WO2021182320A1 - Electrode for lithium ion secondary batteries, and lithium ion secondary battery - Google Patents

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

Electrodes for lithium-ion secondary batteries and lithium-ion secondary batteries

The present invention relates to electrodes for lithium ion secondary batteries and lithium ion secondary batteries.
The present application claims priority based on Japanese Patent Application No. 2020-041660 filed in Japan on March 11, 2020, the contents of which are incorporated herein by reference.

In recent years, as a battery expected to be smaller, lighter, and have a higher capacity, a non-aqueous electrolyte type secondary battery such as a lithium ion battery has been proposed and put into practical use. This lithium ion battery is composed of a positive electrode and a negative electrode having a property of reversibly inserting and removing lithium ions, and a non-aqueous electrolyte.
Lithium composite oxide is used as a positive electrode active material for a lithium secondary battery. Lithium secondary batteries have already been put into practical use as small power sources for mobile phones and notebook computers. Furthermore, application is being attempted to medium- and large-sized power sources such as automobile applications and power storage applications. As the range of application is expanded in this way, extending the life of the lithium secondary battery is an important issue.

As the lithium composite oxide used as the positive electrode active material, an LNMO type composite oxide containing, for example, lithium, nickel, manganese and oxygen is used.
Since the LNMO type composite oxide can be used at a high potential and has high safety, its application to large batteries is progressing, and attempts are being made to increase the capacity.
For example, in order to improve the capacity of a battery using an LNMO type lithium composite oxide, it is described that the content of additives is reduced to produce a dense lithium composite oxide film having few voids (Patent Document). 1).

Here, although the LNMO type composite oxide has an advantage that it can be used at a high potential, there is a problem that a metal element is eluted into the electrolytic solution during high potential operation and the battery is deteriorated. Therefore, an electrode having a coating layer of a water-repellent material on the surface of the electrode active material has been proposed (Patent Document 2).

Japanese Unexamined Patent Publication No. 2014-35909 JP-A-2017-174692

However, it cannot be said that the above-mentioned conventional technique sufficiently suppresses battery deterioration, and there is room for further study and improvement of the composition of the coating layer formed on the surface of the electrode active material.

An object of the present invention is a method for manufacturing an electrode for a lithium ion secondary battery, a lithium ion secondary battery, and a positive electrode for a lithium ion secondary battery, which can further suppress battery deterioration and improve battery characteristics as compared with the conventional case. To provide.

As a result of diligent research, the present inventors can select a molecule having various functions as a coating layer formed on the surface of the electrode active material, and elution or electrolysis of the above metal element depends on the function of a specific molecule. It has been found that the effectiveness of the coating layer against the oxidative decomposition of the liquid is determined. In particular, by using a coating layer suitable for the surface of a positive electrode active material composed of lithium cobalt oxide (LCO), nickel-cobalt-lithium manganate (NCM), or nickel-cobalt-lithium aluminate (NCA), It has been found that the deterioration of the lithium ion secondary battery can be further suppressed and the battery characteristics can be improved.

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

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

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

[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.

[5] The lithium ion battery according to any one of [1] to [4] above, wherein the coating ratio indicating the ratio of the portion of the surface of the electrode active material covered by the coating layer is 80% or more. Electrode for next battery.

[6] The electrode active material has a first electrode active material particle and a second electrode active material particle having a particle size larger than that of the first electrode active material particle.
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 contains an aminosilane compound having an amino group at the end of the molecule.
The lithium ion secondary according to the above [1], wherein the second coating portion contains a compound having a functional group selected from the group consisting of a carboxylic acid, a carboxylic acid salt, a carboxylic acid anhydride, a succinic anhydride and an acylisourea. Battery electrode.

[7] The electrode active material has a first electrode active material particle and a second electrode active material particle having a particle size larger than that of the first electrode active material particle.
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 contains an aminosilane compound having an amino group at the end of the molecule.
The electrode for a lithium ion secondary battery according to the above [1], wherein the second coating portion contains an imide compound.

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

[9] lithium composite oxide having a layered rock-salt _{structure, 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 in (k + l + m = 1 ) The electrode for a lithium ion secondary battery according to the above [8], which is one of the above.

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

[11] A lithium ion secondary battery comprising the electrode for the lithium ion secondary battery according to any one of the above [1] to [10] and an electrolyte.

According to the present invention, it is possible to provide an electrode for a lithium ion secondary battery and a lithium ion secondary battery capable of further suppressing battery deterioration and improving battery characteristics as compared with the conventional case.

FIG. 1 is a diagram showing an example of a configuration of a lithium ion secondary battery according to an embodiment of the present invention. FIG. 2 is a graph showing the cycle characteristics of the lithium ion secondary battery in Examples and Comparative Examples. FIG. 3 is a diagram showing the relationship between the heating time at the time of forming the coating layer and the cycle characteristics in the examples. FIG. 4 is a diagram showing the characteristic evaluation of the positive electrode by the floating test in the example. 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.

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

[Construction of electrodes for lithium-ion secondary batteries]
The electrode for a lithium ion secondary battery according to the present embodiment includes a current collector and an electrode active material-containing layer provided on the current collector.

<Current collector>
The current collector is composed of, for example, a metal foil. Metal foil is suitable for use in batteries of various shapes such as cylindrical, square and laminated. Carbon may be vapor-deposited on the surface of the current collector in order to further enhance the adhesion between the electrode active material and the current collector.

Among the electrodes, for example, an aluminum foil can be used as the current collector on the positive electrode side. The current collector is preferably hydrophilized by surface treatment. Since the surface of the current collector is made hydrophilic, hydrogen bonds are easily formed when the electrode-forming slurry is dried, and an electrode having high adhesive strength can be obtained. Examples of the hydrophilization treatment on the surface of the current collector include a method of irradiating ultraviolet rays (UV) in an _{ozone (O 3 ) atmosphere (UV / O 3} treatment).

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

(Electrode active material)
The electrode active material contains a lithium composite oxide having a layered rock salt type structure. The term "layered" as used herein means a thin sheet-like shape. The "rock salt type structure" is a sodium chloride type structure which is one of the crystal structures, and the face-centered cubic lattice formed by each of the cations and anions is 1 / of the ridge of the unit cell. Refers to a structure that is offset by 2.

The lithium composite oxide having a layered rock-salt structure, for _{example, 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} (k + l + m = 1) of the Any of the above can be mentioned.

Specifically, the lithium composite oxide includes lithium cobalt oxide (hereinafter, also referred to as “LCO”), nickel-cobalt-lithium manganate (hereinafter, also referred to as “NCM”), and nickel-cobalt-lithium aluminate (hereinafter, also referred to as “NCM”). Hereinafter, it is preferably one of (also referred to as "NCA"). Further, nickel-cobalt-lithium manganate (NCM) or nickel-cobalt-lithium aluminate (NCA) is more preferable.

As described above, by using the lithium salt of the above-mentioned ternary transition metal oxide having a layered rock salt type structure as the electrode active material, it is possible to obtain a lithium ion secondary battery having excellent energy density and thermal stability. can.
Further, the particles of the lithium salt of the ternary transition metal oxide such as NCM and NCA have a smaller particle size and a larger specific surface area (about 10 times) than the particles of LCO and the like. As a result, the contact area between the active material particles and the electrolyte can be increased. As a result, the effect of suppressing the reaction between the electrode active material and the electrolyte becomes remarkable due to the coating layer, and the conductivity of lithium ions between the active material particles and the electrolyte is improved as compared with the case where this configuration is not adopted. However, the power of the lithium ion secondary battery can be increased.

Further, by using the lithium salt of the above-mentioned ternary transition metal oxide having a layered rock salt type 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 tends to increase, and the elution of metal elements in the charged state tends to decrease. As a result, the long-term reliability of the lithium ion secondary battery in the charged state can be improved and the cycle characteristics of the lithium ion secondary battery can be improved as compared with the case where this configuration is not adopted.

The electrode active material may have a first electrode active material particle and a second electrode active material particle having a particle size larger than that of the first electrode active material particle. The first electrode active material particles are, for example, primary particles. The second electrode active material particles may be primary particles or secondary particles. As a result, the first electrode active material particles having a small particle size enter into the gaps between the second electrode active material particles having a large particle size and become dense, and the energy density can be increased by increasing the electrode density. Further, by using the first electrode active material particles having a particle size smaller than that of the second electrode active material particles, it is possible to alleviate the destruction of the electrode active material due to volume expansion and contraction.

The particle size of the first electrode active material particles is preferably 0.1 μm or more and 4 μm or less, and more preferably 0.7 μm or more and 2 μm or less. The particle size of the second electrode active material particles is preferably 5 μm or more and 20 μm or less, and more preferably 6 μm or more and 15 μm or less.
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, when 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. Further, 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.

(Coating layer)
The coating layer contains an aminosilane compound having an amino group at the end of the molecule, having at least one ethyleneimine group, or having a dialkylamino group. This coating layer is preferably composed of a self-assembled monolayer (SAM). The self-assembled monomolecular film is formed by using an organic molecule having a functional group forming a predetermined chemical bond as a terminal group with respect to the electrode active material to form a chemical bond with the electrode active material. An anchored organic molecule is regulated from the surface of the electrode active material and interacts with each other to form a monomolecular film in an orderly arrangement.

By providing the coating layer on the surface of the electrode active material in this way, the elution of the metal element into the electrolytic solution can be suppressed by the coating even when the electrode active material is operated at a high voltage, whereby the elution of the metal element into the electrolytic solution can be suppressed. It is possible to suppress a decrease in capacity due to elution of the metal element of the electrode active material. On the surface of the electrode active material, the movement of molecules is restricted to some extent, but if the coating layer contains an aminosilane compound having an amino group at the end of the molecule, the amino group capable of capturing lithium ions by one molecule acts effectively. However, by weakly coordinating lithium ions or solvated lithium ions with amino groups near the surface of the electrode active material, the activation energy required for desolvation of the solvated lithium ions in the electrolytic solution can be obtained. It is possible to reduce the amount and realize high efficiency of lithium ion transport at the interface between the electrode active material and the electrolytic solution. In addition, a monomolecular film with high crystallinity such as a monomolecular film composed of linear molecules such as an alkyl group or a fluoroalkyl group always accompanies the formation of defects at the same time as crystallization, whereas ethyleneimine in the molecular chain. By having a group or a dialkylamino group, self-assembly between adjacent molecular chains is hindered, and the monomolecular film becomes amorphous, so that a dense monomolecular film is formed without the formation of defects. As a result, the direct contact area between the electrode surface and the electrolytic solution is more limited, so that deterioration of battery performance due to decomposition of the electrolytic solution can be prevented more efficiently than in the case of coating a crystalline monomolecular film. Can be done. That is, by forming a monomolecular film of the aminosilane compound having at least one ethyleneimine group or a dialkylamino group on the surface of the electrode active material, further suppression of oxidative decomposition of the electrolytic solution can be realized. In addition, it is possible to improve the efficiency of lithium ion transport at the interface between the electrode active material and the electrolytic solution.

Further, the aminosilane compound is active because it reacts with an acid-base reaction with a carboxylic acid and a salt thereof, a carboxylic acid anhydride, a succinic anhydride or acylisourea nucleophilically, and easily forms an imide bond even at room temperature. The amino group at the molecular end of the monomolecular film coated on the surface of the substance can easily immobilize the compound having the functional group. Therefore, by coating the surface of the active material with the aminosilane compound, a compound having a functional group such as a carboxylic acid and a salt thereof, a carboxylic acid anhydride, a succinic anhydride, or an acylisourea can be immobilized on the surface of the active material. Examples of the compound to be immobilized include secondary electrode active material particles having different particle diameters and morphologies, solid electrolyte particles, strong dielectric particles, polymer binders such as sodium carboxymethyl cellulose and styrene butadiene copolymer, polyacrylic acid, and polyvinylidene fluoride. Examples thereof include, but are not limited to, conductive aids made of carbon materials such as acetylene black, carbon nanotubes, graphene, and hetero-nanocarbon.

The aminosilane compound having an amino group at the molecular terminal is not particularly limited, and examples thereof include (3-aminopropyl) triethoxysilane (hereinafter, also referred to as “APTES”) as shown below.

It is more preferable that the aminosilane compound has an amino group at the molecular terminal and at least one ethyleneimine group. Such an aminosilane compound is not particularly limited, but for example, as shown below, N- (6-aminohexyl) aminomethyltriethoxysilane (hereinafter, also referred to as "AHAMETES"), N- (3-tri), N- (3-tri). Methoxysilylpropyl) diethylenetriamine (hereinafter, also referred to as “DAEAPTS”) can be mentioned.

The aminosilane compound having a dialkylamino group is not particularly limited, and examples thereof include N, N-diethylaminopropyltrimethoxysilane (hereinafter, also referred to as “SID”) as shown below.

In the present embodiment, the reaction related to the deinsertion and insertion of lithium ions is uniformly carried out over the entire surface of the electrode active material. For this reason, it is preferable that 80% or more, more preferably 85% or more, still more preferably 90% or more of the surface of the electrode active material is covered with the coating layer.
The coverage of the coating layer can be measured using a transmission electron microscope (TEM), an energy dispersive X-ray spectrometer (EDX), and an X-ray photoelectron spectroscopy (XPS).
Specifically, for the coating layer formed on the electrode active material, 100 electrode active materials are observed using a transmission electron microscope (TEM) and an energy dispersive X-ray spectroscope (EDX), and the electrodes are electrodelied. The ratio of the portion of the surface of the active material covered by the coating layer is calculated and used as the coverage ratio.
In addition, using X-ray photoelectron spectroscopy (XPS), the ratio of the portion of the surface of the electrode active material covered by the coating layer is calculated from the surface atomic concentration measurement, and the coverage is obtained. Consistency with may be considered.

In the present embodiment, the average thickness of the coating layer is preferably 0.5 nm or more and 0.3 μm or less, more preferably 0.5 nm or more and 0.1 μm or less, and further preferably 0.5 nm or more and 10 nm or less. It is particularly preferably 0.8 nm or more and 6.0 nm or less, and most preferably 1.0 nm or more and 3.0 nm or less. The average thickness can be calculated based on the angle-resolved core level spectrum of the coating layer on the surface of the electrode active material measured by X-ray photoelectron spectroscopy (XPS). Alternatively, it can be measured by wavelength / angle-resolved ellipsometry or an atomic force microscope.

When the electrode active material has a first electrode active material particle and a second electrode active material particle having a particle size different from that of the first electrode active material particle, the coating layer is the first electrode active material. It may have a first coating portion provided on the surface of the material particles and a second coating portion provided on the surface of the second electrode active material particles. At this time, the mass ratio of the first electrode active material particles on which the first coating portion is formed and the second electrode active material particles on which the second coating portion is formed is 15 to 0.5: 85 to 99.5. Is preferable, and 10 to 1: 90 to 99 is more preferable.

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

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 is different from the material constituting the second coating, for example, the first coating contains, for example, an aminosilane compound having an amino group at the molecular terminal, and the second coating is a carboxylic acid. , Carboxylic acid anhydride, succinic anhydride and acylisourea preferably contain a compound having a functional group selected from the group. Examples of succinic anhydride include 3-triethoxysilylpropyl succinic anhydride (hereinafter, also referred to as “TPSA”), as shown below. By using the above material for the second coating portion, the compound having a functional group constituting the second coating portion is easily immobilized on the amino group at the molecular terminal in 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 particles of the second electrode active material particles, and the energy density can be further increased.

When the material constituting the first coating portion is different from the material constituting the second coating portion, for example, the first coating portion contains, for example, an aminosilane compound having an amino group at the molecular terminal, and the second coating portion contains. It is preferably an imide compound. Examples of the imide compound include polyimide (PI), polyetherimide (PEI), and polyamideimide (PAI). By using the above 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 end in the aminosilane compound constituting the first coating portion. As a result, the first electrode active material particles can be selectively accumulated by the gaps between the particles of the second electrode active material particles, and the loss of energy density can be minimized due to the weight increase due to the second coating portion. By suppressing the decomposition reaction of the electrolytic solution, the capacity retention rate associated with the charge / discharge cycle can be maintained at a higher level.

[Method of forming the coating layer]
The coating layer can be formed by surface-treating the electrode active material with the above-mentioned silane coupling agent containing an aminosilane compound. By surface-treating the electrode active material, a coating layer is formed on the surface of the primary particles and / or the 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, but from the viewpoint of uniformly forming the coating layer on the surface of the electrode active material, the coating layer is formed on the surface of the electrode active material before the production of the electrode. It is preferable to form. However, the present invention is not limited to this, and a coating layer may be formed on the surface of the mixture after mixing the electrode active material and another mixture, or an electrode may be manufactured in advance and a coating layer may be formed on the surface of the mixture electrode. It may be formed. As a result, a coating layer containing the above-mentioned aminosilane compound is formed on the surface of the electrode active material, and an electrode for a lithium ion secondary battery having the electrode active material-containing layer can be obtained.

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 the present embodiment, the liquid phase method or the gas phase method is preferable, and the liquid phase method is more preferable. In the liquid phase method, when a high nickel-based electrode active material such as NCM or NCA having a high Ni content is used, a polar solvent such as NMP (N-methylpyrrolidinone) is used, and the electrode having a low Ni content is not so high. When an active material is used, a solvent such as ethanol or isopropanol is used. Examples of the vapor phase method include a vacuum deposition method, a sputtering method, a chemical vapor deposition (CVD) method, and a plasma chemical vapor deposition method.

[Construction of lithium-ion secondary battery]
The lithium ion secondary battery according to the present embodiment includes the above-mentioned electrode for a lithium ion secondary battery and an electrolyte. This secondary battery can have the same configuration as a conventional or known secondary battery except that it has the above electrodes.

FIG. 1 is a diagram showing an example of the configuration of a lithium ion secondary battery according to the present 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.
Further, the lithium ion secondary battery 1 is a stainless steel positive electrode side case 6 in which the separator 5 provided between the positive electrode 2 and the negative electrode 3 and the positive electrode 2, the negative electrode 3 and the electrolyte 4 are housed in cooperation with each other. And the negative electrode side case 7, and the polypropylene gasket 8 between the positive electrode side case 6 and the negative electrode side case 7 and interposed around the outer peripheral portions thereof can be provided.

(Positive electrode)
The electrode active material-containing layer 22 of the positive electrode 2 contains the above-mentioned electrode active material and coating layer according to the present 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 adjusting a positive electrode mixture containing the above lithium composite oxide, a conductive material and a binder.

(Conductive material)
A carbon material can be used as the conductive material of the positive electrode. Examples of the carbon material include graphite powder, carbon black (for example, acetylene black), and fibrous carbon material. Since carbon black is fine and has a large surface area, it is possible to improve the conductivity inside the positive electrode by adding a small amount to the positive electrode mixture to improve charge / discharge efficiency and output characteristics, but if too much is added, it depends on the binder. Both the binding force between the positive electrode mixture and the positive electrode current collector and the binding force inside the positive electrode mixture decrease, which causes an increase in internal resistance.

(binder)
As the binder contained in the positive electrode, a thermoplastic resin can be used. Examples of this thermoplastic resin include polyvinylidene fluoride (hereinafter, also referred to as PVDF), polytetrafluoroethylene (hereinafter, also referred to as PTFE), ethylene tetrafluoride, propylene hexafluoride, vinylidene fluoride-based copolymer, and hexafluoride. Fluororesin such as propylene fluoride / vinylidene fluoride copolymer, ethylene tetrafluoride / perfluorovinyl ether copolymer; polyolefin resin such as polyethylene and polypropylene; can be mentioned.

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

(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 occluding and releasing lithium ions can be used alone or in combination. Examples of compounds capable of occluding and releasing lithium ions include metal materials such as lithium, alloy materials containing titanium, silicon, tin and the like, graphite, coke, calcined organic polymer compounds or carbon materials such as amorphous carbon. Can be mentioned.
Not only these electrode active materials can be used alone, but also a plurality of types of these electrode active materials can be mixed and used. Among these substances, as the electrode active material of the negative electrode 3, titanium-containing oxides (for example, TiO _{2 (B) which is titanium oxide having a bronze structure, Li 4} Ti ₅ O ₁₂ which is lithium titanate), silicon oxide, and the like. It is preferred to use natural graphite, artificial graphite, hard carbon, soft carbon, silicon and alloys containing silicon (eg Si ₈₀ Ti ₂₀ ), tin and the like.
For example, when a lithium foil is used as the negative electrode active material, it can be formed by crimping the lithium foil to the surface of a current collector made of a metal such as copper.

When an alloy material or carbon material is used as the negative electrode active material, the negative electrode active material is mixed with a binder, a conductive auxiliary agent, etc. in a solvent such as water or N-methylpyrrolidone, and then made of a metal such as copper. It can be formed by applying it on a current collector. The binder is preferably formed from a polymer material, and is preferably a material that is chemically and physically stable in the atmosphere inside the lithium secondary battery.

Examples of the binder include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), ethylene-propylene-diene copolymer (EPDM), styrene-butadiene rubber (SBR), and acrylonitrile-butadiene rubber (NBR). , Fluorine rubber and the like.
Examples of the conductive auxiliary agent include Ketjen black, acetylene black, carbon black, graphite, carbon nanotubes, amorphous carbon and the like. Moreover, the conductive polymer polyaniline, polypyrrole, polythiophene, polyacetylene, polyacene and the like can be exemplified.

Further, the electrode active material-containing layer 32 of the negative electrode 3 may include 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 can have the same structure as the coating layer of the electrode active material-containing layer 22 in the positive electrode 2. As a result, deterioration of battery performance due to decomposition of the electrolytic solution can be prevented, and high efficiency of lithium ion transport at the interface between the electrode active material and the electrolytic solution can be realized.

(Electrolyte)
The electrolytic solution is a medium for transporting charged carriers such as ions between the positive electrode and the negative electrode, and is not particularly limited, but is physically, chemically, and electrically stable in an atmosphere in which a lithium ion secondary battery is used. Is desirable.

For example, as the electrolytic solution, LiBF ₄ , LiPF ₆ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) (C ₄ F ₉₎ An electrolytic solution in which one or more selected from SO ₂ ) is used as a supporting electrolyte and this is dissolved in an organic solvent is preferable.

Examples of the organic solvent include propylene carbonate, ethylene carbonate, 1,2-dimethoxyethane, dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate, tetrahydrofuran, 2-methyltetrahydrofuran, tetrahydropyran and the like, and mixtures thereof. Of these, an electrolytic solution containing a carbonate solvent is preferable because it has high stability at high temperatures. Further, a solid polymer electrolyte in which the above electrolyte is contained in a solid polymer such as polyethylene oxide, or a solid electrolyte such as ceramic or glass having lithium ion conductivity can also be used.

It is desirable to insert a separator, which is a member that has both an electrical insulating action and an ionic conduction action, between the positive electrode and the negative electrode. When the electrolyte is liquid, the separator also serves to retain the liquid electrolyte. Examples of the separator include a porous synthetic resin film, particularly a porous film made of a polyolefin polymer (polyethylene, polypropylene) or glass fiber, and a non-woven fabric. Further, the separator preferably adopts a form larger than that of the positive electrode and the negative electrode for the purpose of ensuring the insulation between the positive electrode and the negative electrode.

The positive electrode, the negative electrode, the electrolyte, the separator and the like are generally housed in a case composed of the above-mentioned positive electrode side case 6 and negative electrode side case 7. The case is not particularly limited, and can be made of a known material and form. That is, the lithium secondary battery of the present invention is not particularly limited in its shape, and can be used as a battery having various shapes such as a coin type, a cylindrical type, and a square type. Further, the case of the lithium secondary battery of the present embodiment is not limited, and is used as a battery of various forms such as a case made of metal or resin that can hold its outer shape, a soft case such as a laminate pack, and the like. can.

Hereinafter, examples of the present invention will be described. The present invention is not limited to the following examples.

(Example 1)
<Manufacturing of positive electrodes for lithium-ion secondary batteries>
NCM523 (Ni: 50% by mass, Co: 20% by mass, Mn: 30% by mass) and DB (conductive aid) are weighed, mixed in a mortar, placed in a special container, and mixed (manufactured by Shinky, product name). Kneaded with "Awatori Rentaro") for 2 minutes. After confirming that the mixture was sufficiently stirred, the binder (PVDF / NMP: 10% by mass) was weighed, added dropwise, and kneaded with a mixer for 2 minutes. Next, 100 μL of NMP was added dropwise, kneaded with a mixer for 2 minutes, then further kneaded with a mixer for 2 minutes, and defoamed for 30 seconds.

The obtained material was applied to an aluminum foil and dried at atmospheric pressure at 100 ° C. Then, it was punched to φ14 mm using a punching machine, and vacuum dried at 120 ° C. for 10 hours or more. After completion of vacuum drying, press molding was performed at 50 kN for 1 minute to obtain a positive electrode material. At this time, the mass ratio of the electrode active material: the conductive auxiliary agent: the binder in the positive electrode material was 90: 5: 5, and the tap density was 2.68 g / cm ³ to 2.83 g / cm ³ .

Next, in the dry room, the screw tube and the above electrode material are installed in a closed container made of SUS, and 50 μL of APTES (manufactured by Tokyo Chemical Industry Co., Ltd.) as a surface treatment agent is put in the screw tube and sealed, and the inside of the closed container is sealed. The pressure was 10325 Pa, and the closed container was heated at 120 ° C. for 3 to 15 hours. As a result, a positive electrode for a lithium ion secondary battery having a coating layer formed on the surface of the electrode active material was obtained.

<Making coin cells>
The positive electrode can (positive electrode side case), the positive electrode for the lithium ion secondary battery obtained above, the separator, the gasket, the spacer, the spring and the negative electrode can (negative electrode side case) are laminated in this order, and the electrolytic solution (1M LiPF _{6) is inside.} EC / DMC (1: 2)) was housed to produce a coin cell. Lithium foil was used for the negative electrode.

(Example 2)
A coin cell was produced in the same manner as in Example 1 except that AHAMETES (manufactured by Gelest) was applied onto the aluminum foil instead of APTES to form a coating layer when the positive electrode was produced.

(Example 3)
A coin cell was produced in the same manner as in Example 1 except that DAEAPTS (manufactured by Sigma-Aldrich Co., Ltd.) was applied onto the aluminum foil instead of APTES at the time of producing the positive electrode to form a coating layer.

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

(Comparative Example 2)
A coin cell was produced in the same manner as in Example 1 except that FAS17 (manufactured by Gsrest) was applied onto the aluminum foil instead of APTES at the time of producing the positive electrode to form a coating layer.
FAS17: Heptadecafluoro-1,1,2,2-tetrahydrodecyltrimethoxysilane

[Evaluation of positive electrode cycle characteristics]
The coin cells produced in Examples 1 to 4 and Comparative Examples 1 and 2 were set in a charging / discharging device (manufactured by Hokuto Denko Co., Ltd., product name "HJ1001SD8"), and Li was inserted / discharged into the positive electrode by charging / discharging to perform Li insertion / discharge of the positive electrode. The cycle characteristics were evaluated. Charging and discharging were performed in a cutoff voltage of 2.8 V to 4.2 V, a current density of 0.5 C, a temperature of 50 ° C., and a CCCV-CC mode. This charge / discharge was repeated 100 times, and the discharge capacity and the capacity retention rate at that time were measured. The results are shown in Table 1 and FIG.

From the results of Table 1 and FIG. 2, in each of Examples 1 to 3, the coating layer contains an aminosilane compound having an amino group at the molecular terminal, and the initial discharge capacity is higher than that of Comparative Examples 1 and 2. It was found that the decrease in discharge capacity was small and deterioration could be suppressed. Further, comparing Examples 1 to 3, in Examples 2 to 3, the aminosilane compound has an amino group at the molecular terminal and has one or two ethyleneimine groups, and is more than Example 1. However, it was found that the decrease in discharge capacity became smaller and the deterioration could be further suppressed. In particular, in Example 3, it was found that the aminosilane compound has two ethyleneimine groups, the decrease in discharge capacity is the smallest, and the deterioration can be suppressed most.

On the other hand, in Comparative Examples 1 and 2, the decrease in discharge capacity from the initial discharge capacity was large, and the suppression of deterioration was insufficient.

Further, the coating layer was formed on the surface of the electrode active material in the same manner as in Example 3 except that the heating time at the time of forming the coating layer was changed to 1 hour, 3 hours, 6 hours, 9 hours, and 15 hours, respectively. The formed positive electrode for a lithium ion secondary battery was obtained. Then, each cycle characteristic was evaluated in the same manner as described above. The results are shown in FIG.

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

[Characteristic evaluation of positive electrode by floating test]
A coin cell was produced in the same manner as described above using the positive electrode for a lithium ion secondary battery obtained with the heating time at the time of forming the coating layer set to 1 hour, 3 hours, and 15 hours. Then, a floating test was conducted on these coin cells. In the floating test, the closed circuit voltage (manufactured by Hokuto Denko, product name "HJ1001SD8") in a fully charged state of each coin cell was measured in a constant temperature bath (manufactured by ESPEC, SH222) set at 50 ° C.

As shown in FIG. 4, when the heating time at the time of forming the coating layer is 3 hours or 15 hours, the coverage of the coating layer is 80% and 99%, respectively, the oxidative decomposition of the electrolytic solution is suppressed, and the circuit is closed. It was found that the voltage drop can be sufficiently suppressed.

On the other hand, it was found that when the coating layer was not formed on the electrode active material, the voltage dropped with the passage of time, and the drop in the closed circuit voltage could not be suppressed. Further, it was found that when the heating time at the time of forming the coating layer was set to 1 hour, the coating layer was hardly formed, the voltage dropped with the passage of time, and the drop of the closed circuit voltage could not be suppressed. This is because the current (self-discharge) in the closed circuit increases as the oxidative decomposition of the electrolytic solution increases, causing a voltage drop over time.

(Example 4)
In the dry room, 5 g of primary particles of NCM111 (Ni: 34% by mass, Co: 33% by mass, Mn: 33% by mass) having an average particle size of 1 μm are added to a plastic container by adding 5 g weighing and 60 mL of ethanol, and further. 200 μL of DAEAPTS (manufactured by Sigma-Aldrich) was added as a surface treatment agent, and the container was stirred at room temperature for 12 hours. After stirring, suction filtration was performed using a membrane filter having a pore diameter of 1 μm, and then vacuum drying was performed at 45 ° C. As a result, a material A having a coating layer formed on the surface of the electrode active material (NCM111) was obtained. Further, in a dry room, 5 g of secondary particles of NCM111 (Ni: 34% by mass, Co: 33% by mass, Mn: 33% by mass) having an average particle size of 10 μm were weighed in a plastic container, and 60 mL of ethanol was added. Further, TPSA (manufactured by Tokyo Kasei Co., Ltd.) was added as a surface treatment agent, and the container was stirred at room temperature for 12 hours. As a result, a material B having a coating layer formed on the surface of the electrode active material was obtained.
After that, the material A and the material B are mixed at a mass ratio of 1: 9, and 1% by mass of AB (conductive auxiliary agent) with respect to the total weight of NCM111 is weighed, mixed in a mortar, and placed in a special container. Kneaded with a mixer (manufactured by Shinky Co., Ltd., product name "Awatori Rentaro") for 2 minutes. After confirming that the mixture was sufficiently stirred, a binder (PVDF / NMP: 10% by mass) was weighed in 1% by mass based on the total weight of NCM111, added dropwise, and kneaded with a mixer for 2 minutes. Next, 100 μL of NMP was added dropwise, and the mixture was kneaded with a mixer for 2 minutes and then further kneaded with a mixer for 2 minutes to defoam for 30 seconds to prepare a material C containing the material A and the material B.

The obtained material C was applied to an aluminum foil and dried at a pressure of 20000 pascals and 120 ° C. Then, it was punched to φ14 mm using a punching machine, and vacuum dried at 120 ° C. for 12 hours or more. After completion of vacuum drying, press molding was performed 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: the conductive auxiliary agent: the binder in the positive electrode was 98: 1: 1, and the tap density was 3.0 g / cm ³ to 3.5 g / cm ³ .
Using the obtained positive electrode for a lithium ion secondary battery, a coin cell was produced in the same manner as described above.

(Example 5)
Instead of the material B in which the coating layer (TPSA) is formed on the surface of the electrode active material (NCM111), PI (manufactured by IST, product name "DREAMBOND (registered trademark) 100") is added as a surface treatment agent to perform the electrode activity. A coin cell was produced in the same manner as in Example 4 except that the material D having the coating layer (PI) formed on the surface of the substance (NCM111) was used.

(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 production of the materials A and B.

Using the coin cells obtained in Example 4 and Comparative Example 3, the discharge capacity and the capacity retention rate were measured by the same method as above. The results are shown in Table 2 and FIG.

From the results in Table 2, in Example 4, the coating layer of the primary particles contained an aminosilane compound having an amino group at the molecular terminal and two ethyleneimine groups, and the coating layer of the secondary particles was a highly reactive acid. It was found that when an acid anhydride having an anhydride group was contained, the decrease in discharge capacity from the initial discharge capacity was small and deterioration could be suppressed. Further, when the electron microscope image of the electrode active material used in Example 4 was confirmed, as shown in FIG. 5A, the primary particles uniformly entered the gaps between the secondary particles, and the entire electrode active material was used as an electrode. It was found that the active material particles were extremely densely packed. From this, 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, so that the primary particles are selectively accumulated in the gaps between the secondary particles, resulting in a high capacity. It is presumed that the maintenance rate was obtained.

Further, in Example 5, 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 is an imide having a highly reactive imide group. It was found that when the compound was contained, the decrease in discharge capacity from the initial discharge capacity was very small, and deterioration could be significantly suppressed.

On the other hand, in Comparative Example 3, the coating layer was not formed in either the primary particles or the secondary particles of the electrode active material, the discharge capacity was significantly reduced from the initial discharge capacity, and the deterioration was insufficiently suppressed. .. Further, when the electron microscope image of the electrode active material used in Comparative Example 3 was confirmed, as shown in FIG. 5 (b), the primary particles irregularly entered the gaps between the secondary particles, and the entire electrode active material became It was found that the electrode active material particles were sparsely filled.

1 Lithium-ion secondary battery 2 Positive electrode 3 Negative electrode 4 Electrode 5 Separator 6 Positive electrode side case 7 Negative electrode side case 8 Gasket 21 Current collector 22 Electrode active material-containing layer 31 Current collector 32 Electrode active material-containing layer

Claims (11)

1. A current collector and an electrode active material-containing layer provided on the current collector are provided.
   The electrode active material-containing layer has an electrode active material and a coating layer provided on the surface of the electrode active material.
   An electrode for a lithium ion secondary battery, wherein the coating layer contains an aminosilane compound having an amino group at the molecular terminal, at least one ethyleneimine group, or a dialkylamino group.
2. The electrode for a lithium ion secondary battery according to claim 1, wherein the aminosilane compound has an amino group at the molecular terminal and at least one ethyleneimine group.
3. The electrode for a lithium ion secondary battery according to claim 2, wherein the aminosilane compound is N- (3-trimethoxysilylpropyl) diethylenetriamine.
4. The electrode for a lithium ion secondary battery according to claim 1, wherein the coating layer is composed of a self-assembled monolayer.
5. The electrode for a lithium ion secondary battery according to any one of claims 1 to 4, wherein the coating ratio indicating the ratio of the portion of the surface of the electrode active material covered by the coating layer is 80% or more. ..
6. The electrode active material has a first electrode active material particle and a second electrode active material particle having a particle size larger than that of the first electrode active material particle.
   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 contains an aminosilane compound having an amino group at the end of the molecule.
   The lithium ion secondary battery according to claim 1, 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 anhydride and acylisourea. For electrodes.
7. The electrode active material has a first electrode active material particle and a second electrode active material particle having a particle size larger than that of the first electrode active material particle.
   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 contains an aminosilane compound having an amino group at the end of the molecule.
   The electrode for a lithium ion secondary battery according to claim 1, wherein the second coating portion contains an imide compound.
8. The electrode for a lithium ion secondary battery according to claim 1, wherein the electrode active material contains a lithium composite oxide having a layered rock salt type structure.
9. Lithium composite oxide having a layered rock-salt _{structure, 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} (k + l + m = 1) any of the The electrode for a lithium ion secondary battery according to claim 8.
10. The lithium composite oxide having a layered rock salt structure is one of lithium cobalt oxide (LCO), nickel-cobalt-lithium manganate (NCM) and lithium nickel-cobalt-lithium aluminate (NCA). Item 9. The electrode for a lithium ion secondary battery.
11. A lithium ion secondary battery comprising the electrode for the lithium ion secondary battery according to any one of claims 1 to 10 and an electrolyte.

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