KR101517043B1 - Anode Having Improved Adhesion for Lithium Secondary Battery - Google Patents

Abstract

A negative electrode for a secondary battery in which an electrode mixture containing a negative electrode active material and a binder is coated on a current collector, the negative electrode comprising a first binder lower in affinity for an electrolyte than the second binder below, A first anode mixture layer; And a second negative electrode material mixture layer coated on the first negative electrode material layer, wherein the second negative electrode material mixture layer includes a second binder having an affinity for the electrolyte and higher than that of the first binder; The negative electrode for a secondary battery according to the present invention has an effect of preventing the peeling phenomenon in the electrolyte solution and improving the storage life of the battery.

Description

Anode Having Improved Adhesion for Lithium Secondary Battery

The present invention relates to a negative electrode for a lithium secondary battery, and more particularly, to a negative electrode for a secondary battery in which an electrode mixture containing a negative electrode active material and a binder is coated on a current collector, 1 binder; and a first negative electrode material mixture layer coated on the current collector; And a second negative electrode mixture layer coated on the first negative electrode mixture layer, wherein the second negative electrode mixture layer includes a second binder having an affinity for the electrolyte and higher than that of the first binder .

As technology development and demand for mobile devices are increasing, the demand for secondary batteries as energy sources is rapidly increasing. Among such secondary batteries, lithium secondary batteries having high energy density and voltage have been commercialized. Such a lithium secondary battery generally uses a transition metal oxide of lithium as a cathode active material and uses a graphite based material as a negative electrode active material to repeatedly intercalate / deintercalate lithium ions in the anode, The discharge proceeds.

In general, the theoretical capacity of the battery varies depending on the type of the electrode active material, but the charging and discharging capacities decrease with the progress of the cycle. This phenomenon is due to the fact that the electrode active material or the electrode active material and the current collector are separated from each other due to the change in the volume of the electrode caused by progress of charging and discharging of the battery, It is a big cause. Also, in the process of occlusion and desorption, the lithium ions occluded in the negative electrode can not be properly discharged and the active sites of the negative electrode are reduced. As a result, the charge / discharge capacity and lifetime characteristics of the battery may decrease as the cycle progresses.

Particularly, in order to increase the discharge capacity, materials such as silicon, tin, silicon-tin alloy and silicon-carbon composite material having high discharge capacity are mixed with natural graphite having a theoretical discharge capacity of 372 mAh / g The volume expansion of the material remarkably increases as the charging and discharging progresses. As a result, the electrode mixture is released from the current collector, and eventually the capacity of the battery is rapidly lowered when the cycle is several to several tens of cycles A problem has arisen.

Therefore, a binder and an electrode compound that can improve the structural stability of the electrode and improve the performance of the electrode by controlling the volume expansion of the electrode active material generated during repeated charge and discharge are desperately required in the art.

Polyvinylidene fluoride (PVdF), which is currently widely used as a binder for positive and negative electrodes, is a polymer resin dissolved in an organic solvent such as N-methyl-2-pyrrolidone (NMP). Although PVdF is not an adhesive originally, PVdF has good compatibility with graphite materials, and by adding about 8 to 10% of graphite, it is possible to produce a polar plate having a high adhesive force and is widely used as a binder of an electrode active material.

However, since PVdF covers the active material in the same state as the polymer fibers are filled, the performance of the battery, which is an electrode active material, deteriorates in terms of capacity and efficiency. In addition, PVdF tends to be broken and the cycle characteristics to be degraded when a material having a large specific surface area such as natural graphite or metal-based active material and having a high expansion / shrinkage ratio during charging / discharging is used as an electrode active material due to lack of flexibility. Furthermore, there is a tendency that the electrolyte absorbs and expands in the carbonate-based electrolytic solution, and as the cycle progresses, the output capacity significantly decreases.

Recently, a method of using styrene-butadiene rubber (SBR) has been proposed as a conventional solvent-based binder, polyvinylidene fluoride (PVdF), which is currently used commercially . In the case of such a binder, there is an advantage in that the battery capacity can be increased by reducing the amount of binder used and environmentally friendly. However, in this case, the adhesion persistence is improved by the rubber elasticity,

In this regard, some prior arts have proposed measures for solving the problem of using SBR alone as a binder, Korean Patent Application Publication No. 2003-0033595 discloses a method of mixing a mixed binder of PVdF and SBR at a certain ratio Korean Patent No. 0582518 discloses a binder for a battery containing a composite polymer particle in which two or more polymers having different chemical structures form a heterophase structure, Open Publication No. 2004-0078927 discloses a composite polymer particle of two or more structures containing a polymer, each of which is associated with a battery property, an adhesive force and a coating property.

However, in spite of these various technical proposals, a binder exhibiting a desired level of physical properties has not yet been developed. Therefore, there is a high need for development of a binder having an improved adhesive property and a structural stability of the electrode while improving cycle characteristics of the battery.

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above-mentioned problems of the prior art and the technical problems required from the past.

As a result of intensive research and various experiments, the inventors of the present application have developed a method of using two kinds of binders with different physical properties in order to prevent the peeling phenomenon in the electrolyte, It has been confirmed that when such a negative electrode is used to make a secondary battery, it has a high capacity and initial charge / discharge efficiency as well as a high adhesive force, and has completed the present invention.

According to an aspect of the present invention, there is provided a negative electrode for a secondary battery, comprising:

A first negative electrode mixture layer coated on the current collector, wherein the first negative electrode mixture layer includes a first binder having an affinity to an electrolyte lower than that of the second binder; And

A second negative electrode mixture layer coated on the first negative electrode coater layer, wherein the second negative electrode composition layer includes a second binder having an affinity for an electrolyte higher than that of the first binder;

As shown in Fig.

The negative electrode for a secondary battery according to the present invention exhibits excellent cycle characteristics, rate characteristics, and the like by including a second binder having a relatively high affinity for an electrolyte in a second negative electrode mixture layer which is an electrode outer layer.

Moreover, such a second negative electrode coalescent layer is formed on the first negative electrode mixture layer including the first binder having a relatively low affinity for the electrolyte, whereby the separation phenomenon from the current collector by the high electrolyte swelling can be remarkably suppressed . The reason why the peeling phenomenon can be suppressed in spite of the high electrolyte solution swelling is that the second anode mixture layer containing the second binder is coated on the first anode mixture layer having a relatively higher adhesive force than the current collector, This is because the separation of the second negative electrode material mixture layer is suppressed despite the high electrolyte swelling of the two binders. This is particularly effective in suppressing the peeling phenomenon at a high temperature.

As described above, the second binder has a property of relatively higher electrolyte affinity than the first binder. The difference in affinity of the electrolyte between the first binder and the first binder is, for example, 110 , Preferably from 120 to 500, and more preferably from 130 to 300. [0035]

The first binder and the second binder are not particularly limited as long as they satisfy the conditions of the electrolyte affinity as defined above without causing a chemical reaction in the battery.

The first binder may be, for example, a fluoropolymer such as PVdF or PTFE, and more specifically, at least one compound selected from the group consisting of PVdF homopolymer, PVdF block copolymer, and PVdF graft copolymer .

Examples of the PVdF block copolymer include PVdF-b-CTFE and PVdF-b-HFP. Examples of the PVdF graft copolymer include PVdF-g-PAA and PVdF-g-PVA. no.

The second binder may be, for example, a water-based polymer, and more specifically, an acrylonitrile-butadiene rubber, a styrene-butadiene rubber, an acrylic resin, hydroxyethylcellulose, and carboxymethylcellulose. Lt; / RTI > group.

In one preferred example, the content of the first binder in the first negative electrode coalescent layer is in the range of 2 to 10 wt% based on the total weight of the first negative electrode mixture layer, and the content of the second binder in the second negative electrode mixture layer May be contained in an amount of 1 to 6% by weight based on the total weight of the second negative electrode material mixture layer.

The water-based polymer, which is a specific example of the second binder, generally has an adhesive strength higher than that of the fluorine-based polymer, which is a specific example of the first binder, so that the desired effect can be exhibited even with such a relatively small content.

However, if the content of the binder is too small, it is difficult to expect the effect of the addition. On the other hand, if the content of the binder is too large, the resistance of the negative electrode is increased and the characteristics of the battery are deteriorated.

In one preferred example, the thickness of the first anode mix layer is preferably 10 to 40% of the thickness of the second anode mix layer.

Since the second negative electrode material mixture layer contains the second binder having a relatively high affinity for the electrolyte solution, it is preferable that the thickness of the second negative electrode material mixture layer is set at It is preferable to make it relatively large.

In one specific example, the thickness of the first negative electrode material mixture layer may be 10 to 50 mu m, and preferably 10 to 40 mu m.

The collector generally has a thickness of 3 to 500 mu m. Such an anode current collector is not particularly limited as long as it has conductivity without causing chemical change in the battery, and may be formed of a material such as copper, stainless steel, aluminum, nickel, titanium, fired carbon, surface of copper or stainless steel A surface treated with carbon, nickel, titanium, silver or the like, an aluminum-cadmium alloy, or the like can be used. The current collector may have fine irregularities on the surface thereof to increase the adhesive force of the negative electrode active material, and various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a nonwoven fabric are possible.

The negative electrode active material contained in the first negative electrode mixture layer and the second negative electrode material mixture layer may be the same or different from each other. Examples of such negative electrode active materials include carbon and graphite materials such as natural graphite, artificial graphite, expanded graphite, carbon fiber, hard graphitizable carbon, carbon black, carbon nanotube, fullerene, and activated carbon; Metals such as Al, Si, Sn, Ag, Bi, Mg, Zn, In, Ge, Pb, Pt, and Ti that can be alloyed with lithium and compounds containing these elements; Complexes of metals and their compounds and carbon and graphite materials; Lithium-containing nitrides, and the like. Among them, a carbon-based active material, a tin-based active material, a silicon-based active material, or a silicon-carbon based active material is more preferable, and these may be used singly or in combination of two or more.

The negative electrode material mixture layer may optionally further contain components such as a conductive material, if necessary.

The conductive material is not particularly limited as long as it has electrical conductivity without causing chemical changes in the battery, and examples thereof include graphite such as natural graphite and artificial graphite; Carbon black such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, and summer black; Conductive fibers such as carbon fiber and metal fiber; Metal powders such as carbon fluoride, aluminum, and nickel powder; Conductive whiskey 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. Concrete examples of commercially available conductive materials include acetylene black series such as Chevron Chemical Company, Denka Singapore Private Limited, Gulf Oil Company, etc.), Ketjenblack, EC (Armak Company), Vulcan XC-72 (Cabot Company), and Super P (Timcal).

In some cases, a filler may be optionally added as a component for suppressing the expansion of the electrode. Such a filler is not particularly limited as long as it is a fibrous material without causing a chemical change in the battery, and examples thereof include olefin polymers such as polyethylene and polypropylene; Fibrous materials such as glass fibers and carbon fibers are used.

In addition, other components such as a viscosity adjusting agent, an adhesion promoter and the like may be further included as a selective or a combination of two or more.

The viscosity adjusting agent may be added up to 30% by weight based on the total weight of the electrode mixture, so as to control the viscosity of the electrode mixture so that the mixing process of the electrode mixture and the coating process on the collector may be easy. Examples of such viscosity modifiers include carboxymethylcellulose, polyvinylidene fluoride and the like, but are not limited thereto. In some cases, the above-described solvent may play a role as a viscosity adjusting agent.

The adhesion promoter may be added in an amount of 10% by weight or less based on the binder, for example, oxalic acid, adipic acid, Formic acid, acrylic acid derivatives, itaconic acid derivatives, and the like.

The negative electrode for a secondary battery according to the present invention can be manufactured, for example, by a method including the following process.

(i) applying a slurry containing a first binder and a negative active material to a current collector, and drying the slurry to form a first negative electrode mixture layer;

(ii) applying a slurry containing a second binder and a negative electrode active material to the first negative electrode mixture layer and then drying to form a second negative electrode mixture layer; And

(iii) drying and rolling the first anode mixture layer and the second anode mixture layer,

The coating method, the drying method, the rolling method, and the like can be applied without particular limitation, such as coating, drying, and rolling in a negative electrode manufacturing method known in the art.

In the method of manufacturing a negative electrode according to the present invention, some processes may be changed as necessary, and these are all to be construed as falling within the scope of the present invention. For example, the drying process can be performed in the forming step of each anode mix layer, and the rolling process is the same.

The present invention also provides a secondary battery including the negative electrode for a secondary battery.

The lithium secondary battery has a structure in which a non-aqueous electrolyte solution containing a lithium salt is impregnated in an electrode assembly having a separator interposed between an anode and a cathode.

The anode may be formed by, for example, applying a cathode active material onto a cathode current collector, and drying the cathode active material. The binder, the conductive material, and the components described above with respect to the structure of the anode may be further included.

The cathode current collector generally has a thickness of 3 to 500 mu m. Such a positive electrode current collector is not particularly limited as long as it has high conductivity without causing chemical change in the battery, and may be formed of a material such as stainless steel, aluminum, nickel, titanium, sintered carbon, or a surface of aluminum or stainless steel Treated with carbon, nickel, titanium, silver or the like may be used. The positive electrode current collector may have fine unevenness formed on the surface thereof to increase the adhesive force of the positive electrode active material as in the case of the negative electrode current collector. Alternatively, the positive electrode current collector may have various properties such as a film, sheet, foil, net, porous body, Form is possible.

The cathode active material is not particularly limited as long as it is a substance capable of releasing and inhaling lithium in a charging and discharging process. For example, lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium cobalt- Manganese-based oxide, lithium iron-phosphate-based oxide, and the like, and some transition metals may be aluminum, magnesium, titanium, etc. Substituted materials may also be used.

The binder, the conductive material, and the filler added as needed are the same as those in the negative electrode.

The separator is an insulating thin film interposed between the anode and the cathode and having high ion permeability and mechanical strength. The pore diameter of the separator is generally 0.01 to 10 mu m and the thickness is generally 5 to 300 mu m. As such a separation membrane, for example, a sheet or a nonwoven fabric made of an olefin-based polymer such as polypropylene which is chemically resistant and hydrophobic, glass fiber, polyethylene or the like is used.

In some cases, a gel polymer electrolyte may be coated on the separation membrane to increase the stability of the cell. Representative examples of such gel polymers include polyethylene oxide, polyvinylidene fluoride, and polyacrylonitrile. When a solid electrolyte such as a polymer is used as an electrolyte, the solid electrolyte may also serve as a separation membrane.

The lithium salt-containing non-aqueous electrolyte is composed of an organic solvent electrolyte and a lithium salt. Examples of the electrolyte include N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydroxyfuran, tetrahydrofuran, tetrahydrofuran, tetrahydrofuran, tetrahydrofuran, Dimethylformamide, dioxolane, acetonitrile, nitromethane, formic acid, dimethylformamide, dimethylsulfoxide, dimethyl sulfoxide, 1,3-dioxolane, 4-methyl-1,3-dioxane, diethyl ether, And examples thereof include methyl, methyl acetate, phosphoric acid triester, trimethoxymethane, dioxolane derivatives, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, Ether, methyl pyrophosphate, propionic acid The aprotic organic solvent such as naphthyl can be used.

Examples of the organic solid electrolyte include a polymer electrolyte such as a polyethylene derivative, a polyethylene oxide derivative, a polypropylene oxide derivative, a phosphate ester polymer, an agitation lysine, a polyester sulfide, a polyvinyl alcohol, a polyvinylidene fluoride, A polymer containing an ionic dissociation group and the like may be used.

Examples of the inorganic solid electrolyte include Li ₃ N, LiI, Li ₅ NI ₂ , Li ₃ N-LiI-LiOH, LiSiO ₄ , LiSiO ₄ -LiI-LiOH, Li ₂ SiS ₃ , Li ₄ SiO ₄ , Nitrides, halides and sulfates of Li such as Li ₄ SiO ₄ -LiI-LiOH and Li ₃ PO ₄ -Li ₂ S-SiS ₂ can be used.

The lithium salt is a material that is readily soluble in the non-aqueous electrolyte, for example, LiCl, LiBr, LiI, LiClO 4, LiBF 4, LiB 10 Cl 10, LiPF 6, LiCF 3 SO 3, LiCF 3 CO 2, LiAsF _{6, LiSbF 6, LiAlCl 4,} CH 3 SO 3 Li, CF 3 SO 3 Li, LiSCN, LiC (CF 3 SO 2) 3, (CF 3 SO 2) 2 NLi, chloroborane lithium, lower aliphatic carboxylic acid lithium, 4-phenylborate, imide, and the like can be used.

For the purpose of improving the charge / discharge characteristics and the flame retardancy, the electrolytic solution is preferably mixed with an organic solvent such as pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylenediamine, glyme, Benzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N, N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, . In some cases, a halogen-containing solvent such as carbon tetrachloride or ethylene trifluoride may be further added to impart nonflammability, or a carbon dioxide gas may be further added to improve high-temperature storage characteristics.

In a preferred _{embodiment, LiPF 6, LiClO 4, LiBF} 4, LiN (SO 2 CF 3) 2 , such as a lithium salt, a highly dielectric solvent of DEC, DMC or EMC Fig solvent cyclic carbonate and a low viscosity of the EC or PC of And then adding it to a mixed solvent of linear carbonate to prepare a lithium salt-containing non-aqueous electrolyte.

The secondary battery according to the present invention can be used as a unit battery of a battery pack, which is a power source of a device requiring high temperature safety, long cycle characteristics and high rate characteristics.

Advantageously, the device can be a mobile electronic device, an electric vehicle (EV), a hybrid electric vehicle (HEV) or a plug-in hybrid electric vehicle (HEV) or a power storage device.

INDUSTRIAL APPLICABILITY As described above, the negative electrode of the present invention can exhibit excellent cycle characteristics, rate characteristics, and the like, and can suppress the peeling phenomenon due to the swelling of the electrolyte in the binder, in particular, the peeling phenomenon at high temperature.

Hereinafter, the present invention will be described in detail with reference to Examples and the like, but the scope of the present invention is not limited thereto.

[Example 1]

1-1. Preparation of first negative electrode material mixture slurry

96% by weight of graphite as a negative electrode active material and 4% by weight of PVdF as a binder were mixed and added to N-methyl-2-pyrrolidone (NMP) as a nonaqueous electrolyte solvent to prepare a negative electrode mixture slurry for producing a first negative electrode mixture layer.

1-2. Preparation of Second Anodic Admixture Slurry

98% by weight of graphite as an anode active material and 2% by weight of SBR as a binder were mixed to prepare a negative electrode mixture slurry for preparing a second negative electrode mix layer in a primary distilled water as a water-based electrolyte solvent.

1-3. Cathode manufacturing

The first anode mixture slurry was coated on a current collector of copper foil to a thickness of 20 탆 and dried. Then, the second anode mixture slurry was coated on the first anode mixture layer, dried, and rolled to obtain a cathode having a total thickness of 100 탆 .

[Comparative Example 1]

The same method as in Example 1 was used except that a negative electrode having a total thickness of 100 占 퐉 was produced using only the slurry using the water-based binder in Example 1.

[Experimental Example 1] Adhesive force comparative evaluation

According to the present invention, the following experiment was conducted to evaluate the adhesive strength in a negative electrode using a binder having a double coating structure.

After the negative electrode prepared in Example 1 and Comparative Example 1 was loaded in a solvent of DMC electrolyte solution, the initial adhesion was measured, and the electrode was stored in a 60 ° C chamber for 2 weeks and 4 weeks, and the adhesive strength of the changed electrode was measured again.

<Table 1>

As shown in Table 1, the electrode of Example 1 according to the present invention had almost the same adhesive force under the initial conditions as compared with the electrode of Comparative Example 1 comprising a single layer binder, Which is much better.

[Experimental Example 2] Comparative evaluation of battery characteristics

The negative electrode prepared in Example 1 and Comparative Example 1 was used as a positive electrode of lithium metal and 1M LiPF ₆ EC / EMC electrolytic solution, separator (Celgard ^TM ).

These batteries were charged and discharged under the condition of 0.1 C to measure the initial capacity, and the capacity after 50 cycles at a rate of 0.5 C was measured. The results are shown in Table 2 below.

<Table 2>

As shown in Table 2, the battery of Example 1 according to the present invention had no difference in initial capacity as compared with the electrode of Comparative Example 1 comprising a single layer binder. However, when the battery capacity after 50 cycles was compared, The cycle characteristics are generally excellent.

Those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

Claims (11)

A negative electrode for a secondary battery in which an electrode mixture containing a negative electrode active material and a binder is coated on a current collector,
A first negative electrode mixture layer coated on the current collector, the first negative electrode mixture layer including a first binder having an affinity for an organic solvent electrolyte lower than that of the second binder; And
A second negative electrode material mixture layer coated on the first negative electrode material mixture layer, wherein the second negative electrode material mixture layer includes a second binder having an affinity for an organic solvent electrolyte higher than that of the first binder;
/ RTI &gt;
The content of the first binder in the first negative electrode material mixture layer is 2 to 10% by weight based on the total weight of the first negative electrode material mixture layer, and the content of the second binder in the second negative electrode material mixture layer is the total weight of the second negative electrode material mixture layer And the thickness of the first negative electrode mixture layer is 10 to 40% of the thickness of the second negative electrode material mixture layer,
Wherein the first binder is a fluorine-based polymer and the second binder is an aqueous polymer. delete The negative electrode for a secondary battery according to claim 1, wherein the fluorine-based polymer is at least one selected from the group consisting of PVdF homopolymer, PVdF block copolymer, and PVdF graft copolymer. The water-based polymer according to claim 1, wherein the water-based polymer is at least one selected from the group consisting of acrylonitrile-butadiene rubber, styrene-butadiene rubber, acrylic resin, hydroxyethylcellulose, and carboxymethylcellulose Cathode for secondary battery. delete delete The negative electrode for a secondary battery according to claim 1, wherein the first negative electrode material mixture layer has a thickness of 10 to 40 탆. A method of manufacturing a negative electrode for a secondary battery according to claim 1,
(i) applying a slurry containing a first binder and a negative active material to a current collector, and drying the slurry to form a first negative electrode mixture layer;
(ii) applying a slurry containing a second binder and a negative electrode active material to the first negative electrode mixture layer and then drying to form a second negative electrode mixture layer; And
(iii) drying and rolling the first anode mixture layer and the second anode mixture layer;
Wherein the negative electrode comprises a positive electrode and a negative electrode. The negative electrode for a secondary battery according to claim 1, wherein the negative electrode active material comprises a carbon-based material. A lithium secondary battery comprising a negative electrode for a secondary battery according to any one of claims 1, 3, 4, and 7 to 9. A battery pack comprising a lithium secondary battery according to claim 10 as a unit battery.