JP2001229927A - Lithium ion secondary battery having negative electrode containing carbon material and method of identifying carbon material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high-performance lithium ion secondary battery which has small size, large capacity and improved durability and a method of identifying carbon material, having a chemical structure and geometry suiting a negative electrode thereof. SOLUTION: The lithium ion secondary battery comprises the negative electrode containing the carbon material, the carbon material using a condensed polycyclic aromatic compound, having the highest occupied moving orbital(HOMO) value and the lowest unmoving orbit(LUMO) value in quantum chemical calculation contained in a lithium orbital energy zone or close thereto. The method of identifying the carbon material comprises finding the highest occupied moving orbit(HOMO) value and the lowest unmoving orbit(LUMO) value for the carbon material, in a carbon skeletal structure with condensed carbon aromatic cycles which consists of six-member cycles in quantum chemistry calculation. The condensed polycyclic aromatic compound in the carbon skeletal structure, having these values existing in the lithium orbit energy zone or close to the zone, is selected to be used as the negative electrode carbon material for the lithium ion secondary battery.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention
More specifically, the present invention relates to a lithium ion secondary battery using a carbon material suitable for a negative electrode and a method for identifying the carbon material.

[0002]

2. Description of the Related Art In recent years, the use of lithium ion secondary batteries as batteries for notebook computers, mobile phones, electric vehicles, and the like has been expanding, and accordingly, demands for an increase in capacity, a reduction in size and weight, and a reduction in price have been made. And the performance of the anode material is indispensable. By the way, characteristics required for a negative electrode material of a lithium ion secondary battery include (1) a large amount of occlusion / release of lithium, (2) a high speed of occlusion / release of lithium,
(3) small expansion and contraction during occlusion and release; (4)
High initial charge / discharge efficiency; At present, carbon materials such as graphite, graphitizable carbon (soft carbon), and non-graphitizable carbon (hard carbon) are used as negative electrode materials having these characteristics. However, the structure and mechanism of lithium occlusion in a lithium ion secondary battery have not yet been sufficiently clarified.

[0003] In order to further improve the function of a lithium ion secondary battery, it is essential to clarify the structure and mechanism of lithium occlusion. However, since the carbon material used as the negative electrode material is indefinite, it has been experimentally required. Such analysis is extremely difficult. Therefore, it is required to clarify the chemical structure and morphology of a carbon material that is optimal for lithium ion storage.

[0004]

SUMMARY OF THE INVENTION The present invention has been made in view of the above situation in the prior art. That is, an object of the present invention is to provide a high-performance lithium-ion secondary battery having a small capacity, a large capacity, and excellent durability. Another object of the present invention is to provide a negative electrode of a lithium ion secondary battery that can be reduced in size and improved in performance.
An object of the present invention is to provide an identification method for determining a carbon material having a suitable chemical structure and shape. Further, another object of the present invention is to provide a small and high performance lithium ion secondary battery.
An object of the present invention is to provide a method of using a carbon material having a chemical structure and a shape selected by the above-described identification method for a negative electrode.

[0005]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies to elucidate the shape and structure of a carbon material most suitable for storing lithium ions. We learned the structure and mechanism of lithium occlusion of the negative electrode carbon material of the ion secondary battery, and thereby found a carbon structure suitable as a negative electrode material for improving the performance of the lithium ion secondary battery, and completed the present invention. .

That is, the lithium ion secondary battery of the present invention is a lithium ion secondary battery including a carbon material in the negative electrode, wherein the carbon material has the highest occupied orbital (HOMO) value and a HOMO value by a quantum chemical calculation technique. Lowest free orbit (LUMO)
A condensed polycyclic aromatic compound having a value included in orbital energy range of lithium or a value close thereto is used. The method for identifying a carbon material used for the negative electrode of the lithium ion secondary battery according to the present invention is characterized in that the carbon material has a maximum occupancy of each carbon skeleton structure to which a 6-membered carbon aromatic ring is condensed by a quantum chemical calculation method. Orbit
(HOMO) value and lowest unoccupied orbital (LUMO) value are obtained, and these values have a value included in orbital energy region band of lithium or a value close to the region band. Is selected. Furthermore, the present invention is characterized in that the above-mentioned condensed polycyclic aromatic compound is used as a carbon material for a negative electrode of a lithium ion secondary battery.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail. From the viewpoint that it is necessary to clarify the existence of a chemical structure suitable for a carbon material used for a negative electrode in order to improve the performance of a lithium ion secondary battery, the present inventors have proposed a method of quantum chemical calculation. Was used to determine the chemical structure of the carbon material. In the present invention, in order to find out by calculation the chemical substance having a suitable chemical structure by the method of quantum chemical calculation, a quantum chemical calculation program (Q-Chem Inc.
3-2 of the Restricted Hartree-Fock (RHF) method with the company program: Q-Chem
Although performed using 1G, other known quantum chemistry calculation programs can be used as well.

The method of quantum chemical calculation used in the present invention uses the highest occupancy of various carbon skeleton models in order to obtain a chemical structure suitable for the carbon material of the negative electrode for improving the performance of the lithium ion secondary battery. Orbit (Highest Occupied
Molecular Orbital (HOMO) value and lowest free orbit (Lowes)
t Unoccupied Molecular Orbital (LUMO) value was calculated. As the carbon skeleton model, a HOMO value and a LUMO value of each of the condensed polycyclic aromatic compounds in which six-membered carbon aromatic rings formed only of carbon atoms are continuously and plane-bonded to each other are used. The value was calculated and compared with the orbital energy of lithium. In the calculation of the carbon skeleton model, the energy notation is expressed as 1 au = 1 hartree = 27.2116 eV which is an atomic unit.
= 627.5095 kcal / mol was used.

In the present invention, as a carbon material of the negative electrode for improving the performance of the lithium ion secondary battery, a lithium atom (L) for facilitating occlusion and release of lithium ions is used.
included in the orbital energy zone of i), or
A condensed polycyclic aromatic compound having a chemical structure having a HOMO value and a LUMO value close to the region band is used. The orbital energy range of this lithium atom is
Li: -0.146 au (Single Occupied Molecular Orbit
al: SOMO) and Li +: -0.196 au (LUMO), so that for various carbon skeleton models, their HOMO and LUMO values are calculated to form a chemical structure that meets the above conditions. A condensed polycyclic aromatic compound was determined. Note that Li + is RHF / 3−
In 21G, it is -0.194 au, but RHF / 6-
In 311G, it is -0.196 au.

Tables 1 and 2 below show the chemical formulas of 49 kinds of carbon skeleton models actually calculated. Note that TBC in the table is tetrabenzo [bc, ef, kl, no] coronene, and TBCn (n is a positive integer) has n TBC structures as described later. Means that.

[0011]

[Table 1]

[0012]

[Table 2]

The HOMO and LUMO orbital energies of lithium and the 49 calculated carbon skeleton models are shown in Tables 3 and 4 below.

[Table 3]

[0014]

[Table 4]

FIG. 1 is a graph plotting the relationship between orbital energy values (unit: au) of HOMO and LUMO of the carbon skeleton model actually calculated. The horizontal axis shows the number of carbon atoms in the carbon skeleton model, and the vertical axis shows the orbital energy values of HOMO and LUMO of each carbon skeleton model. In addition, the thing of 0 carbon numbers shows the orbital energy in each state of lithium.

Of the 49 carbon skeleton models, the orbital energy region band of lithium atoms, -0.146 to -0.194
The carbon skeleton model having the smallest structural unit included in the range of (−0.196) au was pyrene. Therefore,
The structural feature of the carbon material required in the present invention lies in the presence of Pyrene as a basic building unit. If pyrene is considered as a basic minimum unit building unit, a structure falling within the range derived from the orbital energy of Li can be easily built. If this pyrene is considered as a basic minimum unit, it is possible to create one having various forms as a two-dimensional development-plane.

When searching for a carbon skeleton model,
Tetramer form of a specific structure (may overlap)
It can be seen that the superconductors specifically overlap with the target orbital energy region band of Li. From these facts, the chemical structure suitable for the carbon material of the negative electrode material for improving the performance of the lithium ion secondary battery in the present invention is shown as pyrene represented by the following (1) or (2) below as a unit structural unit. Has the basic form of tetrabenzo [bc, ef, kl, no] coronene (TBC).

[0018]

Embedded image

[0019]

Embedded image

Therefore, the two-dimensional expansion will be described separately for the two types of pyrene and TBC. First, in the pyrene system, there is pyrene as a minimum unit model. As the next candidate, a tetramer of pyrene or the like can be considered, and these can specifically and significantly reduce the LUMO orbital energy of the carbon skeleton model. However, in the tetramer form having a specific structure, no applicable substance other than pyrene was found up to about 70 carbon atoms.

The carbon skeleton model found next is a tetramer of Coronene. This is because LUMO
0.188 au, which falls within the searched numerical range. However, a larger model resulted in a sudden increase in the carbon number, and could not be calculated due to physical restrictions on computer resources. At the same time, 4
In order to satisfy the monomer form, not only the basic structure is complicated, but also it is thought that it has a discrete carbon number. Therefore, it is presumed that further exploration of this series is extremely difficult.

Next, in the case of the TBC system, there arises a problem of the direction of development of the carbocycle. Recently, in Sony's electrode development research, it was reported that bamboo would be an excellent material for carbon electrodes (Kakodate, November 11, 1999, Nikkei, November 1999.
30, Chemistry and Industry, 1999, 52 (2), 171). Therefore, focusing on the experimental results, it is assumed that the carbon skeleton derived from a certain plant cell extends in a certain direction, and to calculate a model extended in a specific direction. did. In addition, since the naming of a chemical structure becomes complicated, the following notation was used.

[0023]

Embedded image In the above notation, four consecutive six-membered ring portions (notation: 4) and three consecutive six-membered ring portions (notation: 3) are represented by numerical values, and the structure is uniquely defined only by counting the three consecutive portions. It is possible to do. For example, if there are two consecutive portions, the following notation is used. Hereinafter, this notation is used.

[0024]

Embedded image

The calculation results of the TBC system are shown in Table 5 below.
And FIG. Figure 2 shows tetrabenzo [bc, ef, kl, no]
Coronene (tetrabenzo [bc, ef, kl, no] coronene: TBC)
(Where n is the number of TBC structures) and the HOMO-LUMO
FIG. 4 is a diagram showing a relationship between orbital energies (vertical axis). In addition, the thing of length 0 has shown the orbital energy in each state of lithium.

[0026]

[Table 5]

When the number of three continuous parts becomes three or more, the LUMO orbital energy of the carbon skeleton model falls within the numerical range derived from the orbital energy of Li, and when the number of three continuous parts becomes six or more, the carbon skeleton increases. HOMO and L of model
The orbital energy of the UMO falls within the numerical range derived from the orbital energy of Li. All larger model structures fall within this numerical range.
This result indicates the minimum length in the extension direction.
In other words, considering the extended carbon skeleton model,
The continuous part is 6: 28.30mm long (total surface area: 119.64)
It suffices to obtain a carbon skeleton larger than the minimum carbon skeleton model having Å ² ).

From the above results and drawings, the carbon material used for the negative electrode of the lithium ion secondary battery of the present invention has the minimum structural unit falling within the range of the Li orbital energy (-0.146 to -0.196 au). A condensed polycyclic aromatic compound having a carbon skeleton represented by the structural formula (1) or (2) as a basic form is preferable. Among them, a condensation having a bay structure (A) and an edge structure (B) in a chemical structural formula at the same time Polycyclic aromatic compounds are more preferred.

In the present invention, the term “bay structure” refers to a condensed polycyclic aromatic compound in the chemical structure represented by the following chemical structural formula (4): As described above, it refers to a chemical structure in which the uppermost surface of a six-membered carbon ring alternately and regularly forms a convex surface and a concave surface in a lateral direction. In addition, the edge structure is also the same as the surrounded vertical portion shown as the edge structure (B) in the following chemical structural formula (4), in which the carbon atoms on the outermost surface of the six-membered carbon ring are the same. A chemical structure that is regularly arranged on a surface.

[0030]

Embedded image

Among the carbon materials sought in the present invention,
As the condensed polycyclic aromatic compound having both the bay structure (A) and the edge structure (B) in the chemical structural formula, a tetrabenzo [bc, ef, kl, no] coronene structure represented by the structural formula (2) is used. A condensed polycyclic aromatic compound having a minimum unit of 1 to 8 in the chemical structural formula is more preferable, and the most preferable is a tetrabenzo [b represented by the following structural formula (3).
c, ef, kl, no] Compound having six coronene structures (TBC
6).

[0032]

Embedded image

In the process of searching for a carbon skeleton model by HOMO and LUMO in the present invention, knowledge on the chemical structures of several important carbon materials to enter the orbital energy of Li: -0.146 to -0.196 au was obtained. Was.
These are mainly the following six points. 1. The basic unit structure is pyrene. 2. The basic structures for two-dimensional expansion are pyrene and 4
-3-4 type tetrabenzo [bc, ef, kl, no] coronene (TB
C). 3. The tetramer form of a specific structure (which may overlap) is specifically included in the numerical range of Li orbital energies. 4. When pyrene is used as the basic structure, pyrene and tetracoronene are sufficient. 5. Those having the basic structure of tetrabenzo [bc, ef, kl, no] coronene (TBC) of the type 4-3-4, when having a structure elongated in a specific direction, specifically have a Li orbital energy. It can be adjusted to fall within the numerical range. 6. The stretched 4-3-4 type poly TBC system has a length of 6: 28.30Å in 3 consecutive portions (total surface area: 119.64Å ² ), and these figures are the minimum values.

[0034]

According to the present invention, the chemical structure of a condensed polycyclic aromatic compound suitable as a carbon material for use in a negative electrode of a lithium ion secondary battery can be clearly shown. This can greatly contribute to the production of an ion secondary battery. That is, in the present invention, as the carbonaceous material, the carbon skeleton of the condensed polycyclic aromatic compound falling within the range of the orbital energy of the lithium atom could be found. When a laminate of carbonaceous materials obtained by laminating layered bodies formed using a condensed polycyclic aromatic compound is used as a negative electrode material, lithium can be extremely easily transferred, so that even a small size has a large capacity and a long life. It is possible to manufacture a high performance lithium ion secondary battery having the following.

[Brief description of the drawings]

Fig. 1 HOMO and LUMO of various carbon skeleton models
3 is a graph showing the relationship between orbital energies (unit: au).

Fig. 2 Tetrabenzo [bc, ef, kl, no] coronene (TB
The length (horizontal axis) of TBCn and H
It is a graph which shows the relationship of the orbital energy (vertical axis) of OMO-LUMO.

Claims (8)

[Claims] A lithium ion containing a carbon material in a negative electrode
In the secondary battery, as the carbon material, the highest occupied orbit (HOMO) value and the lowest unoccupied orbit (L
A lithium ion secondary battery using a condensed polycyclic aromatic compound having a UMO value that is within or close to the orbital energy range of lithium. 2. A fused polycyclic aromatic compound having, as a structural unit, pyrene represented by the following (1) or tetrabenzo [bc, ef, kl, no] coronene represented by the following (2) as a structural unit. The lithium ion secondary battery according to claim 1, wherein: Embedded image Embedded image 3. The lithium ion secondary battery according to claim 1, wherein the condensed polycyclic aromatic compound is a compound having both a bay structure and an edge structure. 4. The compound having a bay structure and an edge structure simultaneously having 1 to 8 structures having tetrabenzo [bc, ef, kl, no] coronene as a minimum unit. The lithium ion secondary battery according to any one of claims 1 to 3. 5. A compound having both a bay structure and an edge structure is a tetrabenzo [bc, ef, kl,
[no] The lithium ion secondary battery according to any one of claims 1 to 4, wherein the lithium ion secondary battery has a structure having six coronene. Embedded image 6. A method for identifying a carbon material used for a negative electrode of a lithium ion secondary battery, wherein the carbon material comprises:
By the method of quantum chemical calculation, the highest occupied orbital (H
OMO) value and the lowest unoccupied orbital (LUMO) value are determined, and these values are converted to a condensed polycyclic aromatic compound having a carbon skeleton structure having a value included in orbital energy band of lithium or a value close to the band. A method for identifying a carbon material used for a negative electrode of a lithium ion secondary battery, wherein the method comprises sorting. 7. The method for identifying a carbon material used for a negative electrode of a lithium ion secondary battery according to claim 6, wherein the condensed polycyclic aromatic compound is a compound having both a bay structure and an edge structure. 8. The fused polycyclic aromatic compound selected by the method according to claim 6 or 7,
Method to use as negative electrode carbon material of secondary battery.