Negative electrode for lithium ion secondary battery, method for manufacturing the negative electrode, and lithium ion secondary battery including the negative electrode

Abstract

The present invention relates to a negative electrode for a lithium-ion secondary battery, a method for manufacturing the negative electrode, and a lithium-ion secondary battery including the negative electrode. The present invention also provides a negative electrode for a lithium-ion secondary battery, comprising a negative electrode mixture layer on at least one side of a negative electrode current collector, wherein the pores within the negative electrode mixture layer have a Z-tensor value of 0.33 or greater. Furthermore, the present invention provides a method for manufacturing the negative electrode and a lithium-ion secondary battery including the negative electrode.

Description

Technical Field

[0001] The present invention relates to a negative electrode for a lithium ion secondary battery, a method for manufacturing the negative electrode, and a lithium ion secondary battery comprising the negative electrode. Background Art

[0002] Typically, negative electrodes for lithium-ion secondary batteries are manufactured by applying a negative electrode mixture containing a negative electrode active material, a conductive agent, a binder, and a solvent to a negative electrode current collector, drying the mixture, and then pressing the mixture. During this process, the negative electrode active material, which has an anisotropic structure, is primarily oriented parallel to the negative electrode current collector (horizontally). During the charge and discharge process of the lithium-ion secondary battery, lithium ions migrate inward through the horizontal pores formed between the negative electrode active material.

[0003] However, as lithium ions migrate through these horizontally formed pores, the distance they travel inside the electrode increases significantly as the electrode load increases, leading to increased resistance during charging. Especially during high-rate charging (high C-rate), lithium salts (Li-plating) form on the electrode surface, leading to a decrease in battery capacity with repeated cycles.

[0004] At this time, when the negative electrode active material has an orientation perpendicular to the negative electrode current collector (vertical direction), the pores formed can also be formed in the vertical direction, thereby producing an effect in which the length of the channel for moving to the interior of the lithium ions becomes shorter, thereby reducing the resistance of the battery, especially improving the charging and discharging efficiency at high rates, thereby improving the fast charging performance.

[0005] Therefore, a technique has been proposed to apply a magnetic field during the coating of the negative electrode mixture onto the surface of the negative electrode current collector, thereby orienting the negative electrode active material perpendicularly to the negative electrode current collector and controlling the XRD value of the negative electrode active material in the resulting negative electrode within a specific range. However, since the XRD value based on the orientation of the negative electrode active material itself does not directly reflect the migration path of lithium ions, it has the limitation of not being directly related to battery performance. Therefore, an alternative solution is currently needed. Summary of the Invention

[0006] Technical problems to be solved

[0007] The present invention aims to improve battery performance by facilitating the intercalation and deintercalation of lithium ions, and to provide a negative electrode and a lithium ion secondary battery comprising the negative electrode, wherein the pores inside the negative electrode mixture layer of the negative electrode are perpendicularly oriented relative to the negative electrode current collector, thereby directly improving battery performance.

[0008] An object of the present invention is to provide a negative electrode for a lithium ion secondary battery, which allows direct confirmation of battery performance based on the degree of orientation of pores within a negative electrode mixture layer, and a battery including the negative electrode for a lithium ion secondary battery.

[0009] Another object of the present invention is to provide a method for orienting the pores in the negative electrode mixture layer perpendicularly to the negative electrode current collector.

[0010] An object of the present invention is to provide a method for producing a negative electrode for a lithium ion secondary battery, which allows direct confirmation of battery performance based on the degree of orientation of pores within a negative electrode mixture layer.

[0011] Technical Solution

[0012] The present invention relates to a negative electrode for a lithium ion secondary battery and provides a negative electrode for a lithium ion secondary battery, which is a negative electrode for a lithium ion secondary battery comprising a negative electrode mixture layer on at least one side of a negative electrode current collector, wherein the pores inside the negative electrode mixture layer have a Z-tensor value of 0.33 or more.

[0013] The negative electrode mixture layer may include at least one selected from artificial graphite, natural graphite, and silicon as a negative electrode active material.

[0014] The negative electrode active material may be in at least one shape selected from amorphous, plate-like, flake-like, spherical, and fibrous shapes.

[0015] The negative electrode mixture layer may include 94-98 wt % of a negative electrode active material, 0.1-3 wt % of a conductive agent, and 1.5-3 wt % of a binder, relative to the total weight of the negative electrode mixture layer.

[0016] The electrode density of a single surface of the negative electrode mixture layer may be 1.50 g / cm 3 above.

[0017] The present invention also relates to a method for manufacturing a negative electrode for a lithium-ion secondary battery, the method comprising the following steps: coating a negative electrode mixture containing a negative electrode active material on at least one side of a negative electrode current collector to form a negative electrode mixture layer (step A); and applying a magnetic field to the negative electrode mixture layer to change the orientation of the negative electrode active material (step B), wherein the pores inside the negative electrode mixture layer can have a Z-tensor value greater than 0.33.

[0018] The negative electrode mixture was heated at a temperature of 25°C and for 0.1s -1 The viscosity at a shear rate preferably has a range of 5000-30000 cp.

[0019] It is preferable to apply the magnetic field to the negative electrode mixture layer for 1 second or longer and 30 seconds or shorter.

[0020] The magnetic field applied to the negative electrode mixture layer preferably has an intensity of 1000 Gauss or more and 25000 Gauss or less.

[0021] The negative electrode mixture may include 94-98 wt % of a negative electrode active material, 0.1-3 wt % of a conductive agent, and 1.5-3 wt % of a binder relative to the total solid weight of the negative electrode mixture.

[0022] After the step B, a step of drying the negative electrode mixture layer may be further included (step C).

[0023] After step B, the method may further include pressing the negative electrode mixture layer (step D), wherein the pressing is performed so that the single-side electrode density of the negative electrode mixture layer can be 1.50 g / cm 3 above.

[0024] The present invention also provides a negative electrode for a lithium ion secondary battery manufactured by the method.

[0025] The single-side electrode density of the negative electrode mixture layer can be 1.50 g / cm 3 above.

[0026] In addition, the present invention provides a lithium-ion secondary battery comprising: an electrode assembly having the negative electrodes as described above and positive electrodes including a positive electrode mixture layer on at least one side of a positive electrode current collector alternately stacked with a separator as the boundary; and a battery case accommodating and sealing the electrode assembly.

[0027] Beneficial effects

[0028] The pores in the negative electrode mixture layer of the negative electrode for the lithium ion secondary battery of the present invention are oriented perpendicularly to the negative electrode current collector, thereby facilitating the insertion and removal of lithium ions, thereby improving the charging and discharging efficiency and rapid charging performance of the lithium ion secondary battery at high rates.

[0029] According to the method for manufacturing a negative electrode for a lithium ion secondary battery of the present invention, the orientation of the pores inside the negative electrode mixture layer in the vertical direction relative to the negative electrode current collector can be improved, thereby manufacturing a negative electrode with improved charging and discharging efficiency and fast charging performance of a lithium ion secondary battery at a high rate. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 This is a three-dimensional structural image of the negative electrode mixture layer obtained using an X-ray microscope.

[0031] Figure 2 It will Figure 1 An image of the pore structure inside the negative electrode mixture layer is obtained by converting the three-dimensional structural image of the negative electrode mixture layer obtained in FIG.

[0032] Figure 3 It is a schematic diagram showing the concept of the orientation tensor (Orientation tensor) for evaluating the orientation degree of pores inside the negative electrode mixture layer using a three-dimensional structure, (a) shows the case of random orientation in all axial directions of X1, X2 and X3 (3D random), (b) shows the case of orientation in the plane formed by the two axes of X1 and X2 (planar random), and (c) shows the case of aligned orientation in the X1 axis direction.

[0033] Figure 4 Graph showing changes in capacity retention according to the number of cycles of the lithium ion secondary batteries obtained in Examples 1 and 4 and Comparative Examples 1, 4, and 5. DETAILED DESCRIPTION

[0034] The advantages and features of the present invention and the methods for achieving them can be clearly understood with reference to the accompanying drawings and the detailed description of the embodiments. However, the present invention is not limited to the embodiments disclosed below and can be implemented in various different ways. This embodiment is provided to fully disclose the present invention and fully explain the scope of the invention to those skilled in the art. The present invention is limited only by the scope of the claims. Specific embodiments are described in detail with reference to the accompanying drawings. Regardless of the drawings, the same reference numerals refer to the same constituent elements, and "and / or" includes each of the mentioned items and all combinations of more than one item.

[0035] Unless otherwise defined, all terms (including technical and scientific terms) used in this specification may have the same meaning as commonly understood by those skilled in the art in the art to which the present invention belongs. Throughout the specification, unless otherwise specifically stated to the contrary, descriptions of a portion "comprising" or "including" a certain component element mean that other components may also be included, rather than excluding other components. In addition, unless otherwise specifically stated, the singular also includes the plural.

[0036] In this specification, when a layer, film, region, plate or the like is described as being "on" or "upper" another part, this includes not only the case of being "directly" "on" another part, but also the case of having other parts in between.

[0037] The present inventors have noticed that the migration path of lithium ions is through the pores inside the negative electrode mixture layer formed on the negative electrode current collector rather than the negative electrode active material itself, thereby completing the present invention.

[0038] Specifically, the present invention provides a negative electrode for a lithium-ion secondary battery having a specific Z-tensor value, obtained by evaluating the orientation tensor in each axial direction after three-dimensional imaging of the pores within the negative electrode mixture layer formed by applying a magnetic field using an X-ray microscope. Specifically, by confirming the degree of orientation of the pores in the electrode mixture layer, which serve as a pathway for lithium ion migration, a lithium-ion secondary battery can be provided that exhibits improved high-rate charge and discharge efficiency and rapid charging performance.

[0039] In the present invention, "perpendicular direction" refers to a case where the negative electrode is oriented at 90° to the negative electrode current collector, but is not limited to 90° and includes, for example, a case where the orientation is 30° or greater, preferably 45° or greater, and more preferably 60° or greater.

[0040] In addition, the vertical orientation of the pores means that the voids are mainly vertically oriented, rather than that all pores must be vertically oriented. In the present invention, the degree of vertical orientation of the pores relative to the current collector can be expressed as a Z-tensor.

[0041] The negative electrode for a lithium ion secondary battery of the present invention includes a negative electrode mixture layer on at least one side of a negative electrode current collector, and pores inside the negative electrode mixture layer may have a Z-tensor value of 0.33 or more.

[0042] For measuring the Z-tensor value, the pores inside the negative electrode may be imaged using an X-ray microscope, and the Z-tensor value may be derived from the obtained pore image.

[0043] More specifically, when using an X-ray microscope, the inside of the negative electrode mixture layer is observed. Figure 1 As shown in FIG. 1 , the three-dimensional structure of the pores inside the negative electrode mixture layer can be obtained. Moreover, the image of the three-dimensional structure inside the negative electrode mixture layer can be 3D rendered to convert it into a Figure 2 A 3D image of the pore structure inside the negative electrode mixture layer is shown. As an apparatus capable of obtaining such a 3D image, a device including an X-ray source, a detector, and a lens disposed between the detectors that can magnify the source can be used. For example, the Zeiss Xraida 520 Versa can be used. Furthermore, GEODICT can be used as software for the 3D rendering.

[0044] Based on the obtained three-dimensional structure of the pores in the negative electrode mixture layer, the degree of orientation of each axial direction of each pore can be expressed by an orientation tensor. That is, the three-dimensional structure of each pore can be evaluated using three axes, namely, X, Y, and Z. For each pore, the sum of the tensors of the three axes is "1". Figure 3 The concept is schematically shown in FIG.

[0045] Figure 3 The orientation tensor is used to represent the orientation degree of an individual. (a) shows the case of three-dimensional random orientation (3D random) in all axis directions of X1, X2 and X3, (b) shows the case of random orientation on a plane formed by the two axes of X1 and X2, that is, on a plane (planar random), and (c) shows the case of directional orientation in the direction of the X1 axis.

[0046] The larger the orientation tensor value of a particular axis, the more it is oriented toward that axis. That is, the larger the Z-tensor value, the more it can be evaluated as the Z axis (corresponding to Figure 3 The X3 axis) direction is well oriented.

[0047] Reference Figure 3 For example, when the X-tensor value, Y-tensor value, and Z-tensor value of the negative electrode active material are 0.33, the following is shown: Figure 3 In the random orientation shown in (a), when the Z-tensor value is greater than 0.33, it shows that the negative electrode active material is mainly oriented in the Z-axis, that is, perpendicular to the negative electrode current collector. In other words, when the Z-tensor value is greater than 0.33, it can be said that pores mainly oriented in the Z-axis direction are formed in the negative electrode mixture layer.

[0048] In some cases, when a specific axis orientation is developed, that is, when the specific orientation tensor value exceeds 0.33, the shape along the corresponding axis is developed, and thus the length along the axis may increase in an individual.

[0049] In the present invention, the Z-tensor value of the pores within the negative electrode mixture layer is preferably greater than or equal to 0.33. When the Z-tensor value of the pores is greater than or equal to 0.33, the orientation of the pores within the negative electrode mixture layer in a direction perpendicular to the negative electrode current collector increases, thereby shortening the migration path of lithium ions. Therefore, lithium ions can be easily inserted and removed during charging and discharging, thereby improving the charging and discharging efficiency at high rates, thereby improving the fast charging performance. In addition, the diffusion resistance of lithium ions into the electrode during charging and discharging is reduced, thereby suppressing the formation of lithium salts (Li-plating) on the electrode surface.

[0050] In the present invention, to enhance the perpendicular orientation of the pores within the negative electrode mixture layer relative to the negative electrode current collector, a method can be employed in which the negative electrode mixture is applied to the negative electrode current collector and a magnetic field is applied. Applying the magnetic field after the negative electrode mixture is applied to the negative electrode current collector to form the negative electrode mixture layer can align the negative electrode active material within the negative electrode mixture layer perpendicularly, and can also develop pores within the negative electrode mixture layer that are perpendicularly oriented relative to the negative electrode current collector.

[0051] At this time, the application of the magnetic field can be adjusted by confirming the changes in the orientation of the negative electrode active material and the pores depending on the intensity of the magnetic field, the application time of the magnetic field, and the viscosity of the negative electrode mixture.

[0052] The application of the magnetic field can be carried out under the following conditions: a magnetic field with an intensity in the range of 1000 gauss to 25000 gauss, for example, a magnetic field with an intensity in the range of 2000 gauss to 15000 gauss or 2500 gauss to 7500 gauss can be applied for 1 second to 30 seconds, for example, 1 second to 10 seconds.

[0053] At this time, at a temperature of 25°C and 0.1s -1 The negative electrode mixture preferably has a viscosity of 5000cp or more and 50000cp or less when measured under the conditions of a shear rate of . When the viscosity of the negative electrode mixture is less than 5000cp, it is advantageous in terms of the self-orientation of the negative electrode active material, thereby improving the orientation of the pores. However, there is a problem that the active material in the slurry is prone to precipitation due to the low viscosity. When the viscosity of the negative electrode mixture exceeds 50000cp, there is a problem that the negative electrode active material and the pores are difficult to orient due to the application of the magnetic field, and the coating processability deteriorates due to the high viscosity. That is, the negative electrode slurry is at a temperature of 25°C and a speed of 0.1s -1 The viscosity of the composite film is 5000 cp or more and 50000 cp or less under the condition of a shear rate of 1000 cp or less, preferably 30000 cp or less, thereby ensuring both the effect of suppressing the precipitation of the active material and the self-orientation effect of the active material and the pores.

[0054] In addition, within the above-mentioned viscosity range, the lower the viscosity, the lower the flow resistance to the negative electrode mixture even if the magnetic field strength and the magnetic field application time are the same, thereby easily improving the vertical orientation of the negative electrode active material relative to the current collector, and thus it is beneficial to prepare the pores inside the negative electrode mixture layer to have a Z-tensor value of more than 0.33.

[0055] In the present invention, the negative electrode mixture contains a negative electrode active material, a conductive agent, a binder, and water as a solvent, and may further contain a thickener and the like as necessary.

[0056] The negative electrode active material can use a carbon-based negative electrode active material. The carbon-based negative electrode active material can be applied to the present invention as long as it is generally used for manufacturing the negative electrode of a lithium ion secondary battery, and is not particularly limited, but can be artificial graphite or a mixture of artificial graphite and natural graphite. When using artificial graphite or a crystalline carbon-based material as a mixture of artificial graphite and natural graphite as the negative electrode active material, compared with the case of using an amorphous carbon-based active material, the crystallographic characteristics of the particles are more developed, so the orientation characteristics of the carbon material in the electrode plate with respect to an external magnetic field can be further improved, and thus the orientation of the pores can be improved.

[0057] The form of the artificial graphite or natural graphite can be amorphous, plate-like, flaky, spherical or fibrous, and can be a combination of two or more of them. In addition, when the artificial graphite and the natural graphite are used in combination, the mixing ratio can be 70:30 - 95:5 by weight.

[0058] In addition, while the negative electrode active material contains the carbon-based negative electrode active material, it can also contain at least one of a Si-based negative electrode active material, a Sn-based negative electrode active material or a lithium vanadium oxide negative electrode active material. When the negative electrode active material also contains these materials, it can be contained in a range of 1 - 50% by weight based on the weight of the entire negative electrode active material.

[0059] The Si-based negative electrode active material can be Si, a Si-C composite, SiO x (0 < x < 2), a Si-Q alloy, where Q is an element selected from alkali metals, alkaline earth metals, Group 13 elements, Group 14 elements, Group 15 elements, Group 16 elements, transition metals, rare earth elements, and combinations thereof other than Si, and specifically can be selected from Mg, Ca, Sr, Ba, Ra, Sc, Y, Ti, Zr, Hf, Rf, V, Nb, Ta, Db, Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Pb, Ru, Os, Hs, Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, Sn, In, Tl, Ge, P, As, Sb, Bi, S, Se, Te, Po, and combinations thereof.

[0060] The Sn-based negative electrode active material can be Sn, SnO2, or a Sn-R alloy, wherein R is not Sn or Si, but is an element selected from alkali metals, alkaline earth metals, Group 13 elements, Group 14 elements, Group 15 elements, Group 16 elements, transition metals, rare earth elements, and combinations thereof, and specifically can be selected from Mg, Ca, Sr, Ba, Ra, Sc, Y, Ti, Zr, Hf, Rf, V, Nb, Ta, Db, Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Pb, Ru, Os, Hs, Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, In, Tl, Ge, P, As, Sb, Bi, S, Se, Te, Po, and combinations thereof. In addition, at least one of these can be mixed with SiO2.

[0061] The negative electrode active material may be contained in an amount of 94-98 wt % relative to the solid weight of the negative electrode mixture.

[0062] In a specific embodiment, the negative electrode mixture includes a binder. The binder serves to bind the negative electrode active material particles to each other and to bind the negative electrode active material to the negative electrode current collector. The binder can be a water-based binder.

[0063] The water-based adhesive can include styrene-butadiene rubber, acrylated styrene-butadiene rubber, nitrile rubber, acrylate rubber, butyl rubber, ethylene-propylene copolymer, polyepichlorohydrin, polyphosphazene, polyacrylonitrile, polystyrene, ethylene-propylene-diene copolymer, polyvinyl pyridine, chlorosulfonated polyethylene, latex, polyester resin, acrylic resin, phenolic resin, epoxy resin, polyvinyl alcohol resin, acrylic resin or a combination thereof.

[0064] The content of the binder in the negative electrode active material layer may be 1.5-3 wt % relative to the solid weight of the negative electrode mixture.

[0065] Along with the binder, a thickener may also be included to impart viscosity. Examples of such thickeners include cellulose-based compounds, such as a mixture of one or more of carboxymethyl cellulose, hydroxypropyl methyl cellulose, methyl cellulose, or alkali metal salts thereof. Alkali metals such as sodium, potassium, or lithium may be used. The thickener may be present in an amount of 0.1 to 3 parts by weight relative to 100 parts by weight of the negative electrode active material.

[0066] The conductive agent is used to impart conductivity to the electrode, and any conductive agent commonly used in lithium-ion secondary batteries may be used without restriction. For example, carbon-based substances such as natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, and carbon fiber; metal-based substances such as metal powders or metal fibers such as copper, nickel, aluminum, and silver; conductive polymers such as polyphenylene derivatives; or conductive materials containing mixtures thereof may be used.

[0067] The conductive agent may be used in an amount of 0.1-3 wt % relative to the weight of the solid content of the negative electrode mixture.

[0068] The negative electrode current collector can be selected from copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, a polymer substrate coated with a conductive metal, and combinations thereof. The thickness of the negative electrode current collector is not particularly limited, and can be, for example, 5-30 μm.

[0069] As described above, the negative electrode mixture is coated on at least one side of the negative electrode collector, a magnetic field is applied to align the negative electrode active material and pores, and then dried and pressed, thereby manufacturing a negative electrode having a negative electrode mixture layer formed on the negative electrode collector.

[0070] The drying process is used to remove the solvent contained in the negative electrode mixture. The drying method is not particularly limited, and a common drying method can be used. For example, heating drying such as hot air drying can be mentioned.

[0071] The drying process may be performed at a temperature of 60-180° C., preferably 70-150° C., for 20-300 seconds, for example, 40-240 seconds or 60-200 seconds, but is not particularly limited.

[0072] After the drying process, a pressing process may be performed. The thickness or density of the negative electrode mixture layer may be adjusted by the pressing process. The pressing process may be performed using a common method such as a roll pressing method or a flat plate pressing method. The pressing process may achieve a single-side thickness of the negative electrode mixture layer of 20 μm to 120 μm, for example, 40 μm to 100 μm, or 60 μm to 80 μm.

[0073] On the other hand, the negative electrode for lithium ion secondary batteries of the present invention can be applied to a negative electrode mixture layer having a density of 1.5 g / cm 3 High-density electrodes above 1.5 g / cm 3 Above, for example, can be pressed into 1.5g / cm 3 Above and 2.2g / cm 3 Below, or 1.5g / cm 3 Above and 2.0g / cm3 the following.

[0074] The negative electrode of the present invention makes it easy for lithium ions to diffuse into the interior of the electrode through the negative electrode active material and pores inside the negative electrode mixture layer, especially by orienting the pores in a vertical direction relative to the negative electrode current collector, thereby improving the charging and discharging efficiency at high rates, thereby improving the fast charging performance.

[0075] A lithium-ion secondary battery can be manufactured by alternately stacking the negative electrode and the positive electrode with the separator as the boundary to produce an electrode assembly, which is then inserted into a battery case for sealing and injected with an electrolyte, wherein the negative electrode is a negative electrode obtained by the present invention that contains pores with developed vertical orientation.

[0076] The positive electrode is described in more detail below. The positive electrode is not particularly limited. The positive electrode is formed by coating a positive electrode mixture on at least one side of a positive electrode current collector, drying, and pressing the mixture to form a positive electrode mixture layer. Any positive electrode commonly used in lithium-ion secondary batteries is applicable to the present invention.

[0077] The positive electrode mixture includes a positive electrode active material, a binder, and a solvent, and may include a conductive agent as needed, and may further include a thickener.

[0078] The positive electrode active material may be a compound that can reversibly intercalate and deintercalate lithium (lithiated intercalation compound), specifically, one or more composite oxides of lithium and a metal selected from cobalt, manganese, nickel, and combinations thereof.

[0079] As a more specific example, the positive electrode active material can be represented by the general formula LiMO2, and a layered lithium transition metal compound (oxide) can be listed, wherein M contains at least one of transition metal elements such as Ni, Co, and Mn, and can further contain other metal elements or non-metallic elements. As the composite oxide, for example, a single lithium transition metal composite oxide containing one of the transition metal elements, a binary lithium transition metal composite oxide containing two of the transition metal elements, a ternary lithium transition metal composite oxide containing transition metal elements such as Ni, Co and Mn as constituent elements can be listed, preferably such as Li(Ni) 1 / 3 Co 1 / 3 Mn 1 / 3 )O2, etc., a ternary lithium transition metal composite oxide.

[0080] In addition, the positive electrode active material can be a lithium transition metal compound (oxide) represented by the general formula Li2MO3, wherein M contains at least one of transition metal elements such as Mn, Fe, Co, and can further contain other metal elements or non-metallic elements, for example, Li2MnO3, Li2PtO3, etc.

[0081] In addition, the positive electrode active material may be a solid solution of the LiMO 2 and the Li 2 MO 3 , for example, a solid solution represented by 0.5LiNiMnCoO 2 -0.5Li 2 MnO 3 .

[0082] In addition, the positive electrode active material may also be used with a coating on the surface of the positive electrode active material, and the above-mentioned compound and the compound with the coating may also be mixed and used. The coating may contain at least one coating element compound selected from the group consisting of oxides, hydroxides, oxyhydroxides, oxycarbonates and hydroxycarbonates of the coating element. The compounds forming these coatings may be amorphous or crystalline. The coating elements contained in the coating may be Mg, Al, Co, K, Na, Ca, Si, Ti, V, Sn, Ge, Ga, B, As, Zr or mixtures thereof.

[0083] In the positive electrode, the positive electrode active material may account for 90-98 wt % relative to the solid weight of the positive electrode mixture.

[0084] The binder serves to bind the positive electrode active material particles to each other and to bind the positive electrode active material to the positive electrode current collector, and may be present in an amount of 1.5-5% by weight relative to the solid weight of the positive electrode mixture. Examples of the binder include polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, cellulose diacetate, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, and polyamide fiber.

[0085] The binder may be combined with a thickener to impart viscosity. The thickener may be the same as that in the negative electrode mixture and may be present in an amount of 0.1 to 3 parts by weight relative to 100 parts by weight of the positive electrode active material.

[0086] The conductive agent is used to impart conductivity to the positive electrode. Any conductive material commonly used in the positive electrode of a lithium-ion secondary battery can be used, including the conductive agent used in the negative electrode mixture. The conductive agent can be used in an amount of 0.1-5% by weight based on the solid weight of the positive electrode mixture.

[0087] The solvent may be an aqueous solvent such as water or a non-aqueous solvent. The non-aqueous solvent may be used in the present invention as long as it is commonly used to manufacture a positive electrode mixture for a lithium ion secondary battery. For example, N-methyl-2-pyrrolidone (NMP) may be mentioned, but it is not limited thereto.

[0088] The positive electrode current collector can be made of a metal with good conductivity, such as aluminum, nickel, titanium, stainless steel, etc., and can be in various shapes such as sheet, thin, mesh, etc. The thickness of the positive electrode current collector is not particularly limited, and can be, for example, 5-30 μm.

[0089] As described above, the positive electrode mixture is applied to at least one side of the positive electrode collector, dried, and pressed, thereby manufacturing a positive electrode having a positive electrode mixture layer formed on the positive electrode collector.

[0090] The drying and pressing process can be performed by the same method as that used in manufacturing the negative electrode, and thus detailed description thereof will be omitted.

[0091] The separator between the positive electrode and the negative electrode is a porous sheet, non-woven fabric, etc., and can be polyethylene, polypropylene, polyvinylidene fluoride, or a multilayer film of two or more layers thereof, a two-layer mixed multilayer film of polyethylene / polypropylene, a three-layer mixed multilayer film of polyethylene / polypropylene / polyethylene, a three-layer mixed multilayer film of polypropylene / polyethylene / polypropylene, etc., and can be a separator having a porous heat-resistant layer on one or both sides of the porous sheet, non-woven fabric, etc. The separator is not particularly limited, and for example, a separator having a thickness of about 10-40 μm can be used.

[0092] The electrolyte comprises a non-aqueous organic solvent and a lithium salt. The non-aqueous organic solvent acts as a medium for the movement of ions involved in the electrochemical reaction of the battery. Commonly used solvents in lithium-ion secondary batteries may be used, such as carbonate-based solvents, ester-based solvents, ether-based solvents, ketone-based solvents, alcohol-based solvents, or aprotic solvents. The organic solvents may be used alone or as a mixture of more than one.

[0093] The lithium salt is a substance dissolved in an organic solvent and plays the following role, that is, it is used as a supply source of lithium ions in the battery, so that the lithium ion secondary battery can basically operate and promote the movement of lithium ions between the positive electrode and the negative electrode. For example, it can be selected from LiPF6, LiBF4, LiSbF6, LiAsF6, LiN(SO2C2F5)2, Li(CF3SO2)2N, LiN(SO3C2F5)2, LiC4F9SO3, LiClO4, LiAlO2, LiAlCl4, LiN(C x F 2x+1 SO2)(CyF 2y+1 SO2) (wherein x and y are each independently an integer of 1-20), one or more of LiCl, LiI and LiB(C2O4)2 (lithium bis(oxalato)borate (LiBOB). The concentration of the lithium salt is not particularly limited, and a concentration within the range of 0.1M-2.0M can be used.

[0094] To improve battery life, the electrolyte may further include vinylene carbonate or an ethylene carbonate-based compound, as needed. Specifically, the pores within the negative electrode mixture layer of the negative electrode of the present invention are primarily Z-oriented, i.e., the Z-tensor value of the pores within the negative electrode mixture layer is 0.33 or greater. During charging and discharging, lithium ions readily diffuse into the electrode, thereby reducing battery resistance and, in particular, improving high-rate charge and discharge efficiency, thereby enhancing cycle life characteristics and rapid charging performance.

[0095] Example

[0096] The present invention will be described in more detail below by giving examples. However, the following examples are merely examples of the present invention and are not intended to limit the present invention.

[0097] Examples 1 to 4 and Comparative Examples 1 to 5

[0098] 89.3 wt% of artificial graphite, 5 wt% of silicon dioxide, 1.5 wt% of styrene-butadiene rubber, 1.2 wt% of carboxymethyl cellulose, and 3 wt% of carbon nanotubes (CNTs) were mixed in water to prepare a negative electrode mixture. -1 The viscosity of the negative electrode mixture at the shear rate is shown in Table 1.

[0099] The prepared negative electrode mixture was applied to the upper and lower surfaces of a negative electrode current collector made of Cu foil, and the current collector was passed between a pair of neodymium magnets generating a 4000 gauss magnetic field. The movement speed of the negative electrode current collector coated with the negative electrode mixture was varied, and the duration of the magnetic field application was adjusted as shown in Table 1. The negative electrode was then dried and pressed to produce a negative electrode. However, no magnetic field was applied in Comparative Example 1.

[0100] For the manufactured negative electrode, the 3D Z-tensor value of pores was evaluated using an X-ray microscope (Xraida 520 Versa manufactured by Zeiss, 3D rendering software: GEODICT). The results are shown in Table 1.

[0101] 96 wt% of Li(Ni) was mixed in N-methylpyrrolidone solvent. 0.8 Co 0.1 Mn 0.1 )O2 positive electrode active material, 2 wt% CNT conductive agent and 2 wt% polyvinylidene fluoride binder to prepare a positive electrode mixture. The prepared negative electrode mixture was coated on both sides of the positive electrode current collector of Al foil, dried and pressed to produce a positive electrode.

[0102] The negative electrode and the positive electrode are alternately stacked with the separator as the boundary and inserted into a soft package. After sealing, a mixed solvent of ethylene carbonate and diethyl carbonate dissolved with 1M LiPF6 (volume ratio of 50:50) is injected to manufacture a lithium ion secondary battery.

[0103] For each of the manufactured lithium ion secondary batteries, high rate (2.5C) charge and discharge (0.3C) were repeated, and the capacity retention rate at 100 cycles and 300 cycles was measured. The results are shown in Table 1. In addition, for the lithium ion secondary batteries of Example 1, Example 3 and Comparative Examples 1, Comparative Examples 4, and Comparative Examples 5, the change in capacity retention rate according to the number of cycles is shown in Table 1. Figure 4 middle.

[0104] [Table 1]

[0105]

[0106]

[0107] As shown in Table 1, the Z-tensor values for Comparative Example 1, where no magnetic field was applied, and Comparative Example 2, where the magnetic field application time was as short as 0.5 seconds, were as low as 0.24 and 0.25, respectively, for the negative electrode mixtures having the same viscosity of 29290 cp. Meanwhile, in Examples 1 to 4, where the magnetic field was applied for 1 second or longer, the Z-tensor values for the pores within the negative electrode mixture layer were as high as 0.33 or higher. Furthermore, Examples 1 to 4 demonstrated high capacity retention rates of 98% or higher up to 300 cycles.

[0108] On the other hand, the high-rate charge and discharge cycle results show that the lithium-ion secondary batteries of Comparative Examples 1 and 2, which included negative electrodes with low Z-tensor values of pores within the negative electrode mixture layer (less than 0.33), showed capacity retention rates of less than 80% and 85%, respectively, after 300 cycles, significantly decreasing to below 95%. This confirms that the pores within the negative electrode mixture layer, which serve as lithium ion migration pathways, are well-aligned perpendicularly to the current collector in Examples 1 to 3, resulting in improved battery performance.

[0109] In addition, from Figure 4 It can also be seen that the lithium-ion secondary battery of Example 1, which includes the negative electrode of the present invention, maintained a nearly constant capacity retention rate over 300 cycles of charge and discharge. Meanwhile, the lithium-ion secondary battery of Comparative Example 1, to which no magnetic field was applied, experienced a sharp decrease in capacity retention from the initial stages of the charge and discharge cycles, reaching a low of 80% after 100 cycles. Therefore, no further capacity changes were observed after 100 cycles.

[0110] The results described above can be explained as follows: as the magnetic field is applied, the vertical orientation of the negative electrode active material and the degree of orientation of the pores in the Z-axis direction inside the negative electrode mixture layer are improved, which shortens the movement distance of lithium ions, reduces the resistance to the diffusion of lithium ions into the electrode during charging, and inhibits the generation of lithium salts under high-rate charging.

[0111] In particular, compared to the negative electrode of Example 2 manufactured by applying a magnetic field for the same time and the negative electrode of Example 3 manufactured by applying a magnetic field for a longer time, the negative electrode of Example 4 manufactured using a low-viscosity negative electrode mixture has a higher Z-tensor value, and the capacity retention rate of the lithium-ion secondary battery of Example 4 after 300 cycles is at the level of 99.5%, which can be confirmed to be very high compared to the lithium-ion secondary batteries of Examples 2 and 3. In addition, from Figure 4 It can be seen that the capacity retention rate of the secondary battery of Example 4 including the negative electrode of the present invention remains almost constant during 300 cycles of charge and discharge.

[0112] The results described above are because the battery performance results depend on the difference in Z-tensor values. Even if the magnetic field of the same intensity is applied for a shorter time, the flow resistance of the negative electrode mixture is reduced due to the low viscosity of the negative electrode mixture, and thus the orientation of the negative electrode active material and pores inside the negative electrode mixture layer in the vertical direction relative to the current collector is further improved.

[0113] On the other hand, the negative electrodes of Comparative Examples 3 to 5, which were manufactured using a negative electrode mixture having a viscosity as high as 50,000 cp or more, were subjected to a magnetic field of the same intensity and for the same time as in Example 4 to vertically orient the negative electrode active material, but the Z-tensor value showed a value less than 0.33, and therefore it was evaluated that the orientation of the pores in the negative electrode mixture layer in the vertical direction relative to the negative electrode current collector was not sufficiently developed.

[0114] The lithium ion secondary batteries of Comparative Examples 3 to 5 including the negative electrodes described above had a capacity retention rate of 90.2% or less after 300 cycles, and thus it was confirmed that the capacity retention rate was significantly deteriorated. Figure 4 It can be seen that the capacity retention of the lithium ion secondary batteries of Comparative Examples 4 and 5 begins to decrease rapidly after 100 cycles. Comparative Example 4 shows a capacity retention of less than 85% at 200 cycles without further confirmation of capacity change, but it is estimated that it will have a capacity retention of less than 80% at 300 cycles.

[0115] In addition, a comparison between Comparative Examples 4 and 5 shows that the Z-tensor value is not affected even when a mixture having the same viscosity is used and the magnetic field application time is increased.

[0116] Examples 5 to 6 and Comparative Examples 6 to 7

[0117] A negative electrode was manufactured by the same method as in Example 1 except that the viscosity of the negative electrode mixture and the magnetic field application time were adjusted according to Table 2. The Z-tensor value was evaluated and the results are shown in Table 2.

[0118] Furthermore, the XRD of the produced negative electrode was measured to confirm the degree of orientation of the negative electrode active material with respect to the surface of the negative electrode current collector. The results are shown in Table 2.

[0119] Furthermore, positive electrodes and lithium ion secondary batteries were manufactured in the same manner as in Example 1. The capacity retention rate of each of the manufactured lithium ion secondary batteries was measured in the same manner as in Example 1. The results are shown in Table 2.

[0120] [Table 2]

[0121]

[0122] When I(110) / I(002) is 0.5(%) or more, it indicates that the negative electrode active material is vertically oriented with respect to the negative electrode current collector, and the closer it is to 0(%), the more horizontally oriented it is.

[0123] In Table 2, when comparing Example 5 and Comparative Example 6, the I(110) / I(002) value was 0.15% before self-orientation and 0.17% after self-orientation. These values were both below 0.5%, which is a value at which the negative electrode active material was not observed to be oriented perpendicularly to the negative electrode current collector even when a magnetic field was applied.

[0124] However, the Z-tensor value of the negative electrode of Example 5 increased by 0.18 compared to the negative electrode of Comparative Example 6, indicating that the application of the magnetic field developed pores within the negative electrode mixture layer perpendicular to the negative electrode current collector. This increase in pore orientation due to the application of the magnetic field cannot be confirmed by XRD analysis, which indicates the degree of orientation of the negative electrode active material, but can be confirmed by evaluating the Z-tensor value.

[0125] At this time, the Z-tensor value of the negative electrode of Example 5 is 0.48, which shows that the voids inside the negative electrode mixture layer are mainly oriented in the vertical direction of the negative electrode current collector. The capacity retention rate of the lithium-ion secondary battery including this negative electrode is 99.5%, which shows a significantly higher capacity retention rate compared with Comparative Example 6.

[0126] On the other hand, comparing Example 6 and Comparative Example 7 in Table 2, in the case of I(110) / I(002), the ratio before self-orientation is 0.17%, while it is 0.78% after self-orientation. Therefore, it can be seen that the value of I(110) / I(002) becomes more than 0.5% after self-orientation.

[0127] Furthermore, the Z-tensor value of the negative electrode of Example 6 increased by 0.1 compared to the negative electrode of Comparative Example 7, indicating that the application of the magnetic field has led to the development of pores perpendicular to the negative electrode current collector. The Z-tensor value of the negative electrode of Example 6 was 0.42, indicating that the voids within the negative electrode mixture layer are primarily oriented perpendicular to the negative electrode current collector. The lithium-ion secondary battery containing this negative electrode achieved a capacity retention rate of 98.8%, significantly higher than that of Comparative Example 7.

[0128] According to the results of Example 5, Example 6 and Comparative Example 6, Comparative Example 7 as described above, it can be seen that by applying a magnetic field to change the orientation degree of the negative electrode active material, and analyzing the changes in the XRD value and the Z-tensor value, it can be seen that as the magnetic field is applied, the orientation degree of the negative electrode active material changes, when expressed as I(110) / I(002) derived from XRD measurement, the I(110) / I(002) value is not necessarily directly related to the battery performance.

[0129] On the other hand, it was confirmed that when pores oriented perpendicular to the negative electrode current collector are developed within the negative electrode mixture layer by applying a magnetic field, the migration path for lithium ions is shortened, thereby improving battery performance. In other words, it was found that the characteristic directly related to battery performance is the Z-tensor value, which is related to the orientation of the pores within the negative electrode mixture layer, which serve as the migration path for lithium ions, rather than the orientation of the negative electrode active material within the negative electrode mixture layer.

Claims

1. A negative electrode for a lithium ion secondary battery, comprising a negative electrode mixture layer on at least one side of a negative electrode current collector, wherein pores inside the negative electrode mixture layer have a Z-tensor value of 0.33 or greater, The negative electrode active material of the negative electrode mixture layer has an I(110) / I(002) value of 0.5% or less, The negative electrode mixture layer includes a water-based binder, The Z-tensor value is measured by the following steps: Using an X-ray microscope, the three-dimensional structure inside the negative electrode mixture layer is obtained. performing three-dimensional rendering on the image of the three-dimensional structure inside the negative electrode mixture layer to convert it into a three-dimensional image of the pore structure inside the negative electrode mixture layer, and Use the X, Y, and Z axes to evaluate the three-dimensional structure of each pore; The I(110) / I(002) is measured by XRD.

2. The negative electrode for a lithium ion secondary battery according to claim 1, wherein The water-based binder is present in an amount of 1.5-3 wt % relative to the total weight of the negative electrode mixture layer.

3. The negative electrode for a lithium ion secondary battery according to claim 1, wherein The negative electrode mixture layer contains at least one selected from artificial graphite, natural graphite, and silicon as a negative electrode active material.

4. The negative electrode for a lithium ion secondary battery according to claim 3, wherein The negative electrode active material is at least one selected from amorphous, plate-like, flake-like, spherical and fibrous shapes.

5. The negative electrode for a lithium ion secondary battery according to claim 1, wherein The negative electrode mixture layer includes 94-98 wt % of a negative electrode active material, 0.1-3 wt % of a conductive agent, and 1.5-3 wt % of a water-based binder relative to the total weight of the negative electrode mixture layer.

6. The negative electrode for a lithium ion secondary battery according to claim 1, wherein described 7. A method for manufacturing a negative electrode for a lithium ion secondary battery, comprising the following steps: A step A of coating a negative electrode mixture containing a negative electrode active material on at least one side of a negative electrode current collector to form a negative electrode mixture layer; and step B of applying a magnetic field to the negative electrode mixture layer to change the orientation of the negative electrode active material, wherein the pores inside the negative electrode mixture layer have a Z-tensor value of 0.33 or more, The negative electrode active material of the negative electrode mixture layer has an I(110) / I(002) value of 0.5% or less, The negative electrode mixture layer includes a negative electrode active material and a water-based binder, The Z-tensor value is measured by the following steps: Using an X-ray microscope, the three-dimensional structure inside the negative electrode mixture layer is obtained. performing three-dimensional rendering on the image of the three-dimensional structure inside the negative electrode mixture layer to convert it into a three-dimensional image of the pore structure inside the negative electrode mixture layer, and Use the X, Y, and Z axes to evaluate the three-dimensional structure of each pore; The I(110) / I(002) is measured by XRD.

8. The method for producing a negative electrode for a lithium ion secondary battery according to claim 7, wherein: The water-based binder is present in an amount of 1.5-3 wt % relative to the total weight of the negative electrode mixture layer.

9. The method for producing a negative electrode for a lithium ion secondary battery according to claim 7, wherein: The negative electrode mixture was heated at a temperature of 25°C and for 0.1s -1 The viscosity at the shear rate is in the range of 5000-30000cp.

10. The method for producing a negative electrode for a lithium ion secondary battery according to claim 7, wherein: A magnetic field is applied to the negative electrode mixture layer for 1 second or longer and 30 seconds or shorter.

11. The method for producing a negative electrode for a lithium ion secondary battery according to claim 7, wherein: The magnetic field applied to the negative electrode mixture layer is 1000 Gauss or more and 25000 Gauss or less.

12. The method for producing a negative electrode for a lithium ion secondary battery according to claim 7, wherein: The negative electrode mixture comprises 94-98 wt % of a negative electrode active material, 0.1-3 wt % of a conductive agent, and 1.5-3 wt % of a water-based binder relative to the total solid weight of the negative electrode mixture.

13. The method for producing a negative electrode for a lithium ion secondary battery according to claim 7, wherein: After the step B, the method further includes the step C of drying the negative electrode mixture layer.

14. The method for producing a negative electrode for a lithium ion secondary battery according to any one of claims 7 to 13, wherein: After step B, the negative electrode mixture layer is further compressed to obtain a single-side electrode density of 1.50 g / cm 3 above.

15. A lithium-ion secondary battery comprising: An electrode assembly comprising a negative electrode according to any one of claims 1 to 6 and a positive electrode including a positive electrode mixture layer on at least one side of a positive electrode current collector, alternately stacked with a separator as a boundary; and A battery case houses and seals the electrode assembly.

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