Electrode composite material for sodium-ion secondary batteries, electrodes for sodium-ion secondary batteries, and all-solid-state sodium-ion secondary batteries

Abstract

The present invention provides an electrode composite material for sodium-ion secondary batteries that can achieve a high level of both electronic and ionic conductivity, and can achieve a high level of both cycle characteristics and output characteristics. [Solution] An electrode composite for a sodium-ion secondary battery comprising a positive electrode active material containing a sodium transition metal phosphate crystal containing Na, M (where M is at least one transition metal element selected from Cr, Fe, Mn, Co, V, and Ni), P, and O, and a conductive additive containing fibrous carbon and granular carbon.

Description

[Technical Field]

[0001] The present invention relates to an electrode composite material for sodium-ion secondary batteries, an electrode for a sodium-ion secondary battery using the sodium-ion electrode composite material, and an all-solid-state sodium-ion secondary battery using the sodium-ion electrode. [Background technology]

[0002] Lithium-ion rechargeable batteries have established themselves as essential high-capacity, lightweight power sources for mobile devices, electric vehicles, and other applications. However, current lithium-ion rechargeable batteries primarily use flammable organic electrolytes, raising concerns about the risk of fire. To address this issue, development is underway on all-solid-state lithium-ion batteries that use a solid electrolyte instead of organic electrolytes. Furthermore, due to concerns about rising global raw material costs for lithium, the development of all-solid-state batteries is also progressing.

[0003] Patent Document 1 below contains Na x (Fe 1-a M a ) y P2O z A positive electrode material for an energy storage device is disclosed, comprising a positive electrode active material represented by (M is at least one transition metal element selected from the group consisting of Cr, Mn, Co, and Ni, 1.2≦x≦2.8, 0.95≦y≦1.6, 0≦a≦0.9, 7≦z≦8), a sodium ion conductive solid electrolyte, and conductive carbon. Patent Document 1 states that acetylene black, which has high electronic conductivity, is preferred as the conductive carbon. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] International Publication No. 2016 / 084573 [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] However, even in the positive electrode material for a power storage device of Patent Document 1, it is difficult to achieve both high electron conductivity and high ion conductivity at the same time, and there is a problem that it is still difficult to sufficiently improve battery characteristics.

[0006] An object of the present invention is to provide an electrode mixture for a sodium-ion secondary battery, an electrode for a sodium-ion secondary battery using the electrode mixture for a sodium-ion secondary battery, and an all-solid sodium-ion secondary battery using the electrode for a sodium-ion secondary battery, which can achieve both high electron conductivity and high ion conductivity, and can achieve both high cycle characteristics and high output characteristics.

Means for Solving the Problems

[0007] The electrode mixture for a sodium-ion secondary battery according to Aspect 1 of the present invention includes a positive electrode active material containing sodium transition metal phosphate crystals containing Na, M (M is at least one transition metal element selected from Cr, Fe, Mn, Co, V, and Ni), P, and O, fibrous carbon, and a conductive auxiliary agent containing granular carbon.

[0008] The electrode mixture for a sodium-ion secondary battery according to Aspect 2 is the electrode mixture for a sodium-ion secondary battery of Aspect 1, where the BET specific surface area of the granular carbon is S P (m 2 / g), the content of the granular carbon with respect to the electrode mixture for a sodium-ion secondary battery is W P (mass %), the BET specific surface area of the fibrous carbon is S F (m 2 / g), the content of the fibrous carbon with respect to the electrode mixture for a sodium-ion secondary battery is W F (mass %), and when the surface area S C of the conductive auxiliary agent per 1 g of the electrode mixture for a sodium-ion secondary battery represented by the following formula (1) is 1 m 2 / g or more and 5.5 m 2 / g or less, it is preferable.

[0009] S c =S p ×W p / 100+S f ×W f / 100···(1)

[0010] The electrode composite material for sodium-ion secondary batteries according to Embodiment 3 is the electrode composite material for sodium-ion secondary batteries according to Embodiment 1 or Embodiment 2, and it is preferable that the fibrous carbon is carbon nanotubes.

[0011] The electrode mixture for sodium-ion secondary batteries according to Embodiment 4 is an electrode mixture for sodium-ion secondary batteries according to any one of Embodiments 1 to 3, and it is preferable that the granular carbon is acetylene black.

[0012] The electrode mixture for sodium-ion secondary batteries according to Embodiment 5 is an electrode mixture for sodium-ion secondary batteries according to any one embodiment of Embodiments 1 to 4, and more preferably contains a solid electrolyte.

[0013] The electrode composite material for sodium-ion secondary batteries according to embodiment 6 is the electrode composite material for sodium-ion secondary batteries according to embodiment 5, and it is preferable that the solid electrolyte contains at least one selected from the group consisting of β-alumina, β''-alumina, and NASICON-type crystals.

[0014] The electrode composite material for sodium-ion secondary batteries according to Embodiment 7 is an electrode composite material for sodium-ion secondary batteries according to any one embodiment of Embodiments 1 to 6, and is preferably for use in an all-solid-state sodium-ion secondary battery.

[0015] The electrode for a sodium-ion secondary battery according to Embodiment 8 is characterized by containing a sodium-ion secondary battery electrode composite material according to any one of Embodiments 1 to 7.

[0016] The all-solid-state sodium-ion secondary battery according to Embodiment 9 is characterized by having the sodium-ion secondary battery electrode of Embodiment 8 as the positive electrode. [Effects of the Invention]

[0017] According to the present invention, it is possible to provide an electrode composite material for sodium-ion secondary batteries that can achieve a high level of both electronic conductivity and ionic conductivity, and a high level of both cycle characteristics and output characteristics, an electrode for sodium-ion secondary batteries using the sodium-ion secondary battery electrode composite material, and an all-solid-state sodium-ion secondary battery using the electrode for sodium-ion secondary batteries. [Brief explanation of the drawing]

[0018] [Figure 1] Figure 1 is a schematic cross-sectional view showing an all-solid-state sodium-ion secondary battery according to an embodiment of the present invention. [Figure 2] Figure 2 is a schematic cross-sectional view showing the solid electrolyte layer in the all-solid-state sodium-ion secondary battery shown in Figure 1. [Modes for carrying out the invention]

[0019] Preferred embodiments are described below. However, the following embodiments are merely illustrative, and the present invention is not limited to these embodiments. In addition, in each drawing, components having substantially the same function may be referred to by the same reference numerals.

[0020] The electrode composite material for sodium-ion secondary batteries of the present invention comprises a positive electrode active material and a conductive additive.

[0021] The positive electrode active material includes sodium transition metal phosphate crystals containing Na, M (where M is at least one transition metal element selected from Cr, Fe, Mn, Co, V, and Ni), P, and O. Specific examples include Na2FeP2O7, Na4Fe3(PO4)2(P2O7), Na4Fe5(PO4)2(P2O7)2, and Na 3.64 Fe 2.18 (P2O7)2, Na3Fe2(PO4)(P2O7), NaFePO4, Na2MnP2O7, Na4Mn3(PO4)2(P2O7), Na4Mn5(PO4)2(P2O7)2, Na3.64 Mn 2.18 (P2O7)2, Na3V2(PO4)3, NaNiPO4, Na2NiP2O7, Na4Ni3(PO4)2(P2O7), Na4Ni5(PO4)2(P2O7)2, Na 3.64 Ni 2.18 (P2O7)2, Na4Ni7(PO4)6, Na3Ni3(PO4)2(P2O7), NaCoPO4, Na2CoP2O7, Na4Co3(PO4)2(P2O7), Na4Co5(PO4)2(P2O7)2, Na 3.5 Cr 1.5 Co 0.5 (PO4)3, Na 3.64 Co 2.18 Examples include (P2O7)2. These sodium transition metal phosphate crystals are preferred because they have high capacity and excellent chemical stability. Among them, triclinic crystals belonging to space group P1 or P-1 are particularly preferred, especially those with the general formula Na x M y P2O z Crystals represented by (1.2≦x≦2.8, 0.95≦y≦1.6, 6.5≦z≦8) are preferred because they have excellent cycle characteristics.

[0022] The positive electrode active material preferably consists of the above-mentioned positive electrode active material crystals and an amorphous phase. The positive electrode active material may be formed, for example, by calcining a positive electrode active material precursor powder such as glass powder. By calcining the positive electrode active material precursor powder, positive electrode active material crystals are precipitated. In addition, an amorphous phase is formed together with the positive electrode active material crystals by calcination. The formation of the amorphous phase can improve the alkali ion conductivity within the electrode layer. It can also improve the adhesion between the current collector and the electrode layer.

[0023] The lower limit of the content of positive electrode active material in the electrode mixture for sodium-ion secondary batteries is not particularly limited, but is preferably 30% or more by mass, preferably 40% or more, 50% or more, and especially preferably 60% or more. The upper limit of the content of positive electrode active material is not particularly limited, but is preferably 99.9% or less by mass, preferably 95% or less, and especially preferably 90% or less. When the content of positive electrode active material in the electrode mixture for sodium-ion secondary batteries is within the above ranges, the battery capacity can be increased even more effectively.

[0024] The conductive additive includes fibrous carbon and granular carbon. Examples of fibrous carbon include vapor-phase carbon fiber conductive additive (VGCF), carbon nanotubes, and carbon nanofibers. Among these, carbon nanotubes are preferred as the fibrous carbon because they can improve electronic conductivity without reducing the ionic conductivity of the electrode. Examples of granular carbon include acetylene black, carbon black, and Ketjen black. Among these, acetylene black is preferred as the granular carbon because it can improve the electronic conductivity of the electrode and improve the battery's cycle characteristics.

[0025] The BET specific surface area of ​​carbon nanotubes is preferably 50 m². 2 / g or more, comfortably 100m 2 It is 1 / g or more. The upper limit of the BET specific surface area of ​​carbon nanotubes is not particularly limited, but is preferably 2000m 2 / g or less, more preferably 1500m 2 The value is less than / g. When the BET specific surface area of ​​carbon nanotubes is within the above range, the electronic conductivity of the electrode composite material for secondary batteries can be further improved with a smaller amount of additive. The BET specific surface area of ​​carbon nanotubes can be measured by the BET multipoint method using nitrogen as the adsorbate.

[0026] The length of the carbon nanotubes is preferably 0.5 μm or more, more preferably 1 μm or more, even more preferably 3 μm or more, preferably 50 μm or less, more preferably 40 μm or less, and even more preferably 30 μm or less. The diameter of the carbon nanotubes is preferably 1 nm or more, more preferably 3 nm or more, preferably 30 nm or less, and even more preferably 20 nm or less. When the length and diameter of the carbon nanotubes are within the above ranges, the electronic conductivity of the electrode composite material for secondary batteries can be further enhanced with a smaller amount of additive.

[0027] The length and diameter of carbon nanotubes can be determined, for example, by measuring the length and diameter of 200 carbon nanotubes individually using a scanning electron microscope and deriving the average value from these measurements.

[0028] The BET specific surface area of ​​granular carbon is preferably 20 m². 2 / g or more, more preferably 40m 2 It is 1 / g or more. The upper limit of the BET specific surface area of ​​granular carbon is not particularly limited, but is preferably 200m 2 Less than / g, more preferably 150m 2 It is less than / g. When the BET specific surface area of ​​granular carbon is within the above range, the electronic conductivity of the electrode composite material for secondary batteries can be further improved with a smaller amount of additive. The BET specific surface area of ​​granular carbon can be measured by the BET multipoint method using nitrogen as the adsorbate.

[0029] In the electrode mixture for sodium-ion secondary batteries, the lower limit of the conductive additive content is preferably 0.1% or more by mass, and particularly preferably 0.2% or more. The upper limit of the conductive additive content is preferably 20% or less by mass, and particularly preferably 10% or less, and particularly preferably 5% or less. When the conductive additive content in the electrode mixture for sodium-ion secondary batteries is within the above range, it is possible to further improve ionic conductivity while ensuring high electronic conductivity in the electrode mixture for sodium-ion secondary batteries, thereby more effectively improving battery characteristics.

[0030] The electrode composite material for sodium-ion secondary batteries has a BET specific surface area of ​​granular carbon. p (m 2 ( / g), the granular carbon content in the electrode composite material for sodium-ion secondary batteries is W p (mass%), the BET specific surface area of ​​fibrous carbon is S f (m 2 ( / g), the content of the fibrous carbon in the electrode composite material for sodium-ion secondary batteries is W f When expressed as (mass %), the conductive additive surface area S per gram of electrode material for sodium-ion secondary batteries is given by the following formula (1). C is 1m 2 It is preferable that it be 1.5m or more per gram. 2 / g or more, 2m 2 / g or more, especially 2.5m 2 It is preferable that it is 1 / g or more. C If S is too small, both the cycle characteristics and output characteristics of the resulting battery tend to deteriorate. C 5.5m 2 It is preferable that the amount be less than or equal to 5m 2 / g or less, 4.5m 2 / g or less, especially 4m 2 It is preferable that it is less than or equal to / g. C If the value is too large, the output characteristics of the resulting battery tend to deteriorate.

[0031] S c =S p ×W p / 100+S f ×W f / 100···(1)

[0032] The electrode composite material for sodium-ion secondary batteries preferably further contains a solid electrolyte. The solid electrolyte can be the one described in the "Solid Electrolyte Layer" section below. When the electrode composite material for sodium-ion secondary batteries contains a solid electrolyte, the lower limit of the solid electrolyte content in the electrode composite material is preferably 0.1% or more by mass, 5% or more, and particularly preferably 10% or more. The upper limit of the solid electrolyte content in the electrode composite material for sodium-ion secondary batteries is preferably 70% or less by mass, 60% or less, 50% or less, 40% or less, and particularly preferably 30% or less. When the solid electrolyte content in the electrode composite material for sodium-ion secondary batteries is within the above ranges, the positive electrode active material and the solid electrolyte can be integrated. As a result, ionic conductivity can be further improved, and battery characteristics can be more effectively enhanced.

[0033] (All-solid-state sodium-ion secondary battery) Figure 1 is a schematic cross-sectional view showing an all-solid-state sodium-ion secondary battery according to one embodiment of the present invention.

[0034] As shown in Figure 1, the all-solid-state sodium-ion secondary battery 1 comprises a solid electrolyte layer 2, a positive electrode layer 3, a negative electrode layer 4, a first current collector layer 5, and a second current collector layer 6.

[0035] The solid electrolyte layer 2 has opposing first main surface 2a and second main surface 2b. A positive electrode layer 3 is provided on the first main surface 2a of the solid electrolyte layer 2. In this embodiment, the positive electrode layer 3 and the solid electrolyte layer 2 are in contact. A first current collector layer 5 is provided on the main surface of the positive electrode layer 3 opposite to the solid electrolyte layer 2. A negative electrode layer 4 is provided on the second main surface 2b of the solid electrolyte layer 2. In this embodiment, the negative electrode layer 4 and the solid electrolyte layer 2 are in contact. The negative electrode layer 4 is provided in a position that overlaps with the positive electrode layer 3 in a plan view. Furthermore, a second current collector layer 6 is provided on the main surface of the negative electrode layer 4 opposite to the solid electrolyte layer 2. Note that the first current collector layer 5 and the second current collector layer 6 may be omitted.

[0036] Lead electrodes may be connected to the first current collector layer 5 and the second current collector layer 6 of the all-solid-state sodium-ion secondary battery 1. The lead electrodes electrically connect the all-solid-state sodium-ion secondary battery 1 to the outside.

[0037] The details of each layer in the all-solid-state sodium-ion secondary battery of the present invention will be described below.

[0038] (Solid electrolyte layer) Figure 2 is a schematic cross-sectional view showing the solid electrolyte layer in the embodiment shown in Figure 1.

[0039] The solid electrolyte layer 2 comprises a first solid electrolyte layer 7 and a pair of second solid electrolyte layers 8. The first solid electrolyte layer 7 has a third main surface 7a and a fourth main surface 7b. The third main surface 7a and the fourth main surface 7b face each other. One of the pair of second solid electrolyte layers 8 is provided on the third main surface 7a of the first solid electrolyte layer 7. The other second solid electrolyte layer 8 is provided on the fourth main surface 7b of the first solid electrolyte layer 7.

[0040] The first solid electrolyte layer 7 is specifically a dense layer. On the other hand, the second solid electrolyte layer 8 is specifically a porous layer. The second solid electrolyte layer 8 has three-dimensionally interconnected voids. It is desirable that the first solid electrolyte layer 7 and the second solid electrolyte layer 8 are integrated into one unit.

[0041] The first solid electrolyte layer 7 has a denser structure than the second solid electrolyte layer 8. As a result, the first solid electrolyte layer 7 not only has the function of conducting ions, but also functions as a base layer to ensure the mechanical strength of the solid electrolyte layer 2.

[0042] As shown in Figure 2, in this embodiment, the second solid electrolyte layer 8 is provided on both the third main surface 7a and the fourth main surface 7b of the first solid electrolyte layer 7. However, the second solid electrolyte layer 8 may be provided on either the third main surface 7a or the fourth main surface 7b of the first solid electrolyte layer 7. Alternatively, the solid electrolyte layer 2 may consist only of the dense first solid electrolyte layer 7.

[0043] In this embodiment, a positive electrode layer 3 is provided on one of the second solid electrolyte layers 8. A negative electrode layer 4 is provided on the other second solid electrolyte layer 8. In this case, an anchoring effect acts between each of the porous second solid electrolyte layers 8 and the positive electrode layer 3 and the negative electrode layer 4.

[0044] More specifically, when the positive electrode layer 3 is formed on the surface of one of the second solid electrolyte layers 8, the active material powder constituting the positive electrode layer 3 easily penetrates into the voids. Similarly, when the negative electrode layer 4 is formed on the surface of the other second solid electrolyte layer 8, the active material powder constituting the negative electrode layer 4 easily penetrates into the voids. As a result, an anchoring effect acts between each second solid electrolyte layer 8 and the formed positive electrode layer 3 and negative electrode layer 4. Therefore, the adhesion between the solid electrolyte layer 2 and the positive electrode layer 3 and negative electrode layer 4 can be improved. This makes it possible to lower the contact resistance between the solid electrolyte layer 2 and the positive electrode layer 3 and negative electrode layer 4.

[0045] The porosity of the first solid electrolyte layer 7 is preferably smaller than that of the second solid electrolyte layer 8. The porosity is defined by the following equation (1). In equation (1), p is the bulk density and p0 is the true density.

[0046] Porosity = (1-p / p0)×100(%)…Equation (1)

[0047] The porosity of the first solid electrolyte layer 7 is preferably 20% or less, more preferably 10% or less, and even more preferably 5% or less. The lower limit of the porosity of the first solid electrolyte layer 7 is not particularly limited, but can be, for example, 0.1%.

[0048] The porosity of the second solid electrolyte layer 8 is preferably 25% or more, more preferably 30% or more, and even more preferably 40% or more. On the other hand, the porosity of the second solid electrolyte layer 8 is preferably 97% or less, more preferably 95% or less, and even more preferably 90% or less. When the porosity of the second solid electrolyte layer 8 is within the above range, three-dimensionally interconnected voids can be formed more easily, and the adhesion between the solid electrolyte layer 2 and the positive electrode layer 3 and the negative electrode layer 4 can be further improved.

[0049] The diameter of the pores constituting the voids in the second solid electrolyte layer 8 is preferably 0.1 μm or more, more preferably 0.5 μm or more, even more preferably 1 μm or more, preferably 100 μm or less, more preferably 80 μm or less, and even more preferably 60 μm or less. In this case, shrinkage of the solid electrolyte layer 2 can be further suppressed during the firing process when forming the solid electrolyte layer.

[0050] The pore diameter can be measured by performing 3D structural observation using X-ray CT and analyzing the images. It can also be determined by mercury intrusion or 3D reconstruction using SEM-FIB.

[0051] Furthermore, it is preferable that the second solid electrolyte layer 8 has pores with a diameter larger than the thickness of the first solid electrolyte layer 7. In this case, shrinkage of the solid electrolyte layer 2 can be further suppressed during the firing process when forming the solid electrolyte layer.

[0052] The arithmetic mean roughness Ra of the second solid electrolyte layer 8 is preferably 0.1 μm or more, more preferably 1 μm or more, even more preferably 5 μm or more, preferably 300 μm or less, more preferably 200 μm or less, and even more preferably 100 μm or less. In this case, the adhesion between the solid electrolyte layer 2 and the positive electrode layer 3 or the negative electrode layer 4 can be further improved.

[0053] The second solid electrolyte layer 8 may be composed of multiple layers with different porosities. In this case, it is preferable that the multiple layers with different porosities are arranged such that the porosity decreases as the layer is closer to the first solid electrolyte layer 7. The number of multiple layers with different porosities is preferably two or more, more preferably three or more, even more preferably four or more, particularly preferably five or more, preferably 200 or fewer, more preferably 150 or fewer, even more preferably 100 or fewer, particularly preferably 50 or fewer, even more preferably 20 or fewer, and most preferably 10 or fewer.

[0054] The thickness of the first solid electrolyte layer 7 is preferably 0.01 μm or more, more preferably 0.1 μm or more, even more preferably 1 μm or more, and particularly preferably 5 μm or more. On the other hand, the thickness of the first solid electrolyte layer 7 is preferably 1000 μm or less, more preferably 500 μm or less, even more preferably 300 μm or less, and particularly preferably 100 μm or less.

[0055] If the thickness of the first solid electrolyte layer 7 is too thin, the mechanical strength may decrease, warping may occur, or a short circuit may occur between the positive electrode layer 3 and the negative electrode layer 4. On the other hand, if the thickness of the first solid electrolyte layer 7 is too thick, the ionic conductivity in the first solid electrolyte layer 7 tends to decrease. In addition, the energy density per unit volume of the all-solid-state sodium-ion secondary battery 1 tends to decrease.

[0056] The thickness of the second solid electrolyte layer 8 is preferably 1 μm or more, preferably 2 μm or more, more preferably 5 μm or more, and particularly preferably 10 μm or more. On the other hand, the thickness of the second solid electrolyte layer 8 is preferably 100 μm or less, and more preferably 80 μm or less.

[0057] If the thickness of the second solid electrolyte layer 8 is too thin, the amount of material constituting the positive electrode layer 3 and the negative electrode layer 4 that can penetrate the voids in the second solid electrolyte layer 8 will decrease. As a result, the contact area between the solid electrolyte layer 2 and the positive electrode layer 3 and the negative electrode layer 4 will decrease, and adhesion will tend to decline. In this case, the number of ion conduction paths at the interface between the solid electrolyte layer 2 and the positive electrode layer 3 and the negative electrode layer 4 will decrease, and the internal resistance of the all-solid-state sodium-ion secondary battery 1 will tend to increase. Consequently, the rapid charge and discharge characteristics of the all-solid-state sodium-ion secondary battery 1 will tend to deteriorate.

[0058] On the other hand, if the thickness of the second solid electrolyte layer 8 is too thick, it becomes difficult to fill the entire void of the second solid electrolyte layer 8 with the material constituting the positive electrode layer 3 or the negative electrode layer 4. As a result, the energy density per unit volume of the all-solid-state sodium-ion secondary battery 1 decreases. In addition, the amount of shrinkage during the formation of the second solid electrolyte layer 8 increases, making the second solid electrolyte layer 8 more prone to delamination at the interface with the first solid electrolyte layer 7.

[0059] In this embodiment, when the first solid electrolyte layer 7 as a dense layer and the second solid electrolyte layer 8 as a porous layer are integrated, it is desirable that the thickness of the second solid electrolyte layer 8 be greater than the thickness of the first solid electrolyte layer 7, but this is not particularly limited.

[0060] In the solid electrolyte layer 2, the thickness ratio of the second solid electrolyte layer 8 to the first solid electrolyte layer 7 (second solid electrolyte layer 8 / first solid electrolyte layer 7) is preferably 1.01 or more, more preferably 1.1 or more, preferably 1000 or less, and more preferably 100 or less. In this case, shrinkage of the solid electrolyte layer 2 during the firing process when forming the solid electrolyte layer can be further suppressed. In addition, the adhesion between the solid electrolyte layer 2 and the electrode layer can be further improved.

[0061] The thickness of the solid electrolyte layer 2, that is, the combined thickness of the first solid electrolyte layer 7 and the second solid electrolyte layer 8, is preferably 10 μm or more, more preferably 12 μm or more, even more preferably 15 μm or more, and particularly preferably 17 μm or more. On the other hand, the combined thickness of the first solid electrolyte layer 7 and the second solid electrolyte layer 8 is preferably 1200 μm or less, more preferably 700 μm or less, even more preferably 500 μm or less, and particularly preferably 200 μm or less.

[0062] When the thickness of the solid electrolyte layer 2 is greater than or equal to the lower limit, the mechanical strength can be further improved. In addition, short circuits between the positive electrode layer 3 and the negative electrode layer 4 can be made less likely to occur. On the other hand, when the thickness of the solid electrolyte layer 2 is less than or equal to the upper limit, the distance required for ion conduction within the solid electrolyte layer 2 becomes shorter, and the ionic conductivity is further improved. In addition, the energy density per unit volume of the all-solid-state sodium-ion secondary battery 1 can be further increased.

[0063] The flatness of the solid electrolyte layer 2 is preferably 200 μm or less, more preferably 100 μm or less, and even more preferably 50 μm or less. In this case, the handling properties of the solid electrolyte layer 2 can be further improved, and cracks during electrode formation can be made less likely to occur. The lower limit of the flatness of the solid electrolyte layer 2 is not particularly limited, but for example, it can be 0.1 μm or more.

[0064] In JIS B 0621-1984, flatness is defined as "the magnitude of the deviation of a planar feature from a geometrically correct plane." The flatness of the solid electrolyte layer 2 indicates the value of the gap created when one side of the sheet is sandwiched between parallel planes.

[0065] The average crystal grain size of the solid electrolyte in the first solid electrolyte layer 7 is preferably 0.01 μm or more, more preferably 0.05 μm or more, even more preferably 0.1 μm or more, preferably 10 μm or less, more preferably 5 μm or less, even more preferably 3 μm or less, even more preferably 1 μm or less, and particularly preferably 0.6 μm or less. In this case, the uniformity and smoothness of the solid electrolyte in the first solid electrolyte layer 7 during green sheet molding can be further improved, and the adhesion during lamination and the density during firing can be improved, thereby improving the handling properties of the final solid electrolyte layer 2.

[0066] The average crystal grain size of the solid electrolyte in the second solid electrolyte layer 8 is preferably 0.1 μm or more, more preferably 0.2 μm or more, even more preferably 0.5 μm or more, particularly preferably 0.8 μm or more, most preferably 1.0 μm or more, preferably 100 μm or less, more preferably 50 μm or less, even more preferably 10 μm or less, even more preferably 5 μm or less, particularly preferably 3 μm or less, and most preferably 2 μm or less. This improves the uniformity and smoothness of the solid electrolyte in the second solid electrolyte layer 8 during green sheet molding, improves adhesion during lamination and density of the solid electrolyte skeleton during firing, and thus improves the handling of the final solid electrolyte layer 2.

[0067] The average crystal grain size of the solid electrolyte in the first solid electrolyte layer 7 and the second solid electrolyte layer 8 can be measured, for example, as follows.

[0068] First, a sample of the solid electrolyte layer 2 is folded to form a cross-section. Next, the sample is heat-treated. This heat treatment is preferably performed by thermal etching in an electric furnace. The heat treatment temperature can be, for example, 900°C or higher and 1600°C or lower. The heat treatment time can be, for example, 1 minute or higher and 60 minutes or lower.

[0069] Next, the cross-section of the heat-treated sample is observed using a scanning electron microscope (SEM). For example, for any 200 particles in the obtained SEM image, the size of each particle is counted using image analysis software, and the average particle size is determined. This allows the average crystal grain size of the solid electrolyte to be determined. In the image analysis, the area circle equivalent diameter of the crystal grain is used as the particle size.

[0070] The same material can be used for the first solid electrolyte layer 7 and the second solid electrolyte layer 8 in the solid electrolyte layer 2. Preferably, the solid electrolyte layer 2 is made of an oxide solid electrolyte.

[0071] The oxide solid electrolyte used in the solid electrolyte layer 2 of the all-solid-state sodium-ion secondary battery 1 is a sodium ion conductive oxide. Examples of sodium ion conductive oxides include compounds containing at least one selected from Al, Y, Zr, Si, and P, Na, and O. Specific examples of sodium ion conductive oxides include beta-alumina or NASICON-type crystals, which exhibit excellent sodium ion conductivity. In particular, it is preferable that the sodium ion conductive oxide is at least one sodium ion conductive oxide selected from the group consisting of β''-alumina, β-alumina, and NASICON-type crystals. It is more preferable that the sodium ion conductive oxide is β-alumina or β''-alumina, as these exhibit even better sodium ion conductivity. For these reasons, it is preferable that the oxide solid electrolyte constituting the solid electrolyte layer 2 includes at least one selected from the group consisting of β-alumina, β''-alumina, and NASICON-type crystals.

[0072] There are two crystal forms of beta-alumina, namely β-alumina and β''-alumina. The theoretical composition formula of β-alumina is Na2O·11Al2O3. The theoretical composition formula of β''-alumina is Na2O·5.3Al2O3. Since β''-alumina is a metastable substance, usually, those added with stabilizers such as Li2O or MgO are used. Because β''-alumina has higher sodium ion conductivity than β-alumina, it is preferable to use β''-alumina alone or a mixture of β''-alumina and β-alumina, and it is more preferable to use Li2O-stabilized β''-alumina or MgO-stabilized β''-alumina.

[0073] Specific examples of β''-alumina include trigonal MgO-stabilized β''-alumina such as (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 and trigonal Li2O-stabilized β''-alumina such as 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 etc.

[0074] Specific 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 1.63 O), NaAl 11 O 17 、(Al 10.32 Mg0.68 O 16 )(Na 1.68 Examples include O).

[0075] NASICON type crystals are generally formulated with the formula Na s A1 t A2 u O v It is preferable that the compound consists of the following elements: (A1 is at least one selected from Al, Y, Yb, Nd, Nb, Ti, Hf, Zr, Mg, Ca, Zn, Sc, La, Ce, and Gd; A2 is at least one selected from Si and P; s=1.4~5.2, t=1~2.9, u=2.8~4.1, v=9~14). Here, it is preferable that A1 is at least one selected from Y, Nb, Ti, and Zr. By doing so, crystals with superior ionic conductivity can be obtained.

[0076] The preferred ranges for each coefficient in the above general formula are as follows:

[0077] The value of s is preferably 1.4 to 5.2, more preferably 2.5 to 3.5, and even more preferably 2.8 to 3.1. If s is too small, the amount of sodium ions decreases, which tends to reduce the ionic conductivity of the solid electrolyte. On the other hand, if s is too large, the excess sodium forms compounds that do not contribute to ionic conduction, such as sodium phosphate and sodium silicate, which also tends to reduce the ionic conductivity of the solid electrolyte.

[0078] t is preferably 1 to 2.9, more preferably 1 to 2.5, and even more preferably 1.3 to 2. If t is too small, the three-dimensional network structure in the crystal decreases, which tends to reduce the ionic conductivity of the solid electrolyte. On the other hand, if t is too large, compounds that do not contribute to ionic conduction, such as zirconia and alumina, are formed, which also tends to reduce the ionic conductivity of the solid electrolyte.

[0079] The value of u is preferably 2.8 to 4.1, more preferably 2.8 to 4, even more preferably 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, which tends to reduce the ionic conductivity of the solid electrolyte. On the other hand, if u is too large, crystals that do not contribute to ionic conduction are formed, which also tends to reduce the ionic conductivity of the solid electrolyte.

[0080] v is preferably 9 to 14, more preferably 9.5 to 12, and even more preferably 11 to 12. If v is too small, A1 (e.g., aluminum component) will be in a low valence state, which tends to reduce electrical insulation. On the other hand, if v is too large, a peroxide state will occur, and sodium ions will be bound to the lone pair of electrons of the oxygen atom, which tends to reduce the ionic conductivity of the solid electrolyte.

[0081] The NASICON-type crystal is preferably a monoclinic, hexagonal, or trigonal crystal, and more preferably a monoclinic or trigonal crystal. In this case, the ionic conductivity of the solid electrolyte can be further improved.

[0082] A specific example of a NASICON-type crystal is Na3Zr2Si2PO 12 kaNa 3.2 Zr 1.3 Si 2.2 P 0.8 O 10.5 Na3Zr 1.6 Ti 0.4 Si2er 12 Na3Hf2Si2PO 12 kaNa 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 Si2er 12 kaNa 3.6 Ti 0.2 Y 0.8 Si 2.8 O9, Na3Zr1.88 Y 0.12 Si2PO 12 、Na 3.12 Zr 1.88 Y 0.12 Si2PO 12 、Na 3.05 Zr2Si 2.06 P 0.95 About 12 、Na 3.4 Zr2Si 2.4 P 0.6 About 12 、Na 3.4 Zr 1.9 Zn 0.1 Yes 2.4 P 0.6 About 12 、Na 3.4 Zr 1.9 Mg 0.1 Yes 2.4 P 0.6 About 12 、Na 3.4 Zr 1.9 Zn 0.1 Yes 2.2 P 0.8 About 12 、Na 3.4 Zr 1.9 Mg 0.1 Yes 2.2 P 0.8 About 12 、Na 2.8 Zr2Si 2.4 P 0.6 About 12 、Na 3.6 Zr 0.13 Yb 1.67 Yes 0.11 P 2.9 About 12 、Na5YSi4O 12 、Na 3.1 Zr 1.95 Mg 0.05 Si2PO 12 、Na 3.1 Zr 1.9 The 0.1 Si2PO 12 、Na 3.1 Zr 1.9 Nd 0.1 Si2PO 12 、Na 3.1 Zr 1.9 Y 0.1 Si2PO12 、Na 3.256 Zr 1.872 Mg 0.128 Si2PO 12 、Na 3.2 Zr 1.9 Ca 0.1 Si2PO 12 、Na 3.2 Zr 1.9 Mg 0.1 Si2PO 12 、Na 3.2 Zr2Si 2.2 P 0.8 About 12 、Na 3.38 Zr 1.80 Al 0.26 Yes 2.06 P 0.88 About 12 、Na 3.43 Zr 1.83 Zn 0.22 Yes 1.93 P 1.02 About 12 、Na 3.4 Sc 0.4 Zr 1.6 Si2PO 12 、Na 3.4 Zr 1.8 Mg 0.2 Si2PO 12 、Na 3.4 Zr 1.9 Zn 0.1 Yes 2.2 P 0.8 About 12 、Na 3.57 Zr 1.72 The 0.21 Yes 2.08 P 0.92 About 12 、Na3Zr 1.98 Nb 0.08 Si2PO 12 、Na3Zr 1.9 Yes 0.1 Si2PO 12 、Na3Zr 1.9 Yes 0.1 Si2PO 12 、Na3Zr 1.9 Gd 0.1 Si2PO 12 、Na3Zr 1.9 You 0.1 Si2PO12 Na3Zr 1.9 Yb 0.1 Si2er 12 Examples of crystals include those listed above. These may be used individually or in combination of multiple types. Among them, the NASICON type crystal is Na3Zr2Si2PO 12 kaNa 3.4 Zr2Si 2.4 P 0.6 O 12、 or Na 3.05 Zr2Si 2.06 P 0.95 O 12 It is preferable that Na3Zr2Si2PO 12 This is more preferable. In this case, the ionic conductivity of the NASICON-type crystal can be further improved.

[0083] Furthermore, a metal layer may be provided on the surface of the second solid electrolyte layer 8. When the negative electrode layer 4 formed on the second solid electrolyte layer 8 is made of metallic sodium or the like, the adhesion between the second solid electrolyte layer 8, the metal layer, and the negative electrode layer 4 can be increased. This reduces interfacial resistance, thereby increasing the discharge capacity.

[0084] The metals that make up the metal layer are not particularly limited, but for example, Sn, Ti, Bi, Au, Al, Cu, Sb, Pb, etc. can be used. These metals that make up the metal layer may be used individually or in combination of two or more. The metal layer may also be composed of an alloy of these metals.

[0085] The thickness of the metal layer is preferably 3 nm or more, more preferably 5 nm or more, even more preferably 10 nm or more, and particularly preferably 20 nm or more. On the other hand, the thickness of the metal layer is preferably 5 μm or less, more preferably 3 μm or less, even more preferably 1 μm or less, and particularly preferably 500 nm or less.

[0086] Methods for forming the metal layer include, for example, physical vapor phase methods such as vapor deposition or sputtering, or chemical vapor phase methods such as thermal CVD, MOCVD, or plasma CVD. Alternatively, liquid phase film deposition methods such as plating, sol-gel method, or spin coating may be used to form the metal layer. Among these, vapor deposition or sputtering is preferred for forming the metal layer. In this case, the thinning of the metal layer is easy, and the above-mentioned effects of providing a metal layer are easily obtained.

[0087] (Positive electrode layer) The positive electrode layer 3 shown in Figure 1 consists of an electrode for a sodium-ion secondary battery containing the aforementioned sodium-ion secondary battery electrode composite material.

[0088] The lower limit of the thickness of the positive electrode layer 3 is preferably 10 μm or more, 50 μm or more, and particularly preferably 100 μm or more. When the thickness of the positive electrode layer 3 is greater than or equal to the above lower limit, the charge and discharge capacity of the all-solid-state sodium-ion secondary battery 1 can be further increased. On the other hand, the upper limit of the thickness of the positive electrode layer 3 is preferably 1000 μm or less, and particularly preferably 700 μm or less. If the thickness of the positive electrode layer 3 is too thick, the resistance to electron conduction increases, which may reduce the discharge capacity of the all-solid-state sodium-ion secondary battery 1. Alternatively, the operating voltage of the all-solid-state sodium-ion secondary battery 1 may decrease.

[0089] (Negative electrode layer) The negative electrode active material contained in the negative electrode layer 4 is not particularly limited, but for example, carbon electrode materials such as hard carbon or soft carbon can be used. The carbon electrode material is preferably hard carbon. However, if the all-solid-state sodium-ion secondary battery 1 is an all-solid-state sodium-sodium-ion secondary battery, the negative electrode active material may include alloy-based negative electrode active materials that can absorb sodium, such as tin, bismuth, lead, or phosphorus, or metallic sodium. It is preferable that the negative electrode layer 4 is not metallic sodium or a negative electrode layer containing metallic sodium.

[0090] The lower limit of the negative electrode active material content is not particularly limited, but is preferably 50% or more by mass, and particularly preferably 60% or more. The upper limit of the negative electrode active material content is not particularly limited, but is preferably 99.9% or less by mass, preferably 95% or less, and particularly preferably 90% or less. When the negative electrode active material content in the negative electrode layer 4 is within the above ranges, the battery capacity can be increased even more effectively.

[0091] The negative electrode layer 4 may contain a solid electrolyte. The solid electrolyte described in the "Solid Electrolyte Layer" section can be used. When the negative electrode layer 4 contains a solid electrolyte, the lower limit of the solid electrolyte content is preferably 0.1% or more by mass, 5% or more, and particularly preferably 10% or more. The upper limit of the solid electrolyte content is preferably 40% or less by mass, 35% or less, and particularly preferably 30% or less. When the solid electrolyte content is within the above range, integration of the negative electrode active material and the solid electrolyte can be achieved. Alternatively, if the negative electrode layer 4 is in contact with the solid electrolyte layer 2, integration of the two can be achieved. As a result, ionic conductivity can be further improved, and the battery characteristics of the sodium-ion secondary battery can be more effectively enhanced.

[0092] The negative electrode layer 4 may contain a conductive additive. For example, the conductive additive described in the "positive electrode layer" section can be used. When the negative electrode layer 4 contains a conductive additive, the lower limit of the conductive additive content is preferably 0.1% or more by mass, and particularly preferably 0.2% or more. The upper limit of the conductive additive content is preferably 5% or less by mass, 3% or less, and particularly preferably 2% or less. When the content of the conductive additive in the negative electrode layer 4 is within the above range, it is possible to further improve ionic conductivity while ensuring high electronic conductivity in the negative electrode layer 4, and to further effectively improve the battery characteristics of the sodium-ion secondary battery.

[0093] The lower limit of the thickness of the negative electrode layer 4 is preferably 0.3 μm or more, more preferably 3 μm or more, and more preferably 10 μm or more. When the thickness of the negative electrode layer 4 is greater than or equal to the above lower limit, the charge and discharge capacity of the all-solid-state sodium-ion secondary battery 1 can be increased even further. On the other hand, the upper limit of the thickness of the negative electrode layer 4 is preferably 500 μm or less, and more preferably 300 μm or less. If the thickness of the negative electrode layer 4 is too thick, the resistance to electron conduction increases, which may reduce the discharge capacity of the all-solid-state sodium-ion secondary battery 1. Alternatively, the operating voltage of the all-solid-state sodium-ion secondary battery 1 may decrease.

[0094] (First current collector layer and second current collector layer) In this embodiment, the first current collector layer 5 and the second current collector layer 6 are not particularly limited as long as they are electrically conductive. Examples of current collector materials include metallic materials such as aluminum, titanium, silver, copper, stainless steel, or alloys thereof. These metallic materials may be used individually or in combination. These alloys are alloys containing at least one of the above-mentioned metals. These metallic materials have high electronic conductivity and are less prone to chemical reactions during the charging and discharging of sodium-ion secondary batteries, thus effectively increasing the capacity of the sodium-ion secondary battery and exhibiting excellent cycle characteristics due to charging and discharging.

[0095] In this regard, it is preferable that the first current collector layer 5 and the second current collector layer 6 are made of aluminum or an alloy containing aluminum. Since aluminum or an alloy containing aluminum has a low density among metallic materials, it can effectively increase the capacity of the sodium-ion secondary battery. Furthermore, it is preferable that the current collector made of aluminum or an alloy containing aluminum is carbon coated on its surface. By doing so, it is possible to prevent the formation of a passive oxide film on the surface of the first current collector layer 5 and the second current collector layer 6 during electrode firing, resulting in excellent cycle characteristics due to charge and discharge of the sodium-ion secondary battery.

[0096] The first current collector layer 5 and the second current collector layer 6 are preferably made of metal foil. Because metal foil is flexible, it can increase the contact area with the electrode layer and integrate with the extraction electrode when used as a sodium-ion secondary battery, thereby effectively increasing the capacity of the sodium-ion secondary battery and providing excellent cycle characteristics due to charge and discharge.

[0097] Furthermore, the first current collector layer 5 and the second current collector layer 6 are preferably made of foamed metal. Because foamed metal has a high specific surface area, it can increase the contact area with the positive electrode layer 3 and the negative electrode layer 4, resulting in excellent cycle characteristics due to charging and discharging of the sodium-ion secondary battery.

[0098] The thicknesses of the first current collector layer 5 and the second current collector layer 6 are preferably 10 nm or more and 100 μm or less, respectively. The thicknesses of the first current collector layer 5 and the second current collector layer 6 are preferably 50 μm or less, and more preferably 30 μm or less, respectively. In this case, the energy density of the sodium-ion secondary battery can be further increased. Alternatively, the thicknesses of the first current collector layer 5 and the second current collector layer 6 are preferably 30 nm or more, and more preferably 50 nm or more, respectively. In this case, the decrease in discharge capacity due to an increase in the internal resistance of the battery caused by a decrease in conductivity, and the resulting decrease in gravimetric energy density and volumetric energy density can be further suppressed.

[0099] The following describes an example of a manufacturing method for producing the all-solid-state sodium-ion secondary battery 1 according to the present invention.

[0100] (Formation of a solid electrolyte layer) (a) Preparation of the first green sheet for forming the solid electrolyte layer First, a slurry is prepared by adding an organic vehicle containing a binder to the first solid electrolyte powder and at least one of the raw material powders for the first solid electrolyte powder. The raw material powder referred to here is the powder that will react in the subsequent calcination process to become a solid electrolyte. The binder is a material used to bind powdered materials together.

[0101] Next, the slurry is applied to the substrate and dried to produce a first solid electrolyte layer-forming green sheet. The drying temperature of the slurry can be, for example, 40°C or higher and 100°C or lower. The drying time of the slurry can be, for example, 3 minutes or higher and 24 hours or lower. After that, the first solid electrolyte layer-forming green sheet is peeled off from the substrate.

[0102] As the first solid electrolyte powder, at least one can be selected from the group consisting of, for example, β''-alumina, β-alumina, and NASICON-type crystals. The same materials as those described in the "Solid Electrolyte Layer" section above can be used as β''-alumina, β-alumina, and NASICON-type crystals.

[0103] As the raw material powder for the first solid electrolyte powder, for example, if the raw material powder is β''-alumina, a powder containing, in molar percentages, Al2O 365%~98%, Na2O 2%~20%, MgO+Li2O 0.3%~15%, ZrO 20%~20%, and Y2O 30%~5% can be prepared. The reason for limiting the composition as described above will be explained below.

[0104] Al2O3 is the main component of β''-alumina. The Al2O3 content is preferably 65% ​​to 98% in mole percent, and more preferably 70% to 95%. If the amount of Al2O3 is too low, the ionic conductivity of the solid electrolyte tends to decrease. On the other hand, if the amount of Al2O3 is too high, α-alumina that does not have sodium ion conductivity remains, and the ionic conductivity of the solid electrolyte tends to decrease.

[0105] Na2O is a component that imparts sodium ion conductivity to solid electrolytes. The Na2O content is preferably 2% to 20% in mole percent, more preferably 3% to 18%, and even more preferably 4% to 16%. If the amount of Na2O is too low, it becomes difficult to obtain the above effect. On the other hand, if the amount of Na2O is too high, the excess sodium forms compounds such as NaAlO2 that do not contribute to ionic conductivity, so the ionic conductivity of the solid electrolyte tends to decrease.

[0106] MgO and Li2O are components that stabilize the structure of β''-alumina, i.e., stabilizers. The content of MgO + Li2O is preferably 0.3% to 15% in mole percent, more preferably 0.5% to 10%, and even more preferably 0.8% to 8%. If the amount of MgO + Li2O is too low, α-alumina will remain in the solid electrolyte, and the ionic conductivity will tend to decrease. On the other hand, if the amount of MgO + Li2O is too high, MgO or Li2O that did not function as a stabilizer will remain in the solid electrolyte, and the ionic conductivity will tend to decrease.

[0107] ZrO2 and Y2O3 have the effect of suppressing abnormal grain growth of β''-alumina during calcination and improving the adhesion of each β''-alumina particle. As a result, the ionic conductivity of the solid electrolyte is more easily improved. The ZrO2 content is preferably 0% to 20% in mole percent, more preferably 0% to 15%, even more preferably 1% to 13%, and particularly preferably 2% to 10%. The Y2O3 content is preferably 0% to 5% in mole percent, more preferably 0.01% to 4%, and even more preferably 0.02% to 3%. If there is too much ZrO2 or Y2O3, the amount of β''-alumina produced will decrease, and the ionic conductivity of the solid electrolyte will easily decrease.

[0108] As the raw material powder for the first solid electrolyte powder, for example, if it is a raw material powder of the NASICON type crystal, it can be prepared containing, in molar percentages, 17.5% to 50% Na2O, 12% to 45% Al2O3 + Y2O3 + Yb2O3 + Nd2O3 + Nb2O5 + TiO2 + HfO2 + ZrO2, and 24% to 54% SiO2 + P2O5.

[0109] Preferably, the average particle size of at least one of the first solid electrolyte powder and the raw material powder for the first solid electrolyte powder is 0.01 μm or more, more preferably 0.05 μm or more, and even more preferably 0.1 μm or more. On the other hand, preferably, the average particle size of at least one of the first solid electrolyte powder and the raw material powder for the first solid electrolyte powder is 10 μm or less, more preferably 5 μm or less, even more preferably 3 μm or less, even more preferably 1 μm or less, and particularly preferably 0.6 μm or less. In this case, peeling of the porous second solid electrolyte layer 8 from the first solid electrolyte layer 7 can be made less likely to occur in the subsequent firing process.

[0110] In this specification, the average particle diameter refers to the average particle diameter D measured by a laser diffraction particle size distribution analyzer. 50 It refers to.

[0111] As a binder, for example, a resin binder such as polypropylene carbonate can be used. Alternatively, as a resin binder, for example, polyvinyl alcohol (PVA), polyvinyl butyral (PVB) or other polyvinyl acetals, polyvinylidene fluoride (PVDF), polytetrafluoroethylene, acrylic resin, ethyl methylcellulose, carboxymethylcellulose, ethylcellulose, alginic acid, polyethylene glycol, polyethylene carbonate or polypropylene carbonate or other polycarbonate resins, or copolymers thereof can be used. These binders may be used individually or in combination. Among these, polyvinyl butyral (PVB), acrylic resin, polyethylene carbonate or polypropylene carbonate or other polycarbonate resins are preferred as the binder.

[0112] Alternatively, the binder may be a glass binder. For example, sodium ion conductive glass powder can be used as the glass binder.

[0113] Organic vehicles may contain solvents, plasticizers, and other substances in addition to binders. For example, water or organic solvents such as ethanol or acetone can be used as solvents. However, when water is used as a solvent, alkaline components such as sodium may leach 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 organic solvents.

[0114] Preferably, the content of at least one of the first solid electrolyte powder and the raw material powder of the first solid electrolyte powder contained in the slurry is 10% by mass or more, and more preferably 30% by mass or more. On the other hand, preferably, the content of at least one of the first solid electrolyte powder and the raw material powder of the first solid electrolyte powder contained in the slurry is 80% by mass or less, and more preferably 50% by mass or less. The content of the binder contained in the slurry can be, for example, 5% by mass or more and 50% by mass or less.

[0115] The substrate to which the slurry is applied is not particularly limited, but for example, a resin film such as PET (polyethylene terephthalate) film can be used.

[0116] (b) Preparation of a green sheet for forming a second solid electrolyte layer First, a slurry is prepared by adding an organic vehicle containing a binder to a mixed powder containing the second solid electrolyte powder, at least one of the raw material powders for the second solid electrolyte powder, and a polymer powder. The polymer powder is a material for forming voids within the second solid electrolyte layer 8. Specifically, the voids are formed when the polymer powder is burned and removed in a later firing process.

[0117] Next, a second green sheet for forming a solid electrolyte layer is prepared by applying the slurry onto the substrate and drying it. The drying temperature of the slurry can be, for example, 40°C or higher and 100°C or lower. The drying time of the slurry can be, for example, 5 minutes or higher and 24 hours or lower. After that, the second green sheet for forming a solid electrolyte layer is peeled off the substrate.

[0118] As the second solid electrolyte powder and the raw material powder for the second solid electrolyte powder, at least one of them can be the same powder as the first solid electrolyte powder and the raw material powder for the first solid electrolyte powder described above.

[0119] Preferably, the average particle size of at least one of the powders from the second solid electrolyte powder and the raw material powder for the second solid electrolyte powder is 0.1 μm or more, more preferably 0.2 μm or more, even more preferably 0.5 μm or more, and particularly preferably 0.8 μm or more. On the other hand, preferably, the average particle size of at least one of the powders from the second solid electrolyte powder and the raw material powder for the second solid electrolyte powder is 100 μm or less, more preferably 50 μm or less, even more preferably 10 μm or less, even more preferably 5 μm or less, and particularly preferably 3 μm or less. In this case, peeling of the porous second solid electrolyte layer 8 from the first solid electrolyte layer 7 can be made less likely to occur in the subsequent firing process.

[0120] Examples of polymer powder materials include acrylic resin, polyacrylonitrile, polymethacrylonitrile, or polystyrene. A polymer powder consisting of one of these materials may be used alone, or multiple polymer powders consisting of different materials may be used in combination.

[0121] The average particle size of the polymer powder is preferably 0.1 μm or more, more preferably 1 μm or more, even more preferably 5 μm or more, and particularly preferably 10 μm or more. On the other hand, the average particle size of the polymer powder is preferably 100 μm or less, more preferably 80 μm or less, even more preferably 70 μm or less, and particularly preferably 50 μm or less. If the average particle size of the polymer powder is too small, it becomes difficult to form three-dimensionally connected voids in the resulting second solid electrolyte layer 8. On the other hand, if the average particle size of the polymer powder is too large, the sintering of the resulting second solid electrolyte layer 8 may be insufficient, and the ionic conductivity may decrease.

[0122] The mixing ratio of the second solid electrolyte powder and at least one of the raw material powders for the second solid electrolyte powder to the polymer powder is preferably 75:25 to 3:97 by volume, 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 connected voids in the resulting second solid electrolyte layer 8. On the other hand, if the polymer powder content is too high, the sintering of the resulting second solid electrolyte layer 8 may be insufficient, and the ionic conductivity may decrease.

[0123] Furthermore, the mixing ratio of the second solid electrolyte powder and at least one of the raw material powders for the second solid electrolyte powder to the polymer powder is preferably 95:5 to 20:80 by mass, more preferably 90:10 to 30:70, and even more preferably 80:20 to 40:60.

[0124] For the binder, solvent, plasticizer, and other components in the organic vehicle, the same components as those described in the section "Preparation of the Green Sheet for Forming the First Solid Electrolyte Layer" can be used.

[0125] Preferably, the content of at least one of the second solid electrolyte powder and the raw material powder of the second solid electrolyte powder contained in the slurry is 5% by mass or more, and more preferably 10% by mass or more. On the other hand, preferably, the content of at least one of the second solid electrolyte powder and the raw material powder of the second solid electrolyte powder contained in the slurry is 80% by mass or less, and more preferably 60% by mass or less. The content of the binder contained in the slurry can be, for example, 5% by mass or more and 30% by mass or less.

[0126] The substrate to which the slurry is applied is not particularly limited, but for example, a resin film such as PET (polyethylene terephthalate) film can be used.

[0127] (c) Fabrication of laminated sheets Next, a laminated sheet is obtained by laminating a second solid electrolyte layer-forming green sheet onto the main surfaces on both sides of the first solid electrolyte layer-forming green sheet. Alternatively, the laminated sheet may be obtained by laminating each green sheet and then pressing it, such as by heating and pressing. In this case, the adhesion between each green sheet can be further improved.

[0128] Alternatively, a slurry containing a second solid electrolyte powder, a mixed powder containing at least one of the raw material powders for the second solid electrolyte powder and a polymer powder, and an organic vehicle containing a binder may be applied to both main surfaces of the first solid electrolyte layer forming green sheet. A laminated sheet may then be obtained by drying the slurry.

[0129] (d) Firing of the laminated sheets Next, the resulting laminated sheet is fired. This forms a first solid electrolyte layer 7 and a second solid electrolyte layer 8. In this way, a solid electrolyte layer 2 can be obtained, in which the second solid electrolyte layer 8, which is a porous layer, is provided on the main surfaces on both sides of the first solid electrolyte layer 7, which is a dense layer.

[0130] Furthermore, when forming the first solid electrolyte layer 7, it is desirable to remove the binder from the green sheet for forming the first solid electrolyte layer. When forming the second solid electrolyte layer 8, it is desirable to remove the binder and polymer powder from the green sheet for forming the second solid electrolyte layer.

[0131] The firing temperature can be appropriately selected depending on the type of solid electrolyte powder or raw material powder used. When the solid electrolyte powder contains β-alumina or β''-alumina, the firing temperature is preferably 1400°C or higher, more preferably 1450°C or higher, and even more preferably 1500°C or higher. On the other hand, the firing temperature is preferably 1750°C or lower, and more preferably 1700°C or lower. 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 form the desired crystals. On the other hand, if the firing temperature is too high, the amount of evaporation of sodium components, etc. increases, and heterogeneous crystals precipitate, which tends to reduce the ionic conductivity of the resulting solid electrolyte layer 2.

[0132] When the solid electrolyte powder contains NASICON-type crystals, the firing temperature is preferably 1200°C or higher, and more preferably 1210°C or higher. On the other hand, the firing temperature is preferably 1400°C or lower, and more preferably 1300°C or lower. 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 form the desired crystals. On the other hand, if the firing temperature is too high, the amount of evaporation of sodium components, etc. increases, and heterogeneous crystals precipitate, which tends to reduce the ionic conductivity of the solid electrolyte layer 2.

[0133] The firing time is adjusted as appropriate to ensure sufficient sintering. Specifically, for example, the sintering time may be 10 to 120 minutes. However, a sintering time of 20 to 80 minutes is particularly preferred.

[0134] It should be noted that a laminated sheet is not necessarily required when forming the solid electrolyte layer 2. For example, a slurry containing a second solid electrolyte powder, a mixed powder containing at least one of the raw material powders of the second solid electrolyte powder and a polymer powder, and an organic vehicle containing a binder may be applied to both main surfaces of the first solid electrolyte layer 7. The slurry may then be dried to obtain a laminate of the first solid electrolyte layer 7 and the second solid electrolyte green sheet. The solid electrolyte layer 2 may then be obtained by firing the laminate.

[0135] In this embodiment, a second solid electrolyte layer 8, which is a porous layer, is formed on both main surfaces of the first solid electrolyte layer 7, which is a dense layer. However, the second solid electrolyte layer 8 may be formed on only one main surface of the first solid electrolyte layer 7.

[0136] (Sodium ion conductive solid electrolyte precursor and its solution) When the sodium-conducting solid electrolyte is beta-alumina, the sodium-conducting solid electrolyte precursor can be obtained, for example, by mixing aluminum nitrate, sodium nitrate, and lithium nitrate. At this time, the ratio of each of the above materials is adjusted to the composition ratio of the desired sodium ion-conducting solid electrolyte.

[0137] The sodium ion-conducting solid electrolyte is a NASICON-type crystal, or Na5XSi4O 12In the case of a type crystal in which X is at least one selected from group 3 transition metal elements or rare earth elements, the sodium-conducting solid electrolyte precursor solution includes a solution containing sodium elements and transition metal elements that constitute the sodium-conducting solid electrolyte, and carbonate ions. In this solution, the sodium element is contained in the form of sodium ions, and the transition metal elements are contained in the form of transition metal ions. The sodium-ion-conducting solid electrolyte precursor consists, for example, of a gelled or dried product of the sodium-ion-conducting solid electrolyte precursor solution. The sodium-ion-conducting solid electrolyte consists of a calcined product of the sodium-ion-conducting solid electrolyte precursor. Note that the sodium-ion-conducting solid electrolyte is Na5XSi4O 12 If it is a type crystal, it is preferable that X is at least one element selected from rare earth elements.

[0138] As a sodium ion-conducting solid electrolyte precursor solution, a solution containing nitrate ions instead of carbonate ions can also be used.

[0139] In a sodium ion-conducting solid electrolyte precursor solution, it is preferable that carbonate ions are bidentately coordinated to the transition metal element. In this case, the transition metal element is more likely to exist stably in the solution.

[0140] In a sodium ion-conducting solid electrolyte precursor solution, the counterion of sodium ions is of the general formula NR4. + It is preferable that the ion is represented by such that each R is independently a substituent selected from the group consisting of H, CH3, C2H5, and CH2CH2OH. In this case, the transition metal element is more likely to exist stably in the solution.

[0141] A sodium ion-conducting solid electrolyte precursor solution can be obtained, for example, by mixing water glass, sodium tripolyphosphate, and an aqueous solution of zirconium ammonia carbonate. Specifically, the water glass is sodium silicate.

[0142] (Formation of the positive electrode layer) A paste is prepared containing a positive electrode active material precursor, and optionally containing solid electrolyte powder and a conductive additive. The paste may optionally contain a binder, plasticizer, or solvent.

[0143] Examples of binders that can be used include cellulose derivatives such as carboxymethylcellulose, hydroxypropylmethylcellulose, hydroxypropylcellulose, ethylcellulose, hydroxyethylcellulose, and hydroxymethylcellulose, or water-soluble polymers such as polyvinyl alcohol; thermosetting resins such as thermosetting polyimide, phenolic resin, epoxy resin, urea resin, melamine resin, unsaturated polyester resin, and polyurethane; polycarbonate resins such as polypropylene carbonate; and polyvinylidene fluoride.

[0144] Next, the paste is applied to one main surface of the solid electrolyte layer 2 and the paste is dried. The drying temperature of the paste is not particularly limited, but for example, it can be 30°C or higher and 150°C or lower. The drying time of the paste is not particularly limited, but for example, it can be 5 minutes or higher and 600 minutes or lower. By drying the paste, an electrode composite layer is formed on one main surface of the solid electrolyte layer 2. This gives a solid electrolyte layer with electrode composite. The electrode composite layer may be in the form of compacted powder.

[0145] Subsequently, the solid electrolyte layer with electrode composite material is fired. It is preferable that the atmosphere during firing be a reducing atmosphere. The maximum temperature during firing can be, for example, 400°C to 600°C. The holding time at this temperature can be, for example, 5 minutes or more and less than 3 hours. By firing as described above, a positive electrode layer 3 is obtained as the first electrode active material layer.

[0146] (Positive electrode active material precursor and its preparation) The positive electrode active material precursor is, specifically, a positive electrode active material precursor powder. It is preferable that the positive electrode active material precursor powder consists of an amorphous oxide material that generates active material crystals upon firing. When the positive electrode active material precursor powder consists of an amorphous oxide material, active material crystals are generated during firing, and it becomes possible to form a dense positive electrode layer 3 through softening and flow.

[0147] In this invention, the term "amorphous oxide material" is not limited to completely amorphous oxide materials, but also includes oxide materials that contain some crystals. Oxide materials that contain some crystals are, for example, oxide materials with a crystallinity of 10% or less.

[0148] The positive electrode active material precursor powder preferably contains Na2O 25% to 55%, Fe2O3 + Cr2O3 + MnO + CoO + V2O5 + NiO 10% to 30%, and P2O5 25% to 55% in molar percentages in terms of oxides as follows. The reason for limiting the composition in this way is explained below. In the following explanation of the content of each component, unless otherwise specified, "%" means "molar percent".

[0149] Na2O is the general formula Na x M y P2O z The formula is expressed as follows: 1≦x≦2.8, 0.95≦y≦1.6, 6.5≦z≦8, and M is the main component of the active material crystal, which is at least one transition metal element selected from Cr, Fe, Mn, Co, V, and Ni. The Na₂O content is preferably 25% to 55%, and more preferably 30% to 50%. When the Na₂O content is within the above range, the charge and discharge capacity of the all-solid-state sodium-ion secondary battery 1 can be further increased.

[0150] Fe2O3, Cr2O3, MnO, CoO, V2O5, and NiO are also based on the general formula Na x M y P2O zThis is the main component of the active material crystal represented by the above formula. The content of Fe2O3+Cr2O3+MnO+CoO+V2O5+NiO is preferably 10% to 30%, and more preferably 15% to 25%. When the content of Fe2O3+Cr2O3+MnO+CoO+V2O5+NiO is above the lower limit, the charge and discharge capacity of the all-solid-state sodium-ion secondary battery 1 can be further increased. On the other hand, when the content of Fe2O3+Cr2O3+MnO+CoO+V2O5+NiO is below the upper limit, it is possible to make it difficult for unwanted crystals such as Fe2O3, Cr2O3, MnO, CoO, V2O5, or NiO to precipitate. In this specification, for example, when the content of a+b+c+... is described, it means the total amount of a, b, and c.

[0151] To further improve the cycle characteristics of the all-solid-state sodium-ion secondary battery 1, it is preferable to actively include Fe2O3. The Fe2O3 content is preferably 1% to 30%, more preferably 5% to 30%, even more preferably 10% to 30%, and particularly preferably 15% to 25%. The content of each component, Cr2O3, MnO, CoO, V2O5, and NiO, is preferably 0% to 30%, more preferably 10% to 30%, and even more preferably 15% to 25%, respectively. When at least two or more components selected from Fe2O3, Cr2O3, MnO, CoO, V2O5, and NiO are included, their combined amount is preferably 10% to 30%, and more preferably 15% to 25%.

[0152] P2O5 is also, in general formula Na x M y P2O z It is the main component of the active material crystal represented by the above. The P2O5 content is preferably 25% to 55%, and more preferably 30% to 50%. When the P2O5 content is within the above range, the charge and discharge capacity of the all-solid-state sodium-ion secondary battery 1 can be further increased.

[0153] The positive electrode active material precursor powder may also contain Nb2O5, MgO, Al2O3, TiO2, ZrO2, or Sc2O3 in addition to the above components. These components have the effect of increasing conductivity. Conductivity, in this context, specifically refers to electronic conductivity. Therefore, the inclusion of these components in the positive electrode active material precursor powder makes it easier to improve the rapid charge and discharge characteristics of the all-solid-state sodium-ion secondary battery 1. The total content of the above components is preferably 0% to 25%, and more preferably 0.2% to 10%. When the content of the above components is below the above upper limit, heterogeneous crystals that do not contribute to the battery characteristics are less likely to form, and the charge and discharge capacity of the all-solid-state sodium-ion secondary battery 1 can be further increased.

[0154] The positive electrode active material precursor powder may also contain SiO2, B2O3, GeO2, Ga2O3, Sb2O3, or Bi2O3 in addition to the components listed above. When the raw materials for the positive electrode active material precursor powder contain these components, the glass formation ability is further improved when obtaining the positive electrode active material precursor powder. This makes it easier to obtain a more homogeneous positive electrode active material precursor powder. The total content of the above components in the positive electrode active material precursor powder is preferably 0% to 25%, and more preferably 0.2% to 10%. Since these components do not contribute to the battery characteristics, if their content is too high, the charge and discharge capacity of the all-solid-state sodium-ion secondary battery 1 tends to decrease.

[0155] The positive electrode active material precursor powder is preferably prepared by melting and molding a batch of raw materials. This preparation method makes it easier to obtain amorphous positive electrode active material precursor powder with excellent homogeneity. Specifically, the positive electrode active material precursor powder can be prepared as follows.

[0156] First, raw materials are prepared to obtain a raw material batch to achieve the desired composition. Next, the obtained raw material batch is melted. The melting temperature can be adjusted as appropriate to ensure that the raw material batch is melted homogeneously. For example, a melting temperature of 800°C or higher is preferable, and 900°C or higher is more preferable. There is no particular upper limit to the melting temperature. However, if the melting temperature is too high, it can lead to energy loss and evaporation of sodium components, etc. For this reason, a melting temperature of 1500°C or lower is preferable, and 1400°C or lower is more preferable.

[0157] Next, the resulting molten material is molded. The molding method is not particularly limited; for example, the molten material may be poured between a pair of cooling rolls and molded into a film while rapidly cooling. Alternatively, the molten material may be poured into a mold and molded into an ingot.

[0158] Next, the obtained molded body is crushed to obtain a positive electrode active material precursor powder. The average particle size of the positive electrode active material precursor powder is preferably 0.01 μm or more and less than 0.7 μm, more preferably 0.03 μm or more and 0.6 μm or less, even more preferably 0.05 μm or more and 0.6 μm or less, and particularly preferably 0.1 μm or more and 0.5 μm or less.

[0159] (Formation of the negative electrode layer) The negative electrode layer 4 can be prepared, for example, using a paste containing a negative electrode active material precursor powder and, if necessary, a solid electrolyte powder and a conductive additive. Binders, plasticizers, solvents, etc., may be added to the paste as needed.

[0160] Furthermore, when manufacturing an all-solid-state sodium-ion secondary battery as an all-solid-state sodium-ion secondary battery 1, it is preferable to use a sodium-ion conductive solid electrolyte precursor solution when obtaining the paste. Specifically, the sodium-ion conductive solid electrolyte precursor solution and the negative electrode active material precursor are mixed and then dried. This yields a powder mixture of the sodium-ion conductive solid electrolyte precursor and the negative electrode active material precursor. However, by mixing the sodium-ion conductive solid electrolyte precursor and the negative electrode active material precursor, a powder mixture of the sodium-ion conductive solid electrolyte precursor and the negative electrode active material precursor can be obtained without going through the drying process.

[0161] Next, the resulting mixture is ground into a powder and then mixed with a conductive additive and a binder in an organic solvent. If the negative electrode active material of the negative electrode layer 4 to be formed is hard carbon, hard carbon may be added to the powder mixture and mixed. For example, N-methyl-2-pyrrolidone can be used as the organic solvent. This yields a paste.

[0162] If the paste contains a binder, the binder described in the section on "Formation of the positive electrode layer" can be used.

[0163] The negative electrode layer 4 can be formed by applying the paste to one main surface of the solid electrolyte layer 2, drying it, and then firing it.

[0164] When firing the laminate of the solid electrolyte layer 2 and the paste, it is preferable to fire it in an N2 atmosphere at a temperature above 600°C and below 1300°C. The above firing is preferably carried out in an inert atmosphere. For example, the above firing may be carried out in an Ar, Ne, or He atmosphere, or in a vacuum. Alternatively, it is preferable to carry out the above firing in a reducing atmosphere containing H2. When firing is carried out in an inert atmosphere or a reducing atmosphere, the initial charge-discharge efficiency of the all-solid-state sodium-ion secondary battery 1 can be further improved.

[0165] Furthermore, a small amount of oxygen may be present in the atmosphere, provided that the negative electrode active material does not oxidize or decompose during firing. The oxygen concentration can be, for example, 1000 ppm or less, but is not limited to this.

[0166] Alternatively, the negative electrode layer 4 may be formed by applying the paste onto a substrate such as PET (polyethylene terephthalate), drying it to create a green sheet, and then firing this green sheet.

[0167] As described above, the negative electrode layer 4 may be a metal film or an alloy film. In this case, the negative electrode layer 4 can be formed using, for example, a physical vapor phase method such as vapor deposition or sputtering, a chemical vapor phase method such as thermal CVD, MOCVD, or plasma CVD, or a liquid phase film deposition method such as plating, sol-gel method, or spin coating. Among these, it is preferable to use vapor deposition or sputtering to form the negative electrode layer 4. In this case, it is easier to improve the adhesion of the negative electrode layer 4 to the solid electrolyte layer 2.

[0168] (Negative electrode active material precursor) When using sugars as the anode active material precursor, examples include cellulose, D-glucose, and sucrose. When using biomass as the anode active material precursor, examples include corn stalks, sorghum stalks, pine cones, mangosteen, argan shells, rice husks, dandelions, grain straw cores, ramie fibers, cotton, kelp, and coconut endocarp. When using polymers as the anode active material precursor, examples include polyacrylonitrile (PAN), pitch, polyvinyl chloride (PVC), nanofibers, polyaniline, sodium polyacrylate, tires (tire polymers), and phosphorus-doped PAN.

[0169] (First current collector layer and second current collector layer) The method for forming the first current collector layer 5 and the second current collector layer 6 is not particularly limited, and for example, metal foil may be used, or physical vapor phase methods such as vapor deposition or sputtering may be used, or chemical vapor phase methods such as thermal CVD, MOCVD, or plasma CVD may be used. Alternatively, liquid phase film deposition methods such as plating, sol-gel method, or spin coating may be used as the method for forming the first current collector layer 5 and the second current collector layer 6.

[0170] The present invention will be described in more detail below based on specific examples. The present invention is not limited in any way to the following examples, and can be implemented with appropriate modifications without changing its essence.

[0171] (Example 1) (a) Preparation of the solid electrolyte layer As the first solid electrolyte layer, β''-alumina (1 mm thick, 12 mm square) manufactured by Ionotec was used. The second solid electrolyte layer was prepared by the following procedure: The raw materials were mixed to achieve the composition of β''-alumina, calcined at 1250°C, and pulverized. After calcination, the raw materials, binder, plasticizer, dispersant, solvent, etc. were mixed to prepare a paste. This paste was coated onto the first solid electrolyte layer to a thickness of 100 μm and 10 mm square. After drying in a constant temperature bath at 70°C for 2 hours, it was fired at 1550°C for 30 minutes to form a second solid electrolyte layer of 10 mm square and 100 μm thick on the first solid electrolyte layer.

[0172] (b) Preparation of positive electrode paste A glass film was prepared by melting raw materials, which were blended to have a molar ratio of 40Na2O-20Fe2O3-40P2O5, in air at 1250°C for 45 minutes, and then forming it with a cooled twin roller. The resulting glass film was then pulverized using a ball mill and a planetary ball mill to obtain an average particle size, D50 of 0.2 μm and a BET specific surface area of ​​30 m². 2 A positive electrode active material precursor powder (glass powder) of / g was obtained.

[0173] The obtained glass powder was 83.2% by mass, and β''-alumina (BET specific surface area is 45 m²) was used as a solid electrolyte. 2 (12.4% by mass / g) and carbon nanotubes as fibrous carbon (C-nano Corporation, product number "LB116", BET specific surface area: 300m²) 2 (1.0 mass%) of acetylene black (Timcal, part number "SUPER P", BET specific surface area: 62 m²) as granular carbon, and acetylene black as granular carbon. 2 A positive electrode composite powder was prepared by mixing 3.4% by mass of ( / g) with the obtained positive electrode composite powder. To 100% by mass of the obtained positive electrode composite powder, 20% by mass of polypropylene carbonate (PPC) was added as a binder, and N-methyl-2-pyrrolidone was added as a solvent to bring the concentration of the positive electrode composite powder to 50% by mass. A positive electrode paste was prepared by mixing this in a rotary mixer.

[0174] (c) Formation of the positive electrode A positive electrode paste was applied to the center of one main surface of the solid electrolyte layer to a thickness of 100 μm and a square shape of 10 mm. After drying in a constant temperature bath at 70°C for 1 hour, firing was performed in an N2 / H2 (96 / 4 vol%) atmosphere at 500°C for 30 minutes to form an electrode composite material (positive electrode) for a sodium-ion secondary battery.

[0175] (d) Formation of current collector and assembly of coin cell A 100 nm thick gold vapor-deposited film was formed on the entire surface of the positive electrode as a current collector. Then, in an argon atmosphere with a dew point of -60°C or lower, metallic sodium, which would serve as the counter electrode, was pressed onto the other surface of the solid electrolyte layer. After placing it on the lower cover of the coin cell, the upper cover was placed over it to fabricate a CR2032 type test battery.

[0176] (Examples 2 to 23 and Comparative Examples 1 to 2) An all-solid-state sodium-ion secondary battery was fabricated in the same manner as in Example 1, except that the composition of the electrode composite for the sodium-ion secondary battery was adjusted to match the composition shown in Table 1 below.

[0177] [Table 1]

[0178] [evaluation] The fabricated batteries were charged and discharged under conditions of 60°C and 0.1C, and their cycle characteristics were measured by determining the discharge capacity retention rate after 100 cycles. Furthermore, the output characteristics were measured by determining the ratio of the discharge capacity under conditions of 60°C and 20C to the discharge capacity under conditions of 60°C and 0.1C. The results are shown in Table 2 below.

[0179] [Table 2]

[0180] Thus, in Examples 1 to 23, where carbon nanotubes as fibrous carbon and acetylene black as granular carbon were present together, the discharge capacity retention rate after 100 cycles was 80% or higher, and the ratio of 20C rate discharge capacity to 0.1C rate discharge capacity was 34% or higher. On the other hand, in Comparative Example 1, which did not contain carbon nanotubes as fibrous carbon in the conductive additive, the ratio of 20C rate discharge capacity to 0.1C rate discharge capacity was low at 33%, and in Comparative Example 2, which did not contain acetylene black as granular carbon in the conductive additive, the discharge capacity retention rate after 100 cycles was low at 75%. [Explanation of Symbols]

[0181] 1… All-solid-state sodium-ion secondary battery 2...Solid electrolyte layer 2a, 2b… First and second principal surfaces 3…Positive electrode layer 4…Negative electrode layer 5…First current collector layer 6…Second current collector layer 7…First solid electrolyte layer 8…Second solid electrolyte layer 7a, 7b… Third and fourth main surfaces

Claims

1. A positive electrode active material comprising a sodium transition metal phosphate crystal containing Na, M (where M is at least one transition metal element selected from Cr, Fe, Mn, Co, V, and Ni), P, and O, An electrode mixture for sodium-ion secondary batteries comprising fibrous carbon and a conductive additive containing granular carbon.

2. Let the BET specific surface area of the granular carbon be S ｐ (m ２ / g), the content of the granular carbon in the electrode composite material for the sodium-ion secondary battery be W ｐ (mass%), the BET specific surface area of the fibrous carbon be S ｆ (m ２ / g), the content of the fibrous carbon in the electrode composite material for the sodium-ion secondary battery be W ｆ (mass%), when the conductive aid surface area S Ｃ per gram of the electrode composite material for the sodium-ion secondary battery represented by the following formula (1) is 1 m ２ / g or more and 5.5 m ２ / g or less, the electrode composite material for the sodium-ion secondary battery according to claim 1. S ｃ =S ｐ ×W ｐ / 100+S ｆ ×W ｆ / 100・・・(1)

3. The electrode composite material for sodium-ion secondary batteries according to claim 1 or 2, wherein the fibrous carbon is a carbon nanotube.

4. The electrode composite material for sodium-ion secondary batteries according to claim 1 or 2, wherein the granular carbon is acetylene black.

5. Furthermore, the electrode composite material for a sodium-ion secondary battery according to claim 1 or 2, further comprising a solid electrolyte.

6. The electrode material for a sodium-ion secondary battery according to claim 5, wherein the solid electrolyte comprises at least one selected from the group consisting of β-alumina, β''-alumina, and NASICON-type crystals.

7. The electrode composite material for a sodium-ion secondary battery according to claim 1 or 2, which is for use in an all-solid-state sodium-ion secondary battery.

8. An electrode for a sodium-ion secondary battery, comprising the electrode composite material for a sodium-ion secondary battery described in claim 1 or 2.

9. An all-solid-state sodium-ion secondary battery comprising the electrode for sodium-ion secondary batteries described in claim 8 as the positive electrode.

Images