Electrode composition for lithium ion secondary battery negative electrode and negative electrode

Abstract

To provide an electrode composition for a negative electrode of a lithium ion secondary battery, and an electrode, which have excellent electrode properties.SOLUTION: An electrode composition for a lithium ion secondary battery negative electrode includes a negative electrode active material containing graphite and a composite oxide, carbon nanotubes, a composite adhesive containing water-soluble cellulose, and a foil adhesive containing a synthetic resin, and the carbon nanotubes have a G band position wavenumber of 1580 cm-1 or more in a Raman spectrum, a total heavy metal content of less than 10,000 ppm, and a total inorganic anion content of less than 100 ppm, and the negative electrode active material contains 10 mass% or more of the composite oxide relative to the total amount of the negative electrode active material.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to an electrode composition for a negative electrode of a lithium-ion secondary battery.

Background Art

[0002] In recent years, with the spread of electronic devices and mobility considering environmental aspects, the lithium-ion battery market has been booming. A lithium-ion battery includes a negative electrode and a positive electrode containing an active material that reversibly allows lithium ions to enter and exit, and an electrolyte that immerses them. The electrodes are manufactured by applying a slurry composed of an active material, a conductive material, a binder, etc. to a current collector plate made of aluminum foil or the like, drying it, and pressing it.

[0003]

[0004] Patent Document 1 discloses a carbon nanotube dispersion liquid that contains at least a carbon nanotube, a water-soluble polymer material, and a dispersion medium, and the Y of the XYZ color system of a dry coating film produced by applying a dispersion liquid adjusted to a carbon nanotube concentration of 2% by mass with an applicator having a gap of 50 μm is 6.0 or less.

[0005]

Prior Art Documents

Patent Documents

[0006] [Patent Document 1] JP 2023-29200 (Claims, Detailed Description of the Invention) [Patent Document 2] JP 2023-28960 (Claims, Detailed Description of the Invention) [Patent Document 3] JP 2023-13185 (Detailed description of the invention, Examples) Summary of the Invention [Problem to be solved by the invention]

[0007] Electrodes for lithium-ion secondary batteries are required to have electrode properties such as a large discharge capacity as a capacity characteristic, a high resistance to high-power charge and discharge as an output characteristic, and a high resistance to repeated charge and discharge as a cycle characteristic.

[0008] The problem to be solved by the present invention is to provide an electrode composition for a negative electrode of a lithium ion secondary battery, or an electrode for a negative electrode of a lithium ion secondary battery, which has excellent electrode characteristics as a lithium ion secondary battery electrode, such as capacity characteristics, output characteristics, and cycle characteristics. [Means for solving the problem]

[0009] The present inventors have pursued the composition of an electrode composition for a negative electrode of a lithium ion secondary battery, and as a result have completed the present invention.

[0010] That is, the present invention provides a composition containing a negative electrode active material including graphite and a composite oxide, carbon nanotubes, a composite adhesive including water-soluble cellulose, and a foil adhesive including a synthetic resin, The carbon nanotube has a G band position in the Raman spectrum at a wave number of 1580 cm -1 The total content of heavy metals is less than 10,000 ppm, and the total content of inorganic anions is less than 100 ppm. The electrode composition for a lithium ion secondary battery negative electrode is characterized in that the negative electrode active material contains a composite oxide in an amount of 10 mass % or more relative to the total amount of the negative electrode active material. [Effects of the Invention]

[0011] The electrode composition (hereinafter also referred to as composite material) of the present invention provides a lithium ion secondary battery negative electrode having excellent electrode characteristics as a lithium ion secondary battery negative electrode, such as capacity characteristics, output characteristics, and cycle characteristics. Specifically, the negative electrode of a lithium ion secondary battery having the electrode composition of the present invention has a large discharge capacity during charge and discharge as a capacity characteristic. Furthermore, the electrode composition of the present invention has a high resistance to high-power charge and discharge as an output characteristic. Furthermore, the electrode composition of the present invention has a high resistance to repeated charge and discharge as a cycle characteristic. DETAILED DESCRIPTION OF THE INVENTION

[0012] The present invention relates to an electrode composition for a negative electrode of a lithium ion secondary battery. The electrode composition of the present invention is a solid composition formed on a current collector foil, the solid composition including at least a negative electrode active material, carbon nanotubes, a composite adhesive, and a foil adhesive.

[0013] <Negative electrode active material> The negative electrode active material to be blended in the electrode composition for a lithium ion secondary battery negative electrode of the present invention is a combination of graphite and a metal oxide, and the blending ratio of the two in the negative electrode active material is such that the content of the metal oxide is 10 mass % or more relative to the total amount of the negative electrode active material.

[0014] The graphite to be blended as the negative electrode active material may be artificial graphite, natural graphite, kish graphite, expanded graphite, expanded graphite, etc. As the artificial graphite, graphite produced by incomplete combustion of hydrocarbons such as heavy aromatic oils and gases may be used, and preferably, natural graphite such as flake natural graphite may be used.

[0015] Examples of metal oxides that can be blended as negative electrode active materials include metal oxide-based active material particles and silicon-based active material particles. The metal oxide-based negative electrode active material particles may be, for example, titanium oxide. The titanium oxide is not limited as long as it can absorb and release lithium. For example, spinel-type lithium titanate, ramsdellite-type lithium titanate, titanium-containing metal composite oxide, titanium dioxide (TiO(B)) having a monoclinic crystal structure, and anatase-type titanium dioxide may be suitably used. As the silicon-based active material particles, metal silicon and silicon monoxide can be used.

[0016] Spinel-type lithium titanate includes Li 4+x Ti5O 12 (x varies in the range of -1≦x≦3 depending on the charge / discharge reaction). Ramsdellite-type lithium titanate includes Li 2+y Ti3O7 (where y varies in the range of -1≦y≦3 depending on the charge / discharge reaction). TiO2(B) and anatase-type titanium dioxide include Li 1+z Examples include TiO2 (where z changes in the range of -1≦z≦0 depending on the charge / discharge reaction).

[0017] Examples of titanium-containing metal composite oxides include metal composite oxides containing Ti and at least one element selected from the group consisting of P, V, Sn, Cu, Ni, and Fe. Examples of metal composite oxides containing Ti and at least one element selected from the group consisting of P, V, Sn, Cu, Ni, Nb, and Fe include TiO2-P2O5, TiO2-V2O5, TiO2-P2O5-SnO2, and TiO2-P2O5-MeO (Me represents at least one element selected from the group consisting of Cu, Ni, Nb, and Fe).

[0018] Such a metal composite oxide preferably has a microstructure with low crystallinity, in which a crystalline phase and an amorphous phase coexist, or in which an amorphous phase exists alone, which can further improve cycle performance. As the negative electrode active material, components other than graphite and metal oxide may be added.

[0019] <Carbon nanotubes> The carbon nanotubes (CNTs) to be blended in the electrode composition for a lithium ion secondary battery negative electrode of the present invention are preferably carbon nanotubes having a shape substantially formed by rolling one surface of graphite into a cylindrical shape, and either single-walled carbon nanotubes, in which one surface of graphite is rolled into one layer, or multi-walled carbon nanotubes, in which one surface of graphite is rolled into two or more layers, can be used.

[0020] In the electrode composition of the present invention, the wave number of the G band position in the Raman spectrum is 1580 cm -1 As described above, carbon nanotubes having a total heavy metal content of less than 10,000 ppm and a total inorganic anion content of less than 100 ppm are used.

[0021] The carbon nanotubes in the carbon nanotube slurry of the present invention have a G band position of 1580 cm in Raman spectroscopy. -1 Above 1580cm, preferably 1580cm -1 ~1586cm -1 Here, the G band position is the position at 1570 cm in the Raman spectrum. -1 ~1620cm -1 The Raman spectrum can be measured by detecting Raman scattered light in a coating film obtained by drying the carbon nanotube slurry using a Raman spectrometer.

[0022] The wave number of the G band position is an index of the degree of unraveling of the carbon nanotubes in the carbon nanotube slurry and the electrode composition. -1 This indicates that the carbon nanotubes are adequately dissolved in the slurry. The wave number of the G band position of the carbon nanotubes in the carbon nanotube slurry is 1580 cm -1If the content is less than 100%, the carbon nanotubes are not sufficiently dissolved in the slurry, and a negative electrode made of an electrode composition containing such carbon nanotubes does not exhibit good electrical properties.

[0023] Carbon nanotubes generally contain large amounts of heavy metals derived from the manufacturing process. In particular, when Fe and Co are used as manufacturing catalysts, commercially available carbon nanotubes often contain large amounts of Fe and Co, for example, a total of 10,000 ppm (mg / kg, the same applies below) or more. In addition, small amounts of inorganic anions such as sulfate ions and chloride ions may also be contained.

[0024] The carbon nanotubes blended in the electrode composition of the present invention preferably have a smaller content of heavy metals and inorganic anions, specifically, a total content of heavy metals in the carbon nanotubes of less than 10,000 ppm and a total content of inorganic anions of less than 100 ppm.

[0025] Here, the total amount of heavy metals refers to the sum of the content of each heavy metal, such as iron (Fe), lead (Pb), gold (Au), platinum (Pt), silver (Ag), copper (Cu), chromium (Cr), cadmium (Cd), mercury (Hg), zinc (Zn), manganese (Mn), cobalt (Co), nickel (Ni), molybdenum (Mo), tungsten (W), tin (Sn), bismuth (Bi), etc. The content of heavy metals can be measured using an inductively coupled plasma (ICP) emission spectrometer.

[0026] The total content of inorganic anions refers to the total content of inorganic anions such as halogen ions, sulfate ions, nitrate ions, phosphate ions, etc. The content of inorganic anions can be measured using ion chromatography.

[0027] The average outer diameter of the carbon nanotubes is preferably 1 nm or more and 30 nm or less, more preferably 3 nm or more and 15 nm or less, from the viewpoints of viscosity, conductivity, and stability of the composite. Here, the average outer diameter of the carbon nanotubes refers to the arithmetic mean value of the outer diameters of a sufficient number n of carbon nanotubes measured using an image of a transmission electron microscope at a magnification of 100,000 times or more.

[0028] Examples of the form of carbon nanotubes include, but are not limited to, graphite whiskers, filamentous carbon, graphite fibers, ultrafine carbon tubes, carbon tubes, carbon fibrils, carbon microtubes, and carbon nanofibers, and these may be used alone or in combination of two or more types (hereinafter simply referred to as "at least one type").

[0029] <Adhesive for composite materials> The electrode composition for a lithium-ion secondary battery negative electrode of the present invention contains a composite adhesive containing water-soluble cellulose. The composite adhesive binds the electrode composition (composite) formed on the current collector foil and prevents it from collapsing. The composite adhesive is preferably blended in an amount of 0.3 to 4.0 mass %, more preferably 0.5 to 3.5 mass %, and even more preferably 1.0 to 3.0 mass %, based on the total amount of the electrode composition.

[0030] Water-soluble cellulose is used as the composite adhesive. Suitable water-soluble celluloses include carboxymethyl cellulose (CMC) and cellulose nanofibers. Carboxymethyl cellulose has a structure in which the hydroxyl groups in the glucose units constituting cellulose are substituted with carboxymethyl ether groups. In addition to acid-type carboxymethyl cellulose, carboxymethyl cellulose may also include metal salt forms such as sodium salts and ammonium salts. Cellulose nanofibers may have a cellulose structure modified by TEMPO oxidation or the like, or may be in a solubilized form obtained by physically defibrating the cellulose structure.

[0031] Carboxymethyl cellulose can be produced by adding an etherifying agent such as monochloroacetic acid or sodium monochloroacetate to cellulose in the presence of an alkali hydroxide such as sodium hydroxide or potassium hydroxide to cause an etherification reaction. The raw cellulose may be any of native cellulose, regenerated cellulose, fine cellulose, and microcrystalline cellulose from which the amorphous region has been removed. The carboxymethyl cellulose used in the present invention is preferably one that has been purified to reduce the content of inorganic anions.

[0032] <Foil adhesive> The foil adhesive is an adhesive that helps the electrode composition to adhere firmly to the current collector foil, and is preferably blended in an amount of 0.5 to 5.0 mass %, more preferably 1.0 to 4.0 mass %, based on the total amount of the electrode composition.

[0033] Synthetic resins are preferably used as foil adhesives. For example, polyethylene, polypropylene, ethylene-propylene terpolymer, butadiene rubber, styrene-butadiene rubber, butyl rubber, polytetrafluoroethylene, poly(meth)acrylate, polyvinylidene fluoride, polyethylene oxide, polypropylene oxide, polyepichlorohydrin, polyphosphazene, and polyacrylonitrile can be used. Among these, elastomers such as butadiene rubber, styrene-butadiene rubber, and butyl rubber are preferred, and styrene-butadiene rubber (SBR) is particularly preferred. The foil adhesive can be blended into the electrode slurry as an aqueous emulsion.

[0034] <Slurry preparation> The electrode composition of the present invention can be formed by preparing an electrode slurry by dispersing at least a negative electrode active material, carbon nanotubes, a composite adhesive, and a foil adhesive in a solvent, and then applying the obtained electrode slurry to a current collector foil and drying it.

[0035] When preparing the electrode slurry, it is preferable to add carbon nanotubes to a solvent, preferably in the presence of a dispersant, and prepare a carbon nanotube slurry using a mixer or disperser, and then blend the obtained carbon nanotube slurry with components such as a negative electrode active material, a composite adhesive, and a foil adhesive, and disperse the mixture using a mixer or disperser to prepare the electrode slurry. The solvent used in preparing the carbon nanotube slurry and the electrode slurry is preferably water.

[0036] When preparing the electrode slurry, other components may be added as desired within the scope of the present invention. For example, preservatives, pH adjusters, viscosity adjusters, electrolytes, etc. may be added.

[0037] A disperser is used to prepare the electrode slurry. Examples of dispersers that can be used include homomixers, high-pressure homogenizers, ultra-high-pressure homogenizers, ultrasonic dispersion processors, planetary mixers, combimixers, kneaders, planetary mixers, kneaders, planetary mixers, kneaders, planetary mixers, kneaders, kneaders, Henschel mixers, ball mills, bead mills, thin film rotary high-speed agitators, screw mixers, paddle mixers, disperser mixers, turbine mixers, propeller mixers, blenders, ultrasonic homogenizers, colloid mills, pebble mills, beaters, disc refiners, conical refiners, double disc refiners, and grinders. It is also preferable to use two or more types of dispersers in combination.

[0038] <Coating and pressing> The prepared electrode slurry is applied to a current collector foil, dried, and then the electrode composition formed on the current collector foil is pressed to produce a negative electrode for a lithium ion secondary battery. The current collector foil may be a foil made of copper, aluminum, nickel, stainless steel, titanium, or an alloy. In the present invention, it is preferable to use copper or aluminum foil as the current collector foil.

[0039] The thickness of the current collector foil is preferably 1 to 50 μm, more preferably 3 to 30 μm, and even more preferably 10 to 20 μm. With a foil thickness within this range, both quality and productivity as an electrode for a lithium ion secondary battery negative electrode can be achieved. [Example]

[0040] Manufacturing Example 1 A mixture of 1.5 parts by mass of carbon nanotubes 1, 0.5 parts by mass of polyvinylpyrrolidone (PVP; manufactured by Nippon Shokubai Co., Ltd., K-30) as a dispersant, and 98.0 parts by mass of ion-exchanged water as a solvent was mixed and mixed at 2000 rpm for 2 minutes using a mixer (manufactured by Thinky Corporation, Awatori Rentaro ARE-310). The mixture was then kneaded for 50 minutes (abbreviated as long time) using a bead mill (manufactured by Shinmaru Enterprises Co., Ltd., Dynomill KD-L) with φ1.0 mm zirconia beads, a filling rate of 70%, and a peripheral speed of 12 m / s (abbreviated as high intensity) to obtain a carbon nanotube slurry.

[0041] Manufacturing Examples 2 to 13 As shown in Table 1, carbon nanotube slurries were obtained by replacing carbon nanotube 1 with any of carbon nanotubes 2 to 10 and changing the dispersion conditions. Dispersion conditions used included φ1.0 mm zirconia beads, a filling rate of 70%, a peripheral speed of 12 m / s (high intensity) and 50 minutes (long time), as well as φ1.0 mm zirconia beads, a filling rate of 70%, a peripheral speed of 8 m / s (low intensity) and 15 minutes (short time).

[0042] Manufacturing Examples 1 to 13 The heavy metal contents of the carbon nanotube slurries obtained in Production Examples 1 to 13 were measured and converted into the solid amount of carbon nanotubes to determine the sum of the heavy metal contents of the carbon nanotubes, which are shown in Table 1. The heavy metal contents were determined by burning a predetermined amount of carbon nanotube slurry, extracting the ash with an aqueous nitric acid solution, and quantifying it with an ICP device. Heavy metals other than Fe and Co were hardly contained.

[0043] The anion content of the carbon nanotube slurries obtained in Production Examples 1 to 13 was measured and converted into the solid amount of carbon nanotubes to determine the anion content of the carbon nanotubes, which is shown in Table 1. The anion content was determined by subjecting a predetermined amount of the carbon nanotube slurry to ion chromatography to quantify the inorganic anions. Almost no inorganic anions other than sulfate ions and chloride ions were contained.

[0044] For the carbon nanotube slurries obtained in Production Examples 1 to 13, coating films were prepared by drying with an applicator, and the Raman spectrum of the carbon nanotubes was measured to determine the wave number at the G band position. The properties of the carbon nanotubes dispersed in the carbon nanotube slurry, as well as the composition and preparation conditions of the carbon nanotube slurries containing such carbon nanotubes, are shown in Table 1.

[0045] Examples 1 to 7, Comparative Examples 1 to 6 40.0 parts by mass of any of the carbon nanotube slurries obtained in Production Examples 1 to 13, 45.0 parts by mass of artificial graphite and 5.0 parts by mass of silicon monoxide as active materials, 1.5 parts by mass (solids content equivalent) of sodium carboxymethylcellulose (CMC) as a composite adhesive, 1.5 parts by mass (solids content equivalent) of styrene butadiene rubber (SBR) as a foil adhesive, and ion-exchanged water as a solvent were blended to a total of 100 parts by mass, and mixed using a planetary mixer to prepare a slurry for an electrode.

[0046] The obtained electrode slurry was applied to a copper foil current collector with a thickness of 25 μm using an applicator so that the basis weight was 5 mg / cm. 2 The electrode sheet was then applied in a press so that the electrode density became 2.1 g / cm. 3 The electrode sheets were pressed into a shape similar to that of the electrode sheet, and then punched out with a Thomson blade. The counter electrode was made of lithium metal, and a single-electrode coin-type cell was fabricated to produce a lithium battery. Three lithium batteries were fabricated for each electrode sheet, and various battery characteristics were evaluated.

[0047] Comparative Example 7 An electrode slurry was prepared in the same manner as above, except that sodium carboxymethylcellulose was not used as the composite adhesive and was replaced with styrene-butadiene rubber. Three lithium batteries were fabricated using the resulting electrode slurry in the same manner as above, and various battery characteristics were evaluated.

[0048] Comparative Example 8 An electrode slurry was prepared in the same manner as above, except that the proportion of silicon monoxide as an active material was reduced. Three lithium batteries were fabricated using the obtained electrode slurry in the same manner as above, and various battery characteristics were evaluated.

[0049] Battery characteristics evaluation method [Capacitance characteristics] The three coin-type cells were initially charged and discharged at a C rate of 0.2C, and then the discharge capacity was measured when they were charged and discharged at 1C, and evaluated according to the following criteria.

[0050] Evaluation criteria: ◎: Discharge capacity is 500mAh / g or more ○: Discharge capacity is 450mAh / g or more △: Discharge capacity is 400mAh / g or more ×: Discharge capacity is less than 400mAh / g

[0051] [Output characteristics] To test the durability against high-power charge and discharge, three coin-type cells were initially charged and discharged at a C-rate of 0.2C, and then the discharge capacity was measured when the C-rate was changed to 5C. The percentage of the discharge capacity at 5C relative to the discharge capacity at 0.2C was calculated as an average and evaluated according to the following criteria.

[0052] Evaluation criteria: ◎: Discharge capacity at 5C is 90% or more of the discharge capacity at 0.2C ○: Discharge capacity at 5C is 80% or more of the discharge capacity at 0.2C △: Discharge capacity at 5C is 60% or more of the discharge capacity at 0.2C ×: Discharge capacity at 5C is less than 60% of the discharge capacity at 0.2C

[0053] [Cycle characteristics] As a durability test for repeated charge / discharge, the 5C charge / discharge capacity retention rate was evaluated for three coin-type cells, and then a cycle of charge / discharge at 1C was repeated in an environment of 25°C. The discharge capacity after 30 cycles was measured, and the percentage of the discharge capacity relative to the theoretical capacity was calculated as an average value and evaluated according to the following criteria.

[0054] Evaluation criteria: ◎: Discharge capacity is 98% or more of the theoretical capacity ○: Discharge capacity is 95% or more of the theoretical capacity △: Discharge capacity is 85% or more of the theoretical capacity ×: Discharge capacity is less than 85% of the theoretical capacity

[0055] Table 2 shows the composition of the electrode slurry and the evaluation results of the electrode properties of the negative electrode prepared from the electrode slurry.

[0056] [Table 1]

[0057] [Table 2]

[0058] As is clear from the results in Table 2, the negative electrodes of Examples 1 to 7, which satisfied the various conditions stipulated in the present invention, exhibited excellent electrode properties.

[0059] On the other hand, in Comparative Examples 1 to 4, which were prepared using the carbon nanotube slurries of Production Examples 8 to 11 containing carbon nanotubes with a total heavy metal content exceeding 10,000 ppm or carbon nanotubes with a total inorganic anion content exceeding 100 ppm, no negative electrodes with good electrode properties were obtained. -1In Comparative Examples 5 and 6, which used the carbon nanotube slurries of Production Examples 12 and 13, in which the content of the carbon nanotubes was less than 10% by mass, negative electrodes with good electrode properties were not obtained. This is presumably because the carbon nanotubes were not sufficiently dissolved in the electrode composition. Furthermore, in Comparative Example 7, in which a composite adhesive containing water-soluble cellulose was not used, and in Comparative Example 8, in which the content of the metal oxide was less than 10% by mass with respect to the total amount of the negative electrode active material, negative electrodes with good electrode properties were not obtained. [Industrial Applicability]

[0060] The electrode composition of the present invention can be used for the negative electrode of a lithium ion secondary battery.

Claims

1. A composition comprising a negative electrode active material including graphite and a composite oxide, carbon nanotubes, a composite adhesive including water-soluble cellulose, and a foil adhesive including a synthetic resin, The carbon nanotube has a G band position in a Raman spectrum with a wave number of 1580 cm －１ or more, the total content of heavy metals is less than 10,000 ppm, and the total content of inorganic anions is less than 100 ppm; 1. An electrode composition for a lithium ion secondary battery negative electrode, comprising a negative electrode active material containing a composite oxide in an amount of 10 mass % or more based on the total amount of the negative electrode active material.

2. 2. The electrode composition according to claim 1, wherein the composite adhesive is carboxymethyl cellulose or a salt thereof.

3. 10. The electrode composition of claim 1, wherein the foil adhesive is an elastomer.

4. An electrode for a lithium ion secondary battery negative electrode, comprising the electrode composition according to any one of claims 1 to 3 formed on a current collector foil.

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