Solid-state secondary batteries

Abstract

The present invention addresses the problem of providing a solid-state secondary battery capable of suppressing the uneven depositing of metal in a negative electrode interface in the solid-state secondary battery, and capable of improving cycling characteristics. The means for solving the problem is a solid-state secondary battery having a positive electrode layer, a negative electrode layer including at least a negative electrode current collector, a solid electrolyte layer containing a solid electrolyte material, and an intermediate layer provided between the negative electrode layer and the solid electrolyte layer. The voidage of the intermediate layer is greater than the voidage of the solid electrolyte layer.

Description

[Technical Field]

[0001] The present invention relates to a solid secondary battery. [Background technology]

[0002] In recent years, research and development into secondary batteries that contribute to energy efficiency has been conducted to ensure that more people have access to affordable, reliable, sustainable, and advanced energy. Among secondary batteries, solid-state batteries have attracted particular attention due to their superior safety as solid electrolytes are non-flammable and have higher energy density.

[0003] In solid-state batteries, repeated charge and discharge cycles can cause the deposition of metals such as lithium, which are used as a charge transfer medium, between the solid electrolyte layer and the negative electrode layer. The deposition of such metals can reduce the adhesiveness at the interface, potentially resulting in a deterioration in the electrical characteristics of the solid-state battery. To address this issue, a technology is known that provides a layer on the negative electrode current collector that can deposit lithium metal, thereby depositing the lithium metal substantially uniformly on the surface of the coating layer, thereby making it difficult for dead lithium to occur (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2018-129159 Summary of the Invention [Problem to be solved by the invention]

[0005] Patent Document 1 discloses that the coating layer is made of a metal that can form an alloy with lithium. However, since solid-state batteries undergo volume changes during charge and discharge, if the intermediate layer is made of a material that does not easily follow the volume change, there is a risk that the cycle characteristics will deteriorate due to a decrease in interfacial adhesion.

[0006] The present invention has been made in view of the above, and has an object to provide a solid secondary battery that can suppress uneven metal deposition on the negative electrode interface of the solid secondary battery and can improve cycle characteristics. [Means for solving the problem]

[0007] (1) The present invention relates to a solid secondary battery comprising a positive electrode layer, a negative electrode layer including at least a negative electrode current collector, a solid electrolyte layer including a solid electrolyte material, and an intermediate layer provided between the negative electrode layer and the solid electrolyte layer, wherein the porosity of the intermediate layer is greater than the porosity of the solid electrolyte layer.

[0008] According to the invention (1), it is possible to provide a solid secondary battery that can suppress non-uniform deposition of metal on the negative electrode interface of the solid secondary battery and improve the cycle characteristics.

[0009] (2) The solid secondary battery according to (1), wherein the particle diameter of the particles constituting the intermediate layer is smaller than the particle diameter of the particles of the solid electrolyte material.

[0010] According to the invention (2), the adhesion between the solid electrolyte layer and the intermediate layer can be improved, and the contact area between the solid electrolyte layer and the intermediate layer can be increased.

[0011] (3) The solid secondary battery according to (1) or (2), wherein the porosity of the intermediate layer is 40 to 70%.

[0012] According to the invention of (3), the charge transfer medium can easily pass through the intermediate layer and the structure of the intermediate layer can be easily maintained, so that metal is less likely to deposit at the interface between the intermediate layer and the solid electrolyte layer.

[0013] (4) The solid secondary battery according to any one of (1) to (3), wherein the intermediate layer contains amorphous carbon.

[0014] According to the invention (4), the electron conductivity of the intermediate layer can be ensured, and the particles constituting the intermediate layer and the charge transfer medium can be prevented from reacting with each other to form an alloy.

[0015] (5) The solid secondary battery according to any one of (1) to (4), wherein the intermediate layer contains a binder.

[0016] According to the fifth aspect of the present invention, the adhesion between the particles constituting the intermediate layer and between the intermediate layer and the solid electrolyte layer can be improved, so that the structure of the intermediate layer can be easily maintained.

[0017] (6) The solid secondary battery according to any one of (1) to (5), wherein the intermediate layer contains metal nanoparticles, and the content of the metal nanoparticles in the intermediate layer is greater than 0 mass % and not more than 30 mass %.

[0018] According to the invention (6), the volume expansion of the intermediate layer can be reduced, the structural destruction of the intermediate layer and the non-uniform deposition of the charge transfer medium can be suppressed, and the electronic conductivity of the intermediate layer can be improved. [Brief explanation of the drawings]

[0019] [Figure 1] 1 is a cross-sectional view schematically illustrating the structure of a solid secondary battery according to an embodiment of the present invention. [Figure 2] FIG. 2 is an enlarged view of a main part of FIG. 1, showing the structure of the solid secondary battery before charging and discharging. [Figure 3] FIG. 2 is an enlarged view of a main part of FIG. 1, showing the structure of the solid secondary battery after charging and discharging. [Figure 4A] 1 is a micrograph of a main part of a solid secondary battery according to a comparative example. [Figure 4B] 1 is a micrograph of a main part of a solid secondary battery according to a comparative example. [Figure 5A] 1 is a micrograph of a main part of a solid secondary battery according to an example. [Figure 5B] 1 is a micrograph of a main part of a solid secondary battery according to an example. [Figure 6] 1 is a graph showing the relationship between the number of cycles and the capacity retention rate of solid secondary batteries according to Examples and Comparative Examples. [Figure 7]1 is a graph showing the relationship between the number of cycles and the capacity retention rate of solid secondary batteries according to Examples and Comparative Examples. DETAILED DESCRIPTION OF THE INVENTION

[0020] <Solid secondary battery> As shown in Fig. 1, the solid secondary battery 1 according to this embodiment is configured by laminating a positive electrode layer 20, a solid electrolyte layer 40, an intermediate layer 50, and an anode layer 30 in this order. Since Fig. 1 is a diagram schematically illustrating the configuration of the solid secondary battery 1 after charge and discharge, a metal deposition layer 60 is formed between the intermediate layer 50 and the anode layer 30.

[0021] (positive electrode layer) The positive electrode layer 20 is a layer made up of a positive electrode current collector 21 and a positive electrode active material layer 22 containing at least a positive electrode active material.

[0022] The positive electrode current collector 21 is not particularly limited as long as it has the function of collecting current from the positive electrode layer, and examples thereof include aluminum, aluminum alloys, stainless steel, nickel, iron, and titanium, with aluminum, aluminum alloys, and stainless steel being preferred. The positive electrode current collector may be in the form of a foil, a plate, or the like.

[0023] The positive electrode active material contained in the positive electrode active material layer 22 can be the same as that used in the positive electrode layer of a general solid-state battery, and is not particularly limited. For example, in the case of a lithium-ion battery, examples of the positive electrode active material include a layered active material containing lithium, a spinel-type active material, and an olivine-type active material. Specific examples of the positive electrode active material include lithium cobalt oxide (LiCoO), lithium nickel oxide (LiNiO), and LiNi p Mn q Co r O2(p+q+r=1), LiNi p Al q Co rExamples of such an element-substituted Li-Mn spinel include lithium manganate (LiMnO), LiMnO (p+q+r=1), lithium manganate (LiMnO), LiMnMyO (x+y=2, M=at least one selected from Al, Mg, Co, Fe, Ni, and Zn), lithium titanate (oxide containing Li and Ti), and lithium metal phosphate (LiMPO, M=at least one selected from Fe, Mn, Co, and Ni).

[0024] The positive electrode active material layer 22 may optionally contain a solid electrolyte from the viewpoint of improving the conductivity of the charge transfer medium. It may also optionally contain a conductive additive to improve the conductivity. Furthermore, it may also optionally contain a binder from the viewpoint of exhibiting flexibility, etc. The solid electrolyte, conductive additive, and binder may be those generally used in solid-state batteries.

[0025] (negative electrode layer) The negative electrode layer 30 is a layer made up of a negative electrode current collector 31 and a negative electrode active material layer 32 containing at least a negative electrode active material.

[0026] The negative electrode current collector 31 is not particularly limited as long as it has the function of collecting current from the negative electrode layer, and examples of the material for the negative electrode current collector include nickel, copper, stainless steel, etc. The shape of the negative electrode current collector can be, for example, a foil shape, a plate shape, etc.

[0027] The negative electrode active material contained in the negative electrode active material layer 32 can be appropriately selected from known materials capable of absorbing and releasing charge transfer media such as lithium ions. Examples of such materials include lithium transition metal oxides such as lithium titanate, transition metal oxides such as TiO2, Nb2O3, and WO3, Si, SiO2, metal sulfides, metal nitrides, carbon materials such as artificial graphite, natural graphite, graphite, soft carbon, and hard carbon, as well as metallic lithium, metallic indium, and lithium alloys. Metallic lithium is preferred as the negative electrode active material. This is because the solid secondary battery 1 according to this embodiment can effectively suppress dendrite precipitation when metallic lithium is used as the negative electrode active material. The negative electrode active material may be in the form of a powder or a thin film.

[0028] The negative electrode active material layer 32 may optionally contain a solid electrolyte from the viewpoint of improving the conductivity of the charge transfer medium. It may also optionally contain a conductive additive to improve the conductivity. Furthermore, it may also optionally contain a binder from the viewpoint of exhibiting flexibility, etc. The solid electrolyte, conductive additive, and binder may be those generally used in solid-state batteries.

[0029] (solid electrolyte layer) The solid electrolyte layer 40 is a layer laminated between the positive electrode layer 20 and the negative electrode layer 30, and contains at least a solid electrolyte material. Charge transfer medium conduction between the positive electrode active material and the negative electrode active material can be achieved via the solid electrolyte material contained in the solid electrolyte layer.

[0030] The solid electrolyte material is not particularly limited as long as it has charge transfer medium conductivity, and examples thereof include sulfide solid electrolyte materials, oxide solid electrolyte materials, nitride solid electrolyte materials, and halide solid electrolyte materials.

[0031] Examples of sulfide solid electrolyte materials for lithium ion batteries include Li2S-P2S5, Li2S-P2S5-LiI, etc. The term "Li2S-P2S5" above refers to a sulfide solid electrolyte material obtained using a raw material composition containing Li2S and P2S5, and the same applies to other similar terms.

[0032] Examples of oxide solid electrolyte materials for lithium ion batteries include NASICON-type oxides, garnet-type oxides, and perovskite-type oxides. Examples of NASICON-type oxides include oxides containing Li, Al, Ti, P, and O (e.g., Li 1.5 Al 0.5 Ti 1.5 Examples of garnet-type oxides include oxides containing Li, La, Zr, and O (e.g., LiLaZrO 12 Examples of perovskite oxides include oxides containing Li, La, Ti, and O (for example, LiLaTiO3).

[0033] The porosity of the solid electrolyte layer 40 is lower than that of the intermediate layer 50, which will be described later, and is, for example, less than 10%. The particle size of the solid electrolyte material 41 that constitutes the solid electrolyte layer 40 is, for example, 0.5 to 10 μm in median size (D50), and is preferably larger than that of the particles that constitute the intermediate layer, which will be described later.

[0034] The porosity of the solid electrolyte 40 can be calculated, for example, by the following formula (1): In formula (1), the "packing ratio" refers to the percentage of the density of the solid electrolyte layer after molding relative to the true density. Porosity (%)=(100-Filling rate (%)) …(1)

[0035] The method for calculating the porosity is not limited to the above method, and it may be calculated by instrumental analysis such as BET, porosimetry, or gas diffusion, or by image analysis using a scanning electron microscope or the like.

[0036] (middle class) The intermediate layer 50 is a layer laminated between the anode layer 30 and the solid electrolyte layer 40. The intermediate layer 50 can suppress uneven deposition of metal at the interface of the anode layer 30 and can improve interface adhesion. As shown in Figures 2 and 3, the intermediate layer 50 preferably contains amorphous carbon 51 and metal nanoparticles 52.

[0037] The function of the intermediate layer 50 will be described with reference to FIGS. 2 and 3. FIG. 2 is an enlarged view of the area in FIG. 1 where the solid electrolyte layer 40, the intermediate layer 50, and the anode layer 30 are stacked, and is a schematic diagram showing the state of the solid secondary battery 1 immediately after fabrication, before charge and discharge have been performed. FIG. 3 is a diagram corresponding to FIG. 2 and is a schematic diagram showing the state of the solid secondary battery 1 after repeated charge and discharge. The following description will be given assuming that the charge transfer medium of the solid secondary battery 1 is Li ions. In the case of a conventional solid secondary battery without the intermediate layer 50, metallic lithium precipitates at the interface between the solid electrolyte layer and the anode layer as the solid secondary battery is repeatedly charged and discharged. Once metallic lithium precipitates, the electronic conductivity of that area increases, leading to non-uniform deposition of metallic lithium, such as the formation of dendrites. As a result, repeated charge and discharge may cause the anode active material layer 32 to become porous, reducing interfacial adhesion and potentially degrading battery performance.

[0038] On the other hand, the intermediate layer 50 of the solid secondary battery 1 according to this embodiment has electronic conductivity and has voids through which Li ions can pass. Therefore, as shown in FIG. 3 , as the solid secondary battery 1 is repeatedly charged and discharged, Li ions moving from the solid electrolyte layer 40 toward the negative electrode active material layer 32 pass through the intermediate layer 50, and a metal deposit layer 60 (a layer of metallic lithium) is formed between the intermediate layer 50 and the negative electrode active material layer 32. This allows the metal deposit layer 60 to be formed uniformly. Furthermore, since the intermediate layer 50 has flexibility that allows it to follow the volumetric changes of each layer that occur during charge and discharge, interfacial adhesion can be maintained even when the solid secondary battery 1 is repeatedly charged and discharged, and the durability of the solid secondary battery 1 can be improved.

[0039] The porosity of the intermediate layer 50 is higher than the porosity of the solid electrolyte layer 40. This allows voids through which Li ions can pass to be formed inside the intermediate layer 50, and also makes the intermediate layer 50 flexible and able to follow the volumetric changes of the solid secondary battery 1. The porosity of the intermediate layer 50 can be set to, for example, 40 to 70%. The porosity of the intermediate layer 50 can be calculated using the same method as that for calculating the porosity of the solid electrolyte 40.

[0040] The intermediate layer 50 preferably contains amorphous carbon 51. Unlike graphite, for example, amorphous carbon 51 does not react with lithium metal or the like to form an alloy, and therefore can suppress the formation of dendrites and improve the cycle characteristics of the solid secondary battery 1. Examples of amorphous carbon 51 include carbon blacks such as acetylene black, furnace black, and ketjen black, coke, and activated carbon. The amorphous carbon 51 may be easily graphitized carbon (soft carbon), or may be less easily graphitized carbon (hard carbon), CNT (carbon nanotube), fullerene, or graphene.

[0041] The term "amorphous carbon" used here refers to a carbon allotrope that does not exhibit a clear crystalline state, and is not strictly amorphous but is an aggregate of fine graphite crystals. In other words, amorphous carbon refers to all carbon allotropes excluding diamond and graphite.

[0042] The intermediate layer 50 preferably contains metal nanoparticles 52. By including the metal nanoparticles 52 in the intermediate layer 50, the electronic conductivity of the intermediate layer 50 can be improved, and the metal precipitate layer 60 can be formed more uniformly. Furthermore, since the metal nanoparticles 52 have a higher Young's modulus than the amorphous carbon 51, the structure of the intermediate layer 50 can be maintained even when high-pressure pressing is performed during the production of the solid secondary battery 1. Examples of the metal nanoparticles 52 include metal nanoparticles of tin (Sn), silicon (Si), zinc (Zn), magnesium (Mg), gold (Au), platinum (Pt), palladium (Pd), silver (Ag), aluminum (Al), bismuth (Bi), and antimony (Sb). The content of the metal nanoparticles 52 in the intermediate layer 50 is preferably greater than 0% by mass and less than or equal to 30% by mass.

[0043] The particle diameter of the amorphous carbon 51 and metal nanoparticles 52 constituting the intermediate layer 50 is preferably smaller than the particle diameter of the solid electrolyte material 41. This allows the intermediate layer 50 to penetrate into the gaps between the solid electrolyte materials 41 constituting the interface of the solid electrolyte layer 40, thereby increasing the contact area between the solid electrolyte layer 40 and the intermediate layer 50 and improving adhesion. The particle diameter of the amorphous carbon 51 may be, for example, about 0.04 to 0.05 μm in median diameter (D50), and the particle diameter of the metal nanoparticles 52 may be, for example, about 0.07 μm in median diameter (D50).

[0044] In order to maintain the structure of the intermediate layer 50, the intermediate layer 50 preferably contains a binder as a binding material. This improves the adhesion between the particles constituting the intermediate layer 50 and between the intermediate layer 50 and the solid electrolyte layer 40. There are no particular limitations on the binder, and it is possible to use binders that are generally used in solid-state batteries. Examples of the binder include acrylic acid-based polymers, cellulose-based polymers, styrene-based polymers, vinyl acetate-based polymers, urethane-based polymers, fluoroethylene-based polymers, and PVDF-based polymers.

[0045] <Solid secondary battery> The solid secondary battery 1 according to this embodiment is manufactured by stacking a positive electrode layer 20, a solid electrolyte layer 40, an intermediate layer 50, and a negative electrode layer 30 in the order shown in Fig. 1. After the stacking, the layers may be optionally pressed together to form a single unit. Furthermore, a plurality of the structural units shown in Fig. 1 may be stacked as unit batteries.

[0046] Although the preferred embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and modifications and improvements within the scope of achieving the object of the present invention are included in the present invention. [Example]

[0047] The present invention will be described in detail below using examples, but the present invention is not limited to these examples. Example 3 is a reference example.

[0048] Example 1 [Preparation of solid electrolyte layer] The solid electrolyte material used was an argyrodite-type sulfide solid electrolyte.

[0049] [Preparation of positive electrode layer] A slurry was created using lithium nickel cobalt manganese composite oxide (NCM622) as the positive electrode active material, an argyrodite-type sulfide solid electrolyte as the solid electrolyte, butyl butyrate as the solvent, carbon black as a conductive additive, and an SBR (styrene butadiene rubber) binder as the binding material, and the slurry was then coated on aluminum foil as the positive electrode current collector and dried to create the positive electrode layer.

[0050] [Creating the negative electrode layer] Metallic lithium was used as the negative electrode active material and was bonded to SUS foil as the electrode current collector to form the negative electrode layer.

[0051] [Creating the intermediate layer] The intermediate layer was fabricated by creating a slurry using Sn as the metal nanoparticles, acetylene black (particle size 0.05 μm) as the amorphous carbon, NMP (N-methyl-2-pyrrolidone) as the solvent, and a PVDF-based binder as the binding material, and then coating and drying the slurry.

[0052] [Fabrication of solid-state secondary batteries] The positive electrode layer, solid electrolyte layer, intermediate layer, and negative electrode layer obtained above were stacked in this order and pressed to produce a solid secondary battery according to Example 1. The porosity of the intermediate layer after fabrication was 41%. The porosity of the intermediate layer was calculated using the following formula (2). In formula (2), the "packing ratio" refers to the percentage of the intermediate layer density after molding relative to the true density. Porosity (%) = (100-Filling rate (%)) …(2)

[0053] <Examples 2 to 10 and Comparative Examples 1 to 3> Solid secondary batteries according to other examples and comparative examples were fabricated in the same manner as in Example 1, except that the intermediate layer and solid electrolyte layer had the structures shown in Tables 1 and 2. In Comparative Example 1, no intermediate layer was fabricated.

[0054] [Table 1]

[0055] [Table 2]

[0056] [Cross-section observation] The solid secondary batteries according to Example 1 and Comparative Example 1 were subjected to 20 charge-discharge cycles at a C rate of 1 / 3C and 25°C, and the vicinity of the interface of the anode layer or solid electrolyte layer was then observed using a field emission scanning electron microscope (FE-SEM): S-4300SE (manufactured by Hitachi High-Technologies) ( FIGS. 4A and 5A ) and an optical microscope ( FIGS. 4B and 5B ). The results of Comparative Example 1 are shown in FIGS. 4A and 4B , and the results of Example 1 are shown in FIGS. 5A and 5B , respectively.

[0057] 4A and 4B, in the solid secondary battery 1a according to Comparative Example 1, a metal deposit layer 60 was formed at the interface between the solid electrolyte layer 40 and the negative electrode layer, and the metal deposit layer 60 was observed to be porous. In contrast, as shown in Fig. 5A and 5B, in the solid secondary battery 1 according to Example 1, a metal deposit layer 60 was formed at the interface between the intermediate layer 50 and the negative electrode layer, and the metal deposit layer 60 was observed to be not porous.

[0058] [Capacity retention rate measurement] Using the solid secondary batteries according to Example 1 and Comparative Example 1, a cycle test was carried out in which charging and discharging were repeated at an upper limit charging voltage of 4.3 V, a lower limit discharging voltage of 2.65 V, and a C rate of 1 / 3 C, and the capacity retention rate was measured. Fig. 6 shows the test results at 25°C, and Fig. 7 shows the test results at 45°C. The vertical axis of the graphs in Figs. 6 and 7 represents the capacity retention rate (%), and the horizontal axis represents the number of cycles (times).

[0059] As shown in FIGS. 6 and 7, the solid secondary battery according to Example 1 exhibited a more gradual decrease in capacity retention rate with an increase in the number of cycles compared to the solid secondary battery according to Comparative Example 1, and favorable cycle characteristics were obtained.

[0060] [Battery characteristic evaluation] Using the solid-state secondary batteries according to each Example and Comparative Example, 50 charge / discharge cycles were performed at a C rate of 1 / 3C and 45°C, and the discharge capacity retention rate (%) at a C rate of 1 / 10C and 25°C after the cycle test, the average charge / discharge efficiency (%) over the first to 50th cycles, and the Li-transfer state of the intermediate layer were evaluated. The Li-transfer state of the intermediate layer was evaluated according to the following criteria. The results are shown in Tables 1 and 2.

[0061] (Evaluation criteria for middle-class Li passing status) 2 A metal deposition layer is formed between the intermediate layer and the negative electrode layer. 1. No metal deposit layer is formed between the intermediate layer and the negative electrode layer, or a metal deposit layer is formed within the solid electrolyte layer or between the intermediate layer and the solid electrolyte layer.

[0062] The results in Tables 1 and 2 clearly show that the solid secondary batteries according to each Example have a metal deposition layer formed between the intermediate layer and the negative electrode layer. Furthermore, the solid secondary batteries according to each Example have higher discharge capacity retention rates (%) after cycle testing and higher average charge / discharge efficiencies (%) over 1 to 50 cycles than the solid secondary batteries according to each Comparative Example, demonstrating that favorable cycle characteristics can be obtained. [Explanation of symbols]

[0063] 1 Solid state secondary battery 20 Positive electrode layer 30 negative electrode layer 31 Negative electrode current collector 40 Solid electrolyte layer 41 Solid electrolyte materials 50 Middle Class 51 Amorphous carbon 52 Metal nanoparticles

Claims

1. a positive electrode layer; a negative electrode layer including at least a negative electrode current collector; a solid electrolyte layer including a solid electrolyte material; an intermediate layer provided between the negative electrode layer and the solid electrolyte layer, the intermediate layer comprises metal nanoparticles; the metal nanoparticles are nanoparticles of at least one metal selected from the group consisting of tin, silicon, zinc, magnesium, gold, platinum, palladium, silver, aluminum, bismuth, and antimony; A solid secondary battery, wherein the porosity of the intermediate layer is greater than the porosity of the solid electrolyte layer.

2. A positive electrode layer, a negative electrode layer including at least a negative electrode current collector; a solid electrolyte layer including a solid electrolyte material; an intermediate layer provided between the negative electrode layer and the solid electrolyte layer, the intermediate layer includes metal nanoparticles and a binder; A solid secondary battery, wherein the porosity of the intermediate layer is greater than the porosity of the solid electrolyte layer.

3. 3. The solid secondary battery according to claim 1, wherein the particle diameter of the particles constituting the intermediate layer is smaller than the particle diameter of the particles of the solid electrolyte material.

4. 3. The solid secondary battery according to claim 1, wherein the porosity of the intermediate layer is 40 to 70%.

5. The solid secondary battery according to claim 1 or 2, wherein the intermediate layer contains amorphous carbon.

6. The solid secondary battery according to claim 1 , wherein the intermediate layer contains a binder.

7. The solid secondary battery according to claim 1 or 2, wherein the content of the metal nanoparticles in the intermediate layer is more than 0 mass % and 30 mass % or less.

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