technical field
[One]
This application claims the benefit of priority based on Korean Patent Application No. 10-2020-0054232 dated May 7, 2020 and Korean Patent Application No. 10-2021-0045784 dated April 8, 2021. All disclosures are incorporated herein by reference.
[2]
The present invention relates to a high-nickel electrode sheet and a method for manufacturing the same, wherein a slurry of a first positive electrode active material having a high nickel content in a central region of an electrode sheet holding part and a nickel content having a relatively lower nickel content than that of the first positive active material in an edge region of the holding part are provided. It relates to an electrode sheet prepared by applying a second positive electrode active material slurry, and a method for manufacturing the same.
background
[3]
Recently, with the rapid spread of electronic devices using batteries, such as mobile phones, notebook computers, and electric vehicles, the demand for small, lightweight and relatively high-capacity secondary batteries is rapidly increasing. In particular, a lithium secondary battery has been in the spotlight as a driving power source for a portable device because it is lightweight and has a high energy density. Accordingly, research and development efforts for improving the performance of lithium secondary batteries are being actively conducted.
[4]
In a lithium secondary battery, an organic electrolyte or a polymer electrolyte is charged between a positive electrode and a negative electrode made of an active material capable of intercalation and deintercalation of lithium ions, and lithium ions are intercalated/deintercalated from the positive electrode and the negative electrode. Electric energy is produced by a reduction reaction with
[5]
As a positive active material of the lithium secondary battery, lithium cobalt oxide (LiCoO 2), lithium nickel oxide (LiNiO 2), lithium manganese oxide (LiMnO 2 or LiMn 2O 4, etc.), lithium iron phosphate compound (LiFePO 4), etc. were used. In addition, as a method for improving low thermal stability while maintaining the excellent reversible capacity of LiNiO 2 , a lithium composite metal oxide in which a part of nickel (Ni) is substituted with cobalt (Co) and manganese (Mn) (hereinafter simply referred to as ‘NCM-based Lithium composite transition metal oxide') was developed. However, the conventionally developed NCM-based lithium composite transition metal oxide has insufficient capacity characteristics, so there is a limit to its application.
[6]
In order to improve this problem, recently, research to increase the content of nickel in the NCM-based lithium composite transition metal oxide is being made. As the content of nickel increases, the energy density per volume increases, but in the case of a high-concentration nickel positive electrode active material, there is a problem in that the structural stability and chemical stability of the active material are deteriorated, so that thermal stability is rapidly reduced. In addition, nickel ions are irreversibly converted to NiO by reacting with moisture and carbon dioxide present in the storage environment, and as lithium ions escape in this process, lithium by-products present in the form of LiOH and Li 2CO 3 increase, Due to this, there are problems in increasing the resistance of the active material surface, reducing the capacity of the battery, and increasing gas generation during high-temperature storage.
[7]
Accordingly, in order to alleviate moisture content during the manufacture of a high-nickel positive electrode active material electrode having a high nickel content, Japanese Patent Application Laid-Open No. 2019-149269 discloses a first positive electrode active material having a nickel content of 50% to 80% in the central portion. A secondary battery including a positive electrode to which a second positive electrode active material having a nickel content of 20% to 40% is applied to both ends thereof. However, the conventional technique has an effect of suppressing the precipitation of lithium, but is not sufficient to improve the life performance of the battery, and the second positive active material applied to both ends contains only 20% to 40% of nickel, There have been difficulties in realizing the desired energy density.
[8]
Therefore, in a positive electrode having a high nickel content, it is necessary to develop a technology for a battery in which high energy density can be realized and moisture content is alleviated.
DETAILED DESCRIPTION OF THE INVENTION
technical challenge
[9]
The present invention has been devised to solve the above problems, and specifically, it is an object of the present invention to provide an electrode sheet capable of realizing high energy density and a method for manufacturing the same, and to alleviate the reaction of a positive electrode active material having a high nickel content with moisture. do it with
means of solving the problem
[10]
The present invention relates to an electrode sheet of a high-nickel positive electrode, wherein the electrode sheet according to the present invention includes a holding part and an uncoated part coated with a positive electrode mixture layer on at least one surface of a current collector, wherein the holding part has a length of the electrode sheet a first positive electrode mixture layer formed in a central portion along a direction and including a first positive electrode active material of lithium-nickel-cobalt-manganese oxide; and a second positive electrode mixture layer formed on one or both ends of the first positive electrode mixture layer and including a second positive electrode active material having a nickel content lower than that of the first positive electrode active material, wherein the rolling density of the second positive electrode mixture layer (b) is characterized in that it is smaller than the rolling density (a) of the first positive electrode mixture layer.
[11]
In an embodiment of the present invention, the content of nickel in the first positive electrode active material is 70 mol% or more of the total content of the transition metal.
[12]
In one embodiment of the present invention, the content of nickel in the second positive electrode active material may be less than 70 mol% of the total content of the transition metal, preferably 45 to 60 mol%.
[13]
In one embodiment of the present invention, the ratio (b/a) of the rolling density (b) of the second positive electrode mixture layer to the rolling density (a) of the first positive electrode mixture layer is 0.5 to 0.9.
[14]
In an embodiment of the present invention, the rolling density (a) of the first positive electrode mixture layer is 2.5 g/cm 3 to 4.3 g/cm 3 .
[15]
In one embodiment of the present invention, the width length of the second positive electrode mixture layer is 1 to 15% of the width length of the first positive electrode mixture layer.
[16]
The lithium secondary battery according to the present invention includes a positive electrode in which the holding part and the uncoated part of the electrode sheet are punched out according to the shape and size of the unit electrode.
[17]
In the method for manufacturing an electrode sheet according to the present invention, a first positive electrode slurry containing the first positive electrode active material and a second positive electrode slurry containing the second positive electrode active material are applied on a current collector sheet, and the first positive electrode mixture an application process of forming a layer and a second positive electrode mixture layer; drying process; and a rolling process, wherein in the application process, the second positive electrode slurry is parallel to the application direction of the first positive electrode slurry at one or both ends of the first positive electrode slurry based on the width direction of the electrode sheet is applied, and in the rolling process, the rolling density of the second positive electrode mixture layer is rolled to be smaller than the rolling density of the first positive electrode mixture layer.
[18]
In the manufacturing method of an embodiment of the present invention, in the coating process, the content of nickel in the second positive electrode active material may be 45 mol% to 60 mol% of the total content of the transition metal.
[19]
In the manufacturing method of an embodiment of the present invention, in the coating process, a loading amount per unit area of ​​the second positive electrode mixture layer is smaller than a loading amount per unit area of ​​the first positive electrode mixture layer.
[20]
In the manufacturing method of an embodiment of the present invention, in the coating process, the ratio (B) of the loading amount (B) per unit area of ​​the second positive electrode mixture layer to the loading amount (A) per unit area of ​​the first positive electrode mixture layer (B) /A) is 0.7 to 0.99.
[21]
In the manufacturing method according to an embodiment of the present invention, the ratio (b/a) of the rolling density (b) of the second positive electrode mixture layer to the rolling density (a) of the first positive electrode mixture layer is 0.5 to 0.9.
[22]
In the manufacturing method according to an embodiment of the present invention, in the coating process, the width of the second positive electrode mixture layer is 1 to 15% of the width of the first positive electrode mixture layer.
[23]
In the manufacturing method of an embodiment of the present invention, during the application process, the first positive electrode slurry and the second positive electrode slurry may be simultaneously applied.
Effects of the Invention
[24]
The electrode sheet and the method for manufacturing the electrode sheet according to the present invention, the rolling density (b) of the second positive electrode mixture layer formed at the edge of the holding unit having a relatively high possibility of contact with moisture outside the sheet, the first positive electrode mixture formed at the center of the holding unit As a result of reducing the generation of fine powder in the second positive electrode mixture layer by making it smaller than the rolling density (a) of the layer, the increase in the surface area of ​​the positive electrode active material to which moisture can be adsorbed after rolling is suppressed to minimize the reaction with moisture and energy It has the effect of improving the density.
Brief description of the drawing
[25]
1 is a schematic plan view showing an example of an electrode sheet according to an embodiment of the present invention.
Best mode for carrying out the invention
[26]
Hereinafter, the present invention will be described in detail. Prior to this, 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 should properly understand the concept of the term in order to best describe his invention. It should be interpreted as meaning and concept consistent with the technical idea of ​​the present invention based on the principle that it can be defined as
[27]
[28]
The present invention relates to an electrode sheet to which a positive electrode active material having a high nickel content is applied. A first positive electrode mixture layer formed in the central portion along the longitudinal direction of the lithium-nickel-cobalt-manganese oxide including a first positive electrode active material; and a second positive electrode mixture layer formed on one or both ends of the first positive electrode mixture layer and including a second positive electrode active material having a nickel content lower than that of the first positive electrode active material, wherein the rolling density of the second positive electrode mixture layer (b) is characterized in that it is smaller than the rolling density (a) of the first positive electrode mixture layer.
[29]
In general, the electrode sheet, which is coated with the electrode active material slurry on the current collector sheet, dried and rolled, is wound around the core until the electrode is punched for assembly of the battery and stored in a wound state. At this time, moisture permeation is easier at both ends of the holding part than in the central part of the holding part based on the width direction of the electrode sheet, and the moisture content is high by that amount.
[30]
The inventors of the present invention, in the NCM-based lithium composite transition metal oxide, when the content of nickel in the total content of the transition metal is less than 70 mol%, the reaction rate with moisture is slow and the reaction amount is small, paying attention to the fact that the electrode sheet is maintained Moisture needles on one or both edges of the A second positive electrode mixture layer including a second positive electrode active material of a nickel-comalt-manganese transition metal oxide having a nickel content of less than 70 mol% is disposed to relieve penetration, and a high energy density is implemented in the center of the electrode sheet holder A first positive active material layer including a first positive active material of a nickel-cobalt-manganese transition metal oxide having a nickel content of 70 mol% or more for By making it smaller than the rolling density, while maximizing the suppression of moisture reaction, the present invention capable of realizing high energy density was derived.
[31]
Referring to FIG. 1 , the electrode sheet of the present invention includes a holding part 100 and an uncoated part 200 coated with a positive electrode mixture layer on at least one surface of a current collector, and the holding part 100 includes an electrode sheet. It consists of a first positive electrode mixture layer 110 formed in a central portion along the longitudinal direction (y-axis) of the , and a second positive electrode mixture layer 120 formed on both ends of the first positive electrode mixture layer 110 . Here, both sides mean both sides based on the width direction (x-axis) of the electrode sheet. 1 illustrates an embodiment in which the second positive electrode mixture layer is formed on both ends of the first positive electrode mixture layer, but is not limited thereto, and the second positive electrode mixture layer may be formed on one end of the first positive electrode mixture layer .
[32]
The first positive active material of the present invention is a lithium-nickel-cobalt-manganese transition metal oxide, and in order to realize a high energy density, the content of nickel is preferably 70 mol% or more of the total content of the transition metal.
[33]
The second positive active material of the present invention is a lithium-nickel-cobalt-manganese transition metal oxide, which is applied to both edge portions of the holding part, and in order to suppress a reaction with moisture, the content of nickel is 70 mol% of the total content of the electric metal It is preferably less than, and more preferably, 45 mol% to 60 mol%. Conventionally, the nickel content of the second positive electrode active material is 40% or less to suppress reactivity with moisture, but in the present invention, the nickel content in the second positive electrode active material is controlled by controlling the rolling densities of the first positive electrode mixture layer and the second positive electrode mixture layer. to achieve both high energy density and suppression of moisture reactivity.
[34]
Each of the first positive active material and the second positive active material of the present invention is a particle group (powder). The average particle diameter D 50 of the particles of the first positive electrode active material may be 1 to 30 μm, preferably 3 to 20 μm, and most preferably 5 to 15 μm. In the present invention, D 50 may be defined as a particle size corresponding to 50% of the cumulative volume in the particle size distribution curve. The average particle diameter D 50 may be measured using, for example, a laser diffraction method. For example, in the method of measuring the average particle diameter (D 50) of the positive active material, the particles of the positive active material are dispersed in a dispersion medium, and then introduced into a commercially available laser diffraction particle size measuring device (eg, Microtrac MT 3000). After irradiating the 28 kHz ultrasonic wave with the output 60W, the average particle diameter D50 corresponding to 50% of the volume accumulation amount in a measuring apparatus can be computed.
[35]
The average particle diameter D 50 of the particles of the second positive electrode active material may be 1 to 30 μm, preferably 3 to 20 μm, and most preferably 5 to 15 μm.
[36]
The present invention is characterized in that the rolling density (b) of the second positive electrode mixture layer including the second positive electrode active material is smaller than the rolling density (a) of the first positive electrode mixture layer including the first positive electrode active material, wherein the rolling Density is defined as the density of the anode after rolling. The rolling density may be calculated by taking a positive electrode of a specific area and measuring the mass and thickness (volume) of the positive electrode.
[37]
In an embodiment of the present invention, the rolling density of the first positive electrode mixture layer may be 2.5 g/cm 3 to 4.3 g/cm 3 . The rolling density of the second positive electrode mixture layer may be 2.3 g/cm 3 to 4.1 g/cm 3 in a lower range.
[38]
The particle shape of the positive active material particles may be deformed or crushed by the force received during the rolling process. As the particles crumble, the specific surface area of ​​the positive active material particles increases. The available surface area increases. Accordingly, in the present invention, the second positive electrode mixture layer disposed in the edge region of the holding portion, which is highly likely to be in contact with moisture, is applied to the second positive electrode slurry in order to suppress the occurrence of fine powder during the rolling process. By making the loading amount of the positive electrode slurry relatively smaller than the loading amount of the first positive electrode slurry, or by making the rolling ratio of the second positive electrode mixture layer smaller than the rolling ratio of the first positive electrode mixture layer during the rolling process, the second positive electrode mixture layer The rolling density is smaller than the rolling density of the first positive electrode mixture layer.
[39]
In addition, since the first positive electrode mixture layer disposed in the central portion of the holding part is preferable for realizing a high energy density as the rolling density increases, the rolling density of the first positive electrode mixture layer is preferably greater than the rolling density of the second positive electrode mixture layer.
[40]
In one specific example, the ratio (b/a) of the rolling density (b) of the second positive electrode mixture layer to the rolling density (a) of the first positive electrode mixture layer is 0.5 to 0.9, more preferably 0.6 to 0.8 .
[41]
In one specific example, the width length of the second positive electrode mixture layer may be 1 to 15% of the width length of the first positive electrode mixture layer. Referring to FIG. 1 , the width length of the second positive electrode mixture layer may be defined as the sum of the width length W2a of the second positive electrode mixture layer at the left edge of the holding part and the width length W2b of the second positive electrode mixture layer at the right edge of the holding part. and the sum of these lengths (W2a + W2b) may be 1 to 15% of the width and length W1 of the first positive electrode mixture layer. When the width and length of the second positive electrode mixture layer satisfies the above numerical range, it is preferable in terms of realizing a high energy density while suppressing a reaction between nickel and moisture in the positive electrode active material. The width length of the second positive electrode mixture layer may be preferably 3 to 12% of the width of the first positive electrode mixture layer, and more preferably 5 to 10%.
[42]
[43]
Hereinafter, a method for manufacturing an electrode sheet according to the present invention will be described.
[44]
In the method for manufacturing an electrode sheet according to an embodiment of the present invention, a first positive electrode slurry containing the first positive electrode active material and a second positive electrode slurry containing the second positive electrode active material are applied on a current collector sheet, a coating process of forming a first positive electrode mixture layer and a second positive electrode mixture layer; drying process; and a rolling process, wherein in the application process, the second positive electrode slurry is parallel to the application direction of the first positive electrode slurry at one or both ends of the first positive electrode slurry based on the width direction of the electrode sheet is applied, and in the rolling process, the rolling density of the second positive electrode mixture layer is rolled to be smaller than the rolling density of the first positive electrode mixture layer.
[45]
In one specific example, in order to suppress an increase in the surface area of ​​the positive electrode active material in the second positive electrode mixture layer after rolling, the present invention sets the loading amount per unit area of ​​the second positive electrode mixture layer of the first positive electrode mixture layer during the application process. It is preferable to design it to be relatively smaller than the loading amount per unit area. Assuming that the same pressure is applied to the second positive electrode mixture layer and the first positive electrode mixture layer during rolling, by designing the loading amount per unit area of ​​the second positive electrode mixture layer to be smaller than the loading amount per unit area of ​​the first positive electrode mixture layer , generation of fine powder of the positive electrode active material in the second positive electrode mixture layer is relatively suppressed, and the first positive electrode mixture layer may have a predetermined energy density. In addition, such a loading amount design is also a design in consideration of the N/P ratio in the portion corresponding to the second positive electrode mixture layer. By making the loading amount per unit area of ​​the second positive electrode mixture layer smaller than the loading amount per unit area of ​​the first positive electrode mixture layer, lithium precipitation can be effectively prevented. In this case, the ratio (B/A) of the loading amount (B) per unit area of ​​the second positive electrode mixture layer to the loading amount (A) per unit area of ​​the first positive electrode mixture layer may be 0.7 to 0.99, more preferably may be 0.75 to 0.95, most preferably 0.8 to 0.9.
[46]
In another specific example, in order to suppress an increase in the surface area of ​​the positive electrode active material in the second positive electrode mixture layer after rolling, the rolling ratios of the second positive electrode mixture layer and the first positive electrode mixture layer may be set differently. The rolling ratio of the first positive electrode mixture layer is made larger than that of the first positive electrode mixture layer to have a predetermined energy density, while the second positive electrode mixture layer has a lower rolling rate compared to the first positive electrode mixture layer, thereby suppressing the generation of fine powder during the rolling process. can And in the present invention, the rolling rate may be defined as rolling strength. That is, it may be defined as the magnitude of the force per unit area applied to the positive electrode mixture layer during rolling, and the measuring method is not particularly limited as long as the force received per unit area is measured.
[47]
In the electrode sheet manufacturing method of the present invention, each of the first positive electrode active material contained in the first positive electrode mixture layer, the second positive electrode active material contained in the second positive electrode mixture layer, the first positive electrode mixture layer, and the second positive electrode mixture layer is rolled Density is the same as previously described.
[48]
In the coating process, the width of the second positive electrode mixture layer may be 1 to 15% of the width of the first positive electrode mixture layer. When the width and length of the second positive electrode mixture layer satisfies the above numerical range, it is preferable in terms of realizing a high energy density while suppressing a reaction between nickel and moisture in the positive electrode active material.
[49]
In the manufacturing method according to an embodiment of the present invention, in the coating process, the first positive electrode slurry and the second positive electrode slurry may be simultaneously applied, and after the first positive electrode slurry is applied, the second positive electrode slurry is applied it may be doing According to the above-described method, productivity may be improved by simultaneously applying the first positive electrode slurry and the second positive electrode slurry.
[50]
[51]
Hereinafter, the lithium secondary battery of the present invention will be described. Lithium secondary battery of the present invention, the holding portion of the electrode sheet and A positive electrode in which the uncoated region is punched out according to the shape and size of the unit electrode is included.
[52]
The positive electrode includes a positive electrode current collector and a first positive electrode mixture layer and a second positive electrode mixture layer formed on the positive electrode current collector.
[53]
Also, in the present invention, since the rolling ratio of the second positive electrode mixture layer is adjusted to be smaller than that of the first positive electrode mixture layer, the porosity of the second positive electrode mixture layer may be greater than that of the first positive electrode mixture layer. In the present specification, porosity can be defined as follows:
[54]
Porosity = pore volume per unit mass / (specific volume + pore volume per unit mass)
[55]
The measurement of the porosity is not particularly limited, and according to an embodiment of the present invention, for example, using an adsorbed gas such as nitrogen, BEL JAPAN's BELSORP (BET equipment) BET (Brunauer-Emmett-Teller) measurement method or It can be measured by Hg porosimetry.
[56]
The positive electrode current collector is not particularly limited as long as it has conductivity without causing chemical change in the battery, and for example, stainless steel, aluminum, nickel, titanium, fired carbon, or carbon, nickel, titanium on the surface of aluminum or stainless steel. , silver or the like surface-treated may be used. In addition, the positive electrode current collector may typically have a thickness of 3 to 500 μm, and may increase the adhesion of the positive electrode active material by forming fine irregularities on the surface of the positive electrode current collector. For example, it may be used in various forms, such as a film, a sheet, a foil, a net, a porous body, a foam, a non-woven body.
[57]
The first positive electrode mixture layer may include a conductive material and a binder together with the above-described first positive electrode active material, and the second positive electrode mixture layer may also include a conductive material and a binder together with the second positive electrode active material.
[58]
The conductive material is used to impart conductivity to the electrode, and in the configured battery, it can be used without any particular limitation as long as it does not cause chemical change and has electronic conductivity. Specific examples include graphite such as natural graphite and artificial graphite; carbon-based materials such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, summer black, and carbon fiber; metal powders or metal fibers, such as copper, nickel, aluminum, and silver; conductive whiskeys such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; or a conductive polymer such as a polyphenylene derivative, and the like, and one or a mixture of two or more thereof may be used. The conductive material may be included in an amount of 1 to 30 wt % based on the total weight of the cathode material layer.
[59]
The binder serves to improve adhesion between the positive active material particles and the adhesion between the positive active material and the positive electrode current collector. Specific examples include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC) ), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene butadiene rubber (SBR), fluororubber, or various copolymers thereof, and any one of them or a mixture of two or more thereof may be used. The binder may be included in an amount of 1 to 30% by weight based on the total weight of the cathode material layer.
[60]
The positive electrode uses the above-described positive electrode active material, a dual slot-die coater, a first positive electrode mixture layer is applied to the central portion, and a second positive electrode mixture layer is applied to both edges of the central portion, and when rolling, the first positive electrode mixture layer and the second positive electrode mixture layer are applied. 2 Except for varying the rolling ratio of the positive electrode mixture layer, it may be manufactured according to a conventional positive electrode manufacturing method. Specifically, the positive electrode slurry for forming a positive electrode composite layer comprising the above positive electrode active material and, optionally, a binder and a conductive material, is supplied to a dual slot-die coater, and applied on the positive electrode current collector using a dual slot-die coater. Then, it can be prepared by drying and rolling. In this case, the types and contents of the positive electrode material, the binder, and the conductive material are as described above.
[61]
The positive electrode slurry solvent may be a solvent generally used in the art, dimethyl sulfoxide (DMSO), isopropyl alcohol (isopropyl alcohol), N-methylpyrrolidone (NMP), acetone ( acetone) or water, and any one of them or a mixture of two or more thereof may be used. The amount of the solvent used is enough to dissolve or disperse the positive electrode active material, the conductive material and the binder in consideration of the application thickness of the slurry and the production yield, and to have a viscosity capable of exhibiting excellent thickness uniformity during application for the production of the positive electrode thereafter. Do.
[62]
The lithium secondary battery specifically includes a positive electrode, a negative electrode positioned to face the positive electrode, a separator and an electrolyte interposed between the positive electrode and the negative electrode, and the positive electrode is as described above. In addition, the lithium secondary battery may optionally further include a battery container for accommodating the electrode assembly of the positive electrode, the negative electrode, and the separator, and a sealing member for sealing the battery container.
[63]
The negative electrode current collector is not particularly limited as long as it has high conductivity without causing chemical change in the battery, and for example, copper, stainless steel, aluminum, nickel, titanium, fired carbon, copper or stainless steel surface. Carbon, nickel, titanium, silver, etc. surface-treated, aluminum-cadmium alloy, etc. may be used. In addition, the negative electrode current collector may have a thickness of typically 3 to 500 μm, and similarly to the positive electrode current collector, fine concavities and convexities may be formed on the surface of the current collector to strengthen the bonding force of the negative electrode active material. For example, it may be used in various forms, such as a film, a sheet, a foil, a net, a porous body, a foam, a nonwoven body.
[64]
The negative electrode mixture layer optionally includes a binder and a conductive material together with the negative electrode active material. As an example, the negative electrode mixture layer is formed by applying a negative electrode active material, and optionally, a negative electrode forming slurry including a binder and a conductive material on a negative electrode current collector and drying, or casting the negative electrode forming slurry on a separate support. , may be produced by laminating a film obtained by peeling from this support onto a negative electrode current collector.
[65]
As the anode active material, a compound capable of reversible intercalation and deintercalation of lithium may be used. Specific examples include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, and amorphous carbon; metal compounds capable of alloying with lithium, such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloy, Sn alloy, or Al alloy; metal oxides capable of doping and dedoping lithium, such as SiO x (0 < x < 2), SnO 2 , vanadium oxide, and lithium vanadium oxide; Alternatively, a composite including the metallic compound and a carbonaceous material such as a Si-C composite or a Sn-C composite may be used, and any one or a mixture of two or more thereof may be used. In addition, a metal lithium thin film may be used as the negative electrode active material. In addition, as the carbon material, both low crystalline carbon and high crystalline carbon may be used. As low crystalline carbon, soft carbon and hard carbon are representative, and as high crystalline carbon, natural or artificial graphite of amorphous, plate-like, scale-like, spherical or fibrous shape, and Kish graphite graphite), pyrolytic carbon, mesophase pitch based carbon fiber, meso-carbon microbeads, liquid crystal pitches (Mesophase pitches), and petroleum and coal tar pitch (petroleum or coal tar pitch) High-temperature calcined carbon such as derived cokes) is a representative example.
[66]
In addition, the binder and the conductive material may be the same as those described above for the positive electrode.
[67]
On the other hand, in the lithium secondary battery, the separator separates the negative electrode and the positive electrode and provides a passage for lithium ions to move, and as long as it is used as a separator in a lithium secondary battery, it can be used without any particular limitation, especially for the movement of ions in the electrolyte It is preferable to have a low resistance to respect and an excellent electrolyte moisture content. Specifically, a porous polymer film, for example, a porous polymer film made of a polyolefin-based polymer such as ethylene homopolymer, propylene homopolymer, ethylene/butene copolymer, ethylene/hexene copolymer and ethylene/methacrylate copolymer, or these A laminate structure of two or more layers of may be used. In addition, a conventional porous nonwoven fabric, for example, a nonwoven fabric made of high melting point glass fiber, polyethylene terephthalate fiber, etc. may be used. In addition, a coated separator containing a ceramic component or a polymer material may be used to secure heat resistance or mechanical strength, and may optionally be used in a single-layer or multi-layer structure.
[68]
In addition, examples of the electrolyte used in the present invention include organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel polymer electrolytes, solid inorganic electrolytes, and molten inorganic electrolytes, which can be used in the manufacture of lithium secondary batteries, and are limited to these. it is not going to be
[69]
Specifically, the electrolyte may include an organic solvent and a lithium salt.
[70]
The organic solvent may be used without particular limitation as long as it can serve as a medium through which ions involved in the electrochemical reaction of the battery can move. Specifically, as the organic solvent, ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, ε-caprolactone; dibutyl ether or tetrahydrofuran ether solvents such as ydrofuran); ketone solvents such as cyclohexanone; aromatic hydrocarbon-based solvents such as benzene and fluorobenzene; dimethylcarbonate (DMC), diethylcarbonate (DEC), methylethylcarbonate (MEC), ethylmethylcarbonate (EMC), ethylene carbonate (EC), propylene carbonate, carbonate-based solvents such as PC); alcohol solvents such as ethyl alcohol and isopropyl alcohol; nitriles such as R-CN (R is a C2 to C20 linear, branched or cyclic hydrocarbon group, which may include a double bond aromatic ring or an ether bond); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane; Or sulfolane may be used. Among these, a carbonate-based solvent is preferable, and a cyclic carbonate (eg, ethylene carbonate or propylene carbonate, etc.) having high ionic conductivity and high dielectric constant capable of increasing the charge/discharge performance of the battery, and a low-viscosity linear carbonate-based compound ( For example, a mixture of ethyl methyl carbonate, dimethyl carbonate or diethyl carbonate) is more preferable. In this case, when the cyclic carbonate and the chain carbonate are mixed in a volume ratio of about 1:1 to about 1:9, the performance of the electrolyte may be excellent.
[71]
The lithium salt may be used without particular limitation as long as it is a compound capable of providing lithium ions used in a lithium secondary battery. Specifically, the lithium salt is, LiPF 6, LiClO 4, LiAsF 6, LiBF 4, LiSbF 6, LiAl0 4, LiAlCl 4, LiCF 3SO 3, LiC 4F 9SO 3, LiN(C 2F 5SO 3) 2, LiN(C 2F 5SO 2) 2, LiN(CF 3SO 2) 2. LiCl, LiI, or LiB(C 2O 4) 2 and the like may be used. The concentration of the lithium salt is preferably used within the range of 0.1 to 2.0M. When the concentration of the lithium salt is included in the above range, since the electrolyte has appropriate conductivity and viscosity, excellent electrolyte performance may be exhibited, and lithium ions may move effectively.
[72]
In addition to the electrolyte components, the electrolyte includes, for example, haloalkylene carbonate-based compounds such as difluoroethylene carbonate, pyridine, tri Ethyl phosphite, triethanolamine, cyclic ether, ethylene diamine, n-glyme, hexaphosphoric acid triamide, nitrobenzene derivative, sulfur, quinone imine dye, N-substituted oxazolidinone, N,N-substituted imida One or more additives such as jolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxy ethanol or aluminum trichloride may be further included. In this case, the additive may be included in an amount of 0.1 to 5% by weight based on the total weight of the electrolyte.
[73]
As described above, since the lithium secondary battery including the positive electrode according to the present invention stably exhibits excellent discharge capacity, output characteristics and capacity retention rate, it is used in portable devices such as mobile phones, notebook computers, digital cameras, etc. It is useful in the automotive field, etc.
[74]
Accordingly, according to another embodiment of the present invention, a battery module including the lithium secondary battery as a unit cell and a battery pack including the same are provided. The battery module or battery pack is a power tool (Power Tool); electric vehicles, including electric vehicles (EVs), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEVs); Alternatively, it may be used as a power source for any one or more medium and large-sized devices in a system for power storage.
[75]
[76]
Hereinafter, embodiments of the present invention will be described in detail so that those of ordinary skill in the art can easily carry out the present invention. However, the present invention may be embodied in several different forms and is not limited to the embodiments described herein.
[77]
[78]
Example 1
[79]
(Preparation of the first positive electrode slurry for forming the first positive electrode mixture layer)
[80]
As a transition metal oxide of lithium-nickel-cobalt-manganese, a first positive active material having a nickel content of 80 mol% based on the total transition metal (NCM 811, D 50 is 11 μm) is prepared, and a carbon black conductive material and A PVDF binder was mixed in an N-methylpyrrolidone solvent in a weight ratio of 96.5:1.5:2 to prepare a first positive electrode slurry (viscosity: 5000 Pa·s).
[81]
(Preparation of the second positive electrode slurry for forming the second positive electrode mixture layer)
[82]
As a transition metal oxide of lithium-nickel-cobalt-manganese, a second positive electrode active material having a nickel content of 60 mol% based on the total transition metal (NCM 622, D 50 is 11 μm) was prepared, and a carbon black conductive material and A PVDF binder was mixed in an N-methylpyrrolidone solvent in a weight ratio of 96.5:1.5:2 to prepare a second positive electrode slurry (viscosity: 5000 Pa·s).
[83]
(Manufacture and storage of anode sheet)
[84]
The first positive electrode slurry and the second positive electrode slurry are supplied to a dual slot-die coater, and the first positive electrode slurry and the second positive electrode slurry are simultaneously coated on one surface of an aluminum current collector sheet using the coater to form a first positive electrode A mixture layer and a second positive electrode mixture layer were formed. At this time, the first positive electrode slurry was applied to the central portion of the slurry application portion, and the second positive electrode slurry was applied to both edges of the first positive electrode slurry application portion, and the sum of the widths and lengths of the second positive electrode slurry application portion was 1 It was set to be 8% of the width and length of the positive electrode slurry application part. In this case, the loading amount per unit area of ​​the first positive electrode slurry was 700 mg/25 cm 2 , and the loading amount per unit area of ​​the second positive electrode slurry was 560 mg/25 cm 2 , and the second positive electrode mixture with respect to the loading amount (A) of the first positive electrode mixture layer The ratio (B/A) of the loading amount (B) per unit area of ​​the layer was set to 0.8. Thereafter, the positive electrode sheet including the first positive electrode mixture layer and the second positive electrode mixture layer was dried at 130° C. and rolled. As a result of measuring the rolling densities of the first positive electrode mixture layer and the second positive electrode mixture layer after rolling, the rolling density of the first positive electrode mixture layer was 3.8 g/cm 3 , and the rolling density of the second positive electrode mixture layer was 2.47 g/cm 3 . Therefore, the ratio (b/a) of the rolling density (b) of the second positive electrode mixture layer to the rolling density (a) of the first positive electrode mixture layer was 0.65. Here, the rolling density was measured by taking a positive electrode having a specific area from each of the first positive electrode mixture layer and the second positive electrode mixture layer of the electrode sheet, and measuring the mass and volume.
[85]
After the thus-prepared positive electrode sheet was wound in the form of a roll and stored at room temperature of 25° C. for 7 days, the positive electrode tab was notched in the uncoated part of the positive electrode sheet, and the positive electrode sheet was slitted to complete the production of the positive electrode. .
[86]
(Manufacture of battery)
[87]
An electrode assembly was prepared by interposing a porous polyolefin separator between lithium metal as the positive electrode and the negative electrode, and the electrode assembly was placed inside the case, and then the electrolyte was injected into the case to prepare a lithium secondary battery. At this time, it was prepared by dissolving 1.0M concentration of lithium hexafluorophosphate (LiPF 6) in an organic solvent consisting of ethylene carbonate / dimethyl carbonate / ethyl methyl carbonate (mixed volume ratio of EC / DMC / EMC = 3 / 4 / 3).
[88]
[89]
Examples 2 to 3
[90]
In Example 1, a battery was manufactured in the same manner as in Example 1, except that the loading amount per unit area of ​​the second positive electrode slurry and the rolling density of the second positive electrode mixture layer were changed as shown in Table 1.
[91]
[92]
Example 4
[93]
In Example 1, the type of the second positive active material included in the second positive electrode slurry was changed to one having a nickel content of 1/3 (NCM 111 and D 50 were 11 μm), and per unit area of ​​the second positive electrode slurry A battery was manufactured in the same manner as in Example 1, except that the loading amount and the rolling density of the second positive electrode mixture layer were changed as shown in Table 1.
[94]
[95]
Comparative Examples 1 to 2
[96]
In Example 1, a battery was manufactured in the same manner as in Example 1, except that the loading amount per unit area of ​​the second positive electrode slurry and the rolling density of the second positive electrode mixture layer were changed as shown in Table 1.
[97]
[98]
[Table 1]
Loading amount per unit area Rolling density
First positive electrode slurry loading amount (A) Second positive electrode slurry loading amount (B) Ratio of loading amount per unit area (B/A) First positive electrode mixture layer (a) second positive electrode mixture layer (b) Ratio of rolling density ( b/a)
Example 1 700mg/25cm2 560mg/25cm2 0.8 3.8g/cm3 2.47g/cm3 0.65
Example 2 595mg/25cm2 0.85 2.66g/cm3 0.7
Example 3 630mg/25cm2 0.9 3.04g/cm3 0.8
Example 4 595mg/25cm2 0.85 2.85g/cm3 0.75
Comparative Example 1 770mg/25cm2 1.1 4.0g/cm3 1.05
Comparative Example 2 350mg/25cm2 0.5 1.71g/cm3 0.45
[99]
Experimental example: capacity and life characteristics evaluation
[100]
The lithium secondary batteries of Examples 1 to 4 and Comparative Examples 1 to 2 were initially charged and discharged using an electrochemical charger/discharger. At this time, charging was performed by applying a current at a current density of 1/3C C-rate up to a voltage of 4.2V, and discharging was performed up to 2.5V at the same current density. A total of 500 such charging and discharging was performed.
[101]
In the charging/discharging process as described above, the capacity of each battery was measured.
[102]
From this, the capacity retention rate of each battery was calculated as follows, and the results are shown in Table 2.
[103]
Capacity retention rate (%) = (capacity at 500 cycles/initial capacity) X 100
[104]
[105]
[Table 2]
Initial discharge capacity
(mAh/g) capacity retention rate
(%/500cycle)
Example 1 39.4 88.5
Example 2 40.4 90.8
Example 3 42.1 91.5
Example 4 40.9 89.9
Comparative Example 1 44.2 85.1
Comparative Example 2 35.8 87.2
[106]
Referring to Table 2, in Comparative Example 1, compared to Examples 1 to 4, although the initial discharge capacity was larger, it was found that the capacity retention rate after 500 cycles was small. The reason why the initial capacity of Comparative Example 1 was large seems to be because the rolling density of the second positive electrode mixture layer of Comparative Example 1 was relatively larger than that of Examples 1 to 4. And the poorest capacity retention rate of Comparative Example 1 is that the second positive electrode mixture layer of Comparative Example 1 generates the most dust during the rolling process and is relatively vulnerable to moisture. Lifetime performance seems to deteriorate by increasing resistance and gas generation.
[107]
On the other hand, Comparative Example 2 was found to have a small initial discharge capacity and capacity retention rate compared to Examples 1 to 4. The reason that the initial capacity of Comparative Example 2 is the smallest is probably because the rolling density of the second positive electrode mixture layer of Comparative Example 2 is the smallest. In addition, it can be confirmed that when the ratio (b/a) of the rolling density (b) of the second positive electrode mixture layer to the rolling density (a) of the first positive electrode mixture layer is too small, it is rather undesirable in terms of capacity retention. .
[108]
[109]
The present invention has been described in more detail with reference to the drawings and examples above. However, the configuration described in the drawings or embodiments described in the present specification is only one embodiment of the present invention and does not represent all the technical spirit of the present invention, so at the time of the present application, various equivalents and It should be understood that there may be variations.
Claims
[Claim 1]
An electrode sheet comprising a holding part and an uncoated part coated with a positive electrode mixture layer on at least one surface of a current collector, wherein the holding part is formed in a central part along the length direction of the electrode sheet, and is made of lithium-nickel-cobalt-manganese oxide 1 A first positive electrode mixture layer including a positive electrode active material; and a second positive electrode mixture layer formed on one or both ends of the first positive electrode mixture layer and including a second positive electrode active material having a nickel content lower than that of the first positive electrode active material, wherein the rolling density of the second positive electrode mixture layer (b) is smaller than the rolling density (a) of the first positive electrode mixture layer, the electrode sheet characterized in that.
[Claim 2]
The electrode sheet according to claim 1, wherein the content of nickel in the first positive active material is 70 mol% or more of the total content of the transition metal.
[Claim 3]
The electrode sheet according to claim 1, wherein the content of nickel in the second positive active material is less than 70 mol% of the total content of the transition metal.
[Claim 4]
The electrode sheet according to claim 3, wherein the content of nickel in the second positive electrode active material is 45 mol% to 60 mol% based on the total content of the transition metal.
[Claim 5]
The electrode sheet according to claim 1, wherein a ratio (b/a) of the rolling density (b) of the second positive electrode mixture layer to the rolling density (a) of the first positive electrode mixture layer is 0.5 to 0.9.
[Claim 6]
The electrode sheet according to claim 1, wherein the rolling density (a) of the first positive electrode mixture layer is 2.5 g/cm 3 to 4.3 g/cm 3 .
[Claim 7]
The electrode sheet according to claim 1, wherein a width of the second positive electrode mixture layer is 1 to 15% of a width of the first positive electrode mixture layer.
[Claim 8]
A lithium secondary battery comprising a positive electrode in which the holding part and the uncoated part of the electrode sheet according to claim 1 are punched out according to the shape and size of a unit electrode.
[Claim 9]
A method of manufacturing the electrode sheet according to claim 1, wherein a first positive electrode slurry including the first positive electrode active material and a second positive electrode slurry containing the second positive electrode active material are applied on a current collector sheet to form a first positive electrode an application process of forming a mixture layer and a second positive electrode mixture layer; drying process; and a rolling process, wherein in the application process, the second positive electrode slurry is parallel to the application direction of the first positive electrode slurry at one or both ends of the first positive electrode slurry based on the width direction of the electrode sheet coated, and in the rolling process, the rolling density of the second positive electrode mixture layer is rolled to be smaller than the rolling density of the first positive electrode mixture layer.
[Claim 10]
The method according to claim 9, wherein, in the coating process, the content of nickel in the second positive electrode active material is 45 mol% to 60 mol% of the total content of the transition metal.
[Claim 11]
10. The method of claim 9, wherein in the coating process, a loading amount per unit area of ​​the second positive electrode mixture layer is smaller than a loading amount per unit area of ​​the first positive electrode mixture layer.
[Claim 12]
12. The method of claim 11, wherein in the coating process, the ratio (B/A) of the loading amount (B) per unit area of ​​the second positive electrode mixture layer to the loading amount (A) per unit area of ​​the first positive electrode mixture layer is 0.7 to A method of manufacturing an electrode sheet, characterized in that 0.99.
[Claim 13]
10. The method according to claim 9, wherein the ratio (b/a) of the rolling density (a) of the second positive electrode mixture layer to the rolling density (a) of the first positive electrode mixture layer is 0.5 to 0.9. method.
[Claim 14]
10. The method of claim 9, wherein in the coating process, a width of the second positive electrode mixture layer is 1 to 15% of a width of the first positive electrode mixture layer.
[Claim 15]
10. The method of claim 9, wherein in the coating process, the first positive electrode slurry and the second positive electrode slurry are simultaneously applied.