JP2024058258A - Ion conductor, ion conductive sintered body, precursor solution, precursor powder, and methods for producing the same - Google Patents

Abstract

[Problem] To provide an ionic conductor having improved lithium ion conductivity in an oxide based on a β-Li3PO4 type crystal structure. [Solution] An ionic conductor having a β-Li3PO4 type structure crystal phase containing Li, Si, and P in the crystal lattice as the main phase, with a Si/(Si+P) atomic ratio of 0.05 to 0.40. This ionic conductor can be obtained by baking a precursor derived from a colloidal solution obtained by mixing a water-soluble lithium compound, a silicon alkoxide, and a water-soluble phosphorus compound in an aqueous solvent, the Li, Si, and P composition ratios of which are adjusted to a predetermined range, at a temperature of, for example, 150 to 550°C. [Selected Figure] Figure 1

Description

The present invention relates to an oxide-based ion conductor and an ion conductive sintered body that exhibit lithium ion conductivity. It also relates to a precursor solution and precursor powder suitable for synthesizing the ion conductor and the ion conductive sintered body. It also relates to a method for producing them.

One of the oxide-based ion-conductive materials being considered for application to all-solid-state lithium-ion secondary batteries is an oxide called LISICON, which has a γ-Li ₃ PO ₄ type crystal structure. Although the ionic conductivity of γ-Li ₃ PO ₄ itself is low, it is known that the 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 O ₄ (where 0<x<1), in which Li ₄ SiO ₄ is dissolved in the γ-Li 3 PO 4 type crystal structure (for example, Non-Patent Document 1).

Patent Document 1 describes that a precursor mixture was synthesized by hydrothermal reaction at 170° C. and 1.0 MPa using SiO ₂ as a Si source, and the dried precursor mixture was fired at 700° C. to obtain LISICON-type crystal particles of Li _3.5 Si _0.5 P _0.5 O ₄ single phase (Example 1), Li _3.9 Si _0.9 P _0.1 O ₄ single phase (Example 2), and Li _3.1 Si _0.1 P _0.9 O ₄ single phase (Example 3) (Example 1). The discharge capacity was measured for an all-solid-state lithium ion secondary battery (negative electrode is lithium foil) molded by hand pressing using these particles as a solid electrolyte layer (paragraphs 0059 and 0060).

On the other hand, it is known that Li ₃ PO ₄ oxide exists not only in the high-temperature phase γ-Li ₃ PO ₄ but also in the low-temperature phase β-Li ₃ PO ₄ .
Non-Patent Document 2 describes that crystal structure analysis was performed by X-ray diffraction on β-Li ₃ PO ₄ synthesized by a wet reaction at 40° C. and γ-Li ₃ PO ₄ synthesized by a dry method in which a mixed melt containing a lithium compound and a phosphorus compound at 775° C. was quenched. It is said that γ-Li ₃ PO ₄ has higher ionic conductivity than β-Li ₃ PO ₄ .
Non-Patent Document 3 describes an experiment in which β-Li ₃ PO ₄ synthesized by a wet reaction at room temperature was heated to various temperatures to investigate the phase transition to γ-Li ₃ PO _4. According to the experiment, the above phase transition occurs at approximately 450°C, and it is pointed out that a diffraction peak (arrow in Fig. 7) is observed near 2θ = 20 degrees as a characteristic point of the X-ray diffraction pattern using Cu-Kα radiation in the γ-Li ₃ PO ₄ phase. It is also shown that the phase transition from β-Li ₃ PO ₄ to γ-Li ₃ PO ₄ occurs as a continuous structural change (Fig. 8).

JP 2020-102374 A

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. Nur I. P. 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. 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.

In all-solid-state secondary batteries, ion conductors are used as materials for the solid electrolyte layer, and are also used as materials for ion conduction between positive electrode active material particles. Oxide-based ion conductors have the advantage of being able to avoid the risk of toxic gas generation, which is a problem with sulfide-based ion conductors. Since oxide powders are hard, it is generally assumed that a sintering process is carried out in the industrial manufacturing process of all-solid-state lithium-ion secondary batteries using oxide-based ion conductors. In the sintering process, the heating temperature is limited due to the need to prevent adverse effects such as reaction with the electrode active material, and it is desirable to sinter at a temperature range of, for example, about 550°C or less, or even lower, such as 450°C or less. The above-mentioned ion conductive material called LISICON, which has improved ion conductivity by dissolving Li ₄ SiO ₄ in a γ-Li ₃ PO ₄ type crystal structure, needs to be synthesized and sintered at a high temperature, for example, 600°C or more, in order to obtain an oxide structure with good crystallinity. Therefore, it is considered difficult at present to industrially spread all-solid-state lithium ion secondary batteries using oxides based on the γ-Li ₃ PO ₄ type structure.

On the other hand, the low temperature phase, β-Li ₃ PO ₄ , has poor ionic conductivity.
An object of the present invention is to provide an ion conductor having improved lithium ion conductivity in an oxide based on a β-Li ₃ PO ₄ type crystal structure.

As described above, in the high-temperature phase of Li ₃ PO ₄ oxide, γ-Li ₃ PO ₄ , the lithium ion conductivity can be improved by forming an oxide in which Li ₄ SiO ₄ is dissolved in the crystal. However, for the low-temperature phase β-Li ₃ PO ₄ , an oxide synthesis method that realizes the above-mentioned solid solution has not been established. In addition, the ion conduction behavior of such oxides having a solid-solution type β-Li ₃ PO ₄ type crystal structure is not known.

As a result of research, the inventors have found that by using as a precursor a Li, Si, P-containing substance produced by a wet reaction process (aqueous solution synthesis) using silicon alkoxide as a Si supply source, it is possible to synthesize a crystal phase of β-Li ₃ PO ₄ type structure containing Li, Si, and P in the crystal lattice at a low firing temperature, for example, 550° C. or lower. It has also been confirmed that this type of oxide exhibits lithium ion conductivity that is considered to be applicable to the solid electrolyte of an all-solid-state lithium ion secondary battery. The present invention is based on such findings.
This specification discloses the following inventions.

[1] An ionic conductor having a crystal phase of a β-Li ₃ PO ₄ type structure containing Li, Si, and P in the crystal lattice as a main phase, and having a Si/(Si+P) atomic ratio of 0.05 or more and 0.40 or less.
[2] An ion-conductive sintered body using the ion conductor according to the above [1].
[3] A precursor solution of an ionic conductor, which is made of an aqueous solution in which colloidal particles containing Li, Si, and P are suspended, and which has the property of forming a crystalline phase with a β-Li3PO4 structure containing Li, Si, and P in _the crystal lattice when the aqueous solution is evaporated to dryness at 70°C and then heated at 150°C for 2 hours to obtain a solid, and then heat-treated in air at ₄₀₀ °C for 12 hours.
[4] The precursor solution of an ionic conductor according to the above [3], wherein the colloidal particles are a mixture product of an aqueous solvent, a water-soluble lithium compound, a silicon alkoxide, and a water-soluble phosphorus compound.
[5] A precursor powder of an ion conductor, which is composed of particles containing Li, Si, and P, and has the property of forming a crystalline phase having a β- _Li3PO4 type structure containing Li, Si, and P in the crystal lattice when the powder is calcined by heating in air at ₄₀₀ °C for 12 hours.
[6] The precursor powder of the ionic conductor according to the above [5], which is a solvent-removed component of an aqueous solution in which colloidal particles containing Li, Si, and P are suspended.
[7] The precursor powder of the ionic conductor according to the above [5] or [6], wherein the precursor powder satisfies the following formulas (1) and (2) in terms of the contents of Li, Si, and P:
0.05≦Si/(Si+P)≦0.40 ... (1)
0.90(4Si+3P)≦Li≦0.10(4Si+3P) ... (2)
Here, the element symbols Si, P, and Li in formulas (1) and (2) are substituted with the ratios (atomic ratios) of the amounts (moles) of Si atoms, P atoms, and Li atoms to the total amount (moles) of Si atoms, P atoms, and Li atoms present in the powder.
[8] A method for producing an ionic conductor, comprising the steps of: mixing a water-soluble lithium compound, a silicon alkoxide, and a water-soluble phosphorus compound in an aqueous solvent to obtain a suspension of colloidal particles; removing the solvent components from the resulting suspension; and heating and holding the resulting precursor powder in an oxidizing atmosphere at a temperature range of 120°C to 550°C to produce a crystal phase having a β- _Li3PO4 structure that contains Li, _Si , and P in the crystal lattice.
[9] The method for producing an ionic conductor according to the above [8], wherein the precursor powder satisfies the following formulas (1) and (2) in terms of the Li, Si, and P contents:
0.05≦Si/(Si+P)≦0.40 ... (1)
0.90(4Si+3P)≦Li≦0.10(4Si+3P) ... (2)
Here, the element symbols Si, P, and Li in formulas (1) and (2) are substituted with the ratios (atomic ratios) of the amounts (moles) of Si atoms, P atoms, and Li atoms to the total amount (moles) of Si atoms, P atoms, and Li atoms present in the powder.
[10] A method for producing an ion-conductive sintered body, comprising the steps of: mixing a water-soluble lithium compound, a silicon alkoxide, and a water-soluble phosphorus compound in an aqueous solvent to obtain a suspension of colloidal particles; removing the solvent components from the resulting suspension; heating and holding the powder compact containing precursor powder particles in an oxidizing atmosphere at a temperature range of 120°C to 550°C while applying pressure, thereby generating a crystal phase having a β _{- Li3PO4} structure containing Li, Si, and P in the crystal lattice, and obtaining a sintered body.
[11] A firing process in which a precursor powder is formed by removing the 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 the precursor powder is heated and held at a temperature range of 120°C to 550°C in an oxidizing atmosphere to generate a crystal phase having a β-Li ₃ PO ₄ structure containing Li, Si, and P in the crystal lattice, thereby obtaining an ion conductor powder;
a sintering step of heating and holding the powder compact containing the ion conductor powder particles at a temperature range of 120° C. to 550° C. while applying pressure to obtain a sintered body;
The present invention relates to a method for producing an ion-conductive sintered body having an ion-conductive sintered body.
[12] The method for producing an ion-conductive sintered body according to the above [ ₁₁ ], wherein the firing step and the sintering step are carried out under conditions that satisfy T ₁ ≧T ₀ -50 and 120≦T ₁ ≦550, where T 0 is the maximum temperature reached in the firing step (°C) and T ₁ is the maximum temperature reached in the sintering step (°C).
[13] A method for producing a precursor solution of an ion conductor, comprising a step of forming a colloidal solution by mixing, in an aqueous solvent, a water-soluble lithium compound, a silicon alkoxide, and a water-soluble phosphorus compound in quantitative proportions such that the composition of Li, Si, and P satisfies the following formulas (3) and (4):
0.05≦x≦0.45 ... (3)
1.05A≦Si≦1.50A (4)
Here, the value of x in formula (3) and the value of A in formula (4) are determined by the following formulas (5) and (6), respectively.
x = 1-4P / (Li + P) ... (5)
A = xP / (1-x) ... (6)
However, the element symbols Si, P, and Li in formulas (4), (5), and (6) are substituted with the ratios (atomic ratios) of the amounts (moles) of Si atoms, P atoms, and Li atoms to the total amount (moles) of Si atoms, P atoms, and Li atoms present in the powder.
[14] A precursor solution preparation 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 a quantitative ratio such that the composition of Li, Si, and P satisfies the following formulas (3) and (4);
a precursor powder synthesis step of removing a solvent component from the colloidal solution to obtain a solid content;
The present invention relates to a method for producing a precursor powder of an ion conductor.
0.05≦x≦0.45 ... (3)
1.05A≦Si≦1.50A (4)
Here, the value of x in formula (3) and the value of A in formula (4) are determined by the following formulas (5) and (6), respectively.
x = 1-4P / (Li + P) ... (5)
A = xP / (1-x) ... (6)
However, the element symbols Si, P, and Li in formulas (4), (5), and (6) are substituted with the ratios (atomic ratios) of the amounts (moles) of Si atoms, P atoms, and Li atoms to the total amount (moles) of Si atoms, P atoms, and Li atoms present in the powder.

According to the present invention, it is possible to provide an ionic conductor having improved lithium ion conductivity in an oxide based on a β-Li ₃ PO ₄ type crystal structure. This ionic conductor and a sintered body using the same can be obtained in a relatively low temperature range, for example, 120 to 550°C. In addition, because this ionic conductor is an oxide system, the risk of generating toxic gases, which is a problem with conventional sulfide-based ionic conductors, can be avoided. Therefore, the ionic conductor of the present invention is highly useful as a constituent material for all-solid-state lithium ion secondary batteries.

FIG. 1 illustrates X-ray diffraction patterns of calcined powders obtained by calcining a precursor powder (nominal x value=0.2) at various temperature levels between 150 and 700° C. in air for 12 hours.

[Ionic Conductor]
The "ionic conductor" of the present invention is a material having lithium ion conductivity, and can be synthesized by firing a precursor material. This ionic conductor has a β-Li ₃ PO ₄ type structure crystal phase containing Li, Si, and P in the crystal lattice as a main phase. The "β-Li ₃ PO ₄ type structure crystal phase" means a crystal phase in which a diffraction peak from a crystal plane corresponding to the diffraction peak of β-Li ₃ PO ₄ oxide crystal is observed in an X-ray diffraction pattern. The "β-Li ₃ PO ₄ type structure crystal phase containing Li, Si, and P in the crystal lattice" constituting the ionic conductor of the present invention contains Si in addition to Li and P in the crystal lattice. According to the inventors' study, in a comparison between crystal phases of the β-Li ₃ PO ₄ type structure obtained by firing at the same temperature, a tendency was observed in which the volume of the unit cell increases with increasing Si content up to a Si/(Si+P) atomic ratio of about 0.4. In addition, the Li, Si, and P composition ratio of the oxide powder synthesized by the method described below was close to Li3 _{+xSixP1 - xO4 (} where 0<x<1). From this, it is presumed that the "crystalline phase of β- _Li3PO4 type structure" containing Si that is the subject of the present invention is an _oxide phase in which _{Li4SiO4 is} dissolved in β- _{Li3PO4 crystals} .

When the mass ratio of the "β-Li ₃ PO ₄ type crystal phase containing Li, Si, and P in the crystal lattice" accounts for at least 50% of the total mass of the substance that is an ion conductor, it can be said that the β-Li ₃ PO ₄ type crystal phase containing Li, Si, and P in the crystal lattice is the "main phase". When the remainder other than the main phase is called the "heterogeneous phase", the presence of the heterogeneous phase is permitted within a range that does not impair the effects of the present invention (especially ion conductivity and avoidance of the risk of generating toxic gas), but it is preferable that the amount of the heterogeneous phase is small. For example, the mass ratio of the heterogeneous phase is preferably 10% or less. As an embodiment in which the mass ratio of the heterogeneous phase is 10% or less, an ion conductor having a β-Li ₃ PO ₄ type crystal phase containing Li, Si, and P in the crystal lattice as the main phase and an impurity phase that is inevitably mixed in during production as the remainder can be mentioned, and such an ion conductor can be obtained by the production method described below.

The "ionic conductor" of the present invention has an atomic ratio of Si/(Si+P) between 0.05 and 0.40, which represents the atomic ratio of silicon to the total amount of silicon and phosphorus. This atomic ratio of Si/(Si+P) is based on the Si content and P content in the ionic conductor including heterogeneous phases. In a material having a crystal phase of β-Li ₃ PO ₄ type structure as the main phase, when the atomic ratio of Si/(Si+P) 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 amount of Li ₄ SiO ₄ dissolved in the crystal of β-Li ₃ PO ₄ type structure. According to the findings of the inventors, when the atomic ratio of Si/(Si+P) in a material having a crystal phase of β-Li ₃ PO ₄ type structure as the main phase is higher than 0.40, the amount of heterogeneous phases increases, and it was found that Li and Si supplied from the raw materials used are considerably consumed in the formation of the heterogeneous phase. Therefore, the Si/(Si+P) atomic ratio in the ionic conductor is specified here as being 0.05 or more and 0.40 or less, and more preferably 0.15 or more and 0.35 or less.

As a preferred embodiment of the ionic conductor of the present invention, the Li, Si and P contents satisfy the following formulae (1) and (2).
0.05≦Si/(Si+P)≦0.40 ... (1)
0.90(4Si+3P)≦Li≦0.10(4Si+3P) ... (2)
Here, the element symbols Si, P, and Li in formulas (1) and (2) are substituted with the ratios (atomic ratios) of the amounts (moles) of Si atoms, P atoms, and Li atoms to the total amount (moles) of Si atoms, P atoms, and Li atoms present in the powder.
Formula (1) defines the above-mentioned Si/(Si+P) atomic ratio.
In formula (2), "4Si+3P" corresponds to the Li amount (moles) in the stoichiometric composition corresponding to the composition formula Li _3+x Si _x P _1-x O ₄ (where 0<x<1) expressed as a function of the Si amount (moles) and the P amount (moles). Specifically, when the composition formula Li _3+x Si _x P _1-x O ₄ is satisfied, the Li amount (moles):Si amount (moles)=(3+x):x, so the Li amount (moles)=(3+x)×Si amount (moles)/x. Here, by substituting x=Si amount (moles)/(Si amount (moles)+P amount (moles)) into the above formula and rearranging, the Li amount (moles)=4×Si amount (moles)+3×P amount (moles). That is, the above formula (2) indicates that the Li amount in the ionic conductor is allowed to vary within a range of ±10% with respect to the stoichiometric amount determined from the Si amount and the P amount.
When an oxide powder having a β-Li ₃ PO ₄ type structure crystal phase as the main phase is obtained by a method of firing a precursor powder, the precursor powder and the oxide powder derived therefrom have approximately the same composition ratio of Li, Si, and P. Therefore, when a preferred embodiment of an ion conductor satisfying the above formulas (1) and (2) is obtained from a precursor powder, a precursor powder having a composition satisfying the above formulas (1) and (2) may be used.

Even when a precursor substance capable of synthesizing a crystal phase of β-Li ₃ PO ₄ type structure containing Li, Si, and P in the crystal lattice is used, a diffraction peak (see Non-Patent Document 3) specific to crystals of γ-Li ₃ PO ₄ type structure is observed in the X-ray diffraction pattern by Cu-Kα radiation when the firing temperature is increased to about 600° C. Since the phase transition from the β-Li ₃ PO ₄ type structure to the γ-Li ₃ PO ₄ type structure is considered to appear as a continuous structural change, at the very early stage when the structural change to the γ-Li ₃ PO ₄ type structure begins to occur, a very small peak may be observed near 2θ=20 degrees, or a minute intensity change so small that it is unclear whether it can be called a peak. It is known that oxides of the γ-Li ₃ PO ₄ type structure containing Si in the crystal lattice inherently exhibit ionic conductivity, and from the viewpoint of enjoying good ionic conductivity, there is no inconvenience even at the stage where the structural change to the γ-Li ₃ PO ₄ type structure begins to occur. Therefore, when a diffraction peak from a crystal plane corresponding to the diffraction peak of the β-Li ₃ PO ₄ oxide crystal is observed in the X-ray diffraction pattern by Cu-Kα rays, and at the same time, a very small peak or a minute intensity change that is confusingly small as to whether it can be called a peak is observed in the vicinity of 2θ = 20 degrees, it can be considered to have a β-Li ₃ PO ₄ type structure. More specifically, a crystal phase consisting of an oxide phase formed by holding the precursor described below in a temperature range of 550 ° C or less is considered to be a crystal phase of the β-Li ₃ PO ₄ type structure containing Li, Si, and P in the crystal lattice.

Figure 1 shows an example of the X-ray diffraction pattern of the 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 at various temperature levels between 150 and 700°C in air for 12 hours. The meaning of the nominal x value will be described later. X-ray diffraction measurements were performed as follows.

(Measurement of X-ray diffraction pattern)
The X-ray diffraction pattern of the sintered powder sample was measured using an X-ray diffractometer (Rigaku Corporation, SmartLab) under conditions of Cu-Kα radiation, tube voltage of 40 kV, tube current of 30 mA, measurement step of 0.01 degree, and scan speed of 2 degrees/min.

As can be seen from Figure 1, when the sintering temperature was 600°C or higher, a peak thought to be due to the (011) crystal plane of the γ-Li ₃ PO ₄ type structure was observed near 2θ = 20 degrees. It was confirmed that when the sintering temperature was lower than that, an oxide having a β-Li ₃ PO ₄ type structure was obtained. In addition, the presence of a small amount of a different phase thought to be a Li ₂ CO ₃ phase was observed in the sintered powders sintered at each temperature.
Among the X-ray diffraction patterns illustrated in FIG. 1, the examples of 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, respectively, which will be described later.

(nominal x value)
The nominal x value is a value that is uniquely determined as an x value that satisfies the composition formula Li _3+x Si _x P _1-x O ₄ (where 0<x<1) from the amounts of Li and P charged in the aqueous solution used in the synthesis of the precursor. That is, the nominal x value corresponds to the Si/(Si+P) atomic ratio determined from the amounts of Li and P charged when the stoichiometric composition represented by Li _3+x Si _x P 1- _x O ₄ (where 0<x<1) is satisfied. The amount of Si charged may be set to an excess amount (1.3 times the amount in Examples 5 to 8) relative to the above stoichiometric composition in consideration of the amount of Si reduced by volatilization, so the amounts of Li and P charged are used to calculate the nominal x value. A specific example of how to determine the nominal x value in Examples 5 to 8 will be shown below.
The composition formula Li _3+x Si _x P _1-x O ₄ is expressed as Li _4-y Si _1-y P _y O ₄ when y=1-x. 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 charged 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 calculated as x=1-y=0.20.

(Method of manufacturing ionic conductor)
The above-mentioned ionic conductor, which is the subject of the present invention, can be produced by a method of heating and holding a precursor powder described later in an oxidizing atmosphere at a temperature range of 120° C. to 550° C. In this specification, this heat treatment is called "calcination". The precursor powder can be formed by removing the solvent component from 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. The above-mentioned ionic conductor, which is the subject of the present invention, can also be obtained by a manufacturing mode in which the suspension (i.e., the precursor solution described later) is directly heated and held at a temperature range of 120° C. to 550° C. in an oxidizing atmosphere. In this manufacturing mode, it can be considered that a precursor powder from which the solvent component has been removed is formed during the heating and holding in the above temperature range. Therefore, this manufacturing mode is also included in one embodiment of the case where "the precursor powder is heated and held at a temperature range of 120° C. to 550° C. in an oxidizing atmosphere". When forming a layer of the above-mentioned ion conductor, which is the subject of the present invention, on the surface of an active material particle, for example, a method can be used in which a mixed powder of an active material powder and the above-mentioned precursor powder, or a mixture of an active material powder and the above-mentioned suspension (i.e., a precursor solution described below), is subjected to firing in the above-mentioned temperature range.

Air under atmospheric pressure can be used as the oxidizing atmosphere for the firing. Although it is possible to synthesize β-Li ₃ PO ₄ oxide that does not contain Si by a wet process at room temperature, it is desirable to heat to 120°C or higher in order to synthesize an oxide having a β-Li ₃ PO ₄ type structure that contains Si in the crystal lattice. On the other hand, in consideration of industrial production of all-solid-state lithium ion secondary batteries, it is better to have a low firing temperature. According to the inventors' study, a crystal phase having a β-Li ₃ PO ₄ type structure that contains Li, Si, and P in the crystal lattice can be generated in a temperature range of 550°C or less. As described above, when the heating temperature is increased to about 600°C, a crystal phase having a γ-Li ₃ PO ₄ type structure is formed.

The firing may be divided into multiple heating steps. It may also be performed using a heat pattern in which the holding temperature in the range of 120 to 550°C is changed in multiple steps. Examples of the manner in which such multi-step firing is performed include a heat pattern in which the first stage firing is performed by holding the material at a relatively low temperature range of 120°C to 200°C for a relatively short time, such as 1 hour to 3 hours, and then the material is cooled to room temperature, crushed as necessary, and then the second stage firing is performed at a higher temperature and for a longer time than the first stage, and a heat pattern in which the temperature is raised following the first stage firing and the second stage firing is performed at a higher temperature and for a longer time than the first stage. Such a multi-step heat pattern that imparts a relatively low temperature and short heating history in the initial stage of firing can, for example, sufficiently remove volatile components contained in the precursor powder in the initial stage of firing before proceeding with the crystallization reaction, and is effective in obtaining a fired powder with few heterogeneous phases.

The maximum firing temperature is 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 during which the temperature is maintained in the range of 120°C to 550°C, is preferably 2 hours or more, and more preferably 10 hours or more. However, from the perspective of economy, the firing time is preferably set in the range of 30 hours or less, and may be controlled to 20 hours or less. After firing, a crushing process can be performed as necessary to obtain a homogeneous fired powder.

[Ion-conductive sintered body]
A preferred embodiment of the ion conductor used as a constituent material of an all-solid-state lithium ion secondary battery is a sintered body. For example, a sintered body of powder particles of the ion conductor is useful as a member constituting a solid electrolyte layer interposed between a positive electrode layer and a negative electrode layer. In addition, a sintered body of positive electrode active material particles coated with the ion conductor, or a sintered body of positive electrode active material particles and particles of the ion conductor filling the gap therebetween, is useful as a member constituting a positive electrode layer. In an all-solid-state lithium ion secondary battery in which the negative electrode layer, the positive electrode layer, and the solid electrolyte layer therebetween are integrated by sintering, if the ion conductor is contained in any of the layers, the battery corresponds to a sintered body using the ion conductor.

(Method for producing ion-conductive sintered body)
The above-mentioned ion conductive sintered body can be obtained by (i) a method of sintering a powder containing particles of a precursor powder described below, or (ii) a method of sintering a powder containing particles of the above-mentioned ion conductor powder (sintered powder), or the like.
(i) A method of sintering a powder containing particles of a precursor powder In this case, the heating for sintering also serves as the firing for oxide synthesis. Specifically, a method can be applied in which a molded body of a powder containing particles of the precursor powder described below is heated and held in an oxidizing atmosphere at a temperature range of 120°C to 550°C while applying pressure, thereby generating a crystal phase of a β-Li ₃ PO ₄ type structure containing Li, Si, and P in the crystal lattice and obtaining a sintered body. Air under atmospheric pressure can be used as the oxidizing atmosphere. In order to efficiently proceed with sintering, it is more effective to set the heating temperature to 300°C or higher. In addition, the time during which the molded body is held in a temperature range of 120°C to 550°C is preferably 2 hours or more, more preferably 10 hours or more. However, in consideration of economic efficiency, the heating time is preferably set in a range of 30 hours or less, and 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, it is sufficient to apply a molded body made of the precursor powder. When constructing an electrode layer, the above-mentioned molded body may be made of a mixed powder containing, for example, an active material powder and a precursor powder.

(ii) A method of sintering a powder containing particles of an ion conductor powder In this case, a method of obtaining a sintered body by heating and holding a molded body of a powder containing particles of the above-mentioned ion conductor powder (sintered powder) while applying pressure in a temperature range of 120°C to 550°C can be applied. 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 may 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 grain growth of the powder are necessary to efficiently promote sintering, it is effective to set the heating temperature to a temperature range that is not significantly lower than the sintering temperature when the sintered powder was synthesized. Specifically, when the maximum temperature reached in the sintering process is T ₀ (°C) and the maximum temperature reached in this sintering process is T ₁ (°C), it is preferable to set the conditions to satisfy T ₁ ≧T ₀ -50 and 120≦T ₁ ≦550. When constructing a sintered body to be applied to a solid electrolyte layer of an all-solid-state battery, the above-mentioned molded body may be made of the above-mentioned ion conductor powder (sintered powder).When constructing an electrode layer, the above-mentioned molded body may be made of a mixed powder containing, for example, an active material powder and the above-mentioned ion conductor powder (sintered powder).
The pressure applied to the material being heated in the sintering step may be set, for example, in the range of 125 MPa to 1000 MPa.

[Precursor solution]
The ion conductor of the present invention can be obtained by calcining a precursor powder described later. An intermediate product for obtaining the precursor powder can be an aqueous solution in which colloidal particles containing Li, Si, and P are suspended. This aqueous solution is referred to as a "precursor solution" in this specification. The precursor solution is specified by, for example, being an aqueous solution in which colloidal particles containing Li, Si, and P are suspended, and by having a property of forming a β-Li 3 PO 4 type structure crystal phase containing Li, Si, and P in the crystal lattice when the aqueous solution is evaporated to dryness at 70° C. and then heated at 150° C. for 2 hours to obtain a solid, and then subjected to a heat treatment of heating at ₄₀₀ ° C. for 12 hours in air. The colloidal particles are specified as, for example, a mixed product of an aqueous solvent, a water-soluble _lithium compound, a silicon alkoxide, and a water-soluble phosphorus compound.

(Preparation method of precursor solution)
As a result of research, the inventors have found that a precursor substance capable of forming a crystal phase of β-Li ₃ PO ₄ type structure containing Li, Si, and P in the crystal lattice can be obtained by a method of 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 that utilizes a hydrolysis condensation process (sol-gel method) using silicon alkoxide, which is a source of Si. An aqueous solvent is a liquid medium whose main component is water (i.e., the mass ratio of water is 50% or more).

The above stirring and mixing is preferably carried out in the following manner.
First, an aqueous solution in which a water-soluble lithium compound is dissolved is prepared. Silicon alkoxide is added to this aqueous solution and thoroughly stirred. This stirring is preferably performed at a temperature near room temperature (for example, 15°C or higher and 45°C or lower). The atmosphere of the gas phase in contact with the liquid during stirring can be air under atmospheric pressure. Silicon alkoxide is difficult to dissolve in water, but when the hydrolysis of silicon alkoxide progresses due to the presence of lithium ions in the aqueous solution, a solution of active silicon colloid (presumably SiO _x hydrated) and lithium ions is generated. If the silicon alkoxide and water are not mixed sufficiently, an impurity phase is likely to be formed. In order to obtain a solution with as high homogeneity as possible, it is desirable to stir at this stage for 15 minutes or more. Thereafter, a water-soluble phosphorus compound is added to the liquid and dissolved. The water-soluble phosphorus compound may be dissolved in the liquid before adding the silicon alkoxide, but from the viewpoint of suppressing the generation of Li ₃ PO ₄ , it is preferable to add the water-soluble phosphorus compound after mixing the silicon alkoxide.

Next, the solution obtained by the above stirring is heated and stirred to react with the phosphorus compound. As this reaction proceeds, a white suspension containing colloidal particles is produced. This suspension can be used as a precursor solution for obtaining the ionic conductor described above. To allow the reaction to proceed sufficiently, for example, the liquid temperature during stirring is preferably 50 to 95°C. The stirring time is preferably 30 minutes or more. Considering economy, the stirring time is usually set to a range of 120 minutes or less. The gas phase atmosphere that the liquid comes into contact with during stirring can be air under atmospheric pressure. The structure of the colloidal particles produced by this reaction has not yet been clarified, but it is possible that compounds of Li, Si, P, and O are formed at this stage.

The amounts (charge amounts) of the water-soluble lithium compound, silicon alkoxide, and water-soluble phosphorus compound used for the above stirring and mixing are preferably set so that the composition of Li, Si, and P (charge composition) satisfies the following quantitative ratios (3) and (4).
0.05≦x≦0.45 ... (3)
1.05A≦Si≦1.50A (4)
Here, the value of x in formula (3) and the value of A in formula (4) are determined by the following formulas (5) and (6), respectively.
x = 1-4P / (Li + P) ... (5)
A = xP / (1-x) ... (6)
However, the element symbols Si, P, and Li in formulas (4), (5), and (6) are substituted with the ratios (atomic ratios) of the amounts (moles) of Si atoms, P atoms, and Li atoms to the total amount (moles) of Si atoms, P atoms, and Li atoms present in the powder.

The crystal phase of the β-Li ₃ PO ₄ type structure containing Li, Si, and P in the crystal lattice is considered to be represented by the composition formula Li _3+x Si _x P _1-x O ₄ (where 0<x<1). Therefore, it is considered to be advantageous in obtaining a fired product with few heterogeneous phases if the Li, Si, and P composition ratio of the precursor to be fired is as close as possible to the stoichiometric composition of Li ₃₊ x Si _x P _1-x O _4. In the above-mentioned aqueous solution synthesis, the silicon alkoxide serving as the Si source or a part of the substance derived therefrom may disappear from the liquid due to volatilization or the like. Therefore, it is preferable to charge Si in an amount in excess of the above-mentioned stoichiometric composition.

The above formula (5) derives the x value that satisfies the composition formula Li _3+x Si _x P _1-x O ₄ from the charged compositions of Li and P. Considering that it is preferable to charge an amount of Si in excess of the stoichiometric composition, the x value in the charged composition is determined from the charged compositions of Li and P. It is preferable to determine the charged amounts of the water-soluble lithium compound and the water-soluble phosphorus compound so 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 the inventors' studies, it is effective to set the Si charge composition to an excess amount in the range of 1.05 to 1.50 times the A value (i.e., the ratio of Si in the stoichiometric composition). Therefore, it is preferable to determine the charge 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 the following formula (4)'.
1.20A≦Si≦1.40A ... (4)'

Examples of the water-soluble lithium compound include lithium hydroxide monohydrate, lithium acetate, lithium carbonate, and lithium nitrate.
Examples of silicon alkoxides include tetraethoxysilane, trimethoxysilane, tetramethoxysilane, triethoxysilane, tripropoxysilane, tetrapropoxysilane, and tributoxysilane.
Examples of the water-soluble phosphorus compound include ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and phosphoric acid.

[Precursor powder]
A suitable precursor powder for synthesizing the above-mentioned ionic conductor of the present invention is, for example, a precursor powder consisting of particles containing Li, Si, and P, and is specified as having a property of forming a β-Li ₃ PO ₄ type structure crystal phase containing Li, Si, and P in the crystal lattice when the powder is calcined by heating at 400° C. in air for 12 hours. More specifically, an example of the precursor powder is a precursor powder specified as a solvent-removed component of an aqueous solution in which colloidal particles containing Li, Si, and P are suspended when the Si source is silicon alkoxide. The "solvent-removed component of an aqueous solution" refers to the components of the aqueous solution excluding the solvent component, and includes, for example, an evaporated to dryness product and solid components recovered by solid-liquid separation (filtration or centrifugation). The Li, Si, and P composition ratio of the above-mentioned ionic conductor of the present invention almost reflects the Li, Si, and P composition ratio of the precursor powder. Therefore, an example of a preferred embodiment of the precursor powder is one in which the contents of Li, Si, and P satisfy the above-mentioned formulas (1) and (2).

(Method for producing precursor powder)
As a preferred embodiment for obtaining the above precursor powder, the following production method can be mentioned.
a precursor solution preparation step of forming a colloidal solution by stirring and mixing in an aqueous solvent a water-soluble lithium compound, a silicon alkoxide, and a water-soluble phosphorus compound in a quantitative ratio such that the composition of Li, Si, and P satisfies the above-mentioned formulas (3) and (4);
a precursor powder synthesis step of removing a solvent component from the colloidal solution to obtain a solid content;
The present invention relates to a method for producing a precursor powder of an ion conductor.
Examples of the method for removing the solvent component from the colloidal solution to obtain a solid content include evaporation to dryness, filtration, and centrifugation.

[Example 1]
(Precursor solution)
The following water-soluble lithium compound was prepared: lithium hydroxide monohydrate _LiOH.H2O (manufactured by Kojundo Chemical Laboratory, purity 99% or more); the silicon alkoxide was _{tetraethoxysilane} Si( _OC2H5 ) ₄ (manufactured by Fujifilm Wako Pure Chemical Industries, special reagent grade); and the water-soluble phosphorus compound _was ammonium _dihydrogen phosphate _NH4H2PO4 (manufactured by Fujifilm Wako Pure Chemical Industries, special reagent grade).
The amounts of the raw materials charged were 3.462 g _of lithium hydroxide monohydrate, _{2.032 g of tetraethoxysilane ,} and 2.013 g of ammonium dihydrogen phosphate so that the aforementioned nominal x value (x value determined from the charged composition of Li and P in the composition formula Li3+xSixP1-xO4) was 0.3 and the charged amount of Si was 1.3 times the stoichiometric composition.

First, lithium hydroxide monohydrate was dissolved in 50 mL of ion-exchanged water to obtain an aqueous solution of lithium hydroxide. Tetraethoxysilane was added to this aqueous solution of lithium hydroxide, and the solution was stirred at room temperature for 30 minutes using a stirrer. Next, ammonium dihydrogen phosphate was added to the solution after stirring, and the solution temperature was raised to 70° C. and stirred at 70° C. for 1 hour. In this way, a colloidal solution (precursor solution) was obtained.
Table 1 shows the composition of the mixture (the same applies to each of the following examples). In Table 1, the sufficiency of the above-mentioned formulas (3) and (4) is indicated by ◯ if the formula is satisfied, and by × if the formula is not satisfied.

(Precursor Powder)
After stirring at 70° C. for 1 hour, the stirrer was removed from the colloidal solution, which was then heated at 70° C. for 15 hours to evaporate to dryness, and a powder sample (precursor powder) made of the evaporated to dryness product was collected.

(Fired powder)
The collected powder sample (precursor powder) was heated in air at 150°C for 2 hours, then lightly crushed in an agate mortar, and then placed in an alumina crucible and calcined in an electric furnace (FP102, manufactured by Yamato Scientific) at 400°C for 12 hours to obtain a white powder (hereinafter referred to as the "calcined powder"). The maximum calcination temperature in this example was 400°C, and the total calcination time was 14 hours.

The obtained sintered powder was subjected to X-ray diffraction measurement under the conditions described in the above-mentioned "Measurement of X-ray diffraction pattern". From the measured X-ray diffraction pattern, it was confirmed that this sintered powder has a crystal phase of β-Li ₃ PO ₄ type structure as the main phase. A small amount of Li ₂ CO ₃ phase was found to be mixed in as a different phase.
The results are shown in Table 2 (the same applies to each of the following examples). The "firing temperature" shown in Table 2 is the maximum temperature reached. In this example and each of the following examples, heating and holding equivalent to the above-mentioned second stage firing was performed while maintaining the firing temperature (maximum temperature reached) shown in Table 2. In addition, the lattice constant and unit lattice volume of the β-Li ₃ PO ₄ type crystal obtained from the X-ray diffraction pattern are also shown in Table 2 (the same applies to each of the examples described below except Comparative Example 1).

The chemical composition of the resulting sintered powder was examined 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 sufficiency of the above-mentioned formulas (1) and (2) is shown with ◯ when it is satisfied, and with × when it is not satisfied.
The sintered powder of this example satisfies formula (2) and can be evaluated as having a crystal phase in the main phase having a Li, Si, and P composition ratio close to the stoichiometric composition of the composition formula Li _3+x Si _x P _1-x O ₄ .

(Sintered body)
The above-mentioned sintered powder was used as a material and heated in air at 400° C. for 2 hours under a pressure of 375 MPa using a heat press machine (manufactured by AS ONE Corporation) to produce a sintered body.

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, and the resistance value was obtained from the arc of the Nyquist plot, from which the conductivity was calculated. Note that an Au electrode was used as the blocking electrode.
Table 4 shows the electrical conductivity at 200° C. (the same applies to the following Examples 2, 3, 4, and 7, and Comparative Example 3).
The sintered body of this example exhibited an electrical conductivity (lithium ion conductivity) of 3.022×10 ⁻⁶ S/cm at 200° C. In other words, it was confirmed that the sintered powder obtained in this example was an ion conductor.

[Example 2]
An experiment similar to that in Example 1 was carried out, except that in the preparation of the sintered body, the heating temperature was changed from 400°C to 500°C.
The sintered body of this example exhibited an electrical conductivity (lithium ion conductivity) of 7.745×10 ⁻⁶ S/cm at 200° C. In other words, it was confirmed that the sintered powder obtained in this example was an ion conductor.

[Example 3]
In preparing the precursor solution, the amounts of the raw materials were set to 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 charged was 1.3 times the stoichiometric composition. Except for this, the experiment was carried out under the same conditions as in Example 1.
From the X-ray diffraction pattern, it was confirmed that the sintered powder obtained in this example has a β-Li ₃ PO ₄ type structure crystal phase as the main phase. Small amounts of Li ₂ CO ₃ phase and Li ₂ SiO ₃ phase were found to be mixed as heterogeneous phases. As a result of ICP-AES analysis, it can be evaluated that the sintered powder of this example satisfies formula (2) and has a crystalline phase having a Li, Si, and P composition ratio close to the stoichiometric composition of the composition formula Li _3+x Si _x P _1-x O ₄ as the main phase.
The sintered body of this example exhibited an electrical conductivity (lithium ion conductivity) of 7.703×10 ⁻⁶ S/cm at 200° C. In other words, it was confirmed that the sintered powder obtained in this example was an ion conductor.

[Example 4]
In preparing the precursor solution, the amounts of the raw materials were set to 3.252 g of lithium hydroxide monohydrate, 0.677 g of tetraethoxysilane, and 2.589 g of ammonium dihydrogen phosphate so that the nominal x value was 0.1 and the amount of Si charged was 1.3 times the stoichiometric composition. Except for this, the experiment was carried out under the same conditions as in Example 1.
From the X-ray diffraction pattern, it was confirmed that the sintered powder obtained in this example has a β-Li ₃ PO ₄ type structure crystal phase as the main phase. No other phases were observed to be present. As a result of ICP-AES analysis, the sintered powder of this example can be evaluated as having a crystalline phase as the main phase, which satisfies formula (2) and has a Li, Si, and P composition ratio close to the stoichiometric composition of the composition formula Li _3+x Si _x P _1-x O ₄ .
The sintered body of this example exhibited a conductivity (lithium ion conductivity) of 2.859×10 ⁻⁷ S/cm at 200° C. In other words, it was confirmed that the sintered powder obtained in this example was an ion conductor.

[Example 5]
In the preparation of the precursor solution, the amounts of the 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 was 1.3 times the stoichiometric composition. The firing temperature was set to 150° C. A fired powder was prepared under the same conditions as in Example 1 except for the above, and X-ray diffraction measurement and composition analysis were performed in the same manner as in Example 1.
From the X-ray diffraction pattern, it was confirmed that the sintered powder obtained in this example has a β-Li ₃ PO ₄ type structure crystal phase as the main phase. A small amount of Li ₂ CO ₃ phase was found to be mixed in as a different phase. As a result of ICP-AES analysis, the sintered powder of this example can be evaluated as having a crystalline phase as the main phase that satisfies formula (2) and has a Li, Si, and P composition ratio close to the stoichiometric composition of the composition formula Li _3+x Si _x P _1-x O ₄ .

[Example 6]
In the preparation of the precursor solution, the amounts of the 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 was 1.3 times the stoichiometric composition. The firing temperature was set to 300° C. A fired powder was prepared under the same conditions as in Example 1 except for the above, and X-ray diffraction measurement and composition analysis were performed in the same manner as in Example 1.
From the X-ray diffraction pattern, it was confirmed that the sintered powder obtained in this example has a β-Li ₃ PO ₄ type structure crystal phase as the main phase. A small amount of Li ₂ CO ₃ phase was found to be mixed in as a different phase. As a result of ICP-AES analysis, the sintered powder of this example can be evaluated as having a crystalline phase as the main phase that satisfies formula (2) and has a Li, Si, and P composition ratio close to the stoichiometric composition of the composition formula Li _3+x Si _x P _1-x O ₄ .

[Example 7]
In preparing the precursor solution, the amounts of the 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 charged was 1.3 times the stoichiometric composition. Except for this, the experiment was carried out under the same conditions as in Example 1.
From the X-ray diffraction pattern, it was confirmed that the sintered powder obtained in this example has a β-Li ₃ PO ₄ type structure crystal phase as the main phase. A small amount of Li ₂ CO ₃ phase was found to be mixed in as a different phase. As a result of ICP-AES analysis, the sintered powder of this example can be evaluated as having a crystalline phase as the main phase that satisfies formula (2) and has a Li, Si, and P composition ratio close to the stoichiometric composition of the composition formula Li _3+x Si _x P _1-x O ₄ .
The sintered body of this example exhibited a conductivity (lithium ion conductivity) of 9.963×10 ⁻⁷ S/cm at 200° C. In other words, it was confirmed that the sintered powder obtained in this example was an ion conductor.

[Example 8]
In the preparation of the precursor solution, the amounts of the 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 was 1.3 times the stoichiometric composition. The firing temperature was set to 500° C. A fired powder was prepared under the same conditions as in Example 1 except for the above, and X-ray diffraction measurement and composition analysis were performed in the same manner as in Example 1.
From the X-ray diffraction pattern, it was confirmed that the sintered powder obtained in this example has a β-Li ₃ PO ₄ type structure crystal phase as the main phase. A small amount of Li ₂ CO ₃ phase was found to be mixed in as a different phase. As a result of ICP-AES analysis, the sintered powder of this example can be evaluated as having a crystalline phase as the main phase that satisfies formula (2) and has a Li, Si, and P composition ratio close to the stoichiometric composition of the composition formula Li _3+x Si _x P _1-x O ₄ .

[Comparative Example 1]
In preparing the precursor solution, the amounts of the 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 was 1.3 times the stoichiometric composition. The firing temperature was set to 600° C. A fired powder was prepared under the same conditions as in Example 1 except for the above, and X-ray diffraction measurement was performed in the same manner as in Example 1.
From the X-ray diffraction pattern, it was determined that the sintered powder obtained in this example had a crystal phase of γ-Li ₃ PO ₄ type structure as the main phase. A small amount of Li ₂ CO ₃ phase was found to be mixed in as a different phase.

[Comparative Example 2]
In the preparation of the precursor solution, the amounts of the 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 charged was 1.3 times the stoichiometric composition. A sintered powder was prepared under the same conditions as in Example 1, and X-ray diffraction measurements and composition analysis were performed in the same manner as in Example 1.
From the X-ray diffraction pattern, it was confirmed that the sintered powder obtained in this example had a β-Li ₃ PO ₄ type structure crystal phase as the main phase. The presence of Li ₂ CO ₃ and Li ₂ SiO ₃ phases as heterophases was observed, and the amount of these phases was greater than that of Example 3. As a result of ICP-AES analysis, the sintered powder of this example had a Si/(Si+P) of 0.45, which does not satisfy formula (1).

[Comparative Example 3]
Here, the aforementioned β-Li ₃ PO ₄ powder with no added Si and 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 the raw materials charged would be the Li:P ratio that satisfies the composition formula Li ₃ PO _4. The above amount of lithium hydroxide monohydrate was dissolved in 50 mL of ion-exchanged water to obtain an aqueous lithium hydroxide solution. The above amount of ammonium dihydrogen phosphate was added to this aqueous lithium hydroxide solution, the liquid temperature 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. Furthermore, the stirrer was removed from the above colloidal solution after stirring at 70°C for 1 hour, and the solution was heated at 70°C for 15 hours to evaporate and dry. The evaporated and dried product was heated in air at 150°C for 2 hours, and a powder sample was collected.

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 was composed of β-Li ₃ PO ₄ crystals. The crystals may have been formed by the evaporation to dryness described above, but the "calcination temperature" column in Table 2 lists 150°C, which is the maximum temperature reached by the applied thermal history.
Using this powder as the material, a sintered body was produced in the same manner as in Example 1, except that the sintering temperature was 300°C, and the conductivity at 200°C was measured. As a result, the conductivity of the sintered body of this example at 200°C was 3.5 x ^10-8 S/cm. Comparing this example with the above-mentioned Examples 1 to 4 and 7, it is found that although the β-Li ₃ PO ₄ crystal (this example) has poor lithium ion conductivity, the lithium ion conductivity is significantly improved by dissolving Si to form a crystal with a β-Li ₃ PO ₄ type structure containing Li, Si and P in the crystal lattice.

Claims (14)

An ionic conductor having a crystal phase of β-Li ₃ PO ₄ type structure containing Li, Si and P in the crystal lattice as a main phase, and having a Si/(Si+P) atomic ratio of 0.05 or more and 0.40 or less. An ion-conductive sintered body using the ion conductor according to claim 1. A precursor solution for an ionic conductor, which is made of an aqueous solution in which colloidal particles containing Li, Si and P are suspended, and which has the property of forming a crystalline phase with a β-Li3PO4 type structure containing Li, Si and P in the _crystal lattice when the aqueous solution is evaporated to dryness at 70°C and then heated at 150°C for 2 hours to obtain a solid matter, _which is then subjected to a heat treatment in air at 400°C for 12 hours. The precursor solution of the ionic conductor according to claim 3, wherein the colloidal particles are a mixture product of an aqueous solvent, a water-soluble lithium compound, a silicon alkoxide, and a water-soluble phosphorus compound. A precursor powder of an ion conductor, which is composed of particles containing Li, Si, and P, and has the property of forming a crystal phase with a β-Li ₃ PO ₄ type structure containing Li, Si, and P in the crystal lattice when the powder is calcined by heating at 400°C in air for 12 hours. The precursor powder of the ionic conductor according to claim 5, wherein the precursor powder is a solvent-removed component of an aqueous solution in which colloidal particles containing Li, Si, and P are suspended. 6. The precursor powder of an ionic conductor according to claim 5, wherein the precursor powder satisfies the following formulas (1) and (2) in terms of the contents of Li, Si and P:
0.05≦Si/(Si+P)≦0.40 ... (1)
0.90(4Si+3P)≦Li≦0.10(4Si+3P) ... (2)
Here, the element symbols Si, P, and Li in formulas (1) and (2) are substituted with the ratios (atomic ratios) of the amounts (moles) of Si atoms, P atoms, and Li atoms to the total amount (moles) of Si atoms, P atoms, and Li atoms present in the powder. A method for producing an ionic conductor, comprising the steps of: mixing a water-soluble lithium compound, a silicon alkoxide, and a water-soluble phosphorus compound in an aqueous solvent to obtain a suspension of colloidal particles; removing the solvent components from the resulting suspension; and heating and holding the resulting precursor powder in an oxidizing atmosphere at a temperature range of 120°C to 550°C, thereby generating a crystal phase having a β- _Li3PO4 structure that contains Li, _Si , and P in the crystal lattice. The method for producing an ionic conductor according to claim 8 , wherein the precursor powder satisfies the following formulas (1) and (2) in terms of the contents of Li, Si, and P:
0.05≦Si/(Si+P)≦0.40 ... (1)
0.90(4Si+3P)≦Li≦0.10(4Si+3P) ... (2)
Here, the element symbols Si, P, and Li in formulas (1) and (2) are substituted with the ratios (atomic ratios) of the amounts (moles) of Si atoms, P atoms, and Li atoms to the total amount (moles) of Si atoms, P atoms, and Li atoms present in the powder. A method for producing an ion-conductive sintered body, comprising the steps of: mixing a water-soluble lithium compound, a silicon alkoxide, and a water-soluble phosphorus compound in an aqueous solvent to obtain a suspension of colloidal particles; removing the solvent components from the resulting suspension; heating and holding the powder _compact containing precursor powder particles in an oxidizing atmosphere at a temperature range of 120°C to 550°C while applying pressure, thereby generating a crystal phase having a β- _Li3PO4 structure containing Li, Si, and P in the crystal lattice, and obtaining a sintered body. a calcination step in which a precursor powder is formed by removing the solvent 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 the precursor powder is heated and held at a temperature range of 120°C to 550°C in an oxidizing atmosphere to generate a crystal phase having a β- _Li3PO4 structure containing Li, _Si , and P in the crystal lattice, thereby obtaining an ion conductor powder;
a sintering step of heating and holding the powder compact containing the ion conductor powder particles at a temperature range of 120° C. to 550° C. while applying pressure to obtain a sintered body;
The present invention relates to a method for producing an ion-conductive sintered body having an ion-conductive sintered body. The method for producing an ion-conductive sintered body according to claim 11, wherein the firing step and the sintering step are carried out under conditions that satisfy T ₁ ≧T ₀ -50 and 120≦T ₁ ≦550, where T ₀ is the maximum temperature reached in the firing step (°C) and T ₁ is the maximum temperature reached in the sintering step (°C). A method for producing a precursor solution of an ion conductor, comprising the step of forming a colloidal solution by mixing, in an aqueous solvent, a water-soluble lithium compound, a silicon alkoxide, and a water-soluble phosphorus compound in quantitative proportions such that the composition of Li, Si, and P satisfies the following formulas (3) and (4):
0.05≦x≦0.45 ... (3)
1.05A≦Si≦1.50A (4)
Here, the value of x in formula (3) and the value of A in formula (4) are determined by the following formulas (5) and (6), respectively.
x = 1-4P / (Li + P) ... (5)
A = xP / (1-x) ... (6)
However, the element symbols Si, P, and Li in formulas (4), (5), and (6) are substituted with the ratios (atomic ratios) of the amounts (moles) of Si atoms, P atoms, and Li atoms to the total amount (moles) of Si atoms, P atoms, and Li atoms present in the powder. a precursor solution preparation step of forming a colloidal solution by mixing, in an aqueous solvent, a water-soluble lithium compound, a silicon alkoxide, and a water-soluble phosphorus compound in a quantitative ratio such that the composition of Li, Si, and P satisfies the following formulas (3) and (4);
a precursor powder synthesis step of removing a solvent component from the colloidal solution to obtain a solid content;
The present invention relates to a method for producing a precursor powder of an ion conductor.
0.05≦x≦0.45 ... (3)
1.05A≦Si≦1.50A (4)
Here, the value of x in formula (3) and the value of A in formula (4) are determined by the following formulas (5) and (6), respectively.
x = 1-4P / (Li + P) ... (5)
A = xP / (1-x) ... (6)
However, the element symbols Si, P, and Li in formulas (4), (5), and (6) are substituted with the ratios (atomic ratios) of the amounts (moles) of Si atoms, P atoms, and Li atoms to the total amount (moles) of Si atoms, P atoms, and Li atoms present in the powder.

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