KR102018270B1 - Negative Electrode Having Cured Binder for Secondary Battery and Lithium Secondary Battery Comprising the Same - Google Patents

Abstract

According to the present invention, a negative electrode mixture containing a carbon-based active material or a silicon-based active material as a negative electrode active material and a modified polyvinyl alcohol (PVA) as a binder is coated on one or both surfaces of the negative electrode current collector, and the binder is a curing initiator. It includes an (initiator), and to increase the hardness of the negative electrode provides a negative electrode for a secondary battery, characterized in that the negative electrode mixture is applied to the negative electrode current collector and then dried and cured.

Description

Negative Electrode Having Cured Binder for Secondary Battery and Lithium Secondary Battery Comprising the Same

The present invention relates to a negative electrode for a secondary battery including a cured binder and a lithium secondary battery including the same.

Due to the rapid increase in the use of fossil fuels, the demand for the use of alternative energy or clean energy is increasing. As a part of this, the most actively researched fields are power generation and storage using electrochemistry.

A representative example of an electrochemical device using such electrochemical energy is a secondary battery, and its use area is gradually increasing.

Recently, as the development and demand for portable devices such as portable computers, portable telephones, cameras, and the like, the demand for secondary batteries is rapidly increasing, and these secondary batteries exhibit high energy density and operating potential, and have a cycle life. Many studies have been conducted on this long, low self-discharge rate lithium battery and are commercially available and widely used.

In general, in a lithium secondary battery, charging and discharging proceed while repeating a process in which lithium ions of a positive electrode are inserted into and detached from a negative electrode. As the lithium ions are repeatedly inserted and desorbed, the bonding between the electrode active material or the conductive material is loosened, and the contact resistance between particles increases. As a result, the ohmic resistance of the electrode may increase, thereby degrading battery characteristics. Therefore, the binder should be capable of buffering against expansion and contraction of the electrode active material due to insertion and desorption of lithium ions from the electrode, and therefore, the binder is preferably a polymer having elasticity.

In addition, the binder requires an adhesive strength such that the binding force between the electrode active material and the current collector can be maintained during the electrode plate drying process. In particular, in order to increase the discharge capacity, when a material such as silicon, tin, a silicon-tin alloy having a large discharge capacity is mixed with natural graphite having a theoretical discharge capacity of 372 mAh / g, the material is charged and discharged. The volume expansion of is significantly increased, resulting in the release of the negative electrode material, and as a result, the capacity of the battery may be drastically lowered as the repetitive cycle proceeds.

In addition, the lithium ion battery is caused to swell due to the gas generated during the decomposition of the electrolyte in the battery, this phenomenon is called electrolyte swelling. Since decomposition of the electrolyte is promoted at a high temperature, the swelling phenomenon is promoted when the battery is left at a high temperature. This is because when the temperature of the battery rises, the electrolyte is decomposed or a side reaction occurs to generate a gas such as carbon dioxide or carbon monoxide, thereby increasing the thickness of the battery. On the other hand, the increase in thickness in high temperature storage is an important consideration in a small battery, there is a problem of the thickness and stability at high temperature as the temperature of the battery increases rapidly with the use of small devices.

Therefore, the strong adhesion prevents the separation between the electrode active material or between the electrode active material and the current collector during strong electrode manufacturing, and controls the volumetric expansion of the electrode active material generated during repeated charging and discharging with strong physical properties, thereby ensuring the structural stability of the electrode and the resulting battery. There is a high need for development of a binder and an electrode material capable of improving performance.

The present invention aims to solve the problems of the prior art as described above and the technical problems that have been requested from the past.

After extensive research and various experiments, the inventors of the present application have a negative electrode mixture including a carbon-based active material or a silicon-based active material as a negative electrode active material and a modified polyvinyl alcohol (PVA) as a binder. When applied to the negative electrode current collector and then dried and cured, the mechanical strength of the electrode is improved to effectively prevent deformation of the electrode assembly caused by expansion and contraction of the electrode during charging and discharging, and thus has excellent stability and durability. It was confirmed that the secondary battery can be provided, and the present invention has been completed.

In order to achieve the above object, the secondary battery negative electrode according to the present invention,

A negative electrode mixture containing a carbon-based active material or a silicon-based active material as a negative electrode active material and a modified polyvinyl alcohol (PVA) as a binder is coated on one or both sides of the negative electrode current collector,

The binder includes a curing initiator,

In order to increase the hardness of the negative electrode, the negative electrode mixture is applied to the negative electrode current collector, characterized in that the drying and curing treatment.

Here, 'hardening the negative electrode mixture' means a process of increasing the degree of polymerization by polymerizing modified polyvinyl alcohol (PVA) of the binder included in the negative electrode mixture.

Accordingly, the negative electrode for secondary batteries according to the present invention is dried and cured after applying a negative electrode mixture containing a carbon-based active material or a silicon-based active material as a negative electrode active material and a modified polyvinyl alcohol (PVA) as a binder to a negative electrode current collector. As a result, the mechanical strength of the electrode is improved and the deformation of the electrode assembly caused by the expansion and contraction of the electrode during charging and discharging can be effectively prevented, thereby providing an effect of providing a secondary battery having excellent stability and durability.

In one specific example, the carbon-based active material is crystalline artificial graphite, crystalline natural graphite, amorphous hard carbon, low crystalline soft carbon, carbon black, acetylene black, Ketjen black, super P, graphene, and fibrous carbon It may be one or more selected from the group consisting of, and in detail, may be crystalline natural graphite.

In addition, the silicon-based active material may be silicon oxide (SiO).

In addition, the negative electrode current collector may be aluminum, an aluminum alloy or copper, and in detail, may be copper.

In one specific example, some units of the modified polyvinyl alcohol may have a structure represented by the following formula (1).

(1)

(One)

In Formula 1, n is an integer of 1 to 80, R is an acetoacetyl group, a carboxylic acid group, an acrylic group or a urethane group.

Specifically, R in Formula 1 may be an acetoacetyl group, and more specifically, the content of the acetoacetyl group is 0.01 mole based on the total amount of hydroxyl groups of polyvinyl alcohol in the state before modification. % To 15 mol%.

In one specific example, the curing initiator may be a titanate-based coupling agent, an aluminum-based coupling agent, a silane-based coupling agent, or a chromium-based coupling agent, specifically, the titanate-based coupling agent is ethyl It may be a titanate (ethyl titanate), but is not necessarily limited to these coupling agents, it is applicable as long as it causes the curing of the modified polyvinyl alcohol.

In one specific example, curing of the binder may occur by thermal polymerization or photopolymerization, and in detail, curing of the binder may be thermally polymerized at 15 degrees Celsius to 80 degrees Celsius.

On the other hand, in general, polyvinyl alcohol is a hydrophilic material, and has a disadvantage in that it is not suitable as a material of a binder in a lithium secondary battery that causes stability or degradation due to moisture penetrated therein, but the binder of the negative electrode for a secondary battery according to the present invention The bar may increase the blocking property of the water absorption of the polyvinyl alcohol after the curing treatment, it can provide an excellent secondary battery negative electrode by preventing the penetration of moisture.

In one specific example, the binder may be included in an amount of 0.5 wt% to 5 wt% based on the total weight of the negative electrode active material, and in detail, may be 1.0 wt% to 2 wt%, wherein the binder content If it is less than 0.5% by weight, it is difficult to expect a role that can prevent the volume change occurring during charging and discharging.On the contrary, when the content of the binder is more than 5% by weight, the energy density of the electrode may decrease and may cause an increase in internal resistance. have.

In addition, the curing initiator may be included in 0.01% by weight to 1% by weight based on the total weight of the negative electrode active material, in detail, may be included in 0.05% by weight to 0.5% by weight, wherein the content of the curing initiator is 0.01 When the content is less than 1% by weight, it may be difficult to sufficiently harden the binder. On the contrary, when the content is more than 1% by weight, the degree of polymerization of the polyvinyl alcohol may be excessively high, so that it may be difficult to obtain a hardness of the negative electrode.

In one specific example, the modified polyvinyl alcohol may have a degree of polymerization of 3000 or more after curing.

This invention also provides the method of manufacturing the negative electrode for secondary batteries.

As a method of manufacturing the secondary battery negative electrode,

(i) preparing a negative electrode mixture by adding a carbon-based active material or silicon-based active material, modified polyvinyl alcohol (PVA), a dispersant, and a curing initiator into an organic solvent and then mixing them; And

(ii) applying the negative electrode mixture to the negative electrode current collector and then drying and curing the negative electrode mixture;

It characterized in that it comprises a.

In addition, the curing treatment of the binder may be by thermal polymerization or photopolymerization, in detail, the curing of the binder may be thermally polymerized at 15 degrees Celsius to 80 degrees Celsius.

At this time, the curing treatment may be cured until the binder has a degree of polymerization of 3000 or more.

Specifically, the solvent is methanol, dimethyl sulfoxide, N-methyl-2-pyrrolidone, N-methylpyrrolidone, N, N-dimethylformamide, N, N-dimethylacetamide, and methyl ethyl ketone ( methyl ethyl ketone (MEK); may be one or more selected from the group consisting of, specifically, N-methyl-2-pyrrolidone (NMP).

The present invention also provides a lithium secondary battery comprising at least one secondary battery negative electrode, a positive electrode, a separator interposed between the negative electrode and the positive electrode and a nonaqueous electrolyte.

Such a lithium secondary battery may be, for example, a pouch type battery cell in which an electrode assembly having a positive electrode, a separator, and a negative electrode structure is sealed inside the battery case together with an electrolyte solution, and has a plate-like shape having a substantially rectangular parallelepiped structure with a thin thickness to width. consist of. Such a pouch-type battery cell is generally made of a pouch-type battery case, the battery case is an outer coating layer made of a polymer resin having excellent durability; A barrier layer made of a metal material that exhibits barrier properties against moisture, air, and the like; And a laminate sheet structure in which an inner sealant layer made of a polymer resin that can be heat-sealed is sequentially stacked.

The positive electrode is prepared by, for example, applying a mixture of a positive electrode active material, a conductive material, and a binder to a positive electrode current collector, followed by drying, and optionally, a filler is further added to the mixture.

The positive electrode current collector is generally made in a thickness of 3 micrometers to 500 micrometers. Such a positive electrode current collector is not particularly limited as long as it has high conductivity without causing chemical change in the battery. For example, a surface of stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel Surface treated with carbon, nickel, titanium, silver, or the like can be used. The current collector may form fine irregularities on its surface to increase the adhesion of the positive electrode active material, and may be in various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a nonwoven fabric.

The positive electrode active material is a material that can cause the electrochemical reaction, the lithium transition as the metal oxide, and includes two or more transition metals, e.g., lithium cobalt oxide substituted with one or more transition metal (LiCoO _2), lithium Layered compounds such as nickel oxide (LiNiO ₂ ); Lithium manganese oxide substituted with one or more transition metals; Formula LiNi _1-y M _y O ₂ , wherein M = Co, Mn, Al, Cu, Fe, Mg, B, Cr, Zn or Ga and comprises at least one of the above elements, wherein 0.01 ≦ _y ≦ 0.7 Lithium nickel-based oxide represented by; Li _{1 + z} Ni _1/3 Co _1/3 Mn _1/3 O ₂ , Li _{1 + z} Ni _0.4 Mn _0.4 Co _0.2 O _2, etc. Li _{1 + z} Ni _b Mn _c Co _{1- (b + c + d )} M _d O _(2-e) A _e (where -0.5≤z≤0.5, 0.1≤b≤0.8, 0.1≤c≤0.8, 0≤d≤0.2, 0≤e≤0.2, b + c + d Lithium nickel cobalt manganese composite oxide represented by <1, M = Al, Mg, Cr, Ti, Si or Y, and A = F, P or Cl; Formula Li _{1 + x} M _{1-y M'y} PO _4-z X _z , wherein M = transition metal, preferably Fe, Mn, Co or Ni, M '= Al, Mg or Ti, X = Olivine-based lithium metal phosphate represented by F, S, or N, and represented by -0.5≤x≤ + 0.5, 0≤y≤0.5, and 0≤z≤0.1, etc., but is not limited thereto.

The conductive material is typically added in an amount of 1 to 20 wt% based on the total weight of the mixture including the positive electrode active material. Such a conductive material is not particularly limited as long as it has conductivity without causing chemical change in the battery, and examples thereof include graphite such as natural graphite and artificial graphite; Carbon blacks such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and summer black; Conductive fibers such as carbon fibers and metal fibers; Metal powders such as carbon fluoride powder, aluminum powder and nickel powder; Conductive whiskeys such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; Conductive materials such as polyphenylene derivatives and the like can be used.

The filler is optionally used as a component for inhibiting expansion of the positive electrode, and is not particularly limited as long as it is a fibrous material without causing chemical change in the battery. Examples of the filler include olefinic polymers such as polyethylene and polypropylene; Fibrous materials, such as glass fiber and carbon fiber, are used.

The negative electrode current collector is generally made in a thickness of 3 micrometers to 500 micrometers. In addition, like the positive electrode current collector, fine concavities and convexities may be formed on the surface to enhance the bonding strength of the negative electrode active material, and may be used in various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a nonwoven fabric.

The separator is interposed between the anode and the cathode, and an insulating thin film having high ion permeability and mechanical strength is used. The pore diameter of the separator is generally 0.01 micrometer to 10 micrometers, and the thickness is generally 5 micrometers to 300 micrometers. As such a separator, for example, olefin polymers such as chemical resistance and hydrophobic polypropylene; Sheets or non-woven fabrics made of glass fibers or polyethylene are used. When a solid electrolyte such as a polymer is used as the electrolyte, the solid electrolyte may also serve as a separator.

The present invention also provides a battery pack comprising a lithium secondary battery.

The present invention also provides a device including the battery pack as a power source. Specifically, the device may include, for example, a mobile phone, a portable computer, a wearable electronic device, a tablet PC, a smart pad, a netbook, a light electronic vehicle (LEV), an electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, and It may be selected from the group consisting of a power storage device.

Since the structure and fabrication method of such a device are known in the art, detailed description thereof will be omitted herein.

As described above, the secondary battery negative electrode according to the present invention is dried after applying a negative electrode mixture containing a carbon-based active material or silicon-based active material as a negative electrode active material and a modified polyvinyl alcohol (PVA) as a binder to the negative electrode current collector And curing to improve mechanical strength of the electrode, thereby effectively preventing deformation of the electrode assembly caused by expansion and contraction of the electrode during charging and discharging, thereby providing a secondary battery having excellent stability and durability. .

In the following Examples, Comparative Examples and Experimental Examples, the content of the present invention will be described in more detail, but the present invention is not limited thereto.

<Example 1>

<Preparation of 1-1 Electrode>

The negative electrode is 97.0 g of natural graphite (MS46, BTR ENERGY MATERIALS CO., LTD.) By weight of 100 g of total solids, 1.5 g of modified polyvinyl alcohol (PVA) in which some units are substituted with acetoacetyl group as a binder, and carboxy as a dispersant. 1.5 g of sodium methyl cellulose (CMC-Na, BG-L01, GL Chem Co., Ltd.), and 0.15 g of ethyl titanate (KR44, Nippon Synthetic Chemical) as a curing initiator were mixed as a solvent so that the total solid content was 55% by weight. N-methylpyrrolidone (NMP) was added to prepare a slurry for the negative electrode, the prepared slurry was applied to a copper foil, vacuum dried, thermal polymerization was performed at 80 degrees Celsius, and a negative electrode having a thickness of 149 micrometers was prepared. Prepared.

The positive electrode was prepared by using NMP (N-methyl-2-pyrrolidone) as a dispersion medium, mixing 96 g of LiCoO _{2 as an} active material, ₂ g of acetylene black, and 2 g of PVDF binder to prepare a slurry. It was coated with a thickness, dried and then compressed to prepare a positive electrode.

<1-2 Fabrication of Lithium Secondary Battery>

The prepared negative electrode plate was drilled with a surface area of 13.33 cm ² , and the positive electrode plate was drilled with a surface area of 12.60 cm ² to prepare a mono-cell. A tab was attached to the upper part of the positive electrode and the upper part of the negative electrode, and the resultant was loaded into an aluminum pouch with a separator made of a polyolefin microporous membrane between the negative electrode and the positive electrode, and 500 mg of the electrolyte was injected into the pouch. The electrolyte solution was prepared by dissolving a LiPF ₆ electrolyte at a concentration of 1 M using EC (Ethyl carbonate): DEC (Diethyl carbonate): EMC (Ethyl-methyl carbonate) = 4: 3: 3 (volume ratio) mixed solvent. Then, the pouch is sealed using a vacuum packaging machine and maintained at room temperature for 12 hours, followed by a constant voltage charging process that maintains a constant current at a rate of about 0.05 degrees and maintains a voltage until it is about 1/6 of the current. At this time, since gas is generated in the cell, degassing and resealing were performed to complete the lithium secondary battery.

Comparative Example 1

A lithium secondary battery was manufactured in the same manner as in Example 1, except that a curing initiator was not added to the negative electrode mixture when preparing the negative electrode.

<Experimental Example 1> (Measurement of thickness change during high temperature storage of secondary battery)

First, after measuring the thickness of each of the prepared secondary battery Example 1 and Comparative Example 1, the thickness was measured after storing the Example 1 and Comparative Example 1 in a constant temperature chamber set to 90 degrees Celsius for 48 hours, and the thickness change The calculated results are shown in Table 1 below.

Example 1 Comparative Example 1 Early
Cathode thickness (㎛) 149 149 After high temperature storage
Cathode thickness (㎛) 152 156 % Change + 2.01 + 4.70

As shown in Table 1, Example 1 using the secondary battery negative electrode according to the present invention, the amount of change in thickness less than Comparative Example 1, the mechanical strength of the negative electrode is improved due to the expansion and contraction of the negative electrode during charge and discharge It can be expected that the deformation of the electrode assembly caused can be effectively prevented, and the stability and durability of the battery will be improved.

As described above, the secondary battery negative electrode according to the present invention, the negative electrode mixture containing a carbon-based active material or silicon-based active material as a negative electrode active material, and a modified polyvinyl alcohol (PVA) as a binder to the negative electrode current collector and dried and By curing, the mechanical strength of the electrode is improved to effectively prevent the deformation of the electrode assembly caused by the expansion and contraction of the electrode during charging and discharging, thereby providing a secondary battery having excellent stability and durability. .

Those skilled in the art to which the present invention pertains will be able to perform various applications and modifications within the scope of the present invention based on the contents.

Claims (22)

A negative electrode mixture containing a carbon-based active material or a silicon-based active material as a negative electrode active material and a modified polyvinyl alcohol (PVA) as a binder is coated on one or both sides of the negative electrode current collector,
The binder includes a curing initiator,
In order to increase the hardness of the negative electrode negative electrode mixture is applied to the negative electrode current collector is dried and cured,
Curing of the binder occurs by thermal polymerization at 15 degrees Celsius to 80 degrees Celsius,
Some units of the modified polyvinyl alcohol have a structure represented by Formula 1 below, wherein R in Formula 1 is an acetoacetyl group,
The content of the acetoacetyl group is 0.01 mol% to 15 mol% based on the total amount of hydroxyl groups of the polyvinyl alcohol in the state before denaturation,
The curing initiator is ethyl titanate,
The ethyl titanate is a secondary battery negative electrode, characterized in that contained in 0.01% by weight to 1% by weight based on the total weight of the negative electrode active material.

(One)
In Formula 1, n is an integer of 1 to 80. The method of claim 1, wherein the carbon-based active material is crystalline artificial graphite, crystalline natural graphite, amorphous hard carbon, low crystalline soft carbon, carbon black, acetylene black, Ketjen black, super P, graphene, and fibrous carbon A secondary battery negative electrode, characterized in that at least one selected from the group consisting of. The negative electrode of claim 1, wherein the negative electrode current collector is aluminum, an aluminum alloy, or copper. delete delete delete delete delete delete delete The negative electrode for a secondary battery of claim 1, wherein the binder has a property of blocking water absorption of polyvinyl alcohol after curing. The negative electrode of claim 1, wherein the binder is present in an amount of 0.5 wt% to 5 wt% based on the total weight of the negative electrode active material. delete The negative electrode for a secondary battery according to claim 1, wherein the modified polyvinyl alcohol has a degree of polymerization of 3000 or more after curing. A method of manufacturing a negative electrode for a secondary battery according to claim 1,
(i) preparing a negative electrode mixture by adding a carbon-based active material or silicon-based active material, modified polyvinyl alcohol (PVA), a dispersant, and a curing initiator into an organic solvent and then mixing them; And
(ii) applying the negative electrode mixture to the negative electrode current collector and then drying and curing the negative electrode mixture;
Including,
The curing treatment is carried out by thermal polymerization of the modified polyvinyl alcohol contained in the negative electrode mixture at 15 degrees Celsius to 80 degrees Celsius,
Some units of the modified polyvinyl alcohol have a structure represented by Formula 1 below, wherein R in Formula 1 is an acetoacetyl group,
The curing initiator is ethyl titanate,
The ethyl titanate is a method for producing a negative electrode for a secondary battery containing 0.01% by weight to 1% by weight based on the total weight of the negative electrode active material.

(One)
In Formula 1, n is an integer of 1 to 80. delete delete The method of claim 15, wherein the binder is cured until it has a degree of polymerization of 3000 or more. The method of claim 15, wherein the solvent is methanol, dimethyl sulfoxide, N-methyl-2-pyrrolidone, N-methylpyrrolidone, N, N-dimethylformamide, N, N-dimethylacetamide, and Methyl ethyl ketone (MEK); Method for producing a negative electrode for a secondary battery, characterized in that at least one selected from the group consisting of. A lithium secondary battery comprising at least one secondary battery negative electrode, a positive electrode, a separator interposed between the negative electrode and the positive electrode and a nonaqueous electrolyte according to claim 1. A battery pack comprising the lithium secondary battery according to claim 20. A device comprising the battery pack according to claim 21 as a power source.