Silicon oxide powder

Abstract

The present invention addresses the problem of providing a silicon oxide powder that can be used as a negative electrode active material, for lithium ion secondary batteries, which is effective in improving the retention rate of discharge capacity through charge-discharge cycles (cycle characteristics). The silicon oxide powder (which includes metal-element-containing silicon oxide) according to the present invention is a silicon oxide powder expressed as SiOx (0.5<x<1.5), and when the volume-based median diameter of the silicon oxide powder is measured using a laser diffraction particle size measurement device, the median diameter is in the range of 1 μm to 30 μm, and when a dispersion of the silicon oxide powder is passed through a filter having a pore size of 1 μm and the volume-based median diameter of the silicon oxide powder in the dispersion is measured thereafter using a dynamic light-scattering particle size measurement device, the median diameter is in the range of 150 nm to 750 nm.

Description

silicon oxide powder

[0001] The present invention relates to a powder of silicon oxide, particularly silicon oxide (including metal element-containing silicon oxide) used as a negative electrode active material for lithium ion secondary batteries.

[0002] In recent years, interest in lithium-ion secondary batteries has been increasing, and the development of high-energy density lithium-ion batteries has been accelerating. While various high-capacity materials have been developed and put into practical use as positive electrode materials, carbon materials such as graphite are still used as negative electrode materials. However, with the recent trend toward higher capacity positive electrodes, it is becoming increasingly important to also increase the capacity of negative electrodes. Silicon oxide (SiO ｘ ) is gaining popularity. However, silicon oxide has a problem in that its capacity decreases significantly with charge-discharge cycles due to factors such as a large irreversible capacity caused by the formation of an excessive coating at the interface with the electrolyte and a large volume change during charge-discharge. To solve this problem, various proposals have been made in the past aimed at improving the retention rate of discharge capacity during charge-discharge cycles (hereinafter sometimes referred to as "cycle characteristics") (see, for example, JP 2011-60610 A and JP 2021-52014 A).

[0003] JP 2011-60610 A JP 2021-52014 A

[0004] Today, the market for lithium-ion secondary batteries is expanding and their applications are becoming more diverse, making it extremely important to further improve the cycle characteristics of lithium-ion secondary batteries.

[0005] An object of the present invention is to provide a silicon oxide powder that can be used as a negative electrode active material for lithium ion secondary batteries, which is effective in improving the retention rate of discharge capacity through charge-discharge cycles (cycle characteristics).

[0006] The silicon oxide powder (including metal element-containing silicon oxide) according to the present invention is SiO ｘ(0.5<x<1.5). The volume-based median diameter of this silicon oxide powder, as measured with a laser diffraction particle size analyzer, is in the range of 1 μm or more and 30 μm or less. The silicon oxide powder according to the present invention has a volume-based median diameter of 150 nm or more and 750 nm or less when a dispersion of the silicon oxide powder is passed through a filter with a pore size of 1 μm and the volume-based median diameter of the silicon oxide powder in the dispersion is measured with a dynamic light scattering particle size analyzer.

[0007] As a result of extensive research by the present inventors, when silicon oxide powder satisfying the above-mentioned conditions was used as a negative electrode active material for lithium-ion secondary batteries, the capacity retention rate after 50 cycles of coin cells made with that negative electrode active material was improved. This is presumably because the silicon oxide powder (main particles) having a volume-based median diameter of 1 μm to 30 μm and the silicon oxide powder (fine particles) having a volume-based median diameter of 150 nm to 750 nm are mixed together, which allows the fine particles to adhere to the surfaces of the main particles and to enter the gaps between the main particles, thereby suppressing a decrease in Coulomb efficiency due to a reaction between the fine particles with a large surface area and the electrolyte, the formation of an excessive coating at the interface between the main particle surface and the electrolyte, and the disruption of the conductive path due to the expansion and contraction of Si during charge and discharge.

[0008] The silicon oxide powder is preferably doped with a metal element. In this case, the metal element is preferably at least one of Li and Mg. This is because, when the silicon oxide powder is used as a negative electrode active material, a significant decrease in the initial discharge capacity of a lithium ion secondary battery produced using the negative electrode active material can be suppressed.

[0009] Furthermore, it is preferable that at least a portion of the surface of the silicon oxide powder is covered with a conductive carbon coating. In such a case, the mass ratio of carbon in the conductive carbon coating to the mass of the silicon oxide powder is preferably within a range of 0.5 mass% to 20 mass%. This is because, when this silicon oxide powder is used as a negative electrode active material, it is possible to impart good conductivity to the silicon oxide powder while maintaining good charge / discharge capacity and suppress side reactions of the silicon oxide powder.

[0010] 1 is a schematic diagram of an apparatus for producing powdered silicon oxide according to an embodiment of the present invention.

[0011] 5 Vacuum chamber 6 Raw material chamber 7 Precipitation chamber 8 Raw material container 9 Mixed granulated raw materials 10 Heat source 11 Precipitation substrate 12 Silicon oxide

[0012] The silicon oxide powder according to the embodiment of the present invention is SiO ｘ (0.5<x<1.5).

[0013] The silicon oxide powder according to the embodiment of the present invention has a volume-based median diameter (hereinafter referred to as "D50") measured by a laser diffraction particle size distribution analyzer. Ｓ The particle size is preferably in the range of 1.0 μm or more and 30 μm or less, more preferably in the range of 1.0 μm or more and 20 μm or less, and even more preferably in the range of 1.5 μm or more and 10 μm or less. When this silicon oxide powder is used as a negative electrode active material, not only can it suppress a decrease in the coulomb rate, but it can also suppress pulverization and thereby suppress a decrease in the cycle characteristics of the negative electrode.

[0014] Furthermore, the silicon oxide powder according to the embodiment of the present invention has a volume-based median diameter (hereinafter referred to as "D50") of 1 μm when the volume-based median diameter of the silicon oxide powder in the dispersion is measured using a dynamic light scattering particle size distribution analyzer after the dispersion is passed through a filter with a pore size of 1 μm. Ｄ") is preferably in the range of 150 nm or more and 750 nm or less, more preferably in the range of 200 nm or more and 750 nm or less, and even more preferably in the range of 350 nm or more and 750 nm or less.

[0015] The above-mentioned D50 Ｄ can be adjusted by adjusting the amount of grinding aid in the dry grinding process or by sedimentation classification during the drainage in the liquid phase doping process. In the case of a batch-type grinding device, the amount of grinding aid is adjusted by adding a certain ratio of grinding aid to the material to be ground together with the material to be ground. In the case of a continuous grinding device, the material to be ground is continuously fed and added dropwise using a metering pump or the like to achieve a certain weight ratio. In such cases, water, alcohols, alkoxides, etc. are preferably used as the grinding aid, and these grinding aids can be appropriately selected depending on the grinding method, grinding device, etc. In addition, a method for preparing silicon oxide powder by sedimentation classification during drainage involves uniformly dispersing the ground silicon oxide powder in a suitable solvent, leaving it to stand for a certain period of time to allow the powder to settle and be classified, and then removing the solvent. This allows the desired D50 to be obtained. Ｄ The settling time and classification point in the settling classification during drainage can be calculated using the following equations 1 and 2.

[0016]

[0017]

[0018] In the above formulas 1 and 2, u: terminal velocity [m / s] ρ ｐ : Particle density [kg / m ３ ] ・ρ ｆ : Fluid density [kg / m ３ ] d: particle size [m] η: viscosity of fluid [Pa·s] l: distance [m] t: time [s] to fall l (distance [m])

[0019] In addition, it is preferable that the silicon oxide powder according to the embodiment of the present invention is doped with a metal element. In this case, the metal element is preferably at least one of Li and Mg. The Li source for Li doping may be lithium hydride (LiH), lithium oxide (Li ２ O), lithium hydroxide (LiOH), lithium carbonate (Li ２ CO ３ ) is used as the Mg source for Mg doping. ２ ), magnesium oxide (MgO), magnesium hydroxide (Mg(OH) ２ ), magnesium carbonate (MgCO ３ ) are used.

[0020] Furthermore, it is preferable that at least a portion of the surface of the silicon oxide powder according to the embodiment of the present invention is covered with a conductive carbon coating (hereinafter, sometimes referred to as a "C coating"). In such a case, the mass ratio of carbon in the conductive carbon coating relative to the mass of the silicon oxide powder (hereinafter, sometimes referred to as a "carbon coating amount" or a "C coating amount") is preferably in the range of 0.5 mass% to 20 mass%, more preferably in the range of 0.5 mass% to 10 mass%, and even more preferably in the range of 0.5 mass% to 5 mass%. If the carbon coating amount is less than 0.5 mass%, the effect of imparting conductivity is poor, and sufficient charge / discharge characteristics cannot be obtained. On the other hand, if the carbon coating amount exceeds 20 mass%, the SiO 2 content of the entire powder becomes too high. ｘ There is a concern that the capacity may decrease due to a decrease in the weight of the powder. Note that, as a method for forming a conductive carbon coating on at least a part of the surface of the silicon oxide powder, for example, a CVD method or the like is used, but is not limited to this.

[0021] The formation of a conductive carbon coating on at least a portion of the surface of the silicon oxide powder can also be carried out appropriately on the silicon oxide before doping with Li and Mg, the silicon oxide after doping with Li, the silicon oxide after doping with Mg, or the silicon oxide after doping with Li and Mg.

[0022] The silicon oxide powder has a BET specific surface area of ​​1 m ２ / g or more 6m ２ / g or less, and ２ / g or more 5m ２ / g or less, and more preferably 1.5m ２ / g or more 4m ２ / g or less, and more preferably 1.5m ２ / g or more 3m ２ / g or less is particularly preferred. When this silicon oxide powder is used as a negative electrode active material, it is possible to suppress a decrease in Coulomb efficiency while maintaining good output characteristics. The BET specific surface area can be measured, for example, using a Mascorp HM-1201 manufactured by Mountech Co., Ltd. In this measurement, nitrogen is used as the adsorbate, helium is used as the carrier gas, and liquid nitrogen is used as the cooling medium.

[0023] (Regarding the Method for Producing Silicon Oxide) Silicon oxide according to an embodiment of the present invention is produced using a silicon oxide production apparatus as shown in Fig. 1. This apparatus comprises a vacuum chamber 5, a source chamber 6 disposed within the vacuum chamber 5, and a deposition chamber 7 disposed above the source chamber 6.

[0024] The source chamber 6 is formed of a cylindrical body, and in the center thereof, there are disposed a cylindrical source container 8 and a heat source 10 surrounding the source container 8. As the heat source 10, for example, an electric heater can be used.

[0025] The deposition chamber 7 is composed of a cylinder arranged so that its axis coincides with that of the source container 8. A deposition base 11 made of stainless steel is provided on the inner peripheral surface of the deposition chamber 7 for depositing the gaseous silicon oxide generated by sublimation in the source chamber 6.

[0026] A vacuum device (not shown) for discharging atmospheric gas is connected to the vacuum chamber 5 accommodating the source chamber 6 and the deposition chamber 7 , and gas is discharged in the direction of arrow A.

[0027] When producing silicon oxide using the manufacturing apparatus shown in FIG. 1 , a mixed granulated raw material 9 is used, which is prepared by blending silicon powder and silicon dioxide powder, mixing, granulating, and drying the raw material. This mixed granulated raw material 9 is filled into a raw material container 8 and heated in an inert gas atmosphere such as argon gas or in a vacuum to generate (sublimate) SiO. Heating in this case is preferably performed at a temperature between 1000°C and 1600°C, and more preferably between 1200°C and 1400°C. The gaseous SiO gas generated by sublimation rises from the raw material chamber 6 and enters the deposition chamber 7, where it is deposited on the surrounding deposition substrate 11 and precipitates as silicon oxide 12. The deposited silicon oxide 12 is then removed from the deposition substrate 11 to obtain the desired silicon oxide.

[0028] EXAMPLES In the following, examples and comparative examples will be shown to explain the present invention in more detail, but the present invention is not limited to these examples.

[0029] 1. Production of Silicon Oxide Powder The target silicon oxide powder was produced by carrying out the following steps in order.

[0030] (1) Silicon oxide powder preparation step According to the above-mentioned method, a mass of silicon oxide was obtained using the manufacturing apparatus shown in FIG. 1. Specifically, silicon (Si) powder and silicon dioxide (SiO ２ ) powder and silicon dioxide powder were mixed at a molar ratio of Si:O = 1:1 to prepare a mixed granulated raw material. This mixed granulated raw material was then filled into a raw material container and heated at 1400°C under an argon atmosphere to sublimate SiO. The gaseous SiO gas generated by the sublimation was deposited on the surrounding deposition substrate and precipitated as silicon oxide. The silicon oxide was then removed to obtain chunky silicon oxide.

[0031] (2) Pulverization step: The above-mentioned block silicon oxide was pulverized in air using a bead mill to a median diameter of about 5 μm to obtain the desired silicon oxide powder. At this time, 0.5% by weight of ethanol was added as a pulverization aid relative to the silicon oxide added to the bead mill.

[0032] (3) Measurement of the median diameter of silicon oxide powder using a laser diffraction particle size distribution analyzer. The median diameter D50 of the silicon oxide powder obtained in the above-mentioned pulverization process Ｓ was measured using a laser diffraction particle size distribution measuring device (Malvern Mastersizer 3000), and the D50 Ｓ The thickness was 7.5 μm (see Table 2). The measurement conditions were as follows:

[0033] Dispersion medium: isopropyl alcohol (2-propanol) Particle refractive index: 3.500 Particle absorption rate: 1.000 Dispersion medium refractive index: 1.390

[0034] (4) Measurement of the median diameter of silicon oxide powder using a dynamic light scattering particle size distribution analyzer 10 g of the silicon oxide powder after the above-mentioned pulverization process was added to 100 mL of isopropyl alcohol (2-propanol) and dispersed for 10 minutes using an ultrasonic cleaning device (W-113MkII manufactured by Honda Electronics Co., Ltd.) to obtain a dispersion. The dispersion was then filtered using a filter with a pore size of 1 μm, and the filtered dispersion was used as a measurement sample solution. Using this measurement sample solution, the median diameter D50 of the silicon oxide powder in the measurement sample solution was measured using a dynamic light scattering particle size distribution analyzer (Nanotrack particle size distribution analyzer UPA-EX150 manufactured by Nikkiso Co., Ltd. Ｄ When the D50 was measured, Ｄ The wavelength was 351 nm (see Table 2). The measurement conditions were as follows:

[0035] Measurement time: 180 seconds Particle transmittance: Transmitted Particle shape: Aspherical Particle refractive index: 1.47 Particle density: 1.00 Dispersion medium: Isopropyl alcohol (2-propanol) Dispersion medium refractive index: 1.38

[0036] 2. Cycle Characteristics of a Battery Equipped with a Negative Electrode Composed of Lithium-Containing Silicon Oxide Powder (1) Battery Fabrication (1-1) Negative Electrode Fabrication The silicon oxide powder obtained as described above and natural graphite (median diameter 12 μm) were mixed in a mass ratio of 10:90 to form the negative electrode active material. Next, the negative electrode active material, aqueous binder solution, and conductive additive were added to a THINKY MIXER (ARE-310, manufactured by THINKY Corporation) so that the mass ratio of the negative electrode active material, sodium polyacrylate (binder), and Denka Black (acetylene black as a conductive additive) was 92:3:5, and then kneaded to prepare a slurry. Next, the slurry was applied to a 10 μm-thick copper foil, and the coating was pre-dried at 80 °C in air. The slurry-coated copper foil was then punched into a disk shape with a diameter of 11 mm. The disk-shaped slurry-coated copper foil was then dried in vacuum at 150° C. for 12 hours to obtain the desired negative electrode.

[0037] (1-2) Battery Fabrication A coin cell was fabricated using the above-mentioned negative electrode, Li foil as a counter electrode, a separator, and an electrolyte. A 20 μm-thick polyethylene porous film was used as the separator, and the electrolyte was a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 1:1, to which lithium hexafluorophosphate (LiPF ６ ) was dissolved at a concentration of 1 mol / L.

[0038] (2) Cycle Characteristics A charge-discharge test of the coin cell was conducted using a secondary battery charge-discharge tester manufactured by Electrofield Corporation. The capacity retention rate after 50 cycles (the capacity retention rate after 50 cycles is calculated by dividing the 50th discharge capacity by the initial discharge capacity and multiplying the result by 100) was found to be 79.2% (see Table 2). In this charge-discharge test, the initial charge-discharge cycle was performed at a constant current of 0.2 C until the voltage between the two electrodes of the battery reached 0.01 V. After the voltage reached 0.01 V, the battery was charged at a constant potential until the current reached 0.01 C (CC-CV 0.2 C, 0.01 V-0.01 C) (see Table 1). In addition, the initial charge-discharge cycle was performed at a constant current of 0.2 C until the voltage between the two electrodes of the battery reached 1.5 V (CC 0.2 C, 1.5 V cut-off) (see Table 1). In the second and subsequent charge / discharge cycles, charging was performed at a constant current of 1.0 C until the voltage between the two electrodes of the battery reached 0.01 V, and after the voltage reached 0.01 V, constant potential charging was performed until the current reached 0.01 C (CC-CV 1.0 C, 0.01 V-0.01 C) (see Table 1). In the second and subsequent charge / discharge cycles, discharging was performed at a constant current of 1.0 C until the voltage between the two electrodes of the battery reached 1.5 V (CC 1.0 C, 1.5 V cut-off) (see Table 1). Here, the current amount at 1 C was calculated using theoretical capacities calculated assuming that the discharge capacity of natural graphite was 360 mAh / g and the discharge capacity of silicon oxide powder was 1900 mAh / g.

[0039]

[0040] Comparative Example 1 The target silicon oxide powder was obtained according to the method described in Example 1, except that the amount of grinding aid added was changed to 1.0% by weight of ethanol relative to the amount of silicon oxide charged into the bead mill. The D50 of the silicon oxide powder was measured in the same manner as in Example 1. Ｓ and D50 Ｄ When the measurement was performed, D50 Ｓ is 7.3 μm, D50 ＤThe particle diameter was 131 nm. A negative electrode was produced from the silicon oxide powder according to the method described in Example 1. A coin cell was then produced using the negative electrode in the same manner as in Example 1, and the capacity retention rate after 50 cycles was measured using the coin cell. The capacity retention rate was 77.2% (see Table 2).

[0041] Comparative Example 2 The target silicon oxide powder was obtained according to the method described in Example 1, except that no grinding aid was added. The D50 of the silicon oxide powder was measured in the same manner as in Example 1. Ｓ and D50 Ｄ When the measurement was performed, D50 Ｓ is 7.7 μm, D50 Ｄ The average particle diameter was 802 nm. A negative electrode was produced from the silicon oxide powder according to the method described in Example 1. A coin cell was then produced using the negative electrode in the same manner as in Example 1, and the capacity retention rate after 50 cycles was measured using the coin cell. The capacity retention rate was 77.1% (see Table 2).

[0042]

[0043] Agglomerated silicon oxide was produced in the same manner as in Example 1, and the agglomerated silicon oxide was pulverized in a bead mill without adding a grinding aid to produce silicon oxide powder. The following liquid-phase lithium doping step was then performed, and fine powder was removed by sedimentation classification to obtain lithium-containing silicon oxide powder. The liquid-phase lithium doping step and sedimentation classification step are described in detail below.

[0044] (Liquid-phase lithium doping step) A mixed solution was prepared by adding 6.0 mass% of lithium pieces to a solution prepared by dissolving naphthalene in a tetrahydrofuran (hereinafter referred to as "THF") solvent at a concentration of 0.2 mol / L. The silicon oxide powder obtained above was then immersed in the mixed solution at normal pressure and a temperature of 20°C for 20 hours.

[0045] (Sedimentation Classification Step) The silicon oxide powder immersion liquid obtained above was stirred to uniformly disperse the silicon oxide powder in the liquid, obtaining a dispersion. The dispersion was then transferred to a measuring cylinder so that the liquid level was 10 cm high, and left to stand for 191.3 hours to allow the silicon oxide powder dispersed in the liquid to settle. The supernatant was then removed, and the settled silicon oxide powder was recovered.

[0046] (Heat Treatment Step) The obtained silicon oxide compound particles were subjected to heat treatment at 600° C. for 24 hours in an argon atmosphere to obtain lithium-containing silicon oxide powder.

[0047] The lithium-containing silicon oxide powder obtained above was subjected to the method described in Example 1 to obtain a powder of D50 Ｓ and D50 Ｄ When the measurement was performed, D50 Ｓ is 7.7 μm, D50 Ｄ The particle diameter was 198 nm. A negative electrode was produced from the lithium-containing silicon oxide powder according to the method described in Example 1. Thereafter, a coin cell was produced using the negative electrode in the same manner as in Example 1, and the capacity retention rate after 50 cycles was measured using the coin cell, which was found to be 82.4% (see Table 3).

[0048] A lithium-containing silicon oxide powder was obtained according to the method described in Example 2, except that the standing time in the sedimentation classification step was changed to 93.7 hours, and the D50 of the lithium-containing silicon oxide powder was measured in the same manner as in Example 2. Ｓ and D50 Ｄ When the measurement was performed, D50 Ｓ is 7.7 μm, D50 Ｄ The average particle diameter was 429 nm. A negative electrode was produced from the lithium-containing silicon oxide powder according to the method described in Example 2. Thereafter, a coin cell was produced using the negative electrode in the same manner as in Example 2, and the capacity retention rate after 50 cycles was measured using the coin cell, which was found to be 82.5% (see Table 3).

[0049] A lithium-containing silicon oxide powder was obtained according to the method described in Example 2, except that the standing time in the sedimentation classification step was changed to 23.4 hours, and the D50 of the lithium-containing silicon oxide powder was measured in the same manner as in Example 2. Ｓ and D50 Ｄ When the measurement was performed, D50 Ｓ is 7.9 μm, D50 Ｄ The diameter was 750 nm. A negative electrode was produced from the lithium-containing silicon oxide powder according to the method described in Example 2. Thereafter, a coin cell was produced using the negative electrode in the same manner as in Example 2, and the capacity retention rate after 50 cycles was measured using the coin cell, which was found to be 82.6% (see Table 3).

[0050] (Comparative Example 3) A lithium-containing silicon oxide powder was obtained according to the method described in Example 2, except that the standing time in the sedimentation classification step was changed to 240 hours. The D50 of the lithium-containing silicon oxide powder was measured in the same manner as in Example 2. Ｓ and D50 Ｄ When the measurement was performed, D50 Ｓ is 7.6 μm, D50 Ｄ The particle diameter was 121 nm. A negative electrode was produced from the lithium-containing silicon oxide powder according to the method described in Example 2. Thereafter, a coin cell was produced using the negative electrode in the same manner as in Example 2, and the capacity retention rate after 50 cycles was measured using the coin cell, which was found to be 81.1% (see Table 3).

[0051] (Comparative Example 4) The target lithium-containing silicon oxide powder was obtained according to the method described in Example 2, except that the sedimentation classification step was omitted. The D50 of the lithium-containing silicon oxide powder obtained by the same method as in Example 2 was Ｓ and D50 Ｄ When the measurement was performed, D50 Ｓ is 7.9 μm, D50 Ｄ The peak wavelength was 883 nm. A negative electrode was produced from the lithium-containing silicon oxide powder according to the method described in Example 2. Thereafter, a coin cell was produced using the negative electrode in the same manner as in Example 2, and the capacity retention rate after 50 cycles was measured using the coin cell, which was found to be 80.9% (see Table 3).

[0052]

[0053] In the above-mentioned "lithium-containing silicon oxide powder production step", lithium pieces were added in an amount of 10.0 mass % to a solution in which naphthalene was dissolved in a THF solvent at a concentration of 0.2 mol / L, and the same method was used to obtain a lithium-containing silicon oxide powder. Ｓ and D50 Ｄ When the measurement was performed, D50 Ｓ is 7.7 μm, D50 Ｄ The particle diameter was 158 nm. Furthermore, a negative electrode was produced from the lithium-containing silicon oxide powder according to the method described in Example 2. Thereafter, a coin cell was produced using the negative electrode in the same manner as in Example 2, and the capacity retention rate after 50 cycles was measured using the coin cell, and the capacity retention rate was found to be 81.5% (see Table 4).

[0054] A lithium-containing silicon oxide powder was obtained according to the method described in Example 5, except that the standing time in the sedimentation classification step was changed to 93.7 hours. The D50 of the lithium-containing silicon oxide powder obtained by the same method as in Example 5 was Ｓ and D50 Ｄ When the measurement was performed, D50 Ｓ is 7.7 μm, D50 Ｄ The average particle diameter was 521 nm. A negative electrode was produced from the lithium-containing silicon oxide powder according to the method described in Example 5. Thereafter, a coin cell was produced using the negative electrode in the same manner as in Example 5, and the capacity retention rate after 50 cycles was measured using the coin cell, which was found to be 81.6% (see Table 4).

[0055] A lithium-containing silicon oxide powder was obtained according to the method described in Example 5, except that the standing time in the sedimentation classification step was changed to 23.4 hours. The D50 of the lithium-containing silicon oxide powder obtained by the same method as in Example 5 was Ｓ and D50 Ｄ When the measurement was performed, D50 Ｓ is 7.8 μm, D50 ＤThe average particle diameter was 735 nm. A negative electrode was produced from the lithium-containing silicon oxide powder according to the method described in Example 5. Thereafter, a coin cell was produced using the negative electrode in the same manner as in Example 5, and the capacity retention rate after 50 cycles was measured using the coin cell. The capacity retention rate was 81.9% (see Table 4).

[0056] (Comparative Example 5) A lithium-containing silicon oxide powder was obtained according to the method described in Example 5, except that the standing time in the sedimentation classification step was changed to 240 hours. The D50 of the lithium-containing silicon oxide powder obtained by the same method as in Example 5 was Ｓ and D50 Ｄ When the measurement was performed, D50 Ｓ is 7.5 μm, D50 Ｄ The particle diameter was 124 nm. A negative electrode was produced from the lithium-containing silicon oxide powder according to the method described in Example 5. Thereafter, a coin cell was produced using the negative electrode in the same manner as in Example 5, and the capacity retention rate after 50 cycles was measured using the coin cell, which was found to be 80.3% (see Table 4).

[0057] (Comparative Example 6) A lithium-containing silicon oxide powder was obtained according to the method described in Example 5, except that the sedimentation classification step was omitted. The D50 of the lithium-containing silicon oxide powder obtained by the same method as in Example 5 was Ｓ and D50 Ｄ When the measurement was performed, D50 Ｓ is 8.0 μm, D50 Ｄ The diameter was 901 nm. A negative electrode was produced from the lithium-containing silicon oxide powder according to the method described in Example 5. Thereafter, a coin cell was produced using the negative electrode in the same manner as in Example 5, and the capacity retention rate after 50 cycles was measured using the coin cell, which was found to be 79.9% (see Table 4).

[0058]

[0059] A carbon-coated (hereinafter, sometimes referred to as "C-coat") silicon oxide powder was obtained in the same manner as in Example 1, except that a "conductive carbon coating step" was performed after "1. (2) pulverization step" described in Example 1. The "conductive carbon coating step" will be described in detail below.

[0060] (Conductive Carbon Coating Step) The silicon oxide powder obtained in "1. (2) Pulverization Step" described in Example 1 was loaded into a rotary kiln and heated to 700°C under an argon atmosphere. Then, propane gas was injected into the rotary kiln as a carbon source, and the surface of the silicon oxide powder was coated with a carbon coating to obtain a C-coated silicon oxide powder. The mass ratio of carbon in the conductive carbon coating to the mass of the C-coated silicon oxide powder was measured by oxygen stream combustion-infrared absorption spectrometry. A carbon concentration analyzer, CS-400 manufactured by Leco, was used for the measurement. As a result, the mass ratio of carbon in the conductive carbon coating to the mass of the C-coated silicon oxide powder ("carbon coating amount" or "C coating amount") was 3.0% by mass.

[0061] The D50 of the C-coated silicon oxide powder obtained above was measured in the same manner as in Example 1. Ｓ and D50 Ｄ When the measurement was performed, D50 Ｓ is 6.6 μm, D50 Ｄ The average particle diameter was 275 nm. A negative electrode was produced from the lithium-containing silicon oxide powder according to the method described in Example 1. Thereafter, a coin cell was produced using the negative electrode in the same manner as in Example 1, and the capacity retention rate after 50 cycles was measured using the coin cell. The capacity retention rate was 81.0% (see Table 5).

[0062] Comparative Example 7: A C-coated silicon oxide powder was obtained in the same manner as in Example 8, except that the amount of grinding aid added was changed to 1.0% by weight of ethanol relative to the amount of silicon oxide charged into the bead mill. The D50 of the C-coated silicon oxide powder was measured according to the method described in Example 8. Ｓ and D50 Ｄ When the measurement was performed, D50 Ｓ is 6.4 μm, D50 ＤThe average particle diameter was 102 nm. A negative electrode was produced from the C-coated silicon oxide powder according to the method described in Example 8. A coin cell was then produced using the negative electrode in the same manner as in Example 8, and the capacity retention rate after 50 cycles was measured using the coin cell. The capacity retention rate was 79.6% (see Table 5).

[0063]

[0064] A C-coated silicon oxide powder was obtained according to the "conductive carbon coating step" described in Example 8. The amount of C coating was measured in the same manner as in Example 8 and found to be 3.0 mass%. Thereafter, according to the "lithium-containing silicon oxide powder manufacturing step" described in Example 2, the C-coated silicon oxide powder was doped with lithium to obtain a C-coated lithium-containing silicon oxide powder.

[0065] According to the method described in Example 1, the D50 of the C-coated lithium-containing silicon oxide powder Ｓ and D50 Ｄ When the measurement was performed, D50 Ｓ is 6.8 μm, D50 Ｄ The average particle diameter was 164 nm. A negative electrode was produced from the C-coated lithium-containing silicon oxide powder according to the method described in Example 2. Thereafter, a coin cell was produced using the negative electrode in the same manner as in Example 2, and the capacity retention rate after 50 cycles was measured using the coin cell. The capacity retention rate was 85.8% (see Table 6).

[0066] A C-coated lithium-containing silicon oxide powder was obtained according to the method described in Example 9, except that the standing time in the sedimentation classification step was changed to 93.7 hours. The C-coat amount was measured in the same manner as in Example 9, and was found to be 3.0 mass%. Furthermore, the D50 of the C-coated silicon oxide powder was measured according to the method described in Example 9. Ｓ and D50 Ｄ When the measurement was performed, D50 Ｓ is 6.8 μm, D50 ＤThe average particle diameter was 374 nm. A negative electrode was produced from the C-coated silicon oxide powder according to the method described in Example 9. A coin cell was then produced using the negative electrode in the same manner as in Example 9, and the capacity retention rate after 50 cycles was measured using the coin cell. The capacity retention rate was 85.9% (see Table 6).

[0067] A C-coated lithium-containing silicon oxide powder was obtained according to the method described in Example 9, except that the standing time in the sedimentation classification step was changed to 23.4 hours. The C-coat amount was measured in the same manner as in Example 9, and was found to be 3.0 mass%. In addition, the D50 of the silicon oxide powder after C-coating according to the method described in Example 9 was Ｓ and D50 Ｄ When the measurement was performed, D50 Ｓ is 6.9 μm, D50 Ｄ The diameter was 720 nm. A negative electrode was produced from the C-coated lithium-containing silicon oxide powder according to the method described in Example 9. Thereafter, a coin cell was produced using the negative electrode in the same manner as in Example 9, and the capacity retention rate after 50 cycles was measured using the coin cell. The capacity retention rate was 86.1% (see Table 6).

[0068] (Comparative Example 8) A C-coated lithium-containing silicon oxide powder was obtained according to the method described in Example 9, except that the standing time in the sedimentation classification step was changed to 240 hours. The C-coat amount was measured in the same manner as in Example 9, and was found to be 3.0 mass%. In addition, the D50 of the C-coated lithium-containing silicon oxide powder was measured according to the method described in Example 9. Ｓ and D50 Ｄ When the measurement was performed, D50 Ｓ is 6.9 μm, D50 Ｄ The peak diameter was 796 nm. A negative electrode was produced from the C-coated lithium-containing silicon oxide powder according to the method described in Example 9. Thereafter, a coin cell was produced using the negative electrode in the same manner as in Example 9, and the capacity retention rate after 50 cycles was measured using the coin cell. The capacity retention rate was 84.3% (see Table 6).

[0069] (Comparative Example 9) A C-coated lithium-containing silicon oxide powder was obtained according to the method described in Example 9, except that the sedimentation classification step was omitted. The C-coat amount was measured in the same manner as in Example 9, and was found to be 3.0 mass%. In addition, the D50 of the C-coated lithium-containing silicon oxide powder was measured according to the method described in Example 9. Ｓ and D50 Ｄ When the measurement was performed, D50 Ｓ is 6.8 μm, D50 Ｄ The average particle diameter was 144 nm. A negative electrode was produced from the C-coated lithium-containing silicon oxide powder according to the method described in Example 9. Thereafter, a coin cell was produced using the negative electrode in the same manner as in Example 9, and the capacity retention rate after 50 cycles was measured using the coin cell. The capacity retention rate was 83.9% (see Table 6).

[0070]

[0071] A C-coated silicon oxide powder was obtained according to the "conductive carbon coating step" described in Example 8. The amount of C coating was measured in the same manner as in Example 8 and was found to be 3.0 mass%. Thereafter, according to the "lithium-containing silicon oxide powder manufacturing step" described in Example 5, the C-coated silicon oxide powder was doped with lithium. Thereafter, the lithium-doped C-coated silicon oxide powder was subjected to sedimentation classification and heat treatment according to the method described in Example 5 to obtain a C-coated lithium-containing silicon oxide powder.

[0072] According to the method described in Example 1, the D50 of the C-coated lithium-containing silicon oxide powder Ｓ and D50 Ｄ When the measurement was performed, D50 Ｓ is 6.7 μm, D50 Ｄ The particle diameter was 150 nm. A negative electrode was produced from the C-coated lithium-containing silicon oxide powder according to the method described in Example 1. Thereafter, a coin cell was produced using the negative electrode in the same manner as in Example 1, and the capacity retention rate after 50 cycles was measured using the coin cell. The capacity retention rate was 84.8% (see Table 7).

[0073] A C-coated lithium-containing silicon oxide powder was obtained according to the method described in Example 12, except that the standing time in the sedimentation classification step was changed to 93.7 hours, and the C-coat amount was measured in the same manner as in Example 12, and was found to be 3.0 mass%. Ｓ and D50 Ｄ When the measurement was performed, D50 Ｓ is 6.8 μm, D50 Ｄ The peak diameter was 374 nm. A negative electrode was produced from the C-coated lithium-containing silicon oxide powder according to the method described in Example 12. Thereafter, a coin cell was produced using the negative electrode in the same manner as in Example 12, and the capacity retention rate after 50 cycles was measured using the coin cell. The capacity retention rate was 84.6% (see Table 7).

[0074] A C-coated lithium-containing silicon oxide powder was obtained according to the method described in Example 12, except that the standing time in the sedimentation classification step was changed to 23.4 hours. The C-coat amount was measured in the same manner as in Example 12, and was found to be 3.0 mass%. In addition, the D50 of the C-coated lithium-containing silicon oxide powder was measured according to the method described in Example 1. Ｓ and D50 Ｄ When the measurement was performed, D50 Ｓ is 6.9 μm, D50 Ｄ The peak diameter was 736 nm. A negative electrode was produced from the C-coated lithium-containing silicon oxide powder according to the method described in Example 1. Thereafter, a coin cell was produced using the negative electrode in the same manner as in Example 12, and the capacity retention rate after 50 cycles was measured using the coin cell. The capacity retention rate was 84.7% (see Table 7).

[0075] (Comparative Example 10) A C-coated lithium-containing silicon oxide powder was obtained according to the method described in Example 12, except that the standing time in the sedimentation classification step was changed to 240 hours. The C-coat amount was measured in the same manner as in Example 12, and was found to be 3.0 mass%. In addition, the D50 of the C-coated lithium-containing silicon oxide powder was measured according to the method described in Example 12. Ｓ and D50 Ｄ When the measurement was performed, D50Ｓ is 6.6 μm, D50 Ｄ The average particle diameter was 112 nm. A negative electrode was produced from the C-coated lithium-containing silicon oxide powder according to the method described in Example 12. Thereafter, a coin cell was produced using the negative electrode in the same manner as in Example 12, and the capacity retention rate after 50 cycles was measured using the coin cell. The capacity retention rate was 82.5% (see Table 7).

[0076] (Comparative Example 11) A C-coated lithium-containing silicon oxide powder was obtained according to the method described in Example 12, except that the sedimentation classification step was omitted. The C-coat amount was measured in the same manner as in Example 12, and was found to be 3.0 mass%. In addition, the D50 of the C-coated lithium-containing silicon oxide powder was measured according to the method described in Example 1. Ｓ and D50 Ｄ When the measurement was performed, D50 Ｓ is 6.9 μm, D50 Ｄ The average particle diameter was 770 nm. A negative electrode was produced from the C-coated lithium-containing silicon oxide powder according to the method described in Example 1. Thereafter, a coin cell was produced using the negative electrode in the same manner as in Example 12, and the capacity retention rate after 50 cycles was measured using the coin cell. The capacity retention rate was 82.3% (see Table 7).

[0077]

[0078] The following "Mg ２ SiO ４ According to the "Preparation Process", Mg ２ SiO ４ was produced.

[0079] (Mg ２ SiO ４ Preparation process) Si powder, MgO powder and SiO ２ The powders were mixed in a molar ratio of 7:4:3 (Si:Mg:O = 1:0.4:1). This mixed powder was heated to 1350°C at 1 Pa in an argon gas atmosphere, and the generated gas was cooled to 400°C on a deposition plate placed above it, precipitated, and collected.

[0080] The powder thus obtained was subjected to XRD measurement using CuKα radiation, and it was found that Si, MgSiO ３ , Mg２ SiO ４ While the crystal peaks of MgO and SiO ２ The amorphous peak and crystalline peak of MgSi, as well as the crystalline peaks of metallic Mg and MgSi alloy, could not be confirmed.

[0081] As in Example 1, 0.5% by weight of ethanol was added as a grinding aid to the silicon oxide added to the bead mill, and the Mg obtained above was ２ SiO ４ The powder was crushed into Mg ２ SiO ４ A powder was obtained. ２ SiO ４ The powder was used to form a C-coated Mg film according to the "conductive carbon film coating step" described in Example 8. ２ SiO ４ The powder was obtained, and the amount of C coating was measured in the same manner as in Example 8. The amount of C coating was 3.0 mass %. ２ SiO ４ Powder D50 Ｓ and D50 Ｄ When the measurement was performed, D50 Ｓ is 5.2 μm, D50 Ｄ The peak intensity was 374 nm. ２ SiO ４ A negative electrode was produced from the powder, and then a coin cell was produced using the negative electrode in the same manner as in Example 1. The capacity retention rate after 50 cycles was measured using the coin cell, and the capacity retention rate was found to be 82.1% (see Table 8).

[0082] Comparative Example 12: C-coated MgO was prepared in the same manner as in Example 15, except that the amount of grinding aid added was changed to 1.0% by weight of ethanol relative to the amount of silicon oxide added to the bead mill. ２ SiO ４ The powder was obtained, and the amount of C coating was measured in the same manner as in Example 15. The amount of C coating was 3.0% by mass. ２ SiO ４ Powder D50 Ｓand D50 Ｄ When the measurement was performed, D50 Ｓ is 5.1 μm, D50 Ｄ The thickness was 112 nm. ２ SiO ４ A negative electrode was produced from the powder, and then a coin cell was produced using the negative electrode in the same manner as in Example 15. The capacity retention rate after 50 cycles was measured using the coin cell, and the capacity retention rate was found to be 80.3% (see Table 8).

[0083]

[0084] The following "Li ｘ Mg ｙ SiO ｚ According to the "Production Process", Li ｘ Mg ｙ SiO ｚ was prepared. ｘ Mg ｙ SiO ｚ is a SiO2 doped with both Li and Mg and used to form the negative electrode of a lithium secondary battery. ｘ The powder is a powder of this type, where x, y, and z are positive real numbers and satisfy the following conditions: 0.5≦z≦1.5, z / 5≦x+y≦z, and z / 100≦x and z / 100≦y.

[0085] (Li ｘ Mg ｙ SiO ｚ Preparation process) Si powder and SiO ２ powder and LiO as a Li source ２The lithium-magnesium-containing silicon oxide compound was then pulverized in the same manner as in Example 1, using 0.5 wt. % ethanol as a grinding aid relative to the silicon oxide charged into the bead mill, to obtain a pulverized powder. The pulverized powder was then analyzed for the elements Si, O, and Li. The Si and Li contents were determined by ICP atomic emission spectroscopy (PS3520VDD II, manufactured by Hitachi High-Tech Corporation). The content of O was measured by the inert gas fusion infrared absorption method (GFA) using a TC-436 manufactured by Leco. As a result, the composition of the deposit was found to be Li ｘ Mg ｙ SiO ｚ It was found that (x = 0.2, y = 0.2, z = 1).

[0086] The powder obtained after pulverization was carbon-coated (C-coated) at 850°C by thermal CVD using a mixed gas of argon and propane as a carbon source, and C-coated Li ｘ Mg ｙ SiO ｚ (x=0.2, y=0.2, z=1) powder was obtained. ｘ Mg ｙ SiO ｚ The amount of C coating on the powder (x=0.2, y=0.2, z=1) was measured in the same manner as in Example 8, and the amount of C coating was 3.0% by mass.

[0087] Subsequently, the C-coated Li obtained above was subjected to the same method as in Example 1. ｘ Mg ｙ SiO ｚ (x=0.2, y=0.2, z=1) D50 of powder Ｓ and D50 Ｄ When the measurement was performed, D50 Ｓis 5.0 μm, D50 Ｄ The thickness was 153 nm. ｘ Mg ｙ SiO ｚ A negative electrode was produced from the powder (x = 0.2, y = 0.2, z = 1). Thereafter, a coin cell was produced using the negative electrode in the same manner as in Example 1, and the capacity retention rate after 50 cycles was measured using the coin cell. The capacity retention rate was 82.5% (see Table 9).

[0088] Comparative Example 13: A C-coated Li powder was prepared according to the method described in Example 16, except that the amount of grinding aid added was changed to 1.0 wt % of ethanol relative to the amount of silicon oxide added to the bead mill. ｘ Mg ｙ SiO ｚ (x = 0.2, y = 0.2, z = 1) powder was obtained, and the C-coat amount was measured in the same manner as in Example 16, and the C-coat amount was found to be 3.0 mass%. ｘ Mg ｙ SiO ｚ (x=0.2, y=0.2, z=1) D50 of powder Ｓ and D50 Ｄ When the measurement was performed, D50 Ｓ is 5.0 μm, D50 Ｄ The thickness was 135 nm. ｘ Mg ｙ SiO ｚ A negative electrode was produced from the powder (x = 0.2, y = 0.2, z = 1). Then, a coin cell was produced using the negative electrode in the same manner as in Example 16, and the capacity retention rate after 50 cycles was measured using the coin cell. The capacity retention rate was 80.5% (see Table 9).

[0089]

[0090] (Summary) As is clear from Tables 2 to 9 above, the capacity retention rate after 50 cycles of the coin cells using the negative electrodes formed from the silicon oxide powder according to the present invention was improved.

Claims

1. SiO ｘ (0.5<x<1.5), wherein when a volume-based median diameter of the silicon oxide powder is measured with a laser diffraction particle size distribution analyzer, the median diameter is in the range of 1 μm or more and 30 μm or less, and when a dispersion of the silicon oxide powder is passed through a filter having a pore size of 1 μm and then the volume-based median diameter of the silicon oxide powder in the dispersion is measured with a dynamic light scattering particle size distribution analyzer, the median diameter is in the range of 150 nm or more and 750 nm or less.

2. The silicon oxide powder according to claim 1, which is doped with a metal element.

3. The silicon oxide powder according to claim 2, wherein the metal element is at least one of Li and Mg.

4. The silicon oxide powder according to claim 1 or 2, at least a portion of the surface of which is covered with a conductive carbon coating.

5. Silicon oxide powder according to claim 4, wherein the mass ratio of carbon in the conductive carbon coating to the mass of the silicon oxide powder is within the range of 0.5 mass % to 20 mass %.

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