Method for preparing positive electrode

Abstract

The present invention relates to a method for preparing a positive electrode, the method comprising: forming a solid electrolyte by mixing a lithium salt and a polymer for the solid electrolyte in a dry atmosphere; adding a conductive agent and a positive electrode active material to the solid electrolyte in a dry atmosphere and then stirring to form a dry mixture; and pressing after coating a current collector with the dry mixture.

Description

[0001] This application is a divisional application of a patent application for invention with an application date of March 14, 2019, an application number of 201980018146.X, and an invention title of "Method for Preparing a Positive Electrode". Technical Field

[0002] Cross - reference to related applications

[0003] This application claims the benefit of Korean Patent Application No. 10-2018-0029934, filed on March 14, 2018, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference in its entirety. Technical Field

[0005] The present invention relates to a method for preparing a positive electrode, which may specifically include: forming a solid electrolyte by mixing a lithium salt and a polymer for a solid electrolyte in a dry atmosphere; adding a conductive agent and a positive electrode active material to the solid electrolyte in a dry atmosphere and then forming a dry mixture by stirring; and pressing after coating a current collector with the dry mixture. Background Art

[0006] Due to the rapid increase in the use of fossil fuels, the demand for alternative or clean energy has increased. As part of this trend, power generation and electrical energy storage using electrochemical reactions are the most actively studied fields.

[0007] Currently, a typical example of an electrochemical device using electrochemical energy may be a secondary battery, and there is a trend that its use fields are continuously expanding. In recent years, with the technological development and increasing demand for portable devices such as portable computers, mobile phones, and cameras, the demand for secondary batteries as an energy source has also increased significantly. Among these secondary batteries, lithium secondary batteries with a high energy density, that is, a high capacity, have been extensively studied and have been commercialized and widely used.

[0008] Generally, a secondary battery is composed of a positive electrode, a negative electrode, an electrolyte, and a separator. Among them, the positive electrode may include a positive electrode active material and a conductive agent. The conductive agent can play a role in reducing resistance by increasing the conductivity of the positive electrode.

[0009] Organic electrolyte solutions with a high energy density have been used as electrolytes. However, due to problems with the initial stability of organic electrolyte solutions, solid electrolytes that replace organic electrolyte solutions have received attention. The solid electrolyte is added to the electrode paste in a solution state, so that when the electrode paste dries, the solid electrolyte can be contained in the electrode, especially in the active material layer.

[0010] However, in the case where a solvent is used and the solid electrolyte is mixed with the solvent so as to be added to the electrode paste in a solution state (wet process), the solid content of the electrode paste decreases. In addition, during drying of the electrode paste, it is desirable to increase the drying temperature for productivity. However, when drying is performed at a high temperature, since crystallization of the solid electrolyte occurs and unexpected voids are formed in the active material layer, there are limitations in reducing mechanical stability and energy density. Moreover, the porosity of the active material layer included in the prepared electrode must be small in order to increase the energy density. However, in the case where the solid electrolyte is included in the electrode paste by using a solvent, there is a limitation that the porosity may not be reduced to a predetermined level or lower level only by a conventional pressing process.

[0011] Generally, when preparing a composition for forming an electrode including a solid electrolyte, the composition is prepared in a slurry state by using a separate solvent. However, due to the presence of the solvent, the thickness of the active material layer is excessively reduced during the slurry drying process, and it is difficult to prepare an active material layer having a uniform thickness. In addition, since it is very difficult to increase the solid content of the slurry to a predetermined level or higher in consideration of the viscosity suitable for the electrode preparation process, the productivity may be reduced.

[0012] Therefore, a new method for preparing an electrode is needed, which can solve the above limitations caused by using a solvent. Summary of the Invention

[0013] Technical problem

[0014] One aspect of the present invention provides a method for preparing a positive electrode, which can improve productivity and safety because the slurry for preparing the positive electrode includes a solid electrolyte but does not include a solvent.

[0015] Technical solution

[0016] According to one aspect of the present invention, there is provided a method for preparing a positive electrode, the method comprising: forming a solid electrolyte by mixing a lithium salt and a polymer for a solid electrolyte in a dry atmosphere; adding a conductive agent and a positive electrode active material to the solid electrolyte in the dry atmosphere and then forming a dry mixture by stirring; and performing pressing after coating a current collector with the dry mixture.

[0017] Advantageous effects

[0018] According to the method for preparing a positive electrode according to an embodiment of the present invention, since a solid electrolyte in a solid state rather than in a solution state is mixed with a positive electrode active material and a conductive agent to form a dry mixture, and then no separate solvent is added to the dry mixture, the positive electrode can be prepared in a dry atmosphere (solvent-free state). Therefore, since the porosity of the prepared positive electrode is low, the energy density of the positive electrode can be improved. In addition, since it is not necessary to use a positive electrode paste having limitations in solid content and a separate drying process is not required during the manufacturing process, the productivity is improved, and since no toxic solvent is used, the processing safety can be improved. Detailed Description

[0019] Hereinafter, the present invention will be described in more detail so that the present invention can be more clearly understood.

[0020] It should be understood that the words or terms used in the specification and claims should not be construed as having the meanings defined in a commonly used dictionary, and it should also be understood that these words or terms should be construed as having meanings consistent with their meanings in the context of the prior art and the technical concept of the present invention based on the principle that the inventor can appropriately define the meanings of the words or terms to best explain the present invention.

[0021] The terms used in this specification are only for describing exemplary embodiments and are not intended to limit the present invention. Unless otherwise indicated, singular terms may include plural forms.

[0022] It should also be understood that the terms "comprising", "including" or "having" in this specification specify the presence of the specified features, quantities, steps, elements or combinations thereof, but do not exclude the presence of one or more other features, quantities, steps, elements or combinations thereof.

[0023] In this specification, the expression "average particle diameter (D ,

[0025] , ,

[0024] ,

[0023] , 50 , 50 ,

[0022] , )" represents the particle diameter at 50% cumulative volume in the particle size distribution curve. For example, the average particle diameter (D 50 ) can be measured by a laser diffraction method. The laser diffraction method can generally measure particle diameters in the range from the submicron level to several millimeters and can obtain highly reproducible and high-resolution results.

[0024] In this specification, the expression "dry atmosphere" means a process atmosphere in which no solvent is used, and specifically means that no solvent is used during the preparation of the positive electrode.

[0025] The method for preparing a positive electrode according to an embodiment of the present invention may include: forming a solid electrolyte by mixing a lithium salt and a polymer for a solid electrolyte in a dry atmosphere; forming a dry mixture by adding a conductive agent and a positive electrode active material to the solid electrolyte in a dry atmosphere and then stirring; and pressing after coating a current collector with the dry mixture.

[0026] When forming the solid electrolyte, since lithium ions can dissociate from the lithium salt, the lithium ions can be a medium for ionic conduction in the subsequently prepared positive electrode.

[0027] When forming the solid electrolyte, the polymer for the solid electrolyte and the lithium salt can be mixed in a dry atmosphere. That is, the polymer for the solid electrolyte and the lithium salt can be mixed in a state without using a solvent.

[0028] The polymer for the solid electrolyte aggregates with the positive electrode active material and the conductive agent to help form granules.

[0029] The polymer for the solid electrolyte may include at least one selected from the group consisting of polyethylene oxide (PEO), polyacrylonitrile, carboxymethyl cellulose (CMC), polyvinylidene fluoride (PVdF), and styrene-butadiene rubber (SBR), and may specifically be polyethylene oxide. In the case of using the above-mentioned polymer for the solid electrolyte, it is easy to form granules. In addition, since the size of the formed granules can be controlled to an appropriate level, the loading amount and thickness of the positive electrode active material layer can be uniform.

[0030] The polymer for the solid electrolyte may have a weight average molecular weight of 10,000 g / mol to 1,000,000 g / mol, particularly 20,000 g / mol to 500,000 g / mol, and more particularly 30,000 g / mol to 200,000 g / mol. When the weight average molecular weight of the polymer for the solid electrolyte satisfies the above range, since the polymer for the solid electrolyte can aggregate with the positive electrode active material and the conductive agent to easily form granules, the non-aggregated components can be reduced. Therefore, the loading amount and thickness of the positive electrode active material layer can be uniform.

[0031] The lithium salt may include at least one selected from the group consisting of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium hexafluorophosphate (LiPF6), lithium bis(fluorosulfonyl)imide (LiFSI), lithium tetrafluoroborate (LiBF4), and lithium perchlorate (LiClO4). In view of the fact that the lithium salt is easily used as a solid phase, it is suitable for the present invention. Specifically, the lithium salt may be LiTFSI.

[0032] The lithium salt may have an average particle diameter (D 50 ) of 1 μm to 100 μm, particularly 2 μm to 50 μm, and more particularly 5 μm to 20 μm. When the average particle diameter satisfies the above range, since particles of uniform size can be formed, the loading amount and thickness of the positive electrode active material layer can be uniform.

[0033] When forming the solid electrolyte, the polymer and the lithium salt for the solid electrolyte may be mixed at a weight ratio of 5:95 to 50:50, particularly at a weight ratio of 10:90 to 30:70, and more particularly at a weight ratio of 15:85 to 20:80. When the weight ratio satisfies the above range, the ionic conductivity of the prepared positive electrode can be further improved.

[0034] When forming the solid electrolyte, a twin screw mixer can be used to mix the polymer and the lithium salt for the solid electrolyte. When using a twin screw mixer, since a high shear force can be applied to the polymer and the lithium salt for the solid electrolyte, the mixing of the polymer and the lithium salt for the solid electrolyte can be performed more uniformly, and the polymer and the lithium salt for the solid electrolyte can aggregate. In addition, lithium ions can be more easily dissociated from the lithium salt.

[0035] When forming the dry mixture, the conductive agent and the positive electrode active material may be added to the solid electrolyte and then stirred in a dry atmosphere.

[0036] The conductive agent can provide conductivity to the positive electrode. The conductive agent may include graphite such as natural graphite or artificial graphite; carbon-based materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, and carbon fiber; powder or fiber of metals such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide whiskers and potassium titanate whiskers; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives, and any one of them or a mixture of two or more thereof may be used.

[0037] The positive electrode active material may be a commonly used positive electrode active material. Specifically, the positive electrode active material may include a layered compound such as lithium cobalt oxide (LiCoO2) or lithium nickel oxide (LiNiO2) or a compound substituted with one or more transition metals; a lithium iron oxide such as LiFe3O4; a lithium ion battery such as LiFe ... 1+a1 Mn 2-a1 Lithium manganese oxides such as LiO4 (0≤a1≤0.33), LiMnO3, LiMn2O3, and LiMnO2; lithium copper oxides (Li2CuO2); vanadium oxides such as LiV3O8, V2O5, and Cu2V2O7; 0.6 Mn 0.2 Co 0.2 ]O2 or Li[Ni 0.5 Mn 0.3 Co 0.2 ]O2's chemical formula is LiNi 1-a2 M a2 Nickel (Ni)-type lithium nickel oxide represented by O2 (wherein M is at least one selected from the group consisting of cobalt (Co), manganese (Mn), aluminum (Al), copper (Cu), iron (Fe), magnesium (Mg), boron (B) and gallium (Ga), and a2 satisfies 0.01≤a2≤0.5); 2-a3 M a3 O2 (wherein M is at least one selected from the group consisting of Co, Ni, Fe, chromium (Cr), zinc (Zn) and tantalum (Ta), and a3 satisfies 0.01≤a3≤0.1) or Li2Mn3MO8 (wherein M is at least one selected from the group consisting of Fe, Co, Ni, Cu and Zn); and at least one selected from the group consisting of LiMn2O4 in which a portion of lithium (Li) is substituted by alkaline earth metal ions. More specifically, the positive electrode active material may include Li[Ni x Mn y Co z ]O2(0.38 <x<0.84,0.08<y<0.31,0.08<z<0.31)。

[0038] When forming the dry mixture, the solid electrolyte can be included in an amount of 5 wt% to 50 wt%, particularly in an amount of 10 wt% to 40 wt%, and more particularly in an amount of 20 wt% to 35 wt% in the dry mixture. When the amount of the solid electrolyte satisfies the above range, excessive reduction of the flexibility of the positive electrode can be prevented.

[0039] The dry mixture may include granules formed by aggregation of a solid electrolyte, a positive electrode active material, and a conductive agent. Since these granules have good fluidity, the loading amount and thickness of the positive electrode active material layer formed when the dry mixture is applied and coated on the current collector can be uniform.

[0040] The granules may have an average particle diameter (D 50 ) of 20 μm to 500 μm, particularly 45 μm to 300 μm, and more particularly 50 μm to 200 μm. When the average particle diameter of the granules satisfies the above range, since the fluidity of the granules can be further improved, the loading amount and thickness of the positive electrode active material layer can be more uniform.

[0041] Stirring can be performed in a dry atmosphere without using a solvent. In the present invention, since the solid electrolyte is not used in a solution state added to a solvent, and a solvent is not used even during stirring of the positive electrode active material, the conductive agent, and the solid electrolyte, a solvent can be not used throughout the process. Therefore, since the porosity of the prepared positive electrode is lower than the normal level, the energy density of the positive electrode can be improved. In addition, since the preparation process is not affected by the solid content of the positive electrode paste and a separate drying process is not necessary, the productivity can be improved. Moreover, in a conventional process using a solvent, a toxic solvent such as acetonitrile (AN) must be used depending on the type of the solid electrolyte. However, since a toxic solvent can be not used in the present invention, the processing safety can be improved.

[0042] For example, stirring can be performed at a temperature of 20 °C or lower at 300 rpm to 1,000 rpm for 0.2 hours to 1.0 hours using a stirring device.

[0043] In some cases, when forming the dry mixture, a twin screw mixer can be used to perform stirring. When using a twin screw mixer, since a high shear force can be applied to the dry mixture, the solid electrolyte, the conductive agent, and the positive electrode active material can be uniformly dispersed and mixed. Therefore, the energy density can be uniformly formed in the positive electrode active material layer.

[0044] The dry mixture can be coated on the current collector and then can be rolled.

[0045] There is no particular limitation on the current collector as long as it has electrical conductivity and does not cause adverse chemical changes in the battery. For example, stainless steel, aluminum, nickel, titanium, sintered carbon, or aluminum or stainless steel surface-treated with one of carbon, nickel, titanium, silver, or the like can be used. In addition, the positive current collector typically has a thickness of 3 μm to 500 μm, and minute irregularities can be formed on the surface of the current collector to improve the adhesion of the positive active material. The current collector can be used in various shapes such as films, sheets, foils, meshes, porous bodies, foams, non-woven objects, and the like, but the present invention is not limited thereto.

[0046] Coating can be performed in the following manner, but is not necessarily limited thereto. Specifically, the dry mixture can be set on the current collector to a uniform thickness by the sowing method. More specifically, when the sowing method is used, the mixture is transferred by a feeding roller, and when the dry mixture is coated on the current collector, a fixed amount of the dry mixture can be coated using a squeeze roll.

[0047] The dry mixture in powder form is set on the current collector, and the dry mixture is not set on the current collector after being prepared in film form. Since the dry mixture in powder form can be set on the current collector, the contact between the current collector and the dry mixture can be increased compared to the case where the dry mixture is set in the form of a film, and thus the positive adhesion can be further improved.

[0048] The dry mixture can be set on one surface or both surfaces of the current collector. In order to set the dry mixture on both surfaces, after coating one surface of the current collector with the dry mixture and then pressing to form a positive active material layer on the one surface, the dry mixture can be set on the other surface.

[0049] The dry mixture set on the current collector can be formed into a positive active material layer through a pressing process. Specifically, the positive active material layer can be formed by pressing the current collector on which the dry mixture is set, and the pressing can apply spinning to the dry mixture. In the case of applying spinning, since pressing can be performed even with a smaller force than in the case of applying surface pressure, the processability can be improved.

[0050] The pressing can include rolling the current collector on which the dry mixture is set by a roller. Specifically, in a state where two rollers are set on the top and bottom of the current collector on which the dry mixture is set, the roll pressing method includes applying pressure to the current collector on which the dry mixture is set with the roller and simultaneously moving the current collector on which the dry mixture is set in the horizontal direction.

[0051] Through the pressing process, the porosity of the positive electrode active material layer can be in the range of 3.5% to 18%. When this porosity is satisfied, the positive electrode conductivity can be improved, and the thickness of the positive electrode can be appropriate. The porosity can be calculated by the following Equation 1.

[0052] [Equation 1]

[0053] Density of the positive electrode active material layer (g / cm) = Weight of the positive electrode active material layer / (Positive electrode thickness before pressing - Current collector thickness)

[0054] Porosity (%) = 1 - (Density of the positive electrode active material layer / True density of the positive electrode active material layer)

[0055] It is difficult to obtain the above porosity range by a conventional method of preparing a positive electrode using a solvent. Specifically, when a solvent is used, the area where the solvent is located may become macropores due to the removal of the solvent in the drying process. Therefore, since the thickness of the positive electrode from which the solvent has been removed is too large, it is necessary to perform the pressing process several times in order to reduce the thickness and porosity. As a result, the processability of preparing the positive electrode is reduced. In contrast, since no solvent is used in the present invention, the area where the solvent is located does not exist by itself. Therefore, the porosity range can be satisfied even without performing the pressing process several times. Specifically, according to the method of preparing a positive electrode of the present invention, the porosity of 3.5% to 18% can be satisfied even when the number of pressing processes is ten times or less.

[0056] The method of preparing a positive electrode according to another embodiment of the present invention is similar to the method of preparing a positive electrode according to the above embodiment, except that the method of preparing a positive electrode according to another embodiment of the present invention further includes applying a shearing force of 20 N to 500 N to the dry mixture. Therefore, this difference will be described.

[0057] Shearing force can be applied after forming the dry mixture and before coating the current collector. Applying shearing force can include applying shearing force by shearing the dry mixture. Specifically, in the case of using a device for applying shearing force, such as Nobilta (Hosokawa Micron Ltd.) or a twin screw extruder (Twin screw extruder, Thermo KA), shearing force can be applied by shearing the dry mixture with the blade in the device. However, the present invention is not necessarily limited to this method. When shearing force is applied to the dry mixture, the polymers for the solid electrolyte in the dry mixture can be entangled with each other. Therefore, since the positive electrode active material and the conductive agent can be supported by the polymers for the solid electrolyte, the binding force between the positive electrode active material, the conductive agent, and the polymers for the solid electrolyte can be increased. Accordingly, the process of setting the dry mixture on the current collector can be facilitated, and the adhesion of the prepared positive electrode active material layer can be further improved.

[0058] The shearing force can be in the range of 20 N to 500 N, and can specifically be in the range of 25 N to 300 N. When the shearing force meets the above range, entanglement of the polymers for the solid electrolyte can occur smoothly. In addition, when considering the aspect of maintaining the length of the polymers for the solid electrolyte, the shearing force can preferably be in the range of 50 N to 150 N.

[0059] The method for preparing a positive electrode according to another embodiment of the present invention is similar to the method for preparing a positive electrode according to this embodiment described above, but the difference is that when a pressing roller is used in pressing, the roller is in a high temperature state. Therefore, this difference will be described.

[0060] During pressing, the temperature of the roller can be in the range of 10 °C to 60 °C, particularly 10 °C to 40 °C, and more particularly 25 °C to 40 °C. The temperature of the roller is the optimal temperature considering the polymers for the solid electrolyte. When the temperature of the roller is lower than 10 °C, since the polymers for the solid electrolyte do not melt properly thermally, the positive electrode active material layer cannot adhere to the current collector smoothly. In contrast, when the temperature of the roller is higher than 60 °C, since the thermally melted polymers for the solid electrolyte become too hard after pressing, the flexibility of the positive electrode decreases, and thus the cycle characteristics and safety of the battery may deteriorate.

[0061] A positive electrode according to another embodiment of the present invention can be a positive electrode prepared by the method for preparing a positive electrode according to the above embodiment.

[0062] A secondary battery according to another embodiment of the present invention may include a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode. Herein, the positive electrode is the same as the positive electrode of the above embodiment.

[0063] The negative electrode includes a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector.

[0064] There is no particular limitation on the negative electrode current collector as long as it has high electrical conductivity and does not cause adverse chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, sintered carbon, copper or stainless steel surface-treated with one of carbon, nickel, titanium, silver or the like, and aluminum cadmium alloy can be used. In addition, the negative electrode current collector may typically have a thickness of 3 μm to 500 μm, and similar to the positive electrode current collector, minute irregularities may be formed on the surface of the current collector to improve the adhesion of the negative electrode active material. For example, the negative electrode current collector can be used in various shapes such as films, sheets, foils, meshes, porous bodies, foams, non-woven objects, and the like.

[0065] The negative electrode active material layer may selectively include a negative electrode binder, a negative electrode conductive agent, and a negative electrode active material. For example, the negative electrode active material layer can be prepared by coating a composition for forming a negative electrode that selectively includes a negative electrode binder, a negative electrode conductive agent, and a negative electrode active material on the negative electrode current collector and drying the coated negative electrode current collector, or can be prepared by casting a composition for forming a negative electrode on a separate support and then laminating a film separated from the support on the negative electrode current collector.

[0066] Compounds capable of reversibly inserting and extracting lithium can be used as the negative electrode active material. Specific examples of the negative electrode active material can be carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, and amorphous carbon; metal compounds capable of alloying with lithium such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloys, Sn alloys, or Al alloys; such as SiO x(0 < x < 2), metal oxides that can be doped with lithium or not, such as SnO2, vanadium oxides, and lithium vanadium oxides; or composite materials including metal compounds and carbonaceous materials, such as Si-C composite materials or Sn-C composite materials, and any one of them or a mixture of two or more of them can be used. In addition, a thin film of metallic lithium can be used as the negative electrode active material. Moreover, low-crystalline carbon and high-crystalline carbon can be used as carbon materials. Typical examples of low-crystalline carbon can be soft carbon and hard carbon, while typical examples of high-crystalline carbon can be natural graphite or artificial graphite, Kish graphite, pyrolytic carbon, mesophase pitch based carbon fiber, meso-carbon microbeads, mesophase pitches, and high-temperature sintered carbon such as petroleum or coal tar pitch derived cokes, which are irregular, planar, flaky, spherical, or fibrous.

[0067] The negative electrode binder may include at least one selected from the group consisting of polyvinylidene fluoride - hexafluoropropylene (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polyacrylate, ethylene - propylene - diene monomer (EPDM), sulfonated EPDM, styrene - butadiene rubber (SBR), fluororubber, poly acrylic acid, and materials in which hydrogen is replaced by Li, Na, or Ca, or may include various copolymers thereof.

[0068] The negative electrode conductive agent is not particularly limited as long as it has conductivity and does not cause adverse chemical changes in the battery. For example, conductive materials such as graphite such as natural graphite or artificial graphite; carbon blacks such as acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fibers or metal fibers; conductive tubes such as carbon nanotubes; metal powders such as carbon fluoride powder, aluminum powder, and nickel powder; conductive whiskers such as zinc oxide whiskers and potassium titanate whiskers; conductive metal oxides such as titanium oxides; or polyphenylene derivatives. The carbon fiber may be a vapor-grown carbon fiber (VGCF).

[0069] The separator separates the negative electrode and the positive electrode and provides a path for the movement of lithium ions. Any separator can be used as the separator without particular limitation as long as it is typically used in lithium secondary batteries. Specifically, a porous polymer film can be used, such as a porous polymer film made of polyolefin-based polymers such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer, or a laminated structure having two or more layers thereof can be used. In addition, typical porous nonwoven fabrics can be used, such as nonwoven fabrics formed of high-melting-point glass fibers or polyethylene terephthalate fibers. Moreover, a coated separator including ceramic components or polymer materials can be used to ensure heat resistance or mechanical strength, and a separator having a single-layer or multi-layer structure can be selectively used.

[0070] According to another embodiment of the present invention, there is provided a battery module including the secondary battery as a unit cell and a battery pack including the battery module. Since the battery module and the battery pack include a secondary battery having high capacity, high rate capability, and high cycle characteristics, the battery module and the battery pack can be used as a power source for medium-sized and large-sized devices selected from the group consisting of electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and power storage systems.

[0071] Hereinafter, embodiments of the present invention will be described in detail in a manner that enables those of ordinary skill in the art to which the present invention pertains to easily implement the present invention. However, the present invention can be embodied in various different forms and should not be construed as being limited to the embodiments set forth herein.

[0072] Examples and Comparative Examples

[0073] Example 1: Preparation of the positive electrode

[0074] (1) Formation of Solid Electrolyte

[0075] Having an average particle diameter (D of 5 μm 50) LiTFSI is used as the lithium salt, and PEO with a weight-average molecular weight of 100,000 g / mol is used as the polymer for the solid electrolyte.

[0076] 80 g of the lithium salt and 20 g of the polymer for the solid electrolyte are mixed, and a shear force of 100 N is applied for 0.5 hours using a twin screw mixer (Twin screw mixer, RS3000). Thus, the solid electrolyte is formed.

[0077] (2) Preparation of the positive electrode

[0078] Li[Ni 50 with an average particle diameter (D 0.6 Mn 0.2 Co 0.2 O2 is used as the positive electrode active material, and VGCF with an average particle diameter (D 50 ) of 3 μm is used as the conductive agent. After adding 1.5 g of the conductive agent to 28.95 g of the solid electrolyte, 69.55 g of the positive electrode active material is added thereto to obtain a mixture. Thereafter, a dry mixture is prepared by applying a shear force of 100 N to the mixture for 1.0 hour using a twin screw mixer (Twin screw mixer, RS3000). The dry mixture aggregates in the stirring process to form particles (granules) with an average particle diameter (D 50 ) of 100 μm.

[0079] The initial positive electrode is prepared by setting the dry mixture on one surface of an aluminum current collector with a thickness of 15 μm using the spreading method. The initial positive electrode is set on the conveyor belt of a roll press, and then the initial positive electrode is rolled (pressed) ten times at 23 °C at a speed of 0.5 m / min using a roll to prepare the positive electrode of Example 1 including the positive electrode active material layer. The loading amount of the positive electrode active material layer is 40 mg / cm 2 .

[0080] Example 2: Preparation of the positive electrode

[0081] The positive electrode of Example 2 is prepared in the same manner as in Example 1, except that, similar to Example 1, the temperature of the roll for roll pressing is 40 °C, and rolling is performed three times.

[0082] Example 3: Preparation of the positive electrode

[0083] The positive electrode of Example 3 is prepared in the same manner as in Example 1, except that, during the preparation of the dry mixture in Example 1, a shear force of 200 N is applied to the mixture of the solid electrolyte, the conductive agent, and the positive electrode active material using a twin screw mixer.

[0084] Example 4: Preparation of the positive electrode

[0085] The positive electrode of Example 4 was prepared in the same manner as in Example 1, except that during the preparation of the dry mixture in Example 1, a mixture of solid electrolyte, conductive agent, and positive electrode active material was mixed (100 rpm) using an automatic mortar mixer (ALM-50, Nitto Kagaku Co., Ltd.).

[0086] Example 5: Preparation of the positive electrode

[0087] The positive electrode of Example 5 was prepared in the same manner as in Example 1, except that during the preparation of the solid electrolyte in Example 1, 70 g of lithium salt and 30 g of the polymer for the solid electrolyte were mixed while applying a shear force of 100 N for 0.5 hours using a twin-screw mixer.

[0088] Comparative Example 1: Preparation of the positive electrode

[0089] (1) Preparation of electrolyte solution

[0090] LiTFSI having an average particle size (D 50 ) of 5 μm was used as the lithium salt, and PEO having a weight-average molecular weight of 100,000 g / mol was used as the polymer for the solid electrolyte.

[0091] 81 g of the lithium salt and 19 g of the polymer for the solid electrolyte were added to 150 g of xylene, and then mixed using a homogenizing mixer at 25 °C for 24 hours. Thus, an electrolyte solution was formed.

[0092] (2) Preparation of positive electrode slurry

[0093] Li[Ni 50 Mn 0.6 Co 0.2 O2 having an average particle size (D 0.2 of 15 μm was used as the positive electrode active material, and having an average particle size (D 50) VGCF is used as a conductive agent, and acetonitrile is used as a solvent for the positive electrode paste. 1.5 g of the conductive agent is added to 28.95 g of the electrolyte solution, and stirring is performed using a homogenizer at 1000 rpm for 0.2 hours. After that, 20 g of acetonitrile is added to the electrolyte solution containing the conductive agent, and then stirring is performed using a homogenizer at 1000 rpm for 0.2 hours to form a mixture. Subsequently, 69.55 g of the positive electrode active material is added to the mixture, and stirring is performed using a homogenizer at 1,000 rpm for 0.2 hours. After that, 40 g of acetonitrile is added to the mixture containing the positive electrode active material, and then stirring is performed using a homogenizer at 1000 rpm for 0.2 hours to prepare the positive electrode paste.

[0094] (3) Preparation of the positive electrode

[0095] The positive electrode paste is coated on one surface of an aluminum current collector having a thickness of 15 μm with a loading of 40 mg / cm 2 , and then dried. Drying in this case is performed in a convection oven at 25 °C for 24 hours. Subsequently, the current collector coated with the positive electrode paste and dried is rolled at room temperature by a roller and dried in a vacuum oven at 25 °C for 48 hours to prepare the positive electrode of Comparative Example 1. In this case, the loading amount of the positive electrode active material layer is 40 mg / cm 2 .

[0096] Test Example 1: Evaluation of the adhesion of the positive electrode

[0097] After stamping each of the positive electrodes of Examples 1 to 5 and Comparative Example 1 to a size of 20 mm × 150 mm and fixing them to the center of a 25 mm × 75 mm glass slide using tape, the 90-degree peel strength is measured while peeling the current collector using a universal testing machine (UTM). Evaluation is made by measuring the peel strength of 5 or more stamped samples and calculating the average value. The results are shown in Table 1 below.

[0098] Test Example 2: Thickness change of the positive electrode

[0099] For each of the positive electrodes of Examples 1 to 5 and Comparative Example 1, after measuring the thickness of a part of the positive electrode and the thickness of other parts (5 parts) spaced 10 mm from that part by applying a force of 1.0 N using a 5 mm tip (thickness measurement standard: 100 mm), the thickness change is calculated as the average value of the difference between the thickness of that part and the thickness of the other parts. The results are shown in Table 1 below.

[0100] Test Example 3: Porosity evaluation

[0101] For each of the positive electrodes of Examples 1 to 5 and Comparative Example 1, the porosity was evaluated by the following method, and the results are shown in Table 1 below.

[0102] Positive electrode active material layer density (g / cm) = Positive electrode active material layer weight / (Positive electrode thickness before rolling - Current collector thickness)

[0103] Porosity (%) = 1 - (Positive electrode active material layer density / True density of positive electrode active material layer)

[0104] [Table 1]

[0105]

[0106] Referring to Table 1, it can be understood that the positive electrodes of Examples 1 to 5 prepared by the dry method of the present invention have lower thickness variation and porosity than the positive electrode of Comparative Example 1 prepared by the wet method. When comparing Example 1 and Example 2, it can be understood that when the temperature of the roller is set to 40 °C, the thickness variation and porosity of the positive electrode are more greatly improved.

[0107] When comparing Example 1 and Example 3, it can be understood that when an appropriate level of shear force of about 100 N is applied during the preparation of the dry mixture, all of the positive electrode adhesion, thickness variation of the positive electrode, and porosity can be improved.

[0108] When comparing Example 1 and Example 4, it can be understood that when shear force is applied by using a twin-screw mixer, all of the positive electrode adhesion, thickness variation of the positive electrode, and porosity can be improved.

[0109] When comparing Example 1 and Example 5, it can be understood that when the ratio of the lithium salt to the polymer for the solid electrolyte is at a reasonable level of 80:20, all of the positive electrode adhesion, thickness variation of the positive electrode, and porosity can be improved.

[0110] Test Example 4: Evaluation of discharge capacity and initial efficiency

[0111] After using the positive electrodes of Examples 1 to 5 and Comparative Example 1 and preparing secondary batteries by the following method, the discharge capacity and initial efficiency were evaluated, and the results are shown in Table 2 below.

[0112] (1) Preparation of negative electrode

[0113] Natural graphite, carbon black conductive agent, and PVdF binder as negative electrode active materials were mixed in a weight ratio of 85:10:5 in an N-methylpyrrolidone solvent to prepare a composition for forming a negative electrode, and the copper current collector was coated with the composition to prepare a negative electrode.

[0114] (2) Preparation of secondary battery

[0115] An electrode assembly was prepared by disposing a porous polyethylene separator between the negative electrode and each of the positive electrodes prepared in Examples 1 to 5 and Comparative Example 1. The electrode assembly was disposed in a case, and then an electrolyte solution was injected into the case to prepare a lithium secondary battery. In this case, the electrolyte solution was prepared by dissolving 1.0 M lithium hexafluorophosphate (LiPF6) in an organic solvent composed of ethylene carbonate / dimethyl carbonate / ethyl methyl carbonate (EC / DMC / EMC = 3 / 4 / 3 by mixed volume ratio). Thus, each secondary battery was prepared.

[0116] (3) Discharge capacity and initial efficiency evaluation

[0117] The secondary battery was charged and discharged under the following conditions to evaluate the discharge capacity and initial efficiency.

[0118] Charging conditions: Constant current charge at 0.05C to 4.25V, and thereafter, at a current rate of 0.05C, constant voltage charge to a current of 5% of the constant current value

[0119] Discharge conditions: Discharge at a rate of 0.05C to 3.0V

[0120] Initial efficiency (%) = (Discharge capacity after the first discharge / First charge capacity) × 100

[0121] [Table 2]

[0122] Discharge capacity (mAh / g) Initial efficiency (%) Example 1 163 80 Example 2 171 85 Example 3 161 63 Example 4 140 55 Example 5 151 64 Comparative Example 1 105 40

[0123] Referring to Table 2, for the secondary batteries including the positive electrodes of Examples 1 to 5 prepared by the dry method of the present invention, it can be understood that the discharge capacity and initial efficiency are higher than those of the secondary battery including the positive electrode of Comparative Example 1 prepared by the wet method.

Claims

1. A positive electrode, comprising: a positive electrode current collector, and a positive electrode active material layer disposed on the positive electrode current collector and comprising a dry mixture, wherein the positive electrode has a porosity of 3.5% to 18%.

2. The positive electrode according to claim 1, wherein the dry mixture comprises particles, and the particles comprise a solid electrolyte, a positive electrode active material, and a conductive agent.

3. The positive electrode according to claim 2, wherein the solid electrolyte comprises a polymer for the solid electrolyte and a lithium salt.

4. The positive electrode according to claim 3, wherein the polymer for the solid electrolyte comprises at least one selected from the group consisting of polyethylene oxide, polyacrylonitrile, carboxymethyl cellulose, polyvinylidene fluoride, and styrene-butadiene rubber.

5. The positive electrode according to claim 3, wherein the lithium salt comprises at least one selected from the group consisting of lithium bis(trifluoromethanesulfonyl)imide, lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, lithium tetrafluoroborate, and lithium perchlorate.

6. The positive electrode according to claim 3, wherein the polymer for the solid electrolyte has a weight average molecular weight of 20,000 g / mol to 500,000 g / mol.

7. The positive electrode according to claim 3, wherein the lithium salt has an average particle size (D 50 ) of 2 μm to 50 μm.

8. The positive electrode according to claim 3, wherein, In the solid electrolyte, the weight ratio of the polymer for the solid electrolyte to the lithium salt is in the range of 5:95 to 50:

50.

9. The positive electrode according to claim 2, wherein the particles have an average particle size (D 50 ) of 20 μm to 500 μm.

10. The positive electrode according to claim 2, wherein the solid electrolyte is included in the dry mixture in an amount of 10% to 40% by weight.

11. The positive electrode according to claim 1, wherein the positive electrode has a thickness variation of 4 μm to 8 μm.

12. A method for preparing a positive electrode, the method comprising: forming a solid electrolyte by mixing a lithium salt and a polymer for the solid electrolyte in a dry atmosphere; adding a conductive agent and a positive electrode active material to the solid electrolyte in a dry atmosphere and then forming a dry mixture by stirring; and performing pressing after coating the current collector with the dry mixture, wherein the positive electrode has a porosity of 3.5% to 18%.

13. The method according to claim 12, wherein the polymer for the solid electrolyte comprises at least one selected from the group consisting of polyethylene oxide, polyacrylonitrile, carboxymethyl cellulose, polyvinylidene fluoride, and styrene-butadiene rubber.

14. The method according to claim 12, wherein the lithium salt comprises at least one selected from the group consisting of lithium bis(trifluoromethanesulfonyl)imide, lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, lithium tetrafluoroborate, and lithium perchlorate.

15. The method according to claim 12, wherein, In the solid electrolyte, the weight ratio of the polymer for the solid electrolyte to the lithium salt is in the range of 5:95 to 50:

50.

16. The method according to claim 12, wherein the dry mixture comprises particles formed by aggregation of the solid electrolyte, the positive electrode active material, and the conductive agent.

17. The method according to claim 16, wherein the particles have an average particle size (D 50 ) of from 20 μm to 500 μm.

18. The method according to claim 12, wherein When forming the dry mixture, the stirring is performed using a twin-screw mixer.

19. The method according to claim 12, further comprising applying a shearing force of 20 N to 500 N to the dry mixture.

20. The method according to claim 12, wherein the pressing comprises rolling the current collector coated with the dry mixture thereon by a roller.

21. The method according to claim 20, wherein the temperature of the roller is in the range of 10 °C to 60 °C.

Images