Cathode material for energy storage devices and energy storage devices

Abstract

To provide a method for manufacturing a positive electrode material for a power storage device having excellent charge / discharge characteristics, by suppressing excessive reactions between positive electrode active material precursor powders or between a positive electrode active material precursor powder and a solid electrolyte during heat treatment.SOLUTION: A method for manufacturing a positive electrode material for a power storage device, includes a step of heat-treating a raw material containing a positive electrode active material precursor powder made of an amorphous oxide material, and the crystallization temperature of the positive electrode active material precursor powder is 490°C or lower.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a positive electrode material used in a power storage device such as a sodium ion secondary battery.

Background Art

[0002] Lithium ion secondary batteries have established themselves as indispensable high-capacity and lightweight power sources for portable electronic terminals, electric vehicles, etc. As the positive electrode active material, an active material containing an olivine-type crystal represented by the general formula LiFePO4 has attracted attention. However, since there are concerns about problems such as the global increase in raw material prices for lithium, research on sodium ion secondary batteries has been conducted in recent years as an alternative. In Patent Document 1, Na x M y A positive electrode active material composed of P2O7 crystal (M is at least one or more transition metal elements selected from Fe, Cr, Mn, Co, and Ni, 1.20 ≤ x ≤ 2.10, 0.95 ≤ y ≤ 1.60) is disclosed. Further, Patent Document 2 discloses a method for manufacturing an all-solid-state battery using a similar sodium-containing positive electrode active material.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0004] In the positive electrode active material for sodium-ion secondary batteries described in Patent Document 1, it is necessary to reduce the Fe ions in the precursor glass powder from trivalent to divalent by firing at high temperatures in order to exhibit the battery characteristics. However, during firing, the glass powder particles fuse excessively together, forming coarse particles, which reduces the specific surface area of ​​the positive electrode active material and prevents the desired battery characteristics from being obtained.

[0005] Furthermore, when manufacturing all-solid-state batteries, the positive electrode active material precursor powder is integrally fired with a solid electrolyte powder consisting of beta-alumina or NASICON crystals. This improves the adhesion between the positive electrode active material powder and the solid electrolyte powder after firing, resulting in the production of a solid-state secondary battery with excellent discharge characteristics. However, as described in Patent Document 2, there was a problem in that the positive electrode active material precursor powder and the solid electrolyte powder react during firing, causing marisite-type NaFePO4 crystals that do not contribute to charging and discharging to precipitate, thus reducing the charge and discharge capacity.

[0006] Furthermore, during firing, elements contained in the positive electrode active material precursor powder and the solid electrolyte can diffuse with each other, potentially forming a partially high-resistance layer and degrading the rate characteristics of the all-solid-state battery. To suppress the formation of the high-resistance layer, a method has been proposed in which each material is coated with a barrier layer using alkoxide raw materials, but this method has the problem of increasing costs because alkoxide raw materials are expensive.

[0007] In view of the above, the present invention aims to provide a method for producing a positive electrode material for energy storage devices with excellent charge-discharge characteristics by suppressing excessive reactions between positive electrode active material precursor powders and between positive electrode active material precursor powders and solid electrolytes during heat treatment. [Means for solving the problem]

[0008] The present invention relates to a method for manufacturing a positive electrode material for energy storage devices, which includes a step of heat-treating a raw material containing a positive electrode active material precursor powder made of an amorphous oxide material, characterized in that the crystallization temperature of the positive electrode active material precursor powder is 490°C or lower. By using a positive electrode active material precursor powder with a low crystallization temperature of 490°C or lower as a raw material, the crystallization of the positive electrode active material precursor powder can be promoted even when heat-treated (fired) at a low temperature. This allows the heat treatment temperature to be set lower, and excessive reactions between raw materials during heat treatment can be suppressed. As a result, it is possible to manufacture a positive electrode material with excellent charge-discharge characteristics (especially charge-discharge characteristics at relatively high rates of 0.1°C or higher). In this invention, the crystallization temperature is the value measured by DTA (differential thermal analysis).

[0009] In the method for manufacturing the positive electrode material for energy storage devices of the present invention, it is preferable that the heat treatment temperature is 400 to 600°C. By doing so, excessive reactions between the raw materials can be suppressed, making it possible to manufacture a positive electrode material with excellent charge-discharge characteristics.

[0010] In the present invention, the method for manufacturing a positive electrode material for energy storage devices preferably involves a heat treatment time of less than 3 hours. This suppresses excessive reactions between the raw materials, making it possible to manufacture a positive electrode material with excellent charge-discharge characteristics.

[0011] In the method for manufacturing the positive electrode material for energy storage devices of the present invention, it is preferable to perform the heat treatment in a reducing atmosphere. By doing so, the valence of the transition metal elements in the positive electrode active material precursor powder can be controlled to a lower valence. This suppresses the generation of crystalline phases that do not act as active material during heat treatment, making it possible to manufacture a positive electrode material with excellent charge and discharge capacity.

[0012] In the present invention, the method for manufacturing a positive electrode material for energy storage devices preferably involves a positive electrode active material precursor powder with an average particle size of less than 0.01 to 0.7 μm. This allows for a reduction in the crystallization temperature of the positive electrode active material precursor powder. Furthermore, the increased specific surface area of ​​the positive electrode active material precursor powder increases the contact area with the atmospheric gas, making it easier to control the valence state of the transition metal elements in the positive electrode active material precursor powder.

[0013] The present invention relates to a method for producing a positive electrode material for energy storage devices, wherein the positive electrode active material precursor powder preferably contains 25-55% Na2O, 10-30% Fe2O3+Cr2O3+MnO+CoO+NiO, and 25-55% P2O5, in molar percentages in terms of oxides as follows. In this specification, "x+y+···" represents the total amount of each component.

[0014] The method for producing a positive electrode material for energy storage devices of the present invention preferably includes solid electrolyte powder as a raw material.

[0015] In the method for producing a positive electrode material for energy storage devices of the present invention, it is preferable that the solid electrolyte powder is β-alumina, β''-alumina, or NASICON crystal.

[0016] In the method for manufacturing the positive electrode material for energy storage devices of the present invention, it is preferable that the average particle size of the solid electrolyte powder is 0.05 to 3 μm. This facilitates the formation of ion conduction paths in the positive electrode material, thereby improving the charge and discharge characteristics.

[0017] The present invention's method for manufacturing a positive electrode material for energy storage devices preferably includes conductive carbon as a raw material. This facilitates the formation of conductive paths within the positive electrode material, thereby improving charge and discharge characteristics.

[0018] The present invention relates to a method for producing a positive electrode material for energy storage devices, wherein the raw materials preferably contain, by mass%, 30-100% positive electrode active material precursor powder, 0-70% solid electrolyte powder, and 0-20% conductive carbon.

[0019] The positive electrode active material precursor powder for energy storage devices of the present invention is characterized by being made of an amorphous oxide material having a crystallization temperature of 490°C or lower.

[0020] The positive electrode active material precursor powder for the energy storage device of the present invention preferably has an average particle size of less than 0.01 to 0.7 μm.

[0021] The positive electrode active material precursor powder for energy storage devices of the present invention preferably contains 25-55% Na2O, 10-30% Fe2O3+Cr2O3+MnO+CoO+NiO, and 25-55% P2O5, in molar percentages equivalent to the following oxides.

[0022] The positive electrode material for energy storage devices of the present invention comprises a solid electrolyte and a positive electrode active material, and is characterized by having a matrix-domain structure in which the positive electrode active material is a matrix component and the solid electrolyte is a domain component.

[0023] In the positive electrode material for energy storage devices of the present invention, it is preferable that the number of solid electrolyte powder particles with a particle size of 0.5 μm or less per field of view area of ​​1 μm × 1 μm in cross-section is 2 or more.

[0024] The energy storage device of the present invention is characterized by comprising a positive electrode material layer made of the above-described positive electrode material for energy storage devices.

[0025] The energy storage device of the present invention preferably comprises a solid electrolyte layer, wherein the positive electrode material layer is formed on the surface of the solid electrolyte layer.

[0026] In the energy storage device of the present invention, it is preferable that the thickness of the heterogeneous phase at the interface between the positive electrode material layer and the solid electrolyte layer is 1 μm or less.

[0027] The energy storage device of the present invention has an internal resistance per unit area of ​​the positive electrode material layer at 30°C, with a minimum value of 2000 Ωcm² during the discharge process. 2 The following is preferable: [Effects of the Invention]

[0028] According to the present invention, it is possible to manufacture a positive electrode material for energy storage devices with excellent charge-discharge characteristics by suppressing excessive reactions between positive electrode active material precursor powders and between positive electrode active material precursor powders and solid electrolytes during heat treatment. [Brief explanation of the drawing]

[0029] [Figure 1] (a) shows the elemental mapping profile of the cathode material layer in Example 1. (b) shows the elemental mapping profile of the cathode material layer in the comparative example. [Modes for carrying out the invention]

[0030] The present invention relates to a method for producing a positive electrode material for energy storage devices, which includes a step of heat-treating a raw material containing a positive electrode active material precursor powder made of an amorphous oxide material. Each component will be described in detail below.

[0031] (1) Positive electrode active material precursor powder The positive electrode active material precursor powder consists of an amorphous oxide material that generates positive electrode active material crystals through heat treatment. The amorphous oxide material generates positive electrode active material crystals during heat treatment and also softens and flows, forming a dense positive electrode material layer. This is preferable because it allows for the formation of good ion conduction paths. In this invention, the term "amorphous oxide material" is not limited to completely amorphous oxide materials, but also includes materials containing some crystals (e.g., crystallinity of 10% or less).

[0032] The crystallization temperature of the positive electrode active material precursor powder is 490°C or lower, preferably 470°C or lower, and particularly preferably 450°C or lower. If the crystallization temperature of the positive electrode active material precursor powder is too high, it will be necessary to heat-treat the powder at a high temperature to crystallize it. In addition, the heat treatment time (holding time at the maximum temperature) may also be longer. As a result, during heat treatment, the positive electrode active material precursor powder particles may fuse excessively together, forming coarse particles, which reduces the specific surface area of ​​the positive electrode active material and tends to decrease the charge-discharge characteristics. Furthermore, in the case of all-solid-state batteries, the positive electrode active material precursor powder and solid electrolyte powder may react during heat treatment, causing crystals that do not contribute to charge-discharge (such as malisite-type NaFePO4 crystals) to precipitate, potentially reducing the charge-discharge capacity. Alternatively, elements contained in the positive electrode active material precursor powder and solid electrolyte powder may diffuse with each other during heat treatment, forming a partially high-resistance layer, which may reduce the rate characteristics of the all-solid-state battery. The lower limit of the crystallization temperature of the positive electrode active material precursor powder is not particularly limited, but in practice it is 300°C or higher, and even 350°C or higher.

[0033] Furthermore, the crystallization temperature of the positive electrode active material precursor powder varies not only with its composition but also with its particle size. Specifically, as the particle size of the positive electrode active material precursor powder decreases, the specific surface area increases, leading to higher surface energy and making surface crystallization more likely. As a result, the crystallization temperature tends to decrease.

[0034] The positive electrode active material precursor powder preferably contains 25-55% Na2O, 10-30% Fe2O3+Cr2O3+MnO+CoO+NiO, and 25-55% P2O5, in molar percentages based on the oxides listed below. The reason for this limitation of composition is explained below. In the following descriptions of the content of each component, unless otherwise specified, "%" means "molar percent".

[0035] Na2O is the general formula Na x Ma y P2O z(M is at least one transition metal element selected from Fe, Cr, Mn, Co, and Ni, 1.20 ≦ x ≦ 2.10, 0.95 ≦ y ≦ 1.60) is the main component of the cathode active material crystal. The content of Na2O is preferably 25 to 55%, particularly 30 to 50%. If the content of Na2O is too low or too high, the charge-discharge capacity tends to decrease.

[0036] Fe2O3, Cr2O3, MnO, CoO, and NiO are also the main components of the cathode active material crystal represented by the general formula Na x Ma y P2O z The content of Fe2O3 + Cr2O3 + MnO + CoO + NiO is preferably 10 to 30%, particularly 15 to 25%. If the content of Fe2O3 + Cr2O3 + MnO + CoO + NiO is too low, the charge-discharge capacity tends to decrease. On the other hand, if the content of Fe2O3 + Cr2O3 + MnO + CoO + NiO is too high, crystals such as undesirable Fe2O3, Cr2O3, MnO, CoO, or NiO are likely to precipitate. In order to improve the cycle characteristics, it is preferable to actively contain Fe2O3. The content of Fe2O3 is preferably 1 to 30%, 5 to 30%, 10 to 30%, particularly 15 to 25%. The content of each component of Cr2O3, MnO, CoO, and NiO is preferably 0 to 30%, 10 to 30%, particularly 15 to 25% respectively. Also, when containing at least two components selected from Fe2O3, Cr2O3, MnO, CoO, and NiO, the total content is preferably 10 to 30%, particularly 15 to 25%.

[0037] P2O5 is also the main component of the cathode active material crystal represented by the general formula Na x Ma y P2O z The content of P2O5 is preferably 25 to 55%, particularly 30 to 50%. If the content of P2O5 is too low or too high, the charge-discharge capacity tends to decrease.

[0038] In addition to the components listed above, the positive electrode active material precursor powder may also contain V2O5, Nb2O5, MgO, Al2O3, TiO2, ZrO2, or Sc2O3. These components enhance conductivity (electron conductivity), making it easier to improve the high-speed charge-discharge characteristics of the positive electrode active material. The content of the above components is preferably 0-25% in total, and particularly preferably 0.2-10%. If the content of the above components is too high, heterogeneous crystals that do not contribute to the battery characteristics will be formed, and the charge-discharge capacity will tend to decrease.

[0039] In addition to the above components, the material may also contain SiO2, B2O3, GeO2, Ga2O3, Sb2O3, or Bi2O3. Including these components improves the glass-forming ability and makes it easier to obtain a homogeneous positive electrode active material precursor powder. The content of the above components is preferably 0 to 25% in total, and particularly preferably 0.2 to 10%. Since the above components do not contribute to the battery characteristics, if their content is too high, the charge and discharge capacity tends to decrease.

[0040] The positive electrode active material precursor powder is preferably produced by melting and molding a batch of raw materials. This method is preferable because it makes it easier to obtain amorphous positive electrode active material precursor powder with excellent homogeneity. Specifically, the positive electrode active material precursor powder can be produced as follows.

[0041] First, the raw materials are prepared to obtain a raw material batch to achieve the desired composition. Next, the obtained raw material batch is melted. The melting temperature can be adjusted as appropriate to ensure that the raw material batch is melted homogeneously. For example, a melting temperature of 800°C or higher, and particularly 900°C or higher, is preferable. There is no particular upper limit, but if the melting temperature is too high, it can lead to energy loss and evaporation of sodium components, so a melting temperature of 1500°C or lower, and particularly 1400°C or lower, is preferable.

[0042] Next, the resulting molten material is molded. The molding method is not particularly limited; for example, the molten material may be poured between a pair of cooling rolls and molded into a film while rapidly cooling, or the molten material may be poured into a mold and molded into an ingot.

[0043] Next, the obtained molded body is crushed to obtain positive electrode active material precursor powder. The average particle size of the positive electrode active material precursor powder is preferably 0.01 to less than 0.7 μm, 0.03 to less than 0.7 μm, 0.05 to 0.6 μm, and particularly preferably 0.1 to 0.5 μm. If the average particle size of the positive electrode active material precursor powder is too small, the cohesive force between particles becomes strong when used in paste form, making it difficult to disperse in the paste. Also, when mixed with solid electrolyte powder, it becomes difficult to uniformly disperse the positive electrode active material precursor powder in the mixture, which can increase internal resistance and reduce charge / discharge capacity. On the other hand, if the average particle size of the positive electrode active material precursor powder is too large, the crystallization temperature tends to be high. Also, the amount of ion diffusion per unit surface area of ​​the positive electrode material tends to decrease, and the internal resistance tends to increase. Furthermore, when mixed with solid electrolyte powder, the adhesion between the positive electrode active material precursor powder and the solid electrolyte powder decreases, which reduces the mechanical strength of the positive electrode material layer, and consequently tends to reduce charge / discharge capacity. Alternatively, poor adhesion between the positive electrode material layer and the solid electrolyte layer may lead to the positive electrode material layer delaminating from the solid electrolyte layer.

[0044] In this invention, the average particle diameter is D 50 This refers to the average particle diameter (based on volume) and is measured using the laser diffraction scattering method.

[0045] (2) Other raw materials (solid electrolyte powder) Solid electrolyte powder is a component responsible for ion conduction in the positive electrode material layer of all-solid-state energy storage devices.

[0046] Examples of solid electrolyte powders include beta-alumina or NASICON crystals, which have excellent sodium ion conductivity. Beta-alumina exists in two crystalline forms: β-alumina (theoretical composition formula: Na2O·11Al2O3) and β''-alumina (theoretical composition formula: Na2O·5,3Al2O3). Since β''-alumina is a metastable substance, it is usually used with Li2O or MgO added as a stabilizer. Because β''-alumina has higher sodium ion conductivity than β-alumina, it is preferable to use β''-alumina alone or a mixture of β''-alumina and β-alumina, and Li2O-stabilized β''-alumina (Na 1.7 Li 0.3 Al 10.7 O 17 ) or MgO-stabilized β''-alumina ((Al 10.32 Mg 0.68 O 16 )(Na 1.68 Using O)) is more preferable.

[0047] As for NASICON crystals, Na3Zr2Si2PO 12 na 3.2 Zr 1.3 Si 2.2 P 0.7 O 10.5 Na3Zr 1.6 Ti 0.4 Si2er 12 Na3Hf2Si2PO 12 na 3.4 Zr 0.9 Hf 1.4 Al 0.6 Si 1.2 P 1.8 O 12 Na3Zr 1.7 Nb 0.24 Si2er 12 na 3.6 Ti 0.2 Y 0.7 Si 2.8 O9, Na3Zr 1.88 Y 0.12 Si2er 12 na 3.12 Zr 1.88 Y 0.12 Si2er 12na 3.6 Zr 0.13 Yb 1.67 Si 0.11 P 2.9 O 12 These include, in particular, Na 3.12 Zr 1.88 Y 0.12 Si2er 12 This is preferable because it has excellent sodium ion conductivity.

[0048] The average particle size of the solid electrolyte powder is preferably 0.05 to 3 μm, less than 0.05 to 1.8 μm, 0.05 to 1.5 μm, 0.1 to 1.2 μm, and particularly preferably 0.1 to 0.9 μm. If the average particle size of the solid electrolyte powder is too small, it becomes difficult to uniformly mix it with the positive electrode active material precursor powder, and hygroscopic and carbonated ionization may reduce ionic conductivity or promote excessive reaction with the positive electrode active material precursor powder. As a result, the internal resistance of the positive electrode material layer tends to increase, and the voltage characteristics and charge / discharge capacity tend to decrease. On the other hand, if the average particle size of the solid electrolyte powder is too large, it significantly inhibits the softening flow of the positive electrode active material precursor powder, which tends to result in a less smooth positive electrode material layer, reduced mechanical strength, and increased internal resistance.

[0049] (Conductive carbon) Conductive carbon is a component that forms conductive paths in the positive electrode material. When adding conductive carbon, it is preferable to add it when grinding the positive electrode active material precursor powder. Conductive carbon acts as a grinding aid, enabling homogeneous mixing with the positive electrode active material precursor powder. Furthermore, it suppresses excessive fusion of positive electrode active material precursor powder particles during heat treatment, making it easier to ensure conductivity and improving rapid charge-discharge characteristics.

[0050] (Binding agent) A binder is a material used to integrate raw materials (raw material powders). Examples of binders include cellulose derivatives such as carboxymethylcellulose, hydroxypropylmethylcellulose, hydroxypropylcellulose, ethylcellulose, hydroxyethylcellulose, and hydroxymethylcellulose, or water-soluble polymers such as polyvinyl alcohol; thermosetting resins such as thermosetting polyimide, phenolic resin, epoxy resin, urea resin, melamine resin, unsaturated polyester resin, and polyurethane; polycarbonate resins such as polypropylene carbonate; and polyvinylidene fluoride.

[0051] (3) Composition of raw materials The raw materials preferably contain, by mass%, 30-100% positive electrode active material precursor powder, 0-70% solid electrolyte powder, and 0-20% conductive carbon; more preferably 44.5-94.5% positive electrode active material precursor powder, 5-55% solid electrolyte powder, and 0.5-15% conductive carbon; and even more preferably 50-92% positive electrode active material precursor powder, 7-50% solid electrolyte powder, and 1-10% conductive carbon. If the content of positive electrode active material precursor powder is too low, the amount of components that absorb or release sodium ions in the positive electrode material during charging and discharging decreases, which tends to reduce the charge and discharge capacity of the energy storage device. If the content of conductive carbon or solid electrolyte powder is too high, the binding properties of the positive electrode active material precursor powder decrease, increasing the internal resistance, which tends to reduce the voltage characteristics and charge and discharge capacity.

[0052] For mixing the raw materials, mixers such as rotary-orbit mixers and tumbler mixers, as well as general grinders such as mortars, grinders, ball mills, attritors, vibrating ball mills, satellite ball mills, planetary ball mills, jet mills, and bead mills can be used. In particular, the use of a planetary ball mill is preferred. In a planetary ball mill, the pot rotates on its own axis while the base plate revolves around it, efficiently generating very high shear energy, which makes it possible to homogeneously disperse the raw materials.

[0053] (4) Heat treatment conditions The heat treatment temperature (maximum temperature during heat treatment) is preferably 400-600°C, 410-580°C, 420-575°C, and particularly 425-560°C. Furthermore, in relation to the crystallization temperature of the positive electrode active material precursor powder, the heat treatment temperature is preferably +0°C to +200°C, +30°C to +150°C, and particularly +50°C to +120°C relative to the crystallization temperature of the positive electrode active material precursor powder. If the heat treatment temperature is too low, the crystallization of the positive electrode active material precursor powder will be insufficient, and the remaining amorphous phase will become a high-resistance region, which tends to reduce the voltage characteristics and charge / discharge capacity. On the other hand, if the heat treatment temperature is too high, the positive electrode active material precursor powders will fuse excessively together, forming coarse particles, which tends to reduce the specific surface area of ​​the positive electrode active material and decrease the charge / discharge characteristics. Furthermore, in the case of all-solid-state batteries, the positive electrode active material precursor powder and the solid electrolyte powder may react during heat treatment, causing crystals that do not contribute to charging and discharging (such as maliscite-type NaFePO4 crystals) to precipitate, potentially reducing the charge and discharge capacity. Alternatively, elements contained in the positive electrode active material precursor powder and the solid electrolyte powder may diffuse with each other during heat treatment, forming a partially high-resistance layer, which may degrade the rate characteristics of the all-solid-state battery.

[0054] The heat treatment time (holding time at the highest temperature during heat treatment) is preferably less than 3 hours, 2 hours or less, 1 hour or less, and especially 45 minutes or less. If the heat treatment time is too long, the positive electrode active material precursor powders tend to fuse excessively together, forming coarse particles, reducing the specific surface area of ​​the positive electrode active material, and thus degrading the charge-discharge characteristics. In the case of all-solid-state batteries, the positive electrode active material precursor powder and solid electrolyte powder react during heat treatment, and crystals that do not contribute to charge-discharge (such as malisite-type NaFePO4 crystals) may precipitate, potentially reducing the charge-discharge capacity. Alternatively, elements contained in the positive electrode active material precursor powder and solid electrolyte powder may diffuse with each other during heat treatment, forming a partially high-resistance layer, which may degrade the rate characteristics of the all-solid-state battery. On the other hand, if the heat treatment time is too short, the crystallization of the positive electrode active material precursor powder becomes insufficient, and the remaining amorphous phase becomes a high-resistance region, which tends to reduce the voltage characteristics and charge-discharge capacity. Therefore, the heat treatment time is preferably 1 minute or more, and especially 5 minutes or more.

[0055] The atmosphere during heat treatment is preferably a reducing atmosphere. Examples of a reducing atmosphere include an atmosphere containing at least one reducing gas selected from H2, NH3, CO, H2S, and SiH4. From the viewpoint of efficiently reducing Fe ions in the positive electrode active material precursor powder from trivalent to divalent, it is preferable that the atmosphere contains at least one selected from H2, NH3, and CO, and particularly preferable that it contains H2 gas. When using H2 gas, it is preferable to mix it with an inert gas such as N2 to reduce the risk of explosion during heat treatment. Specifically, it is preferable that the reducing gas contains N290-99.9% and H20.1-10% by volume, more preferably N290-99.5% and H20.5-10%, and even more preferably N292-99% and H21-8%.

[0056] For heat treatment, general heat treatment equipment such as electric heating furnaces, rotary kilns, microwave heating furnaces, and high-frequency heating furnaces can be used.

[0057] (5) Characteristics of the positive electrode material layer The positive electrode material layer obtained by the above method preferably has the following characteristics.

[0058] The positive electrode material for energy storage devices of the present invention preferably comprises a solid electrolyte and a positive electrode active material, and has a matrix-domain structure in which the positive electrode active material is a matrix component and the solid electrolyte is a domain component. Here, when the cross-section of the positive electrode material layer is observed with FESEM-EDX (Field Emission Scanning Electron Microscope with Energy Dispersive X-ray Spectrometer), the number of solid electrolyte powder particles with a particle size of 0.5 μm or less per 1 μm × 1 μm field of view is 2 particles / μm 2 In particular, 4 particles / μm 2The above is preferable. This facilitates the formation of ion conduction paths within the positive electrode material layer, thereby increasing the discharge capacity. However, if the number of solid electrolyte powder particles with a particle size of 0.5 μm or less per 1 μm × 1 μm field of view area is too high, the proportion of positive electrode active material in the positive electrode material layer will be relatively small, which may reduce the discharge capacity. Therefore, the upper limit is 30 particles / μm. 2 Below, especially 20 pieces / μm 2 The following is preferable:

[0059] Furthermore, when observing the cross-section of the positive electrode material layer with FESEM-EDX, it is preferable that the area ratio of solid electrolyte powder with a particle size of 0.5 μm or less per 1 μm × 1 μm field of view is 10% or more, and particularly 15% or more. This facilitates the formation of ion conduction paths within the positive electrode material layer, thereby increasing the discharge capacity. However, if the area ratio of solid electrolyte powder with a particle size of 0.5 μm or less per 1 μm × 1 μm field of view is too large, the proportion of positive electrode active material in the positive electrode material layer becomes relatively small, which may reduce the discharge capacity. Therefore, it is preferable that the upper limit be 60% or less, and particularly 50% or less.

[0060] Furthermore, the number and area ratio of the solid electrolyte powders mentioned above can be measured based on the mapping of the elements contained in the solid electrolyte powders.

[0061] A power storage device comprising a positive electrode material layer made of the positive electrode material of the present invention preferably has the following characteristics. The power storage device preferably comprises, for example, a solid electrolyte layer, with the positive electrode material layer formed on the surface of the solid electrolyte layer. Furthermore, it is preferable that the negative electrode material layer is formed on the surface of the solid electrolyte layer opposite to the surface on which the positive electrode material layer is formed.

[0062] If a heterogeneous phase consisting of crystals that do not contribute to charging and discharging (such as mallisite-type NaFePO4 crystals) is formed at the interface between the positive electrode material layer and the solid electrolyte layer, ion conduction paths become difficult to form, and the discharge capacity tends to decrease. Therefore, the thickness of the heterogeneous phase is preferably less than 1 μm, 0.8 μm or less, and particularly 0.6 μm or less, and it is most preferable that no such heterogeneous phase is formed.

[0063] The internal resistance per unit area of ​​the positive electrode material layer at 30°C is 2000 Ωcm, which is the minimum value during the discharge process. 2 Below, 1000Ωcm 2 Below, 600Ωcm 2 Below, 300Ωcm 2 The following, in particular, 100 Ωcm 2 The following is preferable. This improves the output characteristics, making it possible to increase the discharge capacity. [Examples]

[0064] The following describes in detail examples of how the present invention is applied to all-solid-state sodium-ion secondary batteries. However, the present invention is not limited in any way to the following examples.

[0065] Table 1 shows Examples 1-9 and comparative examples.

[0066] [Table 1]

[0067] (a) Preparation of cathode active material precursor powder Sodium metaphosphate (NaPO3), ferric oxide (Fe2O3), and orthophosphate (H3PO4) were used as raw materials. The raw material powders were blended to have a composition of 40% Na2O, 20% Fe2O, and 540% P2O in molar percentages, and melted at 1250°C for 45 minutes in an air atmosphere. The molten material was then poured between a pair of rotating rollers and molded while rapidly cooling to obtain a film-like glass with a thickness of 0.1 to 2 mm. The obtained film-like glass was then ground using a ball mill and a planetary ball mill to obtain glass powder (cathode active material precursor powder) with the particle sizes shown in Table 1. The crystallization temperature was measured using DTA (DTA8410, Rigaku Corporation). Powder X-ray diffraction (XRD) measurements confirmed that all obtained glass powders were amorphous.

[0068] (b) Preparation of solid electrolyte layer and solid electrolyte powder (b-1) Preparation of β''-alumina solid electrolyte layer and β''-alumina solid electrolyte powder Li2O-stabilized β''-alumina (manufactured by Ionotec, chemical formula: Na) 1.7 Li 0.3 Al 10.7 O 17 A solid electrolyte layer was obtained by processing the material into a sheet with a thickness of 0.5 mm. In addition, a solid electrolyte powder having the particle size shown in Table 1 was obtained by grinding the sheet-like Li2O-stabilized β''-alumina using a ball mill and a planetary ball mill.

[0069] (b-2) Preparation of NASICON solid electrolyte layer and NASICON solid electrolyte powder Sodium carbonate (Na2CO3), yttria-stabilized zirconia ((ZrO2)) with a yttrium content of 3.0% 0.97 (Y2O3) 0.03Using silicon dioxide (SiO2) and sodium metaphosphate (NaPO3), raw material powders were prepared to have a composition of Na2O 25.3%, ZrO2 31.6%, Y2O 31.0%, SiO2 33.7%, and P2O 58.4% in molar percentages. Next, the raw material powders were wet-mixed for 4 hours using ethanol as a medium. Then, the ethanol was evaporated, and the raw material powders were calcined at 1100°C for 8 hours, after which they were pulverized and air-classified using an air classifier (MDS-3 model, manufactured by Nippon Pneumatic Mfg. Co., Ltd.). The classified powder was weighed using an acrylic ester copolymer (Oricox KC-7000, manufactured by Kyoeisha Chemical Co., Ltd.) as a binder and benzyl butyl phthalate as a plasticizer, with a mass ratio of raw material powder:binder:plasticizer = 83.5:15:1.5. This mixture was dispersed in N-methylpyrrolidone and thoroughly stirred in a rotating / revolving mixer to form a slurry.

[0070] The slurry obtained above was applied to a PET film and dried at 70°C to obtain a green sheet. The obtained green sheet was pressed using an isotropic press at 90°C and 40 MPa for 5 minutes. The pressed green sheet was fired at 1220°C for 40 hours in an atmosphere with a dew point of -40°C or lower to obtain a solid electrolyte layer containing NASICON crystals.

[0071] Furthermore, the powder obtained after the above classification was molded using a φ20 mm mold at 40 MPa by uniaxial press, and a solid electrolyte containing NASICON crystals was obtained by calcining at 1220°C for 40 hours in an atmosphere with a dew point of -40°C or lower. By pulverizing the obtained solid electrolyte, a solid electrolyte powder having the particle size shown in Table 1 was obtained.

[0072] (c) Preparation of test batteries The cathode active material precursor powder and solid electrolyte powder obtained above, along with acetylene black (TIMCAL SUPER C65) as conductive carbon, were weighed in the proportions shown in Table 1 and mixed for 30 minutes using an agate mortar and pestle. 10 parts by mass of polypropylene carbonate was added to 100 parts by mass of the mixed powder, and then 30 parts by mass of N-methylpyrrolidone was added. The mixture was thoroughly stirred using a rotational / revolving mixer to form a slurry.

[0073] The obtained slurry is spread over an area of ​​1 cm² on one surface of the solid electrolyte layer obtained above. 2 The mixture was coated to a thickness of 80 μm and dried at 70°C for 3 hours. Next, the mixture was placed in a carbon container and heat-treated under the conditions described in Table 1 to form a positive electrode material layer on one surface of the solid electrolyte layer. All of the above operations were performed in an environment with a dew point of -40°C or lower.

[0074] When the powder X-ray diffraction patterns of the obtained cathode material layers were examined, Na2FeP2O7 crystals were confirmed in Examples 1-9, and marisite-type NaFePO4 crystals were confirmed in the comparative example. In addition, crystalline diffraction lines originating from the solid electrolyte powder used were confirmed in all cathode material layers.

[0075] When observing the cross-section of the cathode material layer with FESEM-EDX, the number and area percentage of solid electrolyte powder particles with a diameter of 0.5 μm or less per 1 μm × 1 μm field of view were calculated. These values ​​were measured based on the mapping of elements contained in the solid electrolyte powder. The results are shown in Table 1.

[0076] The cross-sections of the cathode material layer and the solid electrolyte layer were observed using FESEM-EDX, and the elements contained in the interface between the two layers were mapped. The elemental mapping profiles for Example 1 and the comparative example are shown in Figures 1(a) and (b). Comparing the profiles in Figures 1(a) and (b), it can be confirmed that in profile (b), some of the Na element diffuses from the cathode material layer to the solid electrolyte layer. This is thought to originate from a heterogeneous phase (such as a maliscite-type NaFePO4 crystalline phase) formed at the interface between the two layers. The thickness of the heterogeneous phase was determined from the elemental mapping profiles. The results are shown in Table 1.

[0077] Next, a current collector consisting of a 300 nm thick gold electrode was formed on the surface of the positive electrode material layer using a sputtering apparatus (SC-701AT, manufactured by Sanyu Electronics Co., Ltd.). Then, in an argon atmosphere with a dew point of -60°C or lower, metallic sodium, which would serve as the counter electrode, was pressed onto the other surface of the solid electrolyte layer. After placing it on the lower cover of the coin cell, the upper cover was placed over it to fabricate a CR2032 type test battery.

[0078] (d) Charge and discharge test The fabricated test batteries were subjected to charge-discharge tests at 30°C, and their discharge capacity was measured. The results are shown in Table 1. Discharge capacity is defined as the amount of electricity discharged per unit mass of positive electrode active material powder contained in the positive electrode material layer. In the charge-discharge tests, charging was performed by CC (constant current) charging from the open-circuit voltage (OCV) to 4.5V, and discharging was performed by CC discharge from 4.5V to 2V. The C rates tested were 0.02C, 0.1C, 0.2C, and 1C.

[0079] (e) Internal resistance evaluation test The change in internal resistance of the fabricated test battery during charging and discharging at 30°C was determined by 3D impedance measurement using Biologic's VMP-300 and Toyo Technica's software Z-3D, Z-assist, and Z-FIT-analysis. The 3D impedance measurement was performed as follows: In Galvano Electrochemical Impedance Spectroscopy mode, the battery was charged from the open-circuit voltage (OCV) to 4.5V at 0.01C, and impedance measurements were performed while applying current at a frequency of 7MHz to 10mHz so that the response voltage was 5mV. Subsequently, impedance measurements were performed similarly while discharging from 4.5V to 2V at 0.01C. Using the aforementioned software, the change in resistance value of each resistive component constituting the battery during the charging and discharging process was determined from the Nyquist plot obtained from the impedance measurement. Among the resistive components, the lowest resistance value per unit area of ​​the positive electrode material layer during the discharge process is shown in Table 1 as the internal resistance.

[0080] As is clear from Table 1, Examples 1-9 showed excellent discharge capacities: 79-96 mAh / g at 0.02C, 58-92 mAh / g at 0.1C, and 42-87 mAh / g at 0.2C. Furthermore, Examples 1-3, 5, 6, 8, and 9 were able to charge and discharge even when the rate was increased to 1C, exhibiting discharge capacities of 39-75 mAh / g. On the other hand, the comparative examples had low discharge capacities: 68 mAh / g at 0.02C and 35 mAh / g at 0.1C, and were unable to charge or discharge at 0.2C or 1C.

Claims

1. It contains a solid electrolyte and a positive electrode active material, In a field of view area of ​​1 μm × 1 μm in cross-section, the number of solid electrolyte powder particles of 0.5 μm or less is between 2 and 20. A positive electrode material for an energy storage device, characterized in that the positive electrode active material includes a positive electrode active material crystal represented by the general formula Na x My P 2 O 7 (where M is at least one transition metal element selected from Fe, Cr, Mn, Co and Ni, 1.20 ≤ x ≤ 2.10, 0.95 ≤ y ≤ 1.60).

2. It contains a solid electrolyte and a positive electrode active material, In a field of view area of ​​1 μm × 1 μm in cross-section, the area ratio of solid electrolyte powder of 0.5 μm or less is 10% or more and 50% or less. A positive electrode material for an energy storage device, characterized in that the positive electrode active material includes a positive electrode active material crystal represented by the general formula Na x My P 2 O 7 (where M is at least one transition metal element selected from Fe, Cr, Mn, Co and Ni, 1.20 ≤ x ≤ 2.10, 0.95 ≤ y ≤ 1.60).

3. A positive electrode material for an energy storage device according to claim 1 or 2, characterized by having a matrix-domain structure in which the positive electrode active material is a matrix component and the solid electrolyte is a domain component.

4. The positive electrode active material crystal is Na ２ FeP ２ O ７ A positive electrode material for an energy storage device according to any one of claims 1 to 3, characterized in that it is represented as a crystal.

5. A power storage device characterized by comprising a positive electrode material layer made of the positive electrode material for power storage devices described in any one of claims 1 to 4.

6. The energy storage device according to claim 5, comprising a solid electrolyte layer, wherein the positive electrode material layer is formed on the surface of the solid electrolyte layer.

7. The energy storage device according to claim 6, characterized in that the thickness of the heterogeneous phase at the interface between the positive electrode material layer and the solid electrolyte layer is 1 μm or less.

8. The internal resistance per unit area of ​​the positive electrode material layer at 30°C is 2000 Ωcm at its minimum value during the discharge process. ２ The energy storage device according to any one of claims 5 to 7, characterized in that it is as follows:

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