PRODUCTION PROCESS FOR CARBON-COATED SILICON MATERIAL

Abstract

A production process for carbon-coated silicon material includes the steps of: a lamellar-silicon-compound production step of reacting CaSi.sub.2 with an acid to turn the CaSi.sub.2 into a lamellar silicon compound; a silicon-material production step of heating the lamellar silicon compound at 300° C. or more to turn the lamellar silicon compound into a silicon material; a coating step of coating the silicon material with carbon; and a washing step of washing the silicon material, or another silicon material undergone the coating step, with a solvent of which the relative permittivity is 5 or more.

Claims

1. A production process for carbon-coated silicon material, the production process comprising the steps of: a lamellar-silicon-compound production step of reacting CaSi.sub.2 with an acid to turn the CaSi.sub.2 into a lamellar silicon compound; a silicon-material production step of heating the lamellar silicon compound at 300° C. or more to turn the lamellar silicon compound into a silicon material; a coating step of coating the silicon material with carbon; and a washing step of washing the silicon material, or another silicon material undergone the coating step, with a solvent of which the relative permittivity is 5 or more.

2. The production process for carbon-coated silicon material as set forth in claim 1, wherein the carbon-coated silicon material is produced in the order of the silicon-material production step, the washing step, and the coating step.

3. The production process for carbon-coated silicon material as set forth in claim 1, wherein the solvent has a relative permittivity of 15 or more.

4. The production process for carbon-coated silicon material as set forth in claim 1, wherein the solvent comprises one or more members selected from the group consisting of water, methanol, ethanol, n-propanol, i-propanol, n-butanol, i-butanol, sec-butanol, tert-butanol, N-methyl-2-pyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, acetonitrile, ethylene carbonate, and propylene carbonate.

5. The production process for carbon-coated silicon material as set forth in claim 1, wherein the washing step is carried out under such a warming condition as being from 40° C. or more to less than a boiling point of the solvent.

6. The production process for carbon-coated silicon material as set forth in claim 1, wherein the washing step is carried out under a stirring condition.

7. The production process for carbon-coated silicon material as set forth in claim 1, wherein the washing step is carried out while doing an ultrasonic treatment.

8. A manufacturing process for secondary battery comprising a step of manufacturing a negative electrode using a carbon-coated silicon material produced by the production process as set forth in claim 1.

9. A carbon-coated silicon material exhibiting a halogen-ion concentration of 50 ppm or less in water when one gram of the carbon-coated silicon material is stirred in 10-g water for one hour.

10. The carbon-coated silicon material as set forth in claim 9 comprising: nanometer-size silicon agglomerated particles in which plate-shaped silicon bodies are laminated in a plurality of pieces in a thickness direction thereof, the plate-shaped silicon bodies having a structure in which nanometer-size silicon particles are arranged lamellarly; and a carbon layer formed on at least some of a surface of the plate-shaped silicon bodies, and having a thickness falling within a range of from 1 nm to 100 nm.

11. The carbon-coated silicon material as set forth in claim 10 comprising the carbon layer exhibiting an average thickness “R” and a standard deviation “σ” of thicknesses thereof, the average thickness “R” and standard deviation “σ” satisfying Relational Expression (1) set forth below:
(“R”/“3σ”)>1.   Relational Expression (1)

12. A secondary battery comprising the carbon-coated silicon material as set forth in claim 9, the carbon-coated silicon material serving as a negative-electrode active material.

Description

EXAMPLES

[0090] Hereinafter, examples and comparative examples are shown to describe the present invention more concretely. Note that the examples in the following descriptions do not limit the present invention at all. In the following descriptions, the term, “part,” means a part by mass, and the term, “%,” means a percentage by mass, unless otherwise specified especially.

First Example

[0091] A carbon-coated silicon material and lithium-ion secondary battery according to a first example were made as described below.

(i) Lamellar-Silicon-Compound Production Step

[0092] A mixed solution of 7-mL HF aqueous solution with 46%-by-mass concentration and 56-mL HCl aqueous solution with 36%-by-mass concentration was held at 0° C. in an ice bath. In an argon-gas atmosphere, the mixed solution was stirred after adding 3.3-g CaSi.sub.2 to the mixed solution. The mixed solution was subjected to a temperature increase up to room temperature after confirming the completion of bubbling from a reaction liquid therein, and was further stirred for another two hours at room temperature. Thereafter, the mixed solution was furthermore stirred for extra 10 minutes after adding 20-mL distilled water to the mixed solution. On the occasion, a yellow-colored powder floated.

[0093] The obtained reaction liquid was filtered. The residual was washed with 10-mL ethanol after washing the residual with 10-mL distilled water, and was then vacuum dried to obtain 2.5-g lamellar silicon compound. Upon analyzing the lamellar silicon compound by a Raman spectrophotometer, a Raman spectrum in which peaks existed at 341±10 cm.sup.−1, 360±10 cm.sup.−1, 498±10 cm.sup.−1, 638±10 cm.sup.−1 and 734±10 cm.sup.−1 was obtained.

(ii) Silicon-Material Production Step

[0094] The aforementioned lamellar silicon compound was weighed out in an amount of one gram. Then, the lamellar silicon compound was subjected to a heat treatment, which was carried out while retaining the lamellar silicon compound at 500° C. for one hour in an argon-gas atmosphere of which the O.sub.2 volume was 1% by volume or less, to obtain a silicon material. An X-ray diffraction measurement (or XRD measurement) using the CuK.sub.α ray was carried out to the silicon material. A halo, which is believed to be derived from Si fine particles, was observed from the obtained XRD chart. Moreover, regarding Si, a size of the Si crystalline was about 7 nm, which was computed by the Scherrer equation using the half-value width of a diffraction peak of Si (111) plane in the XRD chart.

[0095] Note that, in the aforementioned heat treatment, the Si—H bonds of the lamellar silicon compound were cut off to separate the hydrogen atoms, and the cut-off and recombination of the Si—Si bonds occurred. The recombination of the Si—Si bonds not only occurred within the identical layers, but also was able to arise between the neighboring layers, and accordingly nanometer-size silicon primary particles having diameters at nanometer-size level were generated by the recombination. The nanometer-size silicon primary particles agglomerated each other to generate a silicon material serving as nanometer-size silicon agglomerated particles (or secondary particles). When the obtained silicon material was observed by a scanning-type electron microscope, the silicon material was found out to have a structure which was made by laminating plate-shaped silicon bodies in a plurality of pieces in the thickness direction. The plate-shaped silicon bodies were observed to have a thickness of from about 10 nm to about 100 nm, and were observed to have a length of from 0.1 μm to 50 μm in the major-axis direction.

(iii) Coating Step

[0096] The aforementioned silicon material was put in a rotary kiln-type reactor vessel, and was then subjected to thermal CVD to obtain a carbon-coated silicon material. The thermal CVD was carried out under such conditions as at 850° C. and for 30-minute residence time in a propane-gas flow. The reactor vessel had a furnace core tube arranged in the horizontal direction. The furnace core tube was set to rotate at a revolving speed of one rpm. The furnace core tube had a baffle plate arranged on the inner peripheral wall. Thus, the reactor vessel was constructed so as to let contents, which deposited on the baffle plate as the furnace core tube rotated, fall down from the baffle plate at a predetermined height, and accordingly the contents were stirred by the construction.

[0097] When a cross section of the carbon-coated silicon material was observed by a scanning-type electron microscope, a carbon layer was found out to be formed on a surface of the silicon material.

(iv) Washing Step

[0098] One gram of the aforementioned carbon-coated silicon material was added to 10-g pure water serving as the washing agent. Then, the carbon-coated silicon material and pure water were stirred by operating a mechanical stirrer (e.g., “RW20 DIGITAL” produced by AS-ONE Co., Ltd.) at 400 rpm, for five minutes and at room temperature, and were accordingly turned into a suspension liquid. Thereafter, to the suspension liquid, an ultrasonic treatment was carried out for 60 minutes by operating an ultrasonic washer (e.g., “USK-3R” produced by AS-ONE Co., Ltd.) at an oscillation frequency of 40 kHz. A carbon-coated silicon material according to a first example was obtained by filtering out powdered bodies from the obtained suspension liquid and then reduced-pressure drying the powdered bodies at 80° C. for 12 hours. Note that the used pure water was produced by a pure water producing apparatus (e.g., “AUTOSTILL WS200” produced by YAMATO SCIENTIFIC Co., Ltd.).

(v) Lithium-Ion Secondary Battery

[0099] A slurry was prepared by mixing the following each other: the carbon-coated silicon material according to the first example serving as a negative-electrode active material in an amount of 70 parts by mass; natural graphite serving as another negative-electrode active material in an amount of 15 parts by mass; acetylene black serving as a conductive additive in an amount of 5 parts by mass; and a binder solution in an amount of 33 parts by mass. For the binder solution, a solution comprising a polyamide-imide resin dissolved in N-methyl-2-pyrrolidone in an amount of 30% by mass was used. The aforementioned slurry was coated onto a surface of an electrolyzed copper foil (serving as a current collector) of which the thickness was about 20 μm using a doctor blade, and was then dried to form a negative-electrode active-material layer on the copper foil. Thereafter, the current collector and the negative-electrode active-material layer were adhesion joined firmly by a roll pressing machine. The adhesion-joined substance was vacuum dried at 100° C. for 2 hours to form a negative electrode of which the negative-electrode active-material layer had a thickness of 16 μm.

[0100] Using as an evaluation electrode the negative electrode fabricated through the procedures mentioned above, a lithium-ion secondary battery (i.e., a half cell) was fabricated. A metallic lithium foil with 500 μm in thickness was set as the counter electrode.

[0101] The counter electrode was cut out to φ13 mm, and the evaluation electrode was cut out to φ11 mm. Then, a separator composed of a glass filter produced by HOECHST CELANESE Corporation and “Celgard 2400” produced by CELGARD Corporation was set or held between the two to make an electrode assembly. The electrode assembly was accommodated in a battery case (e.g., a member for CR2032-type coin battery, a product of HOSEN Co., Ltd.). A nonaqueous electrolytic solution was injected into the battery case. Note that the nonaqueous electrolytic solution comprised a mixed solvent composed of ethylene carbonate and diethyl carbonate mixed one another in a ratio of 1:1 by volume, and LiPF.sub.6 dissolved in the mixed solvent in a concentration of 1 M. Then, the battery case was sealed hermetically to obtain a lithium-ion secondary battery according to the first example.

Second Example

[0102] Except that the washing conditions at the washing step were set so that the stirring operation was done at 400 rpm and room temperature for 60 minutes, a carbon-coated silicon material and lithium-ion secondary battery according to a second example were obtained in the same manner as the first example.

Third Example

[0103] Except that the washing conditions at the washing step were set so that the stirring operation was done at 400 rpm and 80° C. for 60 minutes, a carbon-coated silicon material and lithium-ion secondary battery according to a third example were obtained in the same manner as the first example.

Fourth Example

[0104] Except that the washing solvent at the washing step was changed to N-methyl-2-pyrolidone (hereinafter, abbreviated sometimes to “NMP”), a carbon-coated silicon material and lithium-ion secondary battery according to a fourth example were obtained in the same manner as the first example.

Fifth Example

[0105] Except that the washing solvent at the washing step was changed to methanol, and that the time for the stirring operation with the mechanical stirrer was extended to 60 minutes, a carbon-coated silicon material and lithium-ion secondary battery according to a fifth example were obtained in the same manner as the first example.

Sixth Example

[0106] Except that the washing solvent at the washing step was changed to a mixed solvent comprising methanol and water in such a volumetric ratio as 1:1, a carbon-coated silicon material and lithium-ion secondary battery according to a sixth example were obtained in the same manner as the fifth example.

Seventh Example

[0107] Except that the washing solvent at the washing step was changed to ethanol, a carbon-coated silicon material and lithium-ion secondary battery according to a seventh example were obtained in the same manner as the fifth example.

Eighth Example

[0108] Except that the washing solvent at the washing step was changed to a mixed solvent comprising ethanol and water in such a volumetric ratio as 1:1, a carbon-coated silicon material and lithium-ion secondary battery according to an eighth example were obtained in the same manner as the fifth example.

Ninth Example

[0109] Except that the temperature at the washing step was set at 50° C., a carbon-coated silicon material and lithium-ion secondary battery according to a ninth example were obtained in the same manner as the eighth example.

Tenth Example

[0110] Except that the washing solvent at the washing step was changed to n-propanol, a carbon-coated silicon material and lithium-ion secondary battery according to a tenth example were obtained in the same manner as the fifth example.

Eleventh Example

[0111] Except that the washing solvent at the washing step was changed to i-propanol, a carbon-coated silicon material and lithium-ion secondary battery according to an eleventh example were obtained in the same manner as the fifth example.

Twelfth Example

[0112] Except that the washing solvent at the washing step was changed to n-butanol, a carbon-coated silicon material and lithium-ion secondary battery according to a twelfth example were obtained in the same manner as the fifth example.

Thirteenth Example

[0113] Except that the washing solvent at the washing step was changed to i-butanol, a carbon-coated silicon material and lithium-ion secondary battery according to a thirteenth example were obtained in the same manner as the fifth example.

Fourteenth Example

[0114] Except that the washing solvent at the washing step was changed to sec-butanol, a carbon-coated silicon material and lithium-ion secondary battery according to a fourteenth example were obtained in the same manner as the fifth example.

Fifteenth Example

[0115] Except that the washing solvent at the washing step was changed to tert-butanol, a carbon-coated silicon material and lithium-ion secondary battery according to a fifteenth example were obtained in the same manner as the fifth example.

Sixteenth Example

[0116] Except that the washing solvent at the washing step was changed to N,N-dimethylformamide (hereinafter, abbreviated sometimes to “DMF”), a carbon-coated silicon material and lithium-ion secondary battery according to a sixteenth example were obtained in the same manner as the fifth example.

Seventeenth Example

[0117] Except that the washing solvent at the washing step was changed to N,N-dimethylacetamide (hereinafter, abbreviated sometimes to “DMA”), a carbon-coated silicon material and lithium-ion secondary battery according to a seventeenth example were obtained in the same manner as the fifth example.

Eighteenth Example

[0118] Except that the washing solvent at the washing step was changed to dimethyl sulfoxide (hereinafter, abbreviated sometimes to “DMSO”), a carbon-coated silicon material and lithium-ion secondary battery according to an eighteenth example were obtained in the same manner as the fifth example.

Nineteenth Example

[0119] Except that the washing solvent at the washing step was changed to acetonitrile, a carbon-coated silicon material and lithium-ion secondary battery according to a nineteenth example were obtained in the same manner as the fifth example.

Twentieth Example

[0120] Except that the washing solvent at the washing step was changed to propylene carbonate, a carbon-coated silicon material and lithium-ion secondary battery according to a twentieth example were obtained in the same manner as the fifth example.

First Comparative Example

[0121] Except that no washing step was carried out, a carbon-coated silicon material and lithium-ion secondary battery according to a first comparative example were obtained in the same manner as the first example.

Second Comparative Example

[0122] Except that the washing solvent at the washing step was changed to dimethyl carbonate (hereinafter, abbreviated sometimes to “DMC”), a carbon-coated silicon material and lithium-ion secondary battery according to a second comparative example were obtained in the same manner as the first example.

Third Comparative Example

[0123] Except that the washing solvent at the washing step was changed to diethyl carbonate (hereinafter, abbreviated sometimes to “DEC”), a carbon-coated silicon material and lithium-ion secondary battery according to a third comparative example were obtained in the same manner as the first example.

First Evaluative Example

[0124] To the carbon-coated silicon materials according to the first through twentieth example and first comparative example, the following test was carried out.

[0125] One gram of the respective carbon-coated silicon materials was stirred within 10-g water for one hour to turn the carbon-coated silicon materials and water into suspension liquids. After filtering the respective suspension liquids, fluorine- and chlorine-ion concentrations in the obtained filtrates were measured by ion chromatography. Table 2 shows the results. Note that the used water was produced by a pure-water producing apparatus (e.g., “AUTOSTILL WS200” produced by YAMATO SCIENTIFIC Co., Ltd.).

TABLE-US-00002 TABLE 2 Sum of Fluorine- Washing Washing and Chlorine-ion Solvent Conditions Concentrations 1st Ex. Water Room Temp., 5-min 20 ppm Stirring and 60-min Ultrasonic Treatment 2nd Ex. Water Room Temp., and 40 ppm 60-min Stirring 3rd Ex. Water 80° C. and 60- 25 ppm min Stirring 4th Ex. NMP Room Temp., 5-min 48 ppm Stirring and 60-min Ultrasonic Treatment 5th Ex. Methanol Room Temp., 60-min 40 ppm Stirring and 60-min Ultrasonic Treatment 6th Ex. Methanol Room Temp., 60-min 35 ppm and Water Stirring and 60-min Ultrasonic Treatment 7th Ex. Ethanol Room Temp., 60-min 40 ppm Stirring and 60-min Ultrasonic Treatment 8th Ex. Ethanol Room Temp., 60-min 35 ppm and Water Stirring and 60-min Ultrasonic Treatment 9th Ex. Ethanol 50° C., 60-min 25 ppm and Water Stirring and 60-min Ultrasonic Treatment 10th Ex. n-Propanol Room Temp., 60-min 43 ppm Stirring and 60-min Ultrasonic Treatment 11th Ex. i-Propanol Room Temp., 60-min 46 ppm Stirring and 60-min Ultrasonic Treatment 12th Ex. n-Butanol Room Temp., 60 min 40 ppm Stirring and 60-min Ultrasonic Treatment 13th Ex. i-Butanol Room Temp., 60-min 42 ppm Stirring and 60-min Ultrasonic Treatment 14th Ex. sec-Butanol Room Temp., 60-min 48 ppm Stirring and 60-min Ultrasonic Treatment 15th Ex. tert-Butanol Room Temp., 60-min 42 ppm Stirring and 60-min Ultrasonic Treatment 16th Ex. DMF Room Temp., 60-min 44 ppm Stirring and 60-min Ultrasonic Treatment 17th Ex. DMA Room Temp., 60-min 45 ppm Stirring and 60-min Ultrasonic Treatment 18th Ex. DMSO Room Temp., 60-min 45 ppm Stirring and 60-min Ultrasonic Treatment 19th Ex. Acetonitrile Room Temp., 60-min 30 ppm Stirring and 60-min Ultrasonic Treatment 20th Ex. Propylene Room Temp., 60-min 42 ppm Carbonate Stirring and 60-min Ultrasonic Treatment 1st Comp. Not Applicable Not Applicable 350 ppm  Ex.

[0126] The washing step was thus supported to remarkably decline the concentration of the anions which were derived from the acids in the carbon-coated silicon materials.

Second Evaluative Example

[0127] The lithium-ion secondary batteries according to the first through twentieth examples and first through third comparative examples were subjected to a discharging mode or operation which was carried out with a current of 0.2 mA and at a temperature of 25° C., and were subsequently subjected to a charging mode or operation which was carried out with a current of 0.2 mA and at a temperature of 25° C. {(“Charged Capacities”/“Discharged Capacities”)×100} were computed for the charging and discharging modes or operations, and were labeled as “Initial Efficiency (%),” respectively.

[0128] In addition, each of the lithium-ion secondary batteries was subjected to cyclic modes or operations which were carried out repeatedly for 30 cycles as follows: a discharging mode or operation which was carried out with a current of 0.2 mA and at a temperature of 25° C. until a voltage of the evaluation electrode became 0.01 V to the counter electrode; after 10 minutes had passed since the discharging mode or operation, a charging mode or operation which was carried out with a current of 0.2 mA and at a temperature of 25° C. until a voltage of the evaluation electrode became 1 V to the counter electrode; and an intermitting or pausing mode or operation for 10 minutes. Such a value as [100×{(“Post-30-cylcle Charged Capacity”)/(“Post-1-cycle Charged Capacity”)}] was computed, and was labeled as “Capacity Maintained Rate.” Note that, in the second evaluative example, “having Li occlude (or sorb) in the evaluation electrode” is referred to as “discharging,” and “having Li release (or desorb) from the evaluation electrode” is referred to as “charging.” Table 3 shows the results.

TABLE-US-00003 TABLE 3 Capacity Washing Washing Initial Maintained Solvent Conditions Efficiency Rate 1st Ex. Water Room Temp., 5-min Stirring 75% 90% and 60-min Ultrasonic Treat. 2nd Ex. Water Room Temp., and 60-min Stirring 75% 87% 3rd Ex. Water 80° C. and 60-min Stirring 75% 90% 4th Ex. NMP Room Temp., 5-min Stirring 75% 89% and 60-min Ultrasonic Treat. 5th Ex. Methanol Room Temp., 60-min Stirring 75% 91% and 60-min Ultrasonic Treat. 6th Ex. Methanol and Water Room Temp., 60-min Stirring 75% 90% and 60-min Ultrasonic Treat. 7th Ex. Ethanol Room Temp., 60-min Stirring 75% 89% and 60-min Ultrasonic Treat. 8th Ex. Ethanol and Water Room Temp., 60-min Stirring 75% 88% and 60-min Ultrasonic Treat. 9th Ex. Ethanol and Water 50° C., 60-min Stirring 75% 90% and 60-min Ultrasonic Treat. 10th Ex. n-Propanol Room Temp., 60-min Stirring 75% 88% and 60-min Ultrasonic Treat. 11th Ex. i-Propanol Room Temp., 60-min Stirring 75% 87% and 60-min Ultrasonic Treat. 12th Ex. n-Butanol Room Temp., 60 min Stirring 75% 85% and 60-min Ultrasonic Treat. 13th Ex. i-Butanol Room Temp., 60-min Stirring 75% 84% and 60-min Ultrasonic Treat. 14th Ex. sec-Butanol Room Temp., 60-min Stirring 75% 88% and 60-min Ultrasonic Treat. 15th Ex. tert-Butanol Room Temp., 60-min Stirring 75% 88% and 60-min Ultrasonic Treat. 16th Ex. DMF Room Temp., 60-min Stirring 75% 84% and 60-min Ultrasonic Treat. 17th Ex. DMA Room Temp., 60-min Stirring 75% 84% and 60-min Ultrasonic Treat. 18th Ex. DMSO Room Temp., 60-min Stirring 75% 84% and 60-min Ultrasonic Treat. 19th Ex. Acetonitrile Room Temp., 60-min Stirring 75% 88% and 60-min Ultrasonic Treat. 20th Ex. Propylene Room Temp., 60-min Stirring 75% 88% Carbonate and 60-min Ultrasonic Treat. 1st Comp. Not Applicable Not Applicable 73% 83% Ex. 2nd Comp. DMC Room Temp., 5-min Stirring 72% 45% Ex. and 60-min Ultrasonic Treat. 3rd Comp. DEC Room Temp., 5-min Stirring 68% 75% Ex. and 60-min Ultrasonic Treat.

[0129] The lithium-ion secondary batteries according to the first through twentieth examples were superior to the lithium-ion secondary batteries according to the first through third comparative examples in both of the initial efficiency and capacity maintained rate.

[0130] The results of the first evaluative example and second evaluative example supported that the washing step in the production process according to the present invention removes undesirable impurities to make suitable carbon-coated silicon materials obtainable.

Twenty-First Example

[0131] A carbon-coated silicon material and lithium-ion secondary battery according to a twenty-first example were made as described below.

(i) Lamellar-Silicon-Compound Production Step

[0132] A mixed solution of 7-mL HF aqueous solution with 46%-by-mass concentration and 56-mL HCl aqueous solution with 36%-by-mass concentration was held at 0° C. in an ice bath. In an argon-gas atmosphere, the mixed solution was stirred after adding 3.3-g CaSi.sub.2 to the mixed solution. The mixed solution was subjected to a temperature increase up to room temperature after confirming the completion of bubbling from a reaction liquid therein, and was further stirred for another two hours at room temperature. Thereafter, the mixed solution was furthermore stirred for extra 10 minutes after adding 20-mL distilled water to the mixed solution. On the occasion, a yellow-colored powder floated.

[0133] The obtained reaction liquid was filtered. The residual was washed with 10-mL ethanol after washing the residual with 10-mL distilled water, and was then vacuum dried to obtain 2.5-g lamellar silicon compound.

(ii) Silicon-Material Production Step

[0134] The aforementioned lamellar silicon compound was subjected to a heat treatment, which was carried out while retaining the lamellar silicon compound at 500° C. for one hour in an argon-gas atmosphere of which the O.sub.2 volume was 1% by volume or less, to obtain a silicon material.

(iii) Coating Step

[0135] The aforementioned silicon material was put in a rotary kiln-type reactor vessel, and was then subjected to thermal CVD to obtain a carbon-coated silicon material. The thermal CVD was carried out under such conditions as at 850° C. and for 30-minute residence time in a propane-gas flow in a furnace core tube which rotated at a revolving speed of one rpm.

(iv) Washing Step

[0136] 100 g of the aforementioned carbon-coated silicon material was added to 150-mL pure water serving as the washing agent. Then, the carbon-coated silicon material and pure water were stirred by operating a mechanical stirrer (e.g., “RW20 DIGITAL” produced by AS-ONE Co., Ltd.) at 400 rpm, for 60 minutes and at room temperature, and were accordingly turned into a suspension liquid. After filtering the obtained suspension liquid, powdered bodies was reduced-pressure dried at 120° C. for five hours. The post-drying powdered bodies were broken into pieces in a mortar, and were then passed through a sieve to obtain a carbon-coated silicon material according to the twenty-first example. Note that the used pure water was produced by a pure water producing apparatus (e.g., “AUTOSTILL WS200” produced by YAMATO SCIENTIFIC Co., Ltd.).

(v) Lithium-Ion Secondary Battery

[0137] A slurry was prepared by mixing the following each other: the carbon-coated silicon material according to the twenty-first example serving as a negative-electrode active material in an amount of 70 parts by mass; natural graphite serving as another negative-electrode active material in an amount of 15 parts by mass; acetylene black serving as a conductive additive in an amount of 5 parts by mass; and a binder solution in an amount of 33 parts by mass. For the binder solution, a solution comprising a polyamide-imide resin dissolved in N-methyl-2-pyrrolidone in an amount of 30% by mass was used. The aforementioned slurry was coated onto a surface of an electrolyzed copper foil (serving as a current collector) of which the thickness was about 20 μm using a doctor blade, and was then dried to form a negative-electrode active-material layer on the copper foil. Thereafter, the current collector and the negative-electrode active-material layer were adhesion joined firmly by a roll pressing machine. The adhesion-joined substance was vacuum dried at 100° C. for 2 hours to form a negative electrode of which the negative-electrode active-material layer had a thickness of 16 μm.

[0138] A positive electrode was made as described below.

[0139] A slurry was prepared by mixing the following each other: LiNi.sub.bCo.sub.cMn.sub.dO.sub.2 (where “b”+“c”+“d”=1) serving as a positive-electrode active material; acetylene black serving as a conductive additive; polyvinylidene fluoride serving as a binder; and N-methyl-2-pyrrolidone. An aluminum foil with a thickness of 20 μm was readied to serve as a current collector for positive electrode. Onto a surface of the aluminum foil, the aforementioned slurry was coated so as to be in the shape of a film using a doctor blade. The N-methyl-2-pyrrolidone was removed by drying the aluminum foil with the slurry coated thereon at 80° C. for 20 minutes. Thereafter, the aluminum foil with the active-material layer formed thereon was pressed to obtain a joined substance. The obtained joined substance was heat dried at 120° C. for six hours with a vacuum drier to obtain an aluminum foil with the positive-electrode active-material layer formed thereon. The aluminum foil with the positive-electrode active-material layer formed thereon was used as a positive electrode.

[0140] Between the positive electrode and the negative electrode, a rectangle-shaped sheet serving as a separator and comprising a polypropylene/polyethylene/polypropylene three-layered-construction resinous film with 27×32 mm in size and 25 μm in thickness was interposed or held to make a polar-plate subassembly. After covering the polar-plate subassembly with laminated films in which two pieces made a pair and then sealing the laminated films at the three sides, an electrolytic solution was injected into the laminated films which had been turned into a bag shape. As for the electrolytic solution, a solution was used: the solution comprised a solvent in which ethylene carbonate and diethyl carbonate had been mixed with one another in such a volumetric ratio as 3:7; and LiPF.sub.6 dissolved in the solvent so as to make one mol/L. Thereafter, the remaining one side was sealed to obtain a laminated-type lithium-ion secondary battery according to the twenty-first example in which the four sides were sealed air-tightly and in which the polar-plate subassembly and electrolytic solution were closed hermetically. Note that the positive electrode and negative electrode were equipped with a tab connectable electrically with the outside, respectively, and the tabs extended out partially to the outside of the laminated-type lithium-ion secondary battery.

Fourth Comparative Example

[0141] Except that no washing step was carried out, a carbon-coated silicon material and lithium-ion secondary battery according to a fourth comparative example were obtained in the same manner as the twenty-first example.

Third Evaluative Example

[0142] Each of the lithium-ion secondary batteries according to the twenty-first example and fourth comparative example were subjected to charging and discharging modes or operations which were carried out between two voltages, namely, from 2.5 V to 4.5 V, at a rate of 1 C. The thus obtained discharged capacities were labeled an initial capacity of each of the batteries.

[0143] The respective batteries were subjected to a charging mode or operation which was carried out up to 85% of the SOC (or the state of charge). Then, the post-charging respective batteries were held to stand still in a 60° C. constant-temperature chamber, and were then stored therein for 30 days.

[0144] The respective post-storing batteries were subjected to the charging and discharging modes or operations which were carried out between two voltages, namely, from 2.5 V to 4.5 V, at a rate of 1 C. The thus obtained discharged capacities were labeled a post-storing capacity of each of the batteries. A post-storing capacity maintained rate of the respective batteries was computed by the following equation. Table 4 shows the results. Note that the results shown in Table 4 are an average value when N=2, respectively.

Post-storing Capacity Maintained Rate (%)={(Post-storing Capacity)/(Initial Capacity)}×100

TABLE-US-00004 TABLE 4 Post-storing Capacity Maintained Rate Twenty-first Ex. 78% Fourth Comp. Ex. 69%

[0145] A lithium-ion secondary battery comprising a carbon-coated silicon material according to the present invention was supported to excel in the post-storing capacity maintained rate as well.