Electrode and power storage device

Abstract

The present invention provides: an electrode (12) which is capable of reducing a decrease in fluidity during the production; and a power storage device (11). The electrode contains an oxide-based solid electrolyte, the solid electrolyte being a powder (19) that has a garnet-type crystal structure. The powder has a cubic crystal system and a lattice constant of 1.3 nm or more. The powder may have a distance of 0.588 nm or more in a unit lattice between an atom that forms an octahedral coordination with oxygen and an atom that forms a dodecahedral coordination with oxygen. The solid electrolyte may contain Li, La, Zr, Mg, and Sr as components. The power storage device includes the electrode.

Description

Electrodes and energy storage devices

[0001] The present invention relates to an electrode and an electricity storage device including an oxide-based solid electrolyte.

[0002] Patent Document 1 discloses a prior art technique in which a sintered body of a material containing an oxide-based solid electrolyte is disposed as an electrode in an electricity storage device.

[0003] International Publication No. 2018 / 198494

[0004] Prior art requires materials containing solid electrolyte powder to be sintered to the size of the electrode. When oxide-based solid electrolyte powder is placed in an electrode without being sintered, mobile ions diffuse from the powder to the outside, causing uneven distribution of the mobile ions, which can lead to gelation and reduced fluidity. This reduced fluidity can lead to variations in the dispersion of the material or even make it impossible to fabricate an electrode.

[0005] The present invention has been made to solve this problem, and has an object to provide an electrode and an electricity storage device that can reduce the decrease in fluidity during production.

[0006] A first aspect for achieving this object is an electrode including an oxide-based solid electrolyte, the solid electrolyte being a powder having a garnet-type crystal structure, the powder having a cubic crystal system and a lattice constant of 1.3 nm or more.

[0007] In a second aspect, in the powder of the first aspect, the distance in the unit cell between an atom octahedrally coordinated with oxygen and an atom dodecahedrally coordinated with oxygen is 0.588 nm or more.

[0008] In a third aspect, in the first or second aspect, the solid electrolyte contains Li, La, Zr, Mg, and Sr as components.

[0009] A fourth aspect is an electricity storage device, which includes the electrode of any one of the first to third aspects.

[0010] According to the electrode of the present invention, the oxide-based solid electrolyte powder having a garnet-type crystal structure has a cubic crystal system with a lattice constant of 1.3 nm or more, which facilitates diffusion of mobile ions between the powder and the outside of the powder. This reduces gelling of the material caused by uneven distribution of mobile ions, thereby reducing the decrease in fluidity during electrode fabrication.

[0011] It is a cross-sectional view of the electricity storage device in 1st embodiment. It is a figure which shows typically a garnet type crystal structure. It is a cross-sectional view of the electricity storage device in 2nd embodiment.

[0012] 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).

[0013] 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.

[0014] The active material layer 14 contains an oxide-based solid electrolyte powder 19 and an active material 20. The active material layer 14 may contain a conductive additive to reduce the resistance of the active material layer 14. Examples of conductive additives include carbon black, acetylene black, ketjen black, carbon fiber, Ni, Pt, and Ag.

[0015] 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 , LiMn２ 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.

[0016] 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.

[0017] The separator 15 separates the positive electrode 12 and the negative electrode 16 and electrically insulates them from each other. The separator 15 contains a powder 19 and an electrolyte (described below). The separator 15 may further contain a binder.

[0018] 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.

[0019] The active material layer 18 includes a powder 19 and 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. The active material 21 may be Li, a Li-Al alloy, Li, ４ Ti ５ O １２ , graphite, In, Si, a Si—Li alloy, and SiO. As with the separator 15, the active material layers 14 and 18 may contain a binder.

[0020] Examples of the binder include rubber-like polymers such as fluorinated resins, polyolefins, polyimides, polyvinylpyrrolidone, polyvinyl alcohol, cellulose ethers, and styrene-butadiene rubber. Examples of the fluorinated resin include vinylidene fluoride polymers, polychlorotrifluoroethylene, polyvinyl fluoride, tetrafluoroethylene-perfluoroalkyl vinyl ether copolymers, tetrafluoroethylene-hexafluoropropylene copolymers, ethylene-tetrafluoroethylene copolymers, and ethylene-chlorotrifluoroethylene copolymers.

[0021] 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.

[0022] Powder 19 is an oxide-based solid electrolyte having a garnet-type crystal structure. Garnet is represented by the general formula C ３ A ２ B ３ O １２ It is expressed as:

[0023] 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 location that is octahedrally coordinated with an oxygen atom Oa and that becomes a void V. The void V is, for example, a location 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 ７ La ３ Zr ２ O １２ In the garnet-type solid electrolyte having the composition above, La can occupy the C site Sc, Zr can occupy the A site Sa, and Li can occupy the B site Sb and the voids V.

[0024] 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.

[0025] Returning to FIG. 1, powder 19 is typically Li ７ La ３ Zr ２ O １２In the powder 19, some of the constituent elements may be substituted with other elements, or a small amount of other elements may be added without substituting the constituent elements. Examples of the other elements 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).

[0026] The powder 19 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 ３ Zr 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 Nb０．２５ 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 Examples include:

[0027] Powder 19 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 powder 19. (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

[0028] The powder 19 made of solid electrolyte may contain, as components, Li, La, Zr, Mg, and Sr. The components contained in the solid electrolyte can be analyzed by an inductively coupled plasma mass spectrometer (ICP-MS).

[0029] When it is difficult to extract only the powder 19 from the positive electrode 12 or the negative electrode 16, the active material layers 14, 18 can be extracted from the electricity storage device 11, and the powder 19 can be analyzed by powder X-ray diffraction of the materials contained in the active material layers 14, 18. The elemental species of the components contained in the powder 19 can be confirmed by elemental mapping of the cross section of the active material layers 14, 18 using time-of-flight secondary ion mass spectrometry (TOF-SIMS), electron probe microanalyzer (EPMA), Auger electron spectroscopy (AES), or the like, or by point analysis of the portion corresponding to the powder 19 (for example, the portion where La or Zr is present).

[0030] The proportion of the powder 19 appearing on the cross section of the active material layers 14, 18 is preferably 0.1 to 5%. This is to ensure the amount of mobile ions moving between the active materials 20, 21 and the powder 19 without impeding the reactivity of the active materials 20, 21.

[0031] The median diameter of the equivalent circle diameters of the powder 19 appearing on the cross sections of the active material layers 14 and 18 is preferably 0.1 to 10 μm, and more preferably 0.1 to 6 μm, in order to ensure that the surface area of ​​the powder 19 is of an appropriate size and to ensure the amount of movement of mobile ions between the active materials 20 and 21 and the powder 19.

[0032] To determine the proportion and median diameter of the powder 19, first, a scanning electron microscope (SEM) image of the powder 19 appearing on the 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 the powder 19 is calculated by calculating the ratio of the area of ​​the powder 19 to the area of ​​the cross section image. To determine the median diameter, the circle-equivalent diameter is calculated from the area of ​​each particle of the powder 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.

[0033] At least one of the positive electrode 12, the separator 15, and the negative electrode 16 may contain one or more other solid electrolytes in addition to the powder 19 having a garnet-type crystal structure containing Li, La, Zr, and O. Examples of the other solid electrolytes include crystalline or amorphous oxide-based solid electrolytes such as perovskite-type, NASICON-type, and LISICON-type, and hydride-based solid electrolytes.

[0034] The electricity storage device 11 contains an electrolyte solution in which a lithium salt is dissolved in a solvent. The lithium salt is a compound used for transferring cations between the positive electrode 12 and the negative electrode 16. The anion of the lithium salt is a halide ion (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).

[0035] The anion of the lithium salt is a sulfonyl group -S(=O) ２ - N(SO ２ F) ２ － , N(SO ２ CF ３ ) ２ － , N(SO ２ C ２ F ５ )２ － Sulfonylimides such as PF ６ － The sulfonylimide anion is less susceptible to increases in the viscosity of the electrolyte and decreases in ionic conductivity even when the salt concentration is high, while the halophosphate ion has a high degree of dissociation. Both are preferred because they can form a highly stable and low-resistance coating (SEI) that reduces the reductive decomposition of the non-aqueous solvent and widens the reduction-side potential window.

[0036] Examples of the solvent 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. As molecular solvents, aprotic solvents are preferred because they broaden the potential window of the non-aqueous 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 sulfone-based solvents. Mixtures of these solvents are also acceptable.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] The reaction in which an electrolyte dissolves in a molecular solvent and dissociates into free ions proceeds more easily the higher the relative dielectric constant of the solvent and the more easily ions are solvated, so a solvent with a relatively high relative dielectric constant εr (εr>20) is preferred. Examples of molecular solvents with a relative dielectric constant greater than 20 include cyclic esters, nitriles, amides, sulfur compounds, acetone, acetylacetone, and nitro compounds. It is of course possible to mix a solvent with a relative dielectric constant greater than 20 with a solvent with a relative dielectric constant of 20 or less in order to adjust the viscosity of the solvent, etc.

[0041] 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.

[0042] 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:

[0043] 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.

[0044] The lithium salt concentration of the electrolyte solution is preferably 4.0 mol / kg or less. This is because if the salt concentration of the electrolyte solution exceeds 4.0 mol / kg, the viscosity of the electrolyte solution increases, which significantly reduces the ionic conductivity. The salt concentration of the electrolyte solution is determined, for example, as follows. While the separator 15 will be described here, the active material layers 14 and 18 can also be determined in the same manner.

[0045] First, the separator 15 is crushed and immersed in a solvent, and the electrolyte contained in the separator 15 is dissolved in the solvent, and then the separator 15 is separated into a solid component and a liquid component using a centrifuge. The Li content of the separated liquid component is determined by high-frequency inductively coupled plasma spectroscopy (ICP).

[0046] The type of solvent contained in separator 15 is identified by, for example, gas chromatography-mass spectrometry (GC-MS). The identified solvent (hereinafter referred to as a "standard substance") and separator 15 are analyzed by thermogravimetric-differential thermal analysis (TG-DTA), and the analysis results of the standard substance and separator 15 are compared to identify the content of the solvent contained in separator 15. The molar concentration (mol / kg) of the lithium salt in the electrolyte solution is calculated based on the Li content in the liquid component and the content of the solvent in separator 15.

[0047] The electricity storage device 11 is manufactured, for example, as follows: A slurry is prepared by mixing an electrolyte solution, in which a lithium salt is dissolved in a solvent, with the powder 19, and then mixing the mixture with a solution in which a binder is dissolved in a solvent. The slurry is formed into a sheet and then dried to obtain a green sheet (electrolyte sheet) for the separator 15.

[0048] An electrolyte solution in which a lithium salt is dissolved in a solvent is mixed with powder 19, and then an active material 20 is mixed with this, followed by a solution in which a binder is dissolved in a solvent, to form a slurry. The slurry is applied onto current collecting layer 13 and then dried to obtain a green sheet (cathode sheet) for cathode 12.

[0049] An electrolyte solution in which a lithium salt is dissolved in a solvent is mixed with powder 19, and then an active material 21 is mixed with this, followed by a solution in which a binder is dissolved in a solvent to form a slurry. The slurry is applied onto current collecting layer 17 and then dried to obtain a green sheet (negative electrode sheet) for negative electrode 16.

[0050] The electrolyte sheet, positive electrode sheet, and negative electrode sheet are each cut to a predetermined shape, and then stacked in this order, the positive electrode sheet, the electrolyte sheet, and the negative electrode sheet, and then pressed together to form an integrated unit. Terminals (not shown) are connected to the current collecting layers 13 and 17, respectively, and the unit is sealed in a case (not shown), thereby obtaining an electricity storage device 11 including a positive electrode 12, a separator 15, and a negative electrode 16.

[0051] Because powder 19 is highly reactive, lithium ions (mobile ions) easily diffuse from powder 19 into the slurry used to prepare positive electrode sheets, negative electrode sheets, etc. Because the electrolyte solution contained in the slurry also contains a large number of mobile ions, the acid-base equilibrium of the slurry is disrupted, the slurry becomes more basic, and the slurry may gel (become non-fluidized). A decrease in the fluidity of the slurry can result in variations in the dispersion state of the slurry or the inability to prepare sheets.

[0052] To prevent this, powder 19 having a cubic crystal system and a lattice constant of 1.30 nm or more is used, which is presumably because it makes it easier for mobile ions to diffuse between the powder 19 and the outside of the powder 19, reducing gelation of the slurry caused by uneven distribution of mobile ions and thereby reducing the decrease in the fluidity of the slurry.

[0053] It is estimated that the diffusibility of mobile ions is improved when the powder 19 has a cubic crystal system and a lattice constant of 1.30 nm or more. The lattice constant of the powder 19 is preferably 1.32 nm or less to ensure the crystal structure of the garnet-type solid electrolyte.

[0054] In the powder 19, if the distance (interatomic distance) in the unit lattice between the atom octahedrally coordinated with oxygen and the atom dodecahedrally coordinated with oxygen is 0.588 nm or more, the diffusibility of the mobile ions is further improved, which is preferable. For example, Li ７ La ３ Zr ２ O １２ In the garnet-type solid electrolyte having the composition, the atom octahedrally coordinated with oxygen is Zr, which occupies the A site, and the atom dodecahedrally coordinated with oxygen is La, which occupies the C site. That is, it is preferable that the interatomic distance between La and Zr is 0.588 nm or more.

[0055] The crystal system, lattice constant, and interatomic distance of powder 19 can be determined by powder X-ray diffraction. Powder X-ray diffraction is preferably performed using high-energy X-rays with short wavelengths generated by synchrotron radiation in order to accurately measure the reciprocal lattice space. The lattice constant can be calculated by identifying the plane index of the peak in the powder X-ray diffraction pattern using synchrotron X-rays and measuring the interplanar spacing of the peak. The crystal system can be confirmed using the powder X-ray diffraction pattern.

[0056] The positions of peaks that appear in powder X-ray diffraction patterns follow the Bragg equation, which is the physical law of X-ray diffraction and reflection. The Bragg equation is 4 sin ２ θ / λ ２ = 1 / d ２ = (h ２ +k ２ +l ２ ) / a ２ (hkl) are plane indices that specify the type of crystal plane. The left side of the equation can be calculated from the scattering angle 2θ of each peak and the wavelength λ of the incident X-ray. The value of the left side is the sum of the squares of the plane indices h ２ +k ２ +l ２ This is equal to the ratio of the square of the lattice constant a to the value on the left side. Divide the value on the left side by a constant to find a number that gives a series of small integer values. Using this number, you can determine the plane index (hkl) of each peak and calculate the lattice constant a.

[0057] A second embodiment will be described with reference to Fig. 3. In the first embodiment, an electricity storage device 11 using a powder 19 and an electrolytic solution as the electrolyte was described. In the second embodiment, an electricity storage device relating to a liquid-based lithium-ion battery using an electrolytic solution as the electrolyte will be described. The same parts as those described in the first embodiment are given the same reference numerals, and the following description will be omitted. Fig. 3 is a cross-sectional view of an electricity storage device 24 in the second embodiment.

[0058] The power storage device 24 includes, in that order, a positive electrode 12, a separator 25, and a negative electrode 16. These are housed in a case (not shown). The separator 25 is made of a porous material that is durable against the active materials 20, 21 and electrolyte 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 25 include nonwoven fabrics and porous membranes made of cellulose, polypropylene, polyethylene, polyimide, alumina, etc. The nonaqueous electrolyte is the same as that described in the first embodiment, so its description will be omitted.

[0059] In the electricity storage device 24 of the second embodiment, the positive electrode 12 and the negative electrode 16 contain the powder 19, and therefore, similar to the electricity storage device 11 of the first embodiment, the stability of operation is increased.

[0060] The present invention will be explained in more detail with reference to examples, but the present invention is not limited to these examples.

[0061] (Preparation of solid electrolyte) 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 set to about 15 mol% excess in terms of element, taking into account the volatilization of Li during firing. The weighed raw materials and ethanol were placed in a nylon pot together with zirconia balls and ground and mixed in a ball mill for 15 hours. The slurry removed from the pot was dried, placed on an MgO plate, and fired at 900°C for 1 hour, and then further fired at 1200°C for 10 hours to obtain a solid electrolyte.

[0062] (Preparation of negative electrode) Natural graphite and artificial 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 ratio of 97.0:0.7:0.3:1.0:1.0 (volume ratio) to prepare a slurry. The slurry was applied to copper foil and dried at 120 ° C. A square with a side length of 25 mm was cut using a press to obtain a negative electrode.

[0063] Example 1 35 g of solid electrolyte was pulverized for 168 hours in a ball mill at 400 rpm in the presence of 210 mL of methanol having a viscosity of 0.54 cP at 25°C. The viscosity was measured using a vibration viscometer (VM-ICA-M) (the same device was used to measure viscosity in the comparative example). The pulverized solid electrolyte was dried to obtain the powder in this example.

[0064] LiNi ０．６ Mn ０．２ Co ０．２ O ２(active material), acetylene black (conductive additive), powder (solid electrolyte), and polyvinylidene fluoride (binder) dissolved in N-methyl-2-pyrrolidone were weighed out to a volume ratio of 93.5:3.0:0.5:3.0 and mixed to prepare a slurry. The slurry was applied to aluminum foil and dried at 80°C. A square with sides of 20 mm was cut using a press to obtain the positive electrode of Example 1.

[0065] A cell was fabricated by stacking the positive electrode, separator, and negative electrode in this order. Ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate were mixed in a volume ratio of 1:1:1, and 1 wt % of vinylene carbonate was further mixed in the solvent. LiPF 6 was added as a lithium salt (1 mol / L) to the solvent. ６ The cell was immersed in an electrolyte solution containing the above dissolved therein, and then evacuated and sealed to obtain the cell of Example 1.

[0066] (Example 2) LiNi ０．６ Mn ０．２ Co ０．２ O ２ A cell in Example 2 was obtained in the same manner as in Example 1, except that a positive electrode prepared by mixing polyvinylidene fluoride (active material), acetylene black (conductive additive), powder (solid electrolyte), and polyvinylidene fluoride (binder) dissolved in N-methyl-2-pyrrolidone in a volume ratio of 93:3:1:3 was used.

[0067] (Example 3) LiNi ０．６ Mn ０．２ Co ０．２ O ２ A cell in Example 3 was obtained in the same manner as in Example 1, except that a positive electrode prepared by mixing (active material), acetylene black (conductive additive), powder (solid electrolyte), and polyvinylidene fluoride (binder) dissolved in N-methyl-2-pyrrolidone in a volume ratio of 93.9:3.0:0.1:3.0 was used.

[0068] (Example 4) LiNi ０．６ Mn ０．２ Co ０．２ O ２A cell in Example 3 was obtained in the same manner as in Example 1, except that a positive electrode prepared by mixing polyvinylidene fluoride (active material), acetylene black (conductive additive), powder (solid electrolyte), and polyvinylidene fluoride (binder) dissolved in N-methyl-2-pyrrolidone in a volume ratio of 89:3:5:3 was used.

[0069] Comparative Example 1 35 g of a solid electrolyte was pulverized in a ball mill at 400 rpm for 168 hours in the presence of 210 mL of butyl carbitol having a viscosity of 6.6 cP at 25° C., and the pulverized solid electrolyte was dried to obtain a powder for the comparative example.

[0070] LiNi ０．６ Mn ０．２ Co ０．２ O ２ (active material), acetylene black (conductive additive), powder (solid electrolyte), and polyvinylidene fluoride (binder) dissolved in N-methyl-2-pyrrolidone were weighed out to a volume ratio of 93.5:3.0:0.5:3.0, and when mixed, gelation occurred and a slurry could not be produced. A positive electrode could not be obtained, and therefore a cell could not be produced.

[0071] (Comparative Example 2) LiNi ０．６ Mn ０．２ Co ０．２ O ２ (active material), acetylene black (conductive additive), and polyvinylidene fluoride (binder) dissolved in N-methyl-2-pyrrolidone were weighed out to a volume ratio of 94:3:3 and mixed to prepare a slurry. The slurry was applied to aluminum foil and dried at 80°C. A square with a side length of 20 mm was cut using a press to obtain a positive electrode in the comparative example. The positive electrode, separator, and negative electrode were stacked in this order, and a cell in the comparative example was obtained in the same manner as in Example 1.

[0072] (Powder X-ray Diffraction) Powder X-ray diffraction patterns of the powders in the Examples and Comparative Examples were obtained by powder X-ray diffraction using synchrotron radiation. Powder X-ray diffraction was performed at the Aichi Synchrotron Light Center BL5S2 under the following conditions: camera length: 340 mm, beam size: 0.5 mm x 0.5 mm, incident X-ray wavelength: 0.07 nm, capillary: Lindemann glass 0.3 mmΦ, detector: PILATUS 100K. The obtained powder X-ray diffraction patterns were corrected using the powder X-ray diffraction pattern of a standard sample (NIST-Si). Based on the powder X-ray diffraction pattern, the crystal system of the powder, lattice constant, and the distance (interatomic distance) between La and Zr in the unit cell were each determined.

[0073] (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.

[0074] Next, the current density dependence (rate characteristics) of the charge / discharge curve of the cell was evaluated at 25° C. First, the terminal voltage reached 4.2 V during constant current charging at 1 C, and then the potential was maintained at a constant value until the current value reached 0.01 C. Next, the terminal voltage reached 2.5 V during constant current discharging at 0.2 C, and then the potential was maintained at a constant value until the current value reached 0.01 C.

[0075] Similarly, the following cycles were performed in sequence: 1 C constant current charge followed by 0.5 C constant current discharge, 1 C constant current charge followed by 1 C constant current discharge, 1 C constant current charge followed by 2 C constant current discharge, and 1 C constant current charge followed by 3 C constant current discharge, and the discharge capacity was measured for each cycle. The ratio of the discharge capacity at 3 C discharge to the discharge capacity at 0.2 C discharge (capacity retention) was calculated. The crystal system, lattice constant, La-Zr interatomic distance, and capacity retention of the powder are shown in Table 1.

[0076]

[0077] In Table 1, the capacity retention rate for Comparative Example 1 is not recorded because the slurry gelled and it was not possible to prepare a cell necessary for measuring the capacity retention rate. The crystal system, lattice constant, and interatomic distance for Comparative Example 2 are not recorded because the powders corresponding to these factors were not contained in the positive electrode.

[0078] In Comparative Example 1, the lattice constant of the powder was less than 1.30 nm, which resulted in high reactivity of the powder, and it is presumed that the acid-base equilibrium of the slurry was disrupted by lithium ions contained in the electrolyte solution in the slurry as well as lithium ions that diffused from the powder into the slurry, which is thought to have caused the slurry to become more basic and gelled.

[0079] In contrast, in Examples 1-4, in addition to the high lithium ion diffusivity, the lattice constant of the powder is 1.30 nm or more, which allows lithium ions to be incorporated into the crystal lattice of the powder. Because lithium ions are more likely to diffuse between the powder and the outside of the powder, the lithium ion concentration in the slurry can be appropriately adjusted. Therefore, it is presumed that gelation of the slurry, which occurs when the acid-base equilibrium is disrupted due to uneven distribution of lithium ions, was reduced.

[0080] Furthermore, in Example 1-4, it is presumed that the interfacial resistance of the powder (solid electrolyte) can be reduced because lithium ions are more likely to diffuse between the outside of the powder and the powder. As a result, it is presumed that the capacity retention rate of the cell in Example 1-4 was greater than that of the cell in Comparative Example 2. According to Example 1-4, it was confirmed that the capacity retention rate could be increased compared to Comparative Example 2 when the proportion of powder in the active material layer was in the range of 0.1 to 5 vol%.

[0081] Furthermore, in Example 1-4, the interatomic distance between La and Zr in the powder was 0.588 nm or more, which is presumably why voids that lithium ions can occupy were secured in the crystal lattice, resulting in improved diffusibility of lithium ions and a smaller current density dependency of the charge / discharge curve compared to the comparative example.

[0082] The powders in Examples 1-4 had a lower viscosity of the solvent used when wet-milling the powder in a ball mill than the powder in Comparative Example 1. This is thought to have reduced the adhesion and aggregation of the powder during milling, making it easier for the force of the milling media to be applied uniformly to the powder, resulting in fewer defects being introduced into the crystal. As a result, it is thought that the lattice constant of the powder could be made 1.30 nm or more, and the interatomic distance between La and Zr could be made 0.588 nm or more.

[0083] Although the case where the solid electrolyte powder is disposed on the positive electrode has been described in Examples 1-4, it is of course possible to dispose the solid electrolyte powder on the negative electrode. This is because the solid electrolyte powder is electrochemically stable with respect to lithium, and therefore, even when disposed on the negative electrode, it is not reduced and a power storage device that can generate a high electromotive force can be constructed.

[0084] 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.

[0085] 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.

[0086] In the embodiment, the active material layers 14, 18 and the separator 15 all contain the powder 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 power storage device to contain the powder 19.

[0087] In the embodiments, the power storage devices 11 and 24 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 contain the powder 19. Examples of other power storage devices 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 powder 19.

[0088] 11, 24 Electricity storage device 12 Positive electrode (electrode) 16 Negative electrode (electrode) 19 Powder

Claims

1. An electrode including an oxide-based solid electrolyte, wherein the solid electrolyte is a powder having a garnet-type crystal structure, and the powder has a cubic crystal system and a lattice constant of 1.3 nm or more.

2. The electrode according to claim 1, wherein the powder has a unit lattice distance between an atom octahedrally coordinated with oxygen and an atom dodecahedrally coordinated with oxygen of 0.588 nm or more.

3. The electrode of claim 1, wherein said solid electrolyte contains Li, La, Zr, Mg, and Sr as components.

4. An electricity storage device comprising the electrode according to any one of claims 1 to 3.

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