Energy storage elements and all-solid-state secondary batteries

Abstract

To provide a power storage element capable of improving the cycle characteristic at charging and discharging of a secondary battery.SOLUTION: A power storage element 1 includes a solid electrolyte layer 2, an electrode layer 3 provided on a main surface on one side of the solid electrolyte layer 2 and including an electrode active material 5 and solid electrolytes 6a, 6b, and a current collector layer 4 provided on a main surface of the electrode layer 3 on the opposite side of the solid electrolyte layer 2. The porosity of a first electrode layer part 3A disposed on the solid electrolyte layer 2 side relative to a center in a thickness direction of the electrode layer 3 is larger than the porosity of a second electrode layer part 3B disposed on the current collector layer 4 side relative to the center in the thickness direction of the electrode layer 3.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This invention relates to an energy storage element and an all-solid-state secondary battery using the energy storage element. [Background technology]

[0002] Lithium-ion rechargeable batteries have established themselves as essential high-capacity, lightweight power sources for mobile devices, electric vehicles, and other applications. However, current lithium-ion rechargeable batteries primarily use flammable organic electrolytes, raising concerns about the risk of fire. To address this issue, development is underway on all-solid-state lithium-ion batteries that use a solid electrolyte instead of organic electrolytes. However, due to concerns about rising global raw material costs for lithium, development of all-solid-state sodium-ion batteries is also progressing.

[0003] Patent Document 1 discloses an all-solid-state secondary battery comprising a solid electrolyte sheet and an electrode layer formed on the surface of the solid electrolyte sheet. Patent Document 1 describes that the solid electrolyte sheet is formed by forming a second solid electrolyte layer on at least one surface of a first solid electrolyte layer. The second solid electrolyte layer is described as a porous solid electrolyte layer. The first and second solid electrolyte layers can be formed by firing a green sheet. Furthermore, Patent Document 1 describes that the adhesion to the electrode layer can be improved by making a part of the solid electrolyte sheet a porous layer. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] International Publication No. 2021 / 045039 [Overview of the project] [Problems that the invention aims to solve]

[0005] By the way, in sintered all-solid-state secondary batteries, which are formed by firing solid electrolyte layers and electrode layers, there is a problem that the charge-discharge performance deteriorates and the cycle characteristics tend to worsen when the charge-discharge cycle is repeated.

[0006] The object of the present invention is to provide an energy storage element and an all-solid-state secondary battery using the energy storage element that can improve the cycle characteristics during charging and discharging of a secondary battery. [Means for solving the problem]

[0007] The following describes various embodiments of energy storage elements and all-solid-state secondary batteries that solve the above problems.

[0008] The energy storage element according to embodiment 1 of the present invention comprises a solid electrolyte layer and the solid electrolyte layer Solution The electrode comprises an electrode layer provided on one main surface of a mass layer and containing an electrode active material and a solid electrolyte, and a current collector layer provided on the main surface of the electrode layer opposite to the solid electrolyte layer, characterized in that the porosity of the first electrode layer portion located closer to the solid electrolyte layer than the center in the thickness direction of the electrode layer is greater than the porosity of the second electrode layer portion located closer to the current collector layer than the center in the thickness direction of the electrode layer.

[0009] In the energy storage element according to Embodiment 2, it is preferable that the porosity of the electrode layer increases from the current collector layer side toward the solid electrolyte layer side in Embodiment 1.

[0010] The energy storage element according to embodiment 3 is configured in embodiment 1 or embodiment 2, wherein the solid electrolyte layer is a dense layer When a first solid electrolyte layer is used, it is preferable that the electrode layer is configured such that an electrode material containing an electrode active material and a solid electrolyte is arranged within a second solid electrolyte layer which is a porous layer.

[0011] In the energy storage element according to Embodiment 4, it is preferable that the solid electrolyte contains at least one selected from the group consisting of β''-alumina, β-alumina, and NASICON crystal, in any one embodiment from Embodiments 1 to 3.

[0012] In the energy storage element according to Embodiment 5, it is preferable that the solid electrolyte layer and the electrode layer are sintered bodies in any one embodiment from Embodiments 1 to 4.

[0013] The all-solid-state secondary battery according to aspect 6 of the present invention is characterized by comprising an energy storage element according to any one of aspects 1 to 5. [Effects of the Invention]

[0014] According to the present invention, it is possible to provide an energy storage element and an all-solid-state secondary battery using the energy storage element that can improve the cycle characteristics during charging and discharging of a secondary battery. [Brief explanation of the drawing]

[0015] [Figure 1] Figure 1 is a schematic cross-sectional view showing an energy storage element according to one embodiment of the present invention. [Figure 2] Figure 2 is a schematic cross-sectional view of an energy storage element according to one embodiment of the present invention, when the electrode layer is divided into first to third electrode layer portions. [Figure 3] Figure 3 is a schematic diagram showing an example of the porosity distribution of the electrode layer in an energy storage element according to one embodiment of the present invention. [Figure 4] Figure 4 is a schematic cross-sectional view showing an all-solid-state secondary battery according to one embodiment of the present invention. [Modes for carrying out the invention]

[0016] Preferred embodiments are described below. However, the following embodiments are merely illustrative, and the present invention is not limited to these embodiments. In addition, in each drawing, components having substantially the same function may be referred to by the same reference numerals.

[0017] [Energy storage element] Figure 1 is a schematic cross-sectional view showing an energy storage element according to one embodiment of the present invention. As shown in Figure 1, the energy storage element 1 comprises a solid electrolyte layer 2, an electrode layer 3, and a first current collector layer 4. The electrode layer 3 and the first current collector layer 4 are stacked in this order on the first main surface 2a of the solid electrolyte layer 2.

[0018] The electrode layer 3 contains an electrode active material 5 and solid electrolytes 6a and 6b. In this embodiment, the electrode active material 5 is the positive electrode active material, and the electrode layer 3 is the positive electrode layer. However, in the present invention, the electrode active material 5 may be the negative electrode active material, and the electrode layer 3 may be the negative electrode layer.

[0019] The electrode layer 3 has a first electrode layer portion 3A located on the solid electrolyte layer 2 side of the center in the thickness direction of the electrode layer 3, and a second electrode layer portion 3B located on the first current collector layer 4 side of the center in the thickness direction of the electrode layer 3. In this embodiment, the porosity of the first electrode layer portion 3A on the solid electrolyte layer 2 side is greater than the porosity of the second electrode layer portion 3B on the first current collector layer 4 side.

[0020] The porosity of the first electrode layer portion 3A and the second electrode layer portion 3B can be determined by image analysis of the cross-section along the thickness direction of the electrode layer 3. Image analysis is performed, for example, on cross-sectional images of the electrode layer obtained by scanning electron microscopy (SEM) or CT (computed tomography). The porosity can be calculated after binarization (converting to two colors, black and white) using image processing software (e.g., ImageJ). The threshold for binarization is adjusted appropriately while viewing the actual image so that the void areas become black. The porosity is calculated by dividing the area (number of pixels) of the black portion in each electrode layer section by the total area (number of pixels) of the section.

[0021] Since the energy storage element 1 of this embodiment has the above configuration, it can improve the cycle characteristics during charging and discharging of the secondary battery.

[0022] Conventionally, in sintered all-solid-state secondary batteries formed by firing solid electrolyte layers and electrode layers, repeated charging and discharging can cause cracks and other defects to occur inside the energy storage element due to the expansion and contraction of the electrode active material. This can lead to a decrease in charge / discharge performance and a deterioration of cycle characteristics.

[0023] In response to this, the inventors focused on the fact that, in the thickness direction of the electrode layer 3, the charge-discharge reaction proceeds more easily in the area closer to the solid electrolyte layer 2, and the expansion and contraction of the electrode active material 5 is greater there. They found that by making the porosity of the first electrode layer portion 3A on the solid electrolyte layer 2 side larger than the porosity of the second electrode layer portion 3B on the first current collector layer 4 side, the difference in expansion and contraction in the thickness direction of the electrode layer 3 can be reduced. As a result, defects are less likely to occur inside the energy storage element 1 even as the charge-discharge cycle progresses, and the deterioration of charge-discharge performance can be suppressed. Therefore, the energy storage element 1 of this embodiment can provide a secondary battery with excellent cycle characteristics during repeated charge-discharge.

[0024] In this embodiment, the porosity X of the first electrode layer portion 3A is preferably 30 volume% or more, more preferably 50 volume% or more, preferably 90 volume% or less, and more preferably 80 volume% or less. When the porosity X of the first electrode layer portion 3A is within the above range, the cycle characteristics during repeated charging and discharging of the secondary battery can be further improved.

[0025] The porosity Y of the second electrode layer portion 3B is preferably 20 volume% or more, more preferably 30 volume% or more, preferably 80 volume% or less, and more preferably 60 volume% or less. When the porosity Y of the second electrode layer portion 3B is within the above range, the cycle characteristics during repeated charging and discharging of the secondary battery can be further improved.

[0026] The ratio (X / Y) of the porosity X of the first electrode layer portion 3A to the porosity Y of the second electrode layer portion 3B is preferably 1.01 or more, more preferably 1.05 or more, more preferably 10 or less, and more preferably 3 or less. When the ratio (X / Y) is within the above range, the cycle characteristics during repeated charging and discharging of the secondary battery can be further improved.

[0027] Furthermore, as shown in Figure 2, when the electrode layer 3 is divided into three equal parts in the thickness direction, and these are designated as the first electrode layer portion 3C, the second electrode layer portion 3D, and the third electrode layer portion 3E in that order from the solid electrolyte layer 2 side, it is preferable that the porosity increases in the order of the third electrode layer portion 3E, the second electrode layer portion 3D, and the first electrode layer portion 3C. As shown in Figure 3, when the electrode layer 3 has a gradient structure in which the porosity increases from the first current collector layer 4 side to the solid electrolyte layer 2 side, the cycle characteristics during repeated charging and discharging of the secondary battery can be further improved.

[0028] Furthermore, the porosity of the first electrode layer portion 3C, the second electrode layer portion 3D, and the third electrode layer portion 3E can also be determined by image analysis of the cross-section along the thickness direction of the electrode layer 3 as described above.

[0029] The porosity X of the first electrode layer portion 3C is preferably 30 volume% or more, more preferably 50 volume% or more, preferably 90 volume% or less, and more preferably 80 volume% or less.

[0030] The porosity Y of the second electrode layer portion 3D is preferably 25 volume% or more, more preferably 40 volume% or more, preferably 85 volume% or less, and more preferably 70 volume% or less.

[0031] The porosity Z of the third electrode layer portion 3E is preferably 20 volume% or more, more preferably 30 volume% or more, preferably 80 volume% or less, and more preferably 60 volume% or less.

[0032] The following describes the configuration of each layer in the energy storage element of the present invention, such as energy storage element 1.

[0033] (Solid electrolyte layer) The solid electrolyte constituting the solid electrolyte layer 2 is preferably formed from a sodium ion conductive oxide. Examples of the sodium ion conductive oxide include compounds containing at least one kind selected from Al, Y, Zr, Si, and P, Na, and O. Specific examples of the sodium ion conductive oxide include beta-alumina or NASICON crystal having excellent sodium ion conductivity. Among them, the sodium ion conductive oxide is preferably at least one kind of sodium ion conductive oxide selected from the group consisting of β’’-alumina, β-alumina, and NASICON crystal. The sodium ion conductive oxide is more preferably β-alumina or β’’-alumina. These are more excellent in sodium ion conductivity.

[0034] Beta-alumina has two crystal forms, β-alumina (theoretical composition formula: Na2O·11Al2O3) and β’’-alumina (theoretical composition formula: Na2O·5.3Al2O3). Since β’’-alumina is a metastable substance, usually, those added with Li2O or MgO as a stabilizer are used. Since β’’-alumina has a 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 it is more preferable to use 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 O)).

[0035] Examples of NASICON crystal include Na3Zr2Si2PO 12 、Na 3.2 Zr 1.3 Si 2.2 P 0.8 O 10.5 、Na3Zr 1.6 Ti 0.4 Si2PO12 , 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 Si2PO 12 , Na 3.6 Ti 0.2 Y 0.8 Si 2.8 , Na3Zr 1.88 Y 0.12 Si2PO 12 , Na 3.12 Zr 1.88 Y 0.12 Si2PO 12 , Na 3.05 Zr2Si 2.06 P 0.95 O 12 , Na 3.4 Zr2Si 2.4 P 0.6 O 12 , Na 3.4 Zr 1.9 Mg 0.1 Si 2.4 P 0.6 O 12 , Na 3.4 Zr 1.9 Zn 0.1 Si 2.4 P 0.6 O 12 , Na 3.4 Zr 1.9 Mg 0.1 Si 2.2 P 0.8 O 12 , Na 3.4 Zr 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 , Na5YSi4O 12 and the like crystals can be mentioned. Among them, NASICON crystals are Na 3.4Zr2Si 2.4 P 0.6 O 12 or Na 3.05 Zr2Si 2.06 P 0.95 O 12 This is preferable. In this case, sodium ion conductivity can be further improved.

[0036] The solid electrolyte layer 2 can be manufactured by mixing raw material powders, molding the mixed raw material powders, and then firing them. For example, it can be manufactured by slurrying the raw material powders to create a green sheet, and then firing the green sheet. Alternatively, it may be manufactured by the sol-gel method.

[0037] The thickness of the solid electrolyte layer 2 is preferably 5 μm or more, more preferably 10 μm or more, The thickness is preferably 15 μm or more, preferably 1000 μm or less, more preferably 800 μm or less, and even more preferably 500 μm or less. If the thickness of the solid electrolyte layer 2 is too thin, the mechanical strength decreases and it becomes more prone to breakage, making internal short circuits more likely. If the thickness of the solid electrolyte layer 2 is too thick, the sodium ion conduction distance during charging and discharging increases, which increases the internal resistance and makes it easier for the discharge capacity and operating voltage to decrease. In addition, when used in an all-solid-state secondary battery, the energy density per unit volume of the all-solid-state secondary battery also tends to decrease.

[0038] In this embodiment, the solid electrolyte layer 2 is a first solid electrolyte layer as a dense layer. The first solid electrolyte layer has a porosity defined by the following formula (1) preferably 20% or less, more preferably 10% or less, and even more preferably 5% or less. The lower limit of the porosity is not particularly limited, but can be, for example, 0.1%.

[0039] Porosity=(1-p / p0)×100(%)…Equation (1)

[0040] In equation (1), p is the bulk density and p0 is the true density.

[0041] (electrode layer) The electrode layer 3 contains an electrode active material 5 and solid electrolytes 6a and 6b. In this embodiment, the electrode layer 3 is formed by arranging the electrode material within a second solid electrolyte layer which is a porous layer.

[0042] The second solid electrolyte layer is composed of a solid electrolyte 6a and is a porous layer having three-dimensionally interconnected voids. The void ratio of the second solid electrolyte layer, as defined by formula (1) above, is preferably 25% or more, more preferably 30% or more, even more preferably 40% or more, preferably 97% or less, more preferably 95% or less, and even more preferably 90% or less.

[0043] The electrode material also includes an electrode active material 5 and a solid electrolyte 6b. The electrode material may also contain other materials such as conductive additives, if necessary.

[0044] The content of the electrode active material 5 in the electrode layer 3 is not particularly limited, but is preferably 50% or more, more preferably 60% or more, preferably 99.9% or less, more preferably 95% or less, and even more preferably 90% or less by mass. When the content of the electrode active material 5 in the electrode layer 3 is within the above range, the capacity of the secondary battery can be increased even more effectively.

[0045] The content of solid electrolytes 6a and 6b in the electrode layer 3 is not particularly limited, but is preferably 0.1% or more, more preferably 5% or more, preferably 50% or less, more preferably 40% or less, and even more preferably 30% or less by mass%. When the content of solid electrolytes 6a and 6b in the electrode layer 3 is within the above range, the ionic conductivity of the electrode layer 3 can be further improved, and the battery characteristics of the secondary battery can be improved more effectively. The content of solid electrolytes 6a and 6b referred to here is the total content of solid electrolyte 6a and solid electrolyte 6b.

[0046] When the electrode layer 3 contains a conductive assistant, the content of the conductive assistant is preferably 0.1% or more, more preferably 0.2% or more, preferably 20% or less, and more preferably 10% or less in terms of mass%. When the content of the conductive assistant in the electrode layer 3 is within the above range, high electron conductivity in the electrode layer 3 can be ensured while further improving ionic conductivity, and the battery characteristics of the secondary battery can be further effectively improved.

[0047] As the solid electrolytes 6a and 6b, those described in the column of the solid electrolyte layer 2 can be used. It is desirable that the solid electrolytes 6a and 6b are the same type of solid electrolyte, but different types of solid electrolytes may be combined and used.

[0048] As the conductive assistant, for example, conductive carbon can be used. Examples of the conductive carbon include acetylene black, carbon black, ketjen black, and vapor-grown carbon fiber (VGCF). The conductive assistant is preferably a carbon-based conductive assistant composed of the above materials.

[0049] In this embodiment, the electrode active material 5 is a positive electrode active material, and the electrode layer 3 is a positive electrode layer. However, in the present invention, the electrode active material 5 may be a negative electrode active material, and the electrode layer 3 may be a negative electrode layer. Note that both the positive electrode layer and the negative electrode layer may be the electrode layer 3.

[0050] Positive electrode active material; The positive electrode active material is not particularly limited, but is preferably a positive electrode active material composed of crystallized glass containing a crystal represented by the general formula Na x M y P2O z (1 ≦ x ≦ 2.8, 0.95 ≦ y ≦ 1.6, 6.5 ≦ z ≦ 8, and M is at least one selected from the group consisting of Fe, Ni, Co, Mn, and Cr). Among them, the general formula Na xIt is more preferable that the positive electrode active material is made of crystallized glass containing crystals represented by MP2O7 (1 ≤ x ≤ 2, where M is at least one selected from the group consisting of Fe, Ni, Co, Mn, and Cr). Examples of such positive electrode active material crystals include Na2FeP2O7, Na2CoP2O7, Na2NiP2O7, and the like.

[0051] Crystallized glass refers to a precursor glass containing an amorphous phase that has been heated (fired) to precipitate crystals (crystallize). The amorphous phase may all be transformed into the crystalline phase, or some of the amorphous phase may remain. Furthermore, one type of crystal may be precipitated, or two or more types of crystals may be precipitated. For example, crystallized glass can be determined from whether or not it is crystallized glass by the peak angle shown by powder X-ray diffraction (XRD).

[0052] negative electrode active material; The negative electrode active material is not particularly limited, but for example, carbon electrode materials such as hard carbon or soft carbon can be used. Hard carbon is preferred as the carbon electrode material. However, the negative electrode active material may also contain alloy-based negative electrode active materials that can absorb sodium, such as tin, bismuth, lead, or phosphorus, or metallic sodium.

[0053] The following describes an example of a method for manufacturing the electrode layer 3.

[0054] Method for manufacturing an electrode layer; First, a second solid electrolyte layer, which is a porous layer, is formed on a first solid electrolyte layer, which is a dense layer. Such first and second solid electrolyte layers can be formed, for example, according to the method for manufacturing a solid electrolyte sheet described in International Publication No. 2021 / 045039.

[0055] Specifically, the first solid electrolyte layer and the second solid electrolyte layer can be formed, for example, according to a manufacturing method comprising the following steps (a) to (d).

[0056] (a) A step of obtaining a green sheet for forming a first solid electrolyte layer by adding an organic vehicle containing a binder to at least one of the powders of the first solid electrolyte powder and the raw material powder of the first solid electrolyte powder to prepare a slurry, and applying the slurry to a substrate and drying it.

[0057] (b) At least one of the second solid electrolyte powder and the raw material powder of the second solid electrolyte powder A process to obtain a second green sheet for forming a solid electrolyte layer by adding an organic vehicle containing a binder to a mixed powder containing a powder and a polymer powder, preparing a slurry, and applying the slurry to a substrate and drying it.

[0058] (c) A step of obtaining a laminate by laminating a second solid electrolyte layer forming green sheet on at least one main surface of a first solid electrolyte layer forming green sheet.

[0059] (d) A step of forming a dense first solid electrolyte layer and a porous second solid electrolyte layer by firing the laminate.

[0060] Next, an electrode material layer is formed by applying a paste containing an electrode active material precursor, solid electrolyte powder, and optionally a conductive additive to the second solid electrolyte layer, which is a porous layer, and drying it. Thus, the electrode material layer is formed by impregnating the second solid electrolyte layer, which is a porous layer, with the paste. The paste may optionally contain a binder, plasticizer, or solvent. The electrode material layer may also be in the form of compacted powder. Subsequently, the electrode layer 3 can be formed by firing the electrode material layer.

[0061] The drying temperature of the paste is not particularly limited, but for example, it can be between 30°C and 150°C. The drying time of the paste is also not particularly limited, but for example, it can be between 5 minutes and 600 minutes.

[0062] Furthermore, a reducing atmosphere is preferable during firing. The firing temperature (maximum temperature) can be, for example, 400°C to 700°C, and the holding time at that temperature can be, for example, 5 minutes to less than 3 hours.

[0063] In this embodiment, for example, by adjusting the viscosity of the paste, the porosity of the first electrode layer portion 3A on the solid electrolyte layer 2 side can be made larger than the porosity of the second electrode layer portion 3B on the first current collector layer 4 side in the resulting electrode layer 3.

[0064] In this case, it is preferable that the viscosity of the paste be higher than that of conventional pastes. The viscosity of the paste is preferably 6000 mPa·s or higher, more preferably 8000 mPa·s or higher, more preferably 50000 mPa·s or lower, and more preferably 30000 mPa·s or lower. The viscosity of the paste can be measured using a B-type viscometer.

[0065] However, in the present invention, when forming the second solid electrolyte layer, the porosity of the first electrode layer portion 3A on the solid electrolyte layer 2 side may be made larger than the porosity of the second electrode layer portion 3B on the first current collector layer 4 side by applying multiple layers of slurry with different ratios of void-forming material (polymer powder).

[0066] In this embodiment, the electrode active material precursor is a positive electrode active material precursor. However, the electrode active material precursor may also be a negative electrode active material precursor. The following will describe in detail the paste for forming the positive electrode layer when electrode layer 3 is the positive electrode layer, and the paste for forming the negative electrode layer when electrode layer 3 is the negative electrode layer.

[0067] Paste for forming a positive electrode layer; As the paste for forming the positive electrode layer, for example, a paste containing a positive electrode active material precursor and a solid electrolyte powder, and optionally a conductive additive, can be used. The paste may also optionally contain a binder, plasticizer, or solvent.

[0068] The cathode active material precursor (cathode active material precursor powder) is amorphous, meaning that active material crystals are formed by calcination. It is preferable that the material be an amorphous oxide material. When the positive electrode active material precursor powder is made of an amorphous oxide material, active material crystals are formed during firing, and softening and flowing allows for the formation of a dense positive electrode layer. Furthermore, if the positive electrode layer contains a solid electrolyte, integration of the positive electrode active material and the solid electrolyte can be achieved. Alternatively, if the positive electrode layer is in contact with a solid electrolyte layer, integration of the two can be achieved. As a result, a better ion conduction path is formed, which is preferable. In addition, in this invention, "amorphous oxide material" is not limited to a completely amorphous oxide material, but also includes materials that contain some crystals (for example, crystallinity of 10% or less).

[0069] The positive electrode active material precursor powder preferably contains, in molar percentages on an oxide basis, 25% to 55% Na2O, 10% to 30% Fe2O3 + Cr2O3 + MnO + CoO + NiO, and 25% to 55% P2O5. The reason for this limitation of composition is explained below. In the following explanation of the content of each component, unless otherwise specified, "%" means "molar percent".

[0070] Na2O is the general formula Na x M y P2O z The active material crystal is represented by (M is at least one transition metal element selected from Cr, Fe, Mn, Co, and Ni, with 1 ≤ x ≤ 2.8, 0.95 ≤ y ≤ 1.6, and 6.5 ≤ z ≤ 8). The Na2O content is preferably 25% to 55%, and more preferably 30% to 50%. When the Na2O content is within the above range, the charge and discharge capacity of the secondary battery can be further increased.

[0071] Fe2O3, Cr2O3, MnO, CoO, and NiO are also generally formulated with the formula Na x M y P2O zIt is the main component of the active material crystal represented by. The content of Fe2O3 + Cr2O3+MnO+CoO+NiO is preferably 10% - 50%, more preferably 12% - 40%, and most preferably 15% - 30%. When the content of Fe2O3 + Cr2O3+MnO+CoO+NiO is above the above lower limit value, the charge-discharge capacity of the secondary battery can be further increased. On the other hand, when the content of Fe2O3 + Cr2O3+MnO+CoO+NiO is below the above upper limit value, it is possible to make it difficult to precipitate crystals such as undesirable Fe2O3, Cr2O3, MnO, CoO, or NiO. In addition, in order to further improve the cycle characteristics of the secondary battery, it is preferable to actively contain Fe2O3. The content of Fe2O3 is preferably 1% - 30%, more preferably 5% - 30%, still more preferably 10% - 30%, and particularly preferably 15% - 25%. The content of each component of Cr2O3, MnO, CoO, and NiO is preferably 0% - 45%, more preferably 10% - 40%, and still more preferably 15% - 30%. Further, when containing at least two or more components selected from Fe2O3, Cr2O3, MnO, CoO, and NiO, the total amount is preferably 10% - 50%, more preferably 12% - 40%, and most preferably 15% - 30%.

[0072] P2O5 is also the general formula Na x M y P2O z It is the main component of the active material crystal represented by. The content of P2O5 is preferably 25% - 55%, more preferably 30% - 50%. When the content of P2O5 is within the above range, the charge-discharge capacity of the secondary battery can be further increased.

[0073] The positive electrode active material precursor powder may also contain V2O5, Nb2O5, MgO, Al2O3, TiO2, ZrO2, or Sc2O3 in addition to the above components. These components have the effect of increasing conductivity (electron conductivity), which makes it easier to improve the rapid charge and discharge characteristics of the secondary battery. 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 below the above upper limit, heterogeneous crystals that do not contribute to the battery characteristics are less likely to form, and the charge and discharge capacity of the secondary battery can be further increased.

[0074] Furthermore, the positive electrode active material precursor powder may also contain SiO2, B2O3, GeO2, Ga2O3, Sb2O3, or Bi2O3 in addition to the above components. Including these components further improves the glass-forming ability and makes it easier to obtain a more homogeneous positive electrode active material precursor powder. The total content of the above components is preferably 0% to 25%, and more preferably 0.2% to 10%. Since these components do not contribute to the battery characteristics, if their content is too high, the charge and discharge capacity of the secondary battery tends to decrease.

[0075] The positive electrode active material precursor powder is preferably prepared by melting and molding a batch of raw materials. This preparation 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 prepared as follows.

[0076] First, 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, the melting temperature is preferably 800°C or higher, and more preferably 900°C or higher. There is no particular upper limit to the melting temperature, but if the melting temperature is too high, it can lead to energy loss and evaporation of sodium components, so it is preferably 1500°C or lower, and more preferably 1400°C or lower.

[0077] 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.

[0078] Next, the obtained molded body is crushed to obtain a positive electrode active material precursor powder. The average particle size of the positive electrode active material precursor powder is preferably 0.01 μm or more and less than 0.7 μm, more preferably 0.03 μm or more and 0.6 μm or less, even more preferably 0.05 μm or more and 0.6 μm or less, and particularly preferably 0.1 μm or more and 0.5 μm or less.

[0079] A binder is a material used to bind raw materials (raw material powders) together. 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.

[0080] Paste for forming the negative electrode layer; As a paste for forming the negative electrode layer, for example, a paste containing a carbon electrode material precursor and a solid electrolyte precursor can be used. When preparing the paste, first, the solid electrolyte precursor is prepared. At this stage, it is preferable to prepare a solid electrolyte precursor solution. Specific examples of solid electrolyte precursors and their solutions will be described later. Next, a carbon electrode material precursor (a carbon electrode material precursor made of hard carbon) is prepared. Appropriate sugars, biomass, or polymers can be used as the carbon electrode material precursor.

[0081] Next, the solid electrolyte precursor solution and the carbon electrode material precursor are mixed and then dried. This yields a powder mixture of the solid electrolyte precursor and the carbon electrode material precursor. Next, the powder mixture is pulverized and then mixed with a conductive additive and a binder in an organic solvent. For example, N-methyl-2-pyrrolidone can be used as the organic solvent. This yields a paste.

[0082] When using sugars as carbon electrode material precursors, examples include sucrose, cellulose, D-glucose, etc. When using biomass as carbon electrode material precursors, examples include corn stalks, sorghum stalks, pine cones, mangosteen, argan shells, rice husks, dandelions, grain straw cores, ramie fibers, cotton, kelp, coconut endocarp, etc. When using polymers as carbon electrode material precursors, examples include PAN (polyacrylonitrile), pitch, PVC (polyvinyl chloride) nanofibers, polyaniline, sodium polyacrylate, tires (tire polymers), phosphorus-doped PAN, etc.

[0083] When the solid electrolyte is beta-alumina, the solid electrolyte precursor can be obtained, for example, by mixing aluminum nitrate, sodium nitrate, and lithium nitrate. At this time, the ratio of each of these materials is adjusted to achieve the desired composition ratio of the solid electrolyte.

[0084] The solid electrolyte is NASICON crystal or Na5XSi4O 12 When the solid electrolyte is a type crystal (where X is at least one selected from group 3 transition metal elements, preferably rare earth elements), the solid electrolyte precursor solution includes a solution containing sodium and transition metal elements that constitute the solid electrolyte, and carbonate ions. In this solution, the sodium element is contained in the form of sodium ions, and the transition metal elements are contained in the form of transition metal ions. The solid electrolyte precursor consists, for example, of a gelled or dried product of the solid electrolyte precursor solution. The solid electrolyte then consists of a calcined product of the solid electrolyte precursor.

[0085] Furthermore, it is possible to use a solid electrolyte precursor solution that contains nitrate ions instead of carbonate ions.

[0086] Furthermore, it is preferable that the carbonate ions are bidentately coordinated to the transition metal element in the solid electrolyte precursor solution. In this case, the transition metal element is more likely to exist stably in the solution.

[0087] Also, as a counterion for sodium ions, NR 4+ It is preferable that each R in the formula is independently selected from the group consisting of H, CH3, C2H5, and CH2CH2OH, and that this allows the transition metal elements to exist more stably in solution.

[0088] A solid electrolyte precursor solution can be obtained, for example, by mixing water glass (sodium silicate), sodium tripolyphosphate, and an aqueous solution of zirconium ammonia carbonate.

[0089] As a binder, the one described in the section on paste for forming the positive electrode layer can be used.

[0090] Further details; The thickness of the electrode layer 3 is preferably 5 μm or more, more preferably 20 μm or more, even more preferably 50 μm or more, preferably 1000 μm or less, and more preferably 200 μm or less. When the thickness of the electrode layer 3 is greater than or equal to the lower limit above, the charge and discharge capacity of the secondary battery can be further increased. However, if the thickness of the electrode layer 3 is too thick, the resistance to electron conduction increases, which may reduce the discharge capacity and operating voltage of the secondary battery, and the stress due to shrinkage during firing may increase, leading to delamination.

[0091] (First current collector layer) The material of the first current collector layer 4 is not particularly limited, but can be a metallic material such as aluminum, titanium, silver, copper, stainless steel, or an alloy thereof. The above metallic materials may be used individually or in combination. These alloys are alloys containing at least one of the above metals. The thickness of the first current collector layer 4 is not particularly limited, but can be between 0.01 μm and 1000 μm.

[0092] The method for forming the first current collector layer 4 is not particularly limited and includes, for example, physical vapor phase methods such as vapor deposition or sputtering, and chemical vapor phase methods such as thermal CVD, MOCVD, and plasma CVD. Other methods for forming the first current collector layer 4 include liquid phase deposition methods such as plating, sol-gel method, and spin coating. However, it is preferable to form the first current collector layer 4 on the electrode layer 3 by sputtering because it provides excellent adhesion.

[0093] [All-solid-state secondary battery] Figure 4 is a schematic cross-sectional view showing an all-solid-state secondary battery according to one embodiment of the present invention. As shown in Figure 4, the all-solid-state secondary battery 10 comprises an energy storage element 1, a negative electrode layer 20, and a second current collector layer 30.

[0094] As described above, the energy storage element 1 comprises a solid electrolyte layer 2, an electrode layer 3 which is a positive electrode layer and is provided on the first main surface 2a of the solid electrolyte layer 2, and a first current collector layer 4 provided on the electrode layer 3. In this embodiment, a negative electrode layer 20 is provided on the second main surface 2b of the solid electrolyte layer 2, and a second current collector layer 30 is provided on the negative electrode layer 20. The second current collector layer 30 can be the one described in the section on the first current collector layer.

[0095] Thus, in this embodiment, the electrode layer 3 constituting the energy storage element 1 is the positive electrode layer. However, as mentioned above, the electrode layer 3 may also be the negative electrode layer. In that case, the all-solid-state secondary battery 10 only needs to have a positive electrode layer instead of a negative electrode layer 20 on the second main surface 2b of the solid electrolyte layer 2. Furthermore, both the positive electrode layer and the negative electrode layer constituting the all-solid-state secondary battery 10 may have the same configuration as the electrode layer 3. Note that the current collector layers, such as the first current collector layer 4 and the second current collector layer 30, only need to be provided on the side where the electrode layer 3 is located.

[0096] The all-solid-state secondary battery 10 of this embodiment includes an energy storage element 1, and therefore exhibits excellent cycle characteristics during repeated charging and discharging.

[0097] The present invention will be described in more detail below based on specific examples, but the present invention is not limited in any way to the following examples and can be implemented with appropriate modifications without changing the gist of the invention.

[0098] (Example 1) (a) Preparation of the first green sheet for forming the solid electrolyte layer Sodium carbonate (Na2CO3), aluminum oxide (Al2O3), magnesium oxide (MgO), zirconium oxide (ZrO2), and yttrium oxide (Y2O3) were used as raw materials to prepare a raw material powder with molar percentages of Na2O 14.2%, Al2O3 75.4%, MgO 5.4%, ZrO 24.9%, and Y2O 30.1%. This powder was calcined at 1250°C for 4 hours and then pulverized to an average particle size of 2 μm. Next, 100 parts by mass of this calcined raw material powder and 12.5 parts by mass of polyvinyl butyral resin (manufactured by Sekisui Chemical Co., Ltd., product name "BM-SZ") as a binder were weighed and dispersed in N-methylpyrrolidone. The mixture was then thoroughly stirred in a rotary-revolving mixer to form a slurry. The obtained slurry is applied to a polyethylene terephthalate film (PET film) using a doctor blade, dried at 70°C, and then peeled off from the PET film to obtain the first solid electrolytic material. A green sheet for forming a layer was obtained.

[0099] (b) Preparation of a green sheet for forming a second solid electrolyte layer Sodium carbonate (Na2CO3), aluminum oxide (Al2O3), magnesium oxide (MgO), zirconium oxide (ZrO2), and yttrium oxide (Y2O3) were used as raw materials to prepare a raw material powder with molar percentages of Na2O 14.2%, Al2O3 75.4%, MgO 5.4%, ZrO 24.9%, and Y2O 30.1%. This powder was calcined at 1250°C for 4 hours and then pulverized to an average particle size of 2 μm. Next, 35 parts by mass of this calcined raw material powder and 65 parts by mass of cross-linked polymethyl methacrylate particles (manufactured by Sekisui Chemical Co., Ltd., product number "MBX-50", average particle size 50 μm) were weighed and mixed. 100 parts by mass of the obtained mixture and 12.5 parts by mass of polyvinyl butyral resin (manufactured by Sekisui Chemical Co., Ltd., trade name "BM-SZ") as a binder were weighed and dispersed in N-methylpyrrolidone. The mixture was then thoroughly stirred in a rotary-revolving mixer to form a slurry. The obtained slurry was applied to a PET film using a doctor blade, dried at 70°C, and then peeled off the PET film to obtain a green sheet for forming a second solid electrolyte layer.

[0100] (c) Fabrication of the laminate A laminate was fabricated by laminating the obtained second solid electrolyte layer-forming green sheet onto one main surface of the obtained first solid electrolyte layer-forming green sheet and pressing it under isostatic pressure at 90°C and 40 MPa for 5 minutes.

[0101] (d) Firing of the laminate The resulting laminate was fired at 1550°C for 30 minutes to produce a solid electrolyte layer (300 μm thick) in which a second solid electrolyte layer, which is a porous layer, was formed on the main surface of one side of a first solid electrolyte layer, which is a dense layer.

[0102] In the obtained solid electrolyte layer, the thickness of the first solid electrolyte layer was 200 μm, and the thickness of the second solid electrolyte layer was 100 μm. Furthermore, the porosity of the first solid electrolyte layer was 5%, and the porosity of the second solid electrolyte layer was 78%.

[0103] (e) Preparation of paste for forming the positive electrode layer The raw materials are sodium metaphosphate (NaPO3), ferric oxide (Fe2O3), and orthophosphate (H3PO4), with the molar percentages being Na2O 40%, Fe2O3 20%, and P2O5 The raw material powders were mixed to make up 40% of the composition and melted in an air atmosphere at 1250°C for 45 minutes. The molten material was then poured between a pair of rotating rollers and rapidly cooled while being molded to obtain a film-like glass. The obtained film-like glass was then ground using a ball mill and a planetary ball mill to obtain an average particle size (D 50 A glass powder (cathode active material precursor powder) with a diameter of 0.2 μm was obtained. Powder X-ray diffraction (XRD) measurements confirmed that the obtained glass powder was amorphous.

[0104] The obtained cathode active material precursor powder was weighed to a total of 83% by mass, solid electrolyte powder to 13% by mass, and acetylene black (TIMCAL, SUPERC65) as conductive carbon (conductive additive) to 4% by mass. These were then mixed for 30 minutes using an agate mortar and pestle to prepare cathode composite powder. 100 parts by mass of the prepared cathode composite powder were mixed with 10 parts by mass of polypropylene carbonate (PPC), and then 20 parts by mass of N-methylpyrrolidone. These were thoroughly stirred using a rotation-revolution mixer to form a slurry and prepare a paste for cathode layer formation.

[0105] (f) Formation of the positive electrode layer On the main surface of the second solid electrolyte layer, an area of ​​1 cm² 2 , capacity 1mAh / cm 2 So that it becomes so, A paste for forming a positive electrode layer was applied and dried at 70°C for 3 hours to form a positive electrode material layer. The formed positive electrode material layer was then fired in a mixed gas atmosphere of H24 vol% and N296 vol% at 525°C for 30 minutes to form a positive electrode layer on one main surface of the solid electrolyte layer.

[0106] The viscosity of the paste used to form the positive electrode layer was set to 10,000 mPa·s during application. The viscosity was measured using a B-type viscometer at 25°C.

[0107] (g) Porosity measurement The porosity of the positive electrode layer was determined by image analysis of the cross-section along the thickness direction of the positive electrode layer. For the image analysis, the electrode cross-sectional image obtained by scanning electron microscopy (SEM) was binarized (converted to two colors, black and white) using image processing software (ImageJ), and then the porosity was calculated. The threshold value for binarization was adjusted as needed while viewing the actual image so that the void areas would appear black. The porosity was calculated by dividing the area (number of pixels) of the black area for each electrode section by the total area (number of pixels) of the section.

[0108] The porosity of the positive electrode layer was measured for both a sample divided into two sections in the thickness direction (Figure 1) and a sample divided into three sections (Figure 2). For the sample divided into two sections, the porosity X (volume %) of the first electrode layer and the porosity Y (volume %) of the second electrode layer were measured in the thickness direction, starting from the solid electrolyte layer side. For the sample divided into three sections, the porosity X (volume %) of the first electrode layer, the porosity Y (volume %) of the second electrode layer, and the porosity Z (volume %) of the third electrode layer were measured in the thickness direction, starting from the solid electrolyte layer side.

[0109] (h) Formation of the current collector and assembly of the coin cell A 100 nm thick aluminum vapor-deposited film was formed on the entire surface of the positive electrode layer as a current collector. Subsequently, in an argon glove box with a dew point of -60°C or lower, metallic sodium, which would become the negative electrode layer, was pressed onto the main surface of the solid electrolyte layer opposite to the positive electrode layer, and then sealed in a CR2032 coin cell to fabricate an all-solid-state secondary battery.

[0110] (i) Charge-discharge cycle test All-solid-state secondary batteries were vacuum-sealed in an aluminum laminate casing. Next, charge-discharge cycle tests were performed (charging: 0.1C / 4.5V CC / CV 0.02C cutoff, 25℃; discharging: 0.1C CC 1.5V cutoff, 25℃), and the discharge capacity retention rate after 100 cycles relative to the initial discharge capacity (capacity retention rate after 100 cycles) was determined.

[0111] (Comparative Example 1) A solid-state secondary battery was prepared in the same manner as in Example 1, except that the viscosity of the paste for forming the positive electrode layer was set to 5000 mPa·s during application, and a charge-discharge cycle test was performed. In Comparative Example 1, the porosity (volume %) of each positive electrode layer was determined in the same manner as in Example 1.

[0112] (Comparative Example 2) A solid-state secondary battery was prepared in the same manner as in Example 1, except that the viscosity of the paste for forming the positive electrode layer was set to 3000 mPa·s during application, and a charge-discharge cycle test was performed. In Comparative Example 2, the porosity (volume %) of each positive electrode layer was determined in the same manner as in Example 1.

[0113] The results are shown in Table 1 below. In Table 1, the units for X, Y, and Z are in volume percent.

[0114] [Table 1] [Explanation of symbols]

[0115] 1… Energy storage element 2...Solid electrolyte layer 2a, 2b… First and second principal surfaces 3...Electrode layer 3A, 3C... First electrode layer portion 3B,3D...Second electrode layer part 3E...Third electrode layer portion 4…First current collector layer 5...Electrode active material 6a,6b…Solid electrolyte 10…All-solid-state secondary battery 20... Negative electrode layer 30...Second current collector layer

Claims

1. A solid electrolyte layer, An electrode layer comprising an electrode active material and a solid electrolyte, provided on one main surface of the solid electrolyte layer, A current collector layer is provided on the main surface of the electrode layer opposite to the solid electrolyte layer, Equipped with, An energy storage element in which the void ratio of a first electrode layer portion located on the solid electrolyte layer side of the center in the thickness direction of the electrode layer is greater than the void ratio of a second electrode layer portion located on the current collector layer side of the center in the thickness direction of the electrode layer.

2. The energy storage element according to claim 1, wherein the porosity of the electrode layer increases from the current collector layer side toward the solid electrolyte layer side.

3. When the solid electrolyte layer is a dense layer, the first solid electrolyte layer, The energy storage element according to claim 1 or 2, wherein the electrode layer comprises an electrode material containing an electrode active material and a solid electrolyte arranged within a second solid electrolyte layer which is a porous layer.

4. The energy storage element according to claim 1 or 2, wherein the solid electrolyte contains at least one selected from the group consisting of β''-alumina, β-alumina, and NASICON crystal.

5. The energy storage element according to claim 1 or 2, wherein the solid electrolyte layer and the electrode layer are sintered bodies.

6. A solid-state secondary battery comprising the energy storage element described in claim 1 or 2.

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