KR100439164B1 - Method for manufacturing electrode plate for lithium secondary battery having excellent chemical resistance and durability - Google Patents

Abstract

PURPOSE: A method for manufacturing an electrode plate for lithium secondary battery is provided to reduce the reactivity with an electrolyte, to improve the contact degree with active material particles, and to improve the bindability and flexibility as an electrode plate. CONSTITUTION: The method for manufacturing an electrode plate for lithium secondary battery comprises the steps of: adding active material powder and a conductive agent or additive particles to a polymer solution in which a polyvinylidene fluoride-based polymer is dissolved; adding an organic solvent thereto to form slurry; and coating the slurry on a collector. In the method, the coated collector is dried at a drying temperature defined by the formula of £drying temperature = Tm - avertTm -TcC|, wherein Tc is the crystallization temperature, Tm is the melting temperature of crystals and a is a value of 0.5-0.9.

Description

Manufacturing method of electrode plate for lithium secondary battery

The present invention relates to a method for manufacturing a pole plate of a lithium secondary battery that is a rechargeable non-aqueous electrolyte battery, and more particularly, to polyvinylidene fluoride (hereinafter referred to as PVDF) used as a binder in the manufacture of the pole plate. Binding between the active material particles of the electrode plate prepared by changing the drying temperature conditions of the polymer series, adhesion with the metal current collector, chemical resistance with the electrolyte solution, contactable area or volume with the active material particles ( Hereinafter, a method for manufacturing a lithium secondary battery pole plate capable of optimizing conditions such as contact with particles).

Recently, due to the development of electronic devices, light and high energy density secondary batteries are widely used as power sources, and the same characteristics are required in secondary batteries used as power sources as high performance, miniaturization, and portable type of electronic devices are progressed. Among such secondary batteries, lithium ion secondary batteries using a nonaqueous electrolyte as an electrolyte have a high battery voltage and excellent cycle characteristics compared to other secondary batteries.

In general, the electrode plates of such lithium secondary batteries are mixed with electrode active material powder, conductive agent and / or additive particles in a state of dissolving PVDF-based polymer powder mainly used as a binder in a suitable organic solvent, and then made into a slurry. It is produced by coating and drying the current collector.

At this time, the drying conditions are generally performed based on the temperature at which the organic solvent in the slurry is volatilized. However, the volatilization temperature of the organic solvent in the slurry in which the particles and the polymer solution are mixed is not fixed, but a wide temperature range. Due to the slow volatilization, the proper drying temperature is very important for process efficiency.

In addition, in the drying process after coating, if the polymer component of the binder is a semi-crystalline polymer, the organic solvent is volatilized according to the drying temperature, and the crystallinity of the polymer material used as the binder is changed. Such crystallinity is also used as a cathode plate for a lithium secondary battery. It also affects phase chemical resistance, contact with particles, etc., so consideration should be given.

An object of the present invention is to reduce the reactivity with the electrolyte solution by adjusting the drying temperature after coating of the slurry in the production of a lithium secondary battery electrode plate using the PVDF-based polymer as a binder based on the above considerations It is to provide a method for manufacturing a lithium secondary battery pole plate improved contact with the field and improved binding and flexibility as a pole plate.

That is, the present invention adds an active material powder, a conductive agent or an additive particle to a polymer solution in which a polyvinylidene fluoride-based polymer is dissolved, mixes an organic solvent liquid, forms a slurry, and then coats a metal current collector on a lithium secondary battery electrode plate. In manufacturing, to provide a method for manufacturing a lithium secondary battery pole plate characterized in that the drying after coating by setting the drying temperature to the following formula (1).

[Equation 1]

Drying temperature = T _m -a | T _m -T _c |

In the above formula, T _c ; Crystallization temperature (° C.);

T _m ; Melting temperature of the crystal (° C.);

a; It is a value in the range of 0.5-0.9.

1 is a thermogravimetric analysis of a polymer lean solution (sample 1 and sample 2) consisting of polyvinylidene fluoride and 1-methyl-2-pyrrolidinone and an active material slurry (sample 3) made of a lean polymer solution, respectively. Graphical drawing,

FIG. 2 is a polarized light micrograph of magnification of 500 times the crystals on the polymer film formed by increasing the drying temperature of Sample 1. FIG.

Hereinafter, the present invention will be described in more detail with reference to the accompanying drawings.

When manufacturing a lithium secondary battery electrode plate by the method of the present invention, the active material powder and the conductive agent or additive particles are added to the polymer solution in which the polyvinylidene fluoride-based polymer is dissolved, and the organic solvent is mixed to make a slurry and then metal Coating on the current collector to produce a lithium secondary battery pole plate.

At this time, it is preferable to use one kind selected from the group consisting of methylpyrrolidinone, dimethylformamide, toluene, acetone as the organic solvent.

In addition, it is preferable that the active material is a carbon material, the conductive agent and the additive are not used, and the electrode plate manufactured is a negative electrode plate.

At this time, it is necessary to set the drying temperature after coating in the range of the formula (1), where the a value is in the range of 0.5 to 0.9, more preferably in the range of 0.7 to 0.9. If the value of a is outside the range, the effect intended by the present invention cannot be obtained, which is not preferable.

In general, PVDF-based polymers are semicrystalline polymers and exhibit various crystal forms according to their production conditions and post-production conditions (mainly temperature, pressure, electric field, magnetic field, etc.) (AJ Lovinger, "Developments in Crystalline Polymer", Vol. 1, Applied Science Publishers, New York (1982), Chap. 5), the most common crystal forms when excluding the effects of electric and magnetic fields are type α and γ crystals. Form a crystals usually appear when the powder is melted or dissolved in a suitable solvent and then maintained at an appropriate temperature or overcooled. It usually takes the form of a spherulite with a size of 100 microns or more. The γ-type crystals are formed as small spheres at the boundary between the α-type crystals by subjecting the α-type crystals to high temperature annealing.

However, there is a conflicting principle between the above crystal forms and the properties of the electrode plate, which is preferred for the purposes of the present invention. That is, large crystals such as α-type crystals are advantageous for enhancing chemical resistance with the electrolyte, while crystallinity or very small crystals are advantageous in terms of contact with particles. Specifically, in the case of large crystals, the contact with the particles is small, which is disadvantageous in binding the active materials or in contact with the metal current collector and the particles, whereas in the case of low crystallinity or amorphous form, the chemical resistance with the electrolyte is significantly weakened. It is one of the risk factors for stability. Therefore, the optimal crystal state for the plate is that the smallest possible crystals, such as γ-type crystals, are distributed as much and widely as possible.

As a main means of verifying the crystallinity of PVDF-based polymers, which are semi-crystalline polymers, the crystals formed in each polymer film are polarized at a predetermined temperature based on the crystallization temperature ( T _c ) and the crystal melting temperature ( T _m ). Microscopic observation is the easiest way to observe the size and orientation of the crystals. However, in the scope of interest of the present invention, since the crystallinity of PVDF-based polymers is dealt with, the orientation of crystals can be excluded. Thus generally T _c before, T _c close to, to determine the size of each determination between T _c and T _m, T _m and surrounding, T _m Since it is desirable by determining the optimum crystal state are compared with each other. In order to set the optimum drying temperature therefrom, it is necessary to refer to the data on the degree of volatilization of the organic solvent in accordance with the drying temperature in the mixed polymer solution with the organic solvent and select the temperature having the highest process efficiency.

[Experimental example]

1-methyl-2-pyrrolidinone (1-methyl-2-), an organic solvent most commonly used when PVDF-based polymer is used as a binder for three types of samples (sample 1, sample 2 and sample 3) pyrrolidinone (hereinafter referred to as NMP) were mixed together and stirred to form a polymer lean solution, which was then thermogravimetrically analyzed and the results are shown in FIG. 1. Sample 1 is a mixture of 12% by weight of PVDF homopolymer to 88% by weight of reagent grade NMP, Sample 2 is 13% by weight of copolymer of vinylidene fluoride monomer and monoester monomer to 87% by weight of reagent grade NMP. It is mixed. Sample 3 is a slurry made by mixing 50% by weight of sample 1 and 50% by weight of a carbon active material.

As shown in FIG. 1, when all three samples started to evaporate before 50 ° C. and reached about 160 ° C., almost all of the added NMP was volatilized, and at 140 ° C., at least 95% of the total amount of NMP was volatilized. Therefore, based on the volatilization of 95% or more of the total amount of NMP in terms of process efficiency, it is necessary to set the final drying temperature to 140 ℃ or more in the coating process using the sample.

On the other hand, as a result of testing with a differential scanning calorimeter, the PVDF of the homopolymer of the PVDF series polymer used in the present inventionT _c Is 133 to 137 ° C,T _m Was found to be 166 to 170 ° C. This result is consistent with that described in the above documents and other existing documents. Therefore, when using PVDF-based polymer having such crystallization temperature and crystal melting temperature as a binder, 110 ℃ (T _c Previous), 130 ° C (T _c Ambient), 150 ° C (T _c Wow T _m ), 170 ℃ (T _m Ambient), 190 ° C (T _m After that, it is preferable to dry the polymer film and compare the crystal forms.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited by the following Examples.

Example

The PVDF polymer corresponding to Sample 1 had a weight average molecular weight of 316000 and a specific gravity of 1.77. Accurate T _c and T _m were 136.3 ° C. and 172.7 ° C., respectively. 12 g of this PVDF polymer powder and 88 g of reagent grade NMP were mixed and stirred at room temperature to form a uniform polymer lean solution. A polymer plate was coated on a surface of an electrolytic copper foil having a thickness of 10 microns with a doctor blade so as to have a coating thickness of 320 microns to prepare a lithium secondary battery electrode plate.

Subsequently, the coated copper foil was fixed to a glass plate, and then put into a hot air drying furnace maintained at a constant temperature, dried for 10 minutes, taken out, and left to stand at room temperature for about 30 minutes. The copper foil on the back of the film was carefully peeled off, and the remaining polymer film was enlarged 500 times under a polarizing microscope to observe its crystallinity. 2 is a polarization microscope picture of the PVDF polymer film observed for each drying temperature.

In addition, in order to examine the reactivity of the dried films with the electrolyte, lithium hexafluorine at a molar concentration of 1 mole in an electrolyte containing ethylene carbonate / diethyl carbonate / dimethyl carbonate (EC / DEC / DMC) in a volume fraction of 1: 1: 1, respectively. Each polymer film sample was immersed in a non-aqueous electrolyte in which low phosphate (LiPF ₆ ) was dissolved, and left in a glove box under an argon gas atmosphere for one week, and then the remaining film was taken out and observed under a polarization microscope.

In FIG. 2, the part within the cracked area of the unpolarized photographs is a part that appears as a hexagonal cell insulator due to the inflow of the polymer fluid due to the surface tension gradient, not the crystal. The white parts of the polarized photo correspond to crystals, and the black parts are amorphous parts of the polymer chains irregularly distributed. As shown in the polarization photograph of FIG. 2, 110 ° C. is dominant in the amorphous portion, and crystals begin to become apparent at 130 ° C. or higher. It can be seen that as the region of the crystal overlaps with each other as it passes through 170 ° C, when the temperature reaches 190 ° C, the large crystals dominate. Therefore, as mentioned above, it can be seen that the case where a large number of small crystals are distributed at a temperature of T _c or more and T _m or less (130-170 ° C.) or, if possible, is closer to T _c is advantageous in process efficiency.

Also, in the reactivity test with the electrolyte, in the case of dry film at 110 ° C. and 130 ° C., the film melted a considerable amount, making it impossible to observe the microscope.

In view of the above results, it can be considered that the temperature is higher than 150 ° C. except for 130 ° C., but it may be considered to be an appropriate drying temperature range.

Therefore, for PVDF series polymers in which T _c and T _m are slightly different, generalizing by applying the same case, the optimum drying temperature range is a = 0.5 to 0.9 for T _m -a│ T _m - T _c │. It can be confirmed that it is preferable to keep as.

In the case of using PVDF-based polymer as a binder, if the electrode plate is manufactured by applying the above drying temperature range, the lithium secondary battery manufactured by the manufacturing method of the present invention has excellent charge and discharge cycle life because the chemical resistance with electrolyte is excellent. In addition, the degree of contact with the active material particles is also increased to improve the binding force and durability of the electrode plate.

Claims (4)

In preparing a lithium secondary battery electrode plate by adding an active material powder, a conductive agent or an additive particle to a polymer solution in which a polyvinylidene fluoride-based polymer is dissolved, mixing an organic solvent liquid to form a slurry, and then coating a metal current collector, After the coating is to provide a method for manufacturing a lithium secondary battery pole plate characterized in that the drying by setting the drying temperature to the following formula (1). [Equation 1] Drying temperature = T _m -a | T _m -T _c | In the above formula, T _c ; Crystallization temperature (° C.); T _m ; Melting temperature of the crystal (° C.); a; It is a value in the range of 0.5-0.9. The method of claim 1, wherein the organic solvent is one selected from the group consisting of methylpyrrolidinone, dimethylformamide, toluene, and acetone. The method of claim 1, wherein the active material is a carbon material, and a conductive plate is used without using a conductive agent and an additive. The method for manufacturing a cathode plate for a lithium secondary battery according to claim 1, wherein the value a is in the range of 0.7 to 0.9.

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