Lithium secondary battery

Abstract

The present disclosure provides a means capable of suppressing an increase in resistance at high temperatures in a lithium precipitation type lithium secondary battery comprising a negative electrode intermediate layer. Provided is a lithium secondary battery comprising a power generation element that has: a positive electrode; a negative electrode which has a negative electrode collector and in which lithium metal precipitates during charging; a solid electrolyte layer which is interposed between the positive electrode and the negative electrode and which contains a solid electrolyte; and a negative electrode intermediate layer which is interposed between the negative electrode collector and the solid electrolyte layer and which contains a lithium-reactive material and a binder, wherein the porosity of the negative electrode intermediate layer is 45-70%, the lithium-reactive material contains amorphous carbon, and the DBP absorption of the amorphous carbon is 210-265 [mL / 100g].

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 Publication No. 2020-167146 (corresponding to the specification of U.S. Patent Application Publication No. 2020 / 0313164) discloses a technology relating to an all-solid-state secondary battery having, in this order, a positive electrode active material layer, a solid electrolyte layer, and the aforementioned negative electrode active material layer (negative electrode intermediate layer). According to this document, an all-solid-state secondary battery having excellent battery characteristics (particularly cycle characteristics and discharge rate characteristics) is provided by adding 33 mass% or more of amorphous carbon having at least one of a predetermined nitrogen adsorption specific surface area and a predetermined DBP oil absorption to the negative electrode active material layer (negative electrode intermediate layer), and controlling the initial charge capacity of the positive electrode active material layer to be 0.01<b / a<0.5, where a (mAh) is the initial charge capacity of the positive electrode active material layer and b (mAh) is the initial charge capacity of the negative electrode active material layer (negative electrode intermediate layer).

[0005] However, the inventors have conducted research and found that the resistance of the all-solid-state secondary battery described in the above document may increase at high temperatures.

[0006] Therefore, an object of the present invention is to provide a means for suppressing an increase in resistance at high temperatures in 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-mentioned problems and have found that the above-mentioned problems can be solved by incorporating amorphous carbon that exhibits a predetermined DBP absorption amount into the negative electrode intermediate layer and by controlling the porosity of the negative electrode intermediate layer within a predetermined range, 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, 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 porosity of the negative electrode intermediate layer is 45% to 70%, the lithium-reactive material contains amorphous carbon, and the DBP absorption of the amorphous carbon is 210 mL / 100 g to 265 mL / 100 g.

[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, 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 porosity of the negative electrode intermediate layer is 45% to 70%, the lithium-reactive material contains amorphous carbon, and the DBP absorption of the amorphous carbon is 210 mL / 100 g to 265 mL / 100 g. According to this aspect, it is possible to suppress an increase in resistance at high temperatures in a lithium-deposition type lithium secondary battery including the 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 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 restrictions on the material that constitutes the current collector. Examples of materials that can be used for the current collector include metals such as aluminum, nickel, iron, stainless steel, titanium, and copper, as well as conductive resins. There are also no particular restrictions on the thickness of the current collector, but an example is 10 to 100 μm.

[0015] [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 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. 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. By providing such a negative electrode intermediate layer, the precipitation and growth of lithium dendrites are suppressed.

[0017] The lithium secondary battery according to this embodiment essentially contains amorphous carbon, a type of material capable of absorbing lithium during charging, as the lithium-reactive material. Examples of amorphous carbon include, but are not limited to, carbon black (specifically, acetylene black, Ketjen Black (registered trademark), furnace black, channel black, thermal lamp black, etc.), hard carbon, etc. Among these, carbon black is preferred, and acetylene black, Ketjen Black (registered trademark), furnace black, channel black, and thermal lamp black are more preferred. These materials may be used alone or in combination of two or more.

[0018] The lithium secondary battery according to the present embodiment is characterized in that the DBP absorption of the amorphous carbon is 210 mL / 100 g or more and 265 mL / 100 g or less. 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 connections). The DBP absorption of the amorphous carbon in this specification is measured in accordance with JIS K 6217-4:2017, "Carbon Black for Rubber—Fundamental Properties—Part 4: Determination of Oil Absorption (Including Compressed Samples)." When two or more amorphous carbon materials are used, the weighted average of the DBP absorptions of each material is used, weighted by mass. If the DBP absorption is less than 210 mL / 100 g, the amount of binder between the amorphous carbon structures increases, resulting in a larger amount of binder in contact with the deposited lithium. The binder in contact with the deposited lithium is easily reductively decomposed at high temperatures, which may reduce the adhesiveness between the amorphous carbons and between the amorphous carbon and the deposited lithium, resulting in an increase in resistance. If the DBP absorption is more than 265 mL / 100 g, the binder is absorbed into the voids in the amorphous carbon aggregates, reducing the amount of binder that bonds the structures together, which may result in a decrease in the strength of the negative electrode intermediate layer. From the viewpoint of further suppressing an increase in resistance at high temperatures, the DBP absorption is preferably 220 mL / 100 g or more and 265 mL / 100 g or less, more preferably 240 mL / 100 g or more and 265 mL / 100 g or less, and even more preferably 240 mL / 100 g or more and 260 mL / 100 g or less. The DBP absorption value of a carbon material can be changed by changing the manufacturing conditions for the carbon material, and for details, reference can be made to common general technical knowledge in the technical field. The DBP absorption value can also be changed by using two or more materials with different DBP absorption values ​​and adjusting the blending amounts of each material.

[0019] The content of amorphous carbon in the negative electrode intermediate layer (when two or more materials are used in combination, this refers to the total content of those materials) is not particularly limited, but from the viewpoint of further suppressing an increase in resistance at high temperatures, it is preferably 55% by mass or more and 85% by mass or less, more preferably 60% by mass or more and 80% by mass or less, and even more preferably 65% ​​by mass or more and 75% by mass or less, relative to the total mass of the negative electrode intermediate layer.

[0020] The lithium-reactive material may be used alone or in combination with two or more. A preferred embodiment is the use of the amorphous carbon described above in combination with a metal material (e.g., a metal capable of alloying with lithium). That is, according to a preferred embodiment of the present invention, the lithium-reactive material includes the predetermined amorphous carbon and at least one selected from the group consisting of metal materials 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 amorphous carbon (particularly carbon black) exhibiting the predetermined DBP absorption amount in combination with nanoparticles composed of In, Si, Sn, or Ag, and it is more preferable to use amorphous carbon (particularly carbon black) exhibiting the predetermined DBP absorption amount in combination with nanoparticles composed of Ag. When the predetermined amorphous carbon and the metal capable of being alloyed with lithium are used in combination, the compounding ratio (mass ratio) thereof is not particularly limited, but the amorphous carbon:metal capable of being alloyed with lithium (mass ratio) is preferably 10:1 to 1:1, and more preferably 5:1 to 2:1. When the predetermined amorphous carbon and the metal capable of being alloyed with lithium are used in combination, the compounding ratio (volume ratio) thereof is also not particularly limited, but the amorphous carbon:metal capable of being alloyed with lithium (volume ratio) is preferably 99:1 to 70:30, and more preferably 95:5 to 75:25.

[0021] When the amorphous carbon is in a particulate form, its average particle diameter (average primary particle diameter) is, for example, 10 nm to 200 nm, preferably 15 nm to 150 nm, more preferably 20 nm to 100 nm, and even more preferably 25 nm to 70 nm. When the metal is in a 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. When the average particle diameter of the amorphous carbon particles (preferably, metal particles in addition to the amorphous carbon 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 amorphous carbon and metal particles is defined as the 50% cumulative diameter (D) 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). ５０ )

[0022] 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) is not particularly limited, but from the viewpoint of further suppressing an increase in resistance at high temperatures while suppressing the precipitation and growth of dendrites, the content is preferably 85% by mass or more and 97% by mass or less, more preferably 90% by mass or more and 96% by mass or less, and even more preferably 90% by mass or more and 95% by mass or less, relative to the total mass of the negative electrode intermediate layer.

[0023] 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 styrene-butadiene rubber (SBR), polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF) (including compounds in which hydrogen atoms are substituted with other halogen elements), and carboxymethyl cellulose (CMC). Among these, styrene-butadiene rubber, polytetrafluoroethylene, and polyvinylidene fluoride are preferred, and polytetrafluoroethylene and polyvinylidene fluoride are more preferred. These binders may be used alone or in combination of two or more.

[0024] The binder content in the negative electrode intermediate layer is not particularly limited, but is preferably 15% by mass or less, more preferably 10% by mass or less, and even more preferably 7% by mass or less, relative to the total mass of the negative electrode intermediate layer. The lower limit of the binder content is not particularly limited, but is 3% by mass or more. If the binder content is within the above range, the strength of the negative electrode intermediate layer can be maintained while further suppressing an increase in resistance at high temperatures.

[0025] The weight of the negative electrode intermediate layer is not particularly limited, but is preferably 1.0 mg / cm ２ Preferably, it is less than 0.8 mg / cm ２ More preferably, it is 0.5 mg / cm or less. ２ The lower limit of the basis weight is not particularly limited, but is preferably 0.1 mg / cm ２ Preferably, it is greater than 0.3 mg / cm ２ If the basis weight is within the above range, the strength of the negative electrode intermediate layer can be maintained while further suppressing an increase in resistance at high temperatures.

[0026] The lithium secondary battery according to this embodiment is also characterized in that the porosity of the negative electrode intermediate layer is 45% or more and 70% or less. A porosity of less than 45% increases the amount of amorphous carbon adjacent to the deposited lithium, resulting in an increase in the amount of binder (binder binding the amorphous carbon and the deposited lithium) in contact with the deposited lithium. The binder in contact with the deposited lithium is prone to reductive decomposition at high temperatures, which reduces the binding strength between the amorphous carbons and between the amorphous carbon and the deposited lithium, potentially resulting in an increase in resistance. A porosity exceeding 70% may reduce the strength of the negative electrode intermediate layer. From the viewpoint of further suppressing an increase in resistance due to high-temperature storage, the porosity is preferably 50% or more and 70% or less, more preferably 60% or more and 70% or less.

[0027] According to the inventors' investigations, it has been found that in a lithium-deposited lithium secondary battery having a negative electrode intermediate layer, by incorporating amorphous carbon exhibiting a predetermined DBP absorption amount into the negative electrode intermediate layer and controlling the porosity of the negative electrode intermediate layer within a predetermined range, it is possible to significantly suppress the increase in resistance at high temperatures (particularly at low potentials; more specifically, at high SOC values ​​(e.g., 80% or higher, particularly 100%)) compared to conventional techniques. The mechanism by which the lithium secondary battery according to this embodiment exhibits the above-described effects is not completely clear, and the inventors are not bound by any theory, but the following mechanism is presumed. When the inventors investigated the cause of the increase in resistance at high temperatures, they found that one of the causes is believed to be the deterioration of the binder (particularly the binder in contact with the deposited lithium) due to reductive decomposition, which reduces the adhesiveness between the amorphous carbons and between the amorphous carbon and the deposited lithium. Therefore, by incorporating amorphous carbon exhibiting a predetermined DBP absorption amount into the negative electrode intermediate layer and controlling the porosity of the negative electrode intermediate layer within a predetermined range, the amount of binder in contact with the deposited lithium is reduced, making it less susceptible to the effects of reductive decomposition, and thereby significantly suppressing the increase in resistance at high temperatures.

[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 ), Li ６ P.S. 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 intended configuration of the 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 , LiVO ２ 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 (D ５０ ) 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. ５０ The value of 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 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 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 driving power source for 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 or 2 having the features of claim 3; a lithium secondary battery according to any one of claims 1 to 3 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 5 having the features of claim 6; and a lithium secondary battery according to any one of claims 1 to 6 having the features of claim 7.

[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. Note that the following 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. ５ Cl) 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 Cl, average particle diameter (D ５０To 100 parts by mass of a cellulose acylate (0.8 μm), 2 parts by mass of styrene butadiene rubber (SBR) as a binder and mesitylene as a solvent were added and mixed to prepare a solid electrolyte slurry. The solid electrolyte slurry was applied to the surface of a stainless steel foil as a support and dried to obtain a solid electrolyte layer (thickness: 40 μm).

[0049] (Preparation of Negative Electrode Intermediate Layer) Silver nanoparticles (average particle diameter (D ５０ 23.25 parts by mass of silver halide (60 nm) and 69.75 parts by mass of carbon black (DBP absorption: 228 [mL / 100 g]) were weighed (Ag:C = 1:3 (mass ratio)) and mixed. 7 parts by mass of polyvinylidene fluoride (PVDF) as a binder was added to 93 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 (thickness 10 μm) as a negative electrode current collector and dried to form a negative electrode intermediate layer (porosity 56%, basis weight 0.5 mg / cm ２ ) was obtained.

[0050] The porosity of the negative electrode intermediate layer was determined by the following method. First, the negative electrode intermediate layer formed on the stainless steel foil (also referred to as the "first stainless steel foil") was cut to an arbitrary size. A second stainless steel foil (50 μm thick) was placed on the exposed surface of the negative electrode intermediate layer and pressed at 500 MPa for 1 minute by cold isostatic pressing (CIP) (the same pressing conditions as the second pressing step in the <Preparation of Evaluation Cells> described below). Then, a test specimen was prepared by peeling the second stainless steel foil from the negative electrode intermediate layer. The bulk density of the negative electrode intermediate layer was calculated from the mass and thickness of the obtained test specimen and the first stainless steel foil. Then, the porosity was calculated using the true density of each material constituting the negative electrode intermediate layer according to the following formula. In the following formula, "carbon" as "x" refers to amorphous carbon (carbon black in this example), "metal" refers to a metal material that can be alloyed with lithium (silver nanoparticles in this example), and "binder" refers to a binder (PVDF in this example).

[0051]

[0052] <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 welder. The resulting laminate was placed inside an aluminum laminate film and vacuum-sealed, thereby obtaining an evaluation cell that was the lithium deposition-type all-solid-state lithium secondary battery of this example.

[0053] [Example 2] The above (production of the negative electrode intermediate layer) was carried out by the following method; ５０ 23.25 parts by mass of silver halide (60 nm) and 69.75 parts by mass of carbon black (DBP absorption: 247 [mL / 100 g]) were weighed (Ag:C = 1:3 (mass ratio)) and mixed. 7 parts by mass of polyvinylidene fluoride (PVDF) as a binder was added to 93 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 (thickness 30 μm) as a negative electrode current collector and dried to form a negative electrode intermediate layer (porosity 60%, basis weight 0.8 mg / cm ２ Except for this, the same method as in Example 1 was used to obtain an evaluation cell for this example.

[0054] [Example 3] The above (production of the negative electrode intermediate layer) was carried out by the following method; ５０22.50 parts by mass of silver halide (60 nm) and 67.50 parts by mass of carbon black (DBP absorption: 247 mL / 100 g) were weighed (Ag:C = 1:3 (mass ratio)) and mixed. 10 parts by mass of polyvinylidene fluoride (PVDF) as a binder was added to 90 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 (thickness 30 μm) as a negative electrode current collector and dried to form a negative electrode intermediate layer (porosity 56%, basis weight 0.8 mg / cm ２ Except for this, the same method as in Example 1 was used to obtain an evaluation cell for this example.

[0055] [Example 4] The above (production of the negative electrode intermediate layer) was carried out by the following method; ５０ 23.25 parts by mass of silver halide (60 nm) and 69.75 parts by mass of carbon black (DBP absorption: 247 mL / 100 g) were weighed (Ag:C = 1:3 (mass ratio)) and mixed. 7 parts by mass of polyvinylidene fluoride (PVDF) as a binder was added to 93 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 (thickness 30 μm) as a negative electrode current collector and dried to form a negative electrode intermediate layer (porosity 68%, basis weight 0.5 mg / cm ２ Except for this, the same method as in Example 1 was used to obtain an evaluation cell for this example.

[0056] [Example 5] The above (production of the negative electrode intermediate layer) was carried out by the following method; ５０24 parts by mass of silver halide (60 nm) and 72 parts by mass of carbon black (a mixture of 92 parts by mass of carbon black with a DBP absorption of 200 [mL / 100g] and 8 parts by mass of carbon black with a DBP absorption of 360 [mL / 100g], the DBP absorption of the mixture being 213 [mL / 100g]) were weighed (Ag:C = 1:3 (mass ratio)) and mixed. 4 parts by mass of polyvinylidene fluoride (PVDF) as a binder was added to 96 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 a stainless steel foil (thickness 30 μm) as a negative electrode current collector and dried to form a negative electrode intermediate layer (porosity 47%, basis weight 0.5 mg / cm ２ Except for this, the same method as in Example 1 was used to obtain an evaluation cell for this example.

[0057] [Example 6] The above (production of the negative electrode intermediate layer) was carried out by the following method; ５０ 22.50 parts by mass of carbon black (DBP absorption: 200 [mL / 100g] carbon black 60 parts by mass and DBP absorption: 360 [mL / 100g] carbon black 40 parts by mass, the DBP absorption of the mixture is 265 [mL / 100g]) were weighed (Ag:C = 1:3 (mass ratio)) and mixed. 10 parts by mass of polyvinylidene fluoride (PVDF) as a binder was added to 90 parts by mass of the obtained 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 a stainless steel foil (thickness 30 μm) as a negative electrode current collector and dried to form a negative electrode intermediate layer (porosity 62%, basis weight 0.8 mg / cm ２ Except for this, the same method as in Example 1 was used to obtain an evaluation cell for this example.

[0058] [Comparative Example 1] The above (production of the negative electrode intermediate layer) was carried out by the following method; ５０23.25 parts by mass of silver halide (60 nm) and 69.75 parts by mass of carbon black (DBP absorption: 200 [mL / 100 g]) were weighed (Ag:C = 1:3 (mass ratio)) and mixed. 7 parts by mass of polyvinylidene fluoride (PVDF) as a binder was added to 93 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 (thickness 30 μm) as a negative electrode current collector and dried to form a negative electrode intermediate layer (porosity 52%, basis weight 0.25 mg / cm ２ Except for this, the same method as in Example 1 was used to obtain an evaluation cell for this comparative example.

[0059] [Comparative Example 2] The above (production of the negative electrode intermediate layer) was carried out by the following method; ５０ 19 parts by mass of carbon black (DBP absorption: 200 [mL / 100g] carbon black 53 parts by mass and DBP absorption: 360 [mL / 100g] carbon black 47 parts by mass, the DBP absorption of the mixture is 275 [mL / 100g]) 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 obtained 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 a stainless steel foil (thickness 30 μm) as a negative electrode current collector and dried to form a negative electrode intermediate layer (porosity 56%, basis weight 0.25 mg / cm ２ Except for this, the same method as in Example 1 was used to obtain an evaluation cell for this comparative example.

[0060] [Comparative Example 3] The above (production of the negative electrode intermediate layer) was carried out by the following method; ５０19 parts by mass of carbon black (DBP absorption: 200 [mL / 100g] carbon black 75 parts by mass and DBP absorption: 360 [mL / 100g] carbon black 25 parts by mass, the DBP absorption of the mixture is 240 [mL / 100g]) 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 obtained 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 a stainless steel foil (thickness 30 μm) as a negative electrode current collector and dried to form a negative electrode intermediate layer (porosity 42%, basis weight 0.5 mg / cm ２ Except for this, the same method as in Example 1 was used to obtain an evaluation cell for this comparative example.

[0061] [Comparative Example 4] The above (production of the negative electrode intermediate layer) was carried out by the following method; ５０ 24.5 parts by mass of carbon black (DBP absorption: 200 [mL / 100g] carbon black 75 parts by mass and DBP absorption: 360 [mL / 100g] carbon black 25 parts by mass, the DBP absorption of the mixture is 240 [mL / 100g]) were weighed (Ag:C = 1:3 (mass ratio)) and mixed. 2 parts of polyvinylidene fluoride (PVDF) as a binder was added to 98 parts by mass of the obtained 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 a stainless steel foil (thickness 30 μm) as a negative electrode current collector and dried to form a negative electrode intermediate layer (porosity 73%, basis weight 0.5 mg / cm ２ Except for this, the same method as in Example 1 was used to obtain an evaluation cell for this comparative example.

[0062] <Charge / Discharge Test> A positive electrode lead and a negative electrode lead were connected to the positive electrode tab and the negative electrode tab, respectively, of the evaluation cell (before the first charge) prepared above, and a charge / discharge test and a storage test were performed under the following conditions while applying a restraining pressure of 3 MPa in the stacking direction of the evaluation cell using a pressure member.

[0063] (Charge / discharge test conditions, 1st cycle) Evaluation temperature: 333K (60°C) Voltage range: 2.5 to 4.3V Charging process (1): CC (cut off at 25 hours) Charging rate (1): 0.01C Charging process (2): CC Charging rate (2): 0.05C Discharging process: CC Discharging rate: 0.1C After charging and discharging, there was a 30-minute rest period.

[0064] (Charge / discharge test conditions, 2nd cycle) Evaluation temperature: 333K (60°C) Voltage range: 2.5 to 4.3V Charging process: CCCV (cut off at 0.02C) Charging rate: 0.1C Discharging process: CC Discharging rate: 0.1C After charging and discharging, there was a 30 minute rest.

[0065] (Storage test conditions) Evaluation temperature: 333K (60°C) Voltage range: 2.5 to 4.3V Charging process: CCCV (cut off at 0.02C) Charging rate: 0.1C Discharging process: CC Discharging rate: 0.1C.

[0066] <Charge / Discharge Test, First Cycle> Using a charge / discharge tester, the evaluation cell was charged in a thermostatic chamber set to the above evaluation temperature in a constant current (CC) mode at 0.01 C for 25 hours during the charge process (1) (lithium metal precipitates on the negative electrode current collector). Thereafter, during the charge process (2), the cell was charged in constant current (CC) mode at 0.05 C to 4.3 V. Thereafter, during the discharge process (lithium metal dissolves on the negative electrode current collector), the cell was discharged in constant current (CC) mode at 0.1 C to 2.5 V. Here, 1 C refers to the current value at which the battery is fully charged (100% charged) after 1 hour of charging at that current value.

[0067] <Charge-Discharge Test, Second Cycle> After the first cycle of the charge-discharge test, the battery was placed in a thermostatic chamber set at the evaluation temperature and charged at 0.1 C in constant current / low voltage (CCCV) mode (cutoff at 0.02 C). Thereafter, the battery was discharged to 2.5 V at 0.1 C in constant current (CC) mode.

[0068] <Storage Test> After the charge / discharge test, the battery was charged at 0.1 C in a constant current, low voltage (CCCV) mode in a thermostatic chamber set to the evaluation temperature (cutoff at 0.02 C). The battery was then held at the evaluation temperature (333 K (60°C)) for 30 days, and then discharged at 0.1 C in a constant current (CC) mode. The battery was then charged to 50% SOC in a constant current, low voltage (CCCV) mode at 0.1 C (cutoff at 0.02 C) in a thermostatic chamber set to the evaluation temperature (333 K (60°C)). Electrochemical impedance spectroscopy (EIS) measurements were then performed in a thermostatic chamber set to 333 K (60°C) at an amplitude of 10 mV and a frequency of 7 MHz to 100 mHz. The resistance value on the real axis of the Cole-Cole plot at 100 mHz was extracted and multiplied by the positive electrode area to calculate the resistance value (after storage).

[0069] In addition, a blank test was conducted in the same manner as above, except that the 30-day holding period at the evaluation temperature (333 K (60° C.)) was omitted, and the resistance value (before storage) was calculated.

[0070] The resistance value (before storage) was subtracted from the resistance value (after storage) to calculate the resistance value (Δ resistance value) increased by high-temperature storage. These results are shown in Table 1 below.

[0071]

[0072] As shown in Table 1, according to the present invention, by incorporating amorphous carbon exhibiting a predetermined DBP absorption amount into the negative electrode intermediate layer and controlling the porosity of the negative electrode intermediate layer within a predetermined range, it is understood that an increase in resistance at high temperatures can be suppressed in a lithium deposition-type lithium secondary battery equipped with a negative electrode intermediate layer.

[0073] This application is based on Japanese Patent Application No. 2023-220342, filed on December 27, 2023, the disclosure of which is incorporated herein by reference in its entirety.

[0074] REFERENCE SIGNS LIST 10a laminated secondary battery, 11' negative electrode current collector, 11" positive electrode current collector, 13 negative electrode active material layer, 14 negative electrode intermediate layer, 15 positive electrode active material layer, 17 solid electrolyte layer, 19 single cell layer, 21 power generating element, 25 negative electrode current collector, 27 positive electrode current collector, 29 laminate film.

Claims

1. A lithium secondary battery comprising a positive electrode, a negative electrode current collector, a negative electrode 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 porosity of the negative electrode intermediate layer is 45% or more and 70% or less, the lithium reactive material contains amorphous carbon, and the DBP absorption amount of the amorphous carbon is 210 [mL / 100 g] or more and 265 [mL / 100 g] or less.

2. The lithium secondary battery according to claim 1, wherein the porosity is 60% or more and 70% or less, and the DBP absorption amount of the amorphous carbon is 240 [mL / 100 g] or more and 265 [mL / 100 g] or less.

3. The lithium secondary battery according to claim 1 or 2, wherein the content of the binder in the negative electrode intermediate layer is 7% by mass or less.

4. The basis weight of the negative electrode intermediate layer is 0.5 mg / cm ２ The lithium secondary battery according to claim 1 or 2, wherein the basis weight is 0.5 mg / cm or less.

5. The lithium secondary battery according to claim 1 or 2, wherein the lithium reactive material contains a metal material that can alloy with lithium during charging.

6. The lithium secondary battery according to claim 1 or 2, wherein the content of the amorphous carbon in the negative electrode intermediate layer is 55% by mass or more and 85% by mass or less.

7. The lithium secondary battery according to claim 1 or 2, wherein the binder contains at least one selected from styrene-butadiene rubber, polytetrafluoroethylene, and polyvinylidene fluoride.

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