Negative electrode for lithium secondary battery and method for producing same

Abstract

The present invention relates to a negative electrode for a lithium secondary battery and a method for manufacturing the same. The negative electrode includes a negative electrode active layer having a bilayer structure in which the average aspect ratio of a carbon-based negative electrode active material included in each layer and the optical index (O.I.) with respect to a negative electrode current collector are controlled so as to satisfy predetermined ranges, thereby imparting high adhesion to the negative electrode current collector, and thus having excellent lifespan characteristics. In addition, a lithium secondary battery comprising the same has excellent output characteristics and can be charged in a short time even at a 1C rate.

Description

Technical Field

[0001] The present invention relates to a negative electrode for a lithium secondary battery and a method for manufacturing the same.

[0002] This application claims priority to Korean Patent Application No. 10-2023-0089506, filed on July 11, 2023, the disclosure of which is incorporated herein by reference in its entirety. Background Art

[0003] In recent years, lithium secondary batteries have been widely used 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 or power storage devices. In particular, with the increase in environmental problems in recent years, a great deal of research has been conducted on electric vehicles and hybrid electric vehicles, which can replace vehicles using fossil fuels, such as gasoline and diesel vehicles, as the main sources of air pollution.

[0004] Conventional lithium ion batteries are limited to short-range electric vehicles due to their limited energy density. Therefore, technologies for increasing the energy density of lithium secondary batteries have been intensively developed.

[0005] However, the developed automotive lithium secondary batteries have the following problems: the charging time after discharging when driving the vehicle is long. Therefore, as the penetration rate of electric vehicles increases, there is an increasing need to shorten the charging time to an acceptable level for users.

[0006] Meanwhile, a lithium secondary battery is a rechargeable electric energy generating device composed of a stacked structure of a positive electrode, a separator, and a negative electrode. When the lithium secondary battery is charged, a lithium deintercalation reaction is initiated at the positive electrode of the battery, in which lithium contained in the positive electrode active material is oxidized and released, and a lithium intercalation reaction occurs at the negative electrode, in which lithium is reduced and enters the negative electrode active material. Generally, the deintercalation reaction in the positive electrode active material is faster than the intercalation reaction in the negative electrode active material. Therefore, the fast charging and discharging properties of the lithium secondary battery are mainly determined by the negative electrode.

[0007] Materials containing graphite are widely used as the negative electrode active material of the negative electrode. The average potential at which the graphite-containing material releases lithium is about 0.2 V (based on Li / Li + )), and the discharge potential is relatively flat. Therefore, when graphite is used as the negative electrode active material, the voltage of the secondary battery is high and constant.

[0008] The negative electrode active material for the negative electrode is amorphous carbon or crystalline carbon, and crystalline carbon is mainly used because of its high capacity. These crystalline carbons include graphite-based carbons such as natural graphite and artificial graphite.

[0009] Different types of graphitic carbon have different properties. For example, natural graphite is inexpensive and exhibits excellent adhesion to the current collector, but is relatively poor in high-rate charge / discharge performance and life characteristics compared to artificial graphite. However, since artificial graphite has fewer surface defects and functional groups, its adhesion to the current collector is weak, and when propylene carbonate (PC) is mixed into the electrolyte to improve low-temperature performance, propylene carbonate causes delamination and destruction of the layers of the graphite interlayer structure.

[0010] Therefore, attempts have been made to apply hybrid graphite that combines natural graphite and artificial graphite as the negative electrode active material for lithium secondary batteries to utilize their respective advantages. However, the adhesion of this hybrid graphite to the current collector is reduced, and it is difficult to achieve a satisfactory level in terms of life characteristics and shock stability.

[0011] Therefore, there is a great need for a negative electrode technology with high adhesion to the current collector, excellent life characteristics, high output characteristics, and fast charging characteristics to fundamentally solve these problems.

[0012] [Prior Art Documents]

[0013] Korean Patent Publication No. 10-2022-0064389 Summary of the Invention

[0014] [Technical Problem]

[0015] An object of the present invention is to provide a negative electrode for a lithium secondary battery and a method for manufacturing the same, which have excellent life characteristics, excellent output characteristics, and fast charging characteristics due to properties such as high adhesion to the current collector.

[0016] [Technical Solution]

[0017] To solve the above problems, the present invention provides a negative electrode for a lithium secondary battery, which includes:

[0018] A negative electrode current collector;

[0019] A first negative electrode active layer including a first carbon-based negative electrode active material provided on at least one surface of the negative electrode current collector; and

[0020] A second negative electrode active layer including a second carbon-based negative electrode active material provided on the first negative electrode active layer;

[0021] Wherein, the average aspect ratio of the first carbon-based negative electrode active material is greater than 0.85;

[0022] Wherein, the average aspect ratio of the second carbon-based negative electrode active material is 0.85 or less;

[0023] Among them, the optical index (O.I.) of the first carbon-based negative electrode active material in the first negative electrode active layer is 50 or less according to Equation 1:

[0024] [Equation 1]

[0025] O.I. = I 004 / I 110

[0026] In Equation 1,

[0027] I 004 represents the peak area representing the (004) crystal plane in the X-ray diffraction (XRD) spectrum measurement of the negative electrode active layer,

[0028] I 110 represents the peak area representing the (110) crystal plane in the X-ray diffraction (XRD) spectrum measurement of the negative electrode active layer.

[0029] Here, the average aspect ratio of the first carbon-based negative electrode active material can be greater than 0.9 and less than 1.0, and the average aspect ratio of the second carbon-based negative electrode active material can be 0.2 or more and less than 0.8.

[0030] In addition, the optical index (O.I.) of the second carbon-based negative electrode active material in the second negative electrode active layer according to Equation 1 can be 0.01 to 10.

[0031] In addition, each of the first carbon-based negative electrode active material and the second carbon-based negative electrode active material can include at least one of natural graphite and artificial graphite.

[0032] For example, the first carbon-based negative electrode active material can be artificial graphite.

[0033] In addition, the second negative electrode active layer has a porosity of 20% to 30%.

[0034] In addition, based on the average thickness of the entire negative electrode active layer, the thickness ratio of the first negative electrode active layer can be 1% to 100%. In this case, the average thickness of the entire negative electrode active layer can be 50 μm to 500 μm.

[0035] In addition, the present invention provides a method for manufacturing a negative electrode for a lithium secondary battery of the present invention, the manufacturing method including:

[0036] Applying a first negative electrode paste and a second negative electrode paste such that the first negative electrode paste is located on at least one surface of the negative electrode current collector and the second negative electrode paste is located on the first negative electrode paste;

[0037] Applying a magnetic field to the applied first and second negative electrode pastes; and

[0038] Drying the applied first and second negative electrode pastes to which a magnetic field has been applied to form a negative electrode active layer.

[0039] Here, the step of applying a magnetic field can be performed for a time of 1 second to 20 seconds.

[0040] In addition, the step of applying a magnetic field can be performed at a magnetic field strength of 1000 G to 7000 G.

[0041] In addition, the present invention provides a lithium secondary battery, comprising:

[0042] an electrode assembly including a positive electrode, the negative electrode of the present invention, and a separator disposed between the positive electrode and the negative electrode; and

[0043] an electrolyte composition impregnating the electrode assembly.

[0044] Here, the positive electrode may include a positive electrode active layer including one or more positive electrode active materials of lithium metal oxides represented by Chemical Formula 1 and Chemical Formula 2, and disposed on at least one surface of a positive electrode current collector:

[0045] [Chemical Formula 1]

[0046] Li x [Ni y Co z Mn w M 1 v O2

[0047] [Chemical Formula 2]

[0048] LiM 2 p Mn 1-p O4

[0049] In Chemical Formula 1 and Chemical Formula 2,

[0050] M 1 is at least one element 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, or Mo,

[0051] x, y, z, w, and v are 1.0 ≤ x ≤ 1.30, 0.5 ≤ y < 1, 0 < z ≤ 0.3, 0 < w ≤ 0.3, and 0 ≤ v ≤ 0.1, respectively, where y + z + w + v = 1,

[0052] M 2 is Ni, Co, or Fe, and

[0053] p is 0.05 ≤ p ≤ 1.0.

[0054] For example, the positive electrode active material 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 0.1 Mn 0.1 Al 0.1 O2, LiNi 0.7 Mn 1.3 O4, LiNi 0.5 Mn 1.5 O4 and LiNi 0.3 Mn 1.7 One or more of O4.

[0055] Meanwhile, the electrode assembly may be a stacked electrode assembly, a folded electrode assembly, or a fold-stacked electrode assembly.

[0056] [Beneficial Effects]

[0057] The negative electrode for a lithium secondary battery of the present invention includes a negative electrode active layer having a bilayer structure, and by controlling the average aspect ratio of the carbon-based negative electrode active material contained in each layer and the optical index (O.I) with respect to the negative electrode current collector to satisfy a predetermined range, the negative electrode has high adhesion to the negative electrode current collector, and thus has excellent life characteristics. In addition, the lithium secondary battery including this negative electrode has excellent output characteristics and can be charged in a short time even at a 1C rate. Detailed Embodiments

[0058] The present invention can be variously modified and can have various embodiments, some of which are described in detail below.

[0059] However, this is not intended to limit the present invention to any specific embodiment, and it should be understood to include all modifications, equivalents, or alternatives falling within the concept and technical scope of the present invention.

[0060] The term "comprising" or "including" or "having" is used herein to specify the presence of the features, quantities, steps, actions, components or elements or combinations thereof described in the specification, and it should be understood that the possibility of the presence or addition of one or more other features, quantities, steps, actions, components, elements or combinations thereof is not precluded in advance.

[0061] In addition, when a part of a layer, film, region or plate is disposed "on" another part, this includes not only the case where one component is disposed "directly" on another component, but also the case where a third component is inserted therebetween. Conversely, when a part of a layer, film, region or plate is disposed "under" another part, this includes not only the case where one component is disposed "directly" under another component, but also the case where a third component is inserted therebetween. In addition, in the present application, "on" may include not only the case of being disposed on the upper part, but also the case of being disposed on the lower part.

[0062] Furthermore, in the present invention, "comprising... as a main component" may mean containing 50% by weight or more (or 50% by volume or more), 60% by weight or more (or 60% by volume or more), 70% by weight or more (or 70% by volume or more), 80% by weight or more (or 80% by volume or more), 90% by weight or more (or 90% by volume or more) or 95% by weight or more (or 95% by volume or more) of a specified component relative to the total weight (or total volume). For example, "comprising graphite as a main component of the negative electrode active material" may mean containing 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 of graphite based on the total weight of the negative electrode active material, and in some cases may mean that the entire negative electrode active material consists of graphite and contains 100% of graphite.

[0063] In addition, as used herein, "the carbon-based negative electrode active material has an orientation" or "the carbon-based negative electrode active material is aligned" means that a certain crystal plane (e.g., the a-b axis crystal plane of graphite) representing the two-dimensional planar structure of the carbon-based negative electrode active material constituting the negative electrode active material particles is arranged to have a predetermined inclination with respect to the surface of the negative electrode current collector, which may be different from the case where the carbon-based negative electrode active material particles themselves are arranged to have a specific orientation within the negative electrode active layer.

[0064] In addition, "highly oriented carbon-based negative electrode active material" may mean that the frequency of a certain crystal plane (e.g., the a-b axis crystal plane of graphite) representing the two-dimensional planar structure of the carbon-based negative electrode active material contained in the negative electrode active layer having a predetermined inclination with respect to the surface of the negative electrode current collector is high. It may also mean that, in some cases, the crystal planes of the carbon-based negative electrode active material contained in the negative electrode active layer are arranged at a relatively large angle (e.g., close to a right angle, greater than 45°; more specifically, greater than 60°) with respect to the surface of the negative electrode current collector.

[0065] In addition, the "high optical index of the carbon-based negative electrode active material" may refer to a relatively large value of the "optical index (O.I.)" described herein, which means that a certain crystal plane (e.g., the a-b axis crystal plane of graphite) representing the two-dimensional planar structure of the carbon-based negative electrode active material contained in the negative electrode active layer is arranged at a relatively small angle (e.g., less than 45°) with respect to the surface of the negative electrode current collector. On the contrary, the "low optical index of the carbon-based negative electrode active material" may indicate a relatively small value of the "optical index (O.I.)", such that the crystal planes of the carbon-based negative electrode active material contained in the negative electrode active layer are arranged at a relatively large angle (e.g., close to a right angle, greater than 45°; more specifically, greater than 60°) with respect to the surface of the negative electrode current collector.

[0066] In addition, as used herein, the "crystal plane of the carbon-based negative electrode active material" refers to the plane formed by the atoms of the carbon-based negative electrode active material in a crystal shape, which may represent the crystal plane of the plane containing the carbon-based negative electrode active material or the a-axis / b-axis / a-b axis crystal plane of the crystal containing the carbon-based negative electrode active material in the present invention.

[0067] In addition, as used herein, the "average particle size (D 50 )" refers to the particle size at which the sum value of the particle size distribution of the particles is 50%, also known as the median diameter. The average particle size can be measured using a laser diffraction particle size measuring device.

[0068] In addition, as used herein, the "aspect ratio" refers to the ratio of the longest dimension to the shortest dimension passing through the center of the two-dimensional particle when performing cross-sectional structure analysis or projecting the negative electrode active material into two-dimensional particles. In addition, the "average aspect ratio" refers to the number-weighted average of the aspect ratios of each particle in the active material particle group. The aspect ratio can be measured and determined using a particle shape analyzer, or by measuring the shape of the two-dimensional projected particles using a scanning electron microscope (SEM), an energy dispersive spectrometer, etc. and then analyzing the measurement results.

[0069] The present invention will be described in more detail below.

[0070] Negative electrode for lithium secondary battery

[0071] The present invention provides a negative electrode for a lithium secondary battery, comprising:

[0072] A negative electrode current collector;

[0073] A first negative electrode active layer containing a first carbon-based negative electrode active material provided on at least one surface of the negative electrode current collector; and

[0074] A second negative electrode active layer containing a second carbon-based negative electrode active material provided on the first negative electrode active layer;

[0075] Among them, the average aspect ratio of the first carbon-based negative electrode active material is greater than 0.85;

[0076] Among them, the average aspect ratio of the second carbon-based negative electrode active material is 0.85 or less;

[0077] Among them, the optical index (O.I.) of the first carbon-based negative electrode active material of the first negative electrode active layer according to Equation 1 is 50 or less:

[0078] [Equation 1]

[0079] O.I. = I 004 / I 110

[0080] In Equation 1,

[0081] I 004 represents the peak area representing the (004) crystal plane in the X-ray diffraction (XRD) spectrum measurement of the negative electrode active layer,

[0082] I 110 represents the peak area representing the (110) crystal plane in the X-ray diffraction (XRD) spectrum measurement of the negative electrode active layer.

[0083] The negative electrode for a lithium secondary battery of the present invention includes a negative electrode active layer on at least one surface of a negative electrode current collector. The negative electrode active layer is a layer that realizes the electroactivity of the negative electrode and contains a negative electrode active material that realizes an electrochemical oxidation-reduction reaction during the charge and discharge processes of the battery as a main component.

[0084] Here, the negative electrode active material is a carbon-based negative electrode active material, and among them, the carbon-based negative electrode active material is a material mainly composed of carbon atoms. Such a carbon-based negative electrode active material may include graphite. The graphite may include one or more of natural graphite and artificial graphite. In addition, in addition to natural graphite and artificial graphite, the graphite may further contain graphitized mesophase calcined carbon (bulk mesophase) based on tar and pitch, coke (crude coke, green coke, pitch coke, needle coke, petroleum coke, etc.).

[0085] In one example, the negative electrode active layer of the present invention may contain only artificial graphite. The present invention contains only artificial graphite in the negative electrode active layer, significantly improving the life of the negative electrode, which may be advantageous under conditions where frequent charging must be endured for a long time (for example, in an automotive battery). In addition, compared with natural graphite, the advantage of artificial graphite is that it can be charged at a faster rate and has excellent output performance.

[0086] In addition, the negative electrode active layer may have a bilayer structure in which a first negative electrode active layer and a second negative electrode active layer are stacked in sequence on the negative electrode current collector. The bilayer structure of the negative electrode active layer is conducive to controlling the types, shapes, sizes, etc. of the negative electrode active materials contained in each layer. Therefore, according to specific purposes, such as improving the output of the battery or improving the adhesion between the negative electrode active layer and the negative electrode current collector, the performance of the negative electrode can be improved by controlling the composition of the negative electrode active materials contained in each layer. For example, a carbon-based negative electrode active material such as artificial graphite with a large capacity per unit weight / volume or high output efficiency can be optionally applied to the second negative electrode active layer in contact with the positive electrode, and a carbon-based negative electrode active material such as natural graphite with excellent adhesion to the negative electrode current collector can be optionally applied to the first negative electrode active layer in contact with the negative electrode current collector.

[0087] In the present invention, the negative electrode active layer may contain a first carbon-based negative electrode active material having a large average aspect ratio applied to the first negative electrode active layer and a second carbon-based negative electrode active material having a small average aspect ratio applied to the second negative electrode active layer.

[0088] The average aspect ratio of the carbon-based negative electrode active material is one of the indicators of the particle shape. Depending on the shape of the carbon-based negative electrode active material, the electrical properties or adhesion of the negative electrode active layer may vary. By differentially setting the shape / form of the carbon-based negative electrode active materials contained in each layer of the negative electrode active layer, the present invention can improve the adhesion between the negative electrode current collector and the negative electrode active layer while improving the output and fast charging of the negative electrode.

[0089] Specifically, the average aspect ratio of the first carbon-based negative electrode active material contained in the first negative electrode active layer may be greater than 0.85, and the average aspect ratio of the second carbon-based negative electrode active material contained in the second negative electrode active layer may be 0.85 or less.

[0090] For example, the average aspect ratio of the first carbon-based negative electrode active material may be 0.85 to 1.00; 0.85 to 0.99; 0.88 to 0.99; 0.90 to 0.99; 0.92 to 0.98; 0.88 to 0.96; 0.88 to 0.93; greater than 0.90 and less than 1.0. In addition, the average aspect ratio of the second carbon-based negative electrode active material may be 0.10 to 0.85; 0.25 to 0.85; 0.25 to 0.80; 0.25 to 0.70; 0.25 to 0.60; 0.25 to 0.50; 0.25 to 0.40; 0.40 to 0.65; 0.50 to 0.80; 0.50 to 0.65; 0.60 to 0.85; 0.60 to 0.75; 0.70 to 0.85; 0.10 to 0.40; or greater than or equal to 0.2 but less than 0.8.

[0091] In the present invention, an average aspect ratio greater than 0.85 may mean that the particles have a shape close to spherical. Further, in the present invention, an average aspect ratio of 0.85 or less, particularly 0.60 or less, may mean that the particles have an ellipsoidal shape that is stretched in any direction with respect to the center of the particle. In other words, the present invention may include a first negative electrode active layer in contact with a current collector and a second negative electrode active layer in contact with a separator, the first negative electrode active layer including a first carbon-based negative electrode active material having a shape close to spherical, and the second negative electrode active layer including a second carbon-based negative electrode active material having an ellipsoidal shape that is stretched in any direction with respect to the center of the particle. In this case, the adhesion between the first negative electrode active layer and the negative electrode current collector can be improved. Moreover, the second negative electrode active layer in contact with the separator can further shorten the migration path of lithium ions, thereby improving the output characteristics of the negative electrode and shortening the charging time.

[0092] Specifically, when the average aspect ratio of the first carbon-based negative electrode active material is reduced to 0.85 or less, the contact area between the first carbon-based negative electrode active material particles with the a-b axis crystal planes aligned and the negative electrode current collector is significantly reduced, thereby reducing the adhesion between the negative electrode active layer and the negative electrode current collector. Further, when the average aspect ratio of the carbon-based negative electrode active material exceeds 0.85, the conductivity of the negative electrode active material itself can be improved, thereby increasing the capacity of the negative electrode.

[0093] Further, when the average aspect ratio of the second carbon-based negative electrode active material exceeds 0.85, the conductivity of the second negative electrode active layer increases, but the tortuosity at the surface of the negative electrode significantly increases, which reduces the migration rate of lithium ions, resulting in a limitation of low charging rate when charging a secondary battery.

[0094] Further, the deviation in the average aspect ratio between the first carbon-based negative electrode active material and the second carbon-based negative electrode active material may be at least 0.2 and not greater than 0.85. Specifically, the deviation in the average aspect ratio between the first carbon-based negative electrode active material and the second carbon-based negative electrode active material may be from 0.2 to 0.8; from 0.2 to 0.75; from 0.2 to 0.55; from 0.2 to 0.45; from 0.2 to 0.35; from 0.35 to 0.55; from 0.4 to 0.7; from 0.5 to 0.8; or from 0.38 to 0.68.

[0095] By controlling the average aspect ratio and the deviation of the average aspect ratio of the first carbon-based negative electrode active material and the second carbon-based negative electrode active material as described above, the present invention can further improve the output of the negative electrode while further strengthening the adhesion between the negative electrode active layer and the negative electrode current collector. Specifically, when the deviation of the average aspect ratio of the first carbon-based negative electrode active material and the second carbon-based negative electrode active material is 0.2 or less, there is a limitation that the effect of improving the adhesion between the negative electrode active layer and the negative electrode current collector is not significant. In addition, during the charge and discharge of the secondary battery, the migration path of lithium ions in the second negative electrode active layer significantly increases, which may result in poor fast charge and discharge performance. When the deviation of the average aspect ratio exceeds 0.85, the resistance on the surface of the negative electrode active layer increases, so the charge and discharge performance of the negative electrode is poor.

[0096] In addition, the first carbon-based negative electrode active material and the second carbon-based negative electrode active material may each have a shape that satisfies the above average aspect ratio, preferably a rounded particle shape, rather than a plate shape, a sheet shape, a column shape, a needle shape, etc. Here, the rounded particle shape may refer to particles having a non-angular shape. When performing a cross-sectional structure analysis or when projecting into two-dimensional particles, such particles may be spherical or ellipsoidal in shape, and in some cases may have an indefinite shape with a shape that is difficult to define.

[0097] In one example, the first carbon-based negative electrode active material may be graphite having a spherical particle shape. The spherical particles may be processed into a spherical shape / morphology during manufacturing, or may be a spherical graphite assembly formed by aggregation of multiple flaky graphites.

[0098] When the spherical particles are an assembly, one graphite assembly may be formed by 2 to 100 flakes, preferably 3 to 20 flakes of flaky graphite. By controlling the shape of the first carbon-based negative electrode active material as described above, the present invention can further improve the conductivity of the negative electrode active layer, maximize the contact area with the negative electrode current collector, and improve the adhesion between the negative first electrode active layer and the negative electrode current collector.

[0099] As another example, the second carbon-based negative electrode active material may be ellipsoidal. In this case, the second carbon-based negative electrode active material can more easily ensure the migration path of lithium ions in the second negative electrode active layer, so that charging can be completed in a shorter time even when charging is performed under the same conditions.

[0100] In addition, the first carbon-based negative electrode active material and the second carbon-based negative electrode active material may have a predetermined average particle size (D[[ID=]] 50 )。For example, the average particle size (D[[ID=]] 50) can range from 0.1 μm to 20 μm, more specifically, 5 μm to 20 μm; 10 μm to 20 μm; 11 μm to 19 μm; 8 μm to 15 μm; 15 μm to 20 μm; 13 μm to 19 μm; 14 μm to 17 μm; 5 μm to 8 μm; 0.1 μm to 15 μm; 0.1 μm to 10 μm; 0.1 μm to 8 μm; 0.1 μm to 5 μm; 0.1 μm to 3 μm; 0.1 μm to 1 μm; 0.5 μm to 10 μm; 0.5 μm to 5 μm; 0.5 μm to 4.5 μm; 0.5 μm to 3 μm; 1 μm to 4.5 μm; 1 μm to 3 μm; or 0.8 μm to 1.8 μm.

[0101] Within the above ranges, the present invention can suppress the increase in the resistance of the first negative electrode active layer. In addition, within the above average particle size ranges, the first carbon-based negative electrode active material can increase the specific surface area while maximizing the degree of disorder of each particle in the expansion direction, thereby preventing particle expansion caused by the insertion of lithium ions, and further increasing the adhesion between the first negative electrode active layer and the negative electrode current collector. In addition, the advantages of the present invention also lie in that it can increase the conductivity of the second negative electrode active layer containing the second carbon-based negative electrode active material within the above ranges, and at the same time can ensure the migration path of lithium ions.

[0102] At the same time, if the particle sizes of the first carbon-based negative electrode active material and the second carbon-based negative electrode active material are lower than the lower limit of the average particle size (D 50 ), a large amount of binder is required due to the increase in the number of particles per unit volume, and the electrical properties of the negative electrode active layer containing the negative electrode active material may deteriorate. On the other hand, if the particle size is higher than the upper limit of the average particle size (D 50 ), the expansion rate of the negative electrode active material during charge and discharge of the secondary battery increases significantly, such that with the repetition of charge and discharge, the cohesion between the negative electrode active material particles and the adhesion force between the negative electrode active material particles and the current collector decrease, and the cycle characteristics may be significantly reduced.

[0103] In addition, the carbon-based negative electrode active materials contained in the first negative electrode active layer and the second negative electrode active layer of the negative electrode of the present invention can respectively have controlled crystal structure characteristics, thereby achieving a fast charging rate while increasing the adhesion between the first negative electrode active layer and the negative electrode current collector.

[0104] In one example, the optical index (O.I.) of the first carbon-based negative electrode active material of the first negative electrode active layer according to Equation 1 can be 50 or less. In addition, the optical index (O.I.) of the second carbon-based negative electrode active material of the second negative electrode active layer according to Equation 1 can be from 0.02 to 10:

[0105] [Equation 1]

[0106] O.I. = I 004 / I 110

[0107] In Equation 1,

[0108] I 004 represents the peak area representing the (004) crystal plane in the X-ray diffraction (XRD) spectrum measurement of the negative electrode active layer,

[0109] I 110 represents the peak area representing the (110) crystal plane in the X-ray diffraction (XRD) spectrum measurement of the negative electrode active layer.

[0110] The optical index (O.I) of the carbon-based negative electrode active material can be an index of the degree of orientation of the ab-axis crystal plane of the carbon-based negative electrode active material in a specific direction (specifically, relative to the surface of the negative electrode current collector) when measured by X-ray diffraction (XRD). Specifically, in the X-ray diffraction measurement, for the carbon-based negative electrode active material, i.e., graphite, the negative electrode active layer exhibits the following peaks: 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°. These represent the (002), (100), (101)R, (101)H, (004), and (110) planes of graphite. The peak at 2θ = 43.4 ± 0.2° can also be regarded as the overlap of the peak corresponding to the (101)R plane of the carbon-based negative electrode active material and the peak of the (111) plane of the current collector (e.g., copper (Cu)).

[0111] Among them, the area ratio obtained by integrating the corresponding intensities of the peak at 2θ = 54.7 ± 0.2° representing the (004) plane and the peak at 2θ = 77.5 ± 0.2° representing the (110) plane can be used to measure the optical index (O.I) of the carbon-based negative electrode active material.

[0112] Since the peak at 2θ = 54.7 ± 0.2° in the crystal plane of the negative electrode active material represents the crystal plane of the carbon-based negative electrode active material having an inclination with respect to the negative electrode current collector, when the above optical index (O.I) approaches 0, it can indicate that the inclination with respect to the surface of the negative electrode current collector is close to 90°, and when the optical index (O.I) is large, the inclination with respect to the surface of the negative electrode current collector is closer to 0° or 180°. In other words, the arrangement of the negative electrode active layer of the present invention can be such that the angle of the carbon-based negative electrode active material in the layer with respect 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°. Therefore, compared with the case where the carbon-based negative electrode active material is arranged at a lower angle of less than 60°, the optical index (O.I) of the carbon-based negative electrode active material in the negative electrode active layer can be lower. Here, the optical index (O.I.) refers to the degree of arrangement of the ab-axis crystal plane of the carbon-based negative electrode active material. The arrangement of the ab-axis crystal plane of the carbon-based negative electrode active material can also cause the rotation of the carbon-based negative electrode active material particles contained in the negative electrode active layer. However, since the rotation of the particles is affected by the particle shape at this time, it is not equivalent to the arrangement of the ab-axis crystal plane, so the optical index (O.I.) may be difficult to indicate the arrangement of the carbon-based negative electrode active material particles.

[0113] Here, the optical index (O.I 第一 ) of the first carbon-based negative electrode active material contained in the first negative electrode active layer can be 50 or less, more specifically greater than 10 but less than or equal to 50; 11 to 45; 11 to 40; 11 to 35; 11 to 30; 11 to 25; 11 to 20; 11 to 15; 15 to 25; 15 to 20; 15 to 19; 13 to 18; 16 to 24; 21 to 29; or 19 to 26.

[0114] If the optical index (O.I 第一 ) of the first carbon-based negative electrode active material contained in the first negative electrode active layer is less than the above lower limit, the adhesion to the negative electrode current collector may be reduced. Therefore, by adjusting the optical index (O.I 第一 ) of the first carbon-based negative electrode active material contained in the first negative electrode active layer as described above, the adhesion between the first negative electrode active layer and the negative electrode current collector can be improved, and at the same time, the ratio of the crystal plane of the carbon-based negative electrode active material molecules constituting the negative electrode active material particles facing the negative electrode current collector can be increased to a level that does not reduce the fast charging performance of the negative electrode.

[0115] In addition, the optical index (O.I 第二) can be from 0.01 to 10, more specifically from 0.01 to 9; from 0.01 to 7; from 0.01 to 5; from 0.01 to 3; from 1 to 9; from 3 to 9; from 5 to 9; from 3 to 8; from 4 to 7; from 6 to 10; from 4 to 9; from 0.01 to 4; from 0.1 to 2.5; from 0.1 to 2.0; from 0.1 to 1.5; from 0.2 to 1.3; from 0.4 to 1.3; from 0.4 to 1.0; from 0.5 to 1.3; from 1.1 to 1.3; from 0.5 to 0.9; or from 0.4 to 0.6.

[0116] By adjusting the optical index (O.I. 第二 ) of the second carbon-based negative electrode active material contained in the second negative electrode active layer as described above, the present invention can obtain a lithium ion migration channel in which lithium ions can migrate over a shorter distance inside the negative electrode active layer. As a result, the negative electrode of the present invention can prevent an increase in resistance caused by a long migration distance of lithium ions, thereby further increasing the migration speed of lithium ions during charging and discharging, and thus improving the fast charging performance and power output performance with high safety at the same time.

[0117] Specifically, conventional fast charging of lithium secondary batteries has been used to increase the rate by charging under high C-rate conditions exceeding 1C rate in the constant current-constant voltage (CC-CV) method. Generally, in constant current-constant voltage (CC-CV) charging, the diffusion of lithium ions in the electrode during charging occurs during the constant current (CC) charging stage, which inevitably causes concentration polarization due to long-time diffusion. This concentration polarization of lithium ions easily causes lithium precipitation at the negative electrode, especially under high rate conditions where the current amount (A) charged relative to the rated capacity value (Ah) of the secondary battery exceeds the standard value (i.e., the current amount at 1C-rate: 1A), which has the limitation of significantly reducing the safety of the secondary battery. In addition, high rate constant current-constant voltage (CC-CV) charging reaches the upper limit voltage very quickly during the constant current (CC) charging stage, so the current may drop to the preset limit before the active material is completely consumed. This means that the charging time during the constant voltage (CV) charging stage increases significantly, so the reduction in the total charging time of the secondary battery is insignificant.

[0118] However, since the present invention can ensure that the migration path of lithium ions in the negative electrode active layer is shorter, the resistance induced in the negative electrode active layer during charging can be significantly reduced. This reduction in resistance can result in a longer duration of the constant current (CC) charging phase compared to the constant voltage (CV) charging phase in the total charging time when charging by the constant current-constant voltage (CC-CV) method. Here, the constant current (CC) charging phase has the same charging capacity per unit time because the same amount of current flows, but the constant voltage (CV) charging phase tends to reduce the current to maintain the same voltage, that is, the charging capacity per unit time decreases sharply in the constant voltage (CV) charging phase, such that the overall charging time can be significantly reduced as the execution time of the constant current (CC) charging phase increases. Therefore, the present invention can increase the execution time of the constant current (CC) charging phase by controlling the optical index (O.I. 第二 ) of the second carbon-based negative electrode active material, thereby enabling the charging of the secondary battery to be completed in a significantly shorter time.

[0119] In addition, the rapid charging of such a secondary battery can be achieved under standard conditions (such as 1C rate) rather than high C-rate conditions, which has the advantage of overcoming the safety problems of lithium secondary batteries caused by lithium ion concentration polarization induced in the negative electrode active layer during charging.

[0120] Meanwhile, the porosity of the negative electrode active layer, especially the second negative electrode active layer, can be 20% to 30%. More particularly, the porosity of the second negative electrode active layer can be 23% to 30%; 25% to 30%; 26% to 29%; or 24% to 28%.

[0121] At this time, the second negative electrode active layer can exhibit a BET specific surface area of 0.62 m 2 / g or less. Specifically, the BET specific surface area of the second negative electrode active layer can be 0.615 m 2 / g or less, 0.6 m 2 / g or less, 0.5 m 2 / g or less, 0.20 to 0.62 m 2 / g, 0.30 to 0.62 m 2 / g, 0.40 to 0.62 m 2 / g, 0.40 to 0.60 m 2 / g, 0.40 to 0.55 m 2 / g, 0.40 to 0.50 m 2 / g or 0.58 to 0.62 m 2 / g. Here, the specific surface area can be measured by the Brunauer-Emmett-Teller (BET) method. For example, it can be measured by the BET six-point method using the nitrogen adsorption distribution method with a porosimetry analyzer (Porosimetry analyzer; Bell Japan Inc, Belsorp-II mini).

[0122] By adjusting the porosity and / or BET specific surface area of the second negative electrode active layer, which is the outermost layer in each negative electrode active layer and is in direct contact with the electrolyte, to the above range, the energy density of the negative electrode can be further increased without reducing the wettability of the electrolyte in the negative electrode active layer.

[0123] In addition, the average thickness of the entire negative electrode active layer can be 50 μm to 500 μm, more specifically 100 μm to 400 μm; 200 μm to 350 μm; 50 μm to 180 μm; 80 μm to 150 μm; 100 μm to 250 μm; or 130 μm to 190 μm. By adjusting the average thickness of the negative electrode active layer to the above range, the present invention can easily control the crystal characteristics of each carbon-based negative electrode active material contained in the first negative electrode active layer and the second negative electrode active layer. Therefore, the negative electrode of the present invention not only achieves high adhesion between the first negative electrode active layer and the negative electrode current collector, but also has the advantage of fast charging that can complete charging in a short time even under standard conditions (1C rate).

[0124] In addition, the first negative electrode active layer and the second negative electrode active layer can each have the same or different average thicknesses. Specifically, based on the average thickness (D2) of the second negative electrode active layer, the ratio of the average thickness (D1) of the first negative electrode active layer can be 1% to 100%, more specifically, it can be 1% to 50%; 1% to 40%; 1% to 30%; 1% to 20%; 1% to 10%; 1% to 5%; 5% to 10%; 10% to 20%; or 15% to 30%. By adjusting the ratio of the average thickness of the first negative electrode active layer to the above range, the present invention can achieve high charge and discharge capacity while maintaining high durability of the negative electrode, and can maximize the output characteristics.

[0125] In addition, in addition to the negative electrode active material as the main component, the first negative electrode active layer and the second negative electrode active layer of the present invention can also optionally contain a conductive material, a binder, other additives, etc. as needed.

[0126] The conductive material can include one or more of carbon black, acetylene black, Ketjen black, carbon nanotubes, carbon fibers, etc., but is not limited thereto.

[0127] In one example, the negative electrode active layer can contain carbon black, carbon nanotubes, carbon fibers, etc. alone or in combination as the conductive material.

[0128] In this case, based on a total of 100 parts by weight of the negative electrode active layer, the content of the conductive material can be 0.1 to 10 parts by weight, more specifically, 0.1 to 8 parts by weight, 0.1 to 5 parts by weight, 0.1 to 3 parts by weight, 2 to 6 parts by weight, or 0.5 to 2 parts by weight. By controlling the content of the conductive material within the range described above, the present invention can prevent a decrease in the charging capacity due to a low content of the conductive material and an increase in the resistance of the negative electrode, and can prevent a decrease in the charging capacity due to an excessive amount of the conductive material and a decrease in the content of the negative electrode active material, or a decrease in the fast charging characteristics due to an increase in the loading amount of the negative electrode active layer.

[0129] In addition, the binder is a component that helps the binding of the negative electrode active material and the conductive material and the binding to the current collector, and can be appropriately applied to the extent that it does not deteriorate the electrical properties of the electrode. More specifically, the binder may include one or more of the following: vinylidene 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.

[0130] Based on a total of 100 parts by weight of the negative electrode active layer, the content of the binder can be 0.1 to 10 parts by weight, more specifically, 0.1 to 8 parts by weight, 0.1 to 5 parts by weight, 0.1 to 3 parts by weight, or 2 to 6 parts by weight. By controlling the content of the binder in the negative electrode active layer within the above range, the present invention can prevent the adhesion of the active layer from deteriorating due to a low content of the binder, or prevent the electrical properties of the electrode from deteriorating due to an excessive amount of the binder.

[0131] In addition, the negative electrode current collector is not particularly limited 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, and in the case of copper or stainless steel, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, etc. can also be used. In addition, considering the conductivity and total thickness of the negative electrode to be manufactured, the average thickness of the negative electrode current collector can be appropriately set to 1 μm to 500 μm.

[0132] Lithium secondary battery

[0133] In addition, the present invention provides a lithium secondary battery comprising:

[0134] An electrode assembly, the electrode assembly comprising a positive electrode, a negative electrode of the present invention as described above, and a separator disposed between the positive electrode and the negative electrode; and

[0135] An electrolyte composition impregnating the electrode assembly.

[0136] The lithium secondary battery of the present invention includes 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 located therebetween. The lithium secondary battery having the negative electrode of the present invention as described above not only has excellent output characteristics, but can also be charged in a short time even at a 1C rate, and thus can be used as a power source for medium and large-sized devices such as electric vehicles.

[0137] Here, the negative electrode has the same structure as the above-described structure, and thus a detailed description thereof is omitted.

[0138] In addition, the positive electrode may include a positive electrode active layer, the positive electrode active layer including a positive electrode active material on a positive electrode current collector, and optionally, the positive electrode active layer may further optionally include a conductive material, a binder, other additives, etc.

[0139] The positive electrode active material may include one or more lithium metal oxides represented by Chemical Formula 1 and Chemical Formula 2 below, which can undergo an electrochemical reaction on the positive electrode current collector to reversibly intercalate and deintercalate lithium ions:

[0140] [Chemical Formula 1]

[0141] Li x [Ni y Co z Mn w M 1 v O2

[0142] [Chemical Formula 2]

[0143] LiM 2 p Mn q P r O4

[0144] In Chemical Formula 1 and Chemical Formula 2,

[0145] M 1 is at least one element 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.

[0146] x, y, z, w, and v are 1.0 ≤ x ≤ 1.30, 0.5 ≤ y < 1, 0 < z ≤ 0.3, 0 < w ≤ 0.3, and 0 ≤ v ≤ 0.1, respectively, where y + z + w + v = 1,

[0147] M 2 is Ni, Co or Fe,

[0148] p is 0.05 ≤ p ≤ 1.0,

[0149] q is 2 - p, and

[0150] r is 0 or 1.

[0151] The lithium metal oxides represented by Chemical Formula 1 and Chemical Formula 2 above are materials containing high contents of nickel (Ni) and manganese (Mn) respectively, and when used as the positive electrode active material, they have the advantage of being able to stably supply high-capacity and / or high-voltage electricity compared with the commonly used positive electrode active materials (such as lithium iron phosphate (LiFePO4)).

[0152] 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 0.1 Mn 0.1 Al 0.1 O2, etc. In addition, the lithium metal oxide represented by Chemical Formula 2 may include LiNi 0.7 Mn 1.3 O4; LiNi 0.5 Mn 1.5 O4; LiNi 0.3 Mn 1.7 O4, etc. The lithium metal oxides represented by Chemical Formula 1 and / or Chemical Formula 2 can be used alone or in combination.

[0153] In addition, relative to the weight of the positive electrode active layer, the content of the positive electrode active material can be 85 parts by weight or more, and more specifically, its content can be at least 90 parts by weight, at least 93 parts by weight, or at least 95 parts by weight.

[0154] In addition, in addition to the positive electrode active material, the positive electrode active layer may further contain a conductive material, a binder, other additives, etc.

[0155] In this case, the conductive material used to improve the electrical properties of the positive electrode can be any material commonly used in the art, but may specifically include at least one selected from the following: natural graphite, artificial graphite, carbon black, acetylene black, screen printing black, Ketjen black, Super-P, channel black, furnace black, lamp black, thermal cracking carbon black, graphene, and carbon nanotubes.

[0156] In addition, with respect to 100 parts by weight of the positive electrode active layer, the content of the conductive material can be 0.1 part by weight to 5 parts by weight, more specifically, it can be 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.

[0157] In addition, the binder is used to bind the positive electrode active material, the positive electrode additive, and the conductive material to each other, and the binder can be used without limitation as long as it has this function. Specifically, the binder can include one or more resins selected from the following: vinylidene fluoride - hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride (PVdF), polyacrylonitrile, polymethyl methacrylate, and its copolymers. In one example, the binder can include polyvinylidene fluoride.

[0158] In addition, with respect to 100 parts by weight of the positive electrode active layer, the content of the binder can be 1 to 10 parts by weight, more particularly 2 to 8 parts by weight; or 1 to 5 parts by weight.

[0159] The total thickness of the positive electrode active layer is not particularly limited, but can be 50 μm to 300 μm, more specifically, 100 μm to 200 μm; 80 μm to 150 μm; 120 μm to 170 μm; 150 μm to 300 μm; 200 μm to 300 μm; or 150 μm to 190 μm.

[0160] In addition, 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 the positive electrode. 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 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 3 μm to 500 μm.

[0161] In addition, the separator disposed between the positive electrode and the negative electrode of each unit cell is an insulating film with high ionic permeability and mechanical strength, and may include, but is not limited to, for example, one or more polymers commonly used in the art, such as polypropylene, polyethylene, and polyethylene-propylene copolymer, which have chemical resistance and hydrophobicity. The separator may be in the form of a porous polymer substrate, such as a sheet or non-woven fabric containing the above polymers, and in some cases may be in the form of a composite separator in which organic or inorganic particles are coated on a porous polymer substrate through an organic binder. In addition, the average pore size of the separator may be 0.01 to 10 μm, and the average thickness may be 5 to 300 μm.

[0162] Meanwhile, the form of the lithium secondary battery of the present invention may include a secondary battery with a stacked type, a folded type, or a folded-stacked type electrode assembly, without particular limitation. As an example, the lithium secondary battery of the present invention may be a pouch-type secondary battery or a prismatic secondary battery.

[0163] The pouch-type and / or prismatic secondary battery has the advantage of higher availability in terms of energy density because they can be filled with high-density secondary battery unit cells in a limited space.

[0164] In addition, in the lithium secondary battery, the electrolyte composition can be used without limitation as long as it is conventionally applied to lithium secondary batteries.

[0165] Specifically, the electrolyte composition may contain a non-aqueous organic solvent, a lithium salt, and an electrolyte additive.

[0166] Here, the non-aqueous organic solvent may be any organic solvent known in the art for non-aqueous electrolytes, without limitation. For example, the non-aqueous organic solvent may use an aprotic organic solvent, such as N-methyl-2-pyrrolidone, ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate, dimethyl carbonate (DMC), diethyl carbonate (DEC), γ-butyrolactone, 1,2-dimethoxyethane (DME), tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, triphosphate ester, trimethoxymethane, dioxolane derivative, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivative, tetrahydrofuran derivative, ether, methyl pyrophosphate (MP), ethyl propionate (EP), and propyl propionate (PP).

[0167] In addition, the non-aqueous organic solvents used in the present invention can be used alone, or two or more of them can be mixed in any combination or ratio to suit the application. Among them, from the viewpoints of electrochemical stability to redox and chemical stability to heat and reaction with solutes, it is particularly preferred to mix propylene carbonate, ethylene carbonate, fluoroethylene carbonate, diethyl carbonate, dimethyl carbonate, and ethyl methyl carbonate, etc.

[0168] In addition, the lithium salt can be applied to any non-aqueous electrolyte known in the art without limitation. Specifically, the lithium salt can include one or more of LiCl, LiBr, LiI, LiClO4, LiBF4, LiB 10 Cl 10 , LiPF6, LiCF3SO3, LiCF3CO2, LiAsF6, LiSbF6, LiAlCl4, CH3SO3Li, (CF3SO2)2NLi, and (FSO2)2NLi.

[0169] The lower limit of the suitable concentration range of the lithium salt for the appropriate use is 0.5 mol / L or more, more particularly 0.7 mol / L or more, more particularly 0.9 mol / L or more, and the upper limit is 2.5 mol / L or less, more particularly 2.0 mol / L or less, more particularly 1.5 mol / L or less. When the concentration of the lithium salt is lower than 0.5 mol / L, there is a possibility that the cycle characteristics and output characteristics of the non-aqueous electrolyte battery may be reduced due to the decrease in ionic conductivity. In addition, if the concentration of the lithium salt exceeds 2.5 mol / L, the viscosity of the electrolyte of the non-aqueous electrolyte battery increases, which may also reduce the ionic conductivity and may reduce the cycle characteristics and output characteristics of the non-aqueous electrolyte battery.

[0170] In addition, when a large amount of lithium salt is dissolved in the non-aqueous organic solvent at one time, the temperature of the electrolyte may rise due to the heat of dissolution of the lithium salt. In the case of a fluorine-containing lithium salt, if the temperature of the non-aqueous organic solvent rises significantly due to the heat of dissolution of the lithium salt, decomposition may be accelerated and hydrogen fluoride (HF) may be generated. Hydrogen fluoride (HF) is not desirable because it causes a decrease in battery performance. Therefore, the temperature at which the lithium salt is dissolved in the non-aqueous organic solvent is not particularly limited, but can be controlled between -20 and 80 °C, more particularly between 0 and 60 °C.

[0171] In addition, an electrolyte additive can be included as an additional auxiliary component to improve the performance of the electrolyte composition. The electrolyte additive used in the non-aqueous electrolyte of the present invention can be added in any proportion. Specifically, compounds having an overcharge prevention effect, a negative electrode film-forming effect, and a positive electrode protection effect can be used, such as cyclohexylbenzene, biphenyl, tert-butylbenzene, carbonate, vinylene ethylene carbonate, difluorobenzene ether, fluoroethylene carbonate, propane sultone, succinonitrile, dimethyl vinylene carbonate, etc. In addition, the electrolyte solution of the non-aqueous electrolyte battery can be used through a gelling agent or a crosslinked polymer, such as the electrolyte solution for a non-aqueous electrolyte battery called a lithium polymer battery.

[0172] The advantages of the lithium secondary battery of the present invention lie in having the above-described structure, which not only provides excellent output characteristics for the battery, but also allows it to be charged in a short time, even at a 1C rate.

[0173] Method for manufacturing negative electrode

[0174] In addition, the present invention provides a method for manufacturing a negative electrode for a lithium secondary battery, the manufacturing method including:

[0175] Applying a first negative electrode paste and a second negative electrode paste such that the first negative electrode paste is located on at least one surface of the negative electrode current collector and the second negative electrode paste is located on the first negative electrode paste;

[0176] Applying a magnetic field to the applied first and second negative electrode pastes; and

[0177] Drying the applied first and second negative electrode pastes having a magnetic field applied thereto to form a negative electrode active layer.

[0178] The negative electrode manufacturing method of the present invention refers to a method for manufacturing the negative electrode of the present invention as described above. The negative electrode manufacturing method can produce a negative electrode by applying a negative electrode paste to a negative electrode current collector, applying a magnetic field to the surface of the applied negative electrode paste, and drying each negative electrode paste, wherein the negative electrode active material of the negative electrode active layer has controlled crystal characteristics.

[0179] Here, the step of applying the negative electrode paste includes coating the surface of the moving negative electrode current collector by discharging the negative electrode paste containing a carbon-based negative electrode active material. This step can be implemented in any manner commonly used in the art without limitation, but is preferably carried out by a die coating method. The die coating method can be carried out through a slit die having a spacer for controlling the discharging conditions of the negative electrode paste. In this case, by controlling the shape, position, etc. of the spacer, it is possible to easily control the loading amount, coating thickness, etc. of the negative electrode paste applied to the negative electrode current collector.

[0180] Specifically, the present invention uses a dual die head to simultaneously apply a first negative electrode paste and a second negative electrode paste onto the negative electrode current collector. Compared with applying each paste sequentially, this has the advantage of significantly improving the process efficiency.

[0181] Meanwhile, the step of applying a magnetic field to the negative electrode paste can be a step of controlling the crystal characteristics of the negative electrode active material contained in the negative electrode paste. Specifically, this step can apply a magnetic field to the surface of the second negative electrode paste applied onto the negative electrode current collector, so that the a-b axis crystal planes of each carbon-based negative electrode active material contained in the second negative electrode paste and the first negative electrode paste located thereunder are arranged at a relatively high angle with respect to the negative electrode current collector.

[0182] In this case, the application of the magnetic field can be applied by magnetic members provided on the upper and lower portions of the negative electrode current collector, and the negative electrode current collector moves together with the negative electrode paste applied to its surface. In addition, the polarities of the upper and lower magnetic members can be different.

[0183] Furthermore, the optical index (O.I) of the carbon-based negative electrode active material contained in each negative electrode paste can be adjusted by the intensity and application time of the applied magnetic field, etc., so that the step of applying the magnetic field can be carried out under a predetermined magnetic field intensity condition.

[0184] Specifically, the step of applying the magnetic field can include applying a magnetic field of 10,000 G (gauss) or less, more specifically, 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 7,000 G; 2,000 G to 4,000 G; 2,500 G to 3,500 G; 3,000 G to 6,500 G; or 2,700 G to 3,300 G.

[0185] In addition, the step of applying the magnetic field can be carried out for a duration of 1 to 20 seconds, more specifically 1 to 15 seconds; 1 to 10 seconds; 5 to 20 seconds; 10 to 20 seconds; 11 to 18 seconds; 1 to 5 seconds; 7 to 13 seconds; or 6 to 11 seconds.

[0186] In one example, the step of applying the magnetic field can include applying a magnetic field of 3000 ± 50 G to the negative electrode paste for 9 seconds to 11 seconds.

[0187] In addition, as described above, the step of applying a magnetic field can be performed by introducing magnetic members above and below the applied negative electrode paste, but the size of the magnetic members can be adjusted to be larger than the size of the negative electrode paste so that the magnetic field applied to the negative electrode paste can be uniformly applied over the entire surface of the negative electrode paste. For example, based on the length in the width direction of the negative electrode paste, the length ratio of the magnetic members can be 105% to 200%, and more specifically, based on the length in the width direction of the negative electrode paste, the length ratio can be 110% to 180%; 110% to 160%; 110% to 140%; 110% to 130%; 130% to 150%; or 105% to 120%.

[0188] In the step of applying a magnetic field, by controlling the magnetic field intensity, application time, and / or magnetic member size as described above, the present invention can uniformly achieve an optical index (O.I.) of the carbon-based negative electrode active material contained in the negative electrode paste that satisfies a predetermined range.

[0189] In addition, the step of forming the negative electrode active layer may include a step of drying the negative electrode paste; and a step of roll-pressing the dried negative electrode paste.

[0190] In this case, the step of drying the negative electrode paste can be carried out in a manner that can maintain the orientation of the carbon-based negative electrode active material contained in the negative electrode active layer, without particular limitation.

[0191] For example, the drying step can be carried out by applying heat energy to the negative electrode paste using a hot air dryer, a vacuum oven, etc. to dry the negative electrode paste.

[0192] In addition, the step of roll-pressing the dried negative electrode paste is a step of applying pressure to the dried negative electrode paste using a roll press or the like to increase the density of the negative electrode active layer. In this case, the roll-pressing can be carried out under temperature conditions higher than room temperature.

[0193] Specifically, the roll pressing can be carried out at a temperature of 50°C to 100°C, more specifically at the following temperatures: 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. More specifically, the roll pressing can be carried out at a roll pressing speed of 2 m / s to 7 m / s, or more specifically 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. In addition, the roll pressing can be carried out under a pressure condition of 50 MPa to 200 MPa, more specifically, 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.

[0194] By roll pressing the dried negative electrode paste under the above temperature, speed and / or pressure conditions, the present invention can minimize the change in the degree of arrangement of the carbon-based negative electrode active material formed in the negative electrode active layer while increasing the energy density of the negative electrode.

[0195] Hereinafter, the present invention will be described in more detail by way of examples and experimental examples.

[0196] However, the following examples and experimental examples are illustrative of the present invention, and the present invention is not limited to the following examples and experimental examples.

[0197] Examples 1 to 4 and Comparative Examples 1 to 3. Preparation of negative electrodes for lithium secondary batteries

[0198] As the first carbon-based negative electrode active material, a first artificial graphite formed by aggregation of flaky primary particles with an average secondary particle diameter (D 50 ) of 15.5 ± 0.1 μm was prepared; as the second carbon-based negative electrode active material, a second artificial graphite with an average particle diameter (D 50 ) of 16 ± 0.1 μm was prepared. In addition, carbon black as a conductive material, carboxymethyl cellulose (CMC) as a binder, and styrene butadiene rubber (SBR) were prepared.

[0199] Then, 96 parts by weight of the first artificial graphite, 1.5 parts by weight of carboxymethyl cellulose (CMC), and 2.5 parts by weight of styrene butadiene rubber (SBR) were mixed with water to reach 50% solids, thereby preparing the first negative electrode paste.

[0200] In addition, 96 parts by weight of the second artificial graphite, 1.5 parts by weight of carboxymethyl cellulose (CMC), and 2.5 parts by weight of styrene butadiene rubber (SBR) are mixed with water to reach 50% solids, thereby preparing the second negative electrode slurry.

[0201] Once each negative electrode slurry is prepared, the first negative electrode slurry and the second negative electrode slurry are simultaneously cast onto a copper thin sheet (thickness: 10 μm) in a roll-to-roll transfer (transfer speed: 5 m / min) manner using a dual-mode coater.

[0202] Permanent magnets with a length ratio of 110%-120% based on the length of the negative electrode slurry in the width direction are placed above the applied negative electrode slurry and below the negative electrode current collector, and a magnetic field is applied at a magnetic field intensity of 3000 ± 20 G. The magnetic field application time is shown in Table 1. The magnetized negative electrode slurry is dried with hot air to form a negative electrode in the form of a first negative electrode active layer and a second negative electrode active layer stacked in sequence on the negative electrode current collector. The formed negative electrode active layer is roll-pressed at a transfer rate of 3 m / s under a pressure of 100-150 MPa at 50 ± 1 °C to prepare a negative electrode for a lithium secondary battery (the average thicknesses of the first and second negative electrode active layers are 60 ± 5 μm and 160 ± 5 μm respectively, and the loading amount is 5 to 20 mg / cm 2 )

[0203] For each prepared negative electrode, a scanning electron microscope (SEM) analysis is performed on the cross-section of the negative electrode active layer to obtain a spectral image. The obtained image is analyzed to calculate the average aspect ratio of each artificial graphite contained in the first and second negative electrode active layers. This average aspect ratio is the ratio of the shortest dimension to the longest dimension passing through the center of the artificial graphite particles. The results are shown in Table 1 below.

[0204] In addition, for each manufactured negative electrode, X-ray diffraction spectroscopy (XRD) of the first negative electrode active layer and the second negative electrode active layer is performed to measure the spectrum. In the case of the first negative electrode active layer, after performing X-ray diffraction spectroscopy (XRD) on the second negative electrode active layer, the second negative electrode active layer is peeled off to remove the second negative electrode active layer, and X-ray diffraction is measured on the exposed surface of the first negative electrode active layer. In addition, the X-ray diffraction (XRD) measurement conditions are as follows:

[0205] - Target: Cu (Kα ray) graphite monochromator

[0206] - Slit: Divergence slit = 1 degree, receiving slit = 0.1 mm, scattering slit = 1 degree

[0207] - Measurement area: (110) plane: 76.5° < 2θ < 78.5° / (004) plane: 53.5° < 2θ < 56.0°.

[0208] From the spectra measured under the above conditions, the average optical index (O.I.) of the carbon-based active material in each negative electrode active layer was calculated using Equation 1. The results are shown in Table 1:

[0209] [Equation 1]

[0210] O.I. = I 004 / I 110

[0211] In Equation 1,

[0212] I 004 represents the peak area representing the (004) crystal plane in the X-ray diffraction (XRD) spectrum measurement of the negative electrode active layer,

[0213] I 110 represents the peak area representing the (110) crystal plane in the X-ray diffraction (XRD) spectrum measurement of the negative electrode active layer.

[0214] [Table 1]

[0215]

[0216] Comparative Examples 4 to 5. Preparation of Lithium Secondary Batteries

[0217] A negative electrode for a lithium secondary battery was prepared by performing the same method as in Example 2, except that spherical natural graphite or flaky artificial graphite was used as the first carbon-based negative electrode active material. In this case, the average particle size (D 50 ) was 15.5 ± 0.1 μm.

[0218] For the prepared negative electrode, (1) a scanning electron microscope (SEM) analysis was performed on the cross-section of the negative electrode active layer to calculate the average aspect ratio of the carbon-based negative electrode active material contained in the negative electrode active layer. In addition, (2) X-ray diffraction (XRD) of the first and second negative electrode active layers was performed to obtain the optical index (O.I.) of the carbon-based negative electrode active material contained in each layer. The results are shown in Table 2.

[0219] [Table 2]

[0220]

[0221] Examples 5 to 8 and Comparative Examples 6 to 10. Preparation of Lithium Secondary Batteries

[0222] Prepare LiNi with a particle size of 5 μm 0.7 Co 0.1 Mn 0.1 Al 0.1O2 is used as the positive electrode active material and is mixed with a carbon-based conductive material and polyvinylidene fluoride as a binder in a weight ratio of 94:3:3 in N-methylpyrrolidone (NMP) to form a slurry, which is cast on an aluminum thin sheet, dried in a vacuum oven at 120 °C, and roll-pressed to prepare the positive electrode.

[0223] The 1 Ah-class lithium secondary battery is assembled as follows: An 18 μm polypropylene separator is disposed between the positive electrode obtained above and the negative electrodes prepared in Examples 1 to 4 and Comparative Examples 1 to 5, respectively, and it is placed in a battery case, and then an electrolyte composition is injected.

[0224] Herein, the types of negative electrodes applied to the respective lithium secondary batteries are shown in Table 3.

[0225] [Table 3]

[0226] Type of negative electrode applied Example 5 Negative electrode prepared in Example 1 Example 6 Negative electrode prepared in Example 2 Example 7 Negative electrode prepared in Example 3 Example 8 Negative electrode prepared in Example 4 Comparative Example 6 Negative electrode prepared in Comparative Example 1 Comparative Example 7 Negative electrode prepared in Comparative Example 2 Comparative Example 8 Negative electrode prepared in Comparative Example 3 Comparative Example 9 Negative electrode prepared in Comparative Example 4 Comparative Example 10 Negative electrode prepared in Comparative Example 5

[0227] Experimental Example

[0228] In order to evaluate the performance of the negative electrodes of the present invention, the following experiments were conducted on the respective negative electrodes and lithium secondary batteries manufactured in the Examples and Comparative Examples.

[0229] 1) Evaluation of adhesion to the negative electrode current collector

[0230] The negative electrodes prepared in Examples 1 to 4 and Comparative Examples 1 to 5 were cut respectively to prepare specimens such that the transverse and longitudinal lengths were 25 mm and 70 mm, respectively. The prepared specimens were attached to a glass plate using double-sided tape, and they were arranged such that the current collector faced the glass plate. After fixing the specimens to the glass plate in a tensile testing machine, the negative electrode active layer of each negative electrode was pulled down at a speed of 100 mm / minute at 25 °C to form an angle of 90° with the negative electrode current collector. The peel force measured in real time was defined as the interfacial adhesion force between the negative electrode current collector and the negative electrode active layer, and the measurement results are shown in Table 4 below.

[0231] 2) 1C rate fast charging evaluation

[0232] Each lithium secondary battery prepared in Examples 5 to 8 and Comparative Examples 6 to 10 was activated by charging at a rate of 0.3C to 4.2V at 25 °C under CC-CV conditions and discharging at a rate of 0.3C to 2.5V under CC conditions.

[0233] Each activated lithium secondary battery was charged in a constant current-constant voltage (CC-CV) manner at a temperature of 25 °C, and the time required for the state of charge (SOC) to reach 80% was measured. The charging consisted of constant current (CC) charging at a current rate of 1.0 C until the voltage reached 4.2 V, then constant voltage (CV) charging was performed while maintaining 4.2 V, and cut-off was performed when the current reached 0.005 C rate. The measured charging times are shown in Table 4 below.

[0234] [Table 4]

[0235]

[0236] As shown in Table 4, it can be seen that the negative electrode for a lithium secondary battery of the present invention not only has excellent adhesion between the negative electrode active layer and the negative electrode current collector, but also completes charging at a rapid rate under 1C rate conditions.

[0237] Specifically, the first negative electrode active layer of the negative electrode of the example exhibited a high adhesion of greater than 30 gf / cm to the negative electrode current collector. In addition, the secondary battery including these negative electrodes reached a state of charge (SOC) of 80% within less than 25 seconds under standard constant current-constant voltage (CC-CV) charging conditions at 1C rate.

[0238] In contrast, the negative electrode of the comparative example exhibited lower adhesion between the first negative electrode active layer and the negative electrode current collector, which was 28 gf / cm or less. However, the negative electrode of Comparative Example 4 had an adhesion of 30 gf / cm between the first negative electrode active layer and the negative electrode current collector, but it was found that the secondary battery including this negative electrode required more than 30 seconds to reach a state of charge (SOC) of 80%.

[0239] This means that by separately controlling the average aspect ratio and optical index (O.I.) of the first carbon-based negative electrode active material and the first carbon-based negative electrode active material included in the first and second negative electrode active layers, the adhesion of the negative electrode current collector to the negative electrode active layer and the charging rate under standard C-rate conditions can be improved.

[0240] From these results, it can be seen that the negative electrode for a lithium secondary battery of the present invention has excellent life characteristics due to its high adhesion to the negative electrode current collector, and the lithium secondary battery including it has excellent output characteristics and can be charged in a short time even at 1C rate.

[0241] Although the foregoing has been described with reference to preferred embodiments of the present invention, those skilled in the art will understand that various modifications and changes can be made to the present invention without departing from the technical scope of the present invention described in the appended claims.

[0242] Therefore, the technical scope of the present invention is not limited to the content disclosed in the detailed description of the specification, but should be defined by the claims of the patent.

Claims

1. A negative electrode for a lithium secondary battery, comprising: A negative electrode current collector; A first negative electrode active layer containing a first carbon-based negative electrode active material provided on at least one surface of the negative electrode current collector; and A second negative electrode active layer containing a second carbon-based negative electrode active material provided on the first negative electrode active layer; Among them, The average aspect ratio of the first carbon-based negative electrode active material is greater than 0.85; Wherein, the average aspect ratio of the second carbon-based negative electrode active material is 0.85 or less; Wherein, the optical index (O.I.) of the first carbon-based negative electrode active material of the first negative electrode active layer according to Equation 1 is 50 or less: [Equation 1] O.I. = I 004 / I 110 In Equation 1, I 004 represents the peak area of the (004) crystal plane representing the carbon-based negative electrode active material in the measurement of the X-ray diffraction (XRD) spectrum of the negative electrode active layer. I 110 Indicates the peak area of the (110) crystal plane representing the carbon-based negative electrode active material in the X-ray diffraction (XRD) spectrum measurement of the negative electrode active layer.

2. The negative electrode for a lithium secondary battery according to claim 1, wherein, The average aspect ratio of the first carbon-based negative electrode active material is greater than 0.9 and less than 1.0, and the average aspect ratio of the second carbon-based negative electrode active material is 0.2 or more and less than 0.

8.

3. The negative electrode for a lithium secondary battery according to claim 1, wherein The optical index (O.I.) of the second carbon-based negative electrode active material of the second negative electrode active layer according to Equation 1 is 0.01 to 10.

4. The negative electrode for a lithium secondary battery according to claim 1, wherein, Each of the first carbon-based negative electrode active material and the second carbon-based negative electrode active material includes at least one of natural graphite and artificial graphite.

5. The negative electrode for a lithium secondary battery according to claim 1, wherein, The first carbon-based negative electrode active material is artificial graphite.

6. The negative electrode for a lithium secondary battery according to claim 1, wherein, The second negative electrode active layer has a porosity of 20% to 30%.

7. The negative electrode for a lithium secondary battery according to claim 1, wherein Based on the average thickness of the entire negative electrode active layer, the ratio of the thickness of the first negative electrode active layer is 1% to 100%.

8. The negative electrode for a lithium secondary battery according to claim 7, wherein, The average thickness of the entire negative electrode active layer is 50 μm to 500 μm.

9. A method for manufacturing the negative electrode for a lithium secondary battery according to claim 1, the manufacturing method comprising: Applying a first negative electrode paste and a second negative electrode paste such that the first negative electrode paste is on at least one surface of the negative electrode current collector and the second negative electrode paste is on the first negative electrode paste; Applying a magnetic field to the applied first and second negative electrode pastes; and Drying the applied first and second negative electrode pastes with a magnetic field applied thereto to form a negative electrode active layer.

10. The method for manufacturing a negative electrode for a lithium secondary battery according to claim 9, wherein, The step of applying a magnetic field is performed for a time of 1 second to 20 seconds.

11. The method for manufacturing a negative electrode for a lithium secondary battery according to claim 9, wherein, The step of applying a magnetic field is performed at a magnetic field intensity of 1000 G to 7000 G.

12. A lithium secondary battery, comprising: An electrode assembly, the electrode assembly comprising a positive electrode, the negative electrode according to claim 1, and a separator provided between the positive electrode and the negative electrode; And An electrolyte composition impregnating the electrode assembly.

13. The lithium secondary battery according to claim 12, wherein, The positive electrode includes a positive electrode active layer, the positive electrode active layer includes one or more positive electrode active materials of lithium metal oxides represented by Chemical Formula 1 and Chemical Formula 2, and is provided on at least one surface of a positive electrode current collector: [Chemical Formula 1] Li x [Ni y Co z Mn w M 1 v O2 [Chemical Formula 2] LiM 2 p Mn 1-p O4 In Chemical Formula 1 and Chemical Formula 2, M 1 is at least one element 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 or Mo, x, y, z, w, and v are respectively 1.0 ≤ x ≤ 1.30, 0.5 ≤ y < 1, 0 < z ≤ 0.3, 0 < w ≤ 0.3, and 0 ≤ v ≤ 0.1, where y + z + w + v = 1, M 2 is Ni, Co or Fe, and p is 0.05 ≤ p ≤ 1.

0.

14. The lithium secondary battery according to claim 13, wherein, The positive electrode active material includes LiNi 0.8 Co 0.1 Mn 0.1 O2, LiNi 0.6 Co 0.2 Mn 0.2 O2, LiNi 0.9 Co 0.05 Mn 0.05 O2, LiNi 0.6 Co 0.2 Mn 0.1 Al 0.1 O2, LiNi 0.6 Co 0.2 Mn 0.15 Al 0.05 O2, LiNi 0.7 Co 0.1 Mn 0.1 Al 0.1 O2, LiNi 0.7 Mn 1.3 O4, LiNi 0.5 Mn 1.5 O4 and LiNi 0.3 Mn 1.7 one or more of O4.

15. The lithium secondary battery according to claim 12, wherein The electrode assembly is a stacked electrode assembly, a folded electrode assembly, or a fold-stacked electrode assembly.

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