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

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: XAMBYCPDO12 ± 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 Å3 - 1522 Å3, 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

[0001] The present invention relates to a phosphate powder, a solid electrolyte, methods for producing them, and a lithium secondary battery, and more particularly to a phosphate powder suitable for obtaining a solid electrolyte having a Li-Zr-Y-P-O-based Nasicon structure, a solid electrolyte obtained using the same, methods for producing them, and a lithium secondary battery.

[0002] Oxides having a NASICON structure, which are lithium ion conductors, have attracted attention as solid electrolyte materials for lithium secondary batteries.

[0003] Since this Nasicon-type solid electrolyte has poor resistance to reduction when it contains Ti, it is thought that Li-Zr-P-O-based Nasicon-type solid electrolytes containing Zr are more promising.

[0004] Here, Non-Patent Document 1 describes a Zr-based Nasicon-type solid electrolyte in which part of the Zr is substituted with elements such as Ca. In the Li-Zr-P-O-based Nasicon-type solid electrolyte, the crystal structure may undergo transition around room temperature, which leads to a decrease in ionic conductivity. Therefore, by substituting part of the Zr with other elements such as Ca, Al, Y, etc., the transition in the crystal structure is prevented, thereby suppressing the decrease in ionic conductivity.

[0005] Furthermore, Non-Patent Documents 2 and 3 disclose that Nasicon-type solid electrolytes can be stabilized at room temperature by substituting a portion of Zr with Y. Of these, Non-Patent Document 2 investigates the resulting composition ratio of Li, Zr, and Y by ICP analysis. Furthermore, Non-Patent Document 3 describes that when the P composition is insufficient, the resistance is low, i.e., the ionic conductivity is high.

[0006] Furthermore, in Patent Document 1, the general formula: Li １＋ｘ M ｙ P ３ O １２It is described that in a Nasicon-type solid electrolyte represented by the formula (1), 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) M atoms 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, such 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.

[0007] Furthermore, in Patent Document 2, the general formula Li １＋２ｘ Zr ２－ｘ Ca ｘ (PO ４ ) ３ It is said that by substituting a part of the Zr element with Hf and a part of the Ca element with Ba or Sr for a solid electrolyte made of an oxide with a Nasicon structure represented by the formula (1), it is possible to form a path that facilitates lithium ion conduction.

[0008] On the other hand, Patent Document 3 discloses that lithium metaphosphate (LiPO ３ ) and zirconium phosphate ((ZrO) ２ (H.P.O. ４ ) ２ ], and yttrium oxide (Y ２ O ３ ) and mix them, form them into pellets, and then sinter them to obtain a lithium ion conductive material.

[0009] Here, in Patent Document 3, the lithium ion conductive material obtained above is Li ２ O-P ２ O ５ -SiO ２ It is said that a solid electrolyte with excellent lithium ion conductivity can be obtained by mixing and sintering with a base glass (LiPO ３) from the lithium ion conductive material, that is, by reducing Li and P in equimolar amounts, Li ２ O-P ２ O ５ -SiO ２ It is said that the temperature for mixing and sintering with the glass can be reduced to less than 800°C.

[0010] In order to obtain a Nasicon-type solid electrolyte, the aforementioned Patent Document 1 discloses a method for producing lithium carbonate (Li ２ CO ３ ), zirconium oxide (ZrO ２ ), ammonium dihydrogen phosphate (NH ４ H ２ P.O. ４ ), calcium oxide (CaO), yttrium oxide (Y ２ O ３ The raw materials, such as ammonium dihydrogen phosphate, yttrium-stabilized zirconia, are weighed and mixed, then fired at 500°C for 1 hour and at 800°C for 6 hours to remove volatile components, and then crushed. After that, the mixture is heated to remove moisture and then fired at 900-1200°C for 20 hours to obtain a powder of the solid electrolyte. However, because some of the raw materials, such as ammonium dihydrogen phosphate, are in a granular state, they must be crushed thoroughly to be mixed uniformly with the other raw materials.

[0011] In addition, in the above-mentioned Patent Document 3, first, lithium metaphosphate (LiPO ３ ) and zirconium phosphate ((ZrO) ２ (H.P.O. ４ ) ２ ], and yttrium oxide (Y ２ O ３ ) are mixed and pelletized, and the pellets are fired to obtain a lithium ion conductive material. ２ O-P ２ O ５ -SiO ２ In this method, a lithium ion conductive material and a Li-based glass are mixed and sintered to obtain a solid electrolyte having excellent lithium ion conductivity.２ O-P ２ O ５ -SiO ２ However, since the glass is mixed with the sintered glass, the process becomes more involved and requires more time and effort.

[0012] 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. ２ O ７ The crystalline phosphate compound particles such as the above have a higher resistance than the solid electrolyte particles, and since the crystalline phosphate compound particles are crystalline particles with a different crystal structure from the solid electrolyte particles, they are less likely to react with each other, and the solid electrolyte particles maintain a crystalline phase with high ionic conductivity, which results in high ionic conductivity of the solid electrolyte material.

[0013] Here, zirconium phosphate is known to have ion exchange ability (see, for example, Patent Document 5), and as described in Patent Document 4, zirconium phosphate-based materials are sometimes used to obtain a solid electrolyte.

[0014] To obtain such zirconium phosphate, for example, Patent Documents 6 and 7 describe liquid-phase synthesis (wet synthesis) using an organic acid having two or more carboxyl groups, such as oxalic acid or malonic acid, as a chelating agent. Patent Document 8 also discloses a crystalline zirconium phosphate obtained by heat-treating a mixture of zirconium phosphate and an inorganic acid salt, such as yttrium nitrate, at 400°C or higher to immobilize a metal element on the surface of the zirconium phosphate, thereby imparting the properties of the metal element to the intended use of the zirconium phosphate. However, the zirconium phosphate disclosed in Patent Document 8 is not a zirconium phosphate in which yttrium (Y) is solid-solved.

[0015] International Publication No. 2017 / 183255 JP 2018-49701 A JP 2023-19865 A JP 2023-60591 A JP 7-101711 A JP 6-48713 A JP 2006-306677 A JP 2004-284945 A

[0016] Hui Xei, John B. Goodenough, Yutao Li, Journal of Power Sources, 196(2011) 7760-7762Yutao Li, etal.,J.Power Sourses, 240 50-53 (2013)Henghui Xu, etal.,J.Chem Mater, 29 7206-7212 (2017)

[0017] 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 carried out to increase the lithium ion conductivity of Li-Zr-P-O-based Nasicon-type solid electrolytes, but it cannot be said that the results are yet sufficient.

[0018] The present inventors have conducted further research to improve the lithium ion conductivity of Li-Zr-P-O based Nasicon type solid electrolytes, and have found that by making a crystalline oxide with the ratio of triclinic structure to rhombohedral Nasicon type structure as small as possible, the sinterability when making a solid electrolyte is improved, and as a result, a product showing high lithium ion conductivity can be obtained.

[0019] Furthermore, in order to obtain such a solid electrolyte, conventional methods require a lot of preparation and labor, such as thoroughly crushing the raw materials (starting materials) and preparing and firing (sintering) multiple materials. However, surprisingly, Y ２ O ３ It has been found that by preparing zirconium phosphate, which is a phosphate powder in which Zr and Y are not separated (Y-substituted type), as a firing precursor, a solid electrolyte having a Li-Zr-Y-P-O based Nasicon structure can be obtained more easily than by 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, it is possible to obtain a high-purity and high-density one, and thus the present invention has been completed.

[0020] Therefore, an object of the present invention is to provide a solid electrolyte having excellent lithium ion conductivity.

[0021] Another object of the present invention is to provide a phosphate powder from which a high-purity, high-density Li-Zr-Y-P-O based Nasicon type solid electrolyte can be obtained.

[0022] Another object of the present invention is to provide a method for producing such a phosphate powder and a solid electrolyte, and further to provide a lithium secondary battery using the solid electrolyte obtained above.

[0023] 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 The molar ratios of each element, a and b, satisfy the following formulas (1) and (2), and δ indicates the non-stoichiometric amount of oxygen: 0<a≦0.3 (1) 0<b≦0.3 (2) The average intensity at a diffraction angle 2θ of 28.5 to 29.5 degrees determined by X-ray diffraction measurement using Cu-Kα rays is expressed as I １ The average intensity at the diffraction angle 2θ: 30 to 31 degrees is I ２ The average intensity at the diffraction angle 2θ: 10 to 90 degrees is I ３ The maximum intensity at a diffraction angle 2θ of 10 to 90 degrees is I ４ The average intensity at the diffraction angle 2θ: 80 to 90 degrees is I ５ A phosphate powder characterized in that the following relational expressions (3) and (4) are satisfied when 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 a range of more than 900°C and not more than 1100°C in thermogravimetric differential scanning calorimetry (TG-DTA). [3] A phosphate powder having a specific surface area of ​​30 m ２ / g or more. [4] The phosphate powder according to [1], which is a calcination precursor for obtaining a solid electrolyte by calcination. [5] A lithium secondary battery comprising a solid electrolyte obtained by calcining 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 and adjusted to a pH of 3 to 7 to 80°C or higher to remove water, and then calcining the solution at 600°C or higher and 800°C or lower. [7] A method for producing a solid electrolyte, comprising adding a Li raw material containing Li to the phosphate powder obtained in [6] and further calcining the mixture.

[0024] [8] General formula (II): X Ａ M Ｂ Y Ｃ P Ｄ O １２±σ where X is Li or Li and Na, M is Zr or Zr and Ca, δ represents the non-stoichiometric amount of oxygen, the ratio of lattice constants c / a is 2.52 or less, and the lattice volume is 1505 Å. ３ More than 1522Å ３ The compound contains the following rhombohedral Nasicon-type structure: In powder X-ray diffraction measurement using a Cukα radiation source, the peak intensity I appears at a diffraction angle 2θ of 23.3 to 23.5 degrees due to the rhombohedral Nasicon-type structure. Ａ In contrast, the peak intensity I appears at a diffraction angle 2θ of 19.4 to 19.6 degrees due to the triclinic structure. Ｂ Ratio I Ｂ / I Ａ 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] In powder X-ray diffraction measurement using a Cukα radiation source, the peak intensity I appearing at a diffraction angle 2θ of 23.3 to 23.5 degrees due to the rhombohedral Nasicon structure is Ａ In contrast, YPO ４ Diffraction angle 2θ: Peak intensity I of 25.8 to 26.0 degrees C Ratio I C / I Ａ

[10] The solid electrolyte according to [8], wherein the Ca concentration is 10% or less by mass.

[11] A lithium secondary battery using the solid electrolyte according to any one of [8] to

[10] .

[12] A lithium secondary battery using the solid electrolyte according to any one of [8] to

[10] 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] , 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, and adjusted to a pH of 7.0 or less, to remove moisture; a step B of firing the mixed solution in an air atmosphere to obtain an oxide precursor; and a step C of adding a Li raw material containing Li and further firing the mixed solution in an air atmosphere.

[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.

[0025] In the present invention, by controlling the ratio of the triclinic structure to the rhombohedral Nasicon structure within a predetermined range, the sinterability when obtaining a solid electrolyte can be improved, and as a result, a solid electrolyte with excellent lithium ion conductivity can be realized.

[0026] In addition, in order to reduce the ratio of the triclinic structure to the rhombohedral Nasicon structure, the general formula (II): X Ａ M Ｂ Y Ｃ P Ｄ O １２ By setting the molar ratio of each component in the crystalline oxide represented by the formula (I) within a specific range, and particularly by controlling the composition of Li and P (part of Li may be replaced with Na), it becomes possible to obtain a solid electrolyte having excellent lithium ion conductivity as described above.

[0027] Furthermore, in the present invention, by using a specific phosphate powder as a firing precursor for obtaining a solid electrolyte, 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 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.

[0028] 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 Examples 1, 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 solid electrolyte obtained in Comparative Example 9.

[0029] [Phosphate Powder and Method for Producing the Same] The phosphate powder of the present invention is a phosphate powder represented by the general formula (I): H １＋４a－３ｂ Zr ２－ａ Y ｂ P ３ O １２±σ The molar ratios of the constituent elements a and b in the general formula (I) satisfy the relationships of the following formulas (1) and (2). Note that δ represents the non-stoichiometric amount of oxygen: 0<a≦0.3 (1) 0<b≦0.3 (2)

[0030] The above general formula (I) is based on the value obtained by component analysis of the product, not on the charged composition during the production of the phosphate powder. Here, the composition of Zr and Y was determined by normalizing P to 3. 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.

[0031] In the Li-Zr-Y-P-O system Nasicon type structure, if the difference a from the stoichiometric composition becomes large, the Nasicon type structure becomes unstable and other crystal phases are generated. Also, if b, which represents the amount of Y substitution, becomes too large, the solid solubility limit is exceeded, and the ratio of generated phases other than the target Nasicon type structure becomes high. In that case, there is a risk of a decrease in ionic conductivity and a decrease in density during sintering.

[0032] Taking these into consideration, 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 exceeding 0, but is preferably 0.01 or more. 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 exceeding 0, but is preferably 0.05 or more, more preferably 0.10 or more.

[0033] Furthermore, the phosphate powder of the present invention satisfies the following relationship in powder X-ray diffraction (XRD) measurement using a Cu-Kα radiation source (Cu-Kα radiation): 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 I １ The average intensity at the diffraction angle 2θ: 30 to 31 degrees is I ２ The average intensity at the diffraction angle 2θ: 10 to 90 degrees is I ３ The maximum intensity at a diffraction angle 2θ of 10 to 90 degrees is I ４ The average intensity at the diffraction angle 2θ: 80 to 90 degrees is I ５ When the following relations (3) and (4) are satisfied: １ / I ２ )×(I ４ / I ５ )≦500...(3) (I ３ / I ４ ) ≧ 0.3 ... (4)

[0034] First, this relational expression (4) expresses the average intensity I ３ The maximum intensity I at the diffraction angle 2θ: 10 to 90 degrees ４ The maximum intensity I ４Average intensity I ３ This indicates that the value is 30% or more. Generally, when measuring an amorphous material, the sharp peak called the halo peak disappears and the background appears to increase compared to when measuring a crystalline material. Therefore, the average intensity I ３ Maximum intensity I ４ In the present invention, the value obtained by dividing by I becomes large. ３ / I ４ By making the ratio 30% or more (the ratio is 0.3 or more), the amorphous phase is dominant, that is, the phosphate powder is obtained in which the amorphous phase is the main phase. ３ As will be shown in the examples below, in the XRD measurement, measurements were made from 10.0 degrees to 90.0 degrees in 0.02 step increments, and the intensities at all of these measurement points were averaged. Similarly, the maximum intensity I at the diffraction angle 2θ: 10 to 90 degrees ４ is measured from 10.0 degrees to 90.0 degrees in 0.02 steps from the XRD measurement results, and is calculated as the maximum value selected from all of the measurement points.

[0035] In addition, regarding the relational expression (3), I １ / I ２ is the average intensity I of the diffraction angle 2θ: 28.5 to 29.5 degrees １ The average intensity I of the diffraction angle 2θ: 30 to 31 degrees ２ 2θ: 28.5 to 29.5 degrees is yttrium oxide Y ２ O ３ In contrast, 2θ: 30 to 31 degrees is a region where peaks derived from zirconium hydrogen phosphate and yttrium oxide do not appear. Therefore, the amount of yttrium oxide produced can be quantified from the ratio of these average intensities. ４ / I ５ is the maximum intensity I at the diffraction angle 2θ: 10 to 90 degrees ４ The average intensity I of the diffraction angle 2θ: 80 to 90 degrees ５This value is obtained by dividing by 1 / 2, which represents the S / N ratio. In other words, since the peak intensity in the XRD measurement varies depending on the measurement method, it is normalized by multiplying by the S / N ratio. When the value of the relational expression (3) is 500 or less, the generation of yttrium oxide is suppressed (Y ２ O ３ The average intensity I of the diffraction angle 2θ: 28.5 to 29.5 degrees can be obtained as a Y-substituted phosphate powder. １ As will be shown in the examples below, the intensity is measured from 28.50 degrees to 29.5 degrees in 0.02 steps and the intensity at all the measurement points is averaged. Similarly, the average intensity I ２ is measured from 30.00 degrees to 31.00 degrees in 0.02 steps and the intensity at all the measurement points is averaged. ５ is obtained by measuring from 30.00 degrees to 31.00 degrees in 0.02 steps and averaging the intensities at all the measurement points.

[0036] Furthermore, the phosphate powder of the present invention preferably exhibits an exothermic peak in the region 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 appears in the region of 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 solid-dissolved, can react with the lithium raw material in the amorphous phase state as the main phase at the calcination temperature, it is advantageous for obtaining a uniform Nasicon-type solid electrolyte. The temperature range where this crystallization peak appears is more preferably 940°C or more and less than 1100°C. Details of the TG-DTA measurement are as shown in the examples below.

[0037] The phosphate powder in the present invention preferably has a specific surface area of ​​30 m ２ / g or more. ２ / g or more, the primary particle diameter is small, and there is an advantage in that contact with the lithium (Li) raw material described later in vi) increases during mixing, and a uniform solid electrolyte material can be easily produced after firing. The specific surface area was measured as described in the examples described later.

[0038] The phosphate powder of the present invention is preferably produced through i) weighing raw materials, ii) mixing in the liquid phase, iii) heating the liquid phase, iv) washing, and v) heat treatment.

[0039] 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 weighed out so as to obtain a desired composition. Of these, for the Zr raw material, for example, 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, as the P raw material, phosphoric anhydride (P ２ O ５ ), orthophosphate (H ３ P.O. ４ ), pyrophosphate, metaphosphate [(HPO ３ ) ｎ ], and phosphate compounds such as polyphosphoric acid, as well as ammonium salts such as ammonium dihydrogen phosphate and diammonium hydrogen phosphate can be used.

[0040] Zr may contain about 3% Hf due to its raw material. This Hf is present in the hexacoordinated octahedral Zr ４＋ and Hf ４＋Since the ionic radii of Hf and Zr 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 that contain 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 taken into account in the compositional formulas such as the above-mentioned formulas (1) and (2).

[0041] i) Following weighing of the raw materials, ii) in the liquid phase mixing, the Zr raw material, Y raw material, and P raw material are mixed. Here, in order to substitute Y for Zr, the Zr raw material and the Y raw material are first mixed. Then, by adding a chelating agent, the metal ions are stabilized in the liquid, and the recovery rate of the product increases. This makes Zr and Y uniform 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 reason for this is not necessarily clear, the use of malonic acid results in the elimination of Y. ２ O ３ Therefore, a Y-substituted phosphate powder can be obtained in which no Y-substituted phosphate is generated (no Y-substituted phosphate remains).

[0042] Adding phosphorus raw material to the solution causes some of the Zr and Y to precipitate. The resulting precipitate has poor P uniformity relative to the Zr and Y, and the particle shape also varies. Next, aqueous ammonia is added to raise the pH, creating an environment that facilitates the precipitation of Zr and Y. Raising the pH too much can change the degree of acid dissociation of phosphoric acid, potentially altering the resulting composition, so the pH should be 7.0 or less, preferably 3 to 7. Finally, the solution containing the liquid-phase precipitate is heated to improve the uniformity of the composition and particle shape.

[0043] Next, in iii) heating the liquid phase, the mixed solution obtained in ii) above is heated and stirred. The temperature at this time is preferably 70°C or higher and 100°C or lower, preferably 80°C or higher and 98°C or lower, and more preferably 85°C or higher and 98°C or lower. Here, as shown in ii), the pH of the mixed solution to be heated and stirred is set to 7.0 or lower to prevent precipitation of yttrium hydroxide. That is, if yttrium hydroxide precipitates, Zr and Y may become non-uniform. If Zr and Y are non-uniform, the uniformity of the solid electrolyte will decrease when a Li raw material is later added to obtain the solid electrolyte. To prevent this, the pH of the mixed solution is set to 7.0 or lower, preferably pH 3 to 7.

[0044] In the above iii) heating of the liquid phase, a slurry-like oxide such as Zr-Y-P oxide ammonium salt is obtained, and therefore in iv) washing, this slurry is filtered to extract the precipitate, which is then washed with water and dried to remove the moisture.

[0045] Next, in v) heat treatment, the dried material is heat-treated at a temperature of 600°C to 800°C in an air atmosphere. At this time, the dried material may be coarsely pulverized before the heat treatment. By this v) heat treatment, ammonia can be removed from the Zr-Y-P oxide ammonium salt generated by the heating of the liquid phase in iii) above, thereby obtaining Zr-Y-P oxide, i.e., the phosphate powder according to the present invention.

[0046] As described above, the method for producing the phosphate powder of the present invention can be preferably described as follows: That is, 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 the mixture is fired at 600°C or higher and 800°C or lower, thereby obtaining the phosphate powder of the present invention.

[0047] The phosphate powder thus obtained can be used as an oxide precursor (calcination precursor) for obtaining a solid electrolyte containing Li, and can be converted into a solid electrolyte by adding a Li-containing Li raw material and further calcining the mixture. More specifically, the solid electrolyte is produced by further performing vi) mixing with a Li raw material, vii) calcination, and viii) pulverization.

[0048] That is, in vi) mixing with a Li raw material, the phosphate powder (Zr-Y-P oxide) obtained above is mixed with a Li raw material. The Li raw material used here is not particularly limited, and examples thereof 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.

[0049] 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.

[0050] Next, in vii) firing, the mixture mixed with the Li raw material (which may further contain a Ca raw material or a Na raw material) is fired. This firing may be performed in two stages. That is, it is divided into a primary firing (calcination) that promotes gas generation from the mixture, and a secondary firing that performs the main firing. Here, the primary firing can be performed at a temperature of 800°C to 920°C. Next, after gas generation has ceased, the mixture is subjected to secondary firing, for example, by pelletizing the mixture as necessary. The secondary firing can be performed at a temperature of 1100°C to 1300°C. In this case, it is preferable to prevent the pellets from coming into direct contact 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.

[0051] Then, in viii) pulverization, the sintered pellets are pulverized using a roll crusher, pin mill, hammer mill, jet mill, mortar, etc., to obtain a desired solid electrolyte. The degree of pulverization varies depending on the form and application of the lithium secondary battery, and is therefore difficult to specify in general terms, but generally, the pellets are pulverized to a powder (solid electrolyte powder) having a D50 of approximately 30 μm or less.

[0052] 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 solid electrolyte consisting of a crystalline oxide having a Nasicon structure can be obtained with high purity and high density.

[0053] 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 a separator layer between a positive electrode and a negative electrode, which makes it possible to obtain a lithium secondary battery exhibiting excellent lithium ion conductivity.

[0054] [Solid electrolyte and method for producing the same] The solid electrolyte of the present invention is a solid electrolyte represented by the general formula (II): X Ａ M Ｂ Y Ｃ P Ｄ O １２±σ In this general formula, A to D, which represent the molar ratios of the constituent elements, 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)

[0055] In the Li-M-Y-P-O system Nasicon type structure (M=Zr, Ca), if the P composition is 3, which is a stoichiometric composition, the crystal lattice volume will decrease due to the covalent bond of P itself, and as a result, it is thought that the ion diffusion within the crystal will decrease. Therefore, in the present invention, it is thought that by reducing P while maintaining the Nasicon type structure, the ion diffusion will be improved compared to the conventional method.

[0056] That is, as shown in the above formula (10), P is reduced so that "(B+C) / D", which is the ratio of the sum "B+C" of "B" representing the M composition and "C" representing the Y composition to "D" representing the P 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 ZrO, which does not contribute to lithium ion conduction, is formed. ２ and YPO ４ Since an increase in "B" reduces the ionic conductivity, "(B+C) / D" should be 0.8 or less, and preferably 0.77 or less. Incidentally, the stoichiometric composition of "B+C", the sum of "B" representing the M composition and "C" representing the Y composition, is 2, and the stoichiometric composition of "D" representing the P composition is 3 as described above. Therefore, "(B+C) / D" expressed as a combination of stoichiometric compositions is 2 / 3 = 0.667.

[0057] Furthermore, in the present invention, as shown in the above formula (20), the ratio of "A / D", which is the ratio of "D" representing the P composition to "A" representing the Li composition or Li + Na composition, is 0.380 or more and 0.450 or less. This ratio is a value that determines the amount of Li, which is a mobile ion, relative to the P, which is a framework element, in the Li-M-Y-P-O-based Nasicon structure. If the value of "A / D" is small, it is thought that the distance when an ion moves to the neighboring site becomes long, making it difficult for the ion to move. Conversely, if the value of "A / D" is too large, the site to which the ion moves will be filled, which is thought to be an obstacle to the movement. 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.

[0058] Here, the solid electrolyte according to the present invention is represented by the general formula (II) where X is Li or Li and Na. The reason for including Na as in the latter case is as follows: Na has a larger ionic radius than Li, so by substituting a portion of Li with Na, the diffusion path of Li ions can be widened. However, if the proportion of Na becomes too high, the diffusion path of Li ＋ Na with a larger ionic radius ＋However, this will block the diffusion path of Li, and the ionic conductivity of Li will decrease. Therefore, in the total of Li and Na, the proportion of Na on a mass basis 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 will be improved by the effect of expanding the lattice volume rather than by the effect of blocking the diffusion path of Li ions. This effect is due to the ＋ It cannot be obtained by Na alone. ＋ The diffusion path expansion of Li by Li ＋ +Na ＋ However, in the present invention, even if the content of Na in X is 0 and Li is the only element, a solid electrolyte with excellent Li ion conductivity can be obtained by achieving high purity and high density.

[0059] The solid electrolyte of the present invention has a lattice constant ratio c / a of 2.52 or less and a lattice volume of 1505 Å. ３ More than 1522Å ３ The following rhombohedral Nasicon-type structure is included. 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-type 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-type structure, the lattice volume is 1505 Å ３ More than 1522Å ３ However, if the value deviates from this range, it is possible that Zr and Y do not form a solid solution. Therefore, the lattice volume is 1506 Å. ３ It is preferably 1507 Å or more, and more preferably 1507 Å or more. ３ That's all.

[0060] In addition, the solid electrolyte of the present invention has a peak intensity I that appears at a diffraction angle 2θ of 23.3 to 23.5 degrees due to the rhombohedral Nasicon structure in powder X-ray diffraction (XRD) measurement using a Cukα radiation source. ＡIn contrast, the peak intensity I appears at a diffraction angle 2θ of 19.4 to 19.6 degrees due to the triclinic structure. Ｂ Ratio I Ｂ / I Ａ is 10% or less. As mentioned above, in the present invention, while maintaining the Nasicon-type structure of the Li-M-Y-P-O system, the composition of P and Li (or the composition of P and Li + Na) is controlled to improve ion diffusion. Therefore, by making a solid electrolyte containing a crystalline oxide having such a Nasicon-type structure at high purity, the sinterability of the crystalline oxide when obtaining this solid electrolyte is improved, and as a result, it becomes possible to obtain one that exhibits high lithium ion conductivity.

[0061] In the quantitative analysis by the 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 the difference in crystal structure. Therefore, the above ratio I Ｂ / I Ａ If the ratio is 10% or less (the ratio is 0.1 or less), preferably 9% or less (0.09 or less), the ratio of the triclinic structure to the rhombohedral Nasicon structure is minimized, and a crystalline oxide having a highly pure Nasicon structure can be obtained. Ａ is based on the height of the peak in the XRD measurement, as will be shown in the examples below. Similarly, the peak intensity I appearing at a diffraction angle 2θ of 19.4 to 19.6 degrees Ｂ is based on the height of the peak, and the above I is calculated from the intensity ratio of these peaks. Ｂ / I Ａ The above ratio I Ｂ / I Ａ Although a higher purity Nasicon structure can be obtained by using a smaller I, taking into account the manufacturing process described below, Ｂ / I Ａ The actual lower limit is 1% (0.01).

[0062] For the same reason as above, in powder X-ray diffraction measurement using a Cukα radiation source, the peak intensity I Ａ In contrast, YPO ４ Diffraction angle 2θ: Peak intensity I of 25.8 to 26.0 degrees C Ratio I C / I Ａ is preferably 10% or less (0.1 or less), and more preferably 1% or more (0.01 or more) and 9% or less (0.09 or less). C is based on the height of the peak in the XRD measurement, and the above I C / I Ａ The details of the powder X-ray diffraction measurement are as explained in the Examples below.

[0063] In addition, in the solid electrolyte of the present invention, Ca is an element that stabilizes the Nasicon structure, so it can be substituted together with Zr and Y. However, if the amount of Ca substituted is too large, it may lead to a decrease in ionic conductivity, so the Ca concentration in the solid electrolyte should be 2.0% or less by mass, preferably 1.5% or less. Of course, the Ca concentration may be 0% by mass.

[0064] On the other hand, Zr may contain about 3% Hf derived from its raw material. This Hf is present in the Zr at the hexacoordinate octahedral site. ４＋ and Hf ４＋Although the ionic radii of Hf and Zr are similar and therefore do not significantly affect Li ion diffusion, Hf has a larger atomic weight than Zr, and therefore the inclusion of Hf may make the solid electrolyte heavier and reduce the energy density of the battery. However, Zr raw materials that do not contain Hf are very expensive, so Zr raw materials that contain 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).

[0065] The solid electrolyte of the present invention is produced through i) weighing raw materials, ii) mixing in the liquid phase, iii) heating the liquid phase, iv) washing, v) heat treatment, vi) mixing with a Li raw material, vii) firing, and viii) pulverization.

[0066] 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 weighed out so as to obtain a desired composition. Of these, for the Zr raw material, for example, 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, as the P raw material, phosphoric anhydride (P ２ O ５ ), orthophosphate (H ３ P.O. ４ ), pyrophosphate, metaphosphate [(HPO ３ ) ｎ ], and phosphate compounds such as polyphosphoric acid, as well as ammonium salts such as ammonium dihydrogen phosphate and diammonium hydrogen phosphate can be used.

[0067] 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 substitute Y for Zr. 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 substitution of Y for Zr. 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, aqueous ammonia is added to increase 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 less is recommended. Finally, heating the solution containing the liquid-phase precipitate improves the uniformity of the composition and particle shape.

[0068] Next, in iii) heating the liquid phase, the mixed solution obtained in ii) above is heated and stirred. The temperature at this time is preferably 70°C or higher and 100°C or lower, preferably 80°C or higher and 98°C or lower, and more preferably 85°C or higher and 98°C or lower. Here, the pH of the mixed solution to be heated and stirred is set to 7.0 or lower in order to prevent precipitation of yttrium hydroxide. That is, if yttrium hydroxide is precipitated, Zr and Y become non-uniform, which may reduce the uniformity of the solid electrolyte obtained by the subsequent vi) dry mixing with the Li raw material and vii) calcination. To prevent this, the pH of the mixed solution is preferably set to 7.0 or lower.

[0069] In the above iii) heating of the liquid phase, a slurry-like oxide such as Zr-Y-P oxide ammonium salt is obtained, and therefore in iv) washing, this slurry is filtered to extract the precipitate, which is then washed with water and dried to remove the moisture.

[0070] Next, in v) heat treatment, the dried material is heat-treated at a temperature of 600°C to 800°C in an air atmosphere. At this time, the dried material may be coarsely pulverized before the heat treatment. By this v) heat treatment, Zr-Y-P oxide is obtained by removing ammonia from the Zr-Y-P oxide ammonium salt generated by heating the liquid phase in iii) above. The Zr-Y-P oxide obtained by this v) heat treatment corresponds to a precursor (oxide precursor) for obtaining a solid electrolyte containing Li.

[0071] Next, in vi) mixing with a Li raw material, the Zr-Y-P oxide obtained above is mixed with a Li raw material containing Li. The Li raw material used here is not particularly limited, and examples thereof 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.

[0072] Furthermore, vi) when mixing the Li raw material, a Ca raw material such as calcium carbonate, calcium hydroxide, calcium oxide, etc. may be added. Similarly, a Na raw material such as sodium carbonate, sodium hydroxide, etc. may be added.

[0073] Next, in vii) firing, the mixture mixed with the Li raw material is fired. This firing may be performed in two stages. That is, it is divided into a primary firing (calcination) that promotes gas generation from the mixture, and a secondary firing that performs the main firing. Here, the primary firing can be performed at a temperature of 800°C to 920°C. Next, after gas generation has ended, the mixture is subjected to secondary firing, for example, by pelletizing, if necessary. The secondary firing can be performed at a temperature of 1100°C to 1300°C. In this case, it is preferable to prevent the pellets from coming into direct contact 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.

[0074] Here, for Al, the ionic radius of the hexacoordinated octahedral site is Al ３＋ Zr than ４＋Since the lattice volume is large, the inclusion of Al reduces the lattice volume. Therefore, it is better to reduce the amount of Al mixed in. By producing it using the above method, the Al content in the product can be reduced to 0.1 mass% or less, i.e., 1000 ppm or less. The Al content is preferably 0.05 mass% or less, i.e., 500 ppm or less, and more preferably 0.01 mass% or less, i.e., 100 ppm or less.

[0075] It is also better to reduce the amount of Mg mixed in. ２＋ Li ＋ Since the ionic radii of Mg and Mg are close to each other, if the Li site is substituted, the Li diffusion path will be blocked, which may result in a decrease in ionic conductivity. Therefore, the Mg content is preferably 0.1% by mass or less, i.e., 1000 ppm or less, more 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.

[0076] Then, in viii) pulverization, the sintered pellets are pulverized using a roll crusher, pin mill, hammer mill, jet mill, mortar, etc. to obtain the desired solid electrolyte. The degree of pulverization varies depending on the form and application of the lithium secondary battery, and is therefore difficult to specify in general terms, but generally, the pulverization is performed to obtain a powder (solid electrolyte powder) having a D50 (median diameter) of approximately 30 μm or less.

[0077] As described above, the method for producing a solid electrolyte according to the present invention can be generally said to include the following steps A to C. That is, the method includes 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 the mixed solution in an air atmosphere to obtain an oxide precursor; and step C of adding a Li raw material containing Li and firing the mixed solution in an air atmosphere. In this case, the firing in step C is preferably performed separately into the primary firing and secondary firing as described above. Furthermore, in step C, a Ca raw material may be added as needed in addition to the Li raw material.

[0078] In this way, the composition of the resulting solid electrolyte can be made more uniform by obtaining an oxide precursor made of Zr-Y-P oxide through steps A and B. Furthermore, by adding a Li raw material to the oxide precursor and firing it in step C, a solid electrolyte containing a highly pure crystalline oxide having a Nasicon structure can be obtained.

[0079] The solid electrolyte according to the present invention can be preferably used in a lithium secondary battery, and is particularly suitable for use in a separator layer between a positive electrode and a negative electrode, making it possible to obtain a lithium secondary battery exhibiting high lithium ion conductivity.

[0080] The present invention will be described below based on examples, but the present invention is not limited to these examples.

[0081] Examples 1 to 3 Production of Zirconium Hydrogen Phosphate Powder A starting material was a mixture of an aqueous solution of zirconium nitrate and an aqueous solution of yttrium nitrate, to which an aqueous solution of malonic acid was added as a chelating agent. ３ P.O. ４ ) was added dropwise. At this time, the total amount of the aqueous zirconium nitrate solution and the aqueous yttrium nitrate solution was ２ The concentration was adjusted to 0.055 g / mL. The molar ratio of Zr:Y:P was adjusted to 1.91:0.15:3. Furthermore, the molar ratio of malonic acid to Zr+Y was adjusted to 1 / 2. In order to achieve the desired Zr composition, Zr was added in excess within a range of 30% molar ratio.

[0082] 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. After that, the precipitate was removed from the slurry after the heating reaction, washed with water, and then dried at 120°C. The dried product was roughly crushed in a mortar, heated to 700°C in air for 3 hours, and then held for 3 hours for firing. Next, the mixture was classified using a sieve with 45 μm openings to obtain a Y precipitate having a composition of Zr:Y:P=1.85:0.15:3.0. ２ O ３Y-substituted zirconium hydrogen phosphate powders according to Examples 1 to 3 in which no

[0083] The Y-substituted zirconium hydrogen phosphate powder (calcined 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 the amount of Li raw material added when adding a Li raw material (lithium carbonate) and calcining to obtain a solid electrolyte powder was changed (see Table 3 below), as described below, to obtain the solid electrolyte powders according to Examples 1 to 3, respectively.

[0084] First, the specific surface area of ​​the Y-substituted zirconium hydrogen phosphate powders according to Examples 1 to 3 was measured using a sample powder ranging from approximately 0.1 to 0.5 g. The sample was filled to approximately 70% of the sample tube measurement container. After degassing for 60 minutes at 200°C under a nitrogen gas flow, the specific surface area was measured using a surface area measurement device (NOVA Touch manufactured by Anton Paar Japan K.K.) by the BET method (single-point method) using nitrogen gas adsorption.

[0085] Regarding the composition, a predetermined amount of the obtained Y-substituted zirconium hydrogen phosphate powder was collected, 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, H １＋４a－３ｂ Zr ２－ａ Y ｂ P ３ O １２ When the value of a is expressed as

[0086] Furthermore, the Y-substituted zirconium hydrogen phosphate powder was subjected to TGA measurement 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. Here, 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).

[0087] Furthermore, powder X-ray diffraction measurement was performed using a Cukα radiation source, and the average intensity I １ , average intensity I of diffraction angle 2θ: 30 to 31 degrees ２ , diffraction angle 2θ: average intensity I from 10 to 90 degrees ３ , diffraction angle 2θ: maximum intensity I at 10 to 90 degrees ４ , and the average intensity I of the diffraction angle 2θ: 80 to 90 degrees ５ The values ​​of the following relational expressions (3) and (4) relating to the XRD measurement were calculated from the results. Here, the powder X-ray diffraction measurement was performed using a MiniFlex 600 manufactured by Rigaku Corporation, with measurements being performed in the 2θ range of 10-90°, with a step width of 0.02 and a scan speed of 10° / min. At that time, as shown in FIG. 2, by checking the peak at a diffraction angle 2θ of 28.5 to 29.5 degrees, the Y ２ O ３ Here, the average intensity I of the diffraction angle 2θ: 28.5 to 29.5 degrees can be confirmed. １ and average intensity I at diffraction angle 2θ: 30 to 31 degrees ２ The average intensity I of the diffraction angle 2θ from 10 to 90 degrees was calculated by measuring the intensity of all the measurement points in the XRD measurement at 0.02 step intervals. ３ Similarly, in the XRD measurement, a predetermined range of the diffraction angle 2θ was measured in 0.02 steps, and the intensity at all measurement points was averaged to obtain the maximum intensity I at the diffraction angle 2θ: 10 to 90 degrees. ４ In the XRD measurement, a predetermined range of diffraction angles 2θ is measured in 0.02 steps, and the maximum value among all measurement points is taken as the maximum intensity I ４ Furthermore, the average intensity I ５ In the XRD measurement, a predetermined range of diffraction angle 2θ was measured in 0.02 steps, and the intensity at all measurement points was averaged to determine the value. Note that the above relational expression (4) is shown in percentage (%) in Table 1. (I １ / I ２ )×(I ４ / I ５ )≦500...(3) (I ３ / I ４) ≧ 0.3 ... (4)

[0088]

[0089]

[0090] [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.

[0091]

[0092] 0.7 g of the classified product obtained after the primary firing was sampled for impedance measurement and pelletized at φ11 mm and 150 MPa. This pellet was placed in an alumina crucible lined with zirconia beads and subjected to secondary firing in the atmosphere. The pellet was prevented from coming into contact with the alumina crucible. The secondary firing was performed by raising the temperature to 1200°C at a rate of 150°C / h and holding the temperature for 6 hours.

[0093] Next, the pellets obtained after the secondary firing were polished with sandpaper to smooth the surface, and then the thickness and width were measured to calculate the pellet volume. After that, gold was evaporated 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 made at frequencies ranging from 7.0 MHz to 0.5 Hz. The ionic conductivity was calculated from the thickness and cross-sectional area of ​​the pellet, with the diameter of the arc in the Nyquist plot taken as the total resistance.

[0094] Furthermore, in order to confirm the crystalline phase after secondary firing, the pellets after secondary firing were pulverized without the polishing and vapor deposition processes in the pellet processing after secondary firing in the above pellet production process. The pulverization was carried out for 15 minutes in an agate mortar. After pulverization, the pellets were classified using a sieve with a mesh size of 45 μm to obtain a Li-Zr-Y-P-O-based Nasicon-type solid electrolyte powder that fell below the sieve, and powder X-ray diffraction measurement was carried out using a Cukα radiation source. As a result, it was found that the pyrophosphate ZrP ２ O ７ The presence or absence of peaks derived from the nucleotides was confirmed.

[0095] Furthermore, a predetermined weight of the undersized Li-Zr-Y-P-O based Nasicon type solid electrolyte powder obtained above was sampled, and after treatment such as pressurized acid decomposition or heat acid decomposition, it was dissolved in acid, and after the volume was measured, component analysis was performed by atomic absorption spectrometry and ICP atomic emission spectrometry to examine the product composition of the obtained solid electrolyte powder. Li was measured by atomic absorption spectrometry, and other elements were measured by ICP atomic emission spectrometry. The composition shown in Table 4 is the value normalized to Zr + Y = 2.00. Li ＋ , Zr ４＋ , Y ３＋ , P ５＋ Other than the cations, oxygen anions O ２－ However, if the sum of the cation charges and the sum of the anion charges listed in Table 4 do not match, the oxygen anions increase or decrease from the stoichiometric ratio of 12, resulting in a neutral charge.

[0096] Furthermore, the density of the pellets after secondary firing, which had not been subjected to the polishing and vapor deposition processes, was calculated by dividing the pellet weight by the pellet volume. This calculated density was used as the true density of 3.12 g / cm ３ The relative density was calculated by dividing by . The results are summarized in Table 4.

[0097]

[0098] (Examples 4 and 5) When preparing the Y-substituted zirconium hydrogen phosphate powder, the molar ratio of Zr:Y:P was set to 1.91:0.15:3, and the ZrO ２A 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 concentration was set to 0.060 g / mL.

[0099] 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. Various evaluation results are shown in Table 4.

[0100] (Examples 6 and 7) When preparing Y-substituted zirconium hydrogen phosphate powder, the molar ratio of Zr:Y:P was set to 1.95:0.15:3, and the ZrO ２ 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 concentration was set to 0.060 g / mL.

[0101] 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. Various evaluation results are shown in Table 4.

[0102] (Examples 8 and 9) When preparing Y-substituted zirconium hydrogen phosphate powder, the molar ratio of Zr:Y:P was set to 1.91:0.15:3, and the ZrO ２ 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 concentration was set to 0.044 g / mL.

[0103] 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. Various evaluation results are shown in Table 4.

[0104] (Example 10) When preparing a Y-substituted zirconium hydrogen phosphate powder, the molar ratio of Zr:Y:P was set to 1.97:0.10:3, and the ZrO ２ 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 concentration was set to 0.055 g / mL.

[0105] 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. Various evaluation results are shown in Table 4.

[0106] (Examples 11 and 12) When preparing Y-substituted zirconium hydrogen phosphate powder, the molar ratio of Zr:Y:P was set to 1.97:0.10:3, and the ZrO ２ A Y-substituted zirconium hydrogen phosphate powder having a composition of Zr:Y:P=1.94:0.11:3.0 was obtained in the same manner as in Example 1, except that the concentration was set to 0.044 g / mL.

[0107] 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. Various evaluation results are shown in Table 4.

[0108] (Examples 13 to 15) When preparing Y-substituted zirconium hydrogen phosphate powder, the molar ratio of Zr:Y:P was set to 1.97:0.10:3, and the ZrO ２ 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 concentration was set to 0.044 g / mL.

[0109] 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. Various evaluation results are shown in Table 4.

[0110] (Examples 16 to 18) When preparing Y-substituted zirconium hydrogen phosphate powder, the molar ratio of Zr:Y:P was set to 1.97:0.10:3, and the ZrO ２ 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 concentration was set to 0.065 g / mL.

[0111] 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. Various evaluation results are shown in Table 4.

[0112] (Examples 19 and 20) When preparing Y-substituted zirconium hydrogen phosphate powder, the molar ratio of Zr:Y:P was set to 2.06:0.15:3, and the ZrO ２ 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 concentration was set to 0.060 g / mL.

[0113] 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. Various evaluation results are shown in Table 4.

[0114] (Examples 21 to 23) When preparing Y-substituted zirconium hydrogen phosphate powder, the molar ratio of Zr:Y:P was set to 1.86:0.15:3, and the ZrO ２A 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 concentration was set to 0.055 g / mL.

[0115] 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. Various evaluation results are shown in Table 4.

[0116] (Examples 24 and 25) When preparing Y-substituted zirconium hydrogen phosphate powder, the molar ratio of Zr:Y:P was set to 1.93:0.15:3, and the ZrO ２ 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 concentration was set to 0.055 g / mL.

[0117] 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. Various evaluation results are shown in Table 4.

[0118] (Examples 26 and 27) When preparing Y-substituted zirconium hydrogen phosphate powder, the molar ratio of Zr:Y:P was set to 2.00:0.15:3, and the ZrO ２ 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 concentration was set to 0.055 g / mL.

[0119] 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. Various evaluation results are shown in Table 4.

[0120] (Examples 28 and 29) When preparing Y-substituted zirconium hydrogen phosphate powder, the molar ratio of Zr:Y:P was set to 1.91:0.15:3, and the ZrO ２ 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 concentration was adjusted to 0.055 g / mL, and ammonia water was added to adjust the pH to 3.5 and heated. 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) exhibited a cubic NH ４ Zr ２ (PO ４ ) ３ , i.e. Zr ４＋ Y ３＋ The lattice volume increased due to the substitution of NH ４ Zr ２ (PO ４ ) ３ Since the diffraction pattern contains NH in addition to the amorphous component, ４ Zr ２ (PO ４ ) ３ While maintaining the crystal structure, ３ Y-substituted HZr ２ (PO ４ ) ３ It is understood that the material contains cubic crystal components having the basic composition:

[0121] 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. Various evaluation results are shown in Table 4.

[0122] 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.

[0123] The powder X-ray diffraction measurement using a Cukα radiation source was performed on the zirconium hydrogen phosphate powder obtained above. The results are shown in Figures 2 and 3. Peaks at diffraction angles 2θ of 28.5 to 29.5 degrees were confirmed, and the peak positions were similar to those of the PDF pattern, indicating the presence of a cubic NH ４ Zr ２ (PO ４ ) ３ Since the diffraction pattern of the zirconium hydrogen phosphate powder obtained in Comparative Example 1 (and Comparative Example 2) also includes the diffraction pattern of Y ２ O ３ It can be seen that the produced zirconium hydrogen phosphate is a cubic crystal. ２ O ３ The results of TG-DTA measurement of the cubic zirconium hydrogen phosphate powder produced are shown (crystallization peak at 932.09°C).

[0124] 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. Various evaluation results are shown in Table 4.

[0125] (Comparative Examples 3 and 4) In Example 1, the molar ratio of Zr:Y:P used to prepare the Y-substituted zirconium hydrogen phosphate powder was 1.73:0.15:3, and the total amount of ZrO in the aqueous zirconium nitrate solution and the aqueous yttrium nitrate solution was 1.73:0.15:3. ２ A Y-substituted zirconium hydrogen phosphate powder having a composition of Zr:Y:P=1.66:0.15:3.0 was obtained in the same manner as in Example 1, except that the concentration was set to 0.055 g / mL.

[0126] 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. Various evaluation results are shown in Table 4.

[0127] (Comparative Examples 5 and 6) In Example 1, the molar ratio of Zr:Y:P used to prepare the Y-substituted zirconium hydrogen phosphate powder was 1.63:0.15:3, and the total amount of ZrO in the aqueous zirconium nitrate solution and the aqueous yttrium nitrate solution was 1.63:0.15:3. ２ A Y-substituted zirconium hydrogen phosphate powder having a composition of Zr:Y:P=1.58:0.15:3.0 was obtained in the same manner as in Example 1, except that the concentration was set to 0.055 g / mL.

[0128] 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. Various evaluation results are shown in Table 4.

[0129] (Comparative Examples 7 and 8) Without preparing Y-substituted zirconium hydrogen phosphate, lithium carbonate (Wako Reagent Grade), zirconium oxide (specific surface area 30 m ２ ZnO / g), yttrium oxide, and ammonium dihydrogen phosphate (Wako Reagent Special Grade) were added to the mixture, with a molar ratio of Li:Zr:Y:P of 1.15:1.85:0.15:3. The mixture was 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 carried out in the same manner as in Example 1. After secondary firing, pellets and Nasicon-type solid electrolyte powder were obtained and subjected to various evaluations. The results are shown in Table 4.

[0130] 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 a relative density of 77% or more and an ionic conductivity of 8.5×10 －６ In other words, by using the Y-substituted zirconium hydrogen phosphate powder of the present invention, the composition of the resulting solid electrolyte can be made more uniform, and a high-purity, high-density electrolyte can be obtained.

[0131] (Example 30) [Production of Zirconium Hydrogen Phosphate Powder] As a starting material, an aqueous solution of zirconium nitrate and an aqueous solution of yttrium nitrate were mixed, to which an aqueous solution of oxalic acid was added as a chelating agent. ３ P.O. ４ ) was added dropwise. At this time, the total amount of the aqueous zirconium nitrate solution and the aqueous yttrium nitrate solution was ２ The concentration was set to 0.055 g / mL. The molar ratio of Zr:Y:P was set to 2.18:0.18:3. Furthermore, the molar ratio of oxalic acid to Zr+Y was set to 1 / 2. Note that in order to achieve the target Zr composition, Zr was added in excess within a range of 30% molar ratio.

[0132] Ammonia water 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. After that, the precipitate was removed from the slurry after the heating reaction, washed with water, and then 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. Next, the product was classified using a sieve with 45 μm openings to obtain a Y zeolite having a composition of Zr:Y:P=1.83:0.17:3.0. ２ O ３ Cubic zirconium hydrogen phosphate was produced, that is, zirconium hydrogen phosphate not substituted with Y (Zr-Y-P oxide).

[0133] [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.

[0134] 0.7 g of the classified product obtained after the primary firing was sampled for impedance measurement and pelletized at φ11 mm and 150 MPa. This pellet was placed in an alumina crucible lined with zirconia beads and subjected to secondary firing in the atmosphere. The pellet was prevented from coming into contact with the alumina crucible. The secondary firing was performed by raising the temperature to 1200°C at a rate of 150°C / h and holding for 6 hours.

[0135] The pellets obtained after secondary firing were polished with sandpaper to smooth the surface, and then the thickness and width were measured to calculate the pellet volume. Then, gold was vapor-deposited onto the 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 made at frequencies ranging from 7.0 MHz to 0.5 Hz. The ionic conductivity was calculated from the thickness and cross-sectional area of ​​the pellet, with the diameter of the arc of the Nyquist plot taken as the total resistance. Furthermore, to confirm the crystalline phase after secondary firing, the pellets after secondary firing were pulverized without the polishing and vapor deposition steps in the pellet preparation process after secondary firing. Pulverization was carried out in an agate mortar for 15 minutes. After pulverization, the pellets were classified using a sieve with a mesh size of 45 μm, and the Li—Zr—Y—P—O-based Nasicon-type solid electrolyte powder according to Example 30 was obtained.

[0136] The obtained Nasicon-type solid electrolyte powder was subjected to powder X-ray diffraction measurement using a Cukα radiation source. The measurement was performed using a Rigaku MiniFlex 600 device, with a 2θ range of 10-90°, a step width of 0.02, and a scan speed of 10° / min. Furthermore, the peak intensity I appearing at a diffraction angle 2θ of 23.3-23.5°, which is due to the rhombohedral Nasicon-type structure, was Ａ, a peak intensity I appearing at a diffraction angle 2θ of 19.4 to 19.6 degrees due to the triclinic structure Ｂ , YPO ４ Diffraction angle 2θ: Peak intensity I of 25.8 to 26.0 degrees C is calculated, and the peak intensity ratio I Ｂ / I Ａ and peak intensity ratio I C / I Ａ In determining these peak intensity ratios, the peak intensities were not integral values, but were calculated based on the peak heights (peak intensities) appearing within the respective diffraction angle 2θ ranges. The results are shown in Table 7. In this example, Y ２ O ３ It was said that cubic zirconium hydrogen phosphate (zirconium hydrogen phosphate not substituted with Y) was obtained. ２ O ３ The formation of 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.

[0137] 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 row in FIG. 5 indicates that the amount of triclinic structure produced is large, i.e., I Ｂ / I Ａ The peaks at the bottom are derived from the solid electrolyte powder of Comparative Example 9, where the amount of triclinic structure produced is small, i.e., I Ｂ / I Ａ This is due to the peak of the solid electrolyte powder of Example 34, which has a small peak and is mainly composed of a Nasicon-type rhombohedral crystal structure.

[0138] The lattice constants a, b, and c of the obtained solid electrolyte powder were determined by Rietveld analysis using powder X-ray diffraction measurement with a Cukα radiation source. Rietveld analysis is a method for obtaining information on the crystal structure, such as the lattice constant, by approximating the X-ray diffraction pattern obtained from the assumed crystal structure to the actually measured diffraction pattern. The lattice constant ratio c / a and the lattice volume were calculated. In the case of a Nasicon-type rhombohedral crystal structure, the lattice volume is 3 １／２ a ２ It becomes c / 2.

[0139] Furthermore, a predetermined mass of the solid electrolyte powder obtained above is sampled, and after treatment such as pressurized acid decomposition or heated acid decomposition, it is dissolved in acid, and after the volume is determined, component analysis is performed by atomic absorption analysis or ICP emission spectrometry, and it is found that the solid electrolyte powder has the general formula: X Ａ M Ｂ Y Ｃ P Ｄ O １２±σ From the molar ratios expressed as above, "(B + C) / D" corresponding to "(Zr + Y) / P" and "A / D" corresponding to "Li + P" (or "Li + Na / P") were calculated, and the Ca concentration (mass %), Al concentration (mass %), and Mg concentration (mass %) were also calculated. 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.

[0140] Furthermore, for the pellets after secondary firing that had not been subjected to the polishing and vapor deposition processes, the density was calculated by dividing the pellet mass by the pellet volume. This calculated density was used as the true density of 3.12 g / cm ３ The relative density was calculated by dividing by the ratio of Zr to Y. The results are shown in Table 7. Table 9 shows the composition of the solid electrolyte powder obtained. The values ​​in Table 9 are normalized with Zr + Y being 2.00. ＋ , Na ＋ , Zr ４＋ , Y ３＋ , P ５＋ Other than the cations, oxygen anions O ２－ is 12, but if the sum of the cation charges and the sum of the anion charges listed in Table 9 do not match, the oxygen anions increase or decrease from the stoichiometric ratio of 12, resulting in a neutral charge.

[0141]

[0142]

[0143]

[0144]

[0145]

[0146] (Examples 31, 32, and 42) As a starting material, a mixture of an aqueous solution of zirconium nitrate and an aqueous solution of yttrium nitrate was mixed, to which an aqueous solution of malonic acid was added as a chelating agent, and this solution was diluted with orthophosphoric acid (H ３ P.O. ４ ) was added dropwise. At this time, the total amount of the aqueous zirconium nitrate solution and the aqueous yttrium nitrate solution was ２ The concentration was adjusted to 0.055 g / mL. The molar ratio of Zr:Y:P was adjusted to 1.91:0.15:3. Furthermore, the molar ratio of malonic acid to Zr+Y was adjusted to 1 / 2. In this case, to achieve the desired Zr composition, Zr was added in excess within a range of 10% molar ratio.

[0147] Ammonia water 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. After that, the precipitate was removed from the slurry after the heating reaction, washed with water, and then 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. Next, the mixture was classified using a sieve with 45 μm openings to obtain a Y sieve with a composition of Zr:Y:P=1.85:0.15:3.0. ２ O ３ As a result, amorphous zirconium hydrogen phosphate in which no Y was formed, that is, amorphous zirconium hydrogen phosphate in which Y was substituted (Zr-Y-P oxide), was obtained.

[0148] 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, pellets after secondary firing were obtained in the same manner as in Example 30, including the primary and secondary firing. These were then pulverized to obtain Nasicon-type solid electrolyte powders. Various evaluations were carried out on these. The results are shown in Table 7.

[0149] (Examples 33 to 36, 41) As a starting material, a mixture of an aqueous solution of zirconium nitrate and an aqueous solution of yttrium nitrate was mixed, to which an aqueous solution of malonic acid was added as a chelating agent, and the resulting solution was diluted with orthophosphoric acid (H ３ P.O. ４ ) was added dropwise. At this time, the total amount of the aqueous zirconium nitrate solution and the aqueous yttrium nitrate solution was ２ The concentration was adjusted to 0.060 g / mL. The molar ratio of Zr:Y:P was adjusted to 1.91:0.15:3. Furthermore, the molar ratio of malonic acid to Zr+Y was adjusted to 1 / 2. In this case, to achieve the desired Zr composition, Zr was added in excess within a range of 10% molar ratio.

[0150] Ammonia water 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. After that, the precipitate was removed from the slurry after the heating reaction, washed with water, and then 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. Next, the mixture was classified using a sieve with 45 μm openings to obtain a Y sieve with a composition of Zr:Y:P=1.77:0.15:3.0. ２ O ３ As a result, amorphous zirconium hydrogen phosphate in which no Y was formed, that is, amorphous zirconium hydrogen phosphate in which Y was substituted (Zr-Y-P oxide), was obtained.

[0151] 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 performed in a mortar for 15 minutes. In Examples 35 and 36, in addition to lithium carbonate, calcium carbonate (manufactured by Ube Materials) was weighed to obtain the composition shown in Table 5. After that, pellets and Nasicon-type solid electrolyte powder after secondary firing were obtained 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.

[0152] (Example 37) As a starting material, a mixture of an aqueous solution of zirconium nitrate and an aqueous solution of yttrium nitrate was mixed, to which an aqueous solution of malonic acid was added as a chelating agent, and the resulting solution was diluted with orthophosphoric acid (H３ P.O. ４ ) was added dropwise. At this time, the total amount of the aqueous zirconium nitrate solution and the aqueous yttrium nitrate solution was ２ The concentration was 0.060 g / mL. The molar ratio of Zr:Y:P was 1.95:0.15:3. Malonic acid was added at a molar ratio of 1 / 2 to Zr+Y. In order to achieve the desired Zr composition, Zr was added in excess within a range of 10% molar ratio.

[0153] Ammonia water 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. After that, the precipitate was removed from the slurry after the heating reaction, washed with water, and then 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. Next, the mixture was classified using a sieve with 45 μm openings to obtain a Y zeolite having a composition of Zr:Y:P=1.82:0.15:3.0. ２ O ３ As a result, amorphous zirconium hydrogen phosphate in which no Y was formed, that is, amorphous zirconium hydrogen phosphate in which Y was substituted (Zr-Y-P oxide), was obtained.

[0154] 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, pellets and Nasicon-type solid electrolyte powder after secondary firing were obtained in the same manner as in Example 30, including primary and secondary firing, and various evaluations were performed. The results are shown in Table 7.

[0155] (Example 38) As a starting material, a mixture of an aqueous solution of zirconium nitrate and an aqueous solution of yttrium nitrate was mixed, to which an aqueous solution of malonic acid was added as a chelating agent, and the resulting solution was diluted with orthophosphoric acid (H ３ P.O. ４ ) was added dropwise. At this time, the total amount of the aqueous zirconium nitrate solution and the aqueous yttrium nitrate solution was ２The concentration was 0.060 g / mL. The molar ratio of Zr:Y:P was 1.97:0.10:3. Malonic acid was added at a molar ratio of 1 / 2 to Zr+Y. In order to achieve the desired Zr composition, Zr was added in excess within a range of 10% molar ratio.

[0156] Ammonia water 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. After that, the precipitate was removed from the slurry after the heating reaction, washed with water, and then 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. Next, the mixture was classified using a sieve with 45 μm openings to obtain a Y sieve with a composition of Zr:Y:P=1.85:0.10:3.0. ２ O ３ As a result, amorphous zirconium hydrogen phosphate in which no Y was formed, that is, amorphous zirconium hydrogen phosphate in which Y was substituted (Zr-Y-P oxide), was obtained.

[0157] 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, pellets and Nasicon-type solid electrolyte powder after secondary firing were obtained in the same manner as in Example 30, including primary and secondary firing, and various evaluations were performed. The results are shown in Table 7.

[0158] (Example 39, Comparative Examples 11, 12, and 23) As a starting material, a mixture of an aqueous solution of zirconium nitrate and an aqueous solution of yttrium nitrate was mixed, to which an aqueous solution of malonic acid was added as a chelating agent, and the resulting solution was diluted with orthophosphoric acid (H ３ P.O. ４ ) was added dropwise. At this time, the total amount of the aqueous zirconium nitrate solution and the aqueous yttrium nitrate solution was ２ The concentration was 0.060 g / mL. The molar ratio of Zr:Y:P was 1.91:0.15:3. Malonic acid was added at a molar ratio of 1 / 2 to Zr+Y. In order to achieve the desired Zr composition, Zr was added in excess within a range of 10% molar ratio.

[0159] Ammonia water 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. After that, the precipitate was removed from the slurry after the heating reaction, washed with water, and then 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. Next, the mixture was classified using a sieve with 45 μm openings to obtain a Y sieve with a composition of Zr:Y:P=1.71:0.15:3.0. ２ O ３ As a result, amorphous zirconium hydrogen phosphate in which no Y was formed, that is, amorphous zirconium hydrogen phosphate in which Y was substituted (Zr-Y-P oxide), was obtained.

[0160] Next, the Y-substituted amorphous zirconium hydrogen phosphate obtained above and lithium carbonate (Wako Reagent Special Grade) were weighed and mixed to obtain the compositions shown in Tables 5 and 6. Mixing was carried out in a mortar for 15 minutes. After that, pellets and Nasicon-type solid electrolyte powder after secondary firing were obtained in the same manner as in Example 30, including primary and secondary firing, and various evaluations were performed. The results are shown in Tables 7 and 8.

[0161] (Example 40) As a starting material, a mixture of an aqueous solution of zirconium nitrate and an aqueous solution of yttrium nitrate was mixed, to which an aqueous solution of malonic acid was added as a chelating agent, and the resulting solution was diluted with orthophosphoric acid (H ３ P.O. ４ ) was added dropwise. At this time, the total amount of the aqueous zirconium nitrate solution and the aqueous yttrium nitrate solution was ２ The concentration was 0.055 g / mL. The molar ratio of Zr:Y:P was 1.86:0.20:3. Furthermore, the molar ratio of malonic acid to Zr+Y was 1 / 2. In this case, to achieve the desired Zr composition, Zr was added in excess within a range of 10% molar ratio.

[0162] Ammonia water 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, heated to 700°C in air over 3 hours, and then held for 3 hours for firing. The mixture was then classified using a sieve with 45 μm openings to obtain a Y sieve with a composition of Zr:Y:P=1.81:0.20:3.0. ２ O ３ As a result, amorphous zirconium hydrogen phosphate in which no Y was formed, that is, amorphous zirconium hydrogen phosphate in which Y was substituted (Zr-Y-P oxide), was obtained.

[0163] 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, pellets and Nasicon-type solid electrolyte powder after secondary firing were obtained in the same manner as in Example 30, including primary and secondary firing, and various evaluations were performed. The results are shown in Table 7.

[0164] (Examples 43 to 45, Comparative Examples 26 to 27) The same procedures as in Example 31 were 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. ２ O ３ As a result, amorphous zirconium hydrogen phosphate in which no Y was formed, that is, amorphous zirconium hydrogen phosphate in which Y was substituted (Zr-Y-P oxide), was obtained.

[0165] Next, the Y-substituted amorphous zirconium hydrogen phosphate obtained above, lithium carbonate (Wako Reagent Special Grade), and anhydrous sodium carbonate (Wako Reagent First 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.

[0166] 0.7 g of the classified product obtained after the primary firing was collected for impedance measurement, and pellets were prepared 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 content ratio of Na (by mass) in the total of Li and Na is as shown in the product composition of the solid electrolyte powder in Table 9, and is 5% (=0.05 / (1.06+0.05)×100) for Example 43, 9% for Example 44, 16% for Example 45, 6% for Comparative Example 26, and 5% for Comparative Example 27.

[0167] (Comparative Examples 9, 15 to 16) When preparing Y-substituted zirconium hydrogen phosphate, the molar ratio of Zr:Y:P was set to 1.91:0.15:3, and the ZrO ２ The same procedure as in Example 31 was carried out except that the concentration was set to 0.044 g / mL, and 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.

[0168] 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, pellets and Nasicon-type solid electrolyte powder after secondary firing were obtained in the same manner as in Example 30, including primary and secondary firing, and various evaluations were performed. The results are shown in Table 8.

[0169] 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 in preparing Y-substituted zirconium hydrogen phosphate was 1.97:0.10:3. ２ O３ As a result, amorphous zirconium hydrogen phosphate in which no Y was formed, that is, amorphous zirconium hydrogen phosphate in which Y was substituted (Zr-Y-P oxide), was obtained.

[0170] 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, pellets and Nasicon-type solid electrolyte powder after secondary firing were obtained in the same manner as in Example 30, including primary and secondary firing, and various evaluations were performed. The results are shown in Table 8.

[0171] 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. ２ O ３ As a result, amorphous zirconium hydrogen phosphate in which no Y was formed, that is, amorphous zirconium hydrogen phosphate in which Y was substituted (Zr-Y-P oxide), was obtained.

[0172] 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 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.

[0173] 0.7 g of the classified product obtained after the primary firing was collected for impedance measurement, and pellets were prepared at φ11 mm and 150 MPa. This pellet was 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 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 this. The results are shown in Table 8. The obtained solid electrolyte powder contained 0.73 mass% Mg.

[0174] (Comparative Examples 17 to 18) When preparing Y-substituted zirconium hydrogen phosphate, the molar ratio of Zr:Y:P was set to 1.97:0.10:3, and the ZrO ２ The same procedure as in Example 31 was carried out except that the concentration was 0.044 g / mL. ２ O ３ As a result, amorphous zirconium hydrogen phosphate in which no Y was formed, that is, amorphous zirconium hydrogen phosphate in which Y was substituted (Zr-Y-P oxide), was obtained.

[0175] 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, pellets and Nasicon-type solid electrolyte powder after secondary firing were obtained in the same manner as in Example 30, including primary and secondary firing, and various evaluations were performed. The results are shown in Table 8.

[0176] Comparative Examples 19 and 20 The same procedures as in Example 31 were carried out except that when preparing Y-substituted zirconium hydrogen phosphate, the charged Zr:Y:P ratio was set to 1.73:0.15:3 in molar ratio. An 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.

[0177] 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, pellets and Nasicon-type solid electrolyte powder after secondary firing were obtained in the same manner as in Example 30, including primary and secondary firing, and various evaluations were performed. The results are shown in Table 8.

[0178] Comparative Examples 21 and 22 were carried out in the same manner as in Example 31, except that the molar ratio of Zr:Y:P used when preparing Y-substituted amorphous zirconium hydrogen phosphate was 1.63:0.15:3. ２ O ３ As a result, amorphous zirconium hydrogen phosphate in which no Y was formed, that is, amorphous zirconium hydrogen phosphate in which Y was substituted (Zr-Y-P oxide), was obtained.

[0179] 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, pellets and Nasicon-type solid electrolyte powder after secondary firing were obtained in the same manner as in Example 30, including primary and secondary firing, and various evaluations were performed. The results are shown in Table 8.

[0180] (Comparative Examples 24 to 25) Without preparing Y-substituted zirconium hydrogen phosphate, lithium carbonate (Wako Reagent Grade), zirconium oxide (specific surface area 30 m ２ / g), yttrium oxide, and ammonium dihydrogen phosphate (Wako Reagent Special Grade) were charged to a molar ratio of Li:Zr:Y:P of 1.15:1.85:0.15:3, and the mixture was 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 carried out 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.

[0181] As described above, the solid electrolytes according to the examples of the present invention generally have a higher relative density and are superior in ionic conductivity compared to the solid electrolytes of the comparative examples. Specifically, the solid electrolytes according to the examples all have a relative density of 83% or more and an ionic conductivity of 1.2×10 －６S / cm or more. In other words, the solid electrolyte of the present invention has improved sinterability, and as a result, it can be said to have excellent lithium ion conductivity. Therefore, by using the solid electrolyte of the present invention, a lithium secondary battery with excellent ion conductivity can be obtained.

Claims

1. General formula (I): H １＋４a－３ｂ Zr ２－ａ Y ｂ P ３ O １２±σ The molar ratios of each element, a and b, satisfy the following formulas (1) and (2), and δ indicates the non-stoichiometric amount h, where 0<a≦0.3 (1) 0<b≦0.3 (2). The average intensity at a diffraction angle 2θ of 28.5 to 29.5 degrees determined by X-ray diffraction measurement using Cu-Kα rays is expressed as I. １ The average intensity at the diffraction angle 2θ: 30 to 31 degrees is I ２ The average intensity at the diffraction angle 2θ: 10 to 90 degrees is I ３ The maximum intensity at a diffraction angle 2θ of 10 to 90 degrees is I ４ The average intensity at the diffraction angle 2θ: 80 to 90 degrees is I ５ A phosphate powder characterized by satisfying the following relational expressions (3) and (4) when １ / I ２ )×(I ４ / I ５ )≦500...(3) (I ３ / I ４ ) ≧ 0.3 ... (4) 2. The phosphate powder according to claim 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. Specific surface area is 30m ２ 2. The phosphate powder according to claim 1, wherein the phosphate content is 1 / g or more.

4. The phosphate powder according to claim 1, which is a calcination precursor for obtaining a solid electrolyte by calcination.

5. A lithium secondary battery comprising a solid electrolyte obtained by firing the phosphate powder according to claim 1.

6. A method for producing 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 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.

7. A method for producing a solid electrolyte, comprising adding a Li-containing Li raw material to the phosphate powder obtained according to claim 6, and then calcining the mixture.

8. General formula (II): X Ａ M Ｂ Y Ｃ P Ｄ O １２±σ where X is Li or Li and Na, M is Zr or Zr and Ca, δ represents the non-stoichiometric amount of oxygen, the ratio of lattice constants c / a is 2.52 or less, and the lattice volume is 1505 Å. ３ More than 1522Å ３ The compound contains the following rhombohedral Nasicon-type structure: In powder X-ray diffraction measurement using a Cukα radiation source, the peak intensity I appears at a diffraction angle 2θ of 23.3 to 23.5 degrees due to the rhombohedral Nasicon-type structure. Ａ In contrast, the peak intensity I appears at a diffraction angle 2θ of 19.4 to 19.6 degrees due to the triclinic structure. Ｂ Ratio I Ｂ / I Ａ is 10% or less, and 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. In powder X-ray diffraction measurements using a Cukα radiation source, the peak intensity I appears at a diffraction angle 2θ of 23.3 to 23.5 degrees due to the rhombohedral Nasicon structure. Ａ In contrast, YPO ４ Diffraction angle 2θ: Peak intensity I of 25.8 to 26.0 degrees C Ratio I C / I Ａ The solid electrolyte according to claim 8, wherein the content of SiO2 is 10% or less.

10. The solid electrolyte according to claim 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 claims 8 to 10.

12. A lithium secondary battery characterized in that the solid electrolyte according to any one of claims 8 to 10 is used in a separator layer located between a positive electrode and a negative electrode.

13. A method for producing a solid electrolyte according to any one of claims 8 to 10, comprising: 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, the mixed solution being adjusted to a pH of 7.0 or less, to remove moisture; step B of firing the mixed solution in an air atmosphere to obtain an oxide precursor; and step C of adding a Li raw material containing Li, and further firing the mixed solution in an air atmosphere.

14. The method for producing a solid electrolyte according to claim 13, wherein the firing in step C is carried out in two stages: 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.

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