Solid electrolyte, method for manufacturing solid electrolyte, and battery

Abstract

This solid electrolyte contains Na, Mα, Mβ, and F, where Mα is at least one element that forms a trivalent cation, and Mβ is at least one element that forms a tetra- or higher valent cation. Accordingly, a solid electrolyte having high ionic conductivity can be provided. Preferably, Mβ is an element that forms a tetravalent cation, and the solid electrolyte contains a component represented by the compositional formula Na3-xMα1-xMβxF6, where 0＜x＜1 is satisfied.

Description

Solid electrolyte, method for manufacturing solid electrolyte, and battery

[0001] This application claims the benefit of priority from International Patent Application PCT / JP2023 / 35747, filed September 29, 2023, and Japanese Patent Application JP2024-86422, filed May 28, 2024, the entire disclosures of which are incorporated herein by reference.

[0002] In recent years, with the development of portable devices such as personal computers and mobile phones, the demand for batteries as their power sources has expanded significantly. Batteries used for such applications have traditionally used organic electrolytes, in which an electrolyte is dissolved in a flammable organic solvent, as a medium for ion migration. Batteries containing organic electrolytes may pose safety issues. Therefore, to ensure essential safety, development of all-solid-state batteries that use solid electrolytes instead of organic electrolytes is underway. Since the electrolyte in all-solid-state batteries is a non-flammable material, high safety is realized.

[0003] Furthermore, in response to the recent rapid changes in social conditions toward achieving carbon neutrality, demand is rapidly increasing for decarbonized electric vehicles (EVs) and large lithium-ion batteries that ensure a stable supply of renewable energy. In addition to the scarce elements Co and Ni, supply instability and rising costs of Li have also become an issue.

[0004] Therefore, Japanese Patent Laid-Open No. 2023-48155 (Document 1) and Japanese Patent No. 7230772 (Document 2) disclose solid electrolytes that are Na ion conductors. ｘ M １ ２－（ｙ＋Ｚ） M ２ ｙ M ３ ｚ (A.O. ４ ) ３ and M １ , M ２ and M ３are each independently Hf, Mg, Sc, In, Y, Ca or Zr, and A is P, Si, S or a combination thereof. In the production of the solid electrolyte in Document 2, a Na source, a Y source and a Si source are mixed and fired to obtain Na ５ YSi ４ O １２ The sintered body is molded and then sintered. ３ YSi ３ O ９ powder is added.

[0005] However, the production of the solid electrolytes described in References 1 and 2 requires high-temperature sintering (heat treatment) for densification and other purposes. In battery production, when a pre-sintered solid electrolyte is mixed with a positive electrode active material or a negative electrode active material and sintered, the solid electrolyte may react with the active material to increase its resistance. Therefore, there is a need for a novel Na-ion solid electrolyte that can be densified by press molding or other methods and has high ionic conductivity.

[0006] An object of the present invention is to provide a solid electrolyte having high ionic conductivity.

[0007] A first aspect of the invention is a solid electrolyte comprising Na, Mα, Mβ, and F, where Mα is at least one element that forms a trivalent cation, and Mβ is at least one element that forms a tetravalent or higher cation.

[0008] According to the present invention, a solid electrolyte having high ionic conductivity can be provided.

[0009] A second aspect of the present invention is the solid electrolyte of the first aspect, wherein Mβ is an element that becomes a tetravalent cation, and the solid electrolyte is represented by the composition formula Na ３－ｘ Mα １－ｘ Mβ ｘ F ６ and 0<x<1 is satisfied.

[0010] A third aspect of the present invention is the solid electrolyte of the second aspect, wherein the formula satisfies 0.1≦x≦0.85.

[0011] The invention of aspect 4 is the solid electrolyte of aspect 1, wherein the element that becomes a tetravalent cation is Mβ１ , the element that becomes a pentavalent cation is Mβ ２ , and the element that becomes a hexavalent cation is Mβ ３ and the solid electrolyte is represented by the composition formula Na ３－ｘ－２ｙ－３ｚ Mα １－（ｘ＋ｙ＋ｚ） Mβ １ ｘ Mβ ２ ｙ Mβ ３ ｚ F ６ and 0≦x<1, 0≦y<1, 0≦z<1, 0<(x+y+z)<1 are satisfied.

[0012] A fifth aspect of the present invention is the solid electrolyte of the fourth aspect, wherein the formula satisfies 0.1≦x≦0.85.

[0013] A sixth aspect of the present invention is the solid electrolyte according to any one of the first to fifth aspects, in which Mα includes Al or Ga.

[0014] A seventh aspect of the present invention is the solid electrolyte according to any one of the first to sixth aspects, wherein Mβ contains Si, P, or Ge.

[0015] An eighth aspect of the invention is the solid electrolyte of any one of the first to seventh aspects, wherein an X-ray diffraction pattern obtained by X-ray diffraction measurement using CuKα radiation has peaks in the diffraction angle 2θ ranges of 18 to 21°, 21 to 24°, 31 to 34°, and 45 to 48°, respectively.

[0016] A ninth aspect of the present invention is the solid electrolyte according to any one of the first to eighth aspects, wherein the half width of the strongest peak in an X-ray diffraction pattern obtained by X-ray diffraction measurement using CuKα radiation is 0.4° or more.

[0017] A tenth aspect of the present invention is a method for producing a solid electrolyte, the method comprising: a) mixing a substance containing Na, Mβ, and F with Na, where Mα is at least one element that becomes a trivalent cation and Mβ is at least one element that becomes a tetravalent or higher cation; ３ MαF ６ or a mixture of a substance containing Na, Mβ and F with MαF ３and NaF to obtain a mixture; and b) subjecting the mixture to mechanical milling.

[0018] An eleventh aspect of the invention is a battery comprising a positive electrode, a negative electrode, and an electrolyte layer provided between the positive electrode and the negative electrode, wherein the solid electrolyte according to any one of the first to ninth aspects is contained in at least one of the positive electrode, the negative electrode, and the electrolyte layer.

[0019] The above and other objects, features, aspects and advantages will become more apparent from the following detailed description of the invention which proceeds with reference to the accompanying drawings.

[0020] 1 is a longitudinal sectional view showing an all-solid-state secondary battery; FIG. 2 is a diagram showing a flow of manufacturing a solid electrolyte; FIG. 3 is a diagram showing an X-ray diffraction pattern obtained from the solid electrolyte; ２ SiF ６ 1 is a graph showing the relationship between the ratio of the amount of Na and the ionic conductivity at room temperature. ２ SiF ６ 1 is a graph showing the relationship between the ratio of the amounts of substances of the above and the ionic conductivity at 120° C. FIG. 2 is a graph showing the results of a charge-discharge test. FIG. 3 is a graph showing the results of a potential window measurement.

[0021] FIG. 1 is a longitudinal cross-sectional view showing an all-solid-state sodium ion secondary battery 1 (hereinafter simply referred to as "all-solid-state secondary battery 1"). The all-solid-state secondary battery 1 has, from top to bottom in FIG. 1 , a positive electrode 11, an electrolyte layer 13, and a negative electrode 12. That is, the electrolyte layer 13 is provided between the positive electrode 11 and the negative electrode 12. The electrolyte layer 13 is a solid electrolyte layer and also serves as a separator layer. The positive electrode 11 includes a current collector 111 and a positive electrode layer 112. The positive electrode layer 112 includes a positive electrode active material. The negative electrode 12 includes a current collector 121 and a negative electrode layer 122. The negative electrode layer 122 includes a negative electrode active material.

[0022] An example of the positive electrode active material of the positive electrode layer 112 is Na(Fe, Mn)O ２ Another example of the positive electrode active material is a layered compound, NaCoO ２ , NaNiO ２ , NaMnO ２ , Na(Ni,Co,Mn)O ２ , NaFeO ２, Na(Ni, Mn, Fe, Ti)O ２ Spinel-type NaMn ２ O ４ Polyanion type Na ３ V ２ (P.O. ４ ) ３ , Na ２ Fe ２ (SO ４ ) ３ , Na ２ Mn ２ (SO ４ ) ３ , NaFePO ４ , NaMnPO ４ , Na ４ Fe ７ (P.O. ４ ) ６ , Na ３ V (PO ３ ) ３ N, sodium pyrophosphate ２ MnP ２ O ７ , Na ２ FeP ２ O ７ The positive electrode layer 112 includes a solid electrolyte, which will be described later, in addition to a positive electrode active material. The positive electrode layer 112 may further include an electron conduction aid (carbon black, etc.). An example of the positive electrode layer 112 is one in which these materials are integrated by applying pressure or the like.

[0023] An example of the negative electrode active material of the negative electrode layer 122 is hard carbon. Other examples of the negative electrode active material include metals such as Na, In, Sn, and Sb, alloys of these metals with Na, graphite, SiO, and TiO. ２ , Fe ２ O ３ , Na ４／３ Ti ５／３ O ４ , Na ２ Ti ６ O １３ , Na ０．６６ [Li ０．２２ Ti ０．７８ ]O ２ , Na ３ V ２ (P.O. ４ ) ３ , SnO, Li ４Ti ５ O １２ The anode layer 122 includes a solid electrolyte, which will be described later, in addition to the anode active material. The anode layer 122 may further include an electron conduction aid (carbon black, etc.). One example of the anode layer 122 is one in which these materials are integrated by applying pressure or the like.

[0024] The configurations and materials of the positive electrode 11 and the negative electrode 12 of the all-solid-state secondary battery 1 are not limited to those described above, and various other configurations and materials can be adopted.

[0025] The electrolyte layer 13 is made of or includes a solid electrolyte according to one embodiment of the present invention (hereinafter also referred to as the "present solid electrolyte"). The solid electrolyte is a sodium (Na) ion-conducting material. The solid electrolyte includes elemental sodium (Na), at least one element (Mα) that becomes a trivalent cation, at least one element (Mβ) that becomes a tetravalent or higher cation, and elemental fluorine (F). The solid electrolyte may be composed only of Na, Mα, Mβ, and F. Mα is typically a metallic element. Mβ is a metallic or non-metallic element. For example, the amount of substance of Na is greater than the amounts of substances of Mα and Mβ, and the amount of substance of F is greater than the amount of substance of Na. As described below, the solid electrolyte achieves high ionic conductivity (typically sodium ion conductivity). Furthermore, the solid electrolyte is non-flammable and chemically stable. The solid electrolyte is typically non-sulfide-based and, for example, does not include elemental sulfur (S) (although it may contain S as an inevitable impurity).

[0026] When Mβ is an element that becomes a tetravalent cation, the present solid electrolyte is, for example, Na ３ MαF ６ and Na ２ MβF ６ (See Na below. ２ Mβ １ F ６ ) and Na ２ MβF ６ Instead of NaF and MβF ４ A mixture of these in a molar ratio of 2:1 may also be used. ３ MαF６ and Na ２ MβF ６ When the amount of substances is mixed so that the ratio of (1-x):x (where 0<x<1), the solid electrolyte has the composition formula Na ３－ｘ Mα １－ｘ Mβ ｘ F ６ Considering measurement errors, etc., the ratio of the amount of substance of Na, Mα, Mβ, and F is a:b:c:d (i.e., Na ａ Mα ｂ Mβ ｃ F ｄ Components that satisfy the following conditions can be considered to be components represented by the above composition formula: 0.9(3-x)≦a≦1.1(3-x), 0.9(1-x)≦b≦1.1(1-x), 0.9x≦c≦1.1x, and 5.4≦d≦6.6. Since 0<x<1, the above conditions for a to d are equivalent to 1.8<a<3.3, 0<b<1.1, 0<c<1.1, and 5.4≦d≦6.6. In the above composition formula, it is preferable that 0.1≦x≦0.85 is satisfied, and it is more preferable that 0.2≦x≦0.6 is satisfied. This can further improve ionic conductivity. Depending on the method for producing the solid electrolyte, a, b, c, and d may be outside the above ranges. Furthermore, depending on the ionic conductivity required of the solid electrolyte, x may be less than 0.1 or greater than 0.85.

[0027] The element Mα that becomes a trivalent cation is included in the group 13 elements (Al, Ga, etc.), group 3 elements (Sc, etc.), transition metal elements (Fe, Mn, etc.), and rare earth elements (Y, Ho, etc.), and preferably, Mα includes aluminum element (Al) or gallium element (Ga). Mα may include both Al and Ga. ３ MαF ６ An example of this is Na ３ AlF ６ or Na ３ GaF ６When Mβ is an element that becomes a tetravalent cation, Mβ is included in the group 4 elements (Zr, etc.) and group 14 elements (Si, Ge, etc.), and preferably Mβ includes silicon element (Si) or germanium element (Ge). Mβ may include both Si and Ge. ２ MβF ６ An example of this is Na ２ SiF ６ or Na ２ GeF ６ For example, Na ３ AlF ６ and Na ２ SiF ６ In the solid electrolyte containing ３ AlF ６ The reason for this is not entirely clear, but it is thought that by performing the mechanical milling process described below, Al and Si are mixed in the crystal structure containing Na (for example, Al ３＋ In contrast to Si ４＋ One possible reason is that Na deficiency occurs due to the formation of a solid solution. ３ MαF ６ But Na ３ AlF ６ Compounds other than (e.g., Na ３ GaF ６ ), then Na ２ MβF ６ But Na ２ SiF ６ Compounds other than (e.g., Na ２ GeF ６ ), and when Mβ is an element that becomes a cation other than a tetravalent cation (for example, a pentavalent cation, a hexavalent cation, etc.).

[0028] As described above, Mβ is one or more elements that form tetravalent or higher cations, and may include, for example, an element that forms a pentavalent cation or an element that forms a hexavalent cation. １ , the element that becomes a pentavalent cation is Mβ ２ , and the element that becomes a hexavalent cation is Mβ ３ In this case, the composition formula of the solid electrolyte is Na ３－ｘ－２ｙ－３ｚMα １－（ｘ＋ｙ＋ｚ） Mβ １ ｘ Mβ ２ ｙ Mβ ３ ｚ F ６ In the above composition formula, 0≦x<1, 0≦y<1, 0≦z<1, 0<(x+y+z)<1 are satisfied. Taking into account measurement errors, etc., Na, Mα, Mβ １ , Mβ ２ , Mβ ３ , F substance amount ratio a:b:c1:c2:c3:d (i.e., Na ａ Mα ｂ Mβ １ ｃ１ Mβ ２ ｃ２ Mβ ３ ｃ３ F ｄ ) satisfying 0.9(3-x-2y-3z)≦a≦1.1(3-x-2y-3z), 0.9(1-(x+y+z))≦b≦1.1(1-(x+y+z)), 0.9x≦c1≦1.1x, 0.9y≦c2≦1.1y, 0.9z≦c3≦1.1z, and 5.4≦d≦6.6 can be considered to be the component represented by the above composition formula. In the above composition formula, 0.1≦x≦0.85 may be satisfied. An element Mβ that becomes a pentavalent cation ２ is, for example, phosphorus (P), vanadium (V), niobium (Nb), tantalum (Ta), antimony (Sb), etc. An element Mβ that becomes a hexavalent cation ３ is, for example, molybdenum (Mo), tungsten (W), etc. Depending on the method for producing the solid electrolyte, the solid electrolyte may be represented by a composition formula other than the above.

[0029] As described below, the X-ray diffraction pattern obtained by X-ray diffraction measurement using CuKα radiation for this solid electrolyte preferably has peaks in the diffraction angle 2θ ranges of 18 to 21°, 21 to 24°, 31 to 34°, and 45 to 48°. Furthermore, the half-width of the strongest peak in the X-ray diffraction pattern is preferably 0.4° or greater. Such a solid electrolyte has an appropriately low crystallinity, achieving high ionic conductivity. The solid electrolyte may be amorphous.

[0030] FIG. 2 is a diagram showing the flow of manufacturing the solid electrolyte. ３ MαF ６ Na ３ AlF ６ And Na ２ Mβ １ F ６ Na ２ SiF ６ The process for producing the present solid electrolyte by mixing Na ３ AlF ６ A powder of Na is prepared. ３ AlF ６ In preparing the powder, for example, commercially available NaF (sodium fluoride) powder and commercially available AlF ３ The aluminum fluoride powder is weighed and mixed in a ratio of 3:1. The resulting mixture is then heat-treated (for example, at 1050°C) and then pulverized to obtain Na ３ AlF ６ The powder obtained is Na ３ AlF ６ The powder may be produced by other methods.

[0031] Also, Na ２ SiF ６ A powder of Na is prepared. ２ SiF ６ The powder is commercially available. ２ SiF ６ The powder may be produced by known methods. ３ AlF ６ powder and Na ２SiF ６ A mixture is obtained by mixing the powder of Na with the powder of Na (step S11). ３ AlF ６ Amount of substance and Na ２ SiF ６ The amount of Na in the total amount of ２ SiF ６ is greater than 0% and less than 100%. The ratio is preferably 10% or more and 85% or less, and more preferably 20% or more and 60% or less.

[0032] Next, the mixture is subjected to mechanical milling (step S12). Here, a planetary ball mill is used as an example of the mechanical milling. In a planetary ball mill, the pot rotates on its axis while the stage on which the pot is placed revolves, making it possible to generate very high impact energy. The mechanical milling may also be performed using other types of grinders. The mechanical milling produces powder of the present solid electrolyte to be used in the positive electrode layer 112, the negative electrode layer 122, or the electrolyte layer 13. In this example, the mechanical milling is performed at room temperature, but the temperature and other conditions may be changed as appropriate.

[0033] As described above, Mβ may contain an element that becomes a pentavalent cation or an element that becomes a hexavalent cation. １ and compounds consisting of F (e.g., Na ２ Mβ １ F ６ In the above, Na ２ SiF ６ ) or in addition to the compound, Na, Mβ ２ and compounds consisting of F (e.g., NaPF ６ ), Na, Mβ ２ and mixtures containing F (e.g., NaF and NbF ５ mixtures with Na, Mβ ３ and mixtures containing F (e.g., NaF and MoF ６In this way, a substance containing Na, Mβ, and F (typically, a substance consisting of Na, Mβ, and F) can be used. The substance is a compound consisting of Na, Mβ, and F, and / or a mixture containing Na, Mβ, and F (typically, a mixture consisting of Na, Mβ, and F). In step S11, a substance containing Na, Mβ, and F and Na ３ MαF ６ (In the above, Na ３ AlF ６ ) is mixed to obtain a mixture, which is then subjected to mechanical milling in step S12 to produce the present solid electrolyte.

[0034] FIG. 3 is a diagram showing an X-ray diffraction (XRD) pattern obtained by measuring the solid electrolyte by powder X-ray diffraction method. ２ SiF ６ 3 shows the X-ray diffraction pattern of a solid electrolyte (Experimental Example 6 described later) obtained by setting the ratio of the amount of substance of Na to 50%. In this measurement, CuKα radiation is used as the radiation source of the X-ray diffractometer. As shown in FIG. 3, the X-ray diffraction pattern of the solid electrolyte produced by mechanical milling has relatively broad peaks in the diffraction angle 2θ ranges of 18 to 21°, 21 to 24°, 31 to 34°, and 45 to 48°. From FIG. 3, it can be seen that the extremely high impact energy produced by mechanical milling causes the Na ３ AlF ６ and Na ２ SiF ６This is thought to be due to the particles becoming finer and partially becoming amorphous, as well as the crystal structure being disrupted by strain or dislocation within the crystal, which is particularly pronounced at the particle interfaces, resulting in increased ionic conductivity of the present solid electrolyte. Therefore, the present solid electrolyte may be further or completely amorphous. To more reliably improve the ionic conductivity of the present solid electrolyte, the half-width of the strongest peak in the X-ray diffraction pattern (e.g., a peak in the range of 18 to 21°) is preferably 0.4° or greater, more preferably 0.5° or greater. In the X-ray diffraction pattern of FIG. 3, the half-width of the strongest peak is 0.66°. The upper limit of this half-width is not particularly limited, but may be, for example, 3.0°. The half-width of the peak is determined by performing automatic fitting of the X-ray diffraction pattern using the XRD data analysis software JADE.

[0035] Next, solid electrolyte Experimental Examples 1 to 10 will be described. Experimental Examples 2 to 10 are examples of the present invention, and Experimental Example 1 is a comparative example. The following experiments were carried out in a glove box or a dry room with a dew point of −40° C. or lower.

[0036] (Experimental Example 1) Commercially available NaF powder and commercially available AlF were used as raw materials. ３ The powders were prepared as follows: NaF:AlF ３ The mixture was heated at 1050°C and then pulverized in a mortar to obtain Na ３ AlF ６ A powder was obtained. ３ AlF ６ The powder was subjected to mechanical milling using a planetary ball mill to obtain Na ３ AlF ６ A solid electrolyte powder containing only powder was obtained.

[0037] (Experimental Example 2) ３ AlF ６ In addition to powder, commercially available Na ２ SiF ６ Powder was prepared. ３ AlF ６ : Na ２ SiF ６were weighed and mixed so that the ratio of the substances was 90:10, and then mechanically milled using a planetary ball mill to obtain Na ３ AlF ６ -Na ２ SiF ６ The powder was obtained as a solid electrolyte powder. ３ MαF ６ and Na ２ MβF ６ When mixing these materials in a ratio of (1-x):x (where 0<x<1), the composition formula is Na ３－ｘ Mα １－ｘ Mβ ｘ F ６ In this experimental example where Mα and Mβ are Al and Si, and further where x is 0.1, the solid electrolyte powder is represented by the composition formula Na ２．９ Al ０．９ Si ０．１ F ６ It is expressed as:

[0038] (Experimental Example 3) Na ３ AlF ６ : Na ２ SiF ６ The same treatment as in Experimental Example 2 was carried out, except that the ratio of the amounts of the components was 80:20. ２．８ Al ０．８ Si ０．２ F ６ A solid electrolyte powder represented by the formula:

[0039] (Experimental Example 4) Na ３ AlF ６ : Na ２ SiF ６ The same treatment as in Experimental Example 2 was carried out, except that the ratio of the amounts of the components was 70:30. ２．７ Al ０．７ Si ０．３ F ６ A solid electrolyte powder represented by the formula:

[0040] (Experimental Example 5) Na ３ AlF ６ : Na ２ SiF ６The same treatment as in Experimental Example 2 was carried out, except that the ratio of the amounts of the components was 60:40. ２．６ Al ０．６ Si ０．４ F ６ A solid electrolyte powder represented by the formula:

[0041] (Experimental Example 6) Na ３ AlF ６ : Na ２ SiF ６ The same treatment as in Experimental Example 2 was carried out, except that the ratio of the amounts of the components was 50:50. ２．５ Al ０．５ Si ０．５ F ６ A solid electrolyte powder represented by the formula:

[0042] (Experimental Example 7) Na ３ AlF ６ : Na ２ SiF ６ The same treatment as in Experimental Example 2 was carried out, except that the ratio of the amounts of the components was 20:80. ２．２ Al ０．２ Si ０．８ F ６ A solid electrolyte powder represented by the formula:

[0043] (Experimental Example 8) As raw materials, commercially available NaF powder and commercially available GaF ３ (gallium fluoride) powder was prepared. ３ The mixture was heated at 1050°C and then crushed in a mortar to obtain Na ３ GaF ６ A powder was obtained. ３ GaF ６ : Na ２ SiF ６ were weighed out so that the ratio of the amounts of substances was 50:50, and mechanically milled using a planetary ball mill to obtain a compound of the composition formula Na ２．５ Ga ０．５ Si ０．５ F ６ A solid electrolyte powder represented by the formula:

[0044] (Experimental Example 9) The above Na ３ AlF ６ In addition to the powder, commercially available NaPF ６ Powder was prepared. ３ AlF ６ : NaPF ６ The mixture was weighed and mixed so that the ratio of the substances was 67:33, and then mechanically milled using a planetary ball mill to obtain Na ３ AlF ６ -NaPF ６ The solid electrolyte powder was obtained as a powder of the composition formula Na ２．３４ Al ０．６７ P ０．３３ F ６ It is expressed as:

[0045] (Experimental Example 10) Na ３ AlF ６ : NaPF ６ : Na ２ SiF ６ were weighed and mixed in a ratio of 10:80:10 (mass ratio), and mechanically milled using a planetary ball mill to obtain Na ３ AlF ６ -NaPF ６ -Na ２ SiF ６ The solid electrolyte powder was obtained as a powder of the composition formula Na １．３ Al ０．１ Si ０．１ P ０．８ F ６ It is expressed as:

[0046] <Measurement of ionic conductivity> A solid electrolyte powder was placed in a mold consisting of a resin sleeve and upper and lower punches made of stainless steel (SUS), and uniaxial press molding was performed under a pressure of 150 MPa. Lead wires were connected to the upper and lower punches, and impedance measurements were performed at room temperature to 120°C to calculate the ionic conductivity.

[0047] Table 1 shows the Na content in the solid electrolyte powders of Experimental Examples 1 to 10. ３ AlF ６ or Na ３ GaF ６ And Na ２ SiF ６and / or NaPF ６ The figures also show the ratio of the amount of substance to the amount of Na, and the ionic conductivity at room temperature and 120°C. The half-widths of the peaks calculated by the above-mentioned method for the X-ray diffraction patterns obtained from some of the solid electrolyte powders are also shown. ３ AlF ６ and Na ２ SiF ６ The proportion of Na in the total amount of substances ２ SiF ６ 4 shows the relationship between the ratio (mol %) of the amount of substance of the ionic conductivity and the ionic conductivity at room temperature, and FIG.

[0048]

[0049] Na ２ SiF ６ In the solid electrolyte powders of Experimental Examples 2 to 8 containing ２ SiF ６ The solid electrolyte powder of Experimental Example 1, which did not contain Na, was found to be higher than that of Experimental Example 1. ３ AlF ６ In Experimental Examples 1 to 7, which include ２ SiF ６ As the ratio of the amount of substance of increases from 0% to 50%, the ionic conductivity at room temperature and 120°C increases. ２ SiF ６ As the ratio of Na increased further, the ionic conductivity decreased. ２ SiF ６ Even in Experimental Example 7, where the ratio of Na was 80%, the ionic conductivity was sufficiently higher than that of the solid electrolyte powder in Experimental Example 1. ２ SiF ６ is greater than 0% and less than 100% (i.e., 0<x<1 is satisfied), it is believed that a higher ionic conductivity than that of the solid electrolyte powder of Experimental Example 1 is ensured.

[0050] Also, Na ２ SiF ６Considering the results of Experimental Example 2, in which the ratio of the amount of substance of Na is 10%, and Experimental Example 7, in which the ratio is 80%, ２ SiF ６ It is believed that the ionic conductivity will be sufficiently high if the ratio of Na is 10% or more and 85% or less (0.1≦x≦0.85). ２ SiF ６ It is more preferable that the ratio of NaPF is 20% or more and 60% or less (0.2≦x≦0.6), which makes it possible to more reliably achieve sufficiently high ionic conductivity. ６ The solid electrolyte powders of Experimental Examples 9 and 10, which contained the compound (I), also had higher ionic conductivities at room temperature and 120°C than the solid electrolyte powder of Experimental Example 1. For the solid electrolyte powders of Experimental Examples 3 and 6, which were subjected to X-ray diffraction measurements, peaks were present in the X-ray diffraction patterns at diffraction angles 2θ of 18 to 21°, 21 to 24°, 31 to 34°, and 45 to 48°. The half-width of the strongest peak was 0.4° or more.

[0051] <Preparation of all-solid-state battery 1> Na(Fe,Mn)O ２ A positive electrode active material powder, a solid electrolyte powder, and an electron conduction additive powder were weighed and mixed to obtain a positive electrode blended powder. A negative electrode active material powder, a hard carbon, a solid electrolyte powder, and an electron conduction additive powder were weighed and mixed to obtain a negative electrode blended powder. The solid electrolyte powder was placed in a mold consisting of a PEEK resin sleeve and SUS upper and lower punches, and uniaxially pressed under a pressure of 150 MPa. The positive electrode blended powder was placed on top of the pressed solid electrolyte powder and integrated under a pressure of 150 MPa. The negative electrode blended powder was placed on the pressed solid electrolyte powder opposite the positive electrode and integrated under a pressure of 150 MPa. This produced a battery consisting of a positive electrode layer, a solid electrolyte layer, and a negative electrode layer.

[0052] <Charge-Discharge Test 1> After connecting the lead wires to the upper and lower punches, the battery was placed in a thermostatic chamber at 120°C and subjected to a cc-cv (constant current-constant voltage) charge-discharge test. In the charge-discharge test, a cc current density of 300 μA / cm was used. ２, cv current density 30μA / cm ２ The cutoff voltage was set to 4.25-1.50 V. In the battery using the solid electrolyte powder of Experimental Example 1 (Comparative Example), the discharge capacity was 0 mAh / g per positive electrode active material. In other words, the resistance was high and charging and discharging were not possible. In the battery using the solid electrolyte powder of Experimental Example 6 (Example), the discharge capacity was 31 mAh / g per positive electrode active material.

[0053] <Preparation of all-solid-state battery 2> The solid electrolyte powder of Experimental Example 6 and commercially available NaCrO ２ A positive electrode active material powder and a commercially available electron conduction additive powder (acetylene black) were weighed and mixed in a weight ratio of 50:50:3 to obtain a positive electrode powder mixture. 100 mg of solid electrolyte powder was placed in a PEEK resin mold with an inner diameter of 10 mm and press-molded. One punch was removed, and 3.3 mg of the positive electrode powder mixture was placed in and press-molded. The other punch was removed, and a Na metal foil was placed in the mold. The punch was then replaced and secured with screws to construct a battery.

[0054] <Charge / Discharge Test 2> The battery was placed in a thermostatic chamber at 60°C and subjected to a 50 μA cc charge / discharge test. The charge cutoff was 3.6 V and the discharge cutoff was 2.0 V. FIG. 6 shows the results of the charge / discharge test. The vertical axis of FIG. 6 represents voltage, and the horizontal axis represents capacity density. The first through fourth charge curves are labeled L11, L12, L13, and L14, and the first through fourth discharge curves are labeled L21, L22, L23, and L24. As shown in FIG. 6, the battery was able to stably repeat multiple charge / discharge cycles, confirming that the solid electrolyte functioned as a Na solid electrolyte.

[0055] <Measurement of Potential Window> 100 mg of the solid electrolyte powder of Experimental Example 6 was placed in a mold made of PEEK resin with an inner diameter of 10 mm and press-molded. One of the punches was removed, and a Na metal foil was placed thereon. The punch was then replaced and fixed with screws to obtain a cell for measurement. In measuring the potential window, the cell was placed in a thermostatic chamber at 60°C and a voltage of -0.5 to 5 V (vs Na ＋A voltage sweep was performed at 10 mV / sec in the range of 0.1 V to 0.1 V (Na / Na), and cyclic voltammetry was obtained. FIG. 7 shows the results of the potential window measurement. The vertical axis of FIG. 7 represents current, and the horizontal axis represents voltage. As a result of the potential window measurement, no current flowed up to 5 V (the solid electrolyte did not electrolyze), confirming that this solid electrolyte has high electrochemical stability (oxidation resistance).

[0056] In step S11 of FIG. 2, NaF and AlF ３ Na produced from ３ AlF ６ But Na ２ SiF ６ It is mixed with Na ３ AlF ６ without producing NaF and AlF ３ Na ２ SiF ６ In this case, NaF and AlF may be directly mixed. ３ The ratio of the amount of NaF to AlF may be other than 3:1. ３ and Na ２ SiF ６ In the case of mixing Na, by subjecting the mixture to mechanical milling in step S12, it is possible to produce a solid electrolyte having high ionic conductivity. ３ AlF ６ Other than Na ３ MαF ６ (wherein Mα is an element that becomes a trivalent cation) may be used, and Na ２ SiF ６ Other than Na ２ Mβ １ F ６ (However, Mβ １ is an element that becomes a tetravalent cation.) may also be used. ３ AlF ６ Other than Na ３ MαF ６ is, for example, Na ３ GaF ６ , Na ３ CrF ６ , Na ３ LaF ６ etc.２ SiF ６ Other than Na ２ Mβ １ F ６ is, for example, Na ２ GeF ６ , Na ２ TiF ６ , Na ２ ZrF ６ etc. As mentioned above, Mβ ２ is an element that becomes a pentavalent cation, and Mβ ３ As the element that becomes a hexavalent cation, Na ２ Mβ １ F ６ Alternatively, or Na ２ Mβ １ F ６ Along with Na, Mβ ２ and compounds consisting of F (e.g., NaPF ６ , NaVF ６ ), Na, Mβ ３ and a mixture of NaF and MoF ６ or WF ６ Furthermore, AlF ３ Other than MαF ３ and NaF is Na ２ MβF ６ It may be directly mixed with AlF ３ Other than MαF ３ is, for example, GaF ３ , CrF ３ , LaF ３ , BiF ３ , Y.F. ３ etc.

[0057] As described above, the present solid electrolyte contains Na, Mα, Mβ, and F, where Mα is at least one element that forms a trivalent cation, and Mβ is at least one element that forms a tetravalent or higher cation. This makes it possible to provide a solid electrolyte with high ionic conductivity. As a result, it becomes possible to realize an all-solid-state secondary battery with high output.

[0058] Among solid electrolytes, sulfide-based materials are widely known to react with water to generate hydrogen sulfide gas. Meanwhile, oxide-based solid electrolytes that do not generate gases like hydrogen sulfide are also being widely developed. However, these typically require densification by high-temperature sintering to increase ionic conductivity, which can lead to problems with reaction with active materials during battery fabrication, resulting in high resistance. In contrast, the present solid electrolyte is nonflammable and chemically stable, making it possible to provide an essentially safe ion battery. Furthermore, the present solid electrolyte can be densified by press molding pressure rather than sintering, thereby preventing reaction with active materials during battery fabrication. Furthermore, anodes capable of stable charge and discharge can be formed even when using anode active materials with low operating potentials, such as graphite. The present solid electrolyte uses sodium, an inexpensive and resource-free element, making it possible to provide all-solid-state secondary batteries that are inexpensive and can be supplied in large quantities.

[0059] Preferably, Mβ is an element that becomes a tetravalent cation, and the solid electrolyte has a composition formula of Na ａ Mα ｂ Mβ ｃ F ｄ and the following conditions are satisfied: 0.9(3-x)≦a≦1.1(3-x), 0.9(1-x)≦b≦1.1(1-x), 0.9x≦c≦1.1x, 5.4≦d≦6.6, and 0<x<1. This makes it possible to more reliably achieve high ionic conductivity.

[0060] Preferably, the element that becomes a tetravalent cation is Mβ １ , the element that becomes a pentavalent cation is Mβ ２ , and the element that becomes a hexavalent cation is Mβ ３ The solid electrolyte has the composition formula Na ３－ｘ－２ｙ－３ｚ Mα １－（ｘ＋ｙ＋ｚ） Mβ １ ｘ Mβ ２ ｙ Mβ ３ ｚ F ６ and the following conditions are satisfied: 0≦x<1, 0≦y<1, 0≦z<1, 0<(x+y+z)<1. This makes it possible to more reliably achieve high ionic conductivity.

[0061] When confirming whether an unknown solid electrolyte is the present solid electrolyte, chemical analysis is performed on the unknown solid electrolyte to confirm whether the constituent elements are Na, Mα, Mβ, and F. In addition, when confirming whether the unknown solid electrolyte has the above composition formula, for example, Na, Al, Si, and P can be quantified using an ICP-optical emission spectrometer. F can be quantified using an ion chromatograph. When the present solid electrolyte contains an element Mα other than Al and an element Mβ other than Si and P, a measurement method capable of quantifying the elements Mα and Mβ is appropriately selected.

[0062] A preferred method for producing the present solid electrolyte is to combine a substance containing Na, Mβ, and F with Na, where Mα is at least one element that becomes a trivalent cation and Mβ is at least one element that becomes a tetravalent or higher cation. ３ MαF ６ or a mixture of a substance containing Na, Mβ and F with MαF ３ The method includes a step of obtaining a mixture by mixing Mβ and NaF (step S11), and a step of subjecting the mixture to mechanical milling (step S12). This makes it possible to easily provide a solid electrolyte having high ionic conductivity. When Mβ is an element that becomes a tetravalent cation, a preferred manufacturing method is to ３ MαF ６ and Na ２ MβF ６ A mixture of these, or MαF ３ and NaF and Na ２ MβF ６ and a step of mechanically milling the mixture (step S12). This makes it possible to more reliably provide a solid electrolyte having high ionic conductivity.

[0063] The present solid electrolyte, the method for producing the present solid electrolyte, and the battery can be modified in various ways.

[0064] The X-ray diffraction pattern obtained from the present solid electrolyte may not have peaks in all or part of the diffraction angle 2θ ranges of 18 to 21°, 21 to 24°, 31 to 34°, and 45 to 48°. Furthermore, the half-width of the strongest peak in the X-ray diffraction pattern may be less than 0.4°.

[0065] The present solid electrolyte may be mixed with other substances (which may include Na) and used as an electrolyte material. In this case, the present solid electrolyte is preferably the component with the largest mass ratio among the components contained in the electrolyte material, i.e., the main component. The mass ratio of the main component in the electrolyte material is preferably 50 mass% or more, more preferably 60 mass% or more, and even more preferably 70 mass% or more.

[0066] The present solid electrolyte used in the all-solid-state secondary battery 1 does not necessarily need to be contained in all of the positive electrode 11, the negative electrode 12, and the electrolyte layer 13, but may be contained in at least one of the positive electrode 11, the negative electrode 12, and the electrolyte layer 13. The present solid electrolyte may also be used in batteries other than all-solid-state secondary batteries and for applications other than batteries. The present solid electrolyte may be produced by a method other than mechanical milling.

[0067] The configurations in the above-described embodiment and each modification may be combined as appropriate as long as they are not mutually contradictory.

[0068] While the invention has been particularly illustrated and described, it should be understood that the foregoing description is illustrative and not restrictive, and that numerous modifications and variations are possible without departing from the scope of the invention.

[0069] REFERENCE SIGNS LIST 1 all-solid-state sodium ion secondary battery 11 positive electrode 12 negative electrode 13 electrolyte layer S11, S12 steps

Claims

1. A solid electrolyte comprising Na, Mα, Mβ, and F, where Mα is at least one element that is a trivalent cation, and Mβ is at least one element that is a tetravalent or higher cation.

2. The solid electrolyte according to claim 1, wherein Mβ is an element that is a tetravalent cation, and the solid electrolyte has the composition formula Na ３－ｘ Mα １－ｘ Mβ ｘ F ６ A solid electrolyte comprising a component represented by the formula:

3. A solid electrolyte according to claim 2, wherein in said composition formula, 0.1≦x≦0.85 is satisfied.

4. The solid electrolyte according to claim 1, wherein the element that becomes a tetravalent cation is Mβ １ , the element that becomes a pentavalent cation is Mβ ２ , and the element that becomes a hexavalent cation is Mβ ３ The solid electrolyte has the composition formula Na ３－ｘ－２ｙ－３ｚ Mα １－（ｘ＋ｙ＋ｚ） Mβ １ ｘ Mβ ２ ｙ Mβ ３ ｚ F ６ wherein 0≦x<1, 0≦y<1, 0≦z<1, and 0<(x+y+z)<1 are satisfied.

5. A solid electrolyte according to claim 4, wherein in said composition formula, 0.1≦x≦0.85 is satisfied.

6. The solid electrolyte according to claim 1, wherein Mα contains Al or Ga.

7. The solid electrolyte according to claim 1, wherein Mβ contains Si, P or Ge.

8. A solid electrolyte according to claim 1, wherein an X-ray diffraction pattern obtained by X-ray diffraction measurement using CuKα radiation has peaks in the diffraction angle 2θ ranges of 18 to 21°, 21 to 24°, 31 to 34°, and 45 to 48°.

9. The solid electrolyte according to claim 1, wherein in an X-ray diffraction pattern obtained by X-ray diffraction measurement using CuKα radiation, the half-value width of the strongest peak is 0.4° or more.

10. A method for producing a solid electrolyte, comprising: a) mixing a substance containing Na, Mβ, and F, where Mα is at least one element that is a trivalent cation, and Mβ is at least one element that is a tetravalent or higher cation, with Na ３ MαF ６ or a mixture of a substance containing Na, Mβ and F and MαF ３ and NaF to obtain a mixture; and b) subjecting the mixture to a mechanical milling process.

11. A battery comprising: a positive electrode; a negative electrode; and an electrolyte layer provided between the positive electrode and the negative electrode, wherein the solid electrolyte according to any one of claims 1 to 9 is contained in at least one of the positive electrode, the negative electrode, and the electrolyte layer.

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