JP2013117053A - Hollow and copper-cored silicon nanowire and silicon composite copper substrate, and method for manufacturing the same, and lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a silicon nanowire used for negative electrodes or the like of lithium ion secondary batteries including core parts containing copper in the nanowire, and provided with a high charging capacity, a high speed charging and discharging performance, and an excellent cycle characteristic.SOLUTION: The hollow and copper-cored silicon nanowire 1 includes a shell part 5 containing silicon, and core parts 3 containing copper and discontinuously existing in a hollow part formed by the shell part 5. The silicon nanowire 1 can be used as a silicon composite copper substrate having the silicon nanowire 1 on at least one side of a copper substrate, and formed by connecting the silicon nanowire 1 onto the copper substrate, or as a silicon composite copper substrate having a coating film including the silicon nanowire 1 on at least one side of the copper substrate.

Description

  The present invention relates to a silicon nanowire having a core part containing copper in a hollow part.

  Conventionally, lithium-ion secondary batteries using various carbon materials such as natural graphite, artificial graphite, amorphous carbon, and mesophase carbon, lithium titanate, tin alloys, etc. have been put into practical use as negative electrode active materials for lithium-ion secondary batteries. It has become.

  On the other hand, negative electrodes for lithium ion secondary batteries using metals and alloys having a large theoretical capacity as lithium compounds, particularly silicon and its alloys as negative electrode active materials, have been developed with the aim of increasing the capacity. However, since the volume of silicon that occludes lithium ions expands to about 4 times that of silicon before occlusion, a negative electrode using silicon as a negative electrode active material repeats expansion and contraction during a charge / discharge cycle. Therefore, peeling of the negative electrode active material occurred, and there was a problem that the lifetime was extremely short compared to a negative electrode using a conventional carbon-based active material.

  Examples of the negative electrode using silicon include a negative electrode using silicon nanowires (also called silicon fibers or silicon nanotubes) as a negative electrode active material (see, for example, Patent Documents 1 and 2).

  Patent Document 1 discloses a method of obtaining silicon nanotubes by coating silicon on a substrate having zinc oxide (ZnO) nanorods grown in a vertical direction using silane gas and removing the ZnO nanorods.

  Patent Document 2 discloses a method for obtaining silicon fibers by etching a silicon substrate to obtain, for example, a silicon pillar having a length of 100 μm and a diameter of 0.2 μm, and then peeling (detaching) the substrate.

JP 2010-192444 A Special table 2009-523923

  However, the negative electrodes described in Patent Documents 1 and 2 have a problem that it takes a long time to charge and discharge lithium ions due to low electronic conductivity in silicon nanowires. In addition, the negative electrodes described in Patent Documents 1 and 2 have a problem in that the charge / discharge capacity decreases as charge / discharge is repeated, and the cycle characteristics are not sufficient.

  The present invention has been made in view of the above-mentioned problems, and its object is to have a core part containing copper in the nanowire, and have a high charge capacity, high-speed charge / discharge characteristics, and good cycle characteristics. It is obtaining the silicon nanowire used for the negative electrode of a lithium ion secondary battery etc. which combine these.

In order to achieve the above-described object, the present invention has the following features.
(1) A hollow copper core silicon nanowire having a shell part containing silicon and a core part containing copper that is discontinuously present in a hollow part formed by the shell part.
(2) Silicon having the hollow copper core silicon nanowire according to (1) on at least one surface of a copper substrate, wherein the hollow copper core silicon nanowire is connected to the copper substrate. Composite copper substrate.
(3) A silicon composite copper substrate comprising a coating film containing the hollow copper core silicon nanowire according to (1) on at least one surface of the copper substrate.
(4) The hollow copper core silicon nanowire according to (1), wherein the diameter is 10 to 500 nm.
(5) A copper substrate is heated to 300 to 600 ° C. in an atmosphere containing an organic silane and an organic dispersion medium in a pressure vessel to form a hollow copper core silicon nanowire on the copper substrate. A method for manufacturing a silicon composite copper substrate.
(6) The method for producing a silicon composite copper substrate according to (5), wherein the organosilane is phenylsilane, and the copper substrate is heated to 380 to 500 ° C.
(6) A step of heating the copper substrate to 300 to 600 ° C. in an atmosphere containing organosilane and an organic dispersion medium in a pressure vessel, and forming a hollow copper core silicon nanowire on the copper substrate; Separating the hollow copper core silicon nanowire from a copper substrate. A method for producing a hollow copper core silicon nanowire, comprising:
(7) A step of heating the copper substrate to 300 to 600 ° C. in an atmosphere containing organosilane and an organic dispersion medium in a pressure vessel, and forming a hollow copper core silicon nanowire on the copper substrate; Separating the hollow copper core silicon nanowires from the copper substrate, dispersing the hollow copper core silicon nanowires in a slurry, and applying the slurry onto another copper substrate. A method for producing a silicon composite copper substrate.
(8) A lithium ion secondary battery using the silicon composite copper substrate according to (2) or (3) as a negative electrode, and further comprising a positive electrode and a nonaqueous electrolyte.

  According to the present invention, a silicon nanowire having a core part containing copper in a nanowire and having a high charge capacity, high-speed charge / discharge characteristics, and good cycle characteristics is used for a negative electrode of a lithium ion secondary battery. be able to.

Sectional drawing of the silicon nanowire 1 which concerns on embodiment of this invention. The figure which shows the silicon composite copper substrate 7a which concerns on embodiment of this invention. The figure which shows the silicon composite copper substrate 7b which concerns on embodiment of this invention. (A)-(d) The figure which shows the manufacturing method of the silicon nanowire 1 which concerns on embodiment of this invention. (A)-(b) The figure which shows the manufacturing method of the silicon nanowire which concerns on Example 1. FIG. (A) The electron micrograph of the silicon composite copper substrate which concerns on Example 1, (b) The enlarged view in the white line frame in (a). The electron micrograph of the silicon nanowire which concerns on Example 1, and the EDS observation result of copper and silicon in the same visual field.

(Hollow copper core silicon nanowire 1)
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a cross-sectional view of a hollow copper core silicon nanowire 1 according to an embodiment of the present invention. The silicon nanowire 1 has a shell portion 5 containing silicon and a core portion 3 containing copper that is discontinuously present in a hollow portion formed by the shell portion 5.

  The diameter and length of the silicon nanowire 1 can be adjusted by the pressure, temperature, and amount of input raw material depending on the formation conditions of the silicon nanowire 1, but the diameter of the silicon nanowire 1 is about 10 to 500 nm. Moreover, it is preferable that the diameter of the core part 3 is half or less of the diameter of the silicon nanowire 1.

  The length of the silicon nanowire 1 in the major axis direction varies depending on the growth rate and growth time, but is generally 50 μm or less.

  The core part 3 is formed discontinuously in the hollow part formed by the shell part 5. As will be described later, it is considered that the shell portion 5 is formed around the columnar continuous core portion 15 during the synthesis. However, when the core portion 15 and the shell portion 5 contract after cooling after completion of the reaction, the core portion 15 containing copper shrinks more than the shell portion 5 containing silicon due to the difference in thermal expansion coefficient between them. Therefore, it is considered that the core portion 15 is divided and the current non-continuous core portion 3 is formed. The core part including copper may be made of only copper, or may be a copper-silicon solid solution due to, for example, mutual diffusion at the interface between the shell part and the core part when forming silicon nanowires, or may be partially copper In some cases, a copper silicide compound (δ phase, ε phase, η phase), which is a compound of silicon and silicon, may be formed, and these may be mixed in the copper core. The composition of the core part containing copper is also affected by the formation conditions of the silicon nanowires as well as the silicon nanowire dimension and shape. By adjusting these, the composition of the core part containing copper is controlled. be able to. For the above reasons, the “core portion containing copper” in the present invention refers to a case where the core portion is made of only copper, a copper-silicon solid solution, or a copper silicide compound (δ phase, ε phase, η phase). Shall be included.

(Silicon composite copper substrate)
When the silicon nanowire 1 is used for a specific application, it can be used as a silicon composite copper substrate 7a in which the silicon nanowire 1 is connected to at least one surface of the copper substrate 9 as shown in FIG. The silicon composite copper substrate 7a is obtained in a state where the silicon nanowires 1 are grown from the copper substrate 9 and the silicon nanowires 1 are not peeled off. In the silicon composite copper substrate 7a, the end portion of the silicon nanowire 1 is bonded to the copper substrate 9 by a metal bond or a covalent bond.

  The thickness and size of the copper substrate 9 can be appropriately changed according to the application. When the silicon composite copper substrate 7a or the silicon composite copper substrate 7b described later is used for the negative electrode of the lithium ion secondary battery, the copper substrate 9 is preferably a copper foil.

  Alternatively, the silicon nanowire 1 can be used as a silicon composite copper substrate 7b having a coating film 11 including the silicon nanowire 1 on at least one surface of the copper substrate 9, as shown in FIG. The silicon composite copper substrate 7 b is obtained by applying a slurry containing the silicon nanowire 1 to the copper substrate 9. In FIG. 3, the silicon nanowire 1 is illustrated in a simplified manner.

  When using the silicon composite copper substrate 7b for the negative electrode of a lithium ion secondary battery, a slurry obtained by kneading the silicon nanowire 1, the conductive auxiliary agent, the binder, the thickener, the solvent, etc. on the copper foil is applied. Then, a silicon composite copper substrate 7b is formed.

  For dispersion of the silicon nanowires 1 into the slurry, a general kneader can be used, and an apparatus capable of preparing a slurry called a kneader, a stirrer, a disperser, a mixer or the like can be used.

  The slurry can be applied by using a general coating apparatus that can apply the slurry to the copper substrate 9, for example, a roll coater, a coater using a doctor blade, a comma coater, or a die coater.

  In the solid content in the slurry, the silicon nanowire 1 contains 25 to 90% by weight, the conductive additive contains 5 to 70% by weight, the binder contains 1 to 30% by weight, and the thickener contains 0 to 25% by weight.

  When preparing an aqueous slurry, latex (a dispersion of rubber fine particles) such as styrene / butadiene / rubber (SBR) can be used as a binder, and polysaccharides such as carboxymethylcellulose and methylcellulose as thickeners. Etc. are suitably used as one or a mixture of two or more. In preparing an organic slurry, polyvinylidene fluoride (PVdF) or the like can be used as a binder, and N-methyl-2-pyrrolidone can be used as a solvent.

  The conductive assistant is a powder made of at least one conductive material selected from the group consisting of carbon, copper, tin, zinc, nickel, and silver. A single powder of carbon, copper, tin, zinc, nickel, or silver may be used, or a powder of each alloy may be used. For example, general carbon black having an average particle diameter of 1 nm to 1 μm, such as furnace black or acetylene black, can be used.

  The average particle size of the conductive assistant refers to the average particle size of the primary particles. Even when the structure shape is highly developed such as acetylene black (AB), the average particle diameter can be defined by the primary particle diameter here, and the average particle diameter can be obtained by image analysis of the SEM photograph.

  The binder is a resin binder, and a fluororesin such as polyvinylidene fluoride (PVdF) and styrene butadiene rubber (SBR) or a rubber system, and an organic material such as polyimide (PI) or acrylic is used. Can do.

(Manufacturing method of silicon nanowire and silicon composite copper substrate)
The manufacturing method of a silicon nanowire and a silicon composite copper substrate is demonstrated using FIG.

  First, as shown in FIG. 4A, a copper substrate 9 is prepared. The surface of the copper substrate 9 is preferably degreased and cleaned.

  Thereafter, the organosilane solution and the copper substrate 9 are placed in a pressure vessel. At this time, the copper substrate 9 is not immersed in the organic silane solution, and the vapor evaporated from the organic silane and the organic dispersion medium solution is brought into contact. Next, the copper substrate 9 is heated to 300 ° C. or higher and 600 ° C. or lower in a pressure vessel in an organosilane atmosphere. At this time, as the organic silane, the following phenylsilane, tetramethylsilane, silole and the like can be used.

  In addition, the organosilane is introduced into the pressure vessel as a solution with an organic dispersion medium. The organic dispersion medium is preferably one selected from toluene, benzene, hexane, xylene, or a mixture thereof.

  In addition, when using phenylsilane as an organic silane, it is preferable that heating temperature is between 380 degreeC and 500 degreeC.

Organosilane is thermally decomposed into silane (SiH ₄ ) and by-products by a solvothermal reaction in a high temperature and high pressure environment. Further, this silane is deposited as silicon. In this reaction, since the rate of thermal decomposition from organosilane to silane is slower than the rate of decomposition from silane to silicon, silicon deposition is slow in this reaction.

  FIG. 4B shows the state of the copper substrate 9 in the initial reaction. In the initial stage of the reaction, since the concentration of silicon is small, silicon is deposited little and copper silicide is deposited. The deposited copper silicide forms nanodots 13.

  FIG. 4C shows the state of the copper substrate 9 in the late reaction stage. In the second half of the reaction, since the concentration of silicon is high, silicon is deposited on the copper substrate 9 and the shell portion 5 containing silicon grows from the nanodots 13. At the same time, copper derived from the copper substrate 9 is deposited from the nanodots 13 as a copper silicide or a core part 15 containing copper, and a core-shell type nanowire in which the shell part 5 covers the periphery of the core part 15 is obtained. The core portion 15 is continuously connected from the copper substrate 9, and it is considered that there is no cavity inside the shell portion 5.

  Thereafter, when cooled, the copper of the core part 15 is more easily contracted than the silicon of the shell part 5, so that the continuous core part 15 is divided into discontinuous core parts 3, thereby obtaining silicon nanowires 1. It is done. Moreover, the silicon composite copper substrate 7a on which the silicon nanowires 1 are grown from the copper substrate 9 is obtained.

  Furthermore, as shown in FIG.4 (d), when the silicon nanowire 1 is peeled from the silicon composite copper substrate 7a, the silicon nanowire 1 is obtained. The nanowire 1 is peeled off from the copper substrate 9 by a method of peeling with a metal blade such as a knife, a method of applying ultrasonic vibration after being immersed in a dispersion medium, and etching the copper substrate 9 with a chemical. The method of peeling is mentioned.

  Moreover, when the peeled silicon nanowire 1 is applied as a slurry, a coating film 11 including the silicon nanowire 1 is obtained, and a silicon composite copper substrate 7b is obtained.

  In addition, the silicon composite copper substrate 7a which has the silicon nanowire 1 only on one surface can also be produced by masking one surface of the copper substrate 9 with a resin or the like.

  In the embodiment of the present invention, the copper substrate 9 is used. However, from the property of forming an intermetallic compound with silicon, even when using a silver or platinum substrate other than copper, a core containing silver or a core containing platinum It is considered that a hollow silicon nanowire can be obtained.

(Lithium ion secondary battery)
A battery element is formed by disposing a separator between the positive electrode and the negative electrode. After winding or stacking such battery elements into a cylindrical battery case or a rectangular battery case, an electrolyte or an electrolytic solution is injected to obtain a lithium ion secondary battery.

(Positive electrode for lithium ion secondary battery)
First, a positive electrode active material, a conductive additive, a binder, and a solvent are mixed to prepare a positive electrode active material composition. The composition of the positive electrode active material is directly applied on a metal current collector such as an aluminum foil and dried to obtain a positive electrode.

Any positive electrode active material can be used as long as it is generally used. For example, LiCoO ₂ , LiMn ₂ O ₄ , LiMnO ₂ , LiNiO ₂ , LiCo _1/3 Ni _1/3 Mn _1/3. Compounds such as O ₂ and LiFePO ₄ .

  For example, carbon black is used as the conductive assistant, polyvinylidene fluoride (PVdF), a water-soluble acrylic binder is used as the binder, and N-methyl-2-pyrrolidone (NMP) is used as the solvent. Use water, etc. At this time, the contents of the positive electrode active material, the conductive additive, the binder, and the solvent are at levels that are normally used in lithium ion secondary batteries.

(Separator)
Any separator can be used as long as it has a function of insulating electronic conduction between the positive electrode and the negative electrode and is usually used in a lithium ion secondary battery. For example, a microporous polyolefin film can be used.

(Electrolytic solution / electrolyte)
As the electrolyte and electrolyte in the lithium ion secondary battery, an organic electrolyte (non-aqueous electrolyte), an inorganic solid electrolyte, a polymer solid electrolyte, and the like can be used.
Specific examples of the organic electrolyte solvent include carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, diethyl carbonate, dimethyl carbonate, and methyl ethyl carbonate; diethyl ether, dibutyl ether, ethylene glycol dimethyl ether, ethylene glycol diethyl ether, ethylene glycol di Ethers such as butyl ether and diethylene glycol dimethyl ether; aprotic such as benzonitrile, acetonitrile, tetrahydrofuran, 2-methyltetrahydrofuran, γ-butyrolactone, dioxolane, 4-methyldioxolane, N, N-dimethylformamide, dimethylacetamide, dimethylchlorobenzene, nitrobenzene Solvent, or two or more of these solvents Mixed solvent of thereof.

The electrolyte of the organic electrolyte includes LiPF ₆ , LiClO ₄ , LiBF ₄ , LiAlO ₄ , LiAlCl ₄ , LiSbF ₆ , LiSCN, LiCl, LiCF ₃ SO ₃ , LiCF ₃ CO ₃ , LiC ₄ F ₉ SO ₃ , LiN (CF ₃ SO ₂ ) A mixture of one or more electrolytes made of a lithium salt such as ₂ can be used.

  As an additive for the organic electrolyte, it is desirable to add a compound capable of forming an effective solid electrolyte interface film on the surface of the negative electrode active material. For example, a substance having an unsaturated bond in the molecule and capable of reductive polymerization during charging, such as vinylene carbonate (VC), is added.

  Moreover, it can replace with said organic electrolyte solution and can use a solid-state lithium ion conductor. For example, a solid polymer electrolyte in which the lithium salt is mixed with a polymer made of polyethylene oxide, polypropylene oxide, polyethyleneimine, or the like, or a polymer gel electrolyte in which a polymer material is impregnated with an electrolytic solution and processed into a gel shape can be used.

Further, lithium nitride, lithium halide, lithium oxyacid _{salt, Li 4 SiO 4, Li 4} SiO 4 -LiI-LiOH, Li 3 PO 4 -Li 4 SiO 4, Li 2 SiS 3, Li 3 PO 4 -Li Inorganic materials such as ₂ S—SiS ₂ and phosphorus sulfide compounds may be used as the inorganic solid electrolyte.

(Effect of the embodiment of the present invention)
In the embodiment of the present invention, a hollow silicon nanowire having a core part including copper and a shell part including silicon, which is an unconventional structure, can be formed.

  The silicon nanowire according to the embodiment of the present invention can be used as a negative electrode active material of a lithium ion secondary battery, and the silicon composite copper substrate according to the present embodiment can be used as a negative electrode of a lithium ion secondary battery. It is.

  Since the lithium ion secondary battery using the silicon composite copper substrate according to the embodiment of the present invention as the negative electrode has a copper core part inside the silicon shell part, the transfer efficiency of silicon electrons during charge and discharge is high. The charge / discharge rate can be improved. In this embodiment, the copper core portion is discontinuous. However, even if the copper core portion is discontinuous, the copper core portion has a copper core portion, so that electrons can be exchanged compared to silicon nanowires that do not have a copper core portion. High efficiency.

  In addition, when the silicon composite copper substrate according to the embodiment of the present invention is used as a negative electrode of a lithium ion secondary battery, silicon serves as a negative electrode active material, and therefore, compared with a conventional lithium ion secondary battery using graphite. The capacity is large.

  Further, according to the embodiment of the present invention, since the shell portion has a hollow portion, the internal distortion when the lithium ions enter and exit the shell portion and the metal expands and contracts is reduced, and the shell portion is pulverized and cracked. Can be prevented. That is, a lithium ion secondary battery using nanowires as a negative electrode material in this embodiment has a better cycle life than a conventional lithium ion secondary battery.

  Moreover, the silicon nanowire which concerns on embodiment of this invention can be used also as a light-emitting material and a photoreceptor of a solar cell.

Hereinafter, the present invention will be specifically described using examples and comparative examples.
[Example 1]
(Preparation of silicon composite copper substrate 7a)
A copper substrate 9 having a width of 1 cm, a length of 4 cm, and a thickness of 250 μm was immersed in an alkaline solution and further a sulfuric acid solution to perform degreasing and cleaning.
Thereafter, as shown in FIG. 5A, a raw material solution 21 in which phenylsilane and toluene are mixed at a ratio of 1:19 is put into a crucible 23 made of quartz glass, and the copper substrate 9 is fixed to the upper part of the crucible 23. .
Furthermore, the crucible 23 to which the copper substrate 9 was fixed was placed in the container inner cylinder 25 and further installed in an autoclave (pressure vessel) 27.
Then, it heated up to 450 degreeC over 1 hour. The pressure in the autoclave when the temperature was raised to 450 ° C. was about 5 MPa.
After maintaining the temperature for 2 hours, it was cooled for 4 to 5 hours to obtain a silicon composite copper substrate 7a.
It was confirmed that a large amount of yellowish white fibrous material was produced on the crucible 23 side of the copper substrate 9 with which the vaporized raw material 29 was in contact.

  When the cross section of the obtained copper plate was observed with a scanning electron microscope, innumerable nanowires were observed on the surface of the copper plate as shown in FIGS. 6 (a) and 6 (b).

  Thereafter, the nanowire on the surface of the copper substrate was scraped with a knife, observed with a transmission electron microscope, and subjected to EDS (energy dispersive X-ray spectroscopy) mapping. As shown in FIG. And the silicon nanowire which has a core part containing copper in the hollow part was observed. In the central photograph of FIG. 7, the black part is the core part, and the gray part is the shell part.

[Examples 2 to 4, Comparative Examples 1 and 2]
A silicon composite copper substrate was produced in the same manner as in Example 1 except that the temperature during autoclave heating was changed.
The production results are shown in Table 1 below.

  The production amount of nanowires decreased with increasing temperature to a low temperature or a high temperature with a peak at 450 ° C., and no nanowire was produced at 360 ° C. or 550 ° C.

  The preferred embodiments of the present invention have been described above with reference to the accompanying drawings, but the present invention is not limited to such examples. It will be apparent to those skilled in the art that various changes and modifications can be made within the scope of the technical idea disclosed in the present application, and these are naturally within the technical scope of the present invention. Understood.

DESCRIPTION OF SYMBOLS 1 ......... Silicon nanowire 3 ......... Core part 5 ......... Shell part 7a, 7b ......... Silicon composite copper substrate 9 ......... Copper substrate 11 ......... Coating film 13 ......... Nanodot 15 ......... Core part 21 ... …… Raw material solution 23 ………… Crucible 25 ……… Inner cylinder 27 ……… Autoclave 29 ……… Vaporized raw material

Claims (9)

A shell portion containing silicon;
A core part containing copper, which is discontinuously present in the hollow part formed by the shell part;
A hollow copper core silicon nanowire characterized by comprising: At least one side of the copper substrate has the hollow copper core silicon nanowire according to claim 1,
The silicon composite copper substrate, wherein the hollow copper core silicon nanowire is connected to the copper substrate.   A silicon composite copper substrate comprising a coating film containing the hollow copper core silicon nanowire according to claim 1 on at least one surface of the copper substrate.   The hollow copper core silicon nanowire according to claim 1, wherein the diameter is 10 to 500 nm.   A silicon composite characterized in that a copper substrate is heated to 300 to 600 ° C. in an atmosphere containing an organic silane and an organic dispersion medium in a pressure vessel to form a hollow copper core silicon nanowire on the copper substrate. A method for producing a copper substrate.   6. The method for producing a silicon composite copper substrate according to claim 5, wherein the organic silane is phenyl silane, and the copper substrate is heated to 380 to 500 [deg.] C. Heating the copper substrate to 300-600 ° C. in an atmosphere containing organosilane and an organic dispersion medium in a pressure vessel, and forming a hollow copper core silicon nanowire on the copper substrate;
Separating the hollow copper core silicon nanowire from the copper substrate;
A process for producing hollow copper core silicon nanowires, comprising: Heating the copper substrate to 300-600 ° C. in an atmosphere containing organosilane and an organic dispersion medium in a pressure vessel, and forming a hollow copper core silicon nanowire on the copper substrate;
Separating the hollow copper core silicon nanowire from the copper substrate;
Dispersing the hollow copper core silicon nanowires in a slurry;
Applying the slurry onto another copper substrate. A method for producing a silicon composite copper substrate.   A lithium ion secondary battery comprising the silicon composite copper substrate according to claim 2 as a negative electrode, and further comprising a positive electrode and a nonaqueous electrolyte.