Positive electrode, secondary battery including the same, and method for manufacturing the same

Abstract

The present invention relates to a positive electrode including a current collector and a positive electrode active material layer provided on the current collector, in which the positive electrode active material layer includes a positive electrode active material, a binder, and multi-walled carbon nanotubes, wherein the average length of the multi-walled carbon nanotubes is 1 [mu] m to 2 [mu] m, and the standard deviation of length is 0.5 [mu] m or less.

Description

[0001] This application is a divisional application of the application with the filing date of February 1, 2019, application number 201980006999.1, and invention title "Positive Electrode and Secondary Battery Comprising the Same" (PCT / KR2019 / 001479, the date of entry into the national phase is June 29, 2020).

[0002] Cross-reference to related applications

[0003] This application claims the benefit of Korean Patent Application No. 10-2018-0015313, filed on February 7, 2018, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference in its entirety. Technical field

[0004] The present invention relates to a positive electrode and a secondary battery including the positive electrode, the positive electrode including a current collector and a positive electrode active material layer disposed on the current collector, wherein the positive electrode active material layer includes a positive electrode active material, a binder, and multi-walled carbon nanotubes, wherein the average length of the multi-walled carbon nanotubes is 1 μm to 2 μm, and the length standard deviation is 0.5 μm or less. Background art

[0005] In recent years, with the technological development and demand for mobile devices, the demand for batteries as an energy source has increased rapidly. Therefore, various studies on batteries have been conducted to meet various demands. In particular, research on lithium secondary batteries as a power source for these devices with high energy density and excellent life and cycle characteristics is being actively carried out.

[0006] A lithium secondary battery refers to a battery including an electrode assembly, the electrode assembly including: a positive electrode containing a positive electrode active material capable of intercalating / deintercalating lithium ions; a negative electrode containing a negative electrode active material capable of intercalating / deintercalating lithium ions; a microporous separator interposed between the positive electrode and the negative electrode; and a non-aqueous electrolyte containing lithium ions.

[0007] The positive electrode and / or the negative electrode may include a conductive material to improve conductivity. Conventionally, mainly point-type conductive materials such as carbon black have been used, but when the content of the conductive material is increased to improve conductivity, the amount of the positive electrode active material or the negative electrode active material is relatively reduced. Therefore, it is difficult to achieve a high energy density of the battery. Therefore, there is a need to meet the required output and the durability of the required battery even when using only a small amount of conductive material. In particular, in the case of the positive electrode, the positive electrode active material itself has a certain degree of low conductivity, so the above problem is very serious.

[0008] To solve this problem, a method has been introduced that utilizes nanoscale conductive materials such as carbon nanotubes or carbon nanofibers, which have a large specific surface area and can make a large number of conductive contacts with a small amount. However, it is difficult for such nanoscale conductive materials to be smoothly dispersed in the positive electrode paste. Therefore, unless the content of the conductive material in the positive electrode active material layer exceeds an appropriate level, such as 1 wt%, it is difficult to obtain the required conductivity.

[0009] Therefore, it is necessary to develop a positive electrode that can ensure conductivity even when using a small amount of conductive material by improving the dispersibility of the conductive material, and can improve the output and life characteristics of the battery. Summary of the Invention

[0010] Technical problem

[0011] One aspect of the present invention provides a positive electrode and a secondary battery including the positive electrode. The positive electrode can ensure its conductivity even when significantly reducing the content of the conductive material, thereby improving the life characteristics of the battery, and can increase the content of the positive electrode active material, thereby improving the output characteristics of the battery.

[0012] Technical solution

[0013] According to one aspect of the present invention, there is provided a positive electrode including: a current collector and a positive electrode active material layer provided on the current collector, wherein the positive electrode active material layer includes a positive electrode active material, a binder, and multi-walled carbon nanotubes, wherein the average length of the multi-walled carbon nanotubes is 1 μm to 2 μm, and the length standard deviation is 0.5 μm or less.

[0014] According to another aspect of the present invention, there is provided a secondary battery including: the positive electrode; a negative electrode; a separator interposed between the positive electrode and the negative electrode; and an electrolyte.

[0015] Advantageous effect

[0016] According to the present invention, multi-walled carbon nanotubes are used as the conductive material, and the multi-walled carbon nanotubes have an appropriate average length and an appropriate length standard deviation. Therefore, the positive electrode active material can be smoothly electrically connected through the multi-walled carbon nanotubes in the manufactured positive electrode, and at the same time, the multi-walled carbon nanotubes can be uniformly dispersed in the conductive material dispersion liquid and the positive electrode paste, thereby improving the life characteristics of the battery. In addition, due to the improved dispersibility of the multi-walled carbon nanotubes, even when using a small amount of multi-walled carbon nanotubes, the conductivity of the positive electrode can be ensured, thereby relatively increasing the content of the positive electrode active material, and therefore, the output characteristics of the manufactured secondary battery can be improved. Brief Description of the Drawings

[0017] The following drawings attached to the specification illustrate by way of example the preferred embodiments of the present invention and are used to further understand the technical concept of the present invention together with the detailed description of the present invention given below. Therefore, the present invention should not be construed only by the content in these drawings.

[0018] Figure 1 is a graph showing the lengths of multi-walled carbon nanotubes included in the positive electrode used in Example 1 of the present invention;

[0019] Figure 2 is a graph showing the lengths of multi-walled carbon nanotubes included in the positive electrode used in Example 2 of the present invention;

[0020] Figure 3 is a graph showing the lengths of multi-walled carbon nanotubes included in the positive electrode used in Comparative Example 1 of the present invention;

[0021] Figure 4 is a graph showing the lengths of multi-walled carbon nanotubes included in the positive electrode used in Comparative Example 2 of the present invention; and

[0022] Figure 5 is a graph showing the increase in the discharge capacity and resistance of the battery with cycling for the batteries of Example 1 and 2 and Comparative Examples 1 and 2. Detailed Description of the Invention

[0023] Hereinafter, the present invention will be described in more detail to allow for a clearer understanding of the present invention. In this case, it will be understood that the words or terms used in the specification and claims should not be construed as having the meanings defined in a common dictionary, and it will be further understood that, based on the principle that the inventor can appropriately define the meanings of the words or terms to best explain the present invention, these words or terms should be construed as having meanings consistent with the technical concept of the present invention and the context of the related art.

[0024] <Positive Electrode>

[0025] According to one aspect of the present invention, the positive electrode includes a current collector and a positive electrode active material layer provided on the current collector, wherein the positive electrode active material layer includes a positive electrode active material, a binder, and multi-walled carbon nanotubes, wherein the average length of the multi-walled carbon nanotubes is 1 μm to 2 μm, and the length standard deviation is 0.5 μm or less.

[0026] The current collector is not particularly limited as long as it has conductivity and does not cause any chemical changes in the battery. For example, the current collector can be made of copper, stainless steel, aluminum, nickel, titanium, sintered carbon, or aluminum or stainless steel whose surface is surface-treated with carbon, nickel, titanium, silver, etc. Specifically, transition metals that well absorb carbon such as copper and nickel can be used as the current collector.

[0027] The positive electrode active material layer may be provided on one or both sides of the current collector. The positive electrode active material layer may include a positive electrode active material, a binder, and multi-walled carbon nanotubes.

[0028] The positive electrode active material may be the same as the positive electrode active material contained in the positive electrode paste of the above-described embodiment. Specifically, the positive electrode active material may be a commonly used positive electrode active material. Specifically, the positive electrode active material may be: a layered compound such as lithium cobalt oxide (LiCoO 2 ), or lithium nickel oxide (LiNiO 2 ), or a compound substituted with one or more transition metals; a lithium manganese oxide represented by the formula Li 1+y1 Mn 2-y1 O 4 (0 ≤ y1 ≤ 0.33), LiMnO 3 , LiMn 2 O 3 , or LiMnO 2 ; lithium copper oxide (Li 2 CuO 2 ); vanadium oxides such as LiV 3 O 8 , V 2 O 5 , or Cu 2 V 2 O 7 ; Ni-site type lithium nickel oxides represented by, for example, the formula LiNi 1-y2 M1 y2 O 2 (where M1 is Co, Mn, Al, Cu, Fe, Mg, B, or Ga and 0.01 ≤ y2 ≤ 0.3); lithium manganese composite oxides represented by, for example, the formula LiMn 2-y3 M2 y3 O 2 (where M2 is Co, Ni, Fe, Cr, Zn, or Ta and 0.01 ≤ y3 ≤ 0.1) or Li 2 Mn 3 M3O 8 (where M3 is Fe, Co, Ni, Cu, or Zn); or LiMn 2 O 4 , in which part of Li is substituted with alkaline earth metal ions; however, the embodiment is not limited thereto. Specifically, the positive electrode active material may be Li[Ni 0.6 Mn 0.2 Co 0.2 O 2 .

[0029] The positive electrode active material may be included in an amount of 96% to 99% by mass, specifically 97% to 98.5% by weight, based on the total weight of the positive electrode active material layer. When the above range is satisfied, the contents of the multi-walled carbon nanotubes and the binder are not excessively reduced, while the output of the battery can be improved, thereby maintaining the life characteristics of the battery.

[0030] The binder may be the same as the binder contained in the positive electrode paste of the above-described embodiment. Specifically, the binder may include at least any one selected from the group consisting of polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethyl methacrylate, and materials in which their hydrogen atoms are substituted with Li, Na, Ca, etc., or may include various copolymers thereof. For example, the binder may be polyvinylidene fluoride having a functional group such as a carboxyl group or an ether group.

[0031] The multi-walled carbon nanotubes may refer to carbon nanotubes having two or more graphene layers arranged parallel to the axis of the carbon nanotube, that is, may refer to carbon nanotubes having two or more walls. In the present invention, carbon nanotubes having an appropriate length and length standard deviation can be formed during the dispersion process of the conductive material dispersion liquid.

[0032] The average length of the multi-walled carbon nanotubes may be 1 μm to 2 μm, specifically 1.1 μm to 1.4 μm, and more specifically from 1.1 μm to 1.2 μm. When the average length of the multi-walled carbon nanotubes is less than 1 μm, there may be a plurality of multi-walled carbon nanotubes with too short lengths. As a result, the electrical connection between the positive electrode active materials is not smooth, thereby deteriorating the output of the battery. At the same time, when the average length of the multi-walled carbon nanotubes exceeds 2 μm, there may be a plurality of multi-walled carbon nanotubes with too long lengths. As a result, the multi-walled carbon nanotubes may easily tangle in the conductive dispersion liquid and the positive electrode paste. Therefore, the multi-walled carbon nanotubes cannot be uniformly dispersed in the positive electrode active material layer, thereby deteriorating the conductivity of the positive electrode active material layer, and thus deteriorating the output and life characteristics of the battery.

[0033] On the other hand, "the average length exceeds 2 μm" means that the length of the multi-walled carbon nanotubes is not sufficiently shortened, which may be due to the lack of a process for controlling the particle size distribution (such as a milling process). Therefore, when the average length of the multi-walled carbon nanotubes exceeds 2 μm, the length standard deviation of the multi-walled carbon nanotubes also has to be large, specifically has to exceed 0.5 μm.

[0034] The length standard deviation of the multi-walled carbon nanotubes may be 0.5 μm or less, specifically 0.3 μm to 0.5 μm. When the length standard deviation of the multi-walled carbon nanotubes exceeds 0.5 μm, the length difference between the multi-walled carbon nanotubes may be large. As a result, the conductivity in the positive electrode active material layer is non-uniform, thereby deteriorating the output and life characteristics of the battery. Moreover, during the preparation of the conductive material dispersion liquid, due to insufficient particle size improvement processes (such as milling processes) of the multi-walled carbon nanotubes, the standard deviation may be too large. Therefore, the dispersibility of the multi-walled carbon nanotubes in the conductive material dispersion liquid and the positive electrode active material layer may deteriorate, further deteriorating the output and life characteristics of the battery. In addition, when the standard deviation is too large, the viscosities of the conductive material dispersion liquid and the positive electrode paste may increase excessively. Therefore, it is difficult to smoothly apply the positive electrode paste during the formation of the positive electrode active material layer, further deteriorating the output and life characteristics of the battery.

[0035] The length of the multi-walled carbon nanotubes may be 0.5 μm to 3.0 μm, specifically 0.7 μm to 2.5 μm. In this specification, the length does not refer to the average length, but rather the respective lengths of the observed multi-walled carbon nanotubes. When the above range is satisfied, the multi-walled carbon nanotubes can be uniformly dispersed in the conductive material dispersion liquid and the positive electrode paste while maintaining the electrical connection between the positive electrode active materials at an appropriate level. Even when a small amount of multi-walled carbon nanotubes with lengths exceeding 3.0 μm are included in the positive electrode active material layer, aggregation occurs between the multi-walled carbon nanotubes due to these long multi-walled carbon nanotubes. Therefore, it is difficult to uniformly disperse the multi-walled carbon nanotubes, thereby deteriorating the output and life characteristics of the battery. In addition, when there are long multi-walled carbon nanotubes, aggregation occurs and the viscosity of the conductive material dispersion liquid increases. Therefore, the processability deteriorates during the preparation of the positive electrode active material layer, and due to the deteriorated processability, the dispersibility of the multi-walled carbon nanotubes may be further reduced.

[0036] Regarding the multi-walled carbon nanotubes included in the positive electrode active material layer, the average length, length standard deviation, and length of the multi-walled carbon nanotubes can be measured by the following method. First, a certain amount of the positive electrode active material layer is diluted with an NMP solution that is dozens of times its weight, and then the materials constituting the positive electrode active material layer are separated by ultrasonic waves. After that, a part of the upper layer of the solution is extracted and diluted again with an NMP solution that is dozens of times the extraction amount. After that, the respective lengths of a plurality of multi-walled carbon nanotubes (such as 30 or 25 multi-walled carbon nanotubes) are observed by a scanning electron microscope (SEM), and then their average value and standard deviation are calculated, and the average length, length standard deviation, and length of the multi-walled carbon nanotubes are derived.

[0037] The multi-walled carbon nanotubes may be included in an amount of 0.1 wt% to 1 wt% based on the total weight of the positive electrode active material layer, specifically may be included in an amount of 0.2 wt% to 0.9 wt%, and more specifically may be included in an amount of 0.2 wt% to 0.7 wt%. When the above range is satisfied, the conductivity of the positive electrode active material layer can be ensured.

[0038] When the multi-walled carbon nanotubes are included in an amount of 1 wt% or less, especially 0.7 wt% or less, it is difficult to achieve with only carbon nanotubes having general properties. Specifically, when the content of relatively general carbon nanotubes is reduced to 1 wt% or less to increase the content of the positive electrode active material, the electrical connection between the positive electrode active materials cannot be smoothly carried out, and thus there is no choice but to inevitably greatly deteriorate the output characteristics of the secondary battery. In addition, the carbon nanotubes cannot smoothly support the positive electrode active material. As a result, the positive electrode active material is likely to separate from the positive electrode active material layer, or the structure of the positive electrode active material layer is likely to gradually collapse, thereby deteriorating the mechanical stability of the positive electrode. Therefore, the cycle characteristics of the manufactured secondary battery inevitably deteriorate.

[0039] Meanwhile, in the present invention, the positive electrode active material layer includes multi-walled carbon nanotubes having suitable physical properties such as an average length and a length standard deviation in a uniformly dispersed state. Therefore, even when the content of the multi-walled carbon nanotubes is 1 wt% or less, the electrical connection between the positive electrode active materials can be maintained and the mechanical stability of the positive electrode active material layer can be ensured, thereby improving the output and life characteristics of the battery. In addition, since the content of the multi-walled carbon nanotubes is maintained at a low level of 1 wt% or less, the positive electrode active material layer can include a relatively larger amount of the positive electrode active material, thereby further improving the output of the manufactured battery.

[0040] The loading amount of the positive electrode active material layer may be 15 - 40 mg / cm 2 , specifically 20 - 30 mg / cm 2 . When the above range is satisfied, while ensuring the energy density of the positive electrode, the thickness of the positive electrode is not excessively increased, and there are no processability problems during the application of the positive electrode paste.

[0041] <Method for manufacturing a positive electrode>

[0042] According to another aspect of the present invention, a method for manufacturing a positive electrode includes: a step of preparing a conductive material dispersion; a step of forming a positive electrode paste, the positive electrode paste including the conductive material dispersion, a positive electrode active material, a binder, and a solvent; and a step of applying the positive electrode paste onto a current collector and drying it, wherein the conductive material dispersion includes multi-walled carbon nanotubes, a dispersant, and a dispersion medium, and the average length of the multi-walled carbon nanotubes is from 1 μm to 2 μm, and the length standard deviation is 0.5 μm or less.

[0043] The step of preparing the conductive material dispersion may include: mixing bundle-type multi-walled carbon nanotubes, a dispersant, and a dispersion medium to form a mixture; and controlling the particle size distribution of the bundled multi-walled carbon nanotubes.

[0044] The dispersant may be at least one selected from the group consisting of hydrogenated nitrile rubber (H-NBR), polyvinylpyrrolidone (PVP), and carboxymethyl cellulose (CMC).

[0045] The dispersion medium may be at least one of N-methyl-2-pyrrolidone (NMP) and water.

[0046] In the bundle-type multi-walled carbon nanotubes, the bundle-type refers to a secondary shape of a bundle or rope shape, in which the axes in the longitudinal direction of a plurality of carbon nanotube units are arranged in parallel in substantially the same direction, or are entangled in substantially the same direction. The bundle-type multi-walled carbon nanotubes have a shape in which the carbon nanotubes are partially aggregated and have various very different lengths. Therefore, when the bundle-type multi-walled carbon nanotubes are directly used as a conductive material without controlling their shape and length, it is difficult for the carbon nanotubes to be uniformly dispersed in the positive electrode active material layer, and it is difficult to ensure a conduction path therein. Therefore, after mixing the bundle-type multi-walled carbon nanotubes, a dispersant, and a dispersion medium, a process for controlling the particle size distribution (i.e., the shape and length of the bundle-type multi-walled carbon nanotubes) is required.

[0047] In particular, since the carbon nanotubes with a length of 5 μm to 50 μm are randomly aggregated, the bundle-type multi-walled carbon nanotubes just after synthesis have a particle shape with an overall size of several tens of μm. For easy handling such as transportation, storage, and feeding, the bundle-type multi-walled carbon nanotubes are usually used to prepare pellets. Therefore, a milling process is required, which decomposes the pellets to separate the bundle-type carbon nanotubes from each other and make the lengths uniform, so as to be used as a conductive material in the positive electrode.

[0048] The particle size distribution can be controlled by methods such as milling or sonication, preferably by the milling method. Milling can be carried out by a ball mill, spike mill, bead mill, basket mill or attrition mill, etc., specifically by a spike mill.

[0049] The milling of the spike mill can be carried out as follows. A mixture containing bundled carbon nanotubes, a dispersant and a dispersion medium is injected into a spike mill filled with beads while the spike mill is running. During operation, the rotor inside the machine rotates, and the rotational force provides kinetic energy to the beads. Therefore, the bundled carbon nanotubes are dispersed in the mixture. After that, the mixture is discharged through the outlet at a specific discharge rate. This process can be carried out under specific conditions to form the multi-walled carbon nanotubes included in the positive electrode of the present invention. In particular, the size of the beads, the filling amount of the beads, the discharge rate of the mixture and the number of milling times are all main conditions, and a conductive material dispersion used in the present invention can be formed by their appropriate combination. In other words, it is required to appropriately satisfy the combination of the above conditions and the ranges of the above conditions.

[0050] The size of the beads can be 0.5 mm to 2 mm, specifically 0.6 mm to 1 mm, and more specifically 0.6 mm to 0.75 mm. When the size of the beads exceeds 2 mm, the shear force generated by the beads is insufficient, so the dispersion and particle size distribution of the multi-walled carbon nanotubes cannot reach the required level. In addition, when the size of the beads is less than 0.5 mm, the beads and the mixture are discharged together to the outlet. Therefore, there is a problem that the dispersibility of the spike mill cannot be kept constant, and a separation process for separating the beads from the discharged product is required.

[0051] The filling amount of the beads can be 50% to 90%, and specifically 65% to 80%. When the filling amount of the beads exceeds 90%, the pressure inside the spike mill increases greatly, making it difficult to continuously use the spike mill. When the filling amount of the beads is less than 50%, it is difficult to form the appropriate kinetic energy required for dispersion.

[0052] The discharge rate of the mixture can be 1 kg / min to 5 kg / min, and specifically 2 kg / min to 4 kg / min.

[0053] The number of milling times refers to the number of times the mixture is injected into the container. The number of milling times can be 2 to 3 times.

[0054] The viscosity of the conductive material dispersion can be 10,000 cps to 30,000 cps at 30 - 50 °C, and specifically 15,000 cps to 25,000 cps. When the above range is satisfied, the conductive material dispersion can be easily injected during the preparation of the positive electrode paste. In addition, satisfying this viscosity means that the multi-walled carbon nanotubes are smoothly dispersed and the particle size distribution meets the required level.

[0055] In the step of forming the positive electrode paste, the positive electrode paste may include a conductive material dispersion, a positive electrode active material, a binder, and a solvent.

[0056] The positive electrode active material, the binder, the multi-walled carbon nanotubes, and the current collector are the same as those included in the positive electrode of the above-described embodiment, and thus their descriptions will be omitted. In addition, the average length, the length standard deviation, and the length of the multi-walled carbon nanotubes included in the conductive material dispersion can be equally maintained in the positive electrode active material layer.

[0057] The solvent may be at least one selected from the group consisting of dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methyl-2-pyrrolidone (NMP), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP) solution, polyvinylidene fluoride solution, acetone, and water. The solvent may be, for example, NMP.

[0058] The solid content of the positive electrode paste may be 60 wt% to 80 wt% relative to the total weight of the positive electrode paste, and specifically 65 wt% to 75 wt%. When the above range is satisfied, the advantage is that the positive electrode paste can be easily dried while maintaining a viscosity sufficient to apply the positive electrode paste to the current collector, and the preferred viscosity of the positive electrode paste may be 5,000 cps to 25,000 cps.

[0059] Since the positive electrode active material layer is prepared by drying the positive electrode paste and removing the solvent, the respective contents of the positive electrode active material, the binder, and the multi-walled carbon nanotubes included in the solid component relative to the total solid weight of the positive electrode paste are equal to the values relative to the total weight of the positive electrode active material layer included in the positive electrode of the above-described embodiment.

[0060] In the step of applying a positive electrode paste to a current collector and drying the positive electrode paste, the application and drying of the current collector coated with the positive electrode paste can be carried out at a temperature of 100 - 180 °C at a rate of 4 m / min to 80 m / min. A rolling process can also be performed to control the thickness of the dried positive electrode, and an additional drying process can be performed to remove the residual moisture of the positive electrode after rolling.

[0061] <Secondary battery>

[0062] According to another aspect of the present invention, a secondary battery includes: a positive electrode; a negative electrode; a separator interposed between the positive electrode and the negative electrode; and an electrolyte. Hereinafter, the positive electrode is the same as the positive electrode of the above embodiment, and thus, the description of the positive electrode will be omitted.

[0063] The negative electrode may include a negative electrode current collector and a negative electrode active material layer provided on one or both sides of the negative electrode current collector.

[0064] The negative electrode current collector is not particularly limited as long as it has conductivity and does not cause any chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, sintered carbon, or aluminum or stainless steel whose surface is surface-treated with carbon, nickel, titanium, silver, etc. can be used as the negative electrode current collector. Specifically, transition metals that well absorb carbon, such as copper and nickel, can be used as the negative electrode current collector.

[0065] The negative electrode active material layer may include a negative electrode active material, a negative electrode conductive material, and a negative electrode binder.

[0066] The negative electrode active material may be graphite-based active material particles or silicon-based active material particles. The graphite-based active material particles may use at least one selected from the group consisting of artificial graphite, natural graphite, graphitized carbon fiber, and graphitized mesocarbon microbeads, and particularly, when artificial graphite is used, the rate performance can be improved. The silicon-based active material particles may use at least one selected from the group consisting of Si, SiO x (0 < x < 2), Si-C composite materials, and Si-Y alloys (where Y is an element selected from the group consisting of alkali metals, alkaline earth metals, transition metals, group 13 elements, group 14 elements, rare earth elements, and combinations thereof), and particularly, when Si is used, a high-capacity battery can be obtained.

[0067] The negative electrode binder may include at least one selected from the group consisting of poly(vinylidene fluoride - hexafluoropropylene) copolymer (PVDF - co - HFP), polyvinylidene fluoride, polyacrylonitrile, polymethylmethacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, 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, etc., and may also include various copolymers thereof.

[0068] There is no particular limitation on the negative electrode conductive material as long as it has conductivity and does not cause any chemical changes in the battery. For example, graphite such as natural graphite and artificial graphite; carbon black such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fibers and 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 and potassium titanate; conductive metal oxides such as titanium oxide; or conductive materials such as polyphenylene derivatives, etc. can be used as the negative electrode conductive material.

[0069] The separator is used to separate the negative electrode and the positive electrode from each other and provide a transmission channel for lithium ions. Any separator can be used without particular limitation as long as it is commonly used in secondary batteries. In particular, a separator having excellent electrolyte retention ability and low resistance to the transfer of electrolyte ions is preferably used. Specifically, a porous polymer membrane, for example, a porous polymer membrane formed of polyolefin polymers (e.g., ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, ethylene / methacrylate copolymer, etc.), or these polymers having a laminated structure of two or more layers can be used. In addition, conventional porous non - woven fabrics, for example, non - woven fabrics formed of glass fibers or polyethylene terephthalate fibers having a high melting point can be used. Or, a coated separator containing a ceramic component or a polymer material can be used to ensure heat resistance or mechanical strength, and a single - layer structure or a multi - layer structure can be selectively used.

[0070] Examples of the electrolyte may include organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel - type polymer electrolytes, solid inorganic electrolytes, and molten inorganic electrolytes that can be used in the manufacture of lithium secondary batteries, but the present embodiment is not limited thereto.

[0071] Specifically, the electrolyte may include a non-aqueous organic solvent and a metal salt.

[0072] As the non-aqueous organic solvent, for example, an aprotic organic solvent such as N-methyl-2-pyrrolidone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, γ-butyrolactone, 1,2-dimethoxyethane, tetrahydrofuran (franc), 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, triphosphate, trimethoxymethane, dioxolane derivatives, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, methyl propionate, and ethyl propionate, etc. can be used.

[0073] In particular, ethylene carbonate and propylene carbonate as cyclic carbonates in carbonate organic solvents can be preferably used because carbonate organic solvents have high viscosity and high dielectric constant, thus facilitating the dissociation of lithium salts, and when dimethyl carbonate and diethyl carbonate, which are linear carbonates with low viscosity and low dielectric constant, are added to the cyclic carbonate in an appropriate proportion and mixed, an electrolyte with high conductivity can be prepared, and the electrolyte prepared in this way can be more preferably used.

[0074] As the metal salt, a lithium salt can be used, which can be easily dissolved in the non-aqueous electrolyte, and for the anion of the lithium salt, for example, one selected from the group consisting of F - , Cl - , I - , NO 3 - , N(CN) 2 - , BF 4 - , ClO 4 - , PF 6 - , (CF 3 ) 2 , PF 4 - , (CF 3 ) 3 , PF 3 - , (CF 3 ) 4 , PF 2 - , (CF 3 ) 5 , PF - , (CF 3 )6 P - CF 3 SO 3 - CF 3 CF 2 SO 3 - ,(CF 3 SO 2 ) 2 N - CF 3 CF 2 (CF 3 ) 2 CO - ,(CF 3 SO 2 ) 2 CH - , (SF 5 ) 3 C - ,(CF 3 SO 2 ) 3 C - CF 3 (CF 2 ) 7 SO 3 - CF 3 CO 2 - , CH 3 CO 2 - 、SCN - and (CF 3 CF 2 SO 2 ) 2 N - .

[0075] In addition to the electrolyte components, in order to improve the life characteristics of the battery, inhibit the capacity reduction of the battery, and increase the discharge capacity of the battery, the electrolyte may further contain one or more additives, for example, halogenated alkylene carbonate compounds (e.g., difluoroethylene carbonate), pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, glyme, hexaphosphoric acid triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, aluminum chloride, or the like.

[0076] According to another aspect of the present invention, provided are: 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 the secondary battery having high capacity, enhanced rate performance, and high cycle characteristics, the battery module and the battery pack can be used as a power source for medium or large-sized devices selected from the group consisting of electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and power storage systems.

[0077] Hereinafter, the present invention will be described in more detail according to each embodiment. However, the present invention can be implemented in many different forms and should not be construed as being limited to the embodiments set forth herein.

[0078] Examples and Comparative Examples

[0079] Example 1: Manufacturing a battery

[0080] (1) Preparation of conductive material dispersion

[0081] Bundled multi-walled carbon nanotubes in the form of pellets having a size of several centimeters, H-NBR as a dispersant, and NMP as a dispersion medium were mixed at a weight ratio of 4:0.8:95.2 to form a mixture. The mixture was added to a bead mill filled with 80% beads having a size of 0.65 mm and dispersed, and then the mixture was discharged at a discharge rate of 2 kg / min. This process was carried out twice to prepare a conductive material dispersion containing multi-walled carbon nanotubes with a controlled particle size distribution.

[0082] (2) Fabrication of positive electrode

[0083] Li(Ni 0.6 Mn 0.2 Co 0.2 )O 2 was used as the positive electrode active material and PVdF as the binder. The positive electrode active material, the conductive material dispersion, and NMP were mixed to prepare a positive electrode slurry having a solid content of 72% and in which the weight ratio of the positive electrode active material, the binder, the dispersant, and the multi-walled carbon nanotubes was 98:1.52:0.08:0.4.

[0084] The positive electrode slurry was applied to a positive electrode current collector (Al) having a thickness of 20 μm until the solid component weight (loading amount) reached 21 mg / cm 2 and dried. Thereafter, the current collector coated with the positive electrode slurry was roll-pressed by a roll pressing method to control the total thickness to 77 μm. Thereafter, the current collector was dried in a vacuum oven at 110 °C for 6 hours to prepare the positive electrode.

[0085] (3) Fabrication of battery

[0086] Natural graphite as the negative electrode active material, carbon black as the negative electrode conductive material, styrene-butadiene rubber (SBR) as the negative electrode binder, and CMC were mixed with distilled water at a weight ratio of 96.1:0.5:2.3:1.1, respectively, to prepare a negative electrode slurry. The prepared slurry was applied to a negative electrode current collector (Cu) with a thickness of 20 μm until the solid component weight (loading amount) reached 10 mg / cm 2 and dried. Thereafter, the current collector coated with the negative electrode slurry was roll-pressed by a roll-pressing method to control the final thickness (the thickness of the current collector and the active material layer) to 80 μm. Thereafter, the current collector was dried in a vacuum oven at 110 °C for 6 hours to prepare a negative electrode.

[0087] The prepared negative electrode, positive electrode, and porous polyethylene separator were assembled by using the stacking method, and an electrolyte solution (ethylene carbonate (EC) / ethyl methyl carbonate (EMC) = 1 / 2 (volume ratio), lithium hexafluorophosphate (1 mole of LiPF 6 )) was injected into the assembled battery to manufacture a lithium secondary battery.

[0088] Example 2: Manufacturing a battery

[0089] (1) Preparation of conductive material dispersion

[0090] A conductive material dispersion, a positive electrode, and a battery were prepared in the same manner as in Example 1, except that when preparing the conductive material dispersion, the bead size was changed to 1 mm.

[0091] Comparative Example 1: Manufacturing a battery

[0092] (1) Preparation of conductive material dispersion

[0093] A conductive material dispersion, a positive electrode, and a battery were prepared in the same manner as in Example 1, except that when preparing the conductive material dispersion, the bead size was changed to 1 mm and the process was carried out once.

[0094] Comparative Example 2: Manufacturing a battery

[0095] (1) Preparation of conductive material dispersion

[0096] A conductive material dispersion, a positive electrode, and a battery were prepared in the same manner as in Example 1, except that when preparing the conductive material dispersion, the process was carried out 4 times.

[0097] The properties of the multi-walled carbon nanotubes contained in the conductive material dispersions formed in the battery manufacturing processes of Example 1 and 2 and Comparative Examples 1 and 2 were measured by the following method, and the results are shown in Figure 1 (multi-walled carbon nanotubes used in Example 1),Figure 2 (Multi-walled carbon nanotubes used in Example 2) Figure 3 (Multi-walled carbon nanotubes used in Comparative Example 1) Figure 4 (Multi-walled carbon nanotubes used in Comparative Example 2) and in Table 1.

[0098] Specifically, NMP was further added to each conductive material dispersion to prepare a diluted solution, in which the content of multi-walled carbon nanotubes was 0.005% by weight relative to the total weight of the solution. Each diluted solution was dropped on a silicon wafer and dried to prepare various samples, and these samples were observed by SEM. At the same time, the lengths of 25 multi-walled carbon nanotubes observed were measured, and the average length, length standard deviation, maximum length, and minimum length were derived based on the measurement results.

[0099] [Table 1]

[0100]

[0101] Test Example 1: Evaluating discharge capacity and battery resistance according to charge / discharge cycles

[0102] The secondary batteries manufactured in Examples 1 and 2 and Comparative Examples 1 and 2 were charged / discharged to evaluate the discharge capacity and battery resistance, and then the results are shown in Figure 5 the figures and Table 2.

[0103] For the first and second cycles, charging / discharging was carried out at 0.1C, and from the 3rd cycle to the 200th cycle, charging / discharging was carried out at 0.5C.

[0104] Charging conditions: CC (constant current) / CV (constant voltage) (4.2V / 0.05C current cut-off)

[0105] Discharge conditions: CC (constant current) condition with cut-off at 2.7V

[0106] In Figure 5 the figure, the left y-axis represents the discharge capacity (%) of each cycle when the discharge capacity of the first cycle is set to 100%, and the right y-axis represents the increase rate (%) of the battery resistance of each cycle compared to the battery resistance of the first cycle.

[0107] When a current of 3C is applied for 30 seconds while the battery is charged to 50% of the total battery capacity, the battery resistance is due to the reduced voltage.

[0108] [Table 2]

[0109]

[0110] Refer to Figure 5As can be confirmed from Table 2, for the batteries in Examples 1 and 2, the average length of the multi-walled carbon nanotubes contained in the positive electrode active material layer is 1 μm to 2 μm and the length standard deviation is 0.5 μm or less. These batteries have a higher capacity retention rate compared with Comparative Examples 1 and 2, and their battery resistance increases much less.

[0111] Specifically, in Comparative Example 1, the average length of the multi-walled carbon nanotubes satisfies 1 μm to 2 μm, but the length standard deviation is 0.81 μm, which is a value exceeding 0.5 μm. That is to say, it can be considered that Comparative Example 1 includes multi-walled carbon nanotubes with excessively changed lengths. As a result, due to some multi-walled carbon nanotubes with too long lengths, aggregation occurs between the multi-walled carbon nanotubes. Thus, the multi-walled carbon nanotubes cannot be uniformly dispersed in the positive electrode active material layer. Therefore, it can be confirmed that the capacity retention rate and resistance characteristics of the battery deteriorate.

[0112] In Comparative Example 2, the length standard deviation satisfies 0.5 μm or less, but the average length of the multi-walled carbon nanotubes is 0.78 μm, which does not satisfy the range of 1 μm to 2 μm. That is to say, the length of the multi-walled carbon nanotubes is too short. As a result, the electrical connection between the positive electrode active materials cannot be smoothly formed. Therefore, it can be confirmed that the capacity retention rate and resistance characteristics of the battery deteriorate.

[0113] The following corresponds to the original claims in the parent application and is hereby incorporated herein as part of the specification:

[0114] 1. A positive electrode, comprising:

[0115] A current collector; and

[0116] A positive electrode active material layer provided on the current collector,

[0117] wherein the positive electrode active material layer includes a positive electrode active material, a binder, and multi-walled carbon nanotubes,

[0118] wherein the average length of the multi-walled carbon nanotubes is 1 μm to 2 μm, and the length standard deviation is 0.5 μm or less.

[0119] 2. The positive electrode according to item 1,

[0120] wherein the length of the multi-walled carbon nanotubes is 0.5 μm to 3.0 μm.

[0121] 3. The positive electrode according to item 1,

[0122] wherein the multi-walled carbon nanotubes are included in an amount of 0.1% by weight to 1% by weight based on the total weight of the positive electrode active material layer.

[0123] 4. The positive electrode according to item 1,

[0124] The multi-walled carbon nanotubes are included in an amount of 0.2% to 0.7% by weight based on the total weight of the positive electrode active material layer.

[0125] 5. The positive electrode according to item 1,

[0126] wherein the positive electrode active material is included in an amount of 96% to 99% by weight based on the total weight of the positive electrode active material layer.

[0127] 6. The positive electrode according to item 1,

[0128] wherein the loading amount of the positive electrode active material layer is 15 - 40 mg / cm 2 .

[0129] 7. A secondary battery, comprising:

[0130] The positive electrode according to any one of items 1 to 6;

[0131] A negative electrode;

[0132] A separator interposed between the positive electrode and the negative electrode; and

[0133] An electrolyte.

Claims

1. A positive electrode, comprising: a current collector; and a positive electrode active material layer provided on the current collector, wherein the positive electrode active material layer includes a positive electrode active material, a binder, and multi-walled carbon nanotubes, wherein the average length of the multi-walled carbon nanotubes is 1 μm to 2 μm, and the length standard deviation is 0.5 μm or less.

2. The positive electrode according to claim 1, wherein the length of the multi-walled carbon nanotubes is 0.5 μm to 3.0 μm.

3. The positive electrode according to claim 1, wherein the multi-walled carbon nanotubes are included in an amount of 0.1% by weight to 1% by weight based on the total weight of the positive electrode active material layer.

4. The positive electrode according to claim 1, wherein the multi-walled carbon nanotubes are included in an amount of 0.2% by weight to 0.7% by weight based on the total weight of the positive electrode active material layer.

5. The positive electrode according to claim 1, wherein the positive electrode active material is included in an amount of 96% by weight to 99% by weight based on the total weight of the positive electrode active material layer.

6. The positive electrode according to claim 1, wherein the loading amount of the positive electrode active material layer is 15-40 mg / cm 2 .

7. A secondary battery, comprising: the positive electrode according to any one of claims 1 to 6; a negative electrode; a separator interposed between the positive electrode and the negative electrode; and an electrolyte.

8. A method for manufacturing a positive electrode, comprising: a step of preparing a conductive material dispersion; a step of forming a positive electrode paste, the positive electrode paste containing the conductive material dispersion, a positive electrode active material, a binder, and a solvent; and a step of applying the positive electrode paste on a current collector and drying it, wherein the conductive material dispersion contains multi-walled carbon nanotubes, a dispersant, and a dispersion medium, and the multi-walled carbon nanotubes have an average length of 1 μm to 2 μm and a length standard deviation of 0.5 μm or less.

9. The method according to claim 8, wherein the step of preparing the conductive material dispersion comprises: mixing multi-walled carbon nanotubes, a dispersant, and a dispersion medium to form a mixture; and controlling the particle size distribution of the multi-walled carbon nanotubes.

10. The method according to claim 9, wherein the dispersant is at least one selected from the group consisting of hydrogenated nitrile rubber, polyvinylpyrrolidone, and carboxymethyl cellulose.

11. The method according to claim 9, wherein the dispersion medium is at least one of N-methyl-2-pyrrolidone (NMP) and water.

12. The method according to claim 9, wherein controlling the particle size distribution is performed by a milling method.

13. The method according to claim 12, wherein the milling is performed by a pin mill.

14. The method according to claim 8, wherein the viscosity of the conductive material dispersion is 10,000 cps to 30,000 cps at 30 - 50 °C.

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