Negative electrode for lithium secondary battery and method for manufacturing the same

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 them, 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 performed.

[0006] Specifically, the electrodes of a lithium secondary battery can be obtained as electrode sheets. The electrode sheets are formed by applying an electrode slurry containing an electrode active material onto an electrode current collector at a high loading amount during manufacturing, drying the slurry, and then rolling and shaping the dried electrode slurry with a rolling device to form a high-loading and high-density electrode active layer on the electrode current collector. Here, when rolling the sheet, rolling is performed in consideration of the desired density distribution, thickness, thickness distribution, precision, etc. of the electrode active layer during manufacturing.

[0007] However, when the rolled sheet is rolled, the electrode active layer is pressurized, and the electrode active material powder contained in the electrode active layer tends to return to its original state due to the stress accumulated in the powder, that is, the spring-back phenomenon tends to occur severely. Therefore, over time, the thickness of the electrode active layer containing the electrode active material becomes thicker than the desired thickness. This causes the problem of reducing the energy density of the electrode having a high loading amount.

[0008] To improve the spring-back phenomenon, techniques have been developed to perform rolling of the dried electrode slurry multiple times or to use a multi-stage rolling machine. However, these techniques not only cannot completely suppress the spring-back phenomenon but also have limitations in terms of low economic efficiency in terms of energy and process efficiency.

[0009] Therefore, there is a high need for a manufacturing technology for electrodes that can solve the above problems and maintain the desired thickness regardless of the passage of time after rolling. Summary of the Invention Problems to be Solved by the Invention

[0010] An 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 the active layer having a high loading amount, is suppressed, and the desired thickness can be maintained regardless of the passage of a predetermined time during rolling. Means for Solving the Problems

[0011] To solve the above problems, In one embodiment of the present invention, a step of applying a magnetic field to the negative electrode slurry coated on the negative electrode current collector, a step of drying the negative electrode slurry to which the magnetic field is applied to form a negative electrode active layer, and including a step of rolling the formed negative electrode active layer, the negative electrode slurry contains a carbon-based negative electrode active material, the magnetic field is applied at a strength of 3,000 G to 15,000 G, the 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 Formula 1, y represents the rolling load (unit: ton), x represents the intensity of the magnetic field (unit: Gauss) when the magnetic field is applied, A and B satisfy -0.01 ≤ A < 0 and 3 ≤ B ≤ 7.

[0014] At this time, the rolling load can be 1 ton to 7 tons.

[0015] Also, the rolling step includes a primary rolling step of pressurizing the formed negative electrode active layer with the rolling load of [Formula 1], and a secondary rolling step of additionally pressurizing the negatively electrode active layer that has been primarily rolled. 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 may have a ratio of 50% or less of the rolling load during the primary rolling.

[0017] Also, the rolling step can be performed at a temperature of 20°C to 35°C.

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

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

[0020] Furthermore, in one embodiment of the present invention, 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 are included. A negative electrode for a lithium secondary battery manufactured by the manufacturing method according to the present invention described above is provided.

[0021] At this time, the negative electrode active layer may have an alignment degree (O.I) of the carbon-based negative electrode active material represented by the following formula 2 of 0.2 to 1.5:

[0022] [Formula 2] O.I = I 004 / I 110

[0023] In formula 2, I 004 represents the area of the peak indicating the (0, 0, 4) crystal plane during X-ray diffraction spectroscopy (XRD) measurement of the negative electrode active layer, I 110 represents the area of the peak indicating the (1, 1, 0) crystal plane during X-ray diffraction spectroscopy (XRD) measurement of the negative electrode active layer.

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

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

[0026] On the other hand, the carbon-based negative electrode active material contained in the negative electrode active layer may include one or more of natural graphite and artificial graphite.

[0027] Further, the negative electrode active layer may further contain a silicon-based negative electrode active material, and the silicon-based negative electrode active material may contain one or more of Si, SiC, and SiO x (however, 0.8 ≦ x ≦ 2.5).

Advantages of the Invention

[0028] The method for manufacturing a negative electrode for a lithium secondary battery according to the present invention dries the 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, it has an excellent effect of improving the springback phenomenon of the negative electrode active layer.

[0029] In addition, the manufactured negative electrode not only exhibits a high energy density while maintaining a desired thickness even when the loading amount of the negative electrode active layer is large, but also has low rolling stress, improved structural stability of the negative electrode active layer, and suppressed volume change of the negative electrode active layer accompanying charge and discharge, so that there is an advantage that the battery life is improved.

Brief Description of the Drawings

[0030]

Figure 1

Figure 2

Modes for Carrying Out the Invention

[0031] The present invention can be modified in various ways and can have various embodiments, so specific embodiments will be described in detail.

[0032] However, this is not intended to limit the present invention to specific embodiments, and it should be understood to include all modifications, equivalents, or alternatives included in the spirit and technical scope of the present invention.

[0033] In the present invention, terms such as "comprising" and "having" are intended to specify the presence of features, numbers, steps, operations, components, parts, or combinations thereof described in the specification, and can be understood not to preclude in advance the presence or addition of one or more other features, numbers, steps, operations, components, parts, or combinations thereof.

[0034] Also, in the present invention, when a part such as a layer, film, region, or plate is described as being "on" another part, this includes not only the case where it is directly "on" the other part, but also the case where there is another part in between. Conversely, when a part such as a layer, film, region, or plate is described as being "under" another part, it includes not only the case where it is directly "under" the other part, but also the case where there is another part in between. Also, in the present application, being "disposed on" can include not only the upper part but also the case of being disposed on the lower part.

[0035] Also, in this specification, "comprising as a main component" can mean containing 50% by weight or more (or 50% by volume or more), 60% by weight or more (or 60% by volume or more), 70% by weight or more (or 70% by volume or more), 80% by weight or more (or 80% by volume or more), 90% by weight or more (or 90% by volume or more), or 95% by weight or more (or 95% by volume or more) of a component defined with respect to the total weight (or total volume). For example, "comprising graphite as a main component as the negative electrode active material" can mean containing 50% by weight or more, 60% by weight or more, 70% by weight or more, 80% by weight or more, 90% by weight or more, or 95% by weight or more of graphite with respect to the total weight of the negative electrode active material, and in some cases, it can also mean that the entire negative electrode active material consists of graphite and contains graphite at 100% by weight.

[0036] Also, in this specification, the "electrode sheet" can mean an article in a state where a negative electrode slurry is applied on a negative electrode current collector, or in a state where the negative electrode slurry applied on the negative electrode current collector is dried to form a negative electrode active layer.

[0037] In addition, in this specification, "the carbon-based negative electrode active material is oriented" or "the carbon-based negative electrode active material is aligned" means that the crystal planes of the carbon-based negative electrode active material constituting the particles of the negative electrode active material are distributed so as to have a predetermined directionality with respect to the surface of the negative electrode current collector. At this time, the crystal plane is a crystal plane showing the planar structure of the carbon-based negative electrode active material or the two-dimensional plane of graphite, and may mean the (1,0,0) crystal plane or the (1,1,0) crystal plane. Further, "the carbon-based negative electrode active material is oriented" may be different from the case where the particles of the carbon-based negative electrode active material are arranged so as to have a specific direction inside the negative electrode active layer.

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

[0039] In addition, "the degree of alignment of the carbon-based negative electrode active material is high" means that the "degree of alignment (O.I)" mentioned in this specification has a large value, and may mean that the carbon-based negative electrode active material contained in the negative electrode active layer is aligned at a low angle (for example, less than 45°) with respect to the surface of the negative electrode current collector. Conversely, "the degree of alignment of the carbon-based negative electrode active material is low" means that the "degree of alignment (O.I)" has a small value, and may mean that the carbon-based negative electrode active material contained in the negative electrode active layer is aligned at a high angle (for example, more than 45°, specifically 60° or more) with respect to the surface of the negative electrode current collector.

[0040] Furthermore, in this specification, the "crystal plane of the carbon-based negative electrode active material" is a plane on which the atoms of the carbon-based negative electrode 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 negative electrode active material, or a crystal plane including the a-axis / a-b axis of the carbon-based negative electrode active material crystal.

[0041] Hereinafter, the present invention will be described in more detail.

[0042] <Method for manufacturing negative electrode for lithium secondary battery>

[0043] In one embodiment, the present invention provides step S1 of applying a magnetic field to a negative electrode slurry coated on a negative electrode current collector, step S2 of drying the negative electrode slurry applied with the magnetic field to form a negative electrode active layer, and step S3 of rolling the formed negative electrode active layer, and provides a method for manufacturing a negative electrode for a lithium secondary battery including the same.

[0044] In the method for manufacturing a negative electrode according to the present invention, a negative electrode slurry containing a carbon-based negative electrode active material is coated on a negative electrode current collector, and a magnetic field is applied to the surface of the coated negative electrode slurry (S1), whereby the carbon-based negative electrode active material in the negative electrode slurry can be aligned to have a predetermined angle with respect to the surface of the negative electrode current collector. Then, 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] At this time, the present invention can effectively improve the springback phenomenon of the rolled negative electrode active layer by controlling the load during rolling of the negative electrode active layer according to the intensity 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 having a negative electrode active layer arranged thereon with a rolling device in order to increase the energy density. However, in the above negative electrode sheet, a phenomenon (i.e., springback phenomenon) in which the negative electrode sheet tries to return to its original state due to the stress accumulated in the powder of the negative electrode active material occurs severely during rolling, so there is a problem that the thickness of the negative electrode active layer becomes thicker than the desired thickness over time. Such a phenomenon appears more strongly as the loading amount indicating the weight of the negative electrode active material per unit area or unit volume of the negative electrode active layer increases.

[0047] In order to improve such problems, it was necessary to perform rolling multiple times or perform a complex rolling process such as a multi-stage rolling mill. However, even if multiple rolling processes are performed or a rolling process of a complex process such as a multi-stage rolling mill is performed, not only is the effect of improving springback significantly reduced after a predetermined time has elapsed, but there is also a limit in that the effect of improving springback becomes smaller as the loading amount of the negative electrode active material increases.

[0048] On the other hand, the method for manufacturing a negative electrode according to the present invention can exhibit a high springback improvement effect even though the rolling process is performed one or two times, specifically only once, by controlling the load during rolling according to the following formula (1) that depends on the intensity of the magnetic field applied to the negative electrode slurry:

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

[0050] In Formula (1), y represents the rolling load (unit: ton), x represents the intensity of the magnetic field during magnetic field application (unit: Gauss), A and B satisfy -0.01 ≤ A < 0 and 3 ≤ B ≤ 7.

[0051] Hereinafter, the method for manufacturing a negative electrode according to the present invention will be described in more detail for each step.

[0052] First, the method for manufacturing a negative electrode according to the present invention includes a step S1 of applying a magnetic field to the negative electrode slurry coated on the negative electrode current collector. This step S1 refers to a process of orienting the crystal planes of the carbon-based negative electrode active material contained in the negative electrode slurry so as to have 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 among carbon-based negative electrode active materials, it has a crystal phase in which two-dimensional planes are stacked as shown in FIG. 1. In the three-dimensional space where the crystal phase exists, the directions in which the graphite planes are located are the a-axis and the b-axis, and the direction in which the planes are stacked is the c-axis direction. When pressure is applied to such graphite, it will return to its original state due to the stress accumulated in the crystal phase. Among them, when pressure is applied in the c-axis direction of the crystal phase, the expansion due to recovery occurs most significantly.

[0054] Therefore, in order to suppress the springback phenomenon of the carbon-based negative electrode active material after rolling of the negative electrode active layer, before drying the negative electrode slurry, the crystal planes in the c-axis direction of the carbon-based negative electrode active material contained in the negative electrode slurry are made horizontal with the negative electrode current collector, that is, the planes of the carbon-based negative electrode active material are made perpendicular to the negative electrode current collector, and this problem is overcome by alignment.

[0055] Here, the orientation of the carbon-based negative electrode active material can be achieved by applying a magnetic field to the negative electrode slurry coated on the negative electrode current collector before drying. That is, in the step of applying the magnetic field, the magnetic field can be applied by magnet parts arranged above and below the negative electrode current collector on which the negative electrode slurry is coated and moved on the surface.

[0056] In addition, since the degree of orientation of the carbon-based negative electrode active material can be adjusted by factors such as the intensity of the magnetic field applied to the negative electrode slurry and the time of exposure to the magnetic field, the step of applying the magnetic field can be carried out under predetermined magnetic field intensity and time conditions.

[0057] Specifically, in the step of applying the magnetic field, a magnetic field of 3,000 G (gauss) to 15,000 G can be applied. More specifically, the magnetic field can be applied at an intensity of 3,000 G to 12,000 G, 3,000 G to 11,000 G, 3,000 G to 10,000 G, 3,000 G to 9,000 G, 3,000 G to 8,000 G, 3,000 G to 7,500 G, 3,000 G to 6,000 G, 5,000 G to 9,000 G, 10,000 G to 15,000 G, 8,000 G to 12,000 G, 3,000 G to 12,000 G, 4,000 G to 8,000 G, 5,000 G to 8,500 G, 3,000 G to 5,500 G, 3,500 G to 5,500 G, 4,000 G to 5,500 G, 3,500 G to 4,500 G, or 4,500 G to 5,000 G.

[0058] Also, the step of applying the magnetic field can be performed for 5 seconds to 60 seconds. Specifically, it can be performed for 5 seconds to 30 seconds, 10 seconds to 60 seconds, 10 seconds to 30 seconds, 30 seconds to 60 seconds, 40 seconds to 50 seconds, 15 seconds to 35 seconds, 10 seconds to 50 seconds, or 20 seconds to 40 seconds.

[0059] By applying the magnetic field with the above-mentioned magnetic field intensity and time, the present invention can easily orient the crystal plane of the carbon-based negative electrode active material contained in the negative electrode slurry at a high angle close to perpendicular to the surface of the negative electrode current collector.

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

[0061] At this time, the drying of the negative electrode slurry is not particularly limited as long as it can maintain the orientation of the carbon-based negative electrode active material contained in the negative electrode active layer, and can be applied accordingly. 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 to dry the negative electrode slurry.

[0062] In addition, the method for manufacturing a negative electrode according to the present invention includes a step S3 of manufacturing the negative electrode by rolling the formed negative electrode active layer. This step S3 can be performed by a method commonly applied in the art such as roll pressing, whereby the density of the negative electrode active layer can be increased.

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

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

[0065] In formula 1, y represents the rolling load (unit: ton), x represents the intensity of the magnetic field at the time of magnetic field application (unit: Gauss), A and B satisfy -0.01 ≤ A < 0 and 3 ≤ B ≤ 7.

[0066] The above formula 1 is a parameter showing the correlation between the intensity 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 the above formula 1 are constants, and A can be -0.1 or more and less than 0 (that is, -0.1 ≤ A < 0), specifically -0.01 or more and less than 0 (that is, -0.01 ≤ A < 0), -0.001 or more and less than 0 (that is, -0.001 ≤ A < 0), -0.001 to -0.000001 (that is, -0.001 ≤ A ≤ -0.000001), or -0.0005 to -0.000001 (that is, -0.0005 ≤ A ≤ -0.000001). Also, B can be 3 to 7 (that is, 3 ≤ B ≤ 7), specifically 4 to 6 (that is, 4 ≤ B ≤ 6), 4.1 to 4.9 (that is, 4.1 ≤ B ≤ 4.9), 5.1 to 5.9 (that is, 5.1 ≤ B ≤ 5.9), or 4.5 to 5.7 (that is, 4.5 ≤ B ≤ 5.7).

[0067] In the above formula (1), "load" means the force itself applied to the surface of the negative electrode active layer. Since the negative electrode active layer is loaded while being transferred during the rolling process, the "pressure applied during rolling" must reflect the length in the direction perpendicular to the direction in which the negative electrode active layer is transferred (i.e., the length in the width direction of the negative electrode active layer). For example, when applying a negative electrode slurry onto a current collector using a die coater with a coating width of 26.2 cm, drying it, and then rolling the formed negative electrode active layer (width direction length: 26.2 cm) with a load of 5 tons, the linear pressure can be about 0.19 ton / cm.

[0068] According to the above formula (1), the higher the intensity of the magnetic field applied to the negative electrode slurry, the more the load during rolling can be reduced. This means that the load during rolling required to suppress the springback phenomenon of the negative electrode active layer is affected by the degree of orientation of the carbon-based negative electrode active material, and such an orientation of the carbon-based negative electrode active material is affected by the intensity of the magnetic field applied to the negative electrode slurry. The present invention can exhibit a high springback improvement effect even when the rolling process is performed two or fewer times, specifically only once, by controlling the load during rolling of the negative electrode active layer according to the above formula (1).

[0069] In addition, in the present invention, since it is not necessary to apply an excessively high load to the negative electrode active layer in order to improve the springback phenomenon when rolling with the load according to the above formula (1), the rolling stress applied to the manufactured negative electrode can be significantly reduced, and the structural stability of the negative electrode active layer can be ensured. Furthermore, in the present invention, the negative electrode active layer can be rolled even with a relatively low load, and thereafter, there is room to apply an additional external force (such as a rolling load, etc.) without excessive stress to the negative electrode active layer, so the possibility of developing and / or manufacturing a thinner negative electrode is opened. Also, a thinner negative electrode has the advantage of being advantageous in terms of charge-discharge capacity, output, and / or energy density because more electrodes can be stacked in the electrode assembly under the same thickness condition.

[0070] As an example, the negative electrode active layer of the negative electrode manufactured according to the present invention may have an increase rate of less than 4% in the average thickness after 2.5 hours of rolling at room temperature (22 ± 1°C) based on the average thickness immediately after rolling, specifically, it 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 increase rate of less than 10% in the average thickness after 72 hours of rolling at room temperature (22 ± 1°C) based on the average thickness immediately after rolling, specifically, it may be 9.5% or less, or 9% or less.

[0072] The rolling in the present invention may be performed only once as described above, and in some cases, it may be performed twice. In this case, the step of rolling the negative electrode active layer may include a primary rolling step S3-1 of pressurizing the dried negative electrode active layer and a secondary rolling step S3-2 of additionally pressurizing the negatively electrode active layer that has been primarily rolled.

[0073] Specifically, the primary rolling step S3-1 is the main rolling step of increasing the energy density of the dried negative electrode active layer, and may be a step in which the density of the negative electrode active layer substantially increases. Therefore, the primary rolling step S3-1 may be performed with the same load as when only one rolling is performed, and thereby, the rolling load may be controlled according to Equation 1.

[0074] Also, the secondary rolling step S3-2 is a step of additionally pressurizing the negatively electrode active layer that has been primarily rolled, and may be a step of compensating for the slightly occurring springback phenomenon of the negatively electrode active layer and at the same time improving the interfacial characteristics between the negative electrode current collector and the negative electrode active layer. Therefore, 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 have a ratio of 50% or less of the rolling load during primary rolling, and more specifically, it may have a ratio of 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] Also, the load for rolling the negative electrode active layer can be from 1 ton to 7 tons, specifically, it can be from 1 ton to 3 tons, from 2 tons to 6 tons, from 3 tons to 7 tons, from 4 tons to 7 tons, or from 4.5 tons to 5.9 tons.

[0076] The above load can be the load when rolling the negative electrode active layer once, and in some cases, it can be the load applied during the primary rolling step when rolling twice. By performing the rolling load within the above range, the present invention can not only increase the density of the negative electrode active layer, but also reduce the crystal plane alignment of the carbon-based negative electrode active material and prevent the springback phenomenon from not being improved.

[0077] Also, in the above rolling step S3, the temperature and rolling speed during execution can satisfy a predetermined range.

[0078] Specifically, the above rolling step S3 can be performed at a temperature of less than 40°C during execution, specifically, it can be performed at a temperature of 20°C to 30°C, 20°C to 28°C, 25°C to 30°C, or 22°C to 24°C.

[0079] Also, the above rolling step S3 can be performed at a rolling speed of 2 m / s to 7 m / s, more specifically, it can be performed 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 can increase the energy density of the negative electrode while minimizing the change in the alignment degree of the carbon-based negative electrode active material contained in the negative electrode active layer formed by performing the rolling of the dried negative electrode slurry under the above temperature and / or speed conditions.

[0081] On the one hand, in the present invention, the negative electrode slurry can be applied onto the negative electrode current collector in such a manner that the negative electrode slurry containing the carbon-based negative electrode active material is discharged and coated on the surface of the moving negative electrode current collector. Here, the die coating method can be applied as the above coating method.

[0082] The above die coating method can be carried out by a slot die provided with a shim for controlling the discharge conditions of the negative electrode slurry. In this case, by controlling the shape of the shim, etc., the loading amount, coating thickness, etc. of the negative electrode slurry applied onto the negative electrode current collector can be easily controlled.

[0083] The method for manufacturing a negative electrode for a lithium secondary battery according to the present invention has the above-described configuration, and thus, even without performing rolling a plurality of times as in the prior art or performing a complicated rolling process such as multi-stage rolling, the springback phenomenon of the negative electrode active layer can be significantly improved even with a simple and small number of processes, so that it is excellent in workability and economy.

[0084] <Negative electrode for lithium secondary battery>

[0085] Further, in one embodiment of the present invention, it includes 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. There is provided a negative electrode for a lithium secondary battery manufactured by the manufacturing method according to the present invention 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 above negative electrode active layer is a layer that realizes the electrical activity of the negative electrode, and is manufactured by applying a negative electrode slurry containing a negative electrode active material that realizes an electrochemical oxidation-reduction reaction during charging and discharging of the battery onto 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 with a negative electrode active layer disposed thereon using a rolling device in order to increase the energy density. However, in the above negative electrode sheet, due to the stress accumulated in the powder of the negative electrode active material during rolling, a phenomenon (i.e., springback phenomenon) that attempts to return to the original state occurs violently. Therefore, there is a problem that the thickness of the negative electrode active layer becomes thicker than the desired thickness over time. Such a phenomenon appears more strongly 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. In order to improve such a problem, it was necessary to perform rolling multiple times or to perform a complicated rolling process such as a multi-stage rolling machine.

[0088] However, the negative electrode according to the present invention controls the rolling load during rolling 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, and thus has a remarkable feature that the springback phenomenon of the negative electrode active layer after rolling is improved. Therefore, the above negative electrode has advantages such as a high energy density of the negative electrode active layer, high processability and economic efficiency during manufacturing. In addition, the above negative electrode has a low rolling stress and a high structural stability of the negative electrode active layer, so there is an advantage that the volume change of the negative electrode active layer accompanying charge and discharge is reduced and the battery life is improved.

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

[0090] The carbon-based negative electrode active material means a material mainly composed of carbon atoms, and such a carbon-based negative electrode active material may include graphite. The graphite may include any one or more of natural graphite and artificial graphite.

[0091] As an example, the carbonaceous negative electrode active material may include a mixed graphite obtained by mixing natural graphite and artificial graphite. In this case, the mixed graphite may be obtained by mixing natural graphite and artificial graphite at a weight ratio of 10 to 50:50 to 90, or 10 to 30:70 to 90. By adjusting the content ratio of natural graphite and artificial graphite as described above, the mixed graphite can prevent the adhesion between the negative electrode current collector and the negative electrode active layer from being reduced by less than 10 parts by weight of natural graphite with respect to the total weight, and can prevent the charge and discharge capacity of the negative electrode from being reduced by more than 50 parts by weight of natural graphite.

[0092] On the other hand, conventional artificial graphite is excellent in the effect of improving the life characteristics of the battery, but has a lower capacity than natural graphite, so a higher amount must be loaded to manufacture a high-density electrode. However, artificial graphite has low rollability and a strong spring-back phenomenon appears, so it is difficult to fabricate a high-density electrode that satisfies the desired thickness. However, the negative electrode of the present invention can be manufactured by the manufacturing method of the present invention and can be provided with a high-density negative electrode active layer in which the spring-back phenomenon is suppressed even if it contains a high content of artificial graphite in the carbonaceous negative electrode active material.

[0093] Further, the form of the carbonaceous negative electrode active material is not particularly limited, but preferably has a form of spherical graphite granulated material formed by aggregation of a plurality of flaky graphites. Examples of the flaky graphite include, in addition to natural graphite and artificial graphite, mesophase-fired carbon (bulk mesophase) made from tar pitch, cokes (raw coke, green coke, pitch coke, needle coke, petroleum coke, etc.) graphitized, and the like. In particular, as the carbonaceous negative electrode active material, it is preferable to use a plurality of highly crystalline natural graphites assembled. Further, one graphite granulated material can be formed by aggregating 2 to 100, preferably 3 to 20 flaky graphites.

[0094] Further, the carbonaceous 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 diameter (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 diameter of graphite can be more advantageous as the particle diameter is made smaller in order to maximize the degree of disorder in the expansion direction for each particle so as to prevent the expansion of the particles due to the charging of lithium ions. However, when the particle diameter 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 diameter 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] Further, 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 250 mg / 25 cm 2 to 500 mg / 25 cm 2 . Specifically, the negative electrode active layer is 250 mg / 25 cm 2 to 450 mg / 25 cm 2 , 250 mg / 25 cm 2 to 400 mg / 25 cm 2 , 250 mg / 25 cm 2 to 350 mg / 25 cm 2 , 250 mg / 25 cm 2 to 300 mg / 25 cm 2 , 270 mg / 25 cm 2 to 380 mg / 25 cm 2 , 300 mg / 25 cm 2 to 500 mg / 25 cm 2 , 380 mg / 25 cm 2 to 450 mg / 25 cm 2 , or 400 mg / 25 cm 2 to 500 mg / 25 cm 2It may have the loading amount described above. By adjusting the loading amount of the negative electrode active layer within the above range, the present invention can highly embody the capacity and output of the manufactured lithium secondary battery.

[0097] Further, in the negative electrode active layer, a magnetic field may be applied before the drying of the negative electrode slurry, and it may contain a carbon-based negative electrode active material in a state where the crystal planes are aligned at a high angle close to perpendicular to the negative electrode current collector. In the case of graphite among the above carbon-based negative electrode active materials, by aligning the crystal planes in the a-axis and b-axis directions where the graphite plane is located in the three-dimensional space where the crystal phase exists close to perpendicular to the negative electrode current collector, the springback phenomenon due to rolling can be significantly improved, the structural stability can be enhanced, and the volume change of the negative electrode active layer accompanying charge and discharge can be reduced.

[0098] As an example, in the negative electrode active layer, the crystal planes of the carbon-based negative electrode active material are oriented, and the degree of alignment (O.I) represented by the following formula 2 may satisfy a predetermined value:

[0099] [Formula 2] O.I = I 004 / I 110

[0100] In formula 2, I 004 represents the area of the peak indicating the (0, 0, 4) crystal plane during X-ray diffraction spectroscopy (XRD) measurement of the negative electrode active layer, I 110 represents the area of the peak indicating the (1, 1, 0) crystal plane during X-ray diffraction spectroscopy (XRD) measurement of the negative electrode active layer.

[0101] The crystal plane orientation of the carbonaceous negative electrode active material can be determined by crystal plane analysis of the carbonaceous negative electrode active material such as X-ray diffraction. The degree of alignment (O.I) of the carbonaceous negative electrode active material represented by the above formula (2) is an index indicating the degree to which the crystal structure of the carbonaceous negative electrode active material is aligned in the direction during X-ray diffraction (XRD) measurement. More specifically, it indicates the degree to which the crystal plane showing the plane of the carbonaceous negative electrode active material is aligned with respect to the surface of the negative electrode current collector. For example, when the negative electrode active layer contains graphite as the carbonaceous negative electrode active material, peaks are shown 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° during X-ray diffraction measurement with respect to the electrode sheet, which indicate 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 among the crystal planes of the graphite 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 such graphene layers are stacked along the c-axis, resulting in a hexagonal or rhombohedral crystal structure. Here, the above crystal plane peak is a peak indicating the plane characteristics of such a crystal structure. Also, the peak appearing at 2θ = 43.4 ± 0.2° may be considered to be a peak where the (1,0,1)R plane of the carbonaceous negative electrode active material and the (1,1,1) plane of the negative electrode current collector, for example, copper (Cu), overlap, and thus can be excluded when determining the degree of alignment.

[0103] The present invention can measure the degree of alignment (O.I) of a carbon-based negative electrode active material by the area ratio of the peak at 2θ = 54.7 ± 0.2° showing the (0,0,4) plane and the peak at 2θ = 77.5 ± 0.2° showing the (1,1,0) plane, specifically, the ratio of the areas obtained by integrating the intensities of the above peaks. The (0,0,4) plane appearing at 2θ = 54.7 ± 0.2° shows the thickness direction characteristics of the layered structure in which the graphite layers are stacked, and the (1,1,0) plane appearing at 2θ = 77.5 ± 0.2° shows the planar characteristics of the stacked graphite layers. Therefore, the smaller the peak of the (0,0,4) plane showing the thickness direction characteristics of the graphite layer plane, and the larger the peak of the (1,1,0) plane showing the planar characteristics of the graphite layer, the higher the angle at which the graphite plane is aligned with respect to the surface of the negative electrode current collector. That is, the closer the value of the above degree of alignment (O.I) 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 plane of the carbon-based negative electrode active material contained in the negative electrode active layer can be aligned at a high angle with respect to the negative electrode current collector, and the degree of alignment (O.I) 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 degree of alignment (O.I) of the above carbon-based negative electrode active material is 1.5 or less means that when the carbon-based negative electrode active material contains graphite, the crystal plane meaning the graphite layer plane is aligned at a high angle / inclination with respect to the negative electrode current collector, for example, an angle / inclination of 60° or more, 70° or more, 70° to 90°, 80° to 90°, 65° to 85°, or 70° to 85°.

[0105] In addition, the negative electrode active layer has a characteristic of a low springback rate after rolling due to the orientation of the carbon-based negative electrode active material. The springback rate of the negative electrode active layer may mean the increase rate of the average thickness of the negative electrode active layer after a predetermined time has elapsed based on the average thickness of the negative electrode active layer immediately after rolling. In some cases, the average density of the negative electrode active layer may be applied instead of the average thickness of the negative electrode active layer. This is a quantification of the degree to which it tries to return to its original state after rolling, and it means that the lower the numerical value, the greater the degree to which the springback phenomenon of the negative electrode active layer is improved.

[0106] As one example, the negative electrode active layer of the negative electrode according to the present invention may have an increase rate of the average thickness (i.e., the springback rate after 2.5 hours) after 2.5 hours of rolling at room temperature (22 ± 1°C) based on the average thickness immediately after rolling, which is less than 4%, specifically, 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 increase rate of the average thickness (i.e., the springback rate after 72 hours) after 72 hours of rolling at room temperature (22 ± 1°C) based on the average thickness immediately after rolling, which is less than 10%, specifically, 9.5% or less, or 9% or less.

[0108] In addition, the negative electrode according to the present invention has characteristics that not only the springback phenomenon of the negative electrode active layer is improved and the structural stability is excellent, but also the density is high. Specifically, the negative electrode active layer may have a density of 1.2 g / cm 3 ~1.8 g / cm 3 and more specifically, 1.3 g / cm 3 ~1.8 g / cm 3 、1.4 g / cm 3 ~1.8 g / cm 3 、1.5 g / cm 3 ~1.8 g / cm 3 、1.4 g / cm 3 ~1.7 g / cm 3 、or 1.45 g / cm 3 ~1.65 g / cm 3 and may have a density of.

[0109] At this time, 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, it may have an average thickness of 100 μm to 250 μm, or 130 μm to 190 μm. By adjusting the average thickness of the negative electrode active layer within 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 negative electrode according to the present invention may contain a predetermined silicon-based negative electrode active material together with the carbon-based negative electrode active material in the negative electrode active layer. The silicon-based negative electrode active material is a substance containing silicon (Si) as a main component, and may include silicon (Si) particles, silicon monoxide (SiO), silicon dioxide (SiO2) particles, silicon carbide (SiC), or a mixture thereof. In some cases, the silicon (Si)-containing particles may mean those further containing silicon carbide (SiC) particles together with silicon (Si) particles, silicon monoxide (SiO), and / or silicon dioxide (SiO2) particles.

[0111] In addition, the silicon-based negative electrode active material may be contained in an amount of 0.5 parts by weight to 30 parts by weight based on 100 parts by weight of the total negative electrode active material, specifically, it may be contained in an amount of 1 part by weight to 9 parts by weight, 3 parts by weight to 7 parts by weight, 11 parts by weight to 19 parts by weight, 13 parts by weight to 17 parts by weight, 10 parts by weight to 30 parts by weight, 20 parts by weight to 30 parts by weight, 15 parts by weight to 25 parts by weight, 9 parts by weight to 22 parts by weight. By adjusting the content of the carbon-based negative electrode active material and the silicon-based negative electrode active material contained in the negative electrode active material within the above range, the present invention can improve the charge capacity per unit mass while reducing the lithium consumption amount and the irreversible capacity loss during the initial charge and discharge of the battery.

[0112] In addition, the negative electrode active layer may further selectively contain a conductive material, a binder, other additives, etc. as necessary together with the carbon-based negative electrode active material as the main component.

[0113] The above conductive material may include, but is not limited to, one or more of carbon blacks such as acetylene black and ketjen black, carbon nanotubes, carbon fibers, etc.

[0114] As an example, the above negative electrode active layer may contain carbon black, carbon nanotubes, carbon fibers, etc. alone or in combination as the conductive material.

[0115] At this time, the content of the above conductive material may be 0.1 to 10 parts by weight, 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 with respect to 100 parts by weight of the entire negative electrode active layer. By controlling the content of the conductive material within the above range, the present invention can prevent the resistance of the negative electrode from increasing and the charge capacity from decreasing due to a low content of the conductive material, and can prevent problems such as the charge capacity decreasing due to a decrease in the content of the negative electrode active material caused by an excessive amount of the conductive material, or the rapid charging characteristics decreasing due to an increase in the loading amount of the negative electrode active layer.

[0116] In addition, the above binder is a component that helps bind the negative electrode active material and the conductive material, etc. and bind to the current collector, and can be preferably applied within a range that does not reduce the electrical physical properties of the electrode. Specifically, vinylidene fluoride - hexafluoropropylene copolymer (PVDF - co - HFP), polyvinylidene fluoride (PVDF), polyacrylonitrile, polymethylmethacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene - propylene - diene monomer, sulfonated ethylene - propylene - diene monomer, styrene - butadiene rubber (SBR), and fluororubber may include any one or more of them.

[0117] The content of the above binder can be 0.1 part by weight to 10 parts by weight with respect to 100 parts by weight of the entire negative electrode active layer, specifically it can be 0.1 part by weight to 8 parts by weight, 0.1 part by weight to 5 parts by weight, 0.1 part by weight to 3 parts by weight, or 2 parts by weight to 6 parts by weight. By controlling the content of the binder contained in the negative electrode active layer within the above range, the present invention can prevent the adhesive force of the active layer from decreasing due to a low content of the binder or the electrical physical properties of the electrode from decreasing due to an excessive amount of the binder.

[0118] In addition, the above negative electrode active layer may have a structure in which two individual layers are laminated according to the battery model to which the negative electrode of the present invention is applied and the use of the product, 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. At this time, 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 materials contained in each layer may be the same or different. When a negative electrode active layer having a two-layer structure is provided on the negative electrode current collector, the composition of each negative electrode active layer can be easily controlled, so that not only can an active material excellent in the energy efficiency of the battery be applied as the negative electrode active material to improve the electrical performance of the negative electrode, but also problems generated thereby (for example, a decrease in the interfacial adhesive force between the negative electrode current collector and the negative electrode active layer, etc.) can be improved and / or prevented. There is an advantage that a composition of the negative electrode active layer is possible.

[0120] In addition, the above negative electrode current collector is not particularly limited as long as it has high conductivity without inducing a chemical change in the battery. For example, copper, stainless steel, nickel, titanium, fired carbon, etc. can be used. In the case of copper or stainless steel, those surface-treated with carbon, nickel, titanium, silver, etc. can also be used. Further, the average thickness of the above negative electrode current collector can be preferably applied in the range of 1 μm to 500 μm in consideration of the conductivity and the total thickness of the manufactured negative electrode.

[0121] <Lithium secondary battery>

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

[0123] At this time, since the negative electrode has the same configuration as the above-described configuration, a specific description thereof is omitted.

[0124] Further, the positive electrode includes a positive electrode active layer manufactured by applying, drying, and pressing a slurry containing a positive electrode active material on a positive electrode current collector, and may further selectively include a conductive material, a binder, other additives, etc., if necessary.

[0125] The positive electrode active material is a material that can undergo an electrochemical reaction on the positive electrode current collector, and may include one or more of lithium metal oxides represented by the following Chemical Formula 1 and Chemical Formula 2 that 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 Chemical Formula 1 and Chemical Formula 2 above, M 1 is one or more elements among 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, 0 < z ≦ 0.3, 0 < w ≦ 0.3, 0 ≦ v ≦ 0.1, and y + z + w + v = 1, M 2 is Ni, Co, or Fe, p is such that 0.05 ≦ p ≦ 1.0, q is 1 - p or 2 - p, r is 0 or 1.

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

[0130] At this time, examples of the lithium metal oxide represented by Chemical Formula 1 include LiNi 0.8 Co 0.1 Mn 0.1 O2, LiNi 0.6 Co 0.2 Mn 0.2 O2, LiNi 0.9 Co 0.05 Mn 0.05 O2, LiNi 0.6 Co 0.2 Mn 0.1 Al 0.1 O2, LiNi 0.6 Co 0.2 Mn 0.15 Al 0.05 O2, LiNi 0.7 Co 0.1 Mn 0.1 Al 0.1 O2, etc., and the lithium metal oxide represented by Chemical Formula 2 includes LiNi 0.7 Mn 1.3 O4, LiNi 0.5 Mn 1.5 O4, LiNi 0.3 Mn 1.7 O4, LiFePO4, LiFe 0.7 Mn 0.3It may contain PO4 etc., and these can be used alone or in combination.

[0131] Further, the above positive electrode active material may be contained in an amount of 85 parts by weight or more based on the weight of the positive electrode active layer, specifically, it may be contained in an amount of 90 parts by weight or more, 93 parts by weight or more, or 95 parts by weight or more.

[0132] In addition, the above positive electrode active layer may further contain a conductive material, a binder, other additives, etc. together with the positive electrode active material.

[0133] At this time, the above conductive material is used to improve the electrical performance of the positive electrode, and those commonly used in the industry can be applied. Specifically, it may contain one or more of 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] Further, the above conductive material may be contained in an amount of 0.1 part by weight to 5 parts by weight based on the weight of each positive electrode active layer, specifically, it may be contained in an amount of 0.1 part by weight to 4 parts by weight, 2 parts by weight to 4 parts by weight, 1.5 parts by weight to 5 parts by weight, 1 part by weight to 3 parts by weight, 0.1 part by weight to 2 parts by weight, or 0.1 part by weight to 1 part by weight.

[0135] In addition, the above binder serves to bind the cathode active material, the cathode additive, and the conductive material to each other, and any material having such a function can be used without particular limitation. Specifically, examples of the above binder include one or more resins selected from polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride (PVDF), polyacrylonitrile, polymethyl methacrylate, and copolymers thereof. As one example, the above binder may contain polyvinylidene fluoride.

[0136] In addition, the above binder may be contained in an amount of 1 to 10 parts by weight, specifically 2 to 8 parts by weight, or 1 to 5 parts by weight, based on the weight of each cathode active layer.

[0137] The total thickness of the above cathode active layer is not particularly limited, but specifically may be 50 μm to 300 μm, more specifically 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] In addition, for the above cathode, a material having high conductivity without inducing a chemical change in the battery can be used as the cathode current collector. For example, stainless steel, aluminum, nickel, titanium, fired carbon, etc. can be used, and in the case of aluminum or stainless steel, those surface-treated with carbon, nickel, titanium, silver, etc. can also be used. Further, the average thickness of the above current collector can be suitably applied in the range of 3 μm to 500 μm in consideration of the conductivity and the total thickness of the produced cathode.

[0139] On one hand, the separator interposed between the positive electrode and the negative electrode 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 art. Specifically, those containing one or more polymers among chemically resistant and hydrophobic polypropylene, polyethylene, and polyethylene-propylene copolymer can be used. The above separator may have a form of a porous polymer substrate such as a sheet or nonwoven fabric containing the above polymer, and in some cases, may have a form of a composite separator in which organic or inorganic particles are coated on the above porous polymer substrate with an organic binder. Further, the average diameter of the pores of the above separator may be 0.01 μm to 10 μm, and the average thickness may be 5 μm to 300 μm.

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

[0141] Hereinafter, the present invention will be described in more detail with reference to Examples and Experimental Examples.

[0142] However, the following Examples and Experimental Examples illustrate the present invention, and the content of the present invention is not limited to the following Examples and Experimental Examples.

[0143] Examples 1 to 4 and Comparative Examples 1 to 6. Production of negative electrode for lithium secondary battery

[0144] First, natural graphite and artificial graphite were respectively prepared as carbon-based negative electrode active materials, and a negative electrode slurry was produced using the prepared carbon-based negative electrode active materials.

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

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

[0147] Thereafter, permanent magnets were arranged above the applied negative electrode slurry and below the negative electrode current collector, and after applying a magnetic field for 15 seconds to 30 seconds, the negative electrode slurry was dried with hot air to form a 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 X-ray diffraction spectroscopy (XRD) was performed on the negative electrode active layer to measure the spectrum. At this time, the measurement conditions for X-ray diffraction (XRD) are as follows:

[0148] - Target: Cu (Kα-ray) graphite monochromator - Slit: Divergence slit = 1°, Receiving slit = 0.1 mm, Scattering slit = 1° - Measurement region: (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, the areas of the peaks indicating the (0,0,4) crystal plane and the (1,1,0) crystal plane were obtained, and the ratio (I 004 / I 110 ) was calculated to calculate the alignment degree (O.I) of the mixed graphite for each region. The calculated values are shown in Table 1 below.

[0150] Thereafter, the negative electrode active layer was rolled at a transfer speed of 3 m / s at 22 ± 1°C to produce a negative electrode for a lithium secondary battery. At this time, the load during rolling of the negative electrode active layer was adjusted as shown in Table 1 below.

[0151] [Table 1]

[0152] Examples 5 to 8 and Comparative Examples 7 to 12. Manufacture of lithium secondary batteries

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

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

[0155] At this time, the types of negative electrodes applied to each lithium secondary battery are shown in Table 2 below.

[0156] [Table 2]

[0157] Experimental examples

[0158] In order to evaluate the performance of the negative electrode according to the present invention, the following experiments were conducted.

[0159] i) Springback improvement effect of the negative electrode active layer

[0160] The negative electrodes were manufactured in the same manner as in Examples 1 to 4 and Comparative Examples 1 to 6, and the average thickness and density of the negative electrode active layer immediately after rolling were measured. Thereafter, while storing the manufactured negative electrodes at room temperature (22 ± 1°C) for 120 hours, the average thickness of the negative electrode active layer was measured after 0.5 hour, 1 hour, 2.5 hours, 24 hours, 48 hours, and 72 hours had elapsed.

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

[0162] (b) Life characteristic evaluation

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

[0164] [Table 3]

[0165] As shown in Table 3 above, it can be seen that the negative electrode for a lithium secondary battery according to the present invention significantly improves the springback phenomenon of the negative electrode active layer after rolling, improves the structural stability of the negative electrode active layer, and increases the life.

[0166] Specifically, for the negative electrode of the example in which the rolling load was controlled according to Equation 1 after applying a magnetic field to the negative electrode slurry according to the present invention, the springback phenomenon of the negative electrode active layer was improved, and it was confirmed that the springback rate after 2.5 hours from rolling was 2% or less, and it was confirmed that the springback rate after 72 hours from rolling was less than 10%. Further, it was shown that the negative electrode of the example had improved structural stability of the negative electrode active layer and a discharge capacity retention rate of 89% or more even after 100 charge and discharge cycles.

[0167] On the other hand, for the negative electrode of the comparative example in which no magnetic field was applied or rolling was performed with a load that does not satisfy Equation 1 of the present invention after applying a magnetic field, it was confirmed that the thickness of the negative electrode active layer increased significantly from the time when 1 hour had passed after rolling. Specifically, the springback rate after 2.5 hours from rolling was 6% or more, and the springback rate after 72 hours from rolling exceeded 10%. Further, it was shown that the negative electrode of the comparative example had low structural stability of the negative electrode active layer and a discharge capacity retention rate of less than 85% after 100 charge and discharge cycles.

[0168] From these results, it can be seen that the manufacturing method according to the present invention can significantly improve the springback phenomenon of the negative electrode active layer even in a simple process with a small number of steps by drying the negative electrode slurry to which a magnetic field has been applied and then rolling it, and controlling the rolling load according to Equation 1 that depends on the intensity of the magnetic field applied to the negative electrode slurry. Further, the negative electrode manufactured thereby not only exhibits a high energy density while maintaining a 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 accompanying charge and discharge, and improved battery life.

[0169] As described above, the preferred embodiments of the present invention have been described with reference thereto. However, those skilled in the art or those having ordinary knowledge in the technical field can understand that the present invention can be variously modified and changed without departing from the spirit and technical scope of the present invention described in the claims to be described later.

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

Claims

1. Applying a magnetic field to the negative electrode slurry coated on the negative electrode current collector; Drying the negative electrode slurry with the magnetic field applied thereto to form a negative electrode active layer; and Rolling the formed negative electrode active layer, wherein the negative electrode slurry contains a carbon-based negative electrode active material, the magnetic field is applied at an intensity of 3,000 G to 15,000 G, and in the rolling step, the rolling load is controlled according to the following formula 1, a method for manufacturing a negative electrode for a lithium secondary battery: [Formula 1] y = Ax + B In Formula 1, y represents the rolling load (unit: ton), x represents the intensity of the magnetic field (unit: Gauss) at the time of magnetic field application, and A and B satisfy -0.01 ≤ A < 0 and 3 ≤ B ≤ 7.

2. The method for manufacturing a negative electrode for a lithium secondary battery according to Claim 1, wherein the rolling load is 1 ton to 7 tons.

3. The rolling step includes a primary rolling step of pressurizing the formed negative electrode active layer with the rolling load of [Formula 1], and a secondary rolling step of additionally pressurizing the negatively electrode active layer that has been primarily rolled, wherein the rolling load during primary rolling is greater than the rolling load during secondary rolling, the method for manufacturing a negative electrode for a lithium secondary battery according to Claim 1 or 2.

4. The method for manufacturing a negative electrode for a lithium secondary battery according to Claim 3, wherein the rolling load during the secondary rolling has a ratio of 50% or less of the rolling load during the primary rolling.

5. 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.

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

7. 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 seconds to 60 seconds.

8. A negative electrode for a lithium secondary battery, 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, manufactured by the manufacturing method according to Claim 1.

9. The negative electrode for a lithium secondary battery according to Claim 8, wherein the negative electrode active layer has an alignment degree (O.I) of the carbon-based negative electrode active material represented by the following formula 2 of 0.2 to 1.5: [Formula 2] O.I = I ００４ / I １１０ In Formula 2, I ００４ represents the area of the peak indicating the (0, 0, 4) crystal plane during the X-ray diffraction spectroscopy (XRD) measurement for the negative electrode active layer, I １１０ represents the area of the peak indicating the (1, 1, 0) crystal plane during X-ray diffraction spectroscopy (XRD) measurement for the negative electrode active layer.

10. The negative electrode for a lithium secondary battery according to Claim 8, wherein the negative electrode active layer has a springback rate of less than 10%.

11. The negative electrode active layer has an average thickness of 100 μm to 300 μm, and the negative electrode for a lithium secondary battery according to claim 8.

12. The negative electrode active layer has a density of 1.2 g / cm ３ to 1.8 g / cm ３ and is the negative electrode for a lithium secondary battery according to claim 8.

13. The carbon-based negative electrode active material contains one or more of natural graphite and artificial graphite, and the negative electrode for a lithium secondary battery according to claim 8.

14. The negative electrode active layer further contains a silicon-based negative electrode active material. The silicon-based negative electrode active material is Si, SiC, and SiO ｘ (where 0.8 ≦ x ≦ 2.5), and the negative electrode for a lithium secondary battery according to claim 8, which contains one or more of them.

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