JP2002124260A - Negative electrode carbon material for lithium-ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a carbon material, small-sized, lightweight, capable of storing a large volume of lithium, having high performance, and optimum for a negative electrode of a lithium-ion secondary battery. SOLUTION: As this negative electrode carbon material for a lithium-ion secondary battery, fibrous carbon is used having a three-dimensional carbon layer structure provided with air gaps between internal carbon crystal grain layers for storing lithium, lithium ions, and lithium ion clusters, and provided with inlets/outlets allowing lithium ions and lithium ion clusters to easily move therethrough. As the fibrous carbon, tubular hollow yarn is preferable. Preferably, the air gaps capable of storing the lithium, lithium ions, and lithium ion clusters, are cracks and pores.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a carbon material used for a negative electrode of a high performance lithium secondary battery.

[0002]

2. Description of the Related Art The use of lithium ion secondary batteries as batteries for notebook computers, mobile phones, electric vehicles, etc. has been dramatically expanded, and accordingly, large capacity, small size, light weight, low price, etc. Demands are also increasing. In order to meet those demands, it is essential to improve the performance of the negative electrode material. Generally, characteristics required for a negative electrode material of a lithium ion secondary battery include: (1) a large amount of occlusion and release of lithium; (2) a high rate of occlusion and release;
(3) small expansion and contraction during occlusion and release; (4)
High initial charge / discharge efficiency; Therefore, kinetic studies on the insertion and extraction reactions of lithium are required for the carbon material used for the anode of the secondary battery, and the structure of the carbon material and the lithium storage The importance of material design based on detailed analysis of the mechanism is increasing.

For the electrode material on the negative electrode side of a lithium ion secondary battery, active research and development has been conducted on various carbon materials with a view to improving performance such as battery cycle characteristics and safety during handling. Some of them are already in practical use. Examples of the carbon material include graphite, carbon fiber, resin calcined carbon, coke, pyrolytic vapor-grown carbon, polyacene-based organic semiconductor, mesocarbon microbeads (MCMB), mesophase pitch-based carbon fiber, graphite wesker, quasi-isotropic Examples include carbon and a fired body of a natural material. Among them, recently, mainly graphite, MCMB, mesophase pitch-based carbon fiber, and quasi-isotropic carbon have been widely used as negative electrode materials.

[0004] The crystallinity of the carbon material depends on the type of carbon source used and the heat treatment temperature (HT).
T), and there are a number of materials ranging from near amorphous to graphite crystals. Further, the reason why the discharge capacity changes due to the HTT is that the carbon structure changes due to the HTT, and the lithium storage site causes a complicated change. Generally, carbon materials used for the negative electrode can be roughly classified into the following four types. (1) Graphitic material (carbon material with advanced graphitization) (2) Heat treatment at 1,000 to 2,000 ° C as carbonaceous material (carbon material with low graphitization and low crystallinity) Graphitizable carbon: Soft carbon (material that graphitization easily proceeds by heat treatment) (3) Graphitizable carbon hardened by heat treatment at 1,000 to 1,400 ° C: Hard carbon (Graphization proceeds by heat treatment) (Hard material) (4) Low-temperature calcined carbon (non-carbonized material heat-treated at 550 to 1,000 ° C)

At present, a graphite material represented by natural graphite is widely used as a negative electrode of a lithium ion battery, and a lithium graphite intercalation compound (Lithium Carbon Interc) is used.
alation Compound: First stage of Li-CIC (L
In the process of producing iC ₆ ), a maximum discharge capacity value of 372 mAh / g was obtained. The charge / discharge curve of graphite greatly depends on the electrolyte solvent. For example, when a mixed solvent of ethylene carbonate (EC) and a chain carbonate is used as the electrolyte solvent instead of propylene carbonate (PC), Since the irreversible capacity is greatly reduced, it can be used as a practical anode material [(a) AM Dey and BP
Sullivan, J. Electrochem. Soc., 1970, 117,222.,
(B) Z. Jiang, M. Alamgir, KMAbraham, J. Electroc
hem. Soc., 1995, 142, 333].

However, the graphite powder is generally in the form of flakes, and the crystallites are densely transferred to the planes in the flakes, so that the lithium ion insertion and desorption reactions are concentrated on the end faces of the flakes and As a problem, the reaction is difficult to proceed, and when discharging a large current, the capacity is greatly reduced, and
There is a disadvantage that the cycle life is short. Therefore, in order to put graphite powder into practical use, it is necessary to optimize the fine structure, powder shape, and electrode structure of the graphite material to draw out a high capacity.

[0007] Graphitized mesophase pitch-based carbon fibers (MCF) obtained by heat-treating carbon fibers made from mesophase pitch separated and extracted from pitches at a temperature of 2,800 ° C or more are used as high-performance negative electrode materials. It is attracting attention. The negative electrode performance of this graphitized MCF was 300 discharge capacity.
mAh / g, initial charge / discharge efficiency 94%, relatively high (N. Takami, A. Satoh, M. Hara, T. Ohsaki, J. Elec
trochem. Soc., 1995, 142, 2564).
As shown in the electron micrograph of FIG. 1, the cross section of the graphitized MCF fiber has crystallites oriented in a state close to a radial state. The carbon fiber having such crystal orientation smoothly absorbs lithium ions from all directions of the fiber powder.
Because it can be released, it is advantageous for rapid charge and discharge operation,
Since it is a fiber-shaped powder, it is also possible to enhance the negative electrode filling property. Graphitized MCF in a polyethylene oxide polymer electrolyte has excellent high current characteristics as compared with other graphitic materials.

[0008] In general, graphitizable carbon such as coke, MCMB, and MCF heat-treated at 1,000 to 2,000 ° C generally has a charge / discharge curve that changes as the crystallite grows when the heat treatment temperature is increased. I do. For example, when the heat treatment temperature is set to a high temperature of 2,000 ° C., the discharge capacity is reduced as compared with the case where the heat treatment is performed at 1,000 ° C. In addition, 1,000 to 1.5
For graphitizable carbon heat treated at 00 ° C., 200 to 25
Since it was an inexpensive carbon material having a capacity of about 0 mAh / g, a smooth charge-discharge reaction and excellent cycle characteristics, it was first adopted as a negative electrode material.

Further, non-graphitizable carbon is a thermosetting resin.
Is a material obtained by carbonizing
Crystalline structure develops with crystallite orientation disorder maintained
do not do. Especially difficult to heat treated at 1,000 ~ 1,400 ℃
Graphitizable carbon has a density of 1.5 to 1.8 g / cm.³Degree and
It is low and has many nano-sized voids. This
Charge and discharge characteristics of carbon materials
Capacity is higher than that of carbon, and the electrode potential of lithium metal
The characteristic is that a potential flat portion is shown at a potential close to. Inside
However, quasi-isotropic carbon has a potential flat around a charge / discharge curve of 0 V.
Tanbu and LiC ₆(372 mAh / g)
(Naohiro Sonobe, Minoru Ishikawa, Takashi Iwasaki)
Husband, Abstracts of the 35th Battery Symposium, Nagoya, 1994, 2B09, P4
7). The emergence of such a potential flat portion and high capacity has
Due to lithium occlusion reaction into voids and unstructured parts
(N.Takami, A.Satoh, T.Ohsaki,
 DENKI KAGAKU, 1998, 66, 1270).

[0010] The non-graphitizable carbon is effective in increasing the energy density of the battery because it has a high capacity exceeding LiC ₆ and has a good potential flat portion with a small hysteresis of the charge / discharge potential. However, since the true density and the initial charging efficiency are lower than those of the graphite material, it is disadvantageous for achieving a high capacity of the battery, and a device for extracting a high capacity at the time of rapid charge / discharge is required.

Further, the carbon precursor is heated at 550 to 1,000 ° C.
Is heat-treated at low temperature, called low temperature sintering, and has intermediate properties between polymer and carbon. This low-temperature sintered carbon has a discharge capacity exceeding 372 mAh / g.
Above all, polyacene-based organic semiconductors (PAS) obtained by heat-treating a phenol resin at 500 to 700 ° C. have been attracting attention as high-capacity negative electrode carbon (S. Yata, H. Kinoshita, MK).
omori, T. Kashiwamura, T. Harada, K. Tanaka, T. Yamabe,
Synth. Met., 1994, 62, 153).

Low-temperature calcined carbon is an uncarbonized material in which some heteroatoms such as oxygen and nitrogen that are not completely carbonized remain, and for example, have an atomic ratio (H / C) of hydrogen to carbon of 0.1. 26
Thus, perylene calcined carbon heat-treated at 550 ° C. can achieve a high discharge capacity of 804 mAh / g and an initial charging efficiency of 65%. However, since these low-temperature calcined carbons exhibit large hysteresis and irreversible capacity at the discharge potential acting negatively as a negative electrode material,
There is a problem that improvement of the performance is essential.

[0013]

SUMMARY OF THE INVENTION The present invention aims at theoretically elucidating the optimal structure of a carbon material used for a negative electrode, which is indispensable for improving the performance and increasing the capacity of a lithium ion secondary battery. , To develop a high performance lithium ion secondary battery. That is, an object of the present invention is to provide a carbon material that is small and lightweight, and that is optimal for a negative electrode of a high-performance lithium ion secondary battery capable of storing a large amount of lithium.

[0014]

SUMMARY OF THE INVENTION The present inventors have found that it is extremely difficult to experimentally analyze the shape and structure of a carbon material that is optimal for storing lithium ions for a carbon material used for a negative electrode of a lithium ion secondary battery. From the viewpoint that it is difficult, we conducted numerical simulations based on theoretical approaches of quantum mechanics, quantum engineering, solid-state physics and quantum chemistry, and conducted research to elucidate the structure and mechanism of lithium occlusion of carbon materials. The present inventors have found out the structure and shape of a carbon material capable of storing lithium, and have completed the present invention.

That is, the negative electrode carbon material for a lithium ion secondary battery of the present invention has a void between the layers of the crystallite of the carbon material capable of absorbing lithium, lithium ions and lithium ion clusters, and has a lithium, lithium, It is characterized by using fibrous carbon having a three-dimensional carbon structure having an entrance through which lithium ions and lithium ion clusters can easily move. The fibrous carbon used in the present invention is preferably a tubular hollow fiber.
The voids of the fibrous carbon capable of storing lithium, lithium ions, and lithium ion clusters are preferably cracks and pores.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail. In order to improve the performance of a lithium ion secondary battery, it is necessary to investigate in detail the structure of hard carbon (hardly graphitizable carbon) for the carbon material used for the negative electrode. In particular, it is indispensable to clarify the specificity of the laminated structure formed by the carbon of the hard carbon and the randomness of the granular mass. This is because hard carbon has characteristics and unique characteristics not found in other carbon materials such as soft carbon and graphite (graphite).

As a result of a detailed study of one graphite sheet by the present inventors, the existence of an optimal graphite sheet shape was clarified. The following conditions are necessary for the optimized graphite sheet shape. 1. The basic unit structure is Pyrene shown in Fig. 2.
It is. 2. A tetramer of a specific structure (can be overlapped)
Specifically falls within the numerical range of lithium orbital energies. 3. The basic structure for performing two-dimensional expansion is pyrene shown in FIG. 2 and tetrabenzo [bc, ef, kl, no] coronene (TBC) shown in FIG. It is filled. 4. When pyrene is used as a basic structure, it is filled with pyrene shown in FIG. 2 and tetracoronene shown in FIG. 5. In the case of using tetrabenzo [bc, ef, kl, no] coronene (TBC) as a basic structure, it can be adjusted so as to be specifically in the orbital energy range of lithium by extending in a specific direction. 6. As shown in FIG. 5, in a carbocyclic compound having both a bay structure and an edge structure, the state of orbital characteristics of the carbon structure can be directly reflected on the state of orbital characteristics of lithium. Therefore, a bay structure and an edge structure are required at the same time.

The minimum carbon structure satisfying the above conditions is:
Tetrabenzo [bc, ef, kl, no] represented by the chemical structural formula in FIG.
Coronene (Tetrabenzo [bc, ef, kl, no] coronene: TB
C) and a hexamer (n = 6, TBC6) as a unit. Further, as a result of examining the interaction between the graphite sheet and lithium ions, it was found that the operation can be efficiently performed if the flow of lithium ions in and out flows in a certain direction.

Next, the carbon structure and shape capable of controlling the movement of lithium ions in the charge / discharge process of the lithium ion secondary battery were examined in more detail. As shown in FIG. 7, there are carbon materials having various structures. In particular, in the case of hard carbon, the order of arrangement of carbon crystallites therein, that is, the arrangement between carbon crystallites. It is considered that the randomness of the graphite greatly contributes to the behavior of lithium ions. Therefore, graphite (Graphite) and graphite intercala
Discussion Compound (GIC) structure was considered.

First, graphite has a carbon-carbon distance of 1.415 ° and a carbon sheet distance of 3.354 ° as shown in FIG. At this time, it is composed of a structure in which carbons do not overlap each other, that is, an ABA structure. Next, in the GIC doped with lithium, lithium ions are taken in at a wider interval than 3.354 ° which is a distance between graphite carbon sheets. The greatest change at this time is that the carbon has a structure in which carbons overlap each other, that is, an AAA structure. However, in this case, even if a structure in which carbon and carbon overlap each other is formed, since the distance between the carbon sheets is widened by interposing lithium ions therebetween, the force of strong repulsion between the upper and lower carbons is small. It is weakening.

Therefore, in order to theoretically design a carbon material capable of storing a large amount of lithium, considering the above,
It is important to clarify the structure in which hard carbon is doped with lithium. In particular, the form between micropores (voids) formed by carbon crystallites is important, and this micropore (void) is controlled. Is essential. FIG. 10 is a dope model showing an example of a structure in which lithium, lithium ions, and lithium ion clusters are occluded in micropores (voids) between carbon layers (crystallites) of hard carbon used as a carbon material in the present invention. FIG.

In the present invention, a basic model of a new concept is introduced into the laminated structure of the carbon material and considered. First, regarding the graphite structure, which is a representative of carbon materials,
Assuming that the original state of graphite not doped with lithium ions has an ABA structure and the state doped with lithium ions has an AAA structure, a phase change has occurred, and the following explanation can be given.

Lithium has a function as a dielectric in a pair with one graphite sheet, and the graphite sheet sandwiching the lithium can be regarded as a capacitor plate. When this is regarded as the behavior of the condenser at the molecular level, it can be regarded as a molecular condenser as shown in FIG. The feature of this basic model is that the role played by lithium is that lithium and the graphite sheet work together as a capacitor. In other words, the phenomenon of being charged is that a dielectric is inserted between two graphite sheets as in a capacitor containing a normal dielectric, and the work function from the electric field at that time is as follows: It is stored as energy instead of electrical energy.

When this basic model is further extended, the phenomenon on the carbon electrode of the lithium ion battery can be explained as follows with reference to FIG. FIG. 12 is a conceptual diagram illustrating a charging / discharging mechanism. First, lithium ion exists around graphite (hard carbon),
When the hard carbon has the ABA structure, it is in a completely discharged state. Next, to perform charging from this state, an electric field is applied to the entire system, and a work function is supplied from the outside. At this time, one dielectric is formed because the lithium ions exhibit the same behavior as some of the graphite sheets.

In this dielectric, lithium ions are inserted between electrodes formed of two graphite sheets due to a work function supplied from the outside. When this insertion phenomenon occurs, the structure of the graphite system changes from ABA to A
AA changes phase. When the insertion of the dielectric into all the structures is completed, the battery is fully charged. That is, as shown in FIG. 13, the doping pattern of lithium ions between the graphite sheet layers is the same as the phenomenon occurring at the macro level on the negative electrode side of the battery and the phenomenon occurring at the micro level in the molecular capacitor model. , They are continuously linked.

Therefore, the concept considered as a molecular capacitor can be considered to be the same as a general parallel plate capacitor as shown in FIG. In a normal parallel plate capacitor, a set of conductors (electrodes) can store a charge, which is charging. And in a normal parallel plate capacitor, the electric field E, the potential difference V, the electric quantity Q, and the electric capacity C are
Each is expressed by the following relational expressions (1) to (4).

[0027]

(Equation 1) (Where k is a proportionality constant, σ is the charge density, d is the distance between the plates, and S is the area of the electrodes of the parallel plate capacitor.)

The function of the dielectric in the ordinary capacitor is that when a dielectric (insulator) is inserted between the electrode plates of the capacitor, the electric capacity can be made larger than in a vacuum. Therefore, the capacitance C _O and capacitance C at the time of dielectric inserts during vacuum is expressed by the relational expression such as the following equation (5) to (7), between the plates, rather than time of vacuum epsilon _gamma It will have twice the electric capacity.

[0029]

(Equation 2)

Next, based on the concept of the new molecular capacitor described above, the following non-parallel plate capacitor model was set, and the capacity of the capacitor was considered. That is, the non-parallel plate capacitor model is a capacitor that is rectangular (the length of each side is a and b) but is not exactly parallel to the lower capacitor plate. However, while the lower condenser plate has a fixed length, the upper condenser plate has a variable length that can be changed according to the lower condenser plate. The upper condenser plate has an inclination θ in accordance with the fixed length a of the lower condenser plate.
Can have an infinite size for. Here, the capacitance of the condenser was calculated with the inclination of both plates as θ.

FIG. 16 is a conceptual diagram of the above-mentioned non-parallel plate capacitor model. In this non-parallel plate capacitor, the capacitance C (x) = ε as shown in FIG.
It can be considered that micro parallel capacitors of ₀ · b (dx / y) are connected in parallel. Here, since y = d + X · tan θ from FIG. 16, the total capacitance C
(X) becomes the following equation (8).

[0032]

(Equation 3)

Here, when the term of atan θ / 2d is newly set to x, equation (8) is expressed as equation (9).

(Equation 4)
(9)

In equation (9), 1/2 × · ln
When the term of [(1 + x) / (1-x)] is newly set to F (x), the expression shown on the rightmost side in Expression (9) is obtained. In this equation (9), ε ₀ ab / d can be considered as a constant.

(Equation 5) Is a substantial dominant factor in determining the capacitance that a non-parallel plate capacitor can store. That is, F
The control in (x) is a factor that increases the capacitance that the capacitor can store. FIG. 17 focuses on the function of F (x) and draws a graph of F (x) within the range of 0 <x <1. Note that this value of F (x) is equal to ε ₀ ab
The value multiplied by / d is the capacitance that the actual non-parallel plate capacitor can store.

Considering the result from a physical point of view,
Due to the fixed length of the lower condenser plate and the variable length of the upper condenser plate, θ is 0 ° ≦ θ <
Because it can be tilted at 90 °, the upper condenser plate can have infinite size. Further, it can be seen that a certain projection area can be provided to the lower condenser plate, which has a decisive effect on the entire capacitance. And, since the provision of the inclination between the capacitor plates has the same effect as the effect of inserting the dielectric, when doping the hard carbon with lithium ions, the synergistic effect of the inclination effect and the effect of inserting the dielectric is high. Capacitance can be created.

Next, in order to improve the performance of the lithium ion secondary battery, it is necessary to clarify the characteristics of the laminated structure formed by carbon (crystallite) used as the negative electrode material and the randomness of the granular mass. With reference to the above-described non-parallel plate capacitor model, the following non-parallel plate capacitor model in which the upper portion has a fixed length and the lower portion has a variable length was considered. The lower condenser plate has a finite size corresponding to the inclination θ in accordance with the fixed length a of the upper condenser plate. This means
Since the length of an actual carbon crystallite is finite, the length of the lower capacitor corresponds to having a length corresponding to the inclination θ. That is, in the above-mentioned model, the finite length corresponding to the actual carbon crystallite was not supported for the length of the capacitor, but in the model here, the infinite length portion was corrected and the finite length was changed. By changing the model, a model very well suited to a real system as shown in FIG. 10 is obtained. This capacitor has a rectangular shape (the length of each side is a and b) but is not exactly parallel, as in the non-parallel plate capacitor model described above. The capacitance was theoretically calculated for the model.
A conceptual diagram of the non-parallel plate capacitor model in this case is shown in FIG.
FIG.

The capacitance of the non-parallel plate capacitor is shown in FIG.
The capacitance C (x) = ε ₀ b (dx / y) as shown in FIG.
Can be regarded as being connected in parallel, and can be obtained in the same manner as described above.
Here, since y = d + X · tan θ,

(Equation 6)

Here, if the term of asin θ / 2d is newly set to x, equation (10) is expressed as equation (11).

(Equation 7) In equation (11), cos θ / 2x · ln [(1+
x) / (1−x)] is newly expressed as F (x, θ), and the expression shown on the rightmost side of Expression (11) is obtained. In this equation (11), ε ₀ ab / d can be considered as a constant.

This F (x, θ)

(Equation 8) Is a substantial controlling factor that determines the capacitance that can be stored in a non-parallel plate capacitor. That is, F
Control of (x, θ) is a factor that increases the capacitance that can be stored in the capacitor. FIG.
It is a graph drawn in the range of 0 <x <1, paying attention to the function of (x, θ). Note that the F (x, θ) value is a constant ε ₀
The value multiplied by ab / d is the capacitance that the actual non-parallel plate capacitor can store.

Considering this result from a physical point of view,
The term of cos θ of F (x, θ) is a term derived from the area of the upper condenser plate projected on the X-axis side at an angle θ, and is a condenser part where the projected upper condenser can work effectively. Can store electrical energy. Also, 1 / 2x · ln [(1+
The term x) / (1-x)] is an effect that occurs when the capacitor plate is tilted, as in the result of the above-described model. FIG. 19 shows the coefficient (X axis) generated when the capacitor plate is tilted and the degree of tilt (θ axis) of the capacitor plate, and calculates the ratio F (x, θ) of the electrostatic capacity that can be stored in the capacitor. This is taken on the Z axis. Then, in FIG. 19, as shown in Expression (11), F (x,
The value obtained by multiplying the value of θ) by a multiple of the constant ε ₀ ab / d is the capacitance that can be stored in an actual non-parallel plate capacitor.

From the above, it was first clarified that it is important to control the carbon material used for the negative electrode material of the lithium ion battery, particularly the hard particles in the hard carbon. That is, the hard carbon needs to have a structure having fine voids (micropores) capable of storing a large amount of lithium ion clusters in addition to lithium and lithium ions. Is a necessary requirement for large capacity. In other words, the particles in the direction of the agglomerates forming the carbon material, i.e., the individual large and small carbon crystallites, form many differently shaped spaces between the carbon crystallites. It is considered that the control of the lumps makes it possible to increase the capacitance. This will be further described with reference to the drawings. FIG. 20 is an enlarged view of a part of FIG. A void (micropore) portion newly generated between the granules as shown in FIG. 20 and one of which is greatly open like a trumpet is basically the same as the model in FIG. It fits very well with the lithium doping model in carbon. The equation (11) derived from the shape model of FIG. 18 and the equation (12) excluding the constant term part in the equation (11) are important. FIG. 19 is drawn based on the equation (12). The value obtained by multiplying FIG. 19 by the constant term ε ₀ ab / d is the actual capacitance of the capacitor.

The relationship between the molecular condenser and the hard carbon is as follows, assuming that a granular mass as shown in FIG. 20 is a new condenser even if the individual graphite sheets are parallel to each other. They are common in that they can be considered as a larger class capacitor. From this, the control of the morphology of the agglomerates is a direct factor that controls the performance of the condenser, and the reason that the hard carbon has a large capacity is that the crystallites are originally arranged randomly in the hard carbon. Second, it can be considered as a parallel capacitor. In such a parallel capacitor, since the sum of the electric capacities of the respective capacitors, the hard carbon has a large capacity.

Next, the results of investigating the dependence of sites by placing lithium ions on coronene using coronene as a model calculation for a lithium ion secondary battery have already been reported by Yamabe et al. (H. Ago , K.Nagata, K.Yoshi
zawa, K.Tanaka, T.Yamabe, Bull.Chem.Soc.Jpn., 1997,70,
1717.). Therefore, the present inventors placed lithium ions on the coronene and re-examined the site dependence. As a result, they were able to explore the results of Yamabe et al. More deeply and find a new order of how to fill the energetically stable lithium ion sites, which had not been known so far. As shown in FIG. 21 (with a terminal hydrogen cap) and FIG. 22 (without a terminal hydrogen cap), the result is such that the number of lithium-lithium bond combination patterns is minimized according to the graph theory. , Lithium ions are buried from around the coronene,
Finally, it was found that all positions were filled and stabilized in the neutral state. In other words, lithium is packed while moving from the periphery of the graphite sheet to an energy-stable position.

A high-performance lithium-ion battery using graphitized mesophase pitch-based carbon fiber (MCF) has been proposed by the battery development group of Toshiba. From the electron micrograph of the cross-section of the graphitized MCF shown in FIG. 1, it is estimated that the factor for improving the performance of the MCF is due to the fact that cracks in the carbon fiber show an important function. This is because the size of the hole in the region where not only lithium and lithium ions but also lithium ion clusters can enter and exit is extremely important, as in the importance of controlling the agglomerates first. It is clear from the photos. This is the true identity of a high-performance lithium-ion battery, and the same phenomenon occurs in a lithium-ion battery using hard carbon. Although this carbon material can be used for next-generation lithium-ion batteries, it is extremely difficult to artificially control the generation of cracks required for large-capacity batteries using lithium-ion clusters. It is presumed to be insufficient to achieve this.

Further, Mitsui Mining has announced that it will enter the negative electrode material market for lithium ion secondary batteries by 2000 (Nikkan Kogyo Shimbun, Nippon Kogyo Shimbun, Nikkei Sangyo Shimbun, October 28, 1999. ). There is disclosed a simple method for producing a carbon material, but the problem here is that natural graphite is consolidated into a spindle shape, and a crystalline carbon film is formed on the powder surface. It is. This is presumed to have been found by trial and error by experiment, but its meaning is not considered to be theoretically clarified
(M.Yoshio, HY.Wang, K.Fukuda, Y.Hara, Y.Adachi, J.Elect
rochem. Soc., 2000, 147, 1245). However, this manufacturing process has a very important technical meaning. For example, it has been shown that it is important to increase the surface area of a powder surface by performing rearrangement like a quantum dot or a quantum well to which the latest quantum engineering technology is applied. Making the shape of the spindle into a spindle shape by compacting is nothing less than creating a crack. However, in this method, cracks and pores cannot be sufficiently controlled.

Therefore, in the present invention, considering the above theoretical considerations, in order to achieve a large capacity of the carbon material used for the negative electrode of the lithium secondary battery, the internal carbon crystallite layer must be It has a structure with many voids large enough to store lithium, lithium ions and lithium ion clusters, and at the surface of the carbon material, there is an entrance and exit through which the lithium, lithium ions and lithium ion clusters can easily move. It needs to be fibrous carbon having a three-dimensional carbon layer structure. This fibrous carbon is the result of rearrangement based on the results of FIG. 18 and FIGS. 19 and 20 derived therefrom so as to obtain the maximum surface area per unit volume. Originally, the shape having the maximum surface area per unit volume is spherical, but this shape cannot create an entrance used for lithium movement. Therefore,
Next, the shape from which the maximum surface area can be derived is a cylindrical shape, and in this case, the shape can satisfy all the above conditions. As a result of examining from a different viewpoint based on this recognition, cracks were generated artificially by using carbon fiber tubular hollow fibers originally developed as an application of phase separation technology and used in artificial dialysis. It is possible to control it. FIG. 23 shows an electron micrograph of a cross section of a hollow fiber which is an example of the carbon fiber used in the present invention. Such a cylindrical hollow element has an outer surface and an inner surface, and therefore has a large surface area, and is a structure advantageous for entry and exit of lithium, lithium ions, and lithium ion clusters.

As shown in the cross-sectional photograph of FIG. 23, this tubular hollow fiber of carbon fiber forms many cracks which are ideal as a carbon material used for the negative electrode, and has many pores. As a result, a large amount of lithium, lithium ions and lithium ion clusters can be stored in these voids. At the same time, Table 1 below
Lithium structural data shown in Revised 4th edition, Chemical Handbook Basic Edition II, Chemical Society of Japan, Maruzen, 1993, RDShannon, CT
Prewitt, Acta Crystallogr., 1969, B25, 925-946, RDSha
nnon, Acta Crystallogy., A32, 751-767., A. Bondi, J. Phys.
Chem., 1964, 68, 441-451.), The size of the void can be adjusted according to the size of lithium, etc., so the lithium-ion cluster battery has a much higher performance than existing lithium-ion batteries. Can be created.

[Table 1]

The advantages of using the above-described tubular hollow fiber of carbon fiber for the carbon electrode mainly include the following four points. (1) Cracks and pores having optimal sizes can be freely controlled according to the size of lithium, lithium ions or lithium ion clusters. (2) Since the surface area can be further increased by the pores and the hollow fiber also has a surface portion inside, the total surface area is at least twice as large as the external surface area. (3) Due to pores and cracks, a large amount of dangling bonds equal to or greater than hard carbon can be generated. (4) In the hollow fiber, lithium can be doped throughout the entire area of the fiber. That is, doping of a large amount of lithium, lithium ions, and lithium ion clusters is possible.

[0049]

According to the present invention, a carbon material having a structure capable of occluding and releasing a large amount of lithium, lithium ions and lithium ion clusters is used for the negative electrode carbon material.
This makes it possible to create high-capacity, high-capacity lithium-ion cluster batteries that are small in size. Among them, the use of a tubular hollow fiber as the negative electrode carbon material makes it easy to control the voids and, if desired, has a void having the optimal size according to the size of lithium, lithium ion or lithium ion cluster. Can be created freely. Furthermore, since the hollow fiber has a hollow interior, the hollow fiber also has an internal surface, and the surface area is at least twice as large. Therefore, hollow fibers, a large amount can generate hard carbon or dangling bonds, large quantities of lithium, allows lithium ions and doped lithium ion cluster, C ₃ Li or lithium experimental limit the current Therefore, a large capacity lithium ion cluster secondary battery can be realized.

[Brief description of the drawings]

FIG. 1 is an electron micrograph of a cross section of a graphitized MCF.

FIG. 2 is a chemical structural formula of pyrene.

Fig. 3 Tetrabenzo [bc, ef, kl, no] coronene (TB
It is a chemical structural formula of C).

FIG. 4 is a chemical structural formula of tetracoronene.

FIG. 5 is a chemical structure of a carbon material having a bay structure (1) and an edge structure (2).

FIG. 6 is a chemical structural formula of a hexamer (n = 6, TBC6) having tetrabenzo [bc, ef, kl, no] coronene as a unit.

FIG. 7 is a schematic view of a typical structure of a carbon material.

FIG. 8 Structure of graphite (A: carbon-carbon distance: 1.415 °, B: carbon sheet distance: 3.354)
Å).

FIG. 9 shows the structure of a graphite intercalation compound.

FIG. 10 is a model of lithium doping in hard carbon.

FIG. 11 is a conceptual diagram of a molecular condenser.

FIG. 12 is a conceptual diagram of a charging / discharging mechanism.

FIG. 13 is a conceptual diagram of a model of doping lithium between graphite sheet layers.

FIG. 14 is a model of a general parallel plate condenser.

FIG. 15 is a model in which a dielectric is inserted into a general parallel plate capacitor.

FIG. 16 is a conceptual diagram of a non-parallel plate capacitor model.

FIG. 17 is a graph of F (x), a substantial controlling factor of a non-parallel plate capacitor.

FIG. 18 is a conceptual diagram of a non-parallel plate condenser model.

FIG. 19 is a graph of a substantial dominant factor of a non-parallel plate capacitor: F (x, θ). Where the angle θ is
For the convenience of graph creation, the notation was not in radians but in degrees.

FIG. 20 shows a micropore structure formed between crystallites in hard carbon.

FIG. 21 shows the most stable arrangement of lithium when placed on the ovalene in each lithium state in the ovalene having a terminal hydrogen cap.

FIG. 22 shows the most stable arrangement of lithium when placed on ovalene in each lithium state in ovalene having no terminal hydrogen cap.

FIG. 23 is an electron micrograph of a cross section of a hollow fiber of carbon fiber which is an example of a carbon material used in the present invention.

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Claims (3)

[Claims] 1. A three-dimensional structure having voids for storing lithium, lithium ions, and lithium ion clusters between carbon material layers inside, and an entrance through which the lithium, lithium ions, and lithium ion clusters can easily move. A negative electrode carbon material for a lithium secondary battery, which is a fibrous carbon having a carbon structure. 2. The negative electrode carbon material for a lithium ion secondary battery according to claim 1, wherein the fibrous carbon is a hollow cylindrical fiber. 3. The negative electrode carbon material for a lithium ion secondary battery according to claim 1, wherein the voids capable of occluding lithium, lithium ions and lithium ion clusters are cracks and pores.