Negative electrode active material and secondary battery

Abstract

This negative electrode active material is provided with composite particles (10) containing 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 is provided with a negative electrode (26) containing the negative electrode active material according to the present disclosure, a positive electrode (23), and an electrolyte.

Description

Technical Field

[0001] This disclosure relates to negative electrode active materials and secondary batteries. Background Technology

[0002] In recent years, secondary batteries, such as non-aqueous electrolyte batteries, have been anticipated as power sources for small-scale civilian applications, energy storage devices, and electric vehicles due to their high voltage and high energy density. In the pursuit of higher energy density in batteries, materials containing silicon alloyed with lithium are expected to be utilized as the negative electrode active material, possessing a theoretically high capacity density.

[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] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: International Publication No. 2021 / 153078 Summary of the Invention

[0007] The problem the invention aims to solve

[0008] This disclosure provides a negative electrode active material that can improve the charging capacity of a battery.

[0009] Solution for solving the problem

[0010] The negative electrode active material disclosed herein comprises composite particles, wherein the composite particles include:

[0011] matrix, and

[0012] Si-B containing particles,

[0013] The aforementioned Si-B-containing particles are dispersed in the aforementioned matrix.

[0014] The effects of the invention

[0015] According to the technology disclosed herein, a negative electrode active material capable of increasing the charging capacity of a battery can be provided. Attached Figure Description

[0016] Figure 1 This is a cross-sectional view showing the schematic structure of the composite particles 10 of the negative electrode active material in Embodiment 1.

[0017] Figure 2 This is a cross-sectional view showing the general structure of the secondary battery in Embodiment 2.

[0018] Figure 3This is a diagram showing the X-ray diffraction patterns of the composite particles of Example 1 and Comparative Example 1.

[0019] Figure 4 This is a partially enlarged view of the X-ray diffraction patterns of the composite particles of Example 1 and Comparative Example 1. Detailed Implementation

[0020] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. The present disclosure is not limited to the following embodiments.

[0021] (Implementation Method 1)

[0022] The negative electrode active material of Embodiment 1 comprises composite particles containing a matrix and Si-B particles.

[0023] Figure 1 This is a cross-sectional view showing the schematic structure of the composite particles 10 of the negative electrode active material of Embodiment 1. The composite particles 10 are particles having a matrix 1 and Si-B containing particles 2 dispersed in the matrix 1.

[0024] The Si-B-containing particles 2 are particles containing silicon (Si) and boron (B). For example, Si-B-containing particles 2 are silicon particles containing B. By including B, the Si-B-containing particles 2 exhibit improved electronic conductivity and lower volume resistivity (powder resistivity) compared to silicon particles without B. That is, in the composite particles 10, silicon particles with improved electronic conductivity are dispersed in the matrix 1. Through this configuration, the charging and discharging capacity of the battery can be improved.

[0025] Particle 2 containing Si-B is composed of silicon, for example, more than 90 atomic percent. Particle 2 containing Si-B may contain a phase of elemental silicon (Si).

[0026] In the Si-B-containing particles 2, B can be located inside the Si-B-containing particles 2 or on the surface of the Si-B-containing particles 2. For example, B can be dispersed inside the silicon particles or on the surface of the silicon particles.

[0027] In particles 2 containing Si-B, B can be present in any form, such as atoms, elements, or compounds.

[0028] B can be dispersed in particles 2 containing Si-B.

[0029] In the Si-B-containing particles 2, at least a portion of the B can be substituted or inserted into the lattice sites of silicon as B atoms, or it can be substituted. The Si-B-containing particles 2 can contain crystalline silicon with B atoms dissolved in it. That is, in the Si-B-containing particles 2, at least a portion of the B can be B atoms dissolved in crystalline silicon. Based on the above configuration, the charging capacity and discharging capacity of the battery can be improved. In addition, cracks and fractures in the composite particles 10 can also be reduced.

[0030] The Si-B-containing particles 2 may contain at least one component selected from the group consisting of boron oxide and elemental B. The Si-B-containing particles 2 may contain boron oxide. The Si-B-containing particles 2 may contain B atoms dissolved in crystalline silicon, boron oxide, and elemental B, or may contain B atoms dissolved in crystalline silicon and boron oxide. The boron oxide may be dispersed within the silicon particles or located on the surface of the silicon particles.

[0031] The Si-B-containing particles 2 may contain a crystalline phase. For example, as described above, the Si-B-containing particles 2 may contain crystalline silicon. In this specification, the Si-B-containing particles 2 containing crystalline silicon means that a peak originating from the (111) plane of Si is observed in the X-ray diffraction pattern of the composite particles 10 obtained by X-ray diffraction using Cu-Kα rays. The X-ray diffraction peak originating from the (111) plane of Si is, for example, an X-ray diffraction peak with the highest intensity in the range of diffraction angle 2θ between 28.0° and 28.7°.

[0032] The X-ray diffraction pattern of the composite particle 10 can be obtained by X-ray diffraction determination based on the θ-2θ method using Cu-Kα rays with wavelengths of 1.5405 Å and 1.5444 Å, i.e. wavelengths of 0.15405 nm and 0.15444 nm.

[0033] In this specification, a peak refers to a mountain-shaped portion with a signal-to-noise ratio (i.e., the ratio of signal S to background noise N) of 1.3 or higher and a half-width of 10° or less.

[0034] In the X-ray diffraction pattern of the composite particle 10 obtained by X-ray diffraction measurement using Cu-Kα rays, for example, X-ray diffraction peaks originating from the (111) plane of Si can exist in the range of diffraction angle 2θ above 28.42° and below 28.57°. "X-ray diffraction peaks originating from the (111) plane of Si can exist in the range of diffraction angle 2θ above 28.42° and below 28.57°" means that the peak of the X-ray diffraction peak originating from the (111) plane of Si is observed in the range of diffraction angle 2θ above 28.42° and below 28.57°.

[0035] The diffraction angle of a peak in an X-ray diffraction pattern is defined as the angle representing the maximum intensity of the mountain-shaped portion with a S / N ratio greater than 1.3 and a half-value width (WWV) less than 10°. The WWV refers to the angle at which the maximum intensity of the X-ray diffraction peak is set to I. MAX When, the intensity becomes I MAX The width is represented by the difference between two diffraction angles, which is half the value of the diffraction angle.

[0036] Particle 2 containing Si-B can contain an amorphous phase.

[0037] Particle 2 containing Si-B can contain crystalline and amorphous portions. The amorphous portions can originate from either B or Si.

[0038] The proportion of B in Si-B-containing particles 2 can be 0.01% by mass or more and 5.0% by mass, 0.01% by mass or more and 3.0% by mass, or 0.01% by mass or more and 1.0% by mass. The proportion of B in Si-B-containing particles 2 can be 0.05% by mass or more, 0.1% by mass or more, or 1% by mass or more.

[0039] The mass ratio of B in Si-B particles 2 can be determined, for example, by inductively coupled plasma atomic emission spectrometry (ICP-AES).

[0040] The average particle size of the Si-B-containing particles 2 can be greater than 1 nm and less than 1000 nm, greater than 1 nm and less than 100 nm, or greater than 25 nm and less than 65 nm. These fine Si-B-containing particles 2 exhibit small volume changes during charging and discharging, which can improve the structural stability of the composite particles 10. Therefore, it can improve the charging and discharging capacity of the battery.

[0041] The average particle size of the Si-B-containing particles 2 was determined by scanning electron microscopy (SEM) of the cross-section of the negative electrode active material (composite particles 10). That is, the average particle size of the Si-B-containing particles 2 can be determined based on the SEM image of the cross-section of the composite particles 10 exposed by the cross-section of the Si-B-containing particles 2. The average particle size can be calculated by measuring the particle size of 500 Si-B-containing particles 2 randomly extracted from the SEM image of the composite particles 10 and calculating their median particle size. Here, the maximum Ferrette diameter of the Si-B-containing particles 2 in the obtained SEM image can be considered as the particle size of the Si-B-containing particles 2. The "maximum Ferrette diameter" is the maximum length of the perpendicular line obtained by holding the particle between two parallel lines.

[0042] The median particle size of Si-B-containing particles 2 was determined by SEM observation of 500 particles 2.

[0043] The mass ratio of Si in the Si-B-containing particles 2 to the mass of the composite particles 10 can be more than 20% by mass and less than 80% by mass, more than 20% by mass and less than 75% by mass, more than 40% by mass and less than 70% by mass, or more than 50% by mass and less than 66% by mass.

[0044] The mass ratio of Si contained in Si-B particles 2 to the mass of composite particles 10 can be determined using ICP and NMR.

[0045] To achieve high capacity and improved cycle performance, the content of Si-B-containing particles 2 in the composite particles 10 can be 30% by mass or more and 80% by mass or less. By making the content of Si-B-containing particles 2 30% by mass or more, the proportion of the matrix 1 is reduced, making it easier to improve the initial charge-discharge efficiency. By making the content of Si-B-containing particles 2 80% by mass or less, the degree of expansion and contraction of the composite particles 10 during charge and discharge can be easily reduced. The content of Si-B-containing particles 2 in the composite particles 10 can be 40% by mass or more, 50% by mass or more, or 60% by mass or more.

[0046] The matrix 1 may be at least one selected from the group consisting of silicon compound phases, carbon phases, and metal-containing oxide phases. The matrix 1 may be at least one selected from the group consisting of silicon compound phases and carbon phases.

[0047] The silicon compound phase includes at least one selected from the group consisting of silicate phase and silicon oxide phase.

[0048] The silica phase is a phase containing compounds composed of Si and O. The silica phase may contain silicon dioxide (SiO2). The main component of the silica phase can be SiO2. SiO2 can account for, for example, more than 50% by mass or more than 95% by mass of the silica phase.

[0049] The silicon compound phase may include a silicate phase. A silicate phase is a phase comprising a compound (silicate) containing the metal element M1, silicon (Si), and oxygen. The metal element M1 may include at least one element selected from the group consisting of alkali metals and Group 2 elements of the periodic table. Specifically, 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.

[0050] The silicate phase may contain Li as the metal element M1. That is, the silicon compound phase may also contain lithium silicate containing Li, Si, and O. Lithium silicate has excellent lithium ion conductivity.

[0051] The proportion of Li may be 50 atomic % or more with respect to the metal elements other than Si contained in the silicon compound phase. The proportion of Li may be 70 atomic % or more, or may be 90 atomic % or more with respect to the metal elements other than Si contained in the silicon compound phase.

[0052] 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.

[0053] The composition of lithium silicate is represented by, for example, the formula: Li 2z SiO 2+z (0 < z < 2). From the viewpoints of stability, ease of production, lithium ion conductivity, etc., z may satisfy 0 < z < 1, or may satisfy z = 1 / 2. Lithium silicate satisfying z = 1 / 2 may be represented by Li2Si2O5. Lithium silicate preferably contains Li2Si2O5 as a main component, and Li2Si2O5 is preferably the main component of the entire silicon compound phase. Here, the "main component" means a component accounting for 50 mass % or more of the mass of the entire lithium silicate or the entire silicon compound phase. Li2Si2O5 may be a component accounting for 70 mass % or more of the silicon compound phase. In a preferred embodiment of the silicon compound phase, most of Si constitutes lithium silicate.

[0054] In addition to containing Li, Si, and O, the silicon compound phase may further contain an element M2. 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 the element M2.

[0055] The content rate of each element contained in the silicon compound phase can be determined by the following method. In addition, the composition of the silicate phase can be calculated from the content rate of each element. The contents of B, Na, K, and Al are determined by quantitative analysis according to JIS R3105 (1995) (Analytical method for borosilicate glass). The Ca content is determined by quantitative analysis according to JIS R3101 (1995) (Analytical method for soda-lime glass).

[0056] The contents of other elements were determined using the following method. First, the sample of composite particles 10 was completely dissolved in a heated acid solution (a mixture of hydrofluoric acid, nitric acid, and sulfuric acid), filtered, and the carbon residue in the solution was removed. Then, the filtrate was analyzed using ICP-AES to determine the spectral intensity of each element. Next, a standard curve was prepared using commercially available standard solutions of the elements, and the contents of each element contained in composite particles 10 were calculated.

[0057] In composite particles 10, silicon compound phases and Si-B containing particles 2 may be present, but they can be distinguished and quantified by using Si-NMR.

[0058] The carbon phase includes carbonaceous materials. Carbonaceous materials can also be amorphous. The carbon phase can also be composed of amorphous carbonaceous materials. Examples of carbonaceous materials include materials derived from bitumen, amorphous carbon, carbon black, and materials derived from organic polymers. Examples of bitumen include coal tar pitch and petroleum pitch; coal tar pitch includes coal tar pitch.

[0059] Metal-containing oxide phases include, for example, basic aluminate phases. A basic aluminate phase is a phase containing basic aluminates, which are composite oxides 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 basic aluminate contains at least one element selected from the group consisting of Li, Na, K, Rb, Cs, and Fr. Basic aluminates may contain at least one element selected from the group consisting of Li, Na, and K. Basic aluminates may contain at least one element selected from the group consisting of Li and Na, or may contain Li. That is, the basic aluminate may be lithium aluminate.

[0060] The average particle size of the composite particles 10 can be greater than or equal to 1 μm and less than 20 μm, or greater than or equal to 1 μm and less than 10 μm. The average particle size of the composite particles 10 can be measured, for example, in the same manner as the average particle size of the Si-B-containing particles 2.

[0061] Figure 1 The composite particle 10 shown has a capping layer 3 on its surface. The capping layer 3 covers at least a portion of the surface of the particle (hereinafter also referred to as the "parent particle") formed by the matrix 1 and the Si-B containing particle 2.

[0062] The capping layer 3 contains carbon. The capping layer 3 is provided, for example, to improve the conductivity of the composite particles 10. The capping layer 3 is made of, for example, a conductive carbon material. By having a capping layer 3 containing conductive carbon material on its surface, the composite particles 10 are able to significantly improve conductivity.

[0063] Carbon materials include, 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. Carbonaceous materials include, for example, materials derived from bitumen, amorphous carbon, carbon black, and materials derived from organic polymers. Bitumen includes, for example, coal tar pitch and petroleum tar pitch; coal tar pitch includes, for example, coal tar pitch.

[0064] The thickness of the capping layer 3 is preferably thin enough not to substantially affect the average particle size of the composite particles 10. Regarding the thickness of the capping layer 3, from the viewpoint of ensuring conductivity and the diffusion of ions that facilitate charging and discharging, the thickness of the capping layer 3 can be 1 nm or more and 200 nm or less, or 5 nm or more and 100 nm or less. The thickness of the capping layer 3 can be measured, for example, by cross-sectional observation of the negative electrode active material using SEM.

[0065] The composite particles 10 may also not have a covering layer 3. The composite particles 10 may consist only of parent particles.

[0066] In addition to the above-mentioned components, the composite granules 10 may also contain other components. The composite granules 10 may contain other components in a proportion of more than 0% by mass and less than 10% by mass, in a proportion of more than 0% by mass and less than 5% by mass, in a proportion of more than 0% by mass and less than 1% by mass, or in a proportion of more than 0% by mass and less than 0.1% by mass.

[0067] (Method for manufacturing negative electrode active material)

[0068] An example of a method for manufacturing the composite particles 10 of the negative electrode active material of Embodiment 1 will be described.

[0069] An example of the method for manufacturing the negative electrode active material according to Embodiment 1 includes: preparing a matrix; and composited Si-B-containing particles with the matrix. Hereinafter, the preparation of the matrix will be described as step (i), and the composited matrix with Si-B-containing particles and sintered will be described as step (ii). Each step will be described in detail.

[0070] <Process (i)>

[0071] When the matrix is ​​a silicon compound phase containing lithium silicate, a raw material mixture containing Si and Li raw materials in a specified ratio is used. The raw material mixture may further contain at least one element selected from the aforementioned alkali metal elements, Group II elements of the periodic table, and element M2. The mixture of the above raw materials is melted, and the molten liquid is sheeted through a metal roller to synthesize lithium silicate. Then, the sheeted lithium silicate is crystallized by heat treatment in an atmospheric atmosphere at a temperature above the glass transition temperature and below the melting point. It should be noted that the sheeted lithium silicate can also be used without crystallization. Alternatively, lithium silicate can be produced by solid-state reaction by firing at a temperature below the melting point without melting the mixture.

[0072] Silicon (Si) raw materials can include silicon dioxide. Li (Li) raw materials can include, for example, lithium carbonate, lithium oxide, lithium hydroxide, and lithium hydride. They can be used alone or in combination of two or more. Raw materials for alkali metals, group II elements, and element M2 can include oxides, hydroxides, carbonates, hydrides, nitrates, and sulfates of each element.

[0073] Unreacted Si material may remain within lithium silicate. This residual Si material is dispersed within the lithium silicate in the form of a silicon oxide phase.

[0074] <Process (ii)>

[0075] Si-B-containing particles are mixed into the prepared lithium silicate to form a composite. The Si-B-containing particles are, for example, raw silicon containing a specified amount of boron (B). For example, composite particles (master particles) are prepared through the following steps (a) to (c). The method for incorporating B into the silicon is not particularly limited. For example, a specified amount of Si particles and B are added to a quartz crucible and heated to melt it under an argon atmosphere, thereby doping B into the raw silicon.

[0076] [Process(a)]

[0077] For example, silicon powder and lithium silicate powder are mixed at a mass ratio of 20:80 to 95:5. The silicon powder used is, for example, coarse silicon particles with an average particle size of several μm or more but less than tens of μm. The coarse particles contain a specified amount of boron (B).

[0078] [Process (b)]

[0079] Secondly, a pulverizing device such as a ball mill is used to simultaneously micronize and pulverize the mixture of raw silicon and lithium silicate. Alternatively, an organic solvent can be added to the mixture for wet pulverization. The specified amount of organic solvent can be added to the pulverizing container all at once during the initial pulverization stage, or it can be added intermittently in multiple stages during the pulverization process. The organic solvent serves to prevent the material being pulverized from adhering to the inner wall of the pulverizing container.

[0080] As organic solvents, alcohols, ethers, fatty acids, alkanes, cycloalkanes, silicates, metal alkoxides, etc. can be used.

[0081] It should be noted that silicon and lithium silicate can also be micronized separately and then mixed. Alternatively, silicon nanoparticles and amorphous lithium silicate nanoparticles can be prepared without using a pulverizing device and then mixed. Known methods such as gas-phase methods (e.g., plasma methods) and liquid-phase methods (e.g., liquid-phase reduction methods) can be used to prepare nanoparticles.

[0082] [Process (c)]

[0083] The mixture is sintered by pressing it in an inert gas atmosphere (such as argon or nitrogen) and heating it to a temperature of, for example, above 450°C and below 1000°C. Sintering can be performed using sintering apparatus capable of applying pressure in an inert atmosphere, such as hot pressing or discharge plasma sintering. During sintering, lithium silicate softens and flows to fill the gaps between silicon particles. As a result, a dense, blocky sintered body is obtained, with the silicate phase as the seam and the silicon particles as the islands.

[0084] By pulverizing the sintered body, particles (master particles) containing Si-B particles 2 dispersed in matrix 1 can be obtained. By appropriately selecting the pulverization conditions, master particles with a specified average particle size can be obtained.

[0085] Sintering can also be achieved by calendering a pre-formed sheet of powder mixture under heat. Calendering typically involves passing the sheet of mixture between two rotating rollers. By passing the sheet of mixture through the gap between the heated rollers, heating and pressurization of the mixture can be performed simultaneously. The sheet of mixture can be heated before or after passing through the rollers, or it can be heated both before and after passing through the rollers. By performing heat treatment while calendering the sheet-formed mixture, composite particles 10 can be manufactured with high productivity.

[0086] <Process (iii)>

[0087] A coating layer 3 can also be formed by covering at least a portion of the surface of the obtained master particles with a conductive material. The conductive material is preferably electrochemically stable, and preferably a conductive carbon material. As a method for covering at least a portion of the surface of the master particles with a conductive carbon material, a CVD method using hydrocarbon gases such as acetylene or methane as raw materials can be cited. Alternatively, a method of mixing and heating the master particles with coal tar pitch, petroleum tar pitch, phenolic resin, etc., to carbonize them can also be exemplified. The mixture of the master particles and the conductive carbon material such as coal tar pitch, petroleum tar pitch, phenolic resin, etc., is heated, for example, in an inactive atmosphere (e.g., argon, nitrogen, etc.) at a temperature above 700°C and below 950°C. Alternatively, carbon black can be adsorbed onto the surface of the master particles. As described above, composite particles 10 are obtained.

[0088] <Process (iv)>

[0089] A further step can be performed: cleaning the composite particles 10 (which are the master particles if step (iii) is not performed) with acid. For example, by cleaning the composite particles with an acidic aqueous solution, trace amounts of alkaline components that may be generated during the composite formation of raw silicon and lithium silicate can be removed. As the acidic aqueous solution, aqueous solutions of inorganic acids such as hydrochloric acid, hydrofluoric acid, sulfuric acid, nitric acid, phosphoric acid, and carbonic acid, and aqueous solutions of organic acids such as citric acid and acetic acid can be used.

[0090] (Implementation Method 2)

[0091] The secondary battery of Embodiment 2 includes a negative electrode, a positive electrode, and an electrolyte. The negative electrode contains the negative electrode active material of Embodiment 1. For example, the negative electrode includes a negative electrode binder layer containing the negative electrode active material of Embodiment 1 and a negative electrode current collector.

[0092] Figure 2 This is a cross-sectional view showing the schematic configuration of the secondary battery according to Embodiment 2. The secondary battery 100 includes a positive electrode 23, a negative electrode 26, a separator 27, a non-aqueous electrolyte 29, and a casing 28. The positive electrode 23, negative electrode 26, non-aqueous electrolyte 29, and separator 27 are housed in the casing 28. A separator 27 is disposed between the positive electrode 23 and the negative electrode 26. The positive electrode 23 and the negative electrode 26 are opposite to each other with respect to the separator 27. The positive electrode 23 includes a positive electrode flux layer 22 and a positive electrode current collector 21. The positive electrode flux layer 22 is disposed between the positive electrode current collector 21 and the separator 27. The negative electrode 26 includes a negative electrode flux layer 25 and a negative electrode current collector 24. The negative electrode flux layer 25 is disposed between the negative electrode current collector 24 and the separator 27.

[0093] The positive electrode additive layer 22 is composed of a positive electrode additive. The positive electrode additive layer 22 can be formed by coating a positive electrode slurry containing a positive electrode additive dispersed in a dispersion medium onto the surface of the positive electrode current collector 21 and allowing it to dry. Alternatively, the dried coating can be calendered as needed. The positive electrode additive layer 22 can be formed on one surface or on both surfaces of the positive electrode current collector 21.

[0094] The cathode compound contains a positive electrode active material capable of absorbing, storing, 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 lithium-containing transition metal oxides or lithium-containing transition metal phosphates as positive electrode active materials can reduce battery manufacturing costs 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 of these positive electrode active materials can be used.

[0095] Positive electrode additives may also include conductive additives, ion conductors, and binders as needed.

[0096] Conductive additives and ion conductors are used to reduce the resistance of electrodes. Examples of conductive additives include carbon materials and conductive polymers. Examples of carbon materials include carbon black, graphite, acetylene black, carbon nanotubes, carbon nanofibers, graphene, fullerene, and graphite oxide. Examples of conductive polymers include polyaniline, polypyrrole, and polythiophene. At least one of these conductive additives can be used.

[0097] Examples of ion conductors include gel electrolytes such as polymethyl methacrylate and polymethyl methacrylate polymer, organic solid electrolytes such as polyethylene oxide, and Li7La3Zr2O. 12 Inorganic solid electrolytes, etc. At least one selected from these ion conductors can be used.

[0098] Adhesives are used to improve the adhesion of materials constituting electrodes. Examples of adhesives include polyvinylidene fluoride (PVDF), PVDF-hexafluoropropylene copolymer, PVDF-tetrafluoroethylene copolymer, polytetrafluoroethylene (PTFE), carboxymethyl cellulose, polyacrylic acid, styrene-butadiene copolymer rubber, polypropylene, polyethylene, and polyimide. At least one adhesive selected from these materials may be used.

[0099] The positive current collector 21 is a sheet or thin film made of a metal material such as aluminum, aluminum alloy, stainless steel, titanium, or titanium alloy. The sheet or thin film can be porous or non-porous. Metal foil, metal mesh, etc., can be used as the sheet or thin film. Carbon material can be coated on the surface of the positive current collector 21 as a conductive auxiliary material.

[0100] The negative electrode mixture layer 25 is composed of a negative electrode mixture. The negative electrode mixture contains the negative electrode active material of Embodiment 1. The negative electrode mixture layer 25 can be formed, for example, by coating a negative electrode slurry in which the negative electrode mixture is dispersed in a dispersion medium onto the surface of the negative electrode current collector 24 and drying it. The dried coating can also be calendered as needed. The negative electrode mixture layer 25 can be formed on one surface of the negative electrode current collector 24 or on both surfaces. The negative electrode mixture can also contain conductive additives, ion conductors, and binders as needed. As conductive additives, ion conductors, and binders, materials that can be used in the positive electrode mixture layer 22 can also be used in the negative electrode mixture layer 25. The negative electrode mixture can also contain negative electrode active materials other than the negative electrode active material of Embodiment 1.

[0101] The negative current collector 24 is a sheet or thin film made of a metal material such as stainless steel, nickel, nickel alloy, copper, or copper alloy. The sheet or thin film can be porous or non-porous. Metal foil, metal mesh, etc., can be used as the sheet or thin film. Carbon material can be coated on the surface of the negative current collector 24 as a conductive auxiliary material.

[0102] The separator 27 is an electrolyte layer with lithium-ion conductivity. The material of the separator 27 is not particularly limited, as long as lithium ions can pass through. 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, semi-permeable 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 and Li₇La₃Zr₂O. 12 (LLZ) and other oxide solid electrolytes, etc. Gel electrolytes include those containing fluorinated resins such as PVdF. Ion exchange resin membranes include cation exchange membranes and anion exchange membranes. Porous membranes include porous membranes made of polyolefin resins and porous membranes made of cellophane obtained by weaving glass fibers into a nonwoven fabric.

[0103] The non-aqueous electrolyte 29 can be immersed in the positive electrode 23, the negative electrode 26, and the separator 27. The non-aqueous electrolyte 29 can fill the internal space of the casing 28. Through the action of the non-aqueous electrolyte 29, lithium ions can move between the positive electrode 23 and the negative electrode 26. The non-aqueous electrolyte 29 can contain a non-aqueous electrolyte solution, a gel electrolyte, or an ionic liquid.

[0104] Non-aqueous electrolytes may contain non-aqueous solvents and lithium salts.

[0105] As a non-aqueous solvent, cyclic carbonates, chain carbonates, cyclic ethers, chain ethers, cyclic esters, chain esters, fluorinated solvents, nitriles, etc., can be used. Examples of cyclic carbonates include ethylene carbonate, propylene carbonate, butyl carbonate, etc. Examples of chain carbonates include dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, etc. Examples of cyclic ethers include tetrahydrofuran, 1,4-dioxane, 1,3-dioxolane, etc. Examples of chain ethers include 1,2-dimethoxyethane, 1,2-diethoxyethane, etc. Examples of cyclic esters include γ-butyrolactone, etc. Examples of chain esters include methyl acetate, etc. Examples of fluorinated solvents include fluoroethylene carbonate, methyl fluoropropionate, fluorobenzene, ethyl fluorocarbonate, dimethyl fluorocarbonate, etc. Examples of nitriles include acetonitrile, etc. At least one selected from these non-aqueous solvents can be used.

[0106] Examples of lithium salts include lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bisperfluoroethylsulfonylimide (LiN(SO2C2F5)2), LiAsF6, and LiCF3SO4. 3、 Lithium difluoro(oxalate)borate, etc. At least one of these lithium salts can be used.

[0107] Gel electrolytes can be materials obtained by impregnating a non-aqueous electrolyte into a polymer material. Examples of polymer materials include polyethylene oxide, polyacrylonitrile, polyvinylidene fluoride, polymethyl methacrylate, and polymers containing ethylene oxide bonds.

[0108] Examples of cations constituting ionic liquids include aliphatic chain quaternary cations, aliphatic cyclic ammonium cations, and nitrogen-containing heterocyclic aromatic cations. Examples of aliphatic chain quaternary cations include tetraalkylammonium and tetraalkylphosphonium. Examples of aliphatic cyclic ammonium cations include pyrrolidine-onium, morpholinium-onium, imidazoline-onium, tetrahydropyrimidine-onium, piperazine-onium, and piperidinium-onium. Examples of nitrogen-containing heterocyclic aromatic cations include pyridinium and imidazoline-onium. 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 -Etc. Ionic liquids can contain lithium salts.

[0109] The outer casing 28 is made of a material, for example, obtained by laminating a metal foil, such as aluminum foil, with a resin film, such as PET film. The outer casing 28 may also be a container made of resin or metal.

[0110] The shape of the secondary battery 100 is not limited to a stacked type. Other shapes of the secondary battery 100 include coin type, cylindrical type, square type, sheet type, button type, flat type, etc.

[0111] The secondary battery of Embodiment 2 has an increased charging capacity by including the negative electrode active material of Embodiment 1 in the negative electrode.

[0112] (Other implementation methods)

[0113] (appendix)

[0114] Based on the description of the above embodiments, the following technology is disclosed.

[0115] (Technology 1)

[0116] A negative electrode active material comprising composite particles, the composite particles comprising:

[0117] matrix, and

[0118] Si-B containing particles,

[0119] The aforementioned Si-B-containing particles are dispersed in the aforementioned matrix.

[0120] This configuration can improve the charging capacity of the battery.

[0121] (Technology 2)

[0122] According to the negative electrode active material described in Technology 1, the proportion of B in the aforementioned Si-B-containing particles is 0.01% by mass or more and 5.0% by mass or less. With this configuration, the charging capacity of the battery can be improved.

[0123] (Technology 3)

[0124] According to the negative electrode active material described in technique 1 or 2, the aforementioned Si-B-containing particles comprise crystalline silicon with B atoms dissolved in solid solution. This configuration improves the charging capacity of the battery.

[0125] (Technology 4)

[0126] According to any one of techniques 1 to 3, the negative electrode active material comprises at least one selected from the group consisting of boron oxide and elemental B. With this configuration, the charging capacity of the battery can be improved.

[0127] (Technology 5)

[0128] According to any one of techniques 1 to 4, the negative electrode active material contains an amorphous phase in the aforementioned Si-B-containing particles. With this configuration, the charging capacity of the battery can be improved.

[0129] (Technology 6)

[0130] According to any one of techniques 1 to 5, the negative electrode active material wherein the average particle size of the aforementioned Si-B-containing particles is 1 nm or more and 1000 nm or less. With this configuration, the charging capacity of the battery can be improved.

[0131] (Technology 7)

[0132] According to any one of techniques 1 to 6, the negative electrode active material comprises at least one selected from the group consisting of a silicon compound phase and a carbon phase. With this configuration, the charging capacity of the battery can be improved.

[0133] (Technology 8)

[0134] According to the negative electrode active material described in Technology 7, the aforementioned silicon compound phase comprises lithium silicate containing Li, Si, and O as a main component. Lithium silicate has excellent ionic conductivity. With this configuration, the charging capacity of the battery can be improved.

[0135] (Technology 9)

[0136] According to the negative electrode active material described in technique 7 or 8, the aforementioned silicon compound phase further comprises element M2, wherein 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 charging capacity of the battery can be further improved.

[0137] (Technology 10)

[0138] According to any one of techniques 1 to 9, the negative electrode active material has a coating layer on its surface, and the coating layer comprises carbon. By including the coating layer, conductivity is improved. With this configuration, the charging capacity of the battery can be further improved.

[0139] (Technology 11)

[0140] A secondary battery, comprising:

[0141] A negative electrode comprising the negative electrode active material as described in any one of techniques 1 to 10;

[0142] Positive electrode; and

[0143] Electrolytes.

[0144] This configuration allows for an increase in charging capacity.

[0145] Example

[0146] The present disclosure will now be described in more detail using examples. These examples are merely illustrative and are not intended to limit the scope of the disclosure.

[0147] [Preparation of Composite Particles]

[0148] (Example 1)

[0149] Lithium carbonate and silicon dioxide were mixed in a molar ratio of Li₂CO₃:SiO₂ = 34:66. The mixture was heated at 1500°C for 5 hours in an inert gas atmosphere to melt it. The melt was then passed through a metal roller to form thin sheets. The thin sheet product was heated at 750°C for 5 hours to produce lithium silicate. The lithium silicate was then pulverized to an average particle size of 10 μm.

[0150] The obtained lithium silicate with an average particle size of 10 μm and coarse silicon particles (average particle size of 10 μm) containing 1% B at a mass ratio were mixed at a 40:60 mass ratio. The mixture was filled into the jar (SUS, volume: 500 mL) of a planetary ball mill (Fritsch, P-5), 24 SUS balls (20 mm in diameter) were placed in it, the lid was closed, and the mixture was pulverized at 200 rpm for 25 hours in an inactive atmosphere to composite the coarse silicon particles with lithium silicate.

[0151] Then, the pulverized material is pressed into a sheet with a thickness of 4 mm to 5 mm at room temperature to obtain a sheet-shaped shaped body. The shaped body is then calendered between calendering rollers while heated to 800°C to obtain a sintered body. Calendering is performed by applying a pressure of 240 MPa to the shaped body. The sintered body is then pulverized and passed through a 40 μm sieve. As described above, master particles containing Si-B particles dispersed in a lithium silicate phase matrix are obtained.

[0152] The masterbatch was mixed with coal tar pitch (manufactured by JFE Chemical Co., Ltd., MCP250), and the mixture was calcined at 800°C for 5 hours in an inert atmosphere, thereby forming a conductive carbon coating layer on the surface of the masterbatch. At this time, the mass ratio of the coating layer to the total mass of the masterbatch and the coating layer was 5%. Then, using a sieve, composite particles with an average particle size of 5 μm and the coating layer were obtained.

[0153] As described above, the composite particles of Example 1 were obtained. It should be noted that the cross-section of the composite particles was analyzed using SEM-EDX, and the results confirmed that the Si-B-containing particles contained B at a content of 1% by mass.

[0154] (Comparative Example 1)

[0155] Using coarse silicon particles that do not contain B, the composite particles of Comparative Example 1 were obtained in the same manner as in Example 1.

[0156] X-ray diffraction

[0157] The X-ray diffraction patterns of the composite particles of Example 1 and Comparative Example 1 were obtained by powder X-ray diffraction. The measurements were performed using an X-ray diffraction apparatus (RIGAKU, MiniFlex 600). Cu-Kα rays (wavelengths 1.5405 Å and 1.5444 Å) were used as the X-ray source. Figure 3 This is a diagram showing the X-ray diffraction patterns of the composite particles of Example 1 and Comparative Example 1. Figure 4 These are partially enlarged 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 originating from the (111) plane of Si was observed at a diffraction angle of 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 originating from the (111) plane of Si was observed at a diffraction angle of 2θ of 28.41°. Here, it is generally known that if atoms with smaller atomic radii than the atoms constituting the lattice are introduced into the lattice, the peaks shift to the higher angle side. Therefore, based on the above results, it can be confirmed that B is introduced into the silicon lattice in the composite particles of Example 1.

[0158] [Si content]

[0159] The composite particles of Example 1 and Comparative Example 1 were analyzed using ICP and Si-NMR to determine the Si content of the Si-B-containing particles in the composite particles. The results are shown in Table 1.

[0160] [Volume resistivity of silicon particles]

[0161] The volume resistivity (powder resistivity) of the coarse silicon particles containing B used in Example 1 and the coarse silicon particles used in Comparative Example 1 was measured using an automated powder resistivity measurement system (manufactured by Nittoseiko Analytech Co., Ltd.). The volume resistivity of the coarse silicon particles used in Example 1 and Comparative Example 1 at an apparent density of 1.5 g / cc is shown in Table 1.

[0162] [Table 1]

[0163]

[0164] As shown in Table 1, the volume resistivity of the silicon coarse particles containing B used in Example 1 is lower than that used in Comparative Example 1. That is, the composite particles of Example 1 contain silicon particles with improved electronic conductivity.

[0165] [Battery Manufacturing]

[0166] Using the composite particles from Example 1 and Comparative Example 1, evaluation battery cells for Example 1 and Comparative Example 1 were fabricated as follows.

[0167] (Making the negative electrode)

[0168] Composite particles and graphite were mixed at a mass ratio of 20:80 to serve as the negative electrode active material. Water was added to a negative electrode mixture prepared by mixing the negative electrode active material, sodium carboxymethyl cellulose (CMCNa), styrene-butadiene rubber (SBR), and lithium polyacrylate at a mass ratio of negative electrode active material:CMCNa:SBR:lithium polyacrylate = 96.5:1:1.5:1, and then stirring using a mixer to prepare a negative electrode slurry. Next, the slurry was applied to the surface of copper foil at a rate of 1 m² / g. 2 The negative electrode mixture was coated with a negative electrode slurry in a manner that yielded 190g of negative electrode agent. After the coating was dried, it was calendered to produce a coating with a density of 1.5g / cm³ on both sides of the copper foil. 3 The negative electrode of the negative electrode compound layer.

[0169] The negative electrode is cut into a 20mm x 20mm shape with a 5mm x 5mm protrusion. The negative electrode adhesive layer on the protrusion is peeled off to expose the copper foil. Then, the negative electrode tab lead is connected to the exposed part of the copper foil, and a specified area around the negative electrode tab lead is covered with an insulating film.

[0170] (Fabrication of the counter electrode)

[0171] Small pieces of Ni mesh are soldered to the end tabs. The tabs are cut to the specified size, and the mesh is pressed onto a 300μm thick lithium metal foil to create the counter electrode.

[0172] (Preparation of non-aqueous electrolytes)

[0173] A non-aqueous electrolyte was prepared by dissolving LiPF6 in a mixed solvent consisting of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of EC:EMC:DMC = 4:1:15. The concentration of LiPF6 was set to 1.3 mol / L.

[0174] (Battery cell fabrication)

[0175] Using the aforementioned negative electrode and two counter electrodes, a battery cell for evaluating negative electrode limitations was fabricated as follows. The battery cell was fabricated in a dry air atmosphere with a dew point below -60°C. An electrode assembly was fabricated by clamping the negative electrode with a pair of counter electrodes, with the negative electrode binder layer facing the lithium metal foil through a separator. Next, a rectangular Al laminate film was folded in half, and the two ends on the long side were heat-sealed to form a cylindrical shape. Then, the fabricated electrode assembly was placed into the cylinder from one side of the short side of the Al laminate film, aligning the end face of the Al laminate film with the insulating film of each electrode lead and heat-sealing it. Next, 0.3 cm of [unspecified substance] was injected from the unsealed short side of the cylinder. 3 The electrolyte was injected. After injection, the battery was allowed to stand for 3 minutes under reduced pressure of 0.02 MPa, followed by two cycles of returning to atmospheric pressure to allow the non-aqueous electrolyte to permeate into the negative electrode binder layer. Finally, the end face of the cylindrical Al laminate film on the injected side was heat-sealed while the pressure was reduced, resulting in the evaluation battery cell.

[0176] [Battery Review]

[0177] For the evaluation battery cells of Example 1 and Comparative Example 1, the initial charge-discharge characteristics and cycle characteristics were evaluated as described below. The evaluation results are shown in Table 2.

[0178] At an ambient temperature of 25°C, the battery cells were charged at a constant current of 0.1C until the voltage reached 1.0V. Then, they were discharged at a constant current of 0.1C until the cell voltage reached 0.005V. The initial charge-discharge capacity of the evaluation battery cells of Example 1 and Comparative Example 1 was thus evaluated. The above charge-discharge cycle was considered as one cycle, and 100 cycles were performed. The capacity retention rate was calculated using the following formula.

[0179] Capacity retention rate [%] = (Discharge capacity at 100th cycle / Discharge capacity at 1st cycle) × 100

[0180] [Table 2]

[0181]

[0182] (Inspection)

[0183] The battery of Example 1 exhibited higher charging and discharging capacities than that of Comparative Example 1. This is presumably due to the improved electronic conductivity resulting from the inclusion of boron (B) in the silicon particles. Based on the above, by using the negative electrode active material of this disclosure, it is possible to improve the charging and discharging capacity of the battery.

[0184] Industrial availability

[0185] The technology disclosed herein can be used in batteries such as lithium-ion secondary batteries.

Claims

1. A negative electrode active material comprising composite particles, said composite particles comprising: matrix, and Si-B containing particles, The Si-B-containing particles are dispersed in the matrix.

2. The negative electrode active material according to claim 1, wherein, The proportion 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 with B atoms dissolved in it.

4. The negative electrode active material according to claim 1, wherein, The Si-B-containing particles comprise 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 contain 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 greater than 1 nm and less than 1000 nm.

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

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

9. The negative electrode active material according to claim 8, wherein, The silicon compound phase also contains element M2. The element M2 is selected from at least one of 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 their surface. The coating layer contains carbon.

11. A secondary battery, comprising: A negative electrode comprising the negative electrode active material according to any one of claims 1 to 10; Positive electrode; and Electrolytes.

Images