JP2023132317A - Collector and secondary battery - Google Patents

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

This application discloses a current collector and a secondary battery.

Patent Document 1 describes a technique for suppressing expansion of the electrode that occurs when metallic lithium is deposited on the current collector by devising the surface shape of the current collector in a secondary battery equipped with a lithium-deposited electrode. Disclosed. Patent Document 2 discloses a resin current collector for a lithium ion secondary battery that includes a conductive nonporous resin layer and a conductive resin layer laminated thereon. Patent Document 3 discloses a current collector for a bipolar secondary battery that includes an elastomer and a polyolefin resin.

JP 2019-212604 Publication Japanese Patent Application Publication No. 2018-098204 Japanese Patent Application Publication No. 2011-054492

According to the findings of the present inventors, in a secondary battery equipped with a lithium-deposited negative electrode as disclosed in Patent Document 1, metallic lithium is non-uniformly deposited between the electrolyte layer and the negative electrode current collector. There is a possibility that the cycle characteristics of the battery may deteriorate. In this regard, new technology is needed to improve the cycle characteristics of secondary batteries equipped with lithium-deposited negative electrodes.

This application, as one of the means to solve the above problems,
A current collector used in a lithium-deposited negative electrode,
contains a resin and a conductive material, and
A material having a Young's modulus of 17 MPa or more and 60 MPa or less is disclosed.

This application, as one of the means to solve the above problems,
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 is deposited between the electrolyte layer and the negative electrode current collector upon charging,
The negative electrode current collector includes a resin and a conductive material, and has a Young's modulus of 17 MPa or more and 60 MPa or less.

In the secondary battery of the present disclosure, the electrolyte layer may include a solid electrolyte.

According to the technology of the present disclosure, the cycle characteristics of a secondary battery having a lithium-deposited negative electrode can be easily improved.

The configuration of the secondary battery 100 after charging and after discharging is schematically shown. This figure schematically shows the configuration after charging of a secondary battery that employs a current collector with an excessively large Young's modulus in a lithium-precipitated negative electrode. This figure schematically shows the configuration after charging of a secondary battery that employs a current collector with an excessively small Young's modulus in a lithium-precipitated negative electrode.

1. Current Collector The current collector of the present disclosure is a current collector used in a lithium-precipitated 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 term "lithium-deposited negative electrode" refers to a negative electrode in which metallic lithium is deposited during charging.

1.1 Resin The current collector of the present disclosure includes resin. The Young's modulus of the current collector can be arbitrarily adjusted depending on the type of resin. Specific examples of the resin constituting the current collector of the present disclosure include, for example, fluorine resins (polyvinylidene fluoride, polytetrafluoroethylene, etc.), vinyl resins (polyvinyl chloride, polyvinyl acetate, polyvinyl alcohol, polyvinyl butyral, etc.). , polyvinyl acetal, polyvinylpyrrolidone, etc.), polyolefin resins (polyethylene, polypropylene, polymethylpentene, polycycloolefin, etc.), polyester resins (polyethylene terephthalate, etc.), acrylic resins (polymethyl acrylate, polymethyl methacrylate, etc.), Various thermoplastic resins such as these copolymers can be mentioned. Alternatively, engineering plastics such as synthetic rubbers (styrene-butadiene rubber, polyacrylonitrile, etc.), epoxy resins, silicone resins, and polyethernitrile can also be used. One type of resin may be used alone, or two or more types may be used in combination. As far as the present inventor has confirmed, the current collector containing at least a vinyl resin (vinyl resin), especially the current collector containing at least a fluorine resin and a functional group-containing vinyl resin, has a young temperature of 17 MPa or more and 60 MPa or less. It tends to become something with a certain rate.

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

The weight average molecular weight of the resin is preferably 1,000 to 200,000, more preferably 2,000 to 100,000, and more preferably 7,000 to 50,000. Further, when the resin is a vinyl resin, its degree of polymerization is preferably 100 to 4,000, more preferably 100 to 3,000, and particularly preferably 150 to 700. Note that 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, "HLC8120GPC" (trade name, manufactured by Tosoh Corporation) was used as a gel permeation chromatograph, and "TSKgel G-4000HXL", "TSKgel G-3000HXL", "TSKgel G-2500HXL" and "TSKgel G-2500HXL" were used as columns. The measurement can be performed using four units of "TSKgel G-2000HXL" (trade name, all manufactured by Tosoh Corporation) 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.

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, the resin may be contained in an amount of 40% by mass or more and 99% by mass or less, based on the entire mass of the current collector (100% by mass). 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.

1.2 Conductive Material The current collector of the present disclosure includes a conductive material along with the above resin. The Young's modulus of the current collector can be arbitrarily adjusted depending on the type of resin and conductive material. The conductive material may be any material as long as it 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 nanotubes (CNT), carbon nano Examples include fibers (CNF), graphene, etc.) and metal materials (nickel, aluminum, stainless steel, etc.). One type of conductive material may be used alone, or two or more types may be used in combination. The conductive material may be, for example, in the form of particles or fibers, and its size is not particularly limited.

Among these, at least one type of fibrous conductive material selected from carbon nanotubes (CNTs) and carbon nanofibers (CNF) is preferred, and single-layer or multilayer fibrous conductive materials can be suitably used.

The average outer diameter of the 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 determined by observing the morphology of, for example, 100 pieces using a transmission electron microscope, measuring the short axis length, and calculating the number average value.

The average length of the fibrous conductive material is not particularly limited, but is, for example, 0.1 μm or more, preferably 1 μm or more, and 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, and more preferably 60 μm or less. The average length can be determined by observing the morphology of, for example, 100 fibers using a SEM, measuring the fiber length, and calculating the number average value.

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, the conductive material may be contained in an amount of 1% by mass or more and 60% by mass or less, based on the entire mass of the current collector (100% by mass). 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.

1.3 Other Materials The current collector of the present disclosure may be made of other materials in addition to the above-mentioned resins and conductive materials as long as the conductivity necessary for the current collector is ensured and a Young's modulus of 17 MPa or more and 60 MPa or less is achieved. May contain materials. Other materials are not particularly limited.

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-precipitated negative electrode, uneven surface pressure tends to occur during charging, and metallic lithium tends to precipitate unevenly (see FIG. 2). If the Young's modulus of the current collector is too low, when applied to a lithium-precipitated negative electrode, the current collector will undergo excessive plastic deformation due to the precipitation of metallic lithium, causing localized surface pressure loss and uneven reaction. This tends to cause lithium metal to precipitate non-uniformly (see FIG. 3). When metallic lithium is deposited non-uniformly, the cycle characteristics of the secondary battery tend to deteriorate. On the other hand, if the current collector of the present disclosure has a Young's modulus of 17 MPa or more and 60 MPa or less, lithium is likely to be uniformly deposited when applied to a lithium-deposited negative electrode, which improves the cycle characteristics of the secondary battery. is easy 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.

Note that the "Young's modulus" of the current collector refers to Young's modulus at 25° C., and can be determined by a general measuring method. In the present application, Young's modulus is measured by performing a load-unloading test in accordance with JIS Z 2244 using a Vickers hardness tester and determining the slope of the stress-strain curve at the time of unloading.

1.5 Others The current collector of the present disclosure may have a shape and size that can be applied to a lithium-precipitated 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 depending on the planar shape of the electrode. The current collector of the present disclosure can be produced by, for example, applying a conductive paste containing a resin and a conductive material to the surface of a base material (e.g., a release film) and drying it, thereby laminating the base material and the current collector. It may be obtained by obtaining a body and then peeling off the base material from the laminate.

The conductive paste preferably contains at least one type of solvent together with the resin and the conductive material from the viewpoint of the fluidity of the paste and the finishability of the current collector. As the solvent, conventionally known solvents can be used without particular limitation. Specifically, examples thereof include hydrocarbon solvents, aromatic solvents, ketone solvents, ether solvents, ester solvents, alcohol solvents, amide solvents, and the like. These solvents can be used alone or in combination of two or more. Among these, amide solvents are preferred.

The solid content of the 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, from the viewpoint of fluidity and finish of the current collector.

The above-mentioned conductive paste can be prepared using the above-mentioned components, for example, a disper, paint shaker, sand mill, ball mill, pebble mill, LMZ mill, DCP pearl mill, planetary ball mill, homogenizer, twin-screw kneader, thin film rotating high-speed mixer (product name It can be prepared by uniformly mixing and dispersing using a conventionally known dispersing machine such as Clearmix, Filmix, etc.).

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. , 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.

FIG. 1 schematically shows the configuration of a secondary battery 100 according to an embodiment. As shown in FIG. 1, a secondary battery 100 according to an embodiment includes a positive electrode 10, an electrolyte layer 20, a negative electrode current collector 31, and deposits between the electrolyte layer 20 and the negative electrode current collector 31 due to charging. Metal lithium 32 is provided as a negative electrode active material. Here, the negative electrode current collector 31 is the above-described current collector of the present disclosure.

2.1 Positive Electrode The positive electrode 10 includes at least a positive electrode active material. When charging the secondary battery 100, lithium ions arrive from the positive electrode active material through the electrolyte layer 20 and between the electrolyte layer 20 and the negative electrode current collector 31, receive electrons, and are deposited as metallic lithium 32. . Further, when the battery is discharged, the metal lithium 32 between the electrolyte layer 20 and the negative electrode current collector 31 is dissolved (ionized) and returned to the positive electrode 10. The form of the positive electrode 10 may be any form known as a 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.

2.1.1 Positive Electrode Current Collector As the positive electrode current collector 11, any common positive electrode current collector for secondary batteries can be employed. The positive electrode current collector 11 may be a metal foil or a metal mesh. In particular, metal foil has excellent handling properties. The positive electrode current collector 11 may be made of a plurality of metal foils. Examples of metals constituting the positive electrode current collector 11 include Cu, Ni, Cr, Au, Pt, Ag, Al, Fe, Ti, Zn, Co, and stainless steel. 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 kind of coating layer on its surface for the purpose of adjusting resistance or the like. Further, when the positive electrode current collector 11 is made of a plurality of metal foils, it may have some kind of 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, or 1 mm or less or 100 μm or less.

2.1.2 Positive electrode active material layer The positive electrode active material layer 12 contains a positive electrode active material, and may optionally further contain an electrolyte, a conductive aid, a binder, and the like. Furthermore, the positive electrode active material layer 12 may contain various other additives. The content of each of the positive electrode active material, electrolyte, conductive aid, binder, etc. in the positive electrode active material layer 12 may be determined as appropriate depending on the desired battery performance. For example, 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 less than 100% by mass, assuming that the entire positive electrode active material layer 12 (total solid content) is 100% by mass. Alternatively, it may be 90% by mass or less. The shape of the positive electrode active material layer 12 is not particularly limited, and may be, for example, in the form of a substantially flat sheet. The thickness of the positive electrode active material layer 12 is not particularly limited, and may be, for example, 0.1 μm or more, 1 μm or more, 10 μm or more, or 30 μm or more, and 2 mm or less, 1 mm or less, 500 μm or less, or 100 μm or less. There may be.

The positive electrode active material may be any material known as a positive electrode active material for secondary batteries, as long as it can supply lithium to the negative electrode side during charging. For example, various lithium-containing composite oxides such as lithium cobalt oxide, lithium nickel oxide, LiNi _1/3 Co _1/3 Mn _1/3 O ₂ , lithium manganate, and spinel-based lithium compounds can be used as the positive electrode active material. can. 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 in the form of particles, for example, 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 may be secondary particles obtained by agglomerating a plurality of primary particles. 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, or 500 μm or less, 100 μm or less, 50 μm or less, or 30 μm or less.

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 include a composite including the above-described positive electrode active material and a protective layer provided on the surface thereof. This makes it easier to suppress reactions between the positive electrode active material and sulfide (for example, sulfide solid electrolyte described below). Examples of lithium ion conductive oxides include Li ₃ BO ₃ , LiBO ₂ , Li ₂ CO ₃ , LiAlO ₂ , Li ₄ SiO ₄ , Li ₂ SiO ₃ , Li ₃ PO ₄ , Li ₂ SO ₄ , Li ₂ TiO ₃ , Li ₄ Ti ₅ O ₁₂ , Li ₂ Ti ₂ O ₅ , Li ₂ ZrO ₃ , LiNbO ₃ , Li ₂ MoO ₄ and Li ₂ WO ₄ . The coverage ratio (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, or 100 nm or less or 20 nm or less.

The electrolyte that may 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 likely to be further enhanced.

As the solid electrolyte, one known as a solid electrolyte for secondary batteries may be used. The solid electrolyte may be an inorganic solid electrolyte or an organic polymer electrolyte. In particular, inorganic solid electrolytes have excellent ionic conductivity and heat resistance. Examples of the inorganic solid electrolyte include oxide solid electrolytes such as lithium lanthanum zirconate, LiPON, Li _1+X Al _X Ge _2-X (PO ₄ ) ₃ , Li-SiO glass, and Li-Al-S-O glass. ;Li ₂ S-P ₂ S ₅ , Li ₂ S-SiS ₂ , LiI-Li ₂ S-SiS ₂ , LiI-Si ₂ S-P ₂ S ₅ , Li ₂ S-P ₂ S ₅ -LiI-LiBr, Sulfide solids such as LiI-Li ₂ S-P ₂ S ₅ , LiI-Li ₂ SP ₂ O ₅ , LiI-Li ₃ PO ₄ -P ₂ S ₅ , Li ₂ S-P ₂ S ₅ -GeS ₂ An example is an electrolyte. In particular, sulfide solid electrolytes, especially sulfide solid electrolytes 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 in the form of particles, for example. One type of solid electrolyte may be used alone, or two or more types may be used in combination.

The electrolyte 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, one in which a lithium salt is dissolved at a predetermined concentration in a carbonate-based solvent can be used. Examples of carbonate-based solvents include fluoroethylene carbonate (FEC), ethylene carbonate (EC), and dimethyl carbonate (DMC). Examples of the lithium salt include hexafluorophosphate.

Examples of the conductive additive that can be included in the positive electrode active material layer 12 include 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 aid may be, for example, in the form of particles or fibers, and its size is not particularly limited. One type of conductive aid may be used alone, or two or more types may be used in combination.

Examples of binders that can be included in the positive electrode active material layer 12 include butadiene rubber (BR) binders, butylene rubber (IIR) binders, acrylate butadiene rubber (ABR) binders, styrene butadiene rubber (SBR) binders, and polyfluoride binders. Examples include vinylidene (PVdF) binders, polytetrafluoroethylene (PTFE) binders, polyimide (PI) binders, and polyacrylic acid binders. One binder may be used alone, or two or more binders may be used in combination.

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, they are tabs, terminals, etc. The positive electrode 10 can be manufactured by applying a known method. For example, the positive electrode active material layer 12 can be easily formed by dry or wet molding of a positive electrode mixture containing the various components described above. The positive electrode active material layer 12 may be molded together with the positive electrode current collector 11 or separately from the positive electrode current collector 11 .

2.2 Electrolyte layer The electrolyte layer 20 contains at least an electrolyte. The electrolyte layer 20 may contain a solid electrolyte, and may also optionally contain a binder or the like. In this case, the contents of the solid electrolyte, binder, etc. in the electrolyte layer 20 are not particularly limited. Further, the electrolyte layer 20 may contain various additives. Furthermore, the electrolyte layer 20 may include a liquid component as well as a solid electrolyte. Alternatively, the electrolyte layer 20 may contain an electrolytic solution, and may further include 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 may be, for example, 0.1 μm or more or 1 μm or more, or 2 mm or less or 1 mm or less.

The electrolyte included in the electrolyte layer 20 may be appropriately selected from those exemplified as electrolytes that can be included in the above-mentioned 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, making it difficult for the metal lithium 32 to penetrate through the solid electrolyte layer, and the electrolyte layer 20 and negative electrode current collector 31, it is easy to apply pressure to the metal lithium 32. Further, when the electrolyte layer 20 includes a solid electrolyte, the above-mentioned problem related to non-uniform precipitation of metallic lithium tends to occur, but this problem can be solved by employing the current collector of the present disclosure. The binder that may be included in the electrolyte layer 20 may be appropriately selected from among the binders that may be included in the above-mentioned positive electrode active material layer. Each of the electrolytes and binders may be used alone or in combination of two or more. The electrolyte layer 20 can be easily formed, for example, by dry or wet molding of an electrolyte mixture containing the above-mentioned electrolyte, binder, and the like.

On the other hand, when the electrolyte layer 20 includes an electrolytic solution and a separator, the separator may be any separator commonly used in secondary batteries. The separator may be made of resin such as polyethylene (PE), polypropylene (PP), polyester, and polyamide. The separator may have a single layer structure or a multilayer structure. Examples of the multi-layer separator include a PE/PP two-layer separator, a PP/PE/PP or PE/PP/PE three-layer separator, and the like. 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.

2.3 Negative Electrode Current Collector The negative electrode current collector 31 is the current collector of the present disclosure described above. A detailed explanation will be omitted here.

2.4 Metallic lithium as a negative electrode active material In the secondary battery 100, metal lithium 32 as a negative electrode active material is deposited between the electrolyte layer 20 and the negative electrode current collector 31 by charging. Further, as shown in FIG. 1, metallic lithium 32 deposited between the electrolyte layer 20 and the negative electrode current collector 31 is dissolved (ionized) with discharge and returned to the positive electrode 10. In this way, when charging and discharging of the secondary battery 100 is repeated, the precipitation and dissolution of metallic lithium 32 are repeated on the negative electrode side.

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, making surface pressure unevenness likely to occur. Specifically, when a rigid current collector is used in a lithium-deposited electrode, uneven contact between the current collector and the electrolyte layer tends to occur. Furthermore, slight unevenness on the surface of the electrolyte layer tends to cause physical surface pressure unevenness. It is thought that when metallic lithium is deposited between the electrolyte layer and the current collector in such a state, metallic lithium is preferentially deposited in the high surface pressure area due to surface pressure unevenness. At the location where metallic lithium is preferentially deposited, expansion occurs by the size of the deposited metallic lithium, and the surface pressure is likely to increase further. In this case, 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 larger the amount of elastic deformation of the current collector, and the easier it is 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 Figure 3, the current collector undergoes excessive plastic deformation due to the precipitation of metallic lithium during charging, resulting in local surface pressure loss. , rather, reaction unevenness is more likely to occur. As described above, if the Young's modulus of the negative electrode current collector is too low or too high, it tends to have a negative effect on the cycle characteristics of the secondary battery.

On the other hand, as in the secondary battery 100 of the present disclosure, by employing a negative electrode current collector 31 having a Young's modulus of 17 MPa or more and 60 MPa or less, the electrolyte layer 20 and the negative electrode current collector 31 are separated during charging. When the metallic lithium 32 is deposited during this period, as shown in FIG. 1, the metallic lithium 32 tends to be uniformly deposited, and the cycle characteristics of the secondary battery 100 tend to improve.

The amount of metallic lithium 32 deposited between the electrolyte layer 20 and the negative electrode current collector 31 is not particularly limited. It may be adjusted as appropriate depending on the desired battery performance. For example, it is considered that the higher the SOC, the greater the amount of precipitated metallic lithium 32, and the effect of making metallic lithium 32 uniform between the electrolyte layer 20 and the negative electrode current collector 31 is further enhanced. The amount of metal lithium 32 deposited may be such that the charging capacity of the secondary battery 100 is, for example, 1 mAh/cm ² or more and 3 mAh/cm ² or less.

2.5 Other Components The secondary battery 100 may be an all-solid-state battery that does not substantially contain a liquid electrolyte, or may include a part of a liquid component along with a solid electrolyte, or may be an all-solid-state battery that does not contain a liquid electrolyte. It may also be a battery. In particular, as described above, even higher effects can be expected when the secondary battery 100 includes a solid electrolyte as the electrolyte. The secondary battery 100 only needs to have at least each of the above configurations, and may include other members in addition to these. The members described below are examples of other members that the secondary battery 100 may have.

2.5.1 Exterior Body The secondary battery 100 may have each of the above-mentioned components housed inside an exterior body. More specifically, a portion of the secondary battery 100 excluding a tab or terminal for extracting power to the outside may be housed inside the exterior body. As the exterior body, any known exterior body for batteries can be used. For example, a laminate film may be used as the exterior body. Further, a plurality of secondary batteries 100 may be electrically connected and arbitrarily stacked on top of each other to form a battery pack. In this case, the assembled battery may be housed inside a known battery case.

2.5.2 Sealing Resin In the secondary battery 100, each of the above components may be sealed with resin. For example, at least the side surfaces (the surfaces along the stacking direction of each layer) of the positive electrode, electrolyte layer, and negative electrode may be sealed with resin. This makes it easier to prevent moisture from entering each layer. As the sealing resin, a known curable resin or thermoplastic resin may be employed.

2.5.3 Restraint Member The secondary battery 100 may or may not have a restraint member for restraining each of the above components in the thickness direction (direction along the stacking direction of each layer). It's okay. By applying the restraining pressure by the restraining member, the internal resistance of the battery is likely to be reduced. Further, a certain surface pressure is easily applied to the metallic lithium deposited between the electrolyte layer and the negative electrode current collector.

2.6. Method for manufacturing a secondary battery The secondary battery 100 described above can be manufactured, for example, as follows. That is, the method for manufacturing the secondary battery 100 is as follows:
Obtaining a laminate having a positive electrode 10, an 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;
may include. The methods for manufacturing each of the positive electrode 10, electrolyte layer 20, and negative electrode current collector 31 are as described above. The above-mentioned laminate can be obtained by overlapping these. After obtaining the above-mentioned laminate, pressure may be applied to the laminate in the thickness direction (stacking direction). For example, the layers constituting the laminate may be pressed and integrated, or the interfacial resistance may be reduced by eliminating gaps between the layers constituting the laminate. The laminate may be pressurized by known means. For example, the laminate can be pressed in the stacking direction by various pressing methods such as CIP, HIP, roll press, uniaxial press, mold press, etc. In particular, when the laminate is pressed using a horizontal press such as CIP or HIP, it is considered that it is easier to press the laminate surface of the laminate more uniformly. The magnitude of the pressure applied to the stack in the stacking direction can be determined as appropriate depending on the desired performance of the 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 time and temperature for applying pressure to the laminate are not particularly limited. The stacked body may be charged by a method similar to a general battery charging method. 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 stack, lithium ions are conducted from the positive electrode active material included in the positive electrode active material layer 12 to the negative electrode current collector 31 side via the electrolyte layer 20, and the electrolyte layer 20 and the negative electrode current collector 31 are connected. During this time, the lithium ions receive electrons and become metallic lithium 32 and precipitate. The manufacturing method according to this embodiment may include general steps for manufacturing a secondary battery in addition to the steps described above. Examples include a step of accommodating the laminate inside an exterior body such as a laminate film, a step of connecting a current collector tab to the laminate, and the like. Specifically, for example, after connecting a current collector tab to the current collectors 11 and 31 of the laminate (a part of the current collectors 11 and 31 may be made to protrude and be used as a tab), While the laminate is housed in 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 is sealed through the tab outside the laminate film. You may also charge the battery.

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

1. Preparation of negative electrode current collector 1.1 Comparative example 1
SUS304 foil (thickness: 10 μm) was prepared as a negative electrode current collector. Further, when the Young's modulus of the SUS304 foil was measured using a Vickers hardness meter, it was found to be 197,000 MPa.

1.2 Comparative example 2
The following resin current collector foil 1 was prepared as a negative electrode current collector. That is, PVDF (polyvinylidene fluoride: weight average molecular weight approximately 1 million) and VGCF (VGCF-H: trade name, manufactured by Showa Denko K.K., vapor grown carbon fiber, multi-wall type, fiber length 10 to 20 μm, fiber diameter: 150 nm) , specific surface area: 13 m ² /g) in a ratio of 80% by mass: 20% by mass, further diluted with N-methyl-2-pyrrolidone to a solid content of 30%, thoroughly stirred with a disper, and then ultrasonically mixed. It was dispersed to create a conductive paste. Then, it was coated onto a release film (Therapel, WZ, manufactured by Toray Industries, Inc.) using a blade with 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 a resin current collector foil 1. When the Young's modulus of the resin current collector foil 1 was measured using a Vickers hardness meter, it was 95.6 MPa.

1.3 Comparative example 3
PVDF (polyvinylidene fluoride: weight average molecular weight approximately 1 million) and VGCF (VGCF-H: trade name, manufactured by Showa Denko K.K., vapor-grown carbon fiber, multi-wall type, fiber length 10 to 20 μm, fiber Resin current collector foil 2 as a negative electrode current collector was obtained in the same manner as Comparative Example 2, except that the ratio of diameter: 150 nm, specific surface area: 13 m ² /g) was 90% by mass: 10% by mass. Ta. Specifically, the conductive paste used in Comparative Example 2 was adjusted so that PVDF:VGCF = 90% by mass: 10% by mass, and then diluted with N-methyl-2-pyrrolidone to a solid content of 30%. After thorough stirring using a disperser, the paste was dispersed using ultrasonic waves to create a conductive paste. Next, a resin current collector foil 2 was produced in the same manner as in Comparative Example 2. The Young's modulus of the resin current collector foil 2 was measured using a Vickers hardness meter and found to be 75.4 MPa.

1.4 Example 1
The resin constituting the resin current collector foil is a mixed resin of PVDF (polyvinylidene fluoride: weight average molecular weight approximately 1 million) and vinyl resin (polyvinyl butyral: weight average molecular weight 32,000, hydroxyl group 21 mol%, degree of butyralization 77 mol%). was used as a negative electrode current collector in the same manner as in Comparative Example 2, except that CNTs (carbon nanotubes: fiber length of about 8 μm, fiber diameter of about 40 nm, specific surface area of about 90 m ² /g) were used as the conductive material. A resin current collector foil 3 was obtained. Specifically, the mixed resin and CNT are mixed at a ratio of 80% by mass: 20% by mass, further diluted with N-methyl-2-pyrrolidone to a solid content of 30%, stirred thoroughly with a disper, and then mixed with ultrasonic waves. It was dispersed to create a conductive paste. Then, it was coated onto a release film (Therapel, WZ, manufactured by Toray Industries, Inc.) using a blade with 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 a resin current collector foil 3. When the Young's modulus of the resin current collector foil 3 was measured using a Vickers hardness meter, it was 59.1 MPa.

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

1.6 Example 3
A negative electrode was prepared 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%, degree of butyralization 77 mol%) was used alone as the resin constituting the resin current collector foil. A resin current collector foil 5 was obtained as a current collector. Specifically, vinyl resin and VGCF (VGCF-H: trade name, manufactured by Showa Denko Co., Ltd., 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 in a ratio of 80% by mass: 20% by mass, further diluted with N-methyl-2-pyrrolidone to a solid content of 30%, sufficiently stirred with a disper, and then dispersed with ultrasound to create a conductive paste. . Then, it was coated onto a release film (Therapel, WZ, manufactured by Toray Industries, Inc.) using a blade with 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 meter, it was 17.0 MPa.

1.7 Comparative example 4
Vinyl resin (polyvinyl butyral: weight average molecular weight 32,000, hydroxyl group 21 mol%, degree of butyralization 77 mol%) and VGCF (VGCF-H: trade name, manufactured by Showa Denko K.K., vapor grown carbon fiber, multi-layer) in the resin current collector foil. A negative electrode current collector was prepared in the same manner as in Example 3, except that the ratio of wall type, fiber length 10 to 20 μm, fiber diameter: 150 nm, specific surface area: 13 m ² /g was 90% by mass: 10% by mass. A resin current collector foil 6 as a body was obtained. Specifically, vinyl resin: VGCF = 90% by mass: 10% by mass was mixed, further diluted with N-methyl-2-pyrrolidone to a solid content of 30%, thoroughly stirred with a disper, and then dispersed with ultrasound. Then, a conductive paste was created. Next, a resin current collector foil 6 was produced in the same manner as in Example 3. When the Young's modulus of the resin current collector foil 6 was measured using a Pickers hardness meter, it was 6.88 MPa.

1.8 Comparative example 5
Polyethylene resin (PE) is used as the resin constituting the resin current collector foil, and PE and VGCF (VGCF-H: trade name, manufactured by Showa Denko K.K., vapor grown carbon fiber, multi-wall type, fiber length 10 to 20 μm) are used. , fiber diameter: 150 nm, specific surface area: 13 m ² /g), the ratio was set to 50% by mass: 50% by mass, and a resin current collector foil 7 was obtained. Specifically, PE and VGCF are mixed at a ratio of 50% by mass: 50% by mass, further diluted with N-methyl-2-pyrrolidone to a solid content of 30%, thoroughly stirred with a disper, and then dispersed with ultrasound. Then, a conductive paste was created. Then, it was coated onto a release film (Therapel, WZ, manufactured by Toray Industries, Inc.) using a blade with a gap of 200 μm. Then, by drying at 70° 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 a resin current collector foil 7. When the Young's modulus of the resin current collector foil 7 was measured using a Vickers hardness meter, it was 2.14 MPa.

2. Preparation of battery for evaluation 100 mg of a sulfide glass solid electrolyte containing Li, P, and S was weighed, put into a cylindrical cylinder with a diameter of 11.28 mm, and pressure-molded at 6 tons to prepare electrolyte pellets. 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 the pellet was pressed at 1 ton and restrained at 1 MPa to obtain a battery for evaluation.

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 battery at 25°C. Charging (deposition) was performed at a cut-off voltage of -1V, 0.435 mA/cm ² for 10 hours, and subsequent discharge (dissolution) was performed at a cut-off voltage of +1 V, 0.435 mA/cm ² for 20 hours, and charging and discharging were repeated for 10 cycles. Ta. 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 and changes in internal pressure during charging and discharging were measured.

4. Calculation of the deposition area of initially deposited Li A battery was prepared separately from the evaluation of the cycle characteristics described above, charging (deposition) was performed at a cutoff voltage of −1 V, 0.3 mA/cm ² for only 1 hour, and the battery was disassembled. The interface between the electrolyte pellet and the negative electrode current collector was peeled off, and the area ratio of metallic lithium deposited on the negative electrode current collector was determined by surface SEM observation. Specifically, the obtained SEM image was binarized between the negative electrode current collector and the metallic lithium portion, and the ratio of the area where metallic lithium was deposited to the entire area of the negative electrode current collector was determined as the precipitation area ratio. Calculated.

5. Evaluation of presence or absence of plastic deformation The compressive strain of the negative electrode current collector was calculated from the measured Young's modulus and the internal pressure change during metal lithium precipitation. It was determined whether the negative electrode current collector was plastically deformed based on whether the strain was within 5% of the elastic strain of a typical resin.

6. Evaluation Results The results are shown in Table 1 below. Note that in Table 1, the capacity retention rate of Comparative Example 1 is set as 100, and the capacity retention rates of other examples are shown in relative terms. Moreover, the precipitation area ratio of Comparative Example 1 is set as 100, and the precipitation area ratios of other examples are shown relative to each other.

The following can be seen from the results shown in Table 1. First, as in Comparative Example 1, when a negative electrode current collector made of metal foil is used, the area ratio of lithium deposited initially is smaller than when a negative electrode current collector made of resin current collector foil is used. , the capacity retention rate is also likely to deteriorate. This is thought 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-precipitated negative electrode, uneven contact between the current collector and the electrolyte layer tends to occur. Furthermore, slight unevenness on the surface of the electrolyte layer tends to cause physical surface pressure unevenness. If metallic lithium is deposited between the electrolyte layer and the current collector under such conditions, metallic lithium will preferentially deposit in areas with high surface pressure due to uneven surface pressure, and as a result, the deposition area ratio of metallic lithium will decrease. It is thought that the size will become smaller (see Figure 2). At the location where metallic lithium precipitates preferentially, expansion occurs by the precipitated size of metallic lithium, and the surface pressure tends to increase further. In this case, preferential precipitation of metallic lithium is more likely to occur, and reaction unevenness is promoted. As a result, it is thought that the cycle characteristics (capacity retention rate) of the secondary battery were significantly reduced.

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 precipitation area ratio actually decreases. When the Young's modulus of the negative electrode current collector is low, the amount of elastic deformation of the negative electrode current collector is large and the initial displacement of lithium precipitation can be absorbed. However, if the Young's modulus of the negative electrode current collector is too low, the negative electrode current collector becomes plastic. It is thought that this deformation causes surface pressure to be released, which actually promotes uneven reaction (see FIG. 3).

On the other hand, as shown in Table 1, the capacity retention rate of the secondary battery shows a maximum value when the Young's modulus of the negative electrode current collector is in the range of 17 MPa or more and 60 MPa or less, and the precipitation of metallic lithium is uniform. This can be considered to be a manifestation of the effects of 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.

(1) A current collector used in a lithium-precipitated 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, comprising 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, 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.

7. Supplement In the above example, in order to confirm the effect of the negative electrode current collector in the lithium-precipitated negative electrode, an evaluation battery consisting of metallic lithium/electrolyte layer/negative electrode current collector was fabricated. is applicable to various secondary batteries having a lithium-deposited negative electrode. The configurations of the positive electrode and electrolyte layer constituting the secondary battery are not limited to the configurations shown in the above embodiments.

10 Positive electrode 11 Positive electrode current collector 12 Positive electrode active material layer 20 Electrolyte layer 31 Negative electrode current collector 32 Metal lithium 100 Secondary battery

Claims (3)

A current collector used in a lithium-deposited negative electrode,
contains a resin and a conductive material, and
having a Young's modulus of 17 MPa or more and 60 MPa or less,
Current collector. 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 is deposited between the electrolyte layer and the negative electrode current collector upon charging,
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,
Secondary battery. the electrolyte layer includes a solid electrolyte,
The secondary battery according to claim 2.

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