Solid electrolyte and battery

Abstract

This solid electrolyte includes A, M, F, and X. A is at least one element selected from the group consisting of Li, Na, and K. M is a metal element or a metalloid element other than A. X is at least one element selected from the group consisting of Cl, Br, and I. Accordingly, it is possible to provide a solid electrolyte that is highly safe and has high ionic conductivity.

Description

Solid electrolytes and batteries

[0001] This application claims the benefit of priority from International Patent Application PCT / JP2022 / 42227, filed November 14, 2022, the entire disclosure of which is incorporated herein by reference.

[0002] In recent years, development of solid electrolytes for use in all-solid-state batteries and the like has been progressing. For example, Japanese Patent Application Laid-Open No. 2011-129312 (Reference 1) discloses a solid electrolyte made of sulfide and an all-solid-state battery including the solid electrolyte. On the other hand, solid electrolytes made of sulfide may react with moisture in the air and generate toxic hydrogen sulfide gas. Therefore, International Publication Nos. 2021 / 161604 (Reference 2) and 2021 / 186833 (Reference 3) propose solid electrolytes made of fluorine compounds. Specifically, the solid electrolyte in Reference 2 contains Li, Zr, Al, and F. The solid electrolyte in Reference 3 contains Li, Ti, Al, M, and F, where M is Zr or Mg. In addition, in "Solid Halide Electrolytes with High Lithium-Ion Conductivity for Application in 4 V Class Bulk-Type All-Solid-State Batteries" (ADVANCED MATERIALS, 2018, Vol. 30, 1803075) (Reference 4) by Tetsuya Asano and five others, the solid electrolyte material Li ３ YCl ６ It describes:

[0003] Meanwhile, the solid electrolytes of References 2 and 3, which are made of fluorine compounds, improve safety, but the lithium ion conductivity is 10 －６ The resistance is in the order of S / cm, which is not sufficient.

[0004] The present invention is directed to a solid electrolyte, and has an object to provide a solid electrolyte that is highly safe and has high ionic conductivity.

[0005] A first aspect of the invention is a solid electrolyte comprising A, M, F, and X, wherein A is at least one element selected from the group consisting of Li, Na, and K; M is a metal element or a metalloid element other than A; and X is at least one element selected from the group consisting of Cl, Br, and I.

[0006] According to the present invention, it is possible to provide a solid electrolyte that is highly safe and has high ionic conductivity.

[0007] A second aspect of the present invention is the solid electrolyte of the first aspect, in which M contains Ga.

[0008] The invention of aspect 3 is the solid electrolyte of aspect 1 or 2, wherein the composition formula is A ３ Midfielder ６－ａ X ａ where 0<a<6 is satisfied.

[0009] A fourth aspect of the present invention is the solid electrolyte of the first or second aspect, wherein M contains at least one element selected from the group consisting of Mα, which is a trivalent cation, Mβ, which is a tetravalent cation, and Mγ, which is a divalent cation, and the composition formula of the solid electrolyte is A ３－ｂ＋ｃ Mα １－ｂ－ｃ Mβ ｂ Mγ ｃ F ６－ａ X ａ where 0<a<6, 0≦b<1, and 0≦c<1 are satisfied.

[0010] A fifth aspect of the present invention is the solid electrolyte according to any one of the first to fourth aspects, in which M contains Zr.

[0011] A sixth aspect of the present invention is the solid electrolyte according to any one of the first to fifth aspects, wherein M contains Mg.

[0012] A seventh aspect of the invention is a battery including the solid electrolyte according to any one of the first to sixth aspects.

[0013] 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.

[0014] 1 is a longitudinal sectional view showing an all-solid-state lithium ion secondary battery, and FIG. 2 is a diagram showing a discharge curve of the all-solid-state battery.

[0015] FIG. 1 is a longitudinal cross-sectional view showing an all-solid-state lithium-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.

[0016] 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 The positive electrode layer 112 further contains a solid electrolyte and an electron-conducting additive (carbon black or the like) described below in addition to the positive electrode active material. The positive electrode layer 112 in this embodiment is formed by integrating these materials by applying pressure or heat.

[0017] The negative electrode active material of the negative electrode layer 122 is, for example, LTO (Li ４ Ti ５ O １２ ), NTO(Nb ２ TiO ７ ), TiO ２Examples of the negative electrode layer 122 include compounds such as titanium oxide (TiO 2 ), graphite, and silicon monoxide (SiO 2 ). The negative electrode layer 122 includes a solid electrolyte, which will be described later, in addition to the negative electrode active material. The negative electrode layer 122 may further include an electron conduction aid (carbon black, etc.). The negative electrode layer 122 in this embodiment is formed by integrating these materials by applying pressure or heat.

[0018] 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.

[0019] The electrolyte layer 13 is made of or includes a solid electrolyte according to the present invention (hereinafter also referred to as "the present solid electrolyte"). The solid electrolyte is a lithium (Li) ion conductive material. The solid electrolyte includes lithium element (Li), an element (M) that is a metal element or a semimetal element other than Li, fluorine element (F), and an element X described below. M may be only one type of element, or may include two or more types of elements. An example of M is an element that forms a trivalent cation. M preferably includes gallium (Ga), or may consist solely of Ga. M may also include another element that forms a trivalent cation together with Ga, an example of such an element being aluminum (Al).

[0020] In this specification, metalloid elements are boron (B), silicon (Si), germanium (Ge), arsenic (As), antimony (Sb), and tellurium (Te). Metal elements are elements included in Groups 1 to 12 of the periodic table excluding hydrogen, and elements included in Groups 13 to 16 of the periodic table excluding the above-mentioned metalloids and C, N, P, O, S, and Se. In other words, metal elements are a group of elements that can become cations when forming an inorganic compound with a halogen compound.

[0021] X is at least one element selected from the group consisting of chlorine (Cl), bromine (Br), and iodine (I). X more preferably contains Cl, and may be only Cl. Typically, X replaces a portion of F. For example, the amount of X in the present solid electrolyte is equal to or less than the amount of F, but may be greater than the amount of F. The present solid electrolyte does not contain sulfides and does not generate hydrogen sulfide gas. Therefore, a highly safe all-solid-state secondary battery 1 is provided. As will be described later, the present solid electrolyte has a hydrogen sulfide content of 1×10 －５ Therefore, a high lithium ion conductivity of 100 S / cm or more can be achieved.

[0022] A preferred solid electrolyte has the formula: Li ３ Midfielder ６－ａ X ａ ... (1) where 0<a<6 is satisfied. In composition formula (1), it is more preferable that 1<a<6 is satisfied, and it is even more preferable that 1<a<4 is satisfied. Here, M is an element that becomes a trivalent cation, and may contain two or more elements (for example, Ga and Al).

[0023] In another preferred solid electrolyte, the M includes Mα, which is a trivalent cation, and Mβ, which is a tetravalent cation. In this case, the composition formula of the solid electrolyte is: Li ３－ｂ Mα １－ｂ Mβ ｂ F ６－ａ X ａ ... (2) where 0<a<6 and 0<b<1 are satisfied. In composition formula (2), it is more preferable that 1<a<6 is satisfied, and it is even more preferable that 1<a<4 is satisfied. In one example of the solid electrolyte, Mα is Ga and Mβ is zirconium (Zr). For example, the amount of substance of Mα is equal to or greater than the amount of substance of Mβ, but may be less than the amount of substance of Mβ.

[0024] In the present solid electrolyte, the M may contain Mγ, which is a divalent cation. For example, M contains Mα and optionally contains Mβ and / or Mγ. In this case, the composition formula of the solid electrolyte is Li ３－ｂ＋ｃ Mα １－ｂ－ｃ Mβ ｂMγ ｃ F ６－ａ X ａ ... (3) where 0<a<6, 0≦b<1, and 0≦c<1 are satisfied. In composition formula (3), it is more preferable that 1<a<6 is satisfied, and it is even more preferable that 1<a<4 is satisfied. An example of Mγ is magnesium (Mg). For example, the amount of substance of Mα is equal to or greater than the total amount of substance of Mβ and Mγ, but it may be less than this total amount of substance.

[0025] In determining whether an unknown solid electrolyte has composition formula (1), (2), or (3), for example, Li, Ga, Al, Zr, and Mg can be quantified by ICP-emission spectroscopy. F and Cl can be quantified by ion chromatography, for example. When the solid electrolyte contains elements other than those mentioned above, a measurement method capable of quantifying the elements is appropriately selected.

[0026] The molar ratio of Li, M, F, and X in the composition formula (1) is Li:M:F:X = 3:1:6 - a:a. In the molar ratio determined by analysis of an unknown solid electrolyte, if the Li value is 0.90 x 3 or more and 1.10 x 3 or less, it is considered that the composition formula (1) is satisfied for Li. It is more preferable that the Li value is 0.95 x 3 or more and 1.05 x 3 or less. The same applies to M, F, and X. Similarly to the composition formula (1), the composition formula (2) is also considered to satisfy the composition formula (2) if the molar ratios of Li, Mα, Mβ, F, and X determined by analysis are within ±10% (preferably ±5%) of the value of the composition formula (2). The same applies to the composition formula (3) and the composition formulas (4) to (7) described below.

[0027] This solid electrolyte is manufactured, for example, by the following method. First, a powder of a fluoride containing Li, a powder of a fluoride containing M, and a powder of a halide (LiX) containing Li are prepared. Then, these powders are weighed and mixed to a predetermined molar ratio. The fluoride containing Li is, for example, LiF (lithium fluoride). Since Li and F are also contained in other powders, it is possible to omit the fluoride containing Li. When M is Ga, the fluoride containing Ga is, for example, GaF ３ (gallium fluoride). When M is Ga and Al, the fluoride containing Al is, for example, AlF ３ (aluminum fluoride). When M is Ga and Zr, the fluoride containing Zr is, for example, ZrF ４ (zirconium fluoride). When M is Ga and Mg, the fluoride containing Mg is, for example, MgF ２ (magnesium fluoride). Halides containing Li (LiX) include, for example, LiCl (lithium chloride), LiBr (lithium bromide), and LiI (lithium iodide). Halides containing M may also be used as raw materials, and examples of such halides include GaCl ３ (gallium chloride), ZrCl ４ (zirconium chloride), which may be a bromide containing M or an iodide containing M.

[0028] Next, the mixture is subjected to mechanical milling (mechanochemical milling). Here, a planetary ball mill is used as an example of 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 above mechanical milling process produces the present solid electrolyte used in the positive electrode layer 112, the negative electrode layer 122, or the electrolyte layer 13. In this example, the mechanical milling process is performed at room temperature, but the temperature and other conditions may be changed as appropriate. The present solid electrolyte may also be produced by methods other than mechanical milling, such as firing.

[0029] The present solid electrolyte may contain sodium (Na) or potassium (K) instead of Li, or may contain two or more elements from Li, Na, and K. That is, the solid electrolyte contains A, M, F, and X, where A is at least one element selected from the group consisting of Li, Na, and K. M and X are the same as above. When A is Na, high (sodium) ion conductivity is achieved, as described below. The same applies when A is K.

[0030] Considering the case where the present solid electrolyte contains Na or K, the composition formula (1) is expressed as follows: A ３ Midfielder ６－ａ X ａ ...(4) which satisfies 0<a<6, as in the composition formula (1).

[0031] In addition, the composition formula (2) is ３－ｂ Mα １－ｂ Mβ ｂ F ６－ａ X ａ ...(5) Similar to composition formula (2), 0<a<6 and 0<b<1 are satisfied.

[0032] Furthermore, the composition formula (3) is ３－ｂ＋ｃ Mα １－ｂ－ｃ Mβ ｂ Mγ ｃ F ６－ａ X ａ ... (6) which, like composition formula (3), satisfies 0<a<6, 0≦b<1, and 0≦c<1. Note that Na and K can be quantified by ICP-emission spectroscopy or the like.

[0033] In the production of a solid electrolyte in which A is Na, NaF (sodium fluoride), NaCl (sodium chloride), NaBr (sodium bromide), NaI (sodium iodide), etc. are used instead of LiF, the above-mentioned LiX, etc. Similarly, in the production of a solid electrolyte in which A is K, KF (potassium fluoride), KCl (potassium chloride), KBr (potassium bromide), KI (potassium iodide), etc. are used.

[0034] The all-solid-state secondary battery 1 using a solid electrolyte containing Na as A may be an all-solid-state sodium ion secondary battery. In this case, 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 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 ]O２ , Na ３ V ２ (P.O. ４ ) ３ , SnO, Li ４ Ti ５ O １２ The all-solid-state secondary battery 1 may be an all-solid-state potassium ion secondary battery, in which case known materials are used as the positive electrode active material and the negative electrode active material.

[0035] Next, experimental examples of solid electrolytes will be described. The following experiments were carried out in a glove box in an argon (Ar) atmosphere with a dew point of -60°C or less. The conditions and measurement results of Experimental Examples 1 to 21 are shown in Tables 1 and 2. Experimental Examples 2 to 21 are examples of the present invention containing X (here, Cl), and Experimental Example 1 is a comparative example that does not contain X. In the composition formula column of Tables 1 and 2, the composition formula of the solid electrolyte is represented by A ３－ｂ＋ｃ Mα １－ｂ－ｃ Mβ ｂ Mγ ｃ F ６－ａ X ａ Also shown are the values ​​of a, b, and c when Mα is Ga or Ga and Al, Mβ is Zr, and Mγ is Mg.

[0036]

[0037]

[0038] (Experimental Example 1) Commercially available LiF powder and commercially available GaF were used as raw materials. ３ These powders were used as LiF:GaF ３ The components were weighed out so as to have a molar ratio of 3:1, and subjected to mechanical milling using a planetary ball mill to obtain a solid electrolyte.

[0039] (Experimental Example 2) LiF powder and GaF ３ In addition to the powder, commercially available LiCl powder was prepared and used to prepare LiF:LiCl:GaF ３ A solid electrolyte was obtained by the same treatment as in Experimental Example 1, except that the amounts were weighed out so that the molar ratio of the components was 2.9:0.1:1.

[0040] (Experimental Example 3) LiF powder, LiCl powder, and GaF ３ powder, LiF:LiCl:GaF ３ A solid electrolyte was obtained by the same treatment as in Experimental Example 1, except that the amounts were weighed out so as to give a molar ratio of 2.7:0.3:1.

[0041] (Experimental Example 4) LiF powder, LiCl powder, and GaF ３ powder, LiF:LiCl:GaF ３ A solid electrolyte was obtained by the same treatment as in Experimental Example 1, except that the amounts were weighed out so that the molar ratio of the components was 2.4:0.6:1.

[0042] (Experimental Example 5) LiF powder, LiCl powder, and GaF ３ powder, LiF:LiCl:GaF ３ A solid electrolyte was obtained by the same treatment as in Experimental Example 1, except that the amounts were weighed out so as to have a molar ratio of 2:1:1.

[0043] (Experimental Example 6) LiF powder, LiCl powder, and GaF ３ In addition to the powder, commercially available ZrF ４ Powder was prepared and LiF:LiCl:GaF ３ : ZrF ４ A solid electrolyte was obtained by the same treatment as in Experimental Example 1, except that the amounts were weighed out so as to give a molar ratio of 2.2:0.6:0.8:0.2.

[0044] Experimental Example 7: LiF powder, LiCl powder, and GaF ３ In addition to the powder, commercially available AlF ３ Powder was prepared and LiF:LiCl:GaF ３ : AlF ３ A solid electrolyte was obtained by the same treatment as in Experimental Example 1, except that the amounts were weighed out so as to give a molar ratio of 2.4:0.6:0.8:0.2.

[0045] (Experimental Example 8) LiF powder, LiCl powder, GaF ３ Powder and AlF ３ powder, LiF:LiCl:GaF ３ : AlF ３A solid electrolyte was obtained by the same treatment as in Experimental Example 1, except that the amounts were weighed out so as to give a molar ratio of 2.4:0.6:0.5:0.5.

[0046] (Experimental Example 9) LiF powder, LiCl powder, GaF ３ Powder and AlF ３ powder, LiF:LiCl:GaF ３ : AlF ３ A solid electrolyte was obtained by the same treatment as in Experimental Example 1, except that the amounts were weighed out so as to give a molar ratio of 2.4:0.6:0.3:0.7.

[0047] Experimental Example 10: LiF powder, LiCl powder, and GaF ３ powder, LiF:LiCl:GaF ３ A solid electrolyte was obtained by the same treatment as in Experimental Example 1, except that the amounts were weighed out so that the molar ratio of the components was 1.5:1.5:1.

[0048] Experimental Example 11: LiF powder, LiCl powder, and GaF ３ powder, LiF:LiCl:GaF ３ A solid electrolyte was obtained by the same treatment as in Experimental Example 1, except that the amounts were weighed out so as to have a molar ratio of 1:2:1.

[0049] (Experimental Example 12) LiCl powder and GaF ３ powder, LiCl:GaF ３ A solid electrolyte was obtained by the same treatment as in Experimental Example 1, except that the amounts were weighed so that the molar ratio of the components was 3:1.

[0050] (Experimental Example 13) LiF powder, LiCl powder, GaF ３ Powder and AlF ３ powder, LiF:LiCl:GaF ３ : AlF ３ A solid electrolyte was obtained by the same treatment as in Experimental Example 1, except that the amounts were weighed out so as to have a molar ratio of 1:2:0.8:0.2.

[0051] (Experimental Example 14) LiF powder, LiCl powder, GaF ３ Powder and AlF ３ powder, LiF:LiCl:GaF ３ : AlF ３A solid electrolyte was obtained by the same treatment as in Experimental Example 1, except that the amounts were weighed out so as to have a molar ratio of 1:2:0.7:0.3.

[0052] (Experimental Example 15) LiF powder, LiCl powder, GaF ３ Powder and AlF ３ powder, LiF:LiCl:GaF ３ : AlF ３ A solid electrolyte was obtained by the same treatment as in Experimental Example 1, except that the amounts were weighed out so as to give a molar ratio of 1:2:0.6:0.4.

[0053] (Experimental Example 16) LiF powder, LiCl powder, and GaF ３ In addition to the powder, commercially available MgF ２ Powder was prepared and LiF:LiCl:GaF ３ : MgF ２ A solid electrolyte was obtained by the same treatment as in Experimental Example 1, except that the amounts were weighed out so as to give a molar ratio of 1.1:2:0.9:0.1.

[0054] (Experimental Example 17) LiF powder, LiCl powder, GaF ３ powder and MgF ２ powder, LiF:LiCl:GaF ３ : MgF ２ A solid electrolyte was obtained by the same treatment as in Experimental Example 1, except that the amounts were weighed out so as to give a molar ratio of 1.3:2:0.7:0.3.

[0055] (Experimental Example 18) LiF powder, LiCl powder, GaF ３ powder and MgF ２ powder, LiF:LiCl:GaF ３ : MgF ２ A solid electrolyte was obtained by the same treatment as in Experimental Example 1, except that the amounts were weighed out so as to give a molar ratio of 1.4:2:0.6:0.4.

[0056] (Experimental Example 19) Commercially available NaF powder, NaCl powder, and GaF ３ Prepare powder and use NaF:NaCl:GaF ３ A solid electrolyte was obtained by the same treatment as in Experimental Example 1, except that the amounts were weighed out so as to have a molar ratio of 1:2:1.

[0057] (Experimental Example 20) NaCl powder and GaF ３ powder, NaCl:GaF ３ A solid electrolyte was obtained by the same treatment as in Experimental Example 1, except that the amounts were weighed so that the molar ratio of the components was 3:1.

[0058] (Experimental Example 21) NaF powder, NaCl powder, GaF ３ Powder and AlF ３ powder, NaF:NaCl:GaF ３ : AlF ３ A solid electrolyte was obtained by the same treatment as in Experimental Example 1, except that the amounts were weighed out so as to have a molar ratio of 1:2:0.7:0.3.

[0059] <Measurement of ionic conductivity> A solid electrolyte was placed in a mold consisting of a resin sleeve and upper and lower metal punches and subjected to uniaxial press molding. Lead wires were connected to the upper and lower punches, and impedance measurements were performed at room temperature to calculate the ionic conductivity (lithium ion conductivity in Experimental Examples 1 to 18, and sodium ion conductivity in Experimental Examples 19 to 21). In Table 1, the ionic conductivity is shown in the conductivity column.

[0060] <Construction of all-solid-state battery> The solid electrolyte of Experimental Example 14 and the solid electrolyte Li ３ YCl ６ and LiFePO as a positive electrode active material. ４ (LFP), carbon fiber as a conductive additive, and Li as a negative electrode active material. ４ Ti ５ O １２ (LTO) and Li were prepared. ３ YCl ６The positive electrode was prepared based on "Solid Halide Electrolytes with High Lithium-Ion Conductivity for Application in 4 V Class Bulk-Type All-Solid-State Batteries" (ADVANCED MATERIALS, 2018, Vol. 30, 1803075) by Tetsuya Asano and five others (reference 4 mentioned above). LFP, the solid electrolyte of Experimental Example 14, and carbon fiber were weighed out in a weight ratio of 30:30:1 and mixed in a mortar to obtain a positive electrode powder mixture. ３ YCl ６ and carbon fiber were weighed out in a weight ratio of 30:40:1 and mixed in a mortar to obtain a negative electrode mixture powder. ３ YCl ６ One punch was removed, 30 mg of the positive electrode blended powder was added, and the mixture was uniaxially pressed again. Next, the punch on the opposite side to the side where the positive electrode blended powder was added was removed, and 40 mg of the negative electrode blended powder was added, and the mixture was uniaxially pressed. As a result, a positive electrode was formed from the LFP and the solid electrolyte of Experimental Example 14, and the electrolyte layer was formed from Li ３ YCl ６ and the negative electrode is formed by LTO and Li ３ YCl ６ An all-solid-state battery was constructed using the above method.

[0061] <Charge-Discharge Test> Current collecting wires were connected to the upper and lower punches, and a charge-discharge test was carried out at room temperature. The charge conditions were a cut-off voltage of 2.2 V and a current of 300 μA / cm. ２ After constant current (cc) charging of 30 μA / cm ２ The battery was subjected to constant voltage (cv) charging with a cut-off current of 1.0 V and a discharge current of 300 μA / cm. ２ After constant current (cc) discharge of 30 μA / cm ２ A constant voltage (CV) discharge was carried out with a cut-off current of 0.05V.

[0062] FIG. 2 shows the discharge curves obtained in the charge-discharge test. In FIG. 2, the solid line L1 indicates the initial discharge curve, and the dashed line L2 indicates the discharge curve for the 20th cycle. The results of the charge-discharge test showed that the discharge capacity was 46 mAh / g-positive electrode per positive electrode weight. Furthermore, the capacity retention rate at 20 cycles was 99%, meaning that the discharge capacity did not decrease even at the 20th cycle, and charge-discharge could be appropriately repeated. Thus, it was confirmed that the solid electrolyte of Experimental Example 14 functions as a Li-ion solid electrolyte for all-solid-state batteries.

[0063] <Evaluation of Experimental Examples> As shown in Table 1, Experimental Examples 2 to 18, which contained X (here, Cl), had higher lithium ion conductivity than Experimental Example 1, which did not contain X. It is believed that in a Li ion conductive material, the inclusion of an anion with a larger ionic radius weakens the binding force of the Li ion, which is a monovalent cation, and makes the Li ion more mobile, i.e., increases ionic conductivity. The ionic radius of an F ion is 1.33 Å, while the ionic radius of a Cl ion is 1.81 Å. Although the details are not necessarily clear, it is presumed that the solid electrolytes of Experimental Examples 2 to 18 obtained high ionic conductivity by including Cl, which has a larger ionic radius than F (i.e., by replacing some F with X). For the same reason, it is believed that the solid electrolytes of Experimental Examples 19 to 21, which were Na compounds (Na ion conductive materials), also obtained high ionic conductivity.

[0064] As described above, the present solid electrolyte contains A, M, F, and X. A is at least one element selected from the group consisting of Li, Na, and K. M is a metal element or a metalloid element other than A. X is at least one element selected from the group consisting of Cl, Br, and I. In this way, the solid electrolyte contains X having an ionic radius larger than that of F, thereby achieving high ionic conductivity. Furthermore, the solid electrolyte does not contain sulfides and does not generate hydrogen sulfide gas, thereby increasing safety.

[0065] In a preferred solid electrolyte, M contains Ga, as in Experimental Examples 2 to 21. This makes it possible to more reliably improve ionic conductivity.

[0066] A preferred solid electrolyte has the formula: ３ Midfielder ６－ａ X ａ ... (4) where 0<a<6 is satisfied. This makes it possible to suitably realize a solid electrolyte with high ionic conductivity. As in Experimental Examples 10 to 21, it is more preferable that 1<a<6 is satisfied, and it is even more preferable that 1<a<4 is satisfied. This makes it possible to significantly improve ionic conductivity.

[0067] Preferably, as in Experimental Examples 2 to 21, M contains at least one element selected from the group consisting of Mα, which becomes a trivalent cation, Mβ, which becomes a tetravalent cation, and Mγ, which becomes a divalent cation. ３－ｂ＋ｃ Mα １－ｂ－ｃ Mβ ｂ Mγ ｃ F ６－ａ X ａ ... (6) where 0<a<6, 0≦b<1, and 0≦c<1 are satisfied. This makes it possible to more reliably improve the ionic conductivity.

[0068] M may include both Mα, which is a trivalent cation, and Mβ, which is a tetravalent cation. In this case, the composition formula is ３－ｂ Mα １－ｂ Mβ ｂ F ６－ａ X ａ ... (5) where 0<a<6 and 0<b<1 are satisfied. This makes it possible to suitably realize a solid electrolyte having high ionic conductivity, as in Experimental Example 6, in which part of Ga is substituted with Zr.

[0069] Similarly, M may include both Mα, which is a trivalent cation, and Mγ, which is a divalent cation. In this case, the composition formula is ３＋ｃ Mα １－ｃ Mγ ｃ F ６－ａ X ａ... (7) where 0<a<6 and 0<c<1 are satisfied. This makes it possible to suitably realize a solid electrolyte having high ionic conductivity, as in Experimental Examples 16 to 18, in which a portion of Ga was substituted with Mg.

[0070] The present solid electrolyte and all-solid-state secondary battery 1 can be modified in various ways.

[0071] The present solid electrolyte may be represented by a composition formula other than those (1) to (7) above.

[0072] The present solid electrolyte may be mixed with other substances (which may contain A such as Li) 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.

[0073] 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. Furthermore, the present solid electrolyte may be used in batteries other than all solid state secondary batteries, and may be used for applications other than batteries.

[0074] The configurations in the above-described embodiment and each modification may be combined as appropriate as long as they are not mutually contradictory.

[0075] 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.

[0076] 1 All-solid-state lithium-ion secondary battery 11 Positive electrode 12 Negative electrode 13 Electrolyte layer

Claims

1. A solid electrolyte, A, M, F and X, A is at least one element selected from the group consisting of Li, Na and K; M is a metal element or a metalloid element other than A, and includes at least one element selected from the group consisting of Mα which becomes a trivalent cation, Mβ which becomes a tetravalent cation, and Mγ which becomes a divalent cation; X is at least one element selected from the group consisting of Cl, Br and I; The composition formula of the solid electrolyte is represented by A 3-b+c Mα 1-bc Mβ b Mγ c F 6-a X a , A solid electrolyte satisfying 0<a<3, 0≦b<1, and 0≦c<1.

2. The solid electrolyte according to claim 1, A solid electrolyte in which M contains Ga.

3. The solid electrolyte according to claim 1, A solid electrolyte in which M contains Zr.

4. The solid electrolyte according to claim 1, A solid electrolyte in which M comprises Mg.

5. A battery comprising the solid electrolyte according to any one of claims 1 to 4.