Electrode mixture for secondary batteries, electrode for all-solid-state secondary batteries, and all-solid-state secondary battery

Abstract

The present invention provides an electrode mixture for secondary batteries, the electrode mixture being capable of achieving both electron conductivity and ion conductivity at high levels and being capable of effectively improving the battery characteristics. The electrode mixture for secondary batteries contains a positive electrode active material and a conductive aid. The positive electrode active material contains (i) at least one transition metal element that is selected from the group consisting of Cr, Fe, Mn, Co, Ni, Ti, and Nb, (ii) at least one element that is selected from the group consisting of P, Si, and B, and (iii) an element consisting of O, and the conductive aid contains sheet-like carbon.

Description

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

[0001] The present invention relates to an electrode mixture for a secondary battery, an electrode for an all-solid-state secondary battery using the electrode mixture for a secondary battery, and an all-solid-state secondary battery using the electrode for an all-solid-state secondary battery.

[0002] Lithium-ion secondary batteries have established themselves as high-capacity, lightweight power sources essential for mobile devices, electric vehicles, and other applications. However, current lithium-ion secondary batteries primarily use flammable organic electrolytes, raising concerns about the risk of fire. To address this issue, development is underway of all-solid-state lithium-ion secondary batteries, which use solid electrolytes instead of organic electrolytes. Furthermore, due to concerns about the rising cost of lithium raw materials worldwide, development of all-solid-state sodium-ion secondary batteries is also underway.

[0003] In the following Patent Document 1, Na ｘ (Fe １－ａ M ａ ) ｙ P ２ O ｚ (wherein M is at least one transition metal element selected from the group consisting of Cr, Mn, Co, and Ni, and 1.2≦x≦2.8, 0.95≦y≦1.6, 0≦a≦0.9, 7≦z≦8), a sodium ion conductive solid electrolyte, and conductive carbon. Patent Document 1 also describes that acetylene black, which has high electronic conductivity, is preferred as the conductive carbon.

[0004] International Publication No. 2016 / 084573

[0005] However, even with the positive electrode material for an electricity storage device of Patent Document 1, it is difficult to achieve both high levels of electronic conductivity and ionic conductivity, and there is a problem in that it is still difficult to sufficiently improve the battery characteristics.

[0006] An object of the present invention is to provide an electrode composite for a secondary battery, which can achieve high levels of both electronic conductivity and ionic conductivity and can effectively improve battery characteristics at high rates, an electrode for an all-solid-state secondary battery using the electrode composite for a secondary battery, and an all-solid-state secondary battery using the electrode for an all-solid-state secondary battery.

[0007] Each aspect of an electrode mixture for a secondary battery, an electrode for an all-solid-state secondary battery, and an all-solid-state secondary battery that can solve the above problems will be described.

[0008] An electrode composite for a secondary battery according to a first aspect of the present invention includes a positive electrode active material and a conductive additive, wherein the positive electrode active material includes (i) at least one transition metal element selected from the group consisting of Cr, Fe, Mn, Co, Ni, Ti, and Nb, (ii) at least one element selected from the group consisting of P, Si, and B, and (iii) an element consisting of O, and the conductive additive includes sheet carbon.

[0009] In the electrode mixture for a secondary battery of Aspect 2, in Aspect 1, it is preferable that the conductive additive consists of only sheet-like carbon.

[0010] In the electrode mixture for a secondary battery of Aspect 3, in Aspect 1 or Aspect 2, it is preferable that the sheet carbon contains graphene.

[0011] In the electrode mixture for a secondary battery of Aspect 4, in any one of Aspects 1 to 3, it is preferable that the sheet carbon consists of graphene alone.

[0012] Aspect 5 of the electrode mixture for a secondary battery is any one of Aspects 1 to 4, wherein the positive electrode active material is a compound represented by the general formula A ｘ MC ｙ O ｚ (A is at least one selected from the group consisting of Li and Na, M is at least one selected from the group consisting of Cr, Fe, Mn, Co and Ni, C is at least one selected from the group consisting of P, Si and B, and 0.2≦x≦4.2, 0.65≦y≦6.5, and 2.5≦z≦20).

[0013] In the electrode mixture for a secondary battery of Aspect 6, in any one of Aspects 1 to 5, it is preferable that the positive electrode active material further contains Na.

[0014] The electrode mixture for a secondary battery of Aspect 7 is preferably any one of Aspects 1 to 6, further comprising a solid electrolyte.

[0015] In the electrode mixture for a secondary battery of Aspect 8, in Aspect 7, it is preferable that the solid electrolyte contains β-alumina, β″-alumina, or NASICON-type crystals.

[0016] A secondary battery electrode composite of Aspect 9 is any one of Aspects 1 to 8, and preferably contains, in mass %, 30% to 98% of the positive electrode active material, 0.1% to 65% of a solid electrolyte, and 0.1% to 3% of the conductive additive.

[0017] In the electrode mixture for a secondary battery of Aspect 10, in any one of Aspects 1 to 9, it is preferable that the ratio of the content of the conductive additive to the positive electrode active material (conductive additive / positive electrode active material) is 0.001 or more and 0.05 or less in mass ratio.

[0018] In the electrode mixture for a secondary battery of Aspect 11, in any one of Aspects 1 to 10, it is preferable that the ratio of the surface area of ​​the conductive additive to the positive electrode active material (conductive additive / positive electrode active material) is 0.01 or more and 0.5 or less.

[0019] A twelfth aspect of the electrode mixture for a secondary battery is any one of the first to eleventh aspects, wherein the electrode mixture for a secondary battery is preferably for a sodium ion secondary battery.

[0020] A thirteenth aspect of the electrode mixture for a secondary battery is any one of the first to twelfth aspects, wherein the electrode mixture for a secondary battery is preferably for an all-solid-state sodium ion secondary battery.

[0021] An electrode for an all-solid-state secondary battery according to Aspect 14 of the present invention is characterized by including the electrode mixture for a secondary battery according to any one of Aspects 1 to 13.

[0022] An all-solid-state secondary battery according to Aspect 15 of the present invention is characterized by including the all-solid-state secondary battery electrode according to Aspect 14 as a positive electrode.

[0023] According to the present invention, it is possible to provide an electrode composite for a secondary battery, an electrode for an all-solid-state secondary battery using the electrode composite for a secondary battery, and an all-solid-state secondary battery using the electrode for an all-solid-state secondary battery, which can achieve both high levels of electronic conductivity and ionic conductivity and effectively improve battery characteristics at high rates.

[0024] Fig. 1 is a schematic cross-sectional view showing a solid electrolyte layer and an electrode layer formed thereon according to one embodiment of the present invention. Fig. 2 is a schematic cross-sectional view showing an all-solid-state secondary battery according to one embodiment of the present invention. Fig. 3 is a schematic cross-sectional view showing a solid electrolyte layer and an electrode layer formed thereon according to a conventional technology.

[0025] Preferred embodiments will be described below. However, the following embodiments are merely examples, and the present invention is not limited to the following embodiments. In addition, in each drawing, components having substantially the same functions may be referred to by the same reference numerals.

[0026] [Electrode Mix for Secondary Battery] The electrode mix for secondary battery of the present invention includes a positive electrode active material and a conductive additive. The electrode mix for secondary battery of the present invention may further include a solid electrolyte. The positive electrode active material includes (i) at least one transition metal element selected from the group consisting of Cr, Fe, Mn, Co, Ni, Ti, and Nb, (ii) at least one element selected from the group consisting of P, Si, and B, and (iii) an element consisting of O. The conductive additive includes sheet-like carbon.

[0027] The electrode composite for a secondary battery of the present invention has the above-described configuration, and therefore, even when the cathode active material having very low electronic conductivity is used as an electrode for a secondary battery, it can achieve high levels of both electronic conductivity and ionic conductivity, thereby effectively improving the battery characteristics. This can be explained as follows with reference to FIGS. 1 and 3.

[0028] FIG. 1 is a schematic cross-sectional view showing a solid electrolyte layer 2 and an electrode layer 1 formed thereon according to one embodiment of the present invention. The electrode layer 1 is made of an electrode mixture for a secondary battery, which contains a positive electrode active material 3, a solid electrolyte 4, and a conductive additive 5. The conductive additive 5 contains sheet carbon. Preferably, the conductive additive 5 consists solely of sheet carbon. The solid electrolyte layer 2 mainly contains a solid electrolyte.

[0029] 3 is a schematic cross-sectional view showing a conventional solid electrolyte layer and an electrode layer formed thereon. The electrode layer 101 is made of a secondary battery electrode mixture containing a positive electrode active material 103, a solid electrolyte 104, and a conductive additive 105 made of acetylene black.

[0030] However, the active materials used in the electrodes of all-solid-state secondary batteries generally have insufficient electronic and ionic conductivities. Therefore, in addition to the active materials, solid electrolytes and conductive additives are used in the electrodes of all-solid-state secondary batteries. However, if the amount of conductive additive added is increased to increase electronic conductivity, the conductive additive may inhibit ionic conduction, resulting in a decrease in ionic conductivity. Therefore, conventional all-solid-state secondary batteries have a problem in that it is difficult to achieve both high levels of electronic and ionic conductivity.

[0031] Furthermore, when a cathode active material such as that of the present invention is used, if the content of the conductive additive is increased in order to increase the electronic conductivity, the surface of the cathode active material precursor powder is covered with the conductive additive during sintering of the cathode active material precursor powder, which results in a decrease in sinterability and a tendency for the battery characteristics to deteriorate. In particular, when the thickness of the electrode layer is increased in order to increase the battery capacity, there is a problem in that the sinterability further decreases and the battery characteristics are likely to deteriorate.

[0032] In contrast, as shown in FIG. 1 , the electrode layer 1 formed from the electrode composite for a secondary battery of the present invention contains a conductive additive 5 containing sheet-like carbon. Because sheet-like carbon contributes more to electronic conduction than acetylene black, the electrode layer 1 formed from the electrode composite for a secondary battery of the present invention is believed to be more likely to ensure an electronic conduction path than the electrode layer 101 using the roughly spherical acetylene black shown in FIG. 3 . Therefore, the electrode layer 1 formed from the electrode composite for a secondary battery of the present invention can exhibit high electronic conductivity even with a small amount of conductive additive 5 added, making it less likely that the conductive additive will inhibit ionic conduction, thereby enabling high ionic conductivity to be enhanced while maintaining high electronic conductivity. The above effect is particularly enhanced when the conductive additive consists solely of sheet-like carbon.

[0033] Furthermore, since the electrode mixture for a secondary battery of the present invention can reduce the amount of conductive additive 5 added, even when the cathode active material of the present invention is used, the sinterability is less likely to decrease during sintering of the cathode active material precursor, and deterioration of battery characteristics is less likely to occur. In addition, since the sinterability is less likely to decrease during sintering of the cathode active material precursor, it is possible to increase the thickness of electrode layer 1, and the charge / discharge capacity of the resulting secondary battery can be increased.

[0034] Therefore, the electrode material for a secondary battery of the present invention can achieve both high levels of electronic conductivity and high levels of ionic conductivity, thereby effectively improving battery characteristics.

[0035] The electrode material for a secondary battery of the present invention preferably contains, by mass %, 30% to 98% of a positive electrode active material, 0.1% to 65% of a solid electrolyte, and 0.1% to 3% of a conductive additive, which can achieve both higher levels of electronic conductivity and ionic conductivity, thereby effectively improving battery characteristics.

[0036] In the present invention, the content of the positive electrode active material in the electrode mixture for a secondary battery is, in mass %, preferably 30% or more, more preferably 50% or more, and even more preferably 70% or more, and is preferably 98% or less, and more preferably 95% or less. When the content of the positive electrode active material in the electrode mixture for a secondary battery is within the above range, the battery characteristics such as the charge / discharge capacity of the secondary battery can be more effectively improved.

[0037] In the present invention, the content of the solid electrolyte in the electrode mixture for a secondary battery is, in mass %, preferably 0.1% or more, more preferably 5% or more, even more preferably 8% or more, and is preferably 65% ​​or less, more preferably 40% or less, even more preferably 30% or less, and particularly preferably 20% or less. When the content of the solid electrolyte in the electrode mixture for a secondary battery is within the above range, the ionic conductivity can be further improved, and the battery characteristics can be further improved effectively.

[0038] In the present invention, the content of the conductive additive in the electrode mixture for a secondary battery is, in mass %, preferably 0.1% or more, more preferably 0.2% or more, even more preferably 0.3% or more, particularly preferably 0.4% or more, most preferably 0.5% or more, preferably 3% or less, more preferably 2% or less, even more preferably 1% or less, still more preferably 0.9% or less, particularly preferably 0.8% or less, and most preferably 0.7% or less. When the conductive additive in the electrode mixture for a secondary battery contains sheet carbon and the content of the conductive additive is within the above range, it is possible to further improve ionic conductivity while ensuring high electronic conductivity, and to further effectively improve battery characteristics.

[0039] In the present invention, the ratio of the content of the conductive additive to the positive electrode active material (conductive additive / positive electrode active material) is, in mass ratio, preferably 0.001 or more, more preferably 0.002 or more, even more preferably 0.003 or more, particularly preferably 0.004 or more, most preferably 0.005 or more, and preferably 0.05 or less, more preferably 0.03 or less, even more preferably 0.02 or less, even more preferably 0.01 or less, particularly preferably 0.009 or less, and most preferably 0.007 or less. When the conductive additive in the electrode mixture for a secondary battery contains sheet carbon and the content ratio (conductive additive / positive electrode active material) is within the above range, it is possible to further improve ionic conductivity while ensuring high electronic conductivity, and to further effectively improve battery characteristics.

[0040] The content ratio (conductive additive / positive electrode active material) can be determined by dividing the content (mass%) of the conductive additive in the electrode composite for a secondary battery by the content (mass%) of the positive electrode active material in the electrode composite for a secondary battery.

[0041] In the present invention, the surface area ratio of the conductive additive to the positive electrode active material (conductive additive / positive electrode active material) is preferably 0.01 or more, more preferably 0.02 or more, even more preferably 0.03 or more, particularly preferably 0.04 or more, most preferably 0.05 or more, and preferably 1.2 or less, more preferably 1.0 or less, even more preferably 0.9 or less, even more preferably 0.8 or less, particularly preferably 0.7 or less, and most preferably 0.6 or less. When the conductive additive in the electrode mixture for a secondary battery contains sheet carbon and the surface area ratio (conductive additive / positive electrode active material) is within the above range, it is possible to further improve ionic conductivity while ensuring high electronic conductivity, and to further effectively improve battery characteristics.

[0042] The surface area ratio (conductive additive / positive electrode active material) can be calculated by the following formula (1).

[0043] Surface area ratio = (A1 x A2) / (B1 x B2)...Formula (1)

[0044] In formula (1), A1 is the content (mass%) of the conductive additive in the electrode mixture for a secondary battery. A2 is the BET specific surface area (m２ / g). B1 is the content (mass%) of the positive electrode active material in the electrode mixture for a secondary battery. B2 is the BET specific surface area (m ２ The BET specific surface areas of the conductive additive and the positive electrode active material can be measured by a BET multipoint method using nitrogen as an adsorbate.

[0045] The electrode mixture for secondary batteries of the present invention is preferably used in sodium ion secondary batteries, and more preferably in all-solid-state sodium ion secondary batteries.

[0046] Specific examples of the materials constituting the electrode mixture for a secondary battery of the present invention will be described below.

[0047] (Positive Electrode Active Material) The positive electrode active material contains (i) at least one transition metal element selected from the group consisting of Cr, Fe, Mn, Co, Ni, Ti, and Nb, (ii) at least one element selected from the group consisting of P, Si, and B, and (iii) an element consisting of O.

[0048] In the present invention, the positive electrode active material is represented by the general formula A ｘ MC ｙ O ｚ (wherein A is at least one selected from the group consisting of Li and Na, M is at least one selected from the group consisting of Cr, Fe, Mn, Co and Ni, C is at least one selected from the group consisting of P, Si and B, and 0.2≦x≦4.2, 0.65≦y≦6.5 and 2.5≦z≦20), and the crystal represented by the general formula A ｘ MC ｙ O ｚ (A is at least one selected from the group consisting of Li and Na, M is at least one selected from the group consisting of Cr, Fe, Mn, and Co, C is at least one selected from the group consisting of P, Si, and B, and 0.2≦x≦4.2, 0.65≦y≦6.5, and 2.5≦z≦20) is more preferable.

[0049] The positive electrode active material is represented by the general formula Na ｘ M ｙ P２ O ｚ (1≦x≦2.8, 0.95≦y≦1.6, 6.5≦z≦8, M is at least one selected from the group consisting of Cr, Fe, Mn, Co and Ni) is preferred. ｘ MP ２ O ７ It is more preferable that the positive electrode active material is made of crystallized glass containing crystals represented by the formula: (1≦x≦2, M is at least one element selected from the group consisting of Cr, Fe, Mn, Co, and Ni).

[0050] The crystals contained in the positive electrode active material include, for example, Na ２ FeP ２ O ７ , Na ２ CoP ２ O ７、 Na ２ NiP ２ O ７ etc. can be used.

[0051] In the present disclosure, crystallized glass refers to a precursor glass containing an amorphous phase that is heated (fired) to precipitate crystals (crystallization). The entire amorphous phase may be transformed into a crystalline phase, or the amorphous phase may remain. One type of crystal may be precipitated, or two or more types of crystals may be precipitated. For example, whether crystallized glass is crystallized or not can be determined by the peak angle and peak intensity shown by powder X-ray diffraction (XRD).

[0052] (Solid electrolyte) In an all-solid-state secondary battery, a solid electrolyte is a component responsible for ionic conduction in the electrode layer. By including a solid electrolyte, the ionic conductivity of the electrode material for a secondary battery can be increased even without an electrolytic solution.

[0053] The solid electrolyte is preferably formed from a sodium ion conductive oxide. Examples of sodium ion conductive oxides include compounds containing at least one element selected from Al, Y, Zr, Si, and P, Na, and O. Specific examples of sodium ion conductive oxides include beta-alumina or NASICON-type crystal, which have excellent sodium ion conductivity. Of these, the sodium ion conductive oxide is preferably at least one sodium ion conductive oxide selected from the group consisting of β"-alumina, β-alumina, and NASICON-type crystal. The sodium ion conductive oxide is more preferably β-alumina or β"-alumina, which have even better sodium ion conductivity.

[0054] Beta alumina includes β-alumina (theoretical composition formula: Na ２ O.11Al ２ O ３ ) and β''-alumina (theoretical composition formula: Na ２ O 5.3 Al ２ O ３ ) There are two types of crystal forms. β''-alumina is a metastable material, so it is usually ２ O or MgO is added as a stabilizer. Since 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. ２ O-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 It is more preferable to use O).

[0055] The NASICON type crystals include Na ３ Zr ２ Si ２ P.O. １２ , Na ３．２ Zr １．３ Si ２．２P ０．８ O １０．５ 、No ３ Zr １．６ Today ０．４ Yes ２ PO １２ 、No ３ Hf ２ Yes ２ PO １２ 、No ３．４ Zr ０．９ Hf １．４ Al ０．６ Yes １．２ P １．８ O １２ 、No ３ Zr １．７ N﹂ ０．２４ Yes ２ PO １２ 、No ３．６ Today ０．２ Y ０．８ Yes ２．８ O ９ 、No ３ Zr １．８８ Y ０．１２ Yes ２ PO １２ 、No ３．１２ Zr １．８８ Y ０．１２ Yes ２ PO １２ 、No ３．０５ Zr ２ Yes ２．０６ P ０．９５ O １２ 、No ３．４ Zr ２ Yes ２．４ P ０．６ O １２ 、No ３．４ Zr １．９ Mg ０．１ Yes ２．４ P ０．６ O １２ 、No ３．４ Zr １．９ Zn ０．１ Yes ２．４ P ０．６ O １２ 、No ３．４ Zr １．９ Mg ０．１ Yes ２．２ P ０．８ O １２ 、No ３．４Zr １．９ Zn ０．１ Si ２．２ P ０．８ O １２ , Na ３．６ Zr ０．１３ Yb １．６７ Si ０．１１ P ２．９ O １２ , Na ５ YSi ４ O １２ Among them, NASICON type crystals are ３．４ Zr ２ Si ２．４ P ０．６ O １２ or Na ３．０５ Zr ２ Si ２．０６ P ０．９５ O １２ In this case, the sodium ion conductivity can be further improved.

[0056] (Conductive assistant) The conductive assistant is a component that forms a conductive path in the electrode layer. In the present invention, the conductive assistant contains sheet carbon. This makes it easy to achieve both high electronic conductivity and high ionic conductivity as an electrode composite for a secondary battery even with a small amount of conductive assistant. The conductive assistant is preferably made of sheet carbon alone. Examples of the sheet carbon include graphene or reduced graphene oxide. In particular, the sheet carbon preferably contains graphene, and more preferably consists of graphene alone. It is even more preferable that the conductive assistant consists of graphene alone. The number of graphene layers is not particularly limited, and may be one layer or two or more layers.

[0057] The BET specific surface area of ​​the sheet carbon is preferably 100 m ２ / g or more, more preferably 300m ２ / g or more, more preferably 500m ２ The upper limit of the BET specific surface area of ​​the sheet carbon is not particularly limited, but is preferably 5000 m ２ / g or less, more preferably 4000m ２ / g or less, more preferably 3000m ２ / g or less. When the BET specific surface area of ​​the sheet carbon is within the above range, the electronic conductivity of the electrode composite for a secondary battery can be further increased with a smaller amount added. The BET specific surface area of ​​the sheet carbon can be measured by a multipoint BET method using nitrogen as the adsorbate.

[0058] The length of the sheet carbon is preferably 0.5 μm or more, more preferably 1 μm or more, even more preferably 3 μm or more, and preferably 50 μm or less, more preferably 40 μm or less, and even more preferably 30 μm or less. The thickness of the sheet carbon is preferably 1 nm or more, more preferably 3 nm or more, and preferably 30 nm or less, and more preferably 20 nm or less. When the length and thickness of the sheet carbon are within the above ranges, the electronic conductivity of the secondary battery electrode mixture can be further improved with a smaller amount added.

[0059] The length and thickness of the sheet carbon can be determined, for example, by measuring the lengths and thicknesses of 200 pieces of sheet carbon using a scanning electron microscope or the like and averaging the measured values.

[0060] [Electrode for All-Solid State Secondary Battery and All-Solid State Secondary Battery] Fig. 2 is a schematic cross-sectional view showing an all-solid state secondary battery according to one embodiment of the present invention. As shown in Fig. 2, an all-solid state secondary battery 21 includes a solid electrolyte layer 22, a positive electrode layer (electrode layer) 23, a negative electrode layer 24, a first current collector layer 25, and a second current collector layer 26. In this embodiment, the all-solid state secondary battery is an all-solid state sodium ion secondary battery.

[0061] The solid electrolyte layer 22 has a first main surface 22a and a second main surface 22b that face each other. A positive electrode layer 23 is provided on the first main surface 22a of the solid electrolyte layer 22. A first current collector layer 25 is provided on the main surface of the positive electrode layer 23 opposite to the solid electrolyte layer 22. An anode layer 24 is provided on the second main surface 22b of the solid electrolyte layer 22. A second current collector layer 26 is provided on the main surface of the anode layer 24 opposite to the solid electrolyte layer 22. The first current collector layer 25 and the second current collector layer 26 do not necessarily have to be provided. The electrode for an all-solid-state secondary battery of the present invention refers to, for example, the positive electrode layer 23.

[0062] Hereinafter, each layer in the all-solid-state secondary battery 21 of the present invention will be described in detail.

[0063] (Positive Electrode Layer) The positive electrode layer 23 is composed of the electrode composite for a secondary battery of the present invention described above. The positive electrode layer 23 (electrode composite for a secondary battery) can be formed, for example, by forming an electrode material layer containing a positive electrode active material precursor, a conductive additive, and optionally a sodium ion conductive solid electrolyte powder on the first main surface 22a of the solid electrolyte layer 22, and then firing the electrode material layer. The electrode material layer can be obtained, for example, by applying a paste containing a positive electrode active material precursor, a conductive additive, and optionally a solid electrolyte powder, and drying the paste. Note that the paste may contain a binder, a plasticizer, a solvent, or the like, as necessary. Note that the electrode material layer may also be a compact.

[0064] The drying temperature of the paste is not particularly limited, but can be, for example, 30° C. or higher and 150° C. or lower. The drying time of the paste is not particularly limited, but can be, for example, 5 minutes or higher and 600 minutes or lower.

[0065] The firing atmosphere is preferably an inert atmosphere or a reducing atmosphere. The firing temperature (maximum temperature) can be, for example, 400°C to 800°C, and the holding time at that temperature can be, for example, 5 minutes to less than 3 hours.

[0066] Positive electrode active material precursor: The positive electrode active material precursor (positive electrode active material precursor powder) is preferably made of an amorphous oxide material that generates active material crystals upon firing. When the positive electrode active material precursor powder is made of an amorphous oxide material, active material crystals are generated during firing, and the material softens and flows, enabling the formation of a dense positive electrode layer 23. Furthermore, firing can also integrate the positive electrode active material of the positive electrode layer 23 with the solid electrolyte. Furthermore, firing can also integrate the positive electrode layer 23 with the solid electrolyte layer 22. These results are preferable because they result in better formation of ion conduction paths. Furthermore, in the present invention, the term "amorphous oxide material" is not limited to completely amorphous oxide materials, but also includes those that contain some crystals (e.g., a crystallinity of 10% or less).

[0067] The positive electrode active material precursor powder contains, in mole percent in terms of oxide, Na ２ O 10% to 55%, Fe ２ O ３ +Cr ２ O ３ + MnO + CoO + NiO 10% to 50%, and P ２ O ５ It is preferable that the content be 15% to 70%. The reason for limiting the composition in this way will be explained below. In the following explanation of the content of each component, "%" means "mol %" unless otherwise specified.

[0068] Na ２ O is a group of the general formula Na ｘ M ｙ P ２ O ｚ (M is at least one transition metal element selected from the group consisting of Cr, Fe, Mn, Co, and Ni, and 1≦x≦2.8, 0.95≦y≦1.6, 6.5≦z≦8) is the main component of the active material crystal. ２ The O content is preferably 10% to 55%, more preferably 15% to 50%, and even more preferably 30% to 50%. ２ When the O content is within the above range, the charge / discharge capacity can be further increased.

[0069] Fe ２ O ３ , Cr ２ O ３ , MnO, CoO and NiO are also represented by the general formula Na ｘ M ｙ P ２ O ｚ It is the main component of the active material crystal expressed as Fe ２ O ３ +Cr ２ O ３ The content of Fe + MnO + CoO + NiO is preferably 10% to 50%, more preferably 15% to 30%, and even more preferably 15% to 25%. ２ O ３ +Cr ２ O ３When the content of Fe+MnO+CoO+NiO is equal to or greater than the lower limit, the charge / discharge capacity can be further increased. ２ O ３ +Cr ２ O ３ When the content of +MnO + CoO + NiO is less than the above upper limit, unwanted Fe ２ O ３ , Cr ２ O ３ In order to further improve the cycle characteristics, it is possible to make it difficult for crystals such as Fe, MnO, CoO, or NiO to precipitate. ２ O ３ It is preferable to positively contain Fe. ２ O ３ The content of Cr is preferably 1% to 50%, more preferably 5% to 40%, further preferably 10% to 30%, and particularly preferably 15% to 25%. ２ O ３ The content of each of the components Fe, MnO, CoO and NiO is preferably 0% to 30%, more preferably 10% to 30%, and even more preferably 15% to 25%. ２ O ３ , Cr ２ O ３ When at least two or more components selected from the group consisting of MnO, CoO and NiO are contained, the total amount thereof is preferably 10% to 50%, more preferably 10% to 30%, and even more preferably 15% to 25%.

[0070] P ２ O ５ The general formula is Na ｘ M ｙ P ２ O ｚ It is the main component of the active material crystal expressed by P ２ O ５ The content of P is preferably 15% to 70%, more preferably 25% to 55%, and even more preferably 30% to 50%. ２ O ５ When the content is within the above range, the charge / discharge capacity can be further increased.

[0071] The positive electrode active material precursor powder contains, in addition to the above components, V ２ O ５ , Nb ２ O ５ , MgO, Al ２ O ３ , TiO ２ , ZrO ２ , or Sc ２ O ３ These components have the effect of increasing electrical conductivity (electronic conductivity), which facilitates improvement of high-speed charge / discharge characteristics. The total content of the above components is preferably 0% to 25%, and more preferably 0.2% to 10%. When the content of the above components is equal to or less than the upper limit, heterogeneous crystals that do not contribute to the battery characteristics are less likely to be formed, and the charge / discharge capacity can be further increased.

[0072] In addition to the above components, the positive electrode active material precursor powder also contains SiO ２ , B ２ O ３ , GeO ２ , Ga ２ O ３ , Sb ２ O ３ , or Bi ２ O ３ The addition of these components further improves the glass-forming ability, making it easier to obtain a more homogeneous positive electrode active material precursor powder. The total content of the above components is preferably 0% to 37%, and more preferably 0.2% to 25%. Because these components do not contribute to the battery characteristics, if their content is too high, the charge / discharge capacity tends to decrease.

[0073] The positive electrode active material precursor powder is preferably produced by melting and molding a raw material batch. This method is preferred because it makes it easier to obtain an amorphous positive electrode active material precursor powder with excellent homogeneity. Specifically, the positive electrode active material precursor powder can be produced as follows.

[0074] First, raw materials are prepared to have a desired composition to obtain a raw material batch. Next, the obtained raw material batch is melted. The melting temperature may be adjusted as appropriate so that the raw material batch is homogeneously melted. For example, the melting temperature is preferably 800°C or higher, and more preferably 900°C or higher. There are no particular limitations on the upper limit of the melting temperature, but an excessively high melting temperature can lead to energy loss and evaporation of sodium components, etc., so the upper limit is preferably 1500°C or lower, and more preferably 1400°C or lower.

[0075] The molten material is then molded by any method, and 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 being rapidly cooled, or the molten material may be poured into a mold and molded into an ingot.

[0076] The resulting compact is then pulverized to obtain a cathode active material precursor powder, which preferably has an average particle size of 0.01 μm or more and less than 5 μm, more preferably 0.03 μm or more and 3 μm or less, even more preferably 0.03 μm or more and 2 μm or less, particularly preferably 0.03 μm or more and 1 μm or less, and most preferably 0.03 μm or more and 0.7 μm or less.

[0077] In the present invention, the average particle size is D ５０ (volume-based median diameter) and refers to a value measured by a laser diffraction scattering method.

[0078] Solid electrolyte powder: The solid electrolyte powder is obtained, for example, by pulverizing the above-mentioned solid electrolyte. The average particle size of the solid electrolyte powder is preferably 0.01 μm or more and 3 μm or less, more preferably 0.03 μm or more and less than 1.8 μm, even more preferably 0.03 μm or more and 1.5 μm or less, particularly preferably 0.03 μm or more and 1.2 μm or less, and most preferably 0.03 μm or more and 0.7 μm or less. If the average particle size of the solid electrolyte powder is too small, not only is it difficult to uniformly mix it with the active material precursor powder, but moisture absorption and carbonation may cause a decrease in ionic conductivity or promote excessive reaction with the active material precursor powder. As a result, the internal resistance of the 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, the softening and flow of the active material precursor powder is significantly hindered, which tends to result in poor smoothness of the resulting electrode layer, reduced mechanical strength, and increased internal resistance.

[0079] Binder: A binder is a material for binding raw materials (raw material powders) together. Examples of binders include water-soluble polymers such as cellulose derivatives (e.g., carboxymethyl cellulose, hydroxypropyl methyl cellulose, hydroxypropyl cellulose, ethyl cellulose, hydroxyethyl cellulose, and hydroxymethyl cellulose) and polyvinyl alcohol; thermosetting resins (e.g., thermosetting polyimide, phenolic resin, epoxy resin, urea resin, melamine resin, unsaturated polyester resin, and polyurethane); polycarbonate resins (e.g., polypropylene carbonate); and polyvinylidene fluoride.

[0080] The thickness of the positive electrode layer 23 is preferably 10 μm or more, more preferably 30 μm or more, even more preferably 50 μm or more, particularly preferably 70 μm or more, and most preferably 100 μm or more. In this case, the charge / discharge capacity of the all-solid-state secondary battery can be further increased. On the other hand, if the thickness of the positive electrode layer 23 is too thick, resistance to electronic conduction increases, which may reduce the discharge capacity and operating voltage, and stress due to shrinkage during firing may increase, leading to peeling. Therefore, the thickness is preferably 1000 μm or less.

[0081] The amount of the positive electrode active material contained in the positive electrode layer 23 is preferably 1 mg / cm ２ More preferably, 3 mg / cm ２ More preferably, 5 mg / cm ２ More preferably, 7 mg / cm ２ or more, most preferably 10 mg / cm ２ In this case, the charge / discharge capacity of the all-solid-state secondary battery can be further increased. The upper limit of the amount of the positive electrode active material supported is not particularly limited, but is, for example, 500 mg / cm ２ It can be said that:

[0082] (Solid Electrolyte Layer) The solid electrolyte layer 22 can be produced by mixing raw material powders (solid electrolyte powders), molding the mixed raw material powders (solid electrolyte powders), and then firing the molded raw material powders. For example, the solid electrolyte layer 22 can be produced by forming a green sheet from the raw material powders (solid electrolyte powders) into a slurry, and then firing the green sheet. Alternatively, the solid electrolyte layer 22 can be produced by a sol-gel method. Furthermore, the solid electrolyte layer 22 can be formed on a solid electrolyte substrate having a thickness in the range of 0.1 to 3 mm.

[0083] The average particle size of the solid electrolyte powder is preferably 0.01 μm or more and 3 μm or less, more preferably 0.03 μm or more and less than 1.8 μm, even more preferably 0.03 μm or more and 1.5 μm or less, particularly preferably 0.03 μm or more and 1.2 μm or less, and most preferably 0.03 μm or more and 0.7 μm or less. If the average particle size of the solid electrolyte powder is too small, there is a risk of a decrease in ionic conductivity due to moisture absorption or carbonation. As a result, the internal resistance of the 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, the internal resistance tends to increase.

[0084] The thickness of the solid electrolyte layer 22 is preferably 5 μm or more and 1000 μm or less, and more preferably 10 μm or more and 200 μm or less. If the thickness of the solid electrolyte layer 22 is too thin, the mechanical strength decreases and the layer becomes more susceptible to breakage, which makes internal short circuits more likely to occur. If the thickness of the solid electrolyte layer 22 is too thick, the sodium ion conduction distance during charge and discharge increases, resulting in higher internal resistance and a decrease in the discharge capacity and operating voltage of the secondary battery. In addition, the energy density per unit volume of the secondary battery also tends to decrease.

[0085] (Negative Electrode Layer) The negative electrode active material contained in the negative electrode layer 24 is not particularly limited, and may be, for example, a carbon electrode material such as hard carbon or soft carbon. The carbon electrode material is preferably hard carbon. However, the negative electrode active material may contain an alloy-based negative electrode active material capable of absorbing sodium, such as tin, bismuth, lead, or phosphorus, or metallic sodium. It is preferable that the negative electrode layer 24 is not a negative electrode layer consisting of a single phase of metallic sodium.

[0086] The anode layer 24 may further contain a sodium ion conductive solid electrolyte and a conductive additive. The proportions of the respective materials in the anode layer 24 may be, for example, in mass %, 60% to 100% anode active material, 0% to 35% sodium ion conductive solid electrolyte, and 0% to 5% conductive additive. The sodium ion conductive solid electrolyte may be, for example, the solid electrolytes described above. The conductive additive may be, for example, the conductive additive described above.

[0087] The thickness of the negative electrode layer 24 is preferably 0.3 μm or more and 500 μm or less, and more preferably 3 μm or more and 300 μm or less. If the thickness of the negative electrode layer 24 is too thin, the absolute capacity (mAh / g) of the negative electrode tends to decrease. If the thickness of the negative electrode layer 24 is too thick, the resistance increases, and the capacity (mAh / g) tends to decrease.

[0088] The anode layer 24 can be prepared, for example, using a slurry containing an anode active material precursor powder and, as needed, a solid electrolyte powder and a conductive additive. A binder, a plasticizer, a solvent, and the like can be added to the slurry as needed. The anode layer 24 can be prepared by applying the slurry to the second main surface 22b of the solid electrolyte layer 22, drying the slurry, and firing the resulting layer. Alternatively, the anode layer 24 can be prepared by applying the slurry to a substrate such as PET (polyethylene terephthalate), drying the resulting layer, and firing the resulting green sheet. Alternatively, the anode layer 24 can be made of a metal, in which case it can be prepared by sputtering, vapor deposition, or the like.

[0089] (First Current Collector Layer and Second Current Collector Layer) The materials for the first current collector layer 25 and the second current collector layer 26 are not particularly limited, and metal materials such as aluminum, titanium, silver, copper, stainless steel, or alloys thereof can be used, respectively. The above metal materials may be used alone or in combination. Note that these alloys are alloys containing at least one of the above metals. The thicknesses of the first current collector layer 25 and the second current collector layer 26 are not particularly limited, and can be 0.01 μm or more and 1000 μm or less, respectively.

[0090] The method for forming the first current collector layer 25 and the second current collector layer 26 is not particularly limited, and examples thereof include physical vapor deposition methods such as vapor deposition or sputtering, and chemical vapor deposition methods such as thermal CVD, MOCVD, and plasma CVD. Other methods for forming the first current collector layer 25 and the second current collector layer 26 include plating, a sol-gel method, and a liquid phase film formation method using spin coating. However, it is preferable to form the first current collector layer 25 and the second current collector layer 26 on the positive electrode layer 23 and the negative electrode layer 24, respectively, by sputtering, as this provides excellent adhesion.

[0091] In the above embodiment, an all-solid-state sodium-ion secondary battery using a material having sodium ion conductivity has been described, but the electrode composite for a secondary battery of the present invention may be used in other all-solid-state secondary batteries, such as an all-solid-state lithium-ion secondary battery using a material having lithium ion conductivity.

[0092] The present invention will be described in more detail below with reference to specific examples. The present invention is not limited to the following examples, and can be practiced with appropriate modifications within the scope of the present invention.

[0093] Example 1 (a) Preparation of a solid electrolyte layer β"-alumina (1 mm thick, 12 mm square) manufactured by Ionotec was used as the solid electrolyte substrate. Raw materials were mixed to form the β"-alumina composition, calcined at 1250°C, and pulverized. The calcined raw materials, binder, plasticizer, dispersant, solvent, etc. were mixed to prepare a paste. The paste was applied to the aforementioned β"-alumina substrate to a thickness of 100 μm and a size of 10 mm square. After drying in a thermostatic chamber at 70°C for 2 hours, it was fired at 1550°C for 30 minutes, forming a 10 mm square, 100 μm thick solid electrolyte layer on the solid electrolyte substrate.

[0094] (b) Preparation of positive electrode paste The glass composition was 40Na in molar ratio. ２ O-20Fe ２ O ３ -40P ２ O ５The raw materials prepared so as to be: were melted in the atmosphere at 1250°C for 45 minutes, and then cooled and formed with a twin roller to produce a glass film. The obtained glass film was pulverized with a ball mill and a planetary ball mill to produce a glass film having an average particle diameter D50 of 0.2 μm and a BET specific surface area of ​​30 m. ２ / g of a positive electrode active material precursor powder (glass powder) was obtained.

[0095] The resulting glass powder (86.9 mass %) and β''-alumina (specific surface area: 45 m) as a solid electrolyte were mixed. ２ / g) 12.9 mass %, and graphene (manufactured by Jikan Techno Co., Ltd., multi-layer biomass nanocarbon, BET specific surface area: 750 m) as a conductive additive. ２ 0.2 mass% of ZnO (0.15 wt. / g, thickness: 1 to 3 nm, length: 30 μm) was mixed to prepare a positive electrode composite powder. 20 mass% of polypropylene carbonate (PPC) was added as a binder to 100 mass% of the obtained positive electrode composite powder, and N-methyl-2-pyrrolidone was added as a solvent so that the concentration of the positive electrode composite powder was 50 mass%. This was mixed in a planetary mixer to prepare a positive electrode paste.

[0096] (c) Formation of Positive Electrode The positive electrode paste was applied to the center of the main surface of the solid electrolyte layer to a thickness of 100 μm and an area of ​​10 mm square. After drying in a thermostatic chamber at 80° C. for 2 hours, the positive electrode paste was ２ / H ２ The mixture was fired in a 96 / 4 volume % atmosphere at 500° C. for 30 minutes to form an electrode mixture (positive electrode) for a secondary battery.

[0097] (d) Formation of current collector and assembly of coin cell A 100 nm thick gold vapor-deposited film was formed as a current collector on the entire surface of the positive electrode. Then, in an argon glove box, metallic sodium was attached as a negative electrode to the main surface of the solid electrolyte layer opposite the positive electrode, and the resultant was sealed in a CR2032 coin cell to fabricate an all-solid-state battery.

[0098] (Examples 2, 3, and Comparative Example 1) All-solid-state secondary batteries were fabricated in the same manner as in Example 1, except that the composition of the electrode composite for secondary batteries was adjusted to the composition shown in Table 1 below. In Comparative Example 1, acetylene black (manufactured by Timcal Corporation, product number "SUPER P", BET specific surface area: 62 m) was used as a conductive additive. ２ / g) was used.

[0099] The fabricated batteries were charged at 30° C. and 0.02 C, and the initial discharge capacities (hereinafter referred to as discharge capacities) were measured at 30° C. and at rates of 0.02 C, 2 C, and 10 C. The battery characteristics were measured by calculating the ratio of the discharge capacity at 2 C rate to the discharge capacity at 0.02 C rate and the ratio of the discharge capacity at 10 C rate to the discharge capacity at 0.02 C rate. The results are shown in Table 1 below.

[0100]

[0101] Thus, when the graphene content was in the range of 0.2% by mass to 2.0% by mass (Examples 1 to 3), the ratio of the discharge capacity at a 2C rate to the discharge capacity at a 0.02C rate (2C discharge capacity ratio) was 40% or more, and the ratio of the discharge capacity at a 10C rate to the discharge capacity at a 0.02C rate (10C discharge capacity ratio) was high, at 15% or more. In particular, at 0.2% by mass (Example 1), the 2C discharge capacity ratio was 70% and the 10C discharge capacity ratio was 50%. On the other hand, in Comparative Example 1, in which acetylene black was used as the conductive additive, the 2C discharge capacity ratio was high at 63%, but the 10C discharge capacity ratio was low at 0%.

[0102] REFERENCE SIGNS LIST 1 electrode layer 2 solid electrolyte layer 3 positive electrode active material 4 solid electrolyte 5 conductive additive 21 all-solid-state secondary battery 22 solid electrolyte layer 22a, 22b first and second principal surfaces 23 positive electrode layer 24 negative electrode layer 25, 26 first and second current collector layers

Claims

1. An electrode mixture for a secondary battery comprising: a positive electrode active material; and a conductive assistant; wherein the positive electrode active material comprises: (i) at least one transition metal element selected from the group consisting of Cr, Fe, Mn, Co, Ni, Ti, and Nb; (ii) at least one element selected from the group consisting of P, Si, and B; and (iii) an element consisting of O; and the conductive assistant comprises sheet-like carbon.

2. The electrode mixture for a secondary battery according to claim 1, wherein the conductive assistant consists solely of sheet-like carbon.

3. The electrode mixture for a secondary battery according to claim 1 or 2, wherein the sheet carbon includes graphene.

4. The electrode mixture for a secondary battery according to claim 1 or 2, wherein the sheet carbon consists solely of graphene.

5. The positive electrode active material is represented by the general formula A ｘ M.C. ｙ O ｚ 3. The electrode mixture for a secondary battery according to claim 1, comprising a crystal represented by the formula: (A is at least one selected from the group consisting of Li and Na, M is at least one selected from the group consisting of Cr, Fe, Mn, Co and Ni, C is at least one selected from the group consisting of P, Si and B, and 0.2≦x≦4.2, 0.65≦y≦6.5 and 2.5≦z≦20).

6. The electrode mixture for a secondary battery according to claim 1 or 2, wherein the positive electrode active material further contains Na.

7. The electrode mixture for a secondary battery according to claim 1 or 2, further comprising a solid electrolyte.

8. The electrode mixture for a secondary battery according to claim 7, wherein the solid electrolyte contains β-alumina, β″-alumina, or NASICON type crystals.

9. The electrode mixture for a secondary battery according to claim 1 or 2, containing, by mass%, 30% to 98% of the positive electrode active material, 0.1% to 65% of a solid electrolyte, and 0.1% to 3% of the conductive assistant.

10. An electrode mixture for a secondary battery as described in claim 1 or 2, wherein the ratio of the content of the conductive additive to the positive electrode active material (conductive additive / positive electrode active material) is, in mass ratio, 0.001 or more and 0.05 or less.

11. The electrode mixture for a secondary battery according to claim 1 or 2, wherein the ratio of the surface area of ​​the conductive additive to the positive electrode active material (conductive additive / positive electrode active material) is 0.01 or more and 0.5 or less.

12. The electrode mixture for a secondary battery according to claim 1 or 2, which is for a sodium ion secondary battery.

13. The electrode mixture for a secondary battery according to claim 1 or 2, which is for an all-solid-state sodium ion secondary battery.

14. An electrode for an all-solid-state secondary battery, comprising the electrode mixture for a secondary battery according to claim 1 or 2.

15. An all-solid-state secondary battery comprising the electrode for an all-solid-state secondary battery according to claim 14 as a positive electrode.

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