Method for manufacturing a negative electrode for lithium secondary batteries

Abstract

The present invention relates to a negative electrode for a lithium secondary battery and a method for manufacturing the same. The method for manufacturing the negative electrode dries a negative electrode slurry to which a magnetic field is applied and then rolls it, and controls the rolling load according to Formula 1 that depends on the intensity of the magnetic field applied to the negative electrode slurry, so that even in a simple process with a small number of steps, the springback phenomenon of the negative electrode active layer can be remarkably improved, and thus it is excellent in processability and economy. Further, the manufactured negative electrode not only exhibits a high energy density while maintaining a desired thickness, but also has low rolling stress, improved structural stability of the negative electrode active layer, reduced volume change of the negative electrode active layer accompanying charge and discharge, and improved life.

Description

Technical Field

[0001] This application claims the benefit of priority based on Korean Patent Application No. 10-2023-0032292 filed on March 13, 2023, and all the contents disclosed in the literature of the Korean Patent Application are incorporated herein by reference.

[0002] The present invention relates to a negative electrode for a lithium secondary battery and a method for manufacturing the same.

Background Art

[0003] In recent years, secondary batteries have been widely applied not only to small devices such as portable electronic devices but also to medium and large-sized devices such as battery packs or power storage devices for hybrid vehicles and electric vehicles.

[0004] Among these, a lithium secondary battery uses a metal oxide such as LiCoO2 as a positive electrode active material and a carbon material as a negative electrode active material, inserts a polyolefin-based porous separator between the negative electrode and the positive electrode, and injects a non-aqueous electrolyte containing a lithium salt such as LiPF6 to manufacture. During charging, lithium ions in the positive electrode active material are released and inserted into the carbon layer of the negative electrode, and during discharging, conversely, lithium ions in the negative electrode carbon layer are released and inserted into the positive electrode active material. At this time, the non-aqueous electrolyte infiltrated into the positive electrode and the negative electrode serves as a medium for moving lithium ions between the negative electrode and the positive electrode.

[0005] Such a lithium secondary battery increases the loading amount of each electrode in order to realize high capacity and high energy density, and at the same time, an essential process of increasing the rolling density during electrode manufacturing is carried out.

[0006] Specifically, electrodes for lithium secondary batteries can be obtained as electrode sheets. These electrode sheets are manufactured by applying an electrode slurry containing electrode active material to an electrode current collector at a high loading rate and drying it, and then rolling the dried electrode slurry in a rolling mill to form a high-loading and high-density electrode active layer on the electrode current collector. In this process, the rolled sheet is manufactured considering the desired density distribution, thickness, thickness distribution, and precision of the electrode active layer.

[0007] However, rolled sheets tend to exhibit a strong spring-back phenomenon, where the electrode active layer is compressed during rolling, causing stress to accumulate in the electrode active material powder within the layer, leading to a tendency for the sheet to return to its original state. Consequently, over time, the thickness of the electrode active layer containing the electrode active material exceeds the desired thickness. This causes a problem in which the energy density of electrodes with high loading capacity is reduced.

[0008] To improve the springback phenomenon described above, techniques have been developed to roll the dried electrode slurry multiple times or to use a multi-stage rolling mill. However, even with these techniques, the springback phenomenon cannot be completely suppressed, and there are limitations in terms of economic efficiency in terms of energy and process efficiency.

[0009] Therefore, there is a great need for manufacturing technology for electrodes that can solve the above problems and maintain the desired thickness even after time has passed since rolling. [Overview of the project] [Problems that the invention aims to solve]

[0010] The object of the present invention is to provide a negative electrode for a lithium secondary battery and a method for manufacturing the same, in which the spring-back phenomenon of the active layer, particularly an active layer having a high loading amount, is suppressed, and the desired thickness can be maintained even after a predetermined time has elapsed during rolling. [Means for solving the problem]

[0011] To solve the above problem, In one embodiment, the present invention is described as follows: A step of applying a magnetic field to a negative electrode slurry coated on a negative electrode current collector, The steps include drying a negative electrode slurry to which a magnetic field has been applied to form a negative electrode active layer, and The step includes rolling the formed negative electrode active layer, The above negative electrode slurry contains a carbon-based negative electrode active material. The above magnetic field is applied with an intensity of 3,000G to 15,000G. The above rolling step provides a method for manufacturing a negative electrode for a lithium secondary battery in which the rolling load is controlled according to the following formula 1:

[0012] [Formula 1] y = Ax + B

[0013] In Equation 1, y represents the rolling load (unit: tons), x represents the strength of the magnetic field when a magnetic field is applied (unit: Gauss). A and B satisfy -0.01 ≤ A < 0 and 3 ≤ B ≤ 7.

[0014] In this case, the rolling load can range from 1 ton to 7 tons.

[0015] Furthermore, the rolling step described above includes a primary rolling step in which the formed negative electrode active layer is pressurized with the rolling load of [Equation 1], and a secondary rolling step in which the primary rolled negative electrode active layer is further pressurized, and the rolling load during primary rolling may be greater than the rolling load during secondary rolling.

[0016] Specifically, the rolling load during the secondary rolling process may be 50% or less of the rolling load during the primary rolling process.

[0017] Furthermore, the rolling step described above may be carried out at a temperature of 20°C to 35°C.

[0018] The negative electrode active layer formed in this manner may have an average thickness increase rate of less than 10% after 72 hours of rolling at room temperature (22±1℃), based on the average thickness immediately after rolling.

[0019] Furthermore, the step of applying the magnetic field described above may be performed for 5 to 60 seconds.

[0020] Furthermore, in one embodiment of the present invention, The negative electrode current collector and the 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 are included. The present invention provides a negative electrode for a lithium secondary battery manufactured by the manufacturing method described above.

[0021] In this case, the above-mentioned negative electrode active layer may have an alignment degree (OI) of the carbon-based negative electrode active material represented by the following formula 2, which can be 0.2 to 1.5:

[0022] [Formula 2] OI=I 004 / I 110

[0023] In Equation 2, I 004 This represents the area of ​​the peak indicating the (0,0,4) crystal plane when measured by X-ray diffraction spectroscopy (XRD) for the negative electrode active layer. I 110 This represents the area of ​​the peak indicating the (1,1,0) crystal plane when X-ray diffraction (XRD) is measured for the negative electrode active layer.

[0024] Furthermore, the negative electrode active layer contains 1.2 to 1.8 g / cm³ of material. 3 It may have a density and an average thickness of 100 μm to 300 μm.

[0025] Furthermore, the above-mentioned negative electrode active layer may have a springback rate of less than 10%.

[0026] On the other hand, the carbon-based anode active material contained in the above-mentioned anode active layer may contain one or more of natural graphite and artificial graphite.

[0027] Furthermore, the above-mentioned negative electrode active layer may further contain a silicon-based negative electrode active material, and the silicon-based negative electrode active material may be Si, SiC, and SiO x (However, it may include one or more of the following: 0.8 ≤ x ≤ 2.5.) [Effects of the Invention]

[0028] The method for manufacturing a negative electrode for a lithium secondary battery according to the present invention involves drying a negative electrode slurry to which a magnetic field has been applied, then rolling it, and controlling the rolling load according to Equation 1, which depends on the strength of the magnetic field applied to the negative electrode slurry. This method is excellent in improving the springback phenomenon of the negative electrode active layer even with a simple and few-step process.

[0029] Furthermore, the manufactured negative electrode not only exhibits high energy density while maintaining the desired thickness even when the loading amount of the negative electrode active layer is large, but also has the advantage of lower rolling stress, improved structural stability of the negative electrode active layer, and suppression of volume changes in the negative electrode active layer during charging and discharging, thus improving battery life. [Brief explanation of the drawing]

[0030] [Figure 1] This image shows the axial direction in which the crystalline phase of graphite, a carbon-based anode active material, and the expansion of the graphite crystals due to the springback phenomenon during rolling are most pronounced. [Figure 2] This graph shows the rolling load applied according to the magnetic field strength during the manufacturing of the negative electrode according to an embodiment of Formula 1 of the present invention. [Modes for carrying out the invention]

[0031] Since the present invention can be modified in various ways and may have a variety of embodiments, specific embodiments will be described in detail.

[0032] However, this is not intended to limit the present invention to any particular embodiment, but rather should be understood to include all modifications, equivalents, or substitutions that fall within the spirit and technical scope of the present invention.

[0033] In the present invention, terms such as "includes" and "have" are intended to specify the presence of features, numbers, steps, operations, components, parts, or combinations thereof as described in the specification, and do not preemptively exclude the presence or possibility of adding one or more other features, numbers, steps, operations, components, parts, or combinations thereof.

[0034] Furthermore, in this invention, when a part such as a layer, film, region, or plate is described as being "on top" of another part, this includes not only the case where it is "directly on top" of the other part, but also the case where another part is located in between. Conversely, when a part such as a layer, film, region, or plate is described as being "below" another part, this includes not only the case where it is "directly below" the other part, but also the case where another part is located in between. Also, in this application, being "on top" may include being located not only at the top but also at the bottom.

[0035] Furthermore, in this specification, "contains as a main component" may mean that the defined component is contained in an amount of 50% by weight (or 50% by volume), 60% by weight (or 60% by volume), 70% by weight (or 70% by volume), 80% by weight (or 80% by volume), 90% by weight (or 90% by volume), or 95% by weight (or 95% by volume) or more of the total weight (or total volume). For example, "contains graphite as a main component as the negative electrode active material" may mean that graphite is contained in an amount of 50% by weight (or 60% by weight), 70% by weight (or 80% by weight), 90% by weight (or 95% by volume) or more of the total weight of the negative electrode active material, and in some cases, it may also mean that the entire negative electrode active material consists of graphite and contains 100% by weight of graphite.

[0036] Furthermore, in this specification, "electrode sheet" may mean an article in which a negative electrode slurry is applied to a negative electrode current collector, or in which the negative electrode slurry applied to the negative electrode current collector has dried to form a negative electrode active layer.

[0037] Furthermore, in this specification, "the carbon-based anode active material is oriented" or "the carbon-based anode active material is aligned" means that the crystal planes of the carbon-based anode active material constituting the particles of the anode active material are distributed such that they have a predetermined direction with respect to the surface of the anode current collector. In this case, the crystal plane may refer to the planar structure of the carbon-based anode active material, the crystal plane showing the two-dimensional plane of graphite, and may mean a (1,0,0) crystal plane or a (1,1,0) crystal plane. Also, "the carbon-based anode active material is oriented" may differ from the arrangement of the particles of the carbon-based anode active material themselves so that they have a specific direction within the anode active layer.

[0038] Furthermore, "high orientation (or degree of orientation) of carbon-based anode active material" can mean that the carbon-based anode active material contained in the anode active layer is aligned at a high frequency with respect to the surface of the anode current collector, and in some cases, it can mean that the carbon-based anode active material contained in the anode active layer is aligned at a high angle (for example, greater than 45°, specifically 60° or more) with respect to the surface of the anode current collector.

[0039] Furthermore, "high degree of alignment of carbon-based anode active material" means that the "degree of alignment (OI)" referred to herein is large, and may mean that the carbon-based anode active material contained in the anode active layer is aligned at a low angle (e.g., less than 45°) with respect to the surface of the anode current collector. Conversely, "low degree of alignment of carbon-based anode active material" means that the "degree of alignment (OI)" is small, and may mean that the carbon-based anode active material contained in the anode active layer is aligned at a high angle (e.g., greater than 45°, specifically 60° or more) with respect to the surface of the anode current collector.

[0040] Furthermore, in this specification, "crystal plane of carbon-based anode active material" refers to a plane on which the atoms of the carbon-based anode active material form the outer shape of the crystal, and in the present invention, it may mean a crystal plane including the plane of the carbon-based anode active material, or a crystal plane including the a-axis / ab-axis of the carbon-based anode active material crystal.

[0041] The present invention will be described in more detail below.

[0042] <Manufacturing method for negative electrodes in lithium secondary batteries>

[0043] In one embodiment, the present invention is described as follows: Step S1 involves applying a magnetic field to the negative electrode slurry coated on the negative electrode current collector. Step S2 involves drying the negative electrode slurry to which a magnetic field has been applied to form a negative electrode active layer, and The present invention provides a method for manufacturing a negative electrode for a lithium secondary battery, which includes step S3 of rolling the formed negative electrode active layer.

[0044] The method for manufacturing a negative electrode according to the present invention involves applying a negative electrode slurry containing a carbon-based negative electrode active material onto a negative electrode current collector, and applying a magnetic field to the surface of the applied negative electrode slurry (S1), thereby aligning the carbon-based negative electrode active material in the negative electrode slurry to have a predetermined angle with respect to the surface of the negative electrode current collector. Subsequently, the negative electrode slurry is dried to form a negative electrode active layer containing a highly oriented carbon-based negative electrode active material (S2), and the negative electrode can be manufactured by rolling the continuously formed negative electrode active layer (S3).

[0045] In this case, the present invention can effectively improve the springback phenomenon of the negative electrode active layer after rolling by controlling the load during rolling of the negative electrode active layer according to the strength of the magnetic field applied to the negative electrode slurry.

[0046] Generally, the negative electrode of a lithium secondary battery is manufactured by rolling a negative electrode sheet, on which a negative electrode active layer is arranged, in a rolling mill to increase energy density. However, the negative electrode sheet suffers from a phenomenon called springback, where the negative electrode active layer tends to return to its original state due to the stress accumulated in the powder of the negative electrode active material during rolling. This results in a problem where the thickness of the negative electrode active layer becomes thicker than desired over time. This phenomenon becomes more pronounced as the loading amount, which indicates the weight of the negative electrode active material per unit area or unit volume of the negative electrode active layer, increases.

[0047] To improve these problems, it was necessary to perform rolling multiple times or to use complex rolling processes such as multi-stage rolling mills. However, even when multiple rolling processes are performed or complex rolling processes such as multi-stage rolling mills are used, there is a limitation that not only is the effect of improving springback significantly reduced after a certain amount of time has elapsed, but the effect of improving springback becomes only slight as the amount of negative electrode active material loaded increases.

[0048] In contrast, the negative electrode manufacturing method according to the present invention can achieve a high springback improvement effect even though the rolling process is performed two or fewer times, specifically only once, by controlling the load during rolling according to the following formula 1, which depends on the strength of the magnetic field applied to the negative electrode slurry:

[0049] [Formula 1] y = Ax + B

[0050] In Equation 1, y represents the rolling load (unit: tons), x represents the strength of the magnetic field when a magnetic field is applied (unit: Gauss). A and B satisfy -0.01 ≤ A < 0 and 3 ≤ B ≤ 7.

[0051] The method for manufacturing the negative electrode according to the present invention will be described in more detail below, step by step.

[0052] First, the method for manufacturing a negative electrode according to the present invention includes step S1 of applying a magnetic field to a negative electrode slurry coated on a negative electrode current collector. Step S1 refers to the process of orienting the carbon-based negative electrode active material contained in the negative electrode slurry so that it has a predetermined angle with respect to the negative electrode current collector, in order to suppress the springback phenomenon of the carbon-based negative electrode active material after rolling.

[0053] For example, in the case of graphite, a carbon-based negative electrode active material, it has a crystalline phase in which two-dimensional planes are stacked, as shown in Figure 1. In the three-dimensional space in which the above crystalline phase exists, the planes of the graphite are located in the directions of the a-axis and b-axis, and the direction in which the planes are stacked is the c-axis direction. When pressure is applied to such graphite, it returns to its original state due to the stress accumulated in the crystalline phase, but the expansion due to recovery is greatest when pressure is applied in the c-axis direction of the crystalline phase.

[0054] Therefore, in order to suppress the springback phenomenon of the carbon-based anode active material after rolling of the anode active layer, the present invention overcomes this by aligning the carbon-based anode active material contained in the anode slurry so that the crystal plane in the c-axis direction is horizontal to the anode current collector, that is, so that the plane of the carbon-based anode active material is perpendicular to the anode current collector, before drying the anode slurry.

[0055] Here, the orientation of the carbon-based negative electrode active material can be performed by applying a magnetic field to the negative electrode slurry coated on the negative electrode current collector before drying. That is, the step of applying the magnetic field can be performed by applying the magnetic field using magnets positioned above and below the negative electrode current collector, which is moved with the negative electrode slurry coated on its surface.

[0056] Furthermore, the degree of orientation of the carbon-based anode active material can be adjusted by the strength of the magnetic field applied to the anode slurry and the time it is exposed to the magnetic field. Therefore, the step of applying the magnetic field can be performed under predetermined magnetic field strength and time conditions.

[0057] Specifically, the step of applying the above magnetic field may involve applying a magnetic field of 3,000G (Gauss) to 15,000G, more specifically 3,000G to 12,000G, 3,000G to 11,000G, 3,000G to 10,000G, 3,000G to 9,000G, 3,000G to 8,000G, 3,000G to 7,500G, 3,000G to 6,000G, and 5,000G. A magnetic field may be applied with an intensity of G~9,000G, 10,000G~15,000G, 8,000G~12,000G, 3,000G~12,000G, 4,000G~8,000G, 5,000G~8,500G, 3,000G~5,500G, 3,500G~5,500G, 4,000G~5,500G, 3,500G~4,500G, or 4,500G~5,000G.

[0058] Furthermore, the step of applying the magnetic field described above may be performed for 5 to 60 seconds, specifically for 5 to 30 seconds, 10 to 60 seconds, 10 to 30 seconds, 30 to 60 seconds, 40 to 50 seconds, 15 to 35 seconds, 10 to 50 seconds, or 20 to 40 seconds.

[0059] The present invention makes it possible to easily orient the crystal plane of the carbon-based negative electrode active material contained in the negative electrode slurry at a high angle, nearly perpendicular to the surface of the negative electrode current collector, by applying a magnetic field for the aforementioned intensity and duration.

[0060] Furthermore, the method for manufacturing a negative electrode according to the present invention includes step S2, which involves applying a magnetic field to the negative electrode slurry in which the carbon-based negative electrode active material is oriented, and drying it to form a negative electrode active layer.

[0061] In this case, the drying of the negative electrode slurry can be applied without particular limitations, as long as it is a method that can maintain the orientation of the carbon-based negative electrode active material contained in the negative electrode active layer. For example, the drying can be performed by applying thermal energy to the negative electrode slurry using a hot air dryer, a vacuum oven, or the like.

[0062] Furthermore, the method for manufacturing a negative electrode according to the present invention includes step S3 for manufacturing a negative electrode by rolling the formed negative electrode active layer. Step S3 can be carried out using a method commonly applied in the industry, such as a roll press, which can increase the density of the negative electrode active layer.

[0063] At this time, the load applied to the negative electrode active layer during rolling is controlled according to the following equation 1, which depends on the strength of the magnetic field applied to the negative electrode slurry:

[0064] [Formula 1] y = Ax + B

[0065] In Equation 1, y represents the rolling load (unit: tons), x represents the strength of the magnetic field when a magnetic field is applied (unit: Gauss). A and B satisfy -0.01 ≤ A < 0 and 3 ≤ B ≤ 7.

[0066] Equation 1 above is a parameter that shows the correlation between the strength of the magnetic field applied to the negative electrode slurry and the load applied to the negative electrode active layer during rolling. Here, A and B in Equation 1 are constants, and A can be -0.1 or greater and less than 0 (i.e., -0.1 ≤ A < 0), specifically -0.01 or greater and less than 0 (i.e., -0.01 ≤ A < 0), -0.001 or greater and less than 0 (i.e., -0.001 ≤ A < 0), -0.001 to -0.000001 (i.e., -0.001 ≤ A ≤ -0.000001), or -0.0005 to -0.000001 (i.e., -0.0005 ≤ A ≤ -0.000001). Furthermore, B can be between 3 and 7 (i.e., 3 ≤ B ≤ 7), specifically between 4 and 6 (i.e., 4 ≤ B ≤ 6), 4.1 and 4.9 (i.e., 4.1 ≤ B ≤ 4.9), 5.1 and 5.9 (i.e., 5.1 ≤ B ≤ 5.9), or 4.5 and 5.7 (i.e., 4.5 ≤ B ≤ 5.7).

[0067] In Equation 1 above, "load" refers to the force applied to the surface of the negative electrode active layer. Since the negative electrode active layer is subjected to load while being transported during the rolling process, the "pressure applied during rolling" must reflect the length perpendicular to the direction in which the negative electrode active layer is transported (i.e., the widthwise length of the negative electrode active layer). For example, if a negative electrode slurry is applied to a current collector using a die coater with a coating width of 26.2 cm, and the resulting negative electrode active layer (widthwise length: 26.2 cm) is to be rolled with a load of 5 tons, the linear pressure may be approximately 0.19 ton / cm.

[0068] According to Equation 1 above, the stronger the magnetic field applied to the negative electrode slurry, the lower the rolling load can be. This means that the rolling load required to suppress the springback phenomenon of the negative electrode active layer is influenced by the degree of orientation of the carbon-based negative electrode active material, and this orientation of the carbon-based negative electrode active material is influenced by the strength of the magnetic field applied to the negative electrode slurry. The present invention can achieve a high springback improvement effect even when the rolling process is performed two or fewer times, specifically only once, by controlling the rolling load of the negative electrode active layer according to Equation 1 above.

[0069] Furthermore, the present invention eliminates the need to apply an excessively high load to the anode active layer to improve the springback phenomenon when rolling with the load according to Formula 1 above, thereby significantly reducing the rolling stress applied to the manufactured anode and ensuring the structural stability of the anode active layer. Moreover, the present invention allows rolling of the anode active layer even at relatively low loads, and provides room for applying additional external forces (e.g., rolling loads) to the anode active layer without excessive stress thereafter, thus opening up the possibility of developing and / or manufacturing thinner anodes. In addition, thinner anodes have the advantage of being advantageous in terms of charge / discharge capacity, output, and / or energy density because many electrodes can be stacked within the electrode assembly under the same thickness conditions.

[0070] As one example, the negative electrode active layer of the negative electrode manufactured according to the present invention may have an average thickness increase rate of less than 4% after 2.5 hours of rolling at room temperature (22±1℃) based on the average thickness immediately after rolling, and specifically may be 3% or less, 2% or less, or 1.5% or less.

[0071] As another example, the negative electrode active layer of the negative electrode manufactured according to the present invention may have an average thickness increase rate of less than 10% after 72 hours of rolling at room temperature (22±1℃) based on the average thickness immediately after rolling, specifically 9.5% or less, or 9% or less.

[0072] The rolling process in the present invention may be performed only once, as described above, or in some cases twice. In this case, the step of rolling the negative electrode active layer may include a primary rolling step S3-1 in which the dried negative electrode active layer is pressurized, and a secondary rolling step S3-2 in which the primary rolled negative electrode active layer is further pressurized.

[0073] Specifically, the primary rolling step S3-1 described above is a rolling step that increases the energy density of the dried anode active layer, and may be a step in which the density of the anode active layer substantially increases. Therefore, the primary rolling step S3-1 may be performed with a load equivalent to that of performing only one rolling pass, thereby allowing the rolling load to be controlled according to Equation 1.

[0074] Furthermore, the secondary rolling step S3-2 described above is a step in which the primary-rolled negative electrode active layer is further pressurized, and at the same time compensates for the slight springback phenomenon of the negative electrode active layer that has occurred, it may be a step in which the interface characteristics between the negative electrode current collector and the negative electrode active layer are improved. Therefore, the rolling load in the secondary rolling step S3-2 may be smaller than the rolling load applied to the negative electrode active layer during the primary rolling step. Specifically, the rolling load during secondary rolling may be 50% or less of the rolling load during primary rolling, and more specifically, it may be 45% or less, 40% or less, 35% or less, 20% or less, 20-45%, 25-45%, or 30-45% of the rolling load during primary rolling.

[0075] Furthermore, the rolling load of the negative electrode active layer can range from 1 ton to 7 tons, specifically from 1 ton to 3 tons, 2 tons to 6 tons, 3 tons to 7 tons, 4 tons to 7 tons, or 4.5 tons to 5.9 tons.

[0076] The above load may be the load applied when rolling the anode active layer once, and in some cases, it may be the load applied during the primary rolling step when rolling is performed twice. The present invention not only increases the density of the anode active layer by applying the rolling load within the above range, but also prevents the reduction of crystal plane alignment of the carbon-based anode active material and the failure to improve the springback phenomenon.

[0077] Furthermore, the temperature and rolling speed during the rolling step S3 described above can be within a predetermined range.

[0078] Specifically, step S3, which involves rolling, may be performed at a temperature below 40°C, specifically at temperatures of 20°C to 30°C, 20°C to 28°C, 25°C to 30°C, or 22°C to 24°C.

[0079] Furthermore, the rolling step S3 described above may be carried out at a rolling speed of 2 m / s to 7 m / s, more specifically at a rolling speed of 2 m / s to 6.5 m / s, 2 m / s to 6 m / s, 2 m / s to 5.5 m / s, 2 m / s to 5 m / s, 2 m / s to 4.5 m / s, 2 m / s to 4 m / s, 2.5 m / s to 4 m / s, 2.5 m / s to 3.5 m / s, 3.5 m / s to 5 m / s, 5 m / s to 7 m / s, 5.5 m / s to 6.5 m / s, or 6 m / s to 7 m / s.

[0080] The present invention makes it possible to increase the energy density of the anode while minimizing changes in the alignment of the carbon-based anode active material contained in the anode active layer formed by rolling a dried anode slurry under the above temperature and / or speed conditions.

[0081] On the other hand, in the present invention, the negative electrode slurry can be applied to the negative electrode current collector by discharging a negative electrode slurry containing a carbon-based negative electrode active material onto the surface of the moving negative electrode current collector. Here, the above application method may be a die coating method.

[0082] The above die coating method can be performed using a slot die equipped with a shim for controlling the discharge conditions of the negative electrode slurry. In this case, by controlling the shape of the shim, the loading amount and coating thickness of the negative electrode slurry applied to the negative electrode current collector can be easily controlled.

[0083] The method for manufacturing a negative electrode for lithium secondary batteries according to the present invention, having the above-described configuration, can significantly improve the springback phenomenon of the negative electrode active layer with a simple and few steps, without requiring multiple rolling processes or complex rolling processes such as multi-stage rolling as in the conventional method, thus offering excellent processability and cost-effectiveness.

[0084] <Negative electrode for lithium secondary batteries>

[0085] Furthermore, in one embodiment of the present invention, The negative electrode current collector and the 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 are included. The present invention provides a negative electrode for a lithium secondary battery manufactured by the manufacturing method described above.

[0086] The negative electrode for a lithium secondary battery according to the present invention includes a negative electrode active layer containing a carbon-based negative electrode active material on at least one surface of the negative electrode current collector. The negative electrode active layer is a layer that embodies the electrical activity of the negative electrode and is manufactured by applying a negative electrode slurry containing a negative electrode active material that embodies an electrochemical oxidation-reduction reaction during charging and discharging of the battery to at least one surface of the negative electrode current collector, and then drying and rolling it.

[0087] Generally, the negative electrode of a lithium secondary battery is manufactured by rolling a negative electrode sheet, on which a negative electrode active layer is placed, in a rolling mill to increase energy density. However, the aforementioned negative electrode sheet suffers from a phenomenon called springback, where the negative electrode active layer tends to return to its original state due to the stress accumulated in the powder of the negative electrode active material during rolling. This results in a problem where the thickness of the negative electrode active layer exceeds the desired thickness over time. This phenomenon becomes more pronounced as the loading amount, which indicates the weight of the negative electrode active material per unit area or unit volume of the negative electrode active layer, increases. To improve this problem, it was necessary to perform rolling multiple times or to use a complex rolling process such as a multi-stage rolling mill.

[0088] However, the negative electrode according to the present invention has the characteristic that the springback phenomenon of the negative electrode active layer after rolling is significantly improved by controlling the rolling load of the negative electrode active layer formed after applying a magnetic field to the negative electrode slurry by the manufacturing method of the present invention described above, according to Equation 1. Therefore, the above negative electrode has the advantage of high energy density of the negative electrode active layer and high processability and economic efficiency during manufacturing. In addition, the above negative electrode has the advantage of low rolling stress and high structural stability of the negative electrode active layer, which reduces the volume change of the negative electrode active layer during charging and discharging and improves the battery life.

[0089] In this case, the negative electrode active layer contains a carbon-based negative electrode active material as the negative electrode active material in order to exhibit electrical activity through a reversible oxidation-reduction reaction during the charging and discharging of the battery.

[0090] The carbon-based anode active material mentioned above refers to a material whose main component is carbon atoms, and such a carbon-based anode active material may include graphite. The graphite mentioned above may include one or more of either natural graphite or artificial graphite.

[0091] As one example, a carbon-based negative electrode active material may contain mixed graphite, which is a mixture of natural graphite and artificial graphite. In this case, the mixed graphite may be a mixture of natural graphite and artificial graphite in a weight ratio of 10-50:50-90, or 10-30:70-90. By adjusting the content ratio of natural graphite and artificial graphite as described above, it is possible to prevent a reduction in the adhesion between the negative electrode current collector and the negative electrode active layer due to less than 10 parts by weight of natural graphite relative to the total weight, and to prevent a reduction in the charge / discharge capacity of the negative electrode due to more than 50 parts by weight of natural graphite.

[0092] On the other hand, conventional artificial graphite is excellent at improving battery life characteristics, but its capacity is lower compared to natural graphite, so a larger amount must be loaded to manufacture high-density electrodes. However, artificial graphite has poor rollability and exhibits a strong spring-back phenomenon, making it difficult to manufacture high-density electrodes that meet the desired thickness. However, the negative electrode of the present invention, manufactured by the manufacturing method of the present invention, can have a high-density negative electrode active layer with suppressed spring-back phenomenon, even when the carbon-based negative electrode active material contains a high content of artificial graphite.

[0093] Furthermore, while the carbon-based anode active material is not particularly limited in its form, it is preferable that it has the form of a spherical graphite granule formed by the aggregation of multiple flake-shaped graphite particles. Examples of flake-shaped graphite include natural graphite, artificial graphite, mesophase-calcined carbon (bulk mesophase) made from tar and pitch, and graphitized cokes (green coke, green coke, pitch coke, needle coke, petroleum coke, etc.). In particular, the carbon-based anode active material is preferably assembled using multiple highly crystalline natural graphite particles. Also, one graphite granule can be formed by the aggregation of 2 to 100, preferably 3 to 20, flake-shaped graphite particles.

[0094] Furthermore, the above carbon-based negative electrode active material has an average particle size (D) of 0.5 μm to 20 μm. 50) can be shown, specifically, an average particle size (D of 0.5 μm to 15 μm, 0.5 μm to 10 μm, 5 μm to 20 μm, 10 μm to 20 μm, 12 μm to 18 μm, 2 μm to 7 μm, 0.5 μm to 5 μm, or 1 μm to 3 μm 50 ) can be shown.

[0095] The average particle size of graphite can be more advantageous as the particle size is reduced in order to maximize the degree of disorder in the expansion direction for each particle so as to prevent the expansion of particles due to the charging of lithium ions. However, when the particle size of graphite is less than 0.5 μm, a large amount of binder may be required due to an increase in the number of particles per unit volume. On the other hand, when the maximum particle size exceeds 20 μm, the expansion becomes intense, and as charge and discharge are repeated, the inter-particle binding property and the binding property between the particles and the current collector decrease, and the cycle characteristics may be significantly reduced.

[0096] In addition, the negative electrode active layer may have a loading amount indicating the weight of the carbon-based negative electrode active material per unit area / volume within a predetermined range. As an example, the negative electrode active layer may have a loading amount of 25 mg / 25 cm 2 ~50 mg / 25 cm 2 . Specifically, the negative electrode active layer may be 25 mg / 25 cm 2 ~45 mg / 25 cm 2 , 25 mg / 25 cm 2 ~40 mg / 25 cm 2 , 25 mg / 25 cm 2 ~35 mg / 25 cm 2 , 25 mg / 25 cm 2 ~30 mg / 25 cm 2 , 27 mg / 25 cm 2 ~38 mg / 25 cm 2 , 30 mg / 25 cm 2 ~50 mg / 25 cm 2 , 38 mg / 25 cm 2 ~45 mg / 25 cm 2 , or 40 mg / 25 cm 2 ~50 mg / 25 cm 2The loading amount can be such that the present invention can achieve high capacity and output in the manufactured lithium secondary battery by adjusting the loading amount of the negative electrode active layer to the above range.

[0097] Furthermore, the above-mentioned negative electrode active layer may include a carbon-based negative electrode active material in which a magnetic field is applied before the negative electrode slurry dries, and the crystal planes are aligned at a high angle that is nearly perpendicular to the negative electrode current collector. In the case of graphite among the above-mentioned carbon-based negative electrode active materials, the crystal planes in the a-axis and b-axis directions, where the plane of the graphite is located in the three-dimensional space in which the crystal phase exists, are aligned nearly perpendicular to the negative electrode current collector, which can significantly improve the springback phenomenon due to rolling, enhance structural stability, and reduce the volume change of the negative electrode active layer associated with charging and discharging.

[0098] As one example, the above-mentioned negative electrode active layer may have the crystal planes of the carbon-based negative electrode active material oriented such that the degree of alignment (OI) represented by the following formula 2 satisfies a predetermined value:

[0099] [Formula 2] OI=I 004 / I 110

[0100] In Equation 2, I 004 This represents the area of ​​the peak indicating the (0,0,4) crystal plane when measured by X-ray diffraction spectroscopy (XRD) for the negative electrode active layer. I 110 This represents the area of ​​the peak indicating the (1,1,0) crystal plane when X-ray diffraction (XRD) is measured for the negative electrode active layer.

[0101] The crystal plane orientation of the carbon-based anode active material can be determined by crystal plane analysis of the carbon-based anode active material, such as X-ray diffraction. The degree of alignment (OI) of the carbon-based anode active material, expressed in Equation 2 above, can serve as an indicator of the direction in which the crystal structure of the carbon-based anode active material is aligned during X-ray diffraction (XRD) measurement, or more specifically, the degree to which the crystal planes representing the planes of the carbon-based anode active material are aligned with respect to the surface of the anode current collector. For example, when the negative electrode active layer contains graphite as a carbon-based negative electrode active material, X-ray diffraction measurements of the electrode sheet show peaks at 2θ = 26.5±0.2°, 42.4±0.2°, 43.4±0.2°, 44.6±0.2°, 54.7±0.2°, and 77.5±0.2°, which represent the (0,0,2) plane, (1,0,0) plane, (1,0,1)R plane, (1,0,1)H plane, (0,0,4) plane, and (1,1,0) plane of the graphite crystal plane contained in the negative electrode active layer.

[0102] Generally, in the case of graphite, graphene layers are placed on the a-axis and b-axis planes, and these graphene layers are stacked along the c-axis, resulting in a hexagonal or rhombohedral crystal structure. Here, the crystal plane peaks mentioned above are peaks that indicate the plane characteristics of such a crystal structure. Furthermore, the peak that appears at 2θ = 43.4 ± 0.2° may be an overlap between the (1,0,1)R plane of the carbon-based negative electrode active material and the (1,1,1) plane of the negative electrode current collector, such as copper (Cu), and may therefore be excluded when determining the degree of alignment.

[0103] The present invention allows for the measurement of the alignment (OI) of a carbon-based negative electrode active material by the area ratio of the peak at 2θ = 54.7 ± 0.2° representing the (0,0,4) plane and the peak at 2θ = 77.5 ± 0.2° representing the (1,1,0) plane, specifically, the area ratio obtained by integrating the intensity of the above peaks. The (0,0,4) plane appearing at 2θ = 54.7 ± 0.2° indicates the thickness-direction characteristics of a layered structure in which planar graphite layers are stacked, while the (1,1,0) plane appearing at 2θ = 77.5 ± 0.2° indicates the planar characteristics of the stacked graphite layers. Therefore, the smaller the peak of the (0,0,4) plane, which indicates the thickness-direction characteristics of the graphite layer planar characteristics, and the larger the peak of the (1,1,0) plane, which indicates the planar characteristics of the graphite layer, the higher the angle at which the graphite planes are aligned with respect to the surface of the negative electrode current collector. In other words, the closer the alignment degree (OI) is to 0, the closer the angle or inclination of the graphite layer plane with respect to the surface of the negative electrode current collector is to 90°, and the larger the value, the closer the inclination with respect to the surface of the negative electrode current collector is to 0° or 180°.

[0104] For example, in the negative electrode according to the present invention, the crystal planes of the carbon-based negative electrode active material contained in the negative electrode active layer are aligned at a high angle with respect to the negative electrode current collector, and the degree of alignment (OI) can be 0.2 to 1.5, specifically 0.2 to 1.2, 0.2 to 1.0, 0.2 to 0.8, 0.2 to 0.5, 0.4 to 0.7, 0.3 to 0.6, 0.4 to 0.9, 0.5 to 0.9, 0.6 to 1.0, 0.7 to 1.0, 0.9 to 1.2, or 0.8 to 1.1. The fact that the alignment degree (OI) of the carbon-based anode active material is 1.5 or less means that, when graphite is included as the carbon-based anode active material, the crystal planes, which represent the planes of the graphite layer, are aligned to the anode current collector at a high angle / inclination, for example, 60° or more, 70° or more, 70° to 90°, 80° to 90°, 65° to 85°, or 70° to 85°.

[0105] Furthermore, the above-mentioned anode active layer has the characteristic of having a low springback rate after rolling due to the orientation of the carbon-based anode active material. The springback rate of the anode active layer can mean the rate of increase in the average thickness of the anode active layer after a predetermined time has elapsed, based on the average thickness of the anode active layer immediately after rolling, and in some cases, the average density of the anode active layer may be applied instead of the average thickness of the anode active layer. This is a numerical representation of the degree to which the material tries to return to its original state after rolling, and the lower the value, the greater the degree to which the springback phenomenon of the anode active layer has been improved.

[0106] As one example, the negative electrode active layer of the negative electrode according to the present invention may have an average thickness increase rate (i.e., springback rate after 2.5 hours) of less than 4% at room temperature (22±1℃) based on the average thickness immediately after rolling, and specifically may be 3% or less, 2% or less, or 1.5% or less.

[0107] As another example, the negative electrode active layer of the negative electrode according to the present invention may have an average thickness increase rate (i.e., springback rate after 72 hours) of 72 hours after rolling at room temperature (22±1℃) based on the average thickness immediately after rolling, which may be less than 10%, specifically 9.5% or less, or 9% or less.

[0108] Furthermore, the negative electrode according to the present invention not only exhibits improved springback in the negative electrode active layer and superior structural stability, but also has high density. Specifically, the negative electrode active layer has a density of 1.2 g / cm³. 3 ~1.8g / cm 3 It may have a density of 1.3 g / cm³. More specifically, 1.3 g / cm³. 3 ~1.8g / cm 3 1.4 g / cm³ 3 ~1.8g / cm 3 1.5 g / cm³ 3 ~1.8g / cm 3 1.4 g / cm³ 3 ~1.7g / cm 3 , or 1.45 g / cm³ 3 ~1.65g / cm 3 It may have a density of .

[0109] In this case, the thickness of the negative electrode active layer is not particularly limited, but it may have an average thickness of 100 μm to 300 μm, specifically 100 μm to 250 μm, or 130 μm to 190 μm. By adjusting the average thickness of the negative electrode active layer to the above range, the present invention can uniformly align the crystal planes of the carbon-based negative electrode active material, thereby improving the high-rate charge-discharge performance and energy density of the battery including the negative electrode.

[0110] On the other hand, the anode according to the present invention may contain a predetermined silicon-based anode active material together with a carbon-based anode active material in the anode active layer. The silicon-based anode active material is a substance mainly composed of silicon (Si), and may include silicon (Si) particles, silicon monoxide (SiO), silicon dioxide (SiO2) particles, silicon carbide (SiC), or mixtures thereof. In some cases, the silicon (Si)-containing particles may mean a substance further containing silicon carbide (SiC) particles along with silicon (Si) particles, silicon monoxide (SiO), and / or silicon dioxide (SiO2) particles.

[0111] Furthermore, the silicon-based anode active material may be included in an amount of 0.5 to 30 parts by weight per 100 parts by weight of the total anode active material, specifically in amounts of 1 to 9 parts by weight, 3 to 7 parts by weight, 11 to 19 parts by weight, 13 to 17 parts by weight, 10 to 30 parts by weight, 20 to 30 parts by weight, 15 to 25 parts by weight, and 9 to 22 parts by weight. The present invention can improve the charging capacity per unit mass while reducing lithium consumption and irreversible capacity loss during the initial charging and discharging of the battery by adjusting the content of carbon-based anode active material and silicon-based anode active material contained in the anode active material to the above ranges.

[0112] Furthermore, the above-mentioned negative electrode active layer may selectively contain conductive materials, binders, and other additives as needed, along with the carbon-based negative electrode active material, which is the main component.

[0113] The above conductive material may contain, but is not limited to, one or more types of carbon black such as acetylene black or Ketjen black, carbon nanotubes, or carbon fibers.

[0114] As one example, the above-mentioned negative electrode active layer may contain carbon black, carbon nanotubes, carbon fibers, etc., individually or in combination as conductive materials.

[0115] In this case, the content of the conductive material may be 0.1 to 10 parts by weight per 100 parts by weight of the entire negative electrode active layer, specifically 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. By controlling the content of the conductive material within the above range, the present invention can prevent an increase in the resistance of the negative electrode due to a low content of conductive material, thereby reducing the charging capacity, and can also prevent problems such as a decrease in the content of the negative electrode active material due to an excessive amount of conductive material, which reduces the charging capacity, or a decrease in rapid charging characteristics due to an increase in the loading amount of the negative electrode active layer.

[0116] Furthermore, the above-mentioned binder is a component that assists in the bonding of the negative electrode active material to conductive materials and to the current collector, and can be suitably applied within a range that does not degrade the electrical properties of the electrode. Specifically, it may contain one or more of the following: vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride (PVDF), polyacrylonitrile, polymethyl methacrylate, 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.

[0117] The binder content may be 0.1 to 10 parts by weight per 100 parts by weight of the entire negative electrode active layer, specifically 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. By controlling the binder content in the negative electrode active layer within the above range, the present invention can prevent a decrease in the adhesive strength of the active layer due to a low binder content or a decrease in the electrical properties of the electrode due to an excessive amount of binder.

[0118] Furthermore, the negative electrode active layer may have a structure in which two separate layers are stacked, depending on the battery model or product application to which the negative electrode of the present invention is applied, but is not limited thereto.

[0119] Specifically, the negative electrode according to the present invention may have a structure in which a first negative electrode active layer is provided on a negative electrode current collector, and a second negative electrode active layer is provided on the first negative electrode active layer. In this case, the first negative electrode active layer and the second negative electrode active layer each contain a carbon-based negative electrode active material, and the carbon-based negative electrode active material contained in each layer may be the same or different. When a two-layer negative electrode active layer is provided on a negative electrode current collector, the composition of each negative electrode active layer can be easily controlled, which has the advantage that not only can the electrical performance of the negative electrode be improved by applying an active material with excellent energy efficiency for the battery as the negative electrode active material, but it is also possible to compose a negative electrode active layer that can improve and / or prevent problems that arise as a result (for example, a decrease in the interfacial adhesion between the negative electrode current collector and the negative electrode active layer).

[0120] Furthermore, the negative electrode current collector is not particularly limited as long as it has high conductivity without inducing chemical changes in the battery. For example, copper, stainless steel, nickel, titanium, and calcined carbon can be used, and in the case of copper or stainless steel, those with surface treatment with carbon, nickel, titanium, silver, etc. can also be used. The average thickness of the negative electrode current collector can be suitably applied in the range of 1 μm to 500 μm, taking into consideration the conductivity and total thickness of the manufactured negative electrode.

[0121] <Lithium-ion secondary battery>

[0122] Furthermore, in one embodiment of the present invention, The present invention provides a lithium secondary battery comprising an electrode assembly including a positive electrode, the negative electrode described above, and a separator membrane disposed between the positive electrode and the negative electrode.

[0123] In this case, the negative electrode has the same configuration as described above, so a detailed explanation will be omitted.

[0124] Furthermore, the positive electrode comprises a positive electrode active layer manufactured by coating, drying, and pressing a slurry containing positive electrode active material onto a positive electrode current collector, and may optionally further contain conductive materials, binders, and other additives.

[0125] The above positive electrode active material is a substance capable of undergoing electrochemical reactions on the positive electrode current collector and may include one or more lithium metal oxides represented by the following chemical formulas 1 and 2, which are capable of reversible intercalation and deintercalation of lithium ions:

[0126] [Chemical formula 1] Li x [Ni y Co z Mn w M 1 v ]O2

[0127] [Chemical formula 2] LiM 2 p Mn q P r O4

[0128] In the above chemical formulas 1 and 2, M 1 It is one or more elements from W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo. x, y, z, w, and v are such that 1.0 ≤ x ≤ 1.30, 0.5 ≤ y < 1, and 0 respectively. <z≦0.3、0<w≦0.3、0≦v≦0.1であり、かつ、y+z+w+v=1であり、 M 2 It is Ni, Co, or Fe, p is such that 0.05 ≤ p ≤ 1.0, q is either 1-p or 2-p, r is either 0 or 1.

[0129] The lithium metal oxides represented by chemical formulas 1 and 2 above are substances containing high amounts of nickel (Ni) and manganese (Mn), respectively. When used as positive electrode active materials, they have the advantage of being able to supply high capacity and / or high voltage electricity more stably compared to conventionally used positive electrode active materials such as iron oxide phosphate (LiFeO4).

[0130] In this case, the lithium metal oxide represented by the above chemical formula 1 is LiNi 0.8 Co 0.1 Mn 0.1 O2, LiLiLi 0.6 Co 0.2 Mn 0.2 O2, LiLiLi 0.9 Co 0.05 Mn 0.05 O2, LiLiLi 0.6 Co 0.2 Mn 0.1 Al 0.1 O2, LiLiLi 0.6 Co 0.2 Mn 0.15 Al 0.05 O2, LiLiLi 0.7 Co 0.1 Mn 0.1 Al 0.1 The lithium metal oxide represented by the above chemical formula 2 may contain O2, etc., and LiNi 0.7 Mn 1.3 O4, LiSa 0.5 Mn 1.5 O4, LiSa 0.3 Mn 1.7 O4, LiFePO4, LiFe 0.7 Mn 0.3It may contain PO4 and other elements, which can be used alone or in combination.

[0131] Furthermore, the above-mentioned positive electrode active material may be present in 85 parts by weight or more, based on the weight of the positive electrode active layer, specifically in the form of 90 parts by weight or more, 93 parts by weight or more, or 95 parts by weight or more.

[0132] Furthermore, the positive electrode active layer may further contain conductive materials, binders, and other additives along with the positive electrode active material.

[0133] In this case, the conductive material is used to improve the electrical performance of the positive electrode and may be one or more of those commonly used in the industry, specifically natural graphite, artificial graphite, carbon black, acetylene black, Denka black, Ketjen black, Super P, channel black, furnace black, lamp black, thermal black, graphene, and carbon nanotubes.

[0134] Furthermore, the conductive material may be included in amounts of 0.1 to 5 parts by weight based on the weight of each positive electrode active layer, specifically in amounts of 0.1 to 4 parts by weight, 2 to 4 parts by weight, 1.5 to 5 parts by weight, 1 to 3 parts by weight, 0.1 to 2 parts by weight, or 0.1 to 1 part by weight.

[0135] Furthermore, the binder plays a role in binding the positive electrode active material, positive electrode additive, and conductive material to each other, and any binder having such a function can be used without particular limitations. Specifically, the binder may include one or more resins from among polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride (PVDF), polyacrylonitrile, polymethyl methacrylate, and copolymers thereof. As one example, the binder may include polyvinylidene fluoride.

[0136] Furthermore, the above-mentioned binder may be included in an amount of 1 to 10 parts by weight based on the weight of each positive electrode active layer, specifically in an amount of 2 to 8 parts by weight, or 1 to 5 parts by weight.

[0137] The total thickness of the positive electrode active layer described above is not particularly limited, but it may be 50 μm to 300 μm, and more specifically, it may be 100 μm to 200 μm, 80 μm to 150 μm, 120 μm to 170 μm, 150 μm to 300 μm, 200 μm to 300 μm, or 150 μm to 190 μm.

[0138] Furthermore, the positive electrode can be made of a material that has high conductivity without inducing chemical changes in the battery. For example, stainless steel, aluminum, nickel, titanium, or calcined carbon can be used, and in the case of aluminum or stainless steel, materials that have been surface-treated with carbon, nickel, titanium, silver, etc. can also be used. The average thickness of the current collector can be suitably applied in the range of 3 μm to 500 μm, taking into consideration the conductivity and total thickness of the manufactured positive electrode.

[0139] On the other hand, the separation membrane interposed between the positive and negative electrodes of each unit cell is an insulating thin film having high ion permeability and mechanical strength, and is not particularly limited as long as it is commonly used in the industry, but specifically, it may contain one or more polymers from among chemically resistant and hydrophobic polypropylene, polyethylene, and polyethylene-propylene copolymer. The above separation membrane may have the form of a porous polymer substrate such as a sheet or nonwoven fabric containing the above polymer, and in some cases, it may have the form of a composite separation membrane in which organic or inorganic particles are coated with an organic binder on the above porous polymer substrate. Furthermore, the above separation membrane may have an average pore diameter of 0.01 μm to 10 μm and an average thickness of 5 μm to 300 μm.

[0140] On the other hand, the lithium secondary battery according to the present invention is not particularly limited, but may be a secondary battery that includes a stacked type, a zigzag type, or a zigzag-stack type electrode assembly. As one example, the lithium secondary battery according to the present invention may be a pouch-type secondary battery or a prismatic secondary battery.

[0141] The present invention will be described in more detail below with reference to examples and experimental examples.

[0142] However, the following examples and experimental examples are illustrative of the present invention, and the content of the present invention is not limited to the following examples and experimental examples.

[0143] Examples 1-4 and Comparative Examples 1-6. Manufacturing of negative electrodes for lithium secondary batteries.

[0144] First, natural graphite and artificial graphite were prepared as carbon-based anode active materials, and an anode slurry was manufactured using these prepared carbon-based anode active materials.

[0145] Specifically, a mixed graphite, prepared by mixing natural graphite and artificial graphite in a weight ratio of 1-3:7-9, was used as the negative electrode active material. Carbon black was used as the conductive material, and carboxymethylcellulose (CMC) and styrene-butadiene rubber (SBR) were used as binders. Subsequently, 95 parts by weight of the mixed graphite, 1 part by weight of carbon black, 1.5 parts by weight of carboxymethylcellulose (CMC), and 2.5 parts by weight of styrene-butadiene rubber (SBR) were mixed with water to produce a negative electrode slurry with a solid content of 50%.

[0146] After preparing the negative electrode slurry, it was cast onto a thin copper plate (thickness: 10 μm) being transported roll-to-roll (transport speed: 5 m / min) using a die coater. At this time, the average thickness of the cast negative electrode slurry was adjusted to 200 μm.

[0147] Subsequently, permanent magnets were placed on top of the coated negative electrode slurry and below the negative electrode current collector. A magnetic field was applied for 15 to 30 seconds, and then the negative electrode slurry was dried with hot air to form the negative electrode active layer. At this time, the intensity of the magnetic field applied to the negative electrode slurry was adjusted as shown in Table 1 below, and the spectrum was measured by X-ray diffraction spectroscopy (XRD) of the negative electrode active layer. The measurement conditions for X-ray diffraction (XRD) were as follows:

[0148] - Target: Cu(Kα-ray) graphite monochromator - Slit: Divergent slit = 1°, Receiving slit = 0.1 mm, Scattering slit = 1° - Measurement area: (1,1,0) plane: 76.5°<2θ<78.5° / (0,0,4) plane: 53.0°<2θ<57.0°

[0149] From the spectrum measured under the above conditions, determine the area of ​​the peak representing the (0,0,4) crystal plane and the area of ​​the peak representing the (1,1,0) crystal plane, and the ratio of these areas (I 004 / I 110 The degree of alignment (OI) of the mixed graphite in each region was calculated by performing the calculation. The calculated values ​​are shown in Table 1 below.

[0150] Subsequently, the negative electrode active layer was rolled at 22±1℃ at a transfer speed of 3m / s to manufacture a negative electrode for lithium secondary batteries. During this process, the rolling load of the negative electrode active layer was adjusted as shown in Table 1 below.

[0151] [Table 1]

[0152] Examples 5-8 and Comparative Examples 7-12. Manufacturing of Lithium Secondary Batteries

[0153] LiNi with a particle size of 5 μm is used as the positive electrode active material. 0.7 Co 0.1 Mn 0.1 Al 0.1 O2 was prepared and mixed with polyvinylidene fluoride and N-methylpyrrolidone (NMP) in a weight ratio of 94:3:3 as a carbon-based conductive material and binder to form a slurry. This slurry was then cast onto an aluminum sheet, dried in a vacuum oven at 120°C, and then rolled to produce a cathode.

[0154] A separation membrane made of 18 μm polypropylene was interposed between the positive electrode obtained above and the negative electrodes manufactured in Examples 1-4 and Comparative Examples 1-6, respectively. After inserting the membrane into a case, the electrolyte composition was injected to assemble the lithium secondary battery.

[0155] The types of negative electrodes applied to each lithium secondary battery are shown in Table 2 below.

[0156] [Table 2]

[0157] Experimental example

[0158] The following experiment was conducted to evaluate the performance of the negative electrode according to the present invention.

[0159] i) Effect of improving springback in the negative electrode active layer

[0160] Neutral electrodes were manufactured using the same methods as in Examples 1-4 and Comparative Examples 1-6, and the average thickness and density of the negative electrode active layer were measured immediately after rolling. Subsequently, the manufactured negative electrodes were stored at room temperature (22±1℃) for 120 hours, and the average thickness of the negative electrode active layer was measured after 0.5 hours, 1 hour, 2.5 hours, 24 hours, 48 ​​hours, and 72 hours.

[0161] From the measured results, the rate of increase in the average thickness of the negative electrode active layer immediately after rolling was calculated and defined as the springback rate of the negative electrode active layer. The results are shown in Table 3 below.

[0162] (b) Evaluation of lifespan characteristics

[0163] Lithium secondary batteries manufactured in Examples 5-8 and Comparative Examples 7-12 (each equipped with the negative electrodes of Examples 1-4 and Comparative Examples 1-6, respectively) were fully charged (SOC 100%) at room temperature (22±1℃) at a 0.5C-rate, and the capacity (1 cycle capacity) was measured when the fully charged lithium secondary batteries were discharged to 1.5V at a 0.1C-rate. Subsequently, each lithium secondary battery was fully charged again at a 0.5C-rate (SOC 100%) and discharged to 1.5V at a 1.0C-rate. This charge-discharge process was repeated 200 times, and the capacity (200 cycle capacity) was measured when the fully charged lithium secondary batteries were discharged to 1.5V at a 0.1C-rate. The discharge capacity retention rate after 100 cycles was calculated based on the measured 1 cycle capacity, and the results are shown in Table 3 below.

[0164] [Table 3]

[0165] As shown in Table 3 above, the anode for lithium secondary batteries according to the present invention shows a significant improvement in the springback phenomenon of the anode active layer after rolling, resulting in improved structural stability of the anode active layer and increased lifespan.

[0166] Specifically, in the embodiment where the rolling load of the negative electrode slurry according to Equation 1 after magnetic field application was controlled, the springback phenomenon of the negative electrode active layer was improved, and it was confirmed that the springback rate after 2.5 hours after rolling was 2% or less, and the springback rate after 72 hours after rolling was less than 10%. Furthermore, the negative electrode in the embodiment showed improved structural stability of the negative electrode active layer, and it was demonstrated that the discharge capacity retention rate was 89% or more even after 100 charge-discharge cycles.

[0167] On the other hand, in comparative examples where no magnetic field was applied, or where rolling was performed with a load that did not satisfy Formula 1 of the present invention after the application of a magnetic field, it was confirmed that the thickness of the negative electrode active layer increased significantly from 1 hour after rolling. Specifically, the springback rate was 6% or more after 2.5 hours after rolling, and the springback rate exceeded 10% after 72 hours after rolling. Furthermore, the negative electrodes of the comparative examples showed low structural stability of the negative electrode active layer, with a discharge capacity retention rate of less than 85% after 100 charge-discharge cycles.

[0168] These results show that the manufacturing method according to the present invention can significantly improve the springback phenomenon of the negative electrode active layer in a simple and few-step process by rolling a negative electrode slurry after drying it under a magnetic field, and by controlling the rolling load according to Equation 1, which depends on the strength of the magnetic field applied to the negative electrode slurry. Furthermore, the negative electrode manufactured in this way not only exhibits a high energy density while maintaining the desired thickness, but also has the advantages of low rolling stress, improved structural stability of the negative electrode active layer, reduced volume change of the negative electrode active layer during charging and discharging, and improved battery life.

[0169] While preferred embodiments of the present invention have been described above with reference to those skilled in the art or those with ordinary knowledge in the art, it will be understood that the present invention can be modified and altered in various ways without departing from the spirit and technical scope of the invention as described in the claims below.

[0170] Therefore, the technical scope of the present invention is not limited to what is described in the summary of the invention in the specification, but may be defined by the claims.

Claims

1. A step of applying a magnetic field to a negative electrode slurry coated on a negative electrode current collector, The steps include drying a negative electrode slurry to which a magnetic field has been applied to form a negative electrode active layer, and The step includes rolling the formed negative electrode active layer, The aforementioned negative electrode slurry contains a carbon-based negative electrode active material. The aforementioned magnetic field is applied with an intensity of 3,000 G to 15,000 G. The rolling step is performed such that the rolling load is controlled according to the following formula 1. The rolling load is 4.5 tons to 5.9 tons. Method for manufacturing a negative electrode for lithium secondary batteries: [Formula 1] y = Ax + B In Equation 1, y represents the rolling load (unit: ton), x represents the strength of the magnetic field when a magnetic field is applied (unit: Gauss). A and B satisfy -0.01 ≤ A < 0 and 3 ≤ B ≤ 7.

2. The rolling step is, A primary rolling step in which the formed negative electrode active layer is pressed with the rolling load of [Equation 1], and The process includes a secondary rolling step in which the primary rolled negative electrode active layer is further pressurized, A method for manufacturing a negative electrode for a lithium secondary battery according to claim 1, wherein the rolling load during primary rolling is greater than the rolling load during secondary rolling.

3. The method for manufacturing a negative electrode for a lithium secondary battery according to claim 2, wherein the rolling load during the secondary rolling is 50% or less of the rolling load during the primary rolling.

4. The method for manufacturing a negative electrode for a lithium secondary battery according to claim 1, wherein the rolling step is performed at a temperature of 20°C to 35°C.

5. The method for manufacturing a negative electrode for a lithium secondary battery according to claim 1, wherein the negative electrode active layer has an average thickness increase rate of less than 10% after 72 hours of rolling at room temperature, based on the average thickness immediately after rolling.

6. The method for manufacturing a negative electrode for a lithium secondary battery according to claim 1, wherein the step of applying the magnetic field is performed for 5 to 60 seconds.

7. The negative electrode for the lithium secondary battery is A method for manufacturing a negative electrode for a lithium secondary battery according to claim 1, comprising 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.

8. The negative electrode active layer has an alignment degree (O.I) of a carbon-based negative electrode active material represented by the following formula 2, which is 0.2 to 1.5, as described in the method for manufacturing a negative electrode for a lithium secondary battery according to claim 7: [Formula 2] O.I=I ００４ / I １１０ In Equation 2, I ００４ This represents the area of ​​the peak indicating the (0,0,4) crystal plane when X-ray diffraction (XRD) spectroscopy is performed on the negative electrode active layer. I １１０ This represents the area of ​​the peak indicating the (1,1,0) crystal plane when X-ray diffraction (XRD) measurements are taken on the negative electrode active layer.

9. The method for manufacturing a negative electrode for a lithium secondary battery according to claim 7, wherein the negative electrode active layer has an average thickness of 100 μm to 300 μm.

10. The negative electrode active layer is 1.2 g / cm³ ３ ~1.8 g / cm ３ A method for manufacturing a negative electrode for a lithium secondary battery according to claim 7, having the density of [value missing].

11. The method for manufacturing a negative electrode for a lithium secondary battery according to claim 7, wherein the carbon-based negative electrode active material includes one or more of natural graphite and artificial graphite.

12. The aforementioned negative electrode active layer further comprises a silicon-based negative electrode active material, The silicon-based negative electrode active material is Si, SiC, and SiO ｘ A method for manufacturing a negative electrode for a lithium secondary battery according to claim 7, comprising one or more of the following (where 0.8 ≤ x ≤ 2.5).

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