JP2612320B2 - Lithium secondary battery using carbon fiber for both electrodes - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an organic electrolyte lithium secondary battery using carbon fibers having a graphite structure for both the negative electrode and the positive electrode.

2. Description of the Related Art As electronic devices have become smaller and lighter in recent years, there has been an increasing demand for chargeable / dischargeable secondary batteries that have a large discharge capacity, a high voltage, that is, a high energy density, and can be used stably for a long time. ing.

Various batteries using carbon materials for electrodes have been reported as satisfying these requirements. For example, activated carbon,
Alternatively, an active carbon fiber is used for the positive electrode (see
JP-A-55-99714, JP-A-58-2225200, JP-A-59-225200
-138327, JP-A-62-226561). These batteries use an activated carbon or
This is a type of electric double layer capacitor, which is performed using a large surface area of activated carbon fiber, and is characterized by high durability against charge and discharge.

However, the discharge capacity is not always large, and
Since it is a capacitor, the electromotive force is Q = CV (Q: stored amount, C:
As represented by capacitance, V: electromotive force), the battery did not have good discharge characteristics depending on the amount of stored electricity. Further, the electric double layer capacitor has a disadvantage that self-discharge is large and the voltage decreases with time.

Batteries using graphite or a carbon material or carbon fiber with a high degree of graphitization for the positive electrode (JP-A-58-135581, JP-A-61-61581)
-10882 JP, JP-A-62-154564, JP-A-62-1
No. 65857, JP-A-63-58763), batteries used for the negative electrode (JP-A-60-182670, JP-A-62-82669, JP-A-63-24555), and A battery (JP-A-62-103991) used for both electrodes has been studied. These are reactions of doping and undoping lithium ions in the case of a negative electrode, and doping and dedoping of anions in the case of a positive electrode between layers of a carbon material having a layered structure, that is, an electrochemical formation and decomposition reaction of a graphite intercalation compound. It is used for electrodes.

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

Here, the value of n corresponds to the amount of electricity that can be accommodated in the electrode, and the smaller the value of n, the more desirable the electrode. Generally n
Depends on the degree of graphitization of the carbon material used. The degree of graphitization is a concept indicating how close the crystal structure of a carbon material is to graphite. For example, quantification based on an X-ray diffraction pattern or Raman scattering spectrum is generally used. In the case of the above electrode reaction, it is known that as the degree of graphitization increases, n decreases, that is, the discharge capacity increases.

However, it is not possible to judge suitability for an electrode only by the degree of black spotting. Graphite powder having a very high degree of graphitization has poor stability against charge and discharge and is not suitable for practical use. This is because the doping of the anion increases the interplanar spacing of the carbon layer, but the graphite powder has poor structural stability and cannot withstand the expansion in the c-axis direction and collapses.

Therefore, carbon fibers are attracting attention from the viewpoints of high graphitization degree and structural stability.

Among the carbon fibers, pitch-based carbon fibers are most suitable. The PAN-based carbon fiber has the same mechanical strength as the pitch-based carbon fiber, but is inferior in the degree of graphitization. In fact, it is hardly doped with anions and is not suitable for a positive electrode.

However, the pitch-based carbon fibers that have been conventionally studied have not always been satisfactorily optimized from the viewpoint of using a graphite intercalation compound for an electrode, and have a problem that the discharge capacity is small.

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

Although the size of n 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 moderately disordered and moderately graphitized carbon material such as a turbostratic structure.
This is due to the stability of the lithium-doped graphite intercalation compound in the electrolyte. This stability strongly depends on the crystal structure of the carbon material, and the laminated structure is well-ordered, that is, the graphite intercalation compound of the carbon material having a high degree of graphitization is unstable in the electrolytic solution, and therefore self-discharge is strong. While the discharge capacity is still smaller, the graphite intercalation compound of a carbon material having a turbostratic structure has very high stability and can increase the discharge capacity.

As a carbon material having such a turbostratic structure, pyrolytic graphite obtained by graphitizing a carbon thin film formed by utilizing the CVD technique has been reported (Japanese Patent Application Laid-Open Nos. 62-202809 and 63-24).
No. 555). Pyrolytic graphite is characterized by a very high orientation of the carbon layer surface, has a moderately turbostratic structure, and exhibits high charge / discharge stability. However, it cannot be charged / discharged at a high current density because of its thin film shape, and has problems such as deformation of the pole material due to expansion in the c-axis direction due to doping.

On the other hand, the use of carbon fibers with a turbostratic structure for the negative electrode has been studied because of the large surface area and the high mechanical strength, but control over the turbostratic structure is not always performed, The conditions for conversion were not fully grasped. Therefore, there remains a problem regarding the discharge capacity and the 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 positive electrode, the negative electrode has a large discharge capacity using carbon fibers, high stability against repeated charge and discharge, low self-discharge, An object is to provide a lithium secondary battery.

Means for solving the problem The present invention provides a very excellent lithium secondary battery having a large discharge capacity, excellent charge / discharge stability, low self-discharge, and a specific carbon fiber for each of the positive electrode and the negative electrode. It was completed based on the discovery that a secondary battery could be obtained.

The present invention provides, in a lithium secondary battery using an organic electrolyte solution in which a lithium salt is dissolved in an organic solvent, the positive electrode has a high degree of graphitization, and carbon fiber having excellent mechanical strength as an active material. Using an anion doping and dedoping reactions for the electrode reaction, the negative electrode uses a turbostratic carbon fiber having an appropriate degree of graphitization and an appropriate degree of disorder as an active material, and performs lithium ion doping, A lithium secondary battery using carbon fibers for both electrodes, wherein a undoping reaction is used for an electrode reaction.

That is, in the positive electrode, the pitch-based carbon fiber having a high degree of graphitization so that a large amount of electricity can be accommodated, that is, the average plane spacing of the carbon layer surface is 3.40A or less, and the c-axis direction and the crystallite in the a-axis direction The size is 200-800A, using a pitch-based carbon fiber of 200-1000A, respectively, for the negative electrode, moderately disturbed, so-called turbostratic structure, moderately graphitized carbon fiber, that is, the carbon layer surface The average plane spacing is 3.45 to 3.37 A, the crystallite size in the c-axis direction and the a-axis direction is 40 to 500 A, 40, respectively.
In ～700A, and, 1360 cm to the peak intensity of 1580 cm ^-1 in the Raman spectrum using argon laser ^-1
A lithium secondary battery using carbon fibers for both electrodes, characterized by using carbon fibers having a peak intensity ratio of 0.2 to 1.0.

Hereinafter, details regarding the carbon fiber used for the electrode will be described in the order of the positive electrode and the negative electrode.

The carbon fiber having a high degree of graphitization used for the positive electrode and having excellent mechanical strength is coal pitch, or pitch-based carbon fiber obtained from petroleum pitch as a raw material, at 2,000 ° C. or higher in an inert gas, preferably It has been subjected to a graphitization treatment at 2500 ° C or higher.

Here, the graphitization degree and mechanical strength suitable for the positive electrode are defined below.

In general, as an index indicating the degree of graphitization of the carbon material, the interplanar spacing d of the lamination according to the X-ray diffraction method and the crystallite size La in the a-axis direction La,
Three kinds of parameters, that is, the crystallite size Lc in the c-axis direction can be used. And with these metrics,
The ideal graphite is d = 3.354 A, La and Lc are infinite, and d increases as the degree of graphitization decreases.
Lc decreases.

The carbon fiber of the positive electrode used in the present invention has a parameter of the degree of graphitization according to the X-ray diffraction method described above, where d = 3.40 A or less and Lc = 2.
00-800A, which is defined by La = 200-1000A, and more specifically, in the specified range of the degree of graphitization, a carbon fiber having a small interplanar spacing d, a large La, and a small Lc. preferable.

In general, mechanical strength refers to strength based on tensile tests, bending tests, etc., but the mechanical strength expressed here does not necessarily mean mechanical strength corresponding to tensile strength or bending strength. In other words, it means the strength of the carbon fiber against expansion and contraction of the lamination plane interval of the carbon layer due to doping and undoping of the anion. The pitch-based carbon fiber having the above degree of graphitization has sufficient mechanical strength in this sense.

The carbon fibers having the above graphitization degree and mechanical strength are particularly selected for the positive electrode for the following reasons. That is, in the positive electrode, for example, ClO ₄ ^-, such doping fairly large anions carbon layers of the ionic radius, but dedoping reaction is carried out, for this reaction is completely reversible performed as giant anion Therefore, a highly crystalline carbon layer having a firm structure is required. This requires that La be large and consequently that d be small.

On the other hand, the doping and undoping reactions of the anion having a considerably large ionic radius between the carbon layers cause a large change in the plane spacing of the carbon layers, causing distortion in the c-axis direction. In order to increase the reversibility of the reaction, it is necessary to reduce the effect of this distortion, and
Lc is required to be small. In summary, the degree of graphitization required for the carbon material used for the positive electrode is as follows: d: small, La: large, Lc: small.

By the way, these three indices are not independent of each other, but have the following correlation. That is, if d is small, it means that the graphite structure has been developed, so that La and Lc naturally become large. Therefore, the above conditions required for the positive electrode are originally contradictory. One of the features of the present invention is that this condition is realized by using a specific pitch-based carbon fiber made of coal pitch or petroleum pitch as shown below.

In general, pitch-based carbon fibers can have a particularly high degree of graphitization among various carbon fibers. The pitch is originally rich in polycyclic aromaticity, the growth of the carbon layer surface, and
This is because the development of the laminated structure is easy. Further, the size of the crystallite at the time of graphitization can be controlled to some extent depending on the manufacturing process. That is, d, La, and Lc can be controlled.

Therefore, in order to obtain carbon fibers having a degree of graphitization suitable for the positive electrode, for example, the following means may be employed. That is,
If a high graphitizable pitch is used as a raw material and the size of crystallites, particularly Lc, is controlled to be small, the degree of graphitization required for the positive electrode can be positively realized.

Further, the pitch-based carbon fiber can be controlled with respect to the orientation of the carbon layer surface in its cross section. That is, the orientation of the carbon layer surface in the cross section is onion structure, radial structure,
Random structures can be controlled freely. Since doping and undoping of anions use the end face of the carbon layer as a reaction field, the area where the end face of the carbon layer comes into contact with the electrolytic solution must be large in order to facilitate the reaction. Accordingly, the carbon fiber used for the electrode is preferably a pitch-based carbon fiber having the above-mentioned degree of graphitization, particularly one having a radial structure or a random structure in which the orientation of the carbon layer surface in the cross section is particularly large.

Focusing on the degree of freedom of pitch-based carbon fiber that other carbon fibers do not have, that is, control of the degree of graphitization, control of the orientation structure in the cross section, etc. Obtaining an excellent carbon fiber is a very important point in the present invention.

A carbon fiber with a turbostratic structure that has both a suitable degree of graphitization and a suitable degree of turbulence used for the negative electrode is, like the positive electrode, coal pitch, pitch-based carbon fiber made from petroleum pitch, or polyacrylonitrile. Carbon fibers based on various raw materials such as polyacrylonitrile-based carbon fibers (PAN-based carbon fibers), rayon-based carbon fibers, or phenol-based carbon fibers in an inert gas at 2,000 ° C. or higher, preferably 2500 ° C. This is where the graphitization treatment is applied.

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

In general, as an index for defining the degree of graphitization, a spectrum shape in Raman spectroscopy can be used in addition to the three parameters by the X-ray diffraction method as used for defining the degree of graphitization of the positive electrode. Generally an argon laser (wavelength: 5145
Measurement of the Raman spectrum of the carbon material of A) as a light source, 1580 cm ^-1, two peaks appear in the vicinity of 1360 cm ^-1.
The former is derived from a graphite structure, and the latter is derived from a turbostratic structure of a carbon material. Therefore, the degree of graphitization of the carbon material can be known from the relative intensity ratio between these two peaks. The evaluation of the turbostratic structure based on the Raman spectrum is very effective because the peak corresponding to the turbostratic structure appears directly as a peak at 1360 cm ⁻¹ .

Graphitization degree of carbon fiber suitable for the negative electrode in the present invention,
The index in the Y-ray diffraction method is d = 3.45 to 3.37 A, Lc = 40
~ 500A, La = 40 ~ 700A, and
The ratio of the peak intensity of 1360 cm ^-1 to the peak intensity of the above 1580 cm ^-1 is of 0.2 to 1.0.

The reason why the carbon fiber having the turbulent layer structure defined as described above is particularly selected for the negative electrode is as follows. That is, the lithium-doped graphite intercalation compound is essentially unstable in an electrolytic solution using an organic solvent, and this stability is enhanced by destabilizing the carbon material that is the mother crystal.

As is known from the study of the negative electrode reaction of a lithium secondary battery using lithium metal for the negative electrode, lithium metal has extremely high reactivity. Although various organic solvents have been studied as electrolytes, organic solvents completely stable to lithium metal have not been found yet.
Furthermore, it has been clarified that not only an organic solvent but also a lithium salt as an electrolyte is decomposed by a negative electrode reaction.

Such high reactivity of lithium metal is directly applicable to a graphite intercalation compound doped with lithium. The high reactivity of lithium metal is derived from the atomic structure of lithium itself, and it is not essential to be in a metallic state. That is, lithium atoms have high reactivity. Accordingly, lithium existing between layers of graphite does not form a rigid crystal lattice like a metal state, but is considered to have basically the same high reactivity.

For example, when an attempt is made to electrochemically insert lithium into Kish graphite in an electrolyte solution in which LiClO ₄ is dissolved in propylene carbonate, an insertion reaction accompanied by expansion of graphite is initially observed, but thereafter, the insertion of lithium is stopped. It does not proceed, and is stabilized to a state in which bubbles are generated along with 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. Is what you do.

The degree of stability of the lithium-doped graphite intercalation compound, ie, the rate of decomposition, reflects the structure of the graphite matrix used. The higher the crystallinity of graphite, the higher the decomposition rate, and the lower the crystallinity of graphite, the more developed the turbostratic structure, the lower the decomposition rate and the higher the stability. Although the full interpretation of this fact is not always clear, it can be qualitatively considered as follows.

Lithium atoms between layers with a developed graphite structure have very high mobility, and when lithium atoms at the end of the graphite layer in contact with the electrolyte are released from graphite due to decomposition reactions, new lithium atoms in adjacent layers The lithium atoms immediately move to the end face and again contribute to decomposition. Because of the high mobility of lithium atoms between the layers, the decomposition reaction between the lithium atoms and the electrolyte at the end face proceeds one after another, and finally, all the lithium atoms inserted between the layers depend on the decomposition. And release it.

That is, it is considered that a lithium graphite intercalation compound using a carbon material having a high degree of graphitization cannot be stably present in the electrolytic solution.
It should be noted that the potential of the carbon material into which lithium is inserted is located outside the range of the so-called “window of potential that can stably exist” in the electrolytic solution. It 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 proceeds uniformly from near the potential of the lithium metal to near the potential of the carbon material itself.

On the other hand, the carbon material having a turbostratic structure is completely the same as the case where the graphite structure is developed in terms of the reactivity of lithium atoms, but the mobility of lithium atoms between layers is extremely low due to the turbostratic structure. This low mobility allows the lithium graphite intercalation compound to be stably present in the electrolytic solution despite the high reactivity of lithium atoms. That is, after lithium atoms in contact with the electrolyte on the end face are released from the interlayer by a decomposition reaction with the electrolyte, the lithium atoms inside cannot move easily to the end face due to the turbostratic structure, and therefore, the interlayer The lithium intercalation compound having a turbostratic structure can be stably present without the lithium atom and the electrolyte newly contacting with each other.

Here, attention should be paid to the essential difference in the stability of lithium metal and the lithium graphite intercalation compound in the electrolyte.
In the case of metallic lithium, the product resulting from the reaction between the lithium atoms and the organic solvent or electrolyte forms a film, and the lithium and the electrolytic solution may come into direct contact with each other to cover the metallic lithium surface. Disappears and stabilizes. In other words, the fact that lithium forms a solid crystal of a metal makes it possible to form a film.

In contrast, completely liquid lithium such as lithium amalgam, or lithium atoms between layers in a graphite intercalation compound, exist in a state where they can move relatively freely like liquids, and have a solid crystal structure. If not,
The product of the decomposition reaction cannot form a film, and the decomposition proceeds everywhere and is not stabilized. Therefore, in the case of a graphite intercalation compound, stabilization is realized by forming a certain amount of lithium atoms between layers by a turbostratic structure, instead of forming a film.

The reason why carbon fiber is used for the carbon material having a turbostratic structure as described above is essentially the same as that of the positive electrode,
For example, the raw material such as pitch and polyacrylonitrile has a high degree of original graphitization and a wide range of control of the degree of graphitization due to spinning on carbon fiber. Even in the case of a turbostratic structure, the development of a layer surface that can insert lithium atoms between layers is required. For this purpose, a carbon fiber material having a certain degree of graphitization is used. In order to realize a turbostratic structure, graphitization may be suppressed at the raw material stage, or the degree of graphitization may be controlled during the manufacturing process.

Graphitization is exactly the same as for the positive electrode. That is, graphitization treatment is performed in an inert atmosphere at 2000 ° C. or more, preferably 2500 ° C. or more for 1 hour or more. In general, a turbostratic structure can be realized by lowering the heat treatment temperature.However, in order to form an intercalation compound and obtain stability against doping and undoping, the graphitization temperature must be increased to some extent. , A solid carbon layer must be formed, the minimum temperature for which is 2000 ° C.

The present invention relates to a secondary battery system characterized by using carbon fibers having an optimum degree of graphitization for each of a positive electrode and a negative electrode. There is no restriction.

As the electrolyte that can be used in the present invention, the following lithium salts can be exemplified.

Lithium perchlorate: LiClO _4, lithium hexafluoroantimonate: LiSbF _6, lithium hexafluoro acetone sulfonate: LiAsF _6, lithium tetrafluoroborate: LiB
F ₄ , lithium hexachloroantimonate: LiSbCl ₆ , lithium hexafluorophosphate: LiPF ₆ Among these, LiBF ₄ and LiPF ₆ are particularly preferred.

The following conditions regarding anions are required for a lithium salt used as an electrolyte. That is, basically, the stability of the anion in the electrochemical reaction is required. In the present invention, since the insertion and release of anions in the carbon fiber are used for the positive electrode reaction, in addition to the stability in the electrochemical reaction, the insertion and release reaction in the carbon fiber may be further caused. Is required to be a suitable anion.

The conditions of the anion suitable for the insertion-release reaction in carbon fiber are not always clear at this stage, but the stability as a monovalent anion is fundamentally important, The geometry of the anion, or the size of the molecule, involved in the insertion-release reaction is not critical. The series of lithium salts listed above satisfy these two conditions. That is, these lithium salts have a very high degree of ionization, and the anions resulting from ionization are very stable electrochemically, and have very high stability as monovalent anions.

Note here the halogen anion. Halogen has a high electron affinity, and can therefore be expected to have high stability as a monovalent anion. Furthermore, since it has a high electrochemical stability, a halogen anion seems to be the most suitable for the insertion-release reaction in a carbon material. However, as is known in the study of graphite intercalation compounds of iodine and bromine synthesized by a gas phase method, for example, the stability is very poor, and the structure cannot be maintained unless the pressure is halogen gas, and the pressure is reduced. As a result, the halogen atom once inserted easily is released to the outside of the layer. Reflecting such low stability of the halogen graphite intercalation compound, it is difficult to electrochemically synthesize the halogen graphite intercalation compound, and is considered to be unsuitable for the electrode reaction.

The organic solvent as the electrolytic solution used in the present invention is an aprotic organic solvent, and conventionally, as long as it is an organic solvent used in a secondary battery, the organic solvent is not particularly limited. Organic solvents having characteristics such as high dielectric constant, high dipole efficiency, high stability against oxidation / reduction, a wide potential window, and low viscosity are particularly preferable.

Specifically, propylene carbonate, ethylene carbonate, sulfolane, γ-butyrolactone, 1,2-dimethoxyethane,
Tetrahydrofuran, 2 methyl-tetrahydrofuran,
Solvents such as acetonitrile, dimethylsulfoxide, dimethylformamide and the like, and a mixed solvent thereof can be used.

In addition, the concentration of the electrolytic solution cannot be specified unconditionally because it depends on the type of the solvent, the type of the electrolyte, and the electrode material.
It is in the range of 0.1 to 10 mol /. Since a trace amount of oxygen or moisture in the electrolytic solution lowers the performance of the battery, it is necessary to sufficiently purify the solvent and the electrolyte according to a conventional method. In the present invention, if necessary, a porous membrane made of synthetic resin such as polyethylene, polypropylene, Teflon or the like, or a natural fiber may be used as a diaphragm between the two electrodes.

Example 1 As shown in FIG. 1, a positive electrode 4 was obtained by bundling 10 mg of pitch-based carbon fibers 1 heat-treated at 2900 ° C. with a platinum wire 2 having a diameter of 0.1 mm.

The charge / discharge characteristics were measured by pressing a sheet of 10 to 20 mg of metallic lithium on a nickel mesh as shown in Fig. 2 to form a counter electrode (negative electrode) 3 and a small piece of the same metal lithium sheet as the counter electrode (negative electrode) into a nickel wire. The connected electrode was placed in a glass container 7 as a reference electrode 5, and a triode cell using a solution obtained by dissolving LiBF ₄ at a concentration of 2 mol / in propylene carbonate as an electrolytic solution 6 was placed in a glass container 7.
Argon (Ar) gas 8 was introduced, the sample was placed in a closed container in an inert atmosphere, and the potential of the positive electrode with respect to the reference electrode when charge and discharge were repeated at a constant current was measured.

The carbon fiber used for the positive electrode controls the graphitization in the raw material stage and in the manufacturing process, and achieves the degree of graphitization required for the positive electrode. The indices are shown in Table 1. Charge / discharge current value is 10mA
The amount of electricity passed during charging is 300 coulombs / g of carbon material.

As a result of charging and discharging under these conditions, the electricity efficiency of the discharge was at most 50% in the first cycle, but was stable and almost 95% after the second cycle. The charge / discharge potential is very noble, 4.6-4.7
Charging is started from V, and the potential rises with an increase in the amount of charge, and the final potential when charging is completed reaches 5V. The potential change at the time of discharging is almost the same as that at the time of charging, and as the discharging progresses, the potential gradually decreases and drops sharply at 4 V to finish discharging. Thus, the potential available for discharge is in the range of 4-5V. FIG. 3 shows a potential curve at the time of charging and discharging.

Comparative Example 1 Three kinds of carbon materials having different degrees of graphitization: quiche graphite, TORA
Y Company PAN-based carbon fiber M40, UCC Pitch-based carbon fiber P75S
Was measured under the same conditions as in Example 1.

The degree of graphitization of each carbon material by X-ray diffraction is shown in Table 1. Kish graphite has an almost ideal graphite structure with d = 3.354A. PAN-based carbon fiber M40 manufactured by TARAY
It belongs to the carbon fiber with the highest degree of graphitization among the system carbon fibers. The pitch-based carbon fiber P75S manufactured by UCC has a lower degree of graphitization than the carbon fiber used in Example 1 among pitch-based carbon fibers.

The PAN-based carbon fiber starts charging from 4.6 V, and the potential increases rapidly with the increase in the charged amount, and finally reaches 6.1 V. When switching to discharging, the potential drops sharply and hardly discharges with respect to the charged amount. This indicates that the PAN-based carbon fiber has the lowest graphitization degree among the three, and the anion doping reaction hardly occurs due to the low graphitization degree.

P75S starts charging from 4.9V, and the potential slowly increases with the increase in charge amount, and finally reaches 5.1V. The amount of discharge was about 25% of the amount of charge. P75S has a turbostratic structure, and its electricity efficiency is low because of the low degree of graphitization.

Kish graphite starts charging from 4.7V, and the potential increases stepwise as the charged amount increases, and finally reaches 5.0V. This stepwise change in potential, which had a discharge efficiency of about 50%, 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. Kish graphite has a graphitization degree which can be said to be ideal, but is not suitable for reversible electrode reactions such as doping and undoping reactions because both La and Lc are too large. La and Lc must not be too large for reversible reactions. The charge / discharge carp of these three is shown together with Example 1 in FIG.

From the results of Example 1 and Comparative Example 1, it was found that excellent charge / discharge characteristics can be obtained by using pitch-based carbon fibers having a specific degree of graphitization suitable for a positive electrode as in the present invention. You.

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

The carbon fiber used in the negative electrode controls the graphitization in the raw material stage and in the production process, and realizes the degree of graphitization required for the negative electrode. The index of the degree of graphitization of the carbon fiber by X-ray diffraction is shown in Table 2. The Raman spectrum of the used carbon fiber using an argon laser (wavelength: 5145A) as a light source reflects a graphite structure of 1580 cm as shown in FIG.
^-1 and turbostratic and structure coexist two peaks of 1360 cm ^-1 to reflect the ratio of the peak intensity of 1360 cm ^-1 to the peak intensity of 1580 cm ^-1 is 0.5 to 0.6.

The charge / discharge current value was 5 mA / g of carbon material, and the amount of electricity passed during charging was 1300 coulomb / g of carbon material.

The electricity efficiency of the discharge is 70 to 80% in the first cycle, but is stabilized to 93% or more after the second cycle. Regarding the charge, the charge starts charging around 0.5 V with respect to the lithium reference, and the change in the potential becomes very gentle from around 0.2 V, and eventually reaches almost the same potential as lithium metal. Reach. Regarding the discharge, the potential rises very slowly up to around 0.3 V on the basis of lithium, the potential starts to rise rapidly from 0.6 to 0.8 V, and the discharge ends. Therefore, if the range of the potential used in the discharge is set to 0.5 V, a considerably flat potential curve can be obtained. FIG. 5 shows changes in the potential of the negative electrode during charging and discharging.

Comparative Example 2 Kish graphite, a carbon material having a developed graphite structure, was measured under the same conditions as in Example 2.

The index of the degree of graphitization of Kish graphite by X-ray diffraction is also shown in Table 2. The ratio of the two peak intensities according to the argon laser Raman spectrum is almost zero.
From these indices, the degree of graphitization of quiche graphite is high.

Kish graphite with an ideal graphite structure starts charging at 1.0 V with respect to the reference electrode, and even if charging proceeds, the potential hardly changes, and a drop in the potential due to lithium ion doping is observed. Absent. Also, almost no discharge occurs.

This result indicates that the amount of electricity for charging is mostly spent on side reactions due to the high degree of graphitization, and that doping of lithium hardly progresses with quiche graphite. on the other hand,
In the carbon fiber of Example 2 having a turbostratic structure, the potential of the negative electrode was almost the same as that of lithium at a charge amount corresponding to C _6-8 Li, and it was found that the interlayer of the carbon fiber was saturated with lithium. Also, the discharge efficiency is high. This indicates that almost no side reactions occurred. The change in the potential of the negative electrode during charging and discharging is shown in FIG.

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

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

As a result of measuring the constant current charge / discharge, T300 starts charging from 11 V, and the potential decreases with the progress of charging, but the change is not smooth but often discontinuous.
Eventually, it reached 0.1V, but its charging efficiency was less than 10%.

This phenomenon indicates that doping of T300 with or without lithium is unlikely. Further, it lacks stability against repetition. Further, the pitch-based carbon fiber heat-treated at a low temperature draws a potential curve similar to that in Example 2 and lithium doping proceeds, but the discharge efficiency is extremely poor and lacks stability against repeated charge and discharge.

Here, in order to make a comparison between other carbon fibers having a turbostratic structure and carbon fibers having a turbostratic structure claimed by the present invention, the above two kinds of carbon fibers under the same conditions as in Example 2 were used. It is a measurement of carbon fiber.

The carbon fiber used in Example 2 was heat-treated at a very high temperature of 2900 ° C. and then realized a turbostratic structure as shown in Table 2. It is generally known that a pitch-based carbon fiber can realize a turbostratic structure only by lowering the heat treatment temperature.

It is also known that PAN-based carbon fibers also have a turbostratic structure. The experimental results of Example 2 and comparatively 3 show the following. That is, PAN-based carbon fiber T300 is
Since the degree of development of the carbon layer surface is low, the pitch-based carbon fiber that has been heat-treated at a low temperature without doping with lithium has a sufficiently developed carbon layer surface due to the high graphitizing property of the pitch used as the raw material. Although the doping is performed smoothly, the stability of the laminated structure against doping and undoping is lacking due to the low heat treatment temperature.

As described above, the following results were shown from the results of Example 2, Comparative Example 2, and Comparative Example 3. That is, in the carbon material used for the negative electrode,
A carbon material having a very developed crystal structure is unsuitable due to its high degree of graphitization, and a carbon material which is appropriately disordered and moderately graphitized, which is called a turbostratic structure, is suitable. Furthermore, among the turbostratic structures, carbon fibers having a turbostratic structure as defined in the present invention are most suitable, and those having a too low degree of graphitization are unsuitable.

Example 3 The negative electrode characteristics of the pitch-based carbon fiber and the PAN-based carbon fiber were examined in an electrolyte using sulfolane as an organic solvent.

Electrolyte in which carbon fiber bundled with a platinum wire was used as a sample electrode (negative electrode) 4 and lithium metal was used as a counter electrode 3 and a reference station 5 in the same manner as in Example 1 and LiBF ₄ was dissolved in sulfolane at a concentration of 1 mol /. The potential of the negative electrode with respect to the reference electrode when charging and discharging were repeated at a constant current was measured using the three-electrode cell constituted by No. 6.

The pitch-based carbon fibers used for the negative electrode were UCC P75S and P12
0X, PAN-based carbon fiber is M40 manufactured by TORAY. All three types of carbon fibers belong to the range of the graphitization degree of the negative electrode defined in the present invention. The index of the degree of graphitization of these carbon fibers by X-ray diffraction is shown in Table 4. The two peak intensity ratios in the argon laser Raman spectrum are P120X: 0.2 to 0.3, P75S: 0.5 to 0.6, and M40: 0.9 to 1.0.

The charge / discharge current value is 10 mA / g of carbon material, and the amount of electricity passed during charging is 1300 coulomb / g of carbon material.

Discharge electricity efficiency is the first cycle, P120X: 50
6060%, P75S: 60-70%, M40: 50-60%, but after several cycles, all three are stable to 9090% or more. Further, the potential change during charge and discharge is almost the same as in Example 2, which is a measurement in an electrolytic solution using propylene carbonate as an organic solvent. That is, charging starts at around 0.5 V with respect to the reference electrode, the potential decreases relatively sharply from 0.2 to 0.3 V, and the change in the potential becomes very gradual from that point, and finally becomes almost the same as the lithium metal. It reaches the same potential.

The change in potential during discharge was almost the same as in Example 2. In all three, the potential gradually rises after the start of discharge, and 0.6 to 0.8
The potential rises sharply from V. However, the discharge curve
A slight difference in the degree of graphitization of the three carbon fibers was observed.
That is, the pitch-based carbon fiber P120X having a high degree of graphitization has a relatively low discharge end potential and a small difference between the discharge start potential and the discharge end potential, whereas the PAN-based carbon fiber M40 having a low degree of graphitization has a low degree of graphitization.
Have a high discharge end potential, and this difference is large. FIG. 6 shows changes in the potential of the negative electrode during charging and discharging.

Here, in order to investigate the influence of the difference in the degree of graphitization of the carbon fiber on the negative electrode characteristics, the raw material of the carbon fiber raw material included in the range of the degree of graphitization of the carbon fiber suitable for the negative electrode specified in the present invention is used. It is an examination of the negative electrode characteristics of three different types of carbon fibers. 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 it can be determined whether or not the carbon fiber is suitable for the negative electrode by using the index of the degree of graphitization of the carbon fiber specified in the present invention regardless of the raw material.

Further, the carbon fiber suitable for the negative electrode as defined in the present invention has a certain width in the degree of graphitization,
All carbon fibers belonging to this range of graphitization degree exhibited excellent performance as a negative electrode, and it was found that the difference in graphitization degree appeared as a difference in discharge curve. That is, the discharge curve is flatter as the degree of graphitization of carbon fiber is higher and the change in the potential of the negative electrode with the progress of discharge is smaller, whereas the discharge curve is lower as the degree of graphitization of the carbon fiber is lower. The change in the potential of the negative electrode accompanying the progress of is large.

Example 4 A battery was prepared using the same carbon fiber as used in Example 1 as a positive electrode, the carbon fiber used in Example 2 as a positive electrode, and the same carbon fiber as used in Example 1 as an electrolyte. went.

The carbon fiber for the positive electrode is 40 mg, and the carbon fiber for the negative electrode is 10 mg. The current value is 10 mA / g of carbon material, and the charge amount is 100 coulomb / g of carbon material for the negative electrode and 250 coulomb / g of carbon material for the positive electrode.

Charging starts at 3.8V and rises to 4.6V. The discharge voltage varied from 4.3V to 3.5V. The charge / discharge was repeated 100 times, but no remarkable change was observed in the charge / discharge voltage curve. In addition, the electricity efficiency was high and stable at 90% or more. FIG. 7 shows the measurement results when charging and discharging were repeated 100 times.

Effect of the Invention As described above, the present invention relates to a lithium secondary battery using a carbon material for both a positive electrode and a negative electrode and utilizing a doping reaction of anions and lithium ions for an electrode reaction. The greatest feature of the present invention is the positive electrode,
The characteristics of the most suitable carbon material for the reaction of the negative electrode were determined, and this was expressed using an index of the degree of graphitization.
In the case of the positive electrode, it is realized by using pitch-based carbon fibers made of coal pitch or petroleum pitch, and in the case of the negative electrode, it is realized by using carbon fibers having a turbostratic structure. This secondary battery system enables a chargeable / dischargeable secondary battery with a very high voltage and light weight without using metallic lithium as an electrode, and is extremely useful.

[Brief description of the drawings]

FIG. 1 is a schematic view of a carbon fiber electrode. FIG. 2 is a schematic view of an apparatus for measuring electrode characteristics of carbon fibers. FIG. 3 is a characteristic diagram of constant current charging / discharging for a carbon fiber according to the present invention and a positive electrode of an existing carbon material. FIG. 4 is a characteristic diagram showing a Raman spectrum of the carbon fiber according to the present invention. FIG. 5 and FIG. 6 are characteristic diagrams of constant current charging / discharging for the carbon fiber according to the present invention and the negative electrode of the existing carbon material. FIG. 7 is a characteristic diagram of constant current charge / discharge of a battery using the carbon fiber according to the present invention for both electrodes. 1 ... carbon fiber, 2 ... platinum wire, 3 ... counter electrode, 4 ... sample electrode, 5 ... reference electrode, 6 ... electrolyte solution, 7 ... glass container, 8 ... Ar gas.

 ────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kenichi Fujimoto 1618 Ida, Nakahara-ku, Kawasaki-shi, Kanagawa Prefecture New Nippon Steel Corporation First Technical Research Institute (56) References JP-A-60-182670 (JP, A) JP-A-62-103991 (JP, A)

Claims (1)

(57) [Claims] 1. The method according to claim 1, wherein the average distance between the carbon layers is 3.40 A or less,
The size of the crystallites in the axial direction and the a-axis direction is 200
~ 800A, using a pitch-based carbon fiber of 200 ~ 1000A for the positive electrode, the average plane spacing of the carbon layer surface is 3.37 ~ 3.45A, c-axis direction, and the crystallite size in the a-axis direction, 40 ~ 500A, respectively , 40
In ～700A, and, 1360 cm to the peak intensity of 1580 cm ^-1 in the Raman spectrum using argon laser ^-1
A lithium secondary battery using carbon fibers for both electrodes, wherein carbon fibers having a peak intensity ratio of 0.2 to 1.0 are used for the negative electrode.