JP3084256B2 - Material for negative electrode of Li-ion secondary battery - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a material for a negative electrode of a Li-ion secondary battery, and more particularly, to an average particle size of graphite powder obtained by heat-treating a carbon material such as coke at a higher temperature to graphitize. At least two kinds of graphite powders different from each other are blended and filled,
The present invention relates to a material for a negative electrode of a Li-ion secondary battery having an increased discharge capacity as a battery.

[0002]

2. Description of the Related Art Generally, batteries are divided into primary batteries and secondary batteries. A primary battery utilizes the chemical action caused by immersing a metal having a different ionization tendency in a liquid in a liquid as electric energy directly,
This primary battery is a battery that has been conventionally ordinarily used. The primary battery cannot be reused when used, and is discarded as it is. In other words, it is a disposable battery.

On the other hand, recently, with the development of portable devices such as video cameras, boomboxes, portable telephones, and headphone stereos, secondary batteries have been used for these devices. A secondary battery is a battery that can be reused by repeatedly discharging and charging, unlike a primary battery. As the small secondary batteries used in these devices, nickel cadmium batteries having a history of more than 30 years are mainly used.

However, nickel cadmium batteries have a low electromotive force of about 1.2 volts and a small electric capacity, and it is very difficult to improve the density of electric energy. Further, the nickel cadmium battery has a drawback that the capacity is reduced when the battery is repeatedly charged without being completely discharged, has a memory effect, and contains a harmful metal, cadmium. Recently, lithium metal was used as a negative electrode material in place of a nickel cadmium battery in a non-aqueous solvent electrolyte to raise the electromotive force to 3.0 volts or more.
i-ion batteries have been developed.

[0005] Compared to nickel cadmium batteries, Li-ion batteries have an electromotive force that is three times or more, have a stable voltage, a large electric capacity, a small self-discharge, a good storability, and have a good storage capacity. It is very effective for portable equipment.

However, when lithium of an alkali metal is used as a negative electrode material, when charge and discharge are repeated, during the charge and discharge, lithium precipitates in a dendrite shape, that is, a dendritic shape in the lithium of the negative electrode material, and the dendrites are formed. When the material grows, the lithium for the negative electrode and the material for the positive electrode are short-circuited, so that the charge / discharge cycle life is short, and the short-circuit causes ignition and explosion.

On the other hand, with the rapid spread of mobile phones and the like in recent years, demands for smaller and higher performance secondary batteries have increased, and nickel-metal hydride batteries having a large electric capacity have been developed.
Production began in earnest in 1990, and in 1995 the production volume exceeded 300 million. Nickel-metal hydride batteries have an electromotive force of 1.2 volts and are equivalent to nickel-cadmium batteries, but have the advantage of not containing harmful metals, but also have a memory effect and have problems such as capacity not keeping up with market requirements. is there.

[0008] In order to avoid obstacles due to dentite precipitation in Li-ion batteries, lithium oxide such as lithium cobalt oxide (LiCoO ₂ ) is used as a positive electrode material,
Instead of using only lithium or a lithium alloy as a negative electrode material, a carbon material is used as a Li ion storage material. In this Li-ion battery, not the conventional chemical reaction, but an intercalation reaction (topochemical reaction) in which Li ions repeat insertion and desorption in the structural gaps of the graphite particles occurs.

This Li-ion secondary battery has an electromotive force of 3.7 volts larger than the secondary battery developed in advance,
Large capacity and light weight, no memory effect. This has led to a rapid increase in production since it was proposed in 1991. However, even in a secondary battery using such a negative electrode material, it is said that the capacity is determined by the Li ion storage capacity of the negative electrode material, and unless the carbon material itself forming a part of the negative electrode material is improved, it is not necessarily required. The battery capacity cannot be increased.

That is, the secondary battery described in Japanese Patent Application Laid-Open No. 1-204361 is configured by charging a granular coke having an average particle size of 20 to 100 μm in a carbon material constituting a negative electrode material. However, in this secondary battery, the capacity of the battery cannot be increased so much because the carbon material of the negative electrode material is made of carbonaceous granular coke.

In the secondary battery described in Japanese Patent Application Laid-Open No. 4-332465, the carbon material for the negative electrode is composed of 100 parts by weight of a graphitized material, an arithmetic average particle diameter of 70 nm or less, and a chain structure diameter of 100 to 500 nm. Non-graphitizable carbon material 2
-15 parts by weight.

However, in this secondary battery, since a part of the carbon material of the negative electrode material is made of a non-graphitized carbon material, the capacity of the battery cannot be increased, and there are the following problems.

That is, in this secondary battery, the particle size of the graphitized material or the carbon material is on the order of nm (n is a prefix of 10 ^-9 ) and is extremely fine. For this reason, in such a secondary battery, the specific surface area of the negative electrode material is large, and if the contact area with the electrolyte is large, the electrolyte is likely to be decomposed due to repeated charge and discharge, and gas generation and internal pressure of the battery are reduced. Rise and have safety issues.

[0014]

SUMMARY OF THE INVENTION The present invention aims to solve the above-mentioned drawbacks, and among the graphite powders which have been graphitized at an extremely high temperature of 2500 to 3000 ° C., the graphite powder having a small average particle size and the graphite powder having a large average particle size are particularly preferred. A negative electrode material obtained by compounding graphite powder and filling the compounded graphite powder, and adjusting the particle size distribution, filling ratio, compounding ratio, particle size, etc. of the graphite powder of the negative electrode material to improve the battery capacity. We propose a material for the negative electrode of a Li-ion secondary battery that has been greatly increased.

[0015]

That is, the material for a negative electrode according to the present invention is a material for a negative electrode used in a Li-ion secondary battery, and is a graphite powder obtained by pulverizing artificial graphite graphitized at 2500 to 3000 ° C. Among them, a negative electrode material formed by blending small-diameter graphite powder having a small average particle diameter and large-diameter graphite powder and filling the blended graphite powder. This negative electrode material is configured as described above and has the following features.

In the graphite powder in the negative electrode material, the filling rate of the graphite powder is represented by the ratio of the bulk density (ρ _B ) to the true density (ρ _T ), and the filling rate (ρ _B / ρ _T ) is 0.1. It is adjusted within the range of 40 to 0.7.

The compounding ratio of the weight of the small-diameter weight of graphite powder (W _A) and the large径黒lead powder in graphite powder in the negative electrode material _{(W B) (W A /} W B) to 0.40-0.45 Has been adjusted.

The particle size distribution of the graphite powder in which the small-diameter graphite powder and the large-diameter graphite powder are blended is as follows: a particle size of 3 μm or less is 5% by volume or less;
The average particle diameter is adjusted to be 5 μm or more, and the ratio of the average particle diameter of the small-diameter graphite powder to the average particle diameter of the large-diameter graphite powder is adjusted to 0.3 or less.

The material for a negative electrode according to the present invention is used for a Li-ion secondary battery. The material for a negative electrode is prepared by using a starting material such as petroleum pitch or coal at 700 ° C.
The carbon material is heat-treated at 1400 ° C.
This is artificial graphite which has been heat-treated at 00 ° C to 3000 ° C to be graphitized. The artificial graphite thus obtained is pulverized and filled with the graphite powder resulting from the pulverization to constitute a negative electrode material.

That is, unlike the carbon material such as coke, the graphite powder is further graphitized, and the graphite crystal is growing. Therefore, the graphite powder constituting the negative electrode material is softer due to graphitization and has a higher true specific gravity than the carbon material whose crystal growth has not yet been developed. For this reason, the filling rate is increased, and the Li ions eluted at the time of discharge are occluded and retained in the surface of each graphitic particle and in a number of internal layers and fine pores communicating with the surface, so-called occlusion capacity is large. And the capacity as a battery is improved.

The carbon material belongs to the graphite system when viewed from the top of the crystal structure, and both the carbon material and the graphite material belong to the same crystal structure. For this reason, in general, the two are not strictly distinguished. However, according to the degree of development of the crystal structure, the carbon material and the graphite material can be distinguished.

[0022] Despite being distinguishable in this way, in the subsequent research, it was pointed out that a clear transformation point was not detected between the carbon material and the graphite material. It is believed that they are not essentially different, and they are now.

Therefore, although the carbon material indicates a material having a lower degree of crystal development than the graphite material, in the conventional example, a secondary material in which carbon material such as coke is used as the material for the negative electrode is used. Batteries have also been proposed.

On the other hand, the carbon material is 2500 ° C. to 3000 ° C.
Before reaching the graphitized graphite material, there are various intermediates depending on the size of the crystal structure, the degree of its orientation, the difference in the regularity of layer stacking, etc. depending on the degree of crystal structure development . These materials are collectively called carbon materials. In short, the term “carbon material” means that coal, petroleum pitch, etc., which are industrially obtained in large quantities, are 700 to 14
A carbon material such as coke heat-treated at 00 ° C. is shown, and such a carbon material is finely divided and filled to form a conventional negative electrode material.

On the other hand, the negative electrode material according to the present invention
It is constituted by filling artificial graphite powder with sufficiently developed crystals, that is, graphite powder. Graphite powder is produced by developing a crystal structure and transferring it to graphite, and this transfer can be achieved by a heat treatment at 2500 to 3000 ° C.

That is, according to this heat treatment, the properties of carbonaceous or graphitic materials can be controlled in various ways depending on the temperature, and each particle of the graphite powder is usually 2500 ° C., which is called graphitization, and especially 3000 ° C. The crystal structure is sufficiently developed by the high temperature treatment described above, and is graphitized to the extent that it is a so-called graphitized material. Therefore, the graphite powder used in the present invention can be prepared by pulverizing a material that has been graphitized to an extent that is also referred to as artificial graphite.

Furthermore, the transition to graphitization of graphite powder can be achieved depending on the degree of heat treatment as described above. Therefore, the degree of graphitization differs depending on the starting materials. Generally, carbon materials such as petroleum coke are easily graphitized. Such a graphitizable material can sufficiently achieve graphitization by heat treatment at about 2500 ° C. or higher without raising the temperature to about 3000 ° C.

As described above, the carbon powder and the graphite powder have the same crystal structure, but have different properties depending on the degree of crystal growth. From this point, in the present invention, a small-diameter graphite powder having a small average particle diameter and a large-diameter graphite powder are compounded,
Filling the compounded graphite powder to form a material for a negative electrode; filling the compounded graphite powder within a filling ratio of 0.4 to 0.7; two types of graphite powders having different particle diameters; By keeping the compounding ratio of 0.4 to 0.45 in the range of 0.4 to 0.45, the diameter of the two types of graphite powder is adjusted to a fixed relationship, and the like, to obtain an excellent negative electrode material that effectively utilizes the properties as a graphite material. Constitute.

That is, the graphite material or the graphite powder is extremely soft as compared with the carbon material or the carbon powder, and sharp graphite particles having a narrow particle size distribution can be obtained even when finely divided by pulverization. On the other hand, the carbon material is hard and broad carbon particles having a wide particle size distribution can be obtained as fine particles by pulverization. Therefore, in the pulverization of the carbon material, the yield of obtaining a predetermined particle size can be suppressed low.

Next, in order to increase the filling rate, Furna
According to s et al., the following two are reported when two types of powders having different particle sizes are mixed. 1) When mixing coarse and fine powders, the smaller the particle size ratio (= fine particle size / coarse particle size), the lower the porosity. 2) Regardless of the particle size ratio, the packing ratio becomes maximum around 70% of coarse particles and 30% of fine particles. C. C. Furnas, Ind. Eng. Che
m. , 23.1052 (1931), Furna.
s, C.I. C. , Bur. Mines Rep. Inv
est. 7. 2894 (1928); Bur. Mi
nes Bull. , 74, 307 (1929). D
insdale. and Wilkinson, Tr
ans. Brit. Ceram. Soc. , 65,
391 (1966).

Based on this, the present inventors examined the size and mixing ratio of the particle size ratio and the mixing ratio of the graphite powder for the negative electrode material of a Li-ion battery based on the report of Furnas et al. A negative electrode material having a high N was obtained.

When manufacturing batteries, how to load as many electrode materials as possible in a limited volume is one of the titles of increasing battery capacity. For this purpose, it is necessary to fill graphite powder at a high filling rate.

However, in the case of a material for a negative electrode of a Li-ion secondary battery, it does not mean that the filling rate is simply increased. The minimum gap required for the electrolyte to flow around the entire negative electrode material and to store and release Li ions is required.
For this purpose, the bulk density (ρ _B ) will be described later,
And the true density (ρ _T ), and this ratio is 0.40 to 0.40.
The graphite powder is filled so as to obtain a graphite powder. Within such a range, necessary voids can be secured, and as shown in the examples, the discharge capacity can be increased.

The discharge capacity and the like also depend on the specific surface area of the graphite powder particles for the negative electrode, and the specific surface area can be adjusted by the particle size of the graphite particles. The specific surface area can be measured by the BET method, according to the BET method, can calculate the specific surface area from the adsorption amount of the N ₂ gas adsorbed on the wall surface, such as in addition to the fine pore of the surface of the particles, in this way is calculated by the BET method,
The pores inside the particles can also be measured.

In the case of a Li-ion secondary battery, if the specific surface area for the negative electrode material is large, the capacity of the battery increases due to the increase in the fine pores, but this increases the contact area with the electrolyte. When charge and discharge are repeated, the amount of decomposition of the electrolytic solution on the surface of the negative electrode material increases, the amount of generated gas increases, and the internal pressure of the battery is increased.

In general, for powders of the same brand, the smaller the particle size, the larger the specific surface area. On the other hand, the lower limit of the size of the graphite powder is restricted as described above due to safety problems. For this reason, it is preferable that the graphite powder used for the negative electrode material according to the present invention contains as few particles as possible having a particle size of 3 μm or less. In practical use, it is necessary that the average particle size is 5 μm or more, and the content of particles having a size of 3 μm or less contained therein is 5 vol% or less.

With the graphite powder thus adjusted,
From the viewpoint of electric resistance, the specific resistance is extremely small. Incidentally, the specific resistance is 100 × 10 ⁻⁵ Ωcm or less. In contrast,
The specific resistance of the carbon material is, by the way, about 500 × 10 ⁻⁵ Ωcm, which is extremely high. For this reason, when used as a negative electrode material, it is preferable that the specific resistance is low and stable. From this point of view, if the negative electrode material is made of graphite material particles, the voltage is stable and the energy loss is low. Very few secondary batteries can be obtained.

The reason why the above specific surface area can be adjusted by the particle size of the graphite powder is as follows. The graphite material is a graphite material in which crystals are highly developed. When this graphitic graphite material is pulverized, numerous fine pores are formed infinitely inside the graphite powder particles themselves, and the specific surface area involved in the occlusion and release of Li ions is greatly increased.

This increase in the specific surface area is remarkable as compared with a carbon material such as coke, and this is extremely effective when graphite powder is used as a part of the negative electrode material.

By the way, as shown in the embodiment described later,
Starting from a carbon material such as petroleum coke as a starting material, an additional 30
When the specific surface area of each particle of the graphite powder calcined at 00 ° C. was measured by the BET method, the specific surface area was extremely large, being about 2.95 to 5.24 m ² / g. In particular, when the fine particles have an average particle diameter of about 5.1 μm, the specific surface area reaches 5.24 m ² / g, and all the fine pores of the graphite powder particles communicate with the particle surface.
The ability to occlude and release i-ions is greatly enhanced.

That is, as is well known, the BET method is a method of measuring the specific surface area from the amount of sample adsorbed on the wall surface such as micropores in addition to the particle surface. Is also measured. BE like this
The fact that the specific surface area measured by the T method is extremely large means that the number of micropores is extremely large.

Therefore, when the average particle diameter is reduced to about 5.1 μm, the specific surface area is increased to about 5.24 m ² / g, and there are many fine pores. When there are a large number of such fine pores, the eluted Li ions are adsorbed, and the diameter of the fine pores of the graphite material in which the crystal is sufficiently developed by the graphitization has a diameter suitable for the adsorption of the Li ions. Most of the eluted Li ions are adsorbed and held in the graphite particles. From such a point, when graphite powder obtained by reducing graphite material is used as a negative electrode material of a secondary battery, the capacity of the negative electrode battery may be significantly increased.

However, if the particle size of the graphite powder is too small, it becomes difficult to fill the powder as the particle size becomes small, and the performance as a secondary battery is reduced.

From the above, the present invention aims to improve the performance as a Li-ion secondary battery by utilizing such characteristics of graphite powder particles as a material for a negative electrode, and also to improve the particle size of graphite powder particles. In addition, the compounding ratio is appropriately adjusted to further improve the composition.

2500 ° C. to 3000 as described above
The graphite material which has been graphitized at ℃ is crushed and the graphite powder is adjusted and filled as described above.
As taught by rnas et al. (see paragraph 0030), at least two types of graphite powder having different average particle diameters are blended among the particles of the graphite powder. In this case, the average of the average particle size and the weight W _A particle size smaller diameter graphite powder greater atmospheric径黒lead powder weight W _B weight ratio W _A / W _B the mixing ratio of
Then, this ratio is set to 0.40 to 0.45.

In order to improve the capacity of the battery, the filling rate of the graphite powder must be increased. That is, when the filling rate is increased and the void ratio is decreased, the proportion of the graphite powder particles increases, so that the capacity of the battery is inevitably increased. In the case of filling irregularly shaped particles, increasing the filling rate depends on the compounding ratio of the graphite powder particles, and the shape of the graphite powder particles, that is, roundness and sphericity (surface area of the same volume as the particles) / Surface area of the particles).

In this regard, graphite powder graphitized by heat treatment at 2500 ° C. to 3000 ° C. has a higher true specific gravity and is softer than carbonaceous ones such as coke. For this reason, for example, when the particles are pulverized and refined, the proportion of edges generated in the particles is small, the particles are rounded, and the sphericity is increased. For this reason, the space ratio at the time of filling is reduced, and the filling ratio is increased.

Further, when increasing the filling rate by blending at least two types of graphite powders having different particle diameters, the small-diameter graphite powder and the large-diameter graphite powder are blended in a weight ratio of 0.4 to 0.45. Then, the filling rate is further increased for the following reason.

That is, when the graphite powder particles to be filled are composed of only fine powder, the porosity is also increased due to the same particle shape and the same particle diameter, and as shown in the later examples, the porosity is not so large. The filling rate cannot be increased, and in particular, the filling rate decreases as the particle size decreases.

On the other hand, even if the graphite powder has a small particle diameter, the graphite powder having a larger particle diameter is blended.
When the mixing ratio is set to an appropriate ratio, the filling rate is increased.

This ratio is 0.40 to 0.45 by weight.
It is about. That is, the compounding ratio of the small-diameter graphite powder is 0.40.
When it falls below, the filling rate itself falls accordingly. On the other hand, when the compounding ratio of the small-diameter graphite powder exceeds 0.45, the small-diameter graphite powder increases, the filling rate decreases, and the improvement of the capacity as a battery cannot be expected much.

As described above, the graphite powder having a large diameter and a small diameter are mixed, and the particle size of the graphite powder at this time is determined based on the average particle size.

When the particle diameter is determined for each particle of the graphite powder, it can be determined as an equivalent diameter, an effective diameter, or the like, but it is sufficient to determine the average particle diameter.

This average particle size represents the average particle size of a group of particles having a distribution in one particle size range. Therefore, from the relationship with a certain particle size distribution, various average particle size, for example, arithmetic average diameter, average surface area diameter, average volume diameter,
There are a median diameter, a particle diameter showing the maximum frequency in the particle diameter distribution, and the like. However, any of these values can be used, and the same effect can be achieved by using any value, but it is usually sufficient to take the arithmetic mean diameter.

In determining the average particle size in this manner, as described above, the particle size distribution of each particle group of the graphite powder is 5% or less for those having a particle size of 3 μm or less, and the average particle size is 5% or less.
It is preferable that the thickness be at least μm.

That is, in determining the particle size distribution of the graphite powder, it is necessary to determine from the occlusion and release of Li ions eluted from one negative electrode material. From this point, if the particle size distribution is in the above range, the occluded Li ions can be discharged smoothly. On the other hand, when the particle size distribution is below this, especially when the particle size reaches the order of nm, the Li ions occluded in the particles cannot be released smoothly,
Poor discharge efficiency.

Further, in order to increase the charging efficiency as a material for the negative electrode, the average particle diameter (D ₁ ) of the small-diameter graphite powder and the average particle diameter of the large-diameter graphite powder must be adjusted when compounding graphite powders having different average particle diameters. (D ₂₎ the ratio of the (D ₁ / D ₂₎ preferably 0.3 or less.

That is, when the ratio (D ₁ / D ₂ ) is reduced, the filling efficiency is increased, and from this point, the upper limit is preferably about 0.3. However, if the particle size is too small, the particle size (D ₁ ) of the small-diameter graphite powder becomes small, thereby lowering the discharge efficiency. From this viewpoint, the lower limit of the ratio (D ₁ / D ₂ ) is preferably about 0.1. .

Further, the filling rate of the graphite powder depends on the bulk density (ρ _B ) and the true density (ρ _T ) of the graphite powder, and is expressed as ρ _B / ρ _T. This value is preferably set to 0.4 to 0.7 from the viewpoint of improving the capacity of the battery. That is, even if the graphitization proceeds considerably, the bulk density (ρ _B ) is 1.54 g / c.
It is difficult to increase it to m ³ or more, and since the true density (ρ _T ) of graphite is about 2.2 g / cm ³ , the value of ρ _B / ρ _T becomes 0.7 or less, and 0.4 When it becomes below, L
The ability to occlude and release i-ions etc. decreases.

[0060]

EXAMPLE Coke having a coefficient of thermal expansion of ^{2.5.times.10.sup.-6.degree.} C.- ¹ was pulverized so that the average particle size became 5 .mu.m. This powder 1
25 parts by weight of a binder pitch was added to 00 parts by weight, kneaded with a kneader and extruded to obtain a block-shaped molded body.

This is finally reduced to about 3 in a non-oxidizing atmosphere.
It was graphitized at 000 ° C to obtain a block of artificial graphite.

This block was pulverized and classified to have an average particle diameter (arithmetic average diameter) of 25.8 μm, 12.1 μm, 5.1 μm.
m, three types of coarse, medium and fine graphite powders were obtained. The properties of each graphite powder were as shown in Table 1.

[0063]

[Table 1]

Each of these graphite powders, alone or in combination, is filled to prepare a negative electrode material. The characteristics of these materials are summarized below.

[0065]

[Table 2]

As is clear from Table 2, In No. 4, an increase in capacity of about 10% (with respect to Nos. 1 to 3) was recognized, and in particular, a discharge capacity of 300 mAh / g or more was achieved.

In addition, No. Sample No. 3 had a large capacity loss when initially charged.

Further, No. In Nos. 5 and 6, the filling ratio of the graphite powder was No. 5; Despite not increasing as much as 1 and 2, the discharge capacity was increased to the same or better.

[0069]

As described above in detail, the negative electrode material according to the present invention is constituted by blending and filling at least two graphite powders having different average particle diameters among the graphitized graphite powders.

Therefore, since it is made of graphite powder, its resistance is small and it is soft. Therefore, each particle of the powder is rounded by pulverization, the filling rate is increased, and the energy loss at the time of discharging and charging is also reduced. Few. In addition, since graphite powders having different average particle diameters are blended in an appropriate ratio, the filling rate is increased, and accordingly, the capacity as a battery is large, and particularly, the particle size distribution of the graphite powder is within an appropriate range. Therefore, an excellent secondary battery is obtained in which the absorption and release of Li ions are extremely smooth.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-9-27314 (JP, A) JP-A-9-161778 (JP, A) JP-A-9-147860 (JP, A) JP-A-9-127 63586 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) H01M 4/58 H01M 4/02-4/04 H01M 10/40 C01B 31/00-31/36

Claims (1)

(57) [Claims] 1. A graphite powder obtained by pulverizing artificial graphite graphitized at 2500 ° C. to 3000 ° C. is blended with a small-diameter graphite powder having a small average particle diameter and a large-diameter graphite powder, and the blended graphite powder is mixed. In the negative electrode material of a Li-ion secondary battery filled, the compounded and filled graphite powder contains 5% by volume or less of a particle size of 3 μm or less and has a particle size distribution of an average particle size of 5 μm or more and a bulk relative to a true density (ρ _T ). Density (ρ _B ) ratio (ρ _B / ρ _T ) 0.4
And a ratio of the average particle diameter of the small-diameter graphite powder to the average particle diameter of the large-diameter graphite powder is 0.3.
In the following, the compounding ratio (W _A / W _B ) of the weight (W _A ) of the small-diameter graphite powder to the weight (W _B ) of the large-diameter graphite powder is 0.40.
A material for a negative electrode of a Li-ion secondary battery, wherein the thickness is from 0.45 to 0.45.