Method for manufacturing electrodes for secondary batteries

Abstract

The present invention provides a method for manufacturing a secondary battery electrode, which can effectively reduce the amount of residual moisture in the electrode and significantly improve electrode adhesion. The method for manufacturing a secondary battery electrode includes the steps of manufacturing an electrode having an electrode active material layer formed thereon, rolling the electrode, and drying the rolled electrode, and the drying step can be performed such that the drying temperature in at least a portion of the electrode is 170°C to 210°C, which is a temperature higher than the melting point (170°C) of polyvinylidene fluoride (PVDF) binder resin.

Description

[Technical Field]

[0001] This application claims the benefit of priority based on Korean Patent Application No. 10-2022-0108709, filed August 29, 2022, and Korean Patent Application No. 10-2023-0112358, filed August 25, 2023, and all contents disclosed in the documents of said Korean patent applications are incorporated herein by reference.

[0002] The present invention relates to a method for manufacturing an electrode for a secondary battery, and more particularly, to a method for manufacturing an electrode for a secondary battery, which can minimize deterioration of the crystallinity of a binder, effectively reduce the amount of residual moisture in the electrode, and significantly improve the adhesive strength between an electrode active material layer and an electrode current collector. [Background technology]

[0003] In recent years, the rapid increase in the use of fossil fuels has led to an increasing demand for alternative and clean energy sources. As part of this trend, the field of electrochemical power generation and storage has recently become one of the most actively researched areas.

[0004] A typical example of an electrochemical element that utilizes such electrochemical energy is a secondary battery, and its range of use is becoming increasingly wider. In particular, with the recent technological development and increasing demand for portable devices such as portable computers, portable phones, and cameras, the demand for secondary batteries with high energy density, i.e., high-capacity lithium secondary batteries, as an energy source for these devices has been rapidly increasing.

[0005] Among such secondary batteries, much research has been conducted to develop lithium secondary batteries that exhibit high energy density and operating potential, as well as long cycle life and a low self-discharge rate.

[0006] Meanwhile, the lithium secondary battery has a structure in which a chargeable and dischargeable electrode assembly having a positive electrode / separator / negative electrode structure is attached to a battery case, and the positive and negative electrodes are manufactured by coating a slurry containing a mixture of an active material and a binder resin component on one or both sides of a metal current collector to form an electrode active material layer, followed by drying and rolling.

[0007] In the past, after rolling the active material layer, a drying process was typically performed at a low temperature at which typical binder resins, such as polyvinylidene fluoride (PVDF)-based binder resins, would not melt in order to remove moisture remaining inside the electrode. If the drying process was performed at a temperature above the melting point of the PVDF-based binder resin (170°C) after rolling, the PVDF-based binder resin would melt, reducing its crystallinity and making the electrode less brittle, resulting in reduced electrode strength and poor durability. In contrast, if the drying process was performed at a temperature below 170°C, a long drying time would be required to reduce the residual moisture in the electrode to a desired level, resulting in a further problem of poor productivity.

[0008] Therefore, there is a need to develop a new method for manufacturing an electrode that can prevent a decrease in electrode strength during electrode manufacturing, reduce the amount of residual moisture remaining inside the electrode to a desired level, and improve adhesion. Summary of the Invention [Problem to be solved by the invention]

[0009] The present invention aims to solve these problems by providing a method for manufacturing a secondary battery electrode in which the drying process after rolling is performed such that the drying temperature in at least a portion of the process is equal to or higher than the melting point of the binder (170°C), and the total drying time is 5 seconds or less. [Means for solving the problem]

[0010] According to one embodiment, the present invention provides a method for manufacturing an electrode for a secondary battery, the method comprising the steps of: manufacturing an electrode in which an electrode active material layer is formed on an electrode current collector; rolling the electrode; and drying the rolled electrode, wherein the drying step is performed such that a drying temperature in at least a portion of the drying step is 170°C to 210°C.

[0011] In the method, the total drying time of the rolled electrode may be 2 to 5 seconds.

[0012] Also, the step of drying the rolled electrode may be performed by a roll-to-roll process.

[0013] The step of drying the rolled electrode is carried out at a temperature of 130°C or higher, and specifically may include a step of carrying out a first drying step at 130°C to 150°C, a step of carrying out a second drying step at 150°C to 170°C, and a step of carrying out a third drying step at 170°C to 210°C. [Effects of the Invention]

[0014] According to the method for manufacturing a secondary battery electrode of the present invention, during the drying process after rolling, the drying temperature in a portion of the process is set to be equal to or higher than the melting point temperature (170°C) of the binder, and the total drying time is set to 5 seconds or less. This minimizes the deterioration of the binder's crystallinity and effectively reduces the amount of residual moisture in the electrode, thereby significantly improving the adhesion between the electrode active material layer and the electrode current collector, thereby realizing a lithium secondary battery with improved durability.

[0015] The following drawings attached to this specification are intended to illustrate preferred embodiments of the present invention and, together with the above-described content of the invention, serve to further understand the technical concept of the present invention. Therefore, the present invention should not be interpreted as being limited only to the matters shown in these drawings. [Brief explanation of the drawings]

[0016] [Figure 1] 10 is a graph showing the evaluation results of the amount of residual moisture inside the electrode according to Experimental Example 1. [Figure 2] 10 is a graph showing the evaluation results of electrode adhesive strength according to Experimental Example 2. DETAILED DESCRIPTION OF THE INVENTION

[0017] The present invention will be described in more detail below. Terms (including technical and scientific terms) and words used in the present specification and claims may be used in the sense that they can be commonly understood by a person of ordinary skill in the art to which the present invention belongs. Furthermore, terms defined in commonly used dictionaries should not be interpreted ideally or excessively unless clearly defined otherwise.

[0018] The terms used in this specification are for the purpose of describing the embodiments and are not intended to limit the present invention. In this specification, the singular includes the plural unless otherwise stated in the text. The words "comprises" and / or "comprising" used in this specification do not exclude the presence or addition of one or more other elements in addition to the elements mentioned.

[0019] In this specification, when a part is said to include a certain component, this means that it may further include other components, not excluding other components, unless otherwise specified to the contrary.

[0020] The present invention will now be described in more detail.

[0021] Method for manufacturing electrodes for secondary batteries The method for producing an electrode for a secondary battery of the present invention comprises the steps of: (S1) manufacturing an electrode in which an electrode active material layer is formed on an electrode current collector; (S2) rolling the electrode; (S3) drying the rolled electrode; The drying step may be carried out so that the drying temperature is 170°C to 210°C in at least a portion of the step.

[0022] Conventionally, a method has been required in which a slurry containing an active material and a binder resin is applied to one or both sides of a metal current collector to form an electrode with an active material layer, followed by rolling the electrode. To prevent the deterioration of the crystallinity of the binder resin, for example, a polyvinylidene fluoride (PVDF)-based binder resin, a drying process is performed at a low temperature (170°C or less) at which the binder resin does not melt. However, when drying is performed at a low temperature, a long drying time is required to reduce the residual moisture content in the electrode to the desired level, resulting in poor productivity.

[0023] As a result of extensive research to solve these problems, the inventors have found that, in the drying process after rolling, the drying temperature in a portion of the process is set to a temperature equal to or higher than the melting point (170°C) of the polyvinylidene fluoride (PVDF) binder resin, and the total drying time is set to 5 seconds or less, thereby minimizing the deterioration of the binder resin crystallinity and effectively reducing the amount of residual moisture in the electrode, and have completed the present invention.

[0024] This will be explained in detail below.

[0025] (S1) Step of manufacturing electrodes The method for manufacturing an electrode for a secondary battery according to the present invention may include a step of manufacturing an electrode in which an electrode active material layer is formed on an electrode current collector.

[0026] The electrode active material layer can be prepared by mixing or dispersing an electrode active material and a binder in a solvent to prepare an electrode slurry composition, and then coating the electrode slurry composition on an electrode current collector and drying it.

[0027] In this case, the electrode slurry composition may be applied to the electrode current collector by a conventional method known in the art. For example, the electrode slurry composition may be uniformly dispersed using a doctor blade or may be applied by die casting, comma coating, screen printing, or the like.

[0028] The drying step of the electrode slurry composition applied onto the electrode current collector may be carried out by a known ordinary method, for example, a vacuum heat treatment within a certain temperature range or a heat treatment method such as hot air injection.

[0029] The temperature range of the drying process may be 60°C to 130°C, specifically 80°C to 130°C, and more specifically 100°C to 130°C. When the temperature is within this range, the moisture content within the electrode active material layer can be minimized, and volatile components introduced during the process can be sufficiently removed, thereby preventing side reactions and deterioration of battery characteristics due to these components during subsequent battery charge and discharge. In particular, performing the drying process at a temperature of 130°C or less can suppress migration of the electrode active material and binder constituting the electrode active material layer and ensure their uniform distribution throughout the electrode active material layer, thereby preventing an increase in secondary battery resistance and a decrease in adhesion between the current collector and the electrode active material layer.

[0030] The time required for the drying step may be 5 minutes to 3 hours, specifically 5 minutes to 20 minutes, and more specifically 5 minutes to 10 minutes.

[0031] On the other hand, the electrode produced by the method of the present invention may be a positive electrode and / or a negative electrode, and specifically, a positive electrode is preferred.

[0032] Specifically, when the electrode manufactured by the method of the present invention is a positive electrode, as the electrode active material, a positive electrode active material including one or more metals such as cobalt, manganese, nickel, or aluminum capable of reversible intercalation and deintercalation of lithium, and a lithium composite metal oxide containing lithium may be used.

[0033] Specifically, the positive electrode active material is a lithium-cobalt-based oxide (for example, LiCoO2, etc.), a lithium-manganese-based oxide (for example, LiMnO2, LiMn2O4, etc.), a lithium-nickel-based oxide (for example, LiNiO2, etc.), a lithium-nickel-manganese-based oxide (for example, LiNi 1-Y Mn Y O2 (where 0 < Y < 1), LiMn 2-Z Ni Z O4 (where 0 < Z < 2), etc.), a lithium-nickel-cobalt-based oxide (for example, LiNi 1-Y1 Co Y1 O2 (where 0 < Y1 < 1), etc.), a lithium-manganese-cobalt-based oxide (for example, LiCo 1-Y2 Mn Y2 O2 (where 0 < Y2 < 1), LiMn 2-Z1 Co Z1 O4 (where 0 < Z1 < 2), etc.), a lithium-nickel-manganese-cobalt-based oxide (for example, Li(Ni p Co q Mn r1 )O2 (where 0 < p < 1, 0 < q < 1, 0 < r1 < 1, p + q + r1 = 1) or Li(Ni p1 Co q1 Mn r2 )O4 (where 0 < p1 < 2, 0 < q1 < 2, 0 < r2 < 2, p1 + q1 + r2 = 2), etc.), a lithium-nickel-cobalt-transition metal (M) oxide (for example, Li(Ni p2 Co q2 Mn r3 M s2)O2(where M is selected from the group consisting of Al, Fe, V, Cr, Ti, Ta, Mg, Ti, and Mo, and p2, q2, r3, and s2 are atomic fractions of independent elements, respectively, where 0 < p2 < 1, 0 < q2 < 1, 0 < r3 < 1, 0 < s2 < 1, and p2 + q2 + r3 + s2 = 1), etc. may be included, and any one or more of these compounds may be included.

[0034] Among them, from the viewpoint of being able to improve the capacity characteristics and stability of the battery, the positive electrode active material may include at least one selected from the group consisting of lithium-cobalt oxide, lithium-manganese-based oxide, lithium-nickel-manganese-cobalt-based oxide, and lithium-nickel-cobalt-transition metal (M) oxide.

[0035] Specifically, the positive electrode active material may include at least one selected from a lithium-nickel-manganese-cobalt-based oxide having a nickel content of 50 atm% or more, specifically 55 atm% or more, and / or a lithium-nickel-cobalt-transition metal (M) oxide having a nickel content of 50 atm% or more, specifically 55 atm% or more. Specifically, the positive electrode active material may include a lithium-nickel-cobalt-transition metal (M) oxide represented by the following Chemical Formula 1.

[0036] [Chemical Formula 1] Li a Ni x Co y M 1 z M 2 w O2

[0037] In the Chemical Formula 1, the M 1 is Mn, Al, or a combination thereof, and preferably may be Mn, or Mn and Al.

[0038] the M 2is at least one selected from the group consisting of Al, Zr, W, Ti, Mg, Ca, and Sr, preferably one or more selected from the group consisting of Al, Zr, Y, Mg, and Ti, and more preferably may be Al.

[0039] Said a represents the molar ratio of lithium in the lithium-nickel-cobalt-transition metal (M) oxide, and 0.8 ≦ a ≦ 1.2, specifically 0.85 ≦ a ≦ 1.15, or more specifically 0.9 ≦ a ≦ 1.05. When the molar ratio of lithium satisfies the above range, the crystal structure of the lithium-nickel-cobalt-transition metal (M) oxide can be stably formed.

[0040] Said x represents the molar ratio of nickel in all metals excluding lithium in the lithium-nickel-cobalt-transition metal (M) oxide, and 0.55 ≦ x < 1, specifically 0.55 ≦ a ≦ 0.95, more specifically 0.60 ≦ a ≦ 0.95, and even more specifically 0.8 ≦ x ≦ 0.95. When the molar ratio of nickel satisfies the above range, it exhibits a high energy density and high capacity can be realized.

[0041] Said y represents the molar ratio of cobalt in all metals excluding lithium in the lithium-nickel-cobalt-transition metal (M) oxide, and 0 < y ≦ 0.3, preferably 0.001 < y < 0.3, specifically 0.025 ≦ b ≦ 0.20, more specifically 0.01 ≦ y < 0.20, and even more specifically 0.01 ≦ y < 0.15. When the molar ratio of cobalt satisfies the above range, good resistance characteristics and output characteristics can be realized.

[0042] Said z represents the molar ratio of M in all metals excluding lithium in the lithium-nickel-cobalt-transition metal (M) oxide 1 element, and 0 < z ≦ 0.3, preferably 0.001 < z ≦ 0.25, more preferably 0.01 ≦ z ≦ 0.20, more preferably 0.01 ≦ z ≦.20, and even more specifically 0.01 ≦ z ≦ 0.15. M 1When the molar ratio of the elements satisfies the above range, it has excellent structural stability of the cathode active material.

[0043] Said w is M in all metals excluding lithium in the lithium-nickel-cobalt-transition metal (M) oxide. 2 It represents the molar ratio of the elements, and 0 < w ≤ 0.2, specifically 0 < w ≤ 0.1, preferably 0 < w ≤ 0.05 may be applicable.

[0044] Specifically, in order to realize a high-capacity battery, the cathode active material is, for example, Li(Ni 0.65 Mn 0.2 Co 0.15 )O2, Li(Ni 0.7 Mn 0.2 Co 0.1 )O2, Li(Ni 0.8 Mn 0.1 Co 0.1 )O2, Li(Ni 0.8 Co 0.15 Al 0.05 )O2, Li(Ni 0.86 Mn 0.07 Co 0.05 Al 0.02 )O2, or may include a lithium composite transition metal oxide such as Li(Ni 0.90 Mn 0.05 Co 0.05 )O2. Preferably, based on the total number of moles of transition metals, it may include Li(Ni 0.86 Mn 0.07 Co 0.05 Al 0.02 )O2 containing 80 mol% or more of Ni.

[0045] On the other hand, the cathode active material according to the present invention may further include a coating layer containing one or more coating elements selected from the group consisting of Al, Ti, W, B, F, P, Mg, Ni, Co, Fe, Cr, V, Cu, Ca, Zn, Zr, Nb, Mo, Sr, Sb, Bi, Si, and S on the surface of the lithium nickel-based oxide particles, if necessary. Preferably, the coating element may be Al, B, Co, or a combination thereof.

[0046] When a coating layer is present on the surface of the lithium nickel-based oxide particles, the coating layer suppresses contact between the non-aqueous electrolyte and the lithium composite transition metal oxide, thereby reducing the elution of transition metals and the generation of gas due to side reactions with the non-aqueous electrolyte.

[0047] The positive electrode active material may be included in an amount of 80 wt% to 99 wt%, specifically 90 wt% to 99 wt%, based on the total weight of the solid content in the positive electrode slurry composition. When the content of the positive electrode active material satisfies the above range, sufficient positive electrode energy density can be ensured, thereby improving the battery capacity of the positive electrode.

[0048] The binder is a component that plays a role in improving the adhesion between positive electrode active material particles and the adhesive strength between the positive electrode active material and the current collector, and is usually added in an amount of 1 to 30 wt % based on the total weight of the solid content in the positive electrode active material layer. Examples of such binders include polyvinylidene fluoride (PVDF)-based binder resins with a melting point of 170°C or higher.

[0049] According to one embodiment of the present invention, the binder may be included in an amount of 0.5 wt % to 3.5 wt %, specifically 0.5 wt % to 3.0 wt %, and more specifically 0.5 wt % to 1.5 wt %, based on the solid content of the positive electrode slurry composition.

[0050] Meanwhile, the positive electrode slurry composition may be prepared by further mixing or dispersing a conductive material, a dispersant, and the like in addition to the positive electrode active material and the binder in a solvent, if necessary.

[0051] The conductive material is not particularly limited as long as it does not cause a chemical change in the battery and has conductivity, and examples thereof include carbon black such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; graphite powder such as natural graphite, artificial graphite, or graphite with a highly developed crystal structure; conductive fibers such as carbon fiber and metal fiber; metal powders such as carbon fluoride, aluminum, and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and polyphenylene derivatives.

[0052] The conductive material may be contained in an amount of 1.5 wt % or less, specifically 0.5 wt % to 1.0 wt %, more specifically 0.6 wt % to 1.0 wt %, based on the solid content of the positive electrode slurry composition. When the content of the conductive material in the solid content of the positive electrode slurry composition satisfies the above range, a positive electrode conductive network can be secured, thereby improving the electrical conductivity of the positive electrode.

[0053] The dispersant prevents excessive aggregation of the positive electrode active material in the positive electrode slurry composition, ensuring that the positive electrode active material is effectively dispersed in the resulting positive electrode active material layer. The dispersant may include a hydrogenated nitrile copolymer, specifically a copolymer containing structural units derived from an α,β-unsaturated nitrile and structural units derived from a hydrogenated conjugated diene, or a copolymer containing structural units derived from an α,β-unsaturated nitrile, structural units derived from a conjugated diene, and structural units derived from a hydrogenated conjugated diene. Examples of the α,β-unsaturated nitrile monomer include acrylonitrile and methacrylonitrile, and these may be used alone or in a mixture of two or more. Examples of the conjugated diene monomer include conjugated diene monomers having 4 to 6 carbon atoms, such as 1,3-butadiene, isoprene, and 2,3-methylbutadiene, and these may be used alone or in a mixture of two or more.

[0054] More specifically, the hydrogenated nitrile copolymer may be hydrogenated nitrile butadiene rubber (H-NBR).

[0055] The dispersant may be included in an amount of 1.5 wt % or less, specifically 1.2 wt % or less, more specifically 0.1 wt % to 1.0 wt %, based on the solid content of the positive electrode slurry composition. When the content of the dispersant satisfies the above range, aggregation of the conductive material in the solid content of the positive electrode slurry composition may be suppressed, thereby improving the positive electrode conductive network.

[0056] On the other hand, when the electrode produced by the method of the present invention is a positive electrode, the electrode current collector is not particularly limited as long as it is a positive electrode current collector that does not cause chemical changes in the battery and has conductivity. For example, stainless steel, aluminum, nickel, titanium, baked carbon, or aluminum or stainless steel whose surface has been treated with carbon, nickel, titanium, silver, or the like may be used.

[0057] Specifically, the positive electrode current collector has a thickness of 3 μm to 500 μm, and the surface of the positive electrode current collector may be formed with fine irregularities to enhance adhesion to the positive electrode active material layer. For example, the positive electrode current collector can be used in various forms such as a film, sheet, foil, mesh, porous body, foam, or nonwoven fabric.

[0058] The solvent used in preparing the electrode slurry composition may include an organic solvent such as N-methyl-2-pyrrolidone (NMP), and may be used in an amount that provides a suitable viscosity when the positive electrode active material, and optionally a binder and a conductive material, are contained. For example, the active material slurry containing the positive electrode active material, and optionally a binder and a conductive material may be contained so that the solids concentration is 10 wt % to 90 wt %, preferably 30 wt % to 80 wt %.

[0059] (S2) Step of rolling the electrode The method for manufacturing an electrode for a secondary battery according to the present invention may include a step of performing a rolling process on the dried electrode.

[0060] The rolling process may be performed by, but is not limited to, a roll press method, for example, a hot press method.

[0061] Before the rolling step, the porosity of the dried electrode active material layer may be 55% or less, specifically 35% to 55%, more specifically 40% to 55%. If the porosity of the dried electrode active material layer satisfies the above range, it is preferable in that the rolling step of the dried electrode active material layer is easy.

[0062] The rolling step may be performed once or multiple times, specifically 2 to 5 times, more specifically 3 to 5 times, on the dried electrode active material layer. In this case, the rolling step may be performed until the rolling ratio of the electrode active material layer exceeds a specific value. For example, the rolling ratio of the electrode active material layer after the rolling step may be 10% or more, specifically 10% to 17%, more specifically 10% to 15%. When the rolling ratio of the electrode active material layer after the rolling step satisfies the above range, the electrode active material is sufficiently rolled to the extent that the electrode active material layer is not detached, thereby minimizing the number of times of rolling in the second rolling step.

[0063] Furthermore, the thickness change rate after the rolling step may be 3.5% or less, specifically 0.1% to 3.5%, more specifically 0.5% to 3.5%. When the thickness of the electrode active material layer after the rolling step satisfies the above range, detachment of the electrode active material layer is prevented, and a high-density electrode can be produced. When the thickness change rate during the rolling step exceeds 3.5%, weak portions of the electrode active material layer (e.g., portions or edge portions having a thickness thinner than the average thickness) may crack, and the electrode active material layer may detach from the electrode current collector.

[0064] Furthermore, after the rolling step, the porosity of the electrode active material layer may be 20% to 36%, specifically 20% to 34%, more specifically 25% to 34%. The porosity of the electrode active material layer after the rolling step is related to the thickness of the electrode active material layer after the rolling step. Therefore, when the porosity of the electrode active material layer after the rolling step satisfies the above range, detachment of the electrode active material layer is prevented and a high-density positive electrode can be produced.

[0065] (S3) Step of drying the rolled electrode The method for manufacturing an electrode for a secondary battery according to the present invention may include a step of drying the rolled electrode.

[0066] In this case, the drying step may be carried out at a temperature of 130°C or higher, and the drying temperature in at least a portion of the step may be 170°C to 210°C, which is a temperature higher than the melting point (170°C) of polyvinylidene fluoride (PVDF) binder resin.

[0067] The drying process may be performed by a roll-to-roll process, but is not limited thereto.

[0068] Specifically, the step of drying the rolled electrode may include a three-step drying process in which the temperature is sequentially increased, and may include a first drying process at 130°C to 150°C, a second drying process at 150°C to 170°C, and a third drying process at 170°C to 210°C.

[0069] As described above, in the method of the present invention, when the drying process is performed after rolling, the drying temperature is gradually increased from 130°C to 210°C, with the drying temperature in some sections being 170°C to 210°C. This allows heat to be uniformly applied to the active material and binder inside the electrode layer, minimizes deterioration in the crystallinity of the polyvinylidene fluoride (PVDF) binder resin, and effectively reduces the amount of residual moisture in the electrode to a desired level. This prevents an increase in resistance of the secondary battery, further improves the adhesion between the current collector and the electrode active material layer, and realizes an effect of improving productivity.

[0070] On the other hand, if the drying process is performed by gradually decreasing the temperature from a high temperature to a low temperature, for example, by performing the first process at approximately 180°C to 170°C, which is near the melting point of the PVDF binder resin (170°C), and then gradually decreasing the temperature to 130°C, the partially dissolved PVDF binder resin crystals may gradually grow irregularly, increasing the imbalance in electrode physical properties within the electrode and increasing the brittleness of the electrode due to the irregularly grown crystals. Therefore, as in the present invention, by gradually increasing the temperature from 130°C and performing some of the drying processes at a temperature above the melting point of the PVDF binder resin (170°C), the PVDF binder resin can be uniformly dissolved. Therefore, even if a room temperature (rapid) cooling process is performed in the subsequent process, the crystal size of the binder resin is uniform throughout, allowing electrodes with similar overall physical properties to be manufactured. In particular, the small crystals formed by the rapid cooling process can improve the toughness of the electrode.

[0071] Meanwhile, the total drying time of the rolled electrode may be 5 seconds or less, specifically 2 to 5 seconds.

[0072] When the drying time is within the above range, the deterioration of crystallinity can be minimized and the amount of residual moisture in the electrode can be effectively reduced.When the drying time exceeds the above range, the crystallinity of the polyvinylidene fluoride (PVDF) binder can be reduced, which can cause the physical properties (adhesion strength, toughness) of the electrode to change from before drying, and the thickness and size of the electrode can also change.

[0073] Such a positive electrode can be usefully used as a positive electrode for a lithium secondary battery.

[0074] On the other hand, an example of a lithium secondary battery having the above-mentioned positive electrode will be described.

[0075] The lithium secondary battery may further include a case that houses an electrode assembly in which the positive electrode, the separator, and the negative electrode are stacked in order.

[0076] Next, each of the components of the lithium secondary battery of the present invention will be described in more detail.

[0077] The negative electrode may have a structure in which a negative electrode active material layer is formed on one or both sides of a long sheet-shaped negative electrode current collector. The negative electrode active material layer includes a negative electrode active material and may optionally further include a conductive material and / or a binder.

[0078] Specifically, as the negative electrode active material, a compound capable of reversible intercalation and deintercalation of lithium can be used. Specific examples of the negative electrode active material include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, and amorphous carbon; silicon-based materials such as Si, Si-Me alloy (where Me is one or more selected from the group consisting of Al, Sn, Mg, Cu, Fe, Pb, Zn, Mn, Cr, Ti, and Ni), SiOy (where 0 < y < 2), and Si-C composite; lithium metal thin film; metal materials capable of alloying with lithium such as Sn and Al; etc. Any one or a mixture of two or more of these may be used.

[0079] Preferably, the negative electrode active material of the present invention may include at least one of a carbon-based negative electrode active material and a silicon-based negative electrode active material.

[0080] The silicon-based negative electrode active material may be Si, Si-Me alloy (where Me is one or more selected from the group consisting of Al, Sn, Mg, Cu, Fe, Pb, Zn, Mn, Cr, Ti, and Ni), SiOy (where 0 < y < 2), Si-C composite, or a combination thereof, and preferably may be SiOy (where 0 < y < 2). Since the silicon-based negative electrode active material has a high theoretical capacity, when the silicon-based negative electrode active material is included, the capacity characteristics can be improved.

[0081] On the other hand, the silicon-based negative electrode active material may be doped with M b metal, and in this case, the M b metal may be a Group 1 metal element or a Group 2 metal element, specifically, may be Li, Mg, etc. Specifically, the silicon negative electrode active material may be Si, SiOy (where 0 < y < 2), Si-C composite, etc. doped with M b metal. In the case of a metal-doped silicon-based negative electrode active material, although the active material capacity slightly decreases due to the doping element, since it has high efficiency, a high energy density can be realized.

[0082] The silicon-based negative electrode active material may further include a carbon coating layer on the surface of the particles, and the amount of the carbon coating may be 20 wt % or less, preferably 1 wt % to 20 wt %, based on the total weight of the silicon-based negative electrode active material.

[0083] The conductive material is a component for further improving the conductivity of the negative electrode active material and may be added in an amount of 1 wt % to 20 wt % based on the total weight of the solid content in the negative electrode active material layer. Such a conductive material is not particularly limited as long as it does not cause chemical changes in the battery and has conductivity, and examples thereof include graphite such as natural graphite and artificial graphite; carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fiber and metal fiber; conductive powders such as carbon fluoride powder, aluminum powder, and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives.

[0084] The binder is a component that aids in bonding between the conductive material, active material, and current collector, and is typically added in an amount of 1 to 30 wt % based on the total weight of the solids in the negative electrode active material layer. Examples of such binders include fluororesin-based binders including polyvinylidene fluoride or polytetrafluoroethylene; rubber-based binders including styrene-butadiene rubber, acrylonitrile-butadiene rubber, and styrene-isoprene rubber; cellulose-based binders including carboxymethyl cellulose, starch, hydroxypropyl cellulose, and regenerated cellulose; polyalcohol-based binders including polyvinyl alcohol; polyolefin-based binders including polyethylene and polypropylene; polyimide-based binders; polyester-based binders; and silane-based binders.

[0085] The negative electrode may be manufactured by a method known in the art, for example, by dissolving or dispersing a negative electrode active material, and optionally a binder and a conductive material, in a solvent to prepare a negative electrode active material slurry, applying the slurry to a negative electrode current collector, rolling, and drying the slurry to form a negative electrode active material layer, or by casting the negative electrode active material layer on a separate support, peeling off the support, and laminating the resulting film on the negative electrode current collector.

[0086] The negative electrode current collector generally has a thickness of 3 μm to 500 μm. There are no particular limitations on the negative electrode current collector, so long as it does not cause chemical changes in the battery and has high conductivity. Examples of the negative electrode current collector that can be used include copper, stainless steel, aluminum, nickel, titanium, baked carbon, copper or stainless steel surfaces that have been surface-treated with carbon, nickel, titanium, silver, or the like, and aluminum-cadmium alloys. Similarly to the positive electrode current collector, the surface of the negative electrode current collector may be formed with fine irregularities to strengthen the binding force of the negative electrode active material, and various forms are possible, such as a film, sheet, foil, mesh, porous material, foam, or nonwoven fabric.

[0087] The solvent may include water or an organic solvent such as NMP or alcohol, and may be used in an amount that provides a suitable viscosity when the negative electrode active material, and optionally a binder and a conductive material, are contained. For example, the solvent may be contained so that the solids concentration in the active material slurry containing the negative electrode active material, and optionally a binder and a conductive material, is 50 wt % to 75 wt %, preferably 50 wt % to 65 wt %.

[0088] The separator separates the negative electrode and the positive electrode and provides a path for lithium ions to move. Any separator that is commonly used as a separator in a lithium secondary battery can be used without any particular limitation. In particular, a separator that has low resistance to the movement of ions in a non-aqueous electrolyte and has excellent humidifying ability for the non-aqueous electrolyte is preferred.

[0089] Specifically, the separator may be a porous polymer film, such as a porous polymer film made of a polyolefin polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, or an ethylene / methacrylate copolymer, or a laminate structure of two or more layers thereof. Conventional porous nonwoven fabrics, such as nonwoven fabrics made of high-melting-point glass fibers or polyethylene terephthalate fibers, may also be used. To ensure heat resistance or mechanical strength, a coated separator containing a ceramic component or a polymer material may also be used, and may be selectively used as a single-layer or multi-layer structure.

[0090] The electrolyte used in the present invention is not particularly limited to any particular type, and may be any of various electrolytes that can be used in lithium secondary batteries, such as organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, and molten inorganic electrolytes.

[0091] The electrolyte may also include an organic solvent and a lithium salt.

[0092] The organic solvent is not particularly limited as long as it can act as a medium through which ions involved in the electrochemical reaction of the battery can move. Specifically, examples of the organic solvent that may be used include carbonate-based solvents such as dimethylcarbonate (DMC), diethylcarbonate (DEC), methylethylcarbonate (MEC), ethylmethylcarbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC); ester-based solvents such as methyl acetate and ethyl acetate; ether-based solvents such as dibutyl ether and tetrahydrofuran; ketone-based solvents such as cyclohexanone; aromatic hydrocarbon-based solvents such as benzene and fluorobenzene; alcohol-based solvents such as ethyl alcohol and isopropyl alcohol; amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane; and sulfolanes. Among these, carbonate-based solvents are preferred, and more preferred are mixtures of cyclic carbonates (e.g., ethylene carbonate or propylene carbonate) with high ionic conductivity and high dielectric constant, which can improve the charge / discharge performance of batteries, and low-viscosity linear carbonate compounds (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate).

[0093] The lithium salt can be any compound capable of providing lithium ions used in lithium secondary batteries, without particular limitations. Specifically, examples of the lithium salt include LiPF, LiClO, LiAsF, LiBF, LiSbF, LiAlO, LiAlCl, LiCF, SO, LiCF, SO, LiN(CF, SO), LiN(CF, SO), LiN(CF, SO), LiCl, LiI, and LiB(C, O) . The lithium salt concentration is preferably in the range of 0.1 M to 5.0 M, and more preferably 0.1 M to 3.0 M. When the lithium salt concentration is within this range, the electrolyte has appropriate conductivity and viscosity, resulting in excellent electrolyte performance and efficient lithium ion migration.

[0094] In addition to the electrolyte components, the electrolyte may further contain additives for purposes such as improving battery life characteristics, suppressing battery capacity loss, and improving battery discharge capacity. Examples of additives include, but are not limited to, haloalkylene carbonate compounds such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glyme, hexamethylphosphoric triamide, nitrobenzene derivatives, sulfur, quinoneimine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, and aluminum trichloride, which may be used alone or in combination. The additives may be present in an amount of 0.1 wt % to 10 wt %, preferably 0.1 wt % to 5 wt %, based on the total weight of the electrolyte.

[0095] The lithium secondary battery according to the present invention can be usefully used in portable devices such as mobile phones, notebook computers, and digital cameras; and electric vehicles such as hybrid electric vehicles (HEVs).

[0096] The present invention will be described in more detail below with reference to examples. However, the following examples are for illustrative purposes only and are not intended to limit the scope of the present invention.

[0097] [Example] Example 1 (1) Preparation of Positive Electrode Slurry Composition Cathode active material (Li(Ni) 0.86 Mn 0.07 Co 0.05 Al 0.02 )O2), carbon nanotubes as a conductive material, and polyvinylidene fluoride (PVDF) as a binder were added to N-methyl-2-pyrrolidone (NMP) as a solvent in a weight ratio of 97:1:2 to prepare a positive electrode slurry composition (solid content 70 wt%).

[0098] (2) Manufacturing of the positive electrode The positive electrode slurry composition prepared above was applied onto a 15 μm thick aluminum thin film current collector, and then dried at 130° C. for 1 minute to form a positive electrode active material layer (thickness: 218 μm).

[0099] Next, the dried positive electrode active material layer was rolled until the thickness of the positive electrode active material layer reached 164 μm, and then a first drying step was carried out at 130° C., a second drying step was carried out at 150° C., and a third drying step was carried out at 170° C. in this order to manufacture a positive electrode. The total drying time was within 5 seconds.

[0100] Example 2. The positive electrode slurry composition prepared in Example 1 was applied to a 15 μm thick aluminum thin film current collector and then dried at 130° C. for 1 minute to form a positive electrode active material layer (thickness: 218 μm).

[0101] Next, the dried positive electrode active material layer was rolled until the thickness of the positive electrode active material layer reached 164 μm, and then a first drying step was carried out at 150° C., a second drying step was carried out at 170° C., and a third drying step was carried out at 190° C. in this order to manufacture a positive electrode. The total drying time was within 5 seconds.

[0102] Example 3. The positive electrode slurry composition prepared in Example 1 was applied to a 15 μm thick aluminum thin film current collector and then dried at 130° C. for 1 minute to form a positive electrode active material layer (thickness: 218 μm).

[0103] Next, the dried positive electrode active material layer was rolled until the thickness of the positive electrode active material layer reached 164 μm, and then a first drying step was performed at 150° C., a second drying step was performed at 170° C., and a third drying step was performed at 210° C. in this order to manufacture a positive electrode. The total drying time was within 5 seconds.

[0104] Comparative Example 1 The positive electrode slurry composition prepared in Example 1 was applied to a 15 μm thick aluminum thin film current collector and then dried at 130° C. for 1 minute to form a positive electrode active material layer (thickness: 218 μm).

[0105] Next, the dried positive electrode active material layer was rolled until the thickness of the positive electrode active material layer reached 164 μm, and then a first drying step was performed at 110° C., followed by a second drying step at 130° C. The total drying time was set to within 5 seconds.

[0106] Comparative Example 2 The positive electrode slurry composition prepared in Example 1 was applied to a 15 μm thick aluminum thin film current collector and then dried at 130° C. for 1 minute to form a positive electrode active material layer (thickness: 218 μm).

[0107] Next, the dried positive electrode active material layer was subjected to a rolling process until the thickness of the positive electrode active material layer reached 164 μm, and then a drying process was performed at 130° C. for 5 seconds.

[0108] [Experimental Example] Experimental Example 1: Measurement of residual moisture in the positive electrode active material layer The solvent (water) content of each of the positive electrodes prepared in Examples 1 to 3 and Comparative Examples 1 and 2 was measured by Karl Fischer titration. Specifically, each of the positive electrodes prepared in Examples 1 to 3 and Comparative Example 1 was cut into 5 cm x 5 cm pieces in a glove box filled with argon gas. The 5 cm x 5 cm pieces were weighed in a sample vial. The weighed positive electrodes were then measured three times using a Karl Fischer coulometry water content analyzer (831 KF Coulometer, Metrohm, Switzerland), and the average value was calculated. The results are shown in Figure 1.

[0109] Referring to FIG. 1, it can be seen that the positive electrodes prepared in Examples 1 to 3 of the present invention have significantly reduced residual moisture compared to the positive electrodes prepared in Comparative Examples 1 and 2.

[0110] Experimental Example 2: Evaluation of adhesive strength to electrode current collector The positive electrodes produced in Examples 1 to 3 and Comparative Examples 1 and 2 were each cut to a size of 20 mm wide x 125 mm long and fixed to a glass slide, after which the current collector was peeled off and the 90° peel strength was measured. The measurement was repeated three times and the average value was calculated. The results are shown in Figure 2.

[0111] Referring to FIG. 2, it can be seen that the positive electrodes prepared in Examples 1 to 3 of the present invention have significantly increased adhesive strength compared to the positive electrodes prepared in Comparative Examples 1 and 2.

Claims

1. manufacturing an electrode in which an electrode active material layer is formed on an electrode current collector; rolling the electrode; and drying the rolled electrode. The method for manufacturing an electrode for a secondary battery, wherein the drying step is carried out so that the drying temperature in at least a part of the step is 170°C to 210°C.

2. 2. The method for producing an electrode for a secondary battery according to claim 1, wherein the total drying time of the rolled electrode is 2 to 5 seconds.

3. The method for manufacturing an electrode for a secondary battery according to claim 1 , wherein the step of drying the rolled electrode is performed by a roll-to-roll process.

4. The method for manufacturing a secondary battery electrode according to claim 1 , wherein the step of drying the rolled electrode is performed at a temperature of 130° C. or higher.

5. 2. The method for manufacturing a secondary battery electrode according to claim 1, wherein the step of drying the rolled electrode includes a step of carrying out a first drying step at 130°C to 150°C, a step of carrying out a second drying step at 150°C to 170°C, and a step of carrying out a third drying step at 170°C to 210°C, in that order.

6. the electrode active material layer includes an electrode active material and a binder, The method for manufacturing an electrode for a secondary battery according to claim 1 , wherein the electrode active material includes a positive electrode active material or a negative electrode active material.

7. The method for manufacturing an electrode for a secondary battery according to claim 6 , wherein the binder contains a polyvinylidene fluoride-based resin.

8. The method for producing an electrode for a secondary battery according to claim 6 , wherein the electrode active material is a positive electrode active material.

9. 9. The method for manufacturing an electrode for a secondary battery according to claim 8, wherein the positive electrode active material comprises a lithium-nickel-cobalt-transition metal (M) oxide represented by the following chemical formula 1: [Chemical formula 1] Li ａ Ni ｘ Co ｙ M １ ｚ M ２ ｗ O ２ (In the above chemical formula 1, M １ is Mn, Al, or a combination thereof, and M ２ is at least one selected from the group consisting of Al, Zr, W, Ti, Mg, Ca, and Sr, and 0.8≦a≦1.2, 0.55≦x<1, 0<y≦0.3, 0<z≦0.3, 0<w≦0.

2.

10. The method of claim 9, wherein the positive electrode active material has a Ni content of 55 atm % or more.

11. The method for producing an electrode for a secondary battery according to claim 1 , wherein the electrode is a positive electrode.

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