Negative electrode for all-solid-state sodium ion secondary battery, method for manufacturing same, and all-solid-state sodium ion secondary battery

Abstract

Provided are: a negative electrode for an all-solid-state sodium ion secondary battery, the negative electrode being capable of improving the initial charge / discharge efficiency, the conservation characteristics, the output characteristics, and the cycle characteristics of the all-solid-state sodium ion secondary battery; a method for manufacturing the negative electrode; and an all-solid-state sodium ion secondary battery. The negative electrode for an all-solid-state sodium ion secondary battery contains a carbon electrode material but does not contain a sodium ion-conducting solid electrolyte.

Description

A negative electrode for all-solid-state sodium-ion secondary battery and method for manufacturing the same, and all-solid-state sodium-ion secondary battery

[0001] The present invention relates to a negative electrode for an all-solid-state sodium-ion secondary battery, a method for manufacturing the same, and an all-solid-state sodium-ion secondary battery using the negative electrode.

[0002] Lithium-ion rechargeable batteries have advantages such as high capacity, high energy density, and small size and light weight, leading to their use as power sources for mobile devices and electric vehicles, and demand is expanding.

[0003] Current lithium-ion rechargeable batteries use flammable organic electrolytes, raising concerns about the risk of fire. Therefore, development is underway on all-solid-state lithium-ion rechargeable batteries that use solid electrolytes instead of organic ones. However, lithium faces challenges such as resource depletion and rising raw material costs, leading to active development of all-solid-state sodium-ion rechargeable batteries using sodium as an alternative.

[0004] Furthermore, Patent Document 1, described below, discloses a sintered electrode containing a carbon electrode material made of graphite or hard carbon and an alkali ion-conducting solid electrolyte.

[0005] International Publication No. 2022 / 009811

[0006] In batteries equipped with sintered electrodes as described above, when the negative electrode is exposed to a negative potential (low potential) during charging, the alkaline ion-conducting solid electrolyte may undergo reductive decomposition. This reductive decomposition can lead to problems such as the consumption of sodium ions when generating decomposition products, or side reactions between the generated decomposition products and sodium ions, which can easily degrade battery performance, including initial charge / discharge performance, storage performance, output performance, and cycle performance.

[0007] The object of the present invention is to provide an anode for an all-solid-state sodium-ion secondary battery, a method for manufacturing the same, and an all-solid-state sodium-ion secondary battery that can improve the initial charge-discharge efficiency, storage characteristics, output characteristics, and cycle characteristics of an all-solid-state sodium-ion secondary battery by suppressing reductive decomposition and suppressing side reactions between reductive decomposition products and sodium ions.

[0008] This paper describes a negative electrode for an all-solid-state sodium-ion secondary battery that solves the above problems, a method for manufacturing the same, and various embodiments of the all-solid-state sodium-ion secondary battery.

[0009] The negative electrode for an all-solid-state sodium-ion secondary battery according to Embodiment 1 of the present invention is characterized by containing a carbon electrode material and substantially not containing a sodium-ion conductive solid electrolyte.

[0010] In the negative electrode for an all-solid-state sodium-ion secondary battery according to Embodiment 2, it is preferable that the carbon electrode material is hard carbon in Embodiment 1.

[0011] In the negative electrode for an all-solid-state sodium-ion secondary battery according to Embodiment 3, in Embodiment 1 or Embodiment 2, it is preferable that the carbon electrode material is composed of a particulate carbon electrode material and a coated carbon electrode material, and that the coated carbon electrode material coats the particulate carbon electrode material.

[0012] In the negative electrode for an all-solid sodium ion secondary battery according to embodiment 4, the average particle size D of the particulate carbon electrode material is as described in embodiment 3. ５０ It is preferable that the particle size is between 0.1 μm and 30 μm.

[0013] In the negative electrode for an all-solid-state sodium-ion secondary battery according to Embodiment 5, it is preferable that it be a sintered body in any one of Embodiments 1 to 4.

[0014] A method for manufacturing a negative electrode for an all-solid sodium-ion secondary battery according to embodiment 6 of the present invention is a method for manufacturing a negative electrode for an all-solid sodium-ion secondary battery according to any one embodiment from embodiment 3 to embodiment 5, characterized in that it includes a step of firing a negative electrode precursor material obtained by mixing the particulate carbon electrode material and the coated carbon electrode material precursor.

[0015] In the method for manufacturing a negative electrode for an all-solid sodium ion secondary battery according to embodiment 7, in embodiment 6, it is preferable that the coated carbon electrode material precursor softens and flows in the step of firing the negative electrode precursor material.

[0016] The all-solid-state sodium-ion secondary battery according to aspect 8 of the present invention is characterized by comprising a negative electrode for an all-solid-state sodium-ion secondary battery according to any one of aspects 1 to 5.

[0017] According to the present invention, it is possible to provide an anode for an all-solid-state sodium-ion secondary battery, a method for manufacturing the same, and an all-solid-state sodium-ion secondary battery that can improve the initial charge-discharge efficiency, storage characteristics, output characteristics, and cycle characteristics of the all-solid-state sodium-ion secondary battery.

[0018] 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 is a schematic cross-sectional view showing the solid electrolyte layer in the all-solid-state sodium-ion secondary battery of Figure 1.

[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] (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.

[0021] 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.

[0022] 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 are not required.

[0023] 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.

[0024] The details of each layer in the all-solid-state sodium-ion secondary battery of the present invention will be described below.

[0025] (Negative electrode layer) The negative electrode layer 4 is composed of a negative electrode for an all-solid-state sodium-ion secondary battery that contains a carbon electrode material and substantially does not contain a sodium-ion conductive solid electrolyte. The carbon electrode material acts as the negative electrode active material. Since the negative electrode for an all-solid-state sodium-ion secondary battery does not contain a sodium-ion conductive solid electrolyte, when a battery equipped with the negative electrode for an all-solid-state sodium-ion secondary battery is charged, the reductive decomposition of the negative electrode and side reactions between the reductive decomposition products and sodium ions can be suppressed.

[0026] Furthermore, "substantially free of sodium ion conductive solid electrolytes" means, for example, that the solid electrolyte content in the negative electrode layer 4 is less than 1% by mass, or that the content of each component Zr, Si, P, and Al in the negative electrode layer 4 is less than 1% by mass. In addition, when the negative electrode layer 4 is provided on a second solid electrolyte layer 8, which is a porous layer as described later, a part of the negative electrode layer 4 may be provided in a three-dimensionally connected void in the second solid electrolyte layer 8. In this case, the region without the second solid electrolyte layer 8, for example, the region within a depth of 20 μm from the current collector side surface of the negative electrode layer 4, is substantially free of sodium ion conductive solid electrolytes.

[0027] The carbon electrode material is preferably composed of particulate carbon electrode material and coated carbon electrode material. Both of these act as negative electrode active materials. The particulate carbon electrode material has high sodium absorption properties, and the coated carbon electrode material can absorb sodium, but plays a role in binding the particulate carbon electrode material and forming sodium ion conduction paths.

[0028] The lower limit of the carbon electrode material content in the negative electrode for an all-solid-state sodium-ion secondary battery is not particularly limited, but is preferably 50% or more by volume, and particularly preferably 60% or more. The upper limit of the carbon electrode material content in the negative electrode for an all-solid-state sodium-ion secondary battery is not particularly limited, but is preferably 99.9% or less by volume, preferably 95% or less, and particularly preferably 90% or less. When the carbon electrode material content in the negative electrode for an all-solid-state sodium-ion secondary battery is within the above ranges, the battery characteristics can be improved even more effectively.

[0029] The particulate carbon electrode material is preferably hard carbon. The particulate carbon electrode material is preferably spherical. When the particulate carbon electrode material is spherical, the packing density of the carbon electrode material in the negative electrode of the all-solid sodium-ion secondary battery can be improved.

[0030] Average particle size D of particulate carbon electrode material ５０ The lower limit is not particularly limited, but is preferably 0.1 μm or larger, 0.5 μm or larger, and particularly preferably 1 μm or larger. Average particle size D of particulate carbon electrode material ５０The upper limit is not particularly limited, but is preferably 30 μm or less, 20 μm or less, 10 μm or less, and especially preferably 5 μm or less. Average particle size D of particulate carbon electrode material ５０ If the average particle diameter D is within the above range, the contact area between the negative electrode layer 4 and the solid electrolyte layer 2 can be improved. ５０ "50%" refers to the value measured by a laser diffraction particle size distribution analyzer, and represents the particle size at which the cumulative amount, starting from the smallest particle, accounts for 50% of the volume-based cumulative particle size distribution curve measured by the laser diffraction method.

[0031] The lower limit of the content of particulate carbon electrode material in the negative electrode for an all-solid-state sodium-ion secondary battery is not particularly limited, but is preferably 40% or more by volume, and particularly preferably 50% or more. The upper limit of the content of particulate carbon electrode material in the negative electrode for an all-solid-state sodium-ion secondary battery is not particularly limited, but is preferably 90% or less by volume, preferably 85% or less, and particularly preferably 83% or less. When the content of particulate carbon electrode material in the negative electrode for an all-solid-state sodium-ion secondary battery is within the above range, the cycle characteristics and storage characteristics can be further effectively improved.

[0032] The coated carbon electrode material is preferably hard carbon. The coated carbon electrode material is preferably coated over a particulate carbon electrode material. When the coated carbon electrode material is coated over a particulate carbon electrode material, it can bind the particulate carbon electrode material and form sodium ion conduction paths, thereby more effectively improving battery characteristics such as output characteristics.

[0033] The lower limit of the content of the coated carbon electrode material in the negative electrode for an all-solid-state sodium-ion secondary battery is not particularly limited, but is preferably 10% or more by volume, and particularly preferably 15% or more. The upper limit of the content of the coated carbon electrode material in the negative electrode for an all-solid-state sodium-ion secondary battery is not particularly limited, but is preferably 60% or less by volume, preferably 55% or less, and particularly preferably 53% or less. When the content of the coated carbon electrode material in the negative electrode for an all-solid-state sodium-ion secondary battery is within the above range, the output characteristics can be improved even more effectively.

[0034] The negative electrode for an all-solid-state sodium-ion secondary battery is preferably made of a sintered body. When the negative electrode for an all-solid-state sodium-ion secondary battery is made of a sintered body, its reactivity to moisture is low, unlike electrodes made of sodium metal. In addition, since it does not require an organic electrolyte, the risk of ignition is even lower. Therefore, safety can be enhanced.

[0035] The negative electrode for an all-solid-state sodium-ion secondary battery may contain a conductive additive. For example, conductive carbon can be used as the conductive additive. Examples of conductive carbon include acetylene black, carbon black, Ketjenblack, vapor-processed carbon fiber (VGCF), carbon nanotubes, and graphene. The conductive additive is preferably a carbon-based conductive additive made of the above materials. When the negative electrode for an all-solid-state sodium-ion secondary battery 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 10% or less by mass, and particularly preferably 5% or less, and particularly preferably 3% or less. When the content of the conductive additive in the negative electrode for an all-solid-state sodium-ion secondary battery is within the above range, it is possible to further improve ionic conductivity while ensuring high electronic conductivity in the negative electrode for an all-solid-state sodium-ion secondary battery, and to further effectively improve the battery characteristics of the secondary battery.

[0036] The lower limit of the thickness of the negative electrode for the all-solid-state sodium-ion secondary battery 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 for the all-solid-state sodium-ion secondary battery 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 for the all-solid-state sodium-ion secondary battery is preferably 1000 μm or less, more preferably 500 μm or less, and more preferably 300 μm or less. If the thickness of the negative electrode for the all-solid-state sodium-ion secondary battery 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.

[0037] (Solid Electrolyte Layer) Figure 2 is a schematic cross-sectional view showing the solid electrolyte layer in the embodiment shown in Figure 1.

[0038] 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.

[0039] 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 voids that are connected in three dimensions. It is desirable that the first solid electrolyte layer 7 and the second solid electrolyte layer 8 are integrated.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] 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 formula (1). In formula (1), p is the bulk density and p0 is the true density.

[0045] Porosity = (1-p / p0) x 100 (%)...Formula (1)

[0046] 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%.

[0047] 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.

[0048] 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.

[0049] The pore diameter can be measured by performing 3D structural observation using X-ray CT and analyzing the images. In addition, it can be determined by measurement using the mercury intrusion method or by 3D reconstruction using SEM-FIB.

[0050] 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.

[0051] 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.

[0052] 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 approaches 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.

[0053] 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 70 μm or less, and more preferably 50 μm or less.

[0054] 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.

[0055] 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.

[0056] 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 deteriorate. 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.

[0057] 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 when forming 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.

[0058] 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.

[0059] 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.

[0060] 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 170 μm or less, and more preferably 150 μm or less.

[0061] 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.

[0062] 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.

[0063] In addition, flatness is defined in JIS B 0621-1984 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 surface of the sheet is sandwiched between parallel planes.

[0064] 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.

[0065] 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.

[0066] 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.

[0067] 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.

[0068] 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.

[0069] 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.

[0070] The oxide solid electrolyte used in the solid electrolyte layer 2 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 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 crystals. It is more preferable that the sodium ion conductive oxide is β-alumina or β''-alumina, as these exhibit even greater 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 crystals.

[0071] Beta-alumina exists in two crystalline forms: β-alumina and β''-alumina. The theoretical empirical formula for β-alumina is Na２ O·11Al ２ O ３ It is. The theoretical composition formula of β''-alumina is Na ２ O·5.3Al ２ O ３ It is. Since β''-alumina is a metastable substance, usually, those added with Li ２ O or MgO as stabilizers 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. It is more preferable to use Li ２ O-stabilized β''-alumina or MgO-stabilized β''-alumina.

[0072] 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 etc., and trigonal Li 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<​​​​​​​​​​​​​​​​​​​​​​​​​​​​１７ , (Al １０．３２ Mg ０．６８ O １６ ) (Na １．６８ Examples include O).

[0074] NASICON crystals have the general 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, with s = 1.4 to 5.2, t = 1 to 2.9, u = 2.8 to 4.1, and v = 9 to 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.

[0075] The preferred ranges for each coefficient in the above general formula are as follows:

[0076] 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.

[0077] 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.

[0078] 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.

[0079] 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 from the lone pair of electrons of the oxygen atom, which tends to reduce the ionic conductivity of the solid electrolyte.

[0080] The NASICON 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.

[0081] A specific example of a NASICON crystal is Na ３ Zr ２ Si ２ PO １２ Na ３．２ Zr １．３ Si ２．２ P ０．８ O １０．５ Na ３ Zr １．６ Ti ０．４ Si ２ PO １２ Na ３ HF ２ Si ２ PO １２ Na ３．４ Zr ０．９ HF １．４ Al ０．６ Si １．２ P １．８ O １２ Na ３ Zr １．７ Nb ０．２４ Si ２PO １２ 、No ３．６ Today ０．２ Y ０．８ Yes ２．８ O ９ 、No ３ Zr １．８８ Y ０．１２ Yes ２ PO １２ 、No ３．１２ Zr １．８８ Y ０．１２ Yes ２ PO １２ 、No ３．０５ Zr ２ Yes ２．０６ P ０．９５ O １２ 、No ３．４ Zr ２ Yes ２．４ P ０．６ O １２ 、No ３．４ Zr １．９ Zn ０．１ Yes ２．４ P ０．６ O １２ 、No ３．４ Zr １．９ Mg ０．１ Yes ２．４ P ０．６ O １２ 、No ３．４ Zr １．９ Zn ０．１ Yes ２．２ P ０．８ O １２ 、No ３．４ Zr １．９ Mg ０．１ Yes ２．２ P ０．８ O １２ 、No ２．８ Zr ２ Yes ２．４ P ０．６ O １２ 、No ３．６ Zr ０．１３ Yb １．６７ Yes ０．１１ P ２．９ O １２ 、No ５ YSi ４ O １２ 、No ３．１ Zr １．９５ Mg ０．０５ Yes２ PO １２ 、No ３．１ Zr １．９ Yes ０．１ Yes ２ PO １２ 、No ３．１ Zr １．９ N$ ０．１ Yes ２ PO １２ 、No ３．１ Zr １．９ Y ０．１ Yes ２ PO １２ 、No ３．２５６ Zr １．８７２ Mg ０．１２８ Yes ２ PO １２ 、No ３．２ Zr １．９ Ca ０．１ Yes ２ PO １２ 、No ３．２ Zr １．９ Mg ０．１ Yes ２ PO １２ 、No ３．２ Zr ２ Yes ２．２ P ０．８ O １２ 、No ３．３８ Zr １．８０ Al ０．２６ Yes ２．０６ P ０．８８ O １２ 、No ３．４３ Zr １．８３ Zn ０．２２ Yes １．９３ P １．０２ O １２ 、No ３．４ Sc ０．４ Zr １．６ Yes ２ PO １２ 、No ３．４ Zr １．８ Mg ０．２ Yes ２ PO １２ 、No ３．４ Zr １．９ Zn ０．１ Yes ２．２ P ０．８ O １２ 、No ３．５７ Zr１．７２ La ０．２１ Si ２．０８ P ０．９２ O １２ Na ３ Zr １．９８ Nb ０．０８ Si ２ PO １２ Na ３ Zr １．９ Ce ０．１ Si ２ PO １２ Na ３ Zr １．９ Gd ０．１ Si ２ PO １２ Na ３ Zr １．９ Ti ０．１ Si ２ PO １２ Na ３ Zr １．９ Yb ０．１ Si ２ PO １２ Examples of crystals include the following. These may be used individually or in combination of multiple types. Among them, the NASICON crystal is Na ３ Zr ２ Si ２ PO １２ Na ３．４ Zr ２ Si ２．４ P ０．６ O １２、 or Na ３．０５ Zr ２ Si ２．０６ P ０．９５ O １２ It is preferable that Na ３ Zr ２ Si ２ PO １２ This is more preferable. In this case, the ionic conductivity of the NASICON crystal can be further improved.

[0082] 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.

[0083] The metals constituting the metal layer are not particularly limited, but for example, Sn, Ti, Bi, Au, Al, Cu, Sb, Pb, etc. can be used. These metals constituting 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.

[0084] 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.

[0085] 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.

[0086] (Positive Electrode Layer) The positive electrode active material included in the positive electrode layer 3 shown in Figure 1 is not particularly limited, but it is preferably a positive electrode active material containing 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. A specific example is Na ２ FeP ２ O ７ Na ４ Fe ３(2O ４ ) ２ (P ２ O ７ ), Na ４ Fe ５ (2O ４ ) ２ (P ２ O ７ ) ２ 、N ３．６４ Fe ２．１８ (P ２ O ７ ) ２ 、N ３ Fe ２ (2O ４ )(P ２ O ７ ), KFePO ４ 、N ２ MnP ２ O ７ 、N ４ Mn ３ (2O ４ ) ２ (P ２ O ７ ), Na ４ Mn ５ (2O ４ ) ２ (P ２ O ７ ) ２ 、N ３．６４ Mn ２．１８ (P ２ O ７ ) ２ 、N ３ V ２ (2O ４ ) ３ 、NuNiPO ４ 、N ２ NiP ２ O ７ 、N ４ Ni ３ (2O ４ ) ２ (P ２ O ７ ), Na ４ Ni ５ (2O ４ ) ２ (P ２ O ７ ) ２ 、N ３．６４ Ni ２．１８(P ２ O ７ ) ２ Na ４ Ni ７ (PO ４ ) ６ Na ３ Ni ３ (PO ４ ) ２ (P ２ O ７ ), NaCoPO ４ Na ２ CoP ２ O ７ Na ４ Co ３ (PO ４ ) ２ (P ２ O ７ ), Na ４ Co ５ (PO ４ ) ２ (P ２ O ７ ) ２ Na ３．５ Cr １．５ Co ０．５ (PO ４ ) ３ Na ３．６４ Co ２．１８ (P ２ O ７ ) ２ Examples include the following. The sodium transition metal phosphate crystal is preferred because it has high capacity and excellent chemical stability. Among these, triclinic crystals belonging to space group P1 or P-1 are particularly preferred, especially those with the general formula Na ｘ M ｙ P ２ O 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.

[0087] The positive electrode active material preferably contains crystallized glass containing the sodium transition metal phosphate crystals mentioned above, as this provides excellent ion conductivity and improves battery characteristics. Crystallized glass refers to a material obtained by heating a precursor glass containing an amorphous phase and precipitating crystals. In other words, crystallized glass refers to a material obtained by firing a precursor glass containing an amorphous phase and crystallizing the amorphous phase. The entire amorphous phase may have transitioned to a crystalline phase, or some of the amorphous phase may remain. One type of crystal may be precipitated from the amorphous phase, or two or more types of crystals may be precipitated. Whether or not a material is crystallized glass can be determined, for example, based on the peak angle shown by powder X-ray diffraction (XRD).

[0088] The lower limit of the content of the positive electrode active material 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 the 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 the positive electrode active material in the positive electrode layer 3 is within the above range, the battery capacity can be increased even more effectively.

[0089] The positive electrode layer 3 may contain a solid electrolyte. The solid electrolyte described in the "Solid Electrolyte Layer" section can be used. When the positive electrode layer 3 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 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 is within the above range, integration of the positive electrode active material and the solid electrolyte can be achieved. Alternatively, when the positive electrode layer 3 is in contact with the solid electrolyte layer 2, integration of the two can be achieved. As a result, ion conductivity can be further improved, and battery characteristics can be more effectively enhanced.

[0090] The positive electrode layer 3 may contain a conductive additive. For example, the conductive additive described in the "negative electrode layer" section can be used. When the positive electrode layer 3 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 20% or less by mass, and particularly preferably 10% or less, and particularly preferably 5% or less. When the content of the conductive additive in the positive electrode layer 3 is within the above range, it is possible to further improve ionic conductivity while ensuring high electronic conductivity in the positive electrode layer 3, and to further effectively improve battery characteristics.

[0091] 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 increased even further. On the other hand, the upper limit of the thickness of the positive electrode layer 3 is preferably 2000 μm or less, 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.

[0092] (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 electrical conductivity and are less prone to chemical reactions during charging and discharging of secondary batteries, thus effectively increasing the capacity of secondary batteries and exhibiting excellent cycle characteristics due to charging and discharging.

[0093] 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 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 charging and discharging of the secondary battery.

[0094] 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 ejected electrode when used as a secondary battery, thereby effectively increasing the capacity of the secondary battery and providing excellent cycle characteristics due to charge and discharge.

[0095] 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 secondary battery.

[0096] 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 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.

[0097] 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.

[0098] (Formation of the solid electrolyte layer) (a) Preparation of the green sheet for forming the first 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 of the first solid electrolyte powder. The raw material powder referred to here is a 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.

[0099] 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.

[0100] As the first solid electrolyte powder, at least one selected from the group consisting of β''-alumina, β-alumina, and NASICON crystals can be prepared. The same materials as those described in the "Solid Electrolyte Layer" section above can be used as β''-alumina, β-alumina, and NASICON crystals.

[0101] As the raw material powder for the first solid electrolyte powder, for example, if it is a raw material powder of β''-alumina, then in mol% Al ２ O ３ 65% to 98%, Na ２ O 2% to 20%, MgO+Li ２ O 0.3% to 15%, ZrO ２ 0% to 20%, Y ２ O ３ We can prepare formulations containing 0% to 5% of the substance. The reason for limiting the composition as described above is explained below.

[0102] Al ２ O ３ Al is the main component of β''-alumina. ２ O ３The content of is preferably 65% ​​to 98% in mole percent, more preferably 65% ​​to 97.7%, and even more preferably 70% to 95%. ２ O ３ If there is too little of it, the ionic conductivity of the solid electrolyte tends to decrease. On the other hand, Al ２ O ３ If there is too much of it, α-alumina that does not have sodium ion conductivity will remain, and the ionic conductivity of the solid electrolyte will easily decrease.

[0103] Na ２ O is a component that imparts sodium ion conductivity to solid electrolytes. ２ The O content is preferably 2% to 20% in mole percent, more preferably 3% to 18%, and even more preferably 4% to 16%. ２ If there is too little oxygen, it becomes difficult to obtain the above effects. On the other hand, Na ２ If there is too much oxygen, the excess sodium will turn into NaAlO. ２ Because it forms compounds that do not contribute to ionic conductivity, the ionic conductivity of the solid electrolyte tends to decrease.

[0104] MgO and Li ２ O is a component that stabilizes the structure of β''-alumina, i.e., a stabilizer. MgO + Li ２ The O content is preferably 0.3% to 15% in mole percent, more preferably 0.5% to 10%, and even more preferably 0.8% to 8%. MgO + Li ２ If there is too little oxygen, α-alumina will remain in the solid electrolyte, which tends to reduce ionic conductivity. On the other hand, MgO + Li ２ If there is too much oxygen, MgO or Li will not function as a stabilizer. ２ Oxygen can remain in the solid electrolyte, easily leading to a decrease in ionic conductivity.

[0105] ZrO ２ and Y ２ O ３ This has the effect of suppressing abnormal grain growth of β''-alumina during firing and improving the adhesion of each β''-alumina particle. As a result, the ionic conductivity of the solid electrolyte is more easily improved. ZrO２ The content of 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%. ２ O ３ The content of ZrO is preferably 0% to 5% in mole percent, more preferably 0.01% to 4%, and even more preferably 0.02% to 3%. ２ or Y ２ O ３ If there is too much of it, the amount of β''-alumina produced decreases, which can easily reduce the ionic conductivity of the solid electrolyte.

[0106] For example, if the raw material powder for the first solid electrolyte powder is a raw material powder for NASICON crystals, then the amount of Na in mol% is... ２ O 17.5% to 50%, Al ２ O ３ +Y ２ O ３ +Yb ２ O ３ +Nd ２ O ３ +Nb ２ O ５ +TiO ２ +HfO ２ +ZrO ２ 12% to 45%, SiO ２ +P ２ O ５ Products containing 24% to 54% can be prepared.

[0107] The average particle size D of at least one of the first solid electrolyte powder and the raw material powder of the first solid electrolyte powder. ５０ However, it is preferable that the particle size is 0.01 μm or larger, more preferably 0.05 μm or larger, and even more preferably 0.1 μm or larger. On the other hand, the average particle size D of at least one of the powders from the first solid electrolyte powder and the raw material powder of the first solid electrolyte powder. ５０However, it is preferable that the particle size be 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 second solid electrolyte layer 8, which is a porous layer, from the first solid electrolyte layer 7 can be made less likely during the subsequent firing process.

[0108] 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.

[0109] Alternatively, the binder may be a glass binder. For example, sodium ion conductive glass powder can be used as the glass binder.

[0110] 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.

[0111] 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.

[0112] 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.

[0113] (b) Preparation of a green sheet for forming the 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 off in a later firing process.

[0114] 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.

[0115] 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.

[0116] The average particle size D of at least one of the second solid electrolyte powder and the raw material powder of the second solid electrolyte powder. ５０However, it is preferably 0.1 μm or larger, more preferably 0.2 μm or larger, even more preferably 0.5 μm or larger, and particularly preferably 0.8 μm or larger. On the other hand, the average particle size D of at least one of the powders from the second solid electrolyte powder and the raw material powder of the second solid electrolyte powder. ５０ However, it is preferable that the thickness be 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 second solid electrolyte layer 8, which is a porous layer, from the first solid electrolyte layer 7 can be made less likely during the subsequent firing process.

[0117] 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.

[0118] Average particle size D of polymer powder ５０ However, it is preferable that the particle size is 0.1 μm or larger, more preferably 1 μm or larger, even more preferably 5 μm or larger, and particularly preferably 10 μm or larger. On the other hand, the average particle size D of the polymer powder ５０ However, it 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. Average particle size D of polymer powder ５０ If the average particle size D of the polymer powder is too small, it becomes difficult to form three-dimensionally interconnected voids in the resulting second solid electrolyte layer 8. ５０ If the value is too large, the sintering of the resulting second solid electrolyte layer 8 may be insufficient, and the ionic conductivity may decrease.

[0119] 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.

[0120] 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.

[0121] 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.

[0122] 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.

[0123] 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.

[0124] (c) Preparation of laminated sheet Next, a laminated sheet is obtained by laminating the 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 press. In this case, the adhesion between each green sheet can be further improved.

[0125] 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.

[0126] (d) Firing of the laminated sheet Next, the obtained 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.

[0127] 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.

[0128] 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.

[0129] When the solid electrolyte powder contains NASICON 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.

[0130] 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.

[0131] 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.

[0132] 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.

[0133] (Sodium ion-conducting 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.

[0134] The sodium ion-conducting solid electrolyte is a NASICON crystal, or Na ５ XSi ４ O １２ In 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 Na ５ XSi ４ O １２ If it is a type crystal, it is preferable that X is at least one element selected from rare earth elements.

[0135] As a sodium ion-conducting solid electrolyte precursor solution, a solution containing nitrate ions instead of carbonate ions can also be used.

[0136] 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.

[0137] In a sodium ion conductive solid electrolyte precursor solution, the general formula NR is used as the counterion for sodium ions. ４ ＋ This is represented by the equation, where each R is independent of H and CH. ３ , C ２ H ５ and CH ２ CH ２ It is preferable that the mixture contains at least one substituent ion selected from the group consisting of OH groups. In this case, the transition metal element is more likely to exist stably in the solution.

[0138] 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.

[0139] (Formation of the negative electrode layer) The negative electrode layer 4 (negative electrode for all-solid sodium-ion secondary battery) can be prepared using a paste containing, for example, particulate carbon electrode material and coated carbon electrode material precursor, and optionally a conductive additive. Binders, plasticizers, solvents, etc., may be added to the paste as needed.

[0140] It is preferable to pre-fire the particulate carbon electrode material under a nitrogen atmosphere at a temperature of 1400°C to 2000°C in order to remove functional groups from the surface and suppress the adsorption of sodium ions. The firing temperature should be set appropriately according to the oxygen content and specific surface area of ​​the particulate carbon electrode material. If the firing temperature is low, functional groups will remain on the surface of the particulate carbon electrode material, making it easier for sodium ions to be adsorbed. On the other hand, if the firing temperature is high, the reaction area with sodium may decrease due to a reduction in the interlayer spacing within the particulate carbon electrode material and a decrease in the specific surface area.

[0141] To obtain the paste, particulate carbon electrode material and coated carbon electrode material precursor are mixed to obtain a powder of the mixture (negative electrode precursor material). Next, the obtained powder mixture is pulverized and then mixed with a conductive additive and a binder in an organic solvent. For example, N-methyl-2-pyrrolidone can be used as the organic solvent. This yields the paste.

[0142] If the paste contains a binder, the binder may include, for example, 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.

[0143] After applying the paste to one main surface of the solid electrolyte layer 2, drying it and then firing it can form a negative electrode layer 4 (negative electrode for an all-solid sodium ion secondary battery) containing particulate carbon electrode material and coated carbon electrode material. During the firing process, the binder decomposes, the coated carbon electrode material precursor softens and flows, and a portion of it forms the coated carbon electrode material. The softening and flowing of the coated carbon electrode material precursor allows the particulate carbon electrode materials to bond together, forming ion conduction paths.

[0144] When firing the laminate of the solid electrolyte layer 2 and the paste, for example, N ２ It is preferable to perform the firing at a temperature of over 600°C and 1300°C or lower under an inert atmosphere. The above firing is preferably carried out under an inert atmosphere. For example, the above firing is carried out in an Ar, Ne, CO ２ Alternatively, the firing may be carried out in an He atmosphere, or in a vacuum. Alternatively, the above firing may be carried out in an H atmosphere. ２It is preferable to carry out the process in a reducing atmosphere containing [a specific substance]. When firing is performed 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.

[0145] Furthermore, a small amount of oxygen may be present in the atmosphere, provided that the particulate carbon electrode material and the coated carbon electrode material do not oxidize or decompose during firing. The oxygen concentration can be, for example, 1000 ppm or less, but is not limited to this.

[0146] 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.

[0147] (Coated carbon electrode material precursors) When sugars are used as coated carbon electrode material precursors, examples include cellulose, D-glucose, sucrose, etc. When biomass is used as coated carbon electrode material precursors, examples include corn stalks, sorghum stalks, pine cones, mangosteen, argan shells, rice husks, dandelions, grain straw cores, ramie fibers, cotton, kelp, coconut endocarp, etc. When polymers are used as coated carbon electrode material precursors, examples include polyacrylonitrile (PAN), pitch, polyvinyl chloride (PVC), nanofibers, polyaniline, sodium polyacrylate, tires (tire polymers), phosphorus-doped PAN, phenolic resins, etc.

[0148] (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.

[0149] As the binder, the binder described in the section on "Formation of the negative electrode layer" can be used.

[0150] 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.

[0151] 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. The positive electrode layer 3 is obtained by the above firing.

[0152] (Positive electrode active material precursor and its preparation) Specifically, the positive electrode active material precursor is, for example, 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 by 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.

[0153] 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.

[0154] The positive electrode active material precursor powder contains, in the following oxide equivalent mol%, Na ２ O 25% to 55%, Fe ２ O ３ +Cr ２ O ３ +MnO+CoO+V ２ O ５ +NiO 10% to 30%, and P ２ O ５It is preferable that the composition contains 25% to 55%. 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 "mol%".

[0155] Na ２ O is the general formula Na ｘ M ｙ P ２ O ｚ It is represented by , where 1 ≤ x ≤ 2.8, 0.95 ≤ y ≤ 1.6, 6.5 ≤ z ≤ 8, and M is the main component of the active material crystal, being at least one transition metal element selected from Cr, Fe, Mn, Co, V, and Ni. Na ２ The O content is preferably 25% to 55%, and more preferably 30% to 50%. ２ If the O 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.

[0156] Fe ２ O ３ , Cr ２ O ３ , MnO, CoO, V ２ O ５ And NiO also has the general formula Na ｘ M ｙ P ２ O ｚ The main component of the above active material crystal is represented by Fe. ２ O ３ +Cr ２ O ３ +MnO+CoO+V ２ O ５ The +NiO content is preferably 10% to 30%, and more preferably 15% to 25%. Fe ２ O ３ +Cr ２ O ３ +MnO+CoO+V ２ O ５ If the +NiO content is above 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, Fe ２ O ３ +Cr ２ O ３+MnO+CoO+V ２ O ５ If the +NiO content is below the above upper limit, unwanted Fe ２ O ３ , Cr ２ O ３ , MnO, CoO, V ２ O ５ Alternatively, it can make it difficult for crystals such as NiO to precipitate. In this specification, when the content of a+b+c+... is described, for example, it means the total amount of a, b, and c.

[0157] To further improve the cycle characteristics of the all-solid-state sodium-ion secondary battery 1, Fe ２ O ３ It is preferable to actively include Fe. ２ O ３ The content is preferably 1% to 30%, more preferably 5% to 30%, even more preferably 10% to 30%, and particularly preferably 15% to 25%. Cr ２ O ３ , MnO, CoO, V ２ O ５ The content of each component, and NiO, is preferably 0% to 30%, more preferably 10% to 30%, and even more preferably 15% to 25%. ２ O ３ , Cr ２ O ３ , MnO, CoO, V ２ O ５ When at least two components selected from and NiO are included, the total amount is preferably 10% to 30%, and more preferably 15% to 25%.

[0158] P ２ O ５ Also, the general formula Na ｘ M ｙ P ２ O ｚ The main component of the above active material crystal is represented by P. ２ O ５ The content is preferably 25% to 55%, and more preferably 30% to 50%.２ O ５ If the content of is within the above range, the charge and discharge capacity of the all-solid-state sodium-ion secondary battery 1 can be further increased.

[0159] In addition to the above components, the cathode active material precursor powder also contains Nb ２ O ５ MgO, Al ２ O ３ , TiO ２ , ZrO ２ , or Sc ２ O ３ The positive electrode active material precursor powder may contain these 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.

[0160] The positive electrode active material precursor powder contains, in addition to the above components, SiO ２ , B ２ O ３ , GeO ２ Ga ２ O ３ Sb ２ O ３ , or Bi ２ O ３ It may contain these components. When the raw materials for the positive electrode active material precursor powder contain these components, the glass-forming 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.

[0161] 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.

[0162] 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.

[0163] 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.

[0164] Next, the obtained molded body is crushed to obtain a positive electrode active material precursor powder. Average particle size D of the positive electrode active material precursor powder. ５０ The particle size 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.

[0165] (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, as a method for forming the first current collector layer 5 and the second current collector layer 6, liquid phase film formation methods such as plating, sol-gel method or spin coating may be used.

[0166] The present invention will be described in more detail below based on specific examples, but the present invention is not limited in any way to the following examples and can be implemented with appropriate modifications without changing the gist of the invention.

[0167] (Examples 1-4) (a) Preparation of negative electrode paste A hard carbon source sucrose (table sugar), which is a precursor of the coated carbon electrode material, and a hard carbon powder (AT Electrode LN-0001, average particle size D) which is a particulate carbon electrode material. ５０ The hard carbon powder (1 μm) and conductive additive (acetylene black) were weighed so that the composition of the negative electrode was as shown in Table 1 by volume. The hard carbon powder was pre-calcined at 1600°C to remove the functional groups on its surface. A PPC (polypropylene carbonate) binder was added to these in an external proportion of 15 wt%, and the mixture was thoroughly stirred in an N-methylpyrrolidone solvent using a rotation-revolution mixer to form a slurry and prepare a negative electrode paste.

[0168] (b) A β''-alumina solid electrolyte substrate with a thickness of 500 μm was prepared separately for the formation of the negative electrode for the all-solid sodium ion secondary battery. The electrode paste was applied to the first main surface of the solid electrolyte substrate to a thickness of 70 μm and dried in a drying oven at 70°C. After drying, under a nitrogen atmosphere (N ２ A sintered negative electrode (negative electrode for all-solid-state sodium-ion secondary battery) was formed on a β''-alumina solid electrolyte substrate by firing it at 1000°C for 2 hours with a hard carbon powder coating layer. The coating layer was formed by depositing hard carbon, a coating carbon electrode material derived from sucrose. Next, a current collector was formed on the sintered negative electrode by sputtering. The current collector consisted of a thin aluminum film. (c) Test battery assembly A thin gold film was deposited on the side of the β''-alumina solid electrolyte substrate opposite to the side where the sintered negative electrode was formed by vapor deposition. A sodium metal foil was attached thereto and placed on the lower cover of the coin cell. The upper cover of the coin cell was then laminated on top of that to create a test battery. The test battery was assembled in an environment with a dew point temperature of -70°C or lower.

[0169] (Comparative Example 1) (a) In the preparation of the negative electrode paste, in addition to sucrose (cane sugar) as the hard carbon source, hard carbon powder, and a conductive assistant (acetylene black), β″-alumina, which is a sodium ion conductive solid electrolyte, was weighed so that the composition of the negative electrode became the volume % shown in Table 1, and a negative electrode for an all-solid-state sodium ion secondary battery was prepared in the same manner as in Example 1.

[0170] (d) Evaluation of battery characteristics (Initial charge-discharge efficiency) For the obtained test battery, CC (constant current) charging at a rate of 0.1 C was performed from the open circuit voltage to 0.001 V at 30°C, and CV (constant voltage, 0.001 V) charging was performed until the rate reached 0.02 C. Thereafter, the initial charge capacity charged to the negative electrode per unit mass was determined. Next, CC discharge was performed at a rate of 0.02 C from 0.001 V to 2.5 V, the initial discharge capacity discharged from the electrode layer per unit mass was determined, and the initial charge-discharge efficiency was determined. The results are shown in Table 1.

[0171] (Cycle characteristics) Next, after CC charging at a rate of 0.1 C from 2.5 V to 0.001 V at 30°C, CC discharge at a rate of 0.1 C from 0.001 V to 2.5 V was taken as one cycle, and charge-discharge was repeated, and the discharge capacity retention rate, which is the ratio of the discharge capacity at the 100th cycle to the initial discharge capacity, was determined. The results are shown in Table 1.

[0172] (Storage characteristics) Further, CC charging at a rate of 0.1 C from 2.5 V to 0.001 V was performed at 30°C, and CV (0.001 V) charging was performed until the rate reached 0.02 C. After the charged battery was stored in a constant temperature bath at 60°C for 4 weeks, it was residual-discharged by CC discharge at a rate of 0.02 C. Thereafter, CC charging at a rate of 0.1 C from 2.5 V to 0.001 V was performed at 60°C, CV (0.001 V) charging was performed until the rate reached 0.02 C, and the discharge capacity (discharge capacity after storage) when discharged by CC discharge at a rate of 0.02 C was determined. The storage capacity retention rate (%) was calculated from (discharge capacity after storage / initial discharge capacity) × 100. The results are shown in Table 1.

[0173] (Output Characteristics) Also, CC charging was performed at a rate of 0.1 C from 2.5 V to 0.001 V at 30 °C, and then CV (0.001 V) charging was performed until the rate reached 0.02 C. Next, CC discharging was performed at rates of 0.1 C, 1 C, and 5 C from 0.001 V to 2.5 V, and the respective discharge capacities were determined. The results are shown in Table 1.

[0174]

[0175] For the negative electrodes of the all-solid-state sodium-ion secondary batteries of Examples 1 to 4, the initial charge-discharge efficiency was 88% or more, the discharge capacity retention rate after 100 cycles was 91% or more, the storage capacity retention rate was 80% or more, and the discharge capacity at a rate of 5 C was 105 mAh / g or more. On the other hand, for the negative electrode of Comparative Example 1, the initial charge-discharge efficiency was 82%, the discharge capacity retention rate after 100 cycles was 90%, the storage capacity retention rate was 60%, and the discharge capacity at a rate of 5 C was 61 mAh / g.

[0176] (Examples 5 to 8) (a) Preparation of Negative Electrode Paste Sucrose (saccharose), which is a precursor of a carbon electrode material, hard carbon powder (LN-0001 manufactured by AT Electro - de, average particle diameter D ５０ = 1 μm), and a conductive assistant (acetylene black) were weighed so that the composition of the negative electrode became the volume % shown in Table 2. The hard carbon powder was pre-fired at 1600 °C in advance to remove the functional groups on the surface. To these, a PPC (polypropylene carbonate) binder was added so that the external ratio was 15 wt%, and they were sufficiently stirred using a rotation / revolution mixer in an N-methylpyrrolidone solvent to form a slurry and prepare a negative electrode paste.

[0177] (b) Formation of Negative Electrode for All-Solid-State Sodium-Ion Secondary Battery A β''-aluminum solid electrolyte substrate with a thickness of 300 μm was separately prepared. The electrode paste was applied to the first main surface of the solid electrolyte substrate to a thickness of 70 μm and dried in a dryer at 70 °C. After drying, in a nitrogen atmosphere (N ２A sintered negative electrode (negative electrode for all-solid-state sodium-ion secondary battery) was formed on a β''-alumina solid electrolyte substrate by firing at 1000°C for 2 hours with a hard carbon powder coating layer. The coating layer was formed by depositing hard carbon, a coating carbon electrode material derived from sucrose (table sugar).

[0178] (c) Preparation of the positive electrode paste: The glass composition is 40 Na in molar ratio. ２ O-20Fe ２ O ３ -40P ２ O ５ The raw materials, formulated to achieve the desired result, were melted in air at 1250°C for 45 minutes and then molded with a cooled twin roller to produce a glass film. The resulting glass film was then pulverized using a ball mill and a planetary ball mill to obtain a positive electrode active material precursor powder (glass powder).

[0179] 83.2% by mass of the obtained glass powder was mixed with 12.4% by mass of β''-alumina as a solid electrolyte and 4.2% by mass of acetylene black (Timcal, product code "SUPER P") as granular carbon to prepare a cathode composite powder. 20% by mass of polypropylene carbonate (PPC) as a binder was added to 100% by mass of the obtained cathode composite powder, and N-methyl-2-pyrrolidone was added as a solvent to bring the concentration of the cathode composite powder to 50% by mass. This mixture was thoroughly stirred in a rotation / revolution mixer to form a slurry and prepare a cathode paste.

[0180] (d) Formation of the positive electrode for the all-solid sodium ion secondary battery A positive electrode paste was applied to the second main surface of the solid electrolyte layer to a thickness of 70 μm. After drying in a 70°C constant temperature bath for 1 hour, N ２ / H ２ A positive electrode for an all-solid-state sodium-ion secondary battery was formed by firing in a 96 / 4 volume atmosphere at 500°C for 30 minutes.

[0181] (e) Test battery assembly Next, current collectors were formed on both sides of the solid electrolyte by sputtering. The current collectors consisted of a thin aluminum film. They were placed on the lower cover of the coin cell, and the upper cover of the coin cell was stacked on top of them to fabricate a test battery.

[0182] (Comparative Example 2) (a) In the preparation of the negative electrode paste, β''-alumina, a sodium ion conductive solid electrolyte, was weighed in the same manner as in Example 5, except that the composition of the negative electrode was as shown in the volume percentages in Table 2, in addition to sucrose (a hard carbon source), hard carbon powder, and a conductive additive (acetylene black). A test battery was then prepared.

[0183] (Storage Characteristics) The batteries were charged using CC charging at a rate of 0.1C to 4.5V at 30°C, and then charged using CV (0.001V) until the voltage reached 0.02C. After charging, the batteries were stored in a constant temperature bath at 60°C for 4 weeks, and then discharged using CC discharge at a rate of 0.02C. Subsequently, the batteries were charged using CC charging at a rate of 0.1C from 4.5V to 1.5V at 60°C, then charged using CV (0.001V) until the voltage reached 0.02C, and finally discharged using CC discharge at a rate of 0.02C. The discharge capacity (discharge capacity after storage) was determined. The storage capacity retention rate (%) was calculated using the formula: (discharge capacity after storage / initial discharge capacity) × 100. The results are shown in Table 2.

[0184]

[0185] In the negative electrodes for all-solid-state sodium secondary batteries in Examples 5 to 8, the storage capacity retention rate was 79% or higher. On the other hand, the negative electrode for all-solid-state sodium secondary batteries in Comparative Example 2 had a storage capacity retention rate of 62%.

[0186] 1... All-solid sodium-ion secondary battery 2... Solid electrolyte layer 2a, 2b... First and second main 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 negative electrode for an all-solid-state sodium-ion secondary battery, comprising a carbon electrode material and substantially free of a sodium-ion conductive solid electrolyte.

2. The negative electrode for an all-solid sodium-ion secondary battery according to claim 1, wherein the carbon electrode material is hard carbon.

3. The negative electrode for an all-solid sodium-ion secondary battery according to claim 1 or 2, wherein the carbon electrode material is composed of a particulate carbon electrode material and a coated carbon electrode material, and the coated carbon electrode material coats the particulate carbon electrode material.

4. Average particle size D of the particulate carbon electrode material ５０ The negative electrode for an all-solid sodium-ion secondary battery according to claim 3, wherein the diameter is 0.1 μm or more and 30 μm or less.

5. A negative electrode for an all-solid sodium-ion secondary battery according to claim 1 or 2, which is a sintered body.

6. A method for producing a negative electrode for an all-solid-state sodium-ion secondary battery according to claim 3, comprising the step of firing a negative electrode precursor material obtained by mixing the particulate carbon electrode material and the coated carbon electrode material precursor.

7. The method for manufacturing a negative electrode for an all-solid sodium-ion secondary battery according to claim 6, wherein in the step of firing the negative electrode precursor material, the coated carbon electrode material precursor softens and flows.

8. An all-solid-state sodium-ion secondary battery comprising the negative electrode for an all-solid-state sodium-ion secondary battery according to claim 1 or 2.

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