JP3466671B2 - Carbon compound and method for producing the same - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a carbon compound and a method for producing the same.

[0002]

2. Description of the Related Art It is known that a carbon compound having a graphite structure can intercalate different kinds of atoms and molecules between layers of the graphite structure. This intercalation improves the bond strength between layers of graphite structure of carbon compound,
Attempts have been made to control the chemical properties.

For example, alkali metal elements such as Li, alkaline earth elements such as Ba, metal chlorides such as AlCl ₃ and AsF ₅
Fluorides such, there are reports of such halogen molecule such as Br ₂ (MSDresselhaus and G.Dresselhaus, A
dvances in Physics, vol.30, pp139-326 (1981)). However, the atoms and molecules are limited to those exhibiting a strong donor property or acceptor property, and the intercalated carbon compound was unstable in the atmosphere.

[0004]

As described above, it is indispensable to improve the stability of a carbon compound having a graphite structure in practical use, and it has been desired to improve this point. The present invention has been made in consideration of the above points, and by intercalating a stable substance other than a molecule / atom exhibiting a strong donor property or an acceptor property, a stable carbon having an improved bonding force between layers is obtained. It is intended to provide a compound.

[0005]

Means and Actions for Solving the Problems The inventors of the present invention have made earnest studies as to whether a stable substance other than a molecule or an atom having a strong donor property or an acceptor property can be intercalated. Here, the elements such as Hg, Tl, Bi, and H are
Attention was paid to the fact that it can be intercalated by alloying with an alkali metal such as K, Rb, and Cs, and forms a ternary compound such as KBi _0.6 C ₄ . Although the ternary compound itself has poor chemical stability and is a material far from practical use, the present inventors have found that K can be excluded from this ternary compound.

That is, utilizing the difference in vapor pressure between K and Bi,
It was found that only K can be selectively separated by heating in vacuum. As a result, a carbon compound BiC ₄ having a graphite structure can be obtained.

Also, in the case of KHgC ₄ , the vapor pressure of Hg is high, so that K cannot be removed by vacuum heating unlike KBi _0.6 C ₄ . However, since the ion mobility in graphite is significantly higher in K than in Hg, only K can be selectively removed by immersing KHgC ₄ in K using an electrolyte solution of K.

By using an element that can be alloyed with an alkali metal, it is possible to intercalate this element into a carbon compound having a glite structure. Fe, Mn,
Iron group elements such as Co cannot intercalate because they do not form compounds with alkali metals.

Even when forming an alloy with an alkali metal, if the melting point of the alloy is high, the above-mentioned separation by heating cannot be performed. When an alloy having a high melting point is used, the above heat treatment causes reactions other than the desired reaction such as the formation of carbide.

Therefore, the elements that can be intercalated by the above-mentioned vacuum heating, electrochemical method, etc. are limited, and group IIb (Zn, Cd, Hg), group IIIb (Al, Ga, In, Tl), IVb group
(Ge, Sn, Pb), Vb group (Sb, Bi), VIb group (AS, Se, Te), precious metals such as Au and Ag, and elements such as H.

The carbon compound intercalated in this way is remarkably improved in stability as compared with the case of intercalating an element having a strong donor property or acceptor property as in the prior art. Further, since the bond between the graphite layers can be strengthened, the mechanical strength can be improved. Furthermore, it becomes possible to improve various physical and chemical properties by the intercalating element.

[0012]

EXAMPLES Examples of the present invention will be described below. (Example 1) First, Bi and K were vacuum-encapsulated in an equimolar ratio in a Pyrex tube and then heated to 370 ° C to form a BiK alloy.

Then HOPG graphite and the obtained BiK
The alloy and were vacuum sealed in a Pyrex tube and heated under the conditions of 370 ° C. × 2 days to synthesize Bi _0.6 KC _x . The amount of C was set to x because it can be 4, 8 or a mixture thereof depending on the conditions.

As a result of X-ray diffraction, the obtained Bi _0.6 KC _x was a mixture of an α phase having 9.87 A (angstrom) between graphite layers and a β phase having 10.86 A. Next this Bi
_0.6 KC _x was heated in a vacuum of 10 ^-3 mmHg at 250 ° C for 1 day to remove K and synthesize Bi _0.6 C ₈ .

This compound is a carbon compound having a graphite structure in which Bi is intercalated between layers,
It was stable enough at room temperature. (Example 2) Bi _0.6 CsC _x was synthesized in the same manner as in Example 1 except that Cs was used instead of K.

As a result of X-ray diffraction, this compound was found to be a mixture of 10.70 A α phase and 11.50 A β phase between the graphite layers. Next, this Bi _0.6 CsC _x was subjected to the same heat treatment as in Example 1 to remove Cs, whereby Bi _0.6 C ₄ powder could be synthesized.

An example of constructing a Li ion secondary battery using this Bi _0.6 C ₄ powder will be described. First, this Bi _0.6 C
₄ Press the powder on a stainless wire mesh to make a cathode. Next, LiCoO ₂ powder obtained by reacting lithium carbonate and cobalt carbonate at 1000 ° C. is similarly pressed on a stainless wire mesh to form an anode.

A LiClO ₄ / propylene carbonate solution was used as an electrolyte, and an anode and a cathode were arranged opposite to each other in a polytetrafluoroethylene tube via this electrolyte to form a cell.

For comparison, a similar cell was also prepared by using graphite powder instead of Bi _0.6 C ₄ powder. This discharge characteristic is shown in FIG. As is clear from the figure, when using the Bi _0.6 C ₄ powder, the discharge time is about 1.2 times longer and the voltage drop is less. Therefore, it can be seen that the storage energy density is significantly improved in Bi _0.6 C ₄ as compared with the case without intercalation.

The reason for this is that in ordinary graphite, Li ions intercalated in the cathode are saturated at a molar ratio of LiC ₆ , but in Bi _0.6 C ₄ they are saturated with LiBi _0.6 C ₄ , so that a larger amount of Li ions is generated. It is considered that intercalation becomes possible.

When Bi _0.6 C ₄ is used for the cathode, it is 20
It was confirmed that the characteristics were not deteriorated even after repeated charging and discharging, and the stability was excellent. It is considered that this is because the bond strength between the graphite layers is improved and thus the durability is improved.

As a cathode for a Li-ion secondary battery, a carbon cathode which has a high electromotive force and does not form dendrites has been attracting attention, but when graphite is used as a carbon electrode, it can be intercalated. Although there are problems such as a small amount of Li ions and lack of stability due to repeated charge and discharge, the carbon compound having a graphite structure in which Bi or the like is intercalated as in the present invention has such problems. The points can be eliminated.

However, the present invention is not limited to such use as an electrode material for such a Li-ion secondary battery, and can be expected to be applied to various electrode materials, structural materials and the like utilizing electric characteristics. (Example 3) Bi _0.6 obtained in the same manner as in Examples 1 and 2
Mixture of CsC _x and BiK alloy in vacuum at 250 ℃ × 1
Bi and Bi _0.6 C ₄ by heat treatment under the condition of one day
A mixture of

Then, Bi _0.6 C ₄ powder could be obtained by removing Bi by treating with dilute nitric acid. (Example 4) First, Ag and Li were vacuum-encapsulated in a tantalum tube at an equimolar ratio and then heated to 360 ° C to form a LiAg alloy.

Then HOPG graphite and the obtained LiA
The g-alloy was vacuum-sealed in a Pyrex tube and heated under the condition of 350 ° C x 1 day to obtain a graphite compound Li _0.6 Ag.
C ₆ was synthesized.

The obtained Li _0.6 AgC ₆ was finely pulverized in an argon dry box and then press-molded on a stainless wire mesh to obtain an electrode. Using this electrode as the anode,
Was used as a cathode, and an electrochemical cell using LiClO ₄ / propylene carbonate solution as an electrolyte was formed. By energizing this cell, Li was removed from Li _0.6 AgC ₆ .

When the compound formed on the anode was washed with distilled water and then chemically analyzed, the composition ratio (atomic ratio) of the graphite compound was Ag: C = 1: 6.35, and the residual L
i was 0.01% by weight or less.

Although the case of Bi and Ag has been shown in the above-mentioned embodiments, the present invention can be realized by the same processing even in the case of other elements. For example, the carbon compound according to the present invention can be produced by a vacuum heating / electrochemical method using the following alkali metal compounds.

Zn: Zn _x K Cd: Cd _x Li, Cd _x K Hg: Hg _x Li, Hg _x K, Hg _x Cs Al: Al _x Li Ga: Ga _x Li, Ga _x K In: In _x Li, _{In x K Tl: Tl x K} Ge: Ge x Li Sn: Sn x K Pb: Pb x Li Sb: Sb x K Bi: Bi x Li, Bi x K, Bi x Cs S: S x K Se: Se x K Te: Te _x K Au: Au _x Li, Au _x K Ag: Ag _x Li, Ag _x K H: KH For such compounds, vacuum is used when the vapor pressure of the intercalating element is smaller than that of the alkali metal. Heat treatment can be used, and when the ion mobility is low, energization treatment in an electrolyte can be used. Various conditions may be set as appropriate for each compound.

[0030]

As described above, according to the present invention, a carbon compound having a stable graphite structure can be obtained. For example, it is suitable as a cathode material for Li-ion secondary batteries, and it is a great place to contribute industrially.

[Brief description of drawings]

FIG. 1 is a characteristic diagram showing charge / discharge characteristics of a Li-ion secondary battery.

─────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-57-47883 (JP, A) JP-A-4-170310 (JP, A) JP-A-3-153511 (JP, A) Nobuatsu Watanabe, graphite layer Compound, Kyodo Shosha, September 1, 1986, p. 273-274, Go Nakashima, Graphite Intercalation Compound-Functions and Applications-, Ceramics, Japan, 1992, Vo. 27, No. 3, pp. 226-230 (58) Fields investigated (Int. Cl. ⁷ , DB name) C01B 31/04

Claims (2)

(57) [Claims] 1. A carbon compound having a graphite structure, wherein Bi atoms are intercalated. 2. A step of intercalating a low melting point alloy having an alkali metal as a constituent in a carbon compound having a graphite structure, and heat treating or intercalating the alkali metal element of the intercalated low melting point alloy in an electrolyte. And a step of removing the carbon compound from the carbon compound by the energization treatment.