KR20200085587A - Anode and Lithium Secondary Battery Comprising the Same - Google Patents

Abstract

Disclosed are a negative electrode with increased quick charge characteristics and a lithium secondary battery including the same. According to the present invention, the negative electrode comprises a negative electrode active material layer including a negative electrode active material and a binder. The negative electrode active material includes natural graphite and carbon-coated artificial graphite and the content of artificial carbon is 10 wt% or more with respect to the total weight of the negative electrode active material. The artificial graphite is acquired by simultaneously performing granulation and carbon-coating of secondary particles after primary particles are carbonized, the average particle size of the primary particles of the artificial graphite is within a range of 1 to 10 μm, the orientation degree (I004/I110) of the secondary particles of the artificial graphite is 10 or less, the content of the coated carbon is 5 wt% or less with respect to the total weight of the carbon-coated artificial graphite, and the porosity of the negative electrode is 24% or more.

Description

Anode and secondary battery comprising the same{Anode and Lithium Secondary Battery Comprising the Same}

The present invention relates to a negative electrode and a secondary battery comprising the same, specifically, to a negative electrode having improved rapid charging characteristics and a secondary battery comprising the same.

With the development of technology and demand for mobile devices, the demand for rechargeable batteries capable of recharging, miniaturization and high capacity is rapidly increasing. In addition, lithium secondary batteries having high energy density and voltage among secondary batteries have been commercialized and widely used.

The lithium secondary battery consists of a structure in which an electrolyte containing a lithium salt is impregnated in an electrode assembly in which a porous separator is interposed between the positive electrode and the negative electrode, each of which is coated with an active material, and the electrode is an active material, a binder And a slurry in which a conductive material is dispersed in a solvent is applied to a current collector and dried and pressed.

In addition, in recent years, as interest in environmental issues has increased, electric vehicles (EVs) and hybrid electric vehicles (HEVs) that can replace fossil fuel 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. Lithium secondary batteries having high energy density, high discharge voltage, and output stability are mainly researched and used as power sources such as electric vehicles (EV) and hybrid electric vehicles (HEV).

As the use target of the lithium secondary battery is expanded, shortening of the charging time is required to improve the convenience of the battery, and thus high rate discharge characteristics and high rate charging characteristics are becoming important.

One of the rate-rate processes when charging a lithium ion secondary battery is an insertion reaction of lithium ions (Li+) into a negative electrode active material. However, when the rapid charging proceeds, lithium ions are not inserted into the negative electrode active material layer, and a problem arises on the surface of the negative electrode, and the negative electrode active material layer can be relatively increased to avoid this, but in this case, energy density point of view Is disadvantageous in

Currently, in order to expand the electric vehicle (EV) market, an increase in charging speed is continuously required, but due to the limitation that the life characteristics are deteriorated when the charging rate is improved, the charging condition of the existing cylindrical electric vehicle battery is used at around 0.3C. .

Accordingly, there is still a need to develop a secondary battery technology including a negative electrode having an improved charging speed by improving a rapid charging characteristic while simultaneously securing an electric vehicle battery life characteristic.

The present invention is to solve the above problems, one object of the present invention is to provide a negative electrode with improved fast charging characteristics while securing battery life characteristics.

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

The present invention is to solve the above problems, and according to an aspect of the present invention, a negative electrode of the following embodiment is provided.

The first embodiment,

As a negative electrode comprising a negative electrode active material layer containing a negative electrode active material, a conductive material and a binder,

The negative electrode active material includes natural graphite and carbon-coated artificial graphite,

The carbon-coated artificial graphite is present in an amount of 10% by weight or more based on the total amount of the negative electrode active material,

The artificial graphite is obtained by graphitizing the primary particles and then granulating the secondary particles and coating the carbon simultaneously.

The average particle diameter of the artificial graphite primary particles is in the range of 1 to 10㎛,

The artificial graphite secondary particles have an orientation (I004/I110) of 10 or less,

The content of the carbon coated based on the total amount of the carbon-coated artificial graphite is 5% by weight or less,

The cathode has a porosity of 24% or more.

In the second embodiment, in the first embodiment, the carbon-coated artificial graphite is a negative electrode present in an amount of 10 to 30% by weight based on the total amount of the negative electrode active material.

The third embodiment, in the first embodiment or the second embodiment,

It is a negative electrode having a content of 4 to 5% by weight of carbon coated on the artificial graphite.

The fourth embodiment, in any one of the first to third embodiments,

The artificial graphite is a negative electrode having an average particle diameter of 1 to 5㎛ primary particles.

The fifth embodiment, in any one of the first to fourth embodiments,

The artificial graphite is a negative electrode having an average particle diameter of 1 to 2.5㎛ primary particles.

The sixth embodiment according to any one of the first to fifth embodiments,

The artificial graphite is a negative electrode having an orientation of 1 to 10.

The seventh embodiment according to any one of the first to sixth embodiments,

The artificial graphite is a negative electrode having an orientation of 1 to 6.

The eighth embodiment according to any one of the first to seventh embodiments,

The cathode has a porosity of 24 to 26%.

According to an aspect of the present invention, a lithium secondary battery of the following embodiment is provided.

The ninth embodiment is a lithium secondary battery including the negative electrode according to any one of the first to eighth embodiments.

The tenth embodiment is the ninth embodiment,

The secondary battery is a lithium secondary battery capable of rapid charging at 1.0 to 1.2C.

The eleventh embodiment is the ninth or tenth embodiment,

The secondary battery is a lithium secondary battery that is a cylindrical secondary battery.

According to the present invention, natural graphite and carbon-coated artificial graphite are used together as a negative electrode active material, the average particle diameter of the primary particles of the carbon-coated artificial graphite, the degree of orientation of the secondary particles, the content of the coated carbon, the negative electrode By controlling the porosity of the same, it is possible to provide a negative electrode with improved battery charging speed while simultaneously improving battery charging characteristics.

The following drawings attached to the present specification illustrate preferred embodiments of the present invention, and serve to further understand the technical spirit of the present invention together with the detailed description of the invention described below. It should not be interpreted as being limited to.
1 is a graph showing the initial efficiency characteristics of the secondary battery prepared in Example 1 and Comparative Example 1.
2 is a graph showing the life characteristics during low-speed charging and discharging of the secondary batteries prepared in Examples 1 to 2 and Comparative Examples 1 to 7.
3 is a graph showing the life characteristics during high-speed charging and discharging of the secondary batteries prepared in Examples 1 to 2 and Comparative Examples 1 to 7.
4 is a graph showing the life characteristics of the secondary battery prepared in Example 1 during high-speed charging and discharging.

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

The terms used in this specification are only used to describe exemplary embodiments, and are not intended to limit the present invention. Singular expressions include plural expressions unless the context clearly indicates otherwise.

In this specification, the terms “include”, “have” or “have” are intended to indicate the presence of implemented features, numbers, steps, elements, or combinations thereof, one or more other features or It should be understood that the existence or addition possibilities of numbers, steps, elements, or combinations thereof are not excluded in advance.

According to one aspect of the invention,

A negative electrode comprising a negative electrode active material layer containing a negative electrode active material and a binder,

The negative electrode active material includes natural graphite and carbon-coated artificial graphite,

The carbon-coated artificial graphite is present in an amount of 10% by weight or more based on the total amount of the negative electrode active material,

The artificial graphite is obtained by graphitizing the primary particles and then granulating the secondary particles and coating the carbon simultaneously.

The average particle diameter of the primary particles of the artificial graphite is in the range of 1 to 10㎛,

The degree of orientation (I004/I110) of the secondary particles of the artificial graphite is 10 or less,

The content of the carbon coated based on the total amount of the carbon-coated artificial graphite is 5% by weight or less,

A cathode having a porosity of 24% or more is provided.

According to the present invention, natural graphite and carbon-coated artificial graphite are used together as a negative electrode active material, the average particle diameter of the primary particles of the carbon-coated artificial graphite, the degree of orientation of the secondary particles, the content of the coated carbon, the negative electrode By controlling the porosity of the same, it is possible to provide a negative electrode with improved battery charging speed while simultaneously improving battery charging characteristics.

According to an embodiment of the present invention, by graphitizing the carbon-based material to obtain artificial graphite primary particles, the artificial graphite primary particles are aggregated by aggregation, bonding or granulation to obtain artificial graphite secondary particles, wherein The granulation and the carbon coating are carried out simultaneously, and finally the carbon-coated artificial graphite is obtained.

More specifically, the step of pulverizing the carbon-based material to a size of 1㎛ to 10㎛ average particle diameter (D50); Heat-treating the pulverized carbon-based material at a temperature of 2,800°C to 3,000°C to produce artificial graphite primary particles; And after mixing the artificial graphite primary particles and the binder, heat-treating at a temperature of 1,000° C. to 1,600° C. to simultaneously assemble the artificial graphite secondary particles and perform carbon coating; through which the carbon-coated artificial graphite is Can be obtained.

At this time, after mixing the artificial graphite primary particles and a binder, and then heat-treating at a temperature of 1,000°C to 1,600°C, the artificial graphite primary particles aggregate and grow as secondary particles, and at the same time, the carbon layer is formed on the surface of the secondary particles. Coating is done.

In one embodiment of the present invention, the carbon-based material may include coal-based heavy oil, petroleum-based heavy oil, tars, pitches, or coke, and specifically, needle cokes, mosaic coke ( It may include one or more carbon-based materials selected from the group consisting of mosaic cokes and coaltar pitch, and may have a flat shape. More specifically, the carbon-based material may be needle coke.

Artificial graphite used in an embodiment of the present invention is a commercially used MCMB (mesophase carbon microbeads), MPCF (mesophase pitch-based carbon fiber), artificial graphite graphitized in block form, artificial graphite graphitized in powder form There are graphite and the like, and the sphericity of the artificial graphite (artificial graphite secondary particles) may be 0.91 or less, or 0.6 to 0.91, or 0.7 to 0.9.

The average particle diameter of the artificial graphite primary particles may be 1 to 10 μm, or 1 to 5 μm, or 1 to 2.5 μm. When the average particle diameter of the artificial graphite primary particles is less than 1 μm, the surface area of the active material may be excessively large even after the secondary particle formation, resulting in excessive reaction activity to the electrolyte solution. Since this can be reduced, it is difficult to implement a high-capacity battery, and the amount of gas generated on the surface of the electrode increases, which can deteriorate the battery performance. In addition, when the average particle diameter of the artificial graphite primary particles is more than 10 μm, the diffusion distance in the artificial graphite becomes large after lithium ion insertion, and the contact surface of the electrolyte is relatively small, so that the diffusion resistance is large, so that lithium ion insertion and detachment is not easy.

The artificial graphite secondary particles may have an orientation (I004/I110) of 10 or less, or 1 to 10, or 1 to 6.

The degree of orientation (I004/I110) of the artificial graphite secondary particles means a peak intensity ratio (I004/I110) between the (004) plane and the (110) plane by X-ray diffraction analysis of the electrode state, wherein the artificial graphite The peak intensity ratio of graphite is a ratio of the peak intensity of the (004) diffraction line to the peak intensity of the (110) diffraction line of the artificial graphite, and it is possible to evaluate in which direction the artificial graphite crystal is oriented in the electrode. Specifically, the peak intensity ratio may be represented by orientation, and the larger the peak intensity ratio, the (002) plane may be oriented horizontally with the electrode current collector, and the (110) plane may be oriented vertically with the electrode current collector. It may have been done.

The orientation (I004/I110) of the artificial graphite secondary particles is the peak intensity ratio (I004/I110) between the (004) plane and the (110) plane by X-ray diffraction analysis of the electrode state, and the peak intensity ratio is X-ray diffraction It can be obtained through analysis, and the X-ray diffraction analysis can be measured using a Cu-Kα ray using an X-ray diffraction analyzer Bruker D4 Endeavor, and a numerical value can be corrected through a Topas3 fitting program.

At this time, when the peak intensity ratio (I004/I110) of the artificial graphite is greater than 10, the (004) surface showing the inertness of the artificial graphite in the electrode is oriented toward the surface contacting the anode, and lithium ions are inserted and desorbed. Since it is not easy, diffusion resistance increases and lithium precipitation occurs during high rate charging and discharging, which may degrade the life characteristics, and due to the precipitated lithium, thermal stability decreases, and dendrites form, causing short circuits in the battery. There may be a risk of ignition and explosion.

The content of carbon coated on the basis of the carbon-coated artificial graphite may be 5% by weight or less, or 4 to 5% by weight. When the content of the coated carbon is more than 5% by weight, the surface efficiency of the highly reactive surface coating layer lowers the initial efficiency due to side reaction, and since the coating layer does not contribute to the capacity, the capacity per unit weight is also reduced, making it difficult to realize the high capacity of the battery. Can.

In addition, the average particle diameter of the carbon-coated artificial graphite may be 1 to 35 μm, or 10 to 25 μm.

The natural graphite of the present invention is generally a plate-like agglomerate before being processed, and the plate-shaped particles have a spherical surface with a smooth surface through post-treatment processing such as particle crushing and reassembly to be used as an active material for electrode production. It is manufactured in the form.

Natural graphite used in an embodiment of the present invention may have a sphericity of greater than 0.91 and 0.97 or less, or 0.93 to 0.97, or 0.94 to 0.96. In addition, the natural graphite may have an average particle diameter of 5 to 30㎛, or 10 to 25㎛.

The content of the carbon-coated artificial graphite is 10% by weight or more based on the total amount of the negative electrode active material, or may be 10 to 30% by weight. When the content of the carbon-coated artificial graphite is less than 10% by weight, the main factor of the crystal structure of the negative electrode active material to be improved by the present invention is insufficient to be expressed on the final negative electrode, and as a result, the charge transfer resistance and diffusion resistance It is difficult to obtain a reduction effect, and the rapid charging characteristics may be deteriorated.

The porosity of the negative electrode is a parameter indicating the relative ratio of the volume of the pores enclosed from the group consisting of one or more of the active material, the binder, and the conductive material, based on the total volume of the active material layer of the negative electrode. The porosity of the cathode can be calculated by measuring the total amount of mercury permeation (mL/g) with a mercury porosimeter (device name: AutoPore VI 9500, Micromerities, USA).

The porosity of the negative electrode is 24% or more, or may be 24 to 26%.

According to an embodiment of the present invention, the loading amount of the negative electrode may mean the energy density per unit area of the active material layer of the negative electrode, and the electrode weight, the area of the electrode, the capacity of the active material, and the ratio of the active material included in the active material layer may be used. Can be calculated.

The loading amount of the negative electrode according to an embodiment of the present invention is 15 to 28 mg/cm ² based on the dry weight of the negative electrode active material layer. Can be

In one embodiment of the present invention, the thickness of the negative electrode active material layer is not particularly limited. For example, it may be 40 to 300㎛.

The negative electrode according to an embodiment of the present invention is a wet method of coating a slurry for a negative electrode active material layer obtained by dispersing a negative electrode active material, a binder, and a conductive material on a current collector, and a negative electrode active material, a binder, and a conductive material without using a dispersion medium. There may be a dry method of mixing directly in a powder state and selectively pulverizing and then coating the current collector.

For example, in the case of the wet method, a negative electrode active material, a binder, a conductive material, and optionally a thickener may be dispersed in a dispersion medium to prepare a negative electrode slurry, which is coated on at least one surface of a current collector for a negative electrode, dried and rolled. have.

In the method according to an embodiment of the present invention, the current collector for a negative electrode used as a base material for forming a negative electrode 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, copper or stainless steel surface treated with carbon, nickel, titanium, silver, etc., aluminum-cadmium alloy, etc. may be used.

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

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, an immersion coating method, a spray coating method, or the like may be used.

The binder is a component that assists in the bonding of the negative electrode active material and the conductive material and binding to the current collector, and may generally be included in an amount of 0.3 to 5 parts by weight, or 0.5 to 3.5 parts by weight based on 100 parts by weight of the total negative electrode active material.

Examples of the binder, polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HEP), polyvinylidene fluoride (polyvinylidenefluoride), polyacrylonitrile (polyacrylonitrile), polymethyl methacrylate (polymethylmethacrylate) ), polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, styrene butylene rubber (SBR), Various types of binder polymers such as fluorine rubber and various copolymers can be used.

The conductive material is a component for further improving the conductivity of the negative electrode active material, and may be included in 2 parts by weight or less, or 0.1 to 1.5 parts by weight based on 100 parts by weight of the total negative electrode active material.

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, furnace black, lamp black, and thermal black; Conductive fibers such as carbon fibers and metal fibers; Metal oxides such as carbon fluoride, aluminum, and nickel powder; Conductive whiskey such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; Conductive materials such as polyphenylene derivatives, carbon nanotubes, and graphene may be used.

As the solvent, N-methylpyrrolidone, acetone, water, or the like can be used, and the solvent has a concentration of solids in the slurry for the negative electrode active material layer of 50% to 95% by weight, or 70% to 90% by weight. It can be included if possible.

Further, as the thickener that can be selectively used, carboxymethyl cellulose (CMC), carboxyethyl cellulose, polyvinylpyrrolidone, and the like can be used.

The negative electrode active material layer obtained by coating and drying the slurry for the negative electrode active material layer may be rolled, and rolling may be performed by a method commonly used in the art, such as roll pressing, for example, 1 to 20. It can be carried out at a pressure of MPa and a temperature of 15 to 30 ℃.

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

Specifically, the lithium secondary battery may be manufactured by injecting a lithium salt-containing electrolyte into an electrode assembly including an anode, a cathode as described above, and a separator interposed therebetween.

The positive electrode is prepared by mixing a positive electrode active material, a conductive material, a binder, and a solvent to prepare a slurry, and then directly coating it on a metal current collector, or laminating a positive electrode active material film cast on a separate support and peeled from the support to a metal current collector. Thus, an anode can be produced.

Active materials used in the positive electrode include 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 independently of each other Al, Ni, Co, Fe, Mn, V, Cr, Ti, W, Ta, Mg, and any one selected from the group consisting of Mo, x, y and z independently of each other as the atomic fraction of the oxide composition elements 0≤x<0.5, 0≤ y <0.5, 0 ≤ z <0.5, 0 <x + y + z ≤ 1) may include any one of the active material particles or a mixture of two or more of them.

On the other hand, the conductive material, the binder and the solvent may be used in the same way as used in the production of the negative electrode.

The separator is a conventional porous polymer film used as a conventional separator, for example, a polyolefin-based polymer such as ethylene homopolymer, propylene homopolymer, ethylene/butene copolymer, ethylene/hexene copolymer and 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 can be used. The separator may include a safety reinforced separator (SRS) in which a ceramic material is thinly coated on the separator surface. In addition to the conventional porous nonwoven fabric, for example, a high melting point glass fiber, non-woven fabric made of polyethylene terephthalate fiber or the like can be used, but is not limited thereto.

The electrolyte solution includes a lithium salt as an electrolyte and an organic solvent for dissolving it.

The lithium salt may be used without limitation, if the ones commonly used in the secondary battery, the electrolytic solution, for example, is in the lithium salt anion ^{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 ^- can be used.

Organic solvents included in the electrolyte may be used without limitation as long as they are commonly used, and typically propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, dipropyl carbonate, dimethyl sulfoxide One or more selected from the group consisting of side, acetonitrile, dimethoxyethane, diethoxyethane, vinylene carbonate, sulfolane, gamma-butyrolactone, propylene sulfite and tetrahydrofuran can be used.

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

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

In the lithium secondary battery according to an embodiment of the present invention, a separator is disposed between an anode and a cathode to form an electrode assembly, and the electrode assembly is placed in, for example, a pouch, a cylindrical battery case, or a square battery case, and then the electrolyte When injected, the secondary battery can be completed. Alternatively, the lithium secondary battery may be completed by laminating the electrode assembly, impregnating it in an electrolyte, and sealing the resultant product in a battery case.

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

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 for a medium-to-large battery module including a plurality of battery cells. Preferred examples of the medium and large-sized devices include electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, power storage systems, etc., and are particularly useful for hybrid electric vehicles that require high power and batteries for new and renewable energy storage. Can be used.

The present inventors have determined that the largest factors affecting the rapid charging characteristics of the secondary battery are the anode active material crystal structure and the cathode electrode structure. Specifically, in the present invention, the main factors of the anode active material crystal structure include the average particle diameter of the artificial graphite primary particles, the degree of orientation of the artificial graphite secondary particles, and the content of carbon coated on the basis of the carbon-coated artificial graphite, The main factor of the cathode electrode structure is the porosity of the cathode.

In order to realize the improved rapid filling properties, the smaller the average particle diameter of the artificial graphite primary particles is, the smaller the orientation degree of the artificial graphite secondary particles is, the more preferable, and the content of carbon coated on the basis of carbon-coated artificial graphite. The more it is advantageous, the larger the porosity of the negative electrode may be advantageous.

However, even if the factors influencing these rapid charging characteristics satisfy the above conditions, it does not ensure excellent battery life characteristics, so that artificial graphite having crystal characteristics satisfying the above conditions is mixed with natural graphite. By using, the cathode electrode structure is secured to reduce the charge transfer resistance and diffusion resistance of the cathode to facilitate the insertion of lithium into the cathode electrode during charging to relieve the anode overvoltage and to precipitate Li on the cathode surface. It was made to prevent. This enables rapid charging during high-rate charging and improves degradation due to an increase in negative electrode resistance during the cycle of the battery, thereby ensuring long-term life characteristics.

As a result, the secondary battery provided with the negative electrode of the present invention, specifically, the cylindrical secondary battery can be rapidly charged under the conditions of charging and discharging at 1.0 to 1.2C, but can exhibit a value of 90% or higher in capacity retention even under 1,000 cycles or more. have.

Hereinafter, examples will be described in detail to help understanding of the present invention. However, the 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 following examples. The embodiments of the present invention are provided to more fully describe the present invention to those skilled in the art.

Example 1

<Production of carbon-coated artificial graphite>

The needle coke was pulverized using a jet mill, and then sieved to obtain a powder. The powder was heat-treated (graphitized) at 3,000° C. for 20 hours under an inert (Ar) gas atmosphere to prepare artificial graphite primary particles having an average particle diameter (D50) of 2.5 μm.

Thereafter, the artificial graphite primary particles and the binder (petroleum-based pitch) were added at a ratio of 92:8, and then heat treated at 1,500° C. for 10 hours to aggregate the artificial graphite primary particles into secondary particles to assemble. And simultaneously, carbon-coated artificial graphite was prepared while carbon coating was performed.

At this time, the primary average particle diameter of the artificial graphite, the weight ratio of the carbon-coated artificial graphite based on the total amount of the negative electrode active material, the carbon content coated on the artificial graphite, the orientation of the artificial graphite after secondary granulation is shown in Table 1.

<Production of cathode>

Cathode active material (a mixture of natural graphite and carbon-coated artificial graphite), a binder polymer (SBR (styrene-butadiene rubber) and CMC (carboxymethylcellulose)), a mixture of carbon black as a conductive material in a weight ratio of 95:3.5:1.5 And, by using water as the dispersion medium, the weight ratio of the mixture and the dispersion medium was mixed 1:2 to prepare a slurry for an active material layer.

In addition, the weight ratio of SBR (styrene-butadiene rubber) and CMC (carboxymethylcellulose) was 2.3:1.2.

Using the slot die, the slurry for the active material layer was coated on one surface of a copper (Cu) thin film having a thickness of 10 µm as a negative electrode current collector, and dried under vacuum at 130° C. for 1 hour to form an active material layer. The active material layer thus formed was rolled by a roll pressing method to prepare a negative electrode having an active material layer having a thickness of 80 μm. The loading amount was 17 mg/cm ² based on the dry weight of the negative electrode active material layer.

<Production of anode>

Li(Ni _0.8 Mn _0.1 Co _0.1 )O ₂ (NCM-811) as a positive electrode active material, carbon black as a conductive material, and polyvinylidene fluoride (PVdF) as a binder in a weight ratio of 96:2:2 Phosphorus N-methylpyrrolidone (NMP) was added to prepare a positive electrode active material slurry. The slurry was coated on one surface of an aluminum current collector having a thickness of 15 μm, and drying and rolling were performed under the same conditions as the negative electrode to prepare an anode. At this time, the loading amount was 20 mg/cm ² based on the dry weight of the positive electrode active material layer.

<Production of lithium secondary battery>

Ethylene carbonate (EC), propylene carbonate (PC), and ethyl methyl carbonate (EMC) were dissolved in an organic solvent mixed in a composition of 3:1:6 (volume ratio) to dissolve LiPF ₆ to a concentration of 1.0 M to dissolve the non-aqueous electrolyte. It was prepared.

A polyolefin separator was interposed between the positive electrode and the negative electrode prepared above, and then embedded in a cylindrical case, and then the electrolyte was injected to prepare a cylindrical lithium secondary battery.

Example 2

<Production of cathode>

The primary average particle diameter of the artificial graphite shown in Table 1, the weight ratio of the artificial graphite coated with carbon based on the total amount of the negative electrode active material, the carbon content coated on the artificial graphite, the condition of the orientation of the artificial graphite after secondary granulation, the thickness of the negative electrode active material layer A negative electrode was prepared in the same manner as in Example 1, except.

<Production of lithium secondary battery>

A lithium secondary battery was manufactured in the same manner as in Example 1, except that the prepared negative electrode was used.

Comparative Example 1

<Production of cathode>

An anode was prepared in the same manner as in Example 1, except that artificial graphite was not used and the conditions shown in Table 1 were used.

<Production of lithium secondary battery>

A lithium secondary battery was manufactured in the same manner as in Example 1, except that the prepared negative electrode was used.

Comparative Examples 2 to 7

<Production of cathode>

The primary average particle diameter of the artificial graphite shown in Table 1, the weight ratio of the artificial graphite coated with carbon based on the total amount of the negative electrode active material, the carbon content coated on the artificial graphite, the condition of the orientation of the artificial graphite after secondary granulation, the thickness of the negative electrode active material layer A negative electrode was prepared in the same manner as in Example 1, except.

<Production of lithium secondary battery>

A lithium secondary battery was manufactured in the same manner as in Example 1, except that the prepared negative electrode was used.

Characteristic evaluation

(1) Primary average particle size of artificial graphite

The average particle diameter, namely D50, means the particle diameter at 50% of the cumulative distribution of the particle number according to the particle diameter. The average particle diameter of the artificial graphite primary particles to be measured is dispersed in isopropyl alcohol as a dispersion medium, and then introduced into a laser diffraction particle size measuring device (Microtrac S3500) to determine the difference in diffraction patterns according to the average particle diameter when particles pass through the laser beam. The particle size distribution was calculated by measurement. D50 was measured by calculating the particle diameter at a point where 50% of the cumulative distribution of the number of particles according to the particle diameter in the measuring device was obtained.

(2) Orientation of artificial graphite (I004/I110)

The artificial graphite orientation (I004/I110) is the peak intensity ratio (I004/I110) between the (004) plane and the (110) plane by electrode state X-ray diffraction analysis, and the peak intensity ratio is obtained through X-ray diffraction analysis. The X-ray diffraction analysis was performed using a Cu-Kα ray using an X-ray diffraction analyzer Bruker D4 Endeavor, and the numerical value was corrected through a Topas3 fitting program.

(3) Content of carbon coated on the basis of carbon-coated artificial graphite

The content of carbon coated on the basis of carbon-coated artificial graphite was calculated by measuring the total weight of the finally obtained carbon-coated artificial graphite and the total weight of the artificial graphite initially introduced.

Content of carbon coated by weight based on carbon-coated artificial graphite (% by weight)

= {[(The total weight of the final obtained carbon-coated artificial graphite)-(the total weight of the first introduced artificial graphite)]/(the total weight of the final obtained carbon-coated artificial graphite)}X100

(4) Porosity of cathode

The porosity of the cathode was calculated by measuring the total amount of mercury permeation (mL/g) with a mercury porosimeter (device name: AutoPore VI 9500, Micromerities, USA).

Weight ratio of carbon-coated artificial graphite based on the total amount of anode active material (% by weight) Average particle size of artificial graphite primary particles
(㎛) Orientation of artificial graphite secondary particles
(I004/I110) Content of carbon coated by weight based on carbon-coated artificial graphite (% by weight) Thickness of cathode active material layer (㎛) Negative
Porosity (%) Example 1 30 2.5 3.5 4.5 80 24 Example 2 30 2.5 3.5 4.5 81.5 25.5 Comparative Example 1 0 - - - 80.4 24.5 Comparative Example 2 10 12.5 20 0 80.6 24.7 Comparative Example 3 70 5.0 19 0 80 24 Comparative Example 4 30 16 7.5 2 80.4 24.5 Comparative Example 5 87 5.0 12.5 2.5 81 25 Comparative Example 6 30 2.5 3.5 4.5 77.5 21.5 Comparative Example 7 30 2.5 3.5 4.5 79 23

(5) Initial efficiency characteristics of secondary batteries

The secondary battery prepared in Example 1 and Comparative Example 1 was charged at 24° C. with a constant current (CC) of 0.5C until 4.2V, and then charged with a constant voltage (CV) to obtain a charging current of 0.02 C (cut-off). charging) was performed until the current) was reached. Then, it was left for 10 minutes, and then discharged to a constant current (CC) of 0.2 C until 2.5 V was reached. At this time, the charging capacity and the discharging capacity were measured, and the initial efficiency was obtained by the following equation. The results are shown in FIG. 1.

Initial efficiency (%) of the secondary battery = (discharge capacity of 1 cycle / charge capacity of 1 cycle)Х100

Referring to FIG. 1, compared to Example 1, Comparative Example 1 observed a high overvoltage phenomenon in a region of 3.63 V or more during the same C-Rate charging process, and as a result, the battery of Example 1 was compared to Comparative Example 1 In comparison, it was confirmed that the resistance decreases during charging.

(6) Life characteristics of secondary batteries during low-speed charging and discharging

The secondary batteries prepared in Examples 1 to 2 and Comparative Examples 1 to 7 were charged at 24° C. until 0.3C and 4.1V, and discharged to a constant current of 0.5C until they became 2.85V to charge and discharge 1,000 times. The cycle was performed, and the discharge capacity after each cycle was measured compared to the discharge capacity after one cycle. Then, the capacity retention rate was measured according to the following formula. The results are shown in FIG. 2.

Capacity retention rate (%) = {discharge capacity after each cycle / discharge capacity after 1 cycle}X100

(7) Secondary battery life characteristics during high-speed charging and discharging (1.0C)

The secondary batteries prepared in Examples 1 to 2 and Comparative Examples 1 to 7 were charged at 24° C. until 1.0 C and 4.1 V, and discharged to a constant current of 1.0 C until it became 2.85 V to charge and discharge 1,000 times. The cycle was performed, and the discharge capacity after each cycle was measured compared to the discharge capacity after one cycle. Then, the capacity retention rate was measured according to the following formula. The results are shown in FIG. 3.

Capacity retention rate (%) = {discharge capacity after each cycle / discharge capacity after 1 cycle}X100

Referring to FIG. 2, it can be seen that in the low-speed charge/discharge conditions of 0.3C/0.5C, the secondary batteries of Examples 1 to 2 and Comparative Examples 1 to 7 were maintained constant for 1,000 cycles without any significant difference. .

Referring to FIG. 3, it can be seen that, under the conditions of 1.0C/1.0C high-speed charge/discharge, the secondary batteries of Examples 1 to 2 are kept constant for a period of 1,000 cycles, without much difference, similar to low-speed charge/discharge. On the other hand, in the high-speed charge/discharge conditions of 1.0C/1.0C, it was confirmed that the secondary cells of Comparative Examples 1 to 7 had a remarkable drop in capacity retention rate from as little as 100 cycles to as much as 800 cycles. From this, it can be seen that the secondary battery provided with the negative electrode of the present invention secures lifespan characteristics as well as low-speed charging and discharging.

(8) Secondary battery life characteristics during high-speed charging and discharging (1.2C)

The secondary battery prepared in Example 1 was charged at 24°C until it became 1.2C and 4.1V, and discharged until it became 2.85V with a constant current of 1.0C, thereby performing 250 charge and discharge cycles, and discharge after one cycle. The discharge capacity after each cycle compared to the capacity was measured. Then, the capacity retention rate was measured according to the following formula. The results are shown in FIG. 4.

Capacity retention rate (%) = {discharge capacity after each cycle / discharge capacity after 1 cycle}X100

Referring to FIG. 4, it can be seen that the secondary battery of Example 1 maintained a capacity retention rate of 90% or more for 250 cycles even under the ultra-fast charging and discharging conditions of 1.2C/1.0C, thereby significantly improving the rapid charging characteristics.

Claims (11)

As a negative electrode comprising a negative electrode active material layer containing a negative electrode active material, a conductive material and a binder,
The negative electrode active material includes natural graphite and carbon-coated artificial graphite,
The carbon-coated artificial graphite is present in an amount of 10% by weight or more based on the total amount of the negative electrode active material,
The artificial graphite is obtained by graphitizing the primary particles and then granulating the secondary particles and coating the carbon simultaneously.
The average particle diameter of the artificial graphite primary particles is in the range of 1 to 10㎛,
The artificial graphite secondary particles have an orientation (I004/I110) of 10 or less,
The content of the carbon coated based on the total amount of the carbon-coated artificial graphite is 5% by weight or less,
A cathode having a porosity of 24% or more. According to claim 1,
The carbon-coated artificial graphite is present in an amount of 10 to 30% by weight based on the total amount of the negative electrode active material. According to claim 1,
A negative electrode having 4 to 5% by weight of carbon coated on the artificial graphite. According to claim 1,
The cathode having an average particle diameter of the artificial graphite primary particles is 1 to 5㎛. According to claim 1,
The cathode having an average particle diameter of the artificial graphite primary particles is 1 to 2.5㎛. According to claim 1,
The artificial graphite has a negative orientation of 1 to 10. According to claim 1,
The artificial graphite has a negative orientation of 1 to 6. According to claim 1,
A cathode having a porosity of 24 to 26%. A lithium secondary battery comprising the negative electrode according to any one of claims 1 to 8. The method of claim 9,
The secondary battery is a lithium secondary battery capable of rapid charging at 1.0 to 1.2C. The method of claim 9,
A lithium secondary battery, wherein the secondary battery is a cylindrical secondary battery.

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