Thickener dispersion, negative electrode slurry containing the same, negative electrode, and lithium secondary battery

Abstract

The present invention provides a thickener dispersion comprising a thickener containing at least one of carboxymethyl cellulose and a metal salt of the carboxymethyl cellulose, a aging inhibitor, and an aqueous solvent, wherein the aging inhibitor contains at least one selected from the group consisting of phenoxyethanol, sodium azide, paraben, formaldehyde, 5-chloro-2-methyl-4-isothiazolin-3-one, sodium benzoate, ethylhexylglycerin, and 1,2-hexanediol, the thickener is contained in an amount of 0.05% by weight to 3.5% by weight in the thickener dispersion, and the aging inhibitor is contained in an amount of 0.05% by weight to 3.0% by weight in the thickener dispersion.

Description

[Technical Field]

[0001] This application claims the benefit of priority based on Korean Patent Application No. 10-2022-0111714, dated September 2, 2022, the entire contents of which are incorporated herein by reference.

[0002] The present invention relates to a thickener dispersion, a negative electrode slurry containing the thickener dispersion, a negative electrode, and a lithium secondary battery. More specifically, the present invention relates to a thickener dispersion that exhibits little change in viscosity over time, a negative electrode slurry containing the thickener dispersion, and a negative electrode and a lithium secondary battery manufactured using the negative electrode slurry. [Background technology]

[0003]

[0003] Along with technological development and increasing demand for mobile devices, the demand for secondary batteries as an energy source is rapidly increasing. Among these secondary batteries, lithium secondary batteries, which have high energy density and voltage, long cycle life, and low self-discharge rate, have been commercialized and widely used. In addition, research into methods for improving electrode density and manufacturing electrodes with higher energy density per unit volume for such high-capacity lithium secondary batteries is actively being conducted.

[0004] The lithium secondary battery generally includes a positive electrode, a negative electrode, a separator, and an electrolyte, and the positive electrode and negative electrode may be manufactured by coating a positive electrode slurry or a negative electrode slurry on a current collector, followed by drying, rolling, etc. Among them, the negative electrode slurry may be classified as an aqueous negative electrode slurry or an organic negative electrode slurry depending on the type of solvent used, and aqueous negative electrode slurry is commonly used in the battery industry due to process convenience and cost reduction.

[0005] Meanwhile, lithium metal has conventionally been used as the negative electrode of the lithium secondary battery, but there is a problem of short circuit of the battery due to the formation of dendrites, which may result in an explosion. Therefore, attention is being paid to the use of carbon-based active materials that allow reversible intercalation and deintercalation of lithium ions and maintain their structural and electrical properties.

[0006] The carbon-based active material is generally not dispersed in an aqueous solvent (e.g., water), and therefore, in the aqueous negative electrode slurry, a thickener such as carboxymethyl cellulose (CMC) is used to disperse the carbon-based active material in the aqueous solvent, thereby ensuring the viscosity of the aqueous negative electrode slurry.

[0007] In this case, since a thickener such as carboxymethyl cellulose has a relatively low solubility in an aqueous solvent, the thickener may be added to the aqueous negative electrode slurry in the form of a thickener dispersion in which the thickener is previously dissolved in an aqueous solvent.

[0008] Such thickener dispersions may need to be stored for a certain period of time (e.g., one week) depending on various mass production conditions, and during this storage process, problems may arise in that the thickener dispersions undergo changes over time, such as a decrease in viscosity.

[0009] The change in the thickener dispersion over time may cause problems such as a decrease in the processability of the coating process of the negative electrode slurry and a decrease in the quality of the negative electrode. Summary of the Invention [Problem to be solved by the invention]

[0010] An object of the present invention is to provide a thickener dispersion that is easy to store and that is less susceptible to changes over time, such as a decrease in viscosity.

[0011] Another object of the present invention is to provide an anode slurry, an anode, and a lithium secondary battery that are excellent in processability and have improved quality when produced using the above-mentioned thickener dispersion. [Means for solving the problem]

[0012] The present invention provides a thickener dispersion comprising a thickener containing at least one of carboxymethyl cellulose and a metal salt of the carboxymethyl cellulose, a aging inhibitor, and an aqueous solvent, wherein the aging inhibitor contains at least one selected from the group consisting of phenoxyethanol, sodium azide, paraben, formaldehyde, 5-chloro-2-methyl-4-isothiazolin-3-one, sodium benzoate, ethylhexylglycerin, and 1,2-hexanediol, the thickener is contained in an amount of 0.05% by weight to 3.5% by weight in the thickener dispersion, and the aging inhibitor is contained in an amount of 0.05% by weight to 3.0% by weight in the thickener dispersion.

[0013] The present invention also provides a negative electrode slurry containing a negative electrode active material, a binder, the above-mentioned thickener dispersion, and an aqueous solvent.

[0014] The present invention also provides a negative electrode including a negative electrode active material layer formed from the above-described negative electrode slurry.

[0015] The present invention also provides a lithium secondary battery including the above-described negative electrode, a positive electrode, a separator interposed between the negative electrode and the positive electrode, and an electrolyte. [Effects of the Invention]

[0016] The thickener dispersion of the present invention is a thickener dispersion containing a thickener including at least one of carboxymethylcellulose and a metal salt of the carboxymethylcellulose, a aging inhibitor selected from specific compounds, and an aqueous solvent, wherein the thickener and the aging inhibitor are adjusted to specific content ranges. Due to the above characteristics, the thickener dispersion of the present invention can significantly prevent problems with aging such as viscosity reduction.

[0017] Therefore, the thickener dispersion has little risk of deterioration over time when the thickener must be stored for a certain period of time under various mass production conditions, which may improve processability in preparing the anode slurry, thereby enabling the realization of anodes and lithium secondary batteries with improved quality. [Brief explanation of the drawings]

[0018] [Figure 1] 1 is a graph showing the change in Brookfield viscosity over time of the thickener dispersions of Examples 1 to 5 and Comparative Examples 1 to 3. [Figure 2] 1 is a graph showing the adhesive strength of negative electrodes produced using thickener dispersions of Examples 1 to 5 and Comparative Examples 1 to 3. DETAILED DESCRIPTION OF THE INVENTION

[0019] The terms and words used in this specification and claims should not be interpreted in a limited way to their ordinary or dictionary meanings, but should be interpreted in a way that is consistent with the technical idea of ​​the present invention, based on the principle that the inventors can appropriately define the concepts of terms in order to best explain their inventions.

[0020] The terms used in this specification are used only to describe exemplary embodiments and are not intended to limit the present invention. The singular expressions include the plural expressions unless the context clearly indicates otherwise.

[0021] As used herein, the terms "comprises," "comprises," or "having" are intended to specify the presence of embodied features, numbers, steps, components, or combinations thereof, and should be understood as not precluding the presence or possible addition of one or more other features, numbers, steps, components, or combinations thereof.

[0022] <Thickener dispersion> The present invention relates to a thickener dispersion, which can be preferably used for a negative electrode for a lithium secondary battery.

[0023] Specifically, the thickener dispersion contains a thickener including at least one of carboxymethyl cellulose and a metal salt of the carboxymethyl cellulose, a time-dependent change inhibitor, and an aqueous solvent, wherein the time-dependent change inhibitor includes at least one selected from the group consisting of phenoxyethanol, sodium azide, paraben, formaldehyde, 5-chloro-2-methyl-4-isothiazolin-3-one, sodium benzoate, ethylhexylglycerin, and 1,2-hexanediol, the thickener is contained in the thickener dispersion at 0.05% by weight to 3.5% by weight, and the time-dependent change inhibitor is contained in the thickener dispersion at 0.05% by weight to 3.0% by weight.

[0024] The thickener dispersion of the present invention is carboxymethyl cellulose (CMC). and at least one of the metal salts of carboxymethyl cellulose. The thickener dispersion according to the present invention comprises a thickener containing the compound (I), an aging inhibitor having the above-mentioned characteristics, and an aqueous solvent, wherein the thickener and the aging inhibitor are adjusted to have specific content ranges. Due to the above characteristics, the thickener dispersion according to the present invention can significantly prevent problems with aging such as viscosity reduction.

[0025] Therefore, the thickener dispersion is less susceptible to deterioration over time when the thickener must be stored for a certain period of time under various mass production conditions, thereby improving processability in the preparation of anode slurry, thereby enabling the realization of anodes and lithium secondary batteries with improved quality.

[0026] (1) Thickener The thickener may include at least one of carboxymethyl cellulose (CMC) and a metal salt of carboxymethyl cellulose. The thickener may be used to easily disperse a negative electrode active material that is difficult to disperse in an aqueous negative electrode slurry and to adjust the viscosity of the aqueous negative electrode slurry to a level that facilitates application.

[0027] The thickener may include at least one of carboxymethyl cellulose (CMC) and a metal salt of carboxymethyl cellulose, specifically, carboxymethyl cellulose. Meanwhile, the metal salt of carboxymethyl cellulose may be a salt in which at least a portion of the hydrogen atoms of -COOH present in the carboxymethyl cellulose are substituted with a metal ion. The metal ion may be, for example, Li + , Na + , and K. + It may be at least one selected from the group consisting of:

[0028] The weight-average molecular weights of the carboxymethyl cellulose and the metal salt of carboxymethyl cellulose may be, independently of one another, 1,000,000 to 5,000,000, specifically 1,500,000 to 4,000,000, more specifically 1,700,000 to 2,500,000. When the weight-average molecular weights are within the above ranges, the viscosity of the thickener dispersion can be increased to a suitable level, and the problem of the viscosity of the thickener dispersion not being maintained due to an excessively low weight-average molecular weight can be prevented.

[0029] The thickener is contained in the thickener dispersion at 0.05% by weight to 3.5% by weight. When the content is within the above range, the thickener can be easily dispersed or dissolved in the aqueous solvent. The thickener may be contained in the thickener dispersion at specifically 1% by weight to 3% by weight, more specifically 1.0% by weight to 2.3% by weight, and even more specifically 1% by weight to 2% by weight.

[0030] (2) Aging inhibitor The agent for inhibiting change over time may be contained in the thickener dispersion for the purpose of inhibiting change over time, such as a decrease in viscosity, even when the thickener dispersion is stored for a certain period of time (for example, one week).

[0031] The aging inhibitor may include at least one selected from the group consisting of phenoxyethanol, sodium azide, paraben, formaldehyde, 5-chloro-2-methyl-4-isothiazolin-3-one, sodium benzoate, ethylhexylglycerin, and 1,2-hexanediol. Specifically, at least one selected from the group consisting of phenoxyethanol, sodium azide, and 1,2-hexanediol may be included, more specifically, phenoxyethanol, because it has an excellent viscosity-maintaining effect of the thickener dispersion and is less harmful to the human body. Examples of the paraben include butyl paraben (butyl parahydroxybenzoate), propyl paraben (propyl parahydroxybenzoate), ethyl paraben (ethyl parahydroxybenzoate), and methyl paraben (methyl parahydroxybenzoate).

[0032] The thickener dispersion contains the aging inhibitor in an amount of 0.05 wt % to 3.0 wt %. If the thickener dispersion contains the aging inhibitor in an amount of less than 0.05 wt %, the viscosity of the thickener dispersion is maintained and the viscosity reduction prevention effect is minimal. If the thickener dispersion contains the aging inhibitor in an amount of more than 3.0 wt %, problems such as a decrease in energy density and an increase in battery resistance may occur when the thickener dispersion is contained in a negative electrode, which is not preferable.

[0033] The aging inhibitor may be contained in the thickener dispersion in an amount of specifically 0.3 wt % to 2.8 wt %, more specifically 0.7 wt % to 1.7 wt %, and when it is in the above range, it is preferable from the viewpoint of the effect of inhibiting aging of the thickener dispersion, and of improving the energy density and reducing the resistance of a negative electrode and a lithium secondary battery produced using the thickener dispersion.

[0034] (3) Water-based solvents The thickener dispersion of the present invention contains an aqueous solvent as a solvent. As described below, the thickener dispersion of the present invention is suitably used for an aqueous negative electrode slurry, and therefore, it is preferable to use the same aqueous solvent as that used as the aqueous negative electrode slurry solvent as the solvent for the thickener dispersion.

[0035] Specifically, the aqueous solvent may be water. For example, the aqueous solvent may be distilled water or deionized water (DI water).

[0036] In the present invention, the weight ratio of the thickener to the aging inhibitor may be 1:0.025 to 1:1.25, specifically 1:0.15 to 1:1.15, and more specifically 1:0.35 to 1:0.85. When the weight ratio is within the above range, the aging inhibitory effect of the thickener dispersion is preferably realized, and the dissolution or dispersion of the components in the thickener dispersion can be preferably achieved.

[0037] The solid content of the thickener dispersion may be 0.1 wt % to 6.5 wt %, specifically 1.3 wt % to 5.5 wt %, more specifically 2.3 wt % to 4.7 wt %, even more specifically 2.5 wt % to 4.0 wt %, and even more specifically 2.8 wt % to 3.7 wt %. Within this range, the viscosity of the thickener dispersion and the negative electrode slurry to which it is applied can be adjusted to a preferred level, improving processability and the quality of the negative electrode.

[0038] In the present invention, the viscosity change rate (%) of the thickener dispersion at 23°C calculated by the following mathematical formula 1 may be -20% or more and 0% or less, specifically -5% or more and 0% or less, more specifically -3% or more and 0% or less.

[0039] [Mathematical formula 1] Viscosity change rate (%)={(viscosity of the thickener dispersion immediately after production and 7 days later)−(viscosity of the thickener dispersion immediately after production)} / (viscosity of the thickener dispersion immediately after production)×100

[0040] The viscosity change rate (%) is calculated by measuring the viscosity of the thickener dispersion at a temperature of 23°C.

[0041] The viscosity measured in the viscosity change rate may be measured using a Brookfield viscometer. The viscosity may be Brookfield viscosity. Specifically, a Brookfield DV2T viscometer may be used as the Brookfield viscometer.

[0042] In Equation 1, "immediately after preparing the thickener dispersion" means immediately after adding the thickener and the aging inhibitor to the aqueous solvent and dissolving or dispersing them in a mixer. Specifically, the mixer may be a BTM Mixer (e.g., a BTM Mixer from Yunsung F&C Co., Ltd.), and the dissolving (or dispersing) may be performed by stirring for 3 hours at a rotation speed of 50 rpm with an Anchor disper or 1,000 rpm with a Homo disper.

[0043] The present invention also provides a method for producing a thickener dispersion.

[0044] Specifically, the method for producing the thickener dispersion may include the steps of adding a thickener and an anti-aging agent to an aqueous solvent to produce a mixture, and stirring the mixture.

[0045] The aqueous solvent, the thickener, and the aging inhibitor are as described above.

[0046] The mixture can be stirred by any stirring method known in the art without limitation. Specifically, the mixture can be stirred using a BTM Mixer (e.g., a BTM Mixer from Yunsung F&C Co., Ltd.). The stirring conditions for the mixture are not particularly limited, but when using a BTM Mixer, the stirring can be performed at a rotation speed of 10 rpm to 100 rpm for Anchor Mixer, more specifically 40 rpm to 60 rpm, and 100 rpm to 2,000 rpm for Homo Mixer, more specifically 800 rpm to 1,200 rpm.

[0047] The mixture may be stirred under temperature conditions that do not change the physical properties of the mixture, including viscosity, due to evaporation of the aqueous solvent, and may be stirred at a temperature of, for example, 50°C or lower, more specifically, 5°C to 50°C, and even more specifically, room temperature (e.g., 23°C).

[0048] The stirring of the mixture may be appropriately adjusted taking into consideration the mass, volume, etc. of the mixture, and may be carried out for, for example, 30 minutes or more, specifically 30 minutes to 10 hours, and more specifically 1 hour to 5 hours.

[0049] <Negative electrode slurry> The present invention also provides a negative electrode slurry containing the above-described thickener dispersion.

[0050] Specifically, the negative electrode slurry includes a negative electrode active material, a binder, the thickener dispersion, and an aqueous solvent.

[0051] The negative electrode active material is a material capable of lithium intercalation / deintercalation, and may include at least one selected from carbon-based active materials and (quasi)metal-based active materials.

[0052] Specifically, the negative electrode active material may include a carbon-based active material. In particular, the thickener dispersion liquid described above can exhibit excellent effects in a dispersion process of the carbon-based active material in an aqueous solvent.

[0053] The carbon-based active material may include at least one selected from the group consisting of graphite, hard carbon, soft carbon, carbon black, graphene, and fibrous carbon, and more specifically, may include graphite. The graphite may include at least one selected from the group consisting of natural graphite and artificial graphite.

[0054] The (quasi)metallic active material may be at least one (quasi)metal selected from the group consisting of Li, Cu, Ni, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al, V, Ti, and Sn; an alloy of at least one (quasi)metal selected from the group consisting of Cu, Ni, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al, V, Ti, and Sn with lithium; or an alloy of Cu, Ni, Na, The (quasi)metallic active material may contain at least one selected from the group consisting of a composite of at least one (quasi)metal and carbon selected from the group consisting of K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al, V, Ti, and Sn; and an oxide of at least one (quasi)metal selected from the group consisting of Li, Cu, Ni, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al, V, Ti, and Sn. More specifically, the (quasi)metallic active material may contain at least one selected from the group consisting of SiO x (0≦x<2) and silicon-carbon composites.

[0055] The negative electrode active material may be contained in the negative electrode slurry in an amount of 70 to 99 wt %, specifically 80 to 98 wt %, based on the total weight of the solid content of the negative electrode slurry.

[0056] The binder ensures adhesion between active materials or between an active material and a current collector. Any binder commonly used in the art can be used, and the type is not particularly limited. Examples of binders include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, and various copolymers thereof. One of these can be used alone, or two or more can be used in combination. More specifically, the binder can include styrene-butadiene rubber (SBR).

[0057] The binder may be included in the negative electrode slurry in an amount of 1 wt % to 10 wt %, specifically 1 wt % to 5 wt %, based on the total weight of the solid content of the negative electrode slurry.

[0058] Next, the negative electrode slurry contains the above-mentioned thickener dispersion.

[0059] The thickener dispersion can effectively suppress changes over time, such as a decrease in viscosity. According to the present invention, even when the thickener dispersion is stored for a certain period of time under various mass production conditions, the change in viscosity of the thickener dispersion can be minimized, which makes it possible to easily disperse the negative electrode active material in the aqueous negative electrode slurry, thereby improving the processability of the negative electrode and the quality of the negative electrode.

[0060] The thickener, the inhibitor for preventing deterioration over time, and the aqueous solvent contained in the thickener dispersion are as described above.

[0061] The thickener dispersion may be contained in the negative electrode slurry so that the thickener is contained in an amount of 0.5% by weight to 10% by weight, specifically 1% by weight to 5% by weight, based on the total weight of the solid content of the negative electrode slurry.

[0062] The aqueous solvent may be included in the negative electrode slurry to adjust the viscosity of the negative electrode slurry, improve the coating property, etc. The aqueous solvent is used as a solvent for the negative electrode slurry and is distinguished from the aqueous solvent included in the thickener dispersion liquid.

[0063] Specifically, the aqueous solvent may be water. For example, the aqueous solvent may be distilled water or deionized water (DI water).

[0064] The negative electrode slurry may further contain a conductive material in addition to the above components.

[0065] The conductive material is used to further improve the conductivity of the negative electrode active material. The conductive material is not particularly limited as long as it does not cause a chemical change in the battery and has conductivity, and examples thereof include graphite such as natural graphite and artificial graphite; carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fiber and metal fiber; metal powders such as carbon fluoride, aluminum, and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives. Specific examples of commercially available conductive materials include acetylene black-based materials manufactured by Chevron Chemical Company, Denka Singapore Private Limited, and Gulf Oil Company, Ketjenblack, EC-based materials manufactured by Armak Company, Vulcan XC-72 manufactured by Cabot Company, and Super P manufactured by Timcal.

[0066] The conductive material may be included in an amount of 0.5 wt % to 10 wt % based on the total weight of the solid content of the negative electrode slurry.

[0067] The solid content of the negative electrode slurry may be 20 wt% to 70 wt%, preferably 30 wt% to 50 wt%. If the solid content of the negative electrode slurry is less than 30 wt%, the electrode loading may decrease, increasing processing costs, and binder migration may occur, reducing electrode adhesion and resulting in coating defects. On the other hand, if the solid content of the negative electrode slurry exceeds 70 wt%, the viscosity of the negative electrode slurry may become too high, reducing processability and resulting in coating defects.

[0068] <Negative electrode> The present invention also provides a negative electrode including a negative electrode active material layer formed from the above-described negative electrode slurry.

[0069] Specifically, the negative electrode can be manufactured by applying the negative electrode slurry of the present invention and drying it to form a negative electrode active material layer. More specifically, the negative electrode active material layer can be formed by applying the negative electrode slurry to a negative electrode current collector and then drying it, or by applying the negative electrode slurry to a separate support and then peeling it off from the support to obtain a film, which can then be laminated on the negative electrode current collector. If necessary, a rolling process can be further performed after forming the negative electrode active material layer using the above method. In this case, the drying and rolling can be performed under appropriate conditions taking into account the physical properties of the final electrode to be manufactured, and are not particularly limited.

[0070] The negative electrode current collector is not particularly limited as long as it is made of a material that does not cause chemical changes in the battery and has conductivity, and examples thereof include copper, stainless steel, aluminum, nickel, titanium, alloys thereof, those whose surfaces are surface-treated with carbon, nickel, titanium, silver, or the like, and calcined carbon.

[0071] The negative electrode current collector typically has a thickness of 3 μm to 500 μm, and the surface of the current collector may be provided with fine irregularities to strengthen the bonding strength of the negative electrode active material. The negative electrode current collector may be used in various forms, such as a film, sheet, foil, mesh, porous material, foam, or nonwoven fabric.

[0072] [Lithium secondary battery] Next, the lithium secondary battery according to the present invention will be described.

[0073] The secondary battery according to the present invention includes the negative electrode of the present invention described above. Specifically, the lithium secondary battery according to the present invention may include the negative electrode described above, a positive electrode, a separator interposed between the negative electrode and the positive electrode, and an electrolyte.

[0074] The negative electrode has been described above, so a detailed description thereof will be omitted, and only the other components will be described below.

[0075] The positive electrode may be any material commonly used in the art and is not particularly limited. For example, the positive electrode may be manufactured by applying a positive electrode slurry composition to a positive electrode current collector and drying the composition to form a positive electrode active material layer. Specifically, the positive electrode active material layer may be formed by applying the positive electrode slurry composition to a positive electrode current collector and then drying the composition, or by applying the positive electrode slurry composition to a separate support, peeling the composition from the support, and laminating the resulting film on the positive electrode current collector. If necessary, a rolling process may be further performed after the positive electrode active material layer is formed by the above method. In this case, the drying and rolling may be performed under appropriate conditions taking into account the physical properties of the electrode to be finally manufactured, and are not particularly limited.

[0076] The positive electrode current collector is not particularly limited as long as it is a material that does not cause chemical changes and has conductivity. For example, copper, stainless steel, aluminum, nickel, titanium, alloys of these, those whose surfaces have been surface-treated with carbon, nickel, titanium, silver, or the like, or baked carbon can be used.

[0077] The positive electrode current collector typically has a thickness of 3 μm to 500 μm, and may have fine irregularities on its surface to strengthen the bonding strength of the positive electrode active material. The positive electrode current collector may be used in various forms, such as a film, sheet, foil, mesh, porous material, foam, or nonwoven fabric.

[0078] Meanwhile, the positive electrode slurry composition includes a positive electrode active material, a binder, and a solvent, and may further include a conductive material, an additive, and the like, as needed.

[0079] As the positive electrode active material, any positive electrode active material known in the relevant technical field can be used without limitation. For example, lithium cobalt-based oxides, lithium nickel-based oxides, lithium manganese-based oxides, lithium iron phosphate, lithium nickel manganese cobalt-based oxides, or combinations thereof can be used. Specifically, as the positive electrode active material, LiCoO2, LiNiO2, LiMn2O4, LiCoPO4, LiFePO4, and LiNi a Mn b Co c O2 (where 0 < a, b, c < 1) etc. can be used, but are not limited thereto.

[0080] The binder is for ensuring the adhesion force between active materials or between the active material and the current collector, and any general binder used in the relevant technical field can be used, and its type is not particularly limited. Examples of the binder include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene butadiene rubber (SBR), fluorine rubber, or various copolymers thereof. Among these, one type can be used alone, or a mixture of two or more types can be used.

[0081] The binder may be contained at ⑤% by weight or less, preferably ①% - ③% by weight, based on the content of the total solid matter in the positive electrode slurry composition. When the content of the binder satisfies the above range, an increase in the resistance of the electrode can be minimized, and excellent electrode adhesion force can be achieved.

[0082] It should be noted that there are some placeholders like ⑤, ①, ③ in the translation of which seem to be incorrect in the original text. It might be better to double-check the original content for accurate translation. Also, the a , b , c tags are kept as they are as per the requirement.The solvent is used to mix the components in the positive electrode slurry composition and adjust the viscosity, and may be, for example, water, an organic solvent, or a mixture thereof. Examples of the organic solvent include amide polar organic solvents such as dimethylformamide (DMF), diethylformamide, dimethylacetamide (DMAc), and N-methylpyrrolidone (NMP); alcohols such as methanol, ethanol, 1-propanol, 2-propanol (isopropyl alcohol), 1-butanol (n-butanol), 2-methyl-1-propanol (isobutanol), 2-butanol (sec-butanol), 1-methyl-2-propanol (tert-butanol), pentanol, hexanol, heptanol, and octanol; glycols such as ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, 1,3-propanediol, 1,3-butanediol, 1,5-pentanediol, and hexylene glycol; and various solvents such as glycerin, trimethylolpropane, pentaerythritol, and sorbitol. Examples of suitable solvents include hydric alcohols; glycol ethers such as ethylene glycol monomethyl ether, diethylene glycol monomethyl ether, triethylene glycol monomethyl ether, tetraethylene glycol monomethyl ether, ethylene glycol monoethyl ether, diethylene glycol monoethyl ether, triethylene glycol monoethyl ether, tetraethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monobutyl ether, triethylene glycol monobutyl ether, and tetraethylene glycol monobutyl ether; ketones such as acetone, methyl ethyl ketone, methyl propyl ketone, and cyclopentanone; and esters such as ethyl acetate, γ-butyl lactone, and ε-propiolactone, and mixtures of any one or more of these can be used, but the present invention is not limited to these.

[0083] The solvent may be included in an amount that allows the solids content of the positive electrode slurry composition to be 30 wt% to 85 wt%, preferably 30 wt% to 80 wt%. If the solids content of the positive electrode slurry composition is less than 30 wt%, the electrode loading may be reduced, increasing processing costs, and binder migration may occur, reducing electrode adhesion and resulting in coating defects. On the other hand, if the solids content of the positive electrode slurry exceeds 85 wt%, the viscosity of the positive electrode slurry composition may be excessively high, reducing processability and resulting in coating defects.

[0084] The conductive material is a component for further improving conductivity, and conductive materials for secondary batteries known in the art can be used without limitation, for example, graphite such as natural graphite and artificial graphite; carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fiber and metal fiber; carbon fluoride; metal powder such as aluminum and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives.

[0085] The conductive material may be contained in an amount of 10 wt % or less, preferably 0.1 wt % to 10 wt %, and more preferably 0.1 wt % to 5 wt %, based on the total solid content of the positive electrode slurry composition.

[0086] The positive electrode current collector is not particularly limited as long as it does not cause chemical changes in the battery and is conductive. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel whose surface has been surface-treated with carbon, nickel, titanium, silver, or the like may be used. The positive electrode current collector typically has a thickness of 3 μm to 500 μm, and fine irregularities may be formed on the surface of the current collector to enhance the adhesive strength of the positive electrode material. It can be used in various forms, such as a film, sheet, foil, mesh, porous material, foam, or nonwoven fabric.

[0087] The separator separates the negative electrode and the positive electrode and provides a path for lithium ions to move. Any separator commonly used in secondary batteries can be used. Specifically, the separator can be a porous polymer film, such as a porous polymer film made of a polyolefin polymer, such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, or an ethylene / methacrylate copolymer, or a laminate structure of two or more layers thereof. Conventional porous nonwoven fabrics, such as nonwoven fabrics made of high-melting-point glass fibers or polyethylene terephthalate fibers, can also be used. To ensure heat resistance or mechanical strength, a coated separator containing a ceramic component or a polymer material can also be used, and it can be selectively used in a single-layer or multi-layer structure.

[0088] Next, examples of the electrolyte include organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, and molten inorganic electrolytes that can be used in manufacturing lithium secondary batteries, but are not limited to these.

[0089] Specifically, the electrolyte may include a non-aqueous organic solvent and a metal salt.

[0090] Examples of the non-aqueous organic solvent that can be used include aprotic organic solvents such as N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma-butyrolactone, 1,2-dimethoxyethane, tetrahydroxyfuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, phosphoric acid triester, trimethoxymethane, dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, methyl propionate, and ethyl propionate.

[0091] In particular, among the carbonate-based organic solvents, cyclic carbonates such as ethylene carbonate and propylene carbonate are preferred because they are high-viscosity organic solvents with high dielectric constants and can easily dissociate lithium salts. Mixing such cyclic carbonates with linear carbonates having low viscosity and low dielectric constants, such as dimethyl carbonate and diethyl carbonate, in an appropriate ratio can provide an electrolyte with high electrical conductivity, and is therefore preferred.

[0092] The metal salt may be a lithium salt, which is a substance that is easily dissolved in the non-aqueous electrolyte solution. For example, the anion of the lithium salt may be F - , Cl - , I - , NO3 - , N(CN)2 - , BF4 - , ClO4 - , PF6 - , (CF3)2PF4 - , (CF3)3PF3 - , (CF3)4PF2 - , (CF3)5PF - , (CF3)6P - , CF3SO3 -, CF3CF2SO3 - , (CF3SO2)2N - , (FSO2)2N - , CF3CF2(CF3)2CO - , (CF3SO2)2CH - , (SF5)3C - , (CF3SO2)3C - , CF3(CF2)7SO3 - , CF3CO2 - , CH3CO2 - , SCN - , and (CF3CF2SO2)2N - One or more selected from the group consisting of:

[0093] In addition to the constituent components of the electrolyte, the electrolyte may further include one or more additives, such as haloalkylene carbonate compounds such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glyme, hexaphosphoric acid triamide, nitrobenzene derivatives, sulfur, quinoneimine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride, for the purpose of improving the life characteristics of the battery, suppressing a decrease in battery capacity, and improving the discharge capacity of the battery.

[0094] The lithium secondary battery can be usefully used in portable devices such as mobile phones, notebook computers, and digital cameras, and in electric vehicles such as hybrid electric vehicles (HEVs).

[0095] The present invention will be described in more detail below with reference to specific examples.

[0096] Examples and Comparative Examples Example 1: Preparation of thickener dispersion Distilled water as an aqueous solvent, carboxymethyl cellulose (CMC) with a weight-average molecular weight of 2 million as a thickener, and phenoxyethanol as a time-dependent change inhibitor were added to a mixer (manufacturer: Yunsung F&C, product name: BTM MIXER) and stirred at 23°C for 3 hours at a rotation speed of 50 rpm for Anchor disper and 1,000 rpm for Homo disper to prepare a thickener dispersion.

[0097] The thickener dispersion contained 2.00 wt % of the thickener, 0.10 wt % of the aging inhibitor, and the remainder was an aqueous solvent. The total weight of the thickener dispersion was 35 kg.

[0098] Examples 2 to 5, Comparative Examples 1 to 3: Preparation of thickener dispersion The thickener dispersions of Examples 2 to 5 and Comparative Examples 1 to 3 were produced in the same manner as in Example 1, except that the total weight of the thickener dispersion was 35 kg and the contents of the thickener, the aging inhibitor, and the aqueous solvent were adjusted as shown in Table 1 below.

[0099] [Table 1]

[0100] [Experimental Example] Experimental Example 1: Viscosity observation of thickener dispersion over time The thickener dispersion immediately after the stirring step was completed was defined as the thickener dispersion "immediately after production," and the change in viscosity over time was observed from immediately after production.

[0101] The viscosity was measured using a Brookfield viscometer (manufacturer: Brookfield, product name: DV2T viscometer) at a measurement temperature of 23°C.

[0102] Table 2 and FIG. 1 show the viscosity changes over time from immediately after production of the thickener dispersions of Examples 1 to 5 and Comparative Examples 1 to 3.

[0103] [Table 2]

[0104] In addition, the viscosity change rate (%) at 23°C immediately after preparation of the thickener dispersion and after 7 days was calculated using the following mathematical formula 1. The calculated viscosity change rate (%) was rounded up to one decimal place, and the results are shown in Table 3 below.

[0105] [Mathematical formula 1] Viscosity change rate (%)={(viscosity of the thickener dispersion immediately after production and 7 days later)−(viscosity of the thickener dispersion immediately after production)} / (viscosity of the thickener dispersion immediately after production)×100

[0106] [Table 3]

[0107] Referring to Tables 2 and 3 and FIG. 1, it can be seen that the thickener dispersions of Examples 1 to 5 show very little change in viscosity over time, even after 7 days from immediately after production, compared to Comparative Examples 1 and 2.

[0108] On the other hand, the thickener dispersion of Comparative Example 3 showed little change in viscosity over time, but as described below, the adhesive strength of the negative electrode using it was low, the energy density of the negative electrode was low, and the resistance increased, so it was not suitable for use.

[0109] Experimental Example 2: Evaluation of adhesive strength <Production of negative electrodes> A negative electrode slurry was prepared by mixing graphite as a negative electrode active material, styrene-butadiene rubber as a binder, and the "just-prepared" thickener dispersion prepared in Example 1. The weight ratio of graphite to styrene-butadiene rubber to CMC in the thickener dispersion was 95:3:2. The solid content of the negative electrode slurry was 50 wt%.

[0110] The negative electrode slurry was applied to a copper current collector (thickness: 20 μm) as a current collector, and dried and rolled at 130° C. to form a negative electrode active material layer (thickness: 80 μm), thereby producing a negative electrode.

[0111] Furthermore, a negative electrode was produced in the same manner as above, except that the thickener dispersion of Example 1 was replaced with a aged thickener dispersion.

[0112] In addition, negative electrodes of Examples 2 to 5 and Comparative Examples 1 to 3 were produced by the same method as above, except that the thickener dispersions of Examples 2 to 5 and Comparative Examples 1 to 3 were used instead of the thickener dispersion of Example 1.

[0113] <Evaluation of adhesive strength> The negative electrodes of Example 1 were punched out to a size of 2 cm x 15 cm using an electrode punching machine, and the punched negative electrodes were attached to glass slides with double-sided tape to prepare samples. The prepared samples were then rubbed 15 times with a 2 kg load roller to ensure uniform adhesion of the electrode surface to the double-sided tape. Each prepared sample was then attached to the grip of a Universal Testing Machine (UTM) (LF Plus, manufactured by LLOYD) as an electrode adhesion strength tester, and a 5 N (1 lbf) load cell was applied to measure the adhesion strength.

[0114] The adhesive strength of the negative electrodes of Examples 2 to 5 and Comparative Examples 1 to 3 was also measured in the same manner as above.

[0115] The results are shown in Table 4 below and FIG.

[0116] [Table 4]

[0117] Referring to Table 4, it can be seen that in Examples 1 to 5, even when thickener dispersions that had been left for some time after production were used, the adhesive strength of the produced negative electrodes did not decrease.

[0118] However, in Comparative Examples 1 and 2, the viscosity of the thickener dispersion decreased over time, and the negative electrode using the thickener dispersion three days after production showed a decrease in adhesive strength.

[0119] In Comparative Example 3, the viscosity of the thickener dispersion changed little over time as described above, but the adhesive strength itself was low immediately after the production of the thickener compared to the Examples, which is not preferable.

[0120] Experimental Example 3: Resistance Evaluation <Secondary battery manufacturing> 1. Manufacturing the negative electrode A negative electrode slurry was prepared by mixing graphite as a negative electrode active material, styrene-butadiene rubber as a binder, and the thickener dispersion prepared in Example 1 that had been aged for 7 days. The weight ratio of graphite, styrene-butadiene rubber, and CMC in the thickener dispersion was 95:3:2. The solid content of the negative electrode slurry was 50 wt%.

[0121] The negative electrode slurry was applied to a copper current collector (thickness: 20 μm) as a current collector, and dried and rolled at 130° C. to form a negative electrode active material layer (thickness: 80 μm), thereby producing a negative electrode.

[0122] Furthermore, negative electrodes of Examples 2 to 5 and Comparative Examples 1 to 3 were produced by the same method as above, except that the thickener dispersions of Examples 2 to 5 and Comparative Examples 1 to 3 that were produced 7 days ago were used instead of the thickener dispersion of Example 1 that was produced 7 days ago.

[0123] 2. Secondary battery manufacturing An electrode assembly was manufactured by preparing the negative electrode of Example 1 as a working electrode and a lithium metal thin film as a counter electrode, and interposing a polyethylene separator between the negative electrode and the lithium metal thin film.

[0124] The electrode assembly was placed in a coin-type case, and a non-aqueous electrolyte was injected into the case, thereby fabricating a half-cell secondary battery of Example 1.

[0125] The non-aqueous electrolyte used in this experiment was an organic solvent made by mixing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 7:3, to which vinylene carbonate (0.5% by weight of the non-aqueous electrolyte) was added as an additive and LiPF6 (contained in the non-aqueous electrolyte to give a concentration of 1.0 M) as a lithium salt.

[0126] Half-cell secondary batteries of Examples 2 to 5 and Comparative Examples 1 to 3 were manufactured by the same method as above, except that the negative electrodes of Examples 2 to 5 and Comparative Examples 1 to 3 were used instead of the negative electrode of Example 1, respectively.

[0127] <Evaluation of discharge resistance> The half-cell secondary batteries of Examples 1 to 5 and Comparative Examples 1 to 3 prepared above were discharged at 1.5 C for 10 seconds in a state of charge of SOC 50%, and then the discharge resistance was measured. The results are shown in Table 5 below.

[0128] [Table 5]

[0129] Referring to Table 5, it can be seen that the secondary batteries of Examples 1 to 5 exhibit an excellent level of discharge resistance.

[0130] However, Comparative Example 3 is not preferable because the resistance is very high due to the excessive addition of the aging inhibitor.

[0131] On the other hand, Comparative Examples 1 and 2 show resistance at the same level as the Examples, but as mentioned above, in Comparative Examples 1 and 2, the thickener dispersion used changes over time, and the negative electrode adhesive strength is low, which is not preferable.

Claims

1. a thickener containing at least one of carboxymethyl cellulose and a metal salt of the carboxymethyl cellulose; an aging inhibitor; A thickener dispersion comprising: The aging inhibitor includes phenoxyethanol, the thickener is contained in the thickener dispersion in an amount of 0.05% by weight to 3.5% by weight; The thickener dispersion contains the aging inhibitor in an amount of 0.05% by weight to 3.0% by weight.

2. 2. The thickener dispersion according to claim 1, wherein at least one of the carboxymethyl cellulose and the metal salt of carboxymethyl cellulose has a weight average molecular weight of 1,000,000 to 5,000,000.

3. 2. The thickener dispersion according to claim 1, wherein the weight ratio of the thickener to the aging inhibitor is from 1:0.025 to 1:1.

25.

4. 2. The thickener dispersion according to claim 1, wherein the weight ratio of the thickener to the aging inhibitor is from 1:0.15 to 1:1.

15.

5. 2. The thickener dispersion according to claim 1, wherein the solids concentration of the thickener dispersion is 0.1% by weight to 6.5% by weight.

6. The thickener dispersion according to claim 1, wherein the viscosity change rate (%) of the thickener dispersion at 23°C calculated by the following mathematical formula 1 is -20% or more and 0% or less. [Mathematical formula 1] Viscosity change rate (%)={(viscosity of the thickener dispersion immediately after production and 7 days later)−(viscosity of the thickener dispersion immediately after production)} / (viscosity of the thickener dispersion immediately after production)×100

7. a negative electrode active material; Binder and A thickener dispersion according to any one of claims 1 to 6, and an aqueous solvent.

8. The negative electrode slurry according to claim 7 , wherein the negative electrode active material comprises a carbon-based active material.

9. A negative electrode comprising a negative electrode active material layer formed from the negative electrode slurry according to claim 7.

10. The negative electrode according to claim 9 , A positive electrode and a separator interposed between the negative electrode and the positive electrode; and an electrolyte.

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