Electrode composition, electrode, and electrochemical element

Abstract

Provided are an electrode composition (14), an electrode (12), and an electrochemical element (11) that make it possible to improve rate characteristics. The electrode composition includes particles (19) of an oxide. In a frequency distribution of the circularity of particles appearing in a cross-section of an object in which a plurality of the particles are immobilized, the maximum frequency in a range in which the circularity is 0.8 or greater and the maximum frequency in a range in which the circularity is 0.6 or less are greater than the maximum frequency in a range of in which the circularity is greater than 0.6 and less than 0.8. The electrode includes the electrode composition and an active material (20). The electrochemical element includes the electrode.

Description

Electrode composition, electrode, and electrochemical element

[0001] The present invention relates to an electrode composition, an electrode, and an electrochemical device.

[0002] In electrochemical elements such as electricity storage devices, when the current density (rate) during charging and discharging increases, the electrochemical reaction cannot keep up, and the amount of electricity that can be extracted tends to decrease. The prior art disclosed in Patent Document 1 improves the rate characteristics of the electrochemical element by using an electrolyte solution.

[0003] Japanese Patent Application Laid-Open No. 2020-145054

[0004] There is a demand for a technology that improves the rate characteristics of electrochemical devices, as in the prior art.

[0005] The present invention has been made to meet this demand, and an object of the present invention is to provide an electrode composition, an electrode, and an electrochemical device that can improve rate characteristics.

[0006] A first aspect for achieving this object is a composition for electrodes, which contains oxide particles, and in the frequency distribution of the circularity of particles appearing in the cross section of an object to which a plurality of the particles are fixed, the maximum frequency in the range of circularity of 0.8 or more and the maximum frequency in the range of circularity of 0.6 or less are greater than the maximum frequency in the range of circularity of more than 0.6 and less than 0.8.

[0007] In the second aspect, in the first aspect, the ratio of the integrated value of the frequency in the range of circularity of 0.6 or less to the integrated value of the frequency in the entire range of circularity is 30% or more and 70% or less.

[0008] In a third aspect, in the first or second aspect, the particles have a Young's modulus of 140 GPa or more.

[0009] In a fourth aspect, in any one of the first to third aspects, the average equivalent circle diameter of the particles appearing in the cross section is 3 μm or less.

[0010] In a fifth aspect, in any one of the first to fourth aspects, the particles are a solid electrolyte having a garnet-type crystal structure containing Li, La, and Zr.

[0011] A sixth aspect is the fifth aspect, wherein the solid electrolyte further contains Mg and Sr.

[0012] A seventh aspect is an electrode, which comprises the electrode composition of any one of the first to sixth aspects and an active material.

[0013] In an eighth aspect, in the seventh aspect, the ratio of the area of ​​the particles appearing in the cross section of the electrode to the area of ​​the cross section is 0.1% or more and 5% or less.

[0014] A ninth aspect is an electrochemical device, which comprises the electrode of the seventh or eighth aspect.

[0015] A tenth aspect is the ninth aspect, wherein the electrode is a positive electrode.

[0016] When oxide particles of the electrode composition of the present invention are mixed with an active material, particles having a maximum frequency in the range of circularity of 0.6 or less and particles having a maximum frequency in the range of circularity of 0.8 or more assist the function of the active material, thereby reducing the current density dependence of charge and discharge and improving rate characteristics.

[0017] 1 is a cross-sectional view of an electrochemical element according to a first embodiment; FIG. 2 is a diagram schematically showing a garnet-type crystal structure; (a) is a frequency distribution of the circularity of particles, and (b) is a frequency distribution of the circularity of particles in a modified example; and FIG. 3 is a cross-sectional view of an electrochemical element according to a second embodiment.

[0018] Preferred embodiments of the present invention will be described below with reference to the accompanying drawings. FIG. 1 is a schematic cross-sectional view of an electrochemical device 11 according to a first embodiment. The electrochemical device 11 is a device that converts chemical energy into electrical energy and vice versa. The ions (hereinafter referred to as "charge carriers") that contribute to the energy conversion of the electrochemical device 11 are Li. ＋ , Na ＋ , K. ＋ , Mg ２＋ , Cu ＋ , Ag ＋ Examples of cations include:

[0019] Examples of the electrochemical element 11 include secondary batteries such as lithium ion batteries, sodium ion batteries, potassium ion batteries, magnesium ion batteries, and calcium ion batteries, electrochemical capacitors, and metal-air batteries that use metals such as lithium, zinc, aluminum, magnesium, and iron as the negative electrode active material. Examples of the electrochemical capacitor include electric double layer capacitors, redox capacitors that utilize redox reactions of electrodes or redox reactions of ions in a non-aqueous electrolyte, and hybrid capacitors that combine electric double layers and redox reactions, or that combine them with secondary battery materials.

[0020] The electrochemical device 11 includes, in order, a positive electrode 12, a separator 15, and a negative electrode 16. The separator 15 is made of a porous material that is durable against the active materials 20, 21 and the electrolyte solution contained in the positive electrode 12 and the negative electrode 16, and that allows charge carriers to pass through but does not have electronic conductivity. Examples of the separator 15 include nonwoven fabrics and porous films made of cellulose, polypropylene, polyethylene, polyimide, alumina, etc.

[0021] The positive electrode 12 is formed by stacking a current collector 13 and an active material layer 14. The current collector 13 is a conductive member. Examples of materials for the current collector 13 include a metal selected from Ni, Ti, Fe, and Al, an alloy containing two or more of these elements, stainless steel, and a carbon material.

[0022] The active material layer 14 includes an active material 20. The active material 20 is selected appropriately depending on the type of charge carrier and electrochemical device. When the electrochemical device is an electrochemical capacitor, a material capable of reversibly supporting anions is used as the active material 20, and examples of such a material include carbon-based materials such as porous carbon, natural graphite, artificial graphite, graphitizable carbon (hard carbon), non-graphitizable carbon (soft carbon), and carbon fiber. When the electrochemical device is a metal-air battery, oxygen in the atmosphere is used as the positive electrode active material, and therefore the active material layer 14 includes a gas diffusion layer through which air (oxygen) diffuses and a catalyst layer in which an oxygen reduction reaction occurs.

[0023] When the electrochemical device is an ion battery, examples of the active material 20 include metal oxides containing transition metals, sulfur-based active materials, and organic active materials. ＋In this case, the metal oxide containing a transition metal is exemplified by a metal oxide containing Li and one or more elements selected from Mn, Co, Ni, Fe, Cr, and V. The metal oxide containing a transition metal is LiCoO ２ , LiNi ０．８ Co ０．１５ Al ０．０５ O ２ , LiMn ２ O ４ , LiNiVO ４ , LiNi ０．５ Mn １．５ O ２ , LiNi １／３ Mn １／３ Co １／３ O ２ and LiFePO ４ is exemplified.

[0024] The sulfur-based active material is S, TiS ２ , NiS, FeS ２ , Li ２ S, MoS ３ Examples of organic active materials include radical compounds such as 2,2,6,6-tetramethylpiperidinoxyl-4-yl methacrylate and polytetramethylpiperidinoxyl vinyl ether, quinone compounds, radialene compounds, tetraciaquinodimethane, and phenazine oxide.

[0025] The active material layer 14 may contain a conductive additive to reduce the resistance of the active material layer 14. Examples of the conductive additive include carbon black, acetylene black, ketjen black, carbon fiber, Ni, Pt, and Ag.

[0026] The active material layer 14 includes particles 19. The particles 19 are oxides that have the property of promoting ion conduction or have ion conductivity. Examples of the particles 19 include oxides containing elements such as Li, Mg, Al, Si, Ca, Ti, Zr, La, Na, K, Ba, Sr, V, Nb, B, and Ge, and SiO ２ , Al ２ O ３ , AlOOH, MgO, CaO, ZrO ２ , TiO ２ , BaTiO ３The oxide particles 19 have polarity and therefore promote ion conduction.

[0027] The particles 19 may be an oxide-based solid electrolyte having ion conductivity. Examples of the solid electrolyte include oxides having a NASICON structure, oxides having a perovskite structure, and oxides having a garnet structure. The oxides having a NASICON structure include oxides containing at least Li, M (where M is one or more elements selected from Ti, Zr, and Ge), and P, such as Li(Al,Ti). ２ (P.O. ４ ) ３ and Li(Al,Ge) ２ (P.O. ４ ) ３ The oxide having a perovskite structure is an oxide containing at least Li, Ti, and La, for example, La ２／３－Ｘ Li ３Ｘ TiO ３ Examples include:

[0028] The particles 19 are preferably made of a composite oxide containing Li, La, and Zr and having a garnet-type crystal structure. －３ This is because the garnet-type crystal structure has an ionic conductivity of the order of 50 S / cm and is resistant to reduction by metallic lithium. ３ A ２ B ３ O １２ It is expressed as:

[0029] FIG. 2 is a diagram schematically showing a garnet-type crystal structure. In the garnet-type crystal structure, the C site Sc is dodecahedrally coordinated with an oxygen atom Oa, the A site Sa is octahedrally coordinated with an oxygen atom Oa, and the B site Sb is tetrahedrally coordinated with an oxygen atom Oa. In an oxide solid electrolyte, Li can exist in a position that is octahedrally coordinated with an oxygen atom Oa in a normal garnet-type crystal structure, but becomes a void V. The void V is, for example, a position sandwiched between the B site Sb1 and the B site Sb2. The Li present in the void V is octahedrally coordinated with an oxygen atom Oa that constitutes an octahedron including the tetrahedral face Fb1 that forms the B site Sb1 and the tetrahedral face Fb2 that forms the B site Sb2. For example, Li having a garnet-type crystal structure ７La ３ Zr ２ O １２ In the formula, La can occupy the C site Sc, Zr can occupy the A site Sa, and Li can occupy the B site Sb and the void V.

[0030] The garnet-type crystal structure can be identified by X-ray diffraction. The garnet-type crystal structure is identified by X-ray diffraction file No. 422259 (Li) in the Cambridge Structural Database (CSD). ７ La ３ Zr ２ O １２ ) has an XRD pattern similar to that of No. 422259. Compared to No. 422259, the solid electrolyte may differ in the type of constituent elements, Li concentration, etc., and therefore the diffraction angle and intensity ratio may differ. A typical crystal structure of this type is a cubic system (space group Ia-3d (- indicates an overline indicating a reversal operation), JCPDS: 84-1753).

[0031] Solid electrolytes with a garnet-type crystal structure are typically Li ７ La ３ Zr ２ O １２ The solid electrolyte is Li ７ La ３ Zr ２ O １２ A part of the constituent elements may be substituted with another element, or a small amount of another element may be added without substituting the constituent elements. Examples of the other element include at least one element selected from the group consisting of Mg, Al, Si, Ca, Ti, V, Ga, Sr, Y, Nb, Sn, Sb, Ba, Hf, Ta, W, Bi, Rb, and lanthanides (excluding La).

[0032] The solid electrolyte is, for example, Li ６ La ３ Zr １．５ W ０．５ O １２ , Li ６．１５ La ３ Zr １．７５ Ta ０．２５ Al ０．２ O １２ , Li ６．１５ La ３Zr １．７５ Ta ０．２５ Ga ０．２ O １２ , Li ６．２５ La ３ Zr ２ Ga ０．２５ O １２ , Li ６．４ La ３ Zr １．４ Ta ０．６ O １２ , Li ６．５ La ３ Zr １．７５ Te ０．２５ O １２ , Li ６．７５ La ３ Zr １．７５ Nb ０．２５ O １２ , Li ６．９ La ３ Zr １．６７５ Ta ０．２８９ Bi ０．０３６ O １２ , Li ６．４６ Ga ０．２３ La ３ Zr １．８５ Y ０．１５ O １２ , Li ６．８ La ２．９５ Ca ０．０５ Zr １．７５ Nb ０．２５ O １２ , Li ７．０５ La ３．００ Zr １．９５ Gd ０．０５ O １２ , Li ６．２０ Ba ０．３０ La ２．９５ Rb ０．０５ Zr ２ O １２ include

[0033] The garnet-type solid electrolyte preferably contains at least one of Mg and element A (A is at least one element selected from the group consisting of Ca, Sr, and Ba) and the molar ratio of each element satisfies all of the following (1) to (3), or contains both Mg and element A and the molar ratio of each element satisfies all of the following (4) to (6). Element A is preferably Sr, in order to increase the ionic conductivity of the solid electrolyte. (1) 1.33≦Li / (La+A)≦3 (2) 0≦Mg / (La+A)≦0.5 (3) 0≦A / (La+A)≦0.67 (4) 2.0≦Li / (La+A)≦2.6 (5) 0.01≦Mg / (La+A)≦0.14 (6) 0.04≦A / (La+A)≦0.17

[0034] Returning to Fig. 1, the negative electrode 16 is formed by stacking a current collector 17 and an active material layer 18. The current collector 17 is a conductive member. Examples of materials for the current collector 17 include a metal selected from Ni, Ti, Fe, Cu, and Si, an alloy containing two or more of these elements, stainless steel, and a carbon material.

[0035] The active material layer 18 contains an active material 21. To reduce the resistance of the active material layer 18, the active material layer 18 may contain a conductive additive. Examples of the conductive additive include carbon black, acetylene black, ketjen black, carbon fiber, Ni, Pt, and Ag. In this embodiment, the active material layer 18 contains particles 19.

[0036] There is no limitation on the material of the active material 21 as long as it can absorb and release charge carriers. The active material 21 is appropriately selected depending on the type of charge carrier. The active material 21 may be a carbon-based material such as porous carbon, natural graphite, artificial graphite, graphitizable carbon, non-graphitizable carbon, or carbon fiber; Li ４ Ti ５ O １２ , Si, Si-Li alloy, compounds containing Si and O as constituent elements (hereinafter referred to as "SiO ｘ " where 0.5≦x≦1.5), examples include metallic lithium, lithium alloys such as Li-Al alloy, Li-Sn alloy, Li-Si alloy, Li-Mg alloy, and Li-Si alloy, In-Sb alloy, and Si-Li alloy. SiO ｘ is an oxide of Si, amorphous SiO ２Examples include those having a structure in which microcrystalline or amorphous Si is dispersed in a matrix.

[0037] The electrochemical element 11 is manufactured, for example, as follows: The particles 19, the active material 20, and the conductive additive are mixed together, and then a solution in which a binder is dissolved in a solvent is mixed to form a slurry. The slurry is then applied onto the current collector 13 and dried to obtain a positive electrode sheet.

[0038] The particles 19, the active material 21, and the conductive additive are mixed, and then mixed with a solution in which a binder is dissolved in a solvent to form a slurry. The slurry is applied onto the current collector 17 and then dried to obtain a negative electrode sheet.

[0039] A separator 15 separating the positive electrode sheet from the negative electrode sheet, the positive electrode sheet, and the negative electrode sheet are stacked and wound up by a winding machine to produce a cylindrical or rectangular cell. Terminals (not shown) are connected to the current collectors 13 and 17, respectively, and a container (not shown) containing the cell is filled with an electrolyte solution, and the container is then sealed to obtain an electrochemical device 11 including the positive electrode 12, the separator 15, and the negative electrode 16.

[0040] There are no particular limitations on the binder as long as it binds the particles 19 and the active materials 20 and 21. Examples of binders include fluorinated resins, polyolefins, polyimides, polyvinylpyrrolidone, polyvinyl alcohol, cellulose ethers, and rubber-like polymers such as styrene-butadiene rubber. Examples of fluorinated resins include vinylidene fluoride polymers, polychlorotrifluoroethylene, polyvinyl fluoride, tetrafluoroethylene-perfluoroalkyl vinyl ether copolymers, tetrafluoroethylene-hexafluoropropylene copolymers, ethylene-tetrafluoroethylene copolymers, and ethylene-chlorotrifluoroethylene copolymers.

[0041] An electrolyte is a medium through which charge carriers move, and is a solution in which an electrolyte is dissolved in a solvent. Examples of electrolytes include compounds consisting of charge carriers (anions) and cations. Examples of solvents include aqueous solvents and non-aqueous solvents. Non-aqueous solvents are broadly classified into molecular solvents, which are mostly composed of molecules, and ionic liquids, which are composed of cations and anions. Non-aqueous electrolytes containing non-aqueous solvents can have a wider potential window than aqueous electrolytes that use aqueous solvents as the solvent.

[0042] The molecular solvent is preferably an aprotic solvent to widen the potential window of the nonaqueous electrolyte. Examples of aprotic solvents include cyclic esters, chain esters, aliphatic carboxylic acid esters, phosphate esters, nitriles, amides, sulfur compounds, ketones, ethers, nitro compounds, fluorous solvents, and sulfones. Mixtures of these solvents are also acceptable.

[0043] Examples of cyclic esters include carbonate esters such as propylene carbonate, ethylene carbonate, butylene carbonate, vinylene carbonate, vinylethylene carbonate, and fluoroethylene carbonate, and lactones such as β-propiolactone, γ-butyrolactone, δ-valerolactone, α-pyrone, and coumarin. Examples of chain esters include carbonate esters such as dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate. Examples of aliphatic carboxylic acid esters include methyl formate, ethyl formate, methyl acetate, ethyl acetate, and ethyl propionate. Examples of phosphate esters include trimethyl phosphate. Examples of nitriles include acetonitrile, propionitrile, butyronitrile, and benzonitrile.

[0044] Examples of amides include formamide, N-methylformamide, dimethylformamide, N-methylacetamide, dimethylacetamide, N-methylpropioamide, hexamethylphosphoramide, and N-methylpyrrolidone. Examples of sulfur compounds include dimethyl sulfoxide, sulfolane, dimethylthioformamide, and N-methylthiopyrrolidone. Examples of ketones include acetone, 4-methyl-2-pentanone, and acetylacetone.

[0045] Examples of ethers include tetrahydrofuran, 2-methyltetrahydrofuran, 2,5-dimethyltetrahydrofuran, monoglyme, diglyme, triglyme, and tetraglyme. Examples of nitro compounds include nitromethane and nitrobenzene. Fluorous solvents are compounds in which the hydrogen atoms of hydrocarbons are substituted with fluorine atoms, and derivatives thereof. Examples of sulfones include trimethylene sulfone, tetramethylene sulfone (sulfolane), dimethyl sulfone, ethyl methyl sulfone, and ethyl isopropyl sulfone.

[0046] The reaction in which an electrolyte dissolves in a molecular solvent and dissociates into free ions proceeds more easily as the relative dielectric constant of the solvent increases and as the solvation of ions becomes more likely. Therefore, the relative dielectric constant ε ｒ The solvent with a relatively large ｒ >20) is preferred. Examples of molecular solvents with a dielectric constant of greater than 20 include cyclic esters, nitriles, amides, sulfur compounds, acetone, acetylacetone, nitro compounds, and sulfones. It is of course possible to mix a solvent with a dielectric constant of greater than 20 with a solvent with a dielectric constant of 20 or less in order to adjust the viscosity of the solvent, etc.

[0047] Ionic liquids are compounds consisting of cations and anions, and are liquid at room temperature and normal pressure. If the solvent of a non-aqueous electrolyte is an ionic liquid, the flame retardancy of the non-aqueous electrolyte can be improved. The ionic liquid preferably contains one or more cation species selected from the group consisting of ammonium, imidazolium, pyrrolidinium, and piperidinium.

[0048] The anion component of the ionic liquid is not particularly limited. ４ － , N(SO ２ F) ２ － inorganic anions such as B(C ６ H ５ ) ４ － , CH ３ SO ３ － , C.F. ３ SO ３ － , N(SO ２ CF ３ ) ２ － , N(SO ２ C ４ F ９ ) ２ － Examples of organic anions include:

[0049] The ionic liquid may be a solvated ionic liquid, such as a sulfone-based solvent such as sulfolane or a sulfolane derivative, or a glyme-based solvent such as tetraglyme, in which an electrolyte is dissolved.

[0050] The electrolyte concentration of the electrolytic solution is 0.2 mol / kg or more, preferably 0.5 mol / kg or more, and more preferably 1.0 mol / kg or more. As the electrolyte concentration increases, the number of solvent molecules coordinated to the charge carrier increases, resulting in less uncoordinated solvent and predominant coordination with counter anions (so-called ionic association). This suppresses reductive decomposition of the electrolytic solution while increasing the oxidation potential and widening the potential window. The electrolyte concentration is preferably 4.0 mol / kg or less, more preferably 2.0 mol / kg or less. This is because when the electrolyte concentration exceeds 4.0 mol / kg, the ionic conductivity tends to decrease significantly due to an increase in the viscosity of the electrolytic solution.

[0051] In general, as the charge / discharge current density (rate) increases, the diffusion of charge carriers becomes unable to keep up, and the reaction slows down, resulting in a decrease in the utilization rate of the active materials 20, 21 and a decrease in the amount of electricity that can be extracted. Electrochemical device 11 contains particles 19 in active material layers 14, 18 to improve rate characteristics and reduce the rate dependency of the charge / discharge curve.

[0052] 3(a) shows the frequency distribution of the circularity of particles 19 that appears in the cross section of an object having a plurality of particles 19 fixed thereto. Examples of objects having particles 19 fixed thereto include a molded body obtained by applying a force to particles 19 using a press machine to compress the particles 19 without destroying them, an object obtained by mixing particles 19 with a synthetic resin such as a tetrafunctional epoxy resin and then curing the mixture, and active material layers 14, 18 formed using a material containing particles 19. The cross sections of particles 19 appear on the cut surface of an object having particles 19 fixed thereto.

[0053] The circularity of the particles 19 is determined from a scanning electron microscope image (SEM image) of the particles 19 appearing on a cross section of an object to which the particles 19 are fixed. One or more SEM images are acquired so as to obtain cross sections of 30 or more particles 19. After the SEM images are acquired, image analysis is performed using known image analysis software (for example, WinROOF (registered trademark), manufactured by Mitani Shoji Co., Ltd.).

[0054] In image analysis, the size of each SEM image is calibrated based on the scale bar attached to the SEM image, and then the SEM image is subjected to binarization processing in order to extract the edges of the image. In binarization processing, the brightness (brightness) of each pixel of the SEM image is converted into two levels using a predetermined threshold (for example, a threshold of 0-25). By converting the pixels into two levels and eliminating intermediate gradations, a binarized image is obtained in which the edges (grain boundaries) of the particles 19 are emphasized. Using the obtained binarized image, the area S of the particle 19 and the perimeter (total length of the edge) L of the particle 19 with area S are determined using a known image analysis method. The circularity of the particle 19 is 4πS / L ２ The closer the circularity is to 1, the closer the shape of the grain boundary of the particle 19 is to a circle.

[0055] The frequency distribution of the circularity of particles 19 is a graph in which the size of the circularity intervals is set to 0.1 or 0.2, the frequency (number of particles 19) included in each interval where the circularity is from 0 to 1 is tallied, and the number of data points (frequency) for each interval is expressed as a distribution. For particles 19, the maximum frequency in the range of circularity equal to or greater than 0.8 and the maximum frequency in the range of circularity equal to or less than 0.6 are greater than the maximum frequency in the range of circularity greater than 0.6 and less than 0.8. Furthermore, for particles 19, the maximum frequency in the range of circularity equal to or greater than 0.8 is greater than the maximum frequency in the range of circularity equal to or less than 0.6.

[0056] Particles 19 contain more particles with a circularity of 0.6 or less than particles with a circularity of more than 0.6 but less than 0.8. When active materials 20, 21 are mixed with particles 19, the sharp portions of particles with a circularity of 0.6 or less tend to pierce active materials 20, 21, increasing the surface area of ​​active materials 20, 21 by the amount of particles 19 piercing. This facilitates diffusion of charge carriers between active materials 20, 21 and particles 19, and between active materials 20, 21 and the electrolyte, ensuring a sufficient amount of electricity that can be extracted even when the charge / discharge current density is high. This improves rate characteristics.

[0057] Particles 19 contain more particles with a circularity of 0.8 or more than particles with a circularity of more than 0.6 but less than 0.8. This reduces the amount of active material 20, 21 that is destroyed by particles with a circularity of more than 0.6 but less than 0.8 coming into contact with active material 20, 21. When active material 20, 21 is destroyed and becomes finer, electrical connection is easily lost, and the amount of active material 20, 21 that contributes to charging and discharging decreases. However, this can be prevented, ensuring the amount of electricity that can be extracted. Furthermore, particles with a circularity of 0.8 or more can reduce aggregation of particles 19.

[0058] 3B shows the frequency distribution of the circularity of particles 19 appearing in a cross section of an object to which a plurality of particles 19 according to the modified example are fixed. For particles 19, the maximum frequency in the circularity range of 0.8 or more and the maximum frequency in the circularity range of 0.6 or less are greater than the maximum frequency in the circularity range of greater than 0.6 and less than 0.8. Furthermore, for particles 19, the maximum frequency in the circularity range of 0.6 or less is greater than the maximum frequency in the circularity range of 0.8 or more. The rate characteristics of the element can also be improved in the case of particles 19 according to the modified example.

[0059] The ratio of the integrated value of the frequency in the range of circularity of 0.6 or less to the integrated value of the frequency in the entire range of circularity is preferably 30% or more and 70% or less, because when the ratio is in this range, the effect of improving the rate characteristics can be increased and further the aggregation of particles 19 can be reduced.

[0060] The Young's modulus of the particles 19 is preferably 140 GPa or more, because the hard particles 19 can easily penetrate the active materials 20 and 21, and the effect of increasing the surface area of ​​the active materials 20 and 21 penetrated by the particles 19 can be enhanced.

[0061] The average value of the diameter (equivalent circle diameter) of circles having an area equivalent to the area S of particles 19 is smaller than the average value of the equivalent circle diameters of active materials 20 and 21. The average equivalent circle diameter of particles 19 is preferably 3 μm or less. The area S of particles 19 is the area calculated when determining the circularity of particles 19. The average equivalent circle diameter is the value obtained by dividing the sum of the equivalent circle diameters of particles 19 by the number of particles 19. This is because when the equivalent circle diameter of particles 19 is 3 μm or less, particles 19 can improve the diffusibility of charge carriers at the interface between active materials 20 and 21.

[0062] The ratio of the area of ​​particles 19 appearing in the cross section (SEM image) of active material layers 14, 18 to the area of ​​the SEM image is preferably 0.1% or more and 5% or less, because this range of ratio ensures the effect of improving the rate characteristics while also ensuring the capacity obtained when active materials 20, 21 are discharged.

[0063] The ratio (vol %) of the volume of the electrolyte solution to the combined volume of the electrolyte solution and the active material 20 contained in the active material layer 14 is preferably 10% or more and 20% or less. This is to ensure the equilibrium potential of the positive electrode 12 while reducing the interface resistance of the active material 20, which is the reaction field for the charge / discharge reaction.

[0064] The ratio (vol %) of the active material 20 to the electrolyte solution can be determined by analyzing a randomly selected field of view of 5000x magnification from the cross section of the active material layer 14 using an SEM equipped with an energy dispersive X-ray spectrometer (EDS). The analysis involves identifying the element distribution and performing image analysis of the contrast of the backscattered electron image to identify the area of ​​the active material 20 and the area of ​​the electrolyte solution. The ratio (vol %) of the electrolyte solution to the total area of ​​the active material 20 and the electrolyte solution is considered to be the volume ratio.

[0065] The cross section of the active material layer 14 used for analysis is a polished surface, a surface obtained by irradiating with a focused ion beam (FIB), or a surface obtained by ion milling. The polished surface is, for example, a surface obtained by freezing the active material layer 14 or by embedding and solidifying the active material layer 14 in a tetrafunctional epoxy resin or the like and then polishing it.

[0066] A second embodiment will be described with reference to Fig. 4. In the first embodiment, the separator 15 separating the positive electrode 12 and the negative electrode 16 is a nonwoven fabric or porous film made of cellulose, polypropylene, or the like. In contrast, in the second embodiment, an electrochemical element 22 will be described in which the separator 23 separating the positive electrode 12 and the negative electrode 16 contains an electrolyte 24. In the second embodiment, the same parts as in the first embodiment are designated by the same reference numerals, and the following description will be omitted.

[0067] 4 is a cross-sectional view of an electrochemical element 22 according to the second embodiment. The electrochemical element 22 includes, in order, a positive electrode 12, a separator 23, and a negative electrode 16. The separator 23 includes an electrolyte 24. The electrolyte 24 may be, for example, a solid or gel electrolyte having ion conductivity. A composition in which the electrolyte 24 and an electrolytic solution are mixed may be disposed in the separator 23.

[0068] The electrolyte 24 includes at least one selected from sulfide-based, oxide-based, hydride-based, halide-based, and organic-based electrolytes. The sulfide-based electrolytes include crystalline thiolithium-based, Li １０ GeP ２ S １２ type, argyrodite type, Li ７ P ３ S １１ Type, Li ２ S-P ２ S ５ Examples of oxide electrolytes include glass and glass ceramics, such as those listed below. Examples of oxide electrolytes include oxides having a NASICON structure, oxides having a perovskite structure, and oxides having a garnet structure.

[0069] The hydride electrolyte is LiBH ４ and lithium halide compounds (LiI, LiBr, LiCl) and lithium amide (LiNH ２) is an example of a solid solution. ３ YCl ６ Examples of organic solid electrolytes include polyethylene oxide, polypropylene oxide, and polyacrylonitrile.

[0070] The electrochemical element 22 is manufactured, for example, as follows: A mixture of an electrolyte solution and particles 19 is mixed with the active material 20, and then a solvent in which a binder is dissolved is mixed to form a slurry. The slurry is then applied onto the current collector 13 and dried to obtain the active material layer 14.

[0071] A solvent in which a binder is dissolved is mixed with a mixture of the electrolytic solution and the electrolyte 24 to prepare a separator slurry. The separator slurry is applied onto the active material layer 14 and then dried to obtain a positive electrode sheet.

[0072] The mixture of the electrolyte solution and particles 19 is mixed with active material 21, and then a solvent in which a binder is dissolved is mixed to form a slurry. The slurry is applied onto a current collector 17 and then dried to obtain an active material layer 18. A separator slurry is applied onto the active material layer 18 and then dried to obtain a negative electrode sheet.

[0073] After cutting the positive electrode sheet and the negative electrode sheet into a predetermined shape, the positive electrode sheet and the negative electrode sheet are stacked and pressed together to form a cell, so that a separator 23 is formed between the positive electrode 12 and the negative electrode 16. Terminals (not shown) are connected to the current collectors 13 and 17, respectively, and the resulting product is sealed in a container (not shown), thereby obtaining an electrochemical element 22 including the positive electrode 12, the separator 23, and the negative electrode 16.

[0074] The electrochemical device 22 of the second embodiment includes the positive electrode 12 and the negative electrode 16 each containing the particle 19, and therefore can improve the rate characteristics in the same manner as the electrochemical device 11 of the first embodiment.

[0075] The present invention will be explained in more detail with reference to examples, but the present invention is not limited to these examples.

[0076] (Preparation of particles) Li ６．９５ Mg ０．１５ La ２．７５ Sr ０．２５ Zr２．０ O １２ So that Li ２ CO ３ , MgO, La(OH) ３ , SrCO ３ , ZrO ２ was weighed. ２ CO ３ The amount of Li was in excess to take into account the volatilization of Li during firing. The weighed raw materials and non-aqueous solvent were placed in a nylon pot along with zirconia balls and milled and mixed for 15 hours in a ball mill. The slurry removed from the pot was dried and then fired on an MgO plate at 1100°C for 15 hours. The fired powder was crushed, placed in an MgO sagger, and further fired at 1100°C for 4 hours. The fired powder was crushed in a glove box under an argon atmosphere to obtain an oxide-based solid electrolyte (hereinafter referred to as "LLZ"). The crystalline structure of LLZ was confirmed to be garnet-type by powder X-ray diffraction.

[0077] Using a dry jet mill (Aisin Nano Technologies Co., Ltd., Nano Jetmizer (registered trademark) NJ-50 type), LLZ was pulverized in a nitrogen atmosphere at a feed rate of 1 kg / hr to obtain particles A. The median diameter of particles A measured with a laser diffraction particle size distribution analyzer was 0.8 μm.

[0078] The LLZ and the non-aqueous solvent were placed in a nylon pot together with zirconia balls and pulverized in a ball mill. The slurry removed from the pot was dried and then pulverized using a mortar and pestle in a glove box under an argon atmosphere to obtain particles B. The median diameter of particles B measured using a laser diffraction particle size distribution analyzer was 0.8 μm.

[0079] (Preparation of negative electrode) Graphite (active material), acetylene black (conductive additive), carboxymethyl cellulose (thickener), and styrene-butadiene rubber (binder) dissolved in pure water were weighed and mixed in a mass ratio of 97.7:0.3:1.0:1.0 to prepare a slurry. The slurry was applied onto a copper foil and then vacuum-dried to obtain a negative electrode.

[0080] (Example 1) Particles A and B were mixed in a volume ratio of 15:85 to obtain particles in Example 1. SEM images of the cross sections of a molded product obtained by compressing the particles using a press were obtained, and the frequency distribution of the circularity of the particles was obtained by image analysis of the SEM images. The particles had a maximum frequency in the range of circularity of 0.8 or more and a maximum frequency in the range of circularity of 0.6 or less that was greater than the maximum frequency in the range of circularity of more than 0.6 and less than 0.8, and the cumulative percentage of frequencies with a circularity of 0.6 or less was 21%.

[0081] LiNi ０．５ Mn ０．３ Co ０．２ O ２ (active material), acetylene black (conductive additive), particles, and polyvinylidene fluoride (binder) dissolved in N-methyl-2-pyrrolidone were weighed and mixed in a mass ratio of 93:3:1:3 to prepare a slurry. The slurry was applied to an aluminum foil and then vacuum dried to obtain a positive electrode.

[0082] A cell was fabricated by stacking the positive electrode, polyethylene separator, and negative electrode in this order. The cell was then immersed in an electrolyte solution prepared by dissolving lithium hexafluorophosphate (1 mol / kg) as an electrolyte in a solvent prepared by mixing ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate in a volume ratio of 1:1:1 and further mixing 1 wt % vinylene carbonate. The cell was then evacuated and sealed to obtain the cell of Example 1.

[0083] (Example 2) A cell in Example 2 was obtained in the same manner as in Example 1, except that the particles in Example 1 were replaced with the particles in Example 2, which were a mixture of particles A and particles B in a volume ratio of 20:80, and the positive electrode was made of the particles in Example 2.

[0084] SEM images of the cross section of a molded product obtained by compressing the particles in Example 2 using a press were taken, and the frequency distribution of the circularity of the particles was obtained by image analysis of the SEM images. The particles had a maximum frequency in the range of circularity of 0.8 or more and a maximum frequency in the range of circularity of 0.6 or less that was greater than the maximum frequency in the range of circularity of more than 0.6 and less than 0.8, and the cumulative percentage of frequencies with a circularity of 0.6 or less was 24%.

[0085] (Example 3) A cell in Example 3 was obtained in the same manner as in Example 1, except that the particles in Example 1 were replaced with the particles in Example 3, which were a mixture of particles A and particles B in a volume ratio of 25:75, and the positive electrode was made of the particles in Example 3.

[0086] SEM images of the cross section of a molded product obtained by compressing the particles in Example 3 using a press were taken, and the frequency distribution of the circularity of the particles was obtained by image analysis of the SEM images. The particles had a maximum frequency in the range of circularity of 0.8 or more and a maximum frequency in the range of circularity of 0.6 or less that was greater than the maximum frequency in the range of circularity of more than 0.6 and less than 0.8, and the proportion of the cumulative value of the frequency of circularity of 0.6 or less was 30%.

[0087] (Example 4) A cell in Example 4 was obtained in the same manner as in Example 1, except that instead of the particles in Example 1, the particles in Example 4, which were a mixture of particles A and particles B in a ratio of 30:70 (volume ratio), were used for the positive electrode.

[0088] SEM images of the cross section of a molded product obtained by compressing the particles in Example 4 using a press were taken, and the frequency distribution of the circularity of the particles was obtained by image analysis of the SEM images. The particles had a maximum frequency in the range of circularity of 0.8 or more and a maximum frequency in the range of circularity of 0.6 or less, which were greater than the maximum frequency in the range of circularity of more than 0.6 and less than 0.8, and the cumulative proportion of frequencies with a circularity of 0.6 or less was 42%.

[0089] (Example 5) A cell in Example 5 was obtained in the same manner as in Example 1, except that instead of the particles in Example 1, the particles in Example 5, which were a mixture of particles A and particles B in a ratio of 35:65 (volume ratio), were used for the positive electrode.

[0090] The particles in Example 5 were compressed in a press to obtain a cross-section of a molded product, and an SEM image of the molded product was taken. The SEM image was analyzed to obtain a frequency distribution of the circularity of the particles. The particles had a maximum frequency in the range of circularity of 0.8 or more and a maximum frequency in the range of circularity of 0.6 or less, which were greater than the maximum frequency in the range of circularity of more than 0.6 and less than 0.8, and the cumulative percentage of the frequency of circularity of 0.6 or less was 55%.

[0091] (Example 6) A cell in Example 6 was obtained in the same manner as in Example 1, except that the particles in Example 1 were replaced with the particles in Example 6, which were a mixture of particles A and particles B in a ratio of 40:60 (volume ratio), and were used for the positive electrode.

[0092] The particles in Example 6 were compressed in a press to obtain a cross-section of a molded product, and an SEM image of the molded product was taken. The SEM image was analyzed to obtain a frequency distribution of the circularity of the particles. The particles had a maximum frequency in the range of circularity of 0.8 or more and a maximum frequency in the range of circularity of 0.6 or less, which were greater than the maximum frequency in the range of circularity of more than 0.6 and less than 0.8, and the cumulative percentage of the frequency of circularity of 0.6 or less was 59%.

[0093] (Example 7) A cell in Example 7 was obtained in the same manner as in Example 1, except that the particles in Example 1 were replaced with the particles in Example 7, which were a mixture of particles A and particles B in a volume ratio of 45:55, and the positive electrode was made of the particles in Example 7.

[0094] The particles in Example 7 were compressed in a press to obtain a cross-section of a molded product, and the frequency distribution of the circularity of the particles was obtained by image analysis of the SEM image. The particles had a maximum frequency in the range of circularity of 0.8 or more and a maximum frequency in the range of circularity of 0.6 or less, which were greater than the maximum frequency in the range of circularity greater than 0.6 and less than 0.8, and the cumulative percentage of the frequency of circularity of 0.6 or less was 68%.

[0095] (Example 8) An attempt was made to use the particles in Example 8, which were a mixture of particles A and particles B in a volume ratio of 60:40, instead of the particles in Example 1, for the positive electrode. However, the positive electrode slurry gelled (became non-fluidic), and therefore it was not possible to prepare a cell required for evaluation.

[0096] The particles in Example 8 were compressed in a press to obtain a cross-section of a molded product, and an SEM image of the molded product was taken. The SEM image was analyzed to obtain a frequency distribution of the circularity of the particles. The particles had a maximum frequency in the range of circularity of 0.8 or more and a maximum frequency in the range of circularity of 0.6 or less, which were greater than the maximum frequency in the range of circularity of more than 0.6 and less than 0.8, and the cumulative percentage of the frequency of circularity of 0.6 or less was 75%.

[0097] (Comparative example) LiNi ０．５ Mn ０．３ Co ０．２ O ２ A cell in the comparative example was obtained in the same manner as in Example 1, except that a positive electrode (a positive electrode not containing particles) was used, which was prepared using a slurry obtained by weighing and mixing polyvinylidene fluoride (active material), acetylene black (conductive additive), and polyvinylidene fluoride (binder) dissolved in N-methyl-2-pyrrolidone in a ratio of 94:3:3 (mass ratio).

[0098] (Evaluation of Cell Rate Characteristics) First, the initial discharge capacity of the cell was measured at 25° C. First, the terminal voltage reached 4.2 V (charge stop potential) during constant current charging at 0.1 C, and then the constant potential was maintained until the current value reached 0.01 C. Next, the terminal voltage reached 2.5 V (discharge stop potential) during constant current discharging at 0.1 C, and then the constant potential was maintained until the current value reached 0.01 C.

[0099] Next, the current density dependence (rate characteristics) of the charge / discharge curve of the cell was evaluated at 25° C. First, after the terminal voltage reached 4.2 V by constant current charging at 1 C, the cell was held at a constant potential until the current value reached 0.01 C. Next, after the terminal voltage reached 2.5 V by constant current discharging at 0.2 C, the cell was held at a constant potential until the current value reached 0.01 C, and the discharge capacity was measured.

[0100] Similarly, constant current charging at 1 C and constant current discharging at 3 C were performed, and the discharge capacity was measured. The ratio of the discharge capacity at 3 C to the discharge capacity at 0.2 C (capacity retention) was calculated. The capacity retentions of the cells in Examples 1 to 7 and the comparative example are shown in Table 1.

[0101]

[0102] As shown in Table 1, the capacity retention rates of the cells in Examples 1-7 were greater than those of the cells in the comparative examples. Therefore, it was revealed that the current density dependence of the charge and discharge of the cells was reduced by including particles in the positive electrode whose maximum frequency in the circularity range of 0.8 or more and whose maximum frequency in the circularity range of 0.6 or less were greater than the maximum frequency in the circularity range of more than 0.6 and less than 0.8. That is, the rate characteristics of the cells in Examples 1-7 were improved compared to those of the cells in the comparative examples.

[0103] Since the positive electrode slurry containing the particles in Example 8 gelled, it became clear that when producing a positive electrode by wet molding, it is desirable to set the percentage of the cumulative frequency of particles in the range of circularity of 0.6 or less to 70% or less in order to prevent aggregation of the raw materials.

[0104] According to Table 1, it is clear that in order to increase the capacity retention ratio, the percentage of the integrated value of the frequency in the range of circularity of 0.6 or less is preferably 30% or more, and more preferably 40% or more. It is also clear that in order to increase the capacity retention ratio, it is clear that the percentage of the integrated value of the frequency in the range of circularity of 0.6 or less is preferably 70% or more, and more preferably 60% or more.

[0105] Although the examples have been described with the particles disposed on the positive electrode, it is of course possible to dispose the particles on the negative electrode. Because the particles are electrochemically stable with respect to lithium, even when disposed on the negative electrode, they are not reduced and an electrochemical element capable of generating a high electromotive force can be constructed.

[0106] The present invention has been described above based on the embodiments, but the present invention is not limited to the above embodiments, and it can be easily inferred that various improvements and modifications are possible within the scope of the present invention.

[0107] In the embodiments, the electrochemical elements 11, 22 have been described as including a positive electrode 12 having an active material layer 14 provided on one side of a current collector 13, and a negative electrode 16 having an active material layer 18 provided on one side of a current collector 17, but this is not necessarily limited to this. For example, it is of course possible to apply each element in the embodiments to an electrochemical element including electrode layers (so-called bipolar electrodes) having an active material layer 14 and an active material layer 18 provided on both sides of a current collector 13. If bipolar electrodes and separators 15 are alternately stacked and housed in a case (not shown), an electrochemical element with a so-called bipolar structure can be obtained.

[0108] In the embodiment, the active material layers 14, 18 include the particles 19, but this is not necessarily limited to this. It is of course possible to omit the particles 19 from at least one of the active material layers 14, 18.

[0109] In the embodiment, the electrochemical elements 11 and 22 are described as being lithium ion batteries, but the present invention is not necessarily limited to this. It is clear that the electrodes of other electrochemical elements may include the particles 19. Examples of other electrochemical elements include electrochemical capacitors. Examples of electrochemical capacitors include redox capacitors that utilize redox reactions and hybrid capacitors that are asymmetric cells that combine an electric double layer capacitor with the particles 19.

[0110] 11, 22 Electrochemical element 12 Positive electrode (electrode) 14, 18 Active material layer (electrode composition) 16 Negative electrode (electrode) 19 Particles 20, 21 Active material

Claims

1. An electrode composition containing oxide particles, wherein in the frequency distribution of the circularity of the particles appearing in the cross section of an object to which a plurality of the particles are fixed, the maximum frequency in the range of circularity of 0.8 or more and the maximum frequency in the range of circularity of 0.6 or less are greater than the maximum frequency in the range of circularity of more than 0.6 but less than 0.

8.

2. The electrode composition according to claim 1, wherein the ratio of the integrated value of the frequency in the range of circularity of 0.6 or less to the integrated value of the frequency in the entire range of circularity is 30% or more and 70% or less.

3. The electrode composition according to claim 1, wherein the particles have a Young's modulus of 140 GPa or more.

4. The electrode composition according to claim 1, wherein the average equivalent circle diameter of said particles appearing in said cross section is 3 μm or less.

5. The electrode composition according to claim 1, wherein the particles are a solid electrolyte having a garnet-type crystal structure containing Li, La, and Zr.

6. The electrode composition according to claim 5, wherein the solid electrolyte further contains Mg and Sr.

7. An electrode comprising the electrode composition according to any one of claims 1 to 6 and an active material.

8. The electrode according to claim 7, wherein the ratio of the area of ​​said particles appearing in the cross section of said electrode to the area of ​​said cross section is 0.1% or more and 5% or less.

9. An electrochemical device comprising the electrode according to claim 7.

10. The electrochemical device according to claim 9, wherein said electrode is a positive electrode.

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