Composition for forming artificial solid electrolyte interphase layer

Abstract

Provided is a composition for forming an artificial solid electrolyte interphase layer which can be suitably used for forming an artificial solid electrolyte interphase layer constituting a negative electrode of a secondary battery such as an anode-free lithium metal secondary battery. The composition for forming an artificial solid electrolyte interphase layer contains a solvent containing graphene oxide or a salt thereof, an olefin-unsaturated carboxylic acid copolymer or a salt thereof, and water.

Description

Composition for forming artificial solid electrolyte interfacial layer

[0001] The present invention relates to a composition for forming an artificial solid electrolyte interface layer.

[0002] Recently, secondary batteries have come to be used in a wide variety of applications, from mobile phones, wireless home appliances, and electric vehicles to drones and satellite base stations, and expectations for improved performance of secondary batteries are also rising. In particular, reducing the size and weight of secondary batteries and extending their operating time and lifespan are common challenges for all applications, and vigorous efforts are being made to improve these.

[0003] Among them, secondary batteries that charge and discharge by transferring metal ions between a positive electrode and a negative electrode are known to exhibit high voltages and high energy densities, and a typical example is a lithium-ion secondary battery. A typical lithium-ion secondary battery includes a battery that incorporates active materials capable of retaining lithium into the positive electrode and the negative electrode, and that charges and discharges by transferring lithium ions between the positive electrode active material and the negative electrode active material.

[0004] Furthermore, lithium metal secondary batteries (hereinafter sometimes simply referred to as "lithium secondary batteries") have been developed as secondary batteries that use lithium metal in the negative electrode. For example, Patent Document 1 discloses a high-energy density, high-power lithium metal secondary battery that has a volumetric energy density exceeding 1,000 Wh / L and a mass energy density exceeding 350 Wh / kg when discharged at a rate of at least 1 C at room temperature. Patent Document 1 discloses the use of a lithium metal negative electrode with a thickness of approximately 10 μm to 20 μm to realize such a lithium metal secondary battery.

[0005] In addition, Patent Document 2 discloses a lithium secondary battery including a positive electrode, a negative electrode, a separator interposed therebetween, and an electrolyte, in which metal particles are formed on a negative electrode current collector in the negative electrode, and the metal particles are transferred from the positive electrode upon charging to form lithium metal on the negative electrode current collector in the negative electrode. Patent Document 2 discloses that such a lithium secondary battery can solve problems caused by the reactivity of lithium metal and problems that occur during the assembly process, thereby providing a lithium secondary battery with improved performance and lifespan.

[0006] The lithium secondary battery described above is called an anode-free lithium metal secondary battery (hereinafter, sometimes simply referred to as an "anode-free battery"). In this battery, lithium ions migrate from the positive electrode and deposit as metal on the negative electrode during charging, eliminating the need for lithium metal in the negative electrode during manufacturing. This allows for the amount of lithium used to be minimized, and since there is no excess lithium in the battery, improved safety and energy density can be expected. Furthermore, corresponding cost reductions and a simplified manufacturing process can also be expected.

[0007] In general, in lithium-ion batteries, a reaction occurs between active lithium (e.g., lithium metal or lithium graphite) and the electrolyte solvent or electrolyte on the negative electrode surface during charging, forming a solid electrolyte interface (SEI). This inhibits subsequent reactions between the active lithium and the electrolyte solvent or electrolyte on the negative electrode surface, improving the Coulombic efficiency during charging and discharging and improving the charge-discharge cycle life of the lithium-ion battery. However, in lithium metal secondary batteries, the volume of the negative electrode (lithium metal) changes significantly during charging and discharging, and tree-like crystals called lithium dendrites form, destroying the SEI layer. Each time this occurs, a reaction occurs between the active lithium and the electrolyte solvent or electrolyte, causing a decrease in the Coulombic efficiency and a decrease in the discharge capacity retention rate, thereby deteriorating the charge-discharge cycle life characteristics. In the following description, the SEI layer that spontaneously forms during charging is sometimes referred to as the native SEI layer. In response to this, attempts have been made to form an artificial solid electrolyte interface (artificial SEI) layer on the negative electrode in order to form a strong and stable SEI layer on the negative electrode. However, there is room for improvement in terms of the uniformity of the resulting coating film, durability such as electrolyte resistance and mechanical strength, increased resistance due to low lithium ion conductivity, active lithium generation at the artificial solid electrolyte interface layer / separator interface due to low electronic resistance, and reduced energy density due to thicker films.

[0008] Special table 2019-517722 publication Special table 2019-537226 publication

[0009] The present invention has been made in consideration of the above circumstances, and aims to provide a composition for forming an artificial solid electrolyte interface layer that can be suitably used to form an artificial solid electrolyte interface layer that constitutes the negative electrode of a secondary battery such as an anode-free lithium metal secondary battery.

[0010] As a result of extensive research to achieve the above object, the inventors have discovered that a composition for forming an artificial solid electrolyte interface layer, which comprises graphene oxide or a salt thereof, an olefin-unsaturated carboxylic acid copolymer or a salt thereof, and a solvent containing water, can form a uniform coating film and provide an artificial SEI layer with excellent durability, and that a secondary battery produced using an electrode having this artificial SEI layer can obtain excellent battery characteristics, thereby completing the present invention.

[0011] That is, the present invention provides the following compositions for forming an artificial solid electrolyte interface layer. 1. A composition for forming an artificial solid electrolyte interface layer, comprising graphene oxide or a salt thereof, an olefin-unsaturated carboxylic acid copolymer or a salt thereof, and a solvent containing water. 2. The composition for forming an artificial solid electrolyte interface layer of 1, wherein the salt of the graphene oxide is a lithium salt. 3. The composition for forming an artificial solid electrolyte interface layer of 1 or 2, wherein the salt of the olefin-unsaturated carboxylic acid copolymer is an amine salt, ammonium salt, or lithium salt of an ethylene-(meth)acrylic acid copolymer. 4. Any of the compositions for forming an artificial solid electrolyte interface layer of 1 to 3, wherein the graphene oxide contains a carboxyl group or a phenolic hydroxyl group as a surface functional group and has an oxygen element ratio of 20 to 60 atomic %. 5. Any of the compositions for forming an artificial solid electrolyte interface layer of 1 to 4, wherein the graphene oxide is single-layer graphene oxide or multi-layer graphene oxide having a thickness of 5 nm or less. 6. An artificial solid electrolyte interface layer obtained from any of the compositions for forming an artificial solid electrolyte interface layer of 1 to 5. 7. The artificial solid electrolyte interface layer of 6, having a thickness of 1 μm or less. 8. A secondary battery negative electrode comprising the artificial solid electrolyte interface layer of 6 or 7 on a substrate. 9. The secondary battery negative electrode of 8, wherein the substrate is a copper foil. 10. The secondary battery negative electrode of 8 or 9, which is a lithium metal secondary battery negative electrode. 11. The secondary battery negative electrode of 10, which is an anode-free battery negative electrode. 12. A secondary battery comprising the secondary battery negative electrode of 8 or 9. 13. The secondary battery of 12, which is a lithium metal secondary battery. 14. The secondary battery of 13, which is an anode-free battery.

[0012] The composition for forming an artificial solid electrolyte interface layer of the present invention can form a uniform and durable artificial SEI layer. Furthermore, by fabricating a secondary battery using an electrode with the artificial SEI layer, the battery characteristics of the secondary battery are expected to be improved.

[0013] The composition for forming an artificial solid electrolyte interface layer of the present invention (hereinafter, also referred to as "artificial SEI layer forming composition" or "composition") contains graphene oxide or a salt thereof, an olefin-unsaturated carboxylic acid copolymer or a salt thereof, and a solvent containing water.

[0014] [Graphene oxide or salt thereof] Graphene oxide is a compound obtained by oxidizing graphene. In the present invention, the graphene includes single-layer graphene and multi-layer graphene having 2 to 100 layers. Furthermore, single-layer graphene refers to a sheet of carbon molecules having one atomic layer and having π bonds.

[0015] The graphene oxide has oxidized sites in its structure, and these sites are usually bonded with surface functional groups such as carbonyl groups, carboxy groups, hydroxy groups, phenolic hydroxyl groups, and epoxy groups. In the present invention, from the viewpoints of dispersion stability in the composition for forming an artificial solid electrolyte interface layer, improvement of the lithium ion conductivity of the artificial solid electrolyte interface layer, and salt formation, graphene oxides containing carboxy groups or phenolic hydroxyl groups as surface functional groups are preferred, and graphene oxides containing carboxy groups are more preferred.

[0016] In terms of water dispersibility, uniform coating property, mechanical strength, electrolyte resistance, electronic resistance, and lithium ion conductivity, the graphene oxide preferably has an oxygen element ratio of 20 to 60 atm %, more preferably 30 to 50 atm %. The oxygen element ratio can be measured by XPS.

[0017] Furthermore, the graphene oxide generally includes single-layer graphene oxide or multi-layer graphene oxide having 2 to 100 layers. However, from the viewpoints of water dispersibility, uniform coating property, mechanical strength, electrolyte resistance, electronic resistance, and lithium ion conductivity, single-layer graphene oxide or multi-layer graphene oxide having a thickness of 5 nm or less is preferred.

[0018] As the graphene oxide, commercially available products can also be used, and specific examples thereof include EMD Millipore (Merck).

[0019] Examples of the salt of graphene oxide include lithium salt, sodium salt, potassium salt, cesium salt, amine salt, and ammonium salt. When the graphene oxide is used for lithium metal secondary batteries or lithium ion batteries, lithium salt, and amine salts and ammonium salts made of low-molecular-weight amines or ammonia, which are partially or completely volatilized during the process of preparing an artificial SEI layer, are preferred in terms of the lithium purity of the negative electrodes in the lithium metal secondary batteries or lithium ion batteries.

[0020] Furthermore, graphene oxide salts can be obtained by neutralizing surface functional groups, such as carboxyl groups and phenolic hydroxyl groups, contained in graphene oxide using a specific base. Examples of the base include lithium hydroxide, lithium carbonate, lithium bicarbonate, lithium metal, lithium hydride, lithium t-butoxide, sodium lithium (with the lithium moiety replaced by sodium), potassium lithium (with the lithium moiety replaced by potassium), amines, and ammonia. The amine is preferably a tertiary amine with a molecular weight of 200 or less and a boiling point of 200°C or less.

[0021] [Olefin-Unsaturated Carboxylic Acid Copolymer or Salt thereof] In the olefin-unsaturated carboxylic acid copolymer, the olefin preferably has 2 or more carbon atoms and 8 or less carbon atoms, particularly 6 or less carbon atoms. Specific examples include ethylene, propylene, butene, pentene, hexene, heptene, and octene, with ethylene being particularly preferred.

[0022] Examples of the unsaturated carboxylic acid include (meth)acrylic acid, maleic acid, and fumaric acid, with (meth)acrylic acid being particularly preferred.

[0023] Furthermore, examples of the salt of the unsaturated carboxylic acid contained in the copolymer include amine salts, ammonium salts, lithium salts, sodium salts, and potassium salts. In the case of lithium metal secondary batteries and lithium ion batteries, amine salts, ammonium salts, and lithium salts are preferred, and tertiary amine salts are more preferred, from the viewpoint of the lithium purity of the negative electrode.

[0024] Specific examples of the copolymer include an amine salt or ammonium salt of an ethylene-(meth)acrylic acid copolymer, and an amine salt, ammonium salt, or lithium salt of a propylene-(meth)acrylic acid copolymer. An amine salt, ammonium salt, or lithium salt of an ethylene-(meth)acrylic acid copolymer is preferred, and a tertiary amine salt of an ethylene-(meth)acrylic acid copolymer is more preferred.

[0025] The proportion of olefin in the copolymer is preferably 60 to 95 mol % of the structural units derived from all monomers.

[0026] The proportion of the unsaturated carboxylate in the copolymer is preferably 5 to 50 mol % of the structural units derived from all monomers, but this does not exclude the inclusion of a portion of unsaturated carboxylic acid that is not in the form of a salt. The proportion of the unsaturated carboxylate is desirably 10 to 30 mol % of the structural units derived from all monomers.

[0027] The olefin-unsaturated carboxylic acid copolymer or its salt is usually in the form of particles. The average particle size of the particles (hereinafter sometimes referred to as copolymer particles) is preferably 1 μm or less, more preferably 500 nm or less, and even more preferably 100 nm or less, in consideration of improving dispersibility in graphene oxide or its salt, improving the coating area during coating, improving the thickness uniformity of the coating surface, reducing ion conduction resistance, and improving the energy density of the resulting secondary battery. The lower limit is not particularly limited, but is preferably 1 nm or more, more preferably 10 nm or more, in terms of controlling the particle size uniformity. In the present invention, the primary particle size of the copolymer particles is measured by a Cole counter method.

[0028] The olefin-unsaturated carboxylic acid copolymer or a salt thereof can be obtained as a commercially available product, and examples of such commercially available products include ZAIXXEN A, ZAIXXEN A-GH, and ZAIXXEN L (all manufactured by Sumitomo Seika Chemicals Co., Ltd.).

[0029] The content of the olefin-unsaturated carboxylic acid copolymer or its salt is preferably 0.1 to 10 parts by mass, more preferably 0.2 to 5 parts by mass, even more preferably 0.3 to 3 parts by mass, and particularly preferably 0.5 to 2 parts by mass, relative to 1 part by mass of the graphene oxide or its salt. By including the olefin-unsaturated carboxylic acid copolymer or its salt within the above range, improved ionic conductivity, improved flexibility, improved mechanical strength, and improved adhesion to a substrate such as copper foil and to a negative electrode metal formed on the negative electrode are expected. Furthermore, the olefin-unsaturated carboxylic acid copolymer or its salt may be used alone or in combination of two or more types.

[0030] [Solvent] The solvent used in preparing the composition of the present invention includes water, but may also contain a hydrophilic solvent in addition to water. The hydrophilic solvent is an organic solvent that is arbitrarily miscible with water, and examples thereof include organic solvents such as ethers such as tetrahydrofuran (THF); amides such as N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), and N-methyl-2-pyrrolidone (NMP); ketones such as acetone; alcohols such as methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, and t-butanol; glycol ethers such as ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, and propylene glycol monomethyl ether; and glycols such as ethylene glycol and propylene glycol. Among these hydrophilic solvents, alcohols are preferred in the present invention, taking into consideration the improvement of carbon black dispersibility, the improvement of the coating area during coating, and the improvement of the coating film thickness uniformity. Methanol, ethanol, 1-propanol, and 2-propanol are more preferred. These solvents may be used alone or in combination of two or more.

[0031] When an organic solvent is contained, its content in the solvent is preferably 10% by mass or less, more preferably 5% by mass or less, from the viewpoint of solubility and dispersibility of graphene oxide in the solution or dispersion. Although the lower limit is not particularly limited, if the above-mentioned improving effect is low, it is preferable that the solvent does not contain an organic solvent (i.e., 0% by mass) in order to reduce the risk of environmental pollution and reduce equipment costs.

[0032] [Crosslinking Agent] The composition of the present invention may contain a crosslinking agent that crosslinks with the surface functional groups of the graphene oxide or the olefin-unsaturated carboxylic acid copolymer, or a crosslinking agent that self-crosslinks, within a range that does not impair the effects of the present invention. These crosslinking agents are preferably soluble in the solvent used.

[0033] Examples of the crosslinking agent include melamine-based, substituted urea-based, and polymer-based crosslinking agents thereof, and these crosslinking agents can be used alone or in combination of two or more.Preferably, the crosslinking agent has at least two crosslink-forming substituents, and examples thereof include CYMEL (registered trademark), methoxymethylated glycoluril, butoxymethylated glycoluril, methylolated glycoluril, methoxymethylated melamine, butoxymethylated melamine, methylolated melamine, methoxymethylated benzoguanamine, butoxymethylated benzoguanamine, methylolated benzoguanamine, methoxymethylated urea, butoxymethylated urea, methylolated urea, methoxymethylated thiourea, methoxymethylated thiourea, methylolated thiourea, and condensates of these compounds.

[0034] The crosslinking agent can induce a crosslinking reaction with the surface functional groups of the graphene oxide or the olefin-unsaturated carboxylic acid copolymer or its salt. However, a different type of crosslinking agent that reacts with the crosslinking agent can also be added. The different type of crosslinking agent is not particularly limited as long as it is a compound having two or more functional groups reactive with oxazoline groups, such as a carboxyl group, a hydroxyl group, a thiol group, an amino group, a sulfinic acid group, or an epoxy group. However, a compound having two or more carboxyl groups is preferred. Furthermore, compounds having functional groups that undergo crosslinking reactions upon heating during thin film formation or in the presence of an acid catalyst, such as sodium, potassium, lithium, or ammonium salts of carboxylic acids, can also be used as crosslinking agents.

[0035] Examples of self-crosslinking crosslinking agents include compounds having crosslinkable functional groups that react with each other in the same molecule, such as an aldehyde group, epoxy group, vinyl group, isocyanate group, or alkoxy group with a hydroxyl group, an aldehyde group, amino group, isocyanate group, or epoxy group with a carboxyl group, or an isocyanate group or aldehyde group with an amino group, and compounds having a hydroxyl group (dehydration condensation), a mercapto group (disulfide bond), an ester group (Claisen condensation), a silanol group (dehydration condensation), a vinyl group, an acrylic group, or the like, that react with the same crosslinkable functional groups.

[0036] Specific examples of self-crosslinking crosslinking agents include polyfunctional acrylates, tetraalkoxysilanes, block copolymers of monomers having a blocked isocyanate group, and monomers having at least one of a hydroxyl group, a carboxylic acid, and an amino group, which exhibit crosslinking reactivity in the presence of an acid catalyst.

[0037] Such a self-crosslinking crosslinking agent can be obtained as a commercially available product. Examples of such commercially available products include polyfunctional acrylates such as A-9300 (ethoxylated isocyanuric acid triacrylate, manufactured by Shin-Nakamura Chemical Co., Ltd.), A-GLY-9E (ethoxylated glycerine triacrylate (EO 9 mol), manufactured by Shin-Nakamura Chemical Co., Ltd.), and A-TMMT (pentaerythritol tetraacrylate, manufactured by Shin-Nakamura Chemical Co., Ltd.), tetraalkoxysilanes such as tetramethoxysilane (manufactured by Tokyo Chemical Industry Co., Ltd.) and tetraethoxysilane (manufactured by Toyoko Chemical Co., Ltd.), and polymers having a blocked isocyanate group such as Elastron series E-37, H-3, H38, BAP, and NEW Examples include BAP-15, C-52, F-29, W-11P, MF-9, and MF-25K (manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.).

[0038] When a crosslinking agent is contained, the content thereof varies depending on the solvent used, the base material used, the required viscosity, the required film shape, etc., but is preferably 0.001 to 1,000 parts by mass, more preferably 0.001 to 100 parts by mass, and even more preferably 0.001 to 50 parts by mass, relative to 100 parts by mass of the olefin-unsaturated carboxylic acid copolymer or its salt. These crosslinking agents may undergo a crosslinking reaction by self-condensation, but they also undergo a crosslinking reaction with the dispersant, and when crosslinkable substituents are present in the dispersant, the crosslinking reaction is promoted by these crosslinkable substituents.

[0039] [Matrix Polymer] A polymer that serves as a matrix may be added to the composition of the present invention. Examples of the matrix polymer include fluorine-based resins such as polyvinylidene fluoride (PVdF), polytetrafluoroethylene, tetrafluoroethylene-hexafluoropropylene copolymer, vinylidene fluoride-hexafluoropropylene copolymer [P(VDF-HFP)], and vinylidene fluoride-chlorotrifluoroethylene copolymer [P(VDF-CTFE)]; polyvinylpyrrolidone, ethylene-propylene-diene terpolymer, PE (polyethylene), PP (polypropylene), and EVA (ethylene-vinyl acetate). polyolefin resins such as ethylene-vinyl acrylate copolymer and EEA (ethylene-ethyl acrylate copolymer); polystyrene, HIPS (high impact polystyrene), AS (acrylonitrile-styrene copolymer), ABS (acrylonitrile-butadiene-styrene copolymer), MS (methyl methacrylate-styrene copolymer), styrene-butadiene rubber, and other polystyrene resins; polycarbonate resins; vinyl chloride resins; polyamide resins; polyimide resins; (meth)acrylic resins such as sodium polyacrylate and PMMA (polymethyl methacrylate); polyester resins such as PET (polyethylene terephthalate), polybutylene terephthalate, polyethylene naphthalate, polybutylene naphthalate, PLA (polylactic acid), poly-3-hydroxybutyric acid, polycaprolactone, polybutylene succinate, and polyethylene succinate / adipate; polyphenylene ether resins; modified polyphenylene ether resins; polyacetal resins; polysulfone resins; polyphenylene sulfide resins; polyvinyl a Examples of suitable resins include thermoplastic resins such as alcohol resins; polyglycolic acid; modified starch; cellulose acetate, carboxymethyl cellulose, cellulose triacetate; chitin, chitosan; and lignin; conductive polymers such as polyaniline and its semi-oxidized form emeraldine base; polythiophene; polypyrrole; polyphenylene vinylene; polyphenylene; and polyacetylene; and thermosetting resins and photocurable resins such as epoxy resins; urethane acrylates; phenolic resins; melamine resins; urea resins; and alkyd resins.In the conductive carbon material dispersion of the present invention, since it is preferable to use water as the solvent, it is also preferable to use a water-soluble matrix polymer. Examples of water-soluble matrix polymers include sodium polyacrylate, sodium carboxymethyl cellulose, water-soluble cellulose ether, sodium alginate, polyvinyl alcohol, polystyrene sulfonic acid, and polyethylene glycol, with sodium polyacrylate and sodium carboxymethyl cellulose being particularly preferable.

[0040] The matrix polymer may be commercially available. Examples of such commercially available products include sodium polyacrylate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., degree of polymerization 2,700 to 7,500), sodium carboxymethylcellulose (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), sodium alginate (manufactured by Kanto Chemical Co., Ltd., Grade 1), Metrolose SH series (hydroxypropyl methylcellulose, manufactured by Shin-Etsu Chemical Co., Ltd.), Metrolose SE series (hydroxyethyl methylcellulose, manufactured by Shin-Etsu Chemical Co., Ltd.), JC-25 (fully saponified polyvinyl alcohol, manufactured by Japan Vinyl Acetate & Poval Co., Ltd.), JM-17 (intermediately saponified polyvinyl alcohol, manufactured by Japan Vinyl Acetate & Poval Co., Ltd.), JP-03 (partially saponified polyvinyl alcohol, manufactured by Japan Vinyl Acetate & Poval Co., Ltd.), and polystyrene sulfonic acid (manufactured by Aldrich, solids concentration 18% by mass, aqueous solution).

[0041] When a matrix polymer is used, its content is not particularly limited, but is preferably 0.001 to 1,000 parts by mass, more preferably 0.001 to 900 parts by mass, per 100 parts by mass of the olefin-unsaturated carboxylic acid copolymer or its salt.

[0042] [Method for Preparing the Composition] The method for preparing the composition of the present invention is not particularly limited, and may be a method for preparing a dispersion by mixing, in any order, graphene oxide or a salt thereof, an olefin-unsaturated carboxylic acid copolymer or a salt thereof, a solvent, and other components used as needed. Furthermore, from the viewpoint of highly dispersing graphene oxide or a salt thereof to obtain a uniform dispersion, it is preferable to subject the mixture to a dispersion treatment. Examples of dispersion treatment include mechanical treatments such as wet treatments using a ball mill, bead mill, jet mill, or the like, and ultrasonic treatments using a bath-type or probe-type sonicator. In particular, wet treatments using a jet mill and ultrasonic treatments are preferred.

[0043] The time for the dispersion treatment is optional, but is preferably about 1 minute to 10 hours, and more preferably about 5 minutes to 5 hours. During this time, stirring treatment, cooling treatment, heating treatment, etc. may be carried out as necessary.

[0044] When an optional component such as a matrix polymer is used, it may be added after preparing a uniform dispersion containing graphene oxide or a salt thereof, an olefin-unsaturated carboxylic acid copolymer or a salt thereof, a solvent, and, if necessary, a crosslinking agent.

[0045] In the present invention, the solid content concentration of the composition is not particularly limited, but in consideration of forming an artificial SEI layer with a desired basis weight and film thickness, it is preferably 10% by mass or less, more preferably 5% by mass or less, and even more preferably 3% by mass or less. The lower limit is arbitrary, but from a practical viewpoint, it is preferably 0.01% by mass or more, more preferably 0.05% by mass or more, and even more preferably 0.1% by mass or more.

[0046] In consideration of the coatability to substrates such as current collecting substrates, the viscosity of the composition is preferably 0.1 to 20 mPa·s, and more preferably 1 to 10 mPa·s, where the viscosity is measured at 25°C using an E-type viscometer.

[0047] [Artificial solid electrolyte interface layer and negative electrode for secondary battery] The composition described above is applied to one surface of a substrate, such as a negative electrode current collecting substrate or a lithium metal deposition promoting layer formed on the negative electrode current collecting substrate, and then dried naturally or by heating to obtain a thin film (artificial solid electrolyte interface layer), which can be suitably used as an artificial SEI layer for a negative electrode for a secondary battery.

[0048] As the negative electrode current collecting substrate, any of those conventionally used as current collecting substrates for energy storage device electrodes can be used. For example, copper, aluminum, titanium, stainless steel, nickel, gold, silver, and alloys thereof, carbon materials, metal oxides, conductive polymers, etc. can be used. However, metal foils (metal substrates) made of copper, aluminum, nickel, or alloys thereof are preferred, with copper foil being more preferred. Note that, when used as a negative electrode current collecting substrate for a lithium secondary battery, it is preferable to use a substrate that does not react with lithium metal. The thickness of the current collecting substrate is not particularly limited, but in the present invention, a thickness of 1 to 100 μm is preferred.

[0049] Examples of methods for applying the composition include spin coating, dip coating, flow coating, inkjet coating, casting, spray coating, bar coating, gravure coating, slit coating, roll coating, flexographic printing, transfer printing, brush coating, blade coating, air knife coating, and die coating. From the viewpoint of work efficiency, inkjet coating, casting, dip coating, bar coating, blade coating, roll coating, gravure coating, flexographic printing, spray coating, and die coating are preferred. The temperature for heat drying is also optional, but is preferably about 30 to 200°C, and more preferably about 50 to 150°C. The drying time is not particularly limited as long as the solvent can be volatilized, but is usually preferably about 5 to 20 minutes.

[0050] Considering the reduction in internal resistance and film thickness uniformity of the resulting device, the thickness of the SEI layer is preferably 1 μm or less, more preferably 500 nm or less, even more preferably 200 nm or less, and even more preferably 100 nm or less. The lower limit is not particularly limited, but considering the strength of the film, it is preferably 1 nm or more, more preferably 5 nm or more, and even more preferably 10 nm or more. The film thickness of the artificial SEI layer can be determined, for example, by cutting a test piece of an appropriate size from a current collecting substrate on which an artificial SEI layer has been formed, exposing the cross section using a focused ion beam method or the like, and observing the exposed part of the undercoat layer in the cross section using a microscope such as a scanning electron microscope (SEM).

[0051] The negative electrode for a secondary battery can be constructed by forming a negative electrode active material layer, if necessary, on a negative electrode current collector having the above-mentioned artificial SEI layer. The negative electrode for a secondary battery of the present invention can be used as a negative electrode for various secondary batteries, such as electric double layer capacitors, lithium secondary batteries, lithium ion secondary batteries, proton polymer batteries, nickel-metal hydride batteries, aluminum solid capacitors, electrolytic capacitors, and lead-acid batteries. The negative electrode for a secondary battery of the present invention can be suitably used in lithium secondary batteries, particularly anode-free lithium metal secondary batteries (anode-free batteries).

[0052] Furthermore, when the secondary battery to be manufactured is an anode-free battery, since the battery does not have a negative electrode active material layer during manufacturing, the negative electrode current collector having the above-mentioned artificial SEI layer can be used as is. In this case, lithium ions that migrate from the positive electrode side during charging are deposited as lithium metal on the negative electrode current collector substrate, thereby forming a negative electrode active material layer.

[0053] In the present invention, unless otherwise specified, "lithium metal is deposited on the negative electrode current collecting substrate" means that lithium metal is deposited on the surface of the negative electrode current collecting substrate and at least a portion of the surface of the artificial SEI layer formed on the surface of the negative electrode current collecting substrate. Therefore, in a lithium secondary battery, lithium metal may be deposited, for example, on the surface of the negative electrode current collecting substrate (the interface between the negative electrode current collecting substrate and the artificial SEI layer).

[0054] The negative electrode active material layer can be formed by applying a negative electrode slurry (a composition for forming a negative electrode active material layer) prepared by mixing a negative electrode active material, a binder polymer, and, if necessary, a solvent onto a substrate and drying the mixture naturally or by heating. Alternatively, the negative electrode active material layer can be formed by forming a mixture of the negative electrode active material and the binder polymer into a sheet (dry method).

[0055] As the negative electrode active material, at least one element, oxide, sulfide, or nitride selected from alkali metals, alkali alloys, and elements in Groups 4 to 15 of the periodic table that absorb and release lithium ions, or a carbon material that can reversibly absorb and release lithium ions can be used.

[0056] Examples of alkali metals include Li, Na, and K, and examples of alkali metal alloys include Li-Al, Li-Mg, Li-Al-Ni, Na-Hg, and Na-Zn. Examples of the at least one element selected from the elements of Groups 4 to 15 of the periodic table that absorbs and releases lithium ions include silicon, tin, aluminum, zinc, and arsenic. Similarly, examples of oxides include silicon monoxide (SiO), silicon dioxide (SiO), tin silicon oxide (SnSiO), lithium bismuth oxide (LiBiO), lithium zinc oxide (LiZnO), and lithium titanate (LTO, LiTiO). 12 ), titanium oxide, etc. Similarly, examples of sulfides include lithium iron sulfide (Li x FeS2 (0≦x≦3)), lithium copper sulfide (Li x CuS (0≦x≦3)). Similarly, examples of nitrides include lithium-containing transition metal nitrides, specifically Li x M y Examples of carbon materials that can reversibly store and release lithium ions include graphite, carbon black, coke, glassy carbon, carbon fiber, carbon nanotubes, and sintered bodies thereof.

[0057] The binder polymer can be appropriately selected from known materials and used, for example, polyvinylidene fluoride (PVdF), polyvinylpyrrolidone, polytetrafluoroethylene, tetrafluoroethylene-hexafluoropropylene copolymer, vinylidene fluoride-hexafluoropropylene copolymer [P(VDF-HFP)], vinylidene fluoride-chlorotrifluoroethylene copolymer [P(VDF-CTFE)], polyvinyl alcohol, polyimide, ethylene-propylene-diene terpolymer, styrene-butadiene rubber (SBR), carboxymethyl cellulose (CMC), polyacrylic acid (PAA), polyaniline, polyimide, polyamide. The content of the binder polymer is preferably 0.1 to 40 parts by mass, particularly 1 to 30 parts by mass, relative to 100 parts by mass of the negative electrode active material.

[0058] Examples of the solvent include those exemplified as the solvent for the composition above, and may be selected appropriately from among them depending on the type of binder, but NMP is suitable for water-insoluble binders such as PVdF, and water is suitable for water-soluble binders such as SBR-CMC and PAA. When preparing a dry electrode, the solvent does not need to contain a solvent.

[0059] The negative electrode slurry may contain a conductive material such as carbon black, ketjen black, acetylene black, carbon whisker, carbon fiber, natural graphite, artificial graphite, titanium oxide, ruthenium oxide, aluminum, and nickel, with graphite (natural graphite and artificial graphite) being preferred.

[0060] When forming a negative electrode active material layer on an artificial SEI layer, the method for forming the negative electrode active material layer includes a method of press-molding a negative electrode composition prepared without using a solvent onto a substrate (dry method), or a method of preparing a negative electrode slurry using a solvent, coating it on a substrate, and drying it (wet method). These methods are not particularly limited, and various conventionally known methods can be used. For example, wet methods include various printing methods such as offset printing and screen printing, blade coating, dip coating, spin coating, bar coating, slit coating, inkjet printing, and die coating.

[0061] In addition, when the drying by heating is performed, either natural drying or heat drying may be used, but heat drying is preferred from the viewpoint of production efficiency. When the drying by heating is performed, the temperature is preferably about 50 to 400°C, and more preferably about 70 to 150°C.

[0062] The thickness of the negative electrode active material layer (thin film) is not particularly limited, but is preferably about 0.01 to 1,000 μm, more preferably about 5 to 300 μm. When the thin film is used alone as an electrode, the film thickness is preferably 10 μm or more.

[0063] The thickness of the negative electrode active material layer can be determined by a known method such as measurement using a micrometer, for example, by measuring the overall thickness of the fabricated negative electrode and subtracting the thicknesses of the current collecting substrate and the artificial SEI layer from the measured thickness.

[0064] The negative electrode for secondary batteries may be pressed as necessary. A commonly used pressing method can be used, but mold pressing and roll pressing are particularly preferred. The pressing pressure is not particularly limited, but is preferably 1 kN / cm or more, more preferably 2 kN / cm or more, and more preferably 5 kN / cm or more. The upper limit of the pressing pressure is not particularly limited, but is preferably 50 kN / cm or less.

[0065] [Secondary Battery] The secondary battery according to the present invention includes the above-described negative electrode for a secondary battery, and more specifically, includes at least one pair of positive and negative electrodes, a separator interposed between the electrodes, and an electrolyte. Examples of the secondary battery include lithium secondary batteries, lithium ion secondary batteries, lithium-air secondary batteries, lithium ion capacitors, sodium secondary batteries, sodium ion batteries, sodium ion capacitors, potassium secondary batteries, potassium ion secondary batteries, zinc secondary batteries, zinc ion secondary batteries, nickel-zinc secondary batteries, manganese-zinc secondary batteries, zinc-air secondary batteries, magnesium secondary batteries, magnesium ion batteries, magnesium-air batteries, aluminum metal secondary batteries, aluminum ion secondary batteries, aluminum-air secondary batteries, and lead-acid batteries. Lithium secondary batteries are preferred, and anode-free lithium metal secondary batteries (anode-free batteries) are more preferred.

[0066] The secondary battery according to the present invention is characterized in that it uses the above-described negative electrode for a secondary battery as the negative electrode, and therefore other components such as a positive electrode, a separator, and an electrolyte can be appropriately selected from known materials and used.

[0067] The positive electrode is composed of a positive electrode current collecting substrate and a positive electrode active material layer formed on the current collecting substrate.

[0068] The positive electrode current collecting substrate can be one that has been conventionally used as a current collecting substrate for an electrode of an energy storage device. For example, copper, aluminum, titanium, stainless steel, nickel, gold, silver, and alloys thereof, carbon materials, metal oxides, conductive polymers, etc. can be used. However, metal foils (metal substrates) made of copper, aluminum, nickel, or alloys thereof are preferred, with copper foil being more preferred. In the present invention, a material can be selected from these, taking into consideration the combination with the above-mentioned negative electrode current collecting substrate. The thickness of the current collecting substrate is not particularly limited, but in the present invention, a thickness of 1 to 100 μm is preferred.

[0069] The positive electrode active material layer can be formed by applying a positive electrode slurry (composition for forming a positive electrode active material layer) prepared by mixing a positive electrode active material, a binder polymer, and, if necessary, a solvent onto a substrate and drying the mixture naturally or by heating. Alternatively, the positive electrode active material layer can be formed by forming a mixture of the positive electrode active material and the binder polymer into a sheet (dry method).

[0070] The positive electrode active material may be any of various active materials conventionally used in electrodes of energy storage devices. For example, in the case of a lithium secondary battery or a lithium ion secondary battery, the positive electrode active material may be a chalcogen compound capable of adsorbing and releasing lithium ions, a lithium ion-containing chalcogen compound, a polyanionic compound, elemental sulfur, or compounds thereof.

[0071] Examples of such chalcogen compounds capable of absorbing and desorbing lithium ions include FeS, TiS, MoS, V, O, and V. 13 , MnO2, etc. Examples of lithium ion-containing chalcogen compounds include LiCoO2, LiMnO2, LiMn2O4, LiMo2O4, LiV3O8, LiNiO2, Li x Ni y M 1-y O2 (wherein M represents at least one metal element selected from Co, Mn, Ti, Cr, V, Al, Sn, Pb, and Zn, and 0.05≦x≦1.10, 0.5≦y≦1.0). Examples of polyanion compounds include LiFePO4. Examples of sulfur compounds include LiS and rubeanic acid.

[0072] In the case of lithium ion capacitors and sodium ion capacitors, carbonaceous materials can be used as the active material, such as activated carbon with a high specific surface area, for example, activated carbon obtained by carbonizing synthetic resins such as phenolic resins or natural materials such as coconut shells, followed by activation treatment.

[0073] Examples of the binder polymer include those exemplified in the description of the negative electrode slurry. The content of the binder polymer is preferably 0.1 to 40 parts by mass, and more preferably 1 to 30 parts by mass, per 100 parts by mass of the positive electrode active material.

[0074] Examples of the solvent include those exemplified as the solvent for the composition above, and may be selected appropriately from among them depending on the type of binder, but NMP is suitable for water-insoluble binders such as PVdF, and water is suitable for water-soluble binders such as SBR-CMC and PAA. When preparing a dry electrode, the solvent does not need to contain a solvent.

[0075] The positive electrode slurry may contain a conductive material, such as those exemplified in the description of the negative electrode slurry.

[0076] Examples of methods for forming the positive electrode active material layer include a method of pressure-molding a positive electrode composition prepared without using a solvent onto a substrate (dry method), or a method of preparing a positive electrode slurry using a solvent, coating it on a substrate, and drying it (wet method). These methods are not particularly limited, and various conventionally known methods can be used. For example, wet methods include various printing methods such as offset printing and screen printing, blade coating, dip coating, spin coating, bar coating, slit coating, inkjet printing, and die coating.

[0077] In addition, when the drying by heating is performed, either natural drying or heat drying may be used, but heat drying is preferred from the viewpoint of production efficiency. When the drying by heating is performed, the temperature is preferably about 50 to 400°C, and more preferably about 70 to 150°C.

[0078] The thickness of the positive electrode active material layer is preferably 10 to 500 μm, more preferably 30 to 100 μm, from the viewpoints of improving the capacity of the electrode, improving the energy density, and suppressing the electrical resistance.

[0079] The thickness of the positive electrode active material layer can be determined by a known method such as measurement using a micrometer, for example, by measuring the thickness of the entire positive electrode produced and subtracting the thickness of the positive electrode current collecting substrate therefrom.

[0080] The positive electrode for secondary batteries may be pressed as necessary. A commonly used pressing method can be used, but mold pressing and roll pressing are particularly preferred. The pressing pressure is not particularly limited, but is preferably 1 kN / cm or more, more preferably 2 kN / cm or more, and more preferably 5 kN / cm or more. The upper limit of the pressing pressure is not particularly limited, but is preferably 50 kN / cm or less.

[0081] Examples of the separator include a cellulose-based separator, a polyolefin-based separator, and a glass fiber-based separator.

[0082] The electrolyte may be either a liquid electrolyte obtained by dissolving an electrolyte salt in a solvent or a solid electrolyte, and may be either aqueous or non-aqueous. However, the energy storage device electrode of the present invention is preferably applied to a battery using a solid electrolyte, particularly an all-solid-state battery (e.g., an all-solid-state lithium-ion battery).

[0083] Examples of electrolyte salts include lithium salts such as LiPF, iBF, LiN(SOF), LiN(CFSO), LiAsF, LiSbF, LiAlF, LiGaF, LiInF, LiClO, LiN(CFSO), LiCFSO, LiSiF, LiN(CFSO), (CFSO), metal iodides such as LiI, NaI, KI, CsI, and CaI, iodide salts of quaternary imidazolium compounds, iodide salts and perchlorates of tetraalkylammonium compounds, and metal bromides such as LiBr, NaBr, KBr, CsBr, and CaBr. These electrolyte salts may be used alone or in combination of two or more.

[0084] The electrolyte solvent is not particularly limited as long as it does not corrode or decompose the materials constituting the battery, thereby deteriorating their performance, and dissolves the electrolyte salt. Examples of non-aqueous solvents that can be used include cyclic esters such as ethylene carbonate (EC), propylene carbonate, butylene carbonate, and γ-butyrolactone; ethers such as tetrahydrofuran and dimethoxyethane; linear esters such as methyl acetate, dimethyl carbonate (dimethyl carbonate), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC); and nitriles such as acetonitrile. These solvents may be used alone or in combination of two or more.

[0085] To aid in the function of the artificial solid electrolyte interface, the electrolyte may contain additives to promote the formation of a natural SEI layer, such as vinylene carbonate (VC), 1-propene 1,3-sultone (PS), fluoroethylene carbonate (FEC), and 4-(trifluoromethyl)-1,3-dioxolan-2-one.

[0086] As the solid electrolyte, inorganic solid electrolytes such as sulfide-based solid electrolytes and oxide-based solid electrolytes, and organic solid electrolytes such as polymer-based electrolytes can be suitably used. By using these solid electrolytes, an all-solid-state battery can be obtained that does not require an electrolytic solution.

[0087] Examples of sulfide-based solid electrolytes include thiolithium-based materials such as LiS-SiS-lithium compounds (wherein the lithium compound is at least one selected from the group consisting of LiPO, LiI, and LiSiO), LiS-P2S5, LiS-P2O5, LiS-B2S5, and LiS-P2S5-GeS2.

[0088] As the oxide-based solid electrolyte, Li5La3M2O, an oxide with a garnet structure, 12 (M = Nb, Ta) and Li7La3Zr2O 12 , γ-Li3PO4 structure-based oxygen acid salt compounds collectively called LISICON, perovskite type, Li 3.3 P.O.3.8 N 0.22 , sodium / alumina, etc.

[0089] Examples of polymer solid electrolytes include polyethylene oxide materials and polymer compounds obtained by polymerizing or copolymerizing monomers such as hexafluoropropylene, tetrafluoroethylene, trifluoroethylene, ethylene, propylene, acrylonitrile, vinylidene chloride, acrylic acid, methacrylic acid, methyl acrylate, ethyl acrylate, methyl methacrylate, styrene, and vinylidene fluoride.

[0090] The polymer solid electrolyte may contain a supporting salt and a plasticizer. Specific examples of the supporting salt include lithium (fluorosulfonylimide), and examples of the plasticizer include succinonitrile.

[0091] EXAMPLES 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 the following examples.

[0092] The devices used in the examples are as follows: Probe-type ultrasonic irradiation device: UIP1000hd, manufactured by Hielscher Ultrasonics

[0093] The raw materials used are as follows: Graphene oxide: EMD Millipore (now Merck) Zaixen L: Zaixen L (Sumitomo Seika Chemicals Co., Ltd., solid content concentration 24.5 mass %, poly(ethylene-acrylic acid) 2-hydroxyethyldimethylamine salt aqueous dispersion, particle size by Coal counter method less than 0.1 μm) WS-300: WS-300 (Nippon Shokubai Co., Ltd.), aqueous solution containing oxazoline polymer, Epocross (registered trademark) WS-300, weight average molecular weight: 1.2 × 10 5 , solid content concentration: 10.0 mass% Chemipearl S100: Chemipearl S100 (manufactured by Mitsui Chemicals, solid content concentration 27 mass% poly(ethylene-methacrylic acid) partial sodium salt aqueous dispersion, particle size by Cole counter method less than 0.1 μm)

[0094] (1) Preparation of Composition for Forming an Artificial Solid Electrolyte Interface Layer [Example 1-1] 7.00 g of graphene oxide aqueous dispersion paste (manufactured by Merck, solids concentration 1.0% by mass, solids content 0.070 g), 60.82 g of pure water, 1.89 g of lithium hydroxide aqueous solution (solids concentration 1.00% by mass, solids content 0.0189 g), and 0.286 g of ZAIKXEN L (manufactured by Sumitomo Seika Chemicals Co., Ltd., solids concentration 24.5% by mass poly(ethylene-acrylic acid) 2-hydroxyethyldimethylamine salt aqueous dispersion, particle size less than 0.1 μm by Cole counter method) (solids content 0.070 g) were sequentially added to a 100 mL polypropylene bottle and stirred to obtain a black mixture. This black mixture was irradiated with ultrasound at 500 W for 3 minutes using a probe-type ultrasonic irradiation device UIP1000hd (manufactured by Hielsher) while stirring with a magnetic stirrer under ice cooling. After irradiation, the container was removed, and the liquid was shaken to homogenize. This ultrasonic treatment was repeated three times to obtain a black uniform dispersion A-1 (solid content concentration: 0.2% by mass). The amount of lithium hydroxide aqueous solution added was calculated as the total of the graphene oxide neutralization amount of 20 phr (20% by mass of lithium hydroxide relative to the graphene oxide solid content) calculated in advance by neutralization titration of the graphene oxide, and the ZAIXXEN L neutralization amount of 7.0 phr (7.0% by mass of lithium hydroxide relative to the ZAIXXEN L solid content), which is 1 equivalent to the carboxyl groups contained in ZAIXXEN L. Unless otherwise specified, the amount of lithium hydroxide aqueous solution added was calculated in the same manner in the following examples and comparative examples.

[0095] Example 1-2 A black uniform dispersion A-2 (solid content concentration: 1.0 mass %) was obtained in the same manner as in Example 1-1, except that the amount of graphene oxide aqueous dispersion paste used was changed to 35.00 g (solid content: 0.35 g), the amount of pure water used was changed to 24.12 g, the amount of lithium hydroxide aqueous solution used was changed to 9.45 g (solid content: 0.0945 g), and the amount of ZAIXXEN L used was changed to 1.43 g (solid content: 0.35 g).

[0096] Comparative Example 1-1: 0.245 g of Zyxene L (solid content 0.060 g), 23.35 g of pure water, 0.420 g of an aqueous lithium hydroxide solution (solid content 0.0042 g), and 5.99 g of 2-propanol were sequentially added to a 100 mL polypropylene bottle and stirred to obtain a colorless, transparent solution B-1 (solid content concentration 0.2% by mass). 2-Propanol was added to improve the uniformity of the coating surface of the artificial solid electrolyte interface layer in Example 2, which will be described later.

[0097] Comparative Example 1-2: 1.22 g of ZAIKXEN L (solid content 0.30 g), 20.69 g of pure water, 2.10 g of an aqueous lithium hydroxide solution (solid content 0.021 g), and 5.99 g of 2-propanol were added in this order and stirred to obtain a colorless, transparent solution B-2 (solid content concentration 1.0 mass %). 2-Propanol was added to improve the uniformity of the coating surface of the artificial solid electrolyte interface layer in Example 2.

[0098] Comparative Example 1-3 A black uniform dispersion B-3 (solid content concentration 0.2% by mass) was obtained in the same manner as in Example 1-1, except that 0.700 g of Epocross WS-300 (solid content 0.070 g) was used instead of ZAIKXEN L and the lithium hydroxide aqueous solution was not used.

[0099] Comparative Example 1-4 A black uniform dispersion B-4 (solid content concentration 1.0 mass%) was obtained in the same manner as in Example 1-2, except that 3.50 g of Epocross WS-300 (solid content 0.35 g) was used instead of ZAIKXEN L and the lithium hydroxide aqueous solution was not used.

[0100] [Comparative Example 1-5] A black uniform dispersion B-5 (solid content concentration: 0.2% by mass) was obtained in the same manner as in Example 1-1, except that 0.259 g (solid content: 0.070 g) of Chemipearl S-100 was used instead of Zaixen L and the amount of lithium hydroxide solution used was changed to 1.40 g (solid content: 0.014 g). Note that the amount of lithium hydroxide added was only 20 phr (20% by mass relative to the graphene oxide solid content) of graphene oxide neutralization amount.

[0101] Comparative Example 1-6 Black uniform dispersion B-6 (solids concentration 1.0% by mass) was obtained in the same manner as in Example 1-2, except that 1.30 g (solids content 0.35 g) of Chemipearl S-100 (Mitsui Chemicals, Inc., solids content concentration 27% by mass; poly(ethylene-methacrylic acid) partial sodium salt aqueous dispersion, particle size by Cole counter method less than 0.1 μm) was used instead of ZAIKXEN L, and the amount of lithium hydroxide solution used was changed to 7.00 g (solids content 0.070 g). Note that the amount of lithium hydroxide added was only 20 phr (20% by mass relative to the graphene oxide solids content) to neutralize the graphene oxide.

[0102] (2) Formation of an artificial solid electrolyte interface layer [Example 2-1] Dispersion A-1 was spread on copper foil (Fukuda Metal Foil & Powder Co., Ltd., electrolytic copper foil, thickness 15 μm) using a wire bar coater (OSP-6, maximum wet film thickness 6 μm, coating speed 3 m / min). Then, using an oven, it was dried at 120 ° C. for 10 minutes to form an artificial solid electrolyte interface layer, and copper foil C-1 with an artificial solid electrolyte interface layer was obtained. The coating surface of C-1 was uniform when visually observed. The film thickness of the artificial solid electrolyte interface layer was estimated to be an average of 12 nm from the results of SEM observation.

[0103] [Example 2-2] A copper foil C-2 with an artificial solid electrolyte interface layer was obtained using the same method as in Example 2-1, except that dispersion A-1 was changed to dispersion A-2. The coating surface of C-2 was uniform when visually observed. The thickness of the artificial solid electrolyte interface layer was estimated to be 60 nm on average based on SEM observation.

[0104] [Comparative Example 2-1] Copper foil D-1 with an artificial solid electrolyte interface layer was obtained using the same method as in Example 2-1, except that dispersion A-1 was changed to B-1. The coating surface of D-1 was uniform when observed visually. The film thickness of the artificial solid electrolyte interface layer was estimated to be 12 nm on average based on SEM observation.

[0105] [Comparative Example 2-2] A copper foil D-2 with an artificial solid electrolyte interface layer was obtained in the same manner as in Example 2-1, except that dispersion A-1 was changed to dispersion B-2. Visual observation of the coating surface of D-2 confirmed significant unevenness across the entire surface, and it was not suitable for use as an artificial solid electrolyte interface layer, which requires a uniform coating surface without defects.

[0106] [Comparative Example 2-3] Copper foil D-3 with an artificial solid electrolyte interface layer was obtained using the same method as in Example 2-1, except that dispersion A-1 was changed to dispersion B-3. The coating surface of D-3 was uniform when observed visually. The film thickness of the artificial solid electrolyte interface layer was estimated to be an average of 12 nm based on SEM observation.

[0107] [Comparative Example 2-4] Copper foil D-4 with an artificial solid electrolyte interface layer was obtained using the same method as in Example 2-1, except that dispersion A-1 was changed to B-4. The coating surface of D-4 was uniform when observed visually. The film thickness of the artificial solid electrolyte interface layer was estimated to be an average of 60 nm based on SEM observation.

[0108] [Comparative Example 2-5] Copper foil D-5 with an artificial solid electrolyte interface layer was obtained using the same method as in Example 2-1, except that dispersion A-1 was changed to B-5. The coating surface of D-5 was uniform when observed visually. The film thickness of the artificial solid electrolyte interface layer was estimated to be 12 nm on average based on SEM observation.

[0109] [Comparative Example 2-6] Copper foil D-6 with an artificial solid electrolyte interface layer was obtained using the same method as in Example 2-1, except that dispersion A-1 was changed to B-6. The coating surface of D-6 was uniform when observed visually. The film thickness of the artificial solid electrolyte interface layer was estimated to be an average of 60 nm based on SEM observation.

[0110] The copper foils with artificial solid electrolyte interface layers produced in Examples 2-1 to 2-2 and Comparative Examples 2-1 to 2-6 were evaluated for coating uniformity and scratch resistance on the coating surface by the following methods.

[0111] <Coating area and coating uniformity of coating surface> The state of the coating was visually observed immediately after the composition for forming an artificial solid electrolyte interface layer (black dispersion) was spread on a metal foil with a wire bar coater and after drying. Coating uniformity was evaluated based on the state of the coating surface after drying according to the following criteria. The evaluation results are shown in Table 1. <Evaluation criteria for coating uniformity> A: The majority of the coating surface is uniform when visually observed. B: The coating surface has a uniform pattern of unevenness when visually observed. (C: The entire coating surface is clearly uneven when visually observed.) A rating of "A" in the above coating uniformity test was deemed to have passed.

[0112] <Scratch resistance (dry Bemcott peel test)> Contact area with artificial solid electrolyte interface layer: 1 cm 2 A nonwoven wiper (Bencott M-1, manufactured by Asahi Kasei Corporation) was applied at a pressure of 2 kg / cm 2 The wiped portion was moved once horizontally against the metal foil surface while applying a load of 1000 kJ / min. The state of the artificial solid electrolyte interface layer in the wiped area was visually inspected and evaluated according to the following criteria. The results are shown in Table 1. Evaluation Criteria A: No scratches were found on the artificial solid electrolyte interface layer. B: Linear scratches were found on the artificial solid electrolyte interface layer, exposing the metal foil surface. (C: Planar peeling occurred on the artificial solid electrolyte interface layer, exposing the metal foil surface.) In the scratch resistance test, a rating of "A" was considered to be a pass.

[0113] <Water resistance (water Bemcot peeling test)> Contact area with artificial solid electrolyte interface layer: 1 cm 2 A nonwoven wiper (Bencott M-1, manufactured by Asahi Kasei Corporation) moistened with water was applied at a pressure of 2 kg / cm. 2 The wiped portion was moved once horizontally against the metal foil surface while applying a load of 1000 kJ / min. The state of the artificial solid electrolyte interface layer in the wiped area was visually inspected and evaluated according to the following criteria. The results are shown in Table 1. Evaluation criteria: A: No scratches were found on the artificial solid electrolyte interface layer. B: Linear scratches were found on the artificial solid electrolyte interface layer, exposing the metal foil surface. (C: Planar peeling occurred on the artificial solid electrolyte interface layer, exposing the metal foil surface.) In the water resistance test, an "A" rating was considered to be a pass.

[0114]

[0115] The results in Table 1 confirm that the artificial solid electrolyte interface layer of the example has excellent coating uniformity, scratch resistance, and water resistance. Since the electrolyte used in secondary batteries generally contains electrolyte salt and is highly polar, it is expected that the artificial solid electrolyte interface layer, which has excellent water resistance, will also have excellent electrolyte resistance.

[0116] (3) Preparation of an anode-free lithium metal secondary battery [Example 3-1] The copper foil C-1 with the artificial solid electrolyte interface layer prepared in Example 2-1 was punched out to a diameter of 13 mm and dried at 120°C under vacuum for 3 hours to prepare an anode-free negative electrode. Separately, NCM523 (LiNi) prepared on aluminum foil was used. 0.5 Co 0.2 Mn 0.3 O2) / polyvinylidene fluoride (PVdF) / acetylene black (mass ratio 96:2:2, basis weight 25 mg / cm 2 A 10 mm diameter electrode (90 μm thick) was punched out and dried in a vacuum at 120°C for 3 hours to form a positive electrode. In addition to these negative and positive electrodes, two single-layer polypropylene separators (Celgard, #2400) punched out to a diameter of 15 mm were used as separators, and 1M LiPF4 / EC-DEC (1:1) was used as the electrolyte, and a CR2032 coin cell was fabricated according to a standard method. 5 μL of the electrolyte was dropped for each separator layer, for a total of 10 μL. The fabricated coin cell was charged and discharged under the following conditions. The results are shown in Table 2. Current: 0.307 mA (0.391 mA / cm2 for 10 mm diameter electrodes) 2 ) Voltage: Initial charge 0V to 4.2V, initial discharge 4.2V to 3.0V, subsequent charge / discharge 3.0V to 4.2V Number of charge / discharge cycles: 5

[0117] [Example 3-2-1] Coin cells were produced and charge / discharge tests were carried out in the same manner as in Example 3-1, except that C-2 was used instead of C-1, and the number of charge / discharge cycles was 10. The results are shown in Table 2.

[0118] [Example 3-2-2] Coin cells were fabricated and a charge-discharge test was carried out in the same manner as in Example 3-2-1, except that 1M LiPF4 / EC-EMC (3:7) was used as the electrolyte. The results are shown in Table 2.

[0119] [Example 3-2-3] Coin cells were fabricated and a charge-discharge test was carried out in the same manner as in Example 3-2-1, except that a 1M LiPF / EC-EMC (3:7) electrolyte solution was used, and a mixed solution containing 2% by mass of VC and 0.5% by mass of PS was used as an additive. The results are shown in Table 2.

[0120] [Comparative Example 3-1] Coin cells were produced and charge / discharge tests were carried out in the same manner as in Example 3-1, except that plain copper foil not coated with the dispersion was used instead of the copper foil C-1 with the artificial solid electrolyte interface layer. The results are shown in Table 2.

[0121] [Comparative Example 3-2] A coin cell was produced and a charge-discharge test was carried out in the same manner as in Example 3-1, except that D-1 was used instead of C-1, the copper foil with an artificial solid electrolyte interface layer. The results are shown in Table 2.

[0122] [Comparative Example 3-3] A coin cell was produced and a charge / discharge test was carried out in the same manner as in Example 3-1, except that D-3 was used instead of the copper foil with an artificial solid electrolyte interface layer C-1. The results are shown in Table 2.

[0123] [Comparative Example 3-4] A coin cell was produced and a charge-discharge test was carried out in the same manner as in Example 3-1, except that D-4 was used instead of the copper foil with an artificial solid electrolyte interface layer C-1. The results are shown in Table 2.

[0124] [Comparative Example 3-5] A coin cell was produced and a charge-discharge test was carried out in the same manner as in Example 3-1, except that D-5 was used instead of the copper foil with an artificial solid electrolyte interface layer C-1. The results are shown in Table 2.

[0125] [Comparative Example 3-6] A coin cell was produced and a charge-discharge test was carried out in the same manner as in Example 3-1, except that D-6 was used instead of the copper foil with an artificial solid electrolyte interface layer C-1. The results are shown in Table 2.

[0126]

[0127] When a copper foil with an artificial solid electrolyte interface layer was used in the method described in the examples, it was confirmed that the resulting anode-free lithium metal secondary battery was excellent in all of the coulombic efficiency, discharge capacity, and discharge capacity retention rate in charge-discharge tests.

Claims

1. A composition for forming an artificial solid electrolyte interface layer, comprising graphene oxide or a salt thereof, an olefin-unsaturated carboxylic acid copolymer or a salt thereof, and a solvent containing water.

2. The composition for forming an artificial solid electrolyte interface layer according to claim 1, wherein the salt of graphene oxide is a lithium salt.

3. The composition for forming an artificial solid electrolyte interface layer according to claim 1, wherein the salt of the olefin-unsaturated carboxylic acid copolymer is an amine salt, ammonium salt or lithium salt of an ethylene-(meth)acrylic acid copolymer.

4. A composition for forming an artificial solid electrolyte interface layer according to claim 1, wherein the graphene oxide contains a carboxyl group or a phenolic hydroxyl group as a surface functional group and the elemental ratio of oxygen is 20 to 60 atm %.

5. The composition for forming an artificial solid electrolyte interface layer according to claim 1, wherein the graphene oxide is a single-layer graphene oxide or a multi-layer graphene oxide having a thickness of 5 nm or less.

6. An artificial solid electrolyte interface layer obtained from the composition for forming an artificial solid electrolyte interface layer according to any one of claims 1 to 5.

7. The artificial solid electrolyte interface layer of claim 6, having a thickness of 1 μm or less.

8. A negative electrode for a secondary battery comprising the artificial solid electrolyte interface layer according to claim 6 on a substrate.

9. The negative electrode for a secondary battery according to claim 8, wherein the substrate is a copper foil.

10. The negative electrode for a secondary battery according to claim 8, which is a negative electrode for a lithium metal secondary battery.

11. The negative electrode for a secondary battery according to claim 10, which is a negative electrode for an anode-free battery.

12. A secondary battery comprising the negative electrode for secondary batteries according to claim 8.

13. The secondary battery according to claim 12, which is a lithium metal secondary battery.

14. The secondary battery according to claim 13, which is an anode-free battery.

Images