Negative electrode active material and secondary battery

Abstract

To provide a negative electrode active material that yields a secondary battery having high initial coulomb efficiency, which is one of the important properties of the secondary battery, and a secondary battery that includes the negative electrode active material and has high initial efficiency.SOLUTION: The negative electrode active material contains at least one of Si nanoparticles or silicon oxide particles, and carbonaceous phase, and the carbonaceous phase embeds at least one of the Si nanoparticles and the silicon oxide particles, and the interplanar spacing of a carbon 002 plane determined by XRD measurement in the carbonaceous phase is 0.340 nm to 0.38 nm.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a negative electrode active material and a secondary battery containing the negative electrode active material. [Background technology]

[0002] Non-aqueous electrolyte secondary batteries are used in portable devices, hybrid vehicles, electric vehicles, home storage batteries, and the like, and are required to have a good balance of multiple properties such as electrical capacity, safety, and operational stability. As such secondary batteries, lithium ion batteries, which use lithium intercalation compounds that release lithium ions from between layers as the negative electrode material, are mainly known. For example, various lithium ion batteries using carbonaceous materials such as graphite as the negative electrode active material, which can absorb and release lithium ions between layers of crystal planes during charging and discharging, have been developed and are already in practical use. Furthermore, in recent years, with the miniaturization of various electronic and communication devices and the rapid spread of hybrid vehicles, there is a strong demand for the development of secondary batteries that have higher capacity and improved battery characteristics such as cycle characteristics and discharge rate characteristics as the driving power source for these devices.

[0003] As one attempt to improve the performance of secondary batteries, a negative electrode active material for non-aqueous electrolyte secondary batteries containing a silicon compound represented by the general formula SiOx has been described (for example, Patent Document 1). The negative electrode active material particles described in Patent Document 1 are said to have excellent conductivity because at least a portion of the surface of the silicon compound is coated with a carbon film. Furthermore, it is believed that by setting the specific surface area of ​​the carbon film within a specific range, the impregnation of the battery electrolyte is improved, and by setting the compressive resistivity of the carbon film within a specific range, the surface conductivity of the negative electrode active material particles is sufficient and micro-precipitation of Li due to power concentration on the surface is unlikely to occur.

[0004] On the other hand, attempts have been made to improve the performance of secondary batteries by using negative electrode active materials containing silicon particles (for example, Patent Documents 2 to 4). Patent Document 2 describes a battery negative electrode material that includes a silicon material region and a carbon material region made of a carbon material that is formed around the silicon material region at least in part with a gap therebetween, and in which the (002) average interlayer spacing d002 of the carbon material region, as determined by powder X-ray diffraction using Cu-Kα radiation, is 0.365 nm or more and 0.390 nm or less. It also describes that the configuration in Patent Document 2 efficiently suppresses expansion and contraction of silicon during charge and discharge, resulting in a secondary battery with improved specific capacity and cycle durability.

[0005] Patent Document 3 describes sheet-shaped silicon nanoparticles and the SiC4 bonding structure unit that contains them. 29 This paper discloses a negative electrode active material for lithium ion secondary batteries that contains a silicon-based inorganic compound having a Si-NMR peak and an equivalent composition ratio within a specific range. It also describes that the resulting secondary battery has improved charge-discharge characteristics.

[0006] Patent Document 4 discloses core-shell composite particles, which have a core made of a carbon matrix with silicon particles encapsulated in pores of a specific diameter, and a shell made of a non-porous amorphous carbon base. The resulting secondary battery is described as exhibiting high coulombic efficiency and more stable electrochemical behavior in subsequent cycles. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-164870 [Patent Document 2] Japanese Patent Application Publication No. 2019-125435 [Patent Document 3] Patent Publication No. 2021-114483 [Patent Document 4] Patent No. 6523484 Summary of the Invention [Problem to be solved by the invention]

[0008] As mentioned above, attempts have been made to improve the performance of secondary batteries by variously improving negative electrode active materials containing silicon or silicon compounds. However, the performance of the resulting secondary batteries is still not sufficient, and further improvements in negative electrode active materials are required.

[0009] The present inventors focused on Si nanoparticles or silicon oxide particles and the carbonaceous phase that embeds them, and conducted various studies with the aim of improving the electronic conductivity of negative electrode active materials while efficiently suppressing the volumetric expansion and contraction of Si that occurs during charge and discharge, which led to the completion of the present invention. That is, an object of the present invention is to provide a negative electrode active material that provides a secondary battery with a large capacity per weight and excellent initial coulombic efficiency, which are important properties of the secondary battery. [Means for solving the problem]

[0010] The present invention has the following aspects. [1] A negative electrode active material comprising at least one of Si nanoparticles or silicon oxide particles, and a carbonaceous phase, wherein the carbonaceous phase embeds the at least one of the Si nanoparticles or the silicon oxide particles, and wherein the interplanar spacing of carbon 002 planes in the carbonaceous phase, as determined by XRD measurement, is 0.34 nm to 0.38 nm. [2] The negative electrode active material according to [1] above, wherein the mass loss rate at 100 to 800° C. as determined by TG analysis under a dry air flow is 10 to 70%. [3] The negative electrode active material according to [1] or [2] above, wherein the weight gain starting temperature, as determined by TG analysis under dry air flow, is 550° C. or higher. [4] The negative electrode active material according to any one of [1] to [3] above, which contains 0.1 wt % to 19 wt % of a silicon-based material. [5] Specific surface area (BET) is 0.01m 2 / g to 20m 2 / g of the negative electrode active material according to any one of [1] to [4]. [6] The negative electrode active material according to any one of [1] to [5] above, which has an average particle diameter (D50) of 0.5 μm to 10 μm. [7] It has a carbon coating, and has a porosity of 7% or more and 20% or less, and a true density of 1.6 g / cm, as defined by the following formula (1): 3 More than 2.0g / cm 3 The negative electrode active material according to any one of [1] to [6] above, which is: TIFF0007754152000001.tif24161 (In formula (1), V is the porosity (%), ρ is the density of the negative electrode active material (g / cm 3 ), ρ' is the density of the entire negative electrode active material (g / cm 3 ), ρ'' is the density of the carbon coating (g / cm 3 ), and A represents the amount of carbon coating (mass%).

[0011] Furthermore, the present invention has the following aspects. [8] A secondary battery comprising the negative electrode active material according to any one of [1] to [7]. [Effects of the Invention]

[0012] According to the present invention, there are provided a negative electrode active material that provides a secondary battery with high initial coulombic efficiency, which is one of the important properties of a secondary battery, and excellent balance of battery characteristics, and a secondary battery having the negative electrode active material. DETAILED DESCRIPTION OF THE INVENTION

[0013] In the following description, Si represents the same substance as "silicon." The negative electrode active material of the present invention (hereinafter also referred to as "the present negative electrode active material") contains at least one of Si nanoparticles or silicon oxide particles, and a carbonaceous phase, in which the Si nanoparticles are embedded, and in which the interplanar spacing of carbon 002 planes determined by XRD measurement in the carbonaceous phase is 0.34 nm to 0.38 nm.

[0014] The electrochemical reactions that occur during the charging and discharging of secondary batteries can be broadly divided into two types. One is the reaction that occurs during charging and discharging, which is the insertion and desorption reaction of lithium ions. The other is a side reaction that occurs in the solvent in the electrolyte, the electrolyte, and the surface of the active material. This side reaction generates a solid electrolyte interface (SEI), which reduces the initial efficiency of the secondary battery. When this negative electrode active material is used in secondary batteries, it is thought that the electrochemical side reaction on the surface of the active material is suppressed, thereby increasing the initial coulombic efficiency of the secondary battery.

[0015] The Si nanoparticles are nanoparticles of zero-valent Si. Nanoparticles are particles having an average particle size of nanometer order, preferably 10 nm to 300 nm, more preferably 20 nm to 250 nm, and even more preferably 30 nm to 200 nm. Furthermore, from the viewpoint of charge / discharge performance and capacity maintenance when used as a negative electrode active material, the average particle size of the Si nanoparticles is preferably 100 nm or less, more preferably 70 nm or less.

[0016] Here, the average particle size refers to the volume-average particle size, and is the D50 value, which can be measured using a laser diffraction particle size analyzer or the like. D50 can be measured by dynamic light scattering using a laser particle size analyzer or the like. In the particle size distribution of Si nanoparticles, when a volume cumulative distribution curve is drawn from the smallest diameter side, D50 is the particle size at which the cumulative 50% is reached.

[0017] Large Si nanoparticles exceeding 300 nm tend to form large agglomerates, which can easily pulverize during charge and discharge cycles when used as anode active materials, potentially resulting in a decrease in the capacity retention rate of the anode active material. On the other hand, small Si nanoparticles less than 10 nm are too fine and tend to aggregate. This can lead to poor dispersibility of the Si nanoparticles in the anode active material. Furthermore, if the Si nanoparticles are too fine, their surface activation energy increases, which can lead to the formation of by-products on the surface of the Si nanoparticles during high-temperature firing of the anode active material. These factors can potentially lead to a decrease in charge and discharge performance. From this viewpoint, it is preferable that the Si nanoparticles are within the above-mentioned range of average particle size, and that the number of large Si nanoparticles exceeding 300 nm and small Si nanoparticles less than 10 nm is as small as possible.

[0018] The Si nanoparticles can be produced by nano-sizing Si chunks by pulverizing, etc. The presence of these Si nanoparticles can improve the charge / discharge capacity and initial coulombic efficiency when the present negative electrode active material is used in a secondary battery. The Si nanoparticles can be obtained by, for example, pulverizing a zero-valent silicon block to obtain particles having an average particle size within the above range. Examples of mills used for pulverizing the Si agglomerates to form nanoparticles include ball mills, bead mills, jet mills, etc. The pulverization may be wet pulverization using an organic solvent, and suitable organic solvents include, for example, alcohols and ketones, but aromatic hydrocarbon solvents such as toluene, xylene, naphthalene, and methylnaphthalene can also be used. The obtained silicon particles can be made into Si nanoparticles by controlling the bead mill conditions such as bead particle size, blending ratio, rotation speed, or grinding time, and classifying the particles.

[0019] The shape of the Si nanoparticles is not particularly limited, but from the viewpoint of charge-discharge performance when used as a negative electrode active material, the length in the major axis direction is preferably 70 to 300 nm, and the thickness is preferably 15 to 70 nm. From the viewpoint of charge-discharge performance when used as a negative electrode active material, the aspect ratio, which is the ratio of length to thickness, is preferably 0.5 or less. The morphology of Si nanoparticles can be determined by measuring the average particle size using dynamic light scattering, but samples with the above aspect ratios can be identified more easily and precisely using analytical tools such as transmission electron microscopes (TEM) and field emission scanning electron microscopes (FE-SEM). When the present anode active material contains the above Si nanoparticles, the state of the Si nanoparticles can be identified by cutting the sample with a focused ion beam (FIB) and observing the cross section with FE-SEM, or by slicing the sample and observing it with TEM. The aspect ratio of the Si nanoparticles is a calculation result based on 50 particles of the sample present in the main part of the field of view of the TEM image.

[0020] The specific surface area of ​​the Si nanoparticles is 100 m from the viewpoint of capacitance and initial coulombic efficiency. 2 / g to 400m 2 / g is preferred. The specific surface area of ​​Si nanoparticles is 100 m in terms of capacitance and initial Coulombic efficiency. 2 / g to 300m 2 / g is more preferable, and 100m 2 / g to 230m 2 / g is more preferred. The specific surface area is a value determined by the BET method, and can be determined by nitrogen gas adsorption measurement, for example, using a specific surface area measuring device. The specific surface area of ​​Si nanoparticles can be measured as follows: The amount of nitrogen adsorption at liquid nitrogen temperature and a relative pressure of 0.5 or less is determined at multiple points, and the specific surface area is calculated from a BET plot in the range where the heat of adsorption C value is positive and highly linear.

[0021] The shape of the Si nanoparticles may be granular, needle-like, or flake-like, but crystalline is preferred. When the Si nanoparticles are crystalline, the crystallite diameter obtained from the diffraction peak attributed to Si(111) in X-ray diffraction is preferably in the range of 5 to 14 nm from the viewpoint of initial Coulomb efficiency and capacity retention. The crystallite diameter is more preferably 12 nm or less, and even more preferably 10 nm or less.

[0022] The silicon oxide particles are generally a general term for amorphous silicon oxide particles obtained by heating a mixture of silicon dioxide and metallic silicon to produce silicon monoxide gas, which is then cooled and precipitated, and are represented by the following general formula (1): SiOn (1) In the formula (1), n ​​is 0.4 or more and 1.8 or less, and preferably 0.5 or more and 1.6 or less.

[0023] When the present negative electrode active material contains silicon oxide particles, if the average particle size of the silicon oxide particles exceeds 5 μm, the silicon oxide particles form large clumps, and when the present negative electrode active material is used in a negative electrode, the silicon oxide particles cause significant expansion and contraction of the negative electrode active material during charge and discharge. As a result, stress is concentrated in a part of the carbonaceous phase, which makes the negative electrode active material prone to structural collapse and tends to reduce the capacity retention rate of the negative electrode active material. On the other hand, silicon oxide particles smaller than 300 nm are too fine and tend to aggregate. This can reduce the dispersibility of the silicon oxide particles in the negative electrode active material. Furthermore, if the silicon oxide particles are too fine, their specific surface area increases, and high-temperature firing of the negative electrode active material tends to result in the formation of by-products on the surface of the silicon oxide particles. This can lead to a decrease in charge / discharge performance.

[0024] Therefore, from the above viewpoints, the average particle size of the silicon oxide particles is preferably 3 μm or less, more preferably 2 μm or less, and from the viewpoints of particle dispersibility and specific surface area, the average particle size of the silicon oxide particles is preferably 300 nm or more, more preferably 200 nm or more. Here, the average particle size is the D50 value as described above. D50 is as described above.

[0025] The silicon oxide particles can be obtained by pulverizing silicon oxide so that the average particle size falls within the above range. Examples of the mill used for pulverization include a ball mill, a bead mill, a jet mill, etc. The pulverization may be wet pulverization using an organic solvent, and examples of the organic solvent that can be used preferably include alcohols and ketones, but aromatic hydrocarbon solvents such as toluene, xylene, naphthalene, and methylnaphthalene can also be used. The average particle size of the silicon oxide particles obtained can be made to fall within the above range by controlling the bead mill conditions such as bead particle size, blending ratio, rotation speed, or milling time, and classifying the particles.

[0026] The silicon oxide particles may be in the form of granules, needles, or flakes. The morphology of silicon oxide particles can be determined by measuring the average particle size using dynamic light scattering, but samples with the above aspect ratios can be identified more easily and precisely using analytical tools such as transmission electron microscopes (TEM) and field emission scanning electron microscopes (FE-SEM). When the present anode active material contains silicon oxide particles, the state of the Si nanoparticles can be identified by cutting the sample with a focused ion beam (FIB) and observing the cross section with FE-SEM, or by slicing the sample and observing it with TEM. The aspect ratio of the silicon oxide particles is a calculation result based on 50 particles in the main part of the sample within the field of view of the TEM image.

[0027] The present negative electrode active material may contain at least one of the Si nanoparticles or the silicon oxide particles, or may contain both the Si nanoparticles and the silicon oxide particles, and preferably contains both the Si nanoparticles and the silicon oxide particles.

[0028] When the present negative electrode active material contains both the Si nanoparticles and the silicon oxide particles, it is preferable that the Si nanoparticles have silicon oxide on the surface thereof, from the viewpoint of suppressing initial capacity loss and thereby achieving excellent initial coulombic efficiency. When silicon oxide is present on the surface of the Si nanoparticles, it is preferable that the surface of the Si nanoparticles is coated with a silicon dioxide film, which is an oxide film of silicon.

[0029] Examples of the carbonaceous phase of the present negative electrode active material include crystalline carbon and amorphous carbon. Examples of crystalline carbon include natural graphite and artificial graphite, and examples of amorphous carbon include graphitizable carbon and non-graphitizable carbon. The carbonaceous phase is appropriately selected based on the desired performance depending on the application. For example, crystalline carbon is preferably selected from the viewpoint of the energy density of the resulting secondary battery. On the other hand, amorphous carbon is preferably selected from the viewpoint of battery durability due to the expansion and contraction of the active material during charge and discharge. From the viewpoint of the initial efficiency of the secondary battery, the carbonaceous phase is preferably amorphous carbon.

[0030] In the present negative electrode active material, the carbonaceous phase embeds at least one of the Si nanoparticles or the silicon oxide particles. That is, when the present negative electrode active material contains the Si nanoparticles, the carbonaceous phase embeds at least a portion of the Si nanoparticles. When the present negative electrode active material contains the silicon oxide particles, the carbonaceous phase embeds at least a portion of the silicon oxide particles. When the present negative electrode active material contains the Si nanoparticles and the silicon oxide particles, the carbonaceous phase embeds at least a portion of the Si nanoparticles and the silicon oxide particles. "Embedded with a carbonaceous phase" refers to a state in which the Si nanoparticles or the silicon oxide particles are dispersed in the carbonaceous phase. From the viewpoint of the energy density of the resulting secondary battery, it is preferable that the Si nanoparticles or the silicon oxide particles are in as close contact as possible with the carbonaceous phase in the present negative electrode active material. The state in which the carbonaceous phase embeds at least one of the Si nanoparticles or the silicon oxide particles can be confirmed by observing the cross section of the particles using EDS (energy dispersive X-ray spectroscopy) of a SEM (scanning electron microscope) or by an electron probe microanalyzer (EPMA). The Si nanoparticles and the carbonaceous phase preferably satisfy the porosity range described below.

[0031] The amount of the Si nanoparticles or silicon oxide particles in the negative electrode active material is preferably 5% to 70% by mass, more preferably 10% to 60% by mass, based on 100% by mass of the total amount of the Si nanoparticles or silicon oxide particles and the carbonaceous phase. When the negative electrode active material contains both the Si nanoparticles and the silicon oxide particles, the total amount of the Si nanoparticles, the silicon oxide particles, and the carbonaceous phase is preferably 100% by mass, and the total amount of the Si nanoparticles and the silicon oxide particles is preferably within the above range. It is preferable that most of the Si nanoparticles or silicon oxide particles are embedded in the carbonaceous phase, more preferably 60% or more by volume of the total Si nanoparticles or silicon oxide particles are embedded, even more preferably 90% or more.

[0032] The interplanar spacing of the carbon 002 planes in the carbonaceous phase, as determined by XRD measurement, can be measured as follows: A negative electrode active material containing a carbonaceous phase is placed in a sample holder, and an X-ray diffraction pattern is obtained using CuKα radiation as the radiation source. The peak positions in the X-ray diffraction pattern are determined using the 2θ value. The wavelength of CuKα radiation is set to 0.15418 nm, and the carbonaceous phase 002 interplanar spacing is calculated using the Bragg formula shown below. d002=λ / 2·sinθ The closer the carbonaceous phase is to the state of graphite, the higher the crystallinity, and the carbon 002 spacing of the carbonaceous phase approaches 0.3354 nm of ideal graphite. The carbonaceous phase of this negative electrode active material has an amorphous structure, and the interplanar spacing of the carbon 002 plane determined by XRD measurement is 0.34 nm to 0.38 nm.

[0033] When the interplanar spacing of the carbon 002 planes of the carbonaceous phase is 0.34 nm to 0.38 nm, the electronic conductivity of the negative electrode active material is improved, suppressing the isolation of Si nanoparticles due to volume expansion during charging, and as a result, it is possible to reduce the electrical capacity loss of the Si nanoparticles. Furthermore, the carbonaceous phase can also serve as a coating material for the negative electrode active material particles, thereby improving the electronic conductivity between the negative electrode active material particles and suppressing isolation of the negative electrode active material particles due to swelling during charging, thereby enabling an improvement in the capacity retention rate when the battery is made into a secondary battery. The interplanar spacing of the carbon 002 plane determined by XRD measurement is preferably 0.345 nm to 0.375 nm, more preferably 0.350 nm to 0.370 nm, from the viewpoint of Coulomb efficiency.

[0034] The state of the carbonaceous phase can be identified using a thermogravimetric differential thermal analyzer (TG-DTA). The carbonaceous phase is easily thermally decomposed in the atmosphere, and the amount of carbon present can be determined from the amount of thermal weight loss measured in the presence of air. That is, the amount of carbon in the carbonaceous phase can be quantified using TG-DTA. Furthermore, the thermal weight loss behavior from the measurement can easily reveal changes in thermal decomposition temperature behavior, such as the decomposition reaction start temperature, decomposition reaction end temperature, number of thermal decomposition reaction species, and the temperature of maximum weight loss for each thermal decomposition reaction species. The temperature values ​​of these behaviors can be used to determine the state of carbon.

[0035] When the carbon in the carbonaceous phase is amorphous carbon, the carbonaceous phase has properties similar to amorphous carbon and therefore undergoes thermal decomposition in the air at temperatures ranging from approximately 550°C to 900°C. As a result, a rapid weight loss occurs. Although there are no particular limitations on the maximum temperature under TG-DTA measurement conditions, it is preferable to perform TG-DTA measurements in the air at temperatures ranging from approximately 25°C to approximately 1000°C or higher in order to completely complete the thermal decomposition reaction of carbon.

[0036] From the viewpoint of the structure formation of the negative electrode active material, it is preferable that the mass spectrometry reduction rate of the negative electrode active material be 10% to 70% at 100 to 800° C. as determined by TG analysis under a dry air flow. As described above, the mass spectrometry reduction rate can be determined by carrying out TG-DTA measurement under conditions of 100° C. to 800° C. in a dry air flow. The negative electrode active material more preferably has a mass spectrometry reduction rate of 15% to 65%, and even more preferably 20% to 60%, at 100 to 800° C. as determined by TG analysis under a dry air flow.

[0037] The weight gain initiation temperature of this negative electrode active material, as determined by TG analysis under dry air flow, is 550°C or higher, which means that there is a large amount of carbonaceous layer that oxidizes at low temperatures. As a result, the reaction between Si nanoparticles and oxygen is delayed, thereby suppressing electrochemical side reactions on the active material surface, which is preferable from the viewpoint of suppressing the loss of electrical capacity in the initial stage of the secondary battery. From the viewpoint of suppressing initial capacity loss and initial coulomb efficiency, the weight gain starting temperature is more preferably 575°C or higher, and even more preferably 600°C or higher.

[0038] From the viewpoint of electrical capacity per weight, the present negative electrode active material preferably contains 0.1 to 80 parts by mass of a silicon-based material per 100 parts by mass of the present negative electrode active material. Examples of silicon-based materials include silicon and silicon oxycarbide. When the present negative electrode active material contains Si nanoparticles, the silicon material is preferably silicon different from the nanoparticles. When the present negative electrode active material has a carbon coating, as described below, the mass of the present negative electrode active material includes the amount of the carbon coating.

[0039] The specific surface area of ​​this negative electrode active material is 0.01 m 2 / g to 20m 2 From the viewpoint of the amount of solvent absorbed during electrode preparation and the amount of binder used to maintain binding properties, the specific surface area of ​​the negative electrode active material is preferably 1 m 2 / g or more is preferable, and 3m 2 / g or more. The specific surface area of ​​the negative electrode active material is preferably 18 m 2 / g or less is preferable, and 10m 2 / g or less is more preferable. The specific surface area is a value determined by the BET method as described above. As with the Si nanoparticles, the specific surface area of ​​this negative electrode active material can be calculated by determining the nitrogen adsorption amount at multiple points at a relative pressure of 0.5 or less at liquid nitrogen temperature, and then using a BET plot to calculate the specific surface area from the range where the heat of adsorption C value is positive and highly linear.

[0040] The average particle diameter of this negative electrode active material is preferably 0.5 μm to 10 μm, more preferably 2 μm to 8 μm. If the average particle diameter is too small, the specific surface area increases significantly, and when used in a secondary battery, the amount of SEI generated during charge and discharge increases, which can reduce the reversible charge and discharge capacity per unit volume. If the average particle diameter is too large, there is a risk of the material peeling off from the current collector during electrode film production. As mentioned above, the average particle diameter is the volume-average particle diameter, or D50 value. The method for measuring D50 is the same as above. The particle size range of the present electrode active material before classification is preferably 0.1 μm to 30 μm, and the particle size range after fine particles are removed is preferably 0.5 μm to 30 μm. The negative electrode active material may be in the form of particles, needles, or flakes.

[0041] The present negative electrode active material preferably has a carbon coating and a porosity, V, defined by the following formula (1) of 7% or more and 20% or less.

[0042] TIFF0007754152000002.tif24161 In the above formula (1), V is the porosity (%), ρ is the density of the negative electrode active material (g / cm 3 ), ρ' is the density of the entire negative electrode active material (g / cm 3 ), ρ'' is the density of the carbon coating (g / cm 3 ), and A represents the amount of carbon coating (mass %).

[0043] The carbon coating preferably covers at least a portion of the surface of the negative electrode active material, and the carbon coating is preferably a coating made of low-crystalline carbon. From the viewpoint of improving the chemical stability and thermal stability of the present negative electrode active material, the amount of the carbon coating is preferably 0.1% by mass to 30% by mass, more preferably 1% by mass to 25% by mass, and even more preferably 5% by mass to 20% by mass, based on 100% by mass of the present negative electrode active material including the carbon coating. Also, from the viewpoint of improving the chemical stability and thermal stability of the present active material, the average thickness of the carbon coating is preferably 10 nm to 300 nm. From the viewpoint of improving the chemical stability and thermal stability of the present negative electrode active material, the carbon coating preferably covers 1% or more of the surface of the present negative electrode active material, and more preferably covers 10% or more of the surface of the present negative electrode active material. The present negative electrode active material may have the carbon coating continuously or intermittently on its surface. The carbon coating is preferably formed on the surface of the negative electrode active material by chemical vapor deposition.

[0044] As mentioned above, Si nanoparticles have high capacity, but they undergo large volume changes due to the absorption and desorption of large amounts of lithium ions, which is thought to result in poor cycleability. It is believed that this volume change cannot be sufficiently suppressed by a carbon coating alone. Therefore, a method has been proposed in which voids are provided around the Si nanoparticles, which buffer the volume expansion and prevent the destruction of the carbon coating. However, if the voids are not adequate, the buffering effect will not function sufficiently, and the surface area will increase due to cracking of the active material, resulting in increased SEI formation and reduced initial Coulomb efficiency. It is also considered necessary to appropriately control the composition of the negative electrode active material as well as the porosity.

[0045] It is thought that the conventional definition of porosity does not necessarily adequately reflect the state of the voids surrounding the Si nanoparticles. The inventors have found that the porosity defined by the above formula (1) appropriately reflects the state of the voids surrounding the Si nanoparticles. Furthermore, they have found that by using a negative electrode active material whose porosity defined by the above formula (1) falls within a specific range in a secondary battery, the increase in surface area and the generation of SEI due to cracking of the active material are suppressed, resulting in a secondary battery with improved initial Coulombic efficiency.

[0046] The densities ρ, ρ', and ρ'' can all be determined by dry density measurement using a constant volume expansion method. The ρ of the present negative electrode active material is usually about 2.0 to 2.4. The density of the carbon coating may be measured by peeling the carbon coating from the negative electrode active material and directly measuring the true density, or may be calculated, etc. For example, the density of the carbon coating alone may be calculated by plotting the carbon coating content (mass %) versus the density of the negative electrode active material at several points and extrapolating the point where the carbon coating content is 100 mass % by linear approximation. Alternatively, the silicon component may be dissolved from the negative electrode active material, and the true density of the insoluble portion may be directly measured.

[0047] In the formula (1), A represents the amount of the carbon coating, which is expressed as % by mass when the mass of the negative electrode active material including the carbon coating is taken as 100% by mass, as described above. The amount of the carbon coating can be determined by TG-DTA, elemental analysis, etc.

[0048] Conventionally, porosity is the percentage of voids in the entire particle, including pores and internal voids within the particle. The porosity (%) is usually defined by the following formula: Porosity (%) = (1 - apparent density / true density) x 100 In the above formula, the apparent density is the density including the internal voids, and the porosity defined by the above formula is the voids excluding the internal voids of the particles. As mentioned above, in order to prevent the destruction of active material particles having a carbon coating, the gap between the carbon coating and the interior is important, and it is necessary to evaluate the porosity of that part. Various definitions of porosity have been proposed, including the porosity defined in Patent Document 2, but the conventional definition of porosity may not have a sufficient correlation with density. Another method involves observing the cross section of a particle using an electron microscope or the like, visually identifying the voids, and calculating the void ratio. However, this method is highly dependent on the cross-sectional area of ​​the particle, and it is difficult to accurately determine the gap between the carbon coating and its interior.

[0049] On the other hand, the definition of porosity by the above formula (1) differs from the conventional method in that, in a negative electrode active material having a carbon coating, by introducing the amount and density of the carbon coating into formula (1), the porosity of the gap between the carbon coating and its interior can be evaluated more accurately. The porosity, V, is preferably 9% or more, more preferably 11% or more, from the viewpoint of suppressing the influence of expansion due to the insertion of lithium ions, and is preferably 18% or less, more preferably 17% or less, from the viewpoint of improving the energy density of the resulting secondary battery.

[0050] The carbon coating is preferably formed on the surface of the negative electrode active material by chemical vapor deposition. In order to make the negative electrode active material have the true density and porosity, V, within the above ranges, for example, the gas flow rate, treatment time, and treatment temperature during the carbon coating treatment are controlled.

[0051] When the porosity, V, of the present negative electrode active material defined by the formula (1) is 7% or more and 20% or less, the true density of the present negative electrode active material is 1.6 g / cm 3 More than 2.0g / cm 3 From the viewpoint of improving the energy density of the resulting secondary battery, the true density is preferably 1.65 g / cm or less. 3 More preferably, 1.70 g / cm 3 The above is particularly preferred. The true density of this negative electrode active material is 1.95 g / cm3 in relation to the porosity, V. 3 More preferably, 1.90 g / cm 3 The following are particularly preferred: The true density is a value measured using a true density measuring device, and can be calculated by pressurizing a sample chamber containing a sample with helium gas, then opening a valve to diffuse the gas into an expansion chamber, determining the volume of the sample from the pressure change that occurs, and dividing the volume of the sample by the mass of the sample.

[0052] The negative electrode active material may contain a silicate compound in addition to the carbon coating. The silicate compound is preferably a silicate compound of at least one metal selected from the group consisting of Li, K, Na, Ca, Mg and Al. A silicate compound is generally a compound containing an anion having a structure in which one or more silicon atoms are at the center and electronegative ligands surround the silicon atom. A silicate compound that is a salt of at least one metal selected from the group consisting of Li, K, Na, Ca, Mg, and Al with a compound containing the anion is preferred. The compound containing the anion is orthosilicate ion (SiO4 4- ), metasilicate ion (SiO3 2- ), pyrosilicate ion (Si2O7 6- ), cyclic silicate ion (Si3O9 6- or SiO 18 12-) and other silicate ions are known. The silicate compound is preferably a silicate compound that is a salt of metasilicate ion with at least one metal selected from the group consisting of Li, K, Na, Ca, Mg, and Al. Of the metals, Li or Mg is more preferred.

[0053] When a silicate compound contains at least one metal selected from the group consisting of Li, K, Na, Ca, Mg, and Al, it may contain two or more of these metals. When two or more metals are contained, one silicate ion may contain multiple metals, or it may be a mixture of silicate compounds containing different metals. Furthermore, 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 particularly preferably magnesium metasilicate (MgSiO3).

[0054] When silicate compounds are in a crystalline state, they can be detected by powder X-ray diffraction (XRD), and when they are amorphous, they can be detected by powder X-ray diffraction (XRD). 29 This can be confirmed by Si-NMR measurement.

[0055] Since the present negative electrode active material has the carbonaceous phase, in the Raman spectrum of the carbonaceous phase, 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 It is preferable that the scattering peaks have scattering peaks in the vicinity of G band and D band, and that the scattering peak intensity ratio I (G band / D band) is in the range of 0.7 to 2. The scattering peak intensity ratio I is more preferably 0.7 to 1.8.

[0056] When the present negative electrode active material has the coating layer of the low-crystalline carbon, the scattering peak intensity ratio I (G band / D band) of the Raman spectrum of the present negative electrode active material is preferably in the range of 0.9 to 1.1.

[0057] The negative electrode active material may contain other components in addition to the silicon-based material, carbon coating, and silicate compound, as required.

[0058] The present negative electrode active material can be produced, for example, by a method including the following steps 1 to 3. The following steps exemplify a method for containing Si nanoparticles, but the present invention is not limited to this method. When the present negative electrode active material contains silicon oxide, the Si nanoparticles can be converted to silicon oxide particles in the following step 1. When the present negative electrode active material contains Si nanoparticles and silicon oxide particles, the Si nanoparticles and silicon oxide particles can be used in the following step 1. As described above, silicon oxide particles can be produced by heating a mixture of silicon dioxide and metallic silicon to produce silicon monoxide gas, followed by cooling and precipitation. Commercially available silicon oxide may also be used. Step 1: A slurry of Si nanoparticles milled by a wet method is mixed with a carbonaceous phase source, followed by stirring and drying to obtain a precursor. Step 2: The precursor obtained in step 1 is calcined in an inert atmosphere at a maximum temperature ranging from 1000°C to 1180°C to obtain a calcined product. Step 3: The calcined product obtained in step 2 is pulverized to obtain the present negative electrode active material.

[0059] Each step will be described below. <Process 1> (Si (0 valent) slurry) The wet-milled Si (0-valent) slurry used in step 1 can be prepared by milling silicon particles in an organic solvent using a wet powder mill. A dispersant may be used to promote milling of the silicon particles in the organic solvent. The wet milling device is not particularly limited, and examples include a roller mill, a high-speed rotary mill, a container-driven mill, and a bead mill. In wet milling, it is preferable to mill the silicon particles until they become Si nanoparticles.

[0060] The organic solvent used in the wet method is an organic solvent that does not chemically react with silicon. Examples include ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone, and diisobutyl ketone; alcohols such as ethanol, methanol, normal propyl alcohol, and isopropyl alcohol; and aromatics such as benzene, toluene, and xylene.

[0061] The dispersant may be aqueous or non-aqueous. Non-aqueous dispersants are preferred to prevent excessive oxidation of the silicon particle surface. Examples of non-aqueous dispersants include polymeric dispersants such as polyethers, polyalkylene polyamines, and polycarboxylic acid partial alkyl esters; low molecular weight dispersants such as polyhydric alcohol esters and alkyl polyamines; and inorganic dispersants such as polyphosphates. The silicon concentration in the Si (zerovalent) slurry is not particularly limited. However, when the solvent and, if necessary, a dispersant are included, the amount of Si particles is preferably in the range of 5% to 40% by mass, more preferably 10% to 30% by mass, where the total amount of the dispersant and Si particles is 100% by mass.

[0062] (carbonaceous phase source) The carbonaceous phase source used in step 1 is preferably a synthetic resin or natural chemical raw material that is carbonized by high-temperature firing in an inert atmosphere and has an aromatic functional group.

[0063] Examples of synthetic resins include thermoplastic resins such as polyvinyl alcohol and polyacrylic acid, and thermosetting resins such as phenolic resin and furan resin. Examples of natural chemical raw materials include coke and heavy oil, 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.

[0064] (precursor) The carbonaceous phase source and the Si slurry are uniformly mixed and stirred, followed by desolvation and drying to obtain a precursor of the present negative electrode active material (hereinafter also referred to as "precursor"). The mixing is carried out using a device with dispersion and mixing functions. Examples include a stirrer, an ultrasonic mixer, and a premix disperser. In the desolvation and drying process for distilling off the organic solvent, a dryer, a reduced pressure dryer, a spray dryer, etc. can be used.

[0065] The precursor preferably contains 3% to 97% by mass of Si nanoparticles (zero valent) and 3% to 97% by mass of a carbonaceous phase source solid content, and more preferably contains 20% to 80% by mass of silicon particles and 20% to 80% by mass of a carbon source resin solid content. Heat treatment of the precursor of the negative electrode active material, as described below, can reduce the mass and change the ratio of nanosilicon in the negative electrode active material. Therefore, the content of Si nanoparticles in the precursor can be appropriately set based on the desired content of Si nanoparticles in the negative electrode active material.

[0066] <Process 2> In step 2, the precursor obtained in step 1 is calcined in an inert atmosphere at a maximum temperature ranging from 1000°C to 1180°C to completely decompose the thermally decomposable organic components, and the other main components are converted into a calcined product suitable for the present negative electrode active material by precisely controlling the calcination conditions. Specifically, the carbonaceous phase source of the raw material is converted into free carbon by the energy of the high-temperature treatment. In other words, the calcination yields a matrix containing the calcined product of the carbonaceous phase source. The calcined product referred to here is a product in which the composition and structure of the organic compound, such as the carbonaceous phase source, have been partially or completely changed due to the decomposition and conversion at high temperature. In the calcined product of the carbonaceous phase source, the entire carbonaceous phase source may be converted to carbon, or a portion of the carbonaceous phase source may be converted to carbon with the remainder maintaining the structure of the carbonaceous phase source.

[0067] In step 2, the precursor obtained in step 1 is calcined in an inert atmosphere according to a calcination program defined by factors such as the rate of temperature rise and the time required to maintain the temperature. The maximum temperature is the highest temperature that can be set and has a significant effect on the structure and performance of the calcined product, the present anode active material. In the present invention, the maximum temperature is set to 1000°C to 1180°C, which allows for precise control of the microstructure of the present anode active material and prevents oxidation of silicon particles due to excessively high temperature calcination, resulting in superior charge / discharge characteristics.

[0068] The calcination method is not particularly limited, but a reaction device having a heating function in an inert atmosphere may be used, and treatment by a continuous method or a batch method is possible. The calcination device may be appropriately selected depending on the purpose from among a fluidized bed reactor, a rotary furnace, a vertical moving bed reactor, a tunnel furnace, a batch furnace, a rotary kiln, etc.

[0069] <Process 3> Step 3 is a process for obtaining the present negative electrode active material by pulverizing the sintered product obtained in Step 2 and classifying it as necessary. Step 3 also involves forming a carbon coating on the surface of the negative electrode active material by chemical vapor deposition, if necessary. Pulverization can be performed in one step until the desired particle size is reached, or it can be performed in several steps. For example, if the sintered product is in the form of lumps or agglomerates of 10 mm or larger and a 10 μm active material is to be produced, the material is coarsely pulverized using a jaw crusher, roll crusher, or the like to produce particles of approximately 1 mm, then pulverized to 100 μm using a glow mill, ball mill, or the like, and then pulverized to 10 μm using a bead mill, jet mill, or the like. The particles produced by pulverization may contain coarse particles, and classification is performed to remove these and to remove fine powder and adjust the particle size distribution. Classifiers such as air classifiers and wet classifiers are used depending on the purpose. However, to remove coarse particles, sieving is preferred because it reliably achieves the desired goal. Incidentally, if the precursor mixture is controlled to a shape close to the target particle size by spray drying or the like before firing, and the firing is carried out in that shape, it is possible to omit the pulverization step.

[0070] In the manufacturing process, the true density and porosity defined by formula (1) can be controlled within the above ranges by controlling the gas flow rate during the carbon coating treatment and optimizing the treatment time and temperature conditions. For example, by increasing the gas flow rate and treatment time, the amount of carbon coating can be increased, and the porosity defined by formula (1) can be adjusted. Furthermore, by increasing the treatment temperature, the true density can be increased.

[0071] In the manufacturing process, the sintering temperature can be controlled to adjust the lattice spacing of the carbon 002 plane and the specific surface area of ​​the carbonaceous phase of the negative electrode active material, as determined by XRD measurement, to the above ranges. For example, increasing the sintering temperature promotes the carbonization reaction, narrowing the lattice spacing of the carbon 002 plane.

[0072] When the present negative electrode active material contains a silicate compound of at least one metal selected from the group consisting of Li, K, Na, Ca, Mg, and Al, a salt of at least one metal selected from the group consisting of Li, K, Na, Ca, Mg, and Al is added to a suspension obtained by mixing a slurry of Si nanoparticles with a carbonaceous phase source, and then the same procedure as above is repeated to obtain the present active material containing the silicate compound. Examples of salts of at least one metal selected from the group consisting of Li, K, Na, Ca, Mg, and Al include halides such as fluorides, chlorides, and bromides, hydroxides, and carbonates of these metals.

[0073] The salt of the metal may be a salt of two or more kinds of metals, one salt may have a plurality of kinds of metals, or may be a mixture of salts having different metals. The amount of the metal salt added to the suspension is preferably in a molar ratio of 0.01 to 0.4 relative to the number of moles of Si nanoparticles. If the metal salt is soluble in an organic solvent, the metal salt can be dissolved in an organic solvent and added to a suspension of the carbonaceous phase source or, if the active material particles contain Si nanoparticles, to a suspension of Si nanoparticles. If the metal salt is insoluble in an organic solvent, the metal salt particles can be dispersed in an organic solvent and then added to a suspension of the carbonaceous phase source or, if the active material particles contain Si nanoparticles, to a suspension of Si nanoparticles. To improve dispersion efficiency, the metal salt is preferably nanoparticles with an average particle size of 100 nm or less. The organic solvent can be preferably alcohols or ketones, but aromatic hydrocarbon solvents such as toluene, xylene, naphthalene, and methylnaphthalene can also be used.

[0074] When the present negative electrode active material has the carbon coating, the present negative electrode active material having the carbon coating can be obtained by coating at least a portion of the surface of the fired product obtained by the above method with a carbon coating. The carbon coating is preferably an amorphous carbon coating obtained in a chemical vapor deposition apparatus at a temperature in the range of 700°C to 1000°C in a flow of a pyrolytic carbon source gas and a carrier inert gas. Examples of pyrolytic carbon source gases include acetylene, ethylene, acetone, alcohol, propane, methane, and ethane. Examples of the inert gas include nitrogen, helium, and argon, and nitrogen is usually used.

[0075] When the present negative electrode active material contains the silicon-based material, in step 1, a silicon material source that becomes the desired silicon-based material upon firing together with the carbonaceous phase source may be added. Examples of silicon material sources include polyalkoxysilane, polysilsesquioxane, and polysiloxane-containing acrylic resin.

[0076] The present negative electrode active material has excellent initial coulombic efficiency, and therefore a secondary battery using the present negative electrode active material as a battery negative electrode exhibits good charge / discharge characteristics. Specifically, a slurry containing the present negative electrode active material, an organic binder, and optionally other components such as a conductive additive can be applied to a copper foil current collector as a thin film to form a negative electrode. Alternatively, a carbon material can be added to the slurry to form a negative electrode. Examples of carbon materials include natural graphite, artificial graphite, and amorphous carbon such as hard carbon or soft carbon.

[0077] The negative electrode active material and a binder serving as an organic binder are kneaded together with a solvent using a dispersing device such as a stirrer, ball mill, super sand mill, or pressure kneader to prepare a negative electrode material slurry, which is then applied to a current collector to form a negative electrode layer. Alternatively, the negative electrode material paste can be formed into a shape such as a sheet or pellet, which is then integrated with a current collector. The negative electrode thus obtained contains the negative electrode active material, resulting in a secondary battery negative electrode with excellent initial coulombic efficiency. The negative electrode can be obtained, for example, by kneading the negative electrode active material and a binder serving as an organic binder together with a solvent using a dispersing device such as a stirrer, ball mill, super sand mill, or pressure kneader to prepare a negative electrode material slurry, which is then applied to a current collector to form a negative electrode layer. Alternatively, the negative electrode material paste can be formed into a shape such as a sheet or pellet, which is then integrated with a current collector.

[0078] Examples of the organic binder include styrene-butadiene rubber copolymers (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 polymer compounds such as polyvinylidene fluoride, polyethylene oxide, polyepichlorohydrin, polyphosphazene, polyacrylonitrile, polyimide, polyamideimide, and carboxymethyl cellulose (CMC).

[0079] 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 (NMP). 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.

[0080] 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 and contraction during charge and discharge is further suppressed. On the other hand, when the content is 30% by mass or less, an increase in electrode resistance is further suppressed. Within this range, the negative electrode active material of the present invention has high chemical stability and can employ an aqueous binder, making it easy to handle in practical use.

[0081] The negative electrode material slurry may contain a conductive additive, if necessary. Examples of the conductive additive include carbon black, graphite, acetylene black, and conductive oxides and nitrides. The amount of the conductive additive used may be about 1% by mass to 15% by mass of the negative electrode active material of the present invention.

[0082] The current collector may be made of copper, nickel, titanium, stainless steel, or the like in the form of a foil, perforated foil, mesh, or other strip. Porous materials such as porous metal (foamed metal) and carbon paper may also be used.

[0083] 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.

[0084] The negative electrode material slurry can be formed into a sheet or pellet form, and the sheet or pellet can be integrated with the current collector by, for example, rolling, pressing, or a combination thereof.

[0085] The negative electrode layer formed on the current collector and the negative electrode layer integrated with the current collector are preferably heat-treated depending on the organic binder used. For example, when an aqueous styrene-butadiene rubber copolymer (SBR) or the like is used, the heat treatment may be performed at 100 to 130°C. When an organic binder having a polyimide or polyamideimide as the main skeleton is used, the heat treatment is preferably performed at 150 to 450°C.

[0086] This heat treatment removes the solvent and hardens the binder, increasing strength and improving adhesion between particles and between the particles and the current collector. These heat treatments are preferably carried out in an inert atmosphere such as helium, argon, or nitrogen, or in a vacuum atmosphere, to prevent oxidation of the current collector during treatment.

[0087] After the heat treatment, the negative electrode is preferably pressed (pressurized). In the negative electrode using the negative electrode active material of the present invention, the electrode density is 1 g / cm 3 to 1.8 g / cm 3 and preferably 1.1 g / cm 3 to 1.7 g / cm 3 More preferably, it is 1.2 g / cm 3 to 1.6 g / cm 3 With regard to the electrode density, the higher the density, the more the adhesion and the volumetric capacity density of the electrode tend to improve, but if the density is too high, the voids in the electrode decrease, weakening the effect of suppressing the volume expansion of silicon and the like, and resulting in a decrease in the capacity retention rate, so an optimum range must be selected.

[0088] A negative electrode containing the present negative electrode active material has excellent initial coulombic efficiency and is therefore suitable for use in secondary batteries. Secondary batteries having such a negative electrode are preferably non-aqueous electrolyte secondary batteries and solid electrolyte secondary batteries, and the present negative electrode active material exhibits particularly excellent performance when used as a negative electrode for non-aqueous electrolyte secondary batteries.

[0089] When a secondary battery containing the present negative electrode active material is used, for example, as a wet electrolyte secondary battery, it can be constructed by disposing a positive electrode and a negative electrode containing the negative electrode active material of the present invention opposite each other via a separator and injecting an electrolyte solution.

[0090] 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 a strip-shaped current collector made of a metal or alloy such as aluminum, titanium, or stainless steel, in the form of a foil, perforated foil, mesh, or the like.

[0091] The positive electrode material used in the positive electrode layer is not particularly limited. When manufacturing a lithium ion secondary battery among nonaqueous electrolyte secondary batteries, for example, a metal compound, metal oxide, metal sulfide, or conductive polymer material capable of doping or intercalating lithium ions may be used. Examples include 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, VO5, VO 13 , VO2, MnO2, TiO2, MoV2O8, TiS2, V2S5, VS2, MoS2, MoS3, Cr3O8, Cr2O5, olivine-type LiMPO4 (M: Co, Ni, Mn, Fe), conductive polymers such as polyacetylene, polyaniline, polypyrrole, polythiophene, polyacene, porous carbon, etc. can be used alone or in combination.

[0092] The separator may be, for example, a nonwoven fabric, cloth, or microporous film primarily composed of a polyolefin such as polyethylene or polypropylene, or a combination thereof. Note that if the nonaqueous electrolyte secondary battery to be fabricated has a structure in which the positive electrode and the negative electrode are not in direct contact with each other, it is not necessary to use a separator.

[0093] 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 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, either alone or as a mixture of two or more components.

[0094] The structure of a secondary battery containing this negative electrode active material is not particularly limited. However, it is common to form a wound electrode assembly by winding a positive electrode, a negative electrode, and an optional separator into a flat spiral shape, or to stack these electrodes into a laminated electrode assembly, which is then enclosed in an outer casing. The half-cells used in the examples of the present invention were constructed with the negative electrode primarily composed of this negative electrode active material and a counter electrode composed of metallic lithium. This simplified evaluation was performed to more clearly compare the cycle characteristics of the active material itself. Adding a small amount of this negative electrode active material to a mixture primarily composed of a graphite-based active material (capacity approximately 340 mAh / g) can increase the negative electrode capacity to approximately 400 to 700 mAh / g, significantly exceeding the capacity of existing negative electrodes, thereby improving cycle characteristics.

[0095] Secondary batteries containing the present negative electrode active material are not particularly limited and may be used as paper batteries, button batteries, coin batteries, laminated batteries, cylindrical batteries, prismatic batteries, etc. The present negative electrode active material can also be applied to general electrochemical devices that use lithium ion insertion and desorption as a charge / discharge mechanism, such as hybrid capacitors and solid-state lithium secondary batteries.

[0096] As described above, the negative electrode active material of the present invention provides a secondary battery with high initial efficiency, which is one of the important properties of a secondary battery, and with an excellent balance of battery characteristics. Therefore, the negative electrode active material can be suitably used in secondary batteries.

[0097] Although the present negative electrode active material and the secondary battery having the present negative electrode active material have been described above, the present invention is not limited to the configurations of the above-described embodiments. In the configurations of the present negative electrode active material and the secondary battery having the present negative electrode active material according to the above-described embodiments, any other configuration may be added, or any configuration that exhibits the same function may be substituted. [Example]

[0098] The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples. The half-cells used in the examples of the present invention were evaluated using the present negative electrode active material as the negative electrode and metallic lithium as the counter electrode, in order to more clearly compare the cycle characteristics of the active materials themselves. By using this configuration, it is possible to improve cycle characteristics while limiting the negative electrode capacity to approximately 400 to 700 mAh / g, which is significantly higher than the capacity of conventional negative electrodes, by adding a small amount of the present negative electrode active material to a mixture mainly composed of a graphite-based active material with a capacity of approximately 340 mAh / g.

[0099] Example 1 The negative electrode active material was prepared as follows. A lump of silicon (0-valent) was pulverized in a dispersant by wet pulverization using a bead mill to obtain a slurry of silicon nanoparticles. This Si nanoparticle slurry was mixed with phenolic resin so that the mass ratio after firing would be Si / C = 0.5 / 0.5. The resulting precursor was dried under reduced pressure and fired at 1100°C for 6 hours in a nitrogen atmosphere to obtain a black solid containing Si and C.

[0100] The resulting black solid was pulverized in a planetary ball mill, and the resulting black powder was subjected to thermal CVD (chemical vapor deposition) to obtain a carbon-coated negative electrode active material. Thermal CVD was performed in a rotary kiln-type reactor in a nitrogen atmosphere using LPG (liquid propane gas) as the carbon source. The furnace temperature was 900°C, the pressure was 1 atm, and the CVD time was 360 minutes. The resulting negative electrode active material had an average particle size (D50) of 4.7 μm and a pore size (D50) of 13.2 μm. 2 The amount of carbon coating on the obtained negative electrode active material was 25.7% by TG-DTA, and the true density was 1.91 g / cm 3 The calculated porosity was 7.3%. Next, a half battery was fabricated using the negative electrode active material obtained above, and the charge / discharge characteristics were evaluated. The charge / discharge measurement results showed that the initial coulomb efficiency was 83.2%. The evaluation results are shown in Table 1.

[0101] Example 2 A negative electrode active material was obtained in the same manner as in Example 1, except that the thermal CVD time was set to 120 minutes. The evaluation results are shown in Table 1.

[0102] Example 3 The raw coke was crushed and classified to a D50 of 7.9 μm, and the raw coke particles were mixed with silicon dioxide particles as a carbonaceous phase source and dry granulated. The amount of silicon dioxide particles added was 53% by volume, assuming the sum of the volumes of the silicon dioxide particles and the raw coke particles to be 100%. The amount of silicon dioxide particles added was 61% by mass, assuming the sum of the masses of the silicon dioxide particles and the raw coke particles to be 100%. The granulated particles were then carbonized by firing at 1000°C for 5 hours in a nitrogen atmosphere. The resulting black powder was subjected to thermal CVD (chemical vapor deposition) to obtain a carbon-coated negative electrode active material. A rotary kiln-type reactor was used for thermal CVD, and LPG (liquid propane gas) was used as the carbon source. The furnace temperature was 900°C, the pressure was 1 atm, and the CVD time was 260 minutes. The carbon coating amount of the resulting negative electrode active material was 12.4% by TG-DTA, and the true density was 1.78 g / cm. 3The calculated porosity was 10.9%. Next, a half battery was fabricated using the negative electrode active material obtained above, and the charge / discharge characteristics were evaluated. The charge / discharge measurement results showed that the initial coulomb efficiency was 71.0%. The evaluation results are shown in Table 1.

[0103] (Examples 4 to 6) Negative electrode active materials were obtained in the same manner as in Example 3, except that the thermal CVD time in Example 3 was changed to 200 minutes in Example 4, 300 minutes in Example 5, and 360 minutes in Example 6. The evaluation results of half batteries using the obtained negative electrode active materials are shown in Table 1.

[0104] Example 7 A negative electrode active material was obtained in the same manner as in Example 3, except that the baking temperature was changed to 1200°C and the thermal CVD time was changed to 180 minutes. The evaluation results of a half battery using the obtained negative electrode active material are shown in Table 1.

[0105] (Examples 8 to 10) Negative electrode active materials were obtained in the same manner as in Example 7, except that the thermal CVD time in Example 7 was changed to 240 minutes in Example 8, 320 minutes in Example 9, and 400 minutes in Example 10. The evaluation results of half batteries using the obtained negative electrode active materials are shown in Table 1. Example 11 A negative electrode active material was obtained in the same manner as in Example 1, except that the thermal CVD in Example 1 was not carried out. The evaluation results are shown in Table 1.

[0106] Example 12 A negative electrode active material was obtained in the same manner as in Example 3, except that the thermal CVD in Example 3 was not carried out. The evaluation results are shown in Table 1.

[0107] Example 13 A negative electrode active material was obtained in the same manner as in Example 7, except that the thermal CVD of Example 7 was not carried out. Table 1 shows the evaluation results of a half battery using the obtained negative electrode active material.

[0108] Example 14 A negative electrode active material was obtained in the same manner as in Example 3, except that the reaction time of the thermal CVD in Example 3 was changed to 80 minutes. The evaluation results of a half battery using the obtained negative electrode active material are shown in Table 1.

[0109] Example 15 A negative electrode active material was obtained in the same manner as in Example 7, except that the reaction time of the thermal CVD was changed to 90 minutes. The evaluation results of a half battery using the obtained negative electrode active material are shown in Table 1.

[0110] (Comparative Example 1) A negative electrode active material with a carbon coating was obtained by thermal CVD (chemical vapor deposition) on SiO particles with an average particle size of 5 μm. A rotary kiln-type reactor was used for thermal CVD, and LPG (liquid propane gas) was used as the carbon source. The furnace temperature was 900°C, the pressure was 1 atm, and the CVD time was 180 minutes. Because the SiO particles were not embedded in the carbonaceous phase, XRD measurements did not reveal any diffraction peaks attributed to the d002 plane. TG-DTA indicated that the amount of carbon coating on the negative electrode active material was 6.1%, and the true density was 2.23 g / cm. 3 The porosity calculated from this was 1.3%. Table 1 shows the evaluation results of a half battery using the obtained negative electrode active material.

[0111] (Comparative Example 2) A negative electrode active material was obtained in the same manner as in Comparative Example 1, except that the thermal CVD time was changed to 150 minutes. Since the SiO particles were not embedded in the carbonaceous phase in the obtained negative electrode active material, no diffraction peaks attributed to the d002 plane were obtained in XRD measurement. The amount of carbon coating was 5.0% by TG-DTA, and the true density was 2.24 g / cm. 3 The porosity calculated from this was 1.2%. Table 1 shows the evaluation results of a half battery using the obtained negative electrode active material.

[0112] (Comparative Example 3) When the SiO particles of Comparative Example 1 were evaluated, the true density was 2.32 g / cm 3In the obtained negative electrode active material, the SiO particles were not embedded in the carbonaceous phase, so no diffraction peaks attributed to the d002 plane were obtained in the XRD measurement. The evaluation results of a half battery using the obtained negative electrode active material are shown in Table 1.

[0113] [Table 1]

[0114] BET (specific surface area): Measured by nitrogen adsorption measurement using a specific surface area analyzer (BELSORP-mini, manufactured by BELJAPAN). Nitrogen adsorption amounts at liquid nitrogen temperature and below a relative pressure of 0.5 were determined at multiple points, and the specific surface area was calculated from the BET plot in the range where the heat of adsorption C value was positive and highly linear.

[0115] Measurement of d002 plane spacing: Using an Ultima IV manufactured by Rigaku Corporation, a CuKα X-ray source, a goniometer for reflection, and measurements at 2θ in the range of 1 to 70° were performed. The spacing of the carbon 002 plane of the negative electrode active material of the present invention can be evaluated as follows. Specifically, the negative electrode active material containing a carbonaceous phase was placed in a sample holder, and an X-ray diffraction pattern was obtained using CuKα radiation as the radiation source. The peak positions in the X-ray diffraction pattern were determined using 2θ values, and the wavelength of CuKα radiation was set to 0.15418 nm, and the 002 plane spacing of the carbon phase was calculated using the Bragg formula shown below. d002=λ / 2·sinθ

[0116] Thermogravimetric temperature: Using a Rigaku differential thermogravimetric analyzer (ThermoPLUSEVO2), 10 mg of negative electrode active material was placed on an alumina pan and heated to 1000°C at a rate of 10°C / min under a dry air flow of 200 ml / min. The thermogravimetric change during heating was measured, and the weight loss was calculated as a negative value and the weight gain as a positive value. The temperature at which the weight loss changed to a weight gain was defined as the weight gain onset temperature.

[0117] Thermal weight loss rate: Using a differential thermogravimetric analyzer (ThermoPLUSEVO2) manufactured by Rigaku, 10 mg of negative electrode active material was placed on an alumina pan and heated to 1000°C at a rate of 10°C / min in a dry air flow of 200 ml / min, and the change in thermal weight during heating was measured. The weight loss rate was calculated by subtracting the weight percentage at which the weight loss rate reached its minimum from the weight percentage at which the weight loss began, which is thought to be the temperature at which the attached moisture evaporated (100°C or higher).

[0118] True density: Measurement was performed using a true density measuring device (Ultrapyc 5000 micro, manufactured by Anton Paar) using helium gas at a temperature of 25°C and a measurement pressure of 115 kPa.

[0119] Porosity: Calculated based on the above formula (1). ρ'' was 1.6 (g / cm 3 ) was calculated as

[0120] Battery characteristic evaluation: Battery characteristics were measured using a secondary battery charge / discharge tester (manufactured by Hokuto Denko Corporation), and the initial coulombic efficiency was determined as follows at room temperature of 25° C. and in the cutoff voltage range of 0.005 to 1.5 V.

[0121] Initial Coulombic efficiency of negative electrode active material: Electrochemical evaluation was carried out and calculated as follows. A half battery for evaluation using the electrode active material was assembled as follows, and the charge / discharge characteristics were measured. First, the negative electrode active material (8 parts) was mixed with 1 part of the conductive additive acetylene black and 1 part of the organic binder, and stirred for 10 minutes in a planetary rotor mixer to prepare a negative electrode slurry. The organic binder was a mixture of 0.75 parts of styrene-butadiene copolymer rubber (commercially available SBR), 0.25 parts of carboxymethyl cellulose (CMC), and 10 parts of distilled water. This was applied to a 20 μm-thick copper foil using an applicator and then dried under reduced pressure at 110°C to obtain a thin electrode film approximately 40 μm thick. A circular electrode with a diameter of 14 mm was punched out and pressed under a pressure of 20 MPa. In a glove box with an oxygen concentration of less than 10 ppm and a moisture content of -40°C or less as a dew point, a Li foil counter electrode was placed opposite the electrode of the present invention via a 25 μm polypropylene separator, and an electrolyte (Kishida Chemical, 1 mol / L LiPF6, diethyl carbonate:ethylene carbonate = 1:1 (volume ratio)) was adsorbed to prepare a half battery (CR2032 type) for evaluation. Battery characteristics were measured using a secondary battery charge / discharge tester (manufactured by Hokuto Denko Corporation). The battery was tested for charge / discharge characteristics under the following conditions: room temperature 25°C, cutoff voltage range 0.005 to 1.5 V, charge / discharge rate 0.1 C for the first three cycles, and 0.2 C from the fourth cycle onwards. The battery was left in an open circuit for 30 minutes between each charge / discharge cycle. The initial coulombic efficiency was calculated as follows: Initial coulombic efficiency (%) = Initial discharge capacity (mAh / g) / Initial charge capacity (mAh / g)

[0122] As is clear from the above results, the secondary battery using this negative electrode active material has excellent initial coulombic efficiency.

Claims

1. A negative electrode active material comprising at least one of Si nanoparticles or silicon oxide particles and a carbonaceous phase, wherein the carbonaceous phase embeds at least one of the Si nanoparticles or the silicon oxide particles, and wherein the carbonaceous phase has a lattice spacing of carbon 002 planes of 0.34 nm to 0.38 nm as determined by XRD measurement, and a specific surface area of ​​0.01 m 2 / g to 20 m 2 / g.

2. 2. The negative electrode active material according to claim 1, wherein the weight increase starting temperature is 550[deg.] C. or higher as determined by TG analysis under a dry air flow.

3. 2. The negative electrode active material according to claim 1, comprising 0.1% by weight to 80% by weight of a silicon-based material.

4. 2. The negative electrode active material according to claim 1, wherein the average particle size is from 0.5 μm to 10 μm.

5. It has a carbon coating, and has a porosity of 7% or more and 20% or less, and a true density of 1.6 g / cm, as defined by the following formula (1): ３ 2.0g / cm or more ３ The negative electrode active material according to claim 1 , wherein: (In formula (1), V is the porosity (%), ρ is the density (g / cm) of the negative electrode active material. ３ ), ρ' is the density of the entire negative electrode active material (g / cm ３ ), ρ″ is the density of the carbon coating (g / cm ３ ), and A represents the amount of carbon coating (mass %).

6. A secondary battery comprising the negative electrode active material according to claim 1.

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