Oxide, electrolyte composition, and power storage device

Abstract

Provided are an oxide, an electrolyte composition, and a power storage device, all of which can be manufactured by simple methods. The oxide has a garnet-type crystal structure containing Li, La, and Zr, and the crystal structure has at least two tetragonal crystal phases that are different in lattice constant. The crystal structure belongs to a space group I41 / acd, and the occupancy proportion of Li at an 8a site in at least one of the tetragonal crystal phases may be 95% or less. The oxide may contain Sr. The electrolyte composition and the power storage device contain said oxide.

Description

Oxide, electrolyte composition and electricity storage device

[0001] The present invention relates to an oxide containing Li, Zr, and La and having a garnet-type crystal structure, an electrolyte composition, and an electricity storage device.

[0002] Oxides containing Li, Zr, and La and having a garnet-type crystal structure have high chemical stability and lithium ion conductivity. ７ La ３ Zr ２ O １２ (hereinafter referred to as LLZ) is a stable oxide of Li metal, and theoretically, it has a temperature of 10 －４ S / cm to 10 －３ The crystal structure of LLZ is mainly cubic and tetragonal. The tetragonal structure of LLZ, which shows low ionic conductivity, is stable at room temperature. The tetragonal structure is stable at 10 －４ At high temperatures, the LLZ transitions to a cubic crystal that exhibits an ionic conductivity of 50 S / cm or more. In order to obtain an oxide that exhibits high ionic conductivity at room temperature, the prior art disclosed in Patent Document 1 stabilizes the cubic crystal by substituting a specific element that constitutes the LLZ with another element.

[0003] JP 2016-40767 A

[0004] In the prior art, in order to stabilize the cubic crystal, it is necessary to produce the oxide by precisely controlling methods such as material preparation and firing.

[0005] The present invention has been made to solve this problem, and an object of the present invention is to provide an oxide, an electrolyte composition, and an electricity storage device that can be produced by a simple method.

[0006] A first aspect for achieving this object is an oxide having a garnet-type crystal structure containing Li, La, and Zr, the crystal structure including at least two types of tetragonal phases with different lattice constants.

[0007] In the second aspect, the crystal structure of the first aspect is in space group I4 １ / acd, and at least one of the tetragonal phases has an occupancy rate of Li in the 8a site of 95% or less.

[0008] A third aspect is the first or second aspect, further comprising Sr.

[0009] A fourth aspect is an electrolyte composition comprising the oxide of any one of the first to third aspects.

[0010] A fifth aspect is an electric storage device comprising a plurality of electrode layers and a separator separating the plurality of electrode layers, wherein at least one of the plurality of electrode layers and the separator contains the oxide of any one of the first to third aspects.

[0011] A sixth aspect is an electricity storage device comprising a plurality of electrode layers including a current collecting layer and a separator separating the plurality of electrode layers, and further comprising a protective layer provided between the electrode layers and the separator or on the current collecting layer, the protective layer comprising any one of the oxides of the first to third aspects.

[0012] The oxide, electrolyte composition, and electricity storage device of the present invention can be produced by a simple method.

[0013] 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 crystal structure of a conventional tetragonal LLZ; Fig. 3 is a diagram schematically showing an arrangement of Li in one crystal plane of a conventional tetragonal LLZ; Fig. 4 is a diagram schematically showing an arrangement of Li in one crystal plane of an oxide; Fig. 5 is a cross-sectional view of an electricity storage device according to a second embodiment; Fig. 6 is a cross-sectional view of an electricity storage device according to a third embodiment;

[0014] 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 10 according to a first embodiment. The electricity storage device 10 according to this embodiment is a lithium ion solid-state battery (secondary battery) in which the power generating element is made of a solid. "The power generating element is made of a solid" means that the skeleton of the power generating element is made of a solid, and includes a form in which the skeleton is impregnated with a liquid.

[0015] The electricity storage device 10 includes, in order, a positive electrode layer 11, an electrolyte layer 14, and a negative electrode layer 15. The positive electrode layer 11, the electrolyte layer 14, and the negative electrode layer 15 are housed in a case (not shown).

[0016] The positive electrode layer 11 is formed by stacking a current collecting layer 12 and an active material layer 13. The current collecting layer 12 is a conductive member. Examples of materials for the current collecting layer 12 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.

[0017] The active material layer 13 is made of an electrolyte composition containing an oxide 18 and an active material 19. To reduce the resistance of the active material layer 13, the active material layer 13 may contain a conductive additive. Examples of the conductive additive include carbon black, acetylene black, ketjen black, carbon fiber, Ni, Pt, and Ag.

[0018] Examples of the active material 19 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 ４ and LiFePO ４ is exemplified.

[0019] In order to suppress the reaction between the active material 19 and the oxide 18, a coating layer can be provided on the surface of the active material 19. The coating layer is made of Al ２ O ３ , ZrO ２ , LiNbO ３ , Li ４ Ti ５ O １２ , LiTaO ３ , LiNbO ３ , LiAlO ２ , Li ２ ZrO ３ , Li ２ WO４ , Li ２ TiO ３ , Li ２ B ４ O ７ , Li ３ P.O. ４ and Li ２ MoO ４ is exemplified.

[0020] 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.

[0021] The electrolyte layer 14 is made of an electrolyte composition containing an oxide 18. The electrolyte layer 14 is a separator that separates the positive electrode layer 11 and the negative electrode layer 15 and electrically insulates them from each other. The electrolyte layer 14 may contain an electrolyte solution in which an electrolyte salt is dissolved in a solvent, or a binder. The solvent for the electrolyte solution is not particularly limited as long as it dissolves the electrolyte salt. Examples of the solvent include carbonate esters, aliphatic carboxylic acid esters, phosphate esters, γ-lactones, ethers, nitriles, sulfolane, dimethyl sulfoxide, fluorous solvents, and ionic liquids. A mixture of these may also be used.

[0022] There are no particular restrictions on the binder as long as it binds the oxide 18. Examples of the binder include fluorinated resins, polyolefins, polyimides, polyvinylpyrrolidone, polyvinyl alcohol, cellulose ethers, and rubber-like polymers such as 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.

[0023] The negative electrode layer 15 is formed by stacking a current collecting layer 16 and an active material layer 17. The current collecting layer 16 is a conductive member. Examples of materials for the current collecting layer 16 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.

[0024] The active material layer 17 is made of an electrolyte composition containing an oxide 18 and an active material 20. To reduce the resistance of the active material layer 17, the active material layer 17 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 20 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 electrolyte layer 14, the active material layers 13 and 17 may contain an electrolyte solution and a binder.

[0025] The electricity storage device 10 is manufactured, for example, as follows. An example will be given in which the electricity storage device 10 includes an electrolyte solution and a binder. A mixture of an organic solvent in which a lithium salt is dissolved and an oxide 18 is mixed with a solution in which a binder is dissolved in a solvent to form a slurry. After tape casting, the mixture is dried to obtain a green sheet (electrolyte sheet) for the electrolyte layer 14.

[0026] A mixture of an organic solvent in which a lithium salt is dissolved and an oxide 18 is mixed with an active material 19, and then a solution in which a binder is dissolved in a solvent is mixed to form a slurry. After tape casting on the current collecting layer 12, the mixture is dried to obtain a green sheet (positive electrode sheet) for the positive electrode layer 11.

[0027] A mixture of an organic solvent in which a lithium salt is dissolved and an oxide 18 is mixed with an active material 20, and then a solution in which a binder is dissolved in a solvent is mixed to form a slurry. After tape casting on the current collecting layer 16, the mixture is dried to obtain a green sheet (negative electrode sheet) for the negative electrode layer 15.

[0028] 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 12 and 16, respectively, and the device is sealed in a case (not shown), thereby obtaining an electricity storage device 10 including a positive electrode layer 11, an electrolyte layer 14, and a negative electrode layer 15. It is of course possible for at least one of the positive electrode layer 11, the electrolyte layer 14, and the negative electrode layer 15 to be free of at least one of the electrolytic solution and the binder.

[0029] The crystal structure of oxide 18 is similar to that of conventional tetragonal Li ７ La ３ Zr ２ O １２ The crystal structure of (LLZ) and the crystal structure of oxide 18 in this embodiment will be compared for explanation. FIG. 2 is a diagram schematically showing the crystal structure of eight unit cells per formula unit of a conventional tetragonal LLZ. A tetragonal crystal is a crystal in which two of the three mutually perpendicular edges of the unit cell have the same edge lengths a and b, and the length c of the remaining edge is different from a and b. In the conventional tetragonal LLZ, Li exists in three ion sites: the 8a site, the 16f site, and the 32g site. The 8a site is tetrahedral coordinated, and the 16f site and the 32g site are octahedral coordinated. The occupancy of Li in the 8a site, the 16f site, and the 32g site is almost 100%.

[0030] FIG. 3 is a diagram showing a schematic diagram of the arrangement of Li on one crystal plane of conventional tetragonal LLZ. Li present in the 8a ion site is present at the intersection of the ion conduction path. In the Li ion conduction path, Li present in the 16f and 32g ion sites are arranged between Li present in the 8a ion site. Since the occupancy rate of Li in the 8a site is almost 100%, there are almost no vacancies at the intersection of the ion conduction path. Therefore, conventional tetragonal LLZ has low ionic conductivity caused by the movement of Li ions to the vacancy positions. The ionic conductivity of conventional tetragonal LLZ at room temperature is usually 10 －６ It is on the order of S / cm.

[0031] 4 is a diagram showing the arrangement of Li atoms in one crystal plane of oxide 18 (tetragonal LLZ). The crystal structure of oxide 18 belongs to the space group I4 １ / acd. Oxide 18 has Li in four ion sites, namely, 8a, 16f, 32g, and 16e. Li in the 8a and 16e ion sites is present at the intersection of the ion conduction path. In the Li ion conduction path, Li in the 16f, 32g, and 16e ion sites are arranged between Li in the 8a ion site. The 8a and 16e sites are tetrahedral coordinated, and the 16f and 32g sites are octahedral coordinated. The ionic conductivity of oxide 18 at room temperature is 10, which is more than 100 times that of conventional tetragonal LLZ at room temperature. －４ It is on the order of S / cm or more.

[0032] The oxide 18 contains at least two types of tetragonal phases with different lattice constants. Of the two types of tetragonal phases, the one with the larger lattice constant ratio a / c is referred to as the "first phase," and the one with the smaller lattice constant ratio a / c is referred to as the "second phase." For example, the a / c of ​​the first phase is in the range of 1.003 to 1.03, and the a / c of ​​the second phase is in the range of 1.001 to 1.01.

[0033] In oxide 18, the occupancy of Li at the 8a sites of the first phase is higher than the occupancy of Li at the 8a sites of the second phase. The occupancy of Li at the tetrahedral coordination sites consisting of the 8a and 16e sites of the first phase is higher than the occupancy of Li at the tetrahedral coordination sites consisting of the 8a and 16e sites of the second phase.

[0034] It is presumed that the second phase, which has a relatively small Li occupancy rate at the tetrahedral coordination sites consisting of the 8a and 16e ion sites, mainly plays a role in conducting Li ions, while the first phase, which has a relatively large Li occupancy rate at the tetrahedral coordination sites consisting of the 8a and 16e ion sites, mainly plays a role in supplying Li ions. It is presumed that oxide 18 has high ionic conductivity because the first and second phases have such a complementary relationship.

[0035] In the second phase, the occupancy rate of Li in the 8a sites is preferably 30% or more and 95% or less. When the occupancy rate is 95% or less, the probability of vacancies existing at the intersections of ion conduction paths increases, and Li ions move to the vacancy positions, resulting in high ionic conductivity. When the occupancy rate is 30% or more, electrical repulsion between vacancies is unlikely to occur, ensuring Li ion mobility. The occupancy rate of Li in the 8a sites is more preferably 50% or more. This is because when the occupancy rate of Li is less than 50%, the proportion of vacancies increases, and the mobility of Li ions decreases due to electrical repulsion between vacancies, tending to reduce ionic conductivity.

[0036] The 16e site is an ion site whose energy level is easily changed depending on the composition. When the 16e site is stabilized, Li migrates from other ion sites to the 16e site, increasing the occupancy of the 16e site. In the first and second phases, the occupancy of Li in the 16e site is preferably 60% or less (excluding 0%). If the occupancy exceeds 60%, the mobility of Li ions decreases due to electrostatic repulsion with adjacent Li ions. The occupancy of Li in the 16e site is preferably 20% or more. If the occupancy of Li is less than 20%, this indicates that the 16e site is in an unstable state, making it difficult for Li ions to conduct through the 16e site, resulting in a decrease in ionic conductivity.

[0037] Li present in the 16f and 32g sites plays a role in maintaining the crystal structure and ensuring the carrier concentration. In the first and second phases, the occupancy rate of Li in the 16f and 32g sites is preferably 50% or more (including 100%), more preferably 75% or more. This is because when the occupancy rate of Li in the 16f and 32g sites is less than 75%, the carrier concentration tends to decrease and the ionic conductivity tends to decrease. When the occupancy rate is less than 50%, the crystal structure tends to break down due to electrostatic repulsion caused by the continuous presence of vacancies.

[0038] The lattice constants of the first and second phases and the Li occupancy rate in each ion site are determined by acquiring a powder neutron diffraction profile and analyzing the diffraction profile by the Rietveld method. The lattice spacing d of the crystalline phase in the analysis by the powder neutron diffraction and Rietveld method is in the range of 0.5-4.7 Å. The analysis by the Rietveld method uses powder neutron diffraction data with a resolution Δd / d of 0.16% or less. The analysis by the Rietveld method determines that the crystalline structure is in the space group I4 １ / acd, has two tetragonal phases with different lattice constants, and Li can be present in the 8a, 16f, 32g, and 16e sites.

[0039] It can be easily estimated by powder X-ray diffraction and the Rietveld method that oxide 18 contains two types of tetragonal phases with different lattice constants, even without analysis by powder neutron diffraction and the Rietveld method. A characteristic of the tetragonal phase is that the Bragg reflection of the (800) plane in the X-ray diffraction pattern is split into two peaks due to the difference in interplanar spacing. It is estimated that oxide 18 has a Bragg reflection of the (800) plane in the X-ray diffraction pattern that is split into three peaks, and that peaks derived from two or more types of crystalline phases overlap.

[0040] Regarding whether oxide 18 contains two types of tetragonal crystals, the space group is I4 １ This can be estimated by assuming the presence of two tetragonal phases that belong to the lattice constant a > c and determining the lattice constant by the Rietveld method. If the ratios a / c of ​​the lattice constants of the two assumed types of tetragonal crystals both converge to values ​​greater than 1.001, it can be estimated that oxide 18 contains two types of tetragonal phases with different lattice constants.

[0041] The crystal structure of oxide 18 is a tetragonal crystal with a lattice constant close to that of a cubic crystal, and the difference between a and c in the unit cell is small. Therefore, when determining the lattice constant by powder X-ray diffraction, it is necessary to use a powder X-ray diffractometer with high angular resolution. It is desirable for the powder X-ray diffractometer to use synchrotron radiation as the light source. As a guide for the angular resolution of a powder X-ray diffractometer, the standard sample LaB ６When measured by (NIST SRM660c), the full width at half maximum of the Bragg reflection of the (110) plane must be 0.030° or less.

[0042] The oxide 18 is, for example, Li ７ La ３ Zr ２ O １２ Examples of the other elements include those in which some of the constituent elements of (LLZ) are substituted with other elements, and those in which a small amount of other elements is 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). As the other element, at least one of Mg and Sr is preferred, and Sr is particularly preferred.

[0043] Sr has an ionic radius close to that of La and is an element that can easily substitute for La in LLZ. When Sr substitutes for the La site, the difference in valence increases the Li concentration around Sr, which has the effect of increasing the stability of the 16e site. When the amount of Sr substitution is appropriate, it contributes to improving the ionic conductivity of oxide 18.

[0044] Mg is an element that forms a complex oxide with Li. When producing LLZ, the raw materials are prepared so that there is an excess of Li, taking into consideration the volatilization of Li during firing. When Mg is added to the raw materials, if the excess Li does not volatilize sufficiently during firing, the remaining Li and Mg form a complex oxide. This can reduce the interfacial resistance layer, such as lithium carbonate, that is created by the excess Li, and therefore the ionic conductivity of the oxide 18 can be increased.

[0045] The oxide 18 can be produced by using as starting materials a compound containing Li, a compound containing La, a compound containing Zr, and optionally a compound containing other elements (Mg, Sr, etc.), weighing and mixing the starting materials to obtain a desired chemical composition, and then firing the mixture. The oxide 18 is a polycrystalline body. A sintered body containing the oxide 18 can be used as the electrolyte layer 14. Powder of the oxide 18 obtained by pulverizing the sintered body may be placed in the electrolyte layer 14 or the active material layers 13 and 17.

[0046] The electrolyte layer 14 and the active material layers 13 and 17 may contain one or more other solid electrolytes in addition to the oxide 18. 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.

[0047] The perovskite-type solid electrolyte is an oxide containing at least Li, Ti, and La, for example, La ２／３－Ｘ Li ３Ｘ TiO ３ The NASICON type solid electrolyte is an oxide containing at least Li, M (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 LISICON type solid electrolyte is Li １４ Zn(GeO ４ ) ４ Examples of hydride-based solid electrolytes include hydrides of alkali metals or alkaline earth metals, and include at least one element from Group 13 of the Periodic Table of Elements (e.g., B, Al, Ga, In, and Ta). For example, LiBH ４ , LiAlH ４ Examples include:

[0048] A second embodiment will be described with reference to Fig. 5. In the first embodiment, the oxide 18 is used in a secondary battery whose power generating element is made of a solid. In the second embodiment, the oxide 18 is used in a liquid-based lithium-ion battery that uses an organic solvent as the electrolyte. The same parts as those described in the first embodiment are given the same reference numerals, and the following description will be omitted. Fig. 5 is a cross-sectional view of an electricity storage device 21 in the second embodiment.

[0049] The power storage device 21 includes, in this order, a positive electrode layer 11, a separator 22, and a negative electrode layer 15. These are housed in a case (not shown). The separator 22 is made of a porous material that is durable against the active materials 19, 20 and the electrolyte solution contained in the positive electrode layer 11 and the negative electrode layer 15, and that allows lithium ions to pass through but does not have electronic conductivity. Examples of the separator 22 include a nonwoven fabric or porous film made of cellulose, polypropylene, polyethylene, etc.

[0050] In the electricity storage device 21 of the second embodiment, the oxide 18 is contained in the positive electrode layer 11 and the negative electrode layer 15, and therefore, similar to the electricity storage device 11 of the first embodiment, the oxide 18 can be produced by a simple method.

[0051] A third embodiment will be described with reference to Fig. 6. In the first and second embodiments, the case where the oxide 18 is contained in the positive electrode layer 11, the electrolyte layer 14, and the negative electrode layer 15 has been described. In the third embodiment, the case where the oxide 18 is contained in the protective layers 26 and 29 will be described. The same parts as those described in the first and second embodiments are designated by the same reference numerals, and the following description will be omitted. Fig. 6 is a cross-sectional view of an electricity storage device 23 in the third embodiment.

[0052] The power storage device 23 includes, in order, a positive electrode layer 24, a separator 22, and a negative electrode layer 27. These are housed in a case (not shown). The power storage device 23 is a liquid-based lithium-ion battery that uses an organic solvent as an electrolyte.

[0053] The positive electrode layer 24 is formed by stacking the current collecting layer 12 and an active material layer 25. The active material layer 25 contains an active material 19. In order to reduce the resistance of the active material layer 25, the active material layer 25 may contain a conductive additive such as carbon black, acetylene black, ketjen black, carbon fiber, Ni, Pt, or Ag.

[0054] A protective layer 26 is disposed between the separator 22 and the negative electrode layer 27. The protective layer 26 includes an oxide 18.

[0055] The negative electrode layer 27 is formed by stacking an active material layer 28, a protective layer 29, and a current collecting layer 16 in that order. The active material layer 28 is made of, for example, Li, a Li-Al alloy, a Li-Sn alloy, a Li-Si alloy, a Li-Mg alloy, a Li-Si alloy, or a Si-Li alloy. The protective layer 29 contains an oxide 18. The protective layers 26 and 29 are arranged by sheet lamination, coating on the separator 22 or the current collecting layer 16, or the like.

[0056] The oxide 18 having a garnet-type crystal structure containing Li, La, and Zr is resistant to reduction by the metallic lithium of the active material layer 28, thereby increasing the stability of the operation of the electricity storage device 23. Furthermore, the protective layer 26 interposed between the active material layer 28 and the separator 22 suppresses short circuits caused by dendrite growth of metallic lithium. The protective layer 29 interposed between the active material layer 28 and the current collecting layer 16 suppresses deterioration of the current collecting layer 16.

[0057] The present invention will be explained in more detail with reference to examples, but the present invention is not limited to these examples.

[0058] (Preparation of oxides) Li was added to form various theoretical compositions shown in Tables 1 and 2. ２ CO ３ , La(OH) ３ and ZrO ２ and, if necessary, SrCO ３ , MgO were weighed. ２ CO ３ The amount of Li was approximately 15 mol% in excess in terms of element, taking into account the volatilization of Li during firing. The weighed raw materials and organic solvent 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 and then fired on an MgO plate at 900°C for 1 hour, and then further fired on an MgO plate at 1200°C for 15 hours.

[0059] A binder was added to the fired powder, and the mixture was pulverized and mixed in an organic solvent using a ball mill for 15 hours. The slurry was removed from the pot and dried to obtain a powder. The resulting powder was compacted into disk-shaped pellets with a diameter of 12 mm and a thickness of approximately 1.5 mm by press molding, and the pellets were then subjected to cold isostatic pressing (CIP) at 1.5 t / cm. ２The compact was placed on top of powder of the same composition as the compact, and sintered in an inert gas atmosphere at a temperature of 1050°C or higher for 3 to 36 hours, and then cooled at a rate of 10°C / min to obtain sintered bodies of various oxides with different compositions.

[0060] (Measurement of Ionic Conductivity) Using a symmetrical cell in which current collectors were attached to both sides of the obtained sintered body, the ionic conductivity at 25° C. was measured by an AC impedance method.

[0061] (Identification of Crystal Structure and Ion Site Occupancy) The sintered bodies were wet-pulverized using a planetary ball mill in a glove box under an argon atmosphere, and then dried to obtain powders of various oxides. The diffraction profiles of the oxides were obtained using a powder neutron diffraction device. Neutron diffraction was performed at BL20 of the Japan Proton Accelerator Research Complex (J-PARC) under the following conditions: power output 700 kW, double flame (DF), and measurement time 20 minutes.

[0062] The diffraction profile obtained with a back detector bank with a d range of 0.181-5.09 Å and a resolution Δd / d of 0.15% was analyzed by the Rietveld method in the d range of 0.5-4.7 Å to determine the ratio of the first and second phases, the lattice constants (Å) of the first and second phases, and the Li occupancy (%) of each ion site in the first and second phases. The analysis by the Rietveld method was performed using the analysis software "Z-Rietveld," with the prerequisite that the crystal structure is in space group I4. １ It was determined that the material belongs to a tetragonal crystal structure (tetragonal crystal), has two tetragonal phases (first and second phases) with different lattice constants, Li can exist in the 8a site, 16f site, 32g site, and 16e site, the site occupancy rates of La, Zr, and O are 1, and the maximum value of the Li site occupancy rate is 1. The indices of validity were Rwp<15 and S<3.0.

[0063] The theoretical composition of the oxides prepared, the proportion (mass%) of the first phase, the lattice constant (Å) of the first phase, and the occupancy (%) of Li in the 8a, 16f, 32g, and 16e ion sites of the first phase are shown in Table 1. Table 2 shows the theoretical composition of the oxides, the proportion (mass%) of the second phase, the lattice constant (Å) of the second phase, the occupancy (%) of Li in the 8a, 16f, 32g, and 16e ion sites of the second phase, and the ionic conductivity (S / cm).

[0064] The theoretical composition formula of the oxide in Example 4 was the same as that of the oxide in Example 3. For the oxide in Example 4, the ratio of the first phase to the second phase, the lattice constant, and the Li occupancy were determined, but the ionic conductivity was not measured. －３ S / cm or more is judged as "A", and 1.0 x 10 －４ S / cm or more 1.0×10 －３ S / cm or less was judged as "B", and 1.0 x 10 －４ S / cm or less was rated as "C".

[0065]

[0066]

[0067] As shown in Table 1, the crystal structure of the oxides in Comparative Examples 2 and 4 was a cubic crystal with the lattice constants a, b, and c equal. The crystal structure of the oxides in Comparative Examples 1 and 3 was a single tetragonal crystal (space group I4) with the lattice constants a and b equal and c shorter than a. １ / acd), and two tetragonal phases with different lattice constants were not present. As shown in Table 2, the ionic conductivity of Comparative Example 1-4 was judged to be C.

[0068] The crystal structure of the oxide in Examples 1 to 5 had two tetragonal phases (first and second phases) with different lattice constants. In each example, the proportion (mass%) of the first phase was greater than the proportion (mass%) of the second phase, and the lattice constant ratio a / c of ​​the first phase was greater than the lattice constant ratio a / c of ​​the second phase.

[0069] In each example, the occupancy of Li at the 8a sites of the first phase was higher than the occupancy of Li at the 8a sites of the second phase, and the occupancy of Li at the tetrahedral coordination sites consisting of the 8a and 16e sites of the first phase was higher than the occupancy of Li at the tetrahedral coordination sites consisting of the 8a and 16e sites of the second phase. In Example 1-5, the occupancy of Li at the 8a sites of the second phase was 30% or more and 95% or less, and the occupancy of Li at the 16e sites of the first and second phases was 60% or less (excluding 0%).

[0070] In Examples 1-5, the ionic conductivity was judged to be A or B. As a result, the oxide having two tetragonal phases (first phase and second phase) with different lattice constants was found to have an ionic conductivity of 1.0 × 10 －４ It was revealed that the ionic conductivity was as high as 100 S / cm or more.

[0071] In Examples 1-5, a sintered oxide body is obtained by placing a molded body on powder having the same composition as the molded body, firing the molded body, and then lowering the temperature at a rate of 10°C / min. Unlike the prior art, this method does not require complicated operations such as covering the molded body with powder having the same composition as the molded body during firing, and it has become clear that an oxide exhibiting high ionic conductivity can be produced by a simple method.

[0072] 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.

[0073] In the embodiment, the electricity storage device 10 has been described as including a positive electrode layer 11 in which an active material layer 13 is provided on one side of a current collecting layer 12, and an negative electrode layer 15 in which an active material layer 17 is provided on one side of a current collecting layer 16, but this 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 13 and an active material layer 17 are provided on both sides of a current collecting layer 12 (so-called bipolar electrodes). If bipolar electrodes and electrolyte layers 14 are alternately stacked and housed in a case (not shown), an electricity storage device with a so-called bipolar structure can be obtained.

[0074] In the embodiment, the active material layers 13, 17 and the electrolyte layer 14 all contain the oxide 18, but this is not necessarily limited to this. It is sufficient for the power storage device that at least one of the active material layers 13, 17 and the electrolyte layer 14 contains the oxide 18.

[0075] In the third embodiment, the protective layer 26 is present between the active material layer 28 and the separator 22, and the protective layer 29 is present between the current collecting layer 16 and the active material layer 28. However, this is not necessarily limited to this. It is of course possible to omit either the protective layer 26 or the protective layer 29.

[0076] In the embodiment, the oxide 18 has been described using the electricity storage devices 10, 21, and 23 made of lithium ion batteries as examples, but the present invention is not necessarily limited to these. Examples of other electricity storage devices that include the oxide 18 include an electrochemical capacitor that uses a redox reaction of an electrode or a redox reaction of ions in an electrolyte, an electric double layer, and a metal-air battery that uses oxygen in the air as a positive electrode active material.

[0077] 10, 21, 23 Electric storage device 11, 24 Positive electrode layer (electrode layer) 12, 16 Current collecting layer 13, 17 Active material layer (electrolyte composition) 14 Electrolyte layer (electrolyte composition, separator) 15, 27 Negative electrode layer (electrode layer) 18 Oxide 22 Separator 26, 29 Protective layer

Claims

1. An oxide having a garnet-type crystal structure containing Li, La and Zr, The crystal structure includes at least two types of tetragonal phases having different lattice constants, The two types of tetragonal phases are a first phase having a lattice constant ratio a / c in the range of 1.003 to 1.03, and a second phase having a lattice constant ratio a / c in the range of 1.001 to 1.01, An oxide in which the ratio a / c of ​​the lattice constants of the first phase is larger than the ratio a / c of ​​the lattice constants of the second phase.

2. The crystal structure is in space group I4 １ / acd, 2. The oxide according to claim 1, wherein in at least one of the tetragonal phases, the occupancy rate of Li in the 8a site is 95% or less.

3. The oxide according to claim 1 or 2, further comprising Sr.

4. An electrolyte composition comprising the oxide according to claim 1 or 2.

5. An electricity storage device comprising a plurality of electrode layers and a separator separating the plurality of electrode layers, The electric storage device, wherein at least one of the plurality of electrode layers and the separator contains the oxide according to claim 1 .

6. A power storage device comprising a plurality of electrode layers including a current collecting layer and a separator separating the plurality of electrode layers, a protective layer provided between the electrode layer and the separator or on the current collecting layer; The electric storage device, wherein the protective layer comprises the oxide according to claim 1 .