Garnet-type crystalline compounds, methods for producing the same, and lithium secondary batteries

Abstract

The present invention provides a garnet-type crystalline compound that can further stabilize the cubic crystal structure and increase ionic conductivity, and a lithium secondary battery using the same. [Solution] Li (7+x-y)α (La 3-x M x )(Zr 2-y T y )O 12±δ A garnet-type crystalline compound having a garnet-type or garnet-type similar crystalline structure, where M represents at least one element selected from alkaline earth metal elements, T represents at least one element selected from the group consisting of In, Ti, Sn, Bi, Ta, Nb, Sc, Te, W, Mo, Y, Sb, Ce, Ge, and Gd, and the molar ratios x, y, and α are 0.00

Description

[Technical Field]

[0001] This invention relates to a garnet-type crystalline compound, a method for producing the same, and a lithium secondary battery. More specifically, it relates to a garnet-type crystalline compound having a garnet-type crystalline structure containing lithium (Li), lanthanum (La), zirconium (Zr), and oxygen (O), while being lithium-rich (Li-rich), a method for producing the same, and a lithium secondary battery containing a solid electrolyte obtained using this garnet-type crystalline compound. [Background technology]

[0002] In recent years, the widespread use of electronic devices such as personal computers and mobile phones, the development of renewable energy storage technologies, and the development of electric vehicles have all contributed to a growing demand for safe, long-lasting, and high-performance batteries.

[0003] Conventional lithium-ion secondary batteries (sometimes simply referred to as lithium secondary batteries in this specification) primarily use an organic electrolyte solution, which is a lithium salt dissolved in an organic solvent, as the electrolyte layer. However, batteries using such liquid organic electrolytes pose safety concerns due to the risk of leakage, ignition, and explosion of the organic solvent.

[0004] Therefore, to ensure high safety, all-solid-state batteries, which use a solid electrolyte layer instead of a liquid organic electrolyte and have all other battery elements made of solid material, are attracting attention. All-solid-state batteries can simplify the casing required for conventional lithium secondary batteries that use a liquid organic electrolyte, and can be miniaturized by stacking each battery element, thus improving the energy density per unit volume and per unit mass. In particular, all-solid-state lithium secondary batteries containing lithium metal in the negative electrode are expected to achieve high energy density.

[0005] In general, all-solid-state lithium secondary batteries require an electrolyte that is stable with respect to lithium metal and composed of specific materials, due to the high reactivity of lithium metal. Development of such solid electrolytes is progressing, including inorganic solid electrolytes, solid polymer electrolytes, and composite solid electrolytes. Among these, oxide-based solid electrolytes are being investigated extensively because they have high lithium-ion conductivity, a wide potential window, and excellent thermal and mechanical stability.

[0006] For example, Patent Document 1 describes an oxide sintered body having a garnet-type crystal structure containing the main constituent elements Li, La, Zr, and O, and also containing substitutional elements such as Bi, Ta, and Nb.

[0007] This Patent Document 1 states that by substituting some of the Zr sites with other elements such as Bi, Ta, and Nb, and by having the Zr sites in the crystal structure composed of multiple elements, the ionic conductivity can be improved.

[0008] Furthermore, Patent Document 2 describes the production of an oxide having a garnet-type crystal structure containing Li, La, Zr, Sr, and O by the sol-gel method.

[0009] Patent Document 2 states that by employing the sol-gel method, a liquid-phase method, it is possible to obtain a solid electrolyte powder composed of extremely fine particles, thereby improving the packing density when forming the solid electrolyte layer and enhancing the conductivity of the all-solid-state battery. Furthermore, it states that in its production, Sr metal or Sr-containing metal alkoxide and an amphiphilic solvent are used, and since Sr is easily chelated into the solvent, the formation of oxides by Sr combining with other elements such as Zr is suppressed, and a lithium-ion conductive solid electrolyte powder that takes a cubic form at room temperature can be obtained.

[0010] Furthermore, Non-Patent Document 1 describes obtaining an amorphous structure Li-La-Zr-O (a-Li-La-Zr-O) with a high lithium (Li) content using the sol-gel method, and then coating this onto LiCoO2, which is the cathode material, to improve the cycle characteristics of lithium-ion batteries.

[0011] Non-patent document 1 states that the composition of the amorphous structure is unlimited, and when preparing an a-Li-La-Zr-O precursor solution using lithium tert-butoxide, lanthanum(III) nitrate hexahydrate, zirconium n-propoxide, and 2-methoxyethanol, the ratios of Li, La, and Zr are changed to obtain amorphous Li-La-Zr-O (a-Li-La-Zr-O) with a high Li content at various stoichiometric ratios. 11 The ionic conductivity is 3.0 × 10⁻⁶. -8 S / cm, Li 18 La2Zr2O 16 The ionic conductivity is 1.18 × 10⁻⁶. -6 It states that the unit is S / cm. [Prior art documents] [Patent Documents]

[0012] [Patent Document 1] Japanese Patent Publication No. 2015-48280 [Patent Document 2] Japanese Patent Publication No. 2021-147244 [Non-patent literature]

[0013] [Non-Patent Document 1] Tan Tan Bui et.al. Solution Processing of Lithium-Rich Amorphous Li-La-Zr-O Ion Conductor and Its Application for Cycling Durability Improvement of LiCoO2Cathode as Coating Layer, Adv. Mater. Interface. 2021, 8(5), 2001767 Summary of the Invention Problems to be Solved by the Invention

[0014] As in Patent Documents 1 and 2 described above, by substituting some of the elements constituting the complex oxide (also called LLZ) having Li, La, Zr, and O with other elements, the ionic conductivity can be improved. However, with these methods, it cannot be said that the improvement in ionic conductivity is still sufficient. Also, in Non-Patent Document 1, a lithium-rich amorphous structure of Li-La-Zr-O is obtained, but the ionic conductivity is also insufficient for this.

[0015] Under such circumstances, the inventors of the present invention earnestly studied means for further increasing the ionic conductivity of garnet-type crystal compounds having a garnet-type crystal structure. As a result, by substituting La and Zr with predetermined substitution elements respectively to make it lithium-rich with an increased Li content ratio, it was found that the ionic conductivity can be further increased, and the present invention was completed.

[0016] Therefore, an object of the present invention is to provide a garnet-type crystal compound capable of further increasing the ionic conductivity than before.

[0017] Another object of the present invention is to provide a method for producing the above garnet-type crystal compound.

[0018] Furthermore, another object of the present invention is to provide a lithium secondary battery having a solid electrolyte obtained from this garnet-type crystal system compound.

Means for Solving the Problems

[0019] That is, the gist of the present invention is as follows. [1] Li (7+x-y)α (La 3-x M x )(Zr 2-y T y )O 12±δ represented by, a garnet-type crystal system compound having a garnet-type or garnet-type similar crystal structure, M represents at least one element selected from alkaline earth metal elements, T represents at least one element selected from the group consisting of In, Ti, Sn, Bi, Ta, Nb, Sc, Te, W, Mo, Y, Sb, Ce, Ge, and Gd, and the molar ratios x, y, and α are 0.00 < x < 1.00, 0.00 < y < 0.50, 0.20 ≦ x - y ≦ 0.80, and 1.0 ≦ α ≦ 1.20, and δ represents an oxygen non-stoichiometric amount, a garnet-type crystal system compound characterized by that. [2] The garnet-type crystal system compound according to [1], wherein the Li content is 5.8% by mass or more and 7.0% by mass or less. [3] The garnet-type crystal system compound according to [1], wherein the element M is Sr or Ba, and the element T is Bi. [4] The garnet-type crystal system compound according to [1], wherein the crystal structure by XRD measurement is a cubic phase. [5] The garnet-type crystal system compound according to [4], wherein the crystal structure by XRD measurement is substantially only a cubic phase. [6] The garnet-type crystal system compound according to [1], wherein the half-value width of the peak appearing in the range of 2θ = 16 to 第十八条° in the X-ray diffraction pattern using Cu-Kα rays as the X-ray source is 0.17° or less. [7] A lithium secondary battery having a solid electrolyte obtained using the garnet-type crystal system compound according to [1]. [8] The solid electrolyte has an ionic conductivity at room temperature of 1.0 × 10 -3A lithium secondary battery as described in [7], wherein the density is S / cm or higher and the relative density is 85% or higher. A method for producing the garnet-type crystalline compound described in [9] [1], A method for producing a garnet-type crystalline compound, characterized by mixing a Li raw material containing Li, a La raw material containing La, an M raw material containing element M, a Zr raw material containing Zr, and a T raw material containing element T to form a raw material mixture, primary calcining the raw material mixture at a temperature of 800°C to 1100°C, pulverizing it, and then secondary calcining it at a temperature of 800°C to 1100°C. [Effects of the Invention]

[0020] The garnet-type crystalline compound of the present invention makes it possible to obtain a solid electrolyte with even higher ionic conductivity than conventional compounds, and moreover, it is possible to increase its relative density. [Brief explanation of the drawing]

[0021] [Figure 1] Figure 1 shows the results of powder X-ray diffraction (CuKα) measurements of the primary calcined products obtained in Examples 1-2 and Comparative Examples 1-3. [Figure 2] Figure 2 shows the results of powder X-ray diffraction (CuKα) measurements of the secondary calcined products obtained in Examples 1-2 and Comparative Examples 1-3. [Figure 3] Figure 3 shows the impedance spectrum of the LLZ pellet product (sintered body) obtained in Example 2, measured at 25°C (room temperature) under atmospheric conditions. [Modes for carrying out the invention]

[0022] The present invention will be described in detail below. The present invention relates to a garnet-type or garnet-type similar crystalline compound, which has the general formula: Li (7+x-y)α (La 3-x M x )(Zr 2-y T y )O 12±δIt is represented by [formula], where x, y, and α indicating molar ratios have the relationships of 0.00 < x < 1.00, 0.00 < y < 0.50, 0.20 ≤ x - y ≤ 0.80, and 1.0 ≤ α ≤ 1.20. Here, δ indicates the oxygen non-stoichiometric amount.

[0023] Here, M is at least one element selected from alkaline earth metal elements. Since element M is an alkaline earth metal element, 3+ it is close to the ionic radius of La (1.16 Å), and thus replaces element La in the garnet-type crystal system compound represented by the above general formula. Among them, Sr 2+ has an ionic radius of 1.25 Å, and Ba 2+ has an ionic radius of 1.42 Å. Among them, since it is similar to the ionic radius of La 3+ and is effectively easy to substitute, preferably, element M is strontium (Sr) and barium (Ba), and more preferably Sr.

[0024] Also, T is at least one element selected from the group consisting of In, Ti, Sn, Bi, Ta, Nb, Sc, Te, W, Mo, Y, Sb, Ce, Ge, and Gd. Since these elements T are close to the ionic radius of Zr 4+ (0.72 Å), they replace element Zr in the garnet-type crystal system compound represented by the above general formula. Since it is necessary to have a desirable oxidation state and ionic radius as the element substituting at the Zr 4+ site, preferred elements T are Bi, Ta, Sc, Sb, Nb, and Sn. The ionic radii of Bi, Ta, Sc, Sb, Nb, and Sn at the six-coordinate octahedral position are within ±15% of the ionic radius of Zr 4+ . Generally, when substituting elements, it is said that it is easy to substitute if the ionic radii have a difference within 15%. Among them, the ionic radius of Bi 5+ is 0.76 Å, and 0.76 / 0.72 = 1.06, that is, a difference of 6%. Thus, Zr 4+It has an ionic radius very similar to that of [element], and can be effectively and easily substituted. In addition, during the synthesis of the garnet-type crystal system compound in the present invention, since it can react with zirconia (ZrO2) used as a raw material at a low temperature, the most preferable element T is bismuth (Bi).

[0025] In the general formula of the present invention, if the molar ratio x satisfies 0.00 < x < 1.00, the substitution effect by the above-mentioned element M can be exhibited. On the other hand, when x is 1.00 or more, there is a possibility that it may not become a lithium-rich garnet-type crystal system compound having good Li ion conductivity characteristics. From the viewpoint of more surely exhibiting the substitution effect by element M and making it a lithium-rich garnet-type crystal system compound having good Li ion conductivity characteristics, preferably, the molar ratio x is 0.2 < x < 0.8.

[0026] Similarly, if the molar ratio y satisfies 0.00 < y < 0.50, the substitution effect by the above-mentioned element T can be exhibited. On the other hand, when y is 0.50 or more, there is a possibility that it may not become a lithium-rich garnet-type crystal system compound having good Li ion conductivity characteristics. From the viewpoint of more surely exhibiting the substitution effect by element T and making it a lithium-rich garnet-type crystal system compound having good Li ion conductivity characteristics, preferably, the molar ratio y is 0.04 < y < 0.30.

[0027] Also, by having the relationship of 0.20 ≤ x - y ≤ 0.80 for these x and y, a garnet-type crystal system compound with a higher Li content ratio can be obtained. That is, compared with the molar ratio of Li (Li7La3Zr2O 12 ) when there is no substitution by element M and element T, the molar ratio of Li represented by the previous general formula can be made 7.2 or more and 7.8 or less to increase the Li content ratio. Thereby, as shown in the examples described later, the cubic crystal structure of the garnet-type crystal system compound can be further stabilized. Among them, from the viewpoint of further stabilizing the cubic crystal structure, preferably, it has a relationship of 0.3 ≤ x - y ≤ 0.5.

[0028] On the other hand, α is set to 1.0 ≤ α ≤ 1.20. As will be described later, when a solid-phase reaction is employed to obtain the garnet-type crystalline compound in the present invention, the compound actually synthesized may have a different composition from the initial composition of each constituent element used as raw materials. In particular, since Li may volatilize during the synthesis process, an excess of Li raw material is used compared to the stoichiometric ratio. In this case, since the amount of Li that volatilizes is not uniform and varies depending on the reaction conditions, a slightly excess of Li raw material is used so that 1.0 ≤ α ≤ 1.20.

[0029] The garnet-type crystalline compound in the present invention has a higher Li content as described above. Specifically, the Li content is preferably 5.8% by mass or more and 7.0% by mass or less, and more preferably 6.0% by mass or more and 6.95% by mass or less.

[0030] Incidentally, there are two types of garnet-type crystal structures: cubic and tetragonal. To identify these crystal structures, this invention cites ICSD number 183607 for cubic crystals and ICSD number 191528 for tetragonal crystals from the "Inorganic Crystal Structure Database (ICSD)" provided by the Japan Chemical Information Association (organization name). Specifically, Table 3, described later, contains the structural information for ICSD number 183607 for cubic crystals, and Table 4 contains the structural information for ICSD number 191528 for tetragonal crystals. Based on this crystal structure information, Figures 1 and 2 (A) and (B) show at what intensity the XRD diffraction lines appear at at what measurement angle of 2θ when Cu-Kα is used as a source, and this was used to distinguish between cubic and tetragonal crystals by comparing it with the actually measured diffraction profiles.

[0031] Furthermore, the garnet-type crystalline compound of the present invention preferably has a cubic crystal structure as determined by XRD measurement. More preferably, the cubic crystal structure is more stable. Specifically, the crystal structure as determined by XRD measurement preferably consists substantially only of the cubic crystal phase. Here, the cubic crystal structure as determined by XRD measurement means that the peaks mainly observed by XRD measurement belong to the cubic crystal structure of space group Ia-3d. Also, the crystal structure as determined by XRD measurement consisting substantially only of the cubic crystal phase means that no peaks belonging to other secondary crystal phases or impurity crystal phases are detected.

[0032] Furthermore, the garnet-type crystalline compound in the present invention preferably has a full width at half maximum (FWHM) of the peak appearing in the range of 2θ = 16 to 18° in the X-ray diffraction pattern using Cu-Kα rays as the X-ray source, which is 0.17° or less, and more preferably 0.16° or less. The peak observed in this range is characteristic of a cubic garnet-type crystalline compound, and if the FWHM of that peak is 0.17° or less, it can be said to be a higher quality cubic garnet-type crystalline compound. Note that the smaller the FWHM of this peak, the better, but the practical lower limit of this FWHM is 0.10°.

[0033] Furthermore, the garnet-type crystalline compound of the present invention preferably has a lattice constant of 12.974 to 12.987 Å. If the lattice constant is within this range, it can be said to be a higher quality cubic garnet-type crystalline compound.

[0034] The garnet-type crystalline compound of the present invention can be used to obtain a solid electrolyte for lithium secondary batteries. In this case, the garnet-type crystalline compound of the present invention may be used as a solid electrolyte in a sintered state, or it may be used in powder form. These can be appropriately selected depending on the application in the lithium secondary battery, and the shape can be selected based on the obtained state of the garnet-type crystalline compound of the present invention.

[0035] Generally, methods for obtaining composite oxides (LLZ) containing Li, La, Zr, and O include gas-phase synthesis methods such as PVD and CVD, as well as solid-phase reaction methods, spray pyrolysis methods, and wet methods such as coprecipitation and sol-gel methods. However, in this invention, in order to obtain LLZ with a stable cubic crystal structure, the following solid-phase reaction method can preferably be employed.

[0036] First, as raw materials, sulfates, oxides, carbonates, hydroxides, nitrates, acetates, oxalates, halides, etc., of the elements constituting the garnet-type crystalline compound of the present invention, including substitution elements, are prepared. Specifically, Li raw materials containing Li, La raw materials containing La, M raw materials containing element M, Zr raw materials containing Zr, and T raw materials containing element T are prepared. These raw materials are then mixed and subjected to dry or wet grinding using a planetary mill, bead mill, ball mill, etc., to obtain a raw material mixture. Filtration and washing may be performed as needed.

[0037] Next, the raw material mixture obtained above is calcined to synthesize a garnet-type crystalline compound having a cubic crystal structure. In this process, it is preferable to perform a primary calcination (pre-calcination) of the raw material mixture, pulverize it, and then perform a secondary calcination (main calcination) to obtain the garnet-type crystalline compound of the present invention. In other words, a tetragonal crystal structure can be obtained by primary calcination, and a cubic crystal structure can be obtained by secondary calcination. For pulverization after or during calcination, known methods using planetary mills, bead mills, ball mills, etc., can be used.

[0038] Here, the primary firing should be carried out in an atmospheric environment. The temperature for the primary firing should be between 800°C and 1100°C, and the firing time should be between 10 and 15 hours. After the primary firing, the fired material (primary fired material) should be dry-milled and then subjected to secondary firing. The secondary firing should be carried out in an inert atmosphere such as nitrogen or argon. The temperature for the secondary firing should be between 800°C and 1100°C, and the firing time should be between 8 and 15 hours.

[0039] For this secondary firing, the primary firing product obtained in the primary firing may be crushed and then subjected to secondary firing in powder form, or it may be subjected to secondary firing in a pelletized state. When subjected to secondary firing in a pelletized state, sintering proceeds simultaneously, and the garnet-type crystalline compound according to the present invention is obtained as an LLZ pellet product (sintered body). This can be used as a solid electrolyte in lithium secondary batteries, for example, as a separator in all-solid-state lithium secondary batteries, depending on the application.

[0040] On the other hand, if the powder is subjected to secondary calcination, it may be dry-ground to obtain the garnet-type crystalline compound according to the present invention as an LLZ pulverized product (powder). In this case, the pulverized secondary calcined product may be subjected to further tertiary calcination, and the conditions for tertiary calcination can be the same as those for secondary calcination. The particle size of the LLZ pulverized product obtained by secondary calcination (and tertiary calcination if necessary) can be appropriately adjusted according to the application of the solid electrolyte in lithium secondary batteries, but generally, it is preferable to have a D50 (average particle size) of 0.2 μm or more and 20 μm or less. This D50 is the particle size (median diameter) at which the cumulative number of particles in the volume-based cumulative distribution of particle size accounts for 50%.

[0041] The LLZ pulverized product (powder) obtained above can be used, for example, as an additive to the positive or negative electrode of an all-solid-state lithium secondary battery (an additive added together with the active material as an electrode material), or as an additive to the electrodes of a conventional non-aqueous lithium secondary battery. It can also be used as is as a solid electrolyte for a lithium secondary battery, or it may be further sintered to become a solid electrolyte (and then pulverized into a powder). The sintering conditions are not particularly limited and can be carried out in an air atmosphere or an inert atmosphere such as nitrogen or argon, but it is preferable to sinter in an appropriate atmosphere by adjusting the oxygen partial pressure. The sintering temperature is preferably 700°C to 1000°C, and more preferably 800°C to 900°C. The sintering time is preferably 5 hours to 20 hours, and more preferably 10 hours to 15 hours.

[0042] Furthermore, the garnet-type crystalline compound obtained by the present invention has an ionic conductivity of 1.0 × 10⁻⁶ at room temperature. -3 The density is S / cm or greater, preferably 1.5 × 10 -3 The density is S / cm or higher. Furthermore, the relative density at room temperature is 85% or higher, preferably 90% or higher. The relative density was measured using the method described in the following examples. [Examples]

[0043] The present invention will be described in detail below based on the following examples. However, the present invention is not limited to these examples.

[0044] (Examples 1-2, Comparative Examples 1-3) [Preparation of LLZ pulverized product] As raw materials, lithium carbonate (Li2CO3), lanthanum hydroxide (La(OH)3), zirconium oxide (ZrO2), strontium carbonate (SrCO3), and bismuth oxide (Bi2O3) were prepared and weighed to obtain a composite oxide (LLZ) with the composition shown in Table 1. However, the Li raw material was prepared in a molar ratio of 14% excess. These raw materials were mixed in ethanol at 150 rpm for 24 hours using a planetary ball mill (Fritsch Classic Line P-7), and then dried to obtain the raw material mixture.

[0045] Next, the raw material mixture obtained above was subjected to primary calcination in an air atmosphere at 1100°C for 11 hours on a heat-resistant container. The resulting primary calcined material was placed in a nylon pot together with zirconia balls and dry-ground using the same planetary ball mill as above.

[0046] Next, the primary calcined material was dry-ground and subjected to secondary calcination. The secondary calcination was carried out in a tubular furnace under a nitrogen atmosphere at 1100°C for 8 hours. The secondary calcined material obtained from the secondary calcination was then dry-ground using a mortar and pestle to obtain powdered composite oxides (LLZ ground products) according to Examples 1-2 and Comparative Examples 1-3.

[0047] [Table 1]

[0048] [Elemental analysis] The composite oxides obtained in Examples 1-2 and Comparative Examples 1-3 were subjected to elemental analysis to determine their chemical composition. For ICP measurements, elemental concentrations were analyzed using an Agilent 5900 (Agilent Technologies) for ICP emission and a Hitachi High-Tech Z-2310 (Hitachi High-Tech Corporation) for atomic absorption. Li7La3Zr2O 12 Therefore, theoretically, Li is 5.786 mass%, La is 49.627 mass%, and Zr is 21.728 mass%, and the molar ratio of the actually obtained composite oxide was calculated from the atomic weights (mass%) obtained from ICP measurements and the theoretical atomic weights (mass%). The results are shown in Table 2. Note that the molar ratio of oxygen is 12 ± δ, where δ represents the unstoichiometric amount of oxygen. This means the presence of oxygen (O) site vacancies (oxygen defects) or excess oxygen relative to the composition, and although the value of δ is not particularly limited, it is generally between 0 and 1.

[0049] [Table 2]

[0050] [Identification of crystal structure by X-ray diffraction measurement] For Examples 1-2 and Comparative Examples 1-3 described above, powder X-ray diffraction measurements of the primary and secondary calcined products were performed as follows. Specifically, a Rigaku MiniFlex X-ray diffractometer was used. The primary and secondary calcined products were ground into crystalline powders using a mortar and pestle, and then measured with a goniometer radius of 150 mm, an X-ray source of CuKα rays, a tube voltage of 40 kV, and a tube current of 15 mA. The diffraction angle was set to 2θ = 10 to 80°, the scan speed to 2° / min, and the scan step to 0.01°. The results for the primary calcined product are shown in Figure 1, and the results for the secondary calcined product are shown in Figure 2.

[0051] Here, (A) in Figures 1 and 2 shows the peak position and intensity of the tetragonal garnet crystal structure (ICSD number 191528), and (B) shows the peak position and intensity of the cubic garnet crystal structure (ICSD number 183607). These are data obtained using VEST software. Furthermore, Table 3 below shows the parameters of the tetragonal garnet crystal structure (A), and Table 4 shows the parameters of the cubic garnet crystal structure (B).

[0052] [Table 3]

[0053] [Table 4]

[0054] Of these results, first, regarding Figure 1, the primary calcined products of Examples 1-2 and Comparative Examples 1-3 all showed peaks that were almost entirely attributable to a tetragonal crystal structure of space group I41 / acd.

[0055] On the other hand, as can be seen from Figure 2, the crystal structure of the secondary calcined product generally differed significantly from that of the primary calcined product. In the secondary calcined products of Examples 1 and 2, and Comparative Examples 1 and 2 (excluding Comparative Example 3), most of the peaks were attributed to a cubic crystal structure of space group Ia-3d. In Example 2 and Comparative Example 1, it was confirmed that peaks of the impurity phase appeared at 2θ = 20 to 25°. In Comparative Example 3, peaks related to the tetragonal crystal structure at 2θ = 16.9° to 17.3°, 28.6 to 29.1°, 33.5 to 34.5°, and 54.5 to 55.8° were strongly expressed, indicating the formation of a crystal structure other than the cubic crystal structure. In contrast, in Example 1, no peaks corresponding to the secondary crystal phase or impurity crystal phase were detected, and it was found that it was essentially a cubic crystal phase only.

[0056] Furthermore, for the secondary calcined products of Examples 1-2 and Comparative Examples 1-3, the full width at half maximum (FWHM) of the peaks appearing at 2θ = 16-18° was calculated from the powder X-ray diffraction patterns using the analysis software PDXL2 (version 2.8.4.0), based on the X-ray diffraction results shown in Figure 2. In addition, the lattice constants for each crystal structure were calculated based on the X-ray diffraction results also shown in Figure 2. The results are summarized in Table 5.

[0057] [Table 5]

[0058] [Preparation of LLZ pellets (sintered body)] Next, 1 g each of the primary firing products obtained from Examples 1-2 and Comparative Examples 1-3 was taken, placed in a 10φ mold, and held for 2 minutes under uniaxial pressure molding at 14.3 kN (150 MPa) to obtain a molded body. From there, it was sintered under a nitrogen atmosphere at 1100°C for 11 hours to obtain LLZ pellets (sintered bodies). Using these, the relative density and lithium ion conductivity (S / cm) at 298 K (room temperature) were measured as follows.

[0059] First, the LLZ pellets obtained above were polished with sandpaper to make the top and bottom surfaces smooth. Next, the mass of the polished LLZ pellets was measured. The diameter and thickness were also measured at several points using calipers and a micrometer, and the average values ​​were calculated. Then, the volume of the LLZ pellets was determined and the apparent density was calculated. On the other hand, the theoretical density was calculated from the composition of the LLZ pellets, and the relative density was calculated by dividing the apparent density obtained above by the theoretical density and multiplying by 100. The results are shown in Table 4.

[0060] Furthermore, thin films of platinum (Pt) were deposited as electrodes on the upper and lower surfaces of the polished LLZ pellets using ion sputtering deposition. The ion sputtering deposition of the platinum (Pt) film was performed using an MSP-1S magnetron sputtering apparatus. Pt electrodes are a metal with low electrical resistance and high stability.

[0061] As described above, LLZ pellets with Pt electrodes on both sides were used in a solid-state battery evaluation cell, and impedance measurements were performed in a constant-temperature bath. A Biologic SP-200 was used as the impedance measurement device. During this process, AC signals were applied to the LLZ pellets at multiple frequencies, and the AC impedance was measured for each frequency. By plotting the measured Re(Z) / Ω and -Im(Z) / Ω, an impedance plot including a circular arc trajectory was obtained. The impedance arc was then defined as part of a circle, and the diameter of that circle was defined as the resistance value. The reciprocal of the resistance value was multiplied by the thickness (cm) / electrode area (cm) of the LLZ pellet sample used for measurement. 2 The lithium-ion conductivity was calculated by multiplying by ). The results are shown in Table 6. Figure 3 shows the impedance plot of the LLZ pellet obtained in Example 2.

[0062] [Table 6]

[0063] As can be seen from the results of the above examples and comparative examples, the present invention makes it possible to obtain a lithium-rich cubic garnet-type crystalline compound unlike any other, thereby realizing a solid electrolyte with enhanced ionic conductivity. Moreover, since relatively inexpensive metal elements are used as substitution elements, the cubic structure of the garnet-type crystalline compound can be further stabilized, thereby expanding the use of all-solid-state lithium secondary batteries.

Claims

1. Li (７＋ｘ－ｙ)α (La ３－ｘ M ｘ )(Zr ２－ｙ T ｙ )O １２±δ Represented as a garnet-type or garnet-like crystalline compound, A garnet-type crystalline compound characterized in that M represents at least one element selected from alkaline earth metal elements, T represents at least one element selected from the group consisting of In, Ti, Sn, Bi, Ta, Nb, Sc, Te, W, Mo, Y, Sb, Ce, Ge, and Gd, and the molar ratios x, y, and α have the relationships 0.00 < x < 1.00, 0.00 < y < 0.50, 0.20 ≤ x - y ≤ 0.80, and 1.0 ≤ α ≤ 1.20, and δ represents the unstoichiometric amount of oxygen.

2. The garnet-type crystalline compound according to claim 1, wherein the Li content is 5.8% by mass or more and 7.0% by mass or less.

3. The garnet-type crystalline compound according to claim 1, wherein the element M is Sr or Ba, and the element T is Bi.

4. The garnet-type crystalline compound according to claim 1, wherein the crystal structure determined by XRD measurement is in the cubic phase.

5. The garnet-type crystalline compound according to claim 4, wherein the crystal structure determined by XRD measurement is substantially only a cubic phase.

6. The garnet-type crystalline compound according to claim 1, wherein the full width at half maximum of the peak appearing in the range of 2θ = 16 to 18° in the X-ray diffraction pattern using Cu-Kα rays as the X-ray source is 0.17° or less.

7. A lithium secondary battery having a solid electrolyte obtained using the garnet-type crystalline compound described in claim 1.

8. The solid electrolyte has an ionic conductivity of 1.0 × 10⁻⁶ at room temperature. －３ The lithium secondary battery according to claim 7, wherein the density is S / cm or higher and the relative density is 85% or higher.

9. A method for producing the garnet-type crystalline compound described in claim 1, A method for producing a garnet-type crystalline compound, characterized by mixing a Li raw material containing Li, a La raw material containing La, an M raw material containing element M, a Zr raw material containing Zr, and a raw material containing element T to form a raw material mixture, primary calcining the raw material mixture at a temperature of 800°C to 1100°C, pulverizing it, and then secondary calcining it at a temperature of 800°C to 1100°C.

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