Ion conductors, ion-conducting sintered bodies, precursor solutions, precursor powders, and methods for producing the same.

Abstract

To provide an ion conductor comprising oxides based on the β-Li3PO4 type crystal structure, featuring improved lithium ion conductivity.SOLUTION: An ion conductor comprises, in its main phase, a crystal phase of the β-Li3PO4 type crystal structure, which comprises Li, Si, and P in the crystal lattice, with the Si / (Si+P) atomic ratio of 0.05 or more and 0.40 or less. The ion conductor is obtained by firing, at e.g., 150-550°C, a precursor derived from a colloidal solution that is prepared by mixing a water-soluble lithium compound with the Li, Si, and P composition ratio adjusted within a predetermined range, silicon alkoxide, and a water-soluble phosphorus compound in an aqueous solvent.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an oxide-based ionic conductor exhibiting lithium ion conductivity and an ion conductive sintered body. The present invention also relates to a precursor solution and precursor powder suitable for synthesizing the above ionic conductor and ion conductive sintered body. The present invention also relates to methods for producing these. [Background Art]

[0002] One example of an oxide-based ion conductive material that has been studied for application to all-solid-state lithium ion secondary batteries is an oxide called LISICON, which has a γ-Li3PO4-type crystal structure. Although the ion conductivity of γ-Li3PO4 itself is low, it is known that lithium ion conductivity can be improved by forming an oxide of the type represented by the general formula γ-Li 3+x Si x P 1-x O4 (where 0<x<1) in which Li4SiO4 is solid-dissolved in the γ-Li3PO4-type crystal structure (see, for example, Non-Patent Document 1).

[0003] Patent Document 1 describes that by using SiO2 as the Si source, synthesizing a precursor mixture through a hydrothermal reaction at 170°C and 1.0 MPa, and then calcining the dried product of the precursor mixture at 700°C, LISICON-type crystal particles of Li 3.5 Si 0.5 P 0.5 O4 single phase (Example 1), Li 3.9 Si 0.9 P 0.1 O4 single phase (Example 2), and Li 3.1 Si 0.1 P 0.9 O4 single phase (Example 3) were obtained (Example 1). The discharge capacity has been measured for an all-solid-state lithium ion secondary battery (with a lithium foil negative electrode) formed by hand pressing using these particles in the solid electrolyte layer (paragraphs 0059 and 0060).

[0004] On the other hand, it is known that besides γ-Li3PO4 which is a high-temperature phase, Li3PO4 oxide also has β-Li3PO4 which is a low-temperature phase. Non-Patent Document 2 describes that crystal structure analysis by X-ray diffraction was performed on β-Li₃PO₄ synthesized by a wet reaction at 40°C and γ-Li₃PO₄ synthesized by a dry method involving rapid quenching of a mixed melt containing a lithium compound and a phosphorus compound at 775°C. Regarding ionic conductivity, it is stated that γ-Li₃PO₄ has higher ionic conductivity than β-Li₃PO₄. Non-Patent Document 3 describes an experiment investigating the phase transition to γ-Li₃PO₄ by heating β-Li₃PO₄ synthesized by a wet reaction at room temperature to various temperatures. According to the document, the aforementioned phase transition generally occurs around 450°C, and it is pointed out that as a characteristic point of the X-ray diffraction pattern obtained with Cu-Kα radiation, a diffraction peak (arrow in Fig. 7) is observed near 2θ=20° in the γ-Li₃PO₄ phase. It is also shown that the phase transition from β-Li₃PO₄ to γ-Li₃PO₄ occurs as a continuous structural change (Fig. 8). [Prior Art Literature] [Patent Literature]

[0005] [Patent Literature 1] Japanese Unexamined Patent Publication No. 2020-102374 [Non-Patent Literature]

[0006] [Non-Patent Literature 1] Yue Deng, et al. Structural and Mechanistic Insights into Fast Lithium-Ion Conduction in Li4SiO4-Li3PO4 Solid Electrolytes. Journal of the American Chemical Society 2015, 137, 9136-9145. [Non-Patent Literature 2] Nur IP Ayu, et al. Crystal structure analysis of Li3PO4 powder prepared by wet chemical reaction and solid-state reaction by using X-ray diffraction (XRD). Ionics, DOI 10.1007 / s11581-016-1643-z, published online: 25 January 2016. [Non-Patent Document 3] Norikazu Ishigaki, et al. Room temperature synthesis and phase transformation of lithium phosphate Li3PO4 as solid electrolyte. Journal of Asian Ceramic Societies 2021, Vol. 9, No. 2, 452-458. [Overview of the initiative] [Problems that the invention aims to solve]

[0007] In all-solid-state secondary batteries, ion conductors are used not only as constituent materials for the solid electrolyte layer, but also, for example, as materials responsible for ion conduction between positive electrode active material particles. Oxide-based ion conductors have the advantage of avoiding the risk of toxic gas generation that is a problem with sulfide-based ion conductors. Because oxide powders are hard, the industrial manufacturing process of all-solid-state lithium-ion secondary batteries using oxide-based ion conductors generally involves a sintering process. In this sintering process, the heating temperature is limited to prevent adverse effects such as reactions with the electrode active material, and it is desirable to sinter at a temperature range of, for example, 550°C or lower, or even lower, 450°C or lower. The aforementioned ion conductor material called LISICON, which improves ion conductivity by solid-solventing Li4SiO4 in a γ-Li3PO4 type crystal structure, needs to be synthesized and sintered at high temperatures, for example, 600°C or higher, in order to obtain a well-crystallinating oxide structure. Therefore, the industrial widespread adoption of all-solid-state lithium-ion secondary batteries using oxides based on the γ-Li3PO4 type structure is considered difficult at present.

[0008] On the other hand, the low-temperature phase, β-Li3PO4, has poor ionic conductivity. The present invention aims to provide an ion conductor with improved lithium ion conductivity in an oxide based on a β-Li3PO4 type crystal structure. [Means for solving the problem]

[0009] As mentioned above, in the case of γ-Li3PO4, the high-temperature phase of Li3PO4 oxide, lithium ion conductivity can be improved by creating an oxide type in which Li4SiO4 is solid-solved in the crystal. However, for β-Li3PO4, the low-temperature phase, an oxide synthesis method that achieves such solid solution has not been established. Furthermore, the ion conduction behavior of oxides with such a solid-solution type β-Li3PO4 crystal structure is also unknown.

[0010] As a result of their research, the inventors discovered that by using a Li, Si, and P-containing material produced by a wet reaction process (aqueous solution synthesis) using silicon alkoxide as a Si source as a precursor, it is possible to synthesize a crystalline phase with a β-Li3PO4 type structure containing Li, Si, and P in the crystal lattice at a low calcination temperature, for example, below 550°C. Furthermore, it was confirmed that this type of oxide exhibits lithium-ion conductivity that is considered applicable to solid electrolytes in all-solid-state lithium-ion secondary batteries. The present invention is based on these findings. This specification discloses the following inventions.

[0011] [1] An ionic conductor having a β-Li3PO4 type crystalline phase as its main phase, with a Si / (Si+P) atomic ratio of 0.05 to 0.40. [2] An ion-conducting sintered body using the ion conductor described in [1] above. [3] A precursor solution comprising an aqueous solution in which colloidal particles containing Li, Si, and P are suspended, wherein when the aqueous solution is evaporated to dryness at 70°C and then heated at 150°C for 2 hours to obtain a solid, the solid obtained is subjected to a heat treatment by heating at 400°C in air for 12 hours, the crystalline phase having a β-Li3PO4 type structure containing Li, Si, and P in its crystal lattice is formed. [4] The precursor solution of the ion conductor described in [3] above, wherein the colloidal particles are a mixed product of an aqueous solvent, a water-soluble lithium compound, a silicon alkoxide, and a water-soluble phosphorus compound. [5] A precursor powder comprising particles containing Li, Si, and P, wherein when the powder is subjected to calcination by heating at 400°C in air for 12 hours, a crystalline phase with a β-Li3PO4 type structure containing Li, Si, and P in its crystal lattice is formed, which is a precursor powder for an ion conductor. [6] The precursor powder is the precursor powder of the ion conductor described in [5] above, which is a solvent removal component of an aqueous solution in which colloidal particles containing Li, Si, and P are suspended. [7] The precursor powder is the precursor powder of the ion conductor described in [5] or [6] above, wherein the content of Li, Si, and P satisfies the following formulas (1) and (2). 0.05≦Si / (Si+P)≦0.40 …(1) 0.90(4Si+3P)≦Li≦ 1.10 (4Si+3P) …(2) Here, in equations (1) and (2), the element symbols Si, P, and Li are replaced with the ratios (atomic ratios) of the amounts of Si, P, and Li atoms (moles) to the total amount (moles) of Si, P, and Li atoms present in the powder, respectively. [8] A method for producing an ion conductor, comprising: generating a precursor powder by removing solvent components from a suspension of colloidal particles obtained by mixing a water-soluble lithium compound, a silicon alkoxide, and a water-soluble phosphorus compound in an aqueous solvent, and then heating and holding the precursor powder in an oxidizing atmosphere at a temperature range of 120°C to 550°C to produce a crystalline phase of the β-Li3PO4 type structure containing Li, Si, and P in the crystal lattice. [9] The method for producing an ion conductor according to [8] above, wherein the precursor powder satisfies the following formulas (1) and (2) in terms of the content of Li, Si, and P. 0.05≦Si / (Si+P)≦0.40 …(1) 0.90(4Si+3P)≦Li≦ 1.10 (4Si+3P) …(2) Here, in equations (1) and (2), the element symbols Si, P, and Li are replaced with the ratios (atomic ratios) of the amounts of Si, P, and Li atoms (moles) to the total amount (moles) of Si, P, and Li atoms present in the powder, respectively.

[10] A method for producing an ion-conducting sintered body, comprising: a molded powder body containing particles of precursor powder formed by removing solvent components from a suspension of colloidal particles obtained by mixing a water-soluble lithium compound, a silicon alkoxide, and a water-soluble phosphorus compound in an aqueous solvent; heating and holding the molded powder body, which contains these particles, in an oxidizing atmosphere at a temperature range of 120°C to 550°C while applying pressure, thereby generating a crystalline phase of a β-Li3PO4 type structure containing Li, Si, and P in the crystal lattice and obtaining a sintered body.

[11] A calcination step to obtain an ion conductor powder is obtained by a calcination step in which a precursor powder formed by removing solvent components from a suspension of colloidal particles obtained by mixing a water-soluble lithium compound, a silicon alkoxide, and a water-soluble phosphorus compound in an aqueous solvent is heated and held in an oxidizing atmosphere at a temperature range of 120°C to 550°C to generate a crystalline phase of the β-Li3PO4 type structure containing Li, Si, and P in the crystal lattice, and A sintering step is performed to obtain a sintered body by heating and holding a molded powder containing the ion conductor powder particles in a temperature range of 120°C to 550°C while applying pressure. A method for manufacturing an ion-conducting sintered body having [a certain characteristic].

[12] A method for manufacturing an ion-conducting sintered body according to

[11] above, wherein the firing process and the sintering process are carried out under the conditions that T1 ≥ T0 - 50 and 120 ≤ T1 ≤ 550, where T0 (°C) is the highest temperature reached in the firing process and T1 (°C) is the highest temperature reached in the sintering process.

[13] A method for producing a precursor solution for an ion conductor, comprising the step of forming a colloidal solution by mixing a water-soluble lithium compound, a silicon alkoxide, and a water-soluble phosphorus compound in an aqueous solvent in quantitative proportions of Li, Si, and P that satisfy the following formulas (3) and (4). 0.05 ≤ x ≤ 0.45 …(3) 1.05A ≤ Si ≤ 1.50A …(4) Here, the x value in equation (3) and the A value in equation (4) are determined by equations (5) and (6) below, respectively. x = 1 - 4P / (Li + P) …(5) A = xP / (1-x) …(6) However, in equations (4), (5), and (6), the element symbols Si, P, and Li are replaced with the ratios (atomic proportions) of the amounts of Si, P, and Li atoms in the powder to the total amount (moles) of Si, P, and Li atoms present in the powder, respectively.

[14] A precursor solution preparation step involves mixing a water-soluble lithium compound, a silicon alkoxide, and a water-soluble phosphorus compound in an aqueous solvent in quantitative proportions satisfying the following formulas (3) and (4) to form a colloidal solution. A precursor powder synthesis step, in which solvent components are removed from the colloidal solution to obtain a solid, A method for producing an ion conductor precursor powder having the following characteristics. 0.05 ≤ x ≤ 0.45 …(3) 1.05A ≤ Si ≤ 1.50A …(4) Here, the x value in equation (3) and the A value in equation (4) are determined by equations (5) and (6) below, respectively. x = 1 - 4P / (Li + P) …(5) A = xP / (1-x) …(6) However, in equations (4), (5), and (6), the element symbols Si, P, and Li are replaced with the ratios (atomic proportions) of the amounts of Si, P, and Li atoms in the powder to the total amount (moles) of Si, P, and Li atoms present in the powder, respectively. [Effects of the Invention]

[0012] According to the present invention, an ion conductor with improved lithium ion conductivity is provided in an oxide based on a β-Li3PO4 type crystal structure. This ion conductor and sintered bodies using it can be obtained in a relatively low temperature range, for example, 120 to 550°C. Furthermore, since this ion conductor is oxide-based, the risk of toxic gas generation, which is a problem with conventional sulfide-based ion conductors, can be avoided. Therefore, the ion conductor of the present invention is highly useful as a constituent material for all-solid-state lithium-ion secondary batteries. [Brief explanation of the drawing]

[0013] [Figure 1] This figure illustrates the X-ray diffraction pattern of calcined powder obtained by calcining a precursor powder (nominal x-value = 0.2) by holding it in air at various temperature levels from 150 to 700°C for 12 hours. [Modes for carrying out the invention]

[0014] [Ionic conductors] The "ionic conductor" that is the subject of the present invention is a substance having lithium ion conductivity, and can be synthesized by firing a precursor substance. This ionic conductor has, as a main phase, a crystal phase of β-Li3PO4-type structure containing Li, Si and P in its crystal lattice. The "crystal phase of β-Li3PO4-type structure" means a crystal phase in which diffraction peaks from crystal planes corresponding to the diffraction peaks of β-Li3PO4 oxide crystals are observed in an X-ray diffraction pattern. The "crystal phase of β-Li3PO4-type structure containing Li, Si and P in the crystal lattice" constituting the ionic conductor that is the subject of the present invention contains Si in the crystal lattice in addition to Li and P. According to studies by the inventors, in a comparison between crystal phases of β-Li3PO4-type structure obtained by firing at the same temperature, it was observed that as the Si content increases, the volume of the unit cell tends to increase until the Si / (Si+P) atomic ratio reaches approximately 0.4. Further, the Li, Si and P composition ratio of the oxide powder synthesized by the method described later is Li 3+x Si x P 1-x O4 (where 0<x<1), which was close to the above formula. From these results, it is inferred that the "crystal phase of β-Li3PO4-type structure" containing Si that is the subject of the present invention is an oxide phase of a type in which Li4SiO4 forms a solid solution in the β-Li3PO4 crystal.

[0015] When the mass proportion of the "crystal phase of β-Li3PO4-type structure containing Li, Si and P in the crystal lattice" accounts for at least 50% of the total mass of the substance that is the ionic conductor, it can be said that the ionic conductor "has as a main phase" the crystal phase of β-Li3PO4-type structure containing Li, Si and P in the crystal lattice. When the remainder other than the main phase is referred to as a "heterogeneous phase", mixing of heterogeneous phases is permissible within a range that does not impair the effects of the present invention (particularly, the ion conductivity and avoidance of the risk of toxic gas generation), but a smaller amount of heterogeneous phase is preferable. For example, the mass proportion of heterogeneous phases is preferably 10% or less. One embodiment in which the mass proportion of heterogeneous phases is 10% or less is an ionic conductor that has, as a main phase, a crystal phase of β-Li3PO4-type structure containing Li, Si and P in the crystal lattice, and the remainder consists of impurity phases inevitably mixed during production, and such an ionic conductor can be obtained by the production method described later.

[0016] The "ion conductor" that is the subject of the present invention has a Si / (Si+P) atomic ratio, which represents the atomic ratio of the amount of silicon to the total amount of silicon and phosphorus, of 0.05 or more and 0.40 or less. This Si / (Si+P) atomic ratio is based on the Si content and P content in the ion conductor including heterogeneous phases. In a substance having a crystal phase of β-Li₃PO₄-type structure as the main phase, when the Si / (Si+P) atomic ratio is 0.05 or more, a clear improvement in ionic conductivity is observed. On the other hand, it is considered that there is a limit to the solid solution amount of Li₄SiO₄ in crystals of β-Li₃PO₄-type structure. According to the findings obtained by the inventors so far, it has been found that when the Si / (Si+P) atomic ratio in a substance having a β-Li₃PO₄-type crystal phase as the main phase exceeds 0.40, the mixed amount of heterogeneous phases increases, and a large amount of Li and Si supplied from the used raw materials are consumed for forming heterogeneous phases. Therefore, in the present invention, the Si / (Si+P) atomic ratio in the ion conductor is defined as 0.05 or more and 0.40 or less. It is more preferable that the Si / (Si+P) atomic ratio in the ion conductor is 0.15 or more and 0.35 or less.

[0017] As a preferred embodiment of the ion conductor that is the subject of the present invention, those satisfying the following formulas (1) and (2) in the contents of Li, Si and P can be exemplified. 0.05 ≦ Si / (Si+P) ≦ 0.40 …(1) 0.90(4Si+3P) ≦ Li ≦ 1.10 (4Si+3P) …(2) Here, for the positions of the element symbols Si, P and Li in formulas (1) and (2), the value of the ratio (atomic ratio) of the amount (mol) of Si atoms, P atoms and Li atoms to the total amount (mol) of Si atoms, P atoms and Li atoms present in the powder is substituted respectively. Formula (1) defines the Si / (Si+P) atomic ratio described above. "4Si+3P" in formula (2) corresponds to the composition formula Li 3+x Si x P 1-x O₄ (where 0 < x < 1), which represents the amount (mol) of Li in the stoichiometric composition corresponding thereto as a function of the amount (mol) of Si and the amount (mol) of P. Specifically, the composition formula Li3+x Si x P 1-x When O4 is present, the ratio of Li (moles) : Si (moles) = (3 + x) : x, so Li (moles) = (3 + x) × Si (moles) / x. Substituting x = Si (moles) / (Si (moles) + P (moles)) into the above equation and rearranging, we get Li (moles) = 4 × Si (moles) + 3 × P (moles). In other words, equation (2) above indicates that the amount of Li in the ionic conductor can vary within a range of ±10% from the stoichiometric amount determined by the amounts of Si and P. Furthermore, when obtaining an oxide powder having a β-Li3PO4 type crystalline phase as the main phase by calcining a precursor powder, the composition ratio of Li, Si, and P in the precursor powder and the oxide powder derived therefrom will be approximately the same. Therefore, to obtain one preferred embodiment of an ion conductor satisfying equations (1) and (2) above from a precursor powder, it is sufficient to use a precursor powder with a composition that satisfies equations (1) and (2) above.

[0018] Even when using a precursor material that can synthesize a β-Li3PO4 type crystalline phase containing Li, Si, and P in the crystal lattice, raising the calcination temperature to around 600°C reveals a diffraction peak around 2θ=20 degrees (see Non-Patent Literature 3) characteristic of γ-Li3PO4 type crystals in the X-ray diffraction pattern using Cu-Kα rays. Since the phase transition from the β-Li3PO4 type structure to the γ-Li3PO4 type structure is thought to appear as a continuous structural change, a very small peak, or a minute intensity change that is difficult to call a peak, may be observed around 2θ=20 degrees in the very early stages when the structural change to the γ-Li3PO4 type structure begins. Oxides with a γ-Li3PO4 type structure containing Si in the crystal lattice are known to inherently exhibit ionic conductivity, and from the viewpoint of enjoying good ionic conductivity, there is no problem even at the stage when the structural change to the γ-Li3PO4 type structure begins. Therefore, if a diffraction peak corresponding to the diffraction peak of the β-Li3PO4 oxide crystal is observed from the crystal plane in the Cu-Kα X-ray diffraction pattern, and at the same time a very small peak, or a minute intensity change that is so small that it is difficult to call it a peak, is observed around 2θ=20 degrees, then it can be considered to have a β-Li3PO4 type structure. More specifically, a crystalline phase consisting of an oxide phase formed by holding the precursor described later in a temperature range of 550°C or less is considered to be a crystalline phase with a β-Li3PO4 type structure containing Li, Si, and P in the crystal lattice.

[0019] Figure 1 shows an example of the X-ray diffraction pattern of calcined powder, which is an ion conductor obtained by calcining the precursor powder (nominal x-value = 0.2) used in Examples 5 to 8 described later, by holding it in air at various temperature levels from 150 to 700°C for 12 hours. The meaning of the nominal x-value will be explained later. The X-ray diffraction measurement was performed by the following method.

[0020] (Measurement of X-ray diffraction pattern) For calcined powder samples, the X-ray diffraction pattern was measured using an X-ray diffractometer (Rigaku Corporation, SmartLab) under the following conditions: Cu-Kα rays, tube voltage 40kV, tube current 30mA, measurement step 0.01 degrees, and scan speed 2 degrees / minute.

[0021] As can be seen from Figure 1, when the firing temperature is 600°C or higher, a peak that is considered to be attributed to the (011) crystal plane of the γ-Li₃PO₄-type structure was observed around 2θ = 20 degrees. It was confirmed that when the firing temperature is lower than this, an oxide having a β-Li₃PO₄-type structure is obtained. In addition, the presence of a small amount of hetero-phase considered to be a Li₂CO₃ phase was recognized in the fired powder fired at each temperature. Note that among the X-ray diffraction patterns illustrated in Figure 1, the examples for firing temperatures of 150°C, 300°C, 400°C, 500°C and 600°C correspond to Example 5, Example 6, Example 7, Example 8 and Comparative Example 1 described later, respectively.

[0022] (Nominal x value) The nominal x value is calculated from the charged amounts of Li and P in the aqueous solution used for synthesizing the precursor to obtain the composition formula Li 3+x Si x P 1-x O₄ (where 0 < x < 1) is a value uniquely determined as the x value satisfying the above formula. That is, the nominal x value corresponds to the Si / (Si+P) atomic ratio determined from the charged amounts of Li and P when satisfying the stoichiometric composition represented by Li 3+x Si x P 1-x O₄ (where 0 < x < 1). Since the charged amount of Si may be set to an excess amount (1.3 times in Examples 5 to 8) relative to the above stoichiometric composition in consideration of the decrease due to volatilization, the charged amounts of Li and P are used for the calculation of the nominal x value. A specific example of how to obtain the nominal x value in Examples 5 to 8 is given below. Composition formula Li 3+x Si x P 1-x O₄, if y = 1 - x, then Li 4-y Si 1-y P yIt is represented as O4. The molar ratio of Li to P is Li:P=(4-y):y, so y=4P / (Li+P). As shown in Table 1, in the starting compositions of Examples 3 to 8, the molar ratio Li:P=0.751:0.188, so y=4×0.188 / (0.751+0.188)=0.80. Therefore, the nominal x value in this case is found to be x=1-y=0.20.

[0023] (Method for manufacturing ion conductors) The ion conductors that are the subject of the present invention can be produced by a method of heating and holding a precursor powder, as described below, in an oxidizing atmosphere at a temperature range of 120°C to 550°C. In this specification, this heating treatment is referred to as "calcination". As the precursor powder, a suspension of colloidal particles obtained by stirring and mixing a water-soluble lithium compound, a silicon alkoxide, and a water-soluble phosphorus compound in an aqueous solvent, from which solvent components are removed, can be used. Alternatively, the ion conductors that are the subject of the present invention can also be obtained by a production method in which the suspension (i.e., the precursor solution described below) is directly heated and held in an oxidizing atmosphere at a temperature range of 120°C to 550°C. In this production method, it can be considered that a precursor powder from which solvent components have been removed is formed during the heating and holding process in the above temperature range. Therefore, this production method is also included as one of the methods of "heating and holding a precursor powder in an oxidizing atmosphere at a temperature range of 120°C to 550°C". When forming a layer of the ion conductor, which is the subject of the present invention, on the surface of the active material particles, for example, a method can be applied in which a mixture of the active material powder and the precursor powder, or a mixture of the active material powder and the suspension (i.e., the precursor solution described later), is subjected to calcination in the above temperature range.

[0024] Air at atmospheric pressure can be used as the oxidizing atmosphere for calcination. While β-Li3PO4 oxide without Si can be synthesized by a wet process at room temperature, heating to 120°C or higher is desirable to synthesize oxides with a β-Li3PO4 type structure that include Si in the crystal lattice. On the other hand, considering the industrial production of all-solid-state lithium-ion secondary batteries, a lower calcination temperature is preferable. According to the inventors' research, a β-Li3PO4 type crystalline phase containing Li, Si, and P in the crystal lattice can be generated in a temperature range of 550°C or lower. As mentioned above, when the heating temperature rises to around 600°C, a γ-Li3PO4 type crystalline phase is formed.

[0025] The calcination process may be divided into multiple heating stages. Alternatively, it may be carried out using a heat pattern that varies the holding temperature in multiple stages within the range of 120 to 550°C. Examples of these multi-stage calcination methods include a heat pattern in which a first stage of calcination is performed by holding the material at a relatively low temperature range, such as 120°C to 200°C, for a relatively short period of time, such as 1 to 3 hours, after which the material is cooled to room temperature and, if necessary, pulverized, and then a second stage of calcination is performed at a higher temperature and for a longer period than the first stage; or a heat pattern in which the temperature is raised immediately after the first stage of calcination to perform a second stage of calcination at a higher temperature and for a longer period than the first stage. Such multi-stage heat patterns that impart a relatively low temperature and short heating history in the initial stages of calcination are effective in obtaining calcined powder with fewer impurities, for example, by allowing volatile components contained in the precursor powder to be sufficiently removed in the initial stages of calcination before the crystallization reaction can proceed.

[0026] The maximum temperature reached during firing is more preferably in the range of 180°C to 520°C, and even more preferably in the range of 280°C to 420°C. The firing time, i.e., the total time held in the temperature range of 120°C to 550°C, is preferably 2 hours or more, and more preferably 10 hours or more. However, considering economic efficiency, the firing time is preferably set to 30 hours or less, and may be controlled to 20 hours or less. After firing, crushing treatment can be performed as needed to obtain a homogeneous fired powder.

[0027] [Ion-conducting sintered body] A preferred embodiment of using the above-mentioned ion conductor as a constituent material for an all-solid-state lithium-ion secondary battery is a sintered body. For example, a sintered body of powder particles which are the above-mentioned ion conductor is useful as a component of the solid electrolyte layer interposed between the positive electrode layer and the negative electrode layer. Furthermore, a sintered body of positive electrode active material particles coated with the above-mentioned ion conductor, or a sintered body of positive electrode active material particles and particles of the above-mentioned ion conductor filling the spaces between them, is useful as a component of the positive electrode layer. In an all-solid-state lithium-ion secondary battery of the type in which the negative electrode layer, positive electrode layer, and the solid electrolyte layer between them are integrated by sintering, if the above-mentioned ion conductor is included in any of the layers, the battery is considered to be a sintered body using the above-mentioned ion conductor.

[0028] (Method for manufacturing ion-conducting sintered bodies) The above-mentioned ion-conducting sintered body can be obtained by (i) a method of sintering a powder containing particles of the precursor powder described later, or (ii) a method of sintering a powder containing particles of the above-mentioned ion-conducting powder (calcined powder). (i) A method for sintering a powder containing precursor powder particles. In this case, heating for sintering also serves as firing for oxide synthesis. Specifically, a method can be applied in which a molded powder containing particles of the precursor powder described later is heated and held in an oxidizing atmosphere at a temperature range of 120°C to 550°C while applying pressure, thereby generating a crystalline phase with a β-Li3PO4 type structure containing Li, Si, and P in the crystal lattice and obtaining a sintered body. Air at atmospheric pressure can be used as the oxidizing atmosphere. To efficiently advance sintering, it is more effective to set the heating temperature to 300°C or higher. Furthermore, the time for which the molded body is held in the temperature range of 120°C to 550°C is preferably 2 hours or more, and more preferably 10 hours or more. However, considering economics, it is preferable to set the heating time in the range of 30 hours or less, and it may be controlled to 20 hours or less. When constructing a sintered body to be applied to the solid electrolyte layer of an all-solid-state battery, the molded body can be made of the precursor powder as described above. When constructing an electrode layer, the molded body can be, for example, one made of a mixed powder containing active material powder and precursor powder.

[0029] (ii) A method for sintering a powder containing ion-conducting powder particles. In this case, a method can be applied to obtain a sintered body by heating and holding a molded powder containing the aforementioned ion-conducting powder (calcined powder) particles in a temperature range of 120°C to 550°C while applying pressure. Since the synthesis of the oxide has already been completed, this heating is performed for the purpose of promoting sintering. The holding time in the temperature range of 120°C to 550°C should be in the range of 30 minutes to 15 hours, and more preferably in the range of 1 hour to 10 hours. Considering that conditions for particle growth of the powder are necessary to efficiently promote sintering, it is effective to set the heating temperature in a temperature range that does not fall significantly below the calcination temperature when the calcined powder was synthesized. Specifically, when the highest temperature reached in the calcination process is T0 (°C) and the highest temperature reached in this sintering process is T1 (°C), it is preferable to satisfy the conditions T1 ≥ T0 - 50 and 120 ≤ T1 ≤ 550. When constructing a sintered body to be applied to the solid electrolyte layer of an all-solid-state battery, the molded body may consist of the above-mentioned ion conductor powder (calcined powder). When constructing an electrode layer, the molded body may consist of a mixed powder containing, for example, active material powder and the above-mentioned ion conductor powder (calcined powder). The pressure applied to the material during heating in the sintering process should be set within a range of, for example, 125 MPa to 1000 MPa.

[0030] [Precursor solution] The ion conductors targeted by the present invention can be obtained by calcining a precursor powder described later. An intermediate product for obtaining the precursor powder is an aqueous solution in which colloidal particles containing Li, Si, and P are suspended. This aqueous solution is referred to herein as the "precursor solution." This precursor solution is characterized, for example, by being an aqueous solution in which colloidal particles containing Li, Si, and P are suspended, and by having the property that when the solid obtained by evaporating the aqueous solution to dryness at 70°C and then heating it at 150°C for 2 hours is subjected to a heat treatment by heating it in air at 400°C for 12 hours, a crystalline phase with a β-Li3PO4 type structure containing Li, Si, and P in the crystal lattice is formed. The colloidal particles are characterized, for example, by being a mixed product of an aqueous solvent, a water-soluble lithium compound, a silicon alkoxide, and a water-soluble phosphorus compound.

[0031] (Method for producing the precursor solution) As a result of their research, the inventors discovered that a precursor material capable of forming a β-Li3PO4 type crystalline phase containing Li, Si, and P in its crystal lattice can be obtained by stirring and mixing a water-soluble lithium compound, a silicon alkoxide, and a water-soluble phosphorus compound in an aqueous solvent. This method is an aqueous solution synthesis utilizing a hydrolysis condensation process (sol-gel method) by silicon alkoxide, which is the source of Si. An aqueous solvent is a liquid medium whose main component is water (i.e., the mass proportion of water is 50% or more).

[0032] The above stirring and mixing is preferably carried out in the following procedure. First, prepare an aqueous solution containing a water-soluble lithium compound. Add silicon alkoxide to this aqueous solution and stir thoroughly. This stirring is preferably performed at a temperature near room temperature (for example, between 15°C and 45°C). The atmosphere of the gas phase in contact with the liquid during stirring can be air at atmospheric pressure. Although silicon alkoxide is poorly soluble in water, the presence of lithium ions in the aqueous solution promotes hydrolysis of the silicon alkoxide, forming an active silicon colloid (SiO₂). xA solution of lithium ions (which is thought to be a hydrated form of silicon alkoxide) is formed. If the mixing of silicon alkoxide and water is insufficient, an impurity phase is likely to form. To obtain a solution with the highest possible homogeneity, it is desirable to stir at this stage for 15 minutes or more. After that, a water-soluble phosphorus compound is added to the solution and dissolved. Note that the water-soluble phosphorus compound may be dissolved in the solution before adding the silicon alkoxide, but from the viewpoint of suppressing the formation of Li3PO4, it is preferable to add the water-soluble phosphorus compound after mixing the silicon alkoxide.

[0033] Next, the solution obtained by the above stirring is heated and stirred to react with the phosphorus compound. This reaction produces a white suspension containing colloidal particles. This suspension can be used as a precursor solution to obtain the aforementioned ion conductor. To ensure the reaction proceeds sufficiently, the liquid temperature during stirring is preferably 50-95°C. The stirring time is preferably 30 minutes or longer. For economic reasons, the stirring time is usually set within a range of 120 minutes or less. The gas phase atmosphere in contact with the liquid during stirring can be air at atmospheric pressure. The structure of the colloidal particles produced in this reaction is not yet clear, but it is possible that compounds of Li, Si, P, and O are formed at this stage.

[0034] The amounts (charging amounts) of the water-soluble lithium compound, silicon alkoxide, and water-soluble phosphorus compound used for the above stirring and mixing are preferably in quantitative proportions such that the composition of Li, Si, and P (charging composition) satisfies the following formulas (3) and (4). 0.05 ≤ x ≤ 0.45 …(3) 1.05A ≤ Si ≤ 1.50A …(4) Here, the x value in equation (3) and the A value in equation (4) are determined by equations (5) and (6) below, respectively. x = 1 - 4P / (Li + P) …(5) A = xP / (1-x) …(6) However, for the positions of the element symbols Si, P and Li in formulas (4), (5) and (6), the value of the ratio (atomic ratio) of the amount (in moles) of Si atoms, P atoms and Li atoms to the total amount (in moles) of Si atoms, P atoms and Li atoms present in the powder shall be substituted respectively.

[0035] The crystal phase of β-Li3PO4-type structure containing Li, Si and P in its crystal lattice has a composition formula Li 3+x Si x P 1-x O4 (where 0<x<1). Therefore, it is considered that making the Li, Si and P composition ratio of the precursor to be fired as close as possible to the stoichiometric composition of Li 3+x Si x P 1-x O4 is advantageous for obtaining a fired product with few heterogeneous phases. In the aqueous solution synthesis described above, part of the silicon alkoxide serving as the Si source or a substance derived therefrom may disappear from the liquid due to volatilization or other causes. Therefore, it is preferable to set the charged amount of Si in excess relative to the above stoichiometric composition.

[0036] The above formula (5) is used to derive the x value satisfying the composition formula Li 3+x Si x P 1-x O4 from the charged composition of Li and P. In view of the fact that it is preferable to make the charged amount of Si excessive relative to the stoichiometric composition, the x value in the charged composition is determined from the charged composition of Li and P. It is preferable to determine the charged amounts of the water-soluble lithium compound and the water-soluble phosphorus compound such that this x value satisfies the above formula (3). The A value in the above formula (6) represents the atomic ratio of Si to the total of Li, Si and P in the stoichiometric composition when the x value determined from the above formula (5) is adopted. According to studies by the inventors, it is effective to set the charged composition of Si to an excess amount in the range of 1.05 times or more and 1.50 times or less relative to the A value (that is, the proportion of Si in the stoichiometric composition). Therefore, it is preferable to determine the charged amount of silicon alkoxide, which is the Si supply source, so as to satisfy the above formula (4). It is more preferable to determine the amount of silicon alkoxide to be charged so as to satisfy equation (4)' below. 1.20A ≤ Si ≤ 1.40A …(4)'

[0037] Examples of water-soluble lithium compounds include lithium hydroxide monohydrate, lithium acetate, lithium carbonate, and lithium nitrate. Examples of silicon alkoxides include tetraethoxysilane, trimethoxysilane, tetramethoxysilane, triethoxysilane, tripropoxysilane, tetrapropoxysilane, and tripbutoxysilane. Examples of water-soluble phosphorus compounds include ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and phosphoric acid.

[0038] [Precursor powder] A suitable precursor powder for synthesizing the ion conductor targeted by the present invention is, for example, a precursor powder consisting of particles containing Li, Si, and P, which has the property of forming a crystalline phase with a β-Li3PO4 type structure containing Li, Si, and P in its crystal lattice when subjected to calcination by heating at 400°C in air for 12 hours. More specifically, an example can be identified as a solvent removal component in an aqueous solution in which colloidal particles containing Li, Si, and P are suspended when the source of Si is silicon alkoxide. The "solvent removal component of the aqueous solution" is the component of the aqueous solution excluding the solvent component, and includes, for example, the evaporation to dryness product and the solid component recovered by solid-liquid separation (filtration or centrifugation). The Li, Si, and P composition ratio of the ion conductor targeted by the present invention will substantially reflect the Li, Si, and P composition ratio of the precursor powder. Therefore, one preferred embodiment of the precursor powder is one that satisfies the above-mentioned equations (1) and (2) in terms of Li, Si, and P content.

[0039] (Method for producing precursor powder) The following manufacturing method is a preferred embodiment for obtaining the above-mentioned precursor powder. A precursor solution preparation step involves stirring and mixing a water-soluble lithium compound, silicon alkoxide, and water-soluble phosphorus compound in an aqueous solvent in quantitative proportions satisfying the above-mentioned equations (3) and (4) to form a colloidal solution. A precursor powder synthesis step, in which solvent components are removed from the colloidal solution to obtain a solid, A method for producing an ion conductor precursor powder having the following characteristics. Here, methods for removing solvent components from a colloidal solution to obtain solids include, for example, evaporation to dryness, filtration, and centrifugation. [Examples]

[0040] [Example 1] (Precursor solution) We prepared lithium hydroxide monohydrate LiOH·H2O (manufactured by Kojun Chemical Laboratory, purity 99% or higher) as a water-soluble lithium compound, tetraethoxysilane Si(OC2H5)4 (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., reagent grade) as a silicon alkoxide, and ammonium dihydrogen phosphate NH4H2PO4 (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., reagent grade) as a water-soluble phosphorus compound. The aforementioned nominal x value (composition formula Li 3+x Si x P 1-x The x-value (determined from the Li and P charging composition in O4) was 0.3, and the amount of Si charged was 1.3 times the amount in the stoichiometric composition. To achieve this, the amounts of the starting materials were set to 3.462 g of lithium hydroxide monohydrate, 2.032 g of tetraethoxysilane, and 2.013 g of ammonium dihydrogen phosphate.

[0041] First, lithium hydroxide monohydrate was dissolved in 50 mL of deionized water to obtain an aqueous lithium hydroxide solution. Tetraethoxysilane was added to this aqueous lithium hydroxide solution and stirred at room temperature for 30 minutes using a stirrer. Next, ammonium dihydrogen phosphate was added to the stirred solution, the temperature of the solution was raised to 70°C, and the solution was stirred at 70°C for 1 hour. In this way, a colloidal solution (precursor solution) was obtained. Table 1 shows the composition of the starter (the same applies to each example below). In Table 1, the satisfaction of equations (3) and (4) mentioned above is indicated by ○ if satisfied and × if not satisfied.

[0042] (Precursor powder) After stirring the colloidal solution at 70°C for 1 hour, the stirring bar was removed and the solution was heated at 70°C for 15 hours to evaporate to dryness, thereby recovering a powder sample (precursor powder) consisting of the evaporated-to-dryness product.

[0043] (Castration powder) The recovered powder sample (precursor powder) was heated in air at 150°C for 2 hours, then lightly crushed in an agate mortar, and subsequently placed in an alumina crucible. A calcination process was then carried out using an electric furnace (FP102, manufactured by Yamato Scientific Co., Ltd.) at 400°C in air for 12 hours to obtain a white powder (hereinafter referred to as "calcined powder"). In this example, the maximum temperature reached during calcination was 400°C, and the total calcination time was 14 hours.

[0044] The obtained calcined powder was subjected to X-ray diffraction measurements under the conditions described in "Measurement of X-ray Diffraction Pattern" above. From the measured X-ray diffraction pattern, it was confirmed that this calcined powder mainly consists of a β-Li3PO4 type crystalline phase. A small amount of Li2CO3 phase was observed as an admixture. Table 2 shows these results (the same applies to each of the following examples). The "firing temperature" listed in Table 2 is the maximum temperature reached. In this example and each of the following examples, heating and holding equivalent to the second stage of firing described above was performed while maintaining the firing temperature (maximum temperature reached) listed in Table 2. In addition, the lattice constant and unit cell volume of the β-Li3PO4 type crystal, determined from the X-ray diffraction pattern, are also listed in Table 2 (the same applies to each of the following examples except for Comparative Example 1).

[0045] The chemical composition of the obtained calcined powder was determined using an ICP-AES analyzer (Agilent 720-ES, manufactured by Agilent). Table 3 shows the composition ratios of Li, Si, and P based on the measurement results (the same applies to each of the following examples, except for some comparative examples). In addition, in Table 3, the satisfaction of equations (1) and (2) mentioned above is indicated by ○ if satisfied and × if not satisfied. The calcined powder in this example satisfies equation (2), and its compositional formula is Li 3+x Si x P 1-x It can be evaluated that the dominant phase is a crystalline phase with a Li, Si, and P composition ratio close to that of O4's stoichiometric composition.

[0046] (Sintered body) Using the above-mentioned calcined powder as the material, a sintered body was produced by heating it in air at 400°C for 2 hours under a pressure of 375 MPa using a hot press device (manufactured by AS ONE Corporation).

[0047] The impedance of the obtained sintered body was measured at 200°C using a frequency response analyzer (FRA) (Solartron, Model 1260) under the conditions of a frequency of 32 MHz to 100 Hz and an amplitude voltage of 100 mV. The resistance value was determined from the arc of the Nyquist plot, and the conductivity was calculated from this resistance value. Gold electrodes were used as blocking electrodes. Table 4 shows the conductivity at 200°C (the same applies to Examples 2, 3, 4, 7, and Comparative Example 3 below). The sintered body in this example is 3.022 × 10 at 200°C. -6 It exhibited conductivity of S / cm (lithium ion conductivity). In other words, the calcined powder obtained in this example was confirmed to be an ionic conductor.

[0048] [Example 2] In the fabrication of the sintered body, the same experiment as in Example 1 was conducted, except that the heating temperature was changed from 400°C to 500°C. The sintered body in this example is 7.745 × 10 at 200°C. -6 The material exhibited conductivity of S / cm (lithium ion conductivity). In other words, the calcined powder obtained in this example was confirmed to be an ionic conductor.

[0049] [Example 3] In preparing the precursor solution, the starting materials were prepared by adding 3.567 g of lithium hydroxide monohydrate, 2.709 g of tetraethoxysilane, and 1.726 g of ammonium dihydrogen phosphate, so that the nominal x-value was 0.4 and the amount of Si added was 1.3 times the amount of Si relative to the stoichiometric composition. The experiment was carried out under the same conditions as in Example 1. X-ray diffraction patterns confirmed that the calcined powder obtained in this example has a β-Li3PO4 type crystalline phase as its main phase. Small amounts of Li2CO3 and Li2SiO3 phases were observed as heterogeneous phases. ICP-AES analysis revealed that the calcined powder in this example satisfies equation (2), and its compositional formula is Li 3+x Si x P 1-x It can be evaluated that the dominant phase is a crystalline phase with a Li, Si, and P composition ratio close to that of O4's stoichiometric composition. The sintered body in this example is 7.703 × 10 at 200°C. -6 The material exhibited conductivity of S / cm (lithium ion conductivity). In other words, the calcined powder obtained in this example was confirmed to be an ionic conductor.

[0050] [Example 4] In preparing the precursor solution, the starting materials were prepared in the following amounts: lithium hydroxide monohydrate 3.252 g, tetraethoxysilane 0.677 g, and ammonium dihydrogen phosphate 2.589 g, so that the nominal x value was 0.1 and the amount of Si added was 1.3 times the amount of Si relative to the stoichiometric composition. The experiment was conducted under the same conditions as in Example 1. X-ray diffraction patterns confirmed that the calcined powder obtained in this example has a β-Li3PO4 type crystalline phase as its main phase. No other phases were observed. ICP-AES analysis revealed that the calcined powder in this example satisfies equation (2), and its compositional formula is Li 3+x Si x P 1-x It can be evaluated that the dominant phase is a crystalline phase with a Li, Si, and P composition ratio close to that of O4's stoichiometric composition. The sintered body in this example is 2.859 × 10 at 200°C. -7The material exhibited conductivity of S / cm (lithium ion conductivity). In other words, the calcined powder obtained in this example was confirmed to be an ionic conductor.

[0051] [Example 5] In preparing the precursor solution, the amounts of raw materials were set to 3.357 g of lithium hydroxide monohydrate, 1.355 g of tetraethoxysilane, and 2.301 g of ammonium dihydrogen phosphate, so that the nominal x value was 0.2 and the amount of Si added was 1.3 times the amount of the stoichiometric composition. The calcination temperature was set to 150°C. The calcined powder was prepared under the same conditions as in Example 1, and X-ray diffraction measurement and compositional analysis were performed using the same method as in Example 1. X-ray diffraction patterns confirmed that the calcined powder obtained in this example has a crystalline phase with a β-Li3PO4 type structure as its main phase. A small amount of Li2CO3 phase was observed as an alien phase. ICP-AES analysis revealed that the calcined powder in this example satisfies equation (2), and its compositional formula is Li 3+x Si x P 1-x It can be evaluated that the dominant phase is a crystalline phase with a Li, Si, and P composition ratio close to that of O4's stoichiometric composition.

[0052] [Example 6] In preparing the precursor solution, the amounts of raw materials were set to 3.357 g of lithium hydroxide monohydrate, 1.355 g of tetraethoxysilane, and 2.301 g of ammonium dihydrogen phosphate, so that the nominal x value was 0.2 and the amount of Si added was 1.3 times the amount of Si relative to the stoichiometric composition. The calcination temperature was set to 300°C. The calcined powder was prepared under the same conditions as in Example 1, and X-ray diffraction measurement and compositional analysis were performed using the same method as in Example 1. X-ray diffraction patterns confirmed that the calcined powder obtained in this example has a crystalline phase with a β-Li3PO4 type structure as its main phase. A small amount of Li2CO3 phase was observed as an alien phase. ICP-AES analysis revealed that the calcined powder in this example satisfies equation (2), and its compositional formula is Li 3+x Si x P 1-xIt can be evaluated that the dominant phase is a crystalline phase with a Li, Si, and P composition ratio close to that of O4's stoichiometric composition.

[0053] [Example 7] In preparing the precursor solution, the starting materials were prepared in the following amounts: lithium hydroxide monohydrate 3.357 g, tetraethoxysilane 1.355 g, and ammonium dihydrogen phosphate 2.301 g, so that the nominal x value was 0.2 and the amount of Si added was 1.3 times the amount of Si relative to the stoichiometric composition. The experiment was carried out under the same conditions as in Example 1. X-ray diffraction patterns confirmed that the calcined powder obtained in this example has a crystalline phase with a β-Li3PO4 type structure as its main phase. A small amount of Li2CO3 phase was observed as an alien phase. ICP-AES analysis revealed that the calcined powder in this example satisfies equation (2), and its compositional formula is Li 3+x Si x P 1-x It can be evaluated that the dominant phase is a crystalline phase with a Li, Si, and P composition ratio close to that of O4's stoichiometric composition. The sintered body in this example is 9.963 × 10 at 200°C. -7 The material exhibited conductivity of S / cm (lithium ion conductivity). In other words, the calcined powder obtained in this example was confirmed to be an ionic conductor.

[0054] [Example 8] In preparing the precursor solution, the amounts of raw materials were set to 3.357 g of lithium hydroxide monohydrate, 1.355 g of tetraethoxysilane, and 2.301 g of ammonium dihydrogen phosphate, so that the nominal x value was 0.2 and the amount of Si added was 1.3 times the amount of Si relative to the stoichiometric composition. The calcination temperature was set to 500°C. The calcined powder was prepared under the same conditions as in Example 1, and X-ray diffraction measurement and compositional analysis were performed using the same method as in Example 1. X-ray diffraction patterns confirmed that the calcined powder obtained in this example has a crystalline phase with a β-Li3PO4 type structure as its main phase. A small amount of Li2CO3 phase was observed as an alien phase. ICP-AES analysis revealed that the calcined powder in this example satisfies equation (2), and its compositional formula is Li 3+x Si x P1-x It can be evaluated that the dominant phase is a crystalline phase with a Li, Si, and P composition ratio close to that of O4's stoichiometric composition.

[0055] [Comparative Example 1] In preparing the precursor solution, the amounts of raw materials were set to 3.672 g of lithium hydroxide monohydrate, 3.387 g of tetraethoxysilane, and 1.438 g of ammonium dihydrogen phosphate, so that the nominal x value was 0.2 and the amount of Si added was 1.3 times the amount of Si relative to the stoichiometric composition. The calcination temperature was set to 600°C. The calcined powder was prepared under the same conditions as in Example 1, and X-ray diffraction measurements were performed using the same method as in Example 1. X-ray diffraction patterns revealed that the calcined powder obtained in this example primarily consists of a crystalline phase with a γ-Li3PO4 structure. A small amount of the Li2CO3 phase was observed as an adjunct.

[0056] [Comparative Example 2] In preparing the precursor solution, the amounts of raw materials were set to 3.357 g of lithium hydroxide monohydrate, 1.355 g of tetraethoxysilane, and 2.301 g of ammonium dihydrogen phosphate, so that the nominal x value was 0.5 and the amount of Si added was 1.3 times the amount of the stoichiometric composition. Otherwise, the calcined powder was prepared under the same conditions as in Example 1, and X-ray diffraction measurement and compositional analysis were performed in the same manner as in Example 1. X-ray diffraction patterns confirmed that the calcined powder obtained in this example has a β-Li3PO4 type crystalline phase as its main phase. The presence of Li2CO3 and Li2SiO3 phases as heterogeneous phases was observed, and their amounts were greater than those in Example 3 described above. ICP-AES analysis revealed that the calcined powder in this example has a Si / (Si+P) ratio of 0.45, which does not satisfy equation (1).

[0057] [Comparative Example 3] Here, the aforementioned Si-free β-Li3PO4 powder with a nominal x value of 0 was synthesized at room temperature as follows. 3.147 g of lithium hydroxide monohydrate and 2.876 g of ammonium dihydrogen phosphate were weighed out so that the amount of raw materials used would satisfy the Li:P ratio of the compositional formula Li3PO4. The above amount of lithium hydroxide monohydrate was dissolved in 50 mL of deionized water to obtain an aqueous lithium hydroxide solution. The above amount of ammonium dihydrogen phosphate was added to this aqueous lithium hydroxide solution, the temperature was raised to 70°C, and the mixture was stirred at 70°C for 1 hour. A colloidal solution (precursor solution) was thus obtained. Furthermore, the stirring bar was removed from the colloidal solution after stirring at 70°C for 1 hour, and the solution was heated at 70°C for 15 hours to evaporate to dryness. The evaporated dryness product was heated in air at 150°C for 2 hours, and the powder sample was recovered.

[0058] The obtained powder was subjected to X-ray diffraction under the same conditions as in Example 1. As a result, it was confirmed that the powder consisted of β-Li3PO4 crystals. These crystals may have been formed by the evaporation to dryness described above, but the "Castration Temperature" column in Table 2 shows 150°C, which is the highest temperature reached during the applied thermal history. Using this powder as the material, a sintered body was prepared in the same manner as in Example 1, except that the sintering temperature was set to 300°C, and the conductivity at 200°C was measured. As a result, the conductivity of the sintered body in this example at 200°C was 3.5 × 10⁻⁶. -8 The conductivity was S / cm. A comparison of this example with Examples 1-4 and 7 above shows that the β-Li3PO4 crystal (this example) has poor lithium ion conductivity, but by solid-solving Si to create a β-Li3PO4 type crystal structure containing Li, Si, and P in the crystal lattice, lithium ion conductivity is significantly improved.

[0059] [Table 1]

[0060] [Table 2]

[0061] [Table 3]

[0062] Table 4

Claims

1. β-Li containing Li, Si, and P in its crystal lattice ３ PO ４ An ionic conductor having a crystalline phase with a specific structure as its main phase, and a Si / (Si+P) atomic ratio of 0.05 to 0.

40.

2. An ion-conducting sintered body using the ion conductor described in claim 1.

3. A precursor solution consisting of an aqueous solution in which colloidal particles containing Li, Si, and P are suspended, and when the aqueous solution is evaporated to dryness at 70°C and then heated at 150°C for 2 hours to obtain a solid, the solid obtained is subjected to heat treatment by heating at 400°C in air for 12 hours, β-Li containing Li, Si, and P in the crystal lattice. ３ PO ４ A precursor solution of an ion conductor that has the property of forming a crystalline phase with a specific structure.

4. The precursor solution for an ion conductor according to claim 3, wherein the colloidal particles are a mixed product of an aqueous solvent, a water-soluble lithium compound, a silicon alkoxide, and a water-soluble phosphorus compound.

5. A precursor powder consisting of particles containing Li, Si, and P, when the powder is subjected to calcination by heating in air at 400°C for 12 hours, β-Li containing Li, Si, and P in the crystal lattice. ３ PO ４ A precursor powder for ion conductors that has the property of forming a crystalline phase with a specific structure.

6. The precursor powder is a solvent removal component of an aqueous solution in which colloidal particles containing Li, Si, and P are suspended, as described in claim 5.

7. The precursor powder of the ion conductor according to claim 5, wherein the precursor powder satisfies the following formulas (1) and (2) in terms of the content of Li, Si, and P. 0.05≦Si / (Si+P)≦0.40…(1) 0.90 (4Si+3P)≦Li≦1.10 (4Si+3P)…(2) Here, the elemental symbols Si, P, and Li in equations (1) and (2) are replaced with the ratios (atomic ratios) of the amounts of Si, P, and Li atoms in the powder to the total amount (moles) of Si, P, and Li atoms present in the powder, respectively.

8. A precursor powder formed by removing solvent components from a suspension of colloidal particles obtained by mixing a water-soluble lithium compound, a silicon alkoxide, and a water-soluble phosphorus compound in an aqueous solvent is heated and held in an oxidizing atmosphere at a temperature range of 120°C to 550°C to produce β-Li containing Li, Si, and P in the crystal lattice. ３ PO ４ A method for producing an ion conductor that generates a crystalline phase with a specific structure.

9. The method for producing an ion conductor according to claim 8, wherein the precursor powder satisfies the following formulas (1) and (2) in terms of the content of Li, Si, and P. 0.05≦Si / (Si+P)≦0.40…(1) 0.90 (4Si+3P)≦Li≦1.10 (4Si+3P)…(2) Here, the elemental symbols Si, P, and Li in equations (1) and (2) are replaced with the ratios (atomic ratios) of the amounts of Si, P, and Li atoms in the powder to the total amount (moles) of Si, P, and Li atoms present in the powder, respectively.

10. In an aqueous solvent, a powder compact containing precursor powder particles formed by removing solvent components from a suspension of colloidal particles obtained by mixing a water-soluble lithium compound, a silicon alkoxide, and a water-soluble phosphorus compound is heated and held while applying pressure in a temperature range of 120°C to 550°C inclusive in an oxidizing atmosphere, thereby producing β-Li containing Li, Si, and P in its crystal lattice ３ PO ４ A method for producing an ion-conductive sintered body, which comprises generating a crystalline phase having a -type structure and obtaining the sintered body.

11. A precursor powder formed by removing solvent components from a suspension of colloidal particles obtained by mixing a water-soluble lithium compound, a silicon alkoxide, and a water-soluble phosphorus compound in an aqueous solvent is heated and held in an oxidizing atmosphere at a temperature range of 120°C to 550°C to produce β-Li containing Li, Si, and P in the crystal lattice. ３ PO ４ A calcination process is performed to generate a crystalline phase with a specific structure in order to obtain an ion conductor powder. A sintering step is performed to obtain a sintered body by heating and holding a molded powder containing the ion conductor powder particles in a temperature range of 120°C to 550°C while applying pressure. A method for manufacturing an ion-conducting sintered body having [a certain characteristic].

12. The highest temperature reached in the aforementioned firing process is T ０ (°C), the highest temperature reached in the sintering process is T １ When T is (°C), １ ≧T ０ -50 and 120 ≤ T １ A method for producing an ion-conducting sintered body according to claim 11, wherein the firing process and the sintering process are carried out under conditions that satisfy ≤ 550.

13. A method for producing a precursor solution for an ion conductor, comprising the step of forming a colloidal solution by mixing a water-soluble lithium compound, a silicon alkoxide, and a water-soluble phosphorus compound in an aqueous solvent in quantitative proportions of Li, Si, and P that satisfy the following formulas (3) and (4). 0.05 ≤ x ≤ 0.45 …(3) 1.05A≦Si≦1.50A…(4) Here, the x value in equation (3) and the A value in equation (4) are determined by equations (5) and (6) below, respectively. x=1-4P / (Li+P)...(5) A=xP / (1-x)...(6) However, in equations (4), (5), and (6), the element symbols Si, P, and Li are replaced with the ratios (atomic ratios) of the amounts of Si, P, and Li atoms (moles) to the total amount (moles) of Si, P, and Li atoms present in the powder, respectively.

14. A precursor solution preparation step involves forming a colloidal solution by mixing a water-soluble lithium compound, a silicon alkoxide, and a water-soluble phosphorus compound in an aqueous solvent in quantitative proportions satisfying the following formulas (3) and (4) of Li, Si, and P. A precursor powder synthesis step, in which solvent components are removed from the colloidal solution to obtain a solid, A method for producing an ion conductor precursor powder having the following characteristics. 0.05 ≤ x ≤ 0.45 …(3) 1.05A≦Si≦1.50A…(4) Here, the x value in equation (3) and the A value in equation (4) are determined by equations (5) and (6) below, respectively. x=1-4P / (Li+P)...(5) A=xP / (1-x)...(6) However, in equations (4), (5), and (6), the element symbols Si, P, and Li are replaced with the ratios (atomic ratios) of the amounts of Si, P, and Li atoms (moles) to the total amount (moles) of Si, P, and Li atoms present in the powder, respectively.

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