores having a size of 30 nm to 1 μm are distributed.

  Further, the present invention is characterized in that the thickness of the negative electrode side surface layer is about 20 to 30% thicker than the thickness of the positive electrode side surface layer, and more pores are distributed in the negative electrode side surface layer.

  In addition, the method for producing a polyolefin-based microporous membrane according to the present invention includes two different molecular weights selected from paraffins or waxes with respect to a polyethylene mixed resin having a melt index of 0.01 to 0.5. A step of mixing 20 to 80% by weight of the above pore-forming additive, a step of melting and extruding the mixture at a temperature of 200 ° C. to 270 ° C. to form a sheet having a thickness of 300 μm to 600 μm, and a cast roll of the sheet And the other side of the sheet is forced to cool by bringing it into direct contact with a cast roll maintained at a temperature of 40 to 80 ° C., and the other side of the sheet is formed with a bank of about 5 to 15 mm on the cast roll. Providing a temperature and moving speed deviation on both sides of the sheet, and stretching the sheet 4 to 7 times in the direction of the vertical axis and the horizontal axis, respectively, to a thickness of 10 μm Or to a stage of producing a 25μm film, a step of removing the silicon compound by depositing the film in an organic solvent, comprising a.

  According to the present invention, by controlling the size and distribution of pores distributed on both surface layers of the microporous polyolefin-based diaphragm for secondary batteries, the formation of dendrites that close the pores is suppressed, and the ultimate Has the effect of significantly improving the charge and discharge characteristics and life of the secondary battery.

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

  The microporous polyolefin-based diaphragm for a secondary battery according to the present invention is characterized in that the pore characteristics of the negative electrode side surface layer and the positive electrode side surface layer are different from each other. That is, the pores distributed in the negative electrode side surface layer have a pore size of 80 nm to 2 μm of 90 to 97% or more, and the pores distributed in the positive electrode side surface layer have a pore size of 30 nm to 1 μm of 90 to 97%. Thus, the pores in the negative electrode side surface layer are relatively larger than the pores in the positive electrode side surface layer. The remaining 3 to 10% of the pores distributed in the negative electrode side surface layer have a size of less than 80 nm, and the remaining 3 to 10% of the pores distributed in the positive electrode side surface layer have a size of less than 30 nm. In the present invention, the thickness of the surface layer located on the negative electrode side is about 20 to 30% thicker than the thickness of the surface layer located on the positive electrode side, and more pores are distributed in the negative electrode side surface layer than the positive electrode side surface layer. Has been.

  In general, in order to suppress dendrite formation, the size of the pores must be large, but if the pores become infinitely large, charging and discharging proceed so rapidly that they shorten the battery life. It will be. Therefore, when the inventor forms a diaphragm having the pore distribution and size as described above through repeated experiments, the inventor suppresses the formation of dendrites and has excellent charge and discharge characteristics. A new fact has been found to dramatically improve the life expectancy of

  On the other hand, in order to produce the microporous polyolefin diaphragm according to the present invention, first, two or more kinds of polyethylene mixed resins having a melt index of 0.01 to 0.5 are used as main materials, and 20-80 Mix weight percent pore-forming additive. At this time, two or more kinds of compounds having different molecular weights selected from paraffins and waxes are used as additives for forming pores in the diaphragm. In addition to the main material and the pore-forming additive, a usual antioxidant may be added.

  In the present invention, by using two or more pore-forming additives having different molecular weights and physical properties, when the pore-forming additive having a large molecular weight is mixed and melted with the polyethylene resin as a main material, When mixed with the main material polyethylene and melted, the pore-forming additive with a small molecular weight is mixed into the inner layer to form pores with a large pore size. Creates pores.

  Subsequently, the mixture is mixed in an extrusion screw, melted at a temperature of 200 ° C. to 270 ° C., extruded through a T die to form a sheet having a thickness of 300 μm to 600 μm, and then the sheet is continuously cast. Pass through the roll. When the sheet extruded through the T-die, which is one of the features of the present invention, passes through the cast roll, one side of the sheet is immediately cooled by being brought into direct contact with the cast roll cooled to a temperature of 40 to 80 ° C. In addition, by forming a bank of about 5 to 15 mm on the cast roll contacting the other surface of the sheet, a temperature deviation and a difference in moving speed are imparted to both surfaces of the sheet.

  In general, in all molded products formed by injection molding or extrusion molding, if the temperatures applied to both surface layers are different from each other, the material constituting the surface layer and the material constituting the inner layer are mutually separated. Since there is a difference in the degree of flow and solidification, even if the same raw material is used, the surface layer and the inner layer are formed so as to have different structures.

  Based on this principle, in the present invention, a high-temperature disk sheet formed via a T-die is immediately put into a cast roll, and a difference in temperature and moving speed is imparted to both sides of the sheet by the cast roll. Pores having different characteristics are formed in the surface layer and the positive electrode side surface layer, respectively. That is, since the surface layer directly cooled by the cast roll is immediately solidified, a positive electrode-side surface layer having small pores while being thin is formed. The cast roll contacting the surface layer on the opposite side of the positive surface layer is formed with a bank of about 5 to 15 mm to give a slight gradient so that the pores are more efficiently stretched due to the difference in moving speed. Therefore, a negative electrode side surface layer having a relatively large pore size and a thick surface layer as compared with the positive electrode side surface layer is formed. Note that the inside of the diaphragm is gradually solidified most slowly by natural cooling to form a core portion.

  Thereafter, the sheet is continuously biaxially stretched 4 to 7 times in the machine direction and the transverse direction to produce a film having a thickness of 10 μm to 25 μm. The film is made of isopropanol, hexane, pentane, methylene chloride, When the pore-forming additive is extracted by deposition in an organic solvent such as methyl ethyl ketone or dioxane, the microporous polyolefin-based membrane according to the present invention is completed.

  By displaying the positive electrode side or the negative electrode side on both surfaces or one of the surfaces of the microporous diaphragm manufactured according to the present invention, the battery can be assembled more easily.

  Examples of the present invention are as follows.

  A resin mixture having a melt index of 0.05 to 0.1 is prepared as a main material by blending polyethylene resins having different molecular weights, and a wax having a molecular weight of 400 to 500 g / mol and a molecular weight of 900 to 1,000 g / mol. After producing a pore-forming additive mixed with a certain wax in a ratio of 1: 1, 50% by weight of the pore-forming additive is added to the main material. 2% by weight of additive was mixed.

  Next, the mixture is melted at a temperature of 240 to 250 ° C. and extruded through a gear pump and a T die to form a sheet before stretching having a width of 450 mm and a thickness of 500 μm, and then the sheet is continuously put into a cast roll. did. At this time, the temperature of the cast roll in contact with one surface of the sheet was maintained at 45 ° C. and the moving speed was maintained at 8 m / min, and a bank of about 10 mm was formed at the portion in contact with the other surface of the sheet.

  The sheet is biaxially stretched at 120 ° C. at a ratio of 5.5 times in the vertical direction and 7 times in the horizontal direction to obtain a thin film having a thickness of about 12 μm. Then, this thin film was put into a sedimentation tank in which hexane was accommodated, and the waxes mixed in the main material were removed to produce a microporous membrane. In order to improve the thermal and mechanical properties of the microporous membrane, the polyethylene membrane of the present invention was completed by heat setting at a temperature of 110 to 120 ° C.

Test of Charging and Discharging Characteristics The surface of both surfaces of the polyethylene diaphragm manufactured according to the above example was photographed at a magnification of 20,000 using a SEM electron scanning microscope, and the shape of each pore was observed. FIG. 1 is a photomicrograph of the positive electrode surface, and FIG. 2 is a photomicrograph of the negative electrode surface. As a result of confirmation from the micrographs of FIGS. 1 and 2, about 95% of pores having a thickness of 30 nm to 1 μm are distributed in the positive electrode side surface layer, and about 80% to 2 μm of pores are distributed in the negative electrode side surface layer. It was confirmed that the distribution was about 95%.

  A lithium ion battery (model name: 553048) was manufactured using the polyethylene diaphragm manufactured according to the above example, and the charge and discharge characteristics of the lithium battery were evaluated. The result was manufactured by Nippon Asahi Kasei Kogyo Co., Ltd. The results are shown in Table 1 below in comparison with a battery using an SE diaphragm. At this time, the same product was used for the positive electrode material, the negative electrode material, and the electrolytic solution of the battery.

リチウムイオン電池の充電及び放電時の効率特性の比較

Comparison of efficiency characteristics during charging and discharging of lithium-ion batteries

  In Table 1, C1 indicates charging efficiency and D1 indicates discharging efficiency. Rate (2C) is a characteristic obtained by dividing the value when charging and discharging for 30 minutes by the rated capacity value of the battery (1C means 1 hour, 0.5C means 2 hours). Indicates the discharge rate. That is, the larger the Rate (2C) value, the better the charge and discharge characteristics, that is, the battery life is improved.

  As is clear from Table 1, the battery using the diaphragm according to the embodiment of the present invention has a ratio of charge and discharge current to the rated capacity value as compared with the battery using the SE diaphragm manufactured by Asahi Kasei Kogyo Co., Ltd. It can be seen that the C rate characteristic representing the characteristic is excellent. That is, it was confirmed that the battery using the diaphragm of the present invention is excellent in charge and discharge characteristics, and is ultimately excellent in high cycle characteristics, so that the battery life is improved.

It is the microscope picture which image | photographed the positive electrode side surface. It is the microscope picture which image | photographed the negative electrode side surface.

Claims (3)

In a microporous polyolefin diaphragm used for a secondary battery,
90 to 97% of pores having a size of 80 nm to 2 μm are distributed in the negative electrode side surface layer, and 90 to 97% of pores having a size of 30 nm to 1 μm are distributed to the positive electrode side surface layer. A microporous polyolefin diaphragm for batteries.   2. The microporous polyolefin diaphragm for a secondary battery according to claim 1, wherein the thickness of the negative electrode side surface layer is about 20 to 30% thicker than the thickness of the positive electrode side surface layer. Mixing 20 to 80% by weight of two or more pore-forming additives having different molecular weights selected from paraffins or waxes with a polyethylene mixed resin having a melt index of 0.01 to 0.5;
Melting and extruding the mixture mixed in the mixing step at a temperature of 200 ° C. to 270 ° C. to form a sheet having a thickness of 300 μm to 600 μm;
The sheet is put into the cast roll, and one side of the sheet is brought into direct contact with the cast roll maintained at a temperature of 40 to 80 ° C. to forcibly cool, and the other side of the sheet forms a bank of about 5 to 15 mm on the cast roll. Providing a deviation in temperature and moving speed on both sides of the sheet,
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 microporous polyolefin diaphragm for a secondary battery.

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