JP2016094316A - Method for producing laminar silicon-carbon composite, laminar silicon-carbon composite, electrode, and electricity storage device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a laminar silicon-carbon composite more easily.SOLUTION: The method for producing a laminar silicon-carbon composite of the present invention comprises a mixing step where a silicon alloy, a sugar compound, and an acid are mixed. In the mixing step, the silicon compound and the sugar compound are preferably stirred in an acidic solution. As the silicon alloy, for example, calcium silicide (CaSi) and the like can suitably be used. As the sugar compound, for example, sucrose can suitably be used. As the acid, hydrochloric acid can suitably be used. In such a mixing step, there occur synthesis of laminar silicon and compounding of the laminar silicon and the carbon. That is, a laminar silicon-carbon composite is thought to be obtained by a one-step reaction.SELECTED DRAWING: None

Description

  The present invention relates to a method for producing a layered silicon carbon composite, a layered silicon carbon composite, an electrode, and an electricity storage device (for example, a lithium ion battery, an electric double layer capacitor, a hybrid capacitor, a pseudo electric double layer capacitor, etc.).

  As an electrode of a lithium secondary battery, silicon is known to have a high energy density of about 3600 mAh / g at room temperature, and has recently attracted attention as an electrode material for a lithium secondary battery. However, although silicon has a high theoretical capacity, there is a problem that the volume expansion coefficient in the charge / discharge process is large. When the volume expansion coefficient is large, due to the volume change accompanying charge / discharge, for example, cracking of silicon particles or poor contact between the silicon particles and the conductive auxiliary agent, and hence the electrode material made of a material containing the silicon particles and the conductive auxiliary agent, A contact failure with the current collector occurs, and the charge / discharge cycle is shortened. In addition, due to the same cause, the irreversible capacity increases, which may lead to a reduction in battery capacity. For these reasons, several reports have been made on improvements in electrode materials mainly composed of silicon.

  For example, Non-Patent Document 1 proposes forming a nanocomposite with a high porosity in which silicon particles are supported on a carbon skeleton by alternately stacking and supporting silicon and carbon on annealed carbon black by CVD. Has been. Further, Non-Patent Document 2 and Patent Document 1 propose that a silicon-carbon composite is formed by physically mixing layered polysilane and sucrose and firing in an inert atmosphere. For example, Patent Document 2 proposes that the layered polysilane is coated with carbon by a dry film forming method while stirring the layered polysilane powder. Thus, the characteristics have been improved by combining layered silicon and carbon.

JP 2012-59509 A JP 2013-37809 A

Nature Materials 2010 9: 353 Journal of Materials Chemistry, 2011, Vol. 21, 11941

  However, the non-patent document 1 requires a relatively complicated process of alternately stacking and supporting silicon and carbon by the CVD method. In Non-Patent Document 2 and Patent Document 1, synthesis of layered polysilane, kneading with sucrose, firing and multi-step processes are necessary for forming the composite. Moreover, the layered structure of the layered polysilane may be destroyed by firing. In Patent Document 2, carbon is coated on a layered polysilane by a PLD method (Pulsed Laser Deposition) or the like, and a high vacuum and high energy synthesis process is required. For this reason, a composite of layered silicon and carbon that can be obtained more easily has been desired.

  The present invention has been made to solve such problems, and has as its main object to provide a composite of layered silicon and carbon that can be obtained more easily.

  As a result of intensive research to achieve the above-mentioned object, the present inventors have found that a layered silicon carbon composite can be easily obtained when calcium disilicide and sucrose are added to concentrated hydrochloric acid and stirred. The present invention has been completed.

That is, the method for producing the layered silicon carbon composite of the present invention is as follows.
A mixing step of mixing a silicon alloy, a sugar compound and an acid;
Is included.

The layered silicon carbon composite of the present invention is
A diffraction peak derived from layered silicon and a broad peak derived from carbon are observed in the powder X-ray diffraction pattern, and an absorption band derived from -C = C-stretching is observed in the infrared absorption spectrum.

The electrode of the present invention is
The above-described layered silicon carbon composite is used.

  The power storage device of the present invention includes a positive electrode, a negative electrode, and an ion conduction medium interposed between the positive electrode and the negative electrode, and includes the above-described electrode as at least one of the positive electrode and the negative electrode. is there.

  With the method for producing a layered silicon carbon composite and the layered silicon carbon composite of the present invention, a composite of layered silicon and carbon that can be obtained more easily can be provided. Moreover, in the electrode and electrical storage device of this invention, since the layered silicon carbon composite mentioned above is used, the electrode and electrical storage device provided with the layered silicon carbon composite can be provided more easily. The reason why such an effect can be obtained is assumed as follows. That is, it is presumed that when a silicon alloy, a sugar compound, and an acid are mixed, layered silicon is synthesized and carbonization of the sugar compound proceeds.

The schematic diagram showing general layered polysilane. An appearance photograph of CaSi ₂ . An appearance photograph of the Si / C composite of Experimental Example 1. An appearance photograph of the layered silicon of Reference Example 1. The powder X-ray-diffraction pattern of the Si / C composite of Experimental Example 1. The infrared absorption spectrum of the Si / C composite of Experimental Example 1. The Raman spectrum of the Si / C composite of Experimental Example 1. Nitrogen adsorption / desorption isotherm for carbon materials. TG-DTA of Si / C composite of Experimental Example 1. The SEM image and EDX mapping image of the Si / C composite of Experimental Example 1. The charging / discharging curve of the secondary battery which used the Si / C composite of Experimental example 1 for the electrode. 3 is a powder X-ray diffraction pattern of the fired Si / C composite of Experimental Example 1. FIG.

(Method for producing layered silicon carbon composite)
The method for producing a layered silicon carbon composite of the present invention includes a mixing step of mixing a silicon alloy, a sugar compound, and an acid. In this mixing step, the mixing method may be, for example, that the silicon alloy and the sugar compound are stirred in an acid aqueous solution. More specifically, for example, a sugar compound may be dissolved in an acid aqueous solution, and a silicon alloy may be added thereto and stirred. At this time, the sugar compound may be dissolved in a saturated amount in the acid aqueous solution. The mixing ratio of the silicon alloy, the sugar compound, and the acid can be determined empirically so that the composition of the target layered silicon carbon composite is obtained. For example, the Si / C ratio, which is the ratio of the weight of silicon contained in the silicon alloy and the weight of carbon contained in the sugar compound, may be 0.002 or more and 0.6 or less. preferable. The temperature at the time of mixing is not specifically limited, You may carry out at room temperature, for example, you may carry out at 0 degreeC or more and 100 degrees C or less. The pressure at the time of mixing may be under atmospheric pressure, for example, or may be under pressure generated by sealing and heating the reaction vessel. The mixing time may be a time sufficient for the reaction to proceed, and may be, for example, 1 hour to 48 hours. The atmosphere at the time of mixing may be, for example, an inert atmosphere such as an argon atmosphere or a nitrogen atmosphere, or may be an oxidizing atmosphere such as an air atmosphere or an oxygen atmosphere.

The silicon alloy used in the mixing step may be any silicon alloy that can form layered silicon by the reaction of a sugar compound and an acid. Examples thereof include calcium disilicide (CaSi ₂ ) and calcium silicide (CaSi). Of these, calcium disilicide is preferred. This is because it is suitable as a raw material for layered silicon. Calcium disilicide may be obtained, for example, by melting a stoichiometric ratio of silicon and calcium in an inert atmosphere. In this case, the melting temperature may be, for example, 850 ° C. or higher and 1100 ° C. or lower.

  The saccharide compound used in the mixing step may be a monosaccharide, an oligosaccharide in which 2 to 20 molecules of monosaccharide are bonded, or a polysaccharide in which more monosaccharides are bonded. Of these, monosaccharides and oligosaccharides are preferable, and monosaccharides and disaccharides are more preferable. More specifically, the monosaccharide or disaccharide is selected from the group consisting of glucose and fructose, which are monosaccharides, and sucrose, lactose, maltose, trehalose, turanose, cellobiose, and derivatives thereof, which are disaccharides. The above is preferable. These are because the molecular weight is small (180 to 350) and the molecular size is small, so that it is easy to insert between the layers of layered silicon.

  As the acid used in the mixing step, hydrochloric acid, nitric acid, sulfuric acid, acetic acid, a mixture thereof, a mixture with an organic solvent, or the like can be used. Of these, hydrochloric acid is preferable. This is because the sugar compound is easily dissolved and carbonized. The acid is preferably used as an aqueous acid solution. In this case, the concentration is preferably 1M or more and 12M or less, and more preferably 10M or more and 12M or less. This is because the carbonization of the sugar compound and the generation of layered silicon can proceed simultaneously.

  In this mixing step, layered silicon may be synthesized and layered silicon and carbon may be combined. That is, a layered silicon carbon composite may be obtained by a one-step reaction. In such a case, the layered silicon-carbon composite can be synthesized more easily than the case where layered silicon is synthesized in advance and then the layered silicon and carbon are combined.

  This mixing step is preferably a step that does not involve heating. This is because the layered structure of layered silicon may be destroyed by heating, but the structure is more easily maintained if the process does not involve heating. Moreover, heating equipment or the like is unnecessary, and a layered silicon carbon composite can be synthesized more easily.

  The method for producing a layered silicon carbon composite of the present invention may include a baking step after the mixing step. This is because the unreacted sugar compound remaining without reacting in the mixing step, moisture, and the like can be removed by the firing step. The firing temperature may be, for example, 100 ° C. or more and 2000 ° C. or less, and preferably 200 ° C. or more and 1000 ° C. or less. The firing atmosphere may be, for example, an oxidizing atmosphere such as an air atmosphere or an oxygen atmosphere, or an inert atmosphere such as an argon atmosphere or a nitrogen atmosphere, but an inert atmosphere is preferable.

  In the manufacturing method of the layered silicon carbon composite of this invention, it is good also as a thing including a filtration process, a washing | cleaning process, and a drying process after a mixing process and a baking process. In the filtration step, for example, pressure filtration or vacuum filtration may be performed in addition to normal filtration. In the washing step, washing with water, washing with acid, washing with hydrogen halide, washing with an organic solvent, or the like may be performed. These may be performed alone or in combination. Good. In the drying process, heat drying may be performed, air drying may be performed, or vacuum drying may be performed. Drying may be performed, for example, at 20 ° C. or higher and 200 ° C. or lower, and preferably performed at 50 ° C. or higher and 100 ° C. or lower. This is because, in such a temperature range, drying can be performed efficiently and the layered silicon carbon composite is not easily altered. Each process may be performed in an oxidizing atmosphere such as an air atmosphere or an oxygen atmosphere, or may be performed in an inert atmosphere such as an argon atmosphere or a nitrogen atmosphere.

(Layered silicon carbon composite)
The layered silicon carbon composite of the present invention may be obtained by the above-described method for producing a layered silicon carbon composite. Alternatively, in the layered silicon carbon composite of the present invention, a diffraction peak derived from layered silicon and a broad peak derived from carbon are observed in a powder X-ray diffraction pattern, and derived from -C = C-stretching in an infrared absorption spectrum. An absorption band to be observed may be observed. When a diffraction peak derived from layered silicon and a broad peak derived from carbon are observed, it has layered silicon and amorphous carbon. In addition, in the case where an absorption band derived from -C = C-stretching is observed in the infrared absorption spectrum, for example, the amount of carbon is larger than that obtained by coating the layered polysilane with carbon by the PLD method. In the layered silicon carbon composite of the present invention, the layered silicon may be, for example, layered polysilane or layered siloxane. Further, the carbon may be amorphous carbon. Here, the layered polysilane refers to one represented by the general formula Si ₆ H ₆ and having a basic skeleton having a structure in which a plurality of six-membered rings composed of silicon atoms are connected. If it is such, it is not particularly limited, but as shown in FIG. 1, a sheet structure having a basic skeleton having a structure in which a plurality of six-membered rings composed of silicon atoms are continuous is formed in a layer shape. It is preferable that The layered siloxane is represented by the general formula Si ₆ (OH) _6-x H _x (0 ≦ x <6), and is replaced with a part of hydrogen bonded to silicon atoms in the layered polysilane in place of hydroxy. A structure whose basic skeleton is a structure in which a group is bonded to a silicon atom. In the layered polysilane and layered siloxane, the silicon atom is described as having a hydrogen or hydroxy group bonded thereto, but all or a part of the hydrogen (including the hydrogen constituting the hydroxy group) is another element or substituent. It is good also as what is substituted by. The layered silicon may be derived from layered polysilane or layered siloxane. The term “derived from layered polysilane or layered siloxene” refers to a layered polysilane or layered siloxene that has been subjected to some treatment such as baking, and represents a layered silicon compound that is not layered polysilane or layered siloxene itself. In the layered silicon derived from the layered polysilane or the layered siloxane, the layered polysilane or the layered siloxane may partially remain.

In the layered silicon carbon composite of the present invention, the diffraction peak derived from layered silicon in the powder X-ray diffraction pattern may be a diffraction peak characteristic of layered polysilane or layered siloxane. More specifically, for example, in a layered polysilane represented by the general formula Si ₆ H ₆ , a diffraction peak of (001) plane observed near 2θ = 14 ° when CuKα rays are used as a radiation source, A diffraction peak of (100) plane observed near 2θ = 27 ° may be a diffraction peak of (110) plane observed near 2θ = 48 °. The broad peak derived from carbon may be a broad diffraction peak observed at around 2θ = 10 to 30 ° derived from amorphous carbon. That is, in the layered silicon carbon composite of the present invention, the carbon may be amorphous carbon. In the layered silicon carbon composite of the present invention, it is preferable that a diffraction peak (2θ = 28.4 °, 47.3 °, 56.1 °) derived from silicon microcrystals is not observed in the powder X-ray diffraction pattern. This is because in the layered silicon-carbon composite where such a peak is observed, it is considered that the decomposition of the layered silicon proceeds.

In the layered silicon carbon composite of the present invention, the absorption band derived from -C = C-stretching in the infrared absorption spectrum may be, for example, an absorption band observed near 1300 to 1800 cm- ¹ . In this infrared absorption spectrum, an absorption band having a peak in the vicinity of 1100 cm ⁻¹ as an absorption band derived from the Si—O—Si bond, and a peak in the vicinity of 2100 cm ⁻¹ as an absorption band derived from the Si—H bond. It is preferable that an absorption band is observed. In addition, it is preferable that the peak derived from a sugar compound is not observed as much as possible.

In the layered silicon-carbon composite of the present invention, the carbon (carbon compound) contained in the layered silicon-carbon composite, that is, the portion excluding the layered silicon is the type II adsorption / desorption behavior when nitrogen adsorption / desorption measurement is performed. (IUPAC classification). Type II indicates the possibility of no pores or the presence of macropores (pores greater than 50 nm). The carbon has a specific surface area calculated by the BET method of preferably 1 m ² / g or more and 100 m ² / g or less, and more preferably 10 m ² / g or more and 30 m ² / g or less. If it is within such a range, it is considered that when the secondary battery electrode is formed, the electrode density increases and the irreversible capacity can be reduced.

In the layered silicon carbon composite of the present invention, it is preferable that absorption derived from amorphous carbon is observed in the Raman spectrum. For example, a so-called D-band (e.g. absorption with a peak around 1330 cm ^-1), so-called G-band (e.g. absorption with a peak around 1590 cm ^-1) are observed, these peak intensity ratio, i.e. D / G ratio It is good also as what is 0.5-5. In such cases, it is considered that carbon exists as amorphous carbon.

  The layered silicon carbon composite of the present invention may include plate-shaped layered silicon particles and spherical carbon particles. Here, the layered silicon particles may have a lamination interval wider than that of normal layered silicon particles (for example, layered polysilane or layered siloxane), or a part of the layered structure may be peeled off. Further, silicon may be present in the carbon, or carbon distribution may be concentrated on the edge portion of the layered silicon. For example, the layered silicon carbon composite may be formed such that amorphous carbon surrounds the edge portion of the layered silicon.

  In this layered silicon carbon composite, the weight ratio of carbon (carbide) contained in the composite (carbon / layered silicon carbon composite ratio) is preferably 0.1 or more and 0.9 or less, and 0.3 or more and 0. More preferably, it is 5 or less. This is because the irreversible capacity due to carbon can be reduced when a secondary battery electrode is produced. The layered silicon carbon composite preferably has a weight loss in the range of −22 wt% to 62 wt% when thermogravimetric analysis is performed by heating from room temperature to 1000 ° C. More preferably, the weight percent is 22% by weight or more. This is because the carbon / layered silicon carbon composite ratio is considered to be within the above-described range.

(electrode)
The electrode of the present invention uses the above-described layered silicon carbon composite as an electrode material. This electrode is, for example, a layered silicon carbon composite as an electrode material, a conductive material, and a binder mixed, and a paste made by adding an appropriate solvent, applied to the surface of the current collector and dried, You may compress and form so that an electrode density may be raised as needed.

  The conductive material is not particularly limited as long as it is an electron conductive material that does not adversely affect battery performance. For example, graphite such as natural graphite (scale-like graphite, scale-like graphite) or artificial graphite, acetylene black, carbon black, ketjen A mixture of one or more of black, carbon whisker, needle coke, carbon fiber, metal (copper, nickel, aluminum, silver, gold, etc.) can be used. Among these, as the conductive material, carbon black and acetylene black are preferable from the viewpoints of electron conductivity and coatability.

  The binder serves to bind the active material particles and the conductive material particles. For example, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), fluorine-containing resin such as fluorine rubber, or polypropylene, Thermoplastic resins such as polyethylene, ethylene-propylene-dienemer (EPDM), sulfonated EPDM, natural butyl rubber (NBR) and the like can be used alone or as a mixture of two or more. In addition, an aqueous dispersion of cellulose or styrene butadiene rubber (SBR), which is an aqueous binder, can also be used.

  Examples of the solvent for dispersing the carbon material, the conductive material, and the binder include N-methylpyrrolidone, dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, methyl acrylate, diethyltriamine, and N, N-dimethylaminopropylamine. Organic solvents such as ethylene oxide, tetrahydrofuran and chloroform can be used. Moreover, a dispersing agent, a thickener, etc. may be added to water, and an active material may be slurried with latex, such as SBR. As the thickener, for example, polysaccharides such as carboxymethyl cellulose and methyl cellulose can be used alone or as a mixture of two or more. Examples of the application method include roller coating such as applicator roll, screen coating, doctor blade method, spin coating, bar coater, and the like, and any of these can be used to obtain an arbitrary thickness and shape.

  Current collectors include aluminum, titanium, stainless steel, nickel, iron, calcined carbon, conductive polymer, conductive glass, and aluminum, copper, etc. for the purpose of improving adhesion, conductivity, and oxidation resistance. A surface treated with carbon, nickel, titanium, silver or the like can be used. For these, the surface can be oxidized. Examples of the shape of the current collector include foil, film, sheet, net, punched or expanded, lath, porous, foam, and formed fiber group.

(Electric storage device)
The electricity storage device of the present invention includes a positive electrode, a negative electrode, and an ion conductive medium interposed between the positive electrode and the negative electrode. Here, as at least one of the positive electrode and the negative electrode, the electrode described above, that is, the electrode including the layered silicon carbon composite described above can be used.

Hereinafter, a particularly preferable embodiment when the electricity storage device of the present invention is a lithium secondary battery will be described. When the electrode material for an electricity storage device of the present invention is used for a positive electrode, for example, lithium metal can be used for the negative electrode. When the electrode for an electricity storage device of the present invention is used as a negative electrode, the positive electrode includes a compound having a layered rock salt structure such as LiCoO ₂ or LiNiO ₂ , a compound having a spinel structure such as LiMn ₂ O ₄ , LiFePO ₄ A polyanion compound such as can be used.

In this power storage device, the ion conductive medium may be a nonaqueous electrolytic solution, an ionic liquid, a gel electrolyte, a solid electrolyte, or the like in which a supporting salt is dissolved in an organic solvent. Of these, a non-aqueous electrolyte is preferable. Examples of the supporting salt include known LiPF ₆ , LiClO ₄ , LiAsF ₆ , LiBF ₄ , Li (CF ₃ SO ₂ ) ₂ N, Li (CF ₃ SO ₃ ), LiN (C ₂ F ₅ SO ₂ ), and the like. Supporting salts can be used. The concentration of the supporting salt is preferably 0.1 to 2.0M, and more preferably 0.8 to 1.2M. As an organic solvent, for example, ethylene carbonate (EC), propylene carbonate (PC), γ-butyrolactone (γ-BL), diethyl carbonate (DEC), dimethyl carbonate (DMC) and the like are used for conventional secondary batteries and capacitors. An organic solvent is mentioned. These may be used alone or in combination. The ionic liquid is not particularly limited, but N-methyl-N-propylpiperidinium bis (trifluoromethanesulfonyl) imide (PP13-TFSI) or N-methyl-N-propylpyrrolidinium Bis (trifluoromethanesulfonyl) imide (P13-TFSI), 1-methyl-3-propylimidazolium bis (trifluorosulfonyl) imide, 1-ethyl-3-butylimidazolium tetrafluoroborate, or the like can be used. The gel electrolyte is not particularly limited. For example, a polymer such as polyvinylidene fluoride, polyethylene glycol, or polyacrylonitrile, or a saccharide such as an amino acid derivative or sorbitol derivative is added with an electrolyte containing a supporting salt. And a gel electrolyte. Examples of the solid electrolyte include inorganic solid electrolytes and organic solid electrolytes. Well-known inorganic solid electrolytes include, for example, Li nitrides, halides, oxyacid salts, and the like. Among _{them, Li 4 SiO 4, Li 4} SiO 4 -LiI-LiOH, xLi 3 PO 4 - (1-x) Li 4 SiO 4, Li 2 SiS 3, Li 3 PO 4 -Li 2 S-SiS 2, sulfide Examples thereof include phosphorus compounds. These may be used alone or in combination. Examples of the organic solid electrolyte include polyethylene oxide, polypropylene oxide, polyvinyl alcohol, polyvinylidene fluoride, polyphosphazene, polyethylene sulfide, polyhexafluoropropylene, and derivatives thereof. These may be used alone or in combination.

  This power storage device may have a separator between the positive electrode and the negative electrode. The separator is not particularly limited as long as it is a composition that can withstand the usage range of the electricity storage device. For example, a polymer nonwoven fabric such as a polypropylene nonwoven fabric or a polyphenylene sulfide nonwoven fabric, or a microporous film of an olefin resin such as polyethylene or polypropylene is used. Can be mentioned. These may be used alone or in combination.

  The shape of the electricity storage device is not particularly limited, and examples thereof include a coin type, a button type, a sheet type, a laminated type, a cylindrical type, a flat type, and a square type. Moreover, you may apply to the large sized thing used for an electric vehicle etc.

  With the method for producing a layered silicon carbon composite and the layered silicon carbon composite of the present invention, a composite of layered silicon and carbon that can be obtained more easily can be provided. In addition, since the layered silicon carbon composite is used in the electrode and power storage device of the present invention, the electrode and power storage device provided with the layered silicon carbon composite can be provided more easily. The reason why such an effect can be obtained is assumed as follows. That is, by mixing a silicon alloy, a sugar compound, and an acid, “synthesis of a layered silicon compound from a silicon alloy” and “carbonization of a sugar compound on the surface of the layered silicon compound” proceed simultaneously in the same system. This is presumably because a composite of layered silicon (layered silicon compound) and carbon (carbon compound) is produced in the synthesis process in stages. In particular, at this time, with the generation of layered silicon, an intermediate in which a sugar compound is coordinated is formed on the surface of the layered silicon, and a nano-level complex in which the layered silicon is covered with carbon is realized based on them. Inferred.

  In the prior art, in order to combine the layered silicon compound and the carbon compound at the nano level, an expensive organic reagent and a multi-step synthesis process are required, and the yield is extremely low. On the other hand, in the present invention, by simultaneously proceeding “synthesis of a layered silicon compound from a silicon alloy” and “carbonization of a sugar compound on the surface of the layered silicon compound”, the layered silicon is produced at a low cost and in a one-step synthesis process. The synthesis of carbon composite has been realized. In addition, it is assumed that the obtained composite compound has a nano-level composite structure in which carbon is covered with layered silicon. Further, it is possible to form a fired layered silicon carbon composite by firing this composite. Further, a high-capacity secondary battery can be constructed by using the obtained composite compound as an electrode material.

  It should be noted that the present invention is not limited to the above-described embodiment, and it goes without saying that the present invention can be implemented in various modes as long as it belongs to the technical scope of the present invention.

  For example, in the embodiment described above, a conductive material, a binder, and a current collector are used in addition to the layered silicon carbon composite as the electrode of the present invention. It does not have to be. For example, without using a current collector, the layered silicon carbon composite of the present invention, a conductive material, and a binder may be mixed and pressure-molded.

  For example, in the above-described embodiment, the power storage device of the present invention has a particularly preferable form when it is a lithium secondary battery, but is not limited to the lithium secondary battery, and other such as a sodium secondary battery. In addition to the secondary battery, various electric storage devices such as an electric double layer capacitor and an electrochemical capacitor can be used.

  Below, the example which produced the layered silicon carbon composite of this invention concretely is demonstrated as an experiment example.

[Reference Example 1]
Layered silicon (Si ₆ H ₆ ) was synthesized as follows. In this synthesis, 3 g of calcium disilicide (CaSi ₂ ) was added to 100 ml of concentrated hydrochloric acid cooled to −30 ° C., and allowed to stand in a dark room at −30 ° C. for 1 week. This treatment turned the black calcium silicide into yellow. The yellow solid was filtered under pressure in an Ar atmosphere, washed with degassed hydrochloric acid (−30 ° C.), washed with degassed HF (hydrogen fluoride) aqueous solution (−30 ° C.), and further degassed acetone (−30 And then dried under reduced pressure at 110 ° C. overnight to synthesize layered silicon. The chemical reaction formula for the synthesis of this layered silicon is shown in Formula (1).
3CaSi ₂ + 6HCl → Si ₆ H ₆ + 3CaCl ₂

[Experiment 1]
(1) Synthesis of layered silicon carbon composite (hereinafter referred to as Si / C composite) The starting material calcium disilicide (CaSi ₂ ) is obtained by melting silicon and calcium in a stoichiometric ratio at 1000 ° C. in an inert atmosphere. Prepared. Sucrose (4 g) was added to concentrated hydrochloric acid (100 ml) and stirred, and CaSi ₂ (0.6 g) obtained was further added, followed by stirring for 12 hours at room temperature. With this treatment, calcium disilicide having a metallic luster changed to dark brown. The dark brown solid was suction filtered under an argon atmosphere, washed with water, washed with hydrochloric acid, and then dried under reduced pressure at room temperature to obtain a Si / C composite. FIG. 2 shows the appearance of CaSi ₂ . FIG. 3 shows the appearance of the Si / C composite of Experimental Example 1. FIG. 4 shows the appearance of the layered silicon of Reference Example 1 as a reference. The Si / C composite of Experimental Example 1 was a dark brown powder, and the layered silicon of Reference Example 1 was a yellow powder.

(2) Powder X-ray diffraction (XRD) measurement of Si / C composite The obtained Si / C composite was subjected to XRD measurement. As a measuring device, RINT-TTR manufactured by Rigaku Corporation was used. CuKα rays were used as the radiation source. FIG. 5 shows a powder X-ray diffraction pattern of the Si / C composite of Experimental Example 1. FIG. 5 also shows a powder X-ray diffraction pattern of the layered silicon of Reference Example 1. From the Si / C composite of Experimental Example 1, the diffraction peaks (001, 100, 110) characteristic of layered silicon (Si ₆ H ₆ ) and the broad diffraction derived from amorphous carbon (2θ = 10-30). °) was observed. From this, it was confirmed that the Si / C composite of Experimental Example 1 was a composite of layered silicon and carbon. In addition, 001 of the Si / C composite of Experimental Example 1 was broadened to the lower angle side compared to 001 of the layered silicon of Reference Example 1 synthesized without sucrose. This is presumed to indicate that the interlayer of the layered silicon is expanded by the combination with carbon. In the layered silicon (Si ₆ H ₆ ), peaks were observed near 2θ = 14 °, 100 near 2θ = 27 °, and 110 near 2θ = 48 °.

(3) Infrared absorption (IR) spectrum measurement of Si / C composite IR spectrum was measured about the obtained Si / C composite. As a measuring apparatus, an AVATAR360 type Fourier infrared spectrometer manufactured by Nicorey was used. FIG. 6 shows an infrared absorption spectrum of the Si / C composite of Experimental Example 1. FIG. 6 also shows the IR spectrum of the layered silicon of Reference Example 1 and the IR spectrum of sucrose. From the infrared absorption spectrum of the Si / C composite of Experimental Example 1, the absorption band derived from the Si—H group on the surface of the layered silicon of Reference Example 1 is decreased, and the new absorption due to —C═C—stretching is newly obtained. A band was observed. This was presumed to indicate that the decomposition of sucrose progressed to produce a compound having a carbon-carbon double bond.

(4) Raman spectrum measurement of Si / C composite The Raman spectrum was measured about the obtained Si / C composite. As a measuring device, NRS-3300 laser Raman spectrometer manufactured by JASCO Corporation was used. The excitation wavelength was 532 nm and the laser power was 0.01 mW or 0.001 mW. FIG. 7 shows the Raman spectrum of the Si / C composite of Experimental Example 1. FIG. 7 also shows the Raman spectrum of the Si / C composite of Experimental Example 1 that was heat-treated at 500 ° C. for 10 minutes in an argon atmosphere (Si / C composite heat-treated product). From the Raman spectrum of the Si / C composite of Experimental Example 1, absorption derived from amorphous carbon was observed at 1200 to 1600 cm ⁻¹ , although it was weak. Here, it was guessed that the observed peak intensity was weak because of the fluorescence of layered silicon. Therefore, when the sample was heat-treated as described above to quench the fluorescence derived from layered silicon (Si / C composite heat-treated product), the carbon D-band and G-band (1330, 1590 cm ⁻¹ ) were clearly confirmed. did it. From the D / G band ratio, it was confirmed that the carbon contained in the system was amorphous carbon. The D / G band ratio was presumed not to change significantly before and after the heat treatment, and the carbon contained in the Si / C composite of Experimental Example 1 was assumed to be amorphous carbon. From the above results, it was found that by treating CaSi ₂ with sucrose / hydrochloric acid, decomposition / carbonization of sucrose proceeds and amorphous carbon is formed.

(5) Nitrogen adsorption / desorption measurement of carbon contained in Si / C composite The nitrogen adsorption / desorption characteristics and specific surface area of carbon contained in the Si / C composite were examined. First, except that calcium disilicide is not added in the synthesis of (1) Si / C composite, the same operation as in the synthesis of Si / C composite is performed to simulate carbon contained in the Si / C composite. Carbon material was synthesized. The obtained carbon material was a black powder having a particle size of 0.1 to 2 μm. The obtained carbon material was subjected to nitrogen adsorption / desorption measurement. FIG. 8 shows the nitrogen adsorption / desorption isotherm of the carbon material. From FIG. 8, it was found that the carbon material exhibited type II adsorption / desorption behavior, and the amount of nitrogen adsorbed was very small. Moreover, it was guessed from the shape of the adsorption isotherm that a regular pore structure or the like was not formed. The specific surface area of the carbon material calculated by the BET method was about 20 m ² / g. From the above, it was speculated that the carbon contained in the Si / C composite had a specific surface area calculated by the BET method of about 20 m ² / g.

(6) Thermogravimetric analysis of Si / C composite The obtained Si / C composite was subjected to thermogravimetric analysis. FIG. 9 shows the thermogravimetric change (TG) and differential thermal change (DTA) of Si / C due to heating in an air flow atmosphere. From room temperature to 150 ° C., a weight loss of 3.9% accompanying dehydration of adsorbed water was confirmed. Thereafter, the weight temporarily increased at 200 to 280 ° C., thereafter the weight decreased again, and finally the weight decreased by 18.8%. Considering the increase in weight due to oxidation of layered silicon (about 32%: weight increase when TG analysis is similarly performed with layered silicon alone), the maximum amount of carbide in the Si / C composite is 46.9% by mass (= 18.8-3.9 + 32). That is, the weight ratio of carbon / layered silicon carbon composite, which is the ratio of carbon (carbide) to layered silicon carbon composite, was estimated to be 0.469. From the above, it was inferred that layered silicon and carbon formed a composite at a weight ratio of approximately 1: 1.

(7) Electron microscope image and element mapping of Si / C composite The obtained Si / C composite was observed with an electron microscope (SEM). In addition, elemental mapping by energy dispersive X-ray spectroscopy (EDX) was performed. FIG. 10 shows an SEM image and an EDX mapping image of the Si / C composite of Experimental Example 1. 10a and 10b are SEM images of different visual fields, FIG. 10c is an EDX mapping image showing the carbon distribution of the visual field of FIG. 10a, and FIG. 10d is an EDX mapping image showing the silicon distribution of the visual field of FIG. 10a. It is. From the SEM image of the Si / C composite of Experimental Example 1, plate-like layered silicon particles and spherical carbon particles were observed (FIG. 10a). In addition, part of the laminated structure of many layered silicon particles has begun to peel off (FIG. 10b), which is consistent with the results of XRD measurement, that is, broadening of the (001) diffraction peak to the lower angle side. . Moreover, in the EDX mapping image of carbon and silicon, it was found that the distribution positions of both elements overlap each other, and silicon is present in the carbon. In particular, the distribution of carbon tends to concentrate on the edge portion of the layered silicon (thickness portion of the laminated structure), and it is assumed that the amorphous carbon forms the Si / C composite so as to surround the edge portion of the layered silicon. It was done.

(8) Performance Evaluation of Si / C Composite as Secondary Battery Electrode Material The obtained Si / C composite was evaluated for performance as a secondary battery electrode material. Specifically, first, the Si / C composite of Experimental Example 1 was baked at 500 ° C. under an Ar stream (1 L / min) to obtain a baked Si / C composite. An electrode material was prepared by mixing the calcined Si / C composite (0.1 g), a carbon-based conductive additive, and a Teflon binder (Teflon is a registered trademark) at a mass ratio of 80: 15: 5. . The obtained electrode material was pressure-bonded to a copper mesh to produce an electrode. This electrode is used as a working electrode, metallic lithium is used as a counter electrode, a polyethylene microporous membrane is used as a separator, and ethylene carbonate (EC) and diethyl carbonate (DEC) are mixed at a volume ratio of 1: 1, and 1M An evaluation cell was produced using a bipolar cell made by Nippon Tomcell, using LiPF ₆ dissolved as an electrolyte. This evaluation cell was set in a charging / discharging device (HJR-1010SM8 manufactured by Hokuto Denko) and a charging / discharging test was performed. The charge / discharge test was performed by repeating the constant current charge / discharge, which was charged to a maximum potential of 3 V with a constant current of 0.4 mA, after being discharged to a minimum potential of 0.02 V with a constant current of 0.4 mA. FIG. 11 shows a charge / discharge curve of a secondary battery using the Si / C composite of Experimental Example 1 as an electrode material. From FIG. 11, the initial discharge capacity was 3150 mAh / g, but it was found that the charge / discharge capacity of 1300 to 1500 mAh / g became constant by repeating charge and discharge. The observed charge / discharge capacity is 4 to 5 times that of a conventional carbon electrode, and it was found that the synthesized Si / C composite is promising as a high capacity secondary battery negative electrode material.

(9) Powder X-ray diffraction (XRD) measurement of calcined Si / C composite XRD measurement is performed on the calcined Si / C composite in the same manner as “(2) Powder X-ray diffraction (XRD) measurement of Si / C composite”. Went. FIG. 12 shows a powder X-ray diffraction pattern of the fired Si / C composite of Experimental Example 1. FIG. 12 also shows a powder X-ray diffraction pattern of the Si / C composite that was not fired. From the sintered SiC composite, broad diffraction derived from carbide (2θ = 10 to 30 °) was mainly observed, and the intensity of diffraction peaks (001, 100, 110) characteristic of layered silicon was greatly reduced. On the other hand, diffraction peaks (2θ = 28.4 °, 47.3 °, 56.1 °) derived from silicon microcrystals which are decomposition products of layered silicon were not observed. From these results, it was speculated that the laminated structure of the layered silicon contained in the Si / C composite collapsed due to the firing of the Si / C composite, particularly because the strength of 001 decreased. In addition, as a result of these, it was speculated that the silicon skeleton was amorphized due to the decrease in the strength of 100 and 110 in particular.

(10) Experimental results As described above, in Experimental Example 1, “synthesis of the layered silicon compound from the CaSi ₂ alloy” and “carbonization of the sugar compound on the surface of the layered silicon compound” proceed simultaneously in the same system. It was speculated that a composite of layered silicon and carbon was formed in a one-step synthesis process. More specifically, it was inferred that the formation of layered silicon resulted in the formation of an intermediate with sucrose coordinated on the surface, and the formation of nano-level composites in which the layered silicon was covered with carbon. . In the prior art, in order to combine the layered silicon compound and the carbon compound at the nano level, an expensive organic reagent and a multi-step synthesis process are required, and the yield is extremely low. On the other hand, in Experimental Example 1, “synthesis of a layered silicon compound from a CaSi ₂ alloy” and “carbonization of a sugar compound on the surface of the layered silicon compound” proceed at the same time, which is an inexpensive and one-step synthesis process. It was found that synthesis of a composite compound (composite) of layered silicon and carbon can be realized. Further, it was found that the obtained composite compound formed a nano-level composite structure in which carbon is covered with layered silicon. It was also found that a fired layered silicon carbon composite can be formed by firing the obtained composite compound. It was also found that a high-capacity secondary battery can be constructed by using the obtained composite compound as an electrode material.

[Experimental Examples 2 to 4]
Experimental Example 1 except that the firing temperature at 500 ° C. of the Si / C composite was changed to that shown in Table 1 in “(8) Performance Evaluation of Si / C Composite as Secondary Battery Electrode Material”. In the same manner, the performance evaluation of Experimental Examples 2 to 4 was performed. Table 1 shows the initial discharge capacity and the initial charge / discharge efficiency of the unfired Si / C composite and the Si / C composite fired at 200 ° C., 500 ° C., and 900 ° C. The initial charge / discharge efficiency (%) was obtained by multiplying the value obtained by dividing the initial charge capacity (mAh / g) by the initial discharge capacity (mAh / g) by 100. As shown in Table 1, the unfired Si / C composite exhibited an initial discharge capacity of 1840 mAh / g, but the initial charge / discharge efficiency was as low as 20%, and it was found that the irreversible capacity was relatively large. The initial discharge capacity and initial charge / discharge efficiency of the Si / C composite fired at 200 ° C. both showed low values. On the other hand, in the Si / C composite fired at 500 ° C. and 900 ° C., the initial charge / discharge capacity is increased by 1.3 to 1.7 times compared to the unfired Si / C composite, and the initial charge / discharge efficiency is increased. Improved 2.5 to 3 times. From the above results, it was found that heat treatment at 500 ° C. or higher is effective when the Si / C composite is used as a secondary battery electrode material.

  The present invention can be used in the field of the battery industry.

Claims (9)

A mixing step of mixing a silicon alloy, a sugar compound and an acid;
A method for producing a layered silicon carbon composite comprising: The silicon alloy is calcium disilicide (CaSi ₂ ), the acid is hydrochloric acid, and the sugar compound is selected from the group consisting of glucose, fructose, sucrose, lactose, maltose, trehalose, tulanose, cellobiose, and derivatives thereof. The method for producing a layered silicon carbon composite according to claim 1, wherein the number is one or more selected.   The method for producing a layered silicon carbon composite according to claim 1, comprising a firing step after the mixing step.   A layered silicon-carbon composite in which a diffraction peak derived from layered silicon and a broad peak derived from carbon are observed in a powder X-ray diffraction pattern, and an absorption band derived from -C = C-stretching is observed in an infrared absorption spectrum body.   The layered silicon-carbon composite according to claim 4, wherein the layered silicon includes layered polysilane or layered siloxane, the carbon includes amorphous carbon, and the surface of the layered silicon is covered with the carbon.   The layered silicon carbon composite according to claim 4 or 5, wherein the layered silicon carbon composite has a carbon / layered silicon carbon composite weight ratio of 0.1 to 0.9, which is a ratio of carbon to the layered silicon carbon composite. Carbon composite. The layered silicon carbon composite according to any one of claims 4 to 6, wherein the carbon contained in the layered silicon carbon composite has a specific surface area calculated by the BET method of 1 m ² / g or more and 100 m ² / g or less. body.   An electrode using the layered silicon carbon composite according to any one of claims 4 to 7.   An electricity storage device comprising: a positive electrode; a negative electrode; and an ion conductive medium interposed between the positive electrode and the negative electrode, wherein the electrode according to claim 8 is provided as at least one of the positive electrode and the negative electrode.