Negative electrode slurry for lithium ion secondary battery and manufacturing method thereof

Abstract

To provide anode slurry for a lithium secondary battery, which is capable of improving stability of slurry by adsorbing hydrogen gas generated in the slurry to a gas adsorbent, and a manufacturing method therefor.SOLUTION: The anode slurry for a lithium secondary battery comprises: lithium-doped silicon oxide as an anode active material, water as a solvent, a binder, and single-walled carbon nanotubes functioning as a gas adsorbent; where the slurry has a pH of 10 or more.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a negative electrode slurry for a lithium-ion secondary battery and a method for producing the same.

Background Art

[0002] With the progress of technology development and the increase in demand for mobile devices, electric vehicles, etc., the demand for secondary batteries as an energy source has been rapidly increasing. Among such secondary batteries, lithium-ion secondary batteries having a high energy density, a high voltage, a long cycle life, and a low self-discharge rate have been put into practical use and are widely used. Currently, research on increasing the capacity of such lithium-ion secondary batteries is being vigorously promoted.

[0003] Silicon-based materials such as silicon oxide and silicon-based alloys have a higher theoretical capacity density than carbon-based materials such as graphite, which is currently mainstream, and thus are expected as negative electrode active materials for improving the energy density of lithium-ion secondary batteries and have been conventionally studied. Among silicon-based materials, silicon oxide (SiO x (0 < x < 2), for example, SiO) has a relatively low expansion rate and has been partially put into practical use. However, silicon oxide has a problem that the initial efficiency is low due to the irreversible decomposition of the lithium silicate phase formed during the first charge. In order to improve this problem, lithium is pre-doped into silicon oxide. As the amount of lithium pre-doped into silicon oxide increases, the initial efficiency will be further improved.

[0004] When forming a negative electrode using the above-described pre-lithium-doped silicon oxide as the negative electrode active material, a powder of the negative electrode active material is typically mixed with a binder, a solvent such as water, and certain additives (e.g., conductive material, thickener, etc.) to form a slurry, which is then applied to a current collector and dried to form the negative electrode. In this case, if the amount of lithium pre-doped into the silicon oxide is increased to improve initial efficiency, the pH of the slurry increases, and the lithium silicate phase formed inside the particles becomes unstable, resulting in the lithium in the lithium silicate reacting with water and generating hydrogen gas in the slurry. Furthermore, continuous lithium leaching not only changes the viscosity of the slurry, making it impossible to coat, but also reduces initial efficiency even if coating is possible, and furthermore, it reacts with trace amounts of water in the electrolyte, resulting in reduced lifespan and high-temperature storage characteristics.

[0005] Therefore, in order to stabilize the slurry, it has been proposed to form a lithium silicate phase that is stable to moisture as the main phase, or to add silicate or phosphate particles such as aluminum phosphate (see, for example, Patent Document 1).However, these conventional techniques are still insufficient in suppressing the generation of hydrogen gas in the slurry, and also have drawbacks such as a decrease in initial capacity due to the addition of the hydrogen gas. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Patent Publication No. 2021-093239 Summary of the Invention [Problem to be solved by the invention]

[0007] In view of the above-mentioned problems of the conventional art, the problem to be solved by the present invention is to provide a negative electrode slurry for lithium ion secondary batteries, which can simply and effectively deal with the generation of hydrogen gas in the slurry regardless of the type of lithium-doped silicon oxide, and can improve the stability of the slurry, and a method for producing the same. [Means for solving the problem]

[0008] According to one aspect of the present invention, there is provided a negative electrode slurry for a lithium ion secondary battery, the slurry comprising lithium-doped silicon oxide as a negative electrode active material, water as a solvent, a binder, and single-walled carbon nanotubes that function as a gas adsorbent, and the pH of the slurry is 10 or more.

[0009] In the slurry of the above embodiment, the content of the single-walled carbon nanotubes can be 0.1% to 2.0% based on the total mass of the solid content in the slurry.

[0010] The slurry of the above embodiment may further contain a graphite-based material as a negative electrode active material.

[0011] In the embodiment further comprising the graphite-based material, the mass ratio of the graphite-based material to the lithium-doped silicon oxide (graphite-based material:lithium-doped silicon oxide) can be 98:2 to 50:50.

[0012] In the embodiment further comprising the graphite-based material, the content of the single-walled carbon nanotubes can be 0.01% to 0.5% based on the total mass of the solid content in the slurry.

[0013] The slurry of the above embodiment may further contain at least one of a conductive material and a thickener.

[0014] According to another aspect of the present invention, there is provided a method for producing a negative electrode slurry for a lithium ion secondary battery, the method including the steps of: mixing a conductive material and a thickener to form a first mixture; kneading the first mixture with water as a solvent and single-walled carbon nanotubes that function as a gas adsorbent to form a second mixture; kneading the second mixture with lithium-doped silicon oxide as a negative electrode active material to form a third mixture; and mixing the third mixture with a binder to form a negative electrode slurry for a lithium ion secondary battery.

[0015] In the step of forming the first mixture in the method of the above embodiment, a graphite-based material may be further added and mixed as a negative electrode active material.

[0016] In the method of the above embodiment, the single-walled carbon nanotubes serving as the gas adsorbent can adsorb hydrogen gas generated in the slurry.

[0017] According to another aspect of the present invention, there is provided a secondary battery manufactured using the slurry manufactured using the method for manufacturing a negative electrode slurry for a lithium ion secondary battery. [Effects of the Invention]

[0018] By using carbon nanotubes as a gas adsorbent during the production of anode slurry containing lithium-doped silicon oxide and a pH of 10 or higher, the hydrogen gas generated by the reaction of lithium in the lithium silicate phase with water can be adsorbed, maintaining the stability of the slurry. This allows for the production of uniform electrodes using a slurry with a constant viscosity even after a long period of time has passed since the slurry was produced. Furthermore, the occurrence of pinholes on the coated surface due to uncontrolled hydrogen gas release is suppressed. [Brief explanation of the drawings]

[0019] [Figure 1] 1 is a graph showing the change over time in the amount of hydrogen gas generated. [Figure 2] 1 is a graph showing the change over time in the amount of hydrogen gas generated. [Figure 3] 1 is a graph showing the change over time in the amount of hydrogen gas generated. DETAILED DESCRIPTION OF THE INVENTION

[0020] Hereinafter, embodiments of the present invention will be described, but the present invention is not limited to the following embodiments.

[0021] Throughout this application, unless otherwise specified, the "average particle size" refers to the particle size at the integrated value of 50% in the volume-based particle size distribution measured by the laser diffraction scattering method, that is, the median diameter (D 50 50

[0022] [Negative electrode slurry for lithium-ion secondary battery] The negative electrode slurry for a lithium-ion secondary battery according to an embodiment of the present invention includes at least lithium-doped silicon oxide as a negative electrode active material, water as a solvent, a binder, and single-walled carbon nanotubes functioning as a gas adsorbent, and the pH of the slurry is 10 or more. In other embodiments, the slurry may further include other components described below. Hereinafter, each component of the slurry will be described.

[0023] (Negative electrode active material) <1. Lithium-doped silicon oxide> The negative electrode slurry for a lithium-ion secondary battery of the present invention includes silicon oxide as a negative electrode active material. The silicon oxide is represented by the general formula SiO x where x satisfies 0 < x < 2, and for example, it can be silicon monoxide SiO (x = 1). Silicon monoxide is preferred because of its particularly low expansion rate. The silicon oxide has, for example, a structure in which Si microparticles are dispersed in the form of microcrystals or amorphous in an amorphous silicon oxide matrix, and its average composition can be represented by the general formula SiO x . The silicon oxide may contain only SiO x having a specific value of x, or may be a mixture of two or more SiO x having different values of x.

[0024] The silicon oxide is doped with lithium in advance. The lithium is doped into the surface and / or interior of the silicon oxide particles. The method for doping the silicon oxide with lithium is not particularly limited. For example, when using a thermal doping method, the silicon oxide particles are mixed with a lithium source (metallic lithium or a lithium compound (LiH, etc.)) and baked at a high temperature (e.g., about 350°C to 900°C) in an inert atmosphere (e.g., an argon atmosphere or a nitrogen atmosphere), thereby doping the silicon oxide with lithium. Alternatively, lithium can be doped into the silicon oxide using a redox reaction method or an electrochemical method. The doped lithium can react with the silicon oxide to form lithium silicate (e.g., Li2SiO5, Li2SiO3, Li4SiO4, Li4SiO4, etc.).

[0025] The content of doped lithium can be, for example, 1% to 20% or 1% to 10% based on the total mass of the final negative electrode active material (i.e., silicon oxide after lithium doping). The higher the content of doped lithium, the more improved the initial efficiency.

[0026] The lithium-doped silicon oxide may be in the form of particles, and the average particle size (D 50 ) is not particularly limited, but can be, for example, 0.1 μm or more and 10 μm or less, particularly 1 μm or more and 9 μm or less, or 1 μm or more and 2 μm or less.

[0027] <2. Graphite-based materials> According to another embodiment of the present invention, the negative electrode slurry for a lithium ion secondary battery may further include, in addition to the lithium-doped silicon oxide as the negative electrode active material, a graphite-based material as the negative electrode active material (i.e., a graphite-based negative electrode active material). The graphite-based material includes at least one of artificial graphite and natural graphite.

[0028] Artificial graphite is graphite that is industrially produced by firing (i.e., graphitizing) easily graphitizable carbon materials such as coke and coal tar pitch at high temperatures (e.g., about 2800°C). For example, mesocarbon microbeads, mesocarbon fiber, massive artificial graphite, etc. are known as artificial graphite, but the artificial graphite that can be used in the present invention is not particularly limited. In general, artificial graphite is harder than natural graphite, and is known to expand less during charge and discharge than natural graphite when used in the negative electrode of a lithium ion secondary battery.

[0029] Natural graphite is graphite produced by mining graphite ore and subjecting it to processing such as ore dressing and refining. Known natural graphite comes in flake, lump, earthen, etc. forms, but the natural graphite that can be used in the present invention is not particularly limited.

[0030] Average particle size of graphite-based materials (D 50 ) can be, for example, 3 μm or more and 30 μm or less, preferably 5 μm or more and 25 μm or less, or 15 μm or more and 20 μm or less.

[0031] When both a graphite-based material and lithium-doped silicon oxide are used as negative electrode active materials, the mass ratio of the graphite-based material to the lithium-doped silicon oxide in the negative electrode active material can be selected in consideration of the expansion coefficient and capacity of the lithium-doped silicon oxide relative to the graphite-based material. For example, the mass ratio of the graphite-based material to the lithium-doped silicon oxide (graphite-based material:lithium-doped silicon oxide) can be 98:2 to 50:50, 98:2 to 70:30, or 90:10 to 80:20.

[0032] When only lithium-doped silicon oxide is used as the negative electrode active material, the content of the negative electrode active material (i.e., lithium-doped silicon oxide) in the slurry can be, for example, 70% to 99%, preferably 80% to 98%, based on the total mass of the solids in the slurry. When both lithium-doped silicon oxide and a graphite-based material are used as the negative electrode active materials, the content of the negative electrode active materials (i.e., lithium-doped silicon oxide and graphite-based material) in the slurry can be within the same range (70% to 99%) as above, but considering the expansion coefficients and capacities of both materials, the content can be higher than when only lithium-doped silicon oxide is used, for example, 95.5% based on the total mass of the solids in the slurry.

[0033] (solvent) The negative electrode slurry for a lithium ion secondary battery of the present invention contains water as a solvent. The water is preferably pure water, particularly deionized water. The solvent may be contained in an amount such that the content of the solid content (i.e., components other than the solvent) of the slurry is 30 to 85 mass %, preferably 60 to 70 mass %, and in one example, 65 mass %.

[0034] (binder) The binder is added as a component that promotes bonding between the active material and the conductive material, or between the active material and the current collector. Because the solvent used in the present invention is water, the binder of the present invention is a water-soluble aqueous binder. Examples of aqueous binders include styrene-butadiene rubber (SBR), polyvinyl alcohol (PVA), polyacrylonitrile, polyvinylpyrrolidone, polyacrylic acid, acrylamide, polyimide, fluororubber, and urethane rubber. One or a mixture of two or more of these may be used, but the binder is not limited to these. Preferably, styrene-butadiene rubber (SBR) is used, as its environmental impact reduction, improved battery performance, and cost reduction have been particularly proven.

[0035] The binder content may be 0.1% to 30%, preferably 0.5% to 20%, and more preferably 1% to 10% based on the total mass of the solid content of the slurry. When the binder content satisfies the above range, sufficient adhesive strength can be imparted to the electrode while preventing a decrease in the capacity characteristics of the battery.

[0036] (gas absorbent) The negative electrode slurry for lithium-ion secondary batteries of the present invention is characterized by containing single-walled carbon nanotubes (SWCNTs) that function as a gas adsorbent. As described above, in a slurry using lithium-doped silicon oxide as the negative electrode active material, lithium reacts with water to generate hydrogen gas. This reaction is expressed as 2Li + 2H2O → 2LiOH + H2. The gas adsorbent is used to adsorb this hydrogen gas. By adsorbing the hydrogen gas generated in the slurry into the gas adsorbent, the stability of the slurry is maintained, and the viscosity of the slurry remains constant even after the passage of time since the slurry was produced.

[0037] Single-walled carbon nanotubes have a unique hollow structure in which one graphene sheet is rolled into a cylindrical shape. They have a large specific surface area and can adsorb and store large amounts of various gases, particularly hydrogen gas. Hydrogen gas is adsorbed in the internal space and outer surface (outer wall) of the single-walled carbon nanotube. Furthermore, when multiple single-walled carbon nanotubes (e.g., 6 to 20, in one example, 12) form a bundle (nanotube bundle), hydrogen gas is also adsorbed in the interstitial spaces between the carbon nanotubes.

[0038] The single-walled carbon nanotubes may be subjected to an open-end treatment in which the ends of the carbon nanotubes are opened by heating in a vacuum (for example, at a temperature between 600°C and 1000°C, e.g., at 700°C) to promote the adsorption of hydrogen into the interior of the carbon nanotubes. In this way, the single-walled carbon nanotubes of the present invention may be configured to function particularly as gas adsorbents.

[0039] The average diameter of the single-walled carbon nanotubes can be, for example, 0.6 to 10 nm, preferably 0.8 to 5 nm, more preferably 0.8 to 3 nm, such as 1.2 nm or 1.9 nm. The average length of the single-walled carbon nanotubes can be 0.5 to 20 μm, preferably 0.5 to 10 μm, more preferably 0.5 to 5 μm. The average diameter and average length of the single-walled carbon nanotubes can be measured by photographing them with a scanning electron microscope (SEM), and are, for example, the average values ​​of 10 to 100 carbon nanotubes measured in the SEM image.

[0040] Since single-walled carbon nanotubes tend to aggregate due to interactions with each other, in order to prevent excessive aggregate formation, the carbon nanotubes can be provided as a single-walled carbon nanotube dispersion that can properly disperse and retain the single-walled carbon nanotubes. The carbon nanotube dispersion contains single-walled carbon nanotubes, a dispersant, and a solvent (e.g., water). When water is used as the solvent, the resulting dispersion is a single-walled carbon nanotube aqueous dispersion, and the water can be used as a solvent for the slurry as is. Examples of dispersing agents include polyvinylpyrrolidone, polyacrylic acid hydrazide, poly-N-vinyl-5-methoxazolidone, N-alkyl polyimine, N-acetyl polyimine, polyacrylamide, poly-L-lysine hydrobromide, benzyl-dodecyl-dimethylammonium chloride, and polyethyleneimine, and these may be used alone or as a mixture of two or more kinds. The content of single-walled carbon nanotubes in the dispersion is not particularly limited, but can be, for example, 0.01% to 5%, preferably 0.01% to 3%, more preferably 0.1% to 2%, even more preferably 0.1% to 1%, and as an example, 0.4%, based on the total mass of the carbon nanotube dispersion.

[0041] The single-walled carbon nanotubes are included in an amount sufficient to effectively adsorb hydrogen gas generated in the slurry. Generally, the greater the amount of single-walled carbon nanotubes, the greater the amount of hydrogen gas adsorbed. When using only lithium-doped silicon oxide as the negative electrode active material, the content of the single-walled carbon nanotubes in the slurry may be, for example, 0.1% to 10%, 0.1% to 5.0%, or 0.1% to 2.0% based on the total mass of the solids in the slurry. When using both lithium-doped silicon oxide and a graphite-based material as the negative electrode active material, the amount of hydrogen gas generated decreases as the proportion of lithium-doped silicon oxide decreases, so the content of the single-walled carbon nanotubes in the slurry may be reduced accordingly. The content of the single-walled carbon nanotubes in the slurry may be, for example, 0.01% to 1.0%, or 0.01% to 0.5% based on the total mass of the solids in the slurry.

[0042] (Conductive material) In some embodiments, the negative electrode slurry for lithium ion secondary batteries may further contain a conductive material. The conductive material is not particularly limited as long as it is an electrically conductive material that does not induce chemical changes. Examples of conductive materials include amorphous carbon-based materials (carbon-based materials added separately from the graphite-based material used as the negative electrode active material) such as carbon black, acetylene black, ketjen black, denka black, thermal black, channel black, furnace black, and lamp black; metal powders and fibers of aluminum, tin, bismuth, silicon, antimony, nickel, copper, titanium, vanadium, chromium, manganese, iron, cobalt, zinc, molybdenum, tungsten, silver, gold, lanthanum, ruthenium, platinum, and iridium; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive polymers such as polyaniline, polythiophene, polyacetylene, polypyrrole, and polyphenylene derivatives. These materials may be used singly or in combination. However, the conductive materials are not limited to these.

[0043] The content of the conductive material may be 0.1% to 30%, preferably 0.5% to 15%, and more preferably 0.5% to 10%, based on the total mass of the solid content in the slurry. When the content of the conductive material satisfies the above range, sufficient conductivity can be imparted and the amount of negative electrode active material is not reduced, which is advantageous in that battery capacity can be secured.

[0044] (thickener) In some embodiments, the negative electrode slurry for a lithium ion secondary battery may further include a thickener. Specifically, the thickener may be a cellulose-based compound. Examples of the cellulose-based compound include carboxymethyl cellulose (CMC), methyl cellulose (MC), hydroxypropyl cellulose (HPC), methyl hydroxypropyl cellulose (MHPC), ethyl hydroxyethyl cellulose (EHEC), and methyl ethyl hydroxyethyl cellulose (MEHEC). Any one or a combination of two or more of these may be used. The content of the thickener may be, for example, 0.1% to 10%, or 0.5% to 10%, based on the total mass of the solids in the slurry.

[0045] The pH of negative electrode slurry for lithium-ion secondary batteries is determined by the composition and amount of each component in the slurry, and is neutral at around 7 to 8 when graphite or a silicon-based material that is not doped with an alkali such as lithium is used as the active material. When lithium-doped silicon oxide is used as the active material, the pH increases as the amount of doped lithium increases, reaching a high value of 10 or higher.

[0046] [Method for producing negative electrode slurry for lithium ion secondary batteries] Next, a method for producing the negative electrode slurry for a lithium ion secondary battery will be described. The method for producing the negative electrode slurry for a lithium ion secondary battery according to the present invention is characterized by the order in which raw materials are added, specifically, by adding lithium-doped silicon oxide as the negative electrode active material as late as possible in the production process. As a result, kneading is performed in the early stage of the production process in a state where the solid concentration is low due to the absence of lithium-doped silicon oxide, and by adding lithium-doped silicon oxide in the later stage of the production process, it is possible to minimize the generation of hydrogen gas due to the reaction between lithium and water.

[0047] In one embodiment, a method for producing a negative electrode slurry for a lithium-ion secondary battery includes a slurry containing lithium-doped silicon oxide as a negative electrode active material, water as a solvent, a binder, single-walled carbon nanotubes as a gas adsorbent, and a conductive material and an adhesive. The method includes the steps of: mixing the conductive material and a thickener to form a first mixture; adding water and single-walled carbon nanotubes to the first mixture and kneading them to form a second mixture; adding the lithium-doped silicon oxide to the second mixture and kneading them to form a third mixture; and adding the binder to the third mixture and mixing them to form the negative electrode slurry for a lithium-ion secondary battery.

[0048] In another embodiment, the slurry contains lithium-doped silicon oxide as the negative electrode active material, water as the solvent, a binder, and single-walled carbon nanotubes as the gas adsorbent, as well as a graphite-based material as the negative electrode active material, a conductive material, and an adhesive. The method includes the steps of: mixing the graphite-based material as the negative electrode active material, the conductive material, and a thickener to form a first mixture; adding water and the single-walled carbon nanotubes to the first mixture and kneading them to form a second mixture; adding lithium-doped silicon oxide to the second mixture and kneading them to form a third mixture; and adding the binder to the third mixture and mixing them to form a negative electrode slurry for a lithium-ion secondary battery.

[0049] When a binder is used without a thickener, a portion of the binder may be added in place of the thickener in the step of forming the first mixture.

[0050] [Manufacturing of negative electrodes] The negative electrode slurry for a lithium ion secondary battery is applied to a negative electrode current collector, followed by drying and rolling, to produce a negative electrode in which a negative electrode active material layer is formed on the negative electrode current collector. Prior to application of the negative electrode slurry, a solvent may be further added to the kneaded slurry to facilitate application, and then the application may be carried out.

[0051] Alternatively, the negative electrode may be produced by, for example, casting the negative electrode slurry onto a separate support, peeling the film from the support, and laminating the resulting film onto a negative electrode current collector. Alternatively, the negative electrode active material layer may be formed on the negative electrode current collector by any other method.

[0052] Heating may be carried out during the drying, rolling and casting steps in the production of the negative electrode.

[0053] (Negative electrode current collector) The negative electrode current collector used in the negative electrode is not particularly limited as long as it does not induce chemical changes in the battery and has conductivity. For example, the negative electrode current collector may be made of copper, stainless steel, aluminum, nickel, titanium, calcined carbon, aluminum-cadmium alloy, or a surface-treated copper or stainless steel (surface-treated with carbon, nickel, titanium, silver, etc.).

[0054] The negative electrode current collector may have a thickness of 3 μm to 500 μm. Fine irregularities may be formed on the surface of the negative electrode current collector to enhance adhesion to the negative electrode active material. The negative electrode current collector may have various forms, such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0055] [Lithium-ion secondary battery] In addition to the negative electrode, a lithium ion secondary battery also includes a positive electrode, a separator interposed between the negative electrode and the positive electrode, and a non-aqueous electrolyte. The components other than the negative electrode are well known in the art, and therefore will not be described in detail here.

[0056] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to these examples.

[0057] [Example 1] The negative electrode active material (hereinafter referred to as "AM") was lithium-doped SiO2 with an average particle size of 7 μm and a lithium content of 9% by mass. The conductive material was carbon black (hereinafter referred to as "CB"), the gas adsorbent was single-walled carbon nanotubes (hereinafter referred to as "SWCNTs"; a 0.4% solids aqueous dispersion of single-walled carbon nanotubes was used), the binder was styrene-butadiene rubber (hereinafter referred to as "SBR"), and the thickener was carboxymethyl cellulose (hereinafter referred to as "CMC") in a mass ratio of 89.0% AM, 3.3%, 0.1%, 3.6%, and 4.0%. The carbon black and carboxymethyl cellulose were then mixed, followed by the addition of the aqueous dispersion of single-walled carbon nanotubes and water, and thoroughly kneaded. The lithium-doped SiO2 was then added and kneaded. Finally, the styrene-butadiene rubber was added and mixed to produce a negative electrode slurry for lithium-ion secondary batteries. At this time, the pH of the slurry was 12.5.

[0058] [Example 2] A negative electrode slurry for a lithium ion secondary battery was produced under the same conditions as in Example 1, except that the mass ratio of the solid contents of the slurry was AM / CB / SWCNT / SBR / CMC: 89.0% / 3.2% / 0.2% / 3.6% / 4.0%.

[0059] [Example 3] A negative electrode slurry for a lithium ion secondary battery was produced under the same conditions as in Example 1, except that the mass ratio of the solid contents of the slurry was AM / CB / SWCNT / SBR / CMC: 89.0% / 3.1% / 0.3% / 3.6% / 4.0%.

[0060] [Example 4] A negative electrode slurry for a lithium ion secondary battery was produced under the same conditions as in Example 1, except that the mass ratio of the solid contents of the slurry was AM / CB / SWCNT / SBR / CMC, which was 89.0% / 3.0% / 0.4% / 3.6% / 4.0%.

[0061] [Example 5] A negative electrode slurry for a lithium ion secondary battery was produced under the same conditions as in Example 1, except that the mass ratio of the solid contents of the slurry was AM / CB / SWCNT / SBR / CMC: 89.0% / 2.9% / 0.5% / 3.6% / 4.0%.

[0062] [Example 6] The negative electrode active material ("AM") was a mixture of artificial graphite with an average particle size of 20 μm and lithium-doped SiO2 with an average particle size of 7 μm and a Li content of 9% by mass, with a mass ratio (artificial graphite / lithium-doped SiO2) of 85% / 15%. The materials included carbon black ("CB") as a conductive material, single-walled carbon nanotubes ("SWCNTs"; a 0.4% solids aqueous dispersion of single-walled carbon nanotubes) as a gas adsorbent, styrene-butadiene rubber ("SBR") as a binder, and carboxymethyl cellulose ("CMC") as a thickener, with a mass ratio of solids of 95.5% / 0.99% / 0.01% / 2.5% / 1.0% (AM / CB / SWCNT / SBR / CMC). The artificial graphite, carbon black, and carboxymethyl cellulose were then mixed and thoroughly kneaded with water added as needed. The solids concentration at this time was 65%. The carbon nanotube aqueous dispersion was then added to this slurry and kneaded, followed by the addition of lithium-doped SiO2 for a predetermined period of time. The solids concentration at this stage was 50%. Finally, styrene-butadiene rubber was added and mixed to produce a negative electrode slurry for lithium-ion secondary batteries. The pH of the slurry at this stage was 11.9.

[0063] [Example 7] A negative electrode slurry for a lithium ion secondary battery was produced under the same conditions as in Example 6, except that the mass ratio of the solid contents of the slurry was AM / CB / SWCNT / SBR / CMC: 95.5% / 0.97% / 0.03% / 2.5% / 1.0%.

[0064] [Example 8] A negative electrode slurry for a lithium ion secondary battery was produced under the same conditions as in Example 6, except that the mass ratio of the solid contents of the slurry was AM / CB / SWCNT / SBR / CMC: 95.5% / 0.95% / 0.05% / 2.5% / 1.0%.

[0065] [Example 9] A negative electrode slurry for a lithium ion secondary battery was produced under the same conditions as in Example 6, except that the mass ratio of the solid contents of the slurry was AM / CB / SWCNT / SBR / CMC: 95.5% / 0.93% / 0.07% / 2.5% / 1.0%.

[0066] [Comparative Example 1] A negative electrode slurry for a lithium ion secondary battery was produced under the same conditions as in Example 1, except that single-walled carbon nanotubes were not used as a gas adsorbent and the mass ratio of the solid content of the slurry was adjusted to AM / CB / SBR / CMC to 89.0% / 3.4% / 3.6% / 4.0%.

[0067] Comparative Example 2 A negative electrode slurry for a lithium ion secondary battery was produced under the same conditions as in Example 6, except that single-walled carbon nanotubes were not used as a gas adsorbent and the mass ratio of the solid content of the slurry was adjusted to AM / CB / SBR / CMC to 95.5% / 1.0% / 2.5% / 1.0%.

[0068] Comparative Example 3 First, lithium-doped SiO, artificial graphite, carbon black, and carboxymethyl cellulose were mixed and thoroughly kneaded while adding water as needed. The solids concentration at this time was 65%. Next, a carbon nanotube aqueous dispersion was added, and after kneading for a predetermined time, styrene-butadiene rubber was added and mixed. A negative electrode slurry for a lithium ion secondary battery was produced under the same conditions as in Example 6, except that water was added to adjust the final solids content to 50%. The pH of the slurry at this time was 12.1.

[0069] [Experimental example: Measurement of changes in gas generation rate over time] Ten grams of each of the negative electrode slurries for lithium ion secondary batteries produced in Examples 1 to 9 and Comparative Examples 1 to 3 were weighed out, placed in an aluminum laminate pouch, sealed under reduced pressure, and the volume was measured by Archimedes' method. The volume was then measured every 24 hours for one week using the same method. The measurement results for Examples 1 to 5 and Comparative Example 1 are shown in Table 1 and Figure 1 below, and the measurement results for Examples 6 to 9 and Comparative Examples 2 and 3 are shown in Table 2 and Figures 2 and 3 below.

[0070] [Table 1]

[0071] [Table 2]

[0072] As shown in Table 1, Table 2, Figures 1, 2, and 3, the volume increase and change are significantly suppressed in Examples 1 to 9 compared to Comparative Examples 1 and 2, which do not use single-walled carbon nanotubes, demonstrating that the generated hydrogen gas is adsorbed by the single-walled carbon nanotubes in the slurry. Furthermore, when Examples 1 to 5 are compared with each other, and similarly when Examples 6 to 9 are compared with each other, it is demonstrated that the volume increase and change are more suppressed as the amount of single-walled carbon nanotubes increases, which means that a larger amount of hydrogen gas is adsorbed.

[0073] 3, comparing Example 8 with Comparative Example 3, in which lithium-doped silicon oxide was first added and kneaded together with graphite and carbon black during slurry preparation, the lithium-doped silicon oxide remained in the slurry for a long time, and kneading at a high solids concentration physically damaged the surface of the silicon oxide, making it easier for lithium to leach out from within the particles, resulting in a large amount of gas generation. Therefore, it has been demonstrated that gas generation can be suppressed by adding lithium-doped silicon oxide at a low solids concentration after kneading at a high solids concentration has been completed, and then kneading in a short time without damaging the surface, as in Example 8.

Claims

1. A negative electrode slurry for a lithium ion secondary battery, Lithium-doped silicon oxide as a negative electrode active material; Water as a solvent; Binder and and single-walled carbon nanotubes having open ends to function as gas adsorbents; The slurry has a pH of 10 or greater.

2. 2. The slurry of claim 1, wherein the content of the single-walled carbon nanotubes is 0.1% to 2.0% based on the total mass of solids in the slurry.

3. The slurry according to claim 1 , further comprising a graphite-based material as a negative electrode active material.

4. 4. The slurry according to claim 3, wherein a mass ratio of the graphite-based material to the lithium-doped silicon oxide (graphite-based material:lithium-doped silicon oxide) is 98:2 to 50:

50.

5. 5. The slurry according to claim 3, wherein the content of the single-walled carbon nanotubes is 0.01% to 0.5% based on the total mass of the solid content in the slurry.

6. The slurry according to claim 1 , further comprising at least one of a conductive material and a thickener.

7. A method for producing a negative electrode slurry for a lithium ion secondary battery, comprising: mixing a conductive material and a thickener to form a first mixture; kneading the first mixture with water as a solvent and single-walled carbon nanotubes as a gas adsorbent to form a second mixture; kneading the second mixture with lithium-doped silicon oxide as a negative electrode active material to form a third mixture; and mixing the third mixture with a binder to form a negative electrode slurry for a lithium ion secondary battery.

8. The method according to claim 7 , wherein a graphite-based material as a negative electrode active material is further added and mixed in the forming of the first mixture.

9. The method according to claim 7 or 8, wherein the single-walled carbon nanotubes functioning as a gas adsorbent adsorb hydrogen gas generated in the slurry.

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