Negative electrode for secondary battery, secondary battery, and method for producing negative electrode for secondary battery

Abstract

The present invention provides a negative electrode for a secondary battery, which suppresses the generation of dendrites and has high mass capacity density and volume capacity density, a secondary battery, and a method for manufacturing a negative electrode for a secondary battery. The negative electrode (13) for a secondary battery is characterized by comprising: a second three-dimensional current collector (18) composed of a sponge-like structure obtained by self-supporting a second carbon nanotube (17); a negative electrode active material (19) as a metal active material contained in the second three-dimensional current collector (18); and a plurality of seed particles (20) contained in the second three-dimensional current collector (18) and composed of a material different from the negative electrode active material (19), wherein the negative electrode (13) for a secondary battery does not contain a foil of a metal active material. The secondary battery (10) is characterized by comprising a negative electrode (13) for a secondary battery and a positive electrode (12) for a secondary battery whose thickness reversibly changes by charging and discharging, and whose thickness decreases during charging and increases during discharging.

Description

Technical Field

[0001] The present invention relates to a negative electrode for a secondary battery, a secondary battery, and a method for manufacturing the negative electrode for a secondary battery. Background Art

[0002] Metal negative electrodes using metal active materials such as metal Li (Li) have attracted much attention as negative electrodes for secondary batteries with high energy density due to their high theoretical capacity and low negative electrode potential. However, secondary batteries using metal negative electrodes have metal dendrites (Dendrite) growing on the surface of the metal negative electrode due to the dissolution / precipitation of metal accompanying charging and discharging. The grown dendrites penetrate the diaphragm and contact the positive electrode, thus causing a short circuit between the positive electrode and the negative electrode. Therefore, a metal negative electrode that suppresses the formation of dendrites has been proposed (e.g., Non-Patent Document 1).

[0003] Non-patent document 1 describes a metal negative electrode having a Li-doped MWCNT (multiwall carbon nanotubes) layer formed on the surface of a metal Li foil. In the metal negative electrode described in Non-patent document 1, the MWCNT layer regulates the inflow and outflow of Li ions, thereby suppressing the generation of dendrites.

[0004] Prior art literature

[0005] Non-patent literature

[0006] Non-patent document 1: Rodrigo V. Salvatierra et al., Advanced Materials, 30, 1803869 (2018) Summary of the invention

[0007] Problems to be solved by the invention

[0008] In the metal negative electrode of non-patent document 1, a MWCNT layer with a thickness of 25 μm is provided on a metal Li foil with a thickness of 130 μm to 230 μm. In the metal negative electrode of non-patent document 1, the metal Li foil serves as both a current collector and an active material, and contains an excess amount of metal Li that is several tens of times the amount of metal Li that contributes to charging and discharging. Therefore, in the metal negative electrode of non-patent document 1, the mass capacity density and the volume capacity density become low, and the requirements for high energy density cannot be fully met.

[0009] An object of the present invention is to provide a negative electrode for a secondary battery, a secondary battery, and a method for producing a negative electrode for a secondary battery, which can suppress the generation of dendrites and has high mass capacity density and volume capacity density.

[0010] Technical means to solve problems

[0011] According to the negative electrode for secondary batteries of the present invention, it is characterized in that it comprises: a three-dimensional current collector composed of a sponge-like structure obtained by self-supporting carbon nanotubes, a metal active material contained inside the three-dimensional current collector, and a plurality of seed particles contained inside the three-dimensional current collector and composed of a material different from the metal active material, wherein the negative electrode for secondary batteries does not contain a foil of the metal active material.

[0012] The secondary battery of the present invention is characterized by comprising: the above-mentioned negative electrode for secondary battery; and a positive electrode for secondary battery, wherein the thickness of the positive electrode for secondary battery changes reversibly by charge and discharge, and the thickness decreases during charge and increases during discharge.

[0013] The method for producing a negative electrode for a secondary battery of the present invention is characterized in that carbon nanotubes, a metal active material and seed particles are composited.

[0014] Effects of the Invention

[0015] According to the present invention, by providing a plurality of seed particles that become Li precipitation nuclei during charging, the generation of large dendrites that cause short circuits between the positive and negative electrodes can be suppressed. In addition, a metal active material is contained inside the three-dimensional current collector, and by making the foil not contain the metal active material, the mass capacity density and volume capacity density can be improved. Furthermore, the three-dimensional current collector is composed of a sponge-like structure, so that the thickness changes reversibly during charging and discharging, and the space in the secondary battery can be effectively utilized to improve the volume capacity density.

[0016] The interior of the three-dimensional collector composed of a sponge-like structure obtained by self-supporting carbon nanotubes contains metal active materials and multiple seed particles. Since the foil does not contain metal active materials, a negative electrode for a secondary battery, a secondary battery, and a method for manufacturing a negative electrode for a secondary battery can be provided, which can inhibit the formation of dendrites and has high mass capacity density and volume capacity density. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 Schematic diagram showing the structure of the secondary battery during charge and discharge according to the present embodiment.

[0018] Figure 2 This is a flowchart for explaining a first example of the composite film forming step in the method for producing a secondary battery negative electrode according to the present embodiment.

[0019] Figure 3 1 is a flow chart illustrating a second example of the composite film forming step.

[0020] Figure 4 It is a flowchart explaining the third example of the composite film forming process.

[0021] Figure 5This is a flowchart for explaining another method for producing the negative electrode for a secondary battery according to the present embodiment.

[0022] Figure 6 is a schematic diagram of a laminate according to Example 11.

[0023] Figure 7 This is a photograph of the upper surface of the laminated body according to Example 11.

[0024] Figure 8 This is a schematic diagram of a stacked body after the metal active material is electrochemically retained in the first electrode in the test cell of Example 11.

[0025] Fig. 9 This is a photograph of the upper surface of the first electrode after the metal active material was electrochemically retained on the first electrode in the test cell of Example 11.

[0026] Fig.10 This is a photograph of the lower surface of the second electrode after the metal active material was electrochemically retained on the first electrode in the test cell of Example 11.

[0027] Fig.11 This is a photograph of the upper surface of the second electrode after the metal active material is electrochemically retained on the first electrode in the test cell of Example 11.

[0028] Fig.12 This is a photograph of the upper surface of the laminated body according to Example 12.

[0029] Fig.13 This is a photograph of the upper surface of the first electrode after the metal active material was electrochemically retained on the first electrode in the test cell of Example 12.

[0030] Fig.14 This is a photograph of the lower surface of the second electrode after the metal active material was electrochemically retained on the first electrode in the test cell of Example 12.

[0031] Fig.15 This is a graph showing the cycle test results of the test battery of Example 13.

[0032] Fig.16 This is a graph showing the cycle test results of the test battery of Comparative Example 1.

[0033] Fig.17 This is a graph showing the cycle test results of the test battery of Comparative Example 2. DETAILED DESCRIPTION

[0034] Hereinafter, this embodiment will be described in detail with reference to the accompanying drawings.

[0035] 1. Overall structure

[0036] exist Figure 1 In the figure, the secondary battery 10 (10A, 10B) according to the present embodiment includes a separator 11, a positive electrode for a secondary battery (hereinafter referred to as a positive electrode) 12 (12A, 12B), a negative electrode for a secondary battery (hereinafter referred to as a negative electrode) 13 (13A, 13B), an electrolyte (not shown) and a container (not shown).

[0037] The secondary battery 10A during charging includes a positive electrode 12A that shrinks and a negative electrode 13A that expands via a separator 11. The secondary battery 10B during discharging includes a positive electrode 12B that expands and a negative electrode 13B that shrinks via a separator 11. The secondary battery 10 of this embodiment is a lithium ion secondary battery in which lithium (Li) ions move between the positive electrode 12 and the negative electrode 13 via the separator 11 during charging and discharging.

[0038] The secondary battery 10 includes a positive electrode 12 on one surface of a separator 11 and a negative electrode 13 on the other surface of the separator 11. The secondary battery 10 is configured by housing the separator 11, the positive electrode 12, the negative electrode 13, and an electrolytic solution in a container.

[0039] The electrolyte is not particularly limited, and commonly used electrolytes such as non-aqueous electrolytes, ionic liquids, and gel electrolytes can be used. For example, the non-aqueous electrolyte can be prepared by dissolving 1.0 mol / L of LiPF6 in a mixed solution of ethylene carbonate (EC) and dimethyl carbonate (DMC). The volume ratio of EC to DMC is generally about 1:2.

[0040] The container is not particularly limited, and a metal can such as iron, stainless steel, or aluminum generally used as a battery case can be used. From the viewpoint of energy density per unit mass, a metal-resin composite material in which a metal foil and a resin film are laminated is preferred.

[0041] The diaphragm 11 can be composed of a microporous polymer film. As the microporous polymer film, microporous films or non-woven fabrics of polyolefins, polyesters, polyacrylonitrile, polyphenylene sulfide, polyimide or fluororesin can be cited. The diaphragm 11 can also be composed of a sponge-like structure obtained by self-supporting insulating fibers. The sponge-like structure is a film with multiple gaps inside. As the sponge-like structure, for example, non-woven fabric can be cited. The insulating fiber is a boron nitride nanotube (BNNT) or an organic nanofiber. As organic nanofibers, cellulose nanofibers (CNF) and chitin nanofibers can be cited.

[0042] The positive electrode 12 can use various positive electrodes used in general secondary batteries. In particular, if a positive electrode whose thickness changes reversibly due to charging and discharging is used, and whose thickness decreases during charging (12A) and increases during discharging (12B), the space in the secondary battery can be effectively utilized, which is preferred. When the volume of the positive electrode 12 changes due to charging and discharging, the area of ​​the surface in contact with the separator 11 does not substantially change, and it shrinks or expands by changing the thickness. That is, the volume of the positive electrode 12 changes according to the thickness.

[0043] The positive electrode active material 16 (16A, 16B) uses lithium cobalt oxide (LiCoO2), lithium manganese oxide (LiMn2O4), lithium iron phosphate (LiFePO4), NMC (LiNi x Mn y Co z O2) or NCA(LiNi x Co y Al z O2) and other lithium transition metal composite oxides, and sulfur and other active materials that react with lithium to form compounds and change in volume. As the positive electrode active material 16, when using sulfur and other active materials that react with lithium and change in volume, the change in the thickness of the positive electrode 12 during charge and discharge becomes larger, and the expansion and contraction of the positive electrode 12 during charge and discharge and the negative electrode 13 obtained using the metal active material described later are offset, and the change in the thickness of the secondary battery 10 is suppressed. Therefore, as the positive electrode active material 16, it is preferred to use sulfur and other active materials that react with lithium and change in volume. The larger the volume change, the higher the volume capacity density of the active material, preferably an active material with a volume change of more than 1.15 times, more preferably an active material with a volume change of more than 1.3 times, and particularly preferably an active material with a volume change of more than 1.6 times. When using an active material with a volume change of more than 1.15 times, in order to make the volume change reversible, it is preferred that the positive electrode active material 16 is contained inside the first three-dimensional collector 15 composed of a sponge-like structure obtained by self-supporting the first carbon nanotube (CNT) 14.

[0044] The negative electrode (negative electrode for secondary battery) 13 according to the present embodiment is described below. The thickness of the negative electrode 13 changes reversibly due to charging and discharging, and the thickness increases during charging (13A), and the thickness decreases during discharging (13B). When the volume of the negative electrode 13 changes due to charging and discharging, the area of ​​the surface in contact with the separator 11 does not substantially change, and the negative electrode 13 expands or contracts due to the change in thickness. That is, the volume of the negative electrode 13 changes according to the thickness.

[0045] The negative electrode 13 includes: a second three-dimensional current collector 18 having a sponge-like structure obtained by self-supporting a second carbon nanotube (CNT) 17, a negative electrode active material 19 (19A, 19B) as a metal active material contained in the second three-dimensional current collector 18, and a plurality of seed particles 20 contained in the second three-dimensional current collector 18 and composed of a material different from the negative electrode active material 19. Figure 1 It is shown that the negative electrode active material 19 remains during discharge ( 19B), but there are cases where it does not remain during discharge.

[0046] The sponge-like structure of the second three-dimensional current collector 18 is formed by the entanglement of a plurality of second CNTs 17. The length of the second CNTs 17 is preferably 1 μm or more. By setting the length of the second CNTs 17 to be 1 μm or more, a plurality of second CNTs 17 are entangled with each other, thereby ensuring the self-supporting property of the sponge-like structure.

[0047] The diameter of the second CNT 17 is smaller than the diameter of the seed particle 20. The diameter of the second CNT 17 is preferably less than 20 nm, more preferably less than 15 nm, and most preferably less than 10 nm. The smaller the diameter of the second CNT 17, the better the softness of the sponge-like structure. In addition, the smaller the diameter of the second CNT 17, the larger the specific surface area of ​​the second CNT 17, and therefore the more the number of seed particles 20 as Li precipitation nuclei described later.

[0048] The specific surface area of ​​the second CNT17 is 200m 2 / g or more. By making the specific surface area of ​​the second CNT17 200m 2 / g or more, the number of seed particles 20 serving as Li precipitation nuclei increases, thereby further suppressing the generation of dendrites. The specific surface area of ​​the second CNT 17 is preferably 300 m 2 / g or more, particularly preferably 400 m 2 In addition, if the specific surface area of ​​the second CNT 17 is too large, side reactions such as decomposition of the electrolyte may occur. Therefore, the specific surface area of ​​the second CNT 17 is preferably 1200 m 2 / g or less, particularly preferably 800m 2 / g or less.

[0049] The second CNT17 is a carbon nanotube having an average number of layers of more than 1 layer and less than 10 layers. The fewer the average number of layers of the second CNT17, the smaller the diameter of the second CNT17, and the easier it is for multiple second CNT17 to entangle with each other, so that the self-supporting property as a sponge-like structure can be more reliably ensured. In addition, the fewer the average number of layers of the second CNT17, the larger the specific surface area of ​​the second CNT17, and therefore the more the number of seed particles 20 serving as Li precipitation nuclei. However, if the average number of layers of the second CNT17 is too few, the specific surface area of ​​the second CNT17 is too large. The average number of layers of the second CNT17 is preferably more than 1 layer and less than 5 layers, and particularly preferably more than 2 layers and less than 5 layers.

[0050] The negative electrode active material 19 is preferably composed of at least one selected from the group consisting of Li, Na, Mg, Ca, K, Al, and Zn. The material of the negative electrode active material 19 is Li in the present embodiment. The negative electrode active material 19 has a particle-like structure in which Li is precipitated around the seed particles 20 in the present embodiment. The negative electrode active material 19 may also be configured such that the Li precipitated around the plurality of seed particles 20 are combined with each other to fill the voids of the sponge-like structure of the second three-dimensional current collector 18.

[0051] The value obtained by dividing the mass of the negative electrode active material 19 during charging by the mass of the plurality of second CNTs 17 constituting the sponge-like structure of the second three-dimensional current collector 18 is preferably greater than 1. By setting this value to be greater than 1, the mass ratio and volume ratio of the second three-dimensional current collector 18 relative to the mass of the secondary battery 10 can be reduced, and the mass capacity density and volume capacity density can be increased. The above value is more preferably greater than 2, and particularly preferably greater than 4.

[0052] The value obtained by multiplying the mass of the negative electrode active material 19 during charging by the mass reference capacity of the negative electrode active material is preferably 5 times or less relative to the design capacity of a pair of positive and negative electrodes in the secondary battery 10. That is, the value obtained by multiplying the mass of the negative electrode active material 19 during charging by the mass reference capacity of the negative electrode active material and dividing it by the design capacity of a pair of positive and negative electrodes in the secondary battery 10 ([mass of the negative electrode active material 19 during charging] × [mass reference capacity of the negative electrode active material] / [design capacity of a pair of positive and negative electrodes in the secondary battery 10]) is set to 5 times or less, so that the negative electrode active material 19 (Li) in the second three-dimensional current collector 18 will not be excessive, and the mass capacity density and volume capacity density can be improved. The above value is more preferably 3 times or less, and particularly preferably 2 times or less. For example, when the design capacity of the secondary battery is set to 4 mAh / cm per unit electrode area relative to a pair of positive and negative electrodes. 2 In the case of negative electrode, if the negative electrode uses 2 mg / cm2 per unit electrode area, 2The result of multiplying the mass of metal Li per unit electrode area by the mass base capacity of metal Li 3861 mAh / g is 7.72 mAh / cm 2 , so the above value is 1.93.

[0053] The seed particles 20 are preferably composed of at least one selected from the group consisting of C, Mg, Al, Si, Sn, Zn, Cu, Ag, Au, and Pt. These materials are materials that react with Li (negative electrode active material 19) to form an alloy, materials that form a compound with Li, or materials that become Li precipitation nuclei. For example, Mg, Al, Si, Sn, Ag, Au, and Pt among the above materials form an alloy with Li. C forms a compound with Li. Zn and Cu do not form an alloy with Li, but become Li precipitation nuclei. The seed particles 20 are composed of Cu in this embodiment.

[0054] The number of seed particles 20 per unit electrode area is preferably 1×10 8 Pieces / cm 2 Because Li is precipitated around the seed particles 20, the more the number of seed particles 20 per unit electrode area, the less likely it is that Li precipitated around each seed particle 20 will grow, so that the formation of dendrites is suppressed. The charge capacity per unit electrode area of ​​a pair of positive and negative electrodes is 4 mAh / cm 2 When the mass of metal Li deposited per unit electrode area on the negative electrode is 1.04 mg / cm 2 , the volume is 1.94×10 -3 cm 3 / cm 2 Therefore, the amount of metal Li precipitation per single seed particle can be reduced to 1.94×10 -11 cm 3 , i.e. 19.4 μm 3 The number of seed particles 20 per unit electrode area is more preferably 1×10 10 Pieces / cm 2 More preferably, 1×10 12 Pieces / cm 2 The amount of metal Li precipitated per single seed particle can be further reduced to 0.194 μm 3 Below, further reduced to 0.00194μm 3 The reason is that the generation of dendrites can be more effectively suppressed, and the surface area of ​​metal Li can be increased to reduce overvoltage.

[0055] The value obtained by dividing the mass of the negative electrode active material 19 during charging by the mass of the seed particles 20 is preferably 3 or more. By setting this value to 3 or more, the mass ratio and volume ratio of the negative electrode 13 to the mass of the secondary battery 10 can be reduced, and the mass capacity density and volume capacity density can be improved. The above value is more preferably 10 or more, and particularly preferably 30 or more.

[0056] The value obtained by dividing the thickness of the negative electrode 13 during charging by the thickness during discharging is preferably 1.15 or more, more preferably 1.5 or more, and particularly preferably 2.0 or more. As shown in the embodiments described later, the volume change of the active material during charging and discharging is determined by the design capacity of the battery, so the greater the value obtained by dividing the thickness during charging by the thickness during discharging, the thinner the thickness of the negative electrode during discharging and charging, and the volume of the secondary battery can be reduced. In addition, the value obtained by dividing the mass of the negative electrode 13 during charging by the mass during discharging is preferably 1.15 or more, more preferably 1.5 or more, and particularly preferably 2.0 or more. As shown in the embodiments described later, the mass change of the active material during charging and discharging is determined by the design capacity of the battery, so the greater the value of the mass during charging divided by the mass during discharging, the smaller the mass of the negative electrode during discharging and charging, which can make the secondary battery lightweight.

[0057] Since the negative electrode 13 has a second three-dimensional current collector 18 with higher conductivity, it does not contain a foil of a metal active material. When the foil of the metal active material is contained, the mass and volume of the negative electrode become larger, resulting in a decrease in mass capacity density and volume capacity density. In addition, if the foil of the metal active material in contact with the entire surface of the negative electrode is contained, the foil will hinder the volume change of the negative electrode, and stress will be generated between the foil and the negative electrode, which will become the cause of the degradation of the battery characteristics. The negative electrode 13 preferably does not contain another collector foil composed of a material different from the metal active material. In addition, it is preferred that the positive electrode 12 also does not contain a collector foil.

[0058] 2. Manufacturing method

[0059] A method for producing the negative electrode (negative electrode for secondary battery) 13 according to this embodiment will be described. The negative electrode 13 is obtained by compositely forming the second CNT 17, the negative electrode active material 19, and the seed particles 20. An example of a method for producing the negative electrode 13 will be described below.

[0060] The manufacturing method of the negative electrode 13 includes: a composite film forming step of forming a composite film containing seed particles 20 on a second three-dimensional current collector 18 composed of a sponge-like structure obtained by self-supporting the second CNT 17; and a metal active material retaining step of retaining the negative electrode active material 19 as a metal active material in the composite film.

[0061] The first example of the composite film forming process is described. Figure 2As shown, in the composite film forming step, a dispersion 34 is prepared using the second CNT 17 , the seed particles 20 , and the dispersion medium 32 , and the dispersion 34 is used to form a composite film 36 containing the seed particles 20 on the second three-dimensional current collector 18 .

[0062] The second CNT17 can be synthesized by CVD method. For example, Japanese Patent No. 5447367, Japanese Patent No. 5862559, DY Kim, H. Sugime, K. Hasegawa, T. Osawa, and S. Noda, Carbon 49 (6), 1972-1979 (2011)., Z. Chen, DY Kim, K. Hasegawa, T. Osawa, and S. Noda, Carbon 80, 339-350 (2014) and the like record the fluidized bed CVD method. The second CNT17 can also be synthesized by floating catalyst CVD method and substrate supported catalyst CVD method. Thus, a second CNT17 with a long size (diameter of less than 20 nm and length of more than 1 μm) is obtained.

[0063] As the seed particles 20, for example, copper particles are used. The copper particles can be synthesized by a colloidal chemical wet method or by a dry method such as an evaporation method in a gas. As the dispersion medium 32, water or an organic solvent is used. The organic solvent is ethanol, isopropanol, etc. The dispersion 34 is prepared by co-dispersing the second CNT 17 and the seed particles 20 in the dispersion medium 32. The composite film 36 is formed by removing the dispersion medium 32 from the dispersion 34. The dispersion medium 32 is removed from the dispersion 34, for example, by filtering the dispersion 34 using a filter. In the process of removing the dispersion medium 32 from the dispersion 34, the second CNT 17 absorbs the seed particles 20 while forming a network using van der Waals forces and is integrated on the surface of the filter. In this way, the seed particles 20 are taken into the second three-dimensional current collector 18 (see Figure 1 ) in the gap between the two layers, a composite film 36 containing seed particles 20 is formed on the second three-dimensional current collector 18. The composite film 36 is separated from the filter and recovered as a self-supporting film. In addition, the composite film 36 is dried using a dryer before or after separation from the filter as needed. The composite film 36 is annealed after drying. In addition, instead of filtering the dispersion 34 with a filter and drying it, the dispersion 34 can also be applied and dried.

[0064] A second example of the composite film forming process is described. This is not limited to the case where the seed particles 20 are used. Figure 3As shown, in the composite film forming process, after the seed particle material 38 is used to precipitate the seed particle 20 on the second CNT17, the second CNT17 with the precipitated seed particle 20 can be used to form the composite film 36. Specifically, in the composite film forming process, first, the second CNT17 and the seed particle material 38 are placed in the solvent 40, the second CNT17 is dispersed in the solvent 40, and the seed particle material 38 is dissolved in the solvent 40. As the seed particle material 38, for example, copper sulfate, copper hydroxide, and copper acetate are used. Thereafter, a reducing agent (for example, hydrazine, sodium borohydride, polyvinyl pyrrolidone) is added to the solvent 40 containing the second CNT17 and the seed particle material 38, and the seed particle 20 is precipitated on the second CNT17 by a chemical reduction method or a photoreduction method. Then, the composite film 36 is formed by filtering the solvent 40 containing the second CNT17 and the seed particle material 38, for example, with a filter.

[0065] A third example of the composite film forming process is described. Figure 4As shown, in the composite film forming process, after the second three-dimensional current collector 18 is formed, the seed particles 20 can also be precipitated on the second CNT17 of the second three-dimensional current collector 18. Specifically, in the composite film forming process, first, a dispersion 42 in which the second CNT17 is dispersed in a dispersion medium 32 is prepared, and the second three-dimensional current collector 18 is formed using the dispersion 42. The second three-dimensional current collector 18 is formed by removing the dispersion medium 32 from the dispersion 42. The dispersion medium 32 is removed from the dispersion 42, for example, by filtering the dispersion 42 using a filter. By removing the dispersion medium 32, the second CNT17 is integrated on the surface of the filter, and a second three-dimensional current collector 18 consisting of a sponge-like structure obtained by self-supporting the second CNT17 is obtained. The second three-dimensional current collector 18 is separated from the filter and recovered as a self-supporting film. In addition, in the composite film forming process, a solution 44 in which the seed particle material 38 is dissolved in a solvent 40 is prepared. As the solution 44, for example, a copper sulfate aqueous solution or a copper nitrate ethanol solution can be used. The second three-dimensional current collector 18 is immersed in the solution 44, and after the second three-dimensional current collector 18 is taken out from the solution 44, it is dried, so that the seed particle material 38 (for example, copper sulfate or copper nitrate) can be retained on the second three-dimensional current collector 18. By subjecting the second three-dimensional current collector 18 retaining the seed particle material 38 to an annealing treatment (for example, 800° C., 5 minutes) in a reducing atmosphere (for example, a hydrogen-argon mixed gas), the seed particles 20 are precipitated on the second CNT 17 of the second three-dimensional current collector 18, thereby forming a composite film 36. In addition, the second three-dimensional current collector 18 can also be immersed in the solution 44, and the second three-dimensional current collector 18 can be used as an electrode to precipitate the seed particles 20 on the second CNT 17 of the second three-dimensional current collector 18 by electroplating. Then, the second three-dimensional current collector 18 with the precipitated seed particles 20 is taken out from the solution 44 and dried, thereby forming a composite film 36.

[0066] The first example of the metal active material retention process is described. In the metal active material retention process, first, a negative electrode precursor (not shown) is prepared in which a metal foil constituting the negative electrode active material 19 (Li) is stacked on a composite film 36. Next, an electrolyte (not shown) is prepared, in which a negative electrode precursor and an electrode (not shown) serving as the opposite pole of the negative electrode precursor are set. Then, the negative electrode precursor and the electrode are used for charging and discharging. Through charging and discharging, in the negative electrode precursor, the negative electrode active material 19 is precipitated around the seed particles 20 of the composite film 36. That is, the negative electrode active material 19 as a metal active material is retained in the composite film 36. As a result, a negative electrode 13 composited by the second CNT 17, the negative electrode active material 19, and the seed particles 20 is obtained.

[0067] A second example of the metal active material retention process is described. In this example, instead of performing charge and discharge using a negative electrode precursor (not shown), the negative electrode precursor is heated to melt the metal foil constituting the negative electrode active material 19 (Li). The heating temperature is, for example, 200°C. The molten metal enters the voids of the second three-dimensional current collector 18 of the negative electrode precursor and becomes the negative electrode active material 19. As a result, a negative electrode 13 formed by a composite of the second CNT 17, the negative electrode active material 19, and the seed particles 20 is obtained.

[0068] The third example of the metal active material retention process is described. In this example, instead of using a foil of a metal constituting the negative electrode active material 19, a positive electrode (not shown) having a positive electrode active material is used, and the positive electrode active material contains metal ions for constituting the negative electrode active material 19. In the metal active material retention process, an electrolyte (not shown) is first prepared, and a composite film 36 and a positive electrode are set in the electrolyte. Then, by charging using the composite film 36 and the positive electrode, the negative electrode active material 19 is precipitated around the seed particles 20 of the composite film 36. As a result, a negative electrode 13 composed of a second CNT 17, a negative electrode active material 19 and a seed particle 20 is obtained. As a positive electrode having a positive electrode active material containing a metal ion for constituting the negative electrode active material 19, a negative electrode precursor in which a foil of a metal constituting the negative electrode active material 19 (Li) is stacked on the composite film 36 as described in the first example of the metal active material retention process can also be used.

[0069] Another method for manufacturing the negative electrode 13 is described. In this example, the composite film forming step and the metal active material retaining step are not performed. Figure 5 As shown, a dispersion 46 is prepared using a second CNT 17, particles of a negative electrode active material 19, seed particles 20 and a dispersion medium 32, and the negative electrode 13 is formed by removing the dispersion medium 32 from the dispersion 46. The dispersion 46 is prepared by co-dispersing the second CNT 17, particles of the negative electrode active material 19 and seed particles 20 in the dispersion medium 32. For example, the dispersion 46 is filtered using a filter to remove the dispersion medium 32 from the dispersion 46. In the process of removing the dispersion medium 32 from the dispersion 46, the second CNT 17 introduces particles of the negative electrode active material 19 and seed particles 20 while forming a network using van der Waals forces and integrating on the surface of the filter. The particles of the negative electrode active material 19 and the seed particles 20 are introduced into the second three-dimensional current collector 18 (refer to Figure 1 ), a negative electrode 13 containing particles of a negative electrode active material 19 and seed particles 20 is formed inside the second three-dimensional current collector 18. The negative electrode 13 is separated from the filter and recovered as a self-supporting film.

[0070] Another method for manufacturing the negative electrode 13 is described. In this example (not shown), a second CNT 17 having precipitated seed particles 20 is prepared, a dispersion is prepared using the second CNT 17 having precipitated seed particles 20, particles of the negative electrode active material 19, and a dispersion medium, and the negative electrode 13 is formed by removing the dispersion medium from the dispersion. The description of the method for precipitating the seed particles 20 on the second CNT 17 is omitted. When removing the dispersion medium, the dispersion is filtered using a filter, for example. Thus, a negative electrode 13 containing particles of the negative electrode active material 19 and seed particles 20 is formed on the surface of the filter inside the second three-dimensional current collector 18. The negative electrode 13 is separated from the filter and recovered as a self-supporting film.

[0071] 3. Function and effect

[0072] According to the negative electrode 13 of this embodiment, by having a plurality of seed particles 20 that become Li precipitation nuclei during charging, the generation of larger dendrites that cause short circuit between the positive electrode and the negative electrode can be suppressed. In addition, the negative electrode 13 contains a negative electrode active material 19 as a metal active material inside the second three-dimensional current collector 18, and the foil without the metal active material can improve the mass capacity density and volume capacity density. Moreover, the second three-dimensional current collector 18 of the negative electrode 13 is composed of a sponge-like structure, whereby the thickness changes reversibly during charging and discharging, and the space inside the secondary battery 10 can be effectively utilized to improve the volume capacity density.

[0073] The negative electrode 13 includes a plurality of seed particles 20 inside the second three-dimensional current collector 18, thereby reducing the overvoltage of the secondary battery 10. During charging, the plurality of seed particles 20 serve as precipitation nuclei, and Li is precipitated. + Li is introduced into the negative electrode 13 near the reduction potential of the ions, so that the overvoltage is reduced. In addition, since Li is precipitated around the plurality of seed particles 20, the surface area of ​​Li increases, so that the Li per unit surface area of ​​Li can be reduced. + By reducing the overvoltage, the precipitation of Li from outside the seed particles can be suppressed, and the generation of dendrites can be prevented.

[0074] Since the diameter of the second CNT 17 is smaller than that of the seed particle 20, the negative electrode 13 has excellent flexibility as a sponge-like structure and its thickness changes reversibly during charge and discharge. The negative electrode 13 does not contain a foil of a metal active material, so the thickness change (volume change) during charge and discharge is not limited.

[0075] The diameter of the second CNT 17 of the negative electrode 13 is less than 20 nm, and the specific surface area is 200 m 2 / g or more, the flexibility of the sponge-like structure is further improved, and the number of seed particles 20 serving as Li precipitation nuclei increases, so that the generation of dendrites can be reliably suppressed.

[0076] The average number of layers of the second CNTs 17 in the negative electrode 13 is 1 or more and 10 or less, so that the plurality of second CNTs 17 are easily entangled with each other, and thus the self-supporting property as a sponge-like structure can be reliably ensured.

[0077] The number of negative electrodes 13 per unit electrode area is 1×10 8 Pieces / cm 2 As described above, Li precipitated around each seed particle 20 does not grow larger, and thus the generation of dendrites can be more reliably suppressed.

[0078] The thickness of the negative electrode 13 changes reversibly through charge and discharge, increasing during charge and decreasing during discharge, and the value obtained by dividing the thickness during charge by the thickness during discharge is greater than 1.15, thereby effectively utilizing the space in the secondary battery 10 to increase the volume capacity density.

[0079] 4. Modifications

[0080] The present invention is not limited to the above-described embodiments, and can be modified appropriately within the scope of the gist of the present invention.

[0081] For example, the negative electrode 13 can be applied to a secondary battery using a high-concentration aqueous electrolyte instead of an organic electrolyte, an all-solid battery using a solid electrolyte instead of an electrolyte, and an air-metal secondary battery using oxygen in the air as a positive electrode active material. In particular, when the negative electrode 13 is applied to an all-solid battery, since the negative electrode 13 has a plurality of seed particles 20 that serve as Li precipitation nuclei, the interface with the solid electrolyte can be increased, Li is easily introduced into the negative electrode 13, and the generation of dendrites can be reliably suppressed.

[0082] 5. Examples

[0083] 5-1. Calculation of mass-based capacity density and volume-based capacity density

[0084] The structures of the negative electrodes in Examples are summarized in the following Tables 1 and 2. The numerical values ​​in the tables are determined according to predetermined calculation formulas by setting conditions described later.

[0085] Table 1

[0086]

[0087] Table 2

[0088]

[0089] In the negative electrodes of Examples 1 to 10, the material of the negative electrode active material 19 is Li, and the material of the seed particles 20 is Cu. The negative electrodes of Examples 1 to 10 have a negative electrode design capacity per unit electrode area of ​​4 mAh / cm 2 As a premise. When the design capacity is set to n times, it is sufficient to set the materials used, the mass of the electrode and the thickness to n times. Examples 1 to 4 are negative electrodes in which the ratio of the mass of Li metal to the mass of CNT (Li metal mass / CNT mass) is changed. Examples 5, 6, 3, and 7 are negative electrodes in which the ratio of the negative electrode capacity to the negative electrode design capacity (negative electrode capacity / negative electrode design capacity) is changed. Examples 8, 9, 3, and 10 are negative electrodes in which the ratio of the mass of Li metal to the mass of Cu seed particles (Li metal mass / Cu seed mass) is changed.

[0090] Table 1 shows the calculation results of the mass ratio (j) during charge / discharge and the mass-based capacity density (k) of the negative electrodes of Examples 1 to 10.

[0091] The Li metal mass / CNT mass (a) in Table 1 is the value obtained by dividing the Li metal mass during charging by the mass of the CNTs constituting the three-dimensional current collector, and was varied between 1 and 8 in Examples 1 to 4 and was set to 4 in Examples 5 to 10. The negative electrode capacity / negative electrode design capacity (b) is the negative electrode capacity during charging divided by the negative electrode design capacity (4 mAh / cm 2 ), varied between 5 and 1 in Examples 5, 6, 3, and 7, and was set to 2 in Examples 1 to 4 and 8 to 10. Li metal mass / Cu seed mass (c) is a value obtained by dividing the Li metal mass during charging by the Cu seed particle mass, varied between 1 and 30 in Examples 8, 9, 3, and 10, and was set to 10 in Examples 1, 2, and 4 to 7.

[0092] Hereinafter, the calculation method of the mass ratio during charge / discharge (j) and the mass-based capacity density (k) will be described.

[0093] The Li metal mass (d) during charging is calculated using the negative electrode design capacity per unit electrode area (4 mAh / cm 2) and the atomic weight of Li. The mass of Li metal during discharge (e) is calculated by assuming that the ratio of (negative electrode capacity / negative electrode design capacity (b)-1) / (negative electrode capacity / negative electrode design capacity (b)) in the mass of Li metal during charge (d) also remains in the negative electrode during discharge. The mass of CNTs (f) is calculated by dividing the mass of Li metal during charge (d) by the mass of Li metal / the mass of CNTs (a). The mass of Cu seed crystals (g) is calculated by dividing the mass of Li metal during charge (d) by the mass of Li metal / the mass of Cu seed crystals (c). The total mass during charge (h) is the sum of the mass of Li metal during charge (d), the mass of CNTs (f), and the mass of Cu seed crystals (g). The total mass during discharge (i) is the sum of the mass of Li metal during discharge (e), the mass of CNTs (f), and the mass of Cu seed crystals (g).

[0094] The mass ratio during charge / discharge (j) is calculated by dividing the total mass during charge (h) by the total mass during discharge (i). The mass-based capacity density (k) is the negative electrode design capacity per unit electrode area (4 mAh / cm 2 ) divided by the total mass during charging (h).

[0095] It can be seen from Table 1 that, compared with Examples 1 to 4 in which the Li metal mass / CNT mass (a) varies from 1 to 8, the larger the Li metal mass / CNT mass (a), the larger the mass ratio (j) during charge / discharge and the mass-based capacity density (k). In addition, compared with Examples 5, 6, 3, and 7 in which the negative electrode capacity / negative electrode design capacity (b) varies from 5 to 1, it can be seen that the smaller the negative electrode capacity / negative electrode design capacity (b), the larger the mass ratio (j) during charge / discharge and the mass-based capacity density (k). In addition, compared with Examples 8, 9, 3, and 10 in which the Li metal mass / Cu seed mass (c) varies from 1 to 30, it can be seen that the larger the Li metal mass / Cu seed mass (c), the larger the mass ratio (j) during charge / discharge and the mass-based capacity density (k). It can be further seen that the larger the mass ratio (j) during charge / discharge, the smaller the mass of the negative electrode during charge and discharge, and the mass-based capacity density is improved. The mass ratio (j) during charge / discharge is preferably 1.15 or more, more preferably 1.5 or more, and particularly preferably 2.0 or more.

[0096] Table 2 shows the calculation results of the thickness ratio (o) during charge / discharge and the volume-based capacity density (p) of the negative electrodes of Examples 1 to 10.

[0097] The following describes the calculation method of the thickness ratio (o) during charge / discharge and the volume-based capacity density (p).

[0098] The Li metal volume (d') during charge in Table 2 is the value obtained by converting the Li metal mass (d) during charge with the density of Li. The Li metal volume (e') during discharge is calculated by assuming that the ratio of (negative electrode capacity / negative electrode design capacity (b)-1) / (negative electrode capacity / negative electrode design capacity (b)) in the Li metal volume (d') during charge also remains on the negative electrode during discharge. The CNT volume (f') is obtained by converting the CNT mass (f) with the density of CNT. The Cu seed volume (g') is obtained by converting the Cu seed mass (g) with the density of Cu. Porosity (1) is the porosity of the negative electrode, which is set to 0.3 in the negative electrodes of Examples 1 to 10. The total volume (m) during charge is calculated by dividing the total value of the Li metal volume (d'), CNT volume (f') and Cu seed volume (g') during charge by (1-porosity (l)). The total volume (n) during discharge was calculated by dividing the total value of the Li metal volume (e'), the CNT volume (f'), and the Cu seed crystal volume (g') during discharge by (1-porosity (l)).

[0099] The thickness ratio during charge / discharge (o) is calculated by dividing the total volume during charge (m) by the total volume during discharge (n). The volume-based capacity density (p) is the negative electrode design capacity per unit electrode area (4 mAh / cm 2 ) divided by the Li metal volume during charging (d').

[0100] It can be seen from Table 2 that, compared with Examples 1 to 4 in which the Li metal mass / CNT mass (a) varies from 1 to 8, the larger the Li metal mass / CNT mass (a), the larger the thickness ratio (o) during charge / discharge and the volume-based capacity density (p). In addition, compared with Examples 5, 6, 3, and 7 in which the negative electrode capacity / negative electrode design capacity (b) varies from 5 to 1, it can be seen that the smaller the negative electrode capacity / negative electrode design capacity (b), the larger the thickness ratio (o) during charge / discharge and the volume-based capacity density (p). In addition, compared with Examples 8, 9, 3, and 10 in which the Li metal mass / Cu seed mass (c) varies from 1 to 30, it can be seen that the larger the Li metal mass / Cu seed mass (c), the larger the thickness ratio (o) during charge / discharge and the volume-based capacity density (p). It can be further seen that the larger the thickness ratio (o) during charge / discharge, the smaller the volume of the negative electrode during charge and discharge, and the improved volume-based capacity density. The thickness ratio (o) during charge / discharge is preferably 1.15 or more, more preferably 1.5 or more, and particularly preferably 2.0 or more.

[0101] 5-2. Experiment to confirm that the negative electrode does not contain metal active material foil

[0102] When the negative electrode 13 is manufactured using the negative electrode precursor in which the metal foil constituting the negative electrode active material 19 (Li) is stacked on the composite film 36 as described in the first example of the metal active material retaining process and the third example of the metal active material retaining process, it is confirmed that a negative electrode without the metal foil can be obtained. In order to conduct this experiment, two types of test cells were prepared, and each test cell was referred to as Examples 11 and 12.

[0103] Hereinafter, Example 11 will be described. First, a composite film is formed by the method described in the first example of the composite film forming process. The second CNT 17 is a CNT having a diameter of less than 20 nm, a length of more than 1 μm, and an average number of layers of more than 1 layer and less than 5 layers. The seed particle 20 is a copper particle having a diameter of about 25 nm. Isopropyl alcohol is used as the dispersion medium 32. The composite film is formed by drying in a vacuum dryer at 393K for 2 hours. The diameter of the composite film is 12 mm, and the mass density of Cu per unit area is about 0.12 mg / cm 2 The mass density of CNT per unit area is about 0.78 mg / cm 2 In this experiment, two composite films were produced and used for electrodes of a laminated body described later.

[0104] Next, prepare Figure 6 The laminate 50 is shown. The laminate 50 is made by sequentially laminating a first electrode 51, a separator 11, and a second electrode 52. The first electrode 51 is composed of a composite film 53. The second electrode 52 is composed of a composite film 54 and a metal foil 55 (metal active material foil) constituting a negative electrode active material 19 (Li) laminated on the composite film 54. Although the first electrode 51 and the second electrode 52 are Figure 6 The laminate 50 is not shown in the figure, but has a second three-dimensional current collector 18 composed of a sponge-like structure obtained by self-supporting the second CNT 17 and a plurality of seed particles 20 contained in the second three-dimensional current collector 18. The metal foil 55 is a foil with a thickness of 50 μm and a diameter of 12 mm. The separator 11 is made of polypropylene. The photograph of the upper surface (metal foil 55) of the laminate 50 is shown in FIG. Figure 7 As shown. Figure 7 , it was possible to confirm the metallic luster of the metal foil 55. The laminate 50 and the electrolyte solution were placed in a container, and a test cell of Example 11 was produced.

[0105] Next, in the test cell of Example 11, the metal active material was retained by depositing Li on the first electrode. The deposition of Li was carried out at a current density of 0.4 mA / cm 2The process is carried out at a constant current of , and the cut-off voltage is set to 0.1V. In the following description, the deposition of Li on the first electrode and the introduction of Li are referred to as charging, and the stripping of Li from the first electrode and the release of Li from the first electrode are referred to as discharging. By charging, the Li in the metal foil 55 included in the second electrode 52 is dissolved, and the Li ions move to the first electrode 51, and Li is deposited around the seed particles 20 included in the composite film 53. In this embodiment, the first electrode is introduced with a charge equivalent to 8.86mAh / cm 2 After charging, the stacked body 50 has a capacity of Li. Figure 8 As shown, there is no remaining metal foil 55. Figure 8 In the figure, reference numeral 50A denotes the charged stack, reference numeral 51A denotes the charged first electrode, and reference numeral 52A denotes the charged second electrode.

[0106] In fact, the test battery was disassembled and visually confirmed that no metal foil 55 remained. Fig. 9 A photograph of the upper surface (the surface in contact with the separator 11 ) of the first electrode 51A after charging is shown. Fig.10 A photograph of the lower surface (the surface in contact with the separator 11 ) of the second electrode 52A after charging is shown. Fig.11 FIG. 2 shows a photograph of the upper surface (the surface in contact with the metal foil 55) of the second electrode 52A after charging. Fig. 9 It was confirmed that the composite film 53, which was black before charging, turned into white without metallic luster. Li, which is the negative electrode active material 19, precipitated around the seed particles 20 contained in the composite film 53, and the first electrode 51A turned into white without metallic luster. Fig.10 , it was confirmed that the lower surface of the second electrode 52A was almost black and Li was almost absent. Fig.11 , it was confirmed that the upper surface of the second electrode 52A was almost black, no Li foil remained, and Li was almost absent. As described above, it was confirmed that there was no metal active material foil or film with metallic luster on the first electrode 51A and the second electrode 52A. Therefore, an electrode without metal active material foil can be obtained.

[0107] Next, Example 12 is described. First, a composite film of Example 12 is formed by the same method as Example 11, that is, the method described in the first example of the composite film forming step. The mass density of CNTs per unit area of ​​the composite film of Example 12 is about 0.28 mg / cm 2 , which is different from the composite film of Example 11. Next, a laminate of Example 12 was prepared by the same method as in Example 11. Fig.12A photograph of the upper surface (metal foil) of the laminate of Example 12 is shown. Fig.12 , it was possible to confirm the metallic luster of the metal foil 55. The laminate and the electrolyte were placed in a container to prepare a test cell of Example 12.

[0108] Next, in the test cell of Example 12, Li was deposited on the first electrode to retain the metal active material. The deposition of Li was carried out in the same manner as in Example 11 at a current density of 0.4 mA / cm 2 The charge was carried out at a constant current of 0.1 V, and the cut-off voltage was set to 0.1 V. During the charge, the Li in the metal foil contained in the second electrode was dissolved, and the Li ions moved to the first electrode, and Li precipitated around the seed particles contained in the composite film. In this embodiment, the first electrode was introduced with a charge equivalent to 9.13 mAh / cm 2 The capacity of Li.

[0109] In fact, the test battery of Example 12 was disassembled and visually confirmed that no metal foil remained. Fig.13 A photograph of the upper surface (the surface in contact with the separator) of the first electrode after charging is shown. Fig.14 FIG. 2 shows a photograph of the lower surface (the surface in contact with the separator) of the second electrode after charging. Fig.13 It was confirmed that the composite film, which was black before charging, became white without metallic luster. Li, which is the negative electrode active material, precipitated around the seed particles contained in the composite film, and thus the first electrode was observed to be white without metallic luster. Fig.14 , it was confirmed that the lower surface of the second electrode was almost black and Li was almost absent. There was also almost no Li on the upper surface of the second electrode after charging (the surface in contact with the metal foil). As described above, it can be confirmed that there is no metal active material foil or a film with a metallic luster on the first electrode and the second electrode. Therefore, an electrode without a metal active material foil can be obtained.

[0110] 5-3. Cycle test

[0111] A test cell was prepared in the same manner as in Example 12, and the prepared test cell was designated as Example 13. The test cell of Example 13 had the same structure as the test cell of Example 12. The first cycle charge was performed in the same manner as in Examples 11 and 12 at a current density of 0.4 mA / cm 2 The cut-off voltage was set to 0.1 V. A metal Li with a thickness of 50 μm and a diameter of 12 mm is equivalent to about 10 mAh / cm 2Since some Li is consumed in the formation of the SEI (solid electrolyte interface) film during the initial charge, a charge equivalent to about 8.8 mAh / cm is introduced into the first electrode. 2 The first cycle discharge is also at a current density of 0.4 mA / cm 2 The discharge of the first cycle is equivalent to about 2.4 mAh / cm 2 As a result, the second electrode introduced a capacity equivalent to 6.4 mAh / cm 2 In the second and subsequent cycles, the amount of Li transferred between the first and second electrodes is about 4 mAh / cm 2 The capacity of the second to fourth cycles is 0.4 mA / cm 2 , under the condition of constant current with cut-off voltage of 0.1V, after the fifth cycle, at the current density of 1.0mA / cm 2 , and repeated under constant current conditions with a cutoff voltage of 0.3V.

[0112] A test cell having a laminated body in which a first Cu foil (corresponding to the first electrode), a separator, a metal foil, and a second Cu foil are sequentially stacked is prepared, and the prepared test cell is used as Comparative Example 1. The first Cu foil and the second Cu foil used in the test cell of Comparative Example 1 are foils with a thickness of 20 μm and a diameter of 12 mm. The metal foil used in the test cell of Comparative Example 1 has the same structure as the metal foil used in the test cell of Example 13. A test cell having a laminated body in which a first electrode consisting of a self-supporting sponge-like structure of CNTs, a separator, a second electrode consisting of a self-supporting sponge-like structure of CNTs and a metal foil are sequentially stacked is prepared, and the prepared test cell is used as Comparative Example 2. The difference between the test cell of Comparative Example 2 and the test cell of Example 13 is that the first electrode and the second electrode do not contain seed particles. A test cell having a laminated body in which a first electrode consisting of a composite film, a separator, and a second electrode consisting of a composite film and a metal foil with a thickness of 500 μm are sequentially stacked is prepared, and the prepared test cell is used as Comparative Example 3. The thickness of the metal foil in the test battery of Comparative Example 3 is different from that in the test battery of Example 13. The test batteries of Comparative Examples 1 and 2 were subjected to cycle tests. The cycle test of the test battery of Comparative Example 1 was conducted under conditions that were partially different from those of the test battery of Example 13, and the cycle test of the test battery of Comparative Example 2 was conducted under the same conditions as those of the test battery of Example 13. The cycle test of the test battery of Comparative Example 1 is described below. The first cycle of charging was conducted at a current density of 0.4 mA / cm 2The charge was carried out at a constant current of 0.15 V and the cut-off voltage was set to 0.15 V. The charge deposited on the first Cu foil was equivalent to about 8.6 mAh / cm 2 The first cycle discharge was also at a current density of 0.4 mA / cm 2 The first Cu foil retained a charge of approximately 2.3 mAh / cm 2 The discharge was stopped at the state of Li with a capacity of 100 mAh / cm. As a result, a charge of approximately 6.3 mAh / cm 2 In the second and subsequent cycles, the amount of Li transferred between the first Cu foil and the second Cu foil is about 4 mAh / cm 2 The capacity of the second to fourth cycles is 0.4 mA / cm 2 , under the condition of constant current with cut-off voltage of 0.15V, after the fifth cycle, at 1.0mA / cm 2 , and repeated under constant current conditions with a cutoff voltage of 0.3V.

[0113] The results of the cycle test of the test battery of Example 13 are as follows: Fig.15 The cycle test results of the test battery of Comparative Example 1 are shown in Fig.16 The cycle test results of the test battery of Comparative Example 2 are shown in Fig.17 shown. Figures 15 to 17 In the figure, the vertical axis represents voltage and the horizontal axis represents time.

[0114] In the test cell of Example 13, Fig.15 It was confirmed that 94 cycles of operation were possible. Since there are two electrodes, the mass of a single Li foil with a thickness of 50 μm (2.67 mg / cm 2 ) and the mass of the two composite films constituting the first electrode and the second electrode (the mass of Cu in each composite film is 0.12 mg / cm 2 , the mass of CNT is 0.28mg / cm 2 ) has a total mass of 3.47 mg / cm 2 , and the design capacity of the electrode is 4 mAh / cm 2 , so the mass basis capacity density of each single electrode is 2305 mAh / g. Fig.16 As shown in the figure, it is confirmed that the absolute value of the voltage increases sharply in 24 cycles. This is because the density of Li nuclei on the Cu foil is low, and the volume of the Li nuclei changes greatly. Therefore, Li is consumed when SEI is destroyed and reformed repeatedly due to charge and discharge. In addition, the large dendrites are electrically isolated from the Cu foil when they dissolve and cannot contribute to charge and discharge, resulting in the depletion of active Li. Fig.17 As shown, it is confirmed that the absolute value of the voltage increases sharply in 22 cycles. This is because although a sponge-like structure is used in the electrode, the formation of large dendrites cannot be fully suppressed due to the absence of seed particles, resulting in a cycle characteristic that is worse than that of Example 13. In the test battery of Comparative Example 3, since there are two electrodes, the mass of a single Li foil with a thickness of 500 μm (26.7 mg / cm 2 ) and the mass of the two composite films constituting the first electrode and the second electrode (the mass of Cu in each single composite film is 0.12 mg / cm 2 , the mass of CNT is 0.28mg / cm 2 ) has a total mass of 31.1 mg / cm 2 , and the electrode design capacity is 4mAh / cm 2 , so the mass-based capacity density is 257 mAh / g. In the test battery of Comparative Example 3, since the Li that does not contribute to charge and discharge remains in the form of a foil with a thickness of about 450 μm, the mass capacity density and volume capacity density are lower than those of the test battery of Example 13. From the above, it can be confirmed that in the test battery of Example 13, by having a plurality of seed particles that become Li precipitation nuclei during charging, the generation of larger dendrites is suppressed, the consumption of Li is suppressed, and excellent cycle characteristics are obtained.

[0115] Description of Reference Numerals

[0116] 10, 10A, 10B: Secondary battery

[0117] 11: Diaphragm

[0118] 12, 12A, 12B: Positive electrode for secondary batteries

[0119] 13, 13A, 13B: Negative electrode for secondary battery

[0120] 14: The first carbon nanotube

[0121] 15: The first three-dimensional current collector

[0122] 16, 16A, 16B: Positive electrode active material

[0123] 17: Second carbon nanotube

[0124] 18: Second three-dimensional current collector

[0125] 19, 19A, 19B: Negative electrode active material

[0126] 20: Seed particles

Claims

1. A negative electrode for a secondary battery, characterized in that: have: The three-dimensional current collector is composed of a sponge-like structure obtained by self-supporting carbon nanotubes; A metal active material contained in the internal gaps of the sponge-like structure; and a plurality of seed particles, which are contained in gaps inside the sponge-like structure and are distributed throughout the entire interior of the sponge-like structure and are made of a material different from the metal active material; The number of the seed particles per unit electrode area is 1×10 8 Pieces / cm 2 above, The negative electrode for a secondary battery does not contain a foil of the metal active material.

2. The negative electrode for a secondary battery according to claim 1, wherein: The diameter of the carbon nanotube is smaller than the diameter of the seed particle.

3. The negative electrode for a secondary battery according to claim 1, wherein: The metal active material is composed of at least one selected from the group consisting of Li, Na, Mg, Ca, K, Al, and Zn.

4. The negative electrode for a secondary battery according to any one of claims 1 to 3, characterized in that: The carbon nanotubes have a diameter of less than 20 nm and a specific surface area of ​​200 m 2 / g or above.

5. The negative electrode for a secondary battery according to claim 4, characterized in that: The carbon nanotubes have an average number of layers of 1 or more and 5 or less.

6. The negative electrode for a secondary battery according to any one of claims 1 to 3, characterized in that: The seed particles are composed of at least one selected from the group consisting of C, Mg, Al, Si, Sn, Zn, Cu, Ag, Au, and Pt.

7. The negative electrode for a secondary battery according to any one of claims 1 to 3, characterized in that: The thickness of the negative electrode for a secondary battery changes reversibly by charge and discharge, the thickness increases during charge and decreases during discharge, and the value obtained by dividing the thickness during charge by the thickness during discharge is 1.15 or more.

8. A secondary battery, characterized in that: have: The negative electrode for a secondary battery as claimed in claim 7; and The thickness of the positive electrode for a secondary battery changes reversibly by charge and discharge, and the thickness decreases during charge and increases during discharge.

9. A method for manufacturing a negative electrode for a secondary battery, characterized in that: Composite carbon nanotubes, metal active materials and seed particles. The seed particles are obtained by removing the dispersion medium from a dispersion liquid containing the seed particles and the dispersion medium, or by precipitating the seed particles from a solution in which the seed particle material is dissolved in a solvent. The negative electrode for a secondary battery includes a three-dimensional current collector composed of a sponge-like structure obtained by self-supporting the carbon nanotubes. The seed particles are contained in gaps inside the sponge-like structure obtained by self-supporting the carbon nanotubes, and are distributed throughout the entire interior of the sponge-like structure.

10. The method for producing a negative electrode for a secondary battery according to claim 9, wherein: include: A composite film forming step of forming a composite film containing the seed particles on a three-dimensional current collector composed of a sponge-like structure obtained by self-supporting the carbon nanotubes; as well as The metal active material retaining step is to retain the metal active material on the composite film.

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