JP4267634B2 - Microporous polyolefin separator having three-dimensional stretching characteristics and method for producing the same - Google Patents

Description

  The present invention relates to a microporous polyolefin separator having three-dimensional stretching characteristics and a method for manufacturing the same, and more specifically, in manufacturing a separator used in a high-performance secondary battery, the polyolefin sheet is placed on a vertical axis and a horizontal axis. In addition to adding a pore-stretching compound during the extrusion process and stretching in the thickness direction of the polyolefin sheet, a large number of micropores, that is, micropores are uniformly formed inside the separator. As a result, in addition to being excellent in pore characteristics and stretch characteristics, the microporous polyolefin-based separator having excellent mechanical characteristics, thermal characteristics, electrical characteristics, etc. and its It relates to a manufacturing method.

  The development of the electronic industry in recent years is remarkable, and along with this, the demand for high performance secondary batteries such as lithium ion batteries and lithium ion polymer batteries is increasing. For example, mobile phones, portable information terminal devices, PDAs, digital cameras, notebook computers, Bluetooth devices, etc. all use high-performance secondary batteries as a power source, and will be used in various fields such as medical use and military use in the future. The application field of secondary batteries is expected to expand.

  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 separator. Here, the separator membrane has many micropores penetrating the membrane like a sword structure, and is inserted between the positive electrode material and the negative electrode material to block the movement of the electrolytic solution, and the ionic material It plays the role of selectively passing through. In addition, it prevents the short circuit phenomenon that the positive electrode material and the negative electrode material are in direct contact with each other, and when the short circuit occurs, the pores inside the film are blocked to prevent ignition or explosion due to the temperature rise, thereby improving the stability and life of the battery. The function of improving.

  In light of the current technical level, it is recognized that it is almost technically stable or has reached the optimal level for the positive and negative electrode materials and the electrolyte among the components of the secondary battery. In terms of film, quality improvement is still desired in many parts. For this reason, it can be said that one of the important technical factors that determine the life and quality of high-performance secondary batteries currently used is the separator, and the quality and life of the battery depend on the performance of the separator. The current situation is.

  Conventionally, as a main material of a separator for a high performance secondary battery, a polyolefin-based resin that is stable against an electrolytic solution and has excellent shutdown characteristics and electrical insulation characteristics, especially a polyethylene resin having a large molecular weight. It has been used a lot. A conventionally known method for producing a microporous separator is as follows.

  First, a plasticizer, a wax or the like as a pore forming additive is added to a main material, for example, a polyethylene resin in an appropriate ratio. Next, this is charged into an extrusion screw through a hopper and melted, and then passed through a T-die, a casting roll, etc., and a sheet having a predetermined width and thickness ( Sheet) is formed. Next, the sheet is stretched in the vertical and horizontal directions to obtain a separator film having a desired width and thickness, and then the film is deposited in a solvent to remove the additive. Thereby, pores are formed at the positions while the additive is eluted, and as a result, a separator having a microporous structure is obtained. The microporous separator is used as a separator for a secondary battery after being cut into a predetermined size through processes such as heat fixing and corona treatment.

  However, in this type of conventional method, a very low melt viscosity is produced in the process in which the polyethylene resin as the main material and the pore-forming additive are mixed. For this reason, there is a technical limit in forming fine pores so that they are uniformly distributed.

In order to overcome this limitation, the present inventors have proposed a method for producing a microporous separator for a secondary battery in the following patent document. In this method, an amorphous layer (Amorphous layer) having a constant thickness is formed on both end surfaces of a microporous separator to give an amorphous layer / crystalline layer / amorphous layer structure, and the amorphous layer is formed. The porous layer has pores smaller than the crystal layer. According to this method, there is an effect that the pore characteristics can be remarkably improved as compared with the conventional separator. However, a specific method by which a person having ordinary knowledge in the technical field to which the present invention belongs can repeatedly manufacture the separator having the above structure has not been established, and it can be said that the industrial implementation stage has not yet been reached. . In addition, in order to use the separator for a secondary battery, particularly a next-generation secondary battery, the pore characteristics, the stretching characteristics, especially the mechanical characteristics related to the life and stability of the battery, the thermal characteristics, the electrical characteristics. There was still a lot of room for improvement.
Republic of Korea Patent Publication No. 2005-39111 (Publication Date: April 29, 2005)

  Therefore, an object of the present invention is to provide a microporous polyolefin-based separator that is suitable for use as a next-generation high-capacity and thin secondary battery separator and a method for producing the same. This separator has a uniform distribution of fine pores inside the polyolefin-based separator for secondary batteries, so it has excellent pore characteristics and stretching characteristics. As a result, an ultra-thin separator can be manufactured. In addition, mechanical characteristics, thermal characteristics, electrical characteristics, etc. are extremely good.

  In order to achieve this object, the separator used in the high-performance secondary battery includes a polyolefin-based mixed resin having a melt index (Melt Index) of 0.01 or more and 0.5 or less, and a thickness of 6 μm or more. 10 μm or less, and the porosity is 50% or more and 80% or less, and the fine pores in the sheet are three-dimensionally stretched in the thickness direction, the vertical axis, and the horizontal axis in the manufacturing process. In this specification, stretching the polyolefin-based separator sheet in the vertical axis, the horizontal axis, and the thickness direction is referred to as “three-dimensional stretching”.

The microporous polyolefin-based separator according to the present invention is made of a polyolefin-based mixed resin having a melt index of 0.01 or more and 0.5 or less, a thickness of 6 or more and 10 μm or less, and a porosity of 50. % Or more and 80% or less , and the micropores inside the sheet are three-dimensionally stretched in the thickness direction, the vertical axis, and the horizontal axis in the manufacturing process.

  In the present invention, if the melting index (Melt Index) of the polyolefin-based mixed resin constituting the separator is 0.01 or less, the fluidity is too inferior and it becomes difficult to melt, and the selection range of process conditions Is too narrow, on the other hand, if it is 0.5 or more, there is a problem that the molecular weight is excessively reduced and the strength characteristics deteriorate. In addition, an isolation film having a thickness of 6 μm or less cannot be manufactured due to technical limitations, and an isolation film having a thickness of 10 μm or more makes it difficult to increase the capacity and slim the battery. The separator of the present invention has a large number of fine pores penetrating through the membrane like a sword structure. The porosity of such fine pores (the pore volume inside the separator / the whole of the separator) If the volume is less than 50%, the ionic substance cannot smoothly pass through the separator, so that the function of the separator cannot be achieved. There is a drawback that the characteristics, in particular, the puncture strength and the tensile strength are weakened.

  The method for producing a microporous polyolefin-based separator according to the present invention includes adding a wax or an antioxidant as a pore-forming additive to a polyolefin-based mixed resin as a main material, and adding the polyolefin-based mixed resin to the polyolefin-based mixed resin. On the other hand, 0.5% by weight or more and 25% by weight or less of a pore-stretching compound is mixed, and the pore-forming additive contained in the inside of the membrane is added in the thickness direction of the sheet while extruding the mixture into a sheet shape. A step of primary stretching, and after passing the sheet through a casting roll at a speed of 8 m / min to 12 m / min at a temperature of 40 ° C. or higher and 90 ° C. or lower, Next stretching step, forming a sheet having a predetermined width and thickness by using a cast roll, stretching the sheet secondarily in the vertical and horizontal directions, and sinking the stretched sheet in a solvent. By removing the pore-forming additives and characterized the step of obtaining a porous film, the method comprising the porous film tertiary stretching and thermofixing, to include.

  The microporous polyolefin-based separator according to the present invention has a three-dimensional stretch characteristic, and has mechanical characteristics as well as stretch characteristics and pore characteristics as compared with the two-dimensional stretch separator conventionally used. Since it is extremely superior, it is possible to produce an ultra-thin type microporous separator film. As a result, the secondary battery can be made thinner, the ion conduction resistance minimized, and the energy density advanced. There is.

  Hereinafter, the present invention will be described with reference to embodiments, but the scope of the present invention is not limited thereto.

  In order to produce the microporous polyolefin-based separator according to the present invention, first, pore-forming additives, that is, waxes and antioxidants are added to a polyolefin-based resin, preferably a polyethylene resin, and a screw in an extruder is added. Mix well using. At this time, the polyolefin resin used as the main material is obtained by mixing two or more resins having different molecular weights, and a mixed resin having a melt index of 0.01 or more and 0.5 or less is used. Is preferred.

  Next, 0.5% by weight or more and 25% by weight or less of a pore-extensible compound is mixed with the main material, and the main material containing the pore-forming additive and the pore-extensible compound is converted into a gear pump (Gear Pump). ) And a T-die to obtain a sheet. As a result, the pore-stretching compound and the pore-forming additive react to increase the volume, and by increasing the volume, the sheet can be uniformly stretched particularly in the thickness direction.

  As the pore-stretching compound, any one or two of a compound having an epoxy ring, a compound containing chlorine (Cl) or bromine (Br) as a terminal group, a ketal lactone, trioxabicyclooctane, and a spiro ortho compound The above can be used, and among them, spiro orthoester and spiro ortho carbonate can be used.

  The principle of stretching the pore-forming additive contained in the separator sheet in the thickness direction will be described. For example, the pore-stretching compound has two ring structures such as spiro ortho carbonate. In the case of a compound, the volume is greatly increased while a ring-opening reaction occurs in which the ring is opened by heat during the extrusion process of the sheet. In this process, the volume of the pore-forming additive mixed with the pore-stretching compound also increases, and as a result, the pores inside the membrane are activated while the separator membrane rises particularly in the thickness direction.

  At this time, if the added amount of the pore stretchable compound is lower than 0.5% by weight with respect to the polyethylene resin, the stretchability in the thickness direction of the sheet becomes poor. On the other hand, when the added amount exceeds 25% by weight, the pore diameter and distribution are not uniform.

  Thus, the sheet | seat which performed the primary extending | stretching process in the thickness direction obtains a predetermined | prescribed width and thickness through a cast roll process. At this time, the shape and diameter of pores formed inside the sheet are finely controlled by conditions such as a speed difference and a temperature difference between the roll and the roll through which the sheet passes. For example, the cooling efficiency of the sheet and the sheet It is efficient to maintain a temperature of 40 ° C. or higher and 90 ° C. or lower, and a speed of 8 m / min or higher and 12 m / min or lower in consideration of the tension received by the tension.

  The secondary stretched sheet is then deposited in a solvent to extract the pore forming additive. At this time, isopropanol, hexane, pentane, methylene chloride, methyl ethyl ketone, dioxane, or the like can be used as an extraction solvent for the pore-forming additive. Thereby, the pore-forming additive is removed from the sheet, and a porous film in which fine pores are formed at the position is obtained.

  Finally, after depositing the film in which the fine pores are formed in a sedimentation tank in which water and alcohol are mixed, the film is taken out after a predetermined time, and the film is fed into the heat fixing chamber (Chamber) while being fed at a predetermined temperature and a predetermined temperature. Thermal fixing and tertiary stretching are carried out at a stretching ratio of. Thus, a microporous polyolefin-based separator is completed.

  Furthermore, by performing a surface modification treatment such as corona, it is possible to give the characteristics required according to the use of the microporous polyolefin-based separator.

  The microporous polyolefin-based separator thus obtained is made of a polyolefin-based mixed resin having a melt index of 0.01 or more and 0.5 or less, a thickness of 6 μm or more and 10 μm or less, and a porosity of 50% or more. 80% or less, and fine pores inside the sheet are three-dimensionally stretched in the thickness direction, the vertical axis, and the horizontal axis in the manufacturing process.

Hereinafter, an example is given and demonstrated.
<Example>

  A raw material mixture was prepared by mixing 30% by weight of wax and 3% by weight of an acrylate as an antioxidant to a polyethylene mixed resin having a melt index of 0.08 or more and 0.1 or less. After this raw material mixture is melt-blended in a screw (Screw) in the extruder, 8% by weight of a spiro orthoester compound is added to the polyethylene mixed resin in a barrel (Barrel) at the end of the screw, and a gear pump and a T-die are connected. To produce a sheet. In this extrusion process, the sheet was primarily stretched in the thickness direction.

  Next, the sheet was passed through a cast roll continuously at a speed of 10 m / min at a temperature of 80 ° C. or lower to obtain a sheet having a predetermined width and thickness. Subsequently, secondary stretching was performed at a stretching ratio of 7 times in the longitudinal direction and 12 times in the transverse direction at a temperature of 110 ° C. or more and 120 ° C. or less. Further, the stretched sheet was deposited in an isopropanol solvent to remove the pore forming additive to obtain a microporous film.

  Subsequently, after depositing the microporous film in a deposition tank filled with water and alcohol mixed at a ratio of 7: 3 for about 10 minutes, and then removing the microporous film, the microporous film was placed in the heat fixing chamber 3 Next stretching and heat fixing were performed. At this time, the draw ratio was 25%, and the heat fixing temperature was 120 ° C. A microporous separator was thus completed.

The microporous polyolefin-based separator obtained according to the above-described embodiment was measured for pore characteristics, mechanical properties, thermal characteristics, and the like, and the results were compared with the conventional secondary battery separator currently on the market. (Comparative example: SE series product manufactured by Asahi Kasei Kogyo Co., Ltd.) and shown in Table 1 below.

  As is apparent from Table 1, the microporous polyolefin-based separator according to the embodiment of the present invention has a thickness of 6 μm or more and 10 μm or less as compared with the products conventionally used. , Showing excellent results such as porosity (pore volume inside isolation membrane / total volume of isolation membrane) exceeding 50%, and other mechanical and thermal properties showed much better physical properties . In addition, since the microporous polyolefin-based separator of the present invention has an excellent maximum draw ratio of 12 times, an ultra-thin separator can be manufactured, and the next-generation high-capacity and thin battery separator The possibility of being used as was recognized.

Claims (2)

In the separator used for the secondary battery,
It consists of a polyolefin-based mixed resin having a melt index of 0.01 or more and 0.5 or less, a thickness of 6 μm or more and 10 μm or less, and a porosity of 50% or more and 80% or less. A microporous polyolefin-based separator, wherein fine pores in a sheet are three-dimensionally stretched in a thickness direction, a vertical axis direction, and a horizontal axis direction. Adding a wax or an antioxidant as a pore-forming additive to a polyolefin-based mixed resin as a main material;
One or two or more of a compound having an epoxy ring of 0.5% by weight or more and 25% by weight or less , ketal lactone, trioxabicyclooctane, or a spiro ortho compound is mixed with the polyolefin-based mixed resin. a step of lengthened extending the pore-forming additive contained the mixture in the interior of the extrusion while the film into a sheet in the thickness direction of the sheet,
The sheet was passed through a casting roll at a speed of 8 m / min to 12 m / min at a temperature of 40 ° C. or higher and 90 ° C. or lower to form a sheet having a predetermined width and thickness . after the steps of extending extension in the vertical axis and the horizontal axis direction,
Obtaining a porous film by depositing the stretched sheet in a solvent to remove pore-forming additives;
Method for producing a microporous polyolefin separator which comprises a step of extending the extension and heat securing said porous film.