Electrode for lithium ion secondary battery and lithium ion secondary battery

Abstract

This electrode for lithium ion secondary batteries is provided with a collector and an electrode active material-containing layer that is provided on the collector. The electrode active material-containing layer comprises an electrode active material and a coating layer that is provided on the surface of the electrode active material; and the coating layer contains an aminosilane compound that has an amino group, at least one ethyleneimine group or a dialkylamino group at an end of the molecule.

Description

[Technical Field]

[0001] The present invention relates to an electrode for a lithium ion secondary battery and a lithium ion secondary battery. This application claims priority based on Japanese Patent Application No. 2020-041660, filed on March 11, 2020, the contents of which are incorporated herein by reference. [Background technology]

[0002] In recent years, non-aqueous electrolyte secondary batteries such as lithium ion batteries have been proposed and put into practical use as batteries that are expected to be smaller, lighter, and have higher capacities. These lithium ion batteries are composed of a positive electrode and a negative electrode that can reversibly insert and remove lithium ions, and a non-aqueous electrolyte. Lithium composite oxides are used as the positive electrode active material in lithium secondary batteries. Lithium secondary batteries have already been put to practical use as small power sources for mobile phones and laptops. Furthermore, attempts are being made to apply them to medium- and large-sized power sources for automobiles and power storage. As the range of applications expands, extending the life of lithium secondary batteries is becoming an important issue.

[0003] The lithium composite oxide used for the positive electrode active material is, for example, an LNMO type composite oxide containing lithium, nickel, manganese, and oxygen. LNMO-type composite oxides can be used at high potentials and are highly safe, so they are increasingly being applied to large-scale batteries, and attempts are being made to increase their capacity. For example, it is described that in order to improve the capacity of a battery using an LNMO-type lithium composite oxide, the content of additives was reduced to produce a dense lithium composite oxide film with few voids (Patent Document 1).

[0004] Although LNMO-type composite oxides have the advantage of being usable at high potentials, they have the problem that metal elements dissolve into the electrolyte during high-potential operation, causing battery degradation. To address this issue, an electrode has been proposed that includes a coating layer of a water-repellent material on the surface of the electrode active material (Patent Document 2). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-35909 [Patent Document 2] Japanese Patent Application Laid-Open No. 2017-174692 Summary of the Invention [Problem to be solved by the invention]

[0006] However, the above-mentioned conventional techniques cannot be said to be sufficient in suppressing battery deterioration, and there is room for further investigation and improvement of the structure of the coating layer formed on the surface of the electrode active material.

[0007] An object of the present invention is to provide an electrode for a lithium ion secondary battery, a lithium ion secondary battery, and a method for manufacturing a positive electrode for a lithium ion secondary battery, which are capable of further suppressing battery degradation and improving battery characteristics more than ever before. [Means for solving the problem]

[0008] As a result of extensive research, the inventors have found that molecules with various functions can be selected as the coating layer to be formed on the surface of an electrode active material, and that the effectiveness of the coating layer against the elution of the metal elements and the oxidative decomposition of the electrolyte is determined by the function of the specific molecule. In particular, they have found that the use of a coating layer suitable for the surface of a positive electrode active material composed of lithium cobalt oxide (LCO), nickel-cobalt-lithium manganese oxide (NCM), or nickel-cobalt-lithium aluminum oxide (NCA) can further suppress the degradation of lithium-ion secondary batteries and improve their battery performance.

[0009] That is, the gist of the present invention is as follows. [1] A current collector and an electrode active material-containing layer provided on the current collector, the electrode active material-containing layer has an electrode active material and a coating layer provided on a surface of the electrode active material, The electrode for a lithium ion secondary battery, wherein the coating layer contains an aminosilane compound having an amino group, at least one ethyleneimine group, or a dialkylamino group at a molecular terminal.

[0010] [2] The electrode for a lithium ion secondary battery according to the above [1], wherein the aminosilane compound has an amino group and at least one ethyleneimine group at a molecular terminal.

[0011] [3] The electrode for a lithium ion secondary battery according to [2] above, wherein the aminosilane compound is N-(3-trimethoxysilylpropyl)diethylenetriamine.

[0012] [4] The electrode for a lithium ion secondary battery according to the above [1], wherein the coating layer is composed of a self-assembled monolayer.

[0013] [5] The electrode for a lithium ion secondary battery according to any one of the above [1] to [4], wherein a coverage rate, which indicates the proportion of the surface of the electrode active material that is covered with the coating layer, is 80% or more.

[0014] [6] The electrode active material has first electrode active material particles and second electrode active material particles having a particle size larger than that of the first electrode active material particles, the coating layer has a first coating portion provided on the surface of the first electrode active material particles and a second coating portion provided on the surface of the second electrode active material particles, the first coating portion contains an aminosilane compound having an amino group at a molecular terminal, The electrode for a lithium ion secondary battery according to [1] above, wherein the second coating portion contains a compound having a functional group selected from the group consisting of carboxylic acid, carboxylic acid salt, carboxylic acid anhydride, succinic acid anhydride, and acylisourea.

[0015] [7] The electrode active material has first electrode active material particles and second electrode active material particles having a particle size larger than that of the first electrode active material particles, the coating layer has a first coating portion provided on the surface of the first electrode active material particles and a second coating portion provided on the surface of the second electrode active material particles, the first coating portion contains an aminosilane compound having an amino group at a molecular terminal, The electrode for a lithium ion secondary battery according to [1] above, wherein the second coating portion contains an imide-based compound.

[0016] [8] The electrode for a lithium ion secondary battery according to [1] above, wherein the electrode active material contains a lithium composite oxide having a layered rock salt structure.

[0017] [9] The lithium composite oxide having a layered rock salt structure is LiNiO2, LiCoO2, LiNi k Co l Mn m O2 (k+l+m=1) and LiNi k Co l Al m O2(k+l+m=1).

[0018]

[10] The electrode for a lithium ion secondary battery according to the above [9], wherein the lithium composite oxide having a layered rock salt structure is any one of lithium cobalt oxide (LCO), nickel-cobalt-lithium manganese oxide (NCM), and nickel-cobalt-lithium aluminum oxide (NCA).

[0019]

[11] A lithium ion secondary battery comprising the electrode for a lithium ion secondary battery according to any one of [1] to

[10] above and an electrolyte. [Effects of the Invention]

[0020] According to the present invention, it is possible to provide an electrode for a lithium ion secondary battery and a lithium ion secondary battery that can further suppress battery degradation and improve battery characteristics compared to conventional ones. [Brief explanation of the drawings]

[0021] [Figure 1] FIG. 1 is a diagram showing an example of the configuration of a lithium ion secondary battery according to an embodiment of the present invention. [Figure 2] FIG. 2 is a graph showing the cycle characteristics of the lithium ion secondary batteries in the examples and comparative examples. [Figure 3] FIG. 3 is a diagram showing the relationship between the heating time during coating layer formation and cycle characteristics in the examples. [Figure 4] FIG. 4 is a diagram showing the evaluation of the characteristics of the positive electrode by a floating test in the examples. [Figure 5] FIG. 5(a) is an electron microscope image of the electrode active material in the example, and FIG. 5(b) is an electron microscope image of the electrode active material in the comparative example. DETAILED DESCRIPTION OF THE INVENTION

[0022] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[0023] [Electrode structure for lithium-ion secondary batteries] The electrode for a lithium ion secondary battery according to this embodiment includes a current collector and an electrode active material-containing layer provided on the current collector.

[0024] <Current collector> The current collector is made of, for example, a metal foil. Metal foils are suitable for use in batteries of various shapes, such as cylindrical, prismatic, and laminated types. To further enhance the adhesion between the electrode active material and the current collector, carbon may be vapor-deposited on the surface of the current collector.

[0025] The current collector on the positive electrode side of the electrode can be, for example, aluminum foil. It is preferable that the current collector be hydrophilized by surface treatment. Hydrophilizing the current collector surface facilitates the formation of hydrogen bonds when the electrode-forming slurry dries, resulting in an electrode with high adhesive strength. Examples of hydrophilization treatments for the current collector surface include a method of irradiating ultraviolet (UV) rays in an ozone (O) atmosphere (UV / O treatment).

[0026] <Electrode active material containing layer> The electrode active material-containing layer contains an electrode active material and a coating layer provided on the surface of the electrode active material.

[0027] (electrode active material) The electrode active material contains a lithium composite oxide with a layered rock-salt structure. "Layered" here means a thin sheet-like shape. The "rock-salt structure" refers to a sodium chloride structure, a type of crystalline structure, in which the face-centered cubic lattices formed by cations and anions are offset from each other by half the edge of the unit lattice.

[0028] Examples of lithium composite oxides having a layered rock salt structure include LiNiO2, LiCoO2, and LiNi k Co l Mn m O2 (k+l+m=1) and LiNi k Co l Al m O2(k+l+m=1) can be mentioned.

[0029] Specifically, the lithium composite oxide is preferably any one of lithium cobalt oxide (hereinafter also referred to as "LCO"), lithium nickel-cobalt-manganese oxide (hereinafter also referred to as "NCM"), and lithium nickel-cobalt-aluminate (hereinafter also referred to as "NCA"). Furthermore, lithium nickel-cobalt-manganese oxide (NCM) or lithium nickel-cobalt-aluminate (NCA) is more preferred.

[0030] In this way, by using the lithium salt of the above-mentioned ternary transition metal oxide having a layered rock salt structure as the electrode active material, a lithium ion secondary battery excellent in energy density and thermal stability can be obtained. Furthermore, particles of lithium salts of ternary transition metal oxides such as NCM and NCA have smaller particle sizes and larger specific surface areas (approximately 10 times) than particles such as LCO. This allows for a larger contact area between the active material particles and the electrolyte. As a result, the coating layer significantly suppresses the reaction between the electrode active material and the electrolyte, improving the conductivity of lithium ions between the active material particles and the electrolyte compared to when this configuration is not used, thereby increasing the power output of lithium-ion secondary batteries.

[0031] Furthermore, by using a lithium salt of the above-mentioned ternary transition metal oxide having a layered rock salt structure as the electrode active material, the electrode active material contains Ni as a constituent element. In this case, the capacity density of the lithium-ion secondary battery increases, and there is a tendency for the elution of metal elements in the charged state to be reduced. As a result, compared to when this configuration is not adopted, the long-term reliability of the lithium-ion secondary battery in the charged state and the cycle characteristics of the lithium-ion secondary battery can be improved.

[0032] The electrode active material may have first electrode active material particles and second electrode active material particles having a particle size larger than that of the first electrode active material particles. The first electrode active material particles are, for example, primary particles. The second electrode active material particles may be primary particles or secondary particles. This allows the small-sized first electrode active material particles to enter the gaps between the large-sized second electrode active material particles, resulting in densification, and thereby increasing the electrode density and energy density. Furthermore, by using first electrode active material particles having a particle size smaller than that of the second electrode active material particles, destruction of the electrode active material due to volume expansion and contraction can be alleviated.

[0033] The particle size of the first electrode active material particles is preferably 0.1 μm to 4 μm, more preferably 0.7 μm to 2 μm, and the particle size of the second electrode active material particles is preferably 5 μm to 20 μm, more preferably 6 μm to 15 μm. The material constituting the first electrode active material particles may be the same as or different from the material constituting the second electrode active material particles. For example, if the material constituting the first electrode active material particles is NCM, the material constituting the second electrode active material particles may be NCM or NCA. Furthermore, the shape of the first electrode active material particles may be the same as or different from the shape of the second electrode active material particles.

[0034] (covering layer) The coating layer contains an aminosilane compound having an amino group, at least one ethyleneimine group, or a dialkylamino group at the molecular terminal. This coating layer is preferably composed of a self-assembled monolayer (SAM). A self-assembled monolayer is a monolayer formed by forming chemical bonds with an electrode active material using organic molecules having functional groups at their terminals that form specific chemical bonds with the electrode active material. The anchored organic molecules are orderedly arranged due to restrictions from the surface of the electrode active material and interactions between the organic molecules.

[0035] Thus, by providing the coating layer on the surface of the electrode active material, the coating can suppress the elution of metal elements into the electrolyte, even when operated at high voltages, thereby suppressing the decrease in capacity due to the elution of metal elements from the electrode active material into the electrolyte. While molecular movement is somewhat restricted on the surface of the electrode active material, if the coating layer contains an aminosilane compound having amino groups at the molecular end, the amino groups, which can capture lithium ions per molecule, act effectively, and lithium ions or solvated lithium ions weakly coordinate with the amino groups near the surface of the electrode active material, thereby reducing the activation energy required to desolvate lithium ions solvated in the electrolyte and achieving highly efficient lithium ion transport at the interface between the electrode active material and the electrolyte. Furthermore, highly crystalline monomolecular films, such as monomolecular films composed of linear molecules such as alkyl or fluoroalkyl groups, inevitably form defects upon crystallization. However, the inclusion of ethyleneimine groups or dialkylamino groups in the molecular chains prevents self-organization between adjacent molecular chains, resulting in the amorphization of the monomolecular film, resulting in the formation of a dense monomolecular film without the formation of defects. This further limits the area of ​​direct contact between the electrode surface and the electrolyte, thereby more effectively preventing deterioration of battery performance due to decomposition of the electrolyte compared to when a crystalline monolayer is used. In other words, by forming a monolayer of the aminosilane compound having at least one ethyleneimine group or a dialkylamino group on the surface of the electrode active material, it is possible to further suppress oxidative decomposition of the electrolyte and also to achieve highly efficient lithium ion transport at the interface between the electrode active material and the electrolyte.

[0036] Furthermore, the aminosilane compound readily forms imide bonds even at room temperature through acid-base reactions with carboxylic acids and their salts, and nucleophilic reactions with carboxylic anhydrides, succinic anhydride, and acylisourea. Therefore, the amino groups at the molecular terminals of the monolayer coated on the active material surface can easily immobilize compounds having the functional groups. Therefore, by coating the active material surface with the aminosilane compound, compounds having functional groups such as carboxylic acids and their salts, carboxylic anhydrides, succinic anhydride, and acylisourea can be immobilized on the active material surface. Examples of immobilized compounds include, but are not limited to, second electrode active material particles, solid electrolyte particles, ferroelectric particles, polymer binders such as sodium carboxymethylcellulose, styrene-butadiene copolymer, polyacrylic acid, and polyvinylidene fluoride, and conductive additives made of carbon materials such as acetylene black, carbon nanotubes, graphene, and heteronanocarbons.

[0037] The aminosilane compound having an amino group at the molecular end is not particularly limited, but an example thereof is (3-aminopropyl)triethoxysilane (hereinafter also referred to as "APTES"), as shown below.

[0038] [ka]

[0039] The aminosilane compound preferably has an amino group and at least one ethyleneimine group at the molecular terminal. Examples of such aminosilane compounds include, but are not limited to, N-(6-aminohexyl)aminomethyltriethoxysilane (hereinafter also referred to as "AHAMTES") and N-(3-trimethoxysilylpropyl)diethylenetriamine (hereinafter also referred to as "DAEAPTS"), as shown below.

[0040] [ka]

[0041] [ka]

[0042] The aminosilane compound having a dialkylamino group is not particularly limited, but an example thereof is N,N-diethylaminopropyltrimethoxysilane (hereinafter also referred to as "SID"), as shown below.

[0043] [ka]

[0044] In this embodiment, the reaction related to the intercalation and deintercalation of lithium ions is uniformly carried out over the entire surface of the electrode active material, and for this purpose, it is preferable that 80% or more, more preferably 85% or more, and even more preferably 90% or more of the surface of the electrode active material is covered with a coating layer. The coverage of the coating layer can be measured using a transmission electron microscope (TEM), an energy dispersive X-ray spectrometer (EDX), or X-ray photoelectron spectroscopy (XPS). Specifically, the coating layer formed on the electrode active material is observed for 100 electrode active materials using a transmission electron microscope (TEM) and an energy dispersive X-ray spectrometer (EDX), and the proportion of the surface of the electrode active material that is covered with the coating layer is calculated, which is taken as the coverage rate. In addition, X-ray photoelectron spectroscopy (XPS) may be used to measure the surface atomic concentration to calculate the proportion of the surface of the electrode active material that is covered with the coating layer, to determine the coverage, and the consistency with the results calculated by EDX may be considered.

[0045] In this embodiment, the average thickness of the coating layer is preferably 0.5 nm to 0.3 μm, more preferably 0.5 nm to 0.1 μm, even more preferably 0.5 nm to 10 nm, particularly preferably 0.8 nm to 6.0 nm, and most preferably 1.0 nm to 3.0 nm. The average thickness can be calculated based on the spectral data obtained by measuring the angle-resolved core-level spectrum of the coating layer on the surface of the electrode active material using X-ray photoelectron spectroscopy (XPS). Alternatively, the average thickness can be measured using wavelength-angle resolved ellipsometry or atomic force microscopy.

[0046] Furthermore, when the electrode active material comprises first electrode active material particles and second electrode active material particles having a particle size different from that of the first electrode active material particles, the coating layer may comprise a first coating portion provided on the surface of the first electrode active material particles and a second coating portion provided on the surface of the second electrode active material particles. In this case, the mass ratio of the first electrode active material particles having the first coating portion formed thereon to the second electrode active material particles having the second coating portion formed thereon is preferably 15-0.5:85-99.5, more preferably 10-1:90-99.

[0047] The average thickness of the first coating portion may be the same as or different from the average thickness of the second coating portion. The average thickness of the first coating portion is preferably 0.5 nm to 10 nm, more preferably 0.5 nm to 10 nm. The average thickness of the second coating portion is preferably 0.5 nm to 0.3 μm, more preferably 2 nm to 0.1 μm.

[0048] The material constituting the first coating portion may be the same as or different from the material constituting the second coating portion. When the material constituting the first coating portion is different from the material constituting the second coating portion, for example, the first coating portion preferably contains an aminosilane compound having an amino group at the molecular terminal, and the second coating portion preferably contains a compound having a functional group selected from the group consisting of carboxylic acid, carboxylic acid salt, carboxylic acid anhydride, succinic acid anhydride, and acylisourea. Examples of succinic acid anhydride include 3-triethoxysilylpropylsuccinic anhydride (hereinafter also referred to as "TPSA"), as shown below. By using such a material for the second coating portion, the compound having a functional group constituting the second coating portion is easily immobilized to the amino group at the molecular terminal of the aminosilane compound constituting the first coating portion. As a result, the first electrode active material particles can be selectively accumulated in the gaps between the second electrode active material particles, further increasing the energy density.

[0049] [ka]

[0050] Furthermore, when the material constituting the first coating portion is different from the material constituting the second coating portion, it is preferable that the first coating portion contains, for example, an aminosilane compound having an amino group at the molecular terminal, and the second coating portion is an imide-based compound. Examples of imide-based compounds include polyimide (PI), polyetherimide (PEI), and polyamideimide (PAI). By using such a material for the second coating portion, the compound having a functional group constituting the second coating portion is easily immobilized by the amino group at the molecular terminal of the aminosilane compound constituting the first coating portion. As a result, the first electrode active material particles can be selectively accumulated in the gaps between the second electrode active material particles. The weight increase due to the second coating portion minimizes loss of energy density, while suppressing the decomposition reaction of the electrolyte, thereby maintaining a higher capacity retention rate with charge / discharge cycles.

[0051] [Method for forming coating layer] The coating layer can be formed by surface-treating the electrode active material with a silane coupling agent containing the aminosilane compound. By surface-treating the electrode active material, a coating layer is formed on the surfaces of the primary particles and / or secondary particles of the lithium composite oxide. The timing of forming the coating layer on the surface of the electrode active material is not particularly limited. However, from the viewpoint of forming a uniform coating layer on the surface of the electrode active material, it is preferable to form the coating layer on the surface of the electrode active material before manufacturing the electrode. However, without being limited thereto, the coating layer may be formed on the surface of the mixture after mixing the electrode active material with other composite materials, or the electrode may be manufactured in advance and the coating layer may be formed on the surface of the composite electrode. As a result, a coating layer containing the aminosilane compound is formed on the surface of the electrode active material, and a lithium-ion secondary battery electrode having an electrode active material-containing layer is obtained.

[0052] The method for forming the coating layer is not particularly limited, and examples thereof include a coating method, a vapor phase method, and a liquid phase method. In this embodiment, the liquid phase method or the vapor phase method is preferred, and the liquid phase method is more preferred. In the liquid phase method, when a high-nickel electrode active material such as NCM or NCA with a high Ni content is used, a polar solvent such as NMP (N-methylpyrrolidinone) is used, and when an electrode active material with a relatively low Ni content is used, a solvent such as ethanol or isopropanol is used. Examples of the vapor phase method include vacuum deposition, sputtering, chemical vapor deposition (CVD), and plasma-enhanced chemical vapor deposition.

[0053] [Configuration of lithium-ion secondary battery] The lithium ion secondary battery according to this embodiment includes the above-described lithium ion secondary battery electrode and an electrolyte. This secondary battery can have the same configuration as a conventional or well-known secondary battery, except for having the above-described electrode.

[0054] FIG. 1 is a diagram showing an example of the configuration of a lithium ion secondary battery according to this embodiment. As shown in Fig. 1, the lithium ion secondary battery 1 is a coin-type secondary battery and includes a positive electrode 2, a negative electrode 3, and an electrolyte 4. The positive electrode 2 includes a current collector 21 and an electrode active material-containing layer 22 provided on the current collector 21. The negative electrode 3 includes a current collector 31 and an electrode active material-containing layer 32 provided on the current collector 31. The electrolyte 4 is, for example, an electrolytic solution. The lithium ion secondary battery 1 can also include a separator 5 provided between the positive electrode 2 and the negative electrode 3, a stainless steel positive electrode case 6 and a negative electrode case 7 which cooperate with each other to house the positive electrode 2, the negative electrode 3, and the electrolyte 4 therein, and a polypropylene gasket 8 which is disposed between the positive electrode case 6 and the negative electrode case 7 and on the outer periphery thereof.

[0055] (positive electrode) The electrode active material-containing layer 22 of the positive electrode 2 contains the electrode active material and coating layer described above according to this embodiment. The positive electrode 2 is not particularly limited except that it has the electrode active material-containing layer 22. The positive electrode 2 can be produced, for example, by preparing a positive electrode mixture containing the lithium composite oxide, a conductive material, and a binder.

[0056] (Conductive material) Carbon materials can be used as the conductive material in the positive electrode. Examples of carbon materials include graphite powder, carbon black (e.g., acetylene black), and fibrous carbon materials. Because carbon black is a fine particle with a large surface area, adding a small amount to the positive electrode mixture can increase the conductivity within the positive electrode, improving charge / discharge efficiency and output characteristics. However, adding too much carbon black reduces the binding strength between the binder and the positive electrode current collector, as well as the binding strength within the positive electrode mixture, which can actually increase internal resistance.

[0057] (binder) The binder in the positive electrode can be a thermoplastic resin, such as fluororesins such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene-vinylidene fluoride copolymers, hexafluoropropylene-vinylidene fluoride copolymers, and tetrafluoroethylene-perfluorovinyl ether copolymers; or polyolefin resins such as polyethylene and polypropylene.

[0058] When the positive electrode mixture is made into a paste, examples of the organic solvent that can be used include amine-based solvents such as N,N-dimethylaminopropylamine and diethylenetriamine; ether-based solvents such as tetrahydrofuran; ketone-based solvents such as methyl ethyl ketone; ester-based solvents such as methyl acetate; and amide-based solvents such as dimethylacetamide and N-methyl-2-pyrrolidone (hereinafter sometimes referred to as NMP).

[0059] (Negative electrode) The electrode active material-containing layer 32 of the negative electrode 3 contains at least an electrode active material. As the electrode active material of the negative electrode, a compound capable of absorbing and desorbing lithium ions can be used alone or in combination. Examples of the compound capable of absorbing and desorbing lithium ions include metal materials such as lithium, alloy materials containing titanium, silicon, tin, etc., graphite, coke, a fired organic polymer compound, and carbon materials such as amorphous carbon. These electrode active materials can be used alone or in combination. Titanium-containing oxides (e.g., titanium oxide with a bronze structure, TiO2 (B)), lithium titanate, Li4Ti5O4, etc., are preferred as the electrode active material for the negative electrode 3. 12 ), silicon oxide, natural graphite, artificial graphite, hard carbon, soft carbon, silicon and alloys containing silicon (e.g., Si 80 Ti 20 ) or tin is preferably used. For example, when lithium foil is used as the negative electrode active material, the negative electrode can be formed by pressing the lithium foil onto the surface of a current collector made of a metal such as copper.

[0060] When an alloy material or a carbon material is used as the negative electrode active material, the negative electrode active material can be formed by mixing the negative electrode active material with a binder, a conductive additive, etc. in a solvent such as water or N-methylpyrrolidone, and then applying the mixture to a current collector made of a metal such as copper. The binder is preferably made of a polymer material, and is preferably a material that is chemically and physically stable in the atmosphere inside the lithium secondary battery.

[0061] Examples of binders include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), ethylene-propylene-diene copolymer (EPDM), styrene-butadiene rubber (SBR), acrylonitrile-butadiene rubber (NBR), and fluororubber. Examples of the conductive additive include ketjen black, acetylene black, carbon black, graphite, carbon nanotubes, amorphous carbon, etc. Examples of conductive polymers include polyaniline, polypyrrole, polythiophene, polyacetylene, and polyacene.

[0062] Furthermore, the electrode active material-containing layer 32 of the negative electrode 3 may contain an electrode active material and a coating layer provided on the surface of the electrode active material. In this case, the coating layer of the electrode active material-containing layer 32 may have a structure similar to that of the coating layer of the electrode active material-containing layer 22 of the positive electrode 2. This can prevent deterioration of battery performance due to decomposition of the electrolyte solution and also enable highly efficient lithium ion transport at the interface between the electrode active material and the electrolyte solution.

[0063] (electrolyte) The electrolyte is a medium that transports charge carriers such as ions between the positive electrode and the negative electrode, and is not particularly limited, but is preferably one that is physically, chemically, and electrically stable in the atmosphere in which the lithium ion secondary battery is used.

[0064] For example, the electrolyte is preferably one in which one or more supporting electrolytes selected from LiBF4, LiPF6, LiCF3SO3, LiN(CF3SO2)2, LiN(C2F5SO2)2, and LiN(CF3SO2)(C4F9SO2) are dissolved in an organic solvent.

[0065] Examples of organic solvents include propylene carbonate, ethylene carbonate, 1,2-dimethoxyethane, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, tetrahydrofuran, 2-methyltetrahydrofuran, tetrahydropyran, and mixtures thereof. Among these, electrolytes containing carbonate-based solvents are preferred due to their high stability at high temperatures. Solid polymer electrolytes containing the above electrolytes in solid polymers such as polyethylene oxide, and solid electrolytes such as ceramics and glass with lithium ion conductivity can also be used.

[0066] It is desirable to interpose a separator, which is a member that combines electrical insulation and ion conductivity, between the positive electrode and the negative electrode. When the electrolyte is liquid, the separator also plays a role in retaining the liquid electrolyte. Examples of separators include porous synthetic resin membranes, particularly porous membranes made of polyolefin polymers (polyethylene, polypropylene) or glass fibers, and nonwoven fabrics. Furthermore, it is preferable to adopt a separator that is larger than the positive electrode and the negative electrode in order to ensure insulation between the positive electrode and the negative electrode.

[0067] The positive electrode, negative electrode, electrolyte, separator, and the like are generally housed in a case consisting of the above-mentioned positive electrode case 6 and negative electrode case 7. The case is not particularly limited and can be made of known materials and in known shapes. That is, the lithium secondary battery of the present invention is not particularly limited in shape and can be used as batteries of various shapes, such as coin-shaped, cylindrical, and prismatic. The case of the lithium secondary battery of this embodiment is also not limited and can be used as batteries of various shapes, such as metal or resin cases that can maintain the battery's outer shape, and soft cases such as laminate packs. [Example]

[0068] Examples of the present invention will be described below, but the present invention is not limited to the following examples.

[0069] Example 1 <Manufacturing positive electrodes for lithium-ion secondary batteries> NCM523 (Ni: 50% by mass, Co: 20% by mass, Mn: 30% by mass) and DB (conductive additive) were weighed and mixed in a mortar, then placed in a dedicated container and kneaded for 2 minutes with a mixer (Thinky Corporation, product name "Awatori Rentaro"). After confirming that the mixture was sufficiently mixed, a binder (PVDF / NMP: 10% by mass) was weighed and added dropwise, and the mixture was kneaded for 2 minutes with the mixer. Next, 100 μL of NMP was added dropwise, and the mixture was kneaded for 2 minutes with the mixer, and then further kneaded for 2 minutes with the mixer, followed by degassing for 30 seconds.

[0070] The obtained material was applied to aluminum foil and dried at atmospheric pressure at 100°C. It was then punched out to a diameter of 14 mm using a punching machine and vacuum dried at 120°C for 10 hours or more. After vacuum drying, the material was press-molded at 50 kN for 1 minute to obtain a positive electrode material. The mass ratio of the electrode active material:conductive additive:binder in the positive electrode material was 90:5:5, and the tap density was 2.68 g / cm. 3 ~2.83g / cm 3 It was.

[0071] Next, in a dry room, the screw tube and the above electrode material were placed in a sealed SUS container, 50 μL of APTES (manufactured by Tokyo Chemical Industry Co., Ltd.) was added as a surface treatment agent to the screw tube and sealed, the pressure inside the sealed container was set to 10,325 Pa, and the sealed container was heated at 120°C for 3 to 15 hours. As a result, a positive electrode for a lithium ion secondary battery in which a coating layer was formed on the surface of the electrode active material was obtained.

[0072] <Coin cell fabrication> A coin cell was fabricated by stacking a positive electrode can (positive electrode case), the positive electrode for a lithium-ion secondary battery obtained above, a separator, a gasket, a spacer, a spring, and a negative electrode can (negative electrode case) in this order, and filling the inside with an electrolyte (1M LiPF6EC / DMC (1:2)). Lithium foil was used as the negative electrode.

[0073] Example 2 A coin cell was produced in the same manner as in Example 1, except that AHAMTES (manufactured by Gelest) was applied to an aluminum foil instead of APTES to form a coating layer during the production of the positive electrode.

[0074] Example 3 A coin cell was produced in the same manner as in Example 1, except that DAEAPTS (manufactured by Sigma-Aldrich Co.) was applied to an aluminum foil instead of APTES to form a coating layer during the production of the positive electrode.

[0075] (Comparative Example 1) A coin cell was produced in the same manner as in Example 1, except that, in the production of the positive electrode, FAS13 (manufactured by Shin-Etsu Chemical Co., Ltd.) was applied to an aluminum foil to form a coating layer instead of APTES. FAS13: 1H,1H,2H,2H-Perfluorooctyltrimethoxysilane

[0076] (Comparative Example 2) A coin cell was produced in the same manner as in Example 1, except that, in the production of the positive electrode, FAS17 (manufactured by Gslest) was applied to an aluminum foil instead of APTES to form a coating layer. FAS17: Heptadecafluoro-1,1,2,2-tetrahydrodecyltrimethoxysilane

[0077] [Evaluation of positive electrode cycle characteristics] The coin cells prepared in Examples 1 to 4 and Comparative Examples 1 and 2 were set in a charge-discharge device (Hokuto Denko Corporation, product name "HJ1001SD8"), and Li insertion and desorption into and from the positive electrode were performed by charge and discharge to evaluate the cycle characteristics of the positive electrode. Charge and discharge were performed at a cutoff voltage of 2.8 V to 4.2 V, a current density of 0.5 C, a temperature of 50°C, and in CCCV-CC mode. This charge-discharge cycle was repeated 100 times, and the discharge capacity and capacity retention rate were measured. The results are shown in Table 1 and Figure 2.

[0078] [Table 1]

[0079] From the results in Table 1 and Figure 2, it was found that in all of Examples 1 to 3, the coating layer contained an aminosilane compound having an amino group at the molecular terminal, and the decrease in discharge capacity from the initial discharge capacity was small compared to Comparative Examples 1 and 2, and deterioration could be suppressed. Furthermore, when comparing Examples 1 to 3, it was found that in Examples 2 and 3, the aminosilane compound had an amino group at the molecular terminal and one or two ethyleneimine groups, and the decrease in discharge capacity was even smaller than in Example 1, and deterioration could be further suppressed. In particular, it was found that in Example 3, the aminosilane compound had two ethyleneimine groups, and the decrease in discharge capacity was the smallest, and deterioration could be suppressed the most.

[0080] On the other hand, in Comparative Examples 1 and 2, the discharge capacity decreased significantly from the initial discharge capacity, and the suppression of deterioration was insufficient.

[0081] In addition, positive electrodes for lithium ion secondary batteries in which a coating layer was formed on the surface of the electrode active material were obtained in the same manner as in Example 3, except that the heating time during coating layer formation was changed to 1 hour, 3 hours, 6 hours, 9 hours, and 15 hours. Then, the cycle characteristics of each electrode were evaluated in the same manner as above. The results are shown in Figure 3.

[0082] As shown in FIG. 3, it was found that as the heating time in the closed container increased, a coating layer was more effectively formed on the surface of the electrode active material, further improving the cycle characteristics.

[0083] [Evaluation of cathode characteristics through floating test] Coin cells were fabricated in the same manner as described above using the lithium-ion secondary battery positive electrodes obtained by heating the coating layer for 1 hour, 3 hours, and 15 hours. A floating test was then conducted on these coin cells. In the floating test, the closed-circuit voltage (HJ1001SD8, manufactured by Hokuto Denko Corporation) of each coin cell in a fully charged state was measured in a thermostatic chamber (SH222, manufactured by Espec Corporation) set at 50°C.

[0084] As shown in Figure 4, when the heating time during coating layer formation was 3 hours or 15 hours, the coverage of the coating layer was 80% and 99%, respectively, and it was found that the oxidative decomposition of the electrolyte was suppressed and the drop in closed-circuit voltage was sufficiently suppressed.

[0085] On the other hand, when no coating layer was formed on the electrode active material, the voltage dropped over time, and it was found that the drop in closed-circuit voltage could not be suppressed. Furthermore, when the heating time for forming the coating layer was set to one hour, the coating layer was barely formed, and the voltage dropped over time, making it impossible to suppress the drop in closed-circuit voltage. This was because the current (self-discharge) in the closed circuit increased with the increase in oxidative decomposition of the electrolyte, resulting in a drop in voltage over time.

[0086] Example 4 In a dry room, 5 g of primary particles of NCM111 (Ni: 34% by mass, Co: 33% by mass, Mn: 33% by mass) with an average particle size of 1 μm were weighed and added to a plastic container with 60 mL of ethanol. 200 μL of DAEAPTS (Sigma-Aldrich) as a surface treatment agent was then added, and the container was stirred at room temperature for 12 hours. After stirring, the mixture was suction filtered using a membrane filter with a pore size of 1 μm and then vacuum dried at 45°C. This yielded Material A, in which a coating layer was formed on the surface of the electrode active material (NCM111). In a dry room, 5 g of secondary particles of NCM111 (Ni: 34% by mass, Co: 33% by mass, Mn: 33% by mass) with an average particle size of 10 μm were weighed and added to a plastic container with 60 mL of ethanol. TPSA (Tokyo Chemical Industry Co., Ltd.) was then added as a surface treatment agent, and the container was stirred at room temperature for 12 hours. This yielded Material B, in which a coating layer was formed on the surface of the electrode active material. Next, materials A and B were mixed in a mass ratio of 1:9. Furthermore, 1% by mass of AB (conductive additive) based on the total weight of NCM111 was weighed and mixed in a mortar. The mixture was then placed in a dedicated container and kneaded for 2 minutes with a mixer (Thinky Corporation, product name "Awatori Rentaro"). After confirming that the mixture was sufficiently mixed, a binder (PVDF / NMP: 10% by mass) was weighed and added dropwise at 1% by mass based on the total weight of NCM111, and the mixture was kneaded for 2 minutes with the mixer. Next, 100 μL of NMP was added dropwise, and the mixture was kneaded for 2 minutes with the mixer. After that, the mixture was kneaded for another 2 minutes with the mixer and degassed for 30 seconds to produce material C, which contained materials A and B.

[0087] The obtained material C was applied to aluminum foil and dried at 120°C under a pressure of 20,000 Pa. Then, a φ14 mm punch was used to cut the foil, and the foil was vacuum dried at 120°C for 12 hours or more. After vacuum drying, the foil was press-molded at 50 kN for 3 minutes to obtain a positive electrode for a lithium-ion secondary battery. At this time, the mass ratio of the electrode active material:conductive additive:binder in the positive electrode was 98:1:1, and the tap density was 3.0 g / cm. 3 ~3.5g / cm 3 It was. Using the obtained positive electrode for a lithium ion secondary battery, a coin cell was produced in the same manner as above.

[0088] Example 5 A coin cell was produced in the same manner as in Example 4, except that material B, in which a coating layer (TPSA) was formed on the surface of an electrode active material (NCM111), was replaced with material D, in which PI (manufactured by IST, product name "DREAMBOND (registered trademark) 100") was added as a surface treatment agent and a coating layer (PI) was formed on the surface of an electrode active material (NCM111).

[0089] (Comparative Example 3) A positive electrode for a lithium ion secondary battery and a coin cell were obtained in the same manner as in Example 4, except that no surface treatment agent was used in the preparation of either Material A or Material B.

[0090] The discharge capacity and capacity retention rate were measured in the same manner as above using the coin cells obtained in Example 4 and Comparative Example 3. The results are shown in Table 2 and FIG.

[0091] [Table 2]

[0092] The results in Table 2 indicate that in Example 4, when the coating layer of the primary particles contains an aminosilane compound having an amino group at the molecular terminal and two ethyleneimine groups, and the coating layer of the secondary particles contains an acid anhydride having a highly reactive acid anhydride group, the decrease in discharge capacity from the initial discharge capacity is small, and degradation can be suppressed. Furthermore, when electron microscope images of the electrode active material used in Example 4 were examined, it was found that, as shown in Figure 5(a), the primary particles uniformly filled the gaps between the secondary particles, and the electrode active material particles were extremely densely packed throughout the electrode active material. From this, it is inferred that the amino groups in the coating layer of the primary particles react with the acid anhydride groups in the coating layer of the secondary particles, causing the primary particles to selectively accumulate in the gaps between the secondary particles, resulting in a high capacity retention rate.

[0093] Furthermore, in Example 5, it was found that when the coating layer of the primary particles contains an aminosilane compound having an amino group at the molecular end and two ethyleneimine groups, and the coating layer of the secondary particles contains an imide compound having a highly reactive imide group, the decrease in discharge capacity from the initial discharge capacity is very small, and deterioration can be significantly suppressed.

[0094] On the other hand, in Comparative Example 3, no coating layer was formed on either the primary particles or the secondary particles of the electrode active material, and the discharge capacity decreased significantly from the initial discharge capacity, resulting in insufficient suppression of degradation. Furthermore, when an electron microscope image of the electrode active material used in Comparative Example 3 was examined, it was found that the primary particles irregularly filled the gaps between the secondary particles, and the electrode active material particles were sparsely packed throughout the electrode active material, as shown in Figure 5(b). [Explanation of symbols]

[0095] 1. Lithium-ion secondary battery 2 Positive electrode 3 negative electrode 4 Electrolytes 5 Separator 6 Positive electrode case 7 Negative electrode case 8 Gaskets 21 Current collector 22 Electrode active material containing layer 31 Current collector 32 Electrode active material containing layer

Claims

1. A current collector and an electrode active material-containing layer provided on the current collector, the electrode active material-containing layer has an electrode active material and a coating layer provided on a surface of the electrode active material, the electrode active material has first electrode active material particles and second electrode active material particles having a particle size larger than that of the first electrode active material particles, the coating layer has a first coating portion provided on the surface of the first electrode active material particles and a second coating portion provided on the surface of the second electrode active material particles, the first coating portion contains an aminosilane compound having an amino group at a molecular terminal, An electrode for a lithium ion secondary battery, wherein the second coating portion contains a compound having a functional group selected from the group consisting of carboxylic acid, carboxylic acid salt, carboxylic acid anhydride, succinic acid anhydride, and acylisourea.

2. A current collector and an electrode active material-containing layer provided on the current collector, the electrode active material-containing layer has an electrode active material and a coating layer provided on a surface of the electrode active material, the electrode active material has first electrode active material particles and second electrode active material particles having a particle size larger than that of the first electrode active material particles, the coating layer has a first coating portion provided on the surface of the first electrode active material particles and a second coating portion provided on the surface of the second electrode active material particles, the first coating portion contains an aminosilane compound having an amino group at a molecular terminal, The electrode for a lithium ion secondary battery, wherein the second coating portion contains an imide-based compound.

3. 3. The electrode for a lithium ion secondary battery according to claim 1, wherein the aminosilane compound has an amino group and at least one ethyleneimine group at a molecular terminal.

4. 4. The electrode for a lithium ion secondary battery according to claim 3, wherein the aminosilane compound is N-(3-trimethoxysilylpropyl)diethylenetriamine.

5. 3. The electrode for a lithium ion secondary battery according to claim 1, wherein the coating layer is composed of a self-assembled monolayer.

6. 6. The electrode for a lithium ion secondary battery according to claim 1, wherein a coverage rate, which indicates the proportion of the surface of the first electrode active material particle that is covered with the first coating portion, is 80% or more.

7. 3. The electrode for a lithium ion secondary battery according to claim 1, wherein the electrode active material comprises a lithium composite oxide having a layered rock salt structure.

8. The lithium composite oxide having a layered rock salt structure is LiNiO ２ , LiCoO ２ , LiNi ｋ Co ｌ Mn ｍ O ２ (k+l+m=1) and LiNi ｋ Co ｌ Al ｍ O ２ 8. The electrode for a lithium ion secondary battery according to claim 7, wherein k is any one of (k+l+m=1).

9. 9. The electrode for a lithium ion secondary battery according to claim 8, wherein the lithium composite oxide having a layered rock salt structure is any one of lithium cobalt oxide (LCO), nickel-cobalt-lithium manganese oxide (NCM), and nickel-cobalt-lithium aluminum oxide (NCA).

10. A lithium ion secondary battery comprising the electrode for a lithium ion secondary battery according to any one of claims 1 to 9 and an electrolyte.

Images