Electricity storage device and method for manufacturing the same

Abstract

To improve battery characteristics in a power storage device using an electrode structure having a plurality of cells, which are spaces formed by being surrounded by a wall.SOLUTION: The power storage device includes an electrode structure, a counter electrode, and a separation membrane. The electrode structure has a wall portion including an electrode active material and a plurality of cells, each of which is a space formed by being surrounded by the wall portion. The counter electrode contains a counter electrode active material and is filled in the cells. The separation membrane has ion conductivity and insulation properties, is formed on the surface of the wall portion, separates the electrode structure and the counter electrode, contains particles in a range of 70 volume % or more or 96 volume % or less, and having a thickness t in a range of 10 μm<t<70 μm.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present specification discloses an electricity storage device and a method for manufacturing an electricity storage device. [Background technology]

[0002] A conventional electricity storage device has been proposed that includes, for example, a first electrode, a first active material fluid electrically connected to the first electrode and containing a first active material and a supporting salt, which is flowable, and a second electrode formed with a second active material and including a structure that is immersed in the first active material fluid or that contains the first active material fluid, with a separation membrane that is ionically conductive and insulating formed between the first active material fluid and the structure (see, for example, Patent Document 1). This electricity storage device can further improve battery characteristics. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2018-73573 Summary of the Invention [Problem to be solved by the invention]

[0004] However, although the battery characteristics of the electricity storage device of Patent Document 1 can be further improved, they are still not sufficient, and there is a demand for further improvements in, for example, resistance and energy density per unit volume.

[0005] The present disclosure has been made in consideration of such problems, and its main purpose is to provide a novel energy storage device and a method for manufacturing an energy storage device that can further improve battery characteristics in an energy storage device that uses an electrode structure having multiple cells that are spaces formed by being surrounded by walls. [Means for solving the problem]

[0006] As a result of intensive research to achieve the above object, the present inventors have found that, in an electrode structure having a plurality of cells, when a separation membrane containing particles is used, battery characteristics can be further improved, and thus have completed the invention disclosed in this specification.

[0007] That is, the power storage device disclosed in this specification has an electrode structure having a plurality of cells, which are spaces formed by being surrounded by wall portions containing an electrode active material, a counter electrode containing a counter electrode active material and filled in the cells, and a separation membrane having ion conductivity and insulation, formed on the surface of the wall portion, separating the electrode structure and the counter electrode, containing particles in the range of 70% to 96% by volume, and having a thickness t in the range of 10 μm < t < 70 μm. It is provided with the above.

[0008] The method for manufacturing the power storage device disclosed in this specification includes a separation membrane forming step of forming, on the wall portion of an electrode structure having a plurality of cells, which are spaces formed by being surrounded by wall portions containing an electrode active material, a separation membrane having ion conductivity and insulation, containing particles in the range of 70% to 96% by volume, and having a thickness t in the range of 10 μm < t < 70 μm, and a counter electrode forming step of filling a counter electrode composite material containing a counter electrode active material into the cell in which the separation membrane is formed to form a counter electrode. It includes the above.

Advantages of the Invention

[0009] The present disclosure provides a novel power storage device and a manufacturing method thereof that further improves battery characteristics. The reason for this effect is believed to be as follows. For example, the space filling rate in the close-packed structure of spherical particles is 74% by volume in the hexagonal close-packed structure and the face-centered cubic close-packed structure, with the remaining 26% by volume being void. These voids can be used to fill with resin to form a separation membrane. Furthermore, when a separation membrane is configured to contain particles, the separation membrane can be made porous. For example, it is believed that by allowing an electrolyte solution to permeate the fine voids, ionic conductivity can be further improved. Therefore, it is believed that adding a predetermined amount of particles to the separation membrane can further improve battery characteristics, such as further reducing resistance. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a schematic diagram showing an example of an electricity storage device 10. FIG. [Figure 2] FIG. 2 is an explanatory diagram showing an example of the shape of a small compartment space 14 and the inscribed circle diameter r. [Figure 3] Measurement results showing the resistance values ​​of each experimental example. [Figure 4] 10 is a diagram showing the relationship between the distance from the center of the small compartment space 14 and the resistance value. [Figure 5] Measurement results showing the energy density of each experimental example. [Figure 6] Relationship between the ratio r / t and resistance value. [Figure 7] Relationship between the ratio r / t and energy density. [Figure 8] The relationship between the ratio r / t and the slope of the increase in energy density in Fig. 7 . DETAILED DESCRIPTION OF THE INVENTION

[0011] Next, the electricity storage device disclosed in this embodiment will be described with reference to the drawings. FIG. 1 is a schematic diagram showing an example of an electricity storage device 10. FIG. 2 is an explanatory diagram showing an example of the shape and inscribed circle radius r of a small compartment space 14, with FIG. 2A being an explanatory diagram of a hexagonal small compartment space 14, FIG. 2B being a rectangle, and FIG. 2C being a triangle. The electricity storage device 10 may be, for example, an electric double layer capacitor, a hybrid capacitor, a pseudo-electric double layer capacitor, an alkali metal secondary battery, an alkali metal ion battery, or the like. Examples of carrier ions of the electricity storage device 10 include alkali metal ions such as lithium ions, sodium ions, and potassium ions, and Group 2 ions such as magnesium ions, strontium ions, and calcium ions. The electricity storage device 10 includes an electrode structure 11, a separation membrane 17, a counter electrode 19, and a battery case 30. In this electricity storage device 10, the negative electrode 16 is composed of an electrode structure 11 containing an electrode active material and an electrode current collector 15, and the positive electrode 18 is composed of a counter electrode 19 containing a counter electrode active material, a counter electrode current collector 21, and a current collecting terminal 22. In the electricity storage device 10, the negative electrode may be the counter electrode 19, and the positive electrode may be the electrode structure 11. In the electricity storage device 10, one or more of the electrode structure 11, the separator 17, and the positive electrode 18 may contain an electrolyte. In addition, the electrode structure 11 and the counter electrode 19 may have a current collecting member such as a current collecting wire embedded therein, or may not have such a current collecting member. For ease of explanation, the electrode structure 11 is a negative electrode containing a negative electrode active material, the counter electrode 19 is a positive electrode containing a positive electrode active material, and a lithium ion secondary battery using lithium ions as a carrier will be described below as a main example of the electricity storage device 10.

[0012] The negative electrode 16 is composed of an electrode structure 11 and an electrode current collector 15. The electrode structure 11 is a structure having walls 12 containing an electrode active material and a plurality of small compartment spaces 14 (cells) that are spaces surrounded by the walls 12. In the electrode structure 11, the small compartment spaces 14 are bottomed holes with bottoms 13 formed at their ends. The electrode structure 11 is a conductive member formed to contain an electrode active material. In the electrode structure 11, a counter electrode 19 is accommodated in the small compartment space 14. This electrode structure 11 may have a structure in which the counter electrode 19 is filled in the small compartment space 14. This electrode structure 11 has the small compartment space 14 that is a bottomed hole having a bottom 13 formed therein, and the bottom 13 may have a filling hole formed therein to be used for filling the counter electrode 19. This filling hole may be closed after the small compartment space 14 is filled with the counter electrode 19. Although the bottom 13 is formed at the end of the small compartment space 14, the bottom 13 may not be provided as long as short-circuit prevention between the negative electrode 16 and the positive electrode 18 is ensured.

[0013] In this electrode structure 11, the shape of the compartment spaces 14 perpendicular to the longitudinal direction is preferably a shape that optimizes the volume ratio with respect to the wall portions 12. For example, a polygonal cross section is preferred, such as a hexagon, rectangle, octagon, or triangle. Of these, a hexagonal honeycomb structure is more preferred. Here, the cross-sectional shape of the compartment spaces 14 will be explained. The shape with the shortest circumference for the same area is a circle. On the other hand, if multiple compartment spaces 14 with circular cross sections are arranged in the electrode structure 11, the wall portions 12 will have a distorted shape, making it inappropriate to arrange compartment spaces 14 with circular cross sections. As shown in Figure 2, shapes that can be filled in a plane include a triangle, a square, and a hexagon, but the shape with the shortest circumference is a regular hexagon. If the cross-sectional shape of the compartment spaces 14 is a hexagon, the area (volume) of the partition portion can be minimized due to the shortest circumference, making it possible to design the highest energy density per unit volume. Furthermore, because a hexagon is close to a circle, the distance from the center to the outer periphery is the shortest even with the same area, and the distance to the outer periphery can be averaged, thereby improving the lithium reaction rate, i.e., the rate performance, and reducing reaction non-uniformity. Therefore, a hexagon is most preferable as the cross-sectional shape of the compartment space 14. The length A of one side of the opening of the compartment space 14 is preferably, for example, in the range of 40 μm to 500 μm, and more preferably in the range of 50 μm to 250 μm. Furthermore, the thickness B of the wall portion 12 is, for example, preferably in the range of 5 μm to 200 μm, and more preferably in the range of 7 μm to 70 μm. Within this range, a favorable balance between the electrode active material and the counter electrode active material is achieved. The longitudinal length L of the compartment space 14 may be determined appropriately depending on the battery design, such as the battery capacity of the power storage device 10.

[0014] The electrode structure 11 is formed by including an electrode active material. However, if the electrode active material is not conductive, it may be mixed with a conductive material such as a carbon material and molded. The electrode structure 11 may be formed by mixing an electrode active material with, if necessary, a conductive material and a binder and molding the mixture. Examples of the electrode active material include materials capable of absorbing and desorbing lithium as a carrier. Examples of the electrode active material include inorganic compounds such as lithium, lithium alloys, and tin compounds; carbonaceous materials capable of absorbing and desorbing lithium ions; composite oxides containing multiple elements; and conductive polymers. Examples of the carbonaceous material include cokes, glassy carbons, graphites, non-graphitizable carbons, pyrolytic carbons, and carbon fibers. Among these, graphites such as artificial graphite and natural graphite are preferred. Examples of composite oxides include lithium-titanium composite oxide and lithium-vanadium composite oxide. The conductive material is not particularly limited as long as it is an electron-conductive material that does not adversely affect battery performance. For example, graphite such as natural graphite (scale graphite, flake graphite) or artificial graphite, acetylene black, carbon black, ketjen black, carbon whisker, needle coke, carbon fiber, and metals (copper, nickel, aluminum, silver, gold, etc.) can be used alone or in combination. Of these, carbon black and acetylene black are preferred as conductive materials. The average particle size of the conductive material is preferably in the range of 0.02 μm to 0.2 μm, and more preferably in the range of 0.05 μm to 0.1 μm. Within this range, good fluidity is achieved. The binder serves to bind the electrode active material particles and conductive material particles together to maintain a predetermined shape, and can be, for example, a fluorine-containing resin such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), or fluorine rubber, or a thermoplastic resin such as polypropylene or polyethylene, ethylene propylene diene monomer (EPDM) rubber, sulfonated EPDM rubber, or natural butyl rubber (NBR), either alone or as a mixture of two or more. Also usable are aqueous binders such as cellulose-based binders and aqueous dispersions of styrene butadiene rubber (SBR).

[0015] In the electrode structure 11, the content of the electrode active material is preferably higher, and is preferably in the range of 70% by mass to 99% by mass, more preferably 75% by mass to 98% by mass, and even more preferably 80% by mass to 95% by mass, based on the entire electrode structure 11. The content of the conductive material is preferably in the range of 0% by mass to 20% by mass, and more preferably 2% by mass to 10% by mass, based on the entire electrode structure 11. The content of the binder is preferably in the range of 0% by mass to 5% by mass, and more preferably 0.1% by mass to 1% by mass, based on the entire electrode structure 11.

[0016] The electrode current collector 15 is made of a conductive material. Examples of materials that can be used for this electrode current collector 15 include carbon paper, aluminum, copper, titanium, stainless steel, nickel, iron, platinum, baked carbon, conductive polymers, and conductive glass. For the purpose of improving adhesion, conductivity, and oxidation (reduction) resistance, it is also possible to use aluminum or copper whose surface has been treated with carbon, nickel, titanium, silver, platinum, or gold. The electrode current collector 15 may be in the form of, for example, a foil, a film, a sheet, a net, a punched or expanded material, a lath, a porous material, a foam, or a fiber group. The thickness T of the electrode current collector 15 may be, for example, 100 μm to 1 cm.

[0017] The positive electrode 18 is composed of a counter electrode 19, a counter electrode current collector 21, and a current collecting terminal 22. The counter electrode 19 contains a counter electrode active material and is filled in the small compartment space 14. This counter electrode 19 may be made of, for example, a counter electrode composite material containing a counter electrode active material and, as necessary, a conductive material and a binder. The counter electrode 19 may be a solid material in which the counter electrode active material is a solid active material, or may be a fluid slurry containing the counter electrode active material and an electrolyte. Examples of the counter electrode active material include compounds containing lithium and a transition metal, such as oxides containing lithium and a transition metal element, and phosphate compounds containing lithium and a transition metal element. Specifically, a compound having a basic composition formula of Li (1-x)MnO2 (where 0 < x < 1, the same applies hereinafter), Li (1-x) Lithium manganese composite oxides such as Mn2O4, with the basic composition formula Li (1-x) Lithium cobalt composite oxides such as CoO2, with the basic composition formula Li (1-x) Lithium nickel composite oxides such as NiO2, with the basic composition formula Li (1-x) Co a Ni b Mn c Lithium cobalt nickel manganese composite oxides such as O2 (where a > 0, b > 0, c > 0, a + b + c = 1), lithium vanadium composite oxides with the basic composition formula LiV2O3, transition metal oxides with the basic composition formula V2O5, etc. can be used. Also, lithium iron phosphate compounds with the basic composition formula LiFePO4, etc. can be used as the positive electrode active material. Among these, lithium cobalt nickel manganese composite oxides, for example, LiCo 1 / 3 Ni 1 / 3 Mn 1 / 3 O2 and LiNi 0.4 Co 0.3 Mn 0.3 O2, etc. are preferred. Note that the "basic composition formula" means that it may contain components of other elements, such as Al and Mg. The average particle size of the counter electrode active material is preferably in the range of, for example, 0.1 μm or more and 50 μm or less, and more preferably in the range of 1 μm or more and 20 μm or less. In such a range, the filling property is good.

[0018] The conductive material and binder contained in the counter electrode 19 can be those described for the negative electrode 16, as appropriate. The content of the counter electrode active material is preferably higher, preferably in the range of 70% by mass to 98% by mass, and more preferably in the range of 80% by mass to 95% by mass, based on the entire counter electrode 19. Within such a range, the battery capacity can be made suitable. The amount of conductive material added is preferably in the range of 2% by mass to 30% by mass, and more preferably in the range of 5% by mass to 20% by mass, based on the entire counter electrode 19. Within such a range, a decrease in battery capacity can be suppressed and sufficient conductivity can be imparted. Here, the average particle size of powder such as the active material is determined by observing the powder with a scanning electron microscope (SEM) and averaging the diameters of the powder particles measured in the observed image.

[0019] The counter electrode current collector 21 is made of a conductive material. The counter electrode current collector 21 can be made of any of the materials and shapes listed above for the electrode current collector 15. The thickness T' of the counter electrode current collector 21 can be set to the same range as the thickness T of the electrode current collector 15. A current collecting terminal 22 is disposed on the underside of the counter electrode current collector 21 and is inserted into the opening of the small compartment space 14. This current collecting terminal 22 may be made of any conductive material, and may be made of the same material as the counter electrode current collector 21 or a different material from the counter electrode current collector 21. The length of the current collecting terminal 22 can be set appropriately depending on the conductivity of the counter electrode 19, etc. Furthermore, if the conductivity of the counter electrode 19 is sufficient, the current collecting terminal 22 may be omitted.

[0020] The separator 17 has ion conductivity for carrier ions (e.g., lithium ions) and insulation properties, and insulates and separates the electrode structure 11 from the counter electrode 19. The separator 17 is formed on the entire surface of the wall 12 of the compartment space 14 facing the counter electrode 19, preventing short-circuiting between the negative electrode 16 and the positive electrode 18. The separator 17 includes a membrane 31 having ion conductivity and insulation properties and non-conductive (insulating) particles 32. The membrane 31 may be made of, for example, a resin. Examples of resins for the separator 17 include polyvinylidene fluoride (PVdF), a copolymer of PVdF and hexafluoropropylene (PVdF-HFP), polymethyl methacrylate (PMMA), and a copolymer of PMMA and an acrylic polymer. For example, in the case of a copolymer of PVdF and HFP, a portion of the electrolyte swells and gels the membrane, forming an ion-conductive membrane. The thickness t of the separation membrane 17 is, for example, preferably greater than 10 μm, more preferably 12 μm or more, even more preferably 15 μm or more, and may be 20 μm or more. The thickness t is preferably less than 70 μm, more preferably 65 μm or less, even more preferably 60 μm or less, and may be 50 μm or less. A thicker thickness t of greater than 10 μm is preferred to ensure insulation. A thickness t of less than 70 μm of the separation membrane 17 is preferred to prevent a decrease in ion conductivity and to further reduce the volume it occupies in the cell. A thickness t in the range of 20 to 50 μm provides favorable ion conductivity and insulation.

[0021] The particles 32 form a skeleton inside the separation membrane 17. The particles 32 are not particularly limited as long as they are insulating, and may be inorganic or organic particles. Examples of inorganic particles include ceramic particles. Examples of the particles 32 include one or more of aluminum oxide (alumina), silicon oxide (silica), titanium oxide (titania), aluminum hydroxide oxide (boehmite), zirconium oxide (zirconia), silicon carbide, and silicon nitride. Among these, alumina, silica, titania, and boehmite are preferred, and alumina is more preferred. The particles 32 preferably have an average particle size ranging from 0.4 μm to 4 μm, and more preferably from 0.44 μm to 3.5 μm. A smaller average particle size is preferable to further improve ionic conductivity, and a particle size of 1 μm or less is even more preferred. The average particle size can be determined by measuring the longest length of each particle using an image captured by a scanning electron microscope and averaging these values. The particles 32 are contained in a range of 70% by volume or more of the total including the membrane 31. When the content of the particles 32 is 70% by volume or more, it is possible to suppress a decrease in ionic conductivity while further suppressing an increase in membrane thickness. The content of the particles 32 is more preferably 75% by volume or more, and even more preferably 80% by volume or more. The content of the particles 32 may be 96% by volume or less, preferably 95% by volume or less, and even more preferably 90% by volume or less. The upper limit of this content is ultimately determined based on a balance with battery performance, but if it is 96% by volume or less, the membrane 31 can bind the particles 32 to the wall portion 12. The particles 32 are contained in a range of 85% by mass or more of the total including the membrane 31, more preferably 87.5% by mass or more, and even more preferably 90% by mass or more. The content of the particles 32 may be 98% by mass or less, preferably 97.5% by mass or less, and even more preferably 95% by mass or less.

[0022] The electricity storage device 10 may contain an electrolyte solution in one or more of the negative electrode 16, the separator 17, and the positive electrode 18. The electrolyte solution may contain, for example, a supporting salt. Examples of the solvent for the electrolyte solution include solvents for non-aqueous electrolyte solutions. Examples of such solvents include carbonates, esters, ethers, nitriles, furans, sulfolanes, and dioxolanes, which may be used alone or in combination. Specific examples of carbonates include cyclic carbonates such as ethylene carbonate, propylene carbonate, vinylene carbonate, butylene carbonate, and chloroethylene carbonate; chain carbonates such as dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, ethyl-n-butyl carbonate, methyl-t-butyl carbonate, di-i-propyl carbonate, and t-butyl-i-propyl carbonate; cyclic esters such as γ-butyl lactone and γ-valerolactone; chain esters such as methyl formate, methyl acetate, ethyl acetate, and methyl butyrate; ethers such as dimethoxyethane, ethoxymethoxyethane, and diethoxyethane; nitriles such as acetonitrile and benzonitrile; furans such as tetrahydrofuran and methyltetrahydrofuran; sulfolanes such as sulfolane and tetramethylsulfolane; and dioxolanes such as 1,3-dioxolane and methyldioxolane. Among these, a combination of cyclic carbonates and chain carbonates is preferred, as this combination not only provides excellent cycle characteristics, which represent the battery characteristics during repeated charge and discharge, but also allows for a well-balanced viscosity of the electrolyte, the electric capacity of the resulting battery, and the battery output.

[0023] The supporting salt contained in the electrolyte solution contains, for example, ions that serve as carriers for the power storage device 10. Examples of supporting salts include LiPF, LiBF, LiAsF, LiCFSO, LiN(CFSO), LiC(CFSO), LiSbF, LiSiF, LiAlF, LiSCN, LiClO, LiCl, LiF, LiBr, LiI, and LiAlCl. From the viewpoint of electrical properties, it is preferable to use a combination of one or more salts selected from the group consisting of inorganic salts such as LiPF, LiBF, LiAsF, and LiClO, and organic salts such as LiCFSO, LiN(CFSO), and LiC(CFSO). The concentration of this supporting salt in the electrolyte solution is preferably 0.1 mol / L to 5 mol / L, and more preferably 0.5 mol / L to 2 mol / L. When the concentration of the dissolving supporting electrolyte is 0.1 mol / L or more, a sufficient current density can be obtained, and when it is 5 mol / L or less, the electrolyte can be made more stable.

[0024] In this energy storage device 10, as shown in FIG. 2 , the ratio r / t of the radius r of the inscribed circle R inscribed in the unit U partitioned at the center of the wall portion 12 to the thickness t of the separation membrane 17 may be 10 or less, more preferably 7.5 or less, and even more preferably 5 or less. A smaller ratio r / t tends to further reduce resistance, which is preferable. From the viewpoint of energy density, a larger ratio r / t is preferable, and is preferably 2 or more, more preferably 2.4 or more, and even more preferably 3 or more. A ratio r / t in the range of 2.4 to 5 is preferable because it can further optimize energy density and resistance. It is presumed that the smaller the ratio r / t, the greater the effect of the cross-sectional shape of the small compartment space 14. For example, it is more preferable that the cross-sectional shape of the small compartment space 14 is hexagonal and the ratio r / t is in the range of 2.4 to 5, from the viewpoint of further increasing the effect of increasing energy density. Furthermore, the range of the radius r of the inscribed circle R may be determined as an appropriate value from the range of the ratio r / t and the range of the thickness t, for example.

[0025] Battery case 30 is a member that houses first electrode 14, electrode structure 11, etc. This battery case 30 may be made of any material as long as it is strong enough to prevent short-circuiting between first electrode 14 and second electrode 18 and to protect electrode structure 11, etc., and may be made of, for example, an insulating resin. The shape of electricity storage device 10 is not particularly limited, and examples include a coin type, button type, sheet type, laminated type, cylindrical type, flat type, and rectangular type.

[0026] (Method of manufacturing an electricity storage device) Next, a method for manufacturing the electricity storage device 10 will be described. This manufacturing method may include a separation membrane forming step and a counter electrode forming step. Furthermore, this manufacturing method may include, after the counter electrode forming step, an accommodation step of accommodating the electrode structure 11 in a battery case 30, and a current collector connecting step of connecting the electrode current collector 15 and the counter electrode current collector 21. Furthermore, this manufacturing method may include a molding step of molding the electrode structure 11 before the separation membrane forming step. Note that in this manufacturing method, the material, shape, size, content, and the like of the electricity storage device 10 described above are appropriately adopted, and detailed description thereof will be omitted.

[0027] In the separation membrane formation process, a separation membrane 17 having ion conductivity and insulating properties and containing particles 32 in a range of 70% to 96% by volume is formed on the wall 12 of an electrode structure 11 having a wall 12 containing an electrode active material and a plurality of compartment spaces 14 (cells) surrounded by the wall 12. In this process, an electrode structure 11 is used in which the ratio r / t of the radius r of the inscribed circle R inscribed in the unit U partitioned at the center of the wall 12 to the thickness t of the separation membrane 17 is within a predetermined range. As described above, the ratio r / t can be set arbitrarily, for example, within a range of 2 to 10, and more preferably 5 or less. In this process, an electrode structure 11 in which the cross sections of the compartment spaces 14 perpendicular to the longitudinal direction are polygonal may be used. More preferably, the electrode structure 11 has compartment spaces 14 with hexagonal cross sections. The separation membrane 17 may be formed, for example, by using a raw material solution containing the raw material of the membrane 31 and the particles 32, pouring the raw material solution into the compartment space 14, and then removing the excess. Alternatively, the electrode structure 11 may be immersed in the raw material solution and coated on its surface. The solvent for the raw material solution is preferably one that can dissolve the resin that is the raw material of the membrane 31, such as N-methylpyrrolidone (NMP). The particles 32 are insulating, and are preferably ceramic particles, such as alumina particles. The content of the particles 32 is in the range of 70% to 96% by volume, and may be 75% to 95% by volume, based on the total content of the separation membrane 17. The content of the particles 32 is in the range of 85% to 98% by mass, and may be 90% to 95% by mass, based on the total content of the separation membrane 17. The thickness t of the separation membrane 17 can be, for example, in the range of more than 10 μm and less than 70 μm, preferably in the range of 15 μm to 60 μm, and may be in the range of 20 μm to 50 μm.

[0028] In the counter electrode formation step, a counter electrode mixture containing a counter electrode active material is filled into the compartment spaces 14 (cells) of the electrode structure 11 on which the separation membrane 17 has been formed, to form the counter electrode 19. The counter electrode mixture may be, for example, a counter electrode mixture paste or slurry containing the counter electrode active material and, if necessary, a conductive material or binder, and imparted fluidity with a solvent. In this step, after the counter electrode mixture is placed in the compartment spaces 14, the solvent may be removed to form a solid counter electrode 19, or the solvent may remain to form a fluid counter electrode 19. In this manner, the electricity storage device 10 can be produced.

[0029] The electricity storage device 10 and its manufacturing method described above can provide a novel electricity storage device and its manufacturing method that further improves battery characteristics. The reason for this effect is believed to be as follows. For example, the space filling rate in the close-packed structure of spherical particles is 74% by volume in the hexagonal close-packed structure and the face-centered cubic close-packed structure, with the remaining 26% by volume being void. These voids can be filled with resin to form the separation membrane 17. Furthermore, when the separation membrane is configured to contain particles, the separation membrane can be made porous. For example, by allowing an electrolyte to permeate the fine voids, ionic conductivity can be further improved. Furthermore, when the separation membrane 17 is configured to contain particles 32, ionic conduction is promoted at the interface of the particles 32, which is believed to be the so-called Soggy-Sand effect, thereby improving ionic conductivity. Therefore, it is believed that adding a predetermined amount of particles 32 to the separation membrane 17 can further improve battery characteristics, such as reducing resistance. Furthermore, in a separation membrane 17 containing 85% by mass or more of particles 32, for example, when impregnated with an electrolyte solution, the membrane body 31 in the gaps between the particles swells, but this does not affect the increase in volume of the separation membrane 17. Therefore, the structure of the electrode structure 11 is less likely to change, and durability can be further improved.

[0030] Furthermore, since the cross-sectional shape of the compartment space 14 is hexagonal and has a short perimeter, the area (volume) of the separation membrane 17 can be minimized, allowing for a design with a higher energy density. Furthermore, in the electricity storage device 10, the counter electrode active material and the electrode active material are in close proximity to each other, reducing unevenness in the salt concentration in the electrode membrane thickness direction and enabling rapid charging. Furthermore, in the electricity storage device 10, as much active material as possible can contribute to the battery reaction, the battery capacity can be increased.

[0031] It goes without saying that the present invention is not limited to the above-described embodiment, and can be embodied in various forms as long as they fall within the technical scope of the present invention.

[0032] For example, in the above-described embodiment, a lithium ion secondary battery has been described in which the counter electrode has a counter electrode active material capable of absorbing and releasing lithium ions, but the present invention is not particularly limited thereto and may be used in other power storage devices such as a hybrid capacitor. The counter electrode active material may be a carbon material that adsorbs and desorbs carrier ions. The carbon material is not particularly limited, but examples thereof include activated carbons, cokes, glassy carbons, graphites, non-graphitizable carbons, pyrolytic carbons, carbon fibers, carbon nanotubes, and polyacenes. Among these, activated carbons exhibiting a high specific surface area are preferred. Activated carbons as carbon materials have a specific surface area of ​​1000 m 2 / g or more, and 1500m 2 / g or more is more preferable. 2 / g or more, the discharge capacity can be further increased. The specific surface area of ​​this activated carbon is 3000 m 2 / g or less, and 2 It is more preferable that the capacitance is 1 / g or less. It is considered that such a counter electrode 19 stores electricity by adsorbing and desorbing at least one of anions and cations contained in the ionically conductive medium, but it may also store electricity by inserting and desorbing at least one of anions and cations contained in the ionically conductive medium.

[0033] In the above-described embodiment, the electrode structure 11 is the negative electrode and the counter electrode 19 is the positive electrode, but this is not particularly limited, and the counter electrode 19 may be the negative electrode and the electrode structure 11 may be the positive electrode. Furthermore, in the above-described embodiment, the carrier of the electricity storage device 10 is lithium ion, but this is not particularly limited, and the carrier may be alkali ion such as sodium ion or potassium ion, or Group 2 element ion such as calcium ion or magnesium ion. Furthermore, although the electrolyte solution is a nonaqueous electrolyte solution, it may also be an aqueous electrolyte solution. [Example]

[0034] Specific examples of fabricating the above-described electricity storage device will be described below as experimental examples. Experimental examples 7, 15, and 18 are comparative examples, and experimental examples 1 to 6, 8 to 14, 16, and 17 are examples of the present disclosure. First, a separation membrane was examined as a reference example.

[0035] (Separation membranes of Reference Examples 1 to 5) A self-supporting resin membrane (separation membrane) containing ceramic particles and exhibiting ion conductivity and insulation properties was fabricated. Alumina powder (Al2O3, manufactured by Sumitomo Chemical Co., Ltd.) with an average particle size of 0.44 μm as ceramic particles was mixed with N-methylpyrrolidone (NMP) and subjected to ultrasonic treatment for 30 minutes to obtain a dispersion. Polyvinylidene fluoride-co-hexafluoropropylene (PVdF-HFP) was mixed with this dispersion and stirred overnight or longer to dissolve the PVdF-HFP in the dispersion. The amount of alumina was adjusted to 87 mass%, 90 mass%, 93 mass%, 70 mass%, and 78 mass% relative to the total amount of alumina and PVdF-HFP, respectively, to obtain the separation membranes of Reference Examples 1 to 5. The alumina addition amounts (volume %) in Reference Examples 1 to 5 correspond to 75 volume%, 80 volume%, 85 volume%, 50 volume%, and 60 volume%.

[0036] (Evaluation of separation membranes) The ionic conductivity and membrane thickness increase rate of the separation membrane were evaluated as follows. A predetermined amount of the above-mentioned ceramic particle-PVdF-HFP slurry was weighed into a polytetrafluoroethylene dish, and the NMP was evaporated to obtain a 100 μm-thick separation membrane. This membrane was immersed in a nonaqueous electrolyte for at least two days. The nonaqueous electrolyte used was a 1M solution of LiPF6 in a mixed solvent of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 30 / 40 / 30. The membrane thickness before and after immersion was compared, and the membrane thickness increase rate (%) was calculated. In addition, a measurement cell was fabricated by sandwiching the immersed separation membrane between two Ni electrodes, and the conductivity of the separation membrane was evaluated by AC impedance analysis. The measurement cell prepared above was measured using an AC impedance analyzer (Agilent 4294A) at an open circuit voltage amplitude of ±500 mV, a frequency range of 1 Hz to 100 kHz, and a measurement temperature of 25°C, and the ionic conductivity (mS / cm) was calculated from the resistance between the current collectors.

[0037] (Results and Discussion) Table 1 summarizes the ceramic particle type, particle size (μm), addition ratio (mass%), separation membrane conductivity (mS / cm), and membrane thickness increase rate (%) for Reference Examples 1 to 5. As shown in Table 1, in Reference Examples 1 to 3, where the alumina ratio was 70% by volume or more, the membrane thickness increase rate was low at 1%, and the ionic conductivity was also high. On the other hand, in Reference Examples 4 and 5, where the alumina ratio was less than 70% by volume, the membrane thickness increase rate was large at 5% or more, and the ionic conductivity was also low. Thus, when the alumina addition ratio was 70% by volume or more (the closest packed volume), the polymer was contained between the alumina particles, resulting in excellent electrolyte impregnation. Even when the resin swelled in a nonaqueous electrolyte, the membrane thickness increase rate was suppressed, and battery performance was presumably improved. Furthermore, it was presumed that the Li ion conductivity was improved due to the so-called Soggy-Sand effect, in which cation conduction was promoted on the surfaces of nano- to submicron-sized particles. Since this effect is thought to be manifested by the above-mentioned mechanism, it is presumed that the effect is manifested regardless of the particle size and type of resin.

[0038] [Table 1]

[0039] (Fabrication of electricity storage devices) An electricity storage device having an electrode structure, a counter electrode, and a separator, as shown in Figure 1, was fabricated. The electrode structure contained an electrode active material and a binder, and the composite material was mixed at a mass ratio of 98.5:1.5. The composite was extruded from a honeycomb mold that formed cells as multiple internal spaces, and the electrode structure was fabricated by heat treatment (firing and drying). The electrode active material was graphite, and the binder was carboxymethyl cellulose (CMC). The cross-sectional shape of the cells perpendicular to the axial direction was either a regular hexagon, a square (rectangle), or an equilateral triangle. The density of the obtained electrode structure was 1.3 g / cm 3 The separation membrane had a structure in which polyvinylidene fluoride (PVdF: #8500 manufactured by Kureha) contained 90 mass % alumina particles (average particle size 0.44 μm), and was formed on the surface of the electrode structure to a thickness of 10 to 70 μm. This separation membrane was formed by mixing PVdF and alumina particles, dissolving and dispersing the mixture in N-methyl-2-pyrrolidone (NMP), applying the solution to the inner wall surface of the cell of the electrode structure by dipping or spraying, and then drying (solvent removal). The counter electrode was made of LiCo as the counter electrode active material. 1 / 3 Ni 1 / 3 Mn 1 / 3 The counter electrode active material consisted of O2, acetylene black (AB) as a conductive material, and PVdF as a binder. The counter electrode active material, conductive material, and binder were mixed in a mass ratio of 92:6:2, and NMP was used as a solvent to prepare a counter electrode composite slurry. This slurry was then filled into the cell (space) formed in the electrode structure, and the solvent was removed (dried). The composite density of the counter electrode composite was 2.5 g / cm. 3The electrolyte used was a 1M solution of LiPF6 in a mixed solvent of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 1:1:1. The electrode structure with the separator formed and the counter electrode composite filled therein was placed in a battery case, and the required amount of electrolyte was poured into it. This allowed the electrolyte to permeate the gaps between the electrode structure, separator, and counter electrode composite. In each electrode structure shown in Figure 2, the "unit" was a polygonal region separated by a line in the center of the partition wall, and the radius r of the inscribed circle was 43 to 185 μm. The electrode structure and counter electrode composite layer in each unit were designed to have the same volume (cross-sectional area). Based on the configuration and density of the electrode structure (negative electrode) and counter electrode, the capacity ratio (negative electrode capacity / positive electrode capacity) was set to 1.1.

[0040] (Experimental Examples 1 to 6) An electricity storage device fabricated in such a manner that the shape of the cell (small compartment space) was a regular hexagon, the unit inscribed circle diameter r was 54 μm, the wall thickness B was 11.6 μm, the separation membrane thickness t was 20 μm, and the separation membrane was made of PVdF with 90 mass% alumina particles added was designated Experimental Example 1. An electricity storage device fabricated in the same manner as Experimental Example 1 except that the unit inscribed circle diameter r was 70 μm and the wall thickness B was 21 μm was designated Experimental Example 2. An electricity storage device fabricated in the same manner as Experimental Example 1 except that the unit inscribed circle diameter r was 100 μm and the wall thickness B was 38.6 μm was designated Experimental Example 3. An electricity storage device fabricated in the same manner as Experimental Example 1 except that the unit inscribed circle diameter r was 70 μm, the wall thickness B was 11 μm, and the separation membrane thickness t was 30 μm was designated Experimental Example 4. An electricity storage device prepared in the same manner as in Experimental Example 1 except that the unit inscribed circle diameter r was 120 μm, the wall thickness B was 20.3 μm, and the separation membrane thickness t was 50 μm was designated as Experimental Example 5. An electricity storage device prepared in the same manner as in Experimental Example 1 except that the unit inscribed circle diameter r was 185 μm, the wall thickness B was 58.4 μm, and the separation membrane thickness t was 50 μm was designated as Experimental Example 6.

[0041] (Experimental Example 7) An electricity storage device was prepared as Experimental Example 7 in the same manner as in Experimental Example 1, except that the separation membrane was a non-porous membrane made of a copolymer of PVdF and hexafluoropropylene (HFP).

[0042] (Experimental Examples 8-11) Experimental Example 8 was an electricity storage device fabricated in the same manner as Experimental Example 1 except that the shape of the compartment space was a square, the unit inscribed circle diameter r was 50 μm, and the wall thickness B was 9.3 μm. Experimental Example 9 was an electricity storage device fabricated in the same manner as Experimental Example 1 except that the shape of the compartment space was an equilateral triangle, the unit inscribed circle diameter r was 44 μm, and the wall thickness B was 5.8 μm. Experimental Example 10 was an electricity storage device fabricated in the same manner as Experimental Example 1 except that the shape of the compartment space was a square, the unit inscribed circle diameter r was 93 μm, and the wall thickness B was 34.5 μm. Experimental Example 11 was an electricity storage device fabricated in the same manner as Experimental Example 1 except that the shape of the compartment space was an equilateral triangle, the unit inscribed circle diameter r was 82 μm, and the wall thickness B was 28 μm.

[0043] (Experimental Examples 12-18) Experimental Example 12 was an electricity storage device fabricated in the same manner as Experimental Example 1, except that the unit inscribed circle diameter r was 150 μm and the wall thickness B was 9.3 μm. Experimental Example 13 was an electricity storage device fabricated in the same manner as Experimental Example 1, except that the shape of the compartment space was square, the unit inscribed circle diameter r was 140 μm, and the wall thickness B was 62 μm. Experimental Example 14 was an electricity storage device fabricated in the same manner as Experimental Example 1, except that the shape of the compartment space was equilateral triangle, the unit inscribed circle diameter r was 123 μm, and the wall thickness B was 52.1 μm. Experimental Example 15 was an electricity storage device fabricated in the same manner as Experimental Example 1, except that the unit inscribed circle diameter r was 43 μm, the wall thickness B was 15.2 μm, and the separation membrane thickness t was 10 μm. Experimental Example 16 was an electricity storage device fabricated in the same manner as Experimental Example 1, except that the shape of the compartment space was square, the unit inscribed circle diameter r was 112 μm, the wall thickness B was 15.6 μm, and the separation membrane thickness t was 50 μm. An electricity storage device prepared in the same manner as in Experimental Example 1 except that the shape of the small compartment space was an equilateral triangle, the unit inscribed circle diameter r was 98 μm, the wall thickness B was 7.4 μm, and the separation membrane thickness t was 50 μm was designated Experimental Example 17. An electricity storage device prepared in the same manner as in Experimental Example 1 except that the unit inscribed circle diameter r was 140 μm, the wall thickness B was 12 μm, and the separation membrane thickness t was 70 μm was designated Experimental Example 18.

[0044] (Energy density and resistance measurements) The energy density and 1 kHz resistance of the fabricated energy storage device were measured. The energy density was measured at a current density of 0.8 mA / cm. 2 Three charge / discharge cycles were repeated under conditions of a potential window of 3.0 to 4.1 V, and the discharge capacity at the third cycle was measured and calculated as the volumetric energy density (Wh / L) per unit volume of the unit cell. The resistance was measured by charging the unit cell to a remaining capacity SOC of 50% and then measuring the 1 kHz resistance using a measuring device (3555 BATTERY HITESTER manufactured by Hioki E.E. Corporation).

[0045] (Results and Discussion) Table 1 summarizes the results for each experimental example, including the shape of the compartment space (cell), unit inscribed circle diameter r (μm), wall thickness B (μm), type of separation membrane, separation membrane thickness t (μm), ratio r / t (-), energy density per unit volume (Wh / L), and resistance value (standard value) of the single battery (cell). The resistance values ​​for each experimental example were normalized with the value for Experimental Example 7 set to 100. Figure 3 shows the measurement results for the resistance values ​​of each experimental example. Figure 3A shows the measurement results for Experimental Examples 1, 8, and 9; Figure 3B shows the measurement results for Experimental Examples 3, 10, and 11; Figure 3C shows the measurement results for Experimental Examples 5, 16, and 17; and Figure 3D shows the measurement results for Experimental Examples 12 to 14. Figure 4 shows the relationship between the distance from the center of the compartment space 14 and resistance value. Figure 5 shows the relationship between the ratio r / t and resistance value. FIG. 6 shows the measurement results showing the energy density of each experimental example, with FIG. 6A showing the measurement results of experimental examples 1, 8, and 9, FIG. 6B showing the measurement results of experimental examples 3, 10, and 11, FIG. 6C showing the measurement results of experimental examples 5, 16, and 17, and FIG. 6D showing the measurement results of experimental examples 12 to 14.

[0046] (Consideration of the effect of separation membrane) It was found that Experimental Example 1 showed a lower cell resistance value than the cells of Experimental Example 7. The reason is that, for example, although the separation membrane of Experimental Example 7 is "non-porous", by using a separation membrane with a large amount of alumina particles added, the electrolyte can penetrate into the voids formed in the separation membrane, and it is speculated that the ion conductivity is improved and the conductivity can be increased. In Experimental Example 1, it was found that an approximately three-fold resistance reduction effect can be obtained compared to Experimental Example 7. Also, for the 20-μm non-porous membrane of Experimental Example 7, in the case of the alumina-containing separation membrane, if it is less than 70 μm, a lower resistance value can be obtained. On the other hand, as shown in Experimental Example 15, when filling the counter electrode slurry, if the separation membrane is thin, fine powder may penetrate through the micropores and cause a short circuit. It was found that a range of 10 (μm) < t < 70 μm for the thickness t of the separation membrane is more effective.

[0047] (Consideration of the Shape of the Small Compartment Space) When comparing Experimental Example 1 with a hexagonal small compartment space shape, Experimental Example 8 with a rectangular shape, and Experimental Example 9 with a triangular shape, as shown in Table 1 and Figure 3A, it was found that Experimental Example 1 showed a lower resistance value. Also, as shown in Figure 3B, it was found that Experimental Example 3 showed a lower resistance value than Experimental Examples 10 and 11. Additionally, as shown in Figure 3C, it was found that Experimental Example 5 showed a lower resistance value than Experimental Examples 16 and 17. Figure 4 is a graph plotted with the maximum value of the distance between the center and the outer periphery of the cross-sectional shape of the small compartment space on the horizontal axis and the resistance value on the vertical axis for each membrane thickness of the separation membrane. As shown in Figure 4, it was found that both the distance between the center and the outer periphery and the resistance value uniquely correspond to each membrane thickness of each separation membrane regardless of the shape. The reason is speculated to be that the hexagonal honeycomb shape is the closest to a circle, so the distance can be short, and thus the resistance can be lowered even when the unit area (volume) is the same.

[0048] Furthermore, when comparing Experimental Example 1, in which the shape of the compartment space is hexagonal, with Experimental Example 8, in which the shape is rectangular, and Experimental Example 9, in which the shape is triangular, it was found that Experimental Example 1 exhibited a higher energy density, as shown in Table 1 and Figure 5A. This is because, among the triangles, squares, and hexagons that can be filled in a plane, the regular hexagon has the shortest circumference relative to its diameter, and therefore the area (volume) of the partition wall can be minimized, and it is inferred that, in principle, it is possible to configure the highest energy density. Furthermore, as shown in Figure 5B, it is inferred that the same reason is why Experimental Example 3 was able to achieve a higher volumetric energy density than Experimental Examples 10 and 11. Furthermore, as shown in Figure 5C, it is inferred that the same reason is why Experimental Example 5 was able to achieve a higher volumetric energy density than Experimental Examples 16 and 17.

[0049] (Consideration of the ratio r / t between the separation membrane thickness t and the inscribed circle diameter r) As mentioned above, a hexagonal shape can increase energy density more than a rectangular or triangular shape. However, as the ratio (r / t) of the inscribed circle radius r to the separation membrane thickness t increases, the effect (difference) decreases. As shown in Figures 3D and 5D, although an effect was observed in Experimental Examples 12 to 14, the difference was not significant. This is presumably the reason. As shown in Figure 6, the plot of the relationship between the ratio r / t and the resistance value revealed a trend in the resistance reduction effect for each unit (1) to (4) with a constant cell volume. Furthermore, as shown in Figure 7, the relationship between the ratio r / t and the energy density clearly showed that the energy density increased with increasing ratio r / t. However, it was found that the rate of increase in energy density gradually decreased as r / t increased. Figure 8 shows the relationship between the ratio r / t and the slope of the increase in energy density shown in Figure 7. As shown in Figure 8, considering the rate of increase in energy density, a greater improvement was obtained when the ratio r / t was 5 or less. Thus, when attempting to more effectively utilize the shape effect of the hexagonal shape, it is inferred that the range of r / t≦5 is particularly recommended.

[0050] [Table 2] [Explanation of symbols]

[0051] 10 energy storage device, 11 electrode structure, 12 wall portion, 13 bottom portion, 14 small compartment space (cell), 15 electrode current collector, 16 negative electrode, 17 separation membrane, 18 counter electrode, 19 counter electrode composite, 21 counter electrode current collector, 22 current collecting terminal, 30 battery case, 31 membrane body, 32 particle, A opening length, B wall thickness, L length, R inscribed circle, r radius, t, T, T' thickness, U unit.

Claims

1. an electrode structure having a wall portion containing an electrode active material and a plurality of cells which are spaces formed by being surrounded by the wall portion; a counter electrode containing a counter electrode active material and filled in the cell; a separation membrane having ion conductivity and insulation, formed on the surface of the wall portion, separating the electrode structure and the counter electrode, containing particles in a range of 70% by volume or more and 96% by volume or less, and having a thickness t in the range of 20 μm<t<70 μm; a ratio r / t of a radius r of an inscribed circle inscribed in the unit partitioned at the center of the wall portion to a thickness t of the separation membrane is 2.4 or more and 5 or less, An electricity storage device having an energy density in the range of 300 W / L or more and 500 W / L or less.

2. The electricity storage device according to claim 1 , wherein the electrode structure has the cells each having a hexagonal cross section.

3. The electricity storage device according to claim 1 or 2, wherein the separation membrane satisfies one or more of (1) to (3). (1) The separation membrane contains aluminum oxide particles. (2) The separation membrane contains a resin selected from the group consisting of polyvinylidene fluoride and vinylidene fluoride-hexafluoropropylene copolymer. (3) The separation membrane contains the particles having an average particle size in the range of 0.4 μm to 4 μm.

4. a separation membrane forming step of forming a separation membrane having ion conductivity and insulation properties and containing particles in an amount of 70% by volume or more and 96% by volume or less on the wall portion of an electrode structure having a wall portion containing an electrode active material and a plurality of cells which are spaces formed by the wall portion, with a thickness t in the range of 20 μm<t<70 μm; a counter electrode forming step of filling a counter electrode mixture containing a counter electrode active material into the cell in which the separation membrane is formed to form a counter electrode, a ratio r / t of a radius r of an inscribed circle inscribed in the unit partitioned at the center of the wall portion to a thickness t of the separation membrane is 2.4 or more and 5 or less, A method for producing an electricity storage device having an energy density in the range of 300 W / L or more and 500 W / L or less.

5. The method for manufacturing an electricity storage device according to claim 4 , wherein the separation membrane forming step uses the electrode structure having the cell with a hexagonal cross section.

6. A method for manufacturing an electricity storage device as described in claim 4 or 5, wherein the separation membrane satisfies one or more of (1) to (3). (1) The separation membrane contains aluminum oxide particles. (2) The separation membrane contains a resin selected from the group consisting of polyvinylidene fluoride and vinylidene fluoride-hexafluoropropylene copolymer. (3) The separation membrane contains the particles having an average particle size in the range of 0.4 μm to 4 μm.

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