JP2013171629A - Negative electrode material for nonaqueous electrolyte secondary battery, and lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a negative electrode material for a nonaqueous electrolyte secondary battery, excellent in discharge rate characteristics by increasing conductivity, while a manufacturing process thereof is simplified, and a lithium ion secondary battery.SOLUTION: A negative electrode material for a nonaqueous electrolyte secondary battery according to the present invention incudes a complex including a silicon oxide SiOx (1≤x≤2) and a silicon nanoparticle Si, and the complex has a structure where the silicon nanoparticle is dispersed in the silicon oxide. At least one element included in group XIII elements and group XV elements in the periodic table is doped in the silicon nanoparticle, as a dopant.

Description

  The present invention relates to a negative electrode material for a non-aqueous electrolyte secondary battery and a lithium ion secondary battery.

  In communication terminals such as mobile phones, non-aqueous electrolyte secondary batteries represented by lithium ion secondary batteries are widely used from the viewpoints of downsizing and weight reduction of devices. In recent years, for the purpose of application to stationary storage batteries and electric vehicles, there is a strong demand for higher capacity of such secondary batteries.

  In addition, as a technique for increasing the capacity of this type of secondary battery, a method of using silicon for a negative electrode material for a nonaqueous electrolyte secondary battery (hereinafter, appropriately shown as a negative electrode material) is being studied. Since silicon has a capacity higher than that of carbon materials currently in practical use, silicon is the most promising material for reducing the size and increasing the capacity of secondary batteries.

  However, although silicon has a high capacity, since the volume expansion change due to charging / discharging is as large as four times, there is a problem that when charging / discharging is repeated, the capacity deteriorates. Therefore, it is desired to improve the cycle characteristics in which the capacity does not decrease even when charging and discharging are repeated.

  In recent years, a negative electrode material in which a carbon layer is formed on a composite of silicon and silicon oxide SiOx (for example, 1 ≦ x ≦ 2) has also been proposed (see, for example, Patent Document 1).

  The negative electrode material disclosed in Patent Document 1 has a structure in which silicon nanoparticles are dispersed in silicon oxide SiOx. Thus, since the negative electrode material containing silicon oxide SiOx has a small volume expansion change compared with the negative electrode material of silicon only, the cycle characteristics of the negative electrode material are enhanced.

  Moreover, in the negative electrode material disclosed in Patent Document 1, conductivity is enhanced by forming a carbon layer on the surface of the charcoal composite. The negative electrode material with improved conductivity in this way can enhance the effect of silicon absorbing and releasing lithium ions, and as a result, is considered to have an effect of improving discharge rate characteristics.

JP 2010-267588 A

  However, in the prior art, in the manufacturing process of the negative electrode material, a process of coating the surface of the composite with a carbon material is required, so that the manufacturing process is desired to be simplified from the viewpoint of manufacturing cost.

  In the prior art, the conductivity is enhanced by coating the surface of the composite with the carbon material. Therefore, when the surface of the composite is not covered with the carbon material, the discharge rate is reduced due to the decrease in the conductivity. A countermeasure has been desired because it may cause deterioration of characteristics.

  The present invention has been made in view of the above-described situation, and by improving conductivity while simplifying the manufacturing process, the negative electrode material for a non-aqueous electrolyte secondary battery excellent in discharge rate characteristics and lithium ion secondary An object is to provide a secondary battery.

  The negative electrode material for a non-aqueous electrolyte secondary battery according to the first feature of the present invention includes a composite containing silicon oxide SiOx (1 ≦ x ≦ 2) and silicon nanoparticles Si, Silicon nanoparticles have a structure in which the silicon nanoparticles are dispersed in the silicon oxide, and the silicon nanoparticles include at least one element included in the group 13 element and the group 15 element in the periodic table as a dopant. However, the main point is that it is doped.

  According to such a negative electrode material for a non-aqueous electrolyte secondary battery, the composite includes silicon nanoparticles and silicon oxide, and the silicon nanoparticles include the elements of Group 13 of the periodic table and the periodic table as dopants. At least one element included in the Group 15 element is doped. That is, the silicon nanoparticles are doped with at least one element out of the Group 13 element as the P-type dopant and the Group 15 element as the N-type dopant.

  According to the negative electrode material for a non-aqueous electrolyte secondary battery, since the silicon nanoparticles contained in the composite are doped with a P-type dopant or an N-type dopant, the sheet resistance of the silicon nanoparticles is suppressed, and the conductivity is reduced. Can be increased. In addition, according to such a negative electrode material for a non-aqueous electrolyte secondary battery, it is not necessary to form a coating film for enhancing conductivity in the manufacturing process, and therefore the manufacturing process can be simplified.

  Thus, according to such a negative electrode material for a nonaqueous electrolyte secondary battery, it is possible to provide a negative electrode material for a nonaqueous electrolyte secondary battery excellent in discharge rate characteristics while simplifying the manufacturing process.

  In the first feature of the present invention, the silicon nanoparticles may have an average particle size of 10 nm or more and less than 100 nm.

  1st characteristic of this invention WHEREIN: The ratio of the said silicon nanoparticle with respect to the said composite is good also as 5 to 60 mass%.

  In the first aspect of the present invention, the dopant may be nitrogen N.

  The gist of the lithium ion secondary battery according to the second feature of the present invention is that it includes a negative electrode including the above-described negative electrode material for a non-aqueous electrolyte secondary battery.

  According to the present invention, it is possible to provide a negative electrode material for a non-aqueous electrolyte secondary battery and a lithium ion secondary battery that are excellent in discharge rate characteristics while simplifying the manufacturing process.

FIG. 1 is a conceptual diagram of a composite used for a negative electrode material for a nonaqueous electrolyte secondary battery according to the first embodiment of the present invention. FIG. 2 is a conceptual diagram showing an example of a lithium ion secondary battery according to the first embodiment of the present invention. FIG. 3 is a flowchart showing a method for manufacturing a composite that the negative electrode material for a non-aqueous electrolyte secondary battery according to the first embodiment of the present invention has. FIG. 4 is a diagram illustrating an example of steps S103 and S104 performed in the manufacturing method according to the first embodiment of the present invention.

  Next, embodiments of the present invention will be described with reference to the drawings. In the following description of the drawings, the same or similar parts are denoted by the same or similar reference numerals. However, it should be noted that the drawings are schematic and ratios of dimensions and the like are different from actual ones. Accordingly, specific dimensions and the like should be determined in consideration of the following description. Moreover, the part from which the relationship and ratio of a mutual dimension differ also in between drawings may be contained.

[First Embodiment]
First, a negative electrode material for a nonaqueous electrolyte secondary battery according to an embodiment of the present invention will be described.

  A negative electrode material for a non-aqueous electrolyte secondary battery according to an embodiment of the present invention (hereinafter simply referred to as a negative electrode material) has a composite containing silicon oxide SiOx (1 ≦ x ≦ 2) and silicon nanoparticles Si. . The composite has a structure in which silicon nanoparticles are dispersed in silicon oxide. Further, the silicon nanoparticles are doped with at least one element included in Group 13 element and Periodic group 15 element as a dopant. That is, the silicon nanoparticles are doped with at least one element out of the Group 13 element as the P-type dopant and the Group 15 element as the N-type dopant. FIG. 1 is a conceptual diagram showing an image structure of the complex according to the present embodiment.

  The composite preferably further includes silicon carbide nanoparticles in addition to silicon oxide SiOx (1 ≦ x ≦ 2) and silicon nanoparticles Si. The structure of the composite will be described in detail below.

(Complex)
The composite can be produced by the following production method. Such a manufacturing method includes a step of synthesizing a silicon source and a carbon source to produce a mixture containing the silicon source and the carbon source, a step of heating and baking the mixture containing the silicon source and the carbon source in an inert atmosphere, And a step of extracting the composite by extracting the generated gas from the atmosphere and rapidly cooling and solidifying the generated gas. Details of the manufacturing method will be described later.

  Further, when the composite was analyzed by an X-ray diffraction method, a peak attributed to silicon (Si), a broad peak attributed to silicon oxide (SiOx (1 ≦ x ≦ 2)), and silicon carbide (SiC) ) Was detected. That is, it was confirmed that the composite contains silicon, silicon oxide, and silicon carbide.

  Specifically, the composite according to the present embodiment further includes silicon carbide nanoparticles in addition to silicon oxide SiOx (1 ≦ x ≦ 2) and silicon nanoparticles Si, and the composition of the composite has X as a dopant. The following chemical formula (Formula 1) is shown.

Si _p O _q C _r X _s (Formula 1)
The dopant X is preferably nitrogen N. In this embodiment, the dopant X is nitrogen N. However, the dopant X is not limited to this. The dopant X may be an element of Group 13 of the periodic table such as boron, aluminum, gallium, or indium. Further, the dopant may be an element of Group 15 of the periodic table such as phosphorus, arsenic, antimony, and bismuth. That is, the dopant may be a trivalent or pentavalent element.

  Moreover, in (Equation 1) mentioned above, each fixed_quantity | quantitative_assay of p, q, r, and s can be calculated with the following method, for example. Hereinafter, a case where nitrogen N is applied as the dopant X will be described as an example.

  For silicon Si, using the dehydrated weight ICP emission spectroscopy combined method, the complex was melted with sodium carbonate, dissolved in hydrochloric acid, evaporated to dryness to dehydrate silicic acid, and then hydrochloric acid was used to remove sodium chloride and the like. Dissolve soluble salts, filter, and heat the precipitate to weigh, add hydrofluoric acid to volatilize silicon dioxide, and then heat again to measure the weight. Ask. Collect the filtrate and determine the dissolved silicon by ICP emission spectroscopy. The total silicon content is calculated from both.

  Carbon C can be measured by a carbon / sulfur simultaneous analyzer manufactured by LECO, and nitrogen N and oxygen O can be measured by an oxygen / nitrogen / hydrogen analyzer manufactured by LECO, and these are calculated above. Each of p, q, r, and s in the above (Formula 1) is calculated by converting the silicon content and C, O, and N from mass fraction to molar fraction.

  The content of N is calculated by converting the mass fraction into the molar fraction based on the above contents. In addition, other elements such as a group 13 element such as boron B and phosphorus P and a group 15 element can be calculated in the same manner by performing a microanalysis with the ICP.

  In (Formula 1), it is preferable that p is 0 <p <0.78. This is due to the following reason. That is, when p is 0.78 or more, too much silicon is contained, which causes deterioration of cycle characteristics.

In (Formula 1), q is preferably 0.22 ≦ q <0.53. This is due to the following reason. That is, when q is 0.22 or more, silicon oxide and silicon are in a composite state, which is considered to contribute to improvement of cycle characteristics in order for silicon oxide to prevent particles from collapsing due to silicon expansion and contraction. However, when q is 0.53 or more, SiO ₂ that does not contribute to charging / discharging exists, which may cause a decrease in the electric capacity of the composite.

  In (Formula 1), r is preferably 0 ≦ r <0.10. This is due to the following reason. That is, if r is 0.10 or more, the presence of silicon carbide that does not contribute to the electric capacity increases, which may cause a reduction in the electric capacity of the composite.

  Here, r may be 0 (zero). That is, it should be noted that the composite may not contain carbon.

  In (Formula 1), s is preferably 0 <s <0.1. This is due to the following reason. In other words, if s is 0.1 or more, it may exist as silicon nitride in addition to silicon carbide. In that case, there is a concern about a decrease in the electric capacity of the composite.

  Moreover, it was confirmed by the electron microscope observation of the particle | grain cross section of this composite_body | complex that the silicon nanoparticle and the silicon carbide nanoparticle were the structure disperse | distributed in the silicon oxide.

  In addition, silicon oxide in the composite was dissolved with hydrofluoric acid, and the silicon nanoparticles and silicon carbide nanoparticles insoluble in hydrofluoric acid were observed with an electron microscope. The average particle diameter of 200 silicon nanoparticles arbitrarily selected by electron microscope observation was 61 nm, and the average particle diameter of 200 silicon carbide nanoparticles was 25 nm.

  The average particle size of the silicon nanoparticles is preferably 10 nm or more and less than 100 nm. This is due to the following reason. When the average particle size of the silicon nanoparticles is less than 10 nm, the effect of increasing the charge / discharge capacity of the silicon nanoparticles is reduced, and when it is 100 nm or more, the silicon nanoparticles are refined (deteriorated) by repeated charge / discharge. This is because the durability becomes low.

  The ratio of the silicon nanoparticles to the composite is preferably 5% by mass or more and 60% by mass or less. This is due to the following reason. This is because if the ratio of the silicon nanoparticles is less than 5% by mass, the high capacity performance of the silicon nanoparticles cannot be fully utilized. This is because when the ratio of the silicon nanoparticles to the composite is larger than 60% by mass, the durability is remarkably deteriorated. Specifically, when the proportion is larger than 60% by mass, the proportion of silicon nanoparticles in the composite becomes too large, and the composite can withstand when the silicon nanoparticles expand / contract due to charge / discharge. It is because it destroys it.

  The average particle diameter of the silicon carbide nanoparticles is preferably 10 nm or more and less than 100 nm. This is due to the following reason. This is because if the average particle diameter of the silicon carbide nanoparticles is smaller than 10 nm, a sufficient conductive path cannot be formed inside the composite, and the effect of increasing conductivity is reduced. If the average particle size of the silicon carbide nanoparticles is 100 nm or more, the silicon nanoparticles may inhibit expansion / contraction during charge / discharge, and the effect of absorbing and releasing lithium ions by the silicon nanoparticles may be reduced. Because.

  The ratio of the silicon carbide nanoparticles to the composite is preferably 0.1% by mass or more and 15% by mass or less. This is due to the following reason. When the ratio of silicon carbide nanoparticles is less than 0.1% by mass, the silicon carbide nanoparticles are not sufficiently dispersed inside the composite. As a result, when the silicon nanoparticles are refined (deteriorated) due to expansion / contraction, the effect of increasing the conductivity inside the composite is reduced. This is because if the proportion of silicon carbide nanoparticles is larger than 15% by mass, the effect of silicon carbide nanoparticles on the proportion of silicon nanoparticles cannot be expected, and conversely the capacity of the composite may be reduced.

The ratio of silicon carbide nanoparticles to silicon nanoparticles is preferably 0.1% by mass or more and 50% by mass or less. This is due to the following reason. When the amount is less than 0.1% by mass, the contact between silicon and silicon carbide becomes insufficient and does not contribute to the improvement of conductivity. This is because when the amount is larger than 50% by mass, there is a greater risk of a decrease in the capacity of the composite than the improvement in conductivity. Note that the mass percent of silicon carbide nanoparticles, the mass percent of silicon oxide, and the mass of silicon nanoparticles. The detection with% can be detected by the following method, for example.

  The mass% of silicon carbide nanoparticles is determined by adding fluoronitric acid to the composite, dissolving silicon oxide and silicon nanoparticles, and evaporating to dryness to determine the quantity of silicon carbide nanoparticles that form a residue. . Moreover, the mass% of silicon carbide nanoparticles can be calculated based on the mass of the measurement result and the mass of the composite.

  The mass% of silicon oxide is obtained by adding hydrofluoric acid to the composite to dissolve the silicon oxide and measuring the quantification of the dissolved silicon oxide using ICP-AES (inductively coupled plasma emission spectroscopy). Based on the quantification of the measurement result, the mass% of silicon oxide can be calculated.

  The mass% of silicon nanoparticles can be calculated by subtracting the mass% of silicon carbide nanoparticles and the mass% of silicon oxide determined above from the entire composite.

Moreover, it is preferable that the specific surface area of a composite_body | complex is 70 m < ² > / g or more. This is due to the following reason. This is because if the specific surface area is less than 70 m ² / g, the interfacial resistance during charging / discharging is large, and it may be difficult to charge or discharge a large current as a battery. As a method for measuring the specific surface area, a BET method or the like can be applied.

  When the composite includes silicon carbide nanoparticles, nitrogen may be doped into the silicon carbide nanoparticles as a dopant. In this case, the conductivity of the composite can be further increased.

(Negative electrode for non-aqueous electrolyte secondary battery)
The present invention uses the above composite as a negative electrode active material for a negative electrode material for a non-aqueous electrolyte secondary battery. Moreover, the negative electrode for lithium ion secondary batteries is producible using the nonaqueous electrolyte secondary battery negative electrode material obtained by this invention for the negative electrode. In addition, the conventional method can be used for the preparation methods of the negative electrode for lithium ion secondary batteries.

  For example, the negative electrode material and the binder of the present invention are mixed with a dispersing device such as a stirrer and a ball mill together with a solvent to prepare a negative electrode material slurry, which is applied to a current collector to form a negative electrode layer, Or it can obtain by shape | molding paste-like negative electrode material slurry in shapes, such as a sheet form and a pellet form, and integrating this with a collector.

  Although it does not specifically limit as a binder, For example, a styrene-butadiene copolymer, methyl (meth) acrylate, ethyl (meth) acrylate, butyl (meth) acrylate, (meth) acrylonitrile, hydroxyethyl (meth) acrylate, etc. Ethylenically unsaturated carboxylic acid ester, acrylic acid, methacrylic acid, itaconic acid, fumaric acid, maleic acid and other ethylenically unsaturated carboxylic acid, polyvinylidene fluoride, polyethylene oxide, polyepichlorohydrin, polyphosphazene, Examples thereof include a polymer compound having high ionic conductivity such as polyacrylonitrile.

  Moreover, you may add the thickener for adjusting a viscosity to a negative electrode material slurry. As the thickener, for example, carboxymethylcellulose, methylcellulose, hydroxymethylcellulose, ethylcellulose, polyvinyl alcohol, polyacrylic acid (salt), oxidized starch, phosphorylated starch, casein and the like can be used.

  Moreover, you may mix a conductive support material with a negative electrode material slurry. Examples of the conductive auxiliary material include carbon black, graphite, acetylene black, or an oxide or nitride that exhibits conductivity.

  In addition, the material and shape of the current collector are not particularly limited, and for example, a strip-like material in which aluminum, copper, nickel, titanium, stainless steel, or the like is formed into a foil shape, a punched foil shape, a mesh shape, or the like is used. That's fine. A porous material such as porous metal (foamed metal) or carbon paper can also be used.

  The method of applying the negative electrode material slurry to the current collector is not particularly limited. For example, a metal mask printing method, an electrostatic coating method, a dip coating method, a spray coating method, a roll coating method, a doctor blade method, a gravure coating method. And known methods such as screen printing. After the application, a rolling process using a flat plate press, a calendar roll or the like is performed as necessary.

  Further, the integration of the negative electrode slurry and the current collector formed into a sheet shape, a pellet shape, or the like can be performed by a conventional method such as a roll, a press, or a combination thereof.

(Lithium ion secondary battery)
A lithium ion secondary battery can be manufactured with the negative electrode produced using the nonaqueous electrolyte secondary battery negative electrode material of the present invention. FIG. 2 shows an example of a lithium ion secondary battery. For example, the lithium ion secondary battery BT can be manufactured by disposing the negative electrode N and the positive electrode P facing each other with the separator S therebetween and injecting the electrolytic solution Es.

  The positive electrode can be obtained by forming a positive electrode layer on the current collector surface in the same manner as the negative electrode described above. In this case, the current collector may be a band-shaped material made of a metal or an alloy such as aluminum, titanium, or stainless steel in a foil shape, a punched foil shape, a mesh shape, or the like.

The positive electrode material used for the positive electrode layer is not particularly limited, and a metal compound, metal oxide, metal sulfide, or conductive polymer material that can be doped or intercalated with lithium ions may be used. For example, lithium cobaltate (LiCoO ₂ ), lithium nickelate (LiNiO ₂ ), lithium manganate (LiMnO ₂ ), and double oxides thereof (LiCoxNiyMnzO ₂ , x + y + z = 1), lithium manganese spinel (LiMn ₂ O ₄ ) , Lithium vanadium compounds, olivine type LiMPO ₄ (M: Co, Ni, Mn, Fe), conductive polymers such as polyacetylene, polyaniline, polypyrrole, polythiophene, polyacene, porous carbon, etc. are used alone or in combination. can do.

  As the separator, for example, a microporous film mainly composed of polyolefin such as polyethylene or polypropylene, a nonwoven fabric, cloth, or a combination thereof can be used. In addition, when it is set as the structure where the positive electrode and negative electrode of the lithium ion secondary battery to produce are not in direct contact, it is not necessary to use a separator.

Examples of the electrolyte include lithium salts such as LiPF ₆ , LiClO ₄ , LiAsF ₆ , LiBF ₄ , LiSO ₃ CF ₃ , ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate, vinylene carbonate, cyclopentanone. , Sulfolane, 3-methylsulfolane, 2,4-dimethylsulfolane, 3-methyl-1,3-oxazolidine-2-one, γ-butyrolactone, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, butyl methyl carbonate , Ethyl propyl carbonate, butyl ethyl carbonate, dipropyl carbonate, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane In addition, a so-called organic electrolytic solution dissolved in a single aqueous solvent such as methyl acetate or ethyl acetate or a mixture of two or more components can be used.

  The structure of the lithium ion secondary battery is not particularly limited. Usually, a positive electrode and a negative electrode, and a separator provided as necessary, are wound into a flat spiral shape to form a wound electrode group, It is possible to form a laminated electrode plate group by laminating as a shape, or to have a structure in which these electrode plate groups are enclosed in an exterior body.

  The shape of the lithium ion secondary battery is not particularly limited, but can be used as a button battery, a coin battery, a stacked battery, a cylindrical battery, or the like.

(Production method)
Next, the manufacturing method of the negative electrode material for nonaqueous electrolyte secondary batteries is demonstrated. As shown in FIG. 3, in step S101, a silicon source is synthesized by synthesizing a silicon source containing at least one or more silicon compounds and a carbon source containing an organic compound that generates carbon by at least one or more heating. And a mixture of carbon sources. In the present embodiment, at least one of the silicon source and the carbon source used for generating the above mixture is liquid.

  For example, a silicon source and a carbon source are synthesized using an acid aqueous solution as a curing agent. Here, as the silicon source, a liquid silicon source and a solid silicon source can be used in combination, but at least one liquid silicon source must be used.

  For example, as a liquid silicon source, alkoxysilane (mono-, di-, tri-, tetra-) and a polymer of tetraalkoxysilane can be used.

  Among the alkoxysilanes, tetraalkoxysilane is preferably used as the liquid silicon source. Specifically, methoxysilane, ethoxysilane, propoxysilane, butoxysilane, or the like can be preferably used. From the viewpoint of handling, ethoxysilane is preferably used as the liquid silicon source.

  In addition, as a liquid silicon source, among tetraalkoxysilane polymers, a low molecular weight polymer (oligomer) having a degree of polymerization of about 2 to 15 and a silicate polymer having a higher degree of polymerization are used in a liquid form. Can be. Silicon oxide can be used as a solid silicon source that can be used in combination with these liquid silicon sources.

  In the present embodiment, silicon oxide includes silica gel (colloidal ultrafine silica-containing liquid, containing OH groups and alkoxyl groups inside), silicon dioxide (silica gel, fine silica, quartz powder) and the like in addition to SiO.

  These silicon sources may be used alone or in combination of two or more. Among these silicon sources, from the viewpoint of good homogeneity and handling properties, it is preferable to use tetraethoxysilane oligomer, a mixture of tetraethoxysilane oligomer and fine powder silica, or the like.

  Here, the substance used as the carbon source is preferably an organic compound containing oxygen in the molecule and remaining carbon by heating.

  Specific examples include phenolic resins, furan resins, epoxy resins, phenoxy resins, monosaccharides such as glucose, oligosaccharides such as sucrose, and various saccharides such as polysaccharides such as cellulose and starch.

  These carbon sources are mainly liquid at room temperature, those that dissolve in a solvent, those that soften by heating such as thermoplasticity or heat melting, or liquids for the purpose of homogeneously mixing with a silicon source. Will be used.

  Of these, resol type phenol resins and novolac type phenol resins are preferred. In particular, a resol type phenol resin is preferably used.

  Here, a hardening | curing agent can be suitably selected according to a carbon source. For example, when the carbon source is a phenol resin or a furan resin, as a curing agent, a toluenesulfonic acid aqueous solution, a toluenecarboxylic acid aqueous solution, an acetic acid aqueous solution, an oxalic acid aqueous solution, a phosphoric acid aqueous solution, a sulfuric acid aqueous solution, boric acid, nitric acid, hydrochloric acid aqueous solution, etc. Weak acid aqueous solution can be used. In such a case, it is preferable to use phosphoric acid, sulfuric acid, hydrochloric acid, or the like as the curing agent.

  This is due to the following reason. When a phenol resin is selected as the carbon source, an acid aqueous solution acting as an N-type dopant is used as a curing agent, so that silicon nanoparticles are doped with elements included in Group 15 elements of the periodic table, and the conductivity of the composite This is because it can be further enhanced. In the present embodiment, nitrogen N, which is an element belonging to Group 15 of the periodic table, is used as an example.

  On the other hand, when the xylene resin other than the phenol resin is used as the carbon source, it is preferable to use boric acid as the curing agent. This is due to the following reason. When a xylene resin is selected as a carbon source, an acid aqueous solution acting as a P-type dopant is used as a curing agent, so that silicon nanoparticles are doped with elements included in Group 13 elements of the periodic table, and the conductivity of the composite This is because it can be further enhanced.

  Furthermore, since the P-type dopant and the N-type dopant described above are also doped into silicon carbide nanoparticles, when silicon carbide nanoparticles are included in the composite, the conductivity of the composite may be further improved. It becomes possible.

  Thus, the acid aqueous solution preferably contains at least one element included in Group 13 element and Periodic group 15 element. In addition, you may include at least 1 sort (s) of elements contained in the periodic table group 13 element and the periodic table group 15 element in either of a silicon source and a carbon source. That is, at least one element included in Group 13 element and Group 15 element in the periodic table may be included in any of the silicon source, the carbon source, and the acid aqueous solution.

  In step S102, the mixture obtained in step S101 is dried at a temperature of 100 ° C to 300 ° C to generate a solid precursor (precursor).

Here, as described above, since at least one of the silicon source and the carbon source contained in the mixture used for the generation of the precursor is in a liquid state, SiO ₂ and C are at the molecular level inside the precursor. And uniformly distributed.

  Moreover, since such a precursor contains many organic components, the precursor may be carbonized at 500 ° C. to 1300 ° C. in a non-oxidizing atmosphere.

  For example, the compounding ratio of carbon and silicon (hereinafter abbreviated as C / Si ratio) in such a precursor is preferably 0.5 to 3.0, and more preferably 0.75 to 1.5.

  In step S103, the carbonized precursor is heated in an inert atmosphere in the region formed by the non-carbon material. Here, the non-carbon material may be a material containing carbon as long as carbon is not exuded on the surface, or may be a material in which C has a sufficiently high sublimation temperature and C does not sublime.

The inert atmosphere indicates a state filled with an inert gas such as Ar, N ₂ or H ₂ . Note that an inert gas such as O ₂ may be contained in the inert atmosphere as long as it is in a trace amount that does not affect the properties of the inert atmosphere.

  Specifically, in step S103, the carbonized precursor may be heated using thermal plasma, a resistance heating device, a laser heating device, arc plasma, or the like.

  For example, as shown in FIG. 4, after the precursor is finely pulverized, a gas (gas) containing the precursor is generated in the chamber 10 using a powder feeder such as a table feeder or a screw feeder. It sprays with respect to the heating area | region 20A by the thermal plasma which is. When nitrogen is used as the N-type dopant, nitrogen gas may be used as the gas.

  Alternatively, after such a precursor is finely pulverized, a gas (gas) containing the precursor is heated by a resistance heating device generated in the chamber 10 using a powder feeder such as a table feeder or a screw feeder. You may spray on the area.

  Alternatively, after the precursor is finely pulverized, a gas (gas) containing the precursor is heated by a laser heating device that is generated in the chamber 10 using a powder feeder such as a table feeder or a screw feeder. You may spray on the area.

  The inside of the chamber 10 is in an inert atmosphere by the plasma gas. Further, the chamber 10 has an inner wall of a non-carbon substance (for example, an inner wall made of stainless steel).

  In such a case, thermal plasma, a resistance heating device, or a laser heating device functions as a heating means, and can heat the precursor to 1300 ° C. or higher, more preferably 1500 ° C. or higher in the heating region 20A. As a result, silicon monoxide (SiO) gas is generated by the chemical reaction shown in (Formula 2).

SiO ₂ + C → SiO + CO (Formula 2)
In step S104, the gas generated by heating the precursor is quenched in an inert atmosphere in the region formed by the non-carbon material.

  Specifically, for example, as shown in FIG. 4, the silicon monoxide gas generated in the heating region 20 </ b> A is put on the air stream as it is and released outside the heating region 20 </ b> A in the chamber 10.

  In this case, since the temperature outside the heating region 20A is less than 1300 ° C., the silicon monoxide gas can be rapidly cooled to less than 1300 degrees. Further, since the outside of the chamber 20 is kept at room temperature, the silicon monoxide gas is then rapidly cooled to room temperature.

  As a result, a mixed powder containing silicon (Si) fine particles, silicon carbide (SiC) fine particles, and silicon oxide (SiOx (1 ≦ x ≦ 2)) by the chemical reaction shown in (Formula 3) and (Formula 4). Is generated.

2SiO → Si + SiO ₂ (Formula 3)
SiO + 2C → SiC + CO (Formula 4)
The mixed powder generated by being discharged from the chamber 20 is collected by a cyclone or a dust collector.

  The collected mixed powder contains the composite of the present invention having a structure in which silicon nanoparticles and silicon carbide nanoparticles are dispersed in silicon oxide. That is, a composite as a negative electrode material for a nonaqueous electrolyte secondary battery can be obtained from the mixed powder.

  Moreover, as a method of taking out the composite from the mixed powder, dry classification and wet classification can be applied as classification.

  Specifically, sedimentation tanks, mechanical classifiers, and centrifugal classifiers can be applied to wet classification, and dry classification includes sieving and air classification. Industrially, sieving is desirable.

(Function and effect)
The negative electrode material for a non-aqueous electrolyte secondary battery according to this embodiment has a composite containing silicon oxide, silicon nanoparticles, and silicon carbide nanoparticles. The silicon nanoparticles are doped with nitrogen N as an N-type dopant.

  Moreover, as a result of measuring the resistance value of the composite by a four-probe method using a resistivity meter manufactured by Mitsubishi Chemical Analytech, the resistance value of the composite is 3 × 10 ^ 4 Ω · cm, It was confirmed that the resistance value of the composite of silicon and silicon oxide was smaller than 5 × 10 6 Ω · cm.

  That is, the negative electrode material for a non-aqueous electrolyte secondary battery according to the present embodiment makes it possible for silicon nanoparticles to smoothly absorb and release lithium ions by improving conductivity. That is, the negative electrode material for a non-aqueous electrolyte secondary battery according to this embodiment can improve the discharge rate characteristics during discharge.

  Moreover, in the negative electrode material for a nonaqueous electrolyte secondary battery according to the present embodiment, a conductive film such as a carbon layer is not formed on the surface of the composite as in the prior art. That is, the negative electrode material for a non-aqueous electrolyte secondary battery according to the present embodiment does not require a step of forming a conductive film, and thus simplifies the manufacturing process compared to the conventional manufacturing method.

  Thus, according to the negative electrode material for a non-aqueous electrolyte secondary battery according to this embodiment, it is possible to improve the discharge rate characteristics by increasing the conductivity while simplifying the manufacturing process.

  The composite has a structure in which silicon nanoparticles and silicon carbide nanoparticles are dispersed in silicon oxide. Here, since the conductivity of silicon carbide nanoparticles is higher than that of silicon nanoparticles or silicon oxide, a conductive path can be formed inside the composite.

  That is, the composite has a structure in which silicon carbide nanoparticles are dispersed in silicon oxide in addition to silicon nanoparticles, so that the volume resistance (bulk resistance) of the entire composite is suppressed, and the conductivity is further increased. it can.

  As described above, the negative electrode material for a non-aqueous electrolyte secondary battery according to the present embodiment makes it possible for silicon nanoparticles to smoothly absorb and release lithium ions by improving conductivity. As a result, it is possible to suppress a decrease in cycle characteristics due to deterioration of expansion / contraction of silicon nanoparticles.

  Furthermore, since the silicon carbide nanoparticles do not expand or contract during charge / discharge, the structural stability of the composite is greatly improved by the dispersive arrangement of the silicon carbide nanoparticles. That is, according to the negative electrode material for a non-aqueous electrolyte secondary battery according to the present embodiment, it is possible to improve cycle characteristics with respect to charge and discharge.

[Example]
Next, in order to further clarify the effect of the present invention, the discharge rate characteristics of the negative electrode according to the example and the negative electrode according to the comparative example were compared.

  The negative electrode material of the present invention was used for the negative electrode according to Example 1. Specifically, a negative electrode material having a composite containing silicon nanoparticles, silicon carbide nanoparticles, and silicon oxide was used.

  In addition, silicon oxide in the composite was dissolved with hydrofluoric acid, and the silicon nanoparticles and silicon carbide nanoparticles insoluble in hydrofluoric acid were observed with an electron microscope. When the particle size of each of 200 silicon nanoparticles and silicon carbide nanoparticles arbitrarily selected from the photograph taken of the composite with a transmission electron microscope (TEM) was measured, the average of the silicon nanoparticles was measured. The particle size was 61 nm, and the average particle size of the silicon carbide nanoparticles was 25 nm.

  The negative electrode material of the present invention was also used for the negative electrode according to Example 2, but the negative electrode material containing no silicon carbide nanoparticles was used. The negative electrode according to Example 2 was different from the negative electrode material according to Example 1 in that the particle size of the particles was different. In addition, as for the negative electrode material of the negative electrode which concerns on Example 1 or 2, all used nitrogen as a dopant. Table 1 shows the particle diameters and other configurations of the negative electrode materials of the negative electrodes according to Examples 1 and 2.

  In addition, the composite of the present invention, 19% by mass of polyimide (PI) as a binder, 6% by mass of acetylene black (AB) as a conductive auxiliary, and N-methyl-2-pyrrolidone (NMP as a solvent) ) A negative electrode material slurry was prepared by mixing 300% by mass. Also, a negative electrode material slurry was applied to a copper foil having a thickness of 18 μm prepared as a current collector, dried, punched out into a shape with a diameter of φ16 mm using a punch, and a hand-pressed negative electrode according to Examples 1 and 2 As produced.

  For the negative electrodes according to Comparative Examples 1 and 2, a composite having a structure in which silicon nanoparticles were dispersed in silicon oxide was used as the negative electrode active material. That is, the one not containing silicon carbide nanoparticles was used. In addition, the negative electrode materials of the negative electrodes according to Comparative Examples 1 and 2 were all those containing no nitrogen as a dopant. The configurations of the negative electrodes according to Comparative Examples 1 and 2 are the same as those of the negative electrode according to Example 1 except for this point. In addition, in the negative electrode material of the negative electrode according to Comparative Examples 1 and 2, the particle diameters and other configurations of the silicon nanoparticles are as shown in Table 1. Moreover, when the negative electrode material of the negative electrode according to Comparative Examples 1 and 2 was measured using an oxygen-nitrogen analyzer manufactured by LECO, a very small amount of nitrogen in the air in the vicinity of 0% was detected. Therefore, 0% is set as shown in Table 1.

Moreover, each half cell was produced using the negative electrode which concerns on Example 1-2, and Comparative Example 1-2. Specifically, the negative electrodes according to Examples 1 and 2 and Comparative Examples 1 and 2 were used as working electrodes, and Li metal having a thickness of 0.5 mm and a diameter of 16 mm was used for the counter electrode (positive electrode). Between the negative electrode and the positive electrode, a φ19 mm Celgard (# 2400) is disposed as a separator, and 1 M LiPF _{6 in} a non-aqueous solvent of ethylene carbonate (EC) / ethyl methyl carbonate (EMC); 3/7 (volume ratio) A non-electrolytic solution to which was added was injected to prepare a half cell.

  As a test method, charging / discharging of the half cell produced using the negative electrodes according to Examples 1 and 2 and Comparative Examples 1 and 2 (hereinafter simply referred to as Examples 1 and 2 and Comparative Examples 1 and 2) was performed.

  In each cycle, charging to each of Examples 1 to 2 and Comparative Examples 1 to 2 is performed by constant current charging at a current corresponding to 0.1 C, and the current value is reduced from 0.05 V to 1/10 of the setting Constant voltage charging was performed until

  In addition, discharge from each of Examples 1 and 2 and Comparative Examples 1 and 2 was performed at a current corresponding to 0.1 C until 0.05 V to 1.5 V per cycle.

  Here, in the third cycle, after carrying out constant-current / constant-voltage charging with a current corresponding to 0.1 C, the discharge capacity when discharged to 1.5 V with currents corresponding to 1 C, 3 C, and 5 C was measured. did. The discharge rate characteristics were evaluated using the discharge capacity retention ratio between the measurement results and the initial discharge capacity.

  On the other hand, the cycle characteristics were also evaluated. For the cycle characteristic evaluation, the ratio of the initial discharge capacity and the discharge capacity at the 300th cycle was evaluated as the capacity retention rate.

  In addition, the discharge capacity measured the discharge capacity in the time of the voltage discharged from each half cell having cut 1.5V.

Table 1 shows the evaluation results of Examples 1 and 2 and Comparative Examples 1 and 2.

  In Table 1, the higher the discharge capacity retention rate, the better the discharge rate. As shown in Table 1, it was proved that the discharge rate characteristics of Examples 1 and 2 were superior to those of Comparative Examples 1 and 2.

  Furthermore, it was proved that the capacity retention rates of Examples 1 and 2 were superior to those of Comparative Examples 1 and 2. That is, it was proved that the cycle characteristics of Examples 1 and 2 were superior to those of Comparative Examples 1 and 2.

[Other Embodiments]
In the above-described embodiment, the negative electrode material of the present invention has been described by taking as an example a case where the negative electrode material is applied to a lithium ion secondary battery, but is not limited thereto. For example, the negative electrode material of the present invention can be applied to all electrochemical devices having a charge / discharge mechanism that inserts and desorbs lithium ions, for example, capacitors.

  Although the present invention has been described in detail using the above-described embodiments, it is obvious to those skilled in the art that the present invention is not limited to the embodiments described in this specification. The present invention can be implemented as modified and changed modes without departing from the spirit and scope of the present invention defined by the description of the scope of claims. Therefore, the description of the present specification is for illustrative purposes and does not have any limiting meaning to the present invention.

DESCRIPTION OF SYMBOLS 1 ... Mixture, 10 ... Chamber, 10A ... Inner wall, 20A ... Heating area, 2 ... Composite, BT ... Lithium ion secondary battery, P ... Positive electrode, N ... Negative electrode, S ... Separator, Es ... electrolyte,

Claims (5)

Having a composite containing silicon oxide SiOx (1 ≦ x ≦ 2) and silicon nanoparticles Si,
The composite has a structure in which the silicon nanoparticles are dispersed in the silicon oxide,
A non-aqueous electrolyte secondary, wherein the silicon nanoparticles are doped with at least one element included in a group 13 element and a group 15 element in the periodic table as a dopant. Negative electrode material for batteries. 2. The negative electrode material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the silicon nanoparticles have an average particle size of 10 nm or more and less than 100 nm. The ratio of the said silicon nanoparticle with respect to the said composite is 5 mass% or more and 60 mass% or less, The negative electrode material for nonaqueous electrolyte secondary batteries as described in any one of Claim 1 thru | or 2 characterized by the above-mentioned. 4. The negative electrode material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the dopant is nitrogen N. 5. A lithium ion secondary battery comprising a negative electrode comprising the negative electrode material for a nonaqueous electrolyte secondary battery according to any one of claims 1 to 4.

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