Lithium secondary battery

Abstract

[Problem] To provide a means capable of improving cycle durability in a lithium deposition type lithium secondary battery provided with a negative electrode intermediate layer. [Solution] This lithium secondary battery comprises a power generation element having: a positive electrode which has a positive electrode active material layer containing a positive electrode active material; a negative electrode which has a negative electrode current collector and in which lithium metal is deposited during charging; a solid electrolyte layer which is interposed between the positive electrode and the negative electrode and contains a solid electrolyte; and a negative electrode intermediate layer which is interposed between the negative electrode current collector and the solid electrolyte layer and contains a lithium reactive material and a binder. In the lithium secondary battery, the lithium reactive material contains a carbon material, the DBP absorption amount of the carbon material is more than 200 [mL / 100 g] but less than 280 [mL / 100 g], and the content of the binder in the negative electrode intermediate layer is 14-34 mass%.

Description

Lithium secondary battery

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

[0002] In recent years, research and development into all-solid-state lithium secondary batteries using oxide- or sulfide-based solid electrolytes has been actively pursued. Solid electrolytes are materials primarily composed of ionic conductors capable of ion conduction in a solid state. Therefore, all-solid-state lithium secondary batteries, in principle, do not encounter the various problems associated with flammable organic electrolytes that occur in conventional liquid-based lithium secondary batteries. In addition, the use of high-potential, high-capacity positive electrode materials and high-capacity negative electrode materials generally leads to significant improvements in the battery's power density and energy density.

[0003] One type of all-solid-state lithium secondary battery is known as a lithium deposition type, in which lithium metal is deposited on a negative electrode current collector during charging. While lithium deposition type all-solid-state lithium secondary batteries have excellent energy density and output characteristics, they suffer from the problem of susceptibility to short circuits due to dendrites from the lithium metal layer. As a means of suppressing dendrite growth, 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 negative electrode current collector.

[0004] For example, Japanese Patent Application Laid-Open No. 2019-96610 discloses a technique for controlling the ratio of the initial charge capacity of a negative electrode active material layer (negative electrode intermediate layer) to the initial charge capacity of a positive electrode active material layer in a lithium deposition-type all-solid-state lithium secondary battery to 0.01 to 0.5. Japanese Patent Application Laid-Open No. 2019-96610 also discloses that the binder content in the negative electrode active material layer (negative electrode intermediate layer) is 0.3 to 15 mass%.

[0005] However, in the lithium secondary battery described in the above document, a short circuit occurs early in the region where the binder content is relatively high in the negative electrode active material layer (negative electrode intermediate layer), and there is a problem that sufficient cycle durability cannot be obtained.

[0006] Therefore, an object of the present invention is to provide a means for improving the cycle durability of a lithium deposition type lithium secondary battery having a negative electrode intermediate layer.

[0007] The present inventors have conducted extensive research in light of the above-described problems and have found that the above-described problems can be solved by incorporating a carbon material that exhibits a predetermined DBP absorption amount into the negative electrode intermediate layer as a lithium-reactive material in a region where the binder content in the negative electrode intermediate layer is relatively high, thereby completing the present invention.

[0008] That is, one aspect of the present invention relates to a lithium secondary battery including a power generating element including a positive electrode having a positive electrode active material layer containing a positive electrode active material, a negative electrode having a negative electrode current collector and on which lithium metal is deposited during charging, a solid electrolyte layer interposed between the positive electrode and the negative electrode and containing a solid electrolyte, and a negative electrode intermediate layer interposed between the negative electrode current collector and the solid electrolyte layer and containing a lithium-reactive material and a binder. The lithium secondary battery is characterized in that the lithium-reactive material contains a carbon material, the DBP absorption of the carbon material is more than 200 [mL / 100g] and less than 280 [mL / 100g], and the content of the binder in the negative electrode intermediate layer is 14 to 34 mass%.

[0009] FIG. 1 is a cross-sectional view schematically illustrating the overall structure of a stacked-type (internal parallel connection type) all-solid-state lithium secondary battery (stacked-type secondary battery) according to one embodiment of the present invention.

[0010] One aspect of the present invention is a lithium secondary battery including a power generating element including a positive electrode having a positive electrode active material layer containing a positive electrode active material, a negative electrode having a negative electrode current collector and on which lithium metal is deposited during charging, a solid electrolyte layer interposed between the positive electrode and the negative electrode and containing a solid electrolyte, and a negative electrode intermediate layer interposed between the negative electrode current collector and the solid electrolyte layer and containing a lithium-reactive material and a binder, wherein the lithium-reactive material contains a carbon material, the DBP absorption of the carbon material is more than 200 [mL / 100 g] and less than 280 [mL / 100 g], and the binder content in the negative electrode intermediate layer is 14 to 34 mass %. The lithium secondary battery according to this aspect enables improved cycle durability in lithium deposition-type lithium secondary batteries including a negative electrode intermediate layer.

[0011] Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings. In the description of the drawings, the same elements are designated by the same reference numerals, and duplicate explanations will be omitted. Furthermore, the dimensional proportions in the drawings are exaggerated for the convenience of explanation and may differ from the actual proportions.

[0012] FIG. 1 is a cross-sectional view schematically illustrating the overall structure of a stacked-type (internal parallel connection type) all-solid-state lithium secondary battery (hereinafter also simply referred to as a "stacked-type secondary battery") according to one embodiment of the present invention. FIG. 1 shows a cross section of the stacked-type secondary battery during charging. The stacked-type secondary battery 10a shown in FIG. 1 has a structure in which a substantially rectangular power-generating element 21, where charge and discharge reactions actually proceed, is sealed inside a laminate film 29, which is a battery exterior. The power-generating element 21 has a structure in which a negative electrode, a solid electrolyte layer 17, and a positive electrode are stacked. The negative electrode has a structure in which a negative electrode current collector 11′ and a negative electrode active material layer 13 made of lithium metal deposited on the surface of the negative electrode current collector 11′ are stacked. A negative electrode intermediate layer 14 is disposed so as to contact 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". As a result, 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 unit cell layer 19. Therefore, it can be said that the stacked secondary battery 10a shown in FIG. 1 has a structure in which a plurality of unit cell layers 19 are stacked and electrically connected in parallel. A negative electrode current collector 25 and a positive electrode current collector 27 that are electrically connected to the respective electrodes (negative and positive electrodes) are attached to the negative electrode current collector 11′ and the positive electrode current collector 11″, respectively, and are configured to be sandwiched between the ends of the laminate film 29 and extend to the outside of the laminate film 29. A restraining pressure is applied to the stacked secondary battery 10a in the stacking direction of the power generating element 21 by a pressure member (not shown). Therefore, the volume of the power generating element 21 is kept constant.

[0013] The main components of the all-solid-state lithium secondary battery according to this embodiment will be described below.

[0014] [Current Collector] The current collector (negative electrode current collector, positive electrode current collector) has the function of mediating the movement of electrons from the electrode active material layer (negative electrode active material layer, positive electrode active material layer). There are no particular limitations on the material constituting the current collector. For example, metals such as aluminum, nickel, iron, stainless steel, titanium, and copper, as well as conductive resins, can be used as the material constituting the current collector. There are also no particular limitations on the thickness of the current collector, but an example is 10 to 100 μm.

[0015] [Negative Electrode Active Material Layer] The all-solid-state lithium secondary battery according to this embodiment is a so-called lithium deposition type battery in which lithium metal is deposited on the negative electrode current collector during charging. The layer of lithium metal deposited on the negative electrode current collector during charging is the negative electrode active material layer of the all-solid-state lithium secondary battery according to this embodiment. Therefore, the thickness of the negative electrode active material layer increases as the charging process progresses, and decreases as the discharging process progresses. Although the negative electrode active material layer does not need to be present during full discharge, in some cases, a negative electrode active material layer composed of a certain amount of lithium metal may be present during full discharge. Furthermore, the thickness of the negative electrode active material layer (lithium metal layer) during full charge is not particularly limited, but is typically 0.1 to 1000 μm.

[0016] [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 and a binder. In the lithium secondary battery according to this embodiment, the lithium reactive material essentially contains a carbon material.

[0017] Carbon materials are a type of material capable of absorbing lithium ions during charging. Specific examples of carbon materials include carbon black (specifically, acetylene black, Ketjen Black (registered trademark), furnace black, channel black, thermal lamp black, etc.), carbon nanotubes (CNT), graphite, hard carbon, etc. Among these, carbon black is preferred, and at least one selected from the group consisting of acetylene black, Ketjen Black (registered trademark), furnace black, channel black, and thermal lamp black is more preferred.

[0018] The lithium secondary battery according to the present embodiment is characterized in that the DBP absorption value of the carbon material described above is greater than 200 mg / 100 g and less than 280 mg / 100 g. Here, "DBP absorption" refers to the amount of DBP (dibutyl phthalate) absorbed per 100 g of carbon black (mL / 100 g) and is used as an indicator of the degree of development of the carbon material's structure (particle-to-particle connection). Furthermore, in this specification, the "DPB absorption" value is calculated in accordance with JIS K 6217-4:2017, "Carbon Black for Rubber—Fundamental Properties—Part 4: Determination of Oil Absorption (Including Compressed Samples)." Furthermore, when two or more materials are used as the carbon material, the weighted average of the DBP absorption values ​​of each material, weighted by mass, is used as the DBP absorption value of the carbon material. From the viewpoint of highly improving cycle durability, the DBP absorption value of the carbon material as the lithium-reactive material contained in the negative electrode intermediate layer is preferably 210 to 270 [mL / 100g], more preferably 215 to 255 [mL / 100g], and even more preferably 220 to 235 [mL / 100g]. The DBP absorption value of the carbon material can be changed by changing the manufacturing conditions for the carbon material, and for details, the common general technical knowledge in this technical field can be appropriately referenced. Alternatively, the DBP absorption value can be changed by using two or more materials having different DBP absorption values ​​and adjusting the blending amounts of each material.

[0019] The lithium-reactive material contained in the negative electrode intermediate layer preferably further contains a metal material in addition to the above-mentioned carbon material. Among these, it is more preferable to contain a metal that can be alloyed with lithium during charging. Examples of metals that can be alloyed with lithium include In, Al, Si, Sn, Mg, Au, Ag, and Zn. Among these, In, Si, Sn, and Ag are preferred, and Ag is more preferred.

[0020] The lithium-reactive material may be used alone or in combination with two or more. A preferred embodiment of using two or more materials in combination is the use of the aforementioned carbon material in combination with a metal capable of alloying 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 predetermined carbon material and a metal material capable of alloying with lithium during charging. This ensures sufficient mechanical strength and lithium ion conductivity of the negative electrode intermediate layer. More specifically, it is preferable to use carbon black (particularly acetylene black) exhibiting the predetermined DBP absorption amount in combination with nanoparticles composed of In, Si, Sn, and Ag, and it is even more preferable to use carbon black (particularly acetylene black) exhibiting the predetermined DBP absorption amount in combination with nanoparticles composed of Ag. When the predetermined carbon material and the metal capable of alloying with lithium are used in combination, the blending ratio (mass ratio) between them is not particularly limited, but the carbon material:metal capable of alloying with lithium (mass ratio) is preferably 10:1 to 1:1, more preferably 5:1 to 2:1.

[0021] When the carbon material is in particulate form, its average particle diameter (average primary particle diameter) is, for example, 10 nm to 200 nm, preferably 15 nm to 150 nm, and more preferably 20 nm to 100 nm. When the metal is in particulate form, its average particle diameter is, for example, 10 nm to 500 nm, preferably 20 nm to 300 nm, more preferably 30 nm to 200 nm, and even more preferably 40 nm to 100 nm. In this specification, the average particle diameter of the carbon material and metal particles refers to the 50% cumulative diameter (D50) of the particle diameters of the particles observed in several to several tens of fields of view when the cross section of a layer containing the particles is observed with a scanning electron microscope (SEM) (the maximum distance between any two points on the outline of the observed particles).

[0022] The negative electrode intermediate layer essentially contains a binder in addition to the lithium-reactive material. The type of binder is not particularly limited, and binders known in the art can be appropriately used. Examples include fluorine-based resins such as polyvinylidene fluoride (PVDF) (including compounds in which hydrogen atoms are substituted with other halogen elements) and polytetrafluoroethylene (PTFE), as well as styrene-butadiene rubber (SBR) and carboxymethyl cellulose (CMC). Among these, the binder of the negative electrode intermediate layer preferably contains a fluorine-based resin, and particularly preferably contains PVDF.

[0023] The content of the lithium-reactive material in the negative electrode intermediate layer (when two or more materials are used in combination, this refers to the total content of those materials; the same applies hereinafter) is not particularly limited, but from the viewpoint of improving the cycle durability of the battery while suppressing the precipitation and growth of dendrites, it is preferably 66% by mass or more and 86% by mass or less, more preferably 68% by mass or more and 86% by mass or less, and even more preferably 70% by mass or more and 79% by mass or less, relative to the total mass of the negative electrode intermediate layer.

[0024] On the other hand, the lithium secondary battery according to this embodiment is also characterized in that the binder content in the negative electrode intermediate layer is 14 to 34 mass% relative to the total mass of the negative electrode intermediate layer. From the viewpoint of improving the cycle durability of the lithium secondary battery, the binder content in the negative electrode intermediate layer is preferably 14 to 32 mass%, more preferably 21 to 30 mass%. The binder content in the negative electrode intermediate layer may be 15 mass% or more (e.g., 15 to 34 mass%, 15 to 32 mass%, 15 to 30 mass%, etc.).

[0025] The thickness of the negative electrode intermediate layer is not particularly limited, but is preferably 1 to 30 μm, more preferably 1 to 20 μm, even more preferably 1 to 10 μm, and particularly preferably 1 to 5 μm. When the thickness of the negative electrode intermediate layer is 1 μm or more, the function of the negative electrode intermediate layer can be fully exhibited. When the thickness of the negative electrode intermediate layer is 30 μm or less, a decrease in energy density can be suppressed.

[0026] The inventors' investigations have revealed that, in a lithium-precipitation-type lithium secondary battery having a negative electrode intermediate layer, the cycle durability of the lithium secondary battery is significantly improved compared to conventional techniques by incorporating a carbon material exhibiting the above-described DBP absorption amount into the negative electrode intermediate layer and further controlling the binder content in the negative electrode intermediate layer to a range of 14 to 34 mass%. The mechanism by which the lithium secondary battery according to this embodiment achieves the above-described effects is not fully understood, and while not bound by any theory, the following mechanism is presumed. First, if the binder content in the negative electrode intermediate layer is too high (more than 34 mass%), sufficient cycle durability cannot be achieved due to the generation and growth of dendrites. This is thought to be because the high binder content causes cracks in the negative electrode intermediate layer during charge and discharge, making it easier for lithium dendrites to reach the solid electrolyte layer. Furthermore, if the binder content in the negative electrode intermediate layer is too low (less than 14 mass%), sufficient cycle durability cannot be achieved due to the generation and growth of dendrites. This is thought to be because an excessively low binder content results in insufficient mechanical strength of the negative electrode intermediate layer, causing cracks in the negative electrode intermediate layer during charge and discharge, making it easier for lithium dendrites to reach the solid electrolyte layer. Even if the binder content in the negative electrode intermediate layer is within an appropriate range (14 to 34 mass%), if the DBP absorption of the carbon material serving as the lithium-reactive material contained in the negative electrode intermediate layer is too low (200 mL / 100 g or less), sufficient cycle durability cannot be achieved. This is thought to be because, when the binder content is within the above range, the binder is relatively high, making the binder susceptible to reductive decomposition at the negative electrode potential. However, if the DBP absorption of the carbon material is too low, this reductive decomposition of the binder is accelerated, facilitating deterioration of the negative electrode intermediate layer. Furthermore, if the DBP absorption of the carbon material serving as the lithium-reactive material contained in the negative electrode intermediate layer is too high (280 mL / 100 g or more), sufficient cycle durability cannot be achieved.This is thought to be because a carbon material with a large DBP absorption amount absorbs the binder during preparation of the negative electrode intermediate layer slurry, resulting in a decrease in the mechanical strength of the negative electrode intermediate layer, which causes cracks in the negative electrode intermediate layer during charge and discharge, making it easier for lithium dendrites to reach the solid electrolyte layer.

[0027] It is known that the DBP absorption value of carbon materials ranges from as low as about 20 to 30 to as high as 380 to 400. The lithium secondary battery according to the present embodiment achieves a significant improvement in cycle durability by controlling the DBP absorption of the carbon material, which has such a wide distribution, to a value within a specific range and by controlling the binder content in the negative electrode intermediate layer to a range relatively higher than conventional values.

[0028] [Solid Electrolyte Layer] The solid electrolyte layer is interposed between the negative electrode and the positive electrode and contains a solid electrolyte (usually as a main component). The solid electrolyte contained in the solid electrolyte layer is not particularly limited, and any solid electrolyte known in the art can be appropriately used. As an example, LPS (Li ２ S-P ２ S ５ ), Li6PS ５ X (wherein X is Cl, Br or I), Li ７ P ３ S １１ , Li ３．２ P ０．９６ S and Li ３ P.S. ４ Examples of sulfide solid electrolytes include sulfide solid electrolytes such as those listed above. These sulfide solid electrolytes are preferably used because they have excellent lithium ion conductivity and a low bulk modulus, allowing them to follow the volume changes of the electrode active material that accompany charge and discharge. These solid electrolytes may be used alone or in combination of two or more. Of course, solid electrolytes other than those listed above may also be used.

[0029] 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 or less.

[0030] 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 above for the negative electrode intermediate layer.

[0031] The thickness of the solid electrolyte layer varies depending on the configuration of the intended 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.

[0032] [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 additive as necessary. The positive electrode active material layer is typically disposed on the surface of a positive electrode current collector as shown in Figure 1. However, if the positive electrode active material layer 15 itself has a certain degree of conductivity, the positive electrode active material layer itself can constitute the positive electrode without using a positive electrode current collector.

[0033] The type of 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. A specific example of the lithium-containing metal oxide is LiCoO ２ , LiMnO ２ , LiNiO ２ , Li(Ni-Mn-Co)O ２ Layered rock salt active materials such as LiMn ２ O ４ , LiNi ０．５ Mn １．５ O ４ Spinel-type active materials such as LiFePO ４ , LiMnPO ４ Olivine type active materials such as Li ２ FeSiO ４ , Li ２ MnSiO ４ Examples of oxide active materials other than those mentioned above include Si-containing active materials such as Li ４ Ti ５ O １２ , LiVO２ Among them, Li(Ni-Mn-Co)O ２ and those in which a part of these transition metals is substituted with 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.

[0034] The shape of the positive electrode active material may be, for example, particulate (spherical, fibrous), thin film, etc. When the positive electrode active material is particulate, its average particle diameter (D50) is, for example, preferably in the range of 1 nm to 100 μm, more preferably in the range of 10 nm to 50 μm, even 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 average particle diameter (D50) of the positive electrode active material can be measured by a laser diffraction scattering method.

[0035] The content of the positive electrode active material is not particularly limited, but 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, and more preferably 80% by mass or more and 99% by mass or less, relative to the total mass of the positive electrode active material layer.

[0036] The positive electrode active material layer may further contain a solid electrolyte, a binder, and / or a conductive additive in addition to the positive electrode active material. Here, the solid electrolyte that can be used in the positive electrode active material layer is the same as that described for the solid electrolyte layer above. The binder that can be used in the positive electrode active material layer is the same as that described for the negative electrode intermediate layer above. Examples of conductive additives 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), and carbon black (specifically, acetylene black, Ketjen Black (registered trademark), furnace black, channel black, thermal lamp black, etc.), but are not limited thereto. Furthermore, a conductive additive can also be used in which particulate ceramic materials or resin materials are coated with the above-mentioned metal materials by plating or the like.

[0037] 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 even more preferably 70 μm or more and 150 μm or less.

[0038] [Positive current collector plate and negative current collector plate] The material constituting the current collector plate (25, 27) is not particularly limited, and known highly conductive materials conventionally used as current collector plates for secondary batteries can be used. Metal materials such as aluminum, copper, titanium, nickel, stainless steel (SUS), and alloys thereof are preferred as constituent materials of the current collector plate. From the viewpoints of light weight, corrosion resistance, and high conductivity, aluminum and copper are more preferred, and aluminum is particularly preferred. The positive current collector plate 27 and the negative current collector plate 25 may be made of the same material or different materials.

[0039] [Positive Electrode Lead and Negative Electrode Lead] Although not shown in the drawings, the current collectors (11", 11') and the current collector plates (27, 25) may be electrically connected via a positive electrode lead or a negative electrode lead. As the constituent materials of the positive electrode and the negative electrode lead, materials used in known lithium secondary batteries can be similarly adopted. Note that the portion removed from the exterior is preferably covered with a heat-resistant, insulating heat-shrinkable tube or the like so as to prevent contact with peripheral devices or wiring, etc., causing electrical leakage and affecting products (e.g., automobile parts, particularly electronic devices, etc.).

[0040] [Battery Exterior Material] As the battery exterior material, a known metal can case can be used. Alternatively, a bag-shaped case using an aluminum-containing laminate film 29 that can cover the power generating element as shown in FIG. 1 can be used. The laminate film can be, for example, a three-layer laminate film formed by laminating PP, aluminum, and nylon in this order, but is not limited thereto. A laminate film is desirable from the viewpoint of achieving high output and excellent cooling performance, making it suitable for use in batteries for large equipment such as EVs and HEVs. Furthermore, an aluminum-containing laminate film is more preferable as the exterior material because it allows for easy adjustment of the collective pressure applied to the power generating element from the outside.

[0041] The all-solid-state lithium secondary battery according to the present embodiment has a configuration in which a plurality of unit cell layers are connected in parallel, and therefore has high capacity and excellent cycle durability, and is therefore suitable for use as a power source for driving EVs and HEVs.

[0042] Although one embodiment of the lithium secondary battery of the present invention has been described above, the present invention is not limited to the configuration described in the above embodiment, and can be modified as appropriate based on the claims.

[0043] For example, the type of battery to which the lithium secondary battery according to the present invention is applied includes a bipolar battery including a bipolar electrode having 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.

[0044] Furthermore, the lithium secondary battery according to the present embodiment does not have to be an all-solid-state type. That is, the solid electrolyte layer may further contain a conventionally known liquid electrolyte (electrolytic solution). There is no particular limitation on the amount of liquid electrolyte (electrolytic solution) that can be contained in the solid electrolyte layer, but it is preferably an amount that allows the shape of the solid electrolyte layer formed by the solid electrolyte to be maintained and prevents leakage of the liquid electrolyte (electrolytic solution).

[0045] The following embodiments are also included within the scope of the present invention: a lithium secondary battery according to claim 1 having the features of claim 2; a lithium secondary battery according to claim 1 having the features of claim 3; a lithium secondary battery according to claim 1 having the features of claim 4; a lithium secondary battery according to any one of claims 1 to 4 having the features of claim 5; a lithium secondary battery according to any one of claims 1 to 4 having the features of claim 6; a lithium secondary battery according to any one of claims 1 to 6 having the features of claim 7; and a lithium secondary battery according to any one of claims 1 to 7 having the features of claim 8.

[0046] The present invention will be described in more detail below with reference to examples. However, the technical scope of the present invention is not limited to the following examples. In the following, the operations were carried out in a glove box with a dew point of -68°C or less. Furthermore, the instruments and devices used in the glove box were thoroughly dried beforehand.

[0047] <Example of Preparation of Evaluation Cell> [Example 1] (Preparation of Positive Electrode) NMC composite oxide (LiNi) ０．８ Mn ０．１ Co ０．１ O ２ ), carbon fiber as a conductive additive, and an argyrodite-type sulfide solid electrolyte (Li ６ P.S. ５Cl) were weighed out to 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. 2 parts by mass of polytetrafluoroethylene (PTFE) as a binder was added to 100 parts by mass of the obtained mixed powder and mixed. The obtained mixture was layered on aluminum foil as a positive electrode current collector and pressed to obtain a positive electrode having a positive electrode active material layer (thickness 100 μm) on the surface of the positive electrode current collector.

[0048] (Preparation of solid electrolyte layer) Argyrodite-type sulfide solid electrolyte (Li ６ P.S. ５ A solid electrolyte slurry was prepared by adding 2 parts by mass of styrene butadiene rubber (SBR) as a binder to 100 parts by mass of ethylenediaminetetraacetic acid (EAC) (average particle diameter (D50): 0.8 μm) and mesitylene as a solvent. The solid electrolyte slurry was applied to the surface of a stainless steel foil support and dried to obtain a solid electrolyte layer (thickness: 40 μm).

[0049] (Preparation of Negative Electrode Intermediate Layer) 19 parts by mass of silver nanoparticles (average particle diameter (D50): 60 nm), a metal material that can be alloyed with lithium during charging, and 57 parts by mass of carbon black (DBP absorption: 228 [mL / 100 g]) were weighed (Ag:C = 1:3 (mass ratio)) and mixed. 24 parts by mass of polyvinylidene fluoride (PVDF) as a binder was added to 76 parts by mass of the resulting mixture, and N-methyl-2-pyrrolidone (NMP) was added as a solvent and mixed to prepare a negative electrode intermediate layer slurry. The negative electrode intermediate layer slurry was applied to the surface of stainless steel foil as a negative electrode current collector and dried to form a negative electrode intermediate layer (basis weight 0.3 mg / cm ２ ) was obtained.

[0050] (Preparation of Evaluation Cell) A positive electrode active material layer formed on the surface of an aluminum foil (positive electrode current collector) and a solid electrolyte layer formed on the surface of a stainless steel foil were stacked so that the exposed surface of the positive electrode active material layer and the exposed surface of the solid electrolyte layer faced each other, and pressed by cold isostatic pressing (CIP) at 700 MPa for 1 minute (first pressing step). This transferred the solid electrolyte layer 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 stacked so that the exposed surface of the solid electrolyte layer and the exposed surface of the negative electrode intermediate layer faced each other, and pressed by cold isostatic pressing (CIP) at 500 MPa for 1 minute (second pressing step). This transferred the negative electrode intermediate layer to the exposed surface of the solid electrolyte layer. Finally, an aluminum positive electrode tab and a nickel negative electrode tab were joined to the aluminum foil (positive electrode current collector) and the stainless steel foil (negative electrode current collector), respectively, using an ultrasonic welding machine, and the resulting laminate was placed inside an aluminum laminate film and vacuum-sealed to produce an evaluation cell of this example, which was a lithium deposition-type all-solid-state lithium secondary battery.

[0051] Example 2 An evaluation cell for this example was produced using the same method as in Example 1 described above (production of a negative electrode intermediate layer), except that the carbon black used was changed to one with a DBP absorption of 265 mL / 100 g (a mixture of 60 parts by mass of carbon black with a DBP absorption of 200 mL / 100 g and 40 parts by mass of carbon black with a DBP absorption of 360 mL / 100 g).

[0052] [Example 3] An evaluation cell for this example was produced using the same method as in Example 1 described above (production of a negative electrode intermediate layer), except that the carbon black used was changed to one with a DBP absorption capacity of 247 [mL / 100 g].

[0053] [Example 4] An evaluation cell for this example was produced using the same method as in Example 1 described above (production of negative electrode intermediate layer), except that the blending ratio of the mixture when preparing the negative electrode intermediate layer slurry was changed to 86 parts by mass and the blending ratio of PVDF was changed to 14 parts by mass.

[0054] [Example 5] An evaluation cell for this example was produced using the same method as in Example 1 described above (production of negative electrode intermediate layer), except that the blending ratio of the mixture when preparing the negative electrode intermediate layer slurry was changed to 66 parts by mass and the blending ratio of PVDF was changed to 34 parts by mass.

[0055] Comparative Example 1 An evaluation cell for this comparative example was prepared using the same method as in Example 1 described above (preparation of the negative electrode intermediate layer), except that the carbon black used was changed to one with a DBP absorption capacity of 200 [mL / 100 g].

[0056] Comparative Example 2 An evaluation cell for this comparative example was produced using the same method as in Example 1 described above (production of negative electrode intermediate layer), except that the blending ratio of the mixture when preparing the negative electrode intermediate layer slurry was changed to 60 parts by mass and the blending ratio of PVDF was changed to 40 parts by mass.

[0057] Comparative Example 3 An evaluation cell for this comparative example was produced using the same method as in Example 1 described above (production of negative electrode intermediate layer), except that the blending ratio of the mixture when preparing the negative electrode intermediate layer slurry was changed to 93 parts by mass and the blending ratio of PVDF was changed to 7 parts by mass.

[0058] Comparative Example 4 An evaluation cell for this comparative example was produced using the same method as in Example 1 described above (production of a negative electrode intermediate layer), except that the carbon black used was changed to one with a DBP absorption of 280 mL / 100 g (a mixture of 50 parts by mass of carbon black with a DBP absorption of 200 mL / 100 g and 50 parts by mass of carbon black with a DBP absorption of 360 mL / 100 g).

[0059] <Cycle Durability Test> A positive electrode lead and a negative electrode lead were connected to the positive electrode current collector and the negative electrode current collector, respectively, of the evaluation cells prepared in the above Examples and Comparative Examples. A cycle durability test was performed while applying a confining pressure of 3 MPa in the stacking direction of the evaluation cell using a pressure member. The cycle durability test was performed at 60°C, with a cell voltage ranging from 3.0 V to 4.3 V, and a charge / discharge rate of 0.5 C (charge CCCV mode (0.01 C cutoff), discharge CC mode) repeated 200 times. The ratio of the 200th discharge capacity to the first discharge capacity was then calculated, and this was defined as the discharge capacity retention rate (%). The results are shown in Table 1 below.

[0060]

[0061] As shown in Table 1, according to the present invention, in a lithium deposition type lithium secondary battery having a negative electrode intermediate layer, by configuring the DBP absorption amount of the carbon material contained in the negative electrode intermediate layer and the binder content to be values ​​within predetermined ranges, it is understood that cycle durability can be significantly improved.

[0062] This application is based on Japanese Patent Application No. 2023-220336, filed on December 27, 2023, the disclosure of which is incorporated herein by reference in its entirety.

[0063] REFERENCE SIGNS LIST 10a laminated secondary battery, 11' negative electrode current collector, 11" positive electrode current collector, 13 negative electrode active material layer, 14 negative electrode intermediate layer, 15 positive electrode active material layer, 17 solid electrolyte layer, 19 single cell layer, 21 power generating element, 25 negative electrode current collector, 27 positive electrode current collector, 29 laminate film.

Claims

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 interposed between the negative electrode current collector and the solid electrolyte layer and containing a lithium reactive material and a binder; A power generation element having the above; the lithium reactive material contains a carbon material; a lithium secondary battery, wherein the DBP absorption amount of the carbon material exceeds 200 [mL / 100 g] and is less than 280 [mL / 100 g], and the content of the binder in the negative electrode intermediate layer is 14 to 34% by mass. 　 The lithium secondary battery according to claim 1, wherein the DBP absorption amount of the carbon material is 210 to 270 [mL / 100 g]. 　 The lithium secondary battery according to claim 1, wherein the DBP absorption amount of the carbon material is 215 to 255 [mL / 100 g]. 　 The lithium secondary battery according to claim 1, wherein the DBP absorption amount of the carbon material is 220 to 235 [mL / 100 g]. 　 The lithium secondary battery according to claim 1 or 2, wherein the content of the binder in the negative electrode intermediate layer is 14 to 32% by mass. 　 The lithium secondary battery according to claim 1 or 2, wherein the content of the binder in the negative electrode intermediate layer is 21 to 30% by mass. 　 The lithium secondary battery according to claim 1 or 2, wherein the lithium reactive material contains a metal material capable of alloying with lithium during charging. 　 The lithium secondary battery according to claim 1 or 2, wherein the binder includes a fluorine-based resin.

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