Solid electrolyte, all-solid-state battery, method for manufacturing solid electrolyte, and method for manufacturing all-solid-state battery

Abstract

To provide a solid electrolyte exhibiting high ion conductivity and capable of being fired at a low temperature, an all-solid battery, a manufacturing method of the solid electrolyte, and a manufacturing method of the all-solid battery.SOLUTION: An oxide-type solid electrolyte contains Li, Ta, and P, the ratio of Li content to 1 mol of P is 0.5 mol or more and 0.95 mol or less, XRD measurement results confirm that the solid electrolyte has a crystal structure attributed to monoclinic.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a solid electrolyte, an all-solid-state battery, a method for producing a solid electrolyte, and a method for producing an all-solid-state battery. [Background technology]

[0002] In recent years, the demand for secondary batteries has been expanding rapidly, and lithium-ion secondary batteries using organic electrolytes have been put to practical use. However, due to concerns about electrolyte leakage and other issues, expectations are growing for safer solid electrolytes, and the development of all-solid-state batteries using solid electrolytes is actively progressing (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2016 / 157751 Summary of the Invention [Problem to be solved by the invention]

[0004] From a safety perspective, oxide-based solid electrolytes, which are stable in air, are preferable to sulfide-based solid electrolytes, which emit toxic gases when exposed to air. However, oxide-based solid electrolytes require high-temperature sintering to achieve ionic conductivity and to form a good interface with the electrode active material. High-temperature sintering poses a challenge due to interdiffusion reactions between the solid electrolyte and the electrode active material.

[0005] Known oxide-based solid electrolytes include Li-La-Zr-O compounds with a garnet structure and their elemental substitution products, Li-Al-Ti-PO and Li-Al-Ge-PO compounds with a NASICON crystal structure, and Li-La-Ti-O compounds with a perovskite crystal structure. In recent years, Li-Ta-PO compounds have also been reported, but all require heat treatment (firing) at relatively high temperatures, and there are concerns about interdiffusion reactions when forming interfaces with the electrode active material.

[0006] In order to address these issues, a method of mixing an oxide-based solid electrolyte with a glass material has been investigated in order to lower the sintering temperature of the oxide-based solid electrolyte. For example, in Patent Document 1, an oxide crystal is used as a first lithium ion conductor, and is mixed with a second lithium ion conductor, which is a glass material that can be sintered at 600°C or less, thereby achieving high lithium ion conductivity at sintering temperatures of 600°C or less.

[0007] Patent Document 1 proposes that a solid electrolyte with high lithium ion conductivity can be obtained by heat treating a first lithium ion conductor and a second lithium ion conductor at 600°C or less. However, because this heat treatment is performed at a relatively low temperature of 600°C or less, if it is desired to further densify the electrolyte layer and solid electrolyte layer or reduce the interfacial resistance between the two layers, heat treatment at a higher temperature is required, raising concerns about reactions due to component diffusion between the two. Heat treatment at 600°C or more is particularly prone to lithium interdiffusion reactions, raising concerns about changes in the crystal structure of the highly crystalline, highly ion-conductive solid electrolyte and a decrease in ionic conductivity.

[0008] The present invention has been made in view of the above-mentioned problems, and aims to provide a solid electrolyte that exhibits high ionic conductivity and can be sintered at a low temperature, an all-solid-state battery, a method for manufacturing a solid electrolyte, and a method for manufacturing an all-solid-state battery. [Means for solving the problem]

[0009] The solid electrolyte according to the present invention is an oxide-type solid electrolyte containing Li, Ta, and P, characterized in that the ratio of the Li content to 1 mol of P is 0.5 mol or more and 0.95 mol or less, and the result of XRD measurement confirms that the solid electrolyte has a crystal structure belonging to a monoclinic system.

[0010] In the solid electrolyte, the molar ratio of Ta / P may be 1.7 or more and 2.3 or less.

[0011] The all-solid-state battery according to the present invention is characterized by comprising: a solid electrolyte layer containing any one of the above solid electrolytes according to the present invention as a first solid electrolyte and containing, as a second solid electrolyte, a lithium-containing material having a lower sintering start temperature than the first solid electrolyte; a first electrode containing an electrode active material and formed on a first main surface of the solid electrolyte layer; and a second electrode containing an electrode active material and formed on a second main surface of the solid electrolyte layer opposite to the first main surface.

[0012] In the all-solid-state battery, a plurality of units may be stacked, each unit including the solid electrolyte layer, the first electrode, and the second electrode.

[0013] In the all-solid-state battery, one of the first electrode and the second electrode may contain a positive electrode active material, and the other may contain a negative electrode active material.

[0014] In the all-solid-state battery, the first solid electrolyte in the solid electrolyte layer may have an average crystal grain size of 1 μm or more and 20 μm or less.

[0015] In the all-solid-state battery, the lithium-containing material may be at least one of a Li-Ge-PO-based compound, a Li-Zr-PO-based compound, a Li-PO-based compound, a Li-BO-based compound, a Li-Si-O-based compound, a Li-Ge-Zr-PO-based compound, a Li-Si-BO-based compound, a Li-Al-Ge-PO-based compound, a Li-La-Zr-PO-based compound, and a Li-Al-PO-based compound.

[0016] The method for producing a solid electrolyte according to the present invention is characterized in that an oxide-type solid electrolyte is synthesized from a raw material containing Li, Ta, and P, in which the Li content per 1 mol of P is 0.5 mol or more and 0.95 mol or less, at a temperature of 950°C or more and 1300°C or less, and the result of XRD measurement confirms that the oxide-type solid electrolyte has a crystal structure belonging to a monoclinic system.

[0017] A method for producing an all-solid-state battery according to the present invention includes the steps of: preparing a laminate including a green sheet containing, as a first solid electrolyte powder, a powder of an oxide-type solid electrolyte that contains Li, Ta, and P, wherein the Li content per 1 mol of P is 0.5 mol or more and 0.95 mol or less, and which is confirmed to have a crystal structure belonging to a monoclinic system as a result of XRD measurement; and a green sheet containing, as a second solid electrolyte powder, a powder of a lithium-containing material that has a lower sintering start temperature than the first solid electrolyte powder; a first electrode layer paste coating formed on a first main surface of the green sheet; and a second electrode layer paste coating formed on a second main surface of the green sheet; and a firing step of firing the laminate.

[0018] In the method for producing an all-solid-state battery, the firing temperature in the firing step may be 500°C or higher and 900°C or lower. [Effects of the Invention]

[0019] According to the present invention, it is possible to provide a solid electrolyte that exhibits high ionic conductivity and can be sintered at a low temperature, an all-solid-state battery, a method for producing a solid electrolyte, and a method for producing an all-solid-state battery. [Brief explanation of the drawings]

[0020] [Figure 1] FIG. 1(a) is a schematic cross-sectional view showing the basic structure of an all-solid-state battery, and FIG. 1(b) is a schematic cross-sectional view of a solid electrolyte layer. [Figure 2] FIG. 1 is a schematic cross-sectional view of an all-solid-state battery according to an embodiment. [Figure 3] FIG. 1 is a schematic cross-sectional view of another all-solid-state battery. [Figure 4] FIG. 1 is a diagram illustrating a flow of a method for manufacturing an all-solid-state battery. [Figure 5] 1A to 1C are diagrams illustrating a lamination process. [Figure 6] FIG. 10 is a diagram illustrating a flow of another method for manufacturing an all-solid-state battery. DETAILED DESCRIPTION OF THE INVENTION

[0021] Hereinafter, embodiments will be described with reference to the drawings.

[0022] (Embodiment) FIG. 1(a) is a schematic cross-sectional view showing the basic structure of an all-solid-state battery 100. As illustrated in FIG. 1(a), the all-solid-state battery 100 has a structure in which a solid electrolyte layer 30 is sandwiched between a first electrode 10 and a second electrode 20. The first electrode 10 is formed on a first main surface of the solid electrolyte layer 30, and has a structure in which a first electrode layer 11 and a first current collector layer 12 are laminated, with the first electrode layer 11 on the solid electrolyte layer 30 side. The second electrode 20 is formed on a second main surface of the solid electrolyte layer 30, and has a structure in which a second electrode layer 21 and a second current collector layer 22 are laminated, with the second electrode layer 21 on the solid electrolyte layer 30 side.

[0023] When the all-solid-state battery 100 is used as a secondary battery, one of the first electrode 10 and the second electrode 20 is used as a positive electrode, and the other is used as a negative electrode. In the present embodiment, as an example, the first electrode 10 is used as a positive electrode, and the second electrode 20 is used as a negative electrode.

[0024] The solid electrolyte layer 30 is mainly composed of an ion-conductive solid electrolyte. As illustrated in FIG. 1(b), the solid electrolyte of the solid electrolyte layer 30 has a structure in which a plurality of first solid electrolyte particles 31 and a plurality of second solid electrolyte particles 32 are mixed. The first solid electrolyte particles 31 are oxide-based crystals containing Li, Ta, and P, such as a Li-Ta-PO-based compound. XRD (X-ray diffraction) measurement confirms that the first solid electrolyte particles 31 have a monoclinic crystal structure. For example, the first solid electrolyte particles 31 are LiTa2PO8-based compounds. The second solid electrolyte particles 32 are lithium-containing materials and have a lower sintering start temperature than the first solid electrolyte particles 31.

[0025] Here, "XRD measurement confirms that the crystal structure is monoclinic" means that XRD measurement using Cu Kα as a radiation source shows a main peak at least at 25.2° to 25.7°, and further shows sub-peaks with diffraction peak intensities of 30% to 70% of the main peak intensity in three ranges: 20.2° to 20.7°, 24.6° to 25.1°, and 34.6° to 35.1°.

[0026] The first solid electrolyte particles 31 have a Li-poor structure rather than a 1:2:1 Li:Ta:P molar ratio. That is, in the first solid electrolyte particles 31, the ratio of Li to 1 mol of P is less than 1 mol. The Li-poor structure of the first solid electrolyte particles 31 facilitates stabilization of the monoclinic crystal structure. Therefore, even when mixed with a low-temperature sinterable lithium-containing material and sintered, reaction between the two components is suppressed, and a heterogeneous phase that could become a high-resistance component is less likely to form at the interface between the two, resulting in high ionic conductivity. As a result, the solid electrolyte layer 30 exhibits high ionic conductivity and can be sintered at a low temperature.

[0027] In the first solid electrolyte particle 31, if the ratio of Li content to 1 mol of P is too small, there is a risk of a decrease in ion conductivity. Therefore, a lower limit is set for the ratio of Li content to 1 mol of P. In this embodiment, the ratio of Li content to 1 mol of P is 0.5 mol or more, preferably 0.6 or more, and more preferably 0.7 or more.

[0028] On the other hand, if the ratio of Li content to 1 mol of P in the first solid electrolyte particle 31 is too high, the monoclinic crystal structure may not be sufficiently stabilized. Therefore, an upper limit is set for the ratio of Li content to 1 mol of P. In this embodiment, the ratio of Li content to 1 mol of P is 0.95 or less, preferably 0.9 or less, and more preferably 0.8 or less.

[0029] If the molar ratio of Ta / P is small in the first solid electrolyte particles 31, there is a risk of secondary phase formation. Therefore, it is preferable to set a lower limit for the molar ratio of Ta / P in the first solid electrolyte particles 31. For example, the molar ratio of Ta / P is preferably 1.7 or more, more preferably 1.8 or more, and even more preferably 1.9 or more.

[0030] If the molar ratio of Ta / P in the first solid electrolyte particles 31 is high, there is a risk of secondary phase formation and reduced ion conductivity. Therefore, it is preferable to set an upper limit on the molar ratio of Ta / P in the first solid electrolyte particles 31. For example, the molar ratio of Ta / P is preferably 2.3 or less, more preferably 2.2 or less, and even more preferably 2.1 or less.

[0031] In the solid electrolyte layer 30, if the average crystal grain size of the first solid electrolyte particles 31 is small, the solid electrolyte layer is prevented from being densified, and the packing rate of the first solid electrolyte cannot be increased, which may result in insufficient ion conductivity. Therefore, it is preferable to set a lower limit for the average crystal grain size of the first solid electrolyte particles 31. For example, the average crystal grain size of the first solid electrolyte particles 31 is preferably 1 μm or more, more preferably 2 μm or more, and even more preferably 3 μm or more.

[0032] In the solid electrolyte layer 30, if the average crystal grain size of the first solid electrolyte particles 31 is large, the thickness of the solid electrolyte layer 30 cannot be reduced, which may make it difficult to ensure sufficient energy density and responsiveness. Therefore, it is preferable to set an upper limit on the average crystal grain size of the first solid electrolyte particles 31. For example, the average crystal grain size of the first solid electrolyte particles 31 is preferably 20 μm or less, more preferably 10 μm or less, and even more preferably 7 μm or less.

[0033] The average crystal grain size of the first solid electrolyte particles 31 can be measured, for example, by measuring the horizontal or vertical Ferret diameter of 50 first solid electrolyte particles 31 identified by EDS mapping of the cross section of the solid electrolyte layer 30 and calculating the average value.

[0034] If the proportion of the first solid electrolyte particles 31 in the solid electrolyte layer 30 is high, it may be difficult to sinter the solid electrolyte layer 30 at a sufficiently low temperature. Therefore, it is preferable to set an upper limit to the proportion of the first solid electrolyte particles 31 in the solid electrolyte layer 30. For example, in the cross section of the solid electrolyte layer 30, the area proportion of the first solid electrolyte particles 31 is preferably 70% or less, more preferably 60% or less, and even more preferably 50% or less.

[0035] On the other hand, if the proportion of the second solid electrolyte particles 32 in the solid electrolyte layer 30 is high, there is a risk that sufficiently high ion conduction may not be obtained. Therefore, it is preferable to set an upper limit to the proportion of the second solid electrolyte particles 32 in the solid electrolyte layer 30. In this embodiment, the area proportion of the second solid electrolyte particles 32 in the cross section of the solid electrolyte layer 30 is preferably 80% or less, more preferably 70% or less, and even more preferably 60% or less.

[0036] In the cross section of the solid electrolyte layer 30, the area ratio of the first solid electrolyte particles 31 to the second solid electrolyte particles 32 is preferably 20:80 to 70:30, more preferably 30:70 to 60:40, and even more preferably 40:60 to 50:50.

[0037] The area ratio of the first solid electrolyte particles 31 to the second solid electrolyte particles 32 in the cross section of the solid electrolyte layer 30 can be measured, for example, by observing the cross section with an SEM and performing EDS element mapping analysis.

[0038] The second solid electrolyte particles 32 are not particularly limited as long as they contain lithium and have a lower sintering start temperature than the first solid electrolyte particles 31. For example, the second solid electrolyte particles 32 may be made of one or more of a Li-Ge-PO-based compound, a Li-Zr-PO-based compound, a Li-PO-based compound, a Li-BO-based compound, a Li-Si-O-based compound, or the like, or may contain a combination of these elements, or may further contain Al, Y, La, or the like. For example, the second solid electrolyte particles 32 may be a Li-Ge-Zr-PO-based compound, a Li-Si-BO-based compound, a Li-Al-Ge-PO-based compound, a Li-La-Zr-PO-based compound, or a Li-Al-PO-based compound. Among these, from the viewpoint of ion conductivity, it is preferable to use a Li-Al-Ge-PO-based compound as the second solid electrolyte particles 32.

[0039] The thickness of the solid electrolyte layer 30 is, for example, in the range of 2 μm to 25 μm, in the range of 4 μm to 20 μm, or in the range of 6 μm to 15 μm.

[0040] The positive electrode active material of the first electrode 10 is not particularly limited, but examples thereof include LiCoPO4, Li2CoP2O7, and Li6Co5(P2O7)4 when a phosphate compound is used for the second solid electrolyte particles 32. For the negative electrode active material of the second electrode 20, reference can be made to conventional techniques related to secondary batteries as appropriate, and examples thereof include compounds such as titanium oxide, lithium titanium composite oxide, lithium titanium composite phosphate, carbon, and lithium vanadium phosphate.

[0041] In the preparation of the first electrode layer 11 and the second electrode layer 21, in addition to these electrode active materials, a solid electrolyte having ion conductivity, or a conductive material (conductive additive) such as carbon or metal may be further added. For these components, an electrode layer paste can be obtained by uniformly dispersing a binder and a plasticizer in water or an organic solvent. Examples of metals that can be used as conductive additives include Pd, Ni, Cu, Fe, and alloys containing these.

[0042] The first current collector layer 12 and the second current collector layer 22 are mainly made of a conductive material. For example, the conductive material for the first current collector layer 12 and the second current collector layer 22 may be a metal, carbon, or the like.

[0043] 2 is a schematic cross-sectional view of a stacked-type all-solid-state battery 100a in which a plurality of battery units are stacked. The all-solid-state battery 100a includes a stacked chip 60 having a substantially rectangular parallelepiped shape. In the stacked chip 60, a first external electrode 40a and a second external electrode 40b are provided so as to contact two side surfaces, which are two of the four surfaces other than the top and bottom surfaces at the ends in the stacking direction. The two side surfaces may be two adjacent side surfaces or two side surfaces facing each other. In this embodiment, the first external electrode 40a and the second external electrode 40b are provided so as to contact two side surfaces facing each other (hereinafter referred to as two end surfaces).

[0044] In the following description, components having the same composition range, thickness range, and particle size distribution range as those of the all-solid-state battery 100 are denoted by the same reference numerals, and detailed description thereof will be omitted.

[0045] In the all-solid-state battery 100a, a plurality of first current collector layers 12 and a plurality of second current collector layers 22 are alternately stacked. Edges of the plurality of first current collector layers 12 are exposed at a first end face of the laminated chip 60, but are not exposed at a second end face. Edges of the plurality of second current collector layers 22 are exposed at a second end face of the laminated chip 60, but are not exposed at the first end face. As a result, the first current collector layers 12 and the second current collector layers 22 are alternately electrically connected to the first external electrode 40a and the second external electrode 40b.

[0046] A first electrode layer 11 is laminated on the first current collector layer 12. A solid electrolyte layer 30 is laminated on the first electrode layer 11. The solid electrolyte layer 30 extends from the first external electrode 40a to the second external electrode 40b. A second electrode layer 21 is laminated on the solid electrolyte layer 30. A second current collector layer 22 is laminated on the second electrode layer 21. Another second electrode layer 21 is laminated on the second current collector layer 22. Another solid electrolyte layer 30 is laminated on the second electrode layer 21. The solid electrolyte layer 30 extends from the first external electrode 40a to the second external electrode 40b. The first electrode layer 11 is laminated on the solid electrolyte layer 30. These laminate units are repeated in the all-solid-state battery 100a. As a result, the all-solid-state battery 100a has a structure in which a plurality of battery units are laminated.

[0047] Furthermore, if the first current collector layer 12 and the two first electrode layers 11 sandwiching it are considered as one electrode, and the second current collector layer 22 and the two second electrode layers 21 sandwiching it are considered as one electrode, then the laminated chip 60 can be said to have a structure in which multiple internal electrodes and multiple solid electrolyte layers are alternately stacked.

[0048] The all-solid-state battery 100a may not include a current collector layer. For example, as illustrated in Fig. 3, the first current collector layer 12 and the second current collector layer 22 may not be provided. In this case, the first electrode 10 is formed by the first electrode layer 11 alone, and the second electrode 20 is formed by the second electrode layer 21 alone.

[0049] Next, a description will be given of a method for manufacturing the all-solid-state battery 100a illustrated in Fig. 2. Fig. 4 is a diagram illustrating the flow of the method for manufacturing the all-solid-state battery 100a.

[0050] (Ceramic raw powder production process) First, a solid electrolyte powder for constituting the solid electrolyte layer 30 is prepared. The raw material powder for the first solid electrolyte particles 31 is synthesized, for example, by solid-phase synthesis. For example, Li3PO4, Ta2O5, and NH4H2PO4 are mixed in a molar ratio of 0.4:1.4:1 and heat-treated in air at approximately 900°C. The resulting reactants are mashed and mixed, and LiOH·H2O is added to the reactants so that the Li content is in excess, followed by mixing. This heat treatment is then performed at 950°C to 1300°C to synthesize a Li-Ta-PO-based compound that is confirmed to have a monoclinic crystal structure by XRD measurement. The Li-Ta-PO-based compound has a Li content ratio of 0.50 mol to 0.95 mol per mol of P in the synthesized compound. The Li-Ta-PO-based compound is then pulverized to the desired particle size using a wet ball mill.

[0051] A glassy precursor, which is a lithium-containing material and has a lower sintering start temperature than that of first solid electrolyte particles 31, is obtained by a conventionally known melt quenching method as the raw material powder for second solid electrolyte particles 32. For example, the raw materials Li2CO3, Al2O3, GeO2, and PO5 are mixed and heated to 1400°C to form a glass melt, which is then cast to produce glass, and the glass is then pulverized to the desired particle size in a dry ball mill.

[0052] (Solid electrolyte green sheet manufacturing process) Next, the resulting powder is uniformly dispersed in an aqueous or organic solvent together with a binder, dispersant, plasticizer, etc., and wet-pulverized to obtain a solid electrolyte slurry with a desired average particle size. A bead mill, wet jet mill, various kneaders, high-pressure homogenizer, etc. can be used for this process. A bead mill is preferred because it allows for simultaneous adjustment of particle size distribution and dispersion. A binder is added to the resulting solid electrolyte slurry to obtain a solid electrolyte paste. A solid electrolyte green sheet can be produced by coating the resulting solid electrolyte paste. The coating method is not particularly limited, and methods such as a slot die method, reverse coating method, gravure coating method, bar coating method, and doctor blade method can be used. The particle size distribution after wet-pulverization can be measured, for example, using a laser diffraction measurement device using a laser diffraction scattering method.

[0053] (Internal electrode paste manufacturing process) Next, an internal electrode paste for producing the first electrode layer 11 and the second electrode layer 21 is prepared. For example, the internal electrode paste can be obtained by uniformly dispersing a conductive additive, an electrode active material, a solid electrolyte material, a binder, a plasticizer, and the like in water or an organic solvent. The solid electrolyte paste described above may be used as the solid electrolyte material. Pd, Ni, Cu, Fe, alloys containing these, various carbon materials, and the like may also be used as the conductive additive. If the first electrode layer 11 and the second electrode layer 21 have different compositions, the respective internal electrode pastes may be prepared separately.

[0054] (Current collector paste manufacturing process) Next, a current collector paste is prepared for producing the above-described first current collector layer 12 and second current collector layer 22. For example, the current collector paste can be obtained by uniformly dispersing Pd powder, carbon black, plate-like graphite carbon, a binder, a dispersant, a plasticizer, and the like in water or an organic solvent.

[0055] (External electrode paste manufacturing process) Next, an external electrode paste for producing the above-described first external electrode 40 a and second external electrode 40 b is prepared. For example, the external electrode paste can be obtained by uniformly dispersing a conductive material, an electrode active material, a solid electrolyte, a binder, a plasticizer, etc. in water or an organic solvent.

[0056] (Lamination process) As illustrated in FIG. 5( a), an internal electrode paste 52 is printed on one side of a solid electrolyte green sheet 51, followed by a current collector paste 53, and then another internal electrode paste 52. A reverse pattern 54 is printed on the areas of the solid electrolyte green sheet 51 where the internal electrode paste 52 and current collector paste 53 are not printed. The reverse pattern 54 may be the same as that of the solid electrolyte green sheet 51. Multiple printed solid electrolyte green sheets 51 are stacked alternately in a shifted pattern, and cover sheets 55 made of multiple laminated solid electrolyte green sheets are pressed onto the top and bottom of the stacking direction to obtain a laminate. In this case, a roughly rectangular parallelepiped laminate is obtained so that pairs of internal electrode paste 52 and current collector paste 53 are exposed alternately on two end faces of the laminate. Next, as illustrated in FIG. 5( b), an external electrode paste 56 is applied to each of the two end faces by a dipping method or the like and dried. This results in a molded body for forming the all-solid-state battery 100a.

[0057] (Firing process) Next, the obtained laminate is fired. The firing conditions are not particularly limited, and may be in an oxidizing atmosphere or a non-oxidizing atmosphere, with the maximum temperature preferably being 500°C to 900°C, more preferably 600°C to 800°C. A step of maintaining the temperature in an oxidizing atmosphere at a temperature lower than the maximum temperature may be provided in order to thoroughly remove the binder before the maximum temperature is reached. In order to reduce process costs, it is desirable to fire at as low a temperature as possible. After firing, a re-oxidation treatment may be performed. Through the above steps, an all-solid-state battery 100a is produced.

[0058] For the all-solid-state battery 100a illustrated in FIG. 2, the step of applying the current collector paste 53 in the step of FIG. 5(a) may be omitted.

[0059] The first external electrode 40a and the second external electrode 40b may be baked after the firing step. Fig. 6 is a flow diagram illustrating a manufacturing method in this case. For example, instead of applying the external electrode paste 56 in the lamination step, the external electrode paste 56 is applied to two end surfaces of the laminated chip 60 obtained in the firing step, and then baked. This allows the first external electrode 40a and the second external electrode 40b to be formed.

[0060] According to this embodiment, Li-poor raw material powder is used for the synthesis of the first solid electrolyte particles 31. In this case, the monoclinic crystal structure is more likely to be stabilized. Therefore, even when mixed with a lithium-containing material that can be sintered at a low temperature and sintered, the reaction between the two components is suppressed, and a heterogeneous phase that could become a high-resistance component is less likely to form at the interface between the two, resulting in high ionic conductivity. As a result, a solid electrolyte layer 30 that exhibits high ionic conductivity can be sintered at a low temperature. [Example]

[0061] Hereinafter, all-solid-state batteries were fabricated according to the embodiments, and their characteristics were investigated.

[0062] Example 1 In Example 1, a crystalline Li-Ta-PO-based compound was synthesized by solid-phase synthesis. Specifically, Li3PO4, Ta2O5, and NH4H2PO4 were mixed in a molar ratio of 0.4:1.4:1 and heat-treated at 900°C in air. The resulting reaction mixture was crushed and mixed, and LiOH·H2O was added to the reaction mixture so that the Li content was 10 mol% excess. The mixture was then heat-treated at 1100°C to synthesize the compound. XRD analysis of the synthesized powder confirmed that it was a monoclinic single phase. In addition, ICP analysis of the synthesized powder revealed that the ratio of Li content to 1 mol of P was 0.88 mol. The synthesized powder was then pulverized in a wet ball mill to a diameter of D50 = 4 μm.

[0063] Next, a non-stoichiometric Li-Al-Ge-PO glass precursor was obtained by a conventional melt-quenching method. Specifically, the raw materials Li2CO3, Al2O3, GeO2, and P2O5 were mixed and molten at 1400°C to form a glass. The glass was then cast and pulverized to a D50 of 2 μm in a dry ball mill. The molar ratio of Li to P in the glass was 0.63.

[0064] A Li-Ta-PO compound and a non-stoichiometric Li-Al-Ge-PO glassy precursor were mixed in a weight ratio of 30:70, crushed, and then pelletized in a uniaxial press to a diameter of 15 mm and a thickness of 0.5 mm. The pellets were then fired at a top temperature of 650°C.

[0065] The ionic conductivity of the sintered body is 6.0×10 -5 The sintered body was fractured, and the cross section was observed with an SEM. EDS elemental mapping analysis revealed that the area ratio of the Li-Ta-PO compound to the Li-Al-Ge-PO compound was approximately 20:80. No reaction products were observed at the particle interfaces between these compounds.

[0066] Example 2 In Example 2, a crystalline Li-Ta-PO-based compound was synthesized by solid-phase synthesis. Specifically, Li3PO4, Ta2O5, and NH4H2PO4 were mixed in a molar ratio of 0.35:1.35:1 and heat-treated at 900°C in air. The resulting reaction mixture was crushed and mixed, and LiOH·H2O was added to the reaction mixture to achieve a 15 mol% excess of Li. The mixture was then heat-treated at 1300°C to synthesize the compound. XRD analysis of the synthesized powder confirmed that it was a monoclinic single-phase crystal. ICP analysis of the synthesized powder also revealed that the ratio of Li to 1 mol of P was 0.79 mol. The synthesized powder was then pulverized in a wet ball mill to a diameter of D50 = 4 μm.

[0067] Next, a non-stoichiometric Li-Al-Ge-PO glass precursor was obtained by a conventional melt-quenching method. Specifically, the raw materials Li2CO3, Al2O3, GeO2, and P2O5 were mixed and molten at 1400°C to form a glass. The glass was then cast and crushed to a D50 of 2 μm in a dry ball mill. The molar ratio of Li to P in the glass was 0.5.

[0068] A Li-Ta-PO compound and a non-stoichiometric Li-Al-Ge-PO glassy precursor were mixed in a weight ratio of 30:70, crushed, and then pelletized in a uniaxial press to a diameter of 15 mm and a thickness of 0.5 mm. The pellets were then fired at a top temperature of 650°C.

[0069] The ionic conductivity of the sintered body is 8.0×10 -5 The sintered body was fractured, and the cross section was observed with an SEM. EDS elemental mapping analysis revealed that the area ratio of the Li-Ta-PO compound to the Li-Al-Ge-PO compound was approximately 20:80. No reaction products were observed at the particle interfaces between these compounds.

[0070] Example 3 In Example 3, a crystalline Li—Ta—PO compound was synthesized and crushed in the same manner as in Example 1.

[0071] Next, a non-stoichiometric Li-Al-Ge-PO glass precursor was obtained by a conventional melt-quenching method. Specifically, the raw materials Li2CO3, Al2O3, GeO2, and P2O5 were mixed and molten at 1400°C to produce a glass. The glass was then cast and pulverized to a D50 of 2 μm in a dry ball mill. The molar ratio of Li to P in the glass was 0.4.

[0072] A Li-Ta-PO compound and a non-stoichiometric Li-Al-Ge-PO glassy precursor were mixed in a weight ratio of 30:70, crushed, and then pelletized in a uniaxial press to a diameter of 15 mm and a thickness of 0.5 mm. The pellets were then fired at a top temperature of 650°C.

[0073] The ionic conductivity of the sintered body is 2.0×10 -5 The sintered body was fractured, and the cross section was observed with an SEM. EDS elemental mapping analysis revealed that the area ratio of LiTa2PO8 to Li-Al-Ge-PO-based compounds was approximately 20:80. No reaction products were observed at the particle interfaces between these compounds.

[0074] Example 4 In Example 4, a crystalline Li-Ta-PO-based compound was synthesized by solid-phase synthesis. Specifically, Li3PO4, Ta2O5, and NH4H2PO4 were mixed in a molar ratio of 0.32:1.32:1 and heat-treated at 900°C in air. The resulting reaction mixture was crushed and mixed, and LiOH·H2O was added to the reaction mixture to achieve a 10 mol% excess of Li. The mixture was then heat-treated at 1300°C to synthesize the compound. XRD analysis of the synthesized powder confirmed that it was a monoclinic single phase. Furthermore, ICP analysis of the synthesized powder revealed that the ratio of Li content to 1 mol of P was 0.70 mol. The synthesized powder was then pulverized in a wet ball mill to a diameter of D50 = 4 μm.

[0075] Next, a non-stoichiometric Li-Al-Ge-PO-based glass precursor was obtained in the same manner as in Example 1.

[0076] A Li-Ta-PO compound and a non-stoichiometric Li-Al-Ge-PO glassy precursor were mixed in a weight ratio of 30:70, crushed, and then pelletized in a uniaxial press to a diameter of 15 mm and a thickness of 0.5 mm. The pellets were then fired at a top temperature of 650°C.

[0077] The ionic conductivity of the sintered body is 8.5×10 -5 The sintered body was fractured, and the cross section was observed with an SEM. EDS elemental mapping analysis revealed that the area ratio of LiTa2PO8 to Li-Al-Ge-PO-based compounds was approximately 20:80. No reaction products were observed at the particle interfaces between these compounds.

[0078] Example 5 In Example 5, a crystalline Li—Ta—PO compound was synthesized and crushed in the same manner as in Example 4.

[0079] Next, a non-stoichiometric Li-Al-Ge-PO-based glass precursor was obtained in the same manner as in Example 2.

[0080] A Li-Ta-PO compound and a non-stoichiometric Li-Al-Ge-PO glassy precursor were mixed in a weight ratio of 30:70, crushed, and then pelletized in a uniaxial press to a diameter of 15 mm and a thickness of 0.5 mm. The pellets were then fired at a top temperature of 650°C.

[0081] The ionic conductivity of the sintered body is 5.3 × 10 -5 The sintered body was fractured, and the cross section was observed with an SEM. EDS elemental mapping analysis revealed that the area ratio of LiTa2PO8 to Li-Al-Ge-PO-based compounds was approximately 20:80. No reaction products were observed at the particle interfaces between these compounds.

[0082] Example 6 In Example 6, a crystalline Li-Ta-PO compound was synthesized and pulverized in the same manner as in Example 1. LiOH·HO was further added to the synthesized powder so that the Li content was 7 mol% in excess, and the mixture was heat-treated at 700°C to incorporate the Li-poor portion into the crystal structure while maintaining the crystal structure. The synthesized powder was pulverized in a wet ball mill to a D50 of 4 μm.

[0083] Next, a non-stoichiometric Li-Al-Ge-PO-based glass precursor was obtained in the same manner as in Example 1.

[0084] A Li-Ta-PO compound and a non-stoichiometric Li-Al-Ge-PO glassy precursor were mixed in a weight ratio of 30:70, crushed, and then pelletized in a uniaxial press to a diameter of 15 mm and a thickness of 0.5 mm. The pellets were then fired at a top temperature of 650°C.

[0085] The ionic conductivity of the sintered body is 9.1 x 10 -5 The sintered body was fractured, and the cross section was observed with an SEM. EDS elemental mapping analysis revealed that the area ratio of LiTa2PO8 to Li-Al-Ge-PO-based compounds was approximately 20:80. No reaction products were observed at the particle interfaces between these compounds.

[0086] Example 7 In Example 7, a crystalline Li—Ta—PO compound was synthesized and crushed in the same manner as in Example 1.

[0087] Next, a non-stoichiometric Li-Al-Ge-PO-based glass precursor was obtained in the same manner as in Example 1.

[0088] A Li-Ta-PO4 compound and a non-stoichiometric Li-Al-Ge-PO4 glass precursor (LAGP-g) were mixed in a weight ratio of 30:70. 2 wt% of Li3PO4 was added to the Li-Al-Ge-PO4 glass precursor (LAGP-g). The resulting mixed powder was pelletized in a uniaxial press to a diameter of 15 mm and a thickness of 0.5 mm, and then fired at a top temperature of 650°C.

[0089] The ionic conductivity of the sintered body is 9.2 × 10 -5 The sintered body was fractured, and the cross section was observed with an SEM. EDS elemental mapping analysis revealed that the area ratio of LiTa2PO8 to Li-Al-Ge-PO-based compounds was approximately 20:80. No reaction products were observed at the particle interfaces between these compounds.

[0090] (Comparative Example 1) In Comparative Example 1, a crystalline Li-Ta-PO-based compound was synthesized by solid-phase synthesis. Specifically, Li3PO4, Ta2O5, and NH4H2PO4 were mixed in a molar ratio of 0.5:1.5:1 and heat-treated at 900°C in air. The resulting reaction mixture was crushed and mixed, and LiOH·H2O was added to the reaction mixture to achieve a 20 mol% excess of Li. The mixture was then heat-treated at 1300°C to synthesize the compound. XRD analysis of the synthesized powder confirmed that Ta2O5 was formed in addition to the synthesized powder LiTa2PO8. Furthermore, ICP analysis of the synthesized powder revealed that the ratio of Li content to 1 mol of P was 1.05 mol. The resulting powder was then pulverized in a wet ball mill to a D50 of 4 μm.

[0091] Next, a non-stoichiometric Li-Al-Ge-PO glass precursor was obtained by a conventional melt-quenching method. Specifically, the raw materials Li2CO3, Al2O3, GeO2, and P2O5 were mixed and molten at 1400°C to form a glass. The glass was then cast and pulverized to a D50 of 2 μm in a dry ball mill. The molar ratio of Li to P in the glass was 0.63.

[0092] A Li-Ta-PO compound and a non-stoichiometric Li-Al-Ge-PO glassy precursor were mixed in a weight ratio of 30:70, crushed, and then pelletized in a uniaxial press to a diameter of 15 mm and a thickness of 0.5 mm. The pellets were then fired at a top temperature of 650°C.

[0093] The ionic conductivity of the sintered body is 5.0×10 -6 The sintered body was fractured, and the cross section was observed with an SEM. EDS elemental mapping analysis revealed that the area ratio of LiTa2PO8 to Li-Al-Ge-PO-based compounds was approximately 20:80. Reaction products were formed at the particle interfaces between these compounds.

[0094] (Comparative Example 2) In Comparative Example 2, a crystalline Li-Ta-PO-based compound was synthesized by solid-phase synthesis. Specifically, Li3PO4, Ta2O5, and NH4H2PO4 were mixed in a molar ratio of 0.46:1.46:1 and heat-treated at 900°C in air. The resulting reaction mixture was crushed and mixed, and LiOH·H2O was added to the reaction mixture so that the Li content was 10 mol% excess. The mixture was then heat-treated at 1000°C to synthesize the compound. XRD analysis of the synthesized powder confirmed that Li(PO4) was formed in addition to the synthesized powder LiTa2PO8. The ratio of Li content to 1 mol of P was 1.04 mol. The obtained powder was pulverized in a wet ball mill to a D50 of 4 μm.

[0095] Next, a non-stoichiometric Li-Al-Ge-PO glass precursor was obtained by a conventional melt-quenching method. Specifically, the raw materials Li2CO3, Al2O3, GeO2, and P2O5 were mixed and molten at 1400°C to form a glass. The glass was then cast and pulverized to a D50 of 2 μm in a dry ball mill. The molar ratio of Li to P in the glass was 0.63.

[0096] A Li-Ta-PO compound and a non-stoichiometric Li-Al-Ge-PO glassy precursor were mixed in a weight ratio of 30:70, crushed, and then pelletized in a uniaxial press to a diameter of 15 mm and a thickness of 0.5 mm. The pellets were then fired at a top temperature of 650°C.

[0097] The ionic conductivity of the sintered body is 8.0×10 -6 The sintered body was fractured, and the cross section was observed with an SEM. EDS elemental mapping analysis revealed that the area ratio of LiTa2PO8 to Li-Al-Ge-PO-based compounds was approximately 20:80. Reaction products were formed at the particle interfaces between these compounds.

[0098] The results of Examples 1 to 7 and Comparative Examples 1 and 2 are shown in Table 1. [Table 1]

[0099] From the above results, high ionic conductivity was obtained in Examples 1 to 7. This is thought to be because reaction products with the Li-Al-Ge-PO-based compound, which can be sintered at low temperatures, were not formed at the interface of the LiTa2PO8 particles. The reason why reaction products were not formed is thought to be because the monoclinic crystal structure was stabilized by making the Li-Ta-PO-based compound lithium-poor relative to LiTa2PO8.

[0100] High ionic conductivity was not obtained in Comparative Examples 1 and 2. This is thought to be because the monoclinic crystal structure was not stabilized due to the lithium-rich Li-Ta-PO compound compared to LiTa2PO8, and reaction products were formed at the particle interfaces between these compounds.

[0101] Although the embodiments of the present invention have been described in detail above, the present invention is not limited to such specific embodiments, and various modifications and variations are possible within the scope of the gist of the present invention as defined in the claims. [Explanation of symbols]

[0102] 10 1st electrode 11 First electrode layer 12 First current collector layer 20 2nd electrode 21 Second electrode layer 22 Second current collector layer 30 Solid electrolyte layer 40a First outer electrode 40b 2nd external electrode 51 Solid electrolyte green sheet 52 Internal electrode paste 53 Current collector paste 54 Reverse Pattern 55 Cover Sheet 56 External electrode paste 100,100a all solid state battery

Claims

1. An oxide-type solid electrolyte containing Li, Ta, and P, which is Li-poor with respect to a stoichiometric composition, The ratio of the Li content to 1 mol of P is 0.5 mol or more and 0.95 mol or less, A solid electrolyte characterized in that the result of XRD measurement confirms that the solid electrolyte has a crystal structure belonging to a monoclinic system.

2. 2. The solid electrolyte according to claim 1, wherein the molar ratio of Ta / P is 1.7 or more and 2.3 or less.

3. A solid electrolyte as described in claim 1 or claim 2, characterized in that the solid electrolyte is a solid electrolyte consisting of Li, Ta, P, and O.

4. A solid electrolyte layer comprising the solid electrolyte according to any one of claims 1 to 3 as a first solid electrolyte and a lithium-containing material having a lower sintering start temperature than the first solid electrolyte as a second solid electrolyte; a first electrode including an electrode active material and formed on a first main surface of the solid electrolyte layer; a second electrode including an electrode active material and formed on a second main surface of the solid electrolyte layer opposite to the first main surface.

5. 5. The all-solid-state battery according to claim 4, wherein a plurality of units each including the solid electrolyte layer, the first electrode, and the second electrode are stacked.

6. 6. The all-solid-state battery according to claim 4, wherein one of the first electrode and the second electrode contains a positive electrode active material, and the other contains a negative electrode active material.

7. 7. The all-solid-state battery according to claim 4, wherein in the solid electrolyte layer, the average crystal grain size of the first solid electrolyte is 1 μm or more and 20 μm or less.

8. 8. The all-solid-state battery according to claim 4, wherein the lithium-containing material is at least one of a Li-Ge-P-O based compound, a Li-Zr-P-O based compound, a Li-P-O based compound, a Li-B-O based compound, a Li-Si-O based compound, a Li-Ge-Zr-P-O based compound, a Li-Si-B-O based compound, a Li-Al-Ge-P-O based compound, a Li-La-Zr-P-O based compound, and a Li-Al-P-O based compound.

9. The all-solid-state battery according to claim 4, wherein the second solid electrolyte is a Li-Al-Ge-P-O based compound.

10. A method for producing a solid electrolyte, comprising synthesizing an oxide-type solid electrolyte, the solid electrolyte being confirmed to have a crystalline structure belonging to a monoclinic system as a result of XRD measurement, at a temperature of 950°C or higher and 1300°C or lower, from a raw material containing Li, Ta, and P, which is Li-poor with respect to a stoichiometric composition, and in which the Li content per 1 mol of P is 0.5 mol or higher and 0.95 mol or lower.

11. preparing a laminate including: a green sheet including, as a first solid electrolyte powder, a powder of an oxide-type solid electrolyte that contains Li, Ta, and P, is Li-poor with respect to a stoichiometric composition, has a Li content of 0.5 mol or more and 0.95 mol or less per 1 mol of P, and is confirmed to have a crystal structure belonging to a monoclinic system as a result of XRD measurement; and a green sheet including, as a second solid electrolyte powder, a powder of a lithium-containing material that has a lower sintering start temperature than the first solid electrolyte powder; a paste coating for a first electrode layer formed on a first main surface of the green sheet; and a paste coating for a second electrode layer formed on a second main surface of the green sheet; and a firing step of firing the laminate.

12. The method for producing an all-solid-state battery according to claim 11, wherein the firing temperature in the firing step is 500°C or higher and 900°C or lower.

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