negative electrode

Abstract

To provide a negative electrode capable of improving the capacity retention of a lithium secondary battery.SOLUTION: A negative electrode for a lithium secondary battery includes a negative electrode current collector and a negative electrode layer. The negative electrode layer has a composite layer containing an alloy of a lithium metal and a dissimilar metal, and a lithium metal single body layer, as a negative electrode active material in an order from the negative electrode current collector side. The dissimilar metal is an element that can form a solid solution together with the lithium metal, or an element that can form an intermetallic compound together with the lithium metal.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a negative electrode. [Background technology]

[0002] Among batteries, lithium secondary batteries have attracted attention because they can provide high output voltages.

[0003] Patent Document 1 discloses a Li metal negative electrode battery in which a metal Mg layer containing metallic magnesium is formed on one surface of a negative electrode current collector or on one surface of a solid electrolyte layer.

[0004] Patent Document 2 discloses a negative electrode material that contains a metal thin film (Au, Mg, Ag) at the interface between a Li metal layer and a current collector. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2020-184513 [Patent Document 2] Patent Publication No. 2021-077640 Summary of the Invention [Problem to be solved by the invention]

[0006] In lithium secondary batteries that use lithium metal, lithium alloys, or the like as negative electrode active materials, there is a problem in that the capacity retention rate decreases due to deactivation of lithium metal caused by volume changes of the lithium metal during charging and discharging, and there is a demand for an improvement in the capacity retention rate.

[0007] The present disclosure has been made in view of the above circumstances, and has as its main object to provide a negative electrode that can improve the capacity retention rate of a lithium secondary battery. [Means for solving the problem]

[0008] The negative electrode of the present disclosure includes a negative electrode current collector and a negative electrode layer, the negative electrode layer has, in this order from the negative electrode current collector side, a composite layer containing an alloy of lithium metal and a different metal, and a lithium metal simple layer, as negative electrode active materials; The different metal is an element capable of forming a solid solution with the lithium metal or an element capable of forming an intermetallic compound with the lithium metal, and is a negative electrode for a lithium secondary battery.

[0009] In the negative electrode of the present disclosure, the ratio Z (Z=X / Y) of the thickness X of the lithium metal layer to the thickness Y of the composite layer may be 0.0001≦Z≦0.4.

[0010] In the negative electrode of the present disclosure, the element ratio of lithium in the alloy may be 30.00 atomic % or more and 99.97 atomic % or less. [Effects of the Invention]

[0011] The present disclosure can provide a negative electrode that can improve the capacity retention rate of a lithium secondary battery. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a cross-sectional view schematically illustrating an example of a lithium secondary battery according to the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, embodiments of the present disclosure will be described. It should be noted that matters other than those specifically mentioned in this specification that are necessary for implementing the present disclosure (for example, the general configuration and manufacturing process of negative electrodes and lithium secondary batteries that do not characterize the present disclosure) can be understood as design matters for those skilled in the art based on prior art in the relevant field. The present disclosure can be implemented based on the contents disclosed in this specification and common general technical knowledge in the relevant field. Furthermore, the dimensional relationships (length, width, thickness, etc.) in the drawings do not reflect the actual dimensional relationships. In this specification, the use of "to" to indicate a range of values means that the values before and after it are included as the lower and upper limits. Any combination of upper and lower limits in the numerical range can be adopted.

[0014] The negative electrode of the present disclosure includes a negative electrode current collector and a negative electrode layer, the negative electrode layer has, in this order from the negative electrode current collector side, a composite layer containing an alloy of lithium metal and a different metal, and a lithium metal simple layer, as negative electrode active materials; The different metal is an element capable of forming a solid solution with the lithium metal or an element capable of forming an intermetallic compound with the lithium metal, and is a negative electrode for a lithium secondary battery.

[0015] In lithium secondary batteries, the capacity retention rate decreases due to factors such as the decomposition reaction of the newly formed Li metal surface, which is continuously produced by the dissolution and precipitation of Li metal during charging and discharging, with the electrolyte, and the isolation of electrons in the negative electrode active material due to cracks in the negative electrode layer caused by volume changes in Li metal during charging and discharging. Generally, it is desirable for an alloy of Li and a dissimilar metal to be uniformly alloyed, as in the prior art. The mismatch in the crystal structure caused by the uneven composition of the Li and dissimilar metals in the alloy induces cracking of the negative electrode layer, resulting in a decrease in the capacity retention rate of the lithium secondary battery. In the case of the negative electrode of the present disclosure, although there is a difference in metal composition between the lithium metal simple layer and the composite layer containing an alloy of lithium metal and a dissimilar metal, the above-mentioned cracking of the negative electrode layer is unlikely to occur, and the lithium metal simple layer suppresses the reaction between the electrolyte or electrolyte and the composite layer, thereby improving the capacity retention rate and storage characteristics of the lithium secondary battery. Furthermore, in the prior art, when a lithium secondary battery operates at low temperatures, the Li conductivity in the electrolytic solution or electrolyte decreases significantly, the resistance due to the electrolytic solution or electrolyte increases, and the low-temperature output characteristics decrease significantly. On the other hand, according to the present disclosure, the increase in the resistance component of the electrolytic solution or electrolyte is mitigated, resulting in excellent low-temperature output. This is presumably because the lithium metal layer becomes thinner with charge and discharge, the reaction resistance of the lithium metal layer decreases, and the lithium metal layer is preferentially used for charge and discharge.

[0016] [Negative electrode] The negative electrode of the present disclosure includes a negative electrode current collector and a negative electrode layer.

[0017] [Negative electrode current collector] The material of the negative electrode current collector may be a material that does not alloy with Li, such as SUS, copper, or nickel. The negative electrode current collector may be in the form of, for example, a foil or a plate. The shape of the negative electrode current collector in plan view is not particularly limited, but may be, for example, a circle, an ellipse, a rectangle, or any polygonal shape. The thickness of the negative electrode current collector varies depending on the shape, but may be, for example, in the range of 1 μm to 50 μm, or in the range of 5 μm to 20 μm.

[0018] [Negative electrode layer] The negative electrode layer has, in this order from the negative electrode current collector side, a composite layer containing an alloy of lithium metal and a different metal, and a layer of simple lithium metal, as negative electrode active materials. The composite layer contains, as a negative electrode active material, an alloy of lithium metal and a different metal. The element ratio of lithium in the alloy may be 30.00 atomic % or more and 99.97 atomic % or less. In the present disclosure, even when the lithium secondary battery is fully charged, the element ratio of lithium element in the alloy may be 30.00 atomic % or more and 99.97 atomic % or less. In this disclosure, a fully charged lithium secondary battery means that the state of charge (SOC) of the lithium secondary battery is 100%. SOC indicates the ratio of the charge capacity to the fully charged capacity of the battery, and the fully charged capacity is SOC 100%. The SOC may be estimated from, for example, the open circuit voltage (OCV) of the lithium secondary battery.

[0019] The dissimilar metal may be any metal other than lithium metal, and may be an element capable of forming a solid solution with lithium metal or an element capable of forming an intermetallic compound with lithium metal, such as one or more elements selected from the group consisting of Mg, Bi, Pd, Sn, Si, Au, Ag, Pt, Zn, Al, In, Sr, Ba, Ga, Ca, and Ge. The composite layer of the present disclosure contains an alloy of lithium metal and a different metal as the main component of the negative electrode active material, and may also contain other conventionally known negative electrode active materials. In the present disclosure, the term "main component" refers to a component that accounts for 50% by mass or more of the total mass of the composite layer, where the total mass of the composite layer is 100% by mass. The lithium metal simple layer may be any layer made of lithium metal.

[0020] The thickness of the negative electrode layer is not particularly limited, but may be, for example, 10 to 100 μm. The ratio Z (Z=X / Y) of the thickness X of the lithium metal layer to the thickness Y of the composite layer may be 0.0001≦Z≦0.4. As a method for forming the negative electrode layer, for example, first, lithium metal and a different metal are simultaneously vacuum-deposited onto one surface of the negative electrode current collector to form a composite layer containing an alloy of lithium metal and the different metal on one surface of the negative electrode current collector, and then Li metal is vacuum-deposited onto the surface of the composite layer to form a layer of lithium metal alone, thereby forming a negative electrode layer consisting of these two layers. Examples of a method for simultaneously vacuum-depositing lithium metal and a dissimilar metal onto one surface of a negative electrode current collector include a method in which two crucibles, one for lithium metal and the other for the dissimilar metal, are prepared, the crucibles are heated by electron beam heating or resistance heating, and the lithium metal and the dissimilar metal are simultaneously volatilized in a vacuum deposition apparatus and vapor-deposited onto the negative electrode current collector.

[0021] The negative electrode of the present disclosure is a negative electrode for a lithium secondary battery. The lithium secondary battery of the present disclosure includes a positive electrode layer, a negative electrode layer, and an electrolyte layer disposed between the positive electrode layer and the negative electrode layer, and utilizes a deposition-dissolution reaction of lithium metal as the negative electrode reaction. In the present disclosure, a lithium secondary battery refers to a battery that utilizes a deposition-dissolution reaction of lithium metal as a reaction at the negative electrode.

[0022] FIG. 1 is a cross-sectional view schematically illustrating an example of a lithium secondary battery according to the present disclosure. 1, the lithium secondary battery 100 includes a positive electrode 16 including a positive electrode layer 12 and a positive electrode current collector 14, a negative electrode 17 including a negative electrode layer 13 and a negative electrode current collector 15, and an electrolyte layer 11 disposed between the positive electrode layer 12 and the negative electrode layer 13. The negative electrode layer 13 includes, in order from the negative electrode current collector 15 side, a composite layer 18 and a lithium metal simple layer 19.

[0023] [Positive electrode] The positive electrode includes a positive electrode layer and a positive electrode current collector.

[0024] [Positive electrode layer] The positive electrode layer contains a positive electrode active material, and may contain optional components such as a solid electrolyte, a conductive material, and a binder.

[0025] There is no particular limitation on the type of positive electrode active material, and any material that can be used as an active material for a lithium secondary battery can be used. Examples of the positive electrode active material include lithium metal (Li), lithium alloys, LiCoO2, and LiNi. 0.8 Co 0.15 Al 0.05 O2, LiNi x Co 1-x O2(0 <x<1)、LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 Examples of the Li-Mn spinel substituted with different elements include O2, LiMnO2, Lithium titanate, lithium metal phosphate, LiCoN, Li2SiO3, and Li4SiO4, transition metal oxides, TiS2, Si, SiO2, Si alloys, and lithium-storing intermetallic compounds. The Li-Mn spinel substituted with different elements is, for example, LiMn 1.5 Ni 0.5 O4, LiMn 1.5 Al 0.5 O4, LiMn 1.5 Mg 0.5 O4, LiMn 1.5 Co0.5 O4, LiMn 1.5 Fe 0.5 O4 and LiMn 1.5 Zn 0.5 O4, etc. Lithium titanate is, for example, Li4Ti5O 12 Examples of lithium metal phosphates include LiFePO4, LiMnPO4, LiCoPO4, and LiNiPO4. Examples of transition metal oxides include V2O5 and MoO3. Examples of lithium-storing intermetallic compounds include Mg2Sn, Mg2Ge, Mg2Sb, and Cu3Sb. Examples of lithium alloys include Li-Au, Li-Mg, Li-Sn, Li-Si, Li-Al, Li-B, Li-C, Li-Ca, Li-Ga, Li-Ge, Li-As, Li-Se, Li-Ru, Li-Rh, Li-Pd, Li-Ag, Li-Cd, Li-In, Li-Sb, Li-Ir, Li-Pt, Li-Hg, Li-Pb, Li-Bi, Li-Zn, Li-Tl, Li-Te, and Li-At. Examples of Si alloys include alloys with metals such as Li, and may also be alloys with at least one metal selected from the group consisting of Sn, Ge, and Al. The shape of the positive electrode active material is not particularly limited, and may be particulate. When the positive electrode active material is particulate, the positive electrode active material may be primary particles or secondary particles. A coating layer containing a Li-ion conductive oxide may be formed on the surface of the positive electrode active material, because this can suppress the reaction between the positive electrode active material and the solid electrolyte. Examples of Li-ion conductive oxides include LiNbO3 and Li4Ti5O 12 , and Li3PO4. The thickness of the coating layer is, for example, 0.1 nm or more, and may be 1 nm or more. On the other hand, the thickness of the coating layer is, for example, 100 nm or less, and may be 20 nm or less. The coating layer may cover, for example, 70% or more, or may cover 90% or more of the surface of the positive electrode active material.

[0026] Examples of the solid electrolyte include the same ones as those exemplified in the solid electrolyte layer described below.

[0027] Known conductive materials can be used, such as carbon materials and metal particles. Examples of the carbon material include at least one material selected from the group consisting of acetylene black, furnace black, VGCF, carbon nanotubes, and carbon nanofibers. From the viewpoint of electron conductivity, at least one material selected from the group consisting of VGCF, carbon nanotubes, and carbon nanofibers may be used. Examples of metal particles include particles of Ni, Cu, Fe, and SUS. The content of the conductive material in the positive electrode layer is not particularly limited.

[0028] Examples of the binder include acrylonitrile butadiene rubber (ABR), butadiene rubber (BR), polyvinylidene fluoride (PVdF), styrene butadiene rubber (SBR), etc. The content of the binder in the positive electrode layer is not particularly limited.

[0029] The thickness of the positive electrode layer is not particularly limited, but may be, for example, 10 to 100 μm, or 10 to 20 μm.

[0030] The positive electrode layer can be formed by a conventionally known method. For example, a positive electrode active material and, if necessary, other components are put into a solvent and stirred to prepare a paste for forming a positive electrode layer, and the paste for forming a positive electrode layer is applied to one surface of a support and dried to obtain a positive electrode layer. Examples of the solvent include butyl acetate, butyl butyrate, mesitylene, tetralin, heptane, and N-methyl-2-pyrrolidone (NMP). The method for applying the paste for forming a positive electrode layer onto one surface of the support is not particularly limited, and examples thereof include a doctor blade method, a metal mask printing method, an electrostatic application method, a dip coating method, a spray coating method, a roll coating method, a gravure coating method, and a screen printing method. The support can be appropriately selected from those having self-supporting properties and is not particularly limited, and examples thereof include metal foils such as Cu and Al foils.

[0031] As another method for forming the positive electrode layer, the positive electrode layer may be formed by pressure molding a powder of a positive electrode mixture containing a positive electrode active material and, if necessary, other components. When pressure molding the powder of the positive electrode mixture, a pressure of about 1 MPa to 2000 MPa is typically applied. The method of applying pressure is not particularly limited, but examples thereof include a method of applying pressure using a plate press, a roll press, or the like.

[0032] [Positive electrode current collector] The positive electrode current collector may be a known metal that can be used as a current collector for lithium secondary batteries. Examples of such metals include metal materials containing one or more elements selected from the group consisting of Cu, Ni, Al, V, Au, Pt, Mg, Fe, Ti, Co, Cr, Zn, Ge, and In. Examples of the positive electrode current collector include SUS, aluminum, nickel, iron, titanium, and carbon. The shape of the positive electrode current collector is not particularly limited, and can be various shapes such as foil, mesh, etc. The thickness of the positive electrode current collector varies depending on the shape, but may be, for example, in the range of 1 μm to 50 μm, or in the range of 5 μm to 20 μm.

[0033] [Electrolyte layer] The electrolyte layer includes at least an electrolyte. The electrolyte may be an aqueous electrolyte solution, a non-aqueous electrolyte solution, a gel electrolyte, a solid electrolyte, etc. These may be used alone or in combination of two or more.

[0034] The solvent of the aqueous electrolyte solution contains water as a main component. That is, based on the total amount (100 mol%) of the solvent (liquid component) constituting the electrolyte solution, water may account for 50 mol% or more, particularly 70 mol% or more, and even 90 mol% or more. Meanwhile, there is no particular upper limit to the proportion of water in the solvent.

[0035] The solvent contains water as a main component, but may contain a solvent other than water. Examples of the solvent other than water include one or more selected from ethers, carbonates, nitriles, alcohols, ketones, amines, amides, sulfur compounds, and hydrocarbons. The solvent other than water may be 50 mol% or less, particularly 30 mol% or less, and even 10 mol% or less, based on the total amount (100 mol%) of the solvents (liquid components) constituting the electrolytic solution.

[0036] The aqueous electrolyte used in the present disclosure contains an electrolyte. A conventionally known electrolyte can be used for the aqueous electrolyte. Examples of the electrolyte include lithium salts, nitrates, acetates, and sulfates of imide acid compounds. Specific electrolytes include lithium bis(fluorosulfonyl)imide (LiFSI; CAS No. 171611-11-3), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI; CAS No. 90076-65-6), lithium bis(pentafluoroethanesulfonyl)imide (LiBETI; CAS No. 132843-44-8), lithium bis(nonafluorobutanesulfonyl)imide (CAS No. 119229-99-1), lithium nonafluoro-N-[(trifluoromethane)sulfonyl]butanesulfonylamide (CAS No. 176719-70-3), lithium N,N-hexafluoro-1,3-disulfonylimide (CAS No. 189217-62-7), CHClO₃, LiPF₆, LiBF₄, LiSO₄, and LiNO₃.

[0037] The concentration of the electrolyte in the aqueous electrolyte solution can be appropriately set according to the desired battery characteristics, as long as it does not exceed the saturated concentration of the electrolyte in the solvent, because if a solid electrolyte remains in the aqueous electrolyte solution, the solid may inhibit the battery reaction. For example, when LiTFSI is used as the electrolyte, the aqueous electrolyte solution may contain 1 mol or more, particularly 5 mol or more, or even 7.5 mol or more of LiTFSI per kg of water. The upper limit is not particularly limited, and may be, for example, 25 mol or less.

[0038] The non-aqueous electrolyte solution generally contains a lithium salt and a non-aqueous solvent. Examples of lithium salts include inorganic lithium salts such as LiPF6, LiBF4, LiClO4, and LiAsF6; and organic lithium salts such as LiCF3SO3, LiN(SO2CF3)2(Li-TFSI), LiN(SO2C2F5)2, and LiC(SO2CF3)3. Examples of non-aqueous solvents include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), γ-butyrolactone, sulfolane, acetonitrile (AcN), dimethoxymethane, 1,2-dimethoxyethane (DME), 1,3-dimethoxypropane, diethyl ether, tetraethylene glycol dimethyl ether (TEGDME), tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl sulfoxide (DMSO), and mixtures thereof. From the viewpoint of ensuring a high dielectric constant and a low viscosity, the solvent may be a mixture of a cyclic carbonate compound having a high dielectric constant and a high viscosity, such as EC, PC, or BC, and a chain carbonate compound having a low dielectric constant and a low viscosity, such as DMC, DEC, or EMC, or a mixture of EC and DEC. The concentration of the lithium salt in the non-aqueous electrolyte may be, for example, 0.3 to 5M.

[0039] A gel electrolyte is generally a gel formed by adding a polymer to a non-aqueous electrolyte solution. Specifically, the gel electrolyte can be obtained by adding a polymer such as polyethylene oxide, polypropylene oxide, polyacrylonitrile, polyvinylidene fluoride (PVdF), polyurethane, polyacrylate, or cellulose to the nonaqueous electrolyte solution described above and gelling the mixture.

[0040] The electrolyte layer may be impregnated with an electrolyte such as the aqueous electrolyte solution described above, and a separator may be used to prevent contact between the positive electrode layer and the negative electrode layer. The separator material is not particularly limited as long as it is a porous film, and examples thereof include resins such as polyethylene (PE), polypropylene (PP), polyester, polyvinyl alcohol, cellulose, and polyamide, among which polyethylene and polypropylene are preferred. The separator may have a single-layer structure or a multi-layer structure. Examples of multi-layer separators include a two-layer structure of PE / PP, or a three-layer structure of PP / PE / PP or PE / PP / PE. The separator may be a nonwoven fabric such as a resin nonwoven fabric or a glass fiber nonwoven fabric.

[0041] [Solid electrolyte layer] The electrolyte layer may be a solid electrolyte layer made of a solid. The solid electrolyte layer includes at least a solid electrolyte. The solid electrolyte contained in the solid electrolyte layer may be any known solid electrolyte that can be used in all-solid-state batteries, and examples of such solid electrolytes include inorganic solid electrolytes such as sulfide-based solid electrolytes, oxide-based solid electrolytes, hydride-based solid electrolytes, halide-based solid electrolytes, and nitride-based solid electrolytes. The sulfide-based solid electrolyte may contain sulfur (S) as the main anion element. The oxide-based solid electrolyte may contain oxygen (O) as the main anion element. The hydride-based solid electrolyte may contain hydrogen (H) as the main anion element. The halide-based solid electrolyte may contain halogen (X) as the main anion element. The nitride-based solid electrolyte may contain nitrogen (N) as the main anion element.

[0042] The sulfide-based solid electrolyte may be sulfide glass, crystallized sulfide glass (glass ceramics), or a crystalline material obtained by a solid-phase reaction treatment of a raw material composition. The crystalline state of the sulfide-based solid electrolyte can be confirmed, for example, by subjecting the sulfide-based solid electrolyte to powder X-ray diffraction measurement using CuKα radiation.

[0043] Sulfide glass can be obtained by subjecting a raw material composition (e.g., a mixture of Li2S and P2S5) to amorphous processing, such as mechanical milling.

[0044] Glass ceramics can be obtained, for example, by heat treating sulfide glass. The heat treatment temperature may be any temperature higher than the crystallization temperature (Tc) of the sulfide glass observed by thermal analysis, and is usually 195° C. or higher. On the other hand, there is no particular upper limit to the heat treatment temperature. The crystallization temperature (Tc) of sulfide glass can be measured by differential thermal analysis (DTA). The heat treatment time is not particularly limited as long as it is a time that allows the desired crystallinity of the glass ceramic to be obtained, but is, for example, in the range of 1 minute to 24 hours, and particularly in the range of 1 minute to 10 hours. The heat treatment method is not particularly limited, and for example, a method using a firing furnace can be mentioned.

[0045] Examples of the oxide-based solid electrolyte include a solid electrolyte containing Li element, Y element (Y is at least one of Nb, B, Al, Si, P, Ti, Zr, Mo, W, S), and O element. Specific examples of the oxide-based solid electrolyte include Li7La3Zr2O 12 , Li 7-x La3(Zr 2-x Nb x )O 12 (0≦x≦2), Li5La3Nb2O 12 and other garnet-type solid electrolytes; perovskite-type solid electrolytes such as (Li,La)TiO3, (Li,La)NbO3, (Li,Sr)(Ta,Zr)O3; NASICON-type solid electrolytes such as Li(Al,Ti)(PO4)3, Li(Al,Ga)(PO4)3; Li-P-O-based solid electrolytes such as Li3PO4, LIPON (a compound in which part of O in Li3PO4 is replaced by N); Li-B-O-based solid electrolytes such as Li3BO3 and a compound in which part of O in Li3BO3 is replaced by C.

[0046] The hydride-based solid electrolyte has, for example, Li and a complex anion containing hydrogen. Examples of the complex anion include (BH4) - , (NH2) - , (AlH4) - , and (AlH6) 3- and the like. Examples of the halide-based solid electrolyte include Li 6-3z Y z X6 (X is at least one of Cl and Br, and z satisfies 0 < z < 2) and the like. Examples of the nitride-based solid electrolyte include Li3N and the like.

[0047] From the viewpoint of good handleability, the shape of the solid electrolyte may be particulate. The average particle size of the solid electrolyte particles is not particularly limited, but may be, for example, 10 nm or more, or 100 nm or more, while the average particle size of the solid electrolyte particles is, for example, 25 μm or less, or may be 10 μm or less.

[0048] In this disclosure, unless otherwise specified, the average particle size of particles is the volume-based median diameter (D50) value measured by laser diffraction / scattering particle size distribution measurement. Furthermore, in this disclosure, the median diameter (D50) is the diameter (volume average diameter) at which the cumulative volume of particles is half (50%) of the total volume when particles are arranged in order from smallest to largest particle size.

[0049] The solid electrolyte may be used alone or in combination of two or more. When two or more solid electrolytes are used, the two or more solid electrolytes may be mixed, or two or more solid electrolyte layers may be formed to form a multilayer structure. The proportion of the solid electrolyte in the solid electrolyte layer is not particularly limited, but is, for example, 50% by mass or more, may be in the range of 60% by mass or more and 100% by mass or less, may be in the range of 70% by mass or more and 100% by mass or less, or may be 100% by mass.

[0050] The solid electrolyte layer may contain a binder from the viewpoint of exhibiting plasticity, etc. Examples of such binders include the materials exemplified as binders used in the positive electrode layer. However, in order to facilitate achieving high output, the binder may be contained in an amount of 5 mass % or less from the viewpoint of preventing excessive aggregation of the solid electrolyte and enabling the formation of a solid electrolyte layer having a uniformly dispersed solid electrolyte.

[0051] The thickness of the solid electrolyte layer is not particularly limited, but is usually 0.1 μm or more and 1 mm or less. Examples of methods for forming the solid electrolyte layer include a method of applying a paste for forming a solid electrolyte layer containing a solid electrolyte to a support and drying it, and a method of press-molding a powder of a solid electrolyte material containing a solid electrolyte. Examples of the support include the same ones as those exemplified for the positive electrode layer. When press-molding the powder of the solid electrolyte material, a pressure of about 1 MPa to 2000 MPa is typically applied. The pressure application method is not particularly limited, but may be any of the pressure application methods exemplified in the formation of the positive electrode layer.

[0052] The lithium secondary battery may optionally include an exterior body that houses a laminate of a positive electrode, an electrolyte layer, and a negative electrode. The material of the exterior body is not particularly limited as long as it is stable to the electrolyte, and examples thereof include polypropylene, polyethylene, and resins such as acrylic resin.

[0053] The lithium secondary battery may be an aqueous lithium secondary battery, a non-aqueous lithium secondary battery, an all-solid-state lithium secondary battery, or the like. Examples of the shape of the lithium secondary battery include coin type, laminate type, cylindrical type, and prismatic type. The uses of the lithium secondary battery are not particularly limited, and examples thereof include power sources for vehicles such as hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), electric vehicles (BEVs), gasoline-powered vehicles, and diesel-powered vehicles. In particular, the lithium secondary battery may be used as a driving power source for hybrid electric vehicles, plug-in hybrid electric vehicles, or electric vehicles. The lithium secondary battery of the present disclosure may also be used as a power source for mobile objects other than vehicles (e.g., trains, ships, and aircraft), and may also be used as a power source for electrical appliances such as information processing devices.

[0054] In a method for manufacturing a lithium secondary battery according to the present disclosure, for example, a positive electrode layer is formed by pressure-molding a powder of a positive electrode mixture containing a positive electrode active material containing lithium element on one side of a positive electrode current collector to obtain a positive electrode. Lithium metal and a different metal are then simultaneously vacuum-deposited onto one side of a negative electrode current collector to form a composite layer containing an alloy of lithium metal and the different metal on one side of the negative electrode current collector. Lithium metal is then vacuum-deposited onto the surface of the composite layer to form a lithium metal elemental layer, forming a negative electrode layer consisting of these two layers. Finally, a negative electrode layer is formed on one side of the negative electrode current collector to obtain a negative electrode. A separator is then prepared, and the separator is placed between the positive electrode and the negative electrode. An electrolyte is then poured into the separator to produce a lithium secondary battery according to the present disclosure. [Example]

[0055] (Comparative Example 1) [Positive electrode production] The positive electrode active material is particulate lithium nickel cobalt manganese composite oxide (layered structure, LiNi 0.33 Co 0.33 Mn 0.33 O2), acetylene black (AB) as a conductive material, and polyvinylidene fluoride (PVdF) as a binder were weighed out so that the mass ratio of the positive electrode active material:AB:PVdF was 80:8:2. Next, these materials were mixed in N-methyl-2-pyrrolidone (NMP) using a planetary mixer so that the solid content was 56 mass% to prepare a positive electrode layer-forming slurry. This positive electrode layer-forming slurry was applied to a strip-shaped aluminum foil (positive electrode current collector) along the longitudinal direction using a die coater and dried at 120°C. The dried positive electrode layer-forming slurry was then pressed together with the aluminum foil. This produced a strip-shaped positive electrode comprising a positive electrode layer on the positive electrode current collector.

[0056] [Negative electrode production] Li metal was evaporated in a vacuum deposition device and deposited on Cu foil (negative electrode current collector), producing a strip-shaped negative electrode with a negative electrode layer consisting of a single layer of Li metal on the negative electrode current collector. [Separator placement] The positive and negative electrodes were placed opposite each other with a strip-shaped separator (a three-layer structure of PP / PE / PP) interposed therebetween and wound longitudinally to produce a wound electrode assembly. A positive current collector was welded to the positive electrode, and a negative current collector was welded to the negative electrode. [Electrolyte preparation] A non-aqueous electrolyte solution was prepared by dissolving LiPF6 as a supporting electrolyte at a concentration of 1.0 M in a mixed solvent containing ethylene carbonate (EC) and dimethyl carbonate (DMC) in a volume ratio of EC:DMC = 1:1. The wound electrode body and a non-aqueous electrolyte solution prepared above were then housed in a battery case to construct a lithium secondary battery.

[0057] (Comparative Example 2) A lithium secondary battery was constructed in the same manner as in Comparative Example 1, except for the following. In the above [Negative electrode preparation], two crucibles were prepared, one for Li metal and the other for In as a dissimilar metal. The crucibles were heated by electron beam heating, and the Li metal and dissimilar metal were simultaneously evaporated in a vacuum deposition apparatus, resulting in the deposition of the Li metal and dissimilar metal on the Cu foil (negative electrode current collector). A strip-shaped negative electrode was prepared on the negative electrode current collector, with a negative electrode layer consisting of a single composite layer containing an alloy of Li metal and the dissimilar metal. The elemental ratio of Li metal in the alloy was 95 atomic %.

[0058] Example 1 A lithium secondary battery was constructed in the same manner as in Comparative Example 2, except for the following. In the above [Negative electrode preparation], two crucibles, one for Li metal and one for In as a dissimilar metal, were prepared. The crucibles were heated by electron beam heating, and the Li metal and dissimilar metal were simultaneously vaporized in a vacuum evaporation apparatus, thereby vapor-depositing the Li metal and dissimilar metal onto Cu foil (negative electrode current collector), forming a composite layer containing an alloy of Li metal and dissimilar metal on the negative electrode current collector. Thereafter, a crucible for Li metal was prepared, and the crucible was heated by electron beam heating, causing the Li metal to vaporize in the vacuum evaporation apparatus, thereby vapor-depositing Li metal onto the composite layer, thereby forming a simple Li metal layer. A strip-shaped negative electrode was prepared on the negative electrode current collector, with a two-layer negative electrode layer consisting of a composite layer and a simple Li metal layer, in that order from the negative electrode current collector side. The elemental ratio of Li metal in the alloy was 95 atomic %. The ratio Z (Z=X / Y) of the thickness X of the lithium metal layer to the thickness Y of the composite layer was 0.1.

[0059] Examples 2 to 16 In Examples 2 to 16, as shown in Table 1, lithium secondary batteries were constructed in the same manner as in Example 1, except that the types of dissimilar metals used were different.

[0060] (Examples 17 to 21) In Examples 17 to 21, lithium secondary batteries were constructed in the same manner as in Example 1, except that the ratio Z (Z=X / Y) of the thickness X of the lithium metal simple layer to the thickness Y of the composite layer was different, as shown in Table 2.

[0061] Examples 22 to 27 In Examples 22 to 27, as shown in Table 3, lithium secondary batteries were constructed in the same manner as in Example 1, except that the element ratio of Li metal in the alloy was different.

[0062] [Output characteristic evaluation] The voltage (open circuit voltage) of the lithium secondary battery was adjusted to 3.70 V in advance. The lithium secondary battery was then discharged at 5 C for 8 seconds in a temperature environment of -5°C. Note that "1 C" here refers to the current value that can charge the battery capacity (Ah) predicted from the theoretical capacity of the active material in 1 hour. The voltage drop ΔV at this time was obtained, and the resistance value was calculated using the following formula (1). Equation (1) Resistance = ΔV / 5C current value The battery resistance of Comparative Example 1 was normalized to 1.0, and the battery resistances of Examples 1 to 27 and Comparative Example 2 relative to the battery resistance of Comparative Example 1 were calculated and the results are shown in Tables 1 to 3. Note that the up arrows in the tables indicate the same as above.

[0063] [Capacity retention rate evaluation] A cycle test was carried out on the lithium secondary battery in a voltage range of 3.3 V to 4.2 V in an environment of 60° C. The charge and discharge were performed using a constant current method, and the current rate was set to 1C. The lithium secondary battery thus fabricated was charged at a constant current (CC) rate of 1 C in a 60°C environment until the voltage reached 4.2 V, and then charged at a constant voltage (CV) rate until the current reached 1 / 50 C. It was then discharged at a constant current (CC) rate of 1 C until the voltage reached 3.3 V. The discharge capacity at this time was defined as the initial discharge capacity. The discharge capacity at the 200th cycle of the cycle test was measured in the same manner as the initial discharge capacity, and the capacity retention rate after charge-discharge cycling was calculated by dividing the discharge capacity at the 200th cycle of the cycle test by the initial discharge capacity. The results are shown in Tables 1 to 3.

[0064] [Evaluation of capacity retention rate after storage] Furthermore, the lithium secondary batteries of Examples 1 to 27 and Comparative Examples 1 and 2 were charged to 3.8 V and stored in a thermostatic chamber in a 60°C environment for 100 days, and the post-storage capacity retention rate (discharge capacity after storage / discharge capacity before storage × 100) was calculated. Charging and discharging were performed using a constant current method, with a current rate of 1 C, in a 60°C environment, and in a voltage range of 3 V to 4.2 V. The results are shown in Tables 1 to 3.

[0065] [Table 1]

[0066] [Table 2]

[0067] [Table 3]

[0068] [Evaluation results] As shown in Tables 1 to 3, it was demonstrated that Examples 1 to 27 had lower battery resistance, higher capacity retention rates after charge / discharge cycles, and higher capacity retention rates after storage than Comparative Examples 1 and 2. As shown in Table 2, it was demonstrated that by adjusting the ratio Z (Z=X / Y) of the thickness X of the lithium metal layer to the thickness Y of the composite layer so that it is within a predetermined range, it is possible to further reduce the battery resistance and further improve the capacity retention rate after charge / discharge cycling and the capacity retention rate after storage. As shown in Table 3, it was demonstrated that by adjusting the element ratio of Li metal in the alloy so that it falls within a specified range, it is possible to further reduce battery resistance and further improve the capacity retention rate after charge / discharge cycling and the capacity retention rate after storage. [Explanation of symbols]

[0069] 11 Electrolyte layer 12 Positive electrode layer 13 Negative electrode layer 14 Positive electrode current collector 15 Negative electrode current collector 16 positive electrode 17 Negative electrode 18 composite layer 19 Lithium metal monolayer 100 Lithium secondary battery

Claims

1. a negative electrode current collector and a negative electrode layer, the negative electrode layer has, in this order from the negative electrode current collector side, a composite layer containing an alloy of lithium metal and a different metal, and a lithium metal simple layer, as negative electrode active materials; the different metal is an element capable of forming a solid solution with the lithium metal or an element capable of forming an intermetallic compound with the lithium metal, a ratio Z (Z=X / Y) of a thickness X of the lithium metal layer to a thickness Y of the composite layer is 0.001≦Z≦0.1; The negative electrode for a lithium secondary battery, wherein the element ratio of lithium element in the alloy is 30.00 atomic % or more and 99.97 atomic % or less.

2. 2. The negative electrode according to claim 1, wherein a ratio Z (Z=X / Y) of a thickness X of the lithium metal layer to a thickness Y of the composite layer is 0.

1.

3. 3. The negative electrode according to claim 2, wherein the elemental ratio of lithium in the alloy is 95.00 atomic %.

4. 3. The negative electrode according to claim 2, wherein the different metal is one or more elements selected from the group consisting of Mg, Bi, Pd, Ag, Pt, Zn, In, Sr, Ba, Ga, Ca, and Ge.

5. 3. The negative electrode according to claim 2, wherein the different metal is one or more elements selected from the group consisting of Bi, Pd, Pt, Zn, Sr, Ba, Ga, Ca, and Ge.

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