Negative electrode for lithium secondary battery, method of manufacturing the same, and lithium secondary battery including the same

Abstract

To provide a negative electrode for lithium secondary battery that has superior electrochemical performance.SOLUTION: With respect to a negative electrode for lithium secondary battery and a lithium secondary battery including the negative electrode, the negative electrode includes a dry type negative electrode active material layer that has a negative electrode active material and N-containing protective layer formed, and includes polytetrafluoroethylene.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a negative electrode for a lithium secondary battery, a method for producing the same, and a lithium secondary battery including the same. [Background technology]

[0002] Lithium secondary batteries are batteries that exhibit excellent discharge voltage and high energy density. In recent years, the industry for medium- to large-sized secondary batteries required for electric vehicles has developed, increasing the need for lithium secondary batteries with higher energy density. Attempts have been made to develop thick-film electrodes to improve the energy density of lithium secondary batteries (see Patent Documents 1 and 2, etc.). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Korean Patent Application Publication No. 2014-0096526 [Patent Document 2] Korean Patent Application Publication No. 2024-0144817 Summary of the Invention [Problem to be solved by the invention]

[0004] However, when manufacturing electrodes using a conventional wet process, binder migration occurs during the drying process, which causes deterioration of the microstructure and makes it difficult to form electrodes with a sufficient thickness.

[0005] Therefore, methods for manufacturing lithium secondary batteries using dry processes have been studied. [Means for solving the problem]

[0006] One embodiment is to provide a negative electrode for a lithium secondary battery that has excellent electrochemical performance.

[0007] Another embodiment provides a method for producing the negative electrode.

[0008] Another embodiment provides a lithium secondary battery including the negative electrode.

[0009] One embodiment provides a negative electrode for a lithium secondary battery, including a dry negative electrode active material layer including a negative electrode active material and a polytetrafluoroethylene binder on which an N-containing protective layer is formed.

[0010] Another embodiment provides a method for manufacturing a negative electrode for a lithium secondary battery, including the steps of: mixing a negative electrode active material and polytetrafluoroethylene, and fiberizing the mixture to prepare a fiberized product; rolling the fiberized product to prepare a dry film; performing a gas-phase reaction treatment on the dry film to prepare a negative electrode active material layer; and positioning the negative electrode active material layer on a current collector.

[0011] Yet another embodiment provides a lithium secondary battery comprising: the negative electrode; a positive electrode; and a non-aqueous electrolyte.

[0012] Other specific details of the embodiments of the present invention are included in the following detailed description.

[0013] The negative electrode for a lithium secondary battery according to one embodiment can provide a battery that exhibits excellent initial efficiency and life characteristics. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is a diagram illustrating a process of forming a protective layer on a negative electrode according to an embodiment; [Figure 2] 1 is a cross-sectional view schematically illustrating a lithium secondary battery according to an embodiment. [Figure 3] 1 is a cross-sectional view schematically illustrating a lithium secondary battery according to an embodiment. [Figure 4] 1 is a cross-sectional view schematically illustrating a lithium secondary battery according to an embodiment. [Figure 5]1 is a cross-sectional view schematically illustrating a lithium secondary battery according to an embodiment. [Figure 6] 1 is a graph showing the initial efficiency of half cells manufactured according to Example 1 and Comparative Example 1. [Figure 7] 1 is a graph showing the life characteristics of half cells produced in Examples 1 to 3 and Comparative Example 1. [Figure 8] 1 is a graph showing the coulombic efficiencies of half cells produced in Examples 1 to 3 and Comparative Example 1. [Figure 9] 1 is a graph showing the life characteristics of half cells produced in Examples 1, 4, and 5 and Comparative Example 1. [Figure 10] 1 is a graph showing the coulombic efficiency of half cells produced in Examples 1, 4, and 5 and Comparative Example 1. [Figure 11] 1 is an XPS measurement graph for the negative electrode of Comparative Example 1. [Figure 12] 1 is an XPS measurement graph for the negative electrode of Comparative Example 2. [Figure 13] 1 is an XPS measurement graph for the negative electrode of Example 1. [Figure 14] 1 is an XPS measurement graph for the negative electrode of Example 2. [Figure 15] 1 is an XPS measurement graph for the negative electrode of Example 3. [Figure 16] 1 is an XPS measurement graph for the negative electrode of Example 4. [Figure 17] 10 is an XPS measurement graph for the negative electrode of Example 5. DETAILED DESCRIPTION OF THE INVENTION

[0015] DETAILED DESCRIPTION OF THE INVENTION The following detailed description of the present invention is provided by way of example only and is not intended to limit the scope of the present invention, which is defined solely by the claims that follow.

[0016] The terms used herein are merely used to describe exemplary embodiments and are not intended to limit the present invention. The singular expressions include the plural expressions unless the context clearly indicates otherwise.

[0017] As used herein, "combinations thereof" refers to mixtures, laminates, composites, copolymers, alloys, blends, reaction products, and the like of compositions.

[0018] It should be understood that the terms "comprise," "comprise," or "have" are intended to specify the presence of embodied features, numbers, steps, components, or combinations thereof, but do not preclude the possibility of the presence or addition of one or more other features, numbers, steps, components, or combinations thereof.

[0019] To clearly illustrate multiple layers and regions in the drawings, thicknesses have been exaggerated, and similar parts have been given the same reference numerals throughout the specification. When a layer, film, region, plate, or other part is said to be "on" or "above" another part, this includes not only the case where it is "directly on" the other part, but also the case where there is another part between them. Conversely, when a part is said to be "directly on" another part, it means that there is no other part between them.

[0020] Here, the term "layer" includes not only shapes formed on the entire surface but also shapes formed on a portion of the surface when observed in a plan view.

[0021] Here, "or" is not to be construed in an exclusive sense; for example, "A or B" is to be construed as including A, B, A+B, etc.

[0022] Unless otherwise specified, particle size or size herein may refer to the average particle size. This average particle size refers to the average particle size (D50), which is the diameter of particles with a cumulative volume of 50% in a particle size distribution. The average particle size (D50) can be measured by methods well known to those skilled in the art, such as using a particle size analyzer, a transmission electron microscope (TEM), or a scanning electron microscope (SEM). Alternatively, measurements can be performed using a measuring device that utilizes dynamic light scattering, and data analysis can be performed to count the number of particles in each particle size range, followed by calculation to obtain the average particle size (D50).

[0023] In one embodiment, the average particle size can be measured by the various methods described above, for example, using a particle size analyzer.

[0024] In one embodiment, the thickness can be measured by any method known in the art, including, but not limited to, SEM or TEM images of a cross section, and the thickness can be an average thickness.

[0025] In this specification, soft carbon refers to a graphitizable carbon material that can be graphitized by heat treatment at a high temperature, for example, 2800°C, and hard carbon refers to a non-graphitizable carbon material that is not graphitized or is substantially not graphitized by heat treatment. Soft carbon and hard carbon are widely known in the art.

[0026] In one embodiment, crystalline carbon and amorphous carbon can be classified by X-ray diffraction analysis. Crystalline carbon includes natural graphite and artificial graphite. Natural graphite refers to naturally occurring graphite obtained by separation from minerals, and has a d002 of 3.350 Å to 3.360 Å in X-ray diffraction analysis. Artificial graphite refers to graphite produced by graphitization, and has a d002 of 3.355 Å to 3.365 Å in X-ray diffraction analysis. Amorphous carbon has a d002 of 3.34 Å or less in X-ray diffraction analysis. X-ray diffraction analysis (XRD) uses CuKα radiation as the target radiation and an X-ray diffraction analyzer, such as an X'Pert (manufactured by Malvern Panalytical), and the monochromator device can be removed for improved peak intensity resolution. The measurement conditions can be set as follows: 2θ=10° to 80°, scan speed (° / S)=0.044 to 0.089, step size (° / step)=0.013 to 0.039.

[0027] According to one embodiment, a negative electrode for a lithium secondary battery includes a dry negative electrode active material layer including a negative electrode active material and a polytetrafluoroethylene binder having an N-containing protective layer formed thereon.

[0028] Such an anode is an anode that does not use a solvent during the manufacture of the anode, that is, an anode that is manufactured using a dry process rather than a wet process.

[0029] A binder according to one embodiment has a protective layer formed on the surface of polytetrafluoroethylene, and the protective layer contains the element N. Such a protective layer containing the element N, for example, an N-containing protective layer, may further contain an element C (carbon), O (oxygen), H (hydrogen), or a combination thereof.

[0030] The N-containing protective layer may contain an N-containing functional group. The N-containing functional group may be an imide group, an amide group, an amine group, a nitrile group, or a combination thereof. The presence of such an N-containing functional group in the N-containing protective layer can be determined from the bonding energy peak of the negative electrode during XPS measurement. For example, if a peak appears at a bonding energy of 399.0 eV to 400 eV, it indicates the presence of an amide group, and if a peak appears at a bonding energy of 400.4 eV to 401 eV, it indicates the presence of an imide group. Furthermore, if a peak appears at a bonding energy of 397 eV to 400.2 eV, it indicates the presence of an amine group or a nitrile group.

[0031] The N-containing protective layer can suppress side reactions of polytetrafluoroethylene and improve the mechanical strength of the anode. Polytetrafluoroethylene has a low LUMO (lowest unoccupied molecular orbital) level and low reduction stability, which can cause side reactions when used in anodes, resulting in reduced binding properties, a decrease in the initial efficiency of the electrode, and irreversible capacity loss. In one embodiment, the N-containing protective layer can improve the reduction stability of polytetrafluoroethylene, thereby preventing side reactions when used in anodes and improving the initial efficiency and lifespan characteristics while maintaining the mechanical properties of the anode.

[0032] Since the negative electrode according to one embodiment relates to a dry negative electrode, the dry negative electrode active material layer is a thick layer having a thickness of 10 μm to 400 μm, 100 μm to 250 μm, or 120 μm to 180 μm, thereby providing a negative electrode having superior energy density.

[0033] In one embodiment, the N-containing protective layer is formed on the surface of the polytetrafluoroethylene, so the entire surface of the negative electrode active material layer is not substantially covered with the N-containing protective layer, which is not suitable because the conductivity would be reduced if the entire surface of the negative electrode active material layer were covered with the N-containing protective layer.

[0034] In one embodiment, the negative electrode active material may also include an N-containing protective layer. For example, an N-containing protective layer may be formed on the surface of the negative electrode active material. When an N-containing protective layer is formed on the entire negative electrode active material together with the polytetrafluoroethylene binder, side reactions of the polytetrafluoroethylene can be effectively controlled and the interface resistance of the active material can also be reduced.

[0035] In one embodiment, the thickness of the N-containing protective layer may be 0.1 nm to 30 nm, 0.5 nm to 20 nm, 1 nm to 10 nm, or 1 nm to 3 nm. When the thickness of the N-containing protective layer satisfies this range, side reactions of polytetrafluoroethylene can be more effectively prevented and the initial efficiency can be further improved. The thickness may refer to an average thickness. In one embodiment, the thickness can be measured using a SEM or TEM.

[0036] In one embodiment, the content of the N-containing protective layer may be 1 wt% to 50 wt%, 2 wt% to 30 wt%, or 5 wt% to 10 wt% relative to 100 wt% of polytetrafluoroethylene. When the content of the N-containing protective layer is within this range, side reactions of polytetrafluoroethylene can be more effectively controlled.

[0037] In one embodiment, the content of the fluoroetherurene binder forming the N-containing protective layer may be 0.1 wt % to 50 wt %, or 1 wt % to 10 wt %, relative to 100 wt % of the negative electrode active material layer. When the binder content is within this range, the negative electrode active material adheres well while maintaining the mechanical properties of the negative electrode, and the negative electrode active material layer can be firmly attached to the current collector without falling off.

[0038] In one embodiment, the dry negative electrode active material layer may further include a non-fibrillated binder. The non-fibrillated binder may be carboxymethyl cellulose, styrene-butadiene rubber, polyvinylidene fluoride, polyacrylic acid, or a combination thereof. An N-containing protective layer may or may not be formed on the surface of the non-fibrillated binder.

[0039] When the dry negative electrode active material layer further contains such a non-fibrous binder, the mixing ratio of the polytetrafluoroethylene on which the N-containing protective layer is formed and the non-fibrous binder may be 1:5 to 1:1 by weight, or may be 1:5 to 1:2 by weight. When the mixing ratio of the polytetrafluoroethylene on which the N-containing protective layer is formed and the non-fibrous binder is within this range, it is appropriate because it can maintain the mechanical properties of the negative electrode and further strengthen the adhesive strength between the negative electrode active materials and to the current collector.

[0040] The polytetrafluoroethylene on which the N-containing protective layer is formed may be fibrillated.

[0041] The negative electrode active material includes a material capable of reversibly inserting / extracting lithium ions, lithium metal, a lithium metal alloy, a material capable of being doped with and dedoped from lithium, or a transition metal oxide.

[0042] The carbon-based negative electrode active material, which can reversibly insert / extract lithium ions, can include, for example, crystalline carbon, amorphous carbon, or a combination thereof. Examples of crystalline carbon include amorphous, plate-like, flake-like, spherical, or fibrous graphite, such as natural graphite or artificial graphite. Examples of amorphous carbon include soft carbon, hard carbon, mesophase pitch carbide, and calcined coke.

[0043] As an alloy of lithium metal, an alloy of lithium and a metal selected from Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al, and Sn can be used.

[0044] As a substance capable of doping and undoping lithium, a Si-based anode active material or a Sn-based anode active material can be used. The Si-based anode active material can be silicon, a silicon-carbon composite, SiO x (0 < x < 2), a Si-Q alloy (where Q is selected from alkali metals, alkaline earth metals, group 13 elements, group 14 elements (excluding Si), group 15 elements, group 16 elements, transition metals, rare earth elements, and combinations thereof), or a combination of these. The Sn-based anode active material can be Sn, SnO2, a Sn-based alloy, or a combination of these. [[ID=~]]

[0045] The silicon-carbon composite may be a composite of silicon and amorphous carbon. According to one embodiment, the silicon-carbon composite may be in a form in which amorphous carbon is coated on the surface of silicon particles. For example, it can include secondary particles (cores) formed by granulating primary silicon particles, and an amorphous carbon coating layer (shell) located on the surface of these secondary particles. Amorphous carbon may also be located between the primary silicon particles. For example, the primary silicon particles may be coated with amorphous carbon. The secondary particles may be dispersed and present in an amorphous carbon matrix.

[0046] The silicon-carbon composite can further contain crystalline carbon. For example, the silicon-carbon composite can include a core containing crystalline carbon and silicon particles, and an amorphous carbon coating layer located on the surface of this core. [[ID=~]]

[0047] The silicon particles may be silicon nanoparticles.

[0048] The particle size of the silicon nanoparticles may be 10 nm to 1,000 nm, and in another embodiment, 10 nm to 200 nm, or 20 nm to 150 nm. When the particle size of the silicon nanoparticles is within this range, excessive volume expansion that occurs during charge and discharge can be suppressed, and conductive path disconnection due to particle crushing during charge and discharge can be prevented.

[0049] In one embodiment of the negative electrode active material, the core silicon-carbon composite may include silicon nanoparticles and an amorphous carbon coating layer disposed on the silicon nanoparticles. The silicon-carbon composite may also include granules formed by granulating at least one silicon nanoparticle and an amorphous carbon coating layer disposed on the granules.

[0050] In the amorphous carbon coating layer, the amorphous carbon may be soft carbon, hard carbon, mesophase pitch carbide, calcined coke, or a combination thereof. The thickness of the amorphous carbon coating layer may be 1 nm to 2 μm, 1 nm to 500 nm, 10 nm to 300 nm, or 20 nm to 200 nm. When the thickness of the amorphous carbon coating layer is within this range, volume expansion of silicon during charge and discharge can be effectively suppressed.

[0051] The crystalline carbon may be amorphous, plate-like, flake-like, spherical or fibrous natural graphite, artificial graphite, or a combination thereof.

[0052] When the silicon-carbon composite contains silicon nanoparticles and an amorphous carbon coating layer, the content of the silicon nanoparticles may be 30 to 70% by weight, or 40 to 65% by weight, relative to 100% by weight of the entire silicon-carbon composite. The content of the amorphous carbon coating layer may be 30 to 70% by weight, or 35 to 60% by weight, relative to 100% by weight of the entire silicon-carbon composite.

[0053] When the silicon-carbon composite further contains crystalline carbon, the content of silicon nanoparticles may be 20% to 70% by weight or 25% to 65% by weight, relative to 100% by weight of the entire silicon-carbon composite. The content of amorphous carbon may be 25% to 70% by weight or 25% to 60% by weight, and the content of crystalline carbon may be 1% to 20% by weight or 5% to 15% by weight, relative to 100% by weight of the entire silicon-carbon composite.

[0054] In the negative electrode active material layer, the content of the negative electrode active material may be 0.1 wt % to 99.9 wt %, or may be 60 wt % to 99 wt %, relative to 100 wt % of the entire negative electrode active material layer.

[0055] The negative electrode active material layer may further include a conductive material. When the negative electrode active material layer includes a conductive material, the content of the conductive material may be more than 0 wt % and 10 wt % or less, based on 100 wt % of the total weight of the negative electrode active material layer.

[0056] The conductive material is used to impart conductivity to the electrodes and can be any material that is electronically conductive and does not cause chemical changes in the battery. Specific examples include carbon-based materials such as natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, carbon fiber, carbon nanofiber, and carbon nanotube; metal-based materials in the form of metal powder or metal fiber, including copper, nickel, aluminum, and silver; conductive polymers such as polyphenylene derivatives; and mixtures thereof.

[0057] The negative electrode according to one embodiment includes a current collector that supports a negative electrode active material layer.

[0058] The current collector can be selected from copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, a polymer substrate coated with a conductive metal, and combinations thereof.

[0059] <Method of manufacturing the negative electrode> The negative electrode active material and polytetrafluoroethylene are mixed and then fiberized to produce a fiberized product. The mixing ratio of the negative electrode active material to polytetrafluoroethylene may be 80:20 to 98:2 by weight, or may be 94:6 to 98:2 by weight.

[0060] The fiberization process can be carried out by mixing the negative electrode active material and polytetrafluoroethylene at a speed of 100 rpm to 50,000 rpm, for example, 3,000 rpm to 14,000 rpm, so that an appropriate shear force is generated. During the fiberization process, the particulate polytetrafluoroethylene is stretched and converted into a fibrous form, i.e., fibrillated, and the fibrillated polytetrafluoroethylene and the negative electrode active material can form a network together.

[0061] In the mixing step, a non-fibrillating binder can also be added, which can be carboxymethyl cellulose, styrene-butadiene rubber, polyvinylidene fluoride, polyacrylic acid, or a combination thereof.

[0062] In the mixing step, a conductive material may be further added. When a conductive material is further added, the fibrillated polytetrafluoroethylene, the negative electrode active material, and the conductive material may form a network together.

[0063] If polytetrafluoroethylene is first fiberized and then the fibrillated polytetrafluoroethylene is mixed with an active material and a conductive material, network formation will not proceed and a dry film cannot be produced.

[0064] The fiberized product is calendered to produce a dry film.

[0065] Subsequently, the obtained dry film is subjected to a gas phase reaction treatment to produce a negative electrode active material layer.

[0066] The gas phase reaction may be chemical vapor deposition, physical vapor deposition, atomic layer deposition, plasma, or a combination thereof.

[0067] More specifically, the gas-phase reaction treatment process can include a gas plasma treatment process and a gas nitriding process of the dry film. The gas plasma treatment process can be performed using O2 gas, or can be performed by mixing an inert gas with O2 gas. The inert gas can be Ar gas, N2 gas, or a combination thereof. When O2 gas and an inert gas are used together, the mixing ratio can be 1:10 to 10:1 by volume, or 1:3 to 3:1 by volume. The gas nitriding process can be performed by chemical vapor deposition using NH3 gas.

[0068] Figure 1 is a schematic diagram showing an O2 plasma treatment process using O2 as gas plasma and a fiberization process using NH3 gas. Figure 1 illustrates a nitridation process using NH3 gas and chemical vapor deposition (CVD) at 300°C, but is not limited thereto.

[0069] As shown in Figure 1, when a dry film is treated with oxygen plasma, oxygen-containing functional groups such as COOH, C=O, C-OH, or a combination thereof are generated on the surface of the binder. In the gas nitriding process, the oxygen-containing functional groups are converted to N-containing functional groups, forming an N-containing protective layer on the surface of the binder.

[0070] The gas nitriding step can be carried out at a temperature of 50° C. to 600° C., or 200° C. to 320° C. When the gas nitriding step is carried out within this temperature range, a good protective layer can be formed without damaging the binder fibers.

[0071] The gas plasma process can be carried out for 1 second to 120 minutes, 30 seconds to 50 minutes, 30 seconds to 10 minutes, or 30 seconds to 5 minutes. When the gas plasma process is carried out for the above time, oxygen-containing functional groups can be formed on the surface of the binder.

[0072] The gas nitriding step can be carried out for 1 minute to 6 hours, 30 minutes to 5 hours, or 1 hour to 4 hours. When the gas nitriding step is carried out for such a time, N-containing functional groups can be formed on the surface of the binder.

[0073] If a binder is first subjected to a gas-phase reaction, such as a gas plasma treatment or a gas nitriding treatment, and then this binder is mixed with a negative electrode active material to proceed with fiberization, the binder will be stretched as it is fiberized, and new untreated surfaces will continue to be generated, resulting in an excess of surfaces that are not subjected to the gas-phase reaction, which will prevent the formation of a protective layer on the binder surface.

[0074] Subsequently, the negative electrode active material layer is disposed on a current collector to manufacture the negative electrode. The disposing step can be performed by a lamination process, but is not limited thereto.

[0075] <Lithium secondary battery> A lithium secondary battery according to another embodiment includes a negative electrode, a positive electrode, and a non-aqueous electrolyte.

[0076] <Positive electrode> The positive electrode includes a current collector and a positive electrode active material layer formed on the current collector. The positive electrode active material layer includes a positive electrode active material and may further include a binder and / or a conductive material.

[0077] For example, the positive electrode may further include an additive that acts as a sacrificial positive electrode.

[0078] The positive electrode active material may be a compound capable of reversibly inserting and extracting lithium (lithiate intercalation compound). Specifically, one or more of composite oxides of lithium and a metal selected from cobalt, manganese, nickel, and combinations thereof may be used.

[0079] The composite oxide may be a lithium transition metal composite oxide, and specific examples thereof include lithium nickel-based oxides, lithium cobalt-based oxides, lithium manganese-based oxides, lithium iron phosphate-based compounds, cobalt-free nickel-manganese-based oxides, or combinations thereof.

[0080] For example, a compound represented by any of the following chemical formulas can be used.

[0081] Li a A 1-b X b O 2-c D c (0.90≦a≦1.8, 0≦b≦0.5, 0≦c≦0.05);Li a Mn 2-b X b O 4-c D c (0.90≦a≦1.8, 0≦b≦0.5, 0≦c≦0.05);Li a Ni 1-b-c Co b X c O 2-α D α (0.90≦a≦1.8, 0≦b≦0.5, 0≦c≦0.5, 0<α<2);Li a Ni 1-b-c Mn b X c O 2-α D α (0.90≦a≦1.8, 0≦b≦0.5, 0≦c≦0.5, 0<α<2);Li a Ni b Co c L 1 d G e O2(0.90≦a≦1.8, 0≦b≦0.9, 0≦c≦0.5, 0≦d≦0.5, 0≦e≦0.1);Lia NiG b O2(0.90≦a≦1.8, 0.001≦b≦0.1);Li a CoG b O2(0.90≦a≦1.8, 0.001≦b≦0.1);Li a Mn 1-b G b O2(0.90≦a≦1.8, 0.001b≦0.1);Li a Mn2G b O4(0.90≦a≦1.8, 0.001≦b≦0.1);Li a Mn 1-g G g PO4(0.90≦a≦1.8, 0≦g≦0.5);Li (3-f) Fe2(PO4)3(0≦f≦2);Li a FePO4(0.90≦a≦1.8).

[0082] In the above chemical formula, A is Ni, Co, Mn, or a combination thereof; X is Al, Ni, Co, Mn, Cr, Fe, Mg, Sr, V, a rare earth element, or a combination thereof; D is O, F, S, P, or a combination thereof; G is Al, Cr, Mn, Fe, Mg, La, Ce, Sr, V, or a combination thereof; L 1 is Mn, Al, or a combination thereof.

[0083] For example, the positive electrode active material may be a high-nickel positive electrode active material in which the nickel content relative to 100 mol% of metals excluding lithium in a lithium transition metal composite oxide is 80 mol% or more, 85 mol% or more, 90 mol% or more, 91 mol% or more, or 94 mol% or more and 99 mol% or less. The high-nickel positive electrode active material can achieve high capacity and can be applied to high-capacity, high-density lithium secondary batteries.

[0084] The content of the positive electrode active material may be 90% by weight to 99.5% by weight relative to 100% by weight of the positive electrode active material layer, and the contents of the binder and conductive material may each be 0.5% by weight to 5% by weight relative to 100% by weight of the positive electrode active material layer.

[0085] The binder serves to firmly adhere the positive electrode active material particles to each other and to firmly adhere the positive electrode active material to the current collector. Representative examples of the binder include, but are not limited to, polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, (meth)acrylate styrene-butadiene rubber, epoxy resin, (meth)acrylic resin, polyester resin, nylon, etc.

[0086] The conductive material is used to impart electrical conductivity to the electrode, and any material that does not cause chemical changes in the constructed battery and is electronically conductive can be used. Examples of conductive agents include carbon-based materials such as natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, carbon fiber, carbon nanofiber, and carbon nanotube; metal-based materials in the form of metal powder or metal fiber containing copper, nickel, aluminum, silver, etc.; conductive polymers such as polyphenylene derivatives; and mixtures thereof.

[0087] The current collector may be made of Al, but is not limited to this.

[0088] <Electrolytes> The electrolyte includes a non-aqueous organic solvent and a lithium salt.

[0089] The non-aqueous organic solvent serves as a medium through which ions involved in the electrochemical reactions of the battery can migrate.

[0090] The non-aqueous organic solvent may be a carbonate, ester, ether, ketone, alcohol solvent, aprotic solvent, or a combination thereof.

[0091] Examples of carbonate solvents that can be used include dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), methyl ethyl carbonate (MEC), ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), etc. Examples of ester solvents that can be used include methyl acetate, ethyl acetate, n-propyl acetate, dimethyl acetate, methyl propionate, ethyl propionate, decanolide, mevalonolactone, valerolactone, caprolactone, etc. Examples of ether solvents that can be used include dibutyl ether, tetraglyme, diglyme, dimethoxyethane, 2-methyltetrahydrofuran, 2,5-dimethyltetrahydrofuran, and tetrahydrofuran. Examples of ketone solvents that can be used include cyclohexanone. Examples of alcohol solvents that can be used include ethyl alcohol and isopropyl alcohol. Examples of aprotic solvents that can be used include nitriles such as R-CN (where R is a linear, branched, or cyclic hydrocarbon group having 2 to 20 carbon atoms, which may contain a double bond, an aromatic ring, or an ether group); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane and 1,4-dioxolane; and sulfolanes.

[0092] The non-aqueous organic solvents can be used alone or in combination of two or more kinds.

[0093] When a carbonate-based solvent is used, a cyclic carbonate and a chain carbonate can be mixed and used, and the cyclic carbonate and the chain carbonate are mixed in a volume ratio of 1:1 to 1:9.

[0094] Lithium salts are substances that dissolve in organic solvents and act as a source of lithium ions within the battery, enabling basic lithium secondary battery operation and facilitating the movement of lithium ions between the positive and negative electrodes. Representative examples of lithium salts include LiPF6, LiBF4, LiSbF6, LiAsF6, LiClO4, LiAlO2, LiAlCl4, LiPO2F2, LiCl, LiI, LiN(SO3C2F5)2, Li(FSO2)2N (lithium bis(fluorosulfonyl)imide (LiFSI)), LiC4F9SO3, and LiN(C x F 2x+1 SO2)(C y F 2y+1 SO2) (x and y are integers of 1 to 20), lithium trifluoromethanesulfonate, lithium tetrafluoroethanesulfonate, lithium difluorobis(oxalato)phosphate (LiDFOB), and lithium bis(oxalato)borate (LiBOB).

[0095] [Separator] Depending on the type of lithium secondary battery, a separator may be present between the positive electrode and the negative electrode. Such a separator may be made of polyethylene, polypropylene, polyvinylidene fluoride, or a multilayer film of two or more of these layers. Of course, a mixed multilayer film such as a polyethylene / polypropylene two-layer separator, a polyethylene / polypropylene / polyethylene three-layer separator, or a polypropylene / polyethylene / polypropylene three-layer separator may also be used.

[0096] The separator can include a porous substrate and a coating layer comprising an organic material, an inorganic material, or a combination thereof located on one or both sides of the porous substrate.

[0097] The porous substrate may be a polymer film formed from any polymer selected from polyolefins such as polyethylene and polypropylene, polyesters such as polyethylene terephthalate and polybutylene terephthalate, polyacetal, polyamide, polyimide, polycarbonate, polyether ketone, polyaryl ether ketone, polyether imide, polyamide imide, polybenzimidazole, polyether sulfone, polyphenylene oxide, cyclic olefin copolymer, polyphenylene sulfide, polyethylene naphthalate, glass fiber, Teflon, and polytetrafluoroethylene, or a copolymer or mixture of two or more of these.

[0098] The organic material may include a polyvinylidene fluoride polymer or a (meth)acrylic polymer.

[0099] The inorganic material may include, but is not limited to, inorganic particles selected from Al2O3, SiO2, TiO2, SnO2, CeO2, MgO, NiO, CaO, GaO, ZnO, ZrO2, Y2O3, SrTiO3, BaTiO3, Mg(OH)2, boehmite, and combinations thereof.

[0100] The organic material and the inorganic material may be mixed in one coating layer, or may be stacked in a coating layer containing an organic material and a coating layer containing an inorganic material.

[0101] Lithium secondary batteries are classified into lithium ion batteries, lithium ion polymer batteries, and lithium polymer batteries depending on the type of separator and electrolyte used, into cylindrical, prismatic, coin, pouch, etc. depending on the shape, and into bulk and thin film types depending on the size. The structure and manufacturing methods of these batteries are widely known in this field, so a detailed description will be omitted.

[0102] Lithium secondary batteries are classified into cylindrical, prismatic, pouch, and coin types depending on their shape. FIGS. 2 to 5 are schematic diagrams showing a lithium secondary battery according to one embodiment, with FIG. 2 showing a cylindrical battery, FIG. 3 showing a prismatic battery, and FIGS. 4 and 5 showing pouch battery types. Referring to FIGS. 2 to 5, a lithium secondary battery 100 may include an electrode assembly 40 having a separator 30 interposed between a positive electrode 10 and a negative electrode 20, and a case 50 housing the electrode assembly 40. The positive electrode 10, the negative electrode 20, and the separator 30 are impregnated with an electrolyte (not shown). The lithium secondary battery 100 may include a sealing member 60 that seals the case 50, as shown in FIG. 1. Also, in FIG. 2, the lithium secondary battery 100 may include a positive electrode lead tab 11, a positive electrode terminal 12, a negative electrode lead tab 21, and a negative electrode terminal 22. As shown in FIGS. 3 and 4, the lithium secondary battery 100 may include electrode tabs 70, i.e., a positive electrode tab 71 and a negative electrode tab 72, which serve as electrical paths for conducting the current generated in the electrode assembly 40 to the outside.

[0103] The lithium secondary battery according to one embodiment of the present invention may be applied to automobiles, mobile phones, and / or various types of electrical devices, but the present invention is not limited thereto.

[0104] Examples of the present invention and comparative examples are described below. The following examples are merely examples of the present invention, and the present invention is not limited to the following examples.

[0105] Example 1 97.5% by weight of natural graphite and 2.5% by weight of polytetrafluoroethylene were mixed and stirred at 10,000 rpm to form fibers. This process produced a fiberized product in which fibrillated polytetrafluoroethylene and natural graphite formed a network.

[0106] The fiberized product was rolled to produce a dry film.

[0107] The dry film was subjected to O2 plasma treatment for 1 minute, and then a gas nitriding process was carried out for 1 hour by chemical vapor deposition while blowing NH3 gas at 300°C to prepare a negative electrode active material layer.

[0108] The negative electrode active material layer was laminated on a copper foil current collector to fabricate a negative electrode. The negative electrode active material layer had a thickness of 140 μm. The negative electrode active material layer contained a negative electrode active material and polytetrafluoroethylene with an N-containing protective layer formed thereon, and the content of the polytetrafluoroethylene with the N-containing protective layer formed thereon was 2.5 wt % relative to 100 wt % of the negative electrode active material layer.

[0109] A half-cell was fabricated using the fabricated anode, lithium metal counter electrode, and electrolyte. The electrolyte was 1M LiPF6 dissolved in a mixed solvent of ethylene carbonate, ethyl methyl carbonate, and diethyl carbonate (volume ratio: 2:1:7).

[0110] Example 2 A negative electrode and half cell were fabricated in the same manner as in Example 1, except that the O2 plasma treatment was performed for 2 minutes. The thickness of the negative electrode active material layer in the fabricated negative electrode was 140 μm. The negative electrode active material layer contained a negative electrode active material and polytetrafluoroethylene with an N-containing protective layer formed thereon, and the content of the polytetrafluoroethylene with the N-containing protective layer formed thereon was 2.5 wt% relative to 100 wt% of the negative electrode active material layer.

[0111] Example 3 A negative electrode and half cell were fabricated in the same manner as in Example 1, except that the O2 plasma treatment was performed for 4 minutes. The thickness of the negative electrode active material layer in the fabricated negative electrode was 140 μm. The negative electrode active material layer contained a negative electrode active material and polytetrafluoroethylene with an N-containing protective layer formed thereon, and the content of the polytetrafluoroethylene with the N-containing protective layer formed thereon was 2.5 wt% relative to 100 wt% of the negative electrode active material layer.

[0112] Example 4 A negative electrode and half cell were fabricated in the same manner as in Example 1, except that the gas nitriding process was performed for 2 hours. The thickness of the negative electrode active material layer in the fabricated negative electrode was 140 μm. The negative electrode active material layer contained a negative active material and polytetrafluoroethylene with an N-containing protective layer formed thereon, and the content of the polytetrafluoroethylene with the N-containing protective layer formed thereon was 2.5 wt % relative to 100 wt % of the active material layer.

[0113] Example 5 A negative electrode and half cell were fabricated in the same manner as in Example 1, except that the gas nitriding process was carried out for 4 hours. The thickness of the negative electrode active material layer in the fabricated negative electrode was 140 μm. The negative electrode active material layer contained a positive electrode active material and polytetrafluoroethylene with an N-containing protective layer formed thereon, and the content of the polytetrafluoroethylene with the N-containing protective layer formed thereon was 2.5 wt % relative to 100 wt % of the positive electrode active material layer.

[0114] (Comparative Example 1) 97.5% by weight of natural graphite and 2.5% by weight of polytetrafluoroethylene were mixed and stirred at 10,000 rpm to form fibers. This process produced a fiberized product in which fibrillated polytetrafluoroethylene and natural graphite formed a network.

[0115] The fiberized product was rolled to produce a dry film.

[0116] The dry film was laminated on a copper foil current collector as a negative electrode active material layer to prepare a negative electrode. The thickness of the negative electrode active material layer was 140 μm.

[0117] A half cell was fabricated in the same manner as in Example 1, except that the prepared negative electrode was used.

[0118] (Comparative Example 2) Natural graphite was subjected to O2 plasma treatment for 1 minute, and then a gas nitriding process was carried out for 1 hour by chemical vapor deposition while blowing NH3 gas at 300°C to prepare a negative electrode active material.

[0119] 97.5 wt % of the prepared negative electrode active material and 2.5 wt % of polytetrafluoroethylene were mixed, and the mixture was rolled to prepare a dry film.

[0120] The dry film was used as a negative electrode active material layer and laminated on a copper foil current collector to produce a negative electrode. The thickness of the negative electrode active material layer was 140 μm.

[0121] A half cell was fabricated in the same manner as in Example 1, except that the prepared negative electrode was used.

[0122] Experimental example 1) Initial efficiency evaluation The half-cells produced in Examples 1 to 5 and Comparative Examples 1 and 2 were charged and discharged once at 0.1 C. The charge-discharge characteristics are shown in FIG. 6, and the initial efficiency, which is the ratio of the discharge capacity to the charge capacity, was determined. The initial efficiency is shown in Table 1.

[0123] [Table 1]

[0124] As shown in Table 1, it can be seen that the initial efficiencies of Examples 1 to 5 are improved compared to Comparative Examples 1 and 2. Furthermore, in Comparative Example 2, in which graphite was subjected to O2 plasma and nitriding treatment, the efficiency was approximately similar to that of Comparative Example 1. This shows that when only the graphite negative electrode active material is subjected to O2 plasma and nitriding treatment, it is not possible to obtain an efficiency improvement effect.

[0125] Experimental example 2) Life and coulombic efficiency evaluation The half-cells manufactured in Examples 1 to 5 and Comparative Examples 1 and 2 were charged and discharged 50 times at 0.2 C. The discharge capacity was determined for each cycle. The results for Examples 1 to 3 and Comparative Example 1 are shown in FIG. 7, and the results for Examples 1, 4, and 5 and Comparative Example 1 are shown in FIG. 9. As shown in FIGS. 7 and 9, it can be seen that the capacity loss was reduced by performing the O2 plasma and gas nitriding treatment processes. In other words, it can be seen that the life characteristics were improved.

[0126] The half-cells manufactured in Examples 1 to 5 and Comparative Examples 1 to 3 were charged to 0.5 mAh / cm 2 Up to 2C (1mAh / cm 2 ) was charged and discharged 50 times, and the coulombic efficiency was measured. The results for Examples 1 to 3 and Comparative Example 1 are shown in FIG. 8, and the results for Examples 1, 4, and 5 and Comparative Example 1 are shown in FIG. 10. As shown in FIGS. 8 and 10, it can be seen that the coulombic efficiency is maintained by performing the O2 plasma and gas nitriding treatment steps.

[0127] Experimental Example 3) Charge transfer resistance (Rct) The half-cells prepared in Examples 1 to 5 and Comparative Examples 1 and 2 were charged and discharged under SOC50 conditions (a state in which the battery is charged to 50% of its charge capacity when the total charge capacity of the battery is 100%, which means that the battery is discharged to 50% when viewed during discharge), and the resistance was measured. The measured resistance was charge transfer resistance (Rct), and the results are shown in Table 2 below. ct It is expressed as:

[0128] [Table 2]

[0129] As shown in Table 2, it can be seen that the charge transfer resistances of Examples 1 to 5 are lower than those of Comparative Examples 1 and 2.

[0130] Experimental Example 4) X-ray Photoelectron Spectroscopy (XPS) Evaluation XPS evaluation was performed on the negative electrodes produced in Examples 1 to 5 and Comparative Examples 1 and 2. Of the results, the XPS analysis result for Comparative Example 1 is shown in FIG. 11. Since Comparative Example 1 was not subjected to nitriding treatment, the peaks shown in FIG. 11 are noise results, and it can be seen that no N-related peaks appear. The results of Comparative Example 2 are shown in FIG. 12. The results of Example 1 are shown in FIG. 13, the results of Example 2 in FIG. 14, the results of Example 3 in FIG. 15, the results of Example 4 in FIG. 16, and the results of Example 5 in FIG. 17.

[0131] 12 to 17, the O2 plasma and gas nitriding processes produced peaks at binding energies of 399.0 eV to 400 eV and 400.4 eV to 401 eV, indicating that amide and imide groups are present on the surface of the resulting negative electrode active material. In Figures 11 to 17, BE stands for binding energy, and the dotted lines indicate the combined area of ​​the peaks corresponding to amide and imide groups.

[0132] The present invention is not limited to the above-described embodiments, and may be manufactured in various different forms, and a person skilled in the art to which the present invention pertains will understand that the present invention may be embodied in other specific forms without changing the technical concept or essential characteristics of the present invention. Therefore, the above-described embodiments should be understood to be illustrative in all respects and not limiting.

Claims

1. Dry negative electrode active material layer including a negative electrode active material and a polytetrafluoroethylene binder on which an N-containing protective layer is formed 1. A negative electrode for a lithium secondary battery comprising:

2. The negative electrode for a lithium secondary battery according to claim 1 , wherein the N-containing protective layer further contains C, O, H, or a combination thereof.

3. 2. The negative electrode for a lithium secondary battery according to claim 1, wherein the dry negative electrode active material layer further contains a non-fibrillating binder selected from carboxymethyl cellulose, styrene-butadiene rubber, polyvinylidene fluoride, polyacrylic acid, and combinations thereof.

4. 2. The negative electrode for a lithium secondary battery according to claim 1, wherein the polytetrafluoroethylene on which the N-containing protective layer is formed is fibrillated.

5. 2. The negative electrode for a lithium secondary battery according to claim 1, wherein the polytetrafluoroethylene on which the N-containing protective layer is formed contains an N-containing functional group.

6. 6. The negative electrode for a lithium secondary battery according to claim 5, wherein the N-containing functional group is an imide group, an amide group, an amine group, a nitrile group, or a combination thereof.

7. 2. The negative electrode for a lithium secondary battery according to claim 1, wherein, when the negative electrode is measured by XPS, a peak appears at a binding energy of 399.0 eV to 400 eV, a binding energy of 400.4 eV to 401 eV, or a binding energy of 397 eV to 400.2 eV.

8. 2. The negative electrode for a lithium secondary battery according to claim 1, wherein the content of the binder on which the N-containing protective layer is formed is 0.1 wt % to 20.0 wt % relative to 100 wt % of the negative electrode active material layer.

9. 2. The negative electrode for a lithium secondary battery according to claim 1, wherein the negative electrode active material has an N-containing protective layer.

10. 2. The negative electrode for a lithium secondary battery according to claim 1, wherein the thickness of the negative electrode active material layer is 10 μm to 400 μm.

11. The negative electrode for a lithium secondary battery according to claim 1 , wherein the dry negative electrode active material layer further contains a non-fibrillating binder.

12. Mixing the negative electrode active material and polytetrafluoroethylene and fiberizing the mixture to produce a fiberized product; calendering the fiberized product to produce a dry film; a gas phase reaction treatment is performed on the dry film to prepare a negative electrode active material layer; a step of disposing the negative electrode active material layer on a current collector; A method for producing a negative electrode for a lithium secondary battery, comprising:

13. 13. The method for manufacturing a negative electrode for a lithium secondary battery according to claim 12, wherein the gas phase reaction is chemical vapor deposition, physical vapor deposition, atomic layer deposition, plasma, or a combination thereof.

14. The method for producing a negative electrode for a lithium secondary battery according to claim 12, wherein the gas phase reaction includes a step of gas plasma treating the dry film and a step of gas nitriding.

15. The gas plasma treatment step is 2 The method for producing a negative electrode for a lithium secondary battery according to claim 14, wherein the step is a plasma treatment step.

16. The gas nitriding step is carried out using NH 3 The method for producing a negative electrode for a lithium secondary battery according to claim 14, wherein the method is carried out by a chemical vapor deposition process in a gas flow.

17. The method for producing a negative electrode for a lithium secondary battery according to claim 14, wherein the gas nitriding step is carried out at a temperature of 50°C to 600°C.

18. 15. The method of claim 14, wherein the gas plasma process is performed for 1 second to 120 minutes.

19. 15. The method of claim 14, wherein the gas nitriding process is performed for 1 minute to 6 hours.

20. The negative electrode according to any one of claims 1 to 11; a positive electrode; and non-aqueous electrolyte A lithium secondary battery comprising:

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