Anode material powder for lithium-ion batteries

Abstract

The present invention relates to a negative electrode material powder for a lithium ion battery, the negative electrode material powder containing Si, Sn, element X (X=Fe, Ni, Cr, Zr, and Ti), and element Y (Y=Cu, Fe, Ni, Cr, Co, Mn, Zr, and Ti), and including a Si phase, a SiX compound phase, and a SnY compound phase which are independently present in a state of being separated from each other with a phase ratio represented by a[Si]-b[SiX]-c[SnY] (a+b+c=100, 10≤a≤95, 1≤b≤90, and 0.07≤c≤50), wherein average particle diameters mdSi, mdSiX, and mdSnY in the respective phases all fall within the range of 0.1-50 μm, and the proportions of mdSi / mdSiX and mdSi / mdSnY all fall within the range of 0.1-5.0.

Description

[Technical Field]

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

[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 negative electrode to charge and discharge the battery. 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 discharge. Conventionally, lithium cobalt oxide (LiCoO2) has generally been used as the active material for the positive electrode, and graphite has been widely used as the active material for the negative electrode. However, the theoretical capacity of graphite as the active material for the negative electrode is only 372 mAh / g, and there has been a demand for even higher capacity. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2017-224499 Summary of the Invention [Problem to be solved by the invention]

[0005] As an alternative to carbon-based electrode materials, metal materials such as silicon, which are expected to increase capacity, are being considered. The theoretical capacity of silicon is 4198 mAh / g, but silicon absorbs lithium ions through an alloying reaction with lithium, and as the lithium ions are absorbed and released, significant volume expansion and contraction occurs. This can cause silicon particles to crack or peel off from the current collector, deteriorating the cycle characteristics, which are the capacity retention characteristics during repeated charge and discharge.

[0006] As a means to solve the above problem, it has been proposed to reduce the expansion of Si by making Si finer, or to alloy Si. For example, Patent Document 1 discloses providing a Si compound phase together with a Si phase in a Si-based alloy particle. The Si compound phase is effective in reducing the expansion of the Si phase and improving cycle characteristics. However, such measures for improving cycle characteristics may result in a decrease in the initial battery characteristics such as the initial discharge capacity and the initial coulombic efficiency, and there is still room for improvement in improving battery characteristics taking into account the initial characteristics and cycle characteristics.

[0007] The present invention has been made in light of the above circumstances, with the object of providing a negative electrode material powder for lithium ion batteries that can improve battery characteristics in consideration of initial characteristics and cycle characteristics. [Means for solving the problem]

[0008] Thus, the present invention provides a method for producing a semiconductor device comprising: A negative electrode material powder for a lithium ion battery, comprising a Si phase, a SiX compound phase, and a SnY compound phase in a phase ratio represented by the following formula (1): the Si phase, the SiX compound phase, and the SnY compound phase exist separately and in separate states, When the particle size at an integrated value of 50% in the particle size distribution of the Si phase, the SiX compound phase, and the SnY compound phase is defined as the average particle size mdSi, mdSiX, and mdSnY, respectively, The average particle sizes of mdSi, mdSiX, and mdSnY are all within the range of 0.1 to 50 μm, The ratios of average particle diameters expressed as mdSi / mdSiX and mdSi / mdSnY are both characterized by being within the range of 0.1 to 5.0. a[Si]-b[SiX]-c[SnY]...Formula (1) However, the element X is one or more elements selected from the group consisting of Fe, Ni, Cr, Zr, and Ti, the element Y is one or more elements selected from the group consisting of Cu, Fe, Ni, Cr, Co, Mn, Zr, and Ti; In the formula (1), a, b, and c respectively represent the contents (mass%) of the Si phase, the SiX compound phase, and the SnY compound phase, where a+b+c=100, 10≦a≦95, 1≦b≦90, and 0.07≦c≦50.

[0009] In the negative electrode material powder for lithium-ion batteries specified in this way, the Si phase, which expands as Li is absorbed, exists independently and separately from the other SiX compound phases and SnY compound phases. This means that a space that allows the expansion of the Si phase is easily formed around the Si phase. This space acts as a buffer region against the expansion of the Si phase, suppressing the collapse of the SiX compound phase, which acts as a skeleton in the electrode, and improving the cycle characteristics.

[0010] If the ratio of the average particle size of the Si phase to the SiX compound phase and the SnY compound phase, which are present separately and independently, is too small or too large, the initial characteristics or cycle characteristics of the lithium ion battery will deteriorate. Therefore, in the present invention, the ratio of the average particle size of the Si phase to the SiX compound phase (mdSi / mdSiX) and the ratio of the average particle size of the Si phase to the SnY compound phase (mdSi / mdSnY) are both set within the range of 0.1 to 5.0, and more preferably within the range of 0.3 to 3.0. By doing so, excessive deterioration of the initial characteristics or cycle characteristics can be avoided, and battery characteristics taking into account the initial characteristics and cycle characteristics can be improved.

[0011] Here, in consideration of the balance between the initial characteristics and the cycle characteristics, it is preferable that a, b, and c in the formula (1) are set to 30≦a≦90, 1≦b≦70, and 0.1≦c≦30, respectively.

[0012] In addition, it is preferable to use Cu as the element Y in order to improve cycle characteristics. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 1 is a schematic diagram showing the configuration of a negative electrode material powder according to one embodiment of the present invention, in which (A) in FIG. 1 is a schematic diagram of a Si alloy particle having a Si phase, a SiX compound phase, and a SnY compound phase, and (B) in FIG. 1 is a schematic diagram of a negative electrode material powder according to one embodiment of the present invention obtained by finely pulverizing the Si alloy particle of (A) in FIG. 1. [Figure 2] FIG. 2 is a schematic diagram for explaining the effect of the negative electrode material powder according to one embodiment of the present invention. [Figure 3] FIG. 3 is a schematic diagram for explaining the effect of the negative electrode material powder according to another embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0014] Next, a negative electrode material powder for a lithium ion battery according to one embodiment of the present invention and a lithium ion battery (hereinafter, sometimes simply referred to as a battery) using this negative electrode material powder in the negative electrode will be specifically described. Note that the symbol "to" indicating a numerical range is used to mean that the numerical values ​​before and after it are included as the lower limit and upper limit, respectively.

[0015] 1. Negative electrode material powder The negative electrode material powder contains Si, Sn, element X, and element Y as its main constituent elements. Here, element X is one or more elements selected from the group consisting of Fe, Ni, Cr, Zr, and Ti, and element Y is one or more elements selected from the group consisting of Cu, Fe, Ni, Cr, Co, Mn, Zr, and Ti. Elements other than the main constituent elements of Si, Sn, element X, and element Y are not included except for unavoidable impurity elements. Examples of unavoidable impurity elements include nitrogen (N), sulfur (S), phosphorus (P), and oxygen (O). The upper limits of each element are N≦0.10 mass%, S≦0.10 mass%, P≦0.10 mass%, and O≦15 mass%.

[0016] The negative electrode material powder contains, as its metal structure, a Si phase, a SiX compound phase, and a SnY compound phase in a phase ratio represented by the following formula (1). a[Si]-b[SiX]-c[SnY]...Formula (1) In formula (1), [Si] means the Si phase, [SiX] means the SiX compound phase, and [SnY] means the SnY compound phase. Furthermore, a, b, and c (where a + b + c = 100) respectively represent the contents (mass%) of the Si phase, SiX compound phase, and SnY compound phase, and in this example, 10≦a≦95, 1≦b≦90, and 0.07≦c≦50. In addition, non-compound simple substance Sn (Sn phase) may be contained as an impurity as long as its proportion in the whole is 5 mass % or less.

[0017] The Si phase is a phase that mainly contains Si and absorbs Li ions. From the viewpoint of increasing the amount of Li absorbed, it is preferable that the Si phase is a single Si phase. However, the Si phase may contain unavoidable impurities.

[0018] In this negative electrode material powder, the proportion of the Si phase (the value of "a" in formula (1)) is 10 to 95 mass %, preferably 30 to 90 mass %. Here, from the viewpoint of obtaining a high initial discharge capacity, the proportion of the Si phase is 10 mass % or more, preferably 30 mass % or more. Furthermore, from the viewpoint of obtaining high cycle characteristics, the proportion of the Si phase is 95 mass % or less, preferably 90 mass % or less.

[0019] On the other hand, the SiX compounds that make up the SiX compound phase have poor lithium storage capacity and expand very little when reacting with Li ions, so the SiX compound phase serves as a framework that maintains the electrode structure.

[0020] The properties of SiX compounds, such as Li absorption and conductivity, may differ depending on which element is selected as the element X. Fe, Ni, Cr, and Zr as the element X are particularly excellent in the low expansion and high conductivity expected of SiX compounds. Furthermore, the element X that is effective in increasing the initial Coulomb efficiency is Ni and Ti. The element X that is effective in increasing the discharge rate characteristics is Ti. In this way, it is preferable to appropriately select the element X according to the desired properties. The SiX compound phase may be composed of only one type of compound, or may be composed of two or more types of compounds, such as a SiFe compound and a SiNi compound.

[0021] On the other hand, the SnY compound that makes up the SnY compound phase has a theoretical capacity lower than that of Si but higher than that of SiX compounds. For example, the theoretical capacity of SiZr compounds, which correspond to SiX compounds, is 100 mAh / g, while the theoretical capacity of SnY compounds is 150-600 mAh / g. This makes it easier to ensure a diffusion path for Li ions through the SnY compound phase in this example. On the other hand, the degree of expansion due to reaction with Li ions is smaller than that of Si or Sn alone, which are highly reactive with Li ions, so the adverse effects of the formation of the SnY compound on cycle performance can be kept low.

[0022] In particular, the SnCu compound formed when Cu is selected as the element Y has excellent electrical conductivity. It is also effective in improving cycle characteristics. Therefore, it is preferable to include Cu as the element Y. Note that this SnY compound phase can be composed of not only one type of compound but also two or more types of compounds.

[0023] As described above, the SiX compound phase and the SnY compound phase play different roles, and the resulting battery characteristics change depending on the ratio of these compound phases. The SnY compound phase expands more than the SiX compound phase upon reaction with Li ions, although to a lesser extent. Therefore, if the ratio of the SnY compound phase is high and the ratio of the SiX compound phase is low, the cycle characteristics will deteriorate. On the other hand, if the ratio of the SiX compound phase is high and the ratio of the SnY compound phase is low, the initial discharge capacity will be low.

[0024] In this example, the proportion of the SiX compound phase (the value of "b" in formula (1)) is 1 to 90 mass%, preferably 1 to 70 mass% or 5 to 90 mass%, more preferably 5 to 70 mass%, and even more preferably 10 to 70 mass%. Note that when satisfying a+b+c=100, if b is 90% by mass, even if a and c take their minimum values ​​of 10% by mass and 0.07% by mass, respectively, the sum of a+b+c will be 100.07% by mass, exceeding 100% by mass. Due to significant digits, this can be considered to be 100% by mass, but if exceeding 100% by mass becomes a problem, the upper limit of b should be 89.93% by mass, not 90% by mass. Similarly, if b is 90% by mass, a and c are 10% by mass and 0.1% by mass, and it is not permissible for a+b+c=100.1% by mass, then the upper limit of b should be 89.9% by mass, not 90% by mass. Similarly, for example, when 30≦a≦90, 1≦b≦70, and 0.1≦c≦30, if b is 70% by mass and a and c are respectively set to their minimum values ​​of 30% by mass and 0.1% by mass, the sum of a+b+c will be 100.1% by mass, which can be regarded as 100% by mass due to significant digits. However, if exceeding 100% by mass becomes a problem, the upper limit of b will be 89.9% by mass rather than 90% by mass.

[0025] The proportion of the SiX compound phase (the value of "b" in formula (1)) is 1 mass % or more, preferably 5 mass % or more, and more preferably 10 mass % or more, from the viewpoint of obtaining high cycle characteristics. Also, from the viewpoint of obtaining a high initial discharge capacity, the proportion of the SiX compound phase is 90 mass % or less, and in some cases 89.97 mass % or less, preferably 89.9 mass % or less, and more preferably 70 mass % or less.

[0026] The proportion of the SnY compound phase (the value of "c" in formula (1)) is 0.07 to 50 mass%, preferably 0.1 to 50 mass%, and more preferably 0.1 to 30 mass%. Here, from the viewpoint of obtaining a high initial discharge capacity, the proportion of the SnY compound phase (the value of "c" in formula (1)) is 0.07 mass% or more, preferably 0.1 mass% or more. Furthermore, from the viewpoint of obtaining high cycle characteristics, the proportion of the SnY compound phase is 50 mass% or less, preferably 30 mass% or less.

[0027] Therefore, the relationship between a, which indicates the proportion of the Si phase, b, which indicates the proportion of the SiX compound phase, and c, which indicates the proportion of the SnY compound phase, is 10≦a≦95, 1≦b≦90, 0.07≦c≦50, and in some cases 10≦a≦95, 1≦b≦89.93, 0.07≦c≦50, and 10≦a≦95, 1≦b≦90, 0.1≦c≦50 is preferred, and 30≦a≦90, 1≦b≦70, 0.1≦c≦30 is more preferred, and in some cases 30≦a≦90, 1≦b≦69.9, 0.1≦c≦30 is more preferred.

[0028] The contents of the main elements in the negative electrode material powder that are suitable for obtaining the above-described constituent phases are as follows: In the following description, unless otherwise specified, "%" means "% by mass."

[0029] The Si content is preferably 50 to 95%, more preferably 60 to 80%, and even more preferably 73 to 79%. From the viewpoint of obtaining a high initial discharge capacity, the Si content is preferably 50% or more, more preferably 60% or more, and even more preferably 73% or more. From the viewpoint of obtaining good cycle characteristics, the Si content is preferably 95% or less, more preferably 80% or less, and even more preferably 79% or less.

[0030] The content of element X is preferably 1.0 to 38%, more preferably 5.0 to 30%, and even more preferably 13 to 23%. From the viewpoint of obtaining good cycle characteristics, the content of element X is preferably 1.0% or more, more preferably 5.0% or more, and even more preferably 13% or more. From the viewpoint of obtaining a high initial discharge capacity, the content of element X is preferably 38% or less, more preferably 30% or less, and even more preferably 23% or less. When two or more elements are contained as element X, the total content thereof is preferably within the above range.

[0031] The Sn content is preferably 0.7 to 30%, more preferably 1.0 to 10%, and even more preferably 1.5 to 5.0%. Here, from the viewpoint of obtaining a better effect as a Li diffusion path, the Sn content is preferably 0.7% or more, more preferably 1.0% or more, and even more preferably 1.5% or more. Furthermore, from the viewpoint of obtaining good cycle characteristics, the Sn content is preferably 30% or less, more preferably 10% or less, and even more preferably 5.0% or less.

[0032] The content of element Y is preferably 1.0 to 15%, more preferably 1.5 to 10%, and even more preferably 1.5 to 4.0%. Here, the content of element Y is preferably 1.0% or more, more preferably 1.5% or more, from the viewpoint of obtaining a better effect as a Li diffusion path. Furthermore, from the viewpoint of obtaining good cycle characteristics, the content of element Y is preferably 15% or less, more preferably 10% or less, and even more preferably 4.0% or less. Note that when two or more elements are contained as element Y, the total content thereof is preferably within the above range.

[0033] FIG. 1B is a schematic diagram of an anode material powder 3 according to one embodiment of the present invention, obtained by pulverizing Si alloy particles containing a Si phase, a SiX compound phase, and a SnY compound phase. As shown in the figure, the anode material powder 3 contains a Si phase 3a, a SiX compound phase 3b, and a SnY compound phase 3c, each of which exists separately. When the average particle diameters of the Si phase 3a, the SiX compound phase 3b, and the SnY compound phase 3c are defined as mdSi, mdSiX, and mdSnY, respectively, the average particle diameters mdSi, mdSiX, and mdSnY are all within the range of 0.1 to 50 μm, preferably 0.3 to 40 μm, and more preferably 0.5 to 30 μm. Furthermore, the average particle diameter mdSi is more preferably 1.0 to 20 μm, and particularly preferably 1.0 to 10 μm. Here, the average particle size of mdSi, mdSiX, and mdSnY is 0.1 μm or more, preferably 0.5 μm or more, and even more preferably 1.0 μm or more. The average particle size of mdSi, mdSiX, and mdSnY is 50 μm or less, preferably 40 μm or less, and more preferably 30 μm or less, and the average particle size of mdSi is more preferably 20 μm or less, and particularly preferably 10 μm or less. Here, "particle size" refers to the diameter of a circle having the same area as that of each phase constituting the negative electrode material powder measured under an electron microscope, i.e., the equivalent circle diameter. Furthermore, "average particle size" refers to the particle size at 50% of the integrated value in the particle size distribution, specifically the average value obtained by analyzing 100 particles from cross-sectional SEM images (5000x magnification) of each of the Si phase, SiX compound phase, and SnY compound phase powders.

[0034] In the present anode material powder thus specified, the amount of expansion of the Si phase is suppressed by the fine particle size, and its collapse is inhibited. Furthermore, because this Si phase exists independently of the other SiX compound phases and SnY compound phases, a space that allows Si expansion is easily formed around the Si phase. This space acts as a buffer region against Si expansion, and inhibits the collapse of the SiX compound phase, which acts as a skeleton in the electrode.

[0035] However, as shown in Figure 2, if the particle size of the Si phase 3a is excessively large relative to the SiX compound phase 3b or the SnY compound phase 3c, the electrode will collapse due to repeated expansion and contraction of the Si phase 3a, resulting in a deterioration in cycle characteristics. In the figure, reference numeral 4 denotes a conductive substrate that constitutes part of the electrode. On the other hand, as shown in FIG. 3, if the grain size of the Si phase 3a is excessively small relative to the SiX compound phase 3b or the SnY compound phase 3c, the Si phase 3a is surrounded by the SiX compound phase 3b or the SnY compound phase 3c, which prevents the Si phase 3a from absorbing and releasing Li ions, resulting in a deterioration in the initial Coulombic efficiency and the initial capacity.

[0036] Therefore, in this example, the average particle size ratios expressed by mdSi / mdSiX and mdSi / mdSnY are both set within the range of 0.1 to 5.0, thereby preventing deterioration of initial characteristics such as initial discharge capacity and initial coulombic efficiency, as well as cycle characteristics. The average particle size ratios are more preferably within the range of 0.3 to 5.0 or 0.1 to 3.0, and even more preferably within the range of 0.3 to 3.0.

[0037] Next, an example of a method for producing the negative electrode material powder will be described.

[0038] Each raw material is weighed out so as to obtain a predetermined chemical composition, and the weighed raw materials are melted using a melting means such as an arc furnace, a high-frequency induction furnace, or a heating furnace, and the resulting molten alloy is then quenched using an atomization method to obtain a Si alloy as a quenched alloy.

[0039] In the atomization method, a molten alloy is poured into an atomization chamber and flows downward continuously (in a rod-like shape). Gases such as N2, Ar, or He are sprayed at high pressures, for example, 1 to 10 MPa, onto the molten alloy, which is then crushed and cooled. The cooled molten alloy, while remaining semi-molten, falls freely through the atomization chamber, gradually becoming spherical, resulting in Si alloy particles such as those shown in Figure 1(A). The structure of the Si alloy particle 1 shown in the figure contains a Si phase, a SiX compound phase, and a SnY compound phase. In the atomization method, high-pressure water may be sprayed instead of gas to improve the cooling effect. In some cases, it is also possible to obtain a Si alloy foil by using a roll quenching method instead of the atomization method.

[0040] Next, the obtained Si alloy particles are finely pulverized to obtain a negative electrode material powder containing an Si phase 3a, an SiX compound phase 3b, and an SnY compound phase 3c, each of which exists independently and in a separated state, as shown in Figure 1 (B). If the Si alloy particles 1 are pulverized by a dry pulverization method, repeated pulverization and aggregation will occur, making it difficult to separate and independent the Si phase 3a, SiX compound phase 3b, and SnY compound phase 3c, and a powder in which these phases are tightly packed together will be formed. Therefore, in this example, it is preferable to use a wet pulverization method to refine the Si alloy particles.

[0041] In this example, the wet milling method can be a wet milling method using a bead mill or a planetary ball mill. In wet milling, a solvent is used together with the Si alloy particles to be milled. Examples of the solvent that can be used include ethanol, methanol, isopropyl alcohol, and naphthesol. It is also possible to add a dispersing agent.

[0042] If there is a large difference in the amounts (proportions) of the Si phase, SiX compound phase, and SnY compound phase within the Si alloy particles to be pulverized, the phase with the larger amount will not be pulverized, and differences in particle size will likely occur among the phases. However, if the Si alloy particles have a chemical composition adjusted so that the Si, SiX compound, and SnY compound phase proportions are as described above, then a negative electrode material powder can be obtained by wet pulverization in which the average particle sizes mdSi, mdSiX, and mdSnY are all within the range of 0.1 to 50 μm and the ratios of the average particle sizes expressed as mdSi / mdSiX and mdSi / mdSnY are all within the range of 0.1 to 5.0.

[0043] In addition, in producing the present negative electrode material powder, instead of the method of crushing Si alloy particles having the above three phases therein, it is also possible to employ a method in which Si particles, SiX compound particles, and SnY compound particles are separately formed directly from a molten metal, these particles are crushed to respective predetermined particle sizes, and then mixed together.

[0044] 2.Battery Next, a battery constructed using a negative electrode containing this negative electrode material powder will be described.

[0045] The negative electrode has a conductive substrate and a conductive film laminated on the surface of the conductive substrate. The conductive film contains at least the present negative electrode material powder described above in a binder. The conductive film may also contain a conductive additive as needed. When the conductive additive is contained, it becomes easier to ensure a conductive path for electrons.

[0046] The conductive film may also contain an aggregate, if necessary, which helps to suppress expansion and contraction of the negative electrode during charge and discharge, thereby preventing the negative electrode from collapsing, thereby further improving cycle characteristics.

[0047] The conductive substrate functions as a current collector. Examples of the material include Cu, Cu alloy, Ni, Ni alloy, Fe, and Fe-based alloy. Preferably, the conductive substrate is Cu or a Cu alloy. Specific examples of the conductive substrate shape include a foil shape and a plate shape. Preferably, the conductive substrate is foil-shaped, which allows for a smaller battery volume and improved freedom of shape.

[0048] Suitable materials for the binder include, for example, polyvinylidene fluoride (PVdF) resin, fluororesins such as polytetrafluoroethylene, polyvinyl alcohol resin, polyimide resin, polyamide resin, polyamideimide resin, styrene-butadiene rubber (SBR), and polyacrylic acid. These materials can be used alone or in combination. Among these, polyimide resin is particularly preferred because it has high mechanical strength, can withstand the volume expansion of the active material, and effectively prevents the conductive film from peeling off from the current collector due to binder destruction.

[0049] Examples of the conductive additive include carbon black such as ketjen black, acetylene black, and furnace black, graphite, carbon nanotubes, and fullerene. These may be used alone or in combination of two or more. Among these, ketjen black and acetylene black are preferably used from the viewpoint of easily ensuring electronic conductivity.

[0050] The content of the conductive additive is preferably 0 to 30 parts by mass, more preferably 4 to 13 parts by mass, relative to 100 parts by mass of the present negative electrode material powder, from the viewpoints of improved conductivity, electrode capacity, etc. The average particle size (d50) of the conductive additive is preferably 10 nm to 1 μm, more preferably 20 to 50 nm, from the viewpoints of dispersibility, ease of handling, etc.

[0051] As the aggregate, a material that does not expand or contract during charge and discharge or that expands or contracts very little can be preferably used. Examples include graphite, alumina, calcia, zirconia, and activated carbon. These may be used alone or in combination of two or more. Of these, graphite is preferably used from the viewpoints of electrical conductivity, Li activity, and the like.

[0052] The content of the aggregate is preferably 10 to 400 parts by mass, more preferably 43 to 100 parts by mass, per 100 parts by mass of the negative electrode material powder, from the viewpoint of improving cycle characteristics, etc. The average particle size of the aggregate is preferably 10 to 50 μm, more preferably 20 to 30 μm, from the viewpoint of functionality as the aggregate and control of electrode film thickness, etc. The average particle size of the aggregate is a value measured using a laser diffraction / scattering particle size distribution analyzer.

[0053] The present negative electrode can be produced, for example, by adding the present negative electrode material powder, and, if necessary, a conductive additive and an aggregate in required amounts to a binder dissolved in an appropriate solvent to form a paste, applying the paste to the surface of a conductive base material, drying it, and, if necessary, subjecting it to compaction, heat treatment, or the like.

[0054] When a lithium ion battery is constructed using the present negative electrode, the basic components of the battery other than the present negative electrode, such as the positive electrode, electrolyte, and separator, are not particularly limited.

[0055] Specific examples of the positive electrode include those in which a layer containing a positive electrode active material such as LiCoO2, LiNiO2, LiFePO4, or LiMnO2 is formed on the surface of a current collector such as aluminum foil.

[0056] Specific examples of the electrolyte include an electrolytic solution in which a lithium salt is dissolved in a non-aqueous solvent, etc. In addition, a lithium salt dissolved in a polymer, a polymer solid electrolyte in which a polymer is impregnated with the electrolytic solution, etc. can also be used.

[0057] 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.

[0058] Specific examples of the lithium salt include LiPF6, LiBF4, LiClO4, LiCF3SO3, LiAsF6, etc. One or more of these may be contained.

[0059] 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.

[0060] 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. [Example]

[0061] The present invention will be described in more detail below with reference to examples. Note that % in the alloy composition is mass % unless otherwise specified.

[0062] 1. Preparation of negative electrode material powder The alloy compositions of the 27 examples and 8 comparative examples of negative electrode material powders are shown in Table 1. The alloy compositions shown in Table 1 are specified so as to obtain the target compositions shown in Tables 2 and 3 below.

[0063] First, each raw material shown in Table 1 was weighed. The weighed raw materials were heated and melted using a high-frequency induction furnace to prepare a molten alloy. Powdered Si alloy particles were produced from the molten alloy by gas atomization. The atmosphere during the preparation of the molten alloy and gas atomization was an argon atmosphere. During 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 obtained Si alloy particles were mechanically pulverized using a wet bead mill to prepare a negative electrode material powder. However, in Comparative Examples 5 and 6, in which the particle diameters of the phases were significantly different, Si particles, SiX compound particles, and SnY compound particles were separately formed directly from the molten metal, and these particles were crushed to respective predetermined particle diameters and then mixed together to obtain a negative electrode material powder.

[0064] [Table 1]

[0065] 2. Preparation of coin-cell batteries for charge-discharge tests 100 parts by mass of the prepared negative electrode material powder as the negative electrode active material, 6 parts by mass of Ketjen Black (manufactured by Lion Corporation) as a conductive additive, and 19 parts by mass of a polyimide (thermoplastic resin) binder as a binding agent were blended, and this was mixed with N-methyl-2-pyrrolidone (NMP) as a solvent to prepare pastes containing each negative electrode material powder.

[0066] Each coin-shaped half-cell was fabricated as follows. Here, for simple evaluation, an electrode fabricated using the negative electrode material powder was used as the test electrode, and Li foil was used as the counter electrode. First, each paste was applied to a 50 μm thickness using a doctor blade method on the surface of stainless steel (SUS) 316L foil (thickness: 20 μm) that served as the negative electrode current collector, and then dried to form each negative electrode active material layer. After formation, the negative electrode active material layer was consolidated using a roll press. In this way, test electrodes were fabricated using the negative electrode material powders of the examples and comparative examples as the negative electrode active material.

[0067] Next, the test electrode was punched out into a disk shape with a diameter of 11 mm to prepare each test electrode.

[0068] Next, Li foil (thickness 500 μm) was punched to approximately the same shape as the test electrode to prepare each counter electrode. Also, LiPF6 was dissolved at a concentration of 1 mol / L in a mixed solvent of equal volumes of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio to prepare a non-aqueous electrolyte.

[0069] Next, each test electrode was placed in a positive electrode can, and a counter electrode was placed in a negative electrode can, with a polyolefin microporous membrane separator between the test electrode and the counter electrode. Note that each test electrode would be the negative electrode in a lithium-ion battery, but when the counter electrode was a Li foil, the Li foil became the negative electrode and the test electrode became the positive electrode.

[0070] Next, the non-aqueous electrolyte solution was poured into each can, and the negative electrode can and the positive electrode can were fixed by crimping.

[0071] 3. Evaluation of negative electrode material powder 3-1. Confirmation of the constituent phases of the negative electrode material powder The negative electrode material powders prepared in each of the examples and comparative examples were analyzed by XRD (X-ray diffraction) and confirmed to contain Si phase, SiX compound phase, and SnY compound phase. The XRD analysis was performed using a Co tube over an angle range of 120° to 20°.

[0072] 3-2. Calculation of the proportions of Si phase, SiX compound phase, and SnY compound phase The method of calculating the proportions of the Si phase, SiX compound phase, and SnY compound phase shown in Tables 2 and 3 below will be explained using Example 1 as an example. (1) First, the constituent phases of the produced negative electrode material powder were confirmed. In the case of Example 1, the XRD analysis confirmed the presence of Si, Si2Fe, and Sn5Cu6. (2) Si2Fe, expressed as a mass percentage, is 50.1 [Si] - 49.9 [Fe]. Correspondingly, the amount of Si that forms the compound is 17.43 × 50.1 / 49.9 = 17.50%. Therefore, the proportion of the SiX compound phase (Si2Fe) is the sum of the amount of Si that forms the compound (17.50%) and the amount of Fe in Table 1 (17.43%), which is 35% in this example. (3) The proportion of the Si phase is the value obtained by subtracting the amount of compounded Si (17.50%) from the total amount of Si (77.57%) in Table 1, and in this example it is 60%. (4) The proportion of the SnY compound phase is the sum of the Sn content (3.05%) and Cu content (1.96%) in Table 1, which is 5% in this example.

[0073] [Table 2]

[0074] [Table 3]

[0075] 3-3. Calculation of the average particle size and average particle size ratio of the negative electrode material powder The presence of the Si phase, SiX compound phase, and SnY compound phase in the resulting negative electrode powder was confirmed by SEM imaging. The particle size (circle equivalent diameter) of 100 randomly selected particles of each of the Si phase, SiX compound phase, and SnY compound phase, each present in a separate state, was measured from cross-sectional SEM images (magnification: 5000x) of the powder, and the average values ​​were taken as the average particle size mdSi, mdSiX, and mdSnY, respectively. The ratios of the average particle sizes, mdSi / mdSiX and mdSi / mdSnY, were also calculated. These results are shown in Tables 2 and 3. In Tables 2 and 3, the average particle sizes mdSi, mdSiX, and mdSnY are shown as Si, SiX, and SnY in the "average particle size (μm)" column, respectively, and the average particle size ratios expressed as mdSi / mdSiX and mdSi / mdSnY are shown as Si / SiX and Si / SnY in the "average particle size ratio" column, respectively.

[0076] 3-4. Charge / discharge test Each coin-type battery was subjected to one cycle of constant current charging and discharging at a current value of 0.2 mA. The initial discharge capacity C0 (mAh / g) was calculated by dividing the capacity (mAh) used during this Li release by the amount of active material (g). The initial coulombic efficiency (%) was calculated by calculating the ratio of the discharge capacity to the charge capacity in the above charge-discharge cycle as a percentage of discharge capacity / charge capacity.

[0077] The initial discharge capacity (mAh / g) was judged as follows: over 400 and 500 or less: "◎", over 350 and 400 or less: "○", over 300 and 350 or less: "△", over 250 and 300 or less: "×", and 250 or less: "XX". The results are shown in Tables 2 and 3.

[0078] The initial coulombic efficiency was judged as follows: over 95% was marked "◎", over 80% to 95% was marked "○", over 70% to 80% was marked "△", over 60% to 70% was marked "×", and under 60% was marked "XX". The results are shown in Tables 2 and 3.

[0079] From the second cycle onward, the charge-discharge test was performed at a 1 / 5C rate. Here, the current value required to charge and discharge the electrode at a C rate, C0, is defined as the current value required to charge and discharge the electrode in 1 hour. That is, at 5C, it takes 12 minutes, and at 1 / 5C, it takes 5 hours. The cycle characteristics were evaluated by performing the above charge-discharge cycle 100 times. The capacity retention rate (discharge capacity after 100 cycles / initial discharge capacity (discharge capacity at the first cycle) × 100) was calculated from each discharge capacity obtained. The capacity retention rate was evaluated as follows: over 70%: "◎", over 50% to 70%: "○", over 40% to 50%: "△", over 30% to 40%: "×", and under 30%: "XX". The results are shown in Tables 2 and 3.

[0080] The overall evaluations shown in Tables 2 and 3 are based on the evaluation results of the initial coulombic efficiency, initial discharge capacity, and cycle characteristics. Specifically, for each example and comparative example, if there was no "xx" evaluation in the above three types of characteristic evaluation, it was rated as "pass," and if there was even one "xx" evaluation, it was rated as "fail."

[0081] The results in Tables 2 and 3 obtained as described above reveal the following. Comparative Example 1 is an example in which the proportion of the Si phase in the negative electrode material powder exceeds the upper limit (95%), the ratio of the average particle size (mdSi / mdSiX and mdSi / mdSnY) is greater than the upper limit (5.0), and the cycle characteristics were evaluated as "XX". Comparative Example 2 is an example in which the proportion of the Si phase in the negative electrode material powder is below the lower limit (10%), and the average particle size ratio (mdSi / mdSiX and mdSi / mdSnY) is smaller than the lower limit (0.1), resulting in an evaluation of the initial coulombic efficiency and initial discharge capacity as "XX".

[0082] In Comparative Example 3, the proportion of the SnY compound phase exceeded the upper limit (50%), and the initial coulombic efficiency and initial discharge capacity were evaluated as "XX." In Comparative Example 4, the proportion of the SnY compound phase was below the lower limit (0.07%), and the cycle characteristics were evaluated as "XX".

[0083] In Comparative Example 5, the ratio of average particle diameters (mdSi / mdSiX and mdSi / mdSnY) was smaller than the lower limit (0.1), and the initial coulomb efficiency and initial discharge capacity were evaluated as "XX". In Comparative Example 6, the ratio of average particle diameters (mdSi / mdSiX and mdSi / mdSnY) was greater than the upper limit (5.0), and the cycle characteristics were evaluated as "XX."

[0084] In Comparative Example 7, the average particle sizes of mdSi, mdSiX, and mdSnY were larger than the upper limit (50 μm), and the cycle characteristics were evaluated as "XX." In Comparative Example 8, the average particle sizes of mdSi, mdSiX, and mdSnY were smaller than the lower limit (0.1 μm), and the initial coulomb efficiency and initial discharge capacity were evaluated as "XX."

[0085] As described above, the initial coulombic efficiency, initial discharge capacity, or cycle characteristics of all the comparative examples were evaluated as "XX," and the battery characteristics taking into consideration the initial characteristics and cycle characteristics were not sufficiently improved.

[0086] In contrast, in the negative electrode material powder, the proportions of the Si phase, SiX compound phase, and SnY compound phase were adjusted to fall within predetermined ranges, and the average particle sizes mdSi, mdSiX, and mdSnY of the independently existing Si phase, SiX compound phase, and SnY compound phase were all within the range of 0.1 to 50 μm, and the average particle size ratios expressed as mdSi / mdSiX and mdSi / mdSnY were all within the range of 0.1 to 5.0. In these examples, there was no particularly low evaluation of "XX," and the battery characteristics were improved in consideration of the initial coulombic efficiency, initial discharge capacity, and cycle characteristics.

[0087] Comparing Examples 1 to 8, in which the ratios of the Si phase and the SiX compound phase were varied, it was found that in Examples 1, 2, 6, 7, and 8, in which the ratio of the Si phase was within the range of 30 to 90% and the ratio of the SiX compound phase was within the range of 1 to 70%, the properties were improved in a balanced manner.

[0088] Furthermore, when Examples 1, 13, 14, 15, and 16, which differ in the element Y, are compared, particularly favorable evaluations are obtained when the element Y is Cu and the SnY compound is an SnCu compound. Here, when Examples 17 to 21, in which the proportion of the SnY compound phase is changed, are compared, it is found that the proportion of the SnY compound phase is preferably within the range of 0.07 to 30%.

[0089] The average particle size ratios (mdSi / mdSiX and mdSi / mdSnY) differ between Examples 22 to 27. It can be seen that the various properties are improved in a balanced manner when the average particle size ratio is in the range of 0.3 to 3.0.

[0090] Although the negative electrode material powder for lithium ion batteries and the lithium ion battery of the present invention have been described in detail above, the present invention is not limited to the above-described embodiments and examples. For example, the negative electrode material powder of the present invention can be applied not only to negative electrode material powders for liquid lithium ion batteries as in the above-described embodiments, but also to negative electrode material powders for all-solid-state lithium ion batteries. Various modifications of the present invention are possible within the scope of the gist of the present invention.

[0091] This application is based on a Japanese patent application (Patent Application No. 2021-161675) filed on September 30, 2021, the contents of which are incorporated herein by reference.

Claims

1. Contains Si, Sn, element X, and element Y, A negative electrode material powder for a lithium ion battery, comprising a Si phase, a SiX compound phase, and a SnY compound phase in a phase ratio represented by the following formula (1): the Si phase, the SiX compound phase, and the SnY compound phase exist separately and in separate states, When the particle diameters at 50% of the integrated value in the particle diameter distribution of the Si phase, the SiX compound phase, and the SnY compound phase are defined as average particle diameters mdSi, mdSiX, and mdSnY, respectively, The average particle sizes of mdSi, mdSiX, and mdSnY are all within the range of 0.1 to 50 μm, A negative electrode material powder for lithium ion batteries, in which the ratios of average particle diameters represented by mdSi / mdSiX and mdSi / mdSnY are both within the range of 0.1 to 5.

0. a[Si]-b[SiX]-c[SnY]...Formula (1) However, the element X is one or more elements selected from the group consisting of Fe, Ni, Cr, Zr, and Ti, the element Y is one or more elements selected from the group consisting of Cu, Fe, Ni, Cr, Co, Mn, Zr, and Ti; In the formula (1), a, b, and c respectively represent the contents (mass%) of the Si phase, the SiX compound phase, and the SnY compound phase, where a+b+c=100, 10≦a≦95, 1≦b≦90, and 0.07≦c≦50.

2. 2. The negative electrode material powder for a lithium ion battery according to claim 1, wherein a, b, and c in formula (1) are 30≦a≦90, 1≦b≦70, and 0.1≦c≦30, respectively.

3. the element Y is Cu, 2. The negative electrode material powder for lithium ion batteries according to claim 1, wherein a, b, and c in formula (1) are 30≦a≦90, 1≦b≦70, and 0.1≦c≦30, respectively.

4. 4. The negative electrode material powder for a lithium ion battery according to claim 1, wherein the ratios of average particle sizes represented by mdSi / mdSiX and mdSi / mdSnY are both within the range of 0.3 to 3.0.

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