CN113302772A - Lithium ion secondary battery - Google Patents

Lithium ion secondary battery Download PDF

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Publication number
CN113302772A
CN113302772A CN202080007188.6A CN202080007188A CN113302772A CN 113302772 A CN113302772 A CN 113302772A CN 202080007188 A CN202080007188 A CN 202080007188A CN 113302772 A CN113302772 A CN 113302772A
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insulating layer
positive electrode
active material
secondary battery
ion secondary
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铃木章弘
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Sekisui Chemical Co Ltd
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Sekisui Chemical Co Ltd
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/052—Li-accumulators
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/056—Accumulators with non-aqueous electrolyte characterised by the materials used as electrolytes, e.g. mixed inorganic/organic electrolytes
    • H01M10/0564—Accumulators with non-aqueous electrolyte characterised by the materials used as electrolytes, e.g. mixed inorganic/organic electrolytes the electrolyte being constituted of organic materials only
    • H01M10/0565—Polymeric materials, e.g. gel-type or solid-type
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/10—Energy storage using batteries

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  • General Chemical & Material Sciences (AREA)
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  • Condensed Matter Physics & Semiconductors (AREA)
  • Dispersion Chemistry (AREA)
  • General Physics & Mathematics (AREA)
  • Inorganic Chemistry (AREA)
  • Secondary Cells (AREA)
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Abstract

The invention provides a lithium ion secondary battery, which is provided with a positive electrode, a negative electrode, an insulating layer arranged between the positive electrode and the negative electrode, and an electrolyte, wherein the insulating layer is a layer containing a polymer solid electrolyte, and the thickness of the insulating layer is 3-7 μm. According to the present invention, there is provided a lithium ion secondary battery including an insulating layer that suppresses an increase in internal resistance of the lithium ion secondary battery by reducing porosity of the insulating layer.

Description

Lithium ion secondary battery
Technical Field
The present invention relates to a lithium ion secondary battery having an insulating layer.
Background
Lithium ion secondary batteries are used for large stationary power sources for storing electric power, power sources for electric vehicles and the like, and in recent years, studies on reduction in size and thickness of batteries have been advanced. Generally, a lithium ion secondary battery generally includes: two electrodes each having an electrode active material layer formed on a surface of a metal foil, and a separator disposed between the two electrodes. The separator serves to prevent a short circuit between the two electrodes and to retain the electrolyte. In order to increase the energy density of the lithium ion secondary battery, the separator is preferably thin.
A secondary battery in which an insulating layer capable of functioning as a separator is provided on an electrode active material layer is known as a conventional technology (for example, refer to patent document 1). The insulating layer has a porous structure, and can be formed by, for example, applying a slurry for an insulating layer containing insulating fine particles, a binder, and a solvent onto the electrode active material layer and drying the slurry. In addition, the surface roughness of the electrode active material layer coated with the slurry for an insulating layer is smoothed to reduce the surface roughness, whereby the insulating layer can be made thin.
Documents of the prior art
Patent document
Patent document 1 International publication No. 2016/104782
Disclosure of Invention
Problems to be solved by the invention
However, with the conventional insulating layer, the possibility of occurrence of a fine short circuit due to the void of the insulating layer becomes high while the thinning process is performed thereon. Therefore, in the case of a conventional insulating layer, it is necessary to reduce the porosity of the insulating layer while performing a thinning process. However, when the porosity of the insulating layer is reduced, the internal resistance of the secondary battery increases, resulting in deterioration of the characteristics of the secondary battery.
Accordingly, an object of the present invention is to provide a lithium ion secondary battery including an insulating layer that can suppress an increase in internal resistance of the lithium ion secondary battery due to a decrease in porosity of the insulating layer.
Means for solving the problems
The inventor finds out through keen research that: the present inventors have found that an increase in internal resistance of a lithium ion secondary battery due to a decrease in porosity of an insulating layer can be suppressed by including a polymer solid electrolyte in the insulating layer, and have completed the present invention. The main contents of the present invention are the following [1] to [5 ].
[1] A lithium ion secondary battery comprising a positive electrode, a negative electrode, an insulating layer disposed between the positive electrode and the negative electrode, and an electrolyte, wherein the insulating layer is a layer containing a polymer solid electrolyte, and the insulating layer has a thickness of 3 to 7 μm.
[2] The lithium-ion secondary battery according to [1], wherein the insulating layer has a porosity of 20% or less.
[3] The lithium-ion secondary battery according to any one of the above [1] and [2], wherein the polymer solid electrolyte is a polyether electrolyte.
[4] The lithium-ion secondary battery according to [3], wherein the polymer constituting the matrix of the polyether electrolyte is a polymer having an ethylene oxide structure.
[5] The lithium-ion secondary battery according to any one of the above [1] to [4], wherein the polymer solid electrolyte contains a lithium salt.
ADVANTAGEOUS EFFECTS OF INVENTION
According to the present invention, there is provided a lithium ion secondary battery including an insulating layer capable of suppressing an increase in internal resistance of the lithium ion secondary battery due to a decrease in porosity of the insulating layer.
Drawings
Fig. 1 is an exploded view of a jig for evaluating battery characteristics.
Fig. 2 is a graph showing the evaluation results of the lithium-ion secondary batteries of example 1 and comparative example 1.
Detailed Description
[ lithium ion Secondary Battery ]
The lithium ion secondary battery of the present invention includes a positive electrode, a negative electrode, an insulating layer disposed between the positive electrode and the negative electrode, and an electrolyte solution. The insulating layer is a layer containing a polymer solid electrolyte, and the thickness of the insulating layer is 3 to 7 μm. The lithium ion secondary battery of the present invention will be described in detail below.
(Positive electrode)
The positive electrode in the lithium ion secondary battery of the present invention preferably has a positive electrode current collector and a positive electrode active material layer laminated on the positive electrode current collector. The positive electrode active material layer generally contains a positive electrode active material and a positive electrode binder.
As the positive electrode active material, a lithium metal oxide compound is exemplified. The lithium metal compound includes lithium cobaltate (LiCoO)2) Lithium nickelate (LiNiO)2) Lithium manganate (LiMn)2O4) And the like. Further, olivine-type lithium iron phosphate (LiFePO) may be used4) And the like. Further, a plurality of metals other than lithium may be used, and so-called ternary NCM (nickel cobalt manganese) oxides, NCA (nickel cobalt aluminum) oxides, and the like may be used.
The average particle size of the positive electrode active material is not particularly limited, but is preferably 0.5 to 50 μm, and more preferably 1 to 30 μm. The average particle diameter is a particle diameter at which the volume accumulation is 50% in the particle size distribution of the positive electrode active material determined by the laser diffraction scattering method (D50). The average particle diameter can be adjusted to a desired value by pulverizing the positive electrode active material using a known method such as a ball mill.
The content of the positive electrode active material in the positive electrode active material layer is preferably 50 to 98.5 mass%, more preferably 60 to 98 mass%, based on the total amount of the positive electrode active material layer.
The positive electrode active material layer may contain a conductive assistant. As the conductive auxiliary agent, a material having higher conductivity than the positive electrode active material is used, and specific examples thereof include carbon materials such as ketjen black, acetylene black, carbon nanotubes, and rod-like carbon.
When the conductive auxiliary agent is contained in the positive electrode active material layer, the content of the conductive auxiliary agent is preferably 1 to 30% by mass, more preferably 2 to 25% by mass, based on the total amount of the positive electrode active material layer.
The binder for the positive electrode is not particularly limited, and examples thereof include polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), fluorine-containing resins such as Polytetrafluoroethylene (PTFE), acrylic resins such as polymethyl acrylate (PMA) and polymethyl methacrylate (PMMA), polyvinyl acetate, Polyimide (PI), Polyamide (PA), polyvinyl chloride (PVC), polyether nitrile (PEN), Polyethylene (PE), polypropylene (PP), Polyacrylonitrile (PAN), acrylonitrile-butadiene rubber, styrene butadiene rubber, poly (meth) acrylic acid, carboxymethyl cellulose, hydroxyethyl cellulose, and polyvinyl alcohol. These binders may be used alone or in combination of two or more. Carboxymethyl cellulose and the like may be used in the form of sodium salt and the like.
The content of the binder for a positive electrode in the positive electrode active material layer is preferably 1.5 to 40% by mass, and more preferably 2.0 to 25% by mass, based on the total amount of the positive electrode active material layer.
The thickness of the positive electrode active material layer is not particularly limited, but is preferably 10 to 200 μm, and more preferably 50 to 150 μm.
Examples of the material constituting the positive electrode current collector include metals having conductivity such as copper, aluminum, titanium, nickel, and stainless steel, and among them, aluminum or copper is preferable, and aluminum is more preferable. The positive electrode current collector generally includes a metal foil, and the thickness thereof is not particularly limited, but is preferably 1 to 50 μm.
(cathode)
The negative electrode of the lithium ion secondary battery of the present invention preferably has a negative electrode current collector and a negative electrode active material layer stacked on the negative electrode current collector. The negative electrode active material layer usually contains a negative electrode active material and a binder for a negative electrode.
Examples of the negative electrode active material used in the negative electrode active material layer include carbon materials such as graphite and hard carbon, a composite of a tin compound and silicon and carbon, Si, and SiO of the general formulaxA compound represented by (wherein x is a number of 0.5 to 1.5), a Si-based material such as a Si-C-based nanocomposite or a Si-SiO-C-based nanocomposite, lithium, or the like, and among them, a carbon material and a Si-based material are preferably used, and graphite and a general formula SiO are more preferably usedx(wherein x is a number of 0.5 to 1.5).
The average particle size of the negative electrode active material is not particularly limited, but is preferably 0.5 to 50 μm, and more preferably 1 to 30 μm. The average particle diameter is a particle diameter at which the volume accumulation is 50% in the particle size distribution of the negative electrode active material determined by the laser diffraction scattering method (D50). The average particle diameter can be adjusted to a desired value by pulverizing the negative electrode active material using a known method such as a ball mill.
The content of the negative electrode active material in the negative electrode active material layer is preferably 50 to 98.5 mass%, more preferably 60 to 98 mass%, based on the total amount of the negative electrode active material layer.
The anode active material layer may contain a conductive assistant. As the conductive aid, a material having higher conductivity than the negative electrode active material is used, and specific examples thereof include carbon materials such as ketjen black, acetylene black, carbon nanotubes, and rod-like carbon.
When the conductive auxiliary is contained in the negative electrode active material layer, the content of the conductive auxiliary is preferably 1 to 30% by mass, more preferably 2 to 25% by mass, based on the total amount of the negative electrode active material layer.
The binder for the negative electrode is not particularly limited, and the same binder as the positive electrode binder described above can be used.
The content of the binder for a negative electrode in the negative electrode active material layer is preferably 1.5 to 40% by mass, and more preferably 2.0 to 25% by mass, based on the total amount of the negative electrode active material layer.
The thickness of the negative electrode active material layer is not particularly limited, but is preferably 10 to 200 μm, and more preferably 50 to 150 μm.
As a material constituting the negative electrode current collector, the same compounds as those used for the positive electrode current collector can be used, but aluminum or copper is preferable, and copper is more preferable. The negative electrode current collector generally includes a metal foil, and the thickness thereof is not particularly limited, but is preferably 1 to 50 μm.
(insulating layer)
The lithium ion secondary battery of the present invention includes an insulating layer disposed between a negative electrode and a positive electrode. The provision of the insulating layer can effectively prevent a short circuit between the positive electrode and the negative electrode. The insulating layer may hold an electrolyte described later. The insulating layer is a layer containing a polymer solid electrolyte.
< Polymer solid electrolyte >
The polymer solid electrolyte is a material mainly composed of a polymer and exhibiting ion conductivity. Examples of the polymer solid electrolyte include a dry polymer electrolyte and a gel polymer electrolyte. In the dry polymer electrolyte, ion conduction is considered to be essentially caused by the movement of the polymer skeleton. On the other hand, the gel-type polymer electrolyte causes ion conduction by an electrolytic solution contained in a large amount. From the viewpoint of high mechanical strength, a dry polymer electrolyte is a preferred polymer solid electrolyte.
From the viewpoint of high ion conductivity and high mechanical strength, and from the viewpoint of extensive studies on molecular design in the past, a preferred dry polymer electrolyte is a polyether electrolyte. The polymer constituting the matrix of the polyether electrolyte preferably has an ethylene oxide structure, a propylene oxide structure, or both of the above structures. Examples of the polymer constituting the matrix of the polyether electrolyte include polyethylene oxide, polypropylene oxide, ethylene oxide-propylene copolymer, and dimethylsiloxane-ethylene oxide copolymer. Further, there are also included comb polymers having polyether side chains having an ethylene oxide structure, copolymers of monomers other than ethylene oxide and ethylene oxide, crosslinked polyethylene oxide or polyether oligomers with a crosslinking agent, branched polyether polymers, and macromonomers having a molecular weight of about several hundred to several thousand, which are thermally or photopolymerized. These polymers may be used alone or in combination of two or more. From the viewpoint of high ion conductivity and high mechanical strength, the polymer constituting the matrix of the polyether electrolyte is more preferably a polymer having at least an ethylene oxide structure, and even more preferably polyethylene oxide. The ethylene oxide structure is composed of a basic unit composed of ethylene and oxygen.
From the viewpoint of improving the ion conductivity of the insulating layer, the content of the polymer solid electrolyte in the insulating layer is preferably 80 vol% or more, more preferably 90 vol% or more, further preferably 95 vol% or more, and particularly preferably 98 vol% or more. The upper limit of the content of the polymer solid electrolyte is 100 vol%.
From the viewpoint of further improving the ion conductivity of the polymer solid electrolyte, the polymer solid electrolyte preferably contains a lithium salt. For example, if the polymer solid electrolyte is a polyether electrolyte, it is considered that cations (lithium ions) of the lithium salt and isolated electron pairs of ether oxygen in the polymer constituting the matrix of the polyether electrolyte are complexed by ion-dipole interaction, and the lithium salt is dissolved in the polymer constituting the matrix. In addition, it is considered that a part of the dissolved lithium salt becomes a dissociated state, and the ion conductivity of the polyether electrolyte becomes higher.
Examples of the lithium salt used for the polymer solid electrolyte include LiCl, LiBr, LiI and LiClO4、LiBF4、LiPF6、LiAlCl4、LiSbF6、LiSCN、LiCF3SO3、LiAsF6、LiB10Cl10Lithium lower aliphatic carboxylate, lithium chloroborane and LiBPh4Lithium tetraphenylborate, LiTFSA (lithium trifluoromethanesulfonate), LiTFSI (lithium bistrifluoromethanesulfonimide), and the like. These lithium salts may be used alone or in combination of two or more. Among them, LiTFSA and LiTFSI are preferably used, and LiTFSI is more preferably used, from the viewpoint of being able to improve the dissociation of the lithium salt in the polymer solid electrolyte.
The amount of the lithium salt is preferably 1 to 100 parts by mass, more preferably 5 to 80 parts by mass, and still more preferably 10 to 50 parts by mass, per 100 parts by mass of the polymer constituting the matrix of the solid polymer electrolyte, from the viewpoints of ion conductivity of the solid polymer electrolyte and mechanical strength of the solid polymer electrolyte.
The thickness of the insulating layer is 3 to 7 μm. If the thickness of the insulating layer is less than 3 μm, fine short-circuits may not be sufficiently prevented. On the other hand, if the thickness of the insulating layer is larger than 7 μm, the distance between the electrodes may not be shortened, and the volume energy density of the lithium ion secondary battery may not be sufficiently increased. From the above viewpoint, the thickness of the insulating layer is preferably 4 to 7 μm, and more preferably 4 to 6 μm.
The porosity of the insulating layer is preferably 20% or less. When the porosity of the insulating layer is 20% or less, generation of lithium dendrites and the like in the pores of the insulating layer can be prevented, and fine short circuits of the insulating layer can be sufficiently suppressed. From the above viewpoint, the porosity of the insulating layer is preferably 15% or less, and more preferably 10% or less. The lower limit of the range of the porosity of the insulating layer is not particularly limited, and may be, for example, 0%. The porosity of the insulating layer can be measured by the method described in the later-described example. Since the insulating layer of the lithium ion secondary battery of the present invention contains a polymer solid electrolyte, a sufficient amount of lithium ions can pass through the insulating layer even if the porosity of the insulating layer is 20% or less. Therefore, it is possible to suppress an increase in the internal resistance of the lithium ion secondary battery due to a decrease in the porosity of the insulating layer.
The insulating layer may contain insulating fine particles as necessary. This can improve the mechanical strength of the insulating layer. From the viewpoint of reducing the porosity of the insulating layer, the content of the insulating fine particles in the insulating layer is preferably 10 vol% or less, more preferably 7 vol% or less, further preferably 5 vol% or less, and further preferably 1 vol% or less, based on the total 100 vol% of the polymer solid electrolyte and the insulating fine particles. From the viewpoint of more reliably reducing the porosity of the insulating layer, it is particularly preferable that the insulating layer does not contain insulating fine particles.
The insulating fine particles are not particularly limited as long as they are insulating, and may be any of organic particles and inorganic particles. Specific examples of the organic particles include particles made of organic compounds such as crosslinked polymethyl methacrylate, crosslinked styrene-acrylic acid copolymer, crosslinked acrylonitrile resin, polyamide resin, polyimide resin, poly (lithium 2-acrylamido-2-methylpropanesulfonate), polyacetal resin, epoxy resin, polyester resin, phenol resin, and melamine resin. Examples of the inorganic particles include silica, silicon nitride, alumina, boehmite, and alumina,Titanium dioxide, zirconium oxide, boron nitride, zinc oxide, tin dioxide, niobium oxide (Nb)2O5) Tantalum oxide (Ta)2O5) And particles of inorganic compounds such as potassium fluoride, lithium fluoride, clay, zeolite, and calcium carbonate. The inorganic particles may be particles composed of a known composite oxide such as a niobium-tantalum composite oxide or a magnesium-tantalum composite oxide.
The insulating fine particles may be particles using one of the above materials alone, or two or more kinds of particles may be used simultaneously. The insulating fine particles may be fine particles containing both an inorganic compound and an organic compound. For example, the inorganic-organic composite particles may be those in which inorganic oxide is coated on the surface of particles containing an organic compound.
Among the above, inorganic particles are preferable, and among them, alumina particles and boehmite particles are preferable.
The average particle size of the insulating fine particles is usually smaller than the average particle size of the electrode active material, and may be, for example, 0.001 to 0.5. mu.m, preferably 0.05 to 0.4. mu.m, and more preferably 0.1 to 0.3. mu.m. By setting the average particle diameter of the insulating fine particles within the above range, the mechanical strength of the insulating layer can be further improved. The average particle diameter is a particle diameter at which the volume accumulation is 50% in the particle size distribution of the insulating fine particles obtained by the laser diffraction scattering method (D50). In addition, one kind of insulating fine particles having an average particle diameter within the above range may be used alone, or two kinds of insulating fine particles having different average particle diameters may be used in combination.
< method for Forming insulating layer >
The insulating layer can be formed by a known method using the composition for an insulating layer. For example, the composition for an insulating layer can be prepared by mixing a raw material monomer of a polymer constituting a matrix of a polymer solid electrolyte, a lithium salt, a solvent, and additives such as a photopolymerization initiator and a curing agent. The film-like insulating layer can be formed by applying the composition for an insulating layer to a release sheet such as a fluorine-based resin sheet, drying and polymerizing the composition, and then releasing the composition from the release sheet. Alternatively, the composition for an insulating layer may be applied to an electrode, dried, and polymerized to form a film-like insulating layer.
In addition, the composition for an insulating layer may be prepared by mixing a polymer constituting a matrix of the polymer solid electrolyte, a lithium salt, and a solvent, and in this case, the composition for an insulating layer may be applied to a release sheet, dried, and then released from the release sheet to form a film-shaped insulating layer. Alternatively, the composition for an insulating layer may be applied to an electrode and dried to form a film-like insulating layer.
When the insulating layer contains insulating fine particles, the composition for an insulating layer contains insulating fine particles. Also, the insulating layer can be formed by the same method as described above.
The pores of the insulating layer can be formed by, for example, a gas mixing method. For example, when raw materials of the composition for an insulating layer are mixed, the mixture is stirred at a high speed, air is mixed, and the mixture mixed with air is dried and polymerized, or dried, thereby forming pores in the insulating layer. The porosity of the insulating layer can be adjusted by adjusting the amount of air to be mixed. The porosity of the insulating layer may be adjusted by performing a defoaming treatment after sufficiently mixing air.
The film-like insulating layer may be disposed only between the positive electrode and the negative electrode. In this case, the film-like insulating layer may be in contact with at least one of the positive electrode and the negative electrode, or may not be in contact with the electrode.
The film-like insulating layer may contact the positive electrode active material layer when contacting the positive electrode. In addition, if a conventional insulating layer is disposed on the positive electrode active material layer, the film-shaped insulating layer may be in contact with the conventional insulating layer. On the other hand, the film-like insulating layer may be in contact with the negative electrode active material layer when contacting the negative electrode. In addition, if a conventional insulating layer is disposed on the negative electrode active material layer, the film-like insulating layer may be in contact with the conventional insulating layer. The conventional insulating layer is formed by applying a slurry for an insulating layer containing insulating fine particles, a binder, and a solvent but not containing a polymer solid electrolyte onto an electrode active material layer and drying the slurry.
Further, the film-like insulating layer may be crimped on at least one of the positive electrode and the negative electrode.
Further, the insulating layer in the form of a coating film may be coated on the surface of one of the positive electrode and the negative electrode. At this time, a coating-like insulating layer may be formed on the surface of the active material layer of the positive electrode or the active material layer of the negative electrode. In addition, if a conventional insulating layer is provided on the positive electrode active material layer or the negative electrode active material layer, a film-like insulating layer may be formed on the surface of the conventional insulating layer.
(electrolyte)
The lithium ion secondary battery of the present invention includes an electrolyte solution. The electrolyte is not particularly limited as long as a known electrolyte for a lithium ion secondary battery is used.
The electrolyte solution may, for example, be an electrolyte solution containing an organic solvent and an electrolyte salt. Examples of the organic solvent include polar solvents such as ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, γ -butyrolactone, sulfolane, dimethyl sulfoxide, acetonitrile, dimethylformamide, dimethylacetamide, 1, 2-dimethoxyethane, 1, 2-diethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, 1, 3-dioxolane, and methyl acetate, and mixtures of two or more of the above solvents. As the electrolyte salt, LiClO may be mentioned4、LiPF6、LiBF4、LiAsF6、LiSbF6、LiCF3CO2、LiPF6SO3、LiN(SO2CF3)2、LiN(SO2CF2CF3)2、LiN(COCF3)2And LiN (COCF)2CF3)2Lithium bis (oxalato) borate (LiB (C)2O4)2) And the like lithium-containing salts. Further, lithium salt of an organic acid-boron trifluoride complex and LiBH are exemplified4And complex hydrides thereof. These salts or complexes may be used singly or in admixture of two or more kinds.
The electrolyte solution may be present between the negative electrode and the positive electrode, and for example, the electrolyte solution is filled in a battery cell in which the negative electrode, the positive electrode, and the insulating layer are accommodated. Alternatively, the electrolyte may be applied to the negative electrode or the positive electrode and disposed between the negative electrode and the positive electrode, for example.
The lithium ion secondary battery may have a multilayer structure in which a plurality of negative electrodes and positive electrodes are stacked. In this case, the negative electrodes and the positive electrodes may be alternately arranged in the stacking direction. The insulating layer may be disposed between each negative electrode and each positive electrode.
(conventional insulating layer)
The positive electrode or the negative electrode may be provided with a conventional insulating layer on the surface of the electrode active material layer. This can more effectively prevent a short circuit between the positive electrode and the negative electrode.
The conventional insulating layer contains insulating fine particles and a binder, but does not contain a polymer solid electrolyte. The conventional insulating layer is a layer formed by bonding insulating fine particles with an adhesive, and has a porous structure.
The thickness of the conventional insulating layer is preferably 1 to 10 μm. By setting the thickness of the conventional insulating layer to 10 μm or less, the volumetric energy density of the lithium ion secondary battery can be improved. Further, by setting the thickness of the conventional insulating layer to 1 μm or more, the coating rate of the conventional insulating layer with respect to the electrode active material layer is increased, thereby improving the effect of suppressing short-circuiting. From the viewpoint of these volumetric energy density and short circuit suppression effect, the thickness of the conventional insulating layer is more preferably 1.5 to 8.5 μm, and still more preferably 3 to 7 μm. In the lithium ion secondary battery of the present invention, since the insulating layer containing the polymer solid electrolyte is provided, the conventional insulating layer can be thinned.
As described above, the conventional insulating layer has a porous structure, but the porosity thereof is preferably 50 to 90%. By setting the porosity to 90% or less, the coverage of the electrode active material layer by the conventional insulating layer is increased, thereby improving the effect of suppressing short circuits. By setting the porosity to 50% or more, an increase in internal resistance of the lithium ion secondary battery can be suppressed. From the viewpoint of suppressing the short-circuit effect and the internal resistance of the lithium ion secondary battery, the porosity of the conventional insulating layer is more preferably 60 to 85%, and still more preferably 70 to 80%.
The insulating fine particles used in the conventional insulating layer are the same particles as those used in the insulating layer described above. Therefore, the description of the insulating fine particles used in the conventional insulating layer is omitted here.
The content of the insulating fine particles contained in the conventional insulating layer is preferably 15 to 95% by mass, more preferably 40 to 90% by mass, and further preferably 60 to 85% by mass, based on the total amount of the insulating layer. When the content of the insulating fine particles is within the above range, the conventional insulating layer can have a uniform porous structure and can provide appropriate insulating properties.
As the binder used in the conventional insulating layer, the same one as the binder for the positive electrode described above can be used.
The content of the binder contained in the conventional insulating layer is preferably 5 to 50% by mass, more preferably 10 to 45% by mass, and further preferably 15 to 40% by mass, based on the total amount of the insulating layer. Within this range, the conventional insulating layer can have a uniform porous structure and can provide appropriate insulating properties.
The conventional insulating layer may contain any other components than the insulating fine particles and the binder within a range not to impair the effects of the present invention. However, the total content of the insulating fine particles and the binder in the total mass of the conventional insulating layer is preferably 85 mass% or more, and more preferably 90 mass% or more.
The distance between the positive electrode active material layer and the negative electrode active material layer can be further shortened without providing a conventional insulating layer on the electrode. Therefore, from the viewpoint of the volumetric energy density of the lithium ion secondary battery, it is preferable not to provide a conventional insulating layer on the electrode.
< method for producing lithium ion Secondary Battery >
The method for producing the lithium ion secondary battery of the present invention is not particularly limited, and the lithium ion secondary battery of the present invention can be produced, for example, by preparing a positive electrode, a negative electrode, and an insulating layer and producing them by the following method.
(preparation of Positive electrode)
The positive electrode can be obtained by applying the composition for a positive electrode active material layer to one or both surfaces of a positive electrode current collector and drying the composition. The composition for a positive electrode active material layer is a slurry-like material containing a positive electrode active material, a positive electrode binder, and at least one solvent selected from an organic solvent and water.
The positive electrode active material layer can be formed by applying the composition for a positive electrode active material layer on a substrate other than the positive electrode current collector and drying the composition. As the substrate other than the positive electrode current collector, a known separator can be mentioned. The positive electrode active material layer formed on the substrate may be peeled off from the substrate and transferred onto the positive electrode current collector. The positive electrode active material layer formed on the positive electrode current collector or the substrate is preferably subjected to pressure pressing.
(preparation of negative electrode)
The negative electrode can be produced by the same method as that for producing the positive electrode described above. That is, the positive electrode may be changed to the negative electrode in the above-described process of preparing the positive electrode.
(preparation of insulating layer)
The insulating layer can be prepared by the method described in the above-mentioned "method for forming an insulating layer".
And sequentially laminating the prepared positive electrode, the insulating layer and the negative electrode. A plurality of positive electrodes and a plurality of negative electrodes may be prepared, and the insulating layers may be stacked so as to be disposed between the positive electrodes and the negative electrodes. The stacked positive electrode, negative electrode, and insulating layer are generally housed in a battery cell. The battery cell may be any one of a square shape, a cylindrical shape, a laminate type, and the like. Further, after the electrolyte is injected into the battery cell, the battery cell is sealed.
Examples
Hereinafter, the present invention will be described in further detail with reference to examples, but the present invention is not limited to the following examples.
The obtained electrode for a lithium ion secondary battery was evaluated by the following evaluation method.
(whether or not it can be charged and discharged)
In a temperature environment of 45 ℃, it was examined whether or not a voltage drop occurred after the voltage at the time of full charge was 60 minutes after full charge in the case of performing the primary charge. In addition, in a temperature environment of 45 ℃, the charge-discharge efficiency of the second charge-discharge was investigated, and the charge-discharge efficiency was calculated by dividing the discharge capacity by the charge capacity. Then, evaluation was performed according to the following criteria.
Can be charged and discharged: can be charged, has a voltage drop amplitude of 0.1V or less, and has a charge-discharge efficiency of 95% or more
And (3) incapability of charging and discharging: the charging is impossible, or the voltage drop amplitude is more than 0.1V, or the charging and discharging efficiency is less than 95 percent
(internal resistance)
The AC resistance value (measurement unit: Ω) of the lithium ion secondary battery at 1kHz was measured at room temperature (25 ℃) using a measuring instrument for measuring AC resistance (trade name: RM3542A, manufactured by Nikkiso Co., Ltd.).
(porosity)
The cross section of the insulating layer is exposed by an ion milling method. Subsequently, the exposed cross section was observed at a magnification capable of observing the entire insulating layer using an FE-SEM (electric field emission scanning electron microscope), and an image of the insulating layer was obtained. The multiple is 5000 to 25000 times. Subsequently, using the Image analysis software "Image J", the obtained Image was subjected to binarization processing so that the real part of the insulating layer was displayed in black and the void part of the insulating layer was displayed in white. Then, the ratio of the area of the white portion was measured. The ratio of the area of this white portion was set as the porosity (%) of the insulating layer. The porosity of the insulating layer was measured in the same manner.
(thickness of insulating layer)
The thickness of the insulating layer was measured from the SEM image.
[ example 1]
(preparation of Positive electrode)
100 parts by mass of an NCA-based oxide (average particle diameter 10 μm) as a positive electrode active material, 4 parts by mass of acetylene black as a conductive auxiliary agent, 4 parts by mass of polyvinylidene fluoride as an electrode binder, and N-methylpyrrolidone (NMP) as a solvent were mixed to obtain a slurry for a positive electrode active material layer in which the solid content concentration was adjusted to 60 mass%. This slurry for a positive electrode active material layer was applied to an aluminum foil having a thickness of 15 μm as a positive electrode current collector, predried, and then vacuum-dried at 120 ℃. Subsequently, the positive electrode current collector coated with the slurry for a positive electrode active material layer was pressure-pressed with a roll at a linear pressure of 400kN/m, followed by die-cutting into a circular shape with an electrode size of 14mm in diameter, which was taken as a positive electrode having a positive electrode active material layer. The thickness of the positive electrode active material layer was 50 μm.
(preparation of negative electrode)
100 parts by mass of graphite (average particle diameter 10 μm) as a negative electrode active material, 1.5 parts by mass of a sodium salt of carboxymethyl cellulose (CMC) as an electrode binder, and 1.5 parts by mass of Styrene Butadiene Rubber (SBR) were mixed with water as a solvent, and a slurry for a negative electrode active material layer with a solid content adjusted to 50 mass% was obtained. The slurry for a negative electrode active material layer was applied to a copper foil having a thickness of 12 μm as a negative electrode current collector, and vacuum-dried at 100 ℃. Subsequently, the anode current collector coated with the slurry for an anode active material layer was subjected to pressure pressing with a roll under a line pressure of 500kN/m, followed by die cutting into a circular shape having an electrode size of 16mm in diameter, which was taken as an anode having an anode active material layer. The thickness of the negative electrode active material layer was 50 μm.
(preparation of electrolyte solution)
LiPF as an electrolyte salt was dissolved in a solvent in which Ethylene Carbonate (EC) and diethyl carbonate (DEC) were mixed in a volume ratio of 3:7 (EC: DEC)6The electrolyte was prepared so as to reach 1 mol/liter.
(preparation of insulating layer)
To a solution obtained by dissolving 2.5 parts by mass of LiTFSI (lithium bistrifluoromethylsulfonimide) in 16 parts by mass of acetonitrile was added 0.1 part by mass of a photopolymerization initiator (trade name: Esacure KTO46, manufactured by Sartomer corporation) to prepare a lithium-containing salt solution. Subsequently, 10 parts by mass of ethylene oxide was added to the lithium-containing salt solution, and the mixture was stirred while mixing with air to prepare a polymer electrolyte solution. The resulting polymer electrolyte solution was applied to a sheet made of teflon (registered trademark), and dried at a drying temperature of 60 ℃ for 30 minutes under a reduced pressure atmosphere. Another teflon (registered trademark) sheet was placed on the dried polymer electrolyte, and the polymer electrolyte was sandwiched between two teflon (registered trademark) sheets. Subsequently, ultraviolet rays were irradiated onto both surfaces of the polymer electrolyte sandwiched between two teflon (registered trademark) sheets through the teflon (registered trademark) sheets to cure the polymer electrolyte, thereby producing a polymer solid electrolyte membrane. The polymer constituting the matrix of the polymer solid electrolyte membrane was polyethylene oxide, and the lithium salt was LiTFSI. Subsequently, the polymer solid electrolyte membrane was die-cut into a circular shape having an electrode diameter of 16mm, to prepare an insulating layer. The thickness (d) of the insulating layer was 5 μm.
The porosity of the insulating layer was adjusted to five values of 0%, 5%, 10%, 15%, and 20% by changing the amount of air mixed during stirring of the lithium salt-containing solution containing ethylene oxide and by performing a defoaming treatment after stirring.
(production of Battery)
The positive electrode, the insulating layer, and the negative electrode were disposed on the battery characteristic evaluation jig 100 shown in fig. 1, and the electrolyte was injected, thereby preparing a battery for characteristic evaluation. Specifically, between the negative electrode member 106 and the positive electrode member 107, the negative electrode 108, the insulating layer 109, the electrode guide 110, the positive electrode 111, the electrode holder 112, and the spring 113 are arranged in this order from the negative electrode member 106 side on the jig 100 for battery characteristic evaluation. Subsequently, the above electrolyte solution was injected into the battery characteristic evaluation jig 100, and a lithium ion secondary battery for characteristic evaluation was prepared.
Comparative example 1
94 parts by mass of alumina particles (product name: low soda alumina, average particle diameter 500nm, manufactured by Nippon light metals Co., Ltd.) as insulating fine particles were added to 6 parts by mass of a polyvinylidene fluoride solution (manufactured by Kureha, Ltd., product name: L #1710, 10% by mass solution, solvent: NMP) while applying a moderate shearing force, and mixed and dispersed, thereby obtaining a slurry.
To this slurry, a prescribed amount of NMP was further added, and the mixture was slowly stirred with a stirrer for 30 minutes to obtain a slurry for an insulating layer.
The slurry for an insulating layer was coated on the surface of the positive electrode active material layer of the positive electrode after press pressing and before punching using a gravure coater, and the coated film was dried at 90 ℃ for 1 minute, thereby preparing a positive electrode plate having a conventional insulating layer on the surface of the positive electrode active material layer. The positive electrode plate having the conventional insulating layer was punched out into a circular shape having an electrode size of 14mm in diameter to prepare a positive electrode having the conventional insulating layer. The thickness (d) of the insulating layer was 5 μm. A lithium ion secondary battery for evaluation of characteristics of comparative example 1 was prepared in the same manner as in example 1, except that the positive electrode having the insulating layer was used and the insulating layer 109 was not disposed at the time of manufacturing the battery.
The porosity of the insulating layer was adjusted to five values of 0%, 5%, 10%, 15%, and 20% by changing the amount of the alumina particles and the polyvinylidene fluoride solution in the slurry. When the porosity is 25% or more, the battery cannot be charged and discharged, and the internal resistance of the battery cannot be measured.
Fig. 2 shows evaluation results of the lithium ion secondary batteries of example 1 and comparative example 1.
As shown in the results of fig. 2, if the porosity of the insulating layer is reduced in order to make the insulating layer thinner and prevent charging and discharging, the internal resistance of the lithium ion secondary battery becomes very high, and thus the thickness cannot be set to 3 to 7 μm by the conventional insulating layer. On the other hand, in the insulating layer containing the polymer solid electrolyte, the internal resistance of the lithium ion secondary battery does not increase excessively even if the thickness of the insulating layer is set to 3 to 7 μm, and therefore the thickness of the insulating layer can be set to 3 to 7 μm.
Description of the figures
Jig for evaluating 100 cell characteristics
106 negative electrode body
107 positive electrode body
108 negative electrode
109 insulating layer
110 electrode lead
111 positive electrode
112 electrode holder
113 spring

Claims (5)

1. A lithium ion secondary battery comprising a positive electrode, a negative electrode, an insulating layer disposed between the positive electrode and the negative electrode, and an electrolytic solution,
the insulating layer is a layer containing a polymer solid electrolyte,
the thickness of the insulating layer is 3-7 mu m.
2. The lithium ion secondary battery according to claim 1, wherein the insulating layer has a porosity of 20% or less.
3. The lithium ion secondary battery according to claim 1 or 2, wherein the polymer solid electrolyte is a polyether electrolyte.
4. The lithium ion secondary battery according to claim 3, wherein the polymer constituting the matrix of the polyether electrolyte is a polymer having an ethylene oxide structure.
5. The lithium ion secondary battery according to any one of claims 1 to4, wherein the polymer solid electrolyte contains a lithium salt.
CN202080007188.6A 2019-04-02 2020-04-01 Lithium ion secondary battery Pending CN113302772A (en)

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