JP2013201058A - Anode active material for nonaqueous electrolyte secondary battery, and nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide anode active material for a nonaqueous electrolyte secondary battery in which a non-graphite carbon coating film is formed on a surface of a particulate graphite to reduce initial resistance.SOLUTION: Anode active material for a nonaqueous electrolyte secondary battery comprises a particulate graphite and a non-graphite carbon coating film. A lattice fringe is detected on the non-graphite carbon coating film by STEM observation. A DDTA integrated value obtained by integrating values on a differential curve of DTA curve (DDTA curve) within a range between 620°C and 640°C, is 8.7-13.0 μV/s when carrying out TG-DTA measurement on the anode active material. When obtaining an approximate equation, y=a×Q, on obtained data within a range of Q=4πsinθ/λ=0.0015-0.01 nmby carrying out X-ray small angle scattering measurement on the anode active material (where, y denotes scattering intensity, θ denotes 1/2 of scattering angle, λ denotes wavelength of the X-ray, and a and b are constants), a fractal dimension denoted by 6-b is 2.175-2.230.

Description

  The present invention relates to a negative electrode active material for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery using the same.

  In non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries, particulate graphite has been widely used as the negative electrode active material. In order to suppress the reactivity with the electrolyte, an amorphous carbon film is provided on the surface of the particulate graphite.

For example, Patent Document 1 has a specific surface area of 200 to 200 when sintered on the surface of particulate graphite for the purpose of providing a non-aqueous electrolyte secondary battery excellent in adhesion and load characteristics of the negative electrode. A negative electrode active for a non-aqueous electrolyte secondary battery, which is coated with a substance which becomes 500 m ² / g, a molecular weight of 300 to 500 amorphous carbon, and whose coating amount is 0.1 to 10% by mass with respect to the graphite. A substance is disclosed (claim 1).

JP 2009-211818

The Crystallographic Society of Japan 41., p.213-226 (1999)

When an amorphous carbon film is formed on the surface of particulate graphite, the performance is improved as compared with particulate graphite alone.
In the nonaqueous electrolyte secondary battery, it is desirable that the initial resistance is low. The state of the carbon film is thought to affect the initial resistance, but the factors involved in the initial resistance are unknown. Since the amount of carbon coating is small, it is difficult to evaluate the state itself.

  The present invention has been made in view of the above circumstances, clarifying the factors involved in the initial resistance in the non-graphitic carbon coating, and the non-graphitic carbon coating is formed on the surface of the particulate graphite, thereby reducing the initial resistance. An object of the present invention is to provide a negative electrode active material for a non-aqueous electrolyte secondary battery.

The negative electrode active material for the non-aqueous electrolyte secondary battery of the present invention is
A negative electrode active material for a non-aqueous electrolyte secondary battery having a particulate graphite and a non-graphite carbon coating formed on the surface of the particulate graphite,
The non-graphitic carbon coating is one in which lattice fringes are confirmed by scanning transmission electron microscope (STEM) observation,
When the TG-DTA measurement of the negative electrode active material is performed, the DDTA total value in the range of 620 to 640 ° C. of the differential curve of the obtained DTA (Differential Thermal Analysis) curve is 8.7 to 13.0 μV / s,
And,
X-ray small angle scattering measurement of the negative electrode active material was performed, and an approximate expression y = a · Q ^−b of the obtained data was determined in the range of Q = 4πsin θ / λ = 0.015 to 0.01 nm ⁻¹ . (Where y is the scattering intensity, θ is 1/2 the scattering angle, λ is the X-ray wavelength, and a and b are constants), the fractal dimension represented by 6-b is 2.175. It is the negative electrode active material for nonaqueous electrolyte secondary batteries which are -2.230.

  The non-aqueous electrolyte secondary battery of the present invention includes a positive electrode, a negative electrode including the negative electrode active material for the non-aqueous electrolyte secondary battery, and a non-aqueous electrolyte.

  According to the present invention, the non-graphite carbon coating has been clarified as to the factors involved in the initial resistance, and the non-graphite carbon coating is formed on the surface of the particulate graphite, so that the initial resistance can be reduced. A negative electrode active material for a secondary battery can be provided.

In general X-ray small angle scattering measurement, it is a graph which shows the relationship of the reciprocal of particle size, scattering intensity, and the particle information obtained. It is an example of a measurement of a DTA curve and a DDTA curve. It is an example of a measurement of X-ray small angle scattering. In Examples 1-3 and Comparative Examples 1-5, it is a graph which shows the relationship between a DDTA total value and an initial stage resistance ratio. It is a graph which shows the fractal dimension of Examples 1-3 and Comparative Examples 1-5.

Hereinafter, the present invention will be described in detail.
The present invention relates to a negative electrode active material for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery using the same.

"Negative electrode active material"
The negative electrode active material for the non-aqueous electrolyte secondary battery of the present invention is
A negative electrode active material for a non-aqueous electrolyte secondary battery having a particulate graphite and a non-graphite carbon coating formed on the surface of the particulate graphite,
The non-graphitic carbon coating is one in which lattice fringes are confirmed by scanning transmission electron microscope (STEM) observation,
When the TG-DTA measurement of the negative electrode active material was performed, the total DDTA value in the range of 620 to 640 ° C. of the differential curve (DDTA curve) of the obtained DTA (Differential Thermal Analysis) curve was 8.7 to 13.0 μV / s,
And,
X-ray small angle scattering measurement of the negative electrode active material was performed, and an approximate expression y = a · Q ^−b of the obtained data was determined in the range of Q = 4πsin θ / λ = 0.015 to 0.01 nm ⁻¹ . (Where y is the scattering intensity, θ is 1/2 the scattering angle, λ is the X-ray wavelength, and a and b are constants), the fractal dimension represented by 6-b is 2.175. It is the negative electrode active material for nonaqueous electrolyte secondary batteries which are -2.230.

  In nonaqueous electrolyte secondary batteries such as lithium ion secondary batteries, carbon materials capable of inserting and extracting lithium ions and the like are widely used as negative electrode active materials. In particular, graphite having high crystallinity has characteristics such as a flat discharge potential, high true density, and good filling properties. Therefore, many negative electrode active materials for commercially available lithium ion secondary batteries are used. It is used as

In the negative electrode active material of the present invention, a non-graphitic carbon film is formed on the surface of particulate graphite in order to suppress reactivity with the electrolyte.
As described in the “Background Art” section, conventionally, the non-graphite carbon coating formed on the surface of particulate graphite is generally “amorphous”.
In the present invention, the non-graphite carbon film formed on the surface of the particulate graphite is not “amorphous” but has “low crystallinity” lower than that of graphite. The negative electrode active material itself having such a non-graphite carbon film having low crystallinity is novel.

The “crystallinity of the carbon material” can be evaluated by STEM observation. When lattice fringes are confirmed in the STEM image of the carbon material, it can be specified that the material has crystallinity.
In the present invention, the low crystalline non-graphite carbon film formed on the surface of particulate graphite has lattice fringes observed in the STEM image, and the crystallite size in the a-axis direction is small, or the lattice fringes are waved. For example, the order of lattice fringes is lower than that of graphite.

By using a negative electrode active material having a non-graphite carbon coating having low crystallinity, the present inventor has a lower initial resistance than using a conventional negative electrode active material having an amorphous non-graphite carbon coating. It has been found that a non-aqueous electrolyte secondary battery can be obtained.
It is considered that the non-graphite carbon coating has low crystallinity, thereby improving the properties such as the conductivity of the coating and reducing the initial resistance.
Note that if the non-graphite carbon coating becomes too high in crystallinity and is equivalent to or very close to that of graphite, there will be no difference from the case of particulate graphite alone, surface irregularities will be too small, and reaction with the electrolyte will occur. It is considered that the site where ions that bear electric conduction such as lithium ions enter is reduced.
Therefore, it is important that the non-graphitic carbon coating has a “low crystallinity” lower than that of graphite.

  The present inventor further found a new factor related to the initial resistance, and by optimizing this, in the negative electrode active material having a carbon film having low crystallinity in which lattice fringes are confirmed by STEM observation, It was found that the initial resistance of the non-aqueous electrolyte secondary battery can be reduced to the level.

In the TG-DTA measurement, as the temperature rises, each particle of the negative electrode active material starts to burn from the surface side. Therefore, in the DTA curve, the first peak derived from the non-graphite carbon film appears on the relatively low temperature side, and the second peak derived from the particulate graphite appears on the relatively high temperature side.
Here, since the amount of the non-graphite carbon coating is much smaller than that of the particulate graphite, the first peak derived from the non-graphite carbon coating is much more than the second peak derived from the particulate graphite. It will be small.

In the DTA curve, a broad peak may appear in a relatively low temperature region of room temperature to about 500 ° C. due to impurities or the like, but this broad peak is ignored.
The first peak on the relatively low temperature side derived from the non-graphite carbon coating and the second peak on the relatively high temperature side derived from the particulate graphite usually appear at about 500 ° C. or more. In the negative electrode active material of the present invention, the first peak on the relatively low temperature side derived from the non-graphite carbon film appears in the range of 620 to 640 ° C., for example, and the first peak on the relatively high temperature side derived from the particulate graphite. The peak of 2 appears at 700 ° C. or higher, for example.

The present inventor performed TG-DTA by changing the production conditions of the negative electrode active material, and as a result of performing the evaluation under various conditions, a differential curve (DDTA curve) of the DTA curve was obtained. When the DDTA total value in the range of 620 to 640 ° C. in which the first peak on the low temperature side appears is obtained, this DDTA total value is related to the initial resistance of the non-aqueous electric field secondary battery, and the DDTA total It was found that when the value was 8.7 to 13.0 μV / s, the initial resistance was improved to a higher level (see “Example”, FIG. 4 to FIG. 5, Table 1 to Table 2 below). reference). The total value of DDTA is particularly preferably 9.0 to 12.5 μV / s.
By the above evaluation, it is possible to analyze the first peak on the low temperature side, which appears relatively small derived from the non-graphite carbon coating, in the DTA curve to a higher degree.
Note that obtaining a differential curve (DDTA curve) of a DTA curve is a conventionally known method, but an evaluation method for obtaining a DDTA total value within a specific temperature range in a DDTA curve is a novel method that has not been found in the past.

There seems to be a correlation between the thermal stability and crystallinity of the non-graphitic carbon coating. The higher the crystallinity of the non-graphitic carbon coating, the higher its thermal stability. In the DTA curve, the first peak on the relatively low temperature side derived from the non-graphitic carbon coating appears on the higher temperature side, and The peak tends to be sharper and the peak area tends to be larger. As the crystallinity of the non-graphitic carbon coating increases, the DDTA total value tends to increase.
It is considered that the higher the DDTA combined value and the higher the crystallinity of the non-graphitic carbon coating, the more improved the properties such as the conductivity of the coating and the lower the initial resistance.

Although it is effective to evaluate the crystallinity of the non-graphite carbon coating by the above DDTA total value, this evaluation alone is insufficient and the effect of reducing the initial resistance may not be sufficiently exhibited.
If the crystallinity of the non-graphite carbon coating becomes too high and is equivalent to or very close to that of graphite, there will be no difference from the case of particulate graphite alone, surface irregularities will be too small, and reactivity with the electric field will be reduced. It is considered that the effect of suppressing the decrease in the number of ions and the site where ions responsible for electrical conduction such as lithium ions enter are reduced.
Therefore, the non-graphitic carbon coating preferably has higher crystallinity while having “low crystallinity” lower than that of graphite.

The inventor has also focused on the fractal dimension that can be evaluated by X-ray small angle scattering measurement. Information on the fractal interface of the non-graphite carbon coating can be obtained by the fractal dimension, and the surface irregularity of the coating can be satisfactorily evaluated. The present inventor performs X-ray small angle scattering measurement by changing the production conditions of the negative electrode active material. As a result of evaluation under various conditions, when an approximate expression y = a · Q ^−b of the obtained data is obtained in a range of Q = 4π sin θ / λ = 0.015 to 0.01 nm ⁻¹ (here Where y is the scattering intensity, θ is 1/2 of the scattering angle, λ is the X-ray wavelength, a and b are constants), and the fractal dimension represented by 6-b is a non-aqueous electric field secondary battery. It was found that the initial resistance is improved to a higher level when the fractal dimension is 2.175 to 2.230 (see [Example], FIG. 4). -See Fig. 5, Table 1-Table 2).

The approximate expression y = a · Q ^−b is obtained, for example, by performing the type of approximation “y = a · x ^−b ” on the obtained data using the spreadsheet software Excel.

  In general, in X-ray small angle scattering measurement, information such as particle shape or particle interface state can be acquired. In general X-ray small angle scattering measurement, the reciprocal of the particle size, the scattering intensity, and the obtained particle information have the relationship shown in the left graph of FIG. 1 (cited from Non-Patent Document 1). The k value on the horizontal axis in this graph corresponds to the inverse of the particle size R, L, or ε. For the particle sizes R, L, and ε, see the right figure in FIG. The Q value in the X-ray small angle scattering measurement is the k value described above.

The present inventor has found that the information on the fractal interface of the non-graphitic carbon coating can be obtained satisfactorily by obtaining the fractal dimension with the Q value in the above range (0.0015 to 0.01 nm ⁻¹ ).

As the surface irregularity of the non-graphitic carbon coating is larger and the fractal dimension is larger, the effect of reducing the initial resistance tends to appear more. This is considered to be due to an increase in the sites where ions responsible for electrical conduction such as lithium ions enter.
However, if the surface irregularity of the non-graphitic carbon coating becomes too large, the crystallinity is expected to be amorphous or close to that of graphite, and the performance tends to decrease.

  The present inventor defines the DDTA summation value in the range of 8.7 to 13.0 μV / s and defines the fractal dimension in the range of 2.175 to 2.230, thereby achieving a higher level. It has been found that the effect of reducing the initial resistance can be stably obtained (see the section “Example”, FIG. 4 to FIG. 5 and Table 1 to Table 2).

The negative electrode active material of the present invention is a carbon material such as pitch produced using oil such as heavy oil, polymer such as polyvinyl alcohol (PVA), coal, petroleum, etc. as a raw material with respect to the surface of particulate graphite. A coating material containing an additive such as a solvent can be coated by a vapor phase method or a liquid phase method, if necessary, and fired in an inert atmosphere.
Examples of the vapor phase method include a CVD (Chemical Vapor Deposition) method.
Examples of the inert atmosphere at the time of firing include an N ₂ atmosphere, a rare gas atmosphere such as Ar, and combinations thereof.

  For example, the DDTA total value and the fractal dimension can be adjusted by adjusting the manufacturing conditions such as the firing profile.

  For example, after firing at a first temperature of 800 ° C. or higher, the firing profile is preferably returned to room temperature (about 20 to 30 ° C.) and then fired at a second temperature higher than the first temperature. The present inventor has found that a negative electrode active material having a DDTA total value and a fractal dimension defined in the present invention can be obtained by using such a firing profile (see the section “Examples” below, FIG. 4 to FIG. 4). 5, see Table 1-Table 2).

In order to form a film having low crystallinity, baking at 900 ° C. or higher is necessary. Moreover, when the maximum firing temperature becomes too high, the disappearance of the carbon material tends to increase.
For example, the first temperature is preferably in the range of 800 to 900 ° C, and the second temperature is preferably in the range of 900 to 1000 ° C.
The difference between the first temperature and the second temperature is not particularly limited, and is preferably 5 ° C. or higher. In the section of “Examples” to be described later, the difference between the first temperature and the second temperature is set within the range of 5 to 25 ° C., and the initial resistance reduction result is obtained favorably.

The amount of non-graphite carbon coating (particulate amount) with respect to particulate graphite is not particularly limited. If the coating amount is too small, the effect of suppressing the reactivity with the electrolyte is not sufficiently exhibited. If the coating amount is too large, a non-graphite carbon layer may be formed only with non-graphite carbon, which may increase the non-uniformity at the macro level, resulting in failure to obtain desired battery characteristics, or a thick coating. There is a risk that the initial resistance will increase due to too much.
The amount of the non-graphitic carbon coating (covering amount) with respect to 100 parts by mass of particulate graphite is preferably 2 to 7 parts by mass, for example.

  A gas phase method is particularly preferable as the coating method because the non-graphitic carbon can be coated uniformly and uniformly on the particulate graphite without forming a lump with only the non-graphitic carbon without coating the graphite.

The BET specific surface area of the negative electrode active material by the N ₂ adsorption method is not particularly limited.
The larger the BET specific surface area, the more active points and the initial resistance tends to decrease, but the battery capacity tends to decrease.
The BET specific surface area by the N ₂ adsorption method of the negative electrode active material is preferably ^{2 to 5} m ² / g, for example.

"Nonaqueous electrolyte secondary battery"
The non-aqueous electrolyte secondary battery of the present invention includes a positive electrode, a negative electrode containing the negative electrode active material for the non-aqueous electrolyte secondary battery of the present invention, and a non-aqueous electrolyte.

  Examples of the non-aqueous electrolyte secondary battery include a lithium ion secondary battery. Hereinafter, main components will be described by taking a lithium ion secondary battery as an example.

<Positive electrode>
The positive electrode can be manufactured by applying a positive electrode active material to a positive electrode current collector such as an aluminum foil by a known method.
Known no particular limitation on the positive electrode active material, for _{example, LiCoO 2, LiMnO 2, LiMn} 2 O 4, LiNiO 2, LiNi x Co (1-x) O 2, and _{LiNi x Co y Mn (1-} x-y ₎ Lithium-containing composite oxides such as O ₂ are listed.
For example, using a dispersant such as N-methyl-2-pyrrolidone, the above positive electrode active material, a conductive agent such as carbon powder, and a binder such as polyvinylidene fluoride (PVDF) are mixed to form a slurry. This slurry can be applied onto a positive electrode current collector such as an aluminum foil, dried, and pressed to obtain a positive electrode.

<Negative electrode>
The negative electrode containing the negative electrode active material for the nonaqueous electrolyte secondary battery of the present invention is used.
As needed, you may use together well-known negative electrode active materials other than the negative electrode active material for said nonaqueous electrolyte secondary batteries of this invention.
The negative electrode active material that can be used in combination is not particularly limited, and those having a lithium storage capacity of 2.0 V or less on the basis of Li / Li + are preferably used. Negative electrode active materials that can be used in combination include lithium metal, lithium alloys, transition metal oxides / transition metal nitrides / transition metal sulfides capable of doping / dedoping lithium ions, and combinations thereof. Etc.

The negative electrode can be produced by applying a negative electrode active material to a negative electrode current collector such as a copper foil by a known method.
For example, using a dispersant such as water, the negative electrode active material described above, a binder such as a modified styrene-butadiene copolymer latex, and a thickener such as carboxymethyl cellulose Na salt (CMC) as necessary. Mixing is performed to obtain a slurry, and this slurry is applied onto a negative electrode current collector such as a copper foil, dried, and pressed to obtain a negative electrode.

<Nonaqueous electrolyte>
As the non-aqueous electrolyte, known ones can be used, and liquid, gel-like or solid non-aqueous electrolytes can be used.
For example, a lithium-containing electrolyte is dissolved in a mixed solvent of a high dielectric constant carbonate solvent such as propylene carbonate or ethylene carbonate and a low viscosity carbonate solvent such as diethyl carbonate, methyl ethyl carbonate, or dimethyl carbonate. A water electrolysis solution is preferably used.
As the mixed solvent, for example, a mixed solvent of ethylene carbonate (EC) / dimethyl carbonate (DMC) / ethyl methyl carbonate (EMC) is preferably used.
Examples of the lithium-containing electrolyte include LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , Li ₂ SiF ₆ , LiOSO ₂ C _k F _{(2k + 1)} (k = 1 to 8), LiPF _n {C _k F _{(2k + 1) )} (6-n) (} n = 1~5 integer, k = 1 to 8 integer) lithium salts such as, and combinations thereof.

<Separator>
The separator may be a film that electrically insulates the positive electrode and the negative electrode and is permeable to lithium ions, and a porous polymer film is preferably used.
As the separator, for example, a porous film made of polyolefin such as a porous film made of PP (polypropylene), a porous film made of PE (polyethylene), or a laminated porous film of PP (polypropylene) -PE (polyethylene) is preferably used. It is done.

<Case>
A well-known case can be used.
The secondary battery type includes a cylindrical type, a coin type, a square type, a film type, and the like, and a case can be selected according to a desired type.

  As described above, according to the present invention, the factors involved in the initial resistance in the non-graphite carbon coating are clarified, the non-graphite carbon coating is formed on the surface of the particulate graphite, and the initial stage of the non-aqueous electrolyte secondary battery A negative electrode active material for a non-aqueous electrolyte secondary battery capable of reducing resistance can be provided.

  Examples and comparative examples according to the present invention will be described.

[Examples 1-3, Comparative Examples 1-5]
In Examples 1 to 3 and Comparative Examples 1 to 5, lithium ion secondary batteries were manufactured under the same conditions except that the manufacturing conditions of the negative electrode active material were changed.

<Positive electrode>
LiNi _1/3 Co _1/3 Mn _1/3 O ₂ was used as the positive electrode active material.
Using N-methyl-2-pyrrolidone as a dispersant, mixing the above positive electrode active material, acetylene black as a conductive agent, and PVDF as a binder at 93/4/3 (mass ratio), An electrode layer forming paste was obtained.
The electrode layer forming paste was applied to both surfaces of an aluminum foil as a current collector by a doctor blade method, dried at 150 ° C. for 30 minutes, and pressed using a press machine to form an electrode layer.
As described above, a positive electrode was obtained. The positive electrode layer had a basis weight of 30 mg / cm ² and a density of 2.8 g / cm ³ per side.

<Negative electrode>
In each example, natural graphite having an average particle diameter (nominal diameter) of 10 μm and a BET specific surface area of 3.5 m ² / g was used as the particulate graphite.
In any example, the surface of the particulate graphite was coated with a pitch as a coating material by a CVD method, and fired in an N ₂ atmosphere to produce a negative electrode active material. The coating amount of the non-graphite carbon coating on the particulate graphite was 5% by mass.

In each example, the negative electrode active material was manufactured under the same conditions except that the firing profile was changed.
In Comparative Example 1 (Sample A), the temperature is raised from room temperature (about 25 ° C.) to 900 ° C. at a temperature rising rate of 10 ° C./min, held at this temperature for 10 hours, and naturally cooled to room temperature (about 25 ° C.). Profile.
In Example 1 (Sample B), it was fired at 900 ° C. for 10 hours, naturally cooled to room temperature (about 25 ° C.), heated to 905 ° C. at a heating rate of 10 ° C./min, and held at this temperature for 10 hours. And a profile that naturally cools to room temperature (about 25 ° C.).
In Comparative Example 2 (Sample C), it was fired at 900 ° C. for 10 hours, naturally cooled to room temperature (about 25 ° C.), heated to 895 ° C. at a heating rate of 10 ° C./min, and held at this temperature for 10 hours. And a profile that naturally cools to room temperature (about 25 ° C.).
In Comparative Example 3 (Sample D), it was fired at 900 ° C. for 10 hours, naturally cooled to room temperature (about 25 ° C.), heated to 890 ° C. at a heating rate of 10 ° C./min, and held at this temperature for 10 hours. And a profile that naturally cools to room temperature (about 25 ° C.).
In Example 2 (Sample E), it was fired at 900 ° C. for 10 hours, naturally cooled to room temperature (about 25 ° C.), heated to 925 ° C. at a heating rate of 10 ° C./min, and held at this temperature for 10 hours. And a profile that naturally cools to room temperature (about 25 ° C.).
In Example 3 (Sample F), it was fired at 800 ° C. for 10 hours, naturally cooled to room temperature (about 25 ° C.), heated to 925 ° C. at a heating rate of 10 ° C./min, and held at this temperature for 10 hours. And a profile that naturally cools to room temperature (about 25 ° C.).
In Comparative Example 4 (Sample G), the temperature was raised from room temperature (about 25 ° C.) to 1000 ° C. at a temperature rising rate of 10 ° C./min, and the profile was maintained at this temperature for 10 hours.
In Comparative Example 5 (Sample H), the temperature was raised from room temperature (about 25 ° C.) to 800 ° C. at a temperature rising rate of 10 ° C./min, and the profile was maintained at this temperature for 10 hours.

In any of these examples, water is used as a dispersant, the negative electrode active material, a modified styrene-butadiene copolymer latex (SBR) as a binder, and a carboxymethyl cellulose Na salt (CMC) as a thickener. Were mixed at 98/1/1 (mass ratio) to obtain a slurry.
The electrode layer forming paste was applied to both surfaces of an aluminum foil as a current collector by a doctor blade method, dried at 150 ° C. for 30 minutes, and pressed using a press machine to form an electrode layer.
A negative electrode was obtained as described above. The negative electrode layer had a basis weight of 18 mg / cm ² and a density of 1.4 g / cm ³ per side.

<Separator>
A commercially available separator made of a PE (polyethylene) porous film was prepared.

<Nonaqueous electrolyte>
A mixed solution of ethylene carbonate (EC) / dimethyl carbonate (DMC) / ethyl methyl carbonate (EMC) = 3/4/3 (volume ratio) was used as a solvent, and LiPF ₆ which is a lithium salt as an electrolyte was 1.1 mol / L. A non-aqueous electrolysis solution was prepared by dissolving at a concentration.

<Manufacture of lithium ion secondary batteries>
Using the above positive electrode, negative electrode, separator, non-aqueous electrolyte, and cylindrical can case, a lithium ion secondary battery was manufactured by a known method.
Table 1 shows the main production conditions for each example.

<BET specific surface area>
About the negative electrode active material obtained in each example, the BET specific surface area was calculated | required by the nitrogen adsorption method.
As the apparatus, Macsorb (HM model-1208) manufactured by MOUNTECH was used.
The evaluation results of the BET specific surface area are shown in Table 1.

<STEM observation>
The negative electrode active material obtained in each example was observed with a scanning transmission electron microscope (STEM).
The negative electrode active material was embedded in an epoxy resin, and a cross section of the active material particles was obtained by FIB (focused ion beam). This was used as a sample. STEM observation is facilitated by filling the resin in the minute voids of the negative electrode active material.
As the apparatus, a scanning transmission electron microscope HD-2700 manufactured by Hitachi High-Technologies Corporation was used. The acceleration voltage was 200 kV.

In each example, the cross-section of the negative electrode active material was observed in a range of 150 nm width × 50 nm depth, and the presence and absence of lattice fringes of the non-graphitic carbon coating and its pattern were observed.
In any of the examples, lattice fringes were observed in the STEM image of the coating. In addition, these lattice fringes were observed to have a lower order of lattice fringes than graphite, such as the crystallite size in the a-axis direction being small, or the lattice fringes waved.

<TG-DTA measurement>
TG-DTA measurement was implemented about the negative electrode active material obtained in each case.
As an apparatus, a smart loader TG-DTA8120 manufactured by Rigaku Corporation was used. The measurement conditions were as follows.
Firing atmosphere: air (supply speed 650 ml / min),
Temperature increase rate: 20 ° C./min,
Measurement temperature range: 30-1000 ° C.

In each example, a differential curve (DDTA curve) of the obtained DTA curve was obtained, and a DDTA total value in the range of 620 to 640 ° C. was obtained.
As a typical example, a DTA curve and a DDTA curve in the negative electrode active material of Example 1 (Sample A) are shown in FIG.
Table 2 shows the evaluation results of TG-DTA.

<X-ray small angle scattering measurement>
The negative electrode active material obtained in each example was subjected to X-ray small angle scattering measurement.
As the apparatus, a small-angle wide-angle X-ray diffraction apparatus manufactured by Rigaku Corporation was used. The measurement conditions were as follows.
X-ray and its wavelength: CuKα 1.541840Å,
Tube voltage: 50 kV,
Tube current: 300 mA,
Measurement range: θ = 0.1-50 °.
In each example, using spreadsheet software Excel, it was obtained by performing an approximation of the type “y = a · x− ^b ” in the range of Q = 4π sin θ / λ = 0.015 to 0.01 nm ⁻¹ . Approximate data y = a · Q ^−b is obtained (where y is the scattering intensity, θ is 1/2 of the scattering angle, λ is the X-ray wavelength, and a and b are constants), 6 The fractal dimension represented by −b was obtained.
As a representative example, a graph showing the relationship between Q and y in the negative electrode active material of Example 1 (Sample A) is shown in FIG.
Table 2 shows the evaluation results of the fractal dimension.

<Initial resistance>
The obtained non-aqueous electrolyte secondary battery was subjected to the charge / discharge test shown in Table 3 to determine the initial resistance.
The initial resistance ratio for the initial resistance of Comparative Example 3 (Sample D) was determined by standardizing the initial resistance data of Comparative Example 3 (Sample D) as 1.0.
Table 2 shows the evaluation results of the initial resistance in each example.

<Summary of results>
In Examples 1 to 3 and Comparative Examples 1 to 5, the relationship between the DDTA total value and the initial resistance ratio is shown in FIG. The fractal dimension of Examples 1-3 and Comparative Examples 1-5 is shown in FIG. In FIG. 5, “◯” is attached to the data of Examples 1 to 3.
As shown in FIG. 4 to FIG. 5 and Table 1 to Table 2, a negative electrode active material having a DDTA total value of 8.7 to 13.0 μV / s and a fractal dimension of 2.175 to 2.230 is used. In Examples 1 to 3, lithium ion secondary batteries having an initial resistance lower than those of Comparative Examples 1 to 5 could be obtained.

  The negative electrode active material of the present invention can be preferably applied to a nonaqueous electrolyte secondary battery such as a lithium ion secondary battery mounted on a plug-in hybrid vehicle (PHV) or an electric vehicle (EV).

Claims (5)

A negative electrode active material for a non-aqueous electrolyte secondary battery having a particulate graphite and a non-graphite carbon coating formed on the surface of the particulate graphite,
The non-graphitic carbon coating is one in which lattice fringes are confirmed by scanning transmission electron microscope (STEM) observation,
When the TG-DTA measurement of the negative electrode active material was performed, the total DDTA value in the range of 620 to 640 ° C. of the differential curve (DDTA curve) of the obtained DTA (Differential Thermal Analysis) curve was 8.7 to 13.0 μV / s,
And,
X-ray small angle scattering measurement of the negative electrode active material was performed, and an approximate expression y = a · Q ^−b of the obtained data was determined in the range of Q = 4πsin θ / λ = 0.015 to 0.01 nm ⁻¹ . (Where y is the scattering intensity, θ is 1/2 the scattering angle, λ is the X-ray wavelength, and a and b are constants), the fractal dimension represented by 6-b is 2.175. The negative electrode active material for nonaqueous electrolyte secondary batteries which is -2.230.   2. The negative electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the amount of the non-graphite carbon coating is 2 to 7 parts by mass with respect to 100 parts by mass of the particulate graphite. The negative electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the negative electrode active material has a BET specific surface area of ² to 5 m ² / g by N ₂ adsorption method.   The nonaqueous electrolyte secondary battery provided with the positive electrode, the negative electrode containing the negative electrode active material for nonaqueous electrolyte secondary batteries in any one of Claims 1-3, and a nonaqueous electrolyte.   The nonaqueous electrolyte secondary battery according to claim 4, which is a lithium ion secondary battery.

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