Lithium secondary battery and method for manufacturing same

Abstract

A lithium secondary battery provided with a negative electrode, a positive electrode, and a separator disposed between the negative electrode and the positive electrode, the negative electrode having a negative electrode current collector, a lithium metal layer comprising lithium metal, and a mixed layer containing lithium metal and a second metal other than the lithium metal, the mixed layer being located between the lithium metal layer and the separator.

Description

Technical Field

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

[0002] Lithium-ion batteries using metallic lithium as the negative electrode have high energy density and are attracting much attention as power sources for mobile devices such as mobile phones and laptops, as well as for large-scale power sources such as bicycle power supplies. Lithium metal has an extremely low electrical potential; therefore, there is a high expectation that lithium-ion batteries can achieve high theoretical capacity density.

[0003] Lithium-ion batteries using metallic lithium as the negative electrode undergo charging and discharging through the deposition and dissolution of metallic lithium. During charging, metallic lithium is deposited at the negative electrode, and during discharging, it dissolves. Sometimes, during charging, metallic lithium deposits in a dendritic pattern, with the deposition starting point as the root; this deposit is also called dendrites. During discharging, if the root of the dendrite dissolves first, the metallic lithium is ionized from the negative electrode. This ionized lithium cannot contribute to subsequent charging and discharging, thus reducing the cycle characteristics of the lithium-ion battery.

[0004] For example, Patent Document 1 discloses a method that suppresses the localized concentration of lithium metal deposition on the electrode surface by uniformly attaching metal powder, which is intended to become the core for lithium metal deposition, to the surface of the lithium metal electrode in advance, thereby suppressing the dendritic deposition of lithium metal.

[0005] Existing technical documents

[0006] Patent documents

[0007] Patent Document 1: Japanese Patent Application Publication No. 5-234585 Summary of the Invention

[0008] The technical problem the invention aims to solve

[0009] However, although the localized deposition of lithium metal can be suppressed, the method described in Patent Document 1 cannot change the deposition of lithium metal on the surface of the lithium metal electrode. Therefore, during repeated charge-discharge cycles, a low-density lithium metal deposition layer gradually accumulates, and the thickness of the negative electrode increases.

[0010] This disclosure was made in view of the above-mentioned technical problems, and its purpose is to provide a lithium secondary battery with minimal thickness change even after repeated charge-discharge cycles and a method for manufacturing the same.

[0011] Means for solving technical problems

[0012] [1] A lithium secondary battery, wherein,

[0013] It comprises: a negative electrode, a positive electrode, and a separator disposed between the negative electrode and the positive electrode.

[0014] The negative electrode comprises: a negative electrode current collector, a lithium metal layer composed of lithium metal, and a mixed layer containing lithium metal and a second metal other than lithium metal.

[0015] The hybrid layer is located between the lithium metal layer and the separator.

[0016] [2] According to the lithium secondary battery described in [1], wherein,

[0017] The hybrid layer is a layer in which particles containing the second metal are dispersed in the lithium metal.

[0018] [3] According to the lithium secondary battery described in [2], wherein,

[0019] The average diameter of the particles is 10–500 nm.

[0020] [4] A lithium secondary battery according to any one of [1] to [3], wherein,

[0021] The area occupied by the second metal relative to 100% of the cross-sectional area of ​​the hybrid layer is 5.0 to 60% of the area.

[0022] [5] A lithium secondary battery according to any one of [1] to [4], wherein,

[0023] The second metal contains at least one selected from copper, silver, gold, aluminum, bismuth, iron, gallium, germanium, indium, magnesium, niobium, nickel, lead, palladium, platinum, silicon, tin, titanium, zinc, and zirconium oxide.

[0024] [6] A lithium secondary battery according to any one of [1] to [5], wherein,

[0025] The thickness of the hybrid layer is 0.10–5.0 μm.

[0026] [7] A lithium secondary battery according to any one of [1] to [6], wherein,

[0027] There is a lithium metal deposition layer between the mixed layer and the lithium metal layer.

[0028] [8] A lithium secondary battery according to any one of [1] to [7], wherein,

[0029] The outermost layer of the diaphragm side of the hybrid layer is composed of metallic lithium.

[0030] [9] A method for manufacturing a lithium secondary battery, comprising manufacturing a lithium secondary battery as described in any one of [1] to [8], wherein,

[0031] The formation process of the negative electrode has the following characteristics:

[0032] The process of preparing the lithium metal layer; and

[0033] The process of forming a mixed layer containing lithium metal and a second metal other than lithium metal on the lithium metal layer.

[0034] The effects of the invention

[0035] According to this disclosure, a lithium secondary battery with minimal thickness change even after repeated charge-discharge cycles and a method for manufacturing the same can be provided. Attached Figure Description

[0036] Figure 1 This is a schematic cross-sectional view of a lithium secondary battery according to one embodiment of the present disclosure.

[0037] Figure 2 This is a schematic cross-sectional view of the negative electrode of one embodiment of this disclosure.

[0038] Figure 3A It is a SEM image of a portion of the cross-section of the negative electrode of the lithium secondary battery obtained in Example 1.

[0039] Figure 3B The SEM image of a portion of the cross-section of the negative electrode of the lithium secondary battery obtained in Example 1 is taken in such a way that the contrast of the second metal in the hybrid layer becomes clear.

[0040] Figure 3C This is a SEM image of a portion of the cross-section of the negative electrode of the lithium secondary battery obtained in Comparative Example 1.

[0041] Symbol Explanation

[0042] 100……Lithium secondary battery, 10……Separator, 20……Positive electrode, 21……Positive electrode current collector, 22……Positive electrode active material layer, 30……Negative electrode, 31……Negative electrode current collector, 32……Negative electrode active material layer, 33……Lithium metal layer, 34……Mixed layer, 34a……Lithium metal, 34b……Particles, 34c……Outermost layer, 35……Deposition layer, 40……Laminated body, 50……Outer body, 52……Metal foil, 54……Resin layer, 60……Terminal, 62……Terminal. Detailed Implementation

[0043] The embodiments of this disclosure (hereinafter referred to as "this embodiment") will be described in detail below, but this disclosure is not limited thereto, and various modifications can be made without departing from its spirit. In addition, for ease of illustration and understanding, the scale and aspect ratio of the accompanying drawings may be adjusted as appropriate, resulting in differences or exaggerations from the actual object.

[0044] 1. Lithium secondary battery

[0045] exist Figure 1 A schematic cross-sectional view of the lithium secondary battery according to this embodiment is shown. Figure 1 As shown, the lithium secondary battery 100 includes a negative electrode 30, a positive electrode 20, and a separator 10 disposed between the negative electrode 30 and the positive electrode 20. The negative electrode 30, the positive electrode 20, and the separator 10 can also be housed together with the electrolyte (not shown) within an outer casing 50 in a laminated state 40. Furthermore, in Figure 1 The diagram shows a laminate 40 in which a membrane 10 is disposed between the negative electrode 30 and the positive electrode 20. Alternatively, the laminate 40 may also be a multilayer structure in which the negative electrode 30 and the positive electrode 20 are alternately disposed, and a membrane 10 is disposed between the negative electrode 30 and the positive electrode 20.

[0046] In lithium-ion batteries, the lithium metal layer is dissolved and deposited repeatedly through charge-discharge cycles. Furthermore, the uneven deposition of lithium metal due to repeated charge-discharge cycles typically promotes the expansion of the negative electrode. Additionally, as charge-discharge cycles continue, some of the deposited lithium metal transforms into lithium powder, further promoting the expansion of the negative electrode. Moreover, since the deposited lithium metal forms a low-density layer with gaps, similar to dendrites, this also contributes to the expansion of the negative electrode.

[0047] In contrast, the negative electrode 30 of this embodiment has a negative current collector 31, a lithium metal layer 33 made of lithium metal, and a mixed layer 34 containing lithium metal and a second metal other than lithium metal, such as Figure 2 As shown, the hybrid layer 34 has a structure located between the lithium metal layer 33 and the separator 10.

[0048] Therefore, during charging, the lithium secondary battery 100 of this embodiment can uniformly deposit lithium metal in the in-plane direction at multiple deposition start points between the lithium metal layer 33 and the mixed layer 34. Furthermore, during charging, the mixed layer 34 can control the uniform deposition and growth of lithium metal in the thickness direction, suppressing the formation of voids while allowing the deposited layer 35 to grow in the thickness direction. As a result, during charging, the lithium metal deposited layer 35 between the lithium metal layer 33 and the mixed layer 34 becomes a higher density layer with fewer voids. Therefore, the thickness change of the negative electrode 30 caused by charging and discharging can be suppressed. Moreover, during discharging, the lithium metal deposited between the lithium metal layer 33 and the mixed layer 34 dissolves.

[0049] As described above, the lithium secondary battery 100 of this embodiment, by having a defined hybrid layer 34, can suppress thickness changes even after repeated charge-discharge cycles. The structure of the lithium secondary battery 100 of this embodiment will be described in detail below.

[0050] 1.1. Negative electrode

[0051] exist Figure 2 The diagram shows a schematic cross-sectional view of the negative electrode. The negative electrode 30 has a negative electrode current collector 31 and a negative electrode active material layer 32 disposed on one side of the negative electrode current collector 31. The planar shape of the negative electrode 30 can be various shapes depending on the shape of the final battery. Furthermore, unless otherwise specified, in this embodiment, the term "cross-section" refers to the side orthogonal to the plate of the negative electrode current collector 31, in other words, the side parallel to the thickness direction of the negative electrode active material layer 32.

[0052] As the negative current collector 31, there are no particular limitations as long as it is a conductive substrate; examples include aluminum, copper, nickel, and stainless steel foils. Copper foil is preferred. By using this negative current collector 31, conductivity is tended to be further improved.

[0053] In addition, such as Figure 2 As shown, the negative electrode active material layer 32 of this embodiment has a lithium metal layer 33 and a mixed layer 34. If necessary, a lithium metal deposition layer 35 may also be provided between the mixed layer 34 and the lithium metal layer 33.

[0054] The negative electrode active material layer 32 can be disposed on one of the two main surfaces of the negative electrode current collector 31, or it can be disposed on both of the two main surfaces, depending on the structure of the laminate 40. The layers constituting the negative electrode active material layer 32 will be described in detail below.

[0055] The thickness of the negative electrode current collector 31 is preferably 2.0–20 μm or 5.0–15 μm. The overall thickness of the negative electrode 30 is preferably 1.0–100 μm, 5.0–75 μm or 10–50 μm.

[0056] 1.1.1. Lithium Metal Layer

[0057] The lithium metal layer 33 is a layer composed of lithium metal and is responsible for the redox reactions that accompany the charging and discharging of the lithium secondary battery. During charging, lithium metal is deposited between the lithium metal layer 33 and the mixed layer 34, and more specifically on the surface of the lithium metal layer 33; during discharging, the lithium metal deposited between the lithium metal layer 33 and the mixed layer 34, and more specifically on the surface of the lithium metal layer 33, dissolves.

[0058] The lithium metal layer 33 can be grounded on the negative electrode current collector 31. As needed, any other layer can also be formed between the lithium metal layer 33 and the negative electrode current collector 31.

[0059] The lithium metal layer 33 is composed of lithium metal and preferably contains virtually no other metals or organic compounds. Furthermore, the porosity of the lithium metal layer 33 can be less than 1%, or it can be free of voids. In this embodiment, the porosity can be calculated based on the area of ​​the lithium metal layer 33 as confirmed by observing the negative electrode cross-section using SEM or similar methods, and the area of ​​the voids contained within the lithium metal layer 33. There are no particular limitations on this lithium metal layer 33; for example, lithium foil can be used.

[0060] The lithium content in the lithium metal layer 33 relative to the total amount of the lithium metal layer 33 is preferably 95-100% by mass, 98-100% by mass, or 99-100% by mass.

[0061] The thickness of the lithium metal layer 33 is preferably 1.0–50 μm, 2.5–4 μm, or 5.0–30 μm.

[0062] 1.1.2. Hybrid Layer

[0063] The mixed layer 34 contains lithium metal and a second metal other than lithium metal. The mixed layer 34 is located between the lithium metal layer 33 and the separator 10. Compared to the lithium metal layer 33, the mixed layer 34 lacks electronic conductivity, and its adhesion to the lithium metal layer 33 is also relatively weak. Therefore, during charging, lithium metal is deposited on the surface of the lithium metal layer 33, which has a more favorable electronic conductivity than the surface of the mixed layer 34, and the interface between the mixed layer 34 and the lithium metal layer 33 becomes more prone to dynamic changes.

[0064] Therefore, during charging, the deposited layer 35 grows in such a way that the mixed layer 34 rises from the surface of the lithium metal layer 33. Furthermore, the deposited layer 35 grown therein, as described above, is a high-density lithium metal layer with few voids. Additionally, during discharging, the deposited layer 35 thus formed dissolves. Therefore, even with repeated charge-discharge cycles, thickness variations can be reduced.

[0065] Examples of mixed layer 34 include a dispersion of one of lithium metal and the second metal in the other, a solid solution formed by lithium metal and the second metal, and an intermetallic compound formed by lithium metal and the second metal. Among these, for example... Figure 2 As shown, the hybrid layer 34 is preferably a layer in which particles 34b containing a second metal are dispersed in lithium metal 34a. This tends to further reduce the change in thickness after repeated charge-discharge cycles.

[0066] The second metal is not particularly limited, and examples include at least one selected from copper, silver, gold, aluminum, bismuth, iron, gallium, germanium, indium, magnesium, niobium, nickel, lead, palladium, platinum, silicon, tin, titanium, zinc, and zirconium oxide. Furthermore, the second metal can be a single metal or a composite of these metals. Copper and nickel are preferred, with copper being more preferred.

[0067] The hybrid layer 34 is expected to have the following strength: it controls the penetration of the deposited layer 35, preventing the deposited layer 35 from precipitating out in a way that raises the hybrid layer 34, so that the deposited layer 35 becomes a high-density lithium metal layer with few voids. In this respect, because the second metal is unlikely to form an interlayer compound between lithium and the second metal, the generation of voids that accompany the formation of interlayer compounds, which is the main cause of the decrease in the physical strength of the hybrid layer 34, can be suppressed. In addition, because lithium and copper as the second metal can be dissolved in solid solution, insufficient adhesion between lithium and copper as the second metal caused by complete insolubility can be avoided. That is, by using the second metal, a hybrid layer 34 with appropriate physical properties can be formed, and therefore, there is a tendency to further reduce the thickness change after repeated charge-discharge cycles.

[0068] The average diameter of the particles 34b containing the second metal is preferably 10–500 nm, 15–450 nm, 20–400 nm, 25–350 nm, 30–300 nm, 35–250 nm, 40–200 nm, 45–150 nm, or 50–100 nm. By having an average diameter in the range of 500 nm or above, the gaps between the particles 34b are reduced, which can suppress the formation of a lithium metal through-layer 34 grown in the deposition layer 35.

[0069] Here, the aforementioned average diameter refers to the diameter of the cross-section of particle 34b, as confirmed by observation of the negative electrode cross-section using a scanning electron microscope (SEM) or similar method. In the distribution of diameters obtained from the cross-section, when divided in two from a certain diameter, the larger and smaller sides have diameters of equal magnitude. Furthermore, if the cross-section of particle 34b is not circular, the diameter can also be measured as the equivalent circular diameter. Here, the equivalent circular diameter refers to the diameter of a circle with an area equal to that of particle 34b.

[0070] The thickness of the mixing layer 34 is preferably 0.10–5.0 μm, 0.20–4.5 μm, 0.30–4.0 μm, 0.30–3.5 μm, 0.40–3.0 μm, 0.50–2.5 μm, 0.60–2.0 μm, or 0.80–1.5 μm. When the thickness of the mixing layer 34 is 0.10 μm or more, it is possible to suppress the penetration of lithium metal grown in the deposition layer 35 through the mixing layer 34. Furthermore, when the thickness of the mixing layer 34 is 5.0 μm or less, lithium ions in the electrolyte can easily diffuse through the mixing layer 34, and there is a tendency for lithium metal to be deposited uniformly and easily.

[0071] The thickness of the mixed layer 34 relative to the thickness of the lithium metal layer 33 is preferably 25% or less, 20% or less, 15% or less, 1.0 to 12.5%, 2.0 to 10%, or 3.0 to 7.5%. When the thickness of the mixed layer 34 is 1.0% or more relative to the thickness of the lithium metal layer 33, it is possible to suppress the penetration of lithium metal grown in the deposition layer 35 through the mixed layer 34. Furthermore, when the thickness of the mixed layer 34 is 25% or less relative to the thickness of the lithium metal layer 33, lithium ions in the electrolyte can easily diffuse through the mixed layer 34, and there is a tendency for lithium metal to be deposited uniformly and easily.

[0072] Furthermore, regarding the thickness of the hybrid layer 34, in a cross-sectional photograph of the negative electrode active material layer 32 of the negative electrode 30, the width of the hybrid layer 34 can be defined by taking a line hanging down from any point on the outermost surface of the hybrid layer 34 toward the negative electrode current collector 31 as a reference line and the two points where the reference line intersects the upper and lower surfaces of the hybrid layer 34.

[0073] The area occupied by particles 34b relative to 100% of the cross-sectional area of ​​the mixed layer 34 is preferably 5.0–60%, 7.5–55%, 10–50%, 15–45%, 20–40%, or 25–35%. Because the area occupied by particles 34b is within the above range, there is a tendency for minimal change in thickness even after repeated charge-discharge cycles.

[0074] The area occupied by particles 34b can be calculated based on the area of ​​the mixed layer 34 confirmed when observing the cross-section of the negative electrode using SEM or similar methods, and the area of ​​the cross-section of particles 34b contained in the mixed layer 34. Furthermore, lithium metal and the second metal are observed with different contrasts in electron microscope images because they are different metal types.

[0075] The outermost layer 34c on the separator 10 side of the mixed layer 34 is preferably made of lithium metal. That is, the particles 34b of the second metal that become the nucleation site for lithium metal deposition are embedded in the negative electrode 30 and are not attached to the surface of the negative electrode 30. As a result, lithium metal deposition on the surface of the mixed layer 34 can be suppressed.

[0076] 1.1.3. Precipitation Layer

[0077] The deposited layer 35 is located between the mixed layer 34 and the lithium metal layer 33. It is mainly formed by the deposition of lithium metal on the lithium metal layer 33 through charging, or it can be a layer remaining after the lithium metal deposited through discharging has not completely dissolved. In this embodiment, during charging, lithium ions in the electrolyte diffuse through the mixed layer 34, depositing lithium metal on the lithium metal layer 33. Through the mixed layer 34, a high-density lithium metal layer with few voids can be formed as the deposited layer 35.

[0078] Typically, the potential of the front end of the deposited lithium metal is negatively lower than the reference potential of lithium metal, leading to the deposition and growth of lithium metal at the front end. As a result, dendrites or a deposited layer with many pores are formed. In this embodiment, when the front end of the deposited lithium metal is in contact with the mixing layer 34, the potential of the front end of the deposited lithium metal rises to the normal reference potential of lithium metal due to the conductivity of the mixing layer 34 containing lithium metal.

[0079] As a result, the precipitation growth at the leading edge of the lithium metal stops, suppressing anisotropic growth, and the precipitation layer 35 further grows isotropically. Therefore, in this embodiment, a high-density precipitation layer 35 with few voids can be formed. In addition, the precipitation layer 35 formed in this way has a low specific surface area, making it less prone to the generation of decomposition products due to the breakage of the deposited lithium metal. As a result, even with repeated charge-discharge cycles, thickness changes can be reduced.

[0080] The lithium metal content in the precipitated layer 35 is preferably 95-100% by mass, 98-100% by mass, or 99-100% by mass relative to the total amount of the precipitated layer 35.

[0081] The porosity of the precipitated layer 35 may be less than 1%, or it may be free of voids. The porosity can be calculated based on the area of ​​the precipitated layer 35 as confirmed by observing the negative electrode cross-section using SEM or similar methods, and the area of ​​voids contained in the precipitated layer 35.

[0082] In addition, both the deposited layer 35 and the lithium metal layer 33 are layers composed of high-density lithium metal. For example, the negative electrode cross section can be distinguished by the patterns obtained by X-ray diffraction, infrared spectroscopy analysis, or Raman spectroscopy analysis.

[0083] In this embodiment, during charging, lithium ions pass through the mixing layer 34 in an amount equal to the amount of lithium metal deposited in the deposited layer 35, forming the deposited layer 35 by pushing it upwards. During discharging, the deposited layer 35 dissolves and thins, allowing lithium ions to pass through the mixing layer 34. Thus, the thickness of the deposited layer 35 varies due to discharging and cannot be uniquely defined; for example, it can vary within the range of 1 to 50 μm. Furthermore, from this viewpoint, the thickness of the deposited layer 35 relative to the thickness of the mixing layer 34 is preferably 1 (0.01 μm) to 1000% (100 μm), or 5 to 750%, and can also vary within the range of 10 (0.1 μm) to 500% (50 μm).

[0084] 1.2. Positive electrode

[0085] The positive electrode 20 has a positive current collector 21 and a positive active material layer 22 disposed on one side of the positive current collector 21. Depending on the structure of the laminate 40, the positive active material layer 22 may be disposed on one side of the positive current collector 21 or on both sides.

[0086] As the positive current collector 21, there are no particular limitations as long as the plate material is conductive. For example, aluminum, copper, nickel, stainless steel and other metal foils can be used.

[0087] The positive electrode active material layer 22 contains positive electrode active material, and may also contain positive electrode conductive additives and positive electrode binders as needed.

[0088] As a positive electrode active material, an active material capable of reversibly performing lithium ion adsorption and release, lithium ion desorption and insertion (intercalation), or lithium ion and lithium ion anti-anion doping and dedoping can be used.

[0089] There are no particular limitations on the active material used as this positive electrode; examples include lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMnO2), lithium manganese spinel (LiMn2O4), and LiNi x Co y Mn z M a O2 compounds (where 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), lithium titanate (Li₄Ti₅O₄). 12 LiNi x Co y Al z O2 (where 0.9 < x + y + z < 1.1) and other composite metal oxides; organic compounds such as polyacetylene, polyaniline, polypyrrole, polythiophene, and polyacene.

[0090] From the viewpoint of improving the electronic conductivity between the positive electrode active materials, conductive additives for the positive electrode can also be added. There are no particular limitations on such conductive additives; examples include carbon powders such as carbon black, acetylene black, and Ketjen black, as well as carbon materials such as carbon nanotubes; metal powders such as copper, nickel, stainless steel, and iron; mixtures of carbon materials and metal powders; and conductive oxides such as ITO. Among these, carbon materials such as carbon black, acetylene black, and Ketjen black are preferred as conductive additives for the positive electrode. By using such conductive additives, there is a tendency to further improve the electronic conductivity between the positive electrode active materials.

[0091] From the viewpoint of forming the positive electrode active material layer 22 by bonding the positive electrode active materials together, a positive electrode binder can also be added. There are no particular limitations on such a positive electrode binder; examples include fluoropolymers such as 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); other examples of binders include vinylidene fluoride-hexafluoropropylene fluororubber (VDF-HFP fluororubber) and vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene fluororubber (VD... Fluororubbers based on vinylidene fluoride (F-HFP-TFE), vinylidene fluoride-pentafluoropropylene (VDF-PFP), vinylidene fluoride-pentafluoropropylene-tetrafluoroethylene (VDF-PFP-TFE), vinylidene fluoride-perfluoromethyl vinyl ether-tetrafluoroethylene (VDF-PFMVE-TFE), vinylidene fluoride-trifluorochloroethylene (VDF-CTFE), etc.; cellulose, styrene-butadiene rubber, ethylene-propylene rubber, polyimide resin, polyamide-imide resin, acrylic resin, and other resins.

[0092] 1.3. Diaphragm

[0093] A separator 10 is disposed between the positive electrode 20 and the negative electrode 30, thereby preventing short circuits between the positive electrode 20 and the negative electrode 30 by isolating them. From this perspective, the separator 10 can also have a shape that extends in-plane along the positive electrode 20 and the negative electrode 30. Furthermore, lithium ions can pass through the separator 10.

[0094] The membrane 10 is not particularly limited to any currently known type of membrane; for example, it can be a microporous resin membrane or nonwoven fabric with electrical insulation and a porous structure, or a solid electrolyte. The membrane 10 can be a single layer or a laminate of these materials.

[0095] The resin constituting the microporous resin membrane is not particularly limited; examples include polyimide resins, polyolefin resins such as polyethylene and polypropylene, etc. The method of forming pores in the microporous resin membrane is not particularly limited; for example, pores can be formed by stretching the resin membrane, or by removing a pore-forming agent from the resin membrane. Furthermore, the microporous resin membrane can be a monolayer or a laminate.

[0096] Furthermore, there are no particular limitations on the fibers that constitute nonwoven fabrics; examples include polyolefin fibers, polyimide fibers, cellulose fibers, polyester fibers, polyamide fibers, polyacrylonitrile fibers, and glass fibers.

[0097] There are no particular limitations on solid electrolytes. Examples include polymeric solid electrolytes, oxide solid electrolytes such as LLZ and LLTO, and sulfide solid electrolytes such as LISICON.

[0098] Furthermore, the separator 10 may also have a layer containing a material other than the aforementioned material on one or both of its main surfaces. There are no particular limitations on such materials; for example, inorganic materials such as alumina, silicon dioxide, zirconium oxide, and titanium dioxide, and organic materials such as polyvinylidene fluoride and carboxymethyl cellulose can be included. By having such a layer, heat resistance is further improved, and the tendency to deposit transition metals dissolved from the positive electrode 20 onto the surface of the negative electrode 30 is suppressed.

[0099] 1.4. Electrolyte

[0100] Electrolytes can also contain both non-aqueous solvents and electrolytes. Electrolytes can dissolve in non-aqueous solvents.

[0101] As a non-aqueous solvent, there are no particular limitations; examples include cyclic carbonates, chain carbonates, and other organic solvents. Cyclic carbonates have the effect of solubilizing electrolytes, while chain carbonates have the effect of reducing the viscosity of cyclic carbonates. One non-aqueous solvent can be used alone, or two or more can be used in combination.

[0102] The cyclic carbonate is not particularly limited, and examples include ethylene carbonate, propylene carbonate, butene carbonate, fluoroethylene carbonate, and vinylene carbonate. Among them, it is preferred that it contains at least propylene carbonate.

[0103] As a chain carbonate, there are no particular limitations; examples include diethyl carbonate, dimethyl carbonate, and methyl ethyl carbonate.

[0104] Other organic solvents are not particularly limited, but can include chain esters such as methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, and propyl propionate; cyclic esters such as γ-butyrolactone; and chain ethers such as 1,2-dimethoxyethane and 1,2-diethoxyethane.

[0105] There are no particular limitations on the electrolyte; examples include lithium salts such as LiPF6, LiClO4, LiBF4, LiCF3SO3, LiCF3CF2SO3, LiC(CF3SO2)3, LiN(CF3SO2)2, LiN(CF3CF2SO2)2, LiN(CF3SO2)(C4F9SO2), LiN(CF3CF2CO)2, LiBOB, and LiN(FSO2)2. One electrolyte can be used alone, or two or more can be used in combination.

[0106] 1.5.Terminal

[0107] Terminals 60 and 62 are connected to the positive terminal 20 and the negative terminal 30, respectively, connecting the inside and outside of the outer casing 50. Terminal 60, connected to the positive terminal 20, is the positive terminal, and terminal 62, connected to the negative terminal 30, is the negative terminal. Terminals 60 and 62 provide electrical connection to the outside.

[0108] The materials used to construct terminals 60 and 62 are not particularly limited; conductive materials such as aluminum, nickel, and copper can be used. For example, terminal 60, which is connected to the positive electrode 20, could be made of an aluminum plate, and terminal 62, which is connected to the negative electrode 30, could be made of a nickel plate or a metal plate made of copper plated with nickel. To prevent short circuits, it is preferable to protect terminals 60 and 62 with insulating tape.

[0109] 1.6. Exterior body

[0110] The outer casing 50 encapsulates the negative electrode, positive electrode, separator, and electrolyte. The outer casing 50 prevents non-aqueous electrolyte from leaking out and moisture from the outside from entering the lithium secondary battery 100.

[0111] The structure of the outer casing 50 is not particularly limited, such as Figure 1 As shown, it may also have a metal foil 52 and resin layers 54 laminated on each side of the metal foil 52.

[0112] There are no particular limitations on the metal foil 52; for example, aluminum foil can be used. Similarly, there are no particular limitations on the resin layer 54; for example, a polymer film such as polypropylene can be used. The materials constituting the resin layer 54 can also be different on the inner and outer sides. For example, a high-melting-point polymer such as polyethylene terephthalate (PET) or polyamide (PA) can be used as the outer material, while polyethylene (PE) or polypropylene (PP) can be used as the material of the polymer film on the inner side.

[0113] 2. Manufacturing method of lithium secondary batteries

[0114] The method for manufacturing a lithium secondary battery according to this embodiment includes a preparation step for preparing a lithium metal layer and a mixed layer forming step for forming a mixed layer containing lithium metal and a second metal other than lithium metal on the lithium metal layer as the negative electrode 30 forming step. In addition, the positive electrode 20 forming step, the laminate 40 fabrication step in which the positive electrode 20, the negative electrode 30 and the separator 10 are stacked, and the laminate 40 and the electrolyte external package 50 sealing step can be performed using currently known methods.

[0115] 2.1. Formation process of the negative electrode

[0116] The negative electrode formation process includes a preparation step for preparing a lithium metal layer and a mixed layer formation step for forming a mixed layer containing lithium metal and a second metal other than lithium metal on the lithium metal layer. In the negative electrode formation process, the mixed layer formation step can be performed while the lithium metal layer 33 is stacked on the negative electrode current collector 31, or the mixed layer formation step can be performed on the prepared lithium metal layer 33, and then the lithium metal layer 33 with the mixed layer 34 formed is bonded to the negative electrode current collector 31.

[0117] 2.1.1. Preparation process

[0118] The preparation process is the process of preparing the lithium metal layer 33. Specifically, the lithium metal layer 33 can be prepared by bonding the lithium foil to the negative electrode current collector 31, or the lithium foil before bonding to the negative electrode current collector 31 can be prepared as the lithium metal layer 33.

[0119] 2.1.2. Mixed Layer Formation Process

[0120] The mixed layer formation process is a process of forming a mixed layer 34 containing lithium metal and a second metal other than lithium metal on the lithium metal layer 33.

[0121] The method for forming the mixed layer 34 is not particularly limited. Examples include electrolytically or chemically depositing a second metal onto the lithium metal layer 33 followed by annealing; physically or chemically depositing a second metal onto the lithium metal layer 33 followed by annealing; or attaching particles containing the second metal to the lithium metal layer 33 and annealing under pressure. Among these methods, by attaching particles containing the second metal to the lithium metal layer 33 and annealing under pressure, a mixed layer 34 in which particles containing the second metal are dispersed in the lithium metal can be formed.

[0122] The preferred annealing temperature is 120–210°C, 140–200°C, 150–185°C, or 160–175°C. This allows particles containing the second metal to be embedded in the surface portion of the heated lithium metal layer 33. The annealing time is not particularly limited; for example, it can be 10–120 minutes.

[0123] When the melting point of the second metal is sufficiently greater than the heating temperature mentioned above, there is a tendency to maintain the particle size of the attached particles containing the second metal in the mixing layer 34.

[0124] Furthermore, the thickness of the mixed layer 34, i.e., the embedding depth of the second metal into the lithium metal layer 33, can also be adjusted by increasing the heating temperature and heating time during annealing, or by increasing the pressure applied. The higher the heating temperature during annealing, the softer the lithium metal becomes, thus making it easier to embed particles containing the second metal. In addition, increasing the pressure also makes it easier to embed particles containing the second metal.

[0125] Alternatively, after pressurization, the heating temperature during annealing can be temporarily increased to near the melting point of lithium metal. As a result, lithium metal flows in a manner that covers and embeds particles containing a second metal, and the outermost layer 34c of the mixed layer 34 on the side near the diaphragm 10 tends to become a layer composed of lithium metal.

[0126] The atmosphere used in the mixed layer formation process is not particularly limited, and examples include oxidizing atmosphere, reducing atmosphere, and inert atmosphere. However, from the viewpoint of avoiding unexpected effects on the lithium metal layer 33, an inert atmosphere is preferred.

[0127] Example

[0128] The present disclosure will now be described based on embodiments and comparative examples. However, the present disclosure is not limited to the following embodiments.

[0129] 1. Manufacturing of lithium secondary batteries

[0130] 1.1. Example 1

[0131] A 20 μm thick lithium metal foil is disposed on one main surface of an 8 μm thick copper foil serving as the negative electrode current collector. Then, copper particles with a median particle size D50 of 70 nm are added at a concentration of 0.32 mg / cm³. 2 After being attached to the exposed surface of the lithium metal foil, the electrode is annealed at 160°C for 60 minutes under pressure. This results in a negative electrode with a mixed layer of copper particles dispersed within the lithium metal foil on the separator side. Furthermore, the copper particles attached through the above treatment are embedded in the lithium metal foil, and the outermost layer of the mixed layer on the separator side becomes a layer composed of lithium metal.

[0132] Furthermore, a positive electrode slurry is coated onto one main surface of an aluminum foil with a thickness of 15 μm, which serves as the positive electrode current collector. The slurry is then dried to form a positive electrode active material layer, thereby obtaining the positive electrode. Additionally, the loading of the positive electrode active material in the positive electrode active material layer is set to 10 mg / cm³. 2 In addition, the positive electrode slurry consists of 95 parts by mass of LiNi as the positive electrode active material. xCo y Mn z M a The mixture is prepared by mixing O2 (x=0.83, y=0.09, z=0.07, a=0.01, M=Al), 2 parts by mass of carbon black as a conductive additive, and 3 parts by mass of polyvinylidene fluoride (PVDF) as a binder in a solvent.

[0133] As a 10 μm thick polypropylene separator, 11 negative electrodes and 10 positive electrodes were alternately stacked to create a laminate with both outermost electrodes being negative electrodes. Then, a nickel negative electrode was connected to the negative electrode of the laminate, and an aluminum positive electrode was installed on the positive electrode. The laminate and a non-aqueous electrolyte were then inserted into an outer casing, and the negative and positive electrodes were sealed under degassing conditions with their tips exposed outside the casing, thus obtaining the lithium secondary battery of Example 1. Furthermore, as the non-aqueous electrolyte, a solution obtained by adding 4 M (mol / L) LiN(FSO2)2 as a lithium salt to a 1,2-dimethoxyethane solvent was used.

[0134] 1.2. Examples 2-30, Comparative Example 1

[0135] As shown in Table 1, except for changing the particles of the second metal used or changing the thickness of the mixed layer, the same procedures were followed as in Example 1 to obtain the lithium secondary batteries of Examples 2 to 30. Furthermore, except for not using particles of the second metal, the same procedures were followed as in Example 1 to obtain the lithium secondary battery of Comparative Example 1.

[0136] 2. Evaluation of the thickness variation of the negative electrode

[0137] Cyclic tests were conducted on lithium-ion batteries using a secondary battery charge-discharge test apparatus (manufactured by Hokuto Electric Co., Ltd.). Specifically, one charge-discharge cycle was defined as charging at 0.2C to 4.3V at 25°C and discharging at 1C to 3.0V at 1C, and 100 cycles were performed. The lithium-ion batteries after 100 cycles were then disassembled, and the thickness of the negative electrode was measured. The thickness change of each negative electrode layer was then calculated using the following formula.

[0138] The thickness change of each layer of the negative electrode = (the thickness of the negative electrode layer after 100 cycles) - (the thickness of the negative electrode layer before the first charge).

[0139] [Table 1]

[0140]

[0141] exist Figure 3A The image shown is a SEM photograph of a portion of the cross-section of the negative electrode of the lithium secondary battery obtained in Example 1. Figure 3C The image shown is a SEM photograph of a portion of the cross-section of the negative electrode of the lithium secondary battery obtained in Comparative Example 1. (See image from...) Figure 3A and Figure 3C It can be seen that, by observing a portion of the cross-section of the negative electrode of the lithium secondary battery obtained in the examples using SEM, compared with Comparative Example 1, the porosity in the precipitated layer of any example is low, generally below 10%.

[0142] like Figure 3A As shown, in the lithium secondary battery of this disclosure, the lithium metal layer and the deposited layer are both high-density lithium metal layers. Furthermore, regarding the distinction between the deposited layer and the lithium metal layer, SEM observation using X-ray diffraction results confirmed that they can be differentiated based on their different crystallinity.

[0143] In addition, Figure 3B The image shows a partial SEM photograph of the cross-section of the negative electrode of the lithium secondary battery obtained in Example 1, taken in such a way that the contrast of the second metal in the hybrid layer becomes clear. Figure 3B The photo became the top and bottom Figure 3A An inverted photo, with the blending layer located below it. Figure 3B In this context, the gray component is lithium, while the white component is the secondary metal (copper). For example... Figure 3B As shown, the outermost layer of the hybrid layer on the membrane side in Example 1 is composed of metallic lithium, with a second metal (copper) embedded in the layer. Furthermore, SEM observations of Example 1 and others confirmed the presence of lithium and the second metal in a solid solution phase, as confirmed by X-ray diffraction.

[0144] Industrial availability

[0145] This disclosure has industrial applicability as a key technology for lithium secondary batteries.

Claims

1. A lithium secondary battery, wherein, It comprises: a negative electrode, a positive electrode, and a separator disposed between the negative electrode and the positive electrode. The negative electrode comprises: a negative electrode current collector, a lithium metal layer composed of lithium metal, and a mixed layer containing lithium metal and a second metal other than lithium metal. The hybrid layer is located between the lithium metal layer and the separator.

2. The lithium secondary battery according to claim 1, wherein, The hybrid layer is a layer in which particles containing the second metal are dispersed in the lithium metal.

3. The lithium secondary battery according to claim 2, wherein, The average diameter of the particles is 10–500 nm.

4. The lithium secondary battery according to claim 1, wherein, The area occupied by the second metal relative to 100% of the cross-sectional area of ​​the hybrid layer is 5.0 to 60% of the area.

5. The lithium secondary battery according to claim 1, wherein, The second metal contains at least one selected from copper, silver, gold, aluminum, bismuth, iron, gallium, germanium, indium, magnesium, niobium, nickel, lead, palladium, platinum, silicon, tin, titanium, zinc, and zirconium oxide.

6. The lithium secondary battery according to claim 1, wherein, The thickness of the hybrid layer is 0.10–5.0 μm.

7. The lithium secondary battery according to claim 1, wherein, There is a lithium metal deposition layer between the mixed layer and the lithium metal layer.

8. The lithium secondary battery according to claim 1, wherein, The outermost layer of the diaphragm side of the hybrid layer is composed of metallic lithium.

9. A method for manufacturing a lithium secondary battery, wherein, This manufacturing method is a method for manufacturing the lithium secondary battery according to any one of claims 1 to 8. The formation process of the negative electrode has the following characteristics: The process of preparing the lithium metal layer; and The process of forming a mixed layer containing lithium metal and a second metal other than lithium metal on the lithium metal layer.

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