Carbon nanotube dispersion liquid, method for preparing same, electrode slurry composition comprising same, electrode comprising same, and lithium secondary battery

Abstract

The present invention relates to a carbon nanotube dispersion liquid, a method for preparing the same, an electrode slurry composition comprising the same, an electrode comprising the same, and a lithium secondary battery comprising the same, the carbon nanotube dispersion liquid comprising: carbon nanotubes having a specific surface area (BET) of 250 m2 / g to 750 m2 / g; a first dispersant having an amide group; and a second dispersant having an aromatic ring, the carbon nanotube dispersion liquid having a viscosity of 1,300 cPs or less at 25 DEG C and a shear rate of 15 sec <-1 >.

Description

Technical Field

[0001] This application claims priority to Korean Patent Application No. 10-2022-0139966, filed with the Korean Intellectual Property Office on October 27, 2022, the contents of which are incorporated herein by reference.

[0002] This disclosure relates to a carbon nanotube dispersion, a method for preparing the same, an electrode paste composition including the same, an electrode including the same, and a lithium secondary battery including the same. Background Art

[0003] A secondary battery is a battery that can be reused through a discharging process of converting chemical energy into electrical energy and a charging process that is the reverse thereof. A secondary battery includes a positive electrode, a negative electrode, an electrolyte, and a separator. The positive electrode and the negative electrode generally include: an electrode current collector; and an electrode active material layer formed on the electrode current collector. The electrode active material layer is formed by coating an electrode paste composition including an electrode active material, a conductive material, a binder, etc. onto the electrode current collector, followed by drying and rolling.

[0004] Conductive materials are used to improve the conductivity of electrode active materials. Conventionally, point-like conductive materials such as carbon black have been mainly used. However, since the effect of point-like conductive materials on improving conductivity is not high, an excessive amount thereof must be used to obtain a sufficient effect, which results in a decrease in the content of electrode active materials, thereby causing a problem of a decrease in battery capacity.

[0005] To improve this problem, active attempts are being made to apply highly conductive carbon nanotubes (CNTs) as conductive materials. Since carbon nanotubes can achieve high conductivity even when used in a small amount, compared with using carbon black, using carbon nanotubes can significantly reduce the content of conductive materials, thereby having the advantage of increasing the capacitance.

[0006] To use carbon nanotubes as a negative electrode conductive material, from a process perspective, it is necessary to prepare a low-viscosity aqueous dispersion.

[0007] Polyvinylpyrrolidone (PVP) is a dispersant having an amide group. As a polymeric surfactant, it is used as a dispersant, an emulsifier, a thickener, etc. in various dispersion systems, and is known to be effective for the dispersion of carbon nanotubes (Patent Document 1).

[0008] In addition, polyacrylic acid or tannic acid belongs to polyacids containing carboxyl groups, and they are also known to be effective for the dispersion of carbon nanotubes.

[0009] Based on the above, it can be speculated that better CNT dispersion effects can be obtained when polyvinylpyrrolidone and polyacid are used in combination. However, in reality, when these substances are used in combination, an overly strong binding force usually occurs between the two substances, forming insoluble substances or aggregates, thus resulting in a problem that the dispersibility of the dispersion decreases instead.

[0010] Prior art documents

[0011] Patent documents

[0012] Patent Document 1: KR No. 10-2011-0118460

[0013] Non-patent documents

[0014] Non-patent Document 1: Toxicol.Res., 2015, 4, 160 - 168 Summary of the invention

[0015] Technical problem

[0016] The present invention relates to a carbon nanotube dispersion liquid with improved dispersibility of carbon nanotubes, a preparation method thereof, an electrode paste composition containing the same, an electrode containing the same, and a lithium secondary battery containing the same.

[0017] Solution to the problem

[0018] One embodiment of the present invention provides a carbon nanotube dispersion liquid, which comprises: carbon nanotubes with a specific surface area (BET) of 250 m 2 / g to 750 m 2 / g; a first dispersant having an amide group; and a second dispersant having an aromatic ring, and the viscosity of the carbon nanotube dispersion liquid at 25 °C and a shear rate of 15 sec-1 is 1,300 cPs or less.

[0019] In addition, another embodiment of the present invention provides a preparation method of the above carbon nanotube dispersion liquid, including the step of mixing carbon nanotubes with a specific surface area (BET) of 250 m 2 / g to 750 m 2 / g; a first dispersant having an amide group; and a second dispersant having an aromatic ring.

[0020] In addition, another embodiment of the present invention provides an electrode paste composition, which comprises the above carbon nanotube dispersion liquid, an electrode active material, and a binder.

[0021] In addition, another embodiment of the present invention provides an electrode, which includes an electrode active material layer formed by the above electrode paste composition.

[0022] In addition, another embodiment of the present invention provides a lithium secondary battery including the above electrodes.

[0023] Effects of the Invention

[0024] The carbon nanotube dispersion of one embodiment of the present invention has the effects of significantly low viscosity and improved dispersibility of carbon nanotubes in the dispersion.

[0025] The carbon nanotube dispersion of one embodiment of the present invention has excellent coating properties and processability when used in electrode manufacturing. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figures 1 to 3 It is the result of Experimental Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0027] The present invention will be described in detail below.

[0028] The carbon nanotube dispersion of one embodiment of the present invention refers to a dispersion containing carbon nanotubes. Specifically, it refers to carbon nanotubes dispersed in the dispersion, which do not aggregate with each other.

[0029] One embodiment of the present invention provides a carbon nanotube dispersion, the carbon nanotube dispersion comprising: carbon nanotubes with a specific surface area (BET) of 250 m 2 / g to 750 m 2 / g; a first dispersant having an amide group; and a second dispersant having an aromatic ring, and the viscosity of the carbon nanotube dispersion at 25°C and a shear rate of 15 sec-1 is 1,300 cPs or less.

[0030] In one embodiment of the present invention, the above carbon nanotube dispersion is characterized in that its viscosity at 25°C and a shear rate of 15 sec-1 is 1,300 cPs or less. The above viscosity may be 1,200 cPs or less, 1,000 cPs or less, 900 cPs or less, 800 cPs or less, or 600 cPs or less. Considering the purpose of the present invention, the lower the above viscosity, the better, so the lower limit is not particularly limited, but may be 1 cPs or more, 3 cPs or more, or 5 cPs or more. When the above viscosity range is satisfied, the carbon nanotubes in the carbon nanotube dispersion do not aggregate with each other, and the processability is improved when used in electrode manufacturing.

[0031] The viscosity of the above-mentioned carbon nanotube dispersion can be measured using methods commonly used in the technical field. For example, a DVNextCP rheometer from Brookfield can be used to measure at a measurement temperature of 25 °C and a shear rate of 15 sec-1. For more accurate measurement, the prepared carbon nanotube dispersion can be stored at 25 °C for 1 week before measurement.

[0032] In one embodiment of the present invention, the above-mentioned carbon nanotube dispersion is characterized in that it contains carbon nanotubes with a specific surface area (BET) of 250 m 2 / g to 750 m 2 / g. The specific surface area (BET) of the above-mentioned carbon nanotubes can be 300 m 2 / g to 700 m 2 / g or 350 m 2 / g to 650 m 2 / g. Within the above numerical range, the carbon nanotubes have excellent conductivity, and the aggregation phenomenon can be controlled.

[0033] Generally speaking, carbon nanotubes are prone to aggregation due to the van der Waals force between them, and a dispersion technology capable of well-dispersing and aggregating carbon nanotubes is required when dispersing in a solvent. When carbon nanotubes aggregate with each other, the viscosity of the dispersion increases, which is a problem. The high viscosity of the dispersion will increase the viscosity of the slurry when manufacturing the secondary battery electrode, which may lead to uneven dispersion with electrode materials such as the negative electrode material and the binder.

[0034] Various types of carbon nanotubes can be selected according to the use / performance. For example, single-walled carbon nanotubes (SWCNT) are difficult to manufacture, expensive, and due to their high specific surface area (>800 m 2 / g), the viscosity of the dispersion is high, and the increase in the solid content is limited. Multi-walled carbon nanotubes (MWCNT) with a low specific surface area (<250 m 2 / g) are easy to increase the content and have a low viscosity, but the aggregation phenomenon of the material itself is large, and the volume expansion generated during the charge and discharge of the electrode cannot be suppressed, thus limiting the battery performance.

[0035] The inventor of the present invention has developed a dispersion using carbon nanotubes with a specific surface area (BET) of 250 m 2 / g to 750 m 2 / g. This dispersion has improved dispersibility and can economically manufacture electrodes.

[0036] In one embodiment of the present invention, the carbon nanotube dispersion is characterized in that it contains a first dispersant having an amide group; and a second dispersant having an aromatic ring. The first dispersant having an amide group is effective for dispersing carbon nanotubes, and the complex structure of the second dispersant having an aromatic ring can effectively reduce the viscosity of the dispersion. In addition, a hydrogen bond is formed between the oxygen atom of the amide group in the first dispersant and the functional group (hydroxyl group or carboxyl group) of the second dispersant, which helps to reduce the viscosity of the dispersion.

[0037] In one embodiment of the present invention, the first dispersant has an amide group, so that a hydrogen bond can be formed with the hydroxyl group or carboxyl group of the second dispersant described later.

[0038] In one embodiment of the present invention, the first dispersant may be polyvinylpyrrolidone, polyester amide, polycarboxylic amide, polyamido amine, thioamido amine, water-soluble nylon compound or a combination thereof. The first dispersant has an amide group, so that an improved viscosity improvement effect and an effect of suppressing the change of viscosity with time can be exerted.

[0039] In one embodiment of the present invention, the weight average molecular weight of the first dispersant may be from 1,000 g / mol to 100,000 g / mol, preferably from 2,000 g / mol to 80,000 g / mol, more preferably from 2,000 g / mol to 30,000 g / mol, and still more preferably from 2,000 g / mol to 15,000 g / mol. If the weight average molecular weight of the first dispersant is less than 1,000 g / mol, the dispersion performance of the carbon nanotubes may decrease, and the problem of elution of the first dispersant may occur during the manufacture of the electrode. If it exceeds 100,000 g / mol, the viscosity of the carbon nanotube dispersion increases, which may lead to a decrease in coatability and processability. Therefore, it is preferably adjusted to the above range.

[0040] In one embodiment of the present invention, the second dispersant may contain a hydroxyl group; or a carboxyl group. By containing a hydroxyl group; or a carboxyl group, the second dispersant can form a hydrogen bond with the amide group of the first dispersant.

[0041] In one embodiment of the present invention, the second dispersant may include two or more aromatic rings. For example, the second dispersant may be one or more selected from baicalin, luteolin, taxifolin, myricetin, quercetin, rutin, catechin, epigallocatechin gallate, butein, resveratrol, and tannic acid, preferably tannic acid, quercetin, epigallocatechin gallate, or a combination thereof.

[0042] In one embodiment of the present invention, the second dispersant may be a phenolic compound. Among the phenolic compounds, at least one of the aromatic rings may include at least one structure selected from the group consisting of a phenol structure, a catechol structure, a gallic acid structure, and a naphthol structure. The phenol structure refers to a structure in which a hydroxyl group is connected to a benzene ring, the catechol structure refers to a structure in which two hydroxyl groups are connected to a benzene ring, the gallic acid structure refers to a structure in which three hydroxyl groups are connected to a benzene ring, and the naphthol structure refers to a structure in which a hydroxyl group is connected to a naphthalene.

[0043] In one embodiment of the present invention, the weight ratio of the first dispersant to the second dispersant may be 1:10 to 10:1, 1:5 to 5:1, or 1:1 to 1:5. When the above range is satisfied, the dispersion effect of the carbon nanotubes is improved, so that a lower dispersion viscosity can be maintained.

[0044] In one embodiment of the present invention, based on the total weight of the dispersion, the content of the first dispersant may be 0.01 wt% to 10 wt%, 0.01 wt% to 5 wt%, or 0.1 wt% to 3 wt%.

[0045] In one embodiment of the present invention, based on the total weight of the dispersion, the content of the second dispersant may be 0.01 wt% to 10 wt%, 0.01 wt% to 5 wt%, or 0.1 wt% to 3 wt%.

[0046] In one embodiment of the present invention, the carbon nanotube dispersion is characterized in that it contains an alkali metal element. By containing the alkali metal element, the dispersion of the substances contained in the dispersion can be improved.

[0047] The above-mentioned first dispersant and second dispersant can contribute to reducing the viscosity of the dispersion by forming hydrogen bonds. However, if these hydrogen bonds are too strong, insoluble substances or aggregates (complex) will be formed, which is a problem. In this case, there are many limitations in the selection of the dispersant. For example, the contents of the first dispersant and the second dispersant must be adjusted, or only specific types of the first dispersant and / or the second dispersant can be used. However, when the above-mentioned carbon nanotube dispersion contains an alkali metal element, the alkali metal element can inhibit the phenomenon of mutual aggregation of the above-mentioned first dispersant and second dispersant, thereby maintaining the low viscosity of the dispersion. The improvement of the above-mentioned aggregation phenomenon can be confirmed by preparing a carbon nanotube dispersion containing the above-mentioned substances, allowing it to stand for a certain period of time, and visually observing whether aggregation occurs.

[0048] In one embodiment of the present invention, the form of the above-mentioned alkali metal is not particularly limited, but it can exist in the form of an alkali metal salt containing an alkali metal.

[0049] In one embodiment of the present invention, the above-mentioned carbon nanotube dispersion may contain an alkali metal salt.

[0050] In one embodiment of the present invention, the above-mentioned alkali metal salt may include one or more selected from KOH, NaOH, LiOH, KOHH 2 O, NaOHH 2 O, LiOHH 2 O, K 2 CO 3 、Na 2 CO 3 and LiCO 3 selected from the above.

[0051] In one embodiment of the present invention, the content of the above-mentioned alkali metal element may be 1 ppm or more and 300 ppm or less, 5 ppm or more and 200 ppm or less, or 5 ppm or more and 150 ppm or less based on the entire carbon nanotube dispersion.

[0052] In one embodiment of the present invention, the content of the above-mentioned alkali metal element can be adjusted according to the type and content of the above-mentioned first dispersant.

[0053] In one embodiment of the present invention, the above-mentioned first dispersant contains polyvinylpyrrolidone resin, and the molar ratio of the above-mentioned alkali metal salt can be 60 mol or less, 30 mol or less, or 25 mol or less based on 100 mol of vinylpyrrolidone monomer contained in the above-mentioned polyvinylpyrrolidone resin. The lower limit of the content is not particularly limited, but can be 0.1 mol or more, 0.2 mol or more, or 0.5 mol or more. When the above range is satisfied, the viscosity of the carbon nanotube dispersion can be adjusted to 1,300 cPs or less at 25 °C and a shear rate of 15 sec-1.

[0054] The molar ratio of the above-mentioned alkali metal salt can be calculated using the molecular weights of the alkali metal salt and the vinylpyrrolidone monomer and the weight percentages of the alkali metal salt and polyvinylpyrrolidone. Specifically, it can be calculated by the following formula 1.

[0055] [Formula 1] Molar ratio of alkali metal salt = {(wt% of alkali metal salt) / (molecular weight of alkali metal salt)} / {(wt% of polyvinylpyrrolidone) / (molecular weight of vinylpyrrolidone monomer)} * 100

[0056] For example, when based on the total weight of the dispersion, the contents of polyvinylpyrrolidone and alkali metal salt (LiOH) are 0.9 wt% and 0.003 wt% respectively, the molecular weight of the alkali metal salt (LiOH) is 24 g / mol, and the molecular weight of the vinylpyrrolidone monomer is 111.14 g / mol, based on 100 mol of vinylpyrrolidone monomer, the molar ratio of the alkali metal salt is calculated to be 1.5 mol [1.5 = {(0.003) / (24)} / {(0.9) / (111.14)} * 100].

[0057] The above-mentioned vinylpyrrolidone monomer refers to a monomer composed of a 5-membered lactam connected by a vinyl group, and is a unit that constitutes the above-mentioned polyvinylpyrrolidone resin. Specifically, it can be the monomer represented by Chemical Formula 2 in the polyvinylpyrrolidone represented by the following Chemical Formula 1.

[0058] [Chemical Formula 1]

[0059]

[0060] [Chemical Formula 2]

[0061]

[0062] In one embodiment of the present invention, the above-mentioned carbon nanotubes are used to improve the conductivity of the electrode, are cylindrical graphite sheets with a nanoscale diameter, and have sp 2Combined structure. At this time, according to the above-mentioned curling angle and structure of the graphite plane, the characteristics of a conductor or a semiconductor are presented. Carbon nanotubes can be classified into single-walled carbon nanotubes (SWCNT, single-walled carbon nanotube), double-walled carbon nanotubes (DWCNT, double-walled carbon nanotube), and multi-walled carbon nanotubes (MWCNT, multi-walled carbon nanotube) according to the number of bonds forming the wall, and these carbon nanotubes can be appropriately selected according to the use of the dispersion liquid. In addition, the above-mentioned carbon nanotubes can have a secondary shape formed by the aggregation or arrangement of multiple carbon nanotubes. For example, they can be bundle-type carbon nanotubes in the shape of a bundle or a rope where multiple carbon nanotubes are arranged side by side or in a specific direction, or they can be entangled-type carbon nanotubes in the shape of a ball or a potato where multiple carbon nanotubes are entangled with each other without a specific direction.

[0063] In one embodiment of the present invention, the above-mentioned carbon nanotubes can be single-walled carbon nanotubes (SWCNT), multi-walled carbon nanotubes (MWCNT), or a combination thereof.

[0064] In one embodiment of the present invention, the above-mentioned carbon nanotubes can include two or more carbon nanotube monomers. The above-mentioned carbon nanotube monomers have a cylindrical graphite sheet with a nanoscale diameter and have sp 2 bonding structure.

[0065] In one embodiment of the present invention, the diameter of the above-mentioned carbon nanotube monomers can be 1 nm or more and 200 nm or less, 1 nm or more and 150 nm or less, or 1 nm or more and 100 nm or less. Within the above numerical range, the dispersibility of the carbon nanotubes can be improved, and when applied to an electrode, an increase in resistance can be prevented.

[0066] In one embodiment of the present invention, the length of the above-mentioned carbon nanotube monomers can be 0.1 μm or more and 200 μm or less, 0.1 μm or more and 150 μm or less, or 0.5 μm or more and 100 μm or less. Within the above numerical range, the dispersibility of the carbon nanotubes can be improved, and when applied to an electrode, an increase in resistance can be prevented.

[0067] In one embodiment of the present invention, the aspect ratio (length / diameter) of the above-mentioned carbon nanotubes can be 5 to 50,000 or 10 to 15,000. Within the above numerical range, the dispersibility of the carbon nanotubes can be improved, and when applied to an electrode, an increase in resistance can be prevented.

[0068] In one embodiment of the present invention, the average particle size (D50) of the carbon nanotubes may be 0.1 μm to 20 μm, 0.5 μm to 1 μm, 1 μm to 5 μm, or 2 μm to 4 μm. The above average particle size (D50) refers to the particle size corresponding to 50% of the cumulative number of particles in the particle size distribution curve of the carbon nanotubes. For example, the above average particle size (D50) can be measured by the laser diffraction method. Within the above range, the carbon nanotubes do not aggregate with each other, and the dispersibility can be improved.

[0069] In one embodiment of the present invention, the content of the carbon nanotubes may be 0.01 wt% to 10 wt%, preferably 0.1 wt% to 8 wt% based on the total weight of the carbon nanotube dispersion. Within the above numerical range, it is possible to prevent an increase in process cost due to a decrease in the filling amount during electrode manufacturing, or to prevent a decrease in adhesive strength due to the migration of the binder during electrode drying, or to prevent an increase in the viscosity of the carbon nanotube dispersion.

[0070] In one embodiment of the present invention, the value represented by the following formula 2 of the carbon nanotube dispersion may be 2 to 10, 2 to 6.5, or 3 to 6. The value represented by the following formula 2 is the shear thinning index of the dispersion, which represents the viscosity ratio measured at different shear rates. When the above range is satisfied, it is possible to prevent a decrease in fluidity due to too high a viscosity in the static state, so that uniform mixing can be achieved during electrode manufacturing. In addition, the storage stability can be improved by preventing the sedimentation of carbon nanotube particles.

[0071] [Formula 2] Shear Thinning index: STI = V low / V high

[0072] In the above formula 2, V low is the viscosity of the dispersion measured at a shear rate of 25 °C and 15 sec-1, and V high is the viscosity of the dispersion measured at a shear rate of 25 °C and 150 sec-1.

[0073] In one embodiment of the present invention, the value calculated by the following formula 3 for the carbon nanotube dispersion can be 1 to 5, 1 to 3, or 1.1 to 2. The value calculated by the following formula 3 represents the relationship between the shear thinning index (STI) characteristic of the dispersion and the average particle size of the carbon nanotubes contained in the dispersion. Generally speaking, if the particle size of the carbon nanotubes (D50) is too small, as the carbon nanotubes agglomerate with each other, the shear thinning index (STI) of the dispersion tends to increase. In addition, if the particle size of the carbon nanotubes (D50) is too large, the carbon nanotubes are not sufficiently dispersed, and the carbon nanotubes form a network structure, thereby increasing the overall viscosity of the dispersion and the shear thinning index (STI).

[0074] However, by adjusting the value calculated by the following formula 3 to the above numerical range for the carbon nanotube dispersion of one embodiment of the present invention, even if the particle size of the carbon nanotubes is small, it has the effect of improving the time stability of the overall viscosity of the dispersion.

[0075] [Formula 3]

[0076]

[0077] In the above formula 3, STI is the shear thinning index (Shear Thinning index: STI) of the dispersion, and D50 is the average particle size (D50) of the above carbon nanotubes.

[0078] In one embodiment of the present invention, the above carbon nanotube dispersion may further contain a solvent. The above solvent is used to pre-disperse the carbon nanotubes and supply them as a carbon nanotube dispersion to prevent agglomeration when the carbon nanotubes are directly mixed with an electrode active material or the like as an electrode paste composition.

[0079] In one embodiment of the present invention, the above solvent may be an aqueous solvent. For example, the above aqueous solvent may be water. In this case, it is easy to adjust the viscosity of the dispersion.

[0080] In one embodiment of the present invention, the pH of the above carbon nanotube dispersion may be 3 to 10. The above pH may be 4 to 9 or 5 to 8. Within the above numerical range, the phenomenon of agglomeration of the above first dispersant and second dispersant can be further inhibited. Specifically, if the pH of the dispersion exceeds the above numerical range, the surface charge intensity of the first dispersant and the second dispersant will become too strong or too weak, resulting in too strong hydrogen bonds, and thus serious agglomeration may occur. At this time, by adjusting the pH of the dispersion to the above numerical range, the phenomenon of agglomeration of the first dispersant and the second dispersant can be further inhibited. The above pH can be measured at 25 °C.

[0081] One embodiment of the present invention provides a method for preparing a carbon nanotube dispersion, including having a specific surface area (BET) of 250m2 from 250 m² / g to 750 m² / g 2 a step of mixing carbon nanotubes having a specific surface area (BET) of from 250 m² / g to 750 m² / g, a first dispersant having an amide group, and a second dispersant having an aromatic ring.

[0082] In one embodiment of the present invention, carbon nanotubes having a specific surface area (BET) of 250 m² 2 from 250 m² / g to 750 m² / g 2 The step of mixing carbon nanotubes having a specific surface area (BET) of from 250 m² / g to 750 m² / g, a first dispersant having an amide group, and a second dispersant having an aromatic ring can be carried out under temperature conditions where physical properties do not change. For example, it can be carried out at 50°C or lower, more specifically, at a temperature of from 5°C to 50°C.

[0083] In one embodiment of the present invention, the step of dispersing the carbon nanotubes can be carried out by methods such as a ball mill, a bead mill, a disc mill, or a basket mill, a high pressure homogenizer, etc. More specifically, it can be carried out by a grinding method using a disc mill or a high pressure homogenizer.

[0084] When using the above disc mill for grinding, the size of the beads can be appropriately determined according to the type and amount of the carbon nanotubes and the type of the dispersant. Specifically, the diameter of the above beads can be from 0.1 mm to 5 mm, more specifically, from 0.5 mm to 4 mm. In addition, the bead milling process can be carried out at a speed of from 2,000 rpm to 10,000 rpm, more specifically, at a speed of from 5,000 rpm to 9,000 rpm.

[0085] The grinding by the above high pressure homogenizer is achieved by, for example, pressurizing the above mixture by a plunger pump of the high pressure homogenizer and pushing it through the gap of the homogenizing valve, thereby generating forces such as cavitation, shear, impact, and explosion when passing through the above gap.

[0086] The grinding by the above high pressure homogenizer can be carried out at a speed of from 2,000 rpm to 10,000 rpm, more specifically, at a speed of from 2,000 rpm to 5,000 rpm.

[0087] The grinding by the above high pressure homogenizer can be carried out under a pressure condition of from 500 bar to 3,000 bar, more specifically, under a pressure condition of from 1,000 bar to 2,000 bar.

[0088] In one embodiment of the present invention, the step of dispersing the carbon nanotubes may be carried out for 10 minutes to 120 minutes, more specifically, for 20 minutes to 90 minutes, so that the carbon nanotubes can be sufficiently dispersed.

[0089] One embodiment of the present invention provides an electrode paste composition comprising the above carbon nanotube dispersion, an electrode active material, and a binder.

[0090] In one embodiment of the present invention, the above electrode active material comprises a silicon-based electrode active material. The silicon-based electrode active material may comprise one or more selected from metallic silicon (Si), silicon oxide (SiOx, where 0 < x < 2), silicon carbide (SiC), and Si-Y alloy (where Y is an element selected from the group consisting of alkali metals, alkaline earth metals, Group 13 elements, Group 14 elements, transition metals, rare earth elements, and combinations thereof, and is not Si). The element Y may be selected from the group consisting of Mg, Ca, Sr, Ba, Ra, Sc, Y, Ti, Zr, Hf, Rf, V, Nb, Ta, Db, Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Pb, Ru, Os, Hs, Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, Sn, In, Ti, Ge, P, As, Sb, Bi, S, Se, Te, Po, and combinations thereof.

[0091] In one embodiment of the present invention, the silicon-based electrode active material exhibits higher capacity characteristics than the carbon-based electrode active material. Therefore, when the silicon-based electrode active material is further included, more excellent capacity characteristics can be obtained. However, the silicon-based electrode active material has a large volume change during charge and discharge, resulting in a rapid deterioration of battery characteristics during repeated charge and discharge, leading to insufficient cycle characteristics and making it difficult to commercialize. However, when using carbon nanotubes as a conductive material as in the present invention, the application of the silicon-based electrode active material can achieve the effect of improving cycle characteristics. Therefore, by using the electrode paste composition of the present invention containing the carbon nanotube dispersion and the silicon-based electrode active material of the present invention, a secondary battery having excellent capacity characteristics and cycle characteristics can be realized.

[0092] In one embodiment of the present invention, the above electrode active material may further comprise other types of electrode active materials in addition to the above silicon-based electrode active material. As the above other types of electrode active materials, for example, carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, and amorphous carbon can be used; metallic compounds that can form alloys with lithium, such as Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloys, Sn alloys, or Al alloys; metal oxides that can be doped and de-doped with lithium, such as SnO 2, vanadium oxides, lithium vanadium oxides, etc.; or composite materials containing metallic compounds and carbonaceous materials, such as Sn-C composites, etc., among which carbonaceous materials are particularly preferred.

[0093] In one embodiment of the present invention, based on the total solid content in the electrode paste composition, the total amount of the above-mentioned silicon-based electrode active material and other types of electrode active materials can be 70 wt% to 99 wt%, preferably 80 wt% to 98 wt%. When the content of the electrode active material meets the above range, excellent capacity characteristics can be achieved.

[0094] In one embodiment of the present invention, the above-mentioned binder is used to ensure the adhesion between active materials or between the active material and the current collector, and general binders used in the technical field can be used, and their types are not particularly limited. As the above-mentioned binder, polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof, etc. can be cited, and one of them can be used alone or a mixture of two or more can be used.

[0095] In one embodiment of the present invention, based on the total solid content in the electrode paste composition, the content of the above-mentioned binder can be 5 wt% or less, preferably 1 wt% to 3 wt%. When the content of the binder meets the above range, the increase in electrode resistance can be minimized, and at the same time, excellent electrode adhesion can be achieved.

[0096] In one embodiment of the present invention, the above electrode paste composition may further contain a solvent as needed to adjust the viscosity and the like. In this case, the above solvent may be water, an organic solvent, or a mixture thereof. As the above organic solvent, examples thereof include amide-based polar organic solvents such as dimethylformamide (DMF), diethylformamide, dimethylacetamide (DMAc), and N-methylpyrrolidone (NMP); alcohols such as methanol, ethanol, 1-propanol, 2-propanol (isopropanol), 1-butanol (n-butanol), 2-methyl-1-propanol (isobutanol), 2-butanol (sec-butanol), 1-methyl-2-propanol (tert-butanol), pentanol, hexanol, heptanol, or octanol; diols such as ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, 1,3-propanediol, 1,3-butanediol, 1,5-pentanediol, or hexanediol; polyols such as glycerol, trimethylolpropane, pentaerythritol, or sorbitol; ethylene glycol ethers such as ethylene glycol monomethyl ether, diethylene glycol monomethyl ether, triethylene glycol monomethyl ether, tetraethylene glycol monomethyl ether, ethylene glycol monoethyl ether, diethylene glycol monoethyl ether, triethylene glycol monoethyl ether, tetraethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monobutyl ether, triethylene glycol monobutyl ether, or tetraethylene glycol monobutyl ether; ketones such as acetone, methyl ethyl ketone, methyl propyl ketone, or cyclopentanone; esters such as ethyl acetate, γ-butyrolactone, or ε-propiolactone, etc. One or a mixture of two or more of them can be used, but it is not limited thereto.

[0097] In one embodiment of the present invention, the above electrode paste composition may further contain additives such as a viscosity modifier and a filler as needed.

[0098] One embodiment of the present invention provides an electrode including an electrode active material layer formed of the above electrode paste composition. Specifically, the above electrode can be manufactured by coating the electrode paste composition of the present invention and drying to form an electrode active material layer. More specifically, the above electrode active material layer can be formed by a method of coating the electrode paste composition on an electrode current collector and then drying, or by a method of coating the electrode paste composition on another support, peeling it off from the support, and laminating the obtained thin film on the electrode current collector. As needed, the electrode active material layer can be formed by the above method, and then a calendering process can be further performed. In this case, drying and calendering can be carried out under appropriate conditions considering the performance of the finally manufactured electrode, and there is no particular limitation.

[0099] In one embodiment of the present invention, the above electrode current collector may be any material that has conductivity and does not cause chemical changes in the battery, and there is no particular limitation. For example, copper, stainless steel, aluminum, nickel, titanium, their alloys, materials surface-treated with carbon, nickel, titanium, silver, etc., or calcined carbon can be used.

[0100] In one embodiment of the present invention, the above-mentioned electrode current collector generally may have a thickness of 3 μm to 500 μm, and fine irregularities may also be formed on the surface of the current collector to enhance the binding force of the electrode active material. Moreover, the above-mentioned electrode current collector can be used in various forms, for example, a thin film, a sheet, a foil, a net, a porous body, a foam body, a non-woven fabric body, etc.

[0101] In one embodiment of the present invention, the above-mentioned electrode may be a negative electrode.

[0102] One embodiment of the present invention provides a lithium secondary battery including the above-mentioned electrode.

[0103] One embodiment of the present invention provides a lithium secondary battery including a positive electrode; a negative electrode; and a separator and an electrolyte disposed between the positive electrode and the negative electrode, wherein at least one of the positive electrode and the negative electrode is the above-mentioned electrode.

[0104] In one embodiment of the present invention, the above-mentioned separator is used to separate the negative electrode and the positive electrode and provide a movement channel for lithium ions, and it can be used without particular limitation as long as it is generally used as a separator in a secondary battery. Specifically, as the above-mentioned separator, a porous polymer membrane can be used, for example, a porous polymer membrane made of polyolefin polymers such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, ethylene / methacrylate copolymer, or a laminated structure of two or more layers thereof. Moreover, a conventional porous non-woven fabric can also be used, for example, a non-woven fabric made of high-melting-point glass fiber, polyethylene terephthalate fiber, etc. Moreover, a coated separator containing a ceramic component or a polymer substance can also be used to ensure heat resistance or mechanical strength, and optionally, it can be used in a single-layer or multi-layer structure.

[0105] In one embodiment of the present invention, examples of the above-mentioned electrolyte include, but are not limited to, organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, molten inorganic electrolytes, etc. that can be used to manufacture secondary batteries. Detailed implementation mode

[0107] The present invention will be described in more detail below through examples.

[0108] <Preparation of carbon nanotubes>

[0109] The following carbon nanotubes were prepared.

[0110] CNT-1 (multi-walled carbon nanotubes, prepared by JEIO Co., product name 6A, specific surface area 643 m 2 / g, average diameter of monomers 5 - 7 nm)

[0111] CNT-2 (specific surface area 375 m2 / g, average diameter of monomers 9 - 10 nm)

[0112] CNT-3 (specific surface area 385 m 2 / g, average diameter of monomers 5 - 9 nm)

[0113] CNT-4 (specific surface area 1,131 m 2 / g, average diameter of monomers 1 - 2 nm)

[0114] CNT-5 (specific surface area 221 m 2 / g, average diameter of monomers 7 - 12 nm)

[0115] CNT-6 (specific surface area 186 m 2 / g, average diameter of monomers 13 nm)

[0116] <Preparation of the first dispersant>

[0117] A polyvinylpyrrolidone (K15) compound obtained from MERK was prepared.

[0118] <Preparation of the second dispersant>

[0119] Tannic acid (TA) obtained from MERK was prepared.

[0120] <Preparation of the dispersion>

[0121] Example 1

[0122] 1.5 wt% of carbon nanotubes (6A, manufactured by JEIO), 0.9 wt% of polyvinylpyrrolidone (K15, MERK) as the first dispersant, 0.3 wt% of tannic acid (MERK) as the second dispersant, and water as the solvent were mixed to prepare 1 kg of a mixture. Then, it was treated with a high-shear online mixer (IM001, K&S) at 3,000 rpm for 30 minutes, and then treated 7 times with a high-pressure homogenizer under a pressure condition of 1,500 bar to prepare a carbon nanotube dispersion.

[0123] At this time, according to the following formula 1, the molar ratio of the alkali metal salt was 1.5 mol based on 100 mol of vinylpyrrolidone monomers [={(0.003) / (24)} / {(0.9) / (111.14)}*100].

[0124] [Formula 1] Molar ratio of alkali metal salt = {(wt% of alkali metal salt) / (molecular weight of alkali metal salt)} / {(wt% of polyvinylpyrrolidone) / (molecular weight of vinylpyrrolidone monomer)}*100

[0125] Examples and Comparative Examples

[0126] For the remaining examples and comparative examples, dispersions were prepared by changing the weights and types of each substance as shown in Tables 1 and 2 below, and their physical properties were tested.

[0127] Experimental Example 1: Confirmation of Dispersant Aggregation

[0128] The carbon nanotube dispersions of Example 1 and the comparative examples were sufficiently diluted with deionized water (Diwater) and then coated on Si wafers. They were fully dried on a hot plate at 60 °C for more than 30 minutes, and then Pt coating was performed using a COXEM SPT-20 ion sputter coater. After that, the surface was observed using a COXEM EM-30 SEM (15 kV). The results are as Figures 1 to 3 shown.

[0129] It was confirmed that there was no aggregation or precipitation in the test solution of Example 4 ( Figure 1 ), but aggregation and even precipitation of the dispersants occurred in the test solutions of Comparative Example 4 and Comparative Example 5 ( Figure 2 : Comparative Example 4, Figure 3 : Comparative Example 5).

[0130] Experimental Example 2: Viscosity Measurement

[0131] Measurements were carried out using a Brookfield DVNextCP rheometer at a temperature of 25 °C and a shear rate of 15 sec-1.

[0132] Experimental Example 3: Average Particle Size Measurement

[0133] The laser diffraction method and a commercially available laser diffraction particle size measuring device (Malvern Mastersizer 3000) were used. Before measurement, the carbon nanotube dispersions of the examples and comparative examples were sufficiently diluted to a carbon nanotube concentration of less than 0.05 wt%, and then measured after being stabilized for 10 minutes. The average particle size (D50) based on the particle size distribution of 50% was calculated by the measuring device.

[0134] Experimental Example 4: pH Measurement

[0135] Measurements were carried out using an OHAUS ST3100 pH meter. After calibrating the electrode with a buffer solution at 25 °C, the electrode was placed in the sample, stirred for 5 seconds, and then waited for 30 seconds until the signal was stable before measuring the pH.

[0136] Experimental Example 5: Solids Content Measurement

[0137] The solid content was measured using an MB95 moisture analyzer from OHAUS. Approximately 3 g of the sample was placed on an aluminum sample dish, and the initial weight was measured. Then, it was heated to 150 °C and the weight was measured. If the weight change was no more than 1 mg within 60 seconds at 150 °C, it was set as the dry weight, and the solid content was calculated using the following formula.

[0138] % DC (solid component) = dry weight / initial weight x 100%

[0139] Experimental Example 6: Evaluation of battery charge and discharge characteristics

[0140] Production of button cells

[0141] The carbon nanotube dispersion of each example and comparative example was mixed with the electrode active material (silicon microparticles and graphite mixed at a weight ratio of 30:70) and water, and mixed for 30 minutes using a paste mixer to prepare an electrode paste.

[0142] The above electrode paste was coated on a Cu foil current collector and dried at 75 °C under vacuum conditions for 4 hours.

[0143] After punching the above dried electrode to a diameter of 14 mm, a button half-cell (2032) was fabricated using Li metal (diameter 15 mm) as the counter electrode. At this time, Celgard 2450 was used as the separator, and LiPF 6 (1 M) in a composition prepared by mixing ethylene carbonate (EC) and diethylene carbonate (DEC) at a volume ratio of 1:1 was used as the electrolyte, and 100 μL was injected.

[0144] Evaluation of charge and discharge characteristics

[0145] The charge and discharge characteristics of the button cell were evaluated at room temperature in the voltage range of 0.005 - 1.5 V.

[0146] Initial efficiency (0.1C) = delithiation capacity (mAh / g) / lithiation capacity (mAh / g)

[0147] Charge and discharge life (0.5C) = number of cycles when reaching 80% retention

[0148] [Table 1]

[0149]

[0150]

[0151] [Table 2]

[0152]

[0153] From the above results, it can be confirmed that in the case of carbon nanotubes with a specific surface area (BET) of 250 m 2 / g to 750 m 2 / g, a first dispersant having an amide group, and a second dispersant having an aromatic ring (Examples 1 to 6), the viscosity of the dispersion liquid is 1,300 cPs or less, and the battery characteristics are excellent. When the dispersion liquid does not contain an alkali metal element (Comparative Example 1), the viscosity of the dispersion liquid exceeds 1,300 cps.

[0154] In addition, when part of the first dispersant or the second dispersant is not included, the viscosity of the dispersion liquid exceeds 1,300 cps (Comparative Examples 2 and 3).

[0155] On the other hand, when the specific surface area exceeds 750 m 2 / g (Comparative Example 4), the viscosity of the dispersion liquid exceeds 1,300 cps, and when the specific surface area (BET) is lower than 250 m 2 / g (Comparative Examples 5 and 6), the battery characteristics are significantly deteriorated.

Claims

1. A carbon nanotube dispersion liquid, characterized in that, Comprising: carbon nanotubes with a specific surface area (BET) of 250 m 2 / g to 750 m 2 / g; a first dispersant having an amide group; and a second dispersant having an aromatic ring, wherein the viscosity of the carbon nanotube dispersion is 1,300 cPs or less at a shear rate of 25 °C and 15 sec-1.

2. The carbon nanotube dispersion liquid according to claim 1, characterized in that, The first dispersant is polyvinylpyrrolidone, polyester amide, polycarboxylic amide, polyamido amine, thioamido amine, water soluble Nylon compound or a combination thereof.

3. The carbon nanotube dispersion liquid according to claim 1, characterized in that, The second dispersant contains a hydroxyl group; or a carboxyl group.

4. The carbon nanotube dispersion liquid according to claim 1, characterized in that, It contains the first dispersant and the second dispersant in a weight ratio of 1:10 to 10:

1.

5. The carbon nanotube dispersion liquid according to claim 1, characterized in that, It contains an alkali metal element.

6. The carbon nanotube dispersion liquid according to claim 1, characterized in that, It contains an alkali metal salt.

7. The carbon nanotube dispersion liquid according to claim 6, characterized in that, The above alkali metal salts include one or more selected from the group consisting of KOH, NaOH, LiOH, KOHH 2 O, NaOHH 2 O, LiOHH 2 O, K 2 CO 3 , Na 2 CO 3 and LiCO 3 and more than one of the group.

8. The carbon nanotube dispersion liquid according to claim 6, characterized in that, The first dispersant contains polyvinylpyrrolidone resin, and the molar ratio of the alkali metal salt is 60 mol or less based on 100 mol of vinylpyrrolidone monomer contained in the polyvinylpyrrolidone resin.

9. The carbon nanotube dispersion liquid according to claim 1, characterized in that, The average particle size (D50) of the carbon nanotubes is 0.1 μm to 20 μm.

10. The carbon nanotube dispersion liquid according to claim 1, characterized in that, Based on the total weight of the carbon nanotube dispersion liquid, the content of the carbon nanotubes is 0.01 wt% to 10 wt%.

11. The carbon nanotube dispersion liquid according to claim 1, characterized in that, The pH is 3 to 10.

12. A method for preparing a carbon nanotube dispersion liquid, characterized in that, Including the step of mixing carbon nanotubes with a BET specific surface area of 250 m 2 / g to 750 m 2 / g, a first dispersant having an amide group, and a second dispersant having an aromatic ring to prepare the carbon nanotube dispersion liquid according to any one of claims 1 to 11.

13. An electrode paste composition, characterized in that, It contains the carbon nanotube dispersion liquid according to any one of claims 1 to 11, an electrode active material and a binder.

14. An electrode, characterized in that, It includes an electrode active material layer formed from the electrode paste composition according to claim 13.

15. The electrode according to claim 14, characterized in that, The electrode is a negative electrode.

16. A lithium secondary battery, characterized in that, It includes the electrode according to claim 15.

Images