Lithium secondary battery and method for manufacturing same

Abstract

This lithium secondary battery comprises: a negative electrode; a positive electrode; and a separator disposed between the negative electrode and the positive electrode. The negative electrode includes: a negative electrode current collector; a metal lithium layer made of metal lithium; and a mixed layer containing metal lithium and a second metal other than the metal lithium. The mixed layer is located between the metal lithium layer and the separator.

Description

Lithium secondary battery and its manufacturing method

[0001] The present disclosure relates to a lithium secondary battery and a method for manufacturing the same.

[0002] Lithium secondary batteries, which use metallic lithium as the anode, have a high energy density and are attracting attention as large-scale power sources for mobile devices such as mobile phones and laptops, as well as for power storage and power sources for electric vehicles. Because metallic lithium has an extremely low potential, lithium secondary batteries are expected to achieve high theoretical capacity densities.

[0003] Lithium secondary batteries, which use metallic lithium in the negative electrode, charge and discharge by depositing and dissolving metallic lithium. During charging, metallic lithium deposits on the negative electrode, and during discharging, metallic lithium dissolves. During charging, metallic lithium may deposit in a tree-like shape, with the root at the starting point of deposition; these deposits are also called dendrites. During discharge, if the root of the dendrite dissolves first and metallic lithium is liberated from the negative electrode, the liberated metallic lithium cannot contribute to subsequent charging and discharging, reducing the cycle performance of the lithium secondary battery.

[0004] Regarding this issue, for example, Patent Document 1 discloses a method in which metal powder that serves as nuclei for the deposition of metallic lithium is uniformly adhered in advance to the surface of a metallic lithium electrode, thereby preventing the deposition of metallic lithium from occurring in a localized concentration on the electrode surface, and as a result, preventing the deposition of metallic lithium in a dendritic form.

[0005] Japanese Patent Application Publication No. 5-234585

[0006] However, although the local deposition of metallic lithium can be suppressed, the method described in Patent Document 1 still results in the deposition of metallic lithium on the surface of the metallic lithium electrode. As a result, as charge-discharge cycles are repeated, a low-density metallic lithium deposit layer gradually accumulates, and the thickness of the negative electrode increases.

[0007] The present disclosure has been made in view of the above-mentioned problems, and aims to provide a lithium secondary battery that undergoes little change in thickness even with repeated charge / discharge cycles, and a method for manufacturing the same.

[0008] [1] A lithium secondary battery comprising a negative electrode, a positive electrode, and a separator disposed between the negative electrode and the positive electrode, wherein the negative electrode has a negative electrode current collector, a metallic lithium layer made of metallic lithium, and a mixed layer containing metallic lithium and a second metal other than the metallic lithium, and the mixed layer is located between the metallic lithium layer and the separator. [2] The lithium secondary battery according to [1], wherein the mixed layer is a layer in which particles containing the second metal are dispersed in the metallic lithium. [3] The lithium secondary battery according to [2], wherein the average diameter of the particles is 10 to 500 nm. [4] The lithium secondary battery according to any one of [1] to [3], wherein the area occupied by the second metal is 5.0 to 60 area% relative to 100 area% of a cross section of the mixed layer. [5] The lithium secondary battery according to any one of [1] to [4], wherein the second metal comprises at least one selected from the group consisting of copper, silver, gold, aluminum, bismuth, iron, gallium, germanium, indium, magnesium, niobium, nickel, lead, palladium, platinum, silicon, tin, titanium, zinc, and zirconia. [6] The lithium secondary battery according to any one of [1] to [5], wherein the mixed layer has a thickness of 0.10 to 5.0 μm. [7] The lithium secondary battery according to any one of [1] to [6], wherein a precipitate layer formed by precipitated metallic lithium is provided between the mixed layer and the metallic lithium layer. [8] The lithium secondary battery according to any one of [1] to [7], wherein the outermost layer of the mixed layer on the separator side is made of metallic lithium. [9] A method for producing the lithium secondary battery according to any one of [1] to [8], comprising, as a negative electrode forming step, a step of preparing a metallic lithium layer, and a step of forming a mixed layer containing metallic lithium and a second metal other than the metallic lithium on the metallic lithium layer.

[0009] According to the present disclosure, it is possible to provide a lithium secondary battery that undergoes little change in thickness even after repeated charge / discharge cycles, and a method for manufacturing the same.

[0010] FIG. 1 is a schematic cross-sectional view of a lithium secondary battery according to an embodiment of the present disclosure. FIG. 2 is a schematic cross-sectional view of a negative electrode according to an embodiment of the present disclosure. FIG. 3A is an SEM photograph of a portion of a cross section of the negative electrode of the lithium secondary battery obtained in Example 1. FIG. 3B is an SEM photograph of a portion of a cross section of the negative electrode of the lithium secondary battery obtained in Example 1, taken so as to clearly show the contrast of the second metal in the mixed layer. FIG. 3C is an SEM photograph of a portion of a cross section of the negative electrode of the lithium secondary battery obtained in Comparative Example 1.

[0011] Hereinafter, an embodiment of the present disclosure (hereinafter referred to as "the present embodiment") will be described in detail, but the present disclosure is not limited thereto, and various modifications are possible without departing from the spirit thereof. Note that in the drawings attached to this specification, for the convenience of illustration and ease of understanding, the scale and aspect ratios may be appropriately changed and exaggerated from those of the actual objects.

[0012] 1. Lithium Secondary Battery FIG. 1 shows a schematic cross-sectional view of a lithium secondary battery according to this embodiment. As shown in FIG. 1, 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 may be housed in an outer casing 50 in the form of a laminate 40 together with an electrolyte (not shown). While FIG. 1 shows the laminate 40 in which the separator 10 is disposed between the negative electrode 30 and the positive electrode 20, the laminate 40 may instead have a multilayer structure in which the negative electrodes 30 and the positive electrodes 20 are alternately disposed and the separator 10 is disposed between the negative electrode 30 and the positive electrode 20.

[0013] In lithium secondary batteries, dissolution and deposition of metallic lithium layers are repeated during charge and discharge. Normally, uneven deposition of metallic lithium due to repeated charge and discharge cycles promotes expansion of the negative electrode. Furthermore, as charge and discharge cycles progress, some of the deposited metallic lithium transforms into lithium powder, further promoting expansion of the negative electrode. Furthermore, the deposited metallic lithium forms a low-density layer with voids, similar to dendrites, which also promotes expansion of the negative electrode.

[0014] In contrast, the negative electrode 30 of the present embodiment has a negative electrode current collector 31, a metallic lithium layer 33 made of metallic lithium, and a mixed layer 34 containing metallic lithium and a second metal other than the metallic lithium, and as shown in FIG. 2 , the mixed layer 34 is located between the metallic lithium layer 33 and the separator 10.

[0015] As a result, the lithium secondary battery 100 of this embodiment can deposit metallic lithium uniformly in the in-plane direction at multiple deposition initiation points between the metallic lithium layer 33 and the mixed layer 34 during charging. Furthermore, during charging, the mixed layer 34 controls the uniform deposition and growth of metallic lithium in the thickness direction, allowing the deposition layer 35 to grow in the thickness direction while suppressing the formation of voids. As a result, during charging, the deposition layer 35 of metallic lithium deposited between the metallic lithium layer 33 and the mixed layer 34 becomes a layer with fewer voids and a higher density. Therefore, thickness changes in the negative electrode 30 due to charge and discharge are suppressed. During discharge, the metallic lithium deposited between the metallic lithium layer 33 and the mixed layer 34 dissolves.

[0016] As described above, the lithium secondary battery 100 of this embodiment can suppress changes in thickness even with repeated charge / discharge cycles by having the predetermined mixed layer 34. The configuration of the lithium secondary battery 100 of this embodiment will be described in detail below.

[0017] 1.1. Negative Electrode Figure 2 shows a schematic cross-sectional view of a negative electrode. The negative electrode 30 has a negative electrode current collector 31 and a negative electrode active material layer 32 provided on the surface of the negative electrode current collector 31. The planar shape of the negative electrode 30 can take various forms depending on the final shape of the battery. In this embodiment, unless otherwise specified, the term "cross section" refers to a plane perpendicular to the plate material of the negative electrode current collector 31, in other words, a plane parallel to the thickness direction of the negative electrode active material layer 32.

[0018] The negative electrode current collector 31 is not particularly limited as long as it is a conductive plate material, but examples thereof include metal foils such as aluminum, copper, nickel, and stainless steel. Among these, copper foil is preferable. Use of such a negative electrode current collector 31 tends to further improve conductivity.

[0019] As shown in FIG. 2 , the negative electrode active material layer 32 of this embodiment has a metallic lithium layer 33 and a mixed layer 34, and may have a precipitated layer 35 in which metallic lithium is precipitated between the mixed layer 34 and the metallic lithium layer 33, as necessary.

[0020] Depending on the configuration of the laminate 40, the anode active material layer 32 may be provided on one or both of the pair of main surfaces of the anode current collector 31. Each layer constituting the anode active material layer 32 will be described in detail below.

[0021] The thickness of the negative electrode current collector 31 is preferably 2.0 to 20 μm, or 5.0 to 15 μm. The total thickness of the negative electrode 30 is preferably 1.0 to 100 μm, or 5.0 to 75 μm, or 10 to 50 μm.

[0022] The metallic lithium layer 33 is a layer made of metallic lithium, and is responsible for the oxidation-reduction reaction that accompanies the charge and discharge of the lithium secondary battery. During charge, metallic lithium precipitates between the metallic lithium layer 33 and the mixed layer 34, more specifically, on the surface of the metallic lithium layer 33. During discharge, the metallic lithium precipitated between the metallic lithium layer 33 and the mixed layer 34, more specifically, on the surface of the metallic lithium layer 33, dissolves.

[0023] The metallic lithium layer 33 may be formed on and in contact with the negative electrode current collector 31, and any other layer may be formed between the metallic lithium layer 33 and the negative electrode current collector 31 as necessary.

[0024] The metallic lithium layer 33 is preferably made of metallic lithium and is substantially free of other metals and organic compounds. The porosity of the metallic lithium layer 33 may be less than 1%, and the metallic lithium layer 33 may have no voids. In this embodiment, the porosity can be calculated from the area of ​​the metallic lithium layer 33 observed when the cross section of the negative electrode is observed using an SEM or the like, and the area of ​​the voids contained in the metallic lithium layer 33. The metallic lithium layer 33 is not particularly limited, and may be, for example, a lithium foil.

[0025] The content of metallic lithium in metallic lithium layer 33 is preferably 95 to 100 mass %, 98 to 100 mass %, or 99 to 100 mass %, relative to the total amount of metallic lithium layer 33 .

[0026] The thickness of the metallic lithium layer 33 is preferably 1.0 to 50 μm, 2.5 to 4 μm, or 5.0 to 30 μm.

[0027] 1.1.2. Mixed Layer The mixed layer 34 contains metallic lithium and a second metal other than metallic lithium. The mixed layer 34 is located between the metallic lithium layer 33 and the separator 10. The mixed layer 34 has poorer electronic conductivity than the metallic lithium layer 33, and also has relatively weak adhesion to the metallic lithium layer 33. Therefore, during charging, metallic lithium tends to precipitate on the surface of the metallic lithium layer 33, which has better electronic conductivity than the surface of the mixed layer 34, and the interface between the mixed layer 34 and the metallic lithium layer 33 is relatively prone to dynamic change.

[0028] Therefore, during charging, the precipitate layer 35 grows so as to lift the mixed layer 34 from the surface of the metallic lithium layer 33. As described above, the precipitate layer 35 thus grown becomes a metallic lithium layer with few voids and high density. During discharging, the precipitate layer 35 thus formed dissolves. This allows for small changes in thickness even with repeated charge-discharge cycles.

[0029] The mixed layer 34 may be formed in various ways, including one in which the metallic lithium and the second metal are dispersed in the other, one in which the metallic lithium and the second metal form a solid solution, or one in which the metallic lithium and the second metal form an intermetallic compound. Among these, the mixed layer 34 is preferably a layer in which particles 34b containing the second metal are dispersed in metallic lithium 34a, as shown in Fig. 2. This tends to reduce the change in thickness after repeated charge-discharge cycles.

[0030] The second metal is not particularly limited, but may be at least one selected from the group consisting of copper, silver, gold, aluminum, bismuth, iron, gallium, germanium, indium, magnesium, niobium, nickel, lead, palladium, platinum, silicon, tin, titanium, zinc, and zirconia. The second metal may be any of these metals alone or a composite of these metals. Among these, copper and nickel are preferred, and copper is more preferred.

[0031] The mixed layer 34 is desirably strong enough to prevent penetration of the precipitate layer 35, which is deposited to lift the mixed layer 34, and to suppress the penetration so that the precipitate layer 35 becomes a high-density metallic lithium layer with few voids. In this regard, the second metal is unlikely to form an intercalation compound between lithium and the second metal, and therefore can suppress the generation of voids associated with the formation of an intercalation compound, which is a factor in reducing the physical strength of the mixed layer 34. Furthermore, because lithium and the second metal, copper, can form a solid solution, it is possible to avoid insufficient adhesion between lithium and the second metal, copper, which would otherwise be caused by the complete lack of solid solution. In other words, the use of the second metal allows the formation of a mixed layer 34 with appropriate physical properties, which tends to reduce thickness change after repeated charge-discharge cycles.

[0032] The average diameter of the particles 34b containing the second metal is preferably 10 to 500 nm, 15 to 450 nm, 20 to 400 nm, 25 to 350 nm, 30 to 300 nm, 35 to 250 nm, 40 to 200 nm, 45 to 150 nm, or 50 to 100 nm. When the average diameter is within the above range, the gaps between the particles 34b become small, and it is possible to prevent metallic lithium grown in the precipitation layer 35 from penetrating the mixed layer 34.

[0033] Here, the average diameter refers to the diameter obtained by measuring the cross-sectional diameter of particles 34b observed when observing the cross-section of the negative electrode using a scanning electron microscope (SEM) or the like, and dividing the obtained cross-sectional diameter distribution into two equal parts from a certain diameter. Note that if the cross-section of particles 34b is not circular, the diameter may be measured as the equivalent circle diameter. Here, the equivalent circle diameter refers to the diameter of a circle having an area equal to the area of ​​particles 34b.

[0034] The thickness of the mixed layer 34 is preferably 0.10 to 5.0 μm, 0.20 to 4.5 μm, 0.30 to 4.0 μm, 0.30 to 3.5 μm, 0.40 to 3.0 μm, 0.50 to 2.5 μm, 0.60 to 2.0 μm, or 0.80 to 1.5 μm. When the thickness of the mixed layer 34 is 0.10 μm or more, metallic lithium grown in the deposition layer 35 can be prevented from penetrating the mixed layer 34. Furthermore, when the thickness of the mixed layer 34 is 5.0 μm or less, lithium ions in the electrolyte solution can easily pass through and diffuse through the mixed layer 34, and metallic lithium tends to be more uniformly deposited.

[0035] The thickness of the mixed layer 34 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% of the thickness (100%) of the metallic lithium layer 33. When the thickness of the mixed layer 34 is 1.0% or more relative to the thickness of the metallic lithium layer 33, metallic lithium grown in the deposition layer 35 can be prevented from penetrating the mixed layer 34. Furthermore, when the thickness of the mixed layer 34 is 25% or less relative to the thickness of the metallic lithium layer 33, lithium ions in the electrolyte solution tend to easily pass through and diffuse through the mixed layer 34, and metallic lithium tends to be more uniformly deposited.

[0036] The thickness of the mixed layer 34 can be defined as the width of two points where the reference line, drawn from any point on the outermost surface of the mixed layer 34 toward the negative electrode current collector 31 in a cross-sectional photograph of the negative electrode active material layer 32 of the negative electrode 30, hits the upper and lower surfaces of the mixed layer 34.

[0037] The area occupied by the particles 34b is preferably 5.0 to 60 area%, 7.5 to 55 area%, 10 to 50 area%, 15 to 45 area%, 20 to 40 area%, or 25 to 35 area% relative to 100 area% of the cross section of the mixed layer 34. When the area occupied by the particles 34b is within the above range, there is a tendency for the thickness to change less even with repeated charge-discharge cycles.

[0038] The area occupied by the particles 34b can be calculated from the area of ​​the mixed layer 34 confirmed when the cross section of the negative electrode is observed using an SEM or the like, and the area of ​​the cross section of the particles 34b contained in the mixed layer 34. Note that, because metallic lithium and the second metal are different metal species, they are observed with different contrasts in an electron microscope image.

[0039] The outermost layer 34c of the mixed layer 34 on the separator 10 side is preferably made of metallic lithium. That is, the particles 34b of the second metal, which serve as nucleation starting points for metallic lithium deposition, are not attached to the surface of the negative electrode 30 but are embedded within the negative electrode 30. This makes it possible to suppress metallic lithium deposition on the surface of the mixed layer 34.

[0040] 1.1.3. Deposit Layer The deposit layer 35 is located between the mixed layer 34 and the metallic lithium layer 33. The deposit layer 35 is a layer formed by depositing metallic lithium on the metallic lithium layer 33 mainly upon charging, but may also be a layer formed by remaining metallic lithium that has been deposited upon discharging but has not completely dissolved. In this embodiment, during charging, lithium ions in the electrolyte diffuse through the mixed layer 34, depositing metallic lithium on the metallic lithium layer 33. The mixed layer 34 allows the formation of a high-density metallic lithium layer with few voids as the deposit layer 35.

[0041] Normally, the potential at the tip of the deposited metallic lithium is more negative than the reference potential of metallic lithium, and further deposition and growth of metallic lithium occurs at the tip, resulting in the formation of dendrites or a deposit layer with many voids. In this regard, in the present embodiment, when the tip of the deposited and growing metallic lithium comes into contact with the mixed layer 34, the electrical conductivity of the mixed layer 34 containing metallic lithium causes the potential at the tip of the deposited metallic lithium to rise to the reference potential of ordinary metallic lithium.

[0042] This stops the deposition growth at the tip of the metallic lithium, suppresses anisotropic growth, and allows the deposition layer 35 to grow more isotropically. Therefore, in this embodiment, a high-density deposition layer 35 with fewer voids can be formed. Furthermore, the deposition layer 35 formed in this manner has a low specific surface area and is less likely to produce decomposition products caused by breakage of the deposited metallic lithium. As a result, thickness change can be reduced even with repeated charge / discharge cycles.

[0043] The content of metallic lithium in the deposit layer 35 is preferably 95 to 100 mass %, 98 to 100 mass %, or 99 to 100 mass %, relative to the total amount of the deposit layer 35 .

[0044] The porosity of the deposit layer 35 may be less than 1%, and the deposit layer 35 may have no voids. The porosity can be calculated from the area of ​​the deposit layer 35 confirmed when the cross section of the negative electrode is observed using an SEM or the like, and the area of ​​the voids contained in the deposit layer 35.

[0045] Furthermore, the precipitate layer 35 and the metallic lithium layer 33 are both layers made of high-density metallic lithium, but they can be distinguished, for example, by the patterns obtained when the cross section of the negative electrode is subjected to X-ray diffraction, infrared spectroscopy, or Raman spectroscopy.

[0046] In this embodiment, during charging, lithium ions pass through the mixed layer 34 in an amount corresponding to the amount of lithium metal precipitated in the precipitate layer 35, forming the precipitate layer 35 so as to push up the mixed layer 34. During discharging, the precipitate layer 35 dissolves and becomes thinner, allowing lithium ions to pass through the mixed layer 34. As such, the thickness of the precipitate layer 35 varies with discharge and is not limited to a specific value, but may vary, for example, from 1 to 50 μm. From this perspective, the thickness of the precipitate layer 35 may preferably vary from 1 (0.01 μm) to 1000% (100 μm), from 5 to 750%, or from 10 (0.1 μm) to 500% (50 μm) of the thickness of the mixed layer 34 (1 μm).

[0047] The positive electrode 20 includes a positive electrode current collector 21 and a positive electrode active material layer 22 provided on a surface of the positive electrode current collector 21. Depending on the configuration of the laminate 40, the positive electrode active material layer 22 may be provided on one surface or both surfaces of the positive electrode current collector 21.

[0048] The positive electrode current collector 21 is not particularly limited as long as it is a conductive plate material, and examples thereof include metal foils such as aluminum, copper, nickel, and stainless steel.

[0049] The positive electrode active material layer 22 contains a positive electrode active material, and may contain a positive electrode conductive additive and a positive electrode binder as needed.

[0050] As the positive electrode active material, an active material that can reversibly absorb and release lithium ions, desorb and insert (intercalate) lithium ions, or dope and dedop the lithium ions and counter anions of the lithium ions can be used.

[0051] Such a positive electrode active material is not particularly limited, but for example, lithium cobalt oxide (LiCoO ２ ), lithium nickel oxide (LiNiO ２ ), lithium manganese oxide (LiMnO ２ ), lithium manganese spinel (LiMn ２ O ４ ), and LiNi ｘ Co ｙ Mn ｚ M ａ O ２ (wherein 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 the group consisting of Al, Mg, Nb, Ti, Cu, Zn, and Cr), lithium vanadium compounds (LiV ２ O ５ ), olivine-type LiMPO ４ (wherein M represents one or more elements selected from the group consisting of Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, and Zr, or VO), lithium titanate (Li ４ Ti ５ O １２ ), LiNi ｘ Coｙ Al ｚ O ２ (wherein 0.9<x+y+z<1.1) and other composite metal oxides; and organic materials such as polyacetylene, polyaniline, polypyrrole, polythiophene, and polyacene.

[0052] The conductive additive for the positive electrode may be added from the viewpoint of improving the electronic conductivity between the positive electrode active materials. Such conductive additives for the positive electrode are not particularly limited, but include, for example, carbon materials such as carbon powders such as carbon black, acetylene black, and ketjen black, and carbon nanotubes; 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. Among these, carbon materials such as carbon black, acetylene black, and ketjen black are preferred as the conductive additive for the positive electrode. By using such a conductive additive for the positive electrode, the electronic conductivity between the positive electrode active materials tends to be further improved.

[0053] The positive electrode binder may be added from the viewpoint of binding the positive electrode active materials together to form the positive electrode active material layer 22. Such a positive electrode binder is not particularly limited, and examples thereof include fluororesins 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-based fluororubber (VDF-HFP-based fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene-based fluororubber, and vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene-based fluororubber. vinylidene fluoride-based fluororubbers such as vinylidene fluoride-pentafluoropropylene-based fluororubbers (VDF-PFP-based fluororubbers), vinylidene fluoride-pentafluoropropylene-tetrafluoroethylene-based fluororubbers (VDF-PFP-TFE-based fluororubbers), vinylidene fluoride-perfluoromethylvinyl ether-tetrafluoroethylene-based fluororubbers (VDF-PFMVE-TFE-based fluororubbers), and vinylidene fluoride-chlorotrifluoroethylene-based fluororubbers (VDF-CTFE-based fluororubbers); and other resins such as cellulose, styrene-butadiene rubber, ethylene-propylene rubber, polyimide resin, polyamide-imide resin, and acrylic resin.

[0054] 1.3 Separator The separator 10 is disposed between the positive electrode 20 and the negative electrode 30, and by isolating the positive electrode 20 and the negative electrode 30, prevents a short circuit between the positive electrode 20 and the negative electrode 30. From this point of view, the separator 10 may 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.

[0055] The separator 10 is not particularly limited as long as it is a conventionally known material, but examples thereof include a microporous resin film or nonwoven fabric having electrical insulation and a porous structure, or a solid electrolyte. The separator 10 may be a single layer or a laminate of these materials.

[0056] The resin constituting the microporous resin film is not particularly limited, but examples thereof include polyimide resins and polyolefin resins such as polyethylene and polypropylene. The method for forming pores in the microporous resin film is not particularly limited, but for example, pores may be formed by stretching the resin film, or by removing a pore-forming agent from the resin film. The microporous resin film may be a single layer or a laminate.

[0057] The fibers constituting the nonwoven fabric are not particularly limited, but examples thereof include polyolefin fibers, polyimide fibers, cellulose fibers, polyester fibers, polyamide fibers, polyacrylonitrile fibers, and glass fibers.

[0058] The solid electrolyte is not particularly limited, but examples thereof include polymer solid electrolytes, oxide-based solid electrolytes such as LLZ and LLTO, and sulfide-based solid electrolytes such as LISICON.

[0059] The separator 10 may further include a layer containing a material other than the above-mentioned materials on one or both of its main surfaces. Examples of such materials include, but are not limited to, inorganic materials such as alumina, silica, zirconia, and titania; and organic materials such as polyvinylidene fluoride and carboxymethyl cellulose. The presence of such a layer further improves heat resistance and tends to suppress the deposition of transition metals eluted from the positive electrode 20 onto the surface of the negative electrode 30.

[0060] 1.4 Electrolyte The electrolyte may include a non-aqueous solvent and an electrolyte. The electrolyte may be dissolved in the non-aqueous solvent.

[0061] The non-aqueous solvent is not particularly limited, but examples thereof include cyclic carbonates, chain carbonates, and other organic solvents. The cyclic carbonates have the effect of solvating the electrolyte, and the chain carbonates have the effect of reducing the viscosity of the cyclic carbonates. The non-aqueous solvents may be used alone or in combination of two or more.

[0062] The cyclic carbonate is not particularly limited, but examples thereof include ethylene carbonate, propylene carbonate, butylene carbonate, fluoroethylene carbonate, vinylene carbonate, etc. Among these, it is preferable to contain at least propylene carbonate.

[0063] The chain carbonate is not particularly limited, but examples thereof include diethyl carbonate, dimethyl carbonate, and ethyl methyl carbonate.

[0064] The other organic solvent is not particularly limited, but examples thereof 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.

[0065] The electrolyte is not particularly limited, but may be, for example, LiPF ６ , LiClO ４ , LiBF ４ , LiCF ３ SO ３ , LiCF ３ CF ２ SO ３ , LiC(CF ３ SO ２ ) ３ , LiN(CF ３ SO ２ ) ２ , LiN(CF ３ CF ２ SO ２ ) ２ , LiN(CF ３ SO ２ ) (C ４ F ９ SO ２ ), LiN(CF３ CF ２ CO) ２ , LiBOB, LiN(FSO ２ ) ２ The electrolyte may be used alone or in combination of two or more kinds.

[0066] 1.5. Terminals The terminals 60 and 62 are connected to the positive electrode 20 and the negative electrode 30, respectively, and communicate between the inside and outside of the exterior body 50. The terminal 60 connected to the positive electrode 20 is a positive electrode terminal, and the terminal 62 connected to the negative electrode 30 is a negative electrode terminal. The terminals 60 and 62 are responsible for electrical connection to the outside.

[0067] The constituent material of the terminals 60, 62 is not particularly limited, but examples thereof include conductive materials such as aluminum, nickel, and copper. For example, the terminal 60 connected to the positive electrode 20 may be an aluminum plate, and the terminal 62 connected to the negative electrode 30 may be a nickel plate or a nickel-plated copper metal plate. The terminals 60, 62 are preferably protected with insulating tape to prevent short circuits.

[0068] The negative electrode, positive electrode, separator, and electrolyte are enclosed in the exterior case 50. The exterior case 50 prevents the non-aqueous electrolyte from leaking to the outside and prevents moisture and other foreign matter from entering the lithium secondary battery 100.

[0069] The configuration of the exterior body 50 is not particularly limited, but may include a metal foil 52 and a resin layer 54 laminated on each side of the metal foil 52, as shown in FIG.

[0070] The metal foil 52 is not particularly limited, but may be, for example, aluminum foil. The resin layer 54 is not particularly limited, but may be, for example, a polymer film such as polypropylene. The materials constituting the inner and outer resin layers 54 may be different. For example, the outer material may be a polymer with a high melting point, such as polyethylene terephthalate (PET) or polyamide (PA), and the inner polymer film may be made of polyethylene (PE), polypropylene (PP), or the like.

[0071] 2. Manufacturing Method of Lithium Secondary Battery The manufacturing method of the lithium secondary battery of this embodiment includes a preparation step of preparing a metallic lithium layer and a mixed layer formation step of forming a mixed layer containing metallic lithium and a second metal other than metallic lithium on the metallic lithium layer, as the process of forming the negative electrode 30. Note that conventionally known methods can be used for the process of forming the positive electrode 20, the process of producing the laminate 40 in which the positive electrode 20, the negative electrode 30, and the separator 10 are stacked, and the process of sealing the laminate 40 and the electrolyte solution in the exterior body 50.

[0072] 2.1 Negative Electrode Formation Process The negative electrode formation process includes a preparation process for preparing a metallic lithium layer and a mixed layer formation process for forming a mixed layer containing metallic lithium and a second metal other than metallic lithium on the metallic lithium layer. In the negative electrode formation process, the mixed layer formation process may be performed with the metallic lithium layer 33 laminated on the negative electrode current collector 31, or the mixed layer formation process may be performed on the prepared metallic lithium layer 33, and then the metallic lithium layer 33 with the mixed layer 34 formed thereon may be joined to the negative electrode current collector 31.

[0073] The preparation step is a step of preparing the metallic lithium layer 33. Specifically, the metallic lithium layer 33 may be prepared by bonding a lithium foil to the negative electrode current collector 31, or the lithium foil before bonding to the negative electrode current collector 31 may be prepared as the metallic lithium layer 33.

[0074] 2.1.2. Mixed Layer Forming Step The mixed layer forming step is a step of forming a mixed layer 34 containing metallic lithium and a second metal other than metallic lithium on the metallic lithium layer 33.

[0075] The method for forming the mixed layer 34 is not particularly limited, and examples thereof include a method of electrolytically or electrolessly plating the second metal onto the metallic lithium layer 33 and then annealing, a method of physically or chemically vapor-depositing the second metal onto the metallic lithium layer 33 and then annealing, and a method of attaching particles containing the second metal to the metallic lithium layer 33 and then annealing under pressure. Among these, by attaching particles containing the second metal to the metallic lithium layer 33 and then annealing under pressure, it is possible to form the mixed layer 34 in which particles containing the second metal are dispersed in metallic lithium.

[0076] The heating temperature during annealing is preferably 120 to 210° C., 140 to 200° C., 150 to 185° C., or 160 to 175° C. This allows particles containing the second metal to be embedded in the surface layer portion of the metallic lithium layer 33 softened by heating. The heating time during annealing is not particularly limited, but may be, for example, 10 to 120 minutes.

[0077] When the melting point of the second metal is sufficiently higher than the heating temperature, the particle size of the particles containing the deposited second metal tends to be maintained in the mixed layer 34 .

[0078] The thickness of the mixed layer 34, i.e., the embedding depth of the second metal into the metallic lithium layer 33, may be adjusted by increasing the heating temperature or heating time during annealing or by increasing the pressure applied by pressing. As the heating temperature during annealing increases, the metallic lithium softens, making it easier to embed particles containing the second metal, and increasing the pressure also tends to make it easier to embed particles containing the second metal.

[0079] Furthermore, after pressurization, the heating temperature during annealing may be temporarily increased to near the melting point of metallic lithium, which causes metallic lithium to flow and cover the embedded particles containing the second metal, and the outermost layer 34c of mixed layer 34 on the separator 10 side tends to become a layer made of metallic lithium.

[0080] The atmosphere for the mixed layer formation step is not particularly limited, but examples thereof include an oxidizing atmosphere, a reducing atmosphere, and an inert atmosphere. Among these, an inert atmosphere is preferred from the viewpoint of avoiding unexpected effects on the metallic lithium of the metallic lithium layer 33.

[0081] The present disclosure will be described below based on examples and comparative examples, but the present disclosure is not limited to the following examples.

[0082] 1. Fabrication of Lithium Secondary Battery 1.1. Example 1 A 20 μm thick lithium metal foil was placed on one main surface of an 8 μm thick copper foil serving as a negative electrode current collector. Then, copper particles having a median diameter D50 of 70 nm were applied to the exposed surface of the lithium metal foil at a concentration of 0.32 mg / cm. ２ After the copper particles were attached to the lithium foil, the foil was annealed at 160°C for 60 minutes under pressure. This resulted in a negative electrode having a mixed layer in which copper particles were dispersed in the lithium foil on the separator side of the lithium foil. The copper particles attached by the above treatment were embedded in the lithium foil, and the outermost layer on the separator side of the mixed layer was a layer made of lithium metal.

[0083] The positive electrode slurry was applied to one main surface of a 15 μm thick aluminum foil serving as a positive electrode current collector, and the positive electrode slurry was dried to form a positive electrode active material layer, thereby obtaining a positive electrode. The amount of the positive electrode active material supported in the positive electrode active material layer was 10 mg / cm. ２ The positive electrode slurry contained 95 parts by mass of LiNi as a positive electrode active material. ｘ Co ｙ Mn ｚ M ａ O ２ (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 were mixed in a solvent to prepare the powder.

[0084] A laminate was produced by alternately stacking 11 negative electrodes and 10 positive electrodes with 10 μm-thick polypropylene separators in between, with the two outermost electrodes both being negative electrodes. A nickel negative electrode terminal was connected to the negative electrode of the laminate, and an aluminum positive electrode terminal was attached to the positive electrode. The laminate and non-aqueous electrolyte were then inserted into an exterior case, and with the tips of the negative and positive electrode terminals facing out of the exterior case, the case was sealed under degassing, thereby producing a lithium secondary battery of Example 1. The non-aqueous electrolyte was a 1,2-dimethoxyethane solvent containing 4 M (mol / L) LiN(FSO ) as a lithium salt. ２ ) ２ The one to which the above was added was used.

[0085] 1.2 Examples 2 to 30 and Comparative Example 1 Lithium secondary batteries of Examples 2 to 30 were obtained in the same manner as Example 1, except that the particles of the second metal used were changed or the thickness of the mixed layer was changed as shown in Table 1. Furthermore, a lithium secondary battery of Comparative Example 1 was obtained in the same manner as Example 1, except that no particles of the second metal were used.

[0086] 2. Evaluation of Thickness Change of Negative Electrode A cycle test of a lithium secondary battery was performed using a secondary battery charge / discharge tester (manufactured by Hokuto Denko Corporation). Specifically, a charge / discharge cycle was performed in an environment of 25°C, where one charge / discharge cycle consisted of constant current and constant voltage charging at 0.2 C to 4.3 V and constant current discharging at 1 C to 3.0 V, and 100 cycles were performed. After 100 cycles, the lithium secondary battery was disassembled and the thickness of the negative electrode was measured. The thickness change per negative electrode layer was calculated using the following formula: Thickness change per negative electrode layer = (thickness of one negative electrode layer after 100 cycles) - (thickness of one negative electrode layer before the first charge)

[0087]

[0088] Fig. 3A shows an SEM photograph of a portion of a cross section of the negative electrode of the lithium secondary battery obtained in Example 1, and Fig. 3C shows an SEM photograph of a portion of a cross section of the negative electrode of the lithium secondary battery obtained in Comparative Example 1. As is clear from Fig. 3A and Fig. 3C , when a portion of a cross section of the negative electrode of the lithium secondary battery obtained in each Example was observed by SEM, the porosity of the deposit layer in each Example was lower than that in Comparative Example 1, and was generally 10% or less.

[0089] 3A, in the lithium secondary battery of the present disclosure, the metallic lithium layer and the precipitate layer are metallic lithium layers with similar densities. It was confirmed that the precipitate layer and the metallic lithium layer can be distinguished by the difference in crystallinity based on the results of X-ray diffraction in SEM observation.

[0090] FIG. 3B shows an SEM image of a portion of the cross section of the negative electrode of the lithium secondary battery obtained in Example 1, taken so as to clearly show the contrast of the second metal in the mixed layer. The image in FIG. 3B is a vertically inverted image of FIG. 3A, with the mixed layer located at the bottom. In FIG. 3B, the gray portion represents lithium, and the white portion represents the second metal (copper). As shown in FIG. 3B, the outermost layer of the mixed layer in Example 1 on the separator side is composed of metallic lithium, and the second metal (copper) is embedded in the layer. Furthermore, when the X-ray diffraction results of the mixed layer in Example 1 and other SEM observations were confirmed, it was confirmed that lithium and the second metal at least partially formed a solid solution phase.

[0091] The present disclosure has industrial applicability as a fundamental technology for lithium secondary batteries.

[0092] DESCRIPTION OF SYMBOLS 100...Lithium secondary battery, 10...Separator, 20...Positive electrode, 21...Positive electrode current collector, 22...Positive electrode active material layer, 30...Anode, 31...Anode current collector, 32...Anode active material layer, 33...Metallic lithium layer, 34...Mixed layer, 34a...Metallic lithium, 34b...Particles, 34c...Outermost layer, 35...Deposit layer, 40...Laminate, 50...Outer case, 52...Metal foil, 54...Resin layer, 60...Terminal, 62...Terminal

Claims

1. A lithium secondary battery comprising a negative electrode, a positive electrode, and a separator disposed between the negative electrode and the positive electrode, wherein the negative electrode has a negative electrode current collector, a metallic lithium layer made of metallic lithium, and a mixed layer containing metallic lithium and a second metal other than the metallic lithium, and the mixed layer is located between the metallic lithium layer and the separator.

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

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

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

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

6. The lithium secondary battery according to claim 1, wherein the mixed layer has a thickness of 0.10 to 5.0 μm.

7. The lithium secondary battery according to claim 1, further comprising a precipitated layer of metallic lithium between the mixed layer and the metallic lithium layer.

8. The lithium secondary battery according to claim 1, wherein the outermost layer of the mixed layer on the separator side is made of metallic lithium.

9. A method for producing a lithium secondary battery according to any one of claims 1 to 8, comprising the steps of: preparing a metallic lithium layer; and forming a mixed layer containing metallic lithium and a second metal other than metallic lithium on the metallic lithium layer, as a negative electrode forming step.

Images