Energy storage elements and all-solid-state secondary batteries

Abstract

To provide a power storage element capable of improving the capacity retention at charging and discharging of a secondary battery with a large current and the cycle characteristic at repeated charging and discharging.SOLUTION: A power storage element 1 includes a solid electrolyte layer 2, an electrode layer 3 provided on a main surface 2a 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 ratio of the content of the solid electrolytes 6a, 6b to the electrode active material 5 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 ratio of the content of the solid electrolytes 6a, 6b to the electrode active material 5 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 Initiative] [Problems that the invention aims to solve]

[0005] Incidentally, all-solid-state secondary batteries generally have a problem in that they cannot fully extract capacity during high-current charging and discharging. Furthermore, sintered all-solid-state secondary batteries, which are formed by firing solid electrolyte and electrode layers, have a problem in that their charge-discharge performance deteriorates and their cycle characteristics are prone to degradation after repeated charging and discharging.

[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 capacity retention rate during high-current charging and discharging of a secondary battery, as well as the cycle characteristics during repeated charging and discharging. [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] A power storage element according to embodiment 1 of the present invention comprises a solid electrolyte layer, an electrode layer provided on one main surface of the solid electrolyte 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 ratio of the solid electrolyte content to the electrode active material in 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 ratio of the solid electrolyte content to the electrode active material in a second electrode layer portion located on the current collector layer side of the center in the thickness direction of the electrode layer.

[0009] In the energy storage element according to embodiment 2, it is preferable that, in embodiment 1, the ratio of the solid electrolyte content to the electrode active material in the electrode layer increases from the current collector layer side to the solid electrolyte side.

[0010] In the energy storage element according to Embodiment 3, in Embodiment 1 or Embodiment 2, when the solid electrolyte layer is a dense first solid electrolyte layer, it is preferable that the electrode layer is an electrode material containing an electrode active material and a solid electrolyte arranged within a porous second solid electrolyte layer.

[0011] In the energy storage element according to Embodiment 4, it is preferable that the porosity of the first electrode layer portion is greater than the porosity of the second electrode layer portion in any one embodiment from Embodiment 1 to Embodiment 3.

[0012] In the energy storage element according to Embodiment 5, 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 4.

[0013] In the energy storage element according to embodiment 6, it is preferable that the solid electrolyte layer and the electrode layer are sintered bodies in any one embodiment from embodiment 1 to embodiment 5.

[0014] The all-solid-state secondary battery according to aspect 7 of the present invention is characterized by comprising an energy storage element according to any one of aspects 1 to 6. [Effects of the Invention]

[0015] 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 capacity retention rate during high-current charging and discharging of a secondary battery, as well as the cycle characteristics during repeated charging and discharging. [Brief explanation of the drawing]

[0016] [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 composition 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 a modified energy storage element. [Figure 5] Figure 5 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]

[0017] 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.

[0018] [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.

[0019] 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.

[0020] 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 ratio of solid electrolyte content to electrode active material in the first electrode layer portion 3A on the solid electrolyte layer 2 side is greater than the ratio of solid electrolyte content to electrode active material in the second electrode layer portion 3B on the first current collector layer 4 side.

[0021] The ratio of solid electrolyte content to electrode active material in the first electrode layer portion 3A and the second electrode layer portion 3B can be determined by elemental analysis of a cross-section along the thickness direction of the electrode layer 3. Elemental analysis can be performed, for example, by using an energy-dispersive X-ray fluorescence spectrometer (EDX) to calculate the molar ratio of, for example, the Fe content in the positive electrode active material, the Al content in the solid electrolyte, and the C content in the conductive additive for each electrode section area. Furthermore, the volume ratio of electrode active material to solid electrolyte in the electrode mixture can be determined from the specific gravities of the electrode active material, solid electrolyte, and conductive additive. Additionally, the porosity and electrode mixture occupancy rate for each electrode layer section area can be calculated using image analysis software (e.g., ImageJ) from electrode cross-sectional images obtained with a scanning electron microscope (SEM). Specifically, the porosity is determined after binarization (conversion to two colors, black and white). The threshold for binarization is adjusted appropriately while viewing the actual image so that the void areas appear black. The porosity is calculated by dividing the area (number of pixels) of the black portion for each electrode layer section by the total area (number of pixels) of the section. Next, the electrode material occupancy rate other than the obtained porosity is determined. By multiplying the calculated electrode material occupancy rate by the volume ratio of electrode active material and solid electrolyte in the electrode material, the volume ratio of electrode active material and solid electrolyte for each electrode layer section can be calculated.

[0022] Since the energy storage element 1 of this embodiment has the above configuration, it is possible to improve the capacity retention rate during charging and discharging of the secondary battery at high currents, and the cycle characteristics during repeated charging and discharging.

[0023] Conventionally, in all-solid-state secondary batteries, the movement resistance of carrier ions within the electrode layer is high due to the numerous voids present in the electrode composite layer and the large contact resistance between solid electrolyte particles in the electrode composite. As a result, during charging and discharging at high currents, a reaction distribution may occur in the thickness direction of the electrode layer (reactions may occur more easily near the solid electrolyte layer than near the current collector layer), which can result in insufficient capacity being obtained.

[0024] Furthermore, sintered all-solid-state secondary batteries, formed by firing solid electrolyte and electrode layers, have a problem in that repeated charging and discharging can cause defects such as cracks to occur inside the energy storage element due to the expansion and contraction of the electrode active material, which reduces charging and discharging performance and easily degrades cycle characteristics.

[0025] In response to this, the present inventors have found that the reaction distribution in the thickness direction of the electrode layer 3 can be mitigated by increasing the ratio of solid electrolytes 6a and 6b in the composition of the electrode mixture near the solid electrolyte layer 2, where the charge-discharge reaction proceeds easily. Specifically, they found that the reaction distribution in the thickness direction of the electrode layer 3 can be mitigated by making the ratio of solid electrolyte content to electrode active material in the first electrode layer portion 3A on the solid electrolyte layer 2 side greater than the ratio of solid electrolyte content to electrode active material in the second electrode layer portion 3B on the first current collector layer 4 side.

[0026] In the energy storage element 1 of this embodiment, the capacity retention rate during charging and discharging of the secondary battery at high currents can be increased by mitigating the reaction distribution in the thickness direction of the electrode layer 3. Furthermore, by mitigating the reaction distribution in the thickness direction of the electrode layer 3, the difference in expansion and contraction in the thickness direction of the electrode layer 3 is reduced, thereby improving the cycle characteristics.

[0027] In this embodiment, the ratio X (solid electrolyte / electrode active material) of solid electrolyte to electrode active material in the first electrode layer portion 3A is preferably 5 volume% or more, more preferably 20 volume% or more, more preferably 200 volume% or less, and more preferably 150 volume% or less. When the ratio X (solid electrolyte / electrode active material) of the first electrode layer portion 3A is within the above range, the capacity retention rate during high-current charging and discharging of the secondary battery and the cycle characteristics during repeated charging and discharging can be further improved.

[0028] The ratio Y (solid electrolyte / electrode active material) of solid electrolyte to electrode active material in the second electrode layer portion 3B is preferably 5% by volume or more, more preferably 10% by volume or more, preferably 200% by volume or less, and more preferably 100% by volume or less. When the ratio Y (solid electrolyte / electrode active material) of the second electrode layer portion 3B is within the above range, the capacity retention rate during high-current charging and discharging of the secondary battery and the cycle characteristics during repeated charging and discharging can be further improved.

[0029] The ratio (X / Y) of the proportion X (solid electrolyte / electrode active material) of the first electrode layer portion 3A to the proportion Y (solid electrolyte / electrode active material) of the second electrode layer portion 3B is preferably 1.01 or more, more preferably 1.05 or more, preferably 3 or less, and more preferably 1.5 or less. When the ratio (X / Y) is within the above range, the capacity retention rate during high-current charging and discharging of the secondary battery, and the cycle characteristics during repeated charging and discharging can be further improved.

[0030] 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 ratio of solid electrolyte content to electrode active material 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 ratio of solid electrolyte content to electrode active material increases from the first current collector layer 4 side to the solid electrolyte layer 2 side, the capacity retention rate during high-current charging and discharging of the secondary battery and the cycle characteristics during repeated charging and discharging can be further improved.

[0031] Furthermore, the ratio of solid electrolyte content to electrode active material in the first electrode layer portion 3C, the second electrode layer portion 3D, and the third electrode layer portion 3E can also be determined by elemental analysis along the thickness direction of the electrode layer 3 as described above.

[0032] The ratio X (solid electrolyte / electrode active material) of the solid electrolyte content to the electrode active material in the first electrode layer portion 3C is preferably 5% by volume or more, more preferably 20% by volume or more, preferably 200% by volume or less, and more preferably 150% by volume or less.

[0033] The ratio Y (solid electrolyte / electrode active material) of the solid electrolyte content to the electrode active material in the second electrode layer portion 3D is preferably 5% by volume or more, more preferably 15% by volume or more, preferably 200% by volume or less, and more preferably 125% by volume or less.

[0034] The ratio Z (solid electrolyte / electrode active material) of the solid electrolyte content to the electrode active material in the third electrode layer portion 3E is preferably 5% by volume or more, more preferably 10% by volume or more, preferably 200% by volume or less, and more preferably 100% by volume or less.

[0035] In addition, in the present invention, as shown in the modified energy storage element 1A in Figure 4, the porosity of the first electrode layer portion 3A may be made greater than that of the second electrode layer portion 3B, and the ratio of the solid electrolyte content to the electrode active material in the first electrode layer portion 3A on the solid electrolyte layer 2 side may be made greater than the ratio of the solid electrolyte content to the electrode active material in the second electrode layer portion 3B on the first current collector layer 4 side.

[0036] 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. For example, the image analysis is performed by using image processing software (e.g., ImageJ) to binarize (convert to two colors, black and white) the electrode layer cross-sectional image obtained by scanning electron microscopy (SEM) or CT (computed tomography), and then determining the porosity. The threshold value 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 division area by the total area (number of pixels) of the division area.

[0037] The porosity 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 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.

[0038] The porosity 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 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.

[0039] Furthermore, it is preferable that the porosity of the electrode layer 3 has a gradient structure in which it increases from the first current collector layer 4 side towards the solid electrolyte layer 2 side. In this case, the cycle characteristics during repeated charging and discharging of the secondary battery can be further improved.

[0040] The following describes the configuration of each layer in the energy storage elements of the present invention, such as energy storage element 1 and energy storage element 1A.

[0041] (Solid electrolyte layer) The solid electrolyte constituting the solid electrolyte layer 2 is preferably formed from a sodium ion conductive oxide. Examples of sodium ion conductive oxides include compounds containing at least one selected from Al, Y, Zr, Si, and P, Na, and O. Specific examples of sodium ion conductive oxides include beta-alumina or NASICON crystals, which have excellent sodium ion conductivity. In particular, 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 crystals. The sodium ion conductive oxide is more preferably β-alumina or β''-alumina, as these exhibit even better sodium ion conductivity.

[0042] Beta alumina has two crystal types: β-alumina (theoretical composition formula: Na₂O·11Al₂O₃) and β''-alumina (theoretical composition formula: Na₂O·5.3Al₂O₃). Since β''-alumina is a metastable substance, those added with Li₂O or MgO as a stabilizer are usually used. β''-alumina has higher sodium ion conductivity than β-alumina, so it is preferable to use β''-alumina alone or a mixture of β''-alumina and β-alumina, 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)) is more preferably used.

[0043] NASICON crystals include Na₃Zr₂Si₂PO 12 , Na 3.2 Zr 1.3 Si 2.2 P 0.8 O 10.5 , Na₃Zr 1.6 Ti 0.4 Si₂PO 12 , Na₃Hf₂Si₂PO 12 , Na 3.4 Zr 0.9 Hf 1.4 Al 0.6 Si 1.2 P 1.8 O 12 , Na₃Zr 1.7 Nb 0.24 Si₂PO 12 , Na 3.6 Ti 0.2 Y 0.8 Si 2.8 O₉, 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 O12 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 Examples of crystals include those made of Na. Among them, NASICON crystals are made of Na 3.4 Zr2Si 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.

[0044] 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.

[0045] The thickness of the solid electrolyte layer 2 is preferably 5 μm or more, more preferably 10 μm or more, even more 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.

[0046] 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%.

[0047] Porosity = (1-p / p0)×100(%)…Equation (1) In equation (1), p is the bulk density and p0 is the true density.

[0048] (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.

[0049] The second solid electrolyte layer is composed of a solid electrolyte 6a and is a porous layer having three-dimensionally interconnected voids. The porosity 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.

[0050] 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.

[0051] 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.

[0052] 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.

[0053] When the electrode layer 3 contains a conductive additive, the content of the conductive additive is preferably 0.1% or more, more preferably 0.2% or more, preferably 20% or less, and more preferably 10% or less by mass. When the content of the conductive additive in the electrode layer 3 is within the above range, it is possible to further improve ionic conductivity while ensuring high electronic conductivity in the electrode layer 3, thereby more effectively improving the battery characteristics of the secondary battery.

[0054] The solid electrolytes 6a and 6b can be those described in the section on solid electrolyte layer 2. While it is preferable that solid electrolytes 6a and 6b be of the same type, different types of solid electrolytes may be used in combination.

[0055] As the conductive auxiliary agent, for example, conductive carbon can be used. Examples of the conductive carbon include acetylene black, carbon black, ketjen black, vapor grown carbon fiber conductive carbon auxiliary agent (VGCF), and the like. The conductive auxiliary agent is preferably a carbon-based conductive auxiliary agent formed of any of the above materials.

[0056] In the present 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. Furthermore, both the positive electrode layer and the negative electrode layer may be the electrode layer 3.

[0057] Positive electrode active material; The positive electrode active material is not particularly limited, but is preferably a positive electrode active material formed of crystallized glass containing a crystal represented by general formula Na x M y P2O 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 Fe, Ni, Co, Mn, and Cr). Among these, a positive electrode active material formed of crystallized glass containing a crystal represented by general formula Na x MP2O7 (1≦x≦2, M is at least one selected from the group consisting of Fe, Ni, Co, Mn, and Cr) is more preferable. As such a positive electrode active material crystal, for example, Na2FeP2O7, Na2CoP2O7, Na2NiP2O7, or the like can be used.

[0058] Here, crystallized glass refers to a product obtained by heating (firing) precursor glass containing an amorphous phase to precipitate (crystallize) crystals. All of the amorphous phase may be transformed into a crystalline phase, or an amorphous phase may remain. Furthermore, one type of crystal may be precipitated, or two or more types of crystals may be precipitated. For example, whether a material is crystallized glass can be determined based on the peak angle indicated by powder X-ray diffraction (XRD).

[0059] 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.

[0060] The following describes an example of a method for manufacturing the electrode layer 3.

[0061] 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 a first solid electrolyte layer and a second solid electrolyte layer can be formed, for example, according to the method for manufacturing a solid electrolyte sheet described in International Publication No. 2021 / 045039.

[0062] 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).

[0063] (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.

[0064] (b) A step of obtaining a green sheet for forming a second solid electrolyte layer by adding an organic vehicle containing a binder to a mixed powder containing a second solid electrolyte powder and at least one of the raw material powders of the second solid electrolyte powder and a polymer powder, preparing a slurry, and applying the slurry to a substrate and drying it.

[0065] (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.

[0066] (d) A step of forming a dense first solid electrolyte layer and a porous second solid electrolyte layer by firing the laminate.

[0067] 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.

[0068] 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.

[0069] 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.

[0070] In this embodiment, by applying multiple layers of paste with different content ratios of electrode active material precursor and solid electrolyte powder during paste application, the ratio of solid electrolyte content to electrode active material in the first electrode layer portion 3A on the solid electrolyte layer 2 side can be made greater than the ratio of solid electrolyte content to electrode active material in the second electrode layer portion 3B on the first current collector layer 4 side. In this case, the paste can be applied in, for example, two to five separate applications.

[0071] However, in the present invention, by adjusting the viscosity of the paste, the porosity of the first electrode layer portion 3A on the solid electrolyte layer 2 side may be made greater than the porosity of the second electrode layer portion 3B on the first current collector layer 4 side in the resulting electrode layer 3. In this case as well, the ratio of solid electrolyte content to electrode active material in the first electrode layer portion 3A on the solid electrolyte layer 2 side can be made greater than the ratio of solid electrolyte content to electrode active material in the second electrode layer portion 3B on the first current collector layer 4 side.

[0072] 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.

[0073] Furthermore, 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 slurries with different ratios of void-forming material (polymer powder). In this case as well, the ratio of solid electrolyte content to electrode active material in the first electrode layer portion 3A on the solid electrolyte layer 2 side can be made larger than the ratio of solid electrolyte content to electrode active material in the second electrode layer portion 3B on the first current collector layer 4 side.

[0074] 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.

[0075] 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.

[0076] 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 by firing. When the positive electrode active material precursor powder is made of an amorphous oxide material, active material crystals are generated during firing, and it is possible to form a dense positive electrode layer through softening and flow. 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 2, integration of the two can be achieved. As a result, a better ion conduction path is formed, which is preferable. In addition, in the present 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).

[0077] The positive electrode active material precursor powder preferably contains Na2O 25% to 55%, Fe2O3 + Cr2O3 + MnO + CoO + NiO 10% to 30%, and P2O 525% to 55%, in molar percentages based on the oxides listed below. The reason for this limitation of composition is explained below. In the following descriptions of the content of each component, unless otherwise specified, "%" means "molar percent".

[0078] Na2O has 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.

[0079] Fe₂O₃, Cr₂O₃, MnO, CoO, and NiO are also represented by the general formula Na x M y P₂O z which is the main component of the active material crystal represented. The content of Fe₂O₃+Cr₂O₃+MnO+CoO+NiO is preferably 10% to 50%, more preferably 12% to 40%, and most preferably 15% to 30%. When the content of Fe₂O₃+Cr₂O₃+MnO+CoO+NiO is not less than the above lower limit, 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 not more than the above upper limit, it is difficult to precipitate undesirable crystals such as Fe₂O₃, Cr₂O₃, MnO, CoO, or NiO. In order to further improve the cycle characteristics of the secondary battery, it is preferable to positively contain Fe₂O₃. The content of Fe₂O₃ is preferably 1% to 30%, more preferably 5% to 30%, still more preferably 10% to 30%, and particularly preferably 15% to 25%. The content of each component of Cr₂O₃, MnO, CoO, and NiO is preferably 0% to 45%, more preferably 10% to 40%, and still more preferably 15% to 30%, respectively. Further, when containing at least two or more components selected from Fe₂O₃, Cr₂O₃, MnO, CoO, and NiO, the total content is preferably 10% to 50%, more preferably 12% to 40%, and most preferably 15% to 30%.

[0080] P₂O₅ is also represented by the general formula Na x M y P₂O z which is the main component of the active material crystal represented. The content of P₂O₅ is preferably 25% to 55%, more preferably 30% to 50%. When the content of P₂O₅ is within the above range, the charge-discharge capacity of the secondary battery can be further increased.

[0081] 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.

[0082] 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.

[0083] 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.

[0084] 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.

[0085] 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.

[0086] 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.

[0087] 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.

[0088] 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.

[0089] 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.

[0090] 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.

[0091] 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.

[0092] 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.

[0093] Furthermore, it is possible to use a solid electrolyte precursor solution that contains nitrate ions instead of carbonate ions.

[0094] 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.

[0095] Also, as a counterion for sodium ions, NR 4+ It is preferable that the formula includes (wherein each R is at least one substituent independently selected from the group consisting of H, CH3, C2H5, and CH2CH2OH). This makes it easier for the transition metal element to exist stably in solution.

[0096] 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.

[0097] As a binder, the one described in the section on paste for forming the positive electrode layer can be used.

[0098] 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 equal to or greater than the above lower limit, 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.

[0099] (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.

[0100] 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.

[0101] [All-solid-state secondary battery] Figure 5 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 5, 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.

[0102] 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.

[0103] 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.

[0104] The all-solid-state secondary battery 10 of this embodiment is equipped with an energy storage element 1, and therefore exhibits excellent capacity retention during high-current charging and discharging, as well as excellent cycle characteristics during repeated charging and discharging.

[0105] 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.

[0106] (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 rotating / revolving mixer to form a slurry. The obtained slurry was 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 a first solid electrolyte layer formation green sheet.

[0107] (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.

[0108] (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.

[0109] (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.

[0110] In the obtained solid electrolyte layers, 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%.

[0111] (e) Preparation of paste for forming the positive electrode layer Sodium metaphosphate (NaPO3), ferric oxide (Fe2O3), and orthophosphate (H3PO4) were used as raw materials, and the raw material powders were mixed to have a composition of Na2O 40%, Fe2O 320%, and P2O 540% in molar percentages. The mixture was melted at 1250°C for 45 minutes in an air atmosphere. The molten material was then poured between a pair of rotating rollers and molded while rapidly cooling to obtain a film-like glass. 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.

[0112] 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.

[0113] (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 A paste for forming the positive electrode layer was applied and dried at 70°C for 3 hours to form the 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 the positive electrode layer on one main surface of the solid electrolyte layer.

[0114] The paste for forming the positive electrode layer was applied by layering pastes with different ratios of electrode active material precursor (positive electrode active material precursor) and solid electrolyte powder. Specifically, after applying the first layer of paste prepared under the above conditions, subsequent layers were applied using pastes in which the ratio of solid electrolyte powder to electrode active material precursor was gradually reduced, resulting in a total of three layers.

[0115] (g) Measurement of the composition distribution of the positive electrode layer The compositional distribution of the positive electrode layer was measured by elemental analysis and image processing of a cross-section along the thickness direction of the positive electrode layer. Specifically, the ratio of solid electrolyte content to electrode active material (positive electrode active material) in the positive electrode layer was measured using an energy-dispersive X-ray fluorescence spectrometer (EDX). For each electrode section area, the molar ratios of Fe content in the electrode active material (positive electrode active material), Al content in the solid electrolyte, and C content in the conductive additive were calculated. Furthermore, the volume ratio of electrode active material to solid electrolyte in the electrode mixture was determined from the specific gravities of the electrode active material, solid electrolyte, and conductive additive, respectively. In addition, the porosity and mixture occupancy rate for each electrode section area were calculated using image analysis from cross-sectional images of the electrode layer (cross-sectional images of the positive electrode layer) obtained with a scanning electron microscope (SEM). The volume ratio of electrode active material to solid electrolyte for each electrode section area was calculated by multiplying the calculated mixture occupancy rate by the volume ratio of electrode active material and solid electrolyte in the electrode mixture.

[0116] The ratio of solid electrolyte to electrode active material in the positive electrode layer (solid electrolyte / electrode active material) 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 ratio X (solid electrolyte / electrode active material) for the first electrode layer and the ratio Y (solid electrolyte / electrode active material) for 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 ratio X (solid electrolyte / electrode active material) for the first electrode layer, the ratio Y (solid electrolyte / electrode active material) for the second electrode layer, and the ratio Z (solid electrolyte / electrode active material) for the third electrode layer were measured in the thickness direction, starting from the solid electrolyte layer side.

[0117] (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.

[0118] (i) High current discharge test All-solid-state secondary batteries were vacuum-sealed in an aluminum laminate casing. Next, high-current discharge tests were performed (charging: 0.1C / 4.5V CC / CV 0.02C cutoff, 25℃; discharging: 0.02C or 0.1C CC 1.5V cutoff, 25℃) to determine the 0.1C retention rate relative to the 0.02C discharge capacity (0.1C / 0.02C capacity retention rate).

[0119] (j) 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.

[0120] (Comparative Example 1) A solid-state secondary battery was fabricated in the same manner as in Example 1, except that only the first solid electrolyte layer, which is a dense layer, was used as the solid electrolyte layer, and the paste for forming the positive electrode layer was applied only once without multi-layer coating. A charge-discharge cycle test was then performed. In Comparative Example 1, the ratio of solid electrolyte content to electrode active material in the positive electrode layer (solid electrolyte / electrode active material) was determined in the same manner as in Example 1.

[0121] (Comparative Example 2) A solid-state secondary battery was fabricated in the same manner as in Example 1, except that the paste for forming the positive electrode layer was applied only once instead of in multiple layers. A charge-discharge cycle test was then performed. In Comparative Example 2, the ratio of solid electrolyte content to electrode active material in the positive electrode layer (solid electrolyte / electrode active material) was determined in the same manner as in Example 1.

[0122] The results are shown in Table 1 below. In Table 1, the units for X, Y, and Z are in volume percent.

[0123] [Table 1] [Explanation of Symbols]

[0124] 1.1A…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, The volume ratio X of the solid electrolyte content to the electrode active material in the first electrode layer portion, which is located on the solid electrolyte layer side of the center in the thickness direction of the electrode layer, is greater than the volume ratio Y of the solid electrolyte content to the electrode active material in the second electrode layer portion, which is located on the current collector layer side of the center in the thickness direction of the electrode layer. An energy storage element in which the ratio X / Y of the volume ratio X of the solid electrolyte content in the first electrode layer portion to the volume ratio Y of the solid electrolyte content in the second electrode layer portion to the electrode active material is 1.01 or more and 3 or less.

2. The energy storage element according to claim 1, wherein the volume ratio of the solid electrolyte content in the electrode layer to the electrode active material increases from the current collector layer side to the solid electrolyte 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 porosity of the first electrode layer portion is greater than the porosity of the second electrode layer portion.

5. 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.

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

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

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