WO2023243327A1 - Oxide sintered body and method for producing oxide sintered body - Google Patents

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

Oxide sintered body and method for producing oxide sintered body

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

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

Recently, it has been reported that 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. Precise crystal structure analysis has revealed that the skeleton structure of LiTa ₂ PO ₈ is constructed from TaO ₆ octahedrons and PO ₄ tetrahedra, and that lithium ions occupy the gaps between these. Since the arrangement of lithium forms a three-dimensional conduction path, LiTa ₂ PO ₈ enables good lithium ion conduction similar to that of garnet-type materials.

On the other hand, attempts have been made to improve the characteristics by elemental substitution of Ta using 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 element substitution. The firing temperature when producing the element-substituted LiTa ₂ PO ₈ sintered body was 1200° C. or lower, similar to the firing 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 the detailed lithium occupied sites and lithium ion conduction paths are Not only was there no mention of the difference between the two, but there was no consideration even of predicting the difference.

Usually, a high temperature firing method is used to produce a LiTa ₂ PO ₈ sintered body. However, LiTa ₂ PO ₈ contains lithium and phosphoric acid as main elements, which easily volatilize at high temperatures. In order to suppress the composition shift due to these volatilization and the generation of impurity phases due to decomposition, the upper limit of the firing temperature when producing LiTa ₂ PO ₈ has been set at 1200°C. Therefore, studies have been conducted to determine what type of lithium-occupied crystal structure the LiTa ₂ PO ₈ sintered body produced by firing at temperatures exceeding 1200°C has, and how the lithium ion conductive properties change. It was impossible to predict that there would be a change in the crystal structure.

Japanese Patent Application Publication No. 2020-194773 Japanese Patent Application Publication No. 2021-38100 JP 2021-38099 Publication

The present application was made in view of these circumstances, and provides an oxide sintered body with a crystal structure that can further increase lithium ion conductivity, and a method for manufacturing the oxide sintered body that can construct the crystal structure. The challenge is to provide.

The inventors of the present application carefully investigated the relationship between the firing temperature and crystal structure of the target LiTa ₂ PO ₈ , as well as starting materials and synthesis methods suitable for high-temperature firing. As a result, it was surprisingly found that calcination at temperatures exceeding 1200°C produces a crystalline phase with a lithium occupation mode that is advantageous for faster lithium ion diffusion. Furthermore, by firing in a closed system that can suppress the volatilization of lithium and phosphoric acid at high temperatures, LiTa ₂ PO ₈ can be fired without decomposing even when fired at 1300°C, and by such high-temperature firing, It has been found that the grain growth of LiTa ₂ PO ₈ becomes remarkable, and a sintered body and single crystal body having high bulk lithium ion conductivity can be obtained.

Furthermore, we continued to study the chemical composition of LiTa ₂ PO ₈ , and found that by substituting a different element in 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 the element, and that it can be heated to 1200 °C. It has been found that sintering properties can be improved by functioning as a sintering aid at higher temperature firings. As a result, we replaced Ta in LiTa ₂ PO ₈ and LiTa ₂ PO ₈ produced by such a manufacturing method with other elements, and changed the Li composition as necessary to produce element-substituted products (hereinafter, Ta is replaced with other elements). The total lithium ion conductivity of the dense sintered body of the high-temperature phase of (sometimes simply referred to as "element substituted product" is an element substituted product in which the Li composition is changed as necessary) is 2. ×10 ⁻⁴ S/cm or more, and the operation of an all-solid-state battery was confirmed, which was equipped with a solid electrolyte produced by low-temperature sintering using powder obtained by pulverizing the dense sintered body.

The oxide sintered body of the present application is an oxide sintered body containing lithium, tantalum, and phosphorus, has a monoclinic crystal structure belonging to space group C2/c, and has a 4b seat at the Wyckoff position. (0.5,0,0) is not occupied by lithium. The method for producing an 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 can be obtained.

FIG. 1 is a conceptual diagram of an all-solid-state lithium ion secondary battery that is an example of the electrochemical device of the present application. Single crystal X-ray diffraction pattern using a single crystal selected from the LiTa ₂ PO ₈ sintered body obtained in Example 1. 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. An electron microscope image of a fractured surface of the LiTa ₂ PO ₈ sintered body obtained in Example 1. An electron microscope image of a single crystal body selected from the LiTa ₂ PO ₈ sintered body obtained in Example 1. A stereoscopic microscope image of the LiTa ₂ PO ₈ single crystal obtained in Example 2. Powder X-ray diffraction patterns and partially enlarged views of LiTa ₂ PO ₈ sintered bodies obtained in Comparative Example 1, Example 3, and Example 4. An electron microscope image of a fractured 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. 3 is a graph showing AC impedance measurement results 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. An electron microscope image of a fractured surface of the LiTa _1.9 Bi _0.1 PO ₈ sintered body obtained in Example 7. 7 is a graph showing AC impedance measurement results 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. 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. 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 embodiment of the present application contains lithium, tantalum, and phosphorus. The oxide sintered body of this embodiment has a monoclinic crystal structure belonging to space group C2/c, and lithium does not occupy the 4b position (0.5, 0, 0) at the Wyckoff position. . Preferably, Lithium occupies only three or more 8f seats at the Wyckoff location. The lithium occupied seats may be occupied in a disorderly manner.

The oxide sintered body of this embodiment is an oxide sintered body represented by the general formula LiTa _2-x M _x PO ₈ (M is Bi or Sb, 0≦x≦0.2 (the same applies hereinafter)). , for example, LiTa _1.9 Bi _0.1 PO ₈ , LiTa _1.8 Bi _0.2 PO ₈ , LiTa _1.9 Sb _0.1 PO ₈ , and LiTa _1.8 Sb _0.2 PO ₈ . Further, as the oxide sintered body of this embodiment, an oxide sintered body represented by the general formula Li _1+y Ta _2-y Hf _y PO ₈ (0≦y≦0.2 (the same applies hereinafter)), for example, , 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 to bismuth, antimony, and hafnium, examples of the element that replaces Ta in LiTa ₂ PO ₈ include titanium, niobium, molybdenum, zirconium, tungsten, boron, aluminum, silicon, germanium, and gallium. Moreover, _LiTa2PO8 is mentioned as an oxide sintered compact of this _embodiment . In order to obtain higher lithium ion conductivity, LiTa ₂ PO ₈ 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, LiTa ₂ PO ₈ is preferably single crystal.

(Method for manufacturing oxide sintered body)
The method for manufacturing 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 "raw material oxide") is fired at a temperature higher than 1200°C and lower than 1400°C. The raw material oxide is an oxide containing lithium, tantalum, and phosphorus. Examples of oxides containing lithium, tantalum, and phosphorus include oxides represented by the general formula LiTa _2-x M _x PO ₈ , oxides 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 material oxide is obtained by dissolving various raw material compounds, such as metal salts and phosphoric acid compounds, in respective solvents, mixing these solutions to precipitate a precipitate, and firing the precipitate. However, as long as the metal components of various raw material compounds can be mixed uniformly at the atomic level and the raw material oxide can be produced, there are no particular restrictions on the method for producing the raw material oxide. For example, solution synthesis such as ordinary solid phase synthesis, coprecipitation method, sol-gel method, complex polymerization method, and hydrothermal synthesis method, as well as vacuum evaporation method, sputtering method, pulsed laser deposition method, and chemical vapor phase reaction method, etc. Raw material oxides can also be produced by gas phase reaction synthesis. The raw material oxide can also be produced by applying a mechanochemical reaction such as ball milling.

The various raw material compounds are not particularly limited as long as they contain lithium, tantalum, or phosphorus. Examples of raw material compounds that can be used include oxides, carbonates, hydroxides, nitrates, ammonium salts, ammonium hydrogen salts, halides such as chlorides, and oxidized chlorides. The raw material oxide can be produced, for example, by the following procedure. First, a 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 the 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 at least room temperature and at most the boiling point of the solvent. You can mix the raw material compound and the solvent and then leave it to stand still to wait for the raw material compound to dissolve in the solvent, or you can stir the mixture of the raw material compound and the solvent using a stirrer or a stirrer to accelerate the dissolution reaction. You may. Next, the respective raw material compound solutions are mixed. For example, pentavalent tantalum chloride, pentavalent niobium chloride, tetravalent hafnium chloride, trivalent bismuth chloride, and antimony chloride dissolve in ethanol, etc., but when mixed with an aqueous solution afterwards, they form a gelled precipitate. bring about things. By heating in this state, the gelled precipitate is dried and a precursor of the raw material oxide is generated. The heating method is not particularly limited, 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 50°C or higher and lower than the boiling point of the solvent, more preferably 80°C or higher and lower than the boiling point of the solvent.

Then, by firing this precursor, the desired raw material oxide is obtained. There are no particular restrictions on the firing method, and firing may be performed using an electrically heated muffle furnace, a mantle heater, or the like. The firing temperature is preferably 350° C. or higher to obtain powder of the raw material oxide, and may be 500° C. or higher and 1000° C. or lower. There are no particular restrictions on the container used for firing, and glass beakers, non-alumina ceramic containers, and the like can be used. For high-temperature firing of 400° C. or higher, gold containers, platinum containers, alumina containers, etc. can be used. The firing atmosphere is not particularly limited, and is usually an oxidizing gas atmosphere such as oxygen or air.

The firing time can be set depending on the firing temperature, etc., as long as residual substances such as nitrogen, chlorine, and carbon derived from various raw material compounds can be volatilized. There are no particular restrictions on the cooling method after firing, but usually natural cooling (in-furnace cooling) or slow cooling may be used. After firing and cooling, the raw material oxide may be pulverized if necessary, and the firing temperature may be changed and re-fired. The degree of pulverization can be adjusted depending on the firing 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.

Note that the raw material oxide may be prepared by calcining a mixture of Li, Ta, P, and an oxide containing only a part of the substitution element. In this mixture of oxides, a plurality of oxides are mixed so as to have a composition ratio of LiTa ₂ PO ₈ and its element substituted product after synthesis of the oxide sintered body. The pre-calcination temperature can be set depending on the composition of the oxide mixture, but is usually 500°C to 1200°C, preferably 900°C to 1000°C. Further, the pre-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 pre-baking time can be set depending on the pre-baking temperature, etc.

Next, the raw material oxide is fired. The main firing temperature is higher than 1200°C and lower than 1400°C, preferably higher than 1205°C and lower than 1300°C. In the main firing, the sintering reaction may proceed in one step at a temperature higher than 1200°C and lower than 1400°C, or the temperature may be raised higher than 1200°C and lower than 1400°C, and then sintered at a temperature lower than 1200°C. The sintering reaction may proceed in two stages. The second stage firing temperature at this time is usually 800°C or more and 1200°C or less, preferably 850°C or more and 1050°C or less. In this two-step sintering method, by using bismuth, gallium, aluminum, or boron, which form a liquid phase at high temperatures, as a replacement element, the first step of high-temperature sintering produces a liquid phase such as lithium salt. is generated, and in the second stage of sintering at a low temperature, this liquid phase such as lithium salt functions as a sintering aid, resulting in a highly dense oxide sintered body.

Further, the main 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 pulverized raw material oxide is placed during main firing is stable at high temperatures of 1200°C or higher, and the volatilization of lithium, phosphoric acid, bismuth, gallium, etc. at high temperatures is suppressed. For example, platinum, alumina, or zirconia.

There are no particular restrictions on the form of the raw material oxide to be fired, and it may be a powder, a plate-shaped compact formed by hydrostatic pressure or uniaxial pressure, or a plate-shaped body produced by coating or film-forming technology. Examples include membrane bodies. Coating techniques include screen printing, electrophoresis (EPD), doctor blading, spray coating, inkjet, and spin coating. Film deposition techniques include vapor deposition, sputtering, chemical vapor deposition (CVD), electrochemical vapor deposition, ion beam, laser ablation, atmospheric plasma deposition, and reduced pressure plasma deposition. can be mentioned.

Alternatively, a plate-like body or a film body of the raw material oxide may be produced using a method such as hot pressing, hot isostatic pressing, or electrical sintering. After the main firing, the obtained oxide sintered body may be pulverized by a known method if necessary, and the main firing may be repeated. The degree of pulverization can be set depending on the main firing temperature and the like. Alternatively, a molded body of the raw material oxide may be produced from a powdered raw material oxide that has been fired once.

(Solid electrolyte material)
The oxide sintered body obtained by the main firing can be used as a solid electrolyte material for electrochemical devices such as all-solid lithium ion secondary batteries. A solid electrolyte material may be produced by pulverizing the oxide sintered body, then molding and firing it again. Further, a molded body may be produced by mixing or compounding the powder of the oxide sintered body and another electrolyte material. On the other hand, if the oxide sintered body is a bulk 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 component of a positive electrode in order to ensure lithium ion conductivity in the electrode. That is, the positive electrode member is a molded product obtained by mixing or combining the oxide sintered body of the present application and the positive electrode material active material. As the positive electrode material active material, materials that are generally used as positive electrode materials for lithium ion secondary batteries can be used. For example, LiCoO ₂ , LiNiO ₂ , Li(Ni,Mn,Co)O ₂ , Li(Ni,Co,Al)O ₂ , Li ₂ MnO ₃ -Li(Ni,Mn,Co)O ₂ , Li(Ni, Oxides _{such as} Mn) _2O4 , Li(Co,Mn) _2O4 , Li(Mn,Al) _2O4 , _LiFePO4 , _LiMnPO4 , _LiCoPO4 , and _{LiNiPO4 ,} sulfur, and lithium sulfide, etc. It is mentioned as a positive electrode material active material.

Further, a material in which a lithium desorption/insertion reaction occurs reversibly at a high voltage of 2 V or higher 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, this positive electrode member may contain polymers, oxides, sulfides, hydrides, or halides. good. Further, for the purpose of improving the electron conductivity in the positive electrode, this positive electrode member may contain a conductive additive such as carbon black, carbon nanotubes, graphite, or titanium oxide.

(electrochemical device)
Since the oxide sintered body of the present application has excellent lithium ion conductivity, it can be used as a solid electrolyte for electrochemical devices such as all-solid 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 the solid electrolyte. Note that the pair of electrodes does not need to directly sandwich the solid electrolyte.

Furthermore, the oxide sintered body of the present application can also be used for a positive electrode member or a negative electrode member. FIG. 1 conceptually shows an all-solid-state lithium ion secondary battery that is an example of an electrochemical device according to an embodiment of the present application. The all-solid 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. There is. 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 the separator 5.

Example 1: Synthesis of LiTa ₂ PO ₈ sintered body with new crystal structure (1205°C firing)
In a dry environment, 1.4328 g of TaCl ₅ (manufactured by Rare Metallic, 99.9% (the same applies hereinafter)) was dissolved in 50 mL of absolute ethanol to obtain a TaCl ₅ solution. 0.2301 g of NH ₄ H ₂ PO ₄ (manufactured by Wako Pure Chemical Industries, Ltd., reagent grade (the same applies hereinafter)) was dissolved in 50 mL of ion-exchanged water to obtain an NH ₄ H ₂ PO ₄ aqueous solution. 0.0923 g of LiOH.H ₂ O (manufactured by Kojundo Kagaku Kenkyusho, 99% up (the same applies 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 TaCl ₅ solution and this NH ₄ H ₂ PO ₄ aqueous solution were sequentially added and mixed at 80°C. Note that this mixed solution contains LiOH in an amount of 1.1 mol times that of the composition LiTa ₂ PO ₈ , that is, an excess of 10 mol %.

This mixed solution was dried at 120° C. for 15 hours, the dried solidified powder was collected, and the solidified powder was lightly ground in an agate mortar. Using a vacuum gas displacement electric furnace (manufactured by Denken High Dental, KDF-75plus (the same applies hereinafter)), this pulverized powder was fired at 600°C in an oxygen atmosphere for 12 hours to obtain _LiTa2 , an amorphous raw material. A white powder of PO ₈ was obtained. This white powder is wet ball milled using a planetary ball mill (manufactured by Fritsch, P-7 (the same applies hereinafter)), and then molded by uniaxial pressure using a tablet molding machine (manufactured by JASCO (the same applies hereinafter)). I got a body. Using a tabletop high temperature muffle furnace (manufactured by Yamada Denki, SSFS-130-S (the same applies hereinafter)), this compact was fired in the air at 1205° C. for 240 hours to obtain a LiTa ₂ PO ₈ sintered body.

The grown grains were selected from this sintered body, and the crystal structure was examined using a single crystal X-ray diffraction device (Rigaku Corporation, R-AXIS RAPID-II (hereinafter the same)). As a result, it was confirmed that the grains were monoclinic and belonged to space group C2/c. The single crystal X-ray diffraction pattern is shown in FIG. 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 arrangement of Non-Patent Document 1 and Non-Patent Document 2 could not be refined, and that lithium did not occupy the 4b seat (0.5, 0, 0) at the Wyckoff position.

Furthermore, it became clear that lithium was arranged in the 8f seat, which is different from Non-Patent Documents 1 and 2. Figure 3 shows 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, using a model that introduced five 8f seats, unlike Non-Patent Documents 1 and 2. show. In addition, the size of the primary particles forming this sintered body was examined using a desktop scanning electron microscope (manufactured by JEOL Ltd., JCM-6000 (hereinafter the same)), and it was found to be about 50 μm to 100 μm. FIG. 4 shows an electron microscope image of the fractured surface of this sintered body, and FIG. 5 shows an electron microscope image of a single crystal body selected from this sintered body. FIG. 4 shows that this sintered body is a bulk body without clear grain boundaries.

Example 2: Synthesis of LiTa ₂ PO ₈ single crystal with a new crystal structure (calcined at 1310°C)
A LiTa ₂ PO ₈ 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 extracted 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 was monoclinic and belonged to space group C2/c. As a result of collecting single crystal X-ray diffraction data and performing crystal structure analysis in the same manner as in Example 1, it was found that the lithium arrangement in Non-Patent Documents 1 and 2 could not be refined, and that the 4b position (0 .5, 0, 0) was confirmed not to be occupied by lithium.

Furthermore, it has become clear that lithium is arranged in the 8f seat, which is different from Non-Patent Documents 1 and 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 previously reported structural models. Further, when the crystal size of this single crystal was examined using a stereomicroscope (manufactured by LEICA, S8APO), it was approximately 100 μm to 500 μm. A stereomicroscopic image of the selected single crystal is shown in FIG. Note that one scale in FIG. 6 is 0.1 mm.

Comparative Example 1: Synthesis of LiTa ₂ PO ₈ sintered body (1050°C firing)
Li ₂ CO ₃ (manufactured by Rare Metallic, 99.9% (the same below)), Ta ₂ O ₅ (manufactured by Rare Metallic, 99.99% (the same below)), and (NH ₄ ) ₂ HPO ₄ (manufactured by Fujifilm Wa (manufactured by Hikari Pure Chemical Industries, Ltd., Reagent Special Grade (hereinafter the same)) were weighed so that the material ratio (so-called molar ratio) of Li:Ta:P was 1.1:2:1. After crushing and mixing them using an agate mortar, they were heated sequentially at 450°C for 4 hours and at 600°C for 4 hours using an electric furnace (Yamato Scientific, FP100 (the same applies hereinafter)) to decompose the ammonium salt. The raw material was obtained.

This raw material was pulverized in anhydrous ethanol, collected and dried, and then pre-calcined for 4 hours at 900°C to 950°C using an electric furnace. The obtained calcined body was ball-milled and dried to obtain a raw material oxide powder. This powder was uniaxially pressed using a tablet molding machine to obtain a molded body. This molded body 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 this sintered body was investigated using a powder X-ray diffractometer (manufactured by Rigaku, SmartLab (hereinafter the same)), it was found that it was monoclinic and had a space group C2/ It was confirmed that it was almost a single phase of LiTa ₂ PO ₈ having a crystal structure belonging to c. This powder X-ray diffraction pattern is shown in FIG. 7 ("1050° C." in the figure). Further, when the particle size of the primary particles of this sintered body was examined using a desktop scanning electron microscope, it was found to be approximately several μm. FIG. 8 shows an electron microscope image of the fractured surface of this sintered body.

For this sintered body, the resistance value was determined from the arc of the Nyquist plot using a frequency response analyzer (FRA) (manufactured by Solartron, Model 1260 (the same applies hereinafter)), and the lithium ion conductivity was calculated from this resistance value. did. The impedance was measured 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 as the blocking electrode (the same applies hereinafter). From the measurement results at room temperature, the total lithium ion conductivity was calculated to be 2.1×10 ⁻⁴ S/cm, which generally agreed with previously reported values.

Example 3: Synthesis of LiTa ₂ PO ₈ sintered body with new crystal structure (calcined at 1205°C, raw material oxide for solid phase synthesis)
Using the solid-phase synthesized temporary sintered body obtained in Comparative Example 1, a sintered body was further synthesized by high-temperature firing. The pre-sintered body of Comparative Example 1 (diameter: approximately 9 mm, thickness: 1.0 to 1.3 mm) was placed in a platinum container and fired in the air at 1205°C for 4 hours in a tabletop high-temperature muffle furnace to obtain LiTa. _{A 2PO8} sintered body was obtained. When the crystal structure of this sintered body was examined using a powder X-ray diffractometer, it was found to be almost a single phase of LiTa ₂ PO ₈ , which has a monoclinic crystal structure belonging to the space group C2/c, and it does not decompose even when fired at high temperatures. It has become clear that these sintered bodies can be synthesized without using The powder X-ray diffraction patterns of these sintered bodies are shown in FIG. 7 ("1205° C." in the figure).

Example 4: Synthesis of LiTa ₂ PO ₈ sintered body with a new crystal structure (calcined at 1310°C, raw material oxide for 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. When the crystal structure of this sintered body was examined using a powder X-ray diffractometer, it was found to be almost a single phase of LiTa ₂ PO ₈ , which has a monoclinic crystal structure belonging to the space group C2/c, and it does not decompose even when fired at high temperatures. It has become clear that these sintered bodies can be synthesized without using The powder X-ray diffraction patterns of these sintered bodies are shown in FIG. 7 ("1310° C." in the figure).

Looking at each XRD pattern shown in FIG. 7 in detail, the peak positions of the sintered bodies of Example 3 and Example 4 are shifted to the lower angle side compared to the peak position of the sintered body of Comparative Example 1. It was confirmed that From the above results, the sintered bodies of Examples 3 and 4 have a skeleton structure formed by tantalum, phosphorus, and oxygen that is unchanged from the sintered body of Comparative Example 1, but the local structure around lithium is This suggests that the lattice volume is expanding. This result confirms that the local arrangement of lithium is changed by firing at a temperature higher than 1200°C.

Example 5: Synthesis of LiTa ₂ PO ₈ sintered body with new crystal structure (calcined at 1208°C, raw material oxide for solid phase synthesis)
First, a molded article was obtained in the same manner as in Comparative Example 1. Next, this molded body was heated to 1208° C. in air using an electric furnace, and then the temperature was lowered and fired at 1050° C. for 6 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 diffractometer, it was confirmed that it was a single phase of LiTa ₂ PO ₈ having a monoclinic crystal structure belonging to space group C2/c. This powder X-ray diffraction pattern is shown in FIG.

Furthermore, using the obtained powder X-ray diffraction data, the crystal structure of this sintered body was analyzed by the Rietveld method (using the program: Rietan-FP), and the detailed lithium arrangement was investigated. As a result, it was confirmed that the lithium arrangement of Non-Patent Document 1 and Non-Patent Document 2 could not be refined, and that lithium did not occupy the 4b seat (0.5, 0, 0) at the Wyckoff position. Furthermore, it has become clear that lithium is arranged in the 8f seat, which is different from those in Non-Patent Document 1 and Non-Patent Document 2. As a result of these structural analyzes, a crystal structure model equivalent to the results of the single crystal X-ray diffraction structure analyzes of Examples 1 and 2 was appropriate for this sintered body.

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

Example 6: Synthesis of LiTa ₂ PO ₈ sintered body with new crystal structure (1208°C firing, raw material oxide 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, fired for 5 minutes, and cooled to obtain a sintered precursor. This sintered precursor was then fired in an oxygen atmosphere at 1000° C. for 12 hours using a vacuum gas displacement electric furnace to obtain a LiTa ₂ PO ₈ sintered body.

When the crystal structure of this sintered body was examined using a powder X-ray diffractometer, it was confirmed that it was almost a single phase of LiTa ₂ PO ₈ with a monoclinic crystal structure belonging to space group C2/c. . This powder X-ray diffraction pattern is shown in FIG. Furthermore, as a result of SEM-EDS analysis (manufactured by JEOL Ltd., JCM-6000 (hereinafter the same)) of this sintered body, Ta and P were detected, and no metal elements other than Ta were detected. Furthermore, quantitative analysis of this sintered body using inductively coupled plasma (ICP) emission spectrometry (manufactured by Agilent, Agilent 5800 (the same applies hereinafter)) revealed that the chemical composition was approximately stoichiometric LiTa ₂ PO ₈ . was confirmed.

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

Example 7: Synthesis of LiTa _1.9 Bi _0.1 PO ₈ sintered body with new crystal structure (raw material oxide for solid phase synthesis)
Li ₂ CO ₃ , Ta ₂ O ₅ , Bi ₂ O ₃ (manufactured by Rare Metallic, 99.99%), and (NH ₄ ) ₂ HPO ₄ were prepared at a material ratio of Li:Ta:Bi:P of 1.1. :1.9:0.1:1. After pulverizing 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. This raw material oxide was pulverized in anhydrous ethanol, recovered and dried, and then uniaxially pressed using a tablet molding machine to obtain a molded product. This molded body was heated to 1208° C. in air using an electric furnace, and then the temperature was lowered and fired at 900° C. for 6 hours to obtain a LiTa _1.9 Bi _0.1 PO ₈ sintered body.

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 LiTa ₂ PO ₈ type crystal structure that was monoclinic and belonged to space group C2/c. On the other hand, it was confirmed that 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 calcination and then functioned as a sintering aid through phase separation. This powder X-ray diffraction pattern is shown in FIG. Further, when the particle size of the primary particles of this sintered body was examined using a tabletop scanning electron microscope, it was found to be approximately several μm to 10 μm. An electron microscope image of the fractured surface of this sintered body is shown in FIG. Compared to the sintered body of Comparative Example 1, although the main sintering temperature was lowered from 1050°C to 900°C, grain growth was remarkable, and BiPO ₄ precipitated at the grain boundaries was used as a sintering aid. It was confirmed that.

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 1.3×10 ⁻³ S/cm, which was higher than the total lithium ion conductivity of the previously reported LiTa ₂ PO ₈ sintered body. This is due to the fact that this sintered body maintains a high-temperature phase crystal structure, the effect of substituting a part of Ta with Bi, and the improvement in sinterability due to _BiPO4 functioning as a sintering aid. it is conceivable that. A Nyquist plot obtained from the results of this impedance measurement is shown in FIG.

Example 8: Synthesis of LiTa _1.9 Bi _0.1 PO ₈ sintered body with new crystal structure (raw material oxide for solution synthesis)
In a dry environment, dissolve 1.3612 g of TaCl ₅ and 0.0631 g of BiCl ₃ (manufactured by Kojundo Kagaku Kenkyusho, 99.99% (the same applies hereinafter)) in 50 mL of absolute ethanol to create a TaCl ₅ /BiCl ₃ solution. Obtained. 0.2301 g of NH ₄ H ₂ PO ₄ was dissolved in 50 mL of ion exchange water to obtain an aqueous NH ₄ H ₂ PO ₄ solution. 0.0923 g of LiOH·H ₂ O was dissolved in 100 mL of ion exchange water to obtain a LiOH aqueous solution. While stirring this LiOH aqueous solution with a stirrer, this TaCl ₅ .BiCl ₃ solution and this NH ₄ H ₂ PO ₄ aqueous solution were sequentially added and mixed at 80°C. Note that this mixed solution contains 1.1 times the amount of LiOH, that is, a 10 mol % excess of LiOH compared to the composition LiTa _1.9 Bi _0.1 PO ₈ .

This mixed solution was dried at 120° C. for 15 hours, the dried solidified powder was collected, and the solidified powder was lightly ground in an agate mortar. This pulverized powder was fired for 12 hours at 500°C in an oxygen atmosphere using a vacuum gas displacement electric furnace to obtain a white powder of LiTa _1.9 Bi _0.1 PO ₈ , which is an amorphous raw material. . This white powder was wet ball milled using a planetary ball mill, and then uniaxially pressed using a tablet molding machine to obtain a molded product. Using an electric furnace, this molded body was heated to 1208° C. in air, fired for 5 minutes, and cooled to obtain a sintered precursor.

This sintered precursor was then fired in an oxygen atmosphere at 1000° C. for 12 hours using a vacuum gas displacement electric furnace to obtain a LiTa _1.9 Bi _0.1 PO ₈ sintered body. 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 LiTa ₂ PO ₈ type crystal structure that was monoclinic and belonged to space group C2/c. On the other hand, it was confirmed that 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 calcination and then functioned as a sintering aid through phase separation.

Example 9: _Synthesis of LiTa _1.8 Bi _0.2 PO ₈ sintered body with new _crystal structure A LiTa _1.8 Bi _0.2 PO ₈ sintered body was obtained in the same manner as in Example 8 except for the following. Note that the intermediate mixed solution contains 1.1 times the amount of LiOH, that is, 10 mol % excess of LiOH compared to the composition LiTa _1.8 Bi _0.2 PO ₈ . 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 LiTa ₂ PO ₈ type crystal structure that was monoclinic and belonged to space group C2/c.

On the other hand, it was confirmed that in this sintered body, as the amount of Bi increased, BiPO ₄ phase was significantly present as an impurity phase. This indicates that this BiPO ₄ phase precipitated as a liquid phase during high-temperature calcination and then functioned as a sintering aid through phase separation. This powder X-ray diffraction pattern is shown in FIG. 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 7.1 × 10 ^-4 S/cm, which is higher than the previously reported total lithium ion conductivity of the LiTa _1.8 Bi _0.2 PO ₈ sintered body. Ta.

Example 10: Synthesis of Li _1.1 Ta _1.9 Hf _0.1 PO ₈ sintered body with new crystal structure 1.3612 g of TaCl ₅ and 0.0641 g of HfCl in 50 mL of absolute ethanol under dry environment ₄ (manufactured by Fuji Film Wako Pure Chemical Industries, Ltd., 99.9% (the same applies hereinafter)) to obtain a TaCl ₅ /HfCl ₄ solution. 0.2301 g of NH ₄ H ₂ PO ₄ was dissolved in 50 mL of ion exchange water to obtain an aqueous NH ₄ H ₂ PO ₄ solution. 0.1016 g of LiOH.H ₂ O 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 TaCl ₅ .HfCl ₄ solution and this NH ₄ H ₂ PO ₄ aqueous solution were sequentially added and mixed at 80°C. Note that this mixed solution contains 1.1 times the amount of LiOH, that is, a 10 mol% excess of LiOH compared to the composition Li _1.1 Ta _1.9 Hf _0.1 PO ₈ .

This mixed solution was dried at 120° C. for 15 hours, the dried solidified powder was collected, and the solidified powder was lightly ground in an agate mortar. Using a vacuum gas displacement electric furnace, this pulverized powder was fired at 600°C in an oxygen atmosphere for 12 hours to obtain a white powder of Li _1.1 Ta _1.9 Hf _0.1 PO ₈ , which is an amorphous raw material. I got a body. This white powder was wet ball milled using a planetary ball mill, and then uniaxially pressed using a tablet molding machine to obtain a molded product. Using an electric furnace, this molded body was heated to 1208° C. in air, fired for 5 minutes, and cooled to obtain a sintered precursor. Then, using a vacuum gas displacement electric furnace, this sintered precursor was fired at 1000°C in an oxygen atmosphere for 12 hours 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 using a powder X-ray diffractometer, it was confirmed that the main phase was a LiTa ₂ PO ₈ type crystal structure that was monoclinic and belonged to space group C2/c. On the other hand, it was confirmed that LiTa ₃ O ₈ phase was generated in this sintered body. This LiTa ₃ O ₈ phase was formed during high temperature firing and functioned as a sintering aid. This powder X-ray diffraction pattern is shown in FIG. 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 9.2×10 ⁻⁴ S/cm, which was higher than the total lithium ion conductivity of the previously reported LiTa ₂ PO ₈ sintered body.

Example 11: Synthesis of Li _1.2 Ta _1.8 Hf _0.2 PO ₈ sintered body having a new crystal structure The amount of TaCl ₅ used was 1.2896 g, the amount of HfCl ₄ used was 0.1281 g, A Li _1.2 Ta _1.8 Hf _0.2 PO ₈ sintered body was obtained in the same manner as in Example 8, except that the amount of LiOH·H ₂ O used was changed to 0.1108 g. Note that the mixed solution in the middle contains 1.1 times the amount of LiOH, that is, 10 mol % excess of LiOH compared to the composition Li _1.2 Ta _1.8 Hf _0.2 PO ₈ .

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

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 was increased 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 previously reported LiTa ₂ PO ₈ sintered body.

The reasons why the total lithium ion conductivity of the sintered bodies of Examples 10 and 11 were high were that the crystal structure of the high temperature phase was maintained, the expansion of the crystal lattice due to Hf substitution, and the carrier concentration (lithium amount). and the improvement in sinterability due to LiTa ₃ O ₈ functioning as a sintering aid. Furthermore, 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. The activation energy of the LiTa ₂ PO ₈ sintered body of No. 6 was smaller than that of the LiTa 2 PO 8 sintered body. That is, it has become clear that substitution of Ta for Hf improves lithium ion conductivity at low temperatures such as -20°C.

Example 12: Synthesis of LiTa _1.8 Sb _0.2 PO ₈ with a new crystal structure 1.2896 g TaCl ₅ and 0.09125 g SbCl ₃ (Fujifilm Wako Pure Chemical Industries, Ltd.) in 50 mL absolute ethanol in a dry environment A TaCl _{5/SbCl 3 solution was obtained by dissolving TaCl 5} /SbCl ₃ solution. 0.2301 g of NH ₄ H ₂ PO ₄ was dissolved in 50 mL of ion exchange water to obtain an aqueous NH ₄ H ₂ PO ₄ solution. 0.0923 g of LiOH·H ₂ O was dissolved in 100 mL of ion exchange water to obtain a LiOH aqueous solution. While stirring this LiOH aqueous solution with a stirrer, this TaCl ₅ .SbCl ₃ solution and this NH ₄ H ₂ PO ₄ aqueous solution were sequentially added and mixed at 80°C. Note that this mixed solution contains 1.1 times as much LiOH, ie, 10 mol% excess, as compared to the composition LiTa _1.8 Sb _0.2 PO ₈ .

This mixed solution was dried at 120° C. for 15 hours, the dried solidified powder was collected, and the solidified powder was lightly ground in an agate mortar. This pulverized powder was fired for 12 hours at 500°C in an oxygen atmosphere using a vacuum gas displacement electric furnace to obtain a white powder of LiTa _1.8 Sb _0.2 PO ₈ , which is an amorphous raw material. . This white powder was wet ball milled using a planetary ball mill, and then uniaxially pressed using a tablet molding machine to obtain a molded product. Using an electric furnace, this molded body was heated to 1208° C. in air, fired for 5 minutes, and cooled to obtain a sintered precursor. This sintered precursor was then fired in an oxygen atmosphere at 1000° C. for 12 hours using a vacuum gas displacement 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 using a powder X-ray diffractometer, it was confirmed that the main phase was a LiTa ₂ PO ₈ type crystal structure that was monoclinic and belonged to space group C2/c. On the other hand, it was confirmed that 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 clear that Sb, which has an ionic radius smaller than Ta, was substituted. became. This powder X-ray diffraction pattern is shown in FIG. 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 4.3 × 10 ^-4 S/cm, which is higher than the previously reported total lithium ion conductivity of the LiTa _1.8 Sb _0.2 PO ₈ sintered body. Ta.

Example 13: Production of composite positive electrode LiTa ₂ PO ₈ (LTPO) sintered body and LiTa _1.9 Bi _0.1 PO ₈ (LTBPO) sintered body obtained in Example 6, Example 7, and Example 10 , Li _1.1 Ta _1.9 Hf _0.1 PO ₈ (LTHPO) sintered bodies were respectively pulverized to produce a composite positive electrode that was used as an electrolyte of the composite positive electrode. LiCoO ₂ (Cellseed C-5H, manufactured by Nihon Kagaku Kogyo) was used as the positive electrode active material. After mixing the electrolyte powder and LiCoO ₂ powder at a weight ratio of 1:1 using an agate mortar, the mixture was mixed using a tablet molding machine. A green compact was produced using uniaxial pressure. This compacted compact was baked at 600° C. for 2 hours in an argon gas atmosphere to produce the composite positive electrode sintered compact.

The resistance value of the produced composite positive electrode molded body 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 in terms of ionic resistance, it is clear that a conductive path is maintained within the composite positive electrode and that an increase in interfacial resistance with the positive electrode active material is suppressed.

FIG. 18 shows a powder X-ray diffraction pattern measured by crushing the composite positive electrode. In both cases, it was confirmed that only the LiTa ₂ PO ₈ type phase having the new crystal structure of the present invention and the diffraction pattern derived from the LiCoO ₂ phase were the main phases, and almost no reaction products were present. From the above, it has become clear that the solid electrolyte of the present invention can be used as a composite positive electrode of an all-solid-state battery.

Example 14: Production of all-solid-state battery The sintered body obtained in Example 1 was pulverized to obtain a white powder. This white powder was wet-ball-milled using a planetary ball mill, and then dried to produce a dense sintered body consisting of an electrolyte layer and a composite electrode layer using LiTa ₂ PO ₈ sintered body powder. That is, this LiTa ₂ PO ₈ 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. and 374 MPa for 2 hours using a heat press machine (manufactured by As One). A disk-shaped dense sintered body with a thickness of about 0.3 mm was obtained.

This dense sintered body and mold constitute a composite electrode layer comprising a solid electrolyte layer and one electrode layer. Furthermore, a lithium ion conductive polymer electrolyte sheet and a metal lithium sheet (thickness: 0.2 mm), which will become the other electrode layer, are successively pasted on the surface of the exposed solid electrolyte layer to create an all-solid-state battery. did. A constant current charge/discharge test (current density 3 mA/g) was conducted on this all-solid battery at 60° C. using a charge/discharge test device (manufactured by Hokuto Denko, HJ1020mSD8). As a result, the capacity corresponding to the charge/discharge reaction was observed, confirming the operation of the all-solid-state battery.

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

Claims (12)

1. An oxide sintered body containing lithium, tantalum, and phosphorus,
   An oxide sintered body having a monoclinic crystal structure belonging to space group C2/c, and in which lithium does not occupy the 4b position (0.5,0,0) at the Wyckoff position.
2. In claim 1,
   An oxide sintered body represented by the general formula LiTa _2-x M _x PO ₈ (M is Bi or Sb, 0≦x≦0.2).
3. In claim 1,
   An oxide sintered body represented by the general formula Li _1+y Ta _2-y Hf _y PO ₈ (0≦y≦0.2).
4. In claim 1,
   An oxide sintered body of LiTa ₂ PO ₈ composed of primary particles with a particle size of 50 μm to 100 μm.
5. In claim 4,
   An oxide sintered body that is single crystal LiTa ₂ PO ₈ .
6. In any one of claims 1 to 5,
   An oxide sintered body in which lithium occupies only three or more 8f seats at the Wyckoff position.
7. In any one of claims 1 to 5,
   An oxide sintered body in which lithium occupancy is disordered.
8. A method for producing an oxide sintered body according to claim 1, comprising:
   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 lower than 1400°C.
9. A method for producing an oxide sintered body according to claim 2, comprising:
   An oxidation process that includes 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. A method for producing a sintered body.
10. A method for producing an oxide sintered body according to claim 3, comprising:
    An oxide sintered body having a sintering process 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 lower than 1400°C. manufacturing method.
11. In any one of claims 8 to 10,
    A method for producing an oxide sintered body, wherein the oxide is amorphous.
12. 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 with

Images