JP3322962B2 - Alkali ion adsorption / desorption material - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an alkali ion adsorbing / desorbing material, and more particularly, to an alkali ion adsorbing / desorbing material suitable for an electrode material for a secondary battery, particularly for an electrode material for a non-aqueous electrolyte secondary battery. .

[0002]

2. Description of the Related Art In recent years, downsizing of electronic equipment has been progressed, and accordingly, high energy density of batteries has been demanded, and various nonaqueous electrolyte batteries have been proposed. For example, conventionally, as a negative electrode for a non-aqueous electrolyte battery, lithium metal has been known mainly for a primary battery, and a lithium alloy represented by an aluminum / lithium alloy, a carbon negative electrode, and the like are also known.
However, when lithium metal is used as a negative electrode of a secondary battery, it is known that cycle stability is inferior due to generation of dendrites and the like. Further, a lithium alloy negative electrode represented by an aluminum / lithium alloy also has improved cycle stability compared to lithium metal, but does not sufficiently bring out the performance of a lithium battery.

[0003] In order to solve such a problem, it has been proposed to use a carbon negative electrode as a material for adsorbing and desorbing lithium, utilizing a fact that a carbon intercalation compound of lithium can be easily electrochemically formed. Carbon-based materials used in conventional Li secondary battery negative electrodes are classified into graphitizable carbon and hardly graphitizable carbon when classified by crystal structure. These classification methods are mainly based on the spacing of D (002) by X-ray diffraction, the crystallite size in the C-axis direction and the a-axis direction, and the ratio of the laminated structure to the turbostratic structure by laser Raman spectrum analysis. Classification methods are used. These two evaluation methods are based on a sufficiently carbonized carbon (calcination temperature 1,50).
0 ° C. or higher). Also, many patents published to date have been filed with carbon structures that define these numerical values.

[0004] There are many and various types of such carbon negative electrodes. For example, carbon materials obtained by calcining crystalline cellulose at 1,800 ° C under a nitrogen gas flow (Japanese Patent Application Laid-Open No. 3-1769693), Coal pitch or petroleum pitch which has been graphitized at 2,500 ° C. or higher in an inert atmosphere (JP-A-2-82466), 2,000 ° C.
Graphite treated at a high temperature exceeding the above is used, and although the capacity is reduced as compared with lithium metal or a lithium alloy, a material having cycle stability is obtained. However, even with such a negative electrode, sufficient cycle stability has not been obtained in charging and discharging at a high current density.

[0005]

Such a carbon anode has a sufficiently mild reaction with water even in a charged state, that is, in a state where lithium is intercalated into carbon, as compared with lithium metal or a lithium alloy. In addition, the formation of dendrites due to charge and discharge was hardly observed, and the product was excellent. However, depending on the type of carbon, one that can hardly be charged and discharged or the theoretical capacity (L
In many cases, the capacity is extremely low as compared to the state of iC ₆ (assuming the maximum capacity). In addition, even if the initial capacity is relatively large, it deteriorates due to repeated charging and discharging, and the capacity rapidly decreases, and even in a carbon anode having a relatively large capacity,
A conventional carbon negative electrode has not obtained a negative electrode having satisfactory performance. For example, a conventional carbon negative electrode has not been able to achieve satisfactory performance as a secondary battery due to severe deterioration due to repeated charging and discharging at a high current density.

For example, the characteristic of graphitizable carbon is that it is excellent in flatness of discharge potential, but its capacity is extremely reduced when the charge / discharge current density is increased (discussed in the 15th New Battery Design Subcommittee Meeting). Society p24-31). Therefore, its application is limited to applications having a relatively low current density, such as memory backup.

On the other hand, the characteristic of non-graphitizable carbon is that it can be charged and discharged at a higher current density than that of graphitizable carbon, although the flatness of the discharge potential is poor. However, it is not sufficient for applications requiring a large current, such as electric vehicles. This is because the charge / discharge reaction of these carbons
This is because it proceeds by the intercalation-deintercalation reaction of Li ions on the square mesh surface. Therefore, intrusion and desorption of Li ions between layers can proceed only from the edge of the crystal (a plane perpendicular to the stacking direction of the hexagonal mesh plane). In addition, the smaller the size of the crystal, the higher the reaction rate. Therefore, the charge and discharge are promptly increased because the crystal is large with carbon that can be evaluated by the conventionally proposed X-ray diffraction method or laser Raman spectrum analysis. It does not progress.

[0008] Further, there is another problem that the amount of occluded alkali ions is limited by the composition ratio of the intercalation compound, and the capacity cannot be increased. Furthermore, since the interlayer increases by about 10% during intercalation and returns to the original state during deintercalation, there is also a problem that repeated charging and discharging causes structural destruction of carbon and lowers cycle stability.

Therefore, the alkali ion occlusion amount is large,
There has been a demand for an alkali ion adsorbing / desorbing material which does not destroy its structure even after repeated occlusion / release and has a high occlusion / desorption reaction rate of alkali ions. The present invention has been made against the background of the above-mentioned conventional technical problems, and has a large alkali ion occlusion amount, no structural change of a compound capable of adsorbing and desorbing alkali ions, and an excellent occlusion / release reaction rate. The purpose is to obtain an alkali ion adsorbing / desorbing material, thereby obtaining a secondary battery electrode material which has a high capacity, excellent cycle stability, and can cope with high output (high current density) charge / discharge. I do.

[0010]

The present invention relates to a carbon-based material having a condensed aromatic structure and a disordered laminated structure, wherein the fractal dimension D is 1.7 ≦ D <2.0. When the material adsorbs an alkali ion and the amount of adsorption is C ₆ A (where A represents an alkali element) in the composition formula, the chemical shift δ of the NMR spectrum of the alkali nucleus is −3 ppm <δ <. It is intended to provide an alkali ion adsorbing / desorbing material characterized by being in the range of 10 ppm.

The carbon-based material of the present invention has a condensed aromatic structure, and the structure is a disordered laminated structure. Here, the disordered laminated structure refers to a structure in which the orientation of the condensed aromatic plane is completely disordered or has no orientation of more than three sheets. If the layered structure is developed instead of the disordered layered structure, alkali ions are intercalated between the layers, and cannot be said to be the adsorption / desorption material of the present invention.

As such a carbon-based material used in the present invention, those having a structure in a graphitization precursor stage are preferable. Since the carbon in the graphitization stage has a low heat treatment temperature, the crystal is still undeveloped, its size is small, and it is suitable for flowing a large current, and the theoretical limit value of the intercalation reaction is shown in the Examples. LiC ₆ (372 Ah / k
Some have a discharge capacity exceeding g). However, the characteristics of carbon in the graphite precursor stage also vary.
However, although carbon in the graphitization precursor stage has not been graphitized by X-ray diffraction and laser Raman spectra, it is difficult to sufficiently analyze its structure.
Therefore, in the present invention, as a method of analyzing the carbon in the graphite precursor stage in more detail, a lattice image is taken with a high-resolution transmission electron microscope (TEM), fractal analysis of various carbons is performed, and fractal dimension is obtained. D.

Here, the fractal theory will be described. That is, when measuring the length of a complicated and complicated line, when measuring with a long ruler and when measuring with a short ruler, the length of the line is longer when measured with a short ruler. Conversely, for a smooth line, changing the length of the ruler hardly changes the length of the line. That is, the more complicated the shape of the line, the more the length of the line depends on the length of the ruler to be measured.
Thus, as a measure of the complexity of the distance, when the length of the ruler is shortened, the rate at which the distance between the two points becomes longer can be expressed by a certain ratio. This ratio corresponds to the fractal dimension D and represents the complexity of the curve. The N-dimensional fractal dimension D is defined by the following equation. _{D = N + logE D / log} (l / r) ( where, r is a measure ratio. Shows the increase in the distance when changing the E _D = scale ratio) In the present invention, the fractal dimension as D, 1 . 7 ≦ D <2.0, good Mashiku is 1.8 ≦
It is a carbon-based material that satisfies D <2.0. A carbon-based material having a fractal dimension in this range can store much more Li than the intercalation reaction because part of Li forms covalent Li during charging.

Such a carbon-based material can be obtained by sintering an organic polymer compound which is essentially a graphitizable material, into which a factor that inhibits lamination has been introduced. And an organic polymer compound having an aromatic structure including at least one structure selected from the group consisting of an o-bond, an m-bond, a branched structure, and a cross-linked structure. The organic polymer compound which is essentially a graphitizable material may be any organic polymer compound as long as it has an aromatic structure. For example, poly (phenylene), poly (phenylenevinylene) (PPV), poly ( Phenylene xylene)
(PPX), polystyrene, novolak resin and the like. Factors that inhibit the laminated structure include a bent structure (o-, m-position bond), a branched structure (branched), and a cross-linked structure as described above. Further, these organic polymer compounds may include an organic polymer compound having between 5 and 7 members.

Among these organic polymer compounds, those having a low crystallinity are preferable, and the half value width of the diffraction peak near 2θ = 20 ° in X-ray diffraction is preferably 0.75 ° or more.
It is more preferable that the angle is 0.95 ° or more. The organic polymer compound preferably contains a quinoid structure. Here, including a quinoid structure means
This can be confirmed by observing the absorption edge in the range of 600 to 900 nm in the diffuse reflection spectrum of the organic polymer compound powder.

Preferred as such an organic polymer compound is polyphenylene having a high degree of polymerization. In the present invention, the R value defined by the following formula is applied as a measure of the degree of polymerization of polyphenylene,
Among them, the R value is preferably 2 or more, and more preferably 2.3 to 20. R = A [δ (para)] / ｛A [δ (mono1)]
+ A [δ (mono2)]｝ where A [δ (para)] is the absorbance of the absorption band in the infrared out-of-plane bending mode around 804 cm ⁻¹ in the infrared absorption spectrum, A [δ (mono1) ], A
[Δ (mono2)] is around 760 cm ^-1 and 6
The absorbance of the terminal phenyl group near 90 cm ^-1 is shown.

[0017] These organic polymer compounds are not so high in temperature and low in limit of electric conductivity, usually,
What is necessary is just to bake at 300-1000 degreeC. Such an organic polymer compound does not melt up to 700 ° C. during firing,
The sintering temperature (preferably 7) of the carbonaceous material which is the sintering product
(About 00 ° C.) is preferably 40% or more,
More preferably, it is desirable to be 70-90%.
Those that melt up to 700 ° C. are not preferred because they do not branch.

The alkali ion adsorption-desorption material of the present invention is a carbon-based material thus obtained, NMR when this material is adsorbed alkali ions only formula C ₆ A
When the chemical shift δ of the spectrum is −3 ppm <δ <10p
pm, preferably -1 ppm <δ <2 ppm. Here, when the alkali element is lithium, the observed nucleus is ⁷ Li, and the standard of the chemical shift is LiC
1 when the peak of 1 mol / l aqueous solution is 0. The fact that the chemical shift is within this range means that the unpaired electron density on the alkali ion is almost 0, which means that the alkali ion is occluded by adsorption rather than intercalation, This indicates that carbon and alkali ions are separated from each other (this is called "spoon compound" by the present inventor).

On the other hand, it has been reported that the intercalated lithium chemical shift takes a positive value (for example,
Tanaka et al., Proceedings of the 1992 Electrochemical Fall Conference p12
9) In the measurement of the present inventors, it has been found that the intercalated lithium chemical shift takes a positive value without exception. The cause of this positive chemical shift is a paramagnetic shift due to radicals in the carbon-based material. At the same time as lithium is absorbed, the carbon-based material receives electrons and maintains electrical neutrality. The carbon that has received the electrons becomes a radical anion. Therefore, a carbon-based material that has absorbed lithium has an unpaired electron, and a chemical shift occurs due to the interaction between the magnetic moment of the electron and the magnetic moment of the nucleus. According to theory, the sign of the chemical shift due to the interaction with the electron spin matches the sign of the hyperfine coupling constant between the electron and the nucleus.
⁷ the sign of the hyperfine coupling constant of Li is positive, a chemical shift of a radical is positive. The magnitude of the chemical shift is proportional to the unpaired electron density on the measurement nucleus. Since the intercalated lithium ion is confined between the graphite layers, when the graphite layer is converted into a radical anion, some unpaired electron density also exists on the lithium ion, resulting in a positive chemical shift.

As described above, those having a chemical shift exceeding the above range occlude alkali ions by intercalation and cannot be said to be the material of the present invention. The alkali ion adsorbing and desorbing material of the present invention is one that adsorbs and desorbs an alkali ion.
It demonstrates excellent performance. Although the alkali ion adsorbing / desorbing material of the present invention has a condensed aromatic, the laminated structure has not been developed and alkali ions such as lithium are not occluded by intercalation. And adsorbs and desorbs on the condensed aromatic surface. Since the adsorption / desorption reaction is not limited by the composition ratio of the intercalation compound, the amount of adsorption / desorption of alkali ions can be extremely increased.

Further, since alkali ions do not enter and exit between the layers, the structural change of the material due to adsorption and desorption is small, and since the laminated structure is not developed, the alkali ions are parallel to the laminating direction. It is not limited to one direction, but can penetrate from any direction, and the adsorption / desorption reaction speed is high. Therefore, by using such an alkali ion adsorbing / desorbing material of the present invention as an electrode material, the battery can have a very high capacity, cycle stability can be improved, and large current charging / discharging can be performed. Is very excellent as a material for an electrode for a secondary battery.

Next, the method for producing the alkali ion adsorbing / desorbing material of the present invention will be specifically described. The alkali ion adsorption / desorption material of the present invention is, as described above, an organic polymer compound having a condensed aromatic structure, usually argon,
In an inert gas such as helium or nitrogen, or a reducing gas such as hydrogen, 300 to 1,000 ° C., preferably 6 ° C.
At a temperature of 00 to 800 ° C, 0 to 6 hours, preferably 0 to
It is obtained by heat treatment for one hour.

As a specific heat treatment method, in thermal analysis, any rate of temperature increase may be used up to the temperature at which the weight of the organic compound as a raw material starts to decrease, and the temperature from the temperature at which the weight starts to decrease to 5% / hour. A method of raising the temperature at a rate of 200 ° C./hour, preferably 20 ° C./hour to 100 ° C./hour. Here, the heat treatment temperature refers to a temperature at which, in a thermal analysis, a weight loss of 70 to 95% by weight with respect to the weight reduced until the weight no longer decreases.

The heat-treated product thus obtained is usually a powder or a solid, and the alkali ion adsorbing / desorbing material is mechanically pulverized to obtain an excellent electrode material for a secondary battery. When a negative electrode is manufactured using this electrode material, the particle size of the electrode material is not necessarily limited. However, by using an electrode material having an average particle size of 5 μm or less, a high-performance negative electrode can be manufactured. In this case, a binder such as polyethylene powder is added to these powders, mixed, and rolled with a roll to produce a negative electrode. The amount of the binder is 5 to 5 with respect to 100 parts by weight of the electrode material.
0 parts by weight, preferably 5 to 30 parts by weight.

Here, any of organic and inorganic binders can be used as the binder. As the organic binder, in addition to the above-mentioned polyethylene, many binders such as a fluororesin such as polytetrafluoroethylene and polyvinylidene fluoride, polyvinyl alcohol, and polyvinyl chloride can be used. Further, as the inorganic binder, a silicon-based binder such as silicon glass can be used, but also in this case, a heat treatment at a temperature exceeding the melting point is required in order to exert the performance as the binder.

The negative electrode body thus obtained can be used as a negative electrode for a lithium battery by supporting lithium or an alkali metal mainly composed of lithium on the negative electrode body.

As a method of carrying the lithium metal, a lithium foil is contacted and thermally diffused, electrochemically doped with lithium in a lithium salt solution, or immersed in molten lithium to diffuse lithium into a carbon material. Any method conventionally used may be used. The alkali ion adsorbing and desorbing material of the present invention can be widely used as a negative electrode of a lithium battery, and is combined with various positive electrodes, for example, a positive electrode using an organic polymer such as oxides such as manganese dioxide and vanadium pentoxide or polypyrrole, and the like. Can be used.

The non-aqueous electrolyte used in the battery using the electrode material comprising the alkali ion adsorbing / desorbing material of the present invention is chemically stable with respect to the positive electrode material and the negative electrode material, and lithium ion is used. Any non-aqueous substance that can move to perform an electrochemical reaction with the positive electrode active material can be used, and in particular, a compound composed of a combination of a cation and an anion, and the cation is Li
⁺ And examples of anions include PF ₆ ⁻ , AsF ₆ ⁻ ,
SbF ₆ ^- Va group element halide anions such _{^{as, I -, I 3 -,}} Br -, Cl - halogen anion, ClO _4, such as ^- perchlorate anions such as, HF _{2
^{-, CF 3 SO 3 -,}} SCN - can be exemplified compounds having an anion such as, but not necessarily limited to these anions. Such a cation,
Specific examples of the electrolyte having an anion include LiPF ₆ ,
LiAsF ₆ , LiSbF ₆ , LiBF ₄ , LiClO
_{4, LiI, LiBr, LiCl,} LiAlCl 4, L
iHF ₂ , LiSCN, LiCF ₃ SO _{3 and the} like. Among these, in particular, LiPF ₆ , LiAsF
_{6, LiSbF 6, LiBF 4,} LiClO 4, LiC
F ₃ SO ₃ is preferred.

The non-aqueous electrolyte is usually used in a state of being dissolved in a solvent. In this case, the solvent is not particularly limited, but a solvent having a relatively large polarity is preferably used. Specifically, propylene carbonate, ethylene carbonate, tetrahydrofuran, 2-methyltetrahydrofuran, dioxolane, dioxane, dimethoxyethane, glymes such as diethylene glycol dimethyl ether, lactones such as γ-butyrolactone, phosphates such as triethyl phosphate, boric acid Borates such as triethyl, sulfolane, sulfur compounds such as dimethyl sulfoxide, nitriles such as acetonitrile, amides such as dimethylformamide and dimethylacetamide, dimethyl sulfate, nitromethane,
One or a mixture of two or more such as nitrobenzene and dichloroethane can be mentioned. Among these, propylene carbonate, ethylene carbonate, butylene carbonate, tetrahydrofuran, 2-
One or a mixture of two or more selected from methyltetrahydrofuran, dioxane, dimethoxyethane, dioxolane and γ-butyrolactone is preferred.

Further, as the non-aqueous electrolyte, the above-mentioned non-aqueous electrolyte is impregnated with a polymer such as polyethylene oxide, polypropylene oxide, a crosslinked isocyanate of polyethylene oxide, and a phosphazene polymer having an ethylene oxide oligomer in a side chain. Organic solid electrolytes, inorganic ion derivatives such as Li ₃ N and LiBCl _4, and inorganic solid electrolytes such as lithium glass such as Li ₄ SiO ₄ and Li ₃ BO ₃ can also be used.

A lithium secondary battery using the electrode material comprising the alkali ion adsorbing / desorbing material of the present invention will be described in more detail with reference to the drawings. That is, in the lithium secondary battery using the electrode material made of the alkali ion adsorbing and desorbing material of the present invention for the negative electrode, as shown in FIG.
Is a button-shaped positive electrode case 10 sealed with a negative electrode cover plate 20
The inside is partitioned by a separator 30 having micropores, and the positive electrode 50 in which the positive electrode current collector 40 is arranged on the positive electrode case 10 side is stored in the partitioned positive electrode side space, while the negative electrode current collector is stored in the negative electrode side space. The negative electrode 70 in which the negative electrode 60 is disposed on the negative electrode cover plate 20 side is housed.

As the separator 30, a nonwoven fabric, a woven fabric, a knitted fabric, or the like made of a synthetic resin such as polytetrafluoroethylene, polypropylene, or polyethylene, which is porous and through which an electrolytic solution can be passed or used, is used. can do. As the positive electrode material used for the positive electrode 50, fired particles such as lithium-containing vanadium pentoxide and lithium-containing manganese dioxide can be used. Reference numeral 80 denotes a polyethylene insulating packing which is provided around the inner peripheral surface of the positive electrode case 10 and insulates and supports the negative electrode cover plate 20.

[0033]

The alkali ion adsorbing / desorbing material of the present invention has a condensed aromatic structure, but the lamination structure is not developed, and alkali ions such as lithium do not intercalate between the layers, but condensed. Adsorbs and desorbs on aromatic surfaces. Since the adsorption / desorption reaction is not limited by the composition ratio of the intercalation compound, the amount of adsorption / desorption of alkali ions can be extremely increased. Therefore, by using this as an electrode material, the battery can have a very high capacity.

Further, since ions do not enter and exit between the layers, the structural change of the material due to adsorption and desorption is small, and when used as an electrode material, the battery has excellent cycle stability.
Furthermore, since the lamination structure is not developed, ions are not limited to one direction parallel to the lamination direction, but can penetrate from any direction, the reaction speed is high, and large current charging / discharging is possible. Therefore, by using the alkali ion adsorbing / desorbing material of the present invention as an electrode material of a secondary battery, particularly a nonaqueous electrolyte liquid secondary battery, for example, a lithium battery, it has high capacity and excellent cycle stability in charge and discharge, and A battery capable of withstanding high current density charge / discharge can be obtained.

[0035]

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples, but the present invention is not limited to these examples. Example 1 Polyphenylene was synthesized by the Kovacic method [Kovacic, P. et al. andKyriak
is, A. , Journal of the Ameri
can Chemical Society 85,4
54-458 (1963)]. That is, cupric chloride (CuCl ₂ ), aluminum chloride (anhydrous) (Al
Cl ₃ ) and benzene (C ₆ H ₆ ) in a molar ratio of 1: 1: 4.
And stirred under an inert gas atmosphere.
The obtained powder is washed several times with hydrochloric acid (6N-HCl), washed with water, further washed with acetone, and washed with water.
Vacuum dried at 0 ° C. The polyphenylene synthesized in this manner contains stacking inhibition factors such as a bent, branched structure and a quinoid structure.

The thus synthesized polyphenylene was fired at 700 ° C. in a hydrogen stream to obtain a carbon-based material. Specifically, the temperature is raised from room temperature to 500 ° C. at a rate of 250 ° C./hour, from 500 ° C. to 700 ° C. at 40 ° C./hour, and when the temperature reaches 700 ° C., the heating is stopped. After cooling to room temperature, a carbon-based material was obtained. The inter-plane distance of the (002) plane obtained by X-ray diffraction measurement of the obtained carbon-based material was about 3.65 °. The diffraction peak of the (002) plane is very wide, and the crystallite size in the C-axis direction is estimated to be 10 ° or less. That is, the number of stacked layers was three or less.

This material, and 370 Ah /
A lattice image was taken with a transmission electron microscope (TEM) while lithium ions corresponding to kg were charged. The result is shown in FIG. (A) is the material after charging, and (b) is after charging the material. The black part is the carbon net surface and the white part is the space.

FIG. 2 shows an FFT power spectrum of the distance between the carbon mesh planes and the direction of the gap. (A) is before charging, and (b) is after charging. The white point at the center is the origin, and the distance from the center indicates the distance between the carbon mesh planes.
The direction indicates the direction of the carbon net plane. It is clear that the direction of the carbon net surface is not regular because the white dots are ring-shaped. This state hardly changes after charging. FIG. 3 shows the distribution of the vertical component of the FFT power spectrum. (A) is before charging, and (b) is after charging. The surface spacing before and after charging is not widened.

To this material, 30% by weight of a polyethylene powder was added as a binder, and the mixture was pressed and formed into an electrode.
This electrode was used as a working electrode, lithium foil was used as a counter electrode and a reference electrode, and PC (propylene carbonate) and DME (dimethoxyethane) (weight ratio = 1: 1) were used as electrolytes.
A half-battery cell was formed by using a solution prepared by dissolving lithium perchlorate at a concentration of 1 mol / l in the above solvent. Using this cell, a current density of 1.6 mA / cm ^{2 and} a final charge potential of 0
The charging and discharging were repeated at mV, a discharge termination potential of 3 V, and a rest time of 20 minutes. This electrode electrochemically absorbed and released lithium ions and functioned as a negative electrode for a lithium secondary battery.

(Capacity) FIG. 4 shows a charge / discharge potential curve. It shows a discharge capacity of 405 Ah / kg. FIG. 5 schematically shows the relationship between the number of layers and the capacity. As can be seen from this figure, this capacity is 372 Ah / k, which is the limit of the intercalation reaction.
g. Further, considering that the number of laminated layers estimated from the X-ray diffraction measurement is at most three, the capacity limit of the intercalation reaction is 248 Ah / kg.
And the high capacity of 405 Ah / kg cannot be explained by the intercalation reaction.

(NMR) 30 Ah / kg, 1
Samples charged at 50 Ah / kg and 300 Ah / kg were prepared, and the solid-state NMR spectrum of the stored lithium was measured. The measurement sample was washed and dried with DME in a dry argon atmosphere glove box, and after removing the electrolytic solution, mixed with KBr and filled in an NMR sample tube. Observation nucleus is ⁷ Li. As a standard of the chemical shift, the peak of a 1 mol / l aqueous solution of LiCl was set to 0 ppm. The spectrum is shown in FIG. As is clear from FIG. 6, the chemical shift is 0 ppm regardless of the amount of stored lithium. About 60p
Peaks appearing symmetrically to the left and right of the maximum peak at pm intervals are spinning sidebands.

[0042] (current density dependence) to the electrode, were charged and discharged by changing the range of the current density of ^{1mA / cm 2 ~15mA / cm 2} , it was determined potential curve. FIG. 7 shows the results. At 15 mA / cm ² , noise appears to be caused by deposition of metallic lithium, but even at a high current density of 10 mA / cm ² , metallic lithium is not precipitated and charging and discharging are possible.

Morita et al. Reported the discharge capacity of various carbon-based materials when the charge / discharge current density was changed. [Morita et al., 15th New Battery Design Working Group Discussion Papers p24-31.
(1992)]. The results are shown in Table 1 together with the measurement results of the inventor for the material of Example 1.

[0044]

[Table 1]

Carbon generally has a higher D value as the graphite structure develops.
(002) becomes narrow, and Lc (the size of the crystallite in the c-axis direction) becomes large. As can be seen from Table 1, as the current density increases, the capacity of the carbon having a more developed graphite structure decreases more rapidly. On the contrary, the capacity of the material of Example 1 increases as the current density increases. This is because the lower the current density, the longer the time required for one cycle, the greater the amount of water entering the evaluation cell, resulting in a reduction in capacity. If the evaluation is performed under the condition that there is no intrusion of moisture, it is estimated that the discharge capacity is almost the same. As described above, the material of Example 1 can be charged and discharged without reducing the capacity even at a high current density.

COMPARATIVE EXAMPLE 1 1 mol / l EC (ethylene carbonate) of lithium perchlorate was used as an electrolyte, using porous carbon as an electrode.
A half-battery cell was constructed in the same manner as in Example 1 except that a + DME (weight ratio = 1: 1) solution was used, and a charge / discharge cycle test was performed. This electrode electrochemically inserted and extracted lithium ions and functioned as a negative electrode for a lithium secondary battery.

[0047] In this electrode, to produce a 200 Ah / kg charged samples were measured high-resolution solid-state NMR spectra of to ⁷ Li in the same manner as in Example 1. FIG. 8 shows the results. The maximum peak shows a chemical shift of 26.1 ppm.
The peak at 264.2 ppm is due to metallic lithium, which indicates that the porous carbon has undergone a lithium plating reaction in addition to the lithium intercalation.

(NMR) In addition to this comparative example, it has been reported that the chemical shift of intercalated lithium takes a positive value (for example, Tanaka et al., P. 129 of the 1992 Electrochemical Fall Conference Abstracts). Also, it has been found that the chemical shift of intercalated lithium takes a positive value without exception in the measurement by the inventors of materials other than the material of the present invention.

The cause of the positive chemical shift is a paramagnetic shift due to radicals of the carbon-based material. At the same time as lithium is absorbed, the carbon-based material receives electrons and maintains electrical neutrality. The carbon that has received the electrons becomes a radical anion. Therefore, a carbon-based material that has absorbed lithium has an unpaired electron, and a chemical shift occurs due to the interaction between the magnetic moment of the electron and the magnetic moment of the nucleus. According to theory, the sign of the chemical shift due to the interaction with the electron spin matches the sign of the hyperfine coupling constant of the electron and the nucleus. ⁷ L
The sign of the hyperfine coupling constant of i is positive, and the chemical shift due to the radical is positive. The magnitude of the chemical shift is proportional to the unpaired electron density on the measurement nucleus. Since the intercalated lithium ion is confined between the graphite layers, when the graphite layer is converted into a radical anion, some unpaired electron density also exists on the lithium ion, resulting in a positive chemical shift.

On the other hand, in the case of the carbon-based material of the present invention, since lithium ions are absorbed by adsorption rather than by intercalation, the distance between carbon and lithium can be large. Therefore, the unpaired electron density on lithium ions is almost zero, and the chemical shift is also zero.

Comparative Example 2 (Dependence on Current Density) Charge / discharge by changing the current density in the same manner as in Example 1 using a vapor grown carbon fiber (VGCF) subjected to graphitization at 2,600 ° C. as a negative electrode A potential curve was determined. FIG. 9 shows the results.
Shown in At a current density of 9 mA / cm ² , a flat portion of the potential occurred due to the deposition of metallic lithium at the time of charging, and at the time of discharging, an elution reaction of metallic lithium occurred first, followed by discharging from carbon.

[0052] From the above, ⁷ Li of solid resolution NM
The chemical shift of R is 0 irrespective of the amount of lithium occlusion.
From the fact that the material of the present invention exhibits a discharge capacity of 5 Ah / kg, it can be seen that the material of the present invention has a storage / release function by absorption / desorption different from the storage / release of lithium ions by the conventional intercalation reaction. Due to this new occlusion and release function, when the material of the present invention is used as an electrode material, the battery has a high charge / discharge capacity and can be charged / discharged at a high current density, and the structural change accompanying charge / discharge No cycle stability.

Example 2 In the same manner as in Example 1, polyphenylene was obtained by the cobatic method. Since the raw material monomer of the polyphenylene synthesized in this way is benzene, the bonding site is not limited to the para-position, and includes a bond, a branch, and a cross-linked structure at the ortho and meta positions. It is known that the quinoid structure is generated by the oxidizing power of the polymerization catalyst in the Cobatic method. The polymer of this example was brown, and an absorption edge was observed at 820 nm in the diffuse reflection spectrum of the powder. FIG. 10 shows the diffuse reflection spectrum. This is absorption by the quinoid structure.

In the infrared absorption spectrum of polyphenylene, an absorption band belonging to the CH out-of-plane bending vibration mode [δ (para)] near 804 cm ⁻¹ , 760 c
In the vicinity of m ^-1 and 690 cm ^-1 , the vibration modes [δ (mono 1)] and [δ [mono
2)] was observed. The absorbance of each absorption is represented by A [δ (para)] and A [δ
(Mono1)] and A [δ (mono2)], R = A [δ (para)] / ｛A [ω (mono1)]
The R value defined by + A [ω (mono2)]｝ is a measure of the degree of polymerization of polyphenylene, and this value was 6.09.

Further, as a result of X-ray diffraction, 2θ = 19.4
° shows a broad diffraction peak with a half-width of 2.372 °, and the crystallinity was low. The polyphenylene obtained by such a cobatic method was fired at 700 ° C. in a hydrogen stream to obtain a carbon-based material. By visual observation during calcination, the polymer did not melt until reaching 700 ° C. The firing yield was 84%.

A lattice image of this carbon-based material was taken at 400 kV by TEM (manufactured by JEOL Ltd.). The result is shown in FIG. This was input to a computer with an image scanner, and binarized to white for carbon and black for space. The result is shown in FIG. The fractal dimension was measured for all the lines in FIG. The result is shown in FIG. From this, the fractal dimension was 1.91.

A 30% by weight polyethylene binder was added to this carbon-based material, and an electrode having a diameter of 10 mm was produced by compacting . PC (propylene carbonate) / DME (dimethoxyethane) (volume ratio = 1: 1)
LiPF ₆ was dissolved at a concentration of 1 mol / l in the above solvent to prepare an electrolytic solution. Li metal was used for the counter electrode and the reference electrode. The evaluation was performed at a charge / discharge current density of 1.6 mA / cm ² , a charge termination potential of 0 V, and a discharge termination potential of 3 V. FIG. 14 shows a charge / discharge curve of this electrode. The fractal dimension D of this carbon is 1.91
And there is no regular stacking of carbon hexagonal meshes,
The discharge capacity became 695 Ah / kg. This is a value far exceeding the theoretical capacity of 372 Ah / kg of a conventionally known intercalation reaction between graphite layers.

Comparative Example 3 Poly (paraphenylene) obtained in the same manner as in Example 2
Was fired at 1,500 ° C. to obtain a carbon-based material. The fractal dimension D of this carbon-based material was 1.6, and it was not a disordered laminated structure. This carbon-based material was evaluated in the same manner as in Example 2. FIG. 15 shows a charge / discharge curve. In this carbon-based material, lithium is charged by an intercalation reaction because the laminated structure of carbon hexagonal mesh planes is regularly arranged to some extent. Therefore, its discharge capacity is 37
2 Ah / kg is the limit.

Comparative Example 4 A Grignard reagent was synthesized by reacting p-dibromobenzene and magnesium in equimolar amounts in tetrahydrofuran. A catalytic amount of nickel chloride-2,2'-bipyridine complex was added thereto and refluxed for 4 hours. The reaction mixture was dropped into dilute hydrochloric acid, stirred, and then filtered. The filtered solid was washed with a solvent in the order of distilled water, ethanol, hot toluene, and ethanol, and then dried at 80 ° C. under vacuum.
As a result, poly (paraphenylene) having only 100% bond at the para position and no branch or bridge was obtained.

Since this method does not use an oxidizing agent, no quinoid structure is formed. The powder of this polymer was light yellow, and the diffuse reflection spectrum had almost no absorption in the visible region, and the absorption edge was observed at 470 nm. FIG. 16 shows the diffuse reflection spectrum. The R value determined from the infrared absorption spectrum was 1.91. X-ray diffraction is 2θ = 1
At 9.6 °, a sharp diffraction peak with a half width of 0.72 ° is shown, indicating high crystallinity. This polymer was fired in the same manner as in Example 2. Melting was observed by visual observation during firing. The firing yield was 38%. The initial discharge capacity determined in the same manner as in the example was 150 Ah / kg.

[0061]

According to the alkali ion adsorbing / desorbing material of the present invention, the amount of occluded alkali ions can be greatly increased, and the structural change during adsorption / desorption can be eliminated. Further, the alkali ion storage / release reaction can be increased. Therefore, this material is very useful as an electrode material,
By using this, it is possible to obtain a secondary battery that has high capacity, excellent cycle stability, and can withstand charge and discharge at a high current density.

[Brief description of the drawings]

FIG. 1 is a photograph of a lattice image of the carbon-based material obtained in Example 1 observed by a transmission electron microscope (TEM).

FIG. 2 is a photograph of an FFT power spectrum in Example 1.

FIG. 3 is a graph showing a distribution of a vertical component of an FFT power spectrum according to the first embodiment.

FIG. 4 is a charge / discharge potential curve in Example 1.

FIG. 5 is a graph showing the relationship between the number of stacked layers and the capacity.

FIG. 6 is an NMR spectrum in Example 1.

FIG. 7 is a charge / discharge potential curve when the current density is changed in Example 1.

FIG. 8 is an NMR spectrum in Comparative Example 1.

FIG. 9 is a charge / discharge potential curve in Comparative Example 2.

FIG. 10 is a diffuse reflection spectrum of the carbon-based material powder obtained in Example 2.

FIG. 11 is a photograph of a lattice image of the carbon-based material obtained in Example 2 observed by a transmission electron microscope (TEM).

FIG. 12 is a photograph of a computer image obtained by binarizing a photograph of the lattice image in Example 2 with a computer.

FIG. 13 is a diagram illustrating an analysis result of a fractal dimension D according to the second embodiment.

14 is a diagram showing a charge / discharge curve of an electrode using a carbon-based material in Example 2. FIG.

FIG. 15 is a diagram showing a charge / discharge curve of an electrode using a carbon-based material in Comparative Example 3.

FIG. 16 is a diffuse reflection spectrum of the carbon-based material powder obtained in Comparative Example 4.

FIG. 17 is a front view including a partial cross-sectional view of a lithium secondary battery using the alkali ion adsorbing / desorbing material of the present invention for a negative electrode.

[Explanation of symbols]

 30 Separator 50 Positive electrode 70 Negative electrode

Continued on the front page (72) Inventor Atsushi Demachi 1-4-1 Chuo, Wako-shi, Saitama Pref. Inside Honda R & D Co., Ltd. (72) Inventor Morinobu Endo 615 Kitahara-cho, Suzaka City, Nagano Prefecture (56) References JP Akira 61-147456 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01M 4/58 H01M 4/02 C01B 31/00-31/36

Claims (5)

(57) [Claims] 1. A carbon-based material having a condensed aromatic structure and a disordered laminated structure, wherein a fractal dimension D is
When the material satisfying 1.7 ≦ D <2.0 adsorbs an alkali ion, and the adsorbed amount is C ₆ A (where A represents an alkali element) in the composition formula, NMR of the alkali nucleus is used.
The chemical shift δ of the spectrum is −3 ppm <δ <10 pp
m, an alkali ion adsorbing / desorbing material. 2. The alkali ion adsorbing / desorbing material according to claim 1, wherein the alkali ion is a lithium ion. 3. A carbon-based material obtained by calcining an organic polymer compound having an aromatic structure including at least one structure selected from the group consisting of an o-bond, an m-bond, a branched structure, and a cross-linked structure. The alkali ion adsorption / desorption material according to claim 1 or 2. 4. The organic polymer compound having an aromatic structure has a half value width of a diffraction peak at around 2θ = 20 ° in X-ray diffraction of 0.75 ° or more, and 700 ° C. during firing.
The alkali ion adsorbing / desorbing material according to claim 3, wherein the material is not melted by the time, and the yield of the fired product at a firing temperature is 40% or more. 5. An electrode material for a secondary battery comprising the alkali ion adsorbing / desorbing material according to claim 1. Description: