Electrode and power storage device

Abstract

Provided are an electrode (12) and a power storage device (11) capable of reducing internal resistance. The electrode includes: an active material (20); particles (19) of an oxide having a garnet-type crystal structure containing Li, La, and Zr; and a first compound (23) and a second compound (24), which are different from each other and are bonded to the surface of the particles. The first compound contains fluorine, phosphorus, and oxygen, and the second compound contains fluorine. The oxide may include a dopant substituted with at least one of lithium ions, lanthanum ions, and zirconium ions. The valence of the dopant differs from the valence of the substituted ion. The power storage device includes the electrode.

Description

Electrodes and energy storage devices

[0001] The present invention relates to an electrode containing an active material and an electricity storage device.

[0002] In an electricity storage device, when the current density during charging and discharging increases, the electrochemical reaction cannot keep up, and the amount of electricity that can be extracted tends to decrease. In order to reduce the current density dependency during charging and discharging of an electricity storage device, it is necessary to reduce the internal resistance. Patent Document 1 discloses a prior art in which the internal resistance of an electricity storage device is reduced by using an additive contained in the electrolyte.

[0003] JP 2015-95289 A

[0004] The prior art has room for improvement in reducing the internal resistance of the electrodes.

[0005] The present invention has been made to meet this demand, and has an object to provide an electrode and an electricity storage device that can reduce internal resistance.

[0006] A first aspect for achieving this object is an electrode comprising an active material, particles of an oxide having a garnet-type crystal structure containing Li, La, and Zr, and a first compound and a second compound that are different from each other and bonded to the surface of the particles, the first compound containing fluorine, phosphorus, and oxygen, and the second compound containing fluorine.

[0007] In a second aspect, the oxide of the first aspect includes a dopant substituted for at least one of lithium ions, lanthanum ions, and zirconium ions, and the valence of the dopant is different from the valence of the substituted ions.

[0008] In a third aspect, in the first aspect, the oxide includes a dopant substituted for at least one of lanthanum ions and zirconium ions, and the valence of the dopant is smaller than the valence of the substituted ions.

[0009] In a fourth aspect, in any one of the first to third aspects, the concentration of the first compound is greater than the concentration of the second compound.

[0010] A fifth aspect is an electricity storage device, comprising the electrode according to any one of the first to fourth aspects, wherein the electrode contains an electrolyte solution in which lithium hexafluorophosphate is dissolved in a non-aqueous solvent.

[0011] The electrode of the present invention includes particles of an oxide containing Li, La, and Zr and having a garnet-type crystal structure, and a first compound containing fluorine, phosphorus, and oxygen and a second compound containing fluorine are bonded to the surface of the particles, thereby reducing the internal resistance of the electrode.

[0012] Fig. 1 is a cross-sectional view of an electricity storage device according to a first embodiment. Fig. 2 is a diagram schematically showing a garnet-type crystal structure. Fig. 3 is a schematic diagram of an electrode. Fig. 4 is a cross-sectional view of an electricity storage device according to a second embodiment. (a) is a Cole-Cole plot obtained by measuring the AC impedance of a positive electrode in an example, and (b) is a Cole-Cole plot obtained by measuring the AC impedance of a positive electrode in a comparative example.

[0013] Preferred embodiments of the present invention will now be described with reference to the accompanying drawings. Fig. 1 is a schematic cross-sectional view of an electricity storage device 11 according to a first embodiment. The electricity storage device 11 in this embodiment is a secondary battery that uses lithium ions as mobile ions. The electricity storage device 11 includes, in order, a positive electrode 12, a separator 15, and a negative electrode 16. The positive electrode 12, the separator 15, and the negative electrode 16 are housed in a case (not shown).

[0014] The positive electrode 12 has a current collecting layer 13 and an active material layer 14 stacked on top of each other. The current collecting layer 13 is a conductive member. Examples of materials for the current collecting layer 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.

[0015] The active material layer 14 includes oxide particles 19 and an active material 20. In order to reduce the resistance of the active material layer 14, the active material layer 14 may also include a conductive additive 21 (see FIG. 3 ). Examples of the conductive additive 21 include carbon black, acetylene black, ketjen black, carbon fiber, Ni, Pt, and Ag.

[0016] Examples of the active material 20 include a metal oxide containing a transition metal, a sulfur-based active material, and an organic active material. Examples of the metal oxide containing a transition metal include a metal oxide containing Li and one or more elements selected from Mn, Co, Ni, Fe, Cr, and V. Examples of the metal oxide containing a transition metal include 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 ４ , LiNi １／３ Mn １／３ Co １／３ O ２ and LiFePO ４ is exemplified.

[0017] 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.

[0018] The separator 15 separates the positive electrode 12 and the negative electrode 16, electrically insulating them from each other. The separator 15 is made of a porous material that is durable against the active materials 20, 26 and the electrolyte solution 22 (see FIG. 3 ) contained in the positive electrode 12 and the negative electrode 16, and that allows lithium ions 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.

[0019] The negative electrode 16 has a current collecting layer 17 and an active material layer 18 superposed thereon. The current collecting layer 17 is a conductive member. Examples of materials for the current collecting layer 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.

[0020] The active material layer 18 includes particles 19 and an active material 26. A conductive additive (not shown) may be included in the active material layer 18 to reduce the resistance of the active material layer 18. Examples of the conductive additive include carbon black, acetylene black, ketjen black, carbon fiber, Ni, Pt, and Ag.

[0021] The active material 26 may be a carbonaceous material such as porous carbon, natural graphite, artificial graphite, graphitizable carbon, non-graphitizable carbon, or carbon fiber; ４ Ti ５ O １２ , Si, Si-Li alloys, 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 ２ Examples include those having a structure in which microcrystalline or amorphous Si is dispersed in a matrix.

[0022] The active material layers 14, 18 may contain a binder. 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.

[0023] Examples of vinylidene fluoride polymers include homopolymers of vinylidene fluoride and copolymers of vinylidene fluoride and copolymerizable monomers. Examples of copolymerizable monomers include halogen-containing monomers (excluding vinylidene fluoride) and non-halogen copolymerizable monomers. Examples of halogen-containing monomers include chlorine-containing monomers such as vinyl chloride; and fluorine-containing monomers such as trifluoroethylene, tetrafluoroethylene, chlorotrifluoroethylene, hexafluoropropylene, and perfluoroalkyl vinyl ether. Examples of non-halogen copolymerizable monomers include olefins such as ethylene and propylene; acrylic monomers such as acrylic acid, methacrylic acid, and esters or salts thereof; and vinyl monomers such as acrylonitrile, vinyl acetate, and styrene. One or more copolymerizable monomers are polymerized with vinylidene fluoride to form a copolymer.

[0024] Particles 19 are oxides containing Li, La, and Zr and having a garnet-type crystal structure. Particles 19 are strongly basic, have high electrochemical stability, and are resistant to reduction by metallic lithium. In order to increase the surface reactivity of particles 19, it is preferable to use oxides synthesized by firing raw materials and then pulverized in a jet mill. The crystal structure of particles 19 is represented by the general formula C ３ A ２ B ３ O １２ It is a garnet type with a cubic crystal structure represented by the formula:

[0025] 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 the garnet-type crystal structure, Li can exist in a position that is octahedrally coordinated with the oxygen atom Oa and that 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, the oxide Li ７ La ３ Zr ２ O１２ 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.

[0026] The garnet-type solid electrolyte is 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. Various elements are substituted in garnet-type solid electrolytes. For example, Ca, Sr, Ba, etc. are substituted at the C site, Nb, Ta, Sn, Hf, etc. are substituted at the A site, and Al, Ga, etc. are substituted at the B site. The amount of lithium changes due to the substitution of elements, and the arrangement, occupancy, and occupied sites of lithium ions within the crystal structure change, which in turn changes the ionic conductivity. The diffraction angle and intensity ratio may differ compared to No. 422259 due to the substitution of elements.

[0027] The oxide constituting the particles 19 includes a dopant substituted with at least one of lithium ions, lanthanum ions, and zirconium ions, and the valence of the dopant is preferably different from that of the substituted ions. A dopant with a valence smaller than that of the substituted ions can increase the number of lithium ions contained in the crystal lattice due to charge compensation, thereby increasing the carrier concentration and improving the ionic conductivity. On the other hand, a dopant with a valence larger than that of the substituted ions generates voids at the lithium ion sites, increasing the mobility of the lithium ions and improving the ionic conductivity.

[0028] An example of a dopant with a small valence is strontium ions substituted with lanthanum ions. An example of a dopant with a large valence is aluminum ions or gallium ions substituted with lithium ions, or tantalum ions or niobium ions substituted with zirconium ions. Since an increase in carrier concentration has a greater effect on improving ion conductivity than an increase in mobility, particles 19 preferably contain a dopant with a valence smaller than that of the substituted ions.

[0029] The particles 19 preferably contain at least one element selected from the group consisting of Ca, Sr, and Ba, and the molar ratio of each element satisfies the following (1) and (2) or (3) and (4): (1) 1.33≦Li / (La+A)≦3 (2) 0≦A / (La+A)≦0.67 (3) 2.0≦Li / (La+A)≦2.6 (4) 0.04≦A / (La+A)≦0.17

[0030] Returning to FIG. 1 , the proportion of particles 19 appearing in the cross section of active material layers 14, 18 is preferably 0.1-5%. This is to ensure the amount of mobile ion movement without impeding the reactivity of active materials 20, 26. The median equivalent circle diameter of particles 19 appearing in the cross section of active material layers 14, 18 is preferably 0.5-10 μm, and more preferably 0.5-6 μm. This is to ensure that the surface area of ​​particles 19 is of an appropriate size.

[0031] To determine the proportion and median diameter of particles 19, first, a scanning electron microscope (SEM) image of particles 19 appearing on a cross section of the electricity storage device 11 (a polished surface, a surface obtained by irradiating with a focused ion beam (FIB), or a surface obtained by ion milling) is analyzed. The proportion of particles 19 is determined by calculating the ratio of the area of ​​particles 19 to the area of ​​the cross section image. To determine the median diameter, a circle-equivalent diameter is calculated from the area of ​​each particle 19, a volume-based particle size distribution is determined, and the circle-equivalent diameter at which the integrated value of the frequency in the particle size distribution is 50% is calculated.

[0032] 3 is a schematic diagram of the positive electrode 12 (active material layer 14). The electricity storage device 11 (see FIG. 1) contains an electrolyte solution 22 in which a lithium salt is dissolved in a non-aqueous solvent. The lithium salt is a compound used for the transfer of cations between the positive electrode 12 and the negative electrode 16. The lithium salt is lithium hexafluorophosphate LiPF ６ Examples include:

[0033] Lithium hexafluorophosphate LiPF ６ Other lithium salts may be contained in the electrolyte solution 22. The anions of the other lithium salts include halide ions (I － , Cl － ,Br － etc.), SCN － , BF ４－ , BF ３ (CF ３ ) － , BF ３ (C ２ F ５ ) － , P.F. ６ － , ClO ４ － , SbF ６ － , N(SO ２ F) ２ － , N(SO ２ CF ３ ) ２ － , N(SO ２ C ２ F ５ ) ２ － , B(C ６ H ５ ) ４ － , B(O ２ C ２ H ４ ) ２ － , C(SO ２ F) ３ － , C(SO ２ CF ３ ) ３ － , C.F. ３ COO － , C.F. ３ SO ２ O － , C ６ F ５ SO ２ O － , B(O ２ C ２ O ２ ) ２ － , RCOO － (R is an alkyl group having 1 to 4 carbon atoms, a phenyl group, or a naphthyl group).

[0034] The nonaqueous solvents constituting the electrolytic solution 22 are broadly classified into molecular solvents, which are mostly composed of molecules, and ionic liquids, which are composed of cations and anions. The molecular solvent is preferably an aprotic solvent, as it widens the potential window of the electrolytic solution 22. 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 sulfone-based solvents. Mixtures of these solvents are also acceptable.

[0035] 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, methyl acetate, and ethyl propionate. Examples of phosphate esters include trimethyl phosphate. Examples of nitriles include acetonitrile, propionitrile, butyronitrile, and benzonitrile.

[0036] 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. Examples of ethers include tetrahydrofuran and monoglyme. Examples of nitro compounds include nitromethane and nitrobenzene. Fluorous solvents are compounds in which the hydrogen atoms of hydrocarbons are replaced with fluorine atoms, and derivatives thereof.

[0037] Examples of sulfone solvents include trimethylene sulfone, sulfolane, difluorosulfolane, dimethyl sulfolane, monofluorosulfolane, 3-methyl sulfolane, ethyl methyl sulfone, and ethyl isopropyl sulfone. Sulfone solvents are preferred because of their high thermal stability.

[0038] The reaction in which a lithium salt dissolves in a molecular solvent and dissociates into free ions proceeds more easily the larger the dielectric constant of the nonaqueous solvent and the more easily ions are solvated, so a nonaqueous solvent with a relatively large dielectric constant εr (εr>20) is preferred. Examples of molecular solvents with a dielectric constant greater than 20 include cyclic esters, nitriles, amides, sulfur compounds, acetone, acetylacetone, and nitro compounds. In order to adjust the viscosity of the nonaqueous solvent, it is of course possible to mix a nonaqueous solvent with a dielectric constant greater than 20 with a nonaqueous solvent with a dielectric constant of 20 or less.

[0039] Ionic liquids are compounds consisting of cations and anions, and are liquid at room temperature and normal pressure. If the non-aqueous solvent is an ionic liquid, it is possible to improve the flame retardancy of the electrolyte solution 22. The ionic liquid is preferably one having one or more cation species selected from the group consisting of ammonium, imidazolium, pyrrolidinium, and piperidinium.

[0040] 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:

[0041] 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 a lithium salt is dissolved.

[0042] The lithium salt concentration of the electrolyte solution 22 is preferably 4.0 mol / kg or less. If the salt concentration of the electrolyte solution 22 exceeds 4.0 mol / kg, the viscosity of the electrolyte solution increases, which significantly reduces the ionic conductivity.

[0043] The electricity storage device 11 is manufactured, for example, as follows: The particles 19, the active material 20, and the conductive additive 21 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 collecting layer 13 and dried to obtain a positive electrode sheet.

[0044] The particles 19, the active material 26, and the conductive additive 21 are mixed together, 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 collecting layer 17 and then dried to obtain a negative electrode sheet.

[0045] The 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 prepare a cylindrical or rectangular cell. Terminals (not shown) are connected to the current collecting layers 13 and 17, respectively, and lithium hexafluorophosphate LiPF is placed in a container (not shown) containing the cell. ６ After filling the container with an electrolyte solution 22 prepared by dissolving the above in a non-aqueous solvent, the container is sealed to obtain an electricity storage device 11 including the positive electrode 12 , the separator 15 and the negative electrode 16 .

[0046] It is known that when moisture is present in the electrolyte 22, impurities are generated due to hydrolysis of anions and subsequent decomposition reactions. The impurities in the electrolyte 22 affect the ionic conductivity and low-temperature characteristics of the electrolyte 22. In addition, LiPF ６ There is a possibility that the amount of water and impurities in the electrolyte solution 22 may react with the residual moisture in the electrolyte solution 22 to generate hydrofluoric acid (acidic impurity). Hydrofluoric acid may corrode the container and circuits of the electricity storage device 11. Therefore, it is necessary to control the amount of moisture and impurities in the electrolyte solution 22 contained in the electricity storage device 11.

[0047] When the positive electrode sheet or the negative electrode sheet is produced, the particles 19 react with a small amount of water (residual moisture) contained in the solvent and can change from a cubic crystal system to a cubic crystal system or a tetragonal crystal system with a larger lattice constant. This change in the crystal structure of the particles 19 can reduce moisture, which causes deterioration of the electrolyte 22, from the positive electrode sheet or the negative electrode sheet.

[0048] When the electrolyte 22 filled in the container contains water, the particles 19 are expected to cause the following reaction, thereby reducing the amount of water and hydrofluoric acid contained in the electrolyte 22.

[0049] LiPF ６ +H ２ O → LiF + PF ５ +H ２ O → LiF+2HF+POF ３ POF ３ +H ２ O → POF ２ (OH) + HF x H ２ O + Li ７ La ３ Zr ２ O １２ → xLiOH + H ｘ Li ７－ｘ La ３ Zr ２ O １２ Furthermore, the particles 19 and the decomposition products of the lithium salt are expected to undergo the following reaction.

[0050] xHF+Li ７ La ３ Zr ２ O １２ → xLiF+H Ｘ Li ７－ｘ La ３ Zr ２ O １２ xPOF ２ (OH) + Li ７ La ３ Zr ２ O １２ →xPO ２ F ２ +H ｘ Li ７―ｘ La ３ Zr ２ O １２ LiPO２ F ２ is an example of the first compound 23, and LiF is an example of the second compound 24. The first compound 23 is Li ｘ P.O. ｙ F ｚ where x, y, and z are integers from 0 to 6.

[0051] By this reaction, a first compound 23 containing fluorine, phosphorus, and oxygen and a second compound 24 containing fluorine are produced on the surface of the particle 19. The first compound 23 and the second compound 24 are different compounds. The third compound 25 provided on the surface of the particle 19 contains lithium hydroxide or lithium carbonate produced by the reaction of the particle 19 with water, carbon dioxide, or the like.

[0052] It is presumed that because the first compound 23 has a relatively high ionic bonding property, some of the solvent molecules coordinated to the lithium ions preferentially coordinate to the first compound 23 in the microscopic region near the particle 19, facilitating desolvation of the lithium ions coordinated to some of the solvent molecules, thereby reducing the solvation resistance at the interface of the active material 20 located near the particle 19. Because the second compound 24 has lithium ion conductivity, the resistance to lithium ion migration between the active material 20 in contact with the second compound 24 and the second compound 24 is reduced. It is presumed that this reduces the internal resistance of the positive electrode 12.

[0053] The particles 19 are electrochemically stable in the negative electrode 16. The mechanism by which the internal resistance of the positive electrode 12 is reduced also applies to the negative electrode 16, so the particles 19 can also reduce the internal resistance of the negative electrode 16.

[0054] The elements contained in the first compound 23 and the second compound 24, except for Li, can be detected by scanning electron microscope-energy dispersive X-ray spectroscopy (SEM-EDX). The presence of the first compound 23 and the second compound 24 can be confirmed by overlaying the SEM image of the cross section of the positive electrode 12 or the negative electrode 16 with the EDX detection results. In the SEM image of the cross section, the area of ​​the first compound 23 is preferably larger than the area of ​​the second compound 24. The area of ​​the first compound 23 corresponds to the concentration of the first compound 23 (ratio to the surface of the particle 19), and the area of ​​the second compound 24 corresponds to the concentration of the second compound 24. A concentration of the first compound 23 larger than the concentration of the second compound 24 is advantageous for reducing desolvation resistance.

[0055] In the cross-sectional SEM image, the combined area of ​​the first compound 23 and the second compound 24 is preferably larger than the area of ​​the third compound 25. This is because it is advantageous for reducing desolvation resistance and lithium ion migration resistance.

[0056] 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 electricity storage device 27 will be described in which the separator 28 separating the positive electrode 12 and the negative electrode 16 contains an electrolyte 29. 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.

[0057] 4 is a cross-sectional view of an electricity storage device 27 according to the second embodiment. The electricity storage device 27 includes, in order, a positive electrode 12, a separator 28, and a negative electrode 16. The separator 28 includes an electrolyte 29. The electrolyte 29 may be, for example, a solid or gel electrolyte having ion conductivity. A composition in which the electrolyte 29 and the electrolytic solution 22 are mixed may be disposed in the separator 28.

[0058] The electrolyte 29 includes at least one selected from sulfide-based, oxide-based, hydride-based, halide-based, and organic-based electrolytes. The sulfide-based electrolyte includes crystalline thiolithium-based, Li １０ GeP ２ S １２ type, argyrodite type, Li７ P ３ S １１ Type, Li ２ S-P ２ S ５ Examples of oxide electrolytes include oxides having a NASICON structure, oxides having a perovskite structure, and oxides having a garnet structure.

[0059] 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.

[0060] The electricity storage device 27 is manufactured, for example, as follows: A mixture of the electrolyte solution 22 and the 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 applied onto the current collecting layer 13 and then dried to obtain the active material layer 14.

[0061] A solvent in which a binder is dissolved is mixed with a mixture of the electrolytic solution 22 and the electrolyte 29 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.

[0062] The mixture of the electrolyte solution 22 and the particles 19 is mixed with the active material 26, and then a solvent in which a binder is dissolved is mixed to form a slurry. The slurry is applied onto the current collecting layer 17 and then dried to obtain the active material layer 18. The separator slurry is applied onto the active material layer 18 and then dried to obtain the negative electrode sheet.

[0063] 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 28 is formed between the positive electrode 12 and the negative electrode 16. Terminals (not shown) are connected to the current collecting layers 13 and 17, respectively, and the resulting product is sealed in a container (not shown), thereby obtaining an electricity storage device 27 including the positive electrode 12, the separator 28, and the negative electrode 16.

[0064] The electricity storage device 27 in the second embodiment includes the positive electrode 12 and the negative electrode 16 containing the particles 19, and therefore, like the electricity storage device 11 in the first embodiment, the internal resistance of the positive electrode 12 and the negative electrode 16 can be reduced.

[0065] The present invention will be explained in more detail with reference to examples, but the present invention is not limited to these examples.

[0066] (Preparation of particles) Li ６．９５ Mg ０．１５ La ２．７５ Sr ０．２５ Zr ２．０ 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.

[0067] The LLZ was pulverized using a dry jet mill (Nano Jetmizer (registered trademark) NJ-50 model, manufactured by Aisin Nano Technologies Co., Ltd.) in a nitrogen atmosphere at a feed rate of 1 kg / hr to obtain particles. The median diameter of the particles measured using a laser diffraction particle size distribution analyzer was 0.8 μm.

[0068] (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.

[0069] (Example) LiNi ０．６ Mn ０．２ 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.

[0070] 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 a cell according to the example.

[0071] (Comparative example) LiNi ０．６ Mn ０．２ Co ０．２ O ２ A positive electrode (a positive electrode not containing particles) 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 mass ratio of 94:3:3. A cell in the comparative example was obtained in the same manner as in the example, except that a positive electrode (a positive electrode not containing particles) was used.

[0072] (AC Impedance Measurement) First, the cell was charged. The cell was charged at a constant current of 0.1 C until the terminal voltage reached 4.2 V (charge stop potential), and then the current was held at a constant potential until the current value reached 0.01 C. Next, a reference electrode was inserted at the boundary between the separator and the negative electrode of the cell, and the cell was stored in a thermostatic chamber at −20° C. The AC impedance of the positive electrode was measured. The conditions for the AC impedance measurement were a voltage of 10 mV and a frequency of 10 kHz to 100 mHz.

[0073] Figure 5(a) is a Cole-Cole plot obtained by measuring the AC impedance of the positive electrode in the example. Figure 5(b) is a Cole-Cole plot obtained by measuring the AC impedance of the positive electrode in the comparative example. Since the diameter of the arc in the Cole-Cole plot in the example shown in Figure 5(a) is smaller than the diameter of the arc in the Cole-Cole plot in the comparative example shown in Figure 5(b), it is clear that the positive electrode in the example has a lower internal resistance than the positive electrode in the comparative example.

[0074] (Cross-section observation of positive electrode) A cell in an example that had not been subjected to charge / discharge operations was cut, and the cross section of the particles contained in the positive electrode was analyzed by SEM-EDX. A first compound containing fluorine, phosphorus, and oxygen and a second compound containing fluorine were detected on the surface of the particles. It is presumed that the internal resistance of the positive electrode in the example was reduced by the first compound containing fluorine, phosphorus, and oxygen and the second compound containing fluorine that were generated on the surface of the particles.

[0075] 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.

[0076] In the embodiment, the case where the particles 19 appearing on the cross section of the positive electrode 12 or the negative electrode 16 are analyzed using SEM-EDX has been described, but the present invention is not necessarily limited to this. For example, the cross section of the particles 19 can be analyzed using transmission electron microscope-energy dispersive X-ray spectroscopy (TEM-EDX), transmission electron microscope-electron energy loss spectroscopy (TEM-EELS), or electron beam microanalyzer-wavelength dispersive X-ray spectroscopy (EPMA-WDX). The surface of the particles 19 may also be analyzed using X-ray photoelectron spectroscopy (XPS).

[0077] In the embodiment, the electricity storage device 11 has been described as including a positive electrode 12 in which an active material layer 14 is provided on one side of a current collecting layer 13, and a negative electrode 16 in which an active material layer 18 is provided on one side of a current collecting layer 17, but the present invention is not necessarily limited to this. For example, it is of course possible to apply each element in the embodiment to an electricity storage device including electrode layers in which an active material layer 14 and an active material layer 18 are provided on both sides of a current collecting layer 13 (so-called bipolar electrodes). If bipolar electrodes and separators 15 are alternately stacked and housed in a case (not shown), an electricity storage device with a so-called bipolar structure can be obtained.

[0078] In the embodiment, the active material layers 14, 18 contain the particles 19, but this is not necessarily limited to this. It is sufficient for at least one of the active material layers 14, 18 of the electricity storage device to contain the particles 19.

[0079] In the embodiments, the power storage devices 11 and 27 are described as being made of lithium ion batteries, but the present invention is not necessarily limited to this. It is clear that the electrodes of other power storage devices may include the particles 19. An example of the other power storage device is an electrochemical capacitor. Examples of the electrochemical capacitor include a redox capacitor that utilizes a redox reaction and a hybrid capacitor that is an asymmetric cell that combines an electric double layer capacitor with the particles 19. Examples of the active material in the electrochemical capacitor include carbon-based materials such as porous carbon, natural graphite, artificial graphite, graphitizable carbon (hard carbon), non-graphitizable carbon (soft carbon), and carbon fiber.

[0080] 11, 27 Electric storage device 12 Positive electrode (electrode) 16 Negative electrode (electrode) 19 Particles 20, 26 Active material 22 Electrolyte 23 First compound 24 Second compound

Claims

1. An electrode comprising an active material, the electrode comprising: particles of an oxide having a garnet-type crystal structure containing Li, La, and Zr; and a first compound and a second compound that are different from each other and bonded to the surfaces of the particles, wherein the first compound contains fluorine, phosphorus, and oxygen, and the second compound contains fluorine.

2. The electrode of claim 1, wherein said oxide comprises a dopant substituted for at least one of lithium ions, lanthanum ions, and zirconium ions, and the valence of said dopant is different from the valence of the substituted ions.

3. The electrode according to claim 1, wherein said oxide contains a dopant substituted for at least one of lanthanum ions and zirconium ions, and the valence of said dopant is less than the valence of the substituted ions.

4. The electrode of claim 1, wherein the concentration of said first compound is greater than the concentration of said second compound.

5. An electric storage device comprising the electrode according to any one of claims 1 to 4, wherein the electrode contains an electrolyte solution in which lithium hexafluorophosphate is dissolved in a non-aqueous solvent.

Images