Lithium secondary battery

Abstract

The present invention provides a means that can improve the initial charge / discharge efficiency of a lithium precipitation-type lithium secondary battery having a negative electrode intermediate layer, while preventing the occurrence of short-circuiting. The present invention relates to a lithium secondary battery comprising a power generation element including: 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 on which lithium metal precipitates during charging; a solid electrolyte layer which is present between the positive electrode and the negative electrode and which contains a solid electrolyte; and a negative electrode intermediate layer which is adjacent to a surface of the solid electrolyte layer that is on the negative electrode current collector side and which contains a carbon material, wherein, in plan view of the power generation element, the area ratio of the negative electrode intermediate layer to the positive electrode active material layer (the area of the negative electrode intermediate layer / the area of the positive electrode active material layer) is more than 1.0 but not more than 1.3.

Description

Lithium secondary battery

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

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

[0003] Conventionally, one type of lithium secondary battery using a solid electrolyte is known as a so-called lithium deposition type, in which lithium metal is deposited on a negative electrode current collector during charging (for example, JP 2020-167146 A). In such lithium deposition type lithium secondary batteries, lithium metal is deposited between the solid electrolyte layer and the negative electrode current collector. At this time, dendrites grow from the lithium metal layer and penetrate the solid electrolyte layer, which can cause a short circuit. To prevent this, a technology has been reported in which a negative electrode active material layer (negative electrode intermediate layer) is provided between the negative electrode current collector and the solid electrolyte layer. However, when such a layer is included, there is a problem that battery characteristics such as cycle characteristics and discharge rate characteristics may not be sufficiently improved.

[0004] This document discloses a technique in which a predetermined amount or more of amorphous carbon is contained in a negative electrode active material layer (negative electrode intermediate layer) between a negative electrode current collector and a solid electrolyte layer, the specific surface area or DBP oil absorption of the amorphous carbon is limited to a certain range, and the ratio of the charge capacity of the negative electrode active material layer (negative electrode intermediate layer) to that of the positive electrode active material layer is controlled to a certain range. This configuration is said to improve battery characteristics such as cycle characteristics and discharge rate characteristics.

[0005] However, the inventors have found through their investigations that when a negative electrode intermediate layer such as that described in the above document is used, the negative electrode intermediate layer has a large irreversible capacity, which can cause a problem that the initial charge / discharge efficiency may decrease.

[0006] Therefore, an object of the present invention is to provide a means for preventing the occurrence of short circuits and suppressing a decrease in the initial charge / discharge efficiency in a lithium deposition type lithium secondary battery.

[0007] The present inventors have conducted extensive research to solve the above-mentioned problems, and as a result, have found that in a lithium secondary battery including a lithium deposition-type power generating element, the above-mentioned problems can be solved by controlling the area ratio of the negative electrode intermediate layer to the positive electrode active material layer to be more than 1.0 but not more than 1.3, 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 present adjacent to a surface of the solid electrolyte layer facing the negative electrode current collector and containing a carbon material. The lithium secondary battery is characterized in that, in a plan view of the power generating element, the ratio of the area of ​​the negative electrode intermediate layer to the area of ​​the positive electrode active material layer (area of ​​the negative electrode intermediate layer / area of ​​the positive electrode active material layer) is greater than 1.0 and not more than 1.3.

[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 adjacent to the surface of the solid electrolyte layer facing the negative electrode current collector and containing a carbon material, wherein, when the power generating element is viewed from above, the ratio of the area of ​​the negative electrode intermediate layer to the area of ​​the positive electrode active material layer (area of ​​the negative electrode intermediate layer / area of ​​the positive electrode active material layer) is greater than 1.0 and less than or equal to 1.3. According to this aspect, in a lithium deposition-type lithium secondary battery having a negative electrode intermediate layer, it is possible to prevent the occurrence of short circuits and suppress a decrease in initial charge / discharge efficiency. The lithium secondary battery according to this aspect may be an all-solid-state battery.

[0011] Hereinafter, a lithium secondary battery according to the present embodiment will be described with reference to the accompanying drawings. The technical scope of the present invention should be determined based on the claims and is not limited to the following embodiments. In the description of the drawings, the same elements are given the same reference numerals, and redundant explanations will be omitted. In addition, 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) lithium secondary battery (hereinafter also referred to simply 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 adjacent to the surface of the negative electrode active material layer 13 facing the solid electrolyte layer 17. The positive electrode has a structure in which a positive electrode active material layer 15 is disposed on the surface of a positive electrode current collector 11". The negative electrode, solid electrolyte layer, and positive electrode are laminated in this order, with the negative electrode intermediate layer 14 and the positive electrode active material layer 15 facing each other with the solid electrolyte layer 17 interposed therebetween. As a result, adjacent negative electrodes, solid electrolyte layers, and positive electrodes constitute one unit cell layer 19. Therefore, the stacked secondary battery 10a shown in FIG. 1 can be said to have a structure in which a plurality of unit cell layers 19 are laminated and electrically connected in parallel. A negative electrode current collector 25 and a positive electrode current collector 27 that are electrically connected to the respective electrodes (negative electrode and positive electrode) are attached to the negative electrode current collector 11' and the positive electrode current collector 11", respectively, and are structured so as to be sandwiched between the ends of the laminate film 29 and extended to the outside of the laminate film 29. A constraining pressure is applied to the stacked secondary battery 10a in the stacking direction of the power generating element 21 by a pressure member (not shown). Therefore, the volume of the power generating element 21 is kept constant.

[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 (SUS), 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 as the negative electrode active material during the charging process. The layer composed of lithium metal deposited 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 that is adjacent to the surface of the solid electrolyte layer that faces the negative electrode current collector. The negative electrode intermediate layer preferably has electrical conductivity as a whole. The volume resistivity of the negative electrode intermediate layer is not particularly limited, but is preferably 10 ２ The volume resistivity of the negative electrode intermediate layer is preferably 10 Ω cm or less, and more preferably 10 Ω cm or less. In this specification, the volume resistivity of the negative electrode intermediate layer is measured using an electrode resistance measurement system (manufactured by Hioki E.E. Corporation, product name: RM2610). Furthermore, lithium metal may precipitate inside the negative electrode intermediate layer during charging.

[0017] The negative electrode intermediate layer contains a lithium-reactive material. The negative electrode intermediate layer essentially contains a carbon material as the lithium-reactive material. When the carbon material is contained in the negative electrode intermediate layer, the occurrence and growth of dendrites can be particularly effectively suppressed.

[0018] 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, with acetylene black being particularly preferred.

[0019] The carbon material may be contained in the negative electrode intermediate layer, for example, in the form of carbon particles. This can further suppress the generation and growth of dendrites. The average primary particle diameter of the carbon particles is, for example, 200 nm or less, preferably 100 nm or less, more preferably 70 nm or less, and even more preferably 50 nm or less. The lower limit of the average primary particle diameter of the carbon particles is not particularly limited, but is preferably 10 nm or more, more preferably 20 nm or more, and even more preferably 30 nm or more. In one embodiment, the carbon material contained in the negative electrode intermediate layer contains particulate carbon and fibrous carbon. In this case, the content of particulate carbon is preferably greater than the content of fibrous carbon. In another embodiment, the carbon material contained in the negative electrode intermediate layer preferably does not contain fibrous carbon, and more preferably contains only particulate carbon. In this specification, the average primary particle diameter of particles refers to the 50% cumulative diameter (D50) of particle diameters of the particles observed within several to several tens of fields of view when a cross section of a layer containing the particles is observed with a scanning electron microscope (SEM) (the maximum distance between any two points on the outline of the observed particles).

[0020] The irreversible capacity of the carbon material (also referred to herein as "carbon irreversible capacity") is, for example, 500 mAh / g (carbon) or less, preferably 300 mAh / g (carbon) or less, more preferably 250 mAh / g (carbon) or less, and particularly preferably 200 mAh / g (carbon) or less. The lower limit of the carbon irreversible capacity is not particularly limited and may be, for example, 10 mAh / g (carbon) or more. The irreversible capacity of the carbon material can be controlled by the average primary particle size, crystallinity, and cohesion of the carbon material. The smaller the average primary particle size of the carbon material, the greater the irreversible capacity of the carbon material. Furthermore, by controlling the crystallinity and cohesion of the carbon material to be low, the irreversible capacity of the carbon material can be increased. In this specification, the irreversible capacity of the carbon material is measured and calculated by the method described in the Examples below.

[0021] The content of the carbon material is, for example, 90% by mass or less, preferably 85% by mass or less, and more preferably 80% by mass or less, relative to the total mass of the negative electrode intermediate layer. The lower limit of the content of the carbon material is not particularly limited, but is, for example, 30% by mass or more, preferably 40% by mass or more, and more preferably 50% by mass or more, relative to the total mass of the negative electrode intermediate layer. When the content of the carbon material is within the above range, the generation and growth of dendrites can be suppressed.

[0022] The negative electrode intermediate layer may further contain a metal material. The metal material is not particularly limited, but examples thereof include In, Al, Si, Sn, Mg, Au, Ag, and Zn. Among these, In, Si, Sn, and Ag are preferred, and Ag is more preferred.

[0023] The content of the metal material is not particularly limited, but is, for example, 0 to 50% by mass, and preferably 5 to 30% by mass, relative to the total mass of the negative electrode intermediate layer.

[0024] When the lithium-reactive material contains both the above-mentioned specified carbon material and metal material, the compounding ratio (mass ratio) thereof is not particularly limited, but the carbon material:metal material (mass ratio) is preferably 10:1 to 1:1, and more preferably 5:1 to 2:1. Within the above range, the battery performance is better. When multiple types of carbon materials are used, the mass of the carbon material is the total mass of the multiple types of carbon materials. Furthermore, when multiple types of metal materials are included, the mass is the total amount of the multiple types of metal materials.

[0025] When the negative electrode intermediate layer is composed of multiple layers, each layer may contain a metal material. When a metal material is contained, the type of the metal material and the compounding ratio with the carbon material may be the same or different for each layer.

[0026] The negative electrode intermediate layer may contain a binder as needed. The type of binder is not particularly limited, and binders known in the art can be appropriately used. Examples of binders 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. Among these, it is preferable that the binder contains a fluorine-based resin.

[0027] The binder content in the negative electrode intermediate layer is not particularly limited, but is preferably within a range of 1 to 20 mass %, and more preferably within a range of 5 to 15 mass %, relative to the total mass of the negative electrode intermediate layer. When the binder content is 1 mass % or more, a negative electrode intermediate layer with sufficient strength can be formed. When the binder content is 20 mass % or less, a negative electrode intermediate layer with sufficient lithium ion conductivity can be formed.

[0028] In one embodiment, the negative electrode intermediate layer preferably contains at least one of a metal material and a binder in addition to the carbon material, and more preferably contains a carbon material, a metal material, and a binder. Alternatively, the negative electrode intermediate layer may consist only of a carbon material, a metal material, and a binder.

[0029] Furthermore, in the lithium secondary battery according to this embodiment, the weight of the negative electrode intermediate layer is 0.1 mg / cm ２ 2.0mg / cm or more ２ When the basis weight is in the above range, the strength of the negative electrode intermediate layer can be maintained while further suppressing an increase in irreversible capacity. The basis weight of the negative electrode intermediate layer is preferably 0.1 mg / cm or less. ２ ~1.0 mg / cm ２ and more preferably 0.1 mg / cm ２ ~0.5 mg / cm ２ and more preferably 0.1 mg / cm ２ ~0.3 mg / cm ２ is.

[0030] The thickness of the negative electrode intermediate layer is not particularly limited, but may be 0.1 μm or more and less than 100 μm. When the thickness of the negative electrode intermediate layer is 0.1 μm or more, the occurrence of a short circuit during charging can be suppressed. Furthermore, when the thickness of the negative electrode intermediate layer is 100 μm or less, an increase in the lithium ion conduction resistance can be suppressed, and a sufficient charge capacity of the secondary battery can be obtained. In one embodiment, the thickness of the negative electrode intermediate layer is preferably 1 μm or more and 20 μm or less, and more preferably 5 μm or more and 15 μm or less.

[0031] [Solid Electrolyte Layer] The solid electrolyte layer is interposed between the negative electrode and the positive electrode and contains a solid electrolyte (usually as a main component). The solid electrolyte contained in the solid electrolyte layer is not particularly limited, and any solid electrolyte known in the art can be appropriately adopted, for example, a sulfide solid electrolyte and an oxide solid electrolyte. This solid electrolyte exhibits excellent lithium ion conductivity, and is therefore preferably a sulfide solid electrolyte containing an S element, more preferably a sulfide solid electrolyte containing an Li element, an M element, and an S element, wherein the M element contains at least one element selected from the group consisting of P, Si, Ge, Sn, Ti, Zr, Nb, Al, Sb, Br, Cl, and I, and even more preferably a sulfide solid electrolyte containing an S element, an Li element, and an P element. One example is LPS (Li ２ S-P ２ S ５ ), Li ６ P.S.５ X (wherein X is Cl, Br or I), Li ７ P ３ S １１ , Li ３．２ P ０．９６ S ４ and Li ３ P.S. ４ These sulfide solid electrolytes are preferably used because they have excellent lithium ion conductivity.

[0032] The ionic conductivity (e.g., Li ion conductivity) of the sulfide solid electrolyte at room temperature (25°C) is, for example, 1 × 10 －５ S / cm or more, and preferably 1×10 －４ The ionic conductivity of the solid electrolyte can be measured by an AC impedance method.

[0033] Examples of the shape of the solid electrolyte include particulate shapes such as spherical shapes and oval spherical shapes, thin films, etc. When the solid electrolyte is particulate, its average particle size (D50) is not particularly limited, but is preferably 0.01 μm or more and 40 μm or less, more preferably 0.1 μm or more and 20 μm or less, and even more preferably 0.5 μm or more and 10 μm or less.

[0034] The content of the solid electrolyte in the solid electrolyte layer is preferably 50 to 100 mass %, more preferably 90 to 100 mass %.

[0035] The solid electrolyte layer may further contain a binder in addition to the solid electrolyte. The binder is not particularly limited, and known binders can be used as appropriate. For example, the binders described above for the negative electrode intermediate layer can be similarly employed. The content of the binder in the solid electrolyte layer is not particularly limited, and is, for example, 1 to 10 mass %.

[0036] The thickness of the solid electrolyte layer varies depending on the intended configuration of the lithium secondary battery, but is usually 0.1 to 1000 μm, preferably 10 to 100 μm.

[0037] [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 needed.

[0038] The type of the positive electrode active material contained in the positive electrode active material layer is not particularly limited, but may be LiCoO ２ , LiMnO ２ , LiNiO ２ , LiVO ２ , Li(Ni-Mn-Co)O ２ Layered rock salt active materials such as LiMn ２ O ４ , LiNi ０．５ Mn １．５ O ４ Spinel-type active materials such as LiFePO ４ , LiMnPO ４ Olivine type active materials such as Li ２ FeSiO ４ , Li ２ MnSiO ４ Examples of oxide active materials other than those mentioned above include Si-containing active materials such as Li ４ Ti ５ O １２ Among them, Li(Ni-Mn-Co)O ２ Also, those in which part of these transition metals has been replaced with other elements (hereinafter simply referred to as "NMC composite oxides") are preferably used as the positive electrode active material.

[0039] In addition, a sulfur-based positive electrode active material is also one of the preferred embodiments. Examples of the sulfur-based positive electrode active material include particles or thin films of organic sulfur compounds or inorganic sulfur compounds, and any material can be used as long as it is capable of releasing lithium ions during charging and absorbing lithium ions during discharging by utilizing the oxidation-reduction reaction of sulfur.

[0040] The content of the positive electrode active material in the positive electrode active material layer is preferably 50 to 100% by mass, more preferably 55 to 95% by mass, and even more preferably 60 to 90% by mass.

[0041] The positive electrode active material layer preferably further contains a solid electrolyte. The specific form of the solid electrolyte contained in the positive electrode active material layer may be the same as that described in the solid electrolyte layer section. A sulfide solid electrolyte is preferably used because it has excellent lithium ion conductivity and a low bulk modulus, allowing it to follow the volumetric changes of the positive electrode active material that occur during charging and discharging. The content of the solid electrolyte in the positive electrode active material layer is not particularly limited, but is, for example, 1 to 50% by mass, preferably 2 to 20% by mass, and more preferably 3 to 10% by mass.

[0042] The binder used in the positive electrode active material layer is not particularly limited, and any known binder can be used as appropriate. For example, the binder described above for the negative electrode intermediate layer can be used. The content of the binder in the positive electrode active material layer is not particularly limited, and is, for example, 1 to 10 mass %.

[0043] The conductive additive used in the positive electrode active material layer is not particularly limited, and for example, carbon such as carbon black (specifically, acetylene black, Ketjen Black (registered trademark), furnace black, channel black, thermal lamp black, etc.) can be used. Fibrous carbon may also be used as the conductive additive. The content of the conductive additive in the positive electrode active material layer is not particularly limited, and is, for example, 1 to 30 mass%.

[0044] The thickness of the positive electrode active material layer varies depending on the intended configuration of the lithium secondary battery, but is usually 0.1 to 1000 μm, preferably 10 to 150 μm.

[0045] In a lithium secondary battery according to one embodiment of the present invention, the ratio of the area of ​​the negative electrode intermediate layer to the area of ​​the positive electrode active material layer (area of ​​the negative electrode intermediate layer / area of ​​the positive electrode active material layer) in a planar view of the power generating element is greater than 1.0 and less than or equal to 1.3. The positive electrode active material layer is disposed such that the outer peripheral edge of the positive electrode active material layer is inside the outer peripheral edge of the negative electrode intermediate layer in a planar view of the power generating element. The mechanism by which the initial charge / discharge efficiency is improved by controlling the area ratio of the negative electrode intermediate layer to the positive electrode active material layer in a planar view within the above range is not fully understood, but the following mechanism is presumed. Specifically, in a lithium secondary battery using a solid electrolyte, lithium metal precipitated by overcharge is mainly precipitated in the area between the negative electrode intermediate layer and the negative electrode current collector, where the positive electrode active material layer and the negative electrode current collector overlap in a planar view of the power generating element. The deposition of lithium metal occurs in this area due to the presence of a large amount of lithium ions in this area on the negative electrode current collector during charging. If the area of ​​the negative electrode intermediate layer is smaller than the area of ​​the positive electrode active material layer, the negative electrode intermediate layer cannot sufficiently cover the portion where the lithium metal is deposited, and the deposited lithium metal directly contacts the solid electrolyte layer, resulting in a short circuit. Furthermore, even if the area of ​​the negative electrode intermediate layer is the same as the area of ​​the positive electrode active material layer, it may be difficult for the negative electrode intermediate layer to reproducibly protect the solid electrolyte layer from the deposited lithium metal. Based on the above, it is believed that the occurrence of short circuits can be efficiently reduced by making the area of ​​the negative electrode intermediate layer larger than the area of ​​the positive electrode active material layer. Note that the above mechanism is based on speculation, and whether it is correct or incorrect does not affect the technical scope of the present invention.

[0046] Here, in a plan view of the power generating element, the lithium ion abundance rate during charging is not high in a region of the negative electrode current collector away from the region where the positive electrode active material is present. Therefore, it can be predicted that increasing the area of ​​the negative electrode intermediate layer will not significantly affect the battery characteristics. According to the inventors' studies, in lithium secondary batteries in which the ratio of the area of ​​the negative electrode intermediate layer to the area of ​​the positive electrode active material layer is 1.3 or less, the irreversible capacity of the battery remains almost unchanged, and the initial charge / discharge efficiency remains high. However, further studies by the inventors have revealed that in batteries in which the area ratio exceeds 1.3, the increase in the irreversible capacity of the battery due to the increase in the area ratio becomes significant, significantly reducing the initial charge / discharge efficiency.

[0047] When the power generating element is viewed in plan, the ratio of the area of ​​the negative electrode intermediate layer to the area of ​​the positive electrode active material layer is preferably 1.01 to 1.30, more preferably 1.02 to 1.20, and even more preferably 1.02 to 1.10. When the above area ratio satisfies such a range, the decrease in the initial charge / discharge efficiency can be further suppressed. In this specification, the area of ​​each layer when the power generating element is viewed in plan means the maximum area of ​​each layer when the power generating element is viewed in plan.

[0048] [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.

[0049] [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 ion 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 (for example, automobile parts, particularly electronic devices, etc.).

[0050] [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.

[0051] The above describes one embodiment of the present invention, but the present invention is not limited to the configuration described in the above embodiment, and can be modified as appropriate based on the description of the claims.

[0052] The following items are also included within the scope of the present invention: Item 1: A lithium secondary battery comprising a power generating element including: a positive electrode having a positive electrode active material layer containing a positive electrode active material; a negative electrode having a negative electrode current collector and on which lithium metal is deposited during charging; a solid electrolyte layer interposed between the positive electrode and the negative electrode and containing a solid electrolyte; and a negative electrode intermediate layer present adjacent to a surface of the solid electrolyte layer facing the negative electrode current collector and containing a carbon material, wherein, when the power generating element is viewed from above, the ratio of the area of ​​the negative electrode intermediate layer to the positive electrode active material layer (area of ​​negative electrode intermediate layer / area of ​​positive electrode active material layer) is greater than 1.0 and 1.3 or less; Item 2: The lithium secondary battery according to Item 1, wherein the area ratio is 1.02 to 1.20; Item 3: The lithium secondary battery according to Item 1 or 2, wherein the area ratio is 1.02 to 1.10; Item 4: The negative electrode intermediate layer has a basis weight of 0.1 to 2.0 mg / cm ２ Item 5: The lithium secondary battery according to any one of Items 1 to 3, wherein the negative electrode intermediate layer has a basis weight of 0.1 to 1.0 mg / cm ２ Item 6: The lithium secondary battery according to any one of Items 1 to 4, wherein the negative electrode intermediate layer has a basis weight of 0.1 to 0.5 mg / cm ２ Item 7: The lithium secondary battery according to any one of Items 1 to 5, wherein the negative electrode intermediate layer has a basis weight of 0.1 to 0.3 mg / cm ２ Item 8: The lithium secondary battery according to any one of Items 1 to 7, wherein the irreversible capacity of the carbon material of the negative electrode intermediate layer is 300 mAh / g (carbon) or less (preferably 200 mAh / g (carbon) or less); Item 9: The lithium secondary battery according to any one of Items 1 to 8, wherein the carbon material is carbon black (preferably acetylene black); Item 10: The lithium secondary battery according to any one of Items 1 to 9, wherein the negative electrode intermediate layer further contains a metal material (preferably In, Si, Sn, Ag, more preferably Ag); Item 11: The lithium secondary battery according to Item 10, wherein the mass ratio of the carbon material to the metal material (carbon material:metal material) is 10:1 to 1:1 (preferably 5:1 to 2:1).

[0053] 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.

[0054] <Measurement of Carbon Irreversible Capacity> Two types of acetylene black with different particle sizes and crystallinity were prepared as the carbon material for the negative electrode intermediate layer in the following examples, and designated as acetylene black a and acetylene black b. Evaluation half cells were fabricated for each of these carbon materials (acetylene black a and b) according to the following fabrication procedure, and the carbon irreversible capacity was measured.

[0055] [Example of Preparation of Half Cell for Measuring Carbon Irreversible Capacity] (Preparation of Negative Electrode) Carbon material and polyvinylidene fluoride (PVDF) as a binder were weighed out so that the carbon material:PVDF ratio was 86:14, 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 as a negative electrode current collector and dried to obtain a negative electrode with a negative electrode intermediate layer formed thereon.

[0056] (Preparation of Measurement Half Cell) A measurement half cell was prepared according to the following procedure.

[0057] (1) Argyrodite-type sulfide solid electrolyte (Li ６ P.S. ５160 mg of powder of 160 mg of 1,000 sintered lithium ion (Li, average particle size (D50): 0.8 μm) was placed inside a Macol and compression-molded using a uniaxial press at a pressure of 100 MPa for 1 minute to obtain a solid electrolyte layer; (2) The negative electrode prepared above was punched out into a circle with a diameter of 10 mm, and the negative electrode was placed so that the negative electrode intermediate layer and the solid electrolyte layer formed in (1) were in contact with each other, and pressure was applied using a uniaxial press at a pressure of 300 MPa for 1 minute to bond them together, thereby obtaining a laminate of a negative electrode current collector / anode intermediate layer / solid electrolyte layer; (3) With Li foil placed on the surface of the solid electrolyte layer opposite to the surface to which the negative electrode intermediate layer was pressed, the laminate obtained in (2) above was pressed using a uniaxial press at 25 MPa for 1 minute to bond them together, thereby obtaining a composite of a negative electrode current collector / anode intermediate layer / solid electrolyte layer / Li foil; (4) The composite obtained in (3) above was restrained at 6 MPa to prepare a measurement half cell.

[0058] [Measurement of Carbon Irreversible Capacity] In the half cell, the Li foil side was used as the negative electrode and the negative electrode intermediate layer side was used as the positive electrode, and charging and discharging was carried out under the following conditions.

[0059] (Charge / discharge test conditions) Evaluation temperature: 333K (60°C) Voltage range: 5mV to 1.0V (vs. Li / Li ＋ ).

[0060] First, 0.41 mA cm －２ Constant current (CC) discharge at 5 mV (vs. Li / Li ＋ ) and then reached 0.041 mA cm －２ The battery was then subjected to constant voltage (CV) discharge up to 0.41 mA cm. －２The battery was charged at a constant current (CC) up to 1.0 V. The obtained discharge capacity and charge capacity (mAh) were divided by the mass of the carbon material in the negative electrode intermediate layer to obtain the carbon discharge capacity (mAh / g) and carbon charge capacity (mAh / g), respectively. These values ​​were used to calculate the irreversible capacity of the carbon material (mAh / g (Carbon)) (carbon discharge capacity - carbon charge capacity). In the half cell, lithium ions enter the carbon during discharge and a reaction occurs in which lithium ions are released during charge. As a result, the carbon irreversible capacity of acetylene black a was 166 mAh / g (Carbon), and the carbon irreversible capacity of acetylene black b was 237 mAh / g (Carbon).

[0061] <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.

[0062] (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 SiO2 (average particle diameter (D50): 0.8 μm) and mesitylene as a solvent, followed by mixing. The solid electrolyte slurry was applied to the surface of polyethylene terephthalate (PET) foil as a coating substrate and dried to obtain a solid electrolyte layer (thickness: 40 μm).

[0063] (Preparation of Negative Electrode Intermediate Layer) Silver nanoparticles (average particle diameter (D50): 60 nm) and acetylene black a as a carbon material (carbon irreversible capacity: 166 mAh / g (Carbon)) were weighed and mixed to a mass ratio of Ag:C = 1:3. To 86 parts by mass of the obtained mixture, 14 parts by mass of polyvinylidene fluoride (PVDF) as a binder was added, and N-methyl-2-pyrrolidone (NMP) as a solvent was added and mixed to prepare a negative electrode intermediate layer slurry. The negative electrode intermediate layer slurry was coated on the surface of stainless steel foil as a coating substrate and dried to obtain a negative electrode intermediate layer. At this time, the basis weight of the negative electrode intermediate layer was 0.2 mg / cm ２ The thickness of the coating film was adjusted so that

[0064] (Preparation of Evaluation Cell) The positive electrode and the negative electrode were each punched into a square so that the area ratio of the negative electrode intermediate layer to the positive electrode active material layer was 1.02. The solid electrolyte layer was made one size larger than the negative electrode intermediate layer. The positive electrode active material layer formed on the surface of an aluminum foil (positive electrode current collector) and the solid electrolyte layer formed on the surface of a PET 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 then bonded together by cold isostatic pressing (CIP; 700 MPa, 25 ° C, 1 minute). After peeling off the PET foil adjacent to the solid electrolyte layer, the solid electrolyte layer and the negative electrode intermediate layer formed on the surface of a stainless steel foil (coated substrate) 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 then bonded together by cold isostatic pressing (CIP; 80 ° C, 500 MPa, 1 minute). After peeling off the stainless steel foil adjacent to the negative electrode intermediate layer, a stainless steel foil serving as a negative electrode current collector was placed on the exposed surface of the negative electrode intermediate layer. Finally, a stainless steel 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, yielding an evaluation cell that was a lithium deposition-type lithium secondary battery.

[0065] Example 2 An evaluation cell for this example was produced in the same manner as in Example 1, except that the punching size was changed so that the area ratio between the positive electrode active material layer and the negative electrode intermediate layer was 1.05.

[0066] Example 3 An evaluation cell for this example was produced in the same manner as in Example 1, except that the punching size was changed so that the area ratio between the positive electrode active material layer and the negative electrode intermediate layer was 1.10.

[0067] Example 4 An evaluation cell for this example was produced in the same manner as in Example 1, except that the punching size was changed so that the area ratio between the positive electrode active material layer and the negative electrode intermediate layer was 1.20.

[0068] Example 5 An evaluation cell for this example was produced in the same manner as in Example 1, except that the punching size was changed so that the area ratio between the positive electrode active material layer and the negative electrode intermediate layer was 1.30.

[0069] Example 6: Negative electrode intermediate layer with a basis weight of 0.25 mg / cm ２ The evaluation cell of this example was fabricated in the same manner as in Example 1, except for the change in the temperature.

[0070] Example 7: Negative electrode intermediate layer with a basis weight of 0.5 mg / cm ２ The evaluation cell of this example was fabricated in the same manner as in Example 1, except for the change in the temperature.

[0071] Example 8: Negative electrode intermediate layer with a basis weight of 1.0 mg / cm ２ The evaluation cell of this example was fabricated in the same manner as in Example 1, except for the change in the temperature.

[0072] Example 9: Negative electrode intermediate layer with a basis weight of 2.0 mg / cm ２ The evaluation cell of this example was fabricated in the same manner as in Example 1, except for the change in the temperature.

[0073] Example 10 An evaluation cell for this example was produced in the same manner as in Example 1, except that acetylene black a in Example 1 was replaced with acetylene black b (irreversible carbon capacity: 237 mAh / g (Carbon)).

[0074] Comparative Example 1 An evaluation cell for this comparative example was produced in the same manner as in Example 1, except that the punching size was changed so that the area ratio between the positive electrode active material layer and the negative electrode intermediate layer was 1.00.

[0075] Comparative Example 2 An evaluation cell for this comparative example was produced in the same manner as in Example 1, except that the punching size was changed so that the area ratio between the positive electrode active material layer and the negative electrode intermediate layer was 1.40.

[0076] <Measurement of initial charge-discharge efficiency and irreversible capacity of evaluation cell> A positive electrode lead and a negative electrode lead were connected to the positive electrode current collector and the negative electrode current collector, respectively, of the evaluation cell (before the initial charge) prepared above, and charge-discharge was performed under the following charge-discharge test conditions: During this test, the following charge-discharge test was performed while applying a restraining pressure of 3 MPa in the stacking direction of the evaluation cell using a pressure member.

[0077] (Charge / discharge test conditions) Evaluation temperature: 333K (60°C) Voltage range: 2.5 to 4.25V 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.

[0078] The evaluation cells were evaluated using a charge / discharge tester in a thermostatic chamber set at the evaluation temperature. The charging process was performed in two stages. Specifically, in the first charging process (where lithium metal precipitates on the negative electrode current collector), the cells were charged in constant current (CC) mode at 0.01 C for 25 hours. Then, in the second charging process (where lithium metal dissolves on the negative electrode current collector), the cells were charged to 4.25 V at 0.05 C in constant current (CC) mode. Then, in the discharge process (where lithium metal dissolves on the negative electrode current collector), the cells were discharged to 2.5 V at 0.1 C in constant current (CC) mode. Here, 1 C refers to the current value at which the battery reaches a full charge (100% charge) state after 1 hour of charging. The initial coulombic efficiency [%] (discharge capacity / charge capacity × 100) was calculated from the capacities obtained by the above charge and discharge. The irreversible capacity [mAh / g] (charge capacity - discharge capacity) of the evaluation cell was calculated from the difference between the charge capacity and the discharge capacity. The results are shown in Table 1 below.

[0079] <Measurement of dendrite-induced short circuit rate> When charging and discharging were performed using the method described above in "Measurement of initial charge / discharge efficiency and irreversible capacity," if the upper limit voltage (4.25V) was not reached within 40 hours during the second charging process, it was determined that a short circuit had occurred. Using the same procedure as described above in "Measurement of initial charge / discharge efficiency and irreversible capacity of evaluation cells," a charge / discharge test was performed on eight evaluation cells, and the percentage of short-circuited test cells was calculated as the initial short circuit rate [%]. The results are shown in Table 1 below.

[0080]

[0081] The results in Table 1 show that in Examples 1 to 10, in which the ratio of the area of ​​the negative electrode intermediate layer to the positive electrode active material layer was greater than 1.0 and 1.3 or less, the occurrence of short circuits could be prevented while suppressing a decrease in the initial charge-discharge efficiency. On the other hand, in Comparative Example 1, in which the ratio of the area of ​​the negative electrode intermediate layer to the positive electrode active material layer was 1.0 or less, the initial short-circuit rate was 75%, indicating that the occurrence of short circuits was not effectively prevented. Furthermore, in Comparative Example 2, in which the ratio of the area of ​​the negative electrode intermediate layer to the positive electrode active material layer exceeded 1.3, the initial charge-discharge efficiency was significantly reduced compared to the Examples.

[0082] 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 power generating element including: a positive electrode having a positive electrode active material layer containing a positive electrode active material; a negative electrode having a negative electrode current collector and on which lithium metal is deposited during charging; a solid electrolyte layer interposed between the positive electrode and the negative electrode and containing a solid electrolyte; and a negative electrode intermediate layer present adjacent to a surface of the solid electrolyte layer facing the negative electrode current collector and containing a carbon material, wherein, when the power generating element is viewed from above, the ratio of the area of ​​the negative electrode intermediate layer to the area of ​​the positive electrode active material layer (area of ​​the negative electrode intermediate layer / area of ​​the positive electrode active material layer) is greater than 1.0 and 1.3 or less.

2. The lithium secondary battery according to claim 1, wherein the area ratio is 1.02 to 1.

20.

3. The lithium secondary battery according to claim 1, wherein the area ratio is 1.02 to 1.

10.

4. The negative electrode intermediate layer has a basis weight of 0.1 to 2.0 mg / cm ２ 2. The lithium secondary battery according to claim 1, wherein 5. The negative electrode intermediate layer has a basis weight of 0.1 to 1.0 mg / cm ２ 2. The lithium secondary battery according to claim 1, wherein 6. The negative electrode intermediate layer has a basis weight of 0.1 to 0.5 mg / cm ２ 2. The lithium secondary battery according to claim 1, wherein 7. The negative electrode intermediate layer has a basis weight of 0.1 to 0.3 mg / cm ２ 2. The lithium secondary battery according to claim 1, wherein 8. The lithium secondary battery according to claim 1, wherein the irreversible capacity of the carbon material in the negative electrode intermediate layer is 200 mAh / g (carbon) or less.

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