Lithium secondary battery

Abstract

To provide means capable of improving both the rapid charging characteristics and the discharge capacity of a lithium secondary battery that comprises a negative electrode intermediate layer 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 interposed between the negative electrode current collector and the solid electrolyte layer, and containing lithium-reactive material. The lithium-reactive material contains two or more types of carbon materials. An R value, which is expressed as a ratio of intensity of a D band with respect to intensity of a G band in a Raman spectrum of the carbon materials contained in the negative electrode intermediate layer, increases gradually in a stacking direction of the power generation element, from the solid electrolyte layer side to the negative electrode current collector side.SELECTED DRAWING: Figure 1

Description

Technical Field

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

Background Art

[0002] In recent years, research and development on all-solid-state lithium secondary batteries using oxide-based or sulfide-based solid electrolytes as electrolytes have been actively carried out. A solid electrolyte is a material mainly composed of an ion conductor capable of ion conduction in a solid. Therefore, in all-solid-state lithium secondary batteries, various problems caused by flammable organic electrolytes as in conventional liquid-based lithium secondary batteries do not occur in principle. Also, generally, when using a high-potential and high-capacity cathode material and a high-capacity anode material, a significant improvement in the output density and energy density of the battery can be achieved.

[0003] As one type of all-solid-state lithium secondary battery, a so-called lithium deposition type in which lithium metal is deposited on the anode current collector during the charging process is known. Although the lithium deposition type all-solid-state lithium secondary battery is excellent in energy density and output characteristics, it has a problem that a short circuit is likely to occur due to dendrites from the lithium metal layer. As a means of suppressing the growth of dendrites, a technique has been proposed in which a negative electrode active material layer (negative electrode intermediate layer) that forms an alloy or compound with lithium is provided between the solid electrolyte layer and the anode current collector (for example, Patent Document 1).

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, according to the study by the present inventors, it has been found that in the lithium secondary battery described in the above literature, it is difficult to simultaneously improve both the rapid charging characteristics and the discharge capacity.

[0006] Therefore, an object of the present invention is to provide a means capable of improving both the rapid charging characteristics and the discharge capacity in a lithium precipitation type lithium secondary battery provided with a negative electrode intermediate layer.

Means for Solving the Problems

[0007] The present inventors have intensively studied to solve the above problems. As a result, in a lithium precipitation type lithium secondary battery provided with a negative electrode intermediate layer, along the stacking direction of the power generation element, a highly crystalline carbon material is contained in the region on the solid electrolyte layer side of the negative electrode intermediate layer, and a low-crystalline carbon material is contained in the region on the negative electrode current collector side. By doing so, it has been found that the above problems can be solved, and the present invention has been completed.

[0008] That is, one aspect of the present invention is 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 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 interposed between the negative electrode current collector and the solid electrolyte layer and containing a lithium reactive material. The power generation element includes a lithium reactive material containing two or more types of carbon materials, and an R value represented by the ratio of the intensity of the D band to the intensity of the G band in the Raman spectrum of the carbon material contained in the negative electrode intermediate layer increases gradually from the side of the solid electrolyte layer toward the side of the negative electrode current collector along the stacking direction of the power generation element. It is a lithium secondary battery.

Effects of the Invention

[0009] According to the present invention, in a lithium precipitation type lithium secondary battery provided with a negative electrode intermediate layer, both the rapid charging characteristics and the discharge capacity can be improved.

Brief Description of the Drawings

[0010]

Figure 1

[0011] One embodiment of the present invention includes 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 interposed between the negative electrode current collector and the solid electrolyte layer and containing a lithium-reactive material. The lithium-reactive material contains two or more types of carbon materials, and the R value, represented by the ratio of the intensity of the D band to the intensity of the G band in the Raman spectrum of the carbon material contained in the negative electrode intermediate layer, increases gradually from the side of the solid electrolyte layer toward the side of the negative electrode current collector along the stacking direction of the power generation element. This is a lithium secondary battery. According to the lithium secondary battery according to this embodiment, both the rapid charging characteristics and the discharge capacity can be improved.

[0012] 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 assigned to the same elements, and duplicate descriptions are omitted. Also, the dimensional ratios in the drawings are exaggerated for convenience of explanation and may be different from the actual ratios.

[0013] 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 structure 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 so as to be in contact with the negative electrode active material layer 13 and the solid electrolyte layer 17 respectively. 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". Thereby, the negative electrode current collector 11', the negative electrode active material layer 13, the negative electrode intermediate layer 14, the solid electrolyte layer 17, the positive electrode active material layer 15, and the positive electrode current collector 11" constitute one single battery layer 19. Therefore, it can also be said that the laminated secondary battery 10a shown in FIG. 1 has a configuration in which a plurality of single battery layers 19 are laminated and electrically connected in parallel. Negative electrode current collector plates 25 and positive electrode current collector plates 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 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.

[0014] Hereinafter, the main constituent members of the lithium secondary battery according to this embodiment will be described.

[0015] [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.

[0016] [Negative electrode active material layer] The lithium secondary battery according to this embodiment is a so-called lithium deposition type in which lithium metal is deposited on the negative electrode current collector during the charging process. The layer composed of lithium metal deposited on the negative electrode current collector during this charging process is the negative electrode active material layer of the lithium secondary 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. 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.

[0017] [Negative electrode intermediate layer] The negative electrode intermediate layer is a layer interposed between the negative electrode current collector and the solid electrolyte layer and contains a lithium-reactive material. In the lithium secondary battery according to this embodiment, the lithium-reactive material essentially contains two or more types of carbon materials.

[0018] The volume resistivity of the negative electrode intermediate layer is not particularly limited, but is preferably 10 2 Ω·cm or less, 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 Electric Co., Ltd., product name: RM2610).

[0019] The carbon material is a type of material capable of occluding lithium ions during charging. Specific examples of the carbon material include carbon black (specifically, acetylene black, Ketjenblack (registered trademark), furnace black, channel black, thermal black, thermal lamp black, etc.), graphitized carbon black obtained by graphitizing these, carbon nanotubes (CNT), graphite, hard carbon, and the like. Among them, carbon black is preferred, and it is more preferably at least one selected from the group consisting of acetylene black, Ketjenblack (registered trademark), furnace black, channel black, thermal black, and thermal lamp black.

[0020] When the above carbon material is in a particle shape, its average particle diameter (average primary particle diameter) is, for example, 10 nm or more and 200 nm or less, preferably 15 nm or more and 150 nm or less, more preferably 20 nm or more and 100 nm or less, and still more preferably 20 nm or more and 70 μm or less. In this specification, the average particle diameter of the carbon material refers to the 50% cumulative diameter (D 50 ) of the particle diameter (the maximum distance among the distances between any two points on the contour line of the observed particle) 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).

[0021] In the lithium secondary battery according to this embodiment, in the negative electrode intermediate layer, along the stacking direction of the power generation element, the R value represented by the ratio of the intensity of the D band to the intensity of the G band in the Raman spectrum of the carbon material contained on the side of the solid electrolyte layer is smaller than the R value of the carbon material contained on the side of the negative electrode current collector. Here, the R value is an index of the crystallinity of the carbon material, and the smaller the R value, the higher the crystallinity.

[0022] Generally, in a lithium secondary battery, the energy density and the rapid charging characteristics are generally in a trade-off relationship. For example, in order to increase the discharge capacity, means such as increasing the positive electrode weight per unit area or increasing the ratio of the positive electrode active material in the positive electrode active material layer can be considered. However, in the former case, especially the electron resistance and the lithium ion diffusion resistance in the thickness direction of the positive electrode active material layer increase, and in the latter case, the electron resistance and the lithium ion diffusion resistance of the entire electrode increase, so the rapid charging characteristics may deteriorate. Even in a lithium secondary battery having a negative electrode intermediate layer, it has been difficult to simultaneously improve both the rapid charging characteristics and the discharge capacity even by improving the material composition of the negative electrode intermediate layer or the structure of the negative electrode intermediate layer itself.

[0023] On the other hand, in the lithium secondary battery according to this embodiment, the R value of the carbon material contained in the negative electrode intermediate layer gradually increases from the side of the solid electrolyte layer toward the side of the negative electrode current collector along the stacking direction of the power generation elements. During charging, the carbon material in the negative electrode intermediate layer receives lithium ions at the interface between the solid electrolyte layer and the carbon material and diffuses them toward the negative electrode current collector side. At this time, the smaller the R value (higher the crystallinity) of the carbon material present on the solid electrolyte layer side, the better the diffusion and the lower the resistance. As a result, the rapid charging characteristics can be improved.

[0024] On the other hand, in a lithium precipitation type lithium secondary battery, the electrode shrinkage during discharge is large, and the pressure of the negative electrode during discharge decreases as the discharge progresses. In particular, the contact area of the interface between metallic lithium and the negative electrode intermediate layer is likely to be insufficient. When the contact between metallic lithium and the negative electrode intermediate layer becomes insufficient, the lithium around it cannot participate in the discharge reaction, so the discharge capacity decreases. In contrast, a low-crystalline carbon material with a large R value has a small amount of deformation with respect to the pressurization by the restraint pressure when operating the battery and a large spring-back amount. Therefore, when a low-crystalline carbon material is disposed on the negative electrode current collector side of the negative electrode intermediate layer, even when the pressure decreases during discharge, the contact between the negative electrode intermediate layer and the lithium metal can be maintained continuously. As a result, the discharge capacity is improved, and thus the energy density can be improved.

[0025] In the lithium secondary battery according to this embodiment, in the negative electrode intermediate layer, the R value of the carbon material contained in the negative electrode intermediate layer gradually increases from the side of the solid electrolyte layer toward the side of the negative electrode current collector along the stacking direction of the power generation element. "The R value gradually increases" means that it gradually becomes larger, and there may be a region where the R value remains constant along the stacking direction of the power generation element. In one embodiment, along the stacking direction of the power generation element, the R value of the carbon material contained on the side of the solid electrolyte layer is smaller than the R value of the carbon material contained on the side of the negative electrode current collector. In one embodiment, in the negative electrode intermediate layer, the R value of the carbon material present near the interface with the solid electrolyte layer is smaller than the R value of the carbon material present near the interface with the negative electrode (negative electrode active material layer or negative electrode current collector). The R value of the carbon material can be measured by the method described in the examples below.

[0026] For example, when the negative electrode intermediate layer is composed of two layers, a layer on the side of the solid electrolyte layer and a layer on the side of the negative electrode current collector, along the stacking direction of the power generation element, the R value of the carbon material contained on the side of the solid electrolyte layer is not particularly limited, but is preferably 1.0 or less, more preferably 0.95 or less, still more preferably 0.9 or less, and even more preferably 0.88 or less. When it is within the above range, the rapid charging characteristics can be further improved. The lower limit value of the R value is not particularly limited, but is, for example, 0.6 or more, preferably 0.7 or more. When it is within the above range, since the crystallinity of the carbon material does not become too high, it is possible to prevent the anisotropy in the direction in which Li penetrates into the carbon material from becoming strong and making it difficult for Li to penetrate into the carbon material. Therefore, high battery performance can be obtained.

[0027] For example, when the negative electrode intermediate layer consists of two layers, namely the layer on the solid electrolyte layer side and the layer on the negative electrode current collector side, along the stacking direction of the power generation element, the R value of the carbon material contained on the negative electrode current collector side is not particularly limited, but is preferably 1.05 or more, more preferably 1.1 or more, and even more preferably 1.25 or more. When within the above range, the discharge capacity can be further improved. The upper limit value of the R value is not particularly limited, but is, for example, 1.48 or less, preferably 1.45 or less, and more preferably 1.40 or less. When within the above range, the Li conduction resistance near the interface with the negative electrode current collector does not become too high, and high battery performance can be obtained.

[0028] The method for controlling the R value of the carbon material contained in the negative electrode intermediate layer is not particularly limited. For example, it can be appropriately selected and combined from known carbon materials for use. As the carbon material, carbon black is preferably used. Among carbon blacks, for example, Ketjenblack (registered trademark) and acetylene black tend to have a low R value, and acetylene black tends to have an even lower R value. On the other hand, furnace black, thermal black, etc. tend to have a relatively high R value. Incidentally, for example, the R value of a known carbon material such as carbon black can be lowered by heat treatment at 2500 to 3500 °C to enhance crystallinity. Also, the R value can be increased by treating a known carbon material such as carbon black with an alkali such as KOH (alkali activation) to reduce crystallinity.

[0029] In a preferred embodiment, in the negative electrode intermediate layer, along the stacking direction of the power generation element, the R value of the carbon material contained on the solid electrolyte layer side is 1.0 or less, and the R value of the carbon material contained on the negative electrode current collector side is 1.05 or more. In a preferred embodiment, along the stacking direction of the power generation element, the R value of the carbon material contained on the solid electrolyte layer side is 0.9 or less. In a preferred embodiment, the R value of the carbon material contained on the negative electrode current collector side is 1.25 or more. In a preferred embodiment, in the negative electrode intermediate layer, along the stacking direction of the power generation element, the R value of the carbon material contained on the solid electrolyte layer side is 0.9 or less, and the R value of the carbon material contained on the negative electrode current collector side is 1.25 or more. When within the above ranges, the effects of the present invention can be obtained more remarkably.

[0030] In a preferred embodiment, two or more types of carbon materials contained in the negative electrode intermediate layer are present such that the R value of the carbon material increases stepwise or continuously from the solid electrolyte layer side toward the negative electrode current collector side along the stacking direction of the power generation element.

[0031] In a preferred embodiment, the negative electrode intermediate layer is composed of a plurality of layers containing different carbon materials, and the R values of the carbon materials contained in the plurality of layers increase in order as they move 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 more remarkably. At this time, the number of stacked layers is not particularly limited, but for example, it is 2 to 5 layers, preferably 2 to 3 layers, and more preferably 2 layers. When within the above ranges, the effects of the present invention can be obtained more effectively. In particular, when the negative electrode intermediate layer is composed of two layers, the effects of the present invention can be obtained more remarkably.

[0032] At this time, for the same reason as above, in the negative electrode intermediate layer, among the plurality of layers, the R value of the carbon material contained in the layer in contact with the solid electrolyte layer is preferably 1.0 or less, preferably 0.95 or less, more preferably 0.9 or less, and even more preferably 0.88 or less. The lower limit value of the R value is, for example, 0.6 or more, preferably 0.7 or more.

[0033] Also, for the same reason as described above, among the plurality of layers, the R value of the carbon material contained in the layer farthest from the solid electrolyte layer is not particularly limited, but is preferably 1.05 or more, preferably 1.1 or more, and more preferably 1.25 or more. When within the above range, the discharge capacity can be further improved. The upper limit value of the R value is, for example, 1.48 or less, preferably 1.45 or less, and more preferably 1.40 or less.

[0034] In a preferred embodiment, in the negative electrode intermediate layer, among the plurality of layers, the R value of the carbon material contained in the layer closest to the solid electrolyte layer (for example, the layer in contact with the solid electrolyte layer) is 1.0 or less, and the R value of the carbon material contained in the layer farthest from the solid electrolyte layer is 1.05 or more. In a preferred embodiment, the R value of the carbon material in the layer closest to the solid electrolyte layer is 0.9 or less. In a preferred embodiment, the R value of the carbon material contained in the layer farthest from the solid electrolyte layer is 1.25 or more. In a preferred embodiment, in the negative electrode intermediate layer, the R value of the carbon material contained in the layer closest to the solid electrolyte layer is 0.9 or less, and the R value of the carbon material contained in the layer farthest from the solid electrolyte layer is 1.25 or more. When within the above range, the effects of the present invention can be obtained more remarkably.

[0035] In a preferred embodiment, among the plurality of layers, the thickness of the layer closest to the solid electrolyte layer is smaller than the thickness of the layer farthest from the solid electrolyte. When the thickness of the layer closest to the solid electrolyte layer is small, the diffusivity of lithium ions is excellent. Further, the greater the thickness of the layer farthest from the solid electrolyte layer (the layer on the negative electrode side), the greater the spring-back amount due to the low-crystalline carbon material. From these facts, the effects of the present invention can be obtained more remarkably. At this time, since the rapid charging characteristics and the discharge capacity can be compatible at a higher level, it is particularly preferable that the negative electrode intermediate layer is composed of two layers. The thicknesses of these layers can be controlled by adjusting the composition of each layer including the type and amount of the carbon material contained in each layer, the coating amount, the pressing conditions, and the like. When the negative electrode intermediate layer is composed of two layers, the thickness of the layer on the solid electrolyte layer side (the second negative electrode intermediate layer) is, for example, 1.5 to 2.3 μm, and the thickness of the layer on the negative electrode current collector side (the first negative electrode intermediate layer) can be, for example, 2.5 to 3 μm, but is not limited thereto. The thickness of each layer can be determined by the method described in the examples.

[0036] For the same reason, among the plurality of layers, it is preferable that the porosity of the layer closest to the solid electrolyte layer is smaller than the porosity of the layer farthest from the solid electrolyte. The porosities of these layers can be controlled by adjusting the composition of each layer including the type and amount of the carbon material contained in each layer, the coating amount, the pressing conditions, and the like. When the negative electrode intermediate layer is composed of two layers, the porosity of the layer on the solid electrolyte layer side (the second negative electrode intermediate layer) is, for example, 37 to 53%, and the porosity of the layer on the negative electrode current collector side (the first negative electrode intermediate layer) can be, for example, 55 to 60%, but is not limited thereto.

[0037] The porosity of the negative electrode intermediate layer is a value calculated from the negative electrode intermediate layer in the discharged state with the restraint of the cell removed and the exterior body removed after the full discharge of the lithium secondary battery, and can be calculated using "3D-SEM".

[0038] Specifically, the power generation element is taken out from the evaluation cell after complete discharge, and a cross-section (laminated cross-section) perpendicular to the surface direction is exposed by ion milling. Using a focused ion beam-scanning electron microscope (FIB-SEM) manufactured by Hitachi High-Technologies Corporation, an SEM photograph of the laminated cross-section of the power generation element as viewed from the front is taken, and a photograph of the surface portion of the negative electrode intermediate layer (a photograph with a size of 5 μm × 5 μm) is taken. Then, the surface of the negative electrode intermediate layer is 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 is taken. The excavation of the surface of the negative electrode intermediate layer and the photographing of the surface of the negative electrode intermediate layer by the above ion beam irradiation are 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 is discriminated, the area of the voids existing in the 2D region is calculated, the area is integrated to calculate the volume of the voids in the 3D region, and the porosity is calculated by calculating the volume of the voids with respect to the volume of the entire 3D region.

[0039] Note that lithium metal in the negative electrode intermediate layer is regarded as a void. Specifically, when lithium metal remains in the voids in the evaluation cell after discharge, the lithium metal is calculated as a void in terms of area. The identification of lithium metal is carried out by identifying lithium metal by high-sensitivity EDS and by the contrast of the identified lithium metal, and its area is calculated.

[0040] The lithium reactive material contained in the negative electrode intermediate layer preferably further contains a metal material in addition to the carbon material described above. Among them, it is more preferable to contain a metal that can alloy with lithium during charging. By containing a metal that can alloy with lithium, lithium metal can be deposited more uniformly on the surface of the current collector. Examples of the metal that can alloy with lithium include In, Al, Si, Sn, Mg, Au, Ag, Zn, etc. Among them, In, Si, Sn, and Ag are preferable, and Ag is more preferable.

[0041] As a lithium-reactive material, it is also a preferred embodiment to use in combination the carbon material described above and a metal that can alloy with lithium. That is, according to a preferred embodiment of the present invention, the lithium-reactive material includes at least one selected from the group consisting of the above-described predetermined carbon material and a metal material that can alloy with lithium during charging. Thereby, the mechanical strength and lithium ion conductivity of the negative electrode intermediate layer can be sufficiently ensured. Further, short circuit can be further suppressed.

[0042] When the above metal material is in a particle shape, its average particle diameter is preferably 500 nm or less, more preferably 300 nm or less, still more preferably 200 nm or less, and particularly preferably 100 nm or less. The lower limit of the average particle diameter of the metal particles is not particularly limited, but is preferably 20 nm or more. When the average particle diameter of the metal particles is within the above range, it becomes easy to control the basis weight of the negative electrode intermediate layer within a predetermined range. In this specification, the average particle diameter of the metal particles is the particle diameter 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). ) about 50% cumulative diameter (D 50 ) means.

[0043] When the lithium-reactive material contains both the above-described predetermined carbon material and a metal that can alloy with lithium, their mixing ratio (mass ratio) is not particularly limited, but the carbon material: metal that can alloy with lithium (mass ratio) is preferably 10:1 to 1:1, more preferably 5:1 to 2:1. When it is within the above range, the battery performance is more excellent. At this time, the mass of the carbon material is the total mass of a plurality of types of carbon materials. Further, when a plurality of types of metals that can alloy with lithium are included, the mass thereof is the total amount of the plurality of types of metals that can alloy with lithium.

[0044] In addition, when the negative electrode active material layer is composed of a plurality of layers, each layer may contain a metal capable of alloying with lithium. When it contains a metal capable of alloying with lithium, the type thereof and the mixing ratio with the carbon material may be the same or different from each other.

[0045] If the negative electrode intermediate layer can form a self-supporting film only with a lithium-reactive material, it may be composed only of the lithium-reactive material, but may contain a binder as required. The type of the binder is not particularly limited, and those known in the art can be appropriately adopted. As an example, in addition to fluorine-based resins such as polyvinylidene fluoride (PVDF) (including compounds in which hydrogen atoms are substituted with other halogen elements) and polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), carboxymethyl cellulose (CMC), etc. can be mentioned. Among them, it is preferable that the binder of the negative electrode intermediate layer contains a fluorine-based resin, more preferably contains PVDF, and even more preferably is PVDF.

[0046] When the negative electrode intermediate layer contains a binder, the content of the binder is preferably more than 10 parts by mass, more preferably 15 parts by mass or more, based on 100 parts by mass in total of the carbon material and the metal capable of alloying with lithium. When the content of the binder is within the above range, the strength of the negative electrode intermediate layer is sufficiently ensured, so that cracks can be suppressed. The upper limit of the content of the binder is not particularly limited, but from the viewpoint of suppressing the increase in resistance, it is preferably 35 parts by mass or less based on 100 parts by mass in total of the carbon material and the metal capable of alloying with lithium.

[0047] The ratio of the total mass of the carbon material, the metal capable of alloying with lithium, and the 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, even more preferably 98% by mass or more, particularly preferably 99% by mass or more, and most preferably 100% by mass.

[0048] In addition, when the negative electrode active material layer is composed of a plurality of layers, each layer may contain a binder. When a binder is included, the type and mixing ratio thereof may be the same or different from each other.

[0049] The method for manufacturing the negative electrode intermediate layer in the lithium precipitation type lithium secondary battery of this embodiment is not particularly limited. For example, when manufacturing a lithium secondary battery, a two-step pressing process may be adopted in which after subjecting the solid electrolyte layer to a pressing process at a predetermined pressure, the solid electrolyte layer and the negative electrode intermediate layer are laminated and then subjected to a pressing process at a predetermined pressure.

[0050] More specifically, a solid electrolyte slurry containing a solid electrolyte is applied to the surface of a support (for example, 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 at a predetermined pressure (first pressing step). Thereby, the arrangement of the solid electrolyte particles on the surface of the solid electrolyte layer adjacent to the support is adjusted, and the unevenness is 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. Further, 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 in a state where the solid electrolyte layer and the positive electrode active material layer are 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 in a state where the solid electrolyte layer and the positive electrode active material layer are overlapped.

[0051] On the other hand, a negative electrode intermediate layer slurry containing a material (lithium reactive material and, if necessary, a binder) contained in the negative electrode intermediate layer is applied to the surface of a negative electrode current collector (for example, a stainless steel foil), and the coating film is dried to obtain a negative electrode intermediate layer formed on the surface of the negative electrode current collector. Then, the support (metal foil) used in the first pressing step is peeled off to expose the solid electrolyte layer, and the exposed surface of the solid electrolyte layer and the exposed surface of the negative electrode intermediate layer are overlapped so as to face each other, and pressed at a predetermined pressure (second pressing step).

[0052] At this time, prepare two or more negative electrode intermediate layer slurries containing carbon materials with different R values, and coat the surface of the negative electrode current collector in order from the negative electrode intermediate layer slurry containing the carbon material with a large R value to obtain a laminate of two or more negative electrode intermediate layers. Then, stack them so that the exposed surface of the solid electrolyte layer faces the exposed surface of the obtained laminate, and perform a second pressing step. As a result, a negative electrode intermediate layer composed of a plurality of layers can be obtained, and the R values of the carbon materials contained in each layer increase in order as they move away from the solid electrolyte layer along the stacking direction of the power generation element.

[0053] Further, after coating a negative electrode intermediate layer slurry containing a carbon material with a relatively large (low crystallinity) R value on the negative electrode current collector, a carbon material with a relatively small (high crystallinity) R value is vapor-deposited from the surface of the obtained coating film, so that there are many high-crystalline carbon materials near the surface of the coating film, and there are many low-crystalline carbon materials as approaching the negative electrode current collector. A coating film having a gradation in the thickness direction of the R value is formed. By stacking the exposed surface of this coating film and the exposed surface of the solid electrolyte layer so as to face each other and performing a second pressing step, a negative electrode intermediate layer in which the R value of the carbon material continuously increases as it moves away from the solid electrolyte layer along the stacking direction of the power generation element can be obtained.

[0054] Note that cold isostatic pressing (CIP) is preferably used for the pressing in the first pressing step and the second pressing step, but it is not limited thereto.

[0055] The pressing pressure in the first pressing step varies depending on the materials contained in the solid electrolyte layer. When the first pressing step is performed with the solid electrolyte layer and the positive electrode active material layer stacked, it varies depending on the materials contained in the solid electrolyte layer and the positive electrode active material layer, 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 even more preferably 500 MPa or more and 700 MPa or less. The pressing temperature in the first pressing step is not particularly limited either. For example, it is 20 to 80°C, preferably 20 to 40°C. The pressing time in the first pressing step is not particularly limited either. For example, it is 10 seconds to 30 minutes, preferably 10 seconds to 10 minutes.

[0056] The pressing pressure in the second pressing step varies depending on the materials contained in the negative electrode intermediate layer and can be appropriately set by those skilled in the art. For example, the pressing pressure in the second pressing step is preferably 100 MPa or more and 700 MPa or less, more preferably 300 MPa or more and 700 MPa or less, and even more preferably 500 MPa or more and 700 MPa or less.

[0057] The pressing temperature in the second pressing step varies depending on the materials contained in the negative electrode intermediate layer 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 20 to 80°C.

[0058] The pressing time in the second pressing step varies depending on the materials contained in the negative electrode intermediate layer and can be appropriately set by those skilled in the art. For example, the pressing time in the second pressing step is preferably 10 seconds to 30 minutes, more preferably 10 seconds to 10 minutes.

[0059] The thickness of the negative electrode intermediate layer (when the negative electrode intermediate layer is composed of a plurality of layers, the total thickness thereof) is not particularly limited, but is preferably 10 μm or less, more preferably 7 μm or less, and still more preferably 6 μm or less. When the thickness of the negative electrode intermediate layer is 10 μm or less, the path of lithium ions moving through the negative electrode intermediate layer during charging does not become too long, so the charging resistance can be kept low. In addition, 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, and still more preferably 4 μm or more.

[0060] [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 employed. As an example, LPS (Li2S-P2S5), Li6PS5X (where X is Cl, Br or I), Li7P3S 11 , Li 3.2 P 0.96 Sulfide solid electrolytes such as S and Li3PS4 can be mentioned. These sulfide solid electrolytes have excellent lithium ion conductivity and are preferably used because they can follow the volume change of the electrode active material accompanying charge and discharge due to their low bulk modulus of elasticity. These solid electrolytes may be used alone or in combination of two or more. Needless to say, solid electrolytes other than the above may be used.

[0061] The ionic conductivity (for example, Li ion conductivity) of the solid electrolyte at room temperature (25 °C) is preferably, for example, 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 AC impedance method.

[0062] 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 even more preferably 0.5 μm or more and 10 μm or less.

[0063] The content of the solid electrolyte in the solid electrolyte layer is preferably 50% by mass or more and 100% by mass or less, and more preferably 90% by mass or more and 99% by mass.

[0064] The solid electrolyte layer may further contain a binder in addition to the solid electrolyte. The binder that can be used in the solid electrolyte layer is the same as that described in the negative electrode intermediate layer above.

[0065] The thickness of the solid electrolyte layer varies depending on the configuration of the target all-solid-state lithium secondary battery, but is usually 0.1 μm or more and 1000 μm or less, and preferably 10 μm or more and 40 μm or less.

[0066] [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 / or a conductive assistant as necessary. The positive electrode active material layer is typically disposed on the surface of the positive electrode current collector as shown in FIG. 1. However, when the positive electrode active material layer 15 itself has a certain conductivity, etc., it is also possible to form the positive electrode without using a positive electrode current collector and with the positive electrode active material layer itself.

[0067] The type of the positive electrode active material contained in the positive electrode active material layer is not particularly limited, but a lithium-containing metal oxide is preferred. That is, according to a preferred embodiment of the present invention, the positive electrode active material contains at least one selected from lithium-containing metal oxides. According to a more preferred embodiment of the present invention, the positive electrode active material is composed of only at least one selected from lithium-containing metal oxides. Specific examples of the lithium-containing metal oxides include layered rock salt type active materials such as LiCoO2, LiMnO2, LiNiO2, 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. Further, examples of the oxide active materials other than the above include, for example, Li4Ti5O 12 , LiVO2. Among them, Li(Ni-Mn-Co)O2 and those in which a part of these transition metals is substituted by other elements (NMC composite oxides) are preferably used as the positive electrode active material. These positive electrode active materials may be used alone or in combination of two or more.

[0068] 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.

[0069] The shape of the positive electrode active material includes, for example, particulate (spherical, fibrous), thin film, etc. When the positive electrode active material is in a particulate shape, its average particle diameter (D 50 ) is preferably in the range of, for example, 1 nm to 100 μm, more preferably in the range of 10 nm to 50 μm, still more preferably in the range of 100 nm to 20 μm, and particularly preferably in the range of 1 to 20 μm. In this specification, the value of the average particle diameter (D 50 ) of the positive electrode active material can be measured by the laser diffraction scattering method.

[0070] Although the content of the positive electrode active material is not particularly limited, from the viewpoint of energy density, it is, for example, 50% by mass or more and 99% by mass or less, preferably 70% by mass or more and 99% by mass or less, more preferably 80% by mass or more and 99% by mass or less, still more preferably 83% by mass or more and 97% by mass or less, and particularly preferably 83% by mass or more and 95% by mass or less, based on the total mass of the positive electrode active material layer.

[0071] In addition to the positive electrode active material, the positive electrode active material layer may further contain a solid electrolyte, a binder, and / or a conductive assistant. Here, the solid electrolyte that can be used for the positive electrode active material layer is the same as that described for the solid electrolyte layer above. The binder that can be used for the positive electrode active material layer is the same as that described for the negative electrode intermediate layer above. Examples of the conductive assistant include metals such as aluminum, stainless steel (SUS), silver, gold, copper, and titanium, alloys or metal oxides containing these metals; carbon fibers (specifically, vapor-grown carbon fibers (VGCF), polyacrylonitrile-based carbon fibers, pitch-based carbon fibers, rayon-based carbon fibers, activated carbon fibers, etc.), carbon nanotubes (CNT), carbon blacks (specifically, acetylene black, ketjen black (registered trademark), furnace black, channel black, thermal lamp black, etc.), but are not limited thereto. Also, those obtained by coating the above metal materials around particulate ceramic materials or resin materials by plating or the like can be used as the conductive assistant.

[0072] The thickness of the positive electrode active material layer varies depending on the configuration of the intended all-solid-state lithium secondary battery, but is, for example, 0.1 μm or more and 1000 μm or less, preferably 30 μm or more and 300 μm or less, more preferably 50 μm or more and 200 μm or less, and still more preferably 70 μm or more and 150 μm or less.

[0073] [Positive Electrode Current Collector and Negative Electrode Current Collector] The material constituting the current collectors (25, 27) is not particularly limited, and known highly conductive materials conventionally used as current collectors for secondary batteries can be used. As the constituent material of the current collector, for example, metal materials such as aluminum, copper, titanium, nickel, stainless steel (SUS), and alloys thereof are preferable. From the viewpoints of light weight, corrosion resistance, and high conductivity, aluminum and copper are more preferable, and aluminum is particularly preferable. Note that the same material may be used for the positive electrode current collector 27 and the negative electrode current collector 25, or different materials may be used.

[0074] [Positive electrode lead and negative electrode lead] Although not shown in the figure, the current collectors (11”, 11’) and the current collectors (27, 25) may be electrically connected via a positive electrode lead or a negative electrode lead. As the constituent material of the positive electrode and negative electrode leads, materials used in known lithium secondary batteries can be similarly adopted. Note that the portion taken out from the exterior is preferably covered with a heat shrinkable tube having heat insulation properties or the like so as not to affect the product (for example, automotive parts, particularly electronic devices, etc.) due to leakage caused by contact with peripheral devices, wiring, etc.

[0075] [Battery exterior material] As the battery exterior material, a known metal can case can be used, and in addition, as shown in FIG. 1, a bag-shaped case using a laminate film 29 containing aluminum that can cover the power generation element can be used. For the laminate film, for example, a laminate film having a three-layer structure formed by laminating PP, aluminum, and nylon in this order can be used, but it is not limited thereto. From the viewpoint of excellent high output and cooling performance and suitability for use in large-sized devices for EV and HEV, a laminate film is desirable. Further, since the external pressure applied to the power generation element can be easily adjusted, a laminate film containing aluminum is more preferable as the exterior material.

[0076] The all-solid-state lithium secondary battery according to this embodiment has a configuration in which a plurality of single battery layers are connected in parallel, and thus has a high capacity and excellent cycle durability. Therefore, the all-solid-state lithium secondary battery according to this embodiment is suitably used as a driving power source for EVs and HEVs.

[0077] 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 the configuration described in the above-described embodiment, and can be appropriately changed based on the description of the claims.

[0078] For example, as the 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 coupled to one surface of a current collector and a negative electrode active material layer electrically coupled to the opposite surface of the current collector can also be mentioned.

[0079] Further, 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.

[0080] 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 2 having the features of claim 3; the lithium secondary battery according to claim 2 or 3 having the features of claim 4; the lithium secondary battery according to any one of claims 2 to 4 having the features of claim 5; the lithium secondary battery according to any one of claims 2 to 5 having the features of claim 6; the lithium secondary battery according to any one of claims 2 to 6 having the features of claim 7.

Examples

[0081] Hereinafter, the present invention will be described in more detail with reference to 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 inside a glove box with a dew point of -68°C or lower. Also, the instruments and devices used inside the glove box were sufficiently dried in advance.

[0082] <Production Example of Evaluation Cell> [Example 1] (Fabrication of Positive Electrode) The NMC composite oxide (LiNi 0.8 Mn 0.1 Co 0.1 O2) as the positive electrode active material, carbon fiber as the conductive assistant, and argyrodite-type sulfide solid electrolyte (Li6PS5Cl) as the solid electrolyte were weighed so as to have a mass ratio of 85:15:5. These were mixed using an agate mortar and then further stirred and mixed using a planetary ball mill. To 100 parts by mass of the obtained mixed powder, 2 parts by mass of polytetrafluoroethylene (PTFE) as a binder was added and mixed. The obtained mixture was stacked on an aluminum foil as the positive electrode current collector and subjected to a pressing treatment to obtain a positive electrode having a positive electrode active material layer (thickness: 100 μm) on the surface of the positive electrode current collector.

[0083] (Fabrication of Solid Electrolyte Layer) To 100 parts by mass of the argyrodite-type sulfide solid electrolyte (Li6PS5Cl, average particle diameter (D 50 ): 0.8 μm) as the solid electrolyte, 2 parts by mass of styrene-butadiene rubber (SBR) as a binder was added, and mesitylene as a solvent was added and mixed to prepare a solid electrolyte slurry. The solid electrolyte slurry was coated on the surface of a stainless steel foil as a support and dried to obtain a solid electrolyte layer (thickness: 40 μm).

[0084] (Fabrication of Negative Electrode Intermediate Layer) Silver nanoparticles (average particle diameter (D 50): 60 nm), and carbon black A (R value: 1.40) were weighed and mixed so that the mass ratio of silver nanoparticles: carbon black A was 1:3. To 86 parts by mass of the obtained mixture, 14 parts by mass of polyvinylidene fluoride (PVDF) as a binder was added, and NMP was added as a solvent and mixed to prepare a first negative electrode intermediate layer slurry. The first negative electrode intermediate layer slurry was applied to the surface of a stainless steel foil as a negative electrode current collector and dried to obtain a first negative electrode intermediate layer (on the negative electrode current collector side, basis weight 0.25 mg / cm 2 ). The thickness of the first negative electrode intermediate layer before pressing was 7.0 μm.

[0085] Subsequently, silver nanoparticles (average particle diameter (D 50 ): 60 nm) and carbon black E (R value: 0.86) were weighed and mixed so that the mass ratio of silver nanoparticles: carbon black E was 1:3. To 86 parts by mass of the obtained mixture, 14 parts by mass of polyvinylidene fluoride (PVDF) as a binder was added, and NMP was added as a solvent and mixed to prepare a second negative electrode intermediate layer slurry. The second negative electrode intermediate layer slurry was applied to the surface of the first layer of the negative electrode intermediate layer and dried to obtain a second negative electrode intermediate layer (on the solid electrolyte layer side, basis weight 0.25 mg / cm 2 ). The thickness of the second negative electrode intermediate layer before pressing was 3.9 μm.

[0086] (Fabrication of Evaluation Cell) The positive electrode active material layer formed on the surface of the aluminum foil (positive electrode current collector) and the solid electrolyte layer formed on the surface of the stainless steel foil were overlapped so that the exposed surface of the positive electrode active material layer faced the exposed surface of the solid electrolyte layer, and pressed at 700 MPa for 1 minute by cold isostatic pressing (CIP) (first pressing step). As a result, the solid electrolyte layer was transferred to the exposed surface of the positive electrode active material layer. After peeling off the stainless steel foil adjacent to the solid electrolyte layer, the solid electrolyte layer and the negative electrode intermediate layer formed on the surface of the stainless steel foil (negative electrode current collector) were overlapped so that the exposed surface of the solid electrolyte layer faced the exposed surface of the second negative electrode intermediate layer, and pressed at 500 MPa for 1 minute by cold isostatic pressing (CIP) (second pressing step). As a result, the negative electrode intermediate layer was transferred to the exposed surface of the solid electrolyte layer. Finally, an aluminum positive electrode tab and a nickel negative electrode tab were joined to each of the aluminum foil (positive electrode current collector) and the stainless steel foil (negative electrode current collector) by an ultrasonic welder, and the obtained laminate was placed inside an aluminum laminate film and vacuum-sealed to fabricate an evaluation cell of this example, which is a lithium precipitation type all-solid-state lithium secondary battery.

[0087] [Example 2] In the above (fabrication of the negative electrode intermediate layer), an evaluation cell of this example was fabricated using the same method as in Example 1 described above, except that the carbon black E used for the second negative electrode intermediate layer was changed to carbon black D (R value: 0.95). The thickness of the second negative electrode intermediate layer before the pressing treatment was 4.6 μm.

[0088] [Example 3] In the above (fabrication of the negative electrode intermediate layer), an evaluation cell of this example was fabricated using the same method as in Example 1 described above, except that the carbon black A used for the first negative electrode intermediate layer was changed to carbon black C (R value: 1.14). The thickness of the first negative electrode intermediate layer before the pressing treatment was 5.4 μm.

[0089] [Comparative Example 1] In the above (fabrication of the negative electrode intermediate layer), carbon black A used for the first negative electrode intermediate layer was changed to carbon black B (R value: 1.20), and carbon black E used for the second negative electrode intermediate layer was changed to carbon black B (R value: 1.20). Except for the above, the evaluation cell of this comparative example was fabricated using the same method as in Example 1 described above. Incidentally, the thickness of the first negative electrode intermediate layer and the thickness of the second negative electrode intermediate layer before the pressing process were 4.3 μm, respectively.

[0090] [Comparative Example 2] In the above (fabrication of the negative electrode intermediate layer), carbon black A used for the first negative electrode intermediate layer was changed to carbon black E (R value: 0.86), and carbon black E used for the second negative electrode intermediate layer was changed to carbon black A (R value: 1.40). Except for the above, the evaluation cell of this comparative example was fabricated using the same method as in Example 1 described above. Incidentally, the thickness of the first negative electrode intermediate layer before the pressing process was 3.9 μm, and the thickness of the second negative electrode intermediate layer before the pressing process was 7.0 μm.

[0091] [Comparative Example 3] In the above (fabrication of the negative electrode intermediate layer), except that carbon black A used for the first negative electrode intermediate layer was changed to carbon black E (R value: 0.86), the evaluation cell of this comparative example was fabricated using the same method as in Example 1 described above. Incidentally, the thickness of the first negative electrode intermediate layer before the pressing process was 3.9 μm.

[0092] [Comparative Example 4] In the above (fabrication of the negative electrode intermediate layer), except that carbon black E used for the second negative electrode intermediate layer was changed to carbon black A (R value: 1.40), the evaluation cell of this comparative example was fabricated using the same method as in Example 1 described above. Incidentally, the thickness of the second negative electrode intermediate layer before the pressing process was 7.0 μm.

[0093] [Measurement of R value of carbon material] The R value of the carbon material contained in the negative electrode intermediate layer was determined by taking out the power generation element from the evaluation cell after full discharge, exposing a cross-section (laminated cross-section) perpendicular to the surface direction by ion milling, and performing microscopic Raman measurement on the negative electrode intermediate layer.

[0094] For the Raman spectrum measurement, a Renishaw inVia confocal Raman microscope (model: inVia Qontor) was used and the measurement was performed at 500 cm -1 ~3500 cm -1 . Baseline correction and smoothing were performed under the following conditions to obtain the peak intensity (height): <Baseline correction> Fit mode: Intelligent fit Baseline type: Intelligent polynomial (polynomial degree: 3) Noise tolerance range: 1.50 <Smoothing> Smoothed window: 17 Degree of polynomial: 2.

[0095] When the peak intensity of the peak (G band) appearing at 1550~1600 cm in the obtained Raman spectrum was I -1 and the peak intensity of the peak (D band) appearing at 1300~1400 cm G was I -1 , the ratio I D / I D / I G of the two intensities was defined as the R value.

[0096] Note that the R value of the carbon material can also be measured with the carbon material used as the sample as it is. It was confirmed that the R value measured with the carbon material used as the sample as it is was consistent with the R value measured in the state of the negative electrode intermediate layer. In this example, for the evaluation cell after the charge-discharge test described later, Raman spectrum measurement was performed in the state of the above negative electrode intermediate layer after full discharge. However, for the evaluation cell before the first charge, similar R values of the carbon material were obtained respectively.

[0097] The R values of the negative electrode intermediate layers (the first negative electrode intermediate layer and the second negative electrode intermediate layer) obtained for each evaluation cell are shown in Table 1 below. In Table 1, "SE layer side" indicates the negative electrode intermediate layer (the second negative electrode intermediate layer) on the solid electrolyte side, and "current collector side" indicates the negative electrode intermediate layer (the first negative electrode intermediate layer) on the negative electrode current collector side, respectively.

[0098] <Measurement of the thickness of the negative electrode intermediate layer> The thickness of the negative electrode intermediate layer was calculated from the negative electrode intermediate layer in the discharged state with the cell restraint removed and the exterior removed after the full discharge of the lithium secondary battery.

[0099] Specifically, after removing the power generation element from the evaluation cell after full discharge, a cross-section (laminated cross-section) perpendicular to the plane direction was exposed by ion milling.

[0100] The above cross-section was observed by SEM, and the thickness was measured at several to several tens of different locations in the negative electrode intermediate layer (the first negative electrode intermediate layer and the second negative electrode intermediate layer), and the arithmetic mean value thereof was taken as the thickness of the negative electrode intermediate layer.

[0101] In this example, for the evaluation cell after the charge-discharge test described later, the thickness of the negative electrode intermediate layer was measured after full discharge. However, for the evaluation cell before the first charge, the same thickness values of the negative electrode intermediate layer were obtained.

[0102] The thicknesses of the negative electrode intermediate layers (the first negative electrode intermediate layer and the second negative electrode intermediate layer) obtained for each evaluation cell are shown in Table 1 below.

[0103] <Charge-discharge test> A positive electrode lead and a negative electrode lead were connected to the positive electrode current collector and the negative electrode current collector of the evaluation cell (before the first charge) fabricated above, and charge-discharge was performed according to the following charge-discharge test conditions. At this time, the following charge-discharge test was performed while applying a constraint pressure of 3 MPa in the lamination direction of the evaluation cell using a pressure member.

[0104] (Charge-discharge test conditions) Evaluation temperature: 333 K (60 °C) Voltage range: 2.5~4.3V Charging process (1): CC (cut-off in 25 hours) Charging rate (1): 0.01C Charging process (2): CC Charging rate (2): 0.05C Discharging process: CC Discharging rate: 0.1C After charge and discharge, rest for 30 minutes respectively.

[0105] For the evaluation cell, in the first charging process (lithium metal precipitates on the negative current collector) in a thermostatic bath set to the above evaluation temperature using a charge-discharge tester, it was in the constant current (CC) mode and charged at 0.01C for 25 hours. Then, in the second charging process, it was also in the constant current (CC) mode and charged at 0.05C until 4.3V. Then, in the discharging process (lithium metal on the negative current collector dissolves), it was in the constant current (CC) mode and discharged at 0.1C until 2.5V. Here, 1C means the current value at which, when charged for 1 hour at that current value, the battery just reaches the fully charged (100% charged) state. Taking the above charge and discharge as the first cycle of charge and discharge, when performing the second cycle of charge and discharge, the discharge capacity (mAh / g) was obtained and shown in Table 1 below.

[0106] <DCR during charging> Following the above charge-discharge test, it was charged to SOC (State Of Charge) 50% at 0.1C. Then, from the state of SOC 50%, it was charged for 10 seconds in sequence at 0.05C, 0.1C, 0.2C, and 0.5C. After charging at each rate, it was discharged at 0.05C by the amount of the charged capacity to adjust to SOC 50%. The current value at each rate and the slope of the voltage increase during the 10-second charging were calculated by the least squares method and taken as DCR (Ω). Then, the charging DCR (Ω·cm 2 ) was calculated from the product of the positive electrode area and DCR (Ω). The results are shown in Table 1 below.

[0107]

Table 1

[0108] As shown in Table 1, in the lithium precipitation type lithium secondary battery having a negative electrode intermediate layer as in Examples 1 to 3, by configuring the R value of the carbon material contained in the negative electrode intermediate layer to increase from the side of the solid electrolyte layer toward the side of the negative electrode current collector along the stacking direction of the power generation element, it can be seen that excellent performance can be obtained in both the discharge capacity and the charging DCR.

Explanation of Signs

[0109] 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 collector plate, 27 Positive electrode current collector 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 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; a negative electrode intermediate layer interposed between the negative electrode current collector and the solid electrolyte layer and containing a lithium reactive material; a power generation element having the above; the lithium reactive material contains two or more types of carbon materials; a lithium secondary battery, wherein an R value represented by a ratio of the intensity of the D band to the intensity of the G band in the Raman spectrum of the carbon material contained in the negative electrode intermediate layer increases gradually from the side of the solid electrolyte layer toward the side of the negative electrode current collector along the stacking direction of the power generation element.

2. The lithium secondary battery according to claim 1, wherein the negative electrode intermediate layer is composed of a plurality of layers containing different carbon materials, and the R values of the carbon materials contained in the plurality of layers increase 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 2, wherein the R value of the carbon material contained in the layer closest to the solid electrolyte layer among the plurality of layers is 1.0 or less, and the R value of the carbon material contained in the layer farthest from the solid electrolyte layer is 1.05 or more.

4. The lithium secondary battery according to claim 3, wherein the R value of the carbon material in the layer closest to the solid electrolyte layer among the plurality of layers is 0.9 or less.

5. The lithium secondary battery according to claim 4, wherein the R value of the carbon material contained in the layer farthest from the solid electrolyte among the plurality of layers is 1.25 or more.

6. The lithium secondary battery according to claim 2, wherein the thickness of the layer closest to the solid electrolyte layer among the plurality of layers is smaller than the thickness of the layer farthest from the solid electrolyte.

7. The lithium secondary battery according to claim 2, wherein the negative electrode intermediate layer is composed of two layers.

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