Lithium secondary battery

Abstract

The present disclosure provides a means capable of reducing cell resistance in a lithium-deposition-type lithium secondary battery equipped with a negative electrode intermediate layer. Provided is a lithium secondary battery comprising a power generation element which has: a positive electrode having a positive electrode active material layer containing a positive electrode active material; a negative electrode which has a negative electrode current collector and in which lithium metal is deposited during charging; a solid electrolyte layer interposed between the positive electrode and the negative electrode, and containing a solid electrolyte; and a negative electrode intermediate layer that is interposed between the negative electrode current collector and the solid electrolyte layer, and contains at least one type of particle selected from the group consisting of metal particles and carbon particles. Therein, the surface roughness (Ra) of a surface X of the negative electrode current collector which faces the negative electrode intermediate layer is 0.2μm or more.

Description

Lithium-ion battery

[0001] This invention relates to a lithium secondary battery.

[0002] In recent years, research and development on all-solid-state lithium secondary batteries using oxide-based or sulfide-based solid electrolytes has been actively pursued. Solid electrolytes are materials mainly composed of ion conductors capable of ion conduction in a solid state. Therefore, all-solid-state lithium secondary batteries have the advantage that, in principle, various problems caused by flammable organic electrolytes, as seen in conventional liquid-based lithium secondary batteries, do not occur. In addition, generally, using high-potential, high-capacity positive electrode materials and high-capacity negative electrode materials can significantly improve the power density and energy density of the battery.

[0003] One type of all-solid-state lithium secondary battery is the so-called lithium deposition type, in which lithium metal is deposited on the negative electrode current collector during the charging process. In such lithium deposition type all-solid-state lithium secondary batteries, it is known that ionization of the lithium metal during the discharge process can create voids in the lithium metal, which can reduce the discharge capacity. Therefore, in lithium deposition type all-solid-state lithium secondary batteries, a restraining pressure is generally applied in the stacking direction to suppress the generation of voids.

[0004] Japanese Patent Publication No. 2020-191202 (corresponding to U.S. Patent Application Publication No. 2020 / 0373609) discloses a technique in which a negative electrode active material layer (negative electrode intermediate layer) containing Ag is provided between the negative electrode current collector and the solid electrolyte layer. With this configuration, during the charging process, Li precipitates as a Li(Ag) alloy in which Ag is dissolved. Then, during the discharge process, only Li dissolves from the Li(Ag) alloy, and the Ag that was dissolved remains, so the generation of voids can be suppressed.

[0005] However, the inventors' investigations revealed that lithium-deposited lithium secondary batteries equipped with a negative electrode intermediate layer may have high cell resistance.

[0006] Therefore, the object of the present invention is to provide a means for reducing cell resistance in a lithium deposition type lithium secondary battery equipped with a negative electrode intermediate layer.

[0007] In view of the above problems, the inventors conducted diligent studies and found that the above problems can be solved by controlling the surface roughness (Ra) of the surface facing the negative electrode intermediate layer of the negative electrode current collector to within a predetermined range, thereby completing the present invention.

[0008] In other words, one embodiment of the present invention relates to a lithium secondary battery comprising a power generation element having: a positive electrode having a positive electrode active material layer containing a positive electrode active material; a negative electrode having a negative electrode current collector 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 at least one selected from the group consisting of metal particles and carbon particles. The lithium secondary battery is characterized in that the surface roughness (Ra) of the surface X of the negative electrode current collector facing the negative electrode intermediate layer is 0.2 μm or more.

[0009] Figure 1 is a schematic cross-sectional view showing the overall structure of a stacked (internal parallel connection type) all-solid-state lithium secondary battery (stacked secondary battery) according to one embodiment of the present invention. Figure 2(a1) is an enlarged cross-sectional view schematically showing a part of a conventional stacked secondary battery (discharge). Figure 2(a2) is an enlarged cross-sectional view schematically showing a part of a conventional stacked secondary battery (charging). Figure 3(b1) is an enlarged cross-sectional view schematically showing a part of a stacked secondary battery (discharge) according to one embodiment of the present invention. Figure 3(b2) is an enlarged cross-sectional view schematically showing a part of a stacked secondary battery (charging) according to one embodiment of the present invention.

[0010] One embodiment of the present invention is a lithium secondary battery comprising a power generation element having: a positive electrode having a positive electrode active material layer containing a positive electrode active material; a negative electrode having a negative electrode current collector 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 at least one selected from the group consisting of metal particles and carbon particles, wherein the surface roughness (Ra) of the surface X of the negative electrode current collector facing the negative electrode intermediate layer is 0.2 μm or more. According to this embodiment, the cell resistance can be reduced in a lithium deposition type lithium secondary battery equipped with a negative electrode intermediate layer.

[0011] The lithium secondary battery according to this embodiment will be described below with reference to the drawings, but the technical scope of the present invention should be determined based on the claims and is not limited to the following embodiments. Note that the dimensional ratios in the drawings are exaggerated for illustrative purposes and may differ from the actual ratios.

[0012] Figure 1 is a schematic cross-sectional view showing the overall structure of a stacked (internal parallel connection type) all-solid-state lithium secondary battery (hereinafter also simply referred to as "stacked secondary battery"), which is one embodiment of the present invention. Figure 1 shows a cross-section of the stacked secondary battery during charging. The stacked secondary battery 10a shown in Figure 1 has a structure in which a substantially rectangular power generation element 21, where the charge and discharge reaction actually proceeds, is sealed inside a laminate film 29, which is the battery casing. Here, the power generation element 21 has a configuration in which a negative electrode, a solid electrolyte layer 17 containing a solid electrolyte, and a positive electrode are stacked.

[0013] 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. Furthermore, a negative electrode intermediate layer 14 is arranged so as to be in contact with the negative electrode active material layer 13 and the solid electrolyte layer 17, respectively.

[0014] The positive electrode has a structure in which a positive electrode active material layer 15 is arranged on the surface of the 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 a single cell layer 19. Therefore, the stacked secondary battery 10a shown in Figure 1 can also be said to have a configuration in which multiple single cell layers 19 are stacked and electrically connected in parallel.

[0015] The negative electrode current collector 11' and the positive electrode current collector 11'' are each fitted with a negative electrode current collector plate 25 and a positive electrode current collector plate 27, which are electrically connected to the respective electrodes (negative and positive electrodes), and are structured to be sandwiched between the edges of the laminate film 29 and led out to the outside of the laminate film 29. In the stacked secondary battery 10a, a restraining pressure is applied in the stacking direction of the power generation elements 21 by a pressurizing member. As a result, the volume of the power generation elements 21 is kept constant.

[0016] The stacked secondary battery according to this embodiment is characterized in that the surface roughness (Ra) of the surface X facing the negative electrode intermediate layer of the negative electrode current collector is 0.2 μm or more. By adopting such a configuration, the cell resistance can be reduced in a lithium deposition type lithium secondary battery equipped with a negative electrode intermediate layer. The inventors speculate that the mechanism by which such effects are achieved is as follows. Figure 2(a1) is an enlarged cross-sectional view schematically showing a part of a conventional stacked secondary battery (during discharge). Figure 2(a2) is an enlarged cross-sectional view schematically showing a part of a conventional stacked secondary battery (during charging). Figure 3(b1) is an enlarged cross-sectional view schematically showing a part of a stacked secondary battery (during discharge) according to one embodiment of the present invention. Figure 3(b2) is an enlarged cross-sectional view schematically showing a part of a stacked secondary battery (during charging) according to one embodiment of the present invention. In the stacked secondary battery shown in Figures 2 and 3, the negative electrode intermediate layer 14 has a structure in which Ag nanoparticles 14a and acetylene black 14b are bound together by polyvinylidene fluoride (not shown) as a binder. During discharge, voids exist between the Ag nanoparticles 14a and acetylene black 14b. During charging, lithium is first inserted into the acetylene black 14b, and then the lithium solidifies into the Ag nanoparticles 14a. Next, lithium metal (negative electrode active material layer) 13 is deposited in the voids between the Ag nanoparticles 14a and acetylene black 14b. Subsequently, lithium metal (negative electrode active material layer) 13 is also deposited between the negative electrode intermediate layer 14 and the negative electrode current collector 11'. In the stacked secondary battery shown in Figures 2(a1) and (a2), the surface roughness (Ra) of the surface X of the negative electrode current collector 11' facing the negative electrode intermediate layer 14 is 0.02 μm. When the surface roughness (Ra) of surface X is small, as shown in Figure 2(a1), the contact area between the negative electrode intermediate layer 14 and the negative electrode current collector 11' becomes small, and the reaction resistance increases. Also, as shown in Figure 2(a2), the contact area between the lithium metal (negative electrode active material layer) 13 and the negative electrode current collector 11' becomes small, and the reaction resistance increases. As a result, the cell resistance of the stacked secondary battery increases. In the stacked secondary battery shown in Figures 3(b1) and (b2), the surface roughness (Ra) of the surface X of the negative electrode current collector 11' facing the negative electrode intermediate layer 14 is 0.2 μm.Thus, when the surface roughness (Ra) of surface X is large, the contact area between the negative electrode intermediate layer 14 and the negative electrode current collector 11' increases, as shown in Figure 3(b1), and the reaction resistance decreases. Also, as shown in Figure 3(b2), the contact area between the lithium metal (negative electrode active material layer) 13 and the negative electrode current collector 11' increases, and the reaction resistance decreases. As a result, the cell resistance in a stacked secondary battery can be reduced. It should be noted that the mechanism described above is based on speculation, and its accuracy does not affect the technical scope of the present invention.

[0017] The main components of the lithium secondary battery according to this embodiment will be described below.

[0018] [Negative Electrode Current Collector] The negative electrode current collector has the function of mediating the movement of electrons from the negative electrode active material layer. There are no particular restrictions on the materials that constitute the current collector, and one or more combinations of metals, carbon materials, and conductive resins can be appropriately adopted. In particular, from the viewpoint of reducing the variation in surface roughness (Ra) of the surface X facing the negative electrode intermediate layer of the negative electrode current collector, it is preferable, and more preferable, that the current collector be composed of substantially only metal. In this specification, "substantially composed of only metal" means that the content of materials other than metal (e.g., carbon materials, conductive resins) is 10% by mass or less, 5% by mass or less, 3% by mass or less, 1% by mass or less, or 0.1% by mass or less, relative to the total mass of the negative electrode current collector. The metals that constitute the negative electrode current collector are not particularly limited, but examples include aluminum (Al), nickel (Ni), iron (Fe), titanium (Ti), copper (Cu), and alloys containing at least one of these. In particular, from the viewpoint of further reducing cell resistance, it is preferable that the negative electrode current collector be made of Cu, Ni, or an alloy containing at least one of Cu and Ni.

[0019] In the lithium secondary battery according to this embodiment, it is characterized in that the surface roughness (Ra) of the surface X of the negative electrode current collector facing the negative electrode intermediate layer is 0.2 μm or more. When the surface roughness (Ra) is less than 0.2 μm, as described above, the contact area between the negative electrode intermediate layer and the negative electrode current collector becomes small, and the reaction resistance becomes large. Also, the contact area between the lithium metal (negative electrode active material layer) and the negative electrode current collector becomes small, and the reaction resistance becomes large. As a result, the cell resistance of the laminated secondary battery may increase. From the viewpoint of further reducing the cell resistance, the surface roughness (Ra) is preferably 0.25 μm or more, and more preferably 0.3 μm or more. The upper limit of the surface roughness (Ra) is not particularly limited, but is preferably 0.6 μm or less, more preferably 0.55 μm or less, and even more preferably 0.5 μm or less. When the surface roughness (Ra) is 0.6 μm or less, it is possible to suppress the formation of voids between the lithium metal (negative electrode active material layer) and the negative electrode current collector and the reduction of the contact area between the lithium metal (negative electrode active material layer) and the negative electrode current collector. The preferable numerical range of the surface roughness (Ra) is 0.2 to 0.6 μm, 0.25 to 0.55 μm, and 0.3 to 0.5 μm.In the present specification, the surface roughness (Ra) of the surface X of the negative electrode current collector refers to the arithmetic mean roughness defined in JIS B 0601:2013. The surface roughness (Ra) is obtained by cutting out the negative electrode current collector into a size of 5 cm square and measuring it at room temperature (25°C) in an air atmosphere using an optical film thickness meter.

[0020] The method for controlling the surface roughness (Ra) of the negative electrode current collector within a predetermined range is not particularly limited, and known techniques can be appropriately employed. For example, it is possible to manufacture a negative electrode current collector having a predetermined surface roughness (Ra) by polishing the surface of a metal foil using a file or the like. Further, by adopting an electrolysis method as the method for manufacturing the negative electrode current collector, a negative electrode current collector having a predetermined surface roughness (Ra) can be easily manufactured. In particular, according to the electrolysis method, the surface irregularities are isotropic and the convex portions are rounded, so that the contact with the surface of the negative electrode intermediate layer is good and the reaction resistance can be further reduced. Therefore, according to an embodiment of the present invention, a lithium secondary battery is provided in which the negative electrode current collector is a metal foil manufactured by an electrolysis method. In the negative electrode current collector formed by the electrolysis method, there is a situation ( "impossible / impractical situation") that it is impossible or approximately impractical to directly specify the shape of the surface irregularities or the like depending on the structure or characteristics of the negative electrode current collector. Therefore, it is reasonable to specify the negative electrode current collector as an "object" by the provision that "the negative electrode current collector is a metal foil manufactured by an electrolysis method".

[0021] The thickness of the current collector is not particularly limited, but as an example, it is 10 to 100 μm.

[0022] [Negative electrode active material layer] The lithium secondary battery according to this embodiment is a so-called lithium deposition type in which lithium metal is deposited on the negative electrode current collector during the charging process. The layer composed of lithium metal deposited on the negative electrode current collector during this charging process is the negative electrode active material layer of the lithium secondary battery according to this embodiment. Therefore, the thickness of the negative electrode active material layer increases as the charging process progresses, and the thickness of the negative electrode active material layer decreases as the discharging process progresses. At the time of complete discharge, the negative electrode active material layer may not exist, but in some cases, a negative electrode active material layer composed of a certain amount of lithium metal may be arranged at the time of complete discharge. Further, the thickness of the negative electrode active material layer (lithium metal layer) at the time of complete charge is not particularly limited, but is usually 0.1 to 1000 μm.

[0023] [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 at least one selected from the group consisting of metal particles and carbon particles. By providing such a negative electrode intermediate layer, the deposition and growth of lithium dendrites are suppressed. It is preferable that the negative electrode intermediate layer as a whole is conductive. The volume resistivity of the negative electrode intermediate layer is not particularly limited, but is preferably 10 ２ The resistivity is Ω·cm or less, and more preferably 10Ω·cm or less. In this specification, the volume resistivity of the negative electrode intermediate layer is the value measured using an electrode resistance measurement system (manufactured by HIOKI E.E. CORPORATION, product name: RM2610).

[0024] The negative electrode intermediate layer preferably contains carbon particles. The inclusion of carbon particles in the negative electrode intermediate layer can suppress the precipitation and growth of lithium dendrites. Specific examples of carbon particles include carbon black (specifically, acetylene black, Ketjenblack®, furnace black, channel black, thermal lamp black, etc.), carbon nanotubes (CNTs), graphite, hard carbon, etc. In particular, the carbon particles preferably include at least one selected from the group consisting of carbon black, and more preferably at least one selected from the group consisting of acetylene black, Ketjenblack®, furnace black, channel black, and thermal lamp black.

[0025] The negative electrode intermediate layer preferably contains metal particles in place of, or in addition to, the carbon particles. The inclusion of metal particles in the negative electrode intermediate layer allows for more uniform deposition of lithium metal on the surface of the negative electrode current collector. Specific examples of metal materials constituting the metal particles include, for example, magnesium (Mg), aluminum (Al), silicon (Si), zinc (Zn), silver (Ag), indium (In), tin (Sn), gold (Au), bismuth (Bi), nickel (Ni), copper (Cu), and alloys containing at least one of these. Among these, it is preferable that the metal particles are composed of a metal that forms a solid solution with lithium. More specifically, the negative electrode intermediate layer preferably contains metal particles containing at least one element selected from Mg, Al, Si, Zn, Ag, In, Sn, Au, and Bi; more preferably contains metal particles containing at least one element selected from Mg, Al, Zn, and Ag; even more preferably contains metal particles containing at least one element selected from Mg, Zn, and Ag; and particularly preferably contains metal particles containing Ag. The deposited lithium metal is highly reactive and may be deactivated by the atmosphere inside the cell. By constructing the negative electrode intermediate layer using metal particles composed of a metal that forms a solid solution with lithium, the amount of deposited lithium metal is reduced, thereby reducing the risk of lithium metal deactivation and suppressing the degradation of the lithium secondary battery.

[0026] The average primary particle diameter of the carbon particles is not particularly limited, but is, for example, 200 nm or less, preferably 150 nm or less, more preferably 100 nm or less, even more preferably 80 nm or less, even more preferably 60 nm or less, even more preferably 50 nm or less, and especially preferably 45 nm or less. The lower limit of the average primary particle diameter of the carbon particles is also not particularly limited, but is preferably 10 nm or more, more preferably 20 nm or more, and even more preferably 25 nm or more. The average primary particle diameter of the metal particles is not particularly limited, but is, for example, 500 nm or less, preferably 300 nm or less, more preferably 200 nm or less, even more preferably 100 nm or less, and especially preferably 80 nm or less. The lower limit of the average primary particle diameter of the metal particles is also not particularly limited, but is, for example, 10 nm or more, preferably 20 nm or more, even more preferably 30 nm or more, and even more preferably 40 nm or more. When the average primary particle diameter of the carbon particles and / or the average primary particle diameter of the metal particles are within the above ranges, the cell resistance can be further reduced. In this specification, the average primary particle diameter of a particle refers to the arithmetic mean of the particle diameters of the particle observed within several to tens of fields of view when a cross-section of the layer containing the particle is observed with a scanning electron microscope (SEM).

[0027] When the negative electrode intermediate layer contains carbon particles and metal particles, the mass ratio of carbon particles to metal particles (carbon particles:metal particles) is preferably 10:1 to 1:1, more preferably 5:1 to 2:1, and even more preferably 4:1 to 2.5:1. The volume ratio of carbon particles to metal particles (carbon particles:metal particles) is preferably 99:1 to 70:30, and more preferably 95:5 to 75:25. When the blending ratio (mass ratio or volume ratio) of carbon particles to metal particles is within the above range, the cell resistance can be further reduced.

[0028] The content of carbon particles and / or metal particles in the negative electrode intermediate layer is not particularly limited, but is preferably 80 to 100% by mass, more preferably 82 to 99% by mass, and even more preferably 84 to 95% by mass, based on 100% by mass of the total solid content in the negative electrode intermediate layer. When the content of carbon particles and / or metal particles is within the above range, the cell resistance can be further reduced.

[0029] The negative electrode intermediate layer preferably contains a binder in addition to carbon particles and / or metal particles. The type of binder is not particularly limited, and any known in the art can be used as appropriate. Examples include styrene-butadiene rubber (SBR), polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF) (including compounds in which hydrogen atoms are substituted with other halogen elements), and carboxymethylcellulose (CMC). Among these, styrene-butadiene rubber, tetrafluoroethylene, and polyvinylidene fluoride are preferred, and tetrafluoroethylene and polyvinylidene fluoride are more preferred. These binders may be used individually or in combination of two or more.

[0030] The binder content in the negative electrode intermediate layer is not particularly limited, but is preferably 8 to 20% by mass, more preferably 10 to 18% by mass, and even more preferably 12 to 16% by mass, based on 100% by mass of the total solid content in the negative electrode intermediate layer. When the binder content is within the above range, the cell resistance can be further reduced.

[0031] In the lithium secondary battery according to this embodiment, the surface roughness (Ra) of the surface Y of the negative electrode current collector of the negative electrode intermediate layer is the same as or greater than the surface roughness (Ra) of the surface X. By adopting such a configuration, the contact between the negative electrode intermediate layer and the negative electrode current collector becomes good, and the reaction resistance can be further reduced. In this specification, the surface roughness (Ra) of the surface Y of the negative electrode intermediate layer refers to the arithmetic mean roughness defined in JIS B 0601:2013. The surface roughness (Ra) is obtained by observing a cross-section of the lithium secondary battery with a scanning electron microscope (SEM) and performing image analysis on 10 obtained observation images. The surface roughness (Ra) of the surface Y of the negative electrode intermediate layer can be controlled by the solid content concentration of the slurry when producing the negative electrode intermediate layer. More specifically, the lower the solid content concentration of the slurry (the higher the proportion of the solvent in the slurry), the greater the surface roughness (Ra) of the surface Y of the negative electrode intermediate layer can be made.

[0032] The thickness of the negative electrode intermediate layer is preferably 1 μm or more and 20 μm or less, more preferably 1 μm or more and 15 μm or less, and even more preferably 1 μm or more and 10 μm or less. When the thickness of the negative electrode intermediate layer is within the above range, it is possible to suppress the precipitation and growth of lithium dendrites and prevent a decrease in charge and discharge capacity.

[0033] [Solid electrolyte layer] The solid electrolyte layer is interposed between the negative electrode and the positive electrode and contains a solid electrolyte (usually as a main component). The solid electrolyte contained in the solid electrolyte layer is not particularly limited, and those known in the art can be appropriately adopted. As an example, LPS (Li ２ S-P ２ S ５ ), Li ６ PS ５ / / Here, X is Cl, Br, or I), Li ７ ３ P １１ S １１ 、Li ３．２ P ０．９６ S and Li ３ PS ４Examples of sulfide solid electrolytes include the following. These sulfide solid electrolytes are preferred because they have excellent lithium-ion conductivity and a low bulk modulus, allowing them to follow the volume changes of the electrode active material during charging and discharging. These solid electrolytes may be used individually or in combination of two or more. Of course, other solid electrolytes may also be used.

[0034] The solid electrolyte content 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.

[0035] 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 for the negative electrode intermediate layer.

[0036] The thickness of the solid electrolyte layer varies depending on the intended configuration of the lithium secondary battery, but is usually between 0.1 μm and 1000 μm, and preferably between 10 μm and 40 μm.

[0037] [Positive Electrode Current Collector] The positive electrode current collector has the function of mediating the movement of electrons from the positive electrode active material layer. There are no particular restrictions on the materials that make up the positive electrode current collector. Examples of materials that can be used to make up the positive electrode current collector include metals such as aluminum, nickel, iron, stainless steel, titanium, and copper, as well as carbon materials and conductive resins. There are also no particular restrictions on the thickness of the positive electrode current collector, but one example is 10 to 100 μm.

[0038] [Positive Electrode Active Material Layer] The positive electrode active material layer must contain a positive electrode active material. The type of positive electrode active material contained in the positive electrode active material layer is not particularly limited, but lithium-containing metal oxides are preferred. A specific example of a lithium-containing metal oxide is LiCoO ２ LiMnO ２ LiNiO ２ , Li(Ni-Mn-Co)O ２ Layered rock salt type active materials such as LiMn ２ O ４ LiNi ０．５ Mn １．５ O ４spinel-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 Si-containing active materials include the above. Other oxide active materials include, for example, Li ４ Ti ５ O １２ LiVO ２ These include Li(Ni-Mn-Co)O ２ Furthermore, those in which some of these transition metals are substituted with other elements (NMC composite oxides) are preferably used as positive electrode active materials. These positive electrode active materials may be used individually or in combination of two or more types.

[0039] Another preferred embodiment involves the use of a sulfur-based positive electrode active material. Examples of sulfur-based positive electrode active materials include particles or thin films of organic sulfur compounds or inorganic sulfur compounds, and any material that can release lithium ions during charging and absorb lithium ions during discharging by utilizing the oxidation-reduction reaction of sulfur is acceptable.

[0040] The shape of the positive electrode active material can be, for example, particulate (spherical, fibrous), thin film, etc. When the positive electrode active material is particulate, its particle size is 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.

[0041] The content of the positive electrode active material in the positive electrode active material layer is not particularly limited, but is preferably 50 to 99% by mass, more preferably 70 to 99% by mass or less, and even more preferably 80 to 99% by mass or less.

[0042] 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. 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. Examples of conductive additives include, but are not limited to, 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 (CNTs), and carbon black (specifically, acetylene black, Ketjenblack®, furnace black, channel black, thermal lamp black, etc.). Furthermore, particulate ceramic materials or resin materials coated with the above metal materials by plating or the like can also be used as conductive additives.

[0043] The thickness of the positive electrode active material layer varies depending on the configuration of the lithium secondary battery, but is, for example, 0.1 to 1000 μm, preferably 30 to 300 μm, more preferably 50 to 200 μm, and even more preferably 70 to 150 μm.

[0044] [Positive electrode current collector plate and negative electrode current collector plate] The material constituting the current collector plates (25, 27) is not particularly limited, and known highly conductive materials conventionally used as current collector plates for secondary batteries can be used. Preferred materials for the current collector plates are, for example, metallic materials such as aluminum, copper, titanium, nickel, stainless steel (SUS), and alloys thereof. From the viewpoint of lightness, corrosion resistance, and high conductivity, aluminum and copper are more preferred, and aluminum is particularly preferred. The positive electrode current collector plate 27 and the negative electrode current collector plate 25 may be made of the same material, or different materials may be used.

[0045] [Positive and Negative Leads] Although not shown in the diagram, the current collectors (11'', 11'') and the current collector plates (27'', 25) may be electrically connected via positive and negative leads. The materials used for the positive and negative leads may be the same as those used in known lithium secondary batteries. It is preferable to cover the parts that are removed from the casing with heat-resistant insulating heat shrink tubing or the like to prevent leakage current from coming into contact with peripheral equipment or wiring and affecting the product (for example, automotive parts, especially electronic equipment).

[0046] [Battery casing material] As the battery casing material, a known metal can case can be used, or, as shown in Figure 1, a bag-shaped case made of a laminate film 29 containing aluminum that can cover the power generation element can be used. For example, a three-layer laminate film made by laminating PP, aluminum, and nylon in that order can be used, but there is no limit to these. Laminate film is desirable from the viewpoint of being able to increase power output and have excellent cooling performance, and can be suitably used for batteries in large equipment for EVs and HEVs. Furthermore, a laminate film containing aluminum is more preferable as the casing material because it allows for easy adjustment of the group pressure applied to the power generation element from the outside.

[0047] The lithium secondary battery according to this embodiment has a configuration in which multiple single cell layers are connected in parallel, resulting in high capacity and excellent cycle durability. Therefore, the lithium secondary battery according to this embodiment is suitable for use as a power source for EVs and HEVs.

[0048] 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-mentioned embodiment, and can be modified as appropriate based on the description of the claims.

[0049] For example, one type of battery to which the lithium secondary battery according to the present invention is applied is a bipolar battery that includes a bipolar electrode having a positive electrode active material layer electrically coupled to one side of a current collector and a negative electrode active material layer electrically coupled to the opposite side of the current collector.

[0050] Furthermore, the lithium secondary battery according to this embodiment does not have to be all-solid type. That is, the solid electrolyte layer may further contain a conventionally known liquid electrolyte (electrolyte). There are no particular restrictions on the amount of liquid electrolyte (electrolyte) that can be contained in the solid electrolyte layer, but it is preferable that the amount is such that the shape of the solid electrolyte layer formed by the solid electrolyte is maintained and leakage of the liquid electrolyte (electrolyte) does not occur.

[0051] The following embodiments are also included in 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; a lithium secondary battery according to any one of claims 1 to 6 having the features of claim 7.

[0052] The present invention will be described in more detail below with reference to examples. However, the technical scope of the present invention is not limited to the following examples. In the following, the preparation of evaluation cells was carried out in a glove box with an argon atmosphere and a dew point of -68°C or lower. Furthermore, the instruments and equipment used in the glove box were thoroughly dried beforehand.

[0053] <Examples of evaluation cell fabrication> [Comparative Example 1] (Fabrication of positive electrode active material layer) As constituent materials of the positive electrode active material layer, NMC composite oxide (LiNi) ０．８ Mn ０．１ Co ０．１ O ２ ) and argyrodite-type sulfide solid electrolytes (Li ６ PS ５Cl) and carbon nanofiber (CNF) (manufactured by Showa Denko K.K., VGCF®) as a conductive additive and polytetrafluoroethylene (PTFE) as a binder were prepared. These constituent materials were weighed in the ratio of positive electrode active material:solid electrolyte:conductive additive:binder = 89:9.8:0.7:0.5 (mass ratio) and kneaded in an agate mortar. After confirming that the binder had fibrillated, the obtained powder composition (mixture for forming the positive electrode active material layer) was supplied to the powder inlet set in a roll press machine. The powder composition was then rolled using the roll press machine (conditions are shown below) to form a sheet. The obtained sheet was folded in half and rolled again using the roll press machine (conditions are shown below) (folded rolling process). By repeating this folding and rolling process three times, a positive electrode active material layer with a thickness of 100 μm was obtained. In the folding and rolling process, the direction in which the sheet was folded in half and the direction in which the rolling process was applied to the folded sheet were randomized. (Conditions for the roll press machine) Roll size: 250 mmφ × 400 mm Roll rotation speed: 1 m / min Roll spacing (gap): 100 μm Pressure: 10 kN (linear pressure: 25 kN / m).

[0054] (Preparation of the solid electrolyte layer) Algyrodite-type sulfide solid electrolyte (Li ６ PS ５ A solid electrolyte slurry was prepared by mixing 95 parts by mass of Cl with a binder solution (5 parts by mass of styrene-butadiene rubber (SBR) dissolved in mesitylene as a solvent). The obtained solid electrolyte slurry was coated onto the surface of a stainless steel foil support using an applicator and dried to obtain a solid electrolyte layer with a thickness of 40 μm.

[0055] (Fabrication of the negative electrode current collector) SUS430LX foil (metal resistance: 6.0 x 10) －８ [Ω・cm] ２ A negative electrode current collector with a surface roughness (Ra) of 0.02 μm was obtained by applying an electric sander fitted with #2000 grit sandpaper to one surface (surface X) of the material and polishing it for one minute.

[0056] (Preparation of the negative electrode intermediate layer) Acetylene black (AB; average primary particle size 35 nm) and silver (Ag) nanoparticles (average primary particle size 60 nm) were weighed in a ratio of AB:Ag = 3:1 (mass ratio) and mixed. To 86 parts by mass of the resulting mixture, 14 parts by mass of polyvinylidene fluoride (PVDF) as a binder was added, and N-methylpyrrolidone was added as a solvent and mixed to prepare a negative electrode intermediate layer slurry with a solid content of 5% by mass. The negative electrode intermediate layer slurry was coated onto one surface (face X) of the negative electrode current collector prepared above, dried, and a laminate was obtained in which a negative electrode intermediate layer (thickness 4 μm) was formed on the surface of the negative electrode current collector. The surface roughness (Ra) of the negative electrode intermediate layer on the surface Y facing surface X of the negative electrode current collector was 0.2 μm.

[0057] (Preparation of evaluation cell) The positive electrode active material layer prepared above was placed on top of the aluminum foil used as the positive electrode current collector. Then, the solid electrolyte layer formed on the surface of the stainless steel foil was placed on top of the positive electrode active material layer so that the exposed surface of the solid electrolyte layer faced the positive electrode active material layer, and the solid electrolyte layer was transferred onto the positive electrode active material layer by cold isostatic pressing (CIP). After peeling off the stainless steel foil adjacent to the solid electrolyte layer, the negative electrode intermediate layer formed on the surface of the negative electrode current collector was placed on top of the transferred solid electrolyte layer 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). As a result, a laminate was obtained in which the positive electrode current collector, positive electrode active material layer, solid electrolyte layer, negative electrode intermediate layer, and negative electrode current collector were stacked in this order. Finally, an aluminum positive electrode tab and a nickel negative electrode tab were joined to the aluminum foil (positive electrode current collector) and negative electrode current collector, respectively, using an ultrasonic welding machine. The resulting laminate was then placed inside an aluminum laminate film and vacuum-sealed to obtain an evaluation cell, which is a lithium deposition type all-solid-state lithium secondary battery of this comparative example.

[0058] [Comparative Example 2] In the above (fabrication of the negative electrode current collector), rolled Cu foil (metal resistance: 0.17 × 10 －８ [Ω・cm] ２A negative electrode current collector (surface roughness (Ra) 0.08 μm of surface X) was obtained by applying an electric sander fitted with #2000 grit sandpaper to one surface (surface X) of the material and polishing it for one minute. Then, the above (fabrication of the negative electrode intermediate layer) was carried out using this negative electrode current collector. Except for this step, the evaluation cell for this comparative example was obtained by the same method as in Comparative Example 1.

[0059] [Example 1] Without performing the above (fabrication of the negative electrode current collector), electrolytic Cu foil (metal resistance: 0.17 × 10 －８ [Ω・cm] ２ A negative electrode current collector with a surface roughness (Ra) of 0.2 μm was prepared. Then, the above (fabrication of the negative electrode intermediate layer) was carried out using this negative electrode current collector. Except for this, the evaluation cell for this embodiment was obtained by the same method as in Comparative Example 1.

[0060] [Example 2] In the above (fabrication of the negative electrode current collector), electrolytic Ni foil (metal resistance: 0.7 × 10) －８ [Ω・cm] ２ A negative electrode current collector (surface roughness (Ra) 0.3 μm of surface X) was obtained by applying an electric sander fitted with #2000 grit sandpaper to one surface (surface X) and polishing for 1 minute. Then, the above (fabrication of the negative electrode intermediate layer) was carried out using this negative electrode current collector. Except for this, the evaluation cell for this embodiment was obtained by the same method as in Comparative Example 1.

[0061] [Example 3] In the above (fabrication of the negative electrode current collector), Ni-plated Cu foil (metal resistance: 0.65 × 10 －８ [Ω・cm] ２ A negative electrode current collector (surface roughness (Ra) 0.6 μm of surface X) was obtained by applying an electric sander fitted with #2000 grit sandpaper to one surface (surface X) of the negative electrode current collector and polishing for 1 minute. The above (fabrication of the negative electrode intermediate layer) was carried out using this negative electrode current collector. The above (fabrication of the negative electrode intermediate layer) was carried out using this negative electrode current collector. Except for this, the evaluation cell for this embodiment was obtained by the same method as in Comparative Example 1 above.

[0062] [Example 4] In the above (fabrication of the negative electrode intermediate layer), a laminate was obtained in which a negative electrode intermediate layer (thickness 4 μm) was formed on the surface of the negative electrode current collector using a negative electrode intermediate layer slurry with a solid content concentration of 4% by mass. The surface roughness (Ra) of the surface Y of the negative electrode intermediate layer facing surface X of the negative electrode current collector was 0.7 μm. The evaluation cell for this example was obtained by the same method as in Example 1, except that the above (fabrication of the evaluation cell) was performed using this laminate.

[0063] [Example 5] In the above (fabrication of the negative electrode intermediate layer), a laminate was obtained in which a negative electrode intermediate layer (thickness 4 μm) was formed on the surface of the negative electrode current collector using a negative electrode intermediate layer slurry with a solid content concentration of 3.5 mass%. The surface roughness (Ra) of the surface Y of the negative electrode intermediate layer facing the surface X of the negative electrode current collector was 1.6 μm. The evaluation cell for this example was obtained by the same method as in Example 1, except that the above (fabrication of the evaluation cell) was performed using this laminate.

[0064] [Comparative Example 3] In this comparative example, the above (fabrication of the negative electrode intermediate layer) was omitted, and in the above (fabrication of the evaluation cell), the negative electrode current collector was placed on top of the solid electrolyte layer and pressed by cold isostatic pressing (CIP). Except for this, the evaluation cell for this comparative example was obtained by the same method as in Example 2.

[0065] [Comparative Example 4] In this comparative example, the above (fabrication of the negative electrode intermediate layer) was omitted, and in the above (fabrication of the evaluation cell), the negative electrode current collector was placed on top of the solid electrolyte layer and pressed by cold isostatic pressing (CIP). Except for this, the evaluation cell for this comparative example was obtained by the same method as in Example 1.

[0066] <Measurement of Cell Resistance (DCR)> <Measurement of Discharge DCR> The evaluation cells prepared above were subjected to the following charge-discharge tests while applying a constraining pressure of 3 MPa in the stacking direction using a pressurizing member. First, under a temperature of 25°C, three charge-discharge cycles were performed using a charge-discharge tester to check for short circuits.

[0067] The evaluation cells that did not experience a short circuit were charged to 50% of the state of charge (SOC). At 50% SOC, charging and discharging (for 30 seconds each) were performed at rates (current values) of 0.05C, 0.1C, 0.2C, and 0.5C. The cell resistance (Direct Current Resistance; DCR) was calculated according to Ohm's law from the current values ​​and voltage changes (values ​​at 10 seconds). The results are shown in Table 1 below.

[0068]

[0069] As shown in Table 1, according to the present invention, by setting the surface roughness (Ra) of the surface X facing the negative electrode intermediate layer of the negative electrode current collector to 0.2 μm or more, it is possible to reduce the cell resistance in a lithium deposition type lithium secondary battery equipped with a negative electrode intermediate layer.

[0070] 10a Stacked secondary battery, 11' Negative electrode current collector, 11'' Positive electrode current collector, 13 Lithium metal (negative electrode active material layer), 14 Negative electrode intermediate layer, 14a Ag nanoparticles, 14b Acetylene black, 15 Positive electrode active material layer, 17 Solid electrolyte layer, 19 Single cell layer, 21 Power generation element, 25 Negative electrode current collector plate, 27 Positive electrode current collector plate, 29 Laminate film X side.

Claims

1. A lithium secondary battery comprising a power generation element having a positive electrode having a positive electrode active material layer containing a positive electrode active material, a negative electrode having a negative electrode current collector 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 at least one selected from the group consisting of metal particles and carbon particles, wherein the surface roughness (Ra) of the surface X of the negative electrode current collector facing the negative electrode intermediate layer is 0.2 μm or more.

2. The lithium secondary battery according to claim 1, wherein the surface roughness (Ra) of the surface X is 0.6 μm or less.

3. The lithium secondary battery according to claim 1 or 2, wherein the surface roughness (Ra) of the surface Y of the negative electrode current collector in the negative electrode intermediate layer that faces the surface X is the same as the surface roughness (Ra) of the surface X, or is greater than the surface roughness (Ra) of the surface X.

4. The lithium secondary battery according to claim 1 or 2, wherein the negative electrode intermediate layer contains metal particles containing at least one element selected from Mg, Al, Si, Zn, Ag, In, Sn, Au, and Bi.

5. The lithium secondary battery according to claim 1 or 2, wherein the negative electrode current collector is composed substantially of metal only.

6. The lithium secondary battery according to claim 5, wherein the negative electrode current collector is made of Cu, Ni, or an alloy containing at least one of Cu and Ni.

7. The lithium secondary battery according to claim 1 or 2, wherein the negative electrode current collector is a metal foil manufactured by an electrolytic method.

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