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

Abstract

This invention relates to a negative electrode for lithium-ion secondary batteries and a method for manufacturing the same. The ratio of the porosity to the orientation index (OI) of the carbon-based negative electrode active material of the negative electrode meets a predetermined range, thereby shortening the lithium-ion migration path in the negative electrode active layer and significantly reducing the diffusion resistance of lithium ions, resulting in excellent fast-charging performance of the negative electrode. Furthermore, the density of the negative electrode active layer is improved without compromising the shortened lithium-ion migration path, thus resulting in excellent energy density of the negative electrode.

Description

Technical Field

[0001] This invention relates to the negative electrode for lithium secondary batteries and its manufacturing method.

[0002] This application claims priority to Korean Patent Application No. 10-2024-0097421, filed on July 23, 2024, and Korean Patent Application No. 10-2024-0190271, filed on December 18, 2024, the entire disclosure of which is incorporated herein by reference. Background Technology

[0003] Lithium-ion batteries are widely used not only in small devices such as portable electronic devices, but also in medium and large devices such as battery packs for hybrid or electric vehicles and energy storage devices. With increasing concern about environmental issues, the demand for high-capacity batteries has expanded due to the growing market for devices using high-capacity batteries (such as electric vehicles and hybrid electric vehicles), which can replace fossil fuel vehicles, such as gasoline and diesel cars, which are major contributors to air pollution.

[0004] A lithium-ion secondary battery is a power generation device capable of being charged and discharged, featuring a stacked structure of positive electrode / separator / negative electrode. When a lithium-ion secondary battery is charged, a lithium deintercalation reaction is induced at the positive electrode, where lithium in the positive electrode active material is oxidized and released, while a lithium intercalation reaction occurs at the negative electrode, where lithium is reduced and enters the negative electrode active material. Because the deintercalation reaction in the positive electrode active material is faster than the intercalation reaction in the negative electrode active material, performance characteristics such as the charging and discharging speed of a lithium-ion secondary battery are primarily determined by the negative electrode.

[0005] Graphite-containing materials are widely used as negative electrode active materials. Graphite has a layered structure and is formed by stacking multiple layers, in which carbon atoms form a network structure in a planar shape and spread out. During charging, lithium ions penetrate at the edge planes of these graphite layers (the planes where these layers are stacked) and diffuse between these layers, and during discharging, lithium ions are de-intercalated and released from these edge planes. Furthermore, since graphite has a lower resistivity in the planar direction of the layers than in the stacking direction, an electron conduction path is formed by meandering along the planar direction of the layers.

[0006] In conventional lithium-ion secondary batteries using graphite, a technique has been proposed to align the graphite in the negative electrode using magnetic means to improve its charging performance. Specifically, during negative electrode formation, the (002) crystal plane of the graphite is oriented substantially perpendicular to the negative electrode current collector in a magnetic field, and this orientation is fixed. In this case, since the edge plane of the graphite layer faces the positive electrode active layer, lithium-ion insertion and extraction proceed smoothly, and the electron conduction path is shortened, thereby improving the electronic conductivity of the negative electrode. Furthermore, a negative electrode containing graphite can significantly improve the charging speed of the secondary battery.

[0007] However, even when a magnetic field is used to align the planar crystal planes of graphite almost perpendicular to the negative electrode current collector, if the negative electrode active layer is rolled to increase the energy density of the negative electrode, the tilt of the graphite crystal planes relative to the negative electrode current collector will decrease, leading to orientation degradation. In other words, there is a limitation that it is difficult to simultaneously improve the fast charging performance and energy density of the negative electrode using a magnetic field.

[0008] Therefore, there is a need to develop technologies for negative electrodes with excellent fast charging performance and high energy density. Summary of the Invention

[0009] [Technical Issues]

[0010] The purpose of this invention is to provide a negative electrode with excellent fast charging performance and high energy density, and a method for manufacturing the same.

[0011] [Technical Solution]

[0012] The present invention provides a negative electrode comprising a negative electrode current collector and a negative electrode active layer, wherein the negative electrode active layer is disposed on at least one surface of the negative electrode current collector and comprises a carbon-based negative electrode active material.

[0013] The carbon-based anode active material has a total pore volume of 1×10⁻⁶. -5 cm 3 / g to 1×10 -1 cm 3 / g of graphite. For example, the total pore volume is 1×10 -5 cm 3 / g to 1×10 -1 cm 3 Graphite with a density of / g can be simply referred to as "low-expansion graphite".

[0014] The negative electrode active layer satisfies the following ratio, calculated by the following mathematical formula 1, which is between 0.10 and 0.16.

[0015] [Mathematical Expression 1]

[0016] Y / X

[0017] In mathematical formula 1, Y represents the orientation index (OI) of the carbon-based anode active material. X represents the porosity (volume %) of the negative electrode active layer, and X is between 22% and 30%.

[0018] The orientation index (OI) of carbon-based anode active materials is defined by the following mathematical formula 2.

[0019] [Mathematical Expression 2]

[0020] OI=I 004 / I 110

[0021] In mathematical formula 2, I 110 This represents the intensity of the peak on the (110) crystal plane of the carbon-based anode active material during X-ray diffraction (XRD) measurements of the anode active layer. I 004 This represents the intensity of the peak representing the (004) crystal plane of the carbon-based anode active material when performing X-ray diffraction (XRD) measurements on the anode active layer.

[0022] The orientation index (OI) of carbon-based anode active materials can range from 1.5 to 8.0.

[0023] In addition, the porosity (X) of the negative electrode active layer can be in the range of 25% to 29.5% on average.

[0024] Based on the total weight of the carbon-based anode active material, the total pore volume is 1×10⁻⁶. -5 cm 3 / g to 1×10 -1 cm 3 The graphite content per gram can be from 25% to 100% by weight.

[0025] In addition, carbon-based anode active materials may also include one or more of the following: natural graphite, artificial graphite, Kish graphite, pyrolytic carbon, carbon microspheres, mesophase calcined carbon using tar or pitch as raw materials, and graphitized coke.

[0026] In addition, the average thickness of the negative electrode active layer can be from 100 μm to 400 μm.

[0027] In addition, the present invention provides a secondary battery including the above-described negative electrode. The secondary battery is a lithium secondary battery, and can be cylindrical, pouch-shaped, or prismatic.

[0028] Furthermore, the present invention provides a method for manufacturing a negative electrode.

[0029] The method for manufacturing a negative electrode according to the present invention includes: applying a negative electrode slurry containing a carbon-based negative electrode active material to at least one surface of a negative electrode current collector (S1), applying a magnetic field to the applied negative electrode slurry (S2), drying the negative electrode slurry with the applied magnetic field to form a negative electrode active layer (S3), and rolling the formed negative electrode active layer (S4).

[0030] In this invention, the carbon-based anode active material has a total pore volume of 1×10⁻⁶. -5 cm 3 / g to 1×10 -1 cm 3 / g of graphite.

[0031] Based on the total weight of the carbon-based anode active material, the total pore volume is 1×10⁻⁶. -5 cm 3 / g to 1×10 -1 cm 3 The graphite content is 25% to 100% by weight per gram.

[0032] The step of controlling the roll forming of the negative electrode active layer (S4) is such that when the orientation index (OI) of the carbon-based negative electrode active material of the roll forming negative electrode active layer is plotted against the porosity, the average slope value in the porosity range of 25% to 27% is zero or positive.

[0033] The negative electrode active layer satisfies the following mathematical formula 1, where the ratio is between 0.10 and 0.16.

[0034] [Mathematical Expression 1]

[0035] Y / X

[0036] In mathematical formula 1, Y represents the orientation index (OI) of the carbon-based anode active material. X represents the porosity (volume %) of the negative electrode active layer, and X is between 22% and 30%.

[0037] Furthermore, the orientation index (OI) of carbon-based anode active materials is defined by the following mathematical formula 2.

[0038] [Mathematical Expression 2]

[0039] OI=I 004 / I 110

[0040] In mathematical formula 2, I 110 This represents the intensity of the peak on the (110) crystal plane of the carbon-based anode active material during X-ray diffraction (XRD) measurements of the anode active layer. I004 This represents the intensity of the peak representing the (004) crystal plane of the carbon-based anode active material when performing X-ray diffraction (XRD) measurements on the anode active layer.

[0041] The roll compression ratio (volume %) of the negative electrode active layer is greater than 30% and less than 42%.

[0042] [Beneficial Effects]

[0043] The negative electrode of the present invention satisfies the porosity to orientation index (OI) ratio of the carbon-based negative electrode active material within a predetermined range, thereby shortening the lithium ion migration path in the negative electrode active layer and significantly reducing the lithium ion diffusion resistance, resulting in excellent fast-charging performance of the negative electrode. Furthermore, the negative electrode has the advantage of excellent negative electrode energy density because the density of the negative electrode active layer is improved without compromising the shortened lithium ion migration path. Attached Figure Description

[0044] Figure 1 This is a graph showing the relationship between porosity and orientation index (OI) of the carbon-based anode active materials of various embodiments and comparative examples. Detailed Implementation

[0045] This invention can have various modifications and various implementations, and specific implementations will be described in detail below.

[0046] The terms “comprising,” “including,” and “having” as used herein specify the presence of the features, quantities, steps, actions, components, or elements or combinations thereof described in the specification, and should be understood not to preclude the possibility of the presence or addition of one or more other features, quantities, steps, actions, components, elements or combinations thereof.

[0047] Furthermore, in this specification, "average particle size (D)" 50 "The average particle size" refers to the particle size at the 50% cumulative value of the particle size distribution, also known as the median diameter. The average particle size can be measured using methods commonly used in the art. For example, the average particle size can be measured using a particle size analyzer or an analyzer employing laser diffraction dispersion particle size distribution analysis, but is not limited to these methods.

[0048] Furthermore, in this specification, "the carbon-based anode active material has an orientation" means that the ab-axis crystal planes of the carbon-based anode active material constituting the anode active material particles are distributed with a predetermined directionality relative to the surface of the anode current collector. This may differ from the carbon-based anode active material particles themselves arranged in a specific orientation within the anode active layer.

[0049] In addition, "the carbon-based anode active material has a high orientation" can mean that the carbon-based anode active material contained in the anode active layer is arranged at a high frequency relative to the surface of the anode current collector, and in some cases, it can mean that the carbon-based anode active material contained in the anode active layer is arranged at a high angle relative to the surface of the anode current collector.

[0050] Furthermore, "the carbon-based anode active material has a high orientation index" means that the "orientation index (OI)" mentioned in this specification has a large value, and may also mean that the carbon-based anode active material contained in the anode active layer is aligned at a low angle relative to the surface of the anode current collector. Conversely, "the carbon-based anode active material has a low orientation index" means that the "orientation index (OI)" has a small value, and may also mean that the carbon-based anode active material contained in the anode active layer is aligned at a high angle relative to the surface of the anode current collector.

[0051] Furthermore, in this invention, "comprising...as a major component" can refer to a defined component comprising at least 50% by weight (or at least 50% by volume), at least 60% by weight (or at least 60% by volume), at least 70% by weight (or at least 70% by volume), at least 80% by weight (or at least 80% by volume), at least 90% by weight (or at least 90% by volume), or at least 95% by weight (or at least 95% by volume) of the total weight (or volume). For example, "comprising carbon-based anode active material as a major component" can refer to a total of 100% by weight of the anode active layer, anode active material, and / or anode slurry, comprising at least 50% by weight, at least 60% by weight, at least 70% by weight, at least 80% by weight, at least 90% by weight, or at least 95% by weight. In some cases, it can mean that the entire anode active layer, anode active material, and / or anode slurry is composed of carbon-based anode active material, i.e., a content of 100% by weight.

[0052] Furthermore, in this specification, "average slope within the interval" can refer to the value obtained by dividing the specified interval into n equal parts, calculating the slopes of n parts, and taking their average. However, n can be 5 or more, and preferably 5 to 10.

[0053] The invention will be described in more detail below.

[0054] Negative electrode for lithium secondary batteries

[0055] The present invention provides a negative electrode comprising a negative electrode current collector and a negative electrode active layer, wherein the negative electrode active layer is disposed on at least one side of the negative electrode current collector and comprises a carbon-based negative electrode active material.

[0056] The negative electrode of the present invention refers to a negative electrode for lithium secondary batteries. The negative electrode includes a negative electrode active layer containing a carbon-based negative electrode active material on at least one side of the negative electrode current collector. The negative electrode active layer is the layer that realizes the electroactivity of the negative electrode, and is manufactured by applying a negative electrode slurry containing a negative electrode active material (which performs an electrochemical oxidation-reduction reaction during the charging and discharging process of the battery) to at least one side of the negative electrode current collector, and then drying and rolling it.

[0057] The negative electrode active layer comprises a carbon-based negative electrode active material composed of carbon atoms as the main component. The carbon-based negative electrode active material is a negative electrode active material that exhibits electrochemical activity, and may contain graphite-based compounds.

[0058] Specifically, the carbon-based negative electrode active material includes low-expansion graphite. In this invention, "low-expansion graphite" refers to graphite that exhibits low expansion characteristics during secondary battery charging. For example, when manufacturing a secondary battery containing graphite as the negative electrode active material, if the expansion characteristics of the negative electrode active layer remain low even after repeated charge-discharge cycles of the secondary battery, the graphite contained in the negative electrode active layer can be referred to as "low-expansion graphite." In this case, the expansion characteristics of the low-expansion graphite can be determined by the change in the thickness of the negative electrode active layer based on charge-discharge cycles.

[0059] Examples of low-expansion graphite include low-expansion natural graphite, low-expansion synthetic graphite, etc. In this invention, low-expansion graphite can be manufactured by cold isostatic pressing (CIP), in which pressure is uniformly applied in each direction of the particles at low temperature. Low-expansion graphite manufactured by CIP is isotropic graphite, which possesses low electrical resistance, excellent thermal shock resistance, and excellent mechanical properties, thereby improving the lifespan characteristics of the anode itself. Furthermore, low-expansion graphite can refer to natural graphite particles coated with carbon. In this case, the low-expansion graphite has a carbon layer, which not only suppresses graphite expansion during the charging process of the secondary battery, but also significantly reduces impurities generated due to physical damage during the manufacturing process of carbon-based anode active materials and / or during the manufacturing of the anode or battery assembly using it. In addition, low-expansion graphite is characterized by high porosity within the graphite particles. Because the high porosity of graphite particles can improve the ability to control the volume expansion of the negative electrode active material itself, the volume expansion characteristics are small during the charging process of the secondary battery, and the orientation index (OI) of the carbon-based negative electrode active material can remain low even after rolling.

[0060] For example, low-expansion graphite can meet the total pore volume within a predetermined range, and specifically, the average total pore volume can be 1×10⁻⁶. -5 cm 3 / g to 1×10 -1 cm 3 / g. More specifically, the total pore volume of low-expansion graphite can be 5 × 10⁻⁶ g. -4 cm 3 / g to 1×10 -1 cm 3 / g; 1×10 -3 cm 3 / g to 1×10 -1 cm 3 / g; 5×10 -3 cm 3 / g to 1×10 -1 cm 3 / g; 1×10 -3 cm 3 / g to 5×10 -2 cm 3 / g; 1×10 -3 cm 3 / g to 1×10 -2 cm 3 / g; 5×10 -3 cm 3 / g to 5×10 -2 cm 3 / g; or 5×10 -3 cm 3 / g to 2×10 -2 cm 3 / g. The total pore volume of low-expansion graphite can be measured using the BET method with nitrogen (N2) adsorption. By meeting the above range, low-expansion graphite can reduce the diffusion resistance of lithium ions and reduce volume changes during secondary battery charging.

[0061] The negative electrode active layer may contain low-expansion graphite as the main component of the carbon-based negative electrode active material. Specifically, based on the total weight of the carbon-based negative electrode active material, the content of low-expansion graphite may be from more than 25% by weight to less than 100% by weight. For example, based on the total weight of the carbon-based negative electrode active material, the content of low-expansion graphite may be 30% by weight to 100% by weight; 40% by weight to 100% by weight; 50% by weight to 95% by weight; 50% by weight to 90% by weight; 50% by weight to 85% by weight; 50% by weight to 80% by weight; 50% by weight to 75% by weight; 50% by weight to 70% by weight; 60% by weight to 100% by weight; 70% by weight to 100% by weight; 80% by weight to 100% by weight; 90% by weight to 100% by weight; 60% by weight to 95% by weight; 70% by weight to 90% by weight; 80% by weight to 99% by weight; 61% by weight to 99% by weight; or 75% by weight to 85% by weight. By adjusting the content of low-expansion graphite contained in the negative electrode active layer within the above-mentioned range, the present invention can maintain the orientation index (OI) of the carbon-based negative electrode active material contained in the negative electrode active layer at a low level even after rolling.

[0062] To achieve a short lithium-ion migration path and high energy density during secondary battery charging, the negative electrode active layer satisfies the following ratio, calculated by Equation 1, which is between 0.10 and 0.16: [Mathematical Expression 1] Y / X In mathematical formula 1, Y represents the orientation index (OI) of the carbon-based anode active material. X represents the porosity (volume %) of the negative electrode active layer, and X is between 22% and 30%.

[0063] In the above mathematical formula 1, the orientation index (OI) corresponding to the Y value is defined by the following mathematical formula 2 and can satisfy a predetermined range.

[0064] [Mathematical Expression 2]

[0065] OI=I 004 / I 110

[0066] In mathematical formula 2, I 110 This represents the intensity of the peak on the (110) crystal plane of the carbon-based anode active material during X-ray diffraction (XRD) measurements of the anode active layer. I 004 This represents the intensity of the peak representing the (004) crystal plane of the carbon-based anode active material when performing X-ray diffraction (XRD) measurements on the anode active layer.

[0067] Specifically, the orientation index (OI) defined by mathematical formula 2 can be a relative index representing the degree of orientation of the crystal planes of the carbon-based anode active material in a specific direction during X-ray diffraction (XRD) measurements, specifically relative to the degree of orientation of the anode current collector surface. For graphite as a carbon-based anode active material, the anode active layer shows peaks at 2θ = 26.5 ± 0.2°, 42.4 ± 0.2°, 43.4 ± 0.2°, 44.6 ± 0.2°, 54.7 ± 0.2° and 77.5 ± 0.2° during X-ray diffraction measurements, and these peaks represent the (002) plane, (100) plane, (101)R plane, (101)H plane, (004) plane and (110) plane. Furthermore, the peak appearing at 2θ = 43.4 ± 0.2° can be considered as an overlap of the peaks corresponding to the (101)R plane of the carbon-based anode active material and the (111) plane of the current collector (e.g., copper (Cu)).

[0068] The orientation index (OI) of the carbon-based anode active material can be measured by the intensity ratio of the peak at 2θ = 54.7 ± 0.2° representing the (004) plane to the peak at 2θ = 77.5 ± 0.2° representing the (110) plane. Here, the peak at 2θ = 54.7 ± 0.2° represents the peak of the (004) plane, which has an inclination relative to the anode current collector in the graphite crystal plane, and the (004) plane represents the ab-axis crystal plane of the carbon-based anode active material. Therefore, the orientation index (OI) can mean that the closer the value is to 0, the closer the inclination is to 90° relative to the anode current collector surface, and the larger the value, the closer the inclination is to 0° or 180° relative to the anode current collector surface. That is, since the negative electrode active layer of the present invention is arranged such that the carbon-based negative electrode active material has a large angle relative to the negative electrode current collector, specifically, the angle relative to the negative electrode current collector is 60° or more, 70° or more, 70° to 90°, 80° to 90°, 65° to 85° or 70° to 85°, the orientation index (OI) of the carbon-based negative electrode active material can be lower compared with the case where the carbon-based negative electrode active material is arranged at a low angle.

[0069] For example, the orientation index (OI) of the negative electrode active layer, as defined by mathematical formula 2, can be adjusted to 1.5 to 8.0. Specifically, the orientation index (OI) of the negative electrode active layer can be 2.0 to 8.0; 3.0 to 8.0; 4.0 to 8.0; 5.0 to 8.0; 1.5 to 7.0; 1.5 to 6.0; 1.5 to 5.0; 3.0 to 7.0; 2.5 to 5.0; 3.0 to 5.0; 3.5 to 5.0; 4.0 to 5.0; 2.5 to 4.0; 2.5 to 3.5; 2.9 to 3.9; or 3.1 to 4.3.

[0070] Carbon-based anode active materials with ab-axis crystal planes oriented at a high angle relative to the anode current collector surface provide low tortuosity for the anode active layer. This shortens the path that lithium ions take during secondary battery charging, thereby increasing the charging speed.

[0071] Furthermore, the negative electrode active layer can be in a rolled form, thus achieving a high energy density. Typically, when the negative electrode active layer is rolled, even if it comprises carbon-based negative electrode active material with oriented ab-axis crystal planes, the ab-axis crystal planes of the oriented carbon-based negative electrode active material collapse during the rolling process, increasing the orientation index (OI) and further increasing the tortuosity of the negative electrode active layer. In other words, because the rolled negative electrode active layer negates the alignment and / or orientation effects of the carbon-based negative electrode active material due to the rolling process, this effect becomes minimal even if fast charging performance is improved.

[0072] However, the negative electrode active layer of the present invention can improve the energy density and fast charging performance of the negative electrode active layer by minimizing the increase in the orientation index (OI) of the carbon-based negative electrode active material contained in the negative electrode active layer during rolling. As an example, the ratio of the porosity of the negative electrode active layer to the orientation index (OI) of the carbon-based negative electrode active material can be within a predetermined value. For example, the negative electrode active layer satisfies a ratio calculated by Formula 1 of 0.10 to 0.16. Specifically, the ratio of the negative electrode active layer calculated by Formula 1 can be 0.10 to 0.16; 0.10 to 0.15; 0.11 to 0.16; 0.10 to 0.15; or 0.10 to 0.14.

[0073] In order to achieve energy density and excellent fast charging performance, the negative electrode active layer of the present invention may have i) a predetermined porosity range and ii) a composition of carbon-based negative electrode active material.

[0074] Specifically, the orientation index (OI) of the carbon-based negative electrode active material can be affected by the degree of rolling pressure of the negative electrode active layer. Therefore, the negative electrode active layer can satisfy a porosity that indirectly represents a predetermined range of rolling pressure. The average porosity of the negative electrode active layer can be in the range of 22% to 30%. For example, the average porosity of the negative electrode active layer can be in the ranges of 25% to 30%, 22% to 28%, 28% to 30%, or 25% to 29.5%.

[0075] Porosity refers to the porosity of the rolled negative electrode active layer, corresponding to the X value in Equation 1, and can be an indirect indicator of the degree of rolling of the negative electrode active layer. As the density of the negative electrode active layer increases due to rolling, the porosity tends to decrease. There are no particular limitations on the measurement method for porosity, and in this invention, it can be measured, for example, by the BET (Brunauer-Emmett-Teller) method or the mercury porosity method (Hg porosimeter).

[0076] By adjusting the porosity of the negative electrode active layer to the aforementioned range, the present invention can prevent the decrease in the orientation index (OI) of the carbon-based negative electrode active material due to porosity below the lower limit of the aforementioned range, thereby preventing the resulting degradation of fast charging performance. Furthermore, the present invention can solve the problem of maintaining a low energy density of the negative electrode active layer due to high porosity exceeding the upper limit of the aforementioned range by adjusting the porosity to the aforementioned range.

[0077] In addition to low-expansion graphite, carbon-based anode active materials can further include anode active materials with carbon components as the main component. Specifically, carbon-based anode active materials can further include one or more of the following: natural graphite, artificial graphite, Kish graphite, pyrolytic carbon, carbon microspheres, mesophase calcined carbon (bulk mesophase, carbon fibers based on liquid crystal pitch, etc.) using tar or pitch as raw materials, and graphitized coke (coarse coke, green coke, pitch coke, needle coke, petroleum coke, coal coke, etc.).

[0078] Carbon-based anode active materials can take the form of assemblies composed of multiple particles. In this case, a graphite assembly can be formed by aggregating 2 to 100, preferably 3 to 20, graphites.

[0079] Meanwhile, the carbon-based anode active material contained in the anode active layer can meet the average particle size requirement within a predetermined range. Specifically, the carbon-based anode active material can exhibit an average particle size (D) of 0.5 μm to 20 μm. 50 For example, carbon-based anode active materials can exhibit average particle sizes (Di) 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 11 μm to 15 μm. 50 ).

[0080] It is advantageous to make the particle size of the carbon-based anode active material smaller to maximize the disorder of each particle in the expansion direction, thereby preventing particle expansion due to lithium-ion charging. However, when the particle size of the carbon-based anode active material is less than 0.5 μm, a large amount of binder may be required due to the increased number of particles per unit volume. On the other hand, when the average particle size exceeds 20 μm, expansion becomes severe, and the adhesion between particles and between particles and the current collector deteriorates during repeated charging and discharging, which may significantly reduce cycle performance.

[0081] This anode active layer containing carbon-based anode active material can have the following structure: for each region, the arrangement and / or orientation of the carbon-based anode active material is controlled.

[0082] In addition to the carbon-based anode active material as the main component, the anode active layer may optionally include silicon-based anode active material, conductive material, binder, other additives, etc., as needed.

[0083] Silicon-based anode active materials are materials containing silicon (Si) as the main component and capable of increasing the charge / discharge capacity of the anode. Examples of such silicon-based anode active materials include silicon (Si), silicon carbide (SiC), silicon monoxide (SiO), or silicon dioxide (SiO2), and these can be included individually or in combination in the anode active layer. When silicon monoxide (SiO) and silicon dioxide (SiO2) are uniformly mixed or composited as silicon-based anode active materials and included in the anode active layer, they can be represented as silicon oxide (SiO2). q (where 0.8≤q≤2.5).

[0084] Furthermore, silicon-based anode active materials can be doped with Li, Mg, Al, Ca, or Ti, or form alloys with Li, Mg, Al, Ca, or Ti. Additionally, when silicon-based anode active materials contain oxygen (O), they can be surface-treated with a carbon coating or similar material to suppress volume expansion during charging and simultaneously improve the conductivity of the anode active material.

[0085] Furthermore, the content of silicon-based anode active material relative to the total weight of the anode active layer can be from 0.1 wt% to 40 wt%. Specifically, the content of silicon-based anode active material relative to the total weight of the anode active layer can be 0.5 wt% to 20 wt%, 1 wt% to 9 wt%, 5 wt% to 15 wt%, 3 wt% to 7 wt%, 11 wt% to 19 wt%, 13 wt% to 17 wt%, 15 wt% to 20 wt%, 10 wt% to 30 wt%, 20 wt% to 40 wt%, 25 wt% to 35 wt%, 15 wt% to 25 wt%, or 9 wt% to 22 wt%. By adjusting the content ratio of silicon-based anode active material contained in the anode active layer to the above range, the present invention can improve the charge capacity per unit mass while reducing lithium consumption and irreversible capacity loss during the initial charge / discharge period of the secondary battery. Furthermore, since the structural stability of the anode active layer can be improved by minimizing the volume change of the anode active layer during the charge / discharge period of the secondary battery, the lifespan of the secondary battery can be increased.

[0086] Conductive materials may include one or more of the following: carbon black, such as acetylene black, denka black, kejjung black, super-P, furnace black, lamp black and thermal cracking carbon black; graphene; carbon nanotubes; and carbon fibers, but are not limited thereto.

[0087] For example, the negative electrode active layer can contain carbon black, carbon nanotubes, carbon fibers, etc., as conductive materials, either individually or in combination.

[0088] Based on a total of 100 parts by weight of the negative electrode active layer, the content of conductive material can be from 0.1 parts by weight to 10 parts by weight. Specifically, based on a total of 100 parts by weight, the conductive material can be from 0.1 parts by weight to 8 parts by weight, 0.1 parts by weight to 5 parts by weight, 0.1 parts by weight to 3 parts by weight, 2 parts by weight to 6 parts by weight, or 0.5 parts by weight to 2 parts by weight. By controlling the content of conductive material within the above range, the present invention can prevent the decrease in charging capacity caused by the increase in negative electrode resistance due to the low content of conductive material. In addition, the present invention can prevent the following problems: the decrease in charging capacity caused by the decrease in the content of negative electrode active material due to the excessive amount of conductive material exceeding the above range, or the increase in resistance caused by the increase in the loading of the negative electrode active layer.

[0089] Adhesives are components that facilitate the adhesion of negative electrode active materials, conductive materials, etc., and to current collectors, and can be appropriately applied within a range that does not degrade the electrical properties of the negative electrode. For example, adhesives may include: polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride (PVdF), polyacrylonitrile, polymethyl methacrylate, 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, or any one or more thereof.

[0090] Based on a total of 100 parts by weight of the negative electrode active layer, the content of the adhesive can be from 0.1 parts by weight to 10 parts by weight. Specifically, based on a total of 100 parts by weight of the negative electrode active layer, the adhesive can be from 0.1 parts by weight to 8 parts by weight, 0.1 parts by weight to 5 parts by weight, 0.1 parts by weight to 3 parts by weight, or 2 parts by weight to 6 parts by weight. By controlling the content of the adhesive contained in the negative electrode active layer within the above range, the present invention can prevent the adhesion of the active layer from decreasing due to a low adhesive content, or prevent the electrical properties of the negative electrode from decreasing due to an excessive amount of adhesive.

[0091] To achieve high charge / discharge capacity and fast charging speed, the negative electrode active layer can have a predetermined average thickness. As the loading of the electrochemically active negative electrode material increases, the negative electrode active layer exhibits a larger charge / discharge capacity. However, in this case, it is difficult to orient the ab-axis crystal plane of the carbon-based negative electrode active material to a predetermined tilt relative to the negative electrode current collector during the fabrication of the negative electrode active layer, thus limiting the low fast-charging performance of the fabricated negative electrode. Therefore, the present invention enables the average thickness of the negative electrode active layer to be adjusted to a predetermined range. Specifically, the average thickness of the negative electrode active layer can be from 100 μm to 400 μm. For example, the average thickness of the negative electrode active layer can be 100 μm to 350 μm; 100 μm to 300 μm; 100 μm to 250 μm; 100 μm to 200 μm; 150 μm to 400 μm; 200 μm to 400 μm; 150 μm to 300 μm; 150 μm to 250 μm; or 150 μm to 220 μm. By adjusting the average thickness of the negative electrode active layer to the above range, the present invention can prevent the effect of improving fast charging performance from becoming minimal because the orientation index (OI) of the carbon-based negative electrode active material becomes high when the average thickness of the negative electrode active layer exceeds the upper limit of the above range. In addition, by adjusting the average thickness of the negative electrode active layer to the above range, the charge / discharge capacity and energy density of the negative electrode can be prevented from decreasing due to the thickness being less than the lower limit of the above range.

[0092] There are no particular restrictions on the negative electrode current collector, as long as it has high conductivity and does not cause chemical changes in the battery. For example, copper, stainless steel, nickel, titanium, calcined carbon, etc., can be used. In the case of copper or stainless steel, materials with surface treatments such as carbon, nickel, titanium, and silver can also be used. Furthermore, considering the conductivity and total thickness of the manufactured negative electrode, the average thickness of the negative electrode current collector can be appropriately applied in the range of 1 μm to 500 μm.

[0093] Because the negative electrode for lithium secondary batteries of the present invention has the above-described structure, the lithium ion movement path in the negative electrode active layer is shortened, and the diffusion resistance of lithium ions is significantly low, resulting in excellent fast-charging performance of the negative electrode. Furthermore, the negative electrode has the advantage of excellent negative electrode energy density because the density of the negative electrode active layer is improved without compromising the shortened lithium ion movement path.

[0094] Lithium secondary batteries

[0095] Furthermore, the present invention provides a lithium secondary battery comprising: An electrode assembly includes a positive electrode, a negative electrode as described above, and a diaphragm disposed between the positive and negative electrodes.

[0096] The lithium secondary battery of the present invention includes an electrode assembly having a structure in which multiple positive electrodes and multiple negative electrodes are arranged alternately and a separator is disposed between them. The lithium secondary battery includes the negative electrode of the present invention as described above, and therefore has the advantages of excellent fast charging performance and high energy density due to improved lithium-ion diffusion capability.

[0097] Since the negative electrode has the same structure as described above, a detailed description of it will be omitted.

[0098] In addition, the positive electrode includes a positive electrode active layer, which includes a positive electrode active material located on the positive electrode current collector, and optionally, as needed, the positive electrode active layer may further include conductive materials, binders, other additives, etc.

[0099] The positive electrode active material is a material capable of inducing an electrochemical reaction on the positive electrode current collector, and may include one or more lithium metal oxides represented by the following chemical formulas 1 and 2, which are capable of reversibly inserting and deintercalating lithium ions: [Chemical Formula 1] Li l [Ni m Co n Mn w M 1 v O2 [Chemical Formula 2] LiM2 p Mn q P r O4 In the above chemical formulas 1 and 2, where M 1 is one or more elements selected from W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo, l, m, n, w, and v are respectively 1.0 ≤ l ≤ 1.30, 0.5 ≤ m < 1, 0 < n ≤ 0.3, 0 < w ≤ 0.3, 0 ≤ v ≤ 0.1, where m + n + w + v = 1, where M 2 is Ni, Co, or Fe, where p is 0.05 ≤ p ≤ 1.0, where q is 2 - p, and where r is 0 or 1.

[0100] The lithium metal oxides represented by the above chemical formulas 1 and 2 are materials containing high contents of nickel (Ni) and manganese (Mn), respectively, and when used as a positive electrode active material, they have the advantage of being able to stably supply high-capacity and / or high-voltage electricity compared to the commonly used positive electrode active materials (such as lithium iron phosphate (LiFePO4)).

[0101] In this case, the lithium metal oxide represented by chemical formula 1 may 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<000​​​​​​​​​​​​O4, LiNi 0.3 Mn 1.7 O4, etc., and these can be used alone or in combination.

[0102] Furthermore, based on 100 parts by weight of the entire positive electrode active layer, the content of the positive electrode active material can be 85 parts by weight or more. Specifically, based on 100 parts by weight of the entire positive electrode active layer, the content of the positive electrode active material can be 90 parts by weight or more, 93 parts by weight or more, or 95 parts by weight or more.

[0103] In addition to the positive electrode active material, the positive electrode active layer may also include conductive materials, adhesives, and other additives.

[0104] The conductive material used to improve the electrical properties of the positive electrode can be a material conventionally used in the art. Specifically, the conductive material can include one or more of the following: natural graphite; artificial graphite; carbon black, such as acetylene black, Denka black, Ketjen black, Super-P, furnace black, lamp black, thermally cracked carbon black, graphene, and carbon nanotubes.

[0105] Furthermore, the content of conductive material relative to 100 parts by weight of the positive electrode active layer can be from 0.1 parts by weight to 5 parts by weight. Specifically, the content of conductive material relative to 100 parts by weight of each positive electrode active layer can be from 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.

[0106] The adhesive serves to bond the positive electrode active material and the conductive material together, and can be used without restriction as long as it possesses this function. Specifically, the adhesive may include one or more of the following resins: vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride (PVdF), polyacrylonitrile, polymethyl methacrylate, and copolymers thereof. For example, the adhesive may include polyvinylidene fluoride.

[0107] In addition, the content of the binder may be 1 to 10 parts by weight relative to 100 parts by weight of the positive electrode active layer. Specifically, the content of the binder may be 2 to 8 parts by weight, or 1 to 5 parts by weight, relative to 100 parts by weight of the positive electrode active layer.

[0108] There is no particular limitation on the total thickness of the positive electrode active layer, but it can specifically be from 50 μm to 300 μm. More specifically, it can be from 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.

[0109] Furthermore, as the positive electrode current collector, a positive electrode current collector with high conductivity that does not cause chemical changes in the battery can be used. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, etc., can be used, and in the case of aluminum or stainless steel, aluminum or stainless steel that has been surface-treated with carbon, nickel, titanium, silver, etc., can also be used. In addition, considering the conductivity and total thickness of the positive electrode to be manufactured, the average thickness of the current collector can be appropriately adopted from 3 μm to 500 μm.

[0110] The separator disposed between the positive and negative electrodes of a lithium secondary battery is an insulating film with high ion permeability and mechanical strength, and is not particularly limited, as long as it is conventionally used in the art. Specifically, a separator comprising one or more polymers can be used, said polymers being selected from: chemically resistant and hydrophobic polypropylene, polyethylene, and polyethylene-propylene copolymers. The separator can be in the form of a porous polymer substrate, such as a sheet or nonwoven fabric containing the above-mentioned polymers. In some cases, it can be in the form of a composite separator in which organic or inorganic particles are coated onto a porous polymer substrate by means of an organic adhesive. Furthermore, the average pore size of the separator can be from 0.01 to 10 μm, and the average thickness can be from 5 to 300 μm.

[0111] Meanwhile, the lithium secondary battery of the present invention is not particularly limited and can be a secondary battery that may include stacked, folded, or fold-stacked electrode assemblies. For example, the lithium secondary battery of the present invention can be a pouch-type secondary battery or a prismatic secondary battery.

[0112] Pouch-type and / or prismatic secondary batteries have the advantage of high availability in terms of energy density because high-density secondary battery cells can be packed into a limited space.

[0113] Manufacturing method of negative electrode for lithium secondary batteries

[0114] Furthermore, the present invention provides a method for manufacturing the negative electrode for a lithium secondary battery as described above.

[0115] The method for manufacturing a negative electrode for a lithium secondary battery according to the present invention includes: applying a negative electrode slurry containing a carbon-based negative electrode active material to at least one surface of a negative electrode current collector (S1), applying a magnetic field to the applied negative electrode slurry (S2), drying the negative electrode slurry with the applied magnetic field to form a negative electrode active layer (S3), and rolling the formed negative electrode active layer (S4).

[0116] Step (S1) refers to the process of releasing and coating a negative electrode slurry containing a carbon-based negative electrode active material onto at least one side of a moving negative electrode current collector.

[0117] Step (S1) can be performed without particular limitations, as long as it is a method conventionally used in the art; however, it is preferable to use a die-casting method. The die-casting method can be performed using a slit die with a gasket for controlling the release conditions of the negative electrode slurry. By controlling the shape, position, etc., of the gasket, the slit die can easily control the loading amount, coating thickness, etc., of the negative electrode slurry applied to the negative electrode current collector.

[0118] The negative electrode slurry includes a carbon-based negative electrode active material as the main component, and the carbon-based negative electrode active material includes "low-expansion graphite". In this invention, "low-expansion graphite" refers to graphite that has low expansion characteristics during secondary battery charging. The expansion characteristics of low-expansion graphite can be determined by the change in the thickness of the negative electrode active layer according to charge-discharge cycles.

[0119] Examples of low-expansion graphite include low-expansion natural graphite and low-expansion synthetic graphite. In this invention, low-expansion graphite can be manufactured by cold isostatic pressing (CIP), in which plate-like natural graphite is spheroidized and then pressure is uniformly applied to each direction of the particles at low temperature. Low-expansion natural graphite manufactured by cold isostatic pressing can be isotropic graphite. Because isotropic graphite has low electrical resistance, excellent thermal shock resistance, and excellent mechanical properties, it can improve the lifetime characteristics of the negative electrode itself.

[0120] Low-expansion graphite refers to natural graphite particles coated with carbon. Due to its carbon layer, low-expansion graphite not only suppresses graphite expansion during the charging process of secondary batteries, but also significantly reduces impurities generated by physical damage during the manufacturing of carbon-based anode active materials and / or the manufacturing of anodes using it or battery assembly. Furthermore, low-expansion graphite is characterized by high porosity within the graphite particles. Because the high porosity of the graphite particles improves the ability to control the volume expansion of the anode active material itself, the volume expansion characteristics are small during the charging process of secondary batteries, and the orientation index (OI) of the carbon-based anode active material remains low even after rolling.

[0121] For a more detailed description of low-expansion graphite, please refer to the description above.

[0122] By adjusting the content of low-expansion graphite contained in the negative electrode slurry within the above-mentioned range, the present invention is able to maintain the orientation index (OI) of the carbon-based negative electrode active material contained in the negative electrode active layer at a low level, even after rolling.

[0123] The negative electrode slurry may also contain carbon-based negative electrode active materials other than low-expansion graphite, and in addition to the negative electrode active material, the negative electrode slurry may also contain conductive materials, binders, and additives. Since the components contained in the negative electrode slurry are the same as those in the negative electrode active layer of the negative electrode for lithium secondary batteries described above, a detailed description of it is omitted.

[0124] Meanwhile, after step (S1), the orientation of the carbon-based anode active material can be induced by applying a magnetic field.

[0125] Specifically, step (S2) refers to the process of aligning and / or oriented the ab-axis crystal planes of the carbon-based anode active material contained in the anode slurry by applying a magnetic field to the anode slurry.

[0126] At this point, the arrangement and / or orientation of the ab-axis crystal planes of the carbon-based anode active material contained in the anode slurry can vary depending on the conditions of the applied magnetic field and the specific intensity of the applied geomagnetic field.

[0127] Here, the strength of the magnetic field applied to the negative electrode slurry can be from 1000 G to 9000 G (Gauss) to enhance the ab-axis crystal plane alignment and / or orientation of the carbon-based negative electrode active material. Specifically, the intensity of the magnetic field applied to the negative electrode slurry can be 1,000 G to 8,000 G; 2,000 G to 8,000 G; 3,000 G to 8,000 G; 5,000 G to 9,000 G; 5,000 G to 7,500 G; 5,500 G to 6,500 G; 1,000 G to 7,000 G; 2,000 G to 6,000 G; 1,500 G to 5,000 G; 1,500 G to 4,500 G; 4,000 G to 8,000 G; 4,500 G to 7,000 G; 3,000 G to 6,500 G; or 3,500 G to 6,500 G.

[0128] Therefore, carbon-based anode active materials can be oriented such that the ab-axis crystal plane has a predetermined tilt relative to the surface of the anode current collector. This can be indirectly confirmed by X-ray diffraction (XRD) analysis of the anode active layer formed after drying the anode slurry containing the carbon-based anode active material.

[0129] Next, step (S3) refers to the process of forming the negative electrode active layer by drying the negative electrode slurry to which a magnetic field has been applied.

[0130] At this point, drying can be carried out without particular restrictions, as long as it is a method conventionally used in the field. For example, drying can be performed by applying heat energy to the negative electrode slurry using a hot air dryer, vacuum oven, or the like.

[0131] Next, step (S4) refers to the process of rolling the dried negative electrode active layer. Rolling refers to the process of increasing the density of the entire negative electrode active layer by applying pressure to the surface of the formed negative electrode active layer using a roller press or the like.

[0132] By using rolling equipment such as a roll press to roll to the target thickness and target porosity under linear pressure conditions and then vacuum drying, a negative electrode with a final negative electrode active material layer formed on the current collector can be manufactured.

[0133] For example, rolling can be performed under the following conditions: the target thickness (i.e., the average thickness of the negative electrode active layer) is 100 μm to 400 μm and the target porosity (i.e., the porosity) is 21% to 30%.

[0134] Specifically, rolling can be performed even under heating conditions, but in terms of suppressing changes in the physical properties of the electrodes, rolling without heating is more suitable. For example, rolling can be performed at temperatures ranging from 18°C ​​to 35°C, and more specifically, at temperatures ranging from 18°C ​​to 30°C, 18°C ​​to 23°C, 22°C to 25°C, 20°C to 25°C, 20°C, or 25°C.

[0135] Furthermore, the rolling can be performed at rolling speeds ranging from 2 m / s to 7 m / s, and more specifically, at the following rolling speeds: 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.

[0136] In addition, the rolling can be carried out under pressure conditions of 50 MPa to 200 MPa. Specifically, the rolling can be carried out under the following pressure conditions: 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.

[0137] In addition, vacuum drying conditions can be implemented for 1 to 15 hours, for example, in the range of 150°C to 250°C, 160°C to 200°C, or 175°C to 185°C.

[0138] By forming the negative electrode active layer through rolling under the above-mentioned temperature, speed and / or pressure conditions, the present invention can maximize the energy density of the negative electrode active layer while minimizing the reduction of the orientation index (OI) of the carbon-based negative electrode active material of the negative electrode active layer.

[0139] Typically, when the negative electrode active layer is rolled, even if the negative electrode active layer includes carbon-based negative electrode active material with oriented ab-axis crystal planes, the ab-axis crystal planes of the aligned carbon-based negative electrode active material will collapse during the rolling process, causing an increase in the orientation index (OI) and a further increase in the tortuosity of the negative electrode active layer. In other words, because the rolled negative electrode active layer negates the alignment and / or orientation effects of the carbon-based negative electrode active material due to the rolling process, even if fast charging performance is improved, this effect becomes minimal.

[0140] However, the method for manufacturing the negative electrode of the present invention can improve the energy density and fast charging performance of the negative electrode active layer by minimizing the increase in orientation index (OI) during the rolling process of the dried negative electrode active layer. For example, when the orientation index (OI) of the carbon-based negative electrode active material of the negative electrode active layer rolled in step (S4) is plotted against porosity, the average slope value can be zero or positive in the porosity range of 25% to 27%. For example, when plotting the orientation index (OI) of a carbon-based anode active material relative to porosity, the average slope of the anode active layer in the porosity range of 25% to 27% can be above 0 and below 0.5; above 0 and below 0.4; above 0 and below 0.3; above 0 and below 0.2; above 0 and below 0.15; above 0 and below 0.11; above 0.05 and below 0.15; above 0.05 and below 0.2; above 0.1 and below 0.3; above 0.2 and below 0.4; above 0.25 and below 0.5; or above 0.01 and below 0.19.

[0141] When the orientation index (OI) of the carbon-based anode active material is plotted against porosity, an average slope of 0 or higher in the porosity range of 25% to 27% indicates that even with increased roll density in this range, the orientation index (OI) of the carbon-based anode active material does not increase; instead, it remains constant or decreases with increasing roll density. This demonstrates that even with increased roll density due to roll pressure, a low orientation index (OI) is achieved for the carbon-based anode active material within this range. This implies that the anode active layer of the present invention exhibits high energy density and excellent fast-charging performance.

[0142] Conversely, when plotting the orientation index (OI) of carbon-based anode active materials relative to porosity, if the average slope is less than 0 in the porosity range of 25% to 27%, the tortuosity increases during rolling, potentially leading to a decrease in fast-charging performance. Furthermore, if the average slope of the anode active layer in the porosity range exceeds the aforementioned upper limit, there is a limitation in the deterioration of contact characteristics at the interface between the anode active layer and the anode current collector, resulting in reduced durability.

[0143] Furthermore, the negative electrode active layer rolled in this manner satisfies the following ratio calculated by the following mathematical formula 1: 0.10 to 0.16.

[0144] [Mathematical Expression 1]

[0145] Y / X

[0146] In mathematical formula 1, Y represents the orientation index (OI) of the carbon-based anode active material. X represents the porosity (volume %) of the negative electrode active layer, and here X is 22% to 30%.

[0147] The more detailed description of mathematical formula 1 is the same as that described above.

[0148] Furthermore, the orientation index (OI) of carbon-based anode active materials is defined by the following mathematical formula 2.

[0149] [Mathematical Expression 2]

[0150] OI=I 004 / I 110

[0151] In mathematical formula 2, I 110 This represents the intensity of the peak on the (110) crystal plane of the carbon-based anode active material during X-ray diffraction (XRD) measurements of the anode active layer. I 004 This represents the intensity of the peak representing the (004) crystal plane of the carbon-based anode active material when performing X-ray diffraction (XRD) measurements on the anode active layer.

[0152] The more detailed description of mathematical formula 2 is the same as that mentioned above.

[0153] The negative electrode active layer of the negative electrode manufactured according to the present invention can improve the energy density and fast charging performance of the negative electrode active layer by adjusting the ratio calculated by the above mathematical formula 1. Furthermore, in order to achieve energy density and excellent fast charging performance, the negative electrode active layer can have a predetermined roll ratio. The roll ratio is a value obtained by dividing the thickness deviation before and after roll forming by the thickness before roll forming, representing the proportion of the negative electrode active layer thickness reduced due to roll forming. Unlike porosity, the roll ratio tends to increase with increasing density of the negative electrode active layer under roll forming. The average roll ratio (volume %) of the negative electrode active layer of the present invention can be in the range of 30% to 42%. Specifically, the average roll ratio of the negative electrode active layer can be in the range of 30% to 40%, 32% to 40%, or 30% to 39%.

[0154] By ensuring that the rolling pressure ratio of the negative electrode active layer meets the aforementioned range, the present invention can prevent the energy density of the negative electrode active layer from decreasing due to the rolling pressure ratio falling below the lower limit of the aforementioned range. Furthermore, by adjusting the rolling pressure ratio to the aforementioned range, the present invention can prevent the orientation index (OI) of the carbon-based negative electrode active material from decreasing due to a high rolling pressure ratio exceeding the upper limit of the aforementioned range, thereby preventing the resulting degradation of fast charging performance.

[0155] The method for manufacturing a negative electrode for lithium secondary batteries according to the present invention has the above-described configuration, thereby producing a negative electrode with low tortuosity of the negative electrode active layer due to the application of a magnetic field and improved energy density due to rolling.

[0156] The invention will be described in more detail below with reference to examples and comparative examples.

[0157] However, the following embodiments and comparative examples are merely illustrative of the present invention, and the content of the present invention is not limited to the following embodiments and comparative examples.

[0158] Examples 1 to 3 and Comparative Examples 1 to 4. Manufacturing of negative electrodes for lithium secondary batteries

[0159] Prepare natural graphite (average particle size (D) 50 ): Approximately 11 μm to 13 μm), artificial graphite (average particle size (D 50 (Approximately 15 μm to 16 μm) and low-expansion natural graphite (average particle size (D) 50 ): Approximately 12 μm to 16 μm, total pore volume: approximately 0.006 cm³ 3 / g~0.012 cm 3 / g) was used as the carbon-based negative electrode active material. Simultaneously, styrene-butadiene rubber (SBR) was prepared as a binder, carboxymethyl cellulose (CMC) as a thickener, and carbon black (Super P) as a conductive material.

[0160] Then, as shown in Table 1 below, (1) a carbon-based negative electrode active material was prepared, and a negative electrode slurry was prepared by mixing 96.85% by weight of the prepared carbon-based negative electrode active material, 1.05% by weight of carboxymethyl cellulose (CMC), 1.6% by weight of styrene-butadiene rubber (SBR), and 0.5% by weight of carbon black with water to make the solid content 50%. At this time, the viscosity of the prepared negative electrode slurry at room temperature (22±3°C) was 7000 cps to 12000 cps.

[0161] Using a die-coating machine, the prepared negative electrode slurry was applied (S1) onto a copper foil (thickness: 6 μm) conveyed in a roller-to-roll manner (conveyor speed: 6 m / min). Then, a magnetic field with an intensity of 8050±50 G (Gauss) was applied (S2) from the bottom of the applied negative electrode slurry for 2 to 11 seconds. (2) The application of the magnetic field was controlled as shown in Table 1 below.

[0162] The negative electrode slurry with applied magnetic field was dried with hot air to form a negative electrode active layer on the negative electrode current collector. The thickness of the formed negative electrode active layer was measured to calculate the average thickness, and the negative electrode current collector with the negative electrode active layer was cut into multiple pieces of the same width. Each cut negative electrode current collector was rolled with a roller press to achieve a porosity of 21% to 30% to manufacture a negative electrode for lithium secondary batteries. The average thickness of each manufactured negative electrode was measured, and the average thickness of each negative electrode was confirmed to be 170 μm to 205 μm. (3) The rolling ratio was calculated from the average thickness of the negative electrode active layer measured before and after rolling. In addition, (4) the porosity of the negative electrode active layer was calculated using the average thickness of the negative electrode active layer measured above. In addition, X-ray diffraction (XRD) analysis was performed on the negative electrode active layer of the manufactured negative electrode to measure the X-ray diffraction of the carbon-based negative electrode active material contained in the negative electrode active layer. The measurement conditions were as follows: - Target: Cu (Kα-ray) graphite monochromator - Slits: Diverging slit = 1°, Receiving slit = 0.1 mm, Scattering slit = 1° - Measurement area: (110) plane: 76.5° < 2θ < 78.5° / (004) plane: 53.5° < 2θ < 56.0° (5) Using the following mathematical formula 2, calculate the orientation index (OI) of the carbon-based anode active material from the measured spectrum: [Mathematical Expression 2] OI=I 004 / I 110 In mathematical formula 2, I 110This represents the intensity of the peak on the (110) crystal plane of the carbon-based anode active material during X-ray diffraction (XRD) measurements of the anode active layer. I 004 This represents the intensity of the peak representing the (004) crystal plane of the carbon-based anode active material when performing X-ray diffraction (XRD) measurements on the anode active layer.

[0163] The orientation index (OI) of the above-mentioned carbon-based anode active material was plotted against the calculated porosity of the anode active layer, and (6) the average slope in the porosity range of 25% to 27% was calculated. All the measurement and calculation results are shown in Table 1 below.

[0164] [Table 1]

[0165] also, Figure 1 A graph showing the relationship between porosity and orientation index (OI) of the carbon-based anode active material in the above embodiments and comparative examples is presented. Figure 1 The calculation results for Examples 1 to 3 and Comparative Examples 1 to 3 are shown in the figure. Furthermore, Figure 1 The cases with porosities of 21% and 33% are also shown.

[0166] Examples 4 to 12 and Comparative Examples 8 to 16. Manufacturing of lithium secondary batteries

[0167] Preparation of LiNi with a particle size of 5 μm 0.7 Co 0.1 Mn 0.1 Al 0.1 O2 is used as the positive electrode active material. It is mixed with polyvinylidene fluoride and carbon-based conductive agent in N-methylpyrrolidone (NMP) in a weight ratio of 94:3:3 to form a slurry. The slurry is cast onto aluminum foil, dried in a vacuum oven at 120°C, and then rolled to manufacture the positive electrode.

[0168] An electrode assembly was manufactured using the positive electrode, the negative electrode manufactured in Examples 1 to 3 and Comparative Examples 1 to 4, an LTO electrode, and a separator. The separator used was made of 14 μm polypropylene. The LTO electrode was manufactured by coating LTO onto copper (C9) wires. The electrode assembly was inserted into a housing, and then an electrolyte composition was injected to manufacture a lithium secondary battery.

[0169] At this point, the type of negative electrode used in each lithium secondary battery is shown in Table 2 below.

[0170] [Table 2]

[0171] Experimental Example

[0172] The following experiments were conducted to evaluate the physical properties and performance of the negative electrode manufactured according to the present invention.

[0173] 1) Evaluation of fast charging performance

[0174] The fast-charging performance of the lithium-ion secondary batteries manufactured in Examples 4 to 12 and Comparative Examples 5 to 16 was evaluated. Specifically, charging and discharging were performed between the positive electrode and LTO to charge the LTO to SOC 50% to match a potential of approximately 1.53 V. It was found that the LTO voltage curve maintained 1.53 V in most of the SOC region. Furthermore, the voltage curve of the negative electrode potential was confirmed when the voltage between the positive electrode and LTO and between the positive electrode and the negative electrode was measured using an EC-lab charge / discharger. The portion of the negative electrode curve that shows a plateau during charging was determined as the depth of charge (DTC). The DTC was confirmed at each C-rate as the C-rate was increased from 0.5C to 3C in 0.5C increments, and then the required charging time was measured. The measured charging times are shown in Table 3 below.

[0175] 2) Measurement of energy density

[0176] The load per unit area of ​​the negative electrode for lithium secondary batteries manufactured in Examples 1 to 3 and Comparative Examples 1 to 4 was measured.

[0177] Subsequently, the energy density of the large-scale secondary battery cell was calculated using the porosity measured above. The large-scale secondary battery cell was then designed to meet a discharge capacity of 40 Ah at 1 / 3C and have dimensions of 99.7 mm × 301.5 mm × 8.2 mm. The results are shown in Table 3 below.

[0178] [Table 3]

[0179] As shown in Table 3 above, it can be seen that the negative electrode of the present invention simultaneously possesses excellent fast-charging performance and high energy density. Specifically, it was found that as the porosity of the lithium secondary battery manufactured in the examples decreased from 29.5% to 25% due to rolling pressure, the energy density increased from approximately 586 Wh / L to approximately 604.3 Wh / L, an increase of approximately 1.3%. Furthermore, it was confirmed that the time required to charge the lithium secondary battery manufactured in the examples from 10% state of charge (SOC) to 80% was less than 30 minutes. In particular, it was found that even when the porosity of the lithium secondary battery decreased from 27% to 25%, the phenomenon of delayed charging time was suppressed.

[0180] These results mean that when the negative electrode of the present invention comprises oriented low-expansion graphite as a carbon-based negative electrode active material and the ratio of the orientation index (OI) of the carbon-based negative electrode active material to the porosity of the negative electrode active layer meets a predetermined range, not only is the energy density of the negative electrode increased, but the increase in the orientation index (OI) of the carbon-based negative electrode active material is also improved.

[0181] Therefore, it can be seen that the negative electrode of the present invention has excellent fast charging performance and energy density.

[0182] Although the invention has been described with reference to preferred embodiments thereof, those skilled in the art will understand that various modifications and variations may be made therein without departing from the scope of the invention as defined by the appended claims.

[0183] Therefore, the scope of this invention should not be limited by the detailed description in the specification, but should be determined by the appended claims.

Claims

1. A negative electrode comprising: A negative electrode current collector and a negative electrode active layer, wherein the negative electrode active layer is disposed on at least one surface of the negative electrode current collector and comprises a carbon-based negative electrode active material. The carbon-based anode active material has a total pore volume of 1×10⁻⁶. -5 cm 3 / g to 1×10 -1 cm 3 / g of graphite, Wherein, the negative electrode active layer satisfies the following condition: the ratio calculated by the following mathematical formula 1 is between 0.10 and 0.

16. [Mathematical Expression 1] Y / X In mathematical formula 1, Y represents the orientation index (OI) of the carbon-based anode active material. X represents the porosity of the negative electrode active layer in volume percentage, and X is between 22% and 30%. The orientation index (OI) of the carbon-based anode active material is defined by the following mathematical formula 2. [Mathematical Expression 2] OI=I 004 / I 110 In mathematical formula 2, I 110 This represents the intensity of the peak on the (110) crystal plane of the carbon-based anode active material during X-ray diffraction (XRD) measurements of the anode active layer. I 004 This represents the intensity of the peak representing the (004) crystal plane of the carbon-based anode active material when performing X-ray diffraction (XRD) measurements on the anode active layer.

2. The negative electrode as described in claim 1, wherein, The orientation index (OI) of the carbon-based anode active material is in the range of 1.5 to 8.0 on average.

3. The negative electrode as described in claim 1, wherein, The porosity (X) of the negative electrode active layer is in the range of 25% to 29.5% on average.

4. The negative electrode as described in claim 1, wherein, Based on the total weight of the carbon-based anode active material, the total pore volume is 1×10⁻⁶. -5 cm 3 / g to 1×10 -1 cm 3 The graphite content is from 25% to 100% by weight per g.

5. The negative electrode as described in claim 1, wherein, The carbon-based anode active material also includes one or more of the following: natural graphite, artificial graphite, Kish graphite, pyrolytic carbon, carbon microspheres, mesophase calcined carbon using tar or pitch as raw material, and graphitized coke.

6. The negative electrode as described in claim 1, wherein, The average thickness of the negative electrode active layer is 100 μm to 400 μm.

7. A secondary battery comprising the negative electrode as described in claim 1.

8. A method for manufacturing a negative electrode, comprising: A negative electrode slurry containing a carbon-based negative electrode active material is applied to at least one surface of the negative electrode current collector (S1). A magnetic field (S2) is applied to the negative electrode slurry. The negative electrode slurry, to which a magnetic field was applied, was dried to form the negative electrode active layer (S3), and The negative electrode active layer (S4) formed by rolling is The carbon-based anode active material has a total pore volume of 1×10⁻⁶. -5 cm 3 / g to 1×10 -1 cm 3 / g of graphite; Based on the total weight of the carbon-based anode active material, the total pore volume is 1×10⁻⁶. -5 cm 3 / g to 1×10 -1 cm 3 The graphite content is from 25% to 100% by weight per g.

9. The method for manufacturing a negative electrode as described in claim 8, wherein, The step (S4) of controlling the roll-pressed negative electrode active layer is such that when the orientation index (OI) of the carbon-based negative electrode active material of the roll-pressed negative electrode active layer is plotted against the porosity, the average slope value is zero or positive in the range of 25% to 27% porosity.

10. The method for manufacturing a negative electrode as described in claim 8, wherein, The negative electrode active layer satisfies the following mathematical formula 1, where the ratio is between 0.10 and 0.

16. [Mathematical Expression 1] Y / X In mathematical formula 1, Y represents the orientation index (OI) of the carbon-based anode active material. X represents the porosity of the negative electrode active layer in volume percentage, and X is between 22% and 30%. The orientation index (OI) of the carbon-based anode active material is defined by the following mathematical formula 2. [Mathematical Expression 2] OI=I 004 / I 110 In mathematical formula 2, I 110 This represents the intensity of the peak on the (110) crystal plane of the carbon-based anode active material during X-ray diffraction (XRD) measurements of the anode active layer. I 004 This represents the intensity of the peak representing the (004) crystal plane of the carbon-based anode active material when performing X-ray diffraction (XRD) measurements on the anode active layer.

11. The method for manufacturing a negative electrode as described in claim 8, wherein, The roll compression ratio of the negative electrode active layer is greater than 30% and less than 42% by volume.

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