All-solid-state battery

Abstract

Provided is a lithium deposition-type all-solid-state battery comprising a means that is capable of improving battery cycle durability. The all-solid-state battery, which comprises a lithium deposition-type power generation element, has a structure in which: a negative electrode intermediate layer including a sheet-like base material containing a fibrous carbon material is placed adjacent to the negative-electrode-collector-side surface of the solid electrolyte layer; and the negative electrode intermediate layer is configured such that a portion of the solid electrolyte invades into the solid-electrolyte-layer-side surface of the sheet-like base material but does not penetrate the sheet-like base material. Such a structure can be produced, for example, by applying a slurry containing a solid electrolyte to one surface of a sheet-like base material containing a fibrous carbon material such that the solid electrolyte does not penetrate the sheet-like base material.

Description

All solid state battery

[0001] The present invention relates to an all-solid-state battery.

[0002] In recent years, research and development on all-solid-state batteries using oxide- or sulfide-based solid electrolytes has been actively conducted. Solid electrolytes are materials primarily composed of ionic conductors that can conduct ions in a solid state. Therefore, all-solid-state batteries have the advantage that, in principle, they do not encounter the various problems associated with flammable organic electrolytes, as occurs in conventional liquid-based batteries that use nonaqueous electrolytes.

[0003] One type of all-solid-state secondary battery is known as a so-called lithium deposition type, in which lithium metal is deposited on a negative electrode current collector during charging. For example, Japanese Patent Application Laid-Open No. 2020-191202 discloses an example of such a lithium deposition type all-solid-state secondary battery. The battery includes a negative electrode active material layer (negative electrode intermediate layer) that forms an alloy or compound with lithium on the surface of the negative electrode current collector, and silver (Ag) is contained in the negative electrode active material layer (negative electrode intermediate layer). According to this document, this configuration provides a battery with excellent discharge capacity without the need for high external pressure.

[0004] However, according to the investigations of the present inventors, it has been found that when an all-solid-state battery is produced by bonding a negative electrode intermediate layer and a solid electrolyte layer by a pressure treatment such as a hydrostatic press, as in the technique described in the above document, the cycle durability of the battery may be reduced.

[0005] Therefore, an object of the present invention is to provide a means for improving the cycle durability of a lithium deposition-type all-solid-state battery.

[0006] The present inventors have conducted extensive research to solve the above-mentioned problems, and as a result, have found that the above-mentioned problems can be solved by disposing an anode intermediate layer including a sheet-like substrate containing a fibrous carbon material adjacent to the surface of the solid electrolyte layer facing the anode current collector in an all-solid-state battery including a lithium deposition-type power generating element, and by configuring the anode intermediate layer so that part of the solid electrolyte penetrates into the surface of the sheet-like substrate facing the solid electrolyte layer without penetrating the sheet-like substrate, thereby completing the present invention.

[0007] That is, one embodiment of the present invention is an all-solid-state battery comprising: a power generating element including: a positive electrode having a positive electrode active material layer containing a positive electrode active material; a negative electrode having a negative electrode current collector and on which lithium metal is deposited during charging; a solid electrolyte layer interposed between the positive electrode and the negative electrode and containing a solid electrolyte; and a negative electrode intermediate layer present adjacent to a surface of the solid electrolyte layer facing the negative electrode current collector and including a sheet-like substrate containing a fibrous carbon material, wherein the negative electrode intermediate layer has a structure in which a portion of the solid electrolyte penetrates into the surface of the sheet-like substrate facing the solid electrolyte layer in a manner not to penetrate the sheet-like substrate.

[0008] FIG. 1 is a cross-sectional view schematically illustrating the overall structure of a stacked-type (internal parallel connection type) all-solid-state lithium secondary battery (stacked-type secondary battery) according to one embodiment of the present invention.

[0009] One aspect of the present invention is an all-solid-state battery comprising a power generating element including: a positive electrode having a positive electrode active material layer containing a positive electrode active material; a negative electrode having a negative electrode current collector on which lithium metal is deposited during charging; a solid electrolyte layer interposed between the positive electrode and the negative electrode and containing a solid electrolyte; and a negative electrode intermediate layer present adjacent to a surface of the solid electrolyte layer facing the negative electrode current collector and including a sheet-like substrate containing a fibrous carbon material, wherein the negative electrode intermediate layer has a structure in which a portion of the solid electrolyte penetrates into the surface of the sheet-like substrate facing the solid electrolyte layer without penetrating the sheet-like substrate. The all-solid-state battery according to this aspect has excellent cycle durability.

[0010] Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings. In the description of the drawings, the same elements are designated by the same reference numerals, and duplicate explanations will be omitted. Furthermore, the dimensional proportions in the drawings are exaggerated for the convenience of explanation and may differ from the actual proportions.

[0011] FIG. 1 is a cross-sectional view schematically illustrating the overall structure of a stacked-type (internal parallel connection type) all-solid-state lithium secondary battery (hereinafter also simply referred to as a "stacked-type secondary battery") according to one embodiment of the present invention. FIG. 1 shows a cross section of the stacked-type secondary battery during charging. The stacked-type secondary battery 10a shown in FIG. 1 has a structure in which a substantially rectangular power generating element 21, where charge and discharge reactions actually proceed, is sealed inside a laminate film 29, which is a battery exterior. The power generating element 21 has a structure in which a negative electrode, a solid electrolyte layer 17, and a positive electrode are stacked. The negative electrode has a structure in which a negative electrode current collector 11' and a negative electrode active material layer 13 made of lithium metal deposited on the surface of the negative electrode current collector 11' are stacked. Furthermore, a negative electrode intermediate layer 14 including a sheet-like substrate containing a fibrous carbon material is disposed adjacent to the surface of the negative electrode active material layer 13 facing the solid electrolyte layer 17. A part of the solid electrolyte constituting the solid electrolyte layer 17 penetrates into the surface of the sheet-like substrate constituting the negative electrode intermediate layer 14 on the side of the solid electrolyte layer 17 without penetrating the sheet-like substrate. The positive electrode has a structure in which a positive electrode active material layer 15 is disposed on the surface of a positive electrode current collector 11". The negative electrode, solid electrolyte layer, and positive electrode are laminated in this order, with the negative electrode intermediate layer 14 and the positive electrode active material layer 15 facing each other with the solid electrolyte layer 17 interposed therebetween. As a result, adjacent negative electrodes, solid electrolyte layers, and positive electrodes constitute one unit cell layer 19. Therefore, the stacked secondary battery 10a shown in FIG. 1 can be said to have a structure in which a plurality of unit cell layers 19 are laminated and electrically connected in parallel. A negative electrode current collector 25 and a positive electrode current collector 27 that are electrically connected to the respective electrodes (negative electrode and positive electrode) are attached to the negative electrode current collector 11' and the positive electrode current collector 11", respectively, and are structured so as to be sandwiched between the ends of the laminate film 29 and extended to the outside of the laminate film 29. A constraining pressure is applied to the stacked secondary battery 10a in the stacking direction of the power generating element 21 by a pressure member (not shown). Therefore, the volume of the power generating element 21 is kept constant.

[0012] The main components of the all-solid-state battery according to this embodiment will be described below.

[0013] [Current Collector] The current collector (negative electrode current collector, positive electrode current collector) has the function of mediating the movement of electrons from the electrode active material layer (negative electrode active material layer, positive electrode active material layer). There are no particular restrictions on the material that constitutes the current collector. Examples of materials that can be used for the current collector include metals such as aluminum, nickel, iron, stainless steel, titanium, and copper, as well as conductive resins. There are also no particular restrictions on the thickness of the current collector, but an example is 10 to 100 μm.

[0014] [Negative Electrode Active Material Layer] The all-solid-state battery according to this embodiment is a so-called lithium deposition type battery in which lithium metal is deposited on the negative electrode current collector during the charging process. The layer of lithium metal deposited on the negative electrode current collector during this charging process is the negative electrode active material layer of the all-solid-state battery according to this embodiment. Therefore, the thickness of the negative electrode active material layer increases as the charging process progresses, and the thickness of the negative electrode active material layer decreases as the discharging process progresses. Although the negative electrode active material layer does not need to be present during full discharge, in some cases, a negative electrode active material layer composed of a certain amount of lithium metal may be present during full discharge. Furthermore, the thickness of the negative electrode active material layer (lithium metal layer) during full charge is not particularly limited, but is typically 0.1 to 1000 μm.

[0015] [Solid Electrolyte Layer] The solid electrolyte layer is interposed between the negative electrode and the positive electrode and contains a solid electrolyte (usually as a main component). The solid electrolyte contained in the solid electrolyte layer is not particularly limited, and any solid electrolyte known in the art can be appropriately adopted, for example, a sulfide solid electrolyte and an oxide solid electrolyte. This solid electrolyte exhibits excellent lithium ion conductivity, and is therefore preferably a sulfide solid electrolyte containing an S element, more preferably a sulfide solid electrolyte containing an Li element, an M element, and an S element, wherein the M element contains at least one element selected from the group consisting of P, Si, Ge, Sn, Ti, Zr, Nb, Al, Sb, Br, Cl, and I, and even more preferably a sulfide solid electrolyte containing an S element, an Li element, and an P element. One example is LPS (Li ２ S-P ２ S ５ ), Li ６ P.S. ５X (wherein X is Cl, Br or I), Li ７ P ３ S １１ , Li ３．２ P ０．９６ S and Li ３ P.S. ４ These sulfide solid electrolytes are preferably used because they have excellent lithium ion conductivity.

[0016] The ionic conductivity (e.g., Li ion conductivity) of the solid electrolyte at room temperature (25°C) is, for example, 1 × 10 －５ S / cm or more, and preferably 1×10 －４ S / cm or more is more preferable. The ionic conductivity value of the solid electrolyte can be measured by an AC impedance method. Examples of the shape of the solid electrolyte include particulate shapes such as spherical and oval spheres, and thin films. When the solid electrolyte is particulate, its average particle diameter (D50) is not particularly limited, but is preferably 0.01 μm or more and 40 μm or less, more preferably 0.1 μm or more and 20 μm or less, and even more preferably 0.5 μm or more and 10 μm or less. The content of the solid electrolyte in the solid electrolyte layer is preferably 50 to 100 mass%, and more preferably 90 to 100 mass%.

[0017] The solid electrolyte layer may further contain a binder in addition to the solid electrolyte. The binder is not particularly limited, and known binders can be used as appropriate. For example, the binders described in the section on the negative electrode intermediate layer can be similarly employed. The content of the binder in the solid electrolyte layer is not particularly limited, but is, for example, 1 to 10 mass %, and preferably 2 to 3 mass %.

[0018] As described above, the all-solid-state battery according to this embodiment is characterized in that a portion of the solid electrolyte constituting the solid electrolyte layer penetrates into the surface of the sheet-like substrate (including the fibrous carbon material) constituting the negative electrode intermediate layer described below, on the solid electrolyte layer side, without penetrating the sheet-like substrate. Details of this will be described later in the section on the negative electrode intermediate layer. The thickness of the solid electrolyte layer (excluding the portion where the solid electrolyte penetrates into the sheet-like substrate constituting the negative electrode intermediate layer) varies depending on the configuration of the intended all-solid-state battery, but is typically 0.1 to 1000 μm, and preferably 10 to 100 μm.

[0019] [Positive Electrode Active Material Layer] The positive electrode active material layer essentially contains a positive electrode active material, and may contain a solid electrolyte, a binder, and a conductive additive as needed.

[0020] The type of the positive electrode active material contained in the positive electrode active material layer is not particularly limited, but may be LiCoO ２ , LiMnO ２ , LiNiO ２ , LiVO ２ , Li(Ni-Mn-Co)O ２ Layered rock salt active materials such as LiMn ２ O ４ , LiNi ０．５ Mn １．５ O ４ Spinel-type active materials such as LiFePO ４ , LiMnPO ４ Olivine type active materials such as Li ２ FeSiO ４ , Li ２ MnSiO ４ Examples of oxide active materials other than those mentioned above include Si-containing active materials such as Li ４ Ti ５ O １２ Among them, Li(Ni-Mn-Co)O ２ Also, those in which part of these transition metals has been replaced with other elements (hereinafter also simply referred to as "NMC composite oxides") are preferably used as the positive electrode active material.

[0021] In addition, a sulfur-based positive electrode active material is also one of the preferred embodiments. Examples of the sulfur-based positive electrode active material include particles or thin films of organic sulfur compounds or inorganic sulfur compounds, and any material can be used as long as it is capable of releasing lithium ions during charging and absorbing lithium ions during discharging by utilizing the oxidation-reduction reaction of sulfur.

[0022] The content of the positive electrode active material in the positive electrode active material layer is not particularly limited, but is preferably, for example, 30 to 99 mass %, more preferably 40 to 90 mass %, and even more preferably 45 to 80 mass %.

[0023] The positive electrode active material layer preferably further contains a solid electrolyte. The specific form of the solid electrolyte contained in the positive electrode active material layer may be the same as that described in the solid electrolyte layer section. A sulfide solid electrolyte is preferably used because it has excellent lithium ion conductivity and a low bulk modulus, allowing it to follow the volumetric changes of the positive electrode active material that occur during charging and discharging. The content of the solid electrolyte in the positive electrode active material layer is not particularly limited, but is, for example, 1 to 70% by mass, preferably 10 to 60% by mass, and more preferably 20 to 55% by mass.

[0024] The binder used in the positive electrode active material layer is not particularly limited, and any known binder can be used as appropriate. For example, the binders described in the negative electrode intermediate layer section can be used. The content of the binder in the positive electrode active material layer is not particularly limited, and is, for example, 1 to 10 mass %.

[0025] The conductive additive used in the positive electrode active material layer is not particularly limited, and may be, for example, carbon such as carbon black (specifically, acetylene black, Ketjen Black (registered trademark), furnace black, channel black, thermal lamp black, etc.). The content of the conductive additive in the positive electrode active material layer is not particularly limited, and is, for example, 10 to 30 mass%.

[0026] The thickness of the positive electrode active material layer varies depending on the intended configuration of the all-solid-state battery, but is usually 0.1 to 1000 μm, and preferably 10 to 300 μm.

[0027] [Negative electrode intermediate layer] The negative electrode intermediate layer is a layer adjacent to the surface of the solid electrolyte layer facing the negative electrode current collector, and includes a sheet-like substrate containing a fibrous carbon material. In this specification, the term "fibrous carbon material" refers to conductive carbon having an aspect ratio of 10 or more and a minimum Feret diameter of 0.3 μm or less in an image obtained by observing a cross section of the negative electrode intermediate layer using a scanning electron microscope (SEM). The aspect ratio is calculated by dividing the maximum Feret diameter by the minimum Feret diameter. The maximum Feret diameter is the maximum distance between two parallel lines when the contour of the fibrous carbon material is sandwiched between the lines, and the minimum Feret diameter is the minimum distance between the lines when the contour of the fibrous carbon material is sandwiched between the lines. The term "cross section of the layer" refers to a cross section of the layer cut in the stacking direction of a stacked secondary battery (i.e., a cross section of the layer cut in the thickness direction).

[0028] The specific form of the fibrous carbon material that is the constituent material of the sheet-like substrate that constitutes the negative electrode intermediate layer is not particularly limited, and conventionally known knowledge can be referred to. The electronic conductivity of the fibrous carbon material is preferably 1 S / m or more, and more preferably 1×10 ２ S / m or more is more preferable, and 1×10 ４ It is more preferable that the density is 1×10 S / m or more. ５ The upper limit of the electronic conductivity of the fibrous carbon material is not particularly limited, but is usually 1×10 ７ The volume resistivity of the negative electrode intermediate layer is not particularly limited, but is preferably 10 ２ In this specification, the volume resistivity of the negative electrode intermediate layer is a value measured using an electrode resistance measurement system (manufactured by Hioki E.E. Corporation, product name: RM2610).

[0029] The type of fibrous carbon material is not particularly limited as long as it has the above-mentioned shape, and examples thereof include carbon fiber (carbon nanofiber), graphene, and carbon nanotubes (single-walled carbon nanotubes and multi-walled carbon nanotubes). Of these, carbon fiber (carbon nanofiber) is preferred. One type of fibrous carbon material may be used alone, or two or more types may be used in combination.

[0030] The average opening diameter of the sheet-like substrate is preferably 100 nm or less, more preferably 90 nm or less, and even more preferably 80 nm or less. When the average opening diameter of the sheet-like substrate is a value within these ranges, the generation of lithium metal dendrites between the solid electrolyte layer and the negative electrode intermediate layer and the resulting decrease in cycle durability are effectively prevented. There is no particular restriction on the lower limit of the average opening diameter of the sheet-like substrate, but it is usually 20 nm or more. In addition, the value of the average opening diameter of the sheet-like substrate is a value measured by the method described in the Examples section below.

[0031] The negative electrode intermediate layer preferably contains at least one selected from the group consisting of metal materials that can be alloyed with lithium. By containing a metal material that can be alloyed with lithium in the negative electrode intermediate layer, lithium metal can be more uniformly deposited on the surface of the negative electrode current collector. Specific examples of metal materials that can be alloyed with lithium include indium (In), aluminum (Al), silicon (Si), tin (Sn), magnesium (Mg), gold (Au), silver (Ag), zinc (Zn), and alloys containing at least one of these.

[0032] On the other hand, the negative electrode intermediate layer may further contain, in addition to the metal material capable of alloying with lithium, a metal material other than the above-mentioned metal material capable of alloying with lithium (a metal material that does not alloy with lithium). Examples of metal materials that do not alloy with lithium include nickel (Ni) and copper (Cu). According to one embodiment, the negative electrode intermediate layer contains at least one selected from the group consisting of carbon materials capable of absorbing lithium ions and a metal material that does not alloy with lithium. In particular, from the viewpoint of more significantly exhibiting the effects of the present invention, the negative electrode intermediate layer preferably contains a simple element selected from the group consisting of Ag, Ni, Zn, Mg, Au, Al, In, and Si, more preferably at least one selected from the group consisting of Ag, Zn, Mg, and Al, even more preferably at least one selected from the group consisting of Ag, Zn, and Mg, and particularly preferably Ag.

[0033] The average primary particle diameter of the metal material is, for example, 500 nm or less, preferably 300 nm or less, more preferably 200 nm or less, even more preferably 100 nm or less, and particularly preferably 80 nm or less. The lower limit of the average primary particle diameter of the metal particles is not particularly limited, but is, for example, 10 nm or more, preferably 20 nm or more, even more preferably 30 nm or more, and even more preferably 40 nm or more. The average primary particle diameter of the carbon particles is, for example, 200 nm or less, preferably 150 nm or less, more preferably 100 nm or less, even more preferably 80 nm or less, even more preferably 60 nm or less, even more preferably 50 nm or less, and particularly preferably 45 nm or less. The lower limit of the average primary particle diameter of the carbon particles is not particularly limited, but is preferably 10 nm or more, more preferably 20 nm or more, and even more preferably 25 nm or more. When the average primary particle diameter of the metal particles and the average primary particle diameter of the carbon particles are within the above ranges, it becomes easy to control the porosity of the negative electrode intermediate layer, described later, within a predetermined range. In this specification, the average primary particle diameter of particles refers to the 50% cumulative diameter (D50) of particle diameters of the particles observed within several to several tens of fields of view when a cross section of a layer containing the particles is observed with a scanning electron microscope (SEM) (the maximum distance between any two points on the outline of the observed particles).

[0034] The negative electrode intermediate layer preferably contains a particulate carbon material in addition to a sheet-like substrate containing a fibrous carbon material. The inclusion of the particulate carbon material in the negative electrode intermediate layer can further suppress the precipitation and growth of lithium dendrites. Specific examples of particulate carbon materials include carbon black (specifically, acetylene black, Ketjen Black (registered trademark), furnace black, channel black, thermal lamp black, etc.), graphite, hard carbon, etc. Among these, the carbon material preferably contains at least one selected from the group consisting of carbon black, and more preferably contains at least one selected from the group consisting of acetylene black, Ketjen Black (registered trademark), furnace black, channel black, and thermal lamp black.

[0035] When the negative electrode intermediate layer contains a metal material, the mass ratio (carbon particles:metal particles) of the carbon material (total of fibrous carbon material and particulate carbon material) to the metal material contained in the negative electrode intermediate layer is preferably 10:1 to 1:1, more preferably 5:1 to 2:1, and even more preferably 4:1 to 2.5:1. When the blending ratio (mass ratio) of the carbon material to the metal material is within the above range, an all-solid-state battery with better charging resistance, cycle characteristics, and charge short circuit rate can be obtained. Furthermore, the volume of the metal material may change significantly (expand) when alloyed with lithium. Therefore, when both a carbon material and a metal material are used, it is preferable that the mass ratio of the carbon material to the mass of the metal material contained in the negative electrode intermediate layer exceeds 1. This makes it easier to maintain a predetermined porosity, and an all-solid-state battery with better cycle characteristics and charge short circuit rate can be obtained.

[0036] From the same viewpoint, it is preferable that the average primary particle diameter of the particulate carbon material is smaller than the average primary particle diameter of the metallic material. The ratio of the average primary particle diameter of the particulate carbon material to the average primary particle diameter of the metallic material is preferably less than 1, more preferably 0.9 or less, even more preferably 0.8 or less, and even more preferably 0.7 or less. This makes it easier to maintain a predetermined porosity, and an all-solid-state battery with even better cycle characteristics and short-circuit rate during charging can be obtained. Furthermore, from the viewpoint of easily controlling the porosity within a predetermined range, the ratio of the primary particle diameter of the particulate carbon material to the average primary particle diameter of the metallic material is, for example, 0.3 or more, more preferably 0.4 or more, and even more preferably 0.5 or more.

[0037] The negative electrode intermediate layer preferably further contains a binder. The type of binder is not particularly limited, and binders known in the art can be appropriately used. Examples of binders include polyvinylidene fluoride (PVDF), compounds in which hydrogen atoms of PVDF are substituted with other halogen elements, polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), and carboxymethyl cellulose (CMC). Among these, from the viewpoint of easily controlling the porosity and thickness of the negative electrode intermediate layer (described later) within a predetermined range, the binder preferably contains polyvinylidene fluoride (PVDF), and more preferably is polyvinylidene fluoride (PVDF).

[0038] The content of the binder in the negative electrode intermediate layer is, for example, 6% by mass or more, preferably 10% by mass or more, and more preferably 12% by mass or more, relative to a total of 100% by mass of the negative electrode intermediate layer (including the constituent materials of the solid electrolyte layer that has penetrated into the negative electrode intermediate layer). When the binder content is within the above range, it becomes easy to control the porosity of the negative electrode intermediate layer, which will be described later, within a predetermined range. There is no particular upper limit for the binder content, but from the viewpoint of suppressing an increase in resistance, it is preferably 20% by mass or less, more preferably 18% by mass or less, and even more preferably 16% by mass or less.

[0039] The porosity of the negative electrode intermediate layer is not particularly limited, but is preferably 10 to 70%, more preferably 10 to 60%, even more preferably 10 to 50%, still more preferably 10 to 40%, particularly preferably 10 to 30%, and most preferably 10 to 20%. This can further improve the cycle durability of the all-solid-state battery.

[0040] The thickness of the negative electrode intermediate layer is not particularly limited, but is preferably 100 μm or less, more preferably 50 μm or less, even more preferably 20 μm or less, particularly preferably 10 μm or less, and most preferably 3 μm or less. The thinner the negative electrode intermediate layer, the longer the path of lithium ions moving through the negative electrode intermediate layer during charging, thereby reducing charging resistance. Furthermore, the energy density of the all-solid-state battery can be improved. For example, changing the thickness of the negative electrode intermediate layer from 100 μm to 3 μm can improve the volumetric energy density by 1.8 times. The lower limit of the thickness of the negative electrode intermediate layer is not particularly limited, but is preferably 1.5 μm or more, more preferably 2 μm or more, from the viewpoint of ensuring the strength of the negative electrode intermediate layer. In this specification, the porosity and thickness of the negative electrode intermediate layer are measured by the following methods.

[0041] (Method for measuring porosity and thickness of negative electrode intermediate layer) The porosity of the negative electrode intermediate layer is a value calculated from the negative electrode intermediate layer in a post-discharge state, which is performed after the all-solid-state battery has been completely discharged, the cell is released from restraints, and the exterior body is removed, and can be calculated using a "3D-SEM."

[0042] Specifically, the power generating element was removed from the evaluation cell after full discharge, and a cross section (laminated cross section) perpendicular to the surface direction was exposed by ion milling. Using a focused ion beam processing-scanning electron microscope (FIB-SEM) manufactured by Hitachi High-Tech Science Corporation, an SEM photograph of the laminated cross section of the power generating element viewed from the front was taken, and a photograph of the surface portion of the negative electrode intermediate layer (5 μm x 5 μm size photograph) was taken. Thereafter, the surface of the negative electrode intermediate layer was irradiated with an ion beam to excavate the surface of the negative electrode intermediate layer, and a photograph of the surface of the negative electrode intermediate layer was taken again. The excavation of the negative electrode intermediate layer surface by the ion beam irradiation and the photographing of the negative electrode intermediate layer surface were repeated to obtain a group of 2D photographs of the surface of the negative electrode intermediate layer. Then, each 2D region in the group of 2D photographs obtained is identified, the area of ​​the voids present in the 2D region is calculated, and the area is integrated to calculate the volume of the voids in the 3D region.Finally, the void volume relative to the total volume of the 3D region is calculated to calculate the porosity.

[0043] In this case, the lithium metal present in the negative electrode intermediate layer is considered to be voids. Specifically, if lithium metal remains in the voids in the evaluation cell after discharge, the lithium metal is considered to be voids and the area is calculated. The lithium metal is identified by high-sensitivity EDS and then identified based on the contrast of the identified lithium metal, and its area is calculated. In addition, the cross section is observed using an SEM, and the thickness is measured at several to several tens of different locations in the negative electrode intermediate layer, and the arithmetic average value of these measurements is used as the thickness of the negative electrode intermediate layer.

[0044] (Characteristics of the Negative Electrode Intermediate Layer) As described above, the all-solid-state battery according to this embodiment is characterized in that the negative electrode intermediate layer, which includes a sheet-like substrate containing a fibrous carbon material, has a structure in which a portion of the solid electrolyte penetrates the surface of the sheet-like substrate facing the solid electrolyte layer without penetrating the sheet-like substrate. As long as the negative electrode intermediate layer has this configuration, there are no particular limitations on the specific form of penetration, the amount of solid electrolyte penetrating, or the degree of penetration. However, if the solid electrolyte penetrates almost the entire thickness of the sheet-like substrate (negative electrode intermediate layer), there is a risk that the solid electrolyte will penetrate the sheet-like substrate in the thickness direction. If the solid electrolyte penetrates the sheet-like substrate in the thickness direction, the adhesion between the negative electrode intermediate layer and the negative electrode current collector will decrease, making it difficult to maintain the potential, and the risk of short-circuiting due to the generation of dendrites will increase. Therefore, when the thickness of the sheet-like substrate (negative electrode intermediate layer) is taken as 100%, the solid electrolyte preferably penetrates to a depth of 1 to 80%, more preferably 10 to 50%, and even more preferably 20 to 40%. Penetration to a depth of 1% or more ensures the effects of the present invention, while penetration to a depth of 80% or less makes the above-mentioned problems less likely to occur. Although the anode intermediate layer having the above-mentioned configuration is novel, the inventors, in the process of completing the present invention, had thought that obtaining an anode intermediate layer having the above-mentioned configuration using the above-mentioned manufacturing method would be difficult because the slurry containing the solid electrolyte would penetrate the sheet-like substrate. However, upon actual experiments, they surprisingly found that it was possible to manufacture an anode intermediate layer without the slurry containing the solid electrolyte layer penetrating the sheet-like substrate, leading to the completion of the present invention.

[0045] (Method for Producing Negative Electrode Intermediate Layer) To produce the negative electrode intermediate layer in the all-solid-state battery according to one embodiment of the present invention, for example, first, a slurry containing the constituent materials of the negative electrode intermediate layer (a fibrous carbon material, and optionally a metal material, a particulate carbon material, and a binder) is applied to the surface of a support and dried to obtain a coating film (negative electrode intermediate layer precursor). In this case, a negative electrode current collector may be used as the support. Next, a slurry containing the constituent materials of the solid electrolyte layer (a solid electrolyte and optionally a binder) is applied to one surface of the coating film (negative electrode intermediate layer precursor) obtained above and dried to obtain a negative electrode intermediate layer. In this case, the amount and degree of penetration of the slurry into the coating film (negative electrode intermediate layer precursor) can be controlled by adjusting the viscosity and coating weight of the solid electrolyte-containing slurry. Therefore, according to another embodiment of the present invention, a method for producing a laminate is also provided, which includes applying a slurry containing a solid electrolyte to one surface of a sheet-like substrate containing a fibrous carbon material in such a manner that the solid electrolyte does not penetrate the sheet-like substrate. According to yet another aspect of the present invention, a laminate is also provided as an article produced by such a production method, the laminate having a solid electrolyte layer containing a solid electrolyte and an anode intermediate layer adjacent to the solid electrolyte layer, the anode intermediate layer having a structure in which a portion of the solid electrolyte penetrates into the surface of a sheet-like substrate containing a fibrous carbon material facing the solid electrolyte layer without penetrating the sheet-like substrate. This laminate can be used to produce an all-solid-state battery. The above-mentioned production method has the advantage of being able to produce an all-solid-state battery according to one aspect of the present invention by a simple technique.

[0046] Furthermore, when a laminate is produced using the above-described production method, the average aperture ratio of the sheet-like substrate and the porosity of the negative electrode intermediate layer can be controlled by changing the type of fibrous carbon material and other materials contained in the slurry containing the constituent materials of the negative electrode intermediate layer. Furthermore, in the above-described production method, the porosity and thickness of the negative electrode intermediate layer can be controlled by further subjecting the laminate to a press treatment.

[0047] There is no particular limitation on the method for producing an all-solid-state battery using the above-mentioned laminate. A power generating element for an all-solid-state battery can be obtained by arranging a separately produced cathode (a laminate of a cathode current collector and a cathode active material layer) so that the exposed surface of the cathode active material layer of the above-mentioned laminate faces the exposed surface of the solid electrolyte layer, and bonding them together using a method such as cold isostatic pressing (CIP).

[0048] <Method of Using All-Solid-State Battery> An all-solid-state battery according to one embodiment of the present invention can be used in the same manner as conventionally known all-solid-state batteries. However, in a preferred embodiment, the all-solid-state battery is intended for use in charge / discharge cycles, including charging until the negative electrode capacity of the battery exceeds the negative electrode capacity of the negative electrode intermediate layer. During this charging / discharging process, the portion of the negative electrode capacity of the battery corresponding to the negative electrode capacity of the negative electrode intermediate layer is absorbed in a material (carbon material or a metal material capable of being alloyed with lithium) corresponding to the negative electrode active material contained in the negative electrode intermediate layer. Meanwhile, the portion of the negative electrode capacity of the battery that exceeds the negative electrode capacity of the negative electrode intermediate layer is precipitated between the negative electrode intermediate layer and the negative electrode current collector. This is preferable because it provides the effect of the present invention, which is to solve the problems associated with lithium deposition-type all-solid-state batteries. Specific charging conditions (current density, charging mode, etc.) are not particularly limited, and conventionally known knowledge can be referenced.

[0049] Although one embodiment of the all-solid-state battery of the present invention has been described above, the present invention is not limited to the configuration described in the above embodiment, and can be modified as appropriate based on the claims.

[0050] For example, the solid electrolyte layer of the all-solid-state battery according to this embodiment may further contain a conventionally known liquid electrolyte (electrolytic solution). There is no particular limitation on the amount of liquid electrolyte (electrolytic solution) that can be contained in the solid electrolyte layer; however, it is preferable that the amount be such that the shape of the solid electrolyte layer formed by the solid electrolyte is maintained and leakage of the liquid electrolyte (electrolytic solution) does not occur. As the liquid electrolyte (electrolytic solution), a solution in the form of a conventionally known lithium salt dissolved in a conventionally known organic solvent is used. The liquid electrolyte (electrolytic solution) may further contain additives other than the organic solvent and the lithium salt. These additives may be used alone or in combination of two or more. Furthermore, when an additive is used in the electrolyte solution, the amount used can be appropriately adjusted.

[0051] The following embodiments are also included within the scope of the present invention: an all-solid-state battery according to claim 1 having the features of claim 2; an all-solid-state battery according to claim 1 or 2 having the features of claim 3; an all-solid-state battery according to any one of claims 1 to 3 having the features of claim 4; an all-solid-state battery according to any one of claims 1 to 4 having the features of claim 5; an all-solid-state battery according to any one of claims 1 to 5 having the features of claim 6; and an all-solid-state battery according to any one of claims 1 to 6 having the features of claim 7.

[0052] The present invention will be described in more detail below with reference to examples. However, the technical scope of the present invention is not limited to the following examples. Note that the following operations were performed in a glove box. Furthermore, the instruments and devices used in the glove box were thoroughly dried beforehand.

[0053] <Preparation of Evaluation Cell> [Comparative Example 1] (Preparation of Positive Electrode) First, LiNi as a positive electrode active material was prepared. ０．８ Mn ０．１ Co ０．１ O ２ , acetylene black as a conductive additive, and an argyrodite-type sulfide solid electrolyte (Li ６ P.S. ５Cl) were weighed out to a mass ratio of 50:30:20, mixed in an agate mortar, and then further mixed and stirred in a planetary ball mill. 2 parts by mass of styrene-butadiene rubber (SBR) was added to 100 parts by mass of the obtained mixed powder, and mesitylene was added as a solvent to prepare a positive electrode active material slurry. Next, the positive electrode active material slurry prepared above was applied to the surface of aluminum foil as a positive electrode current collector, dried, and pressed to form a positive electrode active material layer (thickness 80 μm), thereby producing a positive electrode.

[0054] (Preparation of solid electrolyte layer) Argyrodite-type sulfide solid electrolyte (Li ６ P.S. ５ A solid electrolyte slurry was prepared by adding 3 parts by mass of styrene-butadiene rubber (SBR) to 97 parts by mass of ethylenediaminetetraacetic acid (EPO) (Cl), and adding mesitylene as a solvent. The solid electrolyte slurry prepared above was then applied to the surface of a stainless steel foil as a support, and dried to form a solid electrolyte layer (basis weight when dried: 10 mg / cm) as a free-standing membrane. ２ ) was prepared.

[0055] (Preparation of Negative Electrode Intermediate Layer) Carbon black (CB) and silver (Ag) nanoparticles with an average primary particle diameter of 60 nm were weighed and mixed to a CB:Ag mass ratio of 3:1. To 86 parts by mass of the resulting mixture, 14 parts by mass of polyvinylidene fluoride (PVDF) as a binder was added, and N-methyl-2-pyrrolidone was added as a solvent and mixed to prepare a negative electrode intermediate layer slurry. The negative electrode intermediate layer slurry was applied to the surface of a stainless steel foil as a negative electrode current collector and dried to prepare a negative electrode intermediate layer (thickness 5 μm).

[0056] (Preparation of Evaluation Cell) The solid electrolyte layer prepared above was transferred to the positive electrode active material layer side of the positive electrode prepared above by cold isostatic pressing (CIP; pressure 200 MPa) so that one surface of the solid electrolyte layer faced the positive electrode active material layer. Thereafter, the negative electrode intermediate layer (formed on a stainless steel foil) prepared above was attached by cold isostatic pressing (CIP; pressure 200 MPa) so that the exposed surface of the negative electrode intermediate layer faced the other surface of the solid electrolyte layer, thereby preparing an evaluation cell (lithium deposition type all-solid-state lithium secondary battery) for this comparative example.

[0057] [Example 1] (Preparation of Sheet-Like Substrate) Carbon nanotubes (CNT) with a diameter of 10 to 40 nm and silver nanoparticles with an average primary particle diameter of 60 nm were weighed to a mass ratio of CNT:Ag = 3:1, and a binder solution (PVDF) was added and mixed to a solid content of 5% by mass. The mixture was applied to a stainless steel foil as a negative electrode current collector using an applicator and dried to obtain a CNT sheet. The binder content in the solid content was 14% by mass. The obtained CNT sheet was then pressed using a roll press to prepare a sheet-like substrate (thickness after drying: 3 μm, porosity: 60%, average opening diameter: 100 nm) constituting the negative electrode intermediate layer. The average opening diameter was determined by observing the surface of the sheet-like substrate in several fields using a scanning electron microscope (SEM), calculating the area-equivalent circle diameter for each of the observed openings, and then calculating the arithmetic average of these values.

[0058] (Preparation of Solid Electrolyte Layer) A solid electrolyte slurry was prepared by the same method as in Comparative Example 1. Next, the prepared solid electrolyte slurry was applied to the exposed surface of the sheet-like substrate prepared above, and dried to form a solid electrolyte layer (basis weight when dried: 10 mg / cm ２ After the fabrication, a cross section of the laminate in the stacking direction was photographed using a scanning electron microscope (SEM), and it was observed that a part of the solid electrolyte constituting the solid electrolyte layer had penetrated into the negative electrode intermediate layer (sheet-like substrate) partway in the stacking direction (to a depth of about 30% of the thickness of the negative electrode active material layer).

[0059] (Preparation of Evaluation Cell) The same solid electrolyte layer / negative electrode intermediate layer / negative electrode current collector laminate prepared above was attached to the positive electrode active material layer side of the positive electrode prepared above by cold isostatic pressing (CIP; pressure: 200 MPa) so that the exposed surface of the solid electrolyte layer faced the positive electrode active material layer, thereby preparing an evaluation cell (lithium deposition-type all-solid-state lithium secondary battery) of this example.

[0060] [Example 2] (Preparation of sheet-like substrate) An equal mass mixture of carbon nanofibers (CNF) and carbon black having an average fiber diameter of 300 nm and an average fiber length of 15 μm and silver nanoparticles having an average primary particle diameter of 60 nm was weighed to a mass ratio of (CNF + CB): Ag = 3: 1, and a binder solution (PVDF) was added and mixed to a solid content of 5 mass%, and the mixture was applied to a stainless steel foil as a negative electrode current collector using an applicator and dried to prepare a sheet-like substrate (thickness after drying: 100 μm, porosity: 60%, average opening diameter: 100 nm) made of a CNF sheet. The binder content in the solid content was 14 mass%.

[0061] Except for using the sheet-like substrate prepared above, an evaluation cell (lithium deposition-type all-solid-state lithium secondary battery) for this example was prepared by the same method as in Example 1. After preparation, a cross section of the interface between the solid electrolyte layer and the negative electrode intermediate layer in the stacking direction was photographed using a scanning electron microscope (SEM). It was observed that part of the solid electrolyte constituting the solid electrolyte layer had penetrated into the sheet-like substrate (negative electrode intermediate layer) in the stacking direction (to a depth of about 30% of the thickness of the negative electrode active material layer) (the same applies to Examples 3 and 4 below).

[0062] [Example 3] Argyrodite-type sulfide solid electrolyte (Li ６ P.S. ５ An evaluation cell of this example (lithium deposition-type all-solid-state lithium secondary battery) was produced in the same manner as in Example 1 described above, except that the mass ratio of the polymer electrolyte membrane (PET) to styrene-butadiene rubber (SBR) was changed from 97:3 to 98:2.

[0063] Example 4 Carbon nanotubes (CNTs) having a diameter of 10 to 40 nm were subjected to a densification treatment for 10 minutes at a peripheral speed of 10,000 rpm in an argon atmosphere using a hybridization device (manufactured by Nara Machinery Co., Ltd.).

[0064] Next, the densified CNTs were used, and the argyrodite-type sulfide solid electrolyte (Li ６ P.S. ５An evaluation cell of this example (lithium deposition-type all-solid-state lithium secondary battery) was produced in the same manner as in Example 1 described above, except that the mass ratio of the polymer electrolyte membrane (PET) to styrene-butadiene rubber (SBR) was changed from 97:3 to 98:2.

[0065] <Evaluation of Cycle Durability of Evaluation Cell> A positive electrode lead and a negative electrode lead were connected to the positive electrode current collector and the negative electrode current collector, respectively, of the evaluation cell (before the first charge), and a cycle test was performed while applying a restraining pressure of 2 MPa in the stacking direction of the evaluation cell using a pressure member. The cycle test was performed at 60 °C, with a cell voltage ranging from 3.0 V to 4.25 V, and 20 cycles of charge / discharge (charge CC mode, discharge CC mode) at a charge / discharge rate of 2C. A 30-minute rest period was provided between each charge and discharge. Six identical evaluation cells were prepared according to the method of each experimental example, and the percentage of evaluation cells that could be charged / discharged 20 times was calculated as the survival rate. The results are shown in Table 1 below.

[0066]

[0067] The results shown in Table 1 show that the cycle durability of all-solid-state batteries is improved by employing a method of transferring a solid electrolyte slurry to the surface of a negative electrode intermediate layer made of a sheet-like substrate containing a fibrous carbon material when fabricating a solid electrolyte layer. This is thought to be because, as a result of employing this fabrication method, part of the solid electrolyte penetrates into the solid electrolyte layer-side surface of the sheet-like substrate constituting the negative electrode intermediate layer without penetrating the sheet-like substrate, preventing the formation of lithium metal dendrites between the solid electrolyte layer and the negative electrode intermediate layer and suppressing the occurrence of short circuits as charge-discharge cycles progress.

[0068] REFERENCE SIGNS LIST 10a laminated secondary battery, 11' negative electrode current collector, 11" positive electrode current collector, 13 negative electrode active material layer, 14 negative electrode intermediate layer, 15 positive electrode active material layer, 17 solid electrolyte layer, 19 single cell layer, 21 power generating element, 25 negative electrode current collector, 27 positive electrode current collector, 29 laminate film.

Claims

1. An all-solid-state battery comprising a power generating element including: a positive electrode having a positive electrode active material layer containing a positive electrode active material; a negative electrode having a negative electrode current collector on which lithium metal is deposited during charging; a solid electrolyte layer interposed between the positive electrode and the negative electrode and containing a solid electrolyte; and a negative electrode intermediate layer present adjacent to a surface of the solid electrolyte layer facing the negative electrode current collector and including a sheet-like substrate containing a fibrous carbon material, wherein the negative electrode intermediate layer has a structure in which a portion of the solid electrolyte penetrates into the surface of the sheet-like substrate facing the solid electrolyte layer in a manner such that the portion does not penetrate the sheet-like substrate.

2. The all-solid-state battery according to claim 1, wherein the porosity of the negative electrode intermediate layer is 10 to 60%.

3. The all-solid-state battery according to claim 1 or 2, wherein the average opening diameter of the sheet-like substrate is 100 nm or less.

4. The all-solid-state battery according to claim 1 or 2, wherein the negative electrode intermediate layer has a thickness of 100 μm or less.

5. The all-solid-state battery according to claim 1 or 2, wherein the negative electrode intermediate layer further contains a simple substance of an element selected from the group consisting of Ag, Ni, Zn, Mg, Au, Al, In and Si.

6. The all-solid-state battery according to claim 1 or 2, wherein the solid electrolyte comprises a sulfide solid electrolyte.

7. The all-solid-state battery according to claim 1 or 2, for use in charging and discharging including charging until the negative electrode capacity of the battery exceeds the negative electrode capacity of the negative electrode intermediate layer.

8. A laminate comprising: a solid electrolyte layer containing a solid electrolyte; and an anode intermediate layer adjacent to the solid electrolyte layer, wherein the anode intermediate layer has a structure in which a part of the solid electrolyte penetrates into a surface of a sheet-like substrate containing a fibrous carbon material on the solid electrolyte layer side in a manner not to penetrate the sheet-like substrate.

9. A method for producing a laminate, comprising applying a slurry containing a solid electrolyte to one surface of a sheet-like substrate containing a fibrous carbon material in such a manner that the solid electrolyte does not penetrate the sheet-like substrate.

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