Negative electrode manufacturing method, negative electrode, and lithium secondary battery comprising same

Abstract

A negative electrode according to exemplary embodiments of the present invention comprises: a negative electrode current collector; and a negative electrode active layer provided on at least one surface of the negative electrode current collector and containing a carbon-based negative electrode active material, wherein the negative electrode active layer has a quantified binder ratio (QBR) of 0.8 or less.

Description

Method for manufacturing a negative electrode, a negative electrode, and a lithium secondary battery including the same

[0001] The present invention relates to a method for manufacturing a cathode, a cathode, and a lithium secondary battery including the same.

[0002] This application claims the benefit of priority based on Korean Patent Application No. 10-2025-0000909 filed on January 3, 2025.

[0003]

[0004] Recently, lithium-ion batteries are being widely applied not only to small devices such as portable electronic devices, but also to medium and large devices such as battery packs or power storage devices for hybrid or electric vehicles.

[0005] Generally, a lithium secondary battery is a power generation device capable of charging and discharging, consisting of a stacked structure of a positive electrode, a separator, and a negative electrode. Currently, materials containing graphite are widely used as the negative electrode active material. Graphite has a layered structure and is formed by stacking multiple layers in which carbon atoms form a network structure and are spread out in a planar shape. During charging, lithium ions can penetrate from the edge surfaces (the surfaces where layers overlap) of these graphite layers and diffuse between the layers, and during discharging, lithium ions can be released from the edge surfaces of the layers. In addition, since the electrical resistivity of graphite in the planar direction is lower than that in the stacking direction, a conduction path for electrons that bypasses along the planar direction of the layers is formed.

[0006] In this regard, a technique for magnetically aligning graphite contained in a cathode has been proposed to improve the charging performance of the cathode. Specifically, a cathode slurry is coated onto a cathode current collector, and the cathode active material is oriented by applying a magnetic field to the cathode slurry. However, a cathode in which the cathode active material is oriented by a magnetic field has the disadvantage of having a high interfacial resistance compared to a cathode in which no magnetic field is applied. Therefore, it is necessary to develop a technique to improve the interfacial resistance in a cathode in which the cathode active material is oriented by a magnetic field.

[0007]

[0008] The problem that the technical concept of the present invention aims to solve is to provide a magnetically oriented cathode with excellent interfacial adhesion between the current collector and the cathode active layer.

[0009]

[0010] A cathode according to exemplary embodiments of the present invention is provided. The cathode comprises a cathode current collector and a cathode active layer provided on at least one surface of the cathode current collector and comprising a carbon-based cathode active material, wherein the cathode active layer has a Quantified Binder Ratio (QBR) of 0.8 or less, and the QBR is defined by the following Formula 1.

[0011] [Equation 1]

[0012] QBR = Bs / Bf

[0013] In the above Equation 1, Bs represents the average value of the binder content in the surface region of the cathode active layer from the outermost surface of the cathode active layer up to within 15% of the total thickness of the cathode active layer, and Bf represents the average value of the binder content in the bottom region of the cathode active layer from the interface of the cathode active layer facing the current collector up to within 15% of the total thickness of the cathode active layer.

[0014] In exemplary embodiments, the cathode active layer has a QBR value calculated from Equation 1 in the range of 0.1 to 0.75.

[0015] In exemplary embodiments, the cathode active layer has an alignment degree (OI) calculated from Equation 2 below during X-ray diffraction spectroscopic analysis in the range of 1 to 15.

[0016] [Equation 2]

[0017] OI = I 004 / I 110

[0018] In Equation 2,

[0019] I 004represents the area value of the peak indicating the [0,0,4] crystal plane during X-ray diffraction (XRD) spectroscopic analysis of the cathode active layer, and

[0020] I 110 represents the area value of the peak representing the [1,1,0] crystal plane during X-ray diffraction (XRD) spectroscopic analysis of the cathode active layer.

[0021] In exemplary embodiments, the carbon-based negative electrode active material comprises one or more mixtures selected from natural graphite, artificial graphite, Kish graphite, pyrolytic carbon, carbon microbeads, mesophase calcined carbon made from tar and pitch, and graphitized coke.

[0022] In exemplary embodiments, the cathode active layer further comprises a binder, and the binder is in the range of 0.1 to 5 parts by weight per 100 parts by weight of the entire cathode active layer.

[0023] In exemplary embodiments, the current collector has a contact angle with water of 50 degrees or less.

[0024] According to other embodiments of the present invention, a method for manufacturing a cathode is provided. The method for manufacturing a cathode comprises: a step of modifying the surface of a cathode current collector; a step of applying a cathode slurry containing a carbon-based cathode active material to at least one surface of the cathode current collector; and a step of applying a magnetic field to the applied cathode slurry, wherein the step of modifying the surface of the cathode current collector includes a corona treatment process of the surface of the cathode current collector or a plasma treatment process of the surface of the cathode current collector.

[0025] In exemplary embodiments, the step of modifying the surface of the cathode current collector includes a corona treatment process on the surface of the cathode current collector.

[0026] In exemplary embodiments, the corona treatment is performed under an output condition of 0.5 to 3.5 kW.

[0027] In exemplary embodiments, the corona treatment is performed at a speed of 1 to 10 m / min.

[0028] A method for manufacturing a cathode according to exemplary embodiments further comprises the step of drying a cathode slurry to which a magnetic field is applied; and the step of rolling the dried cathode slurry to form a cathode active layer.

[0029] In exemplary embodiments, in the step of applying the magnetic field, the strength of the magnetic field is in the range of 1,000 to 10,000 G.

[0030] In exemplary embodiments, after the step of modifying the surface of the cathode current collector, the cathode current collector has a contact angle with water of 50 degrees or less, preferably 1 to 40 degrees.

[0031] According to other embodiments of the present invention, a lithium secondary battery is provided. The lithium secondary battery comprises a positive electrode; a negative electrode; a separator; and an electrolyte, wherein the negative electrode is the negative electrode described above.

[0032]

[0033] According to exemplary embodiments of the present invention, the interfacial adhesion between the negative current collector and the negative active layer is excellent, thereby minimizing the phenomenon of reduced adhesion caused by the orientation of the negative active material.

[0034] According to exemplary embodiments of the present invention, a lithium secondary battery with improved high-speed charge / discharge characteristics can be realized by improving the movement path of lithium ions in the cathode due to the orientation of the cathode active material.

[0035]

[0036] FIG. 1 is a flowchart for explaining a method for manufacturing a cathode according to exemplary embodiments of the present invention.

[0037] Figure 2 is a graph showing the Bs and Bf values ​​for each cathode of the examples and comparative examples.

[0038]

[0039] Hereinafter, the present invention will be described in more detail to aid in understanding the invention.

[0040] Terms and words used in this specification and claims shall not be interpreted as being limited to their ordinary or dictionary meanings, but shall be interpreted in a meaning and concept consistent with the technical spirit of the invention, based on the principle that the inventor can appropriately define the concept of the terms to best describe his invention.

[0041] The terms used in this specification are used merely to describe exemplary embodiments and are not intended to limit the invention. The singular expression includes the plural expression unless the context clearly indicates otherwise.

[0042] In this specification, terms such as “comprising,” “comprising,” or “having” are intended to specify the existence of the implemented features, numbers, steps, components, or combinations thereof, and should not be understood as precluding the existence or addition of one or more other features, numbers, steps, components, or combinations thereof.

[0043] In this specification, when a part such as a layer, film, region, or plate is described as being "on" another part, this includes not only cases where it is "immediately above" the other part, but also cases where there is another part in between. Conversely, when a part such as a layer, film, region, or plate is described as being "under" another part, this includes not only cases where it is "immediately below" the other part, but also cases where there is another part in between. Furthermore, in this application, being placed "on" may include cases where it is placed on the lower part as well as on the upper part.

[0044] In this specification, "bottom portion of the cathode active layer" refers to the area near the cathode current collector in the cathode active layer, and "surface portion of the cathode active layer" refers to the area near the outermost surface of the cathode active layer.

[0045] In this specification, "include as a main component" may mean including a defined component in an amount of 50 wt% or more (or 50 volume% or more), 60 wt% or more (or 60 volume% or more), 70 wt% or more (or 70 volume% or more), 80 wt% or more (or 80 volume% or more), 90 wt% or more (or 90 volume% or more), or 95 wt% or more (or 95 volume% or more) with respect to the total weight (or total volume). For example, "include graphite as a main component as a negative electrode active material" may mean including graphite in an amount of 50 wt% or more, 60 wt% or more, 70 wt% or more, 80 wt% or more, 90 wt% or more, or 95 wt% or more with respect to the total weight of the negative electrode active material, and in some cases, may mean that the entire negative electrode active material is made of graphite and includes graphite in an amount of 100 wt%.

[0046] In addition, in this specification, "carbon-based negative electrode active material is oriented" or "carbon-based negative electrode active material is aligned" means that a specific crystal plane (e.g., the ab-axis crystal plane of graphite) representing the two-dimensional planar structure of the carbon-based negative electrode active material constituting the negative electrode active material particles is arranged to have a predetermined inclination with respect to the surface of the negative electrode current collector, which may differ from the carbon-based negative electrode active material particles themselves being arranged to have a specific direction within the negative electrode active layer.

[0047] In this specification, "crystal plane of a carbon-based negative electrode active material" refers to a plane that forms the outer shape of the valence crystal of the carbon-based negative electrode active material. In the present invention, it may refer to a crystal plane including a plane of the carbon-based negative electrode active material, or a crystal plane including the a-axis / b-axis / ab-axis of the carbon-based negative electrode active material crystal.

[0048] In addition, in this specification, "average particle size (D 50 )" refers to the 50% point of the cumulative volume distribution according to particle size (D 50 It refers to the particle size at ). The above average particle size (D 50 ) can be measured in a manner commonly applied in the industry. For example, it can be measured using a particle size analyzer or an analytical instrument utilizing the laser diffraction scattering particle size distribution method, and by calculating the cumulative volume distribution according to particle size and calculating the particle diameter at the point where the cumulative volume distribution according to particle size in the measuring device is 50%, D 50 This can be measured.

[0049] In this specification, the description “A and / or B” means “A or B or both.”

[0050]

[0051] Method for manufacturing a cathode

[0052] FIG. 1 is a flowchart for explaining a method for manufacturing a cathode according to exemplary embodiments of the present invention.

[0053] Referring to FIG. 1, a method for manufacturing a cathode according to exemplary embodiments may include a step of modifying the surface of a cathode current collector (S11), a step of applying a cathode slurry (S12), and a step of applying a magnetic field (S13). According to exemplary embodiments, the step of modifying the surface of the current collector (S11) may include a process of corona treating the surface of the cathode current collector or a process of plasma treating the surface of the cathode current collector.

[0054] Corona treatment or plasma treatment of the surface of the cathode current collector increases the surface roughness. Here, surface roughness refers to the degree of fine irregularities. Since this increase in surface roughness increases the contact area between the cathode current collector and the cathode active layer, the interfacial adhesion between the cathode current collector and the cathode active layer increases.

[0055] In addition, if the surface of the cathode current collector is corona treated or plasma treated, hydrophilic functional groups can be introduced to the surface of the cathode current collector. Since the cathode slurry is generally a solution in which a cathode active material and a binder are dispersed in an aqueous solvent, the bonding strength between the surface-treated cathode current collector and the cathode slurry is higher compared to the bonding strength between the surface-untreated cathode current collector and the cathode slurry.

[0056] When the surface of the cathode current collector is corona treated or plasma treated, the wettability of the current collector to the cathode slurry is improved, causing the cathode active material oriented by the magnetic field to adhere closer to the surface of the current collector, thereby reducing the interfacial resistance between the cathode active material layer and the current collector.

[0057] In particular, the cathode manufactured with a surface-treated current collector according to the present invention exhibits a difference in the distribution of binder within the cathode active layer compared to the cathode manufactured with a non-surface-treated current collector. The binder content in the bottom portion of the cathode active layer of the cathode manufactured according to the present invention is greater than the binder content in the bottom portion of the cathode active layer of the cathode manufactured with a non-surface-treated current collector. The binder content in the surface portion of the cathode active layer of the cathode manufactured according to the present invention is smaller than the binder content in the surface portion of the cathode active layer of the cathode manufactured with a non-surface-treated current collector. This is because, due to the interaction between the hydrophilic functional groups of the surface-treated cathode current collector and the aqueous solvent of the cathode slurry, it is difficult for the binder in the cathode slurry to move toward the surface of the cathode active layer during the drying process. Therefore, the cathode manufactured according to exemplary embodiments of the present invention has excellent interfacial adhesion between the current collector and the cathode active layer, thereby minimizing the phenomenon of reduced adhesion caused by the orientation of the cathode active material.

[0058]

[0059] The step (S11) of modifying the surface of the above-mentioned negative current collector may be to corona treat the surface of the negative current collector or to plasma treat the surface of the negative current collector.

[0060] The above-mentioned negative electrode current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the battery, and for example, copper, stainless steel, nickel, titanium, calcined carbon, etc. may be used, and in the case of copper or stainless steel, surface-treated carbon, nickel, titanium, silver, etc. may be used. In addition, the average thickness of the above-mentioned negative electrode current collector can be appropriately applied from 1 μm to 300 μm, taking into consideration the conductivity and total thickness of the negative electrode being manufactured.

[0061] Corona treatment is a method that uses high-frequency current to ionize molecules in the air, thereby generating high-temperature plasma. Corona treatment requires only simple electrodes and a high-frequency generator, and is typically suitable for mass production using a roll-to-roll method.

[0062] Plasma treatment is a method that uses high voltage to ionize gases, such as argon, oxygen, and nitrogen, into a plasma state, and then applies this plasma to the surface of a material to induce chemical changes. While plasma treatment requires a vacuum chamber or high-voltage equipment, it offers the advantage of excellent uniformity in surface modification.

[0063] Surface modification by plasma treatment has the disadvantage of increasing the temperature of the cathode current collector and increasing the treatment cost. Since applying a cathode slurry while the temperature of the cathode current collector is high affects the physical properties of the cathode slurry, corona treatment may be desirable as a method for surface modification of the cathode current collector.

[0064] The specific method or condition of the above corona treatment is not particularly limited as long as it can increase the surface roughness of the cathode current collector or decrease the contact angle between the cathode current collector and water. The specific method or condition of the above plasma treatment is also not particularly limited as long as it can increase the surface roughness of the cathode current collector or decrease the contact angle between the cathode current collector and water.

[0065] In exemplary embodiments, after the step (S11) of modifying the surface of the cathode current collector, the cathode current collector may have a contact angle with water of 50 degrees or less, preferably 1 to 40 degrees, more preferably 3 to 35 degrees. When the contact angle with water of the cathode current collector is in the above range, the contact area between the cathode current collector and the cathode slurry is increased, and the binder content at the bottom of the cathode active layer may be increased.

[0066] In exemplary embodiments, the output during the corona treatment may be in the range of 0.5 to 3.5 kW, preferably 1 to 3 kW, and more preferably 1.5 to 2.5 kW. When the output range is in the above range, the adhesion of the negative current collector is excellent. If the output range is below the above range, the effect of increasing adhesion may be negligible, and conversely, if the output range exceeds the above range, the uniformity of surface roughness may decrease.

[0067] In exemplary embodiments, the speed of the corona treatment may be in the range of 1 to 10 m / min, preferably 2 to 8 m / min, and more preferably 3 to 6 m / min. When the speed of the corona treatment is in the above range, the fine irregularities generated by the corona treatment are uniformly formed, and the adhesion of the cathode current collector is excellent.

[0068] The step (S12) of applying the cathode slurry is a step of applying the cathode slurry to at least one surface of a cathode current collector. Here, the cathode current collector is a surface-modified cathode current collector. Here, the cathode slurry can be applied to the surface of the moving cathode current collector. The method of applying the cathode slurry can be applied without special limitation as long as it is a method commonly applied in the industry, but preferably, a die coating method can be used. The die coating method can be performed through a slot die equipped with a shim for controlling the discharge conditions of the cathode slurry. In this case, by controlling the shape of the shim, the loading amount and coating thickness of the cathode slurry applied on the cathode current collector can be easily controlled.

[0069] The above-mentioned cathode slurry can be prepared by introducing a cathode active material and a binder into a solvent and mixing them. In addition to the cathode active material and the binder, the cathode slurry may further include a conductive material and / or a thickener as needed. The solvent may be an aqueous solvent, such as water. When the solvent is an aqueous solvent, the phenomenon of the binder migrating toward the surface of the cathode active layer during the drying process can be suppressed by the interaction between the cathode slurry and the hydrophilic functional groups introduced by the surface treatment of the cathode current collector. Accordingly, the value of QBR described later in the prepared cathode can be reduced, thereby maximizing the effect of improving interfacial adhesion between the cathode current collector and the cathode active layer.

[0070] The above-mentioned cathode active material may include a carbon-based cathode active material as a main component. The carbon-based cathode active material may include a graphite-based compound. For example, the above-mentioned carbon-based cathode active material may include one or more mixtures selected from natural graphite, artificial graphite, Kish graphite, pyrolytic carbon, carbon microbeads, mesophase calcined carbon made from tar and pitch (bulk mesophase, liquid crystal pitch-based carbon fiber, etc.), and cokes (raw coke, green coke, pitch coke, needle coke, petroleum coke, coal coke, etc.) that have been graphitized.

[0071] The above-mentioned thickener may be included in an amount ranging from 0.5 wt% to 3 wt%, 0.5 wt% to 2 wt%; 0.5 wt% to 1 wt%; 1 wt% to 3 wt%; 1 wt% to 2 wt%; 1.5 wt% to 3 wt%; 0.5 wt% to 1.9 wt%; or 1.1 wt% to 1.9 wt%, based on the total weight of the solids in the cathode slurry.

[0072] Accordingly, the cathode slurry may have a predetermined viscosity at room temperature (18~27℃). Specifically, the cathode slurry may have a viscosity of 7,000 cps or more at 25℃, and specifically, 7,000 cps to 20,000 cps; 7,000 cps to 19,000 cps; 7,000 cps to 15,000 cps; 7,000 cps to 13,000 cps; 7,000 cps to 11,000 cps; 11,000 cps to 19,000 cps; 13,000 cps to 18,000 cps; 15,000 cps to 18,000 cps; 7,000 cps to 9,000 cps; It may be 8,000 cps to 10,000 cps; 9,000 cps to 11,000 cps; 7,000 cps to 8,500 cps; or 7,100 cps to 7,900 cps.

[0073] The step of applying the magnetic field (S13) may be a process for controlling the crystal characteristics of the cathode active material contained in the cathode slurry to reduce the disorder of the carbon-based cathode active material contained in the cathode slurry. By applying a magnetic field to the cathode slurry coated on the cathode current collector in the step of applying the magnetic field (S13), the ab-axis crystal plane of the carbon-based cathode active material contained in the cathode slurry can be aligned to have a high angle with respect to the cathode current collector.

[0074] In exemplary embodiments, the magnetic field may be applied by magnetic parts each disposed at the top and bottom of a cathode current collector that is transported with the cathode slurry applied thereto. The polarities of the magnetic parts disposed at the top and bottom may be different from each other or may be the same.

[0075] The alignment degree (OI) of the carbon-based cathode active material contained in the cathode slurry can be controlled by the strength of the applied magnetic field, the application time, etc., and accordingly, the step of applying the magnetic field can be performed under conditions of a predetermined magnetic field strength and application time.

[0076] The step (S13) of applying the magnetic field may apply a magnetic field of 10,000 G (Gauss) or less, specifically, a magnetic field may be applied with a strength of 1,000 G to 10,000 G; 3,000 G to 9,000 G; 3,000 G to 8,500 G; 3,500 G to 8,500 G; 4,000 G to 8,200 G; 3,600 G to 4,500 G; 4,500 G to 6,500 G; 5,000 G to 7,000 G; 6,000 G to 8,500 G; 7,000 G to 8,500 G; 6,000 G to 6,500 G; 4,000 G to 5,000 G.

[0077] The step (S13) of applying the magnetic field may be performed for 1 second to 30 seconds, specifically for 1 second to 20 seconds; 1 second to 15 seconds; 1 second to 10 seconds; 5 seconds to 20 seconds; 10 seconds to 20 seconds; 11 seconds to 19 seconds; 14 seconds to 18 seconds; 4 seconds to 9 seconds; or 6 seconds to 11 seconds. However, it is not limited thereto.

[0078] As one example, the step (S13) of applying the magnetic field may apply a magnetic field of 4,700 ± 50 G to the cathode slurry for 3 to 8 seconds.

[0079] As another example, in the step (S13) of applying the magnetic field, a magnetic field of 4,700 ± 50 G may be applied to the cathode slurry for 12 to 17 seconds.

[0080] The step (S13) of applying the magnetic field is performed by a magnetic part introduced at the top and bottom of the coated cathode slurry, and the magnetic part may be performed with the size of the magnetic part adjusted to be larger than the size of the coated cathode slurry. For example, the magnetic part may have a length ratio of 105% to 200% based on the width direction length of the coated cathode slurry, and specifically, may have a length ratio of 110% to 180%; 110% to 160%; 110% to 140%; 110% to 130%; 130% to 150%; or 105% to 120% based on the width direction length of the coated cathode slurry. In this case, since the magnetic field applied to the cathode slurry is applied uniformly to the entire surface of the coated cathode slurry, the alignment degree (OI) of the carbon-based cathode active material can be uniformly realized.

[0081] The method for manufacturing a cathode according to the present invention may further include the step of drying a cathode slurry to which a magnetic field is applied (S14); and the step of rolling the dried cathode slurry to form a cathode active layer (S15). The drying step (S14) and the rolling step (S15) are processes for drying and rolling the cathode slurry to which a magnetic field is applied to firmly fix an oriented carbon-based cathode active material. The drying step (S14) and the rolling step (S15) can be applied without particular limitation as long as they are methods that can maintain the alignment degree (OI) of the carbon-based cathode active material contained in the cathode active layer.

[0082] The drying step (S14) above may dry the cathode slurry by applying thermal energy to the cathode slurry using a hot air dryer, a vacuum oven, etc. However, the drying means is not limited to this.

[0083] The above rolling step (S15) is a step of increasing the density of the cathode active layer by applying pressure to the dried cathode slurry using a roll press or the like. In some embodiments, the above rolling step (S15) may be performed under conditions where the alignment degree (OI) of the carbon-based cathode active material of the dried cathode slurry does not increase.

[0084] The above rolling step (S15) can be performed at a temperature close to room temperature, and more specifically at a temperature of 20°C to 35°C; 20°C to 30°C or 20°C to 25°C.

[0085] The above rolling step (S15) can be performed under pressure conditions of 50 MPa to 200 MPa, and specifically, under pressure conditions of 50 MPa to 150 MPa; 50 MPa to 100 MPa; 100 MPa to 200 MPa; 150 MPa to 200 MPa; or 80 MPa to 140 MPa.

[0086] By performing the above rolling step (S15) under the above temperature and / or pressure conditions, the energy density of the cathode can be increased while minimizing changes in the alignment of the carbon-based cathode active material contained in the cathode active layer.

[0087] The cathode manufactured according to exemplary embodiments may have an alignment degree (OI) of the cathode active layer calculated from Equation 2 below during X-ray diffraction spectroscopic analysis in the range of 1 to 15.

[0088] [Equation 2]

[0089] OI = I 004 / I 110

[0090] In Equation 2,

[0091] I 004 represents the area value of the peak indicating the [0,0,4] crystal plane during X-ray diffraction (XRD) spectroscopic analysis of the cathode active layer, and

[0092] I 110represents the area value of the peak representing the [1,1,0] crystal plane during X-ray diffraction (XRD) spectroscopic analysis of the cathode active layer.

[0093] Here, the alignment degree (OI) is an alignment degree based on the post-rolling state. The post-rolling alignment degree (OI) may be in the range of 1 to 15, more specifically 1 to 12, and more specifically 1 to 10.

[0094] In exemplary embodiments, the degree of alignment (OI) of the cathode before rolling may be in the range of 0.1 to 8, more specifically 0.1 to 6, and more specifically 0.1 to 5.

[0095] The cathode manufactured according to exemplary embodiments of the present invention has excellent rapid charging performance as the alignment degree of the cathode active layer satisfies the range described above. In addition, the adhesion between the cathode current collector and the cathode active layer is excellent, resulting in excellent long-term capacity characteristics, thereby enabling the realization of a high-energy-density lithium secondary battery.

[0096]

[0097] cathode

[0098] A cathode according to exemplary embodiments of the present invention comprises a cathode current collector and a cathode active layer provided on at least one surface of the cathode current collector and comprising a carbon-based cathode active material. The cathode active layer has a Quantified Binder Ratio (QBR) of 0.8 or less, and the QBR can be defined by the following Formula 1.

[0099] [Equation 1]

[0100] QBR = Bs / Bf

[0101] In the above Equation 1, Bs represents the average value of the binder content in the surface region of the cathode active layer from the outermost surface of the cathode active layer up to within 15% of the total thickness of the cathode active layer, and Bf represents the average value of the binder content in the bottom region of the cathode active layer from the interface of the cathode active layer facing the current collector up to within 15% of the total thickness of the cathode active layer.

[0102] The above Bs and Bf can be calculated in the following manner. The cathode sample is immersed in a diluted OsO4 solution (concentration of about 0.1%) for about 30 minutes to stain the binder component with OsO4. Subsequently, the osmium component of the OsO4 staining material in the binder is detected using an energy-dispersive spectral elemental analyzer attached to a scanning electron microscope (SU8020, HITACHI). An EDS mapping image is extracted showing the locations of the detected osmium. The extracted EDS mapping image is applied to image processing to calculate the area of ​​the stained binder and the area of ​​the unstained binder. The region extending from the outermost surface of the cathode active layer to within 15% of the total thickness of the cathode active layer is defined as the "cathode active layer surface region," and the region extending from the interface of the cathode active layer facing the current collector to within 15% of the total thickness of the cathode active layer is defined as the "cathode active layer bottom region." On the surface of the cathode active layer, the ratio value (Os / C) of the area of ​​the dyed binder (Os) to the undyed area (C) was calculated, and the average value of the ratio value (Os / C) for multiple samples was calculated as Bs. For the bottom of the cathode active layer, the ratio value (Os / C) of the area of ​​the dyed binder (Os) to the undyed area (C) was also calculated, and the average value of the ratio value (Os / C) for multiple samples was calculated as Bf.

[0103] A cathode according to exemplary embodiments of the present invention can be manufactured according to the method described above. That is, it is manufactured by applying a cathode slurry to a corona-treated cathode current collector or a plasma-treated cathode current collector, and applying a magnetic field to the applied cathode slurry. Accordingly, the cathode according to the present invention satisfies a QBR of 0.8 or less, preferably 0.1 to 0.75, and more preferably 0.3 to 0.7, calculated according to Equation 1 for the binder content Bf at the bottom portion of the cathode active layer and the binder content Bs at the surface portion of the cathode active layer. As the QBR satisfies the above range, the adhesion between the cathode current collector and the cathode active layer is excellent.

[0104] The above-mentioned cathode active material may include a carbon-based cathode active material as a main component. The above-mentioned carbon-based cathode active material refers to a material having carbon atoms as a main component, and such carbon-based cathode active material may include graphite-based compounds. For example, the above-mentioned carbon-based cathode active material may include one or more mixtures selected from natural graphite, artificial graphite, Kish graphite, pyrolytic carbon, carbon microbeads, mesophase calcined carbon made from tar and pitch (bulk mesophase, liquid crystal pitch-based carbon fiber, etc.), and cokes (raw coke, green coke, pitch coke, needle coke, petroleum coke, coal coke, etc.) that have been graphitized.

[0105] The above carbon-based cathode active material may have a shape such as a plate, sheet, flake, or needle, or may be a round particle. Here, a round particle shape may refer to a particle without sharp edges. When cross-sectional structural analysis is performed or the particle is projected as a two-dimensional particle, the shape of such particles may be spherical or ellipsoidal, and in some cases, may be shapeless, making it difficult to define the shape.

[0106] In one embodiment, the carbon-based negative electrode active material may be graphite having a spherical particle shape. In this case, the spherical particles may be processed to have a spherical shape / form during manufacturing, or may be a spherical secondary graphite assembly formed by aggregating a plurality of flake-like primary graphite particles. By controlling the shape of the carbon-based negative electrode active material as described above, the present invention can increase the electrical conductivity of the negative electrode active layer and improve the adhesion between the negative electrode active layer and the negative electrode current collector by maximizing the contact area with the negative electrode current collector.

[0107] In another embodiment, the carbon-based negative electrode active material may have a particle shape in the form of scales or sheets. In this case, the carbon-based negative electrode active material may reduce the length of the lithium ion movement path within the negative electrode active layer, thereby providing excellent rapid charging performance.

[0108] In one embodiment, the carbon-based negative electrode active material may have the form of an assembly in which a plurality of particles are assembled. In this case, one graphite assembly may be formed by assembling 2 to 100 graphites, preferably 3 to 20 graphites.

[0109] The above carbon-based negative electrode active material may satisfy an average particle size within a predetermined range to achieve high density of the negative electrode active layer. Specifically, the above carbon-based negative electrode active material has an average particle size (D) of 0.5㎛ to 20㎛. 50) can be represented. Specifically, the carbon-based negative electrode active material may have an average particle size (D) of 0.5㎛ to 15㎛; 0.5㎛ to 10㎛; 5㎛ to 20㎛; 10㎛ to 20㎛; 12㎛ to 18㎛; 2㎛ to 7㎛; 0.5㎛ to 5㎛; or 1㎛ to 3㎛. 50 ...can be represented. The average particle size of the carbon-based negative electrode active material may be more advantageous as the particle size is made smaller to maximize the disorder in the expansion direction for each particle, thereby preventing particle expansion caused by lithium ion charging. However, if the particle size of the carbon-based negative electrode active material is less than 0.5 μm, a large amount of binder may be required due to the increase in the number of particles per unit volume. On the other hand, if the maximum particle size exceeds 20 μm, expansion becomes severe, and as charge and discharge cycles are repeated, the inter-particle bonding and the bonding between particles and the current collector decrease, which may lead to a decrease in cycle characteristics.

[0110] The above negative electrode active material may further include a silicon-based negative electrode active material.

[0111] The above silicon-based negative electrode active material is a material containing silicon (Si) as a main component, and can increase the charge / discharge capacity and energy density per unit volume of the negative electrode. Examples of such silicon-based negative electrode active materials include silicon (Si), silicon carbide (SiC), and silicon oxide (SiO₂). x, Only 0 <x<2) 등을 들 수 있으며, 이들은 음극 활성층에 단독으로 포함하거나 병용될 수 있다.

[0112] The silicon-based negative electrode active material may be doped with or alloyed with Li, Mg, Al, Ca and / or Ti, etc. Additionally, if the silicon-based negative electrode active material contains oxygen (O), the surface may be treated with a carbon coating layer or the like to suppress volume expansion during charging and simultaneously improve the electrical conductivity of the negative electrode active material.

[0113] The silicon-based negative electrode active material may be included in a range of 0.1 to 30 weight% based on the total weight of the negative electrode active layer. Specifically, the silicon-based negative electrode active material may be included in an amount of 0.1 to 25 weight%, 20 to 30 weight%, 10 to 30 weight%, 0.5 to 20 weight%, 1 to 9 weight%, 5 to 15 weight%, 3 to 7 weight%, 11 to 19 weight%, 13 to 17 weight%, 15 to 20 weight%, 3 to 13 weight%, or 1 to 15 weight% based on the total weight of the negative electrode active layer. By adjusting the content ratio of the silicon-based negative electrode active material to the above range, the present invention can improve the charge capacity per unit mass while reducing lithium consumption and irreversible capacity loss during the initial charge and discharge of the secondary battery. In addition, by minimizing the volume change of the negative electrode active layer during charging and discharging of the secondary battery, the structural stability of the negative electrode active layer can be improved, thereby extending the lifespan of the secondary battery.

[0114] The above conductive material may include one or more types of carbon black such as acetylene black, Denka black, Ketjen black, Super-P, furnace black, lamp black, thermal black, etc.; graphene; carbon nanotubes and carbon fibers, etc., but is not limited thereto.

[0115] As an example, the above-mentioned cathode active layer may contain carbon black, carbon nanotubes, carbon fibers, etc., as a conductive material, either alone or in combination.

[0116] The content of the conductive material may be 0.1 to 10 parts by weight per 100 parts by weight of the total cathode active layer. Specifically, the conductive material may be 0.1 to 8 parts by weight, 0.1 to 5 parts by weight, 0.1 to 3 parts by weight, 2 to 6 parts by weight, or 0.5 to 2 parts by weight per 100 parts by weight of the total cathode active layer. By controlling the content of the conductive material within the above range, the present invention can prevent the decrease in charging capacity caused by an increase in the resistance of the cathode due to a low content of the conductive material. Furthermore, the present invention can prevent problems such as a decrease in charging capacity due to a decrease in the content of the cathode active material caused by an excessive amount of conductive material exceeding the above range, or an increase in electrical resistance due to an increase in the loading amount of the cathode active layer.

[0117] In addition, the above binder is for the combination of the negative electrode active material and the conductive material, etc., and for the combination with the current collector.

[0118] As a component that assists in synthesis, it can be appropriately applied within a range that does not degrade the electrical properties of the cathode. For example, the binder may include one or more of vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidenefluoride (PVdF), polyacrylonitrile, polymethylmethacrylate, polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene propylene-diene monomer, sulfonated ethylene-propylene-diene monomer, styrene butadiene rubber (SBR), and fluororubber.

[0119] The content of the binder may be in the range of 0.1 to 10 parts by weight, preferably 0.1 to 5 parts by weight, based on 100 parts by weight of the total cathode active layer. Specifically, the binder may be in the range of 0.1 to 8 parts by weight, 0.1 to 5 parts by weight, 0.1 to 3 parts by weight, or 2 to 6 parts by weight based on 100 parts by weight of the total cathode active layer. By controlling the content of the binder contained in the cathode active layer to the above range, the present invention can prevent the adhesion of the active layer from being reduced due to a low content of binder or the electrical properties of the cathode from being reduced due to an excessive amount of binder.

[0120] The cathode active layer may have an average thickness in the range of 100 μm to 400 μm. For example, the cathode active layer may have an average thickness in the range of 100 μm to 350 μm; 100 μm to 300 μm; 100 μm to 250 μm; 100 μm to 200 μm; 150 μm to 400 μm; 200 μm to 400 μm; 150 μm to 300 μm; 150 μm to 250 μm; or 150 μm to 220 μm.

[0121] The cathode according to exemplary embodiments of the present invention may have an adhesion force between the cathode current collector and the cathode active layer of 20 gf / 20 mm or more, specifically in the range of 20 to 50 gf / 20 mm; 23 to 40 gf / 20 mm; and 25 to 35 gf / 20 mm. Accordingly, the cathode of the present invention has excellent long-term capacity and cycle characteristics.

[0122] The cathode according to exemplary embodiments of the present invention has the configuration described above, which shortens the movement path of lithium ions within the cathode active layer and significantly lowers the diffusion resistance of lithium ions, thereby providing excellent rapid charging performance. In addition, the adhesion between the cathode current collector and the cathode active layer is excellent, so the reduction in adhesion caused by the orientation of the cathode active material at a predetermined angle with respect to the cathode current collector is minimized, resulting in excellent durability of the cathode and superior long-term capacity and cycle characteristics.

[0123]

[0124] lithium secondary battery

[0125] Next, a lithium secondary battery according to the present invention will be described.

[0126] A lithium secondary battery according to exemplary embodiments of the present invention comprises a positive electrode; a negative electrode; a separator; and an electrolyte, wherein the negative electrode may be the negative electrode described above. Since the negative electrode has been described in detail above, a redundant description will be omitted.

[0127] The lithium secondary battery of the present invention can be manufactured according to conventional methods known in the art. For example, it can be manufactured by placing a separator between a positive electrode and a negative electrode and introducing an electrolyte.

[0128] Compared to a cathode using an untreated cathode current collector, even if the composition of the cathode active layer is identical, the binder content at the bottom of the cathode active layer is higher, and the binder content at the surface of the cathode active layer is lower. Therefore,

[0129] The above positive electrode may include a positive electrode current collector and a positive electrode active layer formed on the positive electrode current collector and comprising the positive electrode active material.

[0130] In the above-mentioned positive electrode, the positive electrode current collector is not particularly limited as long as it is conductive without causing chemical changes in the battery, and for example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface treated with carbon, nickel, titanium, silver, etc. may be used. In addition, the above-mentioned positive electrode current collector may typically have a thickness of 3 to 500 μm, and fine irregularities may be formed on the surface of the current collector to increase the adhesion of the positive electrode active material. For example, it may be used in various forms such as a film, sheet, foil, net, porous body, foam, nonwoven fabric, etc.

[0131] The above-mentioned cathode active material is not particularly limited, and any compound known in the art capable of reversible intercalation and deintercalation of lithium may be used without limitation. Specifically, the above-mentioned cathode active material is a layered compound such as lithium cobalt oxide (LiCoO2) or lithium nickel oxide (LiNiO2), or a compound substituted with one or more transition metals; chemical formula Li 1+x Mn 2-x Lithium manganese oxides such as O4 (where x is 0 to 0.33), LiMnO3, LiMn2O3, LiMnO2, etc.; lithium copper oxide (Li2CuO2); LiV3O8, LiV3O4, V2O5, Cu2V2O Vanadium oxide of the like; chemical formula LiNi 1-x M x Ni-site type lithium nickel oxide represented by O2 (where M = Co, Mn, Al, Cu, Fe, Mg, B, or Ga, and x = 0.01 ~ 0.3); chemical formula LiMn 2-x M x Lithium manganese composite oxide represented by O2 (where M = Co, Ni, Fe, Cr, Zn, or Ta, and x = 0.01 ~ 0.1) or Li2Mn3MO8 (where M = Fe, Co, Ni, Cu, or Zn); LiNi x Mn 2-x It may include, but is not limited to, a spinel-structured lithium manganese complex oxide represented by O4; LiMn2O4 in which part of the Li of the chemical formula is substituted with an alkaline earth metal ion; a disulfide compound; a lithium iron phosphate represented by LiFePO4; a disulfide compound; Fe2(MoO4)3, etc.

[0132] The above-described positive active material layer may include a positive conductive material and a positive binder together with the positive active material described above.

[0133] The above-mentioned anode conductive material is used to impart conductivity to the electrode and may include carbon black, graphite, carbon fiber, carbon nanotube, metal powder, conductive metal oxide, organic conductive material, etc. Currently commercially available conductive materials include acetylene black series (products from Chevron Chemical Company or Gulf Oil Company, etc.), Ketjen Black EC series (product from Armak Company), Vulcan XC-72 (product from Cabot Company), and Super P (product from MMM). Among these, carbon nanotubes, carbon nanofibers, and carbon black are preferred as the conductive material of the present invention, and carbon nanotubes are most preferred.

[0134] The above-mentioned positive conductive material may be included in the positive active material layer in an amount of 0.1 to 30 weight%, more specifically 0.1 to 10 weight%, and more specifically 0.5 to 5 weight%.

[0135] The above-mentioned anode binder can be any commonly used binder polymer without limitation. For example, various types of binder polymers such as polyvinylidene fluoride-hexafluoropropylene (PVDF-co-HFP), polyvinylidene fluoride (PVDF), polyacrylonitrile, polymethyl methacrylate, styrene butadiene rubber (SBR), and carboxyl methyl cellulose (CMC) can be used.

[0136] The above anode binder may be included in the anode active material layer in an amount of 0.1 to 30 weight%, more specifically 0.1 to 10 weight%, and more specifically 0.5 to 5 weight%.

[0137] The above separator can be any porous substrate commonly used as a separator in lithium secondary batteries, and for example, a polyolefin-based porous membrane or nonwoven fabric may be used, but is not particularly limited thereto. In particular, it is desirable that it has low resistance to ion movement of the electrolyte and excellent electrolyte moisture retention capacity.

[0138] Examples of the above-mentioned polyolefin-based porous membranes include membranes formed from polyolefin-based polymers such as high-density polyethylene, linear low-density polyethylene, low-density polyethylene, ultra-high molecular weight polyethylene, polypropylene, polybutylene, and polypentene, either individually or as a mixture thereof.

[0139] In addition to polyolefin-based nonwoven fabrics, the above nonwoven fabric may be formed from polymers such as polyethyleneterephthalate, polybutyleneterephthalate, polyester, polyacetal, polyamide, polycarbonate, polyimide, polyetheretherketone, polyethersulfone, polyphenyleneoxide, polyphenylenesulfide, and polyethylenenaphthalene, either individually or in a mixture thereof. The structure of the nonwoven fabric may be a spunbond nonwoven fabric or a melt-blown nonwoven fabric composed of long fibers.

[0140] The thickness of the porous substrate is not particularly limited, but may be 5 to 50 μm, and the pore size and porosity present in the porous substrate are also not particularly limited, but may be 0.01 to 50 μm and 10 to 95%, respectively.

[0141] Meanwhile, to improve the mechanical strength of the separator composed of the above porous substrate and to suppress short circuits between the anode and the cathode, a porous coating layer comprising inorganic particles and a binder polymer may be further included on at least one surface of the above porous substrate.

[0142] Meanwhile, in the above lithium secondary battery, the electrolyte may include an organic solvent and a lithium salt commonly used in electrolytes, and is not particularly limited.

[0143] The above organic solvent may be used without special restrictions as long as it can serve as a medium through which ions involved in the electrochemical reaction of the battery can move. Specifically, the above organic solvent may include ester-based solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether-based solvents such as dibutyl ether or tetrahydrofuran; ketone-based solvents such as cyclohexanone; and aromatic hydrocarbon-based solvents such as benzene and fluorobenzene. Carbonate-based solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC) may be used.

[0144] Among these, a carbonate-based solvent is preferred, and a mixture of a cyclic carbonate (e.g., ethylene carbonate or propylene carbonate, etc.) having high ionic conductivity and high dielectric constant that can improve the charge / discharge performance of the battery, and a low-viscosity linear carbonate-based compound (e.g., ethylmethyl carbonate, dimethyl carbonate or diethyl carbonate, etc.) is more preferred.

[0145] The above lithium salt can be used without special limitations as long as it is a compound capable of providing lithium ions used in lithium secondary batteries. Specifically, the lithium salt may be LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, or LiB(C2O4)2. It is preferable that the lithium salt be included in the electrolyte at a concentration of approximately 0.6 mol% to 2 mol%.

[0146] In addition to the above electrolyte components, the above electrolyte may further include one or more additives, such as pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylenediamine, n-glyme, triamide hexaphosphate, nitrobenzene derivative, sulfur, quinone imine dye, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol, or aluminum trichloride, for the purpose of improving the lifespan characteristics of the battery, suppressing the decrease in battery capacity, and improving the discharge capacity of the battery. In this case, the above additives may be included in an amount of 0.1 to 5 weight% based on the total weight of the electrolyte.

[0147] The lithium secondary battery of the present invention can be manufactured by forming an electrode assembly by placing a separator between a positive electrode and a negative electrode, placing the electrode assembly into a cylindrical battery case or a prismatic battery case, and then injecting an electrolyte. Alternatively, the electrode assembly may be manufactured by stacking the electrode assemblies, impregnating them with an electrolyte, placing the resulting product into a battery case, and sealing it.

[0148] When manufacturing the lithium secondary battery of the present invention, the electrode assembly may be dried to remove one or more organic solvents selected from the group consisting of N-methyl-2-pyrrolidone (NMP), acetone, ethanol, propylene carbonate, ethylmethyl carbonate, ethylene carbonate, and dimethyl carbonate used in the manufacture of the anode. If an electrolyte having the same components as the organic solvent used in the manufacture of the anode is used as the electrolyte, the process of drying the electrode assembly may be omitted.

[0149] Unlike the lithium secondary battery described above, the lithium secondary battery according to another embodiment of the present invention may be a solid-state battery.

[0150] The above battery case may be adopted from those commonly used in the field, and there are no restrictions on the external shape according to the application of the battery; for example, it may be a cylindrical shape using a can, a prismatic shape, a pouch shape, or a coin shape.

[0151] Since the lithium secondary battery according to the present invention stably exhibits excellent resistance characteristics, discharge capacity, output characteristics, and capacity retention rate, it is useful in fields such as portable devices like mobile phones, laptop computers, and digital cameras, energy storage systems (ESS), and electric vehicles such as hybrid electric vehicles (HEV).

[0152]

[0153] The present invention will be explained in more detail below through examples. However, the following examples are intended to illustrate the present invention and do not limit the scope of the present invention.

[0154]

[0155] Examples

[0156] A copper foil was prepared as a cathode current collector. Corona treatment was applied to one side and the other side of the copper foil, respectively. During corona treatment, the speed was set to 3 m / min and the output to 1.5 kW.

[0157] Average particle size D as a negative electrode active material in water, which is the solvent 50 A cathode slurry with a solid content of 50% by weight was prepared by adding and mixing flaky artificial graphite with a thickness of 1 to 10 μm, styrene butadiene rubber (SBR) as a binder, and carboxymethylcellulose (CMC) as a thickener. Based on 100 parts by weight of the solid content of the cathode slurry, the cathode active material was 96.5 parts by weight, the binder was 2.5 parts by weight, and the thickener was 1.0 part by weight.

[0158] A cathode slurry was applied onto a corona-treated copper foil. Permanent magnets having a length ratio of 110–120% relative to the width of the cathode slurry were placed on the upper part of the applied cathode slurry and on the lower part of the cathode current collector, and a magnetic field with a magnetic field strength of 4700±20 G was applied for 8 seconds. The cathode slurry to which the magnetic field was applied was hot-air dried and rolled to manufacture a cathode. The average thickness of the cathode active layer of the manufactured cathode was 120±5㎛.

[0159]

[0160] Comparative example

[0161] A cathode was prepared in the same manner as in Example 1, except that corona treatment was not performed on the cathode current collector in Example 1.

[0162]

[0163] Experimental Example 1: QBR

[0164] Five cathodes each of the above examples and comparative examples were prepared. Ion milling was performed on the cross-section of the prepared cathode samples. The ion-milled cathode samples were immersed in a diluted OsO4 solution (about 0.1% concentration) for about 30 minutes to stain the binder components with OsO4. Subsequently, the osmium components of the OsO4 staining material stained in the binder were detected using an energy dispersive spectral elemental analyzer attached to a scanning electron microscope (SU8020, HITACHI). An EDS mapping image was extracted showing the locations of the detected osmium. The extracted EDS mapping image was applied to image processing to calculate the area of ​​the stained binder and the area of ​​the unstained binder. The region extending from the outermost surface of the cathode active layer to within 15% of the total thickness of the cathode active layer was defined as the "cathode active layer surface region," and the region extending from the interface of the cathode active layer facing the current collector to within 15% of the total thickness of the cathode active layer was defined as the "cathode active layer bottom region." In the cathode active layer surface region defined in this way, the ratio value (Os / C) of the area of ​​the dyed binder (Os) to the undyed area (C) was calculated, and the average value of the ratio value (Os / C) for five samples was calculated as Bs. For the cathode active layer bottom region as well, the ratio value (Os / C) of the area of ​​the dyed binder (Os) to the undyed area (C) was calculated, and the average value of the ratio value (Os / C) for five samples was calculated as Bf. The average values ​​of Bs and Bf for each cathode sample of the example and comparative example are shown in FIG. 2.

[0165] Then, the above Bs and Bf were substituted into Equation 1 below to calculate the QBR value, and the result is shown in Table 1.

[0166] [Equation 1]

[0167] QBR = Bs / Bf

[0168]

[0169] Experimental Example 2: Adhesion Strength Evaluation

[0170] In the examples and comparative examples, specimens were prepared by cutting each manufactured cathode to a length of 25 mm and a length of 70 mm. The prepared specimens were attached to a glass plate using double-sided tape, with the current collector positioned to face the glass plate. After fixing the specimens fixed to the glass plate to a tensile testing machine, the cathode active layer of each cathode was detached by pulling it at a speed of 100 mm / min at 25°C so that it formed a 90° angle with the cathode current collector. The peel force measured in real time is shown in Table 1.

[0171]

[0172] Experimental Example 3: Evaluation of Life Characteristics

[0173] D as a positive active material in N-methylpyrrolidone (NMP) solvent 50 This 5㎛ LiNi 0.7 Co 0.1 Mn 0.1 Al 0.1 An anode slurry was prepared by adding O2, carbon black as a conductive material, and polyvinylidene fluoride as a binder in a weight ratio of 94:3:3 and mixing them. An anode was prepared by applying the anode slurry onto an aluminum foil, drying, and rolling.

[0174] A separator made of 18 μm polypropylene was interposed between the anode and each cathode of the example and comparative example, inserted into a case, and then an electrolyte composition was injected to assemble a lithium secondary battery.

[0175] The lithium secondary batteries were each activated by charging them to 3.6V at a rate of 0.3C at 23℃ under CC-CV conditions. Afterward, they were aged at 60℃ for 36 hours and at 23℃ for 48 hours. Subsequently, a degassing process was performed to expel gas from inside the secondary batteries, followed by full charging and full discharging.

[0176] Subsequently, one cycle was defined as charging each lithium secondary battery to 4.2V at a rate of 1.0C at 23℃ under CC-CV conditions and discharging to 2.5V at a rate of 1.0C under CC conditions, and one cycle of charging and discharging was performed for each lithium secondary battery prepared in the examples and comparative examples. At this time, the capacity was measured during charging and discharging to confirm the charge-discharge capacity of the first cycle. After that, 299 charge-discharge cycles were performed for each lithium secondary battery to perform a total of 300 charge-discharge cycles. At this time, the charge-discharge capacity of the 300th cycle was confirmed during the last cycle. The charge-discharge capacity retention rate of the 300th cycle was calculated based on the measured charge-discharge capacity of the first cycle. The results are shown in Table 1.

[0177] Average value of Bs Average value of Bf QBR(Bs / Bf) Adhesion strength (gf / 20mm) Capacity retention rate (%) Example 0.01 09 0.01 76 0.62 27.792 Comparative Example 0.01 40 0.01 67 0.84 19.888

[0178] Referring to Table 1, the average Bf value of the cathode according to the embodiment is greater than the average Bf value of the cathode according to the comparative example, and the average Bs value of the cathode according to the embodiment is smaller than the average Bs value of the cathode according to the comparative example. That is, the binder content at the bottom portion of the cathode active layer of the cathode according to the embodiment is higher than the binder content at the bottom portion of the cathode active layer of the cathode according to the comparative example, and the binder content at the surface portion of the cathode active layer of the cathode according to the embodiment is lower than the binder content at the surface portion of the cathode active layer of the cathode according to the comparative example. Accordingly, the adhesion strength of the cathode according to the embodiment is higher than that of the cathode according to the comparative example. In addition, the cathode according to the embodiment exhibits superior long-term cycle characteristics of the secondary battery compared to the cathode according to the comparative example.

[0179]

[0180] The present invention has been described in more detail above through drawings and embodiments. However, the configurations described in the drawings or embodiments described in this specification are merely one embodiment of the present invention and do not represent all technical concepts of the present invention; therefore, it should be understood that various equivalents and modifications that can replace them may exist at the time of filing this application.

Claims

1. Cathode current collector, and The above-mentioned cathode active layer is provided on at least one surface of the cathode current collector and comprises a carbon-based cathode active material. The above cathode active layer has a Quantified Binder Ratio (QBR) of 0.8 or less, and the cathode is defined by the following Equation 1: [Equation 1] QBR = Bs / Bf In the above Equation 1, Bs represents the average value of the binder content in the surface region of the cathode active layer from the outermost surface of the cathode active layer up to within 15% of the total thickness of the cathode active layer, and Bf represents the average value of the binder content in the bottom region of the cathode active layer from the interface of the cathode active layer facing the current collector up to within 15% of the total thickness of the cathode active layer.

2. In Paragraph 1, The above cathode active layer is a cathode having a QBR value calculated from Equation 1 in the range of 0.1 to 0.

75.

3. In Paragraph 1, The above cathode active layer is a cathode having an alignment degree (OI) in the range of 1 to 15 calculated from Equation 2 below during X-ray diffraction spectroscopic analysis: [Equation 2] OI = I 004 / I 110 In Equation 2, I 004 represents the area value of the peak indicating the [0,0,4] crystal plane during X-ray diffraction (XRD) spectroscopic analysis of the cathode active layer, and I 110 represents the area value of the peak representing the [1,1,0] crystal plane during X-ray diffraction (XRD) spectroscopic analysis of the cathode active layer.

4. In Paragraph 1, The above carbon-based cathode active material comprises one or more mixtures selected from natural graphite, artificial graphite, Kish graphite, pyrolytic carbon, carbon microbeads, mesophase calcined carbon made from tar and pitch, and graphitized coke.

5. In Paragraph 1, The above cathode active layer further comprises a binder, and The above binder is a cathode in the range of 0.1 to 5 parts by weight per 100 parts by weight of the entire cathode active layer.

6. In Paragraph 1, The above current collector is a cathode having a contact angle with water of 50 degrees or less.

7. A step of modifying the surface of the cathode current collector; A step of applying a cathode slurry comprising a carbon-based cathode active material to at least one surface of the above-mentioned cathode current collector; and The step of applying a magnetic field to the coated cathode slurry, and A method for manufacturing a cathode, wherein the step of modifying the surface of the cathode current collector includes a corona treatment process for the surface of the cathode current collector or a plasma treatment process for the surface of the cathode current collector.

8. In Paragraph 7, A method for manufacturing a cathode, wherein the step of modifying the surface of the cathode current collector comprises a corona treatment process on the surface of the cathode current collector.

9. In Paragraph 7, The above corona treatment is a method for manufacturing a cathode performed under output conditions of 0.5 to 3.5 kW.

10. In Paragraph 7, A method for manufacturing a cathode in which the above corona treatment is performed at a speed of 1 to 10 m / min.

11. In Paragraph 7, A step of drying a cathode slurry to which a magnetic field is applied; and A method for manufacturing a cathode, further comprising the step of forming a cathode active layer by rolling a dried cathode slurry.

12. In Paragraph 7, A method for manufacturing a cathode in which, in the step of applying the magnetic field, the strength of the magnetic field is in the range of 1,000 to 10,000 G.

13. In Paragraph 7, A method for manufacturing a cathode, wherein, after the step of modifying the surface of the cathode current collector, the cathode current collector has a contact angle with water of 50 degrees or less.

14. In Paragraph 7, A method for manufacturing a cathode, wherein, after the step of modifying the surface of the above-mentioned cathode current collector, the cathode current collector has a contact angle with water of 1 to 40 degrees.

15. A lithium secondary battery comprising a positive electrode; a negative electrode; a separator; and an electrolyte, A lithium secondary battery in which the above-mentioned negative electrode is a negative electrode according to any one of claims 1 to 6.

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