WO2024048051A1

WO2024048051A1 - Negative electrode active material and secondary battery

Info

Publication number: WO2024048051A1
Application number: PCT/JP2023/024118
Authority: WO
Inventors: 貴規馬場; 達彦荒井; 敢武久; 賢一川瀬
Original assignee: Dic株式会社
Priority date: 2022-09-01
Filing date: 2023-06-29
Publication date: 2024-03-07

Abstract

The present invention provides: a negative electrode active material that yields a secondary battery with high initial coulomb efficiency, which is one important property of a secondary battery; and a secondary battery that includes the negative electrode active material and has high initial efficiency.　The negative electrode active material contains Si nanoparticles and/or silicon oxide particles, and a carbonaceous phase. The Si nanoparticles and/or the silicon oxide particles are embedded in the carbonaceous phase, and the interplanar spacing of a carbon 002 plane in the carbonaceous phase as determined using XRD measurement is 0.340 to 0.38 nm.

Description

Negative active materials and secondary batteries

The present invention relates to a negative electrode active material and a secondary battery containing the negative electrode active material.

Non-aqueous electrolyte secondary batteries are used in mobile devices, hybrid cars, electric cars, household storage batteries, etc., and are required to have a good balance of multiple characteristics such as electrical capacity, safety, and operational stability. ing.
As such a secondary battery, a lithium ion battery is mainly known in which a lithium intercalation compound that releases lithium ions from between layers is used as a negative electrode material. For example, various lithium ion batteries using carbonaceous materials such as graphite as negative electrode active materials, which can intercalate and release lithium ions between crystal planes during charging and discharging, are being developed and put into practical use.
Furthermore, in recent years, with the miniaturization of various electronic devices and communication devices and the rapid spread of hybrid vehicles, etc., batteries with higher capacity and various characteristics such as cycle characteristics and discharge rate characteristics are being used as power sources for driving these devices. There is a strong need for the development of secondary batteries with further improved performance.

As one attempt to improve the performance of secondary batteries, a negative electrode active material for non-aqueous electrolyte secondary batteries having 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 electrical conductivity because at least a portion of the surface of the silicon compound is coated with a carbon film. Furthermore, by setting the specific surface area of the carbon film within a specific range, the impregnability of the battery electrolyte becomes good, and by setting the compression resistivity of the carbon film within a specific range, the surface of the negative electrode active material particles can be improved. It is believed that the conductivity of the oxide film is sufficient and that fine precipitation of Li due to concentration of power on the surface is less likely to occur.

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 includes a silicon material region and a carbon material region made of a carbon material formed at least in part around the silicon material region with a gap in between, and powder X-ray diffraction using Cu-Kα rays. A negative electrode material for a battery is described in which the (002) average interlayer spacing d002 of the carbon material region determined by a method is 0.365 nm or more and 0.390 nm or less. It is described that the structure of Patent Document 2 effectively suppresses the expansion and contraction of silicon during charging and discharging, and provides a secondary battery with improved specific capacity and cycle durability.

Patent Document 3 discloses a lithium sheet containing silicon nanoparticles and a silicon-based inorganic compound containing them, having a ²⁹ Si-NMR peak attributed to the bonding structural unit of SiC ₄ and having an equivalent composition ratio within a specific range. An ion secondary battery negative electrode active material is disclosed. It is described that the resulting secondary battery has improved charge/discharge characteristics.

Patent Document 4 discloses a core-shell composite particle which is a non-porous shell having a matrix of carbon in which silicon particles are enclosed in pores having a specific diameter as a core and amorphous carbon as a shell. ing. It is stated that the resulting secondary battery exhibits high coulombic efficiency and more stable electrochemical behavior in subsequent cycles.

Japanese Patent Application Publication No. 2016-164870 JP 2019-125435 Publication JP 2021-114483 Publication Patent No. 6523484

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 obtained secondary battery is still not sufficient, and further improvement of the negative electrode active material is required.

The present inventors focused on Si nanoparticles or silicon oxide particles and the carbonaceous phase that embeds them, and efficiently suppressed the expansion and contraction of the Si volume due to charge and discharge, while improving the electronic conductivity of the negative electrode active material. We have conducted various studies aimed at improving this, and have completed the present invention.
That is, an object of the present invention is to provide a negative electrode active material that provides a secondary battery that has a large capacity per weight, which is one of the important properties of a secondary battery, and has excellent initial Coulombic efficiency.

The present invention has the following aspects.
[1]
It contains at least one of Si nanoparticles or silicon oxide particles and a carbonaceous phase, the carbonaceous phase embeds at least one of the Si nanoparticles or the silicon oxide particles, and the carbonaceous phase A negative electrode active material whose interplanar spacing between carbon 002 planes determined by XRD measurement is from 0.34 nm to 0.38 nm.
[2]
The negative electrode active material according to the above [1], which has a mass reduction rate of 10 to 70% at 100 to 800°C as determined by TG analysis under dry air circulation.
[3]
The negative electrode active material according to [1] or [2] above, which has a weight increase start temperature of 550° C. or higher as determined by TG analysis under dry air circulation.
[4]
The negative electrode active material according to any one of [1] to [3] above, containing 0.1% to 19% by weight of a silicon-based material.
[5]
The negative electrode active material according to any one of [1] to [4] above, which has a specific surface area (BET) of 0.01 m ² /g to 20 m ² /g.
[6]
The negative electrode active material according to any one of [1] to [5] above, having an average particle diameter (D50) of 0.5 μm to 10 μm.
[7]
From [1] above, which has a carbon coating, has a porosity defined by the following formula (1) of 7% or more and 20% or less, and a true density of 1.6 g/cm ³ or more and 2.0 g/cm ³ or less. 6].

(In formula (1), V is the porosity (%), ρ is the density inside the negative electrode active material (g/cm ³ ), ρ' is the density of the entire negative electrode active material (g/cm ³ ), and ρ'' is The density of the carbon film (g/cm ³ ), A represents the amount of the carbon film (% by mass), respectively)

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] above.

According to the present invention, 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 are provided. provided.

Note that 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, and the carbonaceous phase is the Si nanoparticle. The interplanar spacing of carbon 002 planes determined by XRD measurement of the carbonaceous phase after embedding the particles is 0.34 nm to 0.38 nm.

Electrochemical reactions caused by charging and discharging secondary batteries can be roughly 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 on the surface of the solvent, electrolyte, and active material in the electrolyte. This side reaction generates SEI (Solid Electrolyte Interface), which reduces the initial efficiency of the secondary battery. It is thought that when this negative electrode active material is used in a secondary battery, electrochemical side reactions on the surface of the active material are suppressed and the initial Coulombic efficiency of the secondary battery is increased.

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

Here, the average particle size refers to a volume average particle size, and is a D50 value that 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 small diameter side, this is the particle size when the cumulative volume is 50%.

Large-sized Si nanoparticles exceeding 300 nm form large clumps, and when used as a negative electrode active material, they tend to become pulverized during charging and discharging, so it is assumed that the capacity retention rate of the negative electrode active material tends to decrease. On the other hand, since Si nanoparticles having a small size of less than 10 nm are too fine, the Si nanoparticles tend to aggregate with each other. Therefore, the dispersibility of Si nanoparticles into the negative electrode active material may be reduced. Furthermore, if the Si nanoparticles are too fine, their surface activation energy will be high, and by-products will tend to increase on the surface of the Si nanoparticles during high-temperature firing of the negative electrode active material. These may lead to a decrease in charge/discharge performance.
From the above-mentioned viewpoint, it is preferable that the Si nanoparticles are within the range of the above-mentioned average particle diameter, and that the number of large-sized Si nanoparticles exceeding 300 nm and small-sized Si nanoparticles less than 10 nm is as small as possible.

The Si nanoparticles can be produced by pulverizing Si lumps to form nanoparticles. Due to the presence of the Si nanoparticles, the charge/discharge capacity and initial Coulombic efficiency can be improved when the present negative electrode active material is used as a secondary battery.
The Si nanoparticles can be obtained by, for example, pulverizing a zero-valent silicon lump so that the average particle size falls within the above range.
Examples of the pulverizer used for pulverizing the Si lump into nanoparticles include a ball mill, a bead mill, a jet mill, and the like. Further, the pulverization may be wet pulverization using an organic solvent. As the organic solvent, for example, alcohols, ketones, etc. can be suitably used, but aromatic substances such as toluene, xylene, naphthalene, methylnaphthalene, etc. Group hydrocarbon solvents 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, and grinding time, and classifying them.

The shape of the Si nanoparticles is not particularly limited, but from the viewpoint of charge and 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, it is preferable that the so-called aspect ratio, which is the ratio of thickness to length, is 0.5 or less.
The average particle size of Si nanoparticles can be measured using dynamic light scattering, but it is also possible to determine the morphology of Si nanoparticles by using analysis methods such as transmission electron microscopy (TEM) and field emission scanning electron microscopy (FE-SEM). , samples with the above aspect ratio can be identified more easily and precisely. When the present negative electrode active material contains the Si nanoparticles, the Si nanoparticles can be obtained 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. It is possible to identify the state of
Note that the aspect ratio of the Si nanoparticles is a calculation result based on 50 particles of the sample present in the main part within the field of view reflected in the TEM image.

The specific surface area of the Si nanoparticles is preferably 100 m ² /g to 400 m ² /g from the viewpoint of electric capacity and initial Coulombic efficiency.
The specific surface area of the Si nanoparticles is more preferably from 100 m ² /g to 300 m 2 /g, and even more preferably from 100 ^{m 2} ^/ g to 230 m ² /g, from the viewpoint of electric capacity and initial Coulombic efficiency.
Note that the specific surface area is a value determined by the BET method, and can be determined by nitrogen gas adsorption measurement, for example, by using a specific surface area measuring device.
The specific surface area of Si nanoparticles can be measured as follows. That is, the amount of nitrogen adsorption at a relative pressure of liquid nitrogen temperature of 0.5 or less is determined at multiple points, and the specific surface area is calculated from a BET plot in a range where the heat of adsorption C value is positive and linearity is high.

The shape of the Si nanoparticles may be granular, acicular, or flaky, but crystalline is preferable. 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 nm to 14 nm from the viewpoint of initial Coulombic efficiency and capacity retention rate. The crystallite diameter is more preferably 12 nm or less, and even more preferably 10 nm or less.

The silicon oxide particles are generally a general term for amorphous silicon oxide particles obtained by cooling and precipitating silicon monoxide gas generated by heating a mixture of silicon dioxide and metal silicon, and the following: It is represented by general formula (1).
SiOn (1)
However, in the formula (1), n is 0.4 or more and 1.8 or less, preferably 0.5 or more and 1.6 or less.

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 become large lumps, and when the present negative electrode active material is used as a negative electrode, the silicon oxide particles are formed during charging and discharging. Large expansion and contraction of the negative electrode active material occurs. As a result, stress is concentrated in a part of the carbonaceous phase, which tends to cause structural collapse of the negative electrode active material, and the capacity retention rate of the negative electrode active material tends to decrease.
On the other hand, silicon oxide particles having a small size of less than 300 nm are too small, and therefore tend to aggregate together. Therefore, the dispersibility of silicon oxide particles into the negative electrode active material may be reduced. Furthermore, if the silicon oxide particles are too fine, their specific surface area will increase, and by-products and the like will tend to increase on the surface of the silicon oxide particles during high-temperature firing of the negative electrode active material. These may lead to a decrease in charge/discharge performance.

Therefore, from the above point of view, the average particle diameter of the silicon oxide particles is preferably 3 μm or less, more preferably 2 μm or less. Further, from the viewpoint of particle dispersibility and specific surface area, the average particle diameter of the silicon oxide particles is preferably 300 nm or more, more preferably 200 nm or more.
Here, the average particle diameter is the value of D50 as described above. D50 is as described above.

The silicon oxide particles can be made into particles by, for example, pulverizing silicon oxide so that the average particle size falls within the above range.
Examples of the pulverizer used for pulverization include pulverizers such as a ball mill, a bead mill, and a jet mill. Further, the pulverization may be wet pulverization using an organic solvent. As the organic solvent, for example, alcohols, ketones, etc. can be suitably used, but aromatic substances such as toluene, xylene, naphthalene, methylnaphthalene, etc. Group hydrocarbon solvents can also be used.
The average particle size of the silicon oxide particles can be controlled within the above range by classifying the obtained silicon oxide particles by controlling the bead mill conditions such as the bead particle size, blending ratio, rotation speed, or grinding time.

The shape of the silicon oxide particles may be granular, acicular, or flaky.
The morphology of silicon oxide particles can be determined by measuring the average particle size using dynamic light scattering, but it is also possible to measure the average particle size using a transmission electron microscope (TEM) or field emission scanning electron microscope (FE-SEM). , samples with the above aspect ratio can be identified more easily and precisely. When the present negative electrode active material contains the silicon oxide particles, the Si nanoparticles can be obtained 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. It is possible to identify the state of
Note that the aspect ratio of the silicon oxide particles is a calculation result based on 50 particles of the main part of the sample within the field of view reflected in the TEM image.

The present negative electrode active material only needs to contain at least either the Si nanoparticles or the silicon oxide particles, and may contain both the Si nanoparticles and the silicon oxide particles. The present negative electrode active material preferably contains both the Si nanoparticles and the silicon oxide particles.

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 from the viewpoint of suppressing initial capacity loss and improving initial Coulombic efficiency. .
When the Si nanoparticles have silicon oxide on their surfaces, the surfaces of the Si nanoparticles are preferably coated with a silicon dioxide film, which is a silicon oxide film.

Examples of the carbonaceous phase included in the present negative electrode active material include crystalline carbon and amorphous carbon. Examples of crystalline carbon include natural graphite or artificial graphite, and examples of amorphous carbon include graphitizable carbon and non-graphitizable carbon.
These carbonaceous phases are appropriately selected from the viewpoint of desired performance depending on the intended use. For example, from the viewpoint of the energy density of the resulting secondary battery, it is preferable to select crystalline carbon. On the other hand, from the viewpoint of battery durability due to expansion and contraction of the active material during charging and discharging, it is preferable to select amorphous carbon.
From the viewpoint of the initial efficiency of the secondary battery, the carbonaceous phase is preferably amorphous carbon.

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

The amount of the Si nanoparticles or the silicon oxide particles in the present negative electrode active material is from 5% by mass to 70% by mass, with the total amount of the Si nanoparticles or silicon oxide particles and the carbonaceous phase being 100% by mass. It is preferably 10% by mass to 60% by mass. When the present 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 100% by mass; Preferably, the total amount of silicon oxide particles is within the above range. It is preferable that most of these Si nanoparticles or silicon oxide particles are embedded in the carbonaceous phase, and 60% or more of the total volume of the Si nanoparticles or silicon oxide particles is embedded. is more preferable, and even more preferably 90% or more.

The spacing between the carbon 002 planes determined by XRD measurement in the carbonaceous phase can be measured as follows. That is, a sample holder is filled with a negative electrode active material containing a carbonaceous phase, and an X-ray diffraction pattern is obtained using CuKα radiation as a radiation source. The peak position of the X-ray diffraction pattern is determined by the 2θ value, and the carbon phase 002 plane spacing is calculated using Bragg's formula described below, setting the wavelength of the CuKα ray to 0.15418 nm.
d002=λ/2・sinθ
The carbonaceous phase has higher crystallinity as it approaches a state of graphite, and the carbon 002 plane spacing of the carbonaceous phase approaches 0.3354 nm of ideal graphite.
The carbonaceous phase of the present negative electrode active material has an amorphous structure, and the interplanar spacing between carbon 002 planes determined by XRD measurement is from 0.34 nm to 0.38 nm.

Since the interplanar spacing between the carbon 002 planes of the carbonaceous phase is from 0.34 nm to 0.38 nm, the electron conductivity of the negative electrode active material is improved, and isolation of Si nanoparticles due to volume expansion during charging is suppressed. As a result, it becomes possible to reduce capacitance loss of Si nanoparticles.
In addition, since the carbonaceous phase 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 and suppresses the isolation of the negative electrode active material particles due to swelling during charging. It is possible to improve the capacity retention rate when used as a next battery.
The spacing between carbon 002 planes determined by XRD measurement is preferably from 0.345 nm to 0.375 nm, more preferably from 0.350 nm to 0.370 nm, from the viewpoint of Coulomb efficiency.

The existence 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 thermogravimetric loss measured in the presence of air. That is, the amount of carbon in the carbonaceous phase can be determined using TG-DTA. In addition, changes in thermal decomposition temperature behavior such as the decomposition reaction start temperature, the decomposition reaction end temperature, the number of thermal decomposition reaction species, and the temperature of the maximum weight loss for each thermal decomposition reaction species can also be determined by the thermal weight loss behavior obtained from the above measurements. Easy to understand. The state of carbon can be determined using the temperature values of these behaviors.

When the carbon in the carbonaceous phase is amorphous carbon, the carbonaceous phase has properties similar to those of amorphous carbon, so it is thermally decomposed in the temperature range of about 550°C to 900°C in the atmosphere. As a result, a rapid weight loss occurs. The maximum temperature of the TG-DTA measurement conditions is not particularly limited, but in order to completely complete the carbon thermal decomposition reaction, TG-DTA measurements are performed in the atmosphere from about 25°C to about 1000°C or higher. is preferable.

From the viewpoint of structural formation of the negative electrode active material, it is preferable that the mass analysis reduction rate of the present negative electrode active material at 100 to 800° C. by TG analysis under dry air circulation is 10% to 70%.
As described above, the mass spectrometry reduction rate can be determined by performing TG-DTA measurement under the conditions of dry air circulation and at a temperature of 100° C. to 800° C.
The present negative electrode active material preferably has a mass spectrometry reduction rate of 15% to 65%, more preferably 20% to 60%, at 100 to 800° C. by TG analysis under dry air circulation.

This negative electrode active material has a weight increase starting temperature of 550°C or higher according to TG analysis under dry air circulation, which means that there are many carbonaceous layers that oxidize at low temperatures, and as a result, Si nanoparticles and This is preferable from the viewpoint of suppressing electrochemical side reactions on the surface of the active material by delaying the reaction of oxygen, and suppressing a decrease in capacitance loss in the initial stage of the secondary battery.
From the viewpoint of suppressing initial capacity loss and initial coulombic efficiency, the weight increase start temperature is more preferably 575°C or higher, and even more preferably 600°C or higher.

From the viewpoint of electric capacity per weight, the present negative electrode active material preferably contains 0.1 to 80 parts by mass of a silicon-based material based on 100 parts by mass of the present negative electrode active material. Examples of the silicon-based material include silicon, silicon carbide oxide (silicon oxycarbide), and the like. When the negative electrode active material contains Si nanoparticles, the silicon material is preferably silicon, which is different from the nanoparticles. As described later, when the present negative electrode active material has a carbon film, the mass of the present negative electrode active material is the amount including the carbon film.

The specific surface area of the negative electrode active material is preferably 0.01 m ² /g to 20 m ² /g. From the viewpoint of the amount of solvent absorbed during electrode production and the amount of binder used to maintain binding properties, the specific surface area of the present negative electrode active material is preferably 1 m ² /g or more, more preferably 3 m ² /g or more. preferable. Further, the specific surface area of the present negative electrode active material is preferably 18 m ² /g or less, more preferably 10 m ² /g or less.
Note that 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 the present negative electrode active material is determined by determining the amount of nitrogen adsorbed at multiple points at a relative pressure of 0.5 or less at the liquid nitrogen temperature, and from the BET plot, it is found that the heat of adsorption C value is positive and the linearity is high. The specific surface area can be calculated from a high range.

The average particle diameter of the present negative electrode active material is preferably from 0.5 μm to 10 μm, more preferably from 2 μm to 8 μm. If the average particle size is too small, as the specific surface area increases significantly, the amount of SEI produced during charging and discharging increases when used as a secondary battery, resulting in a decrease in reversible charge/discharge capacity per unit volume. If the average particle size is too large, there is a risk that the particles will peel off from the current collector during electrode film production. Note that the average particle size is the volume average particle size, as described above, and is the value of D50. The method for measuring D50 is the same as described above.
Further, 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 excluding fine powder particles is preferably 0.5 μm to 30 μm.
The shape of the negative electrode active material may be granular, acicular, or flaky.

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

However, in the above formula (1), V is the porosity (%), ρ is the density inside the negative electrode active material (g/cm ³ ), ρ' is the density of the entire negative electrode active material (g/cm ³ ), ρ'' represents the density of the carbon film (g/cm ³ ), and A represents the amount of the carbon film (% by mass).

The carbon coating preferably covers at least a portion of the surface of the present negative electrode active material. The carbon film is preferably a film 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 film is 0.1% by mass or more and 30% by mass, assuming the mass of the present negative electrode active material including the carbon film as 100% by mass. The following is preferable, 1% by weight or more and 25% by weight or less is more preferable, and even more preferably 5% by weight or more and 20% by weight or less. Further, from the viewpoint of improving the chemical stability and thermal stability of the present active material, the average thickness of the carbon film is preferably 10 nm or more and 300 nm or less.
From the viewpoint of improving the chemical stability and thermal stability of the present negative electrode active material, it is preferable that 1% or more of the surface of the present negative electrode active material has a carbon coating, and 10% or more of the surface of the present negative electrode active material has a carbon coating. It is more preferable to have one. The present negative electrode active material may have a carbon coating continuously or intermittently on its surface.
The carbon film is preferably formed on the surface of the negative electrode active material by chemical vapor deposition.

As mentioned above, although Si nanoparticles have a high capacity, large volume changes occur due to occluding and releasing a large amount of lithium ions, and as a result, it is thought that the Si nanoparticles have poor cyclability. It is thought that this volume change cannot be sufficiently suppressed by the carbon coating alone. Therefore, a method has been proposed in which voids are provided around the Si nanoparticles, the voids buffer volume expansion, and the destruction of the carbon film is suppressed. However, if the voids are not appropriate, the buffering effect will not function sufficiently, and the surface area will increase due to cracks in the active material, which will increase the amount of SEI produced and reduce the initial Coulombic efficiency.
It is also considered necessary to appropriately control the composition of the negative electrode active material as well as the porosity.

It is considered that the conventional definition of porosity does not necessarily reflect the situation of voids around Si nanoparticles appropriately.
The present inventors have found that the porosity defined by the above formula (1) appropriately reflects the state of the voids around the Si nanoparticles. Furthermore, by using a negative electrode active material with a porosity within a specific range defined by the above formula (1) in a secondary battery, the increase in surface area due to cracking of the active material and the generation of SEI are suppressed, and the initial Coulombic efficiency is reduced. It has been discovered that an improved secondary battery can be obtained.

Each of the densities ρ, ρ′ and ρ″ can 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 film may be determined by peeling off the carbon film from the present negative electrode active material and directly measuring the true density, but it may also be determined by calculation or the like. For example, create several plots of the content (mass%) of the carbon film and the density of the present negative electrode active material, and extrapolate the point where the content of the carbon film becomes 100% by mass using linear approximation. You may also calculate the density of only
Alternatively, the silicon component may be dissolved from the present negative electrode active material and the true density of the undissolved portion may be directly measured.

In the above formula (1), A is the amount of the carbon film, and similarly to the above, it is mass % when the mass of the present negative electrode active material including the carbon film is 100 mass %. The amount of carbon film can be determined by TG-DTA, elemental analysis, etc.

Traditionally, 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 internal voids, and the porosity defined by the above formula is the voids excluding the internal voids of the particles. As described above, in order to suppress destruction of active material particles having a carbon coating, the gap between the carbon coating and the inside thereof is important, and it is necessary to evaluate the porosity of that portion. Various definitions of porosity have been proposed, including the porosity defined in Patent Document 2, but the conventionally defined porosity may not have a sufficient correlation with density. be.
There is also a method of calculating the porosity by observing the cross section of the particle using an electron microscope, etc., and visually checking the void area, but this method is highly dependent on the cross-sectional area of the particle, and the gap between the carbon coating and its interior can be calculated accurately. It is difficult to ask for

On the other hand, the definition of porosity by the above formula (1) differs from the above conventional method by introducing the amount and density of the carbon film into the formula (1) in the negative electrode active material having a carbon film. The porosity of the gap between the 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 insertion of lithium ions. Further, the porosity, V, is preferably 18% or less, more preferably 17% or less, from the viewpoint of improving the energy density of the obtained secondary battery.

The carbon film is preferably formed on the surface of the negative electrode active material by chemical vapor deposition. In order to bring the true density, porosity, and V of the negative electrode active material within the ranges described above, for example, the gas flow rate, treatment time, and treatment temperature are controlled when carbon coating treatment is performed.

When the porosity, V, defined by the formula (1) of the present negative electrode active material is 7% or more and 20% or less, the true density of the present negative electrode active material is 1.6 g/cm ³ or more and 2.0 g/cm ³ The following are preferred. From the viewpoint of improving the energy density of the obtained secondary battery, the true density is more preferably 1.65 g/cm ³ or more, and particularly preferably 1.70 g/cm ³ or more.
Moreover, the true density of the present negative electrode active material is more preferably 1.95 g/cm ³ or less, particularly preferably 1.90 g/cm ³ or less, from the relationship with the porosity and V.
The true density is a value measured using a true density measuring device, and is the pressure generated when the sample chamber containing the sample is pressurized with helium gas and then the valve is opened to diffuse the gas into the expansion chamber. It can be determined by determining the volume of the sample from the change and dividing the volume of the sample by the sample mass.

The present 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.
Silicate compounds are compounds containing anions that generally have a structure centered around one or several silicon atoms and surrounded by electronegative ligands, but they can be formed from Li, K, Na, Ca, Mg, and Al. A silicate compound which is a salt of at least one metal selected from the group consisting of a compound containing the anion and the above-mentioned anion is preferred.
Examples of compounds containing the anion include orthosilicate ion (SiO ₄ ^4- ), metasilicate ion (SiO ₃ ^2- ), pyrosilicate ion (Si ₂ O ₇ ^6- ), and cyclic silicate ion (Si ₃ O ₉ ^{6- )} . or Si ₆ O ₁₈ ^12- ) and other silicate ions are known. The silicate compound is preferably a salt of metasilicate ion and at least one metal selected from the group consisting of Li, K, Na, Ca, Mg and Al. Among the metals, Li or Mg is more preferable.

When the 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 having two or more types of metals, one silicate ion may have multiple types of metals, or may be a mixture of silicate compounds having different metals. Further, the silicate compound may contain other metals as long as it contains at least one metal selected from the group consisting of Li, K, Na, Ca, Mg and Al.
The silicate compound is preferably a lithium silicate compound or a magnesium silicate compound, more preferably lithium metasilicate (Li ₂ SiO ₃ ) or magnesium metasilicate (MgSiO ₃ ), and particularly preferably magnesium metasilicate (MgSiO ₃ ).

If the silicate compound is in a crystalline state, it can be detected by powder X-ray diffraction measurement (XRD), and if it is amorphous, it can be confirmed by solid-state ²⁹ Si-NMR measurement.

Since the present negative electrode active material has the above-mentioned carbonaceous phase, in the Raman spectrum of the carbonaceous phase, the carbon structure has a scattering peak near 1590 cm ^-1 , which is assigned to the G band of the graphite long-period carbon lattice structure, and disturbances and defects. It has a scattering peak near 1330 cm ^-1 that is assigned to the D band of a certain graphite short-period carbon lattice structure, and the scattering peak intensity ratio I (G band/D band) is in the range of 0.7 to 2. It is preferable that there be. The scattering peak intensity ratio I is more preferably from 0.7 to 1.8.

Further, 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 in the range of 0.9 to 1.1. It is preferable that

The present negative electrode active material may contain other components as necessary in addition to the silicon-based material, carbon film, and silicate compound.

The present negative electrode active material can be produced, for example, by a method including steps 1 to 3 below. Note that, although the following process exemplifies a method in which Si nanoparticles are included, the method is not limited to this method. When the present negative electrode active material contains silicon oxide, the Si nanoparticles may be changed to silicon oxide particles in the following step 1, and when the present negative electrode active material contains Si nanoparticles and silicon oxide particles, the Si Nanoparticles and silicon oxide particles may be used. As described above, silicon oxide particles can be produced by heating a mixture of silicon dioxide and metal silicon, cooling silicon monoxide gas, and precipitating it. Alternatively, commercially available silicon oxide may be used. Step 1: A slurry of wet-pulverized Si nanoparticles is mixed with a carbonaceous phase source, stirred and dried to obtain a precursor. Step 2: The precursor obtained in Step 1 is fired in an inert atmosphere at a maximum temperature within the range of 1000°C to 1180°C to obtain a fired product. Step 3: The fired product obtained in Step 2 is pulverized to obtain the present negative electrode active material.

Each step will be explained below.
<Step 1>
(Si (zero valent) slurry)
The wet-milled Si (zero-valent) slurry used in step 1 can be prepared by using an organic solvent and grinding silicon particles in a wet powder mill. A dispersant may be used to promote the pulverization of silicon particles in an organic solvent. The wet grinding device is not particularly limited, and examples thereof include a roller mill, a high-speed rotary grinder, a container-driven mill, a bead mill, and the like.
In wet pulverization, it is preferable to pulverize silicon particles until they become Si nanoparticles.

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 aromatic compounds such as benzene, toluene, and xylene.

As for the type of dispersant mentioned above, aqueous or non-aqueous dispersants can be used. In order to suppress excessive oxidation of the surface of silicon particles, it is preferable to use a non-aqueous dispersant. Types of non-aqueous dispersants include polymer types such as polyethers, polyalkylenepolyamines, and polycarboxylic acid partial alkyl esters, low-molecular types such as polyhydric alcohol esters and alkylpolyamines, and polyphosphates. The inorganic type is exemplified. The concentration of silicon in the Si (zero-valent) slurry is not particularly limited, but if the solvent and, if necessary, a dispersant are included, the total amount of the dispersant and Si particles is 100% by mass, and the amount of Si particles is 5% by mass. The range is preferably from 10% to 40% by weight, more preferably from 10% to 30% by weight.

(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.

Examples of synthetic resins include thermoplastic resins such as polyvinyl alcohol and polyacrylic acid, and thermosetting resins such as phenol resins and furan resins. Natural chemical raw materials include coke and heavy oil, and tar pitches in particular include coal tar, light tar oil, medium tar oil, heavy tar oil, naphthalene oil, anthracene oil, coal tar pitch, pitch oil, mesophase pitch, and oxygen crosslinking. Examples include petroleum pitch and heavy oil.

(precursor)
After uniformly mixing and stirring the carbonaceous phase source and the Si slurry, the precursor of the present negative electrode active material (hereinafter also referred to as "precursor") is obtained through desolvation and drying. The mixing is performed using a device having dispersion and mixing functions. Examples include a stirrer, an ultrasonic mixer, a premix disperser, and the like. In the desolvation and drying operations aimed at distilling off the organic solvent, a dryer, a vacuum dryer, a spray dryer, etc. can be used.

Preferably, the precursor contains 3% to 97% by mass of Si nanoparticles, which are Si (0 valent), and 3% to 97% by mass of the solid content of the carbonaceous phase source, and the solid content of silicon particles. It is more preferable that the solid content of the carbon source resin is 20% to 80% by mass and the solid content of the carbon source resin is 20% to 80% by mass. By subjecting the precursor of the negative electrode active material to the heat treatment described below, the mass may decrease and the ratio of nanosilicon in the negative electrode active material may change, so the content of Si nanoparticles in the precursor may It may be set as appropriate based on the content of Si nanoparticles in the present negative electrode active material.

<Step 2>
Step 2 is to sinter the precursor obtained in Step 1 above in an inert atmosphere at a maximum temperature range of 1000°C to 1180°C to completely decompose the thermally decomposable organic components and remove other components. This is a process in which the main components are made into a fired product suitable for the present negative electrode active material by precisely controlling the firing conditions. Specifically, the raw carbonaceous phase source is converted to free carbon by the energy of the high temperature treatment. That is, by firing, a matrix containing a fired product of the carbonaceous phase source is obtained. The term "fired product" as used herein refers to a product whose composition or structure has partially or completely changed due to decomposition or conversion of an organic compound such as a carbonaceous phase source at high temperatures.
In the fired product of the carbonaceous phase source, all of the carbonaceous phase source may be converted to carbon, or a part of the carbonaceous phase source may be converted to carbon and the remainder may maintain the structure of the carbonaceous phase source.

In step 2, the precursor obtained in step 1 is fired in an inert atmosphere according to a firing program defined by the temperature increase rate, the holding time at a constant temperature, etc. The maximum temperature reached is the maximum temperature to be set, and it strongly influences the structure and performance of the present negative electrode active material, which is a fired product. In the present invention, by achieving a maximum temperature of 1000°C to 1180°C, the fine structure of the negative electrode active material can be precisely controlled, and oxidation of silicon particles due to excessively high temperature firing can be avoided, resulting in better charge and discharge characteristics. can get.

The firing method is not particularly limited, but a reaction device having a heating function in an inert atmosphere may be used, and continuous or batch processing is possible. As for the firing device, a fluidized bed reactor, a rotary furnace, a vertical moving bed reactor, a tunnel furnace, a batch furnace, a rotary kiln, etc. can be appropriately selected depending on the purpose.

<Step 3>
Step 3 is a step of obtaining the present negative electrode active material by pulverizing the fired product obtained in Step 2 and classifying as necessary. Further, step 3 is a step of forming a carbon film on the surface of the negative electrode active material by chemical vapor deposition, if necessary. The pulverization may be carried out in one stage to reach the desired particle size, or may be carried out in several stages. For example, if the fired product is a lump or agglomerated particle of 10 mm or more and you want to make an active material of 10 μm, coarsely crush it with a jaw crusher, roll crusher, etc. to make particles of about 1 mm, and then use a glow mill, ball mill, etc. to make 100 μm particles. Then, pulverize to 10 μm using a bead mill, jet mill, etc. Particles produced by pulverization may contain coarse particles, and in order to remove them, or to remove fine particles to adjust the particle size distribution, classification is performed. The classifier used is a wind classifier, a wet classifier, etc. depending on the purpose, but when removing coarse particles, a classification method that passes through a sieve is preferable because it can reliably achieve the purpose. Incidentally, if the shape of the precursor mixture is controlled to be around the target particle diameter by spray drying or the like before firing, and the main firing is performed in that shape, it is of course possible to omit the pulverization step.

In the manufacturing process, by controlling the gas flow rate and optimizing the conditions such as treatment time and treatment temperature when performing the carbon coating treatment, the true density of the present negative electrode active material and the density defined by the above formula (1) can be achieved. The porosity can be within the above range. For example, by increasing the gas flow rate and processing time, the amount of carbon to be coated can be increased, and the porosity defined by the above formula (1) can be adjusted. Further, by increasing the processing temperature, the true density can be increased.

Furthermore, in the manufacturing process, by controlling the firing temperature, the carbonaceous phase of the present negative electrode active material can have the interplanar spacing and specific surface area of the carbon 002 plane determined by XRD measurement within the above range. For example, when the firing temperature is increased, the carbonization reaction progresses, and the spacing between the carbon 002 planes becomes narrower.

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, it can be obtained by mixing a slurry of Si nanoparticles with a carbonaceous phase source. A salt of at least one metal selected from the group consisting of Li, K, Na, Ca, Mg, and Al is added to the suspension, and then the active material containing the silicate compound is added by the same operation as above. is obtained.
Salts of at least one metal selected from the group consisting of Li, K, Na, Ca, Mg and Al include halides, hydroxides, carbonates, etc. of these metals such as fluorides, chlorides, and bromides. Can be mentioned.

The metal salt may be a salt of two or more metals, one salt may contain a plurality of metals, or a mixture of salts containing different metals.
When adding the metal salt to the suspension, the amount of the metal salt added is preferably from 0.01 to 0.4 in molar ratio to the number of moles of Si nanoparticles.
When the metal salt is soluble in an organic solvent, the metal salt is dissolved in the organic solvent to form a suspension of the carbonaceous phase source, or a suspension of the Si nanoparticles when the active material particles contain Si nanoparticles. Just add it to the suspension and mix. If the metal salt is insoluble in the organic solvent, the particles of the metal salt are dispersed in the organic solvent, and then a suspension of the carbonaceous phase source or, if the active material particles contain Si nanoparticles, the Si nanoparticles are prepared. can be added to the suspension and mixed. The metal salt is preferably nanoparticles having an average particle size of 100 nm or less from the viewpoint of improving the dispersion effect. As the organic solvent, alcohols, ketones, etc. can be suitably used, but aromatic hydrocarbon solvents such as toluene, xylene, naphthalene, and methylnaphthalene can also be used.

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

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

Since the present negative electrode active material has excellent initial coulombic efficiency, a secondary battery containing the present negative electrode active material and used as a battery negative electrode exhibits good charge/discharge characteristics.
Specifically, a slurry composed of the present negative electrode active material, an organic binder, and other components such as conductive additives as necessary is applied as a thin film onto a current collector copper foil as a negative electrode. Can be used. Moreover, a negative electrode can also be produced by adding a carbon material to the slurry.
Examples of the carbon material include natural graphite, artificial graphite, and amorphous carbon such as hard carbon or soft carbon.

This negative electrode active material and a binder, which is an organic binder, are kneaded together with a solvent using a dispersion device such as a stirrer, ball mill, super sand mill, pressure kneader, etc. to prepare a negative electrode material slurry, and this is used as a current collector. The negative electrode layer can be obtained by applying the negative electrode layer to the negative electrode layer. Alternatively, it can be obtained by forming a paste-like negative electrode material slurry into a sheet, pellet, or the like, and integrating this with a current collector. Since the negative electrode obtained as described above contains the present negative electrode active material, it becomes a negative electrode for a secondary battery having excellent initial Coulombic efficiency. The negative electrode can be prepared by, for example, kneading the present negative electrode active material and a binder, which is an organic binding material, with a solvent using a dispersion device such as a stirrer, a ball mill, a super sand mill, or a pressure kneader to form a negative electrode material slurry. It can be obtained by preparing a negative electrode layer and applying it to a current collector to form a negative electrode layer. Alternatively, it can be obtained by forming a paste-like negative electrode material slurry into a sheet, pellet, or the like, and integrating this with a current collector.

Examples of the organic binder include styrene-butadiene rubber copolymer (SBR); methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, (meth)acrylonitrile, and hydroxyethyl (meth)acrylate. Unsaturated carboxylic acids such as (meth)acrylic copolymers consisting of ethylenically unsaturated carboxylic acid esters such as acrylates, and ethylenically unsaturated carboxylic acids such as acrylic acid, methacrylic acid, itaconic acid, fumaric acid, and maleic acid. Copolymers; high molecular compounds such as polyvinylidene fluoride, polyethylene oxide, polyepichlorohydrin, polyphosphazene, polyacrylonitrile, polyimide, polyamideimide, and carboxymethyl cellulose (CMC) can be mentioned.

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 ratio of the organic binder in the negative electrode layer of the lithium ion secondary battery negative electrode is preferably 1% by mass to 30% by mass, more preferably 2% by mass to 20% by mass, and 3% by mass. More preferably, the amount is from 15% by mass.

When the content ratio of the organic binder is 1% by mass or more, adhesion is better, and destruction of the negative electrode structure due to expansion and contraction during charging and discharging is further suppressed. On the other hand, when the content is 30% by mass or less, the increase in electrode resistance can be further suppressed.
Within this range, the negative electrode active material of the present invention has high chemical stability and can also be used as an aqueous binder, making it easy to handle in terms of practical use.

Furthermore, a conductive additive may be mixed into the negative electrode material slurry, if necessary. Examples of the conductive additive include carbon black, graphite, acetylene black, and oxides and nitrides exhibiting conductivity. The amount of the conductive aid used may be about 1% by mass to 15% by mass based on the negative electrode active material of the present invention.

Regarding the material and shape of the current collector, for example, a band-like material made of copper, nickel, titanium, stainless steel, etc., into a foil shape, perforated foil shape, mesh shape, etc. may be used. Porous materials such as porous metal (foamed metal) and carbon paper can also be used.

Examples of the method for applying the negative electrode material slurry to the current collector include a metal mask printing method, an electrostatic coating method, a dip coating method, a spray coating method, a roll coating method, a doctor blade method, a gravure coating method, and a screen printing method. Examples include. After coating, it is preferable to perform a rolling treatment using a flat plate press, a calendar roll, etc., if necessary.

Furthermore, the negative electrode material slurry can be formed into a sheet or pellet form, and the current collector can be integrated with the slurry by, for example, a roll, a press, or a combination thereof.

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 water-based styrene-butadiene rubber copolymer (SBR) is used, heat treatment at 100 to 130°C is sufficient; when an organic binder with a main skeleton of polyimide or polyamide-imide is used, Preferably, the heat treatment is performed at a temperature of 150 to 450°C.

Through this heat treatment, the solvent is removed and the binder is hardened to increase its strength, and the adhesion between the particles and between the particles and the current collector can be improved. Note that these heat treatments are preferably performed in an inert atmosphere such as helium, argon, nitrogen, etc., or a vacuum atmosphere in order to prevent oxidation of the current collector during the treatment.

Further, after the heat treatment, the negative electrode is preferably pressed (pressure treated). In the negative electrode using the negative electrode active material of the present invention, the electrode density is preferably 1 g/cm ³ to 1.8 g/cm 3 , more preferably 1.1 g/ ^{cm 3} ^to 1.7 g/cm ³ . It is preferably 1.2 g/cm ³ to 1.6 g/cm ³ . Regarding electrode density, the higher the density, the better the adhesion and the volumetric capacity density of the electrode tend to be. However, if the density is too high, the voids in the electrode are reduced, which weakens the effect of suppressing the volumetric expansion of silicon, etc., and reduces the capacitance. Select the optimal range as retention rates will decrease.

A negative electrode containing the present negative electrode active material has excellent initial Coulombic efficiency and is therefore suitable for use in secondary batteries. As a secondary battery having such a negative electrode, a non-aqueous electrolyte secondary battery and a solid electrolyte secondary battery are preferable, and particularly they exhibit excellent performance when used as a negative electrode of a non-aqueous electrolyte secondary battery.

For example, when a secondary battery containing the negative electrode active material of the present invention is used in a wet electrolyte secondary battery, a positive electrode and a negative electrode containing the negative electrode active material of the present invention are placed facing each other with a separator interposed therebetween, and an electrolyte is poured into the secondary battery. It can be constructed by injection.

The positive electrode can be obtained by forming a positive electrode layer on the surface of the current collector in the same manner as the negative electrode. In this case, the current collector may be a band-shaped object made of metal or alloy such as aluminum, titanium, stainless steel, etc., in the form of foil, perforated foil, mesh, or the like.

The positive electrode material used for the positive electrode layer is not particularly limited. Among nonaqueous electrolyte secondary batteries, when producing a lithium ion secondary battery, for example, metal compounds, metal oxides, metal sulfides, or conductive polymer materials that can be doped or intercalated with lithium ions are used. You can use For example, lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ), lithium manganate (LiMnO ₂ ), and composite oxides thereof (LiCoxNiyMnzO ₂ , x+y+z=1), lithium manganese spinel (LiMn ₂ O ₄ ). , _lithium vanadium compound, _V2O5 , _V6O13 , _VO2 , _MnO2 , _TiO2 _, _MoV2O8 _, _TiS2 _, _V2S5 , _VS2 , _MoS2 _, _MoS3 , _Cr3O8 , Cr ₂ O ₅ , olivine-type LiMPO ₄ (M: Co, Ni, Mn, Fe), conductive polymers such as polyacetylene, polyaniline, polypyrrole, polythiophene, polyacene, porous carbon, etc. are used alone or in combination. be able to.

As the separator, for example, a nonwoven fabric, a cloth, a microporous film, or a combination thereof, which is mainly composed of polyolefin such as polyethylene or polypropylene, can be used. Note that if the structure of the non-aqueous electrolyte secondary battery to be manufactured is such that the positive electrode and negative electrode do not come into direct contact with each other, there is no need to use a separator.

Examples of the electrolytic solution include lithium salts such as LiClO ₄ , LiPF ₆ , LiAsF ₆ , LiBF ₄ , and LiSO ₃ CF ₃ , ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, fluoroethylene carbonate, cyclopentanone, and sulfolane. , 3-methylsulfolane, 2,4-dimethylsulfolane, 3-methyl-1,3-oxazolidin-2-one, γ-butyrolactone, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methylpropyl carbonate, butyl methyl carbonate, ethyl For non-aqueous solvents such as propyl carbonate, butylethyl carbonate, dipropyl carbonate, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, methyl acetate, ethyl acetate, etc. alone or in a mixture of two or more components. Dissolved, so-called organic electrolytes can be used.

The structure of a secondary battery containing the present negative electrode active material is not particularly limited, but usually a positive electrode, a negative electrode, and a separator provided as necessary are wound into a flat spiral shape to form a wound type electrode plate group. It is common that these plates are laminated in a flat plate form to form a laminated electrode plate group, and these electrode plate groups are enclosed in an exterior body. Note that the half cell used in the examples of the present invention has a configuration in which the present negative electrode active material is the main component for the negative electrode, and a simple evaluation is performed using metallic lithium for the counter electrode, but this is a simple evaluation based on the cycle characteristics of the active material itself. This is for a clear comparison. Adding a small amount of this negative electrode active material to a mixture mainly composed of graphite-based active material (capacity of approximately 340 mAh/g) increases the negative electrode capacity to approximately 400 to 700 mAh/g, which greatly exceeds existing negative electrode capacity, and improves cycle characteristics. It is possible to do so.

The secondary battery containing the present negative electrode active material is used as, but not limited to, a paper type battery, a button type battery, a coin type battery, a stacked type battery, a cylindrical type battery, a square type battery, etc. The present negative electrode active material described above can also be applied to general electrochemical devices whose charging/discharging mechanism is insertion and extraction of lithium ions, such as hybrid capacitors and solid lithium secondary batteries.

As described above, the negative electrode active material of the present invention has high initial efficiency, which is one of the important properties of a secondary battery, and provides a secondary battery with excellent balance of battery characteristics. therefore. This negative electrode active material can be suitably used for secondary batteries.

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 embodiments described above.
In the configuration of the present negative electrode active material and the secondary battery having the present negative electrode active material, any other configuration may be added to the configuration of the above embodiment, or any configuration that exhibits the same function may be substituted. good.

EXAMPLES Hereinafter, the present invention will be explained in detail with reference to Examples, but the present invention is not limited thereto.
In addition, the half cell used in the examples of the present invention was subjected to a simple evaluation using the present negative electrode active material as the negative electrode and metallic lithium as the counter electrode, but this was done to more clearly compare the cycle characteristics of the active material itself. It's for a reason. With this configuration, by adding a small amount of this negative electrode active material to a mixture mainly composed of graphite-based active material with a capacity of about 340 mAh/g, it is possible to achieve a negative electrode capacity of about 400 to 700 mAh/g, which greatly exceeds the existing negative electrode capacity. It is possible to improve cycle characteristics while keeping the negative electrode capacity low.

(Example 1)
This negative electrode active material was prepared as follows.
A lump of silicon (zero valence) was pulverized in a dispersant by a wet pulverization method using a bead mill to obtain a slurry of silicon nanoparticles.
This slurry of Si nanoparticles was mixed with a phenol resin so that the composition after firing was Si/C = 0.5/0.5 in mass ratio, and the resulting precursor was dried under reduced pressure in a nitrogen atmosphere. By firing at 1100°C for 6 hours, a black solid containing Si and C was obtained.

The obtained black solid was pulverized with a planetary ball mill, and the obtained black powder was subjected to thermal CVD (chemical vapor deposition) to obtain a negative electrode active material provided with a carbon film. At this time, a rotary kiln type reactor was used for thermal CVD in a nitrogen atmosphere and LPG (liquid propane gas) was used as the carbon source, the temperature in the furnace was 900°C, the pressure was 1 atm, and the CVD time was 360 minutes. did. The obtained negative electrode active material had an average particle size (D50) of 4.7 μm and a specific surface area (BET) of 13.2 m ² /g. The amount of the carbon film of the obtained negative electrode active material was 25.7% from TG-DTA, the true density was 1.91 g/cm ³ , and the porosity calculated therefrom was 7.3%.
Next, a half battery was produced using the negative electrode active material obtained above, and its charge/discharge characteristics were evaluated. The initial Coulombic efficiency was 83.2% from the measurement results of charging and discharging. The evaluation results are shown in Table 1.

(Example 2)
A negative electrode active material was obtained in the same manner as in Example 1 except that the thermal CVD time was 120 minutes. The evaluation results are shown in Table 1.

(Example 3)
The raw coke was crushed and classified to have a D50 of 7.9 μm, and dry granulation was performed by mixing raw coke particles and silicon dioxide particles as a carbonaceous phase source. At this time, the amount of silicon dioxide particles added was 53% by volume when the sum of the volumes of silicon dioxide particles and raw coke particles was taken as 100%. The amount of silicon dioxide particles added is 61% by mass when the sum of the masses of silicon dioxide particles and raw coke particles is taken as 100%. Next, the granulated particles were carbonized by firing at 1000° C. for 5 hours in an elementary atmosphere.
A negative electrode active material provided with a carbon film was obtained by subjecting the obtained black powder to thermal CVD (chemical vapor deposition). At this time, a rotary kiln type reactor was used for thermal CVD, LPG (liquid propane gas) was used as a carbon source, the temperature in the furnace was 900° C., the pressure was 1 atm, and the CVD time was 260 minutes. The amount of carbon film of the obtained negative electrode active material was 12.4% from TG-DTA, the true density was 1.78 g/cm ³ , and the porosity calculated therefrom was 10.9%.
Next, a half battery was produced using the negative electrode active material obtained above, and its charge/discharge characteristics were evaluated. The initial Coulombic efficiency was 71.0% from the measurement results of charging and discharging. The evaluation results are shown in Table 1.

(Example 4 to Example 6)
A negative electrode active material was 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. Table 1 shows the evaluation results of a half battery using the obtained negative electrode active material.

(Example 7)
A negative electrode active material was obtained in the same manner as in Example 3, except that the firing temperature in Example 3 was changed to 1200° C. and the thermal CVD time was changed to 180 minutes. Table 1 shows the evaluation results of a half battery using the obtained negative electrode active material.

(Example 8 to Example 10)
A negative electrode active material was 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. Table 1 shows the evaluation results of a half battery using the obtained negative electrode active material.
(Example 11)
A negative electrode active material was obtained in the same manner as in Example 1, except that the thermal CVD of Example 1 was not performed. The evaluation results are shown in Table 1.

(Example 12)
A negative electrode active material was obtained in the same manner as in Example 3 except that the thermal CVD of Example 3 was not performed. The evaluation results are shown in Table 1.

(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 performed. Table 1 shows the evaluation results of a half battery using the obtained negative electrode active material.

(Example 14)
A negative electrode active material was obtained in the same manner as in Example 3 except that the reaction time of thermal CVD in Example 3 was changed to 80 minutes. Table 1 shows the evaluation results of a half battery using the obtained negative electrode active material.

(Example 15)
A negative electrode active material was obtained in the same manner as in Example 7 except that the thermal CVD reaction time in Example 7 was changed to 90 minutes. Table 1 shows the evaluation results of a half battery using the obtained negative electrode active material.

(Comparative example 1)
A negative electrode active material provided with a carbon film was obtained by performing thermal CVD (chemical vapor deposition) on SiO particles having an average particle size of 5 μm. At this time, a rotary kiln type reactor was used for thermal CVD, LPG (liquid propane gas) was used as a carbon source, the temperature in the furnace was 900° C., the pressure was 1 atm, and the CVD time was 180 minutes. Since the SiO particles of the obtained negative electrode active material were not embedded in the carbonaceous phase, no diffraction peak attributed to the d002 plane was obtained in XRD measurement. The amount of carbon film of the obtained negative electrode active material was 6.1% from TG-DTA, the true density was 2.23 g/cm ³ , and the porosity calculated therefrom was 1.3%. Table 1 shows the evaluation results of a half battery using the obtained negative electrode active material.

(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 in Comparative Example 1 was changed to 150 minutes. Since the SiO particles of the obtained negative electrode active material were not embedded in the carbonaceous phase, no diffraction peak attributed to the d002 plane was obtained in XRD measurement. The amount of carbon film was 5.0% from TG-DTA, the true density was 2.24 g/cm ³ , and the porosity calculated therefrom was 1.2%. Table 1 shows the evaluation results of a half battery using the obtained negative electrode active material.

(Comparative example 3)
When the SiO particles of Comparative Example 1 were evaluated, the true density was 2.32 g/cm ³ . Since the SiO particles of the obtained negative electrode active material were not embedded in the carbonaceous phase, no diffraction peak attributed to the d002 plane was obtained in XRD measurement. Table 1 shows the evaluation results of a half battery using the obtained negative electrode active material.

BET (specific surface area): Measured by nitrogen adsorption measurement using a specific surface area measuring device (manufactured by BELJAPAN, BELSORP-mini). The nitrogen adsorption amount at a relative pressure of liquid nitrogen temperature of 0.5 or less was determined at multiple points, and the specific surface area was calculated from a BET plot in a range where the heat of adsorption C value was positive and linearity was high.

Measurement of the spacing of the d002 plane: Using Ultima IV manufactured by Rigaku Co., Ltd. and CuKα as the X-ray source, measurement was performed using a goniometer as the reflection method, and 2θ was measured in the range of 1 to 70°. The spacing between carbon 002 planes of the negative electrode active material of the present invention can be evaluated as follows. That is, a sample holder is filled with a negative electrode active material containing a carbonaceous phase, and an X-ray diffraction pattern is obtained using CuKα radiation as a radiation source. The peak position of the X-ray diffraction pattern was determined by the 2θ value, and the carbon phase 002 plane spacing was calculated using Bragg's formula described below, setting the wavelength of the CuKα ray to 0.15418 nm.
d002=λ/2・sinθ

Thermogravimetric increase temperature: Using a differential thermal gravimetric analyzer (ThermoPLUSEVO2) manufactured by Rigaku, 10 mg of the negative electrode active material was placed on an alumina pan, and the temperature was increased to 1000°C at a heating rate of 10°C/min at a rate of 200ml/min under a dry air flow. The temperature rose. The thermogravimetric change during temperature rise was measured, and calculations were made so that weight loss was negative and weight increase was positive. The temperature at which the weight changed from a decrease to a weight increase was defined as the weight increase start temperature.

Thermogravimetric reduction rate: Using a differential thermogravimetric analyzer (ThermoPLUSEVO2) manufactured by Rigaku, 10 mg of the negative electrode active material was placed on an alumina pan, and the temperature was increased to 1000°C at a heating rate of 10°C/min at 200ml/min under a dry air flow. The temperature was raised, and the thermogravimetric change during the temperature rise was measured. Calculation was made by subtracting the weight percent at which the weight loss rate was minimum from the weight percent at which the weight loss started at a temperature (100° C. or higher) considered to be the evaporation of attached moisture.

True density: Measured using a true density measuring device (manufactured by Anton Paar, Ultrapyc 5000 micro) using helium as the gas at a temperature of 25° C. and a measuring pressure of 115 kPa.

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

Battery characteristics evaluation: Battery characteristics were measured using a secondary battery charge/discharge test device (manufactured by Hokuto Denko Co., Ltd.), and the initial coulombic efficiency was determined as follows at a room temperature of 25°C and a cutoff voltage range of 0.005 to 1.5V. I asked like this.

Initial Coulombic efficiency of negative electrode active material: Electrochemical evaluation was performed and calculated as below.
A half battery for evaluation using a polar active material was assembled as described below, and its charge/discharge characteristics were measured.
First, the negative electrode active material (8 parts), 1 part of acetylene black as a conductive aid, and 1 part of an organic binder were mixed and stirred for 10 minutes using a rotation-revolution type bubble remover Rentaro. A slurry was prepared. 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 coated onto a copper foil with a thickness of 20 μm using an applicator, and then dried under reduced pressure conditions at 110° C. to obtain an electrode thin film with a thickness of about 40 μm. A circular electrode with a diameter of 14 mm was punched out and pressed under a pressure of 20 MPa. In a glow box with an oxygen concentration of less than 10 ppm and a water content of −40° C. or less as a dew point, the electrode of the present invention was placed opposite a Li foil as a counter electrode through a 25 μm polypropylene separator, and an electrolytic solution (manufactured by Kishida Chemical Co., Ltd., A half battery for evaluation (CR2032 type) was prepared by adsorbing 1 mol/L of LiPF ₆ and diethyl carbonate:ethylene carbonate=1:1 (volume ratio).
Battery characteristics were measured using a secondary battery charge/discharge test device (manufactured by Hokuto Denko Co., Ltd.) at a room temperature of 25°C, a cutoff voltage range of 0.005 to 1.5V, and a charge/discharge rate of 1 to 3 cycles. The charging and discharging characteristics were evaluated under the following conditions: 0.1C, 0.2C after 4th cycle, and constant current/constant voltage charging/constant current discharging. At each charge/discharge switch, the circuit was left open for 30 minutes. The initial coulombic efficiency was determined as follows.
Initial coulombic efficiency (%) = Initial discharge capacity (mAh/g) / Initial charge capacity (mAh/g)

As is clear from the above results, the secondary battery using the present negative electrode active material has excellent initial Coulombic efficiency.

Claims

It contains at least one of Si nanoparticles or silicon oxide particles and a carbonaceous phase, the carbonaceous phase embeds at least one of the Si nanoparticles or the silicon oxide particles, and the carbonaceous phase A negative electrode active material whose interplanar spacing between carbon 002 planes determined by XRD measurement is from 0.34 nm to 0.38 nm.
The negative electrode active material according to claim 1, which has a thermogravimetric reduction rate of 10 to 70% by weight from 100°C to 800°C as determined by TG analysis under dry air circulation.
The negative electrode active material according to claim 1 or 2, which has a weight increase starting temperature of 550° C. or higher as determined by TG analysis under dry air circulation.
The negative electrode active material according to claim 1 or 2, comprising 0.1% to 80% by weight of a silicon-based material.
The negative electrode active material according to claim 1 or 2, having a specific surface area of 0.01 m 2 /g to 20 m 2 /g.
The negative electrode active material according to claim 1 or 2, which has an average particle diameter of 0.5 μm to 10 μm.
Claim 1 or 2, which has a carbon coating, has a porosity defined by the following formula (1) of 7% or more and 20% or less, and a true density of 1.6 g/cm 3 or more and 2.0 g/cm 3 or less. Negative electrode active material as described.

(In formula (1), V is the porosity (%), ρ is the density inside the negative electrode active material (g/cm 3 ), ρ' is the density of the entire negative electrode active material (g/cm 3 ), and ρ'' is The density of the carbon film (g/cm 3 ), A represents the amount of the carbon film (% by mass), respectively)
A secondary battery comprising the negative electrode active material according to claim 1 or 2.