Patent ID: 12195345

DESCRIPTION OF EMBODIMENTS

Hereinafter, the embodiments of the present invention will be described.

A method for producing an SiO powder of this embodiment includes an SiO gas generation step generating SiO gas; an SiO deposition step of condensing and depositing to accumulate the generated SiO gas on a cooled deposition base; and an SiO powder collecting step of scraping off the SiO deposit accumulated on the deposition base with a blade and collecting the scraped deposit as an SiO powder. These steps proceed simultaneously in parallel, and the SiO powder collecting step among these steps has a significant feature.

As shown inFIG.1, an SiO powder production device to be used for the method for producing an SiO powder of this embodiment includes a furnace body1, a crucible2placed in the furnace body1, a heater3surrounding the crucible2in order to heat the crucible2, a heat insulator4covering these leaving an upper-side opening portion of the crucible2, a deposition base5disposed above the upper-side opening portion of the crucible2and including a drum-shaped rotator, a blade7disposed from a front surface side of the deposition base5toward the deposition base5for scraping off an SiO deposit accumulated on the outer circumference of the deposition base5, and a saucer8for the SiO powder disposed at the lower portion of the blade7.

To produce the SiO powder, firstly, for example, a mixture material of Si and SiO2as the SiO gas-generating raw material9is loaded into the crucible2, which is a reaction chamber. Then, the inside of the furnace body1is heated by the heater3while a pressure inside the crucible2is reduced. As described above, the pressure inside the crucible2is desirably 10 Pa or less, more desirably 7 Pa or less, and particularly desirably 5 Pa or less. Furthermore, a heating temperature of the inside of the crucible2, that is, a temperature t1inside the reaction chamber is desirably 1000 to 1600° C., more desirably 1100 to 1500° C., and particularly desirably 1100 to 1400° C.

Heating of the inside of the crucible2under reduced pressure generates the SiO gas from the SiO gas-generating raw material9inside the crucible2. This is the SiO gas generation step.

At this time, on the crucible2, the deposition base5including a drum-shaped rotator rotates. A temperature t2of the deposition base5is set to be lower than t1inside the reaction chamber, in more detail, set to be lower than the condensation temperature of the SiO gas. As described above, the temperature t2is desirably 800° C. or less, more desirably 150° C. or more and 750° C. or less, and particularly desirably 150° C. or more and 650° C. or less. Thus, the SiO gas generated from an SiO gas-generating raw material9inside the crucible2is condensed and deposited and accumulated. This is the SiO deposition step.

At the same time, the blade7faces the rotating deposition base5from the front side. What is important herein is that the tip of the blade7is not brought into contact with the surface of the deposition base5, and a predetermined distance g (gap) is secured between the surface of the deposition base5and the tip of the blade7. As described above, this distance g (gap) is desirably 0.1 to 3 mm, more desirably 0.5 to 2.5 mm, and particularly desirably 0.5 to 2 mm

Thus, SiO deposit10accumulated on the surface of the deposition base5is scraped off by the blade7, and collected as the SiO powder11in the saucer8. However, since the tip of the blade7is not brought into contact with the surface of the deposition base5, and a predetermined distance g (gap) is secured between the surface of the deposition base5to the tip of the blade7, the collected SiO powder10is prevented from being contaminated with an impurity due to direct contact between the deposition base5and the blade7is prevented, and the SiO powder11becomes high quality powder having a rounded spherical particulate shape and having a small particle diameter. The reason therefor is as described before.

This is the SiO powder collecting step, and it proceeds simultaneously in parallel with the SiO gas generation step and the SiO accumulation step, and the high-quality SiO powder described above is continuously produced.

The SiO powder collecting step is described more specifically by focusing a certain position in a circumferential direction of the deposition base5, with a scraping position by the blade7as a starting point. When the certain position reaches the scraping position, the SiO deposit10accumulated so far is scraped off, and the Si deposit10having a predetermined thickness remains on the surface of the deposition base5even after scraping. Then, before the certain position reaches the scraping position, the SiO deposit10is accumulated thereon, and the newly accumulated Si deposit10is scraped off at the scraping position. This is repeated. In other words, on the surface of the deposition base5, the Si deposit10having a predetermined thickness continues to remain, and the Si deposit10newly accumulated thereon is scraped off by the blade7. During the unit time, the rate at which the SiO deposit10is accumulated is a growth rate d (μm/min) of the SiO deposit10. The number of rotations in the unit time of the deposition base5is a scraping period n (l/min).

As described above, the relationship d/n (μm) between the growth rate d (μm/min) of the SiO deposit10and the scraping period n (l/min) has a great effect on the property of the collected SiO powder (particle diameter and shape).

Furthermore, the SiO powder thus obtained is not merely a simple spherical particle having high circularity, but is a composite spherical shape like a cauliflower, so to speak, in which a plurality of small spherical satellite parts are integrally combined into a large spherical core part serving as a core. When the particle shape is represented by the average value Dfi of the fractal dimension D of 20 particles, when the value Dfi shows 1.03 or more and 1.50 or less, and particularly shows 1.05 or more and 1.50 or less. The SiO powder is excellent, the cycle characteristics and pulverizability as described above. Furthermore, the circularity is preferably 0.8 or more as described above, from the viewpoint that the powder particle becomes spherical in shape.

The SiO particles preferably have a median diameter of 0.5 to 30 μm. If the particle diameter is too small, the influence of the decomposition reaction of the electrolytic solution on the particle surface increases, which leads to deterioration of the coulomb efficiency and deterioration of the handling properties due to the increase of the cohesiveness and deterioration of the bulk density. When the particle size is too large, expansion of the electrode when Li is occluded increases, and the cycle characteristics are deteriorated.

EXAMPLES

Next, results obtained by actually producing an SiO powder by the above-described device and procedure are described. A deposition base including a drum-shaped rotator is made of stainless steel and subjected to oil cooling, and the blade is a doctor blade made of stainless steel.

Example 1-1

A mixture of Si and SiO2(Si:O=1:1) as an SiO gas-generating raw material was loaded into a crucible as a reaction chamber, the crucible was set in a predetermined position inside a furnace body, after which the pressure inside the furnace body was reduced to 1 Pa, and the inside of the crucible was heated to 1300° C. to generate SiO gas. At the same time, the deposition base on the crucible was rotated while temperature control at 150° C. was performed to condense and deposit the SiO gas on the surface of the deposition base.

The growth rate d of the SiO deposit, that is, the film formation speed in the surface of the deposition base, was 4.8 μm/min at this time, and by adjusting the rotation speed of the deposition base, the scraping period n was made to be 2.4 min−1, and the ratio of both values d/n was made to be 2. Furthermore, a distance g from the surface of the deposition base to the tip end of the blade was 0.5 mm

In the position for scraping by the blade, the SiO deposit was scraped off with an SiO deposition layer having a thickness of 0.5 mm left, and the SiO powder was collected. Thus, the SiO powder was manufactured continuously. Among the manufactured SiO powders, fine powders having 45 μm or less were evaluated as an active material by sieving.

Furthermore, manufacturing capability per unit length of the deposition base of the SiO powder (g/(hr·m)), a yield (weight of collected SiO/amount of weight loss of raw material), collection rate of fine powder (weight amount of the collected SiO by 45-μm sieve/weight of collected SiO) was examined

The particle shape of the produced SiO powder was examined for the circularity (circumferential length of circle having equal projected area/circumferential length of a particle). A circularity measurement method is shown in Table 1.

TABLE 1Type ofmeasurementFlow-type particle image analyzer FPIA-3000 deviceAdjustmentDispersion0.2 wt % sodiumof samplemediumhexametaphosphateaqueous solutionDispersion agent10 wt % Triton X-100(surfactant)aqueous solutionAdjustment ofSample amount: 10 mgsample0.2 wt % Na-HMP 30 g/10 wt %Triton X-100 aqueoussolution: 1 dropPretreatmentUltrasonic bath (150 W)dispersion: 1 minSettingSheath solutionParticle sheath (manufactured byof deviceSYSMEX CORPORATION)Objective lensStandard (10×)Measurement modeLPF modeCounting methodQuantitative countingNumber ofMeasure three timesmeasurementsAnalysisSample measured by the above methodconditionsResults of three measurements totalized andsubjected to data analysis based on numbers

Next, a negative electrode of a lithiu ion secondary battery was prepared by using SiO fine powder of 45 μm or less, which was the final powder product, for a negative electrode active material. Specifically, SiO powder, Ketjen black, and a polyimide precursor, which is a nonaqueous solvent binder, were mixed at a mass ratio of 85:5:10; furthermore, NMP (n methyl pyrrolidone) was added to the mixture, and the obtained mixture was kneaded to prepare slurry. Next, the slurry was applied to a copper foil having a thickness of 40 μm, preliminarily dried at 80° C. for 15 minutes, punched into a diameter of 11 mm, and then subjected to imidization treatment to obtain a negative electrode.

Furthermore, a lithium ion secondary battery was prepared by using the prepared negative electrode. Specifically, a lithium foil was used for the counter electrode in the secondary battery. For the electrolyte, a solution obtained by dissolving LiPF6(lithium hexafluorophosphate) in a solution obtained by mixing ethylene carbonate and diethyl carbonate at a volume ratio of 1:1, so that the ratio was 1 mol/liter, was used. Then, for a separator, a coil cell was prepared using a porous film made of polyethylene and having a thickness of 20 μm.

The prepared lithium ion secondary battery was subjected to a charge/discharge test using a secondary battery charge/discharge test device (manufactured by Nagano, Co., Ltd.). Test conditions in the charge/discharge test are shown in Table 2. With the charge/discharge test, initial charge capacity, initial discharge capacity, and ratio of the initial discharge capacity in relation to the initial charge capacity (initial Coulomb efficiency), the ratio of the 50th discharge capacity in relation to the initial discharge capacity (capacity retention rate after 50 cycles) were obtained, respectively.

TABLE 2ChargeDischarge1stCC-CV 0.1 C5 mV − 0.01 CCC 0.1 C1.5 V2ndCC-CV 0.3 C5 mV − 0.01 CCC 0.3 C1.5 V3rd-50thCC-CV 0.5 C5 mV − 0.01 CCC 0.5 C1.5 V

In order to evaluate the particle shape of the SiO powder, in addition to the measurement of the circularity described above, the above-described negative electrode was subjected to acquirement of a 3D-SEM image and the fractal analysis of the SiO particle cross-section were performed according to the method mentioned below.

(1) A 3D-SEM image was acquired for the electrode.Device for sample preparation and observation: Helios G4 manufactured by FEIFIB processing condition: acceleration voltage 30 kVSEM observation condition: acceleration voltage 2 kV secondary electron imageProcessing area: about 40 μm (width)×about 40 μm (height)Slice Step: 100 nmNumber of Slices: about 400Sample inclination: 52°

(2) Sample processing by FIB, SEM observation, and sample processing were repeated at an interval of about 100 nm (acquiring about 400 SEM images) to continuously obtain thickness information of about 40 μm in the depth direction. Furthermore, the acquired sequential SEM images were corrected in consideration of the stage inclination angle of FIB. After confirming that a series of SEM images were continuously observed in the depth direction, alignment of sequential SEM images was performed, and image series were superimposed to acquire a three-dimensional reconstructed image. Observation range was selected so that 20 particles were included in the observed field of view.

(3) Then, the fractal dimension analysis was performed as follows.

Software Used:

Avizo9.7.0 manufactured by Thermo Fisher ScientificImage-Pro10 manufactured by Nippon Roper
Image Analysis Method:
An area of each of the XY cross-section (the FIB processing direction) of the SiO particles (20 particles) extracted by the 3D-SEM was calculated using Avizo9.7.0. For each particle, the fractal dimension D of each particle was calculated from the XY cross-sectional image having the maximum area using Image-Pro10, and the average values were compared.

Furthermore, beside these measurements, the pulverizability was examined by the following method in consideration of further pulverizing the SiO powder and using the SiO powder as an active material.

(1) The particle size distribution of the collected powders sifted through a sieve having a mesh opening of 45 μm was measured, and the median diameter D50 (hereinafter, referred to as average particle size) based on the volume was obtained. The particle size distribution is measured by using a laser diffraction type particle size distribution measurement apparatus. In this Example, Mastersizer2000 manufactured by Malvern was used. For the solvent, isopropylalcohol was used.

(2) The powder sifted through a 45-μm sieve was pulverized to a particle size of 5 μm using a dry-type attritor. The used device was a dry-type attritor MAID manufactured by NIPPON COKE & ENGINEERING CO., LTD., the ball used was a material of zirconia having a diameter of 5 mm, and the number of rotations was 300 rpm. The time taken to reach the desired particle size (5 μm) was measured.

Various types of examination results, such as specifications of the produced SiO powder, manufacturing properties, battery performance, and pulverizability, are shown in Table 3 together with the production conditions of the SiO powder.

Example 1-2

In Example 1-1, the rotation speed of the deposition base was reduced, the scraping period n was changed from 2.4 min−1to 0.24 min−1, and accordingly, d/n was changed from 2 to 20. The other production conditions and examination methods were the same as those in Example 1-1. Various examination results are shown in Table 3 together with the production conditions of the SiO powder.

Example 1-3

In Example 1-1, the rotation speed of the deposition base was increased, the scraping period n was changed from 2.4 min−1to 48 min−1, and accordingly, d/n was changed from 2 to 0.1. The other production conditions and examination methods were the same as those in Example 1-1. Various examination results are shown in Table 3 together with the production conditions of the SiO powder.

Example 2

In Example 1-1, a distance g from the surface of the deposition base to the tip of the blade was changed from 0.5 mm to 1 mm. The other production conditions and examination methods were the same as those in Example 1-1. Various examination results are shown in Table 3 together with the production conditions of the SiO powder.

Example 2-i

In Example 2, the prepared final product (SiO fine powder) was subjected to heat treatment. Specifically, the final product (SiO fine powder) was loaded into a crucible made of alumina, and heated in an electric furnace in an inert gas atmosphere (Ar gas atmosphere) at 850° C. for two hours. The other conditions were the same as those in Example 2. Various examination results are shown in Table 3 together with the production conditions of the SiO powder.

Example 2-ii

In Example 2-i, the final product (SiO fine powder) after heat treatment was coated with electrically conductive carbon (C-coating). Specifically, the powder after heat treatment was loaded into a rotary kiln, and carbon coating treatment was performed by a thermal CVD using a mixed gas of argon and propane as a carbon source. A carbon coating amount (weight ratio of the C element to the entire powder) was 2 wt %. The other conditions were the same as those in Example 2-i. Various examination results are shown in Table 3 together with the production conditions of the SiO powder.

Example 3

In Example 2, a distance g from the surface of the deposition base to the tip of the blade was changed from 1 mm to 3 mm. The other production conditions and examination methods were the same as those in Example 2. Various examination results are shown in Table 3 together with production conditions of the SiO powder.

Example 4

In Example 3, the temperature of the deposition base was changed from 150° C. to 500° C. Accordingly, the film formation speed was lowered from 4.8 μm/min to 4.5 μm/min, and d/n was lowered from 2 to 1.88. The other production conditions and examination methods are the same as those in Example 3. Various examination results are shown in Table 3 together with the production conditions of the SiO powder.

Example 5

In Example 1, the SiO gas-generating raw material was changed from the mixture of Si and SiO2(Si:O=1:1) to a mixture of Si, SiO2, and lithium silicate (Li:Si:O=0.1:1:1). Furthermore, the distance g from the surface of the deposition base to the tip of the blade was changed from 0.5 mm to 0.1 mm. The other production conditions and examination methods were the same as those in Example 1-1. Various examination results are shown in Table 3 together with the production conditions of the SiO powder.

Example 5-i

In Example 5, the prepared final product (SiO fine powder) was subjected to heat treatment. Specifically, the final product (SiO fine powder) was loaded into a crucible made of alumina and heated in an electric furnace in an inert gas atmosphere (Ar gas atmosphere) at 850° C. for two hours. The other conditions were the same as those in Example 5. Various examination results are shown in Table 3 together with the production conditions of the SiO powder.

Example 5-ii

In Example 5-i, the final product (SiO fine powder) after heat treatment was coated with electrically conductive carbon (C-coating). Specifically, the powder after heat treatment was loaded into a rotary kiln, and carbon coating treatment was performed by a thermal CVD using a mixed gas of argon and propane as a carbon source. A carbon coating amount (weight ratio of the C element in relation to the entire powder) was 2 wt %. The other conditions were the same as those in Example 5-i. Various examination results are shown in Table 3 together with the production conditions of the SiO powder.

Example 6

In Example 5, the SiO gas-generating raw material was changed from a mixture of Si, SiO2, and lithium silicate (Li:Si:O=0.1:1:1) to a mixture of Si, SiO2, and MgO (Mg:Si:O=0.1:1:1). The other production conditions and examination methods were the same as those in Example 5. Various examination results are shown in Table 3 together with production conditions of the SiO powder.

Example 7

In Example 1-1, the rotation speed of the deposition base was reduced, the scraping period n was changed from 2.4 min−1to 0.08 min−1, and accordingly, d/n was changed from 2 to 60. The other production conditions and examination methods were the same as those in Example 1-1. Various examination results are shown in Table 3 together with the production conditions of the SiO powder.

Comparative Example 1

In Example 1-1, a distance g from the surface of the deposition base to the tip of the blade was changed from 0.5 mm to 0 mm. In other words, the tip of the blade was brought into contact with the surface of the deposition base. The other production conditions and examination methods were the same as those in Example 1-1. Various examination results are shown in Table 3 together with the production conditions of the SiO powder.

Comparative Example 2

In Example 7, the distance g from the surface of the deposition base to the tip of the blade was changed from 0.5 mm to 0 mm. In other words, in Example 1-1, the rotation speed of the deposition base was reduced, the scraping period n was changed from 2.4 min−1to 0.08 min−1, and accordingly, d/n was changed from 2 to 60; and the tip of the blade was brought into contact with the surface of the deposition base. The other production conditions and examination methods were the same as those in Example 7 or Example 1-1. Various examination results are shown in Table 3 together with the production conditions of the SiO powder.

TABLE 3Ex. 1-1Ex.1-2Ex.1-3Ex.2Ex.2-iEx.2-iiEx.3Ex.4ARaw materialSi + SiO2Si + SiO2Si + SiO2Si + SiO2←←Si + SiO2Si + SiO2Base material temperature (° C.)150150150150←←150500Pressure (Pa)1111←←11Reaction chamber temperature (° C.)1300130013001300←←13001300Film formation speed d (um/min)4.84.84.84.8←←4.84.5Scraping period n (min−1)2.40.24482.4←←2.42.4d/n (μm)2200.1221.875Distance g between deposition0.50.50.51←←33base and blade (mm)BSiO shapesphericalsphericalsphericalspherical←←sphericalsphericalSpherical SiO manufacturing114114114114←←114114capability (g/(hr · m))Yield96%95%95%89%←←85%84%Powder collection rate95%94%99%97%94%95%Circularity EPIA-30000.9310.9360.9290.9410.9420.951CHeat treatment atmosphereAt 1 atmHeat treatment temperature850° C.Carbon coating concentration2 wt %DO/Si (mol/mol)1.11.11.21.21.21.21.11.0O/O (mol/mol)————————Mg/O (mol/mol)————————EInitial efficiency56.3%57.1%54.8%56.9%62.4%65.2%56.8%64.3%Capacity retention rate after 50 cycles65.4%52.3%67.3%65.4%65.7%80.6%65.1%79.8%FAverage fractal dimension1.0501.0201.2851.220←←1.2001.370Dfi of 20 particlesGAverage particle size through 45-μm25301525//2326sieve before pulverization (μm)Average particle size after55//5pulverization (μm)Time required for pulverization (μm)108//5Ex. 5Ex.5-iEx.5-iiEx.6Ex.7Co.Ex.1Co.Ex.2ARaw materialSi + SiO2+←←Si + SiO2Si + SiO2Si + SiO2Si + SiO2lithiumMcOsilicateBase material temperature (° C.)150←←150150150150Pressure (Pa)1←←1111Reaction chamber temperature (° C.)1300←←1300130013001300Film formation speed d (um/min)4.8←←4.84.84.84.8Scraping period n (min−1)2.4←←2.40.082.40.08d/n (μm)2←←260260Distance g between deposition0.1←←0.10.500base and blade (mm)BSiO shapespherical←←sphericalsubstantiallyscalescalesphericalshapeshapeSpherical SiO manufacturing160←←16057——capability (g/(hr · m))Yield93%←←92%94%96%96%Powder collection rate95%←←94%50%31.0%0.5%Circularity EPIA-30000.9490.9370.8490.7010.732CHeat treatment atmosphereAt 1 atm←Heat treatment temperature850° C.←Carbon coating concentration2 wt %DO/Si (mol/mol)1.01.01.01.01.21.11.1O/O (mol/mol)0.10.10.1————Mg/O (mol/mol)———0.1———EInitial efficiency63.6%73.5%75.1%65.2%50.2%50.1%50.1%Capacity retention rate after 50 cycles71.0%75.5%82.0%65.7%55.1%52.2%51.1%FAverage fractal dimension1.320←←1.2201.0401.0201.010Dfi of 20 particlesGAverage particle size through 45-μm20//223324sieve before pulverization (μm)Average particle size after//5pulverization (μm)Time required for pulverization (μm)//15Ex. = ExampleCo. Ex = Comparative ExampleA: Production conditionsB: Powder production resultC: Post treatment conditionD: Element ratioE: Battery evaluation resultsF: Particle cross-section fractal dimensional analysisG: Pulverizability

As can be seen from Table 3, in Examples of the present invention in which the tip of the blade is apart from the surface of the deposition base, as compared with Comparative Examples in which the tip of the blade was in contact with the surface of the deposition base, the capacity retention rate after 50 cycles as the battery performance is improved. This is considered to be because in Examples of the present invention, the contamination with an impurity by the SiO powder occurring when the tip of the blade is in contact with the surface of the deposition base is reduced.

Furthermore, regarding the particle shape of the SiO powder, the circularity was less than 0.8 and the shape was a scale-shape in Comparative Examples 1 and 2, while in the Examples of the present invention, the circularity was as high as 0.8 or more, and except for Example 7, all of the shapes were spherical shapes having a particularly high circularity of 0.9 or more. A micrograph of SiO powder (collected powder before sieving) produced in Example 1-1 is shown inFIG.2. Furthermore, a micrograph of SiO powder (collected powder before sieving) produced in Comparative Example 1 is shown inFIG.3, and a state of the pulverized powder is shown inFIG.4.

It was found that the SiO powder produced in Example 1-1 was a rounded spherical particulate powder. Furthermore, it was found that the SiO powder produced in Example 1-1 grew to a composite spherical shape like a cauliflower, so to speak, in which a plurality of small spherical satellite parts were integrally combined into a large spherical core part serving as a core. On the other hand, an SiO powder produced in Comparative Example 1 had a clear scale-shape. Even if the SiO powder is pulverized, it does not become rounded spherical particulate powders as those produced in Example 1-1.

The reason why the circularity in Example 7 was lower than the circularity in other Examples and became substantially spherical particulate is that because the rotation speed of the deposition base was slow and therefore the scraping period n was very short at 0.08 min−1, and the time from one scraping to the next scraping was longer, new SiO deposit accumulated on the remaining SiO deposit and the SiO deposit remaining below were integrated with each other, and the SiO powder obtained by scraping became scale-shaped. Consequently, the SiO powder in two forms, that is, a spherical particulate and a relatively large scale-shaped shape, was peeled off from the deposition base, a spherical particulate powder could be collected by sieving, and the circularity of the spherical particle powder was higher than those of Comparative Examples as described above.

Then, since rounded spherical particulate powders were obtained in the other Examples excluding Example 7, the initial efficiency as the battery performance was greatly improved as compared with Comparative Examples and Example 7.

Furthermore, the fine powder collecting rate in the production of the SiO powder was high. This means that fine pulverization had already proceeded in the stage of scraping with a blade and collecting.

In Examples 2,2-i, 2-ii, 3, and 4, as compared with the other Examples, the yield slightly decreased because the distance from the surface of the deposition base to the tip of the blade was not necessarily optimum.

Furthermore, as is seen from Examples 2-i, 2-ii, 5-i, and 5-ii, the heat treatment with respect to the collected SiO powder is effective in the improvement of the initial efficiency, and coating with electrically conductive carbon is effective in the improvement of the capacity retention rate after 50 cycles. In particular, Example 4 shows relatively high initial efficiency and capacity retention rate after 50 cycles although neither heat treatment on the SiO powder nor coating with electrically conductive carbon were performed. This is thought to be because the temperature of the deposition base was higher than the others, and the film formation speed was suppressed, while the organization of the SiO deposit tended to be dense, and this is reflected in the battery evaluation. The organization became dense even with heat treatment on the collected SiO powder (Example 2-i), but more importantly, the battery performance was further improved. This is thought to be because the amount of oxygen to be picked up is less under heating in the state of non-exposure to air than under heating after exposure to air after collection.

Furthermore, as seen in Examples 5 and 6, Li dope and Mg dope in which the raw material includes a dope source are effective in improvement of the battery performance, and heat treatment and coating with electrically conductive carbon thereon are also effective in improvement of the battery performance (Examples 5-i and 5-ii).

When particle shapes in Examples and Comparative Examples are evaluated based on the average value Dfi of the fractal dimension D of 20 particles, the values fall in the range of 1.03 or more and 1.50 or less in the Examples, while they are less than 1.03 in the Comparative Examples. The three-dimensional reconstruction image of one particle in the SiO powder obtained in Example 1-1 is shown inFIG.5A, and the cross-sectional image thereof is shown inFIG.5B. The fractal dimension D of the cross-sectional image is 1.055. Furthermore, the three-dimensional reconstruction image of one particle in the SiO powder obtained in Comparative Example 1-1 is shown inFIG.6A, and the cross-sectional image thereof is shown inFIG.6B. The fractal dimension D of the cross-sectional image is 1.017, which is clearly different from that obtained in Example 1-1.

FIG.7shows the relationship between the circularity and the capacity retention rate (cycle characteristics) after 50 cycles in the Examples and Comparative Examples.FIG.8shows the relationship between the average value Dfi of the fractal dimension D and the capacity retention rate (cycle characteristics) after 50 cycles in the Examples and Comparative Examples.

As can be seen fromFIG.7, Examples having high circularity show improvement of the capacity retention rate, but do not show monotonous correlation. Therefore, it is difficult to say that control is possible with circularity. On the contrary, as can be seen fromFIG.8, the fractal dimension D shows a strong correlation with the capacity retention rate, and the capacity retention rate can be controlled by the fractal dimension D. This is thought to be because the fractal dimension D exactly reflects a characteristic feature of the composite spherical shape like a cauliflower, so to speak, in which a plurality of small spherical satellite parts are integrally combined into a large spherical core part serving as a core.

Furthermore,FIG.9shows the relationship between the average value Dfi of the fractal dimension D and the time required for pulverization (pulverizability) in the Examples and Comparative Examples. As is apparent fromFIG.9, the larger the average value Dfi of the fractal dimension becomes, the more the pulverizability is improved. This also shows that the fractal dimension D is effective in the evaluation of the quantitative evaluation of the characteristic feature of particles having a composite spherical shape like a cauliflower.

Note here that in the present invention, SiO does not mean SiOx (x=1). It means SiO in a broad sense, and encompasses SiO doped with any other elements. If it is represented by the chemical formula, it is MySiOx wherein 0.5≤x≤1.5 and 0≤y≤1 are satisfied. Herein, x, that is, the rate of O atomic weight with respect to the Si atomic weight, is less than 0.5, SiOx is too close to Si, activity with respect to oxygen increases, and safety is deteriorated. On the contrary, when x is more than 1.5, the initial efficiency is deteriorated, and the battery performance may be deteriorated.

x and y further preferably satisfy 0.05≤y/x≤1. When y/x is less than 0.05, the effect of doping M is minimal. When y/x is more than 1, safety is deteriorated.

In each of the above-described Examples and Comparative Examples, each element amount of Si, O, and Li or Mg in the obtained SiO powder was measured. For Si, Li, and Mg, the element amount was measured by the ICP luminescent spectroscopy. For O, an element amount was measured by the Inert gas melting-infrared absorption method (GFA) using TC-436 manufactured by LECO. The O/Si element ratio, the Li/O element ratio, and the Mg/O element ratio in each Example are shown together in Table 3.1furnace body2crucible3heater4heat insulator5deposition base7blade8saucer9SiO gas-generating raw material10SiO deposit11SiO powder