Solid electrolyte, electrode material, lithium-ion battery, and method for producing solid electrolyte

Abstract

This solid electrolyte 10 comprises Li, Ti, M, and F. M is at least one selected from the group consisting of Mg, Ca, Sr, Ba, Sc, Y, Al, Ga, In, Zr, and Sn. In the X-ray diffraction pattern obtained by X-ray diffraction measurement using CuKα rays, the ratio of the intensity of the peak present in the range of diffraction angle 2θ of 40-43° to the intensity of the peak present in the range of diffraction angle 2θ of 19-23° is 1.0-3.3.

Description

Solid electrolyte, electrode material, lithium secondary battery, and method for manufacturing solid electrolyte

[0001] The present disclosure relates to a solid electrolyte, an electrode material, a lithium secondary battery, and a method for producing the solid electrolyte.

[0002] Patent Document 1 discloses a solid electrolyte material containing Li, Ti, M, and F. M is at least one selected from the group consisting of Al and Y. Patent Document 1 also describes that the solid electrolyte material containing fluorine has excellent oxidation resistance.

[0003] International Publication No. 2021 / 186809

[0004] In the prior art, there is a need to improve the ionic conductivity of fluorine-containing solid electrolytes.

[0005] The present disclosure provides a solid electrolyte comprising: Li, Ti, M, and F; M is at least one selected from the group consisting of Mg, Ca, Sr, Ba, Sc, Y, Al, Ga, In, Zr, and Sn; and in an X-ray diffraction pattern obtained by X-ray diffraction measurement using CuKα radiation, the ratio of the intensity of a peak present in a diffraction angle 2θ range of 40° to 43° to the intensity of a peak present in a diffraction angle 2θ range of 19° to 23° is 1.0 or greater and 3.3 or less.

[0006] According to the present disclosure, the ionic conductivity of a fluorine-containing solid electrolyte can be improved.

[0007] FIG. 1 is a diagram showing a solid electrolyte in embodiment 1. FIG. 2 is a process diagram showing a method for producing a solid electrolyte. FIG. 3 is a cross-sectional view showing a schematic configuration of an electrode material in embodiment 2. FIG. 4 is a cross-sectional view showing a schematic configuration of a battery in embodiment 3. FIG. 5 is a cross-sectional view of a lithium secondary battery in embodiment 4. FIG. 6 is a schematic view of a pressure forming die used for measuring ionic conductivity. FIG. 7 is a graph showing X-ray diffraction patterns of solid electrolytes in examples and comparative examples. FIG. 8 is a graph showing the relationship between the ratio I / I and ionic conductivity.

[0008] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. The present disclosure is not limited to the following embodiments.

[0009] (Embodiment 1) Figure 1 shows a solid electrolyte 10 according to embodiment 1. The solid electrolyte 10 contains Li, Ti, M, and F. M is at least one selected from the group consisting of Mg, Ca, Sr, Ba, Sc, Y, Al, Ga, In, Zr, and Sn. In an X-ray diffraction pattern obtained by X-ray diffraction measurement using CuKα radiation, the ratio I / I of the intensity I of a peak present in a diffraction angle 2θ range of 40° to 43° to the intensity I of a peak present in a diffraction angle 2θ range of 19° to 23° is 1.0 to 3.3. The peak intensities I and I represent relative intensities.

[0010] The solid electrolyte 10 has high oxidation resistance because it contains F. This is because F has a high redox potential. On the other hand, F has a high electronegativity, so the bond between F and Li is relatively strong. Therefore, conventional solid electrolytes containing Li and F tend to have low ionic conductivity. In contrast, the solid electrolyte 10 of this embodiment has a peak intensity ratio I / I of 1.0 or more and 3.3 or less in the X-ray diffraction pattern. As a result, the solid electrolyte 10 exhibits higher ionic conductivity than conventional solid electrolytes containing Li, Ti, M, and F. The peak intensity ratio I / I increases by performing long-term annealing during the manufacturing process of the solid electrolyte 10.

[0011] Peaks present in the diffraction angle 2θ range of 19° to 23° are presumed to be peaks attributable to orthorhombic crystals. Peaks present in the diffraction angle 2θ range of 40° to 43° are presumed to be peaks attributable to trigonal crystals. It is presumed that long-term annealing increases the contribution of the trigonal crystal structure relative to the contribution of the orthorhombic crystal structure, resulting in improved ionic conductivity. As the crystal structure of the solid electrolyte 10 approaches a single-phase trigonal crystal, the ratio I / I approaches an upper limit of 3.3. The peak intensity ratio I / I may be 1.8 to 3.0.

[0012] A peak present in the diffraction angle 2θ range of 19° to 23° means a peak with maximum intensity present in that range, and a peak present in the diffraction angle 2θ range of 40° to 43° means a peak with maximum intensity present in that range.

[0013] M may be at least one selected from the group consisting of Mg, Ca, Zr, and Al. These elements are suitable as elements that improve the ionic conductivity of the solid electrolyte 10.

[0014] M is typically Al. Al is inexpensive and is particularly suitable as an element that improves the ionic conductivity of the solid electrolyte 10.

[0015] It is desirable that the solid electrolyte 10 does not contain sulfur. A sulfur-free solid electrolyte does not generate hydrogen sulfide even when exposed to the atmosphere, and therefore is highly safe.

[0016] To increase ionic conductivity, the solid electrolyte 10 may contain anions other than F. The anions other than F are at least one selected from the group consisting of Cl, Br, I, O, and Se.

[0017] The solid electrolyte 10 may consist essentially of Li, Ti, M, and F. Here, "the solid electrolyte 10 consists essentially of Li, Ti, M, and F" means that the molar ratio (i.e., molar fraction) of the total amount of substance of Li, Ti, M, and F to the total amount of substance of all elements constituting the solid electrolyte 10 is 90% or more. As an example, the molar ratio may be 95% or more. The solid electrolyte 10 may consist only of Li, Ti, M, and F.

[0018] However, the solid electrolyte 10 may contain elements that are inevitably mixed in. Examples of such elements include hydrogen, oxygen, and nitrogen. Such elements may be contained in the raw material powder of the solid electrolyte 10 or may be present in the atmosphere used for producing and storing the solid electrolyte 10.

[0019] To further increase the ionic conductivity of the solid electrolyte 10, the ratio of the amount of substance of Li to the total amount of substance of Ti and M may be 1.7 or more and 4.2 or less.

[0020] The solid electrolyte 10 has, for example, a composition represented by the following formula (1): Formula (1) satisfies 0<x<1 and 0<b≦2. When the solid electrolyte 10 has a composition represented by the following formula (1), the solid electrolyte 10 exhibits good ionic conductivity.

[0021] Li 6-(4-x)b (Ti 1-x M x ) b F6...Formula (1)

[0022] To increase the ionic conductivity of the solid electrolyte 10, formula (1) may satisfy 0.1≦x≦0.9.

[0023] In order to increase the ionic conductivity of the solid electrolyte 10, the formula (1) may satisfy 0.8≦b≦1.2.

[0024] As can be seen from the peaks that appear in the X-ray diffraction pattern, the solid electrolyte 10 has a crystalline phase. However, the solid electrolyte 10 may also contain an amorphous phase.

[0025] The shape of the solid electrolyte 10 is not particularly limited. The solid electrolyte 10 may have a particle shape. Examples of particle shapes include needle shapes, spheres, and oval spheres. The solid electrolyte 10 may have a pellet, plate, or thin film shape.

[0026] When the solid electrolyte 10 is particulate, the particles of the solid electrolyte 10 may have a median diameter of 0.01 μm or more and 100 μm or less. This allows the solid electrolyte 10 to exhibit higher ionic conductivity. Furthermore, when the solid electrolyte 10 is mixed with other materials such as active materials, the solid electrolyte 10 and the other materials are well dispersed. The "median diameter" refers to the particle size at which the cumulative volume in the volume-based particle size distribution is 50%. The volume-based particle size distribution is measured, for example, using a laser diffraction particle size distribution analyzer.

[0027] Next, a method for manufacturing the solid electrolyte 10 will be described. Fig. 2 is a process diagram showing the method for manufacturing the solid electrolyte.

[0028] In step S1, a solid electrolyte is synthesized. First, raw material powders are mixed so as to obtain a solid electrolyte having a target composition. The raw material powders are, for example, fluorides of each element. For example, if the target composition is Li, 2.7 Ti 0.3 Al 0.7 In the case of F, LiF, TiF, and AlF are mixed in a molar ratio of 2.7:0.3:0.7. The raw material powders are then reacted with each other using a mixing device such as a planetary ball mill. That is, the raw material powders are reacted with each other by mechanochemical milling. This results in a solid electrolyte having the desired composition.

[0029] Next, in step S2, the obtained solid electrolyte is annealed. The annealing is performed under conditions of 100°C or higher and 10 hours or longer. This results in the solid electrolyte 10 of this embodiment. Annealing for a long period of time at an appropriate temperature increases the peak intensity ratio I / I. As a result, the ionic conductivity of the solid electrolyte 10 is improved.

[0030] The upper limit of the annealing temperature is not particularly limited, and is, for example, 270°C. The upper limit of the annealing time is not particularly limited, and is, for example, 1000 hours. The annealing conditions are preferably 120°C or higher and 250°C or lower, and 10 hours or higher and 200 hours or lower. The atmosphere during annealing may be air or an inert atmosphere. Examples of inert atmospheres include a nitrogen gas atmosphere and a rare gas atmosphere.

[0031] Before or after annealing, a step may be carried out to pulverize the solid electrolyte 10. In this step, the solid electrolyte 10 is pulverized to have a median diameter on the order of nanometers, for example.

[0032] (Embodiment 2) FIG. 3 is a cross-sectional view of an electrode material 20 in embodiment 2. The electrode material 20 includes an active material 30 and a coating layer 40. The coating layer 40 covers at least a portion of the surface of particles of the active material 30. The active material 30 may be secondary particles formed by aggregation of a plurality of primary particles. The coating layer 40 may cover at least a portion of the surface of the secondary particles of the active material 30. The coating layer 40 includes the solid electrolyte 10 of embodiment 1. When the active material 30 is coated with the solid electrolyte 10, direct contact between the active material 30 and other materials such as an electrolyte solution is prevented. This makes it possible to suppress decomposition of other materials such as an electrolyte solution.

[0033] The coating layer 40 may cover only a portion of the surface of the particles of the active material 30. In this case, the particles of the active material 30 come into direct contact with each other via the portions not covered by the coating layer 40, thereby ensuring electronic conductivity between the particles of the active material 30. However, the coating layer 40 may also cover the surfaces of the particles of the active material 30 uniformly.

[0034] The coating layer 40 may contain the solid electrolyte 10 as a main component, or may contain only the solid electrolyte 10. "Main component" refers to the component that is contained in the largest amount by mass. "Containing only the solid electrolyte 10" means that, with the exception of inevitable impurities, no materials other than the solid electrolyte 10 are intentionally added.

[0035] The active material 30 is, for example, a positive electrode active material. When the electrode material 20 is used in the positive electrode of a nonaqueous electrolyte secondary battery, decomposition of the electrolyte in the positive electrode can be suppressed. Therefore, it becomes possible to use a higher-power active material in the positive electrode.

[0036] When the electrode material 20 is used for the positive electrode of a solid-state battery, a second solid electrolyte having a different composition from the solid electrolyte 10 is mixed with the positive electrode. In this case, direct contact between the active material 30 and the second solid electrolyte is prevented, so decomposition of the second solid electrolyte can be suppressed. Examples of the second solid electrolyte include halide solid electrolytes, sulfide solid electrolytes, polymer solid electrolytes, and complex hydride solid electrolytes. Examples of sulfide solid electrolytes include Li2S-P2S5, Li2S-SiS2, Li2S-B2S3, Li2S-GeS2, and Li3.25 Ge 0.25 P 0.75 S4, Li 10 GeP2S 12 Examples of polymer solid electrolytes include compounds of a polymer compound having an ethylene oxide structure and a lithium salt. Examples of lithium salts that can be used include at least one selected from the group consisting of LiPF6, LiBF4, LiSbF6, LiAsF6, LiSO3CF3, LiN(SO2CF3)2, LiN(SO2C2F5)2, LiN(SO2CF3)(SO2C4F9), and LiC(SO2CF3)3. Examples of complex hydride solid electrolytes include LiBH4-LiI and LiBH4-P2S5. Examples of halide solid electrolytes include Li3YX6, Li2MgX4, Li2FeX4, Li(Al, Ga, In)X4, and Li3(Al, Ga, In)X6. "X" represents a halogen element. "(Al, Ga, In)" means at least one selected from the group consisting of Al, Ga, and In. One or a combination of two or more selected from these solid electrolytes can be used as the second solid electrolyte.

[0037] The active material 30 includes a material that has the property of absorbing and releasing metal ions such as lithium ions.

[0038] When the active material 30 is a positive electrode active material, lithium-containing transition metal oxides, lithium-containing transition metal phosphates, transition metal fluorides, polyanionic materials, fluorinated polyanionic materials, transition metal sulfides, transition metal oxysulfides, transition metal oxynitrides, etc. can be used as the active material 30. In particular, using a lithium-containing transition metal oxide or a lithium-containing transition metal phosphate as the active material 30 can reduce the manufacturing cost of the battery and increase the average discharge voltage. Examples of lithium-containing transition metal oxides include lithium cobalt oxide, lithium nickel cobalt aluminum oxide, lithium nickel cobalt manganese oxide, and lithium nickel manganese oxide. Examples of lithium-containing transition metal phosphates include lithium iron phosphate, lithium vanadium phosphate, lithium cobalt phosphate, and lithium nickel phosphate.

[0039] The active material 30 may be a negative electrode active material, such as lithium titanate, graphite, silicon, a silicon compound, or a NiBi alloy.

[0040] The active material 30 has, for example, a particle shape. There are no particular limitations on the shape of the particles of the active material 30. The shape of the particles of the active material 30 can be spherical, oval, scaly, or fibrous.

[0041] The electrode material 20 can be manufactured by the following method.

[0042] A powder of the active material 30 and a powder of the solid electrolyte 10 are mixed in an appropriate ratio to obtain a mixture. The mixture is then milled to impart mechanical energy to the mixture. A mixing device such as a ball mill can be used for the milling process. The milling process may be performed in a dry and inert atmosphere to prevent oxidation of the materials.

[0043] The electrode material 20 may be manufactured by a dry particle compounding method. The treatment by the dry particle compounding method includes applying at least one mechanical energy selected from the group consisting of impact, compression, and shear to the active material 30 and the solid electrolyte 10. The active material 30 and the solid electrolyte 10 are mixed in an appropriate ratio.

[0044] The device used to manufacture the electrode material 20 is not particularly limited and may be any device capable of applying impact, compression, and shear mechanical energy to a mixture of the active material 30 and the solid electrolyte 10. Examples of devices capable of applying mechanical energy include a ball mill and processing devices (particle composite devices) such as "Mechanofusion" (manufactured by Hosokawa Micron Corporation), "Nobilta" (manufactured by Hosokawa Micron Corporation), and "Balance Gran" (manufactured by Freund Turbo Corporation). The electrode material 20 may be manufactured by a liquid phase method such as a sol-gel method.

[0045] The annealing step (step S2) described with reference to FIG. 2 may be carried out in a state where the unannealed solid electrolyte is attached to the surface of the particles of the active material 30.

[0046] A second coating layer may be present between the active material 30 and the coating layer 40. The second coating layer may be formed of an oxide solid electrolyte such as LISICON or lithium niobate.

[0047] It is not essential that the solid electrolyte 10 be used as a coating material for the active material 30. The solid electrolyte 10 may be used in the electrolyte layer of a solid-state battery.

[0048] (Embodiment 3) Figure 4 is a cross-sectional view of a lithium secondary battery 100 in Embodiment 3. The lithium secondary battery 100 includes a positive electrode 53, a negative electrode 56, an electrolyte layer 57, an exterior casing 58, and a non-aqueous electrolyte solution 59. The positive electrode 53 includes a positive electrode current collector 51 and a positive electrode active material layer 52. The positive electrode active material layer 52 is provided on the positive electrode current collector 51. The negative electrode 56 includes a negative electrode current collector 54 and a negative electrode active material layer 55. The negative electrode active material layer 55 is provided on the negative electrode current collector 54. An electrolyte layer 57 is disposed between the positive electrode 53 and the negative electrode 56. The electrolyte layer 57 is a separator. The positive electrode 53, the negative electrode 56, the electrolyte layer 57, and the non-aqueous electrolyte solution 59 are housed in an exterior casing 58.

[0049] The positive electrode 53 or the negative electrode 56 contains the electrode material 20 of embodiment 2. Specifically, the positive electrode active material layer 52 or the negative electrode active material layer 55 contains the electrode material 20 of embodiment 2. This suppresses decomposition of other materials, such as the nonaqueous electrolyte solution 59, in the positive electrode 53 or the negative electrode 56. As a result, the performance of the lithium secondary battery 100, such as the cycle life, is improved.

[0050] The electrode material 20 is typically contained in the positive electrode 53. This can suppress decomposition of the nonaqueous electrolyte solution 59 in the positive electrode 53.

[0051] The positive electrode current collector 51 is a foil made of a metal material such as aluminum, stainless steel, titanium, or an alloy thereof. The negative electrode current collector 54 is a foil made of a metal material such as stainless steel, nickel, copper, or an alloy thereof.

[0052] The positive electrode active material layer 52 and the negative electrode active material layer 54 may contain a conductive additive, an ion conductor, a binder, and the like.

[0053] The non-aqueous electrolyte 59 is impregnated into the positive electrode 53, the negative electrode 56, and the electrolyte layer 57. The non-aqueous electrolyte 59 may fill the internal space of the exterior casing 58.

[0054] The non-aqueous electrolyte 59 contains a non-aqueous solvent and a lithium salt.

[0055] Examples of non-aqueous solvents that can be used include cyclic carbonates, chain carbonates, esters, cyclic ethers, chain ethers, nitriles, amides, etc. One of these solvents may be used alone, or two or more of them may be used in combination.

[0056] Examples of lithium salts that can be used include lithium hexafluorophosphate (LiPF), lithium tetrafluoroborate (LiBF), lithium perchlorate (LiClO), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bisperfluoroethylsulfonylimide (LiN(SO2C2F5)2), LiAsF6, LiCF3SO3, and lithium difluoro(oxalato)borate. One or more of these electrolyte salts may be used alone or in combination.

[0057] The electrolyte layer 57 has lithium ion conductivity. There are no particular limitations on the material of the electrolyte layer 57 as long as it allows the passage of lithium ions. The material of the electrolyte layer 57 can be at least one selected from the group consisting of solid electrolytes, gel electrolytes, ion exchange resin membranes such as lithium cation exchange resins, semipermeable membranes, and porous membranes. If the electrolyte layer 57 is made of these materials, the safety of the lithium secondary battery 100 can be sufficiently ensured. Examples of solid electrolytes include sulfide solid electrolytes such as Li2S-P2S5, Li7La3Zr2O 12Examples of the electrolyte layer 57 include oxide solid electrolytes such as LLZ. Examples of the gel electrolyte include gel electrolytes containing fluororesins such as PVdF. Examples of the ion exchange resin membrane include cation exchange membranes and anion exchange membranes. Examples of the porous membrane include porous membranes made of polyolefin resin and porous membranes made of glass paper obtained by weaving glass fibers into nonwoven fabric. The electrolyte layer 57 has a thickness of, for example, 0.001 μm or more and 500 μm or less.

[0058] The solid electrolyte 10 of the first embodiment may be used for the solid electrolyte of the electrolyte layer 57 .

[0059] There is no particular limitation on the shape of the lithium secondary battery 100. The lithium secondary battery 100 may have various shapes such as a coin type, a cylindrical type, a square type, a sheet type, a button type, a flat type, and a laminated type.

[0060] (Embodiment 4) Figure 5 is a cross-sectional view of a lithium secondary battery 200 according to Embodiment 4. The lithium secondary battery 200 includes a positive electrode 63, a coating layer 68, an electrolyte layer 67, and a negative electrode 66, in this order. The electrolyte layer 67 is disposed between the positive electrode 63 and the negative electrode 66. The positive electrode 63 includes a positive electrode current collector 61 and a positive electrode active material layer 62. The negative electrode 66 includes a negative electrode current collector 64 and a negative electrode active material layer 65. The coating layer 68 is a layer including the solid electrolyte 10 of Embodiment 1, and is disposed between the positive electrode active material layer 62 and the electrolyte layer 67, covering the surface of the positive electrode active material layer 62. Specifically, the coating layer 68 is disposed on the surface of the positive electrode active material layer 62 and is in contact with both the positive electrode active material layer 62 and the electrolyte layer 67.

[0061] The coating layer 68 may contain a binder. The binder may be the same material as that usable for the positive electrode active material layer 62 and the negative electrode active material layer 65. The coating layer 68 has a thickness of, for example, 0.001 μm or more and 100 μm or less.

[0062] The electrolyte layer 67 may be a layer containing a second solid electrolyte. When the electrolyte layer 67 contains the second solid electrolyte, the sum of the thickness of the coating layer 68 and the thickness of the electrolyte layer 67 is, for example, 0.002 μm or more and 500 μm or less. According to this embodiment, in addition to improving ion conductivity, the effect of suppressing decomposition of the second solid electrolyte is obtained.

[0063] The position of the coating layer 68 is not limited to the position shown in Fig. 5 . The coating layer 68 may be additionally provided between the electrolyte layer 67 and the negative electrode active material layer 65. The coating layer 68 may be disposed on the surface of the negative electrode active material layer 65. Alternatively, the coating layer 68 may be disposed only between the electrolyte layer 67 and the negative electrode active material layer 65. The coating layer 68 may be disposed on the surface of at least one of the positive electrode active material layer 62 and the negative electrode active material layer 65.

[0064] The constituent materials of the lithium secondary battery 200 are as described in embodiment 3. The lithium secondary battery 200 may be a solid-state battery that does not use a liquid electrolyte, or may be a secondary battery that uses a nonaqueous electrolyte as described in embodiment 3.

[0065] (Other Embodiments) (Additional Notes) The above description of the embodiments discloses the following techniques.

[0066] (Technology 1) A solid electrolyte comprising Li, Ti, M, and F, wherein M is at least one selected from the group consisting of Mg, Ca, Sr, Ba, Sc, Y, Al, Ga, In, Zr, and Sn, and wherein, in an X-ray diffraction pattern obtained by X-ray diffraction measurement using CuKα radiation, the ratio of the intensity of a peak present in a diffraction angle 2θ range of 40° to 43° to the intensity of a peak present in a diffraction angle 2θ range of 19° to 23° is 1.0 to 3.3.

[0067] This configuration can improve the ionic conductivity of the fluorine-containing solid electrolyte.

[0068] (Technology 2) The solid electrolyte according to Technology 1, wherein M is at least one selected from the group consisting of Mg, Ca, Zr, and Al. These elements are suitable as elements for improving the ionic conductivity of the solid electrolyte.

[0069] (Technology 3) The solid electrolyte according to Technology 1, wherein M is Al. Al is inexpensive and is particularly suitable as an element for improving the ionic conductivity of the solid electrolyte.

[0070] (Technology 4) The solid electrolyte according to Technology 1, wherein the solid electrolyte has a composition represented by the following formula (1): Formula (1) satisfies 0<x<1 and 0<b≦2. With this structure, the solid electrolyte exhibits good ionic conductivity. Li 6-(4-x)b (Ti 1-x M x ) b F6...Formula (1)

[0071] (Technology 5) An electrode material comprising: an active material; and a coating layer covering at least a part of the surface of particles of the active material, wherein the coating layer contains the solid electrolyte according to any one of Technologies 1 to 4.

[0072] This configuration prevents direct contact between the active material and other materials such as the electrolyte, thereby making it possible to suppress decomposition of the other materials such as the electrolyte.

[0073] (Technology 6) A lithium secondary battery comprising: a positive electrode; a negative electrode; and an electrolyte layer disposed between the positive electrode and the negative electrode, wherein the positive electrode or the negative electrode contains the electrode material according to Technology 5.

[0074] This configuration improves the performance of the lithium secondary battery, such as its cycle life.

[0075] (Technology 7) A method for producing a solid electrolyte, comprising: synthesizing a solid electrolyte containing Li, Ti, M, and F; and annealing the solid electrolyte at 100°C or higher for 10 hours or longer, wherein M is at least one selected from the group consisting of Mg, Ca, Sr, Ba, Sc, Y, Al, Ga, In, Zr, and Sn.

[0076] This configuration can improve the ionic conductivity of the fluorine-containing solid electrolyte.

[0077] Comparative Example 1: In an argon atmosphere having a dew point of -60°C or less, raw material powders LiF, TiF, and AlF were weighed out in a molar ratio of LiF:TiF:AlF = 2.7:0.3:0.7. These were pulverized and mixed in a mortar to obtain a mixture. The mixture was then milled for 12 hours at 500 rpm using a φ5 mm zirconia ball and a planetary ball mill (Fritsch, Model P-7). This resulted in LiF, TiF, and AlF being 0.7% by weight. 2.7 Ti 0.3 Al 0.7 A powdered solid electrolyte having the composition of F6 was obtained.

[0078] Example 1 The solid electrolyte of Comparative Example 1 was placed in a sealed container and annealed at 125° C. (ambient temperature) in the air for 40 hours. Thus, the solid electrolyte of Example 1 was obtained.

[0079] Example 2 The solid electrolyte of Comparative Example 1 was placed in a sealed container and annealed at 150° C. (ambient temperature) in the air for 90 hours. Thus, a solid electrolyte of Example 2 was obtained.

[0080] [Measurement of ionic conductivity] Fig. 6 is a schematic diagram showing a pressure molding die 300 used to measure the ionic conductivity of a solid electrolyte. The pressure molding die 300 included an upper punch 301, a frame 302, and a lower punch 303. The upper punch 301 and the lower punch 303 were made of stainless steel. The frame 302 was made of polycarbonate.

[0081] In a dry atmosphere having a dew point of −60° C. or less, solid electrolyte powder 101 was filled into a pressure molding die 300. A pressure of 400 MPa was applied to solid electrolyte powder 101 using upper punch 301 and lower punch 303.

[0082] While pressure was still applied, the upper punch 301 and the lower punch 303 were connected to a potentiostat (Princeton Applied Research, VersaSTAT4) equipped with a frequency response analyzer. The upper punch 301 was connected to a working electrode and a potential measurement terminal. The lower punch 303 was connected to a counter electrode and a reference electrode. The impedance of the solid electrolyte was measured by electrochemical impedance measurement at 25°C and -40°C.

[0083] In the Cole-Cole plot obtained by impedance measurement, the real value of the impedance at the measurement point where the absolute value of the phase of the complex impedance was smallest was regarded as the resistance value to ionic conduction of the solid electrolyte. Using this resistance value, the ionic conductivity was calculated based on the following formula (2). The results are shown in Table 1.

[0084] σ = (R SE × S / t) -1 ...(2)

[0085] In formula (2), σ represents ionic conductivity. S represents the contact area between the solid electrolyte and the upper punch portion 301. S is equal to the cross-sectional area of ​​the hollow portion of the frame mold 302 in FIG. 6. R SE represents the resistance value of the solid electrolyte in the impedance measurement. t represents the thickness of the solid electrolyte. t represents the thickness of the layer of the solid electrolyte powder 101 in FIG. 6 .

[0086] [X-ray Diffraction Measurement] Powder X-ray diffraction measurement was carried out on the solid electrolytes of Comparative Example 1, Example 1, and Example 2. For the X-ray diffraction measurement, a powder X-ray diffractometer (MiniFlex 600, manufactured by Rigaku Corporation) was used.

[0087] Cu-Kα radiation (wavelengths 1.5405 Å and 1.5444 Å) was used as the X-ray source, and measurements were performed by the θ-2θ method. The measurement angle interval was 0.01°. The divergence angle of the divergence slit was 0.25°. The slit width of the longitudinal limiting slit was 5 mm.

[0088] Figure 7 shows the X-ray diffraction pattern obtained by X-ray diffraction measurement. The ratio I / I of the peak intensity I present in the diffraction angle 2θ range of 40° to 43° to the peak intensity I present in the diffraction angle 2θ range of 19° to 23° was calculated. A Gauss-Lorentz composite function was used to fit each peak. The results are shown in Table 1.

[0089]

[0090] The ionic conductivity of the solid electrolyte of Example 2 at 25°C was about 1.5 times that of the solid electrolyte of Comparative Example 1 at 25°C. Furthermore, the order of ionic conductivity of the solid electrolyte of Example 2 at -40°C was one order of magnitude larger than the order of ionic conductivity of the solid electrolyte of Comparative Example 1 at -40°C.

[0091] 8 is a graph showing the relationship between the ratio I2 / I1 and ionic conductivity at -40°C. In the solid electrolyte of Comparative Example 1, the ratio I2 / I1 was 0.45. In the solid electrolyte of Example 1, the ratio I2 / I1 was 1.99. In the solid electrolyte of Example 2, the ratio I2 / I1 was 2.66. As can be seen from these results, the ratio I2 / I1 increased as the annealing time increased.

[0092] Even if the metal M contained in the solid electrolyte is a metal other than Al, if the ionic radius of the metal M is close to the ionic radius of Al, the solid electrolyte is expected to exhibit the same tendency as the solid electrolytes of Examples 1 and 2. Examples of such metal M include Ca, Mg, and Zr.

[0093] The technology of the present disclosure is useful for lithium secondary batteries.

Claims

1. Li, Ti, M, and F; M is at least one selected from the group consisting of Mg, Ca, Sr, Ba, Sc, Y, Al, Ga, In, Zr, and Sn; In an X-ray diffraction pattern obtained by X-ray diffraction measurement using CuKα radiation, the ratio of the intensity of a peak present in a diffraction angle 2θ range of 40° or more and 43° or less to the intensity of a peak present in a diffraction angle 2θ range of 19° or more and 23° or less is 1.0 or more and 3.3 or less. solid electrolyte.

2. M is at least one selected from the group consisting of Mg, Ca, Zr, and Al; The solid electrolyte according to claim 1 .

3. M is Al; The solid electrolyte according to claim 1 .

4. The solid electrolyte has a composition represented by the following formula (1): Li 6-(4-x)b (Ti 1-x M x ) b F 6 ... Formula (1) Formula (1) satisfies 0<x<1 and 0<b≦2. The solid electrolyte according to claim 1 .

5. An active material; a coating layer covering at least a part of the surface of the active material particles; Equipped with The coating layer includes the solid electrolyte according to claim 1. electrode material.

6. A positive electrode and A negative electrode; an electrolyte layer disposed between the positive electrode and the negative electrode; Equipped with The positive electrode or the negative electrode comprises the electrode material according to claim 5. Lithium secondary battery.