JPH0282466A - Lithium secondary battery in which carbon fiber is used for both electrodes - Google Patents

Abstract

PURPOSE:To boost discharge capacity, improve charge/discharge stability, and reduce self-discharge by using an individually specific carbon fiber for a positive and negative electrodes. CONSTITUTION:For a positive electrode, to retain a large quantity of electricity quantity, the following pitch-system carbon fiber is used: one having high graphitization, that is, the pitch-system carbon fiber having the mean surface interval of a carbon layer surface of 3.40A or below and the size of a crystallite in the c-axis and a-axis derections of 200-800A and 200-1000A respectively. For a negative electrode, the following carbon fiber is used: one having a so- called disordered layer structure in which suitable turbulence exists and suitably graphitized, that is, the one having the mean surface interval of a carbon layer surface of 3.45-3.37A, the size of a crystallite in the c-axis and a-axis directions of 40-500A and 40-700A respectively, and the ratio of a pitch strength of 1360cm<-1> to a peak strength of 1580cm<-1> is 0.2 or more and 1.0 or below in the Raman spectrum using an argon laser. This enables discharge capacity to be boosted, the stability for repeated chargedischarge high, and self-discharge reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to an organic electrolyte lithium secondary battery using carbon fibers having a graphite structure for both the negative and positive electrodes.

BACKGROUND OF THE INVENTION As electronic devices have become smaller and lighter in recent years, there has been an increasing demand for rechargeable and dischargeable secondary batteries that have a large discharge capacity, high voltage, or high energy density, and can be used stably for long periods of time. ing.

Various batteries using carbon materials as electrodes have been reported to meet these requirements.
Alternatively, there are those using activated carbon tata for the positive electrode (Japanese Patent Application Laid-open No. 55-99714, Japanese Patent Application Laid-open No. 58-22252).
00, JP-A No. 58-138327, and JP-A No. 62-2285131), these batteries utilize the large surface area of activated carbon or activated carbon fibers to carry out the electrostatic adsorption reaction of anions. It is a type of electric double layer capacitor, and is characterized by its high durability against charging and discharging.

However, its discharge capacity is not necessarily large, and since it is a capacitor, the electromotive force is Q=CV (Q: storage Tjf
fi, C: capacitance, V: electromotive force) depends on the amount of stored Ml, and the battery did not have good discharge characteristics. Also, electric double layer capacitors have a large self-discharge and a high voltage. The disadvantage was that it decreased.

Graphite or carbon material or carbon with a developed degree of graphitization! a! a
Batteries using as the positive electrode (JP-A-58-135581, JP-A-10882, JP-A-62-15
No. 4584, JP-A-82-1851157,
JP-A No. 63-58783), the battery used for the negative electrode (
JP-A No. 182870, JP-A No. 82-82
No. 869, Japanese Patent Application Laid-Open No. 133-24555), and a battery used for both electrodes (Japanese Patent Laid-Open No. 62-103991) has been studied. These are all reactions in which lithium ions are doped and dedoped between layers of a carbon material with a developed crystal structure in the case of a negative electrode, and anions in the case of a positive electrode.
That is, the electrochemical production and decomposition reaction of a graphite intercalation compound is utilized in the electrode.

In the case of a positive electrode, the following reaction is used.

nC+X −−−+ CnX+e− (x: anion species) Here, the value of n corresponds to the amount of electricity that can be accommodated in the electrode,
The smaller the value of n, the more desirable the electrode is.

In general, n depends on the degree of graphitization of the carbon material used. The degree of graphitization is a concept expressing how close the crystal structure of a carbon material is to graphite, and for example, quantification based on an X-ray diffraction pattern or a Raman scattering spectrum is generally used.

In the case of the above electrode reaction, the higher the degree of graphitization, the more n
It is known that the discharge capacity becomes smaller, that is, the discharge capacity becomes larger.

However, it is not possible to judge suitability for electrodes simply by the degree of graphitization. Graphite powder with a very high degree of graphitization has poor stability against charging and discharging, making it unsuitable for practical use.
This is because the surface 11I of the carbon layer is doped with anions.
This is because although IwA increases, graphite powder has poor structural stability and cannot withstand expansion in the C-axis direction and collapses.

Therefore, carbon fiber is attracting attention from the viewpoint of high graphitization degree and structural stability.

Among carbon fibers, pitch-based carbon fibers are most suitable. PAN-based carbon mm is equivalent to pitch-based carbon fiber in mechanical strength, but is inferior in graphitization degree, and in fact is hardly doped with anions, making it unsuitable for positive electrodes.

However, the pitch-based carbon fibers that have been studied so far are
From the perspective of using graphite intercalation compounds in electrodes, optimization has not necessarily been pursued, and there has been a problem that the discharge capacity is small.

In the case of a negative electrode, the following reaction is used.

nC+Li"+e----+ The size of CnLin depends on the carbon material used for the negative electrode, as in the positive electrode,
The structure of the carbon material suitable for the negative electrode is completely different from that of the positive electrode. That is, while the positive electrode requires a high degree of graphitization, the negative electrode requires a carbon material that is moderately disordered and appropriately graphitized, such as having a turbostratic structure. This is due to the stability of the lithium-doped graphite W 171 compound in the electrolyte. This stability strongly depends on the crystal structure of the carbon material, and the graphite intercalation compound of a carbon material with a well-ordered layered structure, that is, a high degree of graphitization, is unstable in the electrolyte, and therefore self-discharge is severe. Furthermore, the discharge capacity is small, whereas the carbon material graphite intercalation compound having a turbostratic structure has very high stability and can increase the discharge capacity.

As a carbon material with such a turbostratic structure, pyrolytic graphite, which is obtained by graphitizing a carbon thin film produced using CvD technology, has been reported (Japanese Patent Laid-Open No. 62-202809, Japanese Patent Laid-open No. 83-24555). Pyrolytic graphite is characterized by a highly oriented carbon layer, and has a moderately disordered F9 structure, and exhibits high charge/discharge stability. "Charging and discharging cannot be performed at the W flow density, and there are problems such as deformation of the electrode material due to expansion in the C-axis direction due to doping.

On the other hand, carbon fibers with a turbostratic structure have been considered for the negative electrode due to their wide surface area and strong mechanical strength, but the turbostratic structure has not necessarily been controlled. Furthermore, the conditions for optimization were not fully understood. Therefore, problems remain regarding discharge capacity and self-discharge.

Problems to be Solved by the Invention The present invention has been made in view of the current situation as described above.
The object of the present invention is to provide a lithium secondary battery that uses carbon fibers for both the positive and negative electrodes, has a large discharge capacity, has high stability against green charging and discharging, and has little self-discharge.

Means for Solving the Problems The present invention provides an extremely excellent lithium secondary battery that has a large discharge capacity, excellent charging/discharging stability, and little self-discharge by using specific carbon fibers for the positive and negative electrodes. It was completed based on the discovery that batteries could be obtained.

The present invention provides a lithium secondary battery using an organic electrolyte in which lithium salt is dissolved in an organic solvent, in which the positive electrode uses carbon fiber with a high degree of graphitization and excellent mechanical strength as an active material. Anion doping and dedoping reactions are used in the electrode reaction, and carbon fibers with a turbostratic structure with an appropriate degree of graphitization and appropriate disorder are used as the active material in the negative electrode, and lithium ion doping is carried out. This is a lithium secondary battery using carbon fiber for both electrodes, which is characterized by utilizing a dedoping reaction in the electrode reaction.

That is, the positive electrode is made of pitch-based carbon fiber with a high degree of graphitization, that is, the average interplanar spacing of the carbon layer planes is 3.40A or less, and crystallites in the C-axis direction and the a-axis direction so as to accommodate a large amount of electricity. The size of each.

200-800 A, 200-1000 A pitch-based carbon 1ala is used, and the negative electrode is made of moderately disordered and moderately graphitized carbon fibers with a so-called turbostratic structure, that is, the average interplanar spacing of the carbon layer planes is 3. .45~3.37A,C
The crystallite sizes in the axial direction and the a-axis direction are each 40
~500 A, 1580c in the Raman spectrum at 40-700 A and using an argon laser
A lithium secondary battery using carbon m-fibers for both electrodes, with carbon fibers having a ratio of 0.2 to 1.0 as the ratio of the peak intensity at 1380cm-' to the peak intensity at m-' being 4.1F. It's a battery.

Hereinafter, details regarding the carbon fibers used for the electrodes will be explained in the order of the positive electrode and the negative electrode.

Carbon fibers with a high degree of graphitization and excellent mechanical strength used in the positive electrode are pitch-based carbon fibers made from coal pitch or petroleum pitch, preferably at 2000°C or higher in an inert gas. is one that has been subjected to graphitization treatment at 2500° C. or higher. The degree of graphitization and mechanical strength suitable for the positive electrode are defined below.

In general, as an index expressing the degree of graphitization of carbon materials, the interplanar spacing d of the layers is determined by X-ray diffraction method: The size of crystallites in the a-axis direction La
, and the crystallite size Lc in the a-axis direction. Using these indicators, ideal graphite has d = 3.354 Ate, La,
Lc is infinite, d increases as the degree of graphitization decreases, and La.

Lc becomes smaller.

The carbon m-fiber of the positive electrode used in the present invention has graphitization degree parameters determined by the above-mentioned X-ray diffraction method: d = 3.40 A or less, Lc = 200-800 A, La = 200-1
000A, and more specifically, carbon m fibers having a small interplanar spacing d, a large La, and a small Lc within the above specified range of graphitization degree are preferable.

In addition, mechanical strength generally refers to strength determined by tensile tests, bending tests, etc., but the mechanical strength expressed here does not necessarily mean mechanical strength that corresponds to tensile strength or bending strength. Carbon F
It means the strength of fibers. The pitch-based carbon mMk having the above graphitization degree has sufficient mechanical strength in this sense.

The reason why carbon fibers having the above graphitization degree and mechanical strength are particularly selected for the positive electrode is as follows.

That is, at the positive electrode, 1, for example, C1! Doping and dedoping reactions occur between carbon layers of anions with a fairly large ionic radius such as 04-, but in order for this reaction to occur completely reversibly, it is necessary to have a rigid structure due to the huge size of the anion. A carbon layer with high crystallinity is required. This requires that La be large and, as a result, that d be small.

On the other hand, doping and dedoping reactions between carbon layers with an anion having a considerably large ionic radius cause distortion in the C-axis direction accompanied by a large change in the interplanar spacing of the carbon layers.

In order to increase the reversibility of the reaction, it is necessary to reduce the influence of this strain, and therefore Lc is required to be small.To summarize, the degree of graphitization required for the carbon material used for the positive electrode is: d: small , La: large, LC: small.

By the way, these three types of indicators are actually not independent from each other but have the following correlation. That is, if d is small, it means that the graphite structure is developed, so naturally La and Lc become large. Therefore, the above conditions required for the positive electrode are originally contradictory. One of the features of the present invention is that, as shown below, a specific pitch-based carbon made from coal pitch or petroleum pitch is used as a raw material. This condition was achieved by using fibers.

In general, pitch-based carbon m fibers can have a particularly high degree of graphitization among various carbon ams.

This is because pitch is originally rich in polycyclic aromaticity, and the growth of the carbon layer surface and the development of the laminated structure are easy. Furthermore,
The size of crystallites upon graphitization can also be controlled to a certain extent depending on the manufacturing process. That is, d, La,
It is possible to control Lc.

Therefore, in order to obtain carbon fibers with a degree of graphitization suitable for positive electrodes, for example, the following method may be adopted: using a highly graphitizable pitch as a raw material, and controlling the crystallite size, especially Lc. If the size is controlled to be small, the degree of graphitization required for the positive electrode can be achieved exactly.

Furthermore, the orientation of the carbon layer plane in the cross section of the pitch-based carbon m-fiber can be controlled.

That is, the orientation of the carbon layer plane in the cross section has an onion structure,
Radial structures and random structures can be freely controlled. In doping and dedoping of anions, the end face of the carbon layer is used as the reaction site, so in order to facilitate the reaction, the contact area between the end face of the carbon layer and the electrolyte must be large. Among pitch-based carbon fibers having the above-mentioned degree of graphitization, the carbon fibers used in the electrode have a radial structure or
A random structure is preferred.

We focused on the wide degree of freedom of pitch-based carbon fibers that other carbon fibers do not have, that is, the control of the degree of graphitization and the control of the orientation structure in the cross section, and utilized this to create the ideal material for positive electrodes. It is a very important point in the present invention to obtain a carbon fiber with a high quality.

The carbon fiber with a turbostratic structure, which has both an appropriate degree of graphitization and appropriate disorder, is used for the negative electrode, and is made of coal pitch,
Alternatively, various types of fibers such as pitch-based carbon m fibers made from petroleum pitch, polyacrylonitrile-based carbon #a fibers made from polyacrylonitrile (PAN-based carbon jam>, rayon-based carbon fibers, or phenol-based carbon fibers) may be used. Carbon fibers based on raw materials are subjected to graphitization treatment at 2000° C. or higher, preferably 2500° C. or higher in an inert gas.

Here, the turbostratic structure having an appropriate degree of graphitization and appropriate disorder suitable for a negative electrode is defined below.

In general, as an index for defining the degree of graphitization, in addition to the three parameters determined by the Xli diffraction method used to define the degree of graphitization of the positive electrode, the spectral shape determined by Raman spectroscopy can be used. Generally, argon laser (wavelength: 5
When the Raman spectrum of the carbon material is measured using 145A) as a light source, two peaks appear near the 1580c layer - 11360 cm-'. The former.

The latter is derived from the graphite structure, and the latter is derived from the turbostratic structure of the carbon material. Therefore, the degree of graphitization of the carbon material can be determined by the relative intensity ratio of these two peaks. The evaluation of the turbostratic structure based on this Raman spectrum is
The peak corresponding to the turbostratic structure is straight vi1360c■'''1
It is very effective because it appears as a peak.

The graphitization degree of carbon fiber suitable for the negative electrode in the present invention is
The index in the line diffraction method is d = 3.45 to 3.3?
A, Lc=40-500 A, La=40-700
1580c above.
The ratio of the peak intensity at 1 360 cm-' to the peak intensity at m-' is 0.2 or more and 1.0 or less.

The reason why the carbon fiber having the turbostratic structure defined in this manner is particularly selected for the negative electrode is as follows.

That is, the graphite intercalation compound doped with lithium is unstable in an electrolytic solution using an organic solvent due to its woody nature, and this stability is increased by turbostratizing the carbon material that is the mother crystal.

As is known from research on the negative electrode reaction of lithium secondary batteries using lithium metal as the negative electrode, lithium metal is extremely reactive, and various organic solvents have been studied as electrolytes so far. Despite this, no organic solvent has yet been found that is completely stable for lithium metal, and furthermore, it has been revealed that not only organic solvents but also lithium salts, which are electrolytes, are decomposed by negative electrode reactions.

The high reactivity of lithium metal also applies to graphite intercalation compounds doped with lithium. The high reactivity of lithium metal originates from the atomic structure of lithium itself, and being in a metallic state is not important in terms of wood quality; in other words, lithium atoms have high reactivity. Therefore, although the lithium present between the layers of graphite does not form a rigid crystal lattice as in the metallic state, it is thought that it basically has the same high reactivity.

For example, when attempting to electrochemically insert lithium into Quisch graphite in an electrolytic solution in which L i Ci 04 is dissolved in propylene carbonate, an insertion reaction accompanied by expansion of graphite is initially observed, but after that, lithium The insertion does not proceed and the state becomes steady in which bubbles are generated as a result of the decomposition reaction of the electrolytic solution. This steady state is due to the equilibrium between the insertion reaction of lithium between the graphite layers and the release reaction of lithium from the graphite layers due to decomposition, and reflects the instability of the lithium-doped graphite intercalation compound in the electrolyte. It is something to do.

Degree of stability of lithium-doped graphite intercalation compound,
That is, the rate of 1 decomposition reflects the structure of the graphite host crystal used. The higher the crystallinity of graphite, the higher the decomposition rate, and the lower the crystallinity of graphite and the more developed the turbostratic structure, the lower the decomposition rate and the higher the stability.

Although the complete interpretation of this fact is not necessarily clear, it can be qualitatively considered as follows.

Lithium atoms between layers with a developed graphite structure have extremely high mobility, and when the lithium atoms at the edge of the graphite layer in contact with the electrolyte are released from the graphite due to a decomposition reaction, new atoms inside the adjacent layer are generated. Lithium atoms immediately move to the end face and again contribute to decomposition. Because of the high mobility of lithium atoms in the living room, the decomposition reaction between the lithium atoms and the electrolyte at the end surfaces progresses one after another, and eventually all the lithium atoms inserted in the living room are decomposed. and release it.

That is, it is considered that a lithium graphite intercalation compound using a carbon material with a high degree of graphitization cannot exist stably in an electrolytic solution. What should be noted here is that 1 decomposition does not occur because the potential of the carbon material into which lithium is inserted is located outside the range of the so-called "stably existing potential window" of the electrolyte; This is due to the high reactivity of lithium atoms. This can be confirmed from the fact that the decomposition of the lithium graphite intercalation compound uniformly progresses from near the potential of the lithium metal to near the potential of the carbon material itself.

On the other hand, carbon materials with a turbostratic structure are no different from those with a developed graphite structure in terms of the reactivity of lithium atoms, but due to the turbostratic structure, the interlayer mobility of lithium atoms is extremely low. This low mobility allows the lithium graphite intercalation compound to exist stably in the electrolyte despite the high reactivity of lithium atoms. In other words, after the lithium atoms in contact with the electrolyte on the end face are released from the living room due to a decomposition reaction with the electrolyte, the lithium atoms inside cannot easily move to the end face due to the turbostratic structure, and therefore, the lithium atoms in the living room There is no new contact between the lithium atoms and the electrolyte, and the graphite intercalation compound with the turbostratic structure can exist stably.

Here, we should pay attention to the woody difference in the stability of metallic lithium and lithium graphite intercalation compounds in electrolytes. In the case of metallic lithium, the product formed as a result of the reaction between lithium atoms and an organic solvent or electrolyte forms a film,
Since the surface of metallic lithium is covered, lithium and the electrolyte no longer come into direct contact with each other, resulting in stabilization. In other words, the fact that lithium is made up of solid metal crystals makes the formation of the film easy.

On the other hand, lithium that is completely liquid, such as lithium amalgam, and lithium atoms between layers in graphite intercalation compounds, exist in a state where they can move relatively freely like a liquid between layers, and have a rigid crystal structure. If it is not removed, the products of the decomposition reaction will not be able to form a film, and the decomposition will proceed forever and will not be stabilized. Therefore, in the case of graphite intercalation compounds, the formation of a turbostratic structure does not occur, but rather the formation of a film. Stabilization is achieved by fixing lithium atoms between the layers to a certain extent.

The reason for using carbon fiber, especially carbon fiber, as a carbon material with a turbostratic structure as mentioned above is essentially the same as that for the positive electrode. For example, the original degree of graphitization of raw materials such as pitch and polyacrylonitrile and the wide control range of the degree of graphitization due to spinning into carbon fibers. Even though it is called a turbostratic structure, the layer plane must be developed to the extent that lithium atoms can be inserted into the living room. For this purpose, carbon fiber raw materials with a certain degree of graphitization are used. In order to achieve a turbostratic structure, graphitization can be suppressed at the raw material stage, or the degree of graphitization can be controlled during the manufacturing process.

Regarding graphitization, it is exactly the same as the positive electrode. That is, under an inert atmosphere at 2000° C. or higher, preferably.

Graphitization treatment at 2500°C or higher for 1 hour or more. A turbostratic structure can generally be achieved by lowering the heat treatment temperature, but in order to form intercalation compounds and to obtain stability against doping and dedoping, the graphitization temperature must be raised as much as possible. It is necessary to form a firm carbon layer, and the minimum temperature for this is 2000°C.

The present invention relates to a secondary battery system characterized by using carbon fibers with an optimal degree of graphitization for each of the positive electrode and the negative electrode. It is not a restriction.

Examples of electrolytes that can be used in the present invention are:

The following lithium salts may be mentioned.

Lithium barchlorate: LiCIO4, lithium hexafluoroantimonate: LiSbFg, lithium hexafluoroacenate: LiAsF6, lithium tetrafluoroporate: LiBF4. Lithium hexachloroantimonate: Li5bCi6, lithium hexafluorophosphate): LiPF Among these, particularly preferred are LiBF, , LiP
It is F6.

The following conditions regarding the anion are required for the lithium salt used as an electrolyte. That is, basically stability of anions in electrochemical reactions is required. In the present invention, insertion and release of anions in carbon fibers is utilized for positive electrode reactions, so stability of anions in electrochemical reactions is required. In addition to stability in carbon fibers, it is also required that the anion be capable of intercalating and releasing reactions in carbon fibers.

The conditions for anions suitable for intercalation and desorption reactions in carbon fibers are not necessarily clear at this stage, but stability as a -valent 7 anion is fundamentally important, and as has been said in the past. The geometric shape of the anion atoms or the size of the two molecules involved in the insertion/extraction reaction is not important; the series of lithium salts listed above satisfy exactly these two conditions. It is. In other words, these 11 lithium products are TI! The anion produced as a result of ionization with a very high degree of strength is electrochemically very stable, and further has very high stability as a monovalent 7 anion.

What should be noted here is the halogen anion. Halogen has a large electron affinity and can therefore be expected to have high stability as a monovalent anion. Furthermore, since it has high electrochemical stability, halogen 7-ion is considered to be optimal for insertion/extraction reactions in carbon materials. however,
For example, as is known from research on graphite intercalation compounds of iodine and bromine synthesized by the gas phase method, their stability is extremely poor, and their MIJ structure cannot be maintained unless they are under the pressure of halogen gas, so it is necessary to reduce the pressure. Once inserted, the halogen atoms are easily released out of the layer. Reflecting the low stability of such a halogen graphite intercalation compound, it is difficult to electrochemically synthesize a halogen graphite intercalation compound, and 'Q! It is considered not suitable for polar reactions.

The organic solvent as the electrolyte used in the present invention is an aprotic organic solvent, and is not particularly limited to organic solvents that have been conventionally used in secondary batteries. , large permittivity, large dipole constant,
Particularly preferred are organic solvents having characteristics such as high stability against redox, wide potential window, and low viscosity.

Specifically, propylene carbonate, ethylene carbonate, sulfolane, γ-butyrolactone,! , 2-dimethoxyethane,
Tetrahydrofuran, 2methyl-tetrahydrofuran,
Solvents such as acetonitrile, dimethylsulfoxide, dimethylformamide, and mixed solvents thereof can be used.

The concentration of the electrolyte also depends on the type of solvent and electrolyte.

Although it cannot be specified unconditionally as it depends on the electrode material, trace amounts of oxygen and moisture in the 0Ml solution, which is usually in the range of 0.1 to 10 mol/vertical, degrade the performance of the battery.
The solvent and electrolyte must be sufficiently purified in advance according to conventional methods. In the present invention, if necessary, a porous membrane made of synthetic resin such as polyethylene, polypropylene, Teflon, etc., or natural m fibers may be used between the two electrodes. It may also be used as a diaphragm.

Examples Example 1 Pitch-based carbon m heat treated at 2900°C as shown in Figure 1
A positive electrode 4 was prepared by bundling 10 mg of the fiber 1 with a platinum wire 2 having a diameter of 0.1 mm.

Measurement of charge/discharge characteristics is carried out at a distance of 10 to 20 m as shown in the f1m2 diagram.
Counter electrode (negative electrode) 3 is a sheet of lithium metal crimped onto a nickel mesh; reference electrode 5 is a small piece of the same lithium metal sheet as the counter electrode (negative electrode) connected to a nickel wire;
A triode cell using a solution of iBFa dissolved in propylene carbonate at a concentration of 2 Peng 01/m as the electrolyte 6 was placed in a glass container 7, and the whole was placed in an inert atmosphere by introducing argon (Ar) gas 8. The potential of the positive electrode with respect to the reference electrode was measured when charging and discharging were repeated at a constant current.

The graphitization of the carbon fiber used in the positive electrode was controlled during the raw material stage and manufacturing process to achieve the degree of graphitization required for the positive electrode. The indicators are shown in Table 1. The charging/discharging current value is 10m
A/1g of carbon material generates 300 watts of electricity when charging.
The amount of carbon material is 1g.

As a result of charging and discharging under such conditions, the electrical efficiency of discharge was at most 50% in the first cycle, but stabilized from the second cycle onwards to approximately 95%. The charging and discharging potential is very noble, 4.8 against the lithium standard.
Charging is started from ~4.7V, and the potential rises as the charge 71iffi increases, and the final potential reaches 5V when charging is completed. The potential change during discharging is almost the same as that during charging, and as discharging progresses, the potential gradually decreases and suddenly drops to 4 V, ending discharging. Therefore, the available potential for discharge is in the range of 4-5V. Figure 3 shows the potential curve during charging and discharging.

Comparative Example 1 Three types of carbon materials with different degrees of graphitization: Quiche graphite, TOR
PAN-based carbon fiber M40 manufactured by AY Corporation and pitch-based carbon @@P75S manufactured by UCC Corporation were measured under the same conditions as in Example 1.

The degree of graphitization of each carbon material determined by X-ray diffraction method is shown in Table 1. Quiche graphite is a carbon fiber with d = 3.354 A and the highest degree of graphitization among the PAN-based carbon taiM40 (i, PAN-based carbon t, mMl) manufactured by TORAY, which has an almost ideal graphite structure. belongs to Among pitch-based carbon fibers, the pitch-based carbon fiber JIP75S manufactured by UCC has a lower degree of graphitization than the carbon fiber used in Example 1.

PAN-based carbon m-fiber starts charging from 4.8V.
As ut increases, the potential increases rapidly and finally reaches 8.
Reach 1 ■. When switching to discharge, the potential drops rapidly and there is almost no discharge compared to the amount of charge.This is due to the lowest degree of graphitization among PAN-based carbon fibers, and due to the low degree of graphitization, there is almost no doping reaction of anions. It means that it will not occur.

P75S starts charging from 4.8V, and the potential increases slowly as the amount of charge increases, eventually reaching 5.1V. The amount of discharge was about 25% of the amount of charge. P7
5S has developed a turbostratic structure, as expected.

The electrical efficiency is low due to the low degree of graphitization.

Quiche graphite starts charging from 4.7V, and as the amount of charge increases, the potential increases stepwise, and eventually S, OV
reach. The discharge efficiency was about 50%. This step-like change in potential corresponds to the so-called stage structure of a graphite intercalation compound, and is a phenomenon that appears due to the high degree of graphitization of Quiche graphite. Quiche graphite has a degree of graphitization that can be said to be ideal, but because both La and Lc are too large, it is not suitable for reversible electrode reactions such as doping and dedoping reactions.La and Lc are too large to be used for reversible reactions. It must not be too much. The charge/discharge curves of these three are shown in FIG. 3 along with Example 1.

The results of Example 1 and Comparative Example 1 show that excellent charge-discharge characteristics can be obtained by using pitch-based carbon fiber with a specific degree of graphitization suitable for the positive electrode as in the present invention. Ru.

Example 2 As in Example 1, pitch-based carbon M&fibers heat-treated at 2900°C and bundled with platinum wire were used as the sample electrode (negative electrode) 4, and metallic lithium was used as the counter electrode 3 and reference electrode 5. Using a three-electrode cell made up of the same electrolytic solution 6 as in Example 1, the potential of the negative electrode with respect to the reference electrode was measured when charging and discharging were repeated at a constant current.

The graphitization of the carbon fiber used in the negative electrode was controlled during the raw material stage and manufacturing process to achieve the degree of graphitization required for the negative electrode. The graphitization degree index determined by X-ray diffraction of this carbon fiber is shown in Table 2.

In addition, the carbon fiber argon laser (wavelength: 5
As shown in Figure 4, the Raman spectrum using 145A) as a light source has two peaks coexisting: 1580cm-', which reflects the graphite structure, and 1360c layer-1, which reflects the turbostratic structure. 136 for peak intensity
The ratio of the peak intensities of 0C11'' is 0.5 to 0.8.

The current value for charging and discharging was 5 mA/g of carbon material, and the amount of electricity applied during charging was 1300 corons/g of carbon material.

The electrical efficiency of discharge is 70-80% in the first cycle.
However, after the second cycle, it stabilizes at 93% or more. In addition, regarding charging, the potential starts charging from around 0.5V with respect to the lithium standard, and the change in potential becomes very gradual from around 0.2V, and eventually reaches almost the same potential as lithium metal. reach up to. Regarding discharge, the potential rises very slowly up to around 0.3 V based on lithium, and then starts to rise rapidly from 0.6 to 0.8 V.

Finish discharging. Therefore, if the potential range used for discharge is set to 0.5V, a fairly flat potential curve can be obtained. The change in the potential of the negative electrode during charging and discharging is
As shown in the figure.

Comparative Example 2 Quiche graphite, which is a carbon material with a developed graphite structure, was measured under the same conditions as in Example 2.

The index of graphitization degree by X-ray diffraction of Quisch graphite is the second
It is also shown in the table. Moreover, the two peak intensity ratios in the argon laser Raman spectrum are approximately 0. These indicators indicate the high degree of graphitization of Quiche graphite.

Quiche graphite with an ideal graphite structure is 1. Even if charging is started at OV and charging progresses, the 7H position hardly changes, no drop in potential due to lithium ion doping is observed, and almost no discharge occurs.

This result shows that most of the charging electricity is spent on side reactions due to the high degree of graphitization, and that lithium doping hardly progresses in Quiche graphite. On the other hand, in the case of the carbon fiber of Example 2 having a turbostratic structure, the potential of the negative electrode became almost the same potential as lithium at the charge amount corresponding to C,...,) Li, and the interlayers of the carbon m fibers were saturated with lithium. It can be seen that the discharge efficiency is also high, which indicates that almost no side reactions occur. Changes in the potential of the negative electrode during charging and discharging are shown in FIG. 5 together with the results of Example 2.

Comparative Example 3 Two types of carbon fibers, PAN-based carbon fiber T300 manufactured by TORAY and pitch-based carbon fiber heat-treated at 1000° C., were measured under the same conditions as in Example 2.

Table 3 shows the index of graphitization degree of these two types of carbon fibers by X-ray diffraction. The index of graphitization degree by Raman spectroscopy is
0 where the former is 0.9 to 1.0 and the latter is 1.0 to 1.1
It can be seen that both have a turbostratic structure.

As a result of measuring constant current charging and discharging, Ta2O starts charging from ~1 V, and a decrease in potential is observed as charging progresses, but the change is not smooth and often discontinuous. Eventually it reaches 0.1v, but the discharge efficiency is 10
% or less.

This phenomenon indicates that lithium doping is difficult to occur or does not occur in Ta2O. Also, it lacks stability against 5-edge turning.

In addition, the pitch-based carbon m-fiber heat-treated at a low temperature draws a potential curve similar to that of Example 2, and lithium doping progresses, but the discharge efficiency is extremely poor and stability against repeated charging and discharging is lacking.

Here, in order to compare other carbon fibers with a turbostratic structure and the carbon fiber with a turbostratic structure claimed by the present invention, the above two types were tested under the same conditions as in Example 2. Carbon fibers were measured.

The carbon fibers used in Example 2 were heat-treated at a very high temperature of 2900° C. to achieve a turbostratic structure as shown in Table 2. It is generally known that pitch-based carbon fibers can achieve a turbostratic structure simply by lowering the heat treatment temperature.

Furthermore, PAN-based carbon m-fibers are also known to have a turbostratic structure. The experimental results of Example 2 and Comparison 3 show the following. That is, PAN-based carbon fiber T30
0 indicates that the degree of development of the carbon layer surface is low, so pitch-based carbon fibers that are not doped with lithium and are heat-treated at low temperatures have a carbon layer surface that is sufficiently developed due to the high graphitizability of the pitch used as a raw material. Therefore, doping is carried out smoothly, but because the heat treatment temperature is low, the a-layer structure lacks stability against dedoping.

Above, Example 2. The results of Comparative Examples 2 and 3 showed the following. In other words, a carbon material with a highly developed crystal structure is not suitable for use in the negative electrode due to its high degree of graphitization, and it has a moderately disordered structure called a turbostratic structure.
A moderately graphitized carbon material is suitable. Furthermore, it has been shown that among the turbostratic structures, carbon fibers having the turbostratic structure defined in the present invention are the most suitable, and those with a too low degree of graphitization are unsuitable.

Example 3 The negative electrode properties of pitch-based carbon fibers and PAN-based carbon fibers were investigated in an electrolytic solution using sulfolane as an organic solvent.

As in Example 1, carbon fibers bound with platinum wire were used as the sample electrode (negative electrode) 4, and metallic lithium was used as the counter electrode 3 and the reference electrode 5.
Using a three-electrode cell configured with electrolyte 6 in which LiB F4 was dissolved in sulfolane at a concentration of 1+wol/mon, the potential of the negative electrode with respect to the 2 & quasi-electrode was measured when charging and discharging were repeated at a constant Tff[. did.

The pitch-based carbon material used for the negative electrode was P75S manufactured by UCC.
and P120X, PAN-based carbon fiber.

It is M40 manufactured by RAY. All three types of carbon fibers are within the graphitization degree range of the negative electrode defined in the present invention. Table 4 shows the index of graphitization degree based on X-ray diffraction of these carbons m and m. In addition, the two peak intensity ratios from the argon laser Raman spectrum are P120X:
0.2~0.3, P75 S: 0.5~O, fi
, M40 + 0.9 to 1.0.

The current value for charging and discharging is 10 mA/tg of carbon material, and the amount of electricity passed during charging is 1300 corons/1 g of carbon material.
It is.

The electrical efficiency of discharge is as follows: The l-th cycle is P1
20X: 50-60%, P75S: 80-70%, M2
O: 50-80%, but after several cycles, both the champions stabilized at ~90% or higher. Further, the potential changes during charging and discharging are almost the same as in Example 2, which was measured in an electrolytic solution using propylene carbonate as an organic solvent. That is, charging starts from around 0.5V with respect to the reference electrode, the potential drops relatively suddenly to 0.2 to 0.3V, and from there, the change in potential becomes very gradual, and eventually It reaches almost the same potential as lithium metal.

The change in potential during discharge was also almost the same as in Example 2.

After the start of discharge, the potential of both champions increases, and o, e −
The potential rises rapidly from o, av. However, in the discharge curves, there was a slight difference in the degree of graphitization of the champion carbon m-fibers. That is, pitch-based carbon rim fir with a high degree of graphitization
120X has a relatively low discharge end potential and a small difference between the discharge start potential and the discharge end potential, whereas PAN-based carbon m-fiber M40 with a low degree of graphitization has a high discharge end potential and a large difference. FIG. 6 shows changes in the potential of the negative electrode during charging and discharging.

Here, in order to investigate the influence of the raw material of carbon m-fibers and the graphitization degree of carbon m-fibers on the negative electrode characteristics, we will examine the effects of the graphitization degree of carbon fibers within the range of carbon fibers suitable for negative electrodes specified in the present invention. This study investigated the negative electrode characteristics of three types of carbon iam made from different raw materials. As a result, it was shown that the difference in the raw material of the carbon fiber used for the negative electrode is not important, and that regardless of the raw material, it is possible to judge whether carbon fiber is suitable for the negative electrode using the index of the degree of graphitization of the carbon fiber specified in the present invention.

Furthermore, carbon fibers suitable for negative electrodes as defined in the present invention have a certain degree of graphitization degree, but all carbon fibers that fall within this range of graphitization degree are excellent as negative electrodes. It was found that the difference in the degree of graphitization appears as a difference in the discharge curve. In other words, carbon fibers with a high degree of graphitization have a flat discharge curve, and the change in negative electrode potential as discharge progresses is small, whereas carbon fibers with a low degree of graphitization have a discharge curve with a slope. , the potential of the negative electrode changes significantly as the discharge progresses.

Example 4 Using the carbon fiber used in Example 1 as the positive electrode, Example? A battery was prepared using the carbon uam used in Example 1 as a negative electrode and the same electrolyte as in Example 1, and charged and discharged at a constant current.

The carbon fiber of the positive electrode is 40 mg, and the carbon fiber of the negative electrode is 10 mg.
It is. The current value is tamA/1g of carbon material, and the charge amount is:
The negative electrode is 1000 coulombs/tg of carbon material, and the positive electrode is 250 coulombs/g of carbon material.

Charging starts from 3.8v and the voltage rises to 4.6v. The voltage of the discharge varied from 4.3v to 3.5v. 1
Although charging and discharging were repeated 00 times, no significant change was observed in the charging and discharging voltage curve.

In addition, the electricity efficiency was high and stable at 90% or more.

! Figure G7 shows the measurement results when charging and discharging were repeated 100 times.

Table 1 Table 2 Table 3 Table 4 Effects of the Invention As explained above in detail, the present invention uses carbon materials for both the positive and negative electrodes, and the doping reaction of anions and lithium ions is applied to the electrode reaction. The greatest feature of the present invention, which relates to the lithium secondary battery utilized, is that the characteristics of the carbon material optimal for the reaction of the positive electrode and the negative electrode were determined, and this was expressed using an index of the degree of graphitization. This characteristic is achieved by using pitch-based carbon fibers made from coal pitch or petroleum pitch in the case of the positive electrode, and by using carbon fibers with a turbostratic structure in the case of the negative electrode. . This secondary battery system makes it possible to create a very high-voltage, lightweight, chargeable and dischargeable secondary battery without using metallic lithium as an electrode, and has extremely high utility value.

[Brief explanation of the drawing]

The t51 diagram is a schematic diagram of a carbon #am electrode. FIG. 2 is a schematic diagram of an apparatus for measuring electrode properties of carbon fibers. FIG. 3 is a constant current charging/discharging characteristic diagram of the carbon fiber according to the present invention and the existing carbon material positive electrode. FIG. 4 is a characteristic diagram showing the Raman spectrum of the carbon fiber according to the present invention. FIGS. 5 and 6 are constant current charging and discharging characteristics diagrams of the carbon m-fiber according to the present invention and the negative electrode of the existing carbon material. FIG. 7 is a constant current charging/discharging characteristic diagram of a battery using carbon m-fibers in both electrodes according to the present invention. 1... Carbon fiber, 2 Cloudy... Platinum wire, 3-... Counter electrode, 4
No. 0・Sample electrode, 5#・Reference electrode, 6・Electrolyte, 71
1. Gold/glass container, 8.--Ar gas.

Claims (1)

[Claims] The average interplanar spacing of the carbon layer planes is 3.40A or less, and the crystallite sizes in the c-axis direction and the a-axis direction are each 200 to 8
00A, 200-1000A pitch-based carbon fiber is used for the positive electrode, and the average interplanar spacing of the carbon layer surfaces is 3.37-3.4.
5A, the crystallite size in the c-axis direction and the a-axis direction is 40-500A and 40-700A, respectively, and 1580A in the Raman spectrum using an argon laser.
1360cm^-^ for peak intensity of cm^-^1
A lithium secondary battery using carbon fibers for both electrodes, characterized in that carbon fibers having a peak intensity ratio of 0.2 or more and 1.0 or less are used for the negative electrodes.