KR102111480B1 - Negative electrode for secondary battery and secondary battery comprising the same - Google Patents

Abstract

The present invention includes a current collector, a first negative electrode active material layer positioned on the current collector, and a second negative electrode active material layer positioned on the first negative electrode active material layer, wherein the first negative electrode active material layer and the second negative electrode active material Each layer independently includes a negative electrode active material, a styrene-butadiene rubber binder and a cellulose compound, and the first negative electrode active material layer includes a styrene-butadiene rubber binder in a higher content than the second negative electrode active material layer, In addition, the cellulose-based compound included in the first cathode active material layer provides a negative electrode for a secondary battery having a lower degree of substitution of a carboxyl group in a structural unit and a secondary battery including the same, compared to the cellulose-based compound included in the second anode active material layer. The negative electrode for the secondary battery includes two layers of active material having an optimized composition according to the position of the negative electrode active material layer, thereby improving adhesion between the negative electrode active material layer and the current collector, and as a result, capacity characteristics, life characteristics and output characteristics of the battery. Can greatly improve.

Description

A cathode for a secondary battery and a secondary battery comprising the same {NEGATIVE ELECTRODE FOR SECONDARY BATTERY AND SECONDARY BATTERY COMPRISING THE SAME}

The present invention relates to a negative electrode for a secondary battery capable of improving capacity characteristics, life characteristics, and output characteristics when applying a battery by improving the adhesion between a negative electrode active material layer and a current collector without increasing the binder during high loading, and a secondary battery including the same will be.

As technology development and demand for mobile devices increase, the demand for secondary batteries as an energy source is rapidly increasing. Among these secondary batteries, lithium secondary batteries having high energy density and voltage, long cycle life, and low self-discharge rate have been commercialized and widely used. In particular, as the market has expanded from a small lithium secondary battery used in portable devices to a large secondary battery used in automobiles, a high capacity and high output technology of an anode active material is required. Therefore, the development of non-carbon-based anode active materials is progressing around silicon, tin, germanium, zinc, and lead, which have a higher theoretical capacity than carbon-based anode active materials.

Generally, a negative electrode of a lithium secondary battery is formed by directly applying and drying a negative electrode active composition and a binder for dispersing a negative electrode in a negative electrode current collector, or by applying and drying a negative electrode forming composition on a separate support. , It is formed by laminating the film peeled from the support on the current collector. At this time, the binder performs a function of maintaining the binding between the negative electrode active material particles as well as the binding between the negative electrode active material particles and the current collector, which greatly affects the performance of the electrode.

Polyvinylidene fluoride (PVdF) polymer, which is usually used as a binder for lithium secondary batteries, is electrochemically stable, but is an environmental problem that needs to be dissolved in an organic solvent such as NMP (N-methyl-2-pyrrolidone) to prepare a composition. There is this.

Accordingly, recently, a method of forming a cathode using styrene-butadiene rubber (SBR) as an aqueous binder using water as a dispersion medium has been used. The SBR-based binder is not only electrochemically stable, but also has excellent adhesiveness, so it can exhibit an equivalent level of adhesion even in small amounts compared to PVdF. However, since the SBR-based binder uses water as a dispersion medium, a cellulose-based compound such as carboxymethylcellulose (CMC) is used as a thickener to control the viscosity of the electrode-forming composition. As described above, when styrene-butadiene rubber (SBR) is used as a binder and a cellulose-based compound is used as a thickener, the risk of battery rupture is reduced, battery capacity can be increased, and solids sedimentation is suppressed to prevent the formation of a negative electrode composition. The viscosity of the upper and lower portions, and the stabilization in which the dispersion state lasts for a long time can be achieved.

On the other hand, in the lithium secondary battery, it is the resistance characteristic of the battery that determines the output performance, and this resistance characteristic is greatly influenced by the dispersion state of the materials in the active material layer of the positive electrode or the negative electrode. When the active material, the conductive material and the binder present in the active material layer do not have a uniform dispersion state and are agglomerated, a channel in which an electric current can flow in the electrode is not formed locally, thereby increasing the resistance inside the battery or generating a current concentration phenomenon. The performance and stability of the battery is deteriorated. Particularly, in the case of the composition for forming an aqueous negative electrode, since water is used as a dispersion medium, dispersibility of the conductive material and the negative electrode active material is significantly low.

In general, the cellulose-based compound used with the SBR-based binder in the composition for forming an aqueous cathode is more advantageous in improving dispersibility of the composition for forming an anode because it exhibits some dispersibility in itself. However, when the content of the cellulose-based compound is increased in order to increase the dispersibility of the active material, the conductive material, and the binder in the composition for forming a negative electrode, the weight ratio of the active material is reduced, resulting in a decrease in electrode capacity and a decrease in battery characteristics. Occurs. In addition, there is a problem in that the content of the solid content is lowered when the viscosity is adjusted by using water to prevent the viscosity from being increased due to the increase in the thickener.

Accordingly, it is required to develop a composition of a composition capable of uniformly dispersing components in a composition for forming a negative electrode and a manufacturing method thereof without deteriorating a solid content and a binding force.

Korea Patent Publication No. 2014-0095804 (2014.08.04 published)

The first technical problem to be solved by the present invention is to improve the adhesion between the negative electrode active material layer and the current collector without increasing the binder during high loading, and for secondary batteries capable of improving capacity characteristics, life characteristics and output characteristics when applying batteries. It is to provide a cathode and a method of manufacturing the same.

The second technical problem to be solved by the present invention is to provide a lithium secondary battery including the negative electrode.

In order to solve the above problems, according to an embodiment of the present invention

Current collector;

A first cathode active material layer positioned on the current collector; And

It includes a second cathode active material layer located on the first cathode active material layer,

The first negative electrode active material layer and the second negative electrode active material layer each independently include a negative electrode active material, a styrene-butadiene rubber binder and a cellulose-based compound,

The first cathode active material layer includes a styrene-butadiene rubber binder in a higher content than the second cathode active material layer,

The cellulose-based compound contained in the first cathode active material layer provides a negative electrode for a secondary battery having a lower degree of substitution (DS) of carboxyl groups in the structural unit than the cellulose-based compound included in the second cathode active material layer. .

In addition, according to another embodiment of the present invention, preparing a first negative electrode active material layer comprising a negative electrode active material, a styrene-butadiene rubber binder and a cellulose-based compound on a current collector; And forming a second negative electrode active material layer including a negative electrode active material, a styrene-butadiene rubber binder, and a cellulose-based compound on the first negative electrode active material layer, wherein the first negative electrode active material layer is a second negative electrode active material layer. The styrene-butadiene-based rubber binder in a higher content than the layer, and the cellulose-based compound contained in the first negative electrode active material layer has a degree of substitution of the carboxyl group in the structural unit compared to the cellulose-based compound contained in the second negative electrode active material layer. It is to provide a method for manufacturing a negative electrode for a secondary battery as described above.

Furthermore, according to another embodiment of the present invention, there is provided a lithium secondary battery including the negative electrode.

The negative electrode for a secondary battery according to the present invention includes two active material layers having an optimized composition according to the position of the negative electrode active material layer, thereby improving adhesion between the negative electrode active material layer and the current collector, and as a result, capacity characteristics and life characteristics of the battery. And the output characteristics can be greatly improved.

The following drawings attached to this specification are intended to illustrate preferred embodiments of the present invention, and serve to further understand the technical idea of the present invention together with the contents of the above-described invention, so the present invention is limited to those described in those drawings. It should not be construed as limited.
1 is a graph showing the results of evaluating the adhesion in the negative electrode prepared in Example 1 and Comparative Examples 1 and 2.
2 is a graph evaluating the effect of improving the capacity characteristics of the negative electrode batteries prepared in Example 1 and Comparative Examples 1 and 2.

Hereinafter, the present invention will be described in more detail to aid understanding of the present invention.

The terms or words used in the present specification and claims should not be interpreted as being limited to ordinary or lexical meanings, and the inventor can appropriately define the concept of terms in order to best describe his or her invention. Based on the principle that it should be interpreted as meanings and concepts consistent with the technical spirit of the present invention.

In the present invention, by implementing a negative electrode having a multi-layer structure using a composition for forming a negative electrode active material layer having a composition optimized according to the thickness of the negative electrode, the adhesive force between the negative electrode active material layer and the current collector, which is a disadvantage of the high loading electrode, is increased without increasing the binder content. It is possible to improve the capacity characteristics, life characteristics and output characteristics when applying the battery.

Specifically, the negative electrode for a secondary battery according to an embodiment of the present invention

Current collector;

A first cathode active material layer positioned on the current collector; And

It includes a second cathode active material layer located on the first cathode active material layer,

The first negative electrode active material layer and the second negative electrode active material layer each independently include a negative electrode active material, a styrene-butadiene rubber binder and a cellulose-based compound,

The first cathode active material layer includes a styrene-butadiene rubber binder in a higher content than the second cathode active material layer,

The cellulose-based compound contained in the first cathode active material layer has a lower degree of substitution of the carboxyl group in the structural unit than the cellulose-based compound contained in the second cathode active material layer.

In the present invention, the structural unit means a repeating unit of a monomer-derived structure forming a central skeleton of a polymer, and for example, in the case of carboxymethyl cellulose, a portion of the hydroxy group is substituted with a carboxymethyl group. The repeating unit of the structure derived from the lanose (glucopyranose) monomer may be a structural unit.

With the high capacity of the lithium secondary battery, it is required to realize the performance of the high loading anode. When a negative electrode active material layer-forming composition was coated with high loading to prepare a negative electrode, the adhesion between the current collector and the active material layer was lowered as the binder was distributed to the electrode surface portion during the drying process. In order to solve this problem, when manufacturing a high loading electrode, the binder ratio was increased, but in this case, there was a problem that the resistance of the electrode increased.

On the other hand, the negative electrode for a secondary battery according to an embodiment of the present invention is located on the current collector, and the content of the binder in the first negative electrode active material layer directly contacting the current collector is increased to increase the adhesion between the current collector and the negative electrode active material layer. In addition, for the second cathode active material layer located on the first cathode active material layer and coming into contact with the electrolyte, the content of the anode active material layer is relatively increased instead of reducing the content of the binder, thereby improving the output characteristics of the cathode. have.

In addition, a cellulose-based compound used as a binder or a thickener in the production of a negative electrode, for example, carboxymethyl cellulose, may have 3 or less carboxymethyl groups per cellulose repeating unit substituted for a hydroxy group present in a repeating structural unit. . The average number of carboxymethyl groups or carboxyl groups substituted with hydroxy groups in the structural unit of cellulose is referred to as degree of substitution (DS). The cellulose-based compound has a different solubility in water depending on the degree of substitution. In general, the higher the degree of substitution, the higher the degree of hydrophilicity, and the solubility in water increases, but as much as it is familiar with water, there is a problem in that migration becomes severe as moisture evaporates during the drying process. On the other hand, the cellulose-based compound having a low degree of substitution may exhibit better adhesion since the phenomenon of migration in the drying process of moisture is small. However, the cellulose-based compound having a low degree of substitution has poor solubility in water and exists in a microgel state. When such a microgel is present on the electrode surface, the surface on which the negative electrode active material is not coated is exposed after drying. Or, it may cause poor quality of the electrode, such as agglomeration of the conductive material and the active material around the microgel. In addition, since the solubility in water is low, a larger amount of water is required to dissolve the same amount of the cellulose-based compound, and as a result, the solid content of the composition for forming a negative electrode active material layer decreases and the dispersibility in the composition decreases. there is a problem.

On the other hand, the negative electrode for a secondary battery according to an embodiment of the present invention is located on the current collector, and in the first negative electrode active material layer directly contacting the current collector, the solubility in water is low and migration is less likely to occur. A low-cellulose-based compound is applied, and located on the first negative electrode active material layer, the second negative electrode active material layer in contact with the electrolyte has high solubility in water and can secure electrode quality. The output characteristics can be improved by applying a system compound.

Furthermore, in the negative electrode according to an embodiment of the present invention, the above-described binder content in the first and second negative electrode active material layers and the degree of substitution of the cellulose-based compound contained in each of the negative electrode active material layers are the thickness of the negative electrode. By optimizing and controlling accordingly, the adhesion in the negative electrode, the resistance characteristic and the loading amount can be increased, and as a result, the output characteristics of the battery can be further improved.

Specifically, in the negative electrode according to an embodiment of the present invention, the first negative electrode active material layer is more than 100 parts by weight and less than 400 parts by weight with respect to 100 parts by weight of the styrene-butadiene rubber binder contained in the second negative electrode active material layer As it may include a styrene-butadiene-based rubber binder, more specifically, it may include a styrene-butadiene-based rubber binder with a content of 120 parts by weight to 300 parts by weight. The first and second negative electrode active material layers respectively contain styrene-butadiene-based rubber so as to have the above-mentioned content difference, thereby increasing the adhesive strength between the current collector and the negative electrode active material layer without increasing the amount of binder used compared to the conventional negative electrode manufacturing. It can show capacity characteristics, life characteristics and output characteristics.

More specifically, the second cathode active material layer may include the above-mentioned styrene-butadiene-based rubber binder in 0.5 to 5% by weight based on the total weight of the second anode active material layer under conditions that satisfy the difference in the binder content. have. If the content of the binder is less than 0.5% by weight, it is difficult to exhibit the effect of the adhesion required in the electrode, and if it exceeds 5% by weight, the effect of improving the capacity characteristics of the battery according to the reduction in the binder content may be negligible.

The styrene-butadiene-based rubber contained in the first and second negative electrode active material layers is an elastic polymer containing a styrene-derived structural unit and a butadiene-derived structural unit. In the styrene-butadiene-based rubber, butadiene exhibits excellent adhesion properties, while controlling the content of the structural unit derived from butadiene in the rubber, while reducing the content of the binder itself, increasing the binding force between the negative electrode active material and the adhesion between the negative electrode active material layer and the current collector, In addition, the structural stability of the negative electrode active material layer can be improved to improve the overall characteristics of the battery. The styrene-butadiene-based rubber may include styrene-derived structural units and butadiene-derived structural units in a weight ratio of 1: 1 to 3: 1, specifically 1: 1 to 2.5: 1. If, in the styrene-butadiene-based rubber, if the proportion of styrene-derived structural units is greater than the above range, the effect of improving adhesion may be insignificant due to the relative decrease in the proportion of butadiene-derived structural units, whereas the proportion of structural units derived from butadiene is If it is larger than the range, there is a fear of lowering the structural stability in the negative electrode active material layer.

In the styrene-butadiene rubber, the structural unit derived from butadiene is specifically 1,3-butadiene, isoprene, 2,3-dimethyl-1,3-butadiene, or 2-ethyl-1,3-butadiene. , 3-butadiene or a derivative thereof.

In addition, the structural unit derived from styrene is styrene, α-methylstyrene, p-methyl styrene, 3-methylstyrene, 4-methylstyrene, 4-propylstyrene, 1-vinylnaphthalene, 4-cyclohexylstyrene, 4- (p -Methylphenyl) may be a structural unit derived from styrene, which is an aromatic vinyl compound such as styrene, 1-vinyl-5-hexylnaphthalene, or derivatives thereof.

In addition, according to an embodiment of the present invention, the styrene-butadiene-based rubber included in the first and second negative electrode active material layers is additionally a structure derived from (meth) acrylic acid ester in addition to the styrene-derived structural unit and butadiene-derived structural unit. It may further include a unit.

Specifically, the structural unit derived from the (meth) acrylic acid ester may be included in an amount of 30% by weight or less, more specifically 25% by weight or less, based on the total weight of the styrene-butadiene rubber. As described above, when the styrene-butadiene-based rubber in the first cathode active material layer further includes a structural unit derived from (meth) acrylic acid ester, the resistance of the styrene-butadiene-based rubber is reduced due to the excellent conductivity of the (meth) acrylic acid ester itself. It can lower the resistance in the negative electrode active material layer.

In addition, the structural unit derived from the (meth) acrylic acid ester is specifically, such as ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, ethyl methacrylate, butyl methacrylate, hexyl methacrylate or 2-ethylhexyl methacrylate, etc. It may be a structural unit derived from a (meth) acrylic acid ester-based compound.

In addition, the styrene-butadiene-based rubber having the above-described configuration may have a weight average molecular weight (Mw) of 500,000 g / mol to 2,000,000 g / mol. If the Mw of the styrene-butadiene-based rubber is less than 500,000 g / mol, the adhesive strength is insignificant, and if it exceeds 2,000,000 g / mol, there is a fear that the viscosity in the composition increases, resulting in decreased coating properties. More specifically, the styrene-butadiene rubber may have a weight average molecular weight (Mw) of 800,000 g / mol to 1,500,000 g / mol. In the present invention, the weight average molecular weight (Mw) is a polystyrene equivalent weight average molecular weight (Mw) measured by gel permeation chromatography (GPC).

In addition, styrene-butadiene-based rubber having the above-described configuration may be used in a particulate form. In the manufacture of a negative electrode, a binder was dissolved in a solvent and used. However, in this case, since the binder surrounds the surface of the negative electrode active material, there is a problem that the contact between the negative electrode active material and the electrolyte is prevented. On the other hand, in the present invention, by using a particulate styrene-butadiene-based rubber, by making point contact with the negative electrode active material, it is possible to increase the possibility of contact between the negative electrode active material and the electrolytic solution while exhibiting adhesion between the active materials.

Specifically, the styrene-butadiene-based rubber may have an average particle diameter (D ₅₀ ) of 0.001 to 0.04 times the average particle diameter (D ₅₀ ) of the negative electrode active material. Of the negative electrode active material average particle diameter (D ₅₀₎ compared to a styrene-butadiene-average particle diameter (D ₅₀₎ of the type rubber is less than 0.001 times the styrene of the fine particulate - the butadiene-based rubber to prevent the contact with the surrounding the negative electrode active material the surface of the active material and the electrolyte There is a concern, and when it exceeds 0.04 times the average particle diameter (D ₅₀ ) of the negative electrode active material, the point-to-tack ratio with the active material is low, making it difficult to provide excellent adhesion. More specifically, it may be 0.005 times to 0.04 times, and more specifically, the styrene-butadiene-based rubber may have an average particle diameter (D ₅₀ ) of ₅₀ nm to 600 nm. If the average particle diameter of the styrene-butadiene rubber exceeds the above range, problems may occur in the process and adhesion due to the large particle diameter, and if it is less than the above range, dispersibility decreases due to agglomeration between fine particles and uniform dispersion within the anode active material layer This is difficult and there is a fear of deterioration of adhesive properties. More specifically, the average particle diameter (D ₅₀ ) of the styrene-butadiene-based rubber may be 90 nm to 400 nm.

In the present invention, the average particle diameter can be defined as a particle size at 50% of the particle size distribution. In addition, the average particle diameter (D ₅₀ ) of the styrene-butadiene-based rubber particles can be measured using, for example, a laser diffraction method, and more specifically, the binder particles are dispersed in a solvent. Then, it can be introduced into a commercially available laser diffraction particle size measuring device (for example, Microtrac MT 3000) to calculate the average particle diameter (D ₅₀ ) at 50% of the particle size distribution in the measuring device.

The styrene-butadiene-based rubbers included in the first and second cathode active material layers may be identical to each other, and the styrene-butadiene-based rubber-derived styrene or butadiene-based monomers have a different structure and content, and different rubber sizes or shapes. It may be

In consideration of the remarkable effect of the properties of the binder required for each layer and the control effect, the first negative electrode active material layer in the negative electrode for a secondary battery according to an embodiment of the present invention is a styrene-derived structural unit and a butadiene-derived structure It may include a first styrene-butadiene-based rubber having an average particle diameter of 50 nm to 600 nm, including a unit in a weight ratio of 1: 1 to 3: 1, specifically 1: 1 to 2.5: 1, and also a second negative electrode active material layer. The structural unit derived from (meth) acrylic acid ester is further included in an amount of 30% by weight or less and more specifically 25% by weight or less based on the total weight of styrene-butadiene rubber together with the structural unit derived from styrene and structural units derived from butadiene. , May include a second styrene-butadiene rubber having an average particle diameter of 50 nm to 600 nm, respectively.

In addition, in the negative electrode according to an embodiment of the present invention, the first negative electrode active material layer specifically includes a low-substituted first cellulose-based compound having a degree of substitution of less than 1 carboxyl group in the structural unit, and a second negative electrode active material The layer may include one or more highly substituted second cellulose-based compounds.

The first cellulose has excellent solubility in water, and thus has good effect as a thickener, and has excellent affinity for an active material, thereby improving binding strength and durability of the electrode. In addition, since the viscosity of the first cellulose-based compound having a low degree of substitution is higher than that of the second cellulose-based compound having a high degree of substitution, the viscosity of the composition may be improved even in a small amount. Accordingly, the amount of the cellulosic compound in the composition for forming the first negative electrode active material layer can be reduced, and as a result, the charge in the electrode and the mobility of lithium ions are increased, so that the resistance in the electrode can be reduced. In addition, since the adhesive strength is also excellent, it is possible to prevent the active material from falling off, and it is possible to impart excellent coating properties to the composition for forming a negative electrode active material layer. The first cellulose-based compound may have a substitution degree of a carboxyl group in a structural unit of less than 1, more specifically, 0.7 to 0.95. When the degree of substitution of the first cellulose-based compound exceeds 1, the improvement effect due to the mixed use with the second cellulose-based compound having a high degree of substitution is negligible.

In addition, the first cellulose-based compound may satisfy the above-described degree of substitution and at the same time have a weight average molecular weight (Mw) of 1,500,000 g / mol to 4,000,000 g / mol. If the Mw of the first cellulose-based compound is less than 1,500,000 g / mol, there is a fear of agglomeration, and if it exceeds 4,000,000 g / mol, there is a fear of electrode failure due to cosmetic debris. More specifically, the first cellulose-based compound may have a weight average molecular weight (Mw) of 2,000,000 g / mol to 4,000,000 g / mol. In the present invention, the weight average molecular weight (Mw) is a polystyrene equivalent weight average molecular weight (Mw) measured by gel permeation chromatography (GPC).

More specifically, the first cellulose-based compound is carboxymethyl cellulose (carboxymethyl cellulose, CMC), carboxyethyl cellulose, methyl cellulose (methyl cellulose), ethyl cellulose, hydroxyethyl cellulose (hydroxylethyl cellulose), hydroxypropyl cellulose (hydroxylpropyl cellulose), or sodium carboxymethylcellulose (CMCNa), and among these, one type alone or a mixture of two or more types may be used. More specifically, the second cellulose-based compound may be carboxymethyl cellulose (CMC) or carboxymethyl cellulose sodium salt (CMCNa). Carboxymethylcellulose and sodium salt of carboxymethylcellulose, especially carboxymethylcellulose, have higher solubility in water and easier ionization than other cellulosic compounds. Accordingly, the first cellulose-based compound may be more specifically carboxymethylcellulose that satisfies all of the optimum conditions of the substitution degree and the weight average molecular weight.

The first cellulose-based compound as described above may be included in 0.5% to 10% by weight relative to the total weight of the first cathode active material layer. When the content of the first cellulose-based compound is less than 0.5% by weight, the effect of increasing the adhesion force is negligible, and when it exceeds 10% by weight, there is a fear that the capacity characteristics of the battery decrease.

On the other hand, the second cellulose-based compound in the second cathode active material layer may have a substitution degree of 1 to 1.2 of the carboxyl group in the structural unit of the polymer constituting the cellulose-based compound. When a second cellulose-based compound having a degree of substitution within the above range is applied, the solubility of cellulose increases in water, thereby improving dispersibility of inorganic particles such as a negative electrode active material and a conductive material, and as a result, decreases internal resistance of the electrode. The output characteristics are improved. If the substitution degree is less than 1 beyond the high degree of carboxyl group substitution in the above range, the adsorption power to the anode active material is low and the dispersibility improvement effect of the anode active material is insignificant. Difficult to increase the resistance in the electrode.

In addition, the second cellulose-based compound may satisfy the above-mentioned conditions of substitution, and may have a weight average molecular weight (Mw) of 500,000 g / mol to 1,500,000 g / mol. If the Mw of the cellulose-based compound is less than 500,000 g / mol, the thickening effect of the composition for forming a negative electrode is negligible, and if it exceeds 1,500,000 g / mol, the effect of dispersing the conductive material may be lowered. More specifically, the second cellulose-based compound may have a weight average molecular weight (Mw) of 800,000 g / mol to 1,500,000 g / mol. In the present invention, the weight average molecular weight (Mw) is a polystyrene equivalent weight average molecular weight (Mw) measured by gel permeation chromatography (GPC).

Specifically, the second cellulose-based compound may be carboxymethyl cellulose (CMC), carboxyethyl cellulose, methyl cellulose, ethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, or sodium carboxymethyl cellulose (CMCNa), One of these may be used alone or a mixture of two or more. More specifically, the second cellulose-based compound may be carboxymethylcellulose (CMC) or carboxymethylcellulose sodium salt (CMCNa), and more specifically, it satisfies all of the optimum conditions of the above substitution degree and weight average molecular weight. Carboxymethylcellulose.

The second cellulose-based compound as described above may be included in 0.5% to 10% by weight relative to the total weight of the first cathode active material layer. If the content of the second cellulose-based compound is less than 0.5% by weight, the effect of increasing the adhesion force is negligible, and if it exceeds 10% by weight, there is a fear of deteriorating the capacity characteristics of the battery.

In addition, in the negative electrode according to an embodiment of the present invention, the average thickness ratio of the second negative electrode active material layer to the average thickness of the first negative electrode active material layer may be greater than one. If the average thickness of the second anode active material layer is 1 or less compared to the average thickness of the first anode active material layer, the effect of reducing migration of the binder in the active material layer due to the formation of the first anode active material layer and thus the adhesion between the current collector and the first cathode active material layer The improvement effect may be negligible. When considering the remarkable improvement effect by optimizing by combining the content of the binder and the degree of substitution of the cellulose-based compound required according to the thickness in the negative electrode, the average thickness ratio of the first and second cathode active material layers is more specifically 1: 9 to 1: 1.5, more specifically 1: 9 to 1: 2, and most specifically 1: 7 to 3: 7.

More specifically, in the negative electrode according to an embodiment of the present invention, the average thickness of the second negative electrode active material layer is in a condition that satisfies the above-mentioned thickness ratio range, specifically 20 μm to 500 μm, more specifically 30 It may be µm to 450 µm, and more specifically 30 µm to 300 µm.

In addition, in the negative electrode according to an embodiment of the present invention, the first and second negative electrode active material layers may further include a negative electrode active material and optionally a conductive material together with the styrene-butadiene rubber and cellulose-based compounds described above. have.

The negative electrode active material included in the first and second negative electrode active material layers is capable of reversible intercalation and deintercalation of lithium, and can be used without particular limitations as long as it is usually used as a negative electrode active material for lithium secondary batteries. Specific examples include crystalline carbon such as amorphous, plate-like, flake-like, spherical or fibrous natural graphite or artificial graphite; Amorphous carbon such as soft carbon (low-temperature calcined carbon), hard carbon, mesophase pitch carbide, calcined coke, and the like; Metallic compounds capable of alloying with lithium, such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloy, Sn alloy or Al alloy; A composite comprising the metallic compound and a carbonaceous material; Or SiOx (0 <x <2) silicon oxide, and the like, or a mixture of two or more of them may be used. Among the negative electrode active materials, when considering the effect of improving the battery properties of the active material itself, it may be a graphite-based material such as natural graphite or artificial graphite.

The negative electrode active material as described above may be specifically included in 70% to 98% by weight relative to the total weight of each negative electrode active material layer. If the content of the negative electrode active material is less than 70% by weight, the output characteristics of the battery may be deteriorated, and if it exceeds 98% by weight, there is a fear that the active material is detached due to a decrease in the binding force in the electrode and peeling with the current collector. More specifically, the negative electrode active material may be included in a weight range of 70% to 96.5% by weight relative to the total weight of the first negative electrode active material layer in the first negative electrode active material layer, and 80% to 97% by weight in the second negative electrode active material layer, respectively. The negative electrode active material included in each layer may be the same or different.

In addition, the conductive material is used to impart conductivity to the positive electrode, and in the battery configured, it can be used without particular limitation as long as it has electronic conductivity without causing chemical changes. Specifically, the conductive material is 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, carbon nanofiber, carbon nanotube, and carbon nanorod; Metal powders or metal fibers such as copper, nickel, aluminum, and silver; Conductive whiskers such as zinc oxide whisker, calcium carbonate whisker, titanium dioxide whisker, silicon oxide whisker, silicon carbide whisker, aluminum borate whisker, magnesium borate whisker, potassium titanate whisker, silicon nitride whisker, silicon carbide whisker, alumina whisker, etc. (Whisker); Conductive metal oxides such as titanium oxide; Or it may be a conductive polymer such as a polyphenylene derivative, one of them may be used alone or a mixture of two or more. Among these, a carbon-based material such as carbon black may be more preferable when considering the remarkable improvement effect due to the use of a conductive material and the high-temperature drying process in the anode manufacturing process.

In addition, in the composition for forming a negative electrode of a lithium secondary battery according to an embodiment of the present invention, the conductive material may have an aspect ratio of more than 1, more specifically, more than 1 and 200 or less. In the present invention, the aspect ratio represents the ratio of the diameter to the length, and when the aspect ratio is 1, it represents a spherical shape, and when it is larger than 1, it represents a fibrous form. By having such a specific shape, it is possible to impart elasticity to the space between the particles, and to compensate for the breakage of the conductive path due to the volume expansion of the negative electrode active material. If the aspect ratio of the conductive material is 1 or less, the electrical conductivity in the longitudinal direction of the conductive material becomes insufficient, and during expansion and contraction accompanying charging and discharging of the negative electrode active material, it is difficult to maintain electrical conduction between the negative electrode active materials, and as a result, cycle characteristics may deteriorate. have. In addition, when the aspect ratio of the conductive material exceeds 200, uniform dispersion in the composition may be difficult, so considering the remarkable effect of using a specific type of conductive material, the conductive material has an aspect ratio of 5 to 200 It may be more preferable in terms of the effect of improving cycle characteristics. Specifically, the conductive material may be any one or a mixture of two or more selected from the group consisting of carbon nanofibers, carbon nanotubes, and carbon nanorods that satisfy the above aspect ratio conditions. have.

The conductive material may be included in an amount of 0.1% to 10% by weight, respectively, based on the total weight of each negative electrode active material layer. When the content of the conductive material is less than 0.1% by weight, the effect of improving the conductivity and improving the cycle characteristics according to the use of the conductive material is negligible, and when it exceeds 10% by weight, the reaction between the conductive material and the electrolyte due to the increase in the specific surface area of the conductive material There is a possibility that the cycle characteristics decrease due to the increase.

On the other hand, in the negative electrode for a secondary battery according to an embodiment of the present invention, the negative electrode current collector as long as it has a high conductivity without causing a chemical change in the battery can be used without particular limitation, specifically, copper, stainless Steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium or silver, aluminum-cadmium alloy, or the like may be used. The negative electrode current collector as described above may have various forms, and specifically, may be in the form of a film, sheet, foil, net, porous body, foam, or nonwoven fabric.

In addition, the current collector may have a thickness of 3 μm to 100 μm, and fine irregularities or patterns may be formed on the surface of the current collector to enhance the bonding force of the negative electrode active material.

In addition, the negative electrode for a secondary battery according to an embodiment of the present invention, preparing a first negative electrode active material layer comprising a negative electrode active material, a styrene-butadiene rubber binder and a cellulose-based compound on a current collector (step 1) ; And forming a second cathode active material layer comprising the anode active material, a styrene-butadiene rubber binder, and a cellulose-based compound on the first cathode active material layer (step 2). . In this case, the first negative electrode active material layer includes a first styrene-butadiene rubber binder in a higher content than the second negative electrode active material layer, and the cellulose-based compound contained in the first negative electrode active material layer has a second negative electrode active material. The degree of substitution of the carboxyl group in the structural unit may be lower than that of the cellulose-based compound included in the layer.

Hereinafter, each step will be described in detail. In the method of manufacturing a negative electrode for a secondary battery according to an embodiment of the present invention, step 1 is a step of forming a first negative electrode active material layer on the current collector.

Specifically, the first negative electrode active material layer is prepared by dissolving or dispersing a negative electrode active material, a styrene-butadiene rubber binder and a cellulose compound in a solvent to prepare a composition for forming a first negative electrode active material layer, and forming the prepared first negative electrode active material layer The composition is applied to the current collector and dried to form directly on the current collector; Alternatively, the composition for forming the first cathode active material layer formed may be coated on a separate support and dried to produce a film, and the formed film may be peeled from the support and then laminated and rolled on a current collector.

At this time, the types and contents of the negative electrode active material, styrene-butadiene rubber binder, and cellulose-based compound usable in the preparation of the composition for forming the first negative electrode active material layer are the same as described in the first negative electrode active material layer.

The solvent can be used without particular limitation, as long as it is usually used in the manufacture of a negative electrode. Specifically, organic solvents such as dimethyl sulfoxide (DMSO), N-methylpyrrolidone (NMP), or acetone, or water-based solvents such as water and isopropyl alcohol These may be used alone, or a mixture of two or more of them may be used. More specifically, the solvent may be an aqueous solvent, and more specifically, water.

In addition, the solvent may be included in an amount to have an appropriate viscosity in consideration of the applicability and fairness of the composition for forming the first cathode active material layer. Specifically, the viscosity (B-type viscometer, 23 ± 5 ° C, 12rpm) of the composition for forming the first negative electrode active material layer may be added in an amount to be 550 cps to 10,000 cps. If the content of the solvent is too large and the viscosity of the composition for forming the first negative electrode active material layer is less than 550 cps, there is a concern that the process time may be long, and when it exceeds 10,000 cps, the applicability decreases, thereby causing the first negative electrode active material layer There is a concern that the thickness nonuniformity of the film may increase.

The above-mentioned negative electrode active material, styrene-butadiene-based rubber binder, cellulose-based compound, and optionally a conductive material and a solvent may be mixed according to a conventional mixing process.

Specifically, a negative electrode active material, a styrene-butadiene rubber binder cellulose-based compound, and optionally a conductive material may be simultaneously added and mixed in a solvent, or after the addition of a cellulose-based compound and optionally a conductive material in a solvent, the first mixture is performed. Subsequently, the styrene-butadiene-based rubber binder may be added for second mixing, and then a negative electrode active material may be added for third mixing to perform a third mixing process. When preparing the composition for forming the first cathode active material layer through the multi-step mixing process as described above, dispersibility of components in the composition may be increased. Specifically, dispersion of the conductive material having low dispersibility can be promoted by pre-dispersing the cellulose-based compound together with the conductive material. In addition, the cellulose-based compound pre-dispersed in the solvent is easily adsorbed to the negative electrode active material added later to maintain repulsion between the negative electrode active materials, thereby uniformly dispersing the negative electrode active material in the composition. In addition, the cellulose-based compound adsorbed on the surface of the negative electrode active material holds more binder around the negative electrode active material, thereby increasing the adhesive force around the negative electrode active material compared to other materials. Accordingly, when the negative electrode active material is an active material having a large volume change due to charging and discharging, such as silicon or silicon oxide, the volume change of the negative electrode active material due to charging and discharging is minimized, and mechanical detachment and cracks in the electrode are prevented to prevent battery life. The properties can be improved.

In addition, the mixing process may be performed by a high-speed rotation at a rotation speed of 1,000 to 25,000rpm and mixing to increase the dispersibility of the negative electrode active material.

In addition, the application of the composition for forming the first cathode active material layer to the current collector or the support may be performed using a conventional slurry coating method. Specific examples of the slurry coating method may include bar coating, spin coating, roll coating, slot die coating, or spray coating, and one or more of these methods may be mixed.

In addition, when applying the composition for forming the first negative electrode active material layer, it may be desirable to apply an appropriate thickness in consideration of the loading amount and thickness of the active material in the first manufactured negative electrode active material layer.

In addition, a drying process may be performed on the coating film of the composition for forming the first cathode active material layer formed on the current collector or the support.

At this time, the drying process may be performed by a method such as heat treatment or hot air injection at a temperature to remove the moisture contained in the negative electrode together with evaporation of the solvent in the composition for forming the first negative electrode active material layer, and at the same time to increase the binding force of the binder. . However, in the drying process, before the solvent in the composition for forming the first cathode active material layer is completely evaporated and removed, specifically, the solvent is less than 100% by weight of the solvent used, more specifically, 10% to 30% by weight of the solvent used It can be carried out until evaporation. If the composition for forming the first cathode active material layer is completely dried after application, and then a subsequent application process of the composition for forming the second anode active material layer is performed, pores in the first cathode active material layer formed through the drying process As a result, the solvent in the composition for forming the second cathode active material layer is impregnated, and thus, the viscosity of the composition for forming the second anode active material layer applied on the first cathode active material layer increases, and there is a fear that the coating property is deteriorated. Specifically, the drying process may be performed at a temperature above the boiling point of the solvent and below the melting point of the binder, more specifically at 100 ° C to 150 ° C, even more specifically at a temperature of 100 ° C to 120 ° C and a pressure of 10 torr or less Under 1 hour to 50 hours.

Through the process in step 1 described above, a negative electrode active material, a styrene-butadiene rubber binder, a cellulose compound, and a first negative electrode active material layer including an electrically conductive material are formed on the current collector.

Next, step 2 in the negative electrode for a secondary battery according to an embodiment of the present invention is a step of forming a second negative electrode active material layer on the first negative electrode active material layer prepared in step 1 above.

Specifically, the second cathode active material layer is prepared by dissolving or dispersing a cathode active material, a styrene-butadiene-based rubber binder, and a cellulose-based compound in a solvent to prepare a composition for forming a second anode active material layer, and forming the prepared second cathode active material layer The composition is applied on the first cathode active material layer and then dried; Alternatively, the composition for forming the prepared second cathode active material layer is coated on a separate support and dried to produce a film, and the formed film is peeled from the support and then laminated and rolled on the first cathode active material layer to be formed. Can be. However, the step of forming the second cathode active material layer may be performed before application of the composition for forming the first cathode active material layer in step 1 and drying is completely performed.

The second cathode active material layer contains a styrene-butadiene rubber binder in a lower content than the content of the styrene-butadiene rubber binder when the first cathode active material layer is formed, and has a higher degree of substitution of the cellulose-based compound. Using the composition for forming a second cathode active material layer containing the same, and applying or laminating the prepared composition on the first cathode active material layer, it is carried out in the same manner as in the production of the first cathode active material layer. Can be. However, when the composition for forming the second cathode active material layer is applied, a thicker coating process may be performed so that the average thickness ratio with the first cathode active material layer in the negative electrode that is finally manufactured is greater than 1. In addition, the types and contents of the negative electrode active material, styrene-butadiene rubber binder and cellulosic compound that can be used when preparing the composition for forming the second negative electrode active material layer are the same as described above.

On the first cathode active material layer through the process in step 2 described above, the anode active material, a styrene-butadiene rubber binder, and optionally a conductive material, and when forming the first cathode active material layer, a styrene-butadiene rubber binder The styrene-butadiene-based rubber binder has a lower content than the content of, and the degree of substitution of the cellulose-based compound is higher than that of the cellulose-based compound in the first cathode active material layer.

The negative electrode manufactured by the above-described manufacturing method includes two active material layers having an optimized composition according to the position of the negative electrode active material layer, thereby improving the adhesion between the negative electrode active material layer and the current collector, and as a result, the output characteristics and life of the battery. Characteristics can be improved.

According to another embodiment of the present invention, an electrochemical device including the cathode is provided. The electrochemical device may be specifically a battery, a capacitor, or the like, and more specifically, a lithium secondary battery.

The lithium secondary battery specifically includes a positive electrode, a negative electrode facing the positive electrode, a separator and an electrolyte interposed between the positive electrode and the negative electrode, and the negative electrode is as described above. In addition, the lithium secondary battery may further include a battery container for storing the electrode assembly of the positive electrode, the negative electrode, and the separator, and a sealing member for sealing the battery container.

In the lithium secondary battery, the positive electrode includes a positive electrode current collector and a positive electrode active material layer located on the positive electrode current collector, and after mixing a positive electrode active material, a conductive material, a binder, and a solvent to prepare a composition for forming a positive electrode active material layer The positive electrode can be prepared by directly coating it on the positive electrode current collector or by laminating the positive electrode active material film peeled from the support on a separate support to the positive electrode current collector.

The positive electrode current collector is not particularly limited as long as it does not cause a chemical change in the battery and has conductivity. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or carbon, nickel, titanium on aluminum or stainless steel surfaces , Surface treatment with silver or the like can be used. In addition, the positive electrode current collector may have a thickness of 3 μm to 500 μm, and may form fine irregularities on the surface of the current collector to increase adhesion of the positive electrode active material. For example, it can be used in various forms such as films, sheets, foils, nets, porous bodies, foams, and nonwoven fabrics.

In addition, the positive electrode active material layer may include a positive electrode active material, a conductive material and a binder.

The positive electrode active material is lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ), Li [Ni _x Co _y Mn _z Mv] O ₂ (wherein M is selected from the group consisting of Al, Ga and In One or two or more of them; 0.3≤x <1.0, 0≤y, z≤0.5, 0≤v≤0.1, x + y + z + v = 1), Li (Li _a M _{ba-b '} M' _{b '} ) O _{2 - c} A _c (In the above formula, 0≤a≤0.2, 0.6≤b≤1, 0≤b'≤0.2, 0≤c≤0.2; M is Mn, Ni, Co , Fe, Cr, V, Cu, Zn, and one or more selected from the group consisting of Ti; M 'is one or more selected from the group consisting of Al, Mg and B, A is P, F, S and N are selected from the group consisting of one or more.) Layered compounds such as compounds substituted with one or more transition metals; Lithium manganese oxides such as the formula Li _{1 + y} Mn _2-y O ₄ (where y is 0-0.33), LiMnO ₃ , LiMn ₂ O ₃ , LiMnO ₂ ; Lithium copper oxide (Li ₂ CuO ₂ ); Vanadium oxides such as LiV ₃ O ₈ , LiFe ₃ O ₄ , V ₂ O ₅ and Cu ₂ V ₂ O ₇ ; Formula _{LiNi 1 - y M y O 2} ( wherein, Co and M, Mn, Al, Cu, Fe, Mg, B or Ga, y = 0.01 - 0.3 Im) Ni site type lithium nickel oxide which is represented by; Formula LiMn _{2 - y} M _y O ₂ (where M = Co, Ni, Fe, Cr, Zn or Ta, y = 0.01-0.1) or Li ₂ Mn ₃ MO ₈ (where M = Fe, Co, Lithium manganese composite oxide represented by Ni, Cu or Zn); LiMn ₂ O _{4 in} which part of Li in the formula is substituted with alkaline earth metal ions; Disulfide compounds; Fe ₂ (MoO ₄ ) ₃ and the like, but are not limited to these.

The conductive material usable in the production of the positive electrode is used to impart conductivity to the positive electrode, and in the battery configured, it can be used without particular limitation as long as it has electronic conductivity without causing chemical changes. Specific examples include graphite such as natural graphite and 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 whiskers such as zinc oxide whisker, calcium carbonate whisker, titanium dioxide whisker, silicon oxide whisker, silicon carbide whisker, aluminum borate whisker, magnesium borate whisker, potassium titanate whisker, silicon nitride whisker, silicon carbide whisker, alumina whisker, etc. (Whisker); Conductive metal oxides such as titanium oxide; Or a conductive polymer, such as a polyphenylene derivative, and the like, or a mixture of two or more of them may be used. Among these, a carbon-based material such as carbon black may be more preferable when considering the remarkable improvement effect due to the use of a conductive material and the high-temperature drying process in the anode manufacturing process. The conductive material may be included in 30% by weight or less, or 1% by weight to 30% by weight based on the total weight of the composition for forming the positive electrode active material layer.

In addition, the binder usable in the production of the positive electrode serves to improve adhesion between the positive electrode active material particles and the adhesion between the positive electrode active material and the current collector. Specific examples include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, Polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene-butadiene rubber, fluorine rubber, or various copolymers thereof, and one of them alone or a mixture of two or more thereof Can be used. The binder may be included in 30% by weight or less, or 1% by weight to 30% by weight based on the total weight of the composition for forming the positive electrode active material layer.

In addition, a solvent generally used in the art may be used as the solvent, dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone ( acetone) or water, and among these, one kind alone or a mixture of two or more kinds may be used.

Subsequently, the coating, drying, and rolling processes of the composition for forming the positive electrode active material layer on the positive electrode current collector may be performed in the same manner as described in the method for manufacturing the negative electrode.

In addition, the positive electrode may also be prepared by applying a composition for forming a positive electrode active material layer on a separate support and then drying and peeling the film for forming a positive electrode active material layer from the support and laminating on the positive electrode current collector.

On the other hand, in the lithium secondary battery, the separator separates the negative electrode from the positive electrode and provides a passage for lithium ions, and is usually used as a separator in a lithium secondary battery, and can be used without particular limitation. It is desirable to have low resistance and excellent electrolyte-moisturizing ability. Specifically, porous polymer films such as ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers and polyolefin polymers such as ethylene / methacrylate copolymers or the like. A laminate structure of two or more layers of may be used. In addition, a conventional porous non-woven fabric, for example, a high-melting point glass fiber, a polyethylene terephthalate fiber or 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 optionally be used in a single layer or multilayer structure.

In addition, examples of the electrolyte used in the present invention include 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. It does not work.

Specifically, the electrolyte may include an organic solvent and a lithium salt.

The organic solvent may be used without particular limitation as long as it can serve as a medium through which ions involved in the electrochemical reaction of the battery can move. Specifically, the organic solvent, methyl acetate (methyl acetate), ethyl acetate (ethyl acetate), γ-butyrolactone (γ-butyrolactone), ε-caprolactone (ε-caprolactone), such as ester-based solvents; Ether-based solvents such as dibutyl ether or tetrahydrofuran; Ketone solvents such as cyclohexanone; Aromatic hydrocarbon-based solvents such as benzene and fluorobenzene; Dimethylcarbonate (DMC), diethylcarbonate (DEC), methylethylcarbonate (MEC), ethylmethylcarbonate (EMC), ethylene carbonate (EC), propylene carbonate (propylene carbonate, PC) may be used. Among them, more specifically, it may be a carbonate-based solvent, and a cyclic carbonate (for example, ethylene carbonate or propylene carbonate) having high ionic conductivity and high dielectric constant that can improve the charge and discharge performance of the battery, and a low-viscosity linearity It may be a mixture of carbonate-based compounds (for example, ethyl methyl carbonate, dimethyl carbonate or diethyl carbonate).

In addition, the lithium salt may be used without particular limitation as long as it is a compound capable of providing lithium ions used in lithium secondary batteries. Specifically, the lithium salt is LiPF ₆ , LiClO ₄ , LiAsF ₆ , LiBF ₄ , LiSbF ₆ , LiAl0 ₄ , LiAlCl ₄ , LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , LiN (C ₂ F ₅ SO ₃ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) ₂ . LiCl, LiI, or LiB (C ₂ O ₄ ) ₂ and the like can be used. The lithium salt may be included in the electrolyte at a concentration of approximately 0.6 mol% to 2 mol%.

In addition to the components of the electrolyte, the electrolyte may include, for example, pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylene diamine, n for the purpose of improving the life characteristics of the battery, suppressing the decrease in battery capacity, and improving the discharge capacity of the battery. -Glyme, hexaphosphate triamide, nitrobenzene derivative, sulfur, quinone imine dye, N-substituted oxazolidinone, N, N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2 -One or more additives such as methoxy ethanol or aluminum trichloride may be included. At this time, the additive may be included in an amount of 0.1 to 5% by weight based on the total weight of the electrolyte.

A lithium secondary battery having the above-described configuration can be manufactured by interposing a separator between an anode and a cathode to manufacture an electrode assembly, placing the electrode assembly inside a case, and injecting an electrolyte into the case. Alternatively, after laminating the electrode assembly, it may be prepared by impregnating the electrolyte solution and sealing the obtained result in a battery case.

 As described above, the lithium secondary battery according to an embodiment of the present invention may exhibit excellent discharge capacity, output characteristics, and life characteristics by including the negative electrode having the above-described multilayer structure. As a result, it is useful for portable devices such as mobile phones, notebook computers, digital cameras, and electric vehicles such as hybrid electric vehicles.

Accordingly, according to another embodiment of the present invention, a battery module including the lithium secondary battery as a unit cell and a battery pack including the same are provided.

The battery module or battery pack includes a power tool; Electric vehicles including electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs); Alternatively, it can be used as a power supply for any one or more medium-to-large devices among power storage systems.

Hereinafter, embodiments of the present invention will be described in detail so that those skilled in the art to which the present invention pertains can easily practice. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein.

[Example 1: Preparation of negative electrode]

Step 1: Preparation of the first cathode active material layer

As a negative electrode active material, natural graphite (average particle diameter (D ₅₀ ): 16 µm), low-substituted carboxymethyl cellulose as shown in Table 1, and styrene-butadiene rubber as shown in Table 1 (average particle diameter (D ₅₀ ) = 110 nm) were watered. Among them, a composition for forming a first cathode active material layer (viscosity: 4,500 cps) was prepared by mixing in a weight ratio of 96.3: 1.2: 2.5. At this time, the viscosity was measured using a B-type viscometer at a shear rate of 12 rpm at 25 ° C.

The prepared composition for forming a first negative electrode active material layer was coated on one surface of a copper foil current collector so that the average thickness after drying was 20 μm, thereby forming a first negative electrode active material layer.

Step 2: Preparation of the second negative electrode active material layer

As a negative electrode active material, natural graphite (average particle diameter (D ₅₀ ): 16 µm), high-substituted carboxymethyl cellulose as shown in Table 1, and styrene-butadiene rubber as shown in Table 1 (average particle diameter (D ₅₀ ) = 110 nm) were watered. Among them, a composition for forming a second cathode active material layer (viscosity: 3,000 cps) was prepared by mixing in a weight ratio of 96.8: 1.2: 2.0.

The prepared composition for forming a second cathode active material layer was coated on the first cathode active material layer prepared in step 1, dried to have an average thickness of 40 μm, and dried to form a second anode active material layer. At this time, the coating amount of the composition for forming the second anode active material layer was 4 times the coating amount of the composition for forming the first anode active material layer, and before the first cathode active material layer was completely dried, the amount of solvent used in the composition for forming the first anode active material layer When about 20% by weight of the was removed, the composition for forming the second cathode active material layer was applied.

[Examples 2 to 4: Preparation of negative electrode]

A negative electrode was prepared in the same manner as in Example 1, except that various types, contents, and amounts of materials used were varied and used as described in Table 1 below.

Cathode active material
Styrene-butadiene rubber Carboxymethylcellulose thickness
(㎛) Kinds content
(Parts by weight) Weight ratio of structural units derived from SM, BD, and BA
(SM: BD: BA) Mw
(g / mol) content
(Parts by weight) Degree of substitution Mw
(g / mol) content
(Parts by weight) Example 1 1st negative electrode active material layer Natural graphite 96.3 1: 1:- 1,200,000 2.5 0.8 2,300,000 1.2 20 Second anode active material layer Natural graphite 96.8 1: 1:- 1,200,000 2.0 1.1 1,260,000 1.2 40 Example 2 1st negative electrode active material layer Natural graphite 96.3 3: 1:- 1,390,000 2.5 0.8 2,300,000 1.2 20 Second anode active material layer Natural graphite 96.8 3: 1: 1 1,520,000 2.0 1.1 1,260,000 1.2 40 Example 3 1st cathode active material layer Natural graphite 96.3 1: 1:- 1,200,000 2.5 0.95 2,450,000 1.2 20 Second anode active material layer Natural graphite 96.8 1: 1:- 1,200,000 2.0 1.2 1,300,000 1.2 40 Example 4 1st cathode active material layer Natural graphite 96.3 1: 1:- 1,200,000 2.5 0.8 2,300,000 1.2 20 Second anode active material layer Natural graphite 96.8 1: 1:- 1,200,000 2.0 1.1 1,260,000 1.2 80

In Table 1, SM means styrene, BD butadiene, and BA means butyl acrylate.

[Comparative Example 1: Preparation of negative electrode]

In Example 1, except that the composition for forming the first cathode active material layer in Example 1 was coated so that the average thickness was 60 µm after drying, and the second cathode active material layer forming process was not performed. A negative electrode was prepared in the same manner as described above.

[Comparative Example 2: Preparation of negative electrode]

In Example 1, except that the composition for forming the second cathode active material layer in Example 1 was coated so that the average thickness after drying was 60 μm, and the first cathode active material layer forming process was not performed. A negative electrode was prepared in the same manner as described above.

[Production Example: Manufacturing lithium secondary battery]

A lithium secondary battery was manufactured using the negative electrode prepared in Example 1 and Comparative Examples 1 and 2, respectively.

Specifically, Li (Ni _0.6 Mn _0.2 Co _0.2 ) O ₂ A positive electrode active material, a carbon black conductive material and a PVdF binder are mixed in a ratio of 90: 5: 5 by weight in an N-methylpyrrolidone solvent to prepare a positive electrode mixture (viscosity: 5000 mPa · s), which is applied to an aluminum current collector. After that, dry rolling was performed to prepare an anode.

A battery assembly was prepared by interposing a porous polyethylene separator between the positive electrode prepared above and each negative electrode prepared in Example 1 and Comparative Examples 1 and 2, and after the battery assembly was placed inside the case, into the case. A lithium secondary battery was prepared by injecting an electrolyte. At this time, the electrolytic solution was prepared by dissolving lithium hexafluorophosphate (LiPF ₆ ) at a concentration of 1.15M in an organic solvent composed of ethylene carbonate / dimethyl carbonate / ethyl methyl carbonate (mixed volume ratio of EC / DMC / EMC = 3/4/3). Did.

Experimental Example 1: Adhesion measurement

The adhesive strength between the current collector and the negative electrode active material layer was evaluated by a 180 ^° peel test for the negative electrode prepared in Example 1 and Comparative Examples 1 and 2 of the present invention.

In addition, the loading and porosity of the active material were measured by measuring the weight and thickness of each cathode. The results are shown in Table 2 and Fig. 1, respectively.

Active material loading (g / 25cm ² ) Porosity (% by volume) Adhesion (gf) Comparative Example 1 0.350 27 49.7 Comparative Example 2 0.350 27 71.5 Example 1 0.350 27 70.4

As a result of the experiment, the negative electrode of Example 1 exhibited excellent adhesive strength of Comparative Example 2 having a high binder content. These results show that in the case of Comparative Examples 1 and 2, especially Comparative Example 2, although the binder content is high, migration of the binder occurs during the drying process of the composition for forming the coated negative electrode active material layer, so that the binder is located on the upper side of the negative electrode. As a result, when measuring adhesive strength, detachment occurred at the interface between the current collector and the active material layer. On the other hand, the negative electrode of Example 1 was made by using a low-substituted cellulose-based compound capable of alleviating binder migration while reducing the thickness of the first negative electrode active material layer in contact with the current collector instead of increasing the content of the binder and applying it. 1 The migration of the binder in the negative electrode active material layer was minimized, and the abundance ratio of the binder in the vicinity of the current collector was increased to show better adhesive properties.

Experimental Example 2: Battery characteristics evaluation of lithium secondary battery

The half cells including the positive electrode active materials in Examples 1 and 2 and Comparative Examples 1 and 2 were prepared, and the cycle characteristics of the cells were evaluated in the following manner.

Specifically, a half battery including the negative electrode prepared in Example 1 and Comparative Examples 1 and 2, respectively, was prepared, charged at a constant current (CC) of 0.1C at 25 ° C. until 0.005 V, and then 0.005 V. The battery was charged with a constant voltage (CV), and the first charging was performed until the charging current became 0.005 mAh. Thereafter, it was allowed to stand for 20 minutes, and then discharged at a constant current of 0.5C until 0.8V, and the discharge capacity at the first cycle was measured. The results are shown in Table 3 and FIG. 2.

0.5C-rate discharge capacity (mAh / g) Comparative Example 1 346.9 Comparative Example 2 316.8 Example 1 344.8

As a result of the experiment, the negative electrode prepared in Example 1 exhibited an excellent discharge capacity equivalent to that of Comparative Example 1, which is a high loading electrode according to a decrease in the binder content. As a result of this, the negative electrode of Example 1 lowered the binder content in the second negative electrode active material in contact with the electrolyte to the level of Comparative Example 1 to increase the active material ratio, and the dispersibility of the constituents in the active material layer using a high-substituted cellulose-based compound By increasing the, it showed better battery characteristics by reducing the resistance.

Claims (14)

Current collector;
A first cathode active material layer positioned on the current collector; And
It includes a second cathode active material layer located on the first cathode active material layer,
The first negative electrode active material layer and the second negative electrode active material layer each independently include a negative electrode active material, a styrene-butadiene rubber binder and a cellulose-based compound,
The first cathode active material layer includes a styrene-butadiene rubber binder in a higher content than the second cathode active material layer,
The cellulose-based compound included in the first cathode active material layer has a lower degree of substitution of the carboxyl group in the structural unit than the cellulose-based compound included in the second cathode active material layer,
The styrene-butadiene rubber binder contained in the second cathode active material layer is a negative electrode for a secondary battery including a structural unit derived from (meth) acrylic acid ester.
According to claim 1,
The first negative electrode active material layer is a negative electrode for a secondary battery comprising a styrene-butadiene-based rubber binder in an amount of more than 100 parts by weight and 400 parts by weight or less with respect to 100 parts by weight of the styrene-butadiene-based rubber binder contained in the second negative electrode active material layer .
According to claim 1,
The styrene-butadiene-based rubber binder includes a styrene-derived structural unit and a butadiene-derived structural unit in a weight ratio of 1: 1 to 3: 1.
According to claim 1,
The styrene-butadiene-based rubber binder included in the first negative electrode active material layer includes a structural unit derived from (meth) acrylic acid ester.
According to claim 1,
The first cathode active material layer includes a first cellulose-based compound having a low degree of substitution with a substitution degree of less than 1 in the structural unit, and the second anode active material layer has a high degree of substitution of a second cellulose-based compound having a substitution degree of 1 or higher. The negative electrode for a secondary battery comprising a.
The method of claim 5,
The first cellulose-based compound is a negative electrode for a secondary battery comprising a carboxymethyl cellulose having a substitution degree of less than 0.7 to 0.95 and a weight average molecular weight of 1,500,000 g / mol to 4,000,000 g / mol in the structural unit.
The method of claim 5,
The first cellulose-based compound is a negative electrode for a secondary battery that is included in 0.5% to 10% by weight relative to the total weight of the first cathode active material layer.
The method of claim 5,
The second cellulose-based compound layer is a negative electrode for a secondary battery comprising a carboxymethyl cellulose having a substitution degree of 1 to 1.2 and a weight average molecular weight of 500,000 g / mol to 1,500,000 g / mol in the structural unit.
The method of claim 5,
The second cellulose-based compound is a negative electrode for a secondary battery comprising 0.5% to 10% by weight relative to the total weight of the second cathode active material layer.
According to claim 1,
The negative electrode for a secondary battery, wherein the ratio of the average thickness of the second negative electrode active material layer to the average thickness of the first negative electrode active material layer is greater than one.
According to claim 1,
The styrene-butadiene-based rubber binder is a secondary battery negative electrode having an average particle size (D ₅₀₎ having an average particle size (D ₅₀₎ 0.005 times to 0.04 times the contrast of the negative electrode active material.
Preparing a first negative electrode active material layer comprising a negative electrode active material, a styrene-butadiene rubber binder, and a cellulose-based compound on a current collector; And
And forming a second negative electrode active material layer comprising a negative electrode active material, a styrene-butadiene rubber binder, and a cellulose-based compound on the first negative electrode active material layer,
The first cathode active material layer includes a styrene-butadiene-based rubber binder in a higher content than the second cathode active material layer,
The method of manufacturing a negative electrode for a secondary battery according to claim 1, wherein the cellulose-based compound contained in the first cathode active material layer has a lower degree of substitution of a carboxyl group in a structural unit than the cellulose-based compound contained in the second cathode active material layer.
The method of claim 12,
The step of preparing the first negative electrode active material layer includes applying a composition for forming a first negative electrode active material layer containing a solvent on the current collector and drying it,
The drying method of the negative electrode for a secondary battery is performed until 10% to 30% by weight of the solvent in the composition for forming the first negative electrode active material layer is evaporated.
A lithium secondary battery comprising the negative electrode according to any one of claims 1 to 11.

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