JP7675334B2 - Electrode mixture for sodium ion secondary battery, electrode layer, and all-solid-state secondary battery - Google Patents

Description

The present invention relates to an electrode mixture used as an electrode material for secondary batteries, etc.

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 LiFePO ₄ have attracted attention. However, due to concerns about problems with lithium, such as a worldwide rise in the price of raw materials, research has been conducted in recent years on sodium ion secondary batteries that use sodium as an alternative, such as Na ₂ FeP ₂ O ₇ crystals and Na ₄ Ni ₃ (PO ₄ ) ₂ (P ₂ O ₇ ) crystals (see, for example, Patent Documents 1 and 2).

In addition, because secondary batteries that use organic electrolytic solutions as electrolytes are subject to 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).

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 the electrode layers) are made of, for example, a sintered body of active material powder. A conductive additive such as acetylene black is added to the electrode layer to form an electronic conduction path and increase electronic conductivity.

Patent No. 5673836 JP 2016-25067 A JP 2010-15782 A

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

In view of the above, the present invention aims to provide an electrode composite material that can increase the discharge capacity of an all-solid-state battery.

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 the electronic conduction path by the conductive assistant in the electrode layer. Therefore, when the active material powder softens and flows by firing, the electronic conduction path by the conductive assistant is less likely to be broken, making it possible to suppress a decrease in the discharge capacity of the all-solid-state battery.

In addition, by including inorganic filler powder, it is possible to form a sufficient electronic 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.

Furthermore, if the thermal expansion coefficients of the electrode layer and solid electrolyte layer obtained after firing are different, stress may occur at the interface between the two layers, causing 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, thereby preventing the above-mentioned problem of electrode layer peeling.

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

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

An electrode mixture according to 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 inorganic filler powder having electrical conductivity in the 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 used.

In the electrode mixture of the present invention, it is preferable that the inorganic filler powder has an average particle size of 0.01 to 30 μm. In this way, the inorganic filler powder is more likely to function as a skeleton for maintaining an electronic conduction path by the conductive assistant in the electrode layer. Alternatively, if the inorganic filler powder is conductive, an electronic conduction path is more likely to be formed by the inorganic filler powder.

In the electrode mixture of the present invention, it is preferable that 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 0.5 to 50. In this way, the inorganic filler powder is more likely to function as a skeleton for maintaining an electronic conduction path by the conductive assistant in the electrode layer. Alternatively, if the inorganic filler powder is conductive, an electronic conduction path is more likely to be formed by the inorganic filler powder.

It is preferable that the electrode mixture of the present invention has an inorganic filler powder content, by volume percent, of 1 to 40%.

In the electrode mixture of the present invention, it is preferable that the active material powder consists of glass powder.

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

In the electrode mixture of the present invention, the active material powder preferably contains, in mole percent 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 .

The electrode layer of the present invention is characterized in that it is made of a sintered body of the above-mentioned electrode mixture.

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

According to the present invention, it is possible to provide an electrode composite material capable of increasing the discharge capacity of an all-solid-state battery.

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

(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% 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 in mole percent of oxide. A positive electrode active material having this composition is suitable for use in a sodium ion secondary battery. 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 the corresponding components.

Na ₂ O 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 Na ₂ O is preferably 8 to 55%, 15 to 45%, and particularly 25 to 35%. If the content of Na ₂ O is too small, the amount of sodium ions that contribute to absorption and release is reduced, so the discharge capacity tends to decrease. On the other hand, if the content of Na ₂ O is too large, heterogeneous crystals such as Na ₃ PO ₄ that do not contribute to charging and discharging are easily precipitated, so the discharge capacity tends to decrease.

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 discharge capacity tends to decrease because fewer sodium ions are absorbed and released. On the other hand, if the amount of CrO+FeO+MnO+CoO+NiO is too large, foreign crystals tend to precipitate and the discharge capacity tends to decrease.

P ₂ O ₅ , SiO ₂ and B ₂ O ₃ form a three-dimensional network structure, and therefore have the effect of stabilizing the structure of the positive electrode active material. In particular, P ₂ O ₅ and SiO ₂ are preferred because they have excellent ion conductivity, and P ₂ O ₅ is the most preferred. The content of P ₂ O ₅ + SiO ₂ + B ₂ O ₃ is 15 to 70%, preferably 20 to 60%, particularly preferably 25 to 45%. If the amount of _{P 2} O ₅ + SiO ₂ + B ₂ O ₃ is too small, the discharge capacity tends to decrease when repeatedly charged and discharged. On the other hand, if the amount of P ₂ O ₅ + SiO ₂ + B ₂ O ₃ is too large, heterogeneous crystals such as P ₂ O ₅ that do not contribute to charging and discharging tend to precipitate. The contents of the P ₂ O ₅ , SiO ₂ and B ₂ O ₃ components are preferably 0 to 70%, 15 to 70%, 20 to 60%, and particularly preferably 25 to 45%, respectively.

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 oxide notation, MgO, CaO, SrO, BaO, ZnO, CuO, Al ₂ O ₃ , GeO ₂ , Nb ₂ O ₅ , ZrO ₂ , and Sb ₂ O ₅ , and in particular, Al ₂ O ₃ , which acts as a network-forming oxide, and V ₂ O _5, which is 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%.

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, those containing, in mole percent of oxide, 0-90% SnO, 0-90% Bi ₂ O ₃ , 0-90% Nb ₂ O ₅ , 0-90% TiO 2, 0-90% Fe _{2 O 3} , 5-75% SiO ₂ +B ₂ O ₃ +P ₂ O ₅ , and 0-80% Na ₂ O. The negative electrode active material powder having this composition is suitable for sodium ion secondary batteries. By making the above configuration, 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 Na ₂ O, the material has even better sodium ion conductivity. As a result, the volume change that occurs when sodium ions are absorbed and released can be suppressed, making it possible to obtain a negative electrode active material with even more excellent cycle characteristics.

The reasons for limiting the composition of the negative electrode active material powder as described above are explained below.

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.

The SnO content is preferably 0-90%, 45-85%, 55-75%, particularly 60-72%.

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

The content of Nb ₂ O ₅ is preferably 0 to 90%, 7 to 79%, 9 to 69%, 11 to 59%, 13 to 49%, particularly preferably 15 to 39%.

The _TiO2 content is preferably 0 to 90%, 5 to 72%, 10 to 68%, 12 to 58%, 15 to 49%, particularly preferably 15 to 39%.

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

The content of SnO+Bi ₂ O ₃ +Nb ₂ O ₅ +TiO ₂ +Fe ₂ O ₃ is preferably 0 to 90%, more preferably 5 to 85%, and even more preferably 10 to 80%.

SiO ₂ , B ₂ O ₃ and P ₂ O ₅ 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, SiO ₂ and P ₂ O ₅ 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.

SiO ₂ +B ₂ O ₃ +P ₂ O ₅ is preferably 5-85%, 6-79%, 7-69%, 8-59%, 9-49%, and particularly preferably 10-39%. If the amount of SiO ₂ +B ₂ O ₃ +P ₂ O ₅ is too small, the volume change of the negative electrode active material component 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 SiO ₂ +B ₂ O ₃ +P ₂ O ₅ is too large, the content of the negative electrode active material component becomes relatively small, and the charge/discharge capacity per unit mass of the negative electrode active material tends to become small.

The preferred ranges of the respective contents of SiO ₂ , B ₂ O ₃ and P ₂ O ₅ are as follows.

The content of _SiO2 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 content of _SiO2 is too high, the discharge capacity tends to decrease.

The content of P ₂ O ₅ is preferably 5 to 75%, 7 to 60%, 10 to 50%, 12 to 40%, and particularly preferably 20 to 35%. If the content of _{P 2} O ₅ is too low, it is difficult to obtain the above-mentioned effects. On the other hand, if the content of P ₂ O ₅ 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 produced, unwanted heterogeneous crystals are generated and the P ₂ O ₅ network is broken, so that the cycle characteristics are likely to decrease.

The content of B ₂ O ₃ 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 content of _{B 2} O ₃ is too high, the discharge capacity is likely to decrease and the chemical durability is likely to decrease.

Na ₂ O 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 reducing the operating voltage of the negative electrode. The content of _{Na 2} O is preferably 0 to 80%, 1 to 70%, and particularly 5 to 60%. If the content of Na ₂ O is too high, a large amount of heterogeneous crystals containing sodium ions (Na ₄ P ₂ O ₇ , NaPO ₄ , etc.) are formed, and the cycle characteristics tend to decrease. In addition, since the content of the active material component becomes relatively small, the discharge capacity tends to decrease.

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, making the operating voltage more likely to decrease, or the electrode density decreases, decreasing the capacity per unit volume of the battery. On the other hand, if the average particle size of the active material powder is too large, the electrode layer tends to be inferior in density and surface smoothness.

In this specification, the average particle size refers to the median size of primary particles, D ₅₀ (50% volume cumulative size), measured by a laser diffraction particle size distribution analyzer.

The specific surface area of the active material powder is preferably 1 to 100 ^{m 2} /g, 3 to 80 m ² /g, 5 to 70 m ² /g, and particularly preferably 10 to 50 m ² /g. If the specific surface area of the active material powder is too small, the denseness 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.

(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 , _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 by the conductive assistant in the electrode layer _. Therefore, when the active material powder is softened and fluidized by firing, the electron conduction path by the conductive assistant is not easily cut, and it is possible to suppress the decrease in the discharge capacity of the all-solid-state battery. By including the inorganic filler powder in the electrode mixture, it is possible to match the thermal expansion coefficient of the electrode layer to the thermal expansion coefficient of the solid electrolyte layer, and it is possible to suppress the problem of peeling of the electrode layer caused by the difference in thermal expansion coefficient.

The surface of the inorganic filler powder may be coated with carbon. This provides electrical conductivity to the inorganic filler powder, thereby increasing the electrical conductivity of the electrode layer and, as a result, improving the battery characteristics.

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 the electrode mixture, it is possible to form an electronic conduction path by the inorganic filler powder itself 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.

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. Furthermore, 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.

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, 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 additive to function as a skeleton for maintaining the 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 is likely to decrease.

The specific surface area of the inorganic filler powder is preferably 1 to 400 m ² /g, 2 to 200 m ² /g, 3 to 100 m ² /g, and particularly preferably 3 to 70 m ² /g. 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 for the conductive additive to function as a skeleton for maintaining an electron conduction path, or for the inorganic filler powder itself to form an electron conduction path.

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 1.15 to 5. If this ratio is too small, it becomes difficult for the conductive additive to function as a skeleton for maintaining the electron conduction path, or for the inorganic filler powder itself to function as a path for forming the 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 is likely to decrease.

The content of inorganic filler powder in the electrode mixture is preferably 1 to 40%, 3 to 30%, and particularly 4 to 25% by volume. If the content of 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 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 inorganic filler powder is too high, the proportion of 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.

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

The ratio of the volume percent content of inorganic filler powder to active material powder (inorganic filler powder content/active material powder content) is preferably 0.01 to 1, 0.05 to 0.8, and particularly 0.1 to 0.5. If this ratio is too small, it becomes difficult to form an electronic conduction path, and as a result, the discharge capacity is likely 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 is likely to decrease.

(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.

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 electronic 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 proportion 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 a conductive assistant. However, even in that case, it is not necessarily prevented from containing a conductive assistant.

(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.

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.

The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples.

Table 1 shows Examples 1 to 4 and Comparative Example 1.

(a) Preparation of Positive Electrode Active Material Precursor Powder Sodium metaphosphate (NaPO ₃ ), ferric oxide (Fe ₂ O ₃ ), and orthophosphoric acid (H ₃ PO ₄ ) were used as raw materials, and raw material powders were mixed to have a molar ratio of 40% Na ₂ O ₃ 20% Fe ₂ O 3 40% P ₂ O ₅ , and melted in an air atmosphere for 45 minutes at 1250° C. Thereafter, the molten glass was poured into a pair of rollers and formed into a film while being quenched, thereby preparing a positive electrode active material precursor.

The obtained positive electrode active material precursor was ball milled for 5 hours using φ20 mm _{Al2O3 balls} , and then ball milled for 100 hours in ethanol using φ5 mm _ZrO2 balls to obtain a positive electrode active material precursor powder with an average particle size _D50 of 0.7 μm.

(b) Preparation of Positive Electrode Composite 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 composite. Note that in Example 4, no conductive additive was added.

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, and then dried at 100°C for about 1 hour. Then, the mixture was baked at 620° _C for 30 minutes in a nitrogen atmosphere to obtain _Al2O3 powder with a carbon-coated surface.

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

(c) Preparation of Test Battery The above slurried positive electrode mixture was applied to one surface of a solid electrolyte sheet (Li ₂ O stabilized β″-alumina manufactured by Ionotec, composition formula: Na _1.7 Li _0.3 Al _10.7 O ₁₇ , 1 cm square, 1 mm thick) to an area of 1 cm ² and a thickness of 200 μm, and dried at 70° C. for 3 hours. Next, the positive electrode mixture was sintered and the positive electrode active material precursor powder was crystallized to form a positive electrode layer by firing at 550° C. for 1 hour in a mixed gas atmosphere of nitrogen and hydrogen (nitrogen 96 vol %, hydrogen 4 vol %). When the X-ray diffraction pattern of the obtained positive electrode layer was confirmed, diffraction lines derived from the active material crystals, Na ₂ FeP ₂ O _7, were confirmed.

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 the counter electrode, and the sheet was placed on the bottom lid of a coin cell, after which the top lid was placed on top to produce a CR2032-type test battery.

(d) Charge and discharge test A charge and discharge test was performed using the above test battery. The results are shown in Table 1. In the charge and discharge test, charging (sodium ion release from the positive electrode active material) was performed by CC (constant current) charging from the open circuit voltage (OCV) to 4.5 V, and discharging (sodium ion absorption into the positive electrode active material) was performed by CC discharging from 4.5 V to 2 V. The C rate was 0.01 C, and the test was performed 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.

As shown in Table 1, in Examples 1 to 5 in which 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 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 (7)

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 . The active material powder contains, in mole percent in terms of oxide, Na ₂ O 8-55%, CrO+FeO+MnO+CoO+NiO 10-70%, P ₂ O ₅ +SiO ₂ +B ₂ O ₃ The electrode mixture for a sodium ion secondary battery according to claim 1, characterized in that it contains 15 to 70%. 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. The electrode mixture for a sodium ion secondary battery according to claim 3 , wherein the active material powder is made of glass powder. 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. 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 . An all-solid-state secondary battery comprising the electrode layer according to claim 6 .