Lithium secondary battery

Abstract

To provide means for improving the discharge rate characteristics of a lithium secondary battery that uses a solid electrolyte and employs lithium deposition.SOLUTION: The lithium secondary battery includes a power generation element having: 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, in 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 intermediate layer adjacent to a surface of the solid electrolyte layer on a side of the negative electrode current collector and containing conductive particles. The negative intermediate layer has a pore volume increasing region in which a pore volume decreases as separating away from the solid electrolyte layer in a stacking direction of a power generation element.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a lithium secondary battery.

Background Art

[0002] In recent years, research and development on secondary batteries using oxide-based or sulfide-based solid electrolytes as electrolytes have been actively conducted. A solid electrolyte is a material mainly composed of an ion conductor capable of ion conduction in a solid.

[0003] As one type of secondary battery using a solid electrolyte, a so-called lithium deposition type in which lithium metal is deposited on a negative electrode current collector during the charging process is known. In such a lithium deposition type lithium secondary battery using a solid electrolyte, it is known that the lithium metal deposited during the charging process ionizes during the discharging process, causing voids in the lithium metal, which can lead to a decrease in the discharge capacity. For the purpose of suppressing the generation of such voids, generally, a restraining pressure is applied in the stacking direction of the battery in a lithium deposition type lithium secondary battery using a solid electrolyte. However, the presence of an end plate or the like for applying the restraining pressure may be an obstacle to the thinning of the lithium deposition type lithium secondary battery using a solid electrolyte.

[0004] As a means of suppressing the generation of voids accompanying discharge without applying a high restraining pressure, Patent Document 1 discloses a technique of providing a negative electrode active material layer (negative electrode intermediate layer) containing Ag between the negative electrode current collector and the solid electrolyte layer. By adopting such a configuration, Li is deposited as a Li(Ag) alloy in which Ag is solid-dissolved during the charging process. Then, only Li dissolves from the Li(Ag) alloy during the discharging process, and the solid-dissolved Ag remains, so that it is said that the generation of voids can be suppressed.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

SUMMARY OF THE INVENTION

PROBLEM TO BE SOLVED BY THE INVENTION

[0006] However, as a result of investigations by the present inventors, it has been found that even in the lithium precipitation type lithium secondary battery provided with the negative electrode intermediate layer described in Patent Document 1 above, the discharge rate characteristics may be insufficient.

[0007] Therefore, an object of the present invention is to provide a means capable of improving the discharge rate characteristics in a lithium precipitation type lithium secondary battery using a solid electrolyte.

MEANS FOR SOLVING THE PROBLEM

[0008] The present inventors have conducted intensive studies to solve the above problems. As a result, in a lithium secondary battery provided with a lithium precipitation type power generation element, a negative electrode intermediate layer containing conductive particles is provided between the negative electrode current collector and the solid electrolyte layer, and the porosity of the negative electrode intermediate layer is controlled so that the porosity decreases as it moves away from the solid electrolyte layer along the stacking direction of the power generation element. Thus, the present inventors have found that the above problems can be solved and have completed the present invention.

[0009] That is, one embodiment of the present invention is a lithium secondary battery 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 in which lithium metal precipitates 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 the surface of the solid electrolyte layer on the negative electrode current collector side and containing conductive particles, wherein the negative electrode intermediate layer has a porosity gradually decreasing region in which the porosity decreases as it moves away from the solid electrolyte layer along the stacking direction of the power generation element.

EFFECTS OF THE INVENTION

[0010] According to the present invention, in a lithium deposition type lithium secondary battery using a solid electrolyte, the discharge rate characteristics can be improved.

Brief Description of Drawings

[0011]

Figure 1

Embodiments for Carrying Out the Invention

[0012] One embodiment of the present invention is a lithium secondary battery 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 the surface of the solid electrolyte layer on the negative electrode current collector side and containing conductive particles, wherein the negative electrode intermediate layer has a porosity gradually decreasing region in which the porosity decreases as it moves away from the solid electrolyte layer along the stacking direction of the power generation element. According to this embodiment, in a lithium deposition type lithium secondary battery using a solid electrolyte, the discharge rate characteristics can be improved.

[0013] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. In the description of the drawings, the same reference numerals are given to the same elements, and redundant descriptions are omitted. Also, the dimensional ratios in the drawings are exaggerated for the convenience of explanation and may be different from the actual ratios.

[0014] FIG. 1 is a cross-sectional view schematically showing the overall structure of a laminated (internally parallel-connected type) all-solid-state lithium secondary battery (hereinafter also simply referred to as "laminated secondary battery") according to an embodiment of the present invention. Note that FIG. 1 shows a cross-section of the laminated secondary battery during charging. The laminated secondary battery 10a shown in FIG. 1 has a structure in which a substantially rectangular power generation element 21 where charge and discharge reactions actually proceed is sealed inside a laminate film 29 which is a battery exterior body. Here, the power generation element 21 has a configuration in which a negative electrode, a solid electrolyte layer 17, and a positive electrode are laminated. 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 laminated. And a negative electrode intermediate layer 14 is disposed adjacent to the surface of the negative electrode active material layer 13 facing the solid electrolyte layer 17. 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". And the negative electrode intermediate layer 14 and the positive electrode active material layer 15 face each other with the solid electrolyte layer 17 interposed therebetween, and the negative electrode, the solid electrolyte layer, and the positive electrode are laminated in this order. Thereby, an adjacent negative electrode, solid electrolyte layer, and positive electrode constitute one single cell layer 19. Therefore, it can be said that the laminated secondary battery 10a shown in FIG. 1 has a configuration in which a plurality of single cell layers 19 are laminated and electrically connected in parallel. A negative electrode current collector plate 25 and a positive electrode current collector plate 27 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 led out to the outside of the laminate film 29 so as to be sandwiched between the ends of the laminate film 29. A restraining pressure is applied to the power generation element 21 in the lamination direction by a pressing member (not shown) to the laminated secondary battery 10a. Therefore, the volume of the power generation element 21 is kept constant.

[0015] Hereinafter, the main constituent members of the lithium secondary battery according to this embodiment will be described.

[0016] [Current Collector] The current collector (negative electrode current collector, positive electrode current collector) has a function of mediating the movement of electrons from the electrode active material layer (negative electrode active material layer, positive electrode active material layer). There is no particular limitation on the material constituting the current collector. As the constituent material of the current collector, for example, metals such as aluminum, nickel, iron, stainless steel, titanium, copper, etc., and conductive resins can be adopted. There is also no particular limitation on the thickness of the current collector, but as an example, it is 10 to 100 μm.

[0017] [Negative electrode active material layer] The lithium secondary battery according to this embodiment is a so-called lithium deposition type that deposits lithium metal on the negative electrode during the charging process. The layer composed of lithium metal deposited on the negative electrode during this charging process is the negative electrode active material layer of the lithium secondary battery according to this embodiment. Therefore, as the charging process progresses, the thickness of the negative electrode active material layer increases, and as the discharging process progresses, the thickness of the negative electrode active material layer decreases. At the time of full discharge, the negative electrode active material layer may not exist, but in some cases, a negative electrode active material layer composed of a certain amount of lithium metal may be arranged at the time of full discharge. Also, the thickness of the negative electrode active material layer (lithium metal layer) at the time of full charge is not particularly limited, but is usually 0.1 to 1000 μm.

[0018] [Negative electrode intermediate layer] The negative electrode intermediate layer is a layer that exists adjacent to the surface of the solid electrolyte layer on the negative electrode current collector side and contains conductive particles. Note that it is preferable that the negative electrode intermediate layer has conductivity as a whole. The volume resistivity of the negative electrode intermediate layer is not particularly limited, but is preferably 10 2 Ω·cm or less, and more preferably 10 Ω·cm or less. In this specification, the volume resistivity of the negative electrode intermediate layer is the value measured using an electrode resistance measurement system (manufactured by Hioki E.E. Corporation, product name: RM2610).

[0019] The negative electrode intermediate layer preferably contains at least one kind of metal particle as the conductive particle. By containing the metal particle in the negative electrode intermediate layer, lithium metal can be deposited more uniformly on the surface of the current collector. Specific examples of the metal material constituting the metal particle include, for example, indium (In), aluminum (Al), silicon (Si), tin (Sn), magnesium (Mg), gold (Au), silver (Ag), zinc (Zn), nickel (Ni), copper (Cu), and alloys containing at least one of these. Among them, the metal particle preferably contains at least one selected from the group consisting of In, Al, Si, Sn, Mg, Au, Ag, and Zn, more preferably contains at least one selected from the group consisting of Ag, Mg, Zn, and Al, still more preferably contains at least one selected from the group consisting of Ag, Mg, and Zn, and particularly preferably contains Ag.

[0020] Instead of or in addition to containing at least one kind of metal particle as the conductive particle, the negative electrode intermediate layer preferably contains at least one kind of carbon particle. By containing the carbon particle in the negative electrode intermediate layer, the precipitation and growth of lithium dendrites can be suppressed. Specific examples of the carbon material constituting the carbon particle include carbon black (specifically, acetylene black, Ketjenblack (registered trademark), furnace black, channel black, thermal lamp black, etc.), carbon nanotube (CNT), graphite, hard carbon, etc. Among them, the carbon particle preferably contains at least one selected from the group consisting of carbon black, more preferably contains at least one selected from the group consisting of acetylene black, Ketjenblack (registered trademark), furnace black, channel black, and thermal lamp black.

[0021] In addition, when the negative electrode intermediate layer is composed of a plurality of layers as described later, the conductive particles contained in each layer may be the same as or different from each other.

[0022] According to a preferred embodiment, the negative electrode intermediate layer contains at least one kind of metal particles and at least one kind of carbon particles. By forming the negative electrode intermediate layer using both metal particles and carbon particles, the effects of the present invention can be obtained more remarkably. When the negative electrode intermediate layer is composed of a plurality of layers as described later, it is preferable that at least one of these layers contains at least one kind of metal particles and / or at least one kind of carbon particles, and it is more preferable that all layers contain at least one kind of metal particles and / or at least one kind of carbon particles. Further, when the negative electrode intermediate layer is composed of a plurality of layers, it is preferable that at least one of these layers contains both metal particles and carbon particles, and it is more preferable that all layers contain both metal particles and carbon particles.

[0023] The ratio of the total mass of the metal particles and the carbon particles to the total mass of the conductive particles in the negative electrode intermediate layer is, for example, 95% by mass or more, preferably 98% by mass or more, more preferably 99% by mass or more, and particularly preferably 100% by mass. When the negative electrode intermediate layer is composed of a plurality of layers, it is preferable that the ratio of the total mass of the metal particles and the carbon particles is within the above range in at least one of these layers, and it is more preferable that the ratio is within the above range in all layers.

[0024] The average primary particle diameter of the metal particles 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, more preferably 30 nm or more, and even more preferably 40 nm or more. Further, 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, still 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 still more preferably 25 nm or more. When the average primary particle diameters of the metal particles and the carbon particles are within the above ranges, it becomes easy to control the porosity in the negative electrode intermediate layer. In the present specification, the average primary particle diameter of the particles refers to the 50% cumulative diameter (D50) of the particle diameters of the particles observed in several to several tens of fields when the cross section of the layer containing the particles is observed by a scanning electron microscope (SEM) (the maximum distance among the distances between any two points on the contour line of the observed particles).

[0025] When the negative electrode intermediate layer is composed of a plurality of layers, it is preferable that the average primary particle diameters of the metal particles and / or the carbon particles contained in at least one of these layers are within the above ranges, and it is more preferable that the average primary particle diameters of the metal particles and / or the carbon particles contained in all the layers are within the above ranges.

[0026] When using both carbon particles and metal particles, the mass ratio of carbon particles to metal particles (carbon particles: metal particles) is preferably from 10:1 to 1:1, more preferably from 5:1 to 2:1, and still more preferably from 4:1 to 2.5:1. The volume ratio of carbon particles to metal particles (carbon particles: metal particles) is preferably from 99:1 to 70:30, more preferably from 95:5 to 75:25. When the mixing ratio (mass ratio or volume ratio) of carbon particles and metal particles is within the above range, the effects of the present invention can be obtained more remarkably. When the negative electrode intermediate layer is composed of a plurality of layers, it is preferable that the mixing ratio (mass ratio or volume ratio) of carbon particles and metal particles contained in at least one of these layers is within the above range, and it is more preferable that the mixing ratio (mass ratio or volume ratio) of carbon particles and metal particles contained in all layers is within the above range.

[0027] In addition, the volume of metal particles may change (expand) significantly when alloyed with lithium. Therefore, when using both carbon particles and metal particles, it is preferable that the ratio of the mass of carbon particles to the mass of metal particles contained in the negative electrode intermediate layer (mass of carbon particles / mass of metal particles) exceeds 1. This makes it easier to maintain the porosity of the negative electrode intermediate layer, and the effects of the present invention can be obtained more remarkably. When the negative electrode intermediate layer is composed of a plurality of layers, it is preferable that the ratio of the mass of carbon particles to the mass of metal particles contained in at least one of these layers is within the above range.

[0028] As described later, in the lithium secondary battery of this embodiment, the porosity of the negative electrode intermediate layer gradually decreases as it moves away from the solid electrolyte layer along the stacking direction of the power generation elements. Along with this, the content of each component constituting the negative electrode intermediate layer may also change along the stacking direction of the power generation elements, but unless otherwise specified, the content of each component means the value of the content of each component contained in the entire negative electrode intermediate layer.

[0029] From the same perspective, it is preferable that the average primary particle diameter of the carbon particles is smaller than that of the metal particles. The ratio of the average primary particle diameter of the carbon particles to the average primary particle diameter of the metal particles (average primary particle diameter of carbon particles / average primary particle diameter of metal particles) is preferably less than 1, more preferably 0.9 or less, still more preferably 0.8 or less, and even more preferably 0.7 or less. Thereby, it is easier to maintain the porosity of the negative electrode intermediate layer, and the effects of the present invention can be obtained more remarkably. Also, from the perspective of facilitating the control of the porosity, the ratio of the average primary particle diameter of the carbon particles to the average primary particle diameter of the metal particles is, for example, 0.3 or more, more preferably 0.4 or more, and still more preferably 0.5 or more. When the negative electrode intermediate layer is composed of a plurality of layers, it is preferable that at least one of these layers has the above configuration, and it is more preferable that all layers have the above configuration.

[0030] According to a preferred embodiment, the ratio of the mass of the carbon particles to the mass of the metal particles (mass of carbon particles / mass of metal particles) contained in the negative electrode intermediate layer exceeds 1, and the ratio of the average primary particle diameter of the carbon particles to the average primary particle diameter of the metal particles (average primary particle diameter of carbon particles / average primary particle diameter of metal particles) is less than 1. Thereby, it is easier to maintain the porosity of the negative electrode intermediate layer, and the effects of the present invention can be obtained more remarkably. When the negative electrode intermediate layer is composed of a plurality of layers, it is preferable that at least one of these layers has the above configuration, and it is more preferable that all layers have the above configuration.

[0031] The negative electrode intermediate layer preferably further contains a binder. The type of the binder is not particularly limited, and those known in the art can be appropriately adopted. Examples of the binder include polyvinylidene fluoride (PVDF), a compound in which a hydrogen atom of PVDF is substituted with another halogen element, polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), and carboxymethyl cellulose (CMC). Among them, from the viewpoint of facilitating the control of the porosity and thickness of the negative electrode intermediate layer described later, the binder preferably contains polyvinylidene fluoride (PVDF), and more preferably is polyvinylidene fluoride (PVDF). When the negative electrode intermediate layer is composed of a plurality of layers, the binders contained in each layer may be the same as or different from each other.

[0032] The content of the binder in the negative electrode intermediate layer is, for example, 8% by mass or more, preferably 10% by mass or more, and more preferably 12% by mass or more with respect to 100% by mass of the total mass of the negative electrode intermediate layer. When the content of the binder is within the above range, it becomes easy to control the porosity of the negative electrode intermediate layer. The upper limit of the content of the binder is not particularly limited, 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 still more preferably 16% by mass or less. When the negative electrode intermediate layer is composed of a plurality of layers, it is preferable that the content of the binder is within the above range in at least one of them, and more preferably within the above range in all layers.

[0033] The ratio of the total surface area of the conductive particles to the mass of the binder contained in the negative electrode intermediate layer (total surface area of the conductive particles / mass of the binder) is not particularly limited, but is preferably 800 m 2 / g or less. When the value of the above (total surface area of the conductive particles / mass of the binder) is 800 m 2 / g or less, since a sufficient amount of the binder exists on the surface of the conductive particles (for example, metal particles and / or carbon particles), the adhesive force between these particles is increased. As a result, it becomes easy to control the porosity of the negative electrode intermediate layer. The value of the above (total surface area of the conductive particles / mass of the binder) is 700 m2 It is more preferably below / g, and 600m 2 It is even more preferably below / g, and 500m 2 It is even more preferably below / g, and 450m 2 It is particularly preferably below / g. Further, the value of (total surface area of conductive particles / mass of binder) is, for example, 10m 2 / g or more, preferably 100m 2 / g or more, more preferably 200m 2 / g or more, even more preferably 300m 2 / g or more. When within the above range, the energy density of the lithium secondary battery can be improved. Note that the value of (total surface area of conductive particles / mass of binder) is calculated as follows: For each type of particle, the volume per particle is determined as a spherical particle having a radius of the average primary particle diameter / 2. Based on the blending ratio and specific gravity of each component contained in the negative electrode intermediate layer, the number of particles contained in the negative electrode intermediate layer per unit mass is calculated, and this is multiplied by the surface area per particle of the above spherical particle to obtain the total surface area for each type of particle. Then, the total surface areas for each type of particle are added together to obtain the total surface area, and this is divided by the mass of the binder contained in the negative electrode intermediate layer per unit mass to obtain the above (total surface area of conductive particles / mass of binder). Specifically, it can be obtained by the method described in the examples below.

[0034] Here, when the negative electrode intermediate layer is composed of a plurality of layers, in at least one of them, it is preferable that the value of (total surface area of conductive particles / mass of binder) is within the above range, and it is more preferable that it is within the above range in all layers.

[0035] In a preferred embodiment, the average primary particle diameter of the metal particles or carbon particles contained in the negative electrode intermediate layer is 30 nm or more, and the content of the binder can be 10 parts by mass or more with respect to 100 parts by mass of the total mass of the negative electrode intermediate layer. When the negative electrode intermediate layer is composed of a plurality of layers, it is preferable that at least one of them has the above configuration, and it is more preferable that all layers have the above configuration.

[0036] The ratio of the mass of the metal particles to the total mass of the negative electrode intermediate layer is, for example, 15 to 50% by mass, preferably 20 to 45% by mass. When the negative electrode intermediate layer is composed of a plurality of layers, it is preferable that the ratio of the mass of the metal particles is within the above range in at least one of these layers, and more preferably within the above range in all layers.

[0037] The ratio of the total mass of the metal particles, carbon particles, and binder to the total mass of the negative electrode intermediate layer is preferably 90% by mass or more, more preferably 95% by mass or more, still more preferably 98% by mass or more, particularly preferably 99% by mass or more, and most preferably 100% by mass. When the negative electrode intermediate layer is composed of a plurality of layers, it is preferable that the ratio of the total mass of the metal particles, carbon particles, and binder to the total mass of the negative electrode intermediate layer is within the above range in at least one of these layers, and more preferably within the above range in all layers.

[0038] The lithium secondary battery according to this embodiment is characterized in that the negative electrode intermediate layer has a porosity gradually decreasing region in which the porosity decreases as it moves away from the solid electrolyte layer along the stacking direction of the power generation elements. In a lithium precipitation type lithium secondary battery, in the discharge process, it is considered that lithium ions are conducted from the negative electrode active material layer through the lithium metal precipitated in the negative electrode intermediate layer and return to the positive electrode side. In the negative electrode intermediate layer having the above porosity gradually decreasing region, since the porosity is relatively low on the negative electrode current collector side, a large number of conductive particles are present at the interface between the negative electrode active material layer and the negative electrode intermediate layer, and the number of contact points between the conductive particles and the lithium metal increases. Therefore, the current density when lithium ions are supplied from the negative electrode active material layer to the negative electrode intermediate layer at the start of discharge can be relaxed, and the lithium ion acceptance can be improved. And in the negative electrode intermediate layer, by making the porosity on the solid electrolyte layer side relatively high, a precipitation path for lithium metal is ensured in the negative electrode intermediate layer. That is, a precipitation path for lithium ions is ensured. As a result, it is considered that the discharge rate characteristics of the battery can be improved.

[0039] The above-mentioned negative electrode intermediate layer may have a porosity decreasing region in at least a part thereof, where the porosity decreases as it moves away from the solid electrolyte layer along the stacking direction of the power generation elements. However, it is preferable that the porosity decreases as it moves away from the solid electrolyte layer along the stacking direction of the power generation elements throughout the entire negative electrode intermediate layer. In this specification, whether the negative electrode intermediate layer has a porosity decreasing region can be confirmed by the method described in the following examples.

[0040] In a preferred embodiment, for the negative electrode intermediate layer, with the thickness of the negative electrode intermediate layer being d, the porosity in the region from the surface on the solid electrolyte layer side to 0.5d in the stacking direction of the power generation elements is larger than the porosity in the region from the surface on the negative electrode current collector side to 0.5d. The porosity of the negative electrode intermediate layer can be confirmed by the method described in the following examples.

[0041] In a preferred embodiment, the porosity decreasing region is composed of a plurality of layers in which the porosity decreases in order as it moves away from the solid electrolyte layer along the stacking direction of the power generation elements. In a preferred embodiment, the negative electrode intermediate layer is composed of a plurality of layers in which the porosity decreases in order as it moves away from the solid electrolyte layer along the stacking direction of the power generation elements. Thereby, a porosity decreasing region in which the porosity decreases stepwise as it moves away from the solid electrolyte layer along the stacking direction of the power generation elements can be efficiently formed. As a result, the effects of the present invention can be obtained more remarkably. At this time, the number of layers constituting the porosity decreasing region of the negative electrode intermediate layer is not particularly limited. For example, it is 2 to 5 layers. Each of these plurality of layers preferably contains conductive particles. The types and mixing ratios of the conductive particles contained in each layer may be the same or different. In one embodiment, each of the above-mentioned plurality of layers preferably contains the same or different metal particles, and more preferably further contains the same or different carbon particles. The specific forms of the conductive particles contained in each layer are as described above.

[0042] When the porosity decreasing region of the negative electrode intermediate layer is composed of a plurality of layers, the porosity of each layer is not particularly limited as long as it is arranged to have a smaller value from the side of the solid electrolyte layer toward the side of the negative electrode current collector. However, it is preferably 40% or less, more preferably 35% or less, still more preferably 30% or less, and even more preferably 25% or less in the layer closest to the negative electrode current collector. By doing so, more conductive particles can come into contact with lithium metal, which is the negative electrode active material, at the interface with the negative electrode active material layer. As a result, since the effect of relaxing the current density is excellent, the effect of the present invention can be obtained more remarkably. The lower limit of the porosity of the layer closest to the negative electrode current collector is not particularly limited, but is, for example, 10% or more, preferably 20% or more. Further, the porosity of the layer closest to the solid electrolyte layer is, for example, 40% or more, preferably more than 40%, more preferably 42% or more, still more preferably 45% or more, and even more preferably 50% or more. By doing so, a migration path for lithium ions can be more easily secured. As a result, the effect of the present invention can be obtained more remarkably. The upper limit of the porosity of the layer closest to the solid electrolyte layer is not particularly limited, but is, for example, 80% or less, for example, 70% or less, and preferably 60% or less. That is, in a preferred embodiment, among the plurality of layers, the porosity of the layer closest to the negative electrode current collector is 10 to 40%, and the porosity of the layer closest to the solid electrolyte layer is more than 40% and 80% or less.

[0043] In one embodiment, when the porosity decreasing region of the negative electrode intermediate layer is composed of a plurality of layers, the difference in porosity between each layer is not particularly limited. However, the porosity of the layer closest to the solid electrolyte layer is preferably 10% or more greater than the porosity of the layer closest to the negative electrode current collector, more preferably 15% or more greater, and still more preferably 20% or more greater. Further, the absolute value of the difference in porosity between adjacent layers is preferably 5% or more, and more preferably 10% or more for each. When within the above range, the effect of the present invention can be obtained more remarkably.

[0044] In one embodiment, when the porosity gradually decreasing region of the negative electrode intermediate layer is composed of a plurality of layers, the ratio of the porosity of the layer closest to the solid electrolyte layer to the porosity of the layer closest to the negative electrode current collector is, for example, 1.2 or more, preferably 1.3 or more, preferably 1.5 or more, and more preferably 1.7 or more. The upper limit value of the above ratio is not particularly limited, but is, for example, 10 or less. When within the above range, the effects of the present invention can be obtained more remarkably.

[0045] When the negative electrode intermediate layer is composed of two layers, the porosity of the layer on the negative electrode current collector side (the second negative electrode intermediate layer) is, for example, 10 to 40%, for example, 20 to 40%, preferably 20 to 35%, and more preferably 25 to 30%. The porosity of the layer on the solid electrolyte layer side (the first negative electrode intermediate layer) is not particularly limited as long as it is larger than the porosity of the second negative electrode intermediate layer, but is, for example, more than 40% and 80% or less, for example, more than 40% and 70% or less, for example, more than 40% and 60% or less, preferably 42 to 55%, and more preferably 45 to 50%. When within the above range, the effects of the present invention can be obtained more remarkably. In this specification, the porosity of the negative electrode intermediate layer adopts the value measured by the method described in the examples below. When the negative electrode intermediate layer is composed of two layers, the ratio of the thickness of the layer on the solid electrolyte layer side (the first negative electrode intermediate layer) to the thickness of the layer on the negative electrode current collector side (the second negative electrode intermediate layer) is not particularly limited, but is, for example, 0.5 to 2, preferably 0.6 to 1.7.

[0046] The method for controlling the porosity of the negative electrode intermediate layer within a desired range is not particularly limited. However, in the manufacture of a lithium secondary battery, a two-step pressing process can be adopted, in which after subjecting the solid electrolyte layer to a pressing treatment, the solid electrolyte layer and the negative electrode intermediate layer are laminated and then subjected to a pressing treatment. More specifically, a solid electrolyte slurry containing a solid electrolyte is coated on the surface of a support (e.g., a metal foil), and the coating film is dried to obtain a solid electrolyte layer formed on the surface of the support. Thereafter, the solid electrolyte layer formed on the surface of the support is pressed (first pressing step). Thereby, the arrangement of the solid electrolyte particles on the surface of the support adjacent to the solid electrolyte layer can be adjusted, and the unevenness can be reduced. Note that after peeling off the support used for forming the solid electrolyte layer, pressing may be performed using another metal foil or the like. Also, before the first pressing step, the exposed surface of the positive electrode active material layer separately formed on the exposed surface of the solid electrolyte layer may be arranged, and the first pressing step may be performed with the solid electrolyte layer and the positive electrode active material layer overlapped. That is, according to a preferred embodiment of the present invention, before the first pressing step, the exposed surface of the positive electrode active material layer separately formed on the exposed surface of the solid electrolyte layer is arranged, and the first pressing step is performed with the solid electrolyte layer and the positive electrode active material layer overlapped. On the other hand, a negative electrode intermediate layer slurry containing materials (such as conductive particles and a binder) contained in the negative electrode intermediate layer is coated on the surface of a support for the negative electrode intermediate layer (e.g., a stainless steel foil), and the coating film formed on the surface of the support for the negative electrode intermediate layer is obtained by drying. Then, the support (metal foil) used in the first pressing step is peeled off to expose the solid electrolyte layer, and they are overlapped and pressed so that the exposed surface of the solid electrolyte layer faces the exposed surface of the coating film for the negative electrode intermediate layer (second pressing step). At this time, the porosity of the negative electrode intermediate layer can be adjusted by adjusting the pressing pressure, pressing temperature, and pressing time of the second pressing step. Also, the porosity achieved by the pressing treatment can be maintained even after releasing the pressing pressure. At this time, further, the porosity of the negative electrode intermediate layer can be more appropriately adjusted by adjusting the type, blending amount of the conductive particles contained in the negative electrode intermediate layer, their average primary particle diameter, the blending amount of the binder, and the like. For example, when increasing the content of metal particles such as Ag, Zn, and Sn as the conductive particles, it tends to be easily compressed and the porosity becomes small.Note that a hydrostatic press is preferred for the presses in the first pressing step and the second pressing step, but the present invention is not limited thereto.

[0047] In addition, in the above manufacturing method, instead of performing the second pressing step with the solid electrolyte layer and the coating film for the negative electrode intermediate layer stacked, the negative electrode intermediate layer may be obtained by subjecting only the coating film for the negative electrode intermediate layer to the pressing process of the second pressing step.

[0048] Here, a plurality of coating films for the negative electrode intermediate layer formed on the surface of the support for the negative electrode intermediate layer are prepared, and the operation of sequentially stacking them on the negative electrode intermediate layer obtained by the above second pressing step and performing the second pressing step is repeated, whereby a negative electrode intermediate layer composed of a plurality of layers can be produced. Further, an operation of forming a coating film for the negative electrode intermediate layer on the negative electrode intermediate layer obtained by subjecting the coating film for the negative electrode intermediate layer formed on the surface of the support for the negative electrode intermediate layer to the pressing process of the second pressing step and performing the second pressing step is performed, and this is repeated to sequentially stack a plurality of layers to obtain a negative electrode intermediate layer composed of a plurality of layers. At this time, the pressing pressure, pressing temperature, pressing time, etc. when forming each layer are adjusted so that the porosity of each layer decreases in order as the distance from the solid electrolyte layer increases.

[0049] The pressing pressure in the first pressing step varies depending on the material contained in the solid electrolyte layer, and also varies depending on the materials contained in the solid electrolyte layer and the positive electrode active material layer when the first pressing step is performed with the solid electrolyte layer and the positive electrode active material layer stacked, and can be appropriately set by those skilled in the art. For example, the pressing pressure in the first pressing step is preferably 300 MPa or more and 1000 MPa or less, more preferably 300 MPa or more and 800 MPa or less, and still more preferably 500 MPa or more and 700 MPa or less. The pressing temperature in the first pressing step is not particularly limited, but is, for example, 20 to 80°C, preferably 20 to 40°C. The pressing time in the first pressing step is not particularly limited, but is, for example, 10 seconds to 30 minutes, preferably 10 seconds to 10 minutes.

[0050] The method for preparing the negative electrode intermediate layer slurry is not particularly limited. For example, a method of mixing conductive particles, a binder, a solvent, etc. can be mentioned. Examples of the mixer include mixers such as a rotation-revolution mixer, a planetary mixer, a homomixer, and dispersers such as an ultrasonic disperser and a jet mill.

[0051] When coating the negative electrode intermediate layer slurry on the support for the negative electrode intermediate layer and drying to obtain a coating film for the negative electrode intermediate layer, the drying temperature is not particularly limited, but for example, it is 20 to 90 °C, preferably 25 to 80 °C. When it is within the above range, the porosity of the negative electrode intermediate layer can be more easily controlled.

[0052] The pressing pressure in the second pressing step varies depending on the materials contained in the negative electrode intermediate layer and the desired porosity, and can be appropriately set by those skilled in the art. For example, the pressing pressure in the second pressing step is, for example, 50 MPa or more and 700 MPa or less, preferably 100 MPa or more and 700 MPa or less. When it is within the above range, the porosity of the negative electrode intermediate layer can be more easily controlled.

[0053] When fabricating a negative electrode intermediate layer composed of multiple layers, it is preferable to fabricate a negative electrode intermediate layer laminate by sequentially laminating each layer on a support for the negative electrode intermediate layer. At this time, it is preferable to gradually decrease the pressing pressure in the second step when forming each layer. That is, the procedure of laminating a coating film for the negative electrode intermediate layer for forming each layer and performing the second pressing step is repeated, and at this time, it is preferable to sequentially decrease the pressing pressure in the second step. At this time, the composition of the coating film for the negative electrode intermediate layer for forming each layer may be the same or different. At this time, the second pressing step may be performed with the surface of the coating film for the negative electrode intermediate layer for forming the outermost layer of the negative electrode intermediate layer laminate facing the exposed surface of the solid electrolyte layer. Thereby, the coating film for the negative electrode intermediate layer for forming the outermost layer is transferred to the surface of the solid electrolyte layer and at the same time densified to form the outermost layer of the negative electrode intermediate layer. For example, when fabricating a negative electrode intermediate layer composed of two layers, the pressing pressure in the second step when forming the layer (the second negative electrode intermediate layer) on the side in contact with the negative electrode current collector is preferably 300 MPa or more and 700 MPa or less, more preferably 400 MPa or more and 600 MPa or less. The pressing pressure in the second step when forming the layer on the solid electrolyte layer side (the first negative electrode intermediate layer, which may also serve as a transfer to the solid electrolyte layer) is preferably 50 MPa or more and 200 MPa or less, more preferably 50 MPa or more and 150 MPa or less. When within the above range, a negative electrode intermediate layer with a smaller porosity can be more easily obtained as the distance from the solid electrolyte layer increases along the stacking direction of the power generation element. In addition, the porosity of each layer can be easily adjusted to a desired value.

[0054] The pressing temperature in the second pressing step varies depending on the materials contained in the negative electrode intermediate layer and the desired porosity, and can be appropriately set by those skilled in the art. For example, the pressing temperature in the second pressing step is preferably 20 to 90 °C, more preferably 25 to 80 °C. When within the above range, the porosity of the negative electrode intermediate layer can be more easily controlled.

[0055] The pressing time in the second pressing step varies depending on the materials contained in the negative electrode intermediate layer and the desired porosity, and can be appropriately set by those skilled in the art. By way of example, the pressing time in the second pressing step is preferably from 10 seconds to 30 minutes, more preferably from 1 to 10 minutes. When within the above range, the porosity of the negative electrode intermediate layer can be more easily controlled.

[0056] In a preferred embodiment of the present invention, the porosity decreasing region in the negative electrode intermediate layer continuously (with a gradient (slope)) decreases as it moves away from the solid electrolyte layer along the stacking direction of the power generation element. With this configuration, the effects of the present invention can be obtained even more remarkably.

[0057] The method for producing the negative electrode intermediate layer having a porosity decreasing region in which the porosity continuously decreases along the stacking direction of the power generation element is not particularly limited. For example, when producing a coating film for the negative electrode intermediate layer using a negative electrode intermediate layer slurry containing conductive particles, a binder, and a solvent, a continuous porosity gradient can be created by migrating the conductive particles and the binder to the surface layer side of the coating film during the drying process. When producing the negative electrode intermediate layer slurry, it is preferable to lower the viscosity of the negative electrode intermediate layer slurry by using, for example, zirconia beads. Further, after applying the negative electrode intermediate layer, the temperature during drying is preferably, for example, 100 to 140°C. By doing so, a negative electrode intermediate layer having a continuous porosity gradient in the stacking direction can be efficiently obtained. At this time, the above negative electrode intermediate layer slurry may be applied to the surface of the solid electrolyte layer to produce a coating film for the negative electrode intermediate layer.

[0058] In a preferred embodiment of the present invention, the negative electrode intermediate layer contains at least one kind of metal particles, and the content of the metal particles has a metal particle content gradually increasing region that increases as it moves away from the solid electrolyte layer along the stacking direction of the power generation element. By reducing the amount of metal particles present on the solid electrolyte layer side, the porosity on the solid electrolyte layer side can be increased. Therefore, the effects of the present invention can be obtained more remarkably. The metal particle content gradually increasing region may be composed of a plurality of layers in which the content of the metal particles increases in order as it moves away from the solid electrolyte layer along the stacking direction of the power generation element. The metal particle content gradually increasing region may have a continuously increasing metal particle content as it moves away from the solid electrolyte layer along the stacking direction of the power generation element.

[0059] For example, when the negative electrode intermediate layer (or the metal particle content gradually increasing region of the negative electrode intermediate layer) is composed of a plurality of layers, the content of the metal particles in the layer closest to the negative electrode current collector is, for example, 30 to 50% by mass, preferably 35 to 45% by mass, more preferably 40 to 45% by mass, based on the total mass of the layer. Also, the content of the metal particles in the layer closest to the solid electrolyte layer is, for example, 10 to 25% by mass, preferably 15 to 25% by mass, more preferably 18 to 24% by mass, based on the total mass of the layer. Within such a range, a negative electrode intermediate layer having a desired porosity gradient can be more easily manufactured, and the effects of the present invention can be obtained more remarkably.

[0060] The thickness of the negative electrode intermediate layer is not particularly limited, but is preferably 10 μm or less, more preferably 7 μm or less, and even more preferably 6 μm or less. When the thickness of the negative electrode intermediate layer is 10 μm or less, the effects of the present invention can be obtained more remarkably. Also, the energy density of the lithium secondary battery can be improved. The lower limit of the thickness of the negative electrode intermediate layer is not particularly limited, but from the viewpoint of ensuring the strength of the negative electrode intermediate layer, it is preferably 1.5 μm or more, more preferably 2 μm or more. When the negative electrode intermediate layer is composed of a plurality of layers, it is preferable that the total thickness is within the above range. In this specification, the thickness of the negative electrode intermediate layer is the value measured by the method described in the examples below.

[0061] [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 those known in the art can be appropriately adopted. For example, sulfide solid electrolytes and oxide solid electrolytes can be mentioned. Since this solid electrolyte exhibits excellent lithium ion conductivity, it is preferably a sulfide solid electrolyte containing S element, more preferably a sulfide solid electrolyte containing Li element, M element and S element, and the M element is at least one element selected from the group consisting of P, Si, Ge, Sn, Ti, Zr, Nb, Al, Sb, Br, Cl and I, and still more preferably a sulfide solid electrolyte containing S element, Li element and P element. As an example, LPS (Li2S-P2S5), Li6PS5X (where X is Cl, Br or I), Li7P3S 11 , Li 3.2 P 0.96 S and sulfide solid electrolytes such as Li3PS4 can be mentioned. Since these sulfide solid electrolytes have excellent lithium ion conductivity, they are preferably used.

[0062] The ionic conductivity (for example, Li ion conductivity) of the solid electrolyte at room temperature (25 ° C) is preferably 1×10 -5 S / cm or more, and more preferably 1×10 -4 S / cm or more. The value of the ionic conductivity of the solid electrolyte can be measured by the alternating current impedance method.

[0063] Examples of the shape of the solid electrolyte include particulate shapes such as true spherical and ellipsoidal, and thin film shapes. 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 still more preferably 0.5 μm or more and 10 μm or less.

[0064] The content of the solid electrolyte in the solid electrolyte layer is preferably 50 to 100% by mass, more preferably 90 to 100% by mass.

[0065] In addition to the solid electrolyte, the solid electrolyte layer may further contain a binder. The binder is not particularly limited, and known binders can be appropriately used. For example, the binders described in the above negative electrode intermediate layer can be similarly adopted. The content of the binder in the solid electrolyte layer is not particularly limited, but is, for example, 1 to 10% by mass.

[0066] The thickness of the solid electrolyte layer varies depending on the configuration of the target lithium secondary battery, but is usually 0.1 to 1000 μm, preferably 10 to 100 μm.

[0067] [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 assistant as necessary.

[0068] The type of the positive electrode active material contained in the positive electrode active material layer is not particularly limited, but includes layered rock salt type active materials such as LiCoO2, LiMnO2, LiNiO2, LiVO2, Li(Ni-Mn-Co)O2, spinel type active materials such as LiMn2O4, LiNi 0.5 Mn 1.5 O4, olivine type active materials such as LiFePO4, LiMnPO4, Si-containing active materials such as Li2FeSiO4, Li2MnSiO4, etc. In addition, examples of the oxide active materials other than the above include Li4Ti5O 12 Among them, Li(Ni-Mn-Co)O2 and those in which a part of these transition metals is substituted by other elements (hereinafter, also simply referred to as "NMC composite oxides") are preferably used as the positive electrode active material.

[0069] Also, it is one of the preferred embodiments that a sulfur-based positive electrode active material is used. 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 that can utilize the oxidation-reduction reaction of sulfur to release lithium ions during charging and occlude lithium ions during discharging may be used.

[0070] The content of the positive electrode active material in the positive electrode active material layer is not particularly limited, but for example, it is preferably 30 to 99% by mass, more preferably 40 to 90% by mass, and even more preferably 45 to 80% by mass.

[0071] The positive electrode active material layer preferably further contains a solid electrolyte. As the specific form of the solid electrolyte contained in the positive electrode active material layer, the forms described in the column of the solid electrolyte layer can be similarly adopted. A sulfide solid electrolyte is preferably used because it has excellent lithium ion conductivity and a low bulk modulus of elasticity, so it can follow the volume change of the positive electrode active material accompanying charge and discharge. The content of the solid electrolyte in the positive electrode active material layer is not particularly limited, but for example, it is 1 to 70% by mass, preferably 10 to 60% by mass, and even more preferably 20 to 55% by mass.

[0072] The binder used for the positive electrode active material layer is not particularly limited, and known binders can be appropriately used. For example, the binders described in the above negative electrode intermediate layer can be used. The content of the binder in the positive electrode active material layer is not particularly limited, but for example, it is 1 to 10% by mass.

[0073] The conductive assistant used for the positive electrode active material layer is not particularly limited either. For example, carbon such as carbon black (specifically, acetylene black, Ketjenblack (registered trademark), furnace black, channel black, thermal lamp black, etc.) can be used. The content of the conductive assistant in the positive electrode active material layer is not particularly limited, but for example, it is 10 to 30% by mass.

[0074] The thickness of the positive electrode active material layer varies depending on the configuration of the intended lithium secondary battery, but is usually 0.1 to 1000 μm, preferably 10 to 300 μm.

[0075] The lithium secondary battery of this embodiment is excellent in high-speed charging characteristics. In vehicle applications such as electric vehicles, hybrid electric vehicles, fuel cell vehicles, and hybrid fuel cell vehicles, higher capacity and larger current are required compared to electric and portable electronic device applications. Therefore, the lithium secondary battery of this embodiment can be suitably used as a vehicle power source, for example, for vehicle drive power sources and auxiliary power sources. In particular, it can be suitably used as a power source for vehicles such as hybrid electric vehicles that require high output during discharge.

[0076] As described above, one embodiment of the lithium secondary battery of the present invention has been described. However, the present invention is not limited to only the configurations described in the foregoing embodiments, and can be appropriately changed based on the description of the claims.

[0077] For example, as a type of battery to which the lithium secondary battery according to the present invention is applied, a bipolar battery including a positive electrode active material layer electrically bonded to one surface of a current collector and a negative electrode active material layer electrically bonded to the opposite surface of the current collector can also be mentioned.

[0078] Also, the lithium secondary battery according to this embodiment is preferably an all-solid-state lithium secondary battery in which the solid electrolyte layer does not contain a liquid electrolyte (electrolyte solution). However, the lithium secondary battery according to this embodiment does not have to be all-solid-state. That is, the solid electrolyte layer may further contain a conventionally known liquid electrolyte (electrolyte solution). There is no particular limitation on the amount of the liquid electrolyte (electrolyte solution) that can be contained in the solid electrolyte layer, but it is preferably an amount such that the shape of the solid electrolyte layer formed by the solid electrolyte is maintained and no liquid leakage of the liquid electrolyte (electrolyte solution) occurs.

[0079] In addition, the following embodiments are also included in the scope of the present invention: the lithium secondary battery according to claim 1 having the features of claim 2; the lithium secondary battery according to claim 1 having the features of claim 3; the lithium secondary battery according to any one of claims 1 to 3 having the features of claim 4; the lithium secondary battery according to claim 4 having the features of claim 5; the lithium secondary battery according to claim 4 or 5 having the features of claim 6; the lithium secondary battery according to claim 2 having the features of claim 7.

Example

[0080] Hereinafter, the present invention will be described in more detail by way of examples. However, the technical scope of the present invention is not limited only to the following examples. In the following, the operations were carried out in a glove box under an argon atmosphere with a dew point of -68°C or lower. Also, the instruments and devices used in the glove box were sufficiently dried in advance.

[0081] <Production Example of Evaluation Cell> [Example 1] (Fabrication of Positive Electrode) First, as the constituent materials of the positive electrode active material layer, the NMC composite oxide (composition = LiNi 0.8 Mn 0.1 Co 0.1 O2), which is a positive electrode active material, the alligatorite-type sulfide solid electrolyte (Li6PS5Cl), which is a solid electrolyte, acetylene black, which is a conductive assistant, and styrene-butadiene rubber (SBR), which is a binder, were prepared.

[0082] In a glove box with an argon atmosphere having a dew point below -68°C, NMC composite oxide, solid electrolyte, and acetylene black were weighed so as to have a mass ratio of 50:30:20, mixed in an agate mortar, and then further mixed and stirred with a planetary ball mill. To 100 parts by mass of the obtained mixed powder, 2 parts by mass of styrene-butadiene rubber (SBR) was added, and xylene was added as a solvent to prepare a positive electrode active material slurry. Next, the positive electrode active material slurry prepared above was coated on the surface of a carbon-coated aluminum foil as a positive electrode current collector and dried to form a positive electrode active material layer (thickness: 200 μm), thereby fabricating a positive electrode.

[0083] (Fabrication of Solid Electrolyte Layer) In a glove box with an argon atmosphere having a dew point below -68°C, argyrodite-type sulfide solid electrolyte (Li6PS5Cl) as a solid electrolyte and SBR as a binder were mixed at a mass ratio of 95:5, and an appropriate amount of xylene was added as a solvent and mixed to prepare a solid electrolyte slurry. The solid electrolyte slurry was coated on one surface of a stainless steel foil as a support and dried to obtain a solid electrolyte layer (thickness: 80 μm).

[0084] (Fabrication of Laminated Body of Positive Electrode Current Collector, Positive Electrode Active Material Layer, and Solid Electrolyte Layer) Next, the solid electrolyte layer formed on the surface of the stainless steel foil was stacked on the positive electrode active material layer formed on the surface of the above positive electrode current collector such that the exposed surface of the positive electrode active material layer and the exposed surface of the solid electrolyte layer faced each other, and isostatic pressing was performed at 700 MPa and 25°C for 1 minute (first pressing step). By this step, the solid electrolyte layer was transferred onto the positive electrode active material layer.

[0085] (Fabrication of Second Negative Electrode Intermediate Layer) Acetylene black (average primary particle diameter (D50): 35 nm) and silver nanoparticles (average primary particle diameter (D50): 60 nm) were weighed and mixed so that the mass ratio of acetylene black (AB):Ag was 3:1. To 86 parts by mass of the obtained mixture, 14 parts by mass of polyvinylidene fluoride (PVDF) as a binder was added, N-methylpyrrolidone was added as a solvent, and the mixture was mixed using a planetary mixer to prepare a negative electrode intermediate layer slurry 1. The negative electrode intermediate layer slurry 1 was coated on the surface of a stainless steel foil as a negative electrode current collector and dried at 80 °C to obtain a coating film 2 for the negative electrode intermediate layer (thickness of one side before pressing: 5 μm). This was pressed at 500 MPa and 80 °C for 1 minute by hydrostatic pressing (second pressing step). As a result, the coating film 2 for the negative electrode intermediate layer was densified to obtain a second negative electrode intermediate layer.

[0086] (Preparation of the first negative electrode intermediate layer) Next, the negative electrode intermediate layer slurry 1 prepared above was coated on the surface of the second negative electrode intermediate layer prepared above and dried at 80 °C to obtain a coating film 1 for the negative electrode intermediate layer (thickness of one side before pressing: 5 μm). On the surface of the negative electrode current collector, a laminate (negative electrode intermediate layer laminate) in which the second negative electrode intermediate layer and the coating film 1 for the negative electrode intermediate layer were laminated in this order was obtained. Next, after peeling off the stainless steel foil adjacent to the solid electrolyte layer on the surface of the laminate of the positive electrode current collector, the positive electrode active material layer, and the solid electrolyte layer prepared above, the solid electrolyte layer and the negative electrode intermediate layer laminate were overlapped so that the exposed surface of the solid electrolyte layer and the exposed surface of the coating film 1 for the negative electrode intermediate layer faced each other, and pressed at 100 MPa and 80 °C for 1 minute by hydrostatic pressing (second pressing step). As a result, the coating film 1 for the negative electrode intermediate layer was transferred to the exposed surface of the solid electrolyte layer, and the porosity was adjusted to obtain a first negative electrode intermediate layer. In this way, a laminate in which a positive electrode active material layer, a solid electrolyte layer, a first negative electrode intermediate layer, a second negative electrode intermediate layer, and a stainless steel foil (negative electrode current collector) were laminated in this order on the surface of the positive electrode current collector was obtained.

[0087] Finally, an aluminum positive tab and a nickel negative tab were ultrasonically welded to each of the aluminum foil (positive current collector) and the stainless-steel foil (negative current collector), and the obtained laminate was placed inside an aluminum laminate film and vacuum-sealed to obtain an evaluation cell, which is a lithium-depositing type lithium secondary battery (all-solid-state battery) of this example.

[0088] [Example 2] In the same manner as in Example 1 above, a laminate of a positive current collector, a positive active material layer, and a solid electrolyte layer was fabricated.

[0089] (Fabrication of negative intermediate layer) Acetylene black (average primary particle diameter (D50): 35 nm) and silver nanoparticles (average primary particle diameter (D50): 60 nm) were weighed and mixed so that the mass ratio of acetylene black (AB):Ag was 3:1. To 86 parts by mass of the obtained mixture, 14 parts by mass of styrene-butadiene rubber (SBR) as a binder was added, xylene was added as a solvent, and zirconia beads (Zr beads) were further added and mixed using a planetary mixer to prepare a negative intermediate layer slurry 2.

[0090] After peeling off the stainless-steel foil adjacent to the solid electrolyte layer on the surface of the laminate of the positive current collector, the positive active material layer, and the solid electrolyte layer fabricated above, the negative intermediate layer slurry 2 was coated on the exposed surface of the solid electrolyte layer and dried at 120 °C to obtain a coating film 3 for the negative intermediate layer (thickness before pressing: 10 μm). This coating film 3 for the negative intermediate layer has a porosity gradient in the thickness direction, and the porosity is adjusted in a subsequent pressing process.

[0091] Next, a stainless-steel foil (negative current collector) was overlaid on the exposed surface of the coating film 3 for the negative intermediate layer and pressed at 500 MPa and 80 °C for 1 minute by hydrostatic pressing (second pressing process). Thereby, a laminate of a positive current collector, a positive active material layer, a solid electrolyte layer, a negative intermediate layer, and a negative current collector, in which the porosity of the negative intermediate layer was adjusted, was obtained.

[0092] Finally, an aluminum positive tab and a nickel negative tab were welded to an aluminum foil (positive current collector) and a stainless-steel foil (negative current collector) respectively by an ultrasonic welder, and the obtained laminate was placed inside an aluminum laminate film and vacuum-sealed to obtain an evaluation cell of the lithium deposition type lithium secondary battery (all-solid-state battery) of this example.

[0093] [Example 3] (Preparation of Negative Electrode Intermediate Layer Slurry 3) Acetylene black (average primary particle diameter (D50): 35 nm) and silver nanoparticles (average primary particle diameter (D50): 60 nm) were weighed and mixed so that the mass ratio of acetylene black (AB):Ag was 1:1. To 86 parts by mass of the obtained mixture, 14 parts by mass of polyvinylidene fluoride (PVDF) as a binder was added, N-methylpyrrolidone was added as a solvent, and the mixture was mixed using a planetary mixer to prepare negative electrode intermediate layer slurry 3.

[0094] In the (preparation of the first negative electrode intermediate layer) of Example 1, an evaluation cell of this example was prepared in the same manner as in Example 1, except that negative electrode intermediate layer slurry 3 was used instead of negative electrode intermediate layer slurry 1.

[0095] [Example 4] In the (preparation of the second negative electrode intermediate layer) of Example 1, an evaluation cell of this example was prepared in the same manner as in Example 1, except that negative electrode intermediate layer slurry 3 was used instead of negative electrode intermediate layer slurry 1.

[0096] [Comparative Example 1] In the preparation of the second negative electrode intermediate layer of Example 1, the coating amount of negative electrode intermediate layer slurry 1 was adjusted so that the thickness of the coating film 1 for the negative electrode intermediate layer before pressing was 10 μm. Also, the first negative electrode intermediate layer was not prepared. An evaluation cell of this comparative example was prepared in the same manner as in Example 1, except for the above.

[0097] (Observation of Change in Porosity of Negative Electrode Intermediate Layer in Laminating Direction of Power Generation Element) The porosity of the negative electrode intermediate layer can be observed using SEM for the negative electrode intermediate layer in the post-discharge state, which is carried out with the cell restraint removed and the exterior removed after complete discharge of the lithium secondary battery.

[0098] Specifically, from the evaluation cell after complete discharge, the power generation element was taken out, and a cross-section (laminated cross-section) perpendicular to the surface direction was exposed by ion milling. Using a focused ion beam-scanning electron microscope (FIB-SEM) manufactured by Hitachi High-Technologies Corporation, the laminated cross-section of the negative electrode intermediate layer was observed by SEM. The laminated cross-sections were observed at several to dozens of different locations in the negative electrode intermediate layer.

[0099] As a result, it was confirmed that in the cells of Examples 1, 3, and 4, the negative electrode intermediate layer is composed of two layers with a decreasing porosity from the solid electrolyte layer side toward the negative electrode current collector side along the lamination direction of the power generation element. In the cell of Example 2, the porosity of the negative electrode intermediate layer continuously decreased from the solid electrolyte layer side toward the negative electrode current collector side along the lamination direction of the power generation element. On the other hand, in the cell of Comparative Example 1, no change in the porosity in the lamination direction of the power generation element of the negative electrode intermediate layer was observed.

[0100] <Measurement of Porosity and Thickness of Negative Electrode Intermediate Layer> The porosity of each layer constituting the negative electrode intermediate layer of Examples 1, 3, and 4 and the porosity of the negative electrode intermediate layer of Comparative Example 1 were measured. The porosity of the negative electrode intermediate layer is a value calculated from the negative electrode intermediate layer in the post-discharge state, which is carried out with the cell restraint removed and the exterior removed after complete discharge of the lithium secondary battery, and can be calculated using "3D-SEM".

[0101] Specifically, the power generation element was taken out from the evaluation cell after full discharge, and a cross-section (laminated cross-section) perpendicular to the surface direction was exposed by ion milling. A scanning electron microscope (SEM) photograph of the laminated cross-section of the power generation element as viewed from the front, specifically, a photograph of the surface portion of the negative electrode intermediate layer (a photograph with a size of 5 μm × 5 μm) was taken using a focused ion beam-scanning electron microscope (FIB-SEM) manufactured by Hitachi High-Technologies Corporation. Then, 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 again, a photograph of the surface of the negative electrode intermediate layer was taken. The excavation of the surface of the negative electrode intermediate layer by the above ion beam irradiation and the photographing of the surface of the negative electrode intermediate layer were repeated to obtain a group of 2D photographs of the surface of the negative electrode intermediate layer. Then, each 2D region of the obtained group of 2D photographs was discriminated, the area of the voids existing in the 2D region was calculated, and the volume of the voids in the 3D region was calculated by integrating the areas. Then, the porosity was calculated by calculating the volume of the voids with respect to the volume of the entire 3D region.

[0102] Note that lithium metal in the negative electrode intermediate layer is regarded as voids. Specifically, when lithium metal remains in the voids in the evaluation cell after discharge, the lithium metal is calculated as the area of the voids. The identification of lithium metal is performed by identifying lithium metal by high-sensitivity EDX (3DSEM-EDX) and specifying it by the contrast of the identified lithium metal, and calculating its area.

[0103] In addition, the above cross-section was observed by SEM, the thickness was measured at several to several tens of different locations in the negative electrode intermediate layer, and the arithmetic mean value thereof was taken as the thickness of the negative electrode intermediate layer. The results are shown in Table 1 below.

[0104] Note that in this example, for the evaluation cell after performing the discharge rate characteristic evaluation described later, after full discharge, the porosity and thickness of the above negative electrode intermediate layer were measured. However, for the evaluation cell before the first charge, similar values of the porosity and thickness of the negative electrode intermediate layer were obtained respectively.

[0105] In the evaluation cells of Examples 1, 3, and 4, it was confirmed that the negative electrode intermediate layer was composed of two layers in which the porosity gradually decreased as the distance from the solid electrolyte layer increased along the stacking direction of the power generation elements. In the negative electrode intermediate layer of the evaluation cell of Example 2, it was confirmed that the porosity continuously decreased as the distance from the solid electrolyte layer increased along the stacking direction of the power generation elements.

[0106] In addition, the contents of metal particles, carbon particles, and binder in the negative electrode intermediate layer can be specified for each component by high-sensitivity EDX (3DSEM-EDX), and the area thereof can be calculated and calculated using the specific gravity of these components. It was confirmed that the content of metal particles (the ratio of the mass of metal particles to the total mass of metal particles, carbon particles, and binder) in the negative electrode intermediate layer in the evaluation cells of each Example and Comparative Example corresponded to the content of metal particles in the solid content of the negative electrode intermediate layer slurry.

[0107] In Table 1, the values of the porosity and the content of metal particles of the evaluation cell of Example 2 are the values of the porosity and the content of metal particles of the entire negative electrode intermediate layer, respectively.

[0108] <Ratio of the total surface area of conductive particles (silver nanoparticles and acetylene black particles) to the mass of the binder contained in the negative electrode intermediate layer> The ratio (m 2 / g) of the total surface area of silver nanoparticles and acetylene black particles to the mass of the binder contained in the first and second negative electrode intermediate layers of Examples 1, 3, and 4 was determined by the following procedure.

[0109] (1) Grasp the mass and specific gravity of acetylene black particles, silver nanoparticles, and binder per unit mass of the negative electrode intermediate layer. (2) Grasp the average primary particle diameter of acetylene black particles and the average primary particle diameter of silver nanoparticles. (3) From the above (1) and (2), assuming that each particle is spherical, calculate the number of acetylene black particles and the number of silver nanoparticles per unit mass of the negative electrode intermediate layer. And 4πr2 Using the formula for the number of × particles (where r is half of the average primary particle diameter of the acetylene black particles or silver nanoparticles), calculate the total surface area of the acetylene black particles and the total surface area of the silver nanoparticles respectively, add them together, and divide this sum by the mass of the binder per unit mass of the negative electrode intermediate layer.

[0110] Here, the specific gravities of the acetylene black particles, silver nanoparticles, and binder are 2.01 g / cm 3 , 11.23 g / cm 3 , and 1.89 g / cm 3 respectively.

[0111] As a result, in the evaluation cell of Example 1, in the first negative electrode intermediate layer and the second negative electrode intermediate layer, the ratio of the total surface area of the silver nanoparticles and acetylene black particles to the mass of the binder contained in the negative electrode intermediate layer was 407 m 2 / g respectively. Also, in the evaluation cells of Examples 3 and 4, in both the first negative electrode intermediate layer and the second negative electrode intermediate layer, the ratio of the total surface area of the silver nanoparticles and acetylene black particles to the mass of the binder contained in the negative electrode intermediate layer was 800 m 2 / g or less.

[0112] <Discharge rate characteristic evaluation> The discharge rate characteristic evaluation was carried out in a constant temperature bath set at 60 °C while applying a restraint pressure of 3 MPa in the stacking direction of the evaluation cell using a pressure member, using a charge-discharge test device (HJ-SD8, manufactured by Hokuto Denko Corporation). The evaluation cell was placed in the constant temperature bath, and after the cell temperature became constant, constant current charging was performed from the fully discharged state to a cell voltage of 4.3 V at 0.1C. Then, constant current discharge was performed at 0.1C to the fully discharged state. Subsequently, constant current charging was performed from the fully discharged state to a cell voltage of 4.3 V at 0.1C. Then, constant current discharge was performed at 3.5C to the fully discharged state. And the ratio (percentage) of the ratio of the constant current discharge capacity at 3.5C to the constant current discharge capacity at 0.1C (constant current discharge capacity at 3.5C / constant current discharge capacity at 0.1C) was defined as the discharge rate characteristic (%). The results are shown in Table 1 below.

[0113]

Table 1

[0114] From the results in Table 1, it was found that excellent discharge rate characteristics can be obtained in the evaluation cells of Examples 1 to 4 in which the porosity of the negative electrode intermediate layer decreases as it moves away from the solid electrolyte layer along the stacking direction of the power generation element. On the other hand, the discharge rate characteristics were insufficient in the evaluation cell of Comparative Example 1 in which the porosity of the negative electrode intermediate layer was constant.

Explanation of symbols

[0115] 10a Stacked 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 generation element, 25 Negative electrode current collecting plate, 27 Positive electrode current collecting plate, 29 Laminate film.

Claims

1. 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; a negative electrode intermediate layer present adjacent to the surface of the solid electrolyte layer on the negative electrode current collector side and containing conductive particles; a lithium secondary battery comprising a power generation element having the above components, wherein the negative electrode intermediate layer has a porosity gradually decreasing region in which the porosity decreases as the distance from the solid electrolyte layer increases along the stacking direction of the power generation element.

2. The lithium secondary battery according to claim 1, wherein the porosity gradually decreasing region is composed of a plurality of layers in which the porosity decreases in order as the distance from the solid electrolyte layer increases along the stacking direction of the power generation element.

3. The lithium secondary battery according to claim 1, wherein in the porosity gradually decreasing region, the porosity continuously decreases as the distance from the solid electrolyte layer increases along the stacking direction of the power generation element.

4. The lithium secondary battery according to claim 1 or 2, wherein the conductive particles contain at least one kind of metal particles.

5. The lithium secondary battery according to claim 4, wherein the conductive particles further contain at least one kind of carbon particles.

6. The lithium secondary battery according to claim 4, wherein the negative electrode intermediate layer has a metal particle content gradually increasing region in which the content of the metal particles increases as the distance from the solid electrolyte layer increases along the stacking direction of the power generation element.

7. The lithium secondary battery according to claim 2, wherein among the plurality of layers, the porosity of the layer closest to the negative electrode current collector is 10 to 40%, and the porosity of the layer closest to the solid electrolyte layer is more than 40% and 80% or less.

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