KR20050118870A - Fabrication method of litium secondary battery having high power - Google Patents

Abstract

The present invention relates to a method for manufacturing a high-output lithium secondary battery of the present invention, and more particularly, comprising a polyvinylidene fluoride polymer having an electrode active material, a conductive agent and a binder, and having a weight average molecular weight of 500,000 to 1,000,000 as the binder. The present invention relates to an electrode and a high output lithium secondary battery including the same.

In the lithium secondary battery according to the present invention, by using a high molecular weight binder, it is possible to maintain a high electrode plate binding force and reduce the electrical resistance of the electrode plate even when using only a small amount, and to increase the content of the electrode active material in the electrode relatively. There is an improvement in battery output and capacity.

Description

Manufacturing method of high output lithium secondary battery {FABRICATION METHOD OF LITIUM SECONDARY BATTERY HAVING HIGH POWER}

The present invention relates to an electrode obtained by using the high molecular weight polyvinylidene fluoride binder of the present invention and a lithium secondary battery having high output characteristics including the electrode.

In general, the mileage of an electric vehicle is determined by the total energy of a battery mounted in the vehicle, and the total energy is determined by the capacity and voltage of the mounted battery. In the case of a vehicle, the space in which a battery is mounted is limited, and the voltage and capacity of a single unit cell capable of charging and discharging electricity are low. Therefore, research on unit cells to meet the voltage and capacity required in a vehicle has been conducted. It is actively underway.

Recently, a lithium-based secondary battery that has a high energy density per unit weight as a unit cell of the electric vehicle and can achieve high capacity has emerged.

A lithium-based secondary battery typically includes a positive electrode, a separator, and a negative electrode, and the positive electrode and the negative electrode cast an active material composition on each current collector to form an active material layer, and place a separator between the obtained positive electrode and the negative electrode. Assemble the battery by winding it using heat or pressure.

The active material composition includes each electrode active material and a conductive agent, and a binder is used to increase the output area of the battery by increasing the contact area of the electrode active material and the conductive agent.

As such a binder, polyvinylidene fluoride having a molecular weight of 280,000 to 350,000 is conventionally used, and its content is used within a conventional range of 5 to 10% by weight based on the total composition.

The content of the binder is designed in consideration of the capacity characteristics of the battery by reducing the content of the electrode active material as a main component. That is, the excessive use of the binder increases the electrical resistance of the electrode plate, and the content of the electrode active material in the electrode active material composition is relatively low, causing a decrease in the output of the battery. When the amount is too small, the electrode plate binding force and the contact between the electrode active material and the conductive agent are too low. In short, this also lowers the output of the battery.

In particular, in recent years, the HEV battery has a tendency to use a positive electrode active material containing Al as a positive electrode, and to use a natural graphite-based negative electrode active material as a negative electrode. Since the two active materials require more binder content than cobalt-based positive electrode or artificial graphite which is mainly used in small batteries, it is essential to control the content of the binder of the HEV battery employing such materials.

In order to manufacture a high capacity lithium secondary battery, in addition to increasing the capacity of the active material per unit weight, it is important to put as much of the active material as possible in the electrode plate.

Therefore, in order to maintain high output and high capacity of the battery, it is preferable to maintain the content of the binder appropriately, and to increase the content of the electrode active material in the active material composition if possible.

An object of the present invention for solving the above problems is to use a high molecular weight binder of more than 500,000, to lower the content of the binder in the active material composition, and to increase the content of the electrode active material relatively to exhibit high output and high capacity characteristics It is to provide a material and a battery comprising the same.

In order to achieve the above object, the present invention provides an electrode comprising an electrode active material, a conductive agent and a binder, the polyvinylidene fluoride polymer having a weight average molecular weight of 500,000 to 1,000,000 as the binder.

In addition, the present invention provides a high output lithium secondary battery provided with a positive electrode comprising the binder, a positive electrode active material and a conductive agent, a negative electrode comprising the binder, a negative electrode active material and a conductive agent, and a separator between the positive electrode and the negative electrode. to provide.

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

The present invention uses a high molecular weight poly (vinylidende fluoride, PVDF) as a binder exhibits a high electrode plate binding force even with a small amount, and relatively increases the content of the electrode active material to increase the output and capacity of the battery Improve properties.

An electrode for a lithium secondary battery according to the present invention includes an electrode active material, a conductive agent and a binder.

The electrode active material may be a positive electrode active material such as a lithium composite oxide or a negative electrode active material such as a carbon-based material.

The positive electrode active material may be any compound capable of reversibly intercalating and de-intercalating lithium ions, for example, LiNi _x Co _y Al _1-xy O ₂ (0 <x ≦ 1, 0 <y ≦ 1), LiCoO ₂ , LiMn ₂ O ₄ , LiMnO ₂ , LiNiO ₂ , and the like, and preferably LiNi _x Co _y Al _1-xy O ₂ .

The negative electrode active material may also be used as long as it is a compound capable of intercalating / deintercalating lithium ions, and representative examples thereof include crystalline carbon active materials such as natural graphite, artificial graphite, and graphite carbon fibers, and petroleum pitch. And amorphous carbon active materials such as coal-based pitches are possible, and natural graphite or artificial graphite is preferably used.

However, since the conduction of electrons between active materials deteriorates with time due to the continuous adsorption and release of ions while the secondary battery repeatedly performs charging and discharging, a conductive agent is added to prevent this. The conductive agent is conventional, and may be powdered super P (MMM), acetylene black, ketjen black, Denka Black, carbon black, and the like. .

In the present invention, polyvinylidene fluoride is used as the binder. The polymer originally does not have a binding force on the material itself, but is used as a binder having high binding strength due to excellent interaction with an electrode active material and a conductive agent. In addition, it can be easily molded into a thermoplastic resin, can appropriately absorb and swell the electrolyte solution in a subsequent step, and has high ionic conductivity, thermal stability, and electrical stability.

In particular, the present invention uses a polyvinylidene fluoride having a weight average molecular weight of 500,000 to 1,000,000 as the binder. Thus, by using a high molecular weight binder, even when a small amount of binder is used, high binding strength can be maintained. As a result, it is possible to increase the content of the electrode active material relatively, it is possible to manufacture a battery of high capacity and high output.

According to a preferred embodiment of the present invention, by using the polyvinylidene fluoride having a weight average molecular weight of 280,000, 350,000, 500,000, 630,000 and 1,000,000 as the binder and measuring the binding force, as shown in Figure 1 As the molecular weight increases, the same or similar binding strength can be obtained even with a small amount of binder. In particular, the binder having a weight average molecular weight of 1,000,000 was able to lower the content of more than 60% compared to that of 280,000 conventionally used.

As a result, it can be seen that even when a small amount of a high molecular weight binder is used, the binding force can be maintained. In addition, the lower the content of the binder is possible to increase the content of the electrode active material in the electrode material to increase the capacity of the active material per unit weight, which means that as much as possible the active material can be injected into the electrode plate.

In the present invention, the binder content is 10 wt% or less, preferably 2 to 10 wt% with respect to the electrode composition, the content of the binder can be lowered to a lower value than the conventionally used content. For example, in the case of the positive electrode, the content of the positive electrode active material, the conductive agent and the binder may be used in the range of 92: 5: 3 to 93: 5: 2, and in the case of the negative electrode, the content of the negative electrode active material, the conductive agent and the binder may be 90: 2. It can be used in the range of: 8 to 92: 2: 6.

The electrode material as described above is prepared by a conventional method, that is, by dissolving a binder in a solvent, then adding an electrode active material and a conductive agent and then mixing.

Solvents available include N-methyl-2-pyrrolidone (NMP), dimethyl formamide, dimethyl acetamide, dimethylsulfonamide and tetrahydro Polar solvents such as tetrahydrofuran, preferably N-methyl-2-pyrrolidone.

In this case, a mixer such as a planetary mixer is used to increase the contact area between the electrode active material and the conductive agent and to achieve uniform mixing.

The prepared electrode material is applied to a current collector in the form of a foil, and then subjected to a drying process to remove the solvent contained in the mixture, followed by pressing (rolling) to prepare a positive electrode and a negative electrode.

The prepared positive electrode and the negative electrode are made of a polyolefin-based porous polymer separator, and the organic electrolyte is injected and then winded to assemble the battery in a jelly roll type or to assemble by lamination.

The separator may be a known polymer such as polyolefin-based polymer membranes such as polyethylene and polypropylene or multiple membranes thereof, microporous films, woven fabrics and nonwoven fabrics.

As the organic electrolyte, an organic electrolyte in which lithium salt is dissolved in an aprotic solvent may be used.

The aprotic solvents include propylene carbonate, ethylene carbonate, butylene carbonate, benzonitrile, acetonitrile, tetrahydrofuran, 2-methyltetrahydrofuran, γ-butyllactone, dioxolane, 4-methyldioxolane, N, N-dimethylformamide, dimethylacetamide, dimethyl sulfoxide, dioxane, 1,2-dimethoxyethane, sulfolane, dichloroethane, chlorobenzene, nitrobenzene, dimethyl carbonate, methyl isopropyl carbonate, ethyl butyl carbonate , Aprotic solvents such as dipropyl carbonate, diisopropyl carbonate, dibutyl carbonate, diethylene glycol, dimethyl ether, or a mixed solvent in which two or more of these solvents are mixed, and particularly propylene carbonate and ethylene carbonate. At least one of butylene carbonate and at the same time dimethyl carbonate, methyl ethyl carbonate, It is preferable to include any one of diethyl carbonate.

The lithium salt may be LiPF ₆ , LiBF ₄ , LiSbF ₆ , LiAsF ₆ , LiClO ₄ , LiCF ₃ SO ₃ , Li (CF ₃ SO ₂ ) ₂ N, LiC ₄ F ₉ SO ₃ , LiSbF ₆ , LiAlO ₄ , LiAlCl ₄ 1 or 2 or more lithium salts selected from the group consisting of LiN (CxF _{2x + 1} SO ₂ ) (CyF _{2y + 1} SO ₂ ) (where x and y are natural numbers), LiCl, LiI, and the like can be exemplified by that, in particular, it is preferred to include any one of LiPF ₆ and LiBF _4. In addition, what is conventionally known as an organic electrolyte of a lithium secondary battery can also be used.

As another example of the organic electrolyte, a polymer such as polyethylene oxide (PEO), polyvinyl alcohol (PVA), or the like, in which any one of the lithium salts is mixed, or a polymer having high swellability impregnated with an organic electrolyte solution An electrolyte can be used.

As the battery according to the present invention exhibits high output characteristics, electric vehicles, hybrid electric vehicles (HEVs), fuel cell electric vehicles (FCEVs), and battery scooters, in addition to small batteries used in mobile phones or notebook computers, etc. It can be preferably applied to a large battery used in a transport device such as.

Hereinafter, preferred examples and comparative examples of the present invention are described. However, the following examples are only preferred embodiments of the present invention and the present invention is not limited by the following examples.

<Example 1>

1. Fabrication of Anode

5 parts by weight of polyvinylidene fluoride having a weight average molecular weight of 500,000 was dissolved in 50 parts by weight of an N-methyl pyrrolidone (NMP) solvent, and then 90 parts by weight of LiCoO ₂ (average particle diameter: 5 μm) was used as an active material for a positive electrode. 5 parts by weight of acetylene black (average particle diameter: 42 nm) was added, and the resulting mixture was cast on a foil-shaped aluminum current collector to prepare a positive electrode.

2. Preparation of Cathode

To prepare the negative electrode, it was carried out similarly to the preparation of the positive electrode.

First, 50 parts by weight of N-methyl pyrrolidone (NMP) solvent was added to the mixer, and then 5 parts by weight of polyvinylidene fluoride having a weight average molecular weight of 500,000 was added and mixed. Subsequently, 90 parts by weight of mesocarbon microbeads were added as an active material for a negative electrode, and 5 parts by weight of acetylene black (average particle diameter: 42 nm) were added as a conductive agent.

The obtained negative electrode mixture was coated with a doctor blade on a foil-shaped copper current collector, dried at 80 ° C. for 2 hours, and then roll-pressed to prepare a negative electrode.

3. Battery Assembly

After placing the separator (film having a three-layer structure of PP / PE / PP) between the prepared positive and negative electrodes, a unit cell was assembled according to a conventional method. At this time, an electrolyte consisting of 1 mol of LiPF ₆ was used as an electrolyte salt dissolved in 1 L of a 1: 1 mixture (volume) of ethylene carbonate and diethyl carbonate.

<Example 2>

A lithium secondary battery was manufactured in the same manner as in Example 1, except that polyvinylidene fluoride having a weight average molecular weight of 630,000 was used as the binder.

<Example 3>

A lithium secondary battery was manufactured in the same manner as in Example 1, except that polyvinylidene fluoride having a weight average molecular weight of 1,000,000 was used as the binder.

Comparative Example 1

A lithium secondary battery was manufactured in the same manner as in Example 1, except that polyvinylidene fluoride having a weight average molecular weight of 280,000 was used as the binder.

Comparative Example 2

A lithium secondary battery was manufactured in the same manner as in Example 1, except that polyvinylidene fluoride having a weight average molecular weight of 350,000 was used as the binder.

<Test Example 1>

In the present invention, even if a small amount of a high molecular weight binder was used as follows to determine that the electrode plate binding force is maintained.

Specifically, an electrode was prepared using polyvinylidene fluoride having a weight average molecular weight of 280,000, 350,000, 500,000, 630,000, and 1,000,000 as a binder, and the binding force was measured. At this time, the content of the binder required for the electrode in order to exhibit the same binding force was measured, and in the case of the weight average molecular weight of 280,000 in Comparative Example 1 for the comparative comparison according to the molecular weight as 100%, the binder of the remaining examples and comparative examples It is shown in Figure 1 by calculating the content of.

1 is a graph showing the required content of a binder in an electrode according to the weight average molecular weight of polyvinylidene fluoride used as a binder.

Referring to Figure 1, it can be seen that as the molecular weight increases, less content is required to show the same binding force. In particular, the binder having a weight average molecular weight of 1,000,000 was able to lower the content of more than 60% compared to that of 280,000 conventionally used.

These results indicate that even when a small amount of a high molecular weight binder is used, the binding force can be maintained, and thus a relatively large amount of the active material can be used for the content of the binder during electrode production. As a result, as the active material in the electrode is contained in a high content, the output of the battery is increased.

As described above, by using the high molecular weight polyvinylidene fluoride binder according to the present invention, it is possible to maintain the electrode plate binding force even in a small amount, so as to lower the content of the binder, The content could be increased.

Therefore, the use of the binder not only increases the capacity of the active material per unit weight, but also injects as much active material into the electrode plate as possible, and has high output and high capacity characteristics, making it suitable for the electric vehicle field. Can be used.

Although described above with reference to a preferred embodiment of the present invention, those skilled in the art will be variously modified and changed within the scope of the invention without departing from the spirit and scope of the invention described in the claims below I can understand that you can.

1 is a graph showing the required content of the binder in the electrode according to the weight average molecular weight of polyvinylidene fluoride to show the same binding force.

Claims (8)

An electrode for a high power lithium secondary battery comprising an electrode active material, a conductive agent, and a binder, wherein the binder comprises a polyvinylidene fluoride polymer having a weight average molecular weight of 500,000 to 1,000,000. The electrode of claim 1, wherein the polyvinylidene fluoride polymer is included in an amount of 2 to 10 wt% or less based on the electrode composition. The method of claim 1, wherein the positive electrode active material is a lithium composite oxide capable of reversibly intercalation (intercalation) and de-intercalation (de-intercalation) of lithium ions, characterized in that for high output lithium secondary battery electrode. The lithium composite oxide of claim 3, wherein the lithium composite oxide is formed of LiNi _x Co _y Al _1-xy O ₂ (0 <x ≦ 1, 0 <y ≦ 1), LiCoO ₂ , LiMn ₂ O ₄ , LiMnO _2, and LiNiO ₂ . High-power lithium secondary battery electrode, characterized in that one selected from the group. The electrode of claim 1, wherein the negative electrode active material is a carbon-based material capable of reversibly intercalating and deintercalating lithium ions. The electrode of claim 5, wherein the carbonaceous material is one selected from the group consisting of natural graphite, artificial graphite, graphite carbon fiber, petroleum pitch and coal pitch. The method of claim 1, wherein the conductive agent is selected from the group consisting of Super P (MMM Co., Ltd.), acetylene black, ketjen black, Denka Black and carbon black It is 1 type, The electrode for high output lithium secondary batteries characterized by the above-mentioned. A high output lithium secondary battery comprising the electrode according to any one of claims 1 to 7.