Current collector and secondary battery

Abstract

To improve the cycle characteristics of a secondary battery with a precipitated lithium negative electrode.SOLUTION: A collector according to the present disclosure is used in a lithium-deposited electrode, and the collector includes a resin and a conductive material, and has a Young's modulus of 17 MPa or more and 60 MPa or less. Further, the secondary battery according to the present disclosure includes a positive electrode, an electrolyte layer, a negative electrode current collector, and a metallic lithium as a negative electrode active material that is deposited between the electrolyte layer and the negative electrode current collector upon charging, and the negative electrode current collector contains a resin and a conductive material, and has a Young's modulus of 17 MPa or more and 60 MPa or less.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] This application discloses a current collector and a secondary battery.

Background Art

[0002] Patent Document 1 discloses a technique for suppressing the expansion of an electrode that occurs when metallic lithium is deposited on a current collector by devising the surface shape of the current collector in a secondary battery provided with a lithium deposition type electrode. Patent Document 2 discloses a resin current collector for a lithium ion secondary battery, which comprises a conductive porous resin layer and a conductive resin layer laminated thereon. Patent Document 3 discloses a current collector for a bipolar secondary battery, which contains an elastomer and a polyolefin resin.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0004] According to the findings of the present inventor, in a secondary battery provided with a lithium deposition type negative electrode as disclosed in Patent Document 1, metallic lithium is non-uniformly deposited between the electrolyte layer and the negative electrode current collector, and there is a risk that the cycle characteristics of the battery may deteriorate. In this regard, for a secondary battery provided with a lithium deposition type negative electrode, a new technique for improving its cycle characteristics is needed.

Means for Solving the Problems

[0005] As one means for solving the above problems, this application A current collector used for a lithium precipitation type negative electrode, which contains a resin and a conductive material, and has a Young's modulus of 17 MPa or more and 60 MPa or less is disclosed.

[0006] As one of the means for solving the above problems, the present application provides a secondary battery including a positive electrode, an electrolyte layer, a negative electrode current collector, and metallic lithium as a negative electrode active material deposited between the electrolyte layer and the negative electrode current collector upon charging, wherein the negative electrode current collector contains a resin and a conductive material and has a Young's modulus of 17 MPa or more and 60 MPa or less is disclosed.

[0007] In the secondary battery of the present disclosure, the electrolyte layer may contain a solid electrolyte.

Advantages of the Invention

[0008] According to the technology of the present disclosure, the cycle characteristics of a secondary battery having a lithium precipitation type negative electrode are likely to be improved.

Brief Description of the Drawings

[0009]

Figure 1

Figure 2

Figure 3

Embodiments for Carrying Out the Invention

[0010] 1. Current Collector The current collector of the present disclosure is a current collector used for a lithium deposition type negative electrode, includes a resin and a conductive material, and has a Young's modulus of 17 MPa or more and 60 MPa or less. The "lithium deposition type negative electrode" refers to a negative electrode accompanied by the deposition of metallic lithium during charging.

[0011] 1.1 Resin The current collector of the present disclosure includes a resin. Depending on the type of resin, the Young's modulus of the current collector can be arbitrarily adjusted. Specific examples of the resin constituting the current collector of the present disclosure include, for example, fluorine-based resins (such as polyvinylidene fluoride, polytetrafluoroethylene, etc.), vinyl-based resins (such as polyvinyl chloride, polyvinyl acetate, polyvinyl alcohol, polyvinyl butyral, polyvinyl acetal, polyvinyl pyrrolidone, etc.), polyolefin-based resins (such as polyethylene, polypropylene, polymethylpentene, polycycloolefin, etc.), polyester-based resins (such as polyethylene terephthalate, etc.), acrylic-based resins (such as polymethyl acrylate, polymethyl methacrylate, etc.), and various thermoplastic resins such as copolymers thereof. Alternatively, synthetic rubbers (such as styrene-butadiene rubber and polyacrylonitrile, etc.), epoxy resins, silicone resins, and engineering plastics such as polyether nitrile can also be used. The resin may be used alone as only one type, or two or more types may be combined and used. As far as the inventor has confirmed, a current collector containing at least a vinyl-based resin (vinyl resin), and among them, a current collector containing at least a fluorine-based resin and a functional group-containing vinyl-based resin is likely to have a Young's modulus of 17 MPa or more and 60 MPa or less.

[0012] From the perspective of improving the dispersibility of the conductive material, the above resin (especially the functional group-containing vinyl-based resin) may have at least one polar functional group selected from the group consisting of an amide group, an imide group, an ether group, a hydroxyl group, a carboxyl group, a sulfonic acid group, a phosphoric acid group, a silanol group, an amino group, and a pyrrolidone group as a functional group, and the functional group concentration in the resin may be 1 to 23 mmol / g. The functional group concentration in the resin is preferably 5 to 23 mmol / g, and more preferably 10 to 23 mmol / g.

[0013] The weight-average molecular weight of the above resin is preferably 1,000 to 200,000, more preferably 2,000 to 100,000, and even more preferably 7,000 to 50,000. When the resin is a vinyl resin, the degree of polymerization is preferably 100 to 4,000, more preferably 100 to 3,000, and particularly preferably 150 to 700. The weight-average molecular weight in this specification is a value obtained by converting the weight-average molecular weight measured using gel permeation chromatography (GPC) based on the molecular weight of standard polystyrene. Specifically, as the gel permeation chromatograph, "HLC8120GPC" (trade name, manufactured by Tosoh Corporation) is used, and as the columns, four columns of "TSKgel G-4000HXL", "TSKgel G-3000HXL", "TSKgel G-2500HXL", and "TSKgel G-2000HXL" (trade names, all manufactured by Tosoh Corporation) are used, and it can be measured under the conditions of a mobile phase of tetrahydrofuran, a measurement temperature of 40°C, a flow rate of 1 mL / min, and a detector RI.

[0014] The content of the resin in the current collector is not particularly limited as long as the conductivity required for the current collector is ensured and a Young's modulus of 17 MPa or more and 60 MPa or less is achieved. For example, based on the total mass of the current collector (100% by mass), the resin may be contained in an amount of 40% by mass or more and 99% by mass or less. The content of the resin may be 45% by mass or more or 50% by mass or more, and may be 95% by mass or less or 90% by mass or less.

[0015] 1.2 Conductive Material The current collector of the present disclosure includes a conductive material together with the above resin. Depending on the types of the resin and the conductive material, the Young's modulus of the current collector can be arbitrarily adjusted. The conductive material may be any material that can impart desired conductivity to the current collector. Specific examples of the conductive material constituting the current collector of the present disclosure include carbon materials (vapor grown carbon fiber (VGCF), acetylene black (AB), ketjen black (KB), carbon nanotube (CNT), carbon nanofiber (CNF), graphene, etc.) and metal materials (nickel, aluminum, stainless steel, etc.). Only one type of the conductive material may be used alone, or two or more types may be used in combination. The conductive material may be, for example, particulate or fibrous, and its size is not particularly limited.

[0016] Among them, at least one fibrous conductive material selected from carbon nanotubes (CNT) and carbon nanofibers (CNF) is preferable, and single-layer or multi-layer fibrous conductive materials can be preferably used.

[0017] The average outer diameter of the above fibrous conductive material is not particularly limited, but is, for example, 1 nm or more, preferably 10 nm or more, more preferably 50 nm or more, and is, for example, 300 nm or less, preferably 250 nm or less, more preferably 200 nm or less. The average outer diameter can be measured by a transmission electron microscope, for example, by observing the morphology of 100 specimens, measuring the length of the minor axis, and calculating the number average value.

[0018] The average length of the above fibrous conductive material is not particularly limited, but is, for example, 0.1 μm or more, preferably 1 μm or more, more preferably 5 μm or more. The upper limit of the average length is not particularly limited, but is, for example, 100 μm or less, preferably 80 μm or less, more preferably 60 μm or less. The average length can be measured by SEM, for example, by observing the morphology of 100 specimens, measuring the length of the fiber, and calculating the number average value.

[0019] The content of the conductive material in the current collector is not particularly limited as long as the conductivity required for the current collector is ensured and a Young's modulus of 17 MPa or more and 60 MPa or less is achieved. For example, based on the total mass of the current collector (100% by mass), the conductive material may be contained in an amount of 1% by mass or more and 60% by mass or less. The content of the conductive material may be 5% by mass or more, or 10% by mass or more, and may be 55% by mass or less, or 50% by mass or less.

[0020] 1.3 Other Materials The current collector of the present disclosure may contain other materials together with the above resin and conductive material as long as the conductivity required for the current collector is ensured and a Young's modulus of 17 MPa or more and 60 MPa or less is achieved. The other materials are not particularly limited.

[0021] 1.4 Young's Modulus The current collector of the present disclosure has a Young's modulus of 17 MPa or more and 60 MPa or less. If the Young's modulus of the current collector is too high, when applied to a lithium precipitation type negative electrode, uneven surface pressure is likely to occur during charging, and metallic lithium is likely to precipitate unevenly (see Figure 2). If the Young's modulus of the current collector is too low, when applied to a lithium precipitation type negative electrode, the current collector will plastically deform excessively with the precipitation of metallic lithium, resulting in local loss of surface pressure and easy occurrence of reaction unevenness, and metallic lithium is likely to precipitate unevenly (see Figure 3). When metallic lithium precipitates unevenly, the cycle characteristics of the secondary battery are likely to deteriorate. On the other hand, if it has a Young's modulus of 17 MPa or more and 60 MPa or less like the current collector of the present disclosure, lithium is likely to precipitate uniformly when applied to a lithium precipitation type negative electrode, and the cycle characteristics of the secondary battery are likely to improve. The Young's modulus of the current collector of the present disclosure may be 20 MPa or more, 25 MPa or more, or 30 MPa or more, and may be 55 MPa or less, 50 MPa or less, or 45 MPa or less.

[0022] The "Young's modulus" of the current collector refers to the Young's modulus at 25°C and can be obtained by a general measurement method. In the present application, the Young's modulus is measured by using a Vickers hardness tester and performing a load-unloading test in accordance with JIS Z 2244, and obtaining the slope of the stress-strain curve during unloading.

[0023] 1.5 Others The current collector of the present disclosure may have any shape and size applicable to a lithium precipitation type negative electrode. The thickness of the current collector of the present disclosure may be, for example, 100 μm or less, 80 μm or less, 60 μm or less, 40 μm or less, or 20 μm or less. The planar shape of the current collector of the present disclosure can be appropriately determined according to the surface shape of the electrode. The current collector of the present disclosure may be obtained, for example, by coating a conductive paste containing a resin and a conductive material on the surface of a substrate (for example, a release film), drying it to obtain a laminate of the substrate and the current collector, and then peeling off the substrate from the laminate.

[0024] From the viewpoints of the fluidity of the paste and the finish of the current collector, the conductive paste preferably contains at least one solvent together with the resin and the conductive material. As the solvent, conventionally known solvents can be used without particular limitation. Specifically, for example, hydrocarbon solvents, aromatic solvents, ketone solvents, ether solvents, ester solvents, alcohol solvents, amide solvents, etc. can be mentioned. These solvents can be used alone or in combination of two or more. Among them, amide solvents are preferred.

[0025] From the viewpoints of fluidity and the finish of the current collector, the solid content of the above conductive paste is preferably 1 to 90% by mass, more preferably 10 to 80% by mass, and even more preferably 20 to 70% by mass.

[0026] The above conductive paste can be prepared by uniformly mixing and dispersing the above-described components using a conventionally known disperser such as a disper, paint shaker, sand mill, ball mill, pebble mill, LMZ mill, DCP pearl mill, planetary ball mill, homogenizer, twin-screw kneader, thin-film swirl-type high-speed mixer (trade names: Clear Mix, Fill Mix, etc.).

[0027] 2. Secondary battery The secondary battery of the present disclosure includes a positive electrode, an electrolyte layer, a negative electrode current collector, and metallic lithium as a negative electrode active material that is deposited between the electrolyte layer and the negative electrode current collector upon charging, and the negative electrode current collector contains a resin and a conductive material and has a Young's modulus of 17 MPa or more and 60 MPa or less.

[0028] FIG. 1 schematically shows the configuration of a secondary battery 100 according to an embodiment. As shown in FIG. 1, the secondary battery 100 according to an embodiment includes a positive electrode 10, an electrolyte layer 20, a negative electrode current collector 31, and metallic lithium 32 as a negative electrode active material that is deposited between the electrolyte layer 20 and the negative electrode current collector 31 upon charging. Here, the negative electrode current collector 31 is the current collector of the present disclosure described above.

[0029] 2.1 Positive electrode The positive electrode 10 includes at least a positive electrode active material. During charging of the secondary battery 100, lithium ions reach between the electrolyte layer 20 and the negative electrode current collector 31 from the positive electrode active material through the electrolyte layer 20, receive electrons, and deposit as metallic lithium 32. Further, during discharging of the battery, the metallic lithium 32 between the electrolyte layer 20 and the negative electrode current collector 31 dissolves (ionizes) and returns to the positive electrode 10. The form of the positive electrode 10 may be any of the forms known as the positive electrode of a secondary battery. For example, as shown in FIG. 1, the positive electrode 10 may include a positive electrode current collector 11 and a positive electrode active material layer 12.

[0030] 2.1.1 Positive electrode current collector The positive electrode current collector 11 can adopt any of the generally used ones as the positive electrode current collector of the secondary battery. The positive electrode current collector 11 may be a metal foil or a metal mesh. In particular, the metal foil is excellent in handleability and the like. The positive electrode current collector 11 may be composed of a plurality of metal foils. Examples of the metal constituting the positive electrode current collector 11 include Cu, Ni, Cr, Au, Pt, Ag, Al, Fe, Ti, Zn, Co, stainless steel, and the like. In particular, from the viewpoint of ensuring oxidation resistance, the positive electrode current collector 11 may contain Al. The positive electrode current collector 11 may have some coating layer on its surface for the purpose of adjusting resistance or the like. Also, when the positive electrode current collector 11 is composed of a plurality of metal foils, there may be some layer between the plurality of metal foils. The thickness of the positive electrode current collector 11 is not particularly limited. For example, it may be 0.1 μm or more or 1 μm or more, and may be 1 mm or less or 100 μm or less.

[0031] 2.1.2 Positive electrode active material layer The positive electrode active material layer 12 contains a positive electrode active material, and further optionally may contain an electrolyte, a conductive assistant, a binder, and the like. Further, the positive electrode active material layer 12 may contain various other additives. The content of each of the positive electrode active material, electrolyte, conductive assistant, binder, etc. in the positive electrode active material layer 12 may be appropriately determined according to the target battery performance. For example, taking the entire positive electrode active material layer 12 (total solid content) as 100% by mass, the content of the positive electrode active material may be 40% by mass or more, 50% by mass or more, or 60% by mass or more, and may be less than 100% by mass or 90% by mass or less. The shape of the positive electrode active material layer 12 is not particularly limited, and for example, it may be in the form of a sheet having a substantially flat surface. The thickness of the positive electrode active material layer 12 is not particularly limited, and for example, it may be 0.1 μm or more, 1 μm or more, 10 μm or more, or 30 μm or more, and may be 2 mm or less, 1 mm or less, 500 μm or less, or 100 μm or less.

[0032] The positive electrode active material may be a known one as the positive electrode active material of the secondary battery, and may be any material that can supply lithium to the negative electrode side during charging. For example, lithium cobaltate, lithium nickelate, LiNi1 / 3 Co 1 / 3 Mn 1 / 3 Various lithium-containing composite oxides such as O2, lithium manganate, and spinel-type lithium compounds can be used. Only one type of positive electrode active material may be used alone, or two or more types may be used in combination. The positive electrode active material may be, for example, in a particulate form, and its size is not particularly limited. The particles of the positive electrode active material may be solid particles or hollow particles. The particles of the positive electrode active material may be primary particles or secondary particles in which a plurality of primary particles are aggregated. The average particle diameter of the particles of the positive electrode active material may be, for example, 1 nm or more, 5 nm or more, or 10 nm or more, and may also be 500 μm or less, 100 μm or less, 50 μm or less, or 30 μm or less.

[0033] The surface of the positive electrode active material may be coated with a protective layer containing a lithium ion conductive oxide. That is, the positive electrode active material layer 12 may contain a composite including the above positive electrode active material and a protective layer provided on its surface. Thereby, reactions between the positive electrode active material and a sulfide (for example, a sulfide solid electrolyte described later) are likely to be suppressed. Examples of the lithium ion conductive oxide include Li3BO3, LiBO2, Li2CO3, LiAlO2, Li4SiO4, Li2SiO3, Li3PO4, Li2SO4, Li2TiO3, Li4Ti5O 12 , Li2Ti2O5, Li2ZrO3, LiNbO3, Li2MoO4, Li2WO4. The coverage rate (area ratio) of the protective layer may be, for example, 70% or more, 80% or more, or 90% or more. The thickness of the protective layer may be, for example, 0.1 nm or more or 1 nm or more, and may also be 100 nm or less or 20 nm or less.

[0034] The electrolyte that can be included in the positive electrode active material layer 12 may be a solid electrolyte, a liquid electrolyte (electrolyte solution), or a combination thereof. In particular, when the positive electrode active material layer 12 contains a solid electrolyte, the effects of the technology of the present disclosure are more likely to be enhanced.

[0035] As the solid electrolyte, those known as the solid electrolyte of a secondary battery may be used. The solid electrolyte may be an inorganic solid electrolyte or an organic polymer electrolyte. In particular, inorganic solid electrolytes are excellent in ionic conductivity and heat resistance. Examples of inorganic solid electrolytes include lithium lanthanum zirconate, LiPON, Li 1+X Al X Ge 2-X (PO4)3, oxide solid electrolytes such as Li-SiO-based glass, Li-Al-S-O-based glass, etc.; sulfide solid electrolytes such as Li2S-P2S5, Li2S-SiS2, LiI-Li2S-SiS2, LiI-Si2S-P2S5, Li2S-P2S5-LiI-LiBr, LiI-Li2S-P2S5, LiI-Li2S-P2O5, LiI-Li3PO4-P2S5, Li2S-P2S5-GeS2, etc. can be exemplified. In particular, sulfide solid electrolytes, especially those containing at least Li, S, and P as constituent elements, have high performance. The solid electrolyte may be amorphous or crystalline. The solid electrolyte may be, for example, in the form of particles. Only one type of solid electrolyte may be used alone, or two or more types may be combined and used.

[0036] The electrolytic solution may contain, for example, lithium ions as carrier ions. The electrolytic solution may be, for example, a non-aqueous electrolytic solution. For example, as the electrolytic solution, a solution in which a lithium salt is dissolved in a carbonate-based solvent at a predetermined concentration can be used. Examples of carbonate-based solvents include fluoroethylene carbonate (FEC), ethylene carbonate (EC), dimethyl carbonate (DMC), etc. Examples of lithium salts include hexafluorophosphate salts, etc.

[0037] Examples of the conductive assistant that can be included in the positive electrode active material layer 12 include carbon materials such as vapor grown carbon fiber (VGCF), acetylene black (AB), ketjen black (KB), carbon nanotube (CNT), and carbon nanofiber (CNF); and metal materials such as nickel, aluminum, and stainless steel. The conductive assistant may be, for example, particulate or fibrous, and its size is not particularly limited. Only one type of conductive assistant may be used alone, or two or more types may be used in combination.

[0038] Examples of the binder that can be included in the positive electrode active material layer 12 include butadiene rubber (BR)-based binders, isobutylene rubber (IIR)-based binders, acrylate butadiene rubber (ABR)-based binders, styrene butadiene rubber (SBR)-based binders, polyvinylidene fluoride (PVdF)-based binders, polytetrafluoroethylene (PTFE)-based binders, polyimide (PI)-based binders, polyacrylic acid-based binders, and the like. Only one type of binder may be used alone, or two or more types may be used in combination.

[0039] 2.1.3 Others In addition to the above configuration, the positive electrode 10 may have a general configuration as a positive electrode of a secondary battery. For example, tabs, terminals, etc. The positive electrode 10 can be manufactured by applying known methods. For example, the positive electrode active material layer 12 can be easily formed by molding a positive electrode mixture containing the above various components in a dry or wet manner. The positive electrode active material layer 12 may be molded together with the positive electrode current collector 11, or may be molded separately from the positive electrode current collector 11.

[0040] 2.2 Electrolyte layer The electrolyte layer 20 contains at least an electrolyte. The electrolyte layer 20 may contain a solid electrolyte, and may further optionally contain a binder or the like. In this case, the content of the solid electrolyte and the binder or the like in the electrolyte layer 20 is not particularly limited. Further, the electrolyte layer 20 may contain various additives. Also, the electrolyte layer 20 may contain a liquid component together with the solid electrolyte. Alternatively, the electrolyte layer 20 may contain an electrolytic solution, and may further have a separator or the like for holding the electrolytic solution and preventing contact between the positive electrode and the negative electrode. The thickness of the electrolyte layer 20 is not particularly limited, and for example, it may be 0.1 μm or more or 1 μm or more, and may be 2 mm or less or 1 mm or less.

[0041] As the electrolyte contained in the electrolyte layer 20, it may be appropriately selected from among those exemplified as the electrolyte that can be contained in the above-described positive electrode active material layer. In particular, when the electrolyte layer 20 contains a solid electrolyte, especially when it contains an inorganic solid electrolyte, the rigidity of the electrolyte layer 20 increases, and it is difficult for metallic lithium 32 to penetrate the solid electrolyte layer. Also, it is easy to apply pressure to the metallic lithium 32 by the electrolyte layer 20 and the negative electrode current collector 31. Further, in a case where the electrolyte layer 20 contains a solid electrolyte, the problem related to the non-uniform precipitation of the above-described metallic lithium is likely to occur, but by adopting the current collector of the present disclosure, the problem can be solved. Regarding the binder that can be contained in the electrolyte layer 20 as well, it may be appropriately selected from among those exemplified as the binder that can be contained in the above-described positive electrode active material layer. Each of the electrolyte and the binder may be used alone or in combination of two or more. The electrolyte layer 20 can be easily formed, for example, by molding a dry or wet electrolyte mixture containing the above-described electrolyte and binder or the like.

[0042] On the one hand, when the electrolyte layer 20 has an electrolytic solution or a separator, the separator may be any separator commonly used in a secondary battery. The separator may be made of a resin such as polyethylene (PE), polypropylene (PP), polyester, or polyamide, for example. The separator may have a single-layer structure or a multilayer structure. Examples of the separator with a multilayer structure include a separator with a two-layer structure of PE / PP, or a separator with a three-layer structure of PP / PE / PP or PE / PP / PE. The separator may be made of a nonwoven fabric such as a cellulose nonwoven fabric, a resin nonwoven fabric, or a glass fiber nonwoven fabric.

[0043] 2.3 Negative electrode current collector The negative electrode current collector 31 is the current collector of the present disclosure described above. Detailed description is omitted here.

[0044] 2.4 Metallic lithium as negative electrode active material In the secondary battery 100, metallic lithium 32 as a negative electrode active material is deposited between the electrolyte layer 20 and the negative electrode current collector 31 upon charging. Also, as shown in FIG. 1, the metallic lithium 32 deposited between the electrolyte layer 20 and the negative electrode current collector 31 dissolves (ionizes) with discharging and returns to the positive electrode 10. Thus, when the charging and discharging of the secondary battery 100 are repeated, the deposition and dissolution of the metallic lithium 32 are repeated on the negative electrode side.

[0045] If the Young's modulus of the negative electrode current collector is too high, as shown in Fig. 2, metallic lithium will precipitate non-uniformly during charging, and surface pressure unevenness is likely to occur. Specifically, when a rigid current collector is used in a lithium precipitation type electrode, unevenness is likely to occur in the contact between the current collector and the electrolyte layer. Also, due to very slight unevenness on the surface of the electrolyte layer, physical surface pressure unevenness is likely to occur. When metallic lithium is precipitated between the electrolyte layer and the current collector in such a state, it is considered that metallic lithium preferentially precipitates in the high surface pressure part due to the surface pressure unevenness. At the location where metallic lithium preferentially precipitates, expansion occurs by the precipitation dimension of metallic lithium, and the surface pressure is likely to increase further. Then, preferential precipitation of metallic lithium is more likely to occur, and reaction unevenness is promoted. On the other hand, the lower the Young's modulus of the negative electrode current collector, the greater the elastic deformation amount of the current collector, and it becomes easier to absorb the initial lithium precipitation displacement. However, if the Young's modulus of the negative electrode current collector is too low, as shown in Fig. 3, during charging, as metallic lithium precipitates, the current collector undergoes excessive plastic deformation, and local surface pressure loss occurs, and rather, reaction unevenness is likely to occur. Thus, whether the Young's modulus of the negative electrode current collector is too low or too high, it is likely to have an adverse effect on the cycle characteristics of the secondary battery.

[0046] In contrast, like the secondary battery 100 of the present disclosure, by adopting a negative electrode current collector 31 having a Young's modulus of 17 MPa or more and 60 MPa or less, when metallic lithium 32 precipitates between the electrolyte layer 20 and the negative electrode current collector 31 during charging, as shown in Fig. 1, metallic lithium 32 is likely to precipitate uniformly, and the cycle characteristics of the secondary battery 100 are likely to be improved.

[0047] The precipitation amount of metallic lithium 32 between the electrolyte layer 20 and the negative electrode current collector 31 is not particularly limited. It may be appropriately adjusted according to the intended battery performance. For example, the higher the SOC, the larger the amount of precipitated metallic lithium 32, and it is considered that the effect of equalizing metallic lithium 32 between the electrolyte layer 20 and the negative electrode current collector 31 is further enhanced. The precipitation amount of metallic lithium 32 is such that the charge capacity of the secondary battery 100 is, for example, 1 mAh / cm 2 or more and 3 mAh / cm 2It may be an amount as follows.

[0048] 2.5 Other members The secondary battery 100 may be an all-solid battery substantially free of a liquid electrolyte, may contain a liquid component in part together with a solid electrolyte, or may be an electrolytic solution battery. In particular, as described above, a higher effect can be expected when the secondary battery 100 contains a solid electrolyte as an electrolyte. The secondary battery 100 only needs to have at least each of the above configurations, and may have other members in addition. The members described below are an example of other members that the secondary battery 100 may have.

[0049] 2.5.1 Exterior body The secondary battery 100 may be one in which each of the above configurations is housed inside an exterior body. More specifically, the portion excluding a tab, a terminal, etc. for taking out electric power from the secondary battery 100 to the outside may be housed inside the exterior body. Any known exterior body for a battery can be adopted as the exterior body. For example, a laminate film may be used as the exterior body. Also, a plurality of secondary batteries 100 may be electrically connected and optionally stacked to form a battery pack. In this case, the battery pack may be housed inside a known battery case.

[0050] 2.5.2 Sealing resin In the secondary battery 100, each of the above configurations may be sealed with a resin. For example, at least the side surfaces (the surfaces along the stacking direction of each layer) of the positive electrode, the electrolyte layer, and the negative electrode may be sealed with a resin. Thereby, it becomes easier to suppress the entry of moisture into each layer. As the sealing resin, known curable resins or thermoplastic resins can be adopted.

[0051] 2.5.3 Restraining member The secondary battery 100 may or may not have a restraining member for restraining each of the above-described components in the thickness direction (the direction along the stacking direction of the respective layers). By applying a restraining pressure by the restraining member, the internal resistance of the battery is likely to be reduced. Further, a certain surface pressure is likely to be applied to the metallic lithium deposited between the electrolyte layer and the negative electrode current collector.

[0052] 2.6. Method for manufacturing secondary battery The above-described secondary battery 100 can be manufactured, for example, as follows. That is, the method for manufacturing the secondary battery 100 is obtaining a laminate having the positive electrode 10, the electrolyte layer 20, and the negative electrode current collector 31 in this order, and charging the laminate to deposit metallic lithium 32 between the electrolyte layer 20 and the negative electrode current collector 31, It may include the following. The manufacturing methods of the positive electrode 10, the electrolyte layer 20, and the negative electrode current collector 31 are as described above. By laminating these, the above-described laminate can be obtained. After obtaining the above laminate, pressure may be applied to the laminate in the thickness direction (lamination direction). For example, each layer constituting the laminate may be pressed and integrated, or the gaps between the layers constituting the laminate may be eliminated to reduce the interfacial resistance. The laminate can be pressurized by known means. For example, the laminate can be pressurized in the lamination direction by various pressurization methods such as CIP, HIP, roll press, uniaxial press, and die press. In particular, when the laminate is pressurized by an isostatic press such as CIP or HIP, it is considered that the lamination surface of the laminate can be more uniformly pressurized. The magnitude of the pressure applied to the laminate in the lamination direction can be appropriately determined according to the performance of the target battery. For example, the pressure may be 10 MPa or more, 50 MPa or more, 100 MPa or more, 150 MPa or more, 200 MPa or more, 250 MPa or more, 300 MPa or more, or 350 MPa or more. The pressurization time and temperature of the laminate are not particularly limited. The laminate may be charged by a method similar to the general charging method of a battery. That is, charging may be performed by connecting an external power source to the positive electrode current collector 11 and the negative electrode current collector 31 of the laminate. By charging the laminate, lithium ions are conducted from the positive electrode active material contained in the positive electrode active material layer 12 to the negative electrode current collector 31 side through the electrolyte layer 20, and between the electrolyte layer 20 and the negative electrode current collector 31, the lithium ions receive electrons and precipitate as metallic lithium 32. The manufacturing method according to the present embodiment may include general steps for manufacturing a secondary battery in addition to the above-described steps. For example, steps such as housing the laminate inside an exterior body such as a laminate film, and connecting a current collecting tab to the laminate. Specifically, for example, after connecting a current collecting tab to the current collectors 11 and 31 of the laminate (a part of the current collectors 11 and 31 may be protruded and used as the tab), while housing the laminate inside a laminate film as an exterior body, the laminate film is sealed with the tab pulled out to the outside of the laminate film, and then the laminate may be charged through the tab outside the laminate film.

Example

[0053] Hereinafter, while showing examples, the technology of the present disclosure will be described in more detail. However, the technology of the present disclosure is not limited to the following examples.

[0054] 1. Preparation of negative electrode current collector 1.1 Comparative Example 1 As the negative electrode current collector, SUS304 foil (thickness 10 μm) was prepared. Also, when the Young's modulus of the SUS304 foil was measured using a Vickers hardness tester, it was 197000 MPa.

[0055] 1.2 Comparative Example 2 As the negative electrode current collector, the following Resin Current Collector Foil 1 was prepared. That is, PVDF (polyvinylidene fluoride: weight average molecular weight of about 1 million) and VGCF (VGCF-H: trade name, manufactured by Showa Denko KK, vapor grown carbon fiber, multi-wall type, fiber length 10 - 20 μm, fiber diameter: 150 nm, specific surface area: 13 m 2 / g) were mixed at 80 mass%: 20 mass%, further diluted to 30% solids with N-methyl-2-pyrrolidone, thoroughly stirred with a disper and then dispersed with ultrasonic waves to prepare a conductive paste. Next, it was coated on a release film (Celapill WZ, manufactured by Toray Industries, Inc.) with a blade having a gap of 200 μm. Then, by drying at 100 °C for 1 hour, a laminate of the conductive film and the release film was obtained. The interface of the laminate was peeled off with tweezers to obtain a conductive film. This conductive film was used as Resin Current Collector Foil 1. When the Young's modulus of Resin Current Collector Foil 1 was measured using a Vickers hardness tester, it was 95.6 MPa.

[0056] 1.3 Comparative Example 3 In the resin current collector foil, PVDF (polyvinylidene fluoride: weight average molecular weight of about 1 million) and VGCF (VGCF-H: trade name, manufactured by Showa Denko KK, vapor grown carbon fiber, multi-wall type, fiber length 10 - 20 μm, fiber diameter: 150 nm, specific surface area: 13 m 2Except that the ratio to / g) was 90% by mass: 10% by mass, a resin current collector foil 2 as a negative electrode current collector was obtained in the same manner as in Comparative Example 2. Specifically, the conductive paste used in Comparative Example 2 was adjusted so that PVDF:VGCF = 90% by mass: 10% by mass, diluted to a solid content of 30% with N-methyl-2-pyrrolidone, sufficiently stirred with a disper, and then dispersed with ultrasonic waves to prepare a conductive paste. Next, a resin current collector foil 2 was produced in the same procedure as in Comparative Example 2. When the Young's modulus of the resin current collector foil 2 was measured using a Vickers hardness tester, it was 75.4 MPa.

[0057] 1.4 Example 1 A mixed resin of PVDF (polyvinylidene fluoride: weight average molecular weight of about 1 million) and a vinyl resin (polyvinyl butyral: weight average molecular weight of 32,000, hydroxyl group of 21 mol%, degree of butyralization of 77 mol%) was used as the resin constituting the resin current collector foil, and CNT (carbon nanotube: fiber length of about 8 μm, fiber diameter of about 40 nm, specific surface area of about 90 m 2 / g) was used. Except for this, a resin current collector foil 3 as a negative electrode current collector was obtained in the same manner as in Comparative Example 2. Specifically, the mixed resin and CNT were mixed at 80% by mass: 20% by mass, further diluted to a solid content of 30% with N-methyl-2-pyrrolidone, sufficiently stirred with a disper, and then dispersed with ultrasonic waves to prepare a conductive paste. Next, it was coated on a release film (Serapill WZ, manufactured by Toray Industries, Inc.) with a blade having a gap of 200 μm. Then, by drying at 100 °C for 1 hour, a laminate of a conductive film and a release film was obtained. The interface of the laminate was peeled off with tweezers to obtain a conductive film. This conductive film was used as the resin current collector foil 3. When the Young's modulus of the resin current collector foil 3 was measured using a Vickers hardness tester, it was 59.1 MPa.

[0058] 1.5 Example 2 In the resin current collector foil, the mixed resin and CNT (carbon nanotube: fiber length of about 8 μm, fiber diameter of about 40 nm, specific surface area of about 90 m 2Except that the ratio with ( / g) was set to 90% by mass: 10% by mass, a resin current collector foil 4 as a negative electrode current collector was obtained in the same manner as in Example 1. Specifically, the mixed resin: CNT = 90% by mass: 10% by mass was mixed, further diluted to a solid content of 30% with N-methyl-2-pyrrolidone, sufficiently stirred with a disper, and then dispersed with ultrasonic waves to prepare a conductive paste. Next, the resin current collector foil 4 was produced in the same procedure as in Example 1. When the Young's modulus of the resin current collector foil 4 was measured using a Vickers hardness tester, it was 41.7 MPa.

[0059] 1.6 Example 3 A resin current collector foil 5 as a negative electrode current collector was obtained in the same manner as in Comparative Example 2, except that vinyl resin (polyvinyl butyral: weight average molecular weight 32,000, hydroxyl group 21 mol%, butyralization degree 77 mol%) was used alone as the resin constituting the resin current collector foil. Specifically, vinyl resin and VGCF (VGCF-H: trade name, manufactured by Showa Denko KK, vapor-grown carbon fiber, multi-wall type, fiber length 10 to 20 μm, fiber diameter: 150 nm, specific surface area: 13 m 2 / g) were mixed at 80% by mass: 20% by mass, further diluted to a solid content of 30% with N-methyl-2-pyrrolidone, sufficiently stirred with a disper, and then dispersed with ultrasonic waves to prepare a conductive paste. Next, it was coated on a release film (Celapil WZ, manufactured by Toray Industries, Inc.) with a blade having a gap of 200 μm. Then, by drying at 100 °C for 1 hour, a laminate of a conductive film and a release film was obtained. The interface of the laminate was peeled off with tweezers to obtain a conductive film. This conductive film was used as the resin current collector foil 5. When the Young's modulus of the resin current collector foil 5 was measured using a Vickers hardness tester, it was 17.0 MPa.

[0060] 1.7 Comparative Example 4 Vinyl resin (polyvinyl butyral: weight average molecular weight 32,000, hydroxyl group 21 mol%, butyralization degree 77 mol%) and VGCF (VGCF-H: trade name, manufactured by Showa Denko KK, vapor-grown carbon fiber, multi-wall type, fiber length 10 to 20 μm, fiber diameter: 150 nm, specific surface area: 13 m 2Except that the ratio with (g) was set to 90% by mass: 10% by mass, a resin current collector foil 6 was obtained in the same manner as in Example 3. Specifically, vinyl resin: VGCF = 90% by mass: 10% by mass was mixed, further diluted to 30% solid content with N-methyl-2-pyrrolidone, sufficiently stirred with a disper, and then dispersed with ultrasonic waves to prepare a conductive paste. Next, the resin current collector foil 6 was produced in the same procedure as in Example 3. When the Young's modulus of the resin current collector foil 6 was measured using a Vickers hardness tester, it was 6.88 MPa.

[0061] 1.8 Comparative Example 5 Polyethylene resin (PE) was used as the resin constituting the resin current collector foil, and the ratio of PE to VGCF (VGCF-H: trade name, manufactured by Showa Denko KK, vapor-grown carbon fiber, multi-wall type, fiber length 10 to 20 μm, fiber diameter: 150 nm, specific surface area: 13 m 2 / g) was set to 50% by mass: 50% by mass to obtain a resin current collector foil 7. Specifically, PE and VGCF were mixed at 50% by mass: 50% by mass, further diluted to 30% solid content with N-methyl-2-pyrrolidone, sufficiently stirred with a disper, and then dispersed with ultrasonic waves to prepare a conductive paste. Next, it was coated on a release film (Celapil, WZ manufactured by Toray Industries, Inc.) with a blade having a gap of 200 μm. Then, a laminate of a conductive film and a release film was obtained by drying at 70 °C for 1 hour. The interface of the laminate was peeled off with tweezers to obtain a conductive film. This conductive film was used as the resin current collector foil 7. When the Young's modulus of the resin current collector foil 7 was measured using a Vickers hardness tester, it was 2.14 MPa.

[0062] 2. Fabrication of Evaluation Battery 100 mg of a sulfide glass solid electrolyte containing Li, P, and S was weighed and put into a cylindrical cylinder with a diameter of φ11.28 mm, and pressure-molded at 6 tons to prepare an electrolyte pellet. A Li foil (thickness 150 μm) was placed on the upper surface of the pellet, and the above-mentioned negative electrode current collector was placed on the lower surface, and pressed at 1 ton and constrained at 1 MPa to obtain an evaluation battery.

[0063] 3. Evaluation of Cycle Characteristics The obtained battery was connected to a charge-discharge tester, and a charge-discharge cycle test was conducted while maintaining the temperature at 25°C. Charging (deposition) was carried out with a cut-off voltage of -1V and a current density of 0.435 mA / cm 2 for 10 hours, and subsequent discharging (dissolution) was carried out with a cut-off voltage of +1V and a current density of 0.435 mA / cm 2 for 20 hours. The charge-discharge process was repeated for 10 cycles. The ratio of the discharge capacity at the 10th cycle to the discharge capacity at the 1st cycle was calculated as the capacity retention rate. In addition, a load cell was inserted into the battery to measure the change in internal pressure during charge and discharge.

[0064] 4. Calculation of the deposition area of initial deposited Li Separate from the evaluation of the above cycle characteristics, a battery was fabricated. Charging (deposition) was carried out with a cut-off voltage of -1V and a current density of 0.3 mA / cm 2 for only 1 hour, and then the battery was disassembled. The interface between the electrolyte pellet and the negative electrode current collector was peeled off, and the area ratio of the metallic lithium deposited on the negative electrode current collector was identified by surface SEM observation. Specifically, the obtained SEM image was binarized between the negative electrode current collector and the metallic lithium part, and the ratio of the area where the deposition of metallic lithium was observed to the total area of the negative electrode current collector was calculated as the deposition area ratio.

[0065] 5. Evaluation of the presence or absence of plastic deformation Based on the measured Young's modulus and the change in internal pressure during the deposition of metallic lithium, the compressive strain of the negative electrode current collector was calculated. It was determined whether the negative electrode current collector underwent plastic deformation based on whether the strain was within 5% of the elastic strain of a general resin.

[0066] 6. Evaluation results The results are shown in Table 1 below. In Table 1, the capacity retention rate of Comparative Example 1 is set to 100, and the capacity retention rates of other examples are shown in relative terms. Also, the deposition area ratio of Comparative Example 1 is set to 100, and the deposition area ratios of other examples are shown in relative terms.

[0067]

Table 1

[0068] From the results shown in Table 1, the following can be understood. First, when a negative electrode current collector made of a metal foil is employed as in Comparative Example 1, the area ratio of lithium deposited initially is smaller than when a negative electrode current collector made of a resin current collector foil is employed, and the capacity retention rate is also likely to deteriorate. This is considered to be due to the following mechanism. That is, when a rigid negative electrode current collector such as a metal foil is used in a lithium deposition type negative electrode, unevenness is likely to occur in the contact between the current collector and the electrolyte layer. Also, due to very slight irregularities on the surface of the electrolyte layer, physical surface pressure unevenness is likely to occur. When metallic lithium is deposited between the electrolyte layer and the current collector in such a state, metallic lithium preferentially deposits in the high surface pressure portions due to the surface pressure unevenness, and as a result, it is considered that the deposition area ratio of metallic lithium becomes smaller (see Fig. 2). At the locations where metallic lithium preferentially deposits, expansion occurs by the deposition dimension of metallic lithium, and the surface pressure is likely to increase further. Then, further preferential deposition of metallic lithium is likely to occur, and the reaction unevenness is promoted. As a result, it is considered that the cycle characteristics (capacity retention rate) of the secondary battery have greatly decreased.

[0069] On the other hand, when a resin current collector foil is employed as the negative electrode current collector, as the Young's modulus of the negative electrode current collector decreases, the area ratio of lithium deposited initially increases. However, when the Young's modulus of the negative electrode current collector is less than 17.0 MPa, the deposition area ratio rather decreases. When the Young's modulus of the negative electrode current collector is low, the elastic deformation amount of the negative electrode current collector is large and can absorb the initial lithium deposition displacement. However, when the Young's modulus of the negative electrode current collector is too low, the negative electrode current collector undergoes plastic deformation, resulting in surface pressure loss, which is considered to rather promote the reaction unevenness (see Fig. 3).

[0070] In contrast, as shown in Table 1, the capacity retention rate of the secondary battery shows a maximum value within the range where the Young's modulus of the negative electrode current collector is 17 MPa or more and 60 MPa or less, and it can be considered that the effect of uniformizing the deposition of metallic lithium appears. From the above results, it can be said that according to the following current collector (1) and secondary battery (2), a secondary battery with excellent cycle characteristics can be obtained.

[0071] (1) A current collector used for a lithium precipitation type negative electrode, which contains a resin and a conductive material and has a Young's modulus of 17 MPa or more and 60 MPa or less. (2) A secondary battery including a positive electrode, an electrolyte layer, a negative electrode current collector, and metallic lithium as a negative electrode active material that precipitates between the electrolyte layer and the negative electrode current collector upon charging, wherein the negative electrode current collector contains a resin and a conductive material and has a Young's modulus of 17 MPa or more and 60 MPa or less.

[0072] 7. Supplementary In addition, in the above embodiments, in order to confirm the effect of the negative electrode current collector in the lithium precipitation type negative electrode, an evaluation battery composed of metallic lithium / electrolyte layer / negative electrode current collector was fabricated. However, the technology of the present disclosure is applicable to various secondary batteries having a lithium precipitation type negative electrode. The configurations of the positive electrode and the electrolyte layer constituting the secondary battery are not limited to the configurations shown in the above embodiments.

Explanation of reference numerals

[0073] 10 Positive electrode 11 Positive electrode current collector 12 Positive electrode active material layer 20 Electrolyte layer 31 Negative electrode current collector 32 Metallic lithium 100 Secondary battery

Claims

1. A current collector used for a lithium precipitation type negative electrode, which is composed of a mixture containing a resin and a conductive material, has a Young's modulus of 17 MPa or more and 60 MPa or less, wherein the resin contains at least a vinyl-based resin, the conductive material is at least one selected from vapor-grown carbon fibers, carbon nanotubes, and carbon nanofibers, based on the total mass of the current collector (100% by mass), the resin is contained in an amount of 40% by mass or more and 99% by mass or less, and the conductive material is contained in an amount of 1% by mass or more and 60% by mass or less, Current collector.

2. A secondary battery, comprising a positive electrode, an electrolyte layer, a negative electrode current collector, and metallic lithium as a negative electrode active material that precipitates between the electrolyte layer and the negative electrode current collector upon charging, wherein the negative electrode current collector is composed of a mixture containing a resin and a conductive material and has a Young's modulus of 17 MPa or more and 60 MPa or less, the resin contains at least a vinyl-based resin, the conductive material is at least one selected from vapor-grown carbon fibers, carbon nanotubes, and carbon nanofibers, based on the total mass of the negative electrode current collector (100% by mass), the resin is contained in an amount of 40% by mass or more and 99% by mass or less, and the conductive material is contained in an amount of 1% by mass or more and 60% by mass or less, Secondary battery.

3. The secondary battery according to claim 2, wherein the electrolyte layer contains a solid electrolyte. ​

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