Negative electrode active material for secondary battery and secondary battery

Abstract

This negative-electrode active material for secondary batteries contains an intermetallic compound having a cage structure. The cage structure is constituted of at least one first atom disposed in a cage and a plurality of second atoms disposed so as to surround the first atom in a cage-like shape. The first atom is a cerium atom, and the plurality of second atoms include 8-17 atoms of silicon.

Description

[Technical Field]

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

[0002] Non-aqueous electrolyte secondary batteries, especially lithium-ion secondary batteries, have high voltage and high energy density, and are therefore expected to be used in small consumer applications, power storage devices, and power sources for electric vehicles. As higher energy densities are required for batteries, 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). However, alloy-based materials are known to undergo large volume changes during charging and discharging.

[0003] On the other hand, intermetallic compounds having a cage structure and containing Sn as the main component are attractive as negative electrode active materials because they undergo small volume changes due to the insertion and desorption of lithium ions. For example, Patent Document 2 discloses an intermetallic compound having an R element, Sn, an M element, and a Z element as essential components as the main phase, and having the general formula: R a Sn b M c T d X e A f Z g where R is at least one element selected from the group consisting of rare earth elements, 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 Si, Al, Sb, and In, A is at least one element selected from the group consisting of Mg, Ca, Sr, and Ba, Z is at least one element selected from the group consisting of C, N, B, and P, and a, b, c, d, e, f, and g are each such that a+b+c+d+e+f+g=100 atomic %, 5≦a≦35, 38≦b≦55, 8≦c≦30, 0≦d≦10, 0≦e≦20, 0≦f≦20, 0 <g≦30である。 [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2016 / 35290 Brochure [Patent Document 2] Japanese Patent Application Laid-Open No. 2011-146388 Summary of the Invention

[0005] The intermetallic compound described in Patent Document 2 contains the heavy element Sn as a main component, which increases the weight of the secondary battery and makes it unsuitable for mobile applications. For example, in electric vehicle applications, the use of lighter elements is required to extend the driving range. Furthermore, because Sn is a rare element, there is a concern that using a material containing Sn as a main component will increase the cost of battery production.

[0006] In view of the above, one aspect of the present disclosure relates to a negative electrode active material for a secondary battery, including an intermetallic compound having a cage structure, wherein the cage structure is composed of at least one first atom arranged in the cage and a plurality of second atoms arranged in a cage shape so as to surround the first atom, wherein the first atom is a cerium atom, and the plurality of second atoms include 8 to 16 silicon atoms.

[0007] Another aspect of the present disclosure 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.

[0008] The present disclosure proposes the use of a lightweight, inexpensive intermetallic compound capable of inserting and extracting lithium ions as a negative electrode active material for a secondary battery. [Brief explanation of the drawings]

[0009] [Figure 1A] FIG. 1A is a diagram showing the crystal structure of a negative electrode active material (CeNi 2 Si 2 ) according to one embodiment of the present disclosure. [Figure 1B] FIG. 1B is a diagram showing the crystal structure of the negative electrode active material (CeSi 2 ) according to one embodiment of the present disclosure. [Figure 2A] FIG. 2A is a diagram showing an X-ray diffraction pattern of an intermetallic compound. [Figure 2B] FIG. 2B shows another X-ray diffraction pattern of the intermetallic compound. [Figure 3] FIG. 3 is a diagram showing charge / discharge curves of the cells of the example and comparative example. [Figure 4] FIG. 4 is a diagram showing the crystal structure of the negative electrode active material (La3Ni2Sn7) of the comparative example. [Figure 5] FIG. 5 is a diagram showing the crystal structure of the negative electrode active material (Ti4Ni4Si7) of the comparative example. [Figure 6] FIG. 6 is a diagram showing the crystal structure of the negative electrode active material (LaMn2Si2) of the comparative example. [Figure 7] FIG. 7 shows yet another X-ray diffraction pattern of the intermetallic compound. [Figure 8] FIG. 8 is a diagram showing another charge / discharge curve of the cell of the example. DETAILED DESCRIPTION OF THE INVENTION

[0010] [Negative electrode active material for secondary batteries] The negative electrode active material for a secondary battery according to an embodiment of the present disclosure includes an intermetallic compound having a cage structure. Here, the cage structure refers to a cage-shaped structure formed by certain atoms within a crystal structure. Specifically, the cage structure is composed of at least one first atom disposed within the cage and multiple second atoms disposed in a cage-like manner to surround the first atom. The first atom may be, for example, a central atom disposed at the center of the cage. The second atoms are disposed to surround the first atom or the central atom, forming a cage. Hereinafter, the second atom is also referred to as a cage atom. A cage structure is typically a structure having a central atom within the space defined by a polyhedron with cage atoms as vertices.

[0011] For the insertion and desorption of lithium ions to proceed stably, the crystal structure must have a cage structure. Furthermore, the capacity exhibited by the insertion and desorption of lithium ions depends greatly on the type of the first and second atoms. When the first atom is a cerium atom and the multiple second atoms contain 8 to 16 silicon atoms, intermetallic compounds with a cage structure have a high capacity density (mAh / g). While the details of the correlation between capacity density and the type of atom are unclear, it is thought to be desirable to select a metal atom with a relatively low electronegativity as the first atom and a metal atom with a relatively high electronegativity as the second atom.

[0012] In an intermetallic compound having a cage structure, it is not necessary for all atoms to form the cage structure. Some of the atoms constituting the intermetallic compound may be arranged at sites other than the cage structure or may form a different structure. Furthermore, it is not necessary for all cage structures contained in the intermetallic compound to contain a cerium atom as the first atom and 8 to 16 silicon atoms as the second atoms. However, it is desirable that more than half of the cage structures contained in the intermetallic compound contain a cerium atom as the first atom and 8 to 16 silicon atoms as the second atoms. As long as the intermetallic compound can exhibit a highly practical capacity (e.g., 80 mAh / g or more), various changes are possible in the crystal structure and the types of elements of the intermetallic compound.

[0013] The intermetallic compound may contain a subphase or impurity phase (e.g., CeSi2, CeNiSi2). The content of the subphase or impurity phase is preferably within a range that allows the intermetallic compound to exhibit a highly practical capacity (e.g., 80 mAh / g or more), but is not limited thereto.

[0014] Hereinafter, an intermetallic compound having a cage structure composed of at least one first atom arranged in the cage and a plurality of second atoms arranged in a cage shape so as to surround the first atom, wherein the first atom is a cerium atom and the plurality of second atoms include 8 to 16 silicon atoms, will also be referred to as an "intermetallic compound CS."

[0015] Silicon (Si) has an atomic weight of 28.09 and is lighter and less expensive than Sn (atomic weight 118.71). Furthermore, the intermetallic compound CS has a capacity density of, for example, 100 mAh / g or more (even 140 mAh / g or more). Therefore, the intermetallic compound CS is extremely attractive as a negative electrode active material for secondary batteries used in mobile applications (e.g., electric vehicles).

[0016] The plurality of second atoms preferably include one or more nickel atoms. When some of the plurality of second atoms are nickel atoms, the capacity density of the intermetallic compound CS is further increased. In this case, the total number of silicon atoms and nickel atoms contained in one cage structure may be 17 or more and 19 or less, and may even be 18. In the plurality of second atoms (cage atoms), the atomic ratio of silicon atoms to nickel atoms (Si / Ni ratio) may be, for example, 1 to 1.3, and may be, for example, 1.25.

[0017] When the plurality of second atoms (cage atoms) does not include Ni, the number of silicon atoms included in the cage atoms may be 11 or more and 13 or less, for example, 12.

[0018] The intermetallic compound CS has the general formula: Ce x Ni y Si z The general formula satisfies x = 1, 0 ≤ y ≤ 3, and 1 ≤ z ≤ 4. Although at least some of the Ce atoms are first atoms, the intermetallic compound CS may contain Ce atoms that are not arranged in cages. Similarly, although at least some of the Ni and Si atoms are second atoms (cage atoms), the intermetallic compound CS may contain Ni or Si atoms that do not form cages. Therefore, the ratio of (y + z) to x may be a value different from the ratio of the number of second atoms to the number of first atoms in the cage structure (e.g., 11 to 19). The ratio of (y + z) to x: (y + z) / x may be, for example, 1.5 to 5, 2 to 4, or, for example, 2 or 4.

[0019] When the plurality of second atoms include nickel atoms, the number of nickel atoms included in the second atoms may be, for example, 7 or more and 9 or less. In this case, the remaining second atoms may all be silicon atoms or atoms of other elements. Note that even when the intermetallic compound CS includes atoms of elements other than Ce, Si, and Ni, the intermetallic compound CS still has at least the general formula: Ce x Ni y Si z It is desirable to have a phase having a composition represented by the formula:

[0020] In one preferred embodiment, the number of silicon atoms contained in the second atoms is, for example, 10, and in another embodiment, it is, for example, 12. Such an intermetallic compound CS can form a stable cage structure, and the overall stability of the crystal structure is also improved. Therefore, when the intermetallic compound CS is used as a negative electrode active material for a secondary battery, it is less likely to deteriorate during charge-discharge cycles.

[0021] Specific examples of the intermetallic compound Cs include at least one phase selected from the group consisting of CeNi2Si2 and CeSi2. Among them, CeNi2Si2 can exhibit a capacity density of 140 mAh / g or more.

[0022] Figure 1A shows a schematic diagram of the crystal structure of CeNi2Si2. In Figure 1A, the unit cell has symmetry belonging to the I4 / mmm space group. Two cage structures are contained near the center of the unit cell. Ce is positioned at the center of the cage as the central atom. Si and Ni are arranged in a cage-like fashion surrounding Ce. In the illustrated example, the number of cage atoms (here, the total number of Si and Ni) is 18, with 10 Si sites and 8 Ni sites, but this is not limited to this. Various modifications are possible within the range in which the intermetallic compound can exhibit a highly practical capacity (e.g., 80 mAh / g or more) or within the range permitted by crystallography.

[0023] Figure 1B shows a schematic diagram of the crystal structure of CeSi2. In Figure 1B, the unit cell has symmetry belonging to the I41 / amd space group. One cage structure is contained near the center of the unit cell. Ce is positioned at the center of the cage as the central atom. Si is arranged in a cage-like structure surrounding Ce. In the illustrated example, the number of cage atoms is 12, but this is not limited to this. Various modifications are possible within the range in which the intermetallic compound can exhibit a highly practical capacity (e.g., 80 mAh / g or more) or within the range permitted by crystallography.

[0024] The presence of the intermetallic compound CS can be confirmed by X-ray diffraction analysis using CuKα radiation. For example, the X-ray diffraction pattern of the intermetallic compound CS, which has a crystal structure of CeNi2Si2 or a crystal structure similar to this, has diffraction peaks at (1) around 2θ = 23.9°, (2) around 2θ = 35.8°, (3) around 2θ = 36.6°, and (4) around 2θ = 44.9°.

[0025] The intermetallic compound CS belongs to the tetragonal or orthorhombic crystal system. For example, when the intermetallic compound CS having a crystal structure of CeNi2Si2 or a crystal structure similar thereto belongs to the tetragonal crystal system, the lattice constants a and c determined by X-ray diffraction satisfy 3.0 Å≦a≦4.5 Å and 7 Å≦c≦15 Å. Furthermore, when such an intermetallic compound CS belongs to the orthorhombic crystal system, the lattice constants a, b, and c determined by X-ray diffraction satisfy 3.0 Å≦a≦4.5 Å, 3.0 Å≦b≦4.5 Å, and 7 Å≦c≦15 Å.

[0026] In a more stable crystal structure, the lattice constants a and b preferably satisfy 3.9 Å≦a, b ≦ 4.2 Å, and further satisfy 4.0 Å ≦ a, b ≦ 4.1 Å. In a more stable crystal structure, the lattice constant c preferably satisfies 8.0 Å ≦ c ≦ 11.0 Å, and further satisfy 9.0 Å ≦ c ≦ 10.0 Å.

[0027] For example, in the X-ray diffraction pattern of a typical intermetallic compound CS having a crystal structure of CeNi2Si2 or a crystal structure similar thereto, the d value of the diffraction peak given by the hkl index of 112 is 2.38 Å or more and 2.5 Å or less, or 2.40 Å or more and 2.50 Å or less.

[0028] The intermetallic compound CS may contain an atom Me different from any of Ce, Ni, and Si. The atom Me may be incorporated into the crystal structure of a phase contained in the intermetallic compound CS. In other words, the intermetallic compound CS may be a solid solution containing the element Me.

[0029] The element Me is not particularly limited, but examples of the element Me include Fe, La, Pr, Cr, Co, Al, C, Ca, Pb, Nd, Mg, Cu, and Zn. When an intermetallic compound CS containing Fe is used as the negative electrode active material for a secondary battery, it is effective in improving the capacity of the secondary battery and improving the workability of materials in the negative electrode manufacturing process. Specifically, Fe improves the capacity of the intermetallic compound CS and suppresses the alkalinity of the intermetallic compound CS. While the reason for the increase in capacity due to Fe is unclear, it is believed that the intermetallic compound CS containing Fe has better conductivity and strength than the intermetallic compound CS without Fe. It is speculated that changes in conductivity and strength are related to changes in capacity. Furthermore, suppressing the alkalinity of the intermetallic compound CS improves the stability of the negative electrode slurry when preparing the negative electrode slurry in which the negative electrode mixture is dispersed in a dispersion medium, as described below.

[0030] The presence of Fe is preferably dispersed in the intermetallic compound CS to such an extent that it cannot be confirmed by X-ray diffraction analysis of the intermetallic compound CS. In other words, the intermetallic compound CS may be a solid solution containing Fe.

[0031] The content of Me elements (e.g., Fe) in the intermetallic compound CS can be measured, for example, by inductively coupled plasma atomic emission spectroscopy (ICP). For example, a sample of the intermetallic compound CS 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 off, and then the sample is analyzed by ICP to measure the spectral intensity of each element. Next, a calibration curve is created using commercially available standard solutions of the elements, and the content of each element in the intermetallic compound CS is calculated.

[0032] The Fe content in the intermetallic compound may be, for example, 1% by mass or less, 0.5% by mass or less, 0.3% by mass or less, or 0.1% by mass or less. When the intermetallic compound contains Fe, the lower limit of the Fe content at which the effect of Fe is obtained is, for example, 0.001% by mass or more, or 0.01% by mass or more. These upper and lower limits can be arbitrarily combined when setting the content range. For example, the Fe content in the intermetallic compound may be 0.001% by mass or more and 0.3% by mass or less, or 0.001% by mass or more and 0.1% by mass or less.

[0033] [Method of manufacturing intermetallic compound CS] The intermetallic compound CS can be produced by weighing raw materials, melting the raw materials to form a molten metal, and then cooling the molten metal.

[0034] The raw materials are not particularly limited, but single materials (metal ingots, metal powders, etc.) of the constituent elements of the intermetallic compound CS can be used. The raw materials are preferably melted using an arc melting furnace in an inert gas atmosphere such as argon. When the molten metal is cooled, a metal ingot containing the constituent elements with the desired composition is obtained.

[0035] In the metal ingot obtained by cooling the molten metal, the crystallization of the intermetallic compounds may not have progressed sufficiently. -3It is desirable to anneal the metal ingot by heating it 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 disclosure includes a positive electrode, a negative electrode, and a non-aqueous electrolyte, and the negative electrode contains the above-described negative electrode active material (intermetallic compound CS). 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 a negative electrode active material (intermetallic compound CS) as an essential component, and may contain optional components such as a binder, a conductive agent, and a thickener.

[0039] In addition to the intermetallic compound CS, 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 graphitized carbon (soft carbon), and non-graphitized 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, such as 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 fiber and metal fiber, 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 salt), 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 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、 Li2MePO4F (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 during 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] [Electrolytes] 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 LiClO4, LiBF4, LiPF6, LiAlCl4, LiSbF6, LiSCN, LiCF3SO3, LiCF3CO2, LiAsF6, and LiB 10 Cl 10 Examples of the lithium salt include lithium carboxylates, lower aliphatic carboxylates, LiCl, LiBr, LiI, borates, imide salts, etc. Examples of the borates include lithium bis(1,2-benzenediolate(2-)-O,O')borate, lithium bis(2,3-naphthalenediolate(2-)-O,O')borate, lithium bis(2,2'-biphenyldiolate(2-)-O,O')borate, and lithium bis(5-fluoro-2-oleate-1-benzenesulfonic acid-O,O')borate. Examples of imide salts include lithium bisfluorosulfonylimide (LiN(FSO2)2), lithium bistrifluoromethanesulfonyl imide (LiN(CF3SO2)2), lithium trifluoromethanesulfonate nonafluorobutanesulfonyl imide (LiN(CF3SO2)(C4F9SO2)), lithium bispentafluoroethanesulfonyl imide (LiN(C2F5SO2)2), etc. One type of lithium salt may be used alone, or two or more types may be used in combination.

[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 disclosure 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 negative electrode active material] As raw materials, Ce, Ni, and Si elemental materials (metal ingots) were weighed so that the atomic ratio of Ce:Ni:Si was 1:2:2, and placed on a water-cooled copper hearth in an arc melting furnace. The atmosphere inside the arc melting furnace was then replaced with argon, and the raw materials were melted and quenched by arc melting to obtain button-shaped metal ingots. Note that, since the surface in contact with the water-cooled copper hearth was always cooled, the sample cooled instantly when the arc irradiation was stopped. The obtained metal ingot was inverted in the furnace using an inverting rod, and again melted and quenched by arc melting. This process of inverting, melting, and quenching was repeated five times, obtaining a metal ingot containing the constituent elements with a composition of CeNi2Si2. Next, the obtained metal ingot was poured into a 2.0 x 10 -3 The metal was placed in a quartz tube under a vacuum of 0.1 Pa or less and annealed at 800°C for 48 hours to allow sufficient crystallization. The annealed metal mass was crushed in a mortar to particles of 45 μm or less and used as the negative electrode active material (intermetallic compound CS).

[0056] [X-ray diffraction analysis] The X-ray diffraction pattern of the obtained intermetallic compound CS is shown in Figure 2A. Diffraction peaks can be observed at 2θ = 23.91°, 35.84°, 36.63°, and 44.88° in the X-ray diffraction pattern.

[0057] Furthermore, analysis of the X-ray diffraction pattern revealed that the obtained intermetallic compound CS belonged to a tetragonal crystal system with a lattice constant a = 4.027 Å and a lattice constant c = 9.557 Å. The d value of the diffraction peak given by the hkl index of 112 was 2.451 Å. These results indicate that the intermetallic compound CS of this example generally has the crystal structure shown in FIG. 1A.

[0058] [Preparation of negative electrode] A negative electrode mixture containing an intermetallic compound CS, 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 was 150 g per sheet. After drying the coating, the sheet was rolled and coated with a density of 2 g / cm. 3 The electrode plate was punched into a circle having a diameter of 12.5 mm and used as a negative electrode.

[0059] [Preparation of counter electrode] A counter electrode was prepared by punching a 300 μm thick lithium metal foil into a circle with a diameter of 17 mm.

[0060] [Preparation of electrolyte] An electrolyte solution 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.

[0061] [Fabrication of coin-shaped cells] A stainless steel cell case with a bottom and an opening was prepared, and the negative electrode and separator were placed inside in that order. The separator was a 0.45 mm thick nonwoven polyphenylene sulfide (PPS) fabric. 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 the sealing plate to complete Cell A1. The cell size was 20 mm in diameter and 3.2 mm in thickness.

[0062] [Charge / discharge curve] After assembly, cell A1 was charged to 0.01 V at 0.1 mA at 25°C, and then discharged to 1.5 V. The first charge / discharge curve is shown in Figure 3. The first discharge capacity and the ratio of discharge capacity to charge capacity (initial efficiency) are shown in Table 1. Figure 3 and Table 1 show that cell A1 has a capacity density of 145 mAh / g. The initial efficiency was also high, at 79%.

[0063] [Table 1]

[0064] <Example 2> [Production of negative electrode active material] As raw materials, Ce and Si simple materials (metal blocks) were weighed so that the atomic ratio of Ce:Si was 1:2, and placed on a water-cooled copper hearth in an arc melting furnace. A negative electrode active material of Example 2 was obtained in the same manner as in Example 1 except for the raw materials.

[0065] [X-ray diffraction analysis] The X-ray diffraction pattern of the obtained intermetallic compound CS is shown in Figure 2B. Diffraction peaks can be observed at 2θ = 22.0°, 28.6°, 32.7°, and 38.7° in the X-ray diffraction pattern.

[0066] Furthermore, analysis of the X-ray diffraction pattern revealed that the obtained intermetallic compound CS belonged to a tetragonal crystal system with a lattice constant a = 4.15 Å and a lattice constant c = 13.88 Å. The d value of the diffraction peak given by the hkl index of 112 was 2.71 Å. These results indicate that the intermetallic compound CS of this example generally has the crystal structure shown in FIG. 1B.

[0067] [Preparation of negative electrode] Except for using the obtained intermetallic compound (CeSi2) as the negative electrode active material, cell A2 was fabricated and evaluated in the same manner as in Example 1. The first charge / discharge curve is shown in Figure 3. The first discharge capacity and initial efficiency are also shown in Table 1.

[0068] From Figure 3 and Table 1, it can be seen that Cell A2 has a capacity density of approximately 105 mAh / g. It also has a flatter discharge curve than Cell B1 (described later). The initial efficiency was high, at approximately 75%.

[0069] <Comparative Example 1> Cell B1 was fabricated and evaluated in the same manner as in Example 1, except that an intermetallic compound (La3Ni2Sn7) having the crystal structure shown in Figure 4 was used as the negative electrode active material. The first charge / discharge curve is shown in Figure 3. The first discharge capacity and initial efficiency are shown in Table 1.

[0070] La3Ni2Sn7 has symmetry belonging to the Cmmm space group. The center of the unit cell contains a cage structure, with the central atom being La, and 12 Sn atoms arranged in a cage shape surrounding the La. Because La3Ni2Sn7 contains Sn as the main component, it has a low capacity density, and it can be seen that Cell A1 of Example 1 is superior to Cell B1 in terms of capacity density and initial efficiency.

[0071] <Comparative Example 2> Cell B2 was fabricated and evaluated in the same manner as in Example 1, except that an intermetallic compound (Ti4Ni4Si7) having the crystal structure shown in Figure 5 was used as the negative electrode active material. The first charge / discharge curve is shown in Figure 3. The first discharge capacity and initial efficiency are shown in Table 1.

[0072] Ti4Ni4Si7 is an intermetallic compound containing Ti instead of Ce, and has a crystal structure significantly different from that of CeNi2Si2. It is also clear that Ti4Ni4Si7 has almost no electrochemical activity. This indicates that simply having the same elements does not result in a high capacity density similar to that of CeNi2Si2.

[0073] <Comparative Example 3> Cell B3 was fabricated and evaluated in the same manner as in Example 1, except that an intermetallic compound (LaMn2Si2) having the crystal structure shown in Figure 6 was used as the negative electrode active material. The first charge / discharge curve is shown in Figure 3. The first discharge capacity and initial efficiency are shown in Table 1.

[0074] LaMn2Si2 has a cage structure, with the central atom being La, surrounded by 10 Si atoms and 8 Ni atoms in a cage-like arrangement. LaMn2Si2 has the same symmetry as CeNi2Si2 and a similar crystal structure. However, LaMn2Si2 has almost no electrochemical activity, which explains why its capacity density is highly dependent on the type of element.

[0075] <Examples 3 and 4> As raw materials, Ce and Si simple materials (metal ingots) were weighed so that the atomic ratio of Ce:Si was 1:2, and a predetermined amount of Fe was further added, followed by charging onto a water-cooled copper hearth in an arc melting furnace. Except for the above, the negative electrode active material of Example 3 (intermetallic compound CS-1) with an Fe content of 0.06 mass% and the negative electrode active material of Example 4 (intermetallic compound CS-2) with an Fe content of 1.03 mass% were obtained in the same manner as in Example 2.

[0076] The X-ray diffraction patterns of the intermetallic compounds CS-1 and CS-2 are shown in Figure 7. As shown in Figure 7, the X-ray diffraction pattern of the Fe-containing intermetallic compound CS is almost the same as the X-ray diffraction pattern of the intermetallic compound CS of Example 2, which did not contain the specified amount of Fe, and no substantial changes were observed. Therefore, it can be said that the intermetallic compounds CS-1 and CS-2 generally have the crystal structure shown in Figure 1B.

[0077] Next, cell A3 of Example 3 and cell A4 of Example 4 were prepared in the same manner as in Example 1, except that intermetallic compound CS-1 and intermetallic compound CS-2 were used as the negative electrode active materials. For assembled cells A3 and A4 and cell A2 of Example 2, the negative electrodes were charged to 0.01 V at 25°C at 0.5 mA, a higher rate than in Examples 1 and 2 and Comparative Examples 1 to 3, and then discharged to 1.5 V. The first charge / discharge curves are shown in FIG. 8 . The first discharge capacity is also shown in Table 2. For A2, Fe was not added when obtaining intermetallic compound CS, so the Fe content in Table 2 is indicated as "-."

[0078] In Examples 1 and 2, in which the intermetallic compound CS without Fe was used, the negative electrode slurry prepared when producing the negative electrode tended to gel, but in Examples 3 and 4, the negative electrode slurry was stable and no gelation was observed.

[0079] [Table 2]

[0080] From Figure 8 and Table 2, it can be seen that the discharge capacity increases when the intermetallic compound CS contains a small amount of Fe. However, as the Fe content of the intermetallic compound CS increases, the discharge capacity decreases. Therefore, it is considered that the Fe content is preferably 0.001 mass% or more and 0.3 mass% or less, and more preferably 0.001 mass% or more and 0.1 mass% or less. On the other hand, the stability of the negative electrode slurry improved with increasing Fe content.

[0081] In the above Examples 3 and 4, CeSi2 of Example 2 was used as the base of the intermetallic compound CS, but even when CeNi2Si2 of Example 1 was used as the base, similar trends in capacity improvement and stability of the negative electrode slurry were confirmed. [Industrial Applicability]

[0082] The negative electrode active material for secondary batteries according to the present disclosure 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. an intermetallic compound having a cage structure, the cage structure is composed of at least one first atom arranged in a cage and a plurality of second atoms arranged in a cage shape so as to surround the first atom, the first atom is a cerium atom; the plurality of second atoms include 8 to 16 silicon atoms and one or more nickel atoms.

2. The intermetallic compound has the general formula: Ce ｘ Ni ｙ Si ｚ and has a phase represented by x=1, 0<y≦3, and The negative electrode active material for a secondary battery according to claim 1 , wherein 1≦z≦4 is satisfied.

3. The negative electrode active material for a secondary battery according to claim 1 , wherein the number of the nickel atoms contained in the plurality of second atoms is 7 or more and 9 or less.

4. 4. The negative electrode active material for a secondary battery according to claim 1, wherein the number of silicon atoms contained in the plurality of second atoms is 9 or more and 12 or less.

5. The intermetallic compound is CeNi ２ The negative electrode active material for a secondary battery according to any one of claims 1 to 4, which contains a Si2 phase.

6. The X-ray diffraction pattern of the intermetallic compound is (1) Around 2θ = 23.9°, (2) Around 2θ = 35.8°, (3) Around 2θ=36.6°, and (4) The negative electrode active material for a secondary battery according to any one of claims 1 to 5, which has a diffraction peak near 2θ = 44.9°.

7. The intermetallic compound belongs to a tetragonal or orthorhombic crystal system, When the crystal belongs to the tetragonal system, the lattice constants a and c determined by X-ray diffraction are: 3.0 Å≦a≦4.5 Å, and 7 Å≦c≦15 Å is satisfied, When the crystal belongs to the orthorhombic system, the lattice constants a, b, and c determined by X-ray diffraction are 3.0 Å≦a≦4.5 Å, 3.0 Å≦b≦4.5 Å, and 7. The negative electrode active material for a secondary battery according to claim 1, wherein 7 Å≦c≦15 Å is satisfied.

8. 8. The negative electrode active material for a secondary battery according to claim 1, wherein in an X-ray diffraction pattern of the intermetallic compound, a d value of a diffraction peak given by an hkl index of 112 is 2.38 Å or more and 2.52 Å or less.

9. The intermetallic compound contains an atom Me that is different from any of Ce, Ni, and Si, 9. The negative electrode active material for a secondary battery according to claim 1, wherein the atom Me contains at least Fe.

10. 10. The negative electrode active material for a secondary battery according to claim 9, wherein the content of Fe contained in the intermetallic compound is 2 mass % or less.

11. 11. The negative electrode active material for a secondary battery according to claim 9, wherein the content of Fe contained in the intermetallic compound is 0.001% by mass or more.

12. A positive electrode, a negative electrode, and an electrolyte, A secondary battery, wherein the negative electrode comprises the negative electrode active material for a secondary battery according to any one of claims 1 to 11.

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