Positive electrode active material for sodium-ion secondary batteries, positive electrode composite material for sodium-ion secondary batteries, positive electrode for all-solid-state sodium-ion secondary batteries, and all-solid-state sodium-ion secondary batteries

Abstract

This invention provides a positive electrode active material for sodium-ion secondary batteries that can effectively increase the capacity of all-solid-state sodium-ion secondary batteries. [Solution] General formula Na 3+2x Mn 1+x (Ti 1-a Nb a ) 1-x A positive electrode active material for a sodium-ion secondary battery, characterized by containing an oxide material represented by (PO4)3 (0≦x≦0.4, 0.1≦a≦0.9), wherein the oxide material contains NASICON-type crystals.

Description

[Technical Field]

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

[0002] Lithium-ion secondary batteries have established themselves as essential high-capacity, lightweight power sources for portable electronic devices and electric vehicles, and active materials containing olivine-type crystals represented by the general formula LiFePO4 are attracting attention as positive electrode active materials. However, due to concerns about the global surge in raw material costs for lithium, research into sodium-ion secondary batteries, which use sodium as an alternative, has been conducted in recent years (see, for example, Patent Document 1). [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2010-018472 [Overview of the Initiative] [Problems that the invention aims to solve]

[0004] NaFePO4, the positive electrode active material described in Patent Document 1, has a marisite-type structure rather than an olivine structure. As a result, it has low sodium ion conductivity and poor structural stability, leading to problems with poor charge-discharge characteristics when used as an all-solid-state sodium ion secondary battery.

[0005] In view of the above, the present invention aims to provide a positive electrode active material for a sodium-ion secondary battery, a positive electrode composite material for a sodium-ion secondary battery, a positive electrode for an all-solid-state sodium-ion secondary battery, and an all-solid-state sodium-ion secondary battery that can effectively increase the capacity of an all-solid-state sodium-ion secondary battery. [Means for solving the problem]

[0006] This document describes various embodiments of a positive electrode active material for sodium-ion secondary batteries, a positive electrode composite material for sodium-ion secondary batteries, a positive electrode for all-solid-state sodium-ion secondary batteries, and an all-solid-state sodium-ion secondary battery that solve the above problems.

[0007] The positive electrode active material for a sodium-ion secondary battery according to Embodiment 1 of the present invention is a general formula Na 3+2x Mn 1+x (Ti 1-a Nb a ) 1-x The material contains an oxide material represented by (PO4)3 (0≦x≦0.4, 0.1≦a≦0.9), and is characterized in that the oxide material contains NASICON-type crystals.

[0008] In the positive electrode active material for a sodium-ion secondary battery according to Embodiment 2 of the present invention, in Embodiment 1, it is preferable that the content of NASICON-type crystals in the oxide material is 10% by mass or more.

[0009] The positive electrode composite material for sodium-ion secondary batteries according to aspect 3 of the present invention is characterized by comprising a positive electrode active material for sodium-ion secondary batteries according to aspect 1 or aspect 2 and a conductive additive.

[0010] In the positive electrode composite material for sodium-ion secondary batteries according to embodiment 4 of the present invention, in embodiment 3, it is preferable that the conductive additive includes fibrous carbon.

[0011] In the positive electrode composite material for sodium-ion secondary batteries according to aspect 5 of the present invention, in aspect 4, it is preferable that the fibrous carbon includes carbon nanotubes.

[0012] In the positive electrode composite material for sodium-ion secondary batteries according to aspect 6 of the present invention, it is preferable that in any one aspect from aspect 3 to aspect 5, the positive electrode active material for sodium-ion secondary batteries is contained in mass 50% to 99.9% and the conductive additive is contained in mass 0.1% to 10%.

[0013] In the positive electrode composite material for sodium-ion secondary batteries according to embodiment 7 of the present invention, it is preferable that it is for an all-solid-state sodium-ion secondary battery in any one embodiment from embodiment 3 to embodiment 6.

[0014] The positive electrode for an all-solid sodium-ion secondary battery according to aspect 8 of the present invention is characterized by containing a sodium-ion secondary battery positive electrode composite material according to any one of aspects 3 to 7.

[0015] The all-solid-state sodium-ion secondary battery according to aspect 9 of the present invention is characterized by comprising a positive electrode for an all-solid-state sodium-ion secondary battery as described in aspect 8.

[0016] The positive electrode active material precursor glass for sodium-ion secondary batteries according to embodiment 10 of the present invention is characterized by containing Na, Mn, Ti, Nb, P, and O.

[0017] The positive electrode active material precursor glass for sodium-ion secondary batteries according to embodiment 11 of the present invention preferably contains, in embodiment 10, 25-50% Na2O, 1-50% MnO, 1-20% TiO2, 0.5-10% Nb2O, 20-35% P2O5, and 10-50% Fe2O3+Cr2O3+MnO+CoO+V2O5+NiO in terms of oxide molar percentages. [Effects of the Invention]

[0018] According to the present invention, it is possible to provide a positive electrode active material for sodium-ion secondary batteries, a positive electrode composite material for sodium-ion secondary batteries, a positive electrode for all-solid-state sodium-ion secondary batteries, and an all-solid-state sodium-ion secondary battery that can effectively increase the capacity of all-solid-state sodium-ion secondary batteries. [Brief explanation of the drawing]

[0019] [Figure 1] Figure 1 is a schematic cross-sectional view showing an all-solid-state sodium-ion secondary battery according to an embodiment of the present invention. [Figure 2] Figure 2 is a schematic cross-sectional view showing a solid electrolyte layer in the all-solid-state sodium-ion secondary battery of Figure 1.

Mode for Carrying Out the Invention

[0020] Hereinafter, preferred embodiments will be described. However, the following embodiments are merely illustrative, and the present invention is not limited to the following embodiments. Also, in each drawing, members having substantially the same function may be referred to by the same reference numerals.

[0021] (Positive Electrode Active Material for Sodium-Ion Secondary Battery) The positive electrode active material for a sodium-ion secondary battery of the present invention has the general formula Na 3+2x Mn 1+x (Ti 1-a Nb a ) 1-x (PO4)3 (0 ≤ x ≤ 0.4, 0.1 ≤ a ≤ 0.9), and the oxide material contains a NASICON-type crystal. By doing so, the capacity of the all-solid-state sodium-ion secondary battery can be effectively increased. Hereinafter, the reasons for defining the values of each coefficient as described above and the preferred ranges will be explained.

[0022] The lower limit of x is 0 or more, preferably 0.05 or more, and particularly preferably 0.1 or more. If x is too small, the number of sodium ions contributing to occlusion and release decreases, so the discharge capacity tends to decrease. On the other hand, the upper limit of x is 0.4 or less, preferably 0.35 or less, and preferably 0.3 or less, 0.25 or less, and particularly preferably 0.2 or less. If x is too large, heterogeneous crystals that do not contribute to charge and discharge, such as Na3PO4, precipitate and the discharge capacity tends to decrease.

[0023] The lower limit of a is 0.1 or greater, preferably 0.2 or greater, preferably 0.3 or greater, and particularly preferably 0.4 or greater. If a is too small, the sodium ion conductivity in the positive electrode active material decreases, which tends to reduce the discharge capacity. On the other hand, the upper limit of a is 0.9 or less, preferably 0.8 or less, preferably 0.7 or less, 0.6 or less, and particularly preferably 0.5 or less. If a is too large, the number of transition metal elements for the redox reaction decreases, which reduces the number of sodium ions that contribute to intercalation and deintercalation, which tends to reduce the discharge capacity.

[0024] Furthermore, the oxide material contains NASICON-type crystals. This improves the sodium ion conductivity within the positive electrode active material, effectively increasing the capacity of the all-solid-state sodium ion secondary battery. The NASICON-type crystals are preferably monoclinic, hexagonal, or trigonal, and more preferably monoclinic or trigonal. In this case, the sodium ion conductivity within the positive electrode active material can be further improved.

[0025] The lower limit of the NASICON-type crystal content in the oxide material is preferably 10% by mass or more, and more preferably 30% by mass or more, 50% by mass or more, 60% by mass or more, 70% by mass or more, and particularly preferably 80% by mass or more. If the NASICON-type crystal content is too low, the sodium ion conductivity in the positive electrode active material decreases, which tends to reduce the discharge capacity. The upper limit of the NASICON-type crystal content is not particularly limited, but is substantially preferably 99.9% by mass or less, 99% by mass or less, or 95% by mass or less.

[0026] The smaller the crystallite size of the NASICON-type crystal, the smaller the average particle size of the positive electrode active material particles can be, thereby improving conductivity. Specifically, a crystallite size of 100 nm or less, and particularly 80 nm or less, is preferable. There is no particular lower limit to the crystallite size, but in reality, it is 1 nm or more, and even more so, 10 nm or more. The crystallite size is determined from the results of powder X-ray diffraction analysis according to Scherrer's formula.

[0027] In the positive electrode active material for sodium-ion secondary batteries of the present invention, the oxide material may be coated with conductive carbon or compounded with conductive carbon. This increases the electronic conductivity and improves the high-speed charge-discharge characteristics. As the conductive carbon, highly conductive carbon black such as acetylene black or Ketjen black, carbon powder such as graphite, carbon fibers, etc., can be used. Among these, acetylene black, which has high electronic conductivity, is preferred.

[0028] One method for coating an oxide material with conductive carbon involves mixing the oxide material with an organic compound that serves as a conductive carbon source, then firing the mixture in an inert or reducing atmosphere to carbonize the organic compound. Any organic compound that remains as carbon during the heat treatment process can be used as the organic compound, but glucose, citric acid, ascorbic acid, phenolic resin, and surfactants are preferred, with surfactants that readily adsorb to the oxide material surface being particularly preferred. Cationic surfactants, anionic surfactants, amphoteric surfactants, and nonionic surfactants may be used, but nonionic surfactants that exhibit excellent adsorption to the oxide material surface are particularly preferred.

[0029] The mixing ratio of oxide material to conductive carbon is preferably 80:20 to 99.5:0.5 by mass ratio, and more preferably 85:15 to 98:2. If the conductive carbon content is too low, the electronic conductivity tends to be poor. On the other hand, if the conductive carbon content is too high, the oxide material content becomes relatively low, which tends to reduce the discharge capacity.

[0030] Furthermore, when the oxide material surface is coated with conductive carbon, the thickness of the conductive carbon film is preferably 1 to 100 nm, and particularly preferably 5 to 80 nm. If the thickness of the conductive carbon film is too small, the conductive carbon film will disappear during the charging and discharging process, which can easily degrade battery performance. On the other hand, if the thickness of the conductive carbon film is too large, it can inhibit ion conduction, which can easily lead to a decrease in discharge capacity and voltage drop.

[0031] The shape of the positive electrode active material for sodium-ion secondary batteries is not particularly limited, but a powder form is preferable because it provides more sites for the absorption and release of sodium ions. In that case, the average particle size D 50 The particle size is preferably 0.05-20 μm, 0.1-10 μm, 0.2-5 μm, and particularly preferably 0.2-2 μm. 99 The particle size is preferably 150 μm or less, 100 μm or less, 75 μm or less, and especially 55 μm or less. 50 or maximum particle size D 99 If the average particle size D is too large, the number of sites for sodium ion absorption and release during charging and discharging decreases, which tends to reduce the discharge capacity. 50 If the size is too small, the dispersion of the powder deteriorates when it is made into a paste, making it difficult to manufacture a uniform electrode.

[0032] Here, "Average particle size D 50 " and "Maximum particle size D 99 " " refers to the value measured by a laser diffraction particle size distribution analyzer, and represents the particle size at which the cumulative amount, when accumulated from the smallest particles to the largest, accounts for 50% and 99% of the volume-based cumulative particle size distribution curve measured by the laser diffraction method.

[0033] (Method for manufacturing positive electrode active material for sodium-ion secondary batteries) The positive electrode active material for sodium-ion secondary batteries of the present invention can be manufactured by the melt-collapse method described below. First, raw material powders are prepared to obtain a raw material batch to achieve a desired composition. Next, the obtained raw material batch is melted and molded to obtain a precursor glass for the positive electrode active material for sodium-ion secondary batteries. The melting temperature can be adjusted as appropriate to ensure that the raw material batch is homogeneous. Specifically, the melting temperature is preferably 800°C or higher, 900°C or higher, and particularly 1000°C or higher. There is no particular upper limit, but if the melting temperature is too high, the sodium component will evaporate and energy loss will occur, so it is preferable that the temperature be 1500°C or lower, and particularly 1400°C or lower.

[0034] By using phosphates such as sodium metaphosphate (NaPO3) or trisodium phosphate (Na3PO4) as the P raw material, it becomes easier to obtain a positive electrode active material with fewer devitrified foreign matter and excellent homogeneity. If this positive electrode active material is used as the positive electrode material, it becomes easier to obtain a secondary battery with stable discharge capacity.

[0035] The positive electrode active material precursor glass for sodium-ion secondary batteries preferably contains Na, Mn, Ti, Nb, P, and O. This allows the general formula Na 3+2x Mn 1+x (Ti 1-a Nb a ) 1-x This makes it easier to obtain a positive electrode active material containing an oxide material represented by (PO4)3 (0≦x≦0.4, 0.1≦a≦0.9).

[0036] The composition of the precursor glass for the positive electrode active material of sodium-ion secondary batteries is preferably such that it contains, in mole percent equivalent to the following oxides: Na2O 25-50%, MnO 1-50%, TiO2 1-20%, Nb2O 50.5-10%, P2O5 20-35%, and Fe2O3+Cr2O3+MnO+CoO+V2O5+NiO 10-50%. 3+2x Mn 1+x (Ti 1-a Nb a ) 1-xThis makes it easier to obtain a positive electrode active material for sodium-ion secondary batteries that contains an oxide material represented by (PO4)3 (0≦x≦0.4, 0.1≦a≦0.9).

[0037] Na2O is the general formula Na 3+2x Mn 1+x (Ti 1-a Nb a ) 1-x This is the main component of the positive electrode active material for sodium-ion secondary batteries, which contains an oxide material represented by (PO4)3 (0≦x≦0.4, 0.1≦a≦0.9). The Na2O content is preferably 25% to 50%, more preferably 30% to 40%, and particularly preferably 32% to 38%. When the Na2O content is within the above range, the charge and discharge capacity of the battery can be further increased.

[0038] MnO also has the general formula Na 3+2x Mn 1+x (Ti 1-a Nb a ) 1-x This is the main component of the positive electrode active material for sodium-ion secondary batteries, which contains an oxide material represented by (PO4)3 (0≦x≦0.4, 0.1≦a≦0.9). The MnO content is preferably 1% to 50%, more preferably 5% to 40%, even more preferably 10% to 30%, and particularly preferably 15% to 28%. When the MnO content is above the lower limit, the charge and discharge capacity of the battery can be further increased. On the other hand, when the MnO content is below the upper limit, it is possible to prevent the precipitation of unwanted crystals such as MnO.

[0039] To further improve the battery's cycle characteristics, it is preferable to include at least one selected from Fe2O3, Cr2O3, CoO, V2O5, and NiO. In this case, the content of Fe2O3+Cr2O3+MnO+CoO+V2O5+NiO is preferably 10% to 50%, more preferably 12% to 40%, even more preferably 15% to 30%, and particularly preferably 20% to 28%. In this specification, for example, when the content of a+b+c+... is described, it means the combined amount of a, b, and c.

[0040] TiO2 also has the general formula Na 3+2x Mn 1+x (Ti 1-a Nb a ) 1-x This is the main component of the positive electrode active material for sodium-ion secondary batteries, which contains an oxide material represented by (PO4)3 (0≦x≦0.4, 0.1≦a≦0.9). The P2O5 content is preferably 1% to 20%, and more preferably 3% to 18%. When the TiO2 content is within the above range, the charge and discharge capacity of the battery can be further increased.

[0041] Nb2O5 is a general formula Na 3+2x Mn 1+x (Ti 1-a Nb a ) 1-x This component is necessary to increase the charge and discharge capacity of the positive electrode active material for sodium-ion secondary batteries, which contains an oxide material represented by (PO4)3 (0≦x≦0.4, 0.1≦a≦0.9). The Nb2O5 content is preferably 0.5% to 10%, and more preferably 1% to 9%.

[0042] P2O5 is also, in general formula Na 3+2x Mn 1+x (Ti 1-a Nb a ) 1-xThis is the main component of the positive electrode active material for sodium-ion secondary batteries, which contains an oxide material represented by (PO4)3 (0≦x≦0.4, 0.1≦a≦0.9). The P2O5 content is preferably 20% to 35%, more preferably 25% to 34%, and particularly preferably 27% to 33%. When the P2O5 content is within the above range, the charge and discharge capacity of the all-solid-state sodium-ion secondary battery 1 can be further increased.

[0043] Furthermore, vitrification can be facilitated by including various components in addition to the above components, as long as the effects of the present invention are not impaired. Examples of such components include MgO, CaO, SrO, BaO, ZnO, CuO, GeO2, and Sb2O5 in oxide form. The content of these components is preferably 0-10%, 0.1-5%, and particularly 0.5-2%, respectively.

[0044] A molded body is obtained by cooling and shaping the resulting molten material. The shaping method is not particularly limited; for example, the molten material may be poured between a pair of cooling rolls and shaped into a film while rapidly cooling, or the molten material may be poured into a mold and shaped into an ingot. The precursor glass for the positive electrode active material of a sodium-ion secondary battery is preferably a completely amorphous material from the viewpoint of homogeneity, but it may contain a partial crystalline phase.

[0045] Next, the obtained sodium-ion secondary battery positive electrode active material precursor glass is pulverized 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.

[0046] The positive electrode active material of the present invention can be obtained by calcining and crystallizing the positive electrode active material precursor powder obtained above.

[0047] The firing temperature for crystallization is preferably above the glass transition temperature of the positive electrode active material precursor powder, and more preferably above the crystallization temperature of the positive electrode active material precursor powder. The glass transition temperature and crystallization temperature can be determined from DSC (differential scanning calorimetry) or DTA (differential thermal analysis). If the firing temperature is too low, the precipitation of the crystalline phase tends to be insufficient. On the other hand, if the firing temperature is too high, the positive electrode active material precursor powders fuse together, reducing the specific surface area, which tends to reduce the discharge capacity of the positive electrode active material. Therefore, the firing temperature is preferably 900°C or lower, 850°C or lower, 800°C or lower, and especially 750°C or lower.

[0048] The firing time is adjusted as appropriate to ensure sufficient crystallization of the positive electrode active material precursor powder. Specifically, it is preferably 20 to 300 minutes, and more preferably 30 to 240 minutes.

[0049] For the above firing process, electric heating furnaces, rotary kilns, microwave heating furnaces, high-frequency heating furnaces, etc., can be used. Furthermore, the reduction and crystallization of transition metal ions in the positive electrode active material precursor powder may be performed simultaneously.

[0050] Furthermore, conductivity may be imparted by grinding and mixing the positive electrode active material precursor powder with conductive carbon, if necessary. Methods for grinding and mixing include using common grinding machines such as mortars, grinders, ball mills, attritors, vibrating ball mills, satellite ball mills, planetary ball mills, jet mills, and bead mills. Among these, the use of a planetary ball mill is preferable. In a planetary ball mill, the pot rotates on its own axis while the base plate revolves around it, efficiently generating very high impact energy. Therefore, it is possible to homogeneously disperse conductive carbon in the positive electrode active material and improve electronic conductivity.

[0051] Furthermore, as previously described, the positive electrode active material precursor powder may be coated with conductive carbon by mixing it with an organic compound that serves as a conductive carbon source, and then firing it in an inert or reducing atmosphere to carbonize the organic compound. This firing may be performed simultaneously with a heat treatment step to reduce transition metal ions or a heat treatment step to crystallize the positive electrode active material precursor powder.

[0052] (Positive electrode composite material for sodium-ion secondary batteries) By mixing the positive electrode active material for sodium-ion secondary batteries of the present invention with a conductive additive, a binder, etc., a positive electrode composite material for sodium-ion secondary batteries can be obtained.

[0053] Examples of conductive additives include highly conductive carbon blacks such as acetylene black and Ketjenblack, powdered carbon such as graphite, or fibrous carbon such as carbon nanotubes. Among these, fibrous carbon, particularly carbon nanotubes, are preferred because they can improve conductivity with only a small amount of addition.

[0054] A binder is a component added to bind the materials constituting the positive electrode composite for sodium-ion secondary batteries together, preventing the positive electrode active material from detaching from the positive electrode due to volume changes associated with charging and discharging. Specific examples of binders include thermoplastic linear polymers such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), fluororubber, and styrene-butanediene rubber (SBR); thermosetting resins such as thermosetting polyimide, polyamideimide, polyamide, polypropylene carbonate, phenolic resin, epoxy resin, urea resin, melamine resin, unsaturated polyester resin, and polyurethane; cellulose derivatives such as carboxymethylcellulose (including carboxymethylcellulose salts such as sodium carboxymethylcellulose; the same applies hereinafter), hydroxypropylmethylcellulose, hydroxypropylcellulose, hydroxyethylcellulose, ethylcellulose, and hydroxymethylcellulose; and water-soluble polymers such as polyvinyl alcohol, polyacrylamide, polyvinylpyrrolidone, and their copolymers. Among these, thermosetting resins, cellulose derivatives, and water-soluble polymers are preferred due to their excellent binding properties, and thermosetting polyimides or carboxymethylcellulose, which are widely used industrially, are more preferred. In particular, carboxymethylcellulose is most preferred because it is inexpensive and has a low environmental impact as it does not require organic solvents when preparing the electrode-forming paste. These binders may be used individually or in mixtures of two or more.

[0055] It is preferable to add a sodium ion conductive solid electrolyte as a component of the positive electrode composite material for sodium ion secondary batteries. The sodium ion conductive solid electrolyte is the component responsible for sodium ion conduction between the positive and negative electrodes in an all-solid-state sodium ion secondary battery. The sodium ion conductive solid electrolyte is preferably a beta-alumina-based solid electrolyte or a NASICON-type solid electrolyte because it has excellent sodium ion conductivity. There are two crystalline forms of beta-alumina: β-alumina (theoretical composition formula: Na2O·11Al2O3) and β''-alumina (theoretical composition formula: Na2O·5,3Al2O3). Since β''-alumina is a metastable substance, it is usually used with Li2O or MgO added as a stabilizer. Since β''-alumina has higher sodium ion conductivity than β-alumina, it is preferable to use β''-alumina alone or a mixture of β''-alumina and β-alumina, and Li2O stabilized β''-alumina (Na 1.6 Li 0.34 Al 10.66 O 17 ) or MgO-stabilized β''-alumina ((Al 10.32 Mg 0.68 O 16 )(Na 1.68 Using O)) is more preferable.

[0056] As for NASICON crystals, the general formula is Na 1+x Zr2P 3-x Si x O 12 Examples include a first compound represented by (0≦x≦3), and a second compound in which a portion of the Zr in the first compound is substituted with at least one element selected from the group consisting of Ca, Mg, Ba, Sr, Al, Sc, Y, In, Ti, and Ga, and which comprises at least one of these compounds. Specifically, Na3Zr2PSi2O 12 Na3Zr 1.6 Ti 0.4 PSi2O 12 Na3Zr 1.88 Y 0.12 PSi2O 12 kaNa 3.12 Zr 1.88 Y0.12 PSi2O 12 These are preferred, and Na is particularly preferred. 3.12 Zr 1.88 Y 0.12 PSi2O 12 This is preferable because it has excellent sodium ion conductivity.

[0057] The positive electrode composite material for sodium-ion secondary batteries of the present invention is used as a positive electrode for a sodium-ion secondary battery by coating it onto a current collector made of metal foil such as aluminum, copper, or gold, and drying it. Alternatively, the positive electrode composite material for sodium-ion secondary batteries of the present invention may be formed into a sheet, and then a current collector made of a metal film may be formed by sputtering, vapor deposition, plating, or the like.

[0058] The positive electrode composite material for sodium-ion secondary batteries of the present invention preferably contains, by mass%, 50% to 99.9% of positive electrode active material and 0.1% to 3% of a conductive additive. In this case, both electronic conductivity and ionic conductivity can be achieved at a higher level, and the battery characteristics can be effectively improved.

[0059] In the present invention, the content of positive electrode active material in the positive electrode composite material for sodium-ion secondary batteries is preferably 50% or more by mass, more preferably 60% or more, even more preferably 70% or more, preferably 99.9% or less, more preferably 98% or less, and even more preferably 95% or less. When the content of positive electrode active material in the positive electrode composite material for sodium-ion secondary batteries is within the above range, the battery characteristics such as the charge and discharge capacity of the secondary battery can be improved even more effectively.

[0060] In the present invention, the content of the conductive additive in the positive electrode mixture for sodium-ion secondary batteries is preferably 0.1% or more by mass, more preferably 0.2% or more, even more preferably 0.3% or more, particularly preferably 0.4% or more, most preferably 0.5% or more, preferably 3% or less, more preferably 2% or less, even more preferably 1% or less, still preferably 0.9% or less, particularly preferably 0.8% or less, and most preferably 0.7% or less. When the conductive additive in the positive electrode mixture for sodium-ion secondary batteries contains fibrous carbon and the content of the conductive additive is within the above range, it is possible to further improve ionic conductivity while ensuring high electronic conductivity, thereby more effectively improving battery characteristics.

[0061] (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. 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.

[0062] The solid electrolyte layer 2 has opposing first main surface 2a and second main surface 2b. A positive electrode layer 3 is provided on the first main surface 2a of the solid electrolyte layer 2. In this embodiment, the positive electrode layer 3 and the solid electrolyte layer 2 are in contact. A first current collector layer 5 is provided on the main surface of the positive electrode layer 3 opposite to the solid electrolyte layer 2. A negative electrode layer 4 is provided on the second main surface 2b of the solid electrolyte layer 2. In this embodiment, the negative electrode layer 4 and the solid electrolyte layer 2 are in contact. The negative electrode layer 4 is provided in a position that overlaps with the positive electrode layer 3 in a plan view. Furthermore, a second current collector layer 6 is provided on the main surface of the negative electrode layer 4 opposite to the solid electrolyte layer 2. Note that the first current collector layer 5 and the second current collector layer 6 may be omitted.

[0063] 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.

[0064] The details of each layer in the all-solid-state sodium-ion secondary battery of the present invention will be described below.

[0065] (solid electrolyte layer) Figure 2 is a schematic cross-sectional view showing the solid electrolyte layer in the embodiment shown in Figure 1.

[0066] 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.

[0067] The first solid electrolyte layer 7 is specifically a dense layer. On the other hand, the second solid electrolyte layer 8 is specifically a porous layer. The second solid electrolyte layer 8 has three-dimensionally interconnected voids. It is desirable that the first solid electrolyte layer 7 and the second solid electrolyte layer 8 are integrated into one unit.

[0068] 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.

[0069] 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.

[0070] 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.

[0071] 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.

[0072] The porosity of the first solid electrolyte layer 7 is preferably smaller than that of the second solid electrolyte layer 8. The porosity is defined by the following equation (1). In equation (1), p is the bulk density and p0 is the true density.

[0073] Porosity=(1-p / p0)×100(%)…Equation (1)

[0074] 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%.

[0075] 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.

[0076] 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.

[0077] The pore diameter can be measured by performing 3D structural observation using X-ray CT and analyzing the images. It can also be determined by mercury intrusion or 3D reconstruction using SEM-FIB.

[0078] 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.

[0079] 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.

[0080] The second solid electrolyte layer 8 may be composed of multiple layers with different porosities. In this case, it is preferable that the multiple layers with different porosities are arranged such that the porosity decreases as the layer is closer to the first solid electrolyte layer 7. The number of multiple layers with different porosities is preferably two or more, more preferably three or more, even more preferably four or more, particularly preferably five or more, preferably 200 or fewer, more preferably 150 or fewer, even more preferably 100 or fewer, particularly preferably 50 or fewer, even more preferably 20 or fewer, and most preferably 10 or fewer.

[0081] 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.

[0082] 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.

[0083] 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.

[0084] If the thickness of the second solid electrolyte layer 8 is too thin, the amount of material constituting the positive electrode layer 3 and the negative electrode layer 4 that can penetrate the voids in the second solid electrolyte layer 8 will decrease. As a result, the contact area between the solid electrolyte layer 2 and the positive electrode layer 3 and the negative electrode layer 4 will decrease, and adhesion will tend to decline. In this case, the number of ion conduction paths at the interface between the solid electrolyte layer 2 and the positive electrode layer 3 and the negative electrode layer 4 will decrease, and the internal resistance of the all-solid-state sodium-ion secondary battery 1 will tend to increase. Consequently, the rapid charge and discharge characteristics of the all-solid-state sodium-ion secondary battery 1 will tend to deteriorate.

[0085] On the other hand, if the thickness of the second solid electrolyte layer 8 is too thick, it becomes difficult to fill the entire void of the second solid electrolyte layer 8 with the material constituting the positive electrode layer 3 or the negative electrode layer 4. As a result, the energy density per unit volume of the all-solid-state sodium-ion secondary battery 1 decreases. In addition, the amount of shrinkage during the formation of the second solid electrolyte layer 8 increases, making the second solid electrolyte layer 8 more prone to delamination at the interface with the first solid electrolyte layer 7.

[0086] 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.

[0087] 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.

[0088] 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.

[0089] 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.

[0090] 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.

[0091] In JIS B 0621-1984, flatness is defined as "the magnitude of the deviation of a planar feature from a geometrically correct plane." The flatness of the solid electrolyte layer 2 indicates the value of the gap created when one side of the sheet is sandwiched between parallel planes.

[0092] 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.

[0093] The oxide solid electrolyte used for the solid electrolyte layer 2 is a sodium ion conductive oxide. Examples of the sodium ion conductive oxide include compounds containing at least one selected from Al, Y, Zr, Si, and P, Na, and O. Specific examples of the sodium ion conductive oxide include beta alumina or NASICON crystal, which are excellent in sodium ion conductivity. Among them, 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 crystal. More preferably, the sodium ion conductive oxide is β-alumina or β''-alumina. These are more excellent in sodium ion conductivity. From these, it is preferable that the oxide solid electrolyte constituting the solid electrolyte layer 2 contains at least one selected from the group consisting of β-alumina, β''-alumina, and NASICON crystal.

[0094] There are two crystal forms of beta alumina, β-alumina and β''-alumina. The theoretical composition formula of β-alumina is Na2O·11Al2O3. The theoretical composition formula of β''-alumina is Na2O·5.3Al2O3. Since β''-alumina is a metastable substance, usually, those added with Li2O or MgO as a stabilizer are used. Since β''-alumina has higher sodium ion conductivity than β-alumina, it is preferable to use β''-alumina alone or a mixture of β''-alumina and β-alumina, and more preferably to use Li2O-stabilized β''-alumina or MgO-stabilized β''-alumina.

[0095] Specific examples of β''-alumina include (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 17Trigonal MgO-stabilized β''-alumina such as, Na 1.49 Li 0.25 Al 10.75 O 17 、Na 1.72 Li 0.3 Al 10.66 O 17 、Na 1.6 Li 0.34 Al 10.66 O 17 Examples include trigonal Li2O-stabilized β''-alumina such as these.

[0096] Specific examples of β-alumina include hexagonal (Al 10.35 Mg 0.65 O 16 )(Na 1.65 O), (Al 10.37 Mg 0.63 O 16 )(Na 1.63 O), NaAl 11 O 17 、(Al 10.32 Mg 0.68 O 16 )(Na 1.68 O), etc.

[0097] NASICON crystals preferably consist of a compound represented by the general formula Na s A1 t A2 u O v (A is at least one selected from Al, Y, Yb, Nd, Nb, Ti, Hf, Zr, Mg, Ca, Zn, Sc, La, Ce, and Gd; A is at least one selected from Si and P; s = 1.4 to 5.2, t = 1 to 2.9, u = 2.8 to 4.1, v = 9 to 14). Here, A is preferably at least one selected from Y, Nb, Ti, and Zr. By doing so, crystals with excellent ion conductivity can be obtained.

[0098] The preferred ranges of the coefficients in the above general formula are as follows.

[0099] 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.

[0100] 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.

[0101] 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.

[0102] v is preferably 9 to 14, more preferably 9.5 to 12, and even more preferably 11 to 12. If v is too small, A1 (e.g., aluminum component) will be in a low valence state, which tends to reduce electrical insulation. On the other hand, if v is too large, a peroxide state will occur, and sodium ions will be bound to the lone pair of electrons of the oxygen atom, which tends to reduce the ionic conductivity of the solid electrolyte.

[0103] 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.

[0104] Specific examples of NASICON crystals include Na3Zr2Si2PO 12 、Na 3.2 Zr 1.3 Si 2.2 P 0.8 O 10.5 、Na3Zr 1.6 Ti 0.4 Si2PO 12 、Na3Hf2Si2PO 12 、Na 3.4 Zr 0.9 Hf 1.4 Al 0.6 Si 1.2 P 1.8 O 12 、Na3Zr 1.7 Nb 0.24 Si2PO 12 、Na 3.6 Ti 0.2 Y 0.8 Si 2.8 O9、Na3Zr 1.88 Y 0.12 Si2PO 12 、Na 3.12 Zr 1.88 Y 0.12 Si2PO 12 、Na 3.05 Zr2Si 2.06 P 0.95 O 12 、Na 3.4 Zr2Si 2.4 P 0.6 O 12 、Na 3.4 Zr 1.9 Zn 0.1 Si 2.4 P 0.6 O 12 、Na 3.4 Zr 1.9 Mg 0.1 Si 2.4 P 0.6 O 12 、Na 3.4 Zr 1.9 Zn 0.1 Si 2.2 P 0.8 O 12 、Na 3.4 Zr 1.9 Mg 0.1 Si 2.2P 0.8 About 12 、Na 2.8 Zr2Si 2.4 P 0.6 About 12 、Na 3.6 Zr 0.13 Yb 1.67 Yes 0.11 P 2.9 About 12 、Na5YSi4O 12 、Na 3.1 Zr 1.95 Mg 0.05 Si2PO 12 、Na 3.1 Zr 1.9 The 0.1 Si2PO 12 、Na 3.1 Zr 1.9 Nd 0.1 Si2PO 12 、Na 3.1 Zr 1.9 Y 0.1 Si2PO 12 、Na 3.256 Zr 1.872 Mg 0.128 Si2PO 12 、Na 3.2 Zr 1.9 Ca 0.1 Si2PO 12 、Na 3.2 Zr 1.9 Mg 0.1 Si2PO 12 、Na 3.2 Zr2Si 2.2 P 0.8 About 12 、Na 3.38 Zr 1.80 Al 0.26 Yes 2.06 P 0.88 About 12 、Na 3.43 Zr 1.83 Zn 0.22 Yes 1.93 P 1.02 About 12 、Na 3.4 Sc 0.4 Zr 1.6 Si2PO 12 、Na 3.4 Zr 1.8 Mg 0.2Si2er 12 kaNa 3.4 Zr 1.9 Zn 0.1 Si 2.2 P 0.8 O 12 kaNa 3.57 Zr 1.72 La 0.21 Si 2.08 P 0.92 O 12 Na3Zr 1.98 Nb 0.08 Si2er 12 Na3Zr 1.9 Ce 0.1 Si2er 12 Na3Zr 1.9 Gd 0.1 Si2er 12 Na3Zr 1.9 Ti 0.1 Si2er 12 Na3Zr 1.9 Yb 0.1 Si2er 12 Examples of crystals include those listed above. These may be used individually or in combination of multiple types. Among them, the NASICON crystal is Na3Zr2Si2PO 12 kaNa 3.4 Zr2Si 2.4 P 0.6 O 12、 or Na 3.05 Zr2Si 2.06 P 0.95 O 12 It is preferable that Na3Zr2Si2PO 12 This is more preferable. In this case, the ionic conductivity of the NASICON crystal can be further improved.

[0105] 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, metallic lithium, 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.

[0106] The metals that make up the metal layer are not particularly limited, but for example, Sn, Ti, Bi, Au, Al, Cu, Sb, Pb, etc. can be used. These metals that make up the metal layer may be used individually or in combination of two or more. The metal layer may also be composed of an alloy of these metals.

[0107] 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.

[0108] 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 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.

[0109] (Positive electrode layer) The positive electrode layer 3 shown in Figure 1 is a positive electrode for an all-solid sodium-ion secondary battery containing the aforementioned sodium-ion secondary battery positive electrode composite material.

[0110] The lower limit of the thickness of the positive electrode layer 3 is preferably 10 μm or more, 50 μm or more, and particularly preferably 100 μm or more. When the thickness of the positive electrode layer 3 is greater than or equal to the above lower limit, the charge and discharge capacity of the all-solid-state sodium-ion secondary battery 1 can be further increased. On the other hand, the upper limit of the thickness of the positive electrode layer 3 is preferably 1000 μm or less, and particularly preferably 700 μm or less. If the thickness of the positive electrode layer 3 is too thick, the resistance to electron conduction increases, which may reduce the discharge capacity of the all-solid-state sodium-ion secondary battery 1. Alternatively, the operating voltage of the all-solid-state sodium-ion secondary battery 1 may decrease.

[0111] (Negative electrode layer) The negative electrode active material contained in the negative electrode layer 4 is not particularly limited, but for example, carbon electrode materials such as hard carbon or soft carbon can be used. Hard carbon is preferred as the carbon electrode material. However, the negative electrode active material of the all-solid-state sodium-ion secondary battery 1 may also contain alloy-based negative electrode active materials that can absorb sodium, such as tin, bismuth, lead, or phosphorus, or metallic sodium. It is preferable that the negative electrode layer 4 is not metallic sodium or a negative electrode layer containing metallic sodium.

[0112] The lower limit of the negative electrode active material content is not particularly limited, but is preferably 50% or more by mass, and particularly preferably 60% or more. The upper limit of the negative electrode active material content is not particularly limited, but is preferably 99.9% or less by mass, preferably 95% or less, and particularly preferably 90% or less. When the negative electrode active material content in the negative electrode layer 4 is within the above ranges, the battery capacity can be increased even more effectively.

[0113] The negative electrode layer 4 may contain a solid electrolyte. The solid electrolyte used is one of those described in the section on "Positive Electrode Compounds for Sodium Ion Secondary Batteries." When the negative electrode layer 4 contains a solid electrolyte, the lower limit of the solid electrolyte content is preferably 0.1% or more by mass, 5% or more, and particularly preferably 10% or more. The upper limit of the solid electrolyte content is preferably 40% or less by mass, 35% or less, and particularly preferably 30% or less. When the solid electrolyte content is within the above range, the negative electrode active material and the solid electrolyte can be integrated. Alternatively, when the negative electrode layer 4 is in contact with the solid electrolyte layer 2, the two can be integrated. As a result, ionic conductivity can be further improved, and the battery characteristics of the secondary battery can be more effectively enhanced.

[0114] The negative electrode layer 4 may contain a conductive additive. For example, the conductive additive described in the "positive electrode layer" section can be used. When the negative electrode layer 4 contains a conductive additive, the lower limit of the conductive additive content is preferably 0.1% or more by mass, and particularly preferably 0.2% or more. The upper limit of the conductive additive content is preferably 5% or less by mass, 3% or less, and particularly preferably 2% or less. When the content of the conductive additive in the negative electrode layer 4 is within the above range, it is possible to further improve ionic conductivity while ensuring high electronic conductivity in the negative electrode layer 4, thereby more effectively improving the battery characteristics of the secondary battery.

[0115] The lower limit of the thickness of the negative electrode layer 4 is preferably 0.3 μm or more, more preferably 3 μm or more, and more preferably 10 μm or more. When the thickness of the negative electrode layer 4 is greater than or equal to the above lower limit, the charge and discharge capacity of the all-solid-state sodium-ion secondary battery 1 can be increased even further. On the other hand, the lower limit of the thickness of the negative electrode layer 4 is preferably 500 μm or less, and more preferably 300 μm or less. If the thickness of the negative electrode layer 4 is too thick, the resistance to electron conduction increases, which may reduce the discharge capacity of the all-solid-state sodium-ion secondary battery 1. Alternatively, the operating voltage of the all-solid-state sodium-ion secondary battery 1 may decrease.

[0116] (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.

[0117] 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.

[0118] The first current collector layer 5 and the second current collector layer 6 are preferably made of metal foil. Because metal foil is flexible, it can increase the contact area with the electrode layer and integrate with the extraction electrode when used as a secondary battery, thereby effectively increasing the capacity of the secondary battery and providing excellent cycle characteristics due to charge and discharge.

[0119] 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.

[0120] 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. Furthermore, 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.

[0121] 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.

[0122] (Formation of a solid electrolyte layer) (a) Preparation of the first green sheet for forming the solid electrolyte layer First, a slurry is prepared by adding an organic vehicle containing a binder to the first solid electrolyte powder and at least one of the raw material powders for the first solid electrolyte powder. The raw material powder referred to here is the powder that will react in the subsequent calcination process to become a solid electrolyte. The binder is a material used to bind powdered materials together.

[0123] 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.

[0124] As the first solid electrolyte powder, at least one can be selected from the group consisting of, for example, β''-alumina, β-alumina, and NASICON crystals. The same materials as those described in the "Solid Electrolyte Layer" section above can be used as β''-alumina, β-alumina, and NASICON crystals.

[0125] As the raw material powder for the first solid electrolyte powder, for example, if the raw material powder is β''-alumina, a powder containing, in molar percentages, Al2O 365%~98%, Na2O 2%~20%, MgO+Li2O 0.3%~15%, ZrO 20%~20%, and Y2O 30%~5% can be prepared. The reason for limiting the composition as described above will be explained below.

[0126] Al2O3 is the main component of β''-alumina. The Al2O3 content is preferably 65% ​​to 98% in mole percent, and more preferably 70% to 95%. If the amount of Al2O3 is too low, the ionic conductivity of the solid electrolyte tends to decrease. On the other hand, if the amount of Al2O3 is too high, α-alumina that does not have sodium ion conductivity remains, and the ionic conductivity of the solid electrolyte tends to decrease.

[0127] Na2O is a component that imparts sodium ion conductivity to solid electrolytes. The Na2O content is preferably 2% to 20% in mole percent, more preferably 3% to 18%, and even more preferably 4% to 16%. If the amount of Na2O is too low, it becomes difficult to obtain the above effect. On the other hand, if the amount of Na2O is too high, the excess sodium forms compounds such as NaAlO2 that do not contribute to ionic conductivity, so the ionic conductivity of the solid electrolyte tends to decrease.

[0128] MgO and Li2O are components that stabilize the structure of β''-alumina, i.e., stabilizers. The content of MgO + Li2O is preferably 0.3% to 15% in mole percent, more preferably 0.5% to 10%, and even more preferably 0.8% to 8%. If the amount of MgO + Li2O is too low, α-alumina will remain in the solid electrolyte, and the ionic conductivity will tend to decrease. On the other hand, if the amount of MgO + Li2O is too high, MgO or Li2O that did not function as a stabilizer will remain in the solid electrolyte, and the ionic conductivity will tend to decrease.

[0129] ZrO2 and Y2O3 have the effect of suppressing abnormal grain growth of β''-alumina during calcination and improving the adhesion of each β''-alumina particle. As a result, the ionic conductivity of the solid electrolyte is more easily improved. The ZrO2 content is preferably 0% to 20% in mole percent, more preferably 0% to 15%, even more preferably 1% to 13%, and particularly preferably 2% to 10%. The Y2O3 content is preferably 0% to 5% in mole percent, more preferably 0.01% to 4%, and even more preferably 0.02% to 3%. If there is too much ZrO2 or Y2O3, the amount of β''-alumina produced will decrease, and the ionic conductivity of the solid electrolyte will easily decrease.

[0130] As the raw material powder for the first solid electrolyte powder, for example, if it is a raw material powder for NASICON crystals, it can be prepared containing, in molar percentages, 17.5% to 50% Na2O, 12% to 45% Al2O3 + Y2O3 + Yb2O3 + Nd2O3 + Nb2O5 + TiO2 + HfO2 + ZrO2, and 24% to 54% SiO2 + P2O5.

[0131] 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.50 However, it is preferable that the thickness 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.

[0132] 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.

[0133] Alternatively, the binder may be a glass binder. For example, sodium ion conductive glass powder can be used as the glass binder.

[0134] 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.

[0135] 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.

[0136] 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.

[0137] (b) Preparation of a green sheet for forming a second solid electrolyte layer First, a slurry is prepared by adding an organic vehicle containing a binder to a mixed powder containing the second solid electrolyte powder, at least one of the raw material powders for the second solid electrolyte powder, and a polymer powder. The polymer powder is a material for forming voids within the second solid electrolyte layer 8. Specifically, the voids are formed when the polymer powder is burned and removed in a later firing process.

[0138] 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.

[0139] 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.

[0140] 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 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 of 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.

[0141] 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.

[0142] 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.

[0143] 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.

[0144] 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.

[0145] 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.

[0146] 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.

[0147] 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.

[0148] (c) Fabrication of laminated sheets Next, a laminated sheet is obtained by laminating a second solid electrolyte layer-forming green sheet onto the main surfaces on both sides of the first solid electrolyte layer-forming green sheet. Alternatively, the laminated sheet may be obtained by laminating each green sheet and then pressing it, such as by heating and pressing. In this case, the adhesion between each green sheet can be further improved.

[0149] 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.

[0150] (d) Firing of the laminated sheets Next, the resulting laminated sheet is fired. This forms a first solid electrolyte layer 7 and a second solid electrolyte layer 8. In this way, a solid electrolyte layer 2 can be obtained, in which the second solid electrolyte layer 8, which is a porous layer, is provided on the main surfaces on both sides of the first solid electrolyte layer 7, which is a dense layer.

[0151] 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.

[0152] 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.

[0153] 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.

[0154] 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.

[0155] 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.

[0156] 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.

[0157] (Sodium ion conductive solid electrolyte precursor and its solution) When the sodium-conducting solid electrolyte is beta-alumina, the sodium-conducting solid electrolyte precursor can be obtained, for example, by mixing aluminum nitrate, sodium nitrate, and lithium nitrate. At this time, the ratio of each of the above materials is adjusted to the composition ratio of the desired sodium ion-conducting solid electrolyte.

[0158] The sodium ion-conducting solid electrolyte is a NASICON crystal or Na5XSi4O 12In the case of a type crystal in which X is at least one selected from group 3 transition metal elements or rare earth elements, the sodium-conducting solid electrolyte precursor solution includes a solution containing sodium elements and transition metal elements that constitute the sodium-conducting solid electrolyte, and carbonate ions. In this solution, the sodium element is contained in the form of sodium ions, and the transition metal elements are contained in the form of transition metal ions. The sodium-ion-conducting solid electrolyte precursor consists, for example, of a gelled or dried product of the sodium-ion-conducting solid electrolyte precursor solution. The sodium-ion-conducting solid electrolyte consists of a calcined product of the sodium-ion-conducting solid electrolyte precursor. Note that the sodium-ion-conducting solid electrolyte is Na5XSi4O 12 If it is a type crystal, it is preferable that X is at least one element selected from rare earth elements.

[0159] As a sodium ion-conducting solid electrolyte precursor solution, a solution containing nitrate ions instead of carbonate ions can also be used.

[0160] 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.

[0161] In a sodium ion-conducting solid electrolyte precursor solution, the counterion of sodium ions is of the general formula NR4. + It is preferable that the ion is represented by such that each R is independently a substituent selected from the group consisting of H, CH3, C2H5, and CH2CH2OH. In this case, the transition metal element is more likely to exist stably in the solution.

[0162] 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.

[0163] (Formation of the positive electrode layer)

[0164] The above-mentioned positive electrode composite material for sodium-ion secondary batteries 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 material. The electrode composite layer may be in the form of compacted powder.

[0165] Subsequently, the solid electrolyte layer with electrode composite material is fired. It is preferable that the atmosphere during firing be a reducing atmosphere. The maximum temperature during firing can be, for example, 400°C to 600°C. The holding time at this temperature can be, for example, 5 minutes or more and less than 3 hours. By firing as described above, a positive electrode layer 3 (positive electrode for all-solid sodium-ion secondary battery) is obtained as the first electrode active material layer.

[0166] (Formation of the negative electrode layer) The negative electrode layer 4 can be prepared, for example, using a paste containing a negative electrode active material precursor powder and, if necessary, a solid electrolyte powder and a conductive additive. Binders, plasticizers, solvents, etc., may be added to the paste as needed.

[0167] Furthermore, when obtaining the paste, it is preferable to use a sodium ion-conducting solid electrolyte precursor solution. Specifically, the sodium ion-conducting solid electrolyte precursor solution and the negative electrode active material precursor are mixed and then dried. This yields a powder mixture of the sodium ion-conducting solid electrolyte precursor and the negative electrode active material precursor. However, by mixing the sodium ion-conducting solid electrolyte precursor and the negative electrode active material precursor, a powder mixture of the sodium ion-conducting solid electrolyte precursor and the negative electrode active material precursor can be obtained without going through the drying process.

[0168] Next, the resulting mixture is ground into a powder and then mixed with a conductive additive and a binder in an organic solvent. If the negative electrode active material of the negative electrode layer 4 to be formed is hard carbon, hard carbon may be added to the powder mixture and mixed. For example, N-methyl-2-pyrrolidone can be used as the organic solvent. This yields a paste.

[0169] 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.

[0170] The negative electrode layer 4 can be formed by applying the paste to one main surface of the solid electrolyte layer 2, drying it, and then firing it.

[0171] When firing the laminate of the solid electrolyte layer 2 and the paste, it is preferable to fire it in an N2 atmosphere at a temperature above 600°C and below 1300°C. The above firing is preferably carried out in an inert atmosphere. For example, the above firing may be carried out in an Ar, Ne, or He atmosphere, or in a vacuum. Alternatively, it is preferable to carry out the above firing in a reducing atmosphere containing H2. When firing is carried out in an inert atmosphere or a reducing atmosphere, the initial charge-discharge efficiency of the all-solid-state sodium-ion secondary battery 1 can be further improved.

[0172] Furthermore, a small amount of oxygen may be present in the atmosphere, provided that the negative electrode active material does not oxidize or decompose during firing. The oxygen concentration can be, for example, 1000 ppm or less, but is not limited to this.

[0173] 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.

[0174] As described above, the negative electrode layer 4 may be a metal film or an alloy film. In this case, the negative electrode layer 4 can be formed using, for example, a physical vapor phase method such as vapor deposition or sputtering, a chemical vapor phase method such as thermal CVD, MOCVD, or plasma CVD, or a liquid phase film deposition method such as plating, sol-gel method, or spin coating. Among these, it is preferable to use vapor deposition or sputtering to form the negative electrode layer 4. In this case, it is easier to improve the adhesion of the negative electrode layer 4 to the solid electrolyte layer 2.

[0175] (Negative electrode active material precursor) When using sugars as the anode active material precursor, examples include cellulose, D-glucose, and sucrose. When using biomass as the anode active material precursor, examples include corn stalks, sorghum stalks, pine cones, mangosteen, argan shells, rice husks, dandelions, grain straw cores, ramie fibers, cotton, kelp, and coconut endocarp. When using polymers as the anode active material precursor, examples include polyacrylonitrile (PAN), pitch, polyvinyl chloride (PVC), nanofibers, polyaniline, sodium polyacrylate, tires (tire polymers), and phosphorus-doped PAN.

[0176] (First current collector layer and second current collector layer) The method for forming the first current collector layer 5 and the second current collector layer 6 is not particularly limited, and for example, metal foil may be used, or physical vapor phase methods such as vapor deposition or sputtering may be used, or chemical vapor phase methods such as thermal CVD, MOCVD, or plasma CVD may be used. Alternatively, liquid phase film deposition methods such as plating, sol-gel method, or spin coating may be used as the method for forming the first current collector layer 5 and the second current collector layer 6.

[0177] 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.

[0178] (Example 1) (a) Preparation of the solid electrolyte layer As the first solid electrolyte layer, β''-alumina (0.2 mm thick, 12 mm square) manufactured by Ionotec was used. The second solid electrolyte layer was prepared by the following procedure: The raw materials were mixed to achieve the composition of β''-alumina, calcined at 1250°C, and pulverized. After calcination, the raw materials, binder, plasticizer, dispersant, solvent, etc. were mixed to prepare a paste. This paste was coated onto the first solid electrolyte layer to a thickness of 100 μm and 10 mm square. After drying in a constant temperature bath at 70°C for 2 hours, it was fired at 1550°C for 30 minutes to form a second solid electrolyte layer of 10 mm square and 100 μm thick on the first solid electrolyte layer.

[0179] (b) Preparation of electrode composites for sodium-ion secondary batteries A glass film (positive electrode active material precursor glass) was produced by mixing raw material powders of sodium carbonate (Na2CO3), sodium metaphosphate (NaPO3), manganese dioxide (MnO2), titanium dioxide (TiO2), niobium oxide (Nb2O5), and orthophosphate (H3PO4) as raw materials to achieve the composition shown in Table 1. The raw materials were melted at 1200°C under a nitrogen atmosphere for 60 minutes and then molded with a cooled twin roller. The resulting glass film was then pulverized using a ball mill and a planetary ball mill to obtain an average particle size D 50 The surface area is 0.2 μm, and the BET specific surface area is 30 m². 2 A positive electrode active material precursor powder was obtained in a quantity of / g.

[0180] The obtained positive electrode active material precursor powder is 86.5% by mass, and β''-alumina (BET specific surface area is 45 m²) is used as a solid electrolyte. 2 ( / g) 12.9 mass%, and carbon nanotubes as fibrous carbon (C-nano Corporation, product number "LB116", BET specific surface area: 300 m²) 20.6 mass% of (g, diameter: 10 nm, length: 20 μm) was mixed to prepare the positive electrode composite powder. 20 mass% of polypropylene carbonate (PPC) as a binder was added to 100 mass% of the obtained positive electrode composite powder, and N-methyl-2-pyrrolidone was added as a solvent so that the concentration of the positive electrode composite powder became 50 mass%. By mixing this with a planetary mixer, a positive electrode paste was prepared.

[0181] (c) Formation of positive electrode The positive electrode paste was coated on the center of one main surface of the solid electrolyte layer to a thickness of 100 μm and a size of 10 mm□. After drying in a constant temperature bath at 70 °C for 1 hour, firing was performed in a N2 atmosphere under the conditions of holding at 600 °C for 60 minutes to form an electrode composite material (positive electrode) for a sodium ion secondary battery.

[0182] (d) Formation of current collector and coin cell assembly A gold vapor deposition film with a thickness of 100 nm was formed as a current collector on the entire surface of the positive electrode. Then, in an argon atmosphere with a dew point of -60 °C or lower, metallic sodium as a counter electrode was pressure-bonded to the other surface of the solid electrolyte layer, placed on the lower lid of the coin cell, and then covered with the upper lid to fabricate a CR2032 type test battery.

[0183] (Comparative Example 1) An all-solid sodium ion secondary battery was fabricated in the same manner as in Example 1, except that the electrode composite material composition for the sodium ion secondary battery was adjusted to the composition shown in Table 1 below.

[0184] [Evaluation] The fabricated battery was subjected to CC charging at a rate of 0.02C from 2.5V to 4.5V at 60 °C, followed by CC discharging at a rate of 0.02C from 4.5V to 1.5V, and the discharge capacity per unit weight of the positive electrode active material was measured. The results are shown in Table 1.

[0185]

Table Ⅰ

[0186] Thus, in Example 1, the discharge capacity was high at 48 mAh / g. On the other hand, in Comparative Example 1, the discharge capacity was low at 18 mAh / g. [Explanation of Symbols]

[0187] 1… All-solid-state sodium-ion secondary battery 2...Solid electrolyte layer 2a, 2b… First and second principal surfaces 3…Positive electrode layer 4…Negative electrode layer 5…First current collector layer 6…Second current collector layer 7…First solid electrolyte layer 8…Second solid electrolyte layer 7a, 7b… Third and fourth main surfaces

Claims

1. General formula Na ３＋２ｘ Mn １＋ｘ (Ti １－ａ Nb ａ ) １－ｘ (PO ４ ) ３ A positive electrode active material for a sodium-ion secondary battery, characterized by containing an oxide material represented by (0 ≤ x ≤ 0.4, 0.1 ≤ a ≤ 0.9), wherein the oxide material contains a NASICON-type crystal.

2. The positive electrode active material for a sodium-ion secondary battery according to claim 1, characterized in that the content of NASICON-type crystals in the oxide material is 10% by mass or more.

3. A positive electrode composite material for a sodium-ion secondary battery, characterized by comprising the positive electrode active material for a sodium-ion secondary battery described in claim 1 and a conductive additive.

4. The positive electrode composite material for sodium-ion secondary batteries according to claim 3, characterized in that the conductive additive contains fibrous carbon.

5. The positive electrode composite material for sodium-ion secondary batteries according to claim 4, characterized in that the fibrous carbon includes carbon nanotubes.

6. The positive electrode composite material for sodium-ion secondary batteries according to any one of claims 3 to 5, characterized in that it contains, by mass, 50% to 99.9% of the positive electrode active material for sodium-ion secondary batteries and 0.1% to 10% of the conductive additive.

7. A positive electrode composite material for a sodium-ion secondary battery according to any one of claims 3 to 5, characterized in that it is for use in an all-solid sodium-ion secondary battery.

8. A positive electrode for a sodium-ion secondary battery, characterized by containing the positive electrode composite material for a sodium-ion secondary battery described in any one of claims 3 to 5.

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

10. A precursor glass for a positive electrode active material for a sodium-ion secondary battery, characterized by containing Na, Mn, Ti, Nb, P, and O.

11. In mol% in terms of oxide, Na ２ O 25 to 50%, MnO 1 to 50%, TiO ２ 1 to 20%, Nb ２ O ５ 0.5 to 10%, P ２ O ５ 20 to 35%, Fe ２ O ３ + Cr ２ O ３ + MnO + CoO + V ２ O ５ + NiO 10% to 50%, characterized in that it is the precursor glass for the positive electrode active material of the sodium ion secondary battery according to claim 10.

Images