Anode active material and manufacturing method for the same, and secondary battery

Abstract

To provide an anode active material and a manufacturing method for the same, capable of forming a secondary battery with excellent initial coulombic efficiency, low anode expansion rate, and excellent capacity retention, and a secondary battery having the anode active material.SOLUTION: The anode active material for secondary batteries is composed of at least a graphitic phase, a phase containing Si (silicon), O (oxygen), and C (carbon), and a nano-silicon phase.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a negative electrode active material, a method for producing the same, and a secondary battery. More specifically, the present invention relates to a negative electrode active material that can be suitably used as a negative electrode active material for secondary batteries, a method for producing the same, and a secondary battery including such a negative electrode active material in its negative electrode. [Background technology]

[0002] In recent years, the demand for small, high-capacity secondary batteries has increased with the advancement of performance and miniaturization in various portable electronic and communication devices. In particular, various lithium-ion batteries, which are non-aqueous electrolyte secondary batteries that use lithium intercalation compounds as the negative electrode active material, which can absorb and release lithium ions between the layers of crystal planes during charging and discharging, have been rapidly deployed in hybrid vehicles, electric vehicles, home storage batteries, and other applications, and their range of use is expanding. Therefore, there is a demand for lithium-ion batteries with higher capacity and improved battery characteristics such as cycle characteristics and discharge rate characteristics. Conventional lithium-ion batteries primarily use graphite as the anode material, but the low theoretical capacity density of graphite (372 mAh / g) limits the development of lithium-ion batteries with higher energy densities. To compensate for the low theoretical capacity density of graphite, anode materials using elements capable of absorbing and releasing lithium ions, such as silicon and tin, or alloys and oxides with other elements, are being investigated. Silicon (Si) has a theoretical capacity (4200 mAh / g) more than 10 times that of graphite, and silicon and silicon-containing anode active materials are attracting attention as next-generation anode materials capable of achieving higher capacities.

[0003] For example, Patent Document 1 discloses a method for producing composite graphite particles for use in the negative electrode of a non-aqueous secondary battery, the method including the steps of obtaining a mixture containing at least graphite, metal particles that can be alloyed with Li, and a polymer containing nitrogen atoms, and applying mechanical energy to the mixture to form a spheroid. It is claimed that a non-aqueous secondary battery equipped with a negative electrode using such composite graphite particles has a high capacity, high cycle characteristics, and high rate characteristics. Patent Document 2 discloses composite graphite particles containing a metal that can be alloyed with Li, flake graphite, and a carbonaceous material, wherein the content of the carbonaceous material relative to the composite graphite particles is less than 20% by mass, and the intensity ratio of a specific peak measured by Raman spectroscopy is less than 0.4. Lithium-ion secondary batteries using a negative electrode material containing the composite graphite particles are said to have well-balanced battery characteristics such as discharge capacity, initial charge / discharge efficiency, and cycle characteristics. Patent Document 3 discloses a negative electrode material for lithium ion secondary batteries that contains silicon particles, graphite particles, and a carbonaceous substance, and that is composed of composite particles that have a specific pore volume and specific surface area and that have a specific peak intensity ratio observed in a Raman spectrum that is within a predetermined range, and that is said to have high capacity and excellent cycleability. Patent Document 4 discloses a negative electrode active material having a granular structure mainly made of graphite with an uneven surface, and a surface layer on at least a portion of the surface of the granular structure, in which silicon particles of a specific average particle size, preferably with a crystallite size of a predetermined value or less as determined by X-ray diffraction analysis, are dispersed in a matrix phase, and the negative electrode active material is said to provide a secondary battery with high initial discharge capacity, high capacity retention rate, and high charge / discharge capacity, and with an excellent balance of these properties. Patent Document 5 describes a polymer containing silicon, oxygen, and carbon, in which the content ratio of oxygen and carbon to silicon is within a specific range, 29 A secondary battery material is disclosed in which the peak area intensity ratio attributable to zero-valent Si and SiO4 bonds in a Si-NMR spectrum is within a predetermined range. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-035317 [Patent Document 2] Japanese Patent Application Laid-Open No. 2005-243508 [Patent Document 3] Japanese Patent Application Laid-Open No. 2012-043546 [Patent Document 4] Special Publication No. 2023-134577 [Patent Document 5] International Publication No. 2023 / 017694 Summary of the Invention [Problem to be solved by the invention]

[0005] Silicon and silicon-containing negative electrode active materials have a low initial efficiency (hereinafter referred to as "initial coulombic efficiency"), meaning that the irreversible capacity is large during the initial absorption and desorption of lithium ions, preventing the capacity of the positive electrode from being fully utilized. This is because silicon oxide (SiO2), which exists on the surface of silicon due to natural oxidation, forms lithium silicate with the lithium ions supplied from the positive electrode during the initial charge, stabilizing and making it irreversible. This reduces the amount of lithium ions released to the positive electrode, resulting in a decrease in the initial charge-discharge efficiency. The composite particles disclosed in Patent Documents 1 to 3 are composed of graphite, silicon or SiOx, and a carbonaceous material that does not contain silicon (Si), and are intended to achieve high capacity through the combination of these materials. However, during charge and discharge, volume changes occur due to expansion and contraction, and cracking occurs, increasing the surface area and the contact area with the electrolyte. As a result, the amount of solid-phase interfacial electrolyte decomposition products produced during charge and discharge increases, reducing the reversible charge and discharge capacity per unit volume and the initial coulombic efficiency. The negative electrode active material disclosed in Patent Document 4 is composed of graphite and a surface layer in which silicon particles are dispersed in a matrix phase, with the surface layer being present on at least a portion of the uneven surface of the graphite. The secondary battery material disclosed in Patent Document 5 can form a matrix phase composed of silicon oxycarbide (a composite oxide consisting of silicon, oxygen, and carbon) produced by sintering methylsiloxanes at high temperatures, and a calcined product of a carbon source resin such as a phenolic resin, and the negative electrode active material is obtained by incorporating silicon particles into this matrix phase. However, improvements in initial coulombic efficiency and capacity retention are still required.

[0006] The inventors of the present invention have studied measures for improving the reduction in the capacity retention rate due to expansion and contraction of a negative electrode containing a silicon-containing negative electrode active material. As a result, it has been found that a negative electrode active material composed of at least a graphite phase, a phase containing Si (silicon), O (oxygen), C (carbon), and a nanosilicon phase can suppress expansion and is effective in improving the initial Coulomb efficiency and the capacity retention rate, and the present invention has been completed. An object of the present invention is to provide a negative electrode active material capable of forming a secondary battery having excellent initial Coulomb efficiency, a small negative electrode expansion rate, and excellent capacity retention rate, a method for producing the same, and a secondary battery having such a negative electrode active material. [Means for Solving the Problems]

[0007] The present invention has the following aspects. [1] A negative electrode active material for a secondary battery, which is composed of at least a graphite phase, a phase containing Si (silicon), O (oxygen), C (carbon), and a nanosilicon phase. [2] The negative electrode active material for a secondary battery according to [1], wherein the phase containing Si (silicon), O (oxygen), and C (carbon) further contains N (nitrogen). [3] The negative electrode active material for a secondary battery according to [1] or [2], wherein the graphite phase contains flaky graphite having a major axis of 0.3 μm or more and 30 μm or less. [4] The negative electrode active material for a secondary battery according to any one of [1] to [3], wherein the nanosilicon phase contains silicon particles having an average particle diameter of 5 nm or more and 100 nm or less. [5] The negative electrode active material for a secondary battery according to any one of [1] to [4], wherein the content ratio of Si, O, and C in the phase containing Si (silicon), O (oxygen), and C (carbon) is SiOxCy (where x and y each represent a positive number of 0.`1 < x < 2 and 0.3 < y < `11) as an atomic ratio. [6] The negative electrode active material for a secondary battery according to any one of [2] to [5], wherein the phase containing Si (silicon), O (oxygen), and C (carbon) contains SiOC (silicon oxycarbide), and the content ratio of N (nitrogen) to the silicon oxycarbide is more than 0.2% by mass and less than 2.5% by mass. [7] The negative electrode active material for a secondary battery according to any one of [1] to [6], further comprising a low-crystalline carbonaceous phase on at least a portion of the surface. [8] A secondary battery comprising the negative electrode active material for a secondary battery according to any one of [1] to [7]. [9] A process of spheronizing a mixture of graphite, silicon particles, and an organosilicon-based polymer material having a polymer structure containing Si (silicon), O (oxygen), and C (carbon); and firing the mixture at 700 to 1200°C in a non-oxidizing atmosphere. A method for producing a negative electrode active material for a secondary battery, the negative electrode active material comprising at least a graphite phase, a phase containing Si, O, and C, and a nanosilicon phase.

[10] The method for producing a negative electrode active material for a secondary battery according to [9], wherein the graphite contains flake graphite having a major axis of 0.3 μm or more and 30 μm or less.

[11] The method for producing a negative electrode active material for a secondary battery according to [9] or

[10] , wherein the silicon particles contain silicon particles having an average particle size of 5 nm or more and 100 nm or less.

[12] The method for producing a negative electrode active material for a secondary battery according to any one of [9] to

[11] , wherein the organosilicon polymer material has a polymer structure consisting of a mixture of a polysiloxane compound and a carbon source resin, or a composite of a polysiloxane compound and a carbon source resin.

[13] The method for producing a negative electrode active material for a secondary battery according to any one of [9] to

[12] , wherein the phase containing Si (silicon), O (oxygen) and C (carbon) further contains N (nitrogen). [Effects of the Invention]

[0008] The present invention can provide a negative electrode active material capable of forming a secondary battery having excellent initial coulombic efficiency, a small negative electrode expansion rate, and an excellent capacity retention rate, a method for producing the same, and a secondary battery having such a negative electrode active material and excellent battery characteristics such as charge-discharge characteristics. DETAILED DESCRIPTION OF THE INVENTION

[0009] <Negative electrode active material for secondary batteries> The negative electrode active material for a secondary battery of the present invention (hereinafter also referred to as "the negative electrode active material") is composed of at least a graphite phase, a phase containing Si (silicon), O (oxygen), and C (carbon) (hereinafter also referred to as "silicon-based matrix phase"), and a nanosilicon phase. The negative electrode active material contains a silicon-based matrix phase with a tough structure in addition to the graphite phase, which can suppress expansion and contraction due to the nanosilicon phase during charge and discharge, thereby maintaining the shape of the negative electrode active material and maintaining its cycle characteristics for a longer period of time.

[0010] The graphitic phase in the present negative electrode active material is preferably composed of graphite. The graphite that can be used includes natural graphite such as amorphous graphite, flake graphite, scaly graphite, and semi-scaly graphite, and artificial graphite produced by graphitizing petroleum coke, coal pitch coke, coal needle coke, mesophase pitch, etc. by heating them to 2500° C. or higher. If necessary, the graphite may be one that has been subjected to impurity removal, or to pulverization and classification by known methods, and may be subjected to various chemical treatments, heat treatments, oxidation treatments, physical treatments, etc. in the liquid, gas, or solid phase before or after the graphitization treatment.

[0011] The volume-average particle diameter (D50) of the graphite is not particularly limited, but is typically preferably 0.1 μm to 100 μm, more preferably 0.2 μm to 90 μm. When the volume-average particle diameter (D50) is within this range, it is easy to suppress a decrease in the initial coulomb efficiency. Furthermore, it is easy to control the pore volume and specific surface area within the negative electrode active material, which makes it easier to maintain lithium ion mobility, resulting in a small irreversible capacity and a high capacity. In this specification, the volume average particle size (D50) refers to the volume-based median size measured by laser diffraction / scattering particle size distribution measurement.

[0012] Examples of the shape of graphite include flakes, spheres, blocks, plates, etc. Among these, flake graphite is preferred from the viewpoints that it is easy to form internal voids during the formation of the present negative electrode active material, the nanosilicon phase and silicon matrix phase described below are more likely to be dispersed on the surface and inside of the present negative electrode active material, and the present negative electrode active material has a small irreversible capacity and is likely to have a high capacity. The major axis of the flake graphite is preferably 0.2 μm or more and 50 μm or less, more preferably 0.3 μm or more and 30 μm or less. The aspect ratio, which is the ratio of the major axis of the flake graphite to the minor axis perpendicular thereto, is usually preferably 3 or more and 100 or less, more preferably 4 or more and 50 or less. When the major axis and aspect ratio of the flake graphite are within the above ranges, internal voids are likely to be formed during the formation of the present negative electrode active material, the nanosilicon phase described below is likely to be more dispersed on and inside the present negative electrode active material, and volume expansion associated with charge and discharge is likely to be alleviated, which is likely to improve cycle characteristics. In this specification, the aspect ratio of flake graphite is the average value of the aspect ratios of a plurality of (50) pieces of graphite measured by observation with a scanning electron microscope.

[0013] Graphite preferably has high crystallinity from the viewpoint of increasing charge / discharge capacity. Specifically, the interplanar spacing (d002) of the 002 plane measured by wide-angle X-ray diffraction is preferably 0.335 nm or more and 0.337 nm or less, and Lc is preferably 90 nm or more. When the interplanar spacing or Lc of the 002 plane is within the above range, the graphite has high crystallinity, the irreversible capacity of the negative electrode active material is small, and the capacity is likely to be high. Note that d002 is a value calculated from the position of the diffraction peak of the (002) plane of flake graphite using CuKα X-rays and high-purity silicon as a standard substance, according to the Gakken method (a measurement method established by the 17th Committee of the Japan Society for the Promotion of Science).

[0014] The specific surface area of ​​graphite is usually 1m 2 / g or more 40m 2 / g or less, and 2 / g or more 35m 2 / g or less is more preferable. When the specific surface area of ​​graphite is within this range, the irreversible capacity of the present negative electrode active material is small and the capacity tends to be high. The specific surface area of ​​graphite is a value determined by the BET method using nitrogen gas adsorption measurement. The true density of graphite is typically 2.1 g / cm 3 It is preferable that the density is equal to or greater than 2.15 g / cm 3 More preferably, true density is 2.1 g / cm 3 When the graphite has such high crystallinity, the irreversible capacity of the negative electrode active material is small and the capacity is likely to be high. The tap density of the graphite is not particularly limited, and is usually 0.1 g / cm. 3 More than 1.0g / cm 3 Preferably, it is: The graphite used may be a commercially available product that is industrially produced and distributed.

[0015] The graphite phase constituting the present negative electrode active material may be composed of flake graphite alone, or may be composed of a particulate or fibrous conductive material such as graphite or graphitic material other than flake graphite, carbon black, or other graphitized material. From the viewpoint of improving the initial coulombic efficiency and capacity retention rate of the present negative electrode active material and reducing the negative electrode expansion rate, the graphite phase is preferably composed of flake graphite in an amount of 80 mass% or more, more preferably 90 mass% or more, and even more preferably 100 mass% flake graphite, i.e., composed solely of flake graphite. In this case, in the graphite phase constituting the present negative electrode active material, a plurality of flake graphite particles are preferably arranged randomly, and more preferably arranged in a cabbage-like or concentric pattern. Furthermore, it is preferable that the graphite phase constituting the present negative electrode active material has no or very few irregularities on its surface.

[0016] The nanosilicon phase and silicon-based matrix phase constituting this negative electrode active material are preferably incorporated into the graphite phase, the nanosilicon phase is not present on the outer surface, and the outer surface is preferably composed of the silicon-based matrix phase or the graphite phase. If the nanosilicon phase is present on the outer surface, it may peel off from the surface of this negative electrode active material as the secondary battery containing this negative electrode active material is charged and discharged, resulting in a decrease in cycle performance. It is preferable that the nanosilicon phase and silicon-based matrix phase are dispersed in the negative electrode active material as a whole. The distribution state of the graphite phase, nanosilicon phase, and silicon-based matrix phase in this negative electrode active material can be identified, for example, by using a transmission electron microscope (TEM) or a field emission scanning electron microscope (FE-SEM) to cut the negative electrode active material with a focused ion beam (FIB) and observe the cross section with the FE-SEM, or by slicing the sample and observing it with the TEM.

[0017] The nanosilicon phase constituting the present negative electrode active material is preferably composed of silicon particles. Such silicon particles are composed of zero-valent Si (silicon) and have an average particle size of preferably 150 nm or less, more preferably 120 nm or less, and even more preferably 100 nm or less. The average particle size of the silicon particles is preferably 1 nm or more, more preferably 2 nm or more, and even more preferably 5 nm or more. When the average particle size of the silicon particles is within the above-mentioned range, it is easy to improve the charge / discharge performance of a secondary battery containing the present negative electrode active material and to easily maintain a high capacity retention rate over a long period of time. Here, the average particle size of silicon particles is the particle size (D50) at which the cumulative volume distribution curve reaches 50% of the total when the particle size distribution is measured by dynamic light scattering using a laser diffraction particle size analyzer or the like, starting from the small diameter side.

[0018] Silicon particles exceeding 300 nm tend to form large agglomerates, which can easily pulverize during charging and discharging in secondary batteries containing this negative electrode active material, resulting in a decrease in capacity retention. Silicon particles less than 10 nm tend to aggregate, reducing the dispersibility of the nanosilicon phase in this negative electrode active material. Furthermore, their surface activation energy increases, which can lead to increased by-products on the silicon particle surface during high-temperature firing of this negative electrode active material, leading to a decrease in charge-discharge performance. Therefore, it is preferable that the content of silicon particles exceeding 300 nm and silicon particles less than 10 nm in this negative electrode active material be as small as possible.

[0019] When the nanosilicon phase is composed of silicon particles, the shape of the silicon particles may be any of granular, needle-like, and flake-like. From the viewpoint of the charge-discharge performance of the present negative electrode active material, the length of the silicon particles in the major axis direction is preferably 5 to 100 nm, the thickness is preferably 1 to 60 nm, and the aspect ratio, which is the ratio of the thickness to the length, is preferably 0.5 or less. Although the average particle size of silicon particles can be measured by dynamic light scattering, silicon particles with the above aspect ratios can be identified more easily and precisely using a transmission electron microscope (TEM) or a field emission scanning electron microscope (FE-SEM). In the case of this anode active material, the sample to be measured can be cut with a focused ion beam (FIB) and the cross section can be observed with an FE-SEM, or the sample can be sliced ​​and the state of the silicon particles can be identified by TEM observation. The aspect ratio of the silicon particles is a calculation result based on the main 50 particles of silicon particles within the field of view of the TEM image. The silicon particles are preferably crystalline, and the crystallite diameter obtained from the diffraction peak assigned to Si(111) in X-ray diffraction is preferably in the range of 5 nm to 14 nm from the viewpoint of initial coulombic efficiency and capacity retention. The specific surface area of ​​silicon particles that make up the nanosilicon phase, as measured by the BET method using nitrogen gas adsorption, is 100 to 400 m from the viewpoint of capacitance and first coulomb efficiency. 2 / g is preferred, and 100 to 300m 2 / g is more preferred. From the viewpoint of achieving a good balance between the initial coulombic efficiency and the capacity retention rate, silicon oxide may be present near the surface of the silicon particles. In this case, it is preferable that the surface of the silicon particles is coated with a silicon dioxide film, which is an oxide film of silicon.

[0020] Furthermore, when the silicon matrix phase contains N (nitrogen) in addition to Si (silicon), O (oxygen), and C (carbon), as described below, at least one lattice structure attributed to silicon nitride may be present near the surface of the silicon particles constituting the nanosilicon phase. In this case, at least one lattice structure attributed to silicon nitride may be directly bonded or adsorbed chemically or physically to the surface of the silicon particles, as long as it is present near the surface of the silicon particles. Note that the vicinity of the surface of the silicon particles is, for example, within 10 nm, preferably within 5 nm from the surface. Here, the lattice structure attributed to silicon nitride is preferably a lattice structure attributed to a silicon nitride crystal. Here, the nitrogen in the lattice structure attributed to silicon nitride is thought to be introduced in the same way as when the compound constituting the silicon-based matrix phase contains silicon, oxygen, carbon, and nitrogen. That is, in the manufacturing method of the present negative electrode active material described below, nitrogen is thought to be introduced due to the polysiloxane compound, phenolic resin, dispersant, other nitrogen compounds, etc. contained as components of the negative electrode active material precursor having nitrogen-containing atomic groups as functional groups in their molecules, and due to the nitrogen gas used in the baking process. The lattice structure attributed to silicon nitride can suppress contact between the silicon particle surface and oxidizing substances such as oxygen gas, thereby suppressing the growth or increase of silicon oxide films. Therefore, it is thought that the generation of lithium silicate during the initial charging operation of a secondary battery containing this negative electrode active material can be reduced, and the initial coulombic efficiency and capacity retention rate can be improved.

[0021] The nanosilicon phase in the present negative electrode active material may be composed of SiO2 or the like in addition to the silicon particles described above, but is preferably composed solely of the silicon particles described above. The content of the nanosilicon phase in the present negative electrode active material is preferably in the range of 0.05 to 11.0 mass% of the total negative electrode active material, and more preferably in the range of 0.1 to 9.0 mass%. When the content of the nanosilicon phase is in this range, the negative electrode expansion coefficient of the present negative electrode active material is small, and the initial coulombic efficiency and capacity retention rate are likely to be excellent. Furthermore, secondary batteries containing the present negative electrode active material are likely to have excellent battery properties such as charge / discharge characteristics.

[0022] The silicon-based matrix phase constituting this negative electrode active material is a phase containing silicon, oxygen, and carbon. The silicon-based matrix phase is preferably composed of a compound containing silicon, oxygen, and carbon, and the compound containing silicon, oxygen, and carbon preferably has a structure containing free carbon and a three-dimensional network structure of the silicon-oxygen-carbon skeleton of SiOC (silicon oxycarbide). Here, free carbon refers to carbon that is not contained in the three-dimensional silicon-oxygen-carbon skeleton of SiOC, and includes carbon present as a carbon phase, carbon bonded to carbon in a carbon phase, and carbon bonded to the silicon-oxygen-carbon skeleton and a carbon phase. When the compounds that make up the silicon-based matrix phase contain silicon, oxygen, and carbon and have a three-dimensional network structure of the silicon-oxygen-carbon skeleton of SiOC and a structure that includes free carbon, the silicon-oxygen-carbon skeleton in the silicon-based matrix phase has high chemical stability, and by forming a composite structure with the free carbon, the electron transition resistance is reduced, making it easier for lithium ions to diffuse.

[0023] In this negative electrode active material, the nanosilicon phase is believed to be dispersed between the graphite phase and the silicon-based matrix phase. It is also presumed that at least a portion of the silicon particles that make up the nanosilicon phase are tightly wrapped in a composite structure of a silicon-oxygen-carbon skeleton and free carbon. In this case, direct contact between the silicon particles and the electrolyte is prevented. When this negative electrode active material is used as a negative electrode, the silicon particles in the negative electrode play a role as a main component in expressing charge / discharge performance, while chemical reactions between the silicon particle surfaces and the electrolyte during charge / discharge are avoided, which is thought to prevent performance degradation of the silicon particles themselves.

[0024] Furthermore, when the silicon-based matrix phase has a three-dimensional network structure of the silicon-oxygen-carbon skeleton of SiOC and a structure containing free carbon, the approach of lithium ions causes fluctuations in the electron distribution within the silicon-oxygen-carbon skeleton of SiOC, forming electrostatic and coordinate bonds between the silicon-oxygen-carbon skeleton of SiOC and the lithium ions. These electrostatic and coordinate bonds allow lithium ions to be stored in the silicon-oxygen-carbon skeleton of SiOC. Meanwhile, because the coordinate bond energy is relatively low, lithium ion desorption reactions occur easily. In other words, it is thought that the silicon-oxygen-carbon skeleton of SiOC can reversibly intercalate and deintercalate lithium ions during charge and discharge.

[0025] The silicon-based matrix phase having the above-mentioned three-dimensional network structure of the silicon-oxygen-carbon skeleton of SiOC and a structure containing free carbon can be formed, for example, by firing an organosilicon-based polymer material (preferably a polysiloxane compound and a carbon source resin) described below. In the Raman spectrum of this active material, the carbon structure exhibits a band at 1590 cm, which is assigned to the G band of the graphite long-period carbon lattice structure. -1 Scattering peaks around 1330 cm and 1330 cm attributed to the D band of the disordered and defective graphitic short-period carbon lattice structure. -1 Preferably, the scattering peak intensity ratio I (G band / D band) is in the range of 0.7 to 2, more preferably 0.7 to 1.8. When the scattered peak peak intensity ratio I is within the above range, it can be said that some C atoms in the silicon-based matrix are mainly bonded to some Si atoms in the SiOC skeleton composed of SiO2C2, SiO3C, and SiO4. Therefore, the electron transfer between the Si atoms inside and on the surface of the SiOC skeleton and free carbon becomes easier, and the insertion and desorption reactions of lithium ions during charge and discharge when used as a secondary battery proceed rapidly, and it is considered that the charge and discharge characteristics are improved. In addition, the expansion and contraction of the negative electrode active material due to the insertion and desorption reactions of lithium ions are considered to be alleviated by the presence of carbon in the vicinity thereof, and the capacity retention rate is greatly improved. Further, due to the presence of free carbon in the silicon-based matrix phase, a resistance reduction effect of this negative electrode active material is expected. When used in a secondary battery negative electrode, the reaction inside this negative electrode active material occurs uniformly and smoothly, and it is considered that it has an excellent balance between charge and discharge performance and capacity retention rate.

[0026] The content ratio of Si (silicon), O (oxygen), and C (carbon) in the silicon-based matrix phase is preferably SiOxCy (where x and y each represent positive numbers of 0.1 < x < 2 and 0.3 < y < 11) as an atomic ratio. Incidentally, it can also be said that x represents the molar ratio (atomic ratio) of oxygen to silicon, and y represents the molar ratio (atomic ratio) of carbon to silicon. When this negative electrode active material is used in a secondary battery, from the viewpoint of achieving an excellent balance between charge and discharge performance and capacity retention rate, 0.1 < x < 1.5 is preferable, 0.1 < x ≦ 1.0 is more preferable, and 0.1 < x ≦ 0.7 is even more preferable. Also, when this negative electrode active material is used in a secondary battery, from the viewpoint of the balance between charge and discharge performance and the initial coulombic efficiency, 0.3 < y ≦ 8 is preferable. x and y can be obtained by measuring the mass content of each element and then converting it to a molar ratio (atomic ratio). At this time, the contents of oxygen and carbon can be quantified by using an inorganic element analyzer, and the content of silicon can be quantified by using an ICP emission analyzer (ICP-OES). Although it is preferable to measure x and y by the above-mentioned method, the negative electrode active material may be locally analyzed, and the content ratio data obtained thereby may be obtained at many measurement points, and the content ratio of the entire negative electrode active material may be inferred from the data. Examples of local analysis include energy dispersive X-ray spectroscopy (SEM-EDX) and electron probe microanalyzer (EPMA).

[0027] The silicon-based matrix phase may contain N (nitrogen) in addition to Si (silicon), O (oxygen), and C (carbon). Nitrogen can be introduced into the silicon-based matrix phase due to nitrogen contained in the precursor of the present negative electrode active material, such as a polysiloxane compound, a phenolic resin, a dispersant, or other nitrogen compounds, which contain nitrogen-containing atomic groups as functional groups in their molecules in the manufacturing method of the present negative electrode active material described below, or due to nitrogen gas used in the firing process. When the silicon-based matrix phase contains nitrogen, the charge / discharge performance and capacity retention rate of the present negative electrode active material tend to be better.

[0028] When the silicon-based matrix phase contains nitrogen in addition to silicon, oxygen, and carbon, the silicon-based matrix phase preferably contains SiOC (silicon oxycarbide), and the N (nitrogen) content relative to the silicon oxycarbide is preferably more than 0.2 mass% and less than 2.5 mass%, and more preferably 1.0 mass% or more and 2.0 mass% or less. When the compound that constitutes the silicon-based matrix phase contains silicon, oxygen, carbon, and nitrogen, the silicon-based matrix phase preferably contains a compound represented by the following formula (1) in terms of atomic ratio. SiOxCyNz (1) In formula (1), x and y have the same meanings as above, and z represents the molar ratio of nitrogen to silicon (ratio of the number of atoms). When the silicon-based matrix phase contains the compound represented by the above formula (1), from the viewpoints of charge-discharge performance, initial Coulomb efficiency, and capacity retention rate when this negative electrode active material is used in a secondary battery, 0.1 < x < 2, 0.3 < y < 11, and 0 < z ≤ 0.5 are preferable, and 0.1 < x < 1.5, 0.3 < y ≤ 8, and 0 < z < 0.4 are more preferable. Note that x, y, and z are all positive numbers.

[0029] In this negative electrode active material, as peak A with a chemical shift value attributed to Si in the solid-state NMR spectrum in the vicinity of -70 to -90 ppm, and as peak B with a chemical shift value attributed to SiO4 in the vicinity of -90 to -130 ppm, it is preferable that the ratio R of peak A to peak B is in the range of 0.2 to 5.0. When the ratio R is within the above range, the negative electrode expansion rate of this negative electrode active material is small, and it is excellent in initial Coulomb efficiency and capacity retention rate. Also, a secondary battery having this negative electrode active material is excellent in battery characteristics such as charge-discharge characteristics. Such a negative electrode active material showing such characteristics can be suitably obtained from a negative electrode active material precursor described later.

[0030] The average particle size of this negative electrode active material is preferably 5 μm or more and 50 μm or less, and more preferably 10 μm or more and 40 μm or less. The average particle size is the D50 value as described above, and the measurement method is also the same as described above. When the average particle size is within the above range, when a secondary battery having this negative electrode active material is made, the negative electrode expansion rate during charge-discharge is small, and it is easy to suppress the generation of solid-phase interface electrolyte decomposition products, and it is easy to prevent a decrease in the reversible charge-discharge capacity per unit volume. Furthermore, it is easy to suppress the peeling of the electrode film from the current collector during electrode film production. The specific surface area of this negative electrode active material is 2 1 m 2 / g or more and 20 m 2 / g or less, and preferably 3 m 2 [[ / g or more and 18 m / g or less. When the specific surface area is within the above range, the absorption amount of the solvent during electrode production can be appropriately maintained, and the amount of the binder used to maintain the binding property can also be appropriately maintained. Note that the specific surface area of this negative electrode active material can be measured by the BET method by nitrogen gas adsorption measurement using, for example, a specific surface area measurement device. Infrared analysis of this negative electrode active material revealed that -1 It is preferable that there is no absorption spectrum derived from the Si-H stretching vibration in the range of 2000 to 2200 cm. -1 If there is no absorption spectrum derived from the Si-H stretching vibration in the region of 900 to 1200 cm, lithium ions can be efficiently absorbed, leading to a reduction in the number of irreversible capacity generating sites, and improving the charge / discharge capacity and the initial coulomb efficiency. -1 The absorption intensity of the absorption spectrum in the range of 2000 to 2200 cm -1 This means that the absorption intensity of the absorption spectrum derived from the Si-H stretching vibration in the above-mentioned compound is 0.1% or less, and more preferably 0.05% or less.

[0031] In addition to the graphite phase, nanosilicon phase, and silicon-based matrix phase, the present negative electrode active material may contain other components as needed, such as a silicate compound of at least one metal selected from the group consisting of Li, K, Na, Ca, Mg, and Al (hereinafter also referred to as the present silicate compound). A silicate compound is generally a compound containing an anion having a structure in which one or several silicon atoms are at the center and electronegative ligands surround it. The silicate compound is a salt of at least one metal selected from the group consisting of Li, K, Na, Ca, Mg, and Al and a compound containing the anion. The compound containing the anion includes orthosilicate ion (SiO4 4- ), metasilicate ion (SiO3 2- ), pyrosilicate ion (Si2O7 6- ), cyclic silicate ion (Si3O9 6- or SiO 18 12- Examples of silicate compounds include compounds containing silicate ions such as SiO 2 , SiO 3 , SiO 4 , and SiO 2 . The silicate compound is preferably a salt of metasilicate ions with at least one metal selected from the group consisting of Li, K, Na, Ca, Mg, and Al. The metal is preferably Li or Mg. The silicate compound contains at least one metal selected from the group consisting of Li, K, Na, Ca, Mg, and Al, and may contain two or more of these metals. When two or more metals are contained, one silicate ion may contain multiple metals, or the silicate compound may be a mixture of silicate compounds containing different metals. As long as the silicate compound contains at least one metal selected from the group consisting of Li, K, Na, Ca, Mg, and Al, it may contain other metals. The silicate compound is preferably a lithium silicate compound or a magnesium silicate compound, more preferably lithium metasilicate (Li2SiO3) or magnesium metasilicate (MgSiO3), and even more preferably magnesium metasilicate (MgSiO3). Note that the silicate compound can be detected by powder X-ray diffraction measurement (XRD) when it is in a crystalline state, and by solid X-ray diffraction measurement (XRD) when it is amorphous. 29 This can be confirmed by Si-NMR measurement.

[0032] The surface of the negative electrode active material may be at least partially coated with a coating material, preferably a material that is expected to have electron conductivity, lithium ion conductivity, and the effect of inhibiting decomposition of the electrolyte. When at least a portion of the surface of the present negative electrode active material is coated with a coating material, the average thickness of the coating layer is preferably 10 nm to 300 nm, more preferably 20 nm to 200 nm. When the present negative electrode active material has a coating layer with this average thickness, silicon particles exposed on the surface of the present negative electrode active material can be protected, which tends to further improve the chemical stability and thermal stability of the present negative electrode active material, and can suppress deterioration in the charge / discharge performance of the resulting secondary battery. When at least a portion of the surface of the present negative electrode active material is coated with a coating material, the content of the coating material is preferably 1 to 30 mass %, more preferably 3 to 25 mass %, based on the total amount of the components of the present negative electrode active material and the coating material, from the viewpoint of improving the chemical stability and thermal stability of the present negative electrode active material. Examples of the coating material include electron-conductive materials such as carbon, titanium, and nickel. Among them, carbon is preferred, and low-crystalline carbon is more preferred, from the viewpoint of improving the chemical stability and thermal stability of the present negative electrode active material. That is, it is preferred that at least a portion of the surface of the present negative electrode active material is coated with low-crystalline carbon. In other words, it is preferred that the present negative electrode active material further has a low-crystalline carbonaceous phase on at least a portion of the surface. Here, low-crystalline carbon refers to carbon with low crystallinity, and more specifically, carbon with a lattice spacing of 0.34 nm to 0.40 nm between the carbon 002 planes as determined by XRD measurement. When the lattice spacing of the carbon 002 planes of low-crystalline carbon is 0.34 nm to 0.40 nm, the electron conductivity of the negative electrode active material is improved, and isolation of silicon particles due to volume expansion during charging is suppressed, which results in a reduction in the electrical capacity loss of the silicon particles. Furthermore, since the low-crystalline carbon can also serve as a coating material for the negative electrode active material particles, it improves the electronic conductivity between the negative electrode active material particles, suppresses isolation of the negative electrode active material particles due to swelling during charging, and improves the capacity retention rate when the battery is made into a secondary battery. The interplanar spacing of the carbon 002 plane of the low-crystalline carbon determined by XRD measurement is preferably 0.34 nm to 0.38 nm, more preferably 0.345 nm to 0.375 nm, and even more preferably 0.35 to 0.37 nm, from the viewpoint of the initial Coulomb efficiency. The average thickness of the low-crystalline carbonaceous phase is preferably 10 nm to 300 nm, and the content of the low-crystalline carbonaceous phase is preferably 1 to 30 mass % based on the total amount of the components constituting the negative electrode active material and the low-crystalline carbonaceous phase. When the coating material is low-crystalline carbon, examples of the coating method include a wet mixing method, a chemical vapor deposition method, a mechanochemical method, etc. From the viewpoints of easy control of the reaction system for forming the coating layer and easy maintenance of the shape of the present negative electrode active material, the chemical vapor deposition method or the wet mixing method is preferred, and the chemical vapor deposition method is more preferred.

[0033] The present invention also provides a method for producing the present anode active material (hereinafter also referred to as "the method"), which includes the steps of spheronizing a mixture (hereinafter also referred to as "the present anode active material precursor") of graphite, silicon particles, and an organosilicon-based polymer material having a polymer structure containing Si (silicon), O (oxygen), and C (carbon) (hereinafter also referred to as "the organosilicon-based polymer material"), and firing the mixture at 700 to 1200°C in a non-oxidizing atmosphere. The present anode active material is preferably obtained by the method.

[0034] The details of the graphite in the negative electrode active material precursor used in this method are the same as those described above for the graphite contained in the graphite phase constituting the negative electrode active material. Flake graphite having a major axis of 0.3 μm or more and 30 μm or less is more preferred.

[0035] The silicon particles in this negative electrode active material precursor are composed of zero-valent silicon, and the details of their average particle size (D50), shape, length in the major axis direction, thickness, and aspect ratio are the same as those described above for the silicon particles contained in the nanosilicon phase that constitutes this negative electrode active material. The silicon particles used in this method can be obtained, for example, by pulverizing silicon chunks into particles and then adjusting the average particle size to fall within the above-mentioned range by classification or the like. Pulverization can be carried out using a pulverizer such as a ball mill, bead mill, or jet mill. In the case of a bead mill, silicon particles having an average particle size within the above-mentioned range can be easily obtained by controlling conditions such as bead particle size, blending ratio, rotation speed, and pulverization time.

[0036] Alternatively, silicon lumps may be wet-pulverized in the presence of an organic solvent using a wet-pulverizing device such as a roller mill, a high-speed rotary pulverizer, a container-driven mill, or a bead mill, and the average particle size may be adjusted to fall within the above-described range, thereby obtaining silicon particles as a slurry solution in the organic solvent. The organic solvent is preferably a solvent that does not react with silicon particles, and examples thereof include ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone, diisobutyl ketone, and cyclohexanone; alcohols such as ethanol, methanol, n-propanol, isopropanol, benzyl alcohol, and diacetone alcohol; aromatic hydrocarbons such as benzene, toluene, and xylene; aliphatic hydrocarbons such as hexane, cyclohexane, octane, and nonane; ethers such as tetrahydrofuran, diethyl ether, and glyme; and esters such as ethyl acetate and butyl acetate. Of these, acetone, methyl ethyl ketone, and toluene are preferred from the standpoints of cost, solvent removal ability, and solubility of the organosilicon polymer material described below.

[0037] In such wet grinding, a dispersant may be present to promote dispersion of the silicon particles obtained by grinding. Both aqueous and non-aqueous dispersants can be used as the dispersant, but non-aqueous dispersants are preferred from the viewpoint of suppressing excessive oxidation on the silicon particle surfaces. Examples of non-aqueous dispersants include polymeric non-aqueous dispersants such as polyethers, polyalkylene polyamines, and polycarboxylic acid partial alkyl esters; low-molecular-weight non-aqueous dispersants such as polyhydric alcohol esters and alkyl polyamines; and polyphosphates.

[0038] In this method, silicon particles that have been previously pulverized may be used, or a slurry solution obtained by the above-mentioned wet pulverization may be used. When a slurry solution obtained by wet pulverization is used, the silicon particle concentration in the slurry solution is not particularly limited, and is usually preferably in the range of 5 to 40 mass % relative to the total amount of the slurry solution, and more preferably in the range of 10 to 30 mass %.

[0039] The organosilicon polymer material in the present negative electrode active material precursor more preferably has a polymer structure consisting of a mixture of a polysiloxane compound and a carbon source resin, or a composite of a polysiloxane compound and a carbon source resin.

[0040] The polysiloxane compound constituting the organosilicon polymer material is preferably a resin containing at least one of a polycarbosilane structure, a polysilazane structure, a polysilane structure, and a polysiloxane structure, but may be a resin containing only these structures, or a composite resin having at least one of these structures as a segment and chemically bonded to other polymer segments. Examples of the composite form include graft copolymerization, block copolymerization, random copolymerization, alternating copolymerization, etc. Examples of composite resins include composite resins having a graft structure in which a polysiloxane segment is chemically bonded to the side chain of a polymer segment, and composite resins having a block structure in which a polysiloxane segment is chemically bonded to the end of a polymer segment. The polysiloxane segment preferably has at least one structural unit represented by the following general formula (S-1) or (S-2): It is particularly preferred that the polysiloxane compound has a carboxy group, an epoxy group, an amino group, or a polyether group on a side chain or terminal of the siloxane bond (Si-O-Si) main skeleton.

[0041] [ka]

[0042] [ka]

[0043] (In the formula, R 1 represents an alkyl group, an aryl group, an epoxy group, or a carboxy group. 2 and R 3 each independently represents an alkyl group, a cycloalkyl group, an aryl group, an aralkyl group, an epoxy group, or a carboxy group. R 1 , R 2 and R 3Examples of the alkyl group represented by each of the formulae include a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, an isobutyl group, a sec-butyl group, a tert-butyl group, a pentyl group, an isopentyl group, a neopentyl group, a tert-pentyl group, a 1-methylbutyl group, a 2-methylbutyl group, a 1,2-dimethylpropyl group, a 1-ethylpropyl group, a hexyl group, an isohexyl group, a 1-methylpentyl group, a 2-methylpentyl group, a 3-methylpentyl group, a 1,1-dimethylbutyl group, a 1,2-dimethylbutyl group, a 2,2-dimethylbutyl group, a 1-ethylbutyl group, a 1,1,2-trimethylpropyl group, a 1,2,2-trimethylpropyl group, a 1-ethyl-2-methylpropyl group, and a 1-ethyl-1-methylpropyl group. R 1 , R 2 and R 3 Examples of the aryl group represented by each of the above include a phenyl group, a naphthyl group, a 2-methylphenyl group, a 3-methylphenyl group, a 4-methylphenyl group, a 4-vinylphenyl group, and a 3-isopropylphenyl group. R 2 and R 3 Examples of the cycloalkyl group represented by each of the above include a cyclopropyl group, a cyclobutyl group, a cyclopentyl group, and a cyclohexyl group. R 2 and R 3 Examples of the aralkyl group represented by each of the above include a benzyl group, a diphenylmethyl group, and a naphthylmethyl group.

[0044] Examples of polymer segments other than polysiloxane segments contained in the polysiloxane compound include vinyl polymer segments such as acrylic polymers, fluoroolefin polymers, vinyl ester polymers, aromatic vinyl polymers, and polyolefin polymers; polyurethane polymer segments, polyester polymer segments, and polyether polymer segments. Of these, vinyl polymer segments are preferred.

[0045] The polysiloxane compound may be a composite resin in which polysiloxane segments and polymer segments are bonded in a structure represented by the following structural formula (S-3), or may have a three-dimensional network polysiloxane structure.

[0046] [ka]

[0047] (In the formula, the carbon atom is a carbon atom that constitutes a polymer segment, and the two silicon atoms are silicon atoms that constitute a polysiloxane segment.) The polysiloxane segment of the polysiloxane compound may have a functional group capable of reacting by heating, such as a polymerizable double bond, in the polysiloxane segment. In this case, by subjecting the polysiloxane compound to heat treatment before baking, the crosslinking reaction can proceed and the compound can be solidified, making baking easy. Examples of such polymerizable double bonds include vinyl groups and (meth)acryloyl groups. Preferably, two or more polymerizable double bonds are present in the polysiloxane segment, more preferably 3 to 200, and even more preferably 3 to 50. When a composite resin having two or more polymerizable double bonds is used as the polysiloxane compound, the crosslinking reaction can be easily promoted.

[0048] The polysiloxane segment may have at least one of a silanol group or a hydrolyzable silyl group. Examples of the hydrolyzable group in the hydrolyzable silyl group include a halogen atom, an alkoxy group, a substituted alkoxy group, an acyloxy group, a phenoxy group, a mercapto group, an amino group, an amide group, an aminooxy group, an iminoxy group, and an alkenyloxy group. Hydrolysis of these groups converts the hydrolyzable silyl group into a silanol group. In parallel with the crosslinking reaction due to the heat treatment, a hydrolysis condensation reaction occurs between the hydroxyl groups in the silanol groups and the hydrolyzable groups in the hydrolyzable silyl groups, thereby obtaining a solid polysiloxane compound. In this specification, a silanol group refers to a silicon-containing group having a hydroxyl group directly bonded to a silicon atom. Also, in this specification, a hydrolyzable silyl group refers to a silicon-containing group having a hydrolyzable group directly bonded to a silicon atom, and specific examples include groups represented by the following general formula (S-4):

[0049] [ka]

[0050] (In the formula, R 4 represents a monovalent organic group, and R 5 represents a halogen atom, an alkoxy group, an acyloxy group, an allyloxy group, a mercapto group, an amino group, an amido group, an aminooxy group, an iminoxy group, or an alkenyloxy group; and b is an integer of 0 to 2.

[0051] R 4 The monovalent organic group represented by may be an alkyl group, an aryl group, an aralkyl group, etc. Specific examples of the alkyl group, the aryl group, and the aralkyl group are those represented by R 2 and R 3 are the same as the alkyl group, aryl group or aralkyl group represented by each of the above. R 5 Examples of the halogen atom represented by include a fluorine atom, a chlorine atom, a bromine atom, and an iodine atom. R 5 Examples of the alkoxy group represented by include a methoxy group, an ethoxy group, a propoxy group, an isopropoxy group, a butoxy group, a sec-butoxy group, and a tert-butoxy group. R 5 Examples of the acyloxy group represented by include a formyloxy group, an acetoxy group, a propanoyloxy group, a butanoyloxy group, a pivaloyloxy group, a pentanoyloxy group, a phenylacetoxy group, an acetoacetoxy group, a benzoyloxy group, and a naphthoyloxy group. R 5 Examples of the aryloxy group represented by include a phenyloxy group and a naphthyloxy group. R 5 Examples of the alkenyloxy group represented by include a vinyloxy group, an allyloxy group, a 1-propenyloxy group, an isopropenyloxy group, a 2-butenyloxy group, a 3-butenyloxy group, a 2-pentenyloxy group, a 3-methyl-3-butenyloxy group, and a 2-hexenyloxy group.

[0052] The polymer segment may contain various functional groups as needed, as long as the effects of the present invention are not impaired. Examples of such functional groups include a carboxyl group, a protected carboxyl group, a carboxylic anhydride group, a tertiary amino group, a hydroxyl group, a protected hydroxyl group, a cyclocarbonate group, an epoxy group, a carbonyl group, a primary amide group, a secondary amide group, a carbamate group, and a functional group represented by the following structural formula (S-5). The polymer segment may also contain a polymerizable double bond such as a vinyl group or a (meth)acryloyl group.

[0053] [ka]

[0054] The polysiloxane compound can be produced, for example, by the following methods (1) to (3). (1) A method in which a polymer segment containing at least one of a silanol group and a hydrolyzable silyl group is prepared in advance as a raw material for the polymer segment, and this polymer segment is mixed with a silane compound having at least one of a silanol group and a hydrolyzable silyl group and a polymerizable double bond, followed by a hydrolysis and condensation reaction. (2) A method in which a polymer segment containing at least one of a silanol group and a hydrolyzable silyl group is prepared in advance as a raw material for the polymer segment. A polysiloxane is also prepared in advance by subjecting a silane compound having both a silanol group and a hydrolyzable silyl group and a polymerizable double bond to a hydrolysis-condensation reaction. The polymer segment and the polysiloxane are then mixed together to carry out the hydrolysis-condensation reaction. (3) A method in which a polymer segment, a silane compound having at least one of a silanol group or a hydrolyzable silyl group and a polymerizable double bond, and a polysiloxane are mixed together, and a hydrolysis condensation reaction is carried out. Alternatively, commercially available polysiloxane compounds may be used, such as the "Ceranate (registered trademark)" series (organic-inorganic hybrid coating resins; manufactured by DIC Corporation) and the "Compoceran (registered trademark) SQ" series (silsesquioxane organic-inorganic hybrid materials; manufactured by Arakawa Chemical Industries, Ltd.).

[0055] The carbon source resin constituting the organosilicon polymer material is preferably a synthetic resin or natural chemical raw material that has good miscibility with polysiloxane compounds and is easily carbonized by high-temperature baking in a non-oxidizing gas atmosphere. Examples of synthetic resins include thermoplastic resins such as polyvinyl alcohol and polyacrylic acid, and thermosetting resins such as phenol resin and furan resin. Examples of natural chemical raw materials include heavy oils, particularly tar pitches such as coal tar, light tar oil, medium tar oil, heavy tar oil, naphthalene oil, anthracene oil, coal tar pitch, pitch oil, mesophase pitch, oxygen-crosslinked petroleum pitch, and heavy oil. Among these, from the viewpoints of price, availability, and impurity exclusion, the carbon source resin is preferably a synthetic resin containing an aromatic hydrocarbon moiety, and is preferably a phenolic resin, an epoxy resin, or a thermosetting resin, and more preferably a resol-type phenolic resin. As the phenolic resin, commercially available products can be used, for example, the "Sumilite Resin (registered trademark)" series (resol type phenolic resin, manufactured by Sumitomo Bakelite Co., Ltd.).

[0056] The polysiloxane compound and the carbon source resin may be used as a mixture, or may be used as a composite in which the polysiloxane compound and the carbon source resin are bonded to each other. The composite in which the polysiloxane compound and the carbon source resin are bonded to each other can be produced, for example, by using a silane compound having an epoxy group and a hydrolyzable silyl group (hereinafter also referred to as an "epoxysilane compound") or a silane compound having an isocyanate group and a hydrolyzable silyl group (hereinafter also referred to as an "isocyanatesilane compound") as part of the raw materials for the polysiloxane compound, and polycondensing the silane compound containing the epoxysilane compound in the presence of a carbon source resin having a substituent (hydroxyl group, amino group, carboxyl group, thiol group, etc.) that can react with the epoxy group or the isocyanate group. Furthermore, a composite in which a polysiloxane compound and a carbon source resin are bonded to each other can also be produced by a method of polymerizing a monomer having a substituent reactive with an epoxy group or an isocyanate group, which can form a carbon source resin, in the presence of a polysiloxane compound having a polymer segment containing an epoxy group or an isocyanate group and at least one type of hydrolyzable silyl group, preferably a structural unit derived from an epoxysilane compound or an isocyanatesilane compound; or a method of polycondensing a silane compound containing an epoxysilane compound or an isocyanatesilane compound and a monomer having a substituent reactive with an epoxy group or an isocyanate group, which can form a carbon source resin, all at once. The bond in the composite in which the polysiloxane compound and the carbon source resin are bonded to each other is not limited to the bond derived from the epoxy group or isocyanate group described above, but may be, for example, an ester bond, an ether bond, or the like.

[0057] The graphite content in the present negative electrode active material precursor is preferably in the range of 40.0 to 98.0 mass %, more preferably in the range of 45.0 to 96.0 mass %. The content of silicon particles in the present negative electrode active material precursor is preferably in the range of 0.05 to 13.0 mass %, more preferably in the range of 0.1 to 10.0 mass %. The content of the organosilicon polymer material in this negative electrode active material precursor is preferably in the range of 0.05 to 60.0% by mass, more preferably 0.1 to 55.0% by mass. When the organosilicon polymer material has a polymer structure consisting of a mixture of a polysiloxane compound and a carbon source resin, or a composite of a polysiloxane compound and a carbon source resin, the content of the polysiloxane compound is preferably in the range of 1 to 50% by mass and the content of the carbon source resin is preferably in the range of 50 to 99% by mass relative to the total amount of the organosilicon polymer material.

[0058] The negative electrode active material precursor can be obtained, for example, by granulating a mixture of graphite, powdered or slurried silicon particles, and an organosilicon polymer material while removing the solvent, thereby forming the mixture into spherical particles. The conditions for removing the solvent are not particularly limited, and the removal of the solvent can be carried out, for example, at a temperature in the range of 80 to 150° C. in an inert gas atmosphere under atmospheric pressure or under reduced pressure. For spheroidization by granulation, a device can be used that applies compressive force or shear force to the powder raw material and performs compounding by mechanochemical treatment or the like. The apparatus used for spheronization by granulation has, for example, a rotor with many blades installed inside a casing, and mechanical actions such as impact compression, friction, and shear force are applied by convection generated by the rotation (high-speed stirring) of the rotor and centrifugal force acting on the negative electrode active material precursor mixture. These actions enable the negative electrode active material precursor to be spheronized. Such equipment generally includes devices capable of granulating or spheronizing powders, such as dry ball mills, wet bead mills, planetary ball mills, vibrating ball mills, Henschel mixers, and pulperizers; Hosokawa Micron Corporation's Mechanofusion®, Nobilta®, and Agromaster®; Nara Machinery Manufacturing's Hybridization System (trade names), Micros®, and MIRALO®; CF Mill (trade name, manufactured by UBE Corporation); Theta Composer (trade name, manufactured by Tokuju Manufacturing Co., Ltd.); Kryptron (trade name, manufactured by Earth Technica Corporation); and FM Mixer (trade name, manufactured by Nippon Coke and Engineering Co., Ltd.). When using these devices for spheronization, the rotation speed and spheronization time can be appropriately set depending on the capacity of the device and the degree of spheronization desired. The rotation speed is preferably in the range of 5.0 to 100.0 m / s, more preferably 10.0 to 80.0 m / s. The time for spheronization by granulation is preferably in the range of 5 to 120 minutes, more preferably in the range of 10 to 80 minutes. If necessary, drying may be carried out after the spheronization step using a known dryer, reduced pressure dryer, spray dryer, etc., at a temperature of 25 to 200°C under an inert gas atmosphere at atmospheric pressure or under reduced pressure for 1 minute to 24 hours.

[0059] The present negative electrode active material is obtained by firing the present negative electrode active material precursor, which has been spherically formed by the above-mentioned method, in a non-oxidizing atmosphere at 700 to 1200° C. In detail, the present negative electrode active material precursor is fired in a non-oxidizing gas atmosphere, preferably at a maximum temperature of 900 to 1200° C., to completely decompose the thermally decomposable organic components, and the firing conditions are precisely controlled to convert the other main components into a fired product suitable for the present negative electrode active material. Specifically, the organosilicon-based polymer material (preferably a polysiloxane compound and a carbon source resin) contained in the present negative electrode active material precursor is converted into a silicon-oxygen-carbon skeleton and free carbon by the energy of the high-temperature treatment in the baking step, thereby forming the above-mentioned silicon-based matrix in the present negative electrode active material. As described above, the compound that constitutes the silicon-based matrix phase may contain nitrogen in addition to silicon, oxygen, and carbon.

[0060] The firing process is carried out according to a firing program that specifies the temperature rise rate, the holding time at a constant temperature, etc. The maximum temperature set in the firing program has a significant effect on the structure and performance of the fired product, which is the present negative electrode active material. In the present invention, by setting the maximum temperature to 900 to 1200°C, it is possible to precisely control the microstructure of the present negative electrode active material, which maintains the chemical bond state between silicon and carbon, and it is also possible to avoid oxidation of silicon particles due to firing at excessively high temperatures, which makes it easier to obtain better charge / discharge characteristics. The calcination method is not particularly limited, and any suitable device having a heating function under a non-oxidizing gas atmosphere, such as a fluidized bed reactor, a rotary furnace, a vertical moving bed reactor, a tunnel furnace, a batch furnace, or a rotary kiln, can be selected, and either a continuous method or a batch method of calcination can be used.

[0061] At least a portion of the surface of the negative electrode active material obtained in the above-mentioned baking step may be coated with a coating material. Specific examples of the coating material, the content thereof, and the preferred range of the thickness of the coating layer are as described above. Examples of the coating method include a wet mixing method, a chemical vapor deposition method, and a mechanochemical method. When obtaining the present negative electrode active material having a low-crystalline carbonaceous phase as a coating material, the carbon source for forming the low-crystalline carbonaceous phase is not particularly limited. For example, in the wet mixing method and mechanochemical method, polymer compounds such as phenolic resins and styrene resins, heavy oils, pitch, and other carbonizable solids can be used in a solid state or as a solution dispersed or dissolved in a solvent. In the chemical vapor deposition method, aliphatic hydrocarbons such as methane, ethane, propane, butane, and hexane, alicyclic hydrocarbons such as cyclopentane and cyclohexane, and aromatic hydrocarbons such as toluene, benzene, xylene, styrene, naphthalene, and anthracene can be used.

[0062] When obtaining the present negative electrode active material further having a low-crystalline carbonaceous phase on at least a portion of its surface, chemical vapor deposition (CVD) or wet mixing is preferred, with chemical vapor deposition being more preferred, from the viewpoints of ease of control of the reaction system for forming the coating layer and ease of maintaining the shape of the present negative electrode active material. Chemical vapor deposition can be performed by treating the present negative electrode active material obtained in the above-described calcination step in a chemical vapor deposition apparatus (e.g., a rotary kiln, manufactured by Takasago Kogyo Co., Ltd.) at a temperature typically in the range of 700 to 1000°C while flowing a gaseous carbon source and a carrier inert gas. This temperature range can improve the deposition and growth rate of amorphous carbon, facilitates control of the coating, and suppresses the formation of silicon carbide.

[0063] <Secondary battery> The negative electrode active material has excellent charge / discharge capacity, initial coulombic efficiency, and capacity retention. Therefore, a secondary battery including the negative electrode active material, specifically a secondary battery having a negative electrode layer including the negative electrode active material, exhibits good charge / discharge characteristics, a small negative electrode expansion rate, and excellent initial coulombic efficiency and capacity retention. For example, the negative electrode active material and an organic binder are mixed together with a solvent using a dispersing device such as a stirrer, a ball mill, a super sand mill, or a pressure kneader to prepare a negative electrode material slurry. This negative electrode material slurry can be applied to a current collector (e.g., copper foil) to form a negative electrode layer.

[0064] Examples of the organic binder include styrene-butadiene rubber copolymers (hereinafter also referred to as "SBR"); unsaturated carboxylic acid copolymers such as ethylenically unsaturated carboxylic acid esters such as methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, (meth)acrylonitrile, and hydroxyethyl (meth)acrylate, and (meth)acrylic copolymers composed of ethylenically unsaturated carboxylic acids such as acrylic acid, methacrylic acid, itaconic acid, fumaric acid, and maleic acid; and polymeric compounds such as polyvinylidene fluoride, polyethylene oxide, polyepichlorohydrin, polyphosphazene, polyacrylonitrile, polyimide, polyamideimide, and carboxymethylcellulose (hereinafter also referred to as "CMC"). Depending on their physical properties, these organic binders may be dispersed or dissolved in water or dissolved in an organic solvent such as N-methyl-2-pyrrolidone.

[0065] The content of the organic binder in the negative electrode layer of the lithium-ion secondary battery negative electrode is preferably 1 to 30% by mass, more preferably 2 to 20% by mass, and even more preferably 3 to 15% by mass. When the content of the organic binder is 1% by mass or more, adhesion is improved and destruction of the negative electrode structure due to expansion or contraction during charge and discharge is likely to be suppressed. On the other hand, when the content is 30% by mass or less, an increase in electrode resistance is more likely to be suppressed. Within this range, the present negative electrode active material has high chemical stability and can also employ an aqueous binder, making it easy to handle in practical terms.

[0066] The negative electrode material slurry may further contain a conductive additive, if necessary. Examples of conductive additives include carbon black, graphite, acetylene black, and conductive oxides and nitrides. When the negative electrode material slurry further contains a conductive additive, the amount thereof is preferably in the range of 1 to 15% by mass relative to the negative electrode active material.

[0067] Examples of materials for the current collector include copper, nickel, titanium, stainless steel, etc. The current collector is preferably in the form of a strip, such as a foil, perforated foil, or mesh. Porous materials such as porous metal (foamed metal), carbon paper, etc. can also be used as the current collector. Examples of methods for applying the negative electrode material slurry to the current collector include metal mask printing, electrostatic coating, dip coating, spray coating, roll coating, doctor blade coating, gravure coating, screen printing, etc. After application, it is preferable to perform a rolling treatment using a flat plate press, a calendar roll, or the like, as necessary.

[0068] Alternatively, the negative electrode layer may be obtained by forming the paste-like negative electrode material slurry into a sheet or pellet, and then integrating it with a current collector by rolling, pressing, or a combination thereof. Furthermore, a carbon material such as natural graphite, artificial graphite, or amorphous carbon such as hard carbon or soft carbon can be added to the negative electrode material slurry to prepare the negative electrode layer.

[0069] The negative electrode layer formed on the current collector or the negative electrode layer integrated with the current collector is preferably heat-treated depending on the type of organic binder, for example, at 100 to 130°C when an aqueous styrene-butadiene rubber copolymer (SBR) is used, or at 150 to 450°C when an organic binder having a polyimide or polyamideimide main skeleton is used. Such heat treatment can remove the solvent derived from the organic binder and also promotes strength increase due to hardening of the organic binder, thereby improving adhesion between particles and between the particles and the current collector. Note that the heat treatment is preferably carried out in a non-oxidizing gas atmosphere such as helium, argon, or nitrogen, or in a vacuum atmosphere, from the viewpoint of preventing oxidation of the current collector during the heat treatment.

[0070] After the heat treatment, the negative electrode layer formed on the current collector or the negative electrode layer integrated with the current collector is preferably subjected to a pressure treatment in order to adjust the electrode density. In the negative electrode using this negative electrode active material, the electrode density is 1 to 1.8 g / cm. 3is preferably 1.1 to 1.7 g / cm 3 More preferably, it is 1.2 to 1.6 g / cm 3 It is more preferable that the electrode density is 100-2000 MPa. The higher the electrode density, the better the adhesion and the volumetric capacity density of the electrode tend to be. However, if the electrode density is too high, the voids in the electrode decrease, weakening the effect of suppressing the volumetric expansion of silicon and the like, which may result in the negative electrode expansion rate not being suppressed and the capacity retention rate decreasing. Therefore, an optimal range of the electrode density is selected.

[0071] The secondary battery of the present invention contains the present negative electrode active material in the negative electrode. Secondary batteries having a negative electrode containing the present negative electrode active material are preferably nonaqueous electrolyte secondary batteries and solid electrolyte secondary batteries, and the present negative electrode active material exhibits excellent performance, particularly when used as the negative electrode of a nonaqueous electrolyte secondary battery. For example, when the secondary battery of the present invention is a wet electrolyte secondary battery, it can be constructed by disposing a positive electrode and a negative electrode containing the present negative electrode active material opposite each other via a separator and injecting an electrolyte solution.

[0072] The positive electrode can be obtained by forming a positive electrode layer on the surface of a current collector in the same manner as the negative electrode. In this case, the current collector can be made of a metal or alloy such as aluminum, titanium, or stainless steel, and can be in the form of a foil, a perforated foil, a mesh, or a strip.

[0073] The positive electrode material used in the positive electrode layer is not particularly limited, and when producing a nonaqueous electrolyte secondary battery, for example, a lithium ion secondary battery, examples of the positive electrode material include metal compounds, metal oxides, metal sulfides, and conductive polymers that can be doped or intercalated with lithium ions. Specifically, lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMnO2) and their composite oxides (LiCoxNiyMnzO2, x+y+z=1); lithium manganese spinel (LiMn2O4), lithium vanadium compounds, V2O5, VO 13, VO2, MnO2, TiO2, MoV2O8, TiS2, V2S5, VS2, MoS2, MoS3, Cr3O8, Cr2O5, olivine-type LiMPO4 (wherein M is Co, Ni, Mn, or Fe); conductive polymers such as polyacetylene, polyaniline, polypyrrole, polythiophene, and polyacene, porous carbon, etc. These may be used alone or in combination of two or more.

[0074] The separator may be a nonwoven fabric, cloth, microporous film, or a combination thereof, whose main component is a polyolefin such as polyethylene or polypropylene. If the nonaqueous electrolyte secondary battery to be fabricated is configured so that the positive electrode and the negative electrode are not in direct contact with each other, it is not necessary to use a separator.

[0075] As the electrolyte, for example, a so-called organic electrolyte can be used, which is obtained by dissolving a lithium salt such as LiClO4, LiPF6, LiAsF6, LiBF4, or LiSO3CF3 in one or a mixture of two or more of a non-aqueous solvent such as ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, fluoroethylene carbonate, cyclopentanone, sulfolane, 3-methylsulfolane, 2,4-dimethylsulfolane, 3-methyl-1,3-oxazolidin-2-one, γ-butyrolactone, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, butyl methyl carbonate, ethyl propyl carbonate, butyl ethyl carbonate, dipropyl carbonate, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, methyl acetate, or ethyl acetate. The structure of the secondary battery of the present invention is not particularly limited, but typically, a positive electrode, a negative electrode, and an optional separator are wound into a flat spiral to form a wound electrode assembly, or these are stacked as flat plates to form a stacked electrode assembly, and these electrode assembly are sealed in an outer casing. Note that the half cells used in the examples of the present invention have a negative electrode mainly composed of the present negative electrode active material, and a simple evaluation was performed using metallic lithium as the counter electrode, in order to clearly compare the initial coulombic efficiency of the present negative electrode active material itself.

[0076] Secondary batteries using the present negative electrode active material are used as, for example, paper-type batteries, button-type batteries, coin-type batteries, laminated-type batteries, cylindrical batteries, square-type batteries, and the like. The present negative electrode active material can also be applied to electrochemical devices in general that use lithium ion insertion / extraction as a charge / discharge mechanism, such as hybrid capacitors and solid-state lithium secondary batteries.

[0077] The negative electrode active material of the present invention, the method for producing the same, and the secondary battery having the negative electrode active material of the present invention have been described above, but the present invention is not limited to the configurations of the above-described embodiments. For example, the negative electrode active material of the present invention and the secondary battery having the negative electrode active material of the present invention may each have any other optional components in addition to the configurations of the above-mentioned embodiments, or may be replaced with any other components that exhibit the same function. Also, the manufacturing method for the negative electrode active material of the present invention may have any other optional step in the configurations of the above-mentioned embodiments, or may be replaced with any other step that exhibits the same effect. [Example]

[0078] The present invention will be specifically described below with reference to examples. However, the present invention is not limited to the following examples. Unless otherwise specified, "parts" and "%" are by mass. The half-cells used in the examples were configured so that the negative electrode was mainly made of the present negative electrode active material, and a simple evaluation was performed using metallic lithium as the counter electrode. This was done to more clearly compare the initial coulombic efficiency of the present negative electrode active material itself.

[0079] The raw materials used in each example and comparative example are described below. "graphite" Graphite 1: Artificial graphite (MCMB, average particle size 12 μm) Graphite 2: Natural graphite (flake graphite, average particle size 5 μm) Graphite 3: Natural graphite (flake graphite, average particle size 0.4 μm) Graphite 4: Natural graphite (flake graphite, average particle size 1 μm) Graphite 5: Natural graphite (flake graphite, average particle size 15 μm) Graphite 6: Natural graphite (flake graphite, average particle size 28 μm)

[0080] <Silicon particle dispersion (slurry)> Synthesis Example 1-1 A 150 ml container of a small bead mill was charged with zirconia beads (YTZ series, manufactured by Nikkato Corporation) with a particle size of 0.1 to 0.2 mm and a 60% filling rate, and 100 ml of methyl ethyl ketone. 21 g of silicon powder ("SIE23PB" (trade name, manufactured by Kojundo Chemical Co., Ltd.) with an average particle size of 5 μm and 9 g of cationic dispersant liquid ("BYK102" (trade name, manufactured by BYK Japan) were then added and wet-milled in a bead mill to obtain a dark brown liquid silicon particle dispersion (hereinafter referred to as "Dispersion 1") with a solids concentration of 23% by mass. TEM observation of the silicon particles in Dispersion 1 revealed flat flake-like shapes and an average particle size (D50) of 30 nm.

[0081] Synthesis Example 1-2 The same procedure as in Synthesis Example 1-1 was carried out except that the grinding time was extended by 15%, to obtain a dark brown liquid dispersion of silicon particles with a solid concentration of 23% by mass (hereinafter referred to as "Dispersion 2"). When the silicon particles in Dispersion 2 were observed by TEM, they were found to have a flat flake shape and an average particle size (D50) of 20 nm. Synthesis Example 1-3 The same procedure as in Synthesis Example 1-1 was carried out except that the grinding time was shortened by 30%, to obtain a dark brown liquid dispersion of silicon particles with a solid concentration of 23% by mass (hereinafter referred to as "Dispersion 3"). When the silicon particles in Dispersion 3 were observed by TEM, they were found to have a flat flake shape and an average particle size (D50) of 50 nm. Synthesis Example 1-4 The same procedure as in Synthesis Example 1-1 was carried out except that the grinding time was shortened by 75%, to obtain a dark brown liquid dispersion of silicon particles with a solid concentration of 23 mass% (hereinafter referred to as "Dispersion 4"). When the silicon particles in Dispersion 4 were observed by TEM, they were found to have a flat flake shape and an average particle size (D50) of 80 nm.

[0082] 《Organosilicon Polymer Materials》 (a) Production example of polysiloxane compound Synthesis Example 2-1: Synthesis of methyltrimethoxysilane condensate (a1) A reaction vessel equipped with a stirrer, thermometer, dropping funnel, condenser, and nitrogen gas inlet was charged with 1,421 parts by mass of methyltrimethoxysilane (MTMS) and heated to 60°C. Next, a mixture of 0.17 parts by mass of isopropyl acid phosphate (Phoslex A-3 (trade name), manufactured by SC Organic Chemicals) and 207 parts by mass of deionized water was added dropwise to the reaction vessel over 5 minutes, and the mixture was then heated to 80°C and stirred for 4 hours to carry out the hydrolysis and condensation reaction of MTMS. The resulting condensate was distilled at a temperature of 40°C to 60°C and under a reduced pressure of 40 kPa to 1.3 kPa to remove the methanol and water produced during the reaction, yielding 1,000 parts by mass of a liquid containing an MTMS condensate (a1) having a number-average molecular weight of 1,000 to 5,000. The phrase "under a reduced pressure of 40 kPa to 1.3 kPa" means that the pressure is reduced to 40 kPa when the distillation of methanol begins and is reduced to 1.3 kPa eventually. The theoretical yield (parts by mass) when all the methoxy groups of MTMS undergo condensation reaction was divided by the actual yield (parts by mass) after the condensation reaction to calculate the conversion rate in the above solution (the proportion of Si-OCH3 groups of MTMS converted to Si-O-Si bonds after hydrolysis), which was 70% by mass.

[0083] Synthesis Example 2-2: Synthesis of polysiloxane compound 1 A reaction vessel equipped with a stirrer, a thermometer, a dropping funnel, a condenser, and a nitrogen gas inlet was charged with 150 parts by mass of isopropanol (hereinafter also referred to as "IPA"), 105 parts by mass of phenyltrimethoxysilane (hereinafter also referred to as "PTMS"), and 277 parts by mass of dimethyldimethoxysilane (hereinafter also referred to as "DMDMS"), in that order, and the temperature was raised to 80°C. Next, a mixture containing 21 parts by mass of methyl methacrylate (hereinafter also referred to as "MMA"), 4 parts by mass of butyl methacrylate (hereinafter also referred to as "BMA"), 3 parts by mass of butyric acid (hereinafter also referred to as "BA"), 2 parts by mass of methacryloyloxypropyltrimethoxysilane (hereinafter also referred to as "MPTS"), 3 parts by mass of IPA, and 0.6 parts by mass of butylperoxy-2-ethylhexanoate (hereinafter also referred to as "TBPEH") was added dropwise to the reaction vessel at 80°C over 6 hours. After completion of the addition, the mixture was further reacted at 80°C for 20 hours to obtain an organic solvent solution of a vinyl polymer (a2) having a number average molecular weight of 10,000 and having hydrolyzable silyl groups. To the organic solvent solution of vinyl polymer (a2) obtained above, a mixture of 0.04 parts by mass of isopropyl acid phosphate ("Phoslex A-3" (trade name), manufactured by SC Organic Chemical Industries, Ltd.) and 112 parts by mass of deionized water was added dropwise at 80°C over 5 minutes, and after the dropwise addition was completed, the mixture was stirred at 80°C for a further 10 hours to cause a hydrolysis and condensation reaction, thereby obtaining a liquid containing a composite resin in which the hydrolyzable silyl groups of the vinyl polymer (a2) were bonded to the hydrolyzable silyl groups and silanol groups of the polysiloxane derived from PTMS and DMDMS. Next, 472 parts by mass of the MTMS condensate (a1) obtained by the method of Synthesis Example 2-1 and 80 parts by mass of deionized water were added to this liquid, and the mixture was stirred for 10 hours at 80° C. to carry out a hydrolysis condensation reaction. The obtained condensate was distilled under the same conditions as in Synthesis Example 2-1 to remove the produced methanol and water, and then 250 parts by mass of IPA was added to obtain 1,000 parts by mass of polysiloxane compound 1 with a nonvolatile content of 60.1% by mass. (b) Carbon source resin Carbon source resin 1: Phenolic resin ("HE100C-30 (trade name)", manufactured by Air Water Performance Chemicals, Inc., average molecular weight 3000)

[0084] 1. Example of negative electrode active material production Example 1 Carbon source resin 1, polysiloxane compound 1 obtained by the method of Synthesis Example 2-2, and melamine were added to methyl ethyl ketone as a solvent in a resin solid mass ratio of 79 / 20 / 1 to obtain a mixed solution of organosilicon polymer material. The mixed solution, graphite 1, and dispersion 1 prepared in Synthesis Example 1-1 were charged into a Henschel mixer equipped with a jacket, and the jacket was heated to 100°C. The mixture was granulated at a peripheral speed of 10 m / sec for 20 minutes and then at a peripheral speed of 70 m / sec for 30 minutes while removing the solvent under reduced pressure of -80 kPa, thereby obtaining a dried spherical negative electrode active material precursor. The obtained dried product was baked in a non-oxidizing gas (nitrogen gas) atmosphere at 1050° C. for 6 hours to obtain a negative electrode active material 1. The content of the graphite phase in the negative electrode active material 1 was 90 mass %, the content of the phase containing Si, O, and C was 4 mass %, and the content of the nanosilicon phase was 6 mass %. The phase containing Si, O, and C in the negative electrode active material 1 was a composite of silicon oxycarbide (SiOC) and carbon (C).

[0085] The obtained negative electrode active material 1 was measured for the following physical properties. (1) Average particle size (D50) Measurement was performed using a laser diffraction particle size distribution analyzer (Malvern Panalytical, Mastersizer 3000). (2) Specific surface area The specific surface area was measured by the BET method using a specific surface area measuring device (BELSORP-mini, manufactured by BELJAPAN) through nitrogen gas adsorption measurement. (3) Solid-state NMR ( 29 Si-NMR) Using a JNM-ECA600 manufactured by JEOL RESONANCE, the peaks attributable to Si and the peaks attributable to SiO4 were measured, and the ratio R of peak A to peak B was calculated. (4) Elemental composition ratio of silicon-based matrix The negative electrode active material film deposited on copper foil (described later) was subjected to cross-sectional milling with an Ar gas beam using a cross-sectional sample measurement device (JEOL IB-19520CCP cross-section polisher). This cross-section was then subjected to SEM (Scanning Electron Microscopy)-EDS (Energy Dispersive X-ray Spectroscopy) (JEOL JSM-7900F) magnification of approximately 3000-5000 times. A point was selected from the film cross-section in the image that corresponded to the matrix layer without silicon particles, and the elemental composition ratio of the silicon-based matrix phase after removing the added silicon particles was determined by composition analysis using the attached EDS function.

[0086] Example 2 The negative electrode active material 1 obtained in Example 1 was brought into contact with a mixed gas of ethylene gas at 0.3 L / min and nitrogen gas at 0.7 L / min in a chemical vapor deposition apparatus (CVD; rotary kiln, manufactured by Takasago Kogyo Co., Ltd.) at a temperature range of 700°C to 1000°C. This resulted in the production of a negative electrode active material 2 in which at least a portion of the surface of the negative electrode active material 1 was coated with low-crystalline carbon.

[0087] Example 3 Negative electrode active material 3 was obtained in the same manner as in Example 1, except that graphite 2 was used instead of graphite 1 as the component constituting the graphite phase. Example 4 The negative electrode active material 3 obtained in Example 3 was further contacted with a mixed gas of ethylene gas at 0.3 L / min and nitrogen gas at 0.7 L / min in a chemical vapor deposition apparatus (CVD; rotary kiln, manufactured by Takasago Kogyo Co., Ltd.) at a temperature range of 700°C to 1000°C. This resulted in the production of negative electrode active material 4, in which at least a portion of the surface of negative electrode active material 3 was coated with low-crystalline carbon.

[0088] Examples 5 to 11 Negative electrode active materials 5 to 11 were obtained by the same procedure as in Example 1, except that the graphite species constituting the graphite phase and the dispersion species used as components forming the nanosilicon phase were changed as shown in Table 1. Examples 12 to 15 Negative electrode active materials 12 to 15 were obtained in the same manner as in Example 1, except that the type of graphite constituting the graphite phase and the blending ratio of the graphite, dispersion 1, and the mixed liquid of the organosilicon-based polymer material were changed, with the contents of the graphite phase, the phase containing Si, O, and C, and the nanosilicon phase being as shown in Table 1.

[0089] Comparative Example 1 Coal tar pitch as a carbon phase, graphite 1, and dispersion 1 prepared in Synthesis Example 1-1 were charged into a Henschel mixer equipped with a jacket, and while the jacket was heated to 100°C and the solvent was removed under reduced pressure of -80 kPa, the mixture was granulated at a peripheral speed of 10 m / sec for 20 minutes and then at a peripheral speed of 70 m / sec for 30 minutes, to obtain a dried spherical negative electrode active material precursor. The resulting dried product was fired at 1050° C. for 6 hours in a non-oxidizing gas (nitrogen gas) atmosphere to obtain a negative electrode active material C1. In the negative electrode active material C1, the content of the graphite phase was 90 mass %, the content of the nanosilicon phase was 6 mass %, and the content of the carbon layer phase was 4 mass %. Comparative Example 2 A negative electrode active material C2 was obtained in the same manner as in Comparative Example 1, except that Graphite 2 was used instead of Graphite 1 as the component constituting the graphite phase.

[0090] 2. Evaluation of negative electrode active materials The negative electrode active materials obtained in the examples and comparative examples were measured for the following physical properties. The results are summarized in Table 1.

[0091] 2-1. Battery characteristic evaluation A slurry was prepared by mixing 80 parts by weight of the negative electrode active material obtained in each Example and Comparative Example, 10 parts by weight of acetylene black as a conductive additive, and 10 parts by weight of a mixture of CMC and SBR as a binder. The resulting slurry was formed into a film on copper foil. After drying under reduced pressure at 110°C, a coin-type lithium-ion battery was fabricated as a half-cell using a Li metal foil as the counter electrode. Using a secondary battery charge / discharge tester (Hokuto Denko Corporation), the charge / discharge characteristics of the fabricated half-cells were evaluated under the following conditions: constant current / constant voltage charge / constant current discharge, at 25°C, with a cutoff voltage range of 0.005 to 1.5 V, and a charge / discharge rate of 0.1 C (1 to 3 cycles) and 0.2 C (after 4 cycles). Between each charge / discharge cycle, the cells were left in an open circuit for 30 minutes. The initial coulombic efficiency was calculated according to the following formula: Initial coulombic efficiency (%) = 100 × initial discharge capacity (mAh / g) / initial charge capacity (mAh / g) Meanwhile, full-cell evaluation was performed as follows. A positive electrode film was fabricated using a single-layer sheet of LiCoO2 as the positive electrode active material and aluminum foil as the current collector. A negative electrode film was fabricated by mixing graphite powder with the negative electrode active material powder obtained in each Example and Comparative Example at a discharge capacity design value of 500 mAh / g. A nonaqueous electrolyte solution prepared by dissolving lithium hexafluorophosphate at a concentration of 1 mol / L in a 1 / 1 volumetric mixture of ethylene carbonate and diethyl carbonate was used as the nonaqueous electrolyte. A laminated lithium-ion secondary battery was fabricated using a 30 μm-thick microporous polyethylene film as the separator. The fabricated laminated lithium-ion secondary battery was charged at room temperature at a constant current of 1.2 mA (0.25 c based on the positive electrode) until the test cell voltage reached 4.2 V. After reaching 4.2 V, the current was reduced to maintain the cell voltage at 4.2 V, and the discharge capacity was calculated. The capacity retention rate after 50 cycles at 45°C was calculated according to the following formula. Capacity retention rate (%) = 100 x 50th discharge capacity (mAh / g) / initial discharge capacity (mAh / g)

[0092] 2-2. Expansion rate of negative electrode The coin-type lithium-ion battery as a half-cell prepared by the method in 2-1 above was disassembled in the initial fully charged state, and the negative electrode was washed with dimethyl carbonate and air-dried, after which the electrode thickness was measured at five points using a thickness gauge. Measurements were carried out at different points three times, for a total of 15 points, and the expansion rate at each point was calculated using the following formula. Expansion rate (%) = 100 × [electrode thickness in initial fully charged state (μm) - electrode thickness before charge / discharge (μm)] / electrode thickness before charge / discharge (μm) The average value of each expansion rate was calculated, and the average expansion rate of the negative electrode was evaluated according to the following criteria. <Evaluation criteria for average expansion rate of negative electrode> 〇: 0% to less than 20% △: 20% to less than 50% ×: 50% or more

[0093] [Table 1]

[0094] The results in Table 1 show that the negative electrode active material of the present invention has excellent initial coulombic efficiency, a high overall capacity retention rate of 82% or more, and an excellent balance of these secondary battery properties. Furthermore, secondary batteries containing the negative electrode active material of the present invention have a small negative electrode expansion rate and excellent battery properties. [Industrial Applicability]

[0095] The negative electrode active material of the present invention can form a secondary battery with excellent initial coulombic efficiency, a small negative electrode expansion rate, and excellent capacity retention. Secondary batteries containing such a negative electrode active material have excellent battery properties such as charge / discharge characteristics, and can be effectively used in portable electronic devices, for example, as paper-type batteries, button-type batteries, coin-type batteries, stacked batteries, cylindrical batteries, and prismatic batteries. The negative electrode active material can also be used in general electrochemical devices that use lithium ion insertion / extraction as a charge / discharge mechanism, such as hybrid capacitors and solid-state lithium secondary batteries.

Claims

1. A negative electrode active material for a secondary battery, comprising at least a graphite phase, a phase containing Si (silicon), O (oxygen) and C (carbon), and a nanosilicon phase.

2. 2. The negative electrode active material for a secondary battery according to claim 1, wherein the phase containing Si (silicon), O (oxygen), and C (carbon) further contains N (nitrogen).

3. 2. The negative electrode active material for a secondary battery according to claim 1, wherein the graphite phase contains flake graphite having a major axis of 0.3 μm or more and 30 μm or less.

4. 2. The negative electrode active material for a secondary battery according to claim 1, wherein the nanosilicon phase contains silicon particles having an average particle size of 5 nm or more and 100 nm or less.

5. 2. The negative electrode active material for a secondary battery according to claim 1, wherein the content ratio of Si, O, and C in the phase containing Si (silicon), O (oxygen), and C is SiOxCy (wherein x and y are positive numbers satisfying the conditions of 0.1<x<2 and 0.3<y<11, respectively) in terms of atomic ratio.

6. 3. The negative electrode active material for a secondary battery according to claim 2, wherein the phase containing Si (silicon), O (oxygen), and C (carbon) contains SiOC (silicon oxycarbide), and a content ratio of N (nitrogen) relative to the silicon oxycarbide is more than 0.2% by mass and less than 2.5% by mass.

7. The negative electrode active material for a secondary battery according to claim 1 , further comprising a low-crystalline carbonaceous phase on at least a portion of the surface thereof.

8. A secondary battery comprising the negative electrode active material for secondary batteries according to any one of claims 1 to 7.

9. a step of spheronizing a mixture of graphite, silicon particles, and an organosilicon-based polymer material having a polymer structure containing Si (silicon), O (oxygen), and C (carbon); and firing the mixture at 700 to 1200°C in a non-oxidizing atmosphere. A method for producing a negative electrode active material for a secondary battery, the negative electrode active material comprising at least a graphite phase, a phase containing Si, O, and C, and a nanosilicon phase.

10. The method for producing a negative electrode active material for a secondary battery according to claim 9 , wherein the graphite contains flake graphite having a major axis of 0.3 μm or more and 30 μm or less.

11. The method for producing a negative electrode active material for a secondary battery according to claim 9 , wherein the silicon particles contain silicon particles having an average particle size of 5 nm or more and 100 nm or less.

12. 10. The method for producing a negative electrode active material for a secondary battery according to claim 9, wherein the organosilicon-based polymer material has a polymer structure consisting of a mixture of a polysiloxane compound and a carbon source resin, or a composite of a polysiloxane compound and a carbon source resin.

13. The method for producing a negative electrode active material for a secondary battery according to any one of claims 9 to 12, wherein the phase containing Si (silicon), O (oxygen), and C (carbon) further contains N (nitrogen).

Images