Negative electrode active material and secondary battery

Abstract

A negative electrode active material according to the present disclosure comprises composite particles 10 that contain a matrix 1 and Si-B-containing particles 2, wherein the Si-B-containing particles 2 are dispersed in the matrix 1. A secondary battery 100 according to the present disclosure comprises a negative electrode 26 that contains the negative electrode active material according to the present disclosure, a positive electrode 23, and an electrolyte.

Description

Negative electrode active material and secondary battery

[0001] The present disclosure relates to a negative electrode active material and a secondary battery.

[0002] In recent years, secondary batteries such as non-aqueous electrolyte secondary batteries have been expected to be used in small consumer applications, power storage devices, and power sources for electric vehicles due to their high voltage and high energy density. As higher energy densities are required for batteries, the use of materials containing silicon, which alloys with lithium, as negative electrode active materials with high theoretical capacity densities is expected.

[0003] Patent Document 1 discloses a negative electrode active material comprising silicate composite particles containing a lithium silicate phase and silicon particles dispersed in the lithium silicate phase, wherein the silicon particles contain at least one element selected from the group consisting of germanium and aluminum.

[0004] International Publication No. 2021 / 153078

[0005] The present disclosure provides a negative electrode active material that can improve the charge capacity of a battery.

[0006] The negative electrode active material of the present disclosure comprises composite particles including: a matrix; and Si—B-containing particles, the Si—B-containing particles being dispersed in the matrix.

[0007] According to the technology of the present disclosure, it is possible to provide a negative electrode active material that can improve the charge capacity of a battery.

[0008] Fig. 1 is a cross-sectional view showing a schematic configuration of a composite particle 10 included in the negative electrode active material according to embodiment 1. Fig. 2 is a cross-sectional view showing a schematic configuration of a secondary battery according to embodiment 2. Fig. 3 is a graph showing X-ray diffraction patterns of the composite particles of example 1 and comparative example 1. Fig. 4 is a partially enlarged graph of the X-ray diffraction patterns of the composite particles of example 1 and comparative example 1.

[0009] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. The present disclosure is not limited to the following embodiments.

[0010] Embodiment 1 A negative electrode active material according to embodiment 1 includes composite particles containing a matrix and Si—B-containing particles.

[0011] 1 is a cross-sectional view showing a schematic configuration of a composite particle 10 included in the negative electrode active material according to Embodiment 1. The composite particle 10 is a particle including a matrix 1 and Si—B-containing particles 2 dispersed in the matrix 1.

[0012] The Si—B-containing particles 2 are particles containing silicon element (Si) and boron element (B). The Si—B-containing particles 2 are, for example, silicon particles containing B. By containing B, the Si—B-containing particles 2 have improved electronic conductivity and a lower volume resistivity (powder resistivity) compared to silicon particles that do not contain B. That is, in the composite particle 10, silicon particles with improved electronic conductivity are dispersed in the matrix 1. With the above configuration, the charge capacity and discharge capacity of the battery can be improved.

[0013] The Si—B-containing particles 2 are composed of, for example, 90 atomic % or more of silicon. The Si—B-containing particles 2 may also contain a phase of simple silicon (Si).

[0014] In the Si—B-containing particle 2, B may be located inside the Si—B-containing particle 2 or on the surface of the Si—B-containing particle 2. For example, B may be dispersed within the silicon particle or may be located on the surface of the silicon particle.

[0015] In the Si—B-containing particles 2, B may be contained in any form such as an atom, a simple substance, or a compound.

[0016] B may be dispersed in the Si—B containing particles 2 .

[0017] In the Si—B-containing particle 2, at least a portion of the B may be substituted or inserted in the form of B atoms into the crystal lattice sites of silicon, or may be substituted. The Si—B-containing particle 2 may contain crystalline silicon in which B atoms are solid-solved. That is, in the Si—B-containing particle 2, at least a portion of the B may be B atoms solid-solved in the crystalline silicon. This configuration can improve the charge capacity and discharge capacity of the battery. It can also reduce cracks and fractures in the composite particle 10.

[0018] The Si—B-containing particle 2 may contain at least one selected from the group consisting of boron oxide and elemental B. The Si—B-containing particle 2 may contain boron oxide. The Si—B-containing particle 2 may contain B atoms solid-dissolved in crystalline silicon, boron oxide, and elemental B, or may contain B atoms solid-dissolved in crystalline silicon and boron oxide. The boron oxide may be dispersed within the silicon particle, or may be located on the surface of the silicon particle.

[0019] The Si—B-containing particle 2 may include a crystalline phase. For example, as described above, the Si—B-containing particle 2 may include crystalline silicon. In this specification, the expression "the Si—B-containing particle 2 includes crystalline silicon" means that a peak derived from the Si (111) plane is observed in an X-ray diffraction pattern obtained by X-ray diffraction measurement of the composite particle 10 using Cu—Kα radiation. The X-ray diffraction peak derived from the Si (111) plane is, for example, an X-ray diffraction peak having the highest intensity within a diffraction angle 2θ range of 28.0° or more and 28.7° or less.

[0020] The X-ray diffraction pattern of the composite particle 10 can be obtained by X-ray diffraction measurement by the θ-2θ method using Cu-Kα rays with wavelengths of 1.5405 Å and 1.5444 Å, that is, wavelengths of 0.15405 nm and 0.15444 nm.

[0021] In this specification, a peak is a mountain-shaped portion having an S / N ratio (i.e., the ratio of signal S to background noise N) of 1.3 or more and a half-width of 10° or less.

[0022] In an X-ray diffraction pattern obtained by X-ray diffraction measurement of composite particle 10 using Cu-Kα radiation, an X-ray diffraction peak derived from the Si (111) plane may be present within a diffraction angle 2θ range of 28.42° or more and 28.57° or less. "An X-ray diffraction peak derived from the Si (111) plane is present within a diffraction angle 2θ range of 28.42° or more and 28.57° or less" means that the peak top of the X-ray diffraction peak derived from the Si (111) plane is observed within a diffraction angle 2θ range of 28.42° or more and 28.57° or less.

[0023] The diffraction angle of a peak in an X-ray diffraction pattern is defined as the angle at which the S / N ratio is 1.3 or more and the peak width at half maximum is 10° or less, showing the maximum intensity. MAX When the intensity is I MAX The width is the difference between the two diffraction angles that are half the value of the square root of the diffraction angle.

[0024] The Si—B-containing particles 2 may include an amorphous phase.

[0025] The Si—B-containing particle 2 may include a crystalline portion and an amorphous portion. The amorphous portion may be derived from B or Si.

[0026] The ratio of B in the Si—B-containing particles 2 may be 0.01% by mass or more and 5.0% by mass or less, 0.01% by mass or more and 3.0% by mass or less, or 0.01% by mass or more and 1.0% by mass or less. The ratio of B in the Si—B-containing particles 2 may be 0.05% by mass or more, 0.1% by mass or more, or 1% by mass or more.

[0027] The mass ratio of B in the Si—B-containing particles 2 can be measured by, for example, inductively coupled plasma atomic emission spectroscopy (ICP-AES).

[0028] The average particle diameter of the Si—B-containing particles 2 may be 1 nm or more and 1,000 nm or less, 1 nm or more and 100 nm or less, or 25 nm or more and 65 nm or less. Such fine Si—B-containing particles 2 have a small volume change during charge and discharge, and can improve the structural stability of the composite particles 10. Therefore, the charge capacity and discharge capacity of the battery can be improved.

[0029] The average particle diameter of the Si—B-containing particles 2 can be determined from an SEM image obtained by observing the cross section of the negative electrode active material (composite particle 10) with a scanning electron microscope (SEM). That is, the average particle diameter of the Si—B-containing particles 2 can be measured from an SEM image of the cross section of the composite particle 10 in which the cross section of the Si—B-containing particles 2 is exposed. The average particle diameter can be determined by measuring the particle diameters of 500 Si—B-containing particles 2 randomly extracted from the SEM image of the composite particle 10 and calculating their median diameter. Here, the maximum Feret diameter of the Si—B-containing particles 2 in the obtained SEM image can be considered as the particle diameter of the Si—B-containing particles 2. The "maximum Feret diameter" is the maximum length of the perpendicular line obtained by sandwiching a particle between two parallel lines.

[0030] The median value of the particle diameters of 500 Si—B containing particles 2 measured by SEM observation, based on the number of particles, is regarded as the median diameter of the Si—B containing particles 2 .

[0031] The mass ratio of Si contained in the Si—B-containing particle 2 to the mass of the composite particle 10 may be 20 mass% or more and 80 mass% or less, 20 mass% or more and 75 mass% or less, 40 mass% or more and 70 mass% or less, or 50 mass% or more and 66 mass% or less.

[0032] The mass ratio of Si contained in the Si—B-containing particle 2 to the mass of the composite particle 10 can be measured using ICP and NMR.

[0033] In order to increase capacity and improve cycle characteristics, the content of Si—B-containing particles 2 in composite particle 10 may be 30% by mass or more and 80% by mass or less. By setting the content of Si—B-containing particles 2 to 30% by mass or more, the proportion occupied by matrix 1 becomes small, and initial charge / discharge efficiency tends to improve. By setting the content of Si—B-containing particles 2 to 80% by mass or less, the degree of expansion and contraction of composite particle 10 during charge / discharge tends to be reduced. The content of Si—B-containing particles 2 in composite particle 10 may be 40% by mass or more, 50% by mass or more, or 60% by mass or more.

[0034] Matrix 1 may be, for example, at least one selected from the group consisting of a silicide phase, a carbon phase, and a metal-containing oxide phase. Matrix 1 may be at least one selected from the group consisting of a silicide phase and a carbon phase.

[0035] The silicon compound phase includes at least one selected from the group consisting of a silicate phase and a silicon oxide phase.

[0036] The silicon oxide phase is a phase consisting of a compound composed of Si and O. The silicon oxide phase may contain silicon dioxide (SiO). The main component of the silicon oxide phase may be SiO. SiO may account for, for example, 50% by mass or more of the silicon oxide phase, or 95% by mass or more of the silicon oxide phase.

[0037] The silicon compound phase may include a silicate phase. The silicate phase is a phase containing a compound (silicate) containing a metal element M1, silicon (Si), and O. The metal element M1 may include at least one element selected from the group consisting of alkali metal elements and Group 2 elements of the periodic table. The metal element M1 may be at least one element selected from the group consisting of Li, K, Na, Cs, Mg, Ca, Sr, and Ba.

[0038] The silicate phase may contain Li as the metal element M1. That is, the silicon compound phase may contain lithium silicate containing Li, Si, and O. Lithium silicate has excellent lithium ion conductivity.

[0039] The ratio of Li to the metal elements other than Si contained in the silicon compound phase may be 50 atomic % or more, 70 atomic % or more, or 90 atomic % or more.

[0040] The silicon compound phase may contain lithium silicate as a main component. Here, "the silicon compound phase contains lithium silicate as a main component" means that the mass ratio of lithium silicate in the silicon compound phase is 50 mass% or more.

[0041] The composition of lithium silicate is, for example, the formula: Li 2z SiO 2+z (0<z<2). From the viewpoints of stability, ease of preparation, lithium ion conductivity, etc., z may satisfy 0<z<1, or z=1 / 2. Lithium silicate satisfying z=1 / 2 can be represented by Li2SiO5. Lithium silicate desirably contains Li2SiO5 as a main component, and it is desirable that Li2SiO5 be the main component of the entire silicon compound phase. Here, "main component" means a component that accounts for 50% by mass or more of the mass of the entire lithium silicate or the entire silicon compound phase. Li2SiO5 may account for 70% by mass or more of the components of the silicon compound phase. In a preferred embodiment of the silicon compound phase, most of the Si constitutes lithium silicate.

[0042] The silicon compound phase may further contain an element M2 in addition to Li, Si, and O. The element M2 may be at least one selected from the group consisting of Na, K, Ca, Mg, Ba, Zr, Nb, Ta, V, Ti, P, Bi, Zn, Sn, Pb, Sb, Co, F, W, Al, B, and rare earth elements. For example, the silicon compound phase may contain Li 2z SiO 2+z (0<z<2) and may contain element M2.

[0043] The content of each element contained in the silicon compound phase can be determined by the following method. The composition of the silicate phase can be calculated from the content of each element. The contents of B, Na, K, and Al are determined by quantitative analysis in accordance with JIS R3105 (1995) (method of analysis of borosilicate glass). The content of Ca is determined by quantitative analysis in accordance with JIS R3101 (1995) (method of analysis of soda-lime glass).

[0044] The contents of other elements are determined by the following method. First, a sample of composite particle 10 is completely dissolved in a heated acid solution (a mixed acid of hydrofluoric acid, nitric acid, and sulfuric acid), and the carbon remaining in the solution is removed by filtration. The resulting filtrate is then analyzed by ICP-AES 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 contained in composite particle 10 is calculated.

[0045] The composite particle 10 may contain a silicon compound phase and Si—B-containing particles 2, and these can be distinguished and quantified using Si-NMR.

[0046] The carbon phase includes a carbonaceous material. The carbonaceous material may be amorphous. The carbonaceous phase may be composed of an amorphous carbonaceous material. The carbonaceous material may be, for example, a pitch-derived material, amorphous carbon, carbon black, or an organic polymer-derived material. The pitch may be, for example, coal pitch or petroleum pitch, and the coal pitch may be, for example, coal tar pitch.

[0047] The metal-containing oxide phase is, for example, an alkali aluminate phase. The alkali aluminate phase is a phase containing alkali aluminate, which is a composite oxide containing at least one alkali metal element and aluminum (Al). The alkali metal element is an element belonging to Group 1 of the periodic table. That is, the alkali aluminate contains at least one element selected from the group consisting of Li, Na, K, Rb, Cs, and Fr. The alkali aluminate may contain at least one element selected from the group consisting of Li, Na, and K. The alkali aluminate may contain at least one element selected from the group consisting of Li and Na, or may contain Li. That is, the alkali aluminate may be lithium aluminate.

[0048] The average particle diameter of the composite particles 10 may be 1 μm or more and 20 μm or less, or may be 1 μm or more and 10 μm or less. The average particle diameter of the composite particles 10 can be measured, for example, in the same manner as the average particle diameter of the Si—B-containing particles 2.

[0049] 1 has a coating layer 3 on its surface. The coating layer 3 covers at least a portion of the surface of a particle (hereinafter also referred to as a "mother particle") formed of a matrix 1 and Si-B-containing particles 2.

[0050] The coating layer 3 contains carbon. The coating layer 3 is provided, for example, to improve the conductivity of the composite particle 10. The coating layer 3 is made of, for example, a conductive carbon material. The conductivity of the composite particle 10 can be dramatically improved by having the coating layer 3 containing a conductive carbon material on the surface thereof.

[0051] The carbon material includes, for example, at least one selected from the group consisting of carbon compounds and carbonaceous materials. Examples of carbon compounds include compounds containing carbon and hydrogen, and compounds containing carbon, hydrogen, and oxygen. Examples of carbonaceous materials include pitch-derived materials, amorphous carbon, carbon black, and organic polymer-derived materials. Examples of pitch include coal pitch and petroleum pitch, and coal pitch is, for example, coal tar pitch.

[0052] The thickness of the coating layer 3 is preferably thin enough not to substantially affect the average particle size of the composite particles 10. From the viewpoints of ensuring electrical conductivity and diffusibility of ions that contribute to charge and discharge, the thickness of the coating layer 3 may be 1 nm or more and 200 nm or less, or 5 nm or more and 100 nm or less. The thickness of the coating layer 3 can be measured, for example, by observing a cross section of the negative electrode active material using an SEM.

[0053] The composite particle 10 may not have the coating layer 3. The composite particle 10 may be composed of only the base particle.

[0054] In addition to the above-mentioned components, the composite particle 10 may further contain other components. The composite particle 10 may contain the other components in a proportion of more than 0% by mass and not more than 10% by mass, more than 0% by mass and not more than 5% by mass, more than 0% by mass and not more than 1% by mass, or more than 0% by mass and not more than 0.1% by mass.

[0055] (Method for Producing Negative Electrode Active Material) An example of a method for producing the composite particles 10 included in the negative electrode active material according to the first embodiment will be described.

[0056] An example of a method for producing a negative electrode active material according to the first embodiment includes preparing a matrix and combining the Si—B-containing particles with the matrix. Hereinafter, preparing the matrix will be referred to as step (i), and combining and sintering the matrix and the Si—B-containing particles will be referred to as step (ii). Each step will be described in detail.

[0057] <Step (i)> When the matrix is ​​a silicon compound phase containing lithium silicate, a raw material mixture containing a Si raw material and a Li raw material in a predetermined ratio is used. The raw material mixture may further contain at least one selected from the above-mentioned alkali metal elements, Group 2 elements of the periodic table, and element M2. The raw material mixture is melted, and the melt is passed through a metal roll to form flakes, synthesizing lithium silicate. The flaked lithium silicate is then crystallized by heat treatment in an air atmosphere at a temperature above the glass transition point and below the melting point. Note that the flaked lithium silicate can also be used without crystallization. Alternatively, the mixture can be fired at a temperature below the melting point without melting, to produce lithium silicate through a solid-state reaction.

[0058] Silicon oxide can be used as the Si raw material. Li raw materials can be, for example, lithium carbonate, lithium oxide, lithium hydroxide, lithium hydride, etc. These can be used alone or in combination of two or more. Raw materials for alkali metal elements, Group 2 elements, and element M2 include oxides, hydroxides, carbonate compounds, hydrides, nitrates, sulfates, etc. of each element.

[0059] Unreacted Si raw material may remain in the lithium silicate, and the remaining Si raw material is dispersed in the lithium silicate as a silicon oxide phase.

[0060] <Step (ii)> Si—B-containing particles are blended with the prepared lithium silicate to form a composite. The Si—B-containing particles are, for example, raw silicon containing a predetermined amount of boron (B). For example, composite particles (mother particles) are prepared through the following steps (a) to (c). The method for incorporating B into silicon is not particularly limited. For example, B can be doped into the raw silicon by placing a predetermined amount of Si particles and B in a quartz crucible and heating and melting it in an argon atmosphere.

[0061] [Step (a)] Raw silicon powder and lithium silicate powder are mixed in a mass ratio of, for example, 20:80 to 95:5. The raw silicon powder is, for example, coarse silicon particles having an average particle size of several μm or more and several tens of μm or less. The coarse particles contain a predetermined amount of B.

[0062] [Step (b)] Next, using a grinding device such as a ball mill, the mixture of raw silicon and lithium silicate is ground into fine particles and composited. An organic solvent may be added to the mixture and wet-ground. A predetermined amount of organic solvent may be added to the grinding vessel all at once at the beginning of grinding, or the organic solvent may be added to the grinding vessel intermittently in multiple batches during the grinding process. The organic solvent serves to prevent the material to be ground from adhering to the inner wall of the grinding vessel.

[0063] As the organic solvent, alcohols, ethers, fatty acids, alkanes, cycloalkanes, silicate esters, metal alkoxides, etc. can be used.

[0064] The raw silicon and lithium silicate may be separately microparticulated and then mixed. Alternatively, silicon nanoparticles and amorphous lithium silicate nanoparticles may be prepared and then mixed without using a pulverizer. The nanoparticles may be produced by known methods such as a gas phase method (e.g., a plasma method) or a liquid phase method (e.g., a liquid phase reduction method).

[0065] [Step (c)] The mixture is sintered by heating it to, for example, 450°C or higher and 1000°C or lower while applying pressure in an inert gas atmosphere (e.g., an argon, nitrogen, or other atmosphere). For sintering, a sintering device capable of applying pressure in an inert atmosphere, such as a hot press or spark plasma sintering, can be used. During sintering, the lithium silicate softens and flows to fill the gaps between the silicon particles. As a result, a dense block-shaped sintered body can be obtained, with the silicate phase forming the sea portion and the silicon particles forming the islands.

[0066] The obtained sintered body is pulverized to obtain particles (mother particles) in which the Si—B-containing particles 2 are dispersed in the matrix 1. By appropriately selecting the pulverization conditions, mother particles having a predetermined average particle size can be obtained.

[0067] Sintering may be performed by rolling the powder of the mixture previously molded into a sheet in a heated state. Rolling is usually performed by passing a sheet of the mixture between two rotating rolls. By passing the sheet of the mixture through the gap between the heated rolls, the mixture can be heated and pressurized simultaneously. The sheet of the mixture may be heated before or after passing through the rolls, or the sheet of the mixture may be heated before and after passing through the rolls. By performing a heat treatment while rolling the mixture molded into a sheet, the composite particles 10 can be produced with high productivity.

[0068] <Step (iii)> At least a portion of the surface of the obtained base particles may be coated with a conductive material to form a coating layer 3. The conductive material is preferably electrochemically stable, and a conductive carbon material is preferred. A method for coating at least a portion of the surface of the base particles with a conductive carbon material includes a CVD method using a hydrocarbon gas such as acetylene or methane as a raw material. Another example is a method in which coal pitch, petroleum pitch, phenolic resin, or the like is mixed with the base particles and heated to carbonize them. A mixture of the base particles and a conductive carbon material raw material such as coal pitch, petroleum pitch, or phenolic resin is heated, for example, in an inert atmosphere (e.g., an argon or nitrogen atmosphere) at a temperature of 700°C or higher and 950°C or lower. Carbon black may also be attached to the surface of the base particles. In this manner, composite particles 10 are obtained.

[0069] <Step (iv)> A step of washing the composite particles 10 (or the base particles if step (iii) is not performed) with an acid may be further carried out. For example, by washing the composite particles with an acidic aqueous solution, it is possible to remove trace amounts of alkaline components that may be generated when the raw silicon and lithium silicate are combined. As the acidic aqueous solution, an aqueous solution of an inorganic acid such as hydrochloric acid, hydrofluoric acid, sulfuric acid, nitric acid, phosphoric acid, or carbonic acid, or an aqueous solution of an organic acid such as citric acid or acetic acid can be used.

[0070] (Embodiment 2) A secondary battery according to embodiment 2 includes a negative electrode, a positive electrode, and an electrolyte. The negative electrode includes the negative electrode active material according to embodiment 1. For example, the negative electrode includes a negative electrode mixture layer including the negative electrode active material according to embodiment 1 and a negative electrode current collector.

[0071] FIG. 2 is a cross-sectional view showing a schematic configuration of a secondary battery according to a second embodiment. The secondary battery 100 includes a positive electrode 23, a negative electrode 26, a separator 27, a non-aqueous electrolyte 29, and an exterior casing 28. The positive electrode 23, the negative electrode 26, the non-aqueous electrolyte 29, and the separator 27 are housed in the exterior casing 28. The separator 27 is disposed between the positive electrode 23 and the negative electrode 26. The positive electrode 23 and the negative electrode 26 face each other with the separator 27 interposed therebetween. The positive electrode 23 includes a positive electrode mixture layer 22 and a positive electrode current collector 21. The positive electrode mixture layer 22 is disposed between the positive electrode current collector 21 and the separator 27. The negative electrode 26 includes a negative electrode mixture layer 25 and a negative electrode current collector 24. The negative electrode mixture layer 25 is disposed between the negative electrode current collector 24 and the separator 27.

[0072] The positive electrode mixture layer 22 is made of a positive electrode mixture. The positive electrode mixture layer 22 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 21 and drying the applied layer. The dried coating may be rolled as necessary. The positive electrode mixture layer 22 may be formed on one surface or both surfaces of the positive electrode current collector 21.

[0073] The positive electrode mixture includes a positive electrode active material capable of absorbing and releasing lithium ions. Examples of positive electrode active materials include lithium-containing transition metal oxides, lithium-containing transition metal phosphates, transition metal fluorides, polyanionic materials, fluorinated polyanionic materials, transition metal sulfides, transition metal oxysulfides, and transition metal oxynitrides. In particular, using a lithium-containing transition metal oxide or a lithium-containing transition metal phosphate as the positive electrode active material can reduce the manufacturing cost of the battery and increase the average discharge voltage. Examples of lithium-containing transition metal oxides include lithium cobalt oxide, lithium nickel cobalt aluminum oxide, lithium nickel cobalt manganese oxide, and lithium nickel manganese oxide. Examples of lithium-containing transition metal phosphates include lithium iron phosphate, lithium vanadium phosphate, lithium cobalt phosphate, and lithium nickel phosphate. At least one selected from these positive electrode active materials can be used.

[0074] The positive electrode mixture may contain a conductive additive, an ion conductor, and a binder, as needed.

[0075] Conductive additives and ion conductors are used to reduce the resistance of the electrodes. Examples of conductive additives include carbon materials and conductive polymer compounds. Examples of carbon materials include carbon black, graphite, acetylene black, carbon nanotubes, carbon nanofibers, graphene, fullerene, and graphite oxide. Examples of conductive polymer compounds include polyaniline, polypyrrole, and polythiophene. At least one selected from these conductive additives can be used.

[0076] Examples of ionic conductors include gel electrolytes such as polymethyl methacrylate and polymethyl methacrylate, organic solid electrolytes such as polyethylene oxide, and Li7La3Zr2O 12 At least one selected from these ion conductors can be used.

[0077] Binders are used to improve the binding properties of the materials that make up the electrode. Examples of binders include polymeric materials such as polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, polytetrafluoroethylene, carboxymethyl cellulose, polyacrylic acid, styrene-butadiene copolymer rubber, polypropylene, polyethylene, and polyimide. At least one selected from these binders can be used.

[0078] The positive electrode current collector 21 is a sheet or film made of a metal material such as aluminum, an aluminum alloy, stainless steel, titanium, or a titanium alloy. The sheet or film may be porous or non-porous. Examples of the sheet or film include metal foil and metal mesh. A carbon material may be applied to the surface of the positive electrode current collector 21 as a conductive auxiliary material.

[0079] The anode mixture layer 25 is composed of an anode mixture. The anode mixture includes the anode active material according to embodiment 1. The anode mixture layer 25 can be formed, for example, by applying an anode slurry, in which the anode mixture is dispersed in a dispersion medium, to the surface of the anode current collector 24 and drying the applied slurry. The dried coating may be rolled as needed. The anode mixture layer 25 may be formed on one or both surfaces of the anode current collector 24. The anode mixture may include a conductive additive, an ion conductor, and a binder as needed. Materials usable for the conductive additive, ion conductor, and binder in the cathode mixture layer 22 may also be used for the anode mixture layer 25. The anode mixture may include an anode active material other than the anode active material according to embodiment 1.

[0080] The negative electrode current collector 24 is a sheet or film made of a metal material such as stainless steel, nickel, a nickel alloy, copper, or a copper alloy. The sheet or film may be porous or non-porous. Examples of the sheet or film include metal foil and metal mesh. A carbon material may be applied to the surface of the negative electrode current collector 24 as a conductive auxiliary material.

[0081] The separator 27 is an electrolyte layer having lithium ion conductivity. There are no particular limitations on the material of the separator 27 as long as it allows the passage of lithium ions. The material of the separator 27 can be at least one selected from the group consisting of solid electrolytes, gel electrolytes, ion exchange resin membranes, semipermeable membranes, and porous membranes. If the separator 27 is made of these materials, the safety of the secondary battery 100 can be sufficiently ensured. Examples of solid electrolytes include sulfide solid electrolytes such as Li2S-P2S5, Li7La3Zr2O 12 Examples of the electrolyte include oxide solid electrolytes such as LLZ. Examples of the gel electrolyte include gel electrolytes containing fluororesins such as PVdF. Examples of the ion exchange resin membrane include cation exchange membranes and anion exchange membranes. Examples of the porous membrane include porous membranes made of polyolefin resins and porous membranes made of glass paper obtained by weaving glass fibers into nonwoven fabric.

[0082] The non-aqueous electrolyte 29 may be impregnated into the positive electrode 23, the negative electrode 26, and the separator 27. The non-aqueous electrolyte 29 may fill the internal space of the exterior casing 28. The non-aqueous electrolyte 29 functions to allow lithium ions to travel between the positive electrode 23 and the negative electrode 26. The non-aqueous electrolyte 29 may include a non-aqueous electrolyte solution, a gel electrolyte, or an ionic liquid.

[0083] The non-aqueous electrolyte solution contains, for example, a non-aqueous solvent and a lithium salt.

[0084] Examples of non-aqueous solvents that can be used include cyclic carbonates, chain carbonates, cyclic ethers, chain ethers, cyclic esters, chain esters, fluorine-containing solvents, and nitriles. Examples of cyclic carbonates include ethylene carbonate, propylene carbonate, and butylene carbonate. Examples of chain carbonates include dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate. Examples of cyclic ethers include tetrahydrofuran, 1,4-dioxane, and 1,3-dioxolane. Examples of chain ethers include 1,2-dimethoxyethane and 1,2-diethoxyethane. Examples of cyclic esters include γ-butyrolactone. Examples of chain esters include methyl acetate. Examples of fluorine-containing solvents include fluoroethylene carbonate, methyl fluoropropionate, fluorobenzene, fluoroethyl methyl carbonate, and fluorodimethylene carbonate. Examples of nitriles include acetonitrile. At least one selected from these non-aqueous solvents can be used.

[0085] Examples of lithium salts include lithium hexafluorophosphate (LiPF), lithium tetrafluoroborate (LiBF), lithium perchlorate (LiClO), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bisperfluoroethylsulfonylimide (LiN(SO2C2F5)2), LiAsF6, LiCF3SO3, and lithium difluoro(oxalato)borate. At least one selected from these lithium salts can be used.

[0086] The gel electrolyte can be a material obtained by impregnating a polymer material with a non-aqueous electrolyte solution, such as polyethylene oxide, polyacrylonitrile, polyvinylidene fluoride, polymethyl methacrylate, or a polymer having an ethylene oxide bond.

[0087] Examples of cations constituting ionic liquids include aliphatic chain quaternary cations, aliphatic cyclic ammonium, and nitrogen-containing heterocyclic aromatic cations. Examples of aliphatic chain quaternary cations include tetraalkylammonium and tetraalkylphosphonium. Examples of aliphatic cyclic ammonium include pyrrolidiniums, morpholiniums, imidazoliniums, tetrahydropyrimidiniums, piperaziniums, and piperidiniums. Examples of nitrogen-containing heterocyclic aromatic cations include pyridiniums and imidazoliums. Examples of anions constituting ionic liquids include PF6 - , BF4 - , SbF6 - , AsF6 - , SO3CF3 - , N(SO2F)2 - , N(SO2CF3)2 - , N(SO2C2F5)2 - , N(SO2CF3)(SO2C4F9) - , C(SO2CF3)3 - The ionic liquid may contain a lithium salt.

[0088] The exterior 28 is made of a material obtained by laminating a metal foil such as an aluminum foil with a resin film such as a PET film, for example. The exterior 28 may also be a resin or metal container.

[0089] The shape of the secondary battery 100 is not limited to the laminated type. Other shapes of the secondary battery 100 include a coin type, a cylindrical type, a square type, a sheet type, a button type, and a flat type.

[0090] In the secondary battery according to the second embodiment, the negative electrode contains the negative electrode active material according to the first embodiment, and thus the charge capacity is improved.

[0091] (Other Embodiments) (Additional Notes) The above description of the embodiments discloses the following techniques.

[0092] (Technology 1) A negative electrode active material comprising composite particles including a matrix and Si—B-containing particles, the Si—B-containing particles being dispersed in the matrix.

[0093] This configuration improves the charging capacity of the battery.

[0094] (Technology 2) The negative electrode active material according to Technology 1, wherein the ratio of B in the Si—B-containing particles is 0.01 mass % or more and 5.0 mass % or less. With this configuration, the charge capacity of the battery can be improved.

[0095] (Technology 3) The negative electrode active material according to Technology 1 or 2, wherein the Si—B-containing particles contain crystalline silicon in which B atoms are dissolved. With this configuration, the charge capacity of the battery can be improved.

[0096] (Technology 4) The negative electrode active material according to any one of Technologies 1 to 3, wherein the Si—B-containing particles contain at least one selected from the group consisting of boron oxide and elemental B. With this configuration, the charge capacity of the battery can be improved.

[0097] (Technology 5) The negative electrode active material according to any one of Technologies 1 to 4, wherein the Si—B-containing particles contain an amorphous phase. With this configuration, the charge capacity of the battery can be improved.

[0098] (Technology 6) The negative electrode active material according to any one of Technologies 1 to 5, wherein the Si—B-containing particles have an average particle size of 1 nm or more and 1000 nm or less. With this configuration, the charge capacity of the battery can be improved.

[0099] (Technology 7) The negative electrode active material according to any one of Technologies 1 to 6, wherein the matrix is ​​at least one selected from the group consisting of a silicon compound phase and a carbon phase. With this configuration, the charge capacity of the battery can be improved.

[0100] (Technology 8) The negative electrode active material according to Technology 7, wherein the silicon compound phase contains, as a main component, lithium silicate containing Li, Si, and O. Lithium silicate has excellent ionic conductivity. With this configuration, the charge capacity of the battery can be improved.

[0101] (Technology 9) The negative electrode active material according to Technology 7 or 8, wherein the silicon compound phase further contains an element M2, and the element M2 is at least one selected from the group consisting of Na, K, Ca, Mg, Ba, Zr, Nb, Ta, V, Ti, P, Bi, Zn, Sn, Pb, Sb, Co, F, W, Al, B, and rare earth elements. With this configuration, the charge capacity of the battery can be further improved.

[0102] (Technology 10) The negative electrode active material according to any one of Technologies 1 to 9, wherein the composite particles have a coating layer on the surface thereof, and the coating layer contains carbon. By including the coating layer, electrical conductivity is improved. With this configuration, the charge capacity of the battery can be further improved.

[0103] (Technology 11) A secondary battery comprising: a negative electrode containing the negative electrode active material according to any one of Technologies 1 to 10; a positive electrode; and an electrolyte.

[0104] This configuration provides the effect of improving the charging capacity.

[0105] The present disclosure will be described in more detail below using examples. The following examples are merely examples of embodiments, and are not intended to limit the scope of the present disclosure.

[0106] [Preparation of Composite Particles] (Example 1) Lithium carbonate and silicon dioxide were mixed to a molar ratio of Li2CO3:SiO2 = 34:66, the mixture was heated to 1500°C for 5 hours in an inert gas atmosphere to melt, the melt was passed through a metal roller to form flakes, and the flake product was heated at 750°C for 5 hours to produce lithium silicate, which was then pulverized to an average particle size of 10 µm.

[0107] The obtained lithium silicate having an average particle size of 10 μm was mixed with coarse silicon particles (average particle size 10 μm) containing 1 mass % B at a mass ratio of 40:60. The mixture was filled into a pot (SUS, volume: 500 mL) of a planetary ball mill (Fritsch, P-5), 24 SUS balls (diameter 20 mm) were placed inside, the lid was closed, and the mixture was milled at 200 rpm for 25 hours in an inert atmosphere to composite the coarse silicon particles and lithium silicate.

[0108] The pulverized material was then pressure-molded into a sheet having a thickness of 4 mm to 5 mm at room temperature to obtain a sheet-like compact. The compact was heated to 800°C and rolled between rolls to obtain a sintered body. The rolling was carried out so that a pressure of 240 MPa was applied to the compact. The sintered body was pulverized and passed through a 40 μm mesh. In this manner, mother particles in which Si—B-containing particles were dispersed in a matrix of a lithium silicate phase were obtained.

[0109] The base particles were mixed with coal pitch (MCP250, manufactured by JFE Chemical Corporation), and the mixture was fired at 800°C for 5 hours in an inert atmosphere to form a conductive carbon coating layer on the surface of the base particles. At this time, the ratio of the mass of the coating layer to the total mass of the base particles and the coating layer was 5%. Then, composite particles with an average particle diameter of 5 µm and a coating layer were obtained using a sieve.

[0110] In this manner, the composite particles of Example 1 were obtained. When the cross section of the composite particles was analyzed by SEM-EDX, it was confirmed that the Si—B-containing particles contained B at a content of 1 mass %.

[0111] Comparative Example 1 Composite particles of Comparative Example 1 were obtained in the same manner as in Example 1, except that silicon coarse particles not containing B were used.

[0112] [X-ray Diffraction] X-ray diffraction patterns of the composite particles of Example 1 and Comparative Example 1 were obtained by powder X-ray diffraction measurement. An X-ray diffractometer (MiniFlex 600, manufactured by RIGAKU Corporation) was used for the measurement. Cu-Kα radiation (wavelengths 1.5405 Å and 1.5444 Å) was used as the X-ray source. FIG. 3 is a graph showing the X-ray diffraction patterns of the composite particles of Example 1 and Comparative Example 1. FIG. 4 is a partially enlarged graph of the X-ray diffraction patterns of the composite particles of Example 1 and Comparative Example 1. In the X-ray diffraction pattern of the negative electrode active material of Example 1, a peak derived from the (111) plane of Si was observed at a diffraction angle 2θ of 28.42°. On the other hand, in the X-ray diffraction pattern of the negative electrode active material of Comparative Example 1, a peak derived from the (111) plane of Si was observed at a diffraction angle 2θ of 28.41°. It is generally known that when an atom with a smaller atomic radius than the atoms constituting the crystal lattice is introduced into the crystal lattice, the peak shifts to a higher angle. Therefore, from the above results, it can be confirmed that B is introduced into the silicon crystal lattice in the composite particles of Example 1.

[0113] [Si Content] The composite particles of Example 1 and Comparative Example 1 were analyzed using ICP and Si-NMR to measure the Si content of the Si-B-containing particles in the composite particles. The measurement results are shown in Table 1.

[0114] [Volume Resistivity of Silicon Particles] The volume resistivity (powder resistivity) of each of the silicon coarse particles containing B used in Example 1 and the silicon coarse particles used in Comparative Example 1 was measured using an automatic powder resistivity measurement system (manufactured by Nitto Seiko Analytech Co., Ltd.) The volume resistivities of the silicon coarse particles used in Example 1 and Comparative Example 1 at an apparent density of 1.5 g / cc are shown in Table 1.

[0115]

[0116] As can be seen from Table 1, the silicon coarse particles containing B used in Example 1 had a lower volume resistivity than the silicon coarse particles used in Comparative Example 1. In other words, the composite particles of Example 1 contain silicon particles with improved electronic conductivity.

[0117] [Fabrication of Battery] Using the composite particles of Example 1 and Comparative Example 1, evaluation cells of Example 1 and Comparative Example 1 were fabricated as follows.

[0118] (Preparation of Negative Electrode) Composite particles and graphite were mixed in a mass ratio of 20:80 and used as the negative electrode active material. The negative electrode active material, carboxymethyl cellulose sodium (CMCNa), styrene-butadiene rubber (SBR), and lithium polyacrylate were mixed in a mass ratio of negative electrode active material:CMCNa:SBR:lithium polyacrylate = 96.5:1:1.5:1. Water was added to the negative electrode mixture, and the mixture was stirred using a mixer to prepare a negative electrode slurry. Next, a 1 m thick layer was applied to the surface of copper foil. 2 The negative electrode slurry was applied to both sides of the copper foil so that the mass of the negative electrode mixture per sheet was 190 g, and the coating was dried and then rolled to form a negative electrode mixture with a density of 1.5 g / cm 3 A negative electrode having the negative electrode mixture layer formed thereon was fabricated.

[0119] The negative electrode was cut into a 20 mm × 20 mm shape with a 5 mm × 5 mm protrusion, and the negative electrode mixture layer at the protrusion was peeled off to expose the copper foil. A negative electrode tab lead was then connected to the exposed copper foil, and a predetermined region around the periphery of the negative electrode tab lead was covered with an insulating film.

[0120] (Preparation of Counter Electrode) A tab was prepared by welding a small piece of Ni mesh to the end thereof. The tab was cut to a predetermined size, and the mesh portion was pressed against a lithium metal foil having a thickness of 300 μm to prepare a counter electrode.

[0121] (Preparation of non-aqueous electrolyte) A non-aqueous electrolyte was prepared by dissolving LiPF in a mixed solvent of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of EC:EMC:DMC = 4:1:15. The LiPF concentration was 1.3 mol / L.

[0122] (Cell Fabrication) Using the above-mentioned negative electrode and two counter electrodes, a cell for evaluating negative electrode regulation was fabricated as follows. The cell was fabricated in a dry air atmosphere with a dew point of −60° C. or less. An electrode group was fabricated by sandwiching the negative electrode between a pair of counter electrodes and arranging the negative electrode mixture layer and lithium metal foil opposite each other via a separator. Next, an Al laminate film cut into a rectangle was folded in half, and the two long edge portions were heat-sealed to form a cylinder. Thereafter, the fabricated electrode group was placed into the cylinder from one of the short edge portions of the Al laminate film, and the end face of the Al laminate film was aligned with the insulating film of each tab lead and heat-sealed. Next, 0.3 cm of electrolyte was poured into the cylinder from the short edge portion of the cylinder that was not heat-sealed. 3 After the injection, the negative electrode mixture layer was impregnated with the nonaqueous electrolyte by leaving the negative electrode mixture layer under a reduced pressure of 0.02 MPa for 3 minutes and then returning the pressure to atmospheric pressure twice. Finally, the end face of the cylindrical Al laminate film on the injected side was heat-sealed while reducing the pressure to obtain an evaluation cell.

[0123] [Battery Evaluation] The initial charge / discharge characteristics and cycle characteristics of the evaluation cells of Example 1 and Comparative Example 1 were evaluated as follows. The evaluation results are shown in Table 2.

[0124] At an ambient temperature of 25°C, constant current charging was performed at a current value of 0.1 C until the voltage reached 1.0 V. Thereafter, constant current discharging was performed at a current value of 0.1 C until the cell voltage reached 0.005 V. This allowed the initial charge / discharge capacities of the evaluation cells of Example 1 and Comparative Example 1 to be evaluated. The above charge / discharge cycle was counted as one cycle, and 100 cycles were performed, and the capacity retention rate was calculated using the following formula: Capacity retention rate [%] = (100th cycle discharge capacity / 1st cycle discharge capacity) x 100

[0125]

[0126] (Discussion) The battery of Example 1 exhibited higher charge and discharge capacities than those of Comparative Example 1. This is presumably due to improved electronic conductivity caused by the silicon particles containing B. From the above, the use of the negative electrode active material of the present disclosure enabled the charge and discharge capacity of the battery to be improved.

[0127] The technology of the present disclosure can be used in batteries such as lithium ion secondary batteries.

Claims

1. A negative electrode active material comprising composite particles including a matrix and Si-B containing particles, the Si-B containing particles being dispersed in the matrix.

2. The negative electrode active material according to claim 1, wherein the ratio of B in the Si-B containing particles is 0.01% by mass or more and 5.0% by mass or less.

3. The negative electrode active material according to claim 1, wherein the Si-B containing particles contain crystalline silicon in which B atoms are dissolved.

4. The negative electrode active material according to claim 1, wherein the Si-B-containing particles contain at least one selected from the group consisting of boron oxides and elemental B.

5. The negative electrode active material according to claim 1, wherein the Si-B containing particles include an amorphous phase.

6. The negative electrode active material according to claim 1, wherein the average particle size of the Si-B containing particles is 1 nm or more and 1000 nm or less.

7. The negative electrode active material according to claim 1, wherein the matrix is ​​at least one selected from the group consisting of a silicon compound phase and a carbon phase.

8. The negative electrode active material according to claim 7, wherein the silicon compound phase contains lithium silicate containing Li, Si, and O as a main component.

9. The negative electrode active material according to claim 8, wherein the silicon compound phase further contains an element M2, and the element M2 is at least one selected from the group consisting of Na, K, Ca, Mg, Ba, Zr, Nb, Ta, V, Ti, P, Bi, Zn, Sn, Pb, Sb, Co, F, W, Al, B, and rare earth elements.

10. The negative electrode active material according to claim 1, wherein the composite particles have a coating layer on a surface thereof, the coating layer including carbon.

11. A secondary battery comprising: a negative electrode containing the negative electrode active material according to any one of claims 1 to 10; a positive electrode; and an electrolyte.

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