JP7706199B2 - Oxide sintered body and method for producing the same - Google Patents

Description

This application relates to an oxide sintered body having high lithium ion conductivity and a method for producing the oxide sintered body.

Lithium-ion secondary batteries are widely used as power sources for small electronic devices such as smartphones and laptop computers. In recent years, lithium-ion secondary batteries are expected to be used as power sources for large-scale applications such as hybrid and electric vehicles, as well as stationary storage batteries. Furthermore, from the standpoint of safety and high energy density, research and development is being conducted on all-solid-state lithium-ion secondary batteries, lithium-air batteries, and all-solid-state lithium-sulfur batteries that do not use flammable electrolytes. The solid electrolytes used in these electrochemical devices are required to have high lithium-ion conductivity.

Recently, it has been reported that the lithium tantalum phosphate compound LiTa ₂ PO ₈ exhibits high lithium ion conductivity (Non-Patent Documents 1 to 3). The crystal structure of LiTa ₂ PO ₈ is different from other lithium ion conductors. A precise crystal structure analysis has revealed that the framework structure of LiTa ₂ PO ₈ is constructed from TaO ₆ octahedra and PO ₄ tetrahedra, and lithium ions occupy the gaps between them. Since the arrangement of lithium forms a three-dimensional conduction path, LiTa ₂ PO ₈ is capable of good lithium ion conduction similar to that of garnet-type materials.

On the other hand, attempts have been made to improve the properties by elemental substitution of Ta with LiTa ₂ PO ₈ as the basic composition (Patent Documents 1 to 3 and Non-Patent Document ₄ ). It has been reported that the bulk lithium ion conductivity of the LiTa ₂ PO ₈ sintered body is improved by this elemental substitution. The sintering temperature when producing the element-substituted LiTa ₂ PO ₈ sintered body was 1200° C. _or less, similar to the sintering temperature when producing the LiTa 2 PO ₈ sintered body. The crystal structure of the element-substituted LiTa ₂ PO 8 sintered body is described as a monoclinic structure, which is the same as the crystal structure of the LiTa ₂ PO ₈ sintered body, and there has been no mention of the detailed differences in lithium occupancy sites and lithium ion conduction paths, and even prediction of the differences has not been considered.

Usually, a high-temperature sintering method is used to prepare LiTa ₂ PO ₈ sintered body. However, LiTa ₂ PO ₈ contains lithium and phosphate as main elements, which are easily volatilized at high temperatures. In order to suppress the composition deviation caused by these volatilization and the generation of impurity phases caused by decomposition, the sintering temperature when preparing LiTa ₂ PO ₈ has been set at 1200°C as the upper limit. Therefore, it has not been considered what kind of lithium occupied site the LiTa ₂ PO ₈ sintered body produced by sintering at a temperature exceeding 1200°C has in its crystal structure, and how the lithium ion conductive properties change, and it was impossible to predict that there would be a change in the crystal structure.

JP 2020-194773 A JP 2021-38100 A JP 2021-38099 A

J. Kim et al. , Journal of Materials Chemistry A, Vol. 6, 22478 (2018) N. Ishigaki et al. , Solid State Ionics, Vol. 351, 115314 (2020) B. Huang et al. , J. Mater. Sci. , Vol. 56, 2425 (2021) R. Kim et al. , Chem. Mater. , Vol. 33, 6909 (2021)

This application has been made in consideration of these circumstances, and aims to provide an oxide sintered body having a crystal structure that can increase lithium ion conductivity, and a manufacturing method for an oxide sintered body that can construct this crystal structure.

The present inventors have thoroughly investigated the relationship between the sintering temperature and the crystal structure of the target LiTa ₂ PO ₈ , and the starting materials and synthesis methods suitable for high-temperature sintering. As a result, they have surprisingly found that a crystal phase with a lithium occupancy mode advantageous for faster lithium ion diffusion is generated by sintering at a temperature exceeding 1200°C. Furthermore, they have found that LiTa ₂ PO ₈ can be sintered at 1300°C without decomposition by sintering in a closed system in which the volatilization of lithium and phosphoric acid at high temperatures can be suppressed, and that such high-temperature sintering causes significant grain growth of LiTa ₂ PO ₈ , resulting in a sintered body and a single crystal body with high bulk lithium ion conductivity.

Furthermore, the inventors have further studied the chemical composition of LiTa ₂ PO ₈ , and have found that by substituting a different element for the Ta site, the amount of lithium can be made favorable for lithium ion diffusion, and that a liquid phase is formed with the addition of an element, which functions as a sintering aid in high-temperature sintering exceeding 1200° C., and improves sinterability. As a result, it has been confirmed that the total lithium ion conductivity of the dense sintered body in the high-temperature phase of LiTa ₂ PO ₈ and element-substituted bodies in which Ta in LiTa ₂ PO ₈ is substituted with other elements and the Li composition amount is changed as necessary (hereinafter, the element-substituted body in which Ta is substituted with other elements and the Li composition amount is changed as necessary may be simply referred to as "element-substituted body") produced by such a production method is 2×10 ⁻⁴ S/cm or more, and that the battery operation of an all-solid-state battery equipped with a solid electrolyte produced by low-temperature sintering using powder obtained by pulverizing the dense sintered body has been confirmed.

The oxide sintered body of the present application is an oxide sintered body containing lithium, tantalum, and phosphorus, and has a crystal structure belonging to the monoclinic 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 includes a sintering step of firing an oxide containing lithium, tantalum, and phosphorus at a temperature higher than 1200°C and lower than 1400°C. The electrochemical device of the present application includes a solid electrolyte including the oxide sintered body of the present application, and a pair of electrodes sandwiching the solid electrolyte.

According to the present application, an oxide sintered body having a crystalline phase that exhibits higher lithium ion conductivity is obtained.

FIG. 1 is a conceptual diagram of an all-solid-state lithium ion secondary battery, which is an example of an electrochemical device according to the present application. 1 shows a single crystal X-ray diffraction pattern obtained by using a single crystal selected from the LiTa ₂ PO ₈ sintered body obtained in Example 1. 1 is a crystal structure model obtained from the results of single crystal X-ray crystal structure analysis using a single crystal selected from the LiTa ₂ PO ₈ sintered body obtained in Example 1. Electron microscope image of a fracture surface of the LiTa ₂ PO ₈ sintered body obtained in Example 1. Electron microscope image of a single crystal selected from the LiTa ₂ PO ₈ sintered body obtained in Example 1. 1 is a stereomicroscope image of the LiTa ₂ PO ₈ single crystal obtained in Example 2. 1 shows powder X-ray diffraction patterns and partial enlarged views of the LiTa ₂ PO ₈ sintered bodies obtained in Comparative Example 1, Example 3, and Example 4. Electron microscope image of a fracture surface of the LiTa ₂ PO ₈ sintered body obtained in Comparative Example 1. Powder X-ray diffraction pattern of the LiTa ₂ PO ₈ sintered body obtained in Example 5. Graph showing the results of measuring the AC impedance of the LiTa ₂ PO ₈ sintered body obtained in Example 5. Powder X-ray diffraction pattern of the LiTa ₂ PO ₈ sintered body obtained in Example 6. Powder X-ray diffraction pattern of the LiTa _1.9 Bi _0.1 PO ₈ sintered body obtained in Example 7. 1 is an electron microscope image of a fracture surface of the LiTa _1.9 Bi _0.1 PO ₈ sintered body obtained in Example 7. 1 is a graph showing the results of measuring the AC impedance of the LiTa _1.9 Bi _0.1 PO ₈ sintered body obtained in Example 7. Powder X-ray diffraction pattern of the LiTa _1.8 Bi _0.2 PO ₈ sintered body obtained in Example 9. 1 shows a powder X-ray diffraction pattern of the Li _1.1 Ta _1.9 Hf _0.1 PO ₈ sintered body obtained in Example 10. Powder X-ray diffraction pattern of the LiTa _1.8 Sb _0.2 PO ₈ sintered body obtained in Example 12. 16 is a powder X-ray diffraction pattern of the composite positive electrode sintered body obtained in Example 13.

(Oxide sintered body)
The oxide sintered body of the present embodiment contains lithium, tantalum, and phosphorus. The oxide sintered body of the present embodiment has a crystal structure belonging to the monoclinic space group C2/c, and lithium does not occupy the 4b site (0.5,0,0) at the Wyckoff site. It is preferable that the lithium occupies only three or more 8f sites at the Wyckoff site. The lithium occupancy may be disordered.

The oxide sintered body of this embodiment is an oxide sintered body represented by the _general formula _{LiTa2 - xMxPO8} (M is Bi or Sb, ₀ ≦ _{x ≦ 0.2} (hereinafter the same) ₎ , such as _{LiTa1.9Bi0.1PO8} , _{LiTa1.8Bi0.2PO8} , _{LiTa1.9Sb0.1PO8} , _{and LiTa1.8Sb0.2PO8 .} In addition, examples of the oxide sintered body of this embodiment include oxide sintered bodies represented by the general formula Li _1+y Ta _2-y Hf _y PO ₈ (0≦y≦0.2 (the same applies below)), such as Li _1.1 Ta _1.9 Hf _0.1 PO ₈ and Li _1.2 Ta _1.8 Hf _0.2 PO ₈ .

In addition, examples of elements substituting Ta in LiTa ₂ PO ₈ include titanium, niobium, molybdenum, zirconium, tungsten, boron, aluminum, silicon, germanium, and gallium, in addition to bismuth, antimony, and hafnium. In addition, LiTa ₂ PO ₈ is an example of the oxide sintered body of this embodiment. In order to obtain a higher lithium ion conductivity, LiTa ₂ PO ₈ is preferably composed of primary particles having a diameter of 50 μm to 100 μm. The particle diameter of the primary particles, that is, the primary particle size, can be measured, for example, using a scanning electron microscope. In order to obtain an even higher lithium ion conductivity, LiTa ₂ PO ₈ is preferably a single crystal.

(Method for producing oxide sintered body)
The method for producing an oxide sintered body according to the embodiment of the present application includes a sintering step. In the sintering step, a raw material oxide (hereinafter sometimes referred to as a "raw material oxide") is fired at a temperature of more than 1200°C and not more 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 an oxide represented by the general formula LiTa _2-x M _x PO ₈ , an oxide represented by the general formula Li _1+y Ta _2-y Hf _y PO ₈ , and amorphous LiTa ₂ PO _8. Amorphous LiTa ₂ PO ₈ has excellent moldability, which is a concept that includes moldability.

The raw oxide is obtained by dissolving various raw compounds, such as metal salts and phosphate compounds, in a solvent, mixing these solutions to precipitate, and firing the precipitate. However, there are no particular limitations on the method for producing the raw oxide, so long as the metal components of the various raw compounds can be mixed uniformly at the atomic level and the raw oxide can be produced. For example, raw oxides can be produced by solution synthesis, such as conventional solid-phase synthesis, coprecipitation, sol-gel, complex polymerization, and hydrothermal synthesis, as well as gas-phase reaction synthesis, such as vacuum deposition, sputtering, pulsed laser deposition, and chemical vapor reaction. Raw oxides can also be produced by applying mechanochemical reactions, such as ball mill grinding.

There are no particular limitations on the various raw material compounds, so long as they contain lithium, tantalum, or phosphorus. Examples of raw material compounds that can be used include oxides, carbonates, hydroxides, nitrates, ammonium salts, hydrogen ammonium salts, halides such as chlorides, and oxide chlorides. The raw material oxides can be prepared, for example, by the following procedure. First, the raw material compounds are dissolved in a solvent. There are no particular limitations on this solvent, so long as it can mix the raw material compounds uniformly. Examples of this solvent include alcoholic solvents such as methanol, ethanol, hexanol, and propanol, organic solvents such as aromatic compounds and ethers, and water.

The temperature at which the raw material compound is dissolved in the solvent may be from room temperature to the boiling point of the solvent. The raw material compound and the solvent may be mixed and then left to stand until the raw material compound dissolves in the solvent, or the mixture of the raw material compound and the solvent may be stirred using a stirrer or agitator to accelerate the dissolution reaction. Next, the solutions of the 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, but then they are mixed with an aqueous solution to produce a gelled precipitate. By heating in this state, the gelled precipitate dries and a precursor of the raw material oxide is produced. There are no particular limitations on the heating method, and heating may be performed using a hot plate, an electrically heated muffle furnace, a mantle heater, or the like. The heating temperature is preferably from 50°C to the boiling point of the solvent, and more preferably from 80°C to the boiling point of the solvent.

The precursor is then fired to obtain the desired raw oxide. There are no particular limitations on the firing method, and firing may be performed using an electrically heated muffle furnace or a mantle heater. The firing temperature is preferably 350°C or higher to obtain powder of the raw oxide, and may be 500°C or higher and 1000°C or lower. There are no particular limitations on the container used for firing, and glass beakers and non-alumina 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 are no particular limitations on the firing atmosphere, and it is usually an oxidizing gas atmosphere such as oxygen or air.

The calcination time can be set according to the calcination temperature, etc., as long as the residual substances such as nitrogen, chlorine, and carbon derived from various raw material compounds can be volatilized. There is no particular restriction on the cooling method after calcination, but it is usually natural cooling (cooling in the furnace) or slow cooling. After calcination and cooling, the raw material oxide may be crushed as necessary, and the calcination temperature may be changed and re-calcined. The degree of crushing can be adjusted according to the calcination temperature, etc. LiTa ₂ PO ₈ or its element-substituted product produced by this aqueous solution synthesis method can be used as the raw material oxide.

The raw oxide may be prepared by calcining a mixture of oxides containing only a part of Li, Ta, P, and the substitution elements. In this mixture of oxides, a plurality of oxides are mixed so that the composition ratio of LiTa ₂ PO ₈ and its element substitution body is obtained after the synthesis of the oxide sintered body. The calcination temperature can be set according to the composition of the oxide mixture, but is usually 500°C to 1200°C, preferably 900°C to 1000°C. 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, etc.

Next, the raw oxide is sintered. The sintering temperature is higher than 1200°C and lower than 1400°C, preferably higher than 1205°C and lower than 1300°C. In the sintering, a one-stage sintering reaction may be carried out at a temperature higher than 1200°C and lower than 1400°C, or a two-stage sintering reaction may be carried out by first raising the temperature to higher than 1200°C and lower than 1400°C, and then sintering at a temperature lower than 1200°C. The sintering temperature in the second stage is usually 800°C to 1200°C, preferably 850°C to 1050°C. In this two-stage sintering method, a liquid phase such as lithium salt is generated in the first high-temperature sintering, and the liquid phase such as lithium salt functions as a sintering aid in the second low-temperature sintering, resulting in an oxide sintered body with high density.

The firing 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 material of the crucible into which the raw oxide, which is crushed as necessary, is placed during firing should be stable at high temperatures of 1200°C or higher and should be able to suppress the volatilization of lithium, phosphate, bismuth, gallium, and the like at high temperatures, and may be, for example, platinum, alumina, or zirconia.

There are no particular limitations on the form of the raw oxide to be sintered, and examples include powders, plate-shaped bodies pressurized by methods such as hydrostatic pressure or uniaxial pressure, and films produced by coating or film-forming techniques. Coating techniques include screen printing, electrophoretic deposition (EPD), doctor blade, spray coating, inkjet, and spin coating. Film-forming techniques include deposition, sputtering, chemical vapor deposition (CVD), electrochemical vapor deposition, ion beam, laser ablation, atmospheric pressure plasma film-forming, and reduced pressure plasma film-forming.

Alternatively, a plate or film of the raw oxide may be produced using a method such as hot pressing, hot isostatic pressing, or electric sintering. After the main firing, the obtained oxide sintered body may be pulverized by a known method as necessary, and the main firing may be repeated. The degree of pulverization may be set according to the main firing temperature, etc. Alternatively, a molded body of the raw oxide may be produced from the powdered raw oxide that has been sintered once.

(Solid electrolyte material)
The oxide sintered body obtained by this firing can be used as a solid electrolyte material for electrochemical devices such as all-solid-state lithium ion secondary batteries. The oxide sintered body may be pulverized, molded and fired again to produce a solid electrolyte material. In addition, a powder of the oxide sintered body may be mixed or composited with another electrolyte material to produce a molded body. On the other hand, if the oxide sintered body is a block sintered body (including a single crystal body), a molded body, or a coated film body, it can be used as a solid electrolyte as it is.

(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 obtained by mixing or combining the oxide sintered body of the present application with a positive electrode active material is a positive electrode member. As the positive electrode active material, a material generally used as a positive electrode material for lithium ion secondary batteries can be used. Examples of _positive electrode active materials _{include 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.

In addition, a material in which lithium desorption/insertion reactions occur reversibly at a high voltage of 2 V or more can be used as the positive electrode active material. Furthermore, in order to improve the bond between the positive electrode and the solid electrolyte material and to improve lithium ion conductivity, the positive electrode member may contain a polymer, an oxide, a sulfide, a hydride, a halide, or the like. In addition, in order to improve the electronic conductivity in the positive electrode, the positive electrode member may contain a conductive assistant such as carbon black, carbon nanotubes, graphite, or titanium oxide.

(Electrochemical Devices)
The oxide sintered body of the present application has excellent lithium ion conductivity and 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, which is 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 the solid electrolyte. The pair of electrodes does not need to directly sandwich the solid electrolyte.

The oxide sintered body of the present application can also be used as a positive electrode member or a negative electrode member. FIG. 1 conceptually shows an all-solid-state lithium ion secondary battery, which is an example of an electrochemical device of an 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 as a part of the positive electrode 4 or the negative electrode 6, or as the separator 5.

Example 1: Synthesis of LiTa ₂ PO ₈ sintered body having a new crystal structure (sintered at 1205°C)
In a dry environment, 1.4328 g of _TaCl5 (made by Rare Metallic, 99.9% (same below)) was dissolved in 50 mL of anhydrous ethanol to obtain a _TaCl5 solution. _{0.2301 g} of _NH4H2PO4 (made by Wako Pure Chemical Industries, special grade reagent (same below)) was dissolved in 50 mL of _ion -exchanged water to obtain an _{NH4H2PO4 aqueous} solution. 0.0923 g of _LiOH.H2O (made by Kojundo Chemical Laboratory, 99% up (same below)) 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 added in sequence and mixed at 80°C. The mixed solution contains 1.1 moles of LiOH, that is, 10 mol % excess, compared to the composition LiTa ₂ PO ₈ .

The mixed solution was dried at 120°C for 15 hours, and the dried and solidified powder was collected and lightly ground in an agate mortar. The ground powder was fired for 12 hours in an oxygen atmosphere at 600°C using a vacuum gas replacement type electric furnace (KDF-75plus (hereinafter the same) manufactured by Denken Hydental) to obtain a white powder of LiTa ₂ PO ₈ , which is an amorphous raw material. The white powder was wet ball milled using a planetary ball mill (P-7 (hereinafter the same) manufactured by Fritsch), and then uniaxially pressed using a tablet molding machine (JASCO (hereinafter the same)) to obtain a compact. The compact was fired for 240 hours in air at 1205°C using a tabletop high-temperature muffle furnace (SSFS-130-S (hereinafter the same) manufactured by Yamada Denki) to obtain a LiTa ₂ PO ₈ sintered body.

Grains that had grown from this sintered body were selected and their crystal structure was examined using a single crystal X-ray diffractometer (Rigaku, R-AXIS RAPID-II (hereinafter the same)). As a result, it was confirmed that the grains were in the monoclinic system and belonged to the space group C2/c. The single crystal X-ray diffraction pattern is shown in Figure 2. Single crystal X-ray diffraction data was 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 arrangement 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) at the Wyckoff position.

Furthermore, it was revealed that lithium is arranged in the 8f site, which is different from Non-Patent Document 1 and Non-Patent Document 2. Unlike Non-Patent Document 1 and Non-Patent Document 2, the crystal structure model of this sintered body, which was determined with an R value of 3.2%, which indicates the reliability of the analysis, by a model that introduces five 8f sites, is shown in Figure 3. In addition, when the primary particle size forming this sintered body was examined using a tabletop scanning electron microscope (JEOL, JCM-6000 (hereinafter the same)), it was about 50 μm to 100 μm. An electron microscope image of the fracture surface of this sintered body is shown in Figure 4, and an electron microscope image of a single crystal selected from this sintered body is shown in Figure 5. Figure 4 shows that this sintered body is a bulk body without clear grain boundaries.

Example 2: Synthesis of LiTa ₂ PO ₈ single crystal having a new crystal structure (calcination at 1310° C.)
A LiTa ₂ PO ₈ single crystal was obtained in the same manner as in Example 1, except that the sintering temperature was changed from 1205°C to 1310°C. A single crystal was taken out by dividing the single crystal, and the crystal structure was examined by a single crystal X-ray diffraction apparatus. 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 was collected, and crystal structure analysis was performed in the same manner as in Example 1. As a result, it was confirmed that the lithium arrangement in Non-Patent Document 1 and Non-Patent Document 2 could not be refined, and lithium was not occupied in the 4b site (0.5,0,0) at the Wyckoff position.

Furthermore, it was revealed that lithium is arranged in 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 LiTa ₂ PO ₈ sintered body obtained by firing at a temperature higher than 1200°C has a new crystal structure that cannot be explained by the previously reported structural model. In addition, when the crystal size of this single crystal was examined using a stereo microscope (LEICA, S8APO), it was about 100 μm to 500 μm. A stereo microscope image of the selected single crystal is shown in Figure 6. Note that one scale in Figure 6 is 0.1 mm.

Comparative Example 1: Synthesis of LiTa ₂ PO ₈ sintered body (sintered at 1050° C.)
_Li2CO3 (Rare _Metallic , 99.9% (same below)), _{Ta2O5 (} Rare Metallic, 99.99% (same below)), and ( _NH4 ) _2HPO4 (FUJIFILM Wako Pure Chemical, special grade reagent (same below) ₎ were weighed out so that the mass ratio of Li:Ta:P (so-called molar ratio) was 1.1:2:1. These were pulverized and mixed in an agate mortar, and then heated sequentially at 450°C for 4 hours and at 600°C for 4 hours in an electric furnace (Yamato Scientific, FP100 (same below)) to decompose the ammonium salt and obtain raw materials.

The raw material was crushed in anhydrous ethanol, collected and dried, and then pre-fired at 900°C to 950°C for 4 hours using an electric furnace. The pre-fired body obtained was ball milled and dried to obtain a powder of the raw oxide. The powder was uniaxially pressed using a tablet press to obtain a compact. The compact was fired in air at 1050°C for 6 hours using an electric furnace to obtain a LiTa ₂ PO ₈ sintered body (see Non-Patent Document 2). When the crystal structure of the sintered body was examined using a powder X-ray diffraction device (Rigaku, SmartLab (hereinafter the same)), it was confirmed that the sintered body was almost a single phase of LiTa ₂ PO ₈ having a crystal structure belonging to the space group C2/c in the monoclinic system, as described in Non-Patent Documents 1 to 3. The powder X-ray diffraction pattern is shown in FIG. 7 ("1050°C" in the figure). In addition, the particle size of the primary particles of the sintered body was examined using a tabletop scanning electron microscope and was about several μm. An electron microscope image of the fracture surface of this sintered body is shown in FIG.

Furthermore, for this sintered body, a frequency response analyzer (FRA) (Solartron, Model 1260 (same below)) was used to determine the resistance value from the arcs of the Nyquist plot, and the lithium ion conductivity was calculated from this resistance value. The impedance was measured under conditions of a frequency of 32 MHz to 100 Hz and an amplitude voltage of 100 mV, and an Au electrode was used as the blocking electrode (same below). From the measurement results at room temperature, the total lithium ion conductivity was calculated to be 2.1 x 10 ^-4 S/cm, which was roughly consistent with the value previously reported.

Example 3: Synthesis of _{LiTa2PO8 sintered} body having a new crystal structure (1205°C firing, raw oxide of solid phase synthesis)
Using the solid-phase synthesis provisional sintered body obtained in Comparative Example 1, a sintered body was further synthesized by high-temperature firing. The provisional sintered body of Comparative Example 1 (diameter: about 9 mm, thickness: 1.0 to 1.3 mm) was placed in a platinum container and fired in a tabletop high-temperature muffle furnace at 1205°C in air for 4 hours to obtain a LiTa ₂ PO ₈ sintered body. When the crystal structure of this sintered body was examined using a powder X-ray diffraction device, it was found to be almost a single phase of LiTa ₂ PO ₈ having a crystal structure belonging to the monoclinic space group C2/c, and it became clear that these sintered bodies could be synthesized without decomposition even at high-temperature firing. The powder X-ray diffraction patterns of these sintered bodies are shown in Figure 7 ("1205°C" in the figure).

Example 4: Synthesis of LiTa ₂ PO ₈ sintered body having a new crystal structure (1310°C firing, raw oxide of solid phase synthesis)
A LiTa ₂ PO ₈ sintered body was obtained in the same manner as in Example 3, except that the firing temperature was changed from 1205° C. to 1310° C. The crystal structure of this sintered body was examined using a powder X-ray diffraction apparatus, and it was found to be an almost single phase of LiTa ₂ PO ₈ having a crystal structure belonging to the monoclinic space group C2/c, and it was revealed that these sintered bodies could be synthesized without decomposition even when fired at high temperatures. The powder X-ray diffraction patterns of these sintered bodies are shown in FIG. 7 ("1310° C." in the figure).

A closer look at each of the XRD patterns shown in Figure 7 confirmed that the peak positions of the sintered bodies of Examples 3 and 4 were shifted to the lower angle side compared to the peak position of the sintered body of Comparative Example 1. From the above results, it was suggested that the sintered bodies of Examples 3 and 4 had the same skeletal structure formed by tantalum, phosphorus, and oxygen as the sintered body of Comparative Example 1, but that the local structure around lithium had changed, resulting in an expansion of the lattice volume. This result supports the idea that the local arrangement of lithium has changed due to firing at temperatures higher than 1200°C.

Example 5: Synthesis of LiTa ₂ PO ₈ sintered body having a new crystal structure (1208°C firing, raw oxide of solid phase synthesis)
First, a molded body was obtained in the same manner as in Comparative Example 1. Next, the molded body was heated to 1208°C in air using an electric furnace, and then cooled and sintered at 1050°C for ₆ hours to obtain a _LiTa2PO8 sintered body. When the crystal structure of this sintered body was examined using a powder X-ray diffraction device, it was confirmed that it was a _single phase of _LiTa2PO8 having a crystal structure belonging to the monoclinic space group C2/c. The powder X-ray diffraction pattern is shown in Figure 9.

Furthermore, using the obtained powder X-ray diffraction data, the crystal structure of this sintered body was analyzed by the Rietveld method (program: Rietan-FP used) to examine the detailed lithium arrangement. As a result, it was confirmed that the lithium arrangement in Non-Patent Document 1 and Non-Patent Document 2 could not be refined, and lithium was not occupied in the 4b site (0.5,0,0) in the Wyckoff position. Furthermore, it was revealed that lithium was arranged in the 8f site, which is different from Non-Patent Document 1 and Non-Patent Document 2. As a result of these structural analyses, it was found that the crystal structure model of this sintered body was appropriate, which was equivalent to the results of the single crystal X-ray diffraction structural analysis in Examples 1 and 2.

The lithium ion conductivity of this sintered body 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 2.1×10 ⁻⁴ S/cm, which was higher than the total lithium ion conductivity of the LiTa ₂ PO ₈ sintered body previously reported in Comparative Example 1. The Nyquist plot obtained from the results of this impedance measurement is shown in FIG. 10. From the above, it has become clear that the crystal structure of the high-temperature phase exceeding 1200° C. is maintained by two-stage sintering in which the temperature is raised to a temperature exceeding 1200° C. once, then the temperature is lowered and sintered at 1050° C., and a sintered body with a higher conductivity can be produced.

Example 6: Synthesis of LiTa ₂ PO ₈ sintered body having a new crystal structure (1208°C firing, solution synthesis raw material oxide)
First, a green body was obtained in the same manner as in Example 1. Next, the green body was heated to 1208°C in air using an electric furnace, sintered for 5 minutes, and cooled to obtain a sintered precursor. Then, the sintered precursor was sintered in an oxygen atmosphere at 1000°C for 12 hours using a vacuum gas replacement type electric furnace to obtain a _{LiTa2PO8 sintered} body.

When the crystal structure of this sintered body was examined by a powder X-ray diffraction apparatus, it was confirmed that it was an almost single phase of LiTa ₂ PO ₈ having a crystal structure belonging to the monoclinic space group C2/c. The powder X-ray diffraction pattern is shown in FIG. 11. Furthermore, as a result of SEM-EDS analysis of this sintered body (manufactured by JEOL, JCM-6000 (hereinafter the same)), 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, Agilent5800 (hereinafter the same)), it was confirmed that the chemical composition was LiTa ₂ PO ₈ with a nearly stoichiometric ratio.

The lithium ion conductivity of this sintered body 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 5.2×10 ⁻⁴ S/cm, which was found to be higher than the total lithium ion conductivity of the previously reported LiTa ₂ PO ₈ sintered body. This is believed to be because the crystal structure of this sintered body forms a lithium arrangement that exhibits higher lithium ion conductive properties.

Example 7: Synthesis of _{LiTa1.9Bi0.1PO8 sintered} body having a new crystal structure (raw oxide for solid _- phase synthesis)
_Li2CO3 , _Ta2O5 , _Bi2O3 (made _by Rare Metallic, 99.99 _% ), and ( _NH4 ) _{2HPO4 were} weighed so that the substance ratio of Li:Ta:Bi:P _was 1.1:1.9:0.1:1. After crushing and mixing them using an agate mortar, they were heated in an electric furnace at 450°C for 4 hours and then at 600°C for 4 hours to decompose the ammonium salt and obtain a raw material oxide. The raw material oxide was crushed in anhydrous ethanol, recovered and dried, and then uniaxially pressed using a tablet molder to obtain a molded body. The molded body was heated to 1208°C in air using an electric furnace, then cooled and fired at ₉₀₀ °C for ₆ hours to obtain a _{LiTa1.9Bi0.1PO8} sintered body.

When the crystal structure of this sintered body was examined by a powder X-ray diffraction apparatus, it was confirmed that the main phase was a LiTa ₂ PO ₈ type crystal structure belonging to the space group C2/c in the monoclinic system. On the other hand, it was confirmed that a BiPO ₄ phase was generated as an impurity phase in this sintered body. This BiPO ₄ phase precipitated as a liquid phase during high-temperature sintering, and then functioned as a sintering aid through phase separation. The powder X-ray diffraction pattern is shown in FIG. 12. In addition, the particle size of the primary particles of this sintered body was examined using a tabletop scanning electron microscope, and was found to be several μm to about 10 μm. An electron microscope image of the fracture surface of this sintered body is shown in FIG. 13. Compared to the sintered body of Comparative Example 1, it was confirmed that the grain growth was remarkable, and that BiPO ₄ precipitated at the grain boundaries served as a sintering aid, even though the main sintering temperature was lowered from 1050°C to 900°C.

The lithium ion conductivity of this sintered body 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 1.3×10 ⁻³ S/cm, which was higher than the total lithium ion conductivity of the LiTa ₂ PO ₈ sintered body previously reported. This is believed to be due to the fact that this sintered body maintains the crystal structure of the high-temperature phase, the effect of substituting a part of Ta with Bi, and the improvement in sinterability due to the function of BiPO ₄ as a sintering aid. The Nyquist plot obtained from the results of this impedance measurement is shown in FIG. 14.

Example 8: Synthesis of _{LiTa1.9Bi0.1PO8 sintered} body having a new crystal structure (raw oxide _for solution synthesis)
In a dry environment, 1.3612 _g of TaCl5 and 0.0631 g of _BiCl3 (manufactured by Kojundo Chemical Laboratory, 99.99% (hereinafter the same)) were dissolved in 50 mL of anhydrous 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 an LiOH aqueous solution. While stirring the _LiOH aqueous solution with a stirrer, the _TaCl5.BiCl3 solution and the _{NH4H2PO4 aqueous} solution were added in sequence and mixed at 80°C. The mixed solution contained 1.1 moles of LiOH compared to the composition LiTa _1.9 Bi _0.1 PO ₈ , that is, 10 mol % excess LiOH.

The mixed solution was dried at 120°C for 15 hours, and the dried and solidified powder was collected and lightly ground in an agate mortar. The ground powder was fired in an oxygen atmosphere at 500°C for 12 hours using a vacuum gas replacement type electric furnace to obtain a white powder of _{LiTa1.9Bi0.1PO8 ,} which is an _amorphous raw material. The white powder was wet ball milled using a planetary ball mill, and then uniaxially pressed using a tablet press to obtain a molded body. The molded body was heated to 1208°C in air using an electric furnace, fired for 5 minutes, and cooled to obtain a sintered precursor.

Then, this sintered precursor was sintered in an oxygen atmosphere at 1000°C for 12 hours using a vacuum gas _replacement type electric furnace to obtain a _{LiTa1.9Bi0.1PO8 sintered} body. When the crystal structure of this sintered body was examined using a powder X-ray diffraction device, it was confirmed that the main phase was a _LiTa2PO8 type crystal structure belonging to the monoclinic system _and space group C2/c. On the other hand, it was confirmed that a _BiPO4 phase was generated as an impurity phase in this sintered body. This _BiPO4 phase precipitated as a liquid phase during high-temperature sintering, and then functioned as a sintering aid through phase separation.

Example 9: Synthesis of _{LiTa1.8Bi0.2PO8 sintered} body having new crystal structure A LiTa1.8Bi0.2PO8 sintered body was obtained in the same manner as in Example 8, except that the amount of _TaCl5 used was changed to 1.2896g and the amount of _BiCl3 used was changed to _0.1261g . _The mixed solution in the middle contains 1.1 moles _of LiOH, i.e., _{10 mol} % excess, compared to the composition _{LiTa1.8Bi0.2PO8} . When the crystal structure of this sintered body was examined by a powder _X -ray diffraction device, it was confirmed that the main phase was a _{LiTa2PO8 type} crystal structure belonging to the space group C2/c in the monoclinic system.

On the other hand, it was confirmed that the _BiPO4 phase was significantly present as an impurity phase in this sintered body with an increase in the Bi content. This indicates that the _BiPO4 phase precipitated as a liquid phase during high-temperature sintering and then functioned as a sintering aid through phase separation. The powder X-ray diffraction pattern is shown in FIG. 15. The lithium ion conductivity of this sintered body 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 _{LiTa1.8Bi0.2PO8 sintered} body.

Example 10: Synthesis of _{Li1.1Ta1.9Hf0.1PO8 sintered} body _with new crystal structure In a dry environment, 1.3612g of _TaCl5 and 0.0641g of _HfCl4 (manufactured by Fujifilm Wako Pure Chemical Industries, 99.9% (same below)) were dissolved in 50mL of _anhydrous ethanol _to obtain a _TaCl5.HfCl4 solution. _0.2301g of _NH4H2PO4 was dissolved in _50mL of ion-exchanged water to obtain an _{NH4H2PO4 aqueous} solution. 0.1016g of _LiOH.H2O was dissolved in 100mL of _ion -exchanged water to obtain an LiOH aqueous solution. While stirring the LiOH aqueous solution with a stirrer, _{the TaCl5.HfCl4} solution and _{the NH4H2PO4 aqueous} solution were added in sequence and mixed at 80° C. This mixed solution contains 1.1 molar times, or 10 mol % excess, LiOH compared to _{the composition Li1.1Ta1.9Hf0.1PO8 .}

The mixed solution was dried at 120°C for 15 hours, and the dried and solidified powder was collected and lightly ground in an agate mortar. The ground powder was fired in an oxygen atmosphere at 600°C for 12 hours using a vacuum gas replacement type electric furnace to obtain a white powder of Li _1.1 Ta _1.9 Hf _0.1 PO ₈ , which is an amorphous raw material. The white powder was wet ball milled using a planetary ball mill, and then uniaxially pressed using a tablet molding machine to obtain a molded body. The molded body was heated to 1208°C in air using an electric furnace, fired for 5 minutes, and cooled to obtain a sintered precursor. The sintered precursor was then fired in an oxygen atmosphere at 1000°C for 12 hours using a vacuum gas replacement type electric furnace to obtain a Li _1.1 Ta _1.9 Hf _0.1 PO ₈ sintered body.

When the crystal structure of this sintered body was examined by a powder X-ray diffraction apparatus, it was confirmed that the main phase was a LiTa ₂ PO ₈ type crystal structure belonging to the monoclinic system and space group C2/c. On the other hand, it was confirmed that a LiTa ₃ O ₈ phase was generated in this sintered body. This LiTa ₃ O ₈ phase was formed by high-temperature sintering and functioned as a sintering aid. The powder X-ray diffraction pattern is shown in FIG. 16. In addition, the lithium ion conductivity of this sintered body 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 LiTa ₂ PO ₈ sintered body previously reported.

Example 11: Synthesis of _{Li1.2Ta1.8Hf0.2PO8 sintered} body having _a new crystal structure _Except for changing the amount of _TaCl5 to 1.2896g, the amount of _HfCl4 to 0.1281g, and the amount of _LiOH.H2O to _0.1108g , a _{Li1.2Ta1.8Hf0.2PO8} sintered body was obtained in the same manner as in Example _8. Note that the mixed solution during the process contains 1.1 moles of LiOH _, i.e., 10 mol% excess _, compared to the _{composition Li1.2Ta1.8Hf0.2PO8 .}

When the crystal structure of this sintered body was examined by a powder X-ray diffraction apparatus, it was confirmed that the main phase was a LiTa ₂ PO ₈ type crystal structure belonging to the monoclinic system and space group C2/c. On the other hand, it was confirmed that the LiTa ₃ O ₈ phase was significantly present in this sintered body with an increase in the amount of Hf substitution. This LiTa ₃ O ₈ phase was formed by high-temperature sintering and functioned as a sintering aid. It was also confirmed that the peak position of the powder X-ray diffraction pattern shifted to the lower angle side with an increase in the amount of Hf substitution. It was revealed that the lattice volume increased due to the Hf substitution of Ta.

As a result of 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 Hf substitution. That is, it was confirmed that not only the lattice volume but also the carrier concentration was increased by Hf substitution of Ta. Furthermore, the lithium ion conductivity of this sintered body 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 LiTa ₂ PO ₈ sintered body previously reported.

The reasons why the total lithium ion conductivity of the sintered bodies of Examples 10 and 11 was high include the maintenance of the crystal structure of the high temperature phase, the effect of the expansion of the crystal lattice and the increase in the carrier concentration (lithium amount) due to the Hf substitution, and the improvement in sinterability due to the function of LiTa ₃ O ₈ as a sintering aid. In addition, 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 LiTa ₂ PO ₈ sintered body of Example 6. In other words, it was revealed that the lithium ion conductivity at low temperatures such as -20°C was improved by the Hf substitution of Ta.

Example 12: Synthesis of _{LiTa1.8Sb0.2PO8} with _a new crystal structure In a dry environment, 1.2896g of _TaCl5 and 0.09125g of _SbCl3 (manufactured by Fujifilm Wako Pure Chemical Industries, special grade reagent) were dissolved in 50mL of _anhydrous ethanol to obtain a _TaCl5.SbCl3 solution. _0.2301g of _NH4H2PO4 was dissolved in _50mL of _ion -exchanged water to obtain _{an NH4H2PO4 aqueous} solution. 0.0923g of _LiOH.H2O was dissolved in 100mL of ion- _exchanged water to obtain an LiOH aqueous solution. While stirring this LiOH aqueous solution with a stirrer, the _{TaCl5.SbCl3 solution} and this _{NH4H2PO4 aqueous} solution were added in sequence and mixed at 80°C. The mixed solution contained 1.1 moles of LiOH, that is, 10 mol % excess, compared to the composition LiTa _1.8 Sb _0.2 PO _8.

The mixed solution was dried at 120°C for 15 hours, and the dried and solidified powder was collected and lightly ground in an agate mortar. The ground powder was fired in an oxygen atmosphere at 500°C for 12 hours using a vacuum gas replacement type electric furnace to obtain a white powder of LiTa _1.8 Sb _0.2 PO ₈ , which is an amorphous raw material. The white powder was wet ball milled using a planetary ball mill, and then uniaxially pressed using a tablet molding machine to obtain a molded body. The molded body was heated to 1208°C in air using an electric furnace, fired for 5 minutes, and cooled to obtain a sintered precursor. The sintered precursor was then fired in an oxygen atmosphere at 1000°C for 12 hours using a vacuum gas replacement type electric furnace to obtain a LiTa _1.8 Sb _0.2 PO ₈ sintered body.

When the crystal structure of this sintered body was examined by a powder X-ray diffraction apparatus, it was confirmed that the main phase was a LiTa ₂ PO ₈ type crystal structure belonging to the space group C2/c in the monoclinic system. On the other hand, it was confirmed that the TaPO ₅ phase was generated as an impurity phase in this sintered body. It was also confirmed that the peak position of this powder X-ray diffraction pattern was shifted to the lower angle side compared to the peak position of LiTa ₂ PO ₈ , and it was revealed that Sb, which has an ionic radius smaller than Ta, was substituted. This powder X-ray diffraction pattern is shown in FIG. 17. In addition, the lithium ion conductivity of this sintered body 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 LiTa _1.8 Sb _0.2 PO ₈ sintered body previously reported.

Example 13: Preparation of composite positive electrode The LiTa ₂ PO ₈ (LTPO) sintered body, LiTa _1.9 Bi _0.1 PO 8 (LTBPO) sintered body, and Li _1.1 Ta _1.9 Hf _0.1 PO ₈ (LTHPO) sintered body obtained in Examples ₆ , 7, and 10 were each pulverized to prepare a composite positive electrode used as an electrolyte for the composite positive electrode. As the positive electrode active material, LiCoO ₂ (manufactured by Nippon Kagaku Kogyo, Cellseed C-5H) was used, and the electrolyte powder and LiCoO ₂ powder were mixed in a weight ratio of 1:1 using an agate mortar, and then uniaxially pressed using a tablet molding machine to prepare a compact. The compact was fired in an argon gas atmosphere at 600 ° C. for 2 hours to prepare the composite positive electrode sintered compact.

The resistance value of the composite positive electrode compact thus produced was calculated from the arcs of the Nyquist plot using a frequency response analyzer. From the measurement results at room temperature, the ionic resistance of the compact was approximately 1×10 ⁷ Ω. Although these values are high for ionic resistance, it became clear that a conductive path was established within the composite positive electrode and that an increase in the interface resistance with the positive electrode active material was suppressed.

The powder X-ray diffraction patterns measured by pulverizing the composite positive electrode are shown in Figure 18. In both cases, the main phases were the LiTa ₂ PO ₈ type phase having the new crystal structure of the present invention and the LiCoO ₂ phase, and it was confirmed that there were almost no reaction products. From the above, it has become clear that the solid electrolyte of the present invention can be used as a composite positive electrode for an all-solid-state battery.

Example 14: Preparation of all-solid-state battery The sintered body obtained in Example 1 was pulverized to obtain a white powder. The white powder was then wet-milled using a planetary ball mill, and dried to obtain a LiTa ₂ PO ₈ sintered body powder, which was then used to prepare a dense sintered body consisting of an electrolyte layer and a composite electrode layer. That is, the LiTa ₂ PO ₈ sintered body powder was filled into a hot press mold (manufactured by AS ONE) with a diameter of 10 mm, and held at 400°C and 374 MPa for 2 hours using a hot press device (manufactured by AS ONE) to obtain a disk-shaped dense sintered body with a thickness of about 0.3 mm.

The dense sintered body and the mold constitute a composite electrode layer comprising a solid electrolyte layer and one of the electrode layers. Furthermore, a lithium ion conductive polymer electrolyte sheet and a metallic lithium sheet (thickness: 0.2 mm) which will be the other electrode layer were sequentially attached to the exposed surface of the solid electrolyte layer to produce an all-solid-state battery. A constant current charge/discharge test (current density 3 mA/g) was performed on this all-solid-state battery at 60°C using a charge/discharge tester (Hokuto Denko, HJ1020mSD8). As a result, the capacity corresponding to the charge/discharge reaction was observed, and the operation of the all-solid-state battery was confirmed.

By using LiTa ₂ PO ₈ having the novel crystal structure of the present invention and its element-substituted products, an electrolyte member having high lithium ion conductivity can be produced.

Claims (12)

An oxide sintered body containing lithium, tantalum, and phosphorus,
A sintered oxide having a crystal structure belonging to the monoclinic space group C2/c, in which lithium does not occupy the 4b site (0.5,0,0) at the Wyckoff position. In claim 1,
A sintered oxide represented by the general formula LiTa _2-x M _x PO ₈ (M is Bi or Sb, 0≦x≦0.2). In claim 1,
A sintered oxide represented by the general formula Li _1+y Ta _2-y Hf _y PO ₈ (0≦y≦0.2). In claim 1,
The oxide sintered body is LiTa ₂ PO ₈ composed of primary particles having a particle size of 50 μm to 100 μm. In claim 4,
A sintered oxide body of single crystal LiTa ₂ PO ₈ . In any one of claims 1 to 5,
An oxide sintered body in which lithium occupies only three or more 8f sites at the Wyckoff positions. In any one of claims 1 to 5,
A sintered oxide in which the lithium sites are disordered. A method for producing the oxide sintered body according to claim 1,
A method for producing an oxide sintered body, 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. The method for producing an oxide sintered body according to claim 2,
A method for producing an oxide sintered body, comprising a sintering step of firing an oxide represented by the general formula LiTa _2-x M _x PO ₈ (M is Bi or Sb, 0≦x≦0.2) at a temperature higher than 1200° C. and lower than 1400° C. The method for producing an oxide sintered body according to claim 3,
A method for producing an oxide sintered body, comprising a sintering step of firing an oxide represented by the general formula Li1 _{+yTa2 - yHfyPO8} (0≦y≦ _0.2 ) at a temperature higher than 1200°C and lower than 1400°C. In any one of claims 8 to 10,
A method for producing an oxide sintered body, wherein the oxide is amorphous. A solid electrolyte comprising the oxide sintered body according to any one of claims 1 to 5;
A pair of electrodes sandwiching the solid electrolyte;
An electrochemical device comprising:

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