Electrode mixture for sodium ion secondary battery, electrode layer, and all-solid-state secondary battery

Abstract

The present invention provides an electrode composite material capable of increasing discharge capacity of a solid-state battery. This electrode composite material is characterized by containing an active substance powder, an inorganic filler powder, and an electroconductive auxiliary agent.

Description

[Technical field]

[0001] The present invention relates to an electrode mixture used as an electrode material for secondary batteries and the like. [Background technology]

[0002] Lithium-ion secondary batteries have established their position as high-capacity, lightweight power sources essential for portable electronic devices, electric vehicles, etc., and as their positive electrode active material, active materials containing olivine-type crystals represented by the general formula LiFePO4 have attracted attention. However, due to concerns about issues with lithium, such as the worldwide rise in the price of raw materials, research has been conducted in recent years into sodium-ion secondary batteries that use sodium as an alternative, such as Na2FeP2O7 crystals and Na4Ni3(PO4)2(P2O7) crystals (see, for example, Patent Documents 1 and 2).

[0003] Furthermore, since secondary batteries that use organic electrolytic solutions as electrolytes have concerns about the risk of fire, etc., all-solid-state sodium ion secondary batteries that use solid electrolytes instead of organic electrolytic solutions have been proposed (see, for example, Patent Document 3).

[0004] An all-solid-state sodium ion secondary battery is composed of a laminate of a positive electrode layer, a solid electrolyte layer, and a negative electrode layer. The positive electrode layer and the negative electrode layer (hereinafter collectively referred to as electrode layers) are made of, for example, a sintered body of an active material powder. A conductive assistant such as acetylene black is added to the electrode layer to form an electronic conduction path and increase electronic conductivity. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 5673836 [Patent Document 2] JP 2016-25067 A [Patent Document 3] JP 2010-15782 A Summary of the Invention [Problem to be solved by the invention]

[0006] The electrode layer is produced, for example, by firing a mixture (electrode mixture) of an active material powder and a conductive assistant. Even if the conductive assistant is uniformly dispersed in the mixture before firing to form an electronic conductive path, when the active material powder softens and flows during firing, the connections between the conductive assistants tend to be cut, cutting the electronic conductive path. As a result, the electronic conductivity of the obtained electrode layer is poor, and the discharge capacity of the all-solid-state battery is likely to decrease.

[0007] In view of the above, an object of the present invention is to provide an electrode mixture capable of increasing the discharge capacity of an all-solid-state battery. [Means for solving the problem]

[0008] The electrode mixture of the present invention is characterized by containing an active material powder, an inorganic filler powder, and a conductive assistant. In this way, the inorganic filler powder functions as a skeleton for maintaining an electronic conduction path by the conductive assistant in the electrode layer. Therefore, when the active material powder is softened and flowed by firing, the electronic conduction path by the conductive assistant is not easily cut, and it is possible to suppress a decrease in the discharge capacity of the all-solid-state battery.

[0009] In addition, by including the inorganic filler powder, it is possible to form a sufficient electron conduction path even if the content of the conductive additive is reduced. By reducing the content of the conductive additive, the sinterability of the electrode mixture is improved, and as a result, the discharge capacity of the all-solid-state battery can be improved.

[0010] Furthermore, if the thermal expansion coefficients of the electrode layer and the solid electrolyte layer obtained after firing are different, stress may be generated at the interface between the two layers, which may cause the electrode layer to peel off. On the other hand, by including inorganic filler powder in the electrode mixture, it is possible to match the thermal expansion coefficient of the electrode layer to that of the solid electrolyte layer, and the above-mentioned problem of peeling of the electrode layer can be suppressed.

[0011] In addition, when the active material powder is made of glass powder, it may exhibit its function as an active material (or its function as an active material may be improved) by crystallizing during firing. In the present invention, such glass powder before crystallization (active material precursor powder) is also considered to be an active material powder.

[0012] In the electrode mixture of the present invention, the inorganic filler powder includes at least one oxide selected from the group consisting of Al, Mg, Si, Zr, Ce, Fe, Ti, Nb, and Y.

[0013] Another aspect of the present invention is an electrode mixture containing an active material powder and an inorganic filler powder, characterized in that the inorganic filler powder has electrical conductivity. In this way, by including an electrically conductive inorganic filler powder in the electrode mixture, it becomes possible to form an electronic conduction path by the inorganic filler powder itself without adding an electrically conductive assistant. As the electrically conductive inorganic filler powder, at least one metal selected from the group consisting of Al, Cu, Ag, and Au can be used.

[0014] In the electrode mixture of the present invention, the inorganic filler powder preferably has an average particle size of 0.01 to 30 μm. This allows the inorganic filler powder to function as a skeleton for maintaining an electron conduction path by the conductive assistant in the electrode layer. Alternatively, when the inorganic filler powder has conductivity, the inorganic filler powder is likely to form an electron conduction path.

[0015] In the electrode mixture of the present invention, the average particle size ratio of the inorganic filler powder to the active material powder (average particle size of the inorganic filler powder / average particle size of the active material powder) is preferably 0.5 to 50. In this way, the inorganic filler powder is more likely to function as a skeleton for maintaining an electron conduction path by the conductive assistant in the electrode layer. Alternatively, when the inorganic filler powder has conductivity, an electron conduction path is more likely to be formed by the inorganic filler powder.

[0016] In the electrode mixture of the present invention, the content of the inorganic filler powder is preferably 1 to 40% by volume.

[0017] In the electrode mixture of the present invention, the active material powder is preferably made of glass powder.

[0018] The electrode mixture of the present invention is preferably for use in a sodium ion secondary battery.

[0019] In the electrode mixture of the present invention, the active material powder preferably contains, in mole percent calculated as oxide, 8 to 55% Na2O, 10 to 70% CrO+FeO+MnO+CoO+NiO, and 15 to 70% P2O5+SiO2+B2O3.

[0020] The electrode layer of the present invention is characterized by comprising a sintered body of the above-mentioned electrode mixture.

[0021] The all-solid-state secondary battery of the present invention is characterized by having the above-mentioned electrode layer. Effect of the Invention

[0022] According to the present invention, it is possible to provide an electrode mixture capable of increasing the discharge capacity of an all-solid-state battery. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] The electrode mixture of the present invention is characterized by containing an active material powder, an inorganic filler powder, and a conductive assistant. Each of the components will be described below.

[0024] (active material powder) The active material powder includes a positive electrode active material powder and a negative electrode active material powder. The positive electrode active material powder includes, for example, at least one of phosphate, silicate, and borate, and is capable of absorbing and releasing alkali ions such as sodium ions, specifically, those containing 8 to 55% Na2O, 10 to 70% CrO+FeO+MnO+CoO+NiO, and 15 to 70% P2O5+SiO2+B2O3 in mole percent of oxide. The positive electrode active material having this composition is suitable for sodium ion secondary batteries. The reason for limiting each component in this way will be explained below. In the following description of the content of each component, "%" means "mol %" unless otherwise specified. In addition, in this specification, "○+○+···" means the total amount of each corresponding component.

[0025] Na2O is 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 Na2O is preferably 8 to 55%, 15 to 45%, and particularly preferably 25 to 35%. If the Na2O content is too low, the amount of sodium ions that contribute to absorption and release is reduced, and the discharge capacity tends to decrease. On the other hand, if the Na2O content is too high, foreign crystals such as Na3PO4 that do not contribute to charging and discharging are easily precipitated, and the discharge capacity tends to decrease.

[0026] CrO, FeO, MnO, CoO, and NiO are components that act as a driving force for the absorption and release of sodium ions by causing a redox reaction due to the change in the valence of each transition element during charging and discharging. Among them, NiO and MnO have a large effect of increasing the redox potential. FeO is particularly easy to stabilize the structure during charging and discharging, and is easy to improve cycle characteristics. The content of CrO+FeO+MnO+CoO+NiO is preferably 10-70%, 15-60%, 20-55%, 23-50%, 25-40%, and particularly 26-36%. If the amount of CrO+FeO+MnO+CoO+NiO is too small, the redox reaction accompanying charging and discharging is difficult to occur, and the amount of sodium ions absorbed 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, heterogeneous crystals tend to precipitate and the discharge capacity tends to decrease.

[0027] P2O5, SiO2 and B2O3 form a three-dimensional network structure, and therefore have the effect of stabilizing the structure of the positive electrode active material. In particular, P2O5 and SiO2 are preferred because they have excellent ion conductivity, and P2O5 is the most preferred. The content of P2O5+SiO2+B2O3 is 15-70%, preferably 20-60%, and particularly preferably 25-45%. If the content of P2O5+SiO2+B2O3 is too low, the discharge capacity tends to decrease when repeatedly charged and discharged. On the other hand, if the content of P2O5+SiO2+B2O3 is too high, heterogeneous crystals such as P2O5 that do not contribute to charging and discharging tend to precipitate. The content of each of the components P2O5, SiO2 and B2O3 is preferably 0-70%, 15-70%, 20-60%, and particularly preferably 25-45%, respectively.

[0028] In addition, various components can be added in addition to the above components to facilitate vitrification within a range that does not impair the effect as a positive electrode active material. Examples of such components include, in terms of oxide, MgO, CaO, SrO, BaO, ZnO, CuO, Al2O3, GeO2, Nb2O5, ZrO2, and Sb2O5, and in particular, Al2O3, which acts as a network-forming oxide, and V2O5, which serves as an active material component, are preferred. The total content of the above components is preferably 0 to 30%, 0.1 to 20%, and particularly preferably 0.5 to 10%.

[0029] The negative electrode active material powder includes at least one of phosphate, silicate, and borate, and is capable of absorbing and releasing alkali ions such as sodium. Specifically, the negative electrode active material powder includes, in terms of oxide mole percent, 0-90% SnO, 0-90% Bi2O3, 50-90% Nb2O, 0-90% TiO2, 0-90% Fe2O3, 55-75% SiO2+B2O3+P2O, and 0-80% Na2O. The negative electrode active material powder having the above composition is suitable for sodium ion secondary batteries. By adopting the above composition, a structure is formed in which the negative electrode active material components Sn ions, Bi ions, Ti ions, Fe ions, or Nb ions are more uniformly dispersed in the oxide matrix containing Si, B, or P. In addition, by containing Na2O, the material becomes even more excellent in sodium ion conductivity. As a result, the volume change during absorbing and releasing sodium ions can be suppressed, and it is possible to obtain a negative electrode active material with even more excellent cycle characteristics.

[0030] The reason for limiting the composition of the negative electrode active material powder as described above will be explained below.

[0031] SnO, Bi2O3, Nb2O5, TiO2 and Fe2O3 are negative electrode active material components that become sites for absorbing and releasing alkali ions such as sodium ions. By including these components, the discharge capacity per unit mass of the negative electrode active material becomes larger, and the charge / discharge efficiency (ratio of discharge capacity to charge capacity) during the initial charge / discharge is more likely to be improved. However, if the content of these components is too high, the volume change accompanying the absorption and release of alkali ions during charge / discharge cannot be alleviated, and cycle characteristics tend to deteriorate. In view of the above, it is preferable to set the content range of each component as follows.

[0032] The SnO content is preferably 0 to 90%, 45 to 85%, 55 to 75%, and particularly preferably 60 to 72%.

[0033] The content of Bi2O3 is preferably 0 to 90%, 10 to 70%, or 15 to 65%, and particularly preferably 25 to 55%.

[0034] The content of Nb2O5 is preferably 0 to 90%, 7 to 79%, 9 to 69%, 11 to 59%, or 13 to 49%, particularly preferably 15 to 39%.

[0035] The content of TiO2 is preferably 0 to 90%, 5 to 72%, 10 to 68%, 12 to 58%, or 15 to 49%, particularly preferably 15 to 39%.

[0036] The content of Fe2O3 is preferably 0 to 90%, 15 to 85%, or 20 to 80%, and particularly preferably 25 to 75%.

[0037] The content of SnO+Bi2O3+Nb2O5+TiO2+Fe2O3 is preferably 0 to 90%, more preferably 5 to 85%, and even more preferably 10 to 80%.

[0038] SiO2, B2O3 and P2O5 are network-forming oxides that surround the alkali ion absorption and release sites in the negative electrode active material components, and have the effect of further improving cycle characteristics. Among them, SiO2 and P2O5 not only further improve cycle characteristics, but also have excellent conductivity of alkali ions (especially sodium ions), and therefore have the effect of further improving rate characteristics.

[0039] SiO2+B2O3+P2O5 is preferably 5-85%, 6-79%, 7-69%, 8-59%, 9-49%, and particularly preferably 10-39%. If the amount of SiO2+B2O3+P2O5 is too small, the volume change of the negative electrode active material components accompanying the absorption and release of alkali ions during charging and discharging cannot be alleviated, causing structural destruction, and the cycle characteristics tend to deteriorate. On the other hand, if the amount of SiO2+B2O3+P2O5 is too large, the content of the negative electrode active material components becomes relatively small, and the charge / discharge capacity per unit mass of the negative electrode active material tends to become small.

[0040] The preferred ranges of the respective contents of SiO2, B2O3 and P2O5 are as follows:

[0041] The SiO2 content is preferably 0 to 75%, 5 to 75%, 7 to 60%, 10 to 50%, 12 to 40%, and particularly preferably 20 to 35%. If the SiO2 content is too high, the discharge capacity tends to decrease.

[0042] The content of P2O5 is preferably 5 to 75%, 7 to 60%, 10 to 50%, 12 to 40%, and particularly preferably 20 to 35%. If the content of P2O5 is too low, it is difficult to obtain the above-mentioned effects. On the other hand, if the content of P2O5 is too high, the discharge capacity is likely to decrease and the water resistance is likely to decrease. In addition, when an aqueous electrode paste is prepared, unwanted heterogeneous crystals are generated and the P2O5 network is broken, which makes it easy for cycle characteristics to decrease.

[0043] The content of B2O3 is preferably 0 to 75%, 5 to 75%, 7 to 60%, 10 to 50%, or 12 to 40%, and particularly preferably 20 to 35%. If the content of B2O3 is too high, the discharge capacity is likely to decrease and the chemical durability is likely to decrease.

[0044] Na2O is a component that improves the initial discharge capacity by making it difficult for sodium ions to be absorbed in the negative electrode active material during the initial charge. It also has the effect of increasing sodium ion conductivity and decreasing the operating voltage of the negative electrode. The Na2O content is preferably 0 to 80%, 1 to 70%, and particularly 5 to 60%. If the Na2O content is too high, a large amount of heterogeneous crystals containing sodium ions (Na4P2O7, NaPO4, etc.) are formed, which makes it easy for cycle characteristics to deteriorate. In addition, since the content of the active material components becomes relatively small, the discharge capacity tends to decrease.

[0045] The average particle size of the active material powder is preferably 0.01 to 15 μm, 0.05 to 10 μm, 0.07 to 5 μm, and particularly preferably 0.1 to 0.7 μm. If the average particle size of the active material powder is too small, the cohesive force between the active material powders becomes strong, and the dispersibility tends to be poor when the powder is made into a paste. As a result, it becomes difficult to obtain a homogeneous electrode layer. As a result, the internal resistance of the battery increases, and the operating voltage tends to decrease, or the electrode density decreases, and the capacity per unit volume of the battery decreases, and other problems may occur. On the other hand, if the average particle size of the active material powder is too large, the denseness and surface smoothness of the electrode layer tend to be poor.

[0046] In this specification, the average particle size is the median diameter of the primary particles, D 50 (50% volume cumulative diameter) and is a value measured using a laser diffraction particle size distribution analyzer.

[0047] The specific surface area of ​​the active material powder is 1 to 100 m 2 / g, 3-80m 2 / g, 5-70m 2 / g, especially 10-50m 2 / g is preferable. If the specific surface area of ​​the active material powder is too small, the density and surface smoothness of the electrode layer tend to be poor. On the other hand, if the specific surface area of ​​the active material powder is too large, the cohesive force between the active material powders becomes strong, and the dispersibility tends to be poor when the active material powder is made into a paste. As a result, it becomes difficult to obtain a homogeneous electrode layer. As a result, the internal resistance of the battery increases, and the operating voltage tends to decrease, or the electrode density decreases, and the capacity per unit volume of the battery decreases, and other problems may occur.

[0048] (Inorganic filler powder) The inorganic filler powder includes at least one oxide selected from the group consisting of Al, Mg, Si, Zr, Ce, Fe, Ti, Nb, and Y, specifically Al2O3, MgO, SiO2, ZrO2, CeO2, Fe2O3, TiO2, Y2O3, Nb2O5, NaNbO3, KNbO3, BaTiO3, and PbZrTiO3. Among them, Al2O3, MgO, SiO2, ZrO2, CeO2, TiO2, and Y2O3 are preferable because they have excellent chemical stability and are not easily deteriorated during charging and discharging. These inorganic filler powders function as a skeleton for maintaining the electron conduction path of the conductive assistant in the electrode layer. Therefore, when the active material powder is softened and flowed by firing, the electron conduction path of the conductive assistant is not easily cut, making it possible to suppress the decrease in the discharge capacity of the all-solid-state battery. By including inorganic filler powder in the electrode mixture, it is possible to match the thermal expansion coefficient of the electrode layer to that of the solid electrolyte layer, thereby suppressing the problem of peeling of the electrode layer caused by the difference in thermal expansion coefficient.

[0049] The surface of the inorganic filler powder may be coated with carbon, which imparts electrical conductivity to the inorganic filler powder, thereby increasing the electrical conductivity of the electrode layer and thus improving the battery characteristics.

[0050] As the inorganic filler powder, it is also possible to use an inorganic filler powder having electrical conductivity. By including an inorganic filler powder having electrical conductivity in an electrode mixture, it becomes possible for the inorganic filler powder itself to form an electronic conduction path without adding a conductive assistant. As the inorganic filler powder having electrical conductivity, at least one metal selected from the group consisting of Al, Cu, Ag, and Au can be mentioned.

[0051] Incidentally, it is possible to incorporate solid electrolyte powders such as beta-alumina powder and NASICON powder into the electrode mixture in order to increase the ionic conductivity of the electrode layer, but these solid electrolyte powders have problems such as extremely low weather resistance, difficulty in handling, and high cost. In contrast, the inorganic filler powder used in the present invention has the advantage that it is stable in the air, easy to handle, and inexpensive. In addition, as shown in the examples below, if the electrode mixture of the present invention is used, it is possible to operate an all-solid-state battery without incorporating the above-mentioned solid electrolyte powder into the electrode mixture.

[0052] The average particle size of the inorganic filler powder is preferably 0.01 to 30 μm, 0.07 to 20 μm, 0.05 to 10 μm, or 0.1 to 5 μm, and particularly preferably 0.1 to 3 μm. If the average particle size of the inorganic filler powder is too small, it becomes difficult for the conductive assistant to function as a skeleton for maintaining an electron conduction path, or for the inorganic filler powder itself to form an electron conduction path. On the other hand, if the average particle size of the inorganic filler powder is too large, the sinterability decreases, making it difficult to obtain a dense sintered body, and the discharge capacity tends to decrease.

[0053] The specific surface area of ​​inorganic filler powder is 1 to 400 m 2 / g, 2-200m 2 / g, 3-100m 2 / g, especially 3-70m 2 / g is preferable. If the specific surface area of ​​the inorganic filler powder is too small, the sinterability decreases, making it difficult to obtain a dense sintered body, and the discharge capacity tends to decrease. On the other hand, if the specific surface area of ​​the inorganic filler powder is too large, it becomes difficult to obtain the function of the conductive assistant as a skeleton for maintaining the electron conduction path, or the function of the inorganic filler powder itself for forming the electron conduction path.

[0054] The average particle size ratio of the inorganic filler powder to the active material powder (average particle size of the inorganic filler powder / average particle size of the active material powder) is preferably 0.5 to 50, 0.7 to 30, 1 to 10, and particularly preferably 1.15 to 5. If this ratio is too small, it becomes difficult for the conductive assistant to function as a skeleton for maintaining an electron conduction path, or for the inorganic filler powder itself to function to form an electron conduction path. On the other hand, if this ratio is too large, the sinterability decreases, making it difficult to obtain a dense sintered body, and the discharge capacity tends to decrease.

[0055] The content of the inorganic filler powder in the electrode mixture is preferably 1 to 40%, 3 to 30%, and particularly preferably 4 to 25% by volume. If the content of the inorganic filler powder is too low, it becomes difficult to obtain the function of the conductive assistant as a skeleton for maintaining the electron conduction path, or the function of the inorganic filler powder itself for forming the electron conduction path. In addition, it becomes difficult to obtain the function of adjusting the thermal expansion coefficient of the electrode layer. On the other hand, if the content of the inorganic filler powder is too high, the ratio of the active material powder in the electrode layer decreases, or the sinterability decreases, making it difficult to obtain a dense sintered body, and as a result, the discharge capacity tends to decrease.

[0056] The content of the active material powder in the electrode mixture is preferably 5 to 70%, 10 to 60%, 20 to 55%, and particularly preferably 30 to 50% by volume. If the content of the active material powder is too low, the discharge capacity is likely to decrease. On the other hand, if the content of the active material powder is too high, it becomes difficult to form an electron conduction path, and as a result, the discharge capacity is likely to decrease.

[0057] The ratio of the content of the inorganic filler powder to the active material powder in volume percent (content of inorganic filler powder / content of active material powder) is preferably 0.01 to 1, 0.05 to 0.8, and particularly preferably 0.1 to 0.5. If this ratio is too small, it becomes difficult to form an electron conduction path, and as a result, the discharge capacity tends to decrease. On the other hand, if this ratio is too large, the sinterability decreases, making it difficult to obtain a dense sintered body, and the discharge capacity tends to decrease.

[0058] (Conductive assistant) Examples of the conductive assistant include highly conductive carbon black such as acetylene black and ketjen black, powdered or fibrous conductive carbon such as graphite, etc. Among these, acetylene black, which has excellent conductivity, is preferred.

[0059] The content of the conductive assistant in the electrode mixture is preferably 1 to 70%, 5 to 65%, 10 to 60%, 20 to 55%, and particularly 30 to 55% by volume. If the content of the conductive assistant is too small, sufficient electron conduction paths are not formed in the electrode layer, and the discharge capacity of the all-solid-state battery tends to be poor. On the other hand, if the content of the conductive assistant is too large, the ratio of the active material powder in the electrode layer decreases, or the sinterability decreases, making it difficult to obtain a dense sintered body, and as a result, the discharge capacity tends to decrease. As mentioned above, when an inorganic filler powder having conductivity is used as the inorganic filler powder, it is not necessary to contain the conductive assistant. However, even in that case, it is not necessarily prevented from containing the conductive assistant.

[0060] (All-solid-state secondary battery) The all-solid-state secondary battery of the present invention includes an electrode layer made of the sintered body of the electrode mixture. Specifically, the all-solid-state secondary battery includes a solid electrolyte layer, a positive electrode layer formed on one of the main surfaces thereof, and a negative electrode layer formed on the other main surface thereof. Here, both the positive electrode layer and the negative electrode layer may be made of the sintered body of the electrode mixture, or only one of the positive electrode layer and the negative electrode layer may be made of the sintered body of the electrode mixture.

[0061] Examples of solid electrolyte layers include beta alumina (β-alumina or β″-alumina) and NASICON crystals. These solid electrolyte layers are suitable for use in all-solid-state alkaline ion secondary batteries. EXAMPLES

[0062] The present invention will be described in detail below based on examples, but the present invention is not limited to these examples.

[0063] Table 1 shows Examples 1 to 4 and Comparative Example 1.

[0064] [Table 1]

[0065] (a) Preparation of positive electrode active material precursor powder Sodium metaphosphate (NaPO3), ferric oxide (Fe2O3), and orthophosphoric acid (H3PO4) were used as raw materials, and raw material powders were mixed to have a molar ratio of Na2O 40%, Fe2O3 20%, and P2O5 40%, and melted in an air atmosphere at 1250°C for 45 minutes. The molten glass was then poured into a pair of rollers and formed into a film while being quenched, to produce a positive electrode active material precursor.

[0066] The obtained positive electrode active material precursor was subjected to ball milling using φ20 mm Al2O3 balls for 5 hours, and then to ball milling in ethanol using φ5 mm ZrO2 balls for 100 hours to obtain an average particle diameter D 50 A positive electrode active material precursor powder of 0.7 μm was obtained.

[0067] (b) Preparation of the positive electrode mixture The above-mentioned positive electrode active material precursor powder was mixed with the inorganic filler powder shown in Table 1 and acetylene black (SUPER C65 manufactured by TIMCAL) as a conductive additive in the proportions shown in Table 1, and the mixture was mixed for about 30 minutes using an agate mortar and pestle to obtain a positive electrode mixture. Note that in Example 4, no conductive additive was added.

[0068] The inorganic filler powder of Example 5 was prepared as follows. 14.2 parts by mass of polyethylene oxide nonylphenyl ether (HLB value: 13.3, mass average molecular weight: 660), which is a nonionic surfactant, was added as a carbon source to 100 parts by mass of Al2O3 powder, and 60 parts by mass of pure water was further added and mixed thoroughly, followed by drying at 100°C for about 1 hour. Then, the mixture was fired at 620°C for 30 minutes in a nitrogen atmosphere to obtain Al2O3 powder with a carbon-coated surface.

[0069] To 100 parts by mass of the obtained positive electrode mixture, 20 parts by mass of N-methylpyrrolidone containing 10% by mass of polypropylene carbonate was added, and the mixture was thoroughly stirred using a planetary centrifugal mixer to form a slurry.

[0070] (c) Preparation of test battery The above slurried positive electrode mixture was placed on a solid electrolyte sheet (Li2O-stabilized β”-alumina, manufactured by Ionotec, composition formula: Na 1.7 Li 0.3 Al 10.7 O 17 , 1cm square, 1mm thick) on one surface of 2 The coating was applied to an area of ​​100 mm and a thickness of 200 μm, and dried at 70° C. for 3 hours. Next, the coating was fired at 550° C. for 1 hour in a mixed gas atmosphere of nitrogen and hydrogen (nitrogen 96% by volume, hydrogen 4% by volume) to sinter the positive electrode composite and crystallize the positive electrode active material precursor powder to form a positive electrode layer. When the X-ray diffraction pattern of the obtained positive electrode layer was confirmed, diffraction lines derived from the active material crystal Na2FeP2O7 were confirmed.

[0071] Next, a 300 nm-thick gold electrode was formed on the surface of the positive electrode layer as a current collector using a sputtering device (SC-701AT manufactured by Sanyu Electronics Co., Ltd.). After that, metallic sodium was pressed onto the other surface of the solid electrolyte sheet as a counter electrode, and the sheet was placed on the bottom cover of a coin cell, followed by placing the top cover on the sheet to prepare a CR2032 test battery.

[0072] (d) Charge / discharge test A charge-discharge test was carried out using the above test battery. The results are shown in Table 1. In the charge-discharge test, charging (release of sodium ions from the positive electrode active material) was carried out by CC (constant current) charging from the open circuit voltage (OCV) to 4.5V, and discharging (sodium ion absorption into the positive electrode active material) was carried out by CC discharging from 4.5V to 2V. The C rate was set to 0.01C, and the test was carried out at 60°C and 30°C. The discharge capacity was the amount of electricity discharged per unit weight of the positive electrode active material contained in the positive electrode layer.

[0073] As shown in Table 1, in Examples 1 to 5 in which an inorganic filler was added to the electrode mixture, the discharge capacity was excellent, at 80 mAh / g or more at 60° C. and 53 mAh / g or more at 30° C. On the other hand, in Comparative Example 1 in which an inorganic filler was not added to the electrode mixture, the discharge capacity was low at 5 mAh / g at 60° C. and 0 mAh / g at 30° C. (i.e., the battery did not work).

Claims

1. 1. An electrode mixture for a sodium ion secondary battery comprising an active material powder, an inorganic filler powder, and a conductive assistant, wherein the inorganic filler powder is at least one oxide selected from the group consisting of Al, Mg, Si, Zr, Ce, Fe, Ti, Nb, and Y, the active material powder is made of glass powder, the inorganic filler powder has an average particle size of 0.1 to 3 μm, and the average particle size ratio of the inorganic filler powder to the active material powder (average particle size of the inorganic filler powder / average particle size of the active material powder) is 1.15 to 5.

2. The electrode mixture for a sodium ion secondary battery as described in claim 1, characterized in that the active material powder contains, in mole percentages calculated as oxides, 8 to 55% Na2O, 10 to 70% CrO+FeO+MnO+CoO+NiO, and 15 to 70% P2O5+SiO2+B2O3.

3. An electrode mixture for a sodium ion secondary battery containing an active material powder and an inorganic filler powder, wherein the inorganic filler powder has electrical conductivity, is at least one metal selected from the group consisting of Al, Cu, Ag and Au, has an average particle size of 0.1 to 3 μm, has an average particle size ratio between the inorganic filler powder and the active material powder (average particle size of inorganic filler powder / average particle size of active material powder) of 1.15 to 5, and contains, in mole % calculated as oxide, 8 to 55% Na 2 O, 10 to 70% CrO+FeO+MnO+CoO+NiO, and 15 to 70% P 2 O 5 +SiO 2 +B 2 O 3.

4. 4. The electrode mixture for a sodium ion secondary battery according to claim 3, wherein the active material powder is made of glass powder.

5. The electrode mixture for a sodium ion secondary battery according to any one of claims 1 to 4, characterized in that the content of the inorganic filler powder is 1 to 40% by volume.

6. An electrode layer comprising a sintered body of the electrode mixture for a sodium ion secondary battery according to any one of claims 1 to 5.

7. An all-solid-state secondary battery comprising the electrode layer according to claim 6.

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