Anode for secondary battery, secondary battery, and method for manufacturing anode for secondary battery

Abstract

To provide a negative electrode for a secondary battery, which is high in mass capacity density and volume capacity density, and superior in charge / discharge cycle characteristic, a secondary battery, and a method for manufacturing such a negative electrode for a secondary battery.SOLUTION: A negative electrode 13 for a secondary battery comprises: a 3D collector (second three dimension collector) 18 composed of a self-supported spongy structure of carbon nanotube (second carbon nanotube) 17; and a plurality of SiO particles 19 included in the 3D collector 18, in which SiO content is 50 mass% or more and 93 mass% or less. A secondary battery 10 comprises: a negative electrode 13 for a secondary battery; and a positive electrode 12 for a secondary battery.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a negative electrode for a secondary battery, a secondary battery, and a method for producing a negative electrode for a secondary battery. [Background technology]

[0002] Research is underway into anodes that use silicon (Si) or metallic lithium (Li) as the anode active material, which have a theoretical capacity approximately 10 times that of graphite (C), in order to realize high-energy-density secondary batteries.

[0003] The present inventors have proposed a secondary battery having a positive electrode in which sulfur (S) as a positive electrode active material is contained within a three-dimensional current collector made of a self-supporting sponge-like structure of carbon nanotubes (CNTs), and a negative electrode in which Si as a negative electrode active material is contained within a three-dimensional current collector made of a self-supporting sponge-like structure of CNTs (Patent Document 1).

[0004] Furthermore, the present inventors have proposed a negative electrode for a secondary battery, in which metallic Li as a negative electrode active material and multiple seed particles that serve as precipitation nuclei for Li are contained inside a three-dimensional current collector made of a self-standing sponge-like structure of CNT (Patent Document 2).

[0005] However, negative electrodes using Si or Li as the negative electrode active material have the problem of deterioration due to volume changes when a large amount of Li is absorbed and released. On the other hand, silicon monoxide (SiO) has a theoretical capacity about five times that of graphite, and although its theoretical capacity is smaller than that of Si or Li, it has excellent charge-discharge cycle characteristics, and has therefore attracted attention as a negative electrode material. Therefore, a negative electrode using SiO as the negative electrode active material has been proposed (Non-Patent Document 1).

[0006] Non-Patent Document 1 describes a negative electrode in which SiO particles, CNT, and polyvinylidene fluoride (PVDF) are combined as a negative electrode active material. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 2018-113108 [Patent Document 2] International Publication No. 2020 / 175488 [Non-patent literature]

[0008] [Non-Patent Document 1] T. Kang, et al., Energy Technology 7, 1800635 (2018). Summary of the Invention [Problem to be solved by the invention]

[0009] In the negative electrode of Non-Patent Document 1, since PVDF is used, the amount of SiO added cannot be increased, making it difficult to achieve a high capacity density.

[0010] Therefore, an object of the present invention is to provide a negative electrode for a secondary battery that has high mass capacity density and volume capacity density and excellent charge / discharge cycle characteristics, a secondary battery, and a method for producing a negative electrode for a secondary battery. [Means for solving the problem]

[0011] The negative electrode for a secondary battery according to the present invention comprises a three-dimensional current collector made of a self-supporting sponge-like structure of carbon nanotubes and a plurality of SiO particles contained within the three-dimensional current collector, and the SiO content is 50% by mass or more and 93% by mass or less.

[0012] The secondary battery according to the present invention includes the above-described negative electrode for secondary batteries and a positive electrode for secondary batteries.

[0013] The method for producing a negative electrode for a secondary battery according to the present invention includes incorporating a plurality of SiO particles into a three-dimensional current collector made of a self-supporting sponge-like structure of carbon nanotubes, and the SiO content is set to 50% by mass or more and 93% by mass or less. [Effects of the Invention]

[0014] According to the present invention, a plurality of SiO particles are encapsulated in a self-supporting sponge-like structure of carbon nanotubes, and the SiO content is 50% by mass or more and 93% by mass or less, thereby making it possible to provide a secondary battery anode having high mass capacity density and volume capacity density and excellent charge-discharge cycle characteristics, a secondary battery, and a method for manufacturing a secondary battery anode. [Brief explanation of the drawings]

[0015] [Figure 1] 1A and 1B are schematic diagrams illustrating the configuration of a secondary battery according to an embodiment of the present invention during charging and discharging. [Figure 2] 1 is a graph showing SiO mass capacity versus SiO mass %. [Figure 3] 1 is a graph showing the electrode mass-based capacity versus SiO mass %. [Figure 4] 1 is a graph showing the electrode volume-based capacity versus SiO mass %. [Figure 5] 1 is a graph showing the initial coulombic efficiency versus SiO mass %. [Figure 6] 1 is a graph showing the SiO mass capacity relative to the SiO mass % when a carbon coating layer is provided and when a carbon coating layer is not provided. [Figure 7] 1 is a graph showing the electrode mass-based capacity versus SiO mass % when a carbon coating layer is provided and when a carbon coating layer is not provided. [Figure 8] 1 is a graph showing the electrode volume-based capacity versus SiO mass % when a carbon coating layer is provided and when a carbon coating layer is not provided. [Figure 9] 1 is a graph showing the initial coulombic efficiency versus SiO mass % when a carbon coating layer is provided and when a carbon coating layer is not provided. [Figure 10] 1 is a graph showing the results of a cycle test. [Figure 11] 1 shows a cross-sectional SEM image of a negative electrode during a cycle test. DETAILED DESCRIPTION OF THE INVENTION

[0016] Hereinafter, this embodiment will be described in detail with reference to the drawings.

[0017] 1. Overall structure In FIG. 1, a secondary battery 10 (10A, 10B) according to this 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).

[0018] During charging, secondary battery 10A includes a contracted positive electrode 12A and an expanded negative electrode 13A, which are provided with separator 11 interposed therebetween. During discharging, secondary battery 10B includes an expanded positive electrode 12B and a contracted negative electrode 13B, which are provided with separator 11 interposed therebetween. Secondary battery 10 of this embodiment is a lithium-ion secondary battery in which lithium (Li) ions move between positive electrode 12 and negative electrode 13 through separator 11 upon charging and discharging.

[0019] The secondary battery 10 has a positive electrode 12 provided on one surface of a separator 11 and a negative electrode 13 provided 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 electrolyte solution in a container.

[0020] The electrolyte is not particularly limited, and commonly used electrolytes such as nonaqueous electrolytes, ionic liquids, and gel electrolytes can be used. The nonaqueous electrolyte can be prepared, for example, by dissolving 1.0 mol / L of LiPF6 in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) in a volume ratio of 1:2. Alternatively, the nonaqueous electrolyte can be prepared, for example, by dissolving 1.0 mol / L of LiTFSI (lithium bis(trifluoromethanesulfonyl)imide) and 0.6 mol / L of LiNO3 (lithium nitrate) in a mixture of DOL (1,3-dioxolane) and DME in a volume ratio of 1:1.

[0021] The container is not particularly limited, and metal cans commonly used as battery cans, such as iron, stainless steel, aluminum, etc., can be used. From the viewpoint of energy density per mass, a metal-resin composite material in which a metal foil and a resin film are laminated is preferred.

[0022] The separator 11 can be made of a microporous polymer film. Examples of the microporous polymer film include polyolefin-based, polyester-based, polyacrylonitrile-based, polyphenylene sulfide-based, polyimide-based, and fluororesin-based microporous membranes and nonwoven fabrics. The separator 11 may also be made of a self-supporting sponge-like structure of insulating fibers. The sponge-like structure is a membrane having multiple gaps therein. Examples of the sponge-like structure include nonwoven fabrics. The insulating fibers are boron nitride nanotubes (BNNTs) or organic nanofibers. Examples of organic nanofibers include cellulose nanofibers (CNFs) and chitin nanofibers.

[0023] The positive electrode 12 can be any of various positive electrodes used in general secondary batteries. In particular, it is preferable to use a positive electrode whose thickness changes reversibly upon charging and discharging, decreasing during charging (12A) and increasing during discharging (12B), because this allows for effective use of the space within the secondary battery. When the volume of the positive electrode 12 changes upon charging and discharging, the area of ​​the surface in contact with the separator 11 does not change substantially, but the thickness changes, resulting in contraction or expansion. In other words, the volume of the positive electrode 12 changes depending on the thickness.

[0024] The positive electrode active material 16 (16A, 16B) is a composite of lithium cobalt oxide (LiCoO2), lithium manganese oxide (LiMn2O4), lithium iron phosphate (LiFePO4), and NMC (LiNi x Mn y Co z O2) and NCA (LiNi x Co y Al zThe positive electrode active material 16 may be a lithium transition metal composite oxide such as SiO2, or an active material such as sulfur that reacts with lithium to form a compound and change its volume. When an active material such as sulfur that changes its volume upon reaction with lithium is used as the positive electrode active material 16, the thickness of the positive electrode 12 changes significantly during charging and discharging. The expansion and contraction of the positive electrode 12 and the negative electrode 13, which uses SiO particles 19 as a negative electrode active material (described later), cancel each other out during charging and discharging, thereby suppressing thickness change in the secondary battery 10. Therefore, the thickness change rate of the positive electrode 12 upon charging and discharging is preferably 10% or more and 300% or less. The thickness change rate of the positive electrode 12 upon charging and discharging can be calculated by subtracting the thickness upon charging from the thickness upon discharging, and dividing the result by the thickness upon charging. The positive electrode active material 16 preferably uses an active material such as sulfur that changes its volume upon reaction with lithium. A material with a larger volume change can achieve a higher volumetric capacity density. To make the volume change reversible, it is preferable to encapsulate the positive electrode active material 16 inside the first three-dimensional current collector 15 made of a freestanding sponge-like structure of the first carbon nanotubes (CNTs) 14 .

[0025] The negative electrode (secondary battery negative electrode) 13 according to this embodiment will be described below. The thickness of the negative electrode 13 changes reversibly upon charge and discharge, increasing during charge (13A) and decreasing during discharge (13B). The rate of change in thickness of the negative electrode 13 upon charge and discharge is 10% to 300%. The rate of change in thickness of the negative electrode 13 upon charge and discharge can be calculated by subtracting the thickness upon discharge from the thickness upon charge and dividing the result by the thickness upon discharge. When the volume of the negative electrode 13 changes upon charge and discharge, the area of ​​the surface in contact with the separator 11 does not change substantially, and the negative electrode 13 expands or contracts due to a change in thickness. That is, the volume of the negative electrode 13 changes depending on the thickness.

[0026] The negative electrode 13 includes a second three-dimensional current collector 18 having a self-supporting sponge-like structure of second carbon nanotubes (CNTs) 17, and a plurality of SiO particles 19 (19A, 19B) contained within the second three-dimensional current collector 18. The SiO particles 19 function as the negative electrode active material of the negative electrode 13. The SiO particles 19 have an average molar composition of Si:O=1:x (0.5≦x≦1.5). In particular, the SiO particles 19 preferably have an average composition of Si:O=1:1.

[0027] The sponge-like structure of the second three-dimensional current collector 18 is formed by a plurality of second CNTs 17 entangled with one another. The average length of the second CNTs 17 is preferably 1 μm or more, more preferably 10 μm or more, and even more preferably 100 μm or more. When the average length of the second CNTs 17 is 1 μm or more, the plurality of second CNTs 17 are entangled with one another, ensuring the independence of the sponge-like structure. The longer the second CNTs 17, the easier it is for the SiO particles 19 to be held in the sponge-like structure formed by the plurality of second CNTs 17 entangled with one another.

[0028] The diameter of the second CNTs 17 is smaller than the diameter of the SiO particles 19. The average diameter of the second CNTs 17 is preferably 20 nm or less, more preferably 15 nm or less, and even more preferably 10 nm or less. The smaller the average diameter of the second CNTs 17, the more flexible the sponge-like structure becomes.

[0029] The specific surface area of ​​the second CNT17 is 200m 2 / g or more is preferable, and 300m 2 / g or more is more preferable. If the specific surface area of ​​the second CNTs 17 is too large, there is a risk of side reactions such as decomposition of the electrolyte solution occurring. 2 / g or less is preferable, and 800m 2 / g or less is more preferable.

[0030] The average number of layers of the second CNTs 17 is preferably 1 to 10, more preferably 1 to 5, and even more preferably 2 to 5. The smaller the average number of layers of the second CNTs 17, the smaller the average diameter of the second CNTs 17, and the more easily the second CNTs 17 become entangled with each other, thereby more reliably ensuring the self-supporting properties of the sponge-like structure. If the average number of layers of the second CNTs 17 is too small, the specific surface area of ​​the second CNTs 17 becomes too large.

[0031] The SiO content in the negative electrode 13 is preferably 50% by mass to 93% by mass, more preferably 65% ​​by mass to 90% by mass, and even more preferably 75% by mass to 90% by mass. In the following description, the SiO content in the negative electrode 13 may also be referred to as SiO mass%.

[0032] The average particle size of the SiO particles 19 is preferably 0.2 μm to 10 μm, more preferably 0.4 μm to 5 μm, and even more preferably 0.6 μm to 3 μm. The SiO particles 19 have a larger particle size than Si particles commonly used as a negative electrode active material. The average particle size of Si particles commonly used as a negative electrode active material is approximately 0.05 μm to 0.1 μm, and the average particle size of the SiO particles 19 is approximately 10 times larger than that of the Si particles. Because the average particle size of the SiO particles 19 is large, even if the amount of the second CNTs 17 in the negative electrode 13 is small, the number of second CNTs 17 in contact with each SiO particle 19 increases, enabling good electrical contact. By reducing the amount of the second CNTs 17 in the negative electrode 13, the SiO content in the negative electrode 13 can be increased. Furthermore, if the average particle size of the SiO particles 19 contained in the negative electrode 13 is large, the surface area of ​​the SiO particles 19 that comes into contact with the electrolyte solution is reduced, thereby suppressing deterioration of the battery characteristics due to side reactions between the negative electrode active material and the electrolyte solution.

[0033] The smaller the diameter of the second CNTs 17, the larger the specific surface area of ​​the second CNTs 17, which increases the contact area between the second CNTs 17 and the SiO particles 19, enabling better electrical contact. Therefore, the smaller the average diameter of the second CNTs 17 and the larger the average particle size of the SiO particles 19, i.e., the larger the value obtained by dividing the average particle size of the SiO particles 19 by the average diameter of the second CNTs 17, the better the electrical contact between the second CNTs 17 and the SiO particles 19. On the other hand, if the average diameter of the second CNTs 17 is too small, the surface area of ​​the second CNTs 17 becomes too large, causing a side reaction on the surface of the second CNTs 17 to decompose the electrolyte. Therefore, the value obtained by dividing the average particle size of the SiO particles 19 by the average diameter of the second CNTs 17 is preferably 10 to 2000, more preferably 30 to 1000, and even more preferably 50 to 500.

[0034] When the length of the second CNTs 17 is longer than the average particle size of the SiO particles 19, each second CNT 17 can connect multiple SiO particles 19 across each other, and the large-sized SiO particles 19 can be reliably held in the sponge-like structure in which multiple second CNTs 17 are entangled with each other. The longer the length of the second CNTs 17, the better the electrical contact between the second CNTs 17 and the SiO particles 19, making it easier to hold the SiO particles 19 in the sponge-like structure of the second CNTs 17. Furthermore, even when the number of second CNTs 17 is small, the CNTs can maintain contact with each other. Therefore, the greater the value obtained by dividing the average length of the second CNTs 17 by the average particle size of the SiO particles 19, the better the electrical conductivity and the stronger the anode 13 can be. The value obtained by dividing the average length of the second CNTs 17 by the average particle size of the SiO particles 19 is preferably 3 to 1,000, more preferably 10 to 500, and even more preferably 50 to 300.

[0035] The SiO particles 19 preferably have a carbon coating layer (not shown) on their surfaces. That is, the SiO particles 19 preferably have a structure in which SiO is encapsulated in the carbon coating layer. The carbon coating layer improves the conductivity of the SiO particles 19 and reduces the contact resistance between the SiO particles 19 and the second CNTs 17. By encapsulating the second CNTs 17, the electrical contact between the second CNTs 17 and the SiO particles 19 is improved even if the number of second CNTs 17 is reduced. Reducing the number of second CNTs 17 allows the SiO content in the negative electrode 13 to be increased. Furthermore, encapsulating SiO in the carbon coating layer suppresses deterioration of battery characteristics due to side reactions between the negative electrode active material and the electrolyte. The SiO particles 19 having a carbon coating layer can be produced by depositing carbon on the surface of SiO using, for example, a chemical vapor deposition (CVD) method as described in Japanese Patent No. 5996802.

[0036] The negative electrode 13 preferably does not contain metal foil because it includes a second three-dimensional current collector 18 with high electrical conductivity. The inclusion of metal foil increases the mass and volume of the negative electrode, leading to a decrease in mass capacity density and volume capacity density. Furthermore, if the negative electrode contains metal foil that is in contact with the entire surface of the negative electrode, the foil inhibits volumetric change of the negative electrode and generates stress between the foil and the negative electrode, causing deterioration of battery characteristics. The positive electrode 12 also preferably does not contain metal foil.

[0037] The negative electrode 13 preferably does not contain an organic polymer binder such as polyvinylidene fluoride (PVDF) because the negative electrode 13 contains a plurality of SiO particles 19 inside the second three-dimensional current collector 18, which is made of a self-supporting sponge-like structure of the second CNTs 17. If the negative electrode 13 contains an organic polymer binder, the mass and volume of the negative electrode increase, leading to a decrease in the mass capacity density and volume capacity density.

[0038] 2. Manufacturing method A method for manufacturing the negative electrode (secondary battery negative electrode) 13 according to this embodiment will be described. The negative electrode 13 is obtained by incorporating a plurality of SiO particles 19 inside a second three-dimensional current collector 18 made of a self-supporting sponge-like structure of second CNTs 17, and setting the SiO content to 50% by mass or more and 93% by mass or less. An example of a method for manufacturing the negative electrode 13 will be described below.

[0039] The negative electrode 13 can be formed by co-dispersing and filtering CNTs, which are the raw materials for the sponge-like structure, and SiO particles, which are the negative electrode active material. Specifically, a dispersion of CNTs and SiO particles dispersed in a dispersion medium such as isopropanol is used to form a free-standing film by filtering. The mass ratio of CNTs to SiO particles is adjusted to match the SiO content of the negative electrode to be manufactured. The porosity of the negative electrode 13 can be adjusted by changing the ratio of CNTs to SiO particles or the dispersion state of the CNTs and SiO particles. Alternatively, the porosity of the negative electrode 13 may be adjusted by performing a process such as pressing after film formation. SiO particles can be obtained by various methods. For example, a method in which a mixture of silicon dioxide and silicon is heated to produce silicon monoxide gas, which is then cooled and precipitated is exemplified.

[0040] The CNTs used are preferably long (average diameter of approximately 1 nm to 15 nm, average length of approximately 10 μm to 1000 μm). Such CNTs can be synthesized by a CVD method. Examples include the fluidized-bed CVD method described in Japanese Patent No. 5447367, Japanese Patent No. 5862559, D.Y. Kim, H. Sugime, K. Hasegawa, T. Osawa, and S. Noda, Carbon 49(6), 1972-1979 (2011), and Z. Chen, D.Y. Kim, K. Hasegawa, T. Osawa, and S. Noda, Carbon 80, 339-350 (2014). CNTs may also be synthesized by a floating catalyst CVD method or a substrate-supported catalyst CVD method.

[0041] The CNTs incorporate SiO particles into a network through van der Waals forces, and multiple SiO particles, serving as the negative electrode active material, are thus incorporated into the gaps in the three-dimensional current collector made of the sponge-like structure of the CNTs, forming the negative electrode.

[0042] Next, we will explain an example of a method for manufacturing a positive electrode 12 using sulfur as the positive electrode active material. The positive electrode 12 can be formed by co-dispersing and filtering CNTs, which serve as the raw material for the sponge-like structure, and Li2S, which serves as the positive electrode active material. Specifically, a dispersion of CNTs and nanoparticle Li2S in a dispersion medium such as ethanol or isopropanol is used, and a free-standing film is formed by filtering. Nanoparticle Li2S can be obtained by a common method, such as pulverizing Li2S powder using a ball mill.

[0043] The CNTs incorporate nanoparticles of Li2S and form a network through van der Waals forces. In this way, Li2S, the positive electrode active material, is incorporated into the gaps in the three-dimensional current collector, which is made of a sponge-like structure of CNTs, forming a positive electrode.

[0044] The positive electrode 12 and negative electrode 13 obtained by the above steps are laminated on one surface and the other surface of the separator 11 to produce an electrode structure. In the produced electrode structure, the ratio of the total volume of the positive electrode to the negative electrode during charge and discharge can be controlled by adjusting the thickness and / or porosity of the positive electrode and negative electrode. As described above, the porosity can be adjusted by changing the porosity of the three-dimensional current collector or the amount of active material. A metal wire for power transmission is placed on the surface of the electrode structure, and the electrode structure is placed in a container together with an electrolyte to produce a secondary battery 10.

[0045] 3. Action and Effects The negative electrode 13 according to this embodiment contains a plurality of SiO particles 19 as a negative electrode active material within the second three-dimensional current collector 18, and the SiO content is 50% by mass or more and 93% by mass or less. Because the SiO content in the negative electrode 13 is high, the amount of the second CNTs 17 is relatively small, resulting in a negative electrode with high mass capacity density and volume capacity density and excellent charge-discharge cycle characteristics.

[0046] In the negative electrode 13, a conductive carbon coating layer is provided on the surface of the SiO particles 19, so there is good electrical contact between the second CNTs 17 and the SiO particles 19. Since the number of second CNTs 17 can be reduced, the SiO content in the negative electrode 13 can be increased, and the mass capacity density and volume capacity density can be further increased.

[0047] Since the negative electrode 13 does not contain an organic polymer binder, the mass and volume can be reduced, and the mass capacity density and volume capacity density can be increased.

[0048] The negative electrode 13 contains a plurality of SiO particles 19 as the negative electrode active material inside the second three-dimensional current collector 18, and does not contain metal foil, thereby enabling the mass capacity density and volume capacity density to be increased.

[0049] Since the second three-dimensional current collector 18 has a sponge-like structure, the thickness of the negative electrode 13 changes reversibly with charging and discharging, and the rate of change in thickness of the negative electrode 13 with charging and discharging is 10% or more and 300% or less, so the space within the secondary battery 10 can be effectively utilized to increase the volumetric capacity density.

[0050] 4. Working Example 4-1. Optimization of SiO mass% to improve capacity density Table 1 summarizes the configuration of the negative electrodes prepared under different conditions.

[0051] [Table 1]

[0052] The negative electrodes under conditions 1 to 14 were circular negative electrodes with a diameter of 12 mm, which were fabricated by forming a free-standing film using the manufacturing method for negative electrode 13 described above and pressing the free-standing film at a predetermined pressure. Multiple samples were fabricated under conditions 5 to 7 and 9. The SiO particles used under conditions 1 to 14 had an average composition of Si:O=1:1 in molar ratio. Under conditions 1 to 14, the negative electrode design capacity per electrode area was 3 mAh / cm. 2 The mass of the SiO particles as the active material was set so that the mass of the SiO particles was constant, and the mass of the CNTs used as the raw material for the sponge-like structure was changed to adjust the dispersion to the SiO particle mass % shown in Table 1. The SiO particle mass % is the mass proportion of the SiO particles in the negative electrode.

[0053] Under conditions 1 to 11, SiO particles having a carbon coating layer on the surface were used. In Table 1, the column for "carbon coating layer" is marked as "present." The SiO particles having a carbon coating layer had an average particle size of 1 μm and a specific surface area of ​​9.7 m 2 / g was used. The mass percentage of SiO in the SiO particles having a carbon coating layer was 97 mass%, and the mass percentage of C was 3 mass%. The mass percentage of SiO shown in Table 1 was calculated by multiplying the mass percentage of SiO in the SiO particles by the mass percentage of SiO in the SiO particles.

[0054] Under conditions 12 to 14, SiO particles without a carbon coating layer on the surface were used. In Table 1, the column for "carbon coating layer" is marked with "none." The SiO particles without a carbon coating layer had an average particle size of 1 μm and a specific surface area of ​​9.7 m 2 The SiO mass % shown in Table 1 is the same value as the SiO particle mass % in the negative electrode.

[0055] The CNTs used as the raw material for the sponge-like structure have a specific surface area of ​​307 m 2 / g, average diameter 10 nm, average length 300 μm, and average number of layers 1 to 5 were used.

[0056] Test cells were prepared by placing the negative electrodes of conditions 1 to 10 and 12 to 14, a polypropylene separator, a 500 μm-thick lithium metal foil, and an electrolyte solution in a container. The electrolyte solution was prepared by dissolving 1.0 mol / L LiTFSI and 0.6 mol / L LiNO3 in a 1:1 volumetric mixture of DOL and DME. Note that the negative electrode of condition 11, with an SiO mass% of 96.0%, exhibited low film strength, making it impossible to prepare a test cell. This is thought to be because the CNT mass% was too low, preventing the formation of a sponge-like structure in which multiple CNTs are entangled with each other and thus unable to retain the SiO particles.

[0057] The fabricated test cells were cycled through the insertion (lithiation) and delithiation (delithiation) of Li into the anode, and the capacity and initial coulombic efficiency of the anode were measured. The lithiation and delithiation cycles were performed at a C rate of 0.1C and a voltage range of 5mV-1.2V.

[0058] Figures 2 to 5 show the results of measurements of the negative electrode capacity and initial coulombic efficiency versus SiO mass % for test cells using negative electrodes with carbon coating layers under conditions 1 to 10. For conditions 5 to 7 and 9, multiple test cells were fabricated for each sample, and measurements were performed on each test cell. Figure 2 is a graph showing the SiO mass-based capacity versus SiO mass %. The horizontal axis shows SiO mass %, and the vertical axis shows the measured negative electrode capacity calculated as the capacity relative to the SiO mass. Figure 3 is a graph showing the electrode mass-based capacity versus SiO mass %. The horizontal axis shows SiO mass %, and the vertical axis shows the measured negative electrode capacity calculated as the capacity relative to the negative electrode mass. Figure 4 is a graph showing the electrode volume-based capacity versus SiO mass %. The horizontal axis shows SiO mass %, and the vertical axis shows the measured negative electrode capacity calculated as the capacity relative to the negative electrode volume. The negative electrode volume was calculated from the area and thickness of the fabricated negative electrode. The negative electrode thickness was measured using a film thickness gauge. FIG. 5 is a graph showing the initial coulombic efficiency versus SiO mass %.

[0059] Figure 2 shows that the SiO mass-based capacitance is almost constant up to approximately 80% SiO mass, but in the region where the SiO mass% is greater than approximately 80%, the capacitance tends to decrease as the SiO mass% increases. This is thought to be because, in the region where the SiO mass% is large, increasing the SiO mass% relatively reduces the amount of CNT, reducing the contact area between the SiO particles and CNT, and thus reducing the conductivity of the electrode. Figure 3 shows that the electrode mass-based capacitance increases as the SiO mass% increases up to approximately 75% SiO mass, but decreases as the SiO mass% increases once the SiO mass% exceeds approximately 80%. When the SiO mass% is low, the amount of CNT increases, resulting in a larger negative electrode mass and a smaller capacity per electrode mass. This explains the tendency for the electrode mass-based capacitance to increase as the SiO mass% increases up to approximately 75% SiO mass. Figure 4 confirms that the electrode volumetric capacity also shows a similar trend to the electrode mass capacity. The electrode volumetric capacity increases as the SiO mass% increases in the range up to approximately 75% SiO mass, reaching a maximum at 82.5% SiO mass, and tends to decrease above that level. When the SiO mass% is low, the amount of CNT increases, which increases the negative electrode volume and reduces the capacity per electrode volume. This is thought to be why the electrode volumetric capacity tends to increase as the SiO mass% increases in the range up to approximately 75% SiO mass. Figure 5 confirms that the initial coulombic efficiency also shows a similar trend to that of the electrode mass-based capacity. The initial coulombic efficiency increases as the SiO mass% increases up to approximately 75% SiO mass, reaching a maximum at 87.3% SiO mass, and beyond that, it shows a tendency to decrease as the SiO mass% increases. The reason it reaches a maximum at a high SiO mass% of 87.3% is thought to be because the small amount of CNT reduces the surface area of ​​the CNTs, reducing side reactions such as the reductive decomposition of the electrolyte.

[0060] 2 to 5, it was confirmed that a negative electrode exhibiting high mass capacity density, volume capacity density, and initial coulombic efficiency can be obtained when the SiO mass% (SiO content in the negative electrode) is in the range of 50 mass% to 93 mass%. In particular, a particularly high mass capacity density, volume capacity density, and initial coulombic efficiency can be obtained when the SiO mass% is in the range of 75 mass% to 90 mass%, and the SiO mass% (SiO content in the negative electrode) is particularly preferably in the range of 75 mass% to 90 mass%.

[0061] Next, the measurement results of the negative electrode capacity and initial coulombic efficiency versus SiO mass% were compared for test cells using negative electrodes under conditions 5 to 7, which used SiO particles with a carbon coating layer, and negative electrodes under conditions 12 to 14, which used SiO particles without a carbon coating layer. Figures 6 to 9 show graphs plotting the measurement results for negative electrodes under conditions 5 to 7, which used SiO particles with a carbon coating layer, and negative electrodes under conditions 12 to 14, which used SiO particles without a carbon coating layer. Figure 6 is a graph showing the SiO mass-based capacity versus SiO mass% and corresponds to Figure 2. Figure 7 is a graph showing the electrode mass-based capacity versus SiO mass% and corresponds to Figure 3. Figure 8 is a graph showing the electrode volume-based capacity versus SiO mass% and corresponds to Figure 4. Figure 9 is a graph showing the initial coulombic efficiency versus SiO mass% and corresponds to Figure 5. In Figures 6 to 9, "SiO / C-CNT" indicates the measurement results for the negative electrode under conditions using SiO particles with a carbon coating layer, and "SiO-CNT" indicates the measurement results for the negative electrode under conditions using SiO particles without a carbon coating layer.

[0062] 6 to 9, high negative electrode capacity and initial coulombic efficiency were confirmed in the test cells using SiO particles with a carbon coating layer. In the SiO mass% range of 75 mass% to 90 mass%, high negative electrode capacity and initial coulombic efficiency were also confirmed in the test cells using SiO particles without a carbon coating layer. When comparing the cases using SiO particles with a carbon coating layer and the cases using SiO particles without a carbon coating layer, it was confirmed that the cases with a carbon coating layer showed higher values ​​for SiO mass-based capacity, electrode mass-based capacity, electrode volume-based capacity, and initial coulombic efficiency than the cases without a carbon coating layer. The SiO mass capacity and electrode mass capacity are approximately 10% higher for SiO particles with a carbon coating layer than for SiO particles without a carbon coating layer. The electrode volume capacity is approximately 25% higher for SiO particles with a carbon coating layer than for SiO particles without a carbon coating layer, demonstrating the significant effect of the carbon coating layer. Furthermore, the initial coulombic efficiency is also approximately 15% higher for SiO particles with a carbon coating layer than for SiO particles without a carbon coating layer, demonstrating the significant effect of the carbon coating layer.

[0063] As mentioned above, the various capacities and initial coulombic efficiencies of the negative electrode relative to SiO mass% are higher in the test cell using SiO particles with a carbon coating layer than in the test cell using SiO particles without a carbon coating layer. This is thought to be due to the carbon coating layer on the surface of the SiO particles, which improves the electrical contact between the SiO particles and the CNTs, resulting in improved conductivity of the negative electrode.

[0064] 4-2. Cycle characteristics Lithiation and delithiation cycle tests were conducted using test cells with a negative electrode (condition 6) containing 82.5% SiO by mass and a negative electrode (condition 7) containing 87.3% SiO by mass. The lithiation and delithiation cycle tests were conducted at a 0.1C C rate of 5 mV-1.2 V and a 100% SiO utilization for the first cycle. From the second cycle onward, the test cell using the negative electrode (condition 6) was operated at a 42% SiO utilization, and the test cell using the negative electrode (condition 7) was operated at a 50% SiO utilization. The cycle test conditions were set so that the SiO utilization would be the above value based on the theoretical capacity of 1710 mAh / g of the SiO negative electrode active material.

[0065] Figure 10 shows the results of the lithiation-delithiation cycle test (hereafter referred to as the cycle test). The horizontal axis represents the number of cycles, and the vertical axis represents the electrode mass capacity. The graph shows the electrode mass capacity (294 mAh / g) of the graphite slurry anode commonly used in commercially available secondary batteries. This value was calculated based on the mass percentage of graphite (79 mass%), the anode active material, and the theoretical capacity of graphite (372 mAh / g). Under condition 6, stable operation was confirmed up to the 140th cycle at a mass capacity of 584 mAh / g, which is 2.0 times the mass capacity of conventional graphite slurry anodes. Under condition 7, stable operation was confirmed up to the 87th cycle at a mass capacity of 746 mAh / g, which is 2.5 times the mass capacity of conventional graphite slurry anodes.

[0066] Next, the cross-sections of the negative electrodes during the cycle test were observed using a scanning electron microscope (SEM). The cycle test was performed under the conditions of 100% SiO utilization at a C rate of 0.1C and 5 mV-1.2 V for the first cycle. From the second cycle onward, the SiO utilization was 48% for the test cell using the negative electrode under condition 6, and 50% for the test cell using the negative electrode under condition 7. Figure 11 shows cross-sectional SEM images of the negative electrodes during the cycle test. Figure 11 shows cross-sectional SEM images of the negative electrodes before the cycle test, after 0.5 cycles (after the first lithiation), after 1 cycle (after the first delithiation), and after 50.5 cycles (after the 51st lithiation). It was confirmed that the negative electrodes under conditions 6 and 7 thickened due to Li insertion after 0.5 cycles, and thinned due to Li desorption after 1 cycle. Even after 50.5 cycles, the thickness increased due to the insertion of Li, and when comparing the thickness and appearance after 0.5 cycles with those after 50.5 cycles, it was confirmed that the structure had not collapsed due to the insertion and desorption of Li. This indicates that the capacity degradation due to the collapse of the structure was suppressed, and good cycle characteristics were obtained. The thickness of the negative electrode under Condition 6 was 20 μm before the cycle test, but after 0.5 cycles it was 3.75 times thicker (75 μm) and after 50.5 cycles it was 4.00 times thicker (80 μm). The change in thickness from before the cycle test was 275% after 0.5 cycles and 300% after 50.5 cycles. The thickness in the lithiated state was approximately 1.42 to 1.51 times thicker than the delithiated state, and the change in thickness from the delithiated state was approximately 42% to 51%. The thickness of the negative electrode under Condition 7 was 25 μm before the cycle test, but after 0.5 cycles it was 2.72 times thicker (68 μm) and 3.04 times thicker (76 μm) after 50.5 cycles. The change in thickness from before the cycle test was 172% after 0.5 cycles and 204% after 50.5 cycles. The thickness in the lithiated state was approximately 1.24 to 1.38 times thicker than the delithiated state, and the change in thickness from the delithiated state was approximately 24% to 38%. [Explanation of symbols]

[0067] 10,10A,10B secondary battery 11 Separator 12,12A,12B Positive electrode for secondary batteries 13,13A,13B Negative electrode for secondary batteries 14 The first carbon nanotube 15 First three-dimensional current collector 16,16A,16B Positive electrode active material 17 The second carbon nanotube 18 Second three-dimensional current collector 19,19A,19B SiO particles (negative electrode active material)

Claims

1. a three-dimensional current collector made of a self-supporting sponge-like structure of carbon nanotubes; a plurality of SiO particles contained inside the three-dimensional current collector; Equipped with The SiO content is 50% by mass or more and 90% by mass or less, The average particle size of the SiO particles is 0.2 μm or more and 10 μm or less. Negative electrode for secondary batteries.

2. 2. The negative electrode for a secondary battery according to claim 1, wherein a carbon coating layer is provided on the surface of the SiO particles.

3. 3. The negative electrode for a secondary battery according to claim 1, which does not contain a metal foil.

4. 4. The negative electrode for a secondary battery according to claim 1, which does not contain an organic polymer binder.

5. The value obtained by dividing the average particle size of the SiO particles by the average diameter of the carbon nanotubes is 30 or more and 1000 or less, The value obtained by dividing the average length of the carbon nanotubes by the average particle size of the SiO particles is 10 or more and 500 or less. The negative electrode for a secondary battery according to any one of claims 1 to 4.

6. The negative electrode for a secondary battery according to any one of claims 1 to 5, Positive electrodes for secondary batteries A secondary battery comprising:

7. A method for manufacturing a negative electrode for a secondary battery, comprising: incorporating a plurality of SiO particles having an average particle size of 0.2 μm or more and 10 μm or less into a three-dimensional current collector made of a self-supporting sponge-like structure of carbon nanotubes; and setting the SiO content to 50 mass % or more and 90 mass % or less.

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