WO2015068195A1

WO2015068195A1 - Negative electrode active material for lithium ion secondary cell, method for manufacturing negative electrode active material for lithium ion secondary cell, and lithium ion secondary cell

Info

Publication number: WO2015068195A1
Application number: PCT/JP2013/079810
Authority: WO
Inventors: 岡井　誠; 京谷　隆; 康人干川; 孝文石井
Original assignee: 株式会社日立製作所
Priority date: 2013-11-05
Filing date: 2013-11-05
Publication date: 2015-05-14

Abstract

Provided is a negative electrode active material for a lithium ion secondary cell capable of preventing electrical isolation of silicon particles. A negative electrode active material for a lithium ion secondary cell having a silicon nanowire and a carbon coating layer formed on the surface of the silicon nanowire, wherein the carbon coating layer has a nanographene structure, the electrical conductivity of the carbon coating layer is, e.g., 1000 S/m or higher, the carbon coating layer is oriented along, e.g., the surface of the silicon nanowire, the thickness of the carbon coating layer is, e.g., 0.2-100 nm, and the diameter of the silicon nanowire is, e.g., 2-100 nm.

Description

Negative electrode active material for lithium ion secondary battery, method for producing negative electrode active material for lithium ion secondary battery, and lithium ion secondary battery

The present invention relates to a negative electrode active material for a lithium ion secondary battery, a method for producing a negative electrode active material for a lithium ion secondary battery, and a lithium ion secondary battery.

Graphite-based carbon materials are widely used as negative electrode active materials for lithium ion secondary batteries. The stoichiometric composition when graphite is filled with lithium ions is LiC ₆ , and its theoretical capacity can be calculated as 372 mAh / g.

On the other hand, the stoichiometric composition when silicon is filled with lithium ions is Li ₁₅ Si ₄ , and its theoretical capacity can be calculated as 3571 mAh / g. Thus, silicon is an attractive material that can store 9.6 times as much lithium as graphite. However, when the silicon particles are filled with lithium ions, the volume expands to about 3.1 times, so that the silicon particles are dynamically destroyed during repeated filling and releasing of lithium ions. When the silicon particles are broken, the broken fine silicon particles are electrically isolated, and a new electrochemical coating layer is formed on the broken surface, whereby the irreversible capacity is increased and the charge / discharge cycle characteristics are remarkably lowered.

As a conventional technique using silicon as a negative electrode active material of a lithium ion secondary battery, Patent Document 1 describes an example in which carbon is coated on the surface of a silicon nanowire. Patent Document 2 relates to a negative electrode active material and a lithium battery that employs the negative electrode active material. The negative electrode active material is formed of a silicon-based nanowire on a crystalline carbon-based core having a silicon-based nanowire disposed on the surface. An example is described that includes primary particles that are coated with an amorphous carbon-based coating layer such that at least a portion is not exposed.

Special table 2012-527735 gazette JP 2013-84601 A

Patent Document 1 describes that PVDF (poly (vinylidene fluoride)) is decomposed and coated with carbon, and the coated carbon is considered to have an amorphous structure. Also in Patent Document 2, the carbon covering the silicon-based nanowire has an amorphous structure. In this case, the carbon coating film has no electrical conductivity, and it is difficult to prevent electrical isolation of silicon particles.

An object of the present invention is to provide a negative electrode active material for a lithium ion secondary battery that can prevent electrical isolation of silicon particles.

The features of the present invention for solving the above problems are as follows, for example.

A negative electrode active material for a lithium ion secondary battery having a silicon nanowire and a carbon coating layer formed on the surface of the silicon nanowire, wherein the carbon coating layer has a nanographene structure .

According to the present invention, a negative electrode active material for a lithium ion secondary battery that can prevent electrical isolation of silicon particles can be provided. Problems, configurations, and effects other than those described above will be clarified by the following description of embodiments.

It is the figure which expressed typically the structure of the carbon covering silicon nanowire concerning one embodiment of the present invention. 2 is a scanning electron micrograph of silicon nanowires grown on a graphite surface. 2 is a transmission electron micrograph of silicon nanowires grown on a graphite surface. It is the figure which expressed typically the structure of the negative electrode active material for lithium ion secondary batteries which concerns on one Embodiment of this invention. 2 is a scanning electron micrograph of silicon nanoparticles grown on a graphite surface. It is a scanning electron micrograph of a carbon covering silicon nanowire. It is a scanning electron microscope enlarged photograph of a carbon covering silicon nanowire. 2 is a transmission electron micrograph of carbon-coated silicon nanowires. 2 is a transmission electron micrograph of carbon-coated silicon nanowires. It is the schematic of the thermal vapor phase growth apparatus for forming the carbon covering silicon nanowire on the surface of a carbon substrate. It is the schematic of the thermal vapor phase growth apparatus for forming the carbon covering silicon nanowire on the surface of a carbon substrate. It is an internal structure of the lithium ion secondary battery which concerns on one Embodiment of this invention. It is the figure which expressed typically the structure of the negative electrode active material for lithium ion secondary batteries which concerns on one Embodiment of this invention. It is the figure which expressed typically the structure of the negative electrode active material for lithium ion secondary batteries which concerns on one Embodiment of this invention.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. The following description shows specific examples of the contents of the present invention, and the present invention is not limited to these descriptions. Various modifications by those skilled in the art are within the scope of the technical idea disclosed in this specification. Changes and modifications are possible. In all the drawings for explaining the present invention, components having the same function are denoted by the same reference numerals, and repeated description thereof may be omitted.

A first embodiment of the present invention will be described with reference to FIG. FIG. 1 is a diagram schematically showing the structure of a carbon-coated silicon nanowire according to an embodiment of the present invention.

The carbon-coated silicon nanowire 202 has a silicon nanowire 101 and a carbon coating layer 102. The surface of the silicon nanowire 101 is partially or entirely covered with the carbon coating layer 102. In other words, the carbon coating layer 102 is formed on the surface of the silicon nanowire 101.

The carbon coating layer 102 has a structure in which nanographene is laminated in multiple layers, and has an electric conductivity of 1000 S / m or more, preferably 10,000 S / m or more. Electrical conductivity can be added to the silicon nanowire 101 by covering the silicon nanowire 101 partially or entirely with the carbon coating layer 102 having a nanographene structure. Thereby, even when the silicon nanowire 101 is broken in the middle or when the silicon nanowire 101 is peeled off from the carbon coating layer 102, electrical conduction with the current collector can be ensured, and electrical isolation can be prevented. The irreversible capacity of the lithium ion secondary battery can be reduced.

The film thickness L _c of the carbon coating layer 102 is preferably 0.2 nm to 100 nm, particularly 1 nm to 30 nm, and more preferably 3 nm to 20 nm. When the film thickness L _c of the carbon coating layer 102 is less than 0.2 nm, the coating strength is insufficient and there is a possibility of partial peeling. In that case, sufficient electrical conductivity can be ensured. Can not. Further, when the film thickness L _c of the carbon coating layer 102 exceeds 100 nm, it is difficult to set the weight of silicon with respect to the total weight to a desired value, for example, 20 wt%, and as a result, sufficient electric capacity can be obtained. Is difficult.

The diameter D _si of the silicon nanowire 101, 2 nm or more 100nm or less, particularly 5nm or 80nm or less, more desirably at 10nm or more 50nm or less. By making D _si sufficiently thin as 100 nm or less, mechanical destruction of the silicon nanowire 101 can be prevented, and the irreversible capacity can be reduced. If the diameter D _si of the silicon nanowire 101 is less than 2 nm, there is a possibility that the whole is naturally oxidized in the atmosphere is SiO _2, in this case, may not function as an anode active material. Further, if the diameter D _si of the silicon nanowire 101 exceeds 100 nm, the volume expansion when filled with lithium ions, mechanically disrupted, there is a possibility that the electric capacity of the lithium ion secondary battery decreases. The length of the silicon nanowire 101 is not limited, but a length of several microns to several tens of microns is considered optimal for the electrode manufacturing process.

FIG. 2 is a scanning electron micrograph of silicon nanowires grown on the graphite surface. It is a pure silicon nanowire 101 in which the carbon coating layer 102 is not formed. Thus, silicon nanowires can be produced very densely on the graphite surface.

FIG. 3 is a transmission electron micrograph of the same sample as FIG. The diameter of the silicon nanowire 101 is 30 nm. Since silicon lattice stripes are seen inside the silicon nanowire 101, the silicon nanowire 101 has crystallinity. In particular, the silicon nanowire 101 is considered to have a polycrystalline structure. When the silicon nanowire 101 has crystallinity, the diffusion rate of lithium ions is increased. As a result, the charging and discharging speed of lithium ions is fast, and the high-speed charge / discharge characteristics are improved. Further, a natural oxide film layer 103 of slightly less than 3 nm exists on the surface of the silicon nanowire.

FIG. 4 is a diagram schematically showing the structure of a negative electrode active material for a lithium ion secondary battery according to an embodiment of the present invention. The negative electrode active material 1 for a lithium ion secondary battery has a carbon substrate 201 and carbon-coated silicon nanowires 202. In this example, carbon-coated silicon nanowires 202 were formed on the surface of the carbon substrate 201. The carbon-coated silicon nanowire 202 is directly bonded to the surface of the carbon substrate 201 and grows from the surface of the carbon substrate 201.

As the carbon substrate 201, graphite, thermally expanded graphite oxide, graphite oxide, carbon nanotubes, carbon nanohorns, ketjen black, acetylene black, various nanocarbon structures produced by a template method, and the like can be used. By using a graphene structure such as ketjen black or acetylene black having an edge as the carbon substrate 201, more silicon nanoparticles can be attached to the carbon substrate 201.

In order to clarify the growth mechanism of the carbon-coated silicon nanowires 202 on the surface of the carbon substrate 201, the growth was interrupted at the initial stage of the growth of the carbon-coated silicon nanowires 202, the sample was taken out, and observed with a scanning electron microscope. FIG. 5 is a scanning electron micrograph of silicon nanoparticles grown on the graphite surface. As can be seen from FIG. 5, the nanoparticles are localized at the edge portion of the graphite. Further, as can be seen from FIG. 5, innumerable silicon nanoparticles having a diameter of several tens of nanometers exist on the surface of the carbon substrate 201. From this fact, it can be inferred that silicon nanoparticles first grow on the surface of the carbon substrate 201 and the silicon nanoparticles grow into carbon-coated silicon nanowires 202.

FIG. 6 is a scanning electron micrograph of carbon-coated silicon nanowires. As described above, the carbon-coated silicon nanowires 202 grow very densely on the surface of the carbon base material 201.

7 is an enlarged photograph of the same sample as FIG. It is considered that the surface of the carbon-coated silicon nanowire 202 is uniformly covered with the carbon coating layer 102.

FIG. 8 is a transmission electron micrograph of carbon-coated silicon nanowires. In the vicinity of the center of the carbon-coated silicon nanowire 202, the silicon nanowire 101 having a lattice pattern can be observed. Further, on the surface of the silicon nanowire 101, the carbon coating layer 102 having a nanographene multilayer structure oriented along the axial direction of the silicon nanowire 101 can be observed. Since the carbon coating layer 102 is oriented along the axial direction of the silicon nanowire 101, the carbon coating layer 102 becomes a strong film that is difficult to peel off. In FIG. 8, the diameter of the silicon nanowire 101 was 18 nm, and the film thickness of the carbon coating layer 102 was 6.44-8.17 nm.

FIG. 9 shows a transmission electron micrograph of a carbon-coated silicon nanowire of a sample different from FIG. The diameter of the silicon nanowire 101 was 18 nm, and the film thickness of the carbon coating layer 102 was 10 nm.

FIG. 10 is a schematic diagram of a thermal vapor deposition apparatus for forming carbon-coated silicon nanowires on the surface of a carbon substrate. Liquid silicon tetrachloride was used as the silicon raw material and was introduced into the reactor by bubbling with hydrogen gas. The vapor pressure of silicon tetrachloride at 20 ° C. is 30 kPa, and when bubbling is introduced, the amount of silicon tetrachloride introduced is 34%. Therefore, when introducing a smaller amount of silicon tetrachloride, it is necessary to cool the silicon tetrachloride or provide another line of hydrogen gas. In FIG. 10, a hydrogen line that is not bubbled is provided separately, joined with the bubbling line, and introduced into the reactor. The growth procedure of the carbon-coated silicon nanowire 202 is as follows.

炭素 Put the carbon base material in the sample boat and install it near the center of the reactor. The reactor is made of quartz and has a diameter of 5 cm and a length of 40 cm. In the upper hydrogen line of FIG. 10, hydrogen is allowed to flow at a flow rate of 200 mL / min, and the lower bubbling hydrogen line is closed, and the growth furnace is heated from room temperature to 1000 ° C. at a rate of 10 ° C./min. did.

Next, when the temperature reached 1000 ° C., the flow rate of the upper hydrogen line was changed to 100 mL / min, and the flow rate of the hydrogen line of the lower bubbling hydrogen line was set to 100 mL / min. Under this condition, 17% silicon tetrachloride can be introduced. After growing at 1000 ° C. for 1 hour, the lower bubbling hydrogen line was closed, the flow rate of the upper hydrogen line was changed to 200 mL / min, and the temperature was maintained at 1000 ° C. for 30 minutes. Thereby, it is possible to produce the silicon nanowire 101 having a diameter of 20 nm on the surface of the carbon substrate 201.

Thereafter, both hydrogen lines were closed, and argon gas (the argon line is not shown in FIG. 10) was allowed to flow at a flow rate of 200 mL / min, the temperature was lowered at a rate of 10 ° C./min, and the temperature was lowered to 800 ° C. When the temperature reached 800 ° C., propylene gas (the propylene line is not shown in FIG. 10) was introduced at a flow rate of 10 mL / min, and at the same time, the flow rate of argon gas was set to 190 mL / min, and the carbon coating layer 102 was kept for 1 hour. grown.

Thereafter, the propylene gas line was closed, and argon gas was allowed to flow at a flow rate of 200 mL / min, kept for 30 minutes, and then naturally cooled. Thereby, the carbon coating layer 102 (film thickness 10 nm) having a nanographene multilayer structure can be formed on the surface of the silicon nanowire 101. In this way, the silicon nanowire 101 and the subsequent carbon coating layer 102 are continuously formed, thereby preventing the formation of a natural oxide film and eliminating the process of reducing and removing the natural oxide film.

In FIG. 10, in order to prevent the surface oxidation of the silicon nanowire 101, the silicon nanowire 101 and the subsequent carbon coating layer 102 were continuously formed. After the silicon nanowire 101 is grown, it can be taken out once in the air and then heat-treated in a reducing atmosphere to remove the natural oxide film on the surface of the silicon nanowire 101, and then the carbon coating layer 102 can be produced. Productivity is improved by performing the growth of the silicon nanowire 101 and the production of the carbon coating layer 102 in separate reaction furnaces. Further, the diameter and growth weight of the silicon nanowire 101 can be changed by changing the temperature at the time of growing the silicon nanowire 101, the amount of silicon tetrachloride introduced, and the growth time. Further, the film thickness of the carbon coating layer 102 can be controlled by changing the growth time of the carbon coating layer 102. In addition to the propylene gas, various hydrocarbon gases such as acetylene gas, propane gas, and methane gas can be used for producing the carbon coating layer 102.

FIG. 11 is a result of calculating the dependence of the negative electrode electric capacity on the weight ratio Si / (Si + C) of silicon with respect to the total weight with respect to the negative electrode active material for a lithium ion secondary battery. For carbon, the stoichiometric composition when filled with lithium ions was assumed to be LiC ₆ and its electric capacity was 372 mAh / g. For silicon, the stoichiometric composition when lithium ions are filled is assumed to be Li ₁₅ Si _4, and the electric capacity is assumed to be 3577 mAh / g, and Li ₂₂ Si ₅ is assumed, It calculated about the case where the electric capacity was 4197 mAh / g.

From the balance with the positive electrode electric capacity, if the negative electrode electric capacity is 1000 mAh / g or more, it is considered that the performance is sufficient for the time being. From the calculation result of FIG. 11, in order to realize the negative electrode electric capacity of 1000 mAh / g or more, the weight ratio of the negative electrode active material for lithium ion secondary battery is 20% or more, particularly 40% or more, and further 80 It is desirable to contain more than% silicon.

FIG. 12 shows the internal structure of a lithium ion secondary battery according to an embodiment of the present invention. 12, 1301 is a positive electrode, 1302 is a separator, 1303 is a negative electrode, 1304 is a battery can, 1305 is a positive electrode current collecting tab, 1306 is a negative electrode current collecting tab, 1307 is an inner lid, 1308 is an internal pressure release valve, 1309 is a gasket, 1310 is a positive temperature coefficient (PTC) resistive element, and 1311 is a battery lid. The battery lid 1311 is an integrated part including an inner lid 1307, an internal pressure release valve 1308, a gasket 1309, and a positive temperature coefficient resistance element 1310.

For example, the positive electrode 1301 can be manufactured by the following procedure. LiMn ₂ O ₄ is used as the positive electrode active material. 7.0 wt% and 2.0 wt% of graphite powder and acetylene black are added as conductive materials to 85.0 wt% of the positive electrode active material, respectively. Further, a solution dissolved in 6.0 wt% polyvinylidene fluoride (hereinafter abbreviated as PVDF) and 1-methyl-2-pyrrolidone (hereinafter abbreviated as NMP) is added as a binder, and the mixture is mixed with a planetary mixer. Further, air bubbles in the slurry are removed under vacuum to prepare a homogeneous positive electrode mixture slurry. This slurry is uniformly and evenly applied to both surfaces of an aluminum foil having a thickness of 20 μm using an applicator. After the application, compression molding is performed by a roll press so that the electrode density is 2.55 g / cm ³ . This is cut with a cutting machine to produce a positive electrode 1301 having a thickness of 100 μm, a length of 900 mm, and a width of 54 mm.

For example, the negative electrode 1303 can be manufactured by the following procedure. As the negative electrode active material, the negative electrode active material for the lithium ion secondary battery in the present invention such as the negative electrode active material for the lithium ion secondary battery of FIG. 4 can be used. A solution obtained by dissolving 5.0 wt% of PVDF as a binder in NMP is added to 95.0 wt% of the negative electrode active material. It is mixed with a planetary mixer, and bubbles in the slurry are removed under vacuum to prepare a homogeneous negative electrode mixture slurry. This slurry is uniformly and evenly applied to both surfaces of a rolled copper foil having a thickness of 10 μm with an applicator. After application, the electrode is compression-molded by a roll press to make the electrode density 1.3 g / cm ³ . This is cut with a cutting machine to produce a negative electrode 1303 having a thickness of 110 μm, a length of 950 mm, and a width of 56 mm.

The positive electrode current collecting tab 1305 and the negative electrode current collecting tab 1306 are ultrasonically welded to the positive electrode 1301 and the uncoated part (current collector exposed surface) of the negative electrode 1303, respectively, which can be produced as described above. The positive electrode current collecting tab 1305 may be an aluminum lead piece, and the negative electrode current collecting tab 1306 may be a nickel lead piece.

Thereafter, a separator 1302 made of a porous polyethylene film having a thickness of 30 μm is inserted into the positive electrode 1301 and the negative electrode 1303, and the positive electrode 1301, the separator 1302, and the negative electrode 1303 are wound. The wound body is accommodated in the battery can 1304, and the negative electrode current collecting tab 1306 is connected to the bottom of the battery can 1304 by a resistance welding machine. The positive electrode current collecting tab 1305 is connected to the bottom surface of the inner lid 1307 by ultrasonic welding.

Before attaching the upper battery lid 1311 to the battery can 1304, a non-aqueous electrolyte can be injected. The solvent of the electrolytic solution includes, for example, ethylene carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate (DEC), and has a volume ratio of 1: 1: 1. The electrolyte is LiPF ₆ at a concentration of 1 mol / L (about 0.8 mol / kg). Such an electrolytic solution is dropped from above the wound body, and the battery lid 1311 is caulked and sealed in the battery can 1304, whereby the lithium ion secondary battery 1000 can be obtained.

Next, a second embodiment of the present invention will be described with reference to FIG. FIG. 13 is a diagram schematically showing the structure of a negative electrode active material for a lithium ion secondary battery according to an embodiment of the present invention.

This embodiment is different from the first embodiment in that carbon nanotubes 301 are used as a carbon base material and carbon-coated silicon nanowires 302 are formed on the surface thereof. By using the carbon nanotubes 301 for the carbon base material, the silicon has a larger specific surface area than the graphite, so that more silicon nanowires 302 can be grown. it can.

Next, a third embodiment of the present invention will be described with reference to FIG. FIG. 14 is a diagram schematically representing the structure of a negative electrode active material for a lithium ion secondary battery according to an embodiment of the present invention.

In this embodiment, first, carbon-coated silicon nanowires 401 are grown on a carbon substrate, and then the carbon substrate and the carbon-coated silicon nanowires 401 are separated, and only the carbon-coated silicon nanowires 401 are used as a negative electrode active for a lithium ion secondary battery. The point used as a substance is different from Example 1 and Example 2. That is, in this example, the negative electrode active material for a lithium ion secondary battery is composed of carbon-coated silicon nanowires 401. As a result, the weight ratio of silicon to the total weight can be greatly increased, so that the electric capacity can be dramatically increased.

DESCRIPTION OF SYMBOLS 1 Negative electrode active material 101 for lithium ion secondary batteries Silicon nanowire 102 Carbon coating layer 103 Oxide layer 201 Carbon base material 202 302 401 Carbon coating silicon nanowire 301 Carbon nanotube 1000 Lithium ion secondary battery 1301 Positive electrode 1302 Separator 1303 Negative electrode 1304 Battery can 1305 Positive current collecting tab 1306 Negative current collecting tab 1307 Inner lid 1308 Pressure release valve 1309 Gasket,
1310 Positive temperature coefficient resistance element 1311 Battery cover

Claims

Silicon nanowires,
A carbon coating layer formed on the surface of the silicon nanowire, and a negative electrode active material for a lithium ion secondary battery,
The carbon coating layer is a negative electrode active material for a lithium ion secondary battery having a nanographene structure.
In claim 1,
The negative electrode active material for a lithium ion secondary battery, wherein the carbon coating layer has an electric conductivity of 1000 S / m or more.
In claim 1,
A negative electrode active material for a lithium ion secondary battery, wherein the carbon coating layer is oriented along the surface of the silicon nanowire.
In claim 1,
The negative electrode active material for lithium ion secondary batteries whose film thickness of the said carbon coating layer is 0.2 nm or more and 100 nm or less.
In any one of Claims 1 thru | or 4,
The negative electrode active material for lithium ion secondary batteries whose diameter of the said silicon nanowire is 2 nm or more and 100 nm or less.
In any one of Claims 1 thru | or 5,
A negative electrode active material for a lithium ion secondary battery, wherein the silicon nanowire has crystallinity.
In any one of Claims 1 thru | or 6.
The negative electrode active material for lithium ion secondary batteries whose weight ratio with respect to the said negative electrode active material for lithium ion secondary batteries of the said silicon nanowire is 20% or more.
In any one of Claims 1 thru | or 7,
The negative electrode active material for a lithium ion secondary battery includes a carbon substrate,
The silicon nanowire is formed on the surface of the carbon substrate,
The carbon base material is a negative electrode active material for a lithium ion secondary battery, which is ketjen black or acetylene black.
In any one of Claims 1 thru | or 7,
The negative electrode active material for a lithium ion secondary battery is a negative electrode active material for a lithium ion secondary battery comprising the silicon nanowire and the carbon coating layer.
Silicon nanowires,
A method for producing a negative electrode active material for a lithium ion secondary battery having a carbon coating layer,
The carbon coating layer has a nanographene structure,
The manufacturing method of the negative electrode active material for lithium ion secondary batteries which produced the said carbon coating layer on the surface of the said silicon nanowire continuously after producing the said silicon nanowire.
Silicon nanowires,
A method for producing a negative electrode active material for a lithium ion secondary battery having a carbon coating layer,
The carbon coating layer has a nanographene structure,
After producing the silicon nanowire, the surface of the silicon nanowire is reduced to remove the natural oxide film formed on the surface of the silicon nanowire,
The manufacturing method of the negative electrode active material for lithium ion secondary batteries which produced the said carbon coating layer on the surface of the said silicon nanowire.
The lithium ion secondary battery containing the negative electrode active material for lithium ion secondary batteries in any one of Claims 1 thru | or 9.