Solid electrolyte, solid electrolyte production method, and battery

Abstract

This solid electrolyte includes M1, M2, and X, wherein: M1 is an element that forms a monovalent cation, and includes at least one element selected from the group consisting of Li, Na, and K; M2 is at least one element that forms a trivalent cation; X is at least one element selected from the group consisting of F, Cl, Br, and I; and the axial angles α, β, and γ in the unit lattice of the crystal structure are 90°. As a result, a solid electrolyte having high ionic conductivity can be provided.

Description

Solid electrolyte, method for producing solid electrolyte, and battery

[0001] This application claims the benefit of priority from International Patent Application PCT / JP2023 / 35748, filed September 29, 2023, and Japanese Patent Application JP2024-86423, 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 transfer. 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. In all-solid-state batteries, the electrolyte is a non-flammable material, making them inherently safe lithium-ion batteries.

[0003] For example, in "All-Solid-State Lithium-Ion Batteries Using the Li3AlF6-Based Composite as the Solid Electrolyte" (ACS Applied Energy Materials, Volume 5, Issue 12, pp. 15365-15372) by Reona Miyazaki and six others, Li ３ AlF ６ -Li ２ SO ４ Furthermore, International Publication No. 2023 / 13390 (Document 2) discloses a solid electrolyte material including a crystalline phase containing Li, Zr, Al, and F. From Figure 2 in Document 2, it is believed that the crystalline structure of the solid electrolyte material is monoclinic.

[0004] The solid electrolytes described in References 1 and 2 do not have sufficient ionic conductivity, and therefore, a new solid electrolyte having high ionic conductivity is desired.

[0005] The present invention is directed to a solid electrolyte, and aims to provide a solid electrolyte having high ionic conductivity.

[0006] A first aspect of the invention is a solid electrolyte comprising M1, M2, and X, wherein M1 is an element that forms a monovalent cation and includes at least one element selected from the group consisting of Li, Na, and K, M2 is at least one element that forms a trivalent cation, X is at least one element selected from the group consisting of F, Cl, Br, and I, and the axial angles α, β, and γ in a unit cell of the crystal structure are all 90°.

[0007] According to the present invention, a solid electrolyte having high ionic conductivity can be provided.

[0008] A second aspect of the invention is the solid electrolyte of the first aspect, wherein an X-ray diffraction pattern obtained by X-ray diffraction measurement using CuKα radiation has peaks in the diffraction angle 2θ ranges of 19 to 23°, 40 to 44°, 52 to 56°, and 64 to 68°.

[0009] A third aspect of the present invention is the solid electrolyte of the first or second aspect, wherein, in an X-ray diffraction pattern obtained by X-ray diffraction measurement using CuKα radiation, I1 / (I1+I2) is greater than 0.30, where I1 is the intensity of the strongest peak in a diffraction angle 2θ range of 40 to 44° and I2 is the intensity of the strongest peak in a diffraction angle 2θ range of 19 to 23°.

[0010] A fourth aspect of the present invention is the solid electrolyte of any one of the first to third aspects, further comprising M3, which is at least one element that forms a tetravalent cation.

[0011] A fifth aspect of the present invention is the solid electrolyte of the fourth aspect, wherein the composition formula is M1 ａ M2 ｂ M3 ｃ X ６ where 2<a<3, 0<b<1, and 0<c<1 are satisfied.

[0012] A sixth aspect of the present invention is the solid electrolyte according to the fourth or fifth aspect, in which M3 contains Ge or Si.

[0013] A seventh aspect of the present invention is the solid electrolyte according to any one of the first to sixth aspects, wherein X contains F.

[0014] An eighth aspect of the present invention is the solid electrolyte according to any one of the first to seventh aspects, in which M2 contains Al.

[0015] A ninth aspect of the present invention is the solid electrolyte according to any one of the first to eighth aspects, in which M1 contains Rb or Cs.

[0016] The present invention is also directed to a method for producing a solid electrolyte and a battery.

[0017] A tenth aspect of the present invention is the method for producing a solid electrolyte according to any one of the first to ninth aspects, comprising the steps of: a) forming a solid electrolyte containing at least one element selected from the group consisting of Li, Na, and K, M1 being an element that can form a monovalent cation; M2 being at least one element that can form a trivalent cation; X being at least one element selected from the group consisting of F, Cl, Br, and I; and M3 being at least one element that can form a tetravalent cation; ３ M2X ６ and M1 ２ M3X ６ A mixture of M1X and M2X, or M1X and M2X ３ and M1 ２ M3X ６ 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 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 of a solid electrolyte; FIG. 4 is a diagram showing an X-ray diffraction pattern of a solid electrolyte of a comparative example; FIG. 5 is a diagram showing an X-ray diffraction pattern of a solid electrolyte of a comparative example; ３ AlF ６ FIG. 1 is an Arrhenius plot of the ionic conductivity of

[0021] FIG. 1 is a longitudinal cross-sectional view showing an all-solid-state battery 1. The all-solid-state battery 1 in this embodiment is a chargeable and dischargeable secondary battery, and is a lithium (Li) ion battery, a sodium (Na) ion battery, or a potassium (K) ion battery. The all-solid-state battery 1 has, from the top of 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.

[0022] 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 and a solid electrolyte, which will be described later. The positive electrode layer 112 may further include a conductive additive (carbon black, etc.). An example of the positive electrode layer 112 is formed by integrating these materials by applying pressure or the like. 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 and a solid electrolyte, which will be described later. The negative electrode layer 122 may further include a conductive additive (carbon black, etc.). An example of the negative electrode layer 122 is formed by integrating these materials by applying pressure or the like.

[0023] When the all-solid-state battery 1 is a Li-ion battery, the positive electrode active material of the positive electrode layer 112 preferably contains a lithium composite oxide. A preferred positive electrode active material is a lithium composite oxide having a layered rock salt structure, such as NCM (Li(Ni, Co, Mn)O ２ The positive electrode active material may be other lithium composite oxides, for example, NCA (Li(Ni, Co, Al)O) having a layered rock salt structure. ２ ), LCO (LiCoO ２ ), LNMO (LiNi) having a spinel structure ０．５ Mn １．５ O ４), LFP (LiFePO) having an olivine structure ４ ) or the like. The negative electrode active material of the negative electrode layer 122 may be, for example, LTO (Li ４ Ti ５ O １２ ), NTO(Nb ２ TiO ７ ), TiO ２ (titanium oxide), graphite, SiO (silicon monoxide), Si, Sn, and other compounds.

[0024] When the all-solid-state battery 1 is a Na-ion battery, 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 ７An 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 ４ Ti ５ O １２ When the all-solid-state battery 1 is a K ion battery, known materials are used as the positive electrode active material and the negative electrode active material.

[0025] The configurations and materials of the positive electrode 11 and the negative electrode 12 of the all-solid-state battery 1 are not limited to those described above, and various other configurations and materials can be adopted.

[0026] 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 an ion-conductive material. The solid electrolyte includes elements M1, M2, and X. The solid electrolyte may consist only of M1, M2, and X. M1 is at least one metal element that forms a monovalent cation and includes at least one element selected from the group consisting of Li, Na, and K. Preferably, M1 is at least one element selected from the group consisting of Li, Na, and K. M1 may further include rubidium (Rb) or cesium (Cs). M2 is a metal element and at least one element that forms a trivalent cation. Examples of M2 include Group 13 elements of the periodic table such as aluminum (Al), gallium (Ga), and indium (In), Group 3 elements such as scandium (Sc), rare earth elements such as iron (Fe), yttrium (Y), and holmium (Ho), and transition metal elements such as manganese (Mn). X is a halogen element and is at least one element selected from the group consisting of fluorine (F), chlorine (Cl), bromine (Br), and iodine (I). As described below, the crystal structure of the solid electrolyte is orthorhombic, cubic, or tetragonal, and the axial angles α, β, and γ in the unit cell are all 90°. This achieves high ionic conductivity.

[0027] M1 preferably contains Li, and may be only Li. M2 preferably contains Al, and may be only Al. X preferably contains F, and may be only F. In such a solid electrolyte, the ionic conductivity can be improved more reliably. Typically, the amount of substance of M1 is greater than the amount of substance of M2, and the amount of substance of X is greater than the amount of substance of M1.

[0028] Preferably, the solid electrolyte further contains element M3. M3 is a metal element or a metalloid element, and is at least one element that becomes a tetravalent cation. As will be described later, a solid electrolyte containing element M3 can easily realize a crystal structure of a unit cell in which the axial angles α, β, and γ are all 90°. M3 preferably contains germanium (Ge) or silicon (Si), and may contain both Ge and Si. M3 may be only one of Ge or Si, or only both Ge and Si. Of course, M3 may be an element other than Ge and Si.

[0029] The preferred solid electrolyte composition formula is: M1 ａ M2 ｂ M3 ｃ X ６ ... (1) In one example of the production of a solid electrolyte, for example, M1 ３ M2X ６ And, M1 ２ M3X ６ The present solid electrolyte is obtained by mixing M1 and M2. ３ M2X ６ and M1 ２ M3X ６ When the molar ratio of these is (1-x):x (where 0<x<1), the solid electrolyte has a composition formula of M1 ３－ｘ M2 １－ｘ M3 ｘ X ６ In other words, in composition formula (1), 2<a<3, 0<b<1, and 0<c<1 are satisfied. In such a solid electrolyte, ionic conductivity can be more reliably improved. In fact, due to the balance of valence, a+3b+4c=6 is also approximately satisfied. Taking into account measurement error, etc., it is possible to consider that a+3b+4c=6 is satisfied if the value of a+3b+4c is within the range of 5.4 to 6.6 (preferably 5.7 to 6.3). It is also possible to consider that the composition formula (1) is satisfied if the molar ratios of M1, M2, M3, and X determined by analysis are within ±10% (preferably ±5%) of the value of the composition formula (1).

[0030] As will be described later, it is preferable that 0.03≦x≦0.80, in which case 2.20≦a≦2.97, 0.20≦b≦0.97, and 0.03≦c≦0.80 are satisfied. It is more preferable that 0.05≦x≦0.65, in which case 2.35≦a≦2.95, 0.35≦b≦0.95, and 0.05≦c≦0.65 are satisfied.

[0031] In determining whether an unknown solid electrolyte satisfies composition formula (1), for example, Li, Na, Rb, K, Al, Ge, and Si can be quantified by ICP-emission spectroscopy or the like. F, Cl, Br, and I can be quantified by ion chromatography, for example. If the solid electrolyte contains elements other than those mentioned above, a measurement method capable of quantifying those elements is appropriately selected. Note that the raw materials for the solid electrolyte may inevitably contain oxygen (O) as an impurity, and in this case, O will also be contained in the solid electrolyte. O contained in the solid electrolyte at 5% by mass or less is an impurity and will not be taken into consideration in determining whether the solid electrolyte satisfies composition formula (1).

[0032] As described below, for a preferred example of this solid electrolyte, in the X-ray diffraction pattern obtained by X-ray diffraction measurement using CuKα radiation, peaks are present in the diffraction angle 2θ ranges of 19 to 23°, 40 to 44°, 52 to 56°, and 64 to 68°. Furthermore, if the intensity of the strongest peak in the diffraction angle 2θ range of 40 to 44° is defined as I1 and the intensity of the strongest peak in the diffraction angle 2θ range of 19 to 23° is defined as I2, then I1 / (I1 + I2) is greater than 0.30. In such a solid electrolyte, the proportion of orthorhombic crystals is high, and high ionic conductivity is more reliably achieved. The diffraction angle 2θ range in which peaks exist and the peak intensity ratio in the X-ray diffraction pattern vary depending on the crystalline structure and constituent elements of the solid electrolyte.

[0033] FIG. 2 is a diagram showing the flow of manufacturing the present solid electrolyte. ３ M2X ６ And, M1 ２ M3X ６The process for producing the solid electrolyte represented by the composition formula (1) by mixing M1 and M2 will be described. ３ M2X ６ A powder of M1 is prepared. ３ M2X ６ In preparing the powder, for example, M1X powder and M2X ３ The powders are weighed and mixed in a molar ratio of 3:1. The resulting mixture is then heat-treated (for example, at 900°C) and then pulverized to obtain M1. ３ M2X ６ A powder of M1 is obtained. ３ M2X ６ The powder of M1 may be produced by other methods. ３ M2X ６ An example of this is Li ３ AlF ６ where M1X is LiF (lithium fluoride) and M2X ３ is AlF ３ (aluminum fluoride).

[0034] Also, M1 ２ M3X ６ A powder of M1 is prepared. ２ M3X ６ For example, Li ２ GeF ６ or Li ２ SiF ６ Li ２ GeF ６ or Li ２ SiF ６ The powder may be commercially available or may be produced by known methods. ３ M2X ６ Powder and M1 ２ M3X ６ A mixture is obtained by mixing the powder of M1 with the powder of M2 (step S11). ３ M2X ６ Amount of substance and M1 ２ M3X ６ The amount of substance in the total of M1 ２ M3X ６The ratio of the amount of substance M1 is greater than 0% and less than 100%. The ratio is preferably 3% or more and 80% or less, and more preferably 5% or more and 65% or less. ３ M2X ６ Li ３ AlF ６ Besides, Li ３ GaF ６ , Li ３ InF ６ , Li ３ ScF ６ , Li ３ YF ６ etc. M1 ２ M3X ６ Li ２ GeF ６ and Li ２ SiF ６ Besides, Li ２ SnF ６ As mentioned above, M1 may be Na or K (e.g., Na ３ AlF ６ , Na ２ GeF ６ , Na ２ SiF ６ and X may be Cl, Br or I. In addition, there may be a plurality of M1, M2, M3 and X which are all or partly different. ３ M2X ６ and / or M1 ２ M3X ６ In this case, some of M1 may be Rb or Cs. As will be described later, M1X and M2X ３ M1 ２ M3X ６ The same applies when directly mixing with

[0035] Next, the mixture is subjected to mechanical milling (also called mechano-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 carrying the pot 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.

[0036] In step S11 of FIG. ３ M2X ６ M1X and M2X without generating ３ is M1 ２ M3X ６ In this case, the mixture may be subjected to mechanical milling in step S12, thereby producing a solid electrolyte having high lithium ion conductivity. M1X may be NaF, KF, or the like, in addition to LiF. M2X ３ is AlF ３ Besides, GaF ３ , InF ３ , ScF ３ , Y.F. ３ The present solid electrolyte may be produced by a method other than mechanical milling, such as sintering.

[0037] Next, experimental examples of solid electrolytes will be described. The following experiments were carried out in a glove box or dry room with a dew point of -40°C or less. The raw material mixing ratios, solid electrolyte composition formulas, and various measurement results for Experimental Examples 1 to 10 are shown in Table 1. Experimental Examples 2 to 8 and 10 are examples of the present invention, and Experimental Examples 1 and 9 are comparative examples.

[0038]

[0039] (Experimental Example 1) Commercially available LiF powder and commercially available AlF ３ These raw materials were prepared as LiF:AlF powder.３ The resulting mixture was heated at 900°C in an Ar (argon) atmosphere and then crushed in a mortar to obtain Li ３ AlF ６ A powder was obtained. ３ AlF ６ The powder was subjected to mechanical milling using a planetary ball mill to obtain Li ３ AlF ６ A solid electrolyte powder containing only powder was obtained.

[0040] (Experimental Example 2) Li of Experimental Example 1 ３ AlF ６ In addition to powder, commercially available Li ２ GeF ６ Powder was prepared. ３ AlF ６ :Li ２ GeF ６ The resulting mixture was subjected to mechanical milling using a planetary ball mill to obtain a mixture of Li, ３ AlF ６ -Li ２ GeF ６ The powder was obtained as a solid electrolyte powder. ３ M2X ６ and M1 ２ M3X ６ When mixing these materials in a ratio of (1-x):x (where 0<x<1), the composition formula is M1 ３－ｘ M2 １－ｘ M3 ｘ X ６ In this experimental example, where M1, M2, M3 and X are Li, Al, Ge and F, and x is 0.1, the solid electrolyte powder has the composition formula Li ２．９ Al ０．９ Ge ０．１ F ６ It is expressed as:

[0041] (Experimental Example 3) Li ３ AlF ６ :Li ２ GeF ６ The same treatment as in Experimental Example 2 was carried out, except that the molar ratio of Li was 8:2.２．８ Al ０．８ Ge ０．２ F ６ A solid electrolyte powder represented by the formula:

[0042] (Experimental Example 4) Li ３ AlF ６ :Li ２ GeF ６ The same treatment as in Experimental Example 2 was carried out, except that the amounts were weighed out so that the molar ratio was 7:3. ２．７ Al ０．７ Ge ０．３ F ６ A solid electrolyte powder represented by the formula:

[0043] (Experimental Example 5) Li ３ AlF ６ :Li ２ GeF ６ The same treatment as in Experimental Example 2 was carried out, except that the amounts were weighed out so that the molar ratio was 5:5. ２．５ Al ０．５ Ge ０．５ F ６ A solid electrolyte powder represented by the formula:

[0044] (Experimental Example 6) Li of Experimental Example 1 ３ AlF ６ In addition to powder, commercially available Li ２ SiF ６ Powder was prepared. ３ AlF ６ :Li ２ SiF ６ The resulting mixture was subjected to mechanical milling using a planetary ball mill to obtain a compound having the composition formula Li ２．９ Al ０．９ Si ０．１ F ６ A solid electrolyte powder represented by the formula:

[0045] (Experimental Example 7) Li ３ AlF ６ :Li ２ SiF ６ The same treatment as in Experimental Example 6 was carried out, except that the molar ratio of Li was 8:2. ２．８ Al ０．８ Si ０．２F ６ A solid electrolyte powder represented by the formula:

[0046] (Experimental Example 8) Li ３ AlF ６ :Li ２ SiF ６ The same treatment as in Experimental Example 6 was carried out, except that the amounts were weighed out so that the molar ratio was 7:3. ２．７ Al ０．７ Si ０．３ F ６ A solid electrolyte powder represented by the formula:

[0047] (Experimental Example 9) Commercially available LiF powder and commercially available TiF ４ These raw materials were mixed in a LiF:TiF ４ The resulting mixture was subjected to mechanical milling using a planetary ball mill to obtain a Li ２ TiF ６ A powder was obtained. ３ AlF ６ Powder and the above Li ２ TiF ６ powder and Li ３ AlF ６ :Li ２ TiF ６ The resulting mixture was subjected to mechanical milling using a planetary ball mill to obtain a compound having the composition formula Li ２．８ Al ０．８ Ti ０．２ F ６ A solid electrolyte powder represented by the formula:

[0048] (Experimental Example 10) Commercially available RbF powder and commercially available AlF ３ These raw materials were prepared as RbF:AlF powder. ３ The mixture was subjected to mechanical milling using a planetary ball mill to obtain Rb ３ AlF ６ Powder was obtained. ３ AlF ６ In addition to powder, commercially available Na ３ AlF６ Powder and Na ２ SiF ６ These raw materials were prepared as Na powder. ３ AlF ６ : Na ２ SiF ６ : Rb ３ AlF ６ The resulting mixture was subjected to mechanical milling using a planetary ball mill to obtain a compound having the composition formula Na １．５ RbAl ０．５ Si ０．５ F ６ A solid electrolyte powder represented by the formula:

[0049] <Measurement of Ionic Conductivity 1> A solid electrolyte powder was placed in a mold consisting of a resin sleeve and upper and lower punches made of SUS (stainless steel), and uniaxial press molding was performed under a pressure of 400 MPa. Conductors were connected to the upper and lower punches, and impedance measurements were performed at room temperature to calculate the ionic conductivity. In Table 1, the ionic conductivity is shown in the "Conductivity 1" column.

[0050] <Ionic Conductivity Measurement 2> A solid electrolyte powder was placed in a mold consisting of upper and lower SUS punches and uniaxially pressed at 150 MPa, after which a pellet with a diameter of 10 mm and a thickness of approximately 1 mm was removed. The pellet was subjected to WIP (Warm Isotactic Pressing) at 150°C and 900 MPa, a higher temperature and pressure than in <Ionic Conductivity Measurement 1>. A current collecting layer was formed on both the front and back surfaces of the pellet by Au (gold) sputtering, and then impedance measurements were performed to calculate the ionic conductivity. In Table 1, the ionic conductivity is shown in the "Conductivity 2" column. It is believed that the WIP process improved the density of the pellet, i.e., promoted densification, thereby improving the ionic conductivity.

[0051] Li ３ AlF ６ NiLi ２ GeF ６ Experimental Examples 2 to 5 in which Li was mixed ３ AlF ６ NiLi ２ SiF ６In all of Experimental Examples 6 to 8, in which the above were mixed, the ionic conductivity (referred to as "Conductivity 1" in Table 1; the same applies hereinafter in this paragraph) was 1.0 × 10 －６ S / cm or more, and Li ２ GeF ６ and Li ２ SiF ６ The ionic conductivity of Experimental Example 1 (5.0 × 10 －８ S / cm). ３ AlF ６ NiLi ２ TiF ６ In Experimental Example 9, the ionic conductivity was 7.1 × 10 －７ S / cm, and Li ２ GeF ６ or Li ２ SiF ６ It can be said that the ionic conductivity is significantly improved by mixing Li ２ GeF ６ Considering the results of Experimental Example 2, in which the ratio of the amount of substance of Li is 10%, and Experimental Example 5, in which the ratio is 50%, ２ GeF ６ It is believed that the ionic conductivity will be sufficiently high if the ratio of Li is 3% or more and 80% or less (0.03≦x≦0.80). ２ GeF ６ It is more preferable that the ratio of Na is 5% or more and 65% or less (0.05≦x≦0.65), which makes it possible to more reliably achieve sufficiently high ionic conductivity. ３ AlF ６ Na ２ SiF ６ and Rb ３ AlF ６ In Experimental Example 10, the ionic conductivity was 1.1 × 10 at room temperature. －６ S / cm, and a high ionic conductivity was obtained.

[0052] <Fabrication of all-solid-state battery> The solid electrolyte powders of Experimental Examples 3 and 7 and Li(Ni, Co, Mn)O ２A positive electrode active material powder (C10), a negative electrode active material powder (graphite), and a conductive additive powder (carbon) were prepared. A positive electrode blend powder was obtained by weighing and mixing a solid electrolyte powder, a positive electrode active material powder, and a conductive additive powder in a volume ratio of 50:50:2. A negative electrode blend powder was obtained by weighing and mixing a solid electrolyte powder, a negative electrode active material powder, and a conductive additive powder in a volume ratio of 50:50:2. 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 blend powder was placed on top of the pressed solid electrolyte powder and integrated under a pressure of 150 MPa. The negative electrode blend 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 composed of a positive electrode layer, a solid electrolyte layer, and a negative electrode layer.

[0053] <Charge-Discharge Test> Conductors were connected to the upper and lower punches, and the battery was placed in a thermostatic chamber at 120°C to conduct a cc-cv (Constant Current-Constant Voltage) charge-discharge test. ２ , cv current density 30μA / cm ２ The cutoff voltage was set to 4.25-1.25 V. In the battery using the solid electrolyte powder of Experimental Example 3, the discharge capacity was 99 mAh / g per weight of the positive electrode. In the battery using the solid electrolyte powder of Experimental Example 7, the discharge capacity was 113 mAh / g per weight of the positive electrode.

[0054] <Powder X-ray Diffraction Measurement> The solid electrolyte powder was analyzed using an X-ray diffractometer (D8-Advance manufactured by Bruker) using a CuKα radiation source to obtain an X-ray diffraction (XRD) pattern. The measurement conditions were 2θ = 10 to 70°, 0.02° / step, and 0.5 sec / step. In the X-ray diffraction pattern of the solid electrolyte powder of each experimental example, the positions (2θ) of the four peaks with locally maximum intensity are shown in the "XRD Peak Position" column in Table 1. In each X-ray diffraction pattern, the intensity (height) of the strongest peak in the diffraction angle 2θ range of 40 to 44° is I1, and the intensity of the strongest peak in the diffraction angle 2θ range of 19 to 23° is I2, and the value of I1 / (I1 + I2) is shown in the "I1 / (I1 + I2)" column. Note that the peak intensity is the height of the bottom line in the X-ray diffraction pattern, i.e., excluding background noise components. The bottom line in the X-ray diffraction pattern can be determined, for example, by the Sonneveld-Visser method or spline interpolation.

[0055] FIG. 3 is a diagram showing the X-ray diffraction pattern of the solid electrolyte powder of Experimental Example 7, FIG. 4 is a diagram showing the X-ray diffraction pattern of the solid electrolyte powder of Experimental Example 1, and FIG. 5 is a diagram showing the X-ray diffraction pattern of the solid electrolyte powder of Experimental Example 9. In FIG. 3, the top row shows the X-ray diffraction pattern of Experimental Example 7, and the second row shows the X-ray diffraction pattern of the powder diffraction database ICDD (International Centre for Diffraction Data) card number: ICDD22-1137 (Li ３ AlF ６ The bottom row shows the peaks of the card number: ICDD52-1151 (Li ３ AlF ６ This is the peak shown by the monoclinic crystal of Li ３ AlF ６ The orthorhombic crystal of Li is a metastable phase, ３ AlF ６ The monoclinic phase is the most stable phase.

[0056] As shown in the second row of Figure 3, the Li ３ AlF ６In the diffraction data of the orthorhombic crystal of Li, peaks with locally maximum intensity exist at diffraction angles 2θ = 21°, 42°, 54°, and 66°. Here, the area around 21° is in the range of 19 to 23°, and the area around 42° is in the range of 40 to 44°. The area around 54° is in the range of 52 to 56°, and the area around 66° is in the range of 64 to 68°. In addition, since the intensity I1 of the strongest peak near 42° and the intensity I2 of the strongest peak near 21° are almost the same, I1 / (I1+I2) is close to 0.50. In contrast, as shown in the bottom row of Figure 3, the Li shown in ICDD52-1151 ３ AlF ６ In the monoclinic diffraction data, there are no peaks near 54° and near 66°. Furthermore, since the intensity I1 of the strongest peak near 42° is smaller than the intensity I2 of the strongest peak near 21°, I1 / (I1+I2) is significantly smaller than 0.50. In the monoclinic diffraction data, a characteristic peak is present near 58°, and as will be described later, in Experimental Example 1, which has a peak with a locally maximum intensity near 58°, I1 / (I1+I2) is 0.25. Therefore, when I1 / (I1+I2) is greater than 0.30 in the X-ray diffraction pattern of a solid electrolyte powder, it can be said that the crystal structure of the solid electrolyte powder is likely to be an orthorhombic crystal.

[0057] As shown in the top row of FIG. ３ AlF ６ In the X-ray diffraction pattern of the solid electrolyte powder of Experimental Example 7 based on , clear peaks were present at diffraction angles 2θ of around 21°, 42°, 54°, and 66°, and I1 / (I1+I2) was 0.50, as shown in Table 1. In the X-ray diffraction patterns of the solid electrolyte powders of Experimental Examples 2 to 6 and 8, peaks were also confirmed at angles of around 21°, 42°, 54°, and 66°, as shown in Table 1, and I1 / (I1+I2) was greater than 0.30. Therefore, the crystal structure (crystalline phase) of the solid electrolyte powders of Experimental Examples 2 to 8 was identified as an orthorhombic crystal.

[0058] On the other hand, in the X-ray diffraction pattern of the solid electrolyte powder of Experimental Example 1, as shown in Table 1, a peak with a locally maximum intensity was present near 58°. Furthermore, as shown in FIG. 4, the intensity I1 of the strongest peak near 42° was small, and I1 / (I1+I2) was a value (0.25) significantly smaller than 0.50. Therefore, the crystal structure of the solid electrolyte powder of Experimental Example 1 was identified as monoclinic. In the X-ray diffraction pattern of the solid electrolyte powder of Experimental Example 9, as shown in FIG. 5, in addition to peaks near 21° and 42°, small peaks appear to be present near 54° and 66°, but these are not necessarily clear. Furthermore, the intensity I1 of the strongest peak near 42° was small, and I1 / (I1+I2) was 0.25, as shown in Table 1. Therefore, the crystal structure of the solid electrolyte powder of Experimental Example 9 was also identified as monoclinic.

[0059] <Considerations on Crystal Structure> As mentioned above, Experimental Examples 2 to 8, which had an orthorhombic crystal structure, exhibited higher ionic conductivity than Experimental Examples 1 and 9, which had a monoclinic crystal structure. Specifically, comparing Experimental Examples 2 to 5, which contained Li, Al, Ge, and F, the larger I1 / (I1+I2), i.e., the more pronounced the orthorhombic characteristics, the higher the ionic conductivity. The same was true for Experimental Examples 6 to 8, which contained Li, Al, Si, and F. While the reason why orthorhombic solid electrolytes exhibit higher ionic conductivity than monoclinic solid electrolytes is unclear, one possible reason is that the orthorhombic crystal structure has a higher symmetry than the monoclinic crystal, which allows Li ions to more easily move within the crystal lattice. Other crystal structures with a higher symmetry than the monoclinic crystal include cubic and tetragonal crystals, which, like the orthorhombic crystal, have axial angles α, β, and γ of 90° in the unit cell. It is believed that improved ionic conductivity can also be achieved in these crystal structures. In fact, in Experimental Example 10, which contains Na instead of Li and further contains Rb in addition to Al, Si, and F, the crystal structure is Rb ３ AlF ６ Therefore, in this solid electrolyte, the axial angles α, β, and γ in the unit cell of the crystal structure need only be 90°, and the crystal structure is not limited to a rectangular crystal.

[0060] In the solid electrolyte having the above crystal structure, as described above, Na or K, which becomes a monovalent cation like Li, may be contained instead of Li or together with Li. Other elements (e.g., Ga, In, Y, etc.) which become a trivalent cation like Al may be contained instead of Al or together with Al. Other elements (e.g., Sn, etc.) which become a tetravalent cation like Ge or Si may be contained instead of Ge or Si or together with Ge or Si. Cl, Br, or I, which become a monovalent anion like F, may be contained instead of F or together with F.

[0061] As described above, the present solid electrolyte may contain M1, M2, M3, and X, where M1 is an element that forms a monovalent cation and includes at least one element selected from the group consisting of Li, Na, and K, M2 is at least one element that forms a trivalent cation, M3 is at least one element that forms a tetravalent cation, and X is at least one element selected from the group consisting of F, Cl, Br, and I. Furthermore, the axial angles α, β, and γ in the unit cell of the crystal structure are all 90°, thereby realizing a solid electrolyte with high ionic conductivity.

[0062] In the above Experimental Examples 2 to 8, Li ３ AlF ６ Li ２ GeF ６ or Li ２ SiF ６ By mixing, Ge or Si, which becomes a tetravalent cation, is converted into Li ３ AlF ６ It is believed that the highly symmetrical crystal structure (the orthorhombic crystal in the above example) is stabilized by the solid solution of Li. The same applies when Na or K is used instead of Li. On the other hand, Li ３ AlF ６ and Na ３ AlF ６ In Li, the monoclinic crystal is the thermodynamically most stable crystal structure, but the orthorhombic and cubic crystals are metastable and exist as high-temperature phases. ３ AlF ６ and Na ３ AlF ６Lithium can have an orthorhombic or cubic crystal structure even when not mixed with other compounds. ２ GeF ６ or Li ２ SiF ６ Lithium having a rectangular or cubic crystal structure without mixing ３ AlF ６ may be used alone as the solid electrolyte, ３ AlF ６ The same is true for the above. Methods for extracting the metastable state at room temperature generally include quenching by rapid cooling and ultra-high pressure and high temperature treatment (for example, 10 GPa, 900°C, etc.). ３ AlF ６ and Na ３ AlF ６ For example, a highly symmetrical crystal structure may be obtained using these techniques. Thus, the present solid electrolyte may contain M1, M2, and X but not M3, as long as the axial angles α, β, and γ in the unit cell of the crystal structure are all 90°. Even if M3 is not contained, a solid electrolyte having high ionic conductivity can be provided.

[0063] Here, Li in which Si is dissolved ３ AlF ６ , i.e., Al ３＋ In contrast to Si ４＋ Li, which has a Li deficiency due to solid solution ３ AlF ６The ionic conductivity of various crystal structures was calculated using neural network molecular dynamics calculations. LAMMPS was used as the molecular dynamics calculation solver. The crystal structures used were mp-15254 (monoclinic), mp-556020 (orthorhombic), and mp-1111291 (cubic) from the Materials Project crystal structure database. From the above crystal structures, a supercell (number of atoms: 468) was created using NanoLabo from AdvanceSoft. Furthermore, NanoLabo was used to randomly replace 25 mol% of the Al sites with Si, and a crystal structure was created in which the same number of Li atoms as Si were randomly reduced for charge compensation. The M3GNet force field was used. After structural optimization, MD calculations (molecular dynamics calculations) were performed. The ensemble was NVT, the time step was 2 femtoseconds, and the calculation time was 400 picoseconds. The set temperature was set between 400K and 1000K for each compound, and 3-4 cases were performed. From the MD calculation trajectory, the diffusion coefficient was calculated from the slope of the mean square displacement of Li ions against the calculation time, and the Nernst-Einstein equation (σ = z ２ F ２ The Li ion conductivity was calculated based on the following equation: cD / RT; σ: conductivity, z: ionic valence, F: Faraday constant, c: ion concentration, D: diffusion constant, R: molar gas constant. The activation energy was determined from the calculated Li ion conductivity at each temperature, and the ion conductivity at 300 K (room temperature) was calculated (see FIG. 6 described later).

[0064] 6 is a diagram showing an Arrhenius plot of ionic conductivity. The change in ionic conductivity of the monoclinic crystal is denoted by L1, the change in ionic conductivity of the orthorhombic crystal is denoted by L2, and the change in ionic conductivity of the cubic crystal is denoted by L3. The ionic conductivity of each crystal structure at 300 K (ionic conductivity based on MD calculation) is 2.4 × 10 for the cubic crystal. －２ S / cm, 6.5 x 10 for rectangular crystals －４ S / cm, 2.7 x 10 for monoclinic crystals －５As shown in Table 1, the ionic conductivity of Experimental Example 7, in which 20 mol % of the Al site was substituted to form an orthorhombic crystal, was greater than the ionic conductivity of Experimental Example 9, in which 20 mol % of the Al site was substituted to form a monoclinic crystal, and the calculation results of the ionic conductivity by MD calculation were consistent with the tendency of the experimental results. ３ AlF ６ In this case, it can be said that the higher the symmetry of the crystal structure, the higher the ionic conductivity at room temperature, in the order of monoclinic, orthorhombic, and cubic.

[0065] When obtaining a solid electrolyte having a cubic crystal structure, a compound having a cubic crystal structure as a stable crystal structure, such as Rb ３ AlF ６ , Cs ３ AlF ６ , NaK ２ AlF ６ , NaPF ６ For example, Li or Na (or K) may be doped to exhibit Li or Na ion conductivity. ３ AlF ６ Na ３ AlF ６ and Na ２ SiF ６ By mixing these, a cubic solid electrolyte with high Na ion conductivity was obtained.

[0066] A preferred solid electrolyte further contains M3 in addition to M1, M2, and X. This makes it possible to easily realize a solid electrolyte having a unit lattice in which the axial angles α, β, and γ are all 90°. More preferably, the composition formula of the solid electrolyte is M1 ａ M2 ｂ M3 ｃ X ６ (the above composition formula (1)), where 2<a<3, 0<b<1, and 0<c<1 are satisfied. This makes it possible to more reliably realize a solid electrolyte having high ionic conductivity.

[0067] Among solid electrolytes, materials such as sulfides 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 manufacturing, resulting in high resistance. In contrast, the present solid electrolyte is a non-flammable and chemically stable material, making it possible to provide an essentially safe ion battery. Furthermore, the present solid electrolyte can be densified by the pressure of press molding rather than sintering, preventing reaction with active materials during battery manufacturing.

[0068] The method for producing the solid electrolyte comprises: ３ M2X ６ and M1 ２ M3X ６ A mixture of M1X and M2X, or M1X and M2X ３ and M1 ２ M3X ６ The method includes a step of obtaining a mixture by mixing the above (Step S11) and a step of subjecting the mixture to mechanical milling (Step S12). This allows a solid electrolyte having high ionic conductivity to be easily produced.

[0069] The present solid electrolyte, all-solid-state battery 1, and method for manufacturing the solid electrolyte can be modified in various ways.

[0070] The present solid electrolyte may be produced by a method other than the production method shown in Fig. 2. Furthermore, the solid electrolyte containing M1, M2, M3, and X may not satisfy the above composition formula (1).

[0071] The present solid electrolyte may be mixed with other substances 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.

[0072] The present solid electrolyte used in the all-solid-state 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. Furthermore, the present solid electrolyte may be used in batteries other than all-solid-state batteries, and may be used for applications other than batteries.

[0073] The configurations in the above-described embodiment and each modification may be combined as appropriate as long as they are not mutually contradictory.

[0074] 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.

[0075] 1 All-solid-state battery 11 Positive electrode 12 Negative electrode 13 Electrolyte layer S11, S12 Step

Claims

1. A solid electrolyte comprising M1, M2 and X, wherein M1 is an element that forms a monovalent cation and contains at least one element selected from the group consisting of Li, Na and K, M2 is at least one element that forms a trivalent cation, X is at least one element selected from the group consisting of F, Cl, Br and I, and the axial angles α, β and γ in a unit cell of a crystal structure are all 90°.

2. The solid electrolyte according to claim 1, wherein in an X-ray diffraction pattern obtained by X-ray diffraction measurement using CuKα radiation, peaks are present in the diffraction angle 2θ ranges of 19 to 23°, 40 to 44°, 52 to 56°, and 64 to 68°.

3. A solid electrolyte as claimed in claim 1, in which, in an X-ray diffraction pattern obtained by X-ray diffraction measurement using CuKα radiation, when the intensity of the strongest peak in a diffraction angle 2θ range of 40 to 44° is I1 and the intensity of the strongest peak in a diffraction angle 2θ range of 19 to 23° is I2, I1 / (I1+I2) is greater than 0.

30.

4. The solid electrolyte according to claim 1, further comprising M3, which is at least one element that is a tetravalent cation.

5. The solid electrolyte according to claim 4, wherein the composition formula is M1 ａ M2 ｂ M3 ｃ X ６ wherein 2<a<3, 0<b<1, and 0<c<1 are satisfied.

6. The solid electrolyte according to claim 4, wherein M3 contains Ge or Si.

7. The solid electrolyte according to claim 1, wherein X comprises F.

8. The solid electrolyte according to claim 1, wherein M2 contains Al.

9. The solid electrolyte according to claim 1, wherein M1 comprises Rb or Cs.

10. A method for producing a solid electrolyte according to any one of claims 1 to 9, comprising the steps of: a) forming a solid electrolyte comprising at least one element selected from the group consisting of Li, Na, and K, M1 being an element that can be a monovalent cation; M2 being at least one element that can be a trivalent cation; X being at least one element selected from the group consisting of F, Cl, Br, and I; and M3 being at least one element that can be a tetravalent cation; ３ M2X ６ and M1 ２ M3X ６ A mixture of M1X and M2X, or M1X and M2X ３ and M1 ２ M3X ６ 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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