KR102308943B1 - Anode Having Double Active Material Layers, Method of Preparing the Same, and Secondary Battery Comprising the Same - Google Patents

Abstract

The present invention is a current collector for a negative electrode; and an active material layer of a double-layer structure located on at least one surface of the current collector for the negative electrode, wherein the active material layer is located on at least one surface of the current collector for the negative electrode, a lower active material layer, and an upper surface of the lower active material layer Consisting of an upper active material layer, the upper active material layer includes graphite particles having a sphericity of 0.91 or less, and the lower active material layer includes graphite particles having a sphericity greater than 0.91, and the porosity of the upper active material layer is Provided are a negative electrode having a higher porosity than the lower active material layer, a method for manufacturing the same, and a lithium secondary battery including the same. In the negative electrode according to the present invention, the porosity of the upper active material layer is greater than that of the lower active material layer, and thus the diffusion rate of lithium ions is increased, thereby further improving battery performance such as capacity, lifespan, and output.

Description

Anode Having Double Active Material Layers, Method of Preparing the Same, and Secondary Battery Comprising the Same

The present invention relates to an anode capable of improving battery performance by increasing a lithium ion diffusion rate, a method for manufacturing the same, and a secondary battery including the same.

As technology development and demand for mobile devices increase, the demand for rechargeable batteries that can be recharged, miniaturized and large-capacity are rapidly increasing. In addition, a lithium secondary battery having a high energy density and voltage among secondary batteries has been commercialized and widely used.

A lithium secondary battery has a structure in which an electrolyte containing lithium salt is impregnated in an electrode assembly with a porous separator interposed between a positive electrode and a negative electrode, each of which is coated with an active material on an electrode current collector, and the electrode is an active material, a binder and applying a slurry in which a conductive material is dispersed in a solvent to a current collector, drying and pressing the slurry.

In addition, capacity, output, and lifespan, which are basic performance characteristics of a lithium secondary battery, are greatly affected by the anode material. In order to maximize battery performance, the anode active material must have an electrochemical reaction potential close to lithium metal, high reversibility with lithium ions, and high diffusion rate of lithium ions in the active material. Graphite is widely used as a material meeting these requirements.

Recently, the negative electrode tends to have a larger area and become thicker for higher capacity and higher density. As the density of the electrode active material layer increases, porosity decreases, thereby reducing the passage of lithium (Li) ions. Accordingly, diffusion of lithium (Li) ions from the electrode surface to the inside of the electrode becomes difficult, resulting in non-uniformity of reaction. When the non-uniformity of reaction increases, side reactions occur on the surface of the electrode, thereby easily generating by-products between the negative electrode and the separator. This phenomenon eventually becomes a factor that deteriorates the performance of the battery.

Therefore, it is necessary to develop a negative electrode capable of increasing the diffusion rate of lithium ions in the pores between the active materials as well as the diffusion rate of lithium ions inside the active material.

The present invention is to solve the above problems, and one object of the present invention is to provide a negative electrode having a high lithium ion diffusion rate from the electrode surface to the inside by increasing the diffusion rate of lithium ions in the pores in the electrode active material layer. .

Another object of the present invention is to provide a method for manufacturing the negative electrode.

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

The present invention is to solve the above problems, according to an aspect of the present invention, a current collector for a negative electrode; and an active material layer of a double-layer structure located on at least one surface of the current collector for the negative electrode, wherein the active material layer is located on at least one surface of the current collector for the negative electrode, a lower active material layer, and an upper surface of the lower active material layer Consisting of an upper active material layer, the upper active material layer includes graphite particles having a sphericity of 0.91 or less, and the lower active material layer includes graphite particles having a sphericity greater than 0.91, and the porosity of the upper active material layer is A negative electrode having a greater porosity than that of the lower active material layer is provided.

The graphite particles included in the upper active material layer may be artificial graphite having a sphericity of 0.6 to 0.91.

The graphite particles included in the lower active material layer may be natural graphite having a sphericity of greater than 0.91 and less than or equal to 0.97.

The upper active material layer may have a greater porosity than that of the lower active material layer in a range of 3 to 10%.

The thickness of the upper active material layer may be 20% or more of the total thickness of the active material layer.

According to another aspect of the present invention, there is provided a method of manufacturing a negative electrode in which an active material layer is formed on at least one surface of a current collector for a negative electrode in a double-layer structure of an upper active material layer and a lower active material layer,

(i) preparing a slurry of graphite particles having a sphericity of greater than 0.91 as an active material, and coating and drying the slurry on at least one surface of a current collector for a negative electrode to form a lower coating layer;

(ii) preparing a slurry of graphite particles having a sphericity of 0.91 or less as an active material, and coating and drying the slurry on the lower coating layer to form an upper coating layer; and

(iii) There is provided a method of manufacturing a negative electrode comprising the step of rolling the upper coating layer and the lower coating layer at the same time.

In the manufacturing method according to the present invention, the step (i) of forming the lower coating layer and the step (ii) of forming the upper coating layer may be performed simultaneously.

According to another aspect of the present invention, there is provided a lithium secondary battery including the negative electrode.

According to the present invention, an efficient method of performing only one rolling process using different active materials for the upper active material layer and the lower active material layer when manufacturing a negative electrode including an active material layer having a double-layer structure, wherein the upper active material layer has a porosity higher than that of the lower active material layer By providing this large negative electrode, the effect of increasing the passage of lithium ions is generated, and the diffusion rate of lithium ions can be increased, thereby further improving battery performance such as capacity, lifespan, and output.

1A and 1B show images taken by a scanning electron microscope (SEM) of a cross-section of a cathode prepared according to Example 1 of the present invention, respectively.
2 is a graph showing the lifespan characteristics of batteries prepared according to Examples 1 to 2 and Comparative Examples 1 to 3;

Hereinafter, the terms or words used in the present specification and claims should not be construed as being limited to conventional or dictionary meanings, and the inventor appropriately defines the concept of the term in order to best describe his invention. Based on the principle that it can be done, it should be interpreted as meaning and concept consistent with the technical idea of the present invention.

One embodiment of the present invention is a current collector for a negative electrode; and an active material layer of a double-layer structure located on at least one surface of the current collector for the negative electrode, wherein the active material layer is located on at least one surface of the current collector for the negative electrode, a lower active material layer, and an upper surface of the lower active material layer Consisting of an upper active material layer, the upper active material layer includes graphite particles having a sphericity of 0.91 or less, and the lower active material layer includes graphite particles having a sphericity greater than 0.91, and the porosity of the upper active material layer is It relates to a negative electrode greater than the porosity of the lower active material layer.

In the present invention, the 'sphericity' of graphite particles is measured by using a device such as a scanning electron microscope (SEM) and observing the image of the graphite particles obtained after the grinding process, etc., and the same area as the projected image of the observed particles. Means the value obtained by measuring the circumference of a circle with

Sphericity = circumference of a circle of equal area / perimeter of the projected image

In addition, the degree of sphericity can be measured using an analysis device, such as FPIA3000 (Malvern).

In the present invention, the 'porosity' refers to the volume of the pores with respect to the total volume of the active material layer, and may be measured through various methods used in the art.

In one embodiment of the present invention, the graphite particles included in the upper active material layer may be artificial graphite. Artificial graphite is generally manufactured by carbonizing raw materials such as coal tar, coal tar pitch, and petroleum heavy products at 2,500° C. or higher, and after such graphitization, particle size adjustment such as grinding and secondary particle formation is performed. It is used as an anode active material. In the case of artificial graphite, the crystals are randomly distributed in the particles, and the sphericity is lower than that of natural graphite and has a rather sharp shape.

Artificial graphite used in the present invention includes commercially widely used MCMB (mesophase carbon microbeads), MPCF (mesophase pitch-based carbon fiber), artificial graphite graphitized in block form, artificial graphite graphitized in powder form, and the like. , preferably artificial graphite having a sphericity of 0.91 or less, preferably 0.6 to 0.91, more preferably 0.7 to 0.9.

In addition, the artificial graphite may have a particle diameter of 5 to 30 μm, preferably 10 to 25 μm.

In one embodiment of the present invention, the graphite particles included in the lower active material layer may be natural graphite. Natural graphite is generally a plate-shaped aggregate before processing, and plate-shaped particles are manufactured in a spherical shape with a smooth surface through post-processing such as particle crushing and re-assembly process in order to be used as an active material for electrode manufacturing. do.

The natural graphite used in the present invention preferably has a sphericity of greater than 0.91 and less than or equal to 0.97, preferably 0.93 to 0.97, more preferably 0.94 to 0.96.

In addition, the natural graphite may have a particle diameter of 5 to 30㎛, preferably 10 to 25㎛.

The negative electrode according to an embodiment of the present invention includes an electrode active material layer having a double-layer structure, and each active material layer exhibits different porosity as it contains graphite particles having different sphericity. That is, the upper active material layer containing graphite particles with a low degree of sphericity does not roll well during the manufacturing process, so the porosity increases, while the lower active material layer containing graphite particles with a high degree of sphericity rolls well compared to the upper active material layer. The porosity is relatively small. Therefore, since the porosity of the upper active material layer, which is the surface of the electrode, is increased compared to the lower active material layer, which is inside the electrode, ions move into the electrode and the diffusion rate of lithium ions is increased. From this, the reaction non-uniformity and consequent side reactions on the electrode surface can be minimized, so that the life performance of the battery can also be improved.

In one embodiment of the present invention, the porosity of the upper active material layer is 25 to 35%, preferably 26 to 33%, and the porosity of the lower active material layer is 20 to 30%, preferably 20 to 28%.

In one embodiment of the present invention, the porosity of the upper active material layer is greater than that of the lower active material layer by 3 to 10%, preferably by 5 to 10%. When the porosity difference is less than 3%, the improvement of the lithium ion diffusion rate is insignificant, and when it exceeds 10%, the thickness of the active material layer having a large porosity increases too large, so that the diffusion distance of lithium ions increases and the diffusion resistance becomes rather large. can

In one embodiment of the present invention, the total thickness of the negative active material layer of the double-layer structure is not particularly limited, but may be, for example, 100 to 300 μm, and the upper active material layer and the lower active material layer each have a thickness in the range of 50 to 150 μm. can

At this time, when the thickness of the upper active material layer is 20% or more of the total active material layer thickness, there is an effect of improving the lithium ion diffusion rate. When the thickness of the upper active material layer is less than 20%, it is difficult to expect an improvement in the lithium ion diffusion rate because voids on the electrode surface are insufficient.

Another embodiment of the present invention relates to a method of manufacturing a negative electrode in which an active material layer is formed on at least one surface of a current collector for a negative electrode in a double-layer structure of an upper active material layer and a lower active material layer.

A method of manufacturing an anode according to an embodiment of the present invention includes the following steps:

(i) preparing a slurry of graphite particles having a sphericity of greater than 0.91 as an active material, and coating and drying the slurry on at least one surface of a current collector for a negative electrode to form a lower coating layer;

(ii) preparing a slurry of graphite particles having a sphericity of 0.91 or less as an active material, and coating and drying the slurry on the lower coating layer to form an upper coating layer; and

(iii) rolling the upper coating layer and the lower coating layer at the same time.

In the method according to an embodiment of the present invention, the current collector for a negative electrode used as a substrate for forming the active material layer is not particularly limited as long as it has conductivity without causing a chemical change in the battery, for example, copper, Stainless steel, aluminum, nickel, titanium, calcined carbon, one in which the surface of copper or stainless steel is surface-treated with carbon, nickel, titanium, silver, etc., an aluminum-cadmium alloy, etc. may be used.

The thickness of the current collector is not particularly limited, but may have a commonly applied thickness of 3 to 500 μm.

In the method according to an embodiment of the present invention, the slurry used for forming the lower coating layer in step (i) includes graphite particles having a sphericity of more than 0.91 as an active material, and used for forming the upper coating layer in step (ii) The slurry contains graphite particles having a sphericity of 0.91 or less as an active material, and the details of each graphite particle are as described above.

Each of the slurries used in steps (i) and (ii) may be prepared by mixing and stirring a binder, a solvent, and, if necessary, a conductive material and a dispersing agent together with the graphite particles as described above.

In the present invention, the step (i) of forming the lower coating layer and the step (ii) of forming the upper coating layer may be performed sequentially or simultaneously.

That is, the lower coating layer-forming slurry is first coated and dried on the current collector, and then the upper coating layer-forming slurry is coated and dried thereon to sequentially form the upper coating layer and the lower coating layer, or using a double slot die, etc. Two types of slurries can be simultaneously coated and dried using the device to form the upper and lower coating layers at once.

The coating method is not particularly limited as long as it is a method commonly used in the art. For example, a coating method using a slot die may be used, and in addition, a Mayer bar coating method, a gravure coating method, a dip coating method, a spray coating method, etc. may be used.

As the binder, polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HEP), polyvinylidenefluoride, polyacrylonitrile, polymethylmethacrylate, Polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, styrene butyrene rubber (SBR), fluororubber , various types of binder polymers such as various copolymers may be used.

As the solvent, N-methylpyrrolidone, acetone, water, or the like may be used.

The conductive material is not particularly limited as long as it has conductivity without causing a chemical change in the battery, and for example, carbon black, acetylene black, Ketjen black, channel black, Farness black, lamp black, thermal black, etc. carbon black; conductive fibers such as carbon fibers and metal fibers; 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; A conductive material such as a polyphenylene derivative may be used.

In addition, the slurry may further include an additive such as a thickener such as carboxymethyl cellulose (CMC), if necessary.

In step (iii), the upper and lower coating layers obtained by coating and drying each slurry are rolled. The rolling may be performed by a method commonly used in the art, such as roll pressing, for example, may be performed at a pressure of 1 to 20 MPa and a temperature of 15 to 30 ℃.

The present invention is characterized in that the upper coating layer and the lower coating layer are rolled simultaneously, that is, at once.

In general, when forming a multi-layer active material layer, the first active material slurry is first coated, dried and rolled in the order of forming a first active material layer, and then the second active material slurry is coated thereon again, dried and rolled. Although a separate rolling process is performed for each layer in such a way as to form two active material layers by performing it sequentially, in the present invention, the coating and drying processes of the upper layer slurry and the lower layer slurry are simultaneously or sequentially performed, and then the rolling process is performed only once.

The porosity of the upper active material layer and the lower active material layer including graphite particles having different degrees of sphericity as an active material can be differently adjusted through such a single rolling process. Specifically, the upper active material layer containing graphite particles with a low degree of sphericity does not roll well, so the porosity increases, while the lower active material layer containing graphite particles with a high degree of sphericity rolls well compared to the upper active material layer. The porosity will decrease.

As such, in manufacturing an electrode having an active material layer having a double-layer structure, when the upper active material layer includes graphite particles having a relatively small sphericity to form a larger porosity, ion movement from the electrode surface to the inside of the electrode is advantageous This increases the diffusion rate of lithium ions. From this, the reaction non-uniformity and consequent side reactions on the electrode surface can be minimized, so that the life performance of the battery can also be improved.

Another embodiment of the present invention relates to a lithium secondary battery including the negative electrode prepared as described above. Specifically, the lithium secondary battery may be manufactured by injecting a lithium salt-containing electrolyte into an electrode assembly including a positive electrode, a negative electrode as described above, and a separator interposed therebetween.

For the positive electrode, a slurry is prepared by mixing a positive electrode active material, a conductive material, a binder, and a solvent, and the slurry is directly coated on a metal current collector, or a positive electrode active material film that is cast on a separate support and peeled from the support is laminated on a metal current collector. Thus, a positive electrode can be manufactured.

The active material used for the positive electrode includes LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ , LiCoPO ₄ , LiFePO ₄ and LiNi _1-xyz Co _x M1 _y M2 _z O ₂ (M1 and M2 are each independently Al, Ni, Co, Fe, Mn, V, Cr, Ti, W, Ta, any one selected from the group consisting of Mg and Mo, x, y and z are each independently 0≤x<0.5, 0≤ as the atomic fraction of the oxide composition elements y<0.5, 0≤z<0.5, 0<x+y+z≤1) may include any one active material particle selected from the group consisting of, or a mixture of two or more thereof.

Meanwhile, the conductive material, the binder, and the solvent may be used in the same manner as used for manufacturing the negative electrode.

The separator is a conventional porous polymer film used as a conventional separator, for example, an ethylene homopolymer, a propylene homopolymer, an ethylene/butene copolymer, an ethylene/hexene copolymer, and an ethylene/methacrylate copolymer. The prepared porous polymer film may be used alone or by laminating them. In addition, an insulating thin film having high ion permeability and mechanical strength may be used. The separator may include a safety reinforced separator (SRS) in which a ceramic material is thinly coated on a surface of the separator. In addition, a conventional porous nonwoven fabric may be used, for example, a nonwoven fabric made of high melting point glass fiber, polyethylene terephthalate fiber, etc., but is not limited thereto.

The electrolyte includes a lithium salt as an electrolyte and an organic solvent for dissolving the same.

The lithium salt may be used without limitation as long as it is commonly used in electrolytes for secondary batteries. For example, as an anion of the lithium salt ^{, F -} , Cl ^- , 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 ^- , One selected from the group consisting of SCN ^- and (CF ₃ CF ₂ SO ₂ ) ₂ N ^{- may be used.}

The organic solvent included in the electrolyte may be used without limitation as long as it is commonly used, and representatively, propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, ethylmethyl carbonate, methylpropyl carbonate, dipropyl carbonate, dimethyl sulfoxide At least one selected from the group consisting of side, acetonitrile, dimethoxyethane, diethoxyethane, vinylene carbonate, sulfolane, gamma-butyrolactone, propylene sulfite and tetrahydrofuran may be used.

In particular, among the carbonate-based organic solvents, ethylene carbonate and propylene carbonate, which are cyclic carbonates, are highly viscous organic solvents and have a high dielectric constant, so they can be used preferably because they dissociate lithium salts in the electrolyte well. When a low-viscosity, low-dielectric constant linear carbonate such as carbonate is mixed in an appropriate ratio, an electrolyte having a high electrical conductivity can be prepared, which can be more preferably used.

Optionally, the electrolyte stored according to the present invention may further include additives such as an overcharge inhibitor included in a conventional electrolyte.

A lithium secondary battery according to an embodiment of the present invention forms an electrode assembly by arranging a separator between a positive electrode and a negative electrode, and places the electrode assembly in, for example, a pouch, a cylindrical battery case or a prismatic battery case, and then the electrolyte When injected, the secondary battery can be completed. Alternatively, a lithium secondary battery may be completed by stacking the electrode assembly, impregnating it with an electrolyte, and sealing the obtained result in a battery case.

According to an embodiment of the present invention, the lithium secondary battery may be a stack type, a wound type, a stack and fold type, or a cable type.

The lithium secondary battery according to the present invention may not only be used in a battery cell used as a power source for a small device, but may also be preferably used as a unit cell in a medium or large-sized battery module including a plurality of battery cells. Preferred examples of the medium and large devices include electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and power storage systems, and are particularly useful in areas requiring high output, such as hybrid electric vehicles and new and renewable energy storage batteries. can be used

Hereinafter, to help the understanding of the present invention, examples will be described in detail. However, the embodiments according to the present invention may be modified in various other forms, and the scope of the present invention should not be construed as being limited to the following examples. The embodiments of the present invention are provided to more completely explain the present invention to those of ordinary skill in the art.

Example One: double layer structural active material layer Preparation of negative electrode and lithium secondary battery having

<Production of cathode>

Natural graphite with an average sphericity of 0.95 as an anode active material, carbon black as a conductive material, SBR as a binder, and a thickener (CMC) were added to water as a solvent in a weight ratio of 94:1:3:2, 1 A negative active material slurry was prepared.

In addition, artificial graphite graphitized in powder form with an average sphericity of 0.9 as an anode active material, carbon black as a conductive material, SBR as a binder, and a thickener (CMC) in a weight ratio of 94:1:3:2 By adding it to water as a solvent, a second negative electrode active material slurry was prepared.

At this time, the average sphericity of each graphite used was measured using FPIA3000 (Malvern) for the sphericity of 10,000 graphite particles, and then the average value was taken.

Using a single slot die, the first negative electrode active material slurry (containing natural graphite) is coated on one surface of a copper (Cu) thin film having a thickness of 10 μm as an anode current collector to a thickness of 100 μm, and at 100° C. for 5 minutes and dried to form a lower coating layer.

Then, the second negative active material slurry (containing artificial graphite) was coated on the lower coating layer to the same thickness as the lower coating layer, and dried at 100° C. for 5 minutes to form an upper coating layer.

The upper and lower coating layers thus formed were simultaneously rolled (pressure: 10 MPa, temperature: 25° C.) by a roll pressing method to prepare an anode having an upper/lower dual active material layer. At this time, the total loading amount of the negative electrode was 6.0 mAh/cm ² .

<Production of anode>

_{Li(Ni 0.5} Mn _0.3 Co _0.2 )O ₂ (NCM-523) as a cathode active material, carbon black as a conductive material, and polyvinylidene fluoride (PVdF) as a binder in a weight ratio of 94:4:2 as a solvent Phosphorus was added to N-methylpyrrolidone (NMP) to prepare a cathode active material slurry. The slurry was coated on one side of an aluminum current collector having a thickness of 15 μm so that the loading amount was 5.0 mAh/cm ² , and drying and rolling were performed under the same conditions as the negative electrode to prepare a positive electrode.

<Manufacture of lithium secondary battery>

In an organic solvent mixed with ethylene carbonate (EC), propylene carbonate (PC) and diethyl carbonate (DEC) in a composition of 3:3:4 (volume ratio), LiPF ₆ was dissolved to a concentration of 1.0M to obtain a non-aqueous electrolyte solution. prepared.

After interposing the polyolefin separator between the positive electrode and the negative electrode prepared above, the electrolyte was injected to prepare a lithium secondary battery.

Example 2: double layer structural active material layer Preparation of negative electrode and lithium secondary battery having

<Production of cathode>

Natural graphite having an average sphericity of 0.93 as an anode active material, carbon black as a conductive material, SBR as a binder, and a thickener (CMC) were added to water as a solvent in a weight ratio of 94:1:3:2, 1 A negative active material slurry was prepared.

In addition, artificial graphite graphitized in powder form with an average sphericity of 0.89 as an anode active material, carbon black as a conductive material, SBR as a binder, and a thickener (CMC) in a weight ratio of 94:1:3:2 By adding it to water as a solvent, a second negative electrode active material slurry was prepared.

At this time, the average sphericity of each graphite used was measured using FPIA3000 (Malvern) for the sphericity of 10,000 graphite particles, and then the average value was taken.

Using a double slot die, the first negative electrode active material slurry (containing natural graphite) is placed on one side of a copper (Cu) thin film having a thickness of 10 μm as the negative electrode current collector, and the second negative electrode active material slurry (containing artificial graphite) is at the bottom. was placed on it and coated at the same time, and the thickness of each coating layer was 100 μm. After drying the formed two-layer coating layer at 100° C. for 5 minutes, rolling (pressure: 10 MPa, temperature: 25° C.) in a roll press method to prepare a negative electrode having an upper/lower dual active material layer did. At this time, the total loading amount of the negative electrode was 6.0 mAh/cm ² .

<Production of anode>

A positive electrode was prepared in the same manner as in Example 1.

<Manufacture of lithium secondary battery>

A lithium secondary battery was prepared in the same manner as in Example 1.

comparative example 1: single active material layer Preparation of negative electrode and lithium secondary battery having

<Production of cathode>

Natural graphite having an average sphericity of 0.95 as an anode active material, carbon black as a conductive material, SBR as a binder, and a thickener (CMC) were added to water as a solvent in a weight ratio of 94:1:3:2 to the anode, An active material slurry was prepared. The slurry was coated on one surface of a copper (Cu) thin film having a thickness of 10 μm, which is a negative electrode current collector, at a loading amount of ^{6.0 mAh/cm 2 .} The formed coating layer was dried and rolled under the same conditions as in Example 1 to prepare a negative electrode.

<Production of anode>

A positive electrode was prepared in the same manner as in Example 1.

<Manufacture of lithium secondary battery>

A lithium secondary battery was prepared in the same manner as in Example 1.

comparative example 2: double layer structural on the active material layer Preparation of negative electrode and lithium secondary battery containing the same active material

<Production of cathode>

Natural graphite having a sphericity of 0.95 as an anode active material, carbon black as a conductive material, SBR as a binder, and a thickener (CMC) were added to water as a solvent in a weight ratio of 94:1:3:2, and the anode active material A slurry was prepared. Using a double slot die, the slurry was coated, dried and rolled on one surface of a copper (Cu) thin film having a thickness of 10 μm as an anode current collector to prepare an anode having an upper/lower dual active material layer. At this time, the coating thickness of each layer, drying and rolling conditions were the same as in Example 2, and the total loading amount of the negative electrode was 6.0 mAh/cm ² .

<Production of anode>

A positive electrode was prepared in the same manner as in Example 1.

<Manufacture of lithium secondary battery>

A lithium secondary battery was prepared in the same manner as in Example 1.

comparative example 3: double layer structural active material layer Manufacturing of negative electrode and lithium secondary battery that has undergone a rolling process at the time of formation

<Production of cathode>

Natural graphite having a sphericity of 0.95 as an anode active material, carbon black as a conductive material, SBR as a binder, and a thickener (CMC) were added to water as a solvent in a weight ratio of 94:1:3:2, and the anode active material A slurry was prepared.

Using a single slot die, the slurry was first coated on one side of a copper (Cu) thin film as a negative current collector having a thickness of 10 μm so that the loading amount was 3.0 mAh/cm ² , and dried at 100° C. for 5 minutes. , the lower active material layer was formed by rolling in a roll press method so that the porosity was 27%. On top of that, the slurry is coated again so that the loading amount is 3.0 mAh/cm ² , dried at 100° C. for 5 minutes, and then rolled in a roll pressing method so that the porosity is 27%. By forming an upper active material layer, a negative electrode was prepared. At this time, the total loading amount of the negative electrode was 6.0 mAh/cm ² .

<Production of anode>

A positive electrode was prepared in the same manner as in Example 1.

<Manufacture of lithium secondary battery>

A lithium secondary battery was prepared in the same manner as in Example 1.

Experimental example 1: Measurement of porosity

For the negative electrode obtained in Examples 1 and 2 and Comparative Examples 1 to 3, the porosity was measured as follows.

First, the binder and the conductive material present in the active material layer of the negative electrode are dyed, the cross-section of the negative electrode is photographed by SEM, and the cross-sectional area ratio of the active material, the binder (and the conductive material), and the voids are image processed through cross-sectional analysis. ) was obtained through

Specifically, the cross section of each cathode was photographed by SEM, and the porosity distribution inside the anode was confirmed through the images obtained therefrom, and the results are shown in Table 1.

Porosity of cathode (%) overall average
(theoretical value) Lower active material layer average
(actual measurement) Upper active material layer average
(actual measurement) Example 1 27 22.1 30.2 Example 2 27 22.8 29.7 Comparative Example 1 27 27.4 27.1 Comparative Example 2 27 27.6 26.9 Comparative Example 3 27 19.1 27.2

In addition, SEM images of the cross-section of the active material layer of the negative electrode according to Example 1 are shown in FIGS. 1A and 1B . In particular, as shown in FIG. 1B, the porosity measured by subdividing into 9 layers from the electrode surface toward the current collector is shown in Table 2.

upper active material layer lower active material layer One 2 3 4 5 6 7 8 9 porosity
(%) 31.0 28.4 31.3 29.7 30.5 26.4 23.8 21.4 16.9

1 and Table 2, it can be seen that the porosity of the upper active material layer close to the electrode surface is large compared to the lower active material layer close to the current collector.

As can be seen in Tables 1 and 2, the negative electrodes prepared according to Examples 1 and 2 contained graphite particles having a low sphericity in the upper active material layer and graphite particles having a high sphericity in the lower active material layer. As the rolling process is performed once, it can be seen that the porosity of the upper active material layer close to the electrode surface is greater than that of the lower active material layer close to the current collector. Accordingly, the movement of lithium ions from the electrode surface to the inside is advantageous, and the ion diffusion rate can be improved.

On the other hand, the negative electrode of Comparative Example 1 was formed as a single layer, and there was little difference in the porosity measured by dividing the upper and lower parts, and the negative electrode of Comparative Example 2 was rolled once in a state including graphite particles of the same sphericity in both the upper and lower active material layers. It was manufactured through a process, and the difference in the porosity of the upper and lower active material layers was insignificant, so the lifespan improvement effect was small.

As the negative electrode of Comparative Example 3 was manufactured through a rolling process for each layer while including graphite particles of the same sphericity in both the upper and lower active material layers, the porosity of the upper active material layer was higher than that of the lower active material layer. Even when the porosity of the lower active material layer was adjusted to the theoretical value of 27% during the secondary rolling, the porosity of the lower active material layer was less than 20% due to further pressurization during the second rolling. As a result, life expectancy worsened. In addition, in the process of performing the rolling process for each layer, there is a problem in that the graphite particles of the lower active material layer are rolled again and damaged.

Experimental example 2: charging and discharging Capacity (life characteristics) evaluation according to cycle

The lithium secondary batteries of Examples 1 to 2 and Comparative Examples 1 to 3 were charged and discharged 150 times at 25 to 0.5C, and the results are shown in FIG. 2 .

From FIG. 2 , it can be confirmed that the batteries manufactured according to Examples 1 and 2 maintain a constant capacity even after repeated charge/discharge times compared to Comparative Examples 1 to 3 .

Claims (11)

current collector for negative electrode; and an active material layer having a double-layer structure located on at least one surface of the current collector for the negative electrode,
The active material layer consists of a lower active material layer positioned on at least one surface of the current collector for the negative electrode, and an upper active material layer positioned on the upper surface of the lower active material layer,
The upper active material layer contains graphite particles in which crystals are randomly distributed in the particles and has a sphericity of 0.91 or less,
The lower active material layer comprises graphite particles having a sphericity of more than 0.91,
The negative electrode in which the porosity of the upper active material layer is greater than the porosity of the lower active material layer. According to claim 1,
The negative electrode, characterized in that the graphite particles contained in the upper active material layer is artificial graphite having a sphericity of 0.6 to 0.91. According to claim 1,
The negative electrode, characterized in that the graphite particles included in the lower active material layer are natural graphite having a sphericity of greater than 0.91 and less than or equal to 0.97. According to claim 1,
The negative electrode, characterized in that the porosity of the upper active material layer is greater than the porosity of the lower active material layer in the range of 3 to 10%. 5. The method of claim 4,
The negative electrode, characterized in that the porosity of the upper active material layer is 25 to 35%, and the porosity of the lower active material layer is 20 to 30%. According to claim 1,
The thickness of the upper active material layer is an anode, characterized in that 20% or more of the total thickness of the active material layer. A method of manufacturing a negative electrode in which an active material layer is formed on at least one surface of a current collector for a negative electrode in a double-layer structure of an upper active material layer and a lower active material layer,
(i) preparing a slurry of graphite particles having a sphericity of greater than 0.91 as an active material, and coating and drying the slurry on at least one surface of a current collector for a negative electrode to form a lower coating layer;
(ii) preparing a slurry of graphite particles in which crystals are randomly distributed in the particles as an active material and having a sphericity of 0.91 or less, and coating and drying the slurry on the lower coating layer to form an upper coating layer; and
(iii) a method of manufacturing a negative electrode comprising the step of simultaneously rolling the upper coating layer and the lower coating layer. 8. The method of claim 7,
Method, characterized in that the step (i) of forming the lower coating layer and the step (ii) of forming the upper coating layer are performed simultaneously. 8. The method of claim 7,
Method, characterized in that the graphite particles used in the upper coating layer is artificial graphite having a sphericity of 0.6 to 0.91. 8. The method of claim 7,
The method, characterized in that the graphite particles used in the lower coating layer are natural graphite having a sphericity of greater than 0.91 and less than or equal to 0.97. A lithium secondary battery comprising the negative electrode according to any one of claims 1 to 6.

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