JPH09320591A - Lithium ion secondary battery negative electrode material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a negative electrode material with large discharge capacity which is suppressed in decomposition of electrolyte and excellent in cycle characteristic by mixing prescribed carbon material and graphite material together. SOLUTION: This negative electrode material is obtained by mixing a pitch carbonaceous fiber mild (A) and a pitch graphite fiber mild (B). (A) is obtained at a thermal treatment temperature of 550-1300 deg.C, and has an average particle size less than 10μm, and the content is 60-98wt.%. (B) is obtained at a thermal treatment temperature of 2400 deg.C or more, and has an average particle size of 10-30μm, and the content is 2-40wt.%. The (A) and (B) are mixed together in a fixed ratio with the particle sizes being regulated, whereby a negative electrode material having a high discharge capacity in which the initial charge and discharge efficiency is improved more than in the independent use of (A), decomposition of electrolyte is suppressed, and reduction in cycle characteristic is also minimized can be provided.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a negative electrode material for a lithium ion secondary battery containing carbonaceous fiber mill and graphite fiber milled as main components. Furthermore, the present invention relates to two or more types of easily graphitizable pitch-based fiber mills having different heat treatment (carbonization / graphitization) temperatures, particularly pitch-based carbonaceous fiber mills (A) carbonized at 550 ° C. or more and 1300 ° C. or less. 2400 ° C
The present invention relates to a negative electrode material for a lithium ion secondary battery, in which the particle size of the graphitized pitch-based graphite fiber milled (B) is adjusted and the mixture is used. Furthermore, the present invention is a lithium ion secondary having a high discharge capacity, which has improved charge / discharge efficiency and excellent charge / discharge cycle characteristics, as compared with the case where the pitch-based carbonaceous fiber milled (A) is used alone. The present invention relates to a negative electrode material for batteries.

[0002]

2. Description of the Related Art In general, a secondary battery using an alkali metal, for example, lithium as a negative electrode active material has a high energy density and a high electromotive force. It has many advantages such as excellent storage, light weight and small size. Therefore, such a non-aqueous electrolyte lithium-ion secondary battery is expected to be put to practical use as a high-performance battery for power supplies for portable electronic devices, electric vehicles, and power storage.

However, the current prototype battery does not sufficiently realize the above-mentioned characteristics expected of a lithium ion secondary battery, and is insufficient in charge / discharge capacity, cycle life, energy density and the like. One of the reasons was the negative electrode used in the secondary battery. For example, when a negative electrode made of metallic lithium is used in a lithium-ion secondary battery, lithium that deposits on the negative electrode surface during charging forms needle-like dendrites, which easily causes a short circuit between the positive electrode and the negative electrode. Was short and the safety was low.

Further, since lithium has a very high reactivity and causes a decomposition reaction of the electrolytic solution in the vicinity of the surface of the negative electrode, the surface of the negative electrode is denatured by this decomposition reaction, which may cause a decrease in battery capacity due to repeated use. was there. Conventionally,
In order to solve the problems in such a lithium ion secondary battery, various negative electrode materials have been studied, and the use of carbonaceous materials (carbon materials and graphite materials) has been considered as one of them. .

As the carbon material, coal, coke, P
AN-based carbon fibers, pitch-based carbon fibers, carbonized (low-temperature heat treated) products of organic substances, etc. are being studied. As graphite materials, natural graphite, artificial graphite, synthetic graphite, mesocarbon micro beads, graphitization of organic substances ( High-temperature heat treated products, graphite fibers, etc. are being studied. Further, generally, the negative electrode material using the carbon material is excellent in the initial charge capacity, but the initial charge / discharge efficiency is low, and the cycle characteristics are inferior, while the negative electrode material using the graphite material is Although it has excellent cycle characteristics as compared with carbon materials, it has problems of low initial charge capacity and slow charge / discharge speed, and in order to complement them, mixed use of both has been studied.

For example, in JP-A-6-111818, the conductivity of an electrode sheet is obtained by mixing an appropriate amount of spherical graphite particles and graphitized carbon short fibers (graphitized vapor-grown carbon fibers). It is disclosed that the discharge capacity can be improved and a high capacity can be expressed, the electrode strength can also be improved, the carbon material can be prevented from falling off and from the current collecting substrate, and the cycle life can be extended. And the effect of mixing was insufficient. Further, in JP-A-5-283061, by combining carbon particles and carbon fibers,
A lithium secondary battery having excellent charge / discharge rate, output density and cycle characteristics is disclosed by improving the conductivity and improving the diffusion of the electrolyte through the pores due to the bulky structure, but the discharge capacity is 270 mAh. It is as low as / g and is still insufficient.

Further, in Japanese Unexamined Patent Publication (Kokai) No. 6-150931, a carbon material of pitch-based carbon fiber is mixed with an amorphous granular graphite material to improve conductivity, which is a drawback of the carbon material, and to improve the graphite material. Although an attempt to improve the charging / discharging speed, which is a drawback of, and the improvement of cycle life is disclosed,
The discharge capacity is still as low as about 200 mAh / g. Further, Japanese Patent Application Laid-Open No. 7-161347 discloses a high-temperature carbonized highly crystalline PAN-based carbon fiber having a small specific resistance,
An equal amount of low-temperature carbonized, low-crystalline carbonized PAN-based carbon fiber having a large specific resistance is mixed to replenish the drawbacks of both, and a negative electrode material having a high discharge capacity and a low initial capacity loss is provided. However, the initial discharge capacity was still low at about 240 mAh / g, and the initial charge / discharge efficiency was still low at about 55%, which was not practical.

Further, JP-A-7-192724 discloses a coexisting body (mixture) of natural or artificial graphite powder and carbon material powder such as non-graphitizable carbon material and / or graphitizable carbon material. It has been disclosed that it has a high true density of graphite and a high-speed diffusibility of lithium ions of a carbon material, and has a characteristic that high charge / discharge characteristics and stability of a positive electrode are not impaired, but it is mentioned as a negative electrode material. Graphite powder is artificial graphite obtained by carbonizing natural graphite or organic material and further heat-treating it at high temperature.The battery characteristics of the negative electrode material in the co-state with the carbon material powder are expressed by a special operation called intermittent charge / discharge. And is not common.

[0009]

DISCLOSURE OF THE INVENTION The present invention is a subject of a negative electrode material in which a carbon material and a graphite material are mixed. The charge / discharge capacity is still small, the initial charge / discharge efficiency is low, and the charge / discharge speed is slow. In addition, the purpose is to solve the problem that the cycle life is short. Generally, the characteristics required for a negative electrode material of a lithium ion secondary battery are as follows: 1) a large charge / discharge capacity, and 2) a small amount of inactivating lithium ions inside or on the surface of the negative electrode material (the irreversible capacity is Small), 3) do not decompose the electrolytic solution, and 4) do not destroy the structure of the negative electrode material itself as the cycle characteristics.

Research and development of carbon-based and graphite-based negative electrode materials satisfying these characteristics and analysis of charge / discharge mechanism of lithium ions of these negative electrode materials have been actively conducted. Carbon-based and graphite-based materials are so-called amorphous carbon,
It has various structures such as graphite having a regular structure, and diamond, and there are various intermediate structures between amorphous carbon and complete graphite crystal, which are extremely complicated.
The structure itself has not been completely revealed. Generally, what is called a carbon-based material is a material that is in the middle of amorphous carbon to a complete graphite crystal in terms of structure, and means that it is a polycrystalline body.

It can be said that the structures of these carbon-based materials differ variously depending on the type of carbon precursor, the treatment method and the like. It can be said that the charging / discharging mechanism of lithium ions also varies depending on the type of carbon material. Further, the carbon-based material changes its structure when it is heat-treated (graphitized) at high temperature, and approaches a graphite structure. The charge / discharge mechanism of lithium ions in a perfect graphite crystal is as follows: intercalation of so-called lithium ions into and out of the hexagonal net plane stacking plane of carbon atoms, between the graphite layers (interlayer distance 0.3354 nm),
It is explained by a mechanism called deintercalation, and its theoretical capacity is the electric capacity in the state of C ₆ Li in which Li is intercalated and stabilized at room temperature and atmospheric pressure,
2 mAh / g.

The graphite material has a large discharge capacity and
It has a good cycle characteristic because the i irreversible capacity is also small, but it is reported that not only the electrolytic solution is decomposed, but also the graphite layers repeatedly expand and contract due to repeated charging and discharging of Li ions, which causes structural destruction. ing. In the case of a carbon material having an undeveloped graphite layer structure, it is said that lithium ions exist in clusters (aggregated state) in a wide interlayer space in charging lithium ions. Also, the carbon material does not decompose the electrolytic solution and does not cause structural destruction,
The charge and discharge efficiency of the first time is low (the irreversible capacity of Li is large), and the cycle characteristics are deteriorated.

This cause is closely related to the structure of the carbon material itself. In other words, since the crystal structure of the carbon material is undeveloped, not only are there more defects in the structure in the surface layer than in the graphite material, but also the carbon atoms on the surface are oxidized by air and moisture, and Oxygen-containing functional groups (hydroxyl group, carboxyl group, etc.) are present. In addition, when there are many oxygen-containing functional groups, it is easy to adsorb moisture in the air. Therefore, during intercalation and deintercalation of Li ions, they are trapped by carbon radicals in their structural defects or react with oxygen-containing functional groups and water to form inorganic substances such as lithium hydroxide and lithium carbonate. However, it is considered that this causes the deterioration of the initial charge / discharge efficiency and cycle characteristics.

Based on this idea, the present inventors reduce the irreversible capacity of Li by reducing the radical concentration of the defect portion existing in the surface layer portion and removing the oxygen-containing functional group. It was found that the charging / discharging efficiency and the cycle characteristics are remarkably improved. Further, by mixing the modified carbon material and the graphite material, the discharge capacity is larger than that in the case where the unmodified carbon material and the graphite material are mixed, and the electrolytic solution is further increased. It has been found that the negative electrode material has excellent cycle characteristics by suppressing decomposition of An object of the present invention is to provide a negative electrode material having a large discharge capacity, suppressing decomposition of an electrolytic solution, and excellent cycle characteristics by mixing a carbon material with a graphite material.

[0015]

Means for Solving the Problems The present inventors have studied the above-mentioned problems variously, selected starting materials, selected appropriate carbon-based and graphite-based materials including their manufacturing methods, their forms, and their combinations. As a result of researching the blending ratio, etc., the problems of the conventional carbon material or the negative electrode material using the graphite material alone can be solved by adjusting the particle size and mixing the pitch-based carbonaceous fiber milled with the pitch-based graphite fiber milled. They have found what they can do and have completed the present invention.

That is, the negative electrode material according to the present invention contains 60 wt% or more and 98 wt% or less of pitch-based carbonaceous fiber milled (A) having an average particle size of less than 10 μm obtained at a heat treatment temperature of 550 ° C. or more and 1300 ° C. or less, and 24
The average particle size obtained at a heat treatment temperature of 00 ° C. or higher is 10 μm
Provided is a negative electrode material for a lithium ion secondary battery, which contains 2 wt% or more and 40 wt% or less of pitch-based graphite fiber milled (B) having a thickness of 30 μm or less. It is also characterized in that (A) is heat-treated in an inert atmosphere containing hydrogen chloride gas. Further, (A) and (B) are also characterized in that mesophase pitch is used as a starting material.

In the present invention, the milled (crushed) fiber is referred to as a fiber milled, and a fiber milled at a temperature of less than 2000 ° C. is a carbonaceous fiber milled, 2000 ° C.
The heat-treated fiber mill is referred to as a graphite fiber mill. Thus, the negative electrode material for a lithium ion secondary battery in the present invention is a pitch-based carbon precursor as a raw material, spun (fiberized), infusibilized, carbonized and graphitized, and Mainly on carbonaceous fiber mill that is milled into fine particles so that the fiber form is hardly retained,
This is a mixture of graphite fiber mills, which are mainly columnar and are milled to have a specific particle size and particle size distribution as they are in fiber form, and have the following advantages.

1) By using pitch, particularly mesophase pitch as a raw material, the orientation of aromatic ring pitch molecules is promoted to the inside of the fiber, and lithium ions in the negative electrode material come in and out easily. 2) Since the carbonaceous fiber milled has a heat treatment temperature of 1300 ° C. or less, the aromatic ring sheets are not overlapped with each other, and lithium ions may exist in clusters (aggregated state) in a wide interlayer space during charging of lithium ions, resulting in high capacity. Can be achieved. Further, since it is milled into fine particles, the packing density of the negative electrode material can be increased. 3) Graphite fiber milled has a well-developed graphite structure, which facilitates intercalation / deintercalation of lithium ions, and also causes distortion in the graphite structure due to infusibilization after molding into a fiber form. However, the graphite structure does not develop as highly as natural graphite and artificial graphite, structural destruction due to expansion and contraction caused by the ingress and egress of lithium ions does not easily occur, and cycle reduction is small. In addition, the graphite fiber milled has high conductivity, and complements the lack of conductivity when the carbonaceous fiber mill is used alone.

Hereinafter, the present invention will be described specifically. (I) Fiber Milled: <Raw Material Pitch> The starting materials for the carbonaceous fiber milled material and the graphite fiber milled material used in the present invention are not limited to petroleum pitch type, coal pitch type, and synthetic pitch type. It is particularly preferable to use a mesophase pitch that is easily graphitizable. The mesophase pitch is not particularly limited as long as it can be spun, but a mesophase content of 100% is desirable. The softening point of the raw material pitch is not particularly limited, but from the relationship with the spinning temperature, one having a low softening point and a high infusible reaction rate,
It is advantageous in terms of manufacturing cost and stability. Therefore,
The softening point of the raw material pitch is 230 ° C. or higher and 350 ° C. or lower, preferably 250 ° C. or higher and 310 ° C. or lower.

<Spinning> The method of melt spinning the raw material pitch is not particularly limited, and various methods such as melt spinning, melt blow, and centrifugal spinning can be used, but the productivity during spinning and the obtained fiber From the viewpoint of quality, the melt blow method is preferable. At this time, the size of the spinning holes is 0.1 mmΦ or more and 0.5 mmΦ or less, preferably 0.15 mmΦ or more and 0.3 mmΦ or less. The spinning speed is 500 m / min or more, preferably 1,500 / min.
m or more, more preferably 2,000 m or more per minute.

The spinning temperature is somewhat changed depending on the raw material pitch used, but it may be any temperature at which the pitch does not change above the softening point of the raw material pitch, such as 300 ° C to 400 ° C.
It is preferably 300 ° C. or higher and 380 ° C. or lower. Among them, the raw material pitch used in the present invention is a mesophase pitch because the graphite layer surface is arranged so as to open on the fiber surface by spinning at a low viscosity of tens of poise or less and cooling at high speed. Most preferred is the melt-blown method. The carbon fiber obtained from the pitch fiber produced in this manner, while the graphite layer surface is opened to the fiber surface, the graphite layer surface is formed on the fiber surface layer a pseudo onion layer along the fiber circumference, charge and discharge rate Even if you speed up, there is little decrease in capacity,
Further, it has an advantage that the capacity hardly decreases even if charging and discharging are repeated.

<Insolubilization> Examples of the infusibilization method include heat treatment in an atmosphere of an oxidizing gas such as nitrogen dioxide and oxygen, a method of treating in an oxidizing aqueous solution of nitric acid, chromic acid, and the like. A polymerization treatment method using γ-rays or the like is also possible. A simpler infusibilizing method is 200 to 35 in air.
This is a method of performing heat treatment at 0 ° C. for a certain period of time, and the average rate of temperature rise at that time is 1 ° C./min or more, preferably 3 ° C./min or more.

<Carbonization, Milling, Graphitization> (a) Carbonization of fibers: The infusibilized pitch fibers are heated to be carbonaceous fibers in an inert gas or in the absence of an oxidizing gas. be able to. The heating rate and the holding time at this time are not particularly limited. Carbonization of pitch fibers can be carried out according to a conventional method in an inert gas atmosphere at a temperature of 250 ° C or higher and lower than 2,000 ° C. However, in the case of the carbonaceous fiber mill of the present invention, the carbonization temperature is
It is necessary to be 550 ° C or higher and 1300 ° C or lower, preferably 650 ° C or higher and 1200 ° C or lower. When the temperature is lower than 550 ° C., the carbonaceous fiber mill itself still contains a large amount of hydrogen, oxygen or other elements other than carbon, is electrochemically unstable, and is inferior in conductivity, which causes a problem in cycle characteristics.

On the other hand, in the case of a carbonaceous fiber mill that has been heat-treated at a high temperature of over 1300 ° C., most of the fibers are carbon elements, which is an excellent carbon material from the viewpoint of chemical stability and conductivity, but the amount of lithium to be received is large. It is small and not preferable for the present invention. When the crystallization degree of the carbonaceous fiber carbonized at 550 ° C. or more and 1300 ° C. or less is shown by X-ray diffraction data, the graphite interlayer distance (d ₀₀₂ ) is 0.350 nm or more, and the crystallite size in the C-axis direction is large. (Lc) is 5 nm or less.

(B) Milling of Fibers: In order to increase the charge / discharge speed of ordinary carbon fibers, it is necessary to increase the fiber surface area per unit weight. Therefore, it is desirable to mill the fibers. To be done. However, particularly in the case of graphite fiber, if the fiber is mischievously finely powdered as described below, the active graphite layer is exposed and reacts with the electrolytic solution to cause demerits such as capacity reduction, so that an appropriate particle size is obtained. It needs to be milled to have a diameter.

(I) Milling of carbonaceous fiber: As a method for producing the pitch-based carbonaceous fiber milled product of the present invention, mesophase pitch is spun to be infusible, and further in an inert gas at a predetermined temperature required. After carbonization, Victory Mill,
Milling with a jet mill, cross flow mill, etc. is effective. In order to efficiently carry out milling of the fibers, it is common to the above methods. For example, by rotating a rotor with a plate attached thereto at high speed, the fibers are cut in a direction perpendicular to the fiber axis. The method is appropriate.

The particle size suitable for the pitch-based carbonaceous fiber mill is controlled by adjusting the number of rotations of the rotor, the number of blades used, the angle of the plate, the size of the meshes of the filters mounted around the rotor, and the like. It is possible to In addition, there is a method using a Henschel mixer, a ball mill, a crusher, etc. for milling, but according to these methods, a pressure force in the direction perpendicular to the fiber axis works,
This is not preferable because the number of vertical cracks in the fiber axis direction increases.
In addition, these methods require a long time for milling, and it cannot be said that they are suitable milling methods.

(Ii) Manufacture of Graphite Fiber Milled: Usually,
In order to efficiently perform heat treatment (particularly graphitization), it is preferable to increase the filling amount per volume. That is, a method of performing graphitization and then milling can be considered, but it is advantageous to perform the graphitizing treatment after performing the milling treatment in order to reduce the firing cost. Therefore, as a method for producing a pitch-based graphitic fiber mill, a method of carbonizing in an inert gas at a temperature of 550 ° C. or higher and 1300 ° C. or lower, and then milling and graphitizing is advantageous. Particularly in the case of the present invention, 550 ° C. or higher and 130
By carbonizing at 0 ° C or lower, milling, and then graphitizing to prevent longitudinal cracking of the fiber after milling, the newly exposed surface of the graphite layer during milling is polycondensed during graphitization at a higher temperature. The progress of the cyclization reaction and the decrease in the activity of the surface are also effective in preventing the decomposition of the electrolytic solution, which is preferable.

When graphitized and then milled, cleavage easily occurs along the graphite layer surface developed in the fiber axis direction,
The ratio of the fracture surface area to the total surface area of the manufactured milled graphite fiber is large, and the active surface having a lattice defect or the oxygen-containing functional group is also large. It is not preferable because the electrolyte solution is decomposed due to the formation of oxides and the localization of electrons. Also, by matching the carbonization temperature and range of the carbonaceous fiber mill, it is possible to simplify process control. The crushing method is the same as the milling of carbonaceous fibers, and the same equipment can be used.

(C) Graphitization: Graphitization of pitch-based carbonaceous fibers is usually carried out at a temperature of 2000 ° C. or higher, but in order to increase the capacity of the battery, as in the present invention. In the case of pitch-based carbon fiber, further graphitization (increasing firing temperature) is required. Therefore, in the present invention, it is preferable to use the pitch-based carbonaceous fiber fired (graphitized) at a temperature of 2400 ° C. or higher, preferably 2500 ° C. or higher. Further, it is preferable that the graphitization is more advanced because the effect of mixing with the carbon-based material is easily exhibited.

As described above, a higher firing (graphitization) temperature is preferable in terms of capacity and the like, but the production cost rapidly increases with the graphitization temperature, and at a firing temperature of higher than 3000 ° C., graphitization takes place. Since it is difficult to produce a commercially stable product from the viewpoint of durability of the furnace material to be used, it is necessary to select it appropriately according to the purpose. Further, in order to further develop the graphitization degree, a method of adding a boron compound or the like to graphitize can also be used.

The degree of crystallization of the pitch-based graphitic fiber mill thus produced is shown by X-ray diffraction data. The graphite interlayer distance (d ₀₀₂ ) is 0.338 nm or less, and the size of the crystallite in the C-axis direction. (Lc) is 35 nm or more, the crystallite size (La) in the a-axis direction is 50 nm or more, and the peak ratio of the (101) diffraction peak to the (100) diffraction peak (P ₁₀₁ /
P ₁₀₀ ) is 1.0 or more. Here, the X-ray diffraction method is a method in which CuKα is used as an X-ray source, high-purity silicon is used as a standard substance, and a diffraction pattern is measured for carbon fibers. From the peak position and half width of the 002 diffraction pattern, the graphite interlayer distance d ₍₀₀₂₎ , the crystallite size Lc ₍₀₀₂₎ in the c-axis direction, and the peak position and half width of the 110 diffraction pattern from the a-axis Direction crystallite size La
Calculate ₍₁₁₀₎ . The calculation method is based on the Gakushin method. To measure the peak ratio of 101/100, a baseline is drawn on the obtained diffraction diagram, and 10 is obtained from this baseline.
The height of each peak of 1 (2θ ≒ 44.5) and 100 (2θ ≒ 42.5) was measured, and the diffraction peak height of 101 was set to 10
It is determined by dividing by zero diffraction peak height.

(D) Particle size and particle size distribution of carbonaceous fiber milled: The average particle size of pitch-based carbonaceous fiber milled (A) by laser diffraction method is less than 10 μm, preferably 5 μm.
It is preferable that the particle size distributions are in the ranges of 1 to 10 μm and 5 to 20 μm at the cumulative diameters of 50% and 90%, respectively, since the performance of the battery can be improved. The pitch-based carbonaceous fiber mill has a graphite interlayer distance larger than that of the pitch-based graphite fiber milled and is 0.350 nm or more, and the diffusion rate of Li ions between layers is fast. In such a carbon material, the smaller the average particle size, the more the number of Li ion entrances and exits. Therefore, in the case of a sheet, if the particle size is small, the packing density can be increased, and the capacity development and charge / discharge efficiency can be improved. Is preferable in terms of improvement of

However, due to the problem of pulverizability, it is currently difficult to extremely reduce the size, and the pulverization efficiency deteriorates and the cost increases, so the average particle size should be 1 μm or more. On the other hand, if the average particle size is 10 μm or more, even if mixed with the graphite fiber milled, the degree of improvement in the initial charge / discharge efficiency, discharge capacity and cycle characteristics will be low, which is not preferable.

(E) Particle size and particle size distribution of graphite fiber milled: Graphite fiber milled (B) by laser diffraction method
Has an average particle size in the range of 10 to 30 μm and a particle size distribution of 8 to 1 for 10%, 50% and 90% cumulative diameters, respectively.
The range of 5 μm, 10 to 20 μm, and 30 to 60 μm is preferable because the performance of the battery can be improved. Pitch-based graphite fiber milled has a graphite interlayer distance of 0.338n
m or less, it is necessary to spread Li ions so as to easily diffuse between the graphite layers on the surface in order to intercalate between the layers, and more energy is required compared with the carbonaceous fiber milled. It becomes easy to stay with.

Therefore, the reaction of Li ions on the graphite surface and the decomposition of the electrolytic solution are likely to occur. Such a phenomenon is promoted as the graphite particles are made finer,
The particle size of the graphite fiber milled must be controlled within an appropriate range. That is, when the average particle size is less than 10 μm, the number of graphite particles having an active portion on the surface and having a small particle size increases, and therefore Li becomes irreversible as an inorganic substance such as lithium hydroxide or lithium carbonate on the surface, and The decomposition of the electrolyte also becomes severe.

As a result, the charging / discharging efficiency is lowered, and the capacity improving effect cannot be expected, which is not preferable. On the other hand, if the average particle size exceeds 30 μm, the carbonaceous fiber milled product of the present invention has an average particle size of less than 10 μm and is in the form of fine particles, so that the effect of imparting conductivity by mixing is weakened, and the initial charge / discharge efficiency and discharge capacity are reduced. Also, the effect of improving cycle characteristics is reduced, which is not preferable. In the present invention, the proportion of graphite fiber milled particles having a particle size of 2 μm or less is 5 wt% of the total fiber milled particles.
Below, preferably not detected, and the fiber length is 1
The proportion of the graphite fiber milled particles having a size of 25 μm or more is 1 wt% or less of the total fiber milled particles, and preferably not detected.

5 graphite fiber mills having a particle size of 2 μm or less
If it is present in excess of wt%, the number of fine particles having an active surface increases, the decomposition of the electrolytic solution becomes vigorous, the initial charge / discharge efficiency decreases, and the cycle characteristics also decrease. If the fiber milled fiber having a fiber length of 125 μm or more is present in an amount of more than 1 wt%, it may cause unevenness in thickness when formed into a sheet, which is not preferable. When adjusting the average particle size and particle size distribution, a treatment with a classifier, a sieve, etc. can be performed at any stage such as milling treatment and mixing treatment, if necessary.
When the precursor carbonaceous fiber mill of the graphite fiber milled (B) is treated with a classifier, a sieve or the like to adjust the particle size, the carbonaceous fiber mill on the fine particle side to be removed is the carbonaceous fiber milled (A). ) Can be used as a part or all.

(F) Surface modification of carbonaceous fiber milled: Generally, on the surface of a carbon material, especially a carbon material having a low heat treatment temperature,
Oxygen-containing functional groups and carbon radicals are present, which cause the capacity of the battery to decrease. Therefore, also in the carbonaceous fiber mill of the present invention, it is preferable to modify the surface by chemical treatment, heat treatment or the like in order to further improve the performance of the battery. In particular, in the present invention, heat treatment at 400 ° C. or higher and 1300 ° C. or lower, preferably 400 ° C. or higher and 1000 ° C. or lower in an inert atmosphere containing hydrogen chloride gas is effective in improving battery performance, cost and productivity. From the perspective of, it is preferable.

This is because hydrogen chloride has the effect of accelerating the condensation polymerization / cyclization reaction with respect to the carbon radicals existing on the surface of the carbon or the surface layer portion thereof, and with the elimination of the oxygen-containing functional group, the carbon radicals. Similarly, polycondensation and
It is considered that this treatment has an effect of promoting cyclization, and therefore, this treatment can reduce carbon radicals and oxygen-containing functional groups that cause the irreversible capacity during charge and discharge. If the heat treatment temperature at this time is lower than 400 ° C., it takes a long time to modify the surface of the carbonaceous fiber milled, which is not preferable. In addition, if the heat treatment temperature is higher than 1300 ° C., the lithium reception amount is reduced as described above, which is not preferable.

As the atmosphere containing hydrogen chloride gas,
An inert atmosphere that does not generate explosive nitrogen chloride or the like is preferable, and a rare gas, especially an argon atmosphere is particularly preferable. The content of hydrogen chloride gas is not particularly limited, but is 0.5 vol% or more and 10 v or more from the viewpoint of treatment efficiency, apparatus corrosivity, exhaust gas treatment and economic efficiency.
It is ol% or less, preferably 1 vol% or more and 5 vol% or less. Further, the present invention has an effect of efficiently removing elements other than carbon, for example, metal elements such as Fe and Ni and hetero elements such as nitrogen and sulfur in the process of carbon radical generation and recombination. The effect of improving the purity and improving the initial charge / discharge efficiency and cycle characteristics is also considered.

(II) Structure of Negative Electrode: Each fiber mill and the mixed fiber mill obtained by the present invention can be used as a negative electrode by a usual method. That is, a binder such as polyethylene or polytetrafluoroethylene is added, and a slurry is formed using an organic solvent or an aqueous solvent, and is applied to one or both sides of a metal foil made of copper, nickel or the like having a thickness of 10 to 50 μm. The method of rolling and drying to obtain a sheet-like material having a thickness of about 50 to 200 μm is widely used.

After that, a method of slitting into a predetermined width and length and winding and making a can together with the positive electrode and the separator is common. As a mixing ratio when the respective fiber mills are mixed and used, the weight ratio of carbonaceous fiber milled (A) / graphitic fiber milled (B) is 98/2 or less 60/40 or more, preferably 95/5 or less 70 / 30 or more. When the mixing ratio (A) / (B) is less than 60/40, the charge / discharge capacity, which is an advantage of the carbonaceous fiber milled, is drastically reduced as the blending ratio of the carbonaceous fiber milled decreases, which is not preferable. On the other hand, when the mixing ratio (A) / (B) exceeds 98/2, the effect of mixing is weakened, and there is no great difference in battery performance from the use of the carbonaceous fiber milled alone, and the initial charge / discharge efficiency decreases, and the cycle The characteristics are also deteriorated, which is not preferable. It should be noted that (A) and (B) may be used alone (one kind) or may be used in two or more kinds as long as the respective conditions are within the scope of the present invention. It may be appropriately selected in consideration of the aspect.

(III) Battery: When a lithium ion secondary battery is produced by using each fiber milled product according to the present invention and a mixed fiber milled product thereof as a negative electrode, the electrolyte solution should be one that can dissolve a lithium salt. However, an aprotic organic solvent having a large dielectric constant is preferable. The negative electrode made of the fiber mill thus produced has a large charge / discharge capacity per unit volume and is suitable for downsizing of batteries.

Examples of the organic solvent include propylene carbonate, ethylene carbonate, tetrahydrofuran, 2-methyltetrahydrofuran, dioxolane, 4-methyl-dioxolane, acetonitrile,
Examples thereof include dimethyl carbonate, methyl ethyl carbonate and diethyl carbonate. These solvents can be used alone or in a suitable mixture. As the electrolyte, a lithium salt that produces a stable anion, for example, lithium perchlorate, lithium borofluoride, lithium hexamonate antimonate, lithium hexafluoroantimonate (LiPF ₆ ) or the like is preferable.

Examples of the positive electrode of the lithium ion secondary battery include metal oxides such as chromium oxide, titanium oxide, cobalt oxide, vanadium pentoxide, lithium manganese oxide (LiMn ₂ O ₄ ), and lithium cobalt oxide. Substance (LiCoO ₂ ), lithium nickel oxide (LiN
A lithium metal oxide such as iO ₂ ); a chalcogen compound of a transition metal such as titanium sulfide or molybdenum sulfide; and a conjugated polymer substance having conductivity such as polyacetylene, polyparaphenylene, or polypyrrole can be used.

A separator made of synthetic fiber or glass fiber non-woven fabric, woven fabric, polyolefin type porous membrane, polytetrafluoroethylene non-woven fabric or the like is provided between the positive electrode and the negative electrode. The secondary battery of the present invention, the separator,
By using battery constituent elements such as a current collector, a gasket, a sealing plate, and a case, and the specific negative electrode of the present invention, a lithium ion secondary battery of a cylindrical type, a square type, a button type or the like can be assembled according to a conventional method. .

[0048]

The present invention will be described in more detail by the following examples, which do not limit the scope of the present invention. (Example 1) A mesophase pitch having an optical anisotropy and a specific gravity of 1.25, which was synthesized using a catalyst such as hydrogen fluoride with naphthalene as a main component, was used as a raw material, and a diameter of 0.2 mm was used in a slit having a width of 3 mm. Using a spinneret having 500 spinning holes in a row, heated air was ejected from the slit to pull the molten pitch to produce pitch fibers having an average diameter of 13 μm. At this time, the spinning temperature is 360 ° C and the discharge rate is 0.8g.
/ H · min. The spun pitch fibers were collected on the belt while suctioning from the back surface of the belt made of a stainless steel wire mesh having a collecting portion of 20 mesh.

The collected mat was placed in air at room temperature for 3 hours.
The temperature was raised to 00 ° C at an average temperature rising rate of 6 ° C / min for infusibilization treatment, followed by carbonization treatment at 650 ° C, and the carbonaceous fiber was crushed by a cross flow mill to obtain a carbonaceous fiber milled ( A). The carbonaceous fiber mill (A) is manufactured by Shimadzu Corporation, and a powder particle size measuring apparatus by laser diffraction method, SALD
-3000, the refractive index of the particles is 1.80-0.20.
As a result of measuring the particle size as i, the average particle size is 3.5 μm, 5
0% and 90% cumulative diameters are 3.5 μm and 6.2, respectively.
μm.

On the other hand, the above carbonaceous fibers were classified by a classifier by changing the operating conditions of the cross flow mill (A) to obtain a carbonaceous fiber milled (C) having a different particle size distribution. The carbonaceous fiber mill (C) was measured for particle size in the same manner using a powder particle size measuring device. As a result, an average particle size of 19.7 μm, 10
%, 50% and 90% cumulative diameters are each 11.5 μm,
The presence of fiber mills of 18.7 μm and 45.6 μm and 2 μm or less was 0.4 wt%, and fiber milled of 120 μm or more was not detected. The carbonaceous fiber mill (C) was heated to 2900 ° C. at a rate of 3 ° C./min, kept at 2900 ° C. for 1 hour, and graphitized to obtain a graphite fiber milled (B).

The graphite fiber milled (B) was measured for particle size in the same manner by using a powder particle size measuring device. As a result, the average particle size was 1
The cumulative diameters of 7.6 μm, 10%, 50% and 90% were changed to 9.9 μm, 16.8 μm and 43.8 μm, respectively, and the presence of graphite fiber milled of 2 μm or less was changed to 1.8 wt%. The carbonaceous fiber milled (A) thus obtained and the graphite fiber milled (B) in a weight ratio of 98: 2,
After uniformly mixing in the ratio of 80:20, 60:40, add 3% of polytetrafluoroethylene to the total fiber milled
After adding and kneading t% to prepare pellets and using them as negative electrodes, a charge / discharge test was performed in a 3-electrode cell.

In the test, metallic lithium was used for the anode, and ethylene carbonate (EC) / dimethyl carbonate (D
MC) in a mixed carbonate solvent adjusted to a volume ratio of 1/1, and lithium perchlorate (LiClO ₄ ) as an electrolyte.
Was carried out in an electrolytic solution dissolved at a concentration of 1 mol, and the charge / discharge capacity characteristics were measured. The charge / discharge capacity characteristic is 100
It was carried out at a constant current of mA / g, and the discharge capacity was the capacity until the battery voltage dropped to 2 V, and the measurement was repeated 10 times. The measurement results are shown in Table 1.

Comparative Example 1 A negative electrode was prepared and charged and discharged in the same manner as in Example 1 except that the carbonaceous fiber mill (A) and the graphite fiber milled (B) obtained in Example 1 were used alone. The test was conducted. The measurement results of the electrode characteristics are also shown in Table 1.

[Table 1]

Example 2 and Comparative Example 2 The carbonaceous fiber milled product (A) obtained in Example 1 was heat-treated at 600 ° C. for 2 hours in an argon gas stream containing 2 vol% hydrogen chloride. The weight ratio of the modified carbonaceous fiber mill (A) and the graphite fiber milled (B) obtained in Example 1 was 8
After uniformly mixing each in a ratio of 0:20, Example 1
A negative electrode material was prepared in the same manner as above, and a charge / discharge test was conducted. Table 2 shows the measurement results of the electrode characteristics. Table 2 also shows the results of charge-discharge tests performed by using the modified carbonaceous fiber milled (A) alone to prepare a negative electrode material in the same manner as in Example 1.

[0055]

[Table 2]

(Example 3, Comparative Example 3) Five carbonaceous fiber mills (A) were prepared in the same manner as in Example 1 except that the heat treatment temperature was changed as shown in Table 3. The average particle size of each carbonaceous fiber mill (A) measured in the same manner as in Example 1 was 3.5 μm. The carbonaceous fiber mill (A) thus obtained and the graphite fiber milled (B) obtained in Example 1 were uniformly mixed in a weight ratio of 80:20, and then each of Example 1 was used. A negative electrode was prepared and a charge / discharge test was conducted in the same manner as in. Table 3 shows the measurement results of the electrode characteristics.

[0057]

[Table 3]

(Example 4, Comparative Example 4) The carbonaceous fibers obtained in Example 3 and having a firing temperature of 750 ° C. were changed in milling conditions, and carbonaceous fibers having an average particle size as shown in Table 4 were obtained. Five types of milled (A) were prepared. The carbonaceous fiber mill (A) thus obtained and the graphite fiber milled (B) obtained in Example 1 were uniformly mixed in a weight ratio of 80:20, and then each of Example 1 was used. A negative electrode was prepared and a charge / discharge test was conducted in the same manner as in. Table 4 shows the measurement results of the electrode characteristics.

[0059]

[Table 4]

(Example 5 and Comparative Example 5) Table 5 was obtained in the same manner as in Example 1 except that the carbonaceous fiber milled (A) obtained in Example 1 and the milling conditions were changed. Four types of graphite fiber mills (B) having average particle diameters as shown were uniformly mixed at a ratio of 80:20, and then a negative electrode was prepared in the same manner as in Example 1 and a charge / discharge test was conducted. . Table 5 shows the measurement results of the electrode characteristics.

[0061]

[Table 5]

[0062]

The pitch-based carbonaceous fiber mill and the pitch-based graphite fiber milled are respectively adjusted in particle size and mixed at a constant ratio, so that the pitch-based carbonaceous fiber milled can be used for the first time rather than used alone. It has become possible to provide a negative electrode material having high discharge capacity with improved discharge efficiency and less deterioration in cycle characteristics.

Claims (3)

[Claims] 1. A pitch-based carbonaceous fiber milled (A) having an average particle size of less than 10 μm obtained at a heat treatment temperature of 550 ° C. or higher and 1300 ° C. or lower is contained in an amount of 60 wt% or more and 98 wt% or less, and at a heat treatment temperature of 2400 ° C. or more. A negative electrode material for a lithium ion secondary battery, characterized by containing 2 wt% or more and 40 wt% or less of pitch-based graphite fiber milled (B) having an average particle size of 10 μm or more and 30 μm or less. 2. The negative electrode material for a lithium ion secondary battery according to claim 1, wherein (A) is heat-treated in an inert atmosphere containing hydrogen chloride gas. 3. The negative electrode material for a lithium ion secondary battery according to claim 1, wherein (A) and (B) use mesophase pitch as a starting material.