Silicon composite containing metallic element

Abstract

One of the problems addressed by the present invention is to provide a negative electrode active material for a lithium-ion battery whereby initial efficiency is improved while maintaining the rate characteristics of the lithium-ion battery. A silicon composite containing a metallic element according to a first aspect of the present invention comprises a silicon phase and a silicate phase. The silicon phase is formed from silicon (Si). The silicate phase is adjacent to the silicon phase. The silicon composite containing a metallic element has a composition of LixMgySiOz (where x > 0, y > 0, and z > 0). The composition satisfies all of the following conditions: (a) (2.5x + 2y) / z > 1; (b) (1.5x + y) / z < 1; and (c) (x + 2y) / z < 1.

Description

[Technical Field]

[0001] This invention relates to silicon-based composites containing metal elements. [Background technology]

[0002] Silicon-based compounds are used as high-capacity negative electrode active materials in lithium-ion batteries. Examples of silicon-based compounds used as negative electrode active materials include silicon oxide (SiO) (see, for example, Japanese Patent Publication No. 6-325765), lithium-doped silicon oxide (see, for example, Japanese Patent Publication No. 6-325765), magnesium-doped silicon oxide (see, for example, Japanese Patent Publication No. 2018-519648), and lithium-magnesium-doped silicon oxide (see, for example, Japanese Patent Publication No. 2017-204374). [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 6-325765 [Patent Document 2] Japanese Patent Publication No. 2017-204374 [Patent Document 3] Special Publication No. 2018-519648 [Overview of the project] [Problems that the invention aims to solve]

[0004] However, none of the silicon-based compounds used as negative electrodes simultaneously satisfied the initial efficiency and rate characteristics of lithium-ion batteries.

[0005] The object of the present invention is to provide a negative electrode active material for lithium-ion batteries that improves initial efficiency while maintaining the rate characteristics of the lithium-ion battery. [Means for solving the problem]

[0006] The silicon-based composite containing a metal element according to one aspect of the present invention includes a silicon phase and a silicate phase. The silicon phase is formed from silicon (Si). The silicate phase is adjacent to the silicon phase. And this silicon-based composite containing a metal element contains Li x Mg y SiO z (where x > 0, y > 0, z > 0). Here, this composition satisfies all of the conditions of (a) (2.5x + 2y) / z > 1, (b) (1.5x + y) / z < 1, and (c) (x + 2y) / z < 1.

[0007] As a result of intensive studies by the inventors of the present application, it has been clarified that when the above-mentioned silicon-based composite containing a metal element is used as the negative electrode active material of a lithium-ion battery, the initial efficiency can be improved while maintaining the rate characteristics of the lithium-ion battery.

[0008] In addition, in the above-mentioned silicon-based composite containing a metal element, it is preferable that 0.5 < z < 1.5. When this silicon-based composite containing a metal element is used as the negative electrode active material of a lithium-ion battery, it is possible to suppress a decrease in the life characteristics of the lithium-ion battery and to suppress a decrease in the charge-discharge capacity of the lithium-ion battery due to an excessive amount of the silicate phase.

[0009] Also, in the above-mentioned silicon-based composite containing a metal element, it is preferable that x > 0.1 and y > 0.05. When x and y satisfy these conditions, when the silicon-based composite containing a metal element is used as the negative electrode active material of a lithium-ion battery, it is possible to suppress the crystal growth of silicon and suppress a decrease in the initial efficiency of the lithium-ion battery, and it is possible to exhibit good water resistance during electrode assembly.

[0010] Also, in the above-mentioned silicon-based composite containing a metal element, it is preferable that the silicate phase does not contain Li4SiO4 and Li2O as crystals. These substances react with carbon dioxide and moisture in the air to generate lithium carbonate and lithium hydroxide, are chemically unstable, and have poor water resistance, which may cause problems during the production of lithium-ion batteries.

[0011] Further, at least a part of the surface of the above-described metal element-containing silicon-based composite is preferably covered with a conductive carbon film. When this metal element-containing silicon-based composite is used as a negative electrode active material, it is possible to maintain a good charge-discharge capacity of the lithium-ion battery, impart good conductivity to the metal element-containing silicon-based composite, and suppress side reactions of silicon oxide.

Brief Description of the Drawings

[0012] [Figure 1] It is a triangular phase diagram of the metal element-containing silicon-based composite according to an embodiment of the present invention. [Figure 2] It is a schematic diagram of a manufacturing apparatus for the metal element-containing silicon-based composite according to an embodiment of the present invention.

Explanation of Reference Numerals

[0013] 100 Evaporation apparatus 110 Crucible 120 Heater 130 Evaporation drum 141 Scraper 143 Granule guide 150 Chamber 151 Chamber main body 152 Recovery section 153 Exhaust pipe 160 Raw material supply hopper 170 Raw material introduction pipe 180 Recovery container 190 Recovery pipe Gg Gas guide OP Opening RM Deposition chamber Sr Molten metal VL1 First valve VL2 Second valve

Embodiments for Carrying Out the Invention

[0014] The metal element-containing silicon-based composite according to an embodiment of the present invention comprises a silicon phase and a silicate phase. The silicon phase is formed from silicon (Si). The silicate phase is adjacent to the silicon phase. This metal element-containing silicon-based composite is Li x Mg y SiO z The composition is (x>0, y>0, z>0). In the above composition, it is presumed that three of the following phases are in equilibrium: Li2Si2O5, Li2SiO3, Mg2SiO4, and Li2MgSiO4. When this equilibrium relationship is represented as a triangular phase diagram, it is as shown in Figure 1. That is, the region shown in hatching in Figure 1 is the equilibrium region. Here, the same composition satisfies all of the following conditions: (a) (2.5x+2y) / z>1, (b) (1.5x+y) / z<1, and (c) (x+2y) / z<1. By satisfying (a), lithium-ion batteries can exhibit excellent initial efficiency and rate characteristics, and by satisfying (b) and (c), the formation of chemically unstable phases such as Li4SiO4 and Li2O can be suppressed. Furthermore, (2.5x+2y) / z may be in the range of greater than 1 and less than 1.75, or greater than 1.1 and less than 1.5. Also, (1.5x+y) / z may be in the range of greater than 0.5 and less than 1, or greater than 0.5 and less than 0.9. Moreover, (x+2y) / z may be in the range of greater than 0.4 and less than 1, greater than 0.5 and less than 0.9, or greater than 0.6 and less than 0.8. In addition, the silicate phase may exist in a crystalline state, an amorphous state, or a partially crystalline state (partially amorphous state). However, the silicate phase does not necessarily coincide with the compositional equilibrium phase. For example, between Li silicate and Mg silicate, Li silicate crystallizes more easily at low temperatures, so both a crystalline phase of Li silicate and a Mg-rich amorphous phase may occur. Also, although the cause is unclear, MgSiO3 crystals, which are not included in the equilibrium phase, may be observed. In particular, when the recovery temperature or heat treatment temperature of metal element-containing silicon-based composites is low, it is presumed that some of them become amorphous, or that slight concentration variations in the silicate phase lead to the formation of MgSiO3 crystals as a metastable phase.

[0015] The composition of the silicate phase according to the embodiment of the present invention can also be expressed as m(Li2O)·n(MgO)·SiO2. In this composition, it is preferable that all of the following conditions are satisfied: (e) 2m + 0.5n > 1, (f) m < 1, and (g) m + n < 2. Here, 2m + 0.5n may be in the range of greater than 1 and less than 1.7, or in the range of greater than 1.1 and less than 1.7. Also, m may be in the range of greater than 0.45 and less than 1, or in the range of greater than 0.45 and less than 0.9, or in the range of greater than 0.45 and less than 0.8. Furthermore, m + n may be in the range of greater than 0.9 and less than 2, or in the range of greater than 0.9 and less than 1.9, or in the range of greater than 0.9 and less than 1.7, or in the range of greater than 0.9 and less than 1.5, or in the range of greater than 0.9 and less than 1.3.

[0016] Incidentally, the shape of the metal element-containing silicon-based composite according to the embodiment of the present invention is not limited and may be in powder form, granular form, lump form, or other form. However, when the metal element-containing silicon-based composite is used as the negative electrode active material of a lithium-ion battery, it is preferable that it be in powder form.

[0017] Incidentally, when the aforementioned metal element-containing silicon-based composite is used as the negative electrode active material for a lithium-ion battery, it is possible to obtain an initial efficiency higher than that of a conventional lithium-ion battery while maintaining a lithium-ion battery rate characteristic equivalent to that of a conventional lithium-ion battery.

[0018] When the above-described metal element-containing silicon-based composite is used as the negative electrode active material of a lithium-ion battery, it is preferable that 0.5 < z < 1.5. This is because it is possible to suppress a decrease in the life characteristics of the lithium-ion battery and to suppress a decrease in the charge-discharge capacity of the lithium-ion battery due to an excessive amount of the silicate phase. Further, z is preferably in the range of more than 0.6 and less than 1.4, more preferably in the range of more than 0.7 and less than 1.3, still more preferably in the range of more than 0.8 and less than 1.2, and particularly preferably in the range of more than 0.9 and less than 1.1.

[0019] Also, when the above-described metal element-containing silicon-based composite is used as the negative electrode active material of a lithium-ion battery, it is preferable that x > 0.1 and y > 0.05. When x and y satisfy these conditions, it is possible to suppress crystal growth of silicon and suppress a decrease in the initial efficiency of the lithium-ion battery, and it is possible to exhibit good water resistance during electrode assembly. Note that x may be more than 0.2, more than 0.3, more than 0.4, more than 0.5, more than 0.6, more than 0.7, more than 0.8, or more than 0.9. Also, y may be more than 0.1, more than 0.2, more than 0.3, more than 0.4, more than 0.5, more than 0.6, or more than 0.7.

[0020] Also, when using the above-described metal element-containing silicon-based composite as the negative electrode active material of a lithium-ion battery, it is preferable that m > 0.1 and n > 0.1. When m and n satisfy these conditions, crystal growth of silicon can be suppressed, the decrease in the initial efficiency of the lithium-ion battery can be suppressed, and good water resistance can be exhibited during electrode assembly. Note that m may be greater than 0.2, may be greater than 0.3, may be greater than 0.4, may be greater than 0.5, may be greater than 0.6, may be greater than 0.7, may be greater than 0.8, or may be greater than 0.9. Also, n may be greater than 0.2, may be greater than 0.3, may be greater than 0.4, may be greater than 0.5, may be greater than 0.6, may be greater than 0.7, may be greater than 0.8, or may be greater than 0.9. Further, even when the composition is represented as m(Li2O)·n(MgO)·SiO2, it is preferable to satisfy 0.5 < O / Si < 1.5. This is because the decrease in the life characteristics of the lithium-ion battery can be suppressed, and the excessive formation of the silicate phase and the decrease in the charge-discharge capacity of the lithium-ion battery can be suppressed. Also, O / Si is preferably in the range greater than 0.6 and less than 1.4, more preferably in the range greater than 0.7 and less than 1.3, even more preferably in the range greater than 0.8 and less than 1.2, and particularly preferably in the range greater than 0.9 and less than 1.1.

[0021] Also, when using the above-described metal element-containing silicon-based composite as the negative electrode active material of a lithium-ion battery, it is preferable that the silicate phase does not contain Li4SiO4 and Li2O as crystals. These substances react with carbon dioxide and moisture in the air to form lithium carbonate and lithium hydroxide, are chemically unstable, and have poor water resistance, which may cause problems during the fabrication of the lithium-ion battery.

[0022] Furthermore, when the above-mentioned metal element-containing silicon-based composite is used as the negative electrode active material for a lithium-ion battery, it is preferable to cover at least a portion of the surface of the metal element-containing silicon-based composite with a conductive carbon film. This is because it is possible to impart good conductivity to the metal element-containing silicon-based composite while maintaining good charge and discharge capacity of the lithium-ion battery, and to suppress side reactions of silicon oxide. In order to enjoy this effect, it is preferable that the mass ratio of carbon in the conductive carbon film to the mass of the metal element-containing silicon-based composite be in the range of 0.5% by mass or more and 20% by mass or less, more preferably in the range of 0.5% by mass or more and 10% by mass or less, and even more preferably in the range of 0.5% by mass or more and 5% by mass or less.

[0023] The following describes in detail a method for producing a metal element-containing silicon-based composite according to an embodiment of the present invention.

[0024] Examples of methods for producing metal element-containing silicon composites according to embodiments of the present invention include (i) a method of doping silicon oxide with lithium and magnesium, (ii) a method of compounding silicon and silicate by mechanical milling, and (iii) a method of mixed deposition of SiO gas, Li gas, and Mg gas (hereinafter sometimes referred to as the "mixed deposition method"). However, in terms of productivity of metal element-containing silicon composites and the uniformity of the resulting metal element-containing silicon composites, Mixed deposition method It is preferable to adopt this approach.

[0025] Incidentally, the raw materials used in the production of metal element-containing silicon oxide are silicon and metal silicates. Here, the metal silicates contain lithium (Li) and magnesium (Mg). That is, a metal silicate containing both lithium and magnesium may be used alone, or a combination of a lithium-containing silicate and a magnesium-containing silicate may be used. For example, lithium disilicate Li2Si2O5 is an example of lithium-containing silicate. For example, magnesium silicate MgSiO3 is an example of magnesium-containing silicate. Furthermore, the metal silicate may be a mixture of metal oxide and silicon oxide. For example, lithium disilicate Li2Si2O5 may be a mixture of Li2O and 2SiO2, and magnesium silicate MgSiO3 may be a mixture of MgO and SiO2. In addition, in order to include metal elements other than lithium and magnesium in the metal element-containing silicon-based composite, oxides containing metal elements other than lithium and magnesium may be included in the metal silicate phase of the raw materials. Such metal elements are not particularly limited, but when a mixed deposition method is used as a method for producing silicon oxide containing metal elements, examples include alkali metals, alkaline earth metals, zinc, boron, etc., depending on the vapor pressure of the metal elements and oxides. Silicon and metal silicates may be in powder form, granular form, lump form, or other forms.

[0026] Furthermore, the mixed deposition method described above is preferably carried out using a deposition apparatus 100 as shown in Figure 2, from the viewpoint of reducing manufacturing costs. For this reason, we will first describe this deposition apparatus 100, and then describe the mixed deposition method described above in detail.

[0027] As shown in Figure 2, the vapor deposition apparatus 100 mainly consists of a crucible 110, a heater 120, a vapor deposition drum 130, a scraper 141, a granular guide 143, a chamber 150, a raw material supply hopper 160, a raw material introduction pipe 170, a recovery container 180, a first valve VL1, and a second valve VL2.

[0028] The crucible 110 is a heat-resistant container with an opening in the center of its top wall, as shown in Figure 2, and is installed in the chamber 150. A through-hole (not shown) is formed at one point around the perimeter of the top wall of the crucible 110, and a raw material introduction pipe 170 is inserted through this through-hole. That is, the raw material in the raw material supply hopper 160 is supplied to the crucible 110 through the raw material introduction pipe 170. A gas guide Gg is also provided on the upper side of the top wall of the crucible 110. This gas guide Gg is a component that guides the raw material gas generated in the crucible 110 to the deposition drum 130, and as shown in Figure 2, it is installed on the upper surface of the top wall so as to surround the central part of the top wall.

[0029] The heater 120 is for heating the crucible 110 to a high temperature and is positioned to cover the outer circumference of the crucible 110.

[0030] The deposition drum 130 is, for example, a cylindrical horizontal drum, and as shown in Figure 2, is positioned above the opening OP in the top wall of the crucible 110, with its lower part surrounded by a gas guide Gg. The deposition drum 130 is rotated in one direction by a drive mechanism (not shown). The deposition drum 130 is equipped with a temperature controller (not shown) to maintain a constant temperature on its outer surface. This temperature controller cools the outer surface temperature of the deposition drum 130 to a temperature suitable for deposition of the deposition source gas using a cooling medium supplied from the outside. Furthermore, the outer surface temperature of the deposition drum 130 can affect the crystallinity of precipitates deposited on top of precipitates remaining on the deposition drum. By controlling the outer surface temperature of the deposition drum 130 within an appropriate range, crystal growth by disproportionation reaction can be achieved. of The progression can be suppressed. The outer surface temperature of the deposition drum 130 is preferably 900°C or lower, more preferably within the range of 800°C or lower, and particularly preferably within the range of 700°C or lower.

[0031] The scraper 141 is a component that plays the role of scraping off the thin film formed on the deposition drum from the deposition drum 130, and is positioned near the deposition drum 130 as shown in Figure 2. The thin film fragments (metal element-containing silicon-based composite particles) scraped off by the scraper 141 fall into the particle guide 143. The material of the scraper 141 also affects the contamination of the metal element-containing silicon-based composite particles with impurities. From the viewpoint of suppressing this effect, the material of the scraper 141 is preferably a high-hardness metal or ceramic, and is particularly preferably a ceramic. Furthermore, it is preferable that the scraper 141 does not come into contact with the outer surface of the deposition drum 130. This is because it is possible to prevent impurity contamination that may occur due to direct contact between the deposition drum 130 and the scraper 141 from being mixed into the recovered metal element-containing silicon-based composite particles.

[0032] The granular guide 143 is, for example, a vibrating transport member, and as shown in Figure 2, is arranged to incline downward as it moves from the vicinity of the deposition drum toward the recovery section 152 of the chamber 150. It receives thin film fragments scraped off by the scraper 141, which is positioned above it, and sends them to the recovery section 152 of the chamber 150.

[0033] As shown in Figure 2, the chamber 150 is mainly composed of a chamber body 151, a recovery section 152, and an exhaust pipe 153. The chamber body 151 is a box-shaped section with a deposition chamber RM inside, as shown in Figure 2, and houses a crucible 110, a heater 120, a deposition drum 130, a scraper 141, and a granular guide 143. The recovery section 152 is a section that protrudes outward from the side wall of the chamber body 151, as shown in Figure 2, and has a space that communicates with the deposition chamber RM of the chamber body 151. As mentioned above, the tip of the granular guide 143 is located in this recovery section 152.

[0034] The raw material supply hopper 160 is a raw material supply source, and as shown in Figure 2, its outlet is connected to the raw material introduction pipe 170. That is, the raw material fed into the raw material supply hopper 160 is supplied to the crucible 110 via the raw material introduction pipe 170 at an appropriate timing. The raw material supplied to the crucible 110 becomes molten Sr and then vaporizes to become raw material gas.

[0035] The raw material introduction pipe 170 is a round-hole nozzle for supplying solid raw materials that have been introduced into the raw material supply hopper 160 to the crucible 110, and is positioned in the central part of the top plate of the crucible 110 so that its opening faces upward.

[0036] The recovery container 180 is a container for recovering thin film fragments that have passed through the first valve VL1 and the second valve VL2.

[0037] The first valve VL1 and the second valve VL2 are used to adjust the amount of thin film fragments collected into the collection container 180 by opening and closing them, and are provided in the collection pipe 190 that connects the collection section 152 of the chamber 150 and the collection container 180.

[0038] The raw materials (mixed powder or granules) are either fed from the raw material supply hopper 160 to the crucible 110 via the raw material introduction pipe 170, or the raw materials are fed directly into the crucible 110. The raw materials for the metal element-containing silicon-based composite are as described above, and are heated to a temperature in the range of 1200°C to 1400°C to generate the raw material gases SiO gas, Li gas, and Mg gas.

[0039] Once the raw materials are placed in the crucible 110, the crucible 110 is heated by the heater 120 while the pressure inside the deposition chamber RM is reduced. Note that if the pressure inside the deposition chamber RM is too high, the reaction that generates SiO gas from the raw materials becomes difficult to occur. Therefore, the pressure inside the deposition chamber RM is preferably 1000 Pa or less, more preferably 750 Pa or less, and particularly preferably 20 Pa or less. Furthermore, the temperature inside the deposition chamber RM affects the reaction rate of SiO; if the temperature is too low, the reaction rate slows down, and if the temperature is too high, there is a concern that side reactions due to the melting of the raw materials will occur, and energy efficiency will decrease. If it's too high Damage to the crucible 110 is also a concern. From this viewpoint, the temperature inside the deposition chamber RM is preferably in the range of 1000°C to 1600°C, more preferably in the range of 1100°C to 1500°C, and particularly preferably in the range of 1100°C to 1400°C.

[0040] As described above, by heating the raw materials under reduced pressure, raw material gas is generated from the raw materials in the crucible 110, and this raw material gas is supplied to the deposition drum 130 through the gas guide Gg. At this time, the deposition drum 130 is rotated by a drive source. The temperature of the outer surface of the deposition drum 130 is set lower than the temperature inside the deposition chamber RM. More specifically, this temperature is set lower than the condensation temperature of the raw material gas. With this setting, the raw material gas generated from the crucible 110 is deposited and deposited on the outer surface of the rotating deposition drum 130. Then, with the scraper 141 in a standby position above, the deposition drum 130 is rotated multiple times to form a laminated film on the deposition drum 130. After that, when the number of rotations of the deposition drum 130 reaches a specified number, the scraper 141 is moved downward, and the laminated film is scraped off the deposition drum 130 by the scraper 141. The scraped-off fragments of the laminated film fall along the outer surface of the deposition drum 130 onto the granular guide 143. Finally, these fragments of the laminated film are crushed to obtain the desired metal element-containing silicon-based composite.

[0041] Incidentally, the metal element-containing silicon composite obtained as described above may be heat-treated. By heat-treating the metal element-containing silicon composite under reduced pressure or an inert gas atmosphere, the crystalline state of the metal element-containing silicon composite can be stabilized. It is desirable to perform the heat treatment at a temperature of 500°C or higher and 900°C or lower. By treating at 500°C or higher, the crystallization of the silicate phase contained in the metal element-containing silicon composite is promoted, and the crystalline structure and physical properties are stabilized. Furthermore, by setting the heat treatment temperature to 900°C or lower, excessive crystallization of the silicon phase contained in the metal element-containing silicon composite is suppressed, and good battery characteristics can be maintained. The heat treatment may be performed at any time after the collection of the laminated film fragments or after the crushing of the fragments. In particular, it is desirable to perform the heat treatment without exposure to the atmosphere after the collection of the laminated film fragments, as this suppresses oxidation reactions and degradation during storage in the atmosphere and during crushing of the fragments. Conductive carbon can also be coated simultaneously with the heat treatment.

[0042] Examples and comparative examples are shown below to illustrate the present invention in more detail, but the present invention is not limited to these examples. [Examples]

[0043] 1. Manufacturing of Li·Mg-containing silicon dioxide composite powder The desired Li·Mg-containing silicon dioxide composite powder was produced by sequentially carrying out the following steps. (1) Raw material powder preparation process Li 0.41 Mg 0.17 A raw material powder with a composition of SiO was obtained.

[0044] (2) Manufacturing process for Li·Mg-containing silicon dioxide composite Using the vapor deposition apparatus 100 shown in FIG. 1, a Li·Mg-containing silicon oxide composite was produced according to the above-described method for producing a metal element-containing silicon-based composite. At that time, the heater 120 was controlled so that the heater temperature, that is, the raw material heating temperature, was 1200°C to 1400°C, and the pressure reducing device was controlled so that the internal pressure of the chamber 150 was 100 Pa or less. Further, the recovered laminate film pieces were heat-treated at 600 to 700°C in an argon atmosphere to stabilize their crystal structures.

[0045] (3) Grinding process The Li·Mg-containing silicon oxide composite obtained as described above was ground using a bead mill in the atmosphere until the average particle size D50 was about 9 μm, and then the particle size was adjusted to obtain the target Li·Mg-containing silicon oxide composite powder. When the average particle size D50 was measured using a laser diffraction type particle size distribution measuring device (Mastersizer 3000 manufactured by Malvern), it was 9.04 μm. The measurement conditions were as follows. ·Dispersion medium: Isopropyl alcohol (2-propanol) ·Particle refractive index: 3.500 ·Particle absorption rate: 1.000 ·Dispersion medium refractive index: 1.390

[0046] 2. Composition analysis The Li·Mg-containing silicon oxide composite powder obtained as described above was analyzed by an ICP emission spectroscopic analyzer (ICP-AES) to determine the respective contents of silicon, lithium, and magnesium. Regarding the oxygen content, it can be determined using an inert gas fusion type oxygen analyzer or the like. However, since the Li·Mg-containing silicon oxide composite powder contains almost no elements other than silicon, lithium, magnesium, and oxygen, and almost no impurities were detected by the ICP emission spectroscopic analyzer, the content of oxygen was determined by considering all other contained elements as oxygen.

[0047] As a result, the composition of the obtained Li·Mg-containing silicon oxide composite powder was Li 0.46 Mg 0.19 SiO 1.11It was identified as Li x Mg y SiO z In this case, (2.5x+2y) / z=1.39, (1.5x+y) / z=0.80, and (x+2y) / z=0.77.

[0048] Furthermore, when the composition was given as m(Li2O)·n(MgO)·SiO2, m was 0.674 and n was 0.569. Therefore, 2m + 0.5n was 1.632, and m + n was 1.243. In this case, the number of oxygen atoms relative to the number of silicon atoms (O / Si) was 1.11.

[0049] Furthermore, X-ray diffraction (XRD) was used to confirm that Li4SiO4 and Li2O were not present as crystals in the Li·Mg-containing silicon oxide composite powder. The measurement was performed using Cu-Kα rays as the characteristic X-ray, with a diffraction angle interval of 0.13°.

[0050] 3. Characteristics of a battery equipped with a negative electrode made of Li·Mg-containing silicon dioxide composite powder. (1) Battery production (1-1) Negative electrode fabrication As described above, the Li·Mg-containing silicon oxide composite powder was placed in a rotary kiln and carbon-coated using thermal CVD with argon and propane gas flowing at 700°C. The mass ratio of carbon to the mass of the Li·Mg-containing silicon oxide composite powder was 2.31% by mass. This mass ratio was calculated from the amount of carbon quantitatively evaluated by analyzing carbon dioxide gas using the oxygen-flow combustion-infrared absorption method with a carbon concentration analyzer (CS400, Leco). Next, the carbon-coated Li·Mg-containing silicon oxide composite powder (hereinafter simply referred to as "carbon-coated powder") and graphite powder were mixed in a mass ratio of 4:1 to form the negative electrode active material. Next, an aqueous slurry was prepared by adding an aqueous dispersion of the negative electrode active material, styrene-butadiene rubber (SBR) as a binder, carboxymethylcellulose (CMC) as a binder, and carbon nanotubes (CNT) as a conductive additive, in a solid mass ratio of 89.8:9:1:0.2, and then kneading them together. Subsequently, the slurry was coated onto a 10 μm thick copper foil, and the coating was pre-dried in air at 80°C. The slurry-coated copper foil was then punched out into a disc shape with a diameter of 11 mm. The disc-shaped slurry-coated copper foil was then dried in a vacuum at 150°C for 12 hours to obtain the desired negative electrode.

[0051] (1-2) Battery construction Coin cells were fabricated using the above-mentioned negative electrode, lithium iron phosphate positive electrode material as the counter electrode, separator, and electrolyte. A 20 μm thick porous polyethylene film was used as the separator, and a solution of lithium hexafluoride phosphate (LiPF6) dissolved at a concentration of 1 mol / L in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 1:1 was used as the electrolyte.

[0052] (2) Rate characteristics Using a secondary battery charge / discharge test device manufactured by Electrofield Co., Ltd., the above coin cell was subjected to charge / discharge tests to determine the initial efficiency (%), and the charge / discharge efficiency (%) for each cycle during 10 charge / discharge cycles was determined, with the average value used as the rate characteristic. During the charge / discharge tests, the conditions for the first charge were set to "CC-CV 0.2C" and "10mV-0.01C", the conditions for the first discharge were set to "CC 0.2C" and "1.5V cut-off", the conditions for the second and subsequent charges were set to "CC-CV 1C" and "10mV-0.5C", and the conditions for the second and subsequent discharges were set to "CC 0.5C" and "1.5V cut-off". Here, the current amount at 1C was calculated using the theoretical capacity calculated assuming a discharge capacity of 360mAh / g for natural graphite and a discharge capacity of 1900mAh / g for Li·Mg-containing silicon dioxide composite powder. Furthermore, the initial efficiency of this coin cell was 86.0%, and its rate characteristic was 99.4%. [Examples]

[0053] Li 0.47 Mg 0.1 Except for obtaining a raw material powder with a composition of SiO, a Li-containing silicon oxide composite powder was obtained by the same method as shown in Example 1, compositional analysis was performed by the same method as shown in Example 1, and a coin cell was prepared by the same method as shown in Example 1. Coincell The initial efficiency (%) and rate characteristics (%) were measured. As a result, the composition of the obtained Li·Mg-containing silicon dioxide composite powder was Li 0.54 Mg 0.11 SiO 1.08 It was identified as Li x Mg y SiO z In this case, (2.5x+2y) / z=1.45, (1.5x+y) / z=0.85, and (x+2y) / z=0.71.

[0054] Furthermore, when the composition was given as m(Li2O)·n(MgO)·SiO2, m was 0.786 and n was 0.324. Therefore, 2m + 0.5n was 1.698, and m + n was 1.092. In this case, the number of oxygen atoms relative to the number of silicon atoms (O / Si) was 1.08.

[0055] Furthermore, X-ray diffraction (XRD) confirmed that Li4SiO4 and Li2O were not present as crystals in the Li·Mg-containing silicon oxide composite powder. The initial efficiency of this coin cell was 85.6%, and the rate characteristic was 99.5%. [Examples]

[0056] Li 0.33 Mg 0.17 Except for obtaining a raw material powder with a composition of SiO, a Li-containing silicon oxide composite powder was obtained by the same method as shown in Example 1, compositional analysis was performed by the same method as shown in Example 1, and a coin cell was prepared by the same method as shown in Example 1. Coincell The initial efficiency (%) and rate characteristics (%) were measured. As a result, the composition of the obtained Li·Mg-containing silicon dioxide composite powder was Li 0.36 Mg 0.18 SiO 1.15 It was identified as Li x Mg y SiO z In this case, (2.5x+2y) / z=1.11, (1.5x+y) / z=0.63, and (x+2y) / z=0.63.

[0057] Furthermore, when the composition was given as m(Li2O)·n(MgO)·SiO2, m was 0.463 and n was 0.464. Therefore, 2m + 0.5n was 1.158, and m + n was 0.927. Also, the number of oxygen atoms relative to the number of silicon atoms (O / Si) was 1.15.

[0058] Furthermore, X-ray diffraction (XRD) confirmed that Li4SiO4 and Li2O were not present as crystals in the Li·Mg-containing silicon oxide composite powder. The initial efficiency of this coin cell was 86.1%, and the rate characteristic was 99.5%.

[0059] (Comparative Example 1) Li 0.4 Except for obtaining a raw material powder with a composition of SiO, a Li-containing silicon oxide composite powder was obtained by the same method as shown in Example 1, compositional analysis was performed by the same method as shown in Example 1, and a coin cell was prepared by the same method as shown in Example 1. Coincell The initial efficiency (%) and rate characteristics (%) were measured. As a result, the composition of the obtained Li·Mg-containing silicon dioxide composite powder was Li 0.40 SiO 0.93 It was identified as Li x Mg y SiO z At (y=0), (2.5x+2y) / z=1.07, (1.5x+y) / z=0.64, and (x+2y) / z=0.43.

[0060] Furthermore, when the composition was given as m(Li2O)·n(MgO)·SiO2, m was 0.548 and n was 0.000. Therefore, 2m + 0.5n was 1.096, and m + n was 0.548. Also, the number of oxygen atoms relative to the number of silicon atoms (O / Si) was 0.93.

[0061] Furthermore, X-ray diffraction (XRD) confirmed that crystalline Li4SiO4 and Li2O were not present in the Li·Mg-containing silicon oxide composite powder. The initial efficiency of this coin cell was 82.8%, and the rate characteristic was 99.1%.

[0062] (Comparative Example 2) Li 0.27 Mg 0.13 Except for obtaining a raw material powder with a composition of SiO, a Li-containing silicon oxide composite powder was obtained by the same method as shown in Example 1, compositional analysis was performed by the same method as shown in Example 1, and a coin cell was prepared by the same method as shown in Example 1. Coincell The initial efficiency (%) and rate characteristics (%) were measured. As a result, the composition of the obtained Li·Mg-containing silicon dioxide composite powder was Li 0.31 Mg 0.16 SiO 1.29 It was identified as Li x Mg y SiO z In this case, (2.5x+2y) / z=0.85, (1.5x+y) / z=0.49, and (x+2y) / z=0.49.

[0063] Furthermore, when the composition was given as m(Li2O)·n(MgO)·SiO2, m was 0.322 and n was 0.324. Therefore, 2m + 0.5n was 0.806, and m + n was 0.646. Also, the number of oxygen atoms relative to the number of silicon atoms (O / Si) was 1.29.

[0064] Furthermore, X-ray diffraction (XRD) confirmed that Li4SiO4 and Li2O were not present as crystals in the Li·Mg-containing silicon oxide composite powder. The initial efficiency of this coin cell was 84.7%, and the rate characteristic was 99.4%.

[0065] The results shown in Examples 1-3 and Comparative Examples 1 and 2 are summarized in the following Tables 1-3.

[0066] As is clear from Table 3, the coin cells shown in Examples 1 to 3 exhibit superior initial efficiency and rate characteristics compared to the coin cell of Comparative Example 1, which has a negative electrode made of a negative electrode active material that does not contain magnesium, and superior initial efficiency compared to the coin cell of Comparative Example 2, which has a negative electrode made of a negative electrode active material with low lithium and magnesium content.

[0067] [Table 1]

[0068] [Table 2]

[0069] [Table 3]

Claims

1. A method for producing a composite material comprising the steps of: a silicon phase formed from silicon; and a silicate phase adjacent to the silicon phase; ｘ Mg ｙ SiO ｚ A metal-element-containing silicon-based composite having a composition of (x>0, y>0, z>0), which satisfies the following conditions: (a) (2.5x+2y) / z>1, (b) (1.5x+y) / z<1, and (c) (x+2y) / z<1.

2. The metal element-containing silicon-based composite according to claim 1, wherein 0.5<z<1.

5.

3. The metal element-containing silicon-based composite according to claim 1, wherein x>0.1 and y>0.

05.

4. The silicate phase is composed of crystalline Li ４ SiO ４ and Li ２ The metal element-containing silicon-based composite according to claim 1 , which does not contain O.

5. The metal element-containing silicon-based composite according to claim 1, at least a portion of the surface of which is covered with a conductive carbon coating.

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