JP7726540B2 - Lithium-ion battery negative electrode material or negative electrode manufacturing device and manufacturing method thereof - Google Patents

Description

The present invention relates to a negative electrode material for lithium ion batteries, lithium ion batteries, and methods and apparatus for manufacturing negative electrodes or negative electrode materials for lithium ion batteries.

Conventionally used lithium-ion batteries have a negative electrode with a composite layer on the negative electrode side, in which graphite (e.g., natural graphite, artificial graphite) as a negative electrode active material (hereinafter also referred to as "negative electrode material") and a negative electrode material are mixed with a binder. The positive electrode side has a positive electrode with a composite layer formed by mixing a positive electrode material and a positive electrode active material, lithium (Li) oxide powder (e.g., _LiCoO2 , _LiNiO2 , _LiMnO2 ), with conductive graphite (mainly carbon black) using a binder (e.g., PVDF: polyvinylidene fluoride). The cell container of the lithium-ion battery is filled with an electrolyte, and a separator (mainly a polyolefin-based porous material or a porous polypropylene sheet) is provided between the negative electrode material and the positive electrode material.

The separator is designed to allow the electrolyte to pass through, allowing lithium ions to migrate, and to separate the electrodes to prevent electrical short circuits.

Lithium-ion batteries are charged and discharged by lithium ions moving between the negative and positive electrode materials in the electrolyte. During charging, lithium ions move to the negative electrode side, and during discharging, lithium ions move to the positive electrode side. Charging is performed through an externally connected power source, and discharging is performed through an externally connected resistance (load).

Recently, in the field of lithium-ion batteries, a technology has been disclosed that uses silicon particles as an alternative to graphite as the negative electrode active material. For example, one example of silicon particles is a powder with a diameter of approximately 38 microns (μm) or less, formed by crushing single-crystal silicon in a mortar and classifying it using a mesh. The powder is then heated in an argon atmosphere at a heating rate of 30°C/min to 150°C (the ultimate temperature) (see Patent Document 1). Another example involves supplying liquid silicon tetrachloride to high-temperature, high-concentration zinc gas and reacting it at a high temperature of 1050°C or higher, thereby reducing the silicon tetrachloride to form silicon particles. The fine silicon is then subjected to crystal growth and aggregation at temperatures below 1000°C, particularly between 500 and 800°C, after which the particle size of the formed silicon particles is adjusted and collected in an aqueous zinc chloride solution. This process produces high-purity silicon particles with a particle size of approximately 1 to 100 μm, and the use of these particles has been disclosed (see Patent Document 2).

Japanese Patent Application Laid-Open No. 2005-032733 JP 2012-101998 A

However, in conventional technology, when silicon particles are used as the negative electrode material, the electrical capacity during charging and discharging can be increased, but at the same time, lithium is absorbed and released into the silicon particles, causing the silicon particles to change volume or even be destroyed. As a result, there is a problem in that the charge-discharge cycle characteristics cannot be maintained.

Furthermore, the silicon particles disclosed in the above-mentioned prior art documents require high-temperature synthesis and collection processes, making the manufacturing process for obtaining silicon particles as anode materials extremely complicated. As a result, productivity declines and manufacturing costs inevitably increase. Therefore, the silicon particles previously disclosed have major issues, such as poor anode properties for lithium-ion batteries and insufficient industrial applicability. In other words, the development of lithium-ion batteries using silicon particles is still in its infancy.

By resolving at least some of the problems associated with conventional anode materials using silicon particles, such as those related to charge/discharge cycle characteristics, the present invention can significantly contribute to the provision of high-performance anode materials for lithium-ion batteries, lithium-ion batteries, anodes for lithium-ion batteries, and methods and apparatus for manufacturing anode materials.

Furthermore, one negative electrode material for a lithium-ion battery of the present invention is formed using silicon microparticles or their aggregates or aggregates, which have a volume distribution in which the mode diameter and median diameter are less than 50 nm and at least a portion of the surface of which is covered with carbon.

Another lithium-ion battery negative electrode material of the present invention is formed using silicon microparticles, or aggregates or conglomerates thereof, at least a portion of whose surfaces are covered with carbon, and includes the aggregates or conglomerates of the silicon microparticles in a multi-layered petal-like or scale-like folded state.

By using the above-mentioned negative electrode materials, it is possible to realize a lithium-ion battery with little change in charge/discharge capacity even after repeated charge/discharge, in other words, with good charge/discharge cycle characteristics. Alternatively, it is possible to realize an increase in charge/discharge capacity.

It is also worth noting that the silicon microparticles or their agglomerates or aggregates that form each of the above-mentioned negative electrode materials can be made from, for example, chips or shavings that are typically treated as industrial waste, produced by cutting a molten and solidified silicon block or ingot with a fixed abrasive wire. Furthermore, the silicon microparticles or their agglomerates or aggregates formed by pulverizing the chips or shavings using a ball mill and/or a bead mill are a suitable embodiment for maintaining a high level of charge/discharge cycle performance of lithium-ion batteries and/or improving those performances.

Furthermore, one lithium-ion battery of the present invention comprises a negative electrode material formed using silicon microparticles or aggregates or aggregates thereof, which have a volume distribution in which the mode diameter and median diameter are less than 50 nm and at least a portion of the surface of which is covered with carbon.

Another lithium-ion battery of the present invention is provided with a negative electrode material formed using silicon microparticles, or aggregates or conglomerates thereof, at least a portion of whose surfaces are covered with carbon, and containing the aggregates or conglomerates of the silicon microparticles in a multi-layered petal-like or scale-like folded state.

The lithium-ion battery described above can reduce changes in charge/discharge capacity even when repeatedly charged and discharged. Alternatively, it can maintain high levels of charge/discharge cycle characteristics and/or improve those characteristics, as well as achieve an increase in charge/discharge capacity.

In addition, one of the lithium-ion battery negative electrode material manufacturing devices of the present invention includes a crushing unit that crushes crystalline silicon to form silicon microparticles or aggregates or conglomerates thereof having a volume distribution with a mode diameter and a median diameter of less than 50 nm, and a coating formation unit that forms carbon to cover at least a portion of the surface of the silicon microparticles or the aggregates or conglomerates.

Another lithium-ion battery negative electrode material manufacturing apparatus of the present invention includes a crushing unit that crushes crystalline silicon, which is chips or shavings removed by a fixed abrasive wire, to form silicon microparticles containing agglomerates or aggregates of silicon microparticles folded into multi-layered petal-like or scale-like structures, and a coating formation unit that forms carbon to cover at least a portion of the surface of the silicon microparticles or the agglomerates or aggregates.

The above-mentioned lithium-ion battery negative electrode material manufacturing device can contribute to the production of lithium-ion batteries that exhibit little change in charge capacity and/or discharge capacity even after repeated charge and discharge, in other words, lithium-ion batteries with good charge and discharge cycle characteristics, or that can achieve increased charge and discharge capacity.

In addition, one lithium-ion battery negative electrode manufacturing apparatus of the present invention includes a crushing unit that crushes crystalline silicon to form silicon microparticles or aggregates or conglomerates thereof that serve as negative electrode material and have a volume distribution with a mode diameter and median diameter of less than 50 nm, and a coating formation unit that forms carbon that covers at least a portion of the surface of the silicon microparticles or the aggregates or conglomerates.

Another lithium-ion battery negative electrode manufacturing apparatus of the present invention includes a crushing unit that crushes crystalline silicon, which is chips or shavings removed by a fixed abrasive wire, to form the silicon microparticles that will become the negative electrode material, so that the silicon microparticles contain agglomerates or aggregates of silicon microparticles that are folded into multi-layered petal-like or scale-like structures; and a coating formation unit that forms carbon to cover at least a portion of the surface of the silicon microparticles or the agglomerates or aggregates.

The above-described lithium-ion battery negative electrode manufacturing device can contribute to the production of lithium-ion batteries that exhibit little change in charge capacity and/or discharge capacity even after repeated charge and discharge, in other words, lithium-ion batteries with good charge and discharge cycle characteristics, or that can achieve increased charge and discharge capacity.

Furthermore, one method for producing a negative electrode material for a lithium-ion battery according to the present invention includes a milling step in which crystalline silicon is milled to form silicon microparticles or aggregates or conglomerates thereof having a volume distribution in which the mode diameter and median diameter are less than 50 nm, and a coating step in which carbon is formed to cover at least a portion of the surface of the silicon microparticles or the aggregates or conglomerates.

Another method of manufacturing a negative electrode material for a lithium-ion battery according to the present invention includes a milling step in which crystalline silicon, which is chips or shavings removed by a fixed abrasive wire, is milled to form silicon microparticles containing agglomerates or aggregates of silicon microparticles folded into multi-layered petal-like or scale-like structures; and a coating step in which carbon is formed to cover at least a portion of the surface of the silicon microparticles or the agglomerates or aggregates.

The above-described method for manufacturing a negative electrode material for lithium-ion batteries can contribute to the production of lithium-ion batteries that exhibit little change in charge capacity and/or discharge capacity even after repeated charge and discharge, in other words, lithium-ion batteries with good charge and discharge cycle characteristics, or that can achieve increased charge and discharge capacity.

In addition, one method for manufacturing a negative electrode for a lithium-ion battery according to the present invention includes a milling step in which crystalline silicon is milled to form silicon microparticles or aggregates or conglomerates thereof, which will serve as the negative electrode material, and which have a volume distribution in which the mode diameter and median diameter are less than 50 nm, and a coating step in which carbon is formed to cover at least a portion of the surface of the silicon microparticles or the aggregates or conglomerates.

Another method of manufacturing a lithium-ion battery negative electrode according to the present invention includes a milling step in which crystalline silicon, which is chips or shavings removed by a fixed abrasive wire, is milled to form silicon microparticles that will become the negative electrode material, so that the silicon microparticles contain agglomerates or aggregates of silicon microparticles that are folded into multi-layered petal-like or scale-like structures; and a coating step in which carbon is formed to cover at least a portion of the surface of the silicon microparticles or the agglomerates or aggregates.

The above-described method for manufacturing a lithium-ion battery negative electrode not only increases charge/discharge capacity, but also minimizes change in charge and/or discharge capacity even after repeated charge/discharge cycles, which can contribute to the production of lithium-ion batteries with excellent charge/discharge cycle characteristics.

The means for covering at least a portion of the surface of each of the above-mentioned silicon microparticles or their aggregates or aggregates with carbon is not particularly limited. Representative examples of such means include a means using a carbon film formed by thermal decomposition of hydrocarbons such as acetylene, a means using a carbon film formed by the furnace method, a means using a carbon film formed by the channel method, or a means using a carbon film formed by plasma treatment.

Note that the crystalline silicon in the above-mentioned inventions includes not only single-crystal silicon but also polycrystalline silicon. Furthermore, metallic silicon can also be selected as the crystalline silicon in the above-mentioned inventions.

One lithium-ion battery anode material of the present invention not only realizes a lithium-ion battery that exhibits little change in charge/discharge capacity even after repeated charging/discharging, in other words, good charge/discharge cycle characteristics, but also realizes an increase in charge/discharge capacity. Furthermore, one lithium-ion battery of the present invention not only reduces change in charge/discharge capacity even after repeated charging/discharging, in other words, improves charge/discharge cycle characteristics, but also realizes an increase in charge/discharge capacity. In addition, one lithium-ion battery manufacturing apparatus and one lithium-ion battery manufacturing method of the present invention can contribute to the manufacture of lithium-ion batteries that exhibit little change in charge/discharge capacity even after repeated charging/discharging, in other words, good charge/discharge cycle characteristics, as well as lithium-ion batteries that can achieve an increase in charge/discharge capacity.

FIG. 1 is a flow chart showing a manufacturing process of a negative electrode material for a lithium ion battery according to a first embodiment. 1 is a schematic diagram showing a manufacturing apparatus and manufacturing process for a negative electrode material for a lithium ion battery according to a first embodiment. FIG. FIG. 10 is a diagram showing a TEM (transmission electron microscope) image of an example of silicon fine particles or their aggregates or aggregates that have undergone the coating formation step (S4) of the first embodiment. 1 is an SEM image of an example of silicon fine particles or aggregates or aggregates thereof according to the first embodiment. FIG. 2 is a diagram showing an SEM image of an example of enlarged silicon microparticles or their aggregates or aggregates in the first embodiment. 1A is a view showing an SEM image of another example of an aggregate or conglomerate of silicon fine particles in the first embodiment, and FIG. 1B is an enlarged view of a portion of FIG. 1A. FIG. 2 is a diagram showing a TEM image of silicon microparticles according to the first embodiment. 1A is a graph showing the crystallite size distribution in the number distribution and the crystallite size distribution in the volume distribution for the crystallite size of silicon fine particles of the first embodiment. 1 is a graph showing the results of X-ray diffraction measurement of silicon fine particles or their aggregates or conglomerates according to the first embodiment ((a) wide range, (b) limited range). FIG. 10 is a schematic configuration diagram of a lithium ion battery according to a second embodiment. 10 is a graph showing an example of charging cycle characteristics of the lithium ion battery according to the second embodiment. 10 is a graph showing an example of discharge cycle characteristics of the lithium ion battery of the second embodiment. 1 is a graph showing the charging cycle characteristics of a lithium ion battery as a reference example. 1 is a graph showing the discharge cycle characteristics of a lithium ion battery as a reference example. 10 is a TEM image of silicon fine particles or aggregates or aggregates thereof in the lithium ion battery of the second embodiment after 100 charge/discharge cycles. FIG. 10 is a schematic diagram showing a manufacturing apparatus and manufacturing process for a negative electrode material for a lithium ion battery according to another embodiment. 10 is a graph showing the charging cycle characteristics of a lithium ion battery according to another embodiment when the amount of FEC in the electrolyte is changed. 10 is a graph showing the discharge cycle characteristics of a lithium ion battery according to another embodiment when the amount of FEC in the electrolyte is changed. 10 is a graph showing the charging cycle characteristics of a lithium ion battery according to another embodiment when the weight ratio of silicon (Si) to carbon (C) is changed. 10 is a graph showing the discharge cycle characteristics of a lithium ion battery according to another embodiment when the weight ratio of silicon (Si) to carbon (C) is changed.

1. Chips, etc. 2. Silicon fine particles 10. Washing machine (washing and preliminary crushing machine)
DESCRIPTION OF SYMBOLS 11 Bowl type 13a Pot 13b Lid 15 Rotating shaft 20 Crusher 21 Inlet 22 Treatment chamber 24 Outlet 25 Filter 30 Dryer 40 Rotary evaporator 50 Oxide film removal tank 55 Hydrofluoric acid or ammonium fluoride aqueous solution 57 Stirrer 58 Centrifuge 60 Coating forming section 70 Mixing section 100 Lithium ion battery negative electrode material and negative electrode manufacturing apparatus 500 Lithium ion battery 510 Container 512 Negative electrode 514 Negative electrode material and negative electrode material 516 Positive electrode 518 Positive electrode material and positive electrode material 520 Separator 530 Electrolyte 540 Power source 550 Resistor

Embodiments of the present invention will be described in detail with reference to the accompanying drawings. In this description, common parts will be designated by common reference symbols throughout the drawings unless otherwise specified. Furthermore, the elements of each embodiment are not necessarily shown to scale. Furthermore, some symbols may be omitted to make each drawing easier to read.

First Embodiment
Fig. 1 is a flow diagram showing the manufacturing process of the negative electrode material for lithium ion batteries according to this embodiment, and Fig. 2 is a schematic diagram showing the manufacturing apparatus and manufacturing process of the negative electrode material for lithium ion batteries according to this embodiment.

The lithium-ion battery anode material of this embodiment, a lithium-ion battery including this anode material, and their manufacturing methods include various steps using, as a starting material, silicon chips, silicon cutting waste, or silicon grinding waste (hereinafter also referred to as "silicon chips, etc." or "chips, etc."), which are typically discarded during silicon cutting processing in the production of silicon wafers used in semiconductor products such as solar cells. The chips, etc. also include fine chips obtained by pulverizing discarded silicon wafers using a known pulverizer. As shown in FIG. 1 , the lithium-ion battery manufacturing method of this embodiment includes the following steps (1), (2), (4), and (5). Furthermore, the lithium-ion battery manufacturing method of this embodiment can include the following step (3) as another possible embodiment.
(1) Cleaning step (S1)
(2) Grinding process (S2)
(3) Oxide film removal process (S3)
(4) Covering formation step (S4)
(5) Negative electrode formation step (S5)

As shown in FIG. 2, the lithium-ion battery anode material and anode manufacturing apparatus 100 of this embodiment primarily includes a washer (washing and pre-crushing machine) 10, a crusher 20, a dryer (not shown), a rotary evaporator 40, a coating forming unit 60 that forms carbon to cover at least a portion of the surface of silicon microparticles or their aggregates or conglomerates (hereinafter collectively referred to as "silicon microparticles"), and a mixing unit 70 that is responsible for part of forming the lithium-ion battery anode. In another possible embodiment, the lithium-ion battery anode material and anode manufacturing apparatus 100 can also include an oxide film removal tank 50 and a centrifuge 58. The crusher 20 alone, or the washer 10 and crusher 20 together, are referred to as the crushing unit in this embodiment.

(1) Cleaning step (S1)
In the cleaning step (S1) of this embodiment, for example, silicon chips formed during the cutting process of single crystal or polycrystalline silicon, i.e., a crystalline silicon chunk or ingot (an n-type crystalline silicon chunk or ingot), are cleaned. Typical silicon chips are chips formed when a silicon ingot is cut using a known wire or the like (typically, a fixed abrasive wire). Therefore, in this embodiment, silicon chips, which have traditionally been considered waste material, are used as the starting material to form silicon microparticles that constitute the negative electrode material of a lithium-ion battery, which is advantageous in terms of production cost and/or ease of procurement of raw materials and resource utilization.

The cleaning process (S1) in this embodiment is primarily intended to remove organic matter that adheres during the formation of the silicon chips, etc., as described above, typically organic matter such as coolants and additives used in the cutting process. In this embodiment, as shown in FIG. 2, the chips, etc. 1 to be cleaned are first weighed, and then the chips, etc. 1, a predetermined first liquid, and balls 11 are introduced into a cylindrical pot 13a with a bottom. After sealing the pot 13a with a lid 13b, the pot 13a is rotated by rotating two cylindrical rotors 15 of a ball mill, which serves as the cleaning machine (cleaning and preliminary crusher) 10. As a result, the chips, etc. 1 to be cleaned are dispersed in the first liquid within the pot 13a, thereby cleaning the chips, etc. 1 and performing preliminary crushing.

The ball mill of this embodiment uses steel balls, magnetic balls, boulders, and the like housed in the pot 13a and lid 13b as ball seeds 11 (grinding media), and applies physical impact force by rotating the pot 13a and lid 13b. A preferred example of the first liquid is acetone. In a more specific example, 300 milliliters (mL) of acetone was added to 100 grams (g) of silicon chips or the like, and the mixture was stirred for approximately one hour in the pot 13a and lid 13b placed on the rotor 15 of the ball mill (Universal BALL MILL, manufactured by Masuda Corporation) to disperse the silicon chips or the like in the acetone. The ball seeds used in the ball mill were alumina balls with a diameter of 10 millimeters (mm) and alumina balls with a diameter of 20 mm. In the cleaning step (S1) of this embodiment, silicon chips and the like are pre-pulverized and stirred in the first liquid in a ball mill to perform a dispersion process. This significantly improves cleaning efficiency compared to a process in which the chips are simply immersed in the first liquid, making it possible to obtain silicon particles that are suitable for improving the negative electrode characteristics of lithium-ion batteries, particularly their charge/discharge cycle characteristics.

After the cleaning step (S1), the lid 13b is opened and the silicon particles are discharged together with the first liquid. The first liquid is then removed by suction filtration using known vacuum filtration means, resulting in waste liquid. Meanwhile, the remaining silicon particles are dried in a known dryer. If necessary, the silicon particles obtained after the drying process are pre-crushed and washed again in the washer (cleaning and pre-crushing machine) 10 using the same process. Examples of other cleaning methods that can be used in the cleaning step (S1) include cleaning using the RCA cleaning method or cleaning with water (including pure water).

(2) Grinding process (S2)
Then, in the pulverization step (S2), a predetermined second liquid is added to the washed silicon particles, and the silicon particles are pulverized in a bead mill. Therefore, in this embodiment, the silicon particles that have been subjected to the washing step (S1) are pulverized further finely by the bead mill used after the ball mill, in other words, after pulverization by the ball mill.

In this embodiment, a suitable example of the second liquid is IPA (isopropyl alcohol). As a pretreatment for the milling process, the second liquid and the silicon particles obtained in the cleaning process (S1) are placed in a pot 13a in a weight ratio of 95% second liquid to 5% silicon particles. Then, a pre-milling process is performed by rotating the washer (cleaning and pre-milling machine) 10. The pre-milled silicon particle-containing slurry is passed through a mesh with 180-micron openings to remove relatively coarse particles. The resulting silicon particle-containing slurry is then further finely milled using a bead mill (in this embodiment, a Star Mill LMZ015 manufactured by Ashizawa Fining Co., Ltd.) in the mill 20. More specifically, the silicon particle-containing slurry, from which silicon chips with particle diameters of 180 microns or larger have been removed, is introduced into the inlet 21 of the mill 20. The slurry is circulated using the pump 28, and finely milled in the bead mill's processing chamber 22. An example of a specific type of bead used in the bead mill is zirconia beads with a diameter of 0.5 mm. After collecting the slurry containing the finely ground silicon particles, the second liquid is removed using a rotary evaporator 40, which automatically performs vacuum distillation, to obtain fine silicon particles as the result of the finely ground process.

In this embodiment, approximately 450 g of zirconia beads with a particle diameter of 0.5 mm are introduced, and the pulverization process is carried out at a rotation speed of 2908 rpm for 4 hours to obtain silicon microparticles. In another embodiment, the pulverization process in the pulverization step (S2) can be carried out using any one of the following pulverizers, or a combination of two or more of them, selected from the group consisting of ball mills, bead mills, jet mills, and shock wave pulverizers. In addition to automatic pulverizers, manual pulverizers may also be used in the pulverization step (S2). However, from the perspective of highly accurately forming agglomerates or aggregates of silicon microparticles in a multi-layered petal-like or scale-like state, as described below, it is preferable to use a pulverization step (S2) that consists of processing using a bead mill, or a pulverization step (S2) that includes processing using a bead mill.

Another suitable embodiment is to further crush the silicon microparticles obtained in the above-described crushing step (S2) using a known crusher (a typical example is the Model 20D crusher manufactured by Ishikawa Kogyo Co., Ltd.). This crushing process improves dispersibility when forming the negative electrode of a lithium-ion battery, thereby highly reliably preventing or suppressing damage to the negative electrode due to the absorption and release of lithium.

(3) Oxide film removal process (S3)
In this embodiment, the oxide film removing step (S3) is performed as a preferred aspect, but at least some of the effects of this embodiment can be achieved even if this oxide film removing step (S3) is not performed.

In the oxide film removal step (S3) of this embodiment, the silicon microparticles 2 obtained in the pulverization step (S2) are brought into contact with hydrofluoric acid or an aqueous ammonium fluoride solution. The silicon microparticles 2 obtained in the pulverization step (S2) are dispersed by immersing them in the hydrofluoric acid or ammonium fluoride solution. Specifically, in the oxide film removal tank 50, the silicon microparticles 2 are dispersed in the hydrofluoric acid or ammonium fluoride solution 55 using an agitator 57, thereby removing oxides (mainly silicon oxide) from the surfaces of the silicon microparticles 2.

Then, a centrifuge 58 separates the silicon microparticles from which some or all of the surface oxide has been removed and the hydrofluoric acid solution. The silicon microparticles are then immersed in a third liquid, such as an ethanol solution. By removing the third liquid, silicon microparticles are obtained from which some or all of the original surface oxide (or oxide film) has been removed. If the process of removing oxides that may be present on the surface of the silicon microparticles 2 is not performed, the silicon microparticles are then processed in the coating formation process (S4) and anode formation process (S5) described below.

In the oxide film removal step (S3) of this embodiment, the silicon microparticles are brought into contact with hydrofluoric acid by immersing them in hydrofluoric acid or an aqueous ammonium fluoride solution. However, other methods for bringing the silicon microparticles into contact with hydrofluoric acid or an aqueous ammonium fluoride solution may also be used. For example, spraying the hydrofluoric acid solution onto the silicon microparticles in a shower-like manner is another possible embodiment.

(4) Covering formation step (S4)
In this embodiment, a coating step is performed in which a carbon film coats part or all of the surface of silicon fine particles that have or have not undergone the oxide film removal step (S3). Therefore, the silicon fine particles that have undergone the coating step (S4) of this embodiment have at least a portion of their surface covered with carbon, resulting in an increased thickness compared to silicon fine particles that are not covered with carbon. The conductive carbon of this embodiment allows for the formation of a conductive anode material. Furthermore, for example, by covering silicon fine particles with carbon having a film thickness of several nanometers to several micrometers, not only is it possible to form a thin anode, but it also facilitates the formation of a relatively thick anode. FIG. 3 shows a transmission electron microscope (TEM) image of an example of silicon fine particles (partially enlarged) or their aggregates or aggregates (partially enlarged) that have undergone the coating step (S4) of this embodiment. The crystalline silicon shown in FIG. 3 was mainly observed to have a crystal plane orientation (111) (also simply referred to as "Si(111)" or "(111)". The same applies to the notations for other plane orientations).

In addition, for example, covering silicon microparticles with carbon can contribute to alleviating internal stress in the negative electrode as a whole, and more microscopically, to alleviating internal stress in individual silicon microparticles. As a result, it can facilitate the manufacture of lithium-ion batteries and also contribute to improving the performance of lithium-ion batteries (e.g., charge/discharge cycle characteristics).

Furthermore, by forming the negative electrode material of a lithium-ion battery using silicon microparticles at least partially coated with carbon, it is possible to increase the charge capacity and discharge capacity values compared to silicon microparticles not coated with carbon. For example, according to research by the present inventors, in the case of silicon microparticles not coated with carbon, when the thickness of the negative electrode material is, for example, approximately 30 μm, the charge capacity and discharge capacity values are less than 500 (mAh/g). However, by using silicon microparticles at least partially coated with carbon, the charge capacity and discharge capacity values can be increased by more than two times (typically, approximately three to five times).

In the coating formation step (S4) of this embodiment, the means for coating part or all of the surface of the silicon microparticles (in other words, at least a portion of the surface) with a carbon film is not particularly limited. For example, a method of covering at least a portion of the surface of the silicon microparticles 2 with carbon using thermal CVD, an example of chemical vapor deposition. Specifically, at least a portion of the surface of the silicon microparticles 2 can be coated with a carbon film by heating the silicon microparticles 2 at a temperature of approximately 600°C to approximately 1200°C in an atmosphere containing organic gases and/or their vapor. Therefore, a thermal CVD apparatus, an example of a chemical vapor deposition apparatus, is one aspect of the coating formation unit 60 that can be used in the coating formation step (S4) of this embodiment. Examples of materials (or raw materials) that can be used in the thermal CVD method include one or more hydrocarbons selected from the group consisting of methane, ethane, acetylene, ethylene, propane, butane, butene, pentane, isobutane, and hexane, and/or one or more known aromatic hydrocarbons. Furthermore, by using copper or a copper alloy as a catalyst, acetylene can be thermally decomposed even at approximately 400°C, which can contribute to lowering the processing temperature.

As mentioned above, typically, hydrocarbon gas is thermally decomposed on or near heated silicon microparticles 2 to carbonize them, thereby coating at least a portion of the surface of the silicon microparticles 2 with a carbon film. This is an example of a suitable carbon film for this embodiment and is a method with excellent productivity and industrial applicability. Note that low-pressure thermal CVD or plasma CVD, which are other examples of chemical vapor deposition methods, may also be used as appropriate.

Another example of a method for coating at least a portion of the surface of silicon microparticles 2 with a carbon film is as follows. First, a known organic compound (such as chlorinated polyethylene elastomer) or a mixture of such an organic compound and graphite powder is attached or adsorbed to silicon microparticles 2 that have been treated at least by the cleaning step (S1) and the pulverization step (S2) of this embodiment in the coating formation unit 60. The organic compound is then heated to a temperature at which it thermally decomposes, carbonizing the organic compound to form silicon microparticles 2 at least partially coated with carbon (typically graphene, graphite, and/or amorphous carbon). A variation of this embodiment also includes additionally and/or priorly performing a step of baking the formed silicon microparticles 2 in a nitrogen or hydrogen atmosphere, for example, at 400°C to 1200°C. From the perspective of being able to reliably prevent, suppress, or remove the formation of an oxide film on the surface of the silicon microparticles 2 or their aggregates or aggregates, heating the silicon microparticles 2 in a hydrogen atmosphere is preferred.

Another possible approach is to use other known methods to form silicon microparticles 2 whose surfaces are at least partially covered with carbon. For example, it is possible to use a carbon film formed by known carbon black manufacturing methods (furnace method or channel method). The furnace method involves partially burning creosote oil, obtained by distilling coal tar, in a designated chamber where the oil is turbulently diffused, and then water-cooling the resulting droplets. The channel method involves depositing carbon by impinging a flame, for example, created by burning natural gas, on the cooled surface of channel steel. However, because this method contains sulfur components derived from petroleum, it is preferable to use the thermal CVD method described above, which is a method that is less susceptible to the inclusion of such impurities. Furthermore, when employing the above-mentioned methods, a preferred embodiment involves applying kinetic energy to the silicon microparticles or their agglomerates or aggregates using a known mechanical structure to coat at least a portion of the surface of the silicon microparticles 2 or their agglomerates or aggregates with a carbon film, which allows for a more uniform (e.g., uniform thickness and/or density) carbon film to be formed on the surface of the silicon microparticles 2 or their agglomerates or aggregates. More specifically, a preferred embodiment involves applying a carbon film to at least a portion of the surface of the silicon microparticles 2 or their agglomerates or aggregates while rotating a chamber or tube containing the silicon microparticles or their agglomerates or aggregates and/or while applying ultrasonic vibrations to the silicon microparticles 2 or their agglomerates or aggregates, which allows for a more uniform carbon film to be formed on the surface of the silicon microparticles 2 or their agglomerates or aggregates.

(4) Negative electrode formation step (S5)
The lithium-ion battery negative electrode material and negative electrode manufacturing apparatus 100 of this embodiment includes a mixing unit 70 that mixes silicon fine particles as the negative electrode material processed in the coating formation step (S4) with a member (e.g., copper foil) that constitutes a part of the negative electrode using a binder (e.g., ammonium carboxylmethyl cellulose (CMC) and styrene butadiene rubber (SBR) or ammonium carboxylmethyl cellulose (CMC) and polyvinyl alcohol (PVA)). The negative electrode is formed using a mixture layer formed by this mixing unit 70.

<Other processes>
The silicon microparticles obtained by the above-mentioned cleaning step (S1), pulverization step (S2), and (4) coating formation step (S4), or by the cleaning step (S1), pulverization step (S2), oxide film removal step (S3), and (4) coating formation step (S4), may be classified, for example, to reduce variations in the number distribution and/or volume distribution of the crystallite diameter of each silicon microparticle.

<Analysis Results of Silicon Microparticles Obtained in the First Embodiment>
1. Analysis of silicon microparticles using SEM and TEM images

Figure 4A is a scanning electron microscope (SEM) image of an example of silicon microparticles or their aggregates or conglomerates after the pulverization step (S2) of the first embodiment. Also, Figure 4B is a diagram showing an enlarged SEM image of an example of silicon microparticles or their aggregates or conglomerates after the pulverization step (S2) of the first embodiment. Also, Figure 4C shows (a) an SEM image of another example of an aggregate or conglomerate of silicon microparticles in the first embodiment, and (b) an enlarged view of a portion of (a). Additionally, Figure 5 is a diagram showing a transmission electron microscope (TEM) image of silicon microparticles of the first embodiment.

As shown in Figure 4A, not only individual silicon microparticles but also silicon microparticles or their aggregates or conglomerates, as shown in Figure 4B and the Z portion of Figures 4C(a) and (b), were confirmed. Interestingly, upon further detailed analysis, it was confirmed that the silicon microparticles or their aggregates were aggregates or conglomerates in which thin-layered silicon microparticles were folded into multi-layered petal-like or scale-like structures, as shown in Figure 4B and the Z portion of Figures 4C(a) and (b). Furthermore, upon closer observation, for example, when the width (minor axis) of one or a group of scale-like silicon microparticles was taken as 1, the range of the length (major axis) was 3.3 to 12.9.

Furthermore, another interesting finding was obtained from the TEM image shown in Figure 5, which focuses on individual silicon microparticles. Specifically, it was confirmed that the individual silicon microparticles indicated by the areas surrounded by white lines in Figure 5 are crystalline, i.e., single-crystal silicon. In addition, it was confirmed that at least a portion of the silicon microparticles are irregular polygonal crystallites measuring approximately 2 nm to 10 nm in size in a cross-sectional view. Note that in Figure 5, the crystal plane orientation is indicated in each area surrounded by white lines.

2. Analysis of crystallite size distribution of silicon fine particles by X-ray diffraction method Figure 6 is a graph showing (a) the crystallite size distribution showing the number distribution and (b) the crystallite size distribution showing the volume distribution with respect to the crystallite size in the Si (111) direction of the silicon fine particles of the first embodiment. Figure 6 shows the results obtained by analyzing the crystallite size distribution of silicon fine particles after the pulverization step (S2) using X-ray diffraction. In both Figure 6 (a) and Figure 6 (b), the horizontal axis represents the crystallite size (nm) and the vertical axis represents the frequency.

From the results in Figures 6(a) and 6(b), the number distribution showed a mode diameter of 1.6 nm and a median diameter (50% crystallite diameter) of 2.6 nm. Furthermore, the volume distribution showed a mode diameter of 6.3 nm and a median diameter of 9.9 nm. Therefore, it was confirmed that both the mode diameter and the median diameter in the number distribution were 5 nm or less, and more specifically, 3 nm or less. It is noteworthy that the volume distribution showed that both the mode diameter and the median diameter were at least 50 nm or less, and particularly 30 nm or less. Furthermore, Figure 6 confirmed that an extremely small value of 10 nm or less was achieved.

From the results of Figures 6(a) and 6(b), it was confirmed that the silicon microparticles obtained after the pulverization step (S2) using the bead mill method have an average crystallite diameter of approximately 20 nm or less, more specifically, approximately 9.8 nm, which achieves an average crystallite diameter of 10 nm or less. The crystallite diameter distribution of the silicon microparticles after the oxide film removal step (S3) is also similar to that shown in Figure 6.

Therefore, by analyzing the results of Figure 6 together with the results of each diagram in Figure 4, it can be said that the aggregates or aggregates of silicon microparticles at least after the crushing step (S2) or the oxide film removal step (S3) are in a state in which thin-layered silicon microparticles with a major axis of approximately 100 nm or less are folded together in a multi-layered petal-like or scale-like manner. Furthermore, as can be seen from Figures 5 and 6, the silicon microparticles are mainly composed of crystallites with a major axis of 10 nm or less.

Furthermore, as shown in Figure 6, the silicon microparticles of this embodiment contain silicon microparticles with a crystallite diameter of 1 nm or less. Interestingly, it was also confirmed that the average crystallite diameter in the volume distribution of the silicon microparticles of this embodiment is approximately 10 nm. This value can be considered very small. Furthermore, as mentioned above, further investigation confirmed that the apparent volume diameter of the silicon microparticles is in the range of approximately 100 nm or less. In particular, by containing a large number of ultrafine silicon particles with a crystallite diameter of 5 nm or less in the major axis, the charge/discharge cycle characteristics achieved by the silicon microparticles used as the negative electrode material for lithium-ion batteries, as described below, are more reliably improved. Furthermore, because at least a portion of the surface of the ultrafine silicon particles is covered with carbon, higher charge capacity values, discharge capacity values, and excellent charge/discharge cycle characteristics can be achieved.

3. Analysis of Crystallite Orientation of Silicon Microparticles by X-ray Diffraction Method Figure 7(a) shows the results of X-ray diffraction measurement (P) of silicon microparticles or their aggregates or conglomerates before the pulverization step (S2) of the first embodiment, and the results of X-ray diffraction measurement (Q) of silicon microparticles or their aggregates or conglomerates after the pulverization step (S2) in a wide angle range. Figure 7(b) is an enlarged view of the result (P) in Figure 7(a), showing the results of X-ray diffraction measurement (R) of silicon microparticles or their aggregates or conglomerates after the pulverization step (S2) in the first embodiment, analyzed in a limited angle range. The peak intensities of the C(002) and C(003) planes in Figure 7(b) indicate that approximately 1 wt % to approximately 3 wt % of graphite particles are contained in the silicon microparticle groups or silicon microparticle aggregates or conglomerates. As an example, the size of the graphite particles on the C(002) plane was approximately 50 nm or less, more specifically, approximately 35 nm, and the size of the graphite particles on the C(003) plane was approximately 100 nm or less, more specifically, approximately 75 nm.

As shown in Figures 7(a) and (b), it was confirmed that the diffraction peak attributable to the Si (111) crystal plane near 2θ = 28.4° after the milling step (S2) of the first embodiment had a larger half-width than the diffraction peak attributable to the Si (111) crystal plane near 2θ = 28.4° before the milling step (S2). The average crystallite diameter calculated using the Scherrer equation from the half-width of the Si (111) peak after the milling step (S2) was 9.8 nm. Interestingly, it was also found that the intensity of the diffraction peak attributable to Si (111) near 2θ = 28.4° after the milling step (S2) was greater than the intensity of other diffraction peaks (e.g., the peak intensity of Si (220) or Si (311)). The arrangement spacing of Si (111) in the crystal lattice of the silicon microparticles after the milling step (S2) was 0.31 nm (3.1 Å), as shown in Figure 5.

Based on the above analytical results, the silicon microparticles obtained after the pulverization step (S2) of this embodiment can be said to be aggregates or aggregates of crystalline silicon microparticles, primarily with a (111) plane orientation, that are multi-layered and petal- or scale-like, and are folded over one another.

By using the silicon fine particles or their agglomerates or aggregates after the crushing step (S2) or the oxide film removal step (S3) of this embodiment as the negative electrode material of a lithium ion battery, a unique effect can be achieved in which, when lithium ions (Li ⁺ ) ionized from the positive electrode material of the lithium ion battery reach the negative electrode, the lithium ions (Li ⁺ ) can easily enter and exit the gaps in the folds of the agglomerates or aggregates, which are in a multi-layered, petal-like or scale-like state.

Second Embodiment
The lithium-ion battery of this embodiment uses, as the negative electrode material, silicon microparticles at least partially coated with carbon, as prepared in the first embodiment. Note that the configuration other than the negative electrode material is the same as that of a conventional CR2032 coin cell lithium-ion battery.

Figure 8 is a schematic diagram of a lithium-ion battery 500 according to this embodiment. The lithium-ion battery 500 according to this embodiment includes, within a CR2032-type coin cell container 510, a negative electrode 512 electrically connected to the negative electrode material 514, a positive electrode 516 electrically connected to the positive electrode material 518, a separator 520 that electronically insulates the negative electrode material 514 from the positive electrode material 518, and an electrolyte 530. The lithium-ion battery 500 according to this embodiment also includes an external circuit including a power source 540 and a resistor 550 connected to the negative electrode 512 and positive electrode 516 to enable charging and discharging.

The manufacturing method for the lithium-ion battery 500 of this embodiment is as follows.

To manufacture the negative electrode, first, approximately 0.3 g of silicon microparticles, at least a portion of whose surface is covered with carbon, as prepared in the first embodiment, are dispersed in approximately 10 mL (milliliters) of a solution consisting of a 1 wt% aqueous CMC binder solution and an aqueous SBR binder dispersion (TRD2001, manufactured by JSR Corporation).

In this embodiment, the silicon microparticles, carbon black, CMC binder aqueous solution, and SBR binder aqueous dispersion are blended in a dry weight ratio of 67:11:13:9 or 50:25:20:5, in that order. In one variation of this embodiment, the silicon microparticles, carbon black, CMC binder aqueous solution, and PVA binder aqueous dispersion are blended in a dry weight ratio of 50:25:20:5, in that order.

Next, the slurry mixed and prepared using an agate mortar is applied to one side of a 15 μm-thick copper foil measuring approximately 9 cm (length) x 10 cm (width) so that the thickness after drying will be approximately 30 μm to approximately 200 μm. It is then dried on a hot plate in air at 80°C for approximately 1 hour. The copper foil and the dried slurry are then punched into a circle with a diameter of 11.3 mm, which corresponds to the battery standard CR2032 coin cell, to form a working electrode. After measuring the weight of this working electrode, it is dried again by vacuum heating at 120°C for 6 hours in a glove box, and then attached to the inner surface of the copper foil negative electrode 512, thereby producing the negative electrode of this embodiment.

Next, for the positive electrode, in order to evaluate the characteristics of the negative electrode material using a lithium ion battery with a half-cell structure, a lithium substrate was punched out into a circle with a diameter of 13 mm and used as the positive electrode 516. Note that for the positive electrode of a lithium ion battery, a known positive electrode can be used instead of the positive electrode 516 described above.

The separator 520 of this embodiment is a porous polypropylene sheet. In addition, the electrolyte 530 of this embodiment is an electrolyte solution in which 1 mol of lithium hexafluorophosphate (LiPF ₆ ) is dissolved in a solvent (1 L) of a 1/1 volumetric mixture of ethylene carbonate (EC) and diethyl carbonate (DEC), or the electrolyte solution to which fluoroethylene carbonate (FEC) has been added, and is injected in an amount sufficient to fill the internal volume (approximately 1 mL) of a CR2032 coin cell.

The above-mentioned positive electrode material 518, positive electrode 516, negative electrode material 514, negative electrode 512, separator 520, and electrolyte 530 were placed in a CR2032 coin cell container 510. Then, in an argon-filled glove box, the components of the separator 520 and electrolyte 530 were sealed in the container 510 while insulating the positive electrode 516 and negative electrode 512 on the outer frame of the coin cell, thereby producing a prototype CR2032 coin cell lithium-ion battery 500.

In this embodiment, examples of the electrolytic solvent constituting the electrolyte solution 530 include a mixed solvent of cyclic carbonates such as ethylene carbonate (EC) and propylene carbonate (PC) (polypropylene sheet), and chain carbonate organic solvents such as dimethyl carbonate (DMC) and diethyl carbonate (DEC). Supporting salts such as lithium hexafluorophosphate (LiPF) and lithium tetrafluoroborate (LiBF) can also be dissolved in the electrolytic solvent and used.

<Charge/Discharge Cycle Characteristics of Lithium-Ion Battery 500>
By employing the lithium ion battery 500 having the above-described configuration, the charge capacity value and discharge capacity value can be increased by more than two times (typically about three to five times) compared to the case where silicon fine particles whose surfaces are not covered with carbon are employed (less than about 500 (mAh/g)). Furthermore, even after 100 or more charge/discharge cycles, it is possible to achieve extremely good charge/discharge cycle characteristics in which neither the charge capacity value nor the discharge capacity value decreases significantly. A specific comparison is as follows.

Figure 9 is a graph showing the charge cycle characteristics of the lithium-ion battery 500 of this embodiment. Also, Figure 10 is a graph showing the discharge cycle characteristics of the lithium-ion battery 500 of this embodiment. In each figure, circles indicate examples in which CMC and PVA are used as binders, and filled squares indicate examples in which CMC and SBR are used as binders.

As shown in Figures 9 and 10, in both the example using CMC and PVA and the example using CMC and SBR, the charge capacity and discharge capacity values remained stable and high up to at least 60 charge/discharge cycles. Furthermore, a slight decrease in the charge capacity and discharge capacity values was observed up to 100 charge/discharge cycles. However, it was also found that in both the example using CMC and PVA and the example using CMC and SBR, high charge capacity values (approximately 1400 to 1500 mAh/g) and discharge capacity values (approximately 1400 to 1500 mAh/g) were maintained even after 100 charge/discharge cycles.

Interestingly, it was confirmed that the example using CMC and PVA was able to maintain higher charge and discharge capacity values than the example using CMC and SBR. This is thought to be because when SBR is used, it is more difficult to mix uniformly with CMC or silicon microparticles than with PVA.

As a reference example, we will now describe the results of measuring the charge/discharge cycle characteristics of a lithium ion battery manufactured in the same manner as lithium ion battery 500 using the silicon microparticles of this embodiment before treatment in the coating formation step (S4). Figure 11 is a graph showing the charge cycle characteristics of the lithium ion battery of the reference example described above. Figure 12 is a graph showing the discharge cycle characteristics of the lithium ion battery of the reference example described above. Note that this reference example uses CMC and PVA as binders.

As shown in Figures 11 and 12, the charge capacity and discharge capacity values are relatively low, at less than 500 (mAh/g).

Therefore, it was confirmed that the use of the silicon microparticles and/or their aggregates (including aggregates) of the first embodiment described above makes it possible to realize a lithium-ion battery with high capacity and excellent charge/discharge cycle characteristics. It is particularly noteworthy that, compared to the results of the reference lithium-ion battery, the lithium-ion battery 500 of the second embodiment, which employs silicon microparticles and/or their aggregates (including aggregates) whose surfaces are at least partially covered with carbon through the coating formation process (S4), can increase its charge and discharge capacities by more than two times (typically about three to five times). Additionally, covering some or all of the silicon microparticles with carbon facilitates the formation of a thick anode material, thereby making the lithium-ion battery 500 easier to manufacture. Even after more than 100 charge/discharge cycles (e.g., several hundred to 1,000), the lithium-ion battery 500 can be said to have significantly better charge and discharge capacities and/or charge/discharge cycle characteristics than a lithium-ion battery employing the silicon microparticles of the present embodiment before the coating formation process (S4).

As shown in Figure 13, a transmission electron microscope (TEM) image of the silicon microparticles or their aggregates or conglomerates constituting the negative electrode material of the lithium-ion battery 500 of this embodiment after 100 charge-discharge cycles was observed. As shown in region "A" in the figure, roughly circular crystals (microcrystals) with a diameter of approximately 4 nm were confirmed. Therefore, another interesting fact is that if a negative electrode material is used that uses silicon microparticles or their aggregates or conglomerates obtained by crushing crystalline silicon, a partial crystalline state is maintained even after at least 100 charge-discharge cycles. The example of the lithium-ion battery 500 shown in Figure 13 uses CMC and PVA as binders, with approximately 5 wt% of fluoroethylene carbonate (FEC), described below, added. The crystalline silicon in region "A" in Figure 13 is believed to primarily have Si(111) orientation.

<Another embodiment (1)>
In the above-described embodiments, silicon chips formed during the cutting process of a single-crystal or polycrystalline silicon block or ingot are used as starting materials. However, other forms of silicon chips may also be used as starting materials. Specifically, silicon chips are not limited to those inevitably formed during the cutting process of a silicon ingot in the semiconductor product production process. They may also be produced by uniformly or randomly cutting a preselected crystalline silicon ingot with a cutting machine. Furthermore, so-called silicon waste, such as silicon chips and silicon grinding shavings, which are usually considered waste, can be used as starting materials for the silicon microparticles of the above-described embodiments. However, such silicon waste can also include fine chips obtained by crushing wafer fragments, discarded wafers, etc. Furthermore, silicon microparticles using metallic silicon chips, metallic silicon grinding shavings, or other metallic silicon particles as starting materials can also be used.

<Another embodiment (2)>
Furthermore, the impurity concentration of the n-type crystalline silicon in each of the above-described embodiments is not particularly limited. Furthermore, not only n-type but also p-type crystalline silicon can be employed. Furthermore, crystalline silicon, which is an intrinsic semiconductor, can also be employed as the crystalline silicon in each of the above-described embodiments. Since the movement of electrons within the negative electrode material of a lithium-ion battery is important, it is more preferable to use crystalline silicon containing n-type impurities. Furthermore, since approximately 1 wt % to approximately 3 wt % of graphite particles, as indicated by the peak intensities of the C(002) plane and the C(003) plane shown in FIG. 7(b) above, are contained within the silicon fine particle group or silicon fine particle aggregate, it should be noted that some or all of this graphite can contribute to improving the conductivity of the negative electrode material.

<Other embodiment (3)>
Furthermore, the silicon microparticles and lithium ion batteries including the same according to the above-described embodiments are not limited to applications in the coin cell structure described in the second embodiment. Therefore, they can be applied to various devices or apparatuses that include or utilize lithium ion batteries with a larger electrical capacity than those in the coin cell structure. Another possible embodiment is to use a mixture of the silicon mixed powder according to the above-described embodiments and graphite (typically, graphite) as the negative electrode material.

<Other embodiment (4)>
Furthermore, as an alternative to the lithium-ion battery anode material and anode manufacturing apparatus 100 shown in FIG. 2 in the first embodiment described above, a lithium-ion battery anode manufacturing apparatus 200 shown in FIG. 14 may be employed. Specifically, from the viewpoint of simplifying the equipment and/or reducing manufacturing costs, in the lithium-ion battery anode manufacturing apparatus 200, a washer 10 that cleans silicon chips and the like formed during the silicon cutting process also serves as a crusher 20 that crushes the cleaned silicon chips and the like to form silicon microparticles. Therefore, in the apparatus/method shown in FIG. 14 , for example, beads with a relatively large diameter are used in the cleaning process and beads with a relatively small diameter are used in the crushing process to obtain silicon microparticles for use as a lithium-ion battery anode material. However, to obtain the silicon microparticles described in the first embodiment with a higher degree of accuracy, it is preferable to form silicon microparticles using a bead mill after processing using a ball mill, as in the first embodiment.

<Other embodiment (5)>
Furthermore, the inventors' analysis revealed that the amount of fluoroethylene carbonate (FEC) added to the electrolyte solution 530 in the second embodiment can affect the charge capacity, discharge capacity, and charge/discharge cycle characteristics. FIG. 15A is a graph showing the charge cycle characteristics of a lithium-ion battery when the amount of FEC in the electrolyte solution 530 is changed. FIG. 15B is a graph showing the discharge cycle characteristics of a lithium-ion battery when the amount of FEC in the electrolyte solution 530 is changed. The amounts of FEC measured were 5 wt%, 10 wt%, and 15 wt%, respectively. For reference, a lithium-ion battery without added FEC was also measured. In FIGS. 15A and 15B, the weight ratio of silicon (Si) to carbon (C) was 1:0.1. 15A and 15B, the charge-discharge cycles were performed at a current value of 180 mA/g up to 5 charge-discharge cycles, and at a current value of 1800 mA/g for 6 charge-discharge cycles or more, in order to form a good (e.g., low-resistance and/or thin) solid electrolyte interface (SEI) on the surface of the negative electrode material.

As shown in Figures 15A and 15B, when the amount of FEC added (wt%) is in the range of more than 5% and not more than 15%, the charge capacity and discharge capacity values are stably maintained at high values (typically about 1500 mAh/g) even after 100 charge/discharge cycles. In particular, it was found that adding approximately 10% FEC stably maintains very high charge capacity and discharge capacity values. Therefore, it was found that adding FEC can achieve high charge capacity and discharge capacity values, especially after 100 charge/discharge cycles.

<Other embodiment (6)>
The present inventors also investigated the correlation between the weight ratio of carbon (C) to silicon (Si) in the negative electrode material employed in the second embodiment and the charge capacity, discharge capacity, and charge/discharge cycle characteristics. Figure 16A is a graph showing the charge cycle characteristics of a lithium-ion battery when the weight ratio of silicon (Si) to carbon (C) is changed. Figure 16B is a graph showing the discharge cycle characteristics of a lithium-ion battery when the weight ratio of silicon (Si) to carbon (C) is changed. The weight ratios of silicon (Si) to carbon (C) measured were the following four examples (1) to (4).
(1) Carbon (C):Silicon (Si) = 0.1:1
(2) Carbon (C):Silicon (Si) = 0.16:1
(3) Carbon (C):Silicon (Si) = 0.21:1
(4) Carbon (C):Silicon (Si) = 0.37:1

In addition, in Figures 16A and 16B, the charge-discharge cycles were performed at a current value of 180 mA/g up to five charge-discharge cycles, and at a current value of 1800 mA/g for six or more charge-discharge cycles. This is to form a good (e.g., low-resistance and/or thin) solid electrolyte interface (SEI) on the surface of the negative electrode material. Note that Figures 16A and 16B show an example in which approximately 10 wt% fluoroethylene carbonate (FEC) was added.

As shown in Figures 16A and 16B, the charge and discharge capacity values for the example with the highest carbon (C) ratio ((4) above) are somewhat low, but the other three examples all maintain stable high charge and discharge capacity values. Therefore, it was revealed that, when silicon (Si) is taken as 1, if the carbon (C) ratio (weight ratio) is between 0.1 and 0.21, particularly high charge and discharge capacity values (typically, approximately 1500 mAh/g) can be stably maintained even after 100 charge and discharge cycles. However, the charge and discharge capacity values for the example (4) above are more than two times (typically, approximately three times) higher than those of a lithium-ion battery in which at least a portion of the surface of the silicon microparticles or their aggregates or aggregates is not covered with carbon.

In this embodiment, the weight ratio of carbon (C) when silicon (Si) is taken as 1 is 0.1, but if the weight ratio of carbon (C) when silicon (Si) is taken as 1 is 0.05 or more, effects roughly equivalent to the results of (1) to (3) above can be achieved. Therefore, if the ratio of carbon (C) when silicon (Si) is taken as 1 is 0.05 or more but less than 0.37, a stable lithium-ion battery with high charge capacity and discharge capacity values can be obtained.

Furthermore, as a result of further detailed analysis by the inventors of this embodiment, TEM (transmission electron microscope) images showed that in the above-mentioned case (1), the film thickness of the portion where carbon (C) adheres to silicon (Si) was approximately 2 nm to approximately 3 nm. Furthermore, in the above-mentioned case (2), the film thickness of the portion where carbon (C) adheres to silicon (Si) was approximately 5 nm to approximately 10 nm. Furthermore, in the above-mentioned case (3), the film thickness of the portion where carbon (C) adheres to silicon (Si) was also approximately 5 nm to approximately 10 nm. Additionally, in the above-mentioned case (4), the film thickness of the portion where carbon (C) adheres to silicon (Si) was approximately 50 nm to approximately 90 nm. Therefore, if the film thickness of the carbon (C) adhered to silicon (Si) is 1 nm or more but less than 50 nm, and particularly if this film thickness is 2 nm or more but less than 10 nm, a stable lithium-ion battery with high charge capacity and discharge capacity values can be obtained.

The disclosure of each of the above-mentioned embodiments has been provided for the purpose of explaining those embodiments, and is not intended to limit the present invention. In addition, modifications within the scope of the present invention, including other combinations of the embodiments, are also intended to be included within the scope of the claims.

The silicon microparticles of the present invention and lithium-ion batteries containing them can be used in a wide variety of devices or apparatuses, including, for example, various power generation or storage devices (including small home power storage devices and large-scale storage systems), smartphones, personal digital assistants, portable electronic devices (mobile phones, portable music players, laptop computers, digital cameras and videos), electric vehicles, hybrid electric vehicles (HEVs) or plug-in hybrid electric vehicles (PHEVs), motor-powered motorcycles, motor-powered tricycles, and other transportation machinery or vehicles.

Claims (8)

a crushing section for crushing silicon waste using a ball mill and a bead mill used after the ball mill to form aggregates or aggregates of silicon fine particles having a volume distribution in which the mode diameter and median diameter of the silicon fine particles are 50 nm or less, the aggregates or aggregates being in a state of being folded into a multi-layered petal-like or scale-like shape;
a coating forming unit that forms carbon to cover at least a portion of the surface of the aggregate or the collection ;
The silicon waste material is at least one selected from the group consisting of silicon cutting chips, silicon polishing chips, wafer fragments, fine chips obtained by crushing waste wafers, and metallic silicon particles.
Equipment for manufacturing negative electrode materials for lithium-ion batteries. The coating formation unit is a chemical vapor deposition apparatus.
The apparatus for producing a negative electrode material for a lithium ion battery according to claim 1. a crushing section for crushing silicon waste using a ball mill and a bead mill used after the ball mill to form aggregates or aggregates of silicon fine particles that become a negative electrode material, the aggregates or aggregates being in a state of being folded into a multi-layered petal-like or scale-like shape, the aggregates or aggregates having a volume distribution in which the mode diameter and median diameter of the silicon fine particles are 50 nm or less;
a coating forming unit that forms carbon to cover at least a portion of the surface of the aggregate or the collection ;
The silicon waste material is at least one selected from the group consisting of silicon cutting chips, silicon polishing chips, wafer fragments, fine chips obtained by crushing waste wafers, and metallic silicon particles.
Lithium-ion battery negative electrode manufacturing equipment. The coating formation unit is a chemical vapor deposition apparatus.
The apparatus for manufacturing a negative electrode for a lithium ion battery according to claim 3. a crushing step of crushing the silicon waste material with a ball mill and then with a bead mill to form aggregates or aggregates of silicon fine particles having a volume distribution in which the mode diameter and median diameter of the silicon fine particles are 50 nm or less, the aggregates or aggregates being in a state of being folded into a multi-layered petal-like or scale-like shape;
a coating formation step of forming carbon that covers at least a portion of the surface of the aggregates or the conglomerates ,
The silicon waste material is at least one selected from the group consisting of silicon cutting chips, silicon polishing chips, wafer fragments, fine chips obtained by crushing waste wafers, and metallic silicon particles.
A method for manufacturing negative electrode materials for lithium-ion batteries. The coating formation step is a step of forming the carbon by chemical vapor deposition.
The method for producing the negative electrode material of a lithium ion battery according to claim 5. a crushing step of crushing the silicon waste material using a ball mill and then using a bead mill to form aggregates or aggregates of silicon fine particles that will become a negative electrode material, the aggregates or aggregates being in a multi-layered petal-like or scale-like state and having a volume distribution in which the mode diameter and median diameter of the silicon fine particles are 50 nm or less;
a coating formation step of forming carbon that covers at least a portion of the surface of the aggregates or the conglomerates ,
The silicon waste material is at least one selected from the group consisting of silicon cutting chips, silicon polishing chips, wafer fragments, fine chips obtained by crushing waste wafers, and metallic silicon particles.
A method for manufacturing the negative electrode of a lithium-ion battery. The coating formation step is a step of forming the carbon by chemical vapor deposition.
The method for producing the negative electrode of the lithium ion battery according to claim 7.