Negative electrode active material for secondary battery, and secondary battery

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

Negative electrode active material for secondary battery and secondary battery

[0001] The present invention mainly relates to a negative electrode active material for a secondary battery.

[0002] Non-aqueous electrolyte secondary batteries, particularly lithium-ion secondary batteries, have high voltages and high energy densities and are therefore expected to be used in small consumer applications, power storage devices, and electric vehicles. As batteries with higher energy densities are required, the use of alloy-based materials containing silicon, which alloys with lithium, as negative electrode active materials with high theoretical capacity densities is anticipated (see, for example, Patent Document 1).

[0003] Patent Document 1 proposes an electrode material for a negative electrode of a lithium secondary battery containing particles of an amorphous M.A.X alloy having a substantially non-stoichiometric composition, in which in the 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 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 component M in the amorphous M.A.X alloy is (M+A+X)=20 to 80 atomic %.

[0004] Patent Document 2 discloses a compound of the general formula (1): A a Si b M c T d X e R fwherein 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, and f satisfy the following relationships: 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, which comprises an alloy of Si and a metal element that does not form an alloy with Li, wherein the proportion of the metal element that does not form an alloy with Li is 15 atomic % or more and 35 atomic % or less in atomic ratio, and wherein, in X-ray diffraction measurement using CuKα rays, the material does not have a diffraction peak with a half-value width of 1.5° or less that is attributable to the alloy components within a diffraction angle 2θ range of 20 to 60°.

[0006] International Publication No. 2000 / 017949 (Patent No. 3733292), JP 2005-100876 A (Patent No. 4703110), International Publication No. 2015 / 060443

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

[0008] One aspect of the present invention relates to a negative electrode active material for a secondary battery, which includes 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, and the metal element Me includes N or more constituent elements Mi (i = 1 to N, 5 ≤ N), and where Ci is the molar fraction of the N or more constituent elements Mi, the condition represented by the formula: 1.5 < -ΣCi lnCi is satisfied.

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

[0010] The present invention provides a negative electrode active material for a secondary battery that exhibits small volume changes due to the insertion and extraction of lithium ions. The novel features of the present invention are set forth in the appended claims, but the present invention, both in terms of structure and content, together with other objects and features of the present invention, will be better understood from the following detailed description taken in conjunction with the drawings.

[0011] 1 is a diagram showing charge-discharge curves of a cell of an example, a diagram showing charge-discharge curves of cells having different Si contents in a negative electrode active material, and a diagram showing an X-ray diffraction pattern of a negative electrode active material.

[0012] Hereinafter, embodiments of the present disclosure will be described using examples, but the present disclosure is not limited to the examples described below. In the following description, specific numerical values, materials, etc. may be exemplified. However, other numerical values, materials, etc. may be applied as long as the effects of the present disclosure are obtained. Note that components other than those characteristic of the present disclosure may be those of known secondary batteries. In this specification, when a "range from numerical value A to numerical value B" is mentioned, the range includes numerical value A and numerical value B. In the following description, when lower and upper limits of numerical values ​​related to specific physical properties or conditions are exemplified, any of the exemplified lower limits and any of the exemplified upper limits can be arbitrarily combined, as long as the lower limit is not equal to or greater than the upper limit. When multiple materials are exemplified, one of the materials may be selected and used alone, or two or more of the materials may be used in combination.

[0013] The present disclosure also encompasses combinations of two or more features arbitrarily selected from the appended claims, i.e., two or more features arbitrarily selected from the appended claims can be combined unless a technical contradiction arises.

[0014] The secondary battery includes at least a non-aqueous electrolyte secondary battery such as a lithium ion battery, an all-solid-state battery, and the like.

[0015] [Composite Material] The negative electrode active material for a secondary battery according to an embodiment of the present invention includes a composite material containing 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 types of silicide. Specifically, the silicide phase contains an intermetallic compound of silicon and a metal element Me other than silicon. The metal element Me contains N or more types of constituent elements Mi (i = 1 to N, 5 ≤ N).

[0017] Here, when the molar fraction of N or more constituent elements Mi is Ci (where ΣCi=1), the condition expressed by the formula: 1.5<−ΣCi·lnCi is satisfied.

[0018] Hereinafter, the condition where the above formula (1.5<-ΣCi·lnCi) is satisfied is also referred to as the high-entropy condition. In this case, the entropy S, which represents the state of N or more constituent elements Mi, is expressed as S = -RΣCi·lnCi, and S > 1.5R is satisfied, where R is the gas constant (J / (mol·K)).

[0019] In a silicide phase, when N or more constituent elements Mi of a metal element Me satisfy the high-entropy condition, the mechanical strength of the silicide phase tends to be significantly increased. In a composite material containing a silicide phase and a silicon phase, when the silicon phase attempts to undergo a large volume change due to the insertion and desorption of lithium ions, the silicide phase significantly suppresses the volume change of the composite material.

[0020] The silicon content in the composite material is, for example, greater than 50 atomic %, and may be 70 atomic % or greater, or even 75 atomic % or greater. The higher the silicon content in the composite material, the greater the capacity. On the other hand, the content of the metal element Me other than silicon in the composite material is preferably 10 atomic % or greater, and may be 20 atomic % or greater. This ensures a significant amount of silicide phase in the composite material, thereby more significantly suppressing volume change in the composite material. When the atomic ratio Si:Me = 1:1, it is believed that most of the composite material will be in the silicide phase, resulting in a small 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 ensured.

[0021] The average atomic weight (average atomic weight) of the N or more constituent elements Mi is, for example, 60 or less. The smaller the average atomic weight, the higher the capacity density per mass of the composite material, making it a promising anode 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 that the average atomic weight be, for example, 50 or more. Although the reason is not clear, 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 of N or more constituent elements Mi and Ci is the molar fraction.

[0023] The N or more constituent elements Mi may be contained in at least one of MiSi and MiSi2. In other words, the silicide phase may be composed of an intermetallic compound such as MeSi or MeSi2.

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

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

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

[0027] The N or more constituent elements Mi may all be elements having an atomic weight of 29 (Cu) or less. In this case, the average atomic weight of the N or more constituent elements Mi becomes even smaller, making it easier to further increase the capacity density of the composite material.

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

[0029] The N or more constituent elements Mi may all be transition metals, which facilitates increasing the charge / discharge efficiency of the composite material.

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

[0031] The composition of the composite material can be measured, for example, by inductively coupled plasma optical emission spectroscopy (ICP). For example, 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), and the solution residue is filtered to remove it. The sample is then analyzed by ICP to measure the spectral intensity of each element. Subsequently, a calibration curve is created using commercially available standard solutions of the elements, and the content of each element contained in the composite material is calculated.

[0032] The presence of 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 with a scanning electron microscope (SEM). Observation of the cross section of a composite particle can be performed, for example, by the following method. First, a cured thermosetting resin filled with particles of the composite material is formed. A cross section of the cured product is formed using a cross-section polisher (CP) and observed with an SEM. Furthermore, quantitative and qualitative analysis of the silicide phase and the silicon phase can be performed by performing elemental mapping analysis using energy dispersive X-rays (EDX) on the cross section of the particles of the composite material.

[0033] Furthermore, it can be confirmed by performing X-ray diffraction measurement of the composite material that substantially all of the N or more constituent elements Mi form intermetallic compounds and that the content of the simple metal elements is substantially 0 mass%.

[0034] [Method for producing a composite material] A composite material can be produced by weighing out metal ingots of each element as raw materials so as to have a predetermined atomic ratio, melting them together to form a molten metal, and then cooling the molten metal. The raw materials are desirably melted using an arc melting furnace in an inert gas atmosphere such as argon. When the molten metal is cooled, a composite material having the desired composition is obtained as a metal ingot.

[0035] In the metal ingot obtained by cooling the molten metal, the crystallization of the silicide (intermetallic compound) may not have progressed sufficiently. －３ It is desirable to heat and anneal the metal ingot in a reduced pressure or vacuum atmosphere of 100 Pa or less. The heating temperature during annealing is not particularly limited, but may be 600° C. or higher and 900° C. or lower. The heating time during annealing is not particularly limited, but may be 10 hours or higher and 120 hours or lower.

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

[0037] [Negative Electrode] The negative electrode may include 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 applying a negative electrode slurry, in which the negative electrode mixture is dispersed in a dispersion medium, to the surface of the negative electrode current collector and drying the applied layer. The dried coating film may be rolled as necessary.

[0038] The negative electrode mixture contains the above composite material as an essential component of the negative electrode active material, and may contain optional components such as a binder, a conductive agent, and a thickener.

[0039] In addition to the composite material, the negative electrode active material may further include a material that electrochemically absorbs and releases lithium ions. Examples of such materials include, but are not limited to, carbon materials. Examples of carbon materials include graphite, easily graphitizable carbon (soft carbon), and hardly graphitizable carbon (hard carbon).

[0040] The negative electrode current collector may be a non-porous conductive substrate (such as a metal foil) or a porous conductive substrate (such as a mesh, net, or punched sheet). Examples of the material for the negative electrode current collector include stainless steel, nickel, nickel alloys, copper, and copper alloys.

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

[0042] Examples of conductive agents include carbons 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. One type of conductive agent may be used alone, or two or more types may be used in combination.

[0043] Examples of thickeners include carboxymethyl cellulose (CMC) and its modified products (including salts such as Na salts), cellulose derivatives such as methyl cellulose (cellulose ethers, etc.), saponified polymers having vinyl acetate units such as polyvinyl alcohol, polyethers (polyalkylene oxides such as polyethylene oxide, etc.), etc. 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 include 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, in which the positive electrode mixture is dispersed in a dispersion medium, to the surface of the positive electrode current collector and drying the applied layer. The dried coating film may be rolled as necessary.

[0045] The positive electrode mixture contains a positive electrode active material as an essential component, and may 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. ａ CoO ２ , Li ａ NiO ２ , Li ａ MnO ２ , Li ａ Co ｂ Ni １－ｂ O ２ , Li ａ Co ｂ Me １－ｂ O ｃ , Li ａ Ni １－ｂ Me ｂ O ｃ , Li ａ Mn ２ O４ , Li ａ Mn ２－ｂ Me ｂ O ４、 LiMePO ４、 Li ２ MeP.O. ４ F (Me is at least one 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. The value a, which indicates the molar ratio of lithium, increases or decreases with charge and discharge.

[0047] The binder and conductive agent may be the same as those exemplified for the negative electrode. As the conductive agent, graphite such as natural graphite or artificial graphite may be used.

[0048] The shape and thickness of the positive electrode current collector can be selected from the shape and range corresponding to those of the negative electrode current collector. Examples of the material of the positive electrode current collector include stainless steel, aluminum, aluminum alloy, and titanium.

[0049] [Electrolyte] The electrolyte contains 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 the above range, an electrolyte having excellent ionic conductivity and appropriate viscosity can be obtained. However, the lithium salt concentration is not limited to the above.

[0050] The solvent may be an aqueous solvent or a non-aqueous solvent. Examples of non-aqueous solvents that can be used include cyclic carbonates, chain carbonates, cyclic carboxylic acid esters, and chain carboxylic acid esters. Examples of cyclic carbonates include propylene carbonate (PC) and ethylene carbonate (EC). Examples of chain carbonates 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 chain carboxylic acid esters include methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, and propyl propionate. One type of non-aqueous solvent may be used alone, or two or more types may be used in combination.

[0051] Examples of lithium salts include LiClO ４ , LiBF ４ , LiPF ６ , LiAlCl ４ , LiSbF ６ , LiSCN, LiCF ３ SO ３ , LiCF ３ CO ２ , LiAsF ６ , LiB １０ Cl １０ , lower aliphatic lithium carboxylates, LiCl, LiBr, LiI, borates, imide salts, etc. Examples of imide salts include LiN(FSO ２ ) ２ , LiN(CF ３ SO ２ ) ２ , LiN(CF ３ SO ２ ) (C ４ F ９ SO ２ ), LiN(C ２ F ５ SO ２ ) ２ The lithium salts may be used alone or in combination of two or more.

[0052] [Separator] It is usually desirable to interpose a separator between the positive electrode and the negative electrode. The separator has high ion permeability and adequate mechanical strength and insulating properties. The separator may be made of a microporous thin film, woven fabric, nonwoven fabric, or the like. The separator is preferably made of polyolefin such as polypropylene or polyethylene.

[0053] An example of the structure of a secondary battery is a structure in which an electrode group and an electrolyte are housed in an exterior body. The electrode group may be a wound type in which a positive electrode and a negative electrode are wound with a separator interposed therebetween, a laminated type in which a positive electrode and a negative electrode are stacked with a separator interposed therebetween, or another form. The secondary battery may be in any form, such as a cylindrical type, a prismatic type, a coin type, a button type, or a laminate type.

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

[0055] Example 1 [Production of Composite Material] A composite material was prepared that satisfied the high-entropy condition. The atomic ratio of Si:Me was 4:1 (i.e., the Si content was 80 atomic %), the constituent elements of Me were equimolar Ti, V, Cr, Zr, and Mo, and the average atomic weight of Me was 68. Specifically, the raw materials were placed on a water-cooled copper hearth in an arc melting furnace. The furnace was purged with an argon atmosphere, and the raw materials were melted and quenched by arc melting to obtain a button-shaped metal ingot. Note that the surface in contact with the water-cooled copper hearth was always cooled, so the sample cooled instantly when the arc irradiation was stopped. The obtained metal ingot was inverted in the furnace using an inversion rod, melted and quenched again by arc melting, and the inversion, melting, and quenching process was repeated five times to obtain a metal ingot of the composite material. The obtained metal ingot was then poured into a 2.0 x 10 －３ The metal was sealed in a quartz tube under a vacuum of 100 Pa or less and annealed at 800°C for 48 hours to allow sufficient crystallization to proceed. The annealed metal mass was crushed in a mortar to particles of 25 to 45 µm in size and used as the negative electrode active material.

[0056] [Preparation of Negative Electrode] A negative electrode mixture containing the composite material, carbon black, SBR, and CMC in a mass ratio of 95:0.5:1.5:3.0 was mixed with an appropriate amount of water to prepare a negative electrode slurry. 2 The negative electrode slurry was applied to one side of the copper foil so that the mass of the negative electrode mixture per sheet was 150 g, the coating was dried, and then rolled to form a sheet with a density of 2 g / cm 3 The electrode plate was punched out into a circle having a diameter of 12.5 mm and used as a negative electrode.

[0057] [Preparation of Counter Electrode] A counter electrode was prepared by punching a metal lithium foil having a thickness of 300 μm into a circle having a diameter of 17 mm.

[0058] [Preparation of non-aqueous electrolyte] LiPF was dissolved in a mixed solvent containing ethylene carbonate and ethyl methyl carbonate in a volume ratio of 1:3. ６ was dissolved at a concentration of 1.0 mol / L to prepare a non-aqueous electrolyte.

[0059] [Preparation of Coin-Shaped Cell] A stainless steel cell case with an opening and a bottom was prepared, and a negative electrode and a separator were placed inside in this order. A 0.45 mm thick polyphenylene sulfide (PPS) nonwoven fabric was used as the separator. A stainless steel sealing plate with a polypropylene gasket around the periphery was prepared, and a lithium foil 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 a sealing plate to complete Cell A1. The cell size was 20 mm in diameter and 3.2 mm in thickness.

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

[0061] Example 2 Using the same method as in Example 1, a composite material was prepared that satisfied the high-entropy condition. The composite had an atomic ratio of Si:Me = 4:1, the constituent elements of Me were equimolar, consisting of five transition metals: Cr, Mn, Fe, Ni, and Co, and the average atomic weight of Me was 56. Cell A2 was then 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 also shown in Table 1. Figure 1 and Table 1 show that Cell A2 had 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 that satisfied the high-entropy condition. The composite had an atomic ratio of Si:Me = 4:1, the constituent elements of Me were equimolar five transition metals: Cr, Mn, Fe, Ni, and Cu, and the average atomic weight of Me was 57. Cell A3 was then 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 also shown in Table 1. From Figure 1 and Table 1, it can be seen that Cell A3 had 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 that satisfied the high-entropy condition. The composite had an atomic ratio of Si:Me = 4:1, the constituent elements of Me were equimolar Al, Mg, Ti, Fe, and Ni, and the average atomic weight of Me was 57. Cell A4 was then 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 also shown in Table 1. From Figure 1 and Table 1, it can be seen that Cell A4 had a capacity density of 1798 mAh / g. The charge / discharge efficiency was 80%.

[0064]

[0065] Example 5 A composite material with an atomic ratio of Si:Me=3:1 (i.e., Si content of 75 atomic %) and the same Me composition as in Example 2 was prepared in the same manner as in Example 1, except that the annealed metal ingot was crushed in a mortar to particles of 25 μm or less, and then used. Cell A5 was assembled and evaluated. The first discharge capacity and the ratio of the discharge capacity to the 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 ingot was crushed in a mortar to particles of 25 μm or less. The first discharge capacity and the ratio of the discharge capacity to the charge capacity (charge / discharge efficiency) are shown in Table 2.

[0067]

[0068] Figure 2 compares the charge-discharge curves of cells A22 and A5. A sufficiently high-capacity composite material was obtained even when the atomic ratio Si:Me = 3:1. Furthermore, when the atomic ratio Si:Me = 4:1, the capacity was significantly greater than when the atomic ratio Si:Me = 3:1. On the other hand, the charge-discharge efficiency was superior when the silicon phase content was lower.

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

[0070] [X-ray Diffraction Analysis] The X-ray diffraction patterns obtained by the X-ray diffraction analysis of Examples 2 and 5 are shown in Figure 3. In the X-ray diffraction patterns, only peaks attributed to the silicon phase and peaks attributed to silicide are observed, and no peaks attributed to the simple substance of Mi, a constituent element of the metal element Me, are observed. From this data, it can be seen that all of the constituent elements Mi of the metal element Me form intermetallic compounds.

[0071] Furthermore, when the cross section of the composite material of Example 2 was observed by SEM using the method described above, it was confirmed that the composite material was essentially composed of only a silicide phase and a silicon phase. Furthermore, it was confirmed that the composite materials of Examples 1 to 5 had extremely high hardness for a silicide and had physical properties that made them less susceptible to volume changes.

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

Claims

1. A composite material including a silicide phase and a silicon phase, wherein 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 includes N or more constituent elements Mi (i = 1 to N, 5 ≤ N), when the molar fraction of the N or more constituent elements Mi is Ci, the condition represented by the formula: 1.5 < -ΣCi·lnCi is satisfied, and it is a negative electrode active material for a secondary battery.

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

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

4. The N or more constituent elements Mi are each contained in at least one of MiSi and MiSi 2 The negative electrode active material for a secondary battery according to any one of claims 1 to 3.

5. The negative electrode active material for a secondary battery according to any one of claims 1 to 3, wherein the content rate of the simple substance of the N or more constituent elements Mi in the composite material is 1 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 N or more constituent elements Mi are elements having an atomic weight of the fourth period or lower in the long-period type periodic table.

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 elements having an atomic weight of 29 (Cu) or lower.

8. 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.

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 5 selected from the group consisting of Al, Mg, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Mo.

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 5 selected from the group consisting of Al, Mg, Ti, V, Cr, Mn, Fe, Co, Ni, and Cu.

11. 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 5 selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, and Cu.

12. A secondary battery including a positive electrode, a negative electrode, and an electrolyte, wherein the negative electrode includes the negative electrode active material for a secondary battery according to any one of claims 1 to 3.