JP2020136130A - Cleaning method for solid electrolyte sheet - Google Patents

Abstract

To provide a cleaning method for solid electrolyte sheet, capable of suppressing breakage and degradation of ion conductivity.SOLUTION: The cleaning method for solid electrolyte sheet includes removing foreign matter adhering to a surface of a solid electrolyte sheet by irradiating it with plasma.SELECTED DRAWING: None

Description

The present invention relates to a method for cleaning a solid electrolyte sheet, which is a component of an all-solid-state battery used in portable electronic devices, electric vehicles, and the like.

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

In addition, since lithium is a concern about the soaring price of raw materials worldwide, sodium is also attracting attention as a material to replace lithium, and as a solid electrolyte, sodium ion conductive crystals composed of NASICON type Na ₃ Zr ₂ Si ₂ PO ₁₂ are used. The sodium ion all-solid-state battery used has been proposed (see, for example, Patent Document 2). In addition, β-alumina (theoretical composition formula: Na ₂ O ・ 11Al ₂ O ₃ ), β ”-alumina (theoretical composition formula: Na ₂ O ・ 5.3 Al ₂ O ₃ ), Li ₂ O stabilized β” -alumina Beta-alumina solids such as (Na _1.7 Li _0.3 Al _10.7 O ₁₇ ) and MgO stabilized β "-alumina ((Al _10.32 Mg _0.68 O ₁₆ ) (Na _1.68 O)). Electrolytes and Na ₅ YSi ₄ O ₁₂ are also known to exhibit high sodium ion conductivity, and these solid electrolytes can also be used for sodium ion all-solid-state batteries.

Although the above solid electrolyte exhibits high ionic conductivity, when stored in the air, it reacts with moisture and carbon dioxide in the air to form a coating of carbonates such as Na ₂ CO ₃ and hydroxides such as NaOH on the surface of the solid electrolyte. There is a problem that interfacial resistance is generated due to the occurrence of carbon dioxide or the adhesion of organic stains, and the battery performance is deteriorated. The most common method for removing surface coatings and organic stains from solid electrolytes is to polish the surface with abrasive paper, etc. In addition, sandblasting or dry ice blasting that sprays sand or dry ice particles onto the surface to clean it. Alternatively, a method of immersing in a cleaning solution such as an acidic aqueous solution or an organic solvent having an adjusted pH has been proposed.

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

In order to improve battery characteristics such as ionic conductivity and energy density, a solid electrolyte molded into a sheet is used. However, since the sheet-shaped solid electrolyte has low mechanical strength, there is a problem that it is easily damaged during cleaning by physical methods such as polishing, sandblasting, and dry ice blasting.

On the other hand, in the cleaning method using an acidic aqueous solution or an organic solvent, the solid electrolyte reacts with the cleaning liquid, the component of the solid electrolyte elutes into the cleaning liquid, or the organic component remains on the surface of the solid electrolyte sheet and forms on the interface resistance layer. There is a risk that the ionic conductivity will decrease due to such factors.

In view of the above, it is an object of the present invention to provide a method for cleaning a solid electrolyte sheet that can suppress breakage and decrease in ionic conductivity.

The method for cleaning the solid electrolyte sheet of the present invention is characterized in that foreign matter adhering to the surface of the solid electrolyte sheet is removed by irradiating with plasma.

According to the present invention, by irradiating the surface of a solid electrolyte sheet with ions contained in plasma and etching the surface, foreign substances such as a film such as carbonate and organic stains can be removed. Alternatively, foreign matter can be removed by a chemical action due to a reaction with an active molecule contained in plasma. That is, according to the present invention, cleaning can be performed without bringing the polishing plate or polishing particles into contact with the surface of the solid electrolyte sheet. Therefore, it is possible to efficiently remove foreign substances such as a carbonate film and organic stains adhering to the surface while suppressing damage such as cracking of the solid electrolyte sheet. In addition, unlike the treatment with a cleaning liquid, plasma can be used for cleaning in a dry process, so that the reaction between the solid electrolyte and the cleaning liquid, the elution of the solid electrolyte component into the cleaning liquid, and the residue caused by the cleaning liquid on the surface of the solid electrolyte sheet. There is an advantage that there is no concern about the generation of things.

In polishing, the object is limited to a flat surface such as a flat plate, and a solid electrolyte sheet having a large surface area (for example, one having a fine surface shape such as an uneven shape or a porous shape) is washed in order to improve battery characteristics. It is difficult. Also, with regard to sandblasting and dry ice blasting, there is a risk that the fine surface shape will be damaged by the impact of the injected abrasive particles. On the other hand, in the method of the present invention, it is possible to wash the solid electrolyte sheet having such a fine surface shape without damaging it.

In the method for cleaning the solid electrolyte sheet of the present invention, the plasma is preferably vacuum plasma or atmospheric pressure plasma.

In the method for cleaning the solid electrolyte sheet of the present invention, examples of the foreign matter include at least one selected from carbon, hydrocarbons and carbonates.

In the method for cleaning a solid electrolyte sheet of the present invention, it is preferable that the solid electrolyte sheet contains at least one selected from β-alumina, β ″ -alumina and NASICON crystals.

In the method for cleaning the solid electrolyte sheet of the present invention, the thickness of the solid electrolyte sheet is preferably 2 mm or less. As described above, when the thickness of the solid electrolyte sheet is small, the mechanical strength is lowered and damage during cleaning is likely to occur, so that the effect of the present invention can be easily enjoyed.

In the method for cleaning the solid electrolyte sheet of the present invention, it is preferable that the solid electrolyte sheet is for a sodium ion secondary battery.

According to the present invention, it is possible to clean the solid electrolyte sheet while suppressing problems such as breakage and decrease in ionic conductivity.

The method for cleaning the solid electrolyte sheet of the present invention is characterized in that foreign matter adhering to the surface of the solid electrolyte sheet is removed by irradiating with plasma.

Examples of the solid electrolyte sheet include those containing at least one sodium ion conductive oxide solid electrolyte selected from β-alumina, β ”-alumina and NASICON crystals. In particular, β” -alumina and NASICON crystals. Is preferable because it has excellent sodium ion conductivity, high electron insulation, and excellent chemical stability.

The smaller the thickness of the solid electrolyte sheet, the shorter the distance required for ion conduction in the solid electrolyte sheet, which is preferable because the ion conductivity is improved. Further, when used as a solid electrolyte sheet for an all-solid-state battery, the energy density per unit volume of the all-solid-state battery becomes high. Therefore, the thickness of the solid electrolyte sheet is preferably 2 mm or less, 1.5 mm or less, 1 mm or less, 500 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 electrode and the negative electrode may be short-circuited. Therefore, the thickness is preferably 5 μm or more, 20 μm or more, and particularly preferably 50 μm or more.

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). 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.

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 0.3. Examples thereof include those containing ~ 15%, ZrO _{20 to} 20%, and Y ₂ O _{30 to} 5%. The reason for limiting the composition as described above will be described below.

Al ₂ O ₃ is a main component constituting β "-alumina. The content of Al ₂ O ₃ is particularly preferably 65 to 98%, particularly 70 to 95%. If Al ₂ O ₃ is too small, it is particularly preferable. 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 sheet is lowered. It will be easier.

Na ₂ O is a component that constitutes β "-alumina and is a component that imparts sodium ion conductivity to the solid electrolyte sheet. The content of Na ₂ O is 2 to 20%, 3 to 18%, and particularly 4 to. It is preferably 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, excess sodium does not contribute to ionic conductivity such as NaAlO _2. Since it is formed, the ionic conductivity tends to decrease.

MgO and Li ₂ O is beta "- content of .MgO + Li ₂ O is a component to stabilize the structure of alumina (stabilizer) is from 0.3 to 15%, 0.5% to 10%, in particular 0. It is preferably 8 to 8%. If the amount of MgO + Li ₂ O is too small, α-alumina remains in the solid electrolyte sheet and the ionic conductivity tends to decrease. On the other hand, if the amount of MgO + Li ₂ O is too large, it is stable. MgO or Li 2 _O, which did not function as an agent that remains in the solid electrolyte in the sheet, the ion 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 sheet tends to decrease.

The NASICON crystalline, has the general formula _{Na s M t X u O v} (M being selected Al, Y, Yb, Nd, Nb, Ti, at least one selected from Hf and Zr, X is 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, M 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.

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 decreases, so that the ionic 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 and 1 to 2.5, 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 be lowered. 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 be lowered. On the other hand, if u is too large, crystals that do not contribute to ionic conductivity are formed, so that ionic conductivity tends to decrease.

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

The NASICON crystal is preferably a monoclinic crystal, a hexagonal crystal or a trigonal crystal, particularly a monoclinic or 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 , and Na ₅ YSi ₄ O ₁₂ 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.

The solid electrolyte sheet can be produced, for example, by the green sheet method. Specifically, (a) a step of adding a binder, a plasticizer, a solvent, etc. to the raw material powder and kneading the mixture to form a slurry, and (b) applying the slurry on a substrate such as PET (polyethylene terephthalate). It can be produced by a method including a step of drying to obtain a green sheet and (c) a step of firing the green sheet. The β-alumina, β ″ -alumina and NASICON crystals described above are usually precipitated in the firing step of the green sheet.

The type of plasma irradiated to the solid electrolyte sheet is not particularly limited, and for example, non-thermal equilibrium plasma generated by glow discharge, thermal equilibrium plasma by arc discharge, or the like can be used. Further, the plasma may be either vacuum plasma or atmospheric pressure plasma, but vacuum plasma is more preferable from the viewpoint of detergency and cleaning speed, and atmospheric pressure plasma that does not require vacuum is more preferable from the viewpoint of cost.

Examples of the foreign matter to be removed by irradiation with plasma include at least one selected from carbon, hydrocarbons and carbonates. However, the present invention is not limited to these, and examples thereof include organic substances containing carbon.

The solid electrolyte sheet in the present invention can be used, for example, 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. 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 , NaFePO ₄ , and so on. 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, etc. 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, triclinic crystals belonging to the space group P1 or P-1, 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 monoclinic crystal is preferable because the capacity is unlikely to decrease even when charged and discharged with a large current. As the monoclinic crystal, NaTi ₂ O _4, etc., and as the hexagonal crystal, Na ₂ TiO ₃ , NaTi ₈ O ₁₃ , NaTIO ₂ , LiNbO ₃ , LiNbO ₂ , Li ₇ NbO ₆ , LiNbO ₂ , 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 preferably 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 active material crystal 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 powder and a solid electrolyte powder. The solid electrolyte powder 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 powder, the same material as the above-mentioned solid electrolyte sheet can be used.

The positive electrode layer and the negative electrode layer preferably further contain a conductive auxiliary agent. The conductive auxiliary agent is a component added to achieve a high capacity and a 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.

Although the method for cleaning the solid electrolyte sheet for the sodium ion secondary battery has been mainly described above, the present invention is used for cleaning the solid electrolyte sheet used for other all-solid-state batteries such as for the lithium ion secondary battery. Can also be applied. Examples of oxide-based solid electrolytes for lithium ion secondary batteries include Li ₇ La ₃ Zr ₂ O ₁₂ (LLZ), Li ₇ La ₃ Zr ₂ · xNb _x O ₁₂ (LLZ · Nb), Li ₇ La ₃ , Zr. ₂・ xTO ₁₂ (LLZ ・ Ta), Li ₅ La ₃ Ta ₂ O ₁₂ , Li _0.33 La _0.55 TiO ₃ (LLT), Li _1.5 Al _0.5 Ge _1.5 P ₃ O ₁₂ ( LAGP), Li _1.3 Al _0.3 Ti _1.7 P ₃ O ₁₂ (LATP), Li ₃ PO ₄ (LiPON), Li ₄ SiO ₄ · Li ₃ PO ₄ , Li ₄ SiO ₄ , Li ₃ BO ₃ And so on.

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

Tables 1 and 2 show Examples 1 to 6 and Comparative Examples 1 to 5.

(Example 1)
(A) Preparation of solid electrolyte sheet Solid electrolyte sheet (Li ₂ O stabilized β ”-alumina manufactured by Ionotec Co., Ltd., composition formula: Na ₁ ” in which foreign substances such as carbonates are precipitated on the surface by storing in the air for one month. _.7 Li _0.3 Al _10.7 O ₁₇ , 10 mm square, 1 mm thick), using an atmospheric plasma device (P500-SM manufactured by Kagami Semiconductor Co., Ltd.), atmospheric pressure plasma (output power: 45 W, gas used) The cleaning treatment was carried out by irradiating while scanning (: N ₂ , flow rate: 10 L / min). The atmospheric pressure plasma was irradiated on both sides of the solid electrolyte sheet for 5 minutes each.

When the presence or absence of foreign matter on the surface of the solid electrolyte sheet after the cleaning treatment was confirmed by an energy dispersive X-ray analyzer (EDX), no foreign matter was detected.

(B) Preparation of all-solid sodium ion secondary battery (b-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 PO ₄ ) is used as a raw material, and raw material powders are prepared so as to have Na ₂ O 40%, Fe ₂ O ₃ 20%, and P ₂ O ₅ 40% in mol%, and an air atmosphere at 1250 ° C. for 45 minutes. Melting was performed inside. Then, molten glass was poured into a pair of rollers 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 Al ₂ O ₃ jade of φ20 mm for 5 hours, and then pulverized by a ball mill in ethanol using ZrO ₂ jade of φ5 mm for 100 hours, and average particles were obtained. A positive electrode active material crystal precursor powder having a diameter of D ₅₀ 0.7 μm was obtained.

(B-2) Preparation of Solid Electrolyte Powder The above solid electrolyte sheet (before storage for 1 month) 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 Frisch Co., Ltd., which was charged with ZrO ₂ boulders having a diameter of 5 mm, and passed through a mesh having a mesh size of 20 μm. Then, by classifying using an air classifier, a solid electrolyte powder containing Li ₂ O stabilized β "-alumina was obtained. The above work was carried out in an environment with a dew point of −40 ° C. or lower.

(B-3) Preparation of test battery Weighed so that the mass% of the positive electrode active material crystal precursor powder was 60%, the solid electrolyte powder was 35%, and the acetylene black (SUPER C65 manufactured by TIMCAL) was 5%, and made of agate. Mixing was performed for about 30 minutes using a mortar and pestle. To 100 parts by mass of the obtained mixed powder, 20 parts by mass of N-methylpyrrolidone containing 10% by mass of polypropylene carbonate was added, and the mixture was sufficiently stirred using a rotation / revolution mixer to form a slurry. The above work was performed in an environment with a dew point of −40 ° C. or lower.

The obtained slurry was applied to one surface of the washed solid electrolyte sheet with an area of 1 cm ² and a thickness of 200 μm, and dried at 70 ° C. for 3 hours. Next, a positive electrode mixture layer was obtained 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 mixture layer was confirmed, diffraction lines derived from Na ₂ FeP ₂ O ₇ which is an active material crystal and Li ₂ O stabilized β ”-alumina which is a solid electrolyte powder were confirmed. It was.

Next, a gold electrode having a thickness of 300 nm, which is a current collector, was formed on the surface of the positive electrode mixture layer by 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 other surface of the solid electrolyte sheet, placed on the lower lid of the coin cell, and then covered with the upper lid to cover the CR2032 type test battery. Made.

(C) Charge / discharge test A charge / discharge test was performed using the above test battery. The results are shown in Table 1. In the charge / discharge test, charging (sodium ion release from the positive electrode active material) is performed by CC (constant current) charging from the open circuit voltage (OCV) to 4.5 V, and discharge (sodium ion storage in the positive electrode active material) is performed. It was carried out by CC discharge from 4.5V to 2V. The C rate was 0.1 C. The discharge capacity was defined as the amount of electricity discharged per unit weight of the positive electrode active material contained in the positive electrode mixture layer.

(Example 2)
A solid electrolyte sheet was prepared, an all-solid-state sodium-ion secondary battery was prepared, and a charge / discharge test was carried out in the same manner as in Example 1 except that the thickness of the solid electrolyte sheet was 200 μm. The results are shown in Table 1.

(Example 3)
Examples except that a vacuum plasma device (desktop type YHS-R manufactured by Kagami Semiconductor Co., Ltd.) was used as the plasma device, and the vacuum plasma (output power 45 W) was irradiated to both sides of the solid electrolyte sheet for 3 minutes each. A solid electrolyte sheet was prepared, an all-solid-state sodium-ion secondary battery was prepared, and a charge / discharge test was performed in the same manner as in 2. The results are shown in Table 1.

(Example 4)
(A) Preparation of solid electrolyte sheet Sodium carbonate (Na ₂ CO ₃ ), yttrium stabilized zirconia ((ZrO ₂ ) _0.97 (Y ₂ O ₃ ) _0.03 )), silicon dioxide (SiO ₂ ), metaphosphate Using sodium (NaPO ₃ ), in mol%, Na ₂ O 25.3%, ZrO ₂ 31.6%, Y ₂ O ₃₁ %, SiO ₂ 33.7%, P ₂ O ₅ 8.4% The raw material powder was prepared so as to have the composition of. The raw material powder was wet-mixed for 4 hours using ethanol as a medium. After evaporating ethanol, an acrylic acid ester copolymer (Oricox 1700 manufactured by Kyoeisha Chemical Co., Ltd.) was used as a binder, and benzylbutyl phthalate was used as a plasticizer. Raw material powder: binder: plasticizer = 83.5: 15: 1. The mixture was weighed to a ratio of .5 (mass ratio), dispersed in N-methylpyrrolidone, and then sufficiently stirred with a rotation / revolution mixer to form a slurry.

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

The obtained green sheet was pressed at 90 ° C. and 40 MPa for 5 minutes using an isotropic pressure press device. The pressed green sheet was calcined at 1220 ° C. for 40 hours to obtain a solid electrolyte sheet (10 mm square, 1 mm thick). When the powder X-ray diffraction pattern was confirmed on the obtained solid electrolyte sheet, NASICON crystals were confirmed.

The above-mentioned solid electrolyte sheet in which foreign substances such as carbonates were precipitated on the surface by storing in the air for one month was washed in the same manner as in Example 1. When the presence or absence of foreign matter on the surface of the solid electrolyte sheet after the cleaning treatment was confirmed by an energy dispersive X-ray analyzer (EDX), no foreign matter was detected.

(B) Preparation of All Solid Sodium Ion Secondary Battery (b-1) Preparation of Positive Electrode Active Material Crystal Precursor Powder The positive electrode active material crystal precursor powder prepared in Example 1 was used.

(B-2) Preparation of Solid Electrolyte Powder A solid electrolyte powder was prepared in the same manner as in Example 1 using the above-mentioned solid electrolyte sheet containing NASICON crystals (one before storage for one month).

(B-3) Preparation of test battery A test battery was prepared in the same manner as in Example 1. When the X-ray diffraction pattern of the obtained positive electrode mixture layer was confirmed, diffraction lines derived from Na ₂ FeP ₂ O ₇ which is an active material crystal and NASICON crystal which is a solid electrolyte powder were confirmed.

(C) Charge / discharge test A charge / discharge test was performed in the same manner as in Example 1. The results are shown in Table 1.

(Example 5)
A solid electrolyte sheet was prepared, an all-solid-state sodium-ion secondary battery was prepared, and a charge / discharge test was performed in the same manner as in Example 4, except that the thickness of the solid electrolyte sheet was set to 200 μm. The results are shown in Table 1.

(Example 6)
The same as in Example 5 except that a vacuum plasma device (desktop type YHS-R manufactured by Kai Semiconductor Co., Ltd.) was used as the plasma device and the vacuum plasma was irradiated to both sides of the solid electrolyte sheet for 3 minutes each. A solid electrolyte sheet was prepared, an all-solid sodium-ion secondary battery was prepared, and a charge / discharge test was performed. The results are shown in Table 1.

(Comparative Example 1)
The solid electrolyte sheet was prepared, the all-solid-state sodium-ion secondary battery was prepared, and the charge / discharge test was carried out in the same manner as in Example 2 except that the solid electrolyte sheet was not washed. The results are shown in Table 1.

(Comparative Example 2)
In Example 2, when the solid electrolyte sheet was polished using a polishing plate (diamond atom wheel (extra-fine) No. 1200) instead of the irradiation of atmospheric pressure plasma, the solid electrolyte sheet was damaged. , The test battery could not be manufactured.

(Comparative Example 3)
In Example 2, when the solid electrolyte sheet was ultrasonically cleaned (40 kHz) in ethanol instead of being irradiated with atmospheric pressure plasma, the solid electrolyte sheet was damaged and a test battery could not be produced. It was.

(Comparative Example 4)
The solid electrolyte sheet was prepared in the same manner as in Example 2 except that the solid electrolyte sheet was immersed in 1 mol% hydrochloric acid instead of being irradiated with atmospheric pressure plasma, and the all-solid sodium ion secondary was prepared. A battery was prepared and a charge / discharge test was performed. The results are shown in Table 1.

(Comparative Example 5)
In Example 5, instead of irradiating the atmospheric pressure plasma, the solid electrolyte sheet was polished using a polishing plate (diamond atom wheel (ultra-fine) No. 1200), but the solid electrolyte sheet was damaged. The test battery could not be manufactured.

As shown in Table 1, in Examples 1 to 6, the solid electrolyte sheet was not damaged by washing, and foreign matter was removed from the surface of the solid electrolyte sheet by irradiation with plasma. Further, the test battery using these solid electrolyte sheets showed a high discharge capacity of 83 mAh / g or more.

On the other hand, in Comparative Examples 1 and 4, the discharge capacity of the test battery showed a low value of 50 mAh / g or less. It is considered that this is because Na ₂ CO ₃ and NaOH having low ionic conductivity adhered to the surface of the solid electrolyte sheet as foreign substances, and the interfacial resistance between the solid electrolyte sheet and the electrode layer increased. In Comparative Examples 2, 3 and 5, a test battery could not be produced because the solid electrolyte sheet was damaged in the cleaning step.

Claims (6)

A method for cleaning a solid electrolyte sheet, which comprises removing foreign substances adhering to the surface of the solid electrolyte sheet by irradiating with plasma. The method for cleaning a solid electrolyte sheet according to claim 1, wherein the plasma is a vacuum plasma or an atmospheric pressure plasma. The method for cleaning a solid electrolyte sheet according to claim 1 or 2, wherein the foreign matter is at least one selected from carbon, hydrocarbons and carbonates. The method for cleaning a solid electrolyte sheet according to any one of claims 1 to 3, wherein the solid electrolyte sheet contains at least one selected from β-alumina, β ″ -alumina and NASICON crystals. .. The method for cleaning a solid electrolyte sheet according to any one of claims 1 to 4, wherein the thickness of the solid electrolyte sheet is 2 mm or less. The method for cleaning a solid electrolyte sheet according to any one of claims 1 to 5, wherein the solid electrolyte sheet is for a sodium ion secondary battery.