Negative electrode material powder for lithium ion battery

Abstract

The present invention relates to a negative electrode material powder for a lithium ion battery, which contains a Si-containing granulated body that contains a Si-based powder and a carbon material, wherein: the average primary particle diameter of the Si-based powder is 0.1 μm or more; the average secondary particle diameter of the Si-containing granulated body is 2 μm or more; the surface of the Si-based powder is covered with the carbon material and the Si-based powder is bonded with the carbon material; and the value represented by the formula (58.1 × average primary particle diameter (μm) + 5.6 × carbon amount (mass%)) of the Si-based powder is more than 15.

Description

Anode material powder for lithium-ion batteries

[0001] The present invention relates to a negative electrode material powder for lithium ion batteries.

[0002] Lithium-ion batteries have the advantages of high capacity, high voltage, and the ability to be miniaturized, and are widely used as power sources for mobile phones, laptops, etc. In recent years, they have also attracted great expectations as a power source for electric vehicles, hybrid vehicles, etc., and their development is actively progressing.

[0003] In this lithium-ion battery, lithium ions (hereinafter sometimes referred to as Li ions) move between the positive electrode and the negative electrode to perform charging and discharging, and on the negative electrode side, Li ions are absorbed into the negative electrode active material during charging and Li ions are released from the negative electrode active material during discharging.

[0004] Among the above, graphite has been widely used as a negative electrode active material, but its theoretical capacity is only 372 mAh / g, and further increases in capacity have been desired. Therefore, silicon, which exhibits a capacity approximately 10 times that of graphite, has been proposed as a negative electrode active material for lithium-ion batteries. However, silicon has issues with its cycle characteristics during repeated charge and discharge.

[0005] In response to this, Patent Document 1 discloses a battery anode material in which a carbon material is formed around a silicon material with a void therebetween by using a thermal chemical vapor deposition (CVD) method. Patent Document 2 discloses a lithium secondary battery anode material in which carbon is present on part or all of the surface of silicon oxide particles. Patent Document 3 also discloses a carbon material in which silicon oxide is surrounded by carbon material particles.

[0006] Japanese Patent No. 6978947 Japanese Patent No. 6615431 Japanese Patent No. 5516529

[0007] In any of the above Patent Documents 1 to 3, in order to obtain the desired negative electrode material or carbon material, the surface of silicon oxide represented by SiOx, which is the active material, is coated with carbon in the manufacturing process. However, according to the investigations of the present inventors, in any of the above Patent Documents 1 to 3, the Si-based powder is easily ignited at a relatively low temperature, and furthermore, reacts with moisture to produce hydrogen (H ２ ) was found to be more likely to occur.

[0008] Therefore, an object of the present invention is to provide a negative electrode material powder for lithium ion batteries that is less likely to catch fire, has excellent safety, and also has good water resistance.

[0009] As a result of intensive research into the above-mentioned problems, the present inventors have found that by coating the Si-based powder itself, which is made up of primary particles containing a Si phase and a SiX compound phase, with a carbon material and adjusting the average primary particle diameter and the degree of coating to satisfy specific parameters, it is possible to achieve good safety by making the powder less likely to ignite, and also to achieve good water resistance with a small amount of hydrogen generated by reaction with moisture, thereby completing the present invention. Further research has revealed that the above-mentioned parameters are also related to cycle characteristics.

[0010] That is, the gist of the present invention is as follows: [1] A negative electrode material powder for a lithium ion battery comprising Si-containing granules, wherein the Si-containing granules comprise a Si-based powder and a carbon material, the Si-based powder is primary particles comprising a Si phase and a SiX compound phase, the SiX compound phase contains Si and an element X, the element X is at least one selected from the group consisting of Fe, Co, Cr, Mn, Ni, Zr, Ti, B, and P, the Si-based powder has an average primary particle size of 0.1 μm or more, the Si-containing granules have an average secondary particle size of 2 μm or more, the Si-based powder has a surface coated with the carbon material and is bonded by the carbon material, and a carbon coating parameter A of the Si-based powder, represented by the following formula, is greater than 15. Carbon coating parameter A = 58.1 × average primary particle size (μm) + 5.6 × carbon content (mass%) [2] The negative electrode material powder for lithium ion batteries according to [1] above, wherein the SiC content in the Si-containing granules is less than 15%. [3] The negative electrode material powder for lithium ion batteries according to [1] above or [2] above, wherein the carbon coating parameter A is greater than 48. [4] The negative electrode material powder for lithium ion batteries according to any one of [1] to [3] above, wherein the Si-based powder has an average primary particle size of 5.0 μm or less. [5] The negative electrode material powder for lithium ion batteries according to any one of [1] to [4] above, wherein the Si-containing granules have an average secondary particle size of 20 μm or less. [6] The negative electrode material powder for lithium ion batteries according to any one of [1] to [5] above, wherein the Si-containing granules have an aspect ratio of 1.0 to 1.8. [7] The negative electrode material powder for a lithium ion battery according to any one of [1] to [6], wherein the Si-based powder is primary particles further comprising at least one selected from the group consisting of a SnCu compound phase and an AlCu compound phase, the SnCu compound phase containing Sn and Cu, the AlCu compound phase containing Al and Cu, and the content of the Si phase in the Si-based powder is 30 mass% or more. [8] The negative electrode material powder for a lithium ion battery according to [7], wherein the total content of the SnCu compound phase and the AlCu compound phase in the Si-based powder is 0.1 to 15 mass%.

[0011] According to the present invention, a negative electrode material powder for lithium ion batteries can be provided that is resistant to ignition, has excellent safety, and also has good water resistance. Therefore, when the negative electrode material powder is used as a negative electrode active material in a lithium ion battery, the safety of the lithium ion battery itself can be improved. Furthermore, deterioration of the lithium ion battery due to moisture can be prevented. Furthermore, by optimizing the conditions, the present invention can also improve various battery characteristics such as cycle characteristics.

[0012] Hereinafter, embodiments of the present invention will be described. However, the present invention is not limited to the embodiments described below. Furthermore, in this specification, the term "to" indicating a numerical range is used to mean that the numerical values ​​before and after it are included as the lower and upper limits. Furthermore, in this specification, mass % and weight % have the same meaning.

[0013] <<Negative electrode material powder for lithium ion batteries>> The negative electrode material powder for lithium ion batteries according to this embodiment includes Si-containing granules. The Si-containing granules are granules having an average secondary particle diameter of 2 μm or more, and include a Si-based powder and a carbon material. The Si-based powder is primary particles having an average primary particle diameter of 0.1 μm or more, and includes a Si phase and an SiX compound phase. The surfaces of the Si-based powder are coated with the carbon material, and the degree of coating can be expressed by a specific carbon coating parameter A. Furthermore, the carbon material bonds the primary particles of the Si-based powder together, thereby forming the Si-containing granules, which are secondary particles.

[0014] <Si-based powder> The Si-based powder contained in the Si-containing granules in this embodiment is a primary particle, and the surface thereof is coated with a carbon material. The coating with the carbon material can be expressed by a carbon coating parameter A expressed by the following formula: Carbon coating parameter A = 58.1 × average primary particle diameter (μm) + 5.6 × carbon content (mass%)

[0015] The carbon coating parameter A has the average primary particle size and carbon content of the Si-based powder as variables, and the coefficient of the average primary particle size, 58.1, and the coefficient of the carbon content, 5.6, are values ​​derived from experimental results.

[0016] It has been found that when the carbon coating parameter A is greater than 15, the Si-containing granules are less likely to ignite and have good water resistance. The reason for this is unclear, but it is thought that when the carbon coating parameter A is greater than 15, the probability of the Si-based powder coming into direct contact with flames is reduced, making the granules less likely to ignite. Similarly, with regard to water resistance, the probability of the Si-based powder coming into direct contact with water is reduced, making it less likely to ignite. ２ Furthermore, phases other than the Si phase and the SiX compound phase are also prevented from being oxidized by contact with water, and as a result, unexpected reactions at the negative electrode can be prevented.

[0017] That is, from the above viewpoint, the value of the carbon coating parameter A is more than 15, preferably 25 or more, and more preferably 35 or more.

[0018] In addition to the above, it has been found that the carbon coating parameter A in this embodiment is also involved in the cycle characteristics when the negative electrode material powder according to this embodiment is applied to a lithium ion battery. Specifically, good cycle characteristics can be achieved by setting the value of the carbon coating parameter A to greater than 48. It is believed that when the surface of the Si-based powder is coated with a carbon material to the extent that the value of the carbon coating parameter A exceeds 48, direct contact between the Si phase or SiX compound phase and the electrolyte can be suitably prevented, and the generation and collapse of SEI during charge and discharge can be suppressed. From the viewpoint of cycle characteristics, the value of the carbon coating parameter A is preferably greater than 48, more preferably 55 or greater, and even more preferably 65 or greater.

[0019] On the other hand, the upper limit of the value of the carbon coating parameter A is not particularly limited, but from the viewpoint of manufacturability, adding a large amount of carbon material is not preferable, and the value is, for example, 200 or less, or may be 150 or less, and preferably 120 or less. That is, the value of the carbon coating parameter A is, for example, preferably more than 15 and 200 or less, and more preferably more than 48 and 120 or less.

[0020] The carbon amount used in the carbon coating parameter A in this embodiment is the carbon content (mass%) relative to the total of the Si-based powder, which is the primary particles, the carbon amount, and other optional components. Specifically, the carbon amount of the Si-based powder in this specification is the carbon concentration (mass%) measured by analyzing the Si-containing granules using a combustion infrared absorption method.

[0021] The fact that the surfaces of the primary particles of the Si-based powder are coated with a carbon material can be confirmed by observation with a transmission electron microscope (TEM). Furthermore, if the Si-containing granules ignite within 10 seconds in a small gas flame ignition test, a method used in the safety evaluation in the Examples described later, it may be determined that the Si-containing granules are not coated.

[0022] The Si-containing granules in this embodiment are formed by granulating Si-based powder, which are primary particles whose surfaces are coated with a carbon material, by bonding them together with the carbon material to form secondary particles, thereby achieving various effects such as safety, water resistance, and cycle characteristics. The same effects as those described above cannot be obtained with granules of primary particles in which the primary particles are bound together with the carbon material but the carbon material is only attached to a very small part of the surface and therefore the surface cannot be said to be coated, or with secondary particles in which the primary particles are granulated by binding together with a binder or the like and only the surface is coated with the carbon material.

[0023] The carbon content in the Si-based powder is preferably 1.0 to 30.0 mass%, more preferably 4.0 to 20.0 mass%, and even more preferably 6.0 to 15.0 mass%. From the viewpoint of suppressing exposure of the Si phase and SiX compound phase constituting the Si-based powder and covering a wide area, the carbon content is preferably 1.0 mass% or more, more preferably 4.0 mass% or more, and even more preferably 6.0 mass% or more. Furthermore, from the viewpoint of suppressing a decrease in Coulomb efficiency, the carbon content is preferably 30.0 mass% or less, more preferably 20.0 mass% or less, and even more preferably 15.0 mass% or less.

[0024] The average primary particle diameter of the Si-based powder in this embodiment is 0.1 μm or more, preferably 0.1 to 1.0 μm, more preferably 0.2 to 0.8 μm, and even more preferably 0.3 to 0.7 μm. From the viewpoints of improving fire resistance and safety, water resistance, and reducing the amount of binder required as a negative electrode material, the average primary particle diameter is 0.1 μm or more, preferably 0.2 μm or more, and more preferably 0.3 μm or more. Furthermore, from the viewpoints of reducing the absolute amount of expansion upon lithium occlusion and achieving good cycle characteristics, the average primary particle diameter is preferably 1.0 μm or less, more preferably 0.8 μm or less, and even more preferably 0.7 μm or less. The average primary particle diameter of the Si-based powder in this specification is the median diameter (D50) of the Si-containing granules, determined from the volume-based particle size distribution measured using a laser diffraction / scattering particle size distribution analyzer on a slurry before granulation. Alternatively, the average primary particle size of the Si-based powder can be obtained by image analysis of an SEM photograph of the Si-containing granules. The average primary particle size of the Si-based powder can also be adjusted by the pulverization method, conditions, etc.

[0025] The Si phase constituting the Si-based powder in this embodiment absorbs and releases Li and plays a role as a negative electrode active material. The Si phase is usually contained in the Si-based powder as Si particles. The Si particles are preferably particles in which 95 mass % or more of the Si phase is composed. When the Si phase is not Si particles, it is preferably SiO ２ It may be included as a Si phase.

[0026] In this embodiment, the content of the Si phase in the Si-based powder is preferably 10% by mass or more but less than 100% by mass, more preferably 30 to 90% by mass. From the viewpoint of obtaining good initial discharge capacity and initial coulombic efficiency, the content is preferably 10% by mass or more, more preferably 20% by mass or more, even more preferably 30% by mass or more, even more preferably 50% by mass or more, and most preferably 55% by mass or more. Furthermore, since the Si-based powder contains a SiX compound phase in addition to the Si phase, the content of the Si phase is less than 100% by mass. From the viewpoint of optimally obtaining the effects of phases other than the Si phase, the content is preferably 90% by mass or less, more preferably 88% by mass or less, and even more preferably 86% by mass or less.

[0027] In this embodiment, the primary particles of the Si-based powder contain a SiX compound phase in addition to the Si phase. The Si-based powder may further contain at least one selected from the group consisting of a SnCu compound phase and an AlCu compound phase. Here, the SiX compound phase contains Si and an element X, and the element X is at least one selected from the group consisting of Fe, Co, Cr, Mn, Ni, Zr, Ti, B, and P. On the other hand, the SnCu compound phase contains Sn and Cu, and the AlCu compound phase contains Al and Cu.

[0028] The SiX compound phase in this embodiment has poor Li absorption capacity and expands very little due to reaction with Li ions. Therefore, the SiX compound phase serves as a skeleton that maintains the structure of the electrode material. In addition, the SiX compound phase has high conductivity, so it is also effective in ensuring conductivity between the Si phase and conductive materials.

[0029] The SiX compound phase is usually contained in the Si-based powder as SiX compound particles. The SiX compound particles are preferably particles consisting of 95 mass % or more of the SiX compound phase, and may be particles consisting of only the SiX compound phase. When the SiX compound phase is not SiX compound particles, it can be contained in the Si-based powder as an OX compound.

[0030] In the SiX compound phase of this embodiment, X is at least one element selected from the group consisting of Fe, Co, Cr, Mn, Ni, Zr, Ti, B, and P. In particular, from the viewpoints of low expansion and high conductivity, X is preferably at least one element selected from the group consisting of Fe, Ni, Zr, Ti, B, and P, and more preferably at least one element selected from the group consisting of Fe, Ni, B, and P.

[0031] The SiX compound in this embodiment is, for example, Si ２ X, Six, Six ２ Examples of the compound include compounds represented by the formula: Si ２ Examples of the compound represented by X include Si ２ Fe, Si ２ Ni, Si ２ Co, Si ２ Ti, Si ２Zr, Si ２ Examples of the SiX compound phase include Cr, etc. The SiX compound phase may be composed of only one type of compound phase, or may be composed of two or more types of phases, such as a SiFe compound phase and a SiB compound phase or a SiP compound phase, or a SiB compound phase and a SiP compound phase.

[0032] In this embodiment, the content of the SiX compound phase in the Si-based powder may be greater than 0% by mass, and is preferably 5 to 70% by mass. From the viewpoint of obtaining good cycle characteristics, the content is preferably 5% by mass or more, more preferably 8% by mass or more, and even more preferably 10% by mass or more. The content is preferably 70% by mass or less, more preferably 50% by mass or less, and even more preferably 30% by mass or less.

[0033] The SnCu compound phase and the AlCu compound phase in this embodiment may be contained in the Si-based powder together with the Si phase and the SiX compound phase.

[0034] The SnCu compound phase is usually contained in the Si-based powder as SnCu compound particles. The SnCu compound particles are preferably particles whose 95 mass % or more is composed of the SnCu compound phase, and may be particles composed only of the SnCu compound phase. When the SnCu compound phase is not composed of SnCu compound particles, it can be contained in the Si-based powder as Sn or Cu.

[0035] The AlCu compound phase is usually contained in the Si-based powder as AlCu compound particles. The AlCu compound particles are preferably particles whose 95 mass % or more is made of the AlCu compound phase, and may be particles made of only the AlCu compound phase. When the AlCu compound phase is not made of AlCu compound particles, it can be contained in the Si-based powder as Al and Cu.

[0036] The theoretical capacity of the SnCu compound phase and the AlCu compound phase is lower than that of Si and higher than that of SiX compounds. For example, the theoretical capacity of a SiZr compound (SiX compound) is 100 mAh / g, while the theoretical capacity of a SnCu compound is about 500 mAh / g and the theoretical capacity of an AlCu compound is about 700 mAh / g. Therefore, by including a SnCu compound phase or an AlCu compound phase in a Si-based powder, a diffusion path for Li ions is easily secured. Furthermore, the degree of expansion due to reaction with Li ions is smaller for SnCu compounds and AlCu compounds than for simple Si and Sn, which are highly reactive with Li ions. Therefore, by including a SnCu compound phase or an AlCu compound phase, adverse effects on cycle characteristics can be minimized. Furthermore, SnCu compounds and AlCu compounds also have the effect of increasing conductivity, similar to the SiX compounds.

[0037] The SnCu compound phase in this embodiment may be an alloy of Sn and Cu, or may be an alloy containing Sn, Cu, and other metals other than Sn and Cu. The AlCu compound phase in this embodiment may be an alloy of Al and Cu, or may be an alloy containing Al, Cu, and other metals other than Al and Cu.

[0038] In this embodiment, the total content of the SnCu compound phase and the AlCu compound phase in the Si-based powder is preferably 0.1 to 15% by mass, more preferably 0.3 to 10% by mass, and even more preferably 0.5 to 8% by mass. From the viewpoint of obtaining a high discharge capacity, the content is preferably 0.1% by mass or more, more preferably 0.3% by mass or more, and even more preferably 0.5% by mass or more. Furthermore, from the viewpoint of the ratio of the actual discharge capacity to the theoretical capacity of the negative electrode active material, i.e., the utilization rate of the active material, the content is preferably 15% by mass or less, more preferably 10% by mass or less, and even more preferably 8% by mass or less.

[0039] When the Si-based powder in this embodiment contains at least one of a SnCu compound phase and an AlCu compound phase, the ratio of the content in the Si-based powder, expressed as {SiX compound phase / (SnCu compound phase+AlCu compound phase)}, is preferably 1 to 300. From the viewpoint of suppressing deterioration of cycle characteristics, the mass ratio is preferably 1 or more, more preferably 3 or more, even more preferably 5 or more, and particularly preferably 7 or more. On the other hand, from the viewpoint of obtaining a high initial discharge capacity, the mass ratio is preferably 300 or less, more preferably 200 or less, and even more preferably 100 or less.

[0040] The Si-based powder in this embodiment may further contain other compound phases in addition to the Si phase, SiX compound phase, optionally SnCu compound phase, and optionally AlCu compound phase, as long as the effects of the present invention are not impaired. Examples of other compound phases include a SiCu compound phase.

[0041] On the other hand, in the Si-based powder of this embodiment, the smaller the total content of elements other than Si, Sn, Al, Cu, and X that constitute the Si phase, SiX compound phase, SnCu compound phase, and AlCu phase, the better, and it is more preferable that no other elements are contained. The other elements may be contained as, for example, inevitable impurities, and specific examples include nitrogen (N), sulfur (S), phosphorus (P), and oxygen (O). In this case, the content of each element is preferably N≦0.10 mass%, S≦0.10 mass%, P≦0.10 mass%, and O≦15 mass%, and the total of these is preferably 5 mass% or less.

[0042] The content of the Si-based powder in the Si-containing granules is preferably 50 to 90% by mass. From the viewpoint of obtaining a good discharge capacity, the content is preferably 50% by mass or more, more preferably 55% by mass or more, and even more preferably 60% by mass or more. From the viewpoint of obtaining good cycle characteristics, the content is preferably 90% by mass or less, more preferably 88% by mass or less, and even more preferably 86% by mass or less.

[0043] <Carbon Material> The carbon material in this embodiment is a material that not only bonds primary particles of the Si-based powder together but also coats the surface of the Si-based powder.

[0044] The carbon material may be, for example, a polymer material containing carbon atoms that has been partially or entirely calcined and carbonized.

[0045] The carbon content (mass %) relative to the total of the Si-based powder, carbon material, and any other components is the carbon amount in the carbon coating parameter A. That is, the carbon material content contributes to the carbon amount, but when the Si-containing granules in this embodiment further contain a polymer material, the amount of carbon constituting the polymer material also contributes to the carbon amount. When the Si-containing granules contain two or more carbon materials, or when they contain a polymer material in addition to a carbon material, the sum of the carbon contents of those materials is the carbon content, i.e., the carbon amount. As described above, the carbon amount is preferably 1.0 to 30.0 mass %, more preferably 4.0 to 20.0 mass %, and even more preferably 6.0 to 15.0 mass %.

[0046] The average thickness of the carbon material coating that coats the surface of the Si-based powder is preferably 1.0 to 25 nm. From the viewpoint of ignition performance, the average thickness is preferably 1.0 nm or more, more preferably 1.5 nm or more, even more preferably 2.0 nm or more, and particularly preferably 2.5 nm or more. From the viewpoint of capacity, the average thickness is preferably 25 nm or less, more preferably 20 nm or less, even more preferably 18 nm or less, and particularly preferably 15 nm or less. The average thickness can be determined by observing the surface of the Si-containing granules at a magnification of 200,000 times using a transmission electron microscope (TEM) and averaging the thicknesses of the carbon material coating at five locations.

[0047] <Polymer material containing carbon atoms> The Si-containing granules in this embodiment may contain a polymer material containing carbon atoms. The polymer material containing carbon atoms is also preferably used as a precursor of the carbon material. That is, primary particles of Si-based powder are coated with a polymer material containing carbon atoms, and the resulting particles are granulated to form secondary particles, which are then fired. This carbonization allows a portion or all of the polymer material to be carbonized, resulting in Si-containing granules in which the surfaces of the Si-based powder are coated with the carbon material and the Si-based powder particles are bonded together by the carbon material. Note that when the entire polymer material is carbonized, the Si-containing granules do not contain any polymer material, and the carbon amount in the carbon coating parameter A is entirely due to the carbon material.

[0048] Examples of polymeric materials containing carbon atoms include phenolic resins, polyvinylidene fluoride resins, polyethylene resins, polypropylene resins, polyvinyl alcohol resins, polystyrene resins, furan resins, cellulose resins, epoxy resins, polyvinyl chloride, polymethyl methacrylate resins, pitch, coke, biowaste, polyfurfuryl alcohol resins, polyimide resins, and any combination thereof.

[0049] The polymer material preferably functions as a binder. From the viewpoint of binding properties, the polymer material is preferably, for example, a phenolic resin, a polyvinylidene fluoride resin, a polyvinyl alcohol resin, a polyimide resin, or the like, with a phenolic resin and a polyvinyl alcohol resin being more preferred. Furthermore, as the phenolic resin, either a novolac resin or a resol resin is preferred, but a resol resin is preferred from the viewpoint of dispersibility in the slurry. The polymer material may be used alone or in combination of two or more types.

[0050] From the viewpoint of electrical conductivity when at least a portion of the polymer material is carbonized by heat treatment to form a carbon material, the polymer material is preferably a phenolic resin, a polyvinylidene fluoride resin, a polyvinyl alcohol resin, or a polyimide resin, and more preferably a phenolic resin or a polyvinyl alcohol resin. As the phenolic resin, both a novolac resin and a resol resin are preferable, but from the viewpoint of dispersibility in a slurry, a resol resin is preferable.

[0051] <SiC> The Si-containing granules in this embodiment may contain SiC produced during the manufacturing process. However, since SiC is a component that leads to a decrease in capacity as a negative electrode material, its content is preferably low. The SiC content in the Si-containing granules is preferably less than 15%, more preferably 10% or less, and even more preferably 8% or less. There is no particular lower limit for the SiC content, and a lower content is preferable, but it is usually 0.1% or more.

[0052] The content of SiC in the Si-containing granules is expressed as a percentage of the ratio of the area of ​​the peaks attributable to SiC to the area of ​​all peaks in the XRD pattern obtained by powder X-ray diffraction (XRD) measurement of the Si-containing granules. The content of SiC can be adjusted by the time, temperature, etc. of the heat treatment performed when coating the surface of the Si-based powder with the carbon material.

[0053] <Si-Containing Granules> The Si-containing granules in this embodiment are secondary particles formed by granulating Si-based powder, which are primary particles whose surfaces are coated with a carbon material, by binding the primary particles with the carbon material. The average secondary particle diameter is 2 μm or more, preferably 2 to 20 μm, more preferably 3 to 18 μm, and even more preferably 4 to 15 μm. From the viewpoints of improving fire resistance and safety, water resistance, and reducing the amount of binder required as a negative electrode material, the average secondary particle diameter is 2 μm or more, preferably 3 μm or more, and even more preferably 4 μm or more. Furthermore, from the viewpoint of achieving good cycle characteristics, the average secondary particle diameter is preferably 20 μm or less, more preferably 18 μm or less, and even more preferably 15 μm or less.

[0054] In this specification, the average secondary particle size of the Si-containing granules refers to the average particle size determined from the results of particle shape distribution measurement using an image particle size distribution analyzer. The image particle size distribution analyzer may be a Morphologi series device manufactured by Malvern Panalytical. Furthermore, when granulation is performed by spray drying a slurry mixture of primary particles, the average secondary particle size of the Si-containing granules can be adjusted by the solids concentration of the slurry, the drying pressure, etc.

[0055] The carbon coverage of the surface of the Si-containing granules, which are secondary particles, is preferably 14 to 99% by mass, more preferably 30 to 95% by mass, and even more preferably 40 to 90% by mass. From the viewpoint of ignition performance, the carbon coverage is preferably 14% by mass or more, more preferably 30% by mass or more, and even more preferably 40% by mass or more. From the viewpoint of capacity reduction, the carbon coverage is preferably 99% by mass or less, more preferably 95% by mass or less, and even more preferably 90% by mass or less. In this specification, the carbon coverage of the surface of the Si-containing granules refers to the proportion of carbon atoms (C) in the spectrum obtained by analyzing the surface of the Si-containing granules using X-ray photoelectron spectroscopy (XPS).

[0056] In this embodiment, the aspect ratio of the Si-containing granules is preferably 1.0 to 1.8, more preferably 1.0 to 1.5, and even more preferably 1.0 to 1.3. From the viewpoint of obtaining good cycle characteristics, the aspect ratio is preferably 1.8 or less, more preferably 1.5 or less, and even more preferably 1.3 or less. The aspect ratio of the Si-containing granules in this specification is a value determined by measuring the Si-containing granules using a scanning electron microscope (SEM). Specifically, the Si-containing granules are observed at a magnification of 400x using an SEM, and the maximum diameter D and the diameter D' perpendicular to the maximum diameter D are measured for each of 10 Si-based powder particles from the obtained image. The average value of the values ​​expressed as D / D' is taken as the aspect ratio. The aspect ratio of the Si-containing granules can be adjusted by changing the spray pressure and feed rate during spray drying.

[0057] The Si-containing granules may contain other components in addition to the Si-based powder, the carbon material coating the surface of the Si-based powder, the polymer material optionally containing carbon atoms, and unintentionally contained SiC. Examples of the other components include SiO ２ etc.

[0058] <<Method for Producing Negative Electrode Material Powder for Lithium-Ion Batteries>> The method for producing the negative electrode material powder for lithium-ion batteries according to this embodiment is not particularly limited, and may include, for example, the following steps in order: Step 1: preparing primary particles containing an Si phase and an SiX compound phase; Step 2: wet-mixing the primary particles containing the Si phase and the SiX compound phase with a polymer material containing carbon atoms to obtain a slurry mixture; Step 3: spray-drying the slurry mixture; and Step 4: heat-treating the spray-dried mixture.

[0059] Hereinafter, each step will be explained in order.

[0060] <Step 1: Preparation of Primary Particles> Step 1 is a step of preparing primary particles containing a Si phase and a SiX compound phase. Specifically, each raw material is weighed to have a desired composition, and melted using a melting means such as an arc furnace, a high-frequency induction furnace, or a heating furnace to obtain a molten alloy. The molten alloy is then quenched, for example, by atomization, to obtain alloy particles containing a Si phase and a SiX compound phase as a quenched metal. When the Si-based powder contains a SnCu compound phase or an AlCu compound phase in addition to the Si phase and the SiX compound phase, raw materials containing Sn, Cu, and Al in addition to Si and X can also be used as the raw materials to obtain alloy particles in which the SnCu compound phase or the AlCu compound phase is dispersed in the Si phase and the SiX compound phase.

[0061] If the average primary particle diameter of the alloy particles obtained above is large, they may be pulverized as necessary to obtain a Si-based powder with a small average primary particle diameter. On the other hand, to prevent the average primary particle diameter of the alloy particles from becoming less than 0.1 μm, for example, a predetermined particle diameter is obtained by measuring the particle diameter at appropriate times while sampling during pulverization.

[0062] Alternatively, metal particles consisting of a Si phase, alloy particles consisting of a SiX compound phase, optionally alloy particles consisting of a SnCu compound phase, and optionally alloy particles consisting of an AlCu compound phase may be obtained independently, and then these may be mixed to obtain a Si-based powder that is a primary particle containing the desired Si phase and SiX compound phase.

[0063] In the atomization method, the molten alloy is poured into the atomization chamber and flows downward continuously (in a rod shape). ２ Gas such as Ar, He, etc. is sprayed at high pressure, for example, 1 to 10 MPa, to crush and cool the molten metal. The cooled molten metal approaches a spherical shape while free-falling in the spray chamber while remaining semi-molten, and powder-like particles are obtained. Furthermore, high-pressure water may be sprayed instead of gas to improve the cooling effect. In some cases, it is also possible to obtain foil-shaped particles by using a roll quenching method instead of the atomization method.

[0064] The particles obtained above are pulverized as necessary. For pulverization, a suitable pulverizing means such as a ball mill, bead mill, disk mill, coffee mill, or mortar pulverization can be used. The pulverization conditions are adjusted so as to obtain the desired average primary particle size.

[0065] When the primary particles containing the Si phase and the SiX compound phase also contain a SnCu compound phase and an AlCu compound phase in addition to the Si phase and the SiX compound phase, and when particles containing each phase are obtained independently and then mixed, the particles may be pulverized individually and then mixed, or they may be mixed and then pulverized together.

[0066] The pulverization may be wet or dry, but when wet pulverization is employed, desolvation, solvent substitution, drying, etc. may be performed. Furthermore, by changing the pulverization means and conditions, the average primary particle size of the primary particles containing the Si phase and the SiX compound phase can be controlled. Furthermore, the predetermined particle size can be obtained by measuring the particle size at appropriate intervals while sampling during pulverization so that the average primary particle size does not become less than 0.1 μm.

[0067] <Step 2: Step of Obtaining a Slurry Mixture> Step 2 is a step of wet-mixing the primary particles containing the Si phase and the SiX compound phase obtained in step 1 with a polymer material containing carbon atoms to obtain a slurry mixture.

[0068] Primary particles containing a Si phase and a SiX compound phase are wet mixed with a polymer material containing carbon atoms, and then, through step 3, the surfaces of the primary particles are coated with the polymer material, and the polymer material acts as a binder to bond the primary particles together to form secondary particles. Then, through carbonization of the polymer material by firing in step 4, Si-containing granules are obtained in which the surfaces of the Si-based powder are coated with a carbon material and the Si-based powder is bonded by the carbon material.

[0069] As described above, examples of the polymer material containing carbon atoms include phenolic resin, polyvinylidene fluoride resin, polyethylene resin, polypropylene resin, polyvinyl alcohol resin, polystyrene resin, furan resin, cellulose resin, epoxy resin, polyvinyl chloride, polymethyl methacrylate resin, pitch, coke, biowaste, polyfurfuryl alcohol resin, polyimide resin, and any combination thereof.

[0070] There are no particular limitations on the solvent used when wet-mixing the primary particles containing a Si phase and a SiX compound phase obtained in step 1 with a polymer material containing carbon atoms. When wet pulverization is used to obtain the primary particles containing a Si phase and a SiX compound phase in step 1, the mixture may be subjected to step 2 without removing the solvent used during pulverization, and a polymer material containing carbon atoms may be added thereto to obtain a slurry.

[0071] Examples of the solvent include ethanol, N-methyl-2-pyrrolidone, isopropyl alcohol, cyclohexane, and toluene.

[0072] The mixing ratio of the primary particles containing a Si phase and the polymer material containing carbon atoms is determined so that the carbon coating parameter A of the resulting Si-based particles is greater than 15, preferably greater than 48. That is, the appropriate mixing ratio of the polymer material containing carbon atoms varies depending on the average primary particle size of the primary particles containing a Si phase and an SiX compound phase.

[0073] When a phenolic resin is used as the polymer material containing carbon atoms, for example, the blending ratio of the phenolic resin is preferably 1 to 100 parts by mass, more preferably 5 to 70 parts by mass, and even more preferably 10 to 50 parts by mass, relative to a total of 100 parts by mass of the Si-based powder contained in the resulting slurry mixture. Here, from the viewpoint of thoroughly covering the surface of the Si-based powder, the blending ratio is preferably 1 part by mass or more, more preferably 5 parts by mass or more, and even more preferably 10 parts by mass or more. Furthermore, from the viewpoint of obtaining a high capacity, the blending ratio is preferably 100 parts by mass or less, more preferably 70 parts by mass or less, and even more preferably 50 parts by mass or less.

[0074] The polymer material containing carbon atoms is partially or entirely carbonized into a carbon material by heat treatment in step 4 described below.

[0075] The average primary particle size of the Si-based powder and the mixing ratio of the Si-based powder and the polymer material containing carbon atoms are determined so that the carbon coating parameter A of the resulting Si-based powder exceeds 15.

[0076] The above-mentioned mixing ratio can be determined based on the value of the carbon coating parameter B expressed by the following formula.

[0077] Carbon coating parameter B = 58.1 × average primary particle diameter (μm) + 1.1 × binder added amount (mass%) + 0.6 × binder residual carbon rate (mass%)

[0078] In the carbon coating parameter B, the average primary particle size (μm) refers to the average primary particle size of the Si-based powder. The binder addition amount refers to the content (mass%) of the polymer material containing carbon atoms in the finally obtained Si-containing granules. The binder residual carbon ratio refers to the proportion (mass%) of the carbon content constituting the polymer material containing carbon atoms in the carbon content of the finally obtained Si-containing granules. In other words, if the heat treatment in step 4 described below is not performed and the surface of the Si-based powder is not coated with a carbon material, the binder residual carbon ratio will be 100 (mass%).

[0079] In the carbon coating parameter B, the coefficient of the average primary particle diameter of the Si-based powder, 58.1, the coefficient of the binder addition amount, 1.1, and the coefficient of the binder residual carbon rate, 0.6, are all values ​​derived from experiments.

[0080] From the viewpoint of safety and water resistance, the value of the carbon coating parameter B is preferably 15 or more, more preferably 35 or more. From the viewpoint of cycle characteristics, the value is preferably 48 or more, more preferably 55 or more. The upper limit of the carbon coating parameter B is not particularly limited, but is usually 200 or less.

[0081] <Step 3: Step of spray-drying the slurry mixture> Step 3 is a step of spray-drying the slurry mixture obtained in step 2, thereby obtaining a granule of secondary particles in which primary particles whose surfaces are coated with a polymer material containing carbon atoms are bonded together.

[0082] As the spray drying method, a conventionally known method can be used, for example, a spray drying method. The spray drying method is a method in which the slurry is made into fine droplets and then instantly dried with hot air to remove the dispersion medium, thereby granulating secondary particles in which primary particles containing an Si phase and an SiX compound phase are bonded together into granular form by a binder. The particle size of the resulting secondary particles can be controlled by adjusting the solid content concentration of the slurry, the pressure during drying, etc.

[0083] By steps 2 and 3, the surfaces of the Si-based powder, which are primary particles, can be widely and uniformly coated with a polymer material containing carbon atoms. Then, by performing step 4 subsequently, at least a portion of the polymer material containing carbon atoms is carbonized while maintaining the coated state. As a result, direct contact between the Si phase and the SiX compound phase and the electrolyte or moisture can be suppressed, and the effects of the present invention can be preferably achieved. Furthermore, since a good conductive path is also ensured, better battery characteristics can be realized.

[0084] <Step 4: Step of Heat-Treatment of Spray-Dried Mixture> Step 4 is a step of heat-treating the mixture spray-dried in step 3. This allows at least a portion of the carbon atom-containing polymer material that has coated the surface of the Si-based powder in step 3 to be carbonized.

[0085] The conditions for the heat treatment are not particularly limited, but for example, a heating temperature of 400 to 1100°C is preferred, more preferably 600 to 1000°C, and even more preferably 700 to 950°C. From the viewpoint of promoting carbonization and ensuring a conductive path, the heating temperature is preferably 400°C or higher, more preferably 600°C or higher, and even more preferably 700°C or higher. Furthermore, from the viewpoint of suppressing the generation of undesired compounds such as SiC, the heating temperature is preferably 1100°C or lower, more preferably 1000°C or lower, and even more preferably 950°C or lower.

[0086] The heat treatment time is not particularly limited, but is preferably 0.1 to 10 hours, more preferably 0.5 to 6 hours, and even more preferably 1 to 4 hours. From the viewpoint of accelerating carbonization and carbonizing the polymer material, the heating time is preferably 0.1 hour or more, more preferably 0.5 hour or more, and even more preferably 1 hour or more. From the viewpoint of industrial feasibility, the heating time is preferably 10 hours or less, more preferably 6 hours or less, and even more preferably 4 hours or less.

[0087] It has been found that when the heat treatment parameter C below satisfies a specific range for the heating temperature and heating time of the heat treatment, Si-containing granules can be obtained that are excellent in safety, water resistance, and battery characteristics: Heat treatment parameter C = -0.15 × heating temperature (°C) + 9.51 × 10 －５× [Heating temperature (℃)] ２ + 0.27 x heating time (hours)

[0088] The heat treatment parameter C is preferably less than −15, more preferably −20 or less, and even more preferably −25 or less. There is no particular lower limit, but it is usually −40 or more.

[0089] The heat treatment is preferably carried out in a nitrogen atmosphere or an argon atmosphere, for example, and at atmospheric pressure.

[0090] The heat treatment method is not particularly limited, and examples thereof include heat treatment using a rotary kiln, heat treatment using a shuttle kiln, etc. Among these, heat treatment using a rotary kiln is preferred from the viewpoint of being able to heat treat the powder uniformly without unevenness.

[0091] <Lithium Ion Battery> <Negative Electrode> In a lithium ion battery, it is preferable to use a negative electrode using the negative electrode material powder for lithium ion batteries according to this embodiment.

[0092] The negative electrode of this embodiment has a conductive substrate and a conductive film laminated on the surface of the conductive substrate. The conductive film contains at least the negative electrode material powder for a lithium ion battery according to this embodiment, and the negative electrode material powder functions as a negative electrode active material.

[0093] The conductive substrate functions as a current collector, and examples of the material thereof include Cu, Cu alloys, Ni, Ni alloys, Fe, and Fe-based alloys, with Cu and Cu alloys being preferred.

[0094] Specific examples of the conductive substrate include a foil, a plate, etc. Among these, a foil is preferred from the viewpoints of reducing the volume of the battery and improving the degree of freedom in shape.

[0095] Examples of binder materials used in forming the conductive film include polyvinylidene fluoride (PVdF) resin, fluororesins such as polytetrafluoroethylene, polyvinyl alcohol resins, polyimide resins, polyamide resins, polyamideimide resins, styrene butadiene rubber (SBR), polyacrylic acid, etc. These may be used alone or in combination of two or more. Among these, polyimide resins are particularly preferred from the viewpoints of mechanical strength, resistance to volume expansion of the negative electrode active material, and prevention of peeling from the conductive substrate.

[0096] The conductive film may contain a conductive additive as needed, which makes it easier to ensure a conductive path for electrons.

[0097] Any known conductive additive can be used, but examples thereof include carbon and metal, which do not undergo volume change or only slight volume change during charging and discharging and whose characteristics do not deteriorate due to charging and discharging.

[0098] The shape of the carbon is not particularly limited, and examples thereof include particulate carbon, fibrous carbon, flat carbon, etc. Among these, at least one of particulate carbon and fibrous carbon is preferred from the viewpoint of forming a conductive network between the Si-containing granules, and a combination of particulate carbon and fibrous carbon is more preferred from the viewpoint of further improving cycle characteristics.

[0099] The type of carbon is not particularly limited, and examples thereof include carbon black such as ketjen black, acetylene black, and furnace black, graphite, carbon nanotubes, carbon nanofibers, fullerenes, graphene, and graphene oxide. Among these, carbon black, graphite, carbon nanotubes, and fullerenes are preferred, and ketjen black and acetylene black are more preferred. These may be used alone or in combination of two or more.

[0100] Examples of particulate carbon include carbon black, graphite, hard carbon, and soft carbon. More specific examples of carbon black include ketjen black, acetylene black, and furnace black.

[0101] Examples of fibrous carbon include carbon nanotubes (CNT), carbon nanofibers (CNF), etc. As the carbon nanotubes, either single-walled carbon nanotubes (SWCNT) or multi-walled carbon nanotubes (MWCNT) can be used.

[0102] Examples of metals used as the conductive additive include Ni, Cr, and SUS (stainless steel) alloys.

[0103] The conductive film may contain an aggregate as needed, which can more easily suppress expansion and contraction of the negative electrode during charge and discharge, thereby suppressing collapse of the negative electrode, thereby further improving cycle characteristics.

[0104] The negative electrode in this embodiment can be produced, for example, by adding a negative electrode active material and, if necessary, a necessary amount of a conductive additive and an aggregate to a binder dissolved in a suitable solvent to form a paste, applying this to the surface of a conductive base material, drying it, and, if necessary, subjecting it to compaction, heat treatment, or the like.

[0105] When a lithium ion battery is constructed using the negative electrode of this embodiment, the basic components of the battery other than the negative electrode, such as the positive electrode, electrolyte, and separator, are not particularly limited, and conventionally known components can be used.

[0106] <Positive Electrode, Electrolyte, etc.> The positive electrode may be, for example, a cathode formed by dissolving LiCoO ２ , LiNiO ２ , LiFePO ４ , LiMnO ２ Examples of the cathode active material include a cathode active material having a layer formed thereon.

[0107] Examples of the electrolyte include an electrolytic solution in which a lithium salt is dissolved in a non-aqueous solvent, and also include a polymer in which a lithium salt is dissolved and a polymer solid electrolyte in which a polymer is impregnated with the electrolytic solution.

[0108] Specific examples of the non-aqueous solvent include ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, etc. These may be contained alone or in combination of two or more.

[0109] Specific examples of the lithium salt include LiPF ６ , LiBF ４ , LiClO ４ , LiCF ３ SO ３ , LiAsF ６ These may be contained alone or in combination of two or more kinds.

[0110] Other battery components include a separator, a can (battery case), a gasket, etc., and any of these can be appropriately combined to form a battery as long as they are materials that are typically used in lithium-ion batteries.

[0111] The shape of the battery is not particularly limited, and may be any shape such as cylindrical, rectangular, or coin-shaped, and can be selected appropriately according to the specific application.

[0112] The present invention will be described in more detail below using examples.

[0113] Examples 1 to 79 and Comparative Examples 1 and 2 1. Preparation of Si-Based Powders Each raw material was weighed so that Si, and optionally Sn, Cu, Al, Fe, Ti, Cr, Mn, Co, Ni, Zr, B, and P, were present in the component ratios (mass%) listed in Table 1. The raw materials were then heated and melted using a high-frequency induction furnace to form a molten alloy. From the molten alloy, alloy particles in which an SiX compound phase was dispersed within the Si phase, or alloy particles in which an SiX compound phase and an SnCu compound phase or an AlCu compound phase were dispersed within the Si phase, were obtained using a gas atomization method. An argon atmosphere was used during the preparation of the molten alloy and the gas atomization. During the gas atomization, high-pressure (4 MPa) argon gas was sprayed onto the molten alloy falling in a rod-like shape within the atomization chamber. The resulting particles were mechanically finely pulverized using a wet bead mill to obtain a slurry in which the Si-based powder was dispersed. Ethanol was used as the solvent. The composition of each phase constituting the obtained Si-based powder is as shown in "Phase ratio (mass%)" in Table 2. In Tables 1 and 2, "-" means that the component is not contained.

[0114] 2. Preparation of Slurry Mixture A resol resin was added as a polymer material containing carbon atoms to the slurry containing the Si-based powder obtained above to obtain a slurry mixture. The amount of resol resin added to the slurry mixture was 1 to 100 parts by mass per 100 parts by mass of the Si-based powder. The solid content of the slurry mixture was 40% by mass.

[0115] The slurry mixture obtained above was sprayed and dried by a spray drying method to obtain secondary particles in which the Si-based powder particles, the surfaces of which were coated with resol resin, were bonded together. The spray drying conditions were a spray temperature of 150°C, a spray pressure of 0.3 MPa, and a supply rate of 1 kg / h.

[0116] The secondary particles obtained above were placed in a rotary kiln and subjected to heat treatment in a nitrogen atmosphere at the temperature and for the time shown in Table 2, thereby carbonizing part or all of the resol resin and obtaining a powder of a negative electrode material for a lithium ion battery. Table 2 also shows the value of the heat treatment parameter C, which is expressed by the following formula.

[0117] <Evaluation: Si-based powder> <Average primary particle size> For the Si-containing granules, the slurry before granulation was measured using a laser diffraction / scattering particle size distribution analyzer, and the median diameter (D50) calculated from the volume-based particle size distribution was taken as the average primary particle size of the Si-based powder. The results are shown in Table 2.

[0118] <Carbon Coating Parameter A> The Si-containing granules were analyzed using a combustion infrared absorption method to determine the carbon content of the Si-based powder, which is the total of the Si-based powder, the carbon material, and any other components, i.e., the carbon content ratio relative to the entire Si-containing granules. The results are shown in Table 2. Table 2 also shows a value represented by the carbon coating parameter A, which is calculated using this carbon content and the average primary particle diameter obtained above.

[0119] <Evaluation: Si-containing granules> <Average secondary particle diameter> The average secondary particle diameter of the Si-containing granules was determined by particle shape distribution measurement using an image particle size distribution analyzer (Malvern Panalytical, Morphologi series, Morphologi 4). The results are shown in Table 3.

[0120] <Aspect Ratio> The Si-containing granules were observed under an SEM at a magnification of 400 times, and the maximum diameter D and the diameter D' perpendicular to the maximum diameter D were measured for each of 10 Si-containing granules from the obtained images. The average value of the values ​​expressed as D / D' was taken as the aspect ratio. The results are shown in Table 3.

[0121] <Carbon Coverage> The surface of the Si-containing granules was analyzed using X-ray photoelectron spectroscopy (XPS). From the spectrum obtained, the proportion of carbon atoms (C) was determined, and the carbon coverage (mass%) of the surface of the Si-containing granules was calculated. The results are shown in Table 3.

[0122] <SiC content> The Si-containing granules were subjected to powder X-ray diffraction (XRD) measurement, and the ratio of the area of ​​the peak attributable to SiC to the area of ​​all peaks in the obtained XRD pattern was calculated to determine the SiC content. The results are shown in Table 3.

[0123] <Evaluation: Safety> The Si-containing granules were subjected to a small gas flame ignition test as a Class 2 hazardous material test under the Fire Service Act. Materials that did not ignite within 10 seconds of being brought close to a small gas flame were deemed non-hazardous and passed the test, indicated by "○" in Table 3, while materials that ignited within 10 seconds were deemed hazardous and failed the test, indicated by "×" in Table 3.

[0124] <Evaluation: Water Resistance> 1 g of the Si-containing granules was placed in 13 mL of pure water, and the total amount of hydrogen generated from the start of measurement until 336 hours had elapsed was measured using a sensor gas chromatograph. Samples in which the amount of hydrogen generated was 100 ppm by mass or less were considered to be acceptable and are indicated by "○" in Table 3, and samples in which the amount of hydrogen generated was more than 100 ppm by mass were considered to be unacceptable and are indicated by "×" in Table 3.

[0125] Evaluation: Battery Characteristics Preparation of Coin-Type Batteries for Charge / Discharge Tests A paste containing the negative electrode material was prepared by blending 100 parts by weight of a lithium-ion battery negative electrode active material (Si-containing granules), 5 parts by weight of acetylene black (manufactured by Denka) as a conductive additive, and 15 parts by weight of polyimide (thermoplastic resin) as a binder, and mixing this with N-methyl-2-pyrrolidone (NMP) as a solvent. The paste was applied to the surface of stainless steel (SUS) 316L foil (thickness: 20 μm) that served as a current collector using a doctor blade method, dried, consolidated using a roll press, and punched into a disk with a diameter of 11 mm to prepare a test electrode.

[0126] Next, a Li foil (500 μm thick) was punched to approximately the same shape as the test electrode to form a counter electrode. The test electrode was housed in a positive electrode can, and the counter electrode was housed in a negative electrode can. A polyolefin-based microporous membrane separator was placed between the test electrode and the counter electrode. Note that the test electrode would be the negative electrode in a lithium-ion battery, but when the counter electrode was Li foil, the Li foil became the negative electrode and the test electrode became the positive electrode.

[0127] In the positive electrode can and the negative electrode can, a mixed solvent of ethylene carbonate (EC):diethyl carbonate (DEC) = 1:1 (volume ratio) was added. ６ A non-aqueous electrolyte solution in which the above compound was dissolved at a concentration of 1 mol / L was poured into the battery, and the negative electrode can and the positive electrode can were fixed by crimping to prepare a coin-type battery for charge / discharge tests.

[0128] <Charge / Discharge Test> The prepared coin-type batteries for charge / discharge test were subjected to one cycle of constant current charge / discharge at a current value of 0.2 mA. The capacity (mAh) used during this Li absorption / desorption was divided by the amount of active material (g) to determine the initial charge capacity or initial discharge capacity, and the ratio of the initial discharge capacity to the initial charge capacity was calculated as the initial coulomb efficiency. The results are shown in Table 3.

[0129] The theoretical capacity of each negative electrode material (each negative electrode active material) was calculated based on the phase ratios of the Si phase, SiX compound phase, SnCu compound phase, and AlCu compound phase in the Si-containing granules. The initial discharge capacity relative to the theoretical capacity was then calculated as the utilization rate (active material utilization rate, %) of the negative electrode active material relative to the theoretical capacity. The results are shown in Table 3.

[0130] From the second cycle onwards, the charge / discharge test was carried out at a 1 / 5C rate, up to 50 cycles. Here, the C rate refers to the current value required to charge / discharge the electrode in 1 hour, which is 1C. Therefore, the 1 / 5C rate means charging or discharging in 5 hours. The ratio of the discharge capacity at the 50th cycle to the initial discharge capacity at the first cycle, i.e., the discharge capacity retention rate, was evaluated as the cycle retention rate (%). The results are shown in Table 3.

[0131]

[0132]

[0133]

[0134] Among the above results, it is clear from a comparison with the results of Comparative Examples 1 and 2 in particular that the negative electrode material powder for a lithium ion battery according to this embodiment is resistant to ignition and has high safety and good water resistance. This is thought to be because the Si-based powder, which is the primary particle, has a certain size or more and its surface is suitably coated with a carbon material, which highly likely prevents the Si phase, SiX compound phase, and optionally other phases such as a SnCu compound phase and an AlCu compound phase constituting the Si-based powder from coming into contact with the electrolyte or oxygen.

[0135] In addition to the above, the results of Examples 73 to 75 show that when the carbon coating parameter A exceeds 48, the cycle characteristics, in particular, of the battery characteristics, are improved. Furthermore, the results of Examples 78 and 79 show that the cycle characteristics are also improved by setting the upper limits of the average primary particle size of the Si-based powder and the average secondary particle size of the Si-containing granules to a certain value or less. The results of Example 72 show that the aspect ratio of the Si-containing granules also contributes to the cycle characteristics. The results of Example 77 show that an increase in the amount of the SnCu phase in the Si-based powder contributes to the cycle characteristics.

[0136] Furthermore, the results of Examples 67 to 69 and 71 show that when the heat treatment temperature is increased and the value of the heat treatment parameter C is -15 or higher, the content of SiC in the Si-containing granules increases, and the utilization rate of the active material, among other battery characteristics, decreases. On the other hand, the results of Example 70 show that when the heat treatment temperature is too low, sufficient carbonization does not proceed, and the initial Coulomb efficiency decreases. In this way, the content of SiC can be controlled by performing an appropriate heat treatment.

[0137] In addition to the above, the results of Example 76 show that when the proportion of the Si phase in the Si-based powder is low, the initial coulombic efficiency decreases.

[0138] The negative electrode material powder for lithium ion batteries according to the present embodiment has been described in detail above, but the present invention is not limited to the above-described embodiments and examples, and various modifications are possible within the scope of the invention.

[0139] Although the present invention has been described in detail and with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the present invention. This application is based on a Japanese patent application (Patent Application No. 2023-219678) filed on December 26, 2023, the contents of which are incorporated herein by reference.

Claims

1. A negative electrode material powder containing Si-containing granulates, wherein the Si-containing granulates contain Si-based powder and a carbon material, the Si-based powder is primary particles containing an Si phase and an SiX compound phase, the SiX compound phase contains Si and an element X, the element X is at least one selected from the group consisting of Fe, Co, Cr, Mn, Ni, Zr, Ti, B, and P, the average primary particle diameter of the Si-based powder is 0.1 μm or more, the average secondary particle diameter of the Si-containing granulates is 2 μm or more, the surface of the Si-based powder is coated with the carbon material and bonded with the carbon material, and the carbon coating parameter A represented by the following formula of the Si-based powder is more than 15. A negative electrode material powder for a lithium ion battery. Carbon coating parameter A = 58.1 × average primary particle diameter (μm) + 5.6 × carbon content (mass%) 2. The negative electrode material powder for a lithium ion battery according to claim 1, wherein the content ratio of SiC in the Si-containing granulates is less than 15%.

3. The negative electrode material powder for a lithium ion battery according to claim 1 or 2, wherein the carbon coating parameter A is more than 48.

4. The negative electrode material powder for a lithium ion battery according to claim 1 or 2, wherein the average primary particle diameter of the Si-based powder is 5.0 μm or less.

5. The negative electrode material powder for a lithium ion battery according to claim 1 or 2, wherein the average secondary particle diameter of the Si-containing granulates is 20 μm or less.

6. The negative electrode material powder for a lithium ion battery according to claim 1 or 2, wherein the aspect ratio of the Si-containing granulates is 1.0 to 1.

8.

7. The Si-based powder is primary particles further containing at least one selected from the group consisting of an SnCu compound phase and an AlCu compound phase, the SnCu compound phase contains Sn and Cu, the AlCu compound phase contains Al and Cu, and the content ratio of the Si phase in the Si-based powder is 30% by mass or more. The negative electrode material powder for a lithium ion battery according to claim 1 or 2.

8. The negative electrode material powder for a lithium ion battery according to claim 7, wherein the total content ratio of the SnCu compound phase and the AlCu compound phase in the Si-based powder is 0.1 to 15% by mass.

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