Lithium secondary battery and method for manufacturing same

Abstract

A lithium secondary battery according to an embodiment of the present invention is provided with: a negative electrode having, in this order, a current collector layer, a first lithium metal layer comprising a lithium metal foil, a second lithium metal layer containing a granular lithium metal, and a porous third lithium metal layer containing a lithium metal; the average particle diameter of the particulate lithium metal is larger than the average pore diameter of the third lithium metal layer.

Description

Technical Field

[0001] This disclosure relates to lithium secondary batteries and methods for manufacturing the same. Background Technology

[0002] Lithium-ion batteries have gained attention and popularity due to their ability to achieve higher capacities than current nickel-metal hydride and lead-acid batteries, serving as power sources for mobile phones, laptops, large-scale energy storage, and automobiles. However, with the increasing functionality of various electronic devices and the growing demand for higher power supplies, further increases in the capacity of lithium-ion batteries are anticipated. Unlike lithium-ion secondary batteries, which charge and discharge by inserting / deintercalating lithium ions into the electrode materials, lithium secondary batteries charge and discharge by depositing and dissolving lithium metal. As a lightweight metal, it possesses several to ten times the capacity of graphite, and its extremely low potential makes it a promising candidate. (Patent Documents 1-3)

[0003] Existing technical documents

[0004] Patent documents

[0005] Patent Document 1: Japanese Patent Application Publication No. 11-214008

[0006] Patent Document 2: Japanese Patent Application Publication No. 2000-133314

[0007] Patent Document 3: Japanese Patent No. 5317435 Summary of the Invention

[0008] The problem that the invention aims to solve

[0009] One problem arising from the use of lithium metal as the negative electrode active material is the uneven formation and growth of lithium metal nuclei on the surface of the negative electrode during charging. As lithium metal is unevenly deposited, the leading edge of the deposited lithium metal is in closest contact with the electrolyte. Therefore, lithium deposition is easier at this stage compared to other areas, and the leading edge of the deposited lithium metal extends further, forming dendritic lithium metal (dendritic crystals). The more dendrite formation occurs, the larger the specific surface area becomes, and consequently, the more reduction and decomposition products are produced.

[0010] In addition, during the discharge process, when lithium metal dissolves, some of the lithium metal precipitated on the dendrites is stripped off, creating a portion that cannot contribute to the discharge (dead lithium), leading to the micronization of lithium metal.

[0011] The accumulation of reduction decomposition products and the micronization of lithium metal develop with repeated cycling, resulting in severe expansion of the negative electrode. This expansion accelerates electrolyte depletion and leads to reduced safety and cycle performance, making it a significant problem.

[0012] This disclosure was made in view of the above-mentioned problems, and provides a lithium secondary battery with excellent charge-discharge cycle characteristics and a method for manufacturing the same.

[0013] Technical solutions for solving the problem

[0014] [1] A lithium secondary battery, comprising a negative electrode and a positive electrode, wherein,

[0015] The negative electrode has the following characteristics in sequence:

[0016] Current collector layer;

[0017] The first lithium metal layer is composed of lithium metal foil;

[0018] A second lithium metal layer comprising granular lithium metal; and

[0019] The third lithium metal layer contains lithium metal and is porous.

[0020] The average particle size of the granular lithium metal is greater than the average pore diameter of the third lithium metal layer.

[0021] [2] According to the lithium secondary battery described in [1], wherein,

[0022] The porosity of the third lithium metal layer is 10-40%.

[0023] [3] According to the lithium secondary battery described in [1] or [2], wherein,

[0024] The average pore diameter of the third lithium metal layer is 0.1–3 μm.

[0025] [4] A lithium secondary battery according to any one of [1] to [3], wherein,

[0026] The average particle size of the granular lithium metal is 1–22 μm.

[0027] [5] A lithium secondary battery according to any one of [1] to [4], wherein,

[0028] The area ratio S obtained by the following formula (1) is 60% or more.

[0029] Area ratio S = {S3 / (S2+S3)} × 100 ··· Equation (1)

[0030] [In equation (1), S2 is the area of ​​the second lithium metal layer on the main surface of the third lithium metal layer side of the negative electrode, and S3 is the area of ​​the third lithium metal layer on the main surface of the third lithium metal layer side of the negative electrode.]

[0031] [6] A method for manufacturing a lithium secondary battery, comprising:

[0032] (a) A step of preparing a lithium secondary battery precursor, said lithium secondary battery precursor having a positive electrode and a negative electrode having a current collector layer and a lithium metal foil; and

[0033] (b) A step of charging the lithium secondary battery precursor at 0.01 to 0.5C until the charge rate becomes 10 to 100%, and then discharging the lithium secondary battery precursor at 0.2C to 2.5C.

[0034] Perform process (b) more than twice.

[0035] Invention Effects

[0036] According to this disclosure, a lithium secondary battery with excellent charge-discharge cycle characteristics and a method for manufacturing the same are provided. Attached Figure Description

[0037] Figure 1 This is a schematic cross-sectional view of a lithium secondary battery according to one embodiment.

[0038] Figure 2 This is a cross-sectional view of the negative electrode 30C according to one embodiment.

[0039] Figure 3 (a) is a SEM image of a portion of the cross-section of the negative electrode of the lithium secondary battery obtained in the embodiment. Figure 3 (b) is a SEM image of the main surface of the third lithium metal layer side of the negative electrode of the lithium secondary battery obtained in the embodiment. Figure 3 (c) is a SEM image of the main surface of the third lithium metal layer side of the negative electrode of the lithium secondary battery obtained in the example. Detailed Implementation

[0040] Preferred embodiments of this disclosure will be described with reference to the accompanying drawings.

[0041] [Lithium-ion rechargeable battery]

[0042] like Figure 1 As shown, the lithium secondary battery 100 of this embodiment includes a power generation element 90 and a casing 80. The power generation element 90 is a stack having multiple negative electrodes 30C, positive electrodes 10, and separators 20. In the power generation element 90, the negative electrodes 30C and positive electrodes 10 are alternately arranged, and the separator 20 is disposed between the negative electrodes 30C and positive electrodes 10. Each layer of the power generation element 90 is impregnated with a non-aqueous electrolyte.

[0043] (Negative electrode 30C)

[0044] Figure 2This is a cross-sectional view of a negative electrode 30C according to one embodiment. The negative electrode 30C sequentially comprises a current collector layer 32, a first lithium metal layer 34 made of lithium metal foil, a second lithium metal layer 36 containing granular lithium metal 36a, and a third lithium metal layer 38 containing lithium metal and being porous. The negative electrode 30C may have the first to third lithium metal layers on a pair of main surfaces of the current collector layer 32, or it may have the first to third lithium metal layers on only one main surface of the current collector layer 32. The negative electrode 30C may have the first to third lithium metal layers on one main surface of the current collector layer 32, or it may have only the first lithium metal layer on the other main surface of the current collector layer 32.

[0045] The current collector layer 32 can be any conductive material, such as a thin sheet (metal foil) of copper, nickel, stainless steel, or alloys thereof. From the viewpoint of conductivity, copper foil is preferred. The copper foil can be rolled copper foil or other types of copper foil. The thickness of the current collector layer 32 is not limited, and can be, for example, 2 to 20 μm.

[0046] The thickness of the first lithium metal layer 34 is not particularly limited; it can be above 2 μm or below 50 μm.

[0047] The porosity of the first lithium metal layer 34 can be less than 1%, or it can be without porosity.

[0048] The second lithium metal layer 36 comprises granular lithium metal 36a. The second lithium metal layer 36 may be an aggregate of granular lithium metal 36a. A portion of adjacent granular lithium metal 36a may be bonded together. The granular lithium metal 36a may be a single crystal.

[0049] The average particle size of the granular lithium metal 36a is larger than the average pore diameter of the third lithium metal layer 38 described later. The average particle size of the granular lithium metal 36a can be 1–22 μm. The average particle size is determined by the method described in the examples described later.

[0050] The porosity of the second lithium metal layer 36 can be 3-40%. The porosity is determined by observing and measuring the area of ​​the cross-section processed by scanning electron microscopy (SEM).

[0051] The thickness of the second lithium metal layer 36 can be 1–25 μm.

[0052] The third lithium metal layer 38 contains lithium metal and is porous. The third lithium metal layer 38 does not contain the granular lithium metal contained in the second lithium metal layer 36.

[0053] The porosity of the third lithium metal layer 38 is preferably 10-40%, more preferably 15-30%. With a porosity of 10% or more, lithium ions can easily pass through. With a porosity of 40% or less, dendrite precipitation tends to be suppressed. Therefore, lithium secondary batteries with porosities within the above range tend to have better cycle characteristics. The porosity is measured using the method described in the examples below.

[0054] The average pore diameter of the third lithium metal layer 38 tends to further improve charge-discharge cycle characteristics; therefore, it is preferably 0.1 to 3 μm, more preferably 0.5 to 1 μm. With an average pore diameter of 0.1 μm or more, lithium ions can easily pass through. With an average pore diameter of 3 μm or less, dendrite precipitation tends to be suppressed. Therefore, lithium secondary batteries with an average pore diameter in the above range tend to have better cycle characteristics. The average pore diameter is measured using the method described in the examples below.

[0055] The lithium metal content in the third lithium metal layer 38 can be 80% or more by mass, 95% or more by mass, 99% or more by mass, or 100% by mass.

[0056] The thickness of the third lithium metal layer 38 can be 2–50 μm.

[0057] The area ratio S of the negative electrode 30C, obtained by the following formula (1), tends to further improve the charge-discharge cycle characteristics. Therefore, it is preferably 60% or more, more preferably 70% or more, and even more preferably 80% or more. The area ratio S is determined by the method described in the examples described later.

[0058] Area ratio S = {S3 / (S2+S3)} × 100... Equation (1)

[0059] [In formula (1), S2 is the area of ​​the second lithium metal layer on the main surface of the third lithium metal layer side of the negative electrode of the lithium secondary battery, and S3 is the area of ​​the third lithium metal layer on the main surface of the third lithium metal layer side of the negative electrode of the lithium secondary battery.]

[0060] The overall thickness of the negative electrode 30C can be set to 1–150 μm.

[0061] The negative electrode 30C may have a main body and a tab protruding from the main body.

[0062] The planar shape of the negative electrode 30C can be varied depending on the final shape of the battery. For example, the negative electrode 30C may not have a tab. In addition, the shape of the main body can be circular, etc.

[0063] (Mechanism of action)

[0064] The lithium secondary battery 100 has a second lithium metal layer 36 and a third lithium metal layer, and the average particle size of the granular lithium metal 36a is larger than the average pore diameter of the third lithium metal layer 38. Therefore, the uneven formation and growth of lithium metal nuclei on the surface of the second lithium metal layer 36 are suppressed by the third lithium metal layer. As a result, the lithium secondary battery 100 suppresses the expansion of the negative electrode and exhibits excellent cycle characteristics.

[0065] (Positive electrode 10)

[0066] The positive electrode 10 has a positive current collector 12 and a positive active material layer 14 disposed on both sides of the positive current collector 12.

[0067] The positive current collector 12 can be any conductive plate material, such as aluminum, copper, nickel, or other metal foils.

[0068] The positive electrode active material layer 14 includes a positive electrode active material, a conductive additive, and a binder.

[0069] The positive electrode active material contains an active material that can reversibly perform lithium ion adsorption and release, lithium ion detachment and insertion (intercalation), or lithium ion doping and dedoping with anti-anions.

[0070] Positive electrode active materials are, for example, composite metal oxides. Examples of composite metal oxides include lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMnO2), lithium manganese spinel (LiMn2O4), and those with the general formula: LiNi x Co y Mn z M a Compounds of O2 (in the general formula, x + y + z + a = 1, 0 ≦ x < 1, 0 ≦ y < 1, 0 ≦ z < 1, 0 ≦ a < 1, and M is one or more elements selected from Al, Mg, Nb, Ti, Cu, Zn, and Cr), lithium vanadium compounds (LiV₂O₅), olivine-type LiMPO₄ (where M represents one or more elements selected from Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, and Zr, or VO), and lithium titanate (Li₄Ti₅O₄). 12 LiNi x Co y Al z O2 (0.9 < x + y + z < 1.1). The positive electrode active material can be an organic compound. Examples of organic positive electrode active materials include polyacetylene, polyaniline, polypyrrole, polythiophene, and poly(benzo[a]benzene].

[0071] Conductive additives improve the electronic conductivity between positive electrode active materials. Examples of conductive additives include carbon powders such as carbon black, acetylene black, and Ketjen black; carbon nanotubes; carbon materials; metal powders such as copper, nickel, stainless steel, and iron; mixtures of carbon materials and metal powders; and conductive oxides such as ITO. Carbon materials such as carbon black, acetylene black, and Ketjen black are preferred conductive additives.

[0072] Adhesives bind active substances together. Adhesives can be made from known materials. Examples of adhesives are fluoropolymers. Examples of fluoropolymers include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), ethylene-tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE), and polyvinyl fluoride (PVF).

[0073] Other examples of adhesives include vinylidene fluoride-hexafluoropropylene fluororubbers (VDF-HFP), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene fluororubbers (VDF-HFP-TFE), vinylidene fluoride-pentafluoropropylene fluororubbers (VDF-PFP), vinylidene fluoride-pentafluoropropylene-tetrafluoroethylene fluororubbers (VDF-PFP-TFE), vinylidene fluoride-perfluoromethyl vinyl ether-tetrafluoroethylene fluororubbers (VDF-PFMVE-TFE), and vinylidene fluoride-trifluorochloroethylene fluororubbers (VDF-CTFE). Further examples of adhesives include cellulose, styrene-butadiene rubber, ethylene-propylene rubber, polyimide resins, polyamide-imide resins, and acrylic resins.

[0074] (Diaphragm 20)

[0075] The diaphragm 20 has electrical insulation and a porous structure. An example of the diaphragm 20 is a microporous membrane made of resin. Examples of resins include polyimide resins; polyolefin resins such as polyethylene and polypropylene. The resin can be a homopolymer, a copolymer, or a mixture of multiple polymers. The microporous membrane made of resin can be manufactured by stretching the resin membrane (dry process) or removing pore-forming agents from the resin membrane (wet process).

[0076] Other examples of the diaphragm 20 are nonwoven fabrics (paper) of various fibers. Examples of fibers include fibers of the resins mentioned above, cellulose fibers, polyester fibers, polyamide fibers, polyacrylonitrile fibers, and glass fibers.

[0077] Further examples of the diaphragm 20 are solid electrolytes. Examples of solid electrolytes include polymeric solid electrolytes, oxide-based solid electrolytes, and sulfide-based solid electrolytes.

[0078] The separator 20 can be a single layer of the aforementioned material, or it can be a stack of any two or more layers of the aforementioned material. The separator 20 may also have layers of materials other than the aforementioned materials on one or two of its main surfaces. Such materials can be inorganic or organic. Examples of inorganic materials include metal oxides such as alumina, silicon dioxide, zirconium oxide, and titanium dioxide, as well as solid electrolytes such as lithium-ion conductors (LISICON, LLZ, LLTO).

[0079] (Non-aqueous electrolyte)

[0080] A non-aqueous electrolyte is encapsulated within an outer casing 50 and immersed in a power generation element 90. The non-aqueous electrolyte may contain, for example, a non-aqueous solvent and an electrolyte. The electrolyte is dissolved in the non-aqueous solvent.

[0081] Non-aqueous solvents may contain, for example, cyclic carbonates and chain carbonates. Cyclic carbonates solubilize the electrolyte. Examples of cyclic carbonates include ethylene carbonate, propylene carbonate, and butenyl carbonate. Preferably, the cyclic carbonate contains at least propylene carbonate. Chain carbonates reduce the viscosity of cyclic carbonates. Examples of chain carbonates include diethyl carbonate, dimethyl carbonate, and methyl ethyl carbonate. In addition, non-aqueous solvents may also include methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, 1,2-dimethoxyethane, 1,2-diethoxyethane, etc.

[0082] Electrolytes can be lithium salts, such as LiPF6, LiClO4, LiBF4, LiCF3SO3, LiCF3CF2SO3, LiC(CF3SO2)3, LiN(FSO2)2, LiN(CF3SO2)2, LiN(CF3CF2SO2)2, LiN(CF3SO2)(C4F9SO2), LiN(CF3CF2CO)2, and LiBOB. A single lithium salt can be used, or two or more can be used in combination.

[0083] (External body 50)

[0084] The outer casing 50 seals the power generation element 90 and the non-aqueous electrolyte inside. The outer casing 50 prevents the non-aqueous electrolyte from leaking out to the outside and prevents moisture and other substances from entering the lithium secondary battery 100 from the outside.

[0085] Although the illustration is omitted, the outer casing 50 may have a metal foil and a resin layer laminated on both sides of the metal foil.

[0086] As a metal foil, aluminum foil can be used, for example. The resin layer can be a polymer film such as polypropylene. The materials constituting the resin layer can be different on the inside and outside. For example, as the material on the outside, a high-melting-point polymer such as polyethylene terephthalate (PET) or polyamide (PA) can be used, while as the material of the polymer film on the inside, polyethylene (PE) or polypropylene (PP) can be used.

[0087] Although not illustrated, the leads, which are typically connected to the tabs of the positive and negative electrodes, connect the inside and outside of the outer casing. The leads can be made of conductive materials such as aluminum, nickel, or nickel-plated copper. The lead connected to the positive electrode 10 side is particularly preferably made of aluminum, while the lead connected to the negative electrode 30C side is preferably made of nickel or nickel-plated copper.

[0088] In this embodiment, the two outermost layers of the power generation element 90 are set as negative electrodes 30C, but it is not limited to this. For example, the two outermost layers of the power generation element 90 can also be positive electrodes.

[0089] There is no particular limitation on the number of layers of the positive (or negative) electrode; for example, it can be 1 layer, or 10 layers, 20 layers, or more.

[0090] [Manufacturing method of lithium secondary battery]

[0091] The lithium secondary battery of this embodiment includes the following steps (a) and (b), wherein step (b) is performed more than twice.

[0092] (a) A process for preparing a lithium secondary battery precursor, the lithium secondary battery precursor having a positive electrode and a negative electrode having a current collector layer and a lithium metal foil.

[0093] (b) A process of charging the lithium secondary battery precursor at 0.01 to 0.5C until the charge rate becomes 10 to 100%, and then discharging the lithium secondary battery precursor at 0.2C to 2.5C.

[0094] (Process (a))

[0095] The lithium secondary battery precursor has a negative electrode, a positive electrode, and a separator disposed between the negative electrode and the positive electrode. These are impregnated with a non-aqueous electrolyte. The current collector layer of the negative electrode, the lithium metal foil, the positive electrode, and the separator can be made of the same materials as the lithium secondary battery 100 of the above embodiment.

[0096] (Process (b))

[0097] By performing step (b) more than twice, the second lithium metal layer and the third lithium metal layer of the above embodiment are formed.

[0098] Example

[0099] The present disclosure will now be described based on embodiments and comparative examples. However, the present disclosure is not limited to the following embodiments.

[0100] [The manufacture of lithium secondary batteries]

[0101] (Example 1)

[0102] A copper foil with a thickness of 8 μm was prepared as the current collector layer. After a lithium metal foil with a thickness of 20 μm was placed on one main surface of the current collector layer, it was cut into a specified shape to obtain a first laminate with a main body and tabs.

[0103] A positive electrode paste is coated on one side of an aluminum foil with a thickness of 15 μm, and then cut into a prescribed shape with the same main body and tabs as the negative electrode. The positive electrode paste is made by mixing a positive electrode active material, a conductive additive, a binder, and a solvent.

[0104] LiNi, as one of the composite metal oxides, was used as the positive electrode active material. x Co y Mn z M a O2 (x = 0.83, y = 0.09, z = 0.07, a = 0.01, M = Al). Carbon black was used as a conductive additive. Polyvinylidene fluoride (PVDF) was used as a binder. The mass ratio of positive electrode active material, conductive additive, and binder was set to 95:2:3. The loading of positive electrode active material in the dried positive electrode active material layer was set to 10 mg / cm³. 2 The solvent is removed from the positive electrode slurry in a drying oven to produce the positive electrode.

[0105] The first laminate and the positive electrode are alternately stacked using a 10μm thick polypropylene diaphragm, resulting in 11 layers of the first laminate and 10 layers of the positive electrode, to create the second laminate. Both outermost electrodes are constructed from the first laminate. Then, using an ultrasonic welding machine, nickel negative electrode leads are attached to the tabs of the first laminate in the second laminate, and aluminum positive electrode leads are attached to the tabs of the positive electrode in the second laminate.

[0106] Next, the second laminate is inserted into the casing and heat-sealed except for one surrounding area, thus forming a closed section. Then, a non-aqueous electrolyte is injected into the casing. The non-aqueous electrolyte is a liquid formed by adding 4 M (mol / L) LiN(FSO2)2 as a lithium salt to a 1,2-dimethoxyethane solvent. Then, the remaining section is depressurized using a vacuum sealing machine while being heat-sealed to produce a lithium secondary battery precursor.

[0107] Next, using a secondary battery charge-discharge test apparatus (manufactured by Hokuto Electric Co., Ltd.), the lithium secondary battery precursor was charged and discharged at 25°C, and the above charge-discharge cycle was performed twice. Thus, a lithium secondary battery was obtained. Charging was performed using a constant current charging method at the C rate shown in Table 1 until the state of charge (SOC) reached the value shown in Table 1. Discharging was performed using a constant current discharge at the C rate shown in Table 1 until the cell voltage reached 3.0V. The negative electrode of the obtained lithium secondary battery has a structure of a current collector layer / Li metal foil (first lithium metal layer) / a second lithium metal layer containing granular lithium metal / a third lithium metal layer containing lithium metal and being porous.

[0108] (Examples 2-10, Comparative Examples 2 and 3)

[0109] Except that the C-rate during charging, the charge rate, and the C-rate during discharging were set to the values ​​shown in Table 1 during the charge-discharge cycle, the process was carried out in the same manner as in Example 1 to obtain a lithium secondary battery. The negative electrode of the obtained lithium secondary battery has a structure of current collector layer / Li metal foil (first lithium metal layer) / second lithium metal layer containing granular lithium metal / third lithium metal layer containing lithium metal and being porous.

[0110] (Comparative Example 1)

[0111] Except that the number of charge-discharge cycles was set to one, the process was carried out in the same manner as in Example 1 to obtain a lithium secondary battery. The negative electrode of the obtained lithium secondary battery has a structure of current collector layer / Li metal foil (first lithium metal layer) / third lithium metal layer containing lithium metal and being porous / second lithium metal layer containing granular lithium metal.

[0112] [evaluate]

[0113] <Examples 1-10, Comparative Examples 1-3>

[0114] (Cyclic characteristics)

[0115] The rated capacity retention of lithium secondary batteries after cycling was determined using a secondary battery charge-discharge test apparatus (manufactured by Hokuto Electric Co., Ltd.). At 25°C, the batteries were charged at a constant current and constant voltage of 0.2C to 4.3V and discharged at a constant current of 1C to 3.0V. This charge-discharge cycle was defined as one cycle, and the number of cycles required to reduce the capacity to below 80% of the initial capacity was measured. The results are shown in Table 1.

[0116] (Thickness variation of the negative electrode)

[0117] The lithium-ion secondary battery after 100 cycles was disassembled, and the thickness change of the negative electrode was measured. The thickness change ratio was calculated as ("thickness of the negative electrode after 100 cycles" - "thickness of the negative electrode before the first charge") / ("thickness of the negative electrode before the first charge") × 100. The results are shown in Table 1.

[0118] (Average pore diameter of the third lithium metal layer)

[0119] The main surface of the third lithium metal layer of the negative electrode of a lithium-ion secondary battery was observed (magnification: 2000x) using a scanning electron microscope (Hitachi High Technology Co., Ltd., trade name "SU8010"). Images were captured. The images were binarized based on hue, thereby identifying voids and areas outside the third lithium metal layer. Voids were extracted from the images, and the diameters of all voids were measured. The average diameter was taken as the mean void diameter. The results are shown in Table 1.

[0120] (Porosity of the third lithium metal layer)

[0121] The main surface of the third lithium metal layer of the negative electrode of a lithium-ion secondary battery was observed and images were captured using a scanning electron microscope (Hitachi High Technology Co., Ltd., trade name "SU8010") (magnification: 2000x). The obtained images were binarized based on brightness, thereby identifying voids and areas outside the third lithium metal layer. The porosity was determined based on the binarized images. The results are shown in Table 1.

[0122] (Average particle size of the second lithium metal layer)

[0123] The cross-section of the second lithium metal layer of the negative electrode of a lithium-ion secondary battery was observed (magnification: 5000x) using a scanning electron microscope (Hitachi High Technology Co., Ltd., trade name "SU8010"). Images were taken. The obtained images were binarized based on brightness to identify the particles in the second lithium metal layer. The particle size of all particles in the images was measured, and the average value was used as the mean particle size. The results are shown in Table 1.

[0124] (Area ratio S)

[0125] The main surface of the third lithium metal layer side of the negative electrode of a lithium secondary battery was observed (magnification: 500x) using a scanning electron microscope (manufactured by Hitachi High Technology Co., Ltd., trade name "SU8010"). Images were taken. The obtained images were binarized based on brightness to identify the second and third lithium metal layers. Based on the binarized images, the areas S2 of the second lithium metal layer and S3 of the third lithium metal layer when viewed from above were calculated. The area ratio S obtained by the following formula (1) was calculated based on S2 and S3. The results are shown in Table 1.

[0126] Area ratio S = {S3 / (S2+S3)} × 100 ··· Equation (1)

[0127] [In formula (1), S2 is the area of ​​the second lithium metal layer on the main surface of the third lithium metal layer side of the negative electrode of the lithium secondary battery, and S3 is the area of ​​the third lithium metal layer on the main surface of the third lithium metal layer side of the negative electrode of the lithium secondary battery.]

[0128] [Table 1]

[0129]

[0130] Figure 3 (a) is a SEM image (magnification: 5000x) of a portion of the cross-section of the negative electrode of the lithium secondary battery obtained in the embodiment. It can be seen that a second lithium metal layer containing granular lithium metal and a porous third lithium metal layer are formed. Figure 3 (b) is a SEM image (magnification: 2000x) of the main surface of the third lithium metal layer side of the negative electrode of the lithium secondary battery obtained in the embodiment. It can be seen that a second lithium metal layer containing granular lithium metal and a porous third lithium metal layer are formed. Figure 3 (c) is a SEM image (magnification: 500x) of the main surface of the third lithium metal layer side of the negative electrode of the lithium secondary battery obtained in the embodiment. It can be seen that there are a second lithium metal layer 36 and a third lithium metal layer 38.

[0131] Explanation of reference numerals in the attached figures

[0132] 10…positive electrode, 30C…negative electrode, 32…current collector layer, 34, 36, 38…lithium metal layer, 36a…granular lithium metal, 100…lithium secondary battery.

Claims

1. A lithium secondary battery, comprising a negative electrode and a positive electrode, wherein, The negative electrode has the following characteristics in sequence: Current collector layer; The first lithium metal layer is composed of lithium metal foil; A second lithium metal layer comprising granular lithium metal; and The third lithium metal layer contains lithium metal and is porous. The average particle size of the granular lithium metal is greater than the average pore diameter of the third lithium metal layer.

2. The lithium secondary battery according to claim 1, wherein, The porosity of the third lithium metal layer is 10-40%.

3. The lithium secondary battery according to claim 1 or 2, wherein, The average pore diameter of the third lithium metal layer is 0.1–3 μm.

4. The lithium secondary battery according to any one of claims 1 to 3, wherein, The average particle size of the granular lithium metal is 1–22 μm.

5. The lithium secondary battery according to any one of claims 1 to 4, wherein, The area ratio S obtained by the following formula (1) is 60% or more. Area ratio S = {S3 / (S2+S3)} × 100 ··· Equation (1) In formula (1), S2 is the area of ​​the second lithium metal layer on the main surface of the third lithium metal layer side of the negative electrode, and S3 is the area of ​​the third lithium metal layer on the main surface of the third lithium metal layer side of the negative electrode.

6. A method for manufacturing a lithium secondary battery, comprising: (a) A step of preparing a lithium secondary battery precursor, said lithium secondary battery precursor having a positive electrode and a negative electrode having a current collector layer and a lithium metal foil; and (b) A step of charging the lithium secondary battery precursor at 0.01 to 0.5C until the charge rate becomes 10 to 100%, and then discharging the lithium secondary battery precursor at 0.2C to 2.5C. Perform process (b) more than twice.

Images