Negative electrode active material for secondary batteries and secondary batteries

Abstract

This negative electrode active material for a secondary battery includes a composite material containing a silicide phase and a silicon phase. The silicide phase includes an intermetallic compound of a silicon element and a metal element Me other than the silicon element. The metal element Me contains at least N types of constituent elements Mi (i = 1 to N, 5 ≤ N). When the mole fraction of the at least N types of constituent elements Mi is defined as Ci, the condition represented by formula: 1.5 < -ΣCi・InCi is satisfied.

Description

[Technical Field]

[0001] This invention primarily relates to a negative electrode active material for secondary batteries. [Background technology]

[0002] Non-aqueous electrolyte secondary batteries, particularly lithium-ion secondary batteries, are expected to be used in small consumer applications, power storage devices, and as power sources for electric vehicles due to their high voltage and high energy density. Amidst the demand for higher energy density in batteries, the use of alloy materials containing silicon alloyed with lithium is anticipated as a negative electrode active material with high theoretical capacity density (for example, Patent Document 1).

[0003] Patent Document 1 proposes a negative electrode material for a lithium secondary battery containing particles made of an amorphous M·A·X alloy with a substantially non-chemical stoichiometric composition, wherein in the above formula M·A·X, M represents at least one element selected from the group consisting of Si, Ge, and Mg, A represents at least one element selected from the transition metal elements, and X represents at least one element selected from the group consisting of O, F, N, Ba, Sr, Ca, La, Ce, P, S, Se, Te, B, Bi, Sb, Al, In, and Zn, and the content of the constituent element M in the amorphous M·A·X alloy is (M+A+X)=20~80 atomic%.

[0004] Patent Document 2 describes general formula (1): A a Si b M c T d X e R fA negative electrode material for a non-aqueous electrolyte secondary battery, characterized by comprising a crystalline alloy represented by the formula, wherein A is at least one element selected from the group consisting of Mg, Ca and Sr, M is at least one element selected from the group consisting of Co, Ni, Fe, Cu, Mn, V and Cr, T is at least one element selected from the group consisting of Ti, Zr, Hf, Nb, Ta, Mo and W, X is at least one element selected from the group consisting of Sn, Al, Sb, Zn and In, R is at least one element selected from rare earth elements, and a, b, c, d, e, f satisfy a+b+c+d+e+f=100 atomic%, 10≦a≦50, 35≦b≦65, 0<c≦30, 0≦d≦10, 0≦e≦25, and 0≦f≦20, respectively.

[0005] Patent Document 3 proposes a negative electrode active material for a non-aqueous electrolyte secondary battery formed of an alloy of Si and a metal element that does not alloy with Li, wherein the proportion of the metal element that does not alloy with Li is 15 atomic% or more and 35 atomic% or less in terms of atomic ratio, and in X-ray diffraction measurement using CuKα radiation, no diffraction peak originating from a component of the alloy and having a half width of 1.5° or less is present in a diffraction angle 2θ range of 20 to 60°. [Prior Art Document] [Patent Documents]

[0006] [Patent Document 1] International Publication No. WO 2000 / 017949 (Japanese Patent No. 3733292 Specification) [Patent Document 2] Japanese Unexamined Patent Publication No. 2005-100876 (Japanese Patent No. 4703110 Specification) [Patent Document 3] International Publication No. WO 2015 / 060443 [Summary of the Invention] [Problem to be Solved by the Invention]

[0007] On the other hand, it is known that alloy-based materials as proposed in Patent Documents 1 to 3 undergo large volume changes accompanying the insertion and desorption of lithium ions (charging and discharging). [Means for Solving the Problem]

[0008] One aspect of the present invention relates to a negative electrode active material for a secondary battery, the negative electrode active material comprising a composite material including a silicide phase and a silicon phase, wherein the silicide phase includes an intermetallic compound of elemental silicon and a metal element Me other than elemental silicon, the metal element Me includes N or more constituent elements Mi (i=1 to N, 5≦N), where Ci is the mole fraction of the N or more constituent elements Mi, and the condition represented by the formula: 1.5 < -ΣCi·lnCi is satisfied.

[0009] Another aspect of the present invention relates to a secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, wherein the negative electrode includes the above-described negative electrode active material for a secondary battery. [Effects of the Invention]

[0010] The present invention provides a negative electrode active material for a secondary battery that exhibits small volume changes accompanying the insertion and desorption of lithium ions. The novel features of the present invention are set forth in the appended claims. The present invention will be better understood from the following detailed description taken with the drawings, both with respect to its constitution and content, together with other objects and features of the invention. [Brief Description of the Drawings]

[0011] [Figure 1] It is a diagram showing the charge-discharge curve of the cell of the example. [Figure 2] It is a diagram showing charge-discharge curves of cells having different Si contents in the negative electrode active material. [Figure 3] It is a diagram showing an X-ray diffraction pattern of a negative electrode active material. [Mode for Carrying Out the Invention]

[0012] The embodiments of this disclosure will be described below with examples, but this disclosure is not limited to the examples described below. In the following description, specific numerical values, materials, etc. may be given as examples, but other numerical values, materials, etc. may be applied as long as the effects of this disclosure are obtained. In addition, components other than those characteristic of this disclosure may be replaced with components of known secondary batteries. In this specification, when "the range of numerical values ​​A to numerical values ​​B" is mentioned, the range includes numerical values ​​A and B. In the following description, when lower and upper limits of numerical values ​​relating to specific physical properties or conditions are given as examples, either of the given lower limits and either of the given upper limits may be arbitrarily combined as long as the lower limit does not exceed the upper limit. When multiple materials are given as examples, one of them may be selected and used alone, or two or more may be used in combination.

[0013] Furthermore, this disclosure encompasses any combination of matters described in two or more claims, which may be arbitrarily selected from the multiple claims set forth in the attached claims. In other words, any combination of matters described in two or more claims, which may be arbitrarily selected from the multiple claims set forth in the attached claims, is possible, provided that no technical inconsistency arises.

[0014] Secondary batteries include, at a minimum, non-aqueous electrolyte secondary batteries such as lithium-ion batteries, and all-solid-state batteries.

[0015] [Composite materials] An embodiment of the present invention provides a negative electrode active material for a secondary battery, which includes a composite material comprising a silicide phase and a silicon phase. The silicide phase is a phase composed of silicide, and the silicon phase is a phase composed of silicon (silicon element).

[0016] The silicide phase according to this embodiment contains a mixture of five or more silicides. Specifically, the silicide phase contains an intermetallic compound of silicon and a metal element other than silicon, Me. The metal element Me contains N or more constituent elements Mi (i=1 to N, 5≦N).

[0017] Here, if we let Ci be the mole fraction of N or more constituent elements Mi (where ΣCi=1), Formula: 1.5<-ΣCi·lnCi The condition expressed by is satisfied.

[0018] Hereafter, the condition under which the above equation (1.5 < -ΣCi·lnCi) is satisfied will also be called the high-entropy condition. In this case, the entropy S representing the state of N or more constituent elements Mi is given by S = -RΣCi·lnCi, and S > 1.5R is satisfied. R is the gas constant (J / (mol·K)).

[0019] In the silicide phase, when the constituent elements Mi of the metallic element Me (N or higher) satisfy the high-entropy condition, the mechanical strength of the silicide phase tends to increase significantly. In a composite material containing both a silicide phase and a silicon phase, when the silicon phase undergoes a large volume change due to the insertion and removal of lithium ions, the silicide phase significantly suppresses the volume change of the composite material.

[0020] The silicon content in the composite material can be, for example, more than 50 atomic percent, and may be 70 atomic percent or more, or even 75 atomic percent or more. The higher the silicon content in the composite material, the greater the capacity. On the other hand, the content of metal elements other than silicon, such as Me, in the composite material is preferably 10 atomic percent or more, and may be 20 atomic percent or more. This ensures a considerable amount of silicide phase in the composite material, thus more significantly suppressing volume changes in the composite material. When the atomic ratio Si:Me = 1:1, it is thought that most of the composite material becomes the silicide phase, resulting in a smaller capacity. On the other hand, when the atomic ratio Si:Me = (3~4):1, a sufficient amount of silicon phase is ensured in the composite material, and a capacity of, for example, 1380 mA / g to 1640 mAh / g can be secured.

[0021] The average atomic weight of the constituent element Mi (Mi) of type N or higher is, for example, 60 or less. The smaller the average atomic weight, the higher the capacity density per unit mass of the composite material, making it promising as a negative electrode active material. If the average atomic weight is 60 or less, the composite material can achieve a higher capacity density than, for example, SiO (silicon oxide). However, from the viewpoint of improving charge-discharge efficiency, it is desirable for the average atomic weight to be, for example, 50 or higher. The reason is not clear, but the charge-discharge efficiency of the composite material tends to be higher as the average atomic weight increases.

[0022] The average atomic weight is expressed as ΣAi·Ci, where Ai is the atomic weight and Ci is the mole fraction of the constituent elements Mi (of type N or higher).

[0023] The constituent element Mi of type N or higher may each be contained in at least one of MiSi and MiSi2. In other words, the silicide phase may be composed of intermetallic compounds such as MeSi and MeSi2.

[0024] It is desirable that substantially all of the constituent elements Mi of type N or higher form intermetallic compounds. In this case, the total content of elemental Mi of type N or higher in the composite material is 1% by mass or less, or 10,000 ppm or less, and may be substantially 0% by mass. It is desirable that all of the constituent elements Mi that satisfy the high-entropy condition form intermetallic compounds in this way in order to increase the strength of the silicide phase.

[0025] The constituent elements Mi of type N or higher may all be elements with atomic weights in the fourth period or lower of the long-period periodic table. In this case, the average atomic weight of the constituent elements Mi of type N or higher becomes relatively small, making it easier to increase the capacity density of the composite material.

[0026] The constituent elements Mi of type N or higher may be, for example, at least five selected from the group consisting of Al, Mg, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Mo.

[0027] The constituent elements Mi of type N or higher are all atomic numberElements with a molecular weight of 29(Cu) or less may also be used. In this case, the average atomic weight of constituent elements Mi of type N or higher becomes even smaller, making it easier to further increase the capacity density of the composite material.

[0028] The constituent elements Mi of type N or higher may be at least five selected from the group consisting of Al, Mg, Ti, V, Cr, Mn, Fe, Co, Ni, and Cu.

[0029] The constituent elements Mi of type N or higher may all be transition metals. In this case, it becomes easier to improve the charge-discharge efficiency of the composite material.

[0030] The constituent elements Mi of type N or higher may be, for example, at least five selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, and Cu.

[0031] The composition of composite materials can be measured, for example, by inductively coupled plasma atomic emission spectroscopy (ICP). For instance, a sample of the composite material is completely dissolved in a heated acid solution (e.g., a mixed acid of hydrofluoric acid, nitric acid, and sulfuric acid), the solution residue is filtered out, and then the material is analyzed by ICP to measure the spectral intensity of each element. Subsequently, a calibration curve is created using commercially available elemental standard solutions, and the content of each element in the composite material is calculated.

[0032] The presence of both a silicide phase and a silicon phase in a composite material can be confirmed by photographing and observing a cross-section of the composite material using a scanning electron microscope (SEM). Cross-sectional observation of composite particles can be performed, for example, by the following method: First, a cured product of a thermosetting resin filled with composite material particles is formed. A cross-section of the cured product is formed using a cross-section polisher (CP) and observed with an SEM. Furthermore, elemental mapping analysis of the cross-section of the composite material particles using energy-dispersive X-ray (EDX) allows for quantitative and qualitative analysis of the silicide phase and the silicon phase.

[0033] Furthermore, it can be confirmed by performing X-ray diffraction measurements of the composite material that virtually all of the constituent elements Mi of type N or higher form intermetallic compounds, and that the content of these elemental metals is substantially 0% by mass.

[0034] [Method for manufacturing composite materials] Composite materials can be manufactured by weighing metal ingots of each element, which are the raw materials, in a predetermined atomic ratio, melting them simultaneously to form a molten metal, and then cooling the molten metal. It is preferable to melt the raw materials in an arc melting furnace under an inert gas atmosphere such as argon. Upon cooling the molten metal, a composite material with the desired composition is obtained as a metal ingot.

[0035] In some cases, the crystallization of silicides (intermetallic compounds) may not have progressed sufficiently within the metal ingot obtained by cooling the molten metal. Therefore, 2.0 × 10 -3 It is preferable to heat and anneal the metal ingot in a reduced pressure or vacuum atmosphere below Pa. The heating temperature during annealing is not particularly limited, but may be between 600°C and 900°C. The heating time during annealing is not particularly limited, but may be between 10 hours and 120 hours.

[0036] [Secondary battery] A secondary battery according to an embodiment of the present invention comprises a positive electrode, a negative electrode, and an electrolyte, wherein the negative electrode includes the above-mentioned composite material as at least a part of the negative electrode active material. Hereinafter, secondary batteries will be described in detail using a lithium-ion secondary battery as an example.

[0037] [Negative electrode] The negative electrode may comprise a negative electrode current collector and a negative electrode mixture layer supported on the surface of the negative electrode current collector. The negative electrode mixture layer can be formed by coating the surface of the negative electrode current collector with a negative electrode slurry, which is obtained by dispersing the negative electrode mixture in a dispersion medium, and drying it. The dried coating may be rolled if necessary.

[0038] The negative electrode mixture contains the above-mentioned composite material as an essential component of the negative electrode active material, and may also contain binders, conductive agents, thickeners, etc., as optional components.

[0039] The negative electrode active material may also include, in addition to composite materials, materials that electrochemically intercalate and release lithium ions. Such materials are not particularly limited, but carbon materials can be cited. Examples of carbon materials include graphite, easily graphitizable carbon (soft carbon), and poorly graphitizable carbon (hard carbon).

[0040] As the negative electrode current collector, non-porous conductive substrates (such as metal foil) and porous conductive substrates (such as mesh, net, or perforated sheet) are used. Examples of materials for the negative electrode current collector include stainless steel, nickel, nickel alloys, copper, and copper alloys.

[0041] Examples of binders include resin materials such as fluororesins like polytetrafluoroethylene and polyvinylidene fluoride (PVDF); polyolefin resins like polyethylene and polypropylene; polyamide resins like aramid resins; polyimide resins like polyimide and polyamideimide; acrylic resins like polyacrylic acid, methyl polyacrylate, and ethylene-acrylic acid copolymers; vinyl resins like polyacrylonitrile and polyvinyl acetate; polyvinylpyrrolidone; polyethersulfone; and rubber-like materials like styrene-butadiene copolymer rubber (SBR). A single binder may be used alone, or two or more may be used in combination.

[0042] Examples of conductive agents include carbon compounds such as acetylene black; conductive fibers such as carbon fibers and metal fibers; carbon fluoride; metal powders such as aluminum; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and organic conductive materials such as phenylene derivatives. Conductive agents may be used individually or in combination of two or more.

[0043] Examples of the thickener include carboxymethyl cellulose (CMC) and modified products thereof (including salts such as Na salts), cellulose derivatives such as methyl cellulose (cellulose ethers, etc.); saponified products of polymers having vinyl acetate units such as polyvinyl alcohol; polyethers (polyalkylene oxides such as polyethylene oxide, etc.); and the like. One type of thickener may be used alone, or two or more types may be used in combination.

[0044] [Positive Electrode] The positive electrode may comprise a positive electrode current collector and a positive electrode mixture layer supported on the surface of the positive electrode current collector. The positive electrode mixture layer can be formed by applying a positive electrode slurry, which is obtained by dispersing a positive electrode mixture in a dispersion medium, onto the surface of the positive electrode current collector and drying the applied slurry. The dried coating film may be rolled if necessary.

[0045] The positive electrode mixture contains a positive electrode active material as an essential component, and can contain a binder, a conductive agent and the like as optional components.

[0046] As the positive electrode active material, for example, a lithium-containing composite oxide can be used. For example, Li a CoO2, Li a NiO2, Li a MnO2, Li a Co b Ni 1-b O2, Li a Co b Me 1-b O c , Li a Ni 1-b Me b O c , Li a Mn2O4, Li a Mn 2-b Me b O 4、 LiMePO 4、One example is Li2MePO4F (where Me is at least one element selected from the group consisting of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, and B). Here, a = 0 to 1.2, b = 0 to 0.9, and c = 2.0 to 2.3. Note that the value of a, which indicates the molar ratio of lithium, increases or decreases with charging and discharging.

[0047] The same binders and conductive agents as those exemplified for the negative electrode can be used. Natural graphite, artificial graphite, or other types of graphite may be used as the conductive agent.

[0048] The shape and thickness of the positive electrode current collector can be selected from the same shape and range as the negative electrode current collector. Examples of materials for the positive electrode current collector include stainless steel, aluminum, aluminum alloy, and titanium.

[0049] [Electrolyte] The electrolyte comprises a solvent and a lithium salt dissolved in the solvent. The concentration of the lithium salt in the electrolyte is preferably, for example, 0.5 mol / L or more and 2 mol / L or less. By setting the lithium salt concentration within this range, an electrolyte with excellent ionic conductivity and appropriate viscosity can be obtained. However, the lithium salt concentration is not limited to the above.

[0050] The solvent used may be an aqueous or non-aqueous solvent. Examples of non-aqueous solvents include cyclic carbonate esters, linear carbonate esters, cyclic carboxylic acid esters, and linear carboxylic acid esters. Examples of cyclic carbonate esters include propylene carbonate (PC) and ethylene carbonate (EC). Examples of linear carbonate esters include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC). Examples of cyclic carboxylic acid esters include γ-butyrolactone (GBL) and γ-valerolactone (GVL). Examples of linear carboxylic acid esters include methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, and propyl propionate. The non-aqueous solvent may be used alone or in combination of two or more types.

[0051] Examples of lithium salts include LiClO4, LiBF4, LiPF6, LiAlCl4, LiSbF6, LiSCN, LiCF3SO3, LiCF3CO2, LiAsF6, LiB 10 Cl 10 Examples include lithium lower aliphatic carboxylates, LiCl, LiBr, LiI, borates, and imide salts. Examples of imide salts include LiN(FSO2)2, LiN(CF3SO2)2, LiN(CF3SO2)(C4F9SO2), and LiN(C2F5SO2)2. Lithium salts may be used individually or in combination of two or more types.

[0052] [Separator] Generally, it is desirable to interpose a separator between the positive and negative electrodes. The separator should have high ion permeability and appropriate mechanical strength and insulating properties. As the separator, a microporous thin film, woven fabric, nonwoven fabric, etc., can be used. As the material of the separator, polyolefins such as polypropylene and polyethylene are preferred.

[0053] One example of a secondary battery structure is one in which an electrode group and an electrolyte are housed in an outer casing. The electrode group may be a wound type in which the positive and negative electrodes are wound around a separator, or a laminated type in which the positive and negative electrodes are stacked around a separator, or it may take any other form. The secondary battery may take any form, such as cylindrical, prismatic, coin-type, button-type, or laminated type.

[0054] The present invention will be described in detail below based on examples and comparative examples, but the present invention is not limited to the following examples.

[0055] <Example 1> [Manufacturing of composite materials] A composite material was prepared that satisfies the high-entropy condition, with an atomic ratio of Si:Me = 4:1 (i.e., Si content of 80 atoms), Me being composed of equimolar amounts of five transition metals: Ti, V, Cr, Zr, and Mo, and an average atomic weight of Me of 68. Specifically, the raw materials were placed on a water-cooled copper hearth in an arc melting furnace, the furnace was replaced with an argon atmosphere, and the raw materials were melted and rapidly cooled by the arc melting method to obtain a button-shaped metal ingot. Since the surface in contact with the water-cooled copper hearth is always in a cooled state, the sample cools instantly when the arc irradiation is stopped. The obtained metal ingot was inverted in the furnace using an inversion rod, melted and rapidly cooled again by the arc melting method, and this process of inversion and melting and rapidly cooling was repeated five times to obtain a composite material metal ingot. Next, the obtained metal ingot was 2.0 × 10⁻⁶ -3 The metal was sealed in a quartz tube under a vacuum of less than Pa and annealed at 800°C for 48 hours to allow sufficient crystallization to proceed. After annealing, the metal mass was crushed in a mortar to a particle size of 25-45 μm and used as the negative electrode active material.

[0056] [Fabrication of the negative electrode] A negative electrode slurry was prepared by mixing a negative electrode mixture containing composite material, carbon black, SBR, and CMC in a mass ratio of 95:0.5:1.5:3.0 with an appropriate amount of water. Next, a 1 m² layer was applied to the surface of the copper foil. 2The negative electrode slurry is applied so that the mass of the negative electrode mixture per unit is 150g. After the coating is dried, it is rolled out to form a copper foil with a density of 2g / cm³ on one side. 3 An electrode plate with a negative electrode mixture layer was fabricated. The electrode plate was punched out into a circle with a diameter of 12.5 mm and used as the negative electrode.

[0057] [Preparation of the opposing pole] A counter electrode was fabricated by punching out a 300 μm thick metallic lithium foil into a 17 mm diameter circle.

[0058] [Preparation of non-aqueous electrolytes] A non-aqueous electrolyte was prepared by dissolving LiPF6 at a concentration of 1.0 mol / L in a mixed solvent containing ethylene carbonate and ethyl methyl carbonate in a volume ratio of 1:3.

[0059] [Fabrication of coin-shaped cells] A bottomed cell case made of stainless steel with an opening was prepared, and the negative electrode and separator were placed inside in that order. A nonwoven fabric made of polyphenylene sulfide (PPS) with a thickness of 0.45 mm was used for the separator. On the other hand, a stainless steel sealing plate with a polypropylene gasket around the periphery was prepared, and the lithium foil for the counter electrode was attached to its inner surface. After injecting the electrolyte into the cell case, the opening of the cell case was sealed with the sealing plate to complete cell A1. The cell size was 20 mm in diameter and 3.2 mm in thickness.

[0060] [Charge / Discharge Curve] For assembled cell A1, the negative electrode was charged to 0.01V at 0.1mA at 25°C, and then discharged to 1.5V. The first charge-discharge curve is shown in Figure 1. The first discharge capacity and the ratio of discharge capacity to charge capacity (charge-discharge efficiency) are shown in Table 1. From Figure 1 and Table 1, it can be seen that cell A1 has a capacity density of 1047mAh / g. Furthermore, the charge-discharge efficiency was high at 87%.

[0061] <Example 2> Using the same method as in Example 1, a composite material was prepared with an atomic ratio of Si:Me = 4:1, where Me is composed of equimolar amounts of five transition metals: Cr, Mn, Fe, Ni, and Co, and the average atomic weight of Me is 56, satisfying the high-entropy condition. Cell A2 was assembled and evaluated. The first charge-discharge curve is shown in Figure 1. The first discharge capacity and the ratio of discharge capacity to charge capacity (charge-discharge efficiency) are shown in Table 1. From Figure 1 and Table 1, it can be seen that cell A2 has a capacity density of 1457 mAh / g. The charge-discharge efficiency was 82%.

[0062] <Example 3> Using the same method as in Example 1, a composite material was prepared with an atomic ratio of Si:Me = 4:1, where Me is composed of equimolar amounts of five transition metals: Cr, Mn, Fe, Ni, and Cu, and the average atomic weight of Me is 57, satisfying the high-entropy condition. Cell A3 was assembled and evaluated. The first charge-discharge curve is shown in Figure 1. The first discharge capacity and the ratio of discharge capacity to charge capacity (charge-discharge efficiency) are shown in Table 1. From Figure 1 and Table 1, it can be seen that cell A3 has a capacity density of 1690 mAh / g. The charge-discharge efficiency was 84%.

[0063] <Example 4> Using the same method as in Example 1, a composite material was prepared with an atomic ratio of Si:Me = 4:1, where Me is composed of equimolar amounts of five transition metals: Al, Mg, Ti, Fe, and Ni, and the average atomic weight of Me is 57, satisfying the high-entropy condition. Cell A4 was assembled and evaluated. The first charge-discharge curve is shown in Figure 1. The first discharge capacity and the ratio of discharge capacity to charge capacity (charge-discharge efficiency) are shown in Table 1. From Figure 1 and Table 1, it can be seen that cell A4 has a capacity density of 1798 mAh / g. The charge-discharge efficiency was 80%.

[0064] [Table 1]

[0065] <Example 5> Except for using a mortar and pestle to grind the annealed metal mass to a particle size of 25 μm or less, a composite material with the same atomic ratio Si:Me = 3:1 (i.e., Si content of 75 atomic%) and the same Me composition as in Example 2 was prepared using the same method as in Example 1, and cell A5 was assembled and evaluated. The first discharge capacity and the ratio of discharge capacity to charge capacity (charge / discharge efficiency) are shown in Table 2.

[0066] <Example 2A> Cell A22 was assembled and evaluated in the same manner as in Example 2, except that the annealed metal mass was crushed to a particle size of 25 μm or less in a mortar. The first discharge capacity and the ratio of discharge capacity to charge capacity (charge / discharge efficiency) are shown in Table 2.

[0067] [Table 2]

[0068] Figure 2 shows a comparison of the charge-discharge curves for cells A22 and A5. A sufficiently high-capacity composite material is obtained even with an atomic ratio of Si:Me = 3:1. Furthermore, the capacity is significantly larger with an atomic ratio of Si:Me = 4:1 than with the Si:Me = 3:1 case. On the other hand, charge-discharge efficiency is better with a lower silicon phase content.

[0069] The difference in volume between cell A2 in Table 1, Figure 1 and cell A22 in Table 2, Figure 2 is thought to be due to the difference in particle size. It is believed that the smaller the particle size, the higher the utilization rate of the volume.

[0070] [X-ray diffraction analysis] Figure 3 shows the X-ray diffraction patterns obtained from the X-ray diffraction analysis of Examples 2 and 5. In the X-ray diffraction patterns, only peaks attributed to the silicon phase and peaks attributed to the silicide are observed, and no peaks attributed to elemental Mi, a constituent element of the metallic element Me, are observed. From this data, it can be understood that all of the constituent elements Mi of the metallic element Me form intermetallic compounds.

[0071] Furthermore, when the cross-section of the composite material of Example 2 was observed using SEM with the method described above, it was confirmed that the composite material was substantially composed only of the silicide phase and the silicon phase. In addition, it was confirmed that the composite materials of Examples 1 to 5 have very high hardness as silicides and possess physical properties that make them resistant to volume changes. [Industrial applicability]

[0072] The negative electrode active material for secondary batteries according to the present invention is a promising negative electrode active material for secondary batteries used in mobile applications (electric vehicles, mobile communication devices, portable electronic devices, etc.).

Claims

1. The composite material includes a silicide phase and a silicon phase. The silicide phase comprises an intermetallic compound of silicon and a metal element other than silicon, Me. The aforementioned metallic element Me includes N or more constituent elements Mi (i = 1 to N, 5 ≤ N), When the mole fraction of the above N or more constituent elements Mi is Ci, Formula: 1.5<-ΣCi・lnCi The condition expressed by is satisfied, A negative electrode active material for a secondary battery, wherein all of the constituent elements Mi of type N or more are elements having atomic weights in the fourth period or lower of the long-period periodic table.

2. The negative electrode active material for a secondary battery according to claim 1, wherein the silicon element content in the composite material is greater than 50 atomic percent.

3. The negative electrode active material for a secondary battery according to claim 1, wherein the average atomic weight of the constituent elements Mi of type N or more is 60 or less.

4. The above N or more constituent elements Mi are MiSi and MiSi, respectively. ２ A negative electrode active material for a secondary battery according to any one of claims 1 to 3, which is included in at least one of the above.

5. The negative electrode active material for a secondary battery according to any one of claims 1 to 3, wherein the content of individual constituent elements Mi of type N or more in the composite material is 1% by mass or less.

6. The negative electrode active material for a secondary battery according to any one of claims 1 to 3, wherein all of the constituent elements Mi of type N or more are elements with atomic numbers of 29 (Cu) or less.

7. The negative electrode active material for a secondary battery according to any one of claims 1 to 3, wherein all of the N or more constituent elements Mi are transition metals.

8. The negative electrode active material for a secondary battery according to any one of claims 1 to 3, wherein the N or more constituent elements Mi are at least five selected from the group consisting of Al, Mg, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Mo.

9. The negative electrode active material for a secondary battery according to any one of claims 1 to 3, wherein the N or more constituent elements Mi are at least five selected from the group consisting of Al, Mg, Ti, V, Cr, Mn, Fe, Co, Ni, and Cu.

10. The negative electrode active material for a secondary battery according to any one of claims 1 to 3, wherein the N or more constituent elements Mi are at least five selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, and Cu.

11. It comprises a positive electrode, a negative electrode, and an electrolyte. The negative electrode comprises a negative electrode active material for a secondary battery as described in any one of claims 1 to 3, in a secondary battery.

12. A composite material comprising a silicide phase and a silicon phase, The silicide phase comprises an intermetallic compound of silicon and a metal element other than silicon, Me. The aforementioned metallic element Me includes N or more constituent elements Mi (i = 1 to N, 5 ≤ N), When the mole fraction of the above N or more constituent elements Mi is Ci, Formula: 1.5<-ΣCi・lnCi The condition expressed by is satisfied, A negative electrode active material for a secondary battery, wherein all of the constituent elements Mi of type N or higher are elements with atomic numbers of 29 (Cu) or less.

13. A composite material comprising a silicide phase and a silicon phase, The silicide phase comprises an intermetallic compound of silicon and a metal element other than silicon, Me. The aforementioned metallic element Me includes N or more constituent elements Mi (i = 1 to N, 5 ≤ N), When the mole fraction of the above N or more constituent elements Mi is Ci, Formula: 1.5<-ΣCi・lnCi The condition expressed by is satisfied, A negative electrode active material for a secondary battery, wherein the N or more constituent elements Mi are at least five selected from the group consisting of Al, Mg, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Mo.

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