JP6498300B2 - Electrodes for electrochemical devices - Google Patents

Description

  The present invention relates to an electrode for an electrochemical element.

  The demand for electrochemical devices such as lithium ion secondary batteries that are small and light, have high energy density, and can be repeatedly charged and discharged is expected to increase in the future from the environmental viewpoint. Lithium ion secondary batteries have a high energy density and are used in fields such as mobile phones and notebook personal computers. Further, with the expansion and development of applications, electrochemical devices are required to have further improved performance such as low resistance and large capacity.

  For example, in Patent Document 1, an electrode is obtained using a binder having a core-shell structure and a specific gel content.

JP 2013-182765 A

In recent years, there has been an increasing demand for shortening the charging time of electrochemical devices as much as possible, and rapid charging by reducing the resistance during charging is increasingly required.
However, the electrode for an electrochemical element obtained by the technique of Patent Document 1 described above can prevent a decrease in charge / discharge efficiency even when repeated charge / discharge is performed, and further has a high capacity due to a high electrode density. However, even when such an electrode for an electrochemical device is used, there are further problems in the increase in internal resistance and cycle characteristics during rapid charging.

  An object of the present invention is to provide an electrode for an electrochemical element having a low internal resistance and an excellent balance between cycle characteristics and peel strength when incorporated in a battery.

  As a result of diligent research to achieve the above object, the inventors of the present invention are excellent in lithium diffusibility when rapidly charged by making the binder in the electrode for an electrochemical element into a specific shape, When incorporated in a battery, it was found that the internal resistance was low and the cycle characteristics were excellent, and the present invention was completed.

That is, the present invention is as follows.
[1] An electrode for an electrochemical device comprising an electrode active material layer containing an electrode active material and a binder,
An electrode for an electrochemical element, wherein at least a part of the binder has a particle shape, and the roundness of the particle is 0.50 to 0.85.
[2] The electrode for an electrochemical element according to [1], wherein an average particle length of the binder particles measured by cross-sectional observation is 100 to 400 nm.
[3] The electrode for an electrochemical element according to [1] or [2], wherein the binder is a (co) polymer containing a double bond.
[4] The electrode for an electrochemical element according to any one of [1] to [3], wherein the binder has a gel content of 90 to 100%.
[5] An electrochemical element comprising the electrochemical element electrode according to any one of [1] to [4].
[6] A lithium ion battery comprising the electrode for an electrochemical element according to any one of [1] to [4], a separator, and an electrolytic solution.
[7] An automobile comprising the lithium ion battery according to [6].

  According to the present invention, it is possible to provide an electrode for an electrochemical element having a low internal resistance when incorporated in a battery, and excellent balance between cycle characteristics and peel strength.

It is an example of the cross-sectional photograph of an electrode active material layer. It is the figure which traced the outline of the binder particle | grains in the cross-sectional photograph of FIG.

Hereinafter, a mode for carrying out the present invention (hereinafter referred to as “the present embodiment”) will be described in detail. The present invention is not limited to this, and various modifications can be made without departing from the scope of the invention.
This embodiment is an electrode for an electrochemical device comprising an electrode active material layer containing an electrode active material and a binder, wherein at least a part of the binder has a particle shape, and the roundness of the particle is 0.00. It is an electrode for electrochemical elements which is 50-0.85.
The electrode for an electrochemical element of the present embodiment may be an electrode used for an electrochemical element, and is not particularly limited to an electrochemical element. For example, a lithium ion secondary battery, an electric double layer capacitor, a hybrid capacitor (lithium And electrodes for various electrochemical devices such as ion capacitors.
In this embodiment, the electrode active material layer which comprises the electrode for electrochemical elements contains an electrode active material and a binder at least. Hereinafter, the electrode active material and the binder constituting the electrode active material layer will be described.

(Electrode active material)
In the present embodiment, the electrode active material is appropriately selected depending on the type of electrochemical element. For example, when the electrode for an electrochemical device is used as a negative electrode for a lithium ion secondary battery, the negative electrode active material may be a low crystal such as graphitizable carbon, non-graphitizable carbon, activated carbon, or pyrolytic carbon. Carbon (amorphous carbon), graphite (natural graphite, artificial graphite), carbon nanowalls, carbon nanotubes, or composite carbon materials of carbons with different physical properties, alloy materials such as tin and silicon, silicon oxidation Oxide, tin oxide, vanadium oxide, oxide such as lithium titanate, polyacene, and the like.
In addition, the electrode active material illustrated above may be used independently according to a use, and may be used in mixture of multiple types.

The shape of the negative electrode active material for a lithium ion secondary battery is preferably a granulated particle, and if the particle shape is spherical, a higher-density electrode can be formed at the time of electrode formation. Moreover, the volume average particle diameter of the negative electrode active material for lithium ion secondary batteries is 0.1-100 micrometers normally, Preferably it is 0.5-50 micrometers, More preferably, it is 0.8-20 micrometers. Further, the tap density of the negative electrode active material for the lithium ion secondary battery is not particularly limited, but a negative electrode having a negative electrode capacity of 0.6 g / cm ³ or more is preferably used.

(Binder)
The binder used in the present invention is not particularly limited as long as it is a compound capable of binding the above-described electrode active materials to each other, but is preferably a (co) polymer having a double bond. The (co) polymer having a double bond is preferably a (co) polymer obtained by polymerizing a monomer containing a conjugated diene, and more preferably contains an ethylenically unsaturated carboxylic acid as the monomer. preferable.
When the binder is a (co) polymer obtained by polymerizing a monomer containing a conjugated diene, in addition to the conjugated diene and the ethylenically unsaturated carboxylic acid contained if necessary, other copolymerizable with these Monomers such as vinyl compounds may be included.
Moreover, the supply form of the binder at the time of electrode manufacture is not specifically limited, For example, it is preferable to use the thing of the form of copolymer latex which the binder particle | grains disperse | distributed to water as a raw material.

  When the binder is a (co) polymer containing a conjugated diene as a monomer, examples of the conjugated diene include 1,3-butadiene, isoprene, 2-chloro-1,3-butadiene, chloroprene and the like alone or Two or more kinds can be combined, and among these, 1,3-butadiene is preferred from the viewpoint of adhesiveness.

When the copolymer contains an ethylenically unsaturated carboxylic acid as a monomer, examples of the ethylenically unsaturated carboxylic acid include fumaric acid, itaconic acid, acrylic acid, and methacrylic acid. It can be used alone or in combination of two or more. Among these, itaconic acid and acrylic acid are desirable from the viewpoint of the stability of the polymerized copolymer latex.
The amount of the ethylenically unsaturated carboxylic acid used is preferably 0.01 or more and 20 parts by mass or less, more preferably 0.01 or more and 15 parts by mass or less when the total of all monomers is 100 parts by mass. More preferably, it is 0.01 or more and 10 parts by mass or less.

Other vinyl compounds that can be copolymerized include aromatic vinyl compounds, (meth) acrylate compounds, vinyl cyanide compounds, and the like.
As the aromatic vinyl compound, for example, styrene, α-methylstyrene, p-methylstyrene, vinyltoluene, chlorostyrene, divinylbenzene and the like may be used alone or in combination of two or more thereof. Styrene is preferred from the viewpoint of the stability of the polymerized copolymer latex.
As for the usage-amount of an aromatic vinyl compound, 30-70 mass parts is preferable, More preferably, it is 35-65 mass parts, More preferably, it is 35-60 mass parts.

Examples of the (meth) acrylate compound include methyl (meth) acrylate, ethyl (meth) acrylate, n-propyl (meth) acrylate, i-propyl (meth) acrylate, n-butyl (meth) acrylate, i-butyl (meth) ) Acrylate, n-amyl (meth) acrylate, i-amyl (meth) acrylate, hexyl (meth) acrylate, 2-hexyl (meth) acrylate, octyl (meth) acrylate, i-nonyl (meth) acrylate, decyl (meth) ) Acrylate, hydroxymethyl (meth) acrylate, hydroxyethyl (meth) acrylate, ethylene glycol (meth) acrylate, etc., can be used alone or in combination of two or more thereof. Of united latex Methyl methacrylate from the viewpoint of qualitative desirable.
The amount of the (meth) acrylate compound used is preferably 0.1 to 30 parts by mass, more preferably 0.1 to 25 parts by mass, and still more preferably 0.1 to 20 parts by mass.

Examples of vinyl cyanide compounds include acrylonitrile, methacrylonitrile, α-chloroacrylonitrile, and the like. These monomers can be used alone or in combination of two or more. Then, it is desirable from the viewpoint of the stability of the copolymer latex obtained by polymerizing acrylonitrile.
The amount of vinyl cyanide compound used is preferably 0.1 to 30 parts by mass, more preferably 0.1 to 20 parts by mass, and still more preferably 0.1 to 15 parts by mass.

As copolymerizable vinyl compounds, in addition to the above, hydroxyl group-containing monomers such as 2-hydroxyethyl acrylate; aminoalkyl esters such as aminoethyl acrylate, dimethylaminoethyl acrylate, diethylaminoethyl acrylate; Pyridines such as 2-vinylpyridine and 4-vinylpyridine; Glycidyl esters such as glycidyl acrylate and glycidyl methacrylate; acrylamide, methacrylamide, N-methylolacrylamide, glycidylmethacrylamide, N, N-butoxymethylacrylamide, etc. Amides; Carboxylic acid esters such as vinyl acetate; Halogenated vinyls such as vinyl chloride; Divinylbenzene, (Poly) ethylene glycol di (meth) acrylate, Hexanediol (Meth) acrylate, 1,4-butanediol di (meth) acrylate, polyfunctional vinyl monomers such as allyl (meth) acrylate.
These can be used alone or in combination of two or more. Usually, the amount is 0.1 to 30 parts by mass.
From the viewpoint of the stability of the obtained copolymer latex, it is preferable to add a hydroxyl group-containing monomer as another copolymerizable monomer, and among these, it is more preferable to add 2-hydroxyethyl acrylate. preferable.

As molecular weight modifiers, halogenated hydrocarbons such as chloroform and carbon tetrachloride; mercaptans such as n-hexyl mercaptan, n-octyl mercaptan, n-dodecyl mercaptan, t-dodecyl mercaptan, thioglycolic acid; dimethylxanthogendi Xanthogens such as sulfide and diisopropylxanthogen disulfide; all that can be used in usual emulsion polymerization such as terpinolene and α-methylstyrene dimer can be used.
The amount of the molecular weight modifier used is preferably from 0 to 5 parts by mass with respect to 100 parts by mass of all monomers, and α-methylstyrene dimer and t-dodecyl mercaptan are preferably used.

When the (co) polymer latex is used as the binder raw material, the average particle diameter of the binder raw material particles in the (co) polymer latex is preferably 100 to 400 nm, more preferably 200 to 350 nm. This particle size is a volume average particle size measured by a dynamic light scattering method.
By making the average particle diameter of the binder raw material particles in the above range, an electrochemical device obtained while maintaining good stability when preparing a slurry-like electrode composition for producing an electrode active material layer The binder particle diameter in the electrode for an electrode can be adjusted to a suitable range, and the strength and flexibility of the electrode for an electrochemical element become better.

In the present embodiment, the gel content of the binder is preferably 90 to 100%, and more preferably 95 to 100%.
The gel content of the binder is a value that represents the molecular weight and the degree of crosslinking of the binder particles in the (co) polymer latex when the (co) polymer latex is used as the raw material of the binder, and is described in the examples. It can be measured by this method.
The higher the gel content, the more the binder particles can be prevented from fusing in the electrode active material layer, that is, the binder can easily maintain the particle shape, and the roundness of the binder particles can be increased. In addition, since the solvent resistance of the copolymer constituting the binder particles is maintained and does not swell by the electrolyte inside the battery, adhesion between the current collector and the electrode active material and between the electrode active material and the electrode active material The decrease in power is suppressed.

  The (co) polymer latex can be obtained by emulsion polymerization of the above monomers. Appropriate seed particles can be used during the polymerization, and the seed particles can also be obtained by ordinary emulsion polymerization. In addition, a known method can be employed for emulsion polymerization, and the emulsion polymerization can be appropriately performed using an emulsifier, a polymerization initiator, a molecular weight regulator, a chelating agent, a PH regulator, and the like in an aqueous medium.

  Here, as the emulsifier, anionic surfactants, nonionic surfactants, amphoteric surfactants, reactive surfactants and the like can be used alone or in combination of two or more.

  Examples of the anionic surfactant include sulfates of higher alcohols, alkylbenzene sulfonates, aliphatic sulfonates, and sulfate esters of polyethylene glycol alkyl ethers. As the alkylbenzene sulfonate, sodium dodecylbenzenesulfonate is preferable.

  As the nonionic surfactant, an alkyl ester type, an alkyl ether type, an alkylphenyl ether type, or the like of polyethylene glycol is used.

  Examples of amphoteric surfactants include betaines such as lauryl betaine and stearyl betaine, and amino acid types such as lauryl-β-alanine, stearyl-β-alanine and lauryl di (aminoethyl) glycine.

  Examples of the reactive surfactant include polyoxyethylene alkylpropenyl phenyl ether and α- [1-[(allyloxy) methyl] -2- (nonylphenoxy) ethyl] -ω-hydroxypolyoxyethylene.

  The amount of the emulsifier used is preferably 0.1 to 10 parts by mass, more preferably 0.1 to 8 parts by mass, and still more preferably 0.1 to 6 parts by mass with respect to 100 parts by mass of the total monomers.

Polymerization initiators include water-soluble polymerization initiators such as sodium persulfate, potassium persulfate, and ammonium persulfate, oil-soluble polymerization initiators such as benzoyl peroxide and lauryl peroxide, and redox polymerization initiators in combination with a reducing agent. Can be used alone or in combination.
As for the usage-amount of a polymerization initiator, 0.1-3 mass parts is preferable with respect to 100 mass parts of all the monomers.

The binder content in the electrode active material layer of the electrode for an electrochemical element is preferably 0.1 to 10 parts by mass, more preferably 0, based on 100 parts by mass of the electrode active material. .3 to 8 parts by mass, more preferably 0.5 to 5 parts by mass. When the binder content is within this range, sufficient adhesion between the electrode active material layer and the current collector can be secured, and the internal resistance can be lowered.
In addition, when the binder content exceeds 10 parts by mass with respect to 100 parts by mass of the electrode active material, the binder particles tend to be fused together, or the roundness of the binder particles having a particle shape tends to be remarkably lowered. There is.

(Electrode composition)
In this embodiment, the electrode composition constituting the electrode active material layer may contain other components as necessary in addition to the binder and the electrode active material.
Examples of such other components include conductive materials, dispersants, additives such as nonionic or anionic surfactants as stabilizers for copolymer latex, and antifoaming agents.

  The conductive material is not particularly limited as long as it is a particulate material having conductivity. For example, conductive carbon black such as furnace black, acetylene black, and ketjen black; graphite such as natural graphite and artificial graphite And carbon fibers such as polyacrylonitrile-based carbon fibers, pitch-based carbon fibers, and vapor-grown carbon fibers. Among these, acetylene black and ketjen black are preferable. The average particle diameter of the conductive material is not particularly limited, but is preferably smaller than the average particle diameter of the electrode active material, usually 0.001 to 10 μm, more preferably 0.05 to 5 μm, and still more preferably 0.01. It is in the range of ˜1 μm. When the average particle diameter of the conductive material is in the above range, sufficient conductivity can be expressed with a smaller amount of use. The amount of the conductive material used in the case of adding the conductive material is not particularly limited as long as the effect of the present invention is not impaired, but is preferably 0.1 to 50 with respect to 100 parts by mass of the electrode active material. It is a mass part, More preferably, it is 0.5-15 mass part, More preferably, it is 1-10 mass part. By setting the content ratio of the conductive material in the above range, the internal resistance can be sufficiently reduced while keeping the capacity of the obtained electrochemical element high.

The dispersing agent is a component having an action of uniformly dispersing each component in the solvent when the electrode active material, the binder, and optional components added as necessary are dispersed or dissolved in a solvent to form a slurry. It is. Specific examples of the dispersant include cellulosic polymers such as carboxymethylcellulose, methylcellulose, ethylcellulose and hydroxypropylcellulose, and ammonium salts or alkali metal salts thereof, alginates such as propylene glycol alginate, and alginates such as sodium alginate. , Polyacrylic acid, and polyacrylic acid (or methacrylic acid) salts such as sodium polyacrylic acid (or methacrylic acid), polyvinyl alcohol, modified polyvinyl alcohol, polyethylene oxide, polyvinyl pyrrolidone, polycarboxylic acid, oxidized starch, phosphoric acid starch , Casein, various modified starches, chitin, chitosan derivatives and the like. Further, a water-soluble polymer (specific group-containing water-soluble polymer) containing one or more, preferably two or more groups such as a carboxyl group, a sulfonic acid group, a fluorine-containing group, a hydroxyl group and a phosphoric acid group is also used as a dispersant. be able to.
These dispersants can be used alone or in combination of two or more. Among these, a cellulose polymer is preferable, and carboxymethyl cellulose or an ammonium salt or an alkali metal salt thereof is particularly preferable.
The content of the dispersant in the case of adding the dispersant is not particularly limited as long as the effect of the present invention is not impaired, but is usually 0.1 to 10 with respect to 100 parts by mass of the electrode active material. It is in the range of mass parts, preferably 0.5-5 mass parts, more preferably 0.8-2 mass parts.

  When a copolymer latex is used as a raw material for the binder, water can be used as a dispersion medium for the copolymer latex. When the binder particles are obtained by emulsion polymerization as described above, the water used during the polymerization is used. The dispersion medium can be used as it is or after being concentrated. Further, the dispersion medium can be used by substituting with an organic solvent optimum for the active material, if necessary. The organic dispersion medium is not particularly limited, and the method of substitution is not particularly limited, for example, a method of adding an organic dispersion medium to a copolymer latex obtained by emulsion polymerization and volatilizing water by vacuum distillation, Examples thereof include a method of volatilizing water from the copolymer latex and redispersing the resulting solid in an organic dispersion medium.

(Method for manufacturing electrode)
Subsequently, the manufacturing method of the electrode for electrochemical devices of this embodiment is demonstrated.
In the case of an electrode for a lithium ion battery, first, a slurry-like electrode composition is formed, the electrode composition is applied on a current collector such as a copper foil, dried, and pressure-molded to form an electrode. It is obtained by forming an active material layer.
The electrode active material layer is provided on the current collector, but the formation method is not limited.
The slurry-like electrode composition comprises an electrode active material, a conductive material and an essential component of a binder, and other dispersants and additives in water or an organic solvent such as N-methyl-2-pyrrolidone or tetrahydrofuran. It can be manufactured by kneading.

  Although the solvent used in order to obtain the composition for electrodes is not specifically limited, When using said dispersing agent, the solvent which can melt | dissolve a dispersing agent is used suitably. Specifically, water is usually used, but an organic solvent may be used, or a mixed solvent of water and an organic solvent may be used. Examples of the organic solvent include alkyl alcohols such as methyl alcohol, ethyl alcohol and propyl alcohol; alkyl ketones such as acetone and methyl ethyl ketone; ethers such as tetrahydrofuran, dioxane and diglyme; diethylformamide, dimethylacetamide and N-methyl- Amides such as 2-pyrrolidone and dimethylimidazolidinone; sulfur solvents such as dimethyl sulfoxide and sulfolane; and the like. Among these, alcohols are preferable as the organic solvent. The slurry is preferably an aqueous slurry using water as a dispersion medium from the viewpoint of ease of drying of the slurry and excellent environmental load. When water and an organic solvent having a lower boiling point than water are used in combination, the drying rate can be increased during spray drying. Further, the dispersibility of the binder or the solubility of the dispersant varies depending on the amount or type of the organic solvent used in combination with water. Thereby, the viscosity and fluidity | liquidity of a slurry can be adjusted and production efficiency can be improved.

  The amount of the solvent used when preparing the electrode composition is such that the solid content concentration is usually in the range of 1 to 90% by mass, preferably 5 to 85% by mass, more preferably 10 to 80% by mass. It is. When the solid content concentration is within this range, each component is preferably dispersed uniformly.

  The method or procedure for dispersing or dissolving the electrode active material, the conductive material, the binder, and other dispersants and additives in the solvent is not particularly limited. For example, the electrode active material, the conductive material, the binder, the other dispersant, Method of adding and mixing the additive; Dissolving the dispersant in the solvent, adding and mixing the binder dispersed in the solvent, and finally adding and mixing the electrode active material and the conductive material; Dispersing in the solvent Examples include a method in which an electrode active material and a conductive material are added to and mixed with the binder, and a dispersant dissolved in a solvent is added to and mixed with the mixture. Examples of the mixing means include mixing equipment such as a ball mill, a sand mill, a bead mill, a pigment disperser, a crusher, an ultrasonic disperser, a homogenizer, a homomixer, and a planetary mixer. Mixing is usually performed in the range of room temperature to 80 ° C. for 10 minutes to several hours.

  The viscosity of the electrode composition is usually in the range of 10 to 100,000 mPa · s, preferably 30 to 50,000 mPa · s, more preferably 50 to 20,000 mPa · s at room temperature. When the viscosity of the electrode composition is within this range, productivity can be increased.

  The method for applying the electrode composition onto the current collector is not particularly limited. Examples thereof include a doctor blade method, a dip method, a reverse roll method, a direct roll method, a gravure method, an extrusion method, and a brush coating method. The coating thickness of the electrode composition is appropriately set according to the thickness of the target electrode active material layer.

  Examples of the drying method include drying by warm air, hot air, low-humidity air, vacuum drying, and drying by irradiation with (far) infrared rays or electron beams. Among these, a drying method by irradiation with far infrared rays is preferable. The drying temperature and the drying time are preferably a temperature and a time at which the solvent in the electrode composition applied to the current collector can be completely removed, and the drying temperature is 50 to 300 ° C, preferably 50 to 250 ° C. The drying time is usually about 3 hours to 100 hours, preferably 5 hours to 50 hours, more preferably 10 hours to 30 hours.

Examples of the pressure molding method include molding methods such as roll pressing and press pressing. The pressure during pressure molding is preferably 1 to 10 t / cm ² , and the density of the electrode active material layer can be further adjusted by appropriately setting the temperature and time during pressure molding.
The density of the electrode active material layer (electrode density) is 0.30 to 2.0 g / cm ³ from the viewpoint that the binder has a particle shape and the roundness of the particles is adjusted to the range defined in the present invention. It is preferably 0.35 to 1.9 g / cm ³ , more preferably 0.40 to 1.8 g / cm ³ . When the electrode density is 2.0 g / cm ³ or less, it is possible to prevent the binders from fusing with each other and the roundness to be remarkably lowered and out of the range defined in the present invention. In addition, as the electrode density of the electrode increases, the battery capacity per volume usually increases. However, if the electrode density is too high, the cycle characteristics tend to deteriorate.
The thickness of the electrode active material layer is not particularly limited, but is usually 5 to 1000 μm, preferably 20 to 500 μm, more preferably 30 to 300 μm.

(Electrochemical element electrode structure)
Next, the structure of the electrode for an electrochemical element of this embodiment will be described. In the present embodiment, the electrode for an electrochemical element has at least an electrode active material layer, and the electrode active material layer may be formed on a current collector such as a copper foil.

  In the present embodiment, the electrode active material layer includes an electrode active material and a binder, and at least a part of the binder exists in the electrode active material layer in a form having the particle shape. And the roundness of the said particle | grain is 0.50-0.85.

Since the binder has a particle shape in the electrode active material layer, a void portion is secured between the binders close to each other. Therefore, when an electrochemical element is obtained using such an electrode for an electrochemical element, lithium ions can easily diffuse into the electrode active material layer by passing through this void, and the internal resistance can be reduced. Can be reduced.
It is preferable that 70% or more of the binder area ratio when tracing the outline of the binder particles in the cross-sectional photograph of the electrode active material layer has a particle shape, more preferably 80% or more, and still more preferably 90% or more. It is.
Specifically, in the same manner as in the measurement of roundness described later, a cross-sectional photograph of the electrode active material layer is taken with an SEM, and a portion determined to be a binder is recognized as having a particle shape. It is divided into parts determined not to have a particle shape and is calculated based on the areas of both.

Furthermore, by controlling the roundness of the binder particles having a particle shape in the electrode active material layer, the internal resistance is reduced by ensuring the bonding area of the active material or current collector and the binder and the void portion. It is possible to improve the peel strength and cycle characteristics.
The roundness of the binder particles having a particle shape in the electrode active material layer is preferably 0.55 to 0.80, and more preferably 0.70 to 0.80.
The roundness of the binder particles having a particle shape in the electrode active material layer is preferably higher in terms of securing the voids, but on the other hand, it is not too high from the viewpoint of cycle characteristics and peel strength. preferable. If the roundness is too high, the active material cannot cope with the expansion and contraction of the active material, and the binder is peeled off to deteriorate the cycle characteristics, or the current collector-electrode active material or the electrode active material-electrode active material is bonded. At this time, it is considered that the peel strength is lowered due to point contact and the adhesion area becomes too small, but the mechanism does not depend on this.

The roundness of the binder particles in the electrode active material layer is, for example, the average particle diameter of the binder particles in the (co) polymer latex that is the binder raw material, the gel content, and the binder content in the electrode active material layer. , And the density of the electrode active material layer.
Specifically, the smaller the average particle diameter is, the less rounded it can be due to the pressure during pressure molding, and the higher the roundness can be. Can be lowered. The higher the gel content, the higher the roundness of the binder particle shape in the electrode active material layer. Further, the greater the binder content, the lower the roundness, and in particular when the amount exceeds 10 parts by mass, the binders are fused together, or the roundness is significantly reduced. Further, as the density of the electrode active material layer is increased, the roundness can be lowered. In particular, when it exceeds 2.0 g / cm ³ , the binder is fused or the roundness is remarkably lowered. It becomes difficult to make the range specified in the above.

Note that the roundness of the binder particles in the electrode active material layer is calculated by the following method.
In the site determined to be the binder in the cross-sectional photograph (FIG. 1) of the electrode active material layer imaged by SEM, the site exhibiting dark contrast and determining the linear and continuous connection is determined as the boundary (contour) of the particle, Trace the outline of the binder particles freehand (Figure 2). From this trace, 100 binder particles having a particle shape are selected at random. At this time, particles having unclear outlines are not selected (not used for calculating roundness). Further, when the observation image does not have a clear dark contrast, it is determined that it does not have a particle shape.
The contour image of each selected particle is processed by image analysis software (A image-kun, imageJ, etc.), and the roundness is calculated by the following formula from the area and circumference of the region surrounded by the contour, and its arithmetic Let the average be roundness. When the contour of 100 binder particles cannot be extracted with one visual field, 100 contours are extracted with multiple visual fields and the roundness is calculated.
Roundness = 4π × area / (circumference ² )

The particle size of the binder particles in the electrode active material layer is preferably 50 to 1000 nm as an average particle major axis measured by cross-sectional observation. When the average particle major axis is in this range, the strength and flexibility of the electrode for an electrochemical element become better.
The average particle major axis is more preferably 100 to 900 nm, still more preferably 200 to 800 nm.
The average particle major axis has a random particle shape, similar to the calculation of the roundness of the binder particles, from the trace (FIG. 2) of the cross-sectional photograph of the electrode active material layer taken by the SEM. 100 binder particles are selected, the contour image of each selected particle is processed by image analysis software (A image-kun, imageJ, etc.), and the arithmetic average of the major axis of the region surrounded by the contour is taken as the average particle major axis .

(Electrochemical element)
The electrode for an electrochemical element of the present embodiment can be used as an electrode in an electrochemical element such as a lithium ion secondary battery, an electric double layer capacitor, a lithium ion capacitor, a sodium battery, or a magnesium battery, and in particular, a lithium ion secondary battery. It can be used suitably in a battery, and can be used suitably especially for the negative electrode of a lithium ion secondary battery.
For example, a lithium ion secondary battery is composed of the electrode for an electrochemical element, a separator, and an electrolytic solution.

(Separator)
A separator will not be specifically limited if it can insulate between the electrodes for electrochemical elements, and can pass a cation and an anion. Specifically, (a) a porous separator having pores, (b) a porous separator having a polymer coat layer formed on one or both sides, or (c) a porous resin coat layer containing an inorganic ceramic powder The formed porous separator is mentioned. Non-limiting examples of these include solids such as polypropylene, polyethylene, polyolefin, or aramid porous separators, polyvinylidene fluoride, polyethylene oxide, polyacrylonitrile, or polyvinylidene fluoride hexafluoropropylene copolymers. A polymer film for a polymer electrolyte or a gel polymer electrolyte, a separator coated with a gelled polymer coating layer, or a separator coated with a porous membrane layer made of an inorganic filler or a dispersant for inorganic filler is used. be able to. A separator is arrange | positioned between a pair of electrode for electrochemical elements arrange | positioned so that an electrode active material layer may oppose, and an element is obtained. Although the thickness of a separator is suitably selected according to a use purpose, it is 1-100 micrometers normally, Preferably it is 10-80 micrometers, More preferably, it is 20-60 micrometers.

(Electrolyte)
The electrolytic solution is not particularly limited. For example, a solution obtained by dissolving a lithium salt as a supporting electrolyte in a non-aqueous solvent can be used. Examples of the lithium salt include LiPF ₆ , LiAsF ₆ , LiBF ₄ , LiSbF ₆ , LiAlCl ₄ , LiClO ₄ , CF ₃ SO ₃ Li, C ₄ F ₉ SO ₃ Li, CF ₃ COOLi, (CF ₃ CO) ₂ NLi , (CF ₃ SO ₂ ) ₂ NLi, (C ₂ F ₅ SO ₂ ) NLi, and other lithium salts. In particular, LiPF ₆ , LiClO ₄ , and CF ₃ SO ₃ Li that are easily soluble in a solvent and exhibit a high degree of dissociation are preferably used. These can be used alone or in admixture of two or more. The amount of the supporting electrolyte is usually 1% by mass or more, preferably 5% by mass or more, and usually 30% by mass or less, preferably 20% by mass or less, with respect to the electrolytic solution. If the amount of the supporting electrolyte is too small or too large, the ionic conductivity is lowered, and the charging characteristics and discharging characteristics of the battery are degraded.

  The solvent used in the electrolytic solution is not particularly limited as long as it can dissolve the supporting electrolyte. Usually, dimethyl carbonate (DMC), ethylene carbonate (EC), diethyl carbonate (DEC), propylene carbonate (PC), butylene. Alkyl carbonates such as carbonate (BC) and methyl ethyl carbonate (MEC); esters such as γ-butyrolactone and methyl formate; ethers such as 1,2-dimethoxyethane and tetrahydrofuran; sulfolane and dimethyl sulfoxide Sulfur-containing compounds are used. In particular, dimethyl carbonate, ethylene carbonate, propylene carbonate, diethyl carbonate, and methyl ethyl carbonate are preferable because high ion conductivity is easily obtained and the use temperature range is wide. These can be used alone or in admixture of two or more. Moreover, it is also possible to use an electrolyte containing an additive. The additive is preferably a carbonate compound such as vinylene carbonate (VC).

  Examples of the electrolytic solution other than the above include a gel polymer electrolyte obtained by impregnating a polymer electrolyte such as polyethylene oxide and polyacrylonitrile with an electrolytic solution, and an inorganic solid electrolyte such as lithium sulfide, LiI, and Li3N.

A secondary battery is obtained by stacking a negative electrode and a positive electrode through a separator, winding this in accordance with the shape of the battery, folding it into a battery container, injecting an electrolyte into the battery container, and sealing. Further, if necessary, an expanded metal, an overcurrent prevention element such as a fuse or a PTC element, a lead plate and the like can be inserted to prevent an increase in pressure inside the battery and overcharge / discharge. The shape of the battery may be any of a laminated cell type, a coin type, a button type, a sheet type, a cylindrical type, a square type, a flat type, and the like.
The lithium ion secondary battery in the present embodiment includes, for example, a mobile phone, a portable computer, a smartphone, a tablet PC, a smart pad, a netbook, a LEV (Light Electronic Vehicle), a UAV (Unmanned Aerial Vehicle), an automobile, and a power storage. It can use suitably for an apparatus etc. Examples of the automobile include an electric vehicle, a hybrid electric vehicle, and a plug-in hybrid electric vehicle.

Hereinafter, the present invention will be described more specifically with reference to examples and comparative examples. Unless otherwise indicated, the part and% in each example are based on mass.
In addition, the definition and evaluation method of each characteristic are as follows.

<Binder particle size>
The particle size of the binder particles (latex) was determined by measuring the volume average particle size by a dynamic light scattering method using a Microtrac Ultrafine Particle Size Analyzer (W) UPA-150 (manufactured by Nikkiso Co., Ltd.). .

<Binder gel content>
Copolymer latex is applied to a glass plate at a thickness of 0.5 mm, and 0.5 g is weighed from a coating film (dried coating film of the copolymer before immersion) obtained by drying by heating at 130 ° C. for 30 minutes. After that, it was immersed in 40 ml of toluene and shaken for 3 hours. The copolymer coating film after shaking was filtered through a 325 mesh stainless steel wire mesh and dried at 130 ° C. for 1 hour to obtain a dried coating film of the copolymer after immersion, which was weighed. The gel content was calculated from the mass of the coating film before and after immersion (dried coating film) by the following formula.

<Density of electrode active material layer (electrode density)>
About the electrode active material layer (negative electrode active material layer) of the negative electrode obtained by the Example and the comparative example, the area C (cm < ² >), thickness D (micrometer), mass A (g) of a collector, preparation From the mass B (g) of the electrochemical device electrode thus obtained, the density was calculated by the following equation.

Density of electrode active material layer (electrode density) (g / cm ³ )
= (B (g) -A (g)) / (C (cm ² ) × D (μm) × 10 ⁻⁴ )

<Roundness of binder particles>
About the cross section of the electrode active material layer (negative electrode active material layer) of the negative electrode obtained by the Example and the comparative example, using a scanning electron microscope (product name "S4700", Hitachi High-Tech Co., Ltd. make) photographing magnification 50,000 times A cross-sectional photograph was taken with an acceleration voltage of 5.0 kV and a detector: reflected electrons, and the roundness of the binder particles in the negative electrode active material layer was measured by the following method.
The cross section of the negative electrode active material layer was prepared by first disassembling a laminated laminate cell-shaped lithium secondary battery in an Ar glove box, washing the collected negative electrode with dimethyl carbonate, and then drying in air. Next, the dried negative electrode was cut into 2 mm squares, subjected to OsO4 dyeing treatment, and then a cross section was made perpendicular to the electrode surface using a cross section polisher (product name “SM-09010”, manufactured by JEOL Ltd.). .
Using a cross-sectional photograph taken by the above method, 100 binder particles are selected at random, and from a portion judged to be a binder, dark contrast is observed in an observation image, and a portion showing a linear and continuous connection is represented by a particle. The boundary (contour) was judged, and the contour of the binder particles was traced freehand. Particles with unclear outlines were not used for calculating roundness. In addition, when the observed image did not have clear dark contrast, it was determined that the particle shape was not present, and “measurement impossible” was determined. The obtained contour image is processed by image analysis software (imageJ), the roundness is calculated from the area and circumference of the region surrounded by the contour, and the arithmetic average is taken as the roundness. . When the contour of 100 binder particles could not be extracted in one visual field, 100 contours were extracted from multiple visual fields and the roundness was calculated.
Roundness = 4π × area / (circumference ² )

<Average particle long diameter of binder particles>
The 100 contour images obtained by the above <Binder particle roundness> were processed by image analysis software (imageJ), and the arithmetic average of the major axis of the region surrounded by the outline was defined as the average particle major axis.

<Peel strength>
The negative electrodes obtained in the examples and comparative examples were cut into rectangular pieces each having a width of 1 cm and a length of 10 cm to form test pieces, which were fixed with the negative electrode active material layer face up, and cellophane tape was applied to the surface of the negative electrode active material layer. After pasting, the stress when the cellophane tape was peeled off from one end of the test piece in the 180 ° direction at a speed of 50 mm / min was measured. And this measurement was performed 10 times, the average value was calculated | required, this was made into peel strength, and the following reference | standard determined. It can be determined that the higher the peel strength, the higher the adhesion strength in the negative electrode active material layer and the adhesion strength between the negative electrode active material layer and the current collector.
A: Peel strength is 8 N / m or more B: Peel strength is 6 N / m or more and less than 8 N / m C: Peel strength is 4 N / m or more and less than 6 N / m D: Peel strength is 2 N / m or more, 4 N / m Less than E: Peel strength is less than 2 N / m

<Internal resistance>
About the lithium secondary battery of the lamination | stacking type laminated cell shape obtained by the Example and the comparative example, it charged with constant current until it became 4.2V by the constant current method which was made into the charge rate 2C at 25 degreeC, and then The battery was charged at a constant voltage at the rated voltage. Thereafter, the discharge rate was set to 2C and discharged to 3.0V. The amount of voltage drop 10 seconds after the start of discharge was taken as ΔV. Then, by changing the discharge rate from 2C to 10C, the voltage drop amount ΔV is measured in the same manner, the discharge current value I (A) and the voltage drop amount ΔV (V) are plotted, and the slope of the straight line is plotted. The internal resistance was determined according to the following criteria.
A: Internal resistance is less than 3.0Ω B: Internal resistance is 3.0Ω or more and less than 3.5Ω C: Internal resistance is 3.5Ω or more and less than 4.0Ω D: Internal resistance is 4.0Ω or more, 4.5Ω Less than E: Internal resistance is 4.5Ω or more

<Charge / discharge cycle characteristics>
The laminated laminated cell-shaped lithium secondary batteries obtained in the examples and comparative examples were charged at a constant current until reaching 4.2 V by a constant current constant voltage charging method of 2C at 60 ° C. A charge / discharge cycle test was performed in which the battery was charged with a voltage and then discharged to 3.0 V with a constant current of 2C. The charge / discharge cycle test was conducted up to 100 cycles, and the ratio of the discharge capacity at the 100th cycle to the initial discharge capacity was defined as the capacity retention rate, and the following criteria were used. It shows that the capacity | capacitance reduction by repeated charging / discharging is so small that this value is large.
A: Capacity maintenance rate is 90% or more B: Capacity maintenance rate is 80% or more and less than 90% C: Capacity maintenance rate is 70% or more and less than 80% D: Capacity maintenance rate is 60% or more and less than 70% E: Capacity maintenance rate is less than 60%

Example 1
<Manufacture of binder for negative electrode>
Initial water (75 parts by mass of ion exchange water, 3.0 parts by mass of itaconic acid, seed (polystyrene latex having a particle size of 35 nm), emulsifier (0.3 parts by mass of sodium dodecylbenzenesulfonate)) was charged into the reactor and stirred. However, the temperature was raised to 80 ° C. and held. Compounding monomer (40 parts by mass of 1,3-butadiene, 49 parts by mass of styrene, 3.0 parts by mass of methyl methacrylate, 3.0 parts by mass of acrylonitrile, 1.0 part by mass of 2-hydroxyethyl acrylate, acrylic acid 1.0 part by mass, 0.1 part by mass of α-methylstyrene dimer, 0.1 part by mass of t-dodecyl mercaptan) were added over 6.5 hours. Simultaneously, catalyst water (24 parts by mass of ion exchange water, 1.2 parts by mass of sodium persulfate, 0.3 parts by mass of caustic soda, 0.15 parts by mass of an emulsifier (sodium dodecylbenzenesulfonate)) was added. After completion of the addition, the temperature was raised to 95 ° C. and reacted for 1 hour to complete the polymerization. The resulting copolymer latex was steam distilled to remove unreacted monomers.
When the obtained copolymer latex was adjusted to pH 7.0 ± 1.0 with caustic potassium, the volume average particle diameter was 300 nm and the gel content was 98%.

<Preparation of positive electrode>
1. As a positive electrode active material, 100 parts of lithium cobaltate having a volume average particle diameter of 8 μm and a 1.5% aqueous solution of carboxymethyl cellulose ammonium (DN-800Hl Daicel Chemical Industries) as a dispersant in an amount of 2. 0 parts, 5 parts of acetylene black (Denka black powder: manufactured by Denki Kagaku Kogyo Co., Ltd.) as a conductive material, an acrylate having a glass transition temperature of −28 ° C. and a number average particle diameter of 0.28 μm as a binder for an electrode composition Prepare a positive electrode composition by mixing a 40% aqueous dispersion of a polymer with a planetary mixer so that the solid content is 3.0 parts and the total solid content is 35%. did.

  The positive electrode composition was applied to a current collector made of an aluminum foil having a thickness of 20 μm on both the front and back surfaces of the current collector at an electrode forming speed of 20 m / min, dried at 120 ° C. for 5 minutes, and then punched into a 5 cm square. A positive electrode having an electrode active material layer with a thickness of 100 μm on one side was obtained.

<Production of negative electrode>
As a negative electrode active material, 100 parts of graphite (KS-6: manufactured by Timcal) having a volume average particle diameter of 3.7 μm and a 1.5% aqueous solution of carboxymethyl cellulose ammonium as a dispersant (DN-800H: Daicel Chemical) (Made by Kogyo Co., Ltd.) 2.0 parts in terms of solid content, 5 parts of acetylene black (Denka Black powder: made by Denki Kagaku Kogyo Co., Ltd.) as the conductive material, and the above-described copolymer latex as the binder for the electrode composition 3.0 parts by weight and ion-exchanged water were mixed so that the total solid content concentration was 35% to prepare a slurry-like negative electrode composition.

Using a comma coater, the negative electrode composition was applied to one side of a current collector made of a copper foil having a thickness of 18 μm so that the film thickness after drying was about 100 μm, and dried at 60 ° C. for 20 hours. Thus, a negative electrode active material layer was formed. Subsequently, it was rolled using a roll press so that the pressing pressure was 2 t / cm ² with respect to the electrode, to obtain a negative electrode having a thickness of 50 μm.

<Manufacture of batteries>
Using the polyethylene microporous film (film thickness: 25 μm) (Hypore manufactured by Asahi Kasei E-Materials Co., Ltd.) as the positive electrode, the negative electrode, and the separator, a laminated laminate cell-shaped lithium ion battery was produced. As the electrolytic solution, a solution obtained by dissolving LiPF ₆ at a concentration of 1.0 mol / liter in a mixed solvent of ethylene carbonate and diethyl carbonate in a mass ratio of 1: 2 was used.

(Example 2)
When forming the negative electrode active material layer, a negative electrode was obtained in the same manner as in Example 1 except that the drying temperature was 100 ° C. and the drying time was 14 hours. A secondary battery was manufactured and evaluated in the same manner. The results are shown in Table 1.

(Example 3)
When forming the negative electrode active material layer, a negative electrode was obtained in the same manner as in Example 1 except that the drying temperature was 150 ° C. and the drying time was 10 hours. A secondary battery was manufactured and evaluated in the same manner. The results are shown in Table 1.

(Comparative Example 1)
In rolling using a roll press, a negative electrode was obtained in the same manner as in Example 1 except that the pressing pressure was 6 t / cm ² with respect to the electrode, and a lithium ion secondary battery was obtained using the obtained negative electrode. Were manufactured and evaluated in the same manner. The results are shown in Table 1.

(Comparative Example 2)
<Manufacture of binder for negative electrode>
Initial water (75 parts by mass of ion exchange water, 3.0 parts by mass of itaconic acid, seed (polystyrene latex having a particle size of 35 nm), emulsifier (0.3 parts by mass of sodium dodecylbenzenesulfonate)) was charged into the reactor and stirred. However, the temperature was raised to 80 ° C. and held. Compounding monomer (40 parts by mass of 1,3-butadiene, 49 parts by mass of styrene, 3.0 parts by mass of methyl methacrylate, 3.0 parts by mass of acrylonitrile, 1.0 part by mass of 2-hydroxyethyl acrylate, acrylic acid 1.0 part by mass, 0.1 part by mass of α-methylstyrene dimer, 0.8 part by mass of t-dodecyl mercaptan) were added over 6.5 hours. Simultaneously, catalyst water (24 parts by mass of ion exchange water, 1.2 parts by mass of sodium persulfate, 0.3 parts by mass of caustic soda, 0.15 parts by mass of an emulsifier (sodium dodecylbenzenesulfonate)) was added. After completion of the addition, the temperature was raised to 95 ° C. and reacted for 1 hour to complete the polymerization. The resulting copolymer latex was steam distilled to remove unreacted monomers. When the obtained copolymer latex was adjusted to pH 7.0 ± 1.0 with caustic potassium, the volume average particle size was 300 nm and the gel content was 75%.

<Production of negative electrode>
A slurry-like negative electrode composition was prepared in the same manner as in Example 1 except that the above-described copolymer latex was used as a binder for an electrode composition, and the same negative electrode composition as in Example 1 was prepared using the obtained negative electrode composition. Thus, a negative electrode was obtained.

<Manufacture of batteries>
A laminated laminate cell-shaped lithium ion battery was prepared in the same manner as in Example 1 except that the above negative electrode was used as the negative electrode, and evaluated in the same manner as in Example 1. When the photograph which image | photographed the cross section of the electrode active material layer with the scanning electron microscope was observed, the binder did not have a particle shape. The results are shown in Table 1.

(Comparative Example 3)
<Manufacture of binder for negative electrode>
Initial water (75 parts by mass of ion exchange water, 3.0 parts by mass of itaconic acid, seed (polystyrene latex having a particle size of 35 nm), emulsifier (0.3 parts by mass of sodium dodecylbenzenesulfonate)) was charged into the reactor and stirred. However, the temperature was raised to 80 ° C. and held. Compounding monomer (40 parts by weight of butadiene, 49 parts by weight of styrene, 3.0 parts by weight of methyl methacrylate, 3.0 parts by weight of acrylonitrile, 1.0 part by weight of 2-hydroxyethyl acrylate, 1.0 part by weight of acrylic acid) Part, α-methylstyrene dimer 0.1 part by mass, t-dodecyl mercaptan 0.05 part by mass) was added over 6.5 hours. Simultaneously, catalyst water (24 parts by mass of ion exchange water, 1.2 parts by mass of sodium persulfate, 0.3 parts by mass of caustic soda, 0.15 parts by mass of an emulsifier (sodium dodecylbenzenesulfonate)) was added. After completion of the addition, the temperature was raised to 95 ° C. and reacted for 1 hour to complete the polymerization. The resulting copolymer latex was steam distilled to remove unreacted monomers. When the obtained copolymer latex was adjusted to pH 7.0 ± 1.0 with caustic potassium, the volume average particle diameter was 300 nm and the gel content was 99%.

<Production of negative electrode>
A slurry-like negative electrode composition was prepared in the same manner as in Example 1 except that the above-described copolymer latex was used as a binder for an electrode composition, and Example 1 was obtained using the obtained negative electrode composition. A negative electrode was obtained in the same manner.

<Manufacture of batteries>
A laminated laminate cell-shaped lithium ion battery was prepared in the same manner as in Example 1 except that the above negative electrode was used as the negative electrode, and evaluated in the same manner as in Example 1. The results are shown in Table 1.

  The electrode for an electrochemical element of the present invention can be used as an electrode (positive electrode / negative electrode) of various electrochemical elements, and can be particularly suitably used as a negative electrode of a lithium ion secondary battery.

  This application is based on a Japanese patent application (Japanese Patent Application No. 2015-160108) filed with the Japan Patent Office on August 14, 2015, the contents of which are incorporated herein by reference.

Claims (5)

An electrode for an electrochemical device comprising an electrode active material layer containing an electrode active material and a binder,
Wherein at least a portion of the binder has a particle shape, the circularity of the particle is Ri der 0.50 to 0.85,
The content of the binder is 0.1 to 10 parts by mass on a dry mass basis with respect to 100 parts by mass of the electrode active material,
The average particle length of the binder particles measured by cross-sectional observation is 50 to 1000 nm,
The gel content of the binder is 90 to 100%,
The density of the electrode active material layer is 0.30~2.0g / cm ^3, an electrode for an electrochemical element. The binder, alone polymer containing a double bond, or a copolymer containing a double bond, an electrode for an electrochemical element according to claim 1. It comprises an electrode for an electrochemical element according to claim 1 or 2, an electrochemical device. The lithium ion battery containing the electrode for electrochemical elements of Claim 1 or 2 , a separator, and electrolyte solution. An automobile comprising the lithium ion battery according to claim 4 .