KR102111482B1 - Electrode for lithium secondary battery and lithium secondary battery comprising the same - Google Patents

Abstract

The present invention is an electrode current 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 active material, wherein a lithium secondary battery including the lithium secondary battery electrode is prepared and charged. When the discharge cycle is performed, the first electrode active material layer relates to an electrode for a lithium secondary battery, characterized in that the expansion rate in a plane direction is smaller than that of the second electrode active material layer, and the electrode for a lithium secondary battery according to the present invention Since the first electrode active material layer having a small expansion coefficient in the surface direction is formed on the silver electrode current collector, and the second electrode active material layer including the transition metal-based active material is formed on the first electrode active material layer, the first electrode One electrode active material layer of the transition metal-based active material contained in the second electrode active material layer generated during charging and discharging of the lithium secondary battery To improve the desorption problem of the electrode active material layer by absorption by distributing the stress due to changes in blood, and can be useful in the production of high-capacity lithium secondary battery.

Description

An electrode for a lithium secondary battery and a lithium secondary battery comprising the same {ELECTRODE FOR LITHIUM SECONDARY BATTERY AND LITHIUM SECONDARY BATTERY COMPRISING THE SAME}

The present invention relates to an electrode for a lithium secondary battery and a lithium secondary battery comprising the same, and more particularly, to a high capacity lithium secondary battery electrode having excellent structural stability and a lithium secondary battery including the same.

As lithium secondary batteries are commercialized, demand for secondary batteries as an energy source is rapidly increasing in the mobile market as technology development and demand for mobile devices increase. Accordingly, it is necessary to develop a lithium secondary battery having a larger capacity, a long cycle life, and a low self-discharge rate among secondary batteries.

In addition, in recent years, as interest in environmental problems has increased, electric vehicles (EVs) and hybrid electric vehicles (HEVs) that can replace fossil-fueled vehicles, such as gasoline vehicles and diesel vehicles, are one of the main causes of air pollution. A lot of research has been conducted on the back.

These electric vehicles (EVs), hybrid electric vehicles (HEVs), etc., use a nickel-metal hydride (Ni-MH) secondary battery or a lithium secondary battery of high energy density, high discharge voltage and output stability as a power source. When used in electric vehicles, it must be used for more than 10 years under severe conditions, along with high energy density and the ability to exhibit large power in a short period of time, so it is superior in energy density, safety, and long life than existing small lithium secondary batteries. Characteristics are inevitably required.

Meanwhile, 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 a lithium secondary battery, and metal lithium, graphite as a negative electrode active material constituting the negative electrode Carbon-based materials such as (graphite) or activated carbon, or materials such as silicon oxide (SiO _x ) are used. Among the negative electrode active materials, metal lithium was mainly used initially, but as charging and discharging cycles progress, lithium atoms grow on the surface of the metal lithium to damage the separator and damage the battery. Recently, carbon-based materials are mainly used. However, the carbon-based material has a disadvantage that the theoretical capacity is only about 400 mAh / g, so that the capacity is small.

Therefore, development of a lithium secondary battery to which a transition metal (Si, Sn, Ge, etc.) having a high theoretical capacity is applied through an alloy reaction with lithium in addition to a carbon-based material having an existing intercalation reaction mechanism This has been going on. Among them, various studies have been conducted to replace the carbon-based material by using silicon (Si) having a high theoretical capacity (4,200 mAh / g) as a negative electrode active material. The reaction formula when lithium is incorporated in silicon is as follows:

[Scheme 1]

22Li + 5Si = Li ₂₂ Si ₅

 However, most silicon negative electrode materials have a disadvantage in that the silicon volume expands up to 400% by lithium insertion, and thus, the negative electrode is destroyed, and thus it does not exhibit high cycle characteristics. Specifically, in the case of silicon, volume expansion occurs by the lithium insertion as the cycle continues, pulverization, contact losses with conducting agents and current collector, and unstable It may exhibit a fading mechanism, such as the formation of a solid-electrolyte-interphase (SEI).

In particular, when viewed from the inside of the electrode structure, the adhesion between the current collector and the active material layer, which is a metal foil during the coating and drying process of the electrode, is lower than that of the active material and the active material. Therefore, there is a need to develop an electrode structure capable of improving desorption by minimizing or dispersing stresses during charging and discharging.

This behavior is a phenomenon that occurs when using a negative electrode active material that reacts with lithium, and the development of a new technology capable of solving the above problems is continuously required.

The problem to be solved by the present invention is to provide an electrode for a high capacity lithium secondary battery having excellent structural stability and improved desorption of an electrode layer.

Another problem to be solved of the present invention is to provide a lithium secondary battery including the lithium secondary battery electrode.

In order to solve the above problems, the present invention

Electrode current 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,

When the lithium secondary battery including the lithium secondary battery electrode is manufactured and subjected to a charge / discharge cycle, the first electrode active material layer has a small expansion coefficient in a plane direction compared to the second electrode active material layer, lithium An electrode for a secondary battery is provided.

In addition, the present invention to solve the other problems,

It provides a lithium secondary battery comprising the electrode for the lithium secondary battery.

In the electrode for a lithium secondary battery according to the present invention, 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 including a transition metal-based active material is formed on the first electrode active material layer Therefore, the first electrode active material layer is distributed at the interface between the current collector and the electrode active material layer by dispersing and absorbing stress due to the volume change of the transition metal-based active material contained in the second electrode active material layer generated during charging and discharging of the lithium secondary battery. It is possible to improve the desorption problem of the electrode active material generated, it can be usefully used in the production of high capacity lithium secondary battery.

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

Hereinafter, the present invention will be described in more detail to aid understanding of the present invention.

The terms or words used in the specification and claims should not be interpreted as being limited to ordinary or lexical meanings, and the inventor can appropriately define the concept of terms in order to explain his or her invention in the best way. Based on the principle that it should be interpreted as meanings and concepts consistent with the technical spirit of the present invention.

The electrode for a lithium secondary battery according to the present invention includes an electrode current 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) system.

In addition, when a lithium secondary battery including the electrode for a lithium secondary battery is manufactured to perform a charge / discharge cycle, the first electrode active material layer is characterized in that the expansion rate is small compared to the second electrode active material layer.

In the present specification, when the first electrode active material layer is said to have a small expansion rate compared to the second electrode active material layer, the expansion rate of the electrode active material layer is a constant size rather than the expansion rate of each electrode active material included in the electrode active material layer. It indicates the expansion coefficient of the entire electrode active material layer having a (dimension).

Therefore, the fact that the expansion rate of the first electrode active material layer is smaller than the expansion rate of the second electrode active material layer is considered for changes in the size of the active material included in the electrode active material layers, the size of the pores, or the change in the porosity. It does not, and is based on a change in size of the entire electrode active material layer.

In the lithium secondary battery according to an example of the present invention, the expansion rate of the first electrode active material layer may have a value of 5% to 100% smaller than the expansion rate of the second electrode active material layer, specifically 5% to 100% It may have a small value less than, and more specifically may have a small value of 20% to 50%.

When the expansion rate of the first electrode active material layer is less than 5% of the expansion rate of the second electrode active material layer, stress generated due to expansion during charging may be effectively relieved, and the expansion rate of the first electrode active material layer may be When it is less than 100% of the expansion rate of the second electrode active material layer, an electrode for a lithium secondary battery may be manufactured within a range in which the capacity loss is not large.

The expansion rate of the first electrode active material layer and the expansion rate of the second electrode active material layer may be represented by the following equation.

[Equation 1]

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

In the above formula, T1 represents the 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 is the first when the lithium secondary battery is in SOC 100 state. The thickness of the one electrode active material layer (second electrode active material layer) is shown.

As described above, the first electrode active material layer includes or includes an electrode active material exhibiting a small expansion rate compared to the second electrode active material layer, so that the expansion value of the first electrode active material layer is smaller than that of the second electrode active material layer. The gap between each of the electrode active materials being made can be increased so that the volume change of each active material is buffered by the space according to the interval.

In one example 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 also 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 diameter of the electrode active material is 0.1 μm or more, the density of the electrode can be prevented from being lowered to have an appropriate capacity per volume, and when the average particle size is 30 μm or less, the first electrode active material layer is formed to a uniform thickness. Can be.

When the first electrode active material layer includes a void therein, and the first electrode active material 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% Can have

When the porosity is 20% or more, the electrolyte can smoothly penetrate the first electrode active material layer, while the volume change of the graphite can be appropriately dispersed, and 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 , specifically, 0.023 to 0.04 g / 25 cm ² .

When the coating amount when the first electrode active material layer includes a graphite-based active material is 0.02 to 0.1 g / 25 cm ² , the first electrode active material satisfactorily satisfies the aforementioned porosity, while the expansion rate of the first electrode active material layer is It may have a lower value than the expansion rate of the two-electrode active material layer.

The graphite-based active material may be low crystalline carbon or high crystalline carbon. Soft carbon and hard carbon are typical examples of the low crystalline carbon, and the high crystalline carbon includes natural graphite, kish graphite, pyrolytic carbon, and a liquid crystal pitch meter. And high-temperature calcined carbon such as mesophase pitch based carbon fibers, meso-carbon microbeads, mesophase pitches, and petroleum or coal tar pitch derived cokes.

The binder may be a conventional binder used in the manufacture of a positive electrode or a negative electrode for a lithium secondary battery, and specific types 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 included in the second electrode active material layer may not penetrate the internal pores 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 inside the first electrode active material layer except for an interface with the first electrode active material.

In another example 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 also include a binder together with the electrode active material.

When the first electrode active material layer of the lithium secondary battery electrode 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 pores properly absorb the volume increase of each of the transition metal-based active materials, and the volume of the active material increases. While preventing stress from occurring, expansion of the first electrode active material layer can be suppressed.

In the specification of the present invention, the porosity of the first electrode active material layer indicates the ratio of the porosity to the total 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 ㎛, it may have a specific average particle size 1 (D ₅₀₎ of 1 to 3 ㎛.

When the average particle diameter of the transition metal-based active material is 0.5 μm or more, the contact surface between the active materials or the contact surface of the active material and the current collector is increased to prevent detachment of the transition metal-based active material due to volume expansion of the transition metal-based active material, and the electrode density is It can be prevented from being lowered to have an appropriate capacity per volume, and when the average particle diameter is 10 μm or less, the first electrode active material layer may be formed with a uniform thickness, and the volume of the transition metal-based active material increases to an excessively large size You can prevent that.

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 , specifically 0.012 to 0.02 g / 25 cm ² .

When the coating amount when the first electrode active material layer includes a transition metal-based active material is 0.01 to 0.1 g / 25 cm ² , the first electrode active material satisfactorily satisfies the aforementioned porosity, while the expansion rate of the first electrode active material layer is It may have a lower value than the expansion rate 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 group IV, V transition metal (Sn, Ge, Pb, P, As, Sb, Bi, etc.)-Based active material , Specific examples of the silicon-based active material is a group consisting of Si, silicon oxide particles (SiO _x , 0 <x≤2), Si-metal alloy, and alloys of Si and silicon oxide particles (SiO _x , 0 <x≤2) One or more selected from, and the transition metal-based active material is Sn, Ge, Pb, P, As, Sb, Bi and their respective oxides, and Sn, Ge, Pb, P, As, Sb, Bi and And one or more selected from the group consisting of each of these alloys. In this case, the silicon oxide particles (SiO _x , 0 <x <2) may be a composite composed of crystalline SiO ₂ and amorphous Si. The transition metal-based active material is specifically 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≤2). It may be one or more selected.

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

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

When the average particle diameter of the transition metal-based active material included in the second electrode active material layer is 0.2 µm or more, the density of the electrode may be prevented from lowering to have an appropriate capacity per volume, and when the average particle diameter is 30 µm or less, the agent The second electrode active material layer may be formed with a uniform thickness, and the transition metal-based active material included in the second electrode active material layer may increase the contact area with the electrode active material included in the first electrode active material layer, so that 1 Adhesion with the electrode active material layer can be increased.

The lithium secondary battery electrode is coated with a first electrode active material slurry for preparing the first electrode active material layer on an electrode current collector and dried to prepare the first electrode active material layer, and then on the first electrode active material layer. A second electrode active material slurry for preparing a second electrode active material layer may be prepared according to a method including drying and drying the second electrode active material layer.

The preparation of the electrode active material layer including the preparation, application and drying of the electrode active material slurry can be performed according to a conventional method known in the art, for example, mixing of electrode active materials to be included in each layer and additives such as binders and conductive materials And after stirring to prepare a first electrode active material slurry and a second electrode active material slurry, it can be prepared by sequentially applying it to a current collector, drying it, and compressing it.

In one example of the present invention, the lithium secondary battery electrode 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-methyl pyrrolidone), DMF (dimethyl formamide), acetone, dimethyl acetamide or water may be used as a solvent for forming the negative electrode. These solvents may be used alone or in combination of two or more. The amount of the solvent 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 maintain the molded body by binding the negative electrode active material particles, and is not particularly limited as long as it is a conventional binder used in preparing a slurry for the negative electrode active material, for example, non-aqueous binders such as polyvinyl alcohol, carboxymethyl cellulose, and hydroxy Propylene cellulose, diacetylene cellulose, polyvinylchloride, polyvinylpyrrolidone, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), polyethylene or polypropylene, etc. can be used, and acrylic as an aqueous binder. Any one selected from the group consisting of ronitrile-butadiene rubber, styrene-butadiene rubber and acrylic rubber, or a mixture of two or more of them may be used. Aqueous binders are more economical and eco-friendly than non-aqueous binders, are harmless to workers' health, and have a better binding effect than non-aqueous binders. Preferably, styrene-butadiene rubber can be used.

The binder may be included in an amount of 10% by weight or less in the total weight of the slurry for a negative electrode active material, specifically 0.1% by weight to 10% by weight. If the content of the binder is less than 0.1% by weight, the effect of using the binder is insignificant and undesirable, and if it exceeds 10% by weight, the capacity per volume may decrease due to a decrease in the relative content of the active material due to the increase in the binder content. not.

The conductive material is not particularly limited as long as it has conductivity without causing a chemical change in the battery, and examples of the conductive material include graphite such as natural graphite and artificial graphite; Carbon blacks such as acetylene black, ketjen black, channel black, furnace black, lamp black, and summer black; Conductive fibers such as carbon fibers and metal fibers; Metal powders such as carbon fluoride, aluminum, and nickel powders; Conductive whiskers such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; Or conductive materials such as polyphenylene derivatives. The conductive material may be used in an amount of 1% by weight to 9% by weight based on the total weight of the slurry for a negative electrode active material.

The negative electrode current collector used in the negative electrode according to an embodiment of the present invention may have a thickness of 3 μm to 500 μm. The negative electrode current collector is not particularly limited as long as it does not cause a chemical change in the battery and has conductivity. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, carbon, nickel on the surface of copper or stainless steel , Titanium, silver or the like, aluminum-cadmium alloy, or the like can be used. In addition, it is possible to form a fine unevenness on the surface to enhance the bonding force of the negative electrode active material, and may be used in various forms such as films, sheets, foils, nets, porous bodies, foams, and nonwoven fabrics.

The lithium secondary battery electrode may be used in a lithium secondary battery, and thus, the present invention provides a lithium secondary battery including the lithium secondary battery electrode.

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 positive electrode can be produced by a conventional method known in the art. For example, a mixture of a solvent, a binder, a conductive material, and a dispersant may be mixed with a positive electrode active material, if necessary, and stirred to prepare a slurry, then coated (coated) on a current collector of a metal material, compressed, and dried to produce a positive electrode. have.

The current collector of the metallic material is a highly conductive metal, and is a metal that can easily adhere to the slurry of the positive electrode active material, and is particularly limited as long as it has a high conductivity without causing a chemical change in the battery in a voltage range of the battery. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or a surface treated with carbon, nickel, titanium, silver, or the like on the surface of aluminum or stainless steel may be used. In addition, it is also possible to increase the adhesion of the positive electrode active material by forming fine irregularities on the surface of the current collector. The current collector may be used in various forms, such as a film, sheet, foil, net, porous body, foam, nonwoven fabric, and may have a thickness of 3 to 500 μm.

The positive electrode active material is, for example, lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ), Li [Ni _x Co _y Mn _z Mv] O ₂ (wherein M is selected from the group consisting of Al, Ga and In Is any one or two or more of them; 0.3≤x <1.0, 0≤y, z≤0.5, 0≤v≤0.1, x + y + z + v = 1), Li (Li _a M _{ba -b '} M' _{b '} ) O _{2 - c} A _c (where 0≤a≤0.2, 0.6≤b≤1, 0≤b'≤0.2, 0≤c≤0.2; M is Mn, Ni , Co, Fe, Cr, V, Cu, Zn and Ti, and at least one selected from the group consisting of M 'is Al, Mg, and one or more selected from the group consisting of B, A is P, F, S, and N. One or more selected from the group consisting of a layered compound such as a compound substituted with one or more transition metals; Lithium manganese oxides such as the formula Li _{1 + y} Mn _{2 - y} O ₄ (where y is 0 to 0.33), LiMnO ₃ , LiMn ₂ O ₃ , 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, Zn or Ta, y = 0.01 to 0.1) or Li ₂ Mn ₃ MO ₈ (where M = Fe, Co, Ni, Cu or Zn) lithium manganese composite oxide; LiMn ₂ O _{4 in} which part of Li in the formula is substituted with alkaline earth metal ions; Disulfide compounds; Fe ₂ (MoO ₄ ) ₃ and the like, but are not limited to these.

Examples of the solvent for forming the positive electrode include organic solvents such as NMP (N-methyl pyrrolidone), DMF (dimethyl formamide), acetone, and dimethyl acetamide or water, and these solvents may be used alone or in combination of two or more. Can be used by mixing. The amount of the solvent used is sufficient to dissolve and disperse the positive electrode active material, the binder, and the conductive material in consideration of the coating thickness of the slurry and the production yield.

As the binder, polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride (polyvinylidenefluoride), polyacrylonitrile (polyacrylonitrile), polymethyl methacrylate (polymethylmethacrylate), Polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropyl cellulose, 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 substituted with hydrogen, Li, Na, or Ca, or Various types of binder polymers such as various copolymers can be used.

The conductive material is not particularly limited as long as it has conductivity without causing a chemical change in the battery. For example, graphite such as natural graphite or artificial graphite; Carbon blacks such as acetylene black, ketjen black, channel black, panes black, lamp black, and thermal black; Conductive fibers such as carbon fibers and metal fibers; Conductive tubes such as carbon nanotubes; Metal powders such as fluorocarbon, aluminum, and nickel powders; Conductive whiskers such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; Conductive materials such as polyphenylene derivatives may be used. The conductive material may be used in an amount of 1% to 20% by weight relative to the total weight of the positive electrode slurry.

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

Further, as the separator, a conventional porous polymer film conventionally used as a separator, such as ethylene homopolymer, propylene homopolymer, ethylene-butene copolymer, ethylene-hexene copolymer and ethylene-methacrylate copolymer, etc. The porous polymer film prepared by can be used alone or by laminating them, or a conventional porous nonwoven fabric, such as a high melting point glass fiber, a polyethylene terephthalate fiber, or the like, may be used, but is not limited thereto.

The lithium salt which can be included as an electrolyte used in the present invention can be used without limitation, those which are commonly used in a lithium secondary battery electrolyte, such as the lithium salt, the anion is 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 ^- may be any one selected from the group consisting of _{^{-, 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 liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel polymer electrolyte, a solid inorganic electrolyte, a molten inorganic electrolyte, and the like, which can be used in manufacturing 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 or a coin shape using a can.

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

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

1 schematically shows the behavior of the electrode active material during charging and discharging of a conventional electrode using a different electrode active material. Referring to FIG. 1, when the electrode active material layer is formed by mixing different types of electrode active materials 10 and 20, the active material 10 having a relatively large degree of expansion during charging is inside the electrode active material layer including the same, and the active material The stress is generated at the interface between the layer and the current collector 40, and the stress is repeatedly generated at every charge, which causes the electrode to detach.

In addition, 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 during charging as an electrode active material. As shown in FIG. 2, when an electrode active material layer including the same is formed by using the active material 10 having a large degree of expansion of the active material during charging, when charging a lithium secondary battery including the electrode, Since the volume is increased, stress is concentrated at the interface between the active material layer and the current collector 40. Since the occurrence of stress between the current collector and the interface between the active material layer is repeated at every charge, the bonding force between the current collector and the active material is weakened to cause electrode detachment.

3 and 4 show a diagram schematically showing the behavior of the electrode active material during charging and discharging of the electrode according to an example of the present invention.

Referring to FIG. 3, the electrode for a lithium secondary battery according to an example of the present invention changes the volume of the first electrode active material layer 100 during charging and discharging in order to solve the problem of volume expansion of the active material of the conventional electrode as described above. It is configured to include a small active material 20, and the second electrode active material layer 200 includes a transition metal-based active material 10 having a large volume change at the time of charging and discharging but having excellent capacity characteristics. 40) can achieve excellent capacity characteristics through the transition metal-based active material 20 while minimizing the occurrence of stress at the interface between the active material layer.

In addition, referring to Figure 4, the electrode for a lithium secondary battery according to another example of the present invention is to solve the problem of the volume expansion of the active material of the conventional electrode as described above of the active material of the first electrode active material layer 100 After being configured to have an appropriate amount of porosity to accommodate and absorb the volume change (as shown in FIG. 4, the first electrode active material layer 100 has a gap between each electrode active material 10, the second electrode active material layer 200) It has a large porosity because it is large compared to), 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 having excellent capacity characteristics, and It is possible to achieve excellent capacity characteristics through the transition metal-based active material while minimizing the 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 phase, the contact surface between them can be enlarged, and thus the bonding between the current collector and the first electrode active material where the particle and plane meet at the contact point Compared to this, it can bond more firmly, and thus stress may be generated at the interface between the first electrode active material layer and the second electrode active material layer, but the influence of stress concentration is not large compared to the interface between the current collector and the first electrode active material.

Example

Hereinafter, examples and experimental examples will be described in more detail in order to specifically describe the present invention, but the present invention is not limited by these examples and experimental examples. Embodiments according to the present invention can be modified in many different 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 more fully describe the present invention to those skilled in the art.

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

95.8% by weight of carbon powder having an average particle diameter of 12 µm as the negative electrode active material, 0.5% by weight of super-p as a conductive material, and 2.5% by weight and 1.2% by weight of styrene butadiene rubber (SBR) and carboxymethylcellulose (CMC) as binders, respectively. By mixing and adding to the solvent water, a negative electrode active material slurry for forming a first electrode active material layer was prepared.

In addition, 95.8% by weight of silicon particles (manufactured by Shin-Etsu Co., Ltd.) having an average particle diameter of 4.9 µm as the negative electrode active material, 0.5% by weight of super-p as a conductive material, and 2.5% of styrene butadiene rubber (SBR) and carboxymethyl cellulose (CMC) as binders, respectively. % And 1.2 wt% were added to NMP as a solvent to prepare a negative electrode active material slurry for forming a second electrode active material layer.

The negative electrode active material slurry 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 μm, and dried and roll pressed to perform a thickness of 20 μm and a loading amount of 0.29 mAh. A layer of / cm ² first electrode active material was formed. 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. On the other hand, by performing a roll press, a second electrode active material layer having a thickness of 27 μm and a loading amount of 1.93 mAh / cm ² was formed to prepare a negative electrode.

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

95.8 wt% of silicon particles (manufactured by Shin-Etsu Co., Ltd.) having an average particle diameter of 4.9 µm as a negative electrode active material, 0.5 wt% of super-p as a conductive material, and 2.5 wt% of styrene butadiene rubber (SBR) and carboxymethyl cellulose (CMC) as binders, respectively. A negative electrode active material slurry was prepared by mixing 1.2% by weight and adding it to the solvent NMP.

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

Next, on the first electrode active material layer, the negative electrode active material slurry was applied again and dried. On the other hand, by performing a roll press, a second electrode active material layer having a thickness of 12 μm and a loading amount of 1.53 mAh / cm ² was formed to prepare a negative electrode.

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

As a negative electrode active material, a mixture of 38.3% by weight of carbon powder having an average particle diameter of 12 µm and 57.5% by weight of silicon particles (manufactured by Shin-Etsu) having an average particle diameter of 4.9 µm, 0.5% by weight of super-p as a conductive material, and styrene butadiene rubber as a binder ( SBR) and carboxymethylcellulose (CMC) 2.5 wt% and 1.2 wt%, respectively, were mixed and added to NMP as a solvent to prepare a negative electrode active material slurry.

The negative electrode active material slurry was coated on a copper (Cu) thin film as a negative electrode current collector having a thickness of 10 μm, dried, and then roll-pressed to prepare a negative electrode having a thickness of 47 μm and a loading amount of 2.22 mAh / cm ² .

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

95.8 wt% of silicon particles (manufactured by Shin-Etsu Co., Ltd.) having an average particle diameter of 4.9 µm as a negative electrode active material, 0.5 wt% of super-p as a conductive material, and 2.5 wt% of styrene butadiene rubber (SBR) and carboxymethyl cellulose (CMC) as binders, respectively. A negative electrode active material slurry was prepared by mixing 1.2% by weight and adding it to the solvent NMP.

The negative electrode active material slurry was applied to a copper (Cu) thin film as a negative electrode current collector having a thickness of 10 μm, dried, and then roll-pressed to prepare a negative electrode having a thickness of 17 μm and a loading amount of 2.22 mAh / cm ² .

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

As a negative electrode active material, 95.8% by weight of silicon particles (Shin-Etsu Co., Ltd.) having an average particle diameter of 4.9 μm, 0.5% by weight of super-p as a conductive material, and 2.5% by weight of styrene butadiene rubber (SBR) and carboxymethylcellulose (CMC) as binders, respectively. A negative electrode active material slurry was prepared by mixing 1.2% by weight and adding it to the solvent NMP.

The negative electrode active material slurry was applied to a copper (Cu) thin film as a negative electrode current collector having a thickness of 10 μm, and dried and roll pressed to form a first electrode active material layer having a thickness of 5 μm and a loading amount of 0.75 mAh / cm ² .

Next, on the first electrode active material layer, the negative electrode active material slurry was applied again and dried. On the other hand, by performing a roll press, a second electrode active material layer having a thickness of 13 μm and a loading amount of 1.47 mAh / cm ² was formed to prepare a negative electrode.

Example 3: Preparation of lithium secondary battery

Li metal was used as a counter electrode, and after interposing a polyolefin separator between the cathode prepared in Example 1 and the Li metal, ethylene carbonate (EC) and diethyl carbonate were mixed at a volume ratio of 30:70. A coin-type half cell was prepared by injecting 1 M LiPF ₆ dissolved electrolyte into a solvent.

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

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

Experimental Example

<Cycle characteristic evaluation>

The cells prepared in Examples 3 and 4 and Comparative Examples 4 to 6 were charged at 25 ° C. with a constant current (CC) of 0.5 C until 4.25 V, and then charged with a constant voltage (CV) to obtain a charging current. The first charging was performed until it became 0.005 C (cut-off current). Then, it was left for 20 minutes, and then discharged at a constant current (CC) of 0.5 C until 3.0 V was reached. This was repeated 1 to 30 cycles, and the degree of deterioration of the discharge capacity was observed. The results are shown in Table 1 below.

Electrode active material Porosity
(%) Coating amount
(g / 25cm 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 mixed 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 batteries of Example 3 exhibited excellent cycle characteristics compared to those of Comparative Examples 4 to 6. Through this, when forming a first electrode active material layer having a small expansion rate as in the present invention on the electrode current collector, and forming a second electrode active material layer on the first electrode active material layer, Comparative Example 1 used in Comparative Example 4 It was confirmed that the cycle performance can be improved compared to the case of mixing different kinds of active materials such as the electrode of. This is because when the electrode was prepared by mixing different active materials like the electrode of Comparative Example 1, the charge / discharge mechanism between the different active materials is different, and this is considered to be because electrode detachment occurs due to reaction non-uniformity and deterioration of life.

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 contain Si as the electrode active material, have the same loading amount (2.22 mAh / cm) as that of the electrode of Comparative Example 1 containing a mixture of different active materials. ² ), the thickness was much thinner (Example 2: 5 + 12 μm, Comparative Example 1: 47 μm), and a similar level of cycle performance was obtained when the battery was prepared using the same.

In addition, the batteries of Example 4 using the electrode of Example 2 and Comparative Examples 5 and 6 using the electrode of Comparative Example 2 and the electrode of Comparative Example 3, respectively, containing only Si as the electrode active material, like the electrode of Example 2, respectively. By comparing the cells of, it can be confirmed that the cells of Example 4 exhibited excellent cycle performance compared to the cells of Comparative Examples 5 and 6. This is, the electrode of Example 2 included in the battery of Example 4, the electrode active material layer is divided into a first electrode active material layer and a second electrode active material layer, but the porosity of the first electrode active material layer formed on the electrode current collector It is judged that, by relatively increasing, the expansion rate of the first electrode active material layer is reduced to dissipate the electrode active material by dispersing stress that can be concentrated on the interface between the current collector and the first electrode active material. This was confirmed by the fact that the battery of Comparative Example 6, in which 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 used, had a slight improvement in cycle performance.

Therefore, in the case of an electrode for a lithium secondary battery to which a high-capacity and high-expansion negative electrode active material is applied, it is determined that excellent performance can be achieved when the electrode structure is configured as in the present invention.

10, 20: electrode active material
30: binder
40: current collector
100: first electrode active material layer
200: second electrode active material layer

Claims (14)

Electrode current 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,
The first electrode active material layer includes a graphite-based active material and a binder,
The second electrode active material layer does not include a carbon-based active material,
The first electrode active material layer includes a void therein, and has a porosity of 20 to 40%,
When a lithium secondary battery including the lithium secondary battery electrode is manufactured and subjected to a charge / discharge cycle, the first electrode active material layer has a small expansion rate compared to the second electrode active material layer, and the lithium secondary battery electrode.
According to claim 1,
The first electrode active material layer has an expansion rate of 5% to 100% smaller than the expansion rate of the second electrode active material layer, a lithium secondary battery electrode.
delete According to claim 1,
The graphite-based active material has an average particle diameter (D ₅₀ ) size of 0.1 to 30 μm, lithium secondary battery electrode.
delete According to claim 1,
The transition metal-based electrode active material included in the second electrode active material layer does not penetrate into the internal pores of the first electrode active material layer, a lithium secondary battery electrode.
delete delete delete delete According to claim 1,
The transition metal-based active material is 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≤2). Or more, or one or more selected from the group consisting of Sn, Ge, Pb, P, As, Sb, Bi and their respective oxides, and Sn, Ge, Pb, P, As, Sb, Bi and their respective alloys , Lithium secondary battery electrode.
According to claim 1,
The transition metal-based active material included in the second electrode active material layer has an average particle diameter (D ₅₀ ) size of 0.2 to 30 μm, a lithium secondary battery electrode.
A lithium secondary battery comprising the electrode for a lithium secondary battery according to claim 1.
The method of claim 13,
The electrode for a lithium secondary battery is a negative electrode, a lithium secondary battery.

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