JPH04237949A - Electrode material - Google Patents

Abstract

PURPOSE:To provide an electrode material with a large electrode capacity and excellent charge/discharge cycle characteristics when used for the negative electrode of a secondary battery. CONSTITUTION:A carbon material with the spectrum obtained by the Raman spectrum analysis via the argon ion laser light with the wavelength 5145Angstrom satisfying all (A), (B), (C) is used as a main component, where (A) has a peak (PA) with the half-value width 25cm<-1> or above in the range 1585-1620cm<-1>, (B) has a peak (PB) with the half-value width 20cm<-1> or above in the range 1350-1370cm<-1>, and (C) has the ratio IB/IA 1.5 or below of the intensity IB of PB against the intensity IA of PA.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrode material, and more particularly to an electrode material for secondary batteries that provides a large capacity and excellent charge/discharge characteristics. Furthermore, the present invention relates to an electrode material suitable for a negative electrode of a secondary battery whose active material is an alkali metal, particularly lithium metal.

0002

2. Description of the Related Art It has been proposed to use conductive polymers such as polyacetylene as electrodes for lithium secondary batteries. However, the amount of doping of Li ions in the conductive polymer,
That is, it lacks electrode capacity and stable charge/discharge characteristics.

[0003]Also, attempts have been made to use lithium metal in the negative electrode of lithium secondary batteries, but in this case the charge/discharge cycle characteristics become extremely poor.

That is, when the battery is discharged, lithium from the negative electrode body becomes Li ions and moves into the electrolyte solution, and during charging, these Li ions become metallic lithium and are deposited on the negative electrode body again, but this charge-discharge cycle When this process is repeated, the metal lithium deposited becomes dendrite-like. Since this dendrite-like metallic lithium is an extremely active substance, it decomposes the electrolyte, resulting in a disadvantage that the charge/discharge cycle characteristics of the battery deteriorate. As this continues to grow, the dendrite-like metal lithium electrodeposit eventually penetrates the separator and reaches the positive electrode body, causing a problem of short-circuiting. In other words, a problem arises in that the charge/discharge cycle life is short.

In order to avoid such problems, a carbonaceous material prepared by firing an organic compound is used as a carrier for the negative electrode.
Attempts have been made to support this with an alkali metal, particularly lithium, as an active material. By using such a negative electrode body, the charge/discharge cycle characteristics of the negative electrode have been dramatically improved, but on the other hand, the negative electrode molded body using this carbonaceous material as a support still has a satisfactory electrode capacity. It wasn't a big deal.

[0006]

[Problems to be Solved by the Invention] Against this technical background, an object of the present invention is to provide an electrode material for a negative electrode for a secondary battery, which has a large electrode capacity and excellent charge/discharge cycle characteristics. do.

[0007]

[Means for Solving the Problems] In order to solve the above-mentioned problems, the present inventors have conducted extensive research on negative electrodes, and as a result, a carbonaceous material having a Raman spectrum as described below has been found to be suitable for the above-mentioned purpose. The present invention was developed based on the discovery that this method is extremely effective.

That is, the electrode material of the present invention has a wavelength of 51
The spectra obtained by Raman spectrum analysis using 45 Å argon ion laser light are shown below (A) and (B).
) and (C), the main component of which is a carbonaceous material. (A) It has a peak (PA) with a half-width at half maximum of 25 cm-1 or more in the range of 1585 to 1620 cm-1. (B) It has a peak (PB) with a half-width at half maximum of 20 cm-1 or more in the range of 1350 to 1370 cm-1. (C) Strength I of PB relative to the strength IA of PA above
The ratio IB /IA of B is 1.5 or less.

The carbonaceous material used in the present invention has a wavelength of 514
In Ramans vector analysis using 5 Å argon ion laser light, it has the following spectral characteristics. Hereinafter, unless otherwise specified, spectra and peaks are Raman spectra under the above conditions.

First, in the range of 1585 to 1620 cm-1, there is a peak (PA) with a half-width at half maximum of 25 cm-1 or more.
has. This half-width at half maximum is represented by the half value (H/2) of the spectrum width (H) at the half value (I/2) of the peak intensity (I) at the peak of the spectrum as shown in FIG. The narrower the half-width at half maximum, that is, the sharper the peak, the more uniform the higher-order structure is.

The peak appearing near the region of 1585 to 1620 cm -1 described above is due to the crystal structure in which the aromatic ring network planes grow and form in layers. For graphite with a perfect crystal structure, the peak is at 1580 cm-
It is located at 1. The closer the crystal structure is to graphite, the closer the peak position is to 1580 cm.

As mentioned above, the spectrum of the carbonaceous material used in the present invention has a peak PA in the range of 1585 to 1620 cm-1. PA position is 1590-162
0 cm<-1> is preferable, 1595-1615 cm<-1> is more preferable, and 1600-1610 cm<-1> is still more preferable. The half width at half maximum of PA is 25cm- as mentioned above.
1 or more, preferably 25 to 70 cm, and 30 to 70 cm
60 cm<-1> is more preferable, and 35-55 cm<-1> is particularly preferable.

Second, the carbonaceous material used in the present invention has 1
In the range of 350 to 1370 cm-1, the half width at half maximum is 20 c
It has a peak (PB) of m-1 or more. The peak occurring in this region corresponds to a disordered amorphous structure, as opposed to the aforementioned crystalline structure formed by stacking the aromatic ring network planes, and is usually located at 1360 cm −1 .

The half width at half maximum of PB is 20c as mentioned above.
m-1 or more. 20 to 150 cm-1 is preferable,
25 to 125 cm is more preferable, and 30 to 115 cm
m-1 is more preferred, 40-110 cm-1 is particularly preferred, and 50-105 cm-1 is most preferred.

Thirdly, in the carbonaceous material used in the present invention, the ratio R between the intensity IA of the peak PA and the intensity IB of the peak PB is 1.5 or less. Here R is the following formula R
=1B/IA. R is preferably 0.5 to 1.5, more preferably 0.75 to 1.3.

Peak P of Raman spectrum of carbonaceous material
When A and PB satisfy both of the above conditions, an electrode using them as an electrode material has a particularly excellent balance between electrode capacity and charge/discharge cycle characteristics.

In addition to the Raman spectrum described above, the carbonaceous material used in the present invention preferably has the following characteristics.

That is, the hydrogen/carbon atomic ratio is preferably less than 0.15, more preferably less than 0.10, even more preferably less than 0.07, particularly preferably 0.05.
less than

Other atoms such as nitrogen, oxygen, and halogen atoms are present in this carbonaceous material in a proportion of preferably 7 mol % or less, more preferably 4 mol % or less, particularly preferably 2 mol % or less. You may do so.

[0020] Also, the interplanar spacing (d002) of the (002) plane determined by X-ray wide-angle diffraction is preferably 3.45 to 3.45.
70 Å, more preferably 3.45 to 3.67 Å, even more preferably 3.46 to 3.60 Å, particularly preferably 3
．． It is 47-3.58 Å.

Furthermore, the crystallite size Lc in the c-axis direction is preferably 10 to 150 Å, more preferably 12 to 7 Å.
The thickness is 0 Å, more preferably 15 to 50 Å, particularly preferably 16 to 40 Å, and most preferably 17 to 35 Å.

[0022] The carbonaceous material used in the present invention can have any shape such as granules or fibers, but granules are preferable. In the case of granules, the volume average particle size is preferably 30
0 μm or less, more preferably 0.2 to 200 μm, still more preferably 0.5 to 150 μm, particularly preferably 2
~100 μm, most preferably 5-30 μm.

[0023] Further, the particles of the carbonaceous material preferably have a specific surface area of 1 to 100 m as measured using the BET method.
2/g, more preferably 2 to 50 m2/g, still more preferably 3 to 30 m2/g.

Furthermore, it is preferable that the particles of the carbonaceous material have pores inside. The total pore volume and the average pore radius (described below) are determined by measuring the amount of gas adsorbed to the sample (or the amount of desorbed gas) using the constant volume method under several equilibrium pressures. It is determined by measuring the amount of gas present.

The total pore volume is the relative pressure P/PO = 0.99, assuming that the pores are filled with liquid nitrogen.
It is determined from the total amount of gas adsorbed in step 5. P: Vapor pressure of adsorbed gas (mmHg) PO: Saturated vapor pressure of adsorbed gas at cooling temperature (mmHg)

[0026]
From the amount of nitrogen gas adsorbed (Vads), the amount of liquid nitrogen filled in the pores (Vliq
) to determine the total pore volume. Vliq = (Patm ・Vads
・Vm)/RT (1)

00
27] Here, Patm and T are atmospheric pressure (k
gf/cm2) and the absolute temperature (K), and Vm is the molecular volume of the adsorbed gas (34.7 cm3/mol for nitrogen).
).

[0028] The carbonaceous material used in the present invention preferably has a total pore volume of 1.5 x 10-3 ml/g or more, determined as described above. More preferably, the total pore volume is 2.0 x 10-3 ml/g or more, and even more preferably 3.
It is 0x10-3 to 8x10-2 ml/g, particularly preferably 4.0x10-3 to 3x10-2 ml/g.

The average pore radius (γP) is calculated from (1) above.
It is calculated by using the following equation (2) from Vliq obtained from the equation and the specific surface area S obtained by the BET method. Note that it is assumed here that the pore is cylindrical. γP =2Vliq/S (2)

[0030] In this way, the average pore radius (γp) of the carbonaceous material determined from the adsorption of nitrogen gas is 8 to 100.
It is preferable that it is Å. More preferably 10 to 80 Å
, more preferably 12 to 60 Å, particularly preferably 1
It is 4 to 40 Å.

Furthermore, the carbonaceous material used in the present invention preferably has a pore volume of 0.0 as measured by a mercury porosimeter.
It is 5 ml/g or more, more preferably 0.10 ml/g or more, still more preferably 0.15 to 2 ml/g, particularly preferably 0.20 to 1.5 ml/g.

The carbonaceous material used in the present invention has a g value in the range of 1.970 to 2.020 determined from the first-order differential absorption curve of the electron spin resonance spectrum measured at 23°C, and a line width (ΔHpp) of 4. It is preferred to have a signal greater than Gauss or no absorption with a linewidth less than 4 Gauss. The line width of the signal is more preferably 5 Gauss or more, still more preferably 5 to 300 Gauss, particularly preferably 5 to 100 Gauss.

Furthermore, the carbonaceous material used in the present invention preferably has a true density of 1.70 g/ml or more, more preferably 1.73 to 2.25 g/ml, and still more preferably 1.75 to 2.15 g/ml. ml, particularly preferably 1.
78-2.10g/ml, most preferably 1.78-2.10g/ml
It is 2.00g/ml.

Furthermore, the carbonaceous material used in the present invention preferably exhibits a temperature of 70% when differential thermal analysis is performed in an air flow at a flow rate of 5 to 15 liters/g.min.
It has an exothermic peak at 0 to 1,000°C. The temperature of the exothermic peak is more preferably 730 to 950°C, still more preferably 750 to 900°C, and particularly preferably 800 to 900°C.
The temperature is 880°C.

The carbonaceous material of the present invention can be prepared by heating an organic compound at 300 to 3,000°C under a normal inert gas flow.
Preferably, it can be obtained by heating and decomposing at a temperature of 500 to 2,000°C and carbonizing it.

Examples of starting organic compounds include cellulose; phenolic resin; acrylic resins such as polyacrylonitrile and poly(α-halogenated acrylonitrile); polyvinyl chloride, polyvinylidene chloride,
Examples include any organic polymer compound such as halogenated vinyl resin such as chlorinated polyvinyl chloride; polyamideimide resin; polyamide resin; conjugated resin such as polyacetylene and poly(p-nynylene).

[0037] Also, a condensed ring formed by condensing two or more monocyclic hydrocarbon compounds with three or more members such as naphthalene, phenanthrene, anthracene, triphenylene, pyrene, chrysene, naphthacene, picene, perylene, pentaphene, and pentacene. or derivatives of the above compounds such as carboxylic acids, carboxylic acid anhydrides, and carboxylic acid imides; various pitches based on mixtures of the above compounds; indole, isoindole, quinoline, isoquinoline, quinoxaline, Heteromonocyclic compounds with 3 or more members such as phthalazine, carbazole, acridine, phenazine, and phenanthridine have at least 2
a fused heterocyclic compound formed by bonding one or more monocyclic hydrocarbon compounds with one or more three-membered rings or more; derivatives of the above compounds such as carboxylic acids, carboxylic acid anhydrides, and carboxylic acid imides Furthermore, aromatic monocyclic hydrocarbons such as benzene, toluene, and xylene, and their derivatives such as carboxylic acids, carboxylic acid anhydrides, and carboxylic acid imides, such as 1,2,4,5-tetracarboxylic acid, Mention may be made of dianhydrides or derivatives thereof such as diimides.

[0038] To explain the above-mentioned pitch in more detail,
Examples include ethylene heavy end pitch produced during the cracking of naphtha, crude oil pitch produced during the cracking of crude oil, coal pitch produced during the thermal cracking of coal, and asphalt cracked pitch produced during the cracking of asphalt. I can do it. Furthermore, these various pitches can be further heated under a flow of inert gas to form a mesophase pitch with a quinoline insoluble content of preferably 80% or more, more preferably 90% or more, and still more preferably 95% or more. .

Furthermore, aliphatic saturated or unsaturated hydrocarbons such as propane and propylene can also be used.

Alternatively, a carbonaceous material such as carbon black or coke as a starting source may be further heated to appropriately advance carbonization to obtain the carbonaceous material used in the present invention.

The carbonaceous material used in the present invention is usually mixed with a polymeric binder such as the one described below to form an electrode material, and then formed into the shape of an electrode. Examples of the polymer binder include the following.

■Resinous polymers such as polyethylene, polypropylene, polyethylene terephthalate, aromatic polyamide, and cellulose. ■ Rubbery polymers such as styrene/butadiene rubber, isoprene rubber, butadiene rubber, and ethylene/propylene rubber. ■ Styrene/butadiene/styrene block copolymer, its hydrogenated product, styrene/isoprene/styrene block copolymer,
Thermoplastic elastomeric polymers such as their hydrogenated substances. (2) Soft resinous polymers such as syndiotactic 1,2-polybutadiene, ethylene/vinyl acetate copolymer, propylene/α-olefin (carbon number 2 or 4 to 12) copolymer. ■ A polymer composition having ion conductivity for alkali metal ions, especially Li ions.

The above-mentioned ion conductive polymer composition (2) is a polymer compound such as polyethylene oxide, polypropylene oxide, polyepichlorohydrin, polyphosphazene, polyvinylidene fluoride, polyacrylonitrile, etc., containing lithium salt or lithium. A system in which an alkali metal salt as a main component is combined, or a system in which an organic compound having a high dielectric constant such as propylene carbonate, ethylene carbonate, or γ-butyrolactone is further blended therein can be used. The polyphosphazene preferably has a polyether chain, particularly a polyoxyethylene chain, in its side chain.

[0044] The ionic conductivity of such an ion-conductive polymer composition at room temperature is preferably 10-8S·c.
m-1 or more, more preferably 10-6 S.cm-1 or more, still more preferably 10-4 S.cm-1 or more, particularly preferably 10-3 S.cm-1 or more.

[0045] The carbonaceous material and the above-mentioned polymeric binder used in the present invention can be mixed in various forms. That is, a form in which both particles are simply mixed, a form in which a fibrous binder is mixed with particles of carbonaceous material, or a form in which a layer of binder is attached to the surface of particles of carbonaceous material, etc. can be mentioned.

When using a fibrous binder, the diameter of the fibers of the binder is preferably fibrils (ultrafine fibers) of 10 μm or less, more preferably 5 μm or less, and fibrid (tentacle-like ultrafine fibers). It is particularly preferable to have fibrils.

The mixing ratio of the carbonaceous material and the binder is preferably 0.00 parts by weight per 100 parts by weight of the carbonaceous material.
1 to 30 parts by weight, more preferably 0.5 to 20 parts by weight,
More preferably, it is 1 to 10 parts by weight.

Furthermore, a metal capable of forming an alloy with the active material, such as alkali metal, especially lithium, such as aluminum, can be mixed into the above mixture. Alternatively, an alloy consisting of such a metal and an alkali metal, particularly lithium, such as a lithium aluminum alloy, may be used in combination. Such a metal or alloy may be in the form of particles, a thin layer coated on the surface of the carbonaceous material, or contained within the carbonaceous material.

[0049] The blending ratio of such metals or alloys is as follows:
The amount of the metal or alloy is preferably 70 parts by weight or less, more preferably 5 to 60 parts by weight, still more preferably 10 to 50 parts by weight, and particularly preferably 15 to 40 parts by weight, based on 100 parts by weight of the carbonaceous material.

The carbonaceous material used in the present invention is a mixture with the above-mentioned binder; or a mixture of a metal capable of forming an alloy with the above-mentioned active material or an alloy of the active material and the metal. An electrode molded body can be obtained by forming an electrode material made of a mixture into an electrode shape by a method such as roll molding or compression molding. Alternatively, these components may be dispersed in a solvent and applied to a metal current collector or the like. The shape of the electrode molded body can be set arbitrarily, such as a sheet shape or a pellet shape.

[0051] An alkali metal, preferably lithium metal, as an active material is added to the electrode molded body thus obtained.
It can be carried prior to or during assembly of the battery.

Methods for supporting the active material on the carrier include chemical methods, electrochemical methods, and physical methods. For example, an electrode molded body is immersed in an electrolytic solution containing a predetermined concentration of alkali metal cations, preferably Li ions,
Further, a method of electrolytically impregnating the electrode molded body using lithium as a counter electrode and using the electrode molded body as an anode, a method of mixing an alkali metal powder, preferably lithium powder, in the process of obtaining the electrode molded body, etc. can be applied.

Alternatively, a method may be used in which the lithium metal and the electrode molded body are brought into electrical contact. In this case, it is preferable that the lithium metal and the carbonaceous material in the electrode molded body are brought into contact via the lithium ion conductive polymer composition.

[0054] The amount of lithium previously supported on the electrode molded body in this way is preferably 0.030 to 0.250 parts by weight, more preferably 0.
．． 060 to 0.200 parts by weight, more preferably 0.0
70 to 0.150 parts by weight, particularly preferably 0.075
~0.120 parts by weight or less, most preferably ~0.080
It is 0.100 parts by weight.

[0055] Electrodes using the electrode material of the present invention usually have
Used as the negative electrode of a secondary battery, facing the positive electrode with a separator in between.

[0056] The material of the positive electrode body is not particularly limited;
For example, it is preferably made of a metal chalcogen compound that releases or acquires alkali metal cations such as Li ions during charging and discharging reactions. Such metal chalcogen compounds include vanadium oxide, vanadium sulfide, molybdenum oxide, molybdenum sulfide, manganese oxide, chromium oxide, titanium oxide, titanium sulfide, and these. Examples include composite oxides and composite sulfides. Preferably Cr3O8, V2O5, V
6 O13, VO2, Cr2O5, MnO2,
TiO2, MoV2 O8; TiS2, V2 S5
, MoS2, MoS3, VS2, Cr0.25
V0.75S2, Cr0.5 V0.5S2, etc. Also, oxides such as LiCoO2 and WO3; CuS, Fe0.25V0.75S2, Na0.1
Sulfides such as CrS2; NiPS3, FePS3
Phosphorus, sulfur compounds such as VSe2, NbSe3
It is also possible to use selenium compounds such as.

[0057] Also, conductive polymers such as polyaniline and polypyrrole can be used.

The separator for holding the electrolytic solution is generally made of a material with excellent liquid holding properties, such as a nonwoven fabric made of polyolefin resin, and is made of propylene carbonate, 1
, 3-dioxolane, 1,2-dimethoxyethane, etc., LiClO4, LiBF4
, LiAsF6, LiPF6, or the like is impregnated with a non-aqueous electrolyte having a predetermined concentration.

Further, a solid electrolyte which is a conductor of alkali metal cations such as Li ions can be interposed between the positive electrode body and the negative electrode body.

[0060]

[Function] In the battery constructed in this way, active material ions are supported on the carrier in the negative electrode during charging.
During discharging, active material ions in the carrier are released, thereby progressing the electrode reaction of charging and discharging.

On the other hand, when a metal chalcogen compound is used in the positive electrode, active material ions are released into the positive electrode body during charging, and the active material ions are supported during discharging, so that the electrode reaction during charging and discharging progresses.

Alternatively, when a conductive polymer such as polyaniline is used for the positive electrode, the counter ions of the active material ions are supported on the positive electrode body during charging, and the counter ions of the active material ions are released from the positive electrode body during discharge. This causes the electrode reaction to proceed.

[0063] As described above, the battery reaction that accompanies charging and discharging of the battery progresses due to the combination of the electrode reactions of the positive electrode body and the negative electrode body.

[0064]

Effects of the Invention The electrode material of the present invention uses the above-mentioned carbonaceous material as a main component, so that when it is shaped into an electrode molded body and used as an electrode for a secondary battery,
A secondary battery with large electrode capacity and excellent charge/discharge cycle characteristics can be obtained.

[0065]

[Examples] The present invention will be explained below with reference to Examples and Comparative Examples. Note that the present invention is not limited to this example. In these examples, all parts are by weight.

Example 1 (1) Synthesis and Raman spectrum analysis of carbonaceous material Pyrene was set in an electric heating furnace and heated at 250°C/250°C under nitrogen gas flow.
The temperature is raised to 500℃ at a heating rate of 1 hour, and then further increased to 500℃.
It was held for 30 minutes.

[0067] The obtained carbonaceous material was subjected to Raman spectrum analysis. That is, using alcone ion laser light with a wavelength of 5145 Å, the optical slit width was set to 10 cm
Ramanance vectors were obtained using a spectrometer adjusted to . The spectrum has peaks at 1600 cm-1 and 1360 cm-1, and the half-width at half maximum of the former (PA) is 40 cm-1
, the half width of the reversal value of the latter (PB) was 60 cm. Furthermore, the ratio IB of both peak intensities (IA, IA)
/IA was found to be 0.90.

(2) Preparation of electrode molded body 7.5 parts of LiClO4 and 92.5 parts of polyphosphazene were added to 100 parts of dimethoxyethane and uniformly mixed and dissolved. However, the polyphosphazene used was composed of the following repeating units.

[0069]

[Chemical formula 1]

[0070] In addition, the carbonaceous material 83 obtained in (1)
2.5 parts were added and mixed to form a paste. This paste was applied to a nickel steel wire and dried, and the dimethoxyethane was completely volatilized until the electrode material had a thickness of 150 μm.
I got a sheet of This was punched out into a disk shape with a diameter of 2 cm to obtain an electrode molded body.

(3) Evaluation of Electrode The electrode molded body produced in (2) was set at the bottom of a cylindrical glass cell with a diameter of 2 mm to serve as one electrode. On top of this, a polypropylene separator impregnated with a propylene carbonate solution of LiClO4 at a concentration of 1 mol/liter is placed, and a lithium metal sheet is set on top of it as the other electrode, held down with a glass rod from above, and a simple battery is inserted. Configured.

Between the electrode made of the above-mentioned electrode molded body and the lithium metal electrode, charging and discharging were repeated between the potential of 1.5V and 0.01V by constant current charging at 2mA and constant current discharging at 2mA. Ta. 5th cycle and 20th cycle from the start of charging
The performance of the cycle was as shown in Table 1.

[0073]

[Table 1]

Comparative Example 1 (1) Synthesis and Raman Spectrum Analysis of Carbonaceous Materials Pyrene was set in an electric heating furnace and heated at 250° C. under nitrogen gas flow.
The temperature is raised to 500℃ at a heating rate of 1 hour, and then further increased to 500℃.
It was held for 30 minutes. Next, this was heated to 1,800°C at a heating rate of 200°C/hour, and then heated at 1,800°C for 3
It was held for 0 minutes.

Raman spectrum analysis of the obtained carbonaceous material was conducted in the same manner as in Example 1. The obtained spectrum had peaks at 1580 cm-1 and 1360 cm-1, and no peak in the range of 1585-1620 cm^1. The half width at half maximum of the peak at 1580 cm-1 is 17c
It was m-1.

(2) Preparation of electrode molded body An electrode molded body was produced in the same manner as in Example 1 except that the carbonaceous material obtained in (1) was used.

(3) Electrode Evaluation Using the electrode molded body produced in (2), a simple battery similar to that in Example 1 was constructed, and the same charging and discharging was repeated. The results were as shown in Table 1.

[Brief explanation of the drawing]

FIG. 1 shows a method for analyzing the half-width at half maximum of a peak in a Raman spectrum.

[Explanation of symbols]

I Peak intensity I/2
Half value H Half value width H/2 Half value half width

Claims (1)

[Claims] Claim 1: An electrode whose main component is a carbonaceous material whose spectrum obtained by Raman spectrum analysis using argon ion laser light with a wavelength of 5145 Å satisfies all of the following (A), (B), and (C): material. (A) It has a peak (PA) with a half-width at half maximum of 25 cm-1 or more in the range of 1585 to 1620 cm-1. (B) It has a peak (PB) with a half-width at half maximum of 20 cm-1 or more in the range of 1350 to 1370 cm-1. (C) Strength I of PB relative to the strength IA of PA above
The ratio IB /IA of B is 1.5 or less.