JP2022100362A - Electrode-attached solid electrolyte sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electrode-attached solid electrolyte sheet by which superior battery characteristics can be achieved even with a reduced thickness.

SOLUTION: A solid electrolyte sheet comprises at least one kind of sodium ion-conducting crystal selected from β''-alumina and NASICON crystal, of which the thickness is 500 μm or less and the flatness is 200 μm or less.

SELECTED DRAWING: Figure 1

COPYRIGHT: (C)2022,JPO&INPIT

Description

The present invention relates to a sodium ion conductive crystal-containing solid electrolyte sheet used in a power storage device such as a sodium ion secondary battery.

Lithium-ion secondary batteries have established themselves as a high-capacity, lightweight power source that is indispensable for mobile devices and electric vehicles. However, since a flammable organic electrolytic solution is mainly used as an electrolyte in the current lithium ion secondary battery, there is a concern about the risk of ignition and the like. As a method for solving this problem, a lithium ion all-solid-state battery using a solid electrolyte instead of an organic electrolyte is being developed (see, for example, Patent Document 1).

However, lithium is a concern for soaring prices of raw materials worldwide. Therefore, sodium is attracting attention as an alternative material to lithium, and a sodium ion all-solid-state battery using a sodium ion conductive crystal composed of NASICON type Na ₃ Zr ₂ Si ₂ PO ₁₂ as a solid electrolyte has been proposed (for example). See Patent Document 2). In addition, beta-alumina-based solid electrolytes such as β-alumina and β''-alumina are also known to exhibit high sodium ion conductivity, and these solid electrolytes are also used as solid electrolytes for sodium-sulfur batteries. ..

In an all-solid-state battery, the thinner the solid electrolyte, the lower the ion transfer resistance in the battery and the higher the energy density per volume, which is preferable. Therefore, it is required to reduce the thickness of the solid electrolyte.

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

When the thickness of the solid electrolyte is reduced, the internal resistance of the battery tends to increase, and there is a problem that the battery characteristics such as the discharge capacity and the operating voltage are deteriorated.

Therefore, an object of the present invention is to provide a solid electrolyte sheet with an electrode capable of obtaining excellent battery characteristics even if the thickness is reduced.

The solid electrolyte sheet with electrodes of the present invention contains at least one sodium ion conductive crystal selected from β''-alumina and NASICON crystals, and has a thickness of 500 μm or less and a flatness of 200 μm or less. It is characterized by including a positive electrode layer or a negative electrode layer formed on one surface of the solid electrolyte sheet.

As a result of the investigation by the present inventors, it was found that the increase in the internal resistance of the battery when the thickness of the solid electrolyte is reduced is due to the flatness of the solid electrolyte. Planarity is defined in JIS as "the magnitude of deviation from a geometrically correct plane of a plane shape". FIG. 1 is a schematic cross-sectional view for explaining the flatness of the solid electrolyte sheet. As shown in FIG. 1, the flatness of the solid electrolyte sheet indicates the value of the gap formed when one surface of the sheet is sandwiched between parallel planes. If the flatness value of the solid electrolyte is large, the electrode material cannot be uniformly applied to the surface of the solid electrolyte, the thickness of the electrode becomes uneven, and there are places where the internal resistance is locally large. Therefore, by reducing the flatness of the solid electrolyte as described above, the thickness of the electrodes can be made uniform, and the internal resistance of the battery can be reduced. As a result, battery characteristics such as discharge capacity and operating voltage can be improved. Further, when the flatness of the solid electrolyte is small, the handleability is improved and the generation of cracks during the production of the battery can be suppressed.

In the solid electrolyte sheet with electrodes of the present invention, the solid electrolyte sheet is mol%, Al _{2O 3} 65 to 95%, Na ₂ O 2 to 20%, MgO + Li ₂ O 0.3 to 15%, ZrO ₂₀ to. It preferably contains ₂₀ % and _Y2O 30-5%. In the present specification, "○ + ○ + ..." Means the total amount of the contents of each of the described components.

In the solid electrolyte sheet with an electrode of the present invention, the solid electrolyte sheet is at least one selected from the general formula _Nas A1 _t A2 _{uO v} (A1 is Al, Y, Yb, Nd, Nb, Ti, Hf and Zr). , A2 is at least one selected from Si and P, s = 1.4 to 5.2, t = 1 to 2.9, u = 2.8 to 4.1, v = 9 to 14). It is preferable to contain the crystals to be formed.

The solid electrolyte sheet with electrodes of the present invention is suitable for an all-solid-state sodium ion secondary battery.

According to the present invention, it is possible to provide a solid electrolyte sheet with an electrode capable of obtaining excellent battery characteristics even if the thickness is reduced.

It is a schematic sectional view for demonstrating the flatness of a solid electrolyte sheet. It is a figure which shows the powder X-ray diffraction pattern of the raw material powder of the solid electrolyte sheet in an Example. It is a figure which shows the powder X-ray diffraction pattern of the raw material powder after the calcination in an Example. It is a figure which shows the powder X-ray diffraction pattern of the solid electrolyte sheet obtained in an Example.

The solid electrolyte sheet of the present invention contains at least one sodium ion conductive crystal selected from β ″ -alumina and NASICON crystals, and is characterized in that the thickness is 500 μm or less and the flatness is 200 μm or less. β ″ -alumina and NASICON crystals are preferable because they have excellent sodium ion conductivity, high electron insulation, and excellent stability.

The smaller the thickness of the solid electrolyte sheet, the shorter the distance required for ionic conduction in the solid electrolyte, and the better the ionic conductivity, which is preferable. Further, when used as a solid electrolyte for an all-solid-state battery, the energy density per unit volume of the all-solid-state battery becomes high. Specifically, the thickness of the solid electrolyte sheet of the present invention is preferably 500 μm or less, preferably 400 μm or less, 300 μm or less, and particularly preferably 200 μm or less. However, if the thickness of the solid electrolyte sheet is too small, the mechanical strength may decrease or the positive and negative electrodes may be short-circuited. Therefore, the thickness may be 5 μm or more, 10 μm or more, 20 μm or more, 30 μm or more, especially 50 μm or more. preferable.

The flatness of the solid electrolyte sheet is 200 μm or less, preferably 150 μm or less, 100 μm or less, and particularly preferably 50 μm or less. If the flatness value is too large, the electrode material cannot be uniformly applied to the surface of the solid electrolyte, the thickness of the electrode becomes uneven, and there are places where the internal resistance is locally large. As a result, battery characteristics such as discharge capacity, operating voltage, and rate characteristics tend to deteriorate. The lower limit of the flatness value is not particularly limited, but in reality, it is 1 μm or more, and further 5 μm or more.

Specific examples of β''-alumina include trigonal (Al _10.35 Mg _0.65 O ₁₆ ) (Na _1.65 O) and (Al _8.87 Mg _2.13 O ₁₆ ) (Na _{3. 13} O), Na _1.67 Mg _0.67 Al _10.33 O ₁₇ , Na _1.49 Li _0.25 Al _10.75 O ₁₇ , Na _1.72 Li _0.3 Al _10.66 O ₁₇ , Na _1.6 Li _0.34 Al _10.66 O ₁₇ can be mentioned. In addition to β''-alumina, β-alumina may be contained. As β-alumina, 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 Examples thereof include ₁₁ O ₁₇ and (Al _10.32 Mg _0.68 O ₁₆ ) (Na _1.68 O).

When the solid electrolyte sheet of the present invention contains β''-alumina, the specific composition is, in mol%, Al ₂ O ₃ 65 to 98%, Na ₂ O 2 to 20%, MgO + Li ₂ O 0. Examples thereof include those containing 3 to 15%, ZrO ₂₀ to 20%, and _{Y2O 30} to 5%. The reason for limiting the composition as described above will be described below.

Al ₂ O ₃ is the main component constituting β''-alumina. The content of Al ₂ O ₃ is particularly preferably 65 to 98%, particularly 70 to 95%. If the amount of Al ₂ O ₃ is too small, the ionic conductivity of the solid electrolyte 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 ion conductivity of the solid electrolyte tends to decrease.

Na ₂ O is a component that imparts sodium ion conductivity to the solid electrolyte. The Na ₂ O content is preferably 2 to 20%, 3 to 18%, and particularly 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 ionic conductivity, so that the ionic conductivity tends to decrease.

MgO and Li _2O are components (stabilizers) that stabilize the structure of β''-alumina. The content of MgO + Li _2O is preferably 0.3 to 15%, 0.5 to 10%, and particularly preferably 0.8 to 8%. If the amount of MgO + Li _2O is too small, α-alumina remains in the solid electrolyte and the ionic conductivity tends to decrease. On the other hand, if the amount of MgO + Li _2O is too large, MgO or Li _2O that did not function as a stabilizer remains in the solid electrolyte, and the ionic conductivity tends to decrease.

ZrO ₂ and Y ₂ O ₃ have the effect of suppressing the abnormal grain growth of β''-alumina during firing and improving the adhesion of each particle of β''-alumina. As a result, the ionic conductivity of the solid electrolyte sheet is likely to be improved. The content of ZrO ₂ is preferably 0 to 15%, 1 to 13%, and particularly preferably 2 to 10%. The content of Y ₂ O ₃ is preferably 0 to 5%, 0.01 to 4%, and particularly preferably 0.02 to 3%. If the amount of ZrO ₂ or Y ₂ O ₃ is too large, the amount of β ″ -alumina produced decreases, and the ionic conductivity of the solid electrolyte tends to decrease.

As the NASICON crystal, the general formula Na _s A1 _t A2 _{uO v} (A1 is at least one selected from Al, Y, Yb, Nd, Nb, Ti, Hf and Zr, and A2 is selected from Si and P. It is preferably composed of at least one compound represented by s = 1.4 to 5.2, t = 1 to 2.9, u = 2.8 to 4.1, v = 9 to 14). Here, A1 is preferably at least one selected from Y, Nb, Ti and Zr. By doing so, a crystal having excellent ionic conductivity can be obtained.

The preferable range of each coefficient in the above general formula is as follows.

The s is preferably 1.4 to 5.2, 2.5 to 3.5, and particularly preferably 2.8 to 3.1. If s is too small, the number of sodium ions is small, so that the ion conductivity tends to decrease. On the other hand, if s is too large, excess sodium forms compounds such as sodium phosphate and sodium silicate that do not contribute to ionic conduction, so that ionic conductivity tends to decrease.

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

u is preferably 2.8 to 4.1, 2.8 to 4, 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 is reduced, so that the ionic conductivity tends to decrease. On the other hand, if u is too large, crystals that do not contribute to ionic conduction are formed, so that ionic conductivity tends to decrease.

v is preferably 9 to 14, 9.5 to 12, particularly preferably 11 to 12. If v is too small, A1 (for example, an aluminum component) has a low valence, so that the electrical insulating property tends to decrease. On the other hand, if v is too large, the sodium ion is bound from the lone electron pair of the oxygen atom in a peroxidized state, so that the ionic conductivity tends to decrease.

The NASICON crystal is preferable if it is a monoclinic crystal, a hexagonal crystal or a trigonal crystal, particularly a monoclinic crystal or a trigonal crystal because it has excellent ionic conductivity.

Specific examples of NASICON crystals include Na ₃ Zr ₂ Si ₂ PO ₁₂ , Na _3.2 Zr _1.3 Si _2.2 P _0.8 O _10.5 , and 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 Crystals such as Si ₂ PO ₁₂ , Na _3.6 Zr _0.13 Yb _1.67 Si _0.11 P _2.9 O ₁₂ , Na ₅ YSi ₄ O ₁₂ and the like can be mentioned. In particular, Na _3.12 Zr _1.88 Y _0.12 Si ₂ PO ₁₂ is preferable because it has excellent sodium ion conductivity.

Next, the method for producing the solid electrolyte sheet of the present invention will be described. The solid electrolyte sheet of the present invention has (a) a step of tentatively firing the raw material powder, (b) a step of making the raw material powder after the tentative firing into a slurry, and (c) applying the slurry onto a substrate and drying it. It can be produced by a method including a step of obtaining a green sheet and (d) a step of producing sodium ion conductive crystals by firing the green sheet.

When the solid electrolyte sheet contains β''-alumina, the raw material powder contains Al ₂ O ₃ as a main component. Specifically, the raw material powder is mol%, Al ₂ O ₃ 65 to 98%, Na ₂ O 2 to 20%, MgO + Li ₂ O 0.3 to 15%, ZrO ₂₀ to 20%, Y ₂ O. It preferably contains ₃₀ to 5%. Since the reason for limiting the composition in this way is as described above, the explanation is omitted.

When the solid electrolyte sheet contains NASICON crystals, the raw material powder is mol%, Na ₂ O 17.5 to 50%, Al ₂ O ₃ + Y ₂ O ₃ + Yb ₂ O ₃ + Nd ₂ O ₃ + Nb ₂ O ₅ + TIO. It preferably contains ₂ + HfO ₂ + ZrO ₂ 12 to 45% and SiO ₂ + P ₂ O ₅ 24 to 54%. By limiting the composition in this way, it becomes possible to precipitate desired NASICON crystals.

The average particle size (D ₅₀ ) of the raw material powder is preferably 10 μm or less. If the average particle size of the raw material powder is too large, the contact area between the raw material powders decreases, and it becomes difficult for the solid phase reaction to proceed sufficiently. In addition, it tends to be difficult to reduce the thickness of the solid electrolyte sheet. The lower limit of the average particle size of the raw material powder is not particularly limited, but is practically 0.1 μm or more.

Preliminary firing of the raw material powder produces a composite oxide (eg β-alumina) as a precursor, and subsequent main firing changes the composite oxide into sodium ion conductive crystals (eg β-alumina β'. By (phase transition to alumina), shrinkage during the main firing can be suppressed, and deterioration of the flatness of the solid electrolyte sheet can be suppressed. In addition, by temporarily firing the raw material powder in advance, the volume shrinkage of the green sheet is reduced because the release of carbonic acid gas from the carbonate raw material and the volatilization of sodium components are reduced during the subsequent main firing. It is possible to suppress deterioration of flatness and generation of cracks in the solid electrolyte sheet. When the solid electrolyte sheet contains β ″ -alumina, the temperature of the preliminary firing is preferably less than 1000 to 1400 ° C, 1100 to 1350 ° C, and particularly preferably 1200 to 1300 ° C. When the solid electrolyte sheet contains NASICON crystals, the temperature of the calcination is preferably 900 to less than 1200 ° C., 1000 to 1180 ° C., particularly preferably 1050 to 1160 ° C. If the temperature of the tentative firing is too low, it becomes difficult to obtain the above effect. On the other hand, if the temperature of the temporary firing is too high, it becomes difficult to sinter during the main firing, so that the solid electrolyte sheet becomes difficult to become dense.

The time for calcination may be appropriately adjusted so that the above effect can be obtained. Specifically, the time for temporary firing is preferably 1 to 20 hours, 2 to 18 hours, 2 to 15 hours, 2 to 10 hours, and particularly preferably 3 to 8 hours. When the raw material powder is aggregated by the temporary firing, it is preferable to pulverize the raw material powder so as to have a desired particle size.

In addition to β-alumina, β ″ -alumina may be produced by calcination. Alternatively, other composite oxides may be produced. Examples of such a composite oxide include the following.

As hexagonal crystals, NaAl 5O ₈ , _NaAl 7O ₁₁ , _{NaAl 5.9 O 9.4} , Na ₂ Al ₂₂ O ₃₄ , Na _2.58 Al _21.81 O ₃₄ , NaAl ₂₃ O ₃₅ , Na ₂ Al. ₂₂ O ₃₃ , Na _1.5 Al _10.83 O ₁₇ , Na _1.22 Al ₁₁ O _17.11 , Na _2.74 Al ₂₂ O ₃₈ , Na ₂ Li _0.35 Al _12.2 O _19.475 , Na _0.45 Li _0.57 Al ₁₁ O ₁₇ , Na _0.47 Li _0.75 Al ₁₁ O _17.11 , Namg ₂ Al ₁₅ O ₂₅ , Na ₂ MgAl ₁₀ O ₁₇ , Na ₂ Mg ₄ Al ₃₀ O ₅₀ , Na _0.47 Mg _0.75 Al ₁₁ O _17.11 .

Examples of the trigonal crystal include Na _1.77 Al ₁₁ O ₁₇ , Na _1.71 Al ₁₁ O ₁₇ , and Na ₂ MgAl ₁₀ O ₁₇ .

Examples of the tetragonal crystal include Na ₂ Al ₂ O ₄ .

Examples of the cubic crystal include Na ₂ Al ₂ O ₄ .

Examples of the orthorhombic crystal include NaAl ₆ O _9.5 , Na _0.67 Al ₆ O _9.33 , and Na ₅ AlO ₄ .

Examples of the monoclinic crystal include Na ₁₇ Al ₅ O ₁₆ and Na ₁₄ Al ₄ O ₁₃ .

Examples of the triclinic crystal include Na ₇ Al ₃ O ₈ .

When the solid electrolyte sheet contains NASICON crystals, the following may be mentioned as the composite oxide as a precursor.

Cubic crystals include Na ₂ ZrO ₃ , (ZrO ₂ ) _0.92 (Na ₂ O) _0.04 , (ZrO ₂ ) _0.95 (Na ₂ O) _0.025 , Na _2.47 Zr _0.13 . PO ₄ can be mentioned.

Examples of the hexagonal crystal include Na ₂ ZrO ₃ .

As monoclinic crystals, Na ₆ Si ₈ O ₁₉ , Na ₆ Si ₂ O ₇ , Na ₆ Si ₈ O ₁₉ , Na ₂ Si ₃ O ₇ , Na ₂ Si (Si ₃ O ₉ ), Na ₂ ZrO ₃ , NaZr ₅ (PO ₄ ) ₇ is mentioned.

Examples of the orthorhombic crystal include (NaPO ₃ ) ₄ , Na ₃ P ₃ O ₉ , Na ₂ Si ₄ O ₉ , and Na ₁₄ Zr ₂ Si ₁₀ O ₃₁ .

Examples of the triclinic crystal include Na ₄ SiO ₄ .

As trigonal crystals, Na _1.3 Zr _1.832 (PO ₄ ) ₃ , Na ₈ Si (Si ₆ O ₁₈ ), NaZr ₂ (PO ₄ ) ₃ , Na ₅ Zr (PO ₄ ) ₃ , NaZr _1.88 ( PO ₄ ) ₃ can be mentioned.

In addition, Na ₄ P ₂ O ₇ , Na (PO ₃ ) ₃ , NaPO ₃ , Na ₃ PO ₄ , Na ₅ P ₃ O ₁₀ , Na ₄ P ₂ O ₇ , (NaPO ₃ ) ₆ , Na ₄ P ₂ O ₆ , Na ₂ SiO ₃ , Na ₂ Si ₄ O ₉ , Na ₂ Si ₃ O ₇ , Na ₂ Si ₂ O ₅ , Na ₂ ZrSiO ₅ , Na ₁₄ Zr ₂ Si ₁₀ O ₃₁ , Na _2.8 Zr _6.5 Si ₂ O _17.8 can be mentioned.

A binder, a plasticizer, a solvent, etc. are added to the raw material powder after calcination and kneaded to form a slurry.

The solvent may be water or an organic solvent such as ethanol or acetone. However, when water is used as the solvent, the sodium component may elute from the raw material powder, the pH of the slurry may rise, and the raw material powder may aggregate. Therefore, it is preferable to use an organic solvent.

Next, the obtained slurry is applied onto a substrate such as PET (polyethylene terephthalate) and dried to obtain a green sheet. The slurry can be applied with a doctor blade, a die coater, or the like. The thickness of the green sheet is preferably 0.01 to 1 mm, 0.02 to 1 mm, and particularly preferably 0.05 to 0.9 mm. If the thickness of the green sheet is too small, the mechanical strength of the solid electrolyte sheet may decrease, or the positive electrode and the negative electrode may be short-circuited. On the other hand, if the thickness of the green sheet is too large, the thickness of the solid electrolyte sheet becomes large, the distance required for ion conduction in the solid electrolyte sheet becomes long, and the energy density per unit cell tends to decrease.

Further, by firing the green sheet, β ″ -alumina is generated to obtain a solid electrolyte sheet. Specifically, β-alumina undergoes a phase change to β ″ -alumina, which has excellent ionic conductivity, by firing. Alternatively, by firing the green sheet, NASICON crystals are generated to obtain a solid electrolyte sheet.

When the solid electrolyte sheet contains β ″ -alumina, the firing temperature is more preferably 1400 ° C. or higher, 1450 ° C. or higher, and particularly preferably 1500 ° C. or higher. If the firing temperature is too low, the sintering of each particle of β ″ -alumina becomes insufficient in the solid electrolyte sheet, and it is difficult to form a dense sheet, so that the ionic conductivity tends to decrease. In addition, the phase change from β-alumina to β ″ -alumina is unlikely to occur, and the ionic conductivity tends to decrease. On the other hand, the upper limit of the firing temperature is preferably 1750 ° C. or lower, particularly preferably 1700 ° C. or lower. If the calcination temperature is too high, the amount of evaporation of sodium components and the like increases, and dissimilar crystals tend to precipitate or the denseness tends to decrease. As a result, the ionic conductivity of the solid electrolyte sheet tends to decrease. The firing time is appropriately adjusted so that the produced β ″ -alumina is sufficiently sintered. Specifically, it is preferably 10 to 120 minutes, particularly 20 to 80 minutes.

When the solid electrolyte sheet contains NASICON crystals, the firing temperature is more preferably 1200 ° C. or higher, particularly preferably 1210 ° C. or higher. If the firing temperature is too low, a dense solid electrolyte sheet cannot be obtained and the ionic conductivity tends to decrease. In addition, NASICON crystals are less likely to precipitate, and ionic conductivity tends to decrease. On the other hand, the upper limit of the firing temperature is preferably 1400 ° C. or lower, particularly preferably 1300 ° C. or lower. If the calcination temperature is too high, the amount of evaporation of sodium components and the like increases, and dissimilar crystals tend to precipitate or the denseness tends to decrease. As a result, the ionic conductivity of the solid electrolyte sheet tends to decrease. The firing time is appropriately adjusted so that a dense sintered body can be obtained.

By performing a press treatment such as an isotropic pressure press before firing the green sheet, the adhesion of each particle such as β''-alumina and NASICON crystals in the fired solid electrolyte sheet is improved, and ion conduction is achieved. It becomes easier to improve the degree.

Further, it is preferable that the green sheet is installed between the upper setter and the lower setter, and firing is performed with a gap between the green sheet and the upper setter. By doing so, it is possible to further suppress deterioration of flatness and generation of cracks due to shrinkage of the green sheet during firing. The gap between the green sheet and the upper setter is preferably 1 to 500 μm, 2 to 400 μm, and particularly preferably 5 to 300 μm. If the gap is too small, it becomes difficult to obtain the above effect. On the other hand, if the gap is too large, waviness due to shrinkage of the green sheet during firing tends to occur, and the flatness value of the solid electrolyte sheet tends to increase.

The solid electrolyte sheet of the present invention is suitable for a sodium ion all-solid-state secondary battery. The sodium ion all-solid-state secondary battery has a positive electrode layer formed on one surface of the solid electrolyte sheet of the present invention and a negative electrode layer formed on the other surface. The positive electrode layer and the negative electrode layer contain an active material. The active material acts as a positive electrode active material or a negative electrode active material, and can occlude and release sodium ions during charging and discharging.

Examples of the positive electrode active material include layered sodium transition metal oxide crystals such as NaCrO ₂ , Na _0.7 MnO ₂ , NaFe _0.2 Mn _0.4 Ni _0.4 O ₂ , Na ₂ FeP ₂ O ₇ , and NaFePO ₄ . Sodium transition metal phosphate containing Na, M (M is at least one transition metal element selected from Cr, Fe, Mn, Co and Ni), P, O, such as Na ₃ V ₂ (PO ₄ ) ₃ . Examples include active material crystals such as crystals.

In particular, crystals containing Na, M, P and O are preferable because they have a high capacity and excellent chemical stability. Among them, the triclinic crystals belonging to the space group P1 or P-1 are particularly represented by the general formula Na _x MyP ₂ O ₇ (1.20 ≦ x ≦ 2.80, 0.95 ≦ y ≦ 1.60). The crystals to be formed are preferable because they have excellent cycle characteristics.

Examples of the negative electrode active material include active material crystals such as a crystal containing at least one selected from Nb and Ti and O, and at least one metal crystal selected from Sn, Bi and Sb.

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, the crystal containing at least one selected from Nb and Ti and O is a monoclinic crystal, a hexagonal crystal, a cubic crystal or a monoclinic crystal, particularly a monoclinic crystal belonging to the space group P21 / m. A cubic crystal is preferable because the capacity does not easily decrease even when charged and discharged with a large current. The orthoclinic crystals include NaTi ₂ O ₄ , and the hexagonal crystals include Na ₂ TiO ₃ , NaTi ₈ O ₁₃ , NaTIO ₂ , LiNbO ₃ , LiNbO ₂ , Li ₇ NbO ₆ , LiNbO ₂ , and Li. ₂ Ti ₃ O ₇ and the like are Na ₂ TiO ₃ , NaNbO ₃ , Li ₄ Ti ₅ O ₁₂ , Li ₃ NbO ₄ and the like as cubic crystals, and Na ₂ Ti ₆ O as a monoclinic crystal. ₁₃ , NaTi ₂ O ₄ , Na ₂ TIO ₃ , Na ₄ Ti ₅ O ₁₂ , Na ₂ Ti ₄ O ₉ , Na ₂ Ti ₉ O ₁₉ , Na ₂ Ti ₃ 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 , LiNb ₃ O ₈ and the like are Na ₂ Ti ₃ O ₇ as monoclinic crystals belonging to the space group P21 / m. And so on.

The crystal containing at least one selected from Nb and Ti and O is preferably further contained 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 active material crystals and improving the sodium ion conductivity.

In addition, glass containing at least one metal crystal selected from Sn, Bi and Sb, or 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.

The positive electrode layer and the negative electrode layer may be an electrode mixture layer made of a composite of an active material and a solid electrolyte. The solid electrolyte acts as a sodium ion conduction path in the electrode mixture, and can improve the discharge capacity and voltage of the battery.

As the solid electrolyte, a powder of the above-mentioned solid electrolyte sheet can be used.

It is preferable that the positive electrode layer and the negative electrode layer further contain a conductive auxiliary agent. The conductive auxiliary agent is a component added to achieve high capacity and high rate of the electrode. Specific examples of the conductive auxiliary agent include highly conductive carbon black such as acetylene black and ketjen black, graphite, coke and the like, and metal powder such as Ni powder, Cu powder and Ag powder. Among them, it is preferable to use any of highly conductive carbon black, Ni powder, and Cu powder, which exhibit excellent conductivity with the addition of a very small amount.

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

Table 1 shows Examples 1 to 6, and Table 2 shows Comparative Examples 1 to 5.

(A-1) Preparation of solid electrolyte sheet A (preparation of slurry)
Made from sodium carbonate (Na ₂ CO ₃ ), aluminum oxide (Al ₂ O ₃ ) and magnesium oxide (MgO), zirconium oxide (ZrO ₂ ), and yttrium oxide (Y ₂ O ₃ ), in mol%, Na ₂ O Raw material powders were prepared so as to be 14.2%, Al ₂ O ₃ 75.4%, Mg O 5.4%, ZrO ₂ 4.9%, and Y ₂ O ₃ 0.1%. FIG. 2 shows a powder X-ray diffraction pattern of the raw material powder. The powder X-ray diffraction pattern was measured using an X-ray diffractometer (RIGKU RINT2000). For Examples 1 to 3 and 5, raw material powders were calcined at 1250 ° C. for 4 hours, crushed and classified. When the powder X-ray diffraction pattern of the raw material powder after tentative firing was confirmed, hexagonal crystals belonging to the space group P63 (β-alumina = (Al _10.35 Mg _0.65 O ₁₆ ) (Na _1.65 O). ) And trigonal crystals belonging to the space group R-3m (β ″ -alumina = (Al _10.35 Mg _0.65 O ₁₆ ) (Na _1.65 O)) were confirmed. FIG. 3 shows a powder X-ray diffraction pattern. Moreover, when the β'' conversion rate was obtained from the X-ray diffraction pattern, it was 54%. The β'' conversion rate was calculated as follows.

β'' conversion rate = Iβ'' / (Iβ + Iβ'') × 100%
Peak intensity of Iβ: β-alumina phase Iβ'': Peak intensity of β''-alumina phase

Iβ is 4.5 times the intensity of the (1,0,7) plane of the β-alumina phase, and Iβ'' is 4.2 times the peak intensity of the (0,2,10) plane of the β''-alumina phase. I used the one that was made. In Comparative Examples 1, 2 and 4, the raw material powder was used as it was without calcining. Next, the raw material powder was wet-mixed for 4 hours using ethanol as a medium. After evaporating ethanol, an acrylic acid ester-based copolymer (Oricox 1700 manufactured by Kyoeisha Chemical Co., Ltd.) was used as a binder, and benzyl butyl phthalate was used as a plasticizer. Raw material powder: Binder: Plasticizer = 83.5: 15: 1. The mixture was weighed to a mass ratio of .5 (mass ratio), dispersed in N-methylpyrrolidone, and then sufficiently stirred with a rotation / revolution mixer to form a slurry.

(Making a green sheet)
The slurry obtained above was applied onto a PET film using a doctor blade and dried at 70 ° C. to obtain a green sheet.

(Pressing and firing of green sheet)
The obtained green sheet was cut into 15 mm squares and pressed at 90 ° C. and 40 MPa for 5 minutes using an isotropic pressure pressing device. The pressed green sheet is placed between the upper setter and the lower setter (both are MgO sheets), and is fired at 1600 ° C. for 30 minutes with a gap of 10 μm between the green sheet and the upper setter. As a result, a solid electrolyte sheet A was obtained. When the powder X-ray diffraction pattern of the solid electrolyte sheet A was confirmed, hexagonal crystals belonging to the space group P63 (β-alumina = (Al _10.35 Mg _0.65 O ₁₆ ) (Na _1.65 O)) and Diffraction lines derived from trigonal crystals (β ″ -alumina = (Al _10.35 Mg _0.65 O ₁₆ ) (Na _1.65 O)) belonging to the space group R-3m were confirmed. And FIG. 4 shows a powder X-ray diffraction pattern. The β'' conversion rate was calculated from the X-ray diffraction pattern and found to be 82%.

(A-2) Preparation of solid electrolyte sheet B (preparation of slurry)
Using sodium carbonate (Na ₂ CO ₃ ), yttrium stabilized zirconia ((ZrO ₂ ) _0.97 (Y ₂ O ₃ ) _0.03 )), silicon dioxide (SiO ₂ ), sodium metaphosphate (NaPO ₃ ) Raw material powder having a composition of Na ₂ O 25.3%, ZrO ₂ 31.6%, Y ₂ O ₃ 1%, SiO ₂ 33.7%, P ₂ O ₅ 8.4% in mol%. Was formulated. For Examples 4 and 6, the raw material powder was calcined at 1100 ° C. for 8 hours, then pulverized and classified. Comparative Examples 3 and 5 were used as they were without calcination of the raw material powder. Next, the raw material powder was wet-mixed for 4 hours using ethanol as a medium. After evaporating ethanol, an acrylic acid ester-based copolymer (Oricox 1700 manufactured by Kyoeisha Chemical Co., Ltd.) was used as a binder, and benzyl butyl phthalate was used as a plasticizer. Raw material powder: Binder: Plasticizer = 83.5: 15: 1. The mixture was weighed to a mass ratio of .5 (mass ratio), dispersed in N-methylpyrrolidone, and then sufficiently stirred with a rotation / revolution mixer to form a slurry.

(Making a green sheet)
The slurry obtained above was applied onto a PET film using a doctor blade and dried at 70 ° C. to obtain a green sheet.

(Pressing and firing of green sheet)
The obtained green sheet was cut into 15 mm squares and pressed at 90 ° C. and 40 MPa for 5 minutes using an isotropic pressure pressing device. The pressed green sheet is placed between the upper setter and the lower setter (both are Pt plates), and is fired at 1220 ° C. for 40 hours with a gap of 10 μm between the green sheet and the upper setter. As a result, a solid electrolyte sheet B was obtained. When the powder X-ray diffraction pattern was confirmed for the solid electrolyte sheet B, NASICON crystals were confirmed.

(B) Measurement of flatness The surface shape of the obtained solid electrolyte sheet in the central portion of 10 mm square was measured using SURFCORDER ET4000AK (manufactured by Kosaka Laboratory Co., Ltd.) under the following conditions. The difference between the maximum value and the minimum value of the height of the surface of the solid electrolyte sheet obtained by the measurement was defined as the flatness.

・ X measurement length: 10 mm
・ X feed speed: 0.1 mm / s
・ Y feed pitch: 200 μm
・ Number of Y lines: 51

(C) Preparation of sodium ion all-solid secondary battery (c-1) Preparation of positive electrode active material crystal precursor powder Sodium metaphosphate (NaPO ₃ ), ferric oxide (Fe ₂ O ₃ ) and orthophosphate (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 in an air atmosphere at 1250 ° C. for 45 minutes. It was melted at. 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 ZrO2 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. rice field.

In order to confirm the precipitated active material crystals, 93% of the obtained positive electrode active material crystal precursor powder and 7% of acetylene black (SUPER C65 manufactured by TIMCAL) were sufficiently mixed in mass%, and then nitrogen and hydrogen were added. Heat treatment was performed at 450 ° C. for 1 hour in a mixed gas atmosphere (96% by volume nitrogen, 4% by volume hydrogen). 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.

(C-2) Preparation of solid electrolyte powder A Sodium carbonate (Na ₂ CO ₃ ), aluminum oxide (Al ₂ O ₃ ) and magnesium oxide (MgO), zirconium oxide (ZrO ₂ ), yttrium oxide (Y ₂ O ₃ ) As a raw material, Na ₂ O 14.2%, Al ₂ O ₃ 75.4%, MgO 5.4%, ZrO ₂ 4.9%, Y ₂ O ₃ 0.1% in mol%. The raw material powder was prepared and baked in an air atmosphere at 1250 ° C. for 4 hours. The powder after firing was pulverized by a ball mill using Al ₂ O ₃ boulders having a diameter of 20 mm for 24 hours. Then, by air classification, a powder having an average particle diameter of D50 of 2.0 μm was obtained. The obtained powder is formed into a cylinder having a diameter of 30 mm at a pressure of 11 MPa using a molding die, and then heat-treated at 1600 ° C. for 30 minutes in an air atmosphere to obtain sodium ion conductive crystals (β ″ -alumina). Got The obtained sodium ion conductive crystals were pulverized using an alumina mortar and pestle and passed through a mesh having an opening of 300 μm. The obtained powder was pulverized at 300 rpm for 30 minutes (rested for 15 minutes every 15 minutes) using a planetary ball mill P6 manufactured by Fritzch, which was charged with ZrO ₂ boulders having a diameter of 5 mm, and passed through a mesh having an opening of 20 μm. Then, by classifying using an air classifier, a solid electrolyte powder A containing β ″ -alumina was obtained. The sodium ion conductive crystals and the sodium ion conductive crystal powder were prepared in an environment with a dew point of −40 ° C. or lower.

(C-3) Preparation of solid electrolyte powder B Sodium carbonate (Na ₂ CO ₃ ), Itria stabilized zirconia ((ZrO ₂ ) _0.97 (Y ₂ O ₃ ) _0.03 )), Silicon dioxide (SiO ₂ ) , Sodium metaphosphate (NaPO ₃ ), in mol%, Na ₂ O 25.3%, ZrO ₂ 31.6%, Y ₂ O ₃ 1%, SiO ₂ 33.7%, P ₂ O ₅ 8 The raw material powder was prepared so as to have a composition of .4%. Next, the raw material powder was wet-mixed for 4 hours using ethanol as a medium. After that, ethanol was evaporated, and the raw material powder was calcined at 1100 ° C. for 8 hours, pulverized, and classified using an air classifier (MDS-3 type manufactured by Nippon Pneumatic Industries Co., Ltd.). The classified powder was molded by a uniaxial press at 11 MPa using a mold having a diameter of 30 mm, and heat-treated at 1220 ° C. for 40 hours to obtain a solid electrolyte containing sodium ion conductive crystals (NASICON crystals).

The obtained solid electrolyte was pulverized using an alumina mortar and pestle and passed through a mesh having an opening of 300 μm. The obtained powder was pulverized at 300 rpm for 30 minutes (rested for 15 minutes every 15 minutes) using a planetary ball mill P6 manufactured by Fritzch, which was charged with ZrO ₂ boulders having a diameter of 5 mm, and passed through a mesh having an opening of 20 μm. Then, by classifying using an air classifier, a solid electrolyte powder B containing sodium ion conductive crystals (NASICON crystals) was obtained. The sodium ion conductive crystals and the sodium ion conductive crystal powder were prepared in an environment with a dew point of −40 ° C. or lower.

(C-4) Preparation of test battery Weighed to be 72% of positive electrode active material crystal precursor powder, 25% of solid electrolyte powder A, and 3% of acetylene black (SUPER C65 manufactured by TIMCAL) by mass%, and manufactured by Menou. Mixing was performed for about 30 minutes using a 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 solid electrolyte sheet A having an opening of 10 mm square and a thickness of 100 μm and having the thickness shown in the table on 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 450 ° 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, the trigonal crystal system crystal (Na ₂ FeP ₂ O ₇ ) belonging to the space group P-1 which is an active material crystal and the space group which is a sodium ion conductive crystal. Hexagonal crystals belonging to P63 (β-alumina = (Al _10.35 Mg _0.65 O ₁₆ ) (Na _1.65 O)) and trigonal crystals belonging to the space group R-3m (β ″ -alumina) = (Al _10.35 Mg _0.65 O ₁₆ ) (Na _1.65 O))-derived diffraction lines were confirmed.

A current collector made 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 manufactured by covering it with an upper lid.

In Example 4 and Comparative Example 3, a test battery was produced in the same manner as above except that the solid electrolyte sheet B having the thickness shown in the table was used instead of the solid electrolyte sheet A. Further, in Example 5 and Comparative Example 4, a test battery was produced in the same manner as described above except that the solid electrolyte powder B was used instead of the solid electrolyte powder A. Further, in Example 6 and Comparative Example 5, the solid electrolyte sheet B having the thickness shown in the table was used instead of the solid electrolyte sheet A, and the solid electrolyte powder B was used instead of the solid electrolyte powder A. A test battery was prepared in the same manner.

(C-5) 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 Tables 1 and 2. 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 0.01C.

The average voltage and discharge capacity in Tables 1 and 2 mean the initial average voltage and discharge capacity evaluated at a C rate of 0.01C, and the rapid charge / discharge characteristics refer to the initial discharge capacity at a C rate of 0.01C. The ratio of the initial discharge capacity of 0.1C ((0.1C / 0.01C) × 100 (%)) is shown respectively.

In Examples 1 to 6, the average voltage is 2.5 to 2.8 V, the discharge capacity is 61 to 73 mAh / g, the rapid charge / discharge characteristics are 64 to 82%, and the energy density is 37.4 to 82.3 mWh / cm ³ . And each characteristic was excellent. On the other hand, in Comparative Examples 1 and 3 to 5, the average voltage was 2.5 V or less, the discharge capacity was 52 mAh / g or less, the rapid charge / discharge characteristics were 46% or less, and the energy density was 32.0 mWh / cm ³ or less, which were inferior. .. In Comparative Example 2, in which the flatness of the solid electrolyte sheet was remarkably large at 487 μm, the battery characteristics could not be measured because the solid electrolyte sheet was cracked during the production of the battery.

The solid electrolyte sheet of the present invention is suitable not only for battery applications such as sodium ion all-solid secondary batteries and sodium-sulfur batteries, but also as solid electrolytes for gas sensors such as CO ₂ sensors and NO ₂ sensors.

Claims (5)

A solid electrolyte sheet containing at least one sodium ion conductive crystal selected from β''-alumina and NASICON crystals, having a thickness of 500 μm or less and a flatness of 200 μm or less.
A solid electrolyte sheet with an electrode, comprising a positive electrode layer or a negative electrode layer formed on one surface of the solid electrolyte sheet. The solid electrolyte sheet with an electrode according to claim 1, further comprising a current collector formed on the surface of the positive electrode layer or the negative electrode layer opposite to the solid electrolyte side. The solid electrolyte sheet is mol%, Al ₂ O ₃ 65 to 95%, Na ₂ O 2 to 20%, MgO + Li ₂ O 0.3 to 15%, ZrO _{20 to 20%, Y 2 O 30} to. The solid electrolyte sheet with an electrode according to claim 1 or 2, wherein the solid electrolyte sheet contains 5%. The solid electrolyte sheet is selected from the general formula _Nas A1 _t A2 _{uO v} (A1 is at least one selected from Al, Y, Yb, Nd, Nb, Ti, Hf and Zr, and A2 is selected from Si and P. It is characterized by containing at least one crystal represented by s = 1.4 to 5.2, t = 1 to 2.9, u = 2.8 to 4.1, v = 9 to 14). The solid electrolyte sheet with an electrode according to claim 1 or 2. The solid electrolyte sheet with an electrode according to any one of claims 1 to 4, which is for an all-solid-state sodium-ion secondary battery.

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