JP6688673B2 - Silicon oxide powder negative electrode material - Google Patents

Description

The present invention relates to a SiO _x powder negative electrode material used for forming a negative electrode of a lithium ion secondary battery, and more particularly to a SiO _x powder negative electrode material that has been subjected to an irreversible capacity cancellation treatment. Note that in this specification, silicon oxide and SiO _x have the same meaning.

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 powder negative electrode material is formed into a thin film negative electrode by mixing a slurry made by mixing SiO _x powder, a conductive additive and a binder, and applying it to a current collector made of copper foil or the like and drying. It The SiO _x powder here is obtained, for example, by heating a mixture of silicon dioxide and silicon, cooling the generated silicon monoxide gas, depositing it, and then crushing it finely. It is known that the SiO _x powder produced by such a precipitation method contains a large amount of an amorphous portion and has a small thermal expansion coefficient to improve cycle characteristics.

A problem with such a SiO _x -based powder negative electrode material is low initial efficiency, and Li doping is known as a method for solving this problem. Li doping is carried out by mixing a SiO _x powder and a powder lithium source and firing the mixture (Patent Documents 1 to 4). When Li-doping is performed on the SiO _x powder particles, a lithium compound (Li ₄ SiO ₄ etc.) that does not contribute to charge and discharge is generated in advance, and by suppressing the generation of this lithium compound at the time of the first charge, The initial efficiency is improved. This processing is called irreversible capacity cancellation processing.

As a means for improving battery performance other than canceling the irreversible capacity, there is a C coat that coats conductive carbon on the particle surface of the SiO _x powder, which improves the cycle characteristics. In Patent Document 3, C coating is performed after Li doping, and in Patent Document 4, Li doping is performed after C coating.

In addition, Mg doping is known as an irreversible capacity canceling treatment other than Li doping (Patent Document 5). When Mg doping is performed on the SiO _x powder, a magnesium compound (Mg ₂ SiO ₄ etc.) that does not contribute to charge and discharge is generated in advance, and the generation of a lithium compound at the time of the first charge is suppressed, thereby reducing the initial efficiency. Improvement is achieved.

In addition, since magnesium silicate such as Mg ₂ SiO ₄ has a larger lattice constant than lithium silicate such as Li ₄ SiO _4, it has a high Li ion conductivity. For this reason, it has been reported that the Mg _x -doped SiO _x powder negative electrode material is superior to the Li-doped SiO _x powder negative electrode material in output characteristics, specifically, rapid charge and discharge characteristics.

However, Mg is heavier than Li. Therefore, there is a problem that the Mg-doped SiO _x powder negative electrode material has a lower capacity per weight than the Li-doped SiO _x powder negative electrode material.

Currently, batteries for information terminal equipment are generally used as the applications of the SiO _x powder negative electrode material, but batteries for transportation equipment such as automobiles are also considered in the future. In the case of batteries for transportation equipment, it is premised that a large-capacity battery is installed, so while high charge / discharge characteristics are a major advantage, low capacity per weight is a major drawback. In addition to high cycle characteristics, high charge / discharge characteristics, and overall battery performance, including high capacity per weight, is required.

Japanese Patent No. 2997741 Japanese Patent No. 4702510 Japanese Patent No. 4985949 Japanese Patent No. 5411781 Japanese Patent No. 4477522

The object of the present invention is to improve not only the initial characteristics and the cycle characteristics but also the characteristics of the rapid charge / discharge characteristics, and further, the SiO _x effective for improving the overall battery performance including the capacity per weight. An object is to provide a powdered negative electrode material.

In order to achieve the above object, the present inventor conducted a detailed comparative study on the advantages and disadvantages of Li-doping and Mg-doping with respect to the SiO _x -based powder negative electrode material. As a result, the following facts were revealed.

The Li-doped SiO _x powder negative electrode material is effective in improving the initial efficiency, but on the other hand, the activity against air, water, and other solvents becomes high, which not only deteriorates the handling property, but also uses a binder. When making a slurry, water that is a solvent reacts with Li when an aqueous binder is used, and when a polyimide that is a non-aqueous solvent binder is used, the polyimide also reacts with Li to form a slurry. Stability is reduced, which causes deterioration of cycle characteristics.

It is considered that this is because a large amount of highly active Li remains on the surface of the Li-doped SiO _x because the reaction between the Li source and SiO _x is a surface reaction. That is, in the surface reaction of the SiO _x and _{Li, Li 2 Si 2 O 5} , Li 2 SiO 3, Li 4 lithium silicate such SiO _4, but further believed that such LiSi alloy occurs, both compared to the SiO _x In addition to being highly active and requiring attention in handling in air, elution and side reactions especially in an aqueous binder become a problem in the current negative electrode manufacturing process and cause deterioration of cycle characteristics in particular.

On the other hand, magnesium silicates such as MgSiO ₃ and Mg ₂ SiO ₄ produced when SiO _x is doped with Mg are more stable to water and organic solvents than lithium silicate, and have the same or higher Li content than lithium silicate. Since it has ionic conductivity, Mg doping to SiO _x cancels the irreversible capacity and enhances the rapid charge and discharge characteristics, and also secures stability when slurried and is an effective means for securing cycle characteristics. it is conceivable that.

  However, as the characteristics of the negative electrode active material itself, Mg doping is deteriorated as compared with the case of Li doping or undoped. This is because, in the case of Mg doping, there is concern that the cycle characteristics may deteriorate due to the structural change of the active material during charging / discharging, and more specifically, magnesium silicate reacts with Li during charging / discharging to react with magnesium oxide. Since it decomposes into lithium silicate, there is a concern that cycle characteristics will deteriorate in the long term.

  In other words, Li doping causes deterioration of cycle characteristics, which is not a characteristic of the active material itself but a special circumstance due to Li reacting with the binder in the process of slurry formation.

Based on these facts, the present inventor has proposed the combined use of Li doping and Mg doping for SiO _x powder. When both Li-doped and Mg-doped are used, the amount of Mg-doped is reduced as compared with the case of only Mg-doped, so that the capacity per weight is increased. Further, the rapid charge / discharge characteristics are improved as compared with the case of only Li doping. And above all, Mg-doping compensates for the high reactivity with respect to the binder, which is the weak point of Li-doping, and draws out the good characteristics as an active material, which is the advantage of Li-doping, so that the cycle is longer than in the case of any single dope. It has been found that the characteristics can be improved, and as a result, not only the height of initial characteristics and cycle characteristics but also the height of rapid charging / discharging characteristics, and further the total capacity including the capacity per weight is comprehensive. It became clear that it was very effective in improving the performance.

  By the way, there is no big difference between the two with respect to the original purpose of the irreversible capacity canceling effect and the initial efficiency.

SiO _x based powder negative electrode material of the present invention has been completed on the basis of these findings, a SiO _x based powder negative electrode material for use in forming the negative electrode of a lithium secondary battery, the Li _x Mg _y SiO _z It is represented by a composition formula, where x, y and z are positive real numbers and satisfy the following conditions, and the median diameter (D ₅₀ ) measured by a laser diffraction type particle size distribution measuring device is 0.5 μm or more and 30 μm or more. It is the following.
0.5 ≦ z ≦ 1.5
z / 5 ≦ x + y ≦ z
z / 100≤x and z / 100≤y

That is, the SiO _x -based powder negative electrode material of the present invention is a SiO _x powder doped with both Li and Mg, and when both Li and Mg are doped, the weight per unit weight is higher than that in the case of Mg doping alone. Capacity increases. Further, as compared with the case of Li doping alone, the Li ion conductivity of the silicic acid compound is increased, so that the rapid charge / discharge characteristics are improved. Above all, the cycle characteristics are improved as compared with the case of any of the dopes.

  Here, z is the O amount (O / Si ratio). If it is less than 0.5, the negative electrode material becomes too close to Si, and the activity against oxygen becomes high and the stability decreases. On the contrary, if it exceeds 1.5, the initial efficiency decreases due to excess oxygen and the battery performance becomes poor. descend. Therefore, z is set to 0.5 or more and 1.5 or less.

  Further, x + y is the total doping amount of Li and Mg. If it is less than z / 5, the irreversible capacity canceling effect due to Li doping and Mg doping is thin, while if it exceeds z, a highly active Li-Si alloy or Mg-Si alloy is generated, which causes a handling problem. The high reactivity with the slurry solvent and the binder may adversely affect the battery performance. Therefore, x + y is set to z / 5 or more and z or less. This means that the irreversible capacity cancellation rate due to both Li and Mg is 20 to 100%.

  Further, both the Li doping amount (Li / Si ratio) x and the Mg doping amount (Mg / Si ratio) y need to be z / 100 or more. If either one is less than z / 100, the combined effect is small. This means that the Li doping amount and the Mg doping amount each bear an irreversible capacity cancellation rate of at least 1%. Both x and y are preferably z / 20 or more.

Regarding the ratio of the Li doping amount and the Mg doping amount, it is desired that the Li doping amount is larger than the Mg doping amount. This is because it is better for SiO _x feature is doped with Li from SiO _x doped with Mg as an active material, Mg is the problems due to high reactivity in the slurry process of the Li even a relatively small amount of dope The reason is that the weight can be effectively suppressed, the weight occupied by the doping element becomes smaller as the proportion of Li increases, and the capacity per weight increases.

A particularly preferred Mg-doped form is to form a limited Mg-doped layer on the surface of SiO _x powder particles. Specifically, the molar ratio ys / zs of Mg to O on the surface of the particles should be 5 times or more the molar ratio yi / zi of Mg to O inside the particles, that is, ys / zs ≧ (yi / zi ) ・ 5. By doing so, Li-doped SiO _x can be efficiently stabilized, and the amount of Mg-doped can be reduced, so that the capacity per weight can be increased and other battery performance, especially cycle characteristics, can be improved. It is possible to improve.

An important factor other than these is the particle size of SiO _x powder. If the particle size is too large, the effect of the expansion of the particles during charging / discharging becomes too large and the cycle characteristics deteriorate, and if too small, the surface area becomes too large and the Coulomb efficiency decreases due to the reaction with the electrolytic solution, and Since it causes an increase in reactivity with air, it is necessary that the median diameter (D ₅₀ ) measured by a laser diffraction particle size distribution measuring device is 0.5 μm or more and 30 μm or less, and 1 μm or more and 15 μm or less. preferable.

Further, it is desirable that at least a part of the surface of the powder particles is coated with conductive carbon from the viewpoint of conductivity in the electrode, and the coating amount is a weight ratio of carbon to the mass of the entire SiO _x powder. It is desirable that the content is 0.5 to 20 wt%. If the coating amount is less than 0.5 wt%, the effect of imparting conductivity is poor and sufficient charge / discharge characteristics cannot be obtained. On the other hand, if the coating amount exceeds 20 wt%, there is a concern that the capacity may be reduced due to the weight of the SiO _x powder occupying the entire powder being reduced.

The SiO _x -based powder negative electrode material of the present invention has a capacity per weight increased as compared with the case of Mg doping alone by the combined use of Li doping and Mg doping, and has a rapid charge / discharge characteristic as compared with the case of Li doping alone. Not only is it improved, but the cycle characteristics are also improved compared to the case of either dope, so that the initial characteristics and cycle characteristics are high, the rapid charge and discharge characteristics are high, and even the capacity per weight is large. It is effective in improving the overall battery performance including the above.

A EDX mapping diagram for Si powder particle cross sections of the SiO _x based powder negative electrode material of the present invention. It is an EDX mapping figure about O similarly. Similarly, it is an EDX mapping figure about Mg. A EDX mapping diagram for Si powder particle cross sections according to another SiO _x based powder negative electrode material of the present invention. It is an EDX mapping figure about O similarly. Similarly, it is an EDX mapping figure about Mg.

Embodiments of the present invention will be described below. The SiO _x powder negative electrode material of this embodiment can be manufactured by the following doping method.

The first doping method is a method of doping SiO _x powder with Li and further doping the powder with Mg. Specifically, after the SiO _x powder and the powder Li source are mixed and fired, the powder Mg source is mixed with the fired powder and fired. Alternatively, after the SiO _x powder is electrochemically doped with Li, the powder is further electrochemically doped with Mg.

The second doping method is a method of doping Mg into the SiO _x powder and further doping the powder with Li. Specifically, after the SiO _x powder and the powder Mg source are mixed and fired, the powder Li source is mixed with the fired powder and fired. Alternatively, after the SiO _x powder is electrochemically doped with Mg, the powder is further electrochemically doped with Li.

The third doping method is a method of simultaneously doping Li and Mg into SiO _x powder. Specifically, the SiO _x powder is mixed with a powder Li source and a powder Mg source and fired.

The fourth doping method is a method of simultaneously mixing Li and Mg in the production process of the SiO _x powder. Specifically, Si, a SiO ₂ -based powder, a powder Li source, and a powder Mg source are mixed, heat-treated under reduced pressure, and vapor-deposited on a substrate.

By any of the doping methods, it is possible to manufacture a SiO _x powder negative electrode material in which Li doping and Mg doping are used in combination, but from the viewpoint of forming the Mg doped layer on the particle surface only, the first to the first Method 3 is preferred.

As the powder Li source, lithium hydride (LiH), lithium oxide (Li ₂ O), lithium hydroxide (LiOH), lithium carbonate (Li ₂ CO ₃ ) or the like can be used. As the powder Mg source, magnesium hydride (MgH ₂ ), magnesium oxide (MgO), magnesium hydroxide (Mg (OH) ₂ ), magnesium carbonate (MgCO ₃ ), or the like can be used.

  The powder before the both dopes, the powder after the both dopes, or the powder between the both dopes is subjected to C coating for coating the conductive carbon film, if necessary. This 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.

(Example 1)
The SiO _x powder negative electrode material of the present invention was manufactured by the fourth doping method. Specifically, it is as follows.

Si powder, SiO ₂ powder, powder Li source LiO ₂ powder, and powder Mg source MgO powder were mixed at a molar ratio of 13: 7: 2: 4. The mixed powder was calcined under reduced pressure at 1400 ° C. to be sublimated. The sublimed gas was deposited on a quartz substrate. The deposit was collected and crushed. The powder obtained by pulverization was subjected to C coating at 850 ° C. by thermal CVD using a mixed gas of argon and propane as a carbon source.

The obtained powder was analyzed for each element of Si, O and Li. The contents of Si and Li were determined by ICP emission spectroscopy. The content of O was measured by an inert gas fusion infrared absorption method (GFA) using TC-436 manufactured by LECO. As a result, it was found that the composition of the deposited material was Li _x Mg _y SiO _z (x = 0.2, y = 0.2, z = 1). The C coating amount was 1.0 wt% and the median diameter D ₅₀ was 6.13 μm.

  The powder obtained is a SiO powder that is Li-doped and Mg-doped and C-coated. A cross section of powder particles was exposed to this powder by the BIB method, and the cross section was observed by a field emission SEM. In addition, EDX mapping was performed for the three elements of Si, O, and Mg at a voltage of 5 kV on the exposed particle cross section. The EDX mapping results for Si, O, and Mg are shown in FIGS. 1, 2, and 3, respectively.

For particle cross sections subjected to EDX mapping, when the diameter of the circumscribed circle and the D _O, Si in the region from the particle surface to D _O / 100, O, seeking the average of 10 points the composition ratio of Mg, particles The molar ratio ys / zs of Mg to O on the surface was determined. Further, the molar ratio yi / zi of Mg to O inside the particles was determined. It was ys / zs = 0.21 and yi / zi = 0.20.

Here, in the EDX mapping of the particle cross section, the particle surface refers to the surface on the particle inner side of the boundary between the inside of the particle and the portion where the Si signal is 0. The composition ratio inside the particle is within the range from the surface of the particle up to D _O / 5 to the surface of the particle up to D _O / 2, and 10 points from the area of the point inside the particle. Randomly selected and sought.

  Then, a negative electrode for a lithium-ion secondary battery was produced using the SiO powder that had been subjected to Li-doping and Mg-doping and also had been subjected to C-coating, and the battery performance thereof was evaluated.

  Specifically, SiO powder, Ketjen black, and a polyimide precursor that is a non-aqueous solvent-based binder are mixed in a mass ratio of 85: 5: 10, and NMP (n-methylpyrrolidone) is further added and kneaded. A slurry was prepared. Then, the slurry was applied onto a copper foil having a thickness of 40 μm, pre-dried at 80 ° C. for 15 minutes, punched out to a diameter of 11 mm, and then imidized to obtain a negative electrode.

A lithium ion secondary battery was produced using the produced negative electrode. Lithium foil was used as the counter electrode in the secondary battery. As the electrolyte, a solution in which ethylene carbonate and diethyl carbonate were mixed at a volume ratio of 1: 1 and LiPF ₆ (lithium phosphorus hexafluoride) was dissolved at a ratio of 1 mol / liter was used. Then, a coin cell was produced by using a polyethylene porous film having a thickness of 30 μm as the separator.

  A charging / discharging test was performed on the manufactured lithium ion secondary battery using a secondary battery charging / discharging test device (manufactured by Nagano Co., Ltd.). Table 1 shows the charging / discharging conditions.

  By this charge / discharge test, the initial charge capacity, the ratio of the initial discharge capacity to the initial charge capacity (initial efficiency), the ratio of the third discharge capacity to the initial discharge capacity (rate characteristic), the 50th discharge to the initial discharge capacity. The capacity ratios (50th retention rate) were determined.

(Example 2)
In Example 1, the mixing molar ratio of Si powder, SiO ₂ powder, LiO ₂ powder as a powder Li source, and MgO powder as a powder Mg source was set to 25: 15: 6: 4. Li-doped / Mg-doped C-coated SiO powder was produced in the same manner as in Example 1 except that this ratio was different.

The composition of the produced SiO powder is Li _x Mg _y SiO _z (x = 0.3, y = 0.1, z = 1). The C coating amount is 1.5 wt% and the median diameter D ₅₀ is 6.05 μm. The molar ratio of Mg to O on the particle surface is ys / zs = 0.12, and the molar ratio of Mg to O inside the particle is yi / zi = 0.13.

(Example 3)
The SiO _x powder negative electrode material of the present invention was produced by the first doping method. Specifically, SiO _x (x = 1) was prepared by a precipitation method and crushed. The SiO powder was C-coated at 850 ° C. by thermal CVD using a mixed gas of argon and propane as a carbon source. Lithium hydride, which is a powder Li source, was mixed with the C-coated SiO powder so that the molar ratio with SiO was Li / O = 0.35, and then the mixture was baked at 600 ° C. for 24 hours to perform Li doping. I went.

Further, magnesium-hydride powder, which is a powder Mg source, is mixed with Li-doped C-coated SiO powder so that the molar ratio is Mg / O = 0.05, and the mixture is baked at 600 ° C. for 24 hours to add Mg-doped powder. went. At this time, the temperature rising rate when the temperature was raised to 600 ° C. was 300 ° C./Hr. At this point, the C coating amount is 0.8 wt% and the median diameter D ₅₀ is 6.26 μm.

  The EDX mapping results for Si, O, and Mg are shown in FIGS. 4, 5, and 6, respectively. The molar ratio of Mg to O on the particle surface obtained from this is ys / zs = 0.3, and the molar ratio of Mg to O inside the particle is yi / zi = 0.001. It is clear from FIG. 6 that the Mg-doped layer is formed only on the surface of the particles.

(Example 4)
In Example 3, lithium hydride as a powder Li source was mixed with the C-coated SiO powder so that the molar ratio with SiO was Li / O = 0.37. Further, magnesium hydride powder as a powder Mg source was mixed with Li-doped SiO powder so that the molar ratio was Mg / O = 0.03. The molar ratio of Mg to O on the particle surface was ys / zs = 0.22, and the molar ratio of Mg to O inside the particle was yi / zi = 0.001, which was obtained from the EDX mapping results of Si, O, and Mg. Is. Others are the same as in the third embodiment.

(Example 5)
In Example 3, lithium hydride as a powder Li source was mixed with the C-coated SiO powder so that the molar ratio with SiO was Li / O = 0.39. Further, magnesium hydride powder as a powder Mg source was mixed with Li-doped SiO powder so that the molar ratio was Mg / O = 0.01. The molar ratio of Mg to O on the particle surface obtained from the EDX mapping results for Si, O, and Mg is ys / zs = 0.09, and the molar ratio of Mg to O inside the particle is yi / zi = 0. . Others are the same as in the third embodiment.

(Comparative Example 1)
C-coated SiO powder was prepared in the same manner as in Example 3. That is, SiO _x (x = 1) was prepared by a precipitation method, pulverized, and then the SiO powder was C-coated at 850 ° C. by thermal CVD using a mixed gas of argon and propane as a carbon source. The C coat amount in the C coat SiO powder is 1 wt%, and the median diameter D ₅₀ is 6.13 μm. The battery performance of this C-coated SiO powder was evaluated by the same method as in Example 1.

(Comparative example 2)
Lithium hydride that is a powder Li source was mixed with the C-coated SiO powder produced in Comparative Example 1 so that the molar ratio with SiO was Li / O = 0.40, and then calcined at 600 ° C. for 24 hours. By doing so, Li doping was performed. The battery performance of this Li-doped SiO powder was evaluated by the same method as in Example 1.

(Comparative example 3)
Si powder, SiO ₂ powder, and MgO powder as a powder Mg source were mixed at a molar ratio of 7: 3: 4 to prepare a raw material. Except for this, the Mg-doped SiO powder was produced in the same manner as in Example 1. The composition of the produced SiO powder is Li _x Mg _y SiO _z (x = 0, y = 0.4, z = 1). The C coating amount is 1.5 wt% and the median diameter D ₅₀ is 6.05 μm. The molar ratio of Mg to O on the particle surface is ys / zs = 0.42, and the molar ratio of Mg to O inside the particle is yi / zi = 0.39. The battery performance of this Mg-doped SiO powder was evaluated by the same method as in Example 1.

  Table 2 shows the initial discharge capacity, initial efficiency, 50th maintenance rate and rate characteristics of each SiO powder obtained in Examples 1 to 5 and Comparative Examples 1 to 3, together with the composition of each powder.

  The SiO powder produced in Comparative Example 1 is a normal C-coated SiO negative electrode material that is neither Li-doped nor Mg-doped. In addition, the SiO powder prepared in Comparative Example 2 is a C-coated SiO negative electrode material that is only Li-doped, and the SiO powder prepared in Comparative Example 3 is a C-coated SiO negative electrode material that is only Mg-doped. is there.

  The Li-doped SiO negative electrode material of Comparative Example 2 is superior in initial efficiency to the normal SiO negative electrode material of Comparative Example 1 due to the irreversible capacity canceling effect. However, due to a side reaction in the slurry forming step, the 50th maintenance rate, that is, the cycle characteristic is significantly deteriorated.

  On the other hand, the Mg-doped SiO negative electrode material of Comparative Example 3 showed the same initial efficiency as that of the Li-doped SiO negative electrode material of Comparative Example 2, while improving the stability in the slurrying process. Since the characteristics are excellent and the Li ion conductivity of magnesium silicate is high, the rate characteristics, which are rapid charge / discharge characteristics, are also excellent. However, since the atomic weight of Mg is large, the initial discharge capacity (capacity per weight) is greatly reduced.

  On the other hand, the SiO powders produced in Examples 1 and 2 are SiO negative electrode materials in which Li-doping and Mg-doping are used in combination, and although the total doping amount is the same, the combined use of Li-doping and Mg-doping results in There is an effect of increasing the initial discharge capacity while ensuring high initial efficiency and rate characteristics, and what is particularly noteworthy is the effect of greatly increasing the 50th maintenance rate. Among them, the SiO negative electrode material of Example 2 has a large effect of increasing the initial discharge capacity by reducing the Mg doping amount.

  The SiO powders produced in Examples 3 to 5 are also SiO negative electrode materials in which both Li doping and Mg doping are used, but Mg is unevenly distributed on the particle surface and coats the surface. Therefore, even if the Mg doping amount is further reduced, the same or higher high 50th maintenance rate is still exhibited, and the initial discharge capacity is further increased by the limitation of the Mg doping amount.

Claims (4)

A silicon oxide-based powder negative electrode material used for forming a negative electrode of a lithium secondary battery, the composition of which is represented by Li _x Mg _y SiO _z , where x, y, and z are positive real numbers and the following conditions are satisfied. A silicon oxide-based powder negative electrode material which is satisfied and has a median diameter (D ₅₀ ) of 0.5 μm or more and 30 μm or less measured by a laser diffraction type particle size distribution measuring device.
0.5 ≦ z ≦ 1.5
z / 5 ≦ x + y ≦ z
z / 100≤x and z / 100≤y   The silicon oxide powder negative electrode material according to claim 1, wherein the composition satisfies x> y. The silicon oxide-based powder negative electrode material according to claim 1, wherein the composition satisfies ys / z s ≧ (yi / zi) · 5.
ys / zs is the molar ratio of Mg to O on the particle surface
yi / zi is the molar ratio of Mg to O inside the particle The silicon oxide-based powder negative electrode material according to claim 1, wherein at least a part of the surface of the powder particles has a conductive carbon coating, and the amount of carbon in the conductive carbon coating is expressed as a ratio to the mass of the entire powder. 0.5-20 wt% silicon oxide powder negative electrode material.