Oxide Sintered Body and Method for Producing Oxide Sintered Body

Abstract

The present invention provides: an oxide sintered body which has a crystal structure that is capable of enhancing the lithium ion conductivity; and a method for producing an oxide sintered body, the method being capable of building the crystal structure.　This oxide sintered body contains lithium, tantalum and phosphorus, while having a crystal structure belonging to the monoclinic space group C2 / c; and lithium atoms do not occupy the Wyckoff position 4b (0.5, 0, 0). This method for producing an oxide sintered body comprises a sintering step in which an oxide that contains lithium, tantalum and phosphorus is fired at a temperature that is higher than 1200°C but not higher than 1400°C.

Description

Technical Field

[0001] The present application relates to an oxide sintered body having high lithium ion conductivity and a method for producing the oxide sintered body.

Background Art

[0002] Lithium ion secondary batteries are widely adopted as power sources for small electronic devices such as smartphones and notebook computers. In recent years, lithium ion secondary batteries are expected to be developed for use as power sources for hybrid automobiles and electric vehicles, which are large-scale applications, and as stationary storage batteries. Furthermore, from the viewpoints of safety and high energy density, research and development of all-solid-state lithium ion secondary batteries, lithium air batteries, and all-solid-state lithium sulfur batteries that do not use a flammable electrolyte are being promoted. High lithium ion conductivity is required for the solid electrolytes used in these electrochemical devices.

[0003] Recently, it has been reported that the lithium tantalum phosphate compound LiTa2PO8 exhibits high lithium ion conductivity (Non-Patent Documents 1 to 3). The crystal structure of LiTa2PO8 is different from that of other lithium ion conductors. It has been clarified by precise crystal structure analysis that the skeletal structure of LiTa2PO8 is constructed from TaO6 octahedra and PO4 tetrahedra, and lithium ions occupy the gaps between them. Since the arrangement of lithium constructs a three-dimensional conduction path, good lithium ion conduction similar to that of garnet-type materials is possible with LiTa2PO8.

[0004] On the one hand, with LiTa2PO8 as the basic composition, attempts have been made to improve the properties by substituting elements of Ta (Patent Documents 1 to 3 and Non-Patent Document 4). It has been reported that the bulk lithium ion conductivity of the LiTa2PO8 sintered body is improved by this element substitution. The firing temperature when producing the element-substituted LiTa2PO8 sintered body was 1200°C or lower, similar to the firing temperature when producing the LiTa2PO8 sintered body. The crystal structure of the element-substituted LiTa2PO8 sintered body is described as the same monoclinic crystal structure as that of the LiTa2PO8 sintered body, and not only is there no mention of the differences in the detailed lithium occupancy sites and lithium ion conduction paths, but the differences have not even been considered for prediction.

[0005] Normally, in order to produce a LiTa2PO8 sintered body, a high-temperature firing method is used. However, LiTa2PO8 contains lithium and phosphoric acid, which are prone to volatilization at high temperatures, as the main elements. In order to suppress the compositional deviation due to these volatilities and the generation of impurity phases due to decomposition, the firing temperature when producing LiTa2PO8 was set with an upper limit of 1200°C. Therefore, it has not been examined what crystal structure with what lithium occupancy sites the LiTa2PO8 sintered body produced by firing at a temperature exceeding 1200°C has, nor how the lithium ion conduction characteristics change, and it was impossible to predict that there would be a crystal structure change.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Patent Document 3

Non-Patent Documents

[0007]

Non-Patent Document 1

Non-Patent Document 2

Non-Patent Document 3

Non-Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0008] In view of such circumstances, the present application has been made, and an object thereof is to provide an oxide sintered body having a crystal structure capable of achieving higher lithium ion conductivity, and a method for producing an oxide sintered body capable of constructing such a crystal structure.

Means for Solving the Problems

[0009] The inventors of the present application examined the relationship between the firing temperature and the crystal structure of the target LiTa2PO8, as well as the starting materials and synthesis methods suitable for high-temperature firing. As a result, surprisingly, it was found that a crystal phase having a lithium occupation mode advantageous for faster lithium ion diffusion is generated by firing at a temperature exceeding 1200°C. Furthermore, by firing in a sealed system capable of suppressing the volatilization of lithium and phosphoric acid at high temperatures, LiTa2PO8 can be fired without decomposition even when fired at 1300°C. It was found that by such high-temperature firing, the grain growth of LiTa2PO8 becomes remarkable, and a sintered body and a single crystal having a high bulk lithium ion conductivity can be obtained.

[0010] Furthermore, the chemical composition of LiTa₂PO₈ was investigated. It was found that by substituting different elements at the Ta site, the lithium content can be adjusted to a level favorable for lithium ion diffusion. Additionally, the formation of a liquid phase occurs with the addition of elements, which functions as a sintering aid during high-temperature firing above 1200 °C, improving the sinterability. As a result, the total lithium ion conductivity of the dense sintered body of the high-temperature phase of LiTa₂PO₈ prepared by such a manufacturing method and the element substitution product obtained by substituting Ta in LiTa₂PO₈ with other elements and changing the Li composition amount as necessary (hereinafter, the element substitution product obtained by substituting Ta with other elements and changing the Li composition amount as necessary may be simply referred to as "element substitution product") is 2×10 -4 S / cm or more. The battery operation of all-solid-state batteries equipped with solid electrolytes fabricated by low-temperature sintering using the powder obtained by pulverizing the dense sintered body was confirmed.

[0011] The oxide sintered body of the present application is an oxide sintered body containing lithium, tantalum, and phosphorus, having a crystal structure belonging to the monoclinic system and the space group C2 / c, and lithium does not occupy the 4b site (0.5, 0, 0) at the Wyckoff position. The manufacturing method of the oxide sintered body of the present application has a sintering step of firing an oxide containing lithium, tantalum, and phosphorus at a temperature higher than 1200 °C and lower than or equal to 1400 °C. The electrochemical device of the present application has a solid electrolyte comprising the oxide sintered body of the present application and a pair of electrodes sandwiching the solid electrolyte.

Advantages of the Invention

[0012] According to the present application, an oxide sintered body having a crystal phase that exhibits higher lithium ion conductivity can be obtained.

Brief Description of the Drawings

[0013]

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Mode for Carrying Out the Invention

[0014] (Oxide Sintered Body) The oxide sintered body of the embodiment of the present application contains lithium, tantalum, and phosphorus. The oxide sintered body of this embodiment has a crystal structure belonging to the monoclinic system and the space group C2 / c, and lithium does not occupy the 4b site (0.5, 0, 0) in the Wyckoff position. It is preferable that the occupied sites of lithium occupy only three or more 8f sites in the Wyckoff position. The occupied sites of lithium may be in a disordered occupation state.

[0015] As the oxide sintered body of this embodiment, the general formula is LiTa 2-x M x PO8 (M is Bi or Sb, 0 ≤ x ≤ 0.2 (the same applies hereinafter)) represents an oxide sintered body, for example, LiTa 1.9 Bi 0.1 PO8, LiTa 1.8 Bi 0.2 PO8, LiTa 1.9 Sb 0.1 PO8, and LiTa 1.8 Sb 0.2 PO8. Further, as the oxide sintered body of this embodiment, the general formula is Li 1+y Ta 2-y Hf y PO8 (0 ≤ y ≤ 0.2 (the same applies hereinafter)) represents an oxide sintered body, for example, Li 1.1 Ta 1.9 Hf 0.1 PO8 and Li 1.2 Ta 1.8 Hf 0.2 PO8.

[0016] In addition to bismuth, antimony, and hafnium, examples of elements that can replace Ta in LiTa₂PO₈ include titanium, niobium, molybdenum, zirconium, tungsten, boron, aluminum, silicon, germanium, and gallium. Examples of the oxide sintered body of the present embodiment include LiTa₂PO₈. In order to obtain higher lithium ion conductivity, LiTa₂PO₈ is preferably composed of primary particles with a diameter of 50 μm to 100 μm. The particle size of the primary particles, that is, the primary particle size, can be measured using, for example, a scanning electron microscope. In order to obtain even higher lithium ion conductivity, LiTa₂PO₈ is preferably a single crystal.

[0017] (Method for manufacturing an oxide sintered body) The method for manufacturing an oxide sintered body according to an embodiment of the present application includes a sintering step. In the sintering step, the raw material oxide (hereinafter sometimes referred to as "raw material oxide") is fired at a temperature higher than 1200 °C and not higher than 1400 °C. The raw material oxide is an oxide containing lithium, tantalum, and phosphorus. Examples of the oxide containing lithium, tantalum, and phosphorus include oxides represented by the general formula LiTa 2-x M x PO₈, oxides represented by the general formula Li 1+y Ta 2-y Hf y PO₈, and amorphous LiTa₂PO₈. Amorphous LiTa₂PO₈ is excellent in moldability, which is a concept including moldability.

[0018] The raw material oxide is obtained by dissolving various raw material compounds, such as metal salts and phosphate compounds, in a solvent respectively, mixing these solutions to precipitate a precipitate, and firing this precipitate. However, there is no particular limitation on the method for producing the raw material oxide as long as the metal components and the like of the various raw material compounds can be uniformly mixed at the atomic level and the raw material oxide can be formed. For example, it can also be produced by ordinary solid-phase synthesis, coprecipitation method, sol-gel method, complex polymerization method, and solution synthesis such as hydrothermal synthesis method, as well as vapor-phase reaction synthesis such as vacuum evaporation method, sputtering method, pulsed laser deposition method, and chemical vapor reaction method. Further, the raw material oxide can also be produced by applying a mechanochemical reaction such as ball milling.

[0019] The various raw material compounds are not particularly limited as long as they are compounds containing lithium, tantalum, or phosphorus. Examples of the raw material compound include oxides, carbonates, hydroxides, nitrates, ammonium salts, hydrogen ammonium salts, halides such as chlorides, and oxychlorides. The raw material oxide can be produced, for example, by the following procedure. First, the raw material compound is dissolved in a solvent. This solvent is not particularly limited as long as it can uniformly mix the raw material compounds. Examples of this solvent include alcohol-based solvents such as methanol, ethanol, hexanol, and propanol, organic solvents such as aromatic compounds and ethers, and water.

[0020] The temperature at which the raw material compound is dissolved in the solvent may be any temperature from room temperature to the boiling point of the solvent. After mixing the raw material compound and the solvent, it may be left to stand to wait for the raw material compound to dissolve in the solvent, or in order to accelerate the dissolution reaction, a stirrer or a mixer may be used to stir the mixture of the raw material compound and the solvent. Next, the solutions of the respective raw material compounds are mixed. For example, pentavalent tantalum chloride, pentavalent niobium chloride, tetravalent hafnium chloride, trivalent bismuth chloride, and antimony chloride dissolve in ethanol or the like, but then, by mixing with an aqueous solution, a gelled precipitate is formed. By heating in that state, the gelled precipitate dries and a precursor of the raw material oxide is produced. There is no particular limitation on the heating method, and heating may be carried out using a hot plate, an electric heating muffle furnace, a mantle heater, or the like. The heating temperature is preferably 50°C or higher and lower than the boiling point of the solvent, and more preferably 80°C or higher and lower than the boiling point of the solvent.

[0021] Then, by firing this precursor, the target raw material oxide can be obtained. There is no particular limitation on the firing method, and firing may be carried out using an electric heating muffle furnace or a mantle heater or the like. The firing temperature is preferably 350°C or higher in order to obtain a powder of the raw material oxide, and may be 500°C or higher and 1000°C or lower. There is no particular limitation on the container used for firing, and glass beakers and non-alumina-based ceramic containers can be used, and for high-temperature firing at 400°C or higher, gold containers, platinum containers, and alumina containers can be used. There is no particular limitation on the firing atmosphere, and usually it is an oxidizing gas atmosphere such as in oxygen or in air.

[0022] The firing time can be set according to the firing temperature and the like as long as residues such as nitrogen, chlorine, and carbon derived from various raw material compounds can volatilize. There is also no particular limitation on the cooling method after firing, but usually natural cooling (cooling in the furnace) or slow cooling may be sufficient. After firing and cooling, if necessary, the raw material oxide may be pulverized and then the firing temperature may be changed and re-fired. The degree of pulverization can be adjusted according to the firing temperature and the like. LiTa2PO8 or its element-substituted body produced by this aqueous solution synthesis method can be used as the raw material oxide.

[0023] Alternatively, a mixture such as an oxide containing only Li, Ta, P, and a part of the substitution elements may be calcined to produce a raw material oxide. This mixture of oxides and the like is a mixture of a plurality of oxides so that the composition ratio of LiTa2PO8 and its element substitution products is obtained after the synthesis of the oxide sintered body. The calcination temperature can be set according to the composition of the mixture of oxides, but is usually 500°C to 1200°C, preferably 900°C to 1000°C. Also, the calcination atmosphere is not particularly limited, and is usually an argon gas atmosphere, a nitrogen gas atmosphere, an oxygen gas atmosphere, or an air atmosphere. The calcination time can be set according to the calcination temperature and the like.

[0024] Next, the raw material oxide is sintered. The sintering temperature is higher than 1200°C and 1400°C or lower, preferably 1205°C or higher and 1300°C or lower. In the sintering, a one-step sintering reaction at a temperature higher than 1200°C and 1400°C or lower may be allowed to proceed, or after once raising the temperature to higher than 1200°C and 1400°C or lower, a two-step sintering reaction in which sintering is performed at a temperature of 1200°C or lower may be allowed to proceed. The sintering temperature in the second step at this time is usually 800°C or higher and 1200°C or lower, preferably 850°C or higher and 1050°C or lower. In this two-step sintering method, by using bismuth, gallium, aluminum, or boron, etc., which generate a liquid phase at a high temperature, as a substitution element, a liquid phase such as a lithium salt is generated in the high-temperature sintering in the first step, and this liquid phase such as the lithium salt functions as a sintering aid in the low-temperature sintering in the second step, and an oxide sintered body having high density can be obtained.

[0025] Also, the sintering atmosphere is not particularly limited, and is usually an argon gas atmosphere, a nitrogen gas atmosphere, an oxygen gas atmosphere, or an air atmosphere. When sintering, the material of the crucible into which the pulverized raw material oxide is put as necessary only needs to be stable at a high temperature of 1200°C or higher and can suppress the volatilization of lithium, phosphoric acid, bismuth, and gallium, etc. at a high temperature, for example, platinum, alumina, or zirconia.

[0026] There are no particular restrictions on the form of the raw material oxide to be sintered, and examples include powders, plate-shaped compacts pressure-molded by methods such as hydrostatic pressing or uniaxial pressing, and film bodies produced by coating techniques or film-forming techniques. Examples of coating techniques include screen printing method, electrophoresis (EPD) method, doctor blade method, spray coating method, inkjet method, and spin coating method. Examples of film-forming techniques include vapor deposition method, sputtering method, chemical vapor deposition (CVD) method, electrochemical vapor deposition method, ion beam method, laser ablation method, atmospheric pressure plasma film-forming method, and reduced pressure plasma film-forming method.

[0027] Also, a plate-shaped body or film body of the raw material oxide may be produced using techniques such as hot pressing, hot isostatic pressing, or electric current sintering. After this sintering, the obtained oxide sintered body may be ground by a known method as necessary, and this sintering may be repeated. The degree of grinding can be set according to the sintering temperature and the like. Also, a molded body of the raw material oxide may be produced from the powder of the raw material oxide that has been sintered once.

[0028] (Solid electrolyte material) The oxide sintered body obtained by this sintering can be used as a solid electrolyte material for electrochemical devices such as all-solid-state lithium-ion secondary batteries. After the oxide sintered body is ground, it may be molded and sintered again to produce a solid electrolyte material. Also, a molded body may be produced by mixing or compounding the powder of the oxide sintered body and other electrolyte materials. On the other hand, if the oxide sintered body is a massive sintered body (including single crystals), a molded body, or a coated film body, it can be used as a solid electrolyte as it is.

[0029] (Positive electrode member) The oxide sintered body of the present application can be used as a constituent member of a positive electrode in order to ensure lithium ion conductivity in the electrode. That is, a molded body in which the oxide sintered body of the present application is mixed or compounded with a positive electrode material active substance is a positive electrode member. As the positive electrode material active substance, materials generally used as positive electrode materials for lithium ion secondary batteries can be used. For example, oxides such as LiCoO2, LiNiO2, Li(Ni,Mn,Co)O2, Li(Ni,Co,Al)O2, Li2MnO3-Li(Ni,Mn,Co)O2, Li(Ni,Mn)2O4, Li(Co,Mn)2O4, Li(Mn,Al)2O4, LiFePO4, LiMnPO4, LiCoPO4, and LiNiPO4, sulfur, and lithium sulfide can be mentioned as positive electrode material active substances.

[0030] In addition, a substance in which a lithium desorption / insertion reaction occurs reversibly at a high voltage of 2 V or more can be used as the positive electrode material active substance. Further, in order to improve the bonding between the positive electrode and the solid electrolyte material and to improve the lithium ion conductivity, this positive electrode member may contain a polymer, an oxide, a sulfide, a hydride, or a halide. Further, for the purpose of improving the electron conductivity in the positive electrode, this positive electrode member may contain a conductive aid such as carbon black, a carbon nanotube, graphite, or a titanium oxide.

[0031] (Electrochemical device) Since the oxide sintered body of the present application is excellent in lithium ion conductivity, it can be used as a solid electrolyte for electrochemical devices such as all-solid-state lithium ion secondary batteries, lithium air batteries, and lithium sulfur batteries. An all-solid-state lithium ion secondary battery as an example of an electrochemical device includes a solid electrolyte including the oxide sintered body of the present application and a pair of electrodes sandwiching this solid electrolyte. Note that the pair of electrodes do not necessarily sandwich the solid electrolyte directly.

[0032] In addition, the oxide sintered body of the present application can also be used for the positive electrode member or the negative electrode member. Fig. 1 conceptually shows an all-solid-state lithium-ion secondary battery, which is an example of the electrochemical device of the embodiment of the present application. The all-solid-state lithium-ion secondary battery includes an exterior 1, a positive electrode tab 2, a positive electrode current collector 3, a positive electrode 4, a separator 5, a negative electrode 6, a negative electrode current collector 7, and a negative electrode tab 8. The oxide sintered body of the present application can be used for a part of the positive electrode 4 or the negative electrode 6, or the separator 5.

Examples

[0033] Example 1: Synthesis of LiTa2PO8 sintered body with a new crystal structure (sintered at 1205 °C) In a dry environment, 1.4328 g of TaCl5 (manufactured by Rare Metallic, 99.9% (the same hereinafter)) was dissolved in 50 mL of anhydrous ethanol to obtain a TaCl5 solution. 0.2301 g of NH4H2PO4 (manufactured by Wako Pure Chemical Industries, reagent grade (the same hereinafter)) was dissolved in 50 mL of ion-exchanged water to obtain an NH4H2PO4 aqueous solution. 0.0923 g of LiOH·H2O (manufactured by High Purity Chemical Laboratory, 99% or more (the same hereinafter)) was dissolved in 100 mL of ion-exchanged water to obtain a LiOH aqueous solution. While stirring this LiOH aqueous solution with a stirrer, this TaCl5 solution and this NH4H2PO4 aqueous solution were sequentially added and mixed at 80 °C. The mixed solution contains 1.1 molar times, that is, 10 mol% excess LiOH compared to the composition LiTa2PO8.

[0034] This mixed solution was dried at 120 °C for 15 hours to recover the dried and solidified powder, and this solidified powder was gently pulverized in an agate mortar. Using a vacuum gas replacement type electric furnace (manufactured by Denken High Dental, KDF-75plus (the same hereinafter)), this pulverized powder was sintered at 600 °C in an oxygen atmosphere for 12 hours to obtain a white powder of LiTa2PO8 which is an amorphous raw material. Using a planetary ball mill (manufactured by Fritsch, P-7 (the same hereinafter)), this white powder was wet ball mill pulverized, and then uniaxially pressed using a tablet molding machine (manufactured by JASCO (the same hereinafter)) to obtain a molded body. Using a tabletop high-temperature muffle furnace (manufactured by Yamada Electric, SSFS-130-S (the same hereinafter)), this molded body was sintered at 1205 °C in the air for 240 hours to obtain a LiTa2PO8 sintered body.

[0035] Particles grown from this sintered body were selected, and their crystal structures were examined using a single-crystal X-ray diffractometer (Rigaku, R-AXIS RAPID-II (the same hereinafter)). As a result, it was confirmed that these particles belong to the monoclinic system and the space group C2 / c. The single-crystal X-ray diffraction pattern is shown in Fig. 2. Single-crystal X-ray diffraction data were collected, and the crystal structure of this sintered body was analyzed using the crystal structure analysis program JANA2006. As a result, it was confirmed that the lithium arrangements in Non-Patent Document 1 and Non-Patent Document 2 could not be refined, and that lithium did not occupy the 4b site (0.5, 0, 0) in the Wyckoff position.

[0036] Furthermore, it was revealed that lithium is arranged at the 8f site, which is different from Non-Patent Document 1 and Non-Patent Document 2. Different from Non-Patent Document 1 and Non-Patent Document 2, the crystal structure model of this sintered body determined with an R value of 3.2%, which indicates the reliability of the analysis, by a model introducing five 8f sites is shown in Fig. 3. Also, when the primary particle size forming this sintered body was examined using a desktop scanning electron microscope (JEOL, JCM-6000 (the same hereinafter)), it was about 50 μm to 100 μm. The electron microscope image of the fracture surface of this sintered body and the electron microscope image of the single crystal selected from this sintered body are shown in Fig. 4 and Fig. 5, respectively. Fig. 4 shows that this sintered body is a bulk body without distinct grain boundaries.

[0037] Example 2: Synthesis of LiTa2PO8 single crystal having a new crystal structure (sintering at 1310 °C) A LiTa2PO8 single crystal was obtained in the same manner as in Example 1, except that the firing temperature was changed from 1205 °C to 1310 °C. A single crystal was taken out by cleaving from this single crystal, and its crystal structure was examined using a single-crystal X-ray diffractometer. As a result, it was confirmed that this single crystal belongs to the monoclinic system and the space group C2 / c. Single-crystal X-ray diffraction data were collected, and as a result of performing crystal structure analysis in the same manner as in Example 1, it was confirmed that the lithium arrangements in Non-Patent Document 1 and Non-Patent Document 2 could not be refined, and that lithium did not occupy the 4b site (0.5, 0, 0) in the Wyckoff position.

[0038] Furthermore, it was revealed that lithium is arranged at the 8f site, which is different from that in Non-Patent Document 1 and Non-Patent Document 2. From the above, it was confirmed that the LiTa₂PO₈ sintered body obtained by firing at a temperature higher than 1200 °C has a new crystal structure that cannot be explained by the reported structural model. In addition, when the crystal size of this single crystal was examined using a stereomicroscope (manufactured by LEICA, S8APO), it was about 100 μm to 500 μm. The stereomicroscope image of the selected single crystal is shown in Fig. 6. Note that one scale in Fig. 6 is 0.1 mm.

[0039] Comparative Example 1: Synthesis of LiTa₂PO₈ Sintered Body (Firing at 1050 °C) Li₂CO₃ (manufactured by Rare Metallic, 99.9% (the same hereinafter)), Ta₂O₅ (manufactured by Rare Metallic, 99.99% (the same hereinafter)), and (NH₄)₂HPO₄ (manufactured by Fujifilm Wako Pure Chemical Industries, reagent special grade (the same hereinafter)) were weighed so that the molar ratio of Li:Ta:P (so-called molar ratio) was 1.1:2:1. After pulverizing and mixing these using an agate mortar, they were sequentially heated at 450 °C for 4 hours and 600 °C for 4 hours using an electric furnace (manufactured by Yamato Scientific, FP100 (the same hereinafter)) to decompose the ammonium salt and obtain a raw material.

[0040] This raw material was pulverized in absolute ethanol, recovered and dried, and then pre-fired at 900 °C to 950 °C for 4 hours using an electric furnace. The obtained pre-sintered body was pulverized with a ball mill and dried to obtain a powder of the raw material oxide. This powder was uniaxially pressed using a tablet molding machine to obtain a molded body. Using an electric furnace, this molded body was fired at 1050 °C in air for 6 hours to obtain a LiTa₂PO₈ sintered body (see Non-Patent Document 2). When the crystal structure of this sintered body was examined using a powder X-ray diffractometer (manufactured by Rigaku, SmartLab (the same hereinafter)), it was confirmed that it was almost a single phase of LiTa₂PO₈ having a crystal structure belonging to the monoclinic system and the space group C2 / c as in Non-Patent Document 1 to Non-Patent Document 3. This powder X-ray diffraction pattern is shown in Fig. 7 (in the figure, "1050 °C"). In addition, when the particle size of the primary particles of this sintered body was examined using a desktop scanning electron microscope, it was about several μm. The electron microscope image of the fracture surface of this sintered body is shown in Fig. 8.

[0041] Also, for this sintered body, using a frequency response analyzer (FRA) (manufactured by Solartron, model 1260 (the same hereinafter)), the resistance value was obtained from the arc of the Nyquist plot, and the lithium ion conductivity was calculated from this resistance value. Note that impedance measurement was performed under the conditions of a frequency of 32 MHz to 100 Hz and an amplitude voltage of 100 mV, and an Au electrode was used for the blocking electrode (the same hereinafter). From the measurement results at room temperature, the total lithium ion conductivity was calculated to be 2.1×10 -4 S / cm, which was generally consistent with the reported values in previous reports.

[0042] Example 3: Synthesis of LiTa2PO8 Sintered Body with a New Crystal Structure (Sintered at 1205 °C, Raw Material Oxides for Solid Phase Synthesis) Using the pre-sintered body obtained by solid phase synthesis in Comparative Example 1, a sintered body was further synthesized by high-temperature sintering. The pre-sintered body of Comparative Example 1 (diameter: about 9 mm, thickness: 1.0 - 1.3 mm) was placed in a platinum container and fired at 1205 °C in air for 4 hours in a tabletop high-temperature muffle furnace to obtain a LiTa2PO8 sintered body. When the crystal structure of this sintered body was examined by a powder X-ray diffractometer, it was found to be almost a single phase of LiTa2PO8 having a crystal structure belonging to the monoclinic system and the space group C2 / c, indicating that these sintered bodies could be synthesized without decomposition even at high-temperature sintering. The powder X-ray diffraction patterns of these sintered bodies are shown in Fig. 7 (in the figure, "1205 °C").

[0043] Example 4: Synthesis of LiTa2PO8 Sintered Body with a New Crystal Structure (Sintered at 1310 °C, Raw Material Oxides for Solid Phase Synthesis) A LiTa2PO8 sintered body was obtained in the same manner as in Example 3, except that the sintering at 1205 °C was changed to sintering at 1310 °C. When the crystal structure of this sintered body was examined by a powder X-ray diffractometer, it was found to be almost a single phase of LiTa2PO8 having a crystal structure belonging to the monoclinic system and the space group C2 / c, indicating that these sintered bodies could be synthesized without decomposition even at high-temperature sintering. The powder X-ray diffraction patterns of these sintered bodies are shown in Fig. 7 (in the figure, "1310 °C").

[0044] Looking closely at each of the XRD patterns shown in Fig. 7, it was confirmed that the peak positions of the sintered compacts of Example 3 and Example 4 were shifted to the low-angle side compared to the peak positions of the sintered compact of Comparative Example 1. From the above results, although the skeletal structure formed by tantalum, phosphorus, and oxygen in the sintered compacts of Example 3 and Example 4 did not change from that of the sintered compact of Comparative Example 1, it was suggested that the lattice volume expansion occurred because the local structure around lithium changed. This result supports that the local arrangement of lithium has changed by firing at a temperature higher than 1200°C.

[0045] Example 5: Synthesis of LiTa2PO8 Sintered Compact with a New Crystal Structure (Firing at 1208°C, Raw Material Oxides for Solid Phase Synthesis) First, a molded body was obtained in the same manner as in Comparative Example 1. Next, using an electric furnace, this molded body was heated to 1208°C in air and then cooled and fired at 1050°C for 6 hours to obtain a LiTa2PO8 sintered compact. When the crystal structure of this sintered compact was examined by a powder X-ray diffractometer, it was confirmed that it was a single phase of LiTa2PO8 having a crystal structure belonging to the monoclinic system and the space group C2 / c. This powder X-ray diffraction pattern is shown in Fig. 9.

[0046] Furthermore, using the obtained powder X-ray diffraction data, the crystal structure of this sintered compact was analyzed by the Rietveld method (program: using Rietan-FP), and the detailed lithium arrangement was examined. As a result, the lithium arrangements in Non-Patent Document 1 and Non-Patent Document 2 could not be refined, and it was confirmed that lithium did not occupy the 4b site (0.5, 0, 0) at the Wyckoff position. Furthermore, it became clear that lithium was arranged at the 8f site different from Non-Patent Document 1 and Non-Patent Document 2. As a result of these structural analyses, it was reasonable that the crystal structure model equivalent to the results of the single crystal X-ray diffraction structural analyses of Example 1 and Example 2 was valid for this sintered compact.

[0047] Also, for this sintered compact, the lithium ion conductivity was calculated in the same manner as in Comparative Example 1. From the measurement results at room temperature, the total lithium ion conductivity was 2.1×10 -4It was calculated to be S / cm and was higher than the total lithium ion conductivity of the previously reported LiTa₂PO₈ sintered body of Comparative Example 1. The Nyquist plot obtained from the results of this impedance measurement is shown in Fig. 10. From the above, it became clear that by two-step sintering in which the temperature is first raised above 1200 °C and then lowered and sintered at 1050 °C, the crystal structure of the high-temperature phase above 1200 °C can be maintained, and a sintered body with higher conductivity can be produced.

[0048] Example 6: Synthesis of LiTa₂PO₈ Sintered Body with New Crystal Structure (Sintered at 1208 °C, Raw Material Oxides for Solution Synthesis) First, a molded body was obtained in the same manner as in Example 1. Next, using an electric furnace, this molded body was heated to 1208 °C in air and sintered for 5 minutes, then cooled to obtain a pre-sintered body. Then, using a vacuum gas replacement type electric furnace, this pre-sintered body was sintered at 1000 °C in an oxygen atmosphere for 12 hours to obtain a LiTa₂PO₈ sintered body.

[0049] When the crystal structure of this sintered body was examined by a powder X-ray diffractometer, it was confirmed that it was almost a single phase of LiTa₂PO₈ having a crystal structure belonging to the monoclinic system and the space group C2 / c. This powder X-ray diffraction pattern is shown in Fig. 11. Also, as a result of SEM-EDS analysis (manufactured by JEOL, JCM-6000 (the same applies hereinafter)) of this sintered body, Ta and P were detected, and no metal elements other than Ta were detected. Furthermore, as a result of quantitative analysis of this sintered body using inductively coupled plasma (ICP) optical emission spectrometry (manufactured by Agilent, Agilent 5800 (the same applies hereinafter)), it was confirmed that the chemical composition was almost stoichiometric LiTa₂PO₈.

[0050] Also, for this sintered body, the lithium ion conductivity was calculated in the same manner as in Comparative Example 1. From the measurement results at room temperature, the total lithium ion conductivity was 5.2×10 -4 It was calculated to be S / cm and was found to be higher than the total lithium ion conductivity of the previously reported LiTa₂PO₈ sintered body. This is presumably because the crystal structure of this sintered body forms a lithium arrangement that exhibits higher lithium ion conduction characteristics.

[0051] Example 7: LiTa with a new crystal structure 1.9 Bi 0.1 Synthesis of PO8 sintered body (raw material oxide for solid-phase synthesis) Li2CO3, Ta2O5, Bi2O3 (manufactured by Rare Metallic, 99.99%), and (NH4)2HPO4 were weighed so that the molar ratio of Li:Ta:Bi:P was 1.1:1.9:0.1:1. After pulverizing and mixing these using an agate mortar, they were sequentially heated using an electric furnace at 450 °C for 4 hours and then at 600 °C for 4 hours to decompose the ammonium salt and obtain the raw material oxide. After pulverizing this raw material oxide in absolute ethanol, collecting, and drying it, a compact was obtained by uniaxially pressing using a tablet press. Using an electric furnace, after heating this compact to 1208 °C in air, it was cooled and fired at 900 °C for 6 hours to obtain a LiTa 1.9 Bi 0.1 PO8 sintered body.

[0052] When the crystal structure of this sintered body was examined using a powder X-ray diffractometer, it was confirmed that the main phase was a LiTa2PO8-type crystal structure belonging to the monoclinic system and the space group C2 / c. On the other hand, it was confirmed that a BiPO4 phase was formed as an impurity phase in this sintered body. This BiPO4 phase precipitated as a liquid phase during high-temperature firing and then functioned as a sintering aid through phase separation. This powder X-ray diffraction pattern is shown in Fig. 12. Also, when the particle size of the primary particles of this sintered body was examined using a desktop scanning electron microscope, it was about several μm to 10 μm. The electron microscope image of the fracture surface of this sintered body is shown in Fig. 13. Compared with the sintered body of Comparative Example 1, although the main firing temperature was lowered from 1050 °C to 900 °C, grain growth was remarkable, and it was confirmed that BiPO4 precipitated at the grain boundaries served as a sintering aid.

[0053] Also, for this sintered body, the lithium ion conductivity was calculated in the same manner as in Comparative Example 1. From the measurement results at room temperature, the total lithium ion conductivity was 1.3×10 -3It was calculated to be S / cm and was higher than the total lithium ion conductivity of the previously reported LiTa2PO8 sintered body. It is considered that this is due to the fact that this sintered body maintains a high-temperature phase crystal structure, the effect of substituting part of Ta with Bi, and the improvement in sinterability due to BiPO4 functioning as a sintering aid. The Nyquist plot obtained from the results of this impedance measurement is shown in Fig. 14.

[0054] Example 8: LiTa with a new crystal structure 1.9 Bi 0.1 Synthesis of BiPO8 sintered body (raw material oxides for solution synthesis) Under a dry environment, 1.3612 g of TaCl5 and 0.0631 g of BiCl3 (manufactured by Kojundo Chemical Laboratory Co., Ltd., 99.99% (the same hereinafter)) were dissolved in 50 mL of absolute ethanol to obtain a TaCl5·BiCl3 solution. 0.2301 g of NH4H2PO4 was dissolved in 50 mL of ion-exchanged water to obtain an NH4H2PO4 aqueous solution. 0.0923 g of LiOH·H2O was dissolved in 100 mL of ion-exchanged water to obtain a LiOH aqueous solution. While stirring this LiOH aqueous solution with a stirrer, this TaCl5·BiCl3 solution and this NH4H2PO4 aqueous solution were sequentially added and mixed at 80 °C. Note that this mixed solution contains 1.1 molar times, that is, 10 mol% excess LiOH compared to the composition LiTa 1.9 Bi 0.1 PO8.

[0055] This mixed solution was dried at 120 °C for 15 hours, and the dried and solidified powder was recovered and gently pulverized in an agate mortar. Using a vacuum gas replacement type electric furnace, this pulverized powder was fired at 500 °C for 12 hours in an oxygen atmosphere to obtain a white powder of LiTa 1.9 Bi 0.1 PO8 which is an amorphous raw material. Using a planetary ball mill, this white powder was wet ball milled and then uniaxially pressed using a tablet molding machine to obtain a molded body. Using an electric furnace, this molded body was heated to 1208 °C in air and fired for 5 minutes, and then cooled to obtain a sintering precursor.

[0056] Then, using a vacuum gas replacement type electric furnace, this sintering precursor was fired at 1000 °C for 12 hours in an oxygen atmosphere to obtain LiTa 1.9 Bi 0.1 PO8 sintered body. When the crystal structure of this sintered body was examined by a powder X-ray diffractometer, it was confirmed that the main phase was a LiTa2PO8-type crystal structure belonging to the monoclinic system and the space group C2 / c. On the other hand, it was confirmed that a BiPO4 phase was formed as an impurity phase in this sintered body. This BiPO4 phase functioned as a sintering aid by phase separation after precipitating as a liquid phase during high-temperature firing.

[0057] Example 9: Synthesis of LiTa 1.8 Bi 0.2 PO8 sintered body Except that the usage amount of TaCl5 was changed to 1.2896 g and the usage amount of BiCl3 was changed to 0.1261 g, LiTa 1.8 Bi 0.2 PO8 sintered body was obtained in the same manner as in Example 8. In the intermediate mixed solution, 1.1 molar times, that is, 10 mol% excess LiOH was contained compared to the composition LiTa 1.8 Bi 0.2 PO8. When the crystal structure of this sintered body was examined by a powder X-ray diffractometer, it was confirmed that the main phase was a LiTa2PO8-type crystal structure belonging to the monoclinic system and the space group C2 / c.

[0058] On the other hand, it was confirmed that a BiPO4 phase was significantly present as an impurity phase in this sintered body with the increase in the amount of Bi. This indicates that this BiPO4 phase functioned as a sintering aid by phase separation after precipitating as a liquid phase during high-temperature firing. The powder X-ray diffraction pattern is shown in Fig. 15. Also, for this sintered body, the lithium ion conductivity was calculated in the same manner as in Comparative Example 1. From the measurement results at room temperature, the total lithium ion conductivity was calculated to be 7.1×10 -4 S / cm, which was higher than the total lithium ion conductivity of the previously reported LiTa 1.8 Bi 0.2 PO8 sintered body.

[0059] Example 10: Li having a new crystal structure1.1 Ta 1.9 Hf 0.1 Synthesis of LiTaHf(PO8) Sintered Body Under a dry environment, 1.3612 g of TaCl5 and 0.0641 g of HfCl4 (manufactured by Fujifilm Wako Pure Chemical Industries, 99.9% (the same hereinafter)) were dissolved in 50 mL of anhydrous ethanol to obtain a TaCl5·HfCl4 solution. 0.2301 g of NH4H2PO4 was dissolved in 50 mL of ion-exchanged water to obtain an NH4H2PO4 aqueous solution. 0.1016 g of LiOH·H2O was dissolved in 100 mL of ion-exchanged water to obtain a LiOH aqueous solution. While stirring this LiOH aqueous solution with a stirrer, this TaCl5·HfCl4 solution and this NH4H2PO4 aqueous solution were sequentially added and mixed at 80 °C. Note that this mixed solution contains Li 1.1 Ta 1.9 Hf 0.1 1.1 molar times, that is, 10 mol% excess LiOH compared to LiTaHf(PO8).

[0060] This mixed solution was dried at 120 °C for 15 hours, and the dried and solidified powder was recovered. This solidified powder was gently ground in an agate mortar. Using a vacuum gas replacement type electric furnace, this ground powder was fired at 600 °C for 12 hours in an oxygen atmosphere to obtain an amorphous raw material Li 1.1 Ta 1.9 Hf 0.1 white powder of LiTaHf(PO8). Using a planetary ball mill, this white powder was wet ball milled, and then uniaxially pressed using a tablet molding machine to obtain a molded body. Using an electric furnace, this molded body was heated to 1208 °C in air and fired for 5 minutes, and then cooled to obtain a pre-sintered body. Then, using a vacuum gas replacement type electric furnace, this pre-sintered body was fired at 1000 °C for 12 hours in an oxygen atmosphere to obtain a Li 1.1 Ta 1.9 Hf 0.1 LiTaHf(PO8) sintered body.

[0061] When the crystal structure of this sintered body was examined by a powder X-ray diffractometer, it was confirmed that the main phase had a LiTa₂PO₈-type crystal structure belonging to the monoclinic system and the space group C2 / c. On the other hand, it was confirmed that a LiTa₃O₈ phase was formed in this sintered body. This LiTa₃O₈ phase was formed by high-temperature firing and functioned as a sintering aid. The powder X-ray diffraction pattern is shown in Fig. 16. Also, for this sintered body, the lithium ion conductivity was calculated in the same manner as in Comparative Example 1. From the measurement results at room temperature, the total lithium ion conductivity was calculated to be 9.2×10 -4 S / cm, which was higher than the total lithium ion conductivity of the previously reported LiTa₂PO₈ sintered body.

[0062] Example 11: Synthesis of a Li 1.2 Ta 1.8 Hf 0.2 PO₈ sintered body A Li 1.2 Ta 1.8 Hf 0.2 PO₈ sintered body was obtained in the same manner as in Example 8, except that the amount of TaCl₅ used was changed to 1.2896 g, the amount of HfCl₄ used was changed to 0.1281 g, and the amount of LiOH·H₂O used was changed to 0.1108 g. Note that the intermediate mixed solution contains 1.1 molar times, that is, 10 mol% excess LiOH compared to Li 1.2 Ta 1.8 Hf 0.2 PO₈.

[0063] When the crystal structure of this sintered body was examined by a powder X-ray diffractometer, it was confirmed that the main phase had a LiTa₂PO₈-type crystal structure belonging to the monoclinic system and the space group C2 / c. On the other hand, it was confirmed that with the increase in the Hf substitution amount, a LiTa₃O₈ phase was significantly present in this sintered body. This LiTa₃O₈ phase was formed by high-temperature firing and functioned as a sintering aid. Also, it was confirmed that with the increase in the Hf substitution amount, the peak position of the powder X-ray diffraction pattern shifted to the low-angle side. It became clear that the lattice volume increased due to the substitution of Ta with Hf.

[0064] As a result of the SEM-EDS analysis of this sintered body, Ta, Hf, and P were detected, and no metal elements other than Ta and Hf were detected. Furthermore, as a result of quantitative analysis of this sintered body using IPC emission spectroscopy, it was confirmed that the amount of Li increased with the substitution of Hf. That is, it was confirmed that the substitution of Ta with Hf not only increased the lattice volume but also increased the carrier concentration. Also, for this sintered body, the lithium ion conductivity was calculated in the same manner as in Comparative Example 1. From the measurement results at room temperature, the total lithium ion conductivity was calculated to be 1.1×10 -3 S / cm, which was higher than the total lithium ion conductivity of the previously reported LiTa2PO8 sintered body.

[0065] The reasons for the high total lithium ion conductivity of the sintered bodies of Example 10 and Example 11 include maintaining the crystal structure of the high-temperature phase, the effects of crystal lattice expansion and increased carrier concentration (lithium amount) due to Hf substitution, and the improvement in sinterability due to the function of LiTa3O8 as a sintering aid. Also, the activation energy estimated from the temperature dependence of the total lithium ion conductivity was calculated to be 0.34 eV for the sintered body of Example 10 and 0.315 eV for the sintered body of Example 11, which was smaller than the activation energy of the LiTa2PO8 sintered body of Example 6. That is, the substitution of Ta with Hf revealed an improvement in lithium ion conductivity at low temperatures such as -20°C.

[0066] Example 12: Synthesis of LiTa 1.8 Sb 0.2 PO8 In a dry environment, 1.2896 g of TaCl5 and 0.09125 g of SbCl3 (manufactured by Fujifilm Wako Pure Chemical Industries, reagent special grade) were dissolved in 50 mL of absolute ethanol to obtain a TaCl5·SbCl3 solution. 0.2301 g of NH4H2PO4 was dissolved in 50 mL of ion-exchanged water to obtain an NH4H2PO4 aqueous solution. 0.0923 g of LiOH·H2O was dissolved in 100 mL of ion-exchanged water to obtain a LiOH aqueous solution. While stirring this LiOH aqueous solution with a stirrer, this TaCl5·SbCl3 solution and this NH4H2PO4 aqueous solution were sequentially added and mixed at 80 °C. Note that this mixed solution contains 1.1 molar times, that is, 10 mol% excess LiOH compared to the composition LiTa 1.8 Sb 0.2 PO8.

[0067] This mixed solution was dried at 120 °C for 15 hours, and the dried and solidified powder was recovered. This solidified powder was gently ground in an agate mortar. Using a vacuum gas replacement type electric furnace, this ground powder was fired at 500 °C for 12 hours in an oxygen atmosphere to obtain a white powder of amorphous raw material LiTa 1.8 Sb 0.2 PO8. Using a planetary ball mill, this white powder was wet ball milled, and then uniaxially pressed using a tablet press to obtain a formed body. Using an electric furnace, this formed body was heated to 1208 °C in air and fired for 5 minutes, and then cooled to obtain a pre-sintered body. Then, using a vacuum gas replacement type electric furnace, this pre-sintered body was fired at 1000 °C for 12 hours in an oxygen atmosphere to obtain a LiTa 1.8 Sb 0.2 PO8 sintered body.

[0068] When the crystal structure of this sintered body was examined by a powder X-ray diffractometer, it was confirmed that the main phase had a crystal structure of the LiTa2PO8 type belonging to the monoclinic system and the space group C2 / c. On the other hand, it was confirmed that a TaPO5 phase was formed as an impurity phase in this sintered body. Further, it was confirmed that the peak position of this powder X-ray diffraction pattern shifted to the low-angle side compared with the peak position of LiTa2PO8, and it became clear that Sb having an ionic radius smaller than that of Ta was substituted. This powder X-ray diffraction pattern is shown in Fig. 17. Also, for this sintered body, the lithium ion conductivity was calculated in the same manner as in Comparative Example 1. From the measurement results at room temperature, the total lithium ion conductivity was calculated to be 4.3×10 -4 S / cm, which was higher than the total lithium ion conductivity of the previously reported LiTa 1.8 Sb 0.2 PO8 sintered body.

[0069] Example 13: Fabrication of Composite Cathode The LiTa2PO8 (LTPO) sintered body, LiTa 1.9 Bi 0.1 PO8 (LTBPO) sintered body, and Li 1.1 Ta 1.9 Hf 0.1 PO8 (LTHPO) sintered body obtained in Example 6, Example 7, and Example 10 were each pulverized, and a composite cathode was fabricated using them as the electrolyte of the composite cathode. As the cathode active material, LiCoO2 (manufactured by Nippon Chemical Industry Co., Ltd., Cellseed C-5H) was used, and the electrolyte powder and LiCoO2 powder were mixed at a weight ratio of 1:1 using an agate mortar, and then a green compact was fabricated by uniaxially pressing using a tablet molding machine. The composite cathode sintered compact was fabricated by firing this green compact at 600°C for 2 hours in an argon gas atmosphere.

[0070] For the fabricated composite cathode compact, the resistance value was calculated from the arc of the Nyquist plot using a frequency response analyzer. From the measurement results at room temperature, the ionic resistance of the green compact was approximately 1×10 7 Ω. Although these values are high as ionic resistance, it became clear that a conductive path was formed in the composite cathode and an increase in the interfacial resistance with the cathode active material was suppressed.

[0071] The powder X-ray diffraction patterns measured by pulverizing the composite positive electrode are shown in Fig. 18. It was confirmed that only the LiTa2PO8-type phase having the new crystal structure of the present invention and the diffraction pattern derived from the LiCoO2 phase are the main phases, and almost no reaction products or the like exist. From the above, it became clear that the solid electrolyte of the present invention can be used as a composite positive electrode of an all-solid-state battery.

[0072] Example 14: Fabrication of all-solid-state battery The sintered body obtained in Example 1 was pulverized to obtain a white powder. Using a planetary ball mill, this white powder was wet ball-milled and then dried. Using the obtained LiTa2PO8 sintered body powder, a dense sintered body composed of an electrolyte layer and a composite electrode layer was fabricated. That is, the LiTa2PO8 sintered body powder was filled into a hot press mold (manufactured by AS ONE) with a diameter of 10 mmΦ, and held at 400 °C under 374 MPa for 2 hours using a hot press apparatus (manufactured by AS ONE) to obtain a disk-shaped dense sintered body with a thickness of about 0.3 mm.

[0073] This dense sintered body and the mold constitute a composite electrode layer including a solid electrolyte layer and one electrode layer. Further, a lithium ion conductive polymer electrolyte sheet and a metal lithium sheet (thickness: 0.2 mm) serving as the other electrode layer were sequentially attached to the surface of the exposed solid electrolyte layer to fabricate an all-solid-state battery. Regarding this all-solid-state battery, a constant current charge / discharge test (current density 3 mA / g) was performed at 60 °C using a charge / discharge test apparatus (manufactured by Hokuto Denko, HJ1020mSD8). As a result, a capacity corresponding to the charge / discharge reaction was observed, and the operation of the all-solid-state battery was confirmed.

Industrial applicability

[0074] By using LiTa2PO8 having the novel crystal structure of the present application and its element substitution products, an electrolyte member having high lithium ion conductivity can be fabricated.

Claims

1. An oxide sintered body containing lithium, tantalum, and phosphorus, having a crystal structure belonging to the monoclinic system and the space group C2 / c, and in which lithium does not occupy the 4b site (0.5, 0, 0) in the Wyckoff position.

2. In Claim 1, General formula LiTa ２－ｘ M ｘ PO ８ (M is Bi or Sb, 0 ≤ x ≤ 0.2) Oxide sintered body represented by

3. In Claim 1, General formula Li １＋ｙ Ta ２－ｙ Hf ｙ PO ８ An oxide sintered body represented by (0 ≤ y ≤ 0.2).

4. In Claim 1, An oxide sintered body composed of primary particles having a particle size of 50 μm to 100 μm of LiTa ２ PO ８ is.

5. In Claim 4, Single-crystalline LiTa ２ PO ８ is an oxide sintered body.

6. In any one of Claims 1 to 5, An oxide sintered body in which the occupied sites of lithium occupy only three or more 8f sites in the Wyckoff position.

7. In any one of Claims 1 to 5, An oxide sintered body in which the occupied sites of lithium are in a disordered occupation state.

8. A method for producing the oxide sintered body of Claim 1, comprising a sintering step of firing an oxide containing lithium, tantalum, and phosphorus at a temperature higher than 1200°C and not higher than 1400°C.

9. A method for producing the oxide sintered body of Claim 2, A method for manufacturing an oxide sintered body having a sintering step of firing an oxide represented by the general formula LiTa ２－ｘ M ｘ PO ８ (where M is Bi or Sb, 0 ≤ x ≤ 0.2) at a temperature higher than 1200°C and not higher than 1400°C.

10. A method for producing the oxide sintered body of Claim 3, A method for manufacturing an oxide sintered body having a sintering step of firing an oxide represented by the general formula Li １＋ｙ Ta ２－ｙ Hf ｙ PO ８ (0 ≤ y ≤ 0.2) at a temperature higher than 1200°C and not higher than 1400°C.

11. In any one of Claims 8 to 10, A method for producing an oxide sintered body in which the oxide is amorphous.

12. A solid electrolyte comprising an oxide sintered body according to any one of Claims 1 to 5, and a pair of electrodes sandwiching the solid electrolyte, and an electrochemical device having the same.

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