JPH10162827A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte battery on which reciprocal capacity in a low electric potential area is increased and which has high capacity and high energy density by using a carbon material, on which an area rate of a quenching fringe area to the whole area observed by a polarization microscope under direct light nicol falls within a specific range and which has a specific fiber shape, as a negative electode. SOLUTION: Coal-tar pitch or the like is carbonized at 700 to 1500 deg.C in an inactive atmosphere, and a carbon material on which an area rate of a quenching fringe area 8 to the whole area observed by a polarization microscope under direct light nicol is 1 to 50%, is used as a negative electrode In this carbon material, since an optical isotropic area is dominant, reciprocal capacity of doping-dedoping lithium up to 0 to 0.5V is large, and strain caused in a grain boundary is also restrained, and a material having high true density is obtained. Since it is formed of a material on which a shape of the quenching fringe area 8 is a fibrous tissue and a maximum dimension of its repetitive distance is 1 to 20μm, a nonaqueous electrolyte secndary battery on which high density can be realized and which has high capacity, can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a high performance non-aqueous electrolyte secondary battery.

[0002]

2. Description of the Related Art In recent years, portable electronic devices have become portable,
Cordless use is rapidly progressing. Currently, nickel cadmium rechargeable batteries or sealed small lead-acid batteries play a role as a power supply for driving these electronic devices. High energy density of secondary battery,
The demand for smaller and lighter is increasing. In recent years, secondary batteries have been attracting attention as power supplies for mobile phones, and demands for longer talk time and improved cycle life have been increasing with rapid market expansion.

A non-aqueous electrolyte secondary battery using lithium as a negative electrode can be expected to have a higher energy density than conventional nickel cadmium batteries and lead storage batteries, and much research has been conducted. However, when metallic lithium is used for the negative electrode, dendrite is generated at the time of charging, and a short circuit is likely to occur, resulting in a low-reliability battery. Under such circumstances, it has been proposed to use a carbon material as a negative electrode active material as a negative electrode.

[0004] Then, as an example of using a carbon material for a negative electrode,
For example, Japanese Patent Application Laid-Open No. 57-208079,
JP-A-58-102464 discloses a graph in which graphitization has developed.
Eight has been proposed. However, the true density is 2.2kg
/ Cm^ThreeWhen using the graphite material of the above as a negative electrode active material,
Theoretically lithium can operate reversibly during charging ₆
Li (6 lithium atoms for 1 carbon atom)
The chemical capacity remains at 372 mAh / g.

[0005] Then, the organic compound is converted to 1 under an inert atmosphere.
It has been reported that a carbon material obtained by heat treatment under a relatively low temperature condition of 500 ° C. or less is capable of reversibly charging and discharging lithium ions having an electric capacity exceeding 372 mAh / g (for example, The 35th Battery Symposium (1994), p. 45). It is known that these carbon materials have two types of structures, an optically anisotropic structure and an optically isotropic structure, each of which has a high reversible lithium-doped / undoped capacity per unit weight. Can be. Further, in order to obtain a high voltage and a high energy density of the lithium ion secondary battery, it is desired that the negative electrode active material has a large reversible lithium doping / undoping capacity at a relatively low potential of 0 to 0.5 V. . 0
Lithium-doped and undoped reversible capacity up to 0.5V is larger in carbon materials having an optically isotropic structure than general carbon materials, and higher capacity can be expected, but it has an optically anisotropic structure At 0 to 0.5 V, the reversible capacity per unit weight of a carbon material is not as large as that of a carbon material having an optically isotropic structure. However, the true density of a carbon material showing optical isotropy shows a low value of about 1.7 g / cm ³ , while the carbon material showing optical anisotropy shows about 1.9 g / cm ^3.
g / cm ³ , and high electrode filling properties can be expected.

[0006]

As described above, each of the carbon materials exhibiting optical anisotropy and optical isotropy,
There is a trade-off relationship in terms of lithium doping / undoping reversible capacity from 0 to 0.5 V and true density. Since a general carbon material has both of these properties, there is a problem that the reversible capacity per unit volume is not always high. The present invention is to solve the conventional problem, by using a carbon material having a high true density and a large reversible capacity of operation in a low potential region per unit volume,
An object of the present invention is to provide an excellent non-aqueous electrolyte secondary battery.

[0007]

In order to solve the above-mentioned problems, a non-aqueous electrolyte secondary battery of the present invention comprises a non-aqueous electrolyte as an electrolyte, a lithium-containing oxide as a positive electrode, and a non-aqueous electrolyte as a negative electrode. A carbon material having an area ratio of an extinction fringe region of 1 to 50% with respect to the entire area observed by a polarizing microscope under optical Nicols is used.

The above means realizes a non-aqueous electrolyte battery having a high capacity and a high energy density.

The quenching fringe region has a fibrous structure with a maximum repetition distance of 1 to 20 μm. The carbon material is made of coal tar pitch, petroleum pitch, or a condensed polycyclic aromatic hydrocarbon compound. Either organic synthetic pitch obtained by polycondensation, organic synthetic pitch obtained by polycondensation of a heteroatom-containing condensed polycyclic aromatic hydrocarbon compound, or coal coke is used as a starting material.
Carbonized at 1500 ° C. is preferred.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The tissue structure of an optically anisotropic region is a region indicated by an extinction fringe observed by a polarizing microscope, and its scale and size are “Introduction to Carbon Materials” (edited by the Society of Carbon Materials: 1984), page 22, Table 3, the following classifications are made.

1. Isotropic 2. Fine mosaic: 10 units of extinction stripe pattern
2. Mosaic pattern of μm or less Coarse mosaic: extinction stripe pattern repeat unit is 10μ
m mosaic pattern 4. Coarse fibrous: Repetition of vertical quenching fringe pattern of fibrous structure is oriented to fiber direction by 10 μm or more 5. Fine fibrous: A fibrous structure that has undergone large deformation is seen, and the size of the extinction fringe pattern is 10 μm or less. Globular: A group of mesophase spheres solidified before coalescence This makes it possible to control the optically anisotropic structure and the optically isotropic structure by observing the extinction fringe region with a polarizing microscope. Therefore, among these carbon structures, the carbon material having a structure in which the area ratio of the extinction fringe region to the entire area is 1 to 50% maintains the lithium-doped / dedoped reversible capacity of 0 to 0.5 V. As it is, the optically anisotropic region and the optically isotropic region are effectively mixed, and the possibility of a densely packed high-density structure has been found. In the case of a fibrous structure having a thickness of 20 μm, it has been clarified that since a mixed state of an optically anisotropic structure and an optically isotropic structure can be present more densely, a remarkable high density is possible.

Starting materials used for synthesizing such a carbon material include coal tar pitch, petroleum pitch, an organic synthetic pitch obtained by polycondensation of a condensed polycyclic aromatic hydrocarbon compound, and a heteroatom-containing condensed polycyclic aromatic. Production of organic synthetic pitch or coal coke obtained by polycondensation of hydrocarbon compounds by removing volatile components at 200 to 600 ° C to make it insoluble and infusible, and then carbonizing at 700 to 1500 ° C under an inert atmosphere. can do. In the case of calcination at 200 to 600 ° C., if a structural change such as a flow structure occurs due to melting, nitric acid, acetyl nitrate, sulfur, or the like may be added, and a heat treatment or an oxidation reaction by adding an oxidizing agent may be appropriately performed.

By properly controlling the structure of these carbon materials, a secondary battery having a high true density and a high energy density can be obtained.

According to the present invention, there is provided a carbon material in which the area ratio of the extinction fringe region to the entire area observed by a polarizing microscope under direct light Nicols is 1 to 50%. This can be implemented by using. Since the carbon material having the area ratio of the extinction fringe region of 1 to 50% is dominated by the optically isotropic region, it is necessary to obtain a carbon material having a large lithium-doped / de-doped reversible capacity from 0 to 0.5 V. Is possible. Furthermore, since the optically anisotropic structure is mixed with the optically anisotropic structure dispersed therein, distortion generated at the grain boundary in the optically isotropic region is suppressed to a low level, so that the optically isotropic region can be densely packed. As a result, 0-0.5
A carbon material having a large reversible capacity up to V and a high true density can be obtained. In a carbon material having a structure in which the area ratio of the extinction fringe region to the entire area exceeds 50%, the optically anisotropic structure is dominant. Is undesirably small. Further, a carbon material having a structure in which the area ratio of the extinction fringe region is 0% is not preferable because it has only an optically isotropic structure and the true density is low. Preferably, the area ratio is 1 to 50%, more preferably 5 to 30%.

Further, as in the second aspect of the present invention, it is preferable to use a carbon material in which the shape of the extinction fringe region is a fibrous structure and the maximum dimension of the repetition distance is 1 to 20 μm. When the extinction fringe structure becomes a fibrous structure, a state in which an optically isotropic structure and an optically anisotropic structure are mixed is possible, and the fibrous optically anisotropic structures are stacked one on another. This is because, since the contact surface increases, they can be densely packed with each other, and the density can be increased. On the other hand, when the extinction fringe structure becomes mosaic, the optically anisotropic structure is widely dispersed and exists without being in contact with each other. Not preferred. In addition, when the film has a globular shape, the optically isotropic and optically anisotropic structures are likely to be separated from each other, and the effect of suppressing the distortion of the grain boundary due to the mixture of the two regions is weakened, which is not preferable. The true density of the fibrous repeating unit tended to be higher at 1 to 20 μm. This is because, when the repeating unit is 20 μm or more, the structure becomes almost globular, and the two regions are easily separated from each other as described above. The fibrous repeating unit is preferably 1 to 20 μm, more preferably 5 to 20 μm.
15 μm.

In the third aspect of the present invention, the condensed polycyclic aromatic hydrocarbon compound may be naphthalene, anthracene, indene, tetralin, phenanthrene or a condensed polycyclic aromatic hydrocarbon compound containing C _n H.
_{2n + 1} or C _n H _{2n-1 (n} is 2-4) is an organic compound side chain is added as indicated by, and as the hetero atom-containing condensed polycyclic aromatic hydrocarbon compounds quinoline, pyrrole or of C _n H _{2n + 1} or C _n H _2n-1 (where n is 1 to 5)
It is preferable to use an organic compound to which the side chain shown in 4) is added.

Further, according to the third aspect of the present invention, 70
By carbonizing in the range of 0 to 1500 ° C., a carbon material having a large reversible lithium-doped / undoped capacity can be obtained. More preferably, it is 900 to 1300 ° C.

A non-aqueous electrolyte secondary battery according to the present invention uses a carbon material according to claim 1 as a negative electrode, a lithium-containing metal oxide for a positive electrode, a separator for separating the positive electrode and the negative electrode, and a non-aqueous electrolyte. It consists of. As the positive electrode material, any lithium-containing metal oxide may be used as long as it can occlude and release lithium, such as LiCoO ₂ , LiNiO ₂ , and LiMnO ₂ .

[0019]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows a longitudinal sectional view of an evaluation battery for evaluating the negative electrode of the present invention. In FIG. 1, reference numeral 1 denotes a battery case made of a stainless steel sheet having resistance to organic electrolytic solution, reference numeral 2 denotes a sealing plate of the same material, and reference numeral 3 denotes a current collector of the same material, which is spot-welded to the inner surface of the battery case 1. 4 is metallic lithium,
It is crimped inside the sealing plate 2. 5 is a negative electrode, 6
Is a separator made of microporous polypropylene, and 7 is a gasket made of polypropylene. The dimensions of the battery for evaluation are 20 mm in diameter and 1.6 mm in total battery height.

A predetermined amount of a mixture obtained by mixing 10 parts by weight of polyvinylidene fluoride as a binder with 90 parts by weight of a carbon material as a negative electrode active material is formed on a current collector 3 to form an electrode. This was dried at 150 ° C. under reduced pressure, and then assembled into a battery as a negative electrode. The electrolyte is ethylene carbonate and 1,3
Lithium perchlorate was dissolved at a concentration of 1 mol / l as a solute in a mixed solvent of equal volume of dimethoxyethane and used. After the battery is assembled, the negative electrode is charged to electrochemically insert lithium ions.

The above-described battery for evaluation has a negative charge / discharge characteristic.
Metallic lithium because it was configured for evaluation
When current is passed in the direction in which 4 discharges, lithium dissolves and
In both cases, lithium ions are occluded in the negative electrode 5 of the present invention and charged.
Is done. In addition, when a current flows in a direction in which the negative electrode 5 discharges,
In this case, lithium ions are released and the lithium metal 4
Lithium deposits on the surface. This battery is metal
Lithium 4 is composed of a large excess, and is substantially
Indicates that the characteristics of the battery for evaluation indicate the characteristics of the negative electrode 5.
Worth it. At room temperature (20 ° C.)
0.5mA / cm ^TwoIn the range of voltage 2.0 to 0V
Perform a charge / discharge test, and measure from 0 to 2.0V and 0 to 0.5V.
The reversible capacity was examined.

Evaluation methods other than the electrochemical characteristics will be described below. The extinction fringe area was measured under direct light Nicols.
It can be determined by observing with a polarizing microscope at 00 times. FIG. 2 shows a model diagram observed by a polarizing microscope. In FIG. 2, reference numeral 8 denotes an extinction fringe area, and reference numeral 9 denotes an area other than the extinction fringe. The existence ratio of the extinction fringe region 8 is an average value of the area% calculated by the point counting method for each observation extinction fringe region selected from several observation regions. The repetition distance of the extinction stripe region is obtained by measuring the maximum dimension of the observed extinction stripe region.

The true density was measured by the butanol method according to the method described in JIS R7212.

The outline of the method for obtaining the carbonaceous material is as follows. Coal tar pitch, petroleum pitch, organic synthetic pitch obtained by polycondensation of condensed polycyclic aromatic hydrocarbon compounds, organic synthetic pitch obtained by polycondensation of heteroatom-containing condensed polycyclic aromatic hydrocarbon compounds or coal coke Either of them is weighed in a predetermined amount, and the resultant is baked at 200 to 600 ° C. in an inert atmosphere of argon gas to remove volatile components to obtain an insoluble infusible material. At this time, when the material is melted to form a flow structure, nitric acid, acetyl nitrate, sulfur, or the like may be appropriately added to perform a heat treatment or an oxidation reaction by adding an oxidizing agent. Thereafter, after carbonization and firing at a temperature of 700 to 1500 ° C. for a predetermined time, the mixture is naturally cooled and crushed to obtain a carbonaceous substance. Next, the resulting carbon material was used as a sample for electrochemical evaluation.

Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples. (Example 1) Bituminous coal having a fuel ratio (fixed carbon / volatile component) of 1.8 is heated at 300 ° C to remove volatile components to form an insoluble infusible material. 50 g of the obtained coal coke was carbonized and fired at a temperature of 1000 ° C. in an inert atmosphere of argon gas.
After 2.5 hours of carbonization and firing, the mixture was naturally cooled and pulverized to obtain a carbonaceous substance. When the obtained carbonaceous material was observed with a polarizing microscope, the quenching fringe region showed a coarse fibrous structure. In addition, the true density was measured and the electrochemical evaluation was performed by the above method.

Example 2 250 anthracites with a fuel ratio of 12
Heat at ℃ to remove volatile components to form an insoluble infusible. 50 g of the obtained coal coke was carbonized and fired at a temperature of 1000 ° C. in an inert atmosphere of argon gas. After 2.5 hours of carbonization and firing, the mixture was naturally cooled and pulverized to obtain a carbonaceous substance. When the obtained carbonaceous material was observed with a polarizing microscope, the quenching fringe region showed a fine fibrous structure. In addition, the true density was measured and the electrochemical evaluation was performed by the above method.

Example 3 A petroleum pitch having a softening point of 90 ° C. is subjected to a crosslinking treatment at 200 ° C. by air oxidation to obtain an insoluble infusible material. 50 g of the obtained petroleum pitch was carbonized and fired at a temperature of 1000 ° C. in an inert atmosphere of argon gas. After 2.5 hours of carbonization and firing, the mixture was naturally cooled and pulverized to obtain a carbonaceous substance. When the obtained carbonaceous material was observed with a polarizing microscope, the quenching fringe region showed a coarse fibrous structure. In addition, the true density was measured and the electrochemical evaluation was performed by the above method.

(Example 4) 50 g of an optically isotropic organic synthetic pitch having a softening point of 170 ° C. obtained by performing polycondensation using naphthalene as a condensed polycyclic aromatic hydrocarbon compound was mixed with an inert atmosphere of argon gas. It was calcined and calcined at a temperature of 1000 ° C. below. After 2.5 hours of carbonization and firing, the mixture was naturally cooled and pulverized to obtain a carbonaceous substance. When the obtained carbonaceous material was observed with a polarizing microscope, the quenching fringe region showed a coarse fibrous structure. In addition, the true density was measured and the electrochemical evaluation was performed by the above method.

Example 5 Polycondensation was carried out using pyrrole as a heteroatom-containing condensed polycyclic aromatic hydrocarbon compound, and 50 g of an optically isotropic organic synthetic pitch having a softening point of 250 ° C. was obtained. 1100 ° C under inert atmosphere
At a temperature of. After 2.5 hours of carbonization and firing, the mixture was naturally cooled and pulverized to obtain a carbonaceous substance. When the obtained carbonaceous material was observed with a polarizing microscope, the quenching fringe region showed a fibrous structure. In addition, the true density was measured and the electrochemical evaluation was performed by the above method.

Comparative Example 1 Lignite with a fuel ratio of 1.0 was added to 200
Remove volatile components at ℃ to make insoluble infusible. 50 g of the obtained coal coke was placed under an inert atmosphere of argon gas for 10 g.
It was carbonized and fired at a temperature of 00 ° C. Then, carbonization firing is performed.
After 5 hours, the mixture was naturally cooled and pulverized to obtain a carbonaceous substance. When the obtained carbonaceous material was observed with a polarizing microscope, the quenching fringe region showed a fibrous structure with a repeating distance of 18 μm, but the area ratio of the quenching fringe region was 75%. In addition, the true density was measured and the electrochemical evaluation was performed by the above method.

Comparative Example 2 350 anthracite with a fuel ratio of 15
Heat at ℃ to remove volatile components to form an insoluble infusible. 50 g of the obtained coal coke was carbonized and fired at a temperature of 1000 ° C. in an inert atmosphere of argon gas. After 2.5 hours of carbonization and firing, the mixture was naturally cooled and pulverized to obtain a carbonaceous substance. When the obtained carbonaceous material was observed with a polarizing microscope, no quenching fringe region was observed, and the carbonaceous material showed optical isotropy. In addition, the true density was measured and the electrochemical evaluation was performed by the above method.

Comparative Example 3 10 parts by weight of concentrated nitric acid was added to 90 parts by weight of coal tar pitch having a softening point of 60 ° C., and the mixture was crosslinked by heating to 200 ° C. to form an insoluble infusible body. 50 g of the obtained pitch was carbonized and fired at a temperature of 1000 ° C. in an inert atmosphere of argon gas. After 2.5 hours of carbonization and firing, the mixture was naturally cooled and pulverized to obtain a carbonaceous substance. When the obtained carbonaceous material was observed with a polarizing microscope, the area ratio of the quenching fringe region was 35%, but the shape showed a fine mosaic structure with a repeating distance of 5 μm. In addition, the true density was measured and the electrochemical evaluation was performed by the above method.

Comparative Example 4 Petroleum pitch 5 having a softening point of 90 ° C.
0 g was carbonized and fired at a temperature of 1000 ° C. in an inert atmosphere of argon gas. After 2.5 hours of carbonization and firing, the mixture was naturally cooled and pulverized to obtain a carbonaceous substance. When the obtained carbonaceous material was observed with a polarizing microscope, the area ratio of the extinction fringe region was 48%, but the shape showed a globular structure with a repetition distance of 50 μm. In addition, the true density was measured and the electrochemical evaluation was performed by the above method.

From the above, the results shown in Table 1 were obtained by observing the carbonaceous materials of Examples 1 to 4 and Comparative Examples 1 to 4 with a polarizing microscope.

[0036]

[Table 1]

In addition, the true densities, lithium-doped and undoped reversible capacities of carbonaceous materials of Examples 1 to 4 and Comparative Examples 1 to 4 (0
-0.5 V or 0-2.0 V) yielded the results shown in Table 2.

[0038]

[Table 2]

From the results shown in Tables 1 and 2, the carbon material having a fibrous structure having an area ratio of the extinction fringe region of 1 to 50% and a repetition distance of 1 to 20 μm has a large true density. It was also found that the reversible capacity of lithium-doping / de-doping became large.

Example 6 A non-aqueous electrolyte secondary battery was prepared in the same manner as in Example 1 except that the carbon material of Example 1 was used as a negative electrode and lithium cobalt oxide was used as a positive electrode. 6. This was subjected to a charge / discharge test (0.5 mA / cm ² ) in the range of 2.5 to 4.2 V. In addition, the reversible capacity and the average discharge voltage were determined. Table 3 shows the results. That is, the battery capacity is 3.0 mAh,
An average discharge voltage of 3.6 V was obtained.

Comparative Example 5 The carbon material of Comparative Example 1 was used as a negative electrode, and lithium cobalt oxide was used as a positive electrode.
A non-aqueous electrolyte secondary battery was manufactured to be Comparative Example 5. The charge-discharge test (0.5 mA / c) was performed in the range of 2.5 to 4.2 V.
m ² ). In addition, the reversible capacity and the average discharge voltage were determined. Table 3 shows the results. That is, the battery capacity was 1.0 mAh, and the average discharge voltage was 3.2 V.

[0042]

[Table 3]

From the results shown in Table 3, it can be seen that Example 6 is superior to Comparative Example 5 in both battery capacity and average discharge voltage.

According to the result of the evaluation battery using such a carbon material, the same effect can be obtained in a cylindrical lithium ion battery or the like. In addition, when the carbon negative electrode described in the other Examples 2 to 5 and the positive electrode using LiCoO ₂ as an active material were combined, the average discharge voltage was 3.1 to 1.0.
A 3.7 V non-aqueous electrolyte secondary battery can be configured.

In the examples of the present invention, a battery for evaluating a negative electrode active material or a battery having a positive electrode of LiCoO ₂ was used. However, in order to construct an actual nonaqueous electrolyte secondary battery, LiCoO ₂ or LiCoO ₂ was used. LiNiO ₂ , LiMnO ₂ , Li
It can be widely applied to so-called rocking chair type non-aqueous electrolyte secondary batteries using a positive electrode active material capable of inserting and extracting lithium such as Mn ₂ O ₄ , and is extremely effective in improving the performance of these batteries. is there.

[0046]

As described above, when the carbon material obtained by the present invention is used as a negative electrode, a non-aqueous electrolyte secondary battery having a high capacity and an excellent high energy density can be provided.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view of an evaluation battery according to the present invention.

FIG. 2 is a polarizing microscope observation (1) of the carbon material in the present invention.
000 times) model diagram

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Battery case 2 Sealing plate 3 Current collector 4 Metal lithium 5 Negative electrode 6 Separator 7 Gasket 8 Extinction stripe area 9 Area other than extinction stripe

Claims (3)

[Claims] A non-aqueous electrolyte is used as the electrolyte, a lithium-containing oxide is used as the positive electrode, and the area ratio of the extinction fringe region to the entire area observed with a polarizing microscope under direct light Nicols is 1 as the negative electrode.
A non-aqueous electrolyte secondary battery using a carbon material of about 50%. 2. The carbon material used for the negative electrode is characterized in that the shape of the extinction fringe region obtained by observation with a polarizing microscope is a fibrous structure, and the maximum length of the repetition distance is 1 to 20 μm. The non-aqueous electrolyte secondary battery according to claim 1. 3. The carbon material is obtained by the polycondensation of coal tar pitch, petroleum pitch, an organic synthetic pitch obtained by polycondensation of a condensed polycyclic aromatic hydrocarbon compound, and a heteroatom-containing condensed polycyclic aromatic hydrocarbon compound. The non-aqueous electrolyte secondary battery according to claim 1, wherein any one of the organic synthetic pitch and coal coke obtained as a starting material is carbonized at 700 to 1500 ° C. in an inert atmosphere. .