Anode and lithium ion secondary battery

Abstract

To provide a negative electrode and the like capable of improving charge-discharge cycle characteristics.SOLUTION: A negative electrode 30C includes a current collector layer 32 and a lithium metal foil 34 placed on the current collector layer 32. The current collector layer 32 is porous, and at least the inner and outer surfaces of the current collector layer are electrically conductive.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a negative electrode and a lithium ion secondary battery. [Background technology]

[0002] Lithium-ion secondary batteries can achieve higher capacities than conventional nickel-metal hydride batteries and lead-acid batteries, and as such, they have been attracting attention and are increasingly being adopted as power sources for mobile phones and laptops, as well as for large-scale power storage and automotive applications. However, with the increasing functionality of various electronic devices and the growing demand for power sources, even higher capacities for lithium-ion secondary batteries are expected. Lithium metal, as a negative electrode active material, has a capacity several to ten times that of graphite, and is therefore attracting great expectations (see Patent Documents 1 and 2). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 11-214008 [Patent Document 2] Japanese Patent Application Laid-Open No. 2000-133314 Summary of the Invention [Problem to be solved by the invention]

[0004] A problem that arises when lithium metal is used as the negative electrode active material is that nucleation and growth of lithium metal occurs unevenly on the surface of the negative electrode during charging, significantly degrading the charge-discharge cycle characteristics of the battery.

[0005] Specifically, the tip of the deposited lithium metal is more likely to come into contact with the electrolyte, making it easier for lithium to deposit there than in other areas. As a result, as the charge-discharge cycle progresses, the tip of the deposited lithium metal continues to elongate as charging at the negative electrode progresses. This causes the deposition of long, thin lithium metal, which causes an internal short circuit between the positive and negative electrodes and prevents the battery from functioning properly.

[0006] Furthermore, when the lithium metal dissolves during discharge, long, thin pieces of lithium metal peel off from the negative electrode, creating areas that cannot contribute to discharge (dead lithium), which reduces the discharge capacity and shortens the battery's lifespan.

[0007] Furthermore, there is also the problem that the thickness of the negative electrode increases as charge-discharge cycles progress due to elongated lithium metal and by-products.

[0008] The present invention has been made in view of the above problems, and an object of the present invention is to provide a negative electrode and the like that can improve charge-discharge cycle characteristics. [Means for solving the problem]

[0009] A negative electrode according to one aspect of the present invention includes a current collector layer and a lithium metal foil disposed on the current collector layer. The current collector layer is porous, and at least the inner and outer surfaces of the current collector layer are electrically conductive.

[0010] The negative electrode may include a pair of the current collector layers, and the lithium metal foil may be disposed between the pair of current collector layers.

[0011] The current collector layer may have a main portion and a tab portion, and the lithium metal foil may be disposed between the main portions of the current collector and between the tab portions of the current collector.

[0012] One surface of the lithium metal foil may be in contact with one of the current collector layers, and the other surface of the lithium metal foil may be in contact with the other current collector layer.

[0013] The negative electrode may include a pair of lithium metal foils between the pair of current collector layers, and a foil of a metal other than lithium may be disposed between the pair of lithium metal foils.

[0014] The current collector layer may have a porous electrically insulating substrate and conductive layers provided on the inner and outer surfaces of the electrically insulating substrate.

[0015] The conductive layer can be a layer of a metal or alloy containing at least one selected from the group consisting of copper, nickel, tin, aluminum, zinc, iron, magnesium, manganese, cobalt, and titanium.

[0016] The current collector layer may have a porosity of 50 to 95%.

[0017] A lithium ion secondary battery according to one aspect of the present invention includes any one of the negative electrodes described above and a positive electrode. [Effects of the Invention]

[0018] According to the present invention, a negative electrode and the like capable of improving charge-discharge cycle characteristics are provided. [Brief explanation of the drawings]

[0019] [Figure 1] FIG. 1 is a cross-sectional view of a negative electrode 30C according to one embodiment. [Figure 2] FIG. 2 is an enlarged cross-sectional view of a portion of current collector 32 in negative electrode 30C of FIG. [Figure 3] 3(a) is an exploded perspective view of the negative electrode 30C of FIG. 1, and FIG. 3(b) is a top view of the negative electrode 30C of FIG. [Figure 4]FIG. 4(a) is a cross-sectional view of a negative electrode 30A according to another embodiment, and FIG. 4(b) is a cross-sectional view of a negative electrode 30B according to another embodiment. [Figure 5] FIG. 5 is a schematic cross-sectional view of a lithium ion secondary battery according to one embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0020] A preferred embodiment of the present invention will be described with reference to the drawings.

[0021] (Negative electrode 30C) An anode 30C according to one embodiment of the present invention is shown in Figure 1. The anode 30C according to this embodiment includes a pair of current collector layers 32 and a lithium metal foil 34 disposed between the pair of current collector layers 32.

[0022] 2, the current collector layer 32 has pores 32b that communicate between the inside and outside of the current collector layer 32. The pores 32b communicate between the opposing outer surfaces of the current collector layer 32. More specifically, the current collector layer 32 includes a porous electrically insulating substrate 32a, a conductive layer 32c provided on the inner surfaces of the pores 32b of the electrically insulating substrate 32a, and a conductive layer 32d provided on the outer surface of the electrically insulating substrate 32a. The surface of the conductive layer 32c forms the conductive inner surface of the current collector layer 32, and the surface of the conductive layer 32d forms the conductive outer surface of the current collector layer 32.

[0023] The material of the electrically insulating substrate 32a may be any electrically insulating material (non-metallic material). An example of an electrically insulating substrate is a microporous resin film. Examples of resins include polyimide resins and polyolefin resins such as polyethylene and polypropylene. The resin may be a homopolymer, a copolymer, or a mixture of multiple polymers. A microporous resin film can be produced by stretching a resin film (dry method) or by removing a pore-forming agent or the like from a resin film (wet method).

[0024] Other examples of the insulating substrate 32a include nonwoven fabrics (paper) made of various fibers. Examples of the fibers include the above-mentioned resin fibers, cellulose fibers, polyester fibers, polyamide fibers, polyacrylonitrile fibers, and glass fibers.

[0025] The electrically insulating substrate 32a may be a single layer of the above materials, or may be a laminate of two or more of the above materials.

[0026] The material of the conductive layers 32c and 32d may be any material that has higher conductivity than the electrically insulating substrate 32a, but is preferably a metal or an alloy.

[0027] The material of the conductive layers 32c and 32d can be a metal or alloy containing at least one selected from the group consisting of copper, nickel, tin, aluminum, zinc, iron, magnesium, manganese, cobalt, and titanium.

[0028] Among these, tin, aluminum, zinc, and magnesium are suitable as materials for the conductive layers 32c and 32d, from the viewpoint of obtaining the effect of increasing the activity of lithium by alloying with lithium and making it easy and smooth to deposit lithium metal from the alloy.

[0029] The thickness of the conductive layers 32c and 32d is not particularly limited, but may be 0.1 to 2 μm, or may be 1.5 μm or less, or may be 1.0 μm or less.

[0030] Such a current collector layer 32 can be obtained by electroless plating a porous electrically insulating substrate 32a.

[0031] The porosity of the insulating base material 32a before plating is preferably 40 to 98%.

[0032] The porosity of the current collector layer 32 after plating may be 30% or more, or 40% or more, and is preferably 50 to 95%. The porosity can be measured by the Archimedes method using liquid substitution of pores, gas adsorption method, mercury intrusion method, or the like.

[0033] The average pore diameter of the current collector layer 32 after plating can be, for example, 0.1 to 3 μm. The average pore diameter of the current collector can be determined by measuring the number of cells (number of pores) per inch in a direction parallel to the thickness direction (vertical) and a direction perpendicular to the thickness direction (horizontal) in a scanning electron microscope photograph of a cross section parallel to the thickness direction of the current collector (or negative electrode), calculating the average value, and then calculating the reciprocal of the average value as the average pore diameter.

[0034] There is no limitation on the thickness of the current collector layer 32, but it can be 5 μm or more, and may be 10 μm or more. The thickness can be 40 μm or less.

[0035] The thickness of the lithium metal foil 34 is not particularly limited, but can be 2 μm or more and 50 μm or less.

[0036] The total thickness of negative electrode 30C can be set to 1 to 150 μm.

[0037] 3(a) and 3(b), the current collector layer 32 has a main body portion 32m and a tab portion 32t protruding from the main body portion 32m. The lithium metal foil 34 has a main body portion 34m and a tab portion 34t protruding from the main body portion 34m.

[0038] The length D1 of the tab portion 34t of the lithium metal foil 34 that protrudes from the main body portion 34m is shorter than the length D2 of the tab portion 32t of the current collector layer 32 that protrudes from the main body portion 32m.

[0039] As shown in FIG. 3(b), the portion between the pair of tab portions 34t where the lithium metal foil 34 is not sandwiched may have a welded portion 32t′ where the tab portions 34t are welded together.

[0040] The negative electrode 30C having such a configuration can be easily manufactured by punching a laminate having a current collector layer / lithium metal foil / current collector layer structure using a die corresponding to the shapes of the main portion and tab portion. Punching this laminate so that the lithium metal foil 34 has the tab portion 34t, i.e., with the end face L2 of the lithium metal foil of the laminate before punching positioned between the planned position of the end face L1 of the main portion 32m of the current collector layer 32 and the planned position of the end face L3 of the tab portion 32t, is preferable because it prevents the area of ​​the main portion 34m of the lithium metal foil 34 from being smaller than the area of ​​the main portion 32m of the current collector layer 32. Furthermore, having a weld 32t' between the tab portions 32t of the current collector layer 32 is preferable because it reduces the electrical resistance of the battery.

[0041] (Mechanism of action) According to this embodiment, the current collector layer 32 is porous and its inner and outer surfaces are conductive, so that the current collector layer 32 has liquid-retaining properties, and furthermore, the inner surface of the current collector layer 32 can be used to cause a redox reaction (charge and discharge) of lithium metal.

[0042] The current density during charging is smoothed because the current collector layer 32 has a large specific surface area due to its porosity, and the deposition of elongated lithium metal is suppressed because lithium metal can be deposited in the pores 32b of the current collector layer 32. This suppresses the generation of dead lithium and short circuits caused by dendrites, enabling improved cycle characteristics.

[0043] Furthermore, lithium metal deposited during charging and various decomposition products generated as the cycles progress can be deposited in pores 32b of current collector layer 32, which can suppress an increase in the thickness of negative electrode 30C with the progress of cycles. The increase in the specific surface area of ​​current collector layer 32 also reduces the thickness of the decomposition product layer formed on current collector layer 32, which can suppress an increase in electrical resistance with the progress of cycles.

[0044] Furthermore, because the lithium metal foil 34 is disposed between a pair of current collector layers 32, the negative electrode 30C can be handled during battery assembly, etc., without the lithium metal foil 34 coming into contact with other components, etc. This is advantageous. This eliminates the need for surface treatment, such as coating, to reduce reactivity on the surface of the lithium metal foil 34, and prevents the lithium metal from impeding its deposition and dissolution rate and reducing its redox reactivity. Contacting the lithium metal foil with other components, such as a device, can easily cause problems due to the adhesiveness of the lithium metal.

[0045] Furthermore, since both sides of the lithium metal foil 34 are in contact with the current collector layers 32, a large amount of lithium metal can be placed by effectively utilizing the gaps between the current collector layers 32, thereby improving the volumetric energy density.

[0046] Furthermore, since the current collector layer 32 has the porous electrically insulating substrate 32a and the conductive layers 32c, 32d provided on the inner and outer surfaces of the electrically insulating substrate 32a, the current collector layer 32 can be made lighter than when the current collector layer 32 is made of a pure porous metal, and the mass energy density can be improved. Furthermore, the current collector layer 32 is made flexible, making it easier to handle.

[0047] (Negative electrode 30A) Next, a negative electrode 30A according to another embodiment will be described with reference to (a) of Fig. 4. The negative electrode 30A shown in (a) of Fig. 4 differs from the negative electrode 30C shown in Fig. 1 in that a pair of lithium metal foils 34 is disposed between a pair of current collector layers 32, and a metal foil 36 other than lithium is provided between the pair of lithium metal foils 34.

[0048] The non-lithium metal foil 36 may be any metal other than lithium metal foil, and specifically may be a foil of a metal or alloy containing at least one selected from the group consisting of copper, nickel, tin, aluminum, zinc, iron, magnesium, manganese, cobalt, and titanium.

[0049] In this embodiment, the lithium metal foil 34 may have a thickness of 1 to 50 μm. The current collector layer 32 may have a thickness similar to that of the negative electrode 30C.

[0050] The thickness of the metal foil 36 other than lithium can be set to 6 μm.

[0051] The total thickness of negative electrode 30A can be set to 2 to 100 μm.

[0052] Such a negative electrode 30A can be obtained by arranging current collector layers 32 on both sides of a laminated foil (clad material) in which lithium metal foil 34 is attached to both sides of metal foil 36. The attachment can be performed by rolling or the like.

[0053] When metal foils 36 made of a material other than lithium, such as Cu, are attached between the lithium metal foils 34 as in this embodiment, wrinkles and breakage of the foils are prevented, and the electric field can be more easily made uniform.

[0054] (Negative electrode 30B) Next, a negative electrode 30B according to another embodiment will be described with reference to FIG. 4(b). The negative electrode 30B shown in FIG. 4(b) differs from the negative electrode 30C shown in FIG. 1 in that the current collector layer 32 is provided only on one side in the thickness direction. This negative electrode 30B is suitable for use as the outermost negative electrode of a battery. This negative electrode 30B is preferably stacked in a battery so that the current collector layer 32 is disposed on the outer side of the lithium metal foil 34.

[0055] (Another embodiment of the negative electrode) In the above embodiment, the current collector layer 32 has a porous electrically insulating substrate 32a and conductive layers 32c, 32d provided on the inner and outer surfaces of the electrically insulating substrate 32a. However, the current collector layer 32 may be a porous body made of only a conductive material without having the electrically insulating substrate 32a.

[0056] For example, a conductive porous body obtained by removing the insulating substrate 32a from the current collector layer 32, which includes the above-mentioned resin porous insulating substrate 32a and conductive layers 32c, 32d provided on the inner and outer surfaces of the insulating substrate 32a, may be used as the current collector layer 32. The substrate can be removed by heating in an oxidizing atmosphere or a non-oxidizing atmosphere.

[0057] Furthermore, a porous metal foil such as a mesh metal foil may be used as the current collector layer 32. Examples of the metal may be the same as the conductive materials described above.

[0058] The planar shapes of the current collector layer and the lithium metal foil can take various forms depending on the final shape of the battery. For example, the current collector layer may have a tab portion and the lithium metal foil may not have a tab portion, or a shape in which neither the current collector layer nor the lithium metal foil has a tab portion is also not excluded. The shape of the main body may also be circular, for example.

[0059] (lithium-ion secondary battery) Next, a lithium ion secondary battery 100 using the above-described negative electrode will be described with reference to FIG.

[0060] The lithium ion secondary battery 100 according to this embodiment includes a power generating element 90 and a case 80. The power generating element 90 is a laminate having a large number of negative electrodes 30C, positive electrodes 10, and separators 20. In the power generating element 90, the negative electrodes 30C and positive electrodes 10 are alternately arranged, and separators 20 are arranged between the negative electrodes 30C and the positive electrodes 10. Each layer of the power generating element 90 is impregnated with a non-aqueous electrolyte solution.

[0061] In this embodiment, both outermost layers of the power generating element 90 are negative electrodes 30C, but this is not limitative, and for example, both outermost layers of the power generating element 90 may be positive electrodes.

[0062] There is no particular limitation on the number of layers of the positive electrode (or negative electrode), and it may be, for example, one layer or multiple layers such as 10 layers or 20 layers.

[0063] The positive electrode 10 has a positive electrode current collector 12 and positive electrode active material layers 14 provided on both surfaces of the positive electrode current collector 12.

[0064] The positive electrode current collector 12 may be a conductive plate material, for example, it can be a metal foil such as aluminum, copper, nickel, etc.

[0065] The positive electrode active material layer 14 contains a positive electrode active material, a conductive assistant, and a binder.

[0066] The positive electrode active material includes an active material capable of reversibly proceeding with the occlusion and release of lithium ions, the desorption and insertion (intercalation) of lithium ions, or the doping and dedoping of lithium ions and counter anions.

[0067] The positive electrode active material is, for example, a composite metal oxide. Examples of the composite metal oxide are lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganate (LiMnO2), lithium manganese spinel (LiMn2O4), and the general formula: LiNi x Co y Mn z M a O2 compounds (in the general formula, x + y + z + a = 1, 0 ≦ x < 1, 0 ≦ y < 1, 0 ≦ z < 1, 0 ≦ a < 1, M is one or more elements selected from Al, Mg, Nb, Ti, Cu, Zn, 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, Zr or VO), lithium titanate (Li4Ti5O 12 ), LiNi x Co y Al z O2 (0.9 < x + y + z < 1.1). The positive electrode active material may also be an organic substance. For example, examples of the organic positive electrode active material are polyacetylene, polyaniline, polypyrrole, polythiophene, and polyacene.

[0068] The conductive additive enhances the electronic conductivity between the positive electrode active materials. Examples of the conductive additive include carbon powders such as carbon black, acetylene black, and ketjen black, carbon nanotubes, carbon materials, metal fine powders such as copper, nickel, stainless steel, and iron, mixtures of carbon materials and metal fine powders, and conductive oxides such as ITO. The conductive additive is preferably a carbon material such as carbon black, acetylene black, or ketjen black.

[0069] The binder binds the active materials together. Known binders can be used. An example of the binder is a fluororesin. Examples of fluororesins 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).

[0070] Other examples of binders are vinylidene fluoride-based fluororubbers such as vinylidene fluoride-hexafluoropropylene-based fluororubber (VDF-HFP-based fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene-based fluororubber (VDF-HFP-TFE-based fluororubber), vinylidene fluoride-pentafluoropropylene-based fluororubber (VDF-PFP-based fluororubber), vinylidene fluoride-pentafluoropropylene-tetrafluoroethylene-based fluororubber (VDF-PFP-TFE-based fluororubber), vinylidene fluoride-perfluoromethylvinyl ether-tetrafluoroethylene-based fluororubber (VDF-PFMVE-TFE-based fluororubber), and vinylidene fluoride-chlorotrifluoroethylene-based fluororubber (VDF-CTFE-based fluororubber). Further examples of the binder include cellulose, styrene-butadiene rubber, ethylene-propylene rubber, polyimide resin, polyamide-imide resin, and acrylic resin.

[0071] The separator 20 has electrical insulation properties and a porous structure. An example of the separator 20 is a microporous resin film. Examples of the resin include polyimide resins and polyolefin resins such as polyethylene and polypropylene. The resin may be a homopolymer, a copolymer, or a mixture of multiple polymers. The microporous resin film can be produced by stretching a resin film (dry method) or by removing a pore-forming agent or the like from the resin film (wet method).

[0072] Other examples of the separator 20 include nonwoven fabrics (paper) made of various fibers. Examples of the fibers include fibers of the above-mentioned resins, cellulose fibers, polyester fibers, polyamide fibers, polyacrylonitrile fibers, and glass fibers.

[0073] Still another example of the separator 20 is a solid electrolyte. Examples of the solid electrolyte include a polymer solid electrolyte, an oxide-based solid electrolyte, and a sulfide-based solid electrolyte.

[0074] The separator 20 may be a single layer of the above materials, or may be a laminate of two or more of any of the above materials.

[0075] (Non-aqueous electrolyte) The non-aqueous electrolyte is sealed in the exterior body 50 and impregnates the power generating element 90. The non-aqueous electrolyte includes, for example, a non-aqueous solvent and an electrolyte. The electrolyte is dissolved in the non-aqueous solvent.

[0076] The non-aqueous solvent contains, for example, a cyclic carbonate and a chain carbonate. The cyclic carbonate solvates the electrolyte. Examples of the cyclic carbonate include ethylene carbonate, propylene carbonate, and butylene carbonate. It is preferable that the cyclic carbonate contains at least propylene carbonate. The chain carbonate reduces the viscosity of the cyclic carbonate. Examples of the chain carbonate include diethyl carbonate, dimethyl carbonate, and ethyl methyl carbonate. The non-aqueous solvent may also contain methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, 1,2-dimethoxyethane, 1,2-diethoxyethane, etc.

[0077] The electrolyte is, for example, a lithium salt. Examples of the electrolyte include 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. One type of lithium salt may be used alone, or two or more types may be used in combination.

[0078] (exterior body) The power generating element 90 and the non-aqueous electrolyte are sealed inside the exterior body 50. The exterior body 50 prevents the non-aqueous electrolyte from leaking to the outside and prevents moisture and the like from entering the lithium ion secondary battery 100 from the outside.

[0079] Although not shown, the exterior body 50 can have a metal foil and resin layers laminated on both sides of the metal foil.

[0080] The metal foil can be, for example, aluminum foil. The resin layer can be a polymer film such as polypropylene. The materials constituting the inner and outer resin layers can be different. For example, the outer material can be a polymer with a high melting point, such as polyethylene terephthalate (PET) or polyamide (PA), while the inner polymer film can be made of polyethylene (PE), polypropylene (PP), or the like.

[0081] Although not shown in the drawings, leads connected to the tabs of the positive and negative electrodes typically connect the inside and outside of the exterior housing. The leads can be made of a conductive material such as aluminum, nickel, or a nickel-plated copper metal plate. In particular, it is preferable to use an aluminum metal plate for the lead connected to the positive electrode 10, and it is preferable to use a nickel metal plate or a nickel-plated copper metal plate for the lead connected to the negative electrode 30C.

[0082] (Other stacking configurations of power generating elements) The lithium ion secondary battery of the present invention is not limited to a laminated form as long as it employs the above-described negative electrode. For example, the negative electrode 30A may be used instead of the negative electrode 30C in the power generating element 90. Furthermore, the outermost negative electrode 30C or negative electrode 30A in the power generating element 90 may be replaced by the negative electrode 30B. [Example]

[0083] Example 1 Fabrication of current collector layer and negative electrode 30C A 20 μm thick polyimide resin with a porosity of 80% was prepared as an insulating substrate. This substrate was brought into contact with an electroless plating solution to form a 0.5 μm thick Cu layer on the inner and outer surfaces of the insulating substrate, resulting in a current collector layer with a porosity of 60%. Lithium metal foil with a thickness of 20 μm was placed on both sides of the current collector layer and then cut into a predetermined shape to obtain negative electrode 30C having a main body and a tab as shown in Figures 1 and 2. Negative electrode 30C has a structure of porous current collector layer / lithium metal foil / porous current collector layer.

[0084] Cathode manufacturing The positive electrode slurry was prepared by mixing a positive electrode active material, a conductive additive, a binder, and a solvent.

[0085] LiNi, a composite metal oxide, is 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) was used. Carbon black was used as a conductive additive. Polyvinylidene fluoride (PVDF) was used as a binder. The mass ratio of the positive electrode active material, conductive additive, and binder was 95:2:3. The amount of positive electrode active material carried in the dried positive electrode active material layer was 20 mg / cm. 2 The solvent was removed from the positive electrode slurry in a drying furnace to prepare a positive electrode.

[0086] (Fabrication of lithium-ion secondary batteries for evaluation) The prepared negative electrode 30C and positive electrode were alternately stacked with 10 μm thick polypropylene separators in between, and a stack was prepared by stacking 11 negative electrodes 30C and 10 positive electrodes. The outermost electrodes were both negative electrodes 30C. Furthermore, a nickel negative electrode lead was attached to the tab portion of the negative electrode of the stack, and an aluminum positive electrode lead was attached to the tab portion of the positive electrode of the stack using an ultrasonic welding machine.

[0087] The laminate was then inserted into a case and heat-sealed except for one peripheral location to form a closed section. A non-aqueous electrolyte was then poured into the case. The non-aqueous electrolyte was a 1,2-dimethoxyethane solvent containing 4M (mol / L) LiN(FSO2)2 as a lithium salt. The remaining location was then heat-sealed while the pressure was reduced using a vacuum sealer, completing a lithium-ion secondary battery (full cell).

[0088] Next, the charge capacity retention rate of the lithium-ion secondary battery after cycling was measured using a secondary battery charge / discharge tester (manufactured by Hokuto Denko Corp.) In an environment of 25°C, one charge / discharge cycle consisted of charging at a constant current and constant voltage of 0.2 C up to 4.3 V and discharging at a constant current of 1 C down to 3.0 V, and the number of cycles required for the capacity to drop to 80% or less of the initial capacity was measured.

[0089] After 100 cycles, the lithium-ion secondary battery was disassembled and the thickness change of the negative electrode was measured. The thickness change rate was calculated by (thickness of negative electrode after 100 cycles - thickness of negative electrode before first charge) / (thickness of negative electrode before first charge) x 100.

[0090] Furthermore, the mass energy density was calculated by multiplying the initial capacity by the nominal voltage of 3.7 V and the mass of the battery.

[0091] Examples 2 to 8 The procedure was the same as in Example 1, except that the conductive layer electrolessly plated on the porous substrate was changed from copper to nickel, tin, aluminum, zinc, iron, magnesium, or titanium.

[0092] Example 9 The same procedures as in Example 1 were carried out except that the insulating substrate had a thickness of 20 μm and a porosity of 47%, and the current collector layer had a porosity of 21%.

[0093] Example 10 The same procedures as in Example 1 were carried out except that the insulating substrate had a thickness of 20 μm and a porosity of 50%, and the current collector layer had a porosity of 28%.

[0094] Example 11 The same procedures as in Example 1 were carried out except that the insulating substrate had a thickness of 20 μm and a porosity of 62%, and the current collector layer had a porosity of 40%.

[0095] Example 12 The same procedures as in Example 1 were carried out except that the insulating substrate had a thickness of 20 μm and a porosity of 75%, and the current collector layer had a porosity of 54%.

[0096] Example 13 The same procedures as in Example 1 were carried out except that the insulating substrate had a thickness of 20 μm and a porosity of 95%, and the current collector layer had a porosity of 78%.

[0097] Example 14 The same procedures as in Example 1 were carried out except that the insulating substrate had a thickness of 20 μm and a porosity of 97%, and the current collector layer had a porosity of 82%.

[0098] Example 15 The same procedures as in Example 1 were carried out except that the thickness of the Cu layers formed on the inner and outer surfaces of the insulating substrate was 0.1 μm and the porosity of the current collector layer was 75%.

[0099] Example 16 The same procedures as in Example 1 were carried out except that the thickness of the Cu layers formed on the inner and outer surfaces of the insulating substrate was 1.0 μm and the porosity of the current collector layer was 43%.

[0100] Example 17 The same procedures as in Example 1 were carried out except that the thickness of the Cu layers formed on the inner and outer surfaces of the insulating substrate was 1.7 μm and the porosity of the current collector layer was 21%.

[0101] Example 18 4(a) was used instead of the negative electrode 30C, the same procedure as in Example 1. The negative electrode 30A had a structure of porous current collector layer / Li metal foil / Cu foil / Li metal foil / porous current collector layer.

[0102] Example 19 The same procedure as in Example 1 was carried out except that 10 negative electrodes 30C and 11 positive electrodes were used, and both outermost electrodes were positive electrodes.

[0103] Example 20 The procedure was the same as in Example 1, except that the negative electrode 30C was replaced with the negative electrode 30B as both outermost electrodes. The negative electrode 30B has a structure of, from the outside, a porous current collector layer / Li metal foil.

[0104] Comparative Example 1 Except for using a non-porous copper foil (thickness 6 μm) instead of a porous current collector, the negative electrode of Comparative Example 1 was the same as in Example 1. That is, the negative electrode of Comparative Example 1 had a structure of lithium metal foil (thickness 20 μm) / non-porous copper foil / lithium metal foil (thickness 20 μm).

[0105] The results are shown in Table 1. [Table 1]

[0106] It was confirmed that the negative electrode according to the example had improved cycle characteristics. [Explanation of symbols]

[0107] 10...positive electrode, 20...separator, 30A, 30B, 30C...negative electrode, 32...current collector layer, 32a...electrically insulating substrate, 32c, 32d...conductive layer, 32t...tab portion of current collector layer, 32m...main body portion of current collector layer, 34...lithium metal foil, 34t...tab portion of lithium metal foil, 34m...main body portion of lithium metal foil, 36...metal foil other than lithium, 100...lithium ion secondary battery.

Claims

1. a current collector layer; and a lithium metal foil disposed on the current collector layer; the current collector layer is porous, and at least the inner surface and the outer surface of the current collector layer are electrically conductive; a pair of current collector layers, and the lithium metal foil is disposed between the pair of current collector layers; the current collector layer has a body portion and a tab portion; The lithium metal foil is disposed between the main portions of the current collector layer and between the tab portions of the current collector layer.

2. 2. The negative electrode according to claim 1, wherein one surface of the lithium metal foil is in contact with one of the current collector layers, and the other surface of the lithium metal foil is in contact with the other of the current collector layers.

3. 2. The negative electrode according to claim 1, wherein the pair of lithium metal foils is disposed between the pair of current collector layers, and a foil of a metal other than lithium is disposed between the pair of lithium metal foils.

4. 4. The negative electrode according to claim 1, wherein the current collector layer comprises a porous electrically insulating substrate and conductive layers provided on inner and outer surfaces of the electrically insulating substrate.

5. 5. The negative electrode according to claim 4, wherein the conductive layer is a layer of a metal or an alloy containing at least one selected from the group consisting of copper, nickel, tin, aluminum, zinc, iron, magnesium, manganese, cobalt, and titanium.

6. 6. The negative electrode according to claim 1, wherein the current collector layer has a porosity of 50 to 95%.

7. A lithium ion secondary battery comprising the negative electrode according to any one of claims 1 to 6 and a positive electrode.

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