Manufacturing method for 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 priority rights under Korean Patent Application No. 10-2022-0108709 dated August 29, 2022, and Korean Patent Application No. 10-2023-0112358 dated August 25, 2023, and all content disclosed in the documents of said Korean Patent Applications is incorporated herein by reference.

[0002] The present invention relates to a method for manufacturing electrodes for secondary batteries, and more specifically, to a method for manufacturing electrodes for secondary batteries that minimizes the decrease in the crystallinity of the binder, effectively reduces the amount of residual moisture in the electrode, and significantly improves the adhesion between the electrode active material layer and the electrode current collector. [Background technology]

[0003] In recent years, the rapid increase in the use of fossil fuels has led to a growing demand for alternative and clean energy sources. As part of this trend, one of the most actively researched fields recently is the field of power generation and energy storage using electrochemistry.

[0004] A prime example of an electrochemical element that utilizes such electrochemical energy is the secondary battery, and its range of applications is steadily expanding. In particular, in recent years, with the increasing technological development and 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 these 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 low self-discharge rate.

[0006] On the other hand, the lithium secondary battery has a structure in which a chargeable and dischargeable electrode assembly having a positive electrode / separator / negative electrode structure is mounted in a battery case. The positive electrode and negative electrode are produced by applying a slurry obtained by mixing an active material and a binder resin component onto one or both sides of a metal current collector to form an electrode active material layer, followed by drying and rolling.

[0007] In this regard, conventionally, after rolling the active material layer, to remove moisture remaining inside the electrode, it has been common to carry out the drying step at a low temperature at which a conventional binder resin, for example, a polyvinylidene fluoride (PVDF)-based binder resin, does not melt. If the drying step is performed after rolling at a temperature equal to or higher than the melting point (170°C) of the polyvinylidene fluoride (PVDF)-based binder resin, the polyvinylidene fluoride (PVDF)-based binder resin melts, the crystallinity thereof decreases, and the brittleness of the electrode is reduced, which causes problems such as decreased electrode strength and inferior durability. On the other hand, when drying is performed at a temperature of 170°C or lower, a long drying time is required to reduce the residual moisture content in the electrode to a desired level, which further causes a problem of inferior productivity.

[0008] Therefore, there is a need for the development of a novel method for producing an electrode that can prevent a decrease in electrode strength during electrode production, reduce the amount of residual moisture remaining inside the electrode to a desired level, and improve adhesive strength. Summary of the Invention Problem to be Solved by the Invention

[0009] The present invention is intended to solve such problems, and an object of the present invention is to provide a method for producing an electrode for a secondary battery, wherein when the drying step is performed after rolling, the drying temperature in at least a partial section is set to be equal to or higher than the melting point (170°C) of the binder, and the total drying time is set to 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, comprising the steps of: manufacturing an electrode having an electrode active material layer formed on an electrode current collector; rolling the electrode; and drying the rolled electrode, wherein the drying step is performed such that the drying temperature in at least a portion of the electrode is 170°C to 210°C.

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

[0012] Furthermore, the step of drying the rolled electrodes may be carried out 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 may specifically 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 electrodes for secondary batteries of the present invention, during the drying process after rolling, the drying temperature of a portion of the electrode is set to be above the melting point temperature of the binder (170°C), and the total drying time is set to 5 seconds or less. This minimizes the decrease in the crystallinity of the binder 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, and thereby realizing a lithium secondary battery with improved durability.

[0015] The following drawings attached to this specification are for illustrating preferred embodiments of the present invention and, together with the above-described content of the invention, serve to provide a better understanding of the technical concept of the present invention. Therefore, the present invention should not be construed as being limited solely to what is shown in these drawings. [Brief explanation of the drawing]

[0016] [Figure 1] This graph shows the evaluation results of the amount of residual moisture inside the electrode related to Experimental Example 1. [Figure 2] This graph shows the evaluation results of electrode adhesion strength related to Experimental Example 2. [Modes for carrying out the invention]

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

[0018] The terms used herein are for illustrative purposes only and are not intended to limit the invention. In this specification, singular nouns include plural nouns unless otherwise specified in the text. The terms “comprises” and / or “comprising” as used in this specification do not preclude the presence or addition of one or more other components in addition to the components mentioned.

[0019] In this specification, when a part is said to include a component, this means that, unless otherwise stated to the contrary, it may include other components rather than excluding them.

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

[0021] Manufacturing method for electrodes for secondary batteries The method for manufacturing electrodes for secondary batteries of the present invention is as follows: (S1) A step of manufacturing an electrode in which an electrode active material layer is formed on an electrode current collector, (S2) The step of rolling the electrode, (S3) The step of drying the rolled electrode, The drying step may be carried out such that the drying temperature in at least a portion of the section is 170°C to 210°C.

[0022] Conventionally, a slurry of active material and binder resin components was applied to one or both sides of a metal current collector to form an electrode active material layer. After rolling the electrode, a drying process was required at a low temperature (below 170°C) that would not melt the binder resin, in order to prevent a decrease in the crystallinity of the binder resin, such as polyvinylidene fluoride (PVDF) binder resin. However, drying at low temperatures requires a long drying time to reduce the amount of residual moisture in the electrode to the desired level, resulting in poor productivity.

[0023] The inventors of the present invention conducted intensive research to solve these problems and, as a result, discovered that by performing the drying process after rolling so that the drying temperature in a certain section is above the melting point (170°C) of the polyvinylidene fluoride (PVDF) binder resin, and the total drying time is 5 seconds or less, the decrease in the crystallinity of the binder resin can be minimized and the amount of residual moisture in the electrode can be effectively reduced, thus completing 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 the 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 manufactured by mixing or dispersing the electrode active material and binder in a solvent to produce an electrode slurry composition, then applying it to an electrode current collector and drying it.

[0027] In this case, the step of applying the electrode slurry composition to the electrode current collector can be carried out by a conventional method known in the industry, for example, by uniformly dispersing it using a doctor blade, or by methods such as die casting, comma coating, or screen printing.

[0028] Furthermore, the drying process of the electrode slurry composition applied to the electrode current collector may be carried out by known conventional methods, such as vacuum heating in a certain temperature range or by heat treatment methods such as hot air injection.

[0029] Furthermore, the temperature range for the drying process may be 60°C to 130°C, specifically 80°C to 130°C, or more specifically 100°C to 130°C. When the temperature is within the above range, the moisture content inside the electrode active material layer can be minimized, and volatile components introduced during the process can be sufficiently removed, preventing the occurrence of side reactions caused by these components and the deterioration of battery characteristics during subsequent charging and discharging of the battery. In particular, by performing the drying process at a temperature of 130°C or lower, migration of the electrode active material and binder constituting the electrode active material layer can be suppressed, and by ensuring uniform distribution within the electrode active material layer, the increase in resistance of the secondary battery and the decrease in adhesion between the current collector and the electrode active material layer can be improved.

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

[0031] On the other hand, the electrode manufactured by 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 produced by the method of the present invention is a positive electrode, as the electrode active material, a positive electrode active material comprising a lithium composite metal oxide containing lithium and one or more metals such as cobalt, manganese, nickel, or aluminum capable of reversible intercalation and deintercalation of lithium may be used.

[0033] Specifically, the positive electrode active material includes lithium-cobalt-based oxides (e.g., LiCoO₂, etc.), lithium-manganese-based oxides (e.g., LiMnO₂, LiMn₂O₄, etc.), lithium-nickel-based oxides (e.g., LiNiO₂, etc.), lithium-nickel-manganese-based oxides (e.g., LiNi 1-Y Mn Y O₂ (where 0<Y<1), LiMn 2-Z Ni Z O₄ (where 0<Z<2), etc.), lithium-nickel-cobalt-based oxides (e.g., LiNi 1-Y1 Co Y1 O₂ (where 0<Y1<1), etc.), lithium-manganese-cobalt-based oxides (e.g., LiCo 1-Y2 Mn Y2 O₂ (where 0<Y2<1), LiMn 2-Z1 Co Z1 O₄ (where 0<Z1<2), etc.), lithium-nickel-manganese-cobalt-based oxides (e.g., 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.), lithium-nickel-cobalt-transition metal (M) oxides (e.g., Li(Ni p2 Co q2 Mn r3 M s2)O₂ (wherein M is selected from the group consisting of Al, Fe, V, Cr, Ti, Ta, Mg, Ti, and Mo, p2, q2, r3, and s2 are each an independent atomic fraction of an element, which satisfy 0<p2<1, 0<q2<1, 0<r3<1, 0<s2<1, and p2+q2+r3+s2=1), and any one or two or more of these compounds may be included.

[0034] Among others, in view of improving the capacity characteristics and stability of a battery, the positive electrode active material may include at least one selected from the group consisting of lithium-cobalt oxides, lithium-manganese-based oxides, lithium-nickel-manganese-cobalt-based oxides, and lithium-nickel-cobalt-transition metal (M) oxides.

[0035] Specifically, the positive electrode active material may include at least one selected from lithium-nickel-manganese-cobalt-based oxides having a nickel content of 50 atm% or more, specifically 55 atm% or more, and / or lithium-nickel-cobalt-transition metal (M) oxides 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 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 may be 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 may satisfy 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 may satisfy 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, the oxide exhibits high energy density and can achieve high capacity.

[0041] Said y represents the molar ratio of cobalt in all metals excluding lithium in the lithium-nickel-cobalt-transition metal (M) oxide, and satisfies 0<y≦0.3, preferably 0.001<y<0.3, specifically 0.025≦ y ≦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, favorable resistance characteristics and output characteristics can be achieved.

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

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

[0044] Specifically, to realize a high-capacity battery, the positive electrode active material may be 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 Li(Ni 0.90 Mn 0.05 Co 0.05 )O2, and may include other lithium composite transition metal oxides; preferably, it includes Li(Ni 0.86 Mn 0.07 Co 0.05 Al 0.02 )O2 containing 80 mol% or more of Ni based on the total number of moles of transition metals.

[0045] On the other hand, the positive electrode active material according to the present invention may further optionally 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 said lithium nickel-based oxide particles. Preferably, said coating element may be Al, B, Co, or a combination thereof.

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

[0047] The positive electrode active material may be present in an amount of 80% to 99% by weight, specifically 90% to 99% by weight, based on the total weight of solids in the positive electrode slurry composition. When the content of the positive electrode active material satisfies the above range, the battery capacity of the positive electrode can be improved by ensuring sufficient positive electrode energy density.

[0048] The binder is a component that plays a role in improving the adhesion between positive electrode active material particles and the adhesion between the positive electrode active material and the current collector, and is usually added at a concentration of 1% to 30% by weight based on the total weight of solids in the positive electrode active material layer. An example of such a binder is a polyvinylidene fluoride (PVDF)-based binder resin with a melting point of 170°C or higher.

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

[0050] On the other hand, the positive electrode slurry composition can be manufactured by further mixing or dispersing conductive materials, dispersants, and other elements in a solvent, in addition to the positive electrode active material and binder, as needed.

[0051] The conductive material is not particularly limited as long as it does not cause a chemical change in the battery and is conductive. Examples include carbon black such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, or thermal black; graphite powder such as natural graphite, artificial graphite, or graphite with a highly developed crystalline structure; conductive fibers such as carbon fibers or metal fibers; 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 included in the solid content of the positive electrode slurry composition in an amount of 1.5% by weight or less, more specifically in an amount of 0.5% to 1.0% by weight, or more specifically in an amount of 0.6% to 1.0% by weight. When the content of the conductive material in the solid content of the positive electrode slurry composition satisfies the above range, the electrical conductivity of the positive electrode can be improved by ensuring a positive electrode conductive network.

[0053] The dispersant suppresses the phenomenon of excessive aggregation of the positive electrode active material in the positive electrode slurry composition, and ensures that the positive electrode active material is effectively dispersed in the manufactured positive electrode active material layer. The dispersant may contain a hydrogenated nitrile copolymer, specifically a copolymer containing structural units derived from α,β-unsaturated nitrile and structural units derived from hydrogenated conjugated diene, or a copolymer containing structural units derived from α,β-unsaturated nitrile, structural units derived from conjugated diene, and structural units derived from hydrogenated conjugated diene. As the α,β-unsaturated nitrile monomer, for example, acrylonitrile or methacrylonitrile can be used, and one of these may be used alone or a mixture of two or more. As the conjugated diene monomer, for example, a conjugated diene monomer having 4 to 6 carbon atoms, such as 1,3-butadiene, isoprene, or 2,3-methylbutadiene, can be used, and one of these may be used alone or 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 present in the solid content of the positive electrode slurry composition in an amount of 1.5% by weight or less, specifically 1.2% by weight or less, and more specifically 0.1% to 1.0% by weight. 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 can be suppressed, and the positive electrode conductive network can be improved.

[0056] On the other hand, when the electrode manufactured by the method of the present invention is the positive electrode, the electrode current collector is not particularly limited as long as it is a positive electrode current collector that does not cause a chemical change in the battery and is conductive. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel with a surface treatment of carbon, nickel, titanium, silver, etc. may be used.

[0057] Specifically, the positive electrode current collector may have a thickness of 3 μm to 500 μm, and the adhesion to the positive electrode active material layer may be enhanced by forming fine irregularities on the surface of the positive electrode current collector. For example, it can be used in various forms such as film, sheet, foil, mesh, porous material, foam, and nonwoven fabric.

[0058] Furthermore, the solvent used in the production of the electrode slurry composition may include an organic solvent such as NMP (N-methyl-2-pyrrolidone), and may be used in an amount that results in a suitable viscosity when the positive electrode active material and selectively include a binder and conductive material. For example, the concentration of solids in the active material slurry containing the positive electrode active material and selectively including a binder and conductive material may be 10% to 90% by weight, preferably 30% to 80% by weight.

[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 carried out by a roll press, but is not limited thereto. For example, the rolling process may be carried out by a hot press.

[0061] Before performing the rolling process, the porosity of the dried electrode active material layer may be 55% or less, specifically 35% to 55%, and more specifically 40% to 55%. It is preferable that the porosity of the dried electrode active material layer satisfies the above range, as this facilitates the rolling process of the dried electrode active material layer.

[0062] The rolling process may be performed once or multiple times on the dried electrode active material layer, specifically two to five times, or more specifically three to five times. In this case, the rolling process may be performed until the rolling rate of the electrode active material layer exceeds a certain value. For example, the rolling rate of the electrode active material layer after the rolling process may be 10% or more, specifically 10% to 17%, or more specifically 10% to 15%. When the rolling rate of the electrode active material layer after the rolling process satisfies the above range, the electrode active material is sufficiently rolled to the extent that the electrode active material layer does not detach, thereby minimizing the number of rolling cycles in the second rolling process.

[0063] Furthermore, the thickness change rate after the rolling process may be 3.5% or less, specifically 0.1% to 3.5%, or more specifically, 0.5% to 3.5%. When the thickness of the electrode active material layer after the rolling process satisfies the above range, detachment of the electrode active material layer is prevented, and high-density electrodes can be manufactured. If the thickness change rate during the rolling process exceeds 3.5%, weaker parts of the electrode active material layer (for example, parts with a thickness thinner than the average thickness or edge parts) may crack, and there is a risk that the electrode active material layer will detach from the electrode current collector.

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

[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 section may be 170°C to 210°C, which is above the melting point (170°C) of the polyvinylidene fluoride (PVDF) binder resin.

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

[0068] Specifically, in the step of drying the rolled electrode, a three-step drying process may be carried out while sequentially increasing the temperature. Specifically, the step of drying the rolled electrode may include a step of carrying out a first drying process at 130°C to 150°C, a step of carrying out a second drying process at 150°C to 170°C, and a step of carrying out a third drying process at 170°C to 210°C.

[0069] Thus, in the method of the present invention, when performing the drying process after rolling, the drying temperature is gradually increased from 130°C to 210°C, so that the drying temperature in some sections is between 170°C and 210°C. This allows heat to be uniformly applied to the active material and binder inside the electrode layer, minimizing the decrease in the crystallinity of the polyvinylidene fluoride (PVDF) binder resin, and effectively reducing the amount of residual moisture in the electrode to a desired level. As a result, an increase in the resistance of the secondary battery can be prevented, the adhesion between the current collector and the electrode active material layer can be further improved, and productivity can be improved.

[0070] On the other hand, if the drying process is carried out while gradually decreasing the temperature from high to low, for example, if the first step is performed at approximately 180°C to 170°C, which is near the melting point (170°C) of the polyvinylidene fluoride (PVDF) binder resin, and then the drying process is carried out while gradually decreasing the temperature to 130°C, the partially melted crystals of the polyvinylidene fluoride (PVDF) binder resin will gradually grow irregularly, increasing the imbalance in electrode properties within the electrode, and there is a high possibility that the electrode will break down (brittleness) due to the irregularly enlarged crystals. Therefore, as in the present invention, by carrying out the drying process while gradually increasing the temperature from 130°C, and performing the drying temperature for some of the drying steps at a temperature above the melting point (170°C) of the polyvinylidene fluoride (PVDF) binder resin, the polyvinylidene fluoride (PVDF) binder resin can be uniformly dissolved. Therefore, even if a room-temperature (rapid) cooling process is performed in a subsequent step, the crystal size of the binder resin is formed uniformly throughout, making it possible to manufacture electrodes with similar physical properties overall. In particular, the small crystals formed by the rapid cooling process can improve the toughness of the electrodes.

[0071] On the other hand, the total drying time for the rolled electrodes may be 5 seconds or less, specifically between 2 and 5 seconds.

[0072] When the drying time falls within the above range, the decrease in crystallinity can be minimized, and the amount of residual moisture in the electrode can be effectively reduced. If the drying time exceeds the above range, the decrease in crystallinity of the polyvinylidene fluoride (PVDF) binder may cause changes in the physical properties of the electrode (adhesion, toughness) compared to before drying, and the thickness and width of the electrode may change.

[0073] Such positive electrodes can be usefully utilized as positive electrodes in lithium-ion batteries.

[0074] On the other hand, let's explain a lithium secondary battery equipped with the aforementioned positive electrode, using an example.

[0075] The lithium secondary battery specifically includes a positive electrode, a negative electrode positioned opposite the positive electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte, wherein the positive electrode is the positive electrode described above. The lithium secondary battery may further include a case that houses an electrode assembly in which the positive electrode, separator, and negative electrode are stacked in order.

[0076] Next, 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-like negative electrode current collector. The negative electrode active material layer contains a negative electrode active material and may optionally further contain 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 alloys (wherein Me is one or more selected from the group consisting of Al, Sn, Mg, Cu, Fe, Pb, Zn, Mn, Cr, Ti, and Ni), SiOy (wherein 0<y<2), and Si-C composites; lithium metal thin films; metallic materials capable of being alloyed with lithium such as Sn and Al; and 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, a Si-Me alloy (wherein Me is one or more selected from the group consisting of Al, Sn, Mg, Cu, Fe, Pb, Zn, Mn, Cr, Ti, and Ni), SiOy (wherein 0<y<2), a Si-C composite, or a combination thereof, and is preferably SiOy (wherein 0<y<2). Since the silicon-based negative electrode active material has a high theoretical capacity, capacity characteristics can be improved when the silicon-based negative electrode active material is included.

[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 is a group 1 metal element or a group 2 metal element, and specifically may be Li, Mg, or the like. Specifically, the silicon negative electrode active material may be M b metal-doped Si, SiOy (wherein 0<y<2), a Si-C composite, or the like. In the case of a metal-doped silicon-based negative electrode active material, although the capacity of the active material is slightly decreased by the doping element, high energy density can be achieved due to the high efficiency of the doped material.

[0082] Furthermore, the silicon-based anode active material may further contain a carbon coating layer on the surface of the particles. In this case, the amount of carbon coating may be 20% by weight or less, preferably 1% to 20% by weight, based on the total weight of the silicon-based anode 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% to 20% by weight 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 a chemical change in the battery and is conductive, and may be used, for example, graphite such as natural graphite or artificial graphite; carbon black such as acetylene black, Ketjen black, channel black, furnace black, lamp black, or thermal black; conductive fibers such as carbon fibers or metal fibers; conductive powders such as carbon fluoride powder, aluminum powder, or nickel powder; conductive whiskers such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; or conductive materials such as polyphenylene derivatives.

[0084] The binder is a component that assists in bonding between the conductive material, the active material, and the current collector, and is usually added at a concentration of 1% to 30% by weight based on the total weight of the solids in the negative electrode active material layer. Examples of such binders include fluororesin binders containing polyvinylidene fluoride or polytetrafluoroethylene; rubber binders containing styrene-butadiene rubber, acrylonitrile-butadiene rubber, and styrene-isoprene rubber; cellulose binders containing carboxymethylcellulose, starch, hydroxypropylcellulose, and regenerated cellulose; polyalcohol binders containing polyvinyl alcohol; polyolefin binders containing polyethylene and polypropylene; polyimide binders; polyester binders; and silane binders.

[0085] The negative electrode can be manufactured by a negative electrode manufacturing method known in the art. For example, the negative electrode can be manufactured by a method in which a negative electrode active material slurry is produced by dissolving or dispersing a negative electrode active material, a binder, and a conductive material in a solvent, and the slurry is applied to a negative electrode current collector and rolled and dried to form a negative electrode active material layer, or by a method in which the negative electrode active material layer is cast onto another support, and the film obtained by peeling off the support is laminated onto the negative electrode current collector.

[0086] The negative electrode current collector generally has a thickness of 3 μm to 500 μm. Such a negative electrode current collector is not particularly limited as long as it does not cause chemical changes in the battery and has high conductivity. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel with surface treatment using carbon, nickel, titanium, silver, etc., and aluminum-cadmium alloy may be used. Also, similar to the positive electrode current collector, the bonding force of the negative electrode active material may be strengthened by forming fine irregularities on the surface of the negative electrode current collector, and various forms such as film, sheet, foil, mesh, porous material, foam, and nonwoven fabric are possible.

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

[0088] The separator separates the negative electrode and the positive electrode and provides a passage for lithium ions to move. It can be used without particular limitations as long as it is a separator commonly used in lithium secondary batteries, and is especially preferred if it has low resistance to ion movement of the non-aqueous electrolyte and excellent moisture-absorbing capacity for the non-aqueous electrolyte.

[0089] Specifically, as separators, porous polymer films, such as porous polymer films made from polyolefin polymers like ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, or ethylene / methacrylate copolymer, or laminated structures of two or more layers thereof, can be used. Alternatively, ordinary porous nonwoven fabrics, such as nonwoven fabrics made from high-melting-point glass fibers or polyethylene terephthalate fibers, may be used. Furthermore, coated separators containing ceramic components or polymeric substances may be used to ensure heat resistance or mechanical strength, and may be used selectively as single-layer or multi-layer structures.

[0090] The electrolyte used in this invention can be any type of electrolyte suitable for lithium secondary batteries, such as organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, or molten inorganic electrolytes, and the type is not particularly limited.

[0091] Furthermore, the electrolyte may contain an organic solvent and a lithium salt.

[0092] The organic solvent can be any organic solvent that can act as a medium through which ions involved in the electrochemical reaction of the battery can move, and is not particularly limited. Specifically, the organic solvents may include carbonate-based solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), ethyl methyl carbonate (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 or 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; or sulfolanes. Among these, carbonate-based solvents are preferred, and more preferably are mixtures of cyclic carbonates (e.g., ethylene carbonate or propylene carbonate) having high ionic conductivity and high dielectric constant that can improve the charge and 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 for use in lithium secondary batteries, and is not particularly limited. Specifically, the lithium salt can be LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, or LiB(C2O4)2. The concentration of the lithium salt is preferably within the range of 0.1M to 5.0M, more preferably 0.1M to 3.0M. When the concentration of the lithium salt falls within this range, the electrolyte has appropriate conductivity and viscosity, exhibiting excellent electrolyte performance and allowing lithium ions to move effectively.

[0094] The electrolyte may further contain additives in addition to the components of the electrolyte, for the purpose of improving the battery's lifespan, suppressing the decrease in battery capacity, and improving the battery's discharge capacity. For example, the additives may be, but are not limited to, haloalkylene carbonate compounds such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glyme, hexamethyl phosphate triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride, either alone or in combination. The additives may be present in an amount of 0.1% to 10% by weight, preferably 0.1% to 5% by weight, relative to the total weight of the electrolyte.

[0095] The lithium secondary battery according to the present invention, as described above, can be usefully used in portable devices such as mobile phones, notebook computers, and digital cameras; and in the field of 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 the scope of the present invention is not limited to these examples.

[0097] [Examples] Example 1. (1) Production of the cathode slurry composition Cathode active material (Li(Ni) 0.86 Mn 0.07 Co 0.05 Al 0.02 A cathode slurry composition (solid content 70% by weight) was prepared by adding carbon nanotubes as a conductive material and polyvinylidene fluoride (PVDF) as a binder in a weight ratio of 97:1:2 to N-methyl-2-pyrrolidone (NMP) as a solvent.

[0098] (2) Manufacturing of the positive electrode The positive electrode slurry composition produced above 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).

[0099] Next, the dried positive electrode active material layer was rolled until its thickness reached 164 μm. Then, a first drying step was performed at 130°C, a second drying step at 150°C, and a third drying step at 170°C to produce the positive electrode. The total drying time was kept within 5 seconds.

[0100] Example 2. The positive electrode slurry composition produced 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 positive electrode was manufactured by sequentially performing a rolling process on the dried positive electrode active material layer until the thickness of the positive electrode active material layer reached 164 μm, followed by a first drying process at 150°C, a second drying process at 170°C, and then a third drying process at 190°C. The total drying time was kept within 5 seconds.

[0102] Example 3. The positive electrode slurry composition produced 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 positive electrode was manufactured by sequentially performing a rolling process on the dried positive electrode active material layer until the thickness of the positive electrode active material layer reached 164 μm, followed by a first drying process at 150°C, a second drying process at 170°C, and a third drying process at 210°C. The total drying time was kept within 5 seconds.

[0104] Comparative Example 1. The positive electrode slurry composition produced 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 positive electrode was manufactured by sequentially performing a rolling process on the dried positive electrode active material layer until the thickness of the positive electrode active material layer reached 164 μm, followed by a first drying process at 110°C and a second drying process at 130°C. The total drying time was kept within 5 seconds.

[0106] Comparative Example 2. The positive electrode slurry composition produced 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, followed by a drying process at 130°C for 5 seconds.

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

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

[0110] Experimental Example 2. Evaluation of Adhesion Force to Electrode Current Collector The positive electrodes manufactured in Examples 1-3 and the positive electrodes manufactured in Comparative Examples 1 and 2 were each cut to 20 mm wide and 125 mm long, and fixed to a glass slide. The current collector was then 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 Figure 2, it can be confirmed that the positive electrodes manufactured in Examples 1 to 3 of the present invention exhibited significantly increased adhesive strength compared to the positive electrodes manufactured in Comparative Examples 1 and 2.

Claims

1. A step of manufacturing an electrode in which an electrode active material layer is formed on an electrode current collector, The step of rolling the electrode, The step of drying the rolled electrode is included, The drying step is carried out such that the drying temperature in at least a portion of the section is 170°C to 210°C. A method for manufacturing electrodes for secondary batteries, wherein the total drying time of the rolled electrodes is 5 seconds or less.

2. A step of manufacturing an electrode having an electrode active material layer formed on an electrode current collector, The step of rolling the electrode, The step of drying the rolled electrode is included, The drying step is carried out such that the drying temperature in at least a portion of the section is 170°C to 210°C. A method for manufacturing electrodes for secondary batteries, wherein the total drying time of the rolled electrodes 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 an electrode for a secondary battery according to claim 1, wherein the step of drying the rolled electrode is performed at a temperature of 130°C or higher.

5. The method for manufacturing an electrode for a secondary battery according to claim 1, wherein the step of drying the rolled electrode is to sequentially perform a first drying step at 130°C to 150°C, a second drying step at 150°C to 170°C, and a third drying step at 170°C to 210°C.

6. The electrode active material layer comprises an electrode active material and a binder. The method for manufacturing an electrode for a secondary battery according to any one of claims 1 to 5, 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 comprises a polyvinylidene fluoride resin.

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

9. The method for manufacturing an electrode for a secondary battery according to claim 8, wherein the positive electrode active material contains 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, 0.8≦a≦1.2, 0.55≦x<1, 0<y≦0.3, 0<z≦0.3, 0<w≦0.2.)

10. The method for manufacturing an electrode for a secondary battery according to claim 9, wherein the positive electrode active material has a Ni content of 55 atm% or more.

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

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