KR20190060713A - Negative electrode for lithium secondary battery and preparing method therof - Google Patents

Abstract

The present invention relates to a negative electrode for a lithium secondary battery to provide excellent bonding force and mechanical properties, and a manufacturing method thereof. According to the present invention, the negative electrode comprises: a negative current collector; a first negative electrode layer formed on the negative current collector; and a second negative electrode layer formed on the first negative electrode layer. The first negative electrode layer comprises a graphite-based active material, a conductive material, and a first binder formed by polymerizing one or more kinds of binder formation monomers in the first negative electrode layer by an initiator. The second negative electrode layer comprises a silicon-based active material, a conductive material, and a second binder formed by polymerizing one or more kinds of binder formation monomers in the second negative electrode layer by an initiator. The first negative electrode layer includes 0.4 to 3.5 wt% of the first binder and the second negative electrode layer includes 2 to 25 wt% of the second binder.

Description

TECHNICAL FIELD [0001] The present invention relates to a negative electrode for a lithium secondary battery, and a method for manufacturing the negative electrode.

The present invention relates to a negative electrode for a lithium secondary battery and a method for producing the negative electrode, and more particularly to a negative electrode for a lithium secondary battery exhibiting excellent adhesion and mechanical properties even with a binder content equal to or less than that of the conventional one.

As technology development and demand for mobile devices have increased, there has been a rapid increase in demand for secondary batteries as energy sources. Among such secondary batteries, lithium secondary batteries, which exhibit high energy density and operating potential, have a long cycle life, Batteries have been commercialized and widely used.

The lithium secondary battery generally comprises a cathode including a cathode active material, a cathode including a cathode active material, a separator, and an electrolyte, and is charged and discharged by intercalation-decalation of lithium ions. The lithium secondary battery has a high energy density, a large electromotive force, and a high capacity, so it is applied to various fields.

On the other hand, a metal oxide such as LiCoO ₂ , LiMnO ₂ , LiMn ₂ O _4, or LiCrO ₂ is used as a positive electrode active material constituting the positive electrode of the lithium secondary battery. Examples of the negative electrode active material constituting the negative electrode include metal lithium, a carbon based meterial such as graphite or activated carbon or a material such as silicon oxide (SiO _x ) is used. Among the above-mentioned negative electrode active materials, metal lithium is mainly used. However, as charging and discharging cycles are progressed, lithium atoms are grown on the surface of metal lithium, thereby damaging the separator and damaging the battery. Recently, carbonaceous materials are mainly used. However, the carbon-based material has a disadvantage in that the theoretical capacity is only about 400 mAh / g and the capacity is small.

Accordingly, various studies have been made to replace the carbonaceous material with silicon (Si) having a high theoretical capacity (4,200 mAh / g) as the negative electrode active material. The reaction formula when lithium is inserted into silicon is as follows:

[Reaction Scheme 1]

₂₂ Li + ₅ Si = Li ₂₂ Si ₅

 However, most of the silicon cathode materials are disadvantageous in that the silicon volume is expanded up to 300% by lithium insertion, and the negative electrode is destroyed thereby failing to exhibit high cycle characteristics. In addition, in the case of silicon, as the cycle continues, the lithium insertion causes a volume expansion and causes contact losses with pulverization, conducting agents, and current collectors, and unstable solid - can exhibit a fading mechanism such as solid-electrolyte-interphase (SEI) formation.

In order to solve such a problem, a method of increasing the content of the binder in the anode can be considered. However, when the content of the binder is increased, there is a problem that the electric capacity becomes small when the negative electrode of the same volume is formed.

Therefore, there is a need for a technique capable of improving the mechanical properties of the negative electrode even with an equivalent binder content.

A problem to be solved by the present invention is to provide a negative electrode for a lithium secondary battery which exhibits excellent adhesion and mechanical properties even when the binder content is equal to or less than that of the prior art.

Another object of the present invention is to provide a method for producing the negative electrode for a lithium secondary battery.

Another object of the present invention is to provide a lithium secondary battery including the negative electrode.

In order to solve the above problems, the present invention provides a negative electrode collector, A first cathode layer formed on the anode current collector; And a second cathode layer formed on the first cathode layer, wherein the first cathode layer comprises a graphite-based active material; Conductive material; And at least one binder-forming monomer comprising a first binder polymerized by an initiator in the first negative electrode layer, the second negative electrode layer comprising a silicon-based active material; Conductive material; And at least one binder-forming monomer comprises a second binder polymerized by an initiator in the second negative electrode layer, wherein the first negative electrode layer comprises 0.4 wt% to 3.5 wt% of the first binder, And the second negative electrode layer comprises 2 wt% to 25 wt% of the second binder.

Further, in order to solve the above-mentioned problems, the present invention also provides a positive electrode active material composition comprising (A) a graphite-based active material on an anode current collector; Conductive material; At least one binder-forming monomer; Applying a first negative electrode slurry comprising an initiator and an initiator; (B) a silicon-based active material on the applied first negative electrode slurry; Conductive material; At least one binder-forming monomer; And a second negative electrode slurry comprising an initiator; And (c) allowing the resultant of step (B) to stand at a temperature above the start temperature to cause the binder-forming monomers contained in the first negative electrode slurry and the second negative electrode slurry to polymerize through the initiator to form first binder and second Wherein the first negative electrode slurry comprises 0.4 wt% to 3.5 wt% of the first binder on a solids content basis and the second negative electrode layer comprises 2 wt% of the second binder on a solids content basis, By weight to 25% by weight, based on the total weight of the negative electrode for a lithium secondary battery.

The negative electrode for a lithium secondary battery of the present invention can be produced by forming a negative electrode layer through a negative electrode slurry containing a binder forming monomer and an initiator in the production of a negative electrode and then converting the binder forming monomer into a binder, But it can exhibit excellent mechanical properties.

Hereinafter, the present invention will be described in detail in order to facilitate understanding of the present invention.

The terms and words used in the present specification and claims should not be construed as limited to ordinary or dictionary terms and the inventor may appropriately define the concept of the term in order to best describe its invention It should be construed as meaning and concept consistent with the technical idea of the present invention.

A negative electrode for a lithium secondary battery according to the present invention comprises a negative electrode collector; A first cathode layer formed on the anode current collector; And a second negative electrode layer formed on the first negative electrode layer.

Wherein the first negative electrode layer and the second negative electrode layer contain different active materials, and the first negative electrode layer comprises a graphite-based active material; Conductive material; And at least one binder-forming monomer comprises a first binder polymerized by an initiator in the first negative electrode layer, the second negative electrode layer comprising a silicon-based active material; Conductive material; And at least one binder-forming monomer is polymerized by an initiator in the second negative electrode layer, wherein the first negative electrode layer comprises 0.4 wt% to 3.5 wt% of the first binder, And the second cathode layer comprises 2 wt% to 25 wt% of the second binder.

The negative electrode for a lithium secondary battery according to the present invention is characterized in that a first negative electrode layer containing a graphite-based active material having a relatively small volume expansion is located between a negative electrode contact and a second negative electrode layer containing a silicon-based active material having a relatively large volume expansion, The layer can be peeled off from the negative electrode current collector due to the volume change, so that it can have a high electric capacity due to the silicon based active material while having excellent mechanical properties.

The negative electrode for a lithium secondary battery of the present invention is characterized in that the first negative electrode layer and the second negative electrode layer each include a binder in which one or more binder forming monomers are polymerized by the initiator in the negative electrode layer, It can be polymerized by crosslinking with an initiator evenly distributed in the negative electrode layer to exhibit a stronger adhesive force and exhibit a better adhesive force even when the binder content is comparable to that of a conventional negative electrode and have excellent mechanical properties .

The graphite active material contained in the first negative electrode layer may be selected from the group consisting of natural graphite, artificial graphite, kish graphite, pyrolytic carbon, mesophase pitch based carbon fiber, carbon microbeads, mesophase pitches, petroleum coke, and coal-based coke, and specifically may be at least one selected from the group consisting of natural graphite and artificial graphite.

The silicon-based active material contained in the second negative electrode layer may include at least one selected from the group consisting of Si, silicon oxide particles (SiO _x , 0 <x? 2), Si-metal alloy, Specifically, silicon oxide particles (SiO _x , 0 < x? 2).

The silicon-based active material may include a carbon coating layer formed on the outer surface of the silicon-based active material.

The carbon coating layer may be formed on the silicon-based active material to prevent or alleviate the pulverization of the silicon-based active material when the volume of the silicon-based active material changes due to insertion and desorption of lithium in the silicon-based active material, The side reaction between the silicon-based active material and the electrolyte can be effectively prevented or mitigated. In addition, the silicon-based active material can have an excellent conductivity and can easily react with lithium.

The carbon coating layer may be formed of amorphous carbon, natural graphite, artificial graphite, activated carbon, MCMB (MesoCarbon MicroBead), carbon fiber, and coal tar pitch, petroleum pitch, and organic and carbon-based materials prepared by heat-treating the material with a heat treatment. Examples of the material include at least one selected from the group consisting of amorphous carbon, natural graphite, artificial graphite, pitch, and activated carbon. can do.

The first cathode layer and the second cathode layer may have a weight ratio of 98: 2 to 20:80, and more specifically, a weight ratio of 98: 2 to 50:50, and more specifically 95: 5 to 80:20 .

When the first negative electrode layer and the second negative electrode layer have the weight ratio range, the first negative electrode layer can exert an excellent adhesive force with the negative electrode current collector while appropriately absorbing the stress corresponding to the volume change of the second negative electrode layer , The second cathode layer can exhibit an appropriate capacity increasing effect.

The binder-forming monomer contained in the first negative electrode layer and the binder-forming monomer contained in the second negative electrode layer are not particularly limited as long as it is a monomer capable of undergoing a crosslinking reaction by heat and capable of producing a binder for a negative electrode of a lithium secondary battery .

The binder-forming monomer may be, for example, (A) a conjugated diene monomer or a conjugated diene polymer, (B) a (meth) acrylic acid ester monomer, (C) a vinyl monomer, (meth) acrylamide monomer and a nitrile monomer (D) at least one monomer selected from the group consisting of unsaturated carboxylic acid monomers, and more specifically, it can be easily dissolved in a solvent capable of forming an electrode, Unsaturated carboxylic acid-based monomer may be included in that the crosslinking reaction can easily occur.

The content of the binder contained in the first negative electrode layer and the second negative electrode layer may be different from each other.

Since the first negative electrode layer includes a graphite-based active material having a relatively small volume change, it may include a relatively small amount of binder, and the second negative electrode layer includes a silicon-based active material having a relatively large volume change, And may include a large amount of binder. However, since the crosslinking reaction may occur at the interface between the first cathode layer and the second cathode layer at the time of drying the first cathode layer and the second cathode layer, the binder content of the first cathode layer may vary depending on the interface with the second cathode layer In order to impart sufficient adhesive force to the adhesive layer.

Specifically, the first cathode layer contains 0.4 wt% to 3.5 wt%, specifically 0.4 wt% to 3 wt%, more specifically 0.5 wt% to 2.5 wt% of the first binder with respect to the first cathode layer And the second cathode layer comprises 2% by weight to 25% by weight, specifically 2.5% by weight to 22% by weight, more specifically 3% by weight to 20% by weight, of the second binder with respect to the second cathode layer, .

When the first cathode layer and the second cathode layer each include the first binder and the second binder within the above range, each of the cathode layers can exhibit an appropriate degree of mechanical strength. In particular, The amount of the binder contained in the entire negative electrode can be adjusted to the optimum range.

Examples of the initiator include benzoyl peroxide, acetyl peroxide, dilauryl peroxide, di-tert-butyl peroxide, t- Organic peroxides such as t-butyl peroxy-2-ethyl-hexanoate, cumyl hydroperoxide and hydrogen peroxide, and hydroperoxides such as hydrogen peroxide Azobis (2-cyanobutane), 2,2'-azobis (methylbutyronitrile), AIBN (2,2'-Azobis (iso-butyronitrile)) and AMVN '-Azobisdimethyl-Valeronitrile), and the like, but the present invention is not limited thereto.

The initiator may be included in the first binder or the second binder in an amount of 0.01 wt% to 10 wt% relative to 100 wt% of the total weight of one or more binder-forming monomers, specifically 0.2 wt% to 8 wt% By weight, and more specifically from 2% by weight to 5% by weight. If the amount of the initiator exceeds the above range, curing may start before or during the application of the slurry, or an excessive amount of the unreacted initiator may remain and adversely affect the performance of the battery later. If the amount of the initiator is less than the above range, The bonding force may not be exerted. Therefore, when the initiator is included in the above range, the first binder and the second binder can more effectively provide bonding force.

On the other hand, the first cathode layer may further include an inorganic oxide.

Wherein the inorganic oxide is selected from the group consisting of Al ₂ O ₃ , BaTiO ₃ , CaO, CeO ₂ , NiO, MgO, SiO ₂ , SnO ₂ , SrTiO ₃ , TiO ₂ , Y ₂ O ₃ , ZnO, ZrO ₂ , Pb (Zr, _{3 (PZT), Pb 1 -} x La x Zr 1 - y Ti y O 3 (PLZT), PB (Mg 3 Nb 2/3) O 3 -PbTiO 3 (PMN-PT) and hafnia (HfO ₂₎ as And at least one selected from the group consisting of

 When the first negative electrode layer further includes an inorganic oxide, the first negative electrode layer may exhibit better thermal stability.

The negative electrode collector may have a thickness of 3 [mu] m to 500 [mu] m. The negative electrode current collector is not particularly limited as long as it has electrical conductivity without causing a chemical change in the battery. The negative electrode current collector may be formed on the surface of copper, gold, stainless steel, aluminum, nickel, titanium, sintered carbon, Carbon, nickel, titanium, silver or the like, an aluminum-cadmium alloy, or the like can be used. In addition, fine unevenness can be formed on the surface to enhance the bonding force of the negative electrode active material, and it can 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 conductive material is not particularly limited as long as it has electrical conductivity without causing chemical changes in the battery, and examples of the conductive material include graphite such as natural graphite and artificial graphite; Carbon black such as 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 whiskers such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; And conductive materials such as polyphenylene derivatives.

The negative electrode for a lithium secondary battery according to an example of the present invention comprises (A) a graphite active material on an anode current collector; Conductive material; At least one binder-forming monomer; Applying a first negative electrode slurry comprising an initiator and an initiator; (B) a silicon-based active material on the applied first negative electrode slurry; Conductive material; At least one binder-forming monomer; And a second negative electrode slurry comprising an initiator; And (c) allowing the resultant of step (B) to stand at a temperature above the start temperature to cause the binder-forming monomers contained in the first negative electrode slurry and the second negative electrode slurry to polymerize through the initiator to form first binder and second Wherein the first negative electrode slurry comprises 0.4 wt% to 3.5 wt% of the first binder on a solids content basis and the second negative electrode layer comprises 2 wt% of the second binder on a solids content basis, By weight to 25% by weight, based on the total weight of the composition.

Wherein the first binder is a binder in which one or more binder-forming monomers contained in the first negative electrode slurry are polymerized by an initiator, and the second binder is a mixture of one or more binder-forming monomers contained in the second negative electrode slurry &Lt; / RTI &gt; Specifically, the first negative electrode slurry may contain 0.4 wt% to 3.5 wt%, specifically 0.4 wt% to 3 wt%, more specifically 0.5 wt% to 2.5 wt%, based on the solids content of the first binder, The second negative electrode slurry may contain 2 wt% to 25 wt%, specifically 2.5 wt% to 22 wt%, more specifically 3 wt% to 20 wt% of the second binder with respect to the second negative electrode layer. The &quot; solids content &quot; represents the total content of solids such as the remaining active material, binder-forming monomer and initiator except for the solvent contained in the first negative electrode slurry or the second negative electrode slurry.

The first negative electrode slurry and the second negative electrode slurry included in the negative electrode for a lithium secondary battery according to an exemplary embodiment of the present invention each include one or more binder-forming monomers; And an initiator, and can be contained as a binder in the negative electrode by polymerizing the binder-forming monomer through an initiator. Accordingly, in the negative electrode for a lithium secondary battery according to an exemplary embodiment of the present invention, the first negative electrode layer and the second negative electrode each have one or more binder-forming monomers in the negative electrode layer, The binder-forming monomer is polymerized by crosslinking by an initiator in a state where the binder-forming monomer is uniformly distributed in the negative electrode layer, and thus a stronger adhesive force can be exhibited. Accordingly, even when the binder- It is possible to exhibit a superior adhesive force and to have excellent mechanical properties.

In the method for manufacturing a negative electrode for a lithium secondary battery according to an example of the present invention, the electrode for a lithium secondary battery sequentially forms the first negative electrode layer on an anode current collector, and forms a second negative electrode layer on the first negative electrode layer . &Lt; / RTI &gt;

First, in step (A), a graphite-based active material is coated on a negative electrode collector; Conductive material; At least one binder-forming monomer; And an initiator is applied to the first negative electrode slurry.

The first negative electrode slurry may further include a graphite active material; Conductive material; At least one binder-forming monomer; And a solvent for dispersing the initiator.

In the step (B), on the applied first negative electrode slurry, a silicon-based active material; Conductive material; At least one binder-forming monomer; And a second negative electrode slurry containing an initiator.

The application of the second negative electrode slurry may be carried out directly after the application of the first negative electrode slurry without the drying step for the first negative electrode slurry. When the application of the second negative electrode slurry is performed immediately after the application of the first negative electrode slurry, the composition of the first negative electrode layer at the interface between the first negative electrode layer and the second negative electrode layer in the negative electrode for a lithium secondary battery, There may be a region where some of the components of the second cathode layer are mixed. When crosslinking is performed in this state, a polymerization reaction occurs together at the interface between the first cathode layer and the second cathode layer and the first cathode layer and the second cathode layer Can be improved.

Wherein the second negative electrode slurry comprises a silicon-based active material; Conductive material; At least one binder-forming monomer; And a solvent for dispersing the initiator.

In the step (C), the result of the step (B) is left in the starting temperature range so that the binder-forming monomers contained in the first negative electrode slurry and the second negative electrode slurry are polymerized through the initiator.

The binder-forming monomers respectively included in the first negative electrode slurry and the second negative electrode slurry are crosslinked and initiated to polymerize through the included initiator, thereby forming a binder and forming a first negative electrode layer and a second negative electrode layer, respectively.

The first negative electrode slurry and the second negative electrode slurry may contain a solvent and the solvent removal process may be performed separately, but the result of step (B) It can be done together during the neglecting process.

Through such a process, a negative electrode for a lithium secondary battery in which a first negative electrode layer and a second negative electrode layer are stacked on an anode current collector can be formed.

The initiation temperature range for allowing the binder-forming monomer to effect cross-linking polymerization may be 40 占 폚 to 180 占 폚, specifically 50 占 폚 to 90 占 폚. Crosslinking polymerization of the binder-forming monomer is carried out in the above temperature range, and drying of a solvent which can be contained in the first negative electrode slurry and the second negative electrode slurry can be effectively performed.

The conductive material may be included in the first negative electrode slurry and the second negative electrode slurry in an amount of 1 wt% to 15 wt% based on the total weight of the first negative electrode slurry or the second negative electrode slurry.

The present invention also provides a lithium secondary battery including the negative electrode for a lithium secondary battery. The lithium secondary battery may include a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode.

The anode may be prepared by a conventional method known in the art. For example, a slurry may be prepared by mixing and stirring a solvent, a binder, a conductive material, and a dispersant as necessary in a cathode active material, and then coating (coating) the mixture on a current collector of a metal material, have.

The current collector of the metal material is a metal having high conductivity and is a metal which can easily adhere to the slurry of the cathode active material and is not particularly limited as long as it has high conductivity without causing chemical change in the battery in the voltage range of the battery But not limited to, stainless steel, aluminum, nickel, titanium, sintered carbon, or aluminum or stainless steel surface treated with carbon, nickel, titanium, silver, or the like. In addition, fine unevenness may be formed on the surface of the current collector to increase the adhesive force of the positive electrode active material. The current collector 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, and may have a thickness of 3 to 500 μm.

The cathode active material is selected from the group consisting of lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ), Li [Ni _x Co _y Mn _z Mv] O ₂ (Li _a M _ba (where x < = x &lt; 1.0, 0 y, z 0,5, 0 v 0,1 and x + y + z + v = 1) _{-b 'M' b ') O} 2 - c a c ( wherein, 0≤a≤0.2, 0.6≤b≤1, 0≤b'≤0.2, 0≤c≤0.2 and; M is Mn and, Ni M, at least one selected from the group consisting of Al, Mg and B, A is at least one selected from the group consisting of P, F, S, and N), or a compound substituted with one or more transition metals; Lithium manganese oxides such as Li _{1 + y} Mn _{2 - y} O ₄ (where y is 0 to 0.33), LiMnO ₃ , LiMn ₂ O ₃ , LiMnO ₂ and the like; Lithium copper oxide (Li ₂ CuO ₂ ); Vanadium oxides such as LiV ₃ O ₈ , LiFe ₃ O ₄ , V ₂ O ₅ and Cu ₂ V ₂ O ₇ ; Formula _{LiNi 1 - y M y O 2} ( where, the M = Co, Mn, Al, Cu, Fe, Mg, B or Ga, y is from 0.01 to 0.3) Ni site type lithium nickel oxide which is represented by; Formula _{LiMn 2-y M y O 2} ( where, M = Co, Ni, Fe , Cr, and Zn, or Ta, y = 0.01 to 0.1 Im) or Li ₂ Mn ₃ MO ₈ (where, M = Fe, Co, Ni, Cu, or Zn); LiMn ₂ O _{4 in} which a part of Li in the formula is substituted with an alkaline earth metal ion; Disulfide compounds; Fe ₂ (MoO ₄ ) ₃ , and the like. However, the present invention is not limited to these.

Examples of the solvent for forming the positive electrode include organic solvents such as NMP (N-methylpyrrolidone), DMF (dimethylformamide), acetone, and dimethylacetamide, and water. These solvents may be used alone or in combination of two or more Can be mixed and used. The amount of the solvent used is sufficient to dissolve and disperse the cathode active material, the binder and the conductive material in consideration of the coating thickness of the slurry and the production yield.

Examples of the binder include polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethylmethacrylate, Polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene-propylene-diene monomer (EPDM) Sulfonated EPDM, styrene butadiene rubber (SBR), fluorine rubber, poly acrylic acid, and polymers in which hydrogen is substituted with Li, Na, or Ca, or Various kinds of binder polymers such as various copolymers can be used.

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 acetylene black, Ketjen black, channel black, panes black, lamp black, and thermal black; Conductive fibers such as carbon fiber and metal fiber; Conductive tubes such as carbon nanotubes; Metal powders such as fluorocarbon, aluminum and nickel powder; Conductive whiskers 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 conductive material may be used in an amount of 1 wt% to 20 wt% based on the total weight of the positive electrode slurry.

The dispersing agent may be an aqueous dispersing agent or an organic dispersing agent such as N-methyl-2-pyrrolidone.

As the separator, a conventional porous polymer film conventionally used as a separator, such as a polyolefin-based polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene-butene copolymer, an ethylene-hexene copolymer and an ethylene-methacrylate copolymer Porous nonwoven fabric such as glass fiber of high melting point, polyethylene terephthalate fiber or the like may be used, but the present invention is not limited thereto.

The lithium salt that can be used as the electrolyte used in the present invention may be any of those commonly used in electrolytes for lithium secondary batteries, and examples thereof include F ^- , Cl ^- , Br ^- , I ^- , NO _{3 ^{-, N (CN) 2 -}} , BF 4 -, ClO 4 -, PF 6 -, (CF 3) 2 PF 4 -, (CF 3) 3 PF 3 -, (CF 3) 4 PF 2 -, (CF ^{_{3) 5 PF -, (CF}} 3) 6 P -, CF 3 SO 3 -, CF 3 CF 2 SO 3 -, (CF 3 SO 2) 2 N -, (FSO 2) 2 N -, CF 3 CF 2 ^{_{(CF 3) 2 CO -,}} (CF 3 SO 2) 2 CH -, (SF 5) 3 C -, (CF 3 SO 2) 3 C -, CF 3 (CF 2) 7 SO 3 -, CF 3 CO ₂ ^- , CH ₃ CO ₂ ^- , SCN ^-, and (CF ₃ CF ₂ SO ₂ ) ₂ N ^- .

Examples of the electrolyte used in the present invention include an organic-based liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel-type polymer electrolyte, a solid inorganic electrolyte, and a molten inorganic electrolyte that can be used in the production of a lithium secondary battery. no.

The external shape of the lithium secondary battery of the present invention is not particularly limited, but may be a cylindrical shape, a square shape, a pouch shape, a coin shape, or the like using a can.

The lithium secondary battery according to the present invention can be used not only in a battery cell used as a power source of a small device but also as a unit cell in a middle or large battery module including a plurality of battery cells.

Preferable examples of the above medium and large-sized devices include, but are not limited to, electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and electric power storage systems.

Example

Hereinafter, the present invention will be described in more detail with reference to Examples and Experimental Examples, but the present invention is not limited by these Examples and Experimental Examples. The embodiments according to the present invention can be modified into various other forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. The embodiments of the present invention are provided to enable those skilled in the art to more fully understand the present invention.

Example 1

97.6% by weight of natural graphite having an average particle diameter (D ₅₀ ) of 15 μm, 1% by weight of carbon black as a conductive material, and 1.4% by weight of a first binder were added to water to prepare a first negative electrode slurry. In the first binder, the amount of the potassium phosphate initiator was 4% by weight based on 100% by weight of the acrylic acid monomer.

A second negative electrode slurry was prepared by adding 85% by weight of SiO 2 coated with 5% by weight of carbon, 10% by weight of carbon black as a conductive material and 5% by weight of a second binder in water with an average particle diameter (D ₅₀ ) of 5 μm . In the second binder, the amount of the potassium phosphate initiator was 4% by weight based on 100% by weight of the acrylic acid monomer.

The first negative electrode slurry was coated on one surface of the copper collector so that the weight ratio of the first negative electrode layer and the second negative electrode layer was 9: 1, and then the second negative electrode slurry was coated without drying operation. Dried and crosslinked in an oven at 80 캜, and rolled to a porosity of 40% to prepare a negative electrode.

Example 2

In the case of the first negative electrode layer, the first negative electrode slurry was prepared in the same manner as in Example 1 except that the ratio of the active material, the conductive material, and the binder was changed. And 87 wt% of active material, 10 wt% of conductive material, and 3 wt% of binder in the case of the second negative electrode layer to prepare a second negative electrode slurry. In the first and second binders, the potassium phosphate initiator was equal to 4% by weight based on 100% by weight of the acrylic acid monomer. The weight ratio between the first negative electrode layer and the second negative electrode layer and the electrode manufacturing process are the same as in the first embodiment.

Example 3

In Example 2, an electrode was manufactured in the same manner, except that the active material and the binder ratio of the second negative electrode layer were 70 wt% and 20 wt%, respectively.

Example 4

In the case of the first negative electrode layer, the first negative electrode slurry was composed of 98.5 wt% of the active material, 1 wt% of the conductive material, and 0.5 wt% of the binder. In the case of the second negative electrode layer, the second negative electrode slurry was prepared in the same manner as in Example 1 except for the ratio of the active material, the conductive material and the binder. In the first and second binders, the potassium phosphate initiator was equal to 4% by weight based on 100% by weight of the acrylic acid monomer. The weight ratio between the first negative electrode layer and the second negative electrode layer and the electrode manufacturing process are the same as in the first embodiment.

Example 5

An electrode was prepared in the same manner as in Example 4, except that the active material and the binder ratio of the first negative electrode layer were changed to 96.5% by weight and 2.5% by weight, respectively.

Comparative Example 1

An electrode was prepared in the same manner as in Example 2 except that the active material and the binder ratio of the second negative electrode layer were changed to 89 wt% and 1 wt%, respectively.

Comparative Example 2

An electrode was prepared in the same manner as in Example 2 except that the active material and the binder ratio of the second negative electrode layer were changed to 60 wt% and 30 wt%, respectively.

Comparative Example 3

In Example 4, an electrode was manufactured in the same manner, except that the active material and the binder ratio of the first negative electrode layer were changed to 98.8 wt% and 0.2 wt%, respectively.

Comparative Example 4

In Example 4, an electrode was manufactured in the same manner, except that the active material and the binder ratio of the first negative electrode layer were changed to 94 wt% and 5 wt%, respectively

Comparative Example 5

91.2% by weight of natural graphite having an average particle diameter (D ₅₀ ) of 15 탆 and 96.3% by weight of a mixture of 5.8% by weight of SiO and an average particle diameter (D ₅₀ ) of 5 탆 coated with 5% by weight of carbon, By weight and 1.8% by weight of a binder were added to water to prepare an anode slurry. In the binder, the amount of the potassium phosphate initiator was 4% by weight based on 100% by weight of the acrylic acid monomer.

The prepared negative electrode slurry was coated on one side of the copper collector, dried and crosslinked in an oven at 80 ° C, and rolled to a porosity of 40% to prepare a negative electrode.

Comparative Example 6

91.2% by weight of natural graphite having an average particle size (D ₅₀ ) of 15 μm, an average particle size (D ₅₀ ) of 5% by weight of carbon coated, and 93.1% by weight of a mixture of 8.8% by weight of SiO 5 of 5 μm, , And 5 wt% of a binder prepared by mixing styrene butadiene rubber (SBR) and carboxymethyl cellulose (CMC) at a weight ratio of 3.5: 1.5 were added to water to prepare an anode slurry.

The prepared negative electrode slurry was coated on one surface of the copper collector, dried in an oven at 80 ° C, and rolled to a porosity of 40% to prepare a negative electrode.

Experimental Example 1: Evaluation of Adhesion

Each of the electrodes prepared in Examples 1 to 5 and Comparative Examples 1 to 6 was taped at 1 cm * 10 cm, fixed on a slide glass with a double-sided tape, and 180 degrees peel strength was continuously measured 5 times using a tensile strength meter And the average value was calculated. Then, when the average adhesive strength of Comparative Example 6 was taken as 100%, the relative adhesive strength was calculated and is shown in Table 1.

Experimental Example 2: Evaluation of Capacity Retention Ratio and Electrode Thickness Change Rate

After the polyolefin separator was interposed between the negative electrode and the Li metal prepared in each of Examples 1 to 5 and Comparative Examples 1 to 6, ethylene carbonate (EC) and diethyl carbonate (EMC) were mixed at a volume ratio of 30:70 A coin type half cell was prepared by injecting an electrolyte in which 1 M LiPF ₆ was dissolved in a solvent.

The battery thus prepared was subjected to charging and discharging to evaluate the capacity retention rate and the rate of change in electrode thickness. The results are shown in Table 1 below.

Specifically, each of the batteries prepared above was charged at a constant current (CC) of 0.1 C at 25 ° C until the voltage reached 5 mV, and then charged at a constant voltage (CV) to obtain a charge current of 0.005 C The first charging was performed. After that, it was left for 20 minutes and then discharged at a constant current (CC) of 0.1 C until it reached 1.5 V. [ Thereafter, charging and discharging were repeated at 0.5 C up to 50 cycles to evaluate the capacity retention rate. After the completion of the cycle test, 51 cycles were finished in the charged state, and the cell was disassembled to measure the thickness, and then the electrode thickness change rate was calculated.

First negative electrode layer binder content
(weight%) Secondary cathode layer binder content
(weight%) Relative adhesion
(%) Capacity retention rate
(%) Thickness increase rate
(%) Example 1 1.4 5 187 65 41 Example 2 1.4 3 175 60 40 Example 3 1.4 20 220 62 38 Example 4 0.5 5 168 58 43 Example 5 2.5 5 192 57 40 Comparative Example 1 1.4 One 172 51 48 Comparative Example 2 1.4 30 251 54 44 Comparative Example 3 0.2 5 162 52 47 Comparative Example 4 5 5 263 50 44 Comparative Example 5 1.8 - 153 47 52 Comparative Example 6 5 - 100 45 55

Referring to Table 1, in Examples 1 to 5 and Comparative Examples 1 to 4, the adhesive strength tends to increase as the overall content of the binder increases. However, when the binder content is high, the structural stability of the electrode is improved because the adhesion strength of the electrode is increased. However, as the ratio of the active material decreases, the capacity exhibited during charging / discharging decreases and the resistance to insertion and desorption of lithium increases The ratio of the binder should be appropriately configured. In Comparative Example 5, when compared with Comparative Example 6, the binder content was low but showed relatively high adhesive strength, and it was evaluated to be an effect of binding the anode active material layer through the chemical crosslinking reaction as a whole. In the case of Comparative Example 5 and Example 1, the binder content of the entire electrode was the same, but in the case of Example 1, the electrode was made to have a high binder content in the second negative electrode layer, thereby improving the electrode adhesion.

In Examples 1 to 5, the binder ratio of each of the first cathode layer and the second cathode layer was appropriately set, whereby it was confirmed that the electrode thickness was increased and the lifetime characteristics were improved while having an appropriate electrode adhesion. In Comparative Examples 1 to 4, when the binder ratio of the first negative electrode layer or the second negative electrode layer was lower than that of Examples 1 to 5, the lifetime characteristics were comparatively poor and the adhesive strength of the electrode was relatively low, . In addition, when the binder ratio of the first negative electrode layer or the second negative electrode layer is relatively high, the adhesion of the electrode is excellent, but the resistance of the electrode is largely affected and the lifetime characteristics are poor.

On the other hand, when Comparative Examples 5 and 6 were compared, the capacity retention rate of Comparative Example 5 was high and the rate of electrode thickness change was low. In Comparative Example 5, the binder content was small compared to Comparative Example 6, The electrode thickness change rate is low and the secondary battery performance is considered to be superior. Comparing Example 1 with Comparative Example 5, the capacity retention rate of Example 1 was high and the rate of electrode thickness change was low. The overall binder content of the electrode is the same, but since the second negative electrode layer contains a relatively large amount of binder in the case of Example 1, the change in the thickness of the electrode is effectively suppressed by effectively suppressing the volume change of the silicon oxide, Could.

Claims (13)

Cathode collector; A first cathode layer formed on the anode current collector; And a second cathode layer formed on the first cathode layer,
Wherein the first negative electrode layer comprises a graphite-based active material; Conductive material; And at least one binder-forming monomer comprising a first binder polymerized by an initiator in the first cathode layer,
Wherein the second negative electrode layer comprises a silicon-based active material; Conductive material; And at least one binder-forming monomer comprises a second binder polymerized by an initiator in the second cathode layer,
Wherein the first negative electrode layer comprises 0.4 wt% to 3.5 wt% of the first binder, and the second negative electrode layer comprises 2 wt% to 25 wt% of the second binder.
The method according to claim 1,
The graphite-based active material contained in the first negative electrode layer may be selected from the group consisting of natural graphite, kish graphite, pyrolytic carbon, mesophase pitch based carbon fiber, meso-carbon microbeads ), Liquid crystal pitches (mesophase pitches), petroleum cokes, and coal-based cokes.
The method according to claim 1,
Wherein the silicon-based active material contained in the second negative electrode layer comprises a mixture of at least one selected from the group consisting of Si, silicon oxide particles (SiO _x , 0 <x? 2), Si-metal alloy, Negative electrode for lithium secondary battery.
The method according to claim 1,
Wherein the silicon-based active material contained in the second negative electrode layer comprises a carbon coating layer formed on a surface thereof.
The method according to claim 1,
Wherein the first negative electrode layer and the second negative electrode layer have a thickness ratio of 10:90 to 80:20.
The method according to claim 1,
The binder-forming monomer may be (a) a conjugated diene-based monomer or conjugated diene-based polymer; (B) a (meth) acrylic acid ester-based monomer; (C) vinyl monomers; At least one monomer selected from the group consisting of (meth) acrylamide-based monomers and nitrile-based monomers; And (d) an unsaturated carboxylic acid-based monomer.
The method according to claim 1,
Wherein the first negative electrode layer comprises 0.5 wt% to 2.5 wt% of the first binder.
The method according to claim 1,
And the second negative electrode layer comprises 3 wt% to 20 wt% of the second binder.
The method according to claim 1,
Wherein the first negative electrode layer further comprises an inorganic oxide.
10. The method of claim 9,
Wherein the inorganic oxide is selected from the group consisting of Al ₂ O ₃ , BaTiO ₃ , CaO, CeO ₂ , NiO, MgO, SiO ₂ , SnO ₂ , SrTiO ₃ , TiO ₂ , Y ₂ O ₃ , ZnO, ZrO ₂ , Pb (Zr, _{3 (PZT), Pb 1 -} x La x Zr 1 - y Ti y O 3 (PLZT), PB (Mg 3 Nb 2/3) O 3 -PbTiO 3 (PMN-PT) and hafnia (HfO ₂₎ as And at least one selected from the group consisting of lithium,
(A) a graphite-based active material on an anode current collector; Conductive material; At least one binder-forming monomer; Applying a first negative electrode slurry comprising an initiator and an initiator;
(B) a silicon-based active material on the applied first negative electrode slurry; Conductive material; At least one binder-forming monomer; And a second negative electrode slurry comprising an initiator; And
(c) leaving the result of step (B) at a temperature above the start temperature to cause the binder-forming monomers contained in the first negative electrode slurry and the second negative electrode slurry to polymerize through the initiator to form a first binder and a second binder The method comprising:
Wherein the first negative electrode slurry comprises 0.4 wt% to 3.5 wt% of the first binder on a solids content basis and the second negative electrode layer comprises 2 wt% to 25 wt% of the second binder on a solids content basis, A method for manufacturing a negative electrode for a lithium secondary battery according to claim 1.
12. The method of claim 11,
Wherein the temperature above the initiation temperature is 40 占 폚 to 180 占 폚.
A lithium secondary battery comprising the negative electrode for a lithium secondary battery according to claim 1.