Negative electrode for lithium secondary battery and method for manufacturing negative electrode for lithium secondary battery

Abstract

The present invention discloses a negative electrode for a lithium secondary battery and a method for manufacturing a negative electrode for a lithium secondary battery. The negative electrode for a lithium secondary battery includes a negative electrode current collector and a negative electrode layer. The negative electrode layer includes a composite layer and a lithium metal layer in order from the negative electrode current collector side. The composite layer includes an alloy of lithium metal and a dissimilar metal as the negative electrode active material. The dissimilar metal is an element that can form a solid solution with the lithium metal or an element that can form an intermetallic compound with the lithium metal.

Description

Technical Field

[0001] The present disclosure relates to a negative electrode for a lithium secondary battery and a method for manufacturing a negative electrode for a lithium secondary battery. Background Art

[0002] Among batteries, lithium secondary batteries have attracted attention due to their ability to obtain high output voltage.

[0003] Japanese Patent Application Laid-Open No. 2020-184513 discloses a lithium metal negative electrode battery in which a metal Mg layer containing metal magnesium is formed on one side of a negative electrode current collector or on one side of a solid electrolyte layer.

[0004] Japanese Patent Application Laid-Open No. 2021-077640 discloses a negative electrode material including a metal thin film (Au, Mg, Ag) at the interface between a lithium metal layer and a current collector. Summary of the Invention

[0005] Lithium secondary batteries using lithium metal or lithium alloy as a negative electrode active material have a problem of reduced capacity retention due to deactivation of lithium metal caused by volume changes associated with charge and discharge, and there is a demand for improved capacity retention.

[0006] The present disclosure provides a negative electrode capable of improving the capacity retention rate of a lithium secondary battery.

[0007] A negative electrode for a lithium secondary battery according to a first embodiment of the present disclosure includes a negative electrode current collector and a negative electrode layer.

[0008] The negative electrode layer includes a composite layer and a lithium metal single substance layer in order from the negative electrode current collector side, wherein the composite layer contains an alloy of lithium metal and a different metal as a negative electrode active material.

[0009] The dissimilar 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.

[0010] In the negative electrode for a lithium secondary battery according to the first aspect of the present disclosure, a ratio Z (Z=X / Y) of a thickness X of the lithium metal single substance layer to a thickness Y of the composite layer may be 0.0001≤Z≤0.4.

[0011] In the negative electrode for a lithium secondary battery according to the first aspect of the present disclosure, the ratio Z may be 0.001≤Z≤0.3.

[0012] In the negative electrode for a lithium secondary battery according to the first aspect of the present disclosure, the element ratio of the lithium element in the alloy may be 30.00 atomic % or more and 99.97 atomic % or less.

[0013] In the negative electrode for a lithium secondary battery according to the first aspect of the present disclosure, the mass of the alloy of the lithium metal and the dissimilar metal may be 50% or more relative to the total mass of the composite layer.

[0014] In the negative electrode for a lithium secondary battery according to the first embodiment of the present disclosure, the dissimilar metal may be one or more elements selected from Mg, Bi, Pd, Sn, Si, Au, Ag, Pt, Zn, Al, In, Sr, Ba, Ga, Ca, and Ge.

[0015] A second embodiment of the present disclosure relates to a method for manufacturing a negative electrode for a lithium secondary battery, comprising the following steps: preparing a negative electrode current collector; vacuum-depositing lithium metal and a dissimilar metal on the negative electrode current collector to form a composite layer comprising an alloy of the lithium metal and the dissimilar metal; and vacuum-depositing lithium metal on the composite layer to form a lithium metal single substance layer.

[0016] The dissimilar 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.

[0017] In the method for producing a negative electrode for a lithium secondary battery according to the second aspect of the present disclosure, the ratio Z (Z=X / Y) of the thickness X of the lithium metal single substance layer to the thickness Y of the composite layer may be 0.0001≤Z≤0.4.

[0018] In the method for producing a negative electrode for a lithium secondary battery according to the second aspect of the present disclosure, the ratio Z may be 0.001≤Z≤0.3.

[0019] In the method for producing a negative electrode for a lithium secondary battery according to the second aspect of the present disclosure, the element ratio of the lithium element in the alloy may be 30.00 atomic % or more and 99.97 atomic % or less.

[0020] The present disclosure can provide a negative electrode capable of improving the capacity retention rate of a lithium secondary battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Features, advantages, and technical and industrial significance of exemplary embodiments of the present invention will be described below with reference to the accompanying drawings, in which like numerals represent like elements, and in which:

[0022] Figure 1 This is a schematic cross-sectional view showing an example of the lithium secondary battery of the present disclosure. DETAILED DESCRIPTION

[0023] The following describes embodiments of the present disclosure. Furthermore, matters necessary for the implementation of the present disclosure (e.g., the general configuration and manufacturing process of the negative electrode and lithium secondary battery that do not characterize the present disclosure) other than those specifically mentioned in this specification can be understood as design matters by those skilled in the art. The present disclosure can be implemented based on the contents disclosed in this specification and common technical knowledge in the field.

[0024] In addition, the dimensional relationships (length, width, thickness, etc.) in the drawings do not reflect actual dimensional relationships.

[0025] In this specification, "to" indicating a numerical range is used to mean that the numerical values ​​described before and after it are included as the lower limit and the upper limit.

[0026] In addition, the upper limit value and the lower limit value in the numerical range can be used in any combination.

[0027] The negative electrode for the lithium secondary battery disclosed herein comprises a negative electrode current collector and a negative electrode layer.

[0028] The negative electrode layer includes a composite layer and a lithium metal single substance layer in order from the negative electrode current collector side, wherein the composite layer contains an alloy of lithium metal and a different metal as a negative electrode active material.

[0029] The dissimilar 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.

[0030] In lithium secondary batteries, the capacity retention rate of lithium secondary batteries is reduced due to the decomposition reaction between the new lithium metal surfaces continuously generated by the dissolution and precipitation of lithium metal accompanying charging and discharging and the electrolyte, and the occurrence of electron isolation of the negative electrode active material due to cracks in the negative electrode layer caused by the volume change of lithium metal accompanying charging and discharging.

[0031] Generally, regarding alloys of Li and dissimilar metals, it is desirable to alloy them uniformly. Due to the mismatch of the crystal structure caused by the uneven composition of Li and dissimilar metals in the alloy, the cracking of the negative electrode layer is induced, and the capacity retention rate of the lithium secondary battery is reduced. In the case of the negative electrode for the lithium secondary battery disclosed herein, there is a difference in the metal composition between the lithium metal single layer and the composite layer comprising the alloy of lithium metal and dissimilar metals, but it is difficult to produce the cracking of the negative electrode layer as described above. The lithium metal single layer suppresses the reaction between the electrolyte or electrolyte and the composite layer, which can improve the capacity retention rate and storage characteristics of the lithium secondary battery.

[0032] Furthermore, when a lithium secondary battery is operated at low temperatures, the Li conductivity in the electrolyte solution or electrolyte decreases significantly, and the resistance of the electrolyte solution or electrolyte increases, which may cause a significant decrease in low-temperature output characteristics.

[0033] On the other hand, according to the present disclosure, the increase in the resistance component of the electrolyte or electrolyte is mitigated, resulting in excellent low-temperature output. This is presumably because the lithium metal layer becomes thinner during charge and discharge, reducing the reaction resistance of the lithium metal layer and allowing it to be preferentially used for charge and discharge.

[0034] [negative electrode]

[0035] The negative electrode of the present disclosure includes a negative electrode current collector and a negative electrode layer.

[0036] [Negative electrode current collector]

[0037] The material of the negative electrode current collector can be a material that does not alloy with Li, and examples thereof include SUS (stainless steel), copper, and nickel. Examples of the form of the negative electrode current collector include foil and plate. The top view shape of the negative electrode current collector is not particularly limited, and examples thereof include circular, elliptical, rectangular, and arbitrary polygonal shapes. In addition, the thickness of the negative electrode current collector varies depending on the shape, but for example, it can be in the range of 1 μm to 50 μm, or in the range of 5 μm to 20 μm.

[0038] [Negative electrode layer]

[0039] The negative electrode layer includes a composite layer and a lithium metal single substance layer in this order from the negative electrode current collector side. The composite layer contains an alloy of lithium metal and a different metal as a negative electrode active material.

[0040] The composite layer contains an alloy of lithium metal and a different metal as the negative electrode active material.

[0041] The element ratio of the lithium element in the alloy may be 30.00 atomic % or more and 99.97 atomic % or less.

[0042] In the present disclosure, even when the lithium secondary battery is fully charged, the element ratio of the lithium element in the alloy can be 30.00 atomic % or more and 99.97 atomic % or less.

[0043] In this disclosure, a fully charged lithium secondary battery means a state of charge (SOC) of 100%. SOC represents the ratio of a battery's charge capacity to its full charge capacity, with the full charge capacity being SOC 100%.

[0044] The SOC can be estimated from, for example, the open circuit voltage (OCV) of the lithium secondary battery.

[0045] The dissimilar metal is a metal other than lithium metal, as long as it is an element capable of forming a solid solution with lithium metal or an element capable of forming an intermetallic compound with the lithium metal. Examples of the dissimilar metal include 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.

[0046] In the composite layer of the present disclosure, if the alloy of lithium metal and a dissimilar metal is included as the main component as the negative electrode active material, other known negative electrode active materials may also be included. In the present disclosure, the so-called main component means a component that comprises 50% by mass or more when the total mass of the composite layer is set to 100% by mass.

[0047] The lithium metal single substance layer may be any layer as long as it is composed of lithium metal.

[0048] The thickness of the negative electrode layer is not particularly limited, and may be, for example, 10 to 100 μm.

[0049] The ratio Z (Z=X / Y) of the thickness X of the lithium metal single substance layer to the thickness Y of the composite layer may be 0.0001≤Z≤0.4. Alternatively, it may be 0.001≤Z≤0.3.

[0050] As a method for forming a negative electrode layer, for example, lithium metal and a heterogeneous metal can be vacuum-deposited on one side of a negative electrode collector at the same time, thereby forming a composite layer comprising an alloy of lithium metal and the heterogeneous metal on one side of the negative electrode collector, and then, lithium metal is vacuum-deposited on the surface of the composite layer to form a lithium metal single substance layer, to produce a negative electrode layer consisting of these two layers.

[0051] As a method for simultaneously vacuum-depositing lithium metal and a dissimilar metal onto one side of a negative electrode current collector, for example, there are methods such as preparing two crucibles, a crucible containing lithium metal and a crucible containing a dissimilar metal, heating the crucibles by electron beam heating or resistance heating, and causing the lithium metal and the dissimilar metal to simultaneously volatilize into a vacuum evaporation device and evaporate onto the negative electrode current collector.

[0052] The negative electrode disclosed herein is a negative electrode for a lithium secondary battery.

[0053] The lithium secondary battery disclosed herein comprises 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 lithium metal precipitation-dissolution reaction as a reaction at the negative electrode.

[0054] In the present disclosure, a lithium secondary battery refers to a battery that utilizes the precipitation-dissolution reaction of lithium metal as a reaction at the negative electrode.

[0055] Figure 1 This is a schematic cross-sectional view showing an example of the lithium secondary battery of the present disclosure.

[0056] like Figure 1 As shown, 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 a composite layer 18 and a lithium metal single substance layer 19 in this order from the negative electrode current collector 15 side.

[0057] [positive electrode]

[0058] The positive electrode includes a positive electrode layer and a positive electrode current collector.

[0059] [Positive electrode layer]

[0060] The positive electrode layer contains a positive electrode active material, and may also contain a solid electrolyte, a conductive material, a binder (adhesive), and the like as optional components.

[0061] 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 positive electrode active materials include lithium metal (Li), lithium alloys, LiCoO2, 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 O2, LiMnO2, heterogeneous element substituted Li-Mn spinel, lithium titanate, lithium metal phosphate, LiCoN, Li2SiO3 and Li4SiO4, transition metal oxides, TiS2, Si, SiO2, Si alloys and lithium storage metal compounds. Heterogeneous element substituted Li-Mn spinel 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 Co 0.5 O4、LiMn 1.5 Fe 0.5 O4 and LiMn 1.5 Zn 0.5 O4, etc. Lithium titanate is, for example, Li4Ti5O 12Examples of lithium metal phosphates include LiFePO4, LiMnPO4, LiCoPO4, and LiNiPO4. Examples of transition metal oxides include V2O5 and MoO3. Examples of lithium storage intermetallic compounds include Mg2Sn, Mg2Ge, Mg2Sb, and Cu3Sb.

[0062] 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 of Si with metals such as Li, and alloys of Si with at least one metal selected from Sn, Ge, and Al.

[0063] The shape of the positive electrode active material is not particularly limited, and may be in the form of particles. When the positive electrode active material is in the form of particles, the positive electrode active material may be in the form of primary particles or secondary particles.

[0064] A coating layer containing a Li-ion conductive oxide may be formed on the surface of the positive electrode active material to suppress the reaction between the positive electrode active material and the solid electrolyte.

[0065] Examples of Li ion conductive oxides include LiNbO3, Li4Ti5O 12 and Li3PO4, etc. The coating layer may have a thickness of, for example, 0.1 nm or greater, and may be 1 nm or greater. Alternatively, the coating layer may have a thickness of, for example, 100 nm or less, and may be 20 nm or less. The coating layer may cover, for example, 70% or greater, or 90% or greater, of the surface of the positive electrode active material.

[0066] Examples of the solid electrolyte include the same solid electrolytes as those exemplified in the solid electrolyte layer described later.

[0067] As the conductive material, known conductive materials can be used, and examples thereof include carbon materials and metal particles. As the carbon material, for example, at least one selected from acetylene black, furnace black, VGCF (vapor-grown carbon fiber), carbon nanotubes, and carbon nanofibers can be cited. Among them, from the viewpoint of electronic conductivity, at least one selected from VGCF, carbon nanotubes, and carbon nanofibers can be cited. As the metal particles, particles of Ni, Cu, Fe, and SUS can be cited.

[0068] The content of the conductive material in the positive electrode layer is not particularly limited.

[0069] Examples of the binder (adhesive) 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.

[0070] The thickness of the positive electrode layer is not particularly limited, and may be, for example, 10 to 100 μm, or 10 to 20 μm.

[0071] The positive electrode layer can be formed using a known method.

[0072] For example, a positive electrode layer-forming paste is prepared by adding a positive electrode active material and, if necessary, other components into a solvent and stirring. This positive electrode layer-forming paste is then applied onto one side of a support and dried to obtain a positive electrode layer.

[0073] Examples of the solvent include butyl acetate, butyl butyrate, mesitylene, tetralin, heptane, and N-methyl-2-pyrrolidone (NMP).

[0074] The method for coating the positive electrode layer-forming paste on one side of the support is not particularly limited, and examples thereof include a doctor blade method, a metal mask printing method, an electrostatic coating method, a dip coating method, a spray coating method, a roll coating method, a gravure coating method, and a screen printing method.

[0075] As the support, a self-supporting support can be appropriately selected and used without particular limitation, and for example, metal foils such as Cu and Al can be used.

[0076] In addition, as another method for forming the positive electrode layer, the positive electrode layer can also be formed by press-molding a powder of a positive electrode mixture containing a positive electrode active material and other components as needed. When the powder of the positive electrode mixture is press-molded, a pressing pressure of 1 MPa or more and 2000 MPa or less is usually applied.

[0077] The pressurizing method is not particularly limited, and examples thereof include a method of applying pressure using a plate press, a roller press, or the like.

[0078] [Positive electrode current collector]

[0079] As the positive electrode current collector, a known metal that can be used as a current collector for a lithium secondary battery can be used. Examples of such metals include metal materials containing one or more elements selected from 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.

[0080] The form of the positive electrode current collector is not particularly limited and can be various forms such as foil and mesh. The thickness of the positive electrode current collector varies depending on the shape and can be, for example, within the range of 1 μm to 50 μm or 5 μm to 20 μm.

[0081] [Electrolyte layer]

[0082] The electrolyte layer contains at least an electrolyte.

[0083] As the electrolyte, an aqueous electrolyte, a non-aqueous electrolyte, a gel electrolyte, a solid electrolyte, etc. can be used, and these can be used alone or in combination of two or more.

[0084] The solvent of the aqueous electrolyte solution contains water as a main component. That is, based on the total amount of the solvent (liquid component) constituting the electrolyte solution (100 mol%), water can account for 50 mol% or more, particularly 70 mol% or more, and further 90 mol% or more. On the other hand, there is no particular upper limit on the proportion of water in the solvent.

[0085] The solvent is a solvent containing water as a main component, but may also contain solvents other than water. Examples of solvents other than water include one or more selected from ethers, carbonates, nitriles, alcohols, ketones, amines, amides, sulfur compounds, and hydrocarbons. Based on the total amount of the solvent (liquid component) constituting the electrolyte solution (100 mol%), the amount of solvent other than water may be 50 mol% or less, particularly 30 mol% or less, and further 10 mol% or less.

[0086] The aqueous electrolyte used in the present disclosure contains an electrolyte. A known electrolyte can be used as the electrolyte for the aqueous electrolyte. Examples of the electrolyte include lithium salts, nitrates, acetates, and sulfates of imidic acid compounds. As specific electrolytes, there can be listed 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]butanesulfonylimide (CAS No. 176719-70-3), lithium N,N-hexafluoro-1,3-disulfonylimide (CAS No. 189217-62-7), CH3COOLi, LiPF6, LiBF4, Li2SO4 and LiNO3, etc.

[0087] The concentration of the electrolyte in the aqueous electrolyte solution can be appropriately set according to the desired battery characteristics within a range that does not exceed the saturation concentration of the electrolyte relative to the solvent. This is because if solid electrolyte remains in the aqueous electrolyte solution, there is a risk that the solid will hinder the battery reaction.

[0088] For example, when LiTFSI is used as the electrolyte, the aqueous electrolyte can contain 1 mol or more of LiTFSI per kg of water, particularly 5 mol or more of LiTFSI per kg of water, and further 7.5 mol or more of LiTFSI per kg of water. The upper limit is not particularly limited, but can be, for example, 25 mol or less.

[0089] As the non-aqueous electrolyte, a non-aqueous electrolyte containing a lithium salt and a non-aqueous solvent is generally used.

[0090] 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.

[0091] Examples of the non-aqueous solvent include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), γ-butyrolactone, cyclopentane, 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 perspective of ensuring high dielectric constant and low viscosity, the non-aqueous solvent may be a mixture of cyclic carbonate compounds such as EC, PC, BC, etc. having high dielectric constant and high viscosity and chain carbonate compounds such as DMC, DEC, EMC, etc. having low dielectric constant and low viscosity, or a mixture of EC and DEC.

[0092] The concentration of the lithium salt in the non-aqueous electrolyte solution can be, for example, 0.3 to 5M.

[0093] A gel electrolyte is generally obtained by adding a polymer to a non-aqueous electrolyte solution and forming a gel.

[0094] Specifically, the gel electrolyte can be obtained by adding a polymer such as polyethylene oxide, polypropylene oxide, polyacrylonitrile, polyvinylidene fluoride (PVdF), polyurethane, polyacrylate, and cellulose to the above-mentioned non-aqueous electrolyte solution and performing gelation.

[0095] The electrolyte layer may be impregnated with an electrolyte such as the aqueous electrolyte solution described above and may use a separator that prevents contact between the positive electrode layer and the negative electrode layer.

[0096] The material of the separator is not particularly limited as long as it is a porous film. Examples include resins such as polyethylene (PE), polypropylene (PP), polyester, polyvinyl alcohol, cellulose, and polyamide, with polyethylene and polypropylene also being acceptable. Furthermore, the separator may be a single-layer structure or a multilayer structure. Examples of multilayer separators include two-layer structures such as PE / PP, or three-layer structures such as PP / PE / PP or PE / PP / PE.

[0097] The separator may be made of non-woven fabric such as resin non-woven fabric or glass fiber non-woven fabric.

[0098] [Solid electrolyte layer]

[0099] The electrolyte layer may be a solid electrolyte layer composed of a solid.

[0100] The solid electrolyte layer contains at least a solid electrolyte.

[0101] As the solid electrolyte contained in the solid electrolyte layer, a known solid electrolyte that can be used for an all-solid-state battery can be appropriately used, and 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 can be listed. Sulfide-based solid electrolytes can contain sulfur (S) as the main component of the anion element. Oxide-based solid electrolytes can contain oxygen (O) as the main component of the anion element. Hydride-based solid electrolytes can contain hydrogen (H) as the main component of the anion element. Halide-based solid electrolytes can contain halogen (X) as the main component of the anion element. Nitride-based solid electrolytes can contain nitrogen (N) as the main component of the anion element.

[0102] The sulfide-based solid electrolyte may be sulfide glass, crystallized sulfide glass (glass ceramics), or a crystalline material obtained by solid-phase reaction treatment of a raw material composition.

[0103] The crystal state of the sulfide-based solid electrolyte can be confirmed by, for example, performing powder X-ray diffraction measurement of the sulfide-based solid electrolyte using CuKα rays.

[0104] Sulfide glass can be obtained by subjecting a raw material composition (e.g., a mixture of Li2S and P2S5) to amorphization. Examples of the amorphization treatment include mechanical milling.

[0105] Glass ceramics can be obtained by, for example, heat-treating sulfide glass.

[0106] The heat treatment temperature may be higher than the crystallization temperature (Tc) observed by thermal analysis of the sulfide glass, and is usually 195° C. or higher. On the other hand, the upper limit of the heat treatment temperature is not particularly limited.

[0107] The crystallization temperature (Tc) of sulfide glass can be measured by differential thermal analysis (DTA).

[0108] The heat treatment time is not particularly limited as long as it is a time that can obtain the desired crystallinity of the glass ceramics, and is, for example, in the range of 1 minute to 24 hours, particularly in the range of 1 minute to 10 hours.

[0109] The heat treatment method is not particularly limited, and an example thereof includes a method using a firing furnace.

[0110] Examples of oxide-based solid electrolytes include solid electrolytes containing Li, Y (Y is at least one of Nb, B, Al, Si, P, Ti, Zr, Mo, W, and S), and O. Specific examples of oxide-based solid electrolytes include: Li7La3Zr2O 12 、Li 7-x La3(Zr 2-x Nb x )O 12 (0≤x≤2), Li5La3Nb2O 12 Garnet-type solid electrolytes such as (Li, La)TiO3, (Li, La)NbO3, (Li, Sr)(Ta, Zr)O3, etc.; perovskite-type solid electrolytes such as Li(Al, Ti)(PO4)3, Li(Al, Ga)(PO4)3, etc.; NASICON (sodium superion conductor) type solid electrolytes such as Li(Al, Ti)(PO4)3, Li(Al, Ga)(PO4)3; Li-PO system solid electrolytes such as Li3PO4, LIPON (a compound obtained by replacing a part of O in Li3PO4 with N); Li-BO system solid electrolytes such as Li3BO3, a compound obtained by replacing a part of O in Li3BO3 with C.

[0111] 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- wait.

[0112] Examples of halide-based solid electrolytes include Li 6-3z Y z X6 (X is at least one of Cl and Br, and z satisfies 0<z<2), etc.

[0113] Examples of the nitride-based solid electrolyte include Li 3 N and the like.

[0114] From the viewpoint of good handleability, the solid electrolyte may be in the form of particles.

[0115] The average particle size of the solid electrolyte particles is not particularly limited, and may be, for example, 10 nm or more, or 100 nm or more. Meanwhile, the average particle size of the solid electrolyte particles may be, for example, 25 μm or less, or 10 μm or less.

[0116] In this disclosure, unless otherwise specified, the average particle size of particles is the volume-based median diameter (D50) 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, arranged in descending order of particle size, accounts for half (50%) of the total volume.

[0117] Regarding solid electrolytes, one type can be used alone, or two or more types of solid electrolytes can be used. In addition, when using two or more types of solid electrolytes, the two or more types of solid electrolytes can be mixed, or two or more types of solid electrolytes can be formed into a multilayer structure.

[0118] The proportion of the solid electrolyte in the solid electrolyte layer is not particularly limited, and may be, for example, 50% by mass or more, 60% by mass or more and 100% by mass or less, 70% by mass or more and 100% by mass or less, or 100% by mass or more.

[0119] From the perspective of achieving plasticity, etc., a binder may be included in the solid electrolyte layer. Examples of such a binder include the materials exemplified as binders used in the positive electrode layer. However, in order to facilitate high output, from the perspective of preventing excessive aggregation of the solid electrolyte and forming a solid electrolyte layer with a uniformly dispersed solid electrolyte, the binder contained in the solid electrolyte layer may be set to 5% by mass or less.

[0120] The thickness of the solid electrolyte layer is not particularly limited, but is usually 0.1 μm or more and 1 mm or less.

[0121] As a method for forming a solid electrolyte layer, there can be mentioned: a method of applying a solid electrolyte layer-forming paste containing a solid electrolyte on a support and drying it, and a method of press-molding a powder of a solid electrolyte material containing a solid electrolyte. The support can be the same as the support exemplified in the positive electrode layer. When the powder of the solid electrolyte material is press-molded, a pressing pressure of 1 MPa or more and 2000 MPa or less is usually applied.

[0122] The pressurizing method is not particularly limited, and the pressurizing methods exemplified in the formation of the positive electrode layer can be mentioned.

[0123] The lithium secondary battery includes an outer casing or the like that houses a stack of a positive electrode, an electrolyte layer, and a negative electrode, as needed.

[0124] The material of the outer casing is not particularly limited as long as it is stable against electrolytes, and examples thereof include resins such as polypropylene, polyethylene, and acrylic resins.

[0125] The lithium secondary battery may be an aqueous lithium secondary battery, a non-aqueous lithium secondary battery, an all-solid lithium secondary battery, or the like.

[0126] Examples of the shape of the lithium secondary battery include a coin type, a laminate type, a cylinder type, and a square type.

[0127] The use of lithium secondary batteries is 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 vehicles, and diesel vehicles. In particular, they can be used as a driving power source for hybrid electric vehicles, plug-in hybrid electric vehicles, or electric vehicles. In addition, the lithium secondary batteries in the present disclosure can also be used as power sources for mobile bodies other than vehicles (such as railway trains, ships, and aircraft), and can also be used as power sources for electrical products such as information processing devices.

[0128] In the manufacturing method of the lithium secondary battery disclosed herein, for example, first, a positive electrode layer is formed by press-molding a powder of a positive electrode mixture containing a positive electrode active material containing a lithium element on one side of a positive electrode collector to obtain a positive electrode. Then, lithium metal and a dissimilar metal are simultaneously vacuum-evaporated on one side of a negative electrode collector, thereby forming a composite layer comprising an alloy of lithium metal and a dissimilar metal on one side of the negative electrode collector. Then, lithium metal is vacuum-evaporated on the surface of the composite layer to form a lithium metal single substance layer, thereby forming a negative electrode layer consisting of these two layers. In this way, a negative electrode layer is formed on one side of the negative electrode collector to obtain a negative electrode. Then, the lithium secondary battery disclosed herein can be manufactured by preparing a separator, arranging the separator between the positive electrode and the negative electrode, and injecting an electrolyte into the separator.

[0129] (Comparative Example 1)

[0130] [Positive electrode production]

[0131] Lithium nickel cobalt manganese composite oxide (layered structure, LiNi) in the form of particles with an average particle size of 10 μm as the positive electrode active material was weighed so that the mass ratio of positive electrode active material: AB: PVdF = 80:8:2 was obtained. 0.33 Co 0.33 Mn 0.33O2), acetylene black (AB) as a conductive material and polyvinylidene fluoride (PVdF) as a binder. Then, a planetary mixer was used to mix these materials in N-methyl-2-pyrrolidone (NMP) in such a way that the solid fraction was 56% by mass, thereby preparing a slurry for forming a positive electrode layer. Using a die coater, the slurry for forming a positive electrode layer was applied along the length direction of a strip of aluminum foil (positive electrode collector) and dried at 120 ° C. Then, the dried slurry for forming a positive electrode layer was pressed together with the aluminum foil. Thus, a strip-shaped positive electrode having a positive electrode layer on a positive electrode collector was produced.

[0132] [Negative electrode production]

[0133] Lithium metal was volatilized in a vacuum deposition apparatus and deposited on a Cu foil (negative electrode current collector) to produce a strip-shaped negative electrode having a negative electrode layer consisting of a single lithium metal layer on the negative electrode current collector.

[0134] [Partition configuration]

[0135] The positive and negative electrodes prepared above were placed facing each other with a strip-shaped separator (three-layer structure of PP / PE / PP) interposed therebetween and wound in the longitudinal direction to produce a wound electrode body. The positive electrode current collector was then welded to the positive electrode, and the negative electrode current collector was welded to the negative electrode.

[0136] [Electrolyte preparation]

[0137] As a non-aqueous electrolyte, an electrolyte solution was prepared in which LiPF 6 as a supporting electrolyte was dissolved at a concentration of 1.0 M in a mixed solvent containing ethylene carbonate (EC) and dimethyl carbonate (DMC) at a volume ratio of EC:DMC=1:1.

[0138] Then, the wound electrode assembly and the non-aqueous electrolyte prepared above were housed in a battery case to construct a lithium secondary battery.

[0139] (Comparative Example 2)

[0140] A lithium secondary battery was constructed in the same manner as in Comparative Example 1 except for the following.

[0141] In the above-mentioned [negative electrode production], two crucibles are prepared, a crucible filled with lithium metal and a crucible filled with In as a dissimilar metal. The crucibles are heated by electron beam heating, so that the lithium metal and the dissimilar metal are simultaneously volatilized into a vacuum evaporation device, and the lithium metal and the dissimilar metal are evaporated on Cu foil (negative electrode collector). A strip-shaped negative electrode is produced, which has a negative electrode layer composed of a composite layer of an alloy of lithium metal and the dissimilar metal on the negative electrode collector.

[0142] The element ratio of lithium metal in the alloy was 95 atomic %.

[0143] (Example 1)

[0144] A lithium secondary battery was constructed in the same manner as in Comparative Example 2 except for the following.

[0145] In the above-mentioned [negative electrode preparation], two crucibles are prepared, namely a crucible filled with lithium metal and a crucible filled with In as a heterogeneous metal. The crucibles are heated by electron beam heating so that the lithium metal and the heterogeneous metal are simultaneously volatilized into a vacuum evaporation device, and the lithium metal and the heterogeneous metal are evaporated on Cu foil (negative electrode collector), forming a composite layer of an alloy containing lithium metal and the heterogeneous metal on the negative electrode collector. Then, a crucible filled with lithium metal is prepared, and the crucible is heated by electron beam heating so that the lithium metal is volatilized into a vacuum evaporation device, and the lithium metal is evaporated on the composite layer to form a lithium metal single layer. A strip-shaped negative electrode is produced having a negative electrode layer on the negative electrode collector with a two-layer structure having a composite layer and a lithium metal single layer in sequence from the negative electrode collector side.

[0146] The element ratio of lithium metal in the alloy was 95 atomic %.

[0147] The ratio Z of the thickness X of the lithium metal single substance layer to the thickness Y of the composite layer (Z=X / Y) is 0.1.

[0148] (Examples 2 to 16)

[0149] In Examples 2 to 16, lithium secondary batteries were constructed in the same manner as in Example 1 except that the types of dissimilar metals were different as shown in Table 1.

[0150] (Examples 17 to 21)

[0151] 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 single substance layer to the thickness Y of the composite layer was different as shown in Table 2.

[0152] (Examples 22 to 27)

[0153] In Examples 22 to 27, lithium secondary batteries were constructed in the same manner as in Example 1, except that the element ratio of lithium metal in the alloy was different as shown in Table 3.

[0154] [Output Characteristics Evaluation]

[0155] 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 5C for 8 seconds at a temperature of -5°C. Here, "1C" refers to the current value that allows the battery capacity (Ah) predicted based on the theoretical capacity of the active material to be charged in 1 hour. The voltage drop ΔV at this time was obtained, and the resistance value was calculated using the following formula (1).

[0156] Formula (1): Resistance = ΔV / 5C current value

[0157] 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 were calculated relative to the battery resistance of Comparative Example 1. The results are shown in Tables 1 to 3. In addition, the upward arrows in the tables have the same meaning as above.

[0158] [Capacity maintenance rate evaluation]

[0159] A cycle test was performed on the lithium secondary battery in an environment of 60° C. within a voltage range of 3.3 V to 4.2 V. Charging and discharging were performed using a constant current method at a current rate of 1C.

[0160] The lithium secondary battery prepared above was charged at a constant current (CC) rate of 1C at 60°C until the voltage reached 4.2V. It was then charged at a constant voltage (CV) rate until the current reached 1 / 50C. The battery was then discharged at a constant current (CC) rate of 1C until the voltage reached 3.3V. The discharge capacity at this point was defined as the initial discharge capacity.

[0161] The discharge capacity at the 200th cycle of the cycle test was measured using the same method as the initial discharge capacity. The capacity retention rate after charge and discharge cycles 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.

[0162] [Evaluation of capacity retention rate after storage]

[0163] The lithium secondary batteries of Examples 1 to 27 and Comparative Examples 1 to 2 were charged to 3.8 V and stored in a thermostatic chamber at 60°C for 100 days. The post-storage capacity retention (100 × discharge capacity after storage / discharge capacity before storage) was calculated. Charge and discharge were performed using a constant current method at a current rate of 1C at 60°C over a voltage range of 3 V to 4.2 V. The results are shown in Tables 1 to 3.

[0164] Table 1

[0165]

[0166] Table 2

[0167]

[0168] Table 3

[0169]

[0170] [Evaluation results]

[0171] As shown in Tables 1 to 3, it was confirmed that Examples 1 to 27 had lower battery resistance, higher capacity retention rates after charge and discharge cycles, and higher capacity retention rates after storage than Comparative Examples 1 and 2.

[0172] As shown in Table 2, it is confirmed that by adjusting the ratio Z (Z = X / Y) of the thickness X of the lithium metal single layer to the thickness Y of the composite layer to be within the specified range, the battery resistance can be further reduced, and the capacity retention rate after charge and discharge cycles and the capacity retention rate after storage can be further improved.

[0173] As shown in Table 3, it was confirmed that by adjusting the element ratio of lithium metal in the alloy to be within a predetermined range, the battery resistance can be further reduced, and the capacity retention rate after charge and discharge cycles and the capacity retention rate after storage can be further improved.

Claims

1. A negative electrode for a lithium secondary battery, characterized in that comprising a negative electrode current collector and a negative electrode layer, wherein: The negative electrode layer includes a composite layer and a lithium metal single substance layer in order from the negative electrode current collector side, the composite layer contains an alloy of lithium metal and a dissimilar metal as a negative electrode active material, the dissimilar metal is an element that can form a solid solution with the lithium metal, or an element that can form an intermetallic compound with the lithium metal, The ratio Z of the thickness X of the lithium metal single substance layer to the thickness Y of the composite layer is 0.001≤Z≤0.1, wherein Z=X / Y, The element ratio of lithium in the alloy is 40.00 atomic % or more and 99.97 atomic % or less, The mass of the alloy of the lithium metal and the dissimilar metal is 50% or more relative to the total mass of the composite layer. The dissimilar metal is one or more elements selected from Mg, Bi, Pd, Sn, Si, Au, Ag, Pt, Zn, Al, In, Sr, Ba, Ga, Ca and Ge.

2. A method for manufacturing a negative electrode for a lithium secondary battery, characterized in that: The following steps are involved: preparing a negative electrode current collector; forming a composite layer including an alloy of lithium metal and the dissimilar metal by vacuum evaporating lithium metal and the dissimilar metal on the negative electrode current collector; and A lithium metal single substance layer is formed by vacuum evaporating lithium metal on the composite layer. in, The dissimilar metal is an element that can form a solid solution with the lithium metal, or an element that can form an intermetallic compound with the lithium metal. The ratio Z of the thickness X of the lithium metal single substance layer to the thickness Y of the composite layer is 0.001≤Z≤0.1, wherein Z=X / Y, The element ratio of lithium in the alloy is 40.00 atomic % or more and 99.97 atomic % or less, The mass of the alloy of the lithium metal and the dissimilar metal is 50% or more relative to the total mass of the composite layer. The dissimilar metal is one or more elements selected from Mg, Bi, Pd, Sn, Si, Au, Ag, Pt, Zn, Al, In, Sr, Ba, Ga, Ca and Ge.

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