JP2018156787A - Electrode and power storage device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electrode having low internal resistance, capable of suppressing peeling of an insulation layer of an active material layer more effectively and also to provide a power storage device equipped with this electrode.SOLUTION: An electrode 1 includes: a current collector made of metal; an active material layer 3 which is made of a porous body containing a plurality of active material particles 31 and arranged on the current collector; and an insulation layer which is made of the porous body containing a plurality of insulator particles and covers an active material particles 311 exposed on the surface of the active material layers 3. Moreover, when the geometric mean diameter of the insulator particles obtained by performing measurement on the cross section in the thickness direction is D[μm], and when the center distance between the active material particles 31 in contact with the insulator particles is L[μm], a value of D/Lis 190 to 560 ppm.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an electrode and a power storage device.

  For example, vehicles such as forklifts, hybrid vehicles, and electric vehicles may incorporate power storage devices such as lithium ion secondary batteries and nickel metal hydride storage batteries. This type of power storage device has an electrode assembly in which a positive electrode and a negative electrode are stacked or wound via a separator. The positive electrode and the negative electrode have a current collector made of metal and an active material layer provided on one side or both sides of the current collector. Moreover, as a separator which insulates a positive electrode and a negative electrode, the microporous film mainly consisting of resin, such as polyolefin, is used.

  In this type of power storage device, for the purpose of suppressing the short circuit between the positive electrode and the negative electrode even when high heat is generated and the separator is melted, In some cases, an insulating layer made of a porous material including a binder that binds to the active material layer is provided on the active material layer. Insulating layers usually apply a slurry in which insulator particles and a binder are dispersed in a solvent, and then apply heat to dry the slurry to remove the solvent and promote the melting of the binder. Thus, the bonding between the insulating layer and the active material layer is strengthened.

  As an example of this type of power storage device, Patent Document 1 discloses that a positive electrode plate and a negative electrode plate mainly composed of an active material that absorbs and releases lithium ions, and a surface on which the positive electrode plate and the negative electrode plate face each other. A lithium ion secondary battery provided with a porous heat-resistant layer is described. A liquid or gel electrolyte is present between the positive electrode plate and the negative electrode plate.

JP 2008-27634 A

  When the adhesiveness between the active material layer and the insulating layer is low, part of the insulating layer may be peeled off from the active material layer, and the active material layer may be exposed in the form of dots. Since a separator is interposed between the positive electrode and the negative electrode, even if a part of the insulating layer is peeled off, there is no immediate problem, but from the viewpoint of improving the reliability of the power storage device, from the active material layer In order to suppress peeling of the insulating layer, it is preferable to improve the adhesion between the active material layer and the insulating layer.

  As a method for improving the adhesion between the active material layer and the insulating layer, for example, a method of increasing the amount of the binder contained in the insulating layer can be considered. However, when the amount of the binder is increased, the porosity of the insulating layer is decreased, which may hinder the movement of ions in the electrolyte, or may cause problems such as the electrolyte becoming difficult to enter the voids in the insulating layer. is there. In addition, an electrode reaction that accompanies the movement of electrons in the active material layer is hindered by a binder having low conductivity, which may increase the internal resistance of the power storage device.

  Thus, in an electrode used for a power storage device, it is required to suppress the peeling of the insulating layer from the active material layer while suppressing an increase in internal resistance of the power storage device.

  The present invention has been made in view of such a background, and is intended to provide an electrode having a low internal resistance and a power storage device including the electrode, which can more effectively suppress peeling of the insulating layer from the active material layer. Is.

One embodiment of the present invention is a current collector made of metal;
An active material layer comprising a porous body containing a large number of active material particles, and disposed on the current collector;
An insulating layer covering the active material particles exposed on the surface of the active material layer, comprising a porous body containing a large number of insulator particles;
Di _i [μm] is the geometric mean diameter of the insulator particles obtained by measuring in the cross section in the thickness direction, and L _a [μm] is the distance between the centers of the active material particles in contact with the insulator particles. when _a], the value of D i / _{L a} is 190～560Ppm, located in the electrode.

Another aspect of the present invention is an electrode assembly in which positive electrodes and negative electrodes are alternately stacked via separators,
A liquid or gel electrolyte interposed between the positive electrode and the negative electrode;
In the power storage device, at least one of the positive electrode and the negative electrode is the electrode of the above embodiment.

The electrode includes an active material layer disposed on the current collector and an insulating layer that covers the active material particles exposed on the surface of the active material layer. Also obtained by measuring the thickness direction of the cross section, the geometric mean diameter of the insulator particles D _{i [μm],} the distance between the centers of the active material particles in contact with the insulating particles L _{a [[mu]} m when _a], the value of D i / _{L a} is 190～560Ppm.

  The electrode can be manufactured by sequentially performing an operation of forming an active material layer on a current collector and an operation of forming an insulating layer on the active material layer. In forming the active material layer, first, a slurry containing active material particles is applied onto the current collector. By drying this slurry and removing the solvent, an active material layer made of a porous material containing active material particles can be formed. Active material particles are exposed on the surface of the active material layer, and irregularities are formed on the surface of the active material layer by the active material particles.

After the operation of forming the active material layer, a slurry containing insulator particles is applied on the surface of the active material layer. By drying this slurry and removing the solvent, an insulating layer can be formed. At this time, by a range of values of D i _/ L _a particular above, when applied to slurry the active material layer, thereby advancing the insulator particles in the slurry in the recess of the surface of the active material layer Can do. Therefore, the active material particles exposed on the surface of the active material layer can be covered with the insulating layer by drying the slurry and removing the solvent.

Thus, the electrode is a value of D i _/ L _a by the range specified above, the active material particles exposed on the surface of the active material layer was covered with an insulating layer, and the active material layer and the insulating layer The gap can be reduced. Therefore, the electrode can suppress peeling of the insulating layer from the active material layer.

  In addition, since the electrode has the specific structure described above, it is possible to suppress the peeling of the insulating layer from the active material layer, thereby obtaining the effect of suppressing the peeling of the insulating layer from the active material layer, The amount of the binder can be reduced.

Furthermore, by setting a range of values of D i _/ L _a particular above, as well as prevent the insulating particles from entering through the gap between the active material particles into the interior of the active material layer, an insulating layer void It can suppress that a rate becomes small. Therefore, in the above electrode, suppression of the electrode reaction by the insulating layer can be avoided.

  As a result, the electrode can suppress peeling of the insulating layer from the active material layer and suppress increase in internal resistance of the power storage device.

FIG. 3 is a partial cross-sectional view showing a main part of an electrode in Example 1. FIG. 2 is an enlarged view of a boundary between an active material layer and an insulating layer in FIG. 1. It is explanatory drawing which shows the calculation method of the major axis of the insulator particle | grains in FIG. It is explanatory drawing which shows the calculation method of the center distance of the active material particle in FIG. FIG. 6 is a cross-sectional view showing a main part of a power storage device in Example 2. FIG. 5 is an exploded perspective view of an electrode assembly in Example 2. It is a photograph which shows an example of the droplet of the slurry dripped on the test piece in an experiment example. It is a photograph which shows an example of the test piece by which the insulating layer was hold | maintained on the negative electrode active material in an experiment example. It is a photograph which shows an example of the droplet of the slurry dripped on the test piece in an experiment example. It is a photograph which shows an example of the test piece which the insulating layer peeled in the experiment example.

  The electrode may be configured as a positive electrode in which a positive electrode active material layer is provided on one side or both sides of a current collector, and is configured as a negative electrode in which a negative electrode active material layer is provided on one side or both sides of a current collector. It may be. Moreover, the said electrode can also be comprised as a bipolar electrode which has a positive electrode active material layer on the surface side of a collector, and has a negative electrode active material layer on a back side surface.

  As the current collector, a known current collector for a power storage device having a form such as a foil, a sheet, a film, a linear shape, a rod shape, or a mesh can be employed. When the current collector is a metal foil, for example, a metal foil having a thickness of 5 to 100 μm can be employed. As a material for the current collector, for example, a metal known as a current collector for a power storage device, such as aluminum, magnesium, zinc, copper, silver, gold, platinum, iron, titanium, nickel, cobalt, and stainless steel, is employed. be able to. In particular, aluminum or copper is preferable as the current collector material from the viewpoints of electrical conductivity, workability, and cost.

  An active material layer made of a porous body containing active material particles is provided on the current collector. The thickness of the active material layer can be set to 60 to 150 μm, for example. Moreover, the porosity of the active material layer can be 8 to 17%, for example. There is no particular limitation on the shape of the active material particles.

  Examples of the active material particles include graphite, coke, vapor-grown carbon fiber, pitch-based carbon fiber, PAN-based carbon fiber, highly oriented graphite, mesocarbon microbead, hard carbon, soft carbon, and other carbon, lithium, sodium, and the like. Particles composed of alkali metals such as, metal compounds, metal oxides such as SiOx (0.5 ≦ x ≦ 1.5), boron-added carbon, and the like can be used. Examples of graphite include powders such as natural graphite, artificial graphite, spherulitic graphite (graphitized mesophase carbon microspheres), and graphite carbon material. Examples of the graphite carbon material include thermal decomposition products of condensed polycyclic hydrocarbon compounds such as pitch and coke. The active material layer may contain one kind of particles among these particles as active material particles, or may contain two or more kinds of particles as active material particles.

  The volume average particle diameter of the active material particles can be set to 80 to 220 μm, for example. Here, the volume average particle diameter is a median diameter (50% cumulative diameter) in the particle diameter distribution of the active material particles displayed on a volume basis.

  Moreover, the geometric average diameter of the active material particles obtained by measuring in the cross section in the thickness direction can be set to 100 to 210 μm, for example. Here, the geometric mean diameter of the active material particles is a value calculated by the following method. First, the active material layer is cut in the thickness direction by a method such as a cross section polisher method (CP method) using an argon ion beam to expose a cross section in the thickness direction. This cross section is observed using an FE-SEM (Electrolytic Emission Scanning Electron Microscope), and an SEM image with a field of view of 500 μm in length and 800 μm in width is acquired. After obtaining SEM images of a plurality of fields of view by changing the observation position, the major axis of each active material particle present in the SEM image is calculated. The geometric mean of the major axis of the active material particles obtained in this way is taken as the geometric mean diameter of the active material particles.

  In addition to the active material particles, the active material layer usually contains a binder that binds the active material particles. When the binder content is excessively low, the moldability of the electrode may be reduced, and when the binder content is excessively high, the energy density of the electrode may be decreased. From the viewpoint of avoiding these problems, the binder content can be, for example, 0.5 to 5 parts by mass with respect to 100 parts by mass of the active material particles.

  Examples of the binder include fluorine-based polymers such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE), thermoplastic resins such as polypropylene and polyethylene, rubbers such as styrene butadiene rubber (SBR), polyimide, polyamideimide, Imide polymers such as polyamide-imide silica hybrid, alkoxysilyl group-containing resins, polyacrylic acid, polymethacrylic acid, and polyitaconic acid can be used. As the binder, a copolymer of acrylic acid and an acid monomer such as methacrylic acid, itaconic acid, fumaric acid, maleic acid or the like can be used. The active material layer may contain one kind of compound selected from these compounds as a binder, and may contain two or more kinds of compounds as a binder.

  The active material layer may contain a conductive additive. As the conductive auxiliary agent, for example, short fiber carbon fibers such as carbon nanofibers, acetylene black, carbon black, graphite, Ketjen Black (registered trademark) and the like can be used. The active material layer may contain one kind of substance selected from these substances as a conductive assistant, and may contain two or more kinds of substances as a conductive assistant.

  In forming the active material layer, first, a slurry in which the above-described active material particles and the like are dispersed in a solvent is prepared. As the solvent for the slurry, for example, an organic solvent such as N-methylpyrrolidone, methanol, methyl isobutyl ketone, or water can be used. The slurry may contain one type of solvent selected from these solvents, and may contain two or more types of solvents. Further, for example, a thickener such as carboxymethyl cellulose may be added to the slurry.

  After applying the slurry on the current collector, the slurry is dried to remove the solvent, whereby an active material layer can be formed on the current collector. After the slurry is dried, the active material layer may be pressed to adhere the active material layer and the current collector.

  On the active material layer, an insulating layer made of a porous body containing insulating particles is disposed. Although there is no restriction | limiting in particular in the thickness of an insulating layer, For example, it can be set as 1-7 micrometers. Moreover, the porosity of an insulating layer can be 40 to 60%, for example. The porosity of the insulating layer can be measured, for example, by the following method. That is, the insulating layer is cut in the thickness direction by a method such as a cross section polisher method (CP method) using an argon ion beam to expose a cross section in the thickness direction. This cross section is observed using an FE-SEM (electrolytic emission scanning electron microscope), and the void ratio in the obtained SEM image can be set as the void ratio of the insulating layer. In addition, the ratio of the space | gap in a SEM image can be calculated using an image processing apparatus, for example.

  Examples of the insulator particles include metal oxides such as aluminum oxide, silicon oxide, titanium oxide, zirconium oxide, cerium oxide, and chromium oxide, cordierite, β sponge clay, aluminum titanate, barium titanate, mullite, and spinel. Perovskite structures such as composite oxides such as silicon carbide, metal carbides such as silicon carbide, metal nitrides such as aluminum nitride and boron nitride, transition metal oxides such as nickel oxide and iron oxide, lanthanum manganate, lanthanum cobaltite and lanthanum chromite Particles composed of oxides or the like can be used. The insulating layer may contain one type of particles selected from these particles as insulator particles, and may contain two or more types of particles as insulator particles.

  The volume average particle diameter of the insulator particles can be set to 0.020 to 0.080 μm, for example. Here, the volume average particle diameter is a median diameter (50% cumulative diameter) in the particle diameter distribution of the insulator particles displayed on a volume basis.

  The geometric average diameter of the insulator particles obtained by measuring in the cross section in the thickness direction of the electrode can be appropriately set in the range of 0.020 to 0.110 μm, for example. The geometric average diameter of the insulator particles can be calculated by the same method as the geometric average diameter of the active material particles, except that the field of view of the SEM image is changed to 0.200 μm in length and 0.300 μm in width.

  In addition to the insulating particles, the insulating layer usually contains a binder that binds the insulating particles together and holds the insulating layer on the surface of the active material layer. When the binder content is excessively low, the moldability of the electrode may be reduced, and when the binder content is excessively high, the energy density of the electrode may be decreased. From the viewpoint of avoiding these problems, the binder content can be, for example, 0.1 to 25 parts by mass with respect to 100 parts by mass of the insulator particles.

  Examples of the binder that can be used include fluorine-containing resins such as polyvinylidene fluoride, polytetrafluoroethylene, and fluorine rubber, and compounds such as aromatic polyamide, aromatic polyimide, aromatic polyamideimide, and polyacrylic acid. As the aromatic polyamide, para-oriented aromatic polyamide or meta-oriented aromatic polyamide can be used. As the aromatic polyamide, it is preferable to use a para-oriented aromatic polyamide that easily forms voids in the insulating layer.

  Here, the para-oriented aromatic polyamide has an amide group bonded through a skeleton structure such as a paraphenylene skeleton, a 4,4′-biphenylene skeleton, a 1,5-naphthalene skeleton, or a 2,6-naphthalene skeleton. Thus, it refers to a polyamide in which an amide group and a skeleton structure are linearly arranged. Specific examples of the para-oriented aromatic polyamide include poly (paraphenylene terephthalamide), poly (parabenzamide), poly (4,4′-benzanilide terephthalamide), and poly (paraphenylene-4,4′-). Biphenylene dicarboxylic acid amide), poly (paraphenylene-2,6-naphthalenedicarboxylic acid amide), poly (2-chloro-paraphenylene terephthalamide), paraphenylene terephthalamide / 2,6-dichloroparaphenylene terephthalamide copolymer Etc. can be used.

  As the aromatic polyimide, a wholly aromatic polyimide obtained by condensation polymerization of an aromatic dianhydride and a diamine can be used. Examples of the aromatic dianhydride include pyromellitic dianhydride, 3,3 ′, 4,4′-diphenylsulfonetetracarboxylic dianhydride, 3,3 ′, 4,4′-benzophenone tetra Carboxylic dianhydride, 2,2′-bis (3,4-dicarboxyphenyl) hexafluoropropane, 3,3 ′, 4,4′-biphenyltetracarboxylic dianhydride and the like can be used. Examples of the diamine include oxydianiline, paraphenylenediamine, benzophenonediamine, 3,3′-methylenedianiline, 3,3′-diaminobenzophenone, 3,3′-diaminodiphenylsulfone, 1,5. '-Naphthalenediamine can be used.

  The insulating layer may contain one kind of compound selected from these compounds as a binder, and may contain two or more kinds of compounds as a binder.

  In forming the insulating layer, first, a slurry in which the above-described insulator particles and the like are dispersed in a solvent is prepared. Examples of the solvent for the slurry include alcohols such as methanol, ethanol, 2-propanol, 1-butanol and 1-hexanol, ketones such as acetone and 2-butanone, and aliphatic hydrocarbons such as pentane, hexane and heptane. Solvents, aromatic hydrocarbon solvents such as benzene, toluene, xylene and ethylbenzene, phenol solvents such as cresol and o-chlorophenol, acetate solvents such as methyl acetate, ethyl acetate and butyl acetate, N, N-dimethyl A polar amide solvent such as formamide or N, N-dimethylacetamide, a polar urea solvent such as tetramethylurea, an organic solvent such as dimethyl sulfoxide, and water can be used. The slurry may contain one type of solvent selected from these solvents, and may contain two or more types of solvents.

  In the slurry, for example, polyethylene glycol, polyacrylic acid, polyvinyl alcohol, synthetic polymers such as vinyl methyl ether-maleic anhydride copolymer, cellulose derivatives such as carboxymethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, xanthan gum , Natural polysaccharides such as welan gum, gellan gum, guar gum and carrageenan, starches such as dextrin and pregelatinized starch, clay minerals such as montmorillonite and hectorite, inorganic oxides such as fumed silica, fumed alumina and fumed titania Or the like. The slurry may contain one kind of substance selected from these substances as a thickener, and may contain two or more kinds of substances as a thickener.

  After applying the slurry containing the insulator particles on the active material layer, the slurry is dried to remove the solvent, whereby the insulating layer can be formed on the active material layer.

The geometric average diameter of the insulator particles obtained by measuring in the cross section in the thickness direction is D _i [μm], and the distance between the centers of the active material particles in contact with the insulator particles is L _a [μm]. when the _value of D i / _{L a} is a 190～560Ppm. Here, the value of the center-to-center distance La of the active material particles is _a value calculated by the following method. First, a cross section in the thickness direction of the boundary portion between the active material layer and the insulating layer is observed using FE-SEM, and an SEM image having a field of view of 300 μm in length and 480 μm in width is acquired. After acquiring the SEM images of a plurality of fields of view by changing the observation position, the center of the active material particles in contact with the insulator particles is determined.

  In determining the center of the active material particles, first, a line segment corresponding to the major axis of the active material particles is set. Specifically, out of the outlines of the active material particles appearing in the cross section, two points that are the farthest away are selected, and a line segment connecting these two points is defined as a line segment corresponding to the major axis of the active material particles. And the midpoint of the line segment corresponding to the long diameter obtained in this way is set as the center of the active material particles.

  After determining the center of all active material particles in contact with the insulator particles by the above-described method, the distance from the center of the active material particle to the center of the active material particle adjacent to the active material particle is calculated. The geometric average of the distance between the centers of the active material particles obtained as described above is defined as the distance between the centers of the active material particles.

When the value of D _i / L _a exceeds 560 ppm, the particle diameter of the insulator particles increases with respect to the distance between the centers of the active material particles. In this case, when the slurry containing the insulator particles is applied on the active material layer, the insulator particles in the slurry are less likely to enter the recesses on the surface of the active material layer. A relatively large gap is easily formed between the two. Therefore, when the value of D _i / L _a exceeds 560 ppm, the insulating layer may be easily peeled from the active material layer.

On the other hand, when the value of D _i / L _a is less than 190 ppm, the particle diameter of the insulator particles is small with respect to the distance between the centers of the active material particles. In this case, when the slurry containing the insulator particles is applied onto the active material layer, the insulator particles do not stay on the surface of the active material layer, but pass through the gaps between the active material particles to the inside of the active material layer. It becomes easy to enter. In addition, the gap between the active material layers may be blocked by the insulating particles that have entered the inside of the active material layer, which may hinder the movement of ions in the electrolyte.

In this case, since the particle diameter of the insulator particles is small, the porosity of the insulator tends to be small. Therefore, it becomes difficult for the electrolyte to enter the active material layer. As a result, when the value of D _i / L _a is less than 190 ppm, the electrode reaction in the active material layer is hindered, which may increase the internal resistance of the power storage device.

Therefore, by setting the value of D _i / L _a to 190 to 560 ppm, the active material layer is suppressed while preventing the insulator particles from entering the active material layer and avoiding an increase in the internal resistance of the power storage device. The effect which suppresses peeling of the insulating layer from can be acquired.

From the viewpoint of more effectively suppressing exfoliation of the insulating layer from the active material layer, the maximum value of the depth of penetration of the insulator particles into the active material layer obtained by measuring in the cross section in the thickness direction is set. when the t [μm], the value of t / _{L a} is preferably a 0.5 to 1. Here, the maximum value of the penetration depth of the insulator particles into the active material layer is a value measured by the following method.

  First, a cross section in the thickness direction of the active material layer is observed using FE-SEM, and an SEM image with a field of view of 300 μm in length and 480 μm in width is acquired. After acquiring SEM images of a plurality of fields of view by changing the observation position, a number of insulator particles are connected, and the maximum depth recognized as white as the surface may be set as the maximum value of the penetration depth of the insulator particles. it can.

Example 1
Examples of the electrodes will be described with reference to the drawings. As shown in FIGS. 1 and 2, the electrode 1 is composed of a current collector 2 made of metal and a porous body including a large number of active material particles 31, and an active material layer 3 disposed on the current collector 2. And an insulating layer that covers the active material particles 311 that are exposed on the surface of the active material layer 3. Also obtained by measuring in the cross section in the thickness direction, the geometric mean diameter of the insulator particles 41 D _{i [μm],} the distance between the centers of the active material particles 31 in contact with the insulating particles 41 L _a When [μm] is set, the value of D _i / L _a is 190 to 560 ppm.

  The current collector 2 in the electrode 1 of this example is specifically a copper foil having a thickness of 20 μm.

  Although not shown in the drawing, the active material layer 3 is provided on both surfaces of the current collector 2. The active material layer 3 is composed of a porous body containing active material particles 31 and a binder. As shown in FIG. 1, active material particles 311 are exposed on the surface of the active material layer 3, and unevenness is formed on the surface of the active material layer 3 by these active material particles 311. Specifically, the active material particles 31 and 311 in this example are particles made of natural graphite and having a volume average particle size of 180 μm. For convenience, the binder is not shown in FIG.

  As shown in FIGS. 1 and 2, the active material particles 311 exposed on the surface of the active material layer 3 are covered with an insulating layer 4. As shown in FIG. 2, the insulating layer 4 is composed of a porous body including insulating particles 41 and a binder. Specifically, the insulator particles 41 in this example are particles made of aluminum oxide and having a volume average particle size of 0.06 μm. For convenience, the insulating layer 4 is simplified in FIG. Moreover, in FIG. 2, while omitting the description of the binder, the insulator particles 41 are simplified. Insulator particles 41 are actually irregular particles as shown in FIG. 3, for example.

Di _i [μm] is the geometric mean diameter of the insulator particles 41 obtained by measurement in the cross section in the thickness direction, and L _a [μm] is the distance between the centers of the active material particles 31 in contact with the insulator particles 41. ], The value of D _i / L _a is 190 to 560 ppm.

The geometric average diameter D _i [μm] of the insulator particles 41 can be calculated by the following method. First, a cross section in the thickness direction of the insulating layer 4 is observed using an FE-SEM, and an SEM image having a field of view of 0.200 μm in length and 0.300 μm in width is acquired. Cross-sectional observation is performed at intervals of 2 mm, and SEM images for 18 fields of view are acquired, and then the major diameter d _i (see FIG. 3) of each insulator particle 41 present in the SEM image is calculated. Then, the geometric mean of the major axis d _i of the insulating particles 41, and the geometric mean diameter D _i of the insulator particles 41. The geometric average diameter D _i of the insulator particles 41 in this example is 0.05 μm.

Further, the center-to-center distance L _a [μm] of the active material particles 31 in contact with the insulator particles 41 can be calculated by the following method. First, a cross section in the thickness direction of the boundary portion between the active material layer 3 and the insulating layer 4 is observed using FE-SEM, and an SEM image having a field of view of 300 μm in length and 480 μm in width is acquired. After observing the cross section at intervals of 2 mm and acquiring SEM images for 18 visual fields, the center C of the active material particles 31 in contact with the insulator particles 41 is determined.

  In determining the center C of the active material particles 31, as shown in FIG. 4, two points P1 and P2 that are farthest apart from each other in the outline of the active material particles 31 appearing in the cross section are determined. The midpoint of the line segment L1 connecting the two points P1 and P2 is defined as the center C of the active material particles.

The distance from the center C of the active material particle 31 to the center C of the active material particle 31 adjacent to the active material particle 31 after determining the center C by the above method for all the active material particles 31 in contact with the insulator particles 41 l _a (see FIG. 4) is calculated. The geometric mean of the center distance l _a of the active material particles 31 obtained by the above, the center distance L _a of the active material particles 31. Center distance _{L a} of the active material particles 31 of the present example is 180 [mu] m.

Further, as shown in FIG. 2, some of the insulating particles 41 in the insulating layer 4 have entered the recesses 32 formed on the surface of the active material layer 3. Obtained by performing a measurement in the cross section in the thickness direction, the maximum value of the penetration depth of the insulating particles 41 to the active material layer 3 is taken as t [[mu] m], the value of t / L _a 0. 5-1.

Next, the effect of the electrode 1 of this example is demonstrated. The electrode 1 includes an active material layer 3 disposed on the current collector 2 and an insulating layer 4 that covers the active material particles 311 exposed on the surface of the active material layer 3. Also obtained by measuring in the cross section in the thickness direction, the geometric mean diameter of the insulator particles 41 D _{i [μm],} the distance between the centers of the active material particles 31 in contact with the insulating particles 41 L _a When [μm] is set, the value of D _i / L _a is 190 to 560 ppm. Therefore, when the slurry containing the insulator particles 41 is applied on the active material layer 3, the insulator particles 41 in the slurry can enter the recesses 32 on the surface of the active material layer 3.

  Then, by drying the slurry and removing the solvent, the insulator particles 41 are arranged along the active material particles 311 exposed on the surface of the active material layer 3, and these active material particles 311 are covered with the insulating layer 4. can do. As a result, the gap between the active material layer 3 and the insulating layer 4 can be reduced, and peeling of the insulating layer 4 from the active material layer 3 can be suppressed.

  Moreover, since the electrode 1 can suppress the peeling of the insulating layer 4 from the active material layer 3 by having the specific structure, the effect of suppressing the peeling of the insulating layer 4 from the active material layer 3 is achieved. Thus, the amount of binder can be reduced.

Furthermore, since the value of D _i / L _a is within the specific range, the insulating particles 41 are prevented from entering the inside of the active material layer 3 through the gaps between the active material particles 31 and are insulated. It can suppress that the porosity of the layer 4 becomes small. Therefore, suppression of the electrode reaction by the insulating layer 4 can be avoided.

  As a result, the electrode 1 can suppress peeling of the insulating layer 4 from the active material layer 3 and can suppress increase in internal resistance of the power storage device.

(Example 2)
This example is an example of the power storage device 5 including electrodes. Of the reference numerals used in and after the present embodiment, the same reference numerals as those used in the above-described embodiments indicate the same constituent elements as those in the above-described embodiments unless otherwise specified. The power storage device 5 of this example can be configured as, for example, a lithium ion secondary battery.

  As shown in FIGS. 5 and 6, the power storage device 5 includes an electrode assembly 11 in which positive electrodes 1 p and negative electrodes 1 n are alternately stacked with separators 13 interposed therebetween. A liquid or gel electrolyte 14 is interposed between the positive electrode 1p and the negative electrode 1n. The negative electrode 1n of this example has the same configuration as that of the electrode 1 of Example 1.

  As shown in FIG. 6, the positive electrode 1p protrudes from one long side of the positive electrode current collector having a rectangular shape, the positive electrode active material layer 3p provided on both surfaces of the positive electrode current collector, and the positive electrode current collector. And a positive electrode tab 12p. The positive electrode current collector and the positive electrode active material layer 3p are accommodated in a separator 13 formed in a bag shape. The positive electrode tab 12p is provided at the same position in each positive electrode 1p.

  The negative electrode 1n includes a rectangular negative electrode current collector, a negative electrode active material layer provided on both sides of the negative electrode current collector, an insulating layer 4 provided on the negative electrode active material layer, and one of the negative electrode current collectors Negative electrode tab 12n protruding from the long side. The negative electrode tab 12n is provided at the same position in each negative electrode 1n. The negative electrode tab 12n is provided in the electrode assembly 11 at a position spaced from the positive electrode tab 12p.

Although not shown in the drawing, the negative electrode active material layer and the insulating layer 4 have the same configuration as in Example 1. That is, the negative electrode active material particles exposed on the surface of the negative electrode active material layer are covered with the insulating layer 4. In addition, the geometric average diameter of the insulator particles obtained by measuring in the cross section in the thickness direction is D _i [μm], and the distance between centers of the negative electrode active material particles in contact with the insulator particles is L _a [μm. when _a], the value of D i / _{L a} is 190～560Ppm.

  By alternately laminating the positive electrode 1p and the negative electrode 1n accommodated in the separator 13, the electrode assembly 11 can be obtained. In the electrode assembly 11, the positive electrode active material layer 3 p and the negative electrode active material layer face each other through the separator 13. Moreover, the some positive electrode tab 12p in the electrode assembly 11 is welded in the state piled up mutually. These positive electrode tabs 12p are electrically connected to a positive electrode terminal 531p (described later) through a current-carrying member 532p. Further, the plurality of negative electrode tabs 12n in the electrode assembly 11 are welded in a state of being overlapped with each other. These negative electrode tabs 12n are electrically connected to negative electrode terminals 531n (described later) through current-carrying members 532n.

  The electrode assembly 11 is accommodated in a box-shaped case 51 having an opening. In addition, a liquid or gel electrolyte 14 is accommodated in the case together with the electrode assembly 11. An insulating resin sheet 52 is interposed between the case 51 and the electrode assembly 11. Further, the opening of the case 51 is closed by a lid portion 53 provided with a positive terminal 531p and a negative terminal 531n. The positive electrode terminal 531p is electrically connected to the positive electrode tab 12p through the energizing member 532p inside the case 51. Similarly, the negative electrode terminal 531n is electrically connected to the negative electrode tab 12n through the energizing member 532n inside the case 51.

The power storage device 5 of this example includes a negative electrode active material layer, and an insulating layer 4 provided on the negative electrode active material layer and covering the negative electrode active material particles exposed on the surface of the negative electrode active material layer, and D _i / L _It has the negative electrode 1n whose value of a is 190-560 ppm. Therefore, peeling of the insulating layer 4 from the negative electrode active material layer can be suppressed, and an increase in internal resistance of the power storage device 5 can be suppressed.

(Experimental example)
In this example, the value of D _i / L _a which is the ratio of the geometric average diameter D _i [μm] of the insulator particles to the center-to-center distance L _a [μm] of the active material particles in contact with the insulator particles. It is the example which evaluated the ease of osmosis | permeation of the slurry containing an insulator particle and the ease of exfoliation of an insulating layer when changing variously.

  In this example, first, a test piece simulating a negative electrode was prepared. The test piece of this example includes a copper foil having a square shape with a side of 30 mm, and a negative electrode active material layer disposed on one surface of the copper foil. The negative electrode active material layer is made of natural graphite, and is made of a porous body containing 100 parts by weight of negative electrode active material particles having a geometric average diameter of 150 μm and 2 parts by weight of a binder.

  Separately from this test piece, a slurry containing insulator particles and a binder was prepared. As the insulator particles, aluminum oxide particles having a geometric average diameter shown in Table 1 were used.

  The test piece was placed on a hot plate set at 80 ° C., and the test piece was allowed to stand until the temperature was stabilized. In a state where the temperature of the test piece was stable, about 0.05 mL of the slurry containing the insulator particles was dropped on the negative electrode active material layer of the test piece, and how the slurry droplet spread was visually observed. The ease of penetration of the slurry was evaluated based on how the droplets spread.

  After the slurry is dropped on the negative electrode active material layer, if the slurry droplet does not spread immediately after dropping, the symbol A is written in the “Slurry permeability” column of Table 1, and the slurry droplet is dropped. In the case of spreading immediately after, the symbol B is described in the same column. In the evaluation of the ease of penetration of the slurry, the case of symbol A in which the slurry droplets did not spread immediately after dropping was judged as acceptable because the slurry remained on the surface of the negative electrode active material layer. Further, in the case of the symbol B in which the slurry droplets spread immediately after the dropping, the permeability of the slurry was excessively high, and it was judged as rejected because it entered the inside of the negative electrode active material layer.

  Further, after dropping the slurry, the test piece was left on a hot plate for 15 minutes, and the slurry was dried to form an insulating layer. The test piece was placed so that the negative electrode active material layer and the vertical plane were parallel, and the presence or absence of peeling of the insulating layer was visually observed.

  When the test piece is erected, if the insulating layer does not peel from the negative electrode active material layer, the symbol A is entered in the column “Ease of peeling of insulating layer” in Table 1, and at least one of the insulating layers. When the part peeled from the negative electrode active material layer, the symbol B was written in the same column. In the evaluation of the ease of peeling of the insulating layer, the case of symbol A where the insulating layer did not peel from the negative electrode active material layer was judged as acceptable because the peeling of the insulating layer could be suppressed. Moreover, since the effect which suppresses peeling of an insulating layer was low, the case of the symbol B which the insulating layer peeled from the negative electrode active material layer was judged to be disqualified.

As shown in Table 1, when the value of D i _/ L _a is greater than or equal 190ppm, the slurry was little spread after instillation in the negative electrode active material layer. As an example, FIG. 7 shows the shape of a droplet when the value of D _i / L _a is 288 ppm. When the value of D _i / L _a was 190 ppm or more, as shown in FIG. 7, the outline of the liquid droplet in a top view was substantially circular, and the liquid droplet shape immediately after the liquid droplet was maintained.

On the other hand, the values of D i _/ L _a is D _{i /} L _a is in the case of less than 190ppm, the slurry tends to penetrate into the negative electrode active material layer, the slurry is spread after instillation in the negative electrode active material layer. As an example, FIG. 8 shows the shape of a droplet when the value of D _i / L _a is 57 ppm. When the value of D _i / L _a was less than 190 ppm, as shown in FIG. 8, the slurry droplets spread non-uniformly, so that the contour of the droplets in the top view was uneven.

Further, the value of D i _/ L _a is when: 560ppm, when made a test piece, peeling of the insulating layer from the negative electrode active material layer did not occur. As an example, FIG. 9 shows a test piece when the value of D _i / L _a is 288 ppm. When the value of D _i / L _a was 560 ppm or less, as shown in FIG. 9, the insulating layer having a substantially circular shape when viewed from above was held on the negative electrode active material layer.

On the other hand, when the value of D i _/ L _a exceeds 560ppm, since a relatively large gap between the anode active material layer and the insulating layer is formed, when made a test piece, the anode active material layer At least part of the insulating layer was peeled off. As an example, FIG. 10 _shows a test piece in case the value of D i / _{L a} is 637Ppm. As shown in FIG. 10, when the value of D _i / L _a was 637 ppm, the entire insulating layer was peeled from the negative electrode active material layer. Although not shown in the figure, the value of D i _/ L _a is in the case of 751ppm, a part from the anode active material layer of the insulating layer is peeled off. Further, in this case, part of the negative electrode active material layer was peeled off from the current collector as the insulating layer was peeled off.

From these results, it is possible to suppress the exfoliation of the insulating layer from the active material layer while suppressing the entry of the slurry into the active material layer by setting the value of D _i / L _a within the above specific range. Can understand.

DESCRIPTION OF SYMBOLS 1 Electrode 2 Current collector 3 Active material layer 31, 311 Active material particle 4 Insulating layer 41 Insulator particle

Claims (5)

A current collector made of metal;
An active material layer comprising a porous body containing a large number of active material particles, and disposed on the current collector;
An insulating layer covering the active material particles exposed on the surface of the active material layer, comprising a porous body containing a large number of insulator particles;
Di _i [μm] is the geometric mean diameter of the insulator particles obtained by measuring in the cross section in the thickness direction, and L _a [μm] is the distance between the centers of the active material particles in contact with the insulator particles. when _a], the value of D i / _{L a} is 190～560Ppm, electrode.   The electrode according to claim 1, wherein a geometric average diameter of the active material particles is 100 to 210 μm.   The electrode according to claim 1 or 2, wherein a geometric average diameter of the insulator particles is 0.020 to 0.110 µm. Obtained by performing a measurement in the thickness direction of the cross section, the maximum penetration depth of the insulation particles to said active material layer is taken as t [[mu] m], the value of t / L _a is 0. The electrode according to any one of claims 1 to 3, wherein the electrode is 5-1. An electrode assembly in which positive and negative electrodes are alternately stacked via separators;
A liquid or gel electrolyte interposed between the positive electrode and the negative electrode;
5. The power storage device, wherein at least one of the positive electrode and the negative electrode is the electrode according to claim 1.