Solid electrolyte sheet for all-solid-state sodium ion secondary battery, all-solid-state secondary battery, and method for manufacturing solid electrolyte sheet for all-solid-state sodium ion secondary battery

Abstract

To provide a solid electrolyte sheet that can enhance adhesion to an electrode layer and can provide excellent discharge capacity.SOLUTION: A solid electrolyte sheet 10 includes a first solid electrolyte layer 1 and a second solid electrolyte layer 2 formed on at least one surface of the first solid electrolyte layer. In the solid electrolyte sheet 10, the second solid electrolyte layer 2 is a porous solid electrolyte layer.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a solid electrolyte sheet, which is a component of an all-solid-state battery used in portable electronic devices, electric vehicles, etc. [Background technology]

[0002] Lithium-ion secondary batteries have established a position as a high-capacity, lightweight power source that is indispensable for mobile devices, electric vehicles, and the like. Current lithium-ion secondary batteries mainly use flammable organic electrolyte solutions as electrolytes, raising concerns about the risk of fire, etc. As a way to solve this problem, development is underway of all-solid-state lithium-ion batteries that use solid electrolytes instead of organic electrolyte solutions (see, for example, Patent Document 1).

[0003] In addition, because of concerns about the global rise in the price of lithium as a raw material, sodium is also attracting attention as an alternative material to lithium, and NASICON-type Na3Zr2Si2PO 12 A sodium ion all-solid-state battery using a sodium ion conductive crystal made of the above has been proposed (see, for example, Patent Document 2). In addition, β-alumina (theoretical composition formula: Na2O·11Al2O3), β"-alumina (theoretical composition formula: Na2O·5.3Al2O3), Li2O-stabilized β"-alumina (Na 1.7 Li 0.3 Al 10.7 O 17 ), MgO-stabilized β''-alumina ((Al 10.32 Mg 0.68 O 16 )(Na 1.68 O)) and beta-alumina-based solid electrolytes such as NaYSiO 12 It is known that these solid electrolytes also exhibit high sodium ion conductivity, and these solid electrolytes can also be used for sodium ion all-solid-state batteries.

[0004] In all-solid-state batteries, it is important to reduce the interfacial resistance between the electrode layer and the solid electrolyte layer in order to increase the discharge capacity. Therefore, a technique for increasing the surface roughness of the solid electrolyte layer has been proposed to improve the adhesion between the two layers (see, for example, Patent Document 3). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 5-205741 [Patent Document 2] Japanese Patent Application Laid-Open No. 2010-15782 [Patent Document 3] International Publication No. 2015 / 128982 Summary of the Invention [Problem to be solved by the invention]

[0006] However, simply increasing the surface roughness of the solid electrolyte layer is not enough to sufficiently increase the discharge capacity. In particular, if the thickness of the electrode layer is increased, there is a risk that the electrode layer will peel off from the solid electrolyte layer during the firing process in the production of the all-solid-state battery, making it impossible to charge and discharge the battery.

[0007] In view of the above, an object of the present invention is to provide a solid electrolyte sheet that can improve adhesion to an electrode layer and can obtain an excellent discharge capacity. [Means for solving the problem]

[0008] As a result of extensive research, the present inventors have found that the above problems can be solved by using a solid electrolyte sheet having a specific structure.

[0009] That is, the solid electrolyte sheet of the present invention is a solid electrolyte sheet in which a second solid electrolyte layer is formed on at least one surface of a first solid electrolyte layer, and the second solid electrolyte layer is a porous solid electrolyte layer.

[0010] In the solid electrolyte sheet of the present invention, the second solid electrolyte layer is preferably a porous solid electrolyte layer having three-dimensionally interconnected pores. This allows the material constituting the electrode layer to easily penetrate the pores in the second solid electrolyte layer when an electrode layer is formed on the second solid electrolyte layer, resulting in strong adhesion between the electrode layer and the solid electrolyte sheet. This increases the contact area between the electrode layer and the solid electrolyte sheet, reducing the interfacial resistance between the electrode layer and the solid electrolyte layer. Furthermore, during the firing process in the production of an all-solid-state battery, the electrode layer is less likely to peel off from the solid electrolyte layer due to the anchor effect. As a result, an all-solid-state battery with excellent discharge capacity can be obtained.

[0011] In the solid electrolyte sheet of the present invention, when a straight line drawn along the surface of the first solid electrolyte layer is taken as a reference line and a curve drawn along the surface of the second solid electrolyte layer is taken as a contour line in a cross-sectional image of the vicinity of the interface between the first solid electrolyte layer and the second solid electrolyte layer, the ratio of the length of the contour line to the length of the reference line (contour line length / reference line length) is preferably 1.3 to 50. The ratio of the length of the contour line to the length of the reference line defined in this manner is a parameter that serves as an indicator of how three-dimensionally interconnected voids are formed in the second solid electrolyte layer. When this ratio is within the above range, three-dimensionally interconnected voids are well formed in the second solid electrolyte layer, enabling the electrode layer and the solid electrolyte sheet to be firmly adhered to each other.

[0012] In the solid electrolyte sheet of the present invention, the second solid electrolyte layer is preferably composed of multiple layers with different porosities. In particular, it is preferable that the multiple layers with different porosities have lower porosity as the layers are closer to the first solid electrolyte layer. In this way, peeling at the interface with the first solid electrolyte layer 1 can be suppressed.

[0013] The solid electrolyte sheet of the present invention has a second solid electrolyte layer of 1 cm in plan view. 2 The surface area per unit is 3cm 2It is preferable that the surface area of ​​the second solid electrolyte layer defined in this manner is also an indicator of how three-dimensionally interconnected voids are formed in the second solid electrolyte layer. When the surface area is within the above range, three-dimensionally interconnected voids are well formed in the second solid electrolyte layer, the contact area between the electrode layer and the solid electrolyte sheet is increased, and the adhesion between them is improved, enabling them to be firmly bonded. Therefore, the interfacial resistance between the electrode layer and the solid electrolyte sheet is reduced, and as a result, a battery with excellent discharge capacity can be obtained.

[0014] In the solid electrolyte sheet of the present invention, the second solid electrolyte layer preferably has an arithmetic mean roughness Ra of 2.5 μm or more, which can further enhance the adhesion between the electrode layer and the solid electrolyte sheet.

[0015] In the solid electrolyte sheet of the present invention, it is preferable that the second solid electrolyte layer is formed on both sides of the first solid electrolyte layer, which allows both the positive electrode layer and the negative electrode layer to be firmly adhered to the solid electrolyte sheet.

[0016] The solid electrolyte sheet of the present invention preferably has a thickness of 2400 μm or less. A smaller thickness of the solid electrolyte sheet is preferable because the distance required for ion conduction in the solid electrolyte is shorter, thereby improving ion conductivity. Furthermore, when used as a solid electrolyte for an all-solid-state battery, the energy density per unit volume of the all-solid-state battery is increased.

[0017] In the solid electrolyte sheet of the present invention, the first solid electrolyte layer and / or the second solid electrolyte layer preferably contains at least one selected from β''-alumina, β-alumina, and NASICON crystals.

[0018] The solid electrolyte sheet of the present invention can be used, for example, for an all-solid-state sodium ion secondary battery.

[0019] The all-solid-state secondary battery of the present invention is characterized by comprising the above-mentioned solid electrolyte sheet and an electrode layer formed on the surface of the second solid electrolyte layer of the solid electrolyte sheet.

[0020] In the all-solid-state secondary battery of the present invention, the material constituting the electrode layer preferably fills the voids in the second solid electrolyte layer, thereby improving the adhesion between the electrode layer and the second solid electrolyte layer.

[0021] The method for producing a solid electrolyte sheet of the present invention is a method for producing the above-mentioned solid electrolyte sheet, and is characterized by comprising the steps of: (a) adding an organic vehicle containing a binder to a solid electrolyte powder and / or a raw material powder of the solid electrolyte powder to prepare a slurry, and applying the slurry to a substrate and drying it to obtain a green sheet for a first solid electrolyte layer; (b) adding an organic vehicle containing a binder to a mixed powder containing the solid electrolyte powder and / or a raw material powder of the solid electrolyte powder and a polymer powder to prepare a slurry, and applying the slurry to a substrate and drying it to obtain a green sheet for a second solid electrolyte layer; (c) laminating a green sheet for a second solid electrolyte layer on at least one surface of the green sheet for the first solid electrolyte layer to obtain a laminate; and (d) firing the laminate to remove the binder from the green sheet for the first solid electrolyte layer to form a first solid electrolyte layer, and to remove the binder and polymer particles from the green sheet for the second solid electrolyte layer to form a second solid electrolyte layer. This makes it possible to easily produce a solid electrolyte sheet in which a porous second solid electrolyte layer having three-dimensionally interconnected pores is formed on at least one surface of a first solid electrolyte layer.

[0022] The method for producing a solid electrolyte sheet of the present invention is a method for producing the above-mentioned solid electrolyte sheet, and is characterized by comprising the steps of: (a) preparing a first solid electrolyte layer; (b) adding an organic vehicle containing a binder to a mixed powder containing a solid electrolyte powder and / or a raw material powder of the solid electrolyte powder and a polymer powder to prepare a slurry; (c) applying the slurry to at least one surface of the first solid electrolyte layer to obtain a laminate in which a slurry layer is formed on the surface of the first solid electrolyte layer; and (d) firing the laminate to remove the binder and polymer particles in the slurry layer to form a second solid electrolyte layer. This production method also makes it possible to easily produce a solid electrolyte sheet in which a porous second solid electrolyte layer having three-dimensionally interconnected pores is formed on at least one surface of the first solid electrolyte layer.

[0023] In the method for producing a solid electrolyte sheet of the present invention, the polymer powder preferably has an average particle size of 0.1 to 100 μm.

[0024] In the method for producing a solid electrolyte sheet of the present invention, the content ratio of the solid electrolyte powder and / or raw material powder of the solid electrolyte powder to the polymer powder is preferably 75:25 to 3:97 by volume. [Effects of the Invention]

[0025] According to the present invention, it is possible to provide a solid electrolyte sheet that can enhance adhesion to an electrode layer and can obtain an excellent discharge capacity. [Brief explanation of the drawings]

[0026] [Figure 1] 1 is a schematic cross-sectional view showing one embodiment of a solid electrolyte sheet of the present invention. [Figure 2]1 shows cross-sectional images of the vicinity of the interface between the first solid electrolyte layer and the second solid electrolyte layer in the solid electrolyte sheet of Example 1. (a) shows a reference line that is a straight line drawn along the surface of the first solid electrolyte layer, and (b) shows a contour line that is a curved line drawn along the surface of the second solid electrolyte layer. DETAILED DESCRIPTION OF THE INVENTION

[0027] Hereinafter, embodiments of the solid electrolyte sheet of the present invention will be described in detail with reference to the drawings.

[0028] 1 is a schematic cross-sectional view showing one embodiment of a solid electrolyte sheet of the present invention. The solid electrolyte sheet 10 of the present invention includes a first solid electrolyte layer 1 and a second solid electrolyte layer 2 formed on one surface thereof. The second solid electrolyte layer is a porous solid electrolyte layer having voids 2v three-dimensionally connected to the solid electrolyte 2s.

[0029] When an all-solid-state battery is fabricated using the solid electrolyte sheet 10, an electrode layer (anode layer or cathode layer) is formed on both sides of the solid electrolyte sheet 10. Specifically, the electrode layer is formed on the surface of the main surface 1b of the first solid electrolyte layer 1 opposite the second solid electrolyte layer 2 and on the surface of the main surface 2a of the second solid electrolyte layer 2 opposite the first solid electrolyte layer 1. Here, the second solid electrolyte layer has three-dimensionally interconnected voids 2v, which facilitates penetration of the material (e.g., active material powder) constituting the electrode layer into the voids 2v, thereby firmly adhering the electrode layer to the second solid electrolyte layer 2. This increases the contact area between the electrode layer and the solid electrolyte sheet 10 (second solid electrolyte layer 2), increasing the ion conduction path and reducing the interfacial resistance between the electrode layer and the solid electrolyte sheet 10. Furthermore, during the firing process in the fabrication of the all-solid-state battery, the electrode layer is less likely to peel off from the solid electrolyte layer 10 due to the anchor effect. As a result, an all-solid-state battery with excellent discharge capacity can be obtained.

[0030] Furthermore, when the electrode layer is made of a low-melting-point material such as metallic sodium, the electrode layer may soften and flow during fabrication of the all-solid-state battery or during charge and discharge, and may infiltrate from the side of the solid electrolyte sheet 10 toward the counter electrode layer, resulting in a short circuit. However, in the solid electrolyte sheet 10 of this embodiment, the softened and flowed low-melting-point material enters the voids 2v in the second solid electrolyte layer 2, which has the advantage of making it less likely for the low-melting-point material to infiltrate into the counter electrode layer and cause a short circuit. Furthermore, because the relatively dense first solid electrolyte layer 1 acts as a barrier, it is less likely for the low-melting-point material to reach the counter electrode layer from inside the solid electrolyte sheet 10 and cause a short circuit.

[0031] In a cross-sectional image of the vicinity of the interface between the first solid electrolyte layer 1 and the second solid electrolyte layer 2, a straight line drawn along the surface of the first solid electrolyte layer 1 is defined as a reference line, and a curve drawn along the surface of the second solid electrolyte layer 2 is defined as a contour line. The ratio of the length of the contour line to the length of the reference line (contour line length / reference line length) is preferably 1.3 to 50, 1.5 to 20, 1.8 to 10, and particularly preferably 2 to 5 (see the Examples and FIG. 2 described later). The ratio of the length of the contour line to the length of the reference line defined in this manner is a parameter that serves as an indicator of the formation of three-dimensionally interconnected voids 2v in the second solid electrolyte layer 2. If this ratio is too small, the three-dimensionally interconnected voids 2v are not sufficiently formed in the second solid electrolyte layer 2, which tends to result in poor adhesion between the electrode layer and the solid electrolyte sheet 10. On the other hand, if this ratio is too large, the mechanical strength of the second solid electrolyte layer 2 tends to be poor.

[0032] 1 cm in plan view of the second solid electrolyte layer 2 The surface area per unit is 3cm 2 More than 5cm 2 More than 7cm 2 Above, especially 10cm 2If the surface area is too small, the three-dimensionally interconnected voids 2v are not sufficiently formed in the second solid electrolyte layer 2, the contact area between the electrode layer and the solid electrolyte sheet 10 is small, and the adhesion between them tends to be poor. On the other hand, if the surface area is too large, the mechanical strength of the second solid electrolyte layer 2 tends to be poor, so it is preferable to set the surface area to 30 cm or more. 2 It is preferable that the surface area is equal to or less than 1000 nm. The surface area can be determined by the method described in the examples below.

[0033] In this embodiment, the second solid electrolyte layer 2 is formed on only one surface of the first solid electrolyte layer 1, but the second solid electrolyte layer 2 may be formed on both surfaces of the first solid electrolyte layer 1. In this way, both surfaces of the solid electrolyte sheet 10 are formed with the second solid electrolyte layer 2, so that both the positive electrode layer and the negative electrode layer can be firmly adhered to the solid electrolyte sheet.

[0034] A smaller thickness of the solid electrolyte sheet 10 is preferable because it shortens the distance required for ion conduction in the solid electrolyte and improves ion conductivity. Furthermore, when used as a solid electrolyte sheet for an all-solid-state battery, the energy density per unit volume of the all-solid-state battery increases. Specifically, the thickness of the solid electrolyte sheet 10 is preferably 2400 μm or less, 2000 μm or less, 1500 μm or less, 1000 μm or less, 500 μm or less, 400 μm or less, 300 μm or less, and particularly preferably 200 μm or less. However, if the thickness of the solid electrolyte sheet 10 is too small, the mechanical strength may decrease or the positive and negative electrodes may short-circuit. Therefore, a thickness of 5 μm or more, 10 μm or more, 20 μm or more, and particularly preferably 30 μm or more is preferable.

[0035] Each component will be described in detail below.

[0036] (First solid electrolyte layer 1) The first solid electrolyte layer 1 mainly serves as a base layer for ensuring the mechanical strength of the solid electrolyte sheet 10. Therefore, it is preferable that the first solid electrolyte layer 1 has a denser structure than the second solid electrolyte layer 2. In other words, it is preferable that the first solid electrolyte layer 1 has a smaller porosity than the second solid electrolyte layer 2. Specifically, it is preferable that the first solid electrolyte layer 1 has a porosity defined by the following formula of 20% or less, 10% or less, particularly 5% or less.

[0037] Porosity=(1-p / p0)×100(%) p: bulk density, p0: true density

[0038] When the solid electrolyte sheet 10 is used for an all-solid-state sodium ion secondary battery, the first solid electrolyte layer 1 preferably contains at least one selected from β"-alumina, β-alumina, and NASICON crystal. Specific examples of β"-alumina include trigonal (Al 10.35 Mg 0.65 O 16 )(Na 1.65 O), (Al 8.87 Mg 2.13 O 16 )(Na 3.13 O), Na 1.67 Mg 0.67 Al 10.33 O 17 , Na 1.49 Li 0.25 Al 10.75 O 17 , Na 1.72 Li 0.3 Al 10.66 O 17 , Na 1.6 Li 0.34 Al 10.66 O 17 In addition to β''-alumina, β-alumina may also be contained. β-alumina includes hexagonal (Al 10.35 Mg 0.65 O 16 )(Na 1.65 O), (Al 10.37 Mg 0.63 O 16 )(Na 1.63O), NaAl 11 O 17 , (Al 10.32 Mg 0.68 O 16 )(Na 1.68 O) are examples.

[0039] A specific composition of β''-alumina may contain, in mole percent, 65 to 98% Al2O3, 2 to 20% Na2O, 0.3 to 15% MgO+Li2O, 20 to 20% ZrO, and 0 to 5% Y2O. The reasons for limiting the composition as above are explained below.

[0040] Al2O3 is the main component constituting β''-alumina. The Al2O3 content is preferably 65 to 98%, and more preferably 70 to 95%. If the Al2O3 content is too low, the ionic conductivity of the solid electrolyte tends to decrease. On the other hand, if the Al2O3 content is too high, α-alumina that does not have sodium ion conductivity remains, and the ionic conductivity of the solid electrolyte tends to decrease.

[0041] Na2O is a component that imparts sodium ion conductivity to the solid electrolyte. The Na2O content is preferably 2 to 20%, 3 to 18%, and particularly preferably 4 to 16%. If the Na2O content is too low, it becomes difficult to obtain the above effects. On the other hand, if the Na2O content is too high, the excess sodium forms compounds such as NaAlO2 that do not contribute to ion conductivity, which tends to reduce ion conductivity.

[0042] MgO and Li2O are components (stabilizers) that stabilize the β''-alumina structure. The content of MgO + Li2O is preferably 0.3 to 15%, 0.5 to 10%, and particularly preferably 0.8 to 8%. If the amount of MgO + Li2O is too small, α-alumina will remain in the solid electrolyte, and ionic conductivity will tend to decrease. On the other hand, if the amount of MgO + Li2O is too large, MgO or Li2O that did not function as a stabilizer will remain in the solid electrolyte, and ionic conductivity will tend to decrease.

[0043] ZrO2 and Y2O3 have the effect of suppressing abnormal grain growth of β''-alumina during firing and improving the adhesion between the individual β''-alumina particles. As a result, the ionic conductivity of the solid electrolyte sheet is likely to be improved. The ZrO2 content is preferably 0 to 15%, 1 to 13%, and particularly 2 to 10%. The Y2O3 content is preferably 0 to 5%, 0.01 to 4%, and particularly 0.02 to 3%. If the amount of ZrO2 or Y2O3 is too large, the amount of β''-alumina produced decreases, and the ionic conductivity of the solid electrolyte is likely to decrease.

[0044] NASICON crystals are of the general formula Na s A1 t A2 u O v (A1 is at least one selected from Al, Y, Yb, Nd, Nb, Ti, Hf and Zr, A2 is at least one selected from Si and P, s=1.4 to 5.2, t=1 to 2.9, u=2.8 to 4.1, v=9 to 14). Here, A1 is preferably at least one selected from Y, Nb, Ti and Zr. By doing so, a crystal with excellent ionic conductivity can be obtained.

[0045] The preferred ranges of the coefficients in the above general formula are as follows:

[0046] Preferably, s is 1.4 to 5.2, 2.5 to 3.5, and particularly preferably 2.8 to 3.1. If s is too small, the amount of sodium ions decreases, which tends to reduce ionic conductivity. On the other hand, if s is too large, the excess sodium forms compounds such as sodium phosphate and sodium silicate that do not contribute to ionic conduction, which tends to reduce ionic conductivity.

[0047] t is preferably 1 to 2.9, 1 to 2.5, and particularly preferably 1.3 to 2. If t is too small, the three-dimensional network structure in the crystal is reduced, which tends to reduce ionic conductivity. On the other hand, if t is too large, compounds such as zirconia and alumina that do not contribute to ionic conduction are formed, which tends to reduce ionic conductivity.

[0048] Preferably, u is 2.8 to 4.1, 2.8 to 4, 2.9 to 3.2, and particularly preferably 2.95 to 3.1. If u is too small, the three-dimensional network structure in the crystal decreases, and ionic conductivity tends to decrease. On the other hand, if u is too large, crystals that do not contribute to ionic conduction are formed, and ionic conductivity tends to decrease.

[0049] v is preferably 9 to 14, 9.5 to 12, and particularly preferably 11 to 12. If v is too small, A1 (for example, aluminum component) will have a low valence, which tends to reduce electrical insulation. On the other hand, if v is too large, a peroxidized state will occur, and sodium ions will be bound by the lone electron pairs of oxygen atoms, which tends to reduce ionic conductivity.

[0050] The NASICON crystal is preferably a monoclinic crystal, a hexagonal crystal, or a trigonal crystal, and particularly preferably a monoclinic or trigonal crystal, because of its excellent ionic conductivity.

[0051] A specific example of NASICON crystal is Na3Zr2Si2PO 12 , Na 3.2 Zr 1.3 Si 2.2 P 0.8 O 10.5 , Na3Zr 1.6 Ti 0.4 SiPO 12 , Na3Hf2Si2PO 12 , Na 3.4 Zr 0.9 Hf 1.4 Al 0.6 Si 1.2 P 1.8 O 12 , Na3Zr 1.7 Nb 0.24 SiPO 12 , Na 3.6 Ti 0.2 Y 0.8 Si 2.8 O9, Na3Zr 1.88 Y 0.12 SiPO 12 , Na 3.12 Zr 1.88 Y0.12 SiPO 12 , Na 3.05 Zr2Si 2.06 P 0.95 O 12 , Na 3.6 Zr 0.13 Yb 1.67 Si 0.11 P 2.9 O 12 , Na5YSi4O 12 Crystals such as Na 3.12 Zr 1.88 Y 0.12 SiPO 12 and Na 3.05 Zr2Si 2.06 P 0.95 O 12 is preferred because it has excellent sodium ion conductivity.

[0052] When the solid electrolyte sheet 10 is used for an all-solid-state lithium ion secondary battery, the first solid electrolyte layer 1 is made of La 0.51 Li 0.34 Ti 2.94 , Li 1.3 Al 0.3 Ti 1.7 (PO4)3, Li7La3Zr2O 12 , Li 1.07 Al 0.69 Ti 1.46 (PO4)3, Li 1.5 Al 0.5 Ge 1.5 It is preferable that the resin contains at least one selected from (PO4)3.

[0053] The thickness of the first solid electrolyte layer 1 is preferably 4 to 400 μm, 10 to 300 μm, or 20 to 200 μm, and particularly preferably 30 to 100 μm. If the thickness of the first solid electrolyte layer 1 is too small, the mechanical strength may decrease and the positive electrode and negative electrode may short-circuit. On the other hand, if the thickness of the first solid electrolyte layer 1 is too large, the ionic conductivity of the solid electrolyte sheet 10 may decrease. In addition, the energy density per unit volume of the all-solid-state battery tends to increase.

[0054] (Second solid electrolyte layer 2) As described above, the second solid electrolyte layer 2 is a porous solid electrolyte layer having three-dimensionally interconnected voids 2v. The porosity of the second solid electrolyte layer 2 is preferably 30% or more, 50% or more, 60% or more, and particularly 70% or more. If the porosity of the second solid electrolyte layer 2 is too small, it becomes difficult to form three-dimensionally interconnected voids 2v, which tends to result in poor adhesion between the electrode layer and the solid electrolyte sheet 10. There is no particular upper limit to the porosity of the second solid electrolyte layer 2, but in practice it is 99% or less, or even 97% or less.

[0055] The degree of porosity of the second solid electrolyte layer 2 can also be evaluated from a different perspective than porosity, using the porosity defined below. The porosity of the second solid electrolyte layer 2 is preferably 20% or more, 25% or more, and particularly 30% or more. If the porosity of the second solid electrolyte layer 2 is too low, it becomes difficult to form three-dimensionally interconnected voids 2v, which tends to result in poor adhesion between the electrode layer and the solid electrolyte sheet 10. The upper limit of the porosity of the second solid electrolyte layer 2 is not particularly limited, but in practice it is 99% or less, and even 97% or less.

[0056] The porosity is defined as follows: A backscattered electron topographic image of the fracture surface of the second solid electrolyte layer 2 in the depth direction is binarized and divided into pore and non-pore areas. The porosity is defined as the ratio of the area of ​​the pores to the total area.

[0057] The arithmetic mean roughness Ra of the second solid electrolyte layer 2 (the arithmetic mean roughness of the main surface 2a) is preferably 2.5 μm or more, 3 μm or more, 4 μm or more, 5 μm or more, and particularly preferably 5.6 μm or more. This further enhances the adhesion between the electrode layer and the solid electrolyte sheet 10. There is no particular upper limit to the arithmetic mean roughness Ra of the second solid electrolyte layer 2, but in practice it is 20 μm or less, and even 15 μm or less.

[0058] When the second solid electrolyte layer 2 is used for a sodium ion secondary battery, it preferably contains at least one selected from β''-alumina, β-alumina, and NASICON crystal, similarly to the first solid electrolyte layer 1. When the second solid electrolyte layer 2 is used for a lithium ion secondary battery, it preferably contains at least one selected from β''-alumina, β-alumina, and NASICON crystal. 0.51 Li 0.34 Ti 2.94 , Li 1.3 Al 0.3 Ti 1.7 (PO4)3, Li7La3Zr2O 12 , Li 1.07 Al 0.69 Ti 1.46 (PO4)3, Li 1.5 Al 0.5 Ge 1.5 (PO4)3. From the viewpoints of increasing the adhesion between the first solid electrolyte layer 1 and the second solid electrolyte layer 2 and reducing the interfacial resistance between the two layers, the first solid electrolyte layer 1 and the second solid electrolyte layer 2 are preferably made of the same material.

[0059] The thickness of the second solid electrolyte layer 2 is preferably 2 to 1000 μm, 10 to 800 μm, or 15 to 600 μm, particularly preferably 20 to 500 μm. If the thickness of the second solid electrolyte layer 2 is too small, the material constituting the electrode layer will not penetrate into the voids of the second solid electrolyte layer 2 in sufficient quantity, resulting in a small contact area between the electrode layer and the solid electrolyte sheet 10 and a decrease in adhesion. In this case, the ion conduction paths at the interface between the electrode layer and the solid electrolyte sheet 10 will be reduced, which will tend to increase the internal resistance of the battery. As a result, the rapid charge / discharge characteristics will be reduced. On the other hand, if the thickness of the second solid electrolyte layer 2 is too large, it will be difficult to fill the entire voids of the second solid electrolyte layer 2 with the electrode layer material, resulting in a decrease in energy density per unit volume. Furthermore, the amount of shrinkage during formation of the second solid electrolyte layer 2 will be large, making the second solid electrolyte layer 2 more likely to peel off at its interface with the first solid electrolyte layer 1.

[0060] The ratio of the thickness of the second solid electrolyte layer 2 to the thickness of the solid electrolyte sheet 10 is preferably 10% or more, 15% or more, and particularly 20% or more. If this ratio is too small, the contact area between the electrode layer and the solid electrolyte sheet 10 becomes small, which reduces ion conductivity and tends to result in poor rapid charge / discharge characteristics. There is no particular upper limit to this ratio, but in reality it is 99% or less, or even 97% or less.

[0061] The second solid electrolyte layer 2 may be composed of multiple layers with different porosities. In this case, the multiple layers with different porosities are preferably arranged so that the porosity decreases with increasing distance from the first solid electrolyte layer 1. In this case, the number of layers in the second solid electrolyte layer 2 is preferably 2 or more, 3 or more, 4 or more, and particularly 5 or more. There is no particular upper limit, but considering production efficiency, it is preferably 200 layers or less, 150 layers or less, 100 layers or less, 50 layers or less, 20 layers or less, or even 10 layers or less.

[0062] As described above, if the thickness of the second solid electrolyte layer 2 is too large, the amount of shrinkage during formation of the second solid electrolyte layer 2 increases, resulting in the problem of peeling at the interface with the first solid electrolyte layer 1. In response to this problem, if the second solid electrolyte layer 2 is provided with two or more layers with different porosities as described above, and the porosity is particularly lower in the layers closer to the first solid electrolyte layer 1, the amount of shrinkage near the interface with the first solid electrolyte layer 1 decreases, and peeling at the interface with the first solid electrolyte layer 1 can be suppressed.

[0063] When the second solid electrolyte layer 2 is composed of multiple layers, the porosity of the layer closest to the first solid electrolyte layer 1 is preferably 50% or less, 45% or less, and particularly 40% or less. This is preferable because it reduces the amount of shrinkage near the interface with the first solid electrolyte layer 1 and suppresses peeling from the first solid electrolyte layer 1.

[0064] When the second solid electrolyte layer 2 is composed of multiple layers, the difference in porosity between the layer closest to the first solid electrolyte layer 1 and the layer furthest from it is preferably 5% or more, 10% or more, particularly 15% or more. This makes it possible to both suppress peeling from the first solid electrolyte layer 1 and improve adhesion between the electrode layer and the solid electrolyte sheet 10.

[0065] Even when the second solid electrolyte layer 2 is composed of multiple layers, the porosity of the second solid electrolyte layer 2 as a whole is preferably 20% or more, 25% or more, and particularly 30% or more, as described above. The thickness of the second solid electrolyte layer 2 as a whole is preferably 2 to 1000 μm, 10 to 800 μm, 15 to 600 μm, and particularly 20 to 500 μm, as described above. The thickness of each layer constituting the second solid electrolyte layer 2 is preferably 2 to 900 μm, 10 to 800 μm, 15 to 600 μm, and particularly 20 to 500 μm.

[0066] It is preferable that a metal layer be provided on one or both surfaces of the second solid electrolyte layer 2. In particular, when the electrode layer formed on the second solid electrolyte layer 2 is made of a material such as metallic sodium or metallic lithium, providing a metal layer between the second solid electrolyte layer 2 and the electrode layer improves the wettability and adhesion between the electrode layer and the second solid electrolyte layer, thereby reducing the interfacial resistance. This allows for an all-solid-state battery with excellent discharge capacity. It is also possible to improve the cycle characteristics of the all-solid-state battery for the following reasons.

[0067] If the adhesion between the electrode layer and the second solid electrolyte layer 2 is poor, the movement of sodium ions and lithium ions during charge and discharge is hindered, and they tend to precipitate as needle-shaped metal crystals (dendrites). These needle-shaped metal crystals become high-resistance sites, which tend to cause variations in the in-plane resistance at the interface between the electrode layer and the second solid electrolyte layer 2, resulting in a decrease in cycle characteristics. On the other hand, providing a metal layer between the second solid electrolyte layer 2 and the electrode layer improves the adhesion between the electrode layer and the second solid electrolyte layer 2, thereby suppressing the precipitation of needle-shaped metal crystals and improving cycle characteristics.

[0068] The metal constituting the metal layer is not particularly limited, but examples thereof include Sn, Ti, Bi, Au, Al, Cu, Sb, and Pb. These metals constituting the metal layer may be used alone or in combination of two or more. The metal layer may also be composed of an alloy of these metals.

[0069] The thickness of the metal layer is preferably 3 nm to 5 μm, 5 nm to 3 μm, 10 nm to 800 nm, or 20 to 500 nm, and particularly preferably 30 to 300 nm. In this way, the above-mentioned effects can be more easily obtained.

[0070] Examples of methods for forming the metal layer include physical vapor deposition methods such as vapor deposition or sputtering, chemical vapor deposition methods such as thermal CVD, MOCVD, and plasma CVD, plating, the sol-gel method, and liquid phase film formation methods such as spin coating. Among these, vapor deposition or sputtering is preferred because it is easy to form a thin metal layer and it is easy to obtain the above-mentioned effects of providing a metal layer.

[0071] (Method of manufacturing the solid electrolyte sheet 10) The method for manufacturing the solid electrolyte sheet 10 will be described in detail below.

[0072] (i) First manufacturing method (a) Preparation of green sheets for the first solid electrolyte layer An organic vehicle containing a binder is added to the solid electrolyte powder to prepare a slurry. Polypropylene carbonate or the like can be used as the binder. In addition to the binder, a solvent, a plasticizer, or the like can be added to the organic vehicle. The solvent can be either water or an organic solvent such as ethanol or acetone. However, if water is used as the solvent, alkaline components such as sodium may be eluted from the raw material powder, increasing the pH of the slurry and potentially causing the raw material powder to aggregate. Therefore, it is preferable to use an organic solvent.

[0073] Instead of the solid electrolyte powder, a raw material powder of the solid electrolyte powder (a powder that reacts to become a solid electrolyte in a subsequent firing step) may be used. Alternatively, the solid electrolyte powder and the raw material powder of the solid electrolyte powder may be mixed and used.

[0074] The average particle diameter (D 50 ) is preferably 10 μm or less, particularly 5 μm or less. If the average particle size of the raw material powder is too large, the contact area between the raw material powder particles decreases, making it difficult to sufficiently sinter the solid electrolyte powder particles and to sufficiently proceed the solid-phase reaction between the raw material powder particles of the solid electrolyte powder. In addition, it tends to be difficult to thin the solid electrolyte sheet 10. There is no particular lower limit on the average particle size of the solid electrolyte powder and the raw material powder of the solid electrolyte powder, but in reality it is 0.05 μm or more, and even 0.1 μm or more.

[0075] The obtained slurry is applied to a substrate such as a PET (polyethylene terephthalate) film, dried, and then peeled off from the substrate to obtain a green sheet for the first solid electrolyte layer.

[0076] (b) Preparation of green sheets for the second solid electrolyte layer A second solid electrolyte layer green sheet is obtained by adding an organic vehicle containing a binder to a mixed powder containing a solid electrolyte powder and / or a raw material powder of the solid electrolyte powder and a polymer powder to prepare a slurry, and then applying the slurry to a substrate and drying it. The manufacturing process for the second solid electrolyte layer green sheet differs from the manufacturing process for the first solid electrolyte layer green sheet only in that a polymer powder is added as a solid content, but otherwise the same materials and methods can be used.

[0077] The polymer powder is a material that is burned and removed in a subsequent firing step to form voids 2v in the second solid electrolyte layer 2. Examples of the polymer powder include acrylic resin, polyacrylonitrile, polymethacrylonitrile, and polystyrene.

[0078] Average particle diameter of polymer powder (D 50 ) is preferably 0.1 to 100 μm, 1 to 80 μm, 5 to 70 μm, and particularly preferably 10 to 50 μm. If the average particle size of the polymer powder is too small, it becomes difficult to form three-dimensionally interconnected voids in the second solid electrolyte layer 2. On the other hand, if the average particle size of the polymer powder is too large, the second solid electrolyte layer 2 is not sintered sufficiently, resulting in a decrease in ion conductivity and, as a result, a decrease in rate characteristics.

[0079] The volume ratio of the solid electrolyte powder and / or raw material powder of the solid electrolyte powder to the polymer powder is preferably 75:25 to 3:97, more preferably 60:40 to 6:94, and even more preferably 40:60 to 9:91. If the polymer powder content is too low, it becomes difficult to form three-dimensionally interconnected voids in the second solid electrolyte layer 2. On the other hand, if the polymer powder content is too high, the second solid electrolyte layer 2 is insufficiently sintered, resulting in a decrease in ion conductivity and, as a result, in a tendency for the rate characteristics to decrease.

[0080] The mass ratio of the solid electrolyte powder and / or raw material powder of the solid electrolyte powder to the polymer powder is preferably 95:5 to 20:80, more preferably 90:10 to 30:70, and even more preferably 80:20 to 40:60. The reasons for the limitations are as described above.

[0081] The second solid electrolyte layer composed of multiple layers with different porosities is preferably produced by laminating two or more types of green sheets each made from a slurry having a different content ratio of solid electrolyte powder and / or raw material powder of the solid electrolyte powder, and polymer powder.

[0082] In the slurry for forming the layer furthest from the first solid electrolyte layer 1, the volume ratio of the solid electrolyte powder and / or raw powder of the solid electrolyte powder to the polymer powder is preferably 75:25 to 3:97, more preferably 60:40 to 6:94, and even more preferably 40:60 to 9:91. The mass ratio is preferably 95:5 to 20:80, more preferably 90:10 to 30:70, and even more preferably 80:20 to 40:60. If the polymer powder content is too low, it becomes difficult to form three-dimensionally interconnected voids. On the other hand, if the polymer powder content is too high, the second solid electrolyte layer 2 is insufficiently sintered, resulting in a decrease in ionic conductivity and, as a result, in a tendency for the rate characteristics to deteriorate.

[0083] In the slurry for forming the layer closest to the first solid electrolyte layer 1, the volume ratio of the solid electrolyte powder and / or raw powder of the solid electrolyte powder to the polymer powder is preferably 95:5 to 20:80, more preferably 80:20 to 30:70, and even more preferably 70:30 to 40:60. The mass ratio is preferably 99:1 to 25:75, more preferably 90:10 to 30:70, and even more preferably 80:20 to 35:65. If the polymer powder content is too low, it becomes difficult to form three-dimensionally interconnected voids. On the other hand, if the polymer powder content is too high, the second solid electrolyte layer 2 is likely to peel off from the first solid electrolyte layer 1 due to shrinkage during formation.

[0084] (c) Fabrication of laminate The second solid electrolyte layer green sheet is laminated on one or both surfaces of the first solid electrolyte layer green sheet obtained above to obtain a laminate. After laminating the green sheets, it is preferable to press (preferably by hot pressing). This improves the adhesion between the green sheets, and also improves the adhesion between the first solid electrolyte layer 1 and the second solid electrolyte layer 2 in the resulting solid electrolyte sheet 10.

[0085] The second solid electrolyte layer, which is composed of multiple layers with different porosities, is preferably produced by stacking green sheets with different content ratios of solid electrolyte powder and / or raw material powder of solid electrolyte powder and polymer powder so that the content ratio changes continuously. In particular, it is preferable to stack green sheets with a higher content ratio of solid electrolyte powder and / or raw material powder of solid electrolyte powder so that they are closer to the green sheets for the first solid electrolyte layer.

[0086] (d) Firing of the laminate The laminate obtained above is fired to remove the binder from the green sheet for the first solid electrolyte layer to form the first solid electrolyte layer 1, and to remove the binder and polymer particles from the green sheet for the second solid electrolyte layer to form the second solid electrolyte layer 2. In this way, a solid electrolyte sheet 10 is obtained.

[0087] The firing temperature may be appropriately selected depending on the type of solid electrolyte used. When the solid electrolyte sheet contains β-alumina or β''-alumina, the firing temperature is preferably 1400°C or higher, 1450°C or higher, and particularly preferably 1500°C or higher. If the firing temperature is too low, sintering tends to be insufficient. Alternatively, the reaction of the raw material powder becomes insufficient, making it difficult to produce the desired crystals. On the other hand, the upper limit of the firing temperature is preferably 1750°C or lower, and particularly preferably 1700°C or lower. If the firing temperature is too high, the amount of evaporation of sodium components and the like increases, causing the precipitation of heterogeneous crystals, which tends to reduce the ionic conductivity of the solid electrolyte sheet 10.

[0088] When the solid electrolyte contains NASICON crystals, the firing temperature is preferably 1200°C or higher, particularly 1210°C or higher. If the firing temperature is too low, sintering tends to be insufficient. Alternatively, the reaction of the raw material powder becomes insufficient, making it difficult to produce the desired crystals. On the other hand, the upper limit of the firing temperature is preferably 1400°C or lower, particularly 1300°C or lower. If the firing temperature is too high, the amount of evaporation of sodium components and the like increases, causing the precipitation of heterogeneous crystals, which tends to reduce the ionic conductivity of the solid electrolyte sheet 10.

[0089] The firing time is adjusted appropriately so that sintering proceeds sufficiently, and is preferably 10 to 120 minutes, and more preferably 20 to 80 minutes.

[0090] (ii) Second manufacturing method (a) Preparation of the first solid electrolyte layer 1 For example, a commercially available solid electrolyte sheet can be used as the first solid electrolyte layer 1. If necessary, the thickness may be adjusted by polishing to a desired thickness.

[0091] Alternatively, the first solid electrolyte layer 1 may be produced by firing a green sheet for the first solid electrolyte layer produced according to the method (a) of the first production method.

[0092] (b) Preparation of slurry A slurry for the second solid electrolyte layer is prepared in the same manner as in the method (b) of the first production method.

[0093] (c) Fabrication of laminate The slurry is applied to one or both surfaces of the first solid electrolyte layer 1, thereby forming a slurry layer on the surface of the first solid electrolyte layer 1 to obtain a laminate.

[0094] (d) Firing of the laminate The laminate obtained above is fired to remove the binder and polymer particles in the slurry layer, thereby forming the second solid electrolyte layer 2. This produces the solid electrolyte sheet 10. The firing time and firing temperature can be the same as those in the first manufacturing method.

[0095] In step (c), a green sheet for the second solid electrolyte layer may be laminated on the surface of the first solid electrolyte layer 1 instead of the slurry layer to obtain a laminate, and then the laminate may be fired to obtain the solid electrolyte sheet 10.

[0096] As in the first manufacturing method, in the second manufacturing method, two or more types of slurries (or green sheets) having different content ratios of solid electrolyte powder and / or raw material powder of solid electrolyte powder and polymer powder may be prepared, and these may be applied to the surface of the first solid electrolyte layer 1, dried repeatedly to form a laminate, and then fired to form a second solid electrolyte layer composed of multiple layers with different porosities. [Example]

[0097] The present invention will be described in detail below based on examples, but the present invention is not limited to these examples.

[0098] Tables 1 and 2 show Examples 1 to 9 and Comparative Examples 1 and 2.

[0099] [Table 1]

[0100] [Table 2]

[0101] (a) Fabrication of solid electrolyte sheet (a-1) Preparation of green sheet for first solid electrolyte layer To 100 parts by mass of the solid electrolyte powder (average particle size 2.5 μm) shown in Tables 1 and 2, 20 parts by mass of polypropylene carbonate (Q-PAC40, Empower Materials) was added as a binder, and the mixture was dispersed in N-methylpyrrolidone, followed by thorough stirring in a planetary centrifugal mixer to form a slurry. The resulting slurry was applied to a PET film using a doctor blade, dried at 70°C, and then peeled off from the PET film to obtain a first solid electrolyte green sheet. The composition of the NASICON crystal used was Na 3.05 Zr2Si 2.06 P 0.95 O 12 is.

[0102] (a-2) Preparation of green sheets for second solid electrolyte layer The solid electrolyte powder and polymer particles were weighed to the volume ratios shown in Tables 1 and 2. The polymer particles used were acrylic polymer particles with an average particle size of 20 μm (Advancell HB2051 manufactured by Sekisui Chemical Co., Ltd.), cross-linked polymethyl methacrylate particles with an average particle size of 20 μm (MBX-20 manufactured by Sekisui Chemical Co., Ltd.), or cross-linked polymethyl methacrylate particles with an average particle size of 8 μm (MBX-8 manufactured by Sekisui Chemical Co., Ltd.). To 100 parts by mass of these mixtures, 20 parts by mass of polypropylene carbonate was added as a binder. The mixture was dispersed in N-methylpyrrolidone and thoroughly stirred in a planetary mixer to form a slurry. The resulting slurry was applied to a PET film using a doctor blade, dried at 70°C, and peeled off from the PET film to obtain a green sheet for the second solid electrolyte layer. For Example 9, two types of green sheets ("First Layer" and "Second Layer" in Table 2) with different solid electrolyte powder and polymer particle content ratios were prepared.

[0103] (a-3) Firing of green sheets Second solid electrolyte green sheets were laminated on both sides of the first solid electrolyte layer green sheet obtained above and hot-pressed, followed by firing at 1600°C for Examples 1 to 3, 5 to 9, and Comparative Examples 1 and 2, and at 1220°C for Example 4, to produce solid electrolyte sheets in which porous second solid electrolyte layers were formed on both sides of a dense first solid electrolyte layer. For Example 9, the second solid electrolyte green sheets for the "first layer" and "second layer" listed in Table 2 were laminated and hot-pressed to obtain laminates, which were then laminated on both sides of the first solid electrolyte layer green sheet, hot-pressed, and fired at 1600°C. The lamination was performed so that the "first layer" second solid electrolyte green sheet faced the first solid electrolyte layer green sheet.

[0104] FIG. 1 shows a cross-sectional image of the vicinity of the interface between the first solid electrolyte layer and the second solid electrolyte layer in the solid electrolyte sheet of Example 1. FIG. 1(a) is a diagram showing a reference line, which is a straight line drawn along the surface of the first solid electrolyte layer, and FIG. 1(b) is a diagram showing a contour line, which is a curved line drawn along the surface of the second solid electrolyte layer. The ratio of the length of the contour line to the length of the reference line (contour line length / reference line length) was determined by image analysis, and the results are shown in Tables 1 and 2. Image analysis software Image J was used for the image analysis.

[0105] (a-4) Measurement of the resistance of the solid electrolyte sheet and calculation of the surface area The green sheets for the first solid electrolyte layer were fired at 1600°C for Examples 1 to 3, 5 to 9 and Comparative Examples 1 and 2, and at 1220°C for Example 4, to produce the first solid electrolyte layer.

[0106] On the surface of the first solid electrolyte layer obtained above, a gold electrode was formed as an ion-blocking electrode within a range of φ4 mm by RF sputtering, and then measured at a frequency of 1 to 10 7 The resistance R1 of the first solid electrolyte layer was calculated from a Cole-Cole plot. The measurement was performed in an environment with a dew point of −40° C. or lower and a temperature of 0° C.

[0107] The resistance R2 of the solid electrolyte sheet (hereinafter simply referred to as the solid electrolyte sheet) prepared in (a-3) in which the second solid electrolyte layer was formed on both sides of the first solid electrolyte layer was determined in the same manner as above.

[0108] Using the resistances R1 and R2 obtained above, the surface area of ​​the second solid electrolyte layer per unit area (specifically, the surface area of ​​the second solid electrolyte layer in an area of ​​1 cm x 1 cm in plan view) was calculated using the following procedure.

[0109] First, the ionic conductivity σ1 of the first solid electrolyte layer is calculated from the following formula (1): where A1 is the surface area per unit area of ​​the first solid electrolyte layer. Since the first solid electrolyte layer is dense and has a smooth surface, A1 is 1 cm 2In addition, t1 is the thickness of the first solid electrolyte layer.

[0110]

number

[0111]

number

[0112] (b) Preparation of the positive electrode layer (b-1) Preparation of positive electrode active material precursor powder Sodium metaphosphate (NaPO), ferric oxide (FeO), and orthophosphoric acid (HPO) were used as raw materials, and raw material powders were mixed to a molar ratio of 40% NaO, 20% FeO, and 40% PO, and melted in an air atmosphere at 1250°C for 45 minutes. The molten glass was then poured between a pair of rollers and rapidly cooled while being formed into a film, producing a positive electrode active material precursor.

[0113] The obtained positive electrode active material precursor was ball milled using φ20 mm Al2O3 balls for 5 hours, then ball milled in ethanol using φ5 mm ZrO2 balls for 100 hours, and further milled at 300 rpm for 5 hours (with a 10-minute break every 10 minutes) using a Fritsch planetary ball mill P6 containing φ0.3 mm ZrO2 balls to obtain an average particle diameter D 50 A 0.2 μm powder of the positive electrode active material precursor was obtained.

[0114] (b-2) Preparation of positive electrode composite The above-mentioned positive electrode active material precursor powder, the solid electrolyte powder shown in Tables 1 and 2, and acetylene black (SUPER C65 manufactured by TIMCAL) as a conductive additive were weighed out in a mass ratio of 83:13:4 and mixed for about 30 minutes using an agate mortar and pestle to obtain a positive electrode mixture. 20 parts by mass of N-methylpyrrolidone containing 10% by mass of polypropylene carbonate was added to 100 parts by mass of the obtained positive electrode mixture, and the mixture was thoroughly stirred using a planetary mixer to form a slurry.

[0115] (c) Preparation of test battery The slurried positive electrode mixture was applied to one surface of the solid electrolyte sheet obtained above by 1 cm 2 The cathode composite was sintered and the cathode active material precursor powder was crystallized by firing at 525°C for 30 minutes in a mixed gas atmosphere of nitrogen and hydrogen (nitrogen 96% by volume, hydrogen 4% by volume), thereby forming a cathode layer having the thickness shown in Tables 1 and 2. When the X-ray diffraction pattern of the obtained cathode layer was examined, diffraction lines derived from the active material crystal, Na2FeP2O7, were confirmed.

[0116] Figure 2 shows cross-sectional images of the vicinity of the interface between the first solid electrolyte layer and the second solid electrolyte layer in the solid electrolyte sheet of Example 1. Figure 2(a) is a diagram showing a reference line, which is a straight line drawn along the surface of the first solid electrolyte layer, and Figure 2(b) is a diagram showing a contour line, which is a curved line drawn along the surface of the second solid electrolyte layer.

[0117] Next, a 300 nm thick gold electrode serving as a current collector was formed on the surface of the positive electrode layer using a sputtering apparatus (SC-701AT manufactured by Sanyu Electronics Co., Ltd.). Then, metallic sodium, which served as a counter electrode, was pressure-bonded to the surface of the solid electrolyte sheet opposite the side on which the positive electrode layer was formed, and the solid electrolyte sheet was placed on the bottom lid of a coin cell. The top lid was then placed on top to produce a CR2032 test battery. For Example 5, a 90 nm thick gold electrode was formed on the surface of the solid electrolyte sheet opposite the side on which the positive electrode layer was formed using a sputtering apparatus (SC-701AT manufactured by Sanyu Electronics Co., Ltd.), and metallic sodium was pressure-bonded to the surface of the gold electrode.

[0118] (d) Charge / discharge test A charge-discharge test was performed using the test battery. The results are shown in Tables 1 and 2. In the charge-discharge test, charging (sodium ion release from the positive electrode active material) was performed by CC (constant current) charging from the open circuit voltage (OCV) to 4.5 V, and discharging (sodium ion absorption into the positive electrode active material) was performed by CC discharging from 4.5 V to 2 V. The C rate was 0.1 C, 0.5 C, or 5 C, and the test was performed at 30°C. The discharge capacity was defined as the amount of electricity discharged per unit weight of the positive electrode active material contained in the positive electrode layer. A cycle test was also performed at 0.5 C. Specifically, the discharge capacity retention rate ((after 300 cycles / after 1 cycle) × 100 (%)) was calculated from the discharge capacities at 0.5 C after the first and 300th cycles.

[0119] As shown in Tables 1 and 2, in Examples 1 to 9, sufficient three-dimensionally interconnected voids were formed within the second solid electrolyte layer, resulting in a low solid electrolyte sheet resistance of 5.6 to 51.0 Ω. Furthermore, the ratio of the contour length to the reference length was increased to 1.5 to 3.8, resulting in a large contact area between the solid electrolyte sheet and the positive electrode layer, resulting in good discharge capacities of 62 to 83 mAh / g at 0.1 C and 32 to 65 mAh / g at 0.5 C. In Example 5, the presence of a metal layer between the solid electrolyte layer and metallic sodium improved the rate characteristics, resulting in a discharge capacity of 25 mAh / g at 5 C and a good discharge capacity retention rate of 90%. In Example 8, the second solid electrolyte layer was thick (118 μm), increasing the contact area between the electrode layer and the solid electrolyte layer, resulting in improved rate characteristics, resulting in a discharge capacity of 13 mAh / g at 10 C. In Example 9, the second solid electrolyte layer had a thick overall thickness of 197 μm, and therefore exhibited a discharge capacity of 31 mAh / g at 5 C and 20 mAh / g at 10 C. In Example 9, the second solid electrolyte layer consisted of two layers with different porosities, and therefore no peeling occurred at the interface with the first solid electrolyte layer even when the second solid electrolyte layer was made as thick as 197 μm.

[0120] On the other hand, in Comparative Examples 1 and 2, only independent voids existed inside the second solid electrolyte layer, and three-dimensionally interconnected voids were not formed, resulting in a high resistance of the solid electrolyte sheet of 117.5 to 125.1 Ω. Furthermore, the ratio of the contour length to the reference length was small at 1.1, resulting in a small contact area between the solid electrolyte sheet and the positive electrode layer. Comparative Example 1 exhibited a relatively good discharge capacity of 74 mAh / g at 0.1 C, but a low discharge capacity of 14 mAh / g at 0.5 C. Furthermore, in Comparative Example 2, the thickness of the positive electrode layer was large at 93 μm, causing the positive electrode layer to peel off from the second solid electrolyte layer during firing, making charge and discharge impossible. [Explanation of symbols]

[0121] 1. First solid electrolyte layer 1a, 1b main surface 2 Second solid electrolyte layer 2a, 2b main surface 2s solid electrolyte 2v air gap 10 Solid electrolyte sheet

Claims

1. A solid electrolyte sheet comprising a first solid electrolyte layer and a second solid electrolyte layer formed on at least one surface of the first solid electrolyte layer, A solid electrolyte sheet, wherein the second solid electrolyte layer is a porous solid electrolyte layer.

2. 2. The solid electrolyte sheet according to claim 1, wherein the second solid electrolyte layer is a porous solid electrolyte layer having three-dimensionally interconnected pores.

3. 3. The solid electrolyte sheet according to claim 1, wherein, in a cross-sectional image of the vicinity of the interface between the first solid electrolyte layer and the second solid electrolyte layer, a straight line drawn along the surface of the first solid electrolyte layer is taken as a reference line and a curved line drawn along the surface of the second solid electrolyte layer is taken as a contour line, a ratio of the length of the contour line to the length of the reference line (contour line length / reference line length) is 1.3 to 50.

4. 4. The solid electrolyte sheet according to claim 1, wherein the second solid electrolyte layer is composed of a plurality of layers having different porosities.

5. 5. The solid electrolyte sheet according to claim 4, wherein the layers with different porosities have lower porosities as the layers are closer to the first solid electrolyte layer.

6. The second solid electrolyte layer has a thickness of 1 cm in plan view. ２ The surface area per ２ The solid electrolyte sheet according to any one of claims 1 to 5, characterized in that:

7. 7. The solid electrolyte sheet according to claim 1, wherein the second solid electrolyte layer has an arithmetic mean roughness Ra of 2.5 μm or more.

8. 8. The solid electrolyte sheet according to claim 1, wherein the second solid electrolyte layer is formed on both sides of the first solid electrolyte layer.

9. 9. The solid electrolyte sheet according to claim 1, wherein the thickness is 2400 μm or less.

10. The solid electrolyte sheet according to any one of claims 1 to 9, wherein the first solid electrolyte layer and / or the second solid electrolyte layer contains at least one selected from β''-alumina, β-alumina, and NASICON crystals.

11. The solid electrolyte sheet according to any one of claims 1 to 10, which is for an all-solid-state sodium ion secondary battery.

12. An all-solid-state secondary battery comprising: the solid electrolyte sheet according to any one of claims 1 to 11; and an electrode layer formed on a surface of the second solid electrolyte layer of the solid electrolyte sheet.

13. 13. The all-solid-state secondary battery according to claim 12, wherein the material constituting the electrode layer fills the voids in the second solid electrolyte layer.

14. A method for producing the solid electrolyte sheet according to any one of claims 1 to 11, comprising: (a) a step of adding an organic vehicle containing a binder to a solid electrolyte powder and / or a raw material powder of the solid electrolyte powder to prepare a slurry, and then applying the slurry to a substrate and drying the substrate to obtain a green sheet for a first solid electrolyte layer; (b) preparing a slurry by adding an organic vehicle containing a binder to a mixed powder containing a solid electrolyte powder and / or a raw material powder of the solid electrolyte powder and a polymer powder, and applying the slurry to a substrate and drying the substrate to obtain a green sheet for a second solid electrolyte layer; (c) laminating the second solid electrolyte layer green sheet on at least one surface of the first solid electrolyte layer green sheet to obtain a laminate; (d) firing the laminate to remove the binder from the first solid electrolyte layer green sheet to form a first solid electrolyte layer, and to remove the binder and the polymer particles from the second solid electrolyte layer green sheet to form a second solid electrolyte layer; A method for manufacturing a solid electrolyte sheet, comprising:

15. A method for producing the solid electrolyte sheet according to any one of claims 1 to 11, comprising: (a) providing a first solid electrolyte layer; (b) preparing a slurry by adding an organic vehicle containing a binder to a mixed powder containing a solid electrolyte powder and / or a raw material powder of the solid electrolyte powder and a polymer powder; (c) applying the slurry to at least one surface of the first solid electrolyte layer to obtain a laminate in which a slurry layer is formed on the surface of the first solid electrolyte layer; and, (d) firing the laminate to remove the binder and the polymer particles in the slurry layer to form a second solid electrolyte layer; A method for producing a solid electrolyte sheet, comprising:

16. 16. The method for producing a solid electrolyte sheet according to claim 14, wherein the polymer powder has an average particle size of 0.1 to 100 μm.

17. 17. The method for producing a solid electrolyte sheet according to claim 14, wherein a content ratio of the solid electrolyte powder and / or raw material powder of the solid electrolyte powder to the polymer powder is 75:25 to 3:97 by volume.

Images