Negative electrode active material and secondary battery

Abstract

A negative electrode active material according to the present disclosure comprises composite particles that include a carbon matrix and silicon particles dispersed in the carbon matrix. The pore volume of the composite particles as measured by a nitrogen gas adsorption method is 0.8 cm3 / g or more. A secondary battery according to the present disclosure comprises: a negative electrode including the negative electrode active material according to the present disclosure; a positive electrode; and an electrolyte.

Description

Negative electrode active material and secondary battery

[0001] This 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 attracting attention for their high voltage and high energy density, making them promising for small-scale consumer applications, power storage devices, and electric vehicle power sources. Amidst the growing demand for higher energy density in batteries, materials containing silicon alloyed with lithium are expected to be used as anode active materials with high theoretical capacity density.

[0003] For example, Patent Document 1 describes a negative electrode active material for a secondary battery in which composite particles having a matrix and a silicon phase dispersed within the matrix are coated with a coating layer formed from a mixture of a carbon material and a fluorine-containing material, as a silicon-containing material used as a negative electrode active material. The negative electrode active material for a secondary battery disclosed in Patent Document 1 has improved electrolyte resistance due to the above configuration, and as a result, the cycle characteristics of the battery can be improved.

[0004] International Publication No. 2023 / 053888

[0005] Neutral electrode active materials containing silicon experience a significant volume increase during charging. Therefore, negative electrodes containing silicon-based negative electrode active materials swell during charging. Consequently, there is a need for technologies that can suppress this swelling during charging in negative electrodes containing silicon-based negative electrode active materials.

[0006] The negative electrode active material for secondary batteries disclosed in Patent Document 1 can reduce the expansion and contraction of the silicon phase during charging and discharging due to the above configuration. Therefore, the negative electrode active material for secondary batteries disclosed in Patent Document 1 can reduce the swelling of the negative electrode during charging to some extent.

[0007] However, conventional silicon-containing negative electrode active materials still have room for improvement in effectively reducing the expansion of the negative electrode during charging.

[0008] Therefore, this disclosure provides a negative electrode active material that can reduce the swelling of the negative electrode during charging.

[0009] The negative electrode active material of this disclosure comprises a composite particle containing a carbon matrix and silicon particles dispersed in the carbon matrix, wherein the pore volume of the composite particle, as measured by nitrogen gas adsorption, is 0.8 cm². 3 It is 1 / g or more.

[0010] The negative electrode active material of this disclosure can reduce the swelling of the negative electrode during charging.

[0011] Figure 1 is a cross-sectional view showing the schematic configuration of composite particles 10 provided in the negative electrode active material according to Embodiment 1. Figure 2 is a cross-sectional view showing the schematic configuration of a secondary battery according to Embodiment 2.

[0012] The embodiments of this disclosure will be described in detail below with reference to the drawings. This disclosure is not limited to the embodiments described below.

[0013] (Embodiment 1) The negative electrode active material according to Embodiment 1 comprises a composite particle containing a carbon matrix and silicon particles dispersed in the carbon matrix. The pore volume of the composite particle, as measured by nitrogen gas adsorption, is 0.8 cm³. 3 It is 1 / g or more.

[0014] In the negative electrode active material according to Embodiment 1, since pores that fill the above-mentioned pore volume are provided inside the composite particles, the expansion of silicon particles during charging is effectively absorbed within the composite particles, thereby suppressing the expansion of the negative electrode active material. Therefore, the negative electrode active material according to Embodiment 1 can reduce the swelling of the negative electrode during charging.

[0015] Figure 1 is a cross-sectional view showing the schematic configuration of the composite particle 10 contained in the negative electrode active material according to Embodiment 1. The composite particle 10 is a particle comprising a carbon matrix 1 and silicon particles 2 dispersed in the carbon matrix 1. With this configuration, the stress associated with the expansion and contraction of the silicon particles 2 during charging and discharging is relieved by the carbon matrix 1, and cracks and fractures of the composite particle 10 are suppressed. Furthermore, as described above, in the negative electrode active material according to Embodiment 1, the pore volume of the composite particle 10, as measured by the nitrogen gas adsorption method, is 0.8 cm³. 3It is 1 / g or more. Because the composite particles 10 have such a pore volume, the expansion of the silicon particles 2 during charging can be effectively absorbed within the composite particles 10, thereby suppressing the expansion of the composite particles 10. Therefore, the negative electrode active material according to Embodiment 1 can reduce the expansion of the negative electrode during charging while achieving high capacity due to the inclusion of silicon. In order to further reduce the expansion of the negative electrode during charging, the pore volume of the composite particles 10, as measured by the nitrogen gas adsorption method, is 0.9 cm³. 3 It may be more than / g, or 1.0 cm 3 It may be more than / g.

[0016] The pore volume of the composite particle 10 is measured by nitrogen gas adsorption. Pores whose volume can be measured by nitrogen gas adsorption are those with a diameter of several hundred nm or less, for example, 100 nm or less. Therefore, the pore volume of the composite particle 10 measured by nitrogen gas adsorption is, for example, the total pore volume of the pores in the composite particle 10 with a diameter of 100 nm or less.

[0017] It is desirable that the ratio of the total volume of micropores and mesopores in the composite particles 10 to the pore volume measured by the nitrogen gas adsorption method of the composite particles 10 be 70% or more. Micropores are pores with a diameter of 2.0 nm or less, and mesopores are pores with a diameter greater than 2.0 nm and less than 50 nm. By having micropores and mesopores account for 70% or more of the pores contained in the composite particles 10, the expansion of silicon particles 1 during charging is absorbed more effectively within the composite particles 10, thereby further suppressing the expansion of the negative electrode active material. Therefore, the negative electrode active material according to Embodiment 1 can further reduce the swelling of the negative electrode during charging. In order to further effectively suppress the expansion of the negative electrode active material, the ratio of the total volume of micropores and mesopores in the composite particles 10 may be 75% or more, or 80% or more. Therefore, the sum of the volumes of micropores and mesopores in the composite particle 10 can be obtained by analyzing the gas adsorption and desorption measurement results obtained by the nitrogen gas adsorption method using the GCMC (Grand Canonical Monte Carlo) method to determine the volume of pores with a pore diameter of less than 50 nm.

[0018] The upper limit of the pore volume measured by the nitrogen gas adsorption method of composite particles 10 is not particularly limited, but for example, 3.0 cm 3 It is less than / g. The composite particle 10 is, for example, 3.0 cm 3 When the pore volume is less than or equal to / g, the negative electrode active material according to Embodiment 1 can realize a negative electrode that exhibits sufficient capacity while reducing swelling during charging.

[0019] The carbon matrix 1 is a matrix containing a carbonaceous material. The carbonaceous material may be, for example, a material capable of absorbing lithium ions. The carbonaceous material may be amorphous. The carbon matrix 1 may be composed of amorphous carbonaceous material. 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.

[0020] The carbon matrix 1 is, for example, a porous carbonaceous material. A porous carbonaceous material has, for example, a porous skeleton formed of carbonaceous material. Hereinafter, such a porous skeleton formed of carbonaceous material may be referred to as a carbon skeleton.

[0021] For example, if the carbon matrix 1 is a porous carbonaceous material as described above, the composite particle 10 can be considered to have a configuration in which silicon particles 2 exist inside the pores of the porous carbonaceous material.

[0022] The carbon matrix 1 may contain elements other than carbon (C). For example, the carbon matrix 1 may further contain at least one element selected from the group consisting of Mg, Ca, Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, and Si. For example, the carbon skeleton constituting the carbon matrix 1 may contain at least one element selected from the group consisting of Mg, Ca, Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, and Si. By including these elements in the carbon matrix 1, the strength of the carbon matrix 1 is improved, and as a result, the strength of the composite particles 10 can be improved. As a result, the negative electrode active material according to Embodiment 1 can improve battery characteristics such as cycle characteristics.

[0023] The total content of Mg, Ca, Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, and Si in the carbon matrix 1 may be, for example, 1% by mass or less, or 0.1% by mass or less. The mass ratio of the other elements in the carbon matrix 1 can be measured, for example, by inductively coupled plasma atomic emission spectrometry (ICP analysis). When performing ICP analysis, for example, the silicon particles 2 are dissolved from the composite particles 10 using a heated NaOH solution, the silicon particles 2 are removed from the composite particles 10, and only the carbon matrix 1 is extracted from the composite particles 10. If the carbon matrix 1 contains the Si element, since the Si element is compounded with carbon in the carbon matrix 1, it can be extracted as a constituent element of the carbon matrix 1 without dissolving in the heated NaOH solution. Subsequently, the other components can be quantified by alkali fusion of the extracted carbon matrix 1 and ICP analysis.

[0024] The silicon particles 2 may be formed in a phase of pure silicon. The silicon particles 2 may also contain other elements besides silicon (Si). The inclusion of other elements may contribute to improved electronic conductivity. When the silicon particles 2 contain other elements besides silicon, the state of the other elements in the silicon particles 2 is not particularly limited. For example, the other elements may be located inside the silicon particles 2 or on the surface of the silicon particles 2. The other elements may be contained in the silicon particles 2 in any form, such as atoms, elements, or compounds. Examples of other elements include Ge, Al, Ni, P, B, Sb, etc. The mass ratio of the above-mentioned other elements in the silicon particles 2 can be measured, for example, by inductively coupled plasma atomic emission spectrometry (ICP-AES).

[0025] The silicon particles 2 may contain at least one selected from the group consisting of a crystalline phase and an amorphous phase. For example, silicon particles 2 may contain crystalline silicon. If silicon particles 2 contain elements other than Si, they may contain crystalline silicon in which atoms of the other elements are dissolved. Silicon particles 2 may be composed of, for example, multiple crystallites. The crystallite size constituting silicon particles 2 may be 10 nm or less, 5 nm or less, or 2 nm or less. Silicon particles 2 having such crystallite sizes are particularly suitable for reducing volume changes due to expansion and contraction of silicon particles 2 during charging and discharging. The crystallite size of silicon particles 2 can be calculated from the full width at half maximum of the diffraction peak originating from the Si(111) plane in the X-ray diffraction pattern obtained by X-ray diffraction measurement using Cu-Kα rays using Scherrer's formula.

[0026] The lower limit of the crystallite size of the silicon particles 2 is not particularly limited, but as an example, it is 1 nm. An example of a suitable crystallite size of the silicon particles 2 may be 1 nm or more and 5 nm or less. The fact that the crystallite size of the silicon particles 2 is 1 nm or more can contribute to, for example, suppressing the surface area of the silicon particles 2 and suppressing the deterioration of the silicon particles 2 accompanied by the generation of irreversible capacity. The fact that the crystallite size is 5 nm or less can contribute to making the expansion and contraction of the silicon particles 2 uniform, and thereby effectively relaxing the stress generated in the composite particles 10.

[0027] The X-ray diffraction pattern of the composite particles 10 can be obtained by X-ray diffraction measurement by the θ-2θ method using Cu-Kα lines having wavelengths of 1.5405 Å and 1.5444 Å, that is, wavelengths of 0.15405 nm and 0.15444 nm.

[0028] In this specification, a peak is a mountain-shaped portion where the value of the signal-to-noise ratio (that is, the ratio of the signal S to the background noise N) is 1.3 or more and the full width at half maximum is 10° or less.

[0029] The diffraction angle of the peak in the X-ray diffraction pattern is defined as the angle indicating the maximum intensity of the mountain-shaped portion where the value of the signal-to-noise ratio is 1.3 or more and the full width at half maximum is 10° or less. The full width at half maximum is the width represented by the difference between two diffraction angles at which the intensity becomes half of the maximum intensity I of the X-ray diffraction peak. MAX when taken as <​​​​​​​​​The silicon particles 2 may be nanoparticles. The average particle diameter of the silicon particles 2 may be 50 nm or more and 300 nm or less, may be 100 nm or more and 200 nm or less, or may be 100 nm or more and 150 nm or less. The average particle diameter of the silicon particles 2 is determined from a SEM image obtained by observing a 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 silicon particles 2 can be measured from the SEM image of the cross-section of the composite particle 10 in which the cross-section of the silicon particles 2 is exposed. The average particle diameter can be obtained by measuring the particle diameters of 500 silicon particles 2 arbitrarily extracted from the SEM image of the composite particle 10 and calculating their median diameter. Here, the maximum Feret diameter of the silicon particles 2 in the obtained SEM image can be regarded as the particle diameter of the silicon particles 2. The "maximum Feret diameter" is the maximum length of the perpendicular line obtained by sandwiching the particle with two parallel lines.

[0033] The median value in terms of the number of particle diameters of 500 silicon particles 2 measured by SEM observation is regarded as the median diameter of the silicon particles 2.

[0034] The average primary particle diameter of the silicon particles 2 may be, for example, 10 nm or less. Such fine silicon particles 2 having such an average primary particle diameter have a small volume change during charge and discharge and can improve the structural stability of the composite particle 10. Therefore, the swelling of the negative electrode during charging can be further reduced, and the cycle characteristics of the battery can also be improved. The average primary particle diameter of the silicon particles 2 can be obtained in the same manner as when obtaining the average particle diameter of the silicon particles 2, by measuring the particle diameters of the primary particles of 500 silicon particles 2 arbitrarily extracted from the SEM image of the cross-section of the composite particle 10 in which the cross-section of the silicon particles 2 is exposed and calculating their median diameter. The maximum Feret diameter of the primary particles of the silicon particles 2 is taken as the primary particle diameter of the silicon particles 2. When the primary particle diameter of the silicon particles 2 is smaller than 10 nm, a cross-sectional image of the composite particle 10 in which the cross-section of the silicon particles 2 is exposed may be obtained using a transmission electron microscope (TEM).

[0035] The content ratio of silicon particles 2 in the composite particle 10, that is, the mass ratio of silicon particles 2 to the mass of the composite particle 10, may be 30% by mass or more and 80% by mass or less. By setting the content ratio of silicon particles 2 to 30% by mass or more, the proportion occupied by the carbon matrix 1 is reduced, making it easier to improve the initial charge and discharge efficiency. By setting the content ratio of silicon particles 2 to 80% by mass or less, it becomes easier to reduce the degree of expansion and contraction of the composite particle 10 during charge and discharge. The content ratio of silicon particles 2 in the composite particle 10 may be 40% by mass or more. The content ratio of silicon particles 2 in the composite particle 10 may be 70% by mass or less.

[0036] The mass ratio of silicon particles 2 in composite particles 10 can be measured by ICP-AES and nuclear magnetic resonance analysis (NMR).

[0037] The average particle diameter of the composite particle 10 may be 1 μm or more and 20 μm or less, or 1 μm or more and 10 μm or less. The average particle diameter of the composite particle 10 can be measured, for example, in the same way as the average particle diameter of the silicon particle 2.

[0038] The composite particle 10 shown in Figure 1 has a coating layer 3 on its surface. The coating layer 3 covers at least a portion of the surface of the particle formed from the carbon matrix 1 and silicon particles 2 (hereinafter also referred to as "mother particle").

[0039] The coating layer 3 contains carbon. The coating layer 3 is provided, for example, to improve the conductivity of the composite particles 10. The coating layer 3 is composed of, for example, a conductive carbon material. The composite particles 10 can have their conductivity enhanced by having a coating layer 3 containing a conductive carbon material on its surface.

[0040] The carbon material constituting the coating layer 3 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.

[0041] 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 viewpoint of ensuring conductivity and ion diffusion that contributes to charging and discharging, 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 cross-sectional observation of the negative electrode active material using a SEM.

[0042] The composite particle 10 does not necessarily have a coating layer 3. The composite particle 10 may consist only of mother particles.

[0043] The composite particle 10 may contain other components in addition to the components described above. The composite particle 10 may contain the other components in a proportion of more than 0% by mass and 10% by mass or less, more than 0% by mass and 5% by mass or less, more than 0% by mass and 1% by mass or less, or more than 0% by mass and 0.1% by mass or less.

[0044] (Method for manufacturing the negative electrode active material) The method for manufacturing the negative electrode active material according to Embodiment 1 includes manufacturing composite particles 10. An example of a method for manufacturing the composite particles 10 contained in the negative electrode active material according to Embodiment 1 is described below.

[0045] An example of a method for producing composite particles 10 includes preparing a carbon matrix and compounding silicon particles with the carbon matrix. Hereinafter, preparing the carbon matrix will be referred to as step (i), and compounding the carbon matrix with silicon particles will be referred to as step (ii). Each step will be described in detail.

[0046] <Step (i)> Prepare a carbon matrix, for example, a carbon skeleton. Prepare a carbon skeleton, for example, porous carbon. Porous carbon can be prepared, for example, by carbonizing a carbon source.

[0047] For example, pitch, tar, and organic polymers can be used as carbon sources for the preparation of the carbon matrix. The carbon source may also be pitch. Examples of pitch include coal pitch and petroleum pitch, and coal pitch is, for example, coal tar pitch.

[0048] Porous carbon can be produced, for example, using a carbon source by a mold removal method or an activation method.

[0049] In the mold removal method, for example, a carbon source and a mold material for forming pores are mixed, and the resulting mixture is fired at a predetermined temperature to thermally decompose and carbonize the carbon source. This yields a composite of the mold material and the carbonaceous material. When preparing a carbon matrix containing elements other than carbon (C), raw materials containing those elements may be added to further prepare the composite. The composite containing the mold material and the carbonaceous material is washed with a liquid capable of removing the used mold material, and the mold material is removed from the composite to obtain porous carbon. The pore volume of the porous carbon can be adjusted by adjusting the volume ratio of the mold material in the composite. Furthermore, the size of the pores (pore diameter) contained in the porous carbon can be adjusted by adjusting the size of the mold material. Examples of mold materials include metal oxides such as magnesium oxide and compounds such as sodium chloride. Depending on the mold material, an appropriate liquid can be selected from acid solutions such as hydrochloric acid and nitric acid, alkaline solutions such as sodium hydroxide, and water for the removal of the mold material.

[0050] In the activation method, for example, the carbon source is thermally decomposed and carbonized by firing it at a predetermined temperature. When preparing a carbon matrix containing elements other than carbon (C), raw materials containing those elements may be added to the carbon source. Subsequently, to adjust the pore size of the obtained carbonaceous material, for example, a chemical activation process or gas activation is performed. For example, the pore volume and pore size (pore diameter) of the carbonaceous material obtained by thermal decomposition can be adjusted by contacting it with water vapor, CO, CO2, KOH, etc., at a temperature of, for example, 600°C or higher and 1200°C or lower. This allows for the production of porous carbon.

[0051] The porous carbon obtained by the above method can be used as a carbon matrix.

[0052] <Process (ii)> The composite formation of a carbon matrix and silicon particles can be carried out, for example, by using a precursor gas containing silicon as a raw material gas and depositing silicon particles in the pores of the carbon matrix by chemical vapor deposition (CVD). The precursor gas used as a raw material gas for the deposition of silicon particles by CVD contains silicon. An example of the precursor gas is silane gas (SiH4). When composited with a carbon matrix silicon particles containing elements other than silicon, a precursor gas containing those elements in addition to silicon may be used.

[0053] In step (ii), for example, silicon particles may be precipitated in the pores of the carbon matrix by CVD at a temperature of 400°C or higher and 700°C or lower. The temperature may be 450°C or higher and 700°C or lower, 500°C or higher and 650°C or lower, 530°C or higher and 650°C or lower, or 530°C or higher and less than 600°C.

[0054] In step (ii), for example, the carbon matrix obtained in step (i) is placed in a reaction vessel, and oxygen is removed by flowing an inert gas (e.g., Ar gas). The reaction vessel is heated to a reaction temperature of, for example, 400°C or higher and 700°C or lower. For example, while flowing a precursor gas diluted with Ar at a predetermined flow rate, the carbon matrix is ​​stirred with a blade inside the vessel to adjust the time required to precipitate the target weight of silicon. In this way, the reaction for silicon particle deposition is carried out.

[0055] Steps (i) and (ii) above yield a parent particle formed from a carbon matrix 1 and silicon particles 2. When manufacturing composite particles 10 having a coating layer 3 on the surface, the following step (iii) may be performed after step (ii) above to further form the coating layer 3.

[0056] <Step (iii)> At least a portion of the surface of the obtained mother 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. As a method for coating at least a portion of the surface of the mother particles with a conductive carbon material, a CVD method using hydrocarbon gases such as acetylene and methane as raw materials can be mentioned. Another example is a method in which coal pitch, petroleum pitch, phenolic resin, etc. are mixed with the mother particles and heated to carbonize them. The mixture of mother particles and raw materials for conductive carbon materials such as coal pitch, petroleum pitch, and phenolic resin is heated, for example, in an inert atmosphere (for example, an atmosphere such as argon or nitrogen) at a temperature of 700°C or higher and 950°C or lower. Carbon black may also be attached to the surface of the mother particles. In this way, composite particles 10 are obtained.

[0057] (Embodiment 2) The secondary battery according to Embodiment 2 comprises a negative electrode, a positive electrode, and an electrolyte. The negative electrode contains the negative electrode active material according to Embodiment 1. For example, the negative electrode includes a negative electrode mixture layer containing the negative electrode active material according to Embodiment 1 and a negative electrode current collector.

[0058] Figure 2 is a cross-sectional view showing the schematic configuration of a secondary battery according to Embodiment 2. The secondary battery 100 comprises a positive electrode 23, a negative electrode 26, a separator 27, a non-aqueous electrolyte 29, and an outer casing 28. The positive electrode 23, negative electrode 26, non-aqueous electrolyte 29, and separator 27 are housed in the outer casing 28. The separator 27 is positioned between the positive electrode 23 and the negative electrode 26. The positive electrode 23 and the negative electrode 26 face each other via the separator 27. The positive electrode 23 comprises a positive electrode mixture layer 22 and a positive electrode current collector 21. The positive electrode mixture layer 22 is positioned between the positive electrode current collector 21 and the separator 27. The negative electrode 26 comprises a negative electrode mixture layer 25 and a negative electrode current collector 24. The negative electrode mixture layer 25 is positioned between the negative electrode current collector 24 and the separator 27.

[0059] The positive electrode mixture layer 22 is composed of a positive electrode mixture. The positive electrode mixture layer 22 can be formed by applying a positive electrode slurry, which is obtained by dispersing the positive electrode mixture in a dispersion medium, to the surface of the positive electrode current collector 21 and drying it. The dried coating may be rolled if necessary. The positive electrode mixture layer 22 may be formed on one surface of the positive electrode current collector 21, or on both surfaces.

[0060] The positive electrode mixture contains a positive electrode active material capable of intercalating 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, when lithium-containing transition metal oxides or lithium-containing transition metal phosphates are used as the positive electrode active material, the manufacturing cost of the battery can be reduced and the average discharge voltage can be increased. 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.

[0061] The positive electrode mixture may optionally contain a conductive additive, an ion conductor, and a binder.

[0062] Conductive additives and ionic conductors are used to reduce electrode resistance. 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 of these conductive additives can be used.

[0063] Examples of ion conductors include gel electrolytes such as polymethyl methacrylate and polymethyl methacrylate, organic solid electrolytes such as polyethylene oxide, and Li7La3Zr2O 12 Examples include inorganic solid electrolytes. At least one of these ion conductors can be used.

[0064] A binder is used to improve the bonding properties of the materials constituting the electrodes. Examples of binders include polymer materials such as polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, polytetrafluoroethylene, carboxymethylcellulose, polyacrylic acid, styrene-butadiene copolymer rubber, polypropylene, polyethylene, and polyimide. At least one of these binders can be used.

[0065] The positive electrode current collector 21 is a sheet or film made of a metallic material such as aluminum, aluminum alloy, stainless steel, titanium, or titanium alloy. The sheet or film may be porous or non-porous. Metal foil, metal mesh, etc., can be used as the sheet or film. A carbon material may be coated on the surface of the positive electrode current collector 21 as a conductive auxiliary material.

[0066] The negative electrode mixture layer 25 is composed of a negative electrode mixture. The negative electrode mixture includes the negative electrode active material according to Embodiment 1. The negative electrode mixture layer 25 can be formed, for example, by coating a negative electrode slurry, which is obtained by dispersing the negative electrode mixture in a dispersion medium, onto the surface of the negative electrode current collector 24 and drying it. The dried coating may be rolled if necessary. The negative electrode mixture layer 25 may be formed on one surface of the negative electrode current collector 24, or on both surfaces. The negative electrode mixture may optionally contain a conductive additive, an ion conductor, and a binder. Materials usable for the positive electrode mixture layer 22 can also be used as the conductive additive, ion conductor, and binder in the negative electrode mixture layer 25. The negative electrode mixture may contain negative electrode active material other than the negative electrode active material according to Embodiment 1.

[0067] The negative electrode current collector 24 is a sheet or film made of a metallic material such as stainless steel, nickel, nickel alloy, copper, or copper alloy. The sheet or film may be porous or non-porous. Metal foil, metal mesh, etc., can be used as the sheet or film. A carbon material may be coated on the surface of the negative electrode current collector 24 as a conductive auxiliary material.

[0068] The separator 27 is an electrolyte layer having lithium ion conductivity. The material of the separator 27 is not particularly limited as long as the passage of lithium ions is permitted. The material of the separator 27 may 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. As for the solid electrolyte, sulfide solid electrolytes such as Li2S-P2S5 and Li7La3Zr2O 12 Examples of oxide solid electrolytes include (LLZ). Examples of gel electrolytes include gel electrolytes containing fluororesins such as PVdF. Examples of ion exchange resin membranes include cation exchange membranes and anion exchange membranes. Examples of porous membranes include porous membranes made of polyolefin resin and porous membranes made of glass paper obtained by weaving glass fibers into a nonwoven fabric.

[0069] 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 outer casing 28. Lithium ions can move between the positive electrode 23 and the negative electrode 26 through the action of the non-aqueous electrolyte 29. The non-aqueous electrolyte 29 may contain a non-aqueous electrolyte, a gel electrolyte, or an ionic liquid.

[0070] The non-aqueous electrolyte includes, for example, a non-aqueous solvent and a lithium salt.

[0071] Non-aqueous solvents that can be used include cyclic carbonate esters, linear carbonate esters, cyclic ethers, linear ethers, cyclic esters, linear esters, fluorinated solvents, and nitriles. Examples of cyclic carbonate esters include ethylene carbonate, propylene carbonate, and butylene carbonate. Examples of linear carbonate esters include dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate. Examples of cyclic ethers include tetrahydrofuran, 1,4-dioxane, and 1,3-dioxolane. Examples of linear ethers include 1,2-dimethoxyethane and 1,2-diethoxyethane. Examples of cyclic esters include γ-butyrolactone. Examples of linear esters include methyl acetate. Examples of fluorinated solvents include fluoroethylene carbonate, methyl fluoropropionate, fluorobenzene, fluoroethyl methyl carbonate, and fluorodimethylene carbonate. Examples of nitriles include acetonitrile. At least one of these non-aqueous solvents can be used.

[0072] Examples of lithium salts include lithium hexafluoride phosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), 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 of these lithium salts can be used.

[0073] Gel electrolytes can be materials obtained by impregnating a polymer material with a non-aqueous electrolyte. Examples of polymer materials include polyethylene oxide, polyacrylonitrile, polyvinylidene fluoride, polymethyl methacrylate, and polymers having ethylene oxide bonds.

[0074] Examples of cations constituting ionic liquids include aliphatic quaternary cations, aliphatic cyclic ammonium compounds, and nitrogen-containing heterocyclic aromatic cations. Examples of aliphatic quaternary cations include tetraalkylammonium compounds and tetraalkylphosphonium compounds. Examples of aliphatic cyclic ammonium compounds include pyrrolidinium compounds, morpholinium compounds, imidazolinium compounds, tetrahydropyrimidinium compounds, piperadinium compounds, and piperidinium compounds. Examples of nitrogen-containing heterocyclic aromatic cations include pyridinium compounds and imidazolium compounds. An example of anion constituting ionic liquids is PF6. - BF4 - SbF6 - AsF6 - SO3CF3 - , N(SO2F)2 - , N(SO2CF3)2 - , N(SO2C2F5)2 - , N(SO2CF3)(SO2C4F9) - , C(SO2CF3)3 - These are some examples. The ionic liquid may also contain a lithium salt.

[0075] The outer casing 28 is made of a material 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 resin or metal container.

[0076] 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, prismatic, sheet-type, button-type, and flat-type.

[0077] The secondary battery according to Embodiment 2 can reduce the expansion of the negative electrode during charging because the negative electrode contains the negative electrode active material according to Embodiment 1. Therefore, the secondary battery according to Embodiment 2 can achieve both increased capacity due to the inclusion of silicon in the negative electrode active material and improved cycle characteristics due to reduced expansion of the negative electrode during charging.

[0078] (Other Embodiments) (Note) The above description of embodiments discloses the following technologies.

[0079] (Technology 1) A composite particle comprising a carbon matrix and silicon particles dispersed in the carbon matrix, wherein the pore volume of the composite particle, as measured by nitrogen gas adsorption, is 0.8 cm². 3 A negative electrode active material with a concentration of 1 / g or more.

[0080] This configuration allows the negative electrode active material of Technology 1 to reduce the expansion of the negative electrode during charging.

[0081] (Technology 2) The negative electrode active material according to Technology 1, wherein the ratio of the total volume of micropores and mesopores in the composite particles to the pore volume of the composite particles is 70% or more.

[0082] This configuration allows the negative electrode active material of Technology 2 to further reduce the expansion of the negative electrode during charging.

[0083] (Technology 3) The negative electrode active material according to Technology 1 or 2, wherein the content ratio of silicon particles in the composite particles is 30% by mass or more and 80% by mass or less.

[0084] This configuration allows the negative electrode active material of Technology 3 to further reduce the expansion of the negative electrode during charging, and to improve the initial charge and discharge efficiency of the battery.

[0085] (Technology 4) The negative electrode active material according to any one of the technologies 1 to 3, wherein the average primary particle diameter of the silicon particles is 10 nm or less.

[0086] This configuration allows the negative electrode active material of technology 4 to further reduce the expansion of the negative electrode during charging.

[0087] (Technical 5) The negative electrode active material according to any one of Technical 1 to 4, wherein the silicon particles include at least one selected from the group consisting of a crystalline phase and an amorphous phase.

[0088] This configuration allows the negative electrode active material of technology 5 to reduce the expansion of the negative electrode during charging.

[0089] (Technical 6) The negative electrode active material according to any one of Technical 1 to 5, wherein the carbon matrix comprises at least one element selected from the group consisting of Mg, Ca, Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, and Si.

[0090] This configuration allows the negative electrode active material of technology 6 to improve battery characteristics.

[0091] (Technical 7) The composite particles have a coating layer on their surface, and the coating layer contains carbon, the negative electrode active material according to any one of Technical 1 to 6.

[0092] The negative electrode active material of Technology 7 can have its conductivity improved by including a coating layer. As a result, the negative electrode active material of Technology 7 can improve the charging capacity of the battery.

[0093] (Technical 8) A negative electrode mixture containing the negative electrode active material described in any one of Technical 1 to 7.

[0094] This configuration allows the negative electrode mixture of technology 8 to reduce the swelling of the negative electrode during charging.

[0095] (Technical 9) A secondary battery comprising a negative electrode containing a negative electrode active material described in any one of Technical 1 to 7, a positive electrode, and an electrolyte.

[0096] This configuration allows the secondary battery of technology 9 to reduce the expansion of the negative electrode during charging.

[0097] The present disclosure will be described in more detail below with reference to examples. The following examples are merely illustrative and not limited to any one aspect.

[0098] [Preparation of Composite Particles] (Example 1) Coal tar pitch was used as the carbon source. Magnesium oxide particles (average particle size 10 nm) were used as the mold material. Coal tar pitch and magnesium oxide particles were mixed in a mass ratio of coal tar pitch:magnesium oxide particles = 1:3, and the resulting mixture was calcined at 900°C for 5 hours. This yielded a composite of the mold material and the carbonaceous material. The mold material was removed by washing this composite with hydrochloric acid to obtain porous carbon. The pore volume of the obtained porous carbon was measured by nitrogen gas adsorption and was found to be 2.0 cm³. 3 The result was / g. The measurement of the pore volume of porous carbon by nitrogen gas adsorption was performed in the same manner as the method for measuring the pore volume of composite particles by nitrogen gas adsorption, as described later.

[0099] Next, 0.05 g of the obtained porous carbon was placed in a quartz cylindrical tubular furnace with an inner diameter of 50 mm using a glass boat, and the atmosphere inside the furnace was replaced with nitrogen by flowing nitrogen through it. Then, after stopping the inflow of nitrogen, the furnace temperature was raised to 600°C while flowing silane gas at a flow rate of 3 sccm as a precursor gas into the furnace, and the decomposition reaction of the precursor gas was carried out for 60 minutes. This caused silicon particles to precipitate inside the pores of the porous carbon, obtaining the composite particles of Example 1. The mass ratio of carbon (C) to silicon (Si) of the obtained composite particles was determined by ICP analysis and found to be C:Si = 1:1. That is, the composite particles contained 50 mass% Si.

[0100] (Example 2) Porous carbon was prepared in the same manner as in Example 1, except that the mass ratio of coal tar pitch to magnesium oxide particles was changed to coal tar pitch:magnesium oxide particles = 1:5. The pore volume of the obtained porous carbon was measured by the nitrogen gas adsorption method in the same manner as in Example 1, and was found to be 2.8 cm³. 3 It was / g.

[0101] Composite particles were prepared by precipitating silicon particles inside porous carbon using the same method as in Example 1. The mass ratio of carbon (C) to silicon (Si) in the obtained composite particles was determined by ICP analysis and found to be C:Si = 1:1. In other words, the composite particles contained 50% by mass of Si.

[0102] (Comparative Example) Porous carbon was prepared in the same manner as in Example 1, except that the mass ratio of coal tar pitch to magnesium oxide particles was changed to coal tar pitch:magnesium oxide particles = 1:1. The pore volume of the obtained porous carbon was measured by the nitrogen gas adsorption method in the same manner as in Example 1, and was found to be 0.5 cm³. 3 It was / g.

[0103] Composite particles were prepared by precipitating silicon particles inside porous carbon using the same method as in Example 1. The mass ratio of carbon (C) to silicon (Si) in the obtained composite particles was determined by ICP analysis and found to be C:Si = 1:1. In other words, the composite particles contained 50% by mass of Si.

[0104] [Battery Fabrication] Using the composite particles of Example 1, Example 2, and the Comparative Example, evaluation cells for Example 1, Example 2, and the Comparative Example were fabricated as follows.

[0105] (Preparation of the negative electrode) A mixture of composite particles and graphite was used as the negative electrode active material, mixed in a mass ratio of 20:80. The negative electrode active material, sodium carboxymethylcellulose (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 mixture, and it was stirred using a mixer to prepare the negative electrode slurry. Next, 1 m was applied to the surface of the copper foil. 2 The negative electrode slurry is applied so that the mass of the negative electrode mixture per unit is 190 g. After the coating is dried, it is rolled out and a density of 1.5 g / cm³ is applied to both sides of the copper foil. 3 A negative electrode was fabricated with a negative electrode mixture layer formed thereon.

[0106] The negative electrode was cut into a 20mm x 20mm shape with a 5mm x 5mm protrusion, and the negative electrode mixture layer was peeled off the protrusion to expose the copper foil. Then, a negative electrode tab lead was connected to the exposed copper foil, and a predetermined area around the outer circumference of the negative electrode tab lead was covered with an insulating film.

[0107] (Preparation of the counter electrode) Tabs were made by welding small pieces of Ni mesh to the ends. The tabs were cut to the required size, and the mesh portion was pressed onto a 300 μm thick lithium metal foil to create the counter electrode.

[0108] (Preparation of non-aqueous electrolyte) A non-aqueous electrolyte was prepared by dissolving LiPF6 in a mixed solvent prepared by mixing ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of EC:EMC:DMC = 4:1:15. The LiPF6 concentration was 1.3 mol / L.

[0109] (Cell Fabrication) Using the above-mentioned negative electrode and two counter electrodes, an evaluation cell for negative electrode regulation was fabricated as follows. The cell was fabricated in a dry air atmosphere with a dew point of -60°C or lower. An electrode group was fabricated by sandwiching the negative electrode between a pair of counter electrodes and facing the negative electrode mixture layer and lithium metal foil via a separator. Next, a rectangular piece of Al laminate film was folded in half, and the two ends on the long side were heat-sealed to form a cylinder. Then, the fabricated electrode group was inserted into the cylinder from one of the short sides of the Al laminate film, and the end face of the Al laminate film and the insulating film of each tab lead were aligned and heat-sealed. Next, 0.3 cm of electrolyte was poured from the unheat-sealed short side of the cylinder. 3 The solution was injected. After injection, the cells were left to stand for 3 minutes under reduced pressure of 0.02 MPa, and then returned to atmospheric pressure. This process was repeated twice to impregnate the negative electrode mixture layer with a non-aqueous electrolyte. Finally, the end face of the cylindrical Al laminate film on the injected side was heat-sealed under reduced pressure to obtain an evaluation cell.

[0110] [Pore Volume of Composite Particles] The pore volume of the composite particles of Example 1, Example 2, and the Comparative Example was determined by nitrogen gas adsorption. Specifically, measurements were taken using the nitrogen gas adsorption method (77K) with Microtrac-Bell Co., Ltd.: BELSORP-MAX-X. The ratio of micropores, mesopores, and macropores in the total pore volume was calculated from the obtained adsorption isotherms and the theoretical adsorption isotherms fitted by the GCMC method. The results are shown in Table 1.

[0111] [Battery Discharge Capacity] Initial charge-discharge tests were performed using the batteries (evaluation cells) of Example 1, Example 2, and the Comparative Example as described below.

[0112] For the batteries of Example 1, Example 2, and the Comparative Example, constant current charging was performed at an ambient temperature of 25°C with a current of 0.1C until the voltage reached 1.0V. Subsequently, constant current discharge was performed at a current of 0.1C until the cell voltage reached 0.005V. This evaluated the initial discharge capacity of the batteries of Example 1, Example 2, and the Comparative Example. The measurement results are shown in Table 1.

[0113] [Evaluation of Negative Electrode Swelling (Measurement of Negative Electrode Mixture Layer Swelling Rate)] For the batteries of Example 1, Example 2, and Comparative Example after the initial charge-discharge tests described above, constant current charging was performed at an ambient temperature of 25°C with a current value of 0.1C until the voltage reached 1.0V. The cells in the fully charged state were disassembled and the negative electrode was removed, and the thickness of the negative electrode mixture layer was determined as the thickness of the negative electrode mixture layer in the fully charged state. The initial thickness of the negative electrode mixture layer in the negative electrode was determined in advance before the test cells were prepared. The thickness of the negative electrode mixture layer was measured using a film thickness gauge. Specifically, the thickness was measured at five arbitrary locations on the negative electrode mixture layer, and the average value calculated from the five obtained measurements was taken as the thickness. The swelling rate of the negative electrode mixture layer was calculated using the following formula.

[0114] (Formula for calculating the expansion rate of the negative electrode mixture layer) Expansion rate of the negative electrode mixture layer (%) = {(thickness of the negative electrode mixture layer in a fully charged state) / (thickness of the negative electrode mixture layer at the beginning)} × 100

[0115] The results are shown in Table 1. Table 1 also shows the relative values ​​when the expansion rate of the comparative example is set as the baseline (100).

[0116]

[0117] As shown in Table 1, the pore volume of Examples 1 and 2 is 0.8 cm³. 3 A negative electrode active material containing composite particles with a concentration of 0.8 / g or more has a pore volume of 0.8 cm². 3 Compared to a comparative example of a negative electrode active material that does not contain composite particles of 1 / g or more, it was possible to reduce the expansion rate of the negative electrode during charging while maintaining the battery's discharge capacity.

[0118] From the above, it has been confirmed that the negative electrode active material of this disclosure can reduce the swelling of the negative electrode during charging while maintaining battery characteristics.

[0119] The technology disclosed herein can be used in batteries such as lithium-ion secondary batteries.

Claims

1. The composite particle comprises a carbon matrix and silicon particles dispersed in the carbon matrix, and the pore volume of the composite particle, as measured by nitrogen gas adsorption, is 0.8 cm³. 3 A negative electrode active material with a concentration of 1 / g or more.

2. The negative electrode active material according to claim 1, wherein the ratio of the total volume of micropores and mesopores in the composite particles to the pore volume of the composite particles is 70% or more.

3. The negative electrode active material according to claim 1, wherein the content ratio of silicon particles in the composite particles is 30% by mass or more and 80% by mass or less.

4. The negative electrode active material according to claim 1, wherein the average primary particle diameter of the silicon particles is 10 nm or less.

5. The negative electrode active material according to claim 1, wherein the silicon particles include at least one selected from the group consisting of a crystalline phase and an amorphous phase.

6. The negative electrode active material according to claim 1, wherein the carbon matrix comprises at least one element selected from the group consisting of Mg, Ca, Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, and Si.

7. The negative electrode active material according to claim 1, wherein the composite particles have a coating layer on their surface, and the coating layer contains carbon.

8. A negative electrode mixture comprising the negative electrode active material according to any one of claims 1 to 7.

9. A secondary battery comprising a negative electrode containing the negative electrode active material described in any one of claims 1 to 7, a positive electrode, and an electrolyte.

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