Carbon-based conductive agent dispersion, negative electrode slurry for secondary battery, negative electrode for secondary battery, and secondary battery

Abstract

This carbon-based conductive agent dispersion comprises: a carbon-based conductive agent; at least one formaldehyde condensate selected from the group consisting of an alkylnaphthalene sulfonic acid-formaldehyde condensate and / or an alkali metal salt thereof, a naphthalene sulfonic acid-formaldehyde condensate and / or an alkali metal salt thereof, and an alkylnaphthalene sulfonic acid-naphthalene sulfonic acid-formaldehyde condensate and / or an alkali metal salt thereof; and a solvent, wherein the content of the formaldehyde condensate is 150-1,500 parts by mass with respect to 100 parts by mass of the carbon-based conductive agent.

Description

Carbon-based conductive agent dispersion, negative electrode slurry for secondary batteries, negative electrode for secondary batteries, secondary battery

[0001] This disclosure relates to a carbon-based conductive agent dispersion, a negative electrode slurry for secondary batteries, a negative electrode for secondary batteries, and a secondary battery.

[0002] The negative electrode for secondary batteries is manufactured, for example, by coating a negative electrode slurry containing a negative electrode active material, a binder, etc., onto a negative electrode current collector such as copper foil to form a negative electrode mixture layer. For negative electrode active materials, it is known that graphite-based negative electrode active materials and silicon-based negative electrode active materials are used in combination to increase capacity. Silicon-based negative electrode active materials expand and contract significantly with charging and discharging, which can break the conductive paths between the active materials and easily lead to a decrease in capacity. To prevent such breakage of conductive paths due to the expansion and contraction of negative electrode active materials (especially silicon-based negative electrode active materials) with charging and discharging, it is desirable to use a carbon-based conductive agent dispersion in which a carbon-based conductive agent is dispersed in a solvent. For example, by mixing a carbon-based conductive agent dispersion with a negative electrode active material, etc., to prepare a negative electrode slurry, the carbon-based conductive agent can be highly dispersed in the slurry. As a result, the carbon-based conductive agent is dispersed relatively uniformly in the negative electrode mixture layer formed by the slurry, and the effect of preventing the aforementioned breakage of conductive paths is enhanced.

[0003] Japanese Patent Publication No. 2021-193660

[0004] However, the inclusion of a carbon-based conductive agent in the negative electrode slurry increases its viscosity, raising concerns about coating defects. Generally, lowering the viscosity of the negative electrode slurry improves coating defects, but this tends to decrease the peel strength of the negative electrode mixture layer formed by the negative electrode slurry. The peel strength of the negative electrode mixture layer refers to the adhesion between the negative electrode mixture layer and the negative electrode current collector.

[0005] Therefore, the present disclosure aims to provide a carbon-based conductive agent dispersion that can suppress the increase in viscosity of the negative electrode slurry and suppress the decrease in the peel strength of the negative electrode mixture layer. Furthermore, the present disclosure aims to provide a negative electrode slurry for secondary batteries that can suppress the increase in viscosity and suppress the decrease in the peel strength of the negative electrode mixture layer. Furthermore, the present disclosure aims to provide a negative electrode for secondary batteries in which the decrease in the peel strength of the negative electrode mixture layer is suppressed, and a secondary battery equipped with said negative electrode for secondary batteries.

[0006] A carbon-based conductive agent dispersion according to one aspect of the present disclosure comprises a carbon-based conductive agent, at least one formaldehyde condensate selected from the group consisting of alkylnaphthalene sulfonic acid-formaldehyde condensate and / or its alkali metal salt, naphthalene sulfonic acid-formaldehyde condensate and / or its alkali metal salt, and alkylnaphthalene sulfonic acid-naphthalene sulfonic acid-formaldehyde condensate and / or its alkali metal salt, and a solvent, wherein the content of the formaldehyde condensate is 150 parts by mass or more and 1500 parts by mass or less per 100 parts by mass of the carbon-based conductive agent.

[0007] Furthermore, a negative electrode slurry for a secondary battery according to one aspect of the present disclosure comprises the carbon-based conductive agent dispersion, a graphite-based negative electrode active material, a silicon-based negative electrode active material, and carboxymethylcellulose and / or its alkali metal salt.

[0008] Furthermore, a negative electrode for a secondary battery according to one aspect of the present disclosure has a negative electrode mixture layer comprising a graphite-based negative electrode active material, a silicon-based negative electrode active material, carboxymethylcellulose and / or its alkali metal salt, a carbon-based conductive agent, and at least one formaldehyde condensate selected from the group consisting of alkylnaphthalenesulfonic acid-formaldehyde condensate and / or its alkali metal salt, naphthalenesulfonic acid-formaldehyde condensate and / or its alkali metal salt, and alkylnaphthalenesulfonic acid-naphthalenesulfonic acid-formaldehyde condensate and / or its alkali metal salt, wherein the content of the formaldehyde condensate is 150 parts by mass or more and 1500 parts by mass or less per 100 parts by mass of the carbon-based conductive agent.

[0009] Furthermore, a secondary battery according to one aspect of this disclosure includes the above-mentioned negative electrode for secondary batteries.

[0010] According to this disclosure, it is possible to provide a carbon-based conductive agent dispersion that can suppress the increase in viscosity of the negative electrode slurry and suppress the decrease in the peel strength of the negative electrode mixture layer. Furthermore, according to this disclosure, it is possible to provide a negative electrode slurry for secondary batteries that can suppress the increase in viscosity and suppress the decrease in the peel strength of the negative electrode mixture layer. Furthermore, according to this disclosure, it is possible to provide a negative electrode for secondary batteries in which the decrease in the peel strength of the negative electrode mixture layer is suppressed, and a secondary battery equipped with said negative electrode for secondary batteries.

[0011] This is a cross-sectional view of a secondary battery, which is an example of an embodiment.

[0012] The following describes examples of embodiments of the carbon-based conductive agent dispersion, negative electrode slurry for secondary batteries, negative electrode for secondary batteries, and secondary batteries related to this disclosure. This disclosure is not limited to the embodiments described below and includes forms that are implemented without altering the gist of this disclosure.

[0013] (Carbon-based conductive agent dispersion) The carbon-based conductive agent dispersion according to this embodiment comprises a carbon-based conductive agent, at least one formaldehyde condensate selected from the group consisting of alkylnaphthalenesulfonic acid-formaldehyde condensate and / or its alkali metal salt, naphthalenesulfonic acid-formaldehyde condensate and / or its alkali metal salt, and alkylnaphthalenesulfonic acid-naphthalenesulfonic acid-formaldehyde condensate and / or its alkali metal salt, and a solvent.

[0014] Carbon-based conductive agents are carbon materials whose main component is carbon, such as carbon black and nanocarbon. Examples of carbon black include acetylene black, furnace black, hollow carbon black, channel black, thermal black, and Ketjen black. Nanocarbon refers to nano-sized carbon materials, such as carbon nanotubes, carbon nanorods, carbon nanohorns, carbon nanobrushes, carbon nanotwists, graphene, and fullerene. Carbon-based conductive agents can be used individually or in combination of two or more. Among these, nanocarbon is preferred, and carbon nanotubes are particularly preferred, because they can effectively prevent the disruption of conductive paths due to the expansion and contraction of the negative electrode active material (especially silicon-based negative electrode active material) during charging and discharging.

[0015] Carbon nanotubes (CNTs) have a cylindrical shape formed by winding planar graphite, and examples include single-walled carbon nanotubes and multi-walled carbon nanotubes. Single-walled carbon nanotubes have a structure in which one layer of graphite is wound. Multi-walled carbon nanotubes have a structure in which two or more layers of graphite are wound. Furthermore, the sidewalls of carbon nanotubes do not have to be graphite structures. For example, carbon nanotubes may have sidewalls with an amorphous structure.

[0016] The shape of carbon nanotubes is not limited, but examples include needle-shaped, cylindrical, fishbone-shaped (fishbone or cup-stacked), playing card-shaped (platelet), and coil-shaped.

[0017] The content of the carbon-based conductive agent in the carbon-based conductive agent dispersion may be, for example, 6.25% by mass or more and 40% by mass or less, based on the total mass of solids in the dispersion.

[0018] The carbon-based conductive agent dispersion may contain other conductive agents in addition to the carbon-based conductive agent described above. Examples of other conductive agents include metal powders or alloys thereof such as gold, silver, copper, nickel, chromium, palladium, rhodium, ruthenium, indium, silicon, aluminum, tungsten, molbutene, and platinum, as well as metal oxide powders such as silver oxide, indium oxide, tin oxide, zinc oxide, and ruthenium oxide.

[0019] Alkylnaphthalenesulfonic acid-formaldehyde condensate and / or its alkali metal salt (hereinafter sometimes referred to as ANSF) is a condensate of alkylnaphthalenesulfonic acid and formaldehyde, or its alkali metal salt, or a mixture thereof. Examples of alkylnaphthalenesulfonic acid include methylnaphthalenesulfonic acid, ethylnaphthalenesulfonic acid, and butylnaphthalenesulfonic acid. Examples of alkali metal salts include sodium salts, lithium salts, and potassium salts. ANSF may also be a monomer co-condensed with an aromatic compound that can co-condense with alkylnaphthalenesulfonic acid, such as hydroxynaphthalene, naphthalenecarboxylic acid, anthracene, phenol, cresol, or derivatives thereof.

[0020] Naphthalene sulfonic acid formaldehyde condensate and / or its alkali metal salt (hereinafter sometimes referred to as NSF) is a condensate of naphthalene sulfonic acid and formaldehyde, or its alkali metal salt, or a mixture of both. Examples of alkali metal salts include sodium salt, lithium salt, potassium salt, etc. NSF may also be a monomer co-condensed with an aromatic compound that can co-condense with naphthalene sulfonic acid, such as hydroxynaphthalene, naphthalenecarboxylic acid, anthracene, phenol, cresol, or derivatives thereof.

[0021] The alkylnaphthalenesulfonic acid-naphthalenesulfonic acid-formaldehyde condensate and / or alkali metal salt thereof (sometimes referred to as ANSNSF) is a condensate of alkylnaphthalenesulfonic acid, naphthalenesulfonic acid and formaldehyde, an alkali metal salt thereof, or a mixture of the foregoing. Examples of the alkali metal salt include sodium salts, lithium salts, potassium salts and the like. Note that ANSNSF may be co-condensed, as a monomer, with an aromatic compound that can be co-condensed with alkylnaphthalenesulfonic acid or naphthalenesulfonic acid, such as hydroxynaphthalene, naphthalenecarboxylic acid, anthracene, phenol, cresol, or derivatives thereof.

[0022] The weight average molecular weight (Mw) of the aforementioned formaldehyde condensate is not particularly limited, and may be, for example, 300 or more and 3000 or less. When the weight average molecular weight is 300 or more, the dispersion performance is improved, and when the weight average molecular weight is 3000 or less, deterioration of productivity can be suppressed and cost increase can be restrained. The weight average molecular weight is measured by gel permeation chromatography (GPC).

[0023] The solvent contains water as a main component, for example. The phrase "contains water as a main component" means that water is the most abundant component in the solvent components. The content of water in the solvent is, for example, preferably 90% by mass or more, more preferably 95% by mass or more, and even more preferably 100% by mass, relative to the total amount of the solvent. The solvent may contain an organic solvent, but it is preferable that the solvent contains as little organic solvent as possible; the content of the organic solvent is more preferably 5% by mass or less, and even more preferably 1% by mass or less relative to the total amount of the solvent.

[0024] In the carbon-based conductive agent dispersion according to the present embodiment, the content of the aforementioned formaldehyde condensate is 150 parts by mass or more and 1500 parts by mass or less, preferably 200 parts by mass or more and 1000 parts by mass or less, and more preferably 200 parts by mass or more and 500 parts by mass or less, relative to 100 parts by mass of the carbon-based conductive agent. When a negative electrode slurry is prepared using a carbon-based conductive agent dispersion in which the content of the aforementioned formaldehyde condensate satisfies the above range, compared to the case where a negative electrode slurry is prepared using a carbon-based conductive agent dispersion that does not contain the aforementioned formaldehyde condensation moiety or whose content is outside the above range, an increase in the viscosity of the negative electrode slurry can be suppressed, and a decrease in the peel strength of the negative electrode mixture layer formed from the negative electrode slurry can be suppressed.

[0025] The carbon-based conductive agent dispersion according to the present embodiment may contain various agents such as leveling agents, defoaming agents, cratering inhibitors, antistatic agents, pigment dispersants, and ultraviolet absorbers, as necessary.

[0026] (Negative Electrode Slurry for Secondary Battery) The negative electrode slurry for a secondary battery according to the present embodiment includes the aforementioned carbon-based conductive agent dispersion, a graphite-based negative electrode active material, a silicon-based negative electrode active material, and carboxymethyl cellulose and / or an alkali metal salt thereof.

[0027] As the graphite-based negative electrode active material, for example, graphite capable of electrochemically intercalating and deintercalating lithium ions may be used, and examples thereof include natural graphite, artificial graphite, a mixture of natural graphite and artificial graphite, and natural graphite coated with artificial graphite. These may be used alone in one type or in combination of two or more types. The particle shape of the graphite-based negative electrode active material is not particularly limited, and may be, for example, spherical, scaly, or the like.

[0028] The average particle diameter (D50) of the graphite-based negative electrode active material is not particularly limited, and may be, for example, 1 μm or more and 50 μm or less, or 5 μm or more and 30 μm or less. The specific surface area of the graphite-based negative electrode active material is not particularly limited, and is for example 0.5 m ２ / g or more and 10 m ２ / g or less, or 1.0 m ２ / g or more and 5.0 m ２ / g is acceptable. In this specification, the average particle size and specific surface area of ​​the negative electrode active material can be measured, for example, by dispersing the negative electrode active material in a medium that does not dissolve it, using a laser scattering particle size distribution analyzer (for example, "Mastersizer 2000" manufactured by Malvern Panalogical).

[0029] The silicon-based negative electrode active material may be, for example, a substance containing silicon (atoms) that can electrochemically intercept and release lithium ions. Examples of silicon-based negative electrode active materials include Si, Si alloys, and Si compounds. Furthermore, for Si compounds, composite particles containing an ion-conducting phase and a silicon phase (silicon particles in one respect) dispersed within the ion-conducting phase are preferred, for example, in terms of battery capacity and charge-discharge cycle characteristics. The ion-conducting phase is a phase that conducts ions and includes, for example, at least one of a silicate phase, a carbon phase, and a silicon oxide phase.

[0030] The carbon phase may be composed of amorphous carbon (unformed carbon), for example. Examples of amorphous carbon constituting the carbon phase include hard carbon, soft carbon, and other amorphous carbons. The average interplanar spacing d of the (002) plane of the amorphous carbon is measured by X-ray diffraction. ００２ It is a carbon material with a wavelength exceeding 0.34 nm.

[0031] The main component of the silicon oxide phase (for example, 95% by mass or more and 100% by mass or less) may be silicon dioxide. The overall composition of the composite particles, which include the silicon oxide phase and the silicon phase dispersed therein, is SiO ｘ It can be expressed as SiO ｘ This is a silicon nanoparticle in amorphous SiO ２ It has a dispersed structure inside. The oxygen content ratio x to silicon is preferably, for example, 0.5 ≤ x < 2.0, and more preferably 0.8 ≤ x ≤ 1.5.

[0032] The silicate phase may satisfy the following condition (1) and / or (2). (1) The silicate phase contains at least one selected from the group consisting of alkali metal elements and Group 2 elements (Group 2 elements in the long-form periodic table). (2) The silicate phase contains an element L. The element L is at least one selected from the group consisting of B, Al, Zr, Nb, Ta, V, lanthanoids, Y, Ti, P, Bi, Zn, Sn, Pb, Sb, Co, Er, F, and W. Note that lanthanoids is a general term for 15 elements from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71.

[0033] Regarding the above condition (1), examples of the alkali metal elements include lithium (Li), potassium (K), and sodium (Na). Examples of the Group 2 elements include magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba). Inclusion of an alkali metal element and / or a Group 2 element may reduce the irreversible capacity of the silicate phase. A silicate phase containing lithium (hereinafter sometimes referred to as a "lithium silicate phase") is preferable, for example, because it has low irreversible capacity and high initial charge-discharge efficiency.

[0034] The lithium silicate phase only needs to be an oxide phase containing Li, Si and O, and may contain other elements. The atomic ratio of O to Si in the lithium silicate phase: O / Si is, for example, greater than 2 and less than 4. Preferably, O / Si is greater than 2 and less than 3. The atomic ratio of Li to Si in the lithium silicate phase: Li / Si is, for example, greater than 0 and less than 4.

[0035] The lithium silicate phase has the formula: Li ２ｚ SiO （２＋ｚ） (0 < z < 2) may be included, or the lithium silicate phase may be composed of the lithium silicate phase represented thereby. z preferably satisfies the relationship 0 < z < 1, and z = 1 / 2 (that is, Li ２ Si ２ O ５ ) is more preferable.

[0036] Furthermore, the Si compound may also include composite particles comprising an ionic conducting phase and a silicon phase dispersed within the ionic conducting phase, and a coating layer covering at least a portion of the surface of the composite particles.

[0037] The coating layer present on the surface of the composite particles includes, for example, a conductive layer. A conductive material containing carbon is preferred as the conductive material constituting the conductive layer. Examples of carbon-containing conductive materials include conductive carbon materials. Examples of conductive carbon materials include carbon black, graphite, and amorphous carbon with low crystallinity. Amorphous carbon is preferred because it provides a large buffering effect against the silicon phase, which undergoes volume changes during charging and discharging. Amorphous carbon may be easily graphitized carbon (soft carbon) or difficult-to-graphitize carbon (hard carbon). Examples of carbon black include acetylene black and Ketjen black. The thickness of the conductive layer may be, for example, in the range of 1 to 200 nm. The thickness of the conductive layer can be measured by cross-sectional observation of the Si-containing material using a SEM or TEM (transmission electron microscope).

[0038] As a silicon-based negative electrode active material, for example, composite particles (hereinafter sometimes referred to as SiC) containing a carbon phase and a silicon phase (from one perspective, silicon particles) dispersed within the carbon phase are preferred in terms of interaction with the aforementioned formaldehyde. The aforementioned formaldehyde condensate is adsorbed onto the surface of the SiC to form a polymer layer. This polymer layer suppresses the expansion and contraction of the SiC during charging and discharging. Furthermore, the aforementioned formaldehyde condensate also functions as a dispersant to prevent particle aggregation of SiC. This promotes uniform dispersion of SiC and suppresses the non-uniformity of the expansion and contraction of SiC.

[0039] The content of the silicon-based anode active material may be, for example, 3% by mass or more and 30% by mass or less, 5% by mass or more and 25% by mass or less, or 8% by mass or more and 20% by mass or less, relative to the total amount of silicon-based anode active material and graphite-based anode active material, in terms of increasing the capacity of secondary batteries. The average particle size (D50) of the silicon-based anode active material may be, for example, 3 μm or more and 15 μm or less, 4 μm or more and 12 μm or less, or 6 μm or more and 10 μm or less.

[0040] The total amount of graphite-based negative electrode active material and silicon-based negative electrode active material in the negative electrode slurry for secondary batteries may be, for example, 85% by mass or more and 99% by mass or less, or 90% by mass or more and 95% by mass or less, based on the total mass of solids in the slurry.

[0041] The content of the carbon-based conductive agent in the negative electrode slurry for secondary batteries is preferably 0.005 parts by mass or more per 100 parts by mass of the total amount of graphite-based negative electrode active material and silicon-based negative electrode active material, for example, in order to suppress a decrease in the negative electrode capacity. The upper limit of the carbon-based conductive agent content may be, for example, 5 parts by mass or less. Furthermore, when nanocarbon such as carbon nanotubes is used as the carbon-based conductive agent, the nanocarbon content is preferably 0.005 parts by mass or more and less than 0.1 parts by mass per 100 parts by mass of the total amount of graphite-based negative electrode active material and silicon-based negative electrode active material, for example, in order to suppress a decrease in the negative electrode capacity.

[0042] The negative electrode slurry for secondary batteries may contain other negative electrode active materials in addition to graphite-based and silicon-based negative electrode active materials. These other negative electrode active materials may be substances capable of electrochemically intercalating and releasing lithium ions, such as Sn, Sn alloys, Sn compounds, and lithium titanate.

[0043] Carboxymethylcellulose and / or its alkali metal salts (hereinafter sometimes referred to as CMC) are carboxymethylcellulose, its metal salts, or mixtures thereof. Examples of alkali metal salts include sodium salts, lithium salts, potassium salts, etc. CMC functions, for example, as a thickening agent for negative electrode slurries in secondary batteries, as well as as a binder for negative electrode active materials and negative electrode current collectors.

[0044] The CMC content is preferably 0.7 parts by mass or more, more preferably 0.7 parts by mass or more and 3.0 parts by mass or less, and more preferably 0.7 parts by mass or more and 2.5 parts by mass or less, based on 100 parts by mass of the total amount of graphite-based anode active material and silicon-based anode active material. Furthermore, the total amount of the aforementioned formaldehyde condensate and CMC is preferably 0.9 parts by mass or more, more preferably 0.9 parts by mass or more and 3.5 parts by mass or less, and more preferably 0.9 parts by mass or more and 3.2 parts by mass or less, based on 100 parts by mass of the total amount of graphite-based anode active material and silicon-based anode active material. By satisfying the above ranges for the CMC content and the total amount of the aforementioned formaldehyde condensate and CMC, it becomes possible, for example, to further suppress the increase in viscosity of the anode slurry or to further suppress the decrease in the peel strength of the anode mixture layer.

[0045] The ratio (B / A) of the content of the formaldehyde condensate (B) to the content of CMC (A) is preferably less than 0.5 by mass, preferably 0.04 or more and 0.4 or less, and more preferably 0.04 or more and 0.33 or less. By satisfying the above range for B / A, it becomes possible to further suppress the increase in viscosity of the negative electrode slurry or to further suppress the decrease in the peel strength of the negative electrode mixture layer.

[0046] The negative electrode slurry for secondary batteries of this embodiment may further contain additives such as binders. Examples of binders other than CMC include fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyimide resins, acrylic resins, polyolefin resins, styrene-butadiene rubber (SBR), nitrile-butadiene rubber (NBR), polyacrylic acid (PAA) or its salts, and polyvinyl alcohol (PVA). These may be used individually or in combination of two or more. The binder content is, for example, 0.1 parts by mass or more and 10 parts by mass or less, per 100 parts by mass of the total amount of graphite-based negative electrode active material and silicon-based negative electrode active material.

[0047] The viscosity of the negative electrode slurry for the secondary battery in this embodiment may be, for example, 2000 mPa·s or more, and 15000 mPa·s or less. The viscosity of the negative electrode slurry for the secondary battery was measured using a BM rotational viscometer at a rotation speed of 20 rpm in a 25°C environment after 1 minute of rotor rotation.

[0048] The thixotropic index (TI value) of the negative electrode slurry for secondary batteries in this embodiment may be, for example, 0.1 or more and 1.0 or less, or 0.3 or more and 0.8 or less. The TI value is defined by the following formula: TI value = Va / Vb In the formula, Va is the viscosity (mPa·s) of the negative electrode slurry for secondary batteries measured by a BM rotational viscometer at a rotation speed of 2 rpm and after 1 minute of rotor rotation at a 25°C environment, and Vb is the viscosity (mPa·s) of the negative electrode slurry for secondary batteries measured at a rotation speed of 20 rpm and after 1 minute of rotor rotation at a 25°C environment.

[0049] The negative electrode slurry for secondary batteries in this embodiment is prepared by stirring various materials such as a carbon-based conductive agent dispersion, a graphite-based negative electrode active material, a silicon-based negative electrode active material, and CMC using a well-known stirring device. Examples of stirring devices include media-type stirring devices such as bead mills, ball mills, planetary ball mills, kneaders, planetary mixers, and disper mixers, as well as media-less stirring devices such as homogenizers.

[0050] (Negative electrode for secondary battery) The negative electrode for secondary battery of this embodiment is manufactured using the negative electrode slurry for secondary battery of this embodiment. For example, the negative electrode slurry for secondary battery of this embodiment is applied to a negative electrode current collector and dried to form a negative electrode composite layer, and then rolled as necessary to obtain the negative electrode for secondary battery.

[0051] The negative electrode current collector can be made of a metal foil that is stable in the negative electrode potential range, such as copper or a copper alloy, or a film with the metal arranged on its surface. The thickness of the negative electrode current collector is, for example, 5 μm to 30 μm.

[0052] For applying the negative electrode slurry for secondary batteries, for example, a slit die coater, reverse roll coater, lip coater, blade coater, knife coater, gravure coater, and dip coater can be used. The negative electrode slurry for secondary batteries may be applied to only one side of the negative electrode current collector or to both sides.

[0053] The rolling process is carried out several times at a predetermined linear pressure using, for example, a roll press, until the negative electrode mixture layer reaches a predetermined thickness. The thickness of the negative electrode mixture layer is, for example, 10 μm or more and 150 μm or less on one side of the negative electrode current collector.

[0054] The negative electrode mixture layer of the secondary battery negative electrode obtained in this embodiment contains a graphite-based negative electrode active material, a silicon-based negative electrode active material, CMC, a carbon-based conductive agent, and the aforementioned formaldehyde condensate. The amount of the aforementioned formaldehyde condensate in the negative electrode mixture layer is 150 parts by mass or more and 1500 parts by mass or less per 100 parts by mass of the carbon-based conductive agent.

[0055] (Secondary Battery) Figure 1 is a cross-sectional view of a secondary battery, which is an example of an embodiment. The secondary battery 10 shown in Figure 1 comprises a wound electrode body 14 in which a positive electrode 11 and a negative electrode 12 are wound around a separator 13, an electrolyte, insulating plates 18 and 19 arranged above and below the electrode body 14, a battery case 15, a positive electrode lead 20, and a negative electrode lead 21. In addition to the wound electrode body 14, other forms of electrode bodies may be used, such as a laminated electrode body in which the positive and negative electrodes are alternately stacked with a separator. The battery case 15 is composed of an outer can 16 having an opening and housing the electrode body 14, etc., and a sealing body 17 that closes the opening of the outer can 16. Examples of battery cases 15 include metal cases such as cylindrical, square, coin-shaped, and button-shaped cases, and resin cases (so-called pouch type) formed by laminating resin sheets.

[0056] The electrolyte, for example, has lithium ion conductivity. The electrolyte may be a liquid electrolyte (electrolyte solution) or a solid electrolyte.

[0057] A liquid electrolyte (electrolyte solution) comprises a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of non-aqueous solvents include esters, ethers, nitriles, amides, and mixtures of two or more of these. Examples of non-aqueous solvents include ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and mixtures thereof. The non-aqueous solvent may also contain halogen-substituted solvents (e.g., fluoroethylene carbonate) in which at least some of the hydrogen atoms in the solvent are replaced with halogen atoms such as fluorine. Examples of electrolyte salts include LiPF4. ６ Lithium salts such as these are used.

[0058] As the solid electrolyte, for example, a solid or gel-like polymer electrolyte, an inorganic solid electrolyte, etc., can be used. As the inorganic solid electrolyte, materials known for all-solid-state lithium-ion secondary batteries, etc. (for example, oxide-based solid electrolytes, sulfide-based solid electrolytes, halogen-based solid electrolytes, etc.) can be used. The polymer electrolyte includes, for example, a lithium salt and a matrix polymer, or a non-aqueous solvent, a lithium salt and a matrix polymer. As the matrix polymer, for example, a polymer material that absorbs a non-aqueous solvent and gels is used. Examples of polymer materials include fluororesins, acrylic resins, polyether resins, etc. Although the electrolytes exemplified above are non-aqueous electrolytes, the electrolyte is not limited to non-aqueous electrolytes and may also be an aqueous electrolyte.

[0059] The outer casing 16 is, for example, a metal container in the shape of a bottomed cylinder. A gasket 28 is provided between the outer casing 16 and the sealing body 17 to further ensure airtightness inside the battery. The outer casing 16 has, for example, a protruding portion 22 that supports the sealing body 17, which is a part of the side surface that protrudes inward. The protruding portion 22 is preferably formed in an annular shape along the circumferential direction of the outer casing 16, and its upper surface supports the sealing body 17.

[0060] The sealing body 17 has a structure in which an internal terminal plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are stacked in order from the electrode body 14 side. Each component constituting the sealing body 17 has, for example, a disc shape or a ring shape, and each component except the insulating member 25 is electrically connected to one another. The lower valve body 24 and the upper valve body 26 are connected to each other at their respective centers, with the insulating member 25 interposed between their respective peripheral edges. When the internal pressure of the secondary battery 10 rises due to heat generation caused by an internal short circuit or the like, for example, the lower valve body 24 deforms and breaks, pushing the upper valve body 26 towards the cap 27, and the current path between the lower valve body 24 and the upper valve body 26 is interrupted. If the internal pressure rises further, the upper valve body 26 breaks, and gas is discharged from the opening of the cap 27.

[0061] In the secondary battery 10 shown in Figure 1, one end of the positive electrode lead 20 is attached to the positive electrode 11. The positive electrode lead 20 attached to the positive electrode 11 extends through a through-hole in the insulating plate 18 towards the sealing body 17 and is connected by welding or other means to the lower surface of the internal terminal plate 23, which is the bottom plate of the sealing body 17. As a result, the cap 27, which is the top plate of the sealing body 17 and is electrically connected to the internal terminal plate 23, becomes the positive electrode terminal. In the secondary battery 10 shown in Figure 1, one end of the negative electrode lead 21 is attached to the negative electrode 12. The negative electrode lead 21 attached to the negative electrode 12 extends through the outside of the insulating plate 19 towards the bottom of the outer casing 16 and is connected by welding or other means to the inner surface of the bottom of the outer casing 16. As a result, the outer casing 16 becomes the negative electrode terminal.

[0062] The positive electrode 11, negative electrode 12, and separator 13 will be described below.

[0063] The negative electrode 12 is the negative electrode for the secondary battery of this embodiment as described above. The configuration of the negative electrode 12 is as previously described, and will not be explained here.

[0064] The positive electrode 11 includes, for example, a positive electrode current collector and a positive electrode mixture layer disposed on the positive electrode current collector. The positive electrode current collector can be made of a metal foil that is stable in the positive electrode potential range, such as aluminum, or a film with the metal disposed on its surface. The thickness of the positive electrode current collector is, for example, 10 μm or more and 30 μm or less.

[0065] The positive electrode mixture layer may be provided on one side of the positive electrode current collector or on both sides. The thickness of the positive electrode mixture layer is, for example, 10 μm or more and 150 μm or less on one side of the positive electrode current collector. The positive electrode mixture layer includes, for example, a positive electrode active material, a conductive agent, and a binder. The positive electrode 11 can be manufactured, for example, by applying a positive electrode slurry containing a positive electrode active material, a conductive agent, a binder, etc., onto the positive electrode current collector, drying the coating film, and then rolling the coating film using a roller or the like.

[0066] Examples of positive electrode active materials include lithium transition metal composite oxides containing transition metal elements such as Co, Mn, and Ni. ｘ CoO ２ Li ｘ NiO ２ Li ｘ MnO ２ Li ｘ Co ｙ Ni １－ｙ O ２ Li ｘ Co ｙ M １－ｙ O ｚ Li ｘ Ni １－ｙ M ｙ O ｚ Li ｘ Mn ２ O ４ Li ｘ Mn ２－ｙ M ｙ O ４ LiMPO ４ Li ２ MPO ４ F (where M is at least one of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, or B, with 0 < x ≤ 1.2, 0 < y ≤ 0.9, and 2.0 ≤ z ≤ 2.3). These may be used individually or in combination of multiple elements.

[0067] Examples of conductive agents include the aforementioned carbon-based conductive agents and other conductive agents.

[0068] Examples of binders include fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyimide resins, acrylic resins, polyolefin resins, styrene-butadiene rubber (SBR), nitrile-butadiene rubber (NBR), carboxymethylcellulose or its salts, polyacrylic acid (PAA) or its salts, and polyvinyl alcohol (PVA). These may be used individually or in combination of two or more types.

[0069] For the separator 13, for example, a porous sheet having ion permeability and insulating properties can be used. Specific examples of porous sheets include microporous thin films, woven fabrics, and nonwoven fabrics. Suitable materials for the separator include polyethylene, olefin resins such as polypropylene, and cellulose. The separator 13 may be a laminate having a cellulose fiber layer and a thermoplastic resin fiber layer such as an olefin resin. Alternatively, it may be a multilayer separator containing a polyethylene layer and a polypropylene layer, or a separator with a material such as aramid resin or ceramic coated on its surface may be used.

[0070] The present disclosure will be further illustrated by the following examples, but the present disclosure is not limited to these examples.

[0071] (Example 1) To 100 parts by mass of water, 0.02 parts by mass of carbon nanotubes (CNTs) and 0.04 parts by mass of naphthalene sulfonic acid formaldehyde condensate (NSF) were added and mixed to obtain a carbon-based conductive agent dispersion. To this carbon-based conductive agent dispersion, 90 parts by mass of graphite (graphite-based negative electrode active material), 10 parts by mass of SiO (silicon-based negative electrode active material), 0.9 parts by mass of carboxymethylcellulose (CMC), 0.44 parts by mass of polyacrylic acid (PAA), and 1 part by mass of styrene-butadiene rubber (SBR) were added and mixed to obtain a negative electrode slurry.

[0072] The viscosity of the obtained negative electrode slurry was measured, and the viscosity Vb (25°C, 20 rpm) was 13,100 mPa·s, Va (25°C, 2 rpm) was 7,334 mPa·s, and the TI value was 0.56.

[0073] The obtained negative electrode slurry was applied to both sides of a negative electrode current collector made of 10 μm thick copper foil and dried, and then the resulting coating was rolled. In this way, a negative electrode was fabricated in which negative electrode mixture layers were formed on both sides of the negative electrode current collector.

[0074] (Example 2) A negative electrode slurry was prepared in the same manner as in Example 1, except that the amount of NSF added was changed to 0.3 parts by mass. The viscosity of the obtained negative electrode slurry was measured, and the viscosity Vb (25°C, 20 rpm) was 14820 mPa·s, Va (25°C, 2 rpm) was 7390 mPa·s, and the TI value was 0.50. Using the obtained negative electrode slurry, a negative electrode was fabricated under the same conditions as in Example 1.

[0075] (Example 3) A negative electrode slurry was prepared in the same manner as in Example 2, except that the amount of CMC added was changed to 0.6 parts by mass. The viscosity of the obtained negative electrode slurry was measured, and the viscosity Vb (25°C, 20 rpm) was 4737 mPa·s, Va (25°C, 2 rpm) was 2698 mPa·s, and the TI value was 0.57. Using the obtained negative electrode slurry, a negative electrode was prepared under the same conditions as in Example 1.

[0076] (Comparative Example 1) A negative electrode slurry was prepared in the same manner as in Example 1, except that NSF was not added. The viscosity of the obtained negative electrode slurry was measured, and the viscosity Vb (25°C, 20 rpm) was 15070 mPa·s, Va (25°C, 2 rpm) was 8204 mPa·s, and the TI value was 0.54. Using the obtained negative electrode slurry, a negative electrode was fabricated under the same conditions as in Example 1.

[0077] (Comparative Example 2) A negative electrode slurry was prepared in the same manner as in Example 1, except that the amount of NSF added was changed to 0.02 parts by mass. The viscosity of the obtained negative electrode slurry was measured, and the viscosity Vb (25°C, 20 rpm) was 3784 mPa·s, Va (25°C, 2 rpm) was 2204 mPa·s, and the TI value was 0.58. A negative electrode was prepared using the obtained negative electrode slurry under the same conditions as in Example 1.

[0078] [Peel Strength of the Negative Electrode Mixture Layer] The negative electrode mixture layer surface of each example or comparative example (100 mm x 25 mm) was attached to a 120 mm x 30 mm acrylic plate using double-sided tape (Nichiban Co., Ltd. Nicetack NW-20). Next, using a small benchtop test machine (FGS-TV and FGP-5) manufactured by Nidec-Shimpo Corporation, 90° peeling was performed at a measurement temperature of 25°C and a tensile speed of 50 mm / min, and the strength of the peeling of the negative electrode mixture layer from the negative electrode current collector (negative electrode mixture layer peel strength) was measured. The results are summarized in Table 1.

[0079]

[0080] As shown in Table 1, in all of Examples 1 to 3, which used a carbon dispersion containing 150 parts by mass or more of NSF per 100 parts by mass of CNT, the viscosity of the negative electrode slurry was lower compared to Comparative Example 1, which used a carbon dispersion without NSF. Furthermore, in Comparative Example 2, which used a carbon dispersion containing less than 150 parts by mass of NSF per 100 parts by mass of CNT, the peel strength of the negative electrode mixture layer decreased compared to Comparative Example 1, but in Examples 1 to 3, the peel strength of the negative electrode mixture layer was equivalent to or better than that of Comparative Example 1.

[0081] Among Examples 1 to 3, Examples 1 and 2, which used a negative electrode slurry with an NSF / CMC mass ratio of less than 0.5, showed higher peel strength of the negative electrode mixture layer compared to Example 3, which used a negative electrode slurry with an NSF / CMC mass ratio of 0.5.

[0082] [Preparation of Test Cells] An electrode body was prepared by spirally winding the negative electrode and metallic lithium of each example or comparative example via a polypropylene separator. This electrode body was housed in an outer casing made of aluminum laminate sheet, a non-aqueous electrolyte was injected, and the opening of the outer casing was sealed to obtain a test cell. The non-aqueous electrolyte was prepared by mixing fluoroethylene carbonate (FEC) and methyl propionate (FMP) in a volume ratio of 1:3, and then adding lithium hexafluoride phosphate (LiPF) ６ It was prepared by dissolving ) to a concentration of 1 mole / liter.

[0083] [Evaluation of Charge / Discharge Cycle Characteristics] The test cells of each example or comparative example were charged with a constant current of 0.5 It at a temperature of 25°C until the voltage reached 4.2V, and then charged with a constant voltage of 4.2V until the current value reached 0.02 It. After that, constant current discharge was performed with a constant current of 0.5 It until the voltage reached 2.5V. This charge / discharge cycle was considered one cycle, and 10 cycles were performed. The capacity retention rate in the charge / discharge cycle was then calculated using the following formula: Capacity retention rate = (Discharge capacity in the 10th cycle / Discharge capacity in the 1st cycle) × 100

[0084] Taking the capacity retention rate of Comparative Example 1 as 100.0, the capacity retention rates of the other examples and comparative examples are shown relatively: the capacity retention rate of Example 1 was 99.9, the capacity retention rates of Examples 2 and 3 were 100.0, and the capacity retention rate of Comparative Example 2 was 100.1. In terms of charge-discharge cycle characteristics, Examples 1 to 3 showed performance equivalent to that of the comparative examples.

[0085] [Note] Composition 1: A carbon-based conductive agent dispersion comprising: a carbon-based conductive agent; at least one formaldehyde condensate selected from the group consisting of alkylnaphthalenesulfonic acid-formaldehyde condensate and / or its alkali metal salt, naphthalenesulfonic acid-formaldehyde condensate and / or its alkali metal salt, and alkylnaphthalenesulfonic acid-naphthalenesulfonic acid-formaldehyde condensate and / or its alkali metal salt; and a solvent, wherein the content of the formaldehyde condensate is 150 parts by mass or more and 1500 parts by mass or less per 100 parts by mass of the carbon-based conductive agent. Composition 2: A negative electrode slurry for a secondary battery comprising the carbon-based conductive agent dispersion described in Composition 1; a graphite-based negative electrode active material; a silicon-based negative electrode active material; and carboxymethylcellulose and / or its alkali metal salt. Configuration 3: The carbon-based conductive agent contains nanocarbon, and the nanocarbon content is 0.005 parts by mass or more and less than 0.1 parts by mass per 100 parts by mass of the total amount of the graphite-based negative electrode active material and the silicon-based negative electrode active material, as described in Configuration 2. Configuration 4: The carboxymethylcellulose and / or alkali metal salt content is 0.7 parts by mass or more per 100 parts by mass of the total amount of the graphite-based negative electrode active material and the silicon-based negative electrode active material, and the total amount of the formaldehyde condensate and the carboxymethylcellulose and / or alkali metal salt is 0.9 parts by mass or more per 100 parts by mass of the total amount of the graphite-based negative electrode active material and the silicon-based negative electrode active material, as described in Configuration 2 or 3. Configuration 5: The negative electrode slurry for a secondary battery according to any one of Configurations 2 to 4, wherein the ratio (B / A) of the content of the formaldehyde condensate (B) to the content of the carboxymethylcellulose and / or alkali metal salt thereof (A) is less than 0.5 by mass ratio. Configuration 6: The negative electrode slurry for a secondary battery according to any one of Configurations 2 to 5, wherein the silicon-based negative electrode active material is a composite particle having an ion conducting phase and a silicon phase dispersed in the ion conducting phase, and the ion conducting phase includes at least one selected from the group consisting of a silicon oxide phase, a silicate phase, and a carbon phase.Configuration 7: A negative electrode for a secondary battery having a negative electrode mixture layer comprising a graphite-based negative electrode active material, a silicon-based negative electrode active material, carboxymethylcellulose and / or its alkali metal salt, a carbon-based conductive agent, and at least one formaldehyde condensate selected from the group consisting of alkylnaphthalenesulfonic acid-formaldehyde condensate and / or its alkali metal salt, naphthalenesulfonic acid-formaldehyde condensate and / or its alkali metal salt, and alkylnaphthalenesulfonic acid-naphthalenesulfonic acid-formaldehyde condensate and / or its alkali metal salt, wherein the content of the formaldehyde condensate is 150 parts by mass or more and 1500 parts by mass or less per 100 parts by mass of the carbon-based conductive agent. Configuration 8: A secondary battery comprising the negative electrode for a secondary battery described in Configuration 7.

[0086] 10 Secondary battery, 11 Positive electrode, 12 Negative electrode, 13 Separator, 14 Electrode body, 15 Battery case, 16 Outer can, 17 Sealing body, 18, 19 Insulating plate, 20 Positive electrode lead, 21 Negative electrode lead, 22 Protruding part, 23 Internal terminal plate, 24 Lower valve body, 25 Insulating member, 26 Upper valve body, 27 Cap, 28 Gasket.

Claims

1. A carbon-based conductive agent dispersion comprising: a carbon-based conductive agent; at least one formaldehyde condensate selected from the group consisting of alkylnaphthalenesulfonic acid-formaldehyde condensate and / or its alkali metal salt, naphthalenesulfonic acid-formaldehyde condensate and / or its alkali metal salt, and alkylnaphthalenesulfonic acid-naphthalenesulfonic acid-formaldehyde condensate and / or its alkali metal salt; and a solvent, wherein the content of the formaldehyde condensate is 150 parts by mass or more and 1500 parts by mass or less per 100 parts by mass of the carbon-based conductive agent.

2. A negative electrode slurry for a secondary battery comprising a carbon-based conductive agent dispersion according to claim 1, a graphite-based negative electrode active material, a silicon-based negative electrode active material, and carboxymethylcellulose and / or an alkali metal salt thereof.

3. The carbon-based conductive agent contains nanocarbon, and the nanocarbon content is 0.005 parts by mass or more and less than 0.1 parts by mass per 100 parts by mass of the total amount of the graphite-based negative electrode active material and the silicon-based negative electrode active material, as described in claim 2.

4. The negative electrode slurry for a secondary battery according to claim 2 or 3, wherein the content of the carboxymethylcellulose and / or its alkali metal salt is 0.7 parts by mass or more with respect to 100 parts by mass of the total amount of the graphite-based negative electrode active material and the silicon-based negative electrode active material, and the total amount of the formaldehyde condensate and the carboxymethylcellulose and / or its alkali metal salt is 0.9 parts by mass or more with respect to 100 parts by mass of the total amount of the graphite-based negative electrode active material and the silicon-based negative electrode active material.

5. The negative electrode slurry for a secondary battery according to claim 2 or 3, wherein the ratio (B / A) of the content of the formaldehyde condensate (B) to the content of the carboxymethylcellulose and / or alkali metal salt thereof (A) is less than 0.5 by mass.

6. The silicon-based negative electrode active material is a composite particle having an ion-conducting phase and a silicon phase dispersed within the ion-conducting phase, wherein the ion-conducting phase comprises at least one selected from the group consisting of a silicon oxide phase, a silicate phase, and a carbon phase, as described in claim 2 or 3.

7. A negative electrode for a secondary battery having a negative electrode mixture layer comprising: a graphite-based negative electrode active material; a silicon-based negative electrode active material; carboxymethylcellulose and / or its alkali metal salt; a carbon-based conductive agent; and at least one formaldehyde condensate selected from the group consisting of alkylnaphthalenesulfonic acid-formaldehyde condensate and / or its alkali metal salt, naphthalenesulfonic acid-formaldehyde condensate and / or its alkali metal salt; and alkylnaphthalenesulfonic acid-naphthalenesulfonic acid-formaldehyde condensate and / or its alkali metal salt, wherein the content of the formaldehyde condensate is 150 parts by mass or more and 1500 parts by mass or less per 100 parts by mass of the carbon-based conductive agent.

8. A secondary battery comprising the negative electrode for a secondary battery as described in claim 7.

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