WO2025177905A1 - Phosphate powder, solid electrolyte, and method for producing same, and lithium secondary battery - Google Patents

Abstract

Provided are a solid electrolyte excellent in lithium ion conductivity, and a method for producing same.　This solid electrolyte is represented by general formula: X_AM_BY_CP_DO_{12 ± σ}, wherein A to D representing molar ratios satisfy a certain relationship, the lattice constant ratio c/a is 2.52 or less, the solid electrolyte has a NASICON-type structure of a rhombohedral crystal system having a lattice volume of 1505 ų - 1522 ų, and the proportion of triclinic structures is reduced as much as possible. The solid electrolyte is produced by: heating a mixture solution to remove moisture, wherein the mixture solution includes a Zr raw material, a Y raw material, a P raw material, and a chelating agent, and the pH of the mixture solution is adjusted to 7.0 or less; firing the mixture in ambient atmosphere to obtain an oxide precursor; adding a Li raw material thereto; and further firing same in ambient atmosphere.

Description

Phosphate powder, solid electrolyte, methods for producing the same, and lithium secondary battery

The present invention relates to phosphate powder, solid electrolytes, methods for producing them, and lithium secondary batteries. More specifically, it relates to phosphate powder suitable for obtaining solid electrolytes having a Li-Zr-Y-P-O-based Nasicon structure, solid electrolytes obtained using this, methods for producing them, and lithium secondary batteries.

Oxides with a NASICON structure, which are lithium ion conductors, are attracting attention as solid electrolyte materials for lithium secondary batteries.

This Nasicon-type solid electrolyte has poor reduction resistance when it contains Ti, so it is thought that Li-Zr-P-O-based Nasicon-type solid electrolytes containing Zr are more promising.

Non-Patent Document 1 describes a Zr-based Nasicon-type solid electrolyte in which part of the Zr is replaced with elements such as Ca. In Li-Zr-P-O-based Nasicon-type solid electrolytes, the crystal structure can undergo a transition near room temperature, which can lead to a decrease in ionic conductivity. Therefore, by replacing part of the Zr with other elements such as Ca, Al, or Y, the crystal structure transition is prevented and the decrease in ionic conductivity is suppressed.

Furthermore, Non-Patent Documents 2 and 3 disclose that Nasicon-type solid electrolytes can be stabilized at room temperature by substituting a portion of the Zr with Y. Of these, Non-Patent Document 2 uses ICP analysis to investigate the resulting composition ratio of Li, Zr, and Y. Furthermore, Non-Patent Document 3 describes that when the P composition is insufficient, the resistance is low, i.e., the ionic conductivity is high.

Furthermore, Patent Document 1 describes that in a Nasicon-type solid electrolyte represented by the general formula: Li _1+x M _y P ₃ O ₁₂ , M is an element such as Zr, Ca, Na, Y, or Al, and y in the formula representing the molar ratio of M is in the range of 2.001≦y≦2.100, thereby obtaining a solid electrolyte with enhanced ionic conductivity. Generally, when y is greater than 2, (y-2) Ms are located on the Li site. Since the ion conduction path is constituted by the Li site, if the Li site is substituted with an element other than the ion-conducting species, it is thought that the ionic conductivity will decrease. However, the invention of Patent Document 1 states that the ionic conductivity of the solid electrolyte can be improved by using an element that becomes a monovalent to tetravalent cation as described above, as in the Li-Ca-Zr-Y-P-O-based solid electrolyte, where M is an element that becomes a monovalent to tetravalent cation as described above.

Furthermore, Patent Document 2 claims that in a solid electrolyte made of an oxide with a Nasicon structure expressed by the general formula Li _1+2x Zr _2-x Ca _x (PO ₄ ) ₃ , by substituting part of the Zr element with Hf and part of the Ca element with Ba or Sr, it is possible to form a path that facilitates lithium ion conduction.

On the other hand, Patent Document 3 describes a method of obtaining a lithium ion conductive material by mixing lithium metaphosphate (LiPO ₃ ) as a raw material glass and zirconium phosphate [(ZrO) ₂ (HPO ₄ ) ₂ ] and yttrium oxide (Y ₂ O ₃ ) as other raw materials, forming the mixture into pellets, and firing the pellets.

Here, Patent Document 3 describes how the lithium ion conductive material obtained above can be mixed and sintered with Li ₂ O—P ₂ O ₅ —SiO ₂ -based glass to obtain a solid electrolyte with excellent lithium ion conductivity. In this case, the temperature for mixing and sintering with Li 2 O—P 2 O 5 —SiO 2-based glass can be lowered to less than 800°C by reducing the composition of the raw material glass (LiPO ₃ ) used to obtain the _lithium ion conductive material from the _{lithium ion} conductive material, that is, by reducing the Li and P in _equimolar amounts.

Incidentally, in order to obtain a Nasicon-type solid electrolyte, in the aforementioned Patent Document 1, raw materials such as lithium carbonate (Li ₂ CO ₃ ), zirconium oxide (ZrO ₂ ), ammonium dihydrogen phosphate (NH ₄ H ₂ PO ₄ ), calcium oxide (CaO), yttrium oxide (Y ₂ O ₃ ), and yttrium-stabilized zirconia are weighed and mixed, and then fired at 500°C for 1 hour and at 800°C for 6 hours to remove volatile components, and then crushed, and then heated to remove moisture, and then fired at 900 to 1200°C for 20 hours to obtain a solid electrolyte powder. However, since the ammonium dihydrogen phosphate, which is part of the raw material, is in a granular state, it needs to be crushed sufficiently to be mixed uniformly with the other raw materials.

Furthermore, the aforementioned Patent Document 3 discloses a method in which lithium metaphosphate (LiPO ₃ ) is first used as a raw material glass, and zirconium phosphate [(ZrO) ₂ (HPO ₄ ) ₂ ] and yttrium oxide (Y ₂ O ₃ ) are used as other raw materials, which are then mixed and pelletized to obtain a lithium ion conductive material. The method then discloses a method in which the obtained lithium ion conductive material is mixed and sintered with Li ₂ O—P ₂ O ₅ —SiO ₂ -based glass to obtain a solid electrolyte with excellent lithium ion conductivity. In this method, the lithium ion conductive material and Li ₂ O—P ₂ O ₅ —SiO _2- based glass are mixed and sintered in order to minimize the firing temperature (sintering temperature) required to finally obtain the solid electrolyte, which results in a complex process and extra labor.

On the other hand, Patent Document 4 discloses a solid electrolyte material containing solid electrolyte particles having a Nasicon structure and crystalline phosphate compound particles having a different crystal structure in a predetermined ratio. In this solid electrolyte material, the crystalline phosphate compound particles such _{as ZrP2O7} have a higher resistance than the solid electrolyte particles, and the crystalline phosphate compound particles have a different crystal structure from the solid electrolyte particles, so they are less likely to react with each other, and the solid electrolyte particles maintain a crystalline phase with high ionic conductivity, resulting in high ionic conductivity of the solid electrolyte material.

It is known that zirconium phosphate has ion exchange ability (see, for example, Patent Document 5), and as described in Patent Document 4 above, zirconium phosphate-based materials are sometimes used to obtain solid electrolytes.

In order to obtain such zirconium phosphate, for example, Patent Documents 6 and 7 describe liquid-phase synthesis (wet synthesis) using an organic acid with two or more carboxyl groups, such as oxalic acid or malonic acid, as a chelating agent. Furthermore, Patent Document 8 discloses crystalline zirconium phosphate in which a metal element is immobilized on the surface of zirconium phosphate by heat-treating a mixture of zirconium phosphate and an inorganic acid salt, such as yttrium nitrate, at 400°C or higher, thereby imparting the properties of the metal element to the zirconium phosphate's intended use. However, the zirconium phosphate disclosed in Patent Document 8 does not contain yttrium (Y) as a solid solution in zirconium phosphate.

International Publication No. 2017/183255 JP 2018-49701 A JP 2023-19865 A JP 2023-60591 A Japanese Unexamined Patent Publication No. 7-101711 Japanese Patent Application Publication No. 6-48713 Japanese Patent Application Laid-Open No. 2006-306677 Japanese Patent Application Laid-Open No. 2004-284945

Hui Xei, John B. Goodenough, Yutao Li, Journal of Power Sources, 196(2011) 7760-7762 Yutao Li, etal.,J.Power Sourses, 240 50-53 (2013) Henghui Xu, etal.,J.Chem Mater, 29 7206-7212 (2017)

In lithium secondary batteries, expectations are high for solid electrolytes in order to realize all-solid-state batteries. As mentioned above, various developments and studies have been conducted to increase the conductivity of lithium ions in Li-Zr-P-O-based Nasicon-type solid electrolytes, but the results are still not sufficient.

As a result of further research into how to increase the lithium ion conductivity of Li-Zr-P-O-based Nasicon-type solid electrolytes, the inventors discovered that by creating a crystalline oxide that minimizes the ratio of triclinic structure to rhombohedral Nasicon-type structure, the sintering properties when making a solid electrolyte are improved, resulting in a material that exhibits high lithium ion conductivity.

Furthermore, in order to obtain such a solid electrolyte, the conventional methods require the raw materials (starting materials) to be thoroughly crushed, or multiple materials to be prepared and fired (sintered), which requires a lot of preparation and labor. However, surprisingly, it has been discovered that by preparing zirconium phosphate, which is a phosphate powder (Y-substituted type) in which Y ₂ O ₃ is not separated, as a firing precursor, a solid electrolyte having a Li-Zr-Y-P-O-based Nasicon structure can be obtained more easily than with conventional methods. Moreover, since this phosphate powder is a homogeneous solid solution of Zr and Y, when a Li-Zr-Y-P-O-based Nasicon-type solid electrolyte is obtained, a high-purity and high-density one can be obtained, which has led to the completion of the present invention.

Therefore, an object of the present invention is to provide a solid electrolyte with excellent lithium ion conductivity.

Another object of the present invention is to provide a phosphate powder that can be used to obtain a high-purity, high-density Li-Zr-Y-P-O-based Nasicon-type solid electrolyte.

Furthermore, another object of the present invention is to provide a method for producing such phosphate powder and solid electrolyte, and further to provide a lithium secondary battery using the solid electrolyte obtained as described above.

That is, the gist of the present invention is as follows.
[1] General formula (I): H _1+4a-3b Zr _2-a Y _b P ₃ O _12±σ , where a and b representing the molar ratios of each element satisfy the following formulas (1) and (2), and δ represents the non-stoichiometric amount of oxygen:
0<a≦0.3 (1)
0<b≦0.3 (2)
The phosphate powder is characterized in that it satisfies the following relational expressions (3) and (4), where _I1 is the average intensity at a diffraction angle 2θ of 28.5 to 29.5 degrees, _I2 is the average intensity at a diffraction angle 2θ of 30 to 31 degrees, _I3 is the average intensity at a diffraction angle 2θ of 10 to 90 degrees, _I4 is the maximum intensity at a diffraction angle 2θ of 10 to 90 degrees, and _I5 is the average intensity at a diffraction angle 2θ of 80 to 90 degrees, as determined by X-ray diffraction measurement using Cu-Kα rays.
(I ₁ /I ₂ )×(I ₄ /I ₅ )≦500 (3)
(I ₃ /I ₄ )≧0.3 (4)
[2] The phosphate powder according to [1], which exhibits an exothermic peak in the range of more than 900°C and not more than 1100°C in thermogravimetric differential scanning calorimetry (TG-DTA).
[3] The phosphate powder according to [1], which has a specific surface area of 30 m ² /g or more.
[4] The phosphate powder according to [1], which is a firing precursor for obtaining a solid electrolyte by firing.
[5] A lithium secondary battery comprising a solid electrolyte obtained by firing the phosphate powder according to [1].
[6] A method for producing a phosphate powder, comprising heating a mixed solution containing a Zr raw material containing Zr, a Y raw material containing Y, a P raw material containing P, and malonic acid, adjusted to a pH of 3 to 7, to 80°C or higher to remove moisture, and then calcining the mixed solution at a temperature of 600°C or higher and 800°C or lower.
[7] A method for producing a solid electrolyte, comprising adding a Li source material containing Li to the phosphate powder obtained in [6] and further calcining the mixture.

[ ₈ ] General formula (II): _{XAMBYCPDO12 ±σ} , where X is Li or Li and Na, M is Zr or Zr and _Ca , δ represents a non-stoichiometric amount of oxygen, the ratio of _lattice constants c/a is 2.52 or less, and the lattice volume is 1505 ^Å3 or more and 1522 ^Å3 or less, and the rhombohedral Nasicon structure is included;
In powder X-ray diffraction measurement using a Cukα radiation source, the ratio I B /I A of the peak intensity I _A appearing at a diffraction angle 2θ of 23.3 to 23.5 degrees due to the rhombohedral Nasicon structure to the peak intensity I _B appearing at a diffraction angle _2θ of 19.4 to 19.6 degrees due to the triclinic _structure is 10% or less,
A solid electrolyte characterized in that A to D, which represent the molar ratios of the constituent elements in the general formula (II), satisfy the relationships of the following formulas (10) and (20):
0.690≦(B+C)/D...(10)
0.380≦A/D≦0.450 (20)
[9] The solid electrolyte according to [8], wherein in powder X-ray diffraction measurement using a Cukα radiation source, the ratio I _C /I _A of the peak intensity I _C at a diffraction angle 2θ of 25.8 to 26.0 degrees attributable to YPO ₄ to the peak intensity I _A appearing at a diffraction angle 2θ of 23.3 to 23.5 degrees attributable to the rhombohedral Nasicon structure is 10% or less.
[10] The solid electrolyte according to [8], wherein the Ca concentration is 2.0% or less by mass.
[11] A lithium secondary battery characterized by using the solid electrolyte according to any one of [8] to [10].
[12] A lithium secondary battery, characterized in that the solid electrolyte according to any one of [8] to [10] is used in a separator layer located between a positive electrode and a negative electrode.
[13] A method for producing the solid electrolyte according to any one of [8] to [10],
a step A of heating a mixed solution containing a Zr raw material containing Zr, a Y raw material containing Y, a P raw material containing P, and a chelating agent to a pH of 7.0 or less to remove moisture;
Step B: calcining the mixture in an air atmosphere to obtain an oxide precursor;
A step C of adding a Li raw material containing Li and further firing under an air atmosphere;
A method for producing a solid electrolyte, comprising:
[14] The method for producing a solid electrolyte according to [13], wherein the firing in step C is divided into a primary firing at a temperature of 800°C or higher and 920°C or lower, and a secondary firing at a temperature of 1100°C or higher and 1300°C or lower.

In the present invention, by keeping the ratio of triclinic structure to rhombohedral Nasicon structure within a specified range, it is possible to improve the sintering properties when obtaining a solid electrolyte, resulting in a solid electrolyte with excellent lithium ion conductivity.

In addition, in order to suppress the ratio of _the triclinic structure to the rhombohedral Nasicon structure, the molar ratio of each component in the crystalline oxide represented by the general _formula ( _II ) _{: XAMBYCPDO12} is set to a specific range, and in particular, by controlling the composition of Li and P (part of Li may be replaced by Na), it becomes possible to obtain a solid electrolyte with excellent lithium ion conductivity as described above.

Furthermore, in this invention, by using a specified phosphate powder as a firing precursor to obtain a solid electrolyte, it is possible to obtain a solid electrolyte having a Li-Zr-Y-P-O-based Nasicon structure more easily than with conventional methods. Moreover, because this phosphate powder is a uniform solid solution of Zr and Y, a high-purity, high-density solid electrolyte can be obtained without reducing conductivity. Therefore, by using the obtained solid electrolyte in a lithium secondary battery, excellent battery characteristics can be achieved.

FIG. 1 shows the results of TG-DTA measurement of the Y-substituted zirconium hydrogen phosphate powder according to Example 1. FIG. 2 shows the results of XRD measurement of the zirconium hydrogen phosphate powders according to Examples 1, 28, and Comparative Example 1. FIG. 3 shows the results of XRD measurement of the zirconium hydrogen phosphate powders according to Example 1, Example 28, and Comparative Example 1. FIG. 4 shows the results of TG-DTA measurement of the zirconium hydrogen phosphate powder according to Comparative Example 1. FIG. 5 shows the results of powder X-ray diffraction measurement of the solid electrolyte obtained in Example 34 and the results of powder X-ray diffraction measurement of the solid electrolyte obtained in Comparative Example 9.

[Phosphate powder and its manufacturing method]
The phosphate powder of the present invention can be represented by the general formula (I): H _1+4a-3b Zr _2-a Y _b P ₃ O _12±σ , where a and b, which represent the molar ratios of the constituent elements in the general formula (I), satisfy the relationships of the following formulas (1) and (2), where δ represents the non-stoichiometric amount of oxygen.
0<a≦0.3 (1)
0<b≦0.3 (2)

The above general formula (I) is based on the values obtained from component analysis of the product, not the charged composition during the production of phosphate powder. Here, P is normalized to 3 to determine the composition of Zr and Y. In this case, the hydrate was calculated from the weight loss up to 300°C in TG-DTA. Specifically, the weight loss from room temperature to 300°C was considered to be the volatile content of the hydrate, and the number of hydrates was determined.

In the Li-Zr-Y-P-O Nasicon-type structure, if a, the difference from the stoichiometric composition, becomes large, the Nasicon-type structure becomes unstable and other crystalline phases are thought to be formed. Furthermore, if b, which represents the amount of Y substitution, becomes too large, the solid solubility limit is exceeded, and the proportion of phases formed other than the desired Nasicon-type structure increases. In this case, there is a risk of a decrease in ionic conductivity and a decrease in density during sintering.

In consideration of these factors, in the present invention, the upper limit of formula (1), which shows the molar ratio relationship, is set to 0.3, but is preferably less than 0.3. The lower limit of formula (1) is a value greater than 0, but is preferably 0.01 or greater. Meanwhile, the upper limit of formula (2), which also shows the molar ratio relationship, is set to 0.3, but is preferably 0.28 or less, more preferably 0.26 or less. The lower limit of formula (2) is a value greater than 0, but is preferably 0.05 or greater, more preferably 0.10 or greater.

The phosphate powder of the present invention satisfies the following relationships in powder X-ray diffraction (XRD) measurement using a CuKα radiation source (Cu-Kα radiation): When the average intensity at a diffraction angle 2θ of 28.5 to 29.5 degrees, determined by X-ray diffraction measurement using Cu-Kα radiation, is defined as _I1 , the average intensity at a diffraction angle 2θ of 30 to 31 degrees is defined as _I2 , the average intensity at a diffraction angle 2θ of 10 to 90 degrees is defined as _I3 , the maximum intensity at a diffraction angle 2θ of 10 to 90 degrees is defined as _I4 , and the average intensity at a diffraction angle 2θ of 80 to 90 degrees is defined as _I5 , the following relationship formulas (3) and (4) are satisfied.
(I ₁ /I ₂ )×(I ₄ /I ₅ )≦500 (3)
(I ₃ /I ₄ )≧0.3 (4)

First, this relational expression (4) is the value obtained by dividing the average intensity _I3 at a diffraction angle 2θ of 10 to 90 degrees by the maximum intensity _I4 at a diffraction angle 2θ of 10 to 90 degrees, and indicates that the average intensity _I3 is 30% or more of the maximum intensity _I4 . Generally, when measuring an amorphous material using XRD, an XRD pattern is obtained in which a sharp peak known as a halo peak disappears and the background appears elevated, compared to when measuring a crystalline material. Therefore, the value obtained by dividing the average intensity _I3 at 10 to 90 degrees by the maximum intensity _I4 becomes large. Therefore, in the present invention, by making this _I3 / _I4 ratio 30% or more (a ratio of 0.3 or more), a phosphate powder is obtained in which the amorphous phase is dominant, i.e., the amorphous phase is the main phase. Here, the average intensity _I3 at diffraction angles 2θ: 10 to 90 degrees was determined by measuring from 10.0 to 90.0 degrees in 0.02 step increments in the XRD measurement, and averaging the intensities at all of those measurement points, as will be shown in the Examples below. Similarly, the maximum intensity _I4 at diffraction angles 2θ: 10 to 90 degrees was determined by measuring from 10.0 to 90.0 degrees in 0.02 step increments in the XRD measurement results, and selecting the maximum value from all of those measurement points.

Furthermore, in relation to the relational expression (3), _I1 / _I2 is the value obtained by dividing the average intensity _I1 at a diffraction angle 2θ of 28.5 to 29.5 degrees by the average intensity I2 at a diffraction angle _2θ of 30 to ₃₁ degrees. 2θ: 28.5 to 29.5 degrees is the region where peaks derived from yttrium oxide _Y2O3 appear. In contrast, 2θ: 30 to 31 degrees is the region where peaks derived from zirconium hydrogen phosphate or yttrium oxide do not appear. Therefore, the amount of yttrium oxide produced can be quantified from the ratio of these average intensities. On the other hand, _I4 / _I5 is the value obtained by dividing the maximum intensity _I4 at a diffraction angle 2θ of 10 to 90 degrees by the average intensity _I5 at a diffraction angle 2θ of 80 to 90 degrees, and this represents the S/N ratio. In other words, since the peak intensity in XRD measurement varies depending on the measurement method, it is normalized by multiplying by the S/N ratio. When the value of relational expression (3) is 500 or less, a Y-substituted phosphate powder can be obtained in which the generation of yttrium oxide is suppressed (Y ₂ O ₃ is not separated). Here, the average intensity _I1 at a diffraction angle 2θ of 28.5 to 29.5 degrees was measured in 0.02 steps from 28.50 degrees to 29.5 degrees, as shown in the examples described below, and the intensity at all of those measurement points was averaged. Similarly, the average intensity _I2 at a diffraction angle 2θ of 30 to 31 degrees was measured in 0.02 steps from 30.00 degrees to 31.00 degrees, and the intensity at all of those measurement points was averaged. Furthermore, the average intensity _I5 at a diffraction angle 2θ of 80 to 90 degrees was determined by measuring from 30.00 to 31.00 degrees in 0.02 steps and averaging the intensities at all of the measurement points.

Furthermore, the phosphate powder of the present invention preferably exhibits an exothermic peak in the range of greater than 900°C and less than 1100°C in thermogravimetric differential scanning calorimetry (TG-DTA). The exothermic peak, i.e., the crystallization peak, observed in TG-DTA measurement is greater than 900°C and less than 1100°C. This indicates that the amorphous phase remains the main phase below the temperature range where the crystallization peak appears. When preparing a solid electrolyte using the phosphate powder of the present invention as a firing precursor, the lithium raw material is mixed and calcined, preferably at 800°C to 920°C. However, if the phosphate powder of the present invention, in which Zr and Y are uniformly dissolved, can react with the lithium raw material in the amorphous phase state at the calcination temperature, this is advantageous for obtaining a uniform Nasicon-type solid electrolyte. The temperature range where this crystallization peak appears is more preferably 940°C to 1100°C. Details of the TG-DTA measurement are as described below in the Examples.

Furthermore, the phosphate powder in the present invention preferably has a specific surface area of 30 m ² /g or more. A phosphate powder with a specific surface area of 30 m ² /g or more has the advantage of having a small primary particle size, which increases contact when mixed with the lithium (Li) raw material (vi) described below, making it easier to produce a uniform solid electrolyte material after firing. The specific surface area was measured as described in the Examples described below.

The phosphate powder of the present invention is preferably produced through i) weighing the raw materials, ii) mixing in the liquid phase, iii) heating the liquid phase, iv) washing, and v) heat treatment.

First, in i) weighing the raw materials, a Zr raw material containing Zr, a Y raw material containing Y, and a P raw material containing P are each weighed to obtain the desired composition. Of these, for the Zr raw material, a nitrate solution such as zirconium nitrate, a sulfate solution such as zirconium sulfate, or a chloride solution such as zirconium oxychloride can be used. Similarly, for the Y raw material, a nitrate solution such as yttrium nitrate, a sulfate solution such as yttrium sulfate, or a chloride solution such as yttrium chloride can be used. On the other hand, for the P raw material, phosphate compounds such as phosphoric anhydride (P ₂ O ₅ ), orthophosphoric acid (H ₃ PO ₄ ), pyrophosphoric acid, metaphosphoric acid [(HPO ₃ ) _n ], and polyphosphoric acid, as well as ammonium salts such as ammonium dihydrogen phosphate and diammonium hydrogen phosphate can be used.

Note that Zr may contain approximately 3% Hf derived from its raw materials. Regarding Hf, since the ionic radii of Zr ⁴⁺ and Hf ⁴⁺ at the hexacoordinate octahedral positions are similar, when a solid electrolyte is obtained, it does not significantly affect Li ion diffusion. However, since Hf has a larger atomic weight than Zr, the inclusion of Hf may make the solid electrolyte heavier, potentially reducing the energy density of the battery. However, since Zr raw materials that do not contain Hf are very expensive, Zr raw materials containing Hf are used except for special applications. Therefore, the examples of the present invention described below describe results obtained using Zr raw materials containing approximately 3% Hf relative to Zr. Furthermore, Hf is not considered in the compositional formulas such as the above-mentioned formulas (1) and (2).

i) After weighing the raw materials, ii) in the liquid phase, the Zr raw material, Y raw material, and P raw material are mixed. Here, the Zr raw material and Y raw material are first mixed to substitute Y for Zr. Then, by adding a chelating agent, the metal ions are stabilized in the liquid, increasing the recovery rate of the product. This homogenizes Zr and Y in the liquid, making it easier to substitute Y for Zr. The chelating agent used in this case is preferably malonic acid. Although the detailed reasons for this are not entirely clear, using malonic acid results in a Y-substituted phosphate powder in which no Y ₂ O ₃ is generated (remains).

When the phosphorus raw material is added, some of the Zr and Y precipitate. The resulting precipitate has poor P uniformity relative to the Zr and Y, and the particle shape also varies. Next, ammonia water is added to raise the pH, creating an environment that is conducive to the precipitation of Zr and Y. Raising the pH too much can change the degree of acid dissociation of the phosphoric acid, potentially altering the resulting composition, so the pH should be kept below 7.0, preferably between 3 and 7. Finally, the solution containing the liquid phase precipitate is heated to improve the uniformity of the composition and particle shape.

Next, in iii) heating the liquid phase, the mixed solution obtained in ii) above is heated and stirred. The temperature during this process should be between 70°C and 100°C, preferably between 80°C and 98°C, and more preferably between 85°C and 98°C. Here, as shown in ii), the pH of the mixed solution being heated and stirred is set to 7.0 or less to prevent the precipitation of yttrium hydroxide. In other words, if yttrium hydroxide precipitates, there is a risk that the Zr and Y will become non-uniform. If the Zr and Y are non-uniform, the uniformity of the solid electrolyte will be reduced when the Li raw material is later added to obtain the solid electrolyte. To prevent this, the pH of the mixed solution should be set to 7.0 or less, preferably between 3 and 7.

In the above iii) liquid phase heating step, a slurry-like oxide such as Zr-Y-P oxide ammonium salt is obtained. Therefore, in iv) washing, this slurry is filtered to extract the precipitate, which is then washed with water and dried to remove the moisture.

Next, in v) heat treatment, the dried material is heat-treated in an air atmosphere at a temperature of 600°C to 800°C. The dried material may be coarsely pulverized before heat treatment. This v) heat treatment removes ammonia from the Zr-Y-P oxide ammonium salt produced by heating the liquid phase in iii) above, thereby obtaining Zr-Y-P oxide, i.e., the phosphate powder of the present invention.

As mentioned above, the method for producing phosphate powder according to the present invention can be preferably described as follows: A mixed solution containing a Zr raw material containing Zr, a Y raw material containing Y, a P raw material containing P, and malonic acid, adjusted to a pH of 3 to 7, is heated to 80°C or higher to remove moisture, and then calcined at a temperature of 600°C to 800°C, thereby obtaining the phosphate powder according to the present invention.

Furthermore, the phosphate powder obtained in this manner can be used as an oxide precursor (calcination precursor) to obtain a solid electrolyte containing Li, and can be made into a solid electrolyte by adding a Li-containing Li raw material and further calcining. In more detail, the solid electrolyte is produced by further steps vi) mixing with a Li raw material, vii) calcining, and viii) pulverization.

That is, vi) In mixing with a Li raw material, the phosphate powder (Zr-Y-P oxide) obtained above is mixed with a Li raw material. There are no particular restrictions on the Li raw material used here, and examples include lithium carbonate, lithium hydroxide, and lithium oxide. Lithium carbonate and lithium hydroxide are preferred. Furthermore, mixing with the phosphate powder may be dry mixing or wet mixing.

Furthermore, vi) when mixing the Li raw material, a Ca raw material such as calcium carbonate, calcium hydroxide, or calcium oxide may be added for reasons described below. Similarly, a Na raw material such as sodium carbonate or sodium hydroxide may be added.

Next, in vii) firing, the mixture mixed with the Li raw material (which may further contain Ca and Na raw materials) is fired. This firing may be carried out in two stages. That is, it is divided into a primary firing (pre-firing) to promote gas generation from the mixture, and a secondary firing to carry out the main firing. Here, the primary firing can be carried out at a temperature of 800°C to 920°C. Next, after gas generation has ceased, the mixture is molded into pellets as necessary and then subjected to secondary firing. The secondary firing can be carried out at a temperature of 1100°C to 1300°C. In this case, it is advisable to avoid direct contact of the pellets with the alumina crucible during the secondary firing, for example by placing the pellets in an alumina crucible lined with zirconia beads and firing the pellets.

In viii) pulverization, the sintered pellets are pulverized using a roll crusher, pin mill, hammer mill, jet mill, mortar, or the like to obtain the desired solid electrolyte. The degree of pulverization varies depending on the form and application of the lithium secondary battery, so it is difficult to specify a specific level, but generally, the material is pulverized into a powder (solid electrolyte powder) with a D50 of approximately 30 μm or less.

In this way, by obtaining a solid electrolyte using the phosphate powder of the present invention, the composition of the solid electrolyte can be made more uniform, and a high-purity, high-density solid electrolyte made of a crystalline oxide having a Nasicon structure can be obtained.

Furthermore, the solid electrolyte obtained by the present invention can be used in lithium secondary batteries. In particular, it is preferable to use such a solid electrolyte in the separator layer located between the positive electrode and negative electrode, resulting in a lithium secondary battery that exhibits excellent lithium ion conductivity.

[Solid electrolyte and its manufacturing method]
The solid electrolyte of the present invention can be represented by the _general formula (II): _{XAMBYCPDO12 ±σ} , and A to D, which represent the molar ratios of the constituent elements in this general formula, satisfy the _{relationships of} the following formulas (10) and (20). Here, X is Li or Li and Na. M is Zr or Zr and Ca. Furthermore, δ represents the oxygen non-stoichiometry.
0.690≦(B+C)/D...(10)
0.380≦A/D≦0.450 (20)

In the Li-M-Y-P-O Nasicon-type structure (M = Zr, Ca), if the P composition is 3, which is a stoichiometric composition, the covalent bonding of the P itself reduces the crystal lattice volume, which is thought to result in a decrease in ion diffusion within the crystal. Therefore, in this invention, it is believed that by reducing the P while maintaining the Nasicon-type structure, ion diffusion can be improved compared to conventional methods.

That is, as shown in the above formula (10), P is reduced so that "(B + C)/D", which is the ratio of "D", which represents the P composition, to the sum "B + C" of "B", which represents the M composition, and "C", which represents the Y composition, is 0.690 or more, preferably 0.695 or more. On the other hand, if P is reduced too much, Zr and Y become excessive, and the ionic conductivity decreases due to the increase in _ZrO2 and _YPO4 , which do not contribute to lithium ion conduction. Therefore, "(B + C)/D" is preferably 0.8 or less, preferably 0.77 or less. Incidentally, the stoichiometric composition of "B + C", the sum "B" which represents the M composition and "C" which represents the Y composition, is 2, and the stoichiometric composition of "D", which represents the P composition, is 3 as described above. Therefore, "(B + C)/D" can be expressed as a combination of stoichiometric compositions as 2/3 = 0.667.

Furthermore, in the present invention, as shown in formula (20) above, "A/D," which is the ratio of "A," which represents the Li composition or Li+Na composition, to "D," which represents the P composition, is 0.380 or more and 0.450 or less. This ratio determines the amount of mobile Li ions relative to the framework element P in the Li-M-Y-P-O Nasicon structure. If the "A/D" value is small, it is thought that the distance an ion must travel to an adjacent site becomes too long, making it difficult for the ion to migrate. Conversely, if the "A/D" value is too large, the site to which the ion migrates will be filled, which is thought to hinder migration. Therefore, the range of "A/D" is as described above, and its upper limit is preferably 0.440 or less, more preferably 0.430 or less, and even more preferably 0.420 or less.

Here, in the solid electrolyte according to the present invention, X in the general formula (II) is Li or Li and Na. The reason for including Na in the latter case is as follows: Na has a larger ionic radius than Li, so replacing some of the Li with Na can expand the diffusion paths of Li ions. However, if the proportion of Na becomes too high, Na ^+, which has an ionic radius larger than Li ⁺ , will block the diffusion paths of Li, thereby reducing the ionic conductivity of Li. Therefore, the proportion of Na by mass in the total of Li and Na is preferably 30% or less, preferably 25% or less, and more preferably 20% or less. If the proportion of Na is within the above range, the ionic conductivity is improved by the effect of expanding the lattice volume rather than by the effect of blocking the diffusion paths of Li ions. Such an effect cannot be obtained with Li ⁺ alone, and the expansion of the Li diffusion path by Na ⁺ can be said to be the effect of the coexistence of Li ⁺⁺ and Na ^+. However, in the present invention, even if only Li is used, in which the Na content in X is 0, a solid electrolyte with excellent Li ion conductivity can be obtained by achieving high purity and high density.

Furthermore, the solid electrolyte of the present invention includes a rhombohedral Nasicon structure having a lattice constant ratio c/a of 2.52 or less and a lattice volume of 1505 Å ³ or more and 1522 Å ³ or less. Such crystal structure parameters can be calculated from powder X-ray diffraction measurements using a Cukα radiation source. Among these, the lattice constant ratio c/a varies depending on the crystal structure, and in the case of a Li-M-Y-P-O-based Nasicon structure, the lattice constant ratio c/a is 2.52 or less, preferably 2.51 or less. Furthermore, the lattice constant ratio c/a is preferably 2.20 or more, preferably 2.30 or more, more preferably 2.40 or more. On the other hand, in the case of a Li-M-Y-P-O-based Nasicon structure, the lattice volume is 1505 Å ³ or more and 1522 Å ³ or less, but if it deviates from this range, it is possible that Zr and Y are not in solid solution. Therefore, the lattice volume is preferably 1506 Å ³ or more, more preferably 1507 Å ³ or more.

Furthermore, in powder X-ray diffraction (XRD) measurements using a Cukα radiation source, the solid electrolyte of the present invention has a ratio I B /I A of the peak intensity I _A appearing at a diffraction angle 2θ of 23.3 to 23.5 degrees due to the rhombohedral Nasicon structure to the peak intensity I _B appearing at a diffraction angle _2θ of 19.4 to 19.6 _degrees due to the triclinic structure, or 10% or less. As mentioned above, the present invention aims to improve ion diffusion by controlling the composition of P and Li (or the composition of P and Li + Na) while maintaining the Nasicon structure of the Li-M-Y-P-O system. Therefore, by making a solid electrolyte containing a crystalline oxide having such a Nasicon structure with high purity, the sinterability of the crystalline oxide when obtaining this solid electrolyte is improved, and as a result, a solid electrolyte exhibiting high lithium ion conductivity can be obtained.

In the quantification by the above XRD measurement, the peak appearing at a diffraction angle 2θ of 23.3 to 23.5 degrees is a rhombohedral Nasicon structure, and the peak appearing at a diffraction angle 2θ of 19.4 to 19.6 degrees is a triclinic structure. Both are Li-M-Y-P-O-based materials, but the diffraction patterns, such as the strongest peak position, differ due to differences in crystal structure. Therefore, if the above ratio I _B /I _A is 10% or less (ratio value is 0.1 or less), preferably 9% or less (same 0.09 or less), the ratio of the triclinic structure to the rhombohedral Nasicon structure is suppressed as much as possible, and a crystalline oxide having a high-purity Nasicon structure can be obtained. Here, the peak intensity I _A appearing at a diffraction angle 2θ of 23.3 to 23.5 degrees is based on the height of the peak in the XRD measurement, as shown in the examples below. Similarly, the peak intensity _IB appearing at a diffraction angle 2θ of 19.4 to 19.6 degrees is based on the height of the peak, and the above _IB / _IA is calculated from the intensity ratio of these. Note that a smaller ratio _IB / _IA can result in a Nasicon structure with higher purity, but considering the manufacturing process described below, the ratio _IB / _IA has a substantial lower limit of 1% (0.01).

For the same reasons as above, in powder X-ray diffraction measurements using a Cukα radiation source, the ratio I _C /I _A of the peak intensity I _A appearing at a diffraction angle 2θ of 23.3 to 23.5 degrees due to the rhombohedral Nasicon structure to the peak intensity I _C at a diffraction angle 2θ of 25.8 to 26.0 degrees due to _YPO4 should be 10% or less (0.1 or less), preferably 1% or more (0.01 or more) and 9% or less (0.09 or less). Here, the peak intensity I _C at a diffraction angle 2θ of 25.8 to 26.0 degrees is based on the height of the peak in the XRD measurement, as described above, and the I _C /I _A is calculated from these intensity ratios. Details of the powder X-ray diffraction measurements are as described in the Examples below.

Furthermore, in the solid electrolyte of the present invention, Ca is an element that stabilizes the Nasicon structure, so it may be substituted along with Zr and Y. However, if the amount of Ca substituted is too large, it may actually lead to a decrease in ionic conductivity, so the Ca concentration in the solid electrolyte should be 2.0% or less by mass, and preferably 1.5% or less. Of course, a solid electrolyte with a Ca concentration of 0% by mass is also possible.

On the other hand, Zr may contain approximately 3% Hf derived from its raw material. Regarding Hf, since the ionic radii of Zr ⁴⁺ and Hf ⁴⁺ at the hexacoordinate octahedral positions are similar, it does not significantly affect Li ion diffusion. However, since Hf has a larger atomic weight than Zr, the inclusion of Hf may make the solid electrolyte heavier and reduce the energy density of the battery. However, since Zr raw materials that do not contain Hf are very expensive, Zr raw materials containing Hf are used except for special applications. Therefore, the examples of the present invention described below describe results obtained using Zr raw materials containing approximately 3% Hf relative to Zr. Note that Hf is not taken into account in the compositional formulas such as the above-mentioned formulas (10) and (20).

The solid electrolyte of the present invention is manufactured through the following steps: i) weighing the raw materials, ii) mixing in the liquid phase, iii) heating the liquid phase, iv) washing, v) heat treatment, vi) mixing with the Li raw material, vii) firing, and viii) pulverization.

First, in i) weighing the raw materials, a Zr raw material containing Zr, a Y raw material containing Y, and a P raw material containing P are each weighed to obtain the desired composition. Of these, for the Zr raw material, a nitrate solution such as zirconium nitrate, a sulfate solution such as zirconium sulfate, or a chloride solution such as zirconium oxychloride can be used. Similarly, for the Y raw material, a nitrate solution such as yttrium nitrate, a sulfate solution such as yttrium sulfate, or a chloride solution such as yttrium chloride can be used. On the other hand, for the P raw material, phosphate compounds such as phosphoric anhydride (P ₂ O ₅ ), orthophosphoric acid (H ₃ PO ₄ ), pyrophosphoric acid, metaphosphoric acid [(HPO ₃ ) _n ], and polyphosphoric acid, as well as ammonium salts such as ammonium dihydrogen phosphate and diammonium hydrogen phosphate can be used.

Next, in ii) liquid-phase mixing, the Zr, Y, and P raw materials are mixed. Here, the Zr and Y raw materials are first mixed to replace Zr with Y. Then, a chelating agent such as oxalic acid or malonic acid is added to stabilize the metal ions in the liquid, increasing the product recovery rate. This homogenizes Zr and Y in the liquid, facilitating the replacement of Zr with Y. Adding the P raw material then causes some Zr and Y to precipitate. The resulting precipitate exhibits poor P homogeneity relative to Zr and Y, resulting in variations in particle shape. Next, ammonia water is added to raise the pH, creating an environment conducive to the precipitation of Zr and Y. Raising the pH too much can alter the degree of acid dissociation of phosphoric acid, potentially altering the resulting composition; therefore, a pH of 7.0 or below is recommended. Finally, heating the solution containing the liquid-phase precipitate improves the uniformity of the composition and particle shape.

Next, in iii) heating the liquid phase, the mixed solution obtained in ii) above is heated and stirred. The temperature during this process should be between 70°C and 100°C, preferably between 80°C and 98°C, and more preferably between 85°C and 98°C. The pH of the heated and stirred mixed solution is set to 7.0 or less to prevent the precipitation of yttrium hydroxide. In other words, if yttrium hydroxide precipitates, the Zr and Y will become non-uniform, which could reduce the uniformity of the solid electrolyte obtained in the subsequent vi) dry mixing with the Li raw material and vii) firing. To prevent this, the pH of the mixed solution should be set to 7.0 or less.

In the above iii) liquid phase heating step, a slurry-like oxide such as Zr-Y-P oxide ammonium salt is obtained. Therefore, in iv) washing, this slurry is filtered to extract the precipitate, which is then washed with water and dried to remove the moisture.

Next, in v) heat treatment, the dried material is heat-treated in an air atmosphere at a temperature of 600°C to 800°C. The dried material may be coarsely pulverized before heat treatment. This v) heat treatment removes ammonia from the Zr-Y-P oxide ammonium salt produced by heating the liquid phase in iii) above, thereby obtaining Zr-Y-P oxide. The Zr-Y-P oxide obtained by this v) heat treatment corresponds to the precursor (oxide precursor) for obtaining a solid electrolyte containing Li.

Next, in step vi) mixing with a Li raw material, the Zr-Y-P oxide obtained above is mixed with a Li raw material containing Li. There are no particular restrictions on the Li raw material used here, and examples include lithium carbonate, lithium hydroxide, and lithium oxide. Lithium carbonate and lithium hydroxide are preferred. Furthermore, mixing with the Zr-Y-P oxide may be dry mixing or wet mixing.

Furthermore, vi) when mixing the Li raw material, a Ca raw material such as calcium carbonate, calcium hydroxide, or calcium oxide may be added. Similarly, a Na raw material such as sodium carbonate or sodium hydroxide may be added.

Next, in vii) firing, the mixture mixed with the Li raw material described above is fired. This firing can be carried out in two stages. That is, it can be divided into a primary firing (pre-firing) to promote gas generation from the mixture, and a secondary firing to carry out the main firing. Here, the primary firing can be carried out at a temperature of 800°C to 920°C. Next, after gas generation has ceased, the mixture is molded into pellets as necessary and then subjected to secondary firing. The secondary firing can be carried out at a temperature of 1100°C to 1300°C. In this case, it is advisable to avoid direct contact of the pellets with the alumina crucible during the secondary firing, for example by placing the pellets in an alumina crucible lined with zirconia beads and firing the pellets.

Here, since the ionic radius of Zr ⁴⁺ at the hexacoordinate octahedral site is larger than that of Al ³⁺ , the inclusion of Al reduces the lattice volume. Therefore, it is preferable to reduce the amount of Al mixed in. By producing the product using the above method, the Al content can be reduced to 0.1% by mass or less, i.e., 1000 ppm or less. The Al content is preferably 0.05% by mass or less, i.e., 500 ppm or less, and more preferably 0.01% by mass or less, i.e., 100 ppm or less.

It is also advisable to reduce the amount of Mg mixed in. Since Mg ²⁺ has an ionic radius similar to that of Li ⁺ , substituting the Li position may block the Li diffusion path and reduce ionic conductivity. Therefore, the Mg content should be 0.1% by mass or less, i.e., 1000 ppm or less, preferably 0.05% by mass or less, i.e., 500 ppm or less, and more preferably 0.01% by mass or less, i.e., 100 ppm or less.

In the viii) pulverization step, the sintered pellets are pulverized using a roll crusher, pin mill, hammer mill, jet mill, mortar, or the like to obtain the desired solid electrolyte. The degree of pulverization varies depending on the form and application of the lithium secondary battery, making it difficult to specify a specific level, but generally, the material is pulverized into a powder (solid electrolyte powder) with a D50 (median diameter) of approximately 30 μm or less.

As described above, the method for producing a solid electrolyte according to the present invention can be generally said to comprise the following steps A to C. That is, it comprises step A of heating a mixed solution containing a Zr raw material containing Zr, a Y raw material containing Y, a P raw material containing P, and a chelating agent, adjusted to a pH of 7.0 or less, to remove moisture; step B of firing in an atmospheric environment to obtain an oxide precursor; and step C of adding a Li raw material containing Li and firing again in an atmospheric environment. In this case, it is preferable to separate the firing in step C into the primary firing and secondary firing described above. Furthermore, in step C, a Ca raw material may be added as needed in addition to the Li raw material.

In this way, by obtaining an oxide precursor consisting of Zr-Y-P oxide through steps A and B, the composition of the resulting solid electrolyte can be made more uniform. Furthermore, by adding a Li raw material to the oxide precursor and firing it in step C, it is possible to obtain a solid electrolyte containing a highly pure crystalline oxide having a Nasicon structure.

The solid electrolyte of the present invention can be preferably used in lithium secondary batteries. In particular, it is preferable to use such a solid electrolyte in the separator layer located between the positive electrode and negative electrode, which makes it possible to obtain a lithium secondary battery that exhibits high lithium ion conductivity.

The following describes the present invention based on examples. However, the present invention is not limited to these examples.

Examples 1 to 3
[Production of zirconium hydrogen phosphate powder]
As a starting material, a malonic acid aqueous solution was added as a chelating agent to a mixture of a zirconium nitrate aqueous solution and a yttrium nitrate aqueous solution, and _diluted orthophosphoric acid ( _H3PO4 ) was added dropwise to this solution. At this time, the _ZrO2 concentration in the total solution volume of the zirconium nitrate aqueous solution and the yttrium nitrate aqueous solution was set to 0.055 g/mL. In addition, the charged Zr:Y:P ratio at this time was set to a molar ratio of 1.91:0.15:3. Furthermore, the molar ratio of malonic acid to Zr + Y was 1/2. In order to achieve the target Zr composition, the Zr was charged in excess within a range of 30% in molar ratio.

Next, ammonia water was added to the mixed solution obtained above to adjust the pH to 6.0, and the mixture was heated and stirred at 95°C for 12 hours using a round-bottom flask. The precipitate was then removed from the slurry after the heating reaction, washed with water, and dried at 120°C. The dried product was roughly crushed in a mortar, heated to 700°C in air over 3 hours, and then held for 3 hours for firing. The powder was then classified using a sieve with a mesh size of 45 μm to obtain Y-substituted zirconium hydrogen phosphate powders according to Examples 1 to 3, each having a composition of Zr:Y:P = 1.85:0.15:3.0 and no Y ₂ O ₃ was produced.

The Y-substituted zirconium hydrogen phosphate powder (calcination precursor) obtained above was subjected to various evaluations as described below. The results are summarized in Table 1. Table 2 also summarizes information regarding the addition and production of Zr when obtaining the Y-substituted zirconium hydrogen phosphate powder. Note that the Y-substituted zirconium hydrogen phosphate powders obtained in Examples 1 to 3 were all the same, but as described below, the amount of Li raw material (lithium carbonate) added when the solid electrolyte powder was obtained by adding and calcining the Li raw material was varied (see Table 3 below), and the solid electrolyte powders obtained in Examples 1 to 3 were obtained, respectively.

First, the specific surface area of the Y-substituted zirconium hydrogen phosphate powders of Examples 1 to 3 was measured using a sample powder ranging from approximately 0.1 to 0.5 g. The sample tube measurement container was filled with the sample to approximately 70% capacity. After degassing at 200°C for 60 minutes under a nitrogen gas flow, the surface area was measured using the BET method (single-point method) by nitrogen gas adsorption using a surface area measurement device (NOVA touch, manufactured by Anton Paar Japan Co., Ltd.).

Regarding the composition, a predetermined amount of the obtained Y-substituted zirconium hydrogen phosphate powder was collected and dissolved in acid, and then subjected to component analysis by ICP atomic emission spectrometry to determine the values of Zr and Y when P in the composition formula was set to 3. H was calculated from the following formula. That is, a and b were calculated when the formula was expressed as general formula (I): H _1+4a-3b Zr _2-a Y _b P ₃ O ₁₂ .

Furthermore, TGA measurement of the above Y-substituted zirconium hydrogen phosphate powder was performed using a thermogravimetric-differential thermal analyzer (TG-DTA), and the mass change curve when heated from room temperature to 1100°C at a heating rate of 5°C/min using a platinum pan was obtained, and the temperature showing the crystallization peak (exothermic peak) was determined. Figure 1 shows the results of TG-DTA measurement of the Y-substituted zirconium hydrogen phosphate powder of Example 1 (crystallization peak was 985.1°C).

Furthermore, powder X-ray diffraction measurements were performed using a Cukα radiation source, and the average intensity I ₁ at a diffraction angle 2θ of 28.5 to 29.5 degrees, the average intensity I ₂ at a diffraction angle 2θ of 30 to 31 degrees, the average intensity I ₃ at a diffraction angle 2θ of 10 to 90 degrees, the maximum intensity I ₄ at a diffraction angle 2θ of 10 to 90 degrees, and the average intensity I ₅ at a diffraction angle 2θ of 80 to 90 degrees were determined, and values related to the following relational expressions (3) and (4) relating to XRD measurements were calculated. Here, the powder X-ray diffraction measurements were performed using a Rigaku MiniFlex 600 instrument, with a 2θ range of 10-90°, a step width of 0.02, and a scan speed of 10°/min. In this case, the presence or absence of Y ₂ O ₃ formation can be confirmed by checking the peak at a diffraction angle 2θ of 28.5 to 29.5 degrees, as shown in FIG. 2. Here, the average intensity _I1 for a diffraction angle 2θ of 28.5 to 29.5 degrees and the average intensity _I2 for a diffraction angle 2θ of 30 to 31 degrees were each determined by measuring a predetermined range of diffraction angles 2θ in 0.02 steps in XRD measurement and averaging the intensities at all measurement points. Similarly, the average intensity _I3 for a diffraction angle 2θ of 10 to 90 degrees was determined by measuring a predetermined range of diffraction angles 2θ in 0.02 steps in XRD measurement and averaging the intensities at all measurement points. Furthermore, the maximum intensity _I4 for a diffraction angle 2θ of 10 to 90 degrees was determined by measuring a predetermined range of diffraction angles 2θ in 0.02 steps in XRD measurement and taking the maximum value among all measurement points as the maximum intensity _I4 . Furthermore, the average intensity _I5 for a diffraction angle 2θ of 80 to 90 degrees was determined by measuring a predetermined range of diffraction angles 2θ in 0.02 steps in XRD measurement and averaging the intensities at all measurement points. In Table 1, the above relational expression (4) is expressed as a percentage (%).
(I ₁ /I ₂ )×(I ₄ /I ₅ )≦500 (3)
(I ₃ /I ₄ )≧0.3 (4)

[Production of Nasicon-type solid electrolyte powder]
Next, the Y-substituted zirconium hydrogen phosphate powder obtained above and lithium carbonate (Wako Reagent Special Grade) were weighed and mixed to obtain the composition shown in Table 3. Mixing was carried out in a mortar for 15 minutes. The obtained mixed powder (mixed raw material) was subjected to primary firing (calcination) at 900°C in air. The heating rate was 150°C/h and the holding time was 6 hours. After primary firing, the mixture was pulverized and classified using a sieve with 45 μm openings. Table 3 also shows the compositions a and b of the Y-substituted zirconium hydrogen phosphate powder used as the firing precursor.

0.7 g of the classified product obtained above after the primary firing was taken for impedance measurement and pellets were made at φ11 mm and 150 MPa. These pellets were placed in an alumina crucible lined with zirconia beads and subjected to secondary firing in the atmosphere. At this time, the pellets were made to avoid contact with the alumina crucible. During secondary firing, the temperature was raised to 1200°C at a rate of 150°C/h and held for 6 hours.

The pellets obtained after the secondary firing were then polished with sandpaper to smooth the surface, after which the thickness and width were measured to calculate the pellet volume. Gold was then vapor-deposited onto the pellets to form a blocking electrode, and the ionic conductivity (S/cm) at 25°C was measured. A Bio-Logic SP-200 measuring device was used, and measurements were taken over a frequency range of 7.0 MHz to 0.5 Hz. The ionic conductivity was calculated from the pellet's thickness and cross-sectional area, with the diameter of the arc in the Nyquist plot taken as the total resistance.

Furthermore, to confirm the crystalline phase after secondary firing, the pellets were crushed without the polishing and vapor deposition processes in the pellet processing after secondary firing in the pellet production process described above. Crushing was carried out in an agate mortar for 15 minutes. After crushing, the pellets were classified using a sieve with 45 μm openings to obtain the Li-Zr-Y-P-O-based Nasicon-type solid electrolyte powder that fell below the sieve, and powder X-ray diffraction measurement was performed using a Cukα radiation source. As a result, the presence or absence of a peak derived from the pyrophosphate ZrP ₂ O ₇ was confirmed.

Furthermore, a predetermined weight of the undersized Li-Zr-Y-P-O-based Nasicon-type solid electrolyte powder obtained above was sampled, and after treatments such as pressurized acid decomposition and thermal acid decomposition, it was dissolved in acid. After volume determination, component analysis was performed using atomic absorption spectrometry and ICP atomic emission spectrometry to determine the product composition of the resulting solid electrolyte powder. Li was measured using atomic absorption spectrometry, and other elements were measured using ICP atomic emission spectrometry. The composition shown in Table 4 is a value normalized to Zr + Y = 2.00. Except for the cations Li ⁺ , Zr ⁴⁺ , Y ³⁺ , and P ⁵⁺ , the oxygen anion O ^2- is 12. However, if the sum of the cation and anion charges listed in Table 4 does not match, the oxygen anion increases or decreases from the stoichiometric ratio of 12, resulting in neutral charge.

Furthermore, for the pellets after secondary firing that had not undergone the polishing and vapor deposition processes, the density was calculated by dividing the pellet weight by the pellet volume. The relative density was calculated by dividing this calculated density by the true density of 3.12 g/ ^cm3 . These results are summarized in Table 4.

(Examples 4 and 5)
A Y-substituted zirconium hydrogen phosphate powder having a composition of Zr:Y:P=1.77:0.15:3.0 was obtained in the same manner as in Example 1, except that the charged Zr:Y: _P ratio in molar ratio when producing the Y-substituted zirconium hydrogen phosphate powder was set to 1.91:0.15:3, and the ZrO2 concentration in the total solution amount of the aqueous zirconium nitrate solution and the aqueous yttrium nitrate solution was set to 0.060 g/mL.

Next, the Y-substituted zirconium hydrogen phosphate obtained above and lithium carbonate (Wako Reagent Special Grade) were weighed and mixed to obtain the composition shown in Table 3, and a Nasicon-type solid electrolyte powder was obtained in the same manner as in Example 1. The various evaluation results are shown in Table 4.

(Examples 6 and 7)
A Y-substituted zirconium hydrogen phosphate powder having a composition of Zr:Y:P=1.82:0.15:3.0 was obtained in the same manner as in Example 1, except that the charged Zr:Y:P ratio in molar ratio when producing the Y-substituted zirconium hydrogen phosphate powder was set to 1.95:0.15:3, and the _ZrO2 concentration in the total solution amount of the zirconium nitrate aqueous solution and the yttrium nitrate aqueous solution was set to 0.060 g/mL.

Next, the Y-substituted zirconium hydrogen phosphate obtained above and lithium carbonate (Wako Reagent Special Grade) were weighed and mixed to obtain the composition shown in Table 3, and a Nasicon-type solid electrolyte powder was obtained in the same manner as in Example 1. The various evaluation results are shown in Table 4.

(Examples 8 and 9)
A Y-substituted zirconium hydrogen phosphate powder having a composition of Zr:Y:P=1.90:0.15:3.0 was obtained in the same manner as in Example 1, except that the charged Zr:Y:P ratio in molar ratio when producing the Y-substituted zirconium hydrogen phosphate powder was set to 1.91:0.15:3, and the _ZrO2 concentration in the total solution amount of the zirconium nitrate aqueous solution and the yttrium nitrate aqueous solution was set to 0.044 g/mL.

Next, the Y-substituted zirconium hydrogen phosphate obtained above and lithium carbonate (Wako Reagent Special Grade) were weighed and mixed to obtain the composition shown in Table 3, and a Nasicon-type solid electrolyte powder was obtained in the same manner as in Example 1. The various evaluation results are shown in Table 4.

Example 10
A Y-substituted zirconium hydrogen phosphate powder having a composition of Zr:Y:P=1.89:0.11:3.0 was obtained in the same manner as in Example 1, except that the charged Zr:Y: _P ratio in molar ratio when producing the Y-substituted zirconium hydrogen phosphate powder was set to 1.97:0.10:3 and the ZrO2 concentration in the total solution amount of the zirconium nitrate aqueous solution and the yttrium nitrate aqueous solution was set to 0.055 g/mL.

Next, the Y-substituted zirconium hydrogen phosphate obtained above and lithium carbonate (Wako Reagent Special Grade) were weighed and mixed to obtain the composition shown in Table 3, and a Nasicon-type solid electrolyte powder was obtained in the same manner as in Example 1. The various evaluation results are shown in Table 4.

(Examples 11 and 12)
A Y-substituted zirconium hydrogen phosphate powder having a composition of Zr:Y:P=1.94:0.11:3.0 was obtained by the same procedure as in Example 1, except that the charged Zr:Y: _P ratio in molar ratio when producing the Y-substituted zirconium hydrogen phosphate powder was set to 1.97:0.10:3 and the ZrO2 concentration in the total solution amount of the zirconium nitrate aqueous solution and the yttrium nitrate aqueous solution was set to 0.044 g/mL.

Next, the Y-substituted zirconium hydrogen phosphate obtained above and lithium carbonate (Wako Reagent Special Grade) were weighed and mixed to obtain the composition shown in Table 3, and a Nasicon-type solid electrolyte powder was obtained in the same manner as in Example 1. The various evaluation results are shown in Table 4.

(Examples 13 to 15)
A Y-substituted zirconium hydrogen phosphate powder having a composition of Zr:Y:P=1.94:0.10:3.0 was obtained in the same manner as in Example 1, except that the charged Zr:Y:P ratio in molar ratio when producing the Y-substituted zirconium hydrogen phosphate powder was set to 1.97:0.10:3, and the _ZrO2 concentration in the total solution amount of the zirconium nitrate aqueous solution and the yttrium nitrate aqueous solution was set to 0.044 g/mL.

Next, the Y-substituted zirconium hydrogen phosphate obtained above and lithium carbonate (Wako Reagent Special Grade) were weighed and mixed to obtain the composition shown in Table 3, and a Nasicon-type solid electrolyte powder was obtained in the same manner as in Example 1. The various evaluation results are shown in Table 4.

(Examples 16 to 18)
A Y-substituted zirconium hydrogen phosphate powder having a composition of Zr:Y:P=1.71:0.10:3.0 was obtained in the same manner as in Example 1, except that the charged Zr:Y:P ratio in molar ratio when producing the Y-substituted zirconium hydrogen phosphate powder was set to 1.97:0.10:3, and the _ZrO2 concentration in the total solution amount of the zirconium nitrate aqueous solution and the yttrium nitrate aqueous solution was set to 0.065 g/mL.

Next, the Y-substituted zirconium hydrogen phosphate obtained above and lithium carbonate (Wako Reagent Special Grade) were weighed and mixed to obtain the composition shown in Table 3, and a Nasicon-type solid electrolyte powder was obtained in the same manner as in Example 1. The various evaluation results are shown in Table 4.

(Examples 19 and 20)
A Y-substituted zirconium hydrogen phosphate powder having a composition of Zr:Y:P=1.94:0.15:3.0 was obtained in the same manner as in Example 1, except that the charged Zr:Y: _P ratio in molar ratio when producing the Y-substituted zirconium hydrogen phosphate powder was set to 2.06:0.15:3, and the ZrO2 concentration in the total solution amount of the zirconium nitrate aqueous solution and the yttrium nitrate aqueous solution was set to 0.060 g/mL.

Next, the Y-substituted zirconium hydrogen phosphate obtained above and lithium carbonate (Wako Reagent Special Grade) were weighed and mixed to obtain the composition shown in Table 3, and a Nasicon-type solid electrolyte powder was obtained in the same manner as in Example 1. The various evaluation results are shown in Table 4.

(Examples 21 to 23)
A Y-substituted zirconium hydrogen phosphate powder having a composition of Zr:Y:P=1.81:0.15:3.0 was obtained in the same manner as in Example 1, except that the charged Zr:Y:P ratio in molar ratio when producing the Y-substituted zirconium hydrogen phosphate powder was set to 1.86:0.15:3, and the _ZrO2 concentration in the total solution amount of the zirconium nitrate aqueous solution and the yttrium nitrate aqueous solution was set to 0.055 g/mL.

Next, the Y-substituted zirconium hydrogen phosphate obtained above and lithium carbonate (Wako Reagent Special Grade) were weighed and mixed to obtain the composition shown in Table 3, and a Nasicon-type solid electrolyte powder was obtained in the same manner as in Example 1. The various evaluation results are shown in Table 4.

(Examples 24 and 25)
A Y-substituted zirconium hydrogen phosphate powder having a composition of Zr:Y:P=1.87:0.15:3.0 was obtained in the same manner as in Example 1, except that the charged Zr:Y:P ratio in molar ratio when producing the Y-substituted zirconium hydrogen phosphate powder was set to 1.93:0.15:3, and the _ZrO2 concentration in the total solution amount of the zirconium nitrate aqueous solution and the yttrium nitrate aqueous solution was set to 0.055 g/mL.

Next, the Y-substituted zirconium hydrogen phosphate obtained above and lithium carbonate (Wako Reagent Special Grade) were weighed and mixed to obtain the composition shown in Table 3, and a Nasicon-type solid electrolyte powder was obtained in the same manner as in Example 1. The various evaluation results are shown in Table 4.

(Examples 26 and 27)
A Y-substituted zirconium hydrogen phosphate powder having a composition of Zr:Y:P=1.96:0.15:3.0 was obtained in the same manner as in Example 1, except that the charged Zr:Y: _P ratio in molar ratio when producing the Y-substituted zirconium hydrogen phosphate powder was set to 2.00:0.15:3, and the ZrO2 concentration in the total solution amount of the zirconium nitrate aqueous solution and the yttrium nitrate aqueous solution was set to 0.055 g/mL.

Next, the Y-substituted zirconium hydrogen phosphate obtained above and lithium carbonate (Wako Reagent Special Grade) were weighed and mixed to obtain the composition shown in Table 3, and a Nasicon-type solid electrolyte powder was obtained in the same manner as in Example 1. The various evaluation results are shown in Table 4.

(Examples 28 and 29)
A Y-substituted zirconium hydrogen phosphate powder having a composition of Zr:Y:P=1.86:0.15:3.0 was obtained by the same procedure as in Example 1, except that the Zr:Y:P ratio in molar ratio when preparing the Y-substituted zirconium hydrogen phosphate powder was set to 1.91:0.15:3, the _ZrO2 concentration in the total solution volume of the zirconium nitrate aqueous solution and the yttrium nitrate aqueous solution was set to 0.055 g/mL, and ammonia water was added to adjust the pH to 3.5 and heated. Note that Figure 2 shows the results of X-ray diffraction measurement of the Y-substituted zirconium hydrogen phosphate powder obtained in Example 28. According to this, the Y-substituted zirconium hydrogen phosphate powder obtained in Example 28 (and Example 29) contains a diffraction pattern of cubic _NH4Zr2 ( _PO4 ) ₃ that is shifted to a lower angle compared to the PDF pattern, i.e., _NH4Zr2 ( _PO4 ) ₃ whose lattice volume has increased due to the substitution of _Zr4 ⁺ with _Y3 ^+. Therefore, it is understood that in addition to amorphous components, the _powder contains cubic components with a basic composition of Y-substituted _HZr2 ( _PO4 ) ₃ , in which _NH3 has been removed from _NH4Zr2 ( _PO4 ) ₃ while maintaining the crystal structure.

Next, the Y-substituted zirconium hydrogen phosphate obtained above and lithium carbonate (Wako Reagent Special Grade) were weighed and mixed to obtain the composition shown in Table 3, and a Nasicon-type solid electrolyte powder was obtained in the same manner as in Example 1. The various evaluation results are shown in Table 4.

(Comparative Examples 1 and 2)
A zirconium hydrogen phosphate powder having a composition of Zr:Y:P=1.83:0.17:3.0 was obtained in the same manner as in Example 1, except that the chelating agent used in obtaining the Y-substituted zirconium hydrogen phosphate powder in Example 1 was changed to oxalic acid and the charged Zr:Y:P ratio was changed to 2.18:0.15:3 in molar ratio.

The results of powder X-ray diffraction measurement using a Cukα radiation source on the zirconium hydrogen phosphate powder obtained above are shown in Figures 2 and 3, in which peaks can be confirmed at diffraction angles 2θ of 28.5 to 29.5 degrees, and the diffraction pattern of cubic NH ₄ Zr ₂ (PO ₄ ) ₃ , which has peak positions similar to those of the PDF pattern, is also included, indicating that the zirconium hydrogen phosphate powder obtained in Comparative Example 1 (and Comparative Example 2) is cubic zirconium hydrogen phosphate in which Y ₂ O ₃ has been produced. Note that Figure 4 shows the results of TG-DTA measurement of the cubic zirconium hydrogen phosphate powder in Comparative Example 1 in which Y ₂ O ₃ has been produced (crystallization peak at 932.09°C).

Next, the zirconium hydrogen phosphate obtained above and lithium carbonate (Wako Reagent Special Grade) were weighed and mixed to obtain the composition shown in Table 3, and a Nasicon-type solid electrolyte powder was obtained in the same manner as in Example 1. The various evaluation results are shown in Table 4.

(Comparative Examples 3 and 4)
A Y-substituted zirconium hydrogen phosphate powder having a composition of Zr:Y:P=1.66:0.15:3.0 was obtained by the same procedure as in Example 1, except that the charged Zr:Y:P ratio in molar ratio when obtaining the Y-substituted zirconium hydrogen phosphate powder in Example 1 was set to 1.73:0.15:3, and the _ZrO2 concentration in the total solution amount of the zirconium nitrate aqueous solution and the yttrium nitrate aqueous solution was set to 0.055 g/mL.

Next, the Y-substituted zirconium hydrogen phosphate obtained above and lithium carbonate (Wako Reagent Special Grade) were weighed and mixed to obtain the composition shown in Table 3, and a Nasicon-type solid electrolyte powder was obtained in the same manner as in Example 1. The various evaluation results are shown in Table 4.

(Comparative Examples 5 and 6)
A Y-substituted zirconium hydrogen phosphate powder having a composition of Zr:Y:P=1.58:0.15:3.0 was obtained by the same procedure as in Example 1, except that the charged Zr:Y:P ratio in molar ratio when obtaining the Y-substituted zirconium hydrogen phosphate powder in Example 1 was set to 1.63:0.15:3, and the _ZrO2 concentration in the total solution amount of the zirconium nitrate aqueous solution and the yttrium nitrate aqueous solution was set to 0.055 g/mL.

Next, the Y-substituted zirconium hydrogen phosphate obtained above and lithium carbonate (Wako Reagent Special Grade) were weighed and mixed to obtain the composition shown in Table 3, and a Nasicon-type solid electrolyte powder was obtained in the same manner as in Example 1. The various evaluation results are shown in Table 4.

(Comparative Examples 7 and 8)
Instead of preparing Y-substituted zirconium hydrogen phosphate, lithium carbonate (Wako Reagent Special Grade), zirconium oxide (specific surface area 30 m ² /g), yttrium oxide, and ammonium dihydrogen phosphate (Wako Reagent Special Grade) were charged and mixed in a mortar for 15 minutes at a molar ratio of Li:Zr:Y:P of 1.15:1.85:0.15:3, followed by primary firing at 900°C in air. The heating rate was 150°C/h and the holding time was 6 hours. After primary firing, the mixture was pulverized and classified using a sieve with a 45 μm opening. The subsequent secondary firing was carried out in the same manner as in Example 1, and pellets and Nasicon-type solid electrolyte powder were obtained after secondary firing, and various evaluations were performed. The results are shown in Table 4.

It can be seen that the solid electrolytes obtained in the above examples generally have higher relative densities and superior ionic conductivities than the solid electrolytes of the comparative examples. Specifically, since the solid electrolytes of the examples use the Y-substituted zirconium hydrogen phosphate powder according to the present invention as a firing precursor, the obtained solid electrolytes all have relative densities of 77% or more and ionic conductivities of 8.5 × 10 ^-6 S/cm or more. In other words, by using the Y-substituted zirconium hydrogen phosphate powder according to the present invention, the composition of the resulting solid electrolyte can be made more uniform, resulting in a high-purity, high-density electrolyte.

(Example 30)
[Production of zirconium hydrogen phosphate powder]
As a starting material, an aqueous oxalic acid solution was added as a chelating agent to a mixture of an aqueous zirconium nitrate solution and an aqueous yttrium nitrate solution, and diluted orthophosphoric acid (H ₃ PO ₄ ) was added dropwise to this solution. The ZrO ₂ concentration in the total solution volume of the aqueous zirconium nitrate solution and the aqueous yttrium nitrate solution was 0.055 g/mL. The molar ratio of the charged Zr:Y:P was 2.18:0.18:3. Furthermore, the molar ratio of oxalic acid to Zr+Y was 1/2. In order to achieve the desired Zr composition, the Zr was charged in excess within a range of 30% by molar ratio.

Aqueous ammonia was added to this mixed solution to adjust the pH to 3.5, and the mixture was heated and stirred at 95°C for 12 hours in a round-bottom flask. The precipitate was then removed from the slurry after the heating reaction, washed with water, and dried at 120°C. The dried product was roughly crushed in a mortar and heated to 700°C in air over 3 hours, then held for ₃ hours for calcination. The precipitate was then classified using a sieve with 45 μm openings to obtain cubic zirconium hydrogen phosphate with a composition of Zr:Y:P=1.83:0.17: _3.0 , i.e., zirconium hydrogen phosphate without Y substitution (Zr-Y-P oxide).

[Production of Nasicon-type solid electrolyte powder]
Next, the zirconium hydrogen phosphate obtained above and lithium carbonate (Wako Reagent Special Grade) were weighed and mixed to obtain the composition shown in Table 5. Mixing was carried out in a mortar for 15 minutes. The obtained mixed powder (mixed raw material) was subjected to primary firing at 900°C in air. The heating rate was 150°C/h and the holding time was 6 hours. After primary firing, the mixture was pulverized and classified using a sieve with 45 μm openings.

0.7 g of the classified product obtained above after the primary firing was taken for impedance measurement and pellets were made at φ11 mm and 150 MPa. These pellets were placed in an alumina crucible lined with zirconia beads and subjected to secondary firing in air. At this time, the pellets were made to avoid contact with the alumina crucible. During secondary firing, the temperature was raised to 1200°C at a rate of 150°C/h and held for 6 hours.

The pellets obtained after secondary firing were polished with sandpaper to smooth the surface, and then their thickness and width were measured to calculate the pellet volume. Gold was then vapor-deposited onto them to form a blocking electrode, and the ionic conductivity (S/cm) at 25°C was measured. A Bio-Logic SP-200 was used as the measuring device, and measurements were taken at frequencies ranging from 7.0 MHz to 0.5 Hz. The ionic conductivity was calculated from the pellet thickness and cross-sectional area, with the diameter of the arc in the Nyquist plot taken as the total resistance. Furthermore, to confirm the crystalline phase after secondary firing, the pellets were pulverized after secondary firing without the polishing and vapor deposition processes in the pellet preparation process described above. The pulverization was carried out in an agate mortar for 15 minutes. After pulverization, the pellets were classified using a sieve with 45 μm openings, yielding the Li-Zr-Y-P-O-based Nasicon-type solid electrolyte powder according to Example 30.

The Nasicon-type solid electrolyte powder obtained above was subjected to powder X-ray diffraction measurement using a Cukα radiation source. Using a Rigaku MiniFlex 600, measurements were taken at a 2θ range of 10-90°, a step width of 0.02, and a scan speed of 10°/min. Furthermore, the peak intensity I A appearing at a diffraction angle 2θ of 23.3-23.5 degrees derived from the rhombohedral Nasicon-type structure, the peak intensity I _B appearing at a diffraction angle 2θ of 19.4-19.6 degrees derived from the triclinic structure, _and the peak intensity IC at a diffraction angle 2θ of 25.8-26.0 degrees derived from YPO ₄ were determined, and the peak intensity ratios I _B /I _A and I _C /I _A were calculated, respectively. When calculating these peak intensity ratios, the peak intensity ratios were calculated based on the peak heights (peak intensities) that appeared within the respective diffraction angle 2θ ranges, rather than the integrated values of _the respective peak intensities. The results are shown in Table 7. In this example, it was assumed that cubic zirconium hydrogen phosphate containing Y ₂ O ₃ (zirconium hydrogen phosphate not substituted with Y) was obtained, but the generation of Y ₂ O ₃ can be confirmed by whether or not a peak appears at 2θ: 28.5 to 29.5 degrees in powder X-ray diffraction measurement using the above-mentioned Cukα radiation source.

5 shows the results of XRD measurement for the solid electrolyte powders obtained in Comparative Example 9 and Example 34, which will be described later. The upper part of Fig. 5 is derived from the peak of the solid electrolyte powder of Comparative Example 9, which produced a large amount of triclinic structure, i.e., the ratio _I / _I is large, and the lower part is derived from the peak of the solid electrolyte powder of Example 34, which produced a small amount of triclinic structure, i.e., the ratio _I / _I is small and the Nasicon-type rhombohedral crystal structure was the main phase.

The lattice constants a, b, and c of the obtained solid electrolyte powder were determined by Rietveld analysis using powder X-ray diffraction measurements with a Cukα radiation source. Rietveld analysis is a method for obtaining information on crystalline structure, such as lattice constants, by approximating the X-ray diffraction pattern obtained from an assumed crystalline structure to the measured diffraction pattern. The lattice constant ratio c/a and lattice volume were calculated. In the case of a Nasicon-type rhombohedral crystal structure, the lattice volume is 3 ^1/2 a ² c/2.

Furthermore, a predetermined mass of the solid electrolyte powder obtained above was sampled, treated with acid decomposition under pressure or acid decomposition under heat, and then dissolved in acid. After volume determination, component analysis was performed using atomic absorption spectrometry or ICP atomic emission spectrometry. From the _molar ratios expressed by the general formula: _{XAMBYCPDO12 ±σ} , the "( _B + _C )/D" corresponding to "(Zr+Y)/P" and the "A/D" corresponding to "Li+P" (or "Li+Na/P") were determined. The Ca concentration (mass%), Al concentration (mass%), and Mg concentration (mass%) were also determined. In this case, Li and Na were measured by atomic absorption spectrometry, and other elements were measured by ICP atomic emission spectrometry. The Al concentration was 0.01 mass% or less, i.e., 100 ppm or less, in all examples and comparative examples described below. On the other hand, the Mg concentration was 0.005 mass% or less, i.e., 50 ppm or less, except for Comparative Example 14.

Furthermore, for pellets after secondary firing that had not undergone the polishing and vapor deposition processes, the density was calculated by dividing the pellet mass by the pellet volume. The relative density was calculated by dividing this calculated density by the true density of 3.12 g/ ^cm3 . The results are shown in Table 7. Table 9 shows the product composition of the obtained solid electrolyte powder. Table 9 shows values normalized with Zr + Y set to 2.00. The oxygen anion ^O2- is 12 for all cations other than ^Li+ , Na ⁺ , Zr4 ⁺ , Y3 ⁺ , and P5 ⁺ . However, if the sum of the cation charges and the sum of the anion charges listed in Table 9 do not match, the oxygen anion increases or decreases from the stoichiometric ratio of 12, resulting in neutral charge.

(Examples 31, 32, and 42)
As a starting material, a malonic acid aqueous solution was added as a chelating agent to a mixture of a zirconium nitrate aqueous solution and a yttrium nitrate aqueous solution, and _diluted orthophosphoric acid ( _H3PO4 ) was added dropwise to this solution. The _ZrO2 concentration in the total solution volume of the zirconium nitrate aqueous solution and the yttrium nitrate aqueous solution was 0.055 g/mL. The molar ratio of Zr:Y:P charged at this time was 1.91:0.15:3. Furthermore, the molar ratio of malonic acid to Zr+Y was 1/2. In order to achieve the desired Zr composition, the Zr was charged in excess within a range of 10% molar ratio.

Aqueous ammonia was added to this mixed solution to adjust the pH to 6.0, and the mixture was heated and stirred at 95°C for 12 hours in a round-bottom flask. The precipitate was then removed from the slurry after the heating reaction, washed with water, and dried at 120°C. The dried product was roughly crushed in a mortar and heated to 700°C in air over 3 hours, then held for 3 hours for calcination. The resulting mixture was then classified using a 45 μm sieve to obtain amorphous zirconium hydrogen phosphate with a composition of Zr:Y:P=1.85:0.15:3.0 and no Y ₂ O ₃ was formed, i.e., amorphous zirconium hydrogen phosphate with Y substitution (Zr-Y-P oxide).

Next, the zirconium hydrogen phosphate obtained above and lithium carbonate (Wako Reagent Special Grade) were weighed and mixed to obtain the composition shown in Table 1. Mixing was carried out in a mortar for 15 minutes. After that, the primary and secondary firings were carried out in the same manner as in Example 30 described above to obtain pellets after secondary firing. These were then crushed to obtain Nasicon-type solid electrolyte powders. Various evaluations were carried out on these. The results are shown in Table 7.

(Examples 33 to 36, 41)
As a starting material, a malonic acid aqueous solution was added as a chelating agent to a mixture of a zirconium nitrate aqueous solution and a yttrium nitrate aqueous solution, and diluted orthophosphoric acid (H ₃ PO ₄ ) was added dropwise to this solution. The ZrO ₂ concentration in the total solution volume of the zirconium nitrate aqueous solution and the yttrium nitrate aqueous solution was 0.060 g/mL. The molar ratio of Zr:Y:P charged at this time was 1.91:0.15:3. Furthermore, the molar ratio of malonic acid to Zr+Y was 1/2. In this case, to achieve the desired Zr composition, Zr was charged in excess within a range of 10% molar ratio.

Aqueous ammonia was added to this mixed solution to adjust the pH to 6.0, and the mixture was heated and stirred at 95°C for 12 hours in a round-bottom flask. The precipitate was then removed from the slurry after the heating reaction, washed with water, and dried at 120°C. The dried product was roughly crushed in a mortar and heated to 700°C in air over 3 hours, then held for 3 hours for calcination. The precipitate was then classified using a sieve with 45 μm openings to obtain amorphous zirconium hydrogen phosphate with a composition of Zr:Y:P=1.77:0.15:3.0 and no Y ₂ O ₃ was formed, i.e., amorphous zirconium hydrogen phosphate with Y substitution (Zr-Y-P oxide).

Next, the Y-substituted amorphous zirconium hydrogen phosphate obtained above and lithium carbonate (Wako Reagent Special Grade) were weighed and mixed to obtain the composition shown in Table 5. Mixing was carried out in a mortar for 15 minutes. In Examples 35 and 36, calcium carbonate (manufactured by Ube Materials) was used in addition to lithium carbonate to obtain the composition shown in Table 5. After that, pellets and Nasicon-type solid electrolyte powder were obtained after secondary firing in the same manner as in Example 30, including the primary and secondary firing, and various evaluations were performed. The results are shown in Table 7.

(Example 37)
As a starting material, a malonic acid aqueous solution was added as a chelating agent to a mixture of a zirconium nitrate aqueous solution and a yttrium nitrate aqueous solution, and _diluted orthophosphoric acid ( _H3PO4 ) was added dropwise to this solution. At this time, the _ZrO2 concentration in the total solution volume of the zirconium nitrate aqueous solution and the yttrium nitrate aqueous solution was 0.060 g/mL. In addition, the charged Zr:Y:P ratio at this time was set to 1.95:0.15:3 in molar ratio. Furthermore, the molar ratio of malonic acid to Zr + Y was set to 1/2. At this time, in order to achieve the target Zr composition, Zr was charged in excess within a range of 10% in molar ratio.

Aqueous ammonia was added to this mixed solution to adjust the pH to 6.0, and the mixture was heated and stirred at 95°C for 12 hours in a round-bottom flask. The precipitate was then removed from the slurry after the heating reaction, washed with water, and dried at 120°C. The dried product was roughly crushed in a mortar and heated to 700°C in air over 3 hours, then held for 3 hours for calcination. The resulting mixture was then classified using a 45 μm sieve to obtain amorphous zirconium hydrogen phosphate with a composition of Zr:Y:P=1.82:0.15:3.0, free of Y ₂ O ₃ , i.e., amorphous zirconium hydrogen phosphate with Y substitution (Zr-Y-P oxide).

Next, the Y-substituted amorphous zirconium hydrogen phosphate obtained above and lithium carbonate (Wako Reagent Special Grade) were weighed and mixed to obtain the composition shown in Table 5. Mixing was carried out in a mortar for 15 minutes. After that, the same procedures as in Example 30 above, including the primary and secondary firing, were carried out to obtain pellets and Nasicon-type solid electrolyte powder after secondary firing, and various evaluations were carried out. The results are shown in Table 7.

(Example 38)
As a starting material, a malonic acid aqueous solution was added as a chelating agent to a mixture of a zirconium nitrate aqueous solution and a yttrium nitrate aqueous solution, and _diluted orthophosphoric acid ( _H3PO4 ) was added dropwise to this solution. At this time, the _ZrO2 concentration in the total solution volume of the zirconium nitrate aqueous solution and the yttrium nitrate aqueous solution was 0.060 g/mL. In addition, the charged Zr:Y:P ratio at this time was set to 1.97:0.10:3 in molar ratio. Furthermore, the molar ratio of malonic acid to Zr + Y was set to 1/2. At this time, in order to achieve the target Zr composition, Zr was charged in excess within a range of 10% in molar ratio.

Aqueous ammonia was added to this mixed solution to adjust the pH to 6.0, and the mixture was heated and stirred at 95°C for 12 hours in a round-bottom flask. The precipitate was then removed from the slurry after the heating reaction, washed with water, and dried at 120°C. The dried product was roughly crushed in a mortar and heated to 700°C in air over 3 hours, then held for 3 hours for calcination. The precipitate was then classified using a sieve with 45 μm openings to obtain amorphous zirconium hydrogen phosphate with a composition of Zr:Y:P=1.85:0.10:3.0 and no Y ₂ O ₃ was formed, i.e., amorphous zirconium hydrogen phosphate with Y substitution (Zr-Y-P oxide).

Next, the Y-substituted amorphous zirconium hydrogen phosphate obtained above and lithium carbonate (Wako Reagent Special Grade) were weighed and mixed to obtain the composition shown in Table 5. Mixing was carried out in a mortar for 15 minutes. After that, the same procedures as in Example 30 above, including the primary and secondary firing, were carried out to obtain pellets and Nasicon-type solid electrolyte powder after secondary firing, and various evaluations were carried out. The results are shown in Table 7.

(Example 39, Comparative Examples 11, 12, and 23)
As a starting material, a malonic acid aqueous solution was added as a chelating agent to a mixture of a zirconium nitrate aqueous solution and a yttrium nitrate aqueous solution, and _diluted orthophosphoric acid ( _H3PO4 ) was added dropwise to this solution. At this time, the _ZrO2 concentration in the total solution volume of the zirconium nitrate aqueous solution and the yttrium nitrate aqueous solution was 0.060 g/mL. In addition, the charged Zr:Y:P ratio at this time was set to a molar ratio of 1.91:0.15:3. Furthermore, the molar ratio of malonic acid to Zr + Y was 1/2. At this time, in order to achieve the target Zr composition, Zr was charged in excess within a range of 10% in molar ratio.

Aqueous ammonia was added to this mixed solution to adjust the pH to 6.0, and the mixture was heated and stirred at 95°C for 12 hours in a round-bottom flask. The precipitate was then removed from the slurry after the heating reaction, washed with water, and dried at 120°C. The dried product was roughly crushed in a mortar and heated to 700°C in air over 3 hours, then held for 3 hours for calcination. The precipitate was then classified using a sieve with 45 μm openings to obtain amorphous zirconium hydrogen phosphate with a composition of Zr:Y:P=1.71:0.15:3.0 and no Y ₂ O ₃ was formed, i.e., amorphous zirconium hydrogen phosphate with Y substitution (Zr-Y-P oxide).

Next, the Y-substituted amorphous zirconium hydrogen phosphate obtained above and lithium carbonate (Wako Reagent Special Grade) were weighed and mixed to obtain the formulation shown in Tables 5 and 6. Mixing was carried out in a mortar for 15 minutes. After that, the same procedures as in Example 30 above, including the primary and secondary firing, were carried out to obtain pellets and Nasicon-type solid electrolyte powder after secondary firing, and various evaluations were carried out. The results are shown in Tables 7 and 8.

(Example 40)
As a starting material, a malonic acid aqueous solution was added as a chelating agent to a mixture of a zirconium nitrate aqueous solution and a yttrium nitrate aqueous solution, and _diluted orthophosphoric acid ( _H3PO4 ) was added dropwise to this solution. At this time, the _ZrO2 concentration in the total solution volume of the zirconium nitrate aqueous solution and the yttrium nitrate aqueous solution was 0.055 g/mL. The charged Zr:Y:P ratio was set to a molar ratio of 1.86:0.20:3. Furthermore, the molar ratio of malonic acid to Zr+Y was set to 1/2. At this time, in order to achieve the desired Zr composition, Zr was charged in excess within a range of 10% molar ratio.

Aqueous ammonia was added to this mixed solution to adjust the pH to 6.0, and the mixture was heated and stirred at 95°C for 12 hours in a round-bottom flask. The precipitate was then removed from the slurry after the heating reaction, washed with water, and dried at 120°C. The dried product was roughly crushed in a mortar and heated to 700°C in air over 3 hours, then held for 3 hours for calcination. The precipitate was then classified using a sieve with 45 μm openings to obtain amorphous zirconium hydrogen phosphate with a composition of Zr:Y:P=1.81:0.20:3.0 and no Y ₂ O ₃ was formed, i.e., amorphous zirconium hydrogen phosphate with Y substitution (Zr-Y-P oxide).

Next, the Y-substituted amorphous zirconium hydrogen phosphate obtained above and lithium carbonate (Wako Reagent Special Grade) were weighed and mixed to obtain the composition shown in Table 5. Mixing was carried out in a mortar for 15 minutes. After that, the same procedures as in Example 30 above, including the primary and secondary firing, were carried out to obtain pellets and Nasicon-type solid electrolyte powder after secondary firing, and various evaluations were carried out. The results are shown in Table 7.

(Examples 43 to 45, Comparative Examples 26 to 27)
The same procedure as in Example 31 was carried out except that the molar ratio of Zr:Y:P used when preparing Y-substituted zirconium hydrogen phosphate was 1.91:0.15:3. As a result, amorphous _zirconium hydrogen phosphate with a composition of Zr:Y:P=1.83:0.15:3.0 and no _Y2O3 formed, i.e., Y-substituted amorphous zirconium hydrogen phosphate (Zr-Y-P oxide), was obtained.

Next, the Y-substituted amorphous zirconium hydrogen phosphate obtained above, lithium carbonate (Wako Reagent Special Grade), and anhydrous sodium carbonate (Wako Reagent Grade 1) were weighed and mixed to obtain the composition shown in Table 5. Mixing was carried out in a mortar for 15 minutes. The resulting mixed powder (mixed raw material) was subjected to primary firing in air at 900°C. The heating rate was 150°C/h and the holding time was 6 hours. After primary firing, the mixture was pulverized and classified using a sieve with 45 μm openings.

0.7 g of the classified product obtained above after the primary firing was taken for impedance measurement, and pellets were made at φ11 mm and 150 MPa. This pellet was heated to 1200°C at a heating rate of 150°C/h and held for 6 hours in the same manner as in Example 30, except that zirconia beads were not used and a zirconia sagger was used instead of an alumina crucible, and secondary firing was performed to obtain pellets after secondary firing. The obtained pellets were then pulverized in the same manner as in Example 30 to obtain Nasicon-type solid electrolyte powder. Various evaluations were performed on the obtained Nasicon-type solid electrolyte powder. The results are shown in Tables 7 and 8. In these examples and comparative examples, the Na content (by mass) of the total of Li and Na is as shown in the product composition of the solid electrolyte powder in Table 9, with Example 43 being 5% (= 0.05/(1.06 + 0.05) × 100), Example 44 being 9%, Example 45 being 16%, Comparative Example 26 being 6%, and Comparative Example 27 being 5%.

(Comparative Examples 9, 15 to 16)
The same procedure as in Example 31 was carried out except that when preparing Y-substituted zirconium hydrogen phosphate, the charged Zr:Y:P ratio was set to a molar ratio of 1.91:0.15:3 and the _ZrO2 concentration in the total solution volume of the zirconium nitrate aqueous solution and the yttrium nitrate aqueous solution was set to 0.044 g/mL. An amorphous Y-doped zirconium hydrogen phosphate (Zr-Y-P oxide) with a composition of Zr:Y:P=1.90:0.15:3.0 was obtained.

Next, the Y-substituted amorphous zirconium hydrogen phosphate obtained above and lithium carbonate (Wako Reagent Special Grade) were weighed and mixed to obtain the composition shown in Table 6. Mixing was carried out in a mortar for 15 minutes. After that, the same procedures as in Example 30 above, including the primary and secondary firing, were carried out to obtain pellets and Nasicon-type solid electrolyte powder after secondary firing, and various evaluations were carried out. The results are shown in Table 8.

(Comparative Examples 10 and 13)
The same procedure as in Example 31 was carried out except that the molar ratio of Zr:Y:P used when preparing Y-substituted zirconium hydrogen phosphate was 1.97:0.10:3. As a result, amorphous zirconium hydrogen phosphate with a composition of Zr:Y:P=1.89:0.11:3.0 and no _Y2O3 formed, i.e., Y-substituted _amorphous zirconium hydrogen phosphate (Zr-Y-P oxide), was obtained.

Next, the Y-substituted amorphous zirconium hydrogen phosphate obtained above and lithium carbonate (Wako Reagent Special Grade) were weighed and mixed to obtain the composition shown in Table 6. Mixing was carried out in a mortar for 15 minutes. After that, the same procedures as in Example 30 above, including the primary and secondary firing, were carried out to obtain pellets and Nasicon-type solid electrolyte powder after secondary firing, and various evaluations were carried out. The results are shown in Table 8.

(Comparative Example 14)
The same procedure as in Example 31 was carried out except that the molar ratio of Zr:Y:P used when preparing Y-substituted zirconium hydrogen phosphate was 1.91:0.15:3. As a result, amorphous _zirconium hydrogen phosphate with a composition of Zr:Y:P=1.83:0.15:3.0 and no _Y2O3 formed, i.e., Y-substituted amorphous zirconium hydrogen phosphate (Zr-Y-P oxide), was obtained.

Next, the Y-substituted amorphous zirconium hydrogen phosphate obtained above and lithium carbonate (Wako Reagent Special Grade) were weighed and mixed to obtain the composition shown in Table 6. Mixing was carried out in a mortar for 15 minutes. The resulting mixed powder (mixed raw material) was subjected to primary firing in air at 900°C. The heating rate was 150°C/h and the holding time was 6 hours. After primary firing, the mixture was pulverized and classified using a sieve with 45 μm openings.

0.7 g of the classified product obtained after the primary firing was taken for impedance measurement, and pellets were made at φ11 mm and 150 MPa. These pellets were subjected to secondary firing at 1200°C in the same manner as in Example 30, except that zirconia beads were not used and a magnesia sagger was used instead of an alumina crucible, to obtain pellets after secondary firing. The resulting pellets were then pulverized in the same manner as in Example 30 to obtain Nasicon-type solid electrolyte powder. Various evaluations were performed on this. The results are shown in Table 8. The obtained solid electrolyte powder contained 0.73 mass% Mg.

(Comparative Examples 17 to 18)
The same procedure as in Example 31 was carried out except that when preparing Y-substituted zirconium hydrogen phosphate, the charged Zr:Y:P ratio was set to 1.97:0.10:3 in molar ratio and the _ZrO2 concentration in the total solution volume of the zirconium nitrate aqueous solution and the yttrium nitrate aqueous solution was set to 0.044 g/mL. Then, amorphous zirconium hydrogen phosphate with a composition of Zr:Y:P=1.94:0.11:3.0 and _{no Y2O3} was generated, i.e., Y-substituted amorphous zirconium hydrogen phosphate (Zr-Y-P oxide), was obtained.

Next, the Y-substituted amorphous zirconium hydrogen phosphate obtained above and lithium carbonate (Wako Reagent Special Grade) were weighed and mixed to obtain the composition shown in Table 6. Mixing was carried out in a mortar for 15 minutes. After that, the same procedures as in Example 30 above, including the primary and secondary firing, were carried out to obtain pellets and Nasicon-type solid electrolyte powder after secondary firing, and various evaluations were carried out. The results are shown in Table 8.

(Comparative Examples 19 to 20)
The same procedure as in Example 31 was carried out except that the molar ratio of Zr:Y:P used when preparing Y-substituted zirconium hydrogen phosphate was 1.73:0.15:3. Then, amorphous zirconium hydrogen phosphate having a composition of Zr:Y:P=1.66:0.15:3.0 and no YO was formed, i.e., Y-substituted amorphous zirconium hydrogen phosphate (Zr-Y-P oxide), was obtained.

Next, the Y-substituted amorphous zirconium hydrogen phosphate obtained above and lithium carbonate (Wako Reagent Special Grade) were weighed and mixed to obtain the composition shown in Table 6. Mixing was carried out in a mortar for 15 minutes. After that, the same procedures as in Example 30 above, including the primary and secondary firing, were carried out to obtain pellets and Nasicon-type solid electrolyte powder after secondary firing, and various evaluations were carried out. The results are shown in Table 8.

(Comparative Examples 21 to 22)
The same procedure as in Example 31 was carried out except that the molar ratio of Zr:Y:P used when preparing Y-substituted amorphous zirconium hydrogen phosphate was 1.63:0.15:3. Thus, amorphous zirconium hydrogen phosphate having a composition of Zr:Y:P=1.58:0.15:3.0 and _{no Y2O3} was formed, i.e., Y-substituted amorphous zirconium hydrogen phosphate (Zr-Y-P oxide), was obtained.

Next, the Y-substituted amorphous zirconium hydrogen phosphate obtained above and lithium carbonate (Wako Reagent Special Grade) were weighed and mixed to obtain the composition shown in Table 6. Mixing was carried out in a mortar for 15 minutes. After that, the same procedures as in Example 30 above, including the primary and secondary firing, were carried out to obtain pellets and Nasicon-type solid electrolyte powder after secondary firing, and various evaluations were carried out. The results are shown in Table 8.

(Comparative Examples 24 to 25)
Instead of preparing Y-substituted zirconium hydrogen phosphate, lithium carbonate (Wako Reagent Special Grade), zirconium oxide (specific surface area 30 m ² /g), yttrium oxide, and ammonium dihydrogen phosphate (Wako Reagent Special Grade) were added to a Li:Zr:Y:P molar ratio of 1.15:1.85:0.15:3, mixed in a mortar for 15 minutes, and then subjected to primary firing at 900°C in air. The heating rate was 150°C/h and the holding time was 6 hours. After primary firing, the mixture was pulverized and classified using a sieve with a mesh size of 45 μm. The subsequent secondary firing was performed in the same manner as in Example 30, and pellets and Nasicon-type solid electrolyte powder were obtained after secondary firing, and various evaluations were performed. The results are shown in Table 8.

As shown above, it can be seen that the solid electrolytes according to the examples of the present invention generally have higher relative densities and superior ionic conductivity than the solid electrolytes of the comparative examples. Specifically, the solid electrolytes according to the examples all have relative densities of 83% or more and ionic conductivities of 1.2×10 ⁻⁶ S/cm or more. In other words, it can be said that the solid electrolyte of the present invention has improved sinterability and, as a result, has superior lithium ion conductivity. Therefore, by using the solid electrolyte of the present invention, a lithium secondary battery with excellent ionic conductivity can be obtained.

Claims (14)

It is represented by the general formula (I): H _1+4a-3b Zr _2-a Y _b P ₃ O _12±σ , where a and b representing the molar ratios of each element satisfy the following formulas (1) and (2), and δ represents a non-stoichiometric amount of h:
0<a≦0.3 (1)
0<b≦0.3 (2)
The phosphate powder is characterized in that it satisfies the following relational expressions (3) and (4), where _I1 is the average intensity at a diffraction angle 2θ of 28.5 to 29.5 degrees, _I2 is the average intensity at a diffraction angle 2θ of 30 to 31 degrees, _I3 is the average intensity at a diffraction angle 2θ of 10 to 90 degrees, _I4 is the maximum intensity at a diffraction angle 2θ of 10 to 90 degrees, and _I5 is the average intensity at a diffraction angle 2θ of 80 to 90 degrees, as determined by X-ray diffraction measurement using Cu-Kα rays.
(I ₁ /I ₂ )×(I ₄ /I ₅ )≦500 (3)
(I ₃ /I ₄ )≧0.3 (4) The phosphate powder according to claim 1, which exhibits an exothermic peak in the range of more than 900°C and less than or equal to 1100°C in thermogravimetric differential scanning calorimetry (TG-DTA). 2. The phosphate powder according to claim 1, which has a specific surface area of 30 m ² /g or more. The phosphate powder according to claim 1, which is a calcination precursor for obtaining a solid electrolyte by calcination. A lithium secondary battery comprising a solid electrolyte obtained by firing the phosphate powder described in claim 1. A method for producing phosphate powder, characterized by heating a mixed solution containing a Zr raw material containing Zr, a Y raw material containing Y, a P raw material containing P, and malonic acid, adjusted to a pH of 3 to 7, to 80°C or higher to remove moisture, and then calcining the mixture at a temperature of 600°C to 800°C. A method for producing a solid electrolyte, characterized by adding a Li-containing Li raw material to the phosphate powder obtained in claim 6 and further firing the mixture. _{The compound} is represented by the general formula (II): _{XAMBYCPDO12 ±σ ,} where X is Li or Li and Na, M is Zr or Zr and Ca, δ represents a non-stoichiometric amount of oxygen, the ratio of lattice constants c/a is 2.52 or less, and the compound has a rhombohedral Nasicon structure with a lattice volume of 1505 ^Å3 or more and 1522 ^Å3 or less;
In powder X-ray diffraction measurement using a Cukα radiation source, the ratio I B /I A of the peak intensity I _A appearing at a diffraction angle 2θ of 23.3 to 23.5 degrees due to the rhombohedral Nasicon structure to the peak intensity I _B appearing at a diffraction angle _2θ of 19.4 to 19.6 degrees due to the triclinic _structure is 10% or less,
A solid electrolyte characterized in that A to D, which represent the molar ratios of the constituent elements in the general formula (II), satisfy the relationships of the following formulas (10) and (20):
0.690≦(B+C)/D...(10)
0.380≦A/D≦0.450 (20) 9. The solid electrolyte according to claim 8, wherein in powder X-ray diffraction measurement using a Cukα radiation source, the ratio I _C /I _A of the peak intensity I _C at a diffraction angle 2θ of 25.8 to 26.0 degrees originating from YPO ₄ to the peak intensity I _A originating from a rhombohedral Nasicon structure at a diffraction angle 2θ of 23.3 to 23.5 degrees is 10% or less. The solid electrolyte according to claim 8, wherein the Ca concentration is 2.0% or less by mass. A lithium secondary battery characterized by using the solid electrolyte described in any one of claims 8 to 10. A lithium secondary battery characterized by using the solid electrolyte according to any one of claims 8 to 10 in a separator layer located between a positive electrode and a negative electrode. A method for producing the solid electrolyte according to any one of claims 8 to 10, comprising:
a step A of heating a mixed solution containing a Zr raw material containing Zr, a Y raw material containing Y, a P raw material containing P, and a chelating agent to a pH of 7.0 or less to remove moisture;
Step B: calcining the mixture in an air atmosphere to obtain an oxide precursor;
A step C of adding a Li raw material containing Li and further firing under an air atmosphere;
A method for producing a solid electrolyte, comprising: 14. The method for producing a solid electrolyte according to claim 13, wherein the firing in step C is divided into a primary firing at a temperature of 800°C or higher and 920°C or lower and a secondary firing at a temperature of 1100°C or higher and 1300°C or lower.