Title of Invention: Anode and Secondary Battery Containing It
technology field
[One]
Mutual Citation with Related Applications
[2]
This application claims the benefit of priority based on Korean Patent Application No. 10-2020-0048940 filed on April 22, 2020, and all contents disclosed in the literature of the Korean patent application are incorporated as part of this specification.
[3]
[4]
technology field
[5]
The present invention includes an anode current collector, a first anode active material layer disposed on the anode current collector, and a second anode active material layer disposed on the first anode active material layer, wherein the second anode active material layer is a second anode active material layer. It includes a negative electrode active material and a second conductive material, wherein the second negative electrode active material includes a silicon-based active material, the silicon-based active material includes SiO X (0≤X<2), and the second conductive material contains 2 to 5,000 A carbon nanotube structure in which single-walled carbon nanotube units are bonded side by side, wherein the carbon nanotube structure is included in the second negative electrode active material layer in an amount of 0.01% to 1.0% by weight for a negative electrode and a secondary battery including the same it's about
background art
[6]
Recently, as technology development and demand for mobile devices increase, the demand for batteries as an energy source is rapidly increasing, and accordingly, various studies on batteries that can meet various needs are being conducted. In particular, research on lithium secondary batteries having high energy density and excellent lifespan and cycle characteristics as a power source for such devices is being actively conducted.
[7]
A lithium secondary battery includes a positive electrode including a positive electrode active material capable of intercalating/deintercalating lithium ions, a negative electrode including a negative electrode active material capable of intercalating/deintercalating lithium ions, and a positive electrode having a microporous separator interposed between the positive electrode and the negative electrode. It means a battery in which a non-aqueous electrolyte containing lithium ions is included in the assembly.
[8]
On the other hand, since the conductivity of the negative electrode cannot be secured only with the negative electrode active material, there is a problem in that the resistance of the battery is excessively high, so the negative electrode usually additionally includes a conductive material. Conventionally, point-shaped conductive materials such as carbon black have been mainly used, and linear conductive materials such as carbon nanotubes and carbon nanofibers have also been used to improve battery capacity by further improving conductivity.
[9]
Single-walled carbon nanotubes are one of the linear conductive materials, and improve conductivity in the negative electrode active material layer due to their elongated shape. Accordingly, conventionally, after preparing an anode slurry through a dispersion in which the single-walled carbon nanotubes are completely dispersed, an anode active material layer is prepared through the anode slurry.
[10]
However, when charging and discharging of the battery are repeated, as the negative active material repeatedly expands and contracts in volume, the single-walled carbon nanotubes are cut, making it difficult to maintain the conductive network in the negative active material layer. . In particular, when a silicon-based active material is used as an anode active material to improve battery capacity, the volume of the silicon-based active material expands excessively due to charging and discharging of the battery, causing the single-walled carbon nanotubes to break. Accordingly, the conductive network is blocked or reduced, which deteriorates the lifespan characteristics of the battery. In addition, since the single-walled carbon nanotubes exist while surrounding the surface of the silicon-based active material, they cannot smoothly perform a role of conductively connecting adjacent negative electrode active materials to each other.
[11]
Meanwhile, when using carbon nanotubes as a conductive material, a carbon nanotube dispersion having a low solid content should be used to uniformly dispose the carbon nanotubes in the negative active material layer. However, when carbon nanotubes with low solid content are used, the binder and conductive material, which are relatively low in density compared to the negative electrode active material, easily move to the upper part of the negative electrode active material layer (direction away from the current collector) during drying of the negative electrode Migration As a result, there is a problem in that the adhesion of the negative electrode and the electrical conductivity are greatly reduced.
[12]
Therefore, the present invention introduces a negative electrode that can be connected to a conductive network even with a large volume change of the negative electrode active material and can minimize problems caused by the bias of the binder.
DETAILED DESCRIPTION OF THE INVENTION
technical challenge
[13]
An object to be solved by the present invention is to provide a negative electrode capable of improving input/output characteristics and lifespan characteristics by minimizing problems caused by the bias of the binder while smoothly maintaining a conductive network.
[14]
Another problem to be solved by the present invention is to provide a secondary battery including the negative electrode.
means of solving the problem
[15]
According to one embodiment of the present invention, a negative electrode current collector, a first negative active material layer disposed on the negative electrode current collector, and a second negative active material layer disposed on the first negative electrode active material layer, wherein the second The anode active material layer includes a second anode active material and a second conductive material, the second anode active material includes a silicon-based active material, the silicon-based active material includes SiO X (0≤X<2), and the second The conductive material includes a carbon nanotube structure in which 2 to 5,000 single-walled carbon nanotube units are bonded side by side, and the carbon nanotube structure is a negative electrode included in 0.01% to 1.0% by weight in the second negative electrode active material layer. Provided.
[16]
According to another embodiment of the present invention, a secondary battery including the negative electrode is provided.
Effects of the Invention
[17]
In the negative electrode according to the present invention, since the second negative electrode active material layer includes a carbon nanotube structure in the form of a long rope in which 2 to 5,000 single-walled carbon nanotube units are bonded side by side during rolling, the carbon nanotube structure The second anode active materials can be connected and strongly held together even with a large volume change of SiO X (0≤X<2) of the second anode active material layer, and the SiO X (0≤X<2) is damaged (eg cracks). (crack) can be inhibited. In addition, since the negative electrode has the first negative electrode active material layer and the second negative electrode active material layer sequentially disposed as respective slurries, the above-described biasing of the binder and the conductive material can be minimized. Furthermore, since the second anode active material layer includes a carbon nanotube structure, adhesion between the first anode active material layer and the second anode active material layer may be enhanced. Accordingly, input/output characteristics and lifespan characteristics of the battery may be improved.
Brief description of the drawing
[18]
1 is a photograph of analyzing the binder distribution of the negative electrode of Comparative Example 1 and the negative electrode of Example 1.
[19]
2 is a SEM photograph of the second negative electrode active material layer of the negative electrode of Example 1.
[20]
3 is a SEM photograph of the second negative electrode active material layer of the negative electrode of Example 2.
[21]
4 is a SEM photograph of a second negative electrode active material layer of the negative electrode of Comparative Example 3.
[22]
5 is a SEM picture of the second negative electrode active material layer of the negative electrode of Example 1.
[23]
6 is a SEM photograph of a second negative electrode active material layer of the negative electrode of Comparative Example 2.
Mode for Carrying Out the Invention
[24]
Terms or words used in this specification and claims should not be construed as being limited to their usual or dictionary meanings, and the inventors may appropriately define the concept of terms in order to best explain their invention. It should be interpreted as a meaning and concept consistent with the technical spirit of the present invention based on the principle that there is.
[25]
Terms used in this specification are only used to describe exemplary embodiments, and are not intended to limit the present invention. Singular expressions include plural expressions unless the context clearly dictates otherwise.
[26]
In this specification, terms such as "comprise", "comprise" or "have" are intended to indicate that there is an embodied feature, number, step, component, or combination thereof, but one or more other features or It should be understood that the presence or addition of numbers, steps, elements, or combinations thereof is not precluded.
[27]
In this specification, "%" means % by weight unless expressly indicated otherwise.
[28]
In this specification, "specific surface area" is measured by the BET method, and can be specifically calculated from the amount of nitrogen gas adsorbed under liquid nitrogen temperature (77K) using BELSORP-mini II of BEL Japan.
[29]
In the present specification, the average particle diameter (D 50 ) may be defined as a particle diameter corresponding to 50% of the cumulative volume in the particle diameter distribution curve of the particles. The average particle diameter (D 50 ) may be measured using, for example, a laser diffraction method. The laser diffraction method is generally capable of measuring particle diameters of several millimeters in the submicron region, and can obtain results with high reproducibility and high resolution.
[30]
In the present invention, the single-walled carbon nanotube unit means a tube-shaped unit with one wall composed of carbon atoms, and the multi-walled carbon nanotube unit is a tube-shaped unit with multiple walls made of carbon atoms in one tube. means
[31]
[32]
Hereinafter, the present invention will be specifically described.
[33]
[34]
cathode
[35]
[36]
A negative electrode according to the present invention includes a negative electrode current collector, a first negative electrode active material layer disposed on the negative electrode current collector, and a second negative electrode active material layer disposed on the first negative electrode active material layer, wherein the second negative electrode active material layer includes a second negative electrode active material and a second conductive material, the second negative electrode active material includes a silicon-based active material, the silicon-based active material includes SiO X (0≤X<2), and the second conductive material includes 2 A carbon nanotube structure in which to 5,000 single-walled carbon nanotube units are bonded side by side, wherein the carbon nanotube structure may be included in the second negative electrode active material layer in an amount of 0.01% to 1.0% by weight.
[37]
[38]
The anode current collector may be any material having conductivity without causing chemical change in the battery, and is not particularly limited. For example, as the anode current collector, copper, stainless steel, aluminum, nickel, titanium, fired carbon, or aluminum or stainless steel surface treated with carbon, nickel, titanium, silver, etc. may be used. . Specifically, a transition metal that adsorbs carbon well, such as copper and nickel, can be used as the negative electrode current collector.
[39]
[40]
The negative electrode may include a negative electrode active material layer. The negative electrode active material layer may be disposed on one or both surfaces of the negative electrode current collector.
[41]
[42]
The negative active material layer may include a first negative active material layer and a second negative active material layer. The first negative active material layer may be disposed on the negative electrode current collector, and may specifically contact the negative electrode current collector. The second negative active material layer may be disposed on the first negative active material layer, and the first negative active material layer may be disposed between the second negative active material layer and the negative current collector.
[43]
In general, when using carbon nanotubes as a conductive material, a carbon nanotube dispersion having a low solid content should be used to uniformly distribute the carbon nanotubes in the negative electrode active material layer. However, in the case of using carbon nanotubes with low solid content, when the anode slurry is dried, the binder and the conductive material, which are relatively low in density compared to the anode active material, are easily concentrated toward the upper layer of the anode active material layer (away from the anode current collector and closer to the surface) (Leaning phenomenon, migration) occurs, and there is a problem that the adhesion of the negative electrode and the electrical conductivity are greatly reduced. On the other hand, since the negative electrode of the present invention has the first negative electrode active material layer and the second negative electrode active material layer sequentially disposed as respective slurries, the above-described biasing of the binder and the conductive material can be minimized. Accordingly, input/output characteristics and lifespan characteristics of the battery may be improved.
[44]
[45]
(1) First negative electrode active material layer
[46]
[47]
The first negative active material layer may include a first negative active material.
[48]
The first negative active material may be a negative active material commonly used in the art, and the type is not particularly limited.
[49]
Specifically, the first negative active material may include at least one of a carbon-based active material and a silicon-based active material, and the carbon-based active material particles are a group consisting of artificial graphite, natural graphite, graphitized carbon fibers, and graphitized mesocarbon microbeads. At least one selected from may be used, and in particular, when artificial graphite is used, rate characteristics can be improved. The silicon - based active material is SiO X (0≤X <2), a Si—C composite, and a Si—Y alloy (where Y is an alkali metal, an alkaline earth metal, a transition metal, a group 13 element, a group 14 element, a rare earth element, and Elements selected from the group consisting of combinations) may be used, and in particular, when SiO X (0≤X <2) is used, high capacity of the battery can be derived. More specifically, the first negative electrode active material may be a carbon-based active material.
[50]
The first negative active material may be included in an amount of 70% to 99.5% by weight, preferably 80% to 99% by weight in the first negative active material layer. When the content of the first negative electrode active material satisfies the above range, energy density of the negative electrode may be improved, adhesion of the negative electrode may be increased, and electrical conductivity in the negative electrode may be improved.
[51]
The first negative active material layer may further include a first conductive material.
[52]
The first conductive material may include at least one selected from the group consisting of carbon nanotube structures, multi-walled carbon nanotube units, graphene, and carbon black. The carbon nanotube structure will be described in detail later.
[53]
The first conductive material may be included in an amount of 0.01 wt% to 2.0 wt%, specifically 0.01 wt% to 1.5 wt%, and more specifically, 0.05 wt% to 1.0 wt% in the first negative active material layer. When the above range is satisfied, the adhesive strength and electrical conductivity of the negative electrode can be greatly improved, and a battery having excellent input/output characteristics and lifespan characteristics of the battery can be achieved even when a small amount of the first conductive material is applied.
[54]
[55]
The first negative active material layer may have a thickness of 1 μm to 100 μm, specifically 5 μm to 90 μm, and more specifically 10 μm to 80 μm. When the above range is satisfied, it is possible to minimize the drifting phenomenon of the above-described conductive material and binder. Accordingly, adhesion and electrical conductivity of the negative electrode are greatly improved, and input/output characteristics and lifespan characteristics of the battery may be improved.
[56]
[57]
(2) Second negative electrode active material layer
[58]
[59]
The second negative active material layer may include a second negative active material and a second conductive material.
[60]
The second negative active material may include a silicon-based active material.
[61]
The silicon-based active material may include SiO X (0≤X<2). The SiO X (0≤X<2) may be specifically SiO. Since the second negative active material includes SiO X (0≤X<2), battery capacity may be improved. In particular, since the second anode active material layer, not the first anode active material layer, contains SiO X (0≤X<2), durability of the anode may be improved and impregnability with the electrolyte solution may be improved. More specifically, there is a problem in that the anode active material is easily separated from the anode due to contraction and expansion of the anode active material during charging and discharging of the battery at the interface between the anode current collector and the anode active material layer, which has the weakest binding force in the anode. When SiO X (0≤X<2) is located close to the negative current collector, the desorption phenomenon is accelerated. Accordingly, the durability of the negative electrode is lowered, and the capacity and lifespan characteristics of the battery are deteriorated.
[62]
On the other hand, during rolling during the manufacturing process of the negative electrode, the density near the surface of the negative electrode becomes excessively high, and thus the electrolyte impregnability is lowered. When SiO X (0≤X<2) is located near the surface of the negative electrode, the battery During initial charging, volume expansion of SiO X (0≤X<2) reduces the density of the negative electrode to an appropriate level and improves the impregnability of the electrolyte solution.
[63]
The silicon-based active material may further include a carbon coating layer formed on SiO X (0≤X<2). The carbon coating layer may be disposed on the SiO X (0≤X<2). The carbon coating layer serves to improve conductivity of the SiO X (0≤X<2) and suppress excessive volume expansion of the SiO X (0≤X<2).
[64]
The carbon coating layer may include at least one of amorphous carbon and crystalline carbon.
[65]
The crystalline carbon may further improve conductivity of the anode active material. The crystalline carbon may include at least one selected from the group consisting of florene, carbon nanotubes, and graphene.
[66]
The amorphous carbon can properly maintain the strength of the coating layer and suppress expansion of the natural graphite. The amorphous carbon may be a carbon-based material formed by using at least one carbide selected from the group consisting of tar, pitch, and other organic materials, or a hydrocarbon as a source of chemical vapor deposition.
[67]
The other organic carbide may be an organic carbide selected from sucrose, glucose, galactose, fructose, lactose, mannose, ribose, aldohexose or ketohexose carbide, and combinations thereof.
[68]
The hydrocarbon may be a substituted or unsubstituted aliphatic or alicyclic hydrocarbon or a substituted or unsubstituted aromatic hydrocarbon. The aliphatic or alicyclic hydrocarbon of the substituted or unsubstituted aliphatic or alicyclic hydrocarbon may be metherine, etherin, ethylene, acetylene, propane, butane, butene, pentane, isobutane or hexane. Aromatic hydrocarbons of the substituted or unsubstituted aromatic hydrocarbons include benzene, toluene, xylene, styrene, ethylbenzene, diphenylmethane, naphthalene, phenol, cresol, nitrobenzene, chlorobenzene, indene, coumaron, pyridine, Anthracene, phenanthrene, etc. are mentioned.
[69]
The average particle diameter (D 50 ) of the silicon-based active material may be 0.1 μm to 20 μm, specifically 1 μm to 10 μm. When the above range is satisfied, a side reaction between the SiO X (0≤X<2) and the electrolyte may be suppressed, and the reaction of forming lithium silicate from the SiO X (0≤X<2) is controlled to reduce the initial efficiency. This can be prevented, and the initial capacity of the battery can be maximized.
[70]
The second negative active material may further include a carbon-based active material. The carbon-based active material may include at least one selected from the group consisting of artificial graphite, natural graphite, and graphitized mesocarbon microbeads. Specifically, the carbon-based active material is preferably artificial graphite in that it can effectively control the volume expansion of the negative electrode while maintaining an electrical network with a carbon nanotube structure to be described later, but is not limited thereto.
[71]
The weight ratio of the silicon-based active material and the carbon-based active material may be 0.5:99.5 to 20:80, specifically 1:99 to 10:90. When the above range is satisfied, the capacity of the battery may be improved while excessive volume expansion of the second negative electrode active material may be suppressed.
[72]
The second negative active material may be included in an amount of 90% to 99% by weight, specifically, 95% to 99% by weight in the second negative electrode active material layer. When the above range is satisfied, the energy density of the negative electrode may be maintained high, and the conductivity and adhesion of the negative electrode may be improved.
[73]
The second conductive material may further include a carbon nanotube structure.
[74]
The carbon nanotube structure may include a plurality of single-walled carbon nanotube units. Specifically, the carbon nanotube structure may be a carbon nanotube structure in which 2 to 5,000 single-walled carbon nanotube units are bonded in parallel to each other. More specifically, considering the durability and the conductive network of the second negative electrode active material layer, the carbon nanotube structure is 2 to 4,500, preferably 2 to 4,000, more preferably 2 to 200 single carbon nanotubes. It may be a carbon nanotube structure in which wall carbon nanotube units are bonded to each other. Considering the improvement of the dispersibility of the carbon nanotube structure and the durability of the negative electrode, the carbon nanotube structure may be a combination of 2 to 50 single-walled carbon nanotube units arranged side by side with each other.
[75]
In the carbon nanotube structure, the single-walled carbon nanotube units are arranged side by side and combined (a cylindrical structure in the form of a bundle having flexibility in which the long axes of the units are coupled in parallel to each other) to form the carbon nanotube structure. can In the second negative active material layer, the carbon nanotube structures may be connected to each other to form a network structure.
[76]
Conventional electrodes containing carbon nanotubes are generally bundled or entangled type carbon nanotubes (single-walled carbon nanotube units or multi-walled carbon nanotube units are attached or entangled with each other) ) is dispersed in a dispersion medium to prepare a conductive material dispersion, and then prepared using the conductive material dispersion. At this time, the carbon nanotubes are completely dispersed in the conventional conductive material dispersion, and exist as a conductive material dispersion in which single-stranded carbon nanotube units are dispersed. In the conventional conductive material dispersion, the carbon nanotube units are easily cut due to an excessive dispersion process, resulting in a shorter length than the initial length. In addition, the carbon nanotube units can be easily cut even during the rolling process of the cathode. During operation of the battery, an excessive volume change of the silicon-based active material causes an additional problem in that the carbon nanotube units (particularly, the single-walled carbon nanotube units) are cut. Accordingly, there is a problem in that the conductivity of the negative electrode is lowered and the life characteristics of the battery are lowered. Moreover, in the case of multi-walled carbon nanotube monomers, structural defects are high due to the mechanism of nodal growth (nodules exist due to defects generated during the growth process rather than smooth linear growth). Therefore, during the dispersion process, the multi-walled carbon nanotube units are more easily cut, and the multi-walled carbon nanotube units cut short are likely to aggregate with each other due to π-π stacking by the carbon of the unit. Accordingly, it is difficult to be more uniformly dispersed in the negative electrode slurry.
[77]
In contrast, in the case of the carbon nanotube structure included in the second negative electrode active material layer of the present invention, 2 to 5,000 single-walled carbon nanotube units maintaining high crystallinity without relatively structural defects are arranged side by side and bonded to each other. Since it has a rope shape, the length of the second negative active material can be smoothly maintained without being cut even when the volume of the second negative electrode active material changes, so that the conductivity of the negative electrode can be maintained even during the continuous charging and discharging process of the battery. In addition, due to the high electrical conductivity of the single-walled carbon nanotube unit having high crystallinity, the conductivity of the anode is increased to reduce anode resistance, and the input/output characteristics and lifespan characteristics of the battery can be greatly improved. In addition, since the carbon nanotube structures may be connected to each other to have a network structure in the second negative electrode active material layer that is directly subjected to pressure during rolling, damage to the second negative electrode active material (eg, cracking, etc.) is suppressed. can do. In addition, even if a crack occurs in the second anode active material, the carbon nanotube structure crosses the crack and connects the second anode active material, so that the conductive network can be maintained. Furthermore, since the carbon nanotube structure is not easily broken and can maintain a long shape, the conductive network can be strengthened throughout the second anode active material layer. In addition, since detachment of the second negative active material is suppressed, negative electrode adhesion may be greatly improved.
[78]
In addition, since the carbon nanotube structure is included in the second negative active material layer, adhesion between the first negative active material layer and the second negative active material layer can be greatly improved. Due to the long rope shape formed by the horizontal bonding of single-walled carbon nanotube units therein, the carbon nanotube structure can well connect the second anode active materials to each other through van der Waals force and firmly configure the anode. have. Moreover, since the carbon nanotube structure and the surface of the carbon-based active material of the first negative electrode active material layer can be more closely bonded by π-π bonding (stacking) occurring between the same kind of carbon, the first negative active material layer and the surface of the carbon-based active material Adhesion between the second negative active material layers may be further strengthened.
[79]
In the carbon nanotube structure, the average diameter of the single-walled carbon nanotube unit may be 0.5 nm to 10 nm, and specifically, 1 nm to 9 nm. When the average diameter is satisfied, there is an effect of maximizing conductivity in the negative electrode even with a very small amount of conductive material. The average diameter corresponds to an average value of the top 100 single-walled carbon nanotube units and the bottom 100 single-walled carbon nanotube units having large diameters when the manufactured negative electrode is observed through TEM.
[80]
In the carbon nanotube structure, the average length of the single-walled carbon nanotube units may be 1 μm to 100 μm, specifically 5 μm to 50 μm. When the average length is satisfied, a long conductive path for conductive connection between the second negative electrode active materials can be formed and a unique network structure can be formed, so that even with a very small amount of conductive material, the conductivity in the negative electrode can be maximized. There are possible effects. The average length corresponds to an average value of the top 100 single-walled carbon nanotube units and the bottom 100 single-walled carbon nanotube units having a large length when the manufactured negative electrode is observed through TEM.
[81]
The specific surface area of ​​the single-walled carbon nanotube unit may be 500 m 2 /g to 1,000 m 2 /g, and specifically, 600 m 2 /g to 800 m 2 /g. When the above range is satisfied, since a conductive path in the negative electrode can be smoothly secured due to a large specific surface area, there is an effect of maximizing the conductivity in the negative electrode even with a very small amount of conductive material. The specific surface area of ​​the single-walled carbon nanotube unit can be specifically calculated from the amount of nitrogen gas adsorption under liquid nitrogen temperature (77K) using BELSORP-mini II of BEL Japan.
[82]
The average diameter of the carbon nanotube structure may be 2 nm to 500 nm, specifically 5 nm to 200 nm, and more specifically 5 nm to 50 nm. When the above range is satisfied, it is effective in forming a conductive network and is advantageous in connecting the second negative electrode active materials, so that excellent electrical conductivity can be realized. The average length corresponds to an average value of diameters of the top 100 carbon nanotube structures and the bottom 100 carbon nanotube structures having large diameters when the manufactured negative electrode is observed through an SEM.
[83]
The average length of the carbon nanotube structure may be 1 μm to 500 μm, specifically 1 μm to 100 μm, and more specifically 2 μm to 50 μm. When the above range is satisfied, it is effective in forming a conductive network and is advantageous in connecting the second negative electrode active materials, so that excellent electrical conductivity can be realized. The average length corresponds to an average value of the lengths of the top 100 carbon nanotube structures and the bottom 100 carbon nanotube structures when the manufactured negative electrode is observed through an SEM.
[84]
The carbon nanotube structure may be included in the second negative active material layer in an amount of 0.01% to 1.0% by weight, specifically 0.01% to 0.5% by weight, more specifically 0.01% to 0.2% by weight. can be included When the above range is satisfied, a conductive path of the second anode active material layer is secured, and lifespan characteristics of the battery may be improved while maintaining a low level of anode resistance. When the conductive material dispersion is prepared, when the bundled carbon nanotubes are completely dispersed (as a general dispersion method, as far as possible, the carbon nanotube units are dispersed as far apart from each other), the carbon nanotube structure does not occur, or even if it occurs unintentionally. It occurs in very small amounts (eg, 0.0005% by weight). That is, the above content range can never be achieved by general methods. Since the carbon nanotube structure has a form in which 2 to 5,000 single-walled carbon nanotube units are arranged side by side and combined, the carbon nanotube structure is not cut and lengthened smoothly even when the volume of the second negative electrode active material changes. can keep Therefore, the conductive network of the second negative active material layer can be maintained, and the conductivity of the second negative active material layer can be smoothly secured due to the high conductivity of the carbon nanotube structure. Accordingly, even when the content of the carbon nanotube structure in the second negative electrode active material layer is low, input/output characteristics and lifespan characteristics of the battery may be excellent.
[85]
On the other hand, in some cases, the single-walled carbon nanotube units may be surface-treated through oxidation treatment or nitration treatment to improve affinity with the dispersant.
[86]
The second conductive material may further include at least one selected from the group consisting of fullerene, carbon black, carbon nanotube units, and graphene. In this case, since a one-dimensional linear conductive network can become a two-dimensional or more conductive network by the composite application of the materials and the carbon nanotube structure, the conductivity of the second negative electrode active material layer is improved, and the input/output of the battery is improved. Characteristics and life characteristics can be improved.
[87]
The second negative active material layer may have a thickness of 1 μm to 100 μm, specifically 5 μm to 90 μm, and more specifically 10 μm to 80 μm. When the above range is satisfied, it is possible to minimize the aforementioned conductive material and binder drifting phenomenon. Accordingly, the adhesion of the negative electrode (adhesion between the negative electrode active material layer and the current collector), the adhesive strength between the first negative electrode active material layer and the second negative electrode active material layer, and the electrical conductivity of the negative electrode are greatly improved, and the input/output characteristics and lifespan characteristics of the battery are improved. can be improved
[88]
[89]
It is preferable that the thickness of the second negative active material layer is equal to or greater than the thickness of the first negative active material layer. The ratio of the thickness of the first negative active material layer to the thickness of the second negative active material layer may be 10:90 to 50:50, specifically 20:80 to 50:50, and more specifically 25:75 to 50:50. When the above range is satisfied, the effect of suppressing the above-described migration of the conductive material and the binder is reduced, and the effect of improving diffusion resistance by improving the porosity of the second negative active material layer is reduced. Even when the thickness of the first negative electrode active material layer is too thin beyond the above range, the effect of suppressing the above-mentioned conductive material and binder drifting phenomenon is reduced, and thus the effect of improving negative electrode adhesion and interface resistance is insignificant.
[90]
[91]
An interface exists between the first negative active material layer and the second negative active material layer. This can be confirmed through the cross-section of the manufactured negative electrode. Conversely, if the anode active material layer is formed in a single-layer structure rather than a multi-layer structure (only one coating is performed through one anode slurry), the interface is not observed.
[92]
[93]
Each of the first negative active material layer and the second negative active material layer may further include a binder, and the binder of the first negative active material layer and the binder of the second negative active material layer may be the same or different. have. The binder is used to secure adhesion between negative active materials or between the negative active material and the current collector, and general binders used in the art may be used, and the type is not particularly limited. As the binder, for example, vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, starch, hydroxypropyl cellulose, regeneration Cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, carboxymethyl cellulose (CMC), styrene butadiene rubber ( SBR), fluororubber, or various copolymers thereof, and the like may be used alone or in a mixture of two or more of them.
[94]
The binder may be included in an amount of 10% by weight or less, preferably 0.1% to 5% by weight, in the first negative active material layer (or the second negative active material layer). When the content of the binder satisfies the above range, excellent negative electrode adhesion may be implemented while minimizing an increase in negative electrode resistance.
[95]
[96]
cathode manufacturing method
[97]
[98]
Next, the negative electrode manufacturing method of the present invention will be described.
[99]
The method for manufacturing a negative electrode of the present invention includes preparing a first negative electrode slurry and a second negative electrode slurry; forming a first negative electrode active material layer on the negative electrode current collector through the first negative electrode slurry; and forming a second anode active material layer on the first anode active material layer through the second anode slurry, wherein the anode slurry includes a second anode active material; and a second conductive material, wherein the second negative electrode active material includes a silicon-based active material, the silicon-based active material includes SiO X (0≤X<2), and the second conductive material contains 2 to 5,000 single-walled carbon atoms. and a carbon nanotube structure in which nanotube units are bonded side by side, and the carbon nanotube structure may be included in an amount of 0.01% to 1.0% by weight in the second negative electrode active material layer. The first negative active material layer, the second negative active material layer, the second negative active material, the second conductive material, and the carbon nanotube structure are the same as those in the above-described embodiment.
[100]
[101]
(1) Preparing a first negative electrode slurry and a second negative electrode slurry
[102]
[103]
The first negative electrode slurry may be the same as a conventional negative electrode slurry manufacturing method. For example, a first negative active material (same as the first negative active material in the above-described embodiment), a first conductive material (same as the first conductive material in the above-described embodiment), and a solvent (a binder may be further included). After preparing a mixture to prepare a first negative electrode slurry by stirring the mixture.
[104]
However, when the first negative electrode slurry includes a carbon nanotube structure, a dispersion of a carbon nanotube structure to be described later must be prepared.
[105]
Examples of the solvent include amide-based polar organic solvents such as water, dimethylformamide (DMF), diethyl formamide, dimethyl acetamide (DMAc), and N-methyl pyrrolidone (NMP); Methanol, ethanol, 1-propanol, 2-propanol (isopropyl alcohol), 1-butanol (n-butanol), 2-methyl-1-propanol (isobutanol), 2-butanol (sec-butanol), 1-methyl alcohols such as -2-propanol (tert-butanol), pentanol, hexanol, heptanol or octanol; glycols such as ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, 1,3-propanediol, 1,3-butanediol, 1,5-pentanediol, or hexylene glycol; polyhydric alcohols such as glycerin, trimethylolpropane, pentaerythritol, or sorbitol; 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 glycol ethers such as 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, methylpropyl ketone, or cyclopentanone; and esters such as ethyl acetate, γ-butyl lactone, and ε-propiolactone, and any one or a mixture of two or more of these may be used, but is not limited thereto. The solvent may be the same as or different from the dispersion medium used in the conductive material dispersion, and preferably may be water.
[106]
[107]
The second anode slurry may be prepared by preparing a mixture including the second anode active material, the carbon nanotube structure dispersion, and a solvent, and then stirring the mixture.
[108]
The carbon nanotube structure dispersion may be prepared as follows.
[109]
The preparation of the carbon nanotube structure dispersion may include preparing a mixed solution including a dispersion medium, a dispersant, and bundled single-walled carbon nanotubes (assemblies or aggregates of single-walled carbon nanotube units) (S1-1); and applying shear force to the mixed solution to disperse the bundled single-walled carbon nanotubes to form a carbon nanotube structure in which 2 to 5,000 single-walled carbon nanotube units are bonded side by side (S1-2). can
[110]
[111]
In step S1-1, the mixed solution may be prepared by introducing bundled single-walled carbon nanotubes and a dispersant into a dispersion medium. The bundled single-walled carbon nanotubes exist in a bundle form by combining the above-mentioned single-walled carbon nanotube units, and include usually 2 or more, substantially 500 or more, for example, 5,000 or more single-walled carbon nanotube units. do.
[112]
The bundled single-walled carbon nanotubes may have a specific surface area of ​​500 m 2 /g to 1,200 m 2 /g, and specifically, 500 m 2 /g to 1,000 m 2 /g. When the above range is satisfied, since a conductive path in the second negative active material layer can be smoothly secured due to a large specific surface area, there is an effect of maximizing conductivity in the second negative active material layer even with a very small amount of conductive material. In addition, in order to enhance the adhesion between the first negative active material layer and the second negative active material layer , it is preferably 500 m 2 /g to 800 m 2 /g.
[113]
The bundled single-walled carbon nanotubes may be included in an amount of 0.1% to 1.0% by weight, specifically, 0.2% to 0.5% by weight in the mixed solution. When the above range is satisfied, the bundled single-walled carbon nanotubes are dispersed at an appropriate level, and a carbon nanotube structure at an appropriate level may be formed, and dispersion stability may be improved.
[114]
Examples of the dispersion medium include amide-based polar organic solvents such as water, dimethylformamide (DMF), diethyl formamide, dimethyl acetamide (DMAc), and N-methyl pyrrolidone (NMP); Methanol, ethanol, 1-propanol, 2-propanol (isopropyl alcohol), 1-butanol (n-butanol), 2-methyl-1-propanol (isobutanol), 2-butanol (sec-butanol), 1-methyl alcohols such as -2-propanol (tert-butanol), pentanol, hexanol, heptanol or octanol; glycols such as ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, 1,3-propanediol, 1,3-butanediol, 1,5-pentanediol, or hexylene glycol; polyhydric alcohols such as glycerin, trimethylolpropane, pentaerythritol, or sorbitol; 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 glycol ethers such as 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, methylpropyl ketone, or cyclopentanone; and esters such as ethyl acetate, γ-butyl lactone, and ε-propiolactone, and any one or a mixture of two or more of these may be used, but is not limited thereto. More specifically, the dispersion medium may be the same as or different from the solvent for preparing the negative electrode slurry, and preferably may be water.
[115]
The dispersant is hydrogenated nitrile butadiene rubber, polyvinylidene fluoride, polystyrene, polyvinylpyrrolidone, polyvinyl alcohol, pyrene butyric acid, pyrene sulfonic acid, tannic acid, pyrene methylamine, sodium dodecyl sulfate, and carboxylate. It may include at least one of methyl cellulose, and specifically, it may be carboxy methyl cellulose, polyvinylidene fluoride, polyvinylpyrrolidone, or hydrogenated nitrile butadene rubber.
[116]
In the carbon nanotube structure dispersion, the weight ratio of the bundled carbon nanotubes to the dispersant may be 1:0.1 to 1:10, specifically 1:1 to 1:10. When the above range is satisfied, the bundled single-walled carbon nanotubes are dispersed at an appropriate level, and a carbon nanotube structure at an appropriate level may be formed, and dispersion stability may be improved.
[117]
The solid content in the mixture may be 0.1% to 20% by weight, specifically 1% to 10% by weight. When the above range is satisfied, the bundled single-walled carbon nanotubes are dispersed at an appropriate level, and a carbon nanotube structure at an appropriate level may be formed, and dispersion stability may be improved. In addition, the second negative electrode slurry (slurry for preparing the second negative electrode active material layer) may have viscosity and elasticity suitable for forming the second negative electrode active material layer, and contribute to increasing the solid content of the second negative electrode slurry.
[118]
In the step S1-2, the step of dispersing the bundled carbon nanotubes in the mixed solution is a homogenizer, bead mill, ball mill, basket mill, attrition mill, universal stirrer, clear mixer, spike mill, TK It may be performed using a mixing device such as a mixer or ultrasonic sonification equipment. Among them, the beads mill method is preferable in that the diameter size of the carbon nanotube structure can be precisely controlled, uniform distribution of the carbon nanotube structure can be achieved, and there are advantages in terms of cost. .
[119]
The bead mill method may be as follows. The mixed solution may be put into a container containing beads, and the bundled single-walled carbon nanotubes may be dispersed by rotating the container.
[120]
At this time, the conditions under which the bead mill method is performed are as follows.
[121]
The beads may have an average particle diameter of 0.5 mm to 1.5 mm, specifically 0.5 mm to 1.0 mm. When the above range is satisfied, the diameter size can be appropriately controlled without breaking the carbon nanotube structure during the dispersion process, and a dispersion solution having a uniform composition can be prepared.
[122]
The rotational speed of the container may be 500 RPM to 10,000 RPM, specifically 2,000 RPM to 6,000 RPM. When the above range is satisfied, the diameter size can be appropriately controlled without breaking the carbon nanotube structure during the dispersion process, and a dispersion solution having a uniform composition can be prepared.
[123]
The time for performing the bead mill may be 0.5 hours to 2 hours, specifically 0.5 hours to 1.5 hours, and more specifically 0.8 hours to 1 hour. When the above range is satisfied, the diameter size can be appropriately controlled without breaking the carbon nanotube structure during the dispersion process, and a dispersion solution having a uniform composition can be prepared. Since the bead mill execution time means the total time during which the bead mill is applied, for example, if the bead mill is performed several times, it means the total time over several times.
[124]
The bead-tight condition is for dispersing the bundled single-walled carbon nanotubes at an appropriate level, and specifically, except for the case where the bundled single-walled carbon nanotubes are completely dispersed into a single strand of single-walled carbon nanotubes. That is, in the bead-tight condition, the bundled single-walled carbon nanotubes are dispersed at an appropriate level, and 2 to 5,000 single-walled carbon nanotube units are bonded side by side to each other in the prepared carbon nanotube structure dispersion. It is for forming a tube structure. This can be achieved only when the composition of the mixed solution, dispersion process (eg, bead mill process) conditions, etc. are strictly controlled.
[125]
Through the above process, a carbon nanotube structure dispersion may be formed.
[126]
[127]
A binder may be further included in the negative electrode slurries (first negative electrode slurry and second negative electrode slurry), if necessary. At this time, the binder of the above-described embodiment may be used as the binder.
[128]
[129]
(2) forming a first negative electrode active material layer on the negative electrode current collector through the first negative electrode slurry and forming a second negative electrode active material layer on the first negative electrode active material layer through the second negative electrode slurry
[130]
[131]
Next, a first anode active material layer is formed through the first anode slurry prepared as described above. Specifically, the first negative electrode active material layer is obtained by applying the first negative electrode slurry on the negative electrode current collector and then drying it, or applying the first negative electrode slurry on a separate support and then peeling it from the support It may be formed through a method of laminating a film on an anode current collector. If necessary, after the first negative active material layer is formed through the above method, a rolling process may be additionally performed. At this time, drying and rolling may be performed under appropriate conditions in consideration of the physical properties of the negative electrode to be finally manufactured, and are not particularly limited.
[132]
Thereafter, a second anode active material layer is formed through the second anode slurry prepared as described above. Specifically, the second negative electrode active material layer is formed by applying the second negative electrode slurry on the first negative electrode active material layer and then drying the second negative electrode slurry, or applying the second negative electrode slurry on a separate support and then removing the second negative electrode slurry from the support. It may be formed through a method of laminating a film obtained by peeling on the first negative active material layer. If necessary, after the second negative electrode active material layer is formed through the above method, a rolling process may be additionally performed. At this time, drying and rolling may be performed under appropriate conditions in consideration of the physical properties of the negative electrode to be finally manufactured, and are not particularly limited.
[133]
[134]
secondary battery
[135]
Next, a secondary battery according to another embodiment of the present invention will be described.
[136]
A secondary battery according to another embodiment of the present invention may include the negative electrode of the above-described embodiment.
[137]
Specifically, the secondary battery may include the negative electrode, the positive electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte, and the negative electrode is the same as the negative electrode of the above-described embodiment. Since the cathode has been described above, a detailed description thereof will be omitted.
[138]
The positive electrode may include a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector and including the positive electrode active material.
[139]
In the positive electrode, the positive electrode current collector is not particularly limited as long as it has conductivity without causing chemical change in the battery. For example, stainless steel, aluminum, nickel, titanium, fired carbon, or carbon on the surface of aluminum or stainless steel. , those surface-treated with nickel, titanium, silver, etc. may be used. In addition, the cathode current collector may have a thickness of typically 3 μm to 500 μm, and adhesion of the cathode active material may be increased by forming fine irregularities on the surface of the current collector. For example, it may be used in various forms such as films, sheets, foils, nets, porous materials, foams, and non-woven fabrics.
[140]
The cathode active material may be a commonly used cathode active material. Specifically, the cathode active material may include layered compounds such as lithium cobalt oxide (LiCoO 2 ) and lithium nickel oxide (LiNiO 2 ), or compounds substituted with one or more transition metals; lithium iron oxides such as LiFe 3 O 4 ; lithium manganese oxides such as Li 1+c1 Mn 2-c1 O 4 (0≤c1≤0.33), LiMnO 3 , LiMn 2 O 3 , LiMnO 2 ; lithium copper oxide (Li 2 CuO 2 ); LiV 3 O 8 , V 2 O 5 , Cu 2 V 2 O 7 and the like; vanadium oxide; Represented by the formula LiNi 1-c2 M c2 O 2 (where M is at least one selected from the group consisting of Co, Mn, Al, Cu, Fe, Mg, B, and Ga, and satisfies 0.01≤c2≤0.3) Ni site-type lithium nickel oxide; Formula LiMn 2-c3 M c3 O 2 (wherein M is at least one selected from the group consisting of Co, Ni, Fe, Cr, Zn, and Ta, and satisfies 0.01≤c3≤0.1) or Li 2 Mn 3 MO 8 (here, M is at least one selected from the group consisting of Fe, Co, Ni, Cu, and Zn). Lithium manganese composite oxide represented by; Examples include LiMn 2 O 4 in which Li in the formula is partially substituted with an alkaline earth metal ion, but is not limited thereto. The anode may be Li-metal.
[141]
The positive electrode active material layer may include a positive electrode conductive material and a positive electrode binder together with the positive electrode active material described above.
[142]
At this time, the positive electrode conductive material is used to impart conductivity to the electrode, and in the battery, any material that does not cause chemical change and has electronic conductivity can be used without particular limitation. Specific examples include graphite such as natural graphite or artificial graphite; carbon-based materials such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, summer black, and carbon fiber; metal powders or metal fibers such as copper, nickel, aluminum, and silver; conductive whiskeys such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives, and the like, and one of these may be used alone or in a mixture of two or more.
[143]
[144]
In addition, the positive electrode binder serves to improve adhesion between particles of the positive electrode active material and adhesion between the positive electrode active material and the positive electrode current collector. Specific examples include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethylcellulose (CMC) ), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene butadiene rubber (SBR), fluororubber, or various copolymers thereof, and the like, and one type alone or a mixture of two or more types thereof may be used.
[145]
As a separator, it separates the negative electrode and the positive electrode and provides a passage for lithium ion movement. If it is normally used as a separator in a secondary battery, it can be used without particular limitation. it is desirable Specifically, a porous polymer film, for example, a porous polymer film made of polyolefin-based polymers such as ethylene homopolymer, propylene homopolymer, ethylene/butene copolymer, ethylene/hexene copolymer, and ethylene/methacrylate copolymer, or these A laminated structure of two or more layers of may be used. In addition, conventional porous non-woven fabrics, for example, non-woven fabrics made of high melting point glass fibers, polyethylene terephthalate fibers, and the like may be used. In addition, a coated separator containing a ceramic component or a polymer material may be used to secure heat resistance or mechanical strength, and may be selectively used in a single-layer or multi-layer structure.
[146]
Examples of the electrolyte include, but are not limited to, organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel polymer electrolytes, solid inorganic electrolytes, and molten inorganic electrolytes that can be used in the manufacture of lithium secondary batteries.
[147]
Specifically, the electrolyte may include a non-aqueous organic solvent and a metal salt.
[148]
As the non-aqueous organic solvent, for example, N-methyl-2-pyrrolidone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma-butyllolactone, 1,2-dimethine Toxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethylsulfoxide, 1,3-dioxorane, formamide, dimethylformamide, dioxorane, acetonitrile, nitromethane, methyl formate, methyl acetate, phosphoric acid Triester, trimethoxy methane, dioxolane derivative, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivative, tetrahydrofuran derivative, ether, methyl propionate, propionic acid An aprotic organic solvent such as ethyl may be used.
[149]
In particular, among the carbonate-based organic solvents, ethylene carbonate and propylene carbonate, which are cyclic carbonates, are high-viscosity organic solvents and have a high dielectric constant, so they can be preferably used because they dissociate lithium salts well, and dimethyl carbonate and diethyl carbonate and When the same low-viscosity, low-dielectric constant linear carbonate is mixed and used in an appropriate ratio, an electrolyte having high electrical conductivity can be made and can be used more preferably.
[150]
The metal salt may be a lithium salt, and the lithium salt is a material that is easily soluble in the non-aqueous electrolyte solution. For example, the anion of the lithium salt is 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 - , (FSO 2 ) 2 N - , CF 3 CF 2 ( CF 3 ) 2 CO - , (CF 3SO 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 ) 2At least one selected from the group consisting of N - may be used.
[151]
In addition to the components of the electrolyte, the electrolyte may include, for example, haloalkylene carbonate-based compounds such as difluoroethylene carbonate, pyridine, and trialkylene carbonate for the purpose of improving battery life characteristics, suppressing battery capacity decrease, and improving battery discharge capacity. Ethylphosphite, triethanolamine, cyclic ether, ethylene diamine, n-glyme, hexaphosphoric acid triamide, nitrobenzene derivative, sulfur, quinone imine dye, N-substituted oxazolidinone, N,N-substituted imida One or more additives such as zolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxy ethanol or aluminum trichloride may be further included.
[152]
[153]
According to another embodiment of the present invention, a battery module including the secondary battery as a unit cell and a battery pack including the same are provided. Since the battery module and the battery pack include the secondary battery having high capacity, high rate and cycle characteristics, a medium or large-sized device selected from the group consisting of an electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, and a power storage system can be used as a power source for
[154]
[155]
Hereinafter, the present invention will be described in more detail through specific examples.
[156]
[157]
Preparation Example 1: Preparation of Carbon Black Dispersion
[158]
0.4 parts by weight of carbon black (Imerys, Super C65) having an average particle diameter of 35 nm and 0.6 parts by weight of carboxymethyl cellulose (weight average molecular weight: 100,000 g/mol, degree of substitution: 1.0) were mixed with 99.0 parts by weight of water as a dispersion medium, and solid content A liquid mixture was prepared so that it would be 1.0% by weight. The mixture was stirred in a beads-mill method to disperse the carbon black in a solvent to prepare a carbon black dispersion. At this time, the particle diameter of the beads was 1 mm, the rotational speed of the stirring vessel containing the beads was 3,000 RPM, and the stirring was performed for 60 minutes.
[159]
In the carbon black dispersion, the carbon black was 0.4% by weight and the carboxy methyl cellulose was 0.6% by weight.
[160]
[161]
Preparation Example 2: Preparation of carbon nanotube structure dispersion
[162]
0.4 parts by weight of bundled carbon nanotubes (specific surface area: 650 m 2 /g) consisting of single-walled carbon nanotube units with an average diameter of 1.5 nm and an average length of 5 μm or more and carboxymethyl cellulose (weight average molecular weight: 100,000 g / mol) , Degree of substitution: 1.0) 0.6 parts by weight was mixed with 99.0 parts by weight of water as a dispersion medium to prepare a mixture so that the solid content was 1.0% by weight.
[163]
The mixture was stirred in a bead-mill method to disperse bundled single-walled carbon nanotubes in a solvent to prepare a carbon nanotube structure dispersion. At this time, the particle diameter of the beads was 1 mm, and the beads were stirred. The rotation speed of the container was 3,000 RPM, and the stirring was performed for 60 minutes The carbon nanotube structure dispersion liquid contained a carbon nanotube structure in which 2 to 5,000 single-walled carbon nanotube units were bonded side by side.
[164]
In the carbon nanotube structure dispersion, the carbon nanotube structure was 0.4% by weight and the carboxymethyl cellulose was 0.6% by weight.
[165]
[166]
Preparation Example 3: Preparation of Carbon Nanotube Structure Dispersion
[167]
A carbon nanotube structure dispersion was prepared in the same manner as in Preparation Example 2, except that the weight average molecular weight of carboxy methyl cellulose in Preparation Example 2 was changed to 400,000 g/mol (substitution degree: 1.0). In the dispersion, the carbon nanotube structure was 0.4% by weight and the carboxymethyl cellulose was 0.6% by weight.
[168]
[169]
Preparation Example 4: Preparation of single-walled carbon nanotube monomer dispersion
[170]
0.2 parts by weight of bundled carbon nanotubes (specific surface area: 650 m 2 /g) consisting of single-walled carbon nanotube units with an average diameter of 1.5 nm and an average length of 5 μm or more and carboxymethyl cellulose (weight average molecular weight: 100,000 g / mol) , Degree of substitution: 1.0) 1.2 parts by weight was mixed with 98.6 parts by weight of water as a dispersion medium to prepare a mixture so that the solid content was 1.4% by weight.
[171]
The mixture was stirred in a bead-mill method to disperse bundled single-walled carbon nanotubes in a solvent to prepare a conductive material dispersion. At this time, the particle diameter of the beads was 1 mm, and the stirring vessel containing the beads The rotation speed was 3,000 RPM. Agitation was performed for 60 minutes under the above conditions as one cycle, and a total of 4 cycles (natural cooling was performed for 60 minutes between each cycle). Through this, a single-walled carbon nanotube monomer dispersion was prepared. In the dispersion, the bundled single-walled carbon nanotubes were completely dispersed, and only the single-walled carbon nanotube units existed as a strand unit, and the above-described carbon nanotube structure was not detected. 0.2% by weight of the single-walled carbon nanotube unit and 1.2% by weight of the carboxymethyl cellulose.
[172]
[173]
Preparation Example 5: Preparation of multi-walled carbon nanotube monomer dispersion
[174]
4.0 parts by weight of bundled carbon nanotubes (specific surface area: 185 m 2 /g) composed of multi-walled carbon nanotube units having an average diameter of 10 nm and an average length of 1 μm and carboxymethyl cellulose (weight average molecular weight: 100,000 g / mol) , Degree of substitution: 1.0) 0.6 parts by weight was mixed with 95.4 parts by weight of water as a dispersion medium to prepare a mixture so that the solid content was 4.6% by weight.
[175]
The mixed solution was put into a spike mill filled with 80% of beads having a size of 0.65 mm, dispersed, and discharged at a discharge rate of 2 kg/min. By performing this process twice, the bundled multi-walled carbon nanotubes were completely dispersed to prepare a multi-walled carbon nanotube monomer dispersion. In the dispersion, the multi-walled carbon nanotube unit (average diameter: 10 nm) was 4.0% by weight and the carboxymethyl cellulose was 0.6% by weight.
[176]
[177]
Examples and Comparative Examples
[178]
[179]
Example 1: Preparation of negative electrode
[180]
(1) Formation of the first negative electrode active material layer
[181]
Carbon black dispersion of Preparation Example 1, artificial graphite having an average particle diameter (D 50 ) of 21 μm, styrene butadiene rubber (SBR) and carboxymethyl cellulose (CMC) as a binder (weight average molecular weight: 100,000 g/mol, degree of substitution: 1.0 ) was mixed with water to prepare a first negative electrode slurry. The negative electrode slurry was applied to a negative electrode current collector (copper (Cu) metal thin film) having a thickness of 20 μm, dried at 130 ° C, and rolled to form a first negative electrode active material layer (thickness: 50 μm)
[182]
In the first negative active material layer, the artificial graphite was included at 94.80% by weight, the SBR at 3.5% by weight, the CMC at 1.2% by weight, and the carbon black at 0.50% by weight.
[183]
(2) Formation of a second negative electrode active material layer
[184]
Carbon nanotube structure dispersion of Preparation Example 2, anode active material ( artificial graphite having an average particle diameter (D 50 ) of 21 μm: weight ratio of SiO having an average particle diameter (D 50 ) of 6.6 μm = 94: 6), binder SBR and carboxymethyl Cellulose (CMC) (weight average molecular weight: 100,000 g/mol, substitution degree: 1.0) was mixed with water to prepare a second negative electrode slurry. After applying the second negative electrode slurry on the first negative electrode active material layer, it was dried at 130° C. and rolled to form a second negative electrode active material layer (thickness: 50 μm).
[185]
In the second negative active material layer, a total of 96.25% by weight of the negative electrode active material, 2.5% by weight of the SBR, 1.2% by weight of the CMC, and 0.05% by weight of the carbon nanotube structure were included.
[186]
[187]
Example 2: Preparation of negative electrode
[188]
A negative electrode was prepared in the same manner as in Example 1, except that the carbon nanotube structure dispersion of Preparation Example 3 was used instead of the carbon nanotube structure dispersion of Preparation Example 2 when the second negative electrode active material layer was formed in Example 1. did
[189]
[190]
Example 3: Preparation of negative electrode
[191]
When forming the first negative electrode slurry in Example 1, an anode was prepared in the same manner as in Example 1, except that the multi-walled carbon nanotube unit dispersion of Preparation Example 5 was used instead of the carbon black dispersion of Preparation Example 1.
[192]
[193]
Example 4: Preparation of negative electrode
[194]
Example 1, except that the carbon nanotube structure dispersion of Preparation Example 2 and the multi-walled carbon nanotube unit dispersion of Preparation Example 5 were used instead of the carbon black dispersion of Preparation Example 1 when the first negative electrode slurry was formed in Example 1. An anode was prepared in the same manner as above. In the prepared first negative active material layer, the weight ratio of the carbon nanotube structure to the multi-walled carbon nanotube was 10:90. In the first negative active material layer, the artificial graphite is 94.80% by weight, the SBR is 3.5% by weight, the CMC is 1.2% by weight, the carbon nanotube structure is 0.05% by weight, and the multi-walled carbon nanotube unit is 0.45% by weight. included as
[195]
[196]
Example 5: Preparation of negative electrode
[197]
When forming the first negative electrode slurry in Example 1, an anode was prepared in the same manner as in Example 1, except that the carbon nanotube structure dispersion of Preparation Example 2 was used instead of the carbon black dispersion of Preparation Example 1.
[198]
The first negative active material layer includes 95.25% by weight of the artificial graphite, 3.5% by weight of the SBR, 1.2% by weight of the CMC, and 0.05% by weight of the carbon nanotube structure.
[199]
[200]
Comparative Example 1: Preparation of negative electrode
[201]
Carbon nanotube structure dispersion of Preparation Example 2, anode active material ( artificial graphite having an average particle diameter (D 50 ) of 21 μm: weight ratio of SiO having an average particle diameter (D 50 ) of 6.6 μm = 94: 6), binder SBR and carboxymethyl An anode slurry was prepared by mixing cellulose (CMC) (weight average molecular weight: 100,000 g/mol, substitution degree: 1.0) with water. The negative electrode slurry was applied to a negative electrode current collector (copper (Cu) metal thin film) having a thickness of 20 μm, dried at 130° C., and then rolled to form a negative electrode active material layer (thickness: 100 μm).
[202]
In the anode active material layer, the anode active material is included in a total of 95.75% by weight, the SBR at 3.0% by weight, the CMC at 1.2% by weight, and the carbon nanotube structure at 0.05% by weight.
[203]
[204]
Comparative Example 2: Preparation of negative electrode
[205]
A negative electrode was prepared in the same manner as in Example 1, except that the single-walled carbon nanotube unit dispersion of Preparation Example 4 was used instead of the carbon nanotube structure of Preparation Example 2 when the second negative electrode active material layer of Example 1 was formed.
[206]
In the second negative electrode active material layer, the negative active material is 96.25% by weight in total ( artificial graphite having an average particle diameter (D 50 ) of 21 μm: weight ratio of SiO having an average particle diameter (D 50 ) of 6.6 μm = 94:6), the SBR 2.5% by weight, 1.2% by weight of the CMC, and 0.05% by weight of the single-walled carbon nanotube unit.
[207]
[208]
Comparative Example 3: Preparation of negative electrode
[209]
A negative electrode was prepared in the same manner as in Example 1, except that the multi-walled carbon nanotube unit dispersion of Preparation Example 5 was used instead of the carbon nanotube structure of Preparation Example 2 when the second negative electrode active material layer of Example 1 was formed.
[210]
In the second negative electrode active material layer, the negative electrode active material is 95.80% by weight in total ( artificial graphite having an average particle diameter (D 50 ) of 21 μm: weight ratio of SiO having an average particle diameter (D 50 ) of 6.6 μm = 94:6), the SBR 2.5% by weight, 1.2% by weight of the CMC, and 0.5% by weight of the multi-walled carbon nanotube unit.
[211]
[212]
[Table 1]
First negative electrode active material layer Second negative electrode active material layer
Carbon black content (% by weight) Content of multi-walled carbon nanotube monomers (% by weight) Carbon nanotube structure content (% by weight) Carbon nanotube structure content (% by weight) Single-walled carbon nanotube monomer (completely dispersed form) content (% by weight) Content of multi-walled carbon nanotube monomers (% by weight)
Example 1 0.5 - - 0.05 - -
Example 2 0.5 - - 0.05 - -
Example 3 - 0.5 - 0.05 - -
Example 4 - 0.45 0.05 0.05 - -
Example 5 - - 0.05 0.05 - -
Comparative Example 1 The negative electrode active material layer is manufactured as a single layer, and the content of the carbon nanotube structure is 0.05% by weight in the entire negative electrode active material layer.
Comparative Example 2 0.5 - - - 0.05 -
Comparative Example 3 0.5 - - - - 0.5
[213]
In Examples 1, 3 to 5, and Comparative Example 1, the average diameter of the carbon nanotube structures was 10 nm and the average length was 8.2 μm. In Example 2, the average diameter of the carbon nanotube structures was 100 nm and the average length was 15.6 μm. In Comparative Example 2, the average diameter of the single-walled carbon nanotube units was 1.6 nm and the average length was 1.8 μm. .
[214]
In the cathodes of Examples 3 and 4 and Comparative Example 3, the average diameter of the multi-walled carbon nanotube units was 10.8 nm and the average length was 1.3 μm.
[215]
The average diameter and average length are the top 100 carbon nanotube structures (or multi-walled carbon nanotube units, or single-walled carbon nanotube units) having a large diameter (or length) when the prepared negative electrode is observed through TEM. It corresponds to the average value of the lower 100 carbon nanotube structures (or multi-walled carbon nanotube units, single-walled carbon nanotube units).
[216]
[217]
Experimental Example 1: Observation of cathode
[218]
(1) Binder distribution
[219]
1 is a photograph of analyzing the binder distribution of the negative electrode of Comparative Example 1 (Comparative Example 1) and the negative electrode of Example 1 (Example 1). The analysis was performed through an osmium oxide (OsO 4 ) staining method. Specifically, the negative electrode was dyed by exposing/leaving it in a fume atmosphere of osmium oxide for 3 days in a sealed special chamber prepared in a glove box, and then replaced with an Ar atmosphere. After 1 day, the cross section of the cathode cut by ion milling was analyzed by image mapping with SEM-EDX.
[220]
1 is an analysis photograph of Comparative Example 1 (left), in which a single-layer negative active material layer is formed, and Example 1 (right), in which two layers of first and second negative electrode active material layers are formed. Looking at the picture on the right, it can be seen that in Example 1, the first and second negative active material layers have different compositions, and the binder distribution ratios for the first and second negative active material layers are also different. In particular, when the first and second anode active material layers are formed as in Example 1, it can be seen that SiO is located only in the second anode active material layer, and the amount of binder in the first anode active material layer is greater. In addition, it can be seen that the distribution of the binder is changed in the thickness direction of the negative electrode active material layer.
[221]
[222]
(2) Confirmation of existence of carbon nanotube structure
[223]
2 is a SEM picture of the second negative active material layer of the negative electrode of Example 1, and FIG. 3 is a SEM picture of the second negative active material layer of the negative electrode of Example 2. 4 is a SEM photograph of a second negative electrode active material layer of the negative electrode of Comparative Example 3. 6 is a SEM photograph of a second negative electrode active material layer of the negative electrode of Comparative Example 2.
[224]
Referring to FIGS. 2 and 3 , it can be confirmed that a long rope-shaped carbon nanotube structure in which a plurality of single-walled carbon nanotube units are arranged side by side and bonded to each other exists. In particular, in the cathode of Example 1 of FIG. 2, it can be seen that a carbon nanotube structure having an average diameter of 10 nm was formed because hydrogenated nitrile butadiene rubber having a relatively low weight average molecular weight was used, and in the case of FIG. 3, an average of 100 nm It can be seen that a carbon nanotube structure having a diameter is formed. 2 and 3, it can be seen that the carbon nanotube structures form a network structure with each other.
[225]
On the other hand, in FIG. 4 , only short-length multi-walled carbon nanotube units were seen, and no carbon nanotube structure was observed. In addition, in FIG. 6, only single-walled carbon nanotube units existing in units of single strands were seen, and no carbon nanotube structure was observed.
[226]
[227]
(3) Confirmation of the network by the second conductive material
[228]
5 is a SEM photograph of the second negative electrode active material layer of the negative electrode of Example 1. Referring to (A) of FIG. 5 , it can be seen that cracking of SiO can be suppressed by the carbon nanotube structure. Referring to (B) of FIG. 5 , it can be seen that the carbon nanotube structure maintains the conductive network on the cracked SiO so as not to be disconnected. Referring to (C) of FIG. 5 , it can be seen that the conductive network is formed long due to the long length of the carbon nanotube structure.
[229]
[230]
Experimental Example 2: Evaluation of negative electrode adhesion
[231]
Negative electrode adhesive strength (adhesive strength between the negative electrode active material layer and the current collector) was measured under dry conditions. Specifically, after attaching the double-sided tape to the slide glass, putting the cathode cut out to 20 mm × 180 mm on it and adhering it back and forth 10 times with a 2 kg roller, pull it at 200 mm/min using a UTM (TA company) machine The peeling force from the slide glass was measured. At this time, the measuring angle between the slide glass and the cathode was 90°. The measurement results are shown in Table 2 below.
[232]
[233]
Experimental Example 3: Evaluation of adhesion between the first negative active material and the second negative active material
[234]
For Examples 1 to 4 and Comparative Examples 1 to 3, shear strength (shear strength, N/mm 2 ) was measured in the following manner . Measurements were made using SAICAS (Surface and Interfacial Charaterization Analysis System) equipment (SAICAS EN-EX, Dipla Wintes Japan). Specifically, the force applied to the blade at the interface between the first negative electrode active material layer and the second negative electrode active material layer while cutting each negative electrode in an inclined state from the surface toward the inside using a micro-sized blade made of diamond material The shear strength was measured through, and it is shown in Table 2.
[235]
[236]
Experimental Example 4: Evaluation of discharge capacity and capacity retention rate according to C-Rate
[237]
Using the negative electrodes of Examples 1 to 5 and Comparative Examples 1 to 3, batteries were prepared as follows, respectively.
[238]
Li[Ni 0.6 Mn 0.2 Co 0.2 ]O 2 was used as a cathode active material . A positive electrode slurry was prepared by mixing the positive electrode active material, carbon black as a conductive material, and polyvinylidene fluoride (PVdF) as a binder in a weight ratio of 94:4:2 with solvent N-methyl-2 pyrrolidone.
[239]
The prepared positive electrode slurry was applied to an aluminum metal thin film as a positive electrode current collector having a thickness of 15 μm and dried. At this time, the temperature of the circulated air was 110°C. Subsequently, it was rolled and dried in a vacuum oven at 130° C. for 2 hours to form a positive electrode active material layer.
[240]
Each of the negative electrodes of Examples 1 to 5 and Comparative Examples 1 to 3, the positive electrode and the porous polyethylene separator prepared above were assembled using a stacking method, and the assembled battery was charged with an electrolyte solution (ethylene carbonate (EC) / ethyl methyl carbonate). (EMC) = 1/2 (volume ratio), a lithium secondary battery was prepared by injecting lithium hexafluorophosphate (LiPF 6 1 mol).
[241]
1) Evaluation of discharge capacity according to C-Rate
[242]
The charge C-rate was fixed at 0.2C, and the discharge C-rate was increased from 0.2C to 2.0C. After measuring the 2.0C discharge capacity (%) against the 0.2C discharge capacity for each lithium secondary battery, the table 2.
[243]
2) Evaluation of capacity retention rate (life characteristics)
[244]
Each lithium secondary battery was charged and discharged under the following conditions.
[245]
Each lithium secondary battery was charged/discharged at 0.33C/0.33C in a voltage range of 4.25V to 2.8V at 45°C as a condition for one cycle, and a total of 100 cycles were performed. Thereafter, the discharge capacity (capacity retention rate) after 100 cycles was evaluated based on 100% of the discharge capacity after 1 cycle, and is shown in Table 2.
[246]
[247]
[Table 2]
Cathode Adhesion (gf/20mm) Adhesion between the first negative active material and the second negative active material
(N/mm 2 ) 2.0C discharge capacity ratio (%) Capacity retention rate (%)
Example 1 38.9 2.2 93.4 96.6
Example 2 38.1 2.1 92.9 95.9
Example 3 37.3 2.2 94.1 97.2
Example 4 36.2 2.4 95.3 98.5
Example 5 40.2 2.5 93.0 96.2
Comparative Example 1 35.6 1.9 92.6 95.8
Comparative Example 2 36.5 0.6 88.4 92.9
Comparative Example 3 36.8 0.8 91.9 94.8
[248]
Referring to Table 2, when the second negative active material layer includes the carbon nanotube structure, the input/output characteristics and lifespan characteristics of the battery are improved, and the negative electrode adhesive strength and the adhesive strength between the first negative active material layer and the second negative active material layer are improved. It can be seen that this can be improved.
claims
[Claim 1]
A negative electrode current collector, a first negative electrode active material layer disposed on the negative electrode current collector, and a second negative active material layer disposed on the first negative electrode active material layer, wherein the second negative electrode active material layer includes a second negative electrode active material, and a second conductive material, wherein the second negative electrode active material includes a silicon-based active material, the silicon-based active material includes SiO X (0≤X<2), and the second conductive material contains 2 to 5,000 single-walled carbon atoms. A negative electrode comprising: a carbon nanotube structure in which nanotube units are bonded side by side, wherein the carbon nanotube structure is contained in an amount of 0.01% to 1.0% by weight in the second negative electrode active material layer.
[Claim 2]
The negative electrode according to claim 1, wherein the carbon nanotube structures are connected to each other to form a network structure in the second negative electrode active material layer.
[Claim 3]
The negative electrode according to claim 1, wherein the single-walled carbon nanotube units are bonded in a state in which long axes of the single-walled carbon nanotube units are arranged parallel to each other in the carbon nanotube structure.
[Claim 4]
The negative electrode according to claim 1, wherein the carbon nanotube structure has an average length of 1 μm to 500 μm.
[Claim 5]
The negative electrode according to claim 1, wherein the carbon nanotube structure has an average length of 2 μm to 50 μm.
[Claim 6]
The cathode according to claim 1, wherein the carbon nanotube structure has an average diameter of 2 nm to 500 nm.
[Claim 7]
The cathode according to claim 1, wherein the carbon nanotube structure has an average diameter of 5 nm to 200 nm.
[Claim 8]
The negative electrode according to claim 1, wherein the average diameter of the single-walled carbon nanotube units in the carbon nanotube structure is 0.5 nm to 10 nm.
[Claim 9]
The negative electrode according to claim 1, wherein the carbon nanotube structure is a carbon nanotube structure in which 2 to 50 single-walled carbon nanotube units are bonded to each other.
[Claim 10]
The method according to claim 1, wherein the first negative active material layer includes a first negative active material and a first conductive material, the first conductive material is the carbon nanotube structure, multi-walled carbon nanotube units, graphene, and carbon black A cathode comprising at least one selected from the group consisting of:
[Claim 11]
The negative electrode according to claim 1, wherein the first negative electrode active material layer has a thickness of 1 μm to 100 μm.
[Claim 12]
The negative electrode according to claim 1, wherein the second negative electrode active material layer has a thickness of 1 μm to 100 μm.
[Claim 13]
A secondary battery comprising the anode of claim 1.