Solid electrolyte sheet and method for producing the same

Abstract

The present invention provides a solid electrolyte sheet capable of obtaining excellent discharge capacity and increasing adhesiveness to an electrode layer.　A solid electrolyte sheet 10 obtained by forming a second solid electrolyte layer 2 on at least one surface of a first solid electrolyte layer 1, the solid electrolyte sheet 10 being characterized in that the second solid electrolyte layer 2 is a porous solid electrolyte layer.

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, and the like.

Background Art

[0002] Lithium-ion secondary batteries have established themselves as essential high-capacity and lightweight power sources for mobile devices, electric vehicles, and the like. In current lithium-ion secondary batteries, a flammable organic electrolyte is mainly used as the electrolyte, so there are concerns about risks such as ignition. As a method for solving this problem, the development of lithium-ion all-solid-state batteries using a solid electrolyte instead of an organic electrolyte has been underway (see, for example, Patent Document 1).

[0003] In addition, since there are concerns about the global price increase of raw materials for lithium, sodium has also attracted attention as a material to replace lithium, and a sodium-ion all-solid-state battery using a sodium-ion conductive crystal composed of NASICON-type Na3Zr2Si2PO 12 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 other beta-alumina-based solid electrolytes, and Na5YSi4O 12 are also known to exhibit high sodium-ion conductivity, and these solid electrolytes can also be used for sodium-ion all-solid-state batteries.

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

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0006] However, simply increasing the surface roughness of the solid electrolyte layer alone makes it difficult to sufficiently increase the discharge capacity. In particular, when the thickness of the electrode layer is increased, there is a risk that the electrode layer may peel off from the solid electrolyte layer during the firing process in the manufacture of the all-solid-state battery, making charge and discharge impossible.

[0007] In view of the above, an object of the present invention is to provide a solid electrolyte sheet capable of enhancing the adhesion to the electrode layer and obtaining an excellent discharge capacity.

Means for Solving the Problems

[0008] As a result of intensive studies by the present inventors, it has been found that the above problems can be solved by 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, it is preferable that the second solid electrolyte layer is a porous solid electrolyte layer having three-dimensionally connected voids. In this way, when an electrode layer is formed on the second solid electrolyte layer, the material constituting the electrode layer easily enters the voids in the second solid electrolyte layer, and the electrode layer and the solid electrolyte sheet adhere firmly. Therefore, the contact area between the electrode layer and the solid electrolyte sheet increases, and the interfacial resistance between the electrode layer and the solid electrolyte layer can be reduced. Also, even in the firing process during the manufacture of the 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, it becomes possible to obtain an all-solid-state battery having excellent discharge capacity.

[0011] In the cross-sectional image near the interface between the first solid electrolyte layer and the second solid electrolyte layer of the solid electrolyte sheet of the present invention, when a straight line drawn along the surface of the first solid electrolyte layer is used as a reference line and a curve drawn along the surface of the second solid electrolyte layer is used as a contour line, the ratio of the length of the contour line to the length of the reference line (length of contour line / length of reference line) 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 way is a parameter that serves as an index for the manner of forming three-dimensionally connected voids in the second solid electrolyte layer. When the ratio is within the above range, three-dimensionally connected voids are well formed in the second solid electrolyte layer, and it becomes possible to firmly adhere the electrode layer and the solid electrolyte sheet.

[0012] In the solid electrolyte sheet of the present invention, it is preferable that the second solid electrolyte layer is composed of a plurality of layers having different porosities. In particular, it is preferable that the plurality of layers having different porosities have a lower porosity in the layer 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 surface area of 3 cm 2 per 1 cm in plan view 2It is preferably as described above. The surface area of the second solid electrolyte layer defined in this way also serves as an index for how voids that are three-dimensionally continuous are formed in the second solid electrolyte layer. When the surface area is within the above range, voids that are three-dimensionally continuous are well formed in the second solid electrolyte layer, the contact area between the electrode layer and the solid electrolyte sheet increases, the adhesion between the two increases, and they can be firmly joined. Therefore, the interfacial resistance between the electrode layer and the solid electrolyte sheet is reduced, and as a result, it becomes possible to obtain a battery with excellent discharge capacity.

[0014] For the solid electrolyte sheet of the present invention, it is preferable that the arithmetic mean roughness Ra of the second solid electrolyte layer is 2.5 μm or more. By doing so, it becomes possible to further enhance the adhesion between the electrode layer and the solid electrolyte sheet.

[0015] For 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. By doing so, it becomes possible to firmly adhere both the positive electrode layer and the negative electrode layer to the solid electrolyte sheet.

[0016] For the solid electrolyte sheet of the present invention, it is preferable that the thickness is 2400 μm or less. The smaller the thickness of the solid electrolyte sheet, the shorter the distance required for ion conduction in the solid electrolyte, and the ion conductivity is improved, which is preferable. Also, when used as a solid electrolyte for an all-solid-state battery, the energy density per unit volume of the all-solid-state battery increases.

[0017] For the solid electrolyte sheet of the present invention, it is preferable that 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.

[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 solid electrolyte sheet and an electrode layer formed on the surface of the second solid electrolyte layer in the solid electrolyte sheet.

[0020] In the all-solid-state secondary battery of the present invention, it is preferable that the material constituting the electrode layer enters the voids in the second solid electrolyte layer. By doing so, the adhesion between the electrode layer and the second solid electrolyte layer can be enhanced.

[0021] The method for manufacturing the solid electrolyte sheet of the present invention is a method for manufacturing the above solid electrolyte sheet, comprising: (a) adding an organic vehicle containing a binder to solid electrolyte powder and / or raw material powder of solid electrolyte powder to prepare a slurry, applying the slurry onto a substrate, and drying to obtain a green sheet for the first solid electrolyte layer; (b) adding an organic vehicle containing a binder to a mixed powder containing solid electrolyte powder and / or raw material powder of solid electrolyte powder and polymer powder to prepare a slurry, applying the slurry onto a substrate, and drying to obtain a green sheet for the second solid electrolyte layer; (c) laminating the green sheet for the second solid electrolyte layer on at least one surface of the green sheet for the first solid electrolyte layer to obtain a laminate; (d) firing the laminate to remove the binder in the green sheet for the first solid electrolyte layer to form the first solid electrolyte layer, and removing the binder and polymer particles in the green sheet for the second solid electrolyte layer to form the second solid electrolyte layer. According to this, it becomes possible to easily manufacture a solid electrolyte sheet in which a porous second solid electrolyte layer having three-dimensionally connected voids is formed on at least one surface of the first solid electrolyte layer.

[0022] The manufacturing method of the solid electrolyte sheet of the present invention is a method for manufacturing the above solid electrolyte sheet, comprising: (a) a step of preparing a first solid electrolyte layer; (b) a step of preparing a slurry by adding an organic vehicle containing a binder to a mixed powder containing solid electrolyte powder and / or raw material powder of solid electrolyte powder and polymer powder; (c) a step of obtaining a laminate in which a slurry layer is formed on at least one surface of the first solid electrolyte layer by applying the slurry to at least one surface of the first solid electrolyte layer; and (d) a step of forming a second solid electrolyte layer by removing the binder and polymer particles in the slurry layer by firing the laminate. According to the manufacturing method, it is possible to easily manufacture a solid electrolyte sheet in which a porous second solid electrolyte layer having three-dimensionally connected voids is formed on at least one surface of the first solid electrolyte layer.

[0023] In the manufacturing method of the solid electrolyte sheet of the present invention, it is preferable that the average particle diameter of the polymer powder is 0.1 to 100 μm.

[0024] In the manufacturing method of the solid electrolyte sheet of the present invention, it is preferable that the content ratio of the solid electrolyte powder and / or the raw material powder of the solid electrolyte powder to the polymer powder is 75:25 to 3:97 by volume ratio.

Effects of the Invention

[0025] According to the present invention, it is possible to provide a solid electrolyte sheet capable of enhancing the adhesion to the electrode layer and obtaining an excellent discharge capacity.

Brief Description of the Drawings

[0026]

Figure 1

Figure 2

Mode for Carrying Out 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] FIG. 1 is a schematic cross-sectional view showing an embodiment of the 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 that communicate three-dimensionally with the solid electrolyte 2s.

[0029] When manufacturing an all-solid-state battery using the solid electrolyte sheet 10, electrode layers (a positive electrode layer or a negative electrode layer) are formed on both surfaces of the solid electrolyte sheet 10. Specifically, the electrode layers are formed on the surface of the main surface 1b of the first solid electrolyte layer 1 opposite to the second solid electrolyte layer 2 and on the surface of the main surface 2a of the second solid electrolyte layer 2 opposite to the first solid electrolyte layer 1. Here, since the second solid electrolyte layer has voids 2v that communicate three-dimensionally, the material (such as active material powder) constituting the electrode layer easily enters the voids 2v, and the electrode layer and the second solid electrolyte layer 2 adhere firmly. Therefore, the contact area between the electrode layer and the solid electrolyte sheet 10 (the second solid electrolyte layer 2) increases, the ion conduction path increases, and the interfacial resistance between the electrode layer and the solid electrolyte sheet 10 can be reduced. Also, even in the firing process during the manufacture 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, it becomes possible to obtain an all-solid-state battery excellent in discharge capacity.

[0030] In addition, when the electrode layer is a low-melting-point material such as metallic sodium, it may soften and flow during the fabrication or charge / discharge of the all-solid-state battery, penetrate from the side surface of the solid electrolyte sheet 10 to the counter electrode layer side, and as a result, may cause a short circuit. However, in the solid electrolyte sheet 10 of the present embodiment, since the softened and flowing low-melting-point material enters the voids 2v in the second solid electrolyte layer 2, there is also an advantage that problems such as penetration to the counter electrode layer side and the resulting short circuit are less likely to occur. Further, since the relatively dense first solid electrolyte layer 1 serves as a barrier, problems such as the low-melting-point material reaching the counter electrode layer side from inside the solid electrolyte sheet 10 and causing a short circuit are also less likely to occur.

[0031] In a cross-sectional image near the interface between the first solid electrolyte layer 1 and the second solid electrolyte layer 2, when a straight line drawn along the surface of the first solid electrolyte layer 1 is used as a reference line and a curve drawn along the surface of the second solid electrolyte layer 2 is used as a contour line, the ratio of the length of the contour line to the length of the reference line (length of the contour line / length of the reference line) is preferably 1.3 to 50, 1.5 to 20, 1.8 to 10, particularly 2 to 5 (see 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 way is a parameter that serves as an index of how the three-dimensionally connected voids 2v are formed in the second solid electrolyte layer 2. If the above ratio is too small, the three-dimensionally connected voids 2v in the second solid electrolyte layer 2 are not sufficiently formed, and the adhesion between the electrode layer and the solid electrolyte sheet 10 tends to be poor. On the other hand, if the above ratio is too large, the mechanical strength of the second solid electrolyte layer 2 tends to be poor.

[0032] The surface area per 1 cm 2 in a plan view of the second solid electrolyte layer is 3 cm 2 or more, 5 cm 2 or more, 7 cm 2 or more, particularly 10 cm 2It is preferably the above. If the surface area is too small, voids 2v that communicate three-dimensionally in the second solid electrolyte layer 2 are not sufficiently formed, the contact area between the electrode layer and the solid electrolyte sheet 10 becomes small, and the adhesion between the two 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 30 cm 2 It is preferably the following. 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 only on 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, since both surfaces of the solid electrolyte sheet 10 are composed of the second solid electrolyte layer 2, it becomes possible to firmly adhere both the positive electrode layer and the negative electrode layer to the solid electrolyte sheet.

[0034] The smaller the thickness of the solid electrolyte sheet 10, the shorter the distance required for ion conduction in the solid electrolyte, and the ion conductivity is improved, which is preferable. Also, 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, 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 electrode and the negative electrode may short-circuit, so it is preferably 5 μm or more, 10 μm or more, 20 μm or more, particularly preferably 30 μm or more.

[0035] Hereinafter, each component will be described in detail.

[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, the first solid electrolyte layer 1 preferably has a denser structure than the second solid electrolyte layer 2. In other words, the first solid electrolyte layer 1 preferably has a smaller porosity than the second solid electrolyte layer 2. Specifically, the first solid electrolyte layer 1 preferably has a porosity of 20% or less, 10% or less, particularly 5% or less, as defined by the following formula.

[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 crystals. 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 and the like. In addition to β''-alumina, β-alumina may also be contained. Examples of β-alumina include 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) can be mentioned.

[0039] Specific compositions of β’’-alumina include those containing, in mol%, 65 - 98% of Al2O3, 2 - 20% of Na2O, 0.3 - 15% of MgO + Li2O, 0 - 20% of ZrO2, and 0 - 5% of Y2O3. The reasons for limiting the composition as above will be explained below.

[0040] Al2O3 is the main component constituting β’’-alumina. The content of Al2O3 is preferably 65 - 98%, particularly preferably 70 - 95%. If the content of Al2O3 is too low, the ionic conductivity of the solid electrolyte tends to decrease. On the other hand, if the content of Al2O3 is too high, α-alumina having no 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 content of Na2O is preferably 2 - 20%, 3 - 18%, particularly preferably 4 - 16%. If the content of Na2O is too low, it is difficult to obtain the above effects. On the other hand, if the content of Na2O is too high, excess sodium forms compounds that do not contribute to ionic conductivity such as NaAlO2, and the ionic conductivity tends to decrease.

[0042] MgO and Li2O are components (stabilizers) that stabilize the structure of β’’-alumina. The content of MgO + Li2O is preferably 0.3 - 15%, 0.5 - 10%, particularly preferably 0.8 - 8%. If the content of MgO + Li2O is too low, α-alumina remains in the solid electrolyte and the ionic conductivity tends to decrease. On the other hand, if the content of MgO + Li2O is too high, MgO or Li2O that does not function as a stabilizer remains in the solid electrolyte, and the ionic conductivity tends to decrease.

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

[0044] As the NASICON crystal, the general formula is 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), and it is preferably composed of a compound represented by this. Here, A1 is preferably at least one selected from Y, Nb, Ti, and Zr. By doing so, a crystal excellent in ionic conductivity can be obtained.

[0045] In addition, the preferable ranges of the respective coefficients in the above general formula are as follows.

[0046] s is preferably 1.4 to 5.2, 2.5 to 3.5, particularly preferably 2.8 to 3.1. If s is too small, the number of sodium ions will be small, so the ionic conductivity is likely to decrease. On the other hand, if s is too large, excess sodium will form compounds that do not contribute to ionic conduction such as sodium phosphate and sodium silicate, so the ionic conductivity is likely to decrease.

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

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

[0049] v is preferably 9 to 14, 9.5 to 12, particularly 11 to 12. If v is too small, A1 (for example, the aluminum component) becomes a lower valence number, making it easy for the electrical insulation to decrease. On the other hand, if v is too large, it becomes a peroxide state and sodium ions are bound by the lone pair of electrons of oxygen atoms, making it easy for the ionic conductivity to decrease.

[0050] The above NASICON crystal is preferably a monoclinic crystal, a hexagonal crystal or a trigonal crystal, particularly a monoclinic or trigonal crystal because it has excellent ionic conductivity.

[0051] Specific examples of the NASICON crystal include Na3Zr2Si2PO 12 , Na 3.2 Zr 1.3 Si 2.2 P 0.8 O 10.5 , Na3Zr 1.6 Ti 0.4 Si2PO 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 Si2PO 12 , Na 3.6 Ti 0.2 Y 0.8 Si 2.8 O9, Na3Zr 1.88 Y 0.12 Si2PO 12 , Na 3.12 Zr 1.88 Y0.12 Si2PO 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 and the like crystals are exemplified. In particular, Na 3.12 Zr 1.88 Y 0.12 Si2PO 12 and Na 3.05 Zr2Si 2.06 P 0.95 O 12 are preferable because they are excellent in 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 contains at least one selected from 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 (PO4)3 is preferably contained.

[0053] The thickness of the first solid electrolyte layer 1 is preferably 4 to 400 μm, 10 to 300 μm, 20 to 200 μm, particularly 30 to 100 μm. If the thickness of the first solid electrolyte layer 1 is too small, the mechanical strength may decrease or the positive and negative electrodes 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 tends to decrease. Also, the energy density per unit volume of the all-solid-state battery tends to increase.

[0054] (The second solid electrolyte layer 2) As described above, the second solid electrolyte layer 2 is a porous solid electrolyte layer having three-dimensionally communicating voids 2v. The porosity of the second solid electrolyte layer 2 is preferably 30% or more, 50% or more, 60% or more, particularly 70% or more. If the porosity of the second solid electrolyte layer 2 is too small, it becomes difficult to form three-dimensionally communicating voids 2v, and the adhesion between the electrode layer and the solid electrolyte sheet 10 tends to be poor. The upper limit of the porosity of the second solid electrolyte layer 2 is not particularly limited, but is realistically 99% or less, and further 97% or less.

[0055] In addition, the degree of porosity of the second solid electrolyte layer 2 can also be evaluated by the pore fraction defined below from a perspective different from the porosity. The pore fraction of the second solid electrolyte layer 2 is preferably 20% or more, 25% or more, particularly 30% or more. If the pore fraction of the second solid electrolyte layer 2 is too low, it becomes difficult to form three-dimensionally communicating voids 2v, and the adhesion between the electrode layer and the solid electrolyte sheet 10 tends to be poor. The upper limit of the pore fraction of the second solid electrolyte layer 2 is not particularly limited, but is realistically 99% or less, and further 97% or less.

[0056] The pore fraction is defined as follows. The reflection electron topographic image of the cross-section in the depth direction of the second solid electrolyte layer 2 is binarized to divide it into a pore part and a non-pore part. The ratio of the area of the pore part to the total area is defined as the pore fraction.

[0057] The arithmetic mean roughness (arithmetic mean roughness of the main surface 2a) Ra of the second solid electrolyte layer 2 is preferably 2.5 μm or more, 3 μm or more, 4 μm or more, 5 μm or more, particularly 5.6 μm or more. By doing so, it becomes possible to further enhance the adhesion between the electrode layer and the solid electrolyte sheet 10. The upper limit of the arithmetic mean roughness Ra of the second solid electrolyte layer 2 is not particularly limited, but is realistically 20 μm or less, and further 15 μm or less.

[0058] When the second solid electrolyte layer 2 is used for a sodium ion secondary battery, similar to the first solid electrolyte layer 1, it preferably contains at least one selected from β''-alumina, β-alumina, and NASICON crystals. When used for a lithium ion secondary battery, 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 (PO4)3, and it is preferably to contain at least one selected from them. From the viewpoint of enhancing 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, it is preferable that the first solid electrolyte layer 1 and the second solid electrolyte layer 2 are 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, 15 to 600 μm, particularly 20 to 500 μm. If the thickness of the second solid electrolyte layer 2 is too small, the amount of the material constituting the electrode layer entering the voids of the second solid electrolyte layer 2 is small, so the contact area between the electrode layer and the solid electrolyte sheet 10 becomes small and the adhesion tends to decrease. In this case, the number of ion conduction paths at the interface between the electrode layer and the solid electrolyte sheet 10 decreases, so the internal resistance of the battery tends to increase. As a result, the rapid charge and discharge characteristics tend to deteriorate. On the other hand, if the thickness of the second solid electrolyte layer 2 is too large, it becomes difficult to fill the entire voids of the second solid electrolyte layer 2 with the electrode layer material, and the energy density per unit volume becomes low. Also, the amount of shrinkage when forming the second solid electrolyte layer 2 becomes large, and the second solid electrolyte layer 2 is likely to peel off at the 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, particularly 20% or more. If the ratio is too small, the contact area between the electrode layer and the solid electrolyte sheet 10 becomes small, and the ionic conductivity decreases. As a result, the rapid charge-discharge characteristics tend to deteriorate. Although the upper limit of the above ratio is not particularly limited, in reality, it is 99% or less, and further 97% or less.

[0061] The second solid electrolyte layer 2 may be composed of a plurality of layers having different porosities. In that case, it is preferable that the plurality of layers having different porosities are provided such that the porosity becomes lower in the layer closer to the first solid electrolyte layer 1. In this case, the number of layers of the second solid electrolyte layer 2 is preferably 2 or more, 3 or more, 4 or more, particularly 5 or more. Although the upper limit is not particularly limited, considering the manufacturing efficiency, it is preferably 200 layers or less, 150 layers or less, 100 layers or less, 50 layers or less, 20 layers or less, and further 10 layers or less.

[0062] As described above, when the thickness of the second solid electrolyte layer 2 is too large, there is a problem that the amount of shrinkage when forming the second solid electrolyte layer 2 becomes large, and it becomes easy to peel off at the interface with the first solid electrolyte layer 1. On the other hand, if two or more layers having different porosities are provided in the second solid electrolyte layer 2 as described above, and particularly the porosity is made lower in the layer closer to the first solid electrolyte layer 1, the amount of shrinkage near the interface with the first solid electrolyte layer 1 becomes small, so that 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 a plurality of layers, the porosity of the layer closest to the first solid electrolyte layer 1 is preferably 50% or less, 45% or less, particularly 40% or less. By doing so, the amount of shrinkage near the interface with the first solid electrolyte layer 1 becomes small, and peeling from the first solid electrolyte layer 1 can be suppressed, which is preferable.

[0064] When the second solid electrolyte layer 2 is composed of a plurality of layers, the difference in porosity between the layer closest to the first solid electrolyte layer 1 and the layer farthest from it is preferably 5% or more, 10% or more, particularly 15% or more. By doing so, it becomes possible to achieve both suppression of peeling from the first solid electrolyte layer 1 and improvement in the adhesion between the electrode layer and the solid electrolyte sheet 10.

[0065] Even when the second solid electrolyte layer 2 is composed of a plurality of layers, the porosity of the second solid electrolyte layer 2 as a whole is preferably 20% or more, 25% or more, particularly 30% or more, as described above. Also, 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, particularly 20 to 500 μm, as described above. In addition, 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, particularly 20 to 500 μm.

[0066] In addition, it is preferable that a metal layer is 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, the provision of a metal layer between the second solid electrolyte layer 2 and the electrode layer improves the wettability between the electrode layer and the second solid electrolyte layer, enhances the adhesion, and enables reduction of the interfacial resistance. Thereby, an all-solid-state battery excellent in discharge capacity can be obtained. Also, for the following reasons, it is possible to improve the cycle characteristics of the all-solid-state battery.

[0067] When the adhesion between the electrode layer and the second solid electrolyte layer 2 is poor, the movement of sodium ions or lithium ions accompanying charge and discharge is inhibited, and there is a tendency to precipitate as needle-shaped metal crystals (dendrites). Since the needle-shaped metal crystals become high-resistance sites, variations are likely to occur in the in-plane resistance of the interface between the electrode layer and the second solid electrolyte layer 2, and as a result, the cycle characteristics tend to deteriorate. On the other hand, the provision of 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, so that the precipitation of needle-shaped metal crystals is suppressed and the cycle characteristics can be improved.

[0068] The metal constituting the metal layer is not particularly limited. For example, Sn, Ti, Bi, Au, Al, Cu, Sb, Pb, etc. can be used. These metals constituting the metal layer may be used alone or two or more of them may be laminated. Further, the metal layer may 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, it becomes easier to obtain the above effects.

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

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

[0072] (i) First manufacturing method (a) Preparation of the first green sheet for the solid electrolyte layer An organic vehicle containing a binder is added to the solid electrolyte powder to prepare a slurry. As the binder, polypropylene carbonate or the like can be used. Solvents, plasticizers, etc. can be added to the organic vehicle in addition to the binder. The solvent can be either water or an organic solvent such as ethanol or acetone. However, when water is used as the solvent, an alkali component such as sodium may elute from the raw material powder, increasing the pH of the slurry and causing the raw material powder to aggregate. Therefore, it is preferable to use an organic solvent.

[0073] Note that 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 process) 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 ) of the solid electrolyte powder and the raw material powder of the solid electrolyte powder is preferably 10 μm or less, particularly preferably 5 μm or less. If the average particle diameter of the raw material powder is too large, the contact area between the raw material powders decreases, making it difficult for the sintering of the solid electrolyte powders and the solid-state reaction between the raw material powders of the solid electrolyte powder to proceed sufficiently. Also, the thinning of the solid electrolyte sheet 10 tends to become difficult. The lower limit of the average particle diameter of the solid electrolyte powder and the raw material powder of the solid electrolyte powder is not particularly limited, but is realistically 0.05 μm or more, and further 0.1 μm or more.

[0075] The obtained slurry is applied onto 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 the green sheet for the second solid electrolyte layer An organic vehicle containing a binder is added to a mixed powder containing the solid electrolyte powder and / or the raw material powder of the solid electrolyte powder and a polymer powder to prepare a slurry, and the slurry is applied onto a substrate and dried to obtain a green sheet for the second solid electrolyte layer. The manufacturing process of the green sheet for the second solid electrolyte layer is different only in that a polymer powder is added as a solid content compared to the manufacturing process of the green sheet for the first solid electrolyte layer, and the same materials and methods can be adopted otherwise.

[0077] The polymer powder is a material for forming voids 2v in the second solid electrolyte layer 2 by being combusted and removed in a subsequent firing process. Examples of the polymer powder include acrylic resins, polyacrylonitrile, polymethacrylonitrile, and polystyrene.

[0078] The average particle diameter (D 50 ) of the polymer powder is preferably 0.1 to 100 μm, 1 to 80 μm, 5 to 70 μm, particularly preferably 10 to 50 μm. If the average particle diameter of the polymer powder is too small, it becomes difficult to form three-dimensionally connected voids in the second solid electrolyte layer 2. On the other hand, if the average particle diameter of the polymer powder is too large, the sintering of the second solid electrolyte layer 2 becomes insufficient and the ionic conductivity decreases, and as a result, the rate characteristics tend to deteriorate.

[0079] The content ratio of the solid electrolyte powder and / or the 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 in terms of volume ratio. If the content of the polymer powder is too small, it becomes difficult to form three-dimensionally connected voids in the second solid electrolyte layer 2. On the other hand, if the content of the polymer powder is too large, the sintering of the second solid electrolyte layer 2 becomes insufficient and the ionic conductivity decreases, and as a result, the rate characteristics tend to deteriorate.

[0080] In addition, the content ratio of the solid electrolyte powder and / or the 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 in terms of mass ratio. The reasons for the limitation are as described above.

[0081] The second solid electrolyte layer composed of a plurality of layers with different porosities is preferably produced by laminating two or more green sheets produced from slurries with different content ratios of the solid electrolyte powder and / or the raw material powder of the solid electrolyte powder and the polymer powder.

[0082] In the slurry for forming the layer farthest from the first solid electrolyte layer 1, the content ratio of the solid electrolyte powder and / or the 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 in terms of volume ratio. In terms of mass ratio, it 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 content of the polymer powder is too small, it becomes difficult to form three-dimensionally connected voids. On the other hand, if the content of the polymer powder is too large, the sintering of the second solid electrolyte layer 2 becomes insufficient, the ionic conductivity decreases, and as a result, the rate characteristics tend to decrease.

[0083] In the slurry for forming the layer closest to the first solid electrolyte layer 1, the content ratio of the solid electrolyte powder and / or the raw material 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 in terms of volume ratio. In terms of mass ratio, it 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 content of the polymer powder is too small, it becomes difficult to form three-dimensionally connected voids. On the other hand, if the content of the polymer powder is too large, it is likely to peel off from the first solid electrolyte layer 1 due to the shrinkage during the formation of the second solid electrolyte layer 2.

[0084] (c) Fabrication of the laminate The green sheet for the second solid electrolyte layer is laminated on one surface or both surfaces of the green sheet for the first solid electrolyte layer obtained above to obtain a laminate. Here, after laminating the green sheets, it is preferable to press (preferably hot press). In this way, the adhesion of each green sheet is improved, and the adhesion between the first solid electrolyte layer 1 and the second solid electrolyte layer 2 can also be improved in the obtained solid electrolyte sheet 10.

[0085] The second solid electrolyte layer composed of a plurality of layers with different porosity is preferably produced by laminating 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 laminate the green sheet with a larger content ratio of solid electrolyte powder and / or raw material powder of solid electrolyte powder closer to the green sheet for the first solid electrolyte layer.

[0086] (d) Firing of the laminate By firing the laminate obtained above, the binder in the green sheet for the first solid electrolyte layer is removed to form the first solid electrolyte layer 1, and the binder and polymer particles in the green sheet for the second solid electrolyte layer are removed to form the second solid electrolyte layer 2. Thereby, the solid electrolyte sheet 10 is obtained.

[0087] The firing temperature may be appropriately selected according to the type of solid electrolyte used. When the solid electrolyte sheet contains β-alumina or β''-alumina, the firing temperature is more preferably 1400 °C or higher, particularly 1450 °C or higher, and especially 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, and it becomes difficult to generate the desired crystals. On the other hand, the upper limit of the firing temperature is preferably 1750 °C or lower, particularly 1700 °C or lower. If the firing temperature is too high, the evaporation amount of sodium components and the like increases, heterogeneous crystals precipitate, and the ionic conductivity of the solid electrolyte sheet 10 tends to decrease.

[0088] When the solid electrolyte contains NASICON crystals, the firing temperature is more 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, and it becomes difficult to generate 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 evaporation amount of sodium components and the like increases, heterogeneous crystals precipitate, and the ionic conductivity of the solid electrolyte sheet 10 tends to decrease.

[0089] In addition, the firing time is appropriately adjusted so that sintering proceeds sufficiently. Specifically, it is preferably 10 to 120 minutes, particularly preferably 20 to 80 minutes.

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

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

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

[0093] (c) Preparation of the laminate A laminate in which a slurry layer is formed on the surface of the first solid electrolyte layer 1 is obtained by applying the slurry to one or both surfaces of the first solid electrolyte layer 1.

[0094] (d) Firing of the laminate By firing the laminate obtained above, the binder and polymer particles in the slurry layer are removed to form the second solid electrolyte layer 2. Thereby, the solid electrolyte sheet 10 is obtained. The firing time and firing temperature can adopt the same conditions as those of the first manufacturing method.

[0095] In addition, in step (c), after obtaining a laminate by laminating a green sheet for the second solid electrolyte layer on the surface of the first solid electrolyte layer 1 instead of the slurry layer, the solid electrolyte sheet 10 can also be obtained by firing the laminate.

[0096] Similarly to the first manufacturing method, in the second manufacturing method as well, two or more slurries (or green sheets) with different mixing ratios of solid electrolyte powder and / or raw material powder of solid electrolyte powder and polymer powder are prepared, and these are applied to the surface of the first solid electrolyte layer 1, dried repeatedly, laminated, and then fired to form a second solid electrolyte layer composed of a plurality of layers with different porosities.

Examples

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

[0102] (a-2) Preparation of the green sheet for the second solid electrolyte layer The solid electrolyte powder and the polymer particles were weighed so as to have the volume ratios shown in Tables 1 and 2. As the polymer particles, acrylic polymer particles with an average particle diameter of 20 μm (Advancel HB2051 manufactured by Sekisui Chemical Co., Ltd.), crosslinked polymethyl methacrylate particles with an average particle diameter of 20 μm (MBX-20 manufactured by Sekisui Chemical Products Co., Ltd.), or crosslinked polymethyl methacrylate particles with an average particle diameter of 8 μm (MBX-8 manufactured by Sekisui Chemical Products Co., Ltd.) were used. To 100 parts by mass of these mixtures, 20 parts by mass of polypropylene carbonate was added as a binder, dispersed in N-methylpyrrolidone, and then sufficiently stirred with a rotary-revolution mixer to form a slurry. The obtained slurry was applied onto a PET film using a doctor blade, dried at 70 °C, and then peeled from the PET film to obtain a green sheet for the second solid electrolyte layer. For Example 9, two types of green sheets (the "first layer" and the "second layer" in Table 2) with different content ratios of the solid electrolyte powder and the polymer particles were prepared.

[0103] (a-3) Firing of the green sheet The green sheet for the second solid electrolyte was laminated on both sides of the green sheet for the first solid electrolyte layer obtained above, and after heat pressing, firing was carried out 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 a solid electrolyte sheet in which a porous second solid electrolyte layer was formed on both sides of a dense first solid electrolyte layer. For Example 9, the laminate obtained by laminating and heat pressing the "first layer" and "second layer" green sheets for the second solid electrolyte described in Table 2 on both sides of the green sheet for the first solid electrolyte layer, heat pressing, and then firing at 1600 °C. Here, the lamination was carried out such that the green sheet for the second solid electrolyte of the "first layer" was on the side of the green sheet for the first solid electrolyte layer.

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

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

[0106] After forming a gold electrode as an ion blocking electrode on the surface of the first solid electrolyte layer obtained above in a range of φ4 mm by RF sputtering, the resistance R1 of the first solid electrolyte layer was determined by the AC impedance method at a frequency of 1 to 10 7 Hz and an applied voltage of 5 mV. 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 also simply referred to as the solid electrolyte sheet) in which the second solid electrolyte layer was formed on both surfaces of the first solid electrolyte layer produced in (a-3) 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 in the range of □1 cm × 1 cm in plan view of the second solid electrolyte layer) was determined by the following procedure.

[0109] First, the ionic conductivity σ1 of the first solid electrolyte layer is obtained from the following formula (1). Here, A1 is the surface area per unit area of the first solid electrolyte layer. Since the first solid electrolyte layer is dense and the surface is smooth, A1 is 1 cm 2It can be regarded as. Also, t1 is the thickness of the first solid electrolyte layer.

[0110]

Number

[0111]

Number

[0112] (b) Fabrication of the positive electrode layer (b-1) Fabrication of the positive electrode active material precursor powder Using sodium metaphosphate (NaPO3), ferric oxide (Fe2O3), and orthophosphoric acid (H3PO4) as raw materials, the raw material powder was formulated so as to be 40% Na2O, 20% Fe2O3, and 40% P2O5 in mol%, and melted in an air atmosphere at 1250 °C for 45 minutes. Then, the molten glass was poured onto a pair of rollers and formed into a film shape while being rapidly cooled to produce a positive electrode active material precursor.

[0113] Regarding the obtained positive electrode active material precursor, ball milling using a φ20 mm Al2O3 grinding ball was performed for 5 hours, and then ball milling in ethanol using a φ5 mm ZrO2 grinding ball was performed for 100 hours. Furthermore, using a planetary ball mill P6 manufactured by Fritsch and into which φ0.3 mm ZrO2 grinding balls were introduced, grinding was performed at 300 rpm for 5 hours (with a 10-minute break every 10 minutes) to obtain a positive electrode active material precursor powder with an average particle diameter D 50 of 0.2 μm.

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

[0115] (c) Fabrication of test battery The above-mentioned slurried positive electrode composite material was applied to one surface of the solid electrolyte sheet obtained above with an area of 1 cm 2 and dried at 70°C for 3 hours. Next, in a mixed gas atmosphere of nitrogen and hydrogen (96% by volume of nitrogen, 4% by volume of hydrogen), the positive electrode composite material was sintered and the positive electrode active material precursor powder was crystallized by firing at 525°C for 30 minutes to form a positive electrode layer having the thicknesses described in Tables 1 and 2. When the X-ray diffraction pattern of the obtained positive electrode layer was confirmed, diffraction lines derived from Na2FeP2O7, which is an active material crystal, were confirmed.

[0116] A cross-sectional image near the interface between the first solid electrolyte layer and the second solid electrolyte layer in the solid electrolyte sheet of Example 1 is shown in FIG. 2. FIG. 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 FIG. 2(b) is a diagram showing a contour line which is a curve drawn along the surface of the second solid electrolyte layer.

[0117] Next, using a sputtering apparatus (SC-701AT manufactured by Sunyu Electronics Co., Ltd.), a gold electrode with a thickness of 300 nm, which is a current collector, was formed on the surface of the positive electrode layer. Then, metallic sodium as the counter electrode was pressure-bonded to the surface of the solid electrolyte sheet on the side opposite to the side where the positive electrode layer was formed, placed on the lower lid of the coin cell, and then covered with the upper lid to fabricate a CR2032 type test cell. For Example 5, a gold electrode with a thickness of 90 nm was formed on the surface of the solid electrolyte sheet on the side opposite to the side where the positive electrode layer was formed using a sputtering apparatus (SC-701AT manufactured by Sunyu 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 conducted using the above test cell. 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 constant current (CC) charging from the open circuit voltage (OCV) to 4.5 V, and discharging (sodium ion intercalation into the positive electrode active material) was performed by CC discharging from 4.5 V to 2 V. The C-rate was set to 0.1C, 0.5C, or 5C, and the test was conducted 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. In addition, a cycle test was conducted at 0.5C. Specifically, the discharge capacity retention rate ((after 300 cycles / after 1 cycle) × 100 (%)) was determined from the discharge capacities of the first cycle and the 300th cycle at 0.5C.

[0119] As shown in Tables 1 and 2, in Examples 1 to 9, voids communicating three-dimensionally were sufficiently formed inside the second solid electrolyte layer, and the resistance of the solid electrolyte sheet was reduced to as low as 5.6 to 51.0 Ω. Also, the ratio of the contour length to the reference length increased to 1.5 to 3.8. As a result, the contact area between the solid electrolyte sheet and the positive electrode layer increased, and good discharge capacities were shown at 0.1C of 62 to 83 mAh / g and at 0.5C of 32 to 65 mAh / g. In Example 5, since a metal layer was provided between the solid electrolyte layer and metallic sodium, the rate performance was improved, showing a discharge capacity of 25 mAh / g at 5C, and the discharge capacity retention rate was also as good as 90%. In Example 8, since the thickness of the second solid electrolyte layer was as large as 118 μm and the contact area between the electrode layer and the solid electrolyte layer increased, the rate performance was improved, showing a discharge capacity of 13 mAh / g at 10C. In Example 9, since the overall thickness of the second solid electrolyte layer was as large as 197 μm, discharge capacities of 31 mAh / g at 5C and 20 mAh / g at 10C were shown. In Example 9, since the second solid electrolyte layer was composed of two layers with different porosities, even when the thickness of the second solid electrolyte layer was made extremely large at 197 μm, peeling did not occur at the interface with the first solid electrolyte layer.

[0120] On the other hand, in Comparative Examples 1 and 2, only independent voids existed inside the second solid electrolyte layer, and three-dimensionally communicating voids were not formed. Therefore, the resistance of the solid electrolyte sheet increased to 117.5 to 125.1 Ω. Also, the ratio of the contour length to the reference length decreased to 1.1. As a result, the contact area between the solid electrolyte sheet and the positive electrode layer decreased. In Comparative Example 1, although a relatively good discharge capacity of 74 mAh / g was shown at 0.1C, the discharge capacity was low at 14 mAh / g at 0.5C. In Comparative Example 2, since the thickness of the positive electrode layer was as large as 93 μm, the positive electrode layer peeled off from the second solid electrolyte layer during firing, and charge and discharge could not be performed.

Explanation of Signs

[0121] 1 First solid electrolyte layer 1a, 1b Main surfaces 2 Second solid electrolyte layer 2a, 2b Main surfaces 2s Solid electrolyte 2v gap 10 solid electrolyte sheet

Claims

1. A solid electrolyte sheet in which a second solid electrolyte layer is formed on at least one surface of a first solid electrolyte layer, wherein the second solid electrolyte layer is a porous solid electrolyte layer having three-dimensionally interconnected voids, and the arithmetic mean roughness Ra of the second solid electrolyte layer is 8.2 μm or more. A solid electrolyte sheet characterized by that.

2. In a cross-sectional image near 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 used as a reference line, and a curve drawn along the surface of the second solid electrolyte layer is used as a contour line. The solid electrolyte sheet according to claim 1, wherein the ratio of the length of the contour line to the length of the reference line (length of the contour line / length of the reference line) is 1.3 to 50.

3. The solid electrolyte sheet according to claim 1 or 2, wherein the second solid electrolyte layer is composed of a plurality of layers having different porosities.

4. The solid electrolyte sheet according to claim 3, wherein among the plurality of layers having different porosities, the layer closer to the first solid electrolyte layer has a lower porosity.

5. The surface area per 1 cm in plan view of the second solid electrolyte layer ２ is 3 cm ２ or more, and the solid electrolyte sheet according to any one of claims 1 to 4.

6. The solid electrolyte sheet according to any one of claims 1 to 5, wherein the second solid electrolyte layer is formed on both surfaces of the first solid electrolyte layer.

7. The solid electrolyte sheet according to any one of claims 1 to 6, characterized in that the thickness is 2400 μm or less.

8. The solid electrolyte sheet according to any one of claims 1 to 7, 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.

9. The solid electrolyte sheet according to any one of claims 1 to 8, characterized in that it is for an all-solid-state sodium ion secondary battery.

10. An all-solid-state secondary battery comprising the solid electrolyte sheet according to any one of claims 1 to 9 and an electrode layer formed on the surface of the second solid electrolyte layer in the solid electrolyte sheet.

11. The all-solid-state secondary battery according to claim 10, wherein the material constituting the electrode layer penetrates into the voids in the second solid electrolyte layer.

12. A method for manufacturing the solid electrolyte sheet according to any one of claims 1 to 9, Step of preparing a first green sheet for a solid electrolyte layer by 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, applying the slurry onto a substrate, and drying it; Step of preparing a second green sheet for a solid electrolyte layer 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, applying the slurry onto a substrate, and drying it; Step of obtaining a laminate by laminating the second green sheet for a solid electrolyte layer on at least one surface of the first green sheet for a solid electrolyte layer; Step of firing the laminate to remove the binder in the first green sheet for a solid electrolyte layer to form a first solid electrolyte layer, and removing the binder and the polymer powder in the second green sheet for a solid electrolyte layer to form a second solid electrolyte layer; A method for manufacturing a solid electrolyte sheet, characterized by comprising the above steps.

13. A method for manufacturing a solid electrolyte sheet according to any one of Claims 1 to 9, comprising: Step of preparing a first solid electrolyte layer; Step of 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; Step of obtaining a laminate in which a slurry layer is formed on at least one surface of the first solid electrolyte layer by applying the slurry onto the at least one surface of the first solid electrolyte layer; and Step of firing the laminate to remove the binder and the polymer powder in the slurry layer to form a second solid electrolyte layer; A method for manufacturing a solid electrolyte sheet, characterized by comprising the above steps.

14. The method for manufacturing a solid electrolyte sheet according to Claim 12 or 13, characterized in that the average particle diameter of the polymer powder is 0.1 to 100 μm.

15. The method for manufacturing a solid electrolyte sheet according to any one of Claims 12 to 14, characterized in that the content ratio of the solid electrolyte powder and / or the raw material powder of the solid electrolyte powder to the polymer powder is 75:25 to 3:97 by volume ratio.

Images