JP6623285B2 - Li-containing silicon oxide powder - Google Patents

Description

The present invention relates to a silicon oxide-based negative electrode material used for forming a negative electrode of a lithium ion secondary battery, and more particularly to a Li-containing silicon oxide powder doped with lithium. In this specification, silicon oxide and SiO _x are synonymous.

It is known that SiO _x has a large electric capacity and is an excellent negative electrode material for lithium ion secondary batteries. This SiO _x negative electrode material is made into a thin film negative electrode by applying a slurry obtained by mixing SiO _x powder, a conductive additive and a binder onto a current collector made of copper foil or the like and drying it. . The SiO _x powder here is obtained by cooling and precipitating silicon monoxide gas generated by heating a mixture of silicon dioxide and silicon under reduced pressure, and then finely crushing. The SiO _x powder produced by such a precipitation method is advantageous because it contains a lot of amorphous parts, reduces the thermal expansion coefficient, and improves cycle characteristics.

As a problem of such a SiO _x negative electrode material, there is a low initial efficiency, and lithium doping (Li doping) is known as a technique for solving this problem. Li doping is carried out by mixing SiO _x powder and a powdered lithium source and firing them (Patent Documents 1 to 4). By performing Li doping on the SiO _x powder particles, the generation of a lithium compound that does not contribute to charge / discharge during the initial charge is suppressed, and the initial efficiency is improved.

Further, in addition to Li doping, carbon coating treatment (C coating) is performed on the particle surface of the SiO _x powder to improve cycle characteristics. In Patent Document 3, C coating is applied after Li doping. In Patent Document 4, Li doping is performed after C coating.

However, on the other hand, the Li dope has a problem that the activity of the SiO _x powder against air and water is increased and the handling property of the powder is deteriorated.

That, SiO _x prior to the Li doped less active against air because it is an oxide, reactivity with water is also extremely low. For this reason, even if it is a fine powder of a micron order, there was an advantage that safety was high compared with the graphite powder etc. which are the conventional negative electrode active materials. In fact, SiO _x having a particle size distribution of D ₁₀ = 2.35, D ₅₀ = 5.76, and D ₉₀ = 11.82 has a result that no explosion occurs in the dust explosion test defined in JISZ8818. On the other hand, regarding SiO _x doped with Li, when Li is doped at a ratio of Li / O (atomic ratio) = 0.4, it is confirmed by the same test that it has dust explosiveness.

The increase in dust explosiveness due to Li doping is because the reaction between the SiO _x powder and the lithium powder source is a surface reaction, and a large amount of highly active Li remains on the particle surface of the Li doped SiO _x powder. Conceivable. In the surface reaction with SiO _x powder and the powder lithium source, lithium silicate such _{Li 2 Si 2 O 5, Li} 2 SiO 3, Li 4 SiO 4, but further believed that such LiSi alloy occurs both SiO _x Compared to the above, the activity is high, and care must be taken in handling in the air. In general, the activity increases as the Li content increases.

Patent Document 5 proposes wet cleaning as a measure for solving the problem of powder activation associated with Li doping. That is, active Li generated on the particle surface by Li doping is removed by wet cleaning. However, in the case of wet cleaning, active Li is removed in the form of lithium silicate or LiSi alloy, so that pure Si is exposed from the inside of the powder particles to the particle surface, and as a result, the reactivity to the electrolyte is increased, The low reactivity with the electrolytic solution, which is the inherent advantage of the SiO _x powder, is sacrificed, resulting in a decrease in initial efficiency, which is one of battery performances.

  In addition, in wet cleaning, when Li / O becomes large, the activity with respect to the cleaning liquid becomes too high, and zero-valent Si responsible for reversible capacity reacts due to cleaning, which may make it impossible to maintain battery performance. is there.

Japanese Patent No. 2999741 Japanese Patent No. 4702510 Japanese Patent No. 4985949 Japanese Patent No. 54117781 Japanese Patent Publication No. 2014-532267

  An object of the present invention is to provide a safe and high-performance Li-containing silicon oxide powder capable of avoiding a decrease in safety associated with Li doping and suppressing a decrease in battery performance that is a problem in wet cleaning.

As described above, the safety in air SiO _x powder Li doped is reduced in comparison with the SiO _x powder before Li doped, be SiO _x powder exists Li on the particle surface touching the air The cause is that Li contributes to the improvement of the initial efficiency, so if Li is removed unnecessarily, the battery performance is lowered. Therefore, the present inventor limitedly forms a layer with a small amount of Li on the particle surface where the SiO _x powder comes into contact with air for the purpose of improving safety while minimizing deterioration in battery performance. And in that layer, we planned to selectively reduce Li without reducing Si, and continued earnest research and study. As a result, the following facts were found.

As an index representing the low Li layer on the particle surface of the Li-containing SiO _x powder, it is effective to introduce the ratio of the Li amount to the sum of the Li amount and the Si amount (Li amount ratio). It is effective to introduce a ratio between the Li amount ratio at the outermost surface (outermost surface Li amount ratio) and the Li amount ratio (near surface Li amount ratio) at the near surface slightly away from the outermost surface to the inside.

Since Li doped is a surface reaction of SiO _x powder, as Li becomes high concentration near the surface of the powder particles in the Li-doped SiO _x powder, result, the ratio of the outermost surface Li amount ratio and the near surface Li content ratio Over 1 When the powder is subjected to wet cleaning, Li is removed on the particle surface in the form of lithium silicate or LiSi alloy, and Si is removed together with Li, so this tendency does not change and is close to the outermost surface Li amount ratio. The ratio of the surface Li amount ratio exceeds 1.

  On the other hand, when the layer in which Li is selectively reduced is limitedly formed on the particle surface, the outermost surface Li amount ratio becomes smaller and the near surface Li amount ratio becomes larger, that is, the outermost surface Li amount ratio. It has been found that the above object is achieved when the ratio of the surface Li amount ratio and the near surface Li amount ratio is less than 1. The selective and limited low Li layer on the particle surface can be formed, for example, by selectively blowing off Li existing on the particle surface by irradiating the particle surface with an electron beam. Further, the outermost surface Li amount ratio and the near surface Li amount ratio can be quantitatively calculated by observing powder particles with a STEM and obtaining the spectral intensity of the outermost surface and the near surface by EELS measurement.

The Li-containing silicon oxide powder of the present invention was developed on the basis of such knowledge, and is a Li-containing SiO _x powder (0.5 <x <1.5) used for a negative electrode material of a lithium secondary battery. There,
Obtaining the spectral intensities of the Li-K edge region and Si-L edge region when EELS measurement is performed in a 1 μm square field of view including the particle surface with respect to the powder particle cross section and having a resolution of 50 pixels × 50 pixels or more,
Of the integrated intensity of one row on the outermost surface of the particle in the visual field, the integrated intensity in the Li-K edge region is I _Li (s), and the integrated intensity in the Si-L edge region is I _Si (s). _Li (s) / (I _Li (s) + I _Si (s)) is the outermost surface Li intensity ratio R (s),
The integrated intensity in the Li-K edge region is I _Li (i) and the integrated intensity in the Si-L edge region is I _Si (i) out of one row of integrated intensity on the near surface 500 nm away from the outermost surface of the particle in the visual field. I _Li (i) / (I _Li (i) + I _Si (i)) as (i) is defined as the near-surface Li intensity ratio R (i),
R (s) / R (i) <1 is satisfied.

  Here, EELS measurement was performed with a 1 μm square field of view including the particle surface and a resolution of 50 pixels × 50 pixels or more, and mapping was performed on the spectral intensity in the Li-K edge region. A set of measurement points along a direction that is nearly parallel to the particle surface of two orthogonal directions that are parallel to the two sides that form a square field of view is “row”, along a direction that is close to vertical A set of measurement points is a “column”, and for each example, when the spectrum intensity is taken from the outside of the particle toward the inside of the particle, the point having the larger intensity of the two points having the largest intensity difference from the nearest point is selected. The outermost surface point of the row is a set of outermost points designated by this method for each row.

  In the Li-containing silicon oxide powder of the present invention, the outermost surface Li intensity ratio R (s) is smaller than the near surface Li intensity ratio R (i) in the vicinity of the surface of the powder particles. This means that a low Li layer in which Li is selectively reduced is limitedly formed on the outermost surface of the powder particles. As a result, the safety of the powder is improved, the increase in dust explosiveness associated with the Li dope is avoided, and the decrease in battery performance, which is a problem in wet cleaning, is suppressed. R (s) / R (i) is desirably 0.9 or less, and more desirably 0.8 or less.

  In addition, the outermost surface Li intensity ratio R (s) is preferably small, specifically 0.6 or less. When R (s) is large, Li is excessive with respect to Si on the outermost surface, and dust explosiveness is increased. Even if R (s) / R (i) <1, if R (s) is large and R (i) is larger than that, the effect of suppressing dust explosiveness is small. When R (s) / R (i) <1 and R (s) ≦ 0.6, dust explosiveness is effectively suppressed.

  An important factor other than R (s) / R (i) is the Li content of the powder. The Li content is desirably 0.2 or more and 0.9 or less in molar ratio Li / O to O. When Li / O is less than 0.2, the amount of Li is insufficient, so that improvement in battery performance due to the inclusion of Li cannot be expected. When Li / O is more than 0.9, the amount of Li becomes excessive, so that a Li—Si alloy is generated, the reactivity of the powder is extremely increased, and the dust explosiveness is further increased. It is a feature of the powder that Li is sufficiently contained for improving the battery performance and still has high safety.

_X in SiO _x , that is, the ratio O / Si of the number of O atoms to the number of Si atoms needs to be more than 0.5 and less than 1.5. When x ≦ 0.5, SiO _x becomes too close to Si, the activity against oxygen is increased, and the safety is lowered. Conversely, if x ≧ 1.5, the initial efficiency is lowered and the battery performance is lowered.

The particle diameter of the powder is preferably 0.5 μm or more and 30 μm or less, and particularly preferably 1 μm or more and 20 μm or less, expressed as a median diameter (D ₅₀ ) measured by a laser diffraction particle size distribution analyzer. If the particle size is too small, the surface area of the powder becomes excessive, resulting in a decrease in coulomb efficiency due to reaction with the electrolyte and an increase in reactivity with air. On the other hand, when the particle diameter is too large, the influence of the expansion of the particles during charging / discharging becomes large and the cycle characteristics are deteriorated.

  In the Li-containing silicon oxide powder of the present invention, at least a part of the powder particles may be coated with a conductive carbon film. The coating of the conductive carbon film improves the electrical conductivity between the powder particles that make up the negative electrode and the electrical conductivity between the negative electrode and the current collector that is the base, improving battery characteristics, especially cycle characteristics. Is possible. The coating of the conductive carbon film here is the C coat.

The coating amount of the conductive carbon film on the particles of the powder is preferably 0.5 wt% or more and 20 wt% or less in terms of the weight ratio of carbon to the mass of the entire silicon oxide powder. When the amount formed is less than 0.5 wt%, the effect of imparting conductivity is small due to the small amount of carbon coating. When it exceeds 20 wt%, there is a concern that the capacity is reduced due to the weight of the SiO _x powder occupying the whole powder and the safety is lowered due to the outermost surface of the particles being covered with a combustible substance.

As a method of satisfying R (s) / R (i) <1, that is, a method of making the outermost surface Li intensity ratio R (s) smaller than the near surface Li intensity ratio R (i), for example, by irradiation with an electron beam There is a method of flipping off Li on the particle surface, but there are other methods, for example, a method of depositing a SiO _x layer on the particle surface, and the method is not limited. When the SiO _x layer is deposited on the particle surface, the Li concentration on the particle surface is lowered and safety is increased.

Although the Li-containing silicon oxide powder of the present invention is a Li-containing SiO _x powder subjected to Li doping, the Li concentration on the outermost surface of the particles is lower than the Li concentration on the near surface slightly away from the outermost surface to the inside. In addition to low dust explosiveness and excellent safety, it is possible to suppress a decrease in battery performance, which is a problem in wet cleaning, and to show high battery performance. The dust explosibility of the powder is a level at which no explosion occurs at 2000 g / m ³ or less in the dust explosion lower limit concentration measurement specified in JISZ8818.

It is a cross-sectional image (TEM observation image) of the whole particle | grains of Li containing silicon oxide powder of this invention. It is an ADF image in the surface vicinity of the same Li containing silicon oxide powder. It is a Li-K edge mapping image in the surface vicinity of the same Li containing silicon oxide powder. It is a Si-L edge mapping image in the surface vicinity of the same Li containing silicon oxide powder. It is a Li-K edge mapping image in the surface vicinity of the conventional Li containing silicon oxide powder. It is a Si-L edge mapping image in the surface vicinity of the same Li containing silicon oxide powder.

  Embodiments of the present invention will be described below. The Li-containing silicon oxide powder of this embodiment is manufactured by the following method.

First, a raw material silicon oxide powder and a powder lithium source to be mixed therewith are prepared. The raw silicon oxide powder is a lower silicon oxide powder represented by the composition formula SiO _x (0.5 <x <1.5), and here, amorphous SiO produced by a precipitation method, that is, SiO _x. (X = 1) is used. The particle size of the SiO _x powder is 0.5 to 30 μm in terms of median diameter.

The source of powder lithium is lithium hydride (LiH), lithium aluminum hydride (LiAlH), lithium oxide (Li ₂ O), lithium hydroxide (LiOH), lithium carbonate (Li ₂ CO ₃ ), etc. Lithium fluoride (LiH) is used.

  C coating for coating the conductive carbon film is performed on the raw silicon oxide powder. The C coating is performed by a thermal CVD method using a hydrocarbon gas as a carbon source, for example, a heat treatment in a mixed gas atmosphere of argon and propane. The C coating amount is 0.5 to 20 wt% expressed as a weight ratio of carbon to the mass of the entire silicon oxide powder.

Li doping is performed on the SiO _x powder after C coating. That is, firing a mixture of a SiO _x powder and the powder lithium source after C coat. The mixing ratio is 0.2 to 0.9 in terms of Li / O molar ratio, preferably 0.2 to 0.6. The firing temperature is in the range of 300 to 800 ° C., more specifically, a temperature range in which disproportionation of the silicon oxide powder does not occur. By this firing, the SiO _x powder is Li-doped to become a Li-containing SiO x powder.

The Li-containing SiO _x powder after Li doping is subjected to Li removal treatment by electron beam irradiation on the powder particle surface. Li can be selectively removed by adjusting the acceleration voltage in electron beam irradiation. That is, only Li is selectively removed leaving Si from the surface of the powder particles. Thereby, Li amount ratio in the outermost surface of powder particle | grains will be 0.6 or less, and becomes smaller than Li amount ratio in the near surface which left | separated slightly from the outermost surface inside.

  That is, since Li dope is a surface reaction of powder particles, Li-containing SiO before Li removal _xIn the powder, the ratio of the Li amount to the sum of the Li amount and the Si amount (Li amount ratio) tends to gradually decrease from the outermost surface of the particle to the inside thereof. As a result, the outermost surface Li amount ratio is more than the near surface Li amount ratio. Although it becomes large, according to the selective removal process of Li by the electron beam irradiation to the powder particle surface, the outermost surface Li amount ratio can be made 0.6 or less and smaller than the near surface Li amount ratio. As a result, the reactivity of the powder, which is a problem with Li doping, is increased, and the increase in dust explosiveness due to this is solved. Further, according to the selective removal of Li, unlike the wet cleaning, Si remains, so that the deterioration of the battery performance which becomes a problem in the wet cleaning is also solved.

  As described above, the outermost surface Li amount ratio and the near surface Li amount ratio can be specifically evaluated by STEM observation on the powder particles and calculation of the spectral intensity of the outermost surface and the near surface by EELS measurement.

(Example 1)
As silicon oxide powder that is a raw material for producing Li-containing silicon oxide powder, amorphous SiO _x powder (x = 1) produced by a precipitation method, that is, SiO powder was prepared. For this raw material SiO powder, as a heat treatment for C coating, a hydrocarbon gas in which argon and propane are mixed at a weight ratio of 1: 1 is used as a carbon source, and this is supplied into the furnace at a flow rate of 1 liter per minute. Then, the SiO powder was heat-treated at 850 ° C. for 30 minutes in the furnace.

Li doping was performed on the SiO powder after C coating. LiH powder was selected as the powder lithium source. Specifically, the SiO powder and LiH powder after C coating were mixed so that the Li / O molar ratio was 0.2, and baked in an argon atmosphere (1 atm, 600 ° C.) in an atmosphere furnace for 24 hours. . The particle size of the SiO powder at this stage is 6.26 μm in terms of the median diameter (D ₅₀ ) measured by a laser diffraction type particle size distribution measuring device. Further, the C coating amount, that is, the coating amount of the conductive carbon film is 0.8 wt%.

  The Li-containing SiO powder obtained by Li doping was irradiated with an electron beam in vacuum and at an acceleration voltage of 300 kV to selectively remove Li existing on the surface of the powder particles. Then, the powder after removing Li was subjected to FIB processing to expose the cross section, and STEM observation was performed. In FIB processing, Hitachi FB-2000A was used, sampling was performed in Ar gas, and an Al grid was used. In STEM observation, JEOL JEM-ARM200F was used, GATA JIF Quantum was used for the EELS analysis filter, and GATA Digital micrograph was used for image acquisition. The observation conditions were a beam diameter of 0.2 mmφ, an acceleration voltage of 200 kV, and an energy resolution of about 0.5 eV FWHM. A cross-sectional image of the whole particle by STEM observation is shown in FIG.

  The cross-sectional image of the entire particle was enlarged in the vicinity of the particle surface, EELS analysis was performed, and Li-K edge mapping and Si-L edge mapping were performed. FIG. 2 shows an ADF image (low-angle annular dark field image) in the vicinity of the particle surface, FIG. 3 shows a Li-K edge mapping image in the vicinity, and FIG. 4 shows a Si-L edge mapping image in the vicinity.

  When analyzing the spectral intensity of Li-K edge, the background is calculated and subtracted from the range of 49.54 to 55.04 eV using the 1st order Log-polynominal model, and the range of 58.34 to 68.34 eV is calculated. The intensity was integrated. When analyzing the spectral intensity of Si-L edge, the power law model is used to calculate and subtract the background from the range of 88.34 to 98.34 eV, and the intensity in the range of 99.84 to 113.24 eV. Was accumulated.

Thus, to calculate the accumulated intensity I _Si integrated intensity I _Li (s) and Si-L edge region of the Li-K edge region for one column of the particle outermost surface (s), the outermost surface Li intensity ratio R ( s), ie, I _Li (s) / (I _Li (s) + I _Si (s)). Moreover, to calculate the accumulated intensity of the integrated intensity I _Li (i) and Si-L edge region of the Li-K edge region for one column of the near surface remote 500nm from the particle outermost surface inward I _Si (i), near The surface Li intensity ratio R (i), that is, I _Li (i) / (I _Li (i) + I _Si (i)) was determined. Then, a ratio between the outermost surface Li intensity ratio R (s) and the near surface Li intensity ratio R (i), that is, R (s) / R (i) was obtained.

  For the Li-containing SiO powder after surface Li removal by electron beam irradiation, a dust explosion lower limit concentration test was also performed by the method specified in JISZ8818.

(Example 2)
In Example 1, the mixing ratio of the SiO powder and the LiH powder after C coating in Li doping was set to 0.4 in terms of the Li / O molar ratio. Others are the same as Example 1.

Example 3
In Example 1, the mixing ratio of the SiO powder and the LiH powder after C coating in Li doping was 0.9 in terms of Li / O molar ratio. Others are the same as Example 1.

(Example 4)
In Example 1, C coating to the SiO powder used for Li dope was abbreviate | omitted. The mixing ratio of SiO powder and LiH powder in Li dope was set to 0.4 in terms of Li / O molar ratio. Others are the same as Example 1.

(Comparative Example 1)
In Example 1, Li removal treatment by electron beam irradiation on Li-containing SiO powder after Li doping was omitted. The mixing ratio of SiO powder and LiH powder in Li dope was set to 0.4 in terms of Li / O molar ratio. Others are the same as Example 1. FIG. 5 shows a Li-K edge mapping image in the vicinity of the surface corresponding to FIG. 3 and FIG. 6 shows a Si-L edge mapping image in the vicinity of the surface corresponding to FIG.

(Comparative Example 2)
In Example 1, Li removal treatment by electron beam irradiation on Li-containing SiO powder after Li doping was omitted. The mixing ratio of SiO powder and LiH powder in Li dope is 0.2 in terms of Li / O molar ratio. Others are the same as Example 1.

(Comparative Example 3)
In Example 1, Li removal treatment by electron beam irradiation on Li-containing SiO powder after Li doping was omitted. C coating on the SiO powder used for Li doping was omitted. The mixing ratio of SiO powder and LiH powder in Li dope was set to 0.4 in terms of Li / O molar ratio. Others are the same as Example 1.

(Comparative Example 4)
In Example 1, the Li-containing SiO powder after Li doping was subjected to water washing treatment at 25 ° C. for 24 hours instead of Li removal treatment by electron beam irradiation. Specifically, 20 g of Li-containing SiO powder after Li doping was put into 400 g of pure water (25 ° C.), stirred for 24 hours with a magnetic stirrer, and then dried in the atmosphere at 120 ° C. for 24 hours. The mixing ratio of SiO powder and LiH powder in Li dope was set to 0.4 in terms of Li / O molar ratio. Others are the same as Example 1.

(Comparative Example 5)
In Example 1, the same water washing treatment as in Comparative Example 4 was performed on the Li-containing SiO powder after Li doping instead of the Li removal treatment by electron beam irradiation. The mixing ratio of the SiO powder and the LiH powder in the Li dope was 0.9 in terms of a Li / O molar ratio. Others are the same as Example 1.

(Battery performance test)
The negative electrode of the lithium ion secondary battery was produced using SiO powder which received C coat and Li dope. Specifically, SiO powder, ketjen black, and a polyimide precursor which is a non-aqueous solvent binder are mixed at a mass ratio of 85: 5: 10, and NMP (n-methylpyrrolidone) is further added and kneaded. The slurry prepared in (1) was applied onto a copper foil having a thickness of 40 μm and pre-dried at 80 ° C. for 15 minutes. Further, after punching out to a diameter of 11 mm, an imidization treatment was performed to obtain a negative electrode.

  The median diameter of the Li-containing SiO powder was 6.26 μm in Example 1, 6.15 μm in Example 2, 6.22 μm in Example 3, 6.31 μm in Example 4, and 6.11 μm in Comparative Example 1. Comparative Example 2 was 6.21 μm, Comparative Example 3 was 6.30 μm, Comparative Example 4 was 6.45 μm, and Comparative Example 5 was 6.01 μm.

A lithium ion secondary battery was produced using the produced negative electrode. Specifically, lithium foil was used for the counter electrode in the secondary battery. As the electrolyte, a solution obtained by dissolving LiPF ₆ (phosphoryllium hexafluoride) at a ratio of 1 mol / liter in a solution in which ethylene carbonate and diethyl carbonate were mixed at a volume ratio of 1: 1 was used. And the coin cell was produced using the 30-micrometer-thick polyethylene porous film for a separator.

  The prepared lithium ion secondary battery was subjected to a charge / discharge test using a secondary battery charge / discharge test apparatus (manufactured by Nagano Co., Ltd.). Table 1 shows the charge / discharge conditions.

  By this charge / discharge test, the initial charge capacity and the initial discharge capacity were measured to determine the initial efficiency as battery performance.

  The dust explosion test results (dust explosion lower limit concentration) and battery performance test results (initial efficiency) of each Li-containing SiO powder obtained in Examples 1 to 4 and Comparative Examples 1 to 5 are the main specifications [C coat Table 2 shows the presence or absence of Li, the presence or absence of Li removal treatment by electron beam irradiation, the outermost surface Li intensity ratio R (s), the near surface Li intensity ratio R (i) and R (s) / R (i)].

The Li-containing SiO powder obtained in Comparative Examples 1 and 2 is a conventional general negative electrode material for a lithium secondary battery, and has not been subjected to Li removal treatment by electron beam irradiation on the powder particle surface after Li doping. . The outermost surface Li intensity ratio R (s) exceeds 0.6, and the ratio between the outermost surface Li intensity ratio R (s) and the near surface Li intensity ratio R (i), that is, R (s) / R (i) is 1 is exceeded. Because Li doping is a surface reaction, the closer to the surface, the higher the Li concentration. Since using easily nonaqueous solvent based binder ensure battery performance compared with aqueous binder, the initial efficiency is over 75%, dust explosion lower limit concentration of the 1700~1800g / m ^3, reach the 2000 g / m ³ do not do.

  Compared to the Li-containing SiO powders obtained in Comparative Examples 1 and 2, the Li-containing SiO powder obtained in Comparative Example 3 has not been subjected to C coating treatment, and therefore has low initial efficiency and cannot maintain battery performance.

The Li-containing SiO powder obtained in Comparative Examples 4 and 5 was subjected to a water washing treatment instead of Li removal treatment by electron beam irradiation on the powder particle surface after Li doping. Although Li is removed from the surface of the powder particles, R (s) / R (i) still exceeds 1 because Si is also removed. Although the dust explosion lower limit concentration reaches 2000 g / m ³ by the water washing treatment, the initial efficiency is lowered due to the decrease of the surface Li amount. In particular, in Comparative Example 5, although the amount of Li in the particles is as large as Li / O = 0.9, the initial efficiency is extremely lowered and the battery performance cannot be maintained. This is because when Li / O becomes large, the activity with respect to the cleaning solution becomes too high, and zero-valent Si that bears the reversible capacity has reacted by the cleaning.

  On the other hand, the Li-containing SiO powder obtained in Examples 1 to 4 is subjected to Li removal treatment by electron beam irradiation on the powder particle surface after Li doping. The outermost surface Li intensity ratio R (s) is 0.6 or less, and the ratio between the outermost surface Li intensity ratio R (s) and the near surface Li intensity ratio R (i), that is, R (s) / R (i) is Less than 1. As a result, the dust explosion lower limit concentration is 2000 g / m. ^ThreeIs over. The initial efficiency, which is battery performance, also exceeds 75%. The near surface Li intensity ratio R (i) hardly changes.

In addition, in Example 3, although the Li doping amount is as large as 0.9 in Li / O, the Li removal effect on the particle surface is remarkable, and high safety is ensured. The initial efficiency is particularly high due to the large amount of Li doping. The difference from Comparative Example 5 is significant. Therefore, in the Li-containing SiO _x powder of the present invention, high Li is allowed, and particularly high initial efficiency is ensured. A particularly desirable Li amount is 0.4 or more in terms of Li / O.

In Example 4, the C coat is omitted, but the influence on the initial efficiency is negligible. This is because the proportion of the C coat layer in the entire powder is very small. The difference from Comparative Example 3 is remarkable. Therefore, in the Li-containing SiO _x powder of the present invention, the C coat can be omitted, and the omission can reduce the manufacturing cost and improve the productivity.

Claims (6)

Li-containing SiO _x powder (0.5 <x <1.5) used for a negative electrode material of a lithium secondary battery,
Obtaining the spectral intensities of the Li-K edge region and Si-L edge region when EELS measurement is performed in a 1 μm square field of view including the particle surface with respect to the powder particle cross section and having a resolution of 50 pixels × 50 pixels or more,
Wherein among the integrated intensity of one row of the particle outermost surface in the field of view, Li-K the integrated intensity of the edge zone and _{I Li (s), Si-} Ledge zone I _Li when the integrated intensity was I _Si (s) of (S) / (I _Li (s) + I _Si (s)) is the outermost surface Li intensity ratio R (s),
Of the integrated intensity of the near surface 500 nm away from the outermost surface of the particle in the visual field, the integrated intensity in the Li-K edge region is I _Li (i), and the integrated intensity in the Si-Ledge region is I _Si ( I _Li (i) / (I _Li (i) + I _Si (i)) when i) is used as the near surface Li intensity ratio R (i),
Li-containing silicon oxide powder satisfying R (s) / R (i) <1.   The Li-containing silicon oxide powder according to claim 1, wherein the Li content is 0.2 to 0.9 in terms of a Li / O molar ratio. In Li-containing silicon oxide powder according to any one of claims 1 to 2, Li-containing silicon oxide powder according outermost Li intensity ratio R (s) is 0.6 or less. In Li-containing silicon oxide powder according to any one of claims 1 to 3, Li-containing silicon oxide powder median diameter determined by a laser diffraction type particle size distribution measuring apparatus (D ₅₀₎ is 0.5μm or more 30μm or less.   The Li-containing silicon oxide powder according to any one of claims 1 to 4, wherein a conductive carbon film is formed on at least a part of the outer surface of the powder particles.   The Li-containing silicon oxide powder according to claim 5, wherein the amount of the conductive carbon film formed is 0.5 wt% or more and 20 wt% or less in terms of the weight ratio of carbon to the total mass of the silicon oxide powder. Powder.