A composite of silicon nanostructures comprising a silicon porous layer and porous silicon nanowires and nanocarbon.

Abstract

To provide a composite of nanocarbons and a silicon nanostructure, the silicon nanostructure including a structure combining a porous structure and silicon nanowires and a structure in which the silicon nanowires are bonded on a porous layer, the composite capable of inhibiting expansion and preventing electrical loss, as well as being a powder and having a high capacity.SOLUTION: A composite consists of a silicon nanostructure 1 and nanocarbons. The silicon nanostructure 1 includes a silicon porous layer 2 having a plurality of first pores 4 and one or more porous silicon nanowires 3 having a plurality of second pores 6 continuously coupled to the silicon porous layer 2.SELECTED DRAWING: Figure 12

Description

[Technical Field]

[0001] This invention relates to a composite of a silicon nanostructure comprising a silicon porous layer and porous silicon nanowires and nanocarbon. [Background technology]

[0002] Lithium-ion rechargeable batteries are in high demand year after year as essential devices for electric vehicles and the autonomous decentralization of information acquisition devices (IoT) for "Society 5.0". To improve performance, it is necessary to use crystalline silicon, which has a large theoretical capacity, as the negative electrode material for lithium-ion batteries. However, the volume increase and delamination caused by alloying silicon and lithium have been a major obstacle to practical application.

[0003] While it has been shown that nanostructuring is key to solving this problem, nanosilicon developed in the semiconductor field exhibits high performance, and even with various structures and methods in place, it is difficult to directly apply conventional technologies to anode materials.

[0004] Conventional nanostructures, being in powder form, require conductive additives, and suffer from reduced cycle performance due to differences in thermal expansion coefficients compared to silicon. Furthermore, the lack of bonding between nanostructures results in high resistance. On the other hand, there are reports of increased capacitance and cycle speed using porous amorphous silicon, but its amorphous, film-like structure leads to problems with electrical and volume losses (see Figures 16(a) and (b)). Nanowire structures have the problem of residual substrate (e.g., Cu substrate) (see Figure 16(c)).

[0005] Furthermore, studies have been reported on the formation of silicides, which are composites of carbon, oxygen, and other metals, in order to improve cycle characteristics. However, while the formation of silicides does lead to improved cycle characteristics, a decrease in discharge capacity is a problem.

[0006] Patent document 1 describes silicon nanowires such as multilayer nanoporous silicon wires with porous and non-porous structures as nanostructures. Non-patent document 1 describes that when crystalline silicon is used, the cycle characteristics deteriorate significantly after 20 cycles.

[0007] On the other hand, from the standpoint of moldability when using a negative electrode material as the negative electrode, the negative electrode material should be in powder form, and the negative electrode material should be made to have a higher capacity. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Special Publication No. 2013-527103 [Non-patent literature]

[0009] [Non-Patent Document 1] Y. Liu, T. Matsumura, N. Imanishi, A. Hirano, T. Ichiwaka, Y. Takeda, Electrochem. Solid-State Lett. 8 (2005) A599. [Overview of the Initiative] [Problems that the invention aims to solve]

[0010] Therefore, the present invention provides a composite of a silicon nanostructure having a structure combining a porous structure and silicon nanowires, or a structure in which silicon nanowires are bonded on a porous layer, and nanocarbon. The objective is to provide a composite of a silicon nanostructure and nanocarbon that can suppress expansion and prevent electrical loss, is in powder form, and has high capacity. [Means for solving the problem]

[0011] The present invention, which solves the above problems, is as follows. (1) A composite of a silicon porous layer having a plurality of first pores, and one or more porous silicon nanowires having a plurality of second pores, continuously bonded to the silicon porous layer, and nanocarbon. The first pores and / or the second pores, and the plurality of first pores and / or the plurality of second pores can be said to form a porous structure. (2) The composite of the silicon nanostructure according to (1) and nanocarbon, characterized in that the porous silicon nanowires are an aggregation of porous silicon nanowires having separated portions. (3) The composite of the silicon nanostructure according to (1) or (2) and nanocarbon, characterized in that the pore diameter of the first pores is such that the average pore is 1 nm to 100 nm. (4) The composite of the silicon nanostructure according to any one of (1) to (3) and nanocarbon, characterized in that the pore diameter of the second pores is such that the average pore is 1 nm to 200 nm. (5) The composite of the silicon nanostructure according to any one of (1) to (4) and nanocarbon, characterized in that the diameter of the porous silicon nanowires is 10 to 1000 nm. (6) The composite of the silicon nanostructure according to any one of (1) to (5) and nanocarbon, characterized in that the thickness of the silicon porous layer is 1 to 100 μm. (7) The composite of the silicon nanostructure according to any one of (1) to (6) and nanocarbon, characterized in that the silicon nanostructure further contains silver. [Advantages of the Invention]

[0012] According to the composite of a silicon nanostructure and nanocarbon according to the present invention (hereinafter, may be simply referred to as "silicon nanostructure-nanocarbon composite"), it is possible to suppress expansion and prevent electrical loss, and it can be used as a negative electrode material for a secondary battery. Further, when the nanocarbon is a carbon nanotube, it may be referred to as "silicon nanostructure-carbon nanotube composite", and when the carbon nanotube is further a single-walled carbon nanotube, it may be referred to as "silicon nanostructure-single-walled carbon nanotube composite".

Brief Description of Drawings

[0013] [Figure 1] (a) A silicon nanostructure constituting a silicon nanostructure-nanocarbon composite which is one embodiment of the present invention, (b) Introduction of solid charge carriers into a porous silicon nanowire structure among the silicon nanostructures, (c) Reduction of the interfacial resistance between Si and Li+ in the nanowire structure, are respectively schematic diagrams (cross-sectional views). [Figure 2] Diagrams (cross-sectional views) showing the mechanisms of non-destructive and high-capacity for (a) the case of a porous structure and (b) the case of a nanowire structure, respectively. [Figure 3] A diagram (cross-sectional view) schematically showing the composite structure of a silicon nanostructure-single-walled carbon nanotube composite. [Figure 4] An overview of the manufacturing method of a silicon nanostructure is divided into (a) production of silver (electroless plating method), (b-1´) production of silicon nanowires (metal-assisted chemical etching), and (b-2) formation of a porous layer, and is respectively a schematic diagram (cross-sectional view). [Figure 5] A diagram showing the type of silicon, experimental conditions, and etching time for each step of the manufacturing method (two-step) of a silicon nanostructure. [Figure 6] A diagram showing each step of the manufacturing method (two-step) of a silicon nanostructure. [Figure 7] (a) A diagram showing the type of silicon, experimental conditions, and plating time / etching time for each step of the manufacturing method (one-step) of a silicon nanostructure. [Figure 8] This diagram shows the steps involved in the manufacturing method of silicon nanostructures (another one-step process). [Figure 9] This figure shows SEM images of (a) a silicon porous layer and porous silicon nanowires continuously bonded thereto, (b) a magnified view of the porous silicon nanowires, (c) a magnified view of the silicon porous layer, and (d) the surface of the porous silicon nanowires, all obtained in a two-step process. [Figure 10] These figures show (a) a porous silicon layer and porous silicon nanowires continuously bonded thereto, and (b) an SEM image of (a) viewed toward the porous silicon nanowires, respectively. [Figure 11] This figure shows a SEM image of a silicon nanostructure-single-walled carbon nanotube composite. [Figure 12] This diagram schematically shows the configuration of the cells used for evaluating cycle characteristics. [Figure 13] This figure shows the appearance of a sheet-like silicon nanostructure / single-walled carbon nanotube composite. [Figure 14] This figure shows (a) the measurement results of the cycle characteristics evaluation of silicon nanostructure / single-walled carbon nanotube composites, and (b) the number of cycles and capacity (mAh / g) for (a). [Figure 15] This figure shows (a) the measurement results of the cycle characteristics evaluation of a silver-containing silicon nanostructure / single-walled carbon nanotube composite, and (b) the number of cycles and capacity (mAh / g) for (a). [Figure 16] This diagram schematically shows examples of conventional silicon nanostructures: (a) nanoparticles (in powder form), (b) nanowires (in powder form), and (c) nanowires (in an oriented form). [Modes for carrying out the invention]

[0014] Embodiments of the present invention will be described below with reference to the drawings. The present invention is not limited to the following embodiments, and modifications, alterations, and improvements may be made without departing from the scope of the invention.

[0015] The nanocarbons used in this invention must have excellent conductivity, a large specific surface area, and be easily mixed with silicon nanostructures. Furthermore, they must be able to undergo structural changes in response to volume changes in the silicon nanostructures, and possess the flexibility to maintain electron conduction paths even during such structural changes. Specifically, the following types of nanocarbons are usable. Examples include single-walled carbon nanotubes, multi-walled carbon nanotubes, carbon nanofibers, and graphene. By the way, in the following explanation, we will mainly use silicon nanostructure-carbon nanotube composites as an example among silicon nanostructure-nanocarbon composites. This is because silicon nanostructure-carbon nanotube composites have the characteristic of being binder-free and capable of forming self-supporting films.

[0016] As shown in Figure 1(a), the silicon nanostructure 1 constituting the silicon nanostructure-carbon nanotube composite 7 (Figure 3) comprises a silicon porous layer 2 having a plurality of first pores 4, and spaced-out porous silicon nanowires 3, 3' having a plurality of second pores 6, which are continuously bonded to the silicon porous layer 2. Since the porous silicon nanowires (hereinafter sometimes referred to as "silicon nanowires") 3, 3' are bonded approximately perpendicular to the silicon porous layer 2, the length L of the porous silicon nanowires 3, 3' forms a porous silicon nanowire layer 13.

[0017] The first pore 4 has an opening on the surface of the silicon porous layer 2 that opens outward from the silicon porous layer 2. Since Figure 1(a) is a cross-sectional view, one of the first pores 4 appears to be a void in the silicon porous layer 2, but as described above, it has an opening. The same applies to the second pore 6.

[0018] For example, Li +If it contains, silicon nanostructure 1 can be used as the negative electrode of a lithium-ion battery. As shown in Figures 2(a) and (b), Li + This is extremely undesirable for lithium-ion batteries because the silicon is absorbed by the silicon material of the silicon porous layer 2 and porous silicon nanowires 3 and 3', forming a LiSi alloy that is then charged, causing the silicon to expand and degrading its properties.

[0019] As shown in Figure 2(a), in the case of a first porous (pore) 4 having an opening 4a (a second porous 6 having an opening 6a), a portion 2a of the silicon porous layer (a portion 3a of the porous silicon nanowire) is Li + Even if it absorbs and expands, the expanded portion 2a of the silicon porous layer (part 3a of the porous silicon nanowire) will fill the first pore 4 (second pore 6), and the portion 2a of the silicon porous layer (part 3a of the porous silicon nanowire) will not be destroyed. Since it is not destroyed in this way, Li will be absorbed into the portion 2a of the silicon porous layer (part 3a of the porous silicon nanowire). + This will enable increased capacity.

[0020] As shown in Figure 2(b), the mechanism for non-destructive and high-capacitance processing in the case of silicon nanowire structures is as follows: A silicon nanowire gap 10 is formed between the porous silicon nanowires 3 and 3'. The silicon nanowire gap 10 realizes the non-destructive and high-capacitance processing mechanism of the silicon nanowire structure through a mechanism similar to that of the first porous 4 described in the porous structure. Therefore, for the separated portions of an aggregate of porous silicon nanowires having separated portions, the larger the separated portion, the more advantageous it is for non-destructive and high-capacitance processing. Furthermore, porous silicon nanowires 3' and 3 can achieve non-destructive and high-capacitance processing by having a second porous 6.

[0021] Specifically, before charging, the combined void ratio of the second porosity 6 and the voids 10 between silicon nanowires is 100%, but after charging, this void ratio becomes 0%, meaning there is no loss of space. This is because the second porosity 6 and the voids 10 between silicon nanowires are filled when swelling due to charging is complete.

[0022] The first porous 4 is preferably pore with an average vacancy of 1 nm to 100 nm, and more preferably 5 to 30 nm, from the viewpoint of silicon absorbing lithium and preventing expansion and delamination. The second porous 6 is preferably pore with an average vacancy of 1 nm to 200 nm, and more preferably 5 to 30 nm, from the viewpoint of silicon absorbing lithium and preventing expansion and delamination.

[0023] From the viewpoint of supporting the silicon nanowires while maintaining it as a film, the thickness t of the silicon porous layer 2 is preferably 1 to 100 μm, and more preferably 1 to 50 μm. From the viewpoint of increasing the surface area and preventing expansion and delamination, the diameter D of the porous silicon nanowire is preferably 10 to 1000 nm, and more preferably 50 to 500 nm.

[0024] As shown in Figure 1(a), when the silicon nanostructure 1 has an electrode substrate 20 on the surface of the silicon porous layer 2 opposite to the porous silicon nanowire 3(3'), a current path to the electrode substrate 20 can be secured. This is because each silicon within the porous silicon nanowire 3(3') is bonded, and the silicon porous layer 2 and the porous silicon nanowire 3(3') are continuously bonded. In other words, by combining various nanostructures, the destruction of the nanostructure is prevented, and electrical loss due to the destruction of the nanostructure can be reduced.

[0025] As shown in Figure 1(b), when the electrolyte 21 is a solid electrolyte, the solid electrolyte is introduced into the gap 10 between silicon nanowires. Li contained in the solid electrolyte + Since it is absorbed by silicon to form a LiSi alloy, the Li is added to the filled second porous 6 portion. +It can be dispersed and charged.

[0026] As shown in Figure 1(c), silicon and Li + Interfacial resistance may be reduced by forming a low-resistance material at the interface. Known materials such as Al2O3 and TiO2 can be used as the low-resistance material.

[0027] As shown in Figure 3, the silicon nanostructure-carbon nanotube composite 7 is a composite of silicon nanostructure 1 and carbon nanotube 8, containing silicon nanostructure 1 and carbon nanotube 8. Figure 3 shows an embodiment in which single-walled carbon nanotubes (SWCNTs) are used as carbon nanotube 8. Multiple single-walled carbon nanotubes (SWCNTs) 8 are intertwined with multiple silicon nanostructures (PSiNWs) 1. Therefore, the silicon nanostructures (PSiNWs) 1 act as current collectors for the single-walled carbon nanotubes (SWCNTs) 8, preventing the single-walled carbon nanotubes (SWCNTs) 8 from detaching from the silicon nanostructures (PSiNWs) 1.

[0028] Silicon nanostructure 1 can be manufactured by first producing silver using the electroless plating method shown in Figure 4(a), and then by producing silicon nanowires (structures) using metal-assisted chemical etching as shown in Figure 4(b). In the process of producing silver, a silicon wafer is immersed in a solution of hydrogen fluoride (HF) and silver nitrate (AgNo3). In this solution, the main reaction shown in Figure 4(a) occurs, and Ag is adsorbed onto the surface of the silicon wafer (Figures (a-1) and (a-3)). An SEM image of the surface viewed from above was observed as shown in Figure 4(a-2).

[0029] In the process of fabricating silicon nanowires, the main reaction shown in Figure 4(b) occurs, and Ag +Oxidation of silicon occurs. That is, the oxidation-reduction reaction between silicon and silver can partially oxidize the silicon, and the silicon disappears in the oxidized areas, so the remaining parts that do not disappear form nanowires (Figure (b-1)). In the SEM image of these nanowires viewed from the side, silicon nanowires could be observed as shown in Figure (b-2). In this case, the etching direction is perpendicular to the p(100) substrate.

[0030] Figure 4(b-1'), which schematically shows Figure 4(b-1), can be modified by performing the following process to form the silicon porous layer 2. The silicon porous layer 2 is formed when the thickness of the silver layer decreases. As etching progresses, the thickness of the silver layer decreases to about 5 nm, at which point the silicon porous layer 2 is formed. [Examples]

[0031] (Manufacturing of silicon nanostructures) There are two methods for manufacturing silicon nanostructure 1: a two-step method and a one-step method. The two-step method involves creating the structure by varying the conditions for forming silicon nanowires and silicon porous layers. On the other hand, the one-step method involves forming silicon nanowires and silicon porous layers under the same conditions. In other words, in the one-step method, porosity is created when the amount of silver decreases in a single solution, while in the two-step method, porosity is forcibly created using a porosizing solution.

[0032] As shown in Figure 5, the two-step process for manufacturing silicon nanostructure 1 includes two steps: one comprising the steps of fabricating silver particles, fabricating a silicon nanowire array, and separating the silicon nanowire array thin film from the silicon substrate (two-step 1); and another comprising the steps of removing silver from the silicon nanowire array thin film in addition to two-step 1 (two-step 2). The silicon type, experimental conditions, and etching time for each step are shown in Figure 5. In all steps, the silicon type was n-type, the resistivity was ≤0.001 Ω*CM, the thickness was 200 μm, and the plane orientation was 100. Comparing two-step 1 and two-step 2, the amount of silver remaining in the silicon nanowire array thin film that was not removed was a catalytic amount (very small amount).

[0033] In the process of preparing silver particles, the experimental conditions were H2O: 40 ml, HF: 10 ml, AgNO3: 120 mg, with an etching time of ≥ 15 s (30 s). In the process of preparing silicon nanowire arrays, the experimental conditions were H2O: 400 ml, HF: 100 ml, H2O2: 9 ml to 80 ml, with an etching time of ≥ 10 min (10 min). In the process of separating the silicon nanowire array thin film from the silicon substrate, the experimental conditions were H2O: 40 ml, HF: 8 ml, H2O2: 8.35 ml, with an etching time of 5 min. In the process of removing silver from the silicon nanowire array thin film, the experimental conditions were HNO3: 10 ml, H2O: 50 ml, with an etching time of 20 mins. Note that the time in parentheses for etching time represents the time in the example.

[0034] Figure 6(a) shows a silicon wafer, Figures 6(b) to (e) show the process from the fabrication of silver particles to the separation of the silicon nanowire array thin film from the silicon substrate, and Figures 6(f) and (g) show the process of removing silver from the silicon nanowire array thin film.

[0035] As shown in Figure 6(a), the one-step process for manufacturing silicon nanostructures comprises the steps of producing silver particles and producing silicon nanowires and separating the silicon nanowires from the silicon substrate. In all steps, the silicon type was n-type, the resistivity was ≤0.001Ω*CM, the thickness was 200μm, and the plane orientation was 100.

[0036] In the process of preparing silver particles, the experimental conditions were H2O: 40 ml, HF: 10 ml, AgNO3: 120 mg, with an etching time of ≥ 15 s (30 s). In the process of preparing silicon nanowires and separating silicon nanowires from silicon substrates, the experimental conditions were H2O: 400 ml, HF: 100 ml, H2O2: 80 ml, with an etching time of ≥ 10 mins.

[0037] Figure 6(a) shows a silicon wafer, and Figures 6(b) to (d) show the process of fabricating silver particles and separating the silicon nanowire array thin film from the silicon substrate (two-step 1). Figures 6(f) and (g) (an additional step as two-step 2) show the process of removing silver from the silicon nanowire array thin film.

[0038] The following is a comparison between manufacturing silicon nanostructure 1 in one step and in two steps. In the one-step process, the length L of the porous silicon nanowire is 10 μm or less. In the two-step process, longer nanowires have a larger surface area, which reduces the resistance associated with solid-to-liquid phase movement of lithium ions, making them suitable for high-power applications. On the other hand, shorter nanowires are less prone to structural collapse associated with lithium ion insertion and removal, which is advantageous for extending their lifespan.

[0039] In a two-step manufacturing method (Figure 6), silicon nanostructure 11 (Experimental Example 1) was manufactured using the following methods: in the silicon nanowire array preparation step, H2O: 400 ml, HF: 100 ml, H2O2: 50.1 ml, etching time 10 min; and in the separation step of the silicon nanowire array thin film from the silicon substrate, H2O: 40 ml, HF: 8 ml, H2O2: 8.35 ml, etching time 5 min. Furthermore, in the one-step manufacturing method (Figure 7), silicon nanostructure 21 (Experimental Example 2) was manufactured with an etching time of 30 seconds in the silver particle fabrication process and an etching time of 10 minutes in the silicon nanowire fabrication process and the separation of silicon nanowires from the silicon substrate.

[0040] In silicon nanostructure 11, silicon nanostructure 31 (Experimental Example 3) was fabricated without removing silver. That is, silicon nanostructure 31 (Experimental Example 3) contains a catalytic amount of silver.

[0041] (Structural analysis of silicon nanostructures) SEM images were obtained for silicon nanostructure 11 (Experimental Example 1) fabricated by metal-assisted etching.

[0042] The silicon nanostructure 11 (Experimental Example 1) shown in Figure 9(a) comprises a silicon porous layer 12 with a thickness d1(t) of 5.48 μm and a porous silicon nanowire layer 23 with a thickness d2 of 3.23 μm. The thickness d2 of the porous silicon nanowire layer 23 is defined as the thickness at L1, which is estimated to be the longest length of the porous silicon nanowire. The diameter d3 of the porous silicon nanowire 13 shown in Figure 9(b) was 75.3 nm. The diameter d4 of the opening of the first pore 14 shown in Figure 9(c) was 16.9 nm, and the spacing d5 between multiple observed first pores was 7.30 nm. As shown in Figure 9(d), the second pore was observed as a dark region between the silicon structures of the porous silicon nanowire in the whitish region, for example, as a second pore 16. The diameter of the opening of the second pore 16 was several nm to 10 nm when compared to a 10 nm scale.

[0043] The silicon nanostructure 21 (experimental example 2) shown in Figure 10(a) comprises a silicon porous layer 12' with a thickness d7(t) of 1.818 μm and a porous silicon nanowire layer 23' with a thickness d8 of 8.455 μm. The thickness d8 of the porous silicon nanowire layer 23' was taken as the thickness at the point where the longest length of the porous silicon nanowire is estimated to be L2. A 4.092 μm porous silicon nanowire was observed around the porous silicon nanowire of length L2, but it is thought that the porous silicon nanowire was broken and shortened when the SEM image was measured. In Figure 10(b), for example, a cluster of porous silicon nanowires observed in a roughly mountain shape with the tip of length L2 as the peak appeared dark. The parts corresponding to the valleys of multiple roughly mountain-shaped clusters of porous silicon nanowires appeared whitish, and these parts corresponded to the porous silicon nanowire side surface of the silicon porous layer 12'.

[0044] (Manufacturing of silicon nanostructure and single-walled carbon nanotube composites) A silicon nanostructure-single-walled carbon nanotube composite 17 (Example 1) was fabricated as follows, using a silicon nanostructure 11 (Experimental Example 1):single-walled carbon nanotube = 1:2 (mass ratio). To prepare the working electrode, silicon nanostructure 11 and single-walled carbon nanotube (Meijo Nanocarbon (EC2.0)) were dispersed in polyvinylidene fluoride binder (10 wt%) and ethanol. The resulting slurry was attached to a Cu foil and vacuum-dried at 80°C for at least 2 hours before use.

[0045] A composite 27 (Example 2) of silicon nanostructure 31 (Experimental Example 3) and single-walled carbon nanotube was fabricated as follows, with a silicon nanostructure 31 (Experimental Example 3):single-walled carbon nanotube ratio of 1:2 (mass ratio). To fabricate the working electrode, silicon nanostructure 31 (Experimental Example 3) and single-walled carbon nanotube were dispersed in polyvinylidene fluoride binder (10 wt%) and ethanol. The resulting slurry was attached to Cu foil and vacuum-dried at 80°C for at least 2 hours before use. In Examples 1 and 2, single-walled carbon nanotube was used as the carbon nanotube from the viewpoint of an integrated structure with the current collector, but as mentioned above, multi-walled carbon tubes, and even carbon nanofibers, graphene, etc., can be used.

[0046] Regarding the electrode composition (silicon nanostructure: nanocarbon) according to the present invention, a higher proportion of carbon is desirable from the viewpoint of maintaining conductivity within the electrode, and a higher proportion of silicon nanostructure is desirable from the viewpoint of improving the overall energy density of the electrode. Considering these balances, the mass ratio of silicon nanostructure to nanocarbon is preferably 9:1 to 1:9, more preferably 4:1 to 1:4, and even more preferably 2:1 to 1:2.

[0047] As shown in the SEM image in Figure 11, in the silicon nanostructure-single-walled carbon nanotube composite 17 (Example 1, PSiNW), multiple single-walled carbon nanotubes (SWCNTs) form a network, and these network-like SWCNTs are intertwined with the PSiNW.

[0048] (Creation of negative electrode) A negative electrode (WE, Example 3) was prepared by thoroughly mixing the silicon nanostructure / single-walled carbon nanotube composite 17 (Example 1) and PVDF (polyvinylidene fluoride) in a mass ratio of 9:1. The amount of active material was 0.186 mg, and the total amount of the electrode was 0.207 mg. In Example 3, a negative electrode (WE, Example 4) was prepared using silicon nanostructure / single-walled carbon nanotube composite 27 (Example 2) instead of silicon nanostructure / single-walled carbon nanotube composite 17 (Example 1). The amount of active material was 0.060 mg, and the total amount of the electrode was 0.0667 mg.

[0049] On the other hand, a negative electrode (WE, Experimental Example 4) was prepared by thoroughly mixing each component in the following ratio: Si (silicon nanostructure 11, Experimental Example 1):carbon black:PVDF (polyvinylidene fluoride) = 6:3:1 (volume ratio). The amount of active material was 0.792 mg, and the total amount of the electrode was 1.980 mg.

[0050] As shown in Figure 12, the configuration of the cell 30 for cycle characteristic evaluation includes an upper housing 32, a lower housing 34, and a pressing section 31. The upper housing 32 has an upper flange 32a and a through hole 32b, the pressing section 31 has a pressing pin 31a, and the lower housing 34 has a lower flange 34a and a pin 34b. The CE (metallic Li, anode electrode) 40 and WE (fabricated electrode, negative electrode) 41, with a separation glass fiber filter 35 in between, are fixed to the upper surface of the lower housing 34 by the pressing pin 31a, which presses downwards a pressing block 33 set on the upper side of the CE. Current is supplied to the cell 30 (charging and discharging) through conductors 38 and 39.

[0051] The WE (the prepared electrode, negative electrode) of Example 3, which was removed from cell 30, was a roughly circular, black, flat film-like object 51 that could be picked up with tweezers 50, as shown in Figure 13.

[0052] (Cycle characteristics of the negative electrode) The downward pressing of the pressing block 33 was performed by screwing the wing nut 41 onto the pin 34b. The pressing of the pressing portion 31 onto the upper housing 32 and the pressing of the upper flange 32a onto the lower flange 34a were performed via the O-rings 36 and 37, respectively. The cycle characteristics were evaluated using 1 M LiPF6 (EC (ethylene carbonate): DEC (diethyl carbonate) = 1:1 (mass ratio)) as the electrolyte solution.

[0053] The negative electrode characteristics of Example 3 were evaluated by constant current charge-discharge measurements. Specifically, after performing discharge-charge at 200 mA / g for 3 cycles on the cell 30, the cycle characteristics were evaluated by performing 100 discharge-charge cycles at 1000 mA / g. As shown in Fig. 14(a), for the charge (lithium release process) at 1000 mA / g, the capacity in the first charge cycle was 1551.6 mAh / g -1 , the charge in the 50th cycle was 1839.9 mAh / g -1 , and the capacity in the 100th cycle was 1965.6 mAh / g -1 . Comparing the charged capacities, the capacity in the 50th cycle / the discharge in the first cycle = 1839.9 mAh / g -1 / 1551.6 mAh / g -1 = 1.19, and the capacity in the 100th cycle / the discharge in the 50th cycle = 1965.6 mAh / g -1 / 1551.6 mAh / g -1 = 1.07. That is, the characteristic retention rate exceeded 100%.

[0054] Furthermore, regarding the result of calculating the Coulomb efficiency (charge capacity / discharge capacity) from the results of discharge (lithium insertion process) and charge (lithium release process), it was as follows. In the charge-discharge (200 mA / g) immediately after constructing the cell, the discharge capacity in the first cycle was 8132.8 mAh / g -1 , and the charge capacity was 2384.0 mAh / g -1 , and the Coulomb efficiency was as low as 29.3%, but this value was eliminated in the second cycle, and the Coulomb efficiency in the second cycle of charge-discharge at 200 mA / g was 69.8% (the discharge capacity was 3281.5 mAh / g -1, charging capacity 2292.1mAhg -1 The efficiency improved to 97.3% (discharge capacity 1965.5 mAhg) during charge / discharge. This is thought to be because, in the first cycle, a portion of the electricity supplied from the external power source was used for the decomposition reaction of the electrolyte in silicon nanowires and single-walled carbon nanotubes (SEI film formation reaction). The improvement in Coulomb efficiency from the second cycle onward is thought to be because the SEI film formed in the first cycle functioned as an insulating film that suppressed the decomposition of the electrolyte. Furthermore, this SEI film is robust, and the Coulomb efficiency transitions to a high level from the third cycle onward. Specifically, in charge / discharge at 1000 mA / g, the average Coulomb efficiency over 100 cycles was 95.7%, and the Coulomb efficiency at the 100th cycle was 97.3% (discharge capacity 1965.5 mAhg). -1 , charging capacity 1912.6mAhg -1 ) was.

[0055] The cycle characteristics of the negative electrode in Example 4 were also evaluated by performing 100 cycles, where each cycle consisted of charging the cell 30 at 1000 mA / g followed by discharging. As shown in Figure 15(a), for charging, the capacity in the first charging cycle is 552.7mAhg. -1 The 50th charge cycle yielded 415.3mAhg -1 And the capacity after 100 cycles is 498.7mAhg -1 The result was: Comparing the charged capacity, the capacity at 50 cycles / the discharge at 1 cycle = 415.3 mAhg -1 / 552.7mAhg -1 = 0.75, Capacity at 100 cycles / Discharge at 1 cycle = 498.7 mAhg -1 / 552.7mAhg -1 = 0.9. In other words, the characteristic retention rate was 90%.

[0056] Figure 15(b) revealed the following: Carbon nanotubes act as immobilizers for the current collector and silicon nanostructures, enabling lithium absorption and release without expansion or delamination of the silicon nanostructures. Furthermore, the carbon nanotubes of the current collector are in contact with the silicon nanostructures at various points, suppressing the increase in resistance. On the other hand, an oxide film is formed to remove silver, which leads to a decrease in capacitance.

[0057] The following can be said about the negative electrodes of Examples 3 and 4 compared to conventional technology. The adhesion between the silicon nanostructure and the current collector has been a challenge, and conventionally, binders have been used to improve adhesion. The carbon nanotube, which is the current collector, and the silicon nanostructure are in close contact, resulting in low electrical loss. In particular, Example 4 showed the following: Because the surface was entirely silicon, it exhibited a higher capacity compared to Example 3, where an oxide film was formed. [Industrial applicability]

[0058] Increasing the capacity of secondary batteries is expected to lead to applications in high-power devices such as electric vehicles. Furthermore, increased capacity allows for smaller batteries, opening up possibilities for applications in IoT devices and the drone market. [Explanation of symbols]

[0059] 1, 11, 21, 31: Silicon nanostructures 2, 12, 12': Silicon porous layer 2a: Part of the silicon porous layer 3, 3', 13: Porous silicon nanowires 3a: Part of a porous silicon nanowire 4, 14: First Pole 4a: First porous opening 6, 16: The second porous 6a: Second porous opening 7, 17, 27: Silicon nanostructure-carbon nanotube composites 8: Carbon nanotubes 10: Inter-nanowire void 20: Electrode substrate 21: Electrolyte 23, 23': Porous silicon nanowire layer 30: Cell 31: Pressing part 31a: Compression pin 32: Upper housing 32a: Upper flange 32b: Through hole 33: Pressure block 34: Lower housing 34a: Lower flange 34b: Pin 35: Separation glass fiber filter 36, 37: O-rings 38, 39: Conductor 40: Anode electrode (metallic Li) 41: Negative electrode (prepared electrode) 50: Tweezers 51; Silicon nanostructure-carbon nanotube composite D: Diameter of porous silicon nanowire L, L1, L2: Length of porous silicon nanowire t: Thickness of the silicon porous layer d1, d7: Thickness of the silicon porous layer d2, d8: Thickness of the porous silicon nanowire layer d3: Diameter of porous silicon nanowire d4: Diameter of the first porous opening d5: Interval between the first porous regions

Claims

1. A mixture of silicon nanostructure and nanocarbon, comprising a silicon porous layer having a plurality of first pores, and one or more porous silicon nanowires having a plurality of second pores, which are continuously bonded to the silicon porous layer, wherein the first pores are substantially uniformly formed throughout the silicon porous layer.

2. A mixture of nanocarbon and the silicon nanostructure according to claim 1, characterized in that the porous silicon nanowire is an aggregate of porous silicon nanowires having separated portions.

3. A mixture of nanocarbon and the silicon nanostructure according to claim 1 or 2, characterized in that the pore size of the first pore is 1 nm to 100 nm in average vacancy size.

4. A mixture of nanocarbon and the silicon nanostructure according to claim 1 or 2, characterized in that the pore size of the second pore is 1 nm to 200 nm in average vacancy size.

5. A mixture of the silicon nanostructure according to claim 3 and nanocarbon, characterized in that the diameter of the porous silicon nanowire is 1 to 1000 nm.

6. A mixture of the silicon nanostructure according to claim 4 and nanocarbon, characterized in that the diameter of the porous silicon nanowire is 1 to 1000 nm.

7. A mixture of a silicon nanostructure according to claim 5, characterized in that the thickness of the silicon porous layer is 1 to 100 μm, and nanocarbon.

8. A mixture of a silicon nanostructure according to claim 6 and nanocarbon, characterized in that the thickness of the silicon porous layer is 1 to 100 μm.

9. A mixture of the silicon nanostructure according to claim 1, characterized in that the silicon nanostructure further contains silver, and nanocarbon.

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