KR100511634B1 - Separator for High Advanced Lithium Polymer Secondary Batteries, The Preparation Method thereof, and Lithium Polymer Secondary Batteries Comprising the Separator - Google Patents

Abstract

The present invention is an organic polymer comprising a copolymer of polyvinylidene fluoride or / and vinylidene fluoride with hexafluoropropylene; And a polyolefin-based fibrous material, and provides a separator for a lithium polymer secondary battery having a microporous structure.

In addition, the present invention comprises the steps of (a) dissolving a copolymer of polyvinylidene fluoride or / and vinylidene fluoride with hexafluoropropylene in an organic solvent in a suitable solvent; (b) dispersing the polyolefin fibrous material in the solution of step a to prepare a mixed solution; (c) casting the mixed solution on a predetermined substrate; And (d) provides a method for producing a separator for a lithium polymer secondary battery comprising the step of melting and drying the solvent of the step a from the cast slurry.

The polymer electrolyte comprising the separator according to the present invention and the lithium polymer secondary battery including the same have a simple manufacturing process, have affinity with a liquid electrolyte, and induce pore closure at a predetermined temperature or more even during overcharging and overdischarging. It is possible to manufacture a high performance secondary battery.

Description

Separator for High Advanced Lithium Polymer Secondary Batteries, The Preparation Method, and Lithium Polymer Secondary Batteries Comprising the Separator

The present invention relates to a separator for a lithium polymer secondary battery and the like, and more particularly, a manufacturing process is simple, affinity with a liquid electrolyte, and lithium polymer capable of inducing pore closure at a predetermined temperature or more even during overcharging and overdischarging. A secondary battery separator and its application and the like.

Recently, with the rapid advancement of information and communication technology, the demand for small secondary batteries used as essential power sources for IT (Information Technology) related electronic devices such as mobile phones, notebook PCs, PDAs, etc. is growing exponentially. have. The secondary battery not only affects the compactness and light weight of the main body of the device, but also accounts for 10-20% of the weight of the battery in various portable information and communication devices and about 50% of the mobile phone. Whether or not use is becoming an important competitive factor for portable information and communication devices, the development of small secondary batteries that can meet the needs of these devices will be a key factor in determining the competitiveness of not only the battery industry but also electronic information and communication products. will be.

According to the change of the transmission speed of mobile communication system, the existing secondary battery (nickel-cadmium battery, nickel hydride battery) is energy consumption according to the high functionalization of portable information communication device and small electronic device (wireless internet and wireless data communication service). Because of this problem, high energy density, high performance, and high stability of small secondary batteries are required.

Recently, the technology development of lithium polymer secondary battery (Lithium Polymer Secondary Batteries) which improved the thickness and safety of battery by replacing the existing liquid electrolyte (organic solvent + lithium salt) with polymer electrolyte (polymer + organic solvent + lithium salt) This is being done. Such a lithium polymer secondary battery is one of the next-generation high-performance new battery cells, and has a higher energy density per unit weight than the existing battery, and can be manufactured in various forms, and it is easy to develop high voltage and large capacity batteries by lamination. It does not use heavy metals that pollute the environment like mercury, so it has the advantage of being environmentally friendly.

Lithium polymer secondary battery is largely composed of an anode (anode), a polymer electrolyte (polymer electrolyte), a cathode (cathode), lithium, carbon, etc. are used as the negative electrode active material, transition metal oxide, metal chalcogen as the positive electrode active material Compounds, conductive polymers and the like are used. A polymer electrolyte is a substance composed of a polymer, a salt, a non-aqueous organic solvent (optional), and other additives, and exhibits an ionic conductivity of about 10 ⁻³ to 10 ⁻⁸ S / cm at room temperature.

Commercially available separators for lithium secondary batteries are polyolefin-based polymers (polyethylene and polypropylene) films having a thickness of 9 to 35 µm and have a microporous structure. Recently, in order to realize the safety of the lithium polymer secondary battery, the mechanical strength of the polymer electrolyte, and the thinning of the battery, a lithium polymer secondary battery incorporating a commercially available polyolefin separator has been developed. (US Patent 5,639,573, Japan). Patent P2000-149905)

However, there is a limit to the development of high-performance separators for commercialization because high production costs are required to induce thinning and pore formation of polyolefin-based separators. In addition, the polyolefin-based separator exhibits a low content of electrolyte due to its low affinity with the liquid electrolyte, resulting in an increase in the overall resistance of the battery, resulting in a continuous reduction in capacity and high rate charge / discharge characteristics. Has become the underlying cause.

The present invention has been made to solve the problems of the prior art as described above, the main object of the present invention is that the manufacturing process is simpler than the conventional polyolefin-based membrane, has affinity with the liquid electrolyte, and at a constant temperature during overcharge and overdischarge The present invention provides a separator for a lithium polymer secondary battery capable of inducing pore closure.

Another object of the present invention to provide a method for producing a separator for a lithium polymer secondary battery.

Another object of the present invention is to provide a method for preparing a polymer electrolyte for lithium polymer secondary batteries prepared by impregnating the separator for a lithium polymer secondary battery in a predetermined liquid electrolyte.

Another object of the present invention to provide a lithium polymer secondary battery comprising the polymer electrolyte.

In order to achieve the above object, the present invention is a copolymer of polyvinylidene fluoride or / and vinylidene fluoride and hexafluoropropylene as an organic polymer; And a polyolefin-based fibrous material, and provides a separator for a lithium polymer secondary battery having a microporous structure.

In addition, the present invention comprises the steps of (a) dissolving a copolymer of polyvinylidene fluoride or / and vinylidene fluoride with hexafluoropropylene in an organic solvent in a suitable solvent; (b) dispersing the polyolefin fibrous material in the solution of step a to prepare a mixed solution; (c) casting the mixed solution onto a predetermined substrate; And (d) provides a method for producing a separator for a lithium polymer secondary battery comprising the step of melting and drying the solvent of the step a from the cast slurry.

The present invention also provides a method for producing a polymer electrolyte for lithium polymer secondary batteries prepared by impregnating the separator in a predetermined liquid electrolyte.

In addition, the present invention provides a lithium polymer secondary battery comprising the polymer electrolyte.

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

The organic polymer contained in the separator is a copolymer of polyvinylidene fluoride or / and vinylidene fluoride and hexafluoropropylene. In the case of using a mixture of polyvinylidene fluoride and a copolymer of vinylidene fluoride and hexafluoropropylene, the composition ratio between these materials does not require special limitation. The polyvinylidene fluoride does not require special limitation, but preferably has a weight average molecular weight of 10,000 to 1,000,000. Although the copolymer of vinylidene fluoride and hexafluoropropylene does not require a special limitation, Preferably the composition ratio of hexafluoropropylene is 1-30 weight%.

Polyolefin-based fibrous material is added to induce pore closure at a certain temperature or more during overcharging and overdischarging. Such polyolefin-based fibrous materials do not require any particular limitation, but preferably polyethylene or / and polypropylene are included here. The polyolefin fibrous material is preferably 0.1-20 탆 in diameter and 0.5-5 mm in length.

Although the composition ratio of the said organic polymer and a polyolefin fibrous substance does not require a special limitation, Preferably it is 1: 9-9: 1. If the polyolefin-based fibrous material is added below the composition, the thermal shutdown function may be lost. If the polyolefin-based fibrous material is added below the composition, the viscosity may be too high to reduce the flowability of the polymer solution, thereby reducing the flow of the polymer solution. It may cause problems.

Figure 1 shows a schematic manufacturing process of the separator according to the present invention.

The manufacturing process of the separator for a lithium polymer secondary battery includes a wet method and a dry method, but the present invention is preferably a wet method, and a phase inversion method is used to obtain a microporous separator.

The solvent for dissolving the organic polymer does not require special limitation as long as it is suitable for dissolution of the selected organic polymer, and examples thereof include organic solvents such as N-methylpyrrolidone, dimethylformamide, dimethyl sulfoxide, and gamma butyrolactone. , At least one solvent selected from the group of tetrahydrofuran and acetone. The organic solvent is preferably used about 100 to 1500% by weight based on the organic polymer.

The mixing reaction tank as shown in FIG. 2 may be used to mix the organic polymer and the polyolefin fibrous material. This mixing reactor includes a rotating stirrer (4), a revolving stirrer (5), a bubble removing device (2), a vent (3) and a water tank circulator for temperature control (1). The solution flows into the hopper and is evenly mixed by the rotational force of the rotating stirrer 4 and the revolving stirrer 5, and bubbles generated during the stirring process are removed through the bubble removing device 2. The water tank circulator 1 is installed to maintain the mixing reactor at a constant temperature, and external air is introduced through the vent 3. Preferably, in order to prevent agglomeration during the initial mixing of the organic polymer, the distance between the inside of the reactor and the paddle 5 'provided in the idler type stirrer may be within 1 mm.

A mechanical stirrer, a planetary stirrer, a high speed disperser, etc. may be used to smoothly disperse the polyolefin fibrous material and dissolve the organic polymer. Preferably, it is preferable to use a vacuum pump to remove bubbles in preparing a solution. good.

The mixture of organic polymer and polyolefinic fibrous material is cast on a predetermined substrate (e.g., Mylar film, Alpha Industries), and the formation of micropores dissolves the solvent of step a from the cast slurry. This can be achieved through the process. This can be accomplished via a device as shown in FIG. 3 using a phase inversion method. The apparatus of FIG. 3 consists of a comma coater (applicator function 11), a coagulation bath 12, a washing bath 13, a dryer 14 and a substrate 15. The solution flows into the coagulation bath 12 in a state in which the solution is coated on the substrate 15 by the comma coater 11, whereby a microporous film is formed, and the impurities are washed through the cleaning bath 13 to remain in the dryer 14. The solvent is completely removed.

The cast substrate may be immersed in a coagulation bath containing a non-solvent to prepare a microporous separator having a thickness of 10 to 60 μm by a phase inversion method between a solvent and a non-solvent in a solution. In this case, the nonsolvent may be water, alcohols (methanol, ethanol, propanol, etc.) or a mixed solvent thereof. The temperature of the coagulation bath is preferably 10 to 60 ° C, and the temperature of the washing bath is preferably 10 to 50 ° C. If the temperature of the non-solvent is less than 10 ° C., the uniformity of microporosity may be lowered. If the temperature of the non-solvent is more than 60 ° C., there may be a problem in the film forming process.

The microporous separator obtained through the above process may preferably completely remove the remaining solvent by vacuum drying at about 100 to 180 ° C.

The lithium polymer secondary battery of the present invention is composed of a positive electrode (for example, LiCoO ₂ , LiNiO ₂ , LiMn ₂ O _4, etc.), a polymer electrolyte and a negative electrode (for example, hard carbon, soft carbon, graphite, etc.), The polymer electrolyte may be prepared by impregnating a separator prepared through the above process into a predetermined liquid electrolyte. Examples of the liquid electrolyte solution that can be used in the above process,

A: at least one organic solvent selected from the group of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, gamma butyrolactone, methyl ethyl carbonate,

B: The mixing composition of 1 or more types of mixed lithium salts chosen from the group of lithium perchlorate, lithium hexafluoro phosphate, lithium triplate, lithium bistrifluoromethylsulfonyl imide, and lithium tetrafluoro borate salt is mentioned.

The organic solvent A is not particularly limited, but preferably 50 to 1500% by weight based on the organic polymer, lithium salt B is preferably 5 to 30% by weight based on the organic polymer.

Figure 4 shows the surface and cross-sectional morphology of the membrane prepared by the above process, having a microporous structure near 100nm. FIG. 5 is DSC thermograms of the membrane, in which a melt peak of polyethylene fibrous material is observed at 132 ° C. Melting of the polyolefin-based fibrous material has proved well that the stability of the battery can be secured by closing the pores of the separator at a predetermined temperature or more during overcharging and overdischarging.

Hereinafter, the content of the present invention will be described in more detail with reference to Examples. However, these examples are only presented to understand the content of the present invention, and the scope of the present invention should not be construed as being limited to these embodiments.

Example 1 Preparation of Membrane

47 g of polyvinylidene fluoride (PVdF) having a weight average molecular weight of 600,000 was dissolved in 344 mL of N-methylpyrrolidone (NMP), and then 5 g of polyethylene fibrous material (fiber Korea) having a diameter of 17 µm and a length of 5 mm were dissolved in the stomach. In a polyvinylidene fluoride solution. In order to facilitate the dispersion of polyethylene fibrous material and the dissolution of polyvinylidene fluoride, dispersion and agitation were carried out using a planetary mixer (Hanyang Machinery), and a vacuum pump was used to remove bubbles when preparing a solution. It was.

The mixed solution is injected into the applicator 11 in a predetermined amount, cast onto a substrate (Mylar film, Alpha Industries), and then a coagulation bath 12 containing water as a non-solvent. It was immersed in the solution to obtain a microporous membrane having a thickness of 53㎛ by a phase inversion method between the solvent and the non-solvent. At this time, the temperature of the coagulation tank 12 was 30 degreeC, and the temperature of the washing tank 13 was kept at 40 degreeC. The microporous separator prepared by the above process was vacuum dried at 160 ° C. to completely remove the residual solvent.

The cross-section and surface morphology of the dried microporous membrane were observed by scanning electron microscopy (SEM), and the image is shown in FIG. 3. In addition, as shown in Figure 4, it was confirmed that the melt peak of the polyethylene microparticles at 132 ℃ from the DSC thermogram of the prepared microporous membrane, the melting closes the pores during overcharge and over-discharge of the battery to induce temperature termination function Can be.

Table 1 shows the thickness, porosity, average pore size, and mechanical properties of the membrane and the control polyethylene membrane prepared according to the above process.

Example 2 Preparation of Polymer Electrolyte and Measurement of Ion Conductivity

The microporous separator prepared in Example 1 was transferred to a glove box in an argon atmosphere and impregnated with a liquid electrolyte [EC / DMC (1/2, w / w) / 1M LiPF ₆ ] to prepare a polymer electrolyte, followed by two stainless steels. Bonded between steel electrodes, sealed with polyethylene coated aluminum packaging, and ionic conductivity was measured. Ionic conductivity values of the microporous membrane prepared in the present invention and the polyethylene membrane as a control group are shown in Table 1 below.

TABLE 1

unit value How to measure Control Example thickness mm 27 53 Porosity % 36 40 ASTM D1622 Average pore size nm 43 -100 Bubble sorption analysis The tensile strength MD: kg / cm ² 990 850 KS M 3054 TD: kg / cm ² 710 640 KS M 3054 Elongation MD:% 250 50 KS M 3054 TD:% 240 65 KS M 3054 Pore Closure Temperature ^o C 135 132 DSC analysis Ion conductivity mS / cm 1 ~ 2 3 FRA

Example 3 Fabrication and Characterization of Unit Cells (Discharge Capacity Characteristics, Cycle Characteristics)

A microporous separator containing liquid electrolyte (EC / DMC (1/2, w / w) / 1M LiPF ₆ ) was simply laminated between a lithium cobalt oxide (LiCoO ₂ ) anode and a graphite cathode to produce a unit cell. At room temperature charge and discharge experiments were performed. In addition, the result of examining the discharge capacity characteristics of the unit cell according to the discharge rate (C rate) was as shown in FIG. (Charging condition: CC / CV to 4.2V at 25 ℃, Discharge condition: CC, 2.8V cut-off at 25 ℃)

Discharge capacity characteristics according to the cycle of the unit cell to which the polyethylene separator as a control and the unit cell to which the microporous separator prepared in the present invention is applied are shown in FIG. 7. From the results of FIG. 7, it was confirmed that the cycle characteristics of the unit cell to which the microporous separator prepared in the present invention was applied were more excellent. (Charging condition: CC / CV 10mA to 4.2V, 2mA cut-off at 25 ℃, Discharge condition: CC, 10mA (1C rate), 2.8V cut-off at 25 ℃)

According to the present invention, a manufacturing process is simpler than that of a conventional polyolefin-based membrane, and has affinity with a liquid electrolyte, and can provide a separator for a lithium polymer secondary battery that can induce pore closure at a predetermined temperature or more even during overcharging and overdischarging. have.

1 is a schematic manufacturing process of the separator according to the present invention.

Figure 2 is an illustration of a mixing reactor constituting an embodiment of the present invention.

3 is a process chart for formation of a microporous structure using a phase inversion method constituting an embodiment of the present invention.

Figure 4 is a surface (a) and cross-section (b) morphology of the separator according to the invention

5 is a DSC thermogram of the separator according to the present invention

6 is a result of examining the discharge capacity characteristics of the unit cell according to the discharge rate (C rate)

7 is a result of the discharge capacity characteristics according to the cycle of the unit cell [1: control, 2: the present invention]

<Code Description of Main Parts of Drawing>

          1: Water tank circulator 2: Bubble removing device

          3: vent 4: rotating stirrer

          5: Static Stirrer 11: Comma coater

          12: coagulation bath 13: washing bath

          14: dryer 15: substrate

Claims (15)

delete delete delete delete delete delete delete delete delete delete delete delete delete Organic polymers selected from copolymers of polyvinylidene fluoride and hexafluoropropylene, copolymers of vinylidene fluoride and hexafluoropropylene, polyvinylidene fluoride and copolymers of vinylidene fluoride and hexafluoropropylene Any one copolymer; And a polyolefin-based fibrous material, wherein the separator for a lithium polymer secondary battery having a microporous structure is impregnated into a liquid electrolyte composed of the following components A and B. A: 50 to 1500% by weight based on organic polymer, at least one organic solvent selected from the group of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, gamma butyrolactone, methyl ethyl carbonate. B: 5 to 30% by weight based on the organic polymer, 1 selected from the group of lithium perchlorate, lithium hexafluoro phosphate, lithium triplate, lithium bistrifluoromethylsulfonylimide, lithium tetrafluoro borate salt Mixed lithium salts of more than one species. A lithium polymer secondary battery comprising the polymer electrolyte prepared by claim 14