KR20170135425A - Electrode for lithium secondary battery and lithium secondary battery comprising the same - Google Patents

Abstract

The present invention relates to an electrode for a lithium secondary battery and a lithium secondary battery including the same. The electrode for a lithium secondary battery includes: an electrode current collector body; a first electrode active material layer formed on the electrode current collector body; and a second electrode active material layer formed on the first electrode active material layer while including a silicon-based (Si) active material. When a lithium secondary battery including the electrode is manufactured to execute a charge-discharge cycle, the first electrode active material layer has a smaller expansion rate of a direction toward a surface than the second electrode active material layer. According to the present invention, the electrode for a lithium secondary battery includes: the first electrode active material layer with a small expansion rate in a direction toward a surface on the electrode current collector body; and a second electrode active material layer including a transition metal-based active material on the first electrode active material layer. The first electrode active material layer can disperse and absorb the stress due to a change in the volume of the transition metal-based active material included in the second electrode active material layer when charging and discharging the lithium secondary battery, thereby reducing the desorption of the electrode active material layers while being used in the production of a high-capacity lithium secondary battery.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrode for a lithium secondary battery and a lithium secondary battery including the electrode,

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrode for a lithium secondary battery and a lithium secondary battery including the same. More particularly, the present invention relates to an electrode for a high capacity lithium secondary battery having excellent structural stability and a lithium secondary battery including the same.

With the commercialization of lithium secondary batteries, the demand for rechargeable batteries as energy sources is rapidly increasing as technology development and demand for mobile devices are increasing in the mobile market. Therefore, it is necessary to develop a lithium secondary battery having a larger capacity, a longer cycle life, and a lower self-discharge rate among the secondary batteries in the future.

In recent years, there has been a growing interest in environmental issues, and as a result, electric vehicles (EVs) and hybrid electric vehicles (HEVs), which can replace fossil-fueled vehicles such as gasoline vehicles and diesel vehicles, And the like.

Such electric vehicles (EV) and hybrid electric vehicles (HEV) use nickel metal hydride (Ni-MH) secondary batteries as a power source or lithium secondary batteries having high energy density, high discharge voltage and output stability. Is required to be used for more than 10 years under harsh conditions in addition to high energy density and high output in a short period of time. Therefore, the energy density, safety and long life Characteristics are inevitably required.

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.

Therefore, the development of a lithium secondary battery employing a transition metal (Si, Sn, Ge, etc.) having a high theoretical capacity through a reaction with lithium in addition to a carbonaceous material having a conventional intercalation reaction mechanism Has progressed. In particular, various studies have been made to replace the carbonaceous material with silicon (Si) having a high theoretical capacity (4,200 mAh / g) as an anode 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 have a disadvantage in that the silicon volume is expanded up to 400% by lithium insertion, resulting in failure of the cathode and high cycle characteristics. Specifically, 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 And can exhibit fading mechanisms such as solid-electrolyte-interphase (SEI) formation.

In particular, in the electrode structure, the adhesion between the collector and the active material layer is lower than the adhesion between the active material and the active material during the coating and drying process of the electrode. Therefore, it is necessary to develop an electrode structure capable of minimizing or dispersing the stress occurring during charging / discharging and improving the desorption.

This phenomenon occurs when an anode active material that reacts with lithium is used, and development of a new technique for solving the above problems is continuously required.

Disclosure of Invention Technical Problem [8] The present invention provides a high-capacity lithium secondary battery electrode having excellent structural stability and improved desorption of an electrode layer.

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

In order to solve the above problems,

Electrode collector; A first electrode active material layer formed on the electrode current collector; And a second electrode active material layer formed on the first electrode active material layer and including a transition metal-based active material,

Wherein the first electrode active material layer has a smaller coefficient of expansion in the planar direction than the second electrode active material layer when a lithium secondary battery including the electrode for a lithium secondary battery is manufactured and subjected to a charge- An electrode for a secondary battery is provided.

Further, in order to solve the above-mentioned other problems,

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

The electrode for a lithium secondary battery according to the present invention is characterized in that a first electrode active material layer having a small expansion rate is formed on an electrode current collector and a second electrode active material layer containing a transition metal based active material is formed on the first electrode active material layer Therefore, the first electrode active material layer disperses and absorbs stress due to the volume change of the transition metal-based active material contained in the second electrode active material layer, which is generated at the time of charge / discharge of the lithium secondary battery, It is possible to improve the problem of desorption of the electrode active material and can be usefully used in the production of high capacity lithium secondary batteries.

FIGS. 1 and 2 are diagrams schematically showing the behavior of charging and discharging of an electrode active material in a conventional electrode.
FIGS. 3 and 4 are views schematically showing the behavior of an electrode active material during charge / discharge of an electrode according to an example of the present invention.

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.

An electrode for a lithium secondary battery according to the present invention includes: an electrode collector; A first electrode active material layer formed on the electrode current collector; And a second electrode active material layer formed on the first electrode active material layer and including a silicon (Si) based material.

In addition, when the lithium secondary battery including the electrode for a lithium secondary battery is manufactured and a charge-discharge cycle is performed, the first electrode active material layer has a smaller expansion rate than the second electrode active material layer.

In the present specification, when the first electrode active material layer has a smaller expansion ratio than the second electrode active material layer, the expansion ratio of the electrode active material layer is not a rate of expansion of each electrode active material contained in the electrode active material layer, and an expansion coefficient of the entire electrode active material layer having a dimension of the electrode active material layer.

Therefore, the fact that the expansion ratio of the first electrode active material layer is smaller than the expansion ratio of the second electrode active material layer means that the change in the size of the active material contained in each of the electrode active material layers, the size of the gap, And is based on a change in size of the entire electrode active material layer.

In the lithium secondary battery according to an embodiment of the present invention, the expansion ratio of the first electrode active material layer may be 5% to 100% smaller than the expansion ratio of the second electrode active material layer, specifically 5% to 100% , And more specifically from 20% to 50%.

When the expansion ratio of the first electrode active material layer is smaller than the expansion ratio of the second electrode active material layer by 5% or more, the stress caused by the expansion upon filling can be effectively eliminated, and the expansion ratio of the first electrode active material layer The electrode for a lithium secondary battery can be manufactured within a range where capacity loss is not large when the expansion ratio of the second electrode active material layer is 100% or less.

The expansion ratio of the first electrode active material layer and the expansion ratio of the second electrode active material layer can be expressed by the following equations.

[Equation 1]

(T2-T1) / T1 占 100 = expansion ratio of the first electrode active material layer (expansion ratio of the second electrode active material layer, unit%)

T1 represents a thickness of the first electrode active material layer (second electrode active material layer) in a dry state before being included in the lithium secondary battery, and T2 represents a thickness of the lithium secondary battery when the lithium secondary battery is in an SOC 100 state. 1 < / RTI > electrode active material layer (second electrode active material layer).

The first electrode active material layer may include an electrode active material exhibiting a smaller expansion coefficient than the second electrode active material layer or may include an electrode active material layer having an expansion coefficient smaller than that of the second electrode active material layer so that the expansion ratio of the first electrode active material layer may be smaller than that of the second electrode active material layer. So that the change in the volume of each active material can be buffered by the space along the gap.

In one embodiment of the present invention, the electrode active material included in the first electrode active material layer may be a graphite based active material, and the first electrode active material layer may further include a binder together with the electrode active material.

The graphite-based active material may have a mean particle size (D ₅₀₎ of a size from 0.1 to 30 ㎛, may have a specifically 1 to 20 ㎛ having an average particle size (D _50).

When the average particle size of the electrode active material is 0.1 mu m or more, the density of the electrode is prevented from being lowered to have an appropriate capacity per volume. When the average particle diameter is 30 mu m or less, the first electrode active material layer is formed to have a uniform thickness .

When the first electrode active material layer includes a graphite-based active material, the first electrode active material layer has a porosity of 20 to 40%, specifically, a porosity of 24 to 33% Lt; / RTI >

When the porosity is 20% or more, the electrolytic solution penetrates smoothly into the first electrode active material layer, and the volume change of the graphite can be appropriately dispersed. When the porosity is 40% or less, the energy density of the first electrode active material May not be too low.

When the first electrode active material layer includes a graphite-based active material, the coating amount of the first electrode active material layer may be 0.02 to 0.1 g / 25 cm ² , and specifically 0.023 to 0.04 g / 25 cm ² .

When the first electrode active material layer contains a graphite-based active material and the coating amount is 0.02 to 0.1 g / 25 cm ² , the first electrode active material suitably satisfies the above porosity, while the expansion ratio of the first electrode active material layer Electrode active material layer is lower than that of the two-electrode active material layer.

The graphite-based active material may be low-crystalline carbon or high-crystalline carbon. Examples of the low crystalline carbon include soft carbon and hard carbon. Examples of the highly crystalline carbon include natural graphite, kish graphite, pyrolytic carbon, High temperature sintered carbon such as mesophase pitch based carbon fiber, meso-carbon microbeads, mesophase pitches and petroleum or coal tar pitch derived cokes.

The binder may be a conventional binder used for preparing a positive electrode or a negative electrode for a lithium secondary battery, and a specific type thereof will be described later.

In the electrode for a lithium secondary battery according to an example of the present invention, the transition metal-based electrode active material contained in the second electrode active material layer may not penetrate the internal void of the first electrode active material layer. Therefore, the transition metal-based electrode active material included in the second electrode active material may not be mixed in the first electrode active material layer except for the interface with the first electrode active material.

In another embodiment of the present invention, the electrode active material included in the first electrode active material layer may be a transition metal-based active material, and the first electrode active material layer may further include a binder together with the electrode active material.

When the first electrode active material layer of the electrode for a lithium secondary battery includes a transition metal-based active material and a binder, the first electrode active material layer may have a porosity of 27 to 45%, specifically 30 to 40%.

When the first electrode active material layer has a porosity of 27% or more, even if the first electrode active material includes a transition metal-based active material, the voids appropriately absorb the volume increase of each of the transition metal-based active materials, It is possible to prevent stress from occurring and to suppress the expansion of the first electrode active material layer.

In the specification of the present invention, the porosity of the first electrode active material layer indicates a ratio of voids to the entire volume of the first electrode active material layer.

The transition metal-based active material may have a mean particle size (D ₅₀₎ of a size from 0.5 to 10 ㎛, may have a specific average particle size 1 (D ₅₀₎ of 1 to 3 ㎛.

When the transition metal-based active material has an average particle diameter of 0.5 m or more, the contact surface between the active materials or the contact surface between the active material and the current collector increases, thereby preventing the transition metal-based active material from being separated due to the volume expansion of the transition metal- The first electrode active material layer may be formed to have a uniform thickness and the volume of the transition metal based active material may be increased to an excessively large size when the average particle diameter is 10 m or less. .

When the first electrode active material layer includes a transition metal-based active material, the coating amount of the first electrode active material layer may be 0.01 to 0.1 g / 25 cm ² , and may be 0.012 to 0.02 g / 25 cm ² .

When the first electrode active material layer contains a transition metal-based active material, when the coating amount is 0.01 to 0.1 g / 25 cm ² , the first electrode active material suitably satisfies the porosity described above, and the expansion ratio of the first electrode active material layer Electrode active material layer is lower than that of the two-electrode active material layer.

In the electrode for a lithium secondary battery according to an example of the present invention, the transition metal-based active material may be a silicon-based active material, a transition metal (Sn, Ge, Pb, P, As, Sb, Bi, etc.) (SiO _x , 0 < x? 2), an Si-metal alloy, and an alloy of Si and silicon oxide particles (SiO _x , 0 <x? 2) Ge, Pb, P, As, Sb, Bi, and oxides thereof, and at least one selected from the group consisting of Sn, Ge, Pb, P, As, Sb, Bi, And at least one selected from the group consisting of these alloys. At this time, the silicon oxide particles (SiO _x , 0 <x <2) may be a composite composed of crystalline SiO ₂ and amorphous Si. Specifically, the transition metal-based active material is selected from the group consisting of Si, silicon oxide particles (SiO _x , 0 <x ≦ 2), Si-metal alloy, and an alloy of Si and silicon oxide particles (SiO _x , 0 <x ≦ 2) It may be at least one selected.

When the first electrode active material layer includes a transition metal-based active material, specific types of the transition metal-based active material included in the first electrode active material layer and the second electrode active material layer may be the same or different.

The transition metal-based active material contained in the second electrode active material layer may have an average particle size (D ₅₀ ) of 0.2 to 30 탆, specifically 1 to 15 탆, more specifically 2 to 10 탆.

When the average particle diameter of the transition metal-based active material contained in the second electrode active material layer is 0.2 탆 or more, the density of the electrode can be prevented from being lowered so that the capacity per unit volume can be appropriately obtained. When the average particle diameter is 30 탆 or less, The second electrode active material layer can be formed with a uniform thickness and the transition metal based active material included in the second electrode active material layer can have a large contact area with the electrode active material contained in the first electrode active material layer, It is possible to increase the adhesive force with the one-electrode active material layer.

The electrode for a lithium secondary battery includes a first electrode active material layer formed by applying a first electrode active material slurry for manufacturing the first electrode active material layer on a current collector and drying the electrode active material layer, And then coating and drying the second electrode active material slurry for producing the second electrode active material layer to prepare the second electrode active material layer.

The electrode active material layer including the electrode active material slurry may be prepared by a conventional method known in the art. For example, an electrode active material to be contained in each layer, additives such as a binder and a conductive material may be mixed And stirring the mixture to prepare a first electrode active material slurry and a second electrode active material slurry, sequentially applying the slurry to a current collector, drying and compressing the slurry.

In one example of the present invention, the electrode for a lithium secondary battery may be a negative electrode for a lithium secondary battery.

When the electrode for the lithium secondary battery is a negative electrode, an organic solvent such as NMP (N-methylpyrrolidone), DMF (dimethylformamide), acetone, dimethylacetamide, or water is used These solvents may be used alone or in admixture of two or more. The amount of the solvent to be used is sufficient to dissolve and disperse the negative electrode active material, the binder and the conductive material in consideration of the coating thickness of the slurry and the production yield.

The binder may be used to bind the negative electrode active material particles to maintain the formed body. Any conventional binder used in preparing the slurry for the negative electrode active material is not particularly limited, and examples thereof include polyvinyl alcohol, carboxymethyl cellulose, Polyvinylidene fluoride (PTFE), polyvinylidene fluoride (PVdF), polyethylene or polypropylene, and the like can be used. In addition, an acrylic resin such as acrylic resin (polyvinyl chloride), polyvinyl pyrrolidone, polytetrafluoroethylene Acrylonitrile-butadiene rubber, styrene-butadiene rubber, styrene-butadiene rubber, and acrylic rubber, or a mixture of two or more thereof. The aqueous binders are economical, environmentally friendly, harmless to the health of workers, and are superior to non-aqueous binders, and have a better binding effect than non-aqueous binders. Thus, the ratio of the active materials of the same volume can be increased and the capacity of the aqueous binders can be increased. Preferably styrene-butadiene rubber can be used.

The binder may be contained in an amount of 10 wt% or less, specifically 0.1 wt% to 10 wt%, based on the total weight of the slurry for the negative electrode active material. If the content of the binder is less than 0.1 wt%, the effect of the binder is insufficient, which is undesirable. If the content of the binder is more than 10 wt%, the relative content of the active material may decrease to increase the binder content. not.

The conductive material is not particularly limited as long as it has 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 conductive material may be used in an amount of 1 wt% to 9 wt% with respect to the total weight of the slurry for the negative electrode active material.

The negative electrode collector used in the negative electrode according to an embodiment of the present invention may have a thickness of 3 to 500 mu m. The negative electrode current collector is not particularly limited as long as it has electrical conductivity without causing chemical changes in the battery. Examples of the negative electrode current collector include carbon, nickel, and the like on the surface of copper, stainless steel, aluminum, nickel, titanium, , A surface treated with titanium or silver, an aluminum-cadmium alloy, or the like. 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 electrode for a lithium secondary battery can be used for a lithium secondary battery. Accordingly, the present invention provides a lithium secondary battery including the electrode for the 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 and LiMnO ₂ ; 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 = 0.01 ~ 0.3 Im) 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 ~ 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 anode 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 high melting point glass fiber, polyethylene terephthalate fiber or the like may be used as the nonwoven fabric, 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 conventionally used in an electrolyte for a lithium secondary battery. For example, the anion of the lithium salt may 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-sized battery module including a plurality of battery cells.

Hereinafter, a method of manufacturing an electrode for a lithium secondary battery according to the present invention will be described in detail with reference to the drawings. However, the drawings are for illustrating the present invention and the scope of the present invention is not limited thereto.

FIG. 1 schematically shows the behavior of an electrode active material during charge / discharge of a conventional electrode using a heterogeneous electrode active material. 1, when an electrode active material layer is formed by mixing different kinds of electrode active materials 10 and 20, the active material 10 having a relatively large degree of expansion at the time of charging is mixed with the inside of the electrode active material layer containing the same, Stress is generated at the interface between the layer and the current collector 40, and the generation of stress is repeated every time it is charged, which causes electrode tearing.

Fig. 2 schematically shows an electrode active material layer of a conventional electrode for a lithium secondary battery, which includes an active material having a large degree of expansion of an active material when filled as an electrode active material. As shown in FIG. 2, when the electrode active material layer including the active material 10 having a large degree of expansion of the active material is filled, the lithium secondary battery including the electrode is filled with the active material Stress is concentrated on the interface between the active material layer and the current collector 40 because the volume is increased. The generation of stress between the current collector and the active material layer is repeated every time it is charged, which weakens the bonding force between the current collector and the active material, thereby causing electrode tearing.

3 and 4 are diagrams schematically showing the behavior of the electrode active material when the electrode is charged / discharged according to an example of the present invention.

Referring to FIG. 3, in order to solve the problem of the volume expansion of the active material of the conventional electrode, the electrode for a lithium secondary battery according to an exemplary embodiment of the present invention includes a first electrode active material layer 100, And the second electrode active material layer 200 includes a transition metal-based active material 10 having a large volume change during charging and discharging but excellent in capacity characteristics, 40 and the active material layer, it is possible to achieve excellent capacity characteristics through the transition metal-based active material 20 while minimizing the occurrence of stress.

4, the electrode for a lithium secondary battery according to another exemplary embodiment of the present invention includes a first electrode active material layer 100 and a second electrode active material layer 100, (The first electrode active material layer 100 is formed such that the gap between the electrode active materials 10 is larger than that of the second electrode active material layer 200 as shown in FIG. 4) The second electrode active material layer 200 has a large volume change during charging and discharging, but has a good capacity characteristic, so that the electrode active material layer 10 at the time of charging has a high porosity, It is possible to achieve excellent capacity characteristics through the transition metal-based active material while minimizing occurrence of stress at the interface between the active material layers.

Since the interface between the first electrode active material layer and the second electrode active material layer is a bond between the active material layers having the same particle size, the contact surface between the active material layers can be made large, and the coupling between the current collector and the first electrode active material, The stress can be generated even at the interface between the one electrode active material layer and the second electrode active material layer, but the influence of the stress concentration is not so great as compared with the interface between the current collector and the first electrode active material.

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: Preparation of negative electrode for lithium secondary battery

95.8 wt% of carbon powder having an average particle size of 12 탆, 0.5 wt% of super-p as a conductive material, 2.5 wt% and 1.2 wt% of styrene butadiene rubber (SBR) and carboxymethyl cellulose (CMC) And the mixture was added to water as a solvent to prepare a negative electrode active material slurry for forming the first electrode active material layer.

Further, 95.8 wt% of silicone particles having an average particle diameter of 4.9 mu m (Shinetsu Co., Ltd.), 0.5 wt% of super-p as a conductive material, 2.5 wt% of styrene butadiene rubber (SBR) and carboxymethyl cellulose % And 1.2 wt% were mixed and added to NMP as a solvent to prepare a negative electrode active material slurry for forming the second electrode active material layer.

The slurry of the negative electrode active material for forming the first electrode active material layer was applied to a copper (Cu) thin film as a negative electrode current collector having a thickness of 10 mu m, followed by drying and roll pressing to form a slurry having a thickness of 20 mu m and a loading amount of 0.29 mAh / cm &lt; ² &gt; of the first electrode active material layer. Next, on the first electrode active material layer, a negative electrode active material slurry for forming the second electrode active material layer was applied and dried. A roll was pressed to prepare a second electrode active material layer having a thickness of 27 mu m and a loading amount of 1.93 mAh / cm &lt; ² &gt; to prepare a negative electrode.

Example 2: Preparation of negative electrode for lithium secondary battery

95.8 wt% of silicon particles having an average particle diameter of 4.9 mu m (Shin-Etsu Co., Ltd.), 0.5 wt% of super-p as a conductive material, 2.5 wt% of styrene-butadiene rubber (SBR) and carboxymethyl cellulose (CMC) And 1.2 wt% were mixed and added to NMP as a solvent to prepare an anode active material slurry.

The negative electrode active material coating the slurry on a negative electrode current collector, copper (Cu) thin film having a thickness of 10 ㎛, dried, and roll press (roll press) to conduct the first electrode active material of 5 ㎛, the loading amount of 0.69 mAh / cm ² thick layer .

Subsequently, the negative electrode active material slurry was coated again on the first electrode active material layer and dried. A roll was pressed to prepare a second electrode active material layer having a thickness of 12 mu m and a loading amount of 1.53 mAh / cm &lt; ² &gt; to prepare a negative electrode.

Comparative Example 1: Preparation of a negative electrode for a lithium secondary battery

A mixture of 38.3% by weight of carbon powder having an average particle diameter of 12 占 퐉 and 57.5% by weight of silicon particles having an average particle diameter of 4.9 占 퐉 (Shin-Etsu Co.) as a negative electrode active material, 0.5% by weight of super-p as a conductive material, SBR) and carboxymethylcellulose (CMC) were added to NMP as a solvent to prepare a negative electrode active material slurry.

The negative electrode active material slurry was applied to a copper (Cu) thin film as an anode current collector having a thickness of 10 mu m, dried and then subjected to roll pressing to prepare a negative electrode having a thickness of 47 mu m and a loading amount of 2.22 mAh / cm &lt; ^{2 &} gt ;.

Comparative Example 2: Preparation of negative electrode for lithium secondary battery

95.8 wt% of silicon particles having an average particle diameter of 4.9 mu m (Shin-Etsu Co., Ltd.), 0.5 wt% of super-p as a conductive material, 2.5 wt% of styrene-butadiene rubber (SBR) and carboxymethyl cellulose (CMC) And 1.2 wt% were mixed and added to NMP as a solvent to prepare an anode active material slurry.

The negative electrode active material coating the slurry on a negative electrode current collector, copper (Cu) thin film having a thickness of 10 ㎛ and to prepare a negative electrode of the dried, subjected to roll press to 17 ㎛ thickness, loading amount 2.22 mAh / cm ^2.

Comparative Example 3: Preparation of a negative electrode for a lithium secondary battery

95.8 wt% of silicon particles having an average particle diameter of 4.9 mu m (Shin-Etsu Co., Ltd.), 0.5 wt% of super-p as a conductive material, 2.5 wt% of styrene-butadiene rubber (SBR) and carboxymethyl cellulose (CMC) And 1.2 wt% were mixed and added to NMP as a solvent to prepare an anode active material slurry.

The slurry of the negative electrode active material was applied to a copper (Cu) thin film as an anode current collector having a thickness of 10 mu m, followed by drying and roll pressing to form a first electrode active material layer having a thickness of 5 mu m and a loading amount of 0.75 mAh / cm &lt; ^{2 &} gt ;.

Subsequently, the negative electrode active material slurry was coated again on the first electrode active material layer and dried. A roll was pressed to prepare a second electrode active material layer having a thickness of 13 mu m and a loading amount of 1.47 mAh / cm &lt; ² &gt; to prepare a negative electrode.

Example 3: Preparation of lithium secondary battery

Li metal was used as a counter electrode. A polyolefin separator was interposed between the cathode and the Li metal prepared in Example 1, and then ethylene carbonate (EC) and diethyl carbonate were mixed at a volume ratio of 30:70 And an electrolytic solution in which 1 M of LiPF ₆ was dissolved was injected into the solvent to prepare a coin type half cell.

Example 4 and Comparative Examples 4 to 6: Preparation of lithium secondary battery

A lithium secondary battery was manufactured in the same manner as in Example 3 except that the negative electrode prepared in Example 2 and Comparative Examples 1 to 3 was used instead of the negative electrode prepared in Example 1, .

Experimental Example

<Evaluation of cycle characteristics>

The batteries prepared in Examples 3 and 4 and Comparative Examples 4 to 6 were charged at a constant current (CC) of 0.5 C at 25 캜 until the voltage became 4.25 V and then charged at a constant voltage (CV) And the first charging was performed until a cut-off current of 0.005 C was reached. After that, it was left to stand for 20 minutes and then discharged at a constant current (CC) of 0.5 C until it reached 3.0 V. [ This was repeated in 1 to 30 cycles to observe the degree of degradation of the discharge capacity. The results are shown together in Table 1 below.

Electrode active material Porosity
(%) Coating amount
(g / 25 cm ² ) Volume
@ 10 cycles (%) Volume
@ 30 cycles (%) Example 3 black smoke 26 0.024 96.1 90.3 Si 26 0.036 Example 4 Si 31 0.013 95.2 85.6 Si 26 0.029 Comparative Example 4 Graphite, Si blend 26 0.06 95.4 86.1 Comparative Example 5 Si 26 0.042 94.5 81.7 Comparative Example 6 Si 26 0.014 94.9 82.9 Si 31 0.028

As shown in Table 1, the battery of Example 3 exhibited excellent cycle characteristics as compared with the batteries of Comparative Examples 4 to 6. As a result, when the first electrode active material layer having a small expansion rate as in the present invention is formed on the electrode current collector and the second electrode active material layer is formed on the first electrode active material layer, The cycle performance can be improved as compared with the case where different kinds of active materials are mixed together. This is because, when an electrode is manufactured by mixing different kinds of active materials as in the electrode of Comparative Example 1, the charging and discharging mechanisms between the active materials are different, so that the electrode non-uniformity occurs due to non-uniformity of reaction,

On the other hand, the electrode of Example 2 in which the first electrode active material layer and the second electrode active material layer both contained Si as an electrode active material had the same loading amount (2.22 mAh / cm) as that of the electrode of Comparative Example 1 including a mixture of different kinds of active materials ^2), while its thickness is much thinner (for example, it carried out when it has a 2: 5 + 12㎛, Comparative example 1: 47㎛) shows the cycle performance of a similar level when producing a battery using it.

The battery of Example 4 using the electrode of Example 2 and the electrode of Comparative Example 2 containing only Si as an electrode active material and the electrodes of Comparative Example 3 were used as Comparative Examples 5 and 6 , It can be seen that the battery of Example 4 exhibited excellent cycle performance as compared with the batteries of Comparative Examples 5 and 6. This is because the electrode of Example 2 including the battery of Example 4 is formed by laminating the electrode active material layer into the first electrode active material layer and the second electrode active material layer and forming the first electrode active material layer having the porosity The expansion rate of the first electrode active material layer is reduced to disperse the stress that can be concentrated on the interface between the current collector and the first electrode active material, thereby suppressing the desorption of the electrode active material. This fact can be confirmed from the fact that the battery of Comparative Example 6 using the electrode of Comparative Example 3 in which the porosity of the second electrode active material layer was increased in place of the first electrode active material layer was less effective in improving the cycle performance.

Therefore, in the case of an electrode for a lithium secondary battery employing a high capacity and high expansion anode active material, excellent performance can be obtained when the electrode structure is constructed as in the present invention.

10, 20: electrode active material
30: binder
40: The whole house
100: First electrode active material layer
200: second electrode active material layer

Claims (14)

Electrode collector; A first electrode active material layer formed on the electrode current collector; And a second electrode active material layer formed on the first electrode active material layer and including a transition metal-based active material,
Wherein the first electrode active material layer has a smaller expansion rate than the second electrode active material layer when a lithium secondary battery including the electrode for a lithium secondary battery is manufactured and a charge and discharge cycle is performed.
The method according to claim 1,
Wherein the expansion ratio of the first electrode active material layer is 5% to 100% smaller than the expansion ratio of the second electrode active material layer.
The method according to claim 1,
Wherein the first electrode active material layer comprises a graphite-based active material and a binder.
The method of claim 3,
Wherein the graphite-based active material has an average particle size (D ₅₀ ) of 0.1 to 30 μm.
The method of claim 3,
Wherein the first electrode active material layer contains voids therein and has a porosity of 20 to 40%.
6. The method of claim 5,
Wherein the transition metal-based electrode active material contained in the second electrode active material layer is not permeable to the internal voids of the first electrode active material layer.
The method according to claim 1,
Wherein the first electrode active material layer includes a transition metal-based active material and a binder,
And the porosity of the first electrode active material layer is 27 to 45%.
8. The method of claim 7,
Wherein the transition metal-based active material has an average particle diameter (D ₅₀ ) of 0.5 to 10 탆.
8. The method of claim 7,
And the coating amount of the first electrode active material layer is 0.01 to 0.1 g / 25 cm ² .
The method according to claim 1,
Wherein the second electrode active material layer does not include a carbonaceous active material.
8. The method of claim 1 or 7,
Wherein the transition metal-based active material is at least one selected from the group consisting of Si, silicon oxide particles (SiO _x , 0 <x? 2), Si-metal alloys and alloys of Si and silicon oxide particles (SiO _x , 0 <x? At least one selected from the group consisting of Sn, Ge, Pb, P, As, Sb, Bi and oxides thereof and Sn, Ge, Pb, P, As, Sb and Bi, , An electrode for a lithium secondary battery.
The method according to claim 1,
Wherein the transition metal-based active material contained in the second electrode active material layer has an average particle size (D ₅₀ ) of 0.2 to 30 μm.
A lithium secondary battery comprising the electrode for a lithium secondary battery according to claim 1.
14. The method of claim 13,
Wherein the electrode for the lithium secondary battery is a negative electrode.

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