All-solid-state secondary batteries

Abstract

This all-solid-state secondary battery comprises a laminate having a positive electrode layer, a negative electrode layer, and a solid electrolyte layer located between the positive electrode layer and the negative electrode layer. The solid electrolyte layer contains a compound represented by formula (1). The negative electrode layer includes an Ag-containing phase. A plurality of gaps are formed in the negative electrode layer. At least some of the gaps are formed next to the Ag-containing phase.　Formula (1): Li3+xSixP1−xO4 [In formula (1), x satisfies 0<x<1.]

Description

[Technical Field]

[0001] This invention relates to an all-solid-state secondary battery. This application claims priority based on Japanese Patent Application No. 2022-058998, filed in Japan on March 31, 2022, and the contents of that application are incorporated herein by reference. [Background technology]

[0002] In recent years, advancements in electronics technology have been remarkable, leading to the miniaturization, weight reduction, thinning, and increased functionality of portable electronic devices. Consequently, there is a strong demand for smaller, lighter, thinner, and more reliable batteries, which power these devices. Currently, lithium-ion rechargeable batteries, commonly used, traditionally utilize electrolytes (electrolytes) such as organic solvents as a medium for ion transfer. However, batteries with this configuration pose a risk of electrolyte leakage. Furthermore, since organic solvents used in electrolytes are flammable, there is a need to further enhance battery safety.

[0003] One measure to improve battery safety is to use a solid electrolyte instead of a liquid electrolyte. Furthermore, development is underway on all-solid-state secondary batteries in which a solid electrolyte is used as the electrolyte, and other components are also made of solid materials (for example, Patent Document 1).

[0004] Solid electrolytes for all-solid-state secondary batteries mainly consist of sulfide-based and oxide-based solid electrolytes. Sulfide-based solid electrolytes generate hydrogen sulfide when they react with water, so batteries must be manufactured in a glove box with a controlled dew point. Furthermore, sulfide-based solid electrolytes are difficult to form into sheets, and there is a need for a thin, miniaturized solid electrolyte that can contribute to the miniaturization, weight reduction, thinning, and multi-functionality of portable electronic devices. For these reasons, the application of oxide-based solid electrolytes is expected to be promising.

[0005] As an oxide-based solid electrolyte, LSPO(Li 3+x Si x P 1-xExamples include O4 (Non-Patent Document 1). LSPO is known to be relatively stable with respect to the active material even when sintered at high temperatures for densification. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 2022-25903 (A) [Non-patent literature]

[0007] [Non-Patent Document 1] Solid State Ionics 283 (2015) 109-114 [Overview of the project] [Problems that the invention aims to solve]

[0008] Oxide-based solid electrolytes require high-temperature sintering to achieve densification, but during this process, they can react with the active material, sometimes forming a reaction layer with low ionic conductivity at the interface. To suppress the formation of such reaction layers, methods utilizing the above-mentioned LSPO as a solid electrolyte are being investigated, for example, using Ag, AgPd alloy, AgPd alloy and lithium titanate (Li4Ti5O) as the negative electrode. 12 It is expected that using materials containing ) will lead to the realization of all-solid-state secondary batteries in which the reaction layer is difficult to form.

[0009] Furthermore, improved cycle characteristics are required for all-solid-state secondary batteries. The cycle characteristics of stacked chip-type all-solid-state secondary batteries deteriorate during charging and discharging due to the expansion and contraction of the active material, which causes cracks and delamination inside the electrodes and at the interface with the solid electrolyte. Suppression of these cracks and delamination is necessary to improve cycle characteristics. When LSPO is used as the solid electrolyte and an Ag-containing phase is used as the negative electrode, for example, Ag, AgPd alloy, AgPd alloy, and lithium titanate, there is a problem of low cycle characteristics.

[0010] The present invention is an invention made in view of the above circumstances, and an object thereof is to provide an all-solid-state secondary battery in which stress due to expansion and contraction of a negative electrode is alleviated and cycle characteristics can be improved.

Means for Solving the Problems

[0011] The present invention provides the following means in order to solve the above problems.

[0012] (1) An all-solid-state secondary battery according to one aspect of the present invention includes a laminate having a positive electrode layer, a negative electrode layer, and a solid electrolyte layer between the positive electrode layer and the negative electrode layer. The solid electrolyte layer contains a compound represented by the following formula (1), the negative electrode layer contains a phase containing Ag, a plurality of voids are formed in the negative electrode layer, and at least a part of the voids is a first void formed in contact with the phase containing Ag. It is an all-solid-state secondary battery. Li 3+x Si x P 1-x O4(1) (Formula (1) satisfies 0 < x < 1.)

[0013] (2) In the all-solid-state secondary battery according to the above aspect, the diameter of the first void may be 0.01 μm or more and 3.0 μm or less.

[0014] (3) In the all-solid-state secondary battery according to the above aspect, the volume ratio of the first voids formed in contact with the phase containing Ag in the negative electrode layer may be 1% or more and 10% or less.

[0015] (4) In the all-solid-state secondary battery according to the above aspect, the aspect ratio of the length of the voids in the extending direction of the negative electrode layer to the length of the first voids in the stacking direction of the laminate may be 3 or more.

[0016] (5) In the all-solid-state secondary battery according to the above aspect, at least a part of the voids formed in contact with the phase containing Ag may be second voids formed at the interface between the solid electrolyte layer and the phase containing Ag.

[0017] (6) In the all-solid-state secondary battery according to the above embodiment, the ratio of the second void to the entire first void may be 80 to 100%.

[0018] (7) In the all-solid-state secondary battery according to the above embodiment, the negative electrode layer may consist of a phase containing Ag.

[0019] (8) In the all-solid-state secondary battery according to the above embodiment, the negative electrode layer comprises a first phase composed of the Ag-containing phase and a second phase composed of lithium titanate sandwiching the first phase in the stacking direction, and the negative electrode layer may have a third void in contact with the interface between the first phase and the second phase.

[0020] (9) In the all-solid-state secondary battery according to the above embodiment, the proportion of the third void to the entire first void may be 50 to 100%. [Effects of the Invention]

[0021] In the all-solid-state secondary battery according to the above embodiment, stress due to expansion and contraction of the negative electrode is alleviated, and the cycle characteristics can be improved. [Brief explanation of the drawing]

[0022] [Figure 1] This is a schematic cross-sectional view showing an all-solid-state secondary battery according to this embodiment. [Figure 2] Figure 1 is a magnified cross-sectional view of the vicinity of the negative electrode layer of the all-solid-state secondary battery. [Figure 3] This is an enlarged cross-sectional view of the vicinity of the negative electrode layer of an all-solid-state secondary battery according to a modified example of Figure 1. [Modes for carrying out the invention]

[0023] The all-solid-state secondary battery of this embodiment will be described in detail below, with reference to the drawings as appropriate. The drawings used in the following description may be enlarged for convenience in order to clearly illustrate the features of the present invention. Therefore, the dimensional ratios of each component may differ from those of the actual components. The materials, dimensions, etc., exemplified in the following description are examples only, and the present invention is not limited to them. It can be implemented with appropriate modifications without changing the essence of the invention.

[0024] [All-solid-state secondary battery] Figure 1 is a schematic cross-sectional view of an all-solid-state secondary battery according to this embodiment. Figure 2 is an enlarged cross-sectional view of the vicinity of the negative electrode layer of the all-solid-state secondary battery of Figure 1, and Figure 2 also shows an enlarged view schematically of the vicinity of one of the voids.

[0025] The all-solid-state secondary battery 10 shown in Figures 1 and 2 comprises a laminate 4, a positive electrode terminal 5, and a negative electrode terminal 6. The positive electrode terminal 5 and the negative electrode terminal 6 are in contact with opposing surfaces of the laminate 4, respectively. The positive electrode terminal 5 and the negative electrode terminal 6 extend in directions that intersect (are perpendicular to) the stacking surface of the laminate 4. In this embodiment, the direction in which the negative electrode layer 2 extends is sometimes referred to as the x-direction, the direction in which the laminate 4 is stacked is sometimes referred to as the z-direction, and the direction perpendicular to the x-direction and the z-direction is sometimes referred to as the y-direction.

[0026] The laminate 4 has a positive electrode layer 1, a negative electrode layer 2, and a solid electrolyte layer 3 sandwiched between the positive electrode layer 1 and the negative electrode layer 2. The laminate 4 is a sintered body formed by laminating the positive electrode layer 1 and the negative electrode layer 2 with the solid electrolyte layer 3 in between and sintering them. The number of positive electrode layers 1 and negative electrode layers 2 included in the laminate 4 may be one layer each, or two or more layers each. As shown in Figure 1, the solid electrolyte layer 3 is present not only between the positive electrode layer 1 and the negative electrode layer 2, but also between the positive electrode layer 1 and the negative electrode terminal 6, and between the negative electrode layer 2 and the positive electrode terminal 5. Also, as shown in Figure 1, one end of the positive electrode layer 1 is connected to the positive electrode terminal 5. One end of the negative electrode layer 2 is connected to the negative electrode terminal 6.

[0027] The all-solid-state secondary battery 10 charges or discharges by transferring ions through the solid electrolyte layer 3 between the positive electrode layer 1 and the negative electrode layer 2. In this embodiment, the laminated all-solid-state secondary battery 10 shown in FIG. 1 will be described as an example.

[0028] <Solid electrolyte layer> The solid electrolyte layer 3 can move ions by an externally applied electric field. The solid electrolyte layer 3, for example, conducts lithium ions and inhibits the movement of electrons. The solid electrolyte layer 3 contains LSPO represented by the following formula (1). Li 3+x Si x P 1-x O4(1) (In formula (1), x is a number where 0 < x < 1.)

[0029] The solid electrolyte layer 3 preferably contains LSPO as a main component and consists of LSPO. Here, in this embodiment, that the solid electrolyte layer 3 contains LSPO as a main component means that the LSPO in the solid electrolyte layer 3 is 80% by volume or more.

[0030] <Positive electrode layer> The positive electrode layer 1 has, for example, a positive electrode current collector 1A and a positive electrode active material layer 1B. As shown in FIG. 1, the positive electrode active material layer 1B may be formed on both sides of the positive electrode current collector 1A or may be formed on only one side.

[0031] (Positive electrode current collector) The positive electrode current collector 1A is made of a material with high conductivity. The positive electrode current collector 1A includes, for example, metals such as silver, palladium, gold, platinum, aluminum, copper, nickel, stainless steel, iron and their alloys, and conductive materials such as conductive resins. The positive electrode current collector 1A may contain a positive electrode active material such as lithium cobalt oxide (LiCoO2) or a lithium vanadium compound (LiV2O5, Li3V2(PO4)3, LiVOPO4). The positive electrode current collector 1A may be in the form of powder, foil, punching, or expansion.

[0032] (Positive electrode active material layer) The positive electrode active material layer 1B contains a positive electrode active material. The positive electrode active material layer 1B may also contain a conductive additive and a solid electrolyte.

[0033] (Cathode active material) The positive electrode active material is not particularly limited, as long as it is capable of reversibly releasing and intercalating lithium ions, and desorption and insertion of lithium ions. For example, positive electrode active materials used in known lithium-ion secondary batteries can be used.

[0034] The positive electrode active material is preferably one or more selected from, for example, composite transition metal oxides and transition metal composite oxides.

[0035] Examples of positive electrode active materials include lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganese spinel (LiMn2O4), and LiNi (General formula: LiNi x Co y Mn z M a A composite metal oxide represented by O2 (x+y+z+a=1, 0≦x≦1, 0≦y≦1, 0≦z≦1, 0≦a≦1, M represents one or more elements selected from Al, Mg, Nb, Ti, Cu, Zn, Cr), lithium titanate (Li4Ti5O 12 ) are some examples.

[0036] A lithium-free positive electrode active material may be used as the positive electrode active material. A lithium-free positive electrode active material can be used by first placing a negative electrode active material doped with metallic lithium and / or lithium ions in the negative electrode layer 2, and then starting the all-solid-state secondary battery 10 from discharge. Examples of lithium-free positive electrode active materials include metal oxides (MnO2, V2O5, etc.).

[0037] (Conductive additive) The conductive additive is not particularly limited as long as it improves the electronic conductivity within the positive electrode active material layer 1B, and known conductive additives can be used. Examples of conductive additives include carbon-based materials such as graphite, carbon black, graphene, and carbon nanotubes; metals such as gold, platinum, silver, palladium, aluminum, copper, nickel, stainless steel, and iron; conductive oxides such as ITO (indium tin oxide); or mixtures thereof. The conductive additive may be in the form of a powder or fibers.

[0038] The solid electrolyte contained in the positive electrode active material layer 1B improves the ionic conductivity within the positive electrode active material layer 1B. One or more known solid electrolytes can be used as a mixture of two or more. It is preferable to use the same compound as the compound constituting the solid electrolyte layer 3 for the solid electrolyte, from the viewpoint of improving the adhesion between the solid electrolyte layer 3 and the positive electrode layer 1.

[0039] <Negative electrode layer> The negative electrode layer 2 shown in Figures 1 and 2 serves as both the negative electrode current collector and the negative electrode active material layer. The negative electrode layer 2 contains a phase 21 containing Ag, either Ag or an AgPd alloy, and further contains lithium titanate (Li4Ti5O 12 ) may also be included. The negative electrode layer 2 is composed of a substrate exhibiting a lower potential than the positive electrode active material. The negative electrode layer 2 has a void V in contact with the Ag-containing phase 21.

[0040] The negative electrode layer 2 has, for example, a plurality of voids V. The plurality of voids V are classified into a first void 41 which is in contact with the Ag-containing phase 21 and voids 42 which are not in contact with the Ag-containing phase 21. The first void 41 is further classified into a second void 41a and a third void 41b. The second void 41a is a void in contact with the interface between the solid electrolyte layer 3 and the Ag-containing phase 21. The third void 41b is a void in contact with the interface between the first phase 21 (Ag-containing phase 21) and the second phase 22 which is composed of lithium titanate. The relationship between the second void 41a and the third void 41b is basically mutually exclusive. Each of the multiple voids V is formed, for example, in a part of the interface between the Ag-containing phase 21 and the solid electrolyte of the solid electrolyte layer 3, and all of the voids V may each be formed in a part of the interface between the Ag-containing phase and the solid electrolyte. That is, each of the multiple voids V is in contact with a part of the interface between the LSPO31 and the Ag-containing phase 21, and it is preferable that all of the voids V are in contact with a part of the interface between the LSPO31 and the Ag-containing phase 21. Furthermore, if the negative electrode layer 2 contains lithium titanate, each of the multiple voids V may be formed, for example, in a part of the interface between the Ag-containing phase and lithium titanate.

[0041] The volume percentage (porosity) of the voids V in the negative electrode layer 2 is, for example, 0.1% or more and 20% or less, preferably 1% or more and 10% or less, and more preferably 1% or more and 5% or less. The volume ratio of the first void 41 in the negative electrode layer 2 may be 1% or more and 10% or less, or 1% or more and 5% or less. The proportion of the second void to the total first void may be between 80% and 100%. The proportion of the third void to the total first void may be 50% to 100%, or 80% to 100%. The volume percentage of voids V in the negative electrode layer 2 is calculated based on electron microscope images. Specifically, the region of voids V in the negative electrode layer 2 is defined and its volume percentage is calculated using the following procedure.

[0042] First, a scanning electron microscope image of a cross-section parallel to the stacking direction of the laminate 4 is obtained, and the region of the negative electrode layer 2 is defined based on this scanning electron microscope image. The negative electrode layer 2 is considered to be the region between the two solid electrolyte layers in the scanning electron microscope image, and is defined by image analysis. The area of ​​the negative electrode layer 2 is calculated by image analysis. Next, the scanning electron microscope image is binarized by brightness to separate the phase corresponding to the void from the phase corresponding to other regions. The binarization of the scanning electron microscope image is performed by creating a density histogram using the mode method, and using the density range of the valley that marks the boundary between the target phase and the other phases as the threshold. In other words, the "dark areas" and "bright areas" are separated by the threshold and binarized. Note that in the density histogram, there may be two or more valleys, but the void is a region of low brightness in the scanning electron microscope image, and the valley between the lowest brightness peak and the next lowest brightness peak can be selected as the boundary valley, and the density value of this valley can be used as the threshold. The darkest region is considered to be a void. The total area of ​​voids in contact with the alloy such as AgPd or the metal such as Ag in the negative electrode layer 2 is calculated by image analysis. Here, the total area of ​​the voids and the negative electrode layer is determined, for example, in the same scanning electron microscope image at a magnification of 1000 to 5000x. In this way, the area of ​​the negative electrode layer 2 and the area of ​​void V are calculated, and the volume ratio of void V in the negative electrode layer 2 (porosity) is calculated by dividing the area of ​​void V by the area of ​​the negative electrode layer 2.

[0043] In this embodiment, the diameter of the void V is assumed to be the same as the area determined to be a void by the binarization process, where the brightness is below a predetermined value, and the diameter of this circle is estimated to be the diameter of the void V.

[0044] Length L of the void V in the stacking direction of the laminate 4 Vz Length L of the void in the direction in which the negative electrode layer 2 extends relative to Vx Aspect ratio (L Vx / L Vz ) is, for example, 1 or more, and preferably 3 or more. The aspect ratio of each void V is calculated by image analysis of the electron microscope image that has been binarized using the same procedure as described above. That is, the length L in the direction in which the negative electrode layer 2 extends is calculated by image analysis of the void V defined by binarizing the electron microscope image. Vx and length L in the stacking direction Vz Next, the length L of each void V was calculated. Vz Length L VzxThe aspect ratio of each void V was calculated by dividing by L. In this embodiment, the aspect ratio of the above void V was calculated for 10 arbitrary void V locations in the electron microscope image, and the arithmetic mean of these was taken as the length L of the void V in the stacking direction of the laminate 4. Vz Length L of the void V in the direction in which the negative electrode layer 2 extends Vx Treat it as the aspect ratio.

[0045] The void V in the negative electrode layer 2 preferably has an axial direction in the direction in which the negative electrode layer 2 extends. For example, the aspect ratio (L Vx / L Vz It is preferable that the value of ) is greater than 1, and that the direction of extension of the void is substantially parallel to the direction of extension of the negative electrode layer 2.

[0046] It is preferable that the multiple voids V in the negative electrode layer 2 are interconnected to form a network structure. Furthermore, it is preferable that the voids V in the negative electrode layer 2 have the above network structure and are in contact with the AgPd alloy particles in the x and z directions. Having voids with such a structure makes it easier to relieve stress in various directions.

[0047] The negative electrode layer 2 may contain, in addition to at least one of the metal particles and alloy particles mentioned above, a conductive additive, a solid electrolyte, and the like.

[0048] (Conductive additive) The conductive additive improves the electronic conductivity of the negative electrode layer 2. The same material as that used for the positive electrode active material layer 1B can be used as the conductive additive.

[0049] (solid electrolyte) The solid electrolyte contained in the negative electrode layer 2 improves the ionic conductivity within the negative electrode layer 2. One or more known solid electrolytes can be used as the solid electrolyte. The solid electrolyte may also contain the same compound as the compound forming the solid electrolyte layer 3 described above.

[0050] [Manufacturing method for all-solid-state secondary batteries] Next, an example of a method for manufacturing an all-solid-state secondary battery 10 will be described. First, the laminate 4 is manufactured. The laminate 4 can be manufactured, for example, by firing, and may be manufactured by either the simultaneous firing method or the sequential firing method. The simultaneous firing method is a method in which the materials forming each layer are laminated and then fired together to manufacture the laminate 4. The sequential firing method is a method in which firing is performed each time a layer is formed. The simultaneous firing method allows the laminate 4 to be manufactured with fewer work steps than the sequential firing method. In addition, the laminate 4 manufactured by the simultaneous firing method is denser than the laminate 4 manufactured using the sequential firing method. The manufacturing method of the laminate 4 when the laminate 4 is manufactured using the simultaneous firing method will be described below.

[0051] First, the materials constituting the laminate 4—the positive electrode current collector 1A, the positive electrode active material layer 1B, the solid electrolyte layer 3, and the negative electrode layer 2—are made into pastes to produce pastes corresponding to the materials of each layer.

[0052] In the manufacturing method of the all-solid-state secondary battery according to this embodiment, multiple voids in contact with the Ag-containing phase are formed in the negative electrode layer 2, and the following means are employed, for example, to control the amount, shape, and size of these voids. To control the amount of voids V in the negative electrode layer 2, for example, an organic filler is added to the negative electrode active material and its amount is changed. To control the shape of the voids in the negative electrode layer 2, for example, the flatness of the organic filler is changed, and to obtain voids with a high aspect ratio, an organic filler with a high degree of flatness is used. Furthermore, to control the direction of the voids in the negative electrode, for example, the organic filler can be arranged in the in-plane direction during coating. Specifically, the doctor blade method is used, and the degree of orientation is controlled by the coating speed and coating thickness. To control the size of the voids in the negative electrode layer, for example, the size of the added organic filler is controlled, and the size of the voids is increased by using a larger one.

[0053] The method for pasteuring each material used in the manufacture of the laminate 4 is not particularly limited, and for example, a method of mixing the powders of each material with a vehicle to obtain a paste can be used. Here, the vehicle is a general term for the medium in the liquid phase. In this embodiment, the vehicle includes, for example, a solvent, a binder, and a plasticizer. As the solvent, for example, dihydroterpineol can be used. As the binder, for example, ethylcellulose can be used.

[0054] Next, green sheets corresponding to each layer constituting the laminate 4 are prepared. The green sheets are obtained, for example, by applying pastes prepared for each material of each layer constituting the laminate 4 onto a substrate such as a PET (polyethylene terephthalate) film, drying them as necessary, and then peeling them off the substrate.

[0055] The method for applying the paste to the substrate is not particularly limited, and known methods such as screen printing, coating, transfer, and doctor blade methods can be used.

[0056] Next, the green sheets corresponding to each layer constituting the laminate 4 are stacked in the desired order and number of layers to create the laminate 4. When stacking the green sheets, alignment, cutting, etc. are performed as necessary. For example, when creating a parallel or series-parallel type battery as an all-solid-state secondary battery 10, alignment is performed so that the end face of the positive electrode current collector 1A and the end face of the negative electrode layer 2 do not coincide, and the green sheets corresponding to each layer are stacked.

[0057] The green sheet that forms the laminated sheet may be a pre-fabricated positive electrode unit and negative electrode unit.

[0058] The positive electrode unit is a green sheet in which a solid electrolyte layer 3, a sheet that will become the positive electrode active material layer 1B, a sheet that will become the positive electrode current collector 1A, and a sheet that will become the positive electrode active material layer 1B are laminated in this order.

[0059] The negative electrode unit can be configured to suit the desired negative electrode layer, but for example, it is a green sheet in which a second negative electrode paste, which is the second phase mainly containing lithium titanate, a first negative electrode paste, which is the first phase mainly containing an Ag-containing phase, and a second negative electrode paste, which is the second phase mainly containing lithium titanate, are laminated in this order.

[0060] When a positive electrode unit is used, the laminated sheet that forms the laminate 4 can be manufactured by laminating the positive electrode unit and the negative electrode unit that forms the negative electrode layer 2 via a solid electrolyte green sheet.

[0061] Next, the fabricated laminated sheets are pressed together to improve the adhesion of each layer and form a laminate. Pressurization can be performed, for example, by a die press, hot water isostatic press (WIP), cold water isostatic press (CIP), or hydrostatic press. It is preferable to perform pressurization while heating. The heating temperature during pressurization can be, for example, 40 to 95°C. Next, the laminate obtained after pressurization is cut using a dicing device to form laminated chips. After that, the obtained laminated chips are fired. This gives a laminate 4 made of a sintered body.

[0062] The debinding and firing process can be carried out, for example, by placing the laminate on a ceramic base. The debinding and firing process can be, for example, a process of heating to 550°C to 1100°C in an air atmosphere. The heating time (firing time) can be, for example, 0.1 hours to 10 hours. The heating temperature and firing time in the debinding and firing process can be appropriately determined according to the composition of each layer constituting the laminate 4.

[0063] The sintered laminate 4 (sintered body) may be polished by placing it in a cylindrical container with an abrasive such as alumina and polishing it by abrasive. This allows for chamfering of the corners of the laminate 4. The polishing of the laminate 4 may also be performed using sandblasting. Sandblasting is preferred because it can remove only specific parts of the surface of the laminate 4.

[0064] Next, positive electrode terminals 5 and negative electrode terminals 6 are formed on the opposing sides of the fabricated laminate 4, respectively. The positive electrode terminals 5 and negative electrode terminals 6 are formed by methods such as sputtering, dipping, screen printing, or spray coating, respectively. The all-solid-state secondary battery 10 is manufactured by the above process.

[0065] According to the all-solid-state secondary battery 10 of this embodiment, as described above, the solid electrolyte layer 3 contains LSPO, the negative electrode layer 2 contains a phase 21 containing Ag, and a void V is formed in the negative electrode layer 2 in contact with the phase 21 containing Ag. This allows the void V to relieve the stress in the negative electrode layer 2 caused by the expansion and contraction of the phase 21 containing Ag during charging and discharging, thereby suppressing the formation of a reaction layer, as well as the occurrence of cracks in the solid electrolyte layer 3 and delamination between layers, resulting in excellent cycle characteristics.

[0066] Although one embodiment of the present invention has been described in detail above, the configurations and combinations thereof in each embodiment are merely examples, and additions, omissions, substitutions, and other modifications to the configurations are possible without departing from the spirit of the present invention.

[0067] For example, Figure 1 shows an all-solid-state secondary battery 10 in which the negative electrode layer 2 serves as both the negative electrode current collector and the negative electrode active material layer. However, an all-solid-state secondary battery may also be constructed in which the negative electrode layer comprises an independent negative electrode current collector and negative electrode active material. Such an all-solid-state secondary battery may be manufactured in which, for example, negative electrode active material layers are provided on both sides of the main surface of the negative electrode current collector.

[0068] Furthermore, for example, the all-solid-state secondary battery according to the above embodiment may have a configuration in which the negative electrode layer 2 has a laminated structure of a negative electrode active material layer and a negative electrode current collector. When the negative electrode layer 2 has a laminated structure of a negative electrode active material layer and a negative electrode current collector, for example, the phase 21 containing Ag is included in the negative electrode active material layer, and a gap V is formed so as to be in contact with the phase 21 containing Ag. In this case, the phase 21 containing Ag may also be included in the negative electrode current collector, and a gap V may be formed so as to be in contact with a part of the phase 21 containing Ag in the negative electrode current collector.

[0069] Furthermore, the battery may have, for example, a first phase composed of a phase 21 containing Ag and a second phase 22 composed of lithium titanate. Figure 3 shows a cross-sectional view of an all-solid-state secondary battery satisfying the above configuration, with an enlarged view of the vicinity of the negative electrode layer. In this case, the negative electrode layer is configured such that, for example in the z direction, the first phase, a phase 21 containing Ag, is sandwiched between the second phase 22, such as lithium titanate, and a void V is formed in contact with the interface between the first phase, the Ag-containing phase 21, and the second phase 22. The first and second phases may also exist adjacent to each other in a layered manner, and a portion of the first phase may be located outward in the z direction relative to the second phase such that a portion of the first phase is closer to the solid electrolyte layer 3 than the second phase, and a void may be formed in contact with the interface between the first and second phases, or a combination of these configurations may be used.

[0070] In such modified examples, the amount and structure of the voids can be the same as in the all-solid-state secondary battery 10 according to the above embodiment. Furthermore, it is preferable that the proportion of voids in contact with the AgPd alloy among the multiple voids contained in the laminate 4 is 50% or more. The void V is preferably located at the boundary between the Ag-containing phase 21 and lithium titanate, or at the boundary between the Ag-containing phase 21 and the solid electrolyte layer 3. If the void is located elsewhere, the stress caused by the expansion and contraction of the negative electrode cannot be relieved, and no improvement in cycle characteristics will be observed. [Examples]

[0071] The following describes embodiments of the present invention. The present invention is not limited to the following embodiments.

[0072] [Example 1] (1) Preparation of solid electrolyte paste First, the solid electrolyte paste was prepared using the following procedure: Li 3+x Si x P 1-x Particles consisting of O4 (where x in formula (1) is 0.5), ethylcellulose, and dihydroterpineol were added to a ball mill and wet-mixed to obtain a solid electrolyte paste.

[0073] (2) Preparation of positive electrode current collector paste A powder consisting of an AgPd alloy with an Ag:Pd ratio of 8:2, ethyl cellulose, and dihydroterpineol were added to a ball mill and wet-mixed to prepare the positive electrode current collector paste of Example 1.

[0074] (3) Preparation of positive electrode active material paste Lithium cobalt oxide (LiCoO2) powder, ethyl cellulose, and dihydroterpineol were added to a ball mill and wet-mixed to prepare the positive electrode active material paste of Example 1.

[0075] (4) Preparation of negative electrode paste A first negative electrode paste was prepared by wet mixing AgPd powder, consisting of an Ag:Pd alloy with an Ag:Pd ratio of 8:2, polymer particles, ethyl cellulose, and dihydroterpineol in a ball mill. The volume ratio of AgPd powder to polymer particles was set to 95:5, and the polymer particles used had an aspect ratio of 1:3, resulting in a particle size of 0.5 μm when converted to a perfect sphere for the same volume. Furthermore, lithium titanate powder, ethylcellulose, and dihydroterpineol were added to a ball mill and wet-mixed to prepare a second negative electrode paste.

[0076] (5) Fabrication of the positive electrode unit which will form the positive electrode layer A positive electrode active material layer paste was applied to a substrate made of PET (polyethylene terephthalate) film to a thickness of 5 μm using screen printing, and dried at 80°C for 5 minutes. After drying, a positive electrode current collector paste was applied to the positive electrode active material layer paste to a thickness of 5 μm using screen printing, and dried at 80°C for 5 minutes. After drying, a positive electrode active material layer paste was applied to the positive electrode current collector paste to a thickness of 5 μm using screen printing, dried at 80°C for 5 minutes, and then peeled off from the substrate. This resulted in a positive electrode unit in which the positive electrode active material layer, positive electrode current collector layer, and positive electrode active material layer were laminated in this order.

[0077] (6) Preparation of a solid electrolyte green sheet A solid electrolyte paste was applied to a substrate made of PET (polyethylene terephthalate) film to a thickness of 20 μm using screen printing, dried at 80°C for 10 minutes, and then peeled off from the substrate. This yielded a solid electrolyte green sheet, which would serve as the solid electrolyte layer.

[0078] (7) Fabrication of the negative electrode unit which will form the negative electrode layer A second negative electrode paste was applied to a substrate made of PET (polyethylene terephthalate) film to a thickness of 5 μm using screen printing, and dried at 80°C for 5 minutes. After drying, a first negative electrode paste was applied to the second negative electrode paste to a thickness of 5 μm using screen printing, and dried at 80°C for 5 minutes. After drying, a second negative electrode paste was applied to the second negative electrode paste to a thickness of 5 μm using screen printing, dried at 80°C for 5 minutes, and then peeled off from the substrate. This resulted in a negative electrode unit in which the second negative electrode layer, the first negative electrode layer, and the second negative electrode layer were laminated in this order.

[0079] (8) Formation of a laminate A laminated sheet was obtained by alternately stacking 25 positive electrode units and 25 negative electrode units with a solid electrolyte green sheet sandwiched in between. The top and bottom surfaces of the laminated sheet were made up of solid electrolyte units. The top and bottom surfaces of the laminated sheet were made up of solid electrolyte green sheets. Furthermore, the units were stacked in a staggered manner so that the positive electrode units extended only to one end face of the laminated sheet, and the negative electrode units extended only to the end face on the opposite side.

[0080] Next, the prepared green sheets were compressed together and cut using a dicing device to produce laminated chips. The laminated chips were then debindered and fired to obtain the laminate of Example 1, which consists of a sintered body. Debindering was performed by heating the laminate at 500°C for 10 hours in an air atmosphere. Firing was carried out at 900°C for 1 hour in an air atmosphere. Debindering and firing can be performed in a single process.

[0081] Next, positive and negative electrode terminals were formed on a pair of opposing sides of the fabricated laminate by the Au sputtering method to obtain the all-solid-state secondary battery of Example 1. Here, the positive and negative electrode terminals were formed such that the positive electrode terminal was joined to the positive electrode unit, and the negative electrode terminal was joined to the negative electrode unit.

[0082] [Example 2] An all-solid-state secondary battery was fabricated in the same manner as in Example 1, except that the particle size of the polymer particles in the first negative electrode paste was changed to 0.005 μm.

[0083] [Example 3] An all-solid-state secondary battery was fabricated in the same manner as in Example 1, except that the particle size of the polymer particles in the first negative electrode paste was changed to 0.05 μm.

[0084] [Example 4] An all-solid-state secondary battery was fabricated in the same manner as in Example 1, except that the particle size of the polymer particles in the first negative electrode paste was changed to 1 μm.

[0085] [Example 5] An all-solid-state secondary battery was fabricated in the same manner as in Example 1, except that the particle size of the polymer particles in the first negative electrode paste was changed to 5 μm.

[0086] [Example 6] An all-solid-state secondary battery was fabricated in the same manner as in Example 1, except that the volume ratio of AgPd powder to polymer particles in the first negative electrode paste was changed to 99.9:0.1.

[0087] [Example 7] An all-solid-state secondary battery was fabricated in the same manner as in Example 1, except that the volume ratio of AgPd powder to polymer particles in the first negative electrode paste was changed to 99:1.

[0088] [Example 8] An all-solid-state secondary battery was fabricated in the same manner as in Example 1, except that the volume ratio of AgPd powder to polymer particles in the first negative electrode paste was changed to 90:10.

[0089] [Example 9] An all-solid-state secondary battery was fabricated in the same manner as in Example 1, except that the volume ratio of AgPd powder to polymer particles in the first negative electrode paste was changed to 80:20.

[0090] [Example 10] An all-solid-state secondary battery was fabricated in the same manner as in Example 1, except that the aspect ratio of the polymer particles in the first negative electrode paste was changed to 1:1.

[0091] [Example 11] An all-solid-state secondary battery was fabricated in the same manner as in Example 1, except that the aspect ratio of the polymer particles in the first negative electrode paste was changed to 1:10.

[0092] [Example 12] An all-solid-state secondary battery was fabricated in the same manner as in Example 1, except that the negative electrode layer was formed without using a second negative electrode paste. This change resulted in the negative electrode layer, which in Examples 1-11 consisted substantially of AgPd alloy and lithium titanate, being changed to a configuration consisting substantially of AgPd alloy.

[0093] [Example 13] An all-solid-state secondary battery was fabricated in the same manner as in Example 12, except that the aspect ratio of the polymer particles in the first negative electrode paste was changed to 1:10. In Example 13, similar to Example 12, the negative electrode layer was substantially composed of an AgPd alloy.

[0094] [Example 14] An all-solid-state secondary battery was fabricated in the same manner as in Example 1, except that the volume ratio of AgPd powder to polymer particles in the first negative electrode paste was changed to 89:11.

[0095] [Example 15] An all-solid-state secondary battery was fabricated in the same manner as in Example 1, except that the particle size of the polymer particles in the first negative electrode paste was changed to 0.01 μm.

[0096] [Example 16] An all-solid-state secondary battery was fabricated in the same manner as in Example 1, except that the particle size of the polymer particles in the first negative electrode paste was changed to 3 μm.

[0097] [Example 17] An all-solid-state secondary battery was fabricated in the same manner as in Example 1, except that the aspect ratio of the polymer particles in the first negative electrode paste was changed to 1:5.

[0098] [Example 18] An all-solid-state secondary battery was fabricated in the same manner as in Example 12, except that the volume ratio of AgPd powder to polymer particles in the first negative electrode paste was changed to 99:1.

[0099] [Example 19] An all-solid-state secondary battery was fabricated in the same manner as in Example 12, except that the volume ratio of AgPd powder to polymer particles in the first negative electrode paste was changed to 90:10.

[0100] [Example 20] An all-solid-state secondary battery was fabricated in the same manner as in Example 12, except that the particle size of the polymer particles in the first negative electrode paste was changed to 0.01 μm.

[0101] [Example 21] An all-solid-state secondary battery was fabricated in the same manner as in Example 12, except that the particle size of the polymer particles in the first negative electrode paste was changed to 3 μm.

[0102] [Comparative Example 1] An all-solid-state secondary battery was prepared in the same manner as in Example 1, except that polymer particles were not added to the first negative electrode paste.

[0103] [Comparative Example 2] Solid electrolyte paste, Li 1.3 Al 0.3 Ti 1.7An all-solid-state secondary battery was prepared in the same manner as in Example 1, except that solid electrolyte particles consisting of (PO4)3 were prepared by wet mixing ethyl cellulose and dihydroterpineol in a ball mill.

[0104] [Comparative Example 3] An all-solid-state secondary battery was prepared in the same manner as in Example 12, except that polymer particles were not added to the first negative electrode paste.

[0105] [Comparative Example 4] Solid electrolyte paste, Li 1.3 Al 0.3 Ti 1.7 An all-solid-state secondary battery was prepared in the same manner as in Example 12, except that solid electrolyte particles consisting of (PO4)3 were prepared by wet mixing ethyl cellulose and dihydroterpineol in a ball mill.

[0106] <Analysis of all-solid-state secondary batteries> The all-solid-state secondary batteries prepared by the above method were analyzed using the following procedure.

[0107] (Cycle testing) Using an electrochemical testing apparatus (manufactured by Hokuto Denko Co., Ltd.), cycle tests were performed on the all-solid-state secondary batteries 10 of Examples 1 to 20 and Comparative Examples 1 to 4 by the method described below. Under a 60°C environment, constant current charging (CC charging) was performed with a constant current of 100 μA until the battery voltage reached 3.9 V, and then discharge (CC discharge) was performed with a constant current of 100 μA until the battery voltage reached 0 V. This process was considered one cycle, and 10 cycles were repeated. The discharge capacity (μAh) after the 10th cycle was defined as the initial capacity Q1. The all-solid-state secondary battery, whose initial capacity Q1 was determined, was again subjected to constant-current charging (CC charging) at a constant current of 100 μA until the battery voltage reached 3.9 V in a 60°C environment, and then discharged at a constant current of 100 μA until the battery voltage reached 0 V (CC discharge). Each of these charge-discharge cycles was counted as one cycle, and 1000 charge-discharge cycles were performed. After that, the discharge capacity Q2 was determined after 1000 charge-discharge cycles. From the discharge capacities Q1 and Q2 obtained in this manner, the capacity retention rate E after 1000 cycles was calculated using the following formula. The results are shown in Table 1. E(%) = (Q2 / Q1) × 100

[0108] (structural analysis) The structure of the all-solid-state secondary batteries of Examples 1-13 and Comparative Example 1 before charging and discharging was analyzed using a scanning electron microscope (Hitachi High-Technologies Corporation, model number: S-4800). Specifically, first, scanning electron microscope images of cross-sections parallel to the stacking direction were obtained for the all-solid-state secondary batteries of Examples 1-13 and Comparative Example 1. The observation conditions for the scanning electron microscope were set to an acceleration voltage of 10kV. In Examples 1-11, all-solid-state secondary batteries having the negative electrode structure shown in Figure 3 were manufactured. In Figure 3, for convenience of explanation, the symbols corresponding to each component described in Figure 1 are shown. In Example 1, where an example of the structure is shown in Figure 3, the negative electrode layer was made from a material mainly composed of a mixture of AgPd alloy and lithium titanate. As a result, the negative electrode layer 2, which has an AgPd alloy as phase 21 containing Ag, sandwiched between phase 22 containing lithium titanate, was sandwiched between solid electrolyte layers 3. Furthermore, in Examples 12 and 13, the negative electrode layer was fabricated from a material mainly composed of AgPd alloy, resulting in an all-solid-state secondary battery having the structure shown in Figures 1 and 2, in which the negative electrode layer 2 is composed of a phase 21 containing Ag.

[0109] These scanning electron microscope images were binarized using the mode method based on brightness to define the regions where voids were formed. Next, the region sandwiched between these solid electrolyte layers was designated as the negative electrode layer. In Examples 1 to 13, most of the voids were in contact with AgPd. In Comparative Example 1, no voids were observed. Furthermore, in Examples 1 to 11, voids were formed in contact with a portion of the interface between the first and second phases, while in Examples 12 and 13, voids were formed in contact with a portion of the interface between the Ag-containing phase and the solid electrolyte.

[0110] Next, based on the binarized scanning electron microscope image, the area of ​​the negative electrode layer and the area of ​​the voids in the same scanning electron microscope image (magnification: 5000x) were determined by image analysis. The void area was divided by the area of ​​the negative electrode layer to determine the porosity of the voids in the negative electrode layer, which was considered to be the volume ratio of the voids in the negative electrode layer. Furthermore, the arithmetic mean of the areas of all voids in this scanning electron microscope image was calculated, and the diameter of a circle with the same area as the arithmetic mean was calculated. In addition, the length of the voids in the stacking direction of the laminate and the length of the voids in the direction in which the negative electrode layer extends were measured, and the aspect ratio (L) of the voids was determined by dividing the length of the voids in the direction in which the negative electrode layer extends by the length of the voids in the stacking direction of the laminate. Vx / L Vz ) was calculated.

[0111] The measurement results for Examples 1-13 and Comparative Example 1 are summarized in Table 1.

[0112] [Table 1]

[0113] Table 1 shows that Examples 1-13, which are all-solid-state secondary batteries in which the solid electrolyte layer contains LSPO and the negative electrode layer contains Ag or an Ag alloy, and in which a void is formed in part of the interface between the LSPO and the Ag-containing phase of the negative electrode layer, can be shown to have significantly improved cycle characteristics compared to the all-solid-state secondary battery of Comparative Example 1, in which no void is formed in part of the interface between the LSPO and the Ag-containing phase. Furthermore, it was confirmed that Examples 1, 7, and 8, in which the porosity of voids in the negative electrode layer was 1% to 10%, obtained higher cycle characteristics compared to Example 6, which had a porosity of 0.05%, and Example 9, which had a porosity of 20%. This suggests that if the porosity of voids in the negative electrode layer is low, the stress relief due to expansion and contraction caused by charging and discharging of the negative electrode is insufficient, and if the voids are high, there is a possibility that the voids will connect with each other due to the stress. In the all-solid-state secondary batteries of Examples 1-9 and 11-13, the aspect ratio of the voids was greater than 1, and electron microscope images confirmed that many of the voids had an axial direction approximately parallel to the direction in which the negative electrode layer extended. This arrangement, where the voids are in contact with the AgPd alloy, efficiently mitigates void expansion and contraction compared to voids with an axis in the stacking direction, thereby improving cycle characteristics. This effect was confirmed by the fact that Example 1, with an aspect ratio of 3, achieved better cycle characteristics than Example 10, with an aspect ratio of 1. [Industrial applicability]

[0114] Compared to conventional batteries, we can provide all-solid-state batteries that are safer and have superior characteristics. [Explanation of Symbols]

[0115] 1. Positive electrode layer 1A positive electrode current collector 1B Cathode active material layer 2. Negative electrode layer 3 Solid electrolyte layer 4 Laminate 5 Positive terminal 6 Negative terminal 10 All-solid-state secondary battery V void L Vx Length of voids in the direction of extension of the negative electrode layer L Vz Length of voids in the stacking direction of the laminate Phase containing 21 Ag (Phase 1) 22 Phase 2 31 LSPO 41 1st cavity 41a 2nd cavity 41b 3rd cavity 42 Voids not in contact with the phase containing Ag

Claims

1. The laminate comprises a positive electrode layer, a negative electrode layer, and a solid electrolyte layer located between the positive electrode layer and the negative electrode layer. The solid electrolyte layer contains a compound represented by the following formula (1): The negative electrode layer includes a phase containing Ag, Multiple voids are formed in the negative electrode layer, At least a portion of the aforementioned void is a first void formed in contact with the phase containing Ag, An all-solid-state secondary battery in which the aspect ratio of the length of the first void in the direction in which the negative electrode layer extends to the length of the first void in the stacking direction of the laminate is 3 or more. Li ３＋ｘ Si ｘ P １－ｘ O ４ (1) (Equation (1) satisfies 0 < x < 1.)

2. The all-solid-state secondary battery according to claim 1, wherein the diameter of the first void is 0.01 μm or more and 3.0 μm or less.

3. The all-solid-state secondary battery according to claim 1 or 2, wherein the volume ratio of the first void formed in contact with the Ag-containing phase in the negative electrode layer is 1% or more and 10% or less.

4. The all-solid-state secondary battery according to claim 1 or 2, wherein at least a portion of the void formed in contact with the Ag-containing phase is a second void formed at the interface between the solid electrolyte layer and the Ag-containing phase.

5. The all-solid-state secondary battery according to claim 4, wherein the proportion of the second void to the entire first void is 80 to 100%.

6. The all-solid-state secondary battery according to claim 1 or 2, wherein the negative electrode layer comprises the Ag-containing phase.

7. The negative electrode layer comprises a first phase composed of the Ag-containing phase and a second phase composed of lithium titanate, sandwiching the first phase in the stacking direction. The all-solid-state secondary battery according to claim 1 or 2, wherein the negative electrode layer has a third void formed in contact with the interface between the first phase and the second phase.

8. The all-solid-state secondary battery according to claim 7, wherein the proportion of the third void to the entire first void is 50 to 100%.

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