JP6870572B2 - Manufacturing method of solid electrolyte sheet - Google Patents

Description

The present invention relates to a method for producing a solid electrolyte sheet.

Lithium-ion secondary batteries are used as high-capacity and lightweight batteries in fields such as mobile devices and electric vehicles. However, in the lithium ion secondary battery, a flammable organic electrolyte is mainly used as the electrolyte. Although the organic electrolytic solution exhibits high ionic conductivity, it is a liquid and flammable, so that there are safety problems such as ignition and leakage when used in a power storage device. In order to solve such a safety problem, the development of an all-solid-state lithium-ion secondary battery using a solid electrolyte instead of the organic electrolytic solution is underway (Patent Document 1).

By the way, lithium used in an all-solid-state lithium-ion secondary battery is concerned about the soaring price of raw materials and the problem of depletion in the world. Therefore, sodium ions are attracting attention as an alternative to lithium ions, and development of an all-solid-state sodium-ion secondary battery using β-alumina or NASICON type crystals as a solid electrolyte is also underway (Patent Document 2).

However, in the all-solid-state sodium-ion secondary battery, as compared with the all-solid-state lithium-ion secondary battery, the reference voltage rises by 0.3 V, so that the operating voltage decreases, or the weight of the electrode active material increases as the atomic weight increases. There is a problem that the energy density is lowered as a result because the per-capacity is lowered.

Therefore, from the viewpoint of improving the energy density per unit volume, it is desirable that the solid electrolyte is thin, and it is desirable that the solid electrolyte is made into a sheet and used as a solid electrolyte sheet.

The solid electrolyte sheet can be produced, for example, by a green sheet method in which a raw material powder of a solid electrolyte is slurried to obtain a green sheet and then calcined (Patent Document 3).

Japanese Unexamined Patent Publication No. 5-205741 Japanese Unexamined Patent Publication No. 2010-15782 Japanese Unexamined Patent Publication No. 2017-37769

However, in the green sheet method as in Patent Document 3, alkali metal ions may evaporate when the green sheet is fired, and an alkali metal ion deficient layer in which alkali metal ions do not exist may be formed on the surface. Therefore, there is a problem that the ionic conductivity of the obtained solid electrolyte sheet is lowered.

In particular, when β-alumina or β ″ -alumina is used as the solid electrolyte, the firing temperature becomes high, so that the ionic conductivity tends to decrease. Further, the thinner the solid electrolyte sheet, the larger the surface area per volume. Therefore, the alkali metal ions are likely to evaporate during firing, and the ionic conductivity is likely to be lowered.

Further, since the solid electrolyte sheet is in the form of a sheet and has low mechanical strength, it is difficult to remove the alkali metal ion-deficient layer. In particular, a solid electrolyte having sodium ion conductivity is excellent in ion conductivity, but sodium carbonate may be formed on the surface due to moisture absorption, and the ion conductivity may decrease. Therefore, there is a problem that it is difficult to wash with water or polish in a wet state, and it is difficult to remove the alkali metal ion-deficient layer.

An object of the present invention is to provide a method for producing a solid electrolyte sheet capable of suppressing evaporation of alkali metal ions.

In the method for producing a solid electrolyte sheet of the present invention, a first green sheet is prepared by applying a slurry of a raw material powder of a first solid electrolyte composed of a sodium ion conductive oxide onto a substrate and drying it. In the preparation step and on the main surface on at least one side of the first green sheet, a restraining layer containing a powder of a second solid electrolyte made of a sodium ion conductive oxide is laminated and pressure-bonded to form a laminate. It is characterized by including a laminate forming step of forming, a firing step of firing the laminate, and a removing step of removing all or a part of the restraint layer after firing from the fired laminate. ..

In the method for producing a solid electrolyte sheet of the present invention, the restraint layer is a second green sheet prepared by applying a slurry containing the powder of the second solid electrolyte onto a substrate and drying it. preferable.

In the method for producing a solid electrolyte sheet of the present invention, it is preferable to tentatively calcin the raw material powder of the first solid electrolyte before the preparation step.

In the method for producing a solid electrolyte sheet of the present invention, it is preferable that the restraint layers are laminated and pressure-bonded on the main surfaces on both sides of the first green sheet in the laminate forming step.

In the method for producing a solid electrolyte sheet of the present invention, it is preferable that the first green sheet and the restraint layer are pressure-bonded by an isotropic pressure press in the laminate forming step.

In the method for producing a solid electrolyte sheet of the present invention, it is preferable to remove a portion of the restraint layer after firing that does not adhere to the first green sheet after firing in the removal step.

In the method for producing a solid electrolyte sheet of the present invention, it is preferable that the first solid electrolyte and the second solid electrolyte have the same composition.

In the method for producing a solid electrolyte sheet of the present invention, it is preferable that the first solid electrolyte and the second solid electrolyte contain at least one of β-alumina, β ″ -alumina and NASICON type crystals, respectively. ..

In the method for producing a solid electrolyte sheet of the present invention, the thickness of the solid electrolyte sheet is preferably 500 μm or less.

In the method for producing a solid electrolyte sheet of the present invention, it is preferable that the solid electrolyte sheet is used for an all-solid-state sodium-ion secondary battery.

According to the present invention, it is possible to provide a method for producing a solid electrolyte sheet capable of suppressing evaporation of alkali metal ions.

It is a schematic sectional drawing for demonstrating the manufacturing method of the solid electrolyte sheet which concerns on one Embodiment of this invention. It is a schematic sectional drawing for demonstrating the manufacturing method of the solid electrolyte sheet which concerns on one Embodiment of this invention. It is a schematic sectional drawing for demonstrating the manufacturing method of the solid electrolyte sheet which concerns on one Embodiment of this invention. It is a schematic sectional drawing for demonstrating the manufacturing method of the solid electrolyte sheet which concerns on one Embodiment of this invention.

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

The method for producing a solid electrolyte sheet of the present invention includes a preparation step of preparing a first green sheet by applying a slurry of raw material powder of the first solid electrolyte on a substrate and drying it, and a first green. A laminate forming step of forming a laminate by laminating a restraint layer containing a powder of a second solid electrolyte on the main surface of at least one side of the sheet and crimping the sheet, and a firing step of firing the laminate. It includes a removing step of removing the restraining layer after firing from the laminated body after firing.

Hereinafter, each step in the method for producing a solid electrolyte sheet according to an embodiment of the present invention will be described in more detail with reference to FIGS. 1 to 4.

(Preparation process)
In the preparation step of the first green sheet 2 shown in FIG. 1, first, the raw material powder 5 of the first solid electrolyte is slurried to obtain a slurry. Next, the first green sheet 2 can be obtained by applying the obtained slurry on a substrate such as polyethylene terephthalate and drying it.

The slurry of the raw material powder 5 of the first solid electrolyte can be obtained, for example, by adding a binder, a plasticizer, and a solvent to the raw material powder 5 of the first solid electrolyte, if necessary, and stirring the mixture. Further, the slurry can be applied by, for example, a doctor blade, a die coater, or the like.

First solid electrolyte;
The first solid electrolyte consists of a sodium ion conductive oxide. In the present invention, the "sodium ion conductive oxide" refers to an oxide having a sodium ion conductivity of ^10-5 S / cm or more, which is determined by the AC impedance method at 25 ° C. 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 thereof include β-alumina, β ”-alumina, and NASICON type crystals. The sodium ion conductive oxide is preferably β-alumina or β” -alumina. These are even better due to sodium ion conductivity.

When the first solid electrolyte is β-alumina or β "-alumina, the raw material powder 5 of the first solid electrolyte is, for example, in mol%, Al ₂ O ₃ : 65% to 98%, Na ₂ O. _{: 2% ~20%, MgO +} Li 2 O:.. include those containing 0.3% to 15% of the composition will be described a reason for limiting thus the following in the following description, especially of otherwise Unless otherwise noted, "%" means "mol%". In addition, "○ + ○ + ..." Means the total amount of each corresponding component.

Al ₂ O ₃ is a main component constituting β-alumina and β ″ -alumina. _{The content of Al 2} O ₃ is preferably 65% to 98%, more preferably 70% to 95%. If the amount of Al ₂ O ₃ is too small, the sodium ion conductivity tends to decrease. On the other hand, if the amount of Al ₂ O ₃ is too large, α-alumina having no sodium ion conductivity remains and the sodium ion conductivity becomes poor. It tends to decrease.

Na ₂ O is a component that imparts sodium ion conductivity to the first solid electrolyte. The content of Na ₂ O is preferably 2% to 20%, more preferably 3% to 18%, still more preferably 4% to 16%. If the amount of Na ₂ O is too small, it becomes difficult to obtain the above effect. On the other hand, if the amount of Na ₂ O is too large, the excess sodium _{forms a compound such as NaAlO 2} that does not contribute to the sodium ion conductivity, so that the sodium ion conductivity tends to decrease.

MgO and Li ₂ O is, beta-alumina and beta "- content of .MgO + Li ₂ O is a component to stabilize the structure of alumina (stabilizing agent), preferably 0.3% to 15%, more preferably 0.5% to 10%, more preferably when the .MgO + Li ₂ O is 0.8% to 8% is too small, the sodium ion conductivity α- alumina in the first solid electrolyte remained in reduced On the other hand, if the amount of MgO + Li _{2 O is too large, Mg O or Li 2} O that did not function as a stabilizer remains in the first solid electrolyte, and the sodium ion conductivity tends to decrease.

The raw material powder 5 of the first solid electrolyte preferably contains _{ZrO 2} and Y ₂ O _{3 in addition to the above components.} ZrO ₂ and Y ₂ O ₃ suppress the abnormal grain growth of β-alumina and / or β ”-alumina when the raw materials are calcined to prepare the first solid electrolyte, and β-alumina and / or β”. -It has the effect of further improving the adhesion of each particle of alumina. The content of ZrO ₂ is preferably 0% to 15%, more preferably 1% to 13%, still more preferably 2% to 10%. The content of Y ₂ O ₃ is preferably 0% to 5%, more preferably 0.01% to 4%, and even more preferably 0.02% to 3%. If the amount of ZrO ₂ or Y ₂ O ₃ is too large, the amount of β-alumina and / or β ”-alumina produced decreases, and the sodium ion conductivity tends to decrease.

At least one first solid electrolyte when a NASICON-type crystal, the general formula _{Na s A1 t A2 u O v} (A1 Al, Y, Yb, Nd, Nb, Ti, are selected from Hf and Zr, A2 Is represented by 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). Those containing crystals can be mentioned. In addition, as a preferable form of the said crystal, A1 is at least one selected from Y, Nb, Ti and Zr, s = 2.5 to 3.5, t = 1 to 2.5, u = 2.8. ~ 4, v = 9.5-12. In this case, more excellent crystals can be obtained due to the sodium ion conductivity. In particular, monoclinic or trigonal NASICON type crystals are preferable because they are even more excellent in sodium ion conductivity.

The general formula _Na s _A1 t _A2 Specific examples of u _{O v} represented by the _{crystal, Na 3 Zr 2 Si 2 PO} 12, Na 3.2 Zr 1.3 Si 2.2 P 0.8 O 10. ₅ , Na ₃ Zr _1.6 Ti _0.4 Si ₂ PO ₁₂ , Na ₃ Hf ₂ Si ₂ PO ₁₂ , Na _3.4 Zr _0.9 Hf _1.4 Al _0.6 Si _1.2 P _1.8 O ₁₂ , Na ₃ Zr _1.7 Nb _0.24 Si ₂ PO ₁₂ , Na _3.6 Ti _0.2 Y _0.8 Si _2.8 O ₉ , Na ₃ Zr _1.88 Y _0.12 Si ₂ PO ₁₂ , Na _3.12 Zr _1.88 Y _0.12 Si ₂ PO ₁₂ , Na _3.6 Zr _0.13 Yb _1.67 Si _0.11 P _2.9 O _{12 and the} like.

The average particle size of the raw material powder 5 of the first solid electrolyte is preferably 15 μm or less, more preferably 10 μm or less, and further preferably 5 μm or less. If the average particle size of the raw material powder 5 of the first solid electrolyte is too large, the contact area between the raw material powders 5 of the first solid electrolyte decreases, so that the solid phase reaction by firing may be difficult to proceed. Therefore, the sodium ion conduction path between the negative electrode active material powder and the solid electrolyte powder is reduced, and the discharge capacity is likely to decrease. In addition, it may be difficult to make the solid electrolyte sheet 1 thinner. Further, the distance required for sodium ion conduction becomes long, and the sodium ion conductivity tends to decrease. The lower limit of the average particle size of the raw material powder 5 of the first solid electrolyte is not particularly limited, but is preferably 0.01 μm or more, more preferably 0.05 μm or more, and 0.1 μm or more, for example. It is more preferable to have.

In the present invention, the average particle size means D50 (volume-based average particle size) and refers to a value measured by a laser diffraction / scattering method.

Further, the raw material powder 5 of the first solid electrolyte may be used after being calcined in advance. When the raw material powder 5 of the first solid electrolyte is tentatively fired, the evaporation of alkali metal ions can be further suppressed in the main firing described later. Further, the expansion of the solid electrolyte sheet 1 due to degassing and evaporation of alkali metal ions can be further suppressed. The temperature of the tentative firing can be, for example, 1000 ° C. or higher and lower than 1400 ° C. The time for temporary firing can be, for example, 1 hour to 20 hours.

binder;
The binder is not particularly limited, and for example, a (meth) acrylic resin, a polyvinyl butyral resin, or the like can be used. These may be used alone or in combination of two or more.

Plasticizer;
The plasticizer is not particularly limited, and for example, dibutyl phthalate, benzyl butyl phthalate, and the like can be used. These may be used alone or in combination of two or more. By using a plasticizer, it is possible to control the drying rate and impart flexibility to the obtained film.

solvent;
Further, as the solvent, for example, water or an organic solvent such as ethanol, acetone or N-methylpyrrolidone can be used. However, when water is used as the solvent, the sodium component may elute from the raw material powder 5 of the first solid electrolyte, the pH of the slurry rises, and the raw material powder 5 of the first solid electrolyte may aggregate. Therefore, it is preferable to use an organic solvent as the solvent.

The thickness of the first green sheet 2 is preferably 20 μm or more, more preferably 60 μm or more, preferably 1000 μm or less, more preferably 800 μm or less, still more preferably 600 μm or less, and particularly preferably 400 μm or less. If the thickness of the first green sheet 2 is too thin, the mechanical strength of the solid electrolyte sheet 1 may decrease. Further, if the thickness of the first green sheet 2 is too thick, the thickness of the obtained solid electrolyte sheet 1 also becomes thick. Therefore, the distance required for ion conduction in the solid electrolyte sheet 1 also becomes long, and the energy density per unit volume tends to decrease.

(Laminate body forming process)
In the laminate forming step, as shown in FIG. 2, first, the first restraint layer 3 is laminated on the first main surface 2a of the first green sheet 2. Further, the second restraint layer 4 is laminated on the second main surface 2b of the first green sheet 2. The first main surface 2a and the second main surface 2b of the first green sheet 2 face each other.

The first restraint layer 3 and the second restraint layer 4 each contain a second solid electrolyte powder 6. The powder 6 of the second solid electrolyte is a powder after firing obtained by firing the raw material powder of the second solid electrolyte. Therefore, the first restraint layer 3 and the second restraint layer 4 are difficult-to-sinter restraint layers.

As the second solid electrolyte, for example, a sodium ion conductive oxide can be used. 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 thereof include β-alumina, β ”-alumina, and NASICON type crystals. The sodium ion conductive oxide is preferably β-alumina or β” -alumina. These are even better due to sodium ion conductivity.

When the second solid electrolyte is β-alumina or β "-alumina, for example, in mol%, Al ₂ O ₃ : 65% to 98%, Na ₂ O: 2% to 20%, MgO + Li ₂ O: 0. Examples include those containing 3% to 15%. The reason for limiting the composition in this way will be described below. In the following description, unless otherwise specified, "%" is "mol%". means. In addition, "○ + ○ + ..." Means the total amount of each corresponding component.

Al ₂ O ₃ is a main component constituting β-alumina and β ″ -alumina. _{The content of Al 2} O ₃ is preferably 65% to 98%, more preferably 70% to 95%. If the amount of Al ₂ O ₃ is too small, the sodium ion conductivity tends to decrease. On the other hand, if the amount of Al ₂ O ₃ is too large, α-alumina having no sodium ion conductivity remains and the sodium ion conductivity becomes poor. It tends to decrease.

Na ₂ O is a component that imparts sodium ion conductivity to the second solid electrolyte. The content of Na ₂ O is preferably 2% to 20%, more preferably 3% to 18%, still more preferably 4% to 16%. If the amount of Na ₂ O is too small, it becomes difficult to obtain the above effect. On the other hand, if the amount of Na ₂ O is too large, the excess sodium _{forms a compound such as NaAlO 2} that does not contribute to the sodium ion conductivity, so that the sodium ion conductivity tends to decrease.

MgO and Li ₂ O is, beta-alumina and beta "- content of .MgO + Li ₂ O is a component to stabilize the structure of alumina (stabilizing agent), preferably 0.3% to 15%, more preferably 0.5% to 10%, more preferably when the .MgO + Li ₂ O is 0.8% to 8% is too small, the sodium ion conductivity α- alumina in the second solid electrolyte remained in reduced On the other hand, if the amount of MgO + Li _{2 O is too large, Mg O or Li 2} O that did not function as a stabilizer remains in the second solid electrolyte, and the sodium ion conductivity tends to decrease.

The second solid electrolyte preferably contains _{ZrO 2} and Y ₂ O ₃ in addition to the above components. ZrO ₂ and Y ₂ O ₃ suppress the abnormal grain growth of β-alumina and / or β ”-alumina when the raw material is fired to prepare a second solid electrolyte, and β-alumina and / or β”. -It has the effect of further improving the adhesion of each particle of alumina. The content of ZrO ₂ is preferably 0% to 15%, more preferably 1% to 13%, still more preferably 2% to 10%. The content of Y ₂ O ₃ is preferably 0% to 5%, more preferably 0.01% to 4%, and even more preferably 0.02% to 3%. If the amount of ZrO ₂ or Y ₂ O ₃ is too large, the amount of β-alumina and / or β ”-alumina produced decreases, and the sodium ion conductivity tends to decrease.

At least one second solid electrolyte when a NASICON-type crystal, the general formula _{Na s A1 t A2 u O v} (A1 Al, Y, Yb, Nd, Nb, Ti, are selected from Hf and Zr, A2 Is represented by 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). Those containing crystals can be mentioned. In addition, as a preferable form of the said crystal, A1 is at least one selected from Y, Nb, Ti and Zr, s = 2.5 to 3.5, t = 1 to 2.5, u = 2.8. ~ 4, v = 9.5-12. In this case, more excellent crystals can be obtained due to the sodium ion conductivity. In particular, monoclinic or trigonal NASICON type crystals are preferable because they are even more excellent in sodium ion conductivity.

The general formula _Na s _A1 t _A2 Specific examples of u _{O v} represented by the _{crystal, Na 3 Zr 2 Si 2 PO} 12, Na 3.2 Zr 1.3 Si 2.2 P 0.8 O 10. ₅ , Na ₃ Zr _1.6 Ti _0.4 Si ₂ PO ₁₂ , Na ₃ Hf ₂ Si ₂ PO ₁₂ , Na _3.4 Zr _0.9 Hf _1.4 Al _0.6 Si _1.2 P _1.8 O ₁₂ , Na ₃ Zr _1.7 Nb _0.24 Si ₂ PO ₁₂ , Na _3.6 Ti _0.2 Y _0.8 Si _2.8 O ₉ , Na ₃ Zr _1.88 Y _0.12 Si ₂ PO ₁₂ , Na _3.12 Zr _1.88 Y _0.12 Si ₂ PO ₁₂ , Na _3.6 Zr _0.13 Yb _1.67 Si _0.11 P _2.9 O _{12 and the} like.

The first solid electrolyte and the second solid electrolyte preferably have the same composition. In this case, the evaporation of alkali metal ions, which will be described later, can be further suppressed.

The average particle size of the powder 6 of the second solid electrolyte is preferably 0.01 μm or more. If the average particle size of the powder 6 of the second solid electrolyte is too small, the raw material powder of the first solid electrolyte is formed at the interface between the first green sheet 2 and the first restraint layer 3 and the second restraint layer 4. The 5 and the powder 6 of the second solid electrolyte may react with each other to form a reaction product, and as a result, the fixed state may be strengthened. In this case, it may be difficult to remove the first restraint layer 3 and the second restraint layer 4, which will be described later, or to remove the reaction product. The upper limit of the average particle size of the powder 6 of the second solid electrolyte is not particularly limited, but is preferably 15 μm or less, for example.

In the present embodiment, the first restraint layer 3 and the second restraint layer 4 are both second green sheets containing the powder 6 of the second solid electrolyte.

When preparing the second green sheet, it can be obtained by first applying a slurry containing the powder 6 of the second solid electrolyte on a substrate such as polyethylene terephthalate and drying it. The slurry of the powder 6 of the second solid electrolyte can be obtained, for example, by adding a binder, a plasticizer, and a solvent to the powder 6 of the second solid electrolyte, if necessary, and stirring the mixture. As for the binder, the plasticizer, and the solvent, those exemplified in the above-mentioned preparation step of the first green sheet 2 can be used. Further, the slurry can be applied by, for example, a doctor blade, a die coater, or the like.

Next, the first restraint layer 3 and the second restraint layer 4, which are the second green sheets, are laminated on the first green sheet 2 and then crimped to be integrated to form the laminated body 10. obtain. The pressure bonding between the first green sheet 2 and the first restraint layer 3 and the second restraint layer 4 is preferably performed by an isotropic pressure press. As described above, when the isotropic pressure press is performed before firing the laminate 10 including the first green sheet 2, the adhesion of each particle of the solid electrolyte is further improved, and the ionic conductivity is further enhanced. Can be done.

The pressure of the isotropic press is not particularly limited, but is preferably 5 MPa or more, more preferably 10 MPa or more. If the pressure during isotropic pressing is too low, the raw material powders may not adhere to each other sufficiently, and a solid-phase reaction may not easily occur during firing. In addition, the adhesion of each particle of the solid electrolyte may be insufficient, and the ionic conductivity may be lowered.

In the present embodiment, the first restraint layer 3 and the second restraint layer 4 are provided on the main surfaces 2a and 2b on both sides of the first green sheet 2, respectively, but in the present invention, the first restraint layer 4 is provided. Either one of the restraint layer 3 and the second restraint layer 4 may be provided. However, from the viewpoint of further suppressing the evaporation of alkali metal ions, which will be described later, the first restraint layer 3 and the second restraint layer 4 are placed on the main surfaces 2a and 2b on both sides of the first green sheet 2, respectively. Is preferably provided.

Further, in the present embodiment, the first restraint layer 3 and the second restraint layer 4 are both the second green sheet containing the powder 6 of the second solid electrolyte, but the first restraint layer 3 And the second restraint layer 4 is substantially composed of only the powder 6 of the second solid electrolyte, and does not have to contain a binder or the like. Further, the first restraint layer 3 and the second restraint layer 4 may be a paste containing the powder 6 of the second solid electrolyte. Further, a layer containing α-alumina may be further provided on the first restraint layer 3 and the second restraint layer 4 (on the surface opposite to the first green sheet 2).

(Baking process)
Next, the laminate 10 is fired. In the present embodiment, the first restraint layer 3 and the second restraint layer 4 are crimped to the main surfaces 2a and 2b on both sides of the first green sheet 2, respectively, and then fired.

Specifically, in the present embodiment, first, firing for debindering is performed. Then, after firing for removing the binder, the main firing is performed. In this case, the binder disappears by firing for debinder, and as shown in FIG. 3, the first green sheet 2 becomes a layer substantially composed of only the raw material powder 5 of the first solid electrolyte. Further, the first restraint layer 3 and the second restraint layer 4 are substantially formed of only the powder 6 of the second solid electrolyte. Then, by further performing the main firing in this state, the solid-phase reaction of the raw material powder 5 of the first solid electrolyte proceeds, and the powder of the first solid electrolyte is obtained. The firing for removing the binder shall be performed at a temperature lower than the temperature of the main firing for a certain period of time.

The firing temperature for removing the binder can be, for example, 350 ° C. to 800 ° C. The firing time for debindering can be, for example, 30 minutes to 120 minutes. Further, the firing temperature of the main firing can be, for example, 1200 ° C to 1750 ° C, and further 140 to 1750 ° C. The firing time of the main firing can be, for example, 10 minutes to 120 minutes.

As in the present embodiment, the firing of the laminated body 10 is preferably performed in two stages, a firing for removing the binder and a main firing. In this case, the evaporation of alkali metal ions, which will be described later, can be further suppressed. However, in the present invention, the firing for removing the binder and the main firing may be performed at the same time in one step.

(Removal process)
Next, as shown in FIG. 4, the first restraint layer 3A and the second restraint layer 4A after firing are separated and removed from the fired laminate. Thereby, the solid electrolyte sheet 1 can be obtained.

When removing the first restraint layer 3A and the second restraint layer 4A after firing, all or part of the first restraint layer 3A and the second restraint layer 4A after firing is removed. When removing a part of the first restraint layer 3A and the second restraint layer 4A after firing, it is desirable to remove, for example, the portion that does not adhere to the first green sheet 2A after firing. In particular, it is desirable to remove only the portion that does not adhere to the first green sheet 2A after firing. That is, it is desirable that the portion of the first restraint layer 3A and the second restraint layer 4A that is attached to the first green sheet 2A is left as it is without being removed.

The method for removing the first restraint layer 3A and the second restraint layer 4A is not particularly limited, but can be removed by, for example, blowing high-pressure air.

As described above, in the production method of the present embodiment, the main surfaces 2a and 2b on both sides of the first green sheet 2 containing the raw material powder 5 of the first solid electrolyte composed of the sodium ion conductive oxide are respectively. The laminate 10 is fired in a state where the first restraint layer 3 and the second restraint layer 4 which are difficult to sinter and contain the powder 6 of the second solid electrolyte made of sodium ion conductive oxide are pressure-bonded. Therefore, the first restraint layer 3 and the second restraint layer 4 can suppress the evaporation of alkaline ions in the first green sheet 2. Therefore, in the obtained solid electrolyte sheet 1, it is difficult for the alkali metal ion-deficient layer to be formed, and the ion conductivity can be enhanced.

Further, when firing in a state where the upper and lower main surfaces 2a and 2b of the first green sheet 2 are crimped by the first restraint layer 3 and the second restraint layer 4, the first restraint layer 3 and the second restraint layer 3 and the second restraint layer 4 are fired. Layer 4 is substantially non-sintered. Therefore, the first restraint layer 3 and the second restraint layer 4 exert a binding force on the upper and lower main surfaces 2a and 2b of the first green sheet 2 when the first green sheet 2 contracts due to firing. As a result, the shrinkage of the first green sheet 2 in the plane direction is suppressed, and the first green sheet 2 shrinks only in the thickness direction. Therefore, the solid electrolyte sheet 1 having excellent flatness can be obtained. From the viewpoint of further improving the flatness of the obtained solid electrolyte sheet 1, it is preferable that the first restraint layer 3 and the second restraint layer 4 are green sheets as in the present embodiment.

Further, since the solid electrolyte sheet 1 obtained by the production method of the present embodiment is excellent in flatness, the electrode layer can be applied more uniformly on the solid electrolyte sheet 1. Therefore, the thickness of the electrode layer is unlikely to be uneven, and a portion having a large internal resistance is unlikely to occur locally. Therefore, when the obtained solid electrolyte sheet 1 is used, the rate characteristics of the all-solid-state sodium-ion secondary battery can be improved.

Further, in the present embodiment, when the crystals of the first solid electrolyte are formed by firing the laminate 10, the first solid electrolyte and the second solid electrolyte react on the surface of the first green sheet 2. To do. Therefore, when the first restraint layer 3 and the second restraint layer 4 are removed, the second solid electrolyte that has reacted with the first solid electrolyte remains on the surface of the solid electrolyte sheet 1. As a result, the surface of the solid electrolyte sheet 1 is roughened. In particular, in the production method of the present embodiment, the surface can be roughened by the second solid electrolyte remaining on the surface, so that the surface is roughened without cracking, unlike the case where the surface is roughened by dry polishing or sandblasting. Can be transformed into.

As described above, in the production method of the present embodiment, the surface of the solid electrolyte sheet 1 can be roughened, so that the specific surface area can be increased. Therefore, when the electrode layer is formed on the surface of the solid electrolyte sheet 1, the exchange of ions with the electrode layer is increased, and the rate characteristics of the all-solid-state sodium-ion secondary battery can be improved.

(Solid electrolyte sheet)
The solid electrolyte sheet of the present invention can be obtained, for example, by the method for producing the solid electrolyte sheet 1 described above.

The thickness of the solid electrolyte sheet 1 is not particularly limited, but is preferably 10 μm or more, more preferably 50 μm or more, still more preferably 150 μm or more, preferably 2000 μm or less, more preferably 500 μm or less, still more preferably 200 μm or less. If the thickness of the solid electrolyte sheet 1 is too thin, the mechanical strength is lowered and the solid electrolyte sheet 1 is liable to be damaged, and an internal short circuit is liable to occur. On the other hand, if the thickness of the solid electrolyte sheet 1 is too thick, the sodium ion conduction distance associated with charging and discharging becomes long, so that the internal resistance becomes large, and the discharge capacity and the operating voltage tend to decrease. In addition, the energy density per unit volume of the all-solid-state sodium-ion secondary battery may also decrease.

The arithmetic mean roughness Ra of the solid electrolyte sheet 1 is preferably 0.05 μm or more, more preferably 0.1 μm or more, still more preferably 0.5 μm or more, preferably 5.5 μm or less, and more preferably 2 μm or less. When the arithmetic mean roughness Ra is at least the above lower limit, the electrode layer is more difficult to peel off from the solid electrolyte sheet 1, and the charge / discharge capacity can be further increased. Further, when the arithmetic mean roughness Ra is not more than the above upper limit, voids are less likely to be generated between the solid electrolyte sheet 1 and the electrode layer, and the charge / discharge capacity can be further increased. The arithmetic mean roughness Ra can be measured in accordance with JIS B 0633-2001.

(All-solid-state sodium-ion secondary battery)
The solid electrolyte sheet obtained by the production method of the present invention is preferably used for an all-solid-state sodium-ion secondary battery. An all-solid-state sodium-ion secondary battery has a positive electrode layer formed on one surface of a solid electrolyte sheet and a negative electrode layer formed on the other surface.

Positive electrode layer;
The positive electrode layer is not particularly limited as long as it contains a positive electrode active material capable of occluding and releasing sodium and functions as a positive electrode layer. The positive electrode active material may be formed by firing, for example, a positive electrode active material precursor powder such as glass powder. By firing the positive electrode active material precursor powder, positive electrode active material crystals are precipitated, and the positive electrode active material crystals act as the positive electrode active material.

Examples of the positive electrode active material crystal that acts as the positive electrode active material include Na, M (M is at least one transition metal element selected from Cr, Fe, Mn, Co, V, and Ni), P, and a sodium transition metal containing O. Phosphate crystals can be mentioned. Specific examples include Na ₂ FeP ₂ O ₇ , NaFePO ₄ , Na ₃ V ₂ (PO ₄ ) ₃ , Na ₂ NiP ₂ O ₇ , Na _3.64 Ni _2.18 (P ₂ O ₇ ) ₂ , Na ₃ Examples thereof include Ni ₃ (PO ₄ ) ₂ (P ₂ O ₇ ), Na _{2 CoP 2} O ₇ , Na _3.64 Co _2.18 (P ₂ O ₇ ) _2. The sodium transition metal phosphate crystal is preferable because it has a high capacity and excellent chemical stability. Among them triclinic crystals belonging to the space group P1 or P1, particularly the general formula _{Na x M y P 2 O z} (1.2 ≦ x ≦ 2.8,0.95 ≦ y ≦ 1.6,6 The crystal represented by .5 ≦ z ≦ 8) is preferable because it has excellent cycle characteristics. Other examples of positive electrode active material crystals that act as positive electrode active materials include layered sodium transition metal oxide crystals such _{as NaCrO 2} , Na _0.7 MnO ₂ , and NaFe _0.2 Mn _0.4 Ni _0.4 O _2. Be done. The positive electrode active material crystal contained in the positive electrode layer may be a single phase in which only one type of crystal is precipitated, or may be a mixed phase in which a plurality of types of crystals are precipitated.

The positive electrode active material precursor powder includes at least one transition metal element selected from (i) Cr, Fe, Mn, Co, Ni, Ti and Nb, and at least one selected from (ii) P, Si and B. Examples include one element and one containing (iii) O.

As the positive electrode active material precursor powder, especially in terms of oxide equivalent mol%, Na ₂ O: 8% to 55%, CrO + FeO + MnO + CoO + NiO: 10% to 70%, P ₂ O ₅ + SiO ₂ + B ₂ O ₃ : 15% Those containing 70% can be mentioned. The reason for limiting each component in this way will be described below. In the following description of the content of each component, "%" means "mol%" unless otherwise specified. In addition, "○ + ○ + ..." Means the total amount of each corresponding component.

Na ₂ O serves as a source of sodium ions that move between the positive electrode active material and the negative electrode active material during charging and discharging. The content of Na ₂ O is preferably 8% to 55%, more preferably 15% to 45%, and even more preferably 25% to 35%. If the amount of Na ₂ O is too small, the amount of sodium ions that contribute to occlusion and release is small, so the discharge capacity tends to decrease. On the other hand, if the amount of Na ₂ O is too large, _{dissimilar crystals such as Na 3} PO ₄ that do not contribute to charging / discharging tend to precipitate, so that the discharge capacity tends to decrease.

CrO, FeO, MnO, CoO, and NiO are components that act as a driving force for the occlusion and release of sodium ions by causing a redox reaction by changing the valence of each transition element during charging and discharging. Among them, NiO and MnO have a large effect of increasing the redox potential. In addition, FeO is particularly easy to stabilize the structure during charging and discharging, and is easy to improve the cycle characteristics. The content of CrO + FeO + MnO + CoO + NiO is preferably 10 to 70%, more preferably 15% to 60%, still more preferably 20% to 55%, still more preferably 23% to 50%, particularly preferably 25% to 40%, most. It is preferably 26% to 36%. If the amount of CrO + FeO + MnO + CoO + NiO is too small, the redox reaction associated with charging and discharging is less likely to occur, and the amount of sodium ions occluded and released is reduced, so that the discharge capacity tends to decrease. On the other hand, if the amount of CrO + FeO + MnO + CoO + NiO is too large, dissimilar crystals tend to precipitate and the discharge capacity tends to decrease.

Since P ₂ O ₅ , SiO ₂ and B ₂ O ₃ form a three-dimensional network structure, they have the effect of stabilizing the structure of the positive electrode active material. In particular, P ₂ O ₅ and SiO ₂ are preferable because they have excellent sodium ion conductivity, and P ₂ O ₅ is more preferable. The content of P ₂ O ₅ + SiO ₂ + B ₂ O ₃ is preferably 15% to 70%, more preferably 20% to 60%, and even more preferably 25% to 45%. If the amount of P ₂ O ₅ + SiO ₂ + B ₂ O ₃ is too small, the discharge capacity tends to decrease during repeated charging and discharging. On the other hand, if the amount of P ₂ O ₅ + SiO ₂ + B ₂ O ₃ is too large, _{heterogeneous crystals such as P 2} O ₅ that do not contribute to charging / discharging tend to precipitate. The content of each component of P ₂ O ₅ , SiO ₂ and B ₂ O ₃ is preferably 0% to 70%, more preferably 15% to 70%, still more preferably 20% to 60%, and particularly preferably. Is 25% to 45%.

Further, vitrification can be facilitated by containing various components in addition to the above components as long as the effect as the positive electrode active material is not impaired. Examples of such components, _MgO oxide _{notation, CaO, SrO, BaO, ZnO} , CuO, Al 2 O 3, GeO 2, Nb 2 O 5, TiO 2, ZrO 2, V 2 O 5, Sb 2 O _{5 is particularly preferable, and Al 2} O _{3 which} acts as a network-forming oxide _{and V 2} O _{5 which} is an active material component are preferable. The content of the above components, in total, is preferably 0% to 30%, more preferably 0.1% to 20%, and even more preferably 0.5% to 10%.

The positive electrode active material precursor powder is preferably one in which an amorphous phase is formed together with the positive electrode active material crystals by firing. By forming the amorphous phase, the sodium ion conductivity in the positive electrode layer and at the interface between the positive electrode layer and the solid electrolyte sheet 1 can be improved.

The average particle size of the positive electrode active material precursor powder is preferably 0.01 μm to 15 μm, more preferably 0.05 μm to 12 μm, and even more preferably 0.1 μm to 10 μm. If the average particle size of the positive electrode active material precursor powder is too small, the cohesive force between the positive electrode active material precursor powders becomes strong, and the dispersibility tends to be poor when made into a paste. As a result, the internal resistance of the battery becomes high and the operating voltage tends to decrease. In addition, the electrode density tends to decrease, and the capacity per unit volume of the battery tends to decrease. On the other hand, if the average particle size of the active material precursor powder is too large, sodium ions are difficult to diffuse and the internal resistance tends to increase. In addition, the surface smoothness of the electrode tends to be inferior.

In the present invention, the average particle size means D50 (volume-based average particle size) and refers to a value measured by a laser diffraction / scattering method.

The thickness of the positive electrode layer is preferably in the range of 3 μm to 300 μm, and more preferably in the range of 10 μm to 150 μm. If the thickness of the positive electrode layer is too thin, the capacity of the all-solid-state sodium-ion secondary battery itself becomes small, so that the energy density may decrease. If the thickness of the positive electrode layer is too thick, the resistance to electron conduction increases, so that the discharge capacity and the operating voltage tend to decrease.

The positive electrode layer may contain a solid electrolyte powder, if necessary. Therefore, the positive electrode layer may be a positive electrode mixture which is a mixture of the positive electrode active material and the solid electrolyte powder. As the solid electrolyte powder, a powder of the same material as the above-mentioned solid electrolyte sheet 1 can be used. By containing the solid electrolyte powder, the sodium ion conductivity in the positive electrode layer and at the interface between the positive electrode layer and the solid electrolyte sheet 1 can be improved. The average particle size of the solid electrolyte powder is preferably 0.01 μm to 15 μm, more preferably 0.05 μm to 10 μm, and even more preferably 0.1 μm to 5 μm.

If the average particle size of the solid electrolyte powder is too large, the distance required for sodium ion conduction becomes long and the sodium ion conductivity tends to decrease. Also, the sodium ion conduction path between the positive electrode active material powder and the solid electrolyte powder tends to decrease. As a result, the discharge capacity tends to decrease. On the other hand, if the average particle size of the solid electrolyte powder is too small, deterioration due to elution of sodium ions and reaction with carbon dioxide gas occurs, and sodium ion conductivity tends to decrease. In addition, since voids are likely to be formed, the electrode density is also likely to decrease. As a result, the discharge capacity tends to decrease.

The volume ratio of the positive electrode active material precursor powder to the solid electrolyte powder is preferably 20:80 to 95: 5, more preferably 30:70 to 90:10, and even more preferably 35:65 to 88:12.

Further, the positive electrode layer may contain a conductive auxiliary agent such as carbon powder or a binder, if necessary. By including the conductive auxiliary agent, the internal resistance of the positive electrode layer can be reduced. The conductive auxiliary agent is preferably contained in the positive electrode layer in an amount of 0% by mass to 20% by mass, and more preferably in a proportion of 1% by mass to 10% by mass.

As the binder, polypropylene carbonate (PPC), which decomposes at low temperature in an inert atmosphere, is preferable. Further, carboxymethyl cellulose (CMC) having excellent sodium ion conductivity is preferable.

Negative electrode layer;
The negative electrode layer is not particularly limited as long as it contains a negative electrode active material capable of occluding and releasing sodium and functions as a negative electrode layer. The negative electrode active material may be formed by firing, for example, a negative electrode active material precursor powder such as glass powder. By firing the negative electrode active material precursor powder, negative electrode active material crystals are precipitated, and the negative electrode active material crystals act as the negative electrode active material.

Examples of the negative electrode active material crystal that acts as the negative electrode active material include at least one selected from Nb and Ti and a crystal containing O, at least one metal crystal selected from Sn, Bi and Sb, or Sn, Bi and An alloy crystal containing at least one selected from Sb can be mentioned.

Crystals containing at least one selected from Nb and Ti and O are preferable because they have excellent cycle characteristics. Further, when the crystal containing at least one selected from Nb and Ti and O contains Na and / or Li, the charge / discharge efficiency (ratio of the discharge capacity to the charge capacity) is increased, and a high charge / discharge capacity is maintained. It is preferable because it can be discharged. Among them, crystals containing at least one selected from Nb and Ti and O belong to oblique crystal crystals, hexagonal crystals, cubic crystals or monoclinic crystals, particularly the space group P2 ₁ / m. A monoclinic crystal is more preferable because the capacity is unlikely to decrease even when charged and discharged with a large current.

Examples of the orthorhombic crystal include NaTi ₂ O _{4 and the} like. Examples of the hexagonal crystal include Na ₂ TiO _{3 , NaTi 8} O ₁₃ , NaTIO ₂ , LiNbO ₃ , LiNbO ₂ , Li ₇ NbO ₆ , Li ₂ Ti ₃ O _{7, and the} like. Examples of cubic crystals include Na ₂ TiO ₃ , NaNbO ₃ , Li ₄ Ti ₅ O ₁₂ , Li ₃ NbO _{4, and the} like. As monoclinic crystals, Na ₂ Ti ₆ O _{13 , Na Ti 2} O ₄ , Na ₂ Ti O ₃ , Na _{4 Ti 5 O 12} , Na ₂ Ti ₄ O ₉ , Na ₂ Ti ₉ O ₁₉ , Na ₂ Ti ₃ Examples thereof include O ₇ , Na ₂ Ti ₃ O ₇ , Li _1.7 Nb ₂ O ₅ , Li _1.9 Nb ₂ O ₅ , Li ₁₂ Nb ₁₃ O _{33 , and Li Nb 3} O _8. Examples of the monoclinic crystal belonging to the space group P2 _{1 / m include Na 2} Ti ₃ O ₇ .

It is preferable that the crystal containing at least one selected from Nb and Ti and O further contains at least one selected from B, Si, P and Ge. These components have the effect of facilitating the formation of an amorphous phase together with the negative electrode active material crystal and further improving the sodium ion conductivity.

In addition, it is selected from Na metal crystals, alloy crystals containing at least Na (for example, Na—Sn alloy, Na—In alloy), and at least one metal crystal selected from Sn, Bi and Sb, Sn, Bi and Sb. Alloy crystals containing at least one of these (for example, Sn—Cu alloy, Bi—Cu alloy, Bi—Zn alloy), and glass containing at least one selected from Sn, Bi, and Sb can be used. These are preferable because they have a high capacity and the capacity is unlikely to decrease even when charged and discharged with a large current.

As the negative electrode active material precursor powder, in mol% in terms of oxide, SnO: 0% to 90%, Bi ₂ O ₃ : 0% to 90%, TiO ₂ : 0% to 90%, Fe ₂ O ₃ : It preferably contains 0% to 90%, Nb ₂ O ₅ : 0% to 90%, SiO ₂ + B ₂ O ₃ + P ₂ O ₅ : 5% to 75%, and Na ₂ O: 0% to 80%. With the above configuration, a structure is formed in which Sn ions, Bi ions, Ti ions, Fe ions or Nb ions, which are negative electrode active material components, are more uniformly dispersed in an oxide matrix containing Si, B or P. Ion. Further, _{by containing Na 2} O, the material becomes more excellent in sodium ion conductivity. As a result, it is possible to suppress the volume change when occluding and releasing sodium ions, and it is possible to obtain a negative electrode active material having more excellent cycle characteristics.

The reason for limiting the composition of the negative electrode active material precursor powder as described above will be described below. In the following description, unless otherwise specified, "%" means "mol%". In addition, "○ + ○ + ..." Means the total amount of each corresponding component.

SnO, Bi ₂ O ₃ , TiO ₂ , Fe ₂ O ₃ and Nb ₂ O ₅ are negative electrode active material components that serve as sites for occluding and releasing alkaline ions. By containing these components, the discharge capacity per unit mass of the negative electrode active material becomes larger, and the charge / discharge efficiency (ratio of the discharge capacity to the charge capacity) at the time of initial charge / discharge tends to be further improved. However, if the content of these components is too large, the volume change associated with the occlusion and release of sodium ions during charging and discharging cannot be alleviated, and the cycle characteristics tend to deteriorate. In view of the above, the content range of each component is preferably as follows.

The SnO content is preferably 0% to 90%, more preferably 45% to 85%, still more preferably 55% to 75%, and particularly preferably 60% to 72%.

The content of Bi ₂ O ₃ is preferably 0% to 90%, more preferably 10% to 70%, still more preferably 15% to 65%, and particularly preferably 25% to 55%.

The content of TiO ₂ is preferably 0% to 90%, more preferably 5% to 72%, still more preferably 10% to 68%, still more preferably 12% to 58%, and particularly preferably 15% to 49%. , Most preferably 15% to 39%.

The content of Fe ₂ O ₃ is preferably 0% to 90%, more preferably 15% to 85%, still more preferably 20% to 80%, and particularly preferably 25% to 75%.

The content of Nb ₂ O ₅ is preferably 0% to 90%, more preferably 7% to 79%, still more preferably 9% to 69%, still more preferably 11% to 59%, and particularly preferably 13% to. It is 49%, most preferably 15% to 39%. In addition, SnO + Bi ₂ O ₃ + TiO ₂ + Fe ₂ O ₃ + Nb ₂ O ₅ is preferably 0% to 90%, more preferably 5% to 85%, and further preferably 10% to 80%.

Further, SiO ₂ , B ₂ O ₃ and P ₂ O ₅ are network-forming oxides, which surround the occlusion and release sites of sodium ions in the negative electrode active material component, and have an effect of further improving the cycle characteristics. Among them, SiO ₂ and P ₂ O ₅ not only further improve the cycle characteristics, but also have an excellent sodium ion conductivity, so that they have an effect of further improving the rate characteristics.

SiO ₂ + B ₂ O ₃ + P ₂ O ₅ is preferably 5% to 85%, more preferably 6% to 79%, still more preferably 7% to 69%, still more preferably 8% to 59%, and particularly preferably. It is 9% to 49%, most preferably 10% to 39%. If SiO ₂ + B ₂ O ₃ + P ₂ O ₅ is too small, the volume change of the negative electrode active material component due to occlusion and release of sodium ions during charging and discharging cannot be mitigated and structural destruction occurs, so the cycle characteristics tend to deteriorate. Become. On the other hand, if the amount of SiO ₂ + B ₂ O ₃ + P ₂ O ₅ is too large, the content of the negative electrode active material component tends to be relatively small, and the charge / discharge capacity per unit mass of the negative electrode active material tends to be small.

The preferable ranges of the contents of SiO ₂ , B ₂ O ₃ and P ₂ O _{5 are as follows.}

The content of SiO ₂ is preferably 0% to 75%, more preferably 5% to 75%, still more preferably 7% to 60%, still more preferably 10% to 50%, and particularly preferably 12% to 40%. , Most preferably 20% to 35%. If _{the content of SiO 2} is too large, the discharge capacity tends to decrease.

The content of P ₂ O ₅ is preferably 5% to 75%, more preferably 7% to 60%, still more preferably 10% to 50%, particularly preferably 12% to 40%, and most preferably 20% to 20%. It is 35%. If _{the content of P 2} O ₅ is too small, it becomes difficult to obtain the above effect. On the other hand, _{if the content of P 2} O ₅ is too large, the discharge capacity tends to decrease and the water resistance tends to decrease. Further, in the case of preparing a water-based electrode paste, because unwanted heterologous crystals are P ₂ O ₅ network is cut occurs, the cycle characteristics are easily lowered.

The content of B ₂ O ₃ is preferably 0% to 75%, more preferably 5% to 75%, still more preferably 7% to 60%, still more preferably 10% to 50%, and particularly preferably 12% to. It is 40%, most preferably 20% to 35%. If _{the content of B 2} O ₃ is too large, the discharge capacity tends to decrease and the chemical durability tends to decrease.

The negative electrode active material precursor powder is preferably one in which an amorphous phase is formed together with the negative electrode active material crystals by firing. By forming the amorphous phase, the sodium ion conductivity in the negative electrode layer and at the interface between the negative electrode layer and the solid electrolyte sheet can be improved.

The average particle size of the negative electrode active material precursor powder is preferably 0.01 μm to 15 μm, more preferably 0.05 μm to 12 μm, and even more preferably 0.1 μm to 10 μm. If the average particle size of the negative electrode active material precursor powder is too small, the cohesive force between the negative electrode active material precursor powders becomes strong, and the dispersibility tends to be poor when made into a paste. As a result, the internal resistance of the battery becomes high and the operating voltage tends to decrease. In addition, the electrode density tends to decrease, and the capacity per unit volume of the battery tends to decrease. On the other hand, if the average particle size of the negative electrode active material precursor powder is too large, sodium ions are difficult to diffuse and the internal resistance tends to increase. In addition, the surface smoothness of the electrode tends to be inferior.

In the present invention, the average particle size means D50 (volume-based average particle size) and refers to a value measured by a laser diffraction / scattering method.

The thickness of the negative electrode layer is preferably in the range of 0.3 μm to 300 μm, and more preferably in the range of 3 μm to 150 μm. If the thickness of the negative electrode layer is too thin, the absolute capacity (mAh) of the negative electrode tends to decrease. If the thickness of the negative electrode layer is too thick, the resistance tends to increase and the capacity (mAh / g) tends to decrease.

The negative electrode layer may contain a solid electrolyte powder, a conductive auxiliary agent, a binder and the like. By containing the solid electrolyte powder to form a negative electrode mixture, the contact interface between the negative electrode active material and the solid electrolyte powder is increased, and it becomes easier to occlude and release sodium ions during charging and discharging, and as a result, the rate characteristics are further improved. Can be improved.

As the solid electrolyte powder, powder of the same material as the first and second solid electrolytes in the above-mentioned solid electrolyte sheet 1 can be used. The average particle size of the solid electrolyte powder is preferably 0.01 μm to 15 μm, more preferably 0.05 μm to 10 μm, and even more preferably 0.1 μm to 5 μm.

If the average particle size of the solid electrolyte powder is too large, the distance required for sodium ion conduction becomes long and the sodium ion conductivity tends to decrease. Also, the sodium ion conduction path between the negative electrode active material powder and the solid electrolyte powder tends to decrease. As a result, the discharge capacity tends to decrease. On the other hand, if the average particle size of the solid electrolyte powder is too small, deterioration due to elution of sodium ions and reaction with carbon dioxide gas occurs, and sodium ion conductivity tends to decrease. In addition, since voids are likely to be formed, the electrode density is also likely to decrease. As a result, the discharge capacity tends to decrease.

The volume ratio of the negative electrode active material precursor powder to the solid electrolyte powder is preferably 20:80 to 95: 5, more preferably 30:70 to 90:10, and even more preferably 35:65 to 88:12.

Examples of the conductive auxiliary agent include carbon powder and the like. By including the conductive auxiliary agent, the internal resistance of the negative electrode layer can be reduced. The conductive auxiliary agent is preferably contained in the negative electrode layer in an amount of 0% by mass to 20% by mass, and more preferably in a proportion of 1% by mass to 10% by mass.

As the binder, polypropylene carbonate (PPC), which decomposes at low temperature in an inert atmosphere, is preferable. Further, carboxymethyl cellulose (CMC) having excellent ionic conductivity is preferable.

Hereinafter, the present invention will be described based on examples, but the present invention is not limited to these examples.

(Example 1)
(A) Preparation of slurry for the first green sheet Using sodium carbonate (Na ₂ CO ₃ ), aluminum oxide (Al ₂ O ₃ ) and magnesium oxide (Mg O) as raw materials, in mol%, Na ₂ O: 13.0 %, Al ₂ O ₃ : 80.2%, MgO: 6.8%, the raw material powder was prepared. The raw material powder was calcined at 1250 ° C. for 4 hours, then crushed and classified (the calcined raw material). Next, the raw material powder, an acrylic ester-based copolymer (manufactured by Kyoeisha Chemical Co., Ltd., "Oricox 1700") as a binder, and benzyl butyl phthalate as a plasticizer were used, and the raw material powder: binder: plasticizer = 83.5. Weighed to a ratio of 14: 2.5 (mass ratio), dispersed in N-methylpyrrolidone, and then sufficiently stirred with a rotation / revolution mixer to form a slurry to prepare a slurry for the first green sheet. did.

(B) Preparation of Solid Electrolyte Powder for Second Green Sheet (Restricting Green Sheet) A compact was prepared by filling a Φ30 mm press die with a calcining raw material and pressing at 40 MPa. Next, this green compact was fired at 1600 ° C. for 30 minutes to obtain a β "-alumina sintered body. By crushing and classifying this sintered body, a solid electrolyte powder of D50: 2.0 μm was obtained. Obtained.

(C) Preparation of Slurry for Second Green Sheet (Restricting Green Sheet) In the same manner as described above, the obtained solid electrolyte powder is used as a solid electrolyte powder: binder: plasticizer = 83.5: 14: 2.5 ( Weighed so as to have a mass ratio), dispersed in N-methylpyrrolidone, and then sufficiently stirred with a rotating / revolving mixer to form a slurry to prepare a slurry for a second green sheet.

(D) Preparation of First Green Sheet and Second Green Sheet By applying the slurry for the first green sheet obtained above using a doctor blade on a PET film and drying at 70 ° C. Obtained the first green sheet. Further, the slurry for the second green sheet obtained above was applied onto the PET film using a doctor blade and dried at 70 ° C. to obtain a second green sheet.

(E) Laminated green sheet The obtained first green sheet and the second green sheet are each cut into 15 mm squares, and the second green sheet is superposed on both main surfaces of the first green sheet. A laminate was obtained by pressing at 90 ° C. and 40 MPa for 5 minutes using an isotropic pressure press device. The laminated body after pressing was placed on an MgO setter, calcined at 500 ° C. for 1 hour for debindering, and further calcined at 1600 ° C. for 30 minutes.

(F) Removal of Restraint Layer The solid electrolyte powder laminated on the surface as the restraint layer was blown off with high-pressure air and removed to obtain a solid electrolyte sheet. The solid electrolyte powder derived from the restraint layer was attached to the surface of the obtained solid electrolyte sheet.

(G) Measurement of Surface Roughness Ra The surface roughness Ra of the obtained solid electrolyte sheet was measured by JIS B 0633-2001 (ISO) using a contact type surface shape measuring instrument (manufactured by Kosaka Laboratory, product number "ET4000A"). It was determined by measuring by the method described in the procedure of 4288-1966).

(H) Preparation of all-solid sodium ion secondary battery (h-1) Preparation of positive electrode active material crystal precursor powder Sodium metaphosphate (NaPO ₃ ), ferric oxide (Fe ₂ O ₃ ) and orthophosphoric acid (H ₃₎ Using PO ₄ ) as a raw material, the raw material powder was prepared so as to have _{Na 2} O: 40%, Fe ₂ O ₃ : 20%, and P ₂ O _{5: 40% in mol%, and at 1250 ° C. for 45 minutes.} Melting was performed in the air atmosphere. Then, the molten glass was poured into a pair of rolls and molded into a film while being rapidly cooled to prepare a positive electrode active material crystal precursor.

The obtained positive electrode active material crystal precursor _{was pulverized by a ball mill using a ZrO 2} ball stone having a diameter of 20 mm for 5 hours and passed through a resin sieve having an opening of 120 μm to obtain a coarse glass powder having an average particle diameter of 3 to 15 μm. .. Next, the crude glass powder was pulverized by a ball mill using ethanol as a pulverizing aid and a ZrO ₂ ball stone having a diameter of 3 mm for 80 hours to obtain a positive electrode active material crystal precursor powder having an average particle diameter of 0.7 μm. It was.

In order to confirm the precipitated active material crystals, after sufficiently mixing the obtained positive electrode active material crystal precursor powder: 93% and acetylene black (manufactured by TIMCAL, “SUPER C65”): 7% in mass%. , Heat treatment was performed at 550 ° C. for 1 hour in a mixed gas atmosphere of nitrogen and hydrogen (nitrogen: 96% by volume, hydrogen: 4% by volume). When the powder X-ray diffraction pattern of the powder after the heat treatment was confirmed, diffraction lines derived from _{triclinic crystals (Na 2} FeP ₂ O _{7) belonging to the space group P-1 were confirmed.} The powder X-ray diffraction pattern was measured using an X-ray diffractometer (RIGAKU, "RINT2000").

(H-2) Preparation of test battery The positive electrode active material crystal precursor powder: 72%, solid electrolyte powder: 25%, acetylene black (manufactured by TIMCAL, "SUPER C65"): 3% by mass%. Weighed and mixed for 30 minutes using an agate mortar and pestle. To 100 parts by mass of the mixed powder, 20 parts by mass of N-methylpyrrolidone containing 10% by mass of polypropylene carbonate (manufactured by Sumitomo Seika Chemical Co., Ltd.) was added, and the mixture was sufficiently stirred using a rotation / revolution mixer to form a slurry. did. All of the above operations were performed in an environment with a dew point of −40 ° C. or lower.

A slurry was applied to the above-mentioned solid electrolyte sheet having a masking seal having an opening of 10 mm square and a thickness of 100 μm attached to the surface using a squeegee, and dried at 70 ° C. for 3 hours. Next, a positive electrode layer was formed on one surface of the solid electrolyte sheet by firing at 550 ° C. for 1 hour in a mixed gas atmosphere of nitrogen and hydrogen (96% by volume of nitrogen, 4% by volume of hydrogen). When the X-ray diffraction pattern of the obtained positive electrode layer was confirmed, it was found that the triclinic crystal system (Na ₂ FeP ₂ O7) belonging to the space group P-1 which is an active material crystal and the space group R which is a sodium ion conductive crystal. Diffraction lines derived from triclinic crystals belonging to -3 m (β "-alumina [(Al _10.32 Mg _0.68 O ₁₆ ) (Na _1.68 O)]) were confirmed.

A current collector composed of a gold electrode having a thickness of 300 nm was formed on the surface of the positive electrode layer using a sputtering device (“SC-701AT” manufactured by Sanyu Electronics Co., Ltd.). Then, in an argon atmosphere with a dew point of -60 ° C. or lower, the counter electrode metal sodium was crimped to the surface opposite to the layer on which the positive electrode layer of the solid electrolyte sheet was formed, and placed on the lower lid of the coin cell. A CR2032 type test battery was produced by covering it with an upper lid.

(H-3) Charge / Discharge Test A charge / discharge test was performed at 30 ° C. using the obtained test battery, and the discharge capacity was measured. The results are shown in Table 1. In the charge / discharge test, charging (release of sodium ions from the positive electrode active material) is performed by CC (constant current) charging from the open circuit voltage (OCV) to 4.3 V, and discharge (storage of sodium ions in the positive electrode active material). Was carried out by CC discharge from 4.3V to 2V. The C rate was 0.01 C.

The average voltage and discharge capacity in Table 1 mean the initial average voltage and discharge capacity evaluated at a C rate of 0.01C, and the rapid charge / discharge characteristics are 0. The ratio of the initial discharge capacity of 1C ((0.1C / 0.01C) × 100 (%)) is shown respectively.

(Comparative Example 1)
In Comparative Example 1, a green sheet made of α-alumina powder was used instead of the restraining green sheet (second green sheet) of Example 1. Specifically, α-alumina powder having an average particle size of 0.4 μm is weighed so that α-alumina: binder: plasticizer = 84: 14: 2 (mass ratio) and dispersed in N-methylpyrrolidone. After that, the mixture was sufficiently stirred with a rotating / revolving mixer to form a slurry, and a green sheet made of α-alumina powder having a thickness of 120 μm was prepared by the doctor blade method. Next, the obtained green sheet made of α-alumina powder was superposed on both main surfaces of the same green sheet for the base material (first green sheet) as in Example 1, and an isotropic pressure press device was installed. It was pressed at 90 ° C. and 40 MPa for 5 minutes to obtain a laminate. In other respects, a solid electrolyte sheet was obtained in the same manner as in Example 1. Moreover, the obtained solid electrolyte sheet was evaluated in the same manner as in Example 1.

The results are shown in Table 1 below.

As shown in Table 1, in Example 1, the restraint layer suppressed the evaporation of alkali metal ions, and good results were obtained in the charge / discharge test. On the other hand, in Comparative Example 1, the battery did not work. It is considered that this is because the sodium ions evaporate when the green sheet is fired to form a sodium ion-deficient layer in which no sodium ions are present on the surface, thereby reducing the ionic conductivity. It is also considered that this is because α-alumina adheres to the surface of the solid electrolyte sheet obtained by firing and a sodium ion-deficient layer of α-alumina is formed, thereby reducing the ionic conductivity.

1 ... Solid electrolyte sheet 2a ... First main surface 2b ... Second main surface 2 ... First green sheet 2A ... First green sheet after firing 3 ... First restraint layer 3A ... First after firing Restraint layer 4 ... Second restraint layer 4A ... Second restraint layer after firing 5 ... Raw material powder of first solid electrolyte 6 ... Powder of second solid electrolyte 10 ... Laminated body

Claims (10)

A preparation step of preparing a first green sheet by applying a slurry of a raw material powder of a first solid electrolyte composed of a sodium ion conductive oxide onto a base material and drying it.
A laminated body in which a restraining layer containing a powder of a second solid electrolyte composed of a sodium ion conductive oxide is laminated on at least one main surface of the first green sheet and pressure-bonded to form a laminated body. The formation process and
A firing step of firing the laminate and
A removal step of removing all or a part of the restraining layer after firing from the laminated body after firing.
A method for producing a solid electrolyte sheet. The method for producing a solid electrolyte sheet according to claim 1, wherein the restraining layer is a second green sheet prepared by applying a slurry containing the powder of the second solid electrolyte onto a substrate and drying the slurry. .. The method for producing a solid electrolyte sheet according to claim 1 or 2, wherein the raw material powder of the first solid electrolyte is calcined before the preparation step. The solid electrolyte sheet according to any one of claims 1 to 3, wherein in the laminate forming step, the restraint layer is laminated and pressure-bonded on the main surfaces on both sides of the first green sheet, respectively. Production method. The method for producing a solid electrolyte sheet according to any one of claims 1 to 4, wherein in the laminate forming step, the pressure bonding between the first green sheet and the restraint layer is performed by an isotropic pressure press. The production of the solid electrolyte sheet according to any one of claims 1 to 5, wherein in the removing step, a portion of the restraint layer after firing that is not attached to the first green sheet after firing is removed. Method. The method for producing a solid electrolyte sheet according to any one of claims 1 to 6, wherein the first solid electrolyte and the second solid electrolyte have the same composition. The invention according to any one of claims 1 to 7, wherein the first solid electrolyte and the second solid electrolyte contain at least one of β-alumina, β "-alumina and NASICON type crystals, respectively. A method for producing a solid electrolyte sheet. The method for producing a solid electrolyte sheet according to any one of claims 1 to 8, wherein the thickness of the solid electrolyte sheet is 500 μm or less. The method for producing a solid electrolyte sheet according to any one of claims 1 to 9, wherein the solid electrolyte sheet is used in an all-solid-state sodium ion secondary battery.

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