Negative electrode for lithium secondary battery and method of manufacturing same

Abstract

The present invention relates to a negative electrode, a method of manufacturing same, and a lithium secondary battery to which the negative electrode is applied. In the negative electrode, CMC binders having different molecular weights are applied to first and second negative electrode active layers, and the degree of orientation of negative electrode active materials is individually controlled. Accordingly, the negative electrode exhibits excellent adhesion between a current collector and the active layer, and improves charging performance when applied to a lithium secondary battery.

Description

Anode for lithium secondary battery and method for manufacturing the same

[0001] This application claims the benefit of priority from Korean Patent Application No. 10-2023-0129455, filed September 26, 2023, the entire contents of which 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.

[0003]

[0004] Recently, lithium secondary batteries have been widely applied not only in small devices such as portable electronic devices, but also in medium and large-sized devices such as battery packs for hybrid and electric vehicles and power storage devices. In particular, with growing concern over environmental issues, research is being conducted on electric vehicles and hybrid electric vehicles that can replace fossil fuel-powered vehicles such as gasoline and diesel vehicles, which are a major cause of air pollution.

[0005] Due to their limited energy density, existing lithium secondary batteries are limited to short-range electric vehicles. Therefore, technology development has focused on further increasing the energy density of lithium secondary batteries.

[0006] However, developed lithium secondary batteries for automotive applications suffer from the long charging times required after discharging during driving. Consequently, as the adoption of electric vehicles increases, demand is growing for shorter charging times to a level acceptable to users. Furthermore, electric vehicles must be able to operate without issues even under high-power conditions, such as rapid acceleration. To achieve this, lithium secondary batteries must exhibit high-rate discharge performance, but currently, no technology exists to meet this requirement.

[0007] Meanwhile, lithium secondary batteries are rechargeable power generation devices composed of a stacked structure of anode / separator / cathode. When charging a lithium secondary battery, a lithium desorption reaction occurs at the cathode, where lithium contained in the cathode active material is oxidized and released, and a lithium insertion reaction occurs at the anode, where lithium is reduced and inserted into the cathode active material. Generally, the desorption reaction in the cathode active material is faster than the insertion reaction in the anode active material, so the rapid charge / discharge performance of a lithium secondary battery is primarily determined by the cathode.

[0008] As the negative electrode active material of the above negative electrode, a material containing graphite is widely used. The average potential when a material containing graphite releases lithium is about 0.2 V (Li / Li + ) and the discharge potential shows a relatively flat pattern. Therefore, when graphite is used as an anode active material, the voltage of the secondary battery has the advantage of being high and constant. However, the electrical capacity per unit mass of the graphite material is small at 372 mAh / g. On the other hand, since the capacity of the current graphite material has been improved to be close to the theoretical capacity, further capacity increase is difficult. In addition, when graphite is used as an anode active material, the lithium ion insertion reaction proceeds at a slow rate, so there is a limitation that the rapid charging performance is low compared to cases where other anode active materials are applied.

[0009] Accordingly, various anode active materials are being studied to improve the high capacity and rapid charging performance of lithium secondary batteries. For example, since it has been known that silicon can reversibly adsorb and release large amounts of lithium through a compound formation reaction with lithium, much research has been conducted recently on this. Silicon has a theoretical maximum capacity of approximately 4020 mAh / g (9800 mAh / cc, specific gravity 2.23), which is much higher than that of graphite-based materials, and thus has the advantage of being useful as a high-energy density and / or high-capacity anode material. However, silicon has limitations in that it not only induces a large volume change (~300%) during charge and discharge, but also does not have high high-rate discharge characteristics, limiting the lifespan and rapid discharge efficiency of lithium secondary batteries containing it.

[0010] Therefore, in order to fundamentally solve these problems, there is a high need for cathode technology that can simultaneously implement high-capacity characteristics and high-rate charge / discharge characteristics.

[0011]

[0012] The purpose of the present invention is to provide a negative electrode for a lithium secondary battery capable of simultaneously implementing high-capacity characteristics and high-rate charge / discharge characteristics, a method for manufacturing the same, and a lithium secondary battery including the negative electrode.

[0013]

[0014] In order to solve the above-described problem, in one embodiment, the negative electrode for a lithium secondary battery according to the present invention,

[0015] negative current collector,

[0016] A first negative electrode active layer provided on at least one surface of the negative electrode current collector and including a first negative electrode active material and a first CMC (carboxyl methylcellulose) binder having a weight average molecular weight in the range of 2.5 million to 5 million, and

[0017] A second negative electrode active layer is provided on the first negative electrode active layer and includes a second negative electrode active material and a second CMC binder having a weight average molecular weight in the range of 800,000 to 2,000,000.

[0018] Specifically, the first negative electrode active material has an average degree of alignment (OI) in the range of 10 to 50 according to the following Equation 1. In addition, the second negative electrode active material has an average degree of alignment (OI) in the range of 0.1 to 9 according to the following Equation 1.

[0019] [Formula 1]

[0020] OI = I 004 / I 110

[0021] In equation 1,

[0022] I 004 represents the area of ​​the peak representing the (0,0,4) crystal plane of the carbon-based negative electrode active material in the X-ray diffraction (XRD) spectroscopy analysis of the negative electrode active layer,

[0023] I 110 represents the area of ​​the peak representing the (1,1,0) crystal plane of the carbon-based negative electrode active material in X-ray diffraction (XRD) spectroscopy analysis of the negative electrode active layer.

[0024] In a specific example, the difference in weight average molecular weight of the first and second CMC binders is 1 million or more.

[0025] In one embodiment, the first and second negative electrode active materials each comprise first and second carbon-based negative electrode active materials. In a specific example, the first and second carbon-based negative electrode active materials each independently comprise at least one of natural graphite and artificial graphite.

[0026] In a more specific embodiment, the second negative electrode active layer further includes a silicon-based negative electrode active material. In addition, the silicon-based negative electrode active material may be silicon (Si), silicon carbide (SiC), and silicon oxide (SiO). q , but includes at least one of the following (0.8≤q≤2.5).

[0027] In one embodiment, the content of the silicon-based negative electrode active material is in the range of 1 wt% to 40 wt% based on the combined weight of the negative electrode active materials included in the first and second negative electrode active layers.

[0028] In one embodiment, the second negative electrode active layer includes a second carbon-based negative electrode active material and a silicon-based negative electrode active material. In a specific example, the average particle diameter (Dc) of the second carbon-based negative electrode active material is in the range of 1 μm to 50 μm, and the average particle diameter (Ds) of the silicon-based negative electrode active material is in the range of 0.1 μm to 10 μm. In addition, the average particle diameter ratio (Dc / Ds) of the second carbon-based negative electrode active material and the silicon-based negative electrode active material can satisfy a range of 2 to 10.

[0029] In another example, the negative electrode for a lithium secondary battery according to the present invention has an SOC range in which Li precipitation occurs of 35% or more under room temperature and 2.5 C charging conditions, and an interfacial adhesive force between the negative electrode current collector and the first negative electrode active layer is an average of 27 gf / 20 mm or more.

[0030]

[0031] In addition, the present invention provides a method for manufacturing the negative electrode for a lithium secondary battery described above. In one embodiment, the negative electrode for a lithium secondary battery according to the present invention includes an active layer forming step of forming first and second negative electrode active layers on a negative electrode current collector; a magnetic field applying step of applying a magnetic field in the range of 1,000 to 10,000 G for 5 to 20 seconds; and a step of drying and rolling the negative electrode.

[0032] In a specific example, the first negative electrode active layer includes a first negative electrode active material and a first CMC (carboxyl methylcellulose) binder having a weight average molecular weight in the range of 2.5 million to 5 million. In addition, the second negative electrode active layer includes a second negative electrode active material and a second CMC binder having a weight average molecular weight in the range of 800,000 to 2 million.

[0033] In one embodiment, the active layer forming step forms a second negative electrode active layer before the first negative electrode active layer formed on the negative electrode current collector is dried.

[0034] In another embodiment, the active layer forming step simultaneously forms first and second negative electrode active layers formed on a negative electrode current collector.

[0035] In one embodiment, the magnetic field applying step is performed by applying a magnetic field in the range of 2,000 to 5,000 G for 7 to 15 seconds.

[0036]

[0037] The present invention also provides a lithium secondary battery comprising the anode for a lithium secondary battery described above. In one embodiment, the lithium secondary battery according to the present invention comprises an electrode assembly comprising a cathode, an anode, and a separator disposed between the cathode and the anode. Here, the anode may include all of the contents described above.

[0038] In one embodiment, the positive electrode includes a positive electrode active layer provided on at least one surface of the positive electrode current collector and including one or more types of positive electrode active materials among lithium metal oxides represented by the following chemical formulas 1 and 2.

[0039] [Chemical Formula 1]

[0040] Li x [Ni y Co z Mn w M 1 v ]O2

[0041] [Chemical Formula 2]

[0042] LiMn p M 2 1-p O4

[0043] In the above chemical formulas 1 and 2,

[0044] M 1is at least one element selected from the group consisting of 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,

[0045] x, y, z, w, and v are 1.0≤x≤1.30, 0.5≤y<1, 0, respectively. <z≤0.3, 0<w≤0.3, 0≤v≤0.1이되, y+z+w+v=1이고,

[0046] M 2 is Ni, Co or Fe,

[0047] p is 0≤p≤0.9.

[0048] In a specific example, the lithium secondary battery according to the present invention is a pouch-type battery.

[0049]

[0050] The negative electrode for a lithium secondary battery according to the present invention controls the orientation of the negative electrode active material in each layer by applying CMC binders having different molecular weights to the first and second negative electrode active layers. Through this, the present invention increases the adhesive strength between the current collector and the active layer of the negative electrode, and improves the charging performance of a lithium secondary battery to which the negative electrode is applied.

[0051]

[0052] Figure 1 is a schematic diagram showing the orientation of the cathode active layer according to Comparative Example 1.

[0053] Figure 2 is a schematic diagram showing the orientation of the negative electrode active layer according to Comparative Example 2.

[0054] Figure 3 is a schematic diagram showing the orientation of the negative active layer according to Example 1.

[0055]

[0056] The present invention can be modified in various ways and has many embodiments, and specific embodiments will be described in detail in the detailed description.

[0057] However, this is not intended to limit the present invention to a specific embodiment, but should be understood to include all modifications, equivalents, or substitutes included in the spirit and technical scope of the present invention.

[0058] In the present invention, it should be understood that terms such as “include” or “have” are intended to specify the presence of a feature, number, step, operation, component, part or combination thereof described in the specification, but do not exclude in advance the possibility of the presence or addition of one or more other features, numbers, steps, operations, components, parts or combinations thereof.

[0059] Additionally, 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 cases where it is "directly above" the other part, but also cases where there is another part in between. Conversely, when a part such as a layer, film, region, or plate is described as being "under" another part, this includes not only cases where it is "directly below" the other part, but also cases where there is another part in between. Furthermore, in the present application, "on" may include cases where it is placed below as well as above.

[0060] In addition, in the present specification, “the negative active material is oriented” or “the negative active material is aligned” means that a specific crystal plane (e.g., the ab-axis crystal plane of graphite) representing a two-dimensional planar structure of the negative active material particles is arranged to have a predetermined inclination with respect to the surface of the negative electrode current collector, which may be different from the negative active material particles themselves being arranged to have a specific direction within the negative electrode active layer.

[0061] In addition, "the orientation of the negative electrode active material is high" may mean that a specific crystal plane (e.g., the ab-axis crystal plane of graphite) representing the two-dimensional planar structure of the negative electrode active material contained in the negative electrode active layer frequently has a predetermined inclination with respect to the surface of the negative electrode current collector. Furthermore, in some cases, it may mean that the crystal plane of the negative electrode active material contained in the negative electrode active layer is arranged at a high angle (e.g., an angle close to vertical, greater than 45°; specifically, greater than 60°) with respect to the surface of the negative electrode current collector.

[0062] In addition, the "degree of alignment of the negative electrode active material is high" means that the "degree of alignment (OI)" mentioned herein has a large value, which may mean that a specific crystal plane (e.g., an ab-axis crystal plane of graphite) representing a two-dimensional planar structure of the carbon-based negative electrode active material contained in the negative electrode active layer is arranged or randomly oriented at a low angle (e.g., 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 (OI)" has a small value, which may mean that the crystal plane of the carbon-based negative electrode active material contained in the negative electrode active layer is arranged at a high angle (e.g., an angle close to vertical, 45° or more; specifically, 60° or more) with respect to the surface of the negative electrode current collector.

[0063] Furthermore, in this specification, the “crystal plane of the negative electrode active material” refers to a plane on which atoms of the negative electrode active material form the outer shape of the crystal, and in the present invention, it may mean a crystal plane including a plane of the negative electrode active material, or a crystal plane including the a-axis / b-axis / ab-axis of the negative electrode active material crystal.

[0064] Also, in this specification, "average particle diameter (D 50 )" refers to the particle size at which the accumulated value is 50% in the particle size distribution, and is also called the median diameter.

[0065]

[0066] Hereinafter, the present invention will be described in more detail.

[0067]

[0068] Cathode for lithium secondary batteries

[0069] In one embodiment, the negative electrode for a lithium secondary battery according to the present invention comprises:

[0070] negative current collector,

[0071] A first negative electrode active layer provided on at least one surface of the negative electrode current collector and including a first negative electrode active material and a first CMC (carboxyl methylcellulose) binder having a weight average molecular weight in the range of 2.5 million to 5 million, and

[0072] A second negative electrode active layer is provided on the first negative electrode active layer and includes a second negative electrode active material and a second CMC binder having a weight average molecular weight in the range of 800,000 to 2,000,000.

[0073] Specifically, the first negative electrode active material has an average degree of alignment (OI) in the range of 10 to 50 according to the following Equation 1. In addition, the second negative electrode active material has an average degree of alignment (OI) in the range of 0.1 to 9 according to the following Equation 1.

[0074] [Formula 1]

[0075] OI = I 004 / I 110

[0076] In equation 1,

[0077] I 004 represents the area of ​​the peak representing the (0,0,4) crystal plane of the carbon-based negative electrode active material in the X-ray diffraction (XRD) spectroscopy analysis of the negative electrode active layer,

[0078] I 110 represents the area of ​​the peak representing the (1,1,0) crystal plane of the carbon-based negative electrode active material in X-ray diffraction (XRD) spectroscopy analysis of the negative electrode active layer.

[0079] The negative electrode for a lithium secondary battery according to the present invention includes a negative electrode active layer on one or both sides of a negative electrode current collector. The negative electrode active layer is a layer that implements electrical activity of the negative electrode, and includes as a main component a negative electrode active material that implements an electrochemical redox reaction during charging and discharging of the battery.

[0080] At this time, the negative electrode active layer may have a two-layer structure in which a first negative electrode active layer and a second negative electrode active layer are sequentially laminated on a negative electrode current collector. The negative electrode active layer having a two-layer structure can easily control the composition of each layer. The negative electrode active layer having such a two-layer structure can improve the performance of the negative electrode by controlling the type or content of components contained in each layer according to a specific purpose, such as increasing the energy efficiency of the battery or improving the adhesion between the active layer and the current collector. For example, the negative electrode active layer may selectively include a silicon-based negative electrode active material having a high charge / discharge capacity of the battery only in the second negative electrode active layer in contact with the positive electrode. In addition, the negative electrode active layer may selectively include natural graphite having good adhesive properties as a negative electrode active material only in the first negative electrode active layer in contact with the negative electrode current collector, or may include a high content of a binder that provides binding properties to the components constituting the active layer.

[0081] In one embodiment of the present invention, the difference in weight average molecular weight of the first and second CMC binders is 1 million or more. Specifically, the difference in weight average molecular weight of the first and second CMC binders may be in the range of 1 million to 4 million, 1 million to 3 million, 1.8 million to 4 million, or 1.5 million to 2.5 million. In the present invention, a high molecular weight CMC binder is applied to the first negative electrode active layer. The first CMC binder included in the first negative electrode active layer has the effect of increasing the adhesive strength between the current collector and the first negative electrode active layer, and further suppressing the orientation of the negative electrode active material when a magnetic field is applied. In response to this, a second CMC binder having a relatively low molecular weight is applied to the second negative electrode active layer. The molecular weight range of the second CMC binder is intended to minimize the inhibition of the orientation of the negative electrode active material when a magnetic field is applied while maintaining the binding force of the second negative electrode active layer. Furthermore, in the present invention, by applying the same type of binder to the first and second negative electrode active layers, the interfacial adhesive force between the first and second negative electrode active layers is maintained at a high level.

[0082] In one embodiment, the first and second negative electrode active materials independently include first and second carbon-based negative electrode active materials. The first and second carbon-based negative electrode active materials include at least one of natural graphite and artificial graphite. Specifically, the first negative electrode active layer includes the first carbon-based negative electrode active material as the negative electrode active material, and the second negative electrode active layer includes the second carbon-based negative electrode active material as the negative electrode active material.

[0083] At this time, the carbon-based negative electrode active material included in each negative electrode active layer may be the same or different in type and / or content. Specifically, the first carbon-based negative electrode active material and the second carbon-based negative electrode active material refer to materials containing carbon atoms as a main component, and such carbon-based negative electrode active material may include graphite. The graphite may include at least one of natural graphite and artificial graphite. For example, the first carbon-based negative electrode active material and the second carbon-based negative electrode active material may include natural graphite or artificial graphite alone, and in some cases, may include a mixture of natural graphite and artificial graphite. In this case, the mixing ratio of natural graphite and artificial graphite may be 5 to 50:50 to 95, or 10 to 30:70 to 90, based on weight. The carbon-based negative electrode active material can contain natural graphite and artificial graphite in the above-mentioned mixing ratio, thereby strengthening the adhesion between the negative electrode current collector and the negative electrode active layer, and improving the orientation of the carbon-based negative electrode active material on the surface of the negative electrode current collector.

[0084] As an example, the first carbon-based negative electrode active material may include artificial graphite and natural graphite in a weight ratio of 30 to 50:50 to 70, and the second carbon-based negative electrode active material may include artificial graphite alone.

[0085] In addition, it is preferable that the first carbon-based negative electrode active material and the second carbon-based negative electrode active material are spherical graphite aggregates formed by aggregating a plurality of flake-shaped graphite. Examples of the flake-shaped graphite include, in addition to natural graphite and artificial graphite, mesophase calcined carbon (bulk mesophase) using tar and pitch as raw materials, and graphitized coke (raw coke, green coke, pitch coke, needle coke, petroleum coke, etc.), and in particular, one aggregated using a plurality of highly crystalline natural graphites is preferable. In addition, one graphite aggregate can be formed by aggregating 2 to 100, preferably 3 to 20, flake-shaped graphite.

[0086] These carbon-based negative electrode active materials may have a spherical particle shape. At this time, the sphericity of the graphite particles may be 0.75 or more, for example, 0.75 to 1.0; 0.75 to 0.95; 0.8 to 0.95; or 0.90 to 0.99. Here, the "sphericity" may mean the ratio of the shortest diameter (minor diameter) to the longest diameter (major diameter) among any diameters passing through the center of the particle. If the sphericity is 1, it means that the particle shape is spherical. The sphericity may be determined by measuring the shape of the particle using a particle shape analyzer, or by analyzing the measured result after measuring the shape of the particle using a scanning electron microscope (SEM) or an energy dispersive spectrometer.

[0087] The present invention can realize high electrical conductivity of the negative electrode active layer by implementing the shapes of the first carbon-based negative electrode active material and the second carbon-based negative electrode active material to be close to spherical. Therefore, the negative electrode including the spherical carbon-based negative electrode active material can improve the capacity of a secondary battery, and has the advantage of improving the adhesive strength between the negative electrode active layer and the current collector by increasing the specific surface area per unit weight of the negative electrode active material.

[0088] In another embodiment, the second negative electrode active layer may further include a silicon-based negative electrode active material. The silicon-based negative electrode active material is a material containing silicon (Si) as a main component and is contained only in the second negative electrode active layer. The present invention can increase the charge / discharge capacity of the negative electrode by containing the silicon-based negative electrode active material in the second negative electrode active layer adjacent to the positive electrode active layer. In addition, since the silicon-based negative electrode active material undergoes a large volume change during charge / discharge, there is a problem in that the durability of the negative electrode is reduced when included in the first negative electrode active layer. However, the present invention can maintain high durability of the negative electrode by applying the silicon-based negative electrode active material to the second negative electrode active layer.

[0089] The above silicon-based negative electrode active material is silicon (Si), silicon carbide (SiC), and silicon oxide (SiO). q , but includes at least one of 0.8≤q≤2.5). Specifically, the silicon-based negative electrode active material may include silicon (Si), silicon carbide (SiC), silicon monoxide (SiO), or silicon dioxide (SiO2), and these may be included alone or in combination in the second negative electrode active layer. When silicon monoxide (SiO) and silicon dioxide (SiO2) are uniformly mixed or composited and included in the negative electrode active layer as the silicon-based negative electrode active material, they are silicon oxide (SiO q , but can be expressed as 0.8≤q≤2.5).

[0090] In addition, the silicon-based negative electrode active material may be doped with Li, Mg, Al, Ca, or Ti, or may form an alloy. In addition, the silicon-based negative electrode active material may be surface-treated with a carbon coating layer or the like to suppress volume expansion during charging and improve electrical conductivity of the negative electrode active material when it contains oxygen (O).

[0091] In addition, the carbon-based negative electrode active material and the silicon-based negative electrode active material contained in each layer of the negative electrode active layer may be included in an amount of 85 parts by weight or more based on the total weight of the negative electrode active layer, and specifically, may be included in an amount of 90 parts by weight or more, 93 parts by weight or more, or 95 parts by weight or more.

[0092] In addition, the silicon-based negative electrode active material may be included in an amount of 0.1 to 40 wt% based on the total weight of the negative electrode active material contained in the first negative electrode active layer and the second negative electrode active layer, and specifically, may be included in an amount of 0.5 to 20 wt%, 1 to 9 wt%, 5 to 15 wt%, 3 to 7 wt%, 11 to 19 wt%, 13 to 17 wt%, 15 to 20 wt%, 10 to 30 wt%, 20 to 40 wt%, 25 to 35 wt%, 15 to 25 wt%, or 9 to 22 wt%. The present invention can reduce lithium consumption and irreversible capacity loss during initial charge / discharge of a secondary battery while improving the charge capacity per unit mass by controlling the total content ratio of carbon-based negative electrode active material included in the negative electrode active layer and the content ratio of silicon-based negative electrode active material included in the entire negative electrode active material within the above ranges. In addition, since the structural stability of the negative electrode active layer can be improved by minimizing the volume change of the negative electrode active layer during charge / discharge of the secondary battery, the lifespan of the secondary battery can be increased.

[0093] Furthermore, the negative electrode according to the present invention can further improve high-rate charge / discharge characteristics by controlling the crystal structure characteristics of negative electrode active materials contained in the second negative electrode active layer adjacent to the positive electrode.

[0094] As an example, the second carbon-based negative electrode active material contained in the second negative electrode active layer may have an alignment index (OI) in the range of 0.1 to 9 according to the following equation 1.

[0095] [Formula 1]

[0096] OI = I 004 / I 110

[0097] In equation 1,

[0098] I 004 represents the area of ​​the peak representing the [0,0,4] crystal plane of the carbon-based negative electrode active material in the X-ray diffraction (XRD) spectroscopy analysis of the negative electrode active layer,

[0099] I 110 represents the area of ​​the peak representing the [1,1,0] crystal plane of the carbon-based negative electrode active material in X-ray diffraction (XRD) spectroscopy analysis of the negative electrode active layer.

[0100]

[0101] The degree of alignment (OI) of the second carbon-based negative electrode active material can be an indicator of the degree to which the ab-axis crystal plane of the carbon-based negative electrode active material is oriented in a certain direction, specifically, with respect to the negative electrode current collector surface, when measured by X-ray diffraction (XRD). Specifically, the second negative electrode active layer exhibits peaks of 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° for graphite, which is the second carbon-based negative electrode active material, when measured by X-ray diffraction. This represents 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 graphite. Here, the peak appearing at 2θ=43.4±0.2° can be seen as an overlap between the peak corresponding to the (1,0,1)R plane of the second carbon-based negative electrode active material and the (1,1,1) plane of a current collector, for example, copper (Cu).

[0102] The degree of alignment (OI) of the second carbon-based negative electrode active material can be measured through the ratio of the areas obtained by integrating the respective intensities 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.

[0103] The peak at 2θ=54.7±0.2° is a peak indicating a crystal plane having an inclination with respect to the negative electrode current collector among the crystal planes of the second carbon-based negative electrode active material, so that the degree of alignment (OI) may mean that the closer the value is to 0, the closer the inclination with respect to the negative electrode current collector surface is to 90°, and the larger the value is, the closer the inclination with respect to the negative electrode current collector surface is to 0° or 180°. That is, the second negative electrode active layer according to the present invention may be aligned so that the second carbon-based negative electrode active material has an angle of 60° or more, 70° or more, 70 to 90°, 80 to 90°, 65 to 85°, or 70 to 85° with respect to the negative electrode current collector. Accordingly, the second negative electrode active layer may have a lower degree of alignment (OI) of the second carbon-based negative electrode active material compared to a case where the second carbon-based negative electrode active material is aligned at a low angle of less than 60°. For example, the second negative electrode active layer may have an alignment degree (OI) of the carbon-based negative electrode active material according to Equation 1 in the range of 0.1 to 9, 0.1 to 8, 0.5 to 9, 1 to 9, 2 to 7, or 3 to 5. Here, the alignment degree (OI) indicates the degree of alignment of the ab-axis crystal plane of the carbon-based negative electrode active material, and does not indicate the degree to which the carbon-based negative electrode active material particles are aligned by self-rotation within the active layer.

[0104] The present invention can secure a shorter ion migration channel within the negative electrode active layer by controlling the degree of alignment (OI) of the second carbon-based negative electrode active material contained in the second negative electrode active layer as described above. Accordingly, the negative electrode of the present invention can prevent an increase in resistance due to a long migration distance of lithium ions, thereby preventing a decline in high-rate charging performance due to electrical resistance, and at the same time improving high-rate discharge efficiency.

[0105] On the other hand, the first carbon-based negative electrode active material contained in the first negative electrode active layer may have an alignment degree (OI) in the range of 10 to 50 according to the aforementioned Equation 1. Specifically, the first negative electrode active layer may have an alignment degree (OI) of the carbon-based negative electrode active material according to Equation 1 in the range of 10 to 45, 10 to 30, 15 to 50, 20 to 50, or 20 to 35. The present invention can increase the interfacial adhesion with a current collector and improve electrode stability by controlling the alignment degree (OI) of the first carbon-based negative electrode active material contained in the first negative electrode active layer as described above. This electrode stability has the effect of improving the cycle characteristics of a secondary battery to which the negative electrode is applied.

[0106] In one embodiment, the negative electrode for a lithium secondary battery according to the present invention has an SOC range in which Li precipitation occurs at room temperature and 2.5 C charging conditions of 35% or more. The room temperature is, for example, 25°C. The inventors of the present invention have confirmed through various and repeated experiments that the negative electrode for a lithium secondary battery according to the present invention has an SOC range in which Li precipitation occurs of 35% or more, specifically, 35 to 40%.

[0107] In addition, the negative electrode for a lithium secondary battery according to the present invention has an interfacial adhesive force between the negative electrode current collector and the first negative electrode active layer of at least 27 gf / 20 mm on average. Specifically, the interfacial adhesive force between the negative electrode current collector and the first negative electrode active layer is in the range of 27 to 50 gf / 20 mm on average, in the range of 27 to 40 gf / 20 mm on average, or in the range of 30 to 35 gf / 20 mm on average. The negative electrode for a lithium secondary battery according to the present invention realizes excellent interfacial adhesive force by simultaneously controlling the molecular weight of the binder in the first negative electrode active layer and the degree of alignment of the negative electrode active material.

[0108] Furthermore, the second carbon-based negative electrode active material and the silicon-based negative electrode active material contained in the second negative electrode active layer may have predetermined particle sizes. Specifically, the average particle size (Dc) of the second carbon-based negative electrode active material may be 1 µm to 50 µm, the average particle size (Ds) of the silicon-based negative electrode active material may be 0.1 µm to 10 µm, and the average particle size ratio (Dc / Ds) of the second carbon-based negative electrode active material and the silicon-based negative electrode active material may be 2 to 10.

[0109] Specifically, the second carbon-based negative electrode active material has an average particle diameter (D) of 1 μm to 50 μm. 50 , Dc) and can be selectively applied within the above range according to the average thickness of the second negative electrode active layer. For example, the second carbon-based negative electrode active material may have a thickness of 1 μm to 40 μm; 1 μm to 30 μm; 10 μm to 40 μm; 15 μm to 30 μm; 25 μm to 50 μm; 11 μm to 19 μm; 15 μm to 25 μm; 20 μm to 30 μm; 1 μm to 20 μm; 1 μm to 10 μm; 5 μm to 15 μm; 10 μm to 20 μm; 15 μm to 30 μm; 15 μm to 20 μm; 21 μm to 26 μm; 25 μm to 30 μm; 11 μm to 17 μm; 16 μm to 23 μm; 2 μm to 7 μm; 0.5 μm to 5 μm; Or an average particle diameter (D) of 1㎛ to 3㎛ 50) can be expressed. The second carbon-based negative electrode active material in a spherical shape may be advantageous in that the particle size is made smaller to maximize the disorder in the expansion direction for each particle so as to prevent expansion of the particles due to charging of lithium ions. However, when the particle size of the second carbon-based negative electrode active material is less than 1.0 ㎛, a large amount of binder is required due to an increase in the number of particles per unit volume, and the spheroidization degree and spheroidization yield may be reduced. On the other hand, when the maximum particle size exceeds 30 ㎛, the expansion rate of the negative electrode active material significantly increases during charge and discharge of the secondary battery, so that as charge and discharge are repeated, the adhesion between the negative electrode active material particles and the adhesion between the negative electrode active material particles and the current collector deteriorate, which may significantly reduce the cycle characteristics.

[0110] In addition, the silicon-based negative electrode active material has an average particle diameter (D) of 0.1 μm to 10 μm. 50 , Ds) and specifically, the average particle diameter (D) of 0.1 ㎛ to 5 ㎛; 0.1 ㎛ to 3 ㎛; 0.1 ㎛ to 1 ㎛; 0.5 ㎛ to 2 ㎛; 0.5 ㎛ to 5 ㎛; 1 ㎛ to 5 ㎛; 3 ㎛ to 7 ㎛; 5 ㎛ to 10 ㎛; 1 ㎛ to 3 ㎛; 4 ㎛ to 9 ㎛; 0.1 ㎛ to 0.9 ㎛; 0.8 ㎛ to 1.2 ㎛; or 0.3 ㎛ to 0.8 ㎛ 50 ) can be expressed.

[0111] When the silicon-based negative electrode active material has a minimum particle size of less than 0.1 μm, it is difficult to uniformly distribute the second negative electrode active layer. Therefore, as the secondary battery is charged and discharged, the oxidation-reduction reaction is induced unevenly in the agglomerated and non-agglomerated regions of the silicon-based negative electrode active material, which may accelerate the deterioration of the second negative electrode active layer. In addition, when the silicon-based negative electrode active material has a maximum particle size of more than 10 μm, it is difficult to control the crystal plane of the silicon-based negative electrode active material, and the expansion rate per unit area of ​​the second negative electrode active layer significantly increases during charge and discharge of the secondary battery. Therefore, there is a problem in that the cycle characteristics significantly decrease as charge and discharge are repeated.

[0112] Furthermore, the average particle size ratio (Dc / Ds) of the second carbon-based negative electrode active material and the silicon-based negative electrode active material may be 2 to 10, and specifically, 2 to 7; 2 to 5; 3 to 7; 4 to 8; 5 to 10; or 6 to 9.

[0113] The present invention can induce a structure in which a silicon-based negative electrode active material is filled in voids formed by a plurality of second carbon-based negative electrode active materials by satisfying the average particle size ratio (Dc / Ds) of the second carbon-based negative electrode active material and the silicon-based negative electrode active material within the above range. At this time, the voids serve as a buffer that buffers the volume expansion of the silicon-based negative electrode active material during charge and discharge of the secondary battery, thereby minimizing the volume expansion of the second negative electrode active layer.

[0114] In addition, when the average particle size ratio (Dc / Ds) is less than the lower limit of the above-described range, the influence of the second carbon-based negative electrode active material on the silicon-based negative electrode active material is reduced, and thus there is a limitation in that it is difficult to control the crystal plane of pure silicon having a crystal phase. In addition, when the average particle size ratio (Dc / Ds) exceeds the upper limit of the above-described range, there is a problem in that the deterioration of the second negative electrode active layer is accelerated during charge / discharge of the secondary battery, thereby shortening the lifespan.

[0115] Meanwhile, the average thickness of the negative electrode active layer may be 100 ㎛ to 300 ㎛, specifically 100 ㎛ to 250 ㎛; 100 ㎛ to 250 ㎛; or 130 ㎛ to 190 ㎛. The present invention can easily control the crystal characteristics of the negative electrode active material contained in the second negative electrode active layer by adjusting the average thickness of the negative electrode active layer within the above range, thereby improving the high-rate charge / discharge characteristics of a secondary battery including the negative electrode of the present invention.

[0116] In addition, the first negative electrode active layer and the second negative electrode active layer may have the same or different average thicknesses. Specifically, the average thickness (D1) of the first negative electrode active layer and the average thickness (D2) of the second negative electrode active layer may have a ratio of 1: 0.5 to 3, and more specifically, a ratio of 1: 0.8 to 2.5, 1: 1 to 2.5, 1: 1.1 to 2, 1: 0.9 to 1.1, or 1: 1.5 to 3. The present invention can maximize the charge / discharge capacity while maintaining high durability of the negative electrode by controlling the average thickness ratio of the first negative electrode active layer and the second negative electrode active layer within the above range.

[0117] In addition, the first cathode active layer and the second cathode active layer according to the present invention may optionally further include a conductive material, a binder, other additives, etc., as needed, along with the cathode active material as the main component.

[0118] The above-mentioned challenge material may include one or more of carbon black, acetylene black, Ketjen black, carbon nanotubes, carbon fibers, etc., but is not limited thereto.

[0119] As an example, the cathode active layer may contain carbon black, carbon nanotubes, carbon fibers, etc. alone or in combination as a conductive material.

[0120] At this time, the content of the conductive material may be 0.1 to 10 parts by weight based on 100 parts by weight of the entire negative electrode active layer, and specifically, may be 0.1 to 8 parts by weight, 0.1 to 7 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 the resistance of the negative electrode from increasing due to a low content of the conductive material, thereby reducing the charging capacity, and can prevent the problem of the content of the negative electrode active material decreasing due to an excessive amount of the conductive material, thereby reducing the charging capacity, or the problem of the rapid charging characteristics from decreasing due to an increase in the loading amount of the negative electrode active layer.

[0121] In addition, the binder may be appropriately applied as a component that assists in the bonding of the negative electrode active material and the conductive material and the bonding to the current collector, and may be applied within a range that does not deteriorate the electrical properties of the electrode. Specifically, the binder may include at least one selected from the group consisting of 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.

[0122] The content of the binder may be 0.1 to 10 parts by weight based on 100 parts by weight of the entire negative electrode active layer, and specifically, may be 0.1 to 8 parts by weight, 0.1 to 7 parts by weight, 0.1 to 3 parts by weight, or 2 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 strength of the active layer from being lowered due to a low content of binder or the electrical properties of the electrode from being lowered due to an excessive amount of binder.

[0123] In addition, the negative electrode current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the battery, and for example, copper, stainless steel, nickel, titanium, calcined carbon, etc. can be used. In the case of copper or stainless steel, a material surface-treated with carbon, nickel, titanium, silver, etc. can also be used. In addition, the average thickness of the negative electrode current collector can be appropriately applied in the range of 1 to 500 ㎛ in consideration of the conductivity and total thickness of the negative electrode to be manufactured.

[0124]

[0125] Method for manufacturing cathode

[0126] In addition, the present invention provides a method for manufacturing a negative electrode for a lithium secondary battery. In one embodiment, the method for manufacturing a negative electrode for a lithium secondary battery according to the present invention comprises:

[0127] An active layer forming step of forming first and second negative electrode active layers on a negative electrode current collector;

[0128] A magnetic field application step of applying a magnetic field in the range of 1,000 to 10,000 G for 5 to 20 seconds; and

[0129] It includes the steps of drying and rolling the cathode.

[0130] In addition, the first negative electrode active layer includes a first negative electrode active material and a first CMC (carboxyl methylcellulose) binder having a weight average molecular weight in the range of 2.5 million to 5 million, and the second negative electrode active layer includes a second negative electrode active material and a second CMC binder having a weight average molecular weight in the range of 800,000 to 2 million.

[0131] The method for manufacturing a negative electrode according to the present invention refers to the method for manufacturing the negative electrode of the present invention described above. The method for manufacturing the negative electrode comprises: applying a negative electrode slurry on a negative electrode current collector, applying a magnetic field to the surface of the applied negative electrode slurry, and then drying each negative electrode slurry, thereby manufacturing a negative electrode having a negative electrode active layer in which the crystal characteristics of the negative electrode active material are controlled.

[0132] The above-described active layer forming step is a step of discharging and coating a negative electrode slurry containing a carbon-based negative electrode active material onto the surface of a moving negative electrode current collector. This step may be applied without particular limitation as long as it is a method commonly applied in the art, but a die coating method may preferably be used. The die coating method may be performed through 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, etc., the loading amount, coating thickness, etc. of the negative electrode slurry applied onto the negative electrode current collector can be easily controlled.

[0133] In one embodiment, the active layer forming step forms a second negative electrode active layer before the first negative electrode active layer formed on the negative electrode current collector is dried (wet on wet method). Conventional two-layer negative electrodes have also been manufactured through a method of forming the second negative electrode active layer after the first negative electrode active layer is dried (wet on dry method). However, this conventional method (wet on dry method) may have problems with the interfacial adhesion between the first and second negative electrode active layers, and the movement of lithium ions at the interface between the first and second negative electrode active layers may be suppressed, which causes a deterioration in the performance of the electrode. In the present invention, by applying a method of forming the second negative electrode active layer before the first negative electrode active layer is dried (wet on wet method), these conventional problems are solved at once.

[0134] In another embodiment, the active layer forming step simultaneously forms the first and second negative electrode active layers on the negative electrode current collector. In the present invention, the first negative electrode slurry and the second negative electrode slurry can be simultaneously applied onto the negative electrode current collector using a dual slot die. In this case, compared to sequentially applying each slurry, there is an advantage in that the process efficiency is significantly increased.

[0135] Meanwhile, the magnetic field applying step may be a step for controlling the crystal characteristics of the negative electrode active material contained in the negative electrode slurry. Specifically, this step aligns the ab-axis crystal plane of the carbon-based negative electrode active material to have a high angle with respect to the negative electrode collector by applying a magnetic field to the surface of the negative electrode slurry applied on the negative electrode collector. In particular, the (1,1,1) crystal plane characteristics of the silicon-based negative electrode active material contained in the second negative electrode slurry can be controlled.

[0136] At this time, a magnetic field may be applied by magnets disposed on the upper and lower portions of the negative electrode current collector, which is moved while the negative electrode slurry is applied to the surface. In addition, the polarities of the magnets disposed on the upper and lower portions may be different from each other.

[0137] In addition, the alignment degree (OI) of the carbon-based negative electrode active material contained in the negative electrode slurry can be controlled by the strength of the applied magnetic field, etc., and accordingly, the step of applying the magnetic field can be performed under predetermined magnetic field strength conditions.

[0138] Specifically, the step of applying the magnetic field may apply a magnetic field of 10,000 G (Gauss) or less, and specifically, the magnetic field may be applied with a strength of 2,000 G to 5,000 G; 2,000 G to 4,500 G; or 2,500 G to 3,500 G.

[0139] Additionally, the step of applying the magnetic field may be performed for 1 second to 20 seconds, specifically, for 7 seconds to 15 seconds; 7 seconds to 12 seconds; or 8 seconds to 12 seconds.

[0140] As an example, in the step of applying the magnetic field, a magnetic field of 3,000±50 G may be applied to the cathode slurry for 9 to 11 seconds.

[0141] Furthermore, the step of applying the magnetic field is performed by magnets introduced to the upper and lower portions of the applied negative electrode slurry as mentioned above, and the size of the magnets may be adjusted to be larger than the size of the negative electrode slurry so that the magnetic field applied to the negative electrode slurry can be uniformly applied to the entire surface of the negative electrode slurry. For example, the magnets may have a length ratio of 105% to 200% based on the width direction length of the negative electrode slurry, and specifically, may have a length ratio of 110% to 180%; 110% to 160%; 110% to 140%; 110% to 130%; 130% to 150%; or 105% to 120% based on the width direction length of the negative electrode slurry.

[0142] The present invention satisfies the alignment index (OI) of the carbon-based negative electrode active material contained in the second negative electrode slurry to 9 or less by controlling the strength of the magnetic field, the application time, and / or the size of the magnet portion as described above in the step of applying the magnetic field.

[0143] In addition, the present invention may include a step of drying and rolling the cathode after the step of applying the magnetic field.

[0144] At this time, the step of drying the negative electrode can be applied without any particular limitation as long as it is a method capable of maintaining the orientation of the carbon-based negative electrode active material contained in the negative electrode active layer.

[0145] For example, the drying step can dry the cathode slurry by applying heat energy to the cathode slurry using a hot air dryer, a vacuum oven, or the like.

[0146] In addition, the step of rolling the dried cathode is a step of increasing the density of the cathode active layer by applying pressure to the dried cathode slurry using a roll press or the like. At this time, the rolling may be performed at a temperature higher than room temperature.

[0147] Specifically, the rolling can be performed at a temperature of 50°C to 100°C, more specifically at a temperature of 60°C to 100°C; 75°C to 100°C; 85°C to 100°C; 50°C to 90°C; 60°C to 80°C or 65°C to 90°C. Specifically, the rolling can be performed 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; The rolling can be performed at a rolling speed of 5 m / s to 7 m / s; 5.5 m / s to 6.5 m / s or 6 m / s to 7 m / s. In addition, the rolling can be performed under a pressure condition of 50 MPa to 200 MPa, and specifically, it can be performed under a pressure condition of 50 MPa to 150 MPa; 50 MPa to 100 MPa; 100 MPa to 200 MPa; 150 MPa to 200 MPa or 80 MPa to 140 MPa.

[0148] The present invention can increase the energy density of a negative electrode while minimizing changes in the alignment of a carbon-based negative electrode active material contained in a negative electrode active layer formed by performing rolling of a dried negative electrode slurry under the above-described temperature, speed, and / or pressure conditions.

[0149]

[0150] lithium secondary battery

[0151] In addition, in one embodiment of the present invention,

[0152] A lithium secondary battery is provided, comprising an electrode assembly including a positive electrode, a negative electrode of the present invention described above, and a separator disposed between the positive electrode and the negative electrode.

[0153]

[0154] A lithium secondary battery according to the present invention comprises an electrode assembly having a structure in which a plurality of positive electrodes and a plurality of negative electrodes are alternately arranged and a separator is positioned between them. The lithium secondary battery, equipped with the negative electrode of the present invention described above, not only has a high charge / discharge capacity but also has excellent high-rate charge / discharge characteristics, and thus can be usefully used as a power source for medium- to large-sized devices such as electric vehicles.

[0155] At this time, since the cathode has the same configuration as the configuration described above, a detailed description is omitted.

[0156] In addition, the positive electrode includes a positive electrode active layer including a positive electrode active material on a positive electrode current collector, and the positive electrode active layer may optionally further include a conductive material, a binder, other additives, etc., as needed.

[0157] The above cathode active material is a material capable of causing an electrochemical reaction on the cathode current collector, and may include at least one lithium metal oxide represented by the following chemical formulas 1 and 2, which is capable of reversibly intercalating and deintercalating lithium ions:

[0158] [Chemical Formula 1]

[0159] Li x [Ni y Co z Mn w M 1 v ]O2

[0160] [Chemical Formula 2]

[0161] LiMn p M 2 1-p O4

[0162] In the above chemical formulas 1 and 2,

[0163] M 1is at least one element selected from the group consisting of 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,

[0164] x, y, z, w, and v are 1.0≤x≤1.30, 0.5≤y<1, 0, respectively. <z≤0.3, 0<w≤0.3, 0≤v≤0.1이되, y+z+w+v=1이고,

[0165] M 2 is Ni, Co or Fe,

[0166] p is 0≤p≤0.9.

[0167] The lithium metal oxides represented by the above chemical formulas 1 and 2 are substances containing high contents of nickel (Ni) and manganese (Mn), respectively, and when used as a cathode active material, they have the advantage of being able to stably supply electricity at a high capacity and / or high voltage compared to cathode active materials such as iron phosphate (LiFeO4) that are conventionally used.

[0168] At this time, the lithium metal oxide represented by the above chemical formula 1 may include LiNi0.8Co0.1Mn0.1O2, LiNi0.6Co0.2Mn0.2O2, LiNi0.9Co0.05Mn0.05O2, LiNi0.6Co0.2Mn0.1Al0.1O2, LiNi0.6Co0.2Mn0.15Al0.05O2, LiNi0.7Co0.1Mn0.1Al0.1O2, etc., and the lithium metal oxide represented by the above chemical formula 2 may include LiNi0.7Mn1.3O4; LiNi0.5Mn1.5O4; LiNi0.3Mn1.7O4, etc., and these may be used alone or in combination.

[0169] In addition, the positive electrode active material may be included in an amount of 85 parts by weight or more based on the weight of the positive electrode active layer, and specifically, may be included in an amount of 90 parts by weight or more, 93 parts by weight or more, or 95 parts by weight or more.

[0170] In addition, the positive electrode active layer may further include a conductive agent, a binder, other additives, etc., along with the positive electrode active material.

[0171] At this time, the conductive material is used to improve the electrical performance of the anode, and can be applied to materials commonly used in the art, but specifically, can include at least one selected from the group consisting of natural graphite, artificial graphite, carbon black, acetylene black, Denka black, Ketjen black, Super-P, channel black, furnace black, lamp black, summer black, graphene, and carbon nanotubes.

[0172] In addition, the conductive material may be included in an amount of 0.1 to 5 parts by weight based on the weight of each positive electrode active layer, and specifically, may be included in an amount 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.

[0173] In addition, the binder serves to bind the positive electrode active material, the positive electrode additive, and the conductive material to each other, and any binder having this function may be used without particular limitation. Specifically, the binder may include at least one resin selected from the group consisting of polyvinylidene fluoride-hexafluoropropylene copolymer (PVdF-co-HFP), polyvinylidene fluoride (PVdF), polyacrylonitrile, polymethylmethacrylate, and copolymers thereof. As an example, the binder may include polyvinylidene fluoride.

[0174] In addition, the 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 2 to 8 parts by weight; or 1 to 5 parts by weight.

[0175] The total thickness of the above-mentioned positive electrode active layer is not particularly limited, but may be specifically 50 µm to 300 µm, and 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.

[0176] In addition, the positive electrode can be used as a positive electrode current collector that has high conductivity without causing chemical changes in the battery. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, etc. can be used, and in the case of aluminum or stainless steel, a material surface-treated with carbon, nickel, titanium, silver, etc. can also be used. In addition, the average thickness of the current collector can be appropriately applied in the range of 3 to 500 ㎛, taking into account the conductivity and total thickness of the positive electrode to be manufactured.

[0177] In addition, the separator 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 art, but specifically, one including at least one polymer selected from the group consisting of polypropylene, polyethylene, and polyethylene-propylene copolymers having chemical resistance and hydrophobicity can be used. The separator may have a porous polymer substrate form such as a sheet or non-woven fabric including the above-described polymer, and in some cases, may have a composite separator form in which organic or inorganic particles are coated on the porous polymer substrate using an organic binder. In addition, the separator may have an average pore diameter of 0.01 to 10 μm, and an average thickness of 5 to 300 μm.

[0178] Meanwhile, the lithium secondary battery according to the present invention is not particularly limited, but may be a secondary battery having a stacked, zigzag, or zigzag-stacked electrode assembly. As an example, the lithium secondary battery according to the present invention may be a pouch-type secondary battery or a square-shaped secondary battery.

[0179] Pouch-type secondary batteries and / or square secondary batteries have the advantage of high utilization in terms of energy density because secondary batteries can be packed at a high density in a limited space.

[0180]

[0181] Hereinafter, the present invention will be described in more detail through examples and experimental examples.

[0182] However, the following examples and experimental examples are only illustrative of the present invention, and the content of the present invention is not limited to the following examples and experimental examples.

[0183]

[0184] Examples 1 to 5 and Comparative Examples 1 to 3. Preparation of negative electrode for lithium secondary battery

[0185] Natural graphite (average particle size: 18±1㎛) and artificial graphite (average particle size: 15±1㎛) were prepared as carbon-based negative electrode active materials, and silicon oxide (SiO2) was prepared as a silicon-based negative electrode active material, and a first negative electrode slurry and a second negative electrode slurry were prepared using the prepared carbon-based negative electrode active materials and silicon-based negative electrode active materials.

[0186] Specifically, a first carbon-based negative electrode active material was prepared by mixing mixed graphite, which was a mixture of natural graphite and artificial graphite in a weight ratio of 3.5 to 4.5:5.5 to 6.5, and carbon black as a conductive material and first carboxymethyl cellulose (CMC) and styrene butadiene rubber (SBR) as binders. The weight average molecular weight of the first carboxymethyl cellulose (CMC) binder is as shown in Table 1 below. Then, 95 parts by weight of mixed graphite, 1 part by weight of carbon black, 3.5 parts by weight of first carboxymethyl cellulose (CMC), and 0.5 parts by weight of styrene butadiene rubber (SBR) were mixed with water to obtain a solid content of 50% to prepare a first negative electrode slurry.

[0187] In addition, a negative electrode active material was prepared by mixing artificial graphite and silicon oxide (SiO2) in a weight ratio of 85 to 90:10 to 15, and carbon black as a conductive material and secondary carboxymethyl cellulose (CMC) and styrene butadiene rubber (SBR) as binders were prepared. Then, 95 parts by weight of mixed graphite, 1 part by weight of carbon black, 3.5 parts by weight of secondary carboxymethyl cellulose (CMC), and 0.5 parts by weight of styrene butadiene rubber (SBR) were mixed with water to obtain a solid content of 45% to prepare a second negative electrode slurry. At this time, the weight average molecular weight of the secondary carboxymethyl cellulose (CMC) binder is as shown in Table 1 below.

[0188] When each cathode slurry was prepared, the first cathode slurry and the second cathode slurry were simultaneously drawn onto a copper foil (thickness: 10 μm) being transported roll-to-roll (transport speed: 5 m / min) using a double slot die.

[0189] A permanent magnet having a length ratio of 110 to 120% of the widthwise length of the negative electrode slurry was placed on the upper side of the applied negative electrode slurry and on the lower side of the negative electrode current collector, and the magnetic field strength was adjusted as shown in Table 1 below and applied for 10 seconds. The negative electrode slurry to which the magnetic field was applied was dried with hot air to form a negative electrode active layer in the form of a first negative electrode active layer and a second negative electrode active layer sequentially stacked on the negative electrode current collector. The formed negative electrode active layer was rolled at 50±1℃, a pressure of 100 to 150 MPa, and a conveying speed of 3 m / s to manufacture a negative electrode for a lithium secondary battery (average thickness of the first negative electrode active layer and the second negative electrode active layer: each 110±5 μm).

[0190] No. 1 CMC binder molecular weight (Mw) 2 CMC binder molecular weight (Mw) Magnetic field strength (Gauss) Example 13 million 1.5 million 3,000 Example 23 million 1 million 3,000 Example 34 million 1.5 million 3,000 Example 43 million 1.5 million 3,500 Comparative Example 11.5 million 1.5 million Not Approved Comparative Example 21.5 million 1.5 million 3,000 Comparative Example 31.5 million 3 million 3,000 Comparative Example 41 million 4 million 3,000 Comparative Example 53 million 1.5 million Not Approved Comparative Example 64 million 1 million Not Approved

[0191] In addition, FIGS. 1 to 3 are cross-sectional schematic diagrams showing the orientation of the negative electrode active layer for the negative electrodes manufactured in comparative examples or examples. Specifically, FIG. 1 is a schematic diagram showing the orientation of the negative electrode active layer according to comparative example 1. The negative electrode of comparative example 1 is manufactured without applying a separate magnetic field. Referring to FIG. 1, the negative electrode of comparative example 1 has both the first and second negative electrode active layers (111, 121) oriented or randomly oriented in the horizontal direction with respect to the negative electrode current collector (101).

[0192] Fig. 2 is a schematic diagram showing the orientation of the negative electrode active layer according to Comparative Example 2. The negative electrode of Comparative Example 2 is a negative electrode manufactured by controlling both the first and second CMC binders to have relatively low molecular weights. Referring to Fig. 2, the negative electrode of Comparative Example 2 has both the first and second negative electrode active layers (112, 122) oriented in a vertical direction with respect to the negative electrode current collector (102).

[0193] Fig. 3 is a schematic diagram showing the orientation of the negative electrode active layer according to Example 1. The negative electrode of Example 1 is a case in which the first CMC binder has a relatively high molecular weight, the second CMC binder has a relatively low molecular weight, and a magnetic field is applied during the manufacturing process. Referring to Fig. 2, the negative electrode of Example 1 has a form in which the first negative electrode active layer (113) is oriented horizontally with respect to the negative electrode current collector (102), and the second negative electrode active layer (123) is oriented vertically.

[0194]

[0195] Experimental Example 1. Measurement of Orientation Index (OI)

[0196] For each manufactured cathode, X-ray diffraction (XRD) was performed on the first and second cathode active layers to measure the spectrum. At this time, the X-ray diffraction (XRD) spectroscopy analysis was measured targeting the cathode active materials contained in the first and second cathode active layers, respectively, and the measurement conditions for X-ray diffraction (XRD) were as follows:

[0197] - Target: Cu(Kα-ray) graphite monochromator

[0198] - Slit: Diverging slit = 1 degree, Receiving slit = 0.1 mm, Scatter slit = 1 degree

[0199] - Measurement area: (1,1,0) plane: 76.5 degrees < 2θ < 78.5 degrees / (0,0,4) plane: 53.5 degrees < 2θ < 56.0 degrees.

[0200]

[0201] Then, from the spectrum measured targeting the second carbon-based negative electrode active material, the integral values ​​of each area of ​​the peak representing the (0,0,4) crystal plane and the peak representing the (1,1,0) crystal plane were obtained, and the ratio of these areas (I 004 / I 110 ) was calculated to derive the degree of alignment (OI) by region. After removing the second cathode active layer through physical peeling, the degree of alignment (OI) was derived for the first cathode active layer using the same method. The derived values ​​are shown in Table 2 below.

[0202] No. Magnetic field strength (Gauss) Alignment degree (OI) of first cathode active layer Alignment degree (OI) of second cathode active layer Example 13,000 25.2 4.6 4 Example 23,000 23.8 3.8 1 Example 33,000 28.9 4.9 3 Example 43,500 24.1 4.3 4 Comparative Example 1 Not applied 35.1 34.9 Comparative Example 23,000 5.8 2 3.8 1 Comparative Example 33,000 6.9 5 21.7 Comparative Example 43,000 5.4 1 24.2 Comparative Example 5 Not applied 35.5 35.9 Comparative Example 6 Not applied 36.1 35.7

[0203] Referring to the results in Table 2, it can be seen that the alignment degree of the first negative electrode active layer of the negative electrodes of Examples 1 to 4 is in the range of 20 to 30, and the alignment degree of the second negative electrode active layer is in the range of 3 to 5. In the present invention, the alignment degree of each layer is effectively controlled through the binder molecular weight of the first and second negative electrode active layers and the application of a magnetic field. In this regard, in the case of Comparative Examples 1, 5, and 6, it can be seen that the alignment degrees of the first and second negative electrode active layers are all over 30. This shows that when a magnetic field is not applied, the binder molecular weight does not have a significant effect on the alignment degree.

[0204] Additionally, in the case of Comparative Example 2, it can be seen that the alignment degrees of the first and second negative electrode active layers are both 6 or less even when a magnetic field is applied. This means that the negative electrode active materials contained in the first and second negative electrode active layers are all oriented in a direction close to vertical.

[0205] The cathodes of Comparative Examples 3 and 4 are cases where the binder included in the second cathode active layer was controlled to a high molecular weight (Mw=3 million, 4 million). It can be seen that the cathodes of Comparative Examples 3 and 4 have an orientation degree of the second cathode active layer greater than 20, which means that the orientation in the vertical direction was not properly achieved.

[0206]

[0207] Experimental Example 2. Adhesion Evaluation

[0208] The adhesive strength of each negative electrode specimen manufactured in Examples 1 to 5 and Comparative Examples 1 to 3 was measured using the 90° peel test method. Each negative electrode specimen had a structure in which a negative electrode active layer was formed on one side of a copper foil as a current collector. Each negative electrode specimen was attached to a slide glass so that the side without the negative electrode active layer was facing each other. The negative electrode specimen was attached to the slide glass using an imide double-sided tape. Then, the specimen was bonded by reciprocating 10 times with a 2 kg roller.

[0209] Then, using a UTM (TA) device, one side of the insulating layer was pulled at 100 mm / min to measure the force required to peel off the slide glass. At this time, the measurement angle between the slide glass and the electrode was 90°.

[0210] At this time, three metal specimens were prepared, and the adhesive strength of each insulating layer was measured through the 90° peel test described above, and the average value was calculated, and the results are shown in Table 3 below.

[0211]

[0212] Experimental Example 3. Measurement of SOC during Li precipitation

[0213] Battery cells were fabricated using the negative electrodes manufactured in Examples 1 to 5 and Comparative Examples 1 to 3. Specifically, a positive electrode was fabricated by applying and drying a positive electrode slurry containing an NCM622 active material onto an aluminum metal film. An electrode assembly was fabricated by interposing a separator between the negative electrode and the positive electrode and laminating them. Thereafter, the cells were fabricated by injecting an electrolyte solution containing 1 M LiPF6 dissolved in a mixed solvent of dimethyl carbonate (DEC) and ethylene carbonate (EC) (DEC:EC = 1:1).

[0214] For each manufactured battery cell, 2.5C CC charging was performed at 25°C. During charging, the SOC (%) at which Li plating occurred was detected in 5% increments through profile analysis. The detection results are presented in Table 3 below.

[0215] No. Electrode adhesion (gf / 20mm) Li precipitation occurrence SOC (%) Example 130 35-40% Example 230 40% or more Example 335 35-40% Example 428 35-40% Comparative Example 125 20-25% Comparative Example 212 35-40% Comparative Example 315 30-35% Comparative Example 410 25-30% Comparative Example 530 20% or less Comparative Example 635 20% or less

[0216] Referring to the results in Table 3, it can be seen that the negative electrodes of Examples 1 to 4 had a controlled adhesive strength with the current collector of 28 or more. At the same time, the SOC for occurrence of Li precipitation was 35% or more. In contrast, the negative electrodes of Comparative Examples 1, 5, and 6 had a satisfactory adhesive strength with the current collector, but the SOC for occurrence of Li precipitation was 25% or less or 20% or less. In addition, the negative electrodes of Comparative Examples 2 to 4 were confirmed to have poor electrode adhesive strength.

[0217] The present invention provides a negative electrode capable of minimizing Li precipitation without impairing negative electrode adhesion, and a lithium secondary battery using the same.

[0218]

[0219] Although the present invention has been described above with reference to preferred embodiments thereof, it will be understood by those skilled in the art or having ordinary knowledge in the art that various modifications and changes to the present invention can be made without departing from the spirit and technical scope of the present invention as set forth in the claims to be described below.

[0220] Therefore, the technical scope of the present invention should not be limited to the contents described in the detailed description of the specification, but should be defined by the patent claims.

[0221]

[0222] [Explanation of symbols]

[0223] 101, 102, 103: Negative current collector

[0224] 111, 112, 113: First cathode active layer

[0225] 121, 122, 123: Second cathode active layer

Claims

1. Negative current collector, A first negative electrode active layer provided on at least one surface of the negative electrode current collector and including a first negative electrode active material and a first CMC (carboxyl methylcellulose) binder having a weight average molecular weight in the range of 2.5 million to 5 million, and A second negative electrode active layer is provided on the first negative electrode active layer and includes a second negative electrode active material and a second CMC binder having a weight average molecular weight in the range of 800,000 to 2,000,000; The above first negative active material has an average alignment index (OI) in the range of 10 to 50 according to the following equation 1, The above second negative electrode active material is a negative electrode for a lithium secondary battery having an average alignment index (OI) in the range of 0.1 to 9 according to the following equation 1: [Formula 1] OI = I 004 / I 110 In equation 1, I 004 represents the area of ​​the peak representing the (0,0,4) crystal plane of the carbon-based negative electrode active material in the X-ray diffraction (XRD) spectroscopy analysis of the negative electrode active layer, I 110 represents the area of ​​the peak representing the (1,1,0) crystal plane of the carbon-based negative electrode active material in X-ray diffraction (XRD) spectroscopy analysis of the negative electrode active layer.

2. In paragraph 1, A negative electrode for a lithium secondary battery, wherein the difference in weight average molecular weight between the first and second CMC binders is more than 1 million.

3. In paragraph 1, The first and second negative electrode active materials include first and second carbon-based negative electrode active materials, respectively. A negative electrode for a lithium secondary battery, wherein the first and second carbon-based negative electrode active materials each independently include at least one of natural graphite and artificial graphite.

4. In paragraph 3, The second cathode active layer further includes a silicon-based cathode active material, The above silicon-based negative electrode active material is silicon (Si), silicon carbide (SiC), and silicon oxide (SiO). q , but, 0.8≤q≤2.5) containing at least one of the following:

5. In paragraph 4, The content of silicon-based negative electrode active material is Based on the combined weight of the negative electrode active materials contained in the first and second negative electrode active layers, A negative electrode for a lithium secondary battery having a range of 1 wt% to 40 wt%.

6. In paragraph 4, The second cathode active layer is It includes a second carbon-based negative electrode active material and a silicon-based negative electrode active material, The average particle diameter (Dc) of the second carbon-based negative electrode active material is 1 ㎛ to 50 ㎛, The average particle diameter (Ds) of the above silicon-based negative electrode active material is 0.1 ㎛ to 10 ㎛, A negative electrode for a lithium secondary battery, wherein the average particle size ratio (Dc / Ds) of the second carbon-based negative electrode active material and the silicon-based negative electrode active material is in the range of 2 to 10.

7. In paragraph 1, Under room temperature and 2.5 C charging conditions, the SOC range where Li precipitation occurs is greater than 35%, A negative electrode for a lithium secondary battery having an average interfacial adhesion between a negative current collector and a first negative active layer of 27 gf / 20 mm or more.

8. An active layer forming step of forming first and second negative electrode active layers on a negative electrode current collector; A magnetic field application step of applying a magnetic field in the range of 1,000 to 10,000 G for 5 to 20 seconds; and It includes the steps of drying and rolling the cathode, The first negative electrode active layer includes a first negative electrode active material and a first CMC (carboxyl methylcellulose) binder having a weight average molecular weight in the range of 2.5 million to 5 million, A method for manufacturing a negative electrode for a lithium secondary battery, wherein the second negative electrode active layer comprises a second negative electrode active material and a second CMC binder having a weight average molecular weight in the range of 800,000 to 2,000,000.

9. In paragraph 8, The active layer formation step is A method for manufacturing a negative electrode for a lithium secondary battery, characterized in that a second negative electrode active layer is formed before a first negative electrode active layer formed on a negative electrode current collector is dried.

10. In paragraph 8, The active layer formation step is A method for manufacturing a negative electrode for a lithium secondary battery, characterized by simultaneously forming first and second negative electrode active layers formed on a negative electrode current collector.

11. In paragraph 8, The magnetic field application step is A method for manufacturing a negative electrode for a lithium secondary battery, the method comprising applying a magnetic field in the range of 2,000 to 5,000 G for 7 to 15 seconds.

12. A lithium secondary battery having an electrode assembly including a positive electrode, a negative electrode according to claim 1, and a separator disposed between the positive electrode and the negative electrode.

13. In paragraph 12, A lithium secondary battery comprising a cathode active layer provided on at least one surface of a cathode current collector and including at least one cathode active material among lithium metal oxides represented by the following chemical formulas 1 and 2: [Chemical Formula 1] Li x [Ni y Co z Mr w M 1 v ]O2 [Chemical Formula 2] LiMn p M 2 1-p O4 In the above chemical formulas 1 and 2, M 1 is at least one element selected from the group consisting of 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 1.0≤x≤1.30, 0.5≤y<1, 0 respectively <z≤0.3, 0<w≤0.3, 0≤v≤0.1이되, y+z+w+v=1이고, M 2 is Ni, Co or Fe, p is 0≤p≤0.

9.

14. In paragraph 12, A lithium secondary battery characterized in that the above lithium secondary battery is a pouch-type battery.

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