Lithium secondary battery

Abstract

The present disclosure is a lithium secondary battery. The lithium secondary battery has a positive electrode and a negative electrode that causes deposition-dissolution reaction of lithium metal. The negative electrode includes a negative electrode layer that contains an alloy of the lithium metal and a foreign metal as a negative electrode active material, and an atomic ratio of lithium element in the alloy is 40.00 atomic % or more and 99.97 atomic % or less when the lithium secondary battery is fully charged.

Description

Technical Field

[0001] This disclosure relates to lithium secondary batteries. Background Technology

[0002] Among batteries, lithium-ion batteries have attracted attention due to their ability to achieve high output voltage.

[0003] Japanese Patent Application Publication No. 2021-077640 discloses a negative electrode material in which a thin metal film (Au, Mg, Ag) is contained at the interface between the Li metal layer and the current collector.

[0004] Japanese Patent Application Publication No. 2020-184513 discloses a Li metal negative electrode battery in which a metal Mg layer containing metallic magnesium is formed on one side of the negative electrode current collector or on one side of the solid electrolyte layer. Summary of the Invention

[0005] In lithium secondary batteries that use lithium metal and lithium alloys as negative electrode active materials, there is a possibility that the capacity retention rate will decrease due to the deactivation of lithium metal caused by the volume change of lithium metal during charging and discharging, which requires improving the capacity retention rate.

[0006] This disclosure provides a lithium secondary battery that can improve capacity retention.

[0007] One aspect of the lithium secondary battery disclosed herein includes a positive electrode and a negative electrode in which a lithium metal deposition-dissolution reaction occurs.

[0008] The negative electrode comprises a negative electrode layer.

[0009] The negative electrode layer contains an alloy of lithium metal and a dissimilar metal as the negative electrode active material.

[0010] When the lithium secondary battery is fully charged, the lithium element content in the alloy is 40.00 atomic% or more and 99.97 atomic% or less.

[0011] In the lithium secondary battery described above, the negative electrode layer may have an oxygen-concentrated section.

[0012] In the lithium secondary battery described above, when the cross-sectional area of ​​the negative electrode layer is set to 100%, the cross-sectional area of ​​the oxygen concentration section can be 1% or more and 60% or less.

[0013] In the lithium secondary battery described above, the dissimilar metal can be 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.

[0014] This disclosure provides a lithium secondary battery that can improve capacity retention. Attached Figure Description

[0015] The features, advantages, and technical and industrial significance of typical embodiments of the present invention will be described below with reference to the accompanying drawings, wherein like reference numerals denote like elements, wherein:

[0016] Figure 1 This is a cross-sectional schematic diagram illustrating an example of the lithium secondary battery of this disclosure. Detailed Implementation

[0017] The following describes embodiments of this disclosure. Furthermore, matters necessary for the implementation of this disclosure, other than those specifically mentioned in this specification (e.g., the general structure and manufacturing process of lithium secondary batteries not characterized by this disclosure), can be grasped as design matters by those skilled in the art. This disclosure can be implemented based on the content disclosed in this specification and common technical knowledge in the field.

[0018] In addition, the dimensional relationships (length, width, thickness, etc.) in the diagram do not reflect the actual dimensional relationships.

[0019] In this specification, the “~” sign indicating a range of values ​​is used to indicate the lower limit and upper limit of the values ​​described before and after it.

[0020] Furthermore, the upper and lower limits of the numerical range can be used in any combination.

[0021] The lithium secondary battery disclosed herein has a positive electrode and a negative electrode in which a lithium metal precipitation-dissolution reaction occurs.

[0022] The negative electrode comprises a negative electrode layer.

[0023] The negative electrode layer contains an alloy of lithium metal and a dissimilar metal as the negative electrode active material.

[0024] When the lithium secondary battery is fully charged, the lithium element content in the alloy is 40.00 atomic% or more and 99.97 atomic% or less.

[0025] In lithium secondary batteries, the capacity retention rate decreases due to the decomposition reaction between the newly formed Li metal surface, which is continuously generated by the dissolution of Li metal during charging and discharging, and the electron isolation of the negative electrode active material caused by the cracks in the negative electrode layer due to the volume change of Li metal during charging and discharging.

[0026] When lithium secondary batteries operate at low temperatures, the conductivity of Li in the electrolyte or electrolyte solution decreases significantly due to the increased resistance of the electrolyte or electrolyte solution, resulting in a significant reduction in low-temperature output characteristics.

[0027] According to this disclosure, the increase in the resistivity component of the electrolyte or electrolyte solution is mitigated, resulting in excellent low-temperature output.

[0028] The negative electrode active material used in this disclosure has a high affinity for the electrolyte-impregnated separator and a high degree of conformity to the separator during the charging and discharging of the lithium secondary battery. Therefore, it is possible to suppress the deactivation of the negative electrode active material caused by local separation of the negative electrode active material from the separator, thereby improving the capacity retention of the lithium secondary battery.

[0029] In the presence of an oxygen concentration zone in the negative electrode layer, the active material of the negative electrode preferentially fractures from this oxygen concentration zone, increasing the number of cracks in the negative electrode layer. In related technologies, the increase in the number of cracks reduces the capacity retention of the lithium secondary battery. However, the cracks generated in this disclosure are electrochemically inactive, thus minimizing the reduction in capacity retention. Furthermore, since cracks preferentially form in the oxygen concentration zone, the generation of cracks that reduce capacity retention is suppressed, thereby improving capacity retention.

[0030] The lithium secondary battery disclosed herein utilizes the precipitation-dissolution reaction of lithium metal as the negative electrode.

[0031] The lithium secondary battery disclosed herein includes a positive electrode layer, a negative electrode layer, and an electrolyte layer disposed between the positive electrode layer and the negative electrode layer.

[0032] Figure 1 This is a cross-sectional schematic diagram illustrating an example of the lithium secondary battery of this disclosure.

[0033] like Figure 1 As shown, the lithium secondary battery 100 includes: a positive electrode 16 comprising a positive electrode layer 12 and a positive current collector 14, a negative electrode 17 comprising a negative electrode layer 13 and a negative current collector 15, and an electrolyte layer 11 disposed between the positive electrode layer 12 and the negative electrode layer 13.

[0034] [negative electrode]

[0035] The negative electrode disclosed herein includes a negative electrode layer and typically also includes a negative electrode current collector.

[0036] [Negative electrode layer]

[0037] The negative electrode layer contains an alloy of lithium metal and dissimilar metals as the negative electrode active material.

[0038] When the lithium secondary battery is fully charged, the lithium element ratio in the alloy is above 40.00 atomic% and below 99.97 atomic%.

[0039] In this disclosure, "fully charged" refers to a lithium secondary battery when its State of Charge (SOC) is 100%. SOC represents the ratio of the battery's charge capacity to its full charge capacity, with a full charge capacity being 100% SOC.

[0040] SOC can be estimated, for example, based on the open circuit voltage (OCV) of a lithium secondary battery.

[0041] As a dissimilar metal, any metal other than lithium metal is acceptable. It can be an element capable of forming a solid solution with lithium metal or an element capable of forming an intermetallic compound with lithium metal. For example, a dissimilar metal can be one or more elements selected from Mg, Bi, Pd, Sn, Si, Au, Ag, Pt, Zn, Al, In, Sr, Ba, Ga, Ca, and Ge.

[0042] In the negative electrode layer of this disclosure, other negative electrode active materials may also be included, as long as the alloy containing lithium metal and dissimilar metals as the main components is used as the negative electrode active material. In this disclosure, the term "main component" refers to a component containing 50% or more by mass when the total mass of the negative electrode layer is set to 100% by mass.

[0043] The thickness of the negative electrode layer is not particularly limited; for example, it can be 10–100 μm.

[0044] Methods for forming the negative electrode layer include methods such as vacuum evaporating lithium metal and dissimilar metals onto the negative electrode current collector to serve as the negative electrode active material.

[0045] As a method for simultaneously vacuum-depositing lithium metal and a dissimilar metal onto a negative electrode current collector, examples include: preparing two crucibles, one containing lithium metal and the other containing a dissimilar metal, heating the crucibles by electron beam heating or resistance heating, so that the lithium metal and the dissimilar metal simultaneously evaporate into the vacuum deposition apparatus and are deposited onto the negative electrode current collector, etc.

[0046] The negative electrode layer may have at least one oxygen-enriched section (oxygen-rich section).

[0047] Methods for forming an oxygen concentration section include: vacuum evaporation of an oxide of a dissimilar metal, which is an oxygen-introducing compound, into a negative electrode current collector during vacuum evaporation of a negative electrode active material into the negative electrode current collector.

[0048] As oxygen-introducing compounds, they can be MgO, Bi2O3, PdO, SnO, SiO2, Au2O3, Ag2O, PtO, ZnO, Al2O3, In2O3, SrO, BaO, Ga2O3, CaO, and GeO2, etc.

[0049] From the viewpoint of improving the capacity retention of lithium secondary batteries, when the cross-sectional area of ​​the negative electrode layer is set to 100%, the cross-sectional area occupied by the oxygen concentration section can be more than 1% and less than 60%.

[0050] The location and thickness of the oxygen concentration section in the negative electrode layer can be controlled by adjusting the timing of oxygen pressurization during evaporation.

[0051] [Negative current collector]

[0052] The material of the negative current collector can be a material that is not alloyed with Li, such as SUS (stainless steel), copper, and nickel. The form of the negative current collector can include, for example, foil or plate. The top view shape of the negative current collector is not particularly limited, and can include, for example, circular, elliptical, rectangular, and any polygonal shape. Furthermore, the thickness of the negative current collector varies depending on the shape, but can be, for example, in the range of 1 μm to 50 μm, or in the range of 5 μm to 20 μm.

[0053] [positive electrode]

[0054] The positive electrode consists of a positive electrode layer and a positive current collector.

[0055] [Positive electrode layer]

[0056] The positive electrode layer contains positive electrode active material, and can also contain any component such as solid electrolyte, conductive material and binder.

[0057] There are no particular restrictions on the type of positive electrode active material; any material suitable for use as an active material in lithium-ion secondary batteries can be used. Examples of positive electrode active materials 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 O2, LiMnO2, dissimilar 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 intermetallic compounds, etc. Dissimilar element-substituted Li-Mn spinel includes, for example, LiMn... 1.5 Ni 0.5 O4, LiMn 1.5 Al 0.5 O4, LiMn1.5 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. 12 Examples of lithium metal phosphates include LiFePO4, LiMnPO4, LiCoPO4, and LiNiPO4. Transition metal oxides include V2O5 and MoO3. Lithium storage intermetallic compounds include Mg2Sn, Mg2Ge, Mg2Sb, and Cu3Sb.

[0058] 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; additionally, alloys of Si with at least one metal selected from Sn, Ge, and Al can also be used.

[0059] The shape of the positive electrode active material is not particularly limited and can be in particle form. When the positive electrode active material is in particle form, it can be either primary particles or secondary particles.

[0060] A coating containing Li-ion-conducting oxides can also be formed on the surface of the positive electrode active material. This is because it can inhibit the reaction between the positive electrode active material and the solid electrolyte.

[0061] Examples of Li-ion-conducting oxides include LiNbO3 and Li4Ti5O. 12 Including Li3PO4, etc. The coating thickness is, for example, 0.1 nm or more, and can be 1 nm or more. On the other hand, the coating thickness is, for example, 100 nm or less, and can be 20 nm or less. The coating can, for example, cover more than 70% of the surface of the positive electrode active material, and can cover more than 90% of the surface of the positive electrode active material.

[0062] As a solid electrolyte, the same solid electrolyte as that illustrated in the solid electrolyte layer described later can be exemplified.

[0063] As a conductive material, known conductive materials can be used, such as carbon materials and metal particles. As a carbon material, at least one selected from acetylene black, furnace black, VGCF (vapor-grown carbon fiber), carbon nanotubes, and carbon nanofibers can be used. From the viewpoint of electronic conductivity, at least one selected from VGCF, carbon nanotubes, and carbon nanofibers can be used. As metal particles, particles such as Ni, Cu, Fe, and SUS can be used.

[0064] The amount of conductive material in the positive electrode layer is not particularly limited.

[0065] Examples of adhesives include acrylonitrile butadiene rubber (ABR), butadiene rubber (BR), polyvinylidene fluoride (PVDF), and styrene butadiene rubber (SBR). The amount of adhesive in the positive electrode layer is not particularly limited.

[0066] The thickness of the positive electrode layer is not particularly limited; for example, it can be 10–100 μm or 10–20 μm.

[0067] Regarding the positive electrode layer, for example, a paste for forming the positive electrode layer is prepared by adding the positive electrode active material and other components as needed into a solvent and stirring. The positive electrode layer is obtained by coating the paste for forming the positive electrode layer onto one side of a support and drying it.

[0068] Solvents include, for example, butyl acetate, butyl butyrate, mesitylene, tetralin, heptane, and N-methyl-2-pyrrolidone (NMP).

[0069] There are no particular limitations on the method of coating the positive electrode layer on one side of the support to form a paste. Examples include doctor blade coating, metal mask printing, electrostatic coating, dip coating, spray coating, roller coating, gravure coating, and screen printing.

[0070] As a support, a self-supporting support can be appropriately selected and used without particular limitation. Metal foils such as Cu and Al can be used.

[0071] Alternatively, another method for forming the positive electrode layer is to press-form a powder containing a positive electrode active material and other components as needed to form the positive electrode layer. When pressing the powder of the positive electrode compound, a pressing pressure of 1 MPa or more and 2000 MPa or less is typically applied.

[0072] There are no particular limitations on the method of applying pressure; for example, methods such as using a flatbed press or a roller press can be used to apply pressure.

[0073] [Positive current collector]

[0074] As the positive electrode current collector, a known metal suitable for use as a current collector in a lithium secondary battery can be used. Examples of such metals include metallic 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 positive electrode current collectors include SUS, aluminum, nickel, iron, titanium, and carbon.

[0075] The shape of the positive current collector is not particularly limited and can be various shapes such as foil or mesh. The thickness of the positive current collector varies depending on the shape, for example, it can be in the range of 1μm to 50μm, or in the range of 5μm to 20μm.

[0076] [Electrolyte layer]

[0077] The electrolyte layer contains at least an electrolyte.

[0078] As electrolytes, aqueous electrolytes, non-aqueous electrolytes, gel electrolytes, and solid electrolytes can be used. They can be used alone or in combination of two or more.

[0079] The solvent of aqueous electrolytes contains water as the main component. That is, based on the total amount of solvent (liquid component) constituting the electrolyte (100 mol%), water can account for more than 50 mol%, especially more than 70 mol%, and even more than 90 mol%. On the other hand, there is no particular upper limit to the proportion of water in the solvent.

[0080] The solvent contains water as its 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 (100 mol%), the solvents other than water may be 50 mol% or less, particularly 30 mol% or less, and further, 10 mol% or less.

[0081] The aqueous electrolyte used in this disclosure contains an electrolyte. The electrolyte used in the aqueous electrolyte is not particularly limited, but examples of electrolytes include lithium salts, nitrates, acetates, sulfates, etc., which are imidic acid compounds. Specific examples of 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]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.

[0082] The concentration of the electrolyte in an aqueous electrolyte solution can be appropriately set according to the required battery characteristics, within a range that does not exceed the saturation concentration of the electrolyte relative to the solvent. This is because there is a concern that residual solid electrolyte in the aqueous electrolyte solution might hinder the battery reaction.

[0083] For example, when LiTFSI is used as an electrolyte, the aqueous electrolyte may contain 1 mol or more of LiTFSI relative to 1 kg of water, particularly 5 mol or more of LiTFSI relative to 1 kg of water, and further, 7.5 mol or more of LiTFSI relative to 1 kg of water. There is no particular upper limit; for example, it may be 25 mol or less relative to 1 kg of water.

[0084] As a non-aqueous electrolyte, a non-aqueous electrolyte containing lithium salt and non-aqueous solvent is usually used.

[0085] 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.

[0086] Examples of non-aqueous solvents include ethylene carbonate (EC), propylene carbonate (PC), butyl carbonate (BC), dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl 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 high dielectric constant and low viscosity, it can be a mixture of cyclic carbonate compounds such as EC, PC, and BC with high dielectric constant and high viscosity and chain carbonate compounds such as DMC, DEC, and EMC with low dielectric constant and low viscosity, or a mixture of EC and DEC.

[0087] The concentration of lithium salt in non-aqueous electrolytes can be, for example, 0.3–5 M.

[0088] Gel electrolytes are typically obtained by adding polymers to non-aqueous electrolytes and then gelling them.

[0089] Specifically, a gel electrolyte can be obtained by adding polymers such as polyethylene oxide, polypropylene oxide, polyacrylonitrile, polyvinylidene fluoride (PVDF), polyurethane, polyacrylate, and cellulose to the above-mentioned non-aqueous electrolyte and then gelling it.

[0090] A separator can be used in the electrolyte layer to contain an electrolyte such as an aqueous electrolyte and to prevent the positive electrode layer from contacting the negative electrode layer.

[0091] The material used for the separator is not particularly limited as long as it is a porous membrane. Examples include resins such as polyethylene (PE), polypropylene (PP), polyester, polyvinyl alcohol, cellulose, and polyamide, among which polyethylene and polypropylene are also possible. Furthermore, the separator can be a single-layer structure or a multi-layer structure. Examples of multi-layer separators include two-layer structures such as PE / PP, or three-layer structures such as PP / PE / PP or PE / PP / PE.

[0092] The partition can be made of nonwoven fabrics such as resin nonwoven fabric and glass fiber nonwoven fabric.

[0093] [Solid electrolyte layer]

[0094] The electrolyte layer can also be a solid electrolyte layer composed of solids.

[0095] The solid electrolyte layer contains at least a solid electrolyte.

[0096] As for the solid electrolyte contained in the solid electrolyte layer, known solid electrolytes suitable for use in all-solid-state batteries can be appropriately used, including 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. Sulfide-based solid electrolytes may contain sulfur (S) as the main anionic element. Oxide-based solid electrolytes may contain oxygen (O) as the main anionic element. Hydride-based solid electrolytes may contain hydrogen (H) as the main anionic element. Halide-based solid electrolytes may contain halogen (X) as the main anionic element. Nitride-based solid electrolytes may contain nitrogen (N) as the main anionic element.

[0097] Sulfide-based solid electrolytes can be sulfide glasses, crystallized sulfide glasses (glass ceramics), or crystalline materials obtained through solid-phase reaction treatment of raw material compositions.

[0098] The crystalline state of sulfide-based solid electrolytes can be confirmed, for example, by powder X-ray diffraction using CuKα rays.

[0099] Chalcogenide glasses can be obtained by amorphous treatment of a raw material composition (e.g., a mixture of Li2S and P2S5). Examples of amorphous treatment include mechanical milling.

[0100] Glass ceramics, for example, can be obtained by heat treatment of sulfide glass.

[0101] The heat treatment temperature can be any temperature higher than the crystallization temperature (Tc) observed by thermal analysis of the sulfide glass, typically above 195°C. On the other hand, there is no particular upper limit to the heat treatment temperature.

[0102] The crystallization temperature (Tc) of sulfide glass can be determined by differential thermal analysis (DTA).

[0103] There is no particular limitation on the heat treatment time as long as it is the time required to achieve the desired crystallinity of the glass ceramic, for example, it can be in the range of 1 minute to 24 hours, of which the range of 1 minute to 10 hours can be listed.

[0104] The heat treatment method is not particularly limited; for example, the use of a firing furnace can be listed.

[0105] Examples of oxide-based solid electrolytes include those containing Li, Y (where Y is at least one of Nb, B, Al, Si, P, Ti, Zr, Mo, W, and S), and O. A specific example of an oxide-based solid electrolyte is Li₇La₃Zr₂O. 12 Li 7-x La3(Zr 2-x Nb x )O 12 (0≤x≤2), Li5La3Nb2O 12 Garnet-type solid electrolytes, etc.; perovskite-type solid electrolytes such as (Li,La)TiO3, (Li,La)NbO3, (Li,Sr)(Ta,Zr)O3, etc.; NASICON (sodium superionic conductor) type solid electrolytes such as Li(Al,Ti)(PO4)3 and Li(Al,Ga)(PO4)3; Li-PO series solid electrolytes such as Li3PO4 and LIPON (a compound obtained by replacing part of the O in Li3PO4 with N); Li-BO series solid electrolytes such as Li3BO3 and a compound obtained by replacing part of the O in Li3BO3 with C, etc.

[0106] Hydride-based solid electrolytes include, for example, those containing Li and hydrogen-containing complex anions. Examples of complex anions include (BH4). - (NH2) - (AlH4) - And (AlH6) 3- wait.

[0107] Examples of halide-based solid electrolytes include Li 6-3z Y z X6 (where X is at least one of Cl and Br, and z satisfies 0 < z < 2), etc.

[0108] Examples of nitride-based solid electrolytes include Li3N.

[0109] From an operational point of view, solid electrolytes can be in the form of particles.

[0110] The average particle size of solid electrolytes is not particularly limited; for example, it can be 10 nm or more, or 100 nm or more. On the other hand, the average particle size of solid electrolytes can be 25 μm or less, or 10 μm or less.

[0111] In this disclosure, unless otherwise specified, the average particle size refers to the median particle size (D50) of the volume reference determined by laser diffraction-scattering particle size distribution measurement. Furthermore, in this disclosure, the median particle size (D50) refers to the diameter (volume-average diameter) when the cumulative volume of the particles, arranged sequentially from the smallest particle size, is half (50%) of the total volume.

[0112] Regarding solid electrolytes, one type or two or more solid electrolytes can be used alone. Furthermore, when using two or more solid electrolytes, they can be mixed, or a multilayer structure can be formed by creating two or more layers of solid electrolytes.

[0113] The proportion of solid electrolyte in the solid electrolyte layer is not particularly limited, for example, it can be 50% or more by mass, it can be in the range of 60% or more and less than 100% by mass, it can be in the range of 70% or more and less than 100% by mass, or it can be 100% by mass.

[0114] From the viewpoint of maximizing plasticity, the solid electrolyte layer may contain a binder. Examples of such binders include materials used in the positive electrode layer. However, to facilitate high output, and from the viewpoint of preventing excessive aggregation of the solid electrolyte and forming a solid electrolyte layer with uniformly dispersed solid electrolyte, the binder content in the solid electrolyte layer can be set to 5% by mass or less.

[0115] The thickness of the solid electrolyte layer is not particularly limited, but is usually above 0.1 μm and below 1 mm.

[0116] Methods for forming a solid electrolyte layer include: applying a paste containing a solid electrolyte onto a support and drying it; and pressing a powder containing a solid electrolyte into shape. The support can be the same as the support exemplified in the positive electrode layer. When pressing a powder containing a solid electrolyte into shape, a pressing pressure of 1 MPa or more and 2000 MPa or less is typically applied.

[0117] There are no particular limitations on the pressurization method; examples of pressurization methods exemplified in the formation of the positive electrode layer can be cited.

[0118] Lithium secondary batteries require an external casing that houses the positive electrode, electrolyte layer, and negative electrode, etc.

[0119] There are no particular restrictions on the material of the outer casing as long as it is a material that is stable for electrolytes. Examples include polypropylene, polyethylene, and acrylic resins.

[0120] Lithium secondary batteries can be aqueous lithium secondary batteries, non-aqueous lithium secondary batteries, and all-solid-state lithium secondary batteries, etc.

[0121] Examples of shapes for lithium secondary batteries include coin-shaped, laminated, cylindrical, and square.

[0122] The applications of lithium secondary batteries are not particularly limited, and examples include power sources for vehicles such as hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), battery electric vehicles (BEVs), gasoline vehicles, and diesel vehicles. In particular, they can be used as power sources for driving hybrid electric vehicles, plug-in hybrid electric vehicles, or electric vehicles. Furthermore, the lithium secondary batteries disclosed herein can also be used as power sources for mobile bodies other than vehicles (e.g., railway trains, ships, and aircraft), and as power sources for electrical appliances such as information processing devices.

[0123] In the method for manufacturing the lithium secondary battery disclosed herein, for example, firstly, a positive electrode is obtained by pressing a powder containing a positive electrode active material of lithium element into one side of a positive electrode current collector to form a positive electrode layer. Then, a negative electrode active material is vacuum-deposited onto a negative electrode current collector to form a negative electrode layer on one side of the negative electrode current collector, thereby obtaining a negative electrode. Subsequently, the lithium secondary battery of this disclosure is manufactured by preparing a separator, placing a separator between the positive and negative electrodes, and injecting an electrolyte into the separator.

[0124] (Comparative Example 1)

[0125] [Positive electrode production]

[0126] The lithium nickel cobalt manganese composite oxide (layered structure, LiNi) particles with an average particle size of 10 μm, used as the positive electrode active material, were weighed to achieve a mass ratio of AB:PVDF = 80:8:2. 0.33 Co 0.33 Mn 0.33 O2), acetylene black (AB) as a conductive material, and polyvinylidene fluoride (PVDF) as a binder. Next, these materials are mixed in N-methyl-2-pyrrolidone (NMP) using a planetary mixer to achieve a solids content of 56% by mass, thereby preparing a slurry for forming the positive electrode layer. Using a die-coating machine, the slurry for forming the positive electrode layer is applied along the length of a strip of aluminum foil (positive electrode current collector) and dried at 120°C. Then, the dried slurry for forming the positive electrode layer is pressed together with the aluminum foil. Thus, a strip of positive electrode with a positive electrode layer on the positive electrode current collector is produced.

[0127] [Negative electrode fabrication]

[0128] Li metal is volatilized in a vacuum evaporation apparatus and deposited onto a Cu foil (negative electrode current collector), thus creating a strip-shaped negative electrode with a negative electrode layer containing Li metal on the negative electrode current collector.

[0129] [Partition Configuration]

[0130] A wound electrode body was fabricated by placing the aforementioned positive and negative electrodes face each other with a strip-shaped separator (a 3-layer structure of PP / PE / PP) and winding them along the length. Then, the positive current collector was welded to the positive electrode, and the negative current collector was welded to the negative electrode.

[0131] [Electrolyte Preparation]

[0132] As a non-aqueous electrolyte, an electrolyte was prepared by dissolving LiPF6, which serves as the supporting electrolyte, in a mixed solvent at a concentration of 1.0 M, wherein the mixed solvent contains ethylene carbonate (EC) and dimethyl carbonate (DMC) in a volume ratio of EC:DMC = 1:1.

[0133] Then, the wound electrode body and non-aqueous electrolyte prepared above are placed in the battery casing to construct a lithium secondary battery.

[0134] (Comparative Examples 2-3, Example 1)

[0135] Except as described below, a lithium secondary battery was constructed using the same method as Comparative Example 1.

[0136] In the aforementioned [anode fabrication], two crucibles are prepared: one containing Li metal and the other containing Mg as a dissimilar metal. The crucibles are heated using an electron beam, causing both the Li metal and the dissimilar metal to evaporate simultaneously into a vacuum deposition apparatus. This allows the Li metal and the dissimilar metal to be deposited onto a Cu foil (anode current collector), creating a strip-shaped anode with an alloy layer comprising Li metal and the dissimilar metal on the anode current collector. The elemental ratio of Li metal in the alloy is controlled by adjusting the temperature inside the crucible and the deposition time of the Li metal.

[0137] The elemental percentage of Li metal in the alloy was 10 atomic percent in Comparative Example 2, 30 atomic percent in Comparative Example 3, and 40 atomic percent in Example 1.

[0138] (Examples 2-5)

[0139] Except as described below, a lithium secondary battery was constructed using the same method as Comparative Example 1.

[0140] In the aforementioned [anode fabrication], three crucibles are prepared: one containing Li metal, one containing Mg as a dissimilar metal, and one containing an oxygen-importing compound (MgO, an oxide of a dissimilar metal element). These crucibles are heated using an electron beam, causing the Li metal, dissimilar metal, and oxygen-importing compound to simultaneously evaporate into a vacuum deposition apparatus. This allows the Li metal, dissimilar metal, and oxygen-importing compound to be deposited onto a Cu foil (anode current collector), creating a strip-shaped anode with an alloy layer containing Li metal and a dissimilar metal, and an oxygen-concentrated portion on the anode current collector. The elemental ratio of Li metal in the alloy is controlled by adjusting the temperature inside the crucible and the deposition time of the Li metal.

[0141] The elemental ratio of Li metal in the alloy was 40 atomic% in Example 2, 70 atomic% in Example 3, 90 atomic% in Example 4, and 99.97 atomic% in Example 5.

[0142] The amount of oxygen-introducing compound vapor deposition is controlled such that the cross-sectional area occupied by the oxygen concentration section (cross-sectional area of ​​oxygen concentration section / cross-sectional area of ​​negative electrode layer × 100) is 2% when the cross-sectional area of ​​the negative electrode layer is set to 100% in the cross-section when the negative electrode layer is cut. The formation position and thickness of the oxygen concentration section are controlled by adjusting the timing of oxygen pressurization during vapor deposition.

[0143] (Examples 6-20)

[0144] Examples 6-20, as shown in Table 1, were constructed using the same method as in Example 5, except for the different types of dissimilar metals. The oxygen-introducing compounds used in Examples 6-20 were oxides of the dissimilar metal elements used in Examples 6-20.

[0145] (Examples 21-25)

[0146] Examples 21-25, as shown in Table 2, differ in the cross-sectional area of ​​the oxygen concentration section (cross-sectional area of ​​oxygen concentration section / cross-sectional area of ​​negative electrode layer × 100) when the cross-sectional area of ​​the negative electrode layer is set to 100% in the cross-section when the negative electrode layer is cut. Otherwise, lithium secondary batteries were constructed using the same method as in Example 5.

[0147] [Output Characteristic Evaluation]

[0148] The open-circuit voltage of the lithium secondary battery was pre-adjusted to 3.70V. Then, the lithium secondary battery was discharged at 5C for 8 seconds at a low temperature of -5°C. Furthermore, here, "1C" refers to the current value that can charge the battery to the capacity (Ah) predicted based on 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).

[0149] Equation (1): Resistance = ΔV / 5C current value

[0150] The battery resistance of Comparative Example 1 was standardized by setting it to 1.0. The results of calculating the battery resistance of Examples 1 to 25 relative to the battery resistance of Comparative Example 1 are shown in Tables 1 to 2. Furthermore, the upward arrows in the tables indicate the same meaning as above.

[0151] [Capacity Maintenance Rate Assessment]

[0152] Regarding the lithium secondary battery, a cycle test was conducted at 60°C within a voltage range of 3.3V to 4.2V. Charge and discharge were performed using a constant current mode with a current rate of 1C.

[0153] For the lithium secondary battery prepared above, it was charged at a constant current (CC) at a rate of 1C to a voltage of 4.2V at 60°C, and then charged at a constant voltage (CV) to a current of 1 / 50C. Next, it was discharged at a constant current (CC) at a rate of 1C to a voltage of 3.3V. The discharge capacity at this point was taken as the initial discharge capacity.

[0154] The discharge capacity of the 200th cycle of the cyclic test was determined using the same method as the initial discharge capacity determination. The capacity retention rate after charge-discharge cycles was calculated by dividing the discharge capacity of the 200th cycle by the initial discharge capacity. The results are shown in Tables 1 and 2.

[0155] [Evaluation of Capacity Retention Rate After Storage]

[0156] In addition, for the lithium secondary batteries of Examples 1-25 and Comparative Examples 1-3, they were charged to 3.8V and stored in a constant temperature bath at 60°C for 100 days. The capacity retention rate after storage was calculated as (discharge capacity after storage / discharge capacity before storage × 100). Charge and discharge were performed using a constant current mode at a current rate of 1C at 60°C within a voltage range of 3V to 4.2V. The results are shown in Tables 1-2.

[0157] Table 1

[0158]

[0159] Table 2

[0160]

[0161] [Evaluation Results]

[0162] As shown in Tables 1-2, it was confirmed that, compared with Comparative Examples 1-3, Examples 1-25 had lower battery resistance and higher capacity retention after charge-discharge cycles and after storage.

[0163] As shown in Tables 1-2, it was confirmed that: Examples 2-25, compared with Example 1 which used a negative electrode layer without an oxygen concentration section, had lower battery resistance and higher capacity retention after charge-discharge cycles and after storage.

Claims

1. A lithium secondary battery, characterized by comprising: A lithium secondary battery including a positive electrode and a negative electrode in which a lithium metal deposition-dissolution reaction occurs, wherein the negative electrode includes a negative electrode layer, the lithium metal and a heterogeneous metal are included as a negative electrode active material in the negative electrode layer, an elemental ratio of lithium in the alloy is 40.00 atomic % or more and 99.97 atomic % or less when the lithium secondary battery is fully charged, the negative electrode layer has an oxygen concentration portion in which oxygen is concentrated, the oxygen concentration portion being formed by vacuum deposition of an oxide of the heterogeneous metal as an oxygen introduction compound to a negative electrode current collector while vacuum depositing the negative electrode active material to the negative electrode current collector, a cross-sectional area of the oxygen concentration portion is 1% or more and 60% or less when a cross-sectional area of the negative electrode layer is 100%.

2. The lithium secondary battery according to claim 1, characterized by the heterogeneous 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.

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