WO2019188590A1 - Nonaqueous electrolyte secondary battery and method for producing nonaqueous electrolyte secondary battery - Google Patents

Abstract

The present invention relates to a nonaqueous electrolyte secondary battery (10) which is provided with: a positive electrode (1) which comprises a positive electrode collector (11) and a positive electrode active material layer (12) that is arranged on the surface of the positive electrode collector (11); a negative electrode (3) which comprises a negative electrode collector (31) and a negative electrode active material layer (32) that is arranged on the surface of the negative electrode collector (31); a nonaqueous electrolyte which contains lithium ions; a separator (2) which is arranged between the positive electrode (1) and the negative electrode (3); and a particle layer (4) which is arranged on one or both of the surface of the positive electrode (1) and the surface of the negative electrode (3). This nonaqueous electrolyte secondary battery (10) is configured such that the ratio of the specific surface areas A of the particles contained in the particle layer (4) to the specific surface areas B of the particles contained in the positive electrode active material layer (12), namely A/B is more than 0.2 but less than 1.5.

Description

Non-aqueous electrolyte secondary battery and method for producing non-aqueous electrolyte secondary battery

The present invention relates to a non-aqueous electrolyte secondary battery and a method for manufacturing a non-aqueous electrolyte secondary battery.
This application claims priority based on Japanese Patent Application No. 2018-058199 filed in Japan on March 26, 2018, the contents of which are incorporated herein by reference.

Lithium ion secondary batteries are characterized by higher energy density and electromotive force than lead acid batteries and nickel metal hydride batteries. For this reason, it is widely used as a power source for mobile phones, notebook computers and the like that are required to be small and light. In lithium ion secondary batteries, those using a non-aqueous electrolyte solution in which a lithium salt is dissolved in an organic solvent are used as an electrolyte.
A lithium ion secondary battery is, for example, a positive electrode in which a positive electrode active material layer is provided on a positive electrode current collector, a negative electrode in which a negative electrode active material layer is provided on a negative electrode current collector, and a position between the positive electrode and the negative electrode. And a non-aqueous electrolyte solution are provided in the exterior body.

There is a lithium ion secondary battery having a porous insulating layer on the surface of an electrode. For example, in the example of Patent Document 1, a nonaqueous electrolyte secondary battery including a particle layer having a thickness of 2 μm on the surface of the positive electrode is described. The particle layer includes inorganic particles, polycarboxylate, and styrene butadiene rubber.
The particle layer provided on the surface of the positive electrode functions as a filter for trapping non-aqueous electrolyte decomposition products generated by the reaction at the positive electrode and elements (elements other than lithium) eluted from the positive electrode active material. For this reason, by having the said particle layer, it can prevent that elements other than the said decomposition product or lithium precipitate on the negative electrode surface or a separator.

Japanese Patent No. 5213534

When this inventor examined, when the particle layer of patent document 1 was provided in the surface of an electrode, the mechanical strength of a lithium ion secondary battery was low, and the problem that cycling characteristics were inferior was discovered.
An object of the present invention is to provide a non-aqueous electrolyte secondary battery having a particle layer on the surface of an electrode, high mechanical strength, and excellent cycle characteristics, and a method for producing the non-aqueous electrolyte secondary battery.

The present invention has the following aspects.
[1] A positive electrode including a positive electrode current collector and a positive electrode active material layer positioned on the surface of the positive electrode current collector, a negative electrode current collector, and a negative electrode active material layer positioned on the surface of the negative electrode current collector A negative electrode, a nonaqueous electrolyte containing lithium ions, a separator positioned between the positive electrode and the negative electrode, and a particle layer positioned on the surface of one or both of the positive electrode and the negative electrode. A nonaqueous electrolyte secondary battery in which A / B, which is a ratio of specific surface area A of particles contained in the particle layer to specific surface area B of particles contained in the material layer, is more than 0.2 and less than 1.5.
[2] The nonaqueous electrolyte secondary battery according to [1], wherein the particle layer includes inorganic particles.
[3] The nonaqueous electrolyte secondary battery according to [2], wherein the inorganic particles are at least one selected from the group consisting of magnesia particles, titania particles, alumina particles, silica particles, and lithium phosphate particles.
[4] The nonaqueous electrolyte secondary battery according to [2] or [3], wherein the inorganic particles have an average particle size of 1.3 μm or less.
[5] The nonaqueous electrolyte secondary battery according to any one of [1] to [4], wherein at least a part of the particle layer is present on the surface of the positive electrode current collector.
[6] The nonaqueous electrolyte secondary battery according to any one of [1] to [4], wherein at least a part of the particle layer is present on the surface of the negative electrode current collector.
[7] The nonaqueous electrolyte secondary battery according to any one of [1] to [6], wherein the particle layer has a thickness of 2 to 20 μm.
[8] The nonaqueous electrolyte secondary battery according to any one of [1] to [7], wherein the particle layer is located on a surface of the positive electrode.
[9] The method for producing a nonaqueous electrolyte secondary battery according to any one of [1] to [8], wherein a coating liquid containing particles and a binder is applied to either the positive electrode or the negative electrode, A ′ / B which is a ratio of the specific surface area A ′ of the particles contained in the coating liquid to the specific surface area B of the particles contained in the positive electrode active material layer is 0. The manufacturing method of the nonaqueous electrolyte secondary battery which is more than 2 and less than 1.5.

The nonaqueous electrolyte secondary battery of the present invention has high mechanical strength and excellent cycle characteristics.

It is a schematic diagram of the cross section of the nonaqueous electrolyte secondary battery which concerns on one Embodiment of this invention. It is a schematic diagram explaining the measuring method of piercing strength. It is a schematic diagram of the cross section of the nonaqueous electrolyte secondary battery which concerns on one Embodiment of this invention. It is a schematic diagram of the cross section of the nonaqueous electrolyte secondary battery which concerns on one Embodiment of this invention.

Hereinafter, embodiments of a nonaqueous electrolyte secondary battery according to the present invention will be described with reference to the drawings. Note that the drawings used in the following description may show the characteristic parts in an enlarged manner for the sake of convenience in order to make the characteristics easy to understand. There is. In addition, the materials, dimensions, and the like exemplified in the following description are examples, and the present invention is not limited to them, and can be appropriately changed and implemented without changing the gist thereof.

[Nonaqueous electrolyte secondary battery]
FIG. 1 is a schematic diagram of a cross section according to an embodiment of a nonaqueous electrolyte secondary battery (hereinafter, also simply referred to as a secondary battery) of the present invention. A secondary battery 10 in FIG. 1 includes a positive electrode 1, a separator 2, a negative electrode 3, a particle layer 4, and an exterior body 5.
The positive electrode 1 and the negative electrode 3 have a flat plate shape that is rectangular in plan view. The positive electrode 1 and the negative electrode 3 are opposed to each other. A separator 2 is located between the positive electrode 1 and the negative electrode 3 facing each other. Thus, the negative electrode 3, the separator 2, the positive electrode 1, the separator 2, and the negative electrode 3 are positioned in this order to form the laminate 20.
The laminate 20 and the non-aqueous electrolyte are located in the exterior body 5. The particle layer 4 is located on the surface of the positive electrode 1. The particle layer 4 is preferably located on the surface of the positive electrode 1 facing the negative electrode 3.

The positive electrode 1 has a plate-like positive electrode current collector 11 and positive electrode active material layers 12 located on both sides thereof. The positive electrode active material layer 12 is located on a part of the surface of the positive electrode current collector 11. A positive electrode current collector exposed portion 13 where the positive electrode active material layer 12 does not exist is located at the edge of the surface of the positive electrode current collector 11. A lead wire (tab) (not shown) is connected to the positive electrode current collector 11 at an arbitrary portion of the exposed edge. The particle layer 4 covers the positive electrode active material layer 12, and a part of the particle layer 4 is located on the surface of the positive electrode current collector exposed portion 13.

The negative electrode 3 has a plate-shaped negative electrode current collector 31 and negative electrode active material layers 32 located on both sides thereof. The negative electrode active material layer 32 is located on a part of the surface of the negative electrode current collector 31. A negative electrode current collector exposed portion 33 where the negative electrode active material layer 32 does not exist is located at the edge of the surface of the negative electrode current collector 31. A lead-out wiring (tab) (not shown) is connected to the negative electrode current collector 31 at an arbitrary portion of the exposed edge.

In the secondary battery 10 of the present embodiment, the laminated body 20 and a non-aqueous electrolyte (not shown) are accommodated in the exterior body 5 and sealed.

<Particle layer>
The particle layer includes particles and a binder. When the particle layer does not contain other components described later, the particles are referred to as particles A, and when the particle layer includes other components described later, a mixture of the particles and other components is referred to as particles A.
The particle layer 4 in the secondary battery 10 is infiltrated with the electrolytic solution. As the electrolytic solution infiltrates into the particle layer 4, a slight gap is formed between the binder and the particles. Since lithium ions and the like permeate through the particle layer 4 through the gap, the particle layer 4 has ion conductivity.

(particle)
The particles contained in the particle layer are preferably particles that do not occlude and release lithium ions. “Occlusion and release of lithium ions” means that a lithium ion secondary battery including the positive electrode 1 and the negative electrode 3 occludes or releases lithium ions to the extent that it interferes with the charge / discharge operation. . The particles may be inorganic particles or organic particles.

A / B, which is the ratio of the specific surface area A of the particles A to the specific surface area B of particles (hereinafter also referred to as “particles B”) contained in a positive electrode active material layer described later, is preferably more than 0.2 and less than 1.5. More than 0.3 and less than 1.1 is more preferable, and more than 0.3 and less than 0.8 is more preferable. If the A / B exceeds the lower limit of the range, the mechanical strength and cycle characteristics of the secondary battery are improved, and the increase in internal resistance is suppressed. When the A / B is less than the upper limit of the range, the mechanical strength and cycle characteristics of the secondary battery are improved.
In the present specification, the “specific surface area” is a BET specific surface area measured by a BET gas adsorption method using nitrogen as an adsorption gas.

The specific surface area of the particles A so long as it has the effect of the present invention is not particularly limited, but is preferably 1 ~ ^30m 2 / g, more preferably 2 ~ ^25m 2 / g, more preferably 3 ~ ^20m 2 / g, 3 ˜8 m ² / g is particularly preferred.

It is preferable that the particles A include inorganic particles in that the peel strength of the particle layer and the mechanical strength of the secondary battery become higher.

The inorganic particles may be particles made of an inorganic material that does not occlude and release lithium ions. One kind of inorganic particles in the particle layer may be used, or two or more kinds may be used in combination.
The inorganic particles are preferably inorganic oxide particles, for example. The inorganic oxide particles include at least one selected from the group consisting of magnesia (magnesium oxide) particles, titania (titanium oxide) particles, alumina (aluminum oxide) particles, silica (silicon dioxide) particles, and lithium phosphate particles. Preferably, at least one selected from the group consisting of magnesia particles, titania particles, alumina particles, and lithium phosphate particles is more preferable, and at least one selected from the group consisting of magnesia particles, titania particles, and alumina particles is more preferable.

The average particle diameter of the inorganic particles is preferably 1.3 μm or less, more preferably 1.0 μm or less, and even more preferably 0.8 μm or less from the viewpoint of improving the mechanical strength of the secondary battery and suppressing the increase in internal resistance. Preferably, 0.7 μm or less is particularly preferable, and 0.6 μm or less is most preferable.
The lower limit of the average particle diameter of the inorganic particles is not particularly limited as long as it has the effect of the present invention, but is preferably 0.1 μm or more, and more preferably 0.3 μm or more.
In addition, the upper limit value and lower limit value mentioned above can be combined arbitrarily.
The combination of the upper limit value and the lower limit value is preferably 0.1 μm or more and 1.3 μm or less, more preferably 0.1 μm or more and 1.0 μm or less, further preferably 0.3 μm or more and 0.8 μm or less, and 0.3 μm or more and 0 or less. 0.7 μm or less is particularly preferable, and 0.3 μm or more and 0.6 μm or less is most preferable.

The particles A may contain particles other than inorganic particles, for example, organic particles. When the particle layer contains organic particles, the internal resistance of the secondary battery can be further reduced.
The organic particles may be particles made of an organic material that does not occlude and release lithium ions. The organic particles in the particle layer may be one kind or a combination of two or more kinds.
Examples of organic materials constituting the organic particles include, for example, poly α-olefin, polyacrylic acid, polyacrylic acid ester, polymethacrylic acid, polymethacrylic acid ester, polysilicone (polymethylsilsesquioxane, etc.), polystyrene, Polydivinylbenzene, styrene-divinylbenzene copolymer, polyamide, polyimide, polycarbonate, urea resin, urethane resin, melamine resin, phenol resin, benzoguanamine-formaldehyde condensate, polysulfone, polyacrylonitrile, polyacetal, thermoplastic polyimide, etc. .
The upper limit of the average particle diameter of the organic particles is preferably 2 μm or less, and more preferably 1 μm or less in consideration of a suitable thickness of the particle layer. The lower limit of the average particle diameter of the organic particles is preferably 0.01 μm or more, more preferably 0.1 μm or more from the viewpoint of dispersibility with respect to the dispersion medium.
In addition, the upper limit value and lower limit value mentioned above can be combined arbitrarily.
The combination of the upper limit value and the lower limit value is preferably 0.01 μm or more and 2 μm or less, and more preferably 0.1 μm or more and 1 μm or less.

The average particle size of the inorganic particles and organic particles is the volume from the small diameter side of the particle size distribution obtained by measurement with a laser diffraction particle size distribution analyzer (for example, Partica LA-960 manufactured by Horiba, Ltd., SALD-3000J manufactured by Shimadzu). It is the particle size (that is, the volume average particle size) at which the cumulative total is 50%. Details of the measurement conditions will be described later in Examples.

The content of inorganic particles with respect to all particles (100 parts by mass) contained in the particle layer is preferably 50 to 100 parts by mass, more preferably 60 to 100 parts by mass, still more preferably 70 to 100 parts by mass, and more preferably 80 to 100 parts by mass. Part by mass is particularly preferred.
When the content of the inorganic particles is at least the lower limit of the above range, the mechanical strength of the secondary battery and the adhesive strength to the separator are further increased. When the content of the inorganic particles is not more than the upper limit of the above range, the liquid retention of the insulating layer is increased, and the increase in internal resistance of the secondary battery can be further reduced.

When the particle layer contains not only inorganic particles but also organic particles, the content of organic particles relative to all particles (100 parts by mass) contained in the particle layer is preferably 50 parts by mass or less, more preferably 40 parts by mass or less, 30 parts by mass or less is more preferable, and 20 parts by mass or less is particularly preferable.
When the content of the organic particles is not more than the above upper limit value, the internal resistance of the secondary battery can be further reduced while maintaining the peel strength of the particle layer and the mechanical strength of the secondary battery.
The lower limit of the content of the organic particles is not particularly limited as long as it has the effect of the present invention, but is, for example, more than 0 parts by mass.
That is, the content of the organic particles is preferably more than 0 parts by mass and 50 parts by mass or less, more preferably more than 0 parts by mass and 40 parts by mass or less, further preferably more than 0 parts by mass and 30 parts by mass or less, and more than 0 parts by mass and more than 20 parts by mass. Part or less is particularly preferable.

The total content of the particles in the particle layer is preferably 70 to 98% by mass, more preferably 85 to 95% by mass with respect to the total mass (100% by mass) of the particle layer.
When the total content of the particles is not less than the lower limit of the above range, the mechanical strength and ionic conductivity of the secondary battery are increased, and the increase in cell resistance is reduced. When the total content of the particles is not more than the upper limit of the above range, the peel strength of the particle layer is further increased.

(binder)
The binder is a polymer that binds the particles to each other in the particle layer.
As the binder constituting the particle layer, those used as binders for electrodes of non-aqueous secondary batteries can be applied. For example, polyacrylic acid (PAA), polyacrylic acid lithium (PAALi), polyvinylidene fluoride (PVDF) , Polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), styrene butadiene rubber (SBR), polyvinyl alcohol (PVA), polyethylene oxide (PEO), polyethylene glycol (PEG), carboxymethyl cellulose (CMC), poly Examples include acrylonitrile (PAN) and polyimide (PI).
The molecular weight of the binder is appropriately set in consideration of the dispersibility and binding properties of the particles.
A binder may be used individually by 1 type and may use 2 or more types together. When using 2 or more types together, the combination and ratio may be appropriately selected according to the purpose.

As the binder, an aqueous binder dispersible in water is preferable. Specific examples of the aqueous binder include CMC, PAA, PAALi, PVA, PEO, and PEG.
When the aqueous binder is used, the dissolution resistance of the particle layer with respect to the non-aqueous electrolyte is improved, and the peel strength when the particle layer is immersed in the electrolyte is further increased.

The binder content is preferably 1.5 to 20 parts by mass and more preferably 4 to 20 parts by mass with respect to 100 parts by mass of the particles in the particle layer.
When the content of the binder is not less than the lower limit of the above range, the binding force and peel strength between the particles are further increased. When the content of the binder is not more than the upper limit of the above range, the mechanical strength of the secondary battery is increased while reducing the cell resistance.

As the thickness of the particle layer increases up to a certain thickness, the peel strength of the particle layer increases. On the other hand, when the particle layer exceeds a certain thickness, the peel strength of the particle layer decreases and the cell resistance tends to increase.
The thickness of the particle layer is preferably 1.5 to 20 μm, more preferably 2 to 15 μm, and particularly preferably 2 to 10 μm.
In the present specification, the “thickness of the particle layer” is a value obtained by observing the thickness of any 10 positions in the cross section of the particle layer with a scanning electron microscope (SEM) and calculating the average.

When the particle layer contains inorganic particles, the relationship between the particle layer thickness T (μm) and the average particle size D (μm) of the inorganic particles is expressed by (average particle size D) / (particle layer thickness T). The ratio (D / T) is, for example, preferably 0.02 to 0.50, more preferably 0.04 to 0.40, and even more preferably 0.05 to 0.30.
When the ratio (D / T) is within the above range, the mechanical strength of the secondary battery is further improved.

The particle layer may contain other components in addition to the particles and the binder as long as the effects of the present invention are not impaired. Examples of other components include polyvinyl pyrrolidone.
When the particle layer contains other components, the total content of the other components is preferably more than 0% by mass and less than 5% by mass, more than 0% by mass and more than 3% by mass with respect to the total mass (100% by mass) of the particle layer. % Or less is more preferable.

[Preparation method of particle A for measurement of specific surface area]
A sample containing the particles A can be obtained by cutting out the part of the particle layer that does not overlap the positive electrode active material in the positive electrode of the secondary battery. Moreover, about the part which overlaps with a positive electrode active material, the sample containing particle | grains A can be obtained by shaving off only the part of a particle layer visually.

Since the sample contains the particles A and the binder, it is necessary to remove the binder. The method for removing the binder is not limited. For example, the binder can be removed from the sample by ultrasonic cleaning in an organic solvent capable of dissolving the binder, solid-liquid separation, and subsequent drying. Examples of the organic solvent include N-methylpyrrolidone. If the steps from ultrasonic cleaning in an organic solvent and solid-liquid separation are one cleaning, the cleaning is usually performed 1 to 10 times, and preferably 2 to 5 times. In addition, the temperature of the organic solvent at the time of washing is usually 20 to 80 ° C., more preferably 40 to 70 ° C.

The drying after the solid-liquid separation may be performed at normal pressure or at reduced pressure. The drying temperature is not particularly limited, but is usually 20 to 200 ° C.
In any case, the particles A are fractionated by appropriately selecting conditions that can remove the binder substantially completely from the above-described conditions.

As the particle layer for performing the above-described sorting of the particles A and the measurement of the specific surface area, the particle layer in the secondary battery immediately after the production of the secondary battery may be used, or the charge / discharge described in the examples described later is performed. The particle layer in the secondary battery after this may be sufficient. Especially, it is preferable to use the particle layer in the secondary battery after charging / discharging.
In addition, before and after charging / discharging, it confirmed separately that the value of the specific surface area of the particle | grains A did not change.

[Method of forming particle layer]
The particle layer can be formed by applying a coating liquid (slurry) to at least one surface of the positive electrode and the negative electrode and then drying to remove the diluting solvent and the like. The coating solution contains particles, a binder, and, if necessary, a dilution solvent and optional other components.
The coating method is not particularly limited, and for example, a doctor blade method, various coater methods, a printing method, and the like are applied.
As the particles contained in the coating solution, the above-described inorganic particles and organic particles are used. Moreover, as a binder, what is used as a binder of the electrode of the above-mentioned non-aqueous secondary battery is used.
The dilution solvent may be any solvent that can disperse the particles and the binder. The use amount of the dilution solvent can be appropriately adjusted according to the coating workability and the like. An example of a diluent solvent is N-methylpyrrolidone.

A ′ / B, which is the ratio of the specific surface area A ′ of the particles (hereinafter also referred to as “particle A ′”) contained in the coating solution to the specific surface area B of particles (particle B) contained in the positive electrode active material layer described later. Is preferably more than 0.2 and less than 1.5, more preferably more than 0.3 and less than 1.1, and still more preferably more than 0.3 and less than 0.8. If A ′ / B exceeds the lower limit of the range, the mechanical strength and cycle characteristics of the secondary battery are improved, and an increase in internal resistance is suppressed. When A ′ / B is less than the upper limit of the range, the mechanical strength and cycle characteristics of the secondary battery are improved.

The content of the particles with respect to 100 parts by mass of the coating solution is preferably 3 to 60 parts by mass, more preferably 8 to 50 parts by mass, and even more preferably 10 to 50 parts by mass.
The content of the binder with respect to 100 parts by mass of the coating solution is preferably 1 to 40 parts by mass, and more preferably 1 to 30 parts by mass.
From the viewpoint of workability, the viscosity of the coating solution is preferably 30 to 3000 cps, more preferably 30 to 2000 cps, and further preferably 100 to 1800 cps.

Drying temperature and drying time are not particularly limited. The drying temperature is usually 60 to 200 ° C, preferably 60 to 150 ° C.

<Positive electrode>
The positive electrode current collector and the positive electrode active material layer are not particularly limited as long as they have the effects of the present invention, and known materials can be used.
As the positive electrode current collector, a conductive metal foil is used. For example, aluminum, stainless steel, nickel, titanium, or an alloy thereof is used.
The positive electrode active material layer includes particles (particle B) and a binder. Examples of the particle B include a positive electrode active material and a conductive additive. When a plurality of particles B are contained, the mixture is referred to as a particle B.
The specific surface area of the particles B is preferably 5 ~ ^20m 2 / g, more preferably 10 ~ ^15m 2 / g. If the specific surface area is not less than the lower limit of the above range, the load characteristics as a cell are further enhanced. If the specific surface area is not more than the upper limit of the above range, the binding property is further enhanced.

Positive electrode active materials include layered rock salt type lithium cobaltate, lithium nickelate, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum compound, spinel type lithium manganate, lithium nickel manganese oxide, olivine type lithium iron phosphate, etc. And one or more selected from the group consisting of these transition metal compounds are preferred.
Examples of the conductive auxiliary agent include acetylene black, ketjen black, and carbon nanofiber.
Examples of the binder include a fluororesin such as polyvinylidene fluoride.
The positive electrode active material layer is formed, for example, by applying a positive electrode slurry in which a positive electrode active material, a conductive additive, and a binder are dispersed in a solvent to the surface of the positive electrode current collector. Examples of the solvent include N-methylpyrrolidone.

The fractionation of the particles B for the specific surface area measurement (sample collection and binder removal) can be performed in the same manner as the above-mentioned [Method for fractionating the particles A for the measurement of the specific surface area]. That is, a sample containing the particles B can be obtained by cutting off a portion of the positive electrode active material layer that does not overlap with the particle layer in the positive electrode in the secondary battery. Moreover, about the part which overlaps with a particle layer, the sample containing particle | grains B can be obtained by shaving only the part of a positive electrode active material layer visually.
Moreover, the removal of the binder from the sample can be performed by the same method as the removal of the binder described in the above-mentioned “Method for separating particles A for measurement of specific surface area”.
As the positive electrode active material layer for separating the particles B and measuring the specific surface area, the positive electrode active material layer in the secondary battery immediately after the production of the secondary battery may be used, or the charge / discharge described in the examples described later. It may be a positive electrode active material layer in the secondary battery after performing the above. Especially, it is preferable to use the positive electrode active material layer in the secondary battery after charging / discharging.
In addition, before and after charging / discharging, it confirmed separately that the value of the specific surface area of the particle | grains B did not change.

<Negative electrode>
The negative electrode current collector and the negative electrode active material layer are not particularly limited, and known materials can be used.
As the negative electrode current collector, a conductive metal foil is used, and for example, copper, stainless steel, nickel, titanium, or an alloy thereof is used.
The negative electrode active material layer is formed, for example, by applying a negative electrode slurry obtained by dispersing a negative electrode active material, a binder, and, if necessary, a conductive additive added in a solvent to the surface of the negative electrode current collector. .
Examples of the negative electrode active material include metal lithium, lithium alloy, carbon-based materials that can occlude and release lithium ions (carbon powder, graphite powder, etc.), and metal oxide materials, and are selected from the group consisting of these materials. It is preferable that it is 1 or more types.
As the conductive auxiliary agent, for example, acetylene black, carbon nanotube, or the like can be used.
Examples of the binder include fluororesins such as polyvinylidene fluoride, styrene butadiene rubber, carboxymethyl cellulose, and the like.

<Separator>
The material of the separator is not particularly limited, and examples thereof include a microporous polymer film or nonwoven fabric made of an olefin resin (polyolefin) or a cellulose material, and a woven fabric or nonwoven fabric made of glass fiber. Especially, from a viewpoint of improving adhesiveness with a particle layer, an olefin resin or a cellulose material is preferable, and an olefin resin is more preferable.

The olefin-based resin may be a single polyolefin or a mixture of two or more different polyolefins (for example, a mixture of polyethylene and polypropylene), or a copolymer of different olefins. Particularly preferred are polyethylene and polypropylene.

The mass average molecular weight (Mw) of the olefin resin is not particularly limited, and is preferably 1 × 10 ⁴ to 1 × 10 ⁷ , for example, from the viewpoint of obtaining sufficient mechanical strength, and is preferably 1 × 10 ⁴ to 15 × 10 ^6. More preferably, 1 × 10 ⁵ to 5 × 10 ⁶ is even more preferable.
In the present specification, the “mass average molecular weight” means a polystyrene equivalent value measured by a gel permeation chromatography (GPC) method.

The air permeability of the separator to which the particle layer adheres is preferably 50 to 200 seconds / 100 cc, and more preferably 120 to 180 seconds / 100 cc.
When the air permeability of the separator is equal to or higher than the lower limit of the above range, the permeability and permeability of the electrolytic solution can be sufficiently obtained. When the air permeability of the separator is not more than the upper limit of the above range, the adhesion strength of the particle layer to the separator is further increased.
The air permeability is determined by measuring with a Gurley type densometer (manufactured by Toyo Seiki etc.).

The thickness of the separator is not particularly limited, and can be set to, for example, 5 μm to 30 μm from the viewpoint of obtaining sufficient mechanical strength.
The size of the separator in the vertical and horizontal directions is preferably equal to or larger than the size of the electrode current collector, and more preferably larger than the size of the electrode current collector, for example, about 0.1 cm to 5 cm.

<Nonaqueous electrolyte>
A known nonaqueous electrolyte can be used in the nonaqueous electrolyte secondary battery.
It may be a non-aqueous electrolyte that is a mixture of an electrolyte and a non-aqueous solvent, or a polymer solid electrolyte that is a mixture of an electrolyte and a polymer. The polymer solid electrolyte includes those containing a non-aqueous solvent as a plasticizer.
As the electrolyte, those used in known lithium ion secondary batteries can be used. For example, lithium hexafluorophosphate (LiPF ₆ ), lithium boron tetrafluoride (LiBF ₄ ), lithium bis (fluorosulfonyl) ) Known lithium salts such as imide (LiN (SO ₂ F) ₂ , LiFSI), lithium bis (trifluoromethanesulfonyl) imide (LiN (SO ₂ CF ₃ ) ₂ , LiTFSI). The electrolyte may be used alone or in combination of two or more.

As the non-aqueous solvent, for example, carbonates, esters, ethers, lactones, nitriles, amides, sulfones and the like can be used. A nonaqueous solvent may be used individually by 1 type, and 2 or more types of mixed solvents may be sufficient as it.
Specific examples include propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, methyl ethyl carbonate, dimethoxyethane, dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, dioxane, acetonitrile, propionitrile, nitromethane, N, N-dimethylformamide, Examples thereof include dimethyl sulfoxide, sulfolane, and γ-butyrolactone.

[Method for producing non-aqueous electrolyte secondary battery]
The non-aqueous electrolyte secondary battery according to the present invention includes a step of forming a particle layer described in [Particle Layer Formation Method] described above. As a process for assembling the members constituting the nonaqueous electrolyte secondary battery, a known assembly process can be applied except that at least one of the positive electrode on which the particle layer is formed and the negative electrode on which the particle layer is formed is used. The assembly process includes, for example, an operation of laminating a negative electrode, a separator, and a positive electrode, an operation of accommodating the laminated body in the exterior body, an operation of filling the exterior body with a nonaqueous electrolyte, and an operation of sealing the exterior body.

<Action and effect>
In the secondary battery of this embodiment, A / B, which is the ratio of the specific surface area A of the particles contained in the particle layer to the specific surface area B of the particles contained in the positive electrode active material layer, is more than 0.2 and less than 1.5. As a result, the mechanical strength is high and the cycle characteristics are excellent.

The internal resistance of the nonaqueous electrolyte secondary battery of the present embodiment, measured by the method described in Examples below, is preferably 90 to 250 mΩ, more preferably 90 to 180 mΩ, and further preferably 90 to 130 mΩ.
The capacity maintenance ratio measured by the method described in the examples described later is preferably 80 to 95%, more preferably 85 to 95%, and still more preferably 90 to 95%.
The puncture strength measured by the method described in the examples described later is preferably 20 to 100N, more preferably 40 to 100N, and further preferably 60 to 100N.

As one aspect of the present invention, the capacity maintenance rate of the secondary battery is preferably 90 to 95%, the puncture strength is preferably 35 to 80 N, and the capacity maintenance rate is 90 to 95%. More preferably, the piercing strength is 40 to 60N.
As another aspect of the present invention, the internal resistance of the secondary battery is preferably 90 to 130 mΩ, the capacity retention is preferably 85 to 95%, the internal resistance is 90 to 130 mΩ, and The capacity retention rate is more preferably 90 to 95%.

In the secondary battery of this embodiment, the particle layer is present not only on the positive electrode active material layer but also on the surface of the positive electrode current collector on the surface of the positive electrode current collector on which the positive electrode active material layer is provided. . That is, as shown in FIG. 1, a part of the particle layer 4 is present on the surface of the positive electrode current collector exposed portion 13. There is a possibility that a state in which no separator is present between the opposing negative electrode active material layer 32 and the positive electrode current collector exposed portion 13 may occur due to thermal contraction or displacement of the separator. In such a case, the negative electrode active material layer 32 and the positive electrode current collector exposed portion 13 can be prevented from coming into contact with each other by the particle layer 4 existing between them.
The larger the region where the particle layer 4 is present in the surface of the positive electrode current collector exposed portion 13, the higher the effect of preventing the short circuit. For example, the distance from the edge of the positive electrode active material layer 12 to the edge of the particle layer 4 (indicated by x in the figure) is preferably 1 mm or more, and more preferably 2 mm or more. Further, the upper limit of x is not particularly limited as long as it has the above effect, but may be, for example, 20 mm or less, or 8 mm or less.
The combination of the upper limit value and the lower limit value is preferably 1 mm or more and 20 mm or less, and more preferably 2 mm or more and 8 mm or less.

<Modification>
In the present embodiment, the particle layer 4 is provided on the surface of the positive electrode 1. However, the same particle layer may be provided on the surface of the negative electrode 3, or may be provided on both the surface of the positive electrode 1 and the surface of the negative electrode 3. 3 shows a secondary battery 10 having a particle layer 4 on the surface of the negative electrode 3 and no particle layer on the surface of the positive electrode 1, and FIG. 4 shows a particle layer 4 on both the surface of the positive electrode 1 and the surface of the negative electrode 3. The secondary battery 10 which has is shown.
3 and 4 are the same as those described in FIG.
Further, when the particle layer is provided on the surface of the negative electrode, a part of the particle layer may be present on the surface of the negative electrode current collector exposed portion as in the case of the positive electrode. The larger the region where the particle layer is present in the surface of the exposed portion of the negative electrode current collector, the higher the effect of preventing the short circuit described above. For example, the distance from the edge of the negative electrode active material layer to the edge of the particle layer (x in FIGS. 3 and 4) is preferably 1 mm or more, and more preferably 2 mm or more. In addition, the upper limit of the distance from the edge of the negative electrode active material layer to the edge of the particle layer is not particularly limited as long as the above effect is obtained, but may be, for example, 20 mm or less, or 8 mm or less.
The combination of the upper limit value and the lower limit value is preferably 1 mm or more and 20 mm or less, and more preferably 2 mm or more and 8 mm or less.
In the secondary battery 10 of this embodiment, one positive electrode 1, two negative electrodes 3, and two separators 2 are stacked as shown in FIG. 1, but a unit in which a negative electrode, a separator, and a positive electrode are stacked in this order. The number of units can be arbitrarily changed.
In the secondary battery 10 of the present embodiment, the positive electrode active material layer 12 and the particle layer 4 are provided on both surfaces of the positive electrode current collector 11, but may be provided only on one surface of the positive electrode current collector 11. When the particle layer is provided on the surface of the negative electrode, the negative electrode active material layer and the particle layer may be provided on both surfaces of the negative electrode current collector, or may be provided only on one surface of the negative electrode current collector.
In general, the conductivity is rate-limiting at the positive electrode and the ion conductivity is often rate-limiting at the negative electrode. For this reason, it is preferable to have the particle layer on the positive electrode surface rather than the negative electrode surface from the viewpoint of smoothly promoting the electrochemical reaction of the secondary battery and suppressing the increase in internal resistance.
The shape of the secondary battery is not limited to the shape of the present embodiment, and can be adjusted to various shapes such as a cylindrical shape, a square shape, a coin shape, and a sheet shape.

Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples.

[Production Example 1]
90 parts by mass of a solid component containing a positive electrode active material, 5 parts by mass of acetylene black as a conductive additive, 5 parts by mass of polyvinylidene fluoride (Kureha # 7200) as a binder, and NMP (N-methylpyrrolidone) as a solvent And a slurry adjusted to a solid content of 45% was obtained. This slurry was applied to both surfaces of an aluminum foil, pre-dried, and then vacuum dried at 120 ° C. The electrode was pressure-pressed at 4 kN, and further punched out to a 40 mm square of the electrode dimensions to produce a positive electrode.
Mixing 98 parts by mass of the solid component containing the negative electrode active material, 1 part by mass of styrene butadiene rubber (SBR) as a binder, 1 part by mass of carboxymethylcellulose Na (CMC), and water as a solvent to a solid content of 50% An adjusted slurry was obtained. This slurry was applied to both sides of a copper foil and vacuum dried at 100 ° C.
The electrode was pressure-pressed at 2 kN, and further punched out to a 42 mm square of electrode dimensions to prepare a negative electrode.

In Production Example 1, the following materials were used.
As the positive electrode active material, olivine type lithium iron phosphate (specific surface area: 10 m ² / g) was used and mixed at the following mass ratio.
Positive electrode active material: Binder (PVdF): Conductive aid = 90: 5: 5
As the negative electrode active material, graphite was used and mixed at the following mass ratio.
Negative electrode active material: binder (CMC): binder (SBR) = 98: 1: 1
Electrolytic solution: Ethylene carbonate (EC): Diethyl carbonate (DEC) was mixed in a volume ratio of 3: 7, and LiPF ₆ was dissolved as an electrolyte to a concentration of 1 mol / liter to prepare a non-aqueous electrolytic solution. did.

[Production Example 2]
In Production Example 1, olivine type lithium iron phosphate was pulverized with a planetary ball mill for 1 hour to adjust the specific surface area to 13 m ² / g. Other than that was carried out similarly to manufacture example 1, and created the positive electrode.

[Production Example 3]
In Production Example 1, olivine-type lithium iron phosphate was pulverized with a planetary ball mill for 6 hours to adjust the specific surface area to 15 m ² / g. Other than that was carried out similarly to manufacture example 1, and created the positive electrode.

The following materials were used as the inorganic particles forming the particle layer: Al ₂ O ₃ -1 (specific surface area: 4 m ² / g, average particle size: 0.3 μm)
Al ₂ O ₃ -2 (specific surface area: 10 m ² / g, average particle size: 0.3 μm)
Al ₂ O ₃ -3 (specific surface area: 11 m ² / g, average particle size: 0.3 μm)
Al ₂ O ₃ -4 (specific surface area: 17 m ² / g, average particle size: 0.3 μm)
Al ₂ O ₃ -5 (specific surface area: 3 m ² / g, average particle size: 0.3 μm)
Al ₂ O ₃ -6 (specific surface area: 4 m ² / g, average particle size: 1.2 μm)
Al ₂ O ₃ -7 (specific surface area: 4 m ² / g, average particle size: 2.0 μm)
Al ₂ O ₃ -8 (specific surface area: 2 m ² / g, average particle size: 0.3 μm)
TiO ₂ -1 (specific surface area: 4 m ² / g, average particle size: 0.4 μm)
MgO-1 (specific surface area: 4 m ² / g, average particle size: 0.7 μm)
Li ₃ PO ₄ -1 (specific surface area: 5 m ² / g, average particle size: 0.3 μm)

[Example 1]
(Formation of particle layer)
A coating solution was prepared by uniformly mixing 100 parts by mass of inorganic particles, 10 parts by mass of polyvinylidene fluoride (Kureha # 7200) and 500 parts by mass of N-methylpyrrolidone. Al ₂ O ₃ -1 was used as the inorganic particles.
The obtained coating solution was applied to both sides of the positive electrode obtained in Production Example 1 and dried to form particle layers on both sides of the positive electrode. The thickness of each particle layer after drying was 5 μm.
As shown in FIG. 1, the particle layer 4 was continuously formed on the positive electrode active material layer 12 of the positive electrode and on the positive electrode current collector exposed portion 13 adjacent thereto. The distance (x) from the edge of the positive electrode active material layer 12 to the edge of the particle layer 4 was 5 mm.
The solid content of the binder in the particle layer was 10 parts by mass with respect to 100 parts by mass of all particles.

(Manufacture of batteries)
As the separator, a polyethylene porous film (melting point: 128 ° C.) was used.
As shown in FIG. 1, two negative electrodes obtained in Production Example 1, one positive electrode on which a particle layer was formed, and two separators were laminated in this order: negative electrode, separator, positive electrode, separator, and negative electrode. A terminal tab is electrically connected to each of the positive electrode current collector exposed portion and the negative electrode current collector exposed portion, and the laminate is sandwiched between aluminum laminate films so that the terminal tab protrudes to the outside. Sealed by laminating. A secondary battery (laminated cell) was manufactured by injecting the electrolytic solution obtained in Production Example 1 from one side left without sealing and vacuum sealing.

<Evaluation>
The performance of the secondary battery produced above was evaluated by the following method. The results are shown in Table 1.
(1) Internal resistance (cell resistance)
In order to evaluate the internal resistance of the secondary battery manufactured above, the resistance (cell resistance) of the laminate cell was measured at room temperature (25 ° C.) using a battery high tester BT3562 (product name, manufactured by Hioki Electric Co., Ltd.). Measured (measurement unit: mΩ).
(2) Capacity maintenance rate The manufactured secondary battery was set | placed on a 40 degreeC thermostat, the charge rate was 1C, the discharge rate was 1C, and the charging / discharging cycle was repeated. The capacity retention rate was determined by comparing the discharge capacity after 100 cycles with the discharge capacity after 10 cycles.
(3) Puncture strength The puncture strength was measured using the apparatus comprised like FIG. The puncture strength is a measure of the mechanical strength of the lithium ion secondary battery. In the figure, reference numeral 41 denotes a negative electrode obtained in Production Example 1, 42 denotes a separator used in Example 1, 43 denotes a nickel piece, 44 denotes a particle layer, and 45 denotes an aluminum foil used in Production Example 1. The particle layer 44 is formed on the aluminum foil 45 under the same conditions as in the first embodiment. Reference numeral 51 denotes a pressing jig that applies pressure in a direction in which the negative electrode 41 and the aluminum foil 45 (positive electrode) approach each other, and this pressure is measured by an autograph. Reference numeral 52 is a receiving plate made of SUS304. The nickel small piece 43 used what was described in the JIS C8714 forced internal short circuit test. When the pressing jig 51 is lowered to increase the pressure for pressing the negative electrode 41 against the aluminum foil 45 (positive electrode), the nickel small piece 43 penetrates the separator 42 and the particle layer 44 to cause conduction (short circuit).
In the test, 2V was applied between the negative electrode 41 and the aluminum foil 45 (positive electrode), the resistance value between the positive electrode and the negative electrode was measured while lowering the pressing jig 51, and the resistance value was 10Ω or less. Judgment was conducted, and the pressure at that time was defined as the piercing strength of the particle layer.
(4) Measurement of specific surface area (i) Sorting of sample and sorting of particle layer After the initial charge / discharge of the manufactured secondary battery, the battery was discharged and it was confirmed that the OCV was 1 V or less. From the positive electrode, the part which does not overlap with a positive electrode active material layer and a separator among the particle layers formed between a positive electrode active material layer and a separator was taken out. Regarding the part of the particle layer that overlaps the positive electrode active material layer, only the part of the particle layer was carefully scraped with a spatula to take out the particles in the particle layer.
-Sorting of positive electrode active material layer After the initial charge / discharge of the manufactured secondary battery, the battery was discharged and it was confirmed that the OCV was 1 V or less. A sample including the positive electrode active material layer was taken out from the positive electrode, and the particle layer formed between the positive electrode active material and the separator was scraped off with a spatula to obtain particles in the positive electrode active material layer.
(Ii) Pretreatment of sample The particles in the particle layer taken out above or the particles in the positive electrode active material layer were immersed in NMP at 60 ° C. Next, ultrasonic cleaning was performed for 10 minutes, and after filtering the solid content, 90% of NMP was removed by vacuum drying at 130 ° C. for 4 hours. After performing the washing | cleaning process by the above-mentioned NMP 3 times, it vacuum-dried at 130 degreeC for 4 hours.
BET gas adsorption method Using the particles in the particle layer after vacuum drying described above or 1 g of the particles in the positive electrode active material layer, the specific surface area of the N ₂ adsorption device (product name BELSORP- manufactured by Microtrac Bell) measured by miniII).
(5) Measurement of average particle diameter of inorganic particles The average particle diameter of the inorganic particles was measured with a laser diffraction particle size distribution analyzer (Partica LA-960 manufactured by Horiba, Ltd.) using NMP as a solvent. The particle size at which the cumulative volume from the small diameter side of the obtained particle size distribution becomes 50% (that is, the volume average particle size) was defined as the average particle size of the inorganic particles. In addition, it confirmed that the average particle diameter of an inorganic particle did not change after the above-mentioned charging / discharging after a raw material and a secondary battery manufacture.
The results are shown in Table 1.

[Examples 2 to 16, Comparative Examples 1 and 2]
Manufacture of a secondary battery in the same manner as in Example 1 except that the inorganic particles shown in Table 1 and the positive electrode were used and the particle layer on the positive electrode surface was made to have the film thickness shown in Table 1 on both sides. And evaluated. The evaluation results are shown in Table 1.

As shown in the results of Table 1, in the secondary batteries of Examples 1 to 16, the ratio of the specific surface area A of the particles contained in the particle layer to the specific surface area B of the particles contained in the positive electrode active material layer is A / Both the capacity retention ratio and the piercing strength were higher than those of Comparative Example 1 in which B was 1.5 or more. In addition, the secondary batteries of Examples 1 to 16 had higher capacity retention and piercing strength and lower internal resistance than Comparative Example 2 in which A / B was 0.2 or less.

DESCRIPTION OF SYMBOLS 1 Positive electrode 2 Separator 3 Negative electrode 4 Particle layer 5 Exterior body 10 Secondary battery 11 Positive electrode collector 12 Positive electrode active material layer 13 Positive electrode collector exposed part 20 Laminated body 31 Negative electrode collector 32 Negative electrode active material layer 33 Negative electrode current collector Body exposed part 41 Negative electrode 42 Separator 43 Nickel child piece 44 Particle layer 45 Aluminum foil 51 Pressing jig 52 Receiving plate

Claims (9)

A positive electrode including a positive electrode current collector and a positive electrode active material layer positioned on a surface of the positive electrode current collector; a negative electrode including a negative electrode current collector and a negative electrode active material layer positioned on a surface of the negative electrode current collector; A non-aqueous electrolyte containing lithium ions, a separator located between the positive electrode and the negative electrode, and a particle layer located on the surface of one or both of the positive electrode and the negative electrode,
A nonaqueous electrolyte secondary battery in which A / B, which is a ratio of a specific surface area A of particles contained in the particle layer to a specific surface area B of particles contained in the positive electrode active material layer, is more than 0.2 and less than 1.5. The non-aqueous electrolyte secondary battery according to claim 1, wherein the particle layer includes inorganic particles. The non-aqueous electrolyte secondary battery according to claim 2, wherein the inorganic particles are at least one selected from the group consisting of magnesia particles, titania particles, alumina particles, silica particles, and lithium phosphate particles. The nonaqueous electrolyte secondary battery according to claim 2 or 3, wherein the inorganic particles have an average particle diameter of 1.3 µm or less. The nonaqueous electrolyte secondary battery according to any one of claims 1 to 4, wherein at least a part of the particle layer is present on a surface of the positive electrode current collector. The nonaqueous electrolyte secondary battery according to any one of claims 1 to 4, wherein at least a part of the particle layer is present on a surface of the negative electrode current collector. The nonaqueous electrolyte secondary battery according to any one of claims 1 to 6, wherein the particle layer has a thickness of 2 to 20 µm. The nonaqueous electrolyte secondary battery according to any one of claims 1 to 7, wherein the particle layer is located on a surface of the positive electrode. A method for producing a nonaqueous electrolyte secondary battery according to any one of claims 1 to 8,
A step of applying a coating liquid containing particles and a binder to the surface of either one or both of the positive electrode and the negative electrode and drying;
Nonaqueous electrolyte secondary in which A ′ / B, which is the ratio of the specific surface area A ′ of the particles contained in the coating liquid to the specific surface area B of the particles contained in the positive electrode active material layer, is more than 0.2 and less than 1.5. Battery manufacturing method.

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