METHOD FOR PRODUCING SILICON OXIDE POWDER AND NEGATIVE ELECTRODE MATERIAL

Abstract

To produce a silicon oxide-based negative electrode material containing Li and having uniform distribution of a Li concentration both inside particles and between particles although a C-coating film is formed on a surface, and yet in which generation of SiC is suppressed. A SiO gas and a Li gas are simultaneously generated by heating a Si-lithium silicate-containing raw material under reduced pressure. The Si-lithium silicate-containing raw material includes Si, Li, and O, in which a part of the Si is present as a Si simple substance and the Li is present as lithium silicate. By cooling the generated gases, Li-containing silicon oxide having an average composition of SiLi.sub.xO.sub.y (0.05<x<y and 0.5<y<1.5 are satisfied) is prepared. After adjusting the particle size, a C-coating film having an average film thickness of 0.5 to 10 nm is formed on a surface of particles at a treatment temperature of 900° C. or less.

Claims

1. A negative electrode material being a powder having an average composition represented by SiLi.sub.xO.sub.y wherein 0.05<x<y, 0.5<y<1.5, and an average particle diameter of 1 μm or more are satisfied, wherein when particles of the powder are subjected to a cross-sectional TEM observation, ten particles each having a minor axis of 1 μm or more are extracted, and a Li concentration L1 in a position at a depth of 50 nm from an outermost surface and a Li concentration L2 in a position at a depth of 400 nm from the outermost surface are measured for each particle, L1/L2 satisfies 0.8<L1/L2<1.2 and a coefficient of variation of L2 is 0.25 or less for all the particles, and further comprising a C-coating film having an average film thickness of 0.5 to 10 nm on a surface of particles, wherein a SiC peak is not present in XRD measurement.

2. The negative electrode material according to claim 1, wherein a Si peak is not present in XRD measurement.

3. A lithium ion secondary battery comprising the negative electrode material according to claim 1 as a negative electrode active material in a negative electrode.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0048] FIG. 1 is an XRD profile diagram of a C-coating Li-containing silicon oxide powder in accordance with an Example of the present invention.

[0049] FIG. 2 is an XRD profile diagram of a C-coating Li-containing silicon oxide powder in accordance with another Example of the present invention.

[0050] FIG. 3 is an XRD profile diagram of a C-coating Li-containing silicon oxide powder in accordance with Comparative Example of the present invention.

DESCRIPTION OF EMBODIMENTS

[0051] Hereinafter, the embodiments of the present invention will be described. A typical method for producing a negative electrode material according to the present invention, that is, a typical method for producing a silicon oxide powder according to the present invention will be described below.

[0052] Firstly, a Si powder and a lithium-silicate powder, for example, a Li.sub.2Si.sub.2O.sub.5 powder, as raw materials containing Si, O, and Li, are mixed with each other. A SiO.sub.2 powder is mixed to adjust the O content if necessary. The mixing ratio of the powders is adjusted so that the average composition SiLi.sub.xO.sub.y of the powder mixture satisfies 0.05<x<y and 0.5<y<1.5, and the element ratio of Li, Si, and O (Li:Si:O) becomes a target value (for example, 1:0.4:1).

[0053] Next, the above powder mixture as the raw material are placed in a reaction vessel, and heated under reduced pressure to generate a gas from, in particular, lithium silicate in the mixed raw material. In the gas-generating reaction herein, a SiO gas and a Li gas are generated simultaneously. With reference to chemical formulae, the general formula is estimated to be the formula (1). When lithium silicate is Li.sub.2Si.sub.2O.sub.5, the formula is estimated to be the formula (2). As described above, lithium silicate is represented by the general formula xLi.sub.2O.ySiO.sub.2.

[Chem. 1]

(x+y)Si+(xLi.sub.2O.ySiO.sub.2).fwdarw.(x+2y)SiO↑+2xLi↑  (1)

3Si+Li.sub.2Si.sub.2O.sub.5.fwdarw.5SiO↑+2Li↑  (2)

[0054] As shown in the formulae (1) and (2), a SiO gas and a Li gas are generated simultaneously from lithium silicate by heating in the coexistence of a Si simple substance. The reaction herein is thought to be a reductive reaction by Si.

[0055] While gases are generated from the raw material in the reaction vessel, the generated gases are deposited by cooling on a surface of a vapor deposition table disposed in an upper portion in the reaction vessel. After the end of the reaction, a deposit is collected from the surface of the vapor deposition table. The collected deposit is a Li-containing silicon oxide material, and this material is pulverized and the particle size is adjusted, following C-coating treatment. Thus, a powder for a negative electrode material is obtained.

[0056] Since a SiO gas and a Li gas are simultaneously generated from the raw material in the reaction vessel, the concentration distribution of a gas mixture of the both gases is uniform. Accordingly, a deposit obtained by cooling the gas mixture on the same surface of a vapor deposition table will also have a uniform concentration distribution. Therefore, a powder obtained by pulverizing the deposit will have a uniform distribution of a Li concentration both between particles of the powder and in individual particles of the powder. When the powder is used as a powder for a negative electrode material, generation of a Li-rich phase is suppressed, and thereby, the reactivity is reduced and battery performance is improved.

[0057] A cooling temperature, that is, a deposition temperature of the simultaneously generated gases is desirably 900° C. or less from the viewpoint of suppressing growth of Si crystal particles. When the cooling temperature is 700° C. or less, Li-containing silicon oxide in which a Si crystal peak is not present in XRD is obtained.

[0058] The C-coating treatment is performed by a thermal CVD method using a hydrocarbon gas as a carbon source, for example, by heat treatment in an atmosphere of a gas mixture of argon and propane. The C-coating treatment herein is performed at 900° C. or less, and preferably at 800° C. or less. A heating atmosphere, a heating temperature, a heating time, and the like, are managed such that an average film thickness is 0.5 to 10 nm. This average film thickness corresponds to treatment time of three hours or less in a case where the treatment temperature is 900 to 800° C.

[0059] Such C-coating treatment improves the electrical conductivity between powder particles constituting a negative electrode, and the electrical conductivity between the negative electrode and its base, i.e., a current collector, resulting in enabling the battery properties, in particular, cycle properties, to be improved. In addition, since the Li-containing silicon oxide powder subjected to the C-coating is prevented from locally increasing the reactivity due to the uniformity of the distribution of a Li concentration, the heating temperature in the C-coating treatment is restricted to be low, and the average film thickness is also restricted to be small, the reaction for forming SiC on the surface of particles is suppressed in the C-coating treatment, and generation of SiC is prevented. In addition, when the treatment temperature in the C-coating treatment is restricted to 700° C. or less, generation of Si crystal is also suppressed.

[0060] In another embodiment, a Si powder is mixed with a LiOH powder. A SiO.sub.2 powder is mixed to adjust the O content if necessary. The resulting powder mixture as a primary raw material is placed in a reaction vessel, and heated and calcined in an atmosphere of Ar. A reaction when LiOH is heated in the coexistence of a Si simple substance is estimated to be represented by the former part of the formula (3).

[Chem. 2]

4Si+4LiOH.fwdarw.3Si+Li.sub.4SiO.sub.4+2H.sub.2↑

3Si+Li.sub.4SiO.sub.4.fwdarw.4SiO↑+4Li↑  (3)

4Si+2Li.sub.2CO.sub.3.fwdarw.3Si+Li.sub.4SiO.sub.4+2CO↑

3Si+Li.sub.4SiO.sub.4.fwdarw.4SiO↑+4Li↑  (4)

[0061] As shown in the former part of the formula (3), by heating and calcining LiOH in the coexistence of a Si simple substance, lithium silicate (Li.sub.4SiO.sub.4) is generated while an undesired element H is removed as a gas component. As a result, the calcined material is a mixture of lithium silicate (Li.sub.4SiO.sub.4) and a residual Si simple substance. This corresponds to the raw material containing Si, O, and Li used in the aforementioned embodiment.

[0062] Then, the resulting calcined material as a secondary raw material is continued to be heated under reduced pressure. Then, as shown in the latter part of the formula (3), in the secondary raw material, by heating lithium silicate (Li.sub.4SiO.sub.4) in the coexistence of a Si simple substance, a Si gas and a Li gas are generated simultaneously from the lithium silicate (Li.sub.4SiO.sub.4). The generated gases herein are cooled and collected on the same surface to obtain a powder for a negative electrode material having a uniform distribution of a Li concentration as in the aforementioned embodiment. Instead of continuously heating the secondary raw material, the secondary raw material may be newly heated again.

[0063] Thus, a raw material including a Si simple substance and lithium silicate (a Si lithium silicate-containing raw material) can be obtained by heating and calcining a primary raw material including a Si simple substance and LiOH. By heating the resulting raw material as a secondary raw material, a SiO gas and a Li gas can be simultaneously obtained.

[0064] Instead of LiOH, LiCO.sub.3 can also be used. In other words, a Si powder is mixed with a Li.sub.2SiO.sub.3 powder. A SiO.sub.2 powder is mixed to adjust the O content, if necessary. The resulting powder mixture as a primary raw material is placed in a reaction vessel and heated and calcined in an atmosphere of Ar. A reaction in which Li.sub.2CO.sub.3 is heated in the coexistence of a Si simple substance is estimated to be represented by the former part of the formula (4).

[0065] As shown in the former part of the formula (4), by heating and calcining Li.sub.2CO.sub.3 in the coexistence of a Si simple substance, lithium silicate (Li.sub.4SiO.sub.4) is generated while an undesired element C is removed as a gas component. As a result, the calcined material is a mixture of lithium silicate (Li.sub.4SiO.sub.4) and a residual Si simple substance. This corresponds to the raw material containing Si, O, and Li used in the aforementioned embodiment.

[0066] Then, the resulting calcined material as a secondary raw material is continued to be heated under reduced pressure. As shown in the latter part of the formula (4), in the secondary raw material, when lithium silicate (Li.sub.4SiO.sub.4) is heated in the coexistence of a Si simple substance, a Si gas and a Li gas are simultaneously generated from the lithium silicate (Li.sub.4SiO.sub.4). When the generated gases herein are cooled and collected on the same surface, a Li-containing silicone oxide powder having a uniform distribution of a Li concentration can be obtained as in the aforementioned embodiment. Instead of continuously heating the secondary raw material, the secondary raw material may be newly heated again.

[0067] As described above, a raw material including a Si simple substance and lithium silicate (a Si lithium silicate-containing raw material) can be obtained also by heating and calcining a primary raw material including a Si simple substance and Li.sub.2CO.sub.3. By heating the resulting raw material as a secondary raw material, a SiO gas and a Li gas can be generated simultaneously. LiOH and Li.sub.2CO.sub.3 can also be used instead of using LiOH or Li.sub.2CO.sub.3.

[0068] The resulting Li-containing silicon oxide powder is subjected to the C-coating treatment is the same as in the first embodiment.

[0069] Note here that the chemical reactions in the embodiments are represented by the chemical formulae (1) to (4), but these merely represent putative reactions in model cases in which these phenomena are simplified. The actual reactions may likely be more complicated due to addition of SiO.sub.2 and the like for adjusting the O content.

EXAMPLES

Example 1

[0070] A Si powder and a Li.sub.2Si.sub.2O.sub.5 powder were mixed in a molar ratio of 3:1. The element ratio of the powder mixture is Si:Li:O=1:0.4:1. This powder mixture as a raw material was placed in a reaction vessel and heated to 1400° C. under reduced pressure of 1 Pa. Generated gases were deposited and collected on a substrate, which had been cooled to 600° C., disposed in the upper portion of the reaction vessel. When the composition of the collected deposit was analyzed by the ICP emission spectroscopy and the infrared absorption method, the deposit was Li-containing silicon oxide having an average composition represented by SiLi.sub.xO.sub.y (x=0.38, y=1.06).

[0071] The collected Li-containing silicon oxide deposit was pulverized into a powder in an agate mortar. Then, the powder was subjected to heat treatment as C-coating treatment in a rotary heating furnace at 850° C. for two hours while propane gas was introduced. When the average particle diameter of a Li-containing SiO powder after the C-coating was examined by the laser diffraction type particle size distribution measurement, D.sub.50 was 5.4 μm. Furthermore, a surface area measured using a BET specific surface area measuring device was 2.4 m.sup.2/g, the C content measured by the infrared absorption method was 4.8 wt %, and the average film thickness of the C-coating measured therefrom was 9.1 nm.

[0072] The resulting C-coating Li-containing SiO powder particles were subjected to the cross-sectional TEM observation, and ten particles each having a minor axis of 1 μm or more were extracted. Then, EELS measurement was performed in a region of 20 nm in the longitudinal direction and 400 nm in the transverse direction in a position at a depth of 50 nm from an outmost surface of each of the particles to obtain a Si spectral intensity and a Li spectral intensity. The ratio of the Li spectral intensity to the Si spectral intensity was defined as a Li concentration L1 on the surface of particles. The ratio of a Li spectral intensity to a Si spectral intensity was obtained also in a position at a depth of 400 nm from the outmost surface of the particles by the similar procedure. This was defined as a Li concentration L2 inside the particles. For each of the ten particles, L1/L2 was determined, and the standard deviation and a coefficient of variation of L2 were determined.

[0073] Furthermore, the resulting C-coating Li-containing SiO powder particles were subjected to XRD measurement by an X-ray diffraction device using a CuKα ray in diffraction angle interval of 0.2°. The XRD profile is shown in FIG. 1. Clear peaks of Si, Li.sub.2Si.sub.2O.sub.5, and Li.sub.2SiO.sub.3 were observed, but a peak of SiC was not present.

Example 2

[0074] The deposit obtained in Example 1 was pulverized into a powder, and then the powder was subjected to heat treatment as C-coating treatment in a rotary heating furnace at 700° C. for 12 hours while propane gas was introduced. When the average particle diameter of the powder after the C-coating was examined by the laser diffraction type particle size distribution measurement, D.sub.50 was 5.3 μm. Furthermore, a surface area measured by a BET specific surface area measuring device was 2.5 m.sup.2/g, the C content measured by the infrared absorption method was 2.6 wt %, and the average film thickness of the C-coating measured therefrom was 4.7 nm.

[0075] Furthermore, the resulting C-coating Li-containing SiO powder particles were subjected to the cross-sectional TEM observation, and ten particles each having a minor axis of 1 μm or more were extracted. Then, each of the particles was subjected to the cross-sectional TEM observation and the EELS measurement in the same manner as Example 1 to obtain L1/L2, and standard deviation and a coefficient of variation of L2.

[0076] In addition, the resulting C-coating Li-containing SiO powder particle was subjected to XRD measurement in the same manner as Example 1. The XRD profile is shown in FIG. 2. Clear peaks of Li.sub.2Si.sub.2O.sub.5 and Li.sub.2SiO.sub.3 were observed, but peaks of SiC and Si were not present.

Example 3

[0077] A Si powder, a SiO.sub.2 powder, and a Li.sub.2Si.sub.2O.sub.5 powder were mixed in a molar ratio of 11:5:2. The element ratio of the powder mixture is Si:Li:O=1:0.2:1. This powder mixture as a raw material was placed in a reaction vessel and heated to 1400° C. under reduced pressure of 1 Pa. Generated gases were deposited and collected on a substrate, which had been cooled to 600° C., disposed in the upper portion of the reaction vessel. When the composition of the collected deposit was analyzed by the ICP emission spectroscopy and the infrared absorption method, the deposit was Li-containing silicon oxide having an average composition of SiLi.sub.xO.sub.y (x=0.18, y=1.02).

[0078] The collected Li-containing silicon oxide deposit was pulverized into a powder in an agate mortar. Then, the powder was subjected to heat treatment as C-coating treatment in a rotary heating furnace at 850° C. for two hours while propane gas was introduced. When the average particle diameter of a Li-containing silicon oxide powder after the C-coating was examined by the laser diffraction type particle size distribution measurement, D.sub.50 was 5.4 μm. Furthermore, a surface area measured using a BET specific surface area measuring device was 2.2 m.sup.2/g, the C content measured by the infrared absorption method was 4.4 wt %, and the average film thickness of the C-coating measured therefrom was 9.1 nm.

[0079] The resulting C-coating Li-containing SiO powder particles were subjected to the cross-sectional TEM observation, and ten particles each having a minor axis of 1 μm or more were extracted. Then, EELS measurement was performed in a region of 20 nm in the longitudinal direction and 400 nm in the transverse direction in a position at a depth of 50 nm from an outmost surface of each of the particles to obtain a Si spectral intensity and a Li spectral intensity. The ratio of the Li spectral intensity to the Si spectral intensity was defined as a Li concentration L1 on the surface of particles. The ratio of a Li spectral intensity to a Si spectral intensity was obtained also in a position at a depth of 400 nm from the outmost surface of the particles by the similar procedure. This was defined as a Li concentration L2 inside the particles. For each of the ten particles, L1/L2 was determined, and the standard deviation and a coefficient of variation of L2 were determined.

[0080] In addition, the resulting C-coating Li-containing SiO powder particles were subjected to XRD measurement by an X-ray diffraction device using a CuKα ray in diffraction angle interval of 0.2°. Clear peaks of Si and Li.sub.2Si.sub.2O.sub.5 were observed, but a peak of SiC was not present.

Example 4

[0081] A Si powder and a Li.sub.2Si.sub.2O.sub.5 powder were mixed in a molar ratio of 2:1. The element ratio of the powder mixture is Si:Li:O=1:0.67:1. This powder mixture as a raw material was placed in a reaction vessel, and heated to 1400° C. under reduced pressure of 1 Pa. Generated gases were deposited and collected on a substrate, which had been cooled to 600° C., disposed in the upper portion of the reaction vessel. When the composition of the collected deposit was analyzed by the ICP emission spectroscopy and the infrared absorption method, the deposit was Li-containing silicon oxide having an average composition of SiLi.sub.xO.sub.y (x=0.95, y=1.05).

[0082] The collected Li-containing silicon oxide deposit was pulverized into a powder in an agate mortar. Then, the resulting powder was subjected to heat treatment as C-coating treatment in a rotary heating furnace at 850° C. for two hours while propane gas was introduced. When the average particle diameter of a Li-containing silicon oxide powder after the C-coating was examined by the laser diffraction type particle size distribution measurement, D.sub.50 was 5.1 μm. Furthermore, a surface area measured using a BET specific surface area measuring device was 2.1 m.sup.2/g, the C content measured by the infrared absorption method was 4.5 wt %, and the average film thickness of the C-coating measured therefrom was 9.7 nm.

[0083] Particles of the resulting C-coating Li-containing SiO powder were subjected to the cross-sectional TEM observation, and ten particles each having a minor axis of 1 μm or more were extracted. Then, EELS measurement was performed in a region of 20 nm in the longitudinal direction and 400 nm in the transverse direction in a position at a depth of 50 nm from an outmost surface of each of the particles to obtain a Si spectral intensity and a Li spectral intensity. The ratio of the Li spectral intensity to the Si spectral intensity was defined as a Li concentration L1 on the surface of particles. The ratio of a Li spectral intensity to a Si spectral intensity was obtained also in a position at a depth of 400 nm from the outmost surface of the particles by the similar procedure. This was defined as a Li concentration L2 inside the particles. For each of the ten particles, L1/L2 was determined, and the standard deviation and a coefficient of variation of L2 were determined.

[0084] In addition, the resulting C-coating Li-containing SiO powder particles were subjected to XRD measurement by an X-ray diffraction device using a CuKα ray in diffraction angle interval of 0.2°. Clear peaks of Si, Li.sub.2SiO.sub.3, and Li.sub.4SiO.sub.4 were observed, but a peak of SiC was not present.

Comparative Example 1

[0085] A Si powder and a SiO.sub.2 powder were mixed in a molar ratio of 1:1. This powder mixture was placed in a reaction vessel and heated to 1400° C. under reduced pressure of 1 Pa. Generated gases were deposited and collected on a substrate, which had been cooled to 600° C., disposed in the upper portion of the reaction vessel. The collected SiO deposit was pulverized in an agate mortar to obtain a SiO powder.

[0086] In order to perform Li-doping to the SiO powder by the solid phase method, the SiO powder and a LiH powder were mixed in a molar ratio of 5:2, and the resulting powder mixture was heated to 600° C. in an atmosphere of Ar. When the composition of the resulting Li-containing SiO powder was analyzed by the ICP emission spectroscopy and the infrared absorption method, the composition was SiLi.sub.xO.sub.y (x=0.42, y=0.96).

[0087] Next, this Li-containing SiO powder was subjected to heat treatment as C-coating treatment in a rotary heating furnace at 1000° C. for one hour while propane gas was introduced. When the average particle diameter of a Li-containing SiO powder after the C-coating was examined by the laser diffraction type particle size distribution measurement, D.sub.50 was 5.7 μm. Furthermore, a surface area measured using a BET specific surface area measuring device was 2.2 m.sup.2/g, the C content measured by the infrared absorption method was 3.0 wt %, and the average film thickness of the C-coating measured therefrom was 6.2 nm.

[0088] Furthermore, the resulting C-coating Li-containing SiO powder particles were subjected to the cross-sectional TEM observation, and ten particles each having a minor axis of 1 μm or more were extracted. Then, each of the particles was subjected to the EELS measurement in the same manner as Example 1 to obtain L1/L2, and standard deviation and a coefficient of variation of L2.

[0089] In addition, the resulting C-coating Li-containing SiO powder was subjected to XRD measurement in the same manner as Example 1. The XRD profile is shown in FIG. 3. A peak of SiC is observed. Besides, peaks of Si, SiO.sub.2, Li.sub.2Si.sub.2O.sub.5, and Li.sub.2SiO.sub.3 are present.

Comparative Example 2

[0090] A SiO powder was obtained from a Si powder and a SiO.sub.2 powder in the same manner as in Comparative Example 1. Then, the resulting SiO powder was subjected to heat treatment as C-coating treatment in a rotary heating furnace at 1000° C. for one hour while propane gas was introduced.

[0091] The SiO powder and a LiH powder were mixed in a molar ratio of 5:2 in order to perform Li-doping to the resulting C-coating SiO powder by the solid phase method, and the resulting powder mixture was heated in an atmosphere of Ar at 600° C. When the composition of the resulting C-coating Li-containing SiO powder was analyzed by the ICP emission spectroscopy and the infrared absorption method, the composition was SiLi.sub.xO.sub.y (x=0.40, y=1.03).

[0092] When the average particle diameter of the Li-containing SiO powder after the C-coating was examined by the laser diffraction type particle size distribution measurement, D.sub.50 was 5.9 μm. Furthermore, a surface area measured using a BET specific surface area measuring device was 2.4 m.sup.2/g, the C content measured by the infrared absorption method was 3.2 wt %, and the average film thickness of the C-coating measured therefrom was 6.1 nm.

[0093] The resulting C-coating Li-containing SiO powder particles were subjected to the cross-sectional TEM observation, and ten particles each having a minor axis of 1 μm or more were extracted. Then, each of the particles was subjected to the EELS measurement in the same manner as Example 1 to obtain L1/L2, and standard deviation and a coefficient of variation of L2.

[0094] Furthermore, the resulting C-coating Li-containing SiO powder was subjected to XRD measurement in the same manner as Example 1. Peaks of Si, Li.sub.2Si.sub.2O.sub.5, and Li.sub.2SiO.sub.3 were observed, but a peak of SiC was not present.

Comparative Example 3

[0095] A Li-containing silicon oxide powder obtained in Example 1 having an average composition represented by SiLi.sub.xO.sub.y (x=0.38, y=1.06) was subjected to heat treatment as C-coating treatment in a rotary heating furnace at 850° C. for six hours while propane gas was introduced.

[0096] When the average particle diameter of the Li-containing silicon oxide powder after the C-coating was examined by the laser diffraction type particle size distribution measurement, D.sub.50 was 5.7 μm. Furthermore, a surface area measured using a BET specific surface area measuring device was 2.7 m.sup.2/g, the C content measured by the infrared absorption method was 15.1 wt %, and the average film thickness of the C-coating measured therefrom was 25.4 nm.

[0097] The resulting C-coating Li-containing SiO powder particles were subjected to the cross-sectional TEM observation, and ten particles each having a minor axis of 1 μm or more were extracted. Then, each particle was subjected to EELS measurement in the same manner as Example 1 to obtain L1/L2, and the standard deviation and the coefficient of variation of L2.

[0098] Furthermore, the resulting C-coating Li-containing silicon oxide powder was subjected to XRD measurement in the same manner as Example 1, clear peaks of SiC, Si, Li.sub.2Si.sub.2O.sub.5, and Li.sub.2SiO.sub.3 were present.

Comparative Example 4

[0099] A Li-containing silicon oxide powder obtained in Example 1 and having an average composition represented by SiLi.sub.xO.sub.y (x=0.38, y=1.06) was subjected to heat treatment as C-coating treatment in a rotary heating furnace at 850° C. for five minutes while propane gas was introduced.

[0100] When the average particle diameter of the Li-containing silicon oxide powder after the C-coating was examined by the laser diffraction type particle size distribution measurement, D.sub.50 was 5.1 μm. Furthermore, a surface area measured using a BET specific surface area measuring device was 2.5 m.sup.2/g, the C content measured by the infrared absorption method was 0.21 wt %, and the average film thickness of the C-coating measured therefrom was 0.38 nm.

[0101] The resulting C-coating Li-containing SiO powder particles were subjected to the cross-sectional TEM observation, and ten particles each having a minor axis of 1 μm or more were extracted. Then, each particle was subjected to EELS measurement in the same manner as Example 1 to obtain L1/L2, and the standard deviation and the coefficient of variation of L2.

[0102] Furthermore, the resulting C-coating Li-containing SiO powder was subjected to XRD measurement in the same manner as Example 1, clear peaks of Si, Li.sub.2Si.sub.2O.sub.5, and Li.sub.2SiO.sub.3 were observed, but a peak of SiC was not present.

[0103] (Battery Evaluation)

[0104] Battery evaluation was performed with respect to the powder samples prepared in Examples 1 to 4 and Comparative Examples 1 to 4 according to the following procedure.

[0105] The powder sample, a PI binder as a nonaqueous (organic) binder, and a KB as an electrically conductive auxiliary agent were mixed in a weight ratio of 80:15:5 and kneaded with an organic solvent NMP to prepare a slurry. The prepared slurry was applied onto a copper foil and subjected to vacuum heat treatment at 350° C. for 30 minutes to obtain a negative electrode. This negative electrode, a counter electrode (a Li foil), an electrolytic solution (EC:DEC=1:1), an electrolyte (1 mol/L of LiPF.sub.6), and a separator (a polyethylene porous film having a film thickness of 30 μm) were combined to prepare a coin cell battery.

[0106] The prepared coin cell battery was subjected to a charge and discharge test. Charge was performed at a constant current of 0.1 C until the voltage across the two electrodes of the battery reached 0.005 v. After the voltage reached 0.005 V, constant-potential charge was performed until the electric current reached 0.01 C. Discharge was performed at a constant current of 0.1 C until the voltage across the two electrodes of the battery reached 1.5 V.

[0107] The initial charge capacity and initial discharge capacity were measured by this charge and discharge test to determine the initial efficiency. Furthermore, the capacity maintenance rate after 50 cycles was examined Results are shown in Table 1 along with the main specifications (presence or absence of SiC peak, presence or absence of Si peak, L1/L2, and standard deviation and a coefficient of variation of L2, and the C-coating film thickness).

TABLE-US-00001 TABLE 1 Thickness Initial Initial of charge discharge Capacity SiC Si L2 L2 C-coating capacity capacity Initial maintenance Composition peak peak L1/L2 SD CV (nm) (mAh/g) (mAh/g) efficiency rate Example 1 SiLi0.38 Not Present 0.92-1.07 0.05 0.09 9.1 2080 1668 80.2% 80.1% O1.06 present Example 2 SiLi0.38 Not Not 0.88-1.11 0.07 0.14 4.7 2106 1687 80.1% 83.5% O1.06 present present Example 3 SiLi0.18 Not Present 0.93-1.07 0.04 0.20 9.1 2195 1699 77.4% 83.9% O1.02 present Example 4 SiLi0.95 Not Present 0.91-1.06 0.05 0.10 9.7 1932 1594 82.5% 74.2% O1.05 present Comparative SiLi0.42 Present Present 1.19-1.57 0.14 0.27 6.2 1788 1318 73.7% 42.0% Example 1 O0.96 Comparative SiLi0.40 Not Present 1.21-1.58 0.15 0.30 6.1 1947 1538 78.7% 70.8% Example 2 O1.03 present Comparative SiLi0.38 Present Present 0.89-1.11 0.05 0.11 25.4 1844 1435 77.8% 69.7% Example 3 O1.06 Comparative SiLi0.38 Not Present 0.95-1.05 0.04 0.09 0.38 2096 1662 79.3% 60.4% Example 4 O1.06 present *1: SD = standard deviation *2: CV = coefficient of variation

[0108] As can be seen from the results of Examples 1 to 4 and Comparative Examples 1 to 4, when a Si-containing silicon oxide powder having uniformly dispersing Li was subjected to thin film C-coating at a low temperature, a negative electrode material having high battery performance was obtained.

[0109] Incidentally, in Comparative Example 1, the C-coating was performed after the Li-doping by the solid phase method. Consequently, the distribution of a Li concentration is nonuniform. As a result, a SiC peak is present, so that the battery performance is largely inferior as compared with Examples 1 to 4.

[0110] In Comparative Example 2, after the C-coating, the Li-doping by the solid phase method was performed. A SiC peak is not present, but the distribution of a Li concentration is nonuniform, and therefore, battery performance is inferior as compared with Examples 1 to 4 although the battery performance is not as inferior as in that of Comparative Example 1.

[0111] In Comparative Example 3, the distribution of a Li concentration is uniform, but a film thickness of the C-coating was thick, and heating conditions are severe, so that a SiC peak is present, and battery performance is inferior as compared with Examples 1 to 4.

[0112] In Comparative Example 4, since the distribution of a Li concentration is uniform, a film thickness of the C-coating is thin, and heating condition is gentle, a SiC peak is not present. However, because the C-coating is too thin, the battery performance was inferior as compared with Examples 1 to 4.